publications

9999

1. Oceanic control on western European climate variability during the Last Glacial

   Olivier Cartapanis, Edouard Bard, Suzanne A.G. Leroy, Manuel Chevalier, Damien Flas, and Devièse Thibaut

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2. Proxy record selection to refine the Holocene temperature reconstruction in Europe

   Jan Esper, Edurne Castillo, Manuel Chevalier, Tito Arosio, and Ulf Büntgen

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3. A multi-model approach to the spatial and temporal characterization of the African Humid Period

   William D. Gosling, Manuel Chevalier, Markus L. Fischer, Marjolein Holewijn, Jemma Finch, Graciela Gil-Romera, Trevor Hill, Alfred Houngnon, Michela Leonardi, Andrea Manica, and Stefanie Kaboth-Bahr

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4. Pollen-based Early Pleistocene East Asian Summer Monsoon Precipitation Variability from the Western Chinese Loess Plateau

   Zhengchuang Hui, Xiao Wei, Zhendong Xue, Xue Lu, Manuel Chevalier, Jun Zhang, Tingjiang Peng, and Peng Chen

   9999
5. Open-Access

   Global biome changes over the last 21,000 years inferred from model-data comparisons

   Chenzhi Li, Anne Dallmeyer, Jian Ni, Manuel Chevalier, Mateo Willeit, Andrej A. Andreev, Xianyong Cao, Laura Schild, Birgit Heim, and Ulrike Herzschuh

   9999

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6. Open-Access

   More is not always better: downscaling climate model outputs from 30 to 5-minute resolution has minimal impact on coherence with Late Quaternary proxies

   Lucy Timbrell, James Blinkhorn, Margherita Colucci, Michela Leonardi, Manuel Chevalier, Matt Grove, Eleanor Scerri, and Andrea Manica

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7. A ready-to-use version of the Eurasian Modern Pollen Database 2 (EMPD 2; Davis et al., 2020) for paleoclimatic reconstructions

   Jena Zumaque, Anne Vernal, Bianca Fréchette, Basil A.S. Davis, and Manuel Chevalier

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2025

1. Open-Access

   Eccentricity-driven glacial climate variability and its influence on speciation in the tropical Andes

   Manuel Chevalier, William D. Gosling, Henry Hooghiemstra, Olivier Cartapanis, Brian M. Chase, and Stefanie Kaboth-Bahr

   Quaternary Science Advances, 2025

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   This study presents a detailed pollen-based climate reconstruction from Lake Fúquene, Colombia, spanning the past 284,000 years and revealing significant changes in temperature and hydroclimate modes of variability, particularly between glacial periods. Our findings highlight the central role of eccentricity in modulating the influence of high-latitude glacial climate variability on the northern Andes. During periods of high eccentricity (∼70-260 ka), we conclude that the region was shielded from northern high-latitude glacial ice-sheet dynamics by an intensified Walker circulation. After ∼70 ka, a weakening of this zonal circulation allowed high-latitude, millennial-scale glacial climate variability to penetrate deeper into the tropics, influencing the position of the ITCZ and the rainbelt. This shift in climate dynamics led to increased regional millennial-scale climate variability. A detrended correspondence analysis (DCA) on fossil pollen data highlights a contemporaneous compositional shift previously masked by glacial-interglacial vegetation changes. The observed ecological shift coincides temporally with predictions of elevated speciation rates in mechanistic simulations. Combined, these results suggest a long-lasting environmental impact from glacial millennial-scale climate dynamics, supporting the hypothesis that increased environmental dynamism during glacial periods may trigger the speciation pump in the Northern Andes more than glacial inceptions or terminations. These findings underscore the evolving nature of regional climate drivers and their complex interplay with ecology, offering new insights into the role of climate variability in shaping biodiversity in this hotspot.

2. Palynological and geochemical evidence for East Asian monsoon evolution from the northern South China Sea during the middle to late Eocene

   Mengtian Gao, Li You, Gaowei Hu, Manuel Chevalier, Xiaohua Teng, Qi Feng, Yabing Chen, Feng Jin, Rui Yang, and Jingyu Zhang

   Palaeogeography, Palaeoclimatology, Palaeoecology, May 2025

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   The evolution of the East Asian monsoon (EAM) profoundly affected the climate and environment of southeast China. This paper presents middle to late Eocene palynological records from the WC36-A borehole in the northern South China Sea (SCS) to improve understanding of EAM evolution and its driving mechanism during this key time interval. Findings reveal that vegetated landscapes transformed from mixed evergreen and de­ ciduous broad-leaved forest in the middle Eocene to the mixed coniferous and broad-leaved forest ecosystem in the late Eocene. Furthermore, palynology-based CREST climate reconstructions suggest the existence of a sub­ tropical monsoon climate and a modern-like EAM in the northern SCS region since the middle Eocene. Coupled with chemical weathering proxies, our analysis reveals the three-stage monsoon climatic evolution from coolsubhumid conditions to warm-humid conditions and then returning to cool-subhumid conditions. Our study suggests that the uplift of the Central-Southern Tibetan Plateau drove the EAM enhancement in the middle to late middle Eocene. The weakened EAM was likely caused by global cooling in the early middle and late Eocene and further promoted by the global sea level fall in the late Eocene. Our study proposes different interaction processes between forcing factors and provides new insight from the northern SCS to reconstruct the EAM evolution during the middle to late Eocene.

2024

1. Open-Access

   Fire drives major Holocene vegetation shifts between subtropical and Mediterranean-type ecosystems: a case study from a biodiversity hotspot in South Africa

   Lynne J. Quick, Brian M. Chase, Manuel Chevalier, B. Adriaan Grobler, and Saúl Manzano

   Ecography, May 2024

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   Fire plays a pivotal role in driving ecological shifts between Mediterranean-type vegetation and subtropical ecosystems in South Africa. This study investigates long-term environmental dynamics and ecological regime changes at the Mediterranean-type vegetation /subtropical boundary using a 6000-year palaeoecological sequence from the Baviaanskloof – a region of South Africa characterized by high levels of biodiversity and climate dynamism. Combining fossil pollen and microcharcoal data from a rock hyrax Procavia capensis midden, we analyse vegetation responses to environmental changes. Our findings reveal that Mediterranean-type vegetation resilience prevailed until ca 2800 cal year BP when a major fire event triggered a transition to a subtropical thicket-dominated environment. This abrupt ecological turnover underscores the significance of fire as a major driver of vegetation change at the Mediterranean-type vegetation /subtropical boundary. Our study emphasizes the vulnerability of Mediterranean-type vegetation ecosystems to global environmental change, suggesting potential implications for similar biome boundaries worldwide. By integrating multi-proxy palaeoecological evidence, we gain insights into the resilience and vulnerability of these ecosystems, aiding in understanding future responses to climate change scenarios.

2. Open-Access

   Discrepancies in lacustrine bacterial lipid temperature reconstructions explained by microbial ecology

   Jie Liang, Manuel Chevalier, Keshao Liu, Amedea Perfumo, Mingda Wang, Haichao Xie, Juzhi Hou, Ulrike Herzschuh, and Fahu Chen

   Communications Earth & Environment, Dec 2024

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   Bacterial lipid branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a valuable tool for reconstructing past temperatures. However, a gap remains regarding the influence of bacterial communities on brGDGT profiles. Here, we identified two distinct patterns of brGDGTs from the surface sediments of 38 Tibetan Plateau lakes using an unsupervised clustering technique. Further investigation revealed that salinity and pH significantly change bacterial community composition, affecting brGDGT profiles and causing brGDGT-based temperatures to be overestimated by up to 2.7 ± 0.7 °C in haloalkaline environments. We subsequently used the trained clustering model to examine the patterns of bacterial assemblages in the global lacustrine brGDGT dataset, confirming the global applicability of our approach. We finally applied our approach to Holocene brGDGT records from the Tibetan Plateau, showing that shifts in bacterial clusters amplified temperature variations over timescales. Our findings demonstrate that microbial ecology can robustly diagnose and constrain site-specific discrepancies in temperature reconstruction.

3. Open-Access

   EUPollMap: the European atlas of contemporary pollen distribution maps derived from an integrated Kriging interpolation approach

   Fabio Oriani, Grégoire Mariethoz, and Manuel Chevalier

   Earth System Science Data, Dec 2024

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   Modern and fossil pollen data are widely used in paleoenvironmental research to characterize past environmental changes in a given location. However, their discrete and discontinuous nature can limit the inferences that can be made from them. Deriving continuous spatial maps of the pollen presence from point-based datasets would enable more robust regional characterization of such past changes. To address this problem, we propose a comprehensive collection of European pollen presence maps including 194 pollen taxa derived from the interpolation of pollen data from the Eurasian Modern Pollen Database (EMPD v2) restricted to the Euro-Mediterranean Basin. To do so, we developed an automatic Kriging-based interpolation workflow to select an optimal geostatistical model describing the spatial variability for each taxon. The output of the interpolation model consists of a series of multivariate predictive maps of Europe at 25 km scale, showing the occurrence probability of pollen taxa, the predicted presence based on diverse probability thresholds, and the interpolation uncertainty for each taxon. Combined visual inspections of the maps and systematic cross-validation tests demonstrated that the ensemble of predictions is reliable even in data-scarce regions, with a relatively low uncertainty, and robust to complex and non-stationary pollen distributions. The maps, freely distributed as GeoTIFF files (https://doi.org/10.5281/zenodo.10015695, Oriani et al., 2023), are proposed as a ready-to-use tool for spatial paleoenvironmental characterization. Since the interpolation model only uses the coordinates of the observation to spatialize the data, the model can also be employed with fossil pollen records (or other presence/absence indicators), thus enabling the spatial characterization of past changes, and possibly their subsequent use for quantitative paleoclimate reconstructions.

2023

1. Open-Access

   Refining data–data and data–model vegetation comparisons using the Earth mover’s distance (EMD)

   Manuel Chevalier, Anne Dallmeyer, Nils Weitzel, Chenzhi Li, Jean-Philippe Baudouin, Ulrike Herzschuh, Xianyong Cao, and Andreas Hense

   Climate of the Past, 2023

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   Comparing temporal and spatial vegetation changes between reconstructions or between reconstructions and model simulations requires carefully selecting an appropriate evaluation metric. A common way of comparing reconstructed and simulated vegetation changes involves measuring the agreement between pollen- or model-derived unary vegetation estimates, such as the biome or plant functional type (PFT) with the highest affinity scores. While this approach based on summarising the vegetation signal into unary vegetation estimates performs well in general, it overlooks the details of the underlying vegetation structure. However, this underlying data structure can influence conclusions since minor variations in pollen percentages modify which biome or PFT has the highest affinity score (i.e. modify the unary vegetation estimate). To overcome this limitation, we propose using the Earth mover’s distance (EMD) to quantify the mismatch between vegetation distributions such as biome or PFT affinity scores. The EMD circumvents the issue of summarising the data into unary biome or PFT estimates by considering the entire range of biome or PFT affinity scores to calculate a distance between the compared entities. In addition, each type of mismatch can be given a specific weight to account for case-specific ecological distances or, said differently, to account for the fact that reconstructing a temperate forest instead of a boreal forest is ecologically more coherent than reconstructing a temperate forest instead of a desert. We also introduce two EMD-based statistical tests that determine (1) if the similarity of two samples is significantly better than a random association given a particular context and (2) if the pairing between two datasets is better than might be expected by chance. To illustrate the potential and the advantages of the EMD as well as the tests in vegetation comparison studies, we reproduce different case studies based on previously published simulated and reconstructed biome changes for Europe and capitalise on the advantages of the EMD to refine the interpretations of past vegetation changes by highlighting that flickering unary estimates, which give an impression of high vegetation instability, can correspond to gradual vegetation changes with low EMD values between contiguous samples (case study 1). We also reproduce data–model comparisons for five specific time slices to identify those that are statistically more robust than a random agreement while accounting for the underlying vegetation structure of each pollen sample (case study 2). The EMD and the statistical tests are included in the paleotools R package (https://github.com/mchevalier2/paleotools, last access: 3 May 2023).

2. Regional but not global temperature variability underestimated by climate models at supradecadal timescales

   Thomas Laepple, Elisa Ziegler, Nils Weitzel, Raphel Hebert, Beatrice Ellerhoff, Patricia Schoch, Belen Martrat, Oliver Bothe, Eduardo Morena-Chamarro, Manuel Chevalier, Annika Herbert, and Kira Rehfeld

   Nature Geoscience, Nov 2023

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   Knowledge of the characteristics of natural climate variability is vital when assessing the range of plausible future climate trajectories in the next decades to centuries. The reliable detection of climate fluctuations on multidecadal to centennial timescales depends on proxy reconstructions and model simulations as the instrumental record extends back only a few decades in most parts of the world. Systematic comparisons between model-simulated and proxy-based inferences of natural variability, however, often seem contradictory. Locally, simulated temperature variability is consistently smaller on multidecadal and longer timescales than is indicated by proxy-based reconstructions, implying that climate models or proxy interpretations might have deficiencies. In contrast, at global scales, studies found agreement between simulated and proxy reconstructed temperature variations. Here we review the evidence regarding the scale of natural temperature variability during recent millennia. We identify systematic reconstruction deficiencies that may contribute to differing local and global model–proxy agreement but conclude that they are probably insufficient to resolve such discrepancies. Instead, we argue that regional climate variations persisted for longer timescales than climate models simulating past climate states are able to reproduce. This would imply an underestimation of the regional variability on multidecadal and longer timescales and would bias climate projections and attribution studies. Thus, efforts are needed to improve the simulation of natural variability in climate models accompanied by further refining proxy-based inferences of variability.

3. Multiple forcing on Late Miocene East Asian Summer Monsoon Precipitation Variability in NE Tibetan Plateau

   Zhengchuang Hui, Jia Liu, Manuel Chevalier, Xiao Wei, Peng Chen, Jun Zhan, Tingjiang Peng, and Xuewen Zhou

   CATENA, Feb 2023

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   Understanding how the East Asian Summer Monsoon (EASM) evolves at different time scales affords a valuable opportunity to reveal the interactions between the hydrosphere, land, oceans and atmosphere. However, the preQuaternary evolutionary history and the driving forces that controlled its variability in response to different boundary conditions remain enigmatic. Here, we focus on the late Miocene (~10.8 to 6.3 Ma) – a period of profound climatic and topographic changes in Asia – and present a quantitative reconstruction of EASM pre­ cipitation using the probabilistic CREST (Climate Reconstruction Software) method with a high temporal reso­ lution pollen record from the Tianshui Basin in NE Tibetan Plateau (TP). Our new EASM precipitation record shows a slowly decreasing long-term trend during the period of ~ 10.8–7.6 Ma, which was followed by a strengthening period from ~ 7.6 to 6.3 Ma with a large amplitude of precipitation variability. We argue the decrease and increase periods of EASM precipitation were primarily response to late Miocene global cooling and TP uplift after ~ 8 Ma, respectively. These results are supported by existing climate model simulations, wherein both global climate and paleotopography play key roles in regulating the long-term evolution of late Miocene EASM. On orbital time scales, the precipitation time series exhibit a dominant ~ 410 kyr eccentricity periodicity, with lower (higher) values intervals corresponding to eccentricity minima (maximum). The synchronous phase of the precipitation and eccentricity records indicate that the eccentricity exerts a dominant influence on the EASM precipitation cycles via its modulation of the precessional amplitude, and the period expansion and contraction of Antarctic ice sheet (AIS) also probably play an important role during that time. Our quantitative late Miocene EASM precipitation records provide new insight into late Miocene EASM precipitation evolution and its relation with global climate, paleotopography, and cryosphere.

4. Middle Miocene evolution of East Asian summer monsoon precipitation in the northeast part of the Tibetan Plateau based on a quantitative analysis of palynological records

   Zhengchuang Hui, Xiao Wei, Zhendong Xue, Xuerong Zhao, Manuel Chevalier, Xue Lu, Jun Zhang, Tingjiang Peng, Yingyong Chen, and Peng Chen

   Palaeogeography, Palaeoclimatology, Palaeoecology, Feb 2023

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   Characterized by elevated pCO2 levels and global warmth, the mid-Miocene climate is a valuable analogue for investigating how the East Asian Summer Monsoon (EASM) may evolve at different time scales. In this paper, we present a quantitative EASM precipitation record with a temporal resolution of ∼14 kyr during the mid-Miocene (∼15.97 to 13.64 Ma) by applying the probabilistic CREST (Climate Reconstruction Software) method to palynological records from the northeast part of the Tibetan Plateau. Reconstructed mid-Miocene EASM precipitation (∼860 mm) was almost twice that of today (∼450 mm), indicating much stronger EASM intensity. The reconstruction shows a gradual long-term decline on which was superimposed a stronger EASM period (∼15.97–14.54 Ma) followed by a relatively stable period (∼14.54–13.84 Ma) and a short period of reduced precipitation (∼13.84–13.64 Ma). The correspondence of EASM precipitation changes with the mid-Miocene climate optimum and west-east thermal gradients in equatorial Pacific suggests these two factors were the main driving forces for EASM evolution from ∼15.97 to 14.54 Ma, whereas the combined impact of global cooling and the northward shift of the Intertropical Convergence Zone is probably responsible for the slight decline of the EASM from ∼14.54 to 13.84 Ma. The weaker EASM between ∼13.84 and 13.64 Ma was most likely a response to the global significant cooling event Mi-3. On orbital time scales, the precipitation records exhibit a dominant ∼400 kyr periodicity, indicating EASM changes were mainly paced by eccentricity via the modulation of precessional amplitude, and the East Antarctic Ice Sheet variations were probably another important driving force.

5. Open-Access

   Regional pollen-based Holocene temperature and precipitation patterns depart from the Northern Hemisphere mean trends

   Ulrike Herzschuh, Thomas Böhmer, Manuel Chevalier, Anne Dallmeyer, Chenzhi Li, Xianyong Cao, Raphaël Hébert, Odile Peyron, Larisa Nazarova, Elena Y. Novenko, Jungjae Park, Natalia A. Rudaya, Frank Schlütz, Lyudmila S. Shumilovskikh, Pavel E. Tarasov, Yongbo Wang, Ruilin Wen, Qinghai Xu, and Zhuo Zheng

   Climate of the Past, 2023

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   A mismatch between model- and proxy-based Holocene climate change, known as the “Holocene conundrum”, may partially originate from the poor spatial coverage of climate reconstructions in, for example, Asia, limiting the number of grid cells for model–data comparisons. Here we investigate hemispheric, latitudinal, and regional mean time series and time-slice anomaly maps of pollen-based reconstructions of mean annual temperature, mean July temperature, and annual precipitation from 1908 records in the Northern Hemisphere extratropics. Temperature trends show strong latitudinal patterns and differ between (sub-)continents. While the circum-Atlantic regions in Europe and eastern North America show a pronounced Middle Holocene temperature maximum, western North America shows only weak changes, and Asia mostly shows a continuous Holocene temperature increase. Likewise, precipitation trends show certain regional peculiarities such as the pronounced Middle Holocene precipitation maximum between 40 and 50◦ N in Asia and Holocene increasing trends in Europe and western North America, which can all be linked with Holocene changes in the regional circulation pattern responding to temperature change. Given a background of strong regional heterogeneity, we conclude that the calculation of global or hemispheric means, which initiated the Holocene conundrum debate, should focus more on understanding the spatiotemporal patterns and their regional drivers.

6. Open-Access

   LegacyClimate 1.0: A dataset of pollen-based climate reconstructions from 2594 Northern Hemisphere sites covering the late Quaternary

   Ulrike Herzschuh, Thomas Böhmer, Chenzhi Li, Manuel Chevalier, Raphaël Hébert, Anne Dallmeyer, Xianyong Cao, Nancy H Bigelow, Larisa Nazarova, Elena Y Novenko, Jungjae Park, Odile Peyron, Natalia A Rudaya, Frank Schlütz, Lyudmila S Shumilovskikh, Pavel E Tarasov, Yongbo Wang, Ruilin Wen, Qinghai Xu, and Zhuo Zheng

   Earth System Science Data, 2023

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   Here we describe LegacyClimate 1.0, a dataset of the reconstruction of the mean July temperature (TJuly), mean annual temperature (Tann), and annual precipitation (Pann) from 2594 fossil pollen records from the Northern Hemisphere, spanning the entire Holocene, with some records reaching back to the Last Glacial Period. Two reconstruction methods, the modern analog technique (MAT) and weighted averaging partial least squares regression (WA-PLS), reveal similar results regarding spatial and temporal patterns. To reduce the impact of precipitation on temperature reconstruction, and vice versa, we also provide reconstructions using tailored modern pollen data, limiting the range of the corresponding other climate variables. We assess the reliability of the reconstructions, using information from the spatial distributions of the root mean squared error in the prediction and reconstruction significance tests. The dataset is beneficial for synthesis studies of proxy-based reconstructions and to evaluate the output of climate models and thus help to improve the models themselves. We provide our compilation of reconstructed TJuly, Tann, and Pann as open-access datasets at PANGAEA (https://doi.org/10.1594/PANGAEA.930512; Herzschuh et al., 2023a). The R code for the reconstructions is provided at Zenodo (https://doi.org/10.5281/zenodo.7887565; Herzschuh et al., 2023b), including the harmonized open-access modern and fossil datasets used for the reconstructions, so that customized reconstructions can be easily established.

2022

1. Open-Access

   crestr: an R package to perform probabilistic climate reconstructions from palaeoecological datasets

   Manuel Chevalier

   Climate of the Past, Apr 2022

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   Statistical climate reconstruction techniques are fundamental tools to study past climate variability from fossil proxy data. In particular, the methods based on probability density functions (or PDFs) can be used in various environments and with different climate proxies because they rely on elementary calibration data (i.e. modern geolocalised presence data). However, the difficulty of accessing and curating these calibration data and the complexity of interpreting probabilistic results have often limited their use in palaeoclimatological studies. Here, I introduce a new R package (crestr) to apply the PDF-based method CREST (Climate REconstruction SofTware) on diverse palaeoecological datasets and address these problems. crestr includes a globally curated calibration dataset for six common climate proxies (i.e. plants, beetles, chironomids, rodents, foraminifera, and dinoflagellate cysts) associated with an extensive range of climate variables (20 terrestrial and 19 marine variables) that enables its use in most terrestrial and marine environments. Private data collections can also be used instead of, or in combination with, the provided calibration dataset. The package includes a suite of graphical diagnostic tools to represent the data at each step of the reconstruction process and provide insights into the effect of the different modelling assumptions and external factors that underlie a reconstruction. With this R package, the CREST method can now be used in a scriptable environment and thus be more easily integrated with existing workflows. It is hoped that crestr will be used to produce the much-needed quantified climate reconstructions from the many regions where they are currently lacking, despite the availability of suitable fossil records. To support this development, the use of the package is illustrated with a step-by-step replication of a 790 000-year-long mean annual temperature reconstruction based on a pollen record from southeastern Africa.

2. A 25,000 year record of climate and vegetation change from the southwestern Cape coast, South Africa

   Lynne J. Quick, Brian M. Chase, Andrew S. Carr, Manuel Chevalier, B. Adriaan Grobler, and Michael E. Meadows

   Quaternary Research, Jan 2022

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   The southwestern Cape of South Africa is a particularly dynamic region in terms of long-term climate change. We analysed fossil pollen from a 25,000 year sediment core taken from a near-coastal wetland at Pearly Beach that revealed that distinct changes in vegetation composition occurred along the southwestern Cape coast. From these changes, considerable variability in temperature and moisture availability are inferred. Consistent with indications from elsewhere in southwestern Africa, variability in Atlantic Meridional Overturning Circulation (AMOC) was identified as a strong determinant of regional climate change. At Pearly Beach, this resulted in phases of relatively drier conditions (~24–22.5 cal ka BP and ~22–18 cal ka BP) demarcated by brief phases of increased humidity from ~24.5–24 cal ka BP and 22.5–22 cal ka BP. During glacial Termination I (~19–11.7 ka), a marked increase in coastal thicket pollen from ~18.5 to 15.0 cal ka BP indicates a substantial increase in moisture availability, coincident, and likely associated with, a slowing AMOC and a buildup of heat in the southern Atlantic. With clear links to glacial and deglacial Earth system dynamics and perturbations, the Pearly Beach record represents an important new contribution to a growing body of data, providing insights into the patterns and mechanisms of southwestern African climate change.

2021

1. An uncertainty-focused database approach to extract spatiotemporal trends from qualitative and discontinuous lake-status histories

   Gijs De Cort, Manuel Chevalier, Sallie L. Burrough, Christine Y. Chen, and Sandy P. Harrison

   Quaternary Science Reviews, Apr 2021

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   Changes in lake status are often interpreted as palaeoclimate indicators due to their dependence on precipitation and evaporation. The Global Lake Status Database (GLSDB) has since long provided a standardised synopsis of qualitative lake status over the last 30,000 14C years. Potential sources of un- certainty however are not recorded in the GLSDB. Here we present an updated and improved relational-database framework that incorporates uncertainty in both chronology and the interpretation of palae- oenvironmental data. The database uses peer-reviewed palaeolimnological studies to produce a consensus on qualitative lake-status histories, whose chronologies are revised and standardized through the recalibration of radiocarbon dates and the application of Bayesian age-depth modelling for strati- graphic archives. Quantitative information on absolute water-level elevation is preserved if available from geomorphological sources. We also propose a new probabilistic analytical framework that accounts for these uncertainties to reconstruct synoptic, integrated environmental signals. The process is based on a Monte Carlo algorithm that iteratively samples individual lake-status histories within the limits of their uncertainties to produce many possible scenarios. We then use Recursively-Subtracted Empirical Orthogonal Function analysis to extract dominant patterns of lake-status variability from these scenarios. As a proof of concept, we apply this framework to 67 sites in eastern and southern Africa whose lake- status histories cover part of the late Pleistocene and/or Holocene. We show that, despite the sometimes large temporal and interpretation uncertainties, and the inclusion of highly discontinuous lake-status time series, identifying the major known millennial-scale climatic phases during the last 20,000 years is possible. Our framework was also able to identify an antiphased response between the lake basins in eastern and interior southern Africa to these changes. We propose that our new database and meth- odology framework serves as a template for efficient lake-status data synthesis, encourages the incor- poration of lake-status data in palaeoclimate syntheses, and expands the possibilities for the use of such data in the evaluation of climate models.

2. Open-Access

   Temperature change in subtropical southeastern Africa during the past 790,000 yr

   Manuel Chevalier, Brian M. Chase, Lynne J. Quick, Lydie M. Dupont, and Thomas C. Johnson

   Geology, Jan 2021

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   Across the glacial-interglacial cycles of the late Pleistocene (∼700 k.y.), temperature variability at low latitudes is often considered to have been negligible compared to changes in precipitation. However, a paucity of quantified temperature records makes this difficult to reliably assess. In this study, we used the Bayesian method CREST (Climate REconstruction SofTware) to produce a 790,000 yr quantified temperature reconstruction from a marine pollen record from southeast Africa. The results reveal a strong similarity between temperature variability in subtropical Africa and global ice volume and CO2 concentrations, indicating that temperature in the region was not controlled by local insolation, but followed global trends at these time scales, with an amplitude of ∼4 °C between glacial minima and interglacial maxima. The data also enabled us to make an assessment of the impact of temperature change on pollen diversity, with results showing there is no link between glacial-age temperatures/CO2 and a loss of diversity in this record.

3. A modern analogue matching approach to characterize fire temperatures and plant species from charcoal

   S. Yoshi Maezumi, William D. Gosling, Judith Kirschner, Manuel Chevalier, Henk L. Cornelissen, Thilo Heinecke, and Crystal N.H. McMichael

   Palaeogeography, Palaeoclimatology, Palaeoecology, Sep 2021

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   Charcoal identification and the quantification of its abundance in sedimentary archives is commonly used to reconstruct fire frequency and the amounts of biomass burning. There are, however, limited metrics to measure past fire temperature and fuel type (i.e. the types of plants that comprise the fuel load), which are important for fully understanding the impact of past fire regimes. Here, we expand the modern reference dataset of charcoal spectra derived from micro-Fourier Transformed Infrared Spectroscopy (FTIR) and apply an analogue matching model to estimate the maximum pyrolysis temperature and the type of plant material burned. We generated laboratory-created reference charcoal from nine plant species that were heated to six temperature categories (100 °C increments between 200 °C–700 °C). The analogue matching approach used on the FTIR spectra of charcoal estimated the maximum pyrolysis temperatures with an accuracy of 57%, which improved to 93% when accuracy was considered ±100 °C. Model accuracy for the type of plant material burned was 38% at the species level, which increased to 67% when species were grouped into trait-based categories. Our results show that analogue matching is an effective approach for estimating pyrolysis temperature and the type of plant material burned, and we suggest that it can also be applied to charcoal found in palaeoecological records, improving our understanding of past fire regimes and fuel dynamics.

4. The resilience of Amazon tree cover to past and present drying

   Tyler Kukla, Anders Ahlström, S. Yoshi Maezumi, Manuel Chevalier, Zhengyao Lu, Matthew J. Winnick, and C. Page Chamberlain

   Global and Planetary Change, Jul 2021

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   The Amazon forest is increasingly vulnerable to dieback and encroachment of grasslands and agricultural fields. Threats to these forested ecosystems include drying, deforestation, and fire, but feedbacks among these make it difficult to determine their relative importance. Here, we reconstruct the central and western Amazon tree cover response to aridity and fire in the mid-Holocene—a time of less intensive human land use and markedly drier conditions than today—to assess the resilience of tree cover to drying and the strength of vegetation-climate feedbacks. We use pollen, charcoal, and speleothem oxygen isotope proxy data to show that Amazon tree cover in the mid-Holocene was resilient to drying in excess of the driest bias-corrected future precipitation projections. Experiments with a dynamic global vegetation model (LPJ-GUESS) suggest tree cover resilience may be owed to weak feedbacks that act to amplify tree cover loss with drying. We also compare these results to observational data and find that, under limited human interference, modern tree cover is likely similarly resilient to mid-Holocene levels of aridification. Our results suggest human-driven fire and deforestation likely pose a greater threat to the future of Amazon ecosystems than drying alone.

5. Miocene East Asia summer monsoon precipitation variability and its possible driving forces

   Zhengchuang Hui, Xuewen Zhou, Manuel Chevalier, Xiao Wei, Yanfang Pan, and Yingyong Chen

   Palaeogeography, Palaeoclimatology, Palaeoecology, Nov 2021

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   The East Asian summer monsoon (EASM) precipitation is vital to hydrology, ecology and societal activities in the densely populated region of East Asia. However, its long-term evolution history and driving forces during the relatively warm Miocene remain unclear, even conflicting in some intervals. Here, we present a new, and quantitative record of EASM precipitation during Miocene using the Bayesian approach of Climate Reconstruction Software (CREST) based on pollen flora from the Tianshui Basin located on the northwestern Tibetan Plateau (TP). The results demonstrate that a strong and relatively stable EASM precipitation period occurred during the Neogene in northern China at ~17.1–13.6 Ma, which was followed by a strong and gradual decreasing period between ~13.6 and 7.4 Ma. This trend was abruptly stopped at ~7.4 Ma with the beginning of a period of large amplitude precipitation increase. The comparison analysis reveals that the gradual decrease of EASM precipitation during the period of ~17.1–7.4 Ma was primarily controlled by the global cooling, whereas the significant increase period after ~7.4 Ma was mainly related to the late Miocene uplift of the TP, supporting climate model simulations, in which both the global temperature and palaeogeography play important roles in regulating the long-term evolution of EASM precipitation.

6. Open-Access

   An atlas of southern African pollen types and their climatic affinities

   Manuel Chevalier, Brian M. Chase, Lynne J. Quick, and Louis Scott

   Palaeoecology of Africa, Nov 2021

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   Interpretations of fossil pollen data are often limited to broad, qualitative assessments of past climatic and environmental conditions (e.g. colder vs. warmer, wetter vs. drier, open vs. closed landscape). These assessments can be particularly imprecise in regions such as southern Africa, where botanical biodiversity is high, and there exists an associated uncertainty regarding the climatic/environmental sensitivities of the plants contributing to a given pollen type. This atlas addresses this limitation by characterising the climate sensitivities of the 140 pollen morphotypes most often recorded in Late Quaternary palaeoecology studies in southern Africa, relying on their parent plant distributions as one of the basic factors that determine their presence. The atlas is designed as a suite of graphical diagnostic tools and photographs together with analyses of the modern geographical distribution of more than 22,000 plant species to identify their primary climatic sensitivities across southern Africa. Together, the elements included span the complete workflow from pollen identification through interpretation and climate reconstruction. The atlas can be accessed from https://doi.org/10.5281/zenodo.4013452.

2020

1. Open-Access

   Pollen-based climate reconstruction techniques for late Quaternary studies

   Manuel Chevalier, Basil A.S. Davis, Oliver Heiri, Heikki Seppä, Brian M. Chase, Konrad Gajewski, Terri Lacourse, Richard J. Telford, Walter Finsinger, Joël Guiot, Norbert Kühl, S. Yoshi Maezumi, John R. Tipton, Vachel A. Carter, Thomas Brussel, Leanne N. Phelps, Andria Dawson, Marco Zanon, Francesca Vallé, Connor Nolan, Achille Mauri, Anne Vernal, Kenji Izumi, Lasse Holmström, Jeremiah Marsicek, Simon Goring, Philipp S. Sommer, Michelle Chaput, and Dmitry Kupriyanov

   Earth-Science Reviews, Nov 2020

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   Fossil pollen records are well-established indicators of past vegetation changes. The prevalence of pollen across environmental settings including lakes, wetlands, and marine sediments, has made palynology one of the most ubiquitous and valuable tools for studying past environmental and climatic change globally for decades. A complementary research focus has been the development of statistical techniques to derive quantitative estimates of climatic conditions from pollen assemblages. This paper reviews the most commonly used statistical techniques and their rationale and seeks to provide a resource to facilitate their inclusion in more palaeoclimatic research. To this end, we first address the fundamental aspects of fossil pollen data that should be considered when undertaking pollen-based climate reconstructions. We then introduce the range of techniques currently available, the history of their development, and the situations in which they can be best employed. We review the literature on how to define robust calibration datasets, produce high-quality reconstructions, and evaluate climate reconstructions, and suggest methods and products that could be developed to facilitate accessibility and global usability. To continue to foster the development and inclusion of pollen climate reconstruction methods, we promote the development of reporting standards. When established, such standards should 1) enable broader application of climate reconstruction techniques, especially in regions where such methods are currently underused, and 2) enable the evaluation and reproduction of individual reconstructions, structuring them for the evolving open-science era, and optimising the use of fossil pollen data as a vital means for the study of past environmental and climatic variability. We also strongly encourage developers and users of palaeoclimate reconstruction methodologies to make associated programming code publicly available, which will further help disseminate these techniques to interested communities.

2. Open-Access

   The Eurasian Modern Pollen Database (EMPD), version 2

   Basil A. S. Davis, Manuel Chevalier, Philipp S. Sommer, Vachel A. Carter, Walter Finsinger, Achille Mauri, Leanne N. Phelps, Marco Zanon, Roman Abegglen, Christine M. Åkesson, Francisca Alba-Sánchez, R. Scott Anderson, Tatiana G. Antipina, Juliana R. Atanassova, Ruth Beer, Nina I. Belyanina, Tatyana A. Blyakharchuk, Olga K. Borisova, Elissaveta Bozilova, Galina Bukreeva, M. Jane Bunting, Eleonora Clò, Daniele Colombaroli, Nathalie Combourieu-Nebout, Stéphanie Desprat, Federico Di Rita, Morteza Djamali, Kevin J. Edwards, Patricia L. Fall, Angelica Feurdean, William J. Fletcher, Assunta Florenzano, Giulia Furlanetto, Emna Gaceur, Arsenii T. Galimov, Mariusz Gałka, Iria García-Moreiras, Thomas Giesecke, Roxana Grindean, Maria Angela Guido, Irina G. Gvozdeva, Ulrike Herzschuh, Kari L. Hjelle, Sergey Ivanov, Susanne Jahns, Vlasta Jankovska, Gonzalo Jiménez-Moreno, Monika Karpińska-Kołaczek, Ikuko Kitaba, Piotr Kołaczek, and 52 more authors

   Earth System Science Data, Oct 2020

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   The Eurasian (née European) Modern Pollen Database (EMPD) was established in 2013 to provide a public database of high-quality modern pollen surface samples to help support studies of past climate, land cover, and land use using fossil pollen. The EMPD is part of, and complementary to, the European Pollen Database (EPD) which contains data on fossil pollen found in Late Quaternary sedimentary archives throughout the Eurasian region. The EPD is in turn part of the rapidly growing Neotoma database, which is now the primary home for global palaeoecological data. This paper describes version 2 of the EMPD in which the number of samples held in the database has been increased by 60 % from 4826 to 8134. Much of the improvement in data coverage has come from northern Asia, and the database has consequently been renamed the Eurasian Modern Pollen Database to reflect this geographical enlargement. The EMPD can be viewed online using a dedicated map-based viewer at \urlhttps://empd2.github.io and downloaded in a variety of file formats at https://doi.pangaea.de/10.1594/PANGAEA.909130 (Chevalier et al., 2019).

3. Open-Access

   Asymmetric response of forest and grassy biomes to climate variability across the African Humid Period: influenced by anthropogenic disturbance?

   Leanne N. Phelps, Manuel Chevalier, Timothy M. Shanahan, Julie C. Aleman, Colin J. Courtney-Mustaphi, Christopher Albert Kiahtipes, Olivier Broennimann, Robert A. Marchant, John Shekeine, Lynne J. Quick, Basil A. S. Davis, Antoine Guisan, and Katie Manning

   Ecography, Aug 2020

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   A comprehensive understanding of the relationship between land cover, climate change, and disturbance dynamics is needed to inform scenarios of vegetation change on the African continent. Although significant advances have been made, large uncertainties exist in projections of future biodiversity and ecosystem change for the World’s largest tropical landmass. To better illustrate the effects of climate-disturbance-ecosystem interactions on continental-scale vegetation change, we apply a novel statistical multivariate envelope approach to subfossil pollen data and climate model outputs (TraCE-21ka). We target paleoenvironmental records across continental Africa, from the African Humid Period (AHP: c. 14,700-5,500 years BP) – an interval of spatially and temporally variable hydroclimatic conditions – until recent times, to improve our understanding of overarching vegetation trends and to compare changes between forest and grassy biomes (savanna and grassland). Our results suggest that although climate variability was the dominant driver of change, forest and grassy biomes responded asymmetrically: (1) the climatic envelope of grassy biomes expanded, or persisted in increasingly diverse climatic conditions, during the second half of the AHP whilst that of forest did not; (2) forest retreat occurred much more slowly during the mid to late Holocene compared to the early AHP forest expansion; and (3) as forest and grassy biomes diverged during the second half of the AHP, their ecological relationship (envelope overlap) fundamentally changed. Based on these asymmetries and associated changes in human land use, we propose and discuss three hypotheses about the influence of anthropogenic disturbance on continental-scale vegetation change.

4. Open-Access

   A global database of Holocene paleotemperature records

   Darrell Kaufman, Nicholas McKay, Cody Routson, Michael Erb, Basil Davis, Oliver Heiri, Samuel Jaccard, Jessica Tierney, Christoph Dätwyler, Yarrow Axford, Thomas Brussel, Olivier Cartapanis, Brian Chase, Andria Dawson, Anne Vernal, Stefan Engels, Lukas Jonkers, Jeremiah Marsicek, Paola Moffa-Sánchez, Carrie Morrill, Anais Orsi, Kira Rehfeld, Krystyna Saunders, Philipp S. Sommer, Elizabeth Thomas, Marcela Tonello, Mónika Tóth, Richard Vachula, Andrei Andreev, Sebastien Bertrand, Boris Biskaborn, Manuel Bringué, Stephen Brooks, Magaly Caniupán, Manuel Chevalier, Les Cwynar, Julien Emile-Geay, John Fegyveresi, Angelica Feurdean, Walter Finsinger, Marie-Claude Fortin, Louise Foster, Mathew Fox, Konrad Gajewski, Martin Grosjean, Sonja Hausmann, Markus Heinrichs, Naomi Holmes, Boris Ilyashuk, Elena Ilyashuk, and 43 more authors

   Scientific Data, Dec 2020

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   A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format.

2019

1. Orbital controls on Namib Desert hydroclimate over the past 50,000 years

   Brian M. Chase, Eva M. Niedermeyer, Arnoud Boom, Andrew S. Carr, Manuel Chevalier, Feng He, Michael E. Meadows, Neil Ogle, and Paula J. Reimer

   Geology, 2019

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   Despite being one of the world’s oldest deserts, and the subject of decades of research, evidence of past climate change in the Namib Desert is extremely limited. As such, there is significant debate regarding the nature and drivers of climate change in the low-latitude drylands of southwestern Africa. Here we present data from stratified accumulations of rock hyrax urine that provide the first continuous high-resolution terrestrial climate record for the Namib Desert spanning the past 50,000 yr. These data, spanning multiple sites, show remarkably coherent variability that is clearly linked to orbital cycles and the evolution and perturbation of global boundary conditions. Contrary to some previous predictions of southwestern African climate change, we show that orbital-scale cycles of hydroclimatic variability in the Namib Desert region are in phase with those of the northern tropics, with increased local summer insolation coinciding with periods of increased aridity. Supported by climate model simulations, our analyses link this to variations in position and intensity of atmospheric pressure cells modulated by hemispheric and land-sea temperature gradients. We conclude that hydroclimatic variability at orbital time scales is driven by the combined influence of direct low-latitude insolation forcing and the influence of remote controls on the South Atlantic anticyclone, with attendant impacts on upwelling and sea-surface temperature variations.

2. Extreme hydroclimate response gradients within the western Cape Floristic region of South Africa since the Last Glacial Maximum

   Brian M. Chase, Arnoud Boom, Andrew S. Carr, Manuel Chevalier, Lynne J. Quick, G. Anthony Verboom, and Paula J. Reimer

   Quaternary Science Reviews, 2019

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   The Cape Floristic Region (CFR) is one of the world’s major biodiversity hotspots, and much work has gone into identifying the drivers of this diversity. Considered regionally in the context of Quaternary climate change, climate stability is generally accepted as being one of the major factors promoting the abundance of species now present in the CFR. However, little direct evidence is available from the region, and responses to changes in global boundary conditions have been difficult to assess. In this paper, we present new high-resolution stable isotope data from Pakhuis Pass, in the species-rich western CFR, and contextualise our findings through comparison with other records from the region. Combined, they indicate clear, coherent changes in regional hydroclimate, which we relate to broader forcing mechanisms. However, while these climate change events share similar timings (indicating shared macro-scale drivers), the responses are distinct between sites, in some cases expressing opposing trends over very short spatial gradients (\textless50 km). We describe the evolution of these trends, and propose that while long-term (105 yr) general climatic stability may have fostered high diversity in the region through low extinction rates, the strong, abrupt changes in hydroclimate gradients observed in our records may have driven a form of allopatric speciation pump, promoting the diversification of plant lineages through the periodic isolation and recombination of plant populations.

3. Open-Access

   PaCTS 1.0: A Crowdsourced Reporting Standard for Paleoclimate Data

   Deborah Khider, J. Emile‐Geay, Nicholas P. McKay, Y. Gil, D. Garijo, V. Ratnakar, M. Alonso‐Garcia, S. Bertrand, Oliver Bothe, P. Brewer, A. Bunn, Manuel Chevalier, Laia Comas‐Bru, A. Csank, E. Dassié, K. DeLong, Thomas Felis, P. Francus, A. Frappier, W. Gray, S. Goring, Lukas Jonkers, M. Kahle, D. Kaufman, Natalie M. Kehrwald, B. Martrat, H. McGregor, J. Richey, A. Schmittner, Nick Scroxton, E. Sutherland, K. Thirumalai, K. Allen, F. Arnaud, Y. Axford, T. Barrows, L. Bazin, S. E. Pilaar Birch, E. Bradley, J. Bregy, E. Capron, Olivier Cartapanis, H.‐W. Chiang, Kim M. Cobb, M. Debret, R. Dommain, J. Du, K. Dyez, S. Emerick, Michael P. Erb, and 43 more authors

   Paleoceanography and Paleoclimatology, Oct 2019

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   The progress of science is tied to the standardization of measurements, instruments, and data. This is especially true in the Big Data age, where analyzing large data volumes critically hinges on the data being standardized. Accordingly, the lack of community-sanctioned data standards in paleoclimatology has largely precluded the benefits of Big Data advances in the field. Building upon recent efforts to standardize the format and terminology of paleoclimate data, this article describes the Paleoclimate Community reporTing Standard (PaCTS), a crowdsourced reporting standard for such data. PaCTS captures which information should be included when reporting paleoclimate data, with the goal of maximizing the reuse value of paleoclimate data sets, particularly for synthesis work and comparison to climate model simulations. Initiated by the LinkedEarth project, the process to elicit a reporting standard involved an international workshop in 2016, various forms of digital community engagement over the next few years, and grassroots working groups. Participants in this process identified important properties across paleoclimate archives, in addition to the reporting of uncertainties and chronologies; they also identified archive-specific properties and distinguished reporting standards for new versus legacy data sets. This work shows that at least 135 respondents overwhelmingly support a drastic increase in the amount of metadata accompanying paleoclimate data sets. Since such goals are at odds with present practices, we discuss a transparent path toward implementing or revising these recommendations in the near future, using both bottom-up and top-down approaches.

4. Open-Access

   straditize: Digitizing stratigraphic diagrams

   Philipp S. Sommer, Dilan Rech, Manuel Chevalier, and Basil A. S. Davis

   Journal of Open Source Software, Feb 2019

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   In an age of digital data analysis, gaining access to data from the pre-digital era – or any data that is only available as a figure on a page – remains a problem and an under- utilized scientific resource. Whilst there are numerous programs available that allow the digitization of scientific data in a simple x-y graph format, we know of no semi-automated program that can deal with data plotted with multiple horizontal axes that share the same vertical axis, such as pollen diagrams (see image below) and other stratigraphic figures that are common in the Earth sciences. Straditize (Stratigraphic Diagram Digitizer) (Sommer, 2019) fills this gap. It is an open-source program that allows stratigraphic figures to be digitized in a single semi-automated operation. It is designed to detect multiple plots of variables analyzed along the same vertical axis, whether this is a sediment core or any similar depth/time series. The program supports mixtures of many different diagram types, such as bar plots, line plots, as well as shaded, stacked, and filled area plots. Other features of straditize include text recognition to interpret the names of the different plotted variables, the automatic and semi-automatic recognition of picture artifacts, as well an automatic measurement finder to exactly reproduce the data that has been used to create the diagram. Straditize is written in the programming language Python and is available for Windows, Linux and MacOS. Being implemented in the visualization framework psyplot (Sommer, 2017) it provides an extensively documented graphical user interface for a point-and- click handling of the semi-automatic process, but can also be scripted or used from the command line. The visualization is based on matplotlib (Hunter, 2007) and most of the detection algorithms use image recognition functions from the scikit-image package (Walt et al., 2014) and numeric routines from scipy (Jones, Oliphant, Peterson, & others, 2001) and numpy (T. E. Oliphant, 2006).

5. Enabling possibilities to quantify past climate from fossil assemblages at a global scale

   Manuel Chevalier

   Global and Planetary Change, Jan 2019

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   The field of quantitative palaeoclimatology has made significant progress in the past decades. However, this progress has been spatially heterogeneous and strong discrepancies – both in terms of quality and density – exist between Europe and North America and the rest of the world. The need to balance this distribution of quantified records has never been stronger, and improving our understanding of past global climate is urgent in order to better evaluate and employ the predictions of climate models. In this paper, it is argued that this gap can be reduced by applying the climate reconstruction method CREST (Climate REconstruction SofTware) calibrated using the open-access GBIF (Global Biodiversity Information Facility) database, which contains hundreds of thousands modern distributions of numerous bio-indicator proxies (e.g. pollen, chironomids, foraminifers, etc.). Using the taxonomical diversity of the GBIF database, CREST can be used to reconstruct various climate and/or environmental parameters from assemblage variety of different records. Independent from the usual re- construction techniques using surface samples for calibration, the CREST/GBIF framework can also be used as an independent tool that can be easily and efficiently applied (1) to complete the global coverage of climate re- constructions, as exemplified with a precipitation reconstruction from Lake Van, Turkey, (2) to revisit existing data sets to obtain new quantitative reconstructions, and (3) to evaluate and/or refine reconstructions based on other methods. The application of this tool promises to foster advances in our understanding of the past climate variability of the Earth system at a global scale.

6. Modern drought conditions in western Sahel unprecedented in the past 1600 years

   Matthieu Carré, Moufok Azzoug, Paul Zaharias, Abdoulaye Camara, Rachid Cheddadi, Manuel Chevalier, Denis Fiorillo, Amadou T. Gaye, Serge Janicot, Myriam Khodri, Alban Lazar, Claire E. Lazareth, Juliette Mignot, Nancy Mitma García, Nicolas Patris, Océane Perrot, and Malick Wade

   Climate Dynamics, Feb 2019

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   As climate model uncertainties remain very large for future rainfall in the Sahel, a multi-centennial perspective is required to assess the situation of current Sahel climate in the context of global warming. We present here the first record of hydroclimatic variability over the past 1600 years in Senegal, obtained from stable oxygen isotope analyses (\delta18O) in archaeological shell middens from the Saloum Delta. During the preindustrial period, the region was relatively humid, with maximum humidity reached during the period from AD 1500 to AD 1800, referred to as the Little Ice Age. A significant negative link is observed at the centennial scale between global temperature and humidity in the Sahel that is at odds with the expected effects of latitudinal shifts of the intertropical convergence zone during the last millennium. In the context of the past 1600 years, the Western Sahel appears to be experiencing today unprecedented drought conditions. The rapid aridification that started ca. AD 1800 and the recent emergence of Sahel drought from the natural variability point to an anthropogenic forcing of Sahel drying trend. This new long-term perspective suggests that the recovery of Sahel rainfall in the last decade may only result from short-term internal variability, and supports climate models that predict an increase of Sahel drought under future greenhouse climate.

2018

1. Climatic controls on Later Stone Age human adaptation in Africa’s southern Cape

   Brian M. Chase, J. Tyler Faith, Alex Mackay, Manuel Chevalier, Andrew S. Carr, Arnoud Boom, Sophak Lim, and Paula J. Reimer

   Journal of Human Evolution, Jan 2018

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   Africa’s southern Cape is a key region for the evolution of our species, with early symbolic systems, marine faunal exploitation, and episodic production of microlithic stone tools taken as evidence for the appearance of distinctively complex human behavior. However, the temporally discontinuous nature of this evidence precludes ready assumptions of intrinsic adaptive benefit, and has encouraged diverse explanations for the occurrence of these behaviors, in terms of regional demographic, social and ecological conditions. Here, we present a new high-resolution multi-proxy record of environmental change that indicates that faunal exploitation patterns and lithic technologies track climatic variation across the last 22,300 years in the southern Cape. Conditions during the Last Glacial Maximum and deglaciation were humid, and zooarchaeological data indicate high foraging returns. By contrast, the Holocene is characterized by much drier conditions and a degraded resource base. Critically, we demonstrate that systems for technological delivery – or provisioning – were responsive to changing humidity and environmental productivity. However, in contrast to prevailing models, bladelet-rich microlithic technologies were deployed under conditions of high foraging returns and abandoned in response to increased aridity and less productive subsistence environments. This suggests that posited links between microlithic technologies and subsistence risk are not universal, and the behavioral sophistication of human populations is reflected in their adaptive flexibility rather than in the use of specific technological systems.

2. A high-resolution record of Holocene climate and vegetation dynamics from the southern Cape coast of South Africa: pollen and microcharcoal evidence from Eilandvlei

   Lynne J. Quick, Brian M. Chase, Michael Wündsch, Kelly L. Kirsten, Manuel Chevalier, Roland Mäusbacher, Michael E. Meadows, and Torsten Haberzettl

   Journal of Quaternary Science, Jul 2018

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   The southern Cape is a particularly dynamic region of South Africa in terms of climate change as it is influenced by both temperate and tropical circulation systems. This paper presents pollen and microcharcoal data generated from a sediment core extracted from the coastal lake Eilandvlei spanning the last ∼8900 years. With an average sample resolution of 57 years, this record represents the highest resolution record of Holocene vegetation change from the region. The data indicate that cool, seasonal and moderately dry conditions characterized the Wilderness Embayment from ∼8900 to 8000 cal a BP. Afrotemperate forests expanded from ∼8000 cal a BP until 4700 cal a BP. This humid period is followed by indications of more arid and seasonal conditions until 3500 cal a BP. A long-term increase in forest taxa suggests steadily increasing moisture availability across the late Holocene. Strong affinities are noted with records from more tropical regions of South Africa, suggesting that tropical systems are of importance in maintaining higher moisture availability in the region. An important mechanism of climate change is the Agulhas Current, which transmits what appears to be a localized signal of tropical variability to the southern Cape coast.

2017

1. Late Pleistocene-Holocene vegetation and climate change in the Middle Kalahari, Lake Ngami, Botswana

   Carlos E. Cordova, Louis Scott, Brian M. Chase, and Manuel Chevalier

   Quaternary Science Reviews, Sep 2017

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   Pollen, spores, and microscopic charcoal from a sediment core from Lake Ngami, in the Middle Kalahari, reflect paleovegetation and paleoclimatic conditions over the last 16,600 cal years BP. The location of Lake Ngami allows for the receipt of moisture sourced from the Indian and/or Atlantic oceans, which may have influenced local rainfall or long distance water transport via the Okavango system. We interpret results of statistical analyses of the pollen data as showing a complex, dynamic system wherein variability in tropical convective systems and local forcing mechanisms influence hydrological changes. Our reconstructions show three primary phases in the regional precipitation regime: 1) an early period of high but fluctuating summer rainfall under relatively cool conditions from ∼16,600–12,500 cal BP, with reduced tree to herb and shrub ratio; 2) an episode of significantly reduced rainfall centered around c. 11,400 cal BP, characterized by an increase in xeric Asteraceae pollen, but persistent aquatic elements, suggesting less rainfall but cool conditions and lower evaporation that maintained water in the basin; and 3) a longer phase of high, but fluctuating rainfall from ∼9000 cal BP to present with more woody savanna vegetation (Vachellia (Acacia) and Combretaceae). We propose a model to relate these changes to increased Indian Ocean-sourced moisture in the late Pleistocene due to a southerly position of the African rain belt, a northerly contraction of tropical systems that immediately followed the Younger Dryas, and a subsequent dominance of local insolation forcing, modulated by changes in the SE Atlantic basin.

2. The dynamic relationship between temperate and tropical circulation systems across South Africa since the last glacial maximum

   Brian M. Chase, Manuel Chevalier, Arnoud Boom, and Andrew S. Carr

   Quaternary Science Reviews, 2017

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   A fundamental and long-standing question of southern African palaeoclimatology is the way tropical and temperate climate system dynamics have influenced rainfall regimes across the subcontinent since the Last glacial maximum. In this paper, we analyse a selection of recently published palaeoclimate reconstructions along a southwest-northeast transect across South Africa. These records span the last 22,000 years, and encompass the transition between the region’s winter and summer rainfall zones. In synthesis, these records confirm broad elements of the dominant paradigm, which proposes an inverse coeval relationship between temperate and tropical systems, with increased precipitation in the winter (summer) rainfall zone during glacial (interglacial) periods. Revealed, however, is a substantially more complex dynamic, with millennial-scale climate change events being strongly – even predominantly – influenced by the interaction and combination of temperate and tropical systems. This synoptic forcing can create same sign anomalies across the South African rainfall zones, contrary to expectations based on the classic model of phase opposition. These findings suggest a new paradigm for the interpretation of southern African palaeoenvironmental records that moves beyond simple binary or additive influences of these systems.

3. Qualitative assessment of PMIP3 rainfall simulations across the eastern African monsoon domains during the mid-Holocene and the Last Glacial Maximum

   Manuel Chevalier, Simon Brewer, and Brian M. Chase

   Quaternary Science Reviews, Jan 2017

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   In this paper we compare a compilation of multiproxy records spanning the eastern African margin with general circulation model simulations of seasonal precipitation fields for the mid-Holocene and the Last Glacial Maximum (LGM) carried out as part of the third phase of the Paleoclimate Modelling Intercomparison Project (PMIP3). Results show good agreement during the mid-Holocene (the ‘6K experiment’), with palaeodata and model outputs correlating well and indicating that changes in insolation drove a stronger northern African monsoon (north of ∼0–5°S) during the terminal “African Humid Period” and a weaker southeast African monsoon. For the LGM (the ‘21K experiment’), however, significant discrepancies exist both between model simulations, and between existing palaeodata and simulated conditions, both in terms of direction and amplitude of change. None of the PMIP3 simulations reflect the pattern inferred from the palaeodata. Two major discrepancies have been identified to explain this: 1) the limited sensitivity of the southern monsoon domain to the colder temperatures of the Indian Ocean (−2 °C), and 2) the absence of changes in the dynamic of the Indian Ocean Walker circulation over the entire basin, despite the exposure of the Sahul and Sunda shelves that weakened convection over the Indo-Pacific Warm Pool during the LGM. These results indicate that some major features of the atmospheric and oceanic teleconnections between the different monsoon regions require further consideration as models evolve.

2016

1. Determining the drivers of long-term aridity variability: a southern African case study

   Manuel Chevalier, and Brian M. Chase

   Journal of Quaternary Science, Feb 2016

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   This paper highlights the importance of differentiating between precipitation amount and moisture availability (‘humidity’/‘aridity’) when considering proxy records of climate change. While the terms are sometimes used interchangeably, moisture availability is determined by both (i) precipitation amount and (ii) temperature, through its influence on potential evapotranspiration. As many palaeoenvironmental proxies reflect changes in this water balance rather than purely precipitation amount, it is important to distinguish between the potential relative influences of precipitation and temperature if those records are to be interpreted in terms of climate mechanisms and/or compared with model outputs. As a case study, we explore how precipitation and temperature have determined moisture availability in South Africa’s summer rainfall zone over the last 45 000 years. Using quantitative reconstructions of mean annual temperature, summer rainfall amount and an aridity index, our analysis reveals strong spatiotemporal variability in the relative influences of precipitation and temperature on aridity. Temperature is shown to have exerted a considerable and even dominant influence on moisture availability, resulting in elevated humidity during the last glacial period, despite significant reductions in precipitation amount.

2. 50,000 years of vegetation and climate change in the southern Namib Desert, Pella, South Africa

   Sophak Lim, Brian M. Chase, Manuel Chevalier, and Paula J. Reimer

   Palaeogeography, Palaeoclimatology, Palaeoecology, Jun 2016

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   This paper presents the first continuous pollen record from the southern Namib Desert spanning the last 50,000 years. Obtained from rock hyrax middens found near the town of Pella, South Africa, these data are used to reconstruct vegetation change and quantitative estimates of temperature and aridity. Results indicate that the last glacial period was characterised by increased water availability at the site relative to the Holocene. Changes in temperature and potential evapotranspiration appear to have played a significant role in determining the hydrologic balance. The record can be considered in two sections: 1) the last glacial period, when low temperatures favoured the development of more mesic Nama-Karoo vegetation at the site, with periods of increased humidity concurrent with increased coastal upwelling, both responding to lower global/regional temperatures; and 2) the Holocene, during which time high temperatures and potential evapotranspiration resulted in increased aridity and an expansion of the Desert Biome. During this latter period, increases in upwelling intensity created drier conditions at the site. Considered in the context of discussions of forcing mechanisms of regional climate change and environmental dynamics, the results from Pella stand in clear contrast with many inferences of terrestrial environmental change derived from regional marine records. Observations of a strong precessional signal and interpretations of increased humidity during phases of high local summer insolation in the marine records are not consistent with the data from Pella. Similarly, while high percentages of Restionaceae pollen has been observed in marine sediments during the last glacial period, they do not exceed 1% of the assemblage from Pella, indicating that no significant expansion of the Fynbos Biome has occurred during the last 50,000 years. These findings pose interesting questions regarding the nature of environmental change in southwestern Africa, and the significance of the diverse records that have been obtained from the region.

3. Temperature Range Shifts for Three European Tree Species over the Last 10,000 Years

   Rachid Cheddadi, Miguel B. Araújo, Luigi Maiorano, Mary Edwards, Antoine Guisan, Matthieu Carré, Manuel Chevalier, and Peter B. Pearman

   Frontiers in Plant Science, 2016

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   We quantified the degree to which the relationship between the geographic distribution of three major European tree species, Abies alba, Fagus sylvatica and Picea abies and January temperature (Tjan) has remained stable over the past 10,000 years. We used an extended data-set of fossil pollen records over Europe to reconstruct spatial variation in Tjan values for each 1000-year time slice between 10,000 and 3000 years BP (before present). We evaluated the relationships between the occurrences of the three species at each time slice and the spatially interpolated Tjan values, and compared these to their modern temperature ranges. Our results reveal that F. sylvatica and P. abies experienced Tjan ranges during the Holocene that differ from those of the present, while A. alba occurred over a Tjan range that is comparable to its modern one. Our data suggest the need for re-evaluation of the assumption of stable climate tolerances at a scale of several thousand years. The temperature range instability in our observed data independently validates similar results based exclusively on modeled Holocene temperatures. Our study complements previous studies that used modeled data by identifying variation in frequencies of occurrence of populations within the limits of suitable climate. However, substantial changes that were observed in the realized thermal niches over the Holocene tend to suggest that predicting future species distributions should not solely be based on modern realized niches, and needs to account for the past variation in the climate variables that drive species ranges.

2015

1. Southeast African records reveal a coherent shift from high- to low-latitude forcing mechanisms along the east African margin across last glacial–interglacial transition

   Manuel Chevalier, and Brian M. Chase

   Quaternary Science Reviews, Oct 2015

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   Late Quaternary climate variability in the southern African subtropics is still only poorly resolved, with significant complexity and apparent contradictions in the regional dataset. To more effectively interpret and synthesize key regional records, we reanalysed the data from 13 pollen sequences from the summer rainfall zone of South Africa spanning the last 45,000 years, obtaining directly comparable quantitative reconstructions of mean annual temperature and summer rainfall. Temperature reconstructions from across the region provide consistent results, with all sites reflecting trends observed in southwest Indian Ocean sea-surface temperatures in the adjacent Mozambique Channel. Precipitation reconstructions are more heterogeneous, with two distinct subregions being identified. In the northeast, long-term trends in precipitation are determined by sea-surface and continental temperature trends, revealing a positive relationship between temperature and rainfall. This long-term pattern appears to be primarily driven by high northern latitude mechanisms, with direct local insolation being subordinate. Their relative impact reversed during terminal glacial period/early Holocene, at which time direct insolation forcing became the main driver of rainfall variability. Further south, in central South Africa, precipitation variability appears also to be influenced by the latitudinal position of the Southern Hemisphere westerlies, which combine with tropical flow to create tropical-temperate trough, advecting moisture into the interior. In this region, periods of maximum precipitation coincide with periods of elevated SSTs and equatorward expansions of the westerly storm track. This study allows for a fully constrained understanding of climate dynamics along the eastern African margin for the last 45,000 years, linking dynamics to drivers and describing how the climate systems evolved across the last glacial–interglacial transition.

2. Influence of tropical easterlies in southern Africa’s winter rainfall zone during the Holocene

   Brian M. Chase, Sophak Lim, Manuel Chevalier, Arnoud Boom, Andrew S. Carr, Michael E. Meadows, and Paula J. Reimer

   Quaternary Science Reviews, Jan 2015

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   South Africa’s southwestern Cape occupies a critical transition zone between Southern Hemisphere temperate (winter) and tropical (summer) moisture-bearing systems. In the recent geological past, it has been proposed that the relative influence of these systems may have changed substantially, but little reliable evidence regarding regional hydroclimates and rainfall seasonality exists to refine or substantiate the understanding of long-term dynamics. In this paper we present a mid-to late Holocene multi-proxy record of environmental change from a rock hyrax midden from Katbakkies Pass, located along the modern boundary between the winter and summer rainfall zones. Derived from stable carbon and nitrogen isotopes, fossil pollen and microcharcoal, these data provide a high resolution record of changes in humidity, and insight into changes in rainfall seasonality. Whereas previous work concluded that the site had generally experienced only subtle environmental change during the Holocene, our records indicate that significant, abrupt changes have occurred in the region over the last 7000 years. Contrary to expectations based on the site’s location, these data indicate that the primary determinant of changes in humidity is summer rather than winter rainfall variability, and its influence on drought season intensity and/or length. These findings are consistent with independent records of upwelling along the southern and western coasts, which indicate that periods of increased humidity are related to increased tropical easterly flow. This substantially refines our understanding of the nature of temperate and tropical circulation system dynamics in SW Africa, and how changes in their relative dominance have impacted regional environments during the Holocene.

3. Evolving southwest African response to abrupt deglacial North Atlantic climate change events

   Brian M. Chase, Arnoud Boom, Andrew S. Carr, Matthieu Carré, Manuel Chevalier, Michael E. Meadows, Joel B. Pedro, J. Curt Stager, and Paula J. Reimer

   Quaternary Science Reviews, 2015

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   Climate change during the last deglaciation was strongly influenced by the ‘bipolar seesaw’, producing antiphase climate responses between the North and South Atlantic. However, mounting evidence demands refinements of this model, with the occurrence of abrupt events in southern low to mid latitudes occurring in-phase with North Atlantic climate. Improved constraints on the north–south phasing and spatial extent of these events are therefore critical to understanding the mechanisms that propagate abrupt events within the climate system. We present a 19,400 year multi-proxy record of climate change obtained from a rock hyrax midden in southernmost Africa. Arid anomalies in phase with the Younger Dryas and 8.2 ka events are apparent, indicating a clear shift in the influence of the bipolar seesaw, which diminished as the Earth warmed, and was succeeded after ∼14.6 ka by the emergence of a dominant interhemispheric atmospheric teleconnection.

2014

1. Open-Access

   CREST (Climate REconstruction SofTware): a probability density function (PDF)-based quantitative climate reconstruction method

   Manuel Chevalier, Rachid Cheddadi, and Brian M. Chase

   Climate of the Past, Nov 2014

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   Abstract. Several methods currently exist to quantitatively reconstruct palaeoclimatic variables from fossil botanical data. Of these, probability density function (PDF)-based methods have proven valuable as they can be applied to a wide range of plant assemblages. Most commonly applied to fossil pollen data, their performance, however, can be limited by the taxonomic resolution of the pollen data, as many species may belong to a given pollen type. Consequently, the climate information associated with different species cannot always be precisely identified, resulting in less-accurate reconstructions. This can become particularly problematic in regions of high biodiversity. In this paper, we propose a novel PDF-based method that takes into account the different climatic requirements of each species constituting the broader pollen type. PDFs are fitted in two successive steps, with parametric PDFs fitted first for each species and then a combination of those individual species PDFs into a broader single PDF to represent the pollen type as a unit. A climate value for the pollen assemblage is estimated from the likelihood function obtained after the multiplication of the pollen-type PDFs, with each being weighted according to its pollen percentage. To test its performance, we have applied the method to southern Africa as a regional case study and reconstructed a suite of climatic variables (e.g. winter and summer temperature and precipitation, mean annual aridity, rainfall seasonality). The reconstructions are shown to be accurate for both temperature and precipitation. Predictable exceptions were areas that experience conditions at the extremes of the regional climatic spectra. Importantly, the accuracy of the reconstructed values is independent of the vegetation type where the method is applied or the number of species used. The method used in this study is publicly available in a software package entitled CREST (Climate REconstruction SofTware) and will provide the opportunity to reconstruct quantitative estimates of climatic variables even in areas with high geographical and botanical diversity.

2013

1. Quantification of climate change for the last 20,000years from Wonderkrater, South Africa: Implications for the long-term dynamics of the Intertropical Convergence Zone

   Loïc Truc, Manuel Chevalier, Charly Favier, Rachid Cheddadi, Michael E. Meadows, Louis Scott, Andrew S. Carr, Gideon F. Smith, and Brian M. Chase

   Palaeogeography, Palaeoclimatology, Palaeoecology, Sep 2013

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   In southeast Africa – a region for which few palaeoenvironmental records are available – the fossil pollen record from the Wonderkrater spring mound has contributed substantially to our understanding of past vegetation change since the Last Glacial Maximum (LGM; 21 ka). Multivariate analysis of the pollen data by Scott and Thackeray (1987) provided environmental reconstructions suggesting relatively mesic LGM conditions, with warm and dry conditions during the early Holocene (11–6 cal kBP). This conforms to predicted patterns of precipitation change in the southern African tropics in response to Northern Hemisphere cooling and orbital forcing. Subsequent data from the Cold Air Cave speleothems and a sea-surface temperature record from the Mozambique Channel, however, indicate that conditions during the early to mid-Holocene may have been wetter than present in the Wonderkrater region. To explore this question further, we have created a series of botanical–climatological transfer functions based on a combination of modern climate and plant distribution data from southern Africa. Applying these to the Wonderkrater fossil pollen sequence, we have derived quantitative estimates for temperatures during the cold and warm quarters, as well as precipitation during the wet and dry quarters. In addition, a species-selection method based on Bayesian statistics is outlined, which provided a parsimonious choice of likely plant species from what are otherwise taxonomically broad pollen-types. We do not propose that our findings invalidate the previous principal component analyses, but they do have the advantage of being based more clearly on the relationship between modern plant distributions and individual climatic variables. Results indicate that temperatures during both the warm and cold seasons were 6 ± 2 °C colder during the LGM and Younger Dryas, and that rainy season precipitation during the Last Glacial Maximum was ~ 50% of that during the mid-Holocene. Our results also imply that changes in precipitation at Wonderkrater generally track changes in Mozambique Channel sea-surface temperatures, with a steady increase following the Younger Dryas to a period of maximum water availability at Wonderkrater ~ 3–7 ka. These findings argue against a dominant role of a shifting Intertropical Convergence Zone in determining long-term environmental trends, and indicate that the northern and southern tropics experienced similar climatic trends during the last 20 kyr.

2012

1. A continuous record of vegetation and climate change over the past 50,000years in the Fujian Province of eastern subtropical China

   Yuanfu Yue, Zhuo Zheng, Kangyou Huang, Manuel Chevalier, Brian M. Chase, Matthieu Carré, Marie-Pierre Ledru, and Rachid Cheddadi

   Palaeogeography, Palaeoclimatology, Palaeoecology, Dec 2012

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   A 425 cm core has been collected from the Gantang subalpine peat bog, in Pingnan County, Fujian Province of Southern China. High-resolution pollen analysis of the core has allowed for the reconstruction of past vegetation and climate changes over the last 50,000 years. Today, Fujian province is located in the eastern part of middle subtropical zone where the dominant vegetation is evergreen broadleaved forest that receives its precipitation from the East Asian Monsoon. The pollen record testifies that the vegetation in Fujian varied between subtropical evergreen and warm temperate deciduous forests during the last glacial-interglacial cycle. It appears that during Marine Oxygen Isotope Stage 3, this area was covered with broadleaved forest suggesting warm and wet subtropical condition, whereas the flora was quite different with many ancient elements. The Last Glacial Maximum is distinguished by an increase in warm temperate deciduous taxa, particularly Fagus and Alnus accompanied by abundant Ericaceae, implying a spatial shift of zonal vegetation during this coldest episode in the world. Local swamp developed soon after the Younger Dryas event coinciding with the formation of peatlands in the Northern Hemisphere possibly linked with atmospheric carbon accumulation. A rapid increase in evergreen broadleaved taxa that dominated the local forest occurred at ~ ca. 8.2 cal ka BP, indicating the inception of the regional thermal maximum during the Holocene, which was generally characterised by more humid conditions. The middle to late Holocene sees a progressive decline in arboreal elements, and an increase in grasses and Ericaceae. These changes beginning from around 4 ka cal BP were concordant with the general weakening of the East Asian Monsoon during the Holocene, corresponding with an orbitally induced reduction of boreal summer insolation. This is the first terrestrial high-resolution record from eastern part of middle subtropical areas that evaluates the forest changes through the last glacial-interglacial cycle.