Quantitative Paleoclimate Reconstructions Research Articles

Quantification of Holocene temperatures in the eastern Hunshandake Sandy Land using δ13C of loess organic matter

The soil organic carbon isotope indicator (δ13Corg) is widely used in paleoecology and paleoclimate reconstruction due to its ability to record the history of paleovegetation change. Although it has been used as a proxy for precipitation reconstruction, increasing evidence suggests that temperature has played a crucial role in influencing the relative abundance of C3/C4 plants in the mid-latitudes during the interglacial period. This opens up the possibility of reconstructing Holocene temperature using δ13Corg. In this study, the variation of δ13Corg values in a Holocene loess profile (GLH profile), located in the eastern Hunshandake Sandy Land, China, was investigated. And we analyzed the correlation between δ13Corg values and modern temperatures for a total of 792 surface soil samples from mid-latitudes of the Northern Hemisphere, which had thermal conditions comparable to the GLH profile. Strong positive correlations were found between δ13Corg values of surface soil samples and both summer temperatures (r = 0.672, p < 0.001) and growing season temperatures (r = 0.669, p < 0.001). Additionally, δ13Corg values showed significant positive correlations with accumulated temperatures above 10 °C (r = 0.641, p < 0.001) and above 0 °C (r = 0.527, p < 0.001). However, the correlation with mean annual temperature was weak (r = 0.280, p < 0.001). The δ13Corg variation in the GLH profile recorded a mixed C3/C4 vegetation composition, with C4 plants comprising between 23.40% and 1.70% of the total abundance during the Holocene. The combined evidence from modern processes and profile variations confirms that warm-season temperature was the main factor driving variations in the δ13Corg values in the GLH profile. Therefore, we used δ13Corg–temperature conversion equations to quantify Holocene seasonal and accumulated temperatures based on the GLH profile. The reconstructed summer temperatures peaked at 9.3 ka, followed by a fluctuating downward trend during the mid- and late-Holocene, which is consistent with variations in summer solar insolation. Mean summer temperatures were relatively high at 23.3 °C and 22.5 °C in the early and mid-Holocene, respectively, while the lowest mean summer temperature of 20.5 °C occurred in the late Holocene. Accumulated temperatures above 10 °C and above 0 °C in the early Holocene were 3722 °C·d and 4641 °C·d, respectively, similar to modern observations in the Beijing area, indicating a southward shift of the temperature zone by nearly three degrees of latitude since the early Holocene. These reconstruction results are supported by modern observations, TraCE21 simulations, and other studies. This study is significant for extending the application of the δ13Corg indicator for quantitative paleoclimate reconstructions. It verifies its effectiveness in quantifying Holocene temperatures in the marginal zone of the East Asian summer monsoon.

Asian monsoon variations over the past 21 ka: An introduction

The Asian monsoon influences the lives of more than a third of the world's population, and understanding its spatial and temporal variability is fundamental to sustainable development. The complex dynamics of this climatic system are therefore of broad scientific and societal interest. In this virtual special issue (VSI), we present 19 papers dealing with the Asian monsoon since the Last Glacial Maximum (LGM, ∼21 kyr BP), with a focus on understanding variability and forcing mechanisms. Findings use recently developed quantitative reconstructions combined with numerical simulations, and present novel insights as follows: (1) analyses of comprehensive datasets of pollen, diatom, brGDGTs, oxygen and hydrogen isotopes and frequency magnetic susceptibility, as well as an innovative algorithm model for the lake energy-water balance, improve the reliability of quantitative reconstructions of Asian monsoon climate variations; (2) studies of quantitative climate parameters in central East Asia show that mean annual temperature (MAT) was ∼5 °C (or ∼ 7.9 °C using brGDGTs) lower during the LGM, ∼1.5 °C (±41% using brGDGTs) higher around the Holocene thermal optimum (HO) and mean annual precipitation (MAP) varied between 30 and 150% with marked seasonality during 8–5 ka; (3) dynamical downscaling palaeoclimatic simulations improve the accuracy of past monsoon variations, revealing strong variability in monsoon precipitation across the LGM and HO. The holistic study of Asian monsoon variations at orbital to centennial timescales in these VSI papers presents reliable observations of monsoon variability and its driving mechanisms, providing high-resolution quantitative paleoclimate reconstructions that fill existing gaps in spatial coverage, as well as offering an improved understanding, thorough numerical simulation, of coupled sea-level change and vegetation feedbacks.

The Quantitative Reconstruction of Paleoclimate in the Huangling Region of the Chinese Loess Plateau during the Middle and Late Holocene

The Huangling region is located in the central part of the Chinese Loess Plateau, which is sensitive to climate change due to the transitional characteristics of the natural environmental zone in which it is located. In this study, we utilized a spore–pollen analysis of the Tianjiahe (TJH) profile in Huangling to apply the pollen–climate factor conversion function method. This approach allowed us to quantitatively reconstruct the paleotemperature and paleoprecipitation of the Huangling area during the Middle and Late Holocene. The results show that the Huangling area experienced four climatic stages during the Middle and Late Holocene, including mild and slightly humid → warm and humid → warm and slightly humid → warm and humid. Except for the period of 5.3–4.72 kaBP, during which the climate was relatively cool and dry compared to the present, the climate in the remaining period (4.72–0.03 kaBP) was warmer and more humid than that of the present. The above results provide an important insight for further exploring the mechanism of paleoclimate change and predicting future climate change.

Effects of varied modern pollen-climate calibration-set establishment approaches on quantitative climate reconstructions

A representative modern calibration dataset is an essential prerequisite for a reliable quantitative reconstruction of paleoclimate. However, there has been a notable lack of systematic research on the optimal filtering of calibration sets. Four representative high-resolution pollen records were selected to repeatedly reconstruct paleoclimate using multiple approaches such as geographical distance, analogues and climate gradient. Comparisons of pollen-based reconstructions indicate that these varied approaches exhibit significant discrepancy in model performance and reconstruction accuracy under different geographical circumstances and time scales. Generally, in terms of calibration-set selection, the geographical distance method demonstrates greater efficiency for the plain region, but exhibit limitations in areas characterized by significant topographic changes. Analogue-calibration has been found to be more suitable for the edge of the database and also has been proven superior to other two methods for reconstructions exceed Holocene. Selecting calibration sets based only on climate gradients is inappropriate for independent applications, but may act as a limitation for other methods. This preliminary research can contribute to a more precise projection of potential future climate changes by providing essential data input or validating hindcasts for Earth system models.

A novel approach for quantitatively distinguishing between anthropogenic and natural effects on paleovegetation.

How to distinguish and quantify past human impacts on vegetation is a significant challenge in paleoecology. Here, we propose a novel method, the error inflection point-discriminant technique. It finds out the inflection points (IPs) of the regression errors of pollen-climate transfer functions using modern pollen spectra from vegetation with different values of the Human Influence Index (HII), which represent the HII threshold values of native/secondary and secondary/artificial vegetation systems. Our results show that the HII value at the native/secondary vegetation IPs is approximately 22 and globally uniform, whereas it varies regionally for the secondary/artificial vegetation IPs. In a case study of the Liangzhu archaeological site in the lower Yangtze River, discriminant functions for pollen spectra from three vegetation types and pollen-climate transfer functions of the native vegetation were established to reconstruct paleovegetation and paleoclimate over the past 6,600 years. Our study demonstrates this method's feasibility for quantitatively distinguishing human impacts on paleovegetation and assessing quantitative paleoclimate reconstructions using pollen data.

Quantitative Holocene climate reconstruction and anthropogenic impact analysis based on the pollen records in peat sediment in Southern China

The quantitative reconstruction of Holocene paleoclimate is pivotal for unraveling the evolution of East Asian Monsoon (EAM). Some recent studies have highlighted the escalating human activities in the late Holocene, which could potentially pose significant challenges to the precise quantitative reconstruction of paleoclimate. In this study, we undertake a quantitative reconstruction of Holocene climate and human influence index (HII) based on pollen data from the Shiwangutian (SWGT) in Southern China. This is achieved by developing and applying a pollen-based calibration model utilizing Locally Weighted Average-Partial Least Squares Regression (LWWA-PLS) and the Modern Analogue Technique (MAT). Our findings reveal that evergreen forests experienced expansion between 11.6 and 9.0 cal ka BP, coinciding with elevated Mean Annual Precipitation (Map) and Mean Annual Temperature (Mat) levels. These patterns signify a progressively warm and wet climate during this period. Subsequently, evergreen forests achieved their maximum abundance between 9.0 and 4.0 cal ka BP, aligning with a notable increase in vegetation richness, indicating a warmer and wetter climate during the mid-Holocene. Notably, during the interval of 6.0–4.0 cal ka BP, the Map, Mat, and Precipitation of Warmest Month (Mpwa) reached the Holocene optimum, approximately 14% higher than modern. Consequently, we deduce that in the early and middle Holocene, when the monsoon was particularly strong, synchronous changes in precipitation fostered the regional expansion of evergreen broad-leaved forests. Furthermore, the record demonstrates a substantial reduction in forest cover and a noteworthy increase in the HII value since 4.0 cal ka BP, likely attributable to anthropogenic impacts. Our analysis suggests that the influence of anthropogenic activities surpassed that of natural climate factors in shaping regional vegetation cover. These findings offer insights into potential drivers of climate change. As a preliminary conclusion, we propose that the summer insolation, ice sheets, and meltwater fluxes were the primary controlling factors for climate variations during the early to mid-Holocene in Southern China.

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

Abstract. 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.

A modern pollen dataset from lake surface sediments on the central and western Tibetan Plateau

Abstract. Modern pollen datasets are essential for pollen-based quantitative paleoclimate (e.g. precipitation) reconstructions, which can aid in better understanding recent climate change and its underlying forcing mechanisms. A modern pollen dataset based on surface sediments from 90 lakes in the shrub, meadow, steppe and desert regions of the central and western Tibetan Plateau (TP) was established to fill geographical gaps left by previous datasets. Ordination analyses of pollen data and climatic parameters revealed that annual precipitation is the dominant factor in modern pollen distribution on the central and western TP. A regional transfer function for annual precipitation was developed with weighted averaging partial least squares (WA-PLS), which suggests a good inference power of the modern pollen dataset for annual precipitation. A case study in which the transfer function was effectively applied to a fossil pollen record from Tangra Yumco on the central TP for paleoprecipitation reconstruction demonstrated the significance of the modern pollen dataset in a lower data region for paleoclimate change studies. Data from this study, including pollen data for each sample and information on the sampled sites (location, altitude and climate data), are openly available via the Zenodo portal (https://doi.org/10.5281/zenodo.8008474, Ma et al., 2023).

Late Quaternary Dynamics of Landscape and Climate in the North of the West Siberian Plain Revealed by Paleoecological Studies of Peat and Lake Sediments

West Siberian mires covering more than 50% of area in the subarctic are still poorly investigated despite their thick peat sediments suitable for paleogeographic research of past long-term landscape and climatic changes. In this research, a combination of paleoecological methods were used, including the analysis of pollen, spores, diatoms, NPPs, and macrofossils, the measurement of peat humification, and quantitative paleoclimate reconstruction. This multi-proxy approach was applied to study a palsa bog (frost peat heave mound) located in the north of western Siberia on the border of the northern taiga and forest–tundra (65°18′56″ N, 72°52′27″ E). Chronology is based on 21 radiocarbon dates, which were calibrated in CLAM. Studies have shown that sediments of palsa bog Nadym of a 1050 cm thickness were formed both in the Holocene and earlier periods of the Quaternary. Radiocarbon dating worked well for peat sequences (610 cm thick), but failed for underlying lacustrine and mineral sediments (440 cm thick). Numerous remains of salt-water diatoms and exotic Neogene pollen were found in the lacustrine sediments (650–850 cm). The oldest sediments (850–1050 cm) have signs of secondary epicryogenic diagenesis in the form of cryogenic iron-enriched granules. Both lacustrine and bottom sediments contain abundant coniferous pollen. At the same time, spore–pollen complexes dated to the Last Glacial Age were not found in low sediments because of failed dates. To explain this, the authors turn to the hypothesis of glyacioisostatic compensation, according to which the study area was uplifted during the Last Glacial Age and the ancient deposits underwent secondary diagenesis in subaerial conditions. Holocene lacustrine sedimentation began to form about 9800 cal. a BP. These lacustrine sediments turned out to be enriched in redeposited Neogene pollen and diatoms. It was interpreted as an influence of excess humid climate in combination with geological subsidence of landscape in the study area during the Early Holocene. This caused lake formation and introduction of exotic microfossils via surface run-off from higher-relief areas in the catchment. Syngenetic sedimentation in the Nadym section is associated only with peat-mire deposits covering the last 8400 cal. a BP. For this time, the dynamic of vegetation cover and quantitative changes in paleoclimate were reconstructed using spore–pollen, macrofossil, humus, and NPP data as well as the information–statistical method of V.A. Klimanov. The spore–pollen analysis revealed four main phases in the development of vegetation cover: 1. Spruce–birch forests with open meadows and lakes (8400–7600 cal. a BP); 2. Dominance of spruce forests and thawed eutrophic (minerotrophic) mires (7600 to 6500 cal. a BP); 3. Coniferous–birch forests and thawed mesotrophic mires (6500 to 4500 cal. a BP); 4. Birch–pine forests and oligotrophic (ombrotrophic) bogs with permafrost mounds—palsa bogs (approx. the last 4500 years). Quantitative reconstructions of paleoclimate based on pollen data show that in most cases the periods of a sharp decrease in mean January and mean July temperatures coincided with episodes of low solar activity. The assumption was made about the determining influence of solar activity on the formation of permafrost in the soils and mires of the study area. Sun minima caused permafrost formation in the mire periodically since 8400 cal. a BP in study peatland, but complete freezing of the peat mire and formation of the palsa bog occurred at c. 2800 cal. a BP.

Quantitative pollen-based paleoclimate reconstructions for the past 18.5 ka in southwestern Yunnan Province, China

Quantitative reconstruction of past climate plays an important role in understanding global and regional climate changes and validating climate models. Although some important progress has been made in quantitative paleoclimate reconstructions based on terrestrial records in the Indian summer monsoon region, high-resolution quantitative studies spanning the last ∼20 ka are still relatively sparse with differing results. This study presents high-resolution quantitative variations in mean temperature of the coldest month (MTCM, the first controlling factor of the regional vegetation), mean annual temperature (MAT, the second controlling factor), and mean annual precipitation (MAP) from Lake Tengchongqinghai in southwestern China, based on an updated modern pollen dataset and the fossil pollen record spanning the last 18.5 ka. The results show that temperature and precipitation increased gradually from 18.5 ka, and peaked from 7.2 to 4.5 ka when MAT was on average 1.0 °C higher than the modern observational value (14.5 °C), corresponding to the mid-Holocene thermal maximum (HTM), and then decreased gradually. The total reconstructed ranges are between −2.2 and 9.2 °C for MTCM, 7.7 and 17.2 °C for MAT, and 840 and 1300 mm for MAP. On top of this overall climate trend, seven abrupt cold and dry events were detected during the periods of 16.2–14.8 ka, 12.8–11.5 ka, ∼11.1 ka, 9.1–8.4 ka, ∼7.7 ka, 4.3–3.7 ka, and 0.68–0.009 ka (1270–1950 CE). The results of this quantitative reconstruction were validated by both statistical and ecological evaluations. We conclude that the trend of climatic change since 18.5 ka in this study area was primarily driven by June, July, August, and September solar insolation and changes in radiative forcing and greenhouse gas concentrations. The abrupt changes may be caused by changes in the Atlantic meridional overturning circulation, solar activity, the position of the Intertropical Convergence Zone, and volcanic activity.

Surface soil pollen and climate in northern Xinjiang, China: Implications for paleoclimate reconstruction

Quantitative paleoclimate reconstruction based on surface soil pollen and climate records on a regional scale is widely applied as a standard approach to investigating past climate changes. This study aimed to explore the quantitative relationships between the distribution of pollen taxa in 438 pollen samples from surface soils at altitudes ranging from 190 m to 3235 m and 19 environmental parameters in northern Xinjiang, China, using ordination and correlation analyses. The results showed that mean annual precipitation (MAP), precipitation in July, mean annual temperature (MAT), mean temperature in July and mean annual wind speed were the main environmental factors influencing the surface pollen distribution. Pinus pollen dispersal and transportation were primarily affected by spring wind, suggesting that exotic pollen played an essential role in surface pollen distribution. Picea pollen, with a percentage and concentration of over 30% and 30000 grains/g, was distributed at sites with MAT and MAP of −5.1–1.2 °C and 212–459 mm, respectively. Artemisia and Chenopodiaceae, with percentages and concentrations of over 50% and 50,000 grains/g and 50% and 10,000 grains/g, respectively, were mainly distributed at sites with MAT and MAP of −1.9–9.1 °C and 145–416 mm, and −3.2–8.9 °C and 129–449 mm, respectively, providing an accurate reference for paleoclimates reconstruction. This study presents information regarding the relationship between surface soil pollen and climate records, which may contribute to the reconstruction of historical climates, promote ecological development in Xinjiang, China, and assist in predicting future climate trends.

Quantitative climatic reconstruction and prehistoric human subsistence strategy evolution since the mid-Holocene in Nenjiang river Basin, Northeastern China

Quantitative paleoclimate reconstructions can provide crucial climate context to test the hypothesis of climatic impact on prehistoric human activities. However, the climate change in Nenjiang River Basin is poorly understood due to the lack of reliable paleoclimate records. In this study, we report paired quantitative temperature (MAAT0) and soil pH records based on the branched glycerol dialkyl glycerol tetraether (brGDGTs) since the mid-Holocene in an alluvial sedimentary sequence near the Honghe Neolithic site in the middle reaches of Nenjiang River Basin. The result of MAAT0 shows an obvious cooling interval at 6.5–4.0 kyr BP, which is consistent with paleotemperature reconstruction results in EASM margin. The strengthened westerlies may play a crucial role in shaping the cooling mid-Holocene in EASM margin. The soil pH variations yield a peak at 6.0–3.5 kyr BP, which is roughly consistent with other precipitation reconstruction from northeastern China. From the middle Neolithic to Bronze period, the prehistoric humans gradually settled in Nenjiang River Basin. Cool-wet climate during 6.0–4.0 kyr BP provide enough water & wildlife resources and broader space for human survival and thus prompted the settlement and cultivation development. With the deterioration of the climate since 4.0 kyr BP, the ancient inhabitants have to move southward along the Nenjiang River to lower reaches to find more suitable places for settlement.

Multiproxy Paleosol Evidence for Jurassic Paleoclimate Fluctuations in the Sichuan Basin, SW China

AbstractPaleoclimate reconstruction of continental environments has been hampered by the limited evidence. A thick sequence of Jurassic continental deposits in the Sichuan Basin, Southwest China yields abundant paleosols that may offer valuable insights regarding Jurassic climate scenarios. A succession of 169 paleosols belonging to Protosols, Calcisols, and Argillisols from 23 detailed stratigraphic sections was recognized and characterized macro‐ and micromorphologically and assessed for mineral and geochemical compositions. Quantitative paleoclimate reconstructions using bulk geochemical proxies, the depth to and the stable oxygen isotopic composition of paleosol carbonates indicated a predominant alternation of semiarid and arid cool/warm‐temperate climatic conditions punctuated by several episodes of subhumid and humid climates that generally prevailed in the Sichuan Basin during the Jurassic. The estimated paleoatmospheric CO2 concentrations (pCO2) from calcic paleosols yielded a low range of ∼104 ± 58 to ∼610 ± 152 ppmv during the Middle Jurassic. The terrestrial paleotemperature changes in the Sichuan Basin coincided with the pCO2 variations, which probably resulted from global geological events (e.g., volcanic activities, magmatic and oceanic events, and the ephemeral caps development) in the Middle Jurassic. Jurassic climatic fluctuations in the basin were likely attributed to true polar wander due to global plate motion, megamonsoon effect linked to global and regional paleogeography, and regional paleotopography.

Application of a topsoil phytolith dataset to quantitative paleoclimate reconstruction in Northeast China

Although phytoliths are recognized as an important proxy for paleoenvironmental reconstruction, the quantitative relationship between phytoliths and climate is still debated. In order to provide an improved basis for phytolith-based paleoclimate reconstructions, a representative modern phytolith dataset is essential. Here, we synthesize a modern topsoil phytolith dataset for Northeast China, analyze its climatic significance, and apply it to a fossil phytolith series from the Hani peat core in Northeast China. The dataset comprises 660 topsoil phytolith assemblages from 289 sample sites. We compiled modern meteorological data to assess the quantitative relationship between the phytolith assemblages and climatic variables. Detrended correspondence analysis (DCA) and Redundancy analysis (RDA) were used to determine the dominant climatic variable influencing the phytolith distributions. The results showed that mean annual temperature (MAT) is the dominant variable controlling the spatial distribution of phytoliths, accounting for 8.91% of the total variance. Transfer function based on inverse deshrinking locally-weighted weighted averaging (LWWA_Inv) was developed for MAT (R2_boot = 0.86, RMSEP = 1.02 °C). Applying the LWWA_Inv transfer function to fossil phytolith records from the Hani peat core enables quantitative inferences to be made about Holocene climate changes in Northeast China. Overall, combined with the LWWA_Inv method, the topsoil phytolith dataset of Northeast China can be used for reliable quantitative MAT reconstruction.

Age-Heterogeneity in Marine Sediments Revealed by Three-Dimensional High-Resolution Radiocarbon Measurements

Marine sedimentary archives are routinely used to reconstruct past environmental changes. In many cases, bioturbation and sedimentary mixing affect the proxy time-series and the age-depth relationship. While idealized models of bioturbation exist, they usually assume homogeneous mixing, thus that a single sample is representative for the sediment layer it is sampled from. However, it is largely unknown to which extent this assumption holds for sediments used for paleoclimate reconstructions. To shed light on 1) the age-depth relationship and its full uncertainty, 2) the magnitude of mixing processes affecting the downcore proxy variations, and 3) the representativity of the discrete sample for the sediment layer, we designed and performed a case study on South China Sea sediment material which was collected using a box corer and which covers the last glacial cycle. Using the radiocarbon content of foraminiferal tests as a tracer of time, we characterize the spatial age-heterogeneity of sediments in a three-dimensional setup. In total, 118 radiocarbon measurements were performed on defined small- and large-volume bulk samples ( ∼ 200 specimens each) to investigate the horizontal heterogeneity of the sediment. Additionally, replicated measurements on small numbers of specimens (10 × 5 specimens) were performed to assess the heterogeneity within a sample volume. Visual assessment of X-ray images and a quantitative assessment of the mixing strength show typical mixing from bioturbation corresponding to around 10 cm mixing depth. Notably, our 3D radiocarbon distribution reveals that the horizontal heterogeneity (up to 1,250 years), contributing to the age uncertainty, is several times larger than the typically assumed radiocarbon based age-model error (single errors up to 250 years). Furthermore, the assumption of a perfectly bioturbated layer with no mixing underneath is not met. Our analysis further demonstrates that the age-heterogeneity might be a function of sample size; smaller samples might contain single features from the incomplete mixing and are thus less representative than larger samples. We provide suggestions for future studies, optimal sampling strategies for quantitative paleoclimate reconstructions and realistic uncertainty in age models, as well as discuss possible implications for the interpretation of paleoclimate records.

Relationship between modern pollen assemblages and vegetation in the Bashang typical steppe region of North China

Accurate knowledge of the relationship between modern vegetation and the corresponding surface pollen assemblages is the key to the quantitative reconstruction of paleovegetation and paleoclimate. The pollen assemblages of steppe regions are affected by many factors and distinguishing the pollen assemblages of different plant community types, especially Artemisia and Poaceae communities is a major difficulty in modern pollen studies in these regions. In this study we used RDA analysis to analyze modern vegetation and pollen percentage data from 119 sampling sites in different plant communities in the Bashang region of North China, which has a typical steppe vegetation. We used the results to quantitatively determine the distinguishing features of various plant community types and the relationship between the percentages of the dominant pollen types and the corresponding vegetation cover. The major results are as follows: 1) there is a clear distinction between Artemisia and Poaceae communities; in the Artemisia community, Artemisia pollen percentages are typically > 50%, and Poaceae pollen percentages are typically < 3%. In the Poaceae community, Poaceae pollen percentages are typically > 3%, and Artemisia pollen percentages are typically < 40%. 2) The Artemisia, Chenopodiaceae, Poaceae and Asteraceae are the most common vegetation types and their pollen percentages are the four most abundant types in the pollen assemblages. Their respective pollen percentages and vegetation cover show varying degrees of correlation and representativeness. Artemisia and Chenopodiaceae pollen are over-represented; that is, when the vegetation cover of Artemisia and Chenopodiaceae is < 5%, or even zero %, Artemisia and Chenopodiaceae pollen percentages can still reach 30–40%. By contrast, Poaceae and Asteraceae are under-represented in the pollen assemblages, and even where they have a high vegetation cover, their pollen percentages are rarely > 10%.

Middle Jurassic climate oscillations from paleosol records of the Sichuan Basin, SW China

The Middle Jurassic is characterized by major plate tectonic changes, variable atmospheric CO2 concentrations (pCO2), and climate oscillations in both marine and terrestrial realms. However, the limited evidence from the terrestrial sedimentary records hampers a comprehensive understanding of global climate state in this period. Well-exposed strata of the Shaximiao Formation in the Sichuan Basin, Southwest China, preserve successions of paleosols that offer the possibility of evaluating the terrestrial paleoclimatic changes during the Middle Jurassic (Bajocian to Callovian). A total of 151 paleosol profiles belonging to four paleosol types (Entisol-like, Inceptisol-like, Aridisol-like and argillic Inceptisol-like paleosols) were identified and characterized. The quantitative paleoclimate reconstructions based on the bulk geochemistry of paleosol horizons and depths of carbonate nodules indicate a generally semiarid–subhumid climate alternating with arid–humid and cool/warm–temperate climates during this time interval. This dynamic climate terminated in a dry, subhumid, humid/perhumid and superhumid moisture regime characterized by steppe/wet or rain forest floral provinces. We suggest that these climatic fluctuations may have been related to global geodynamic (e.g., the fragmentation of Pangea, “megamonsoonal” circulation, and bolide impacts) and regional tectonic uplift (e.g., the uplift of the Longmenshan Mountains in the South China Block) during the Middle Jurassic. The pCO2 estimates suggest relatively low pCO2 levels within the range of ∼100 ppm–∼890 ppm during the Bajocian to Callovian. Our results coincide with the threshold for the initiation of glaciation and may help to explain the occurrence of cool intervals in the generally warm Jurassic. These pCO2 fluctuations, which coincide with paleotemperature variations in the low paleolatitudes of the Northern Hemisphere, are possibly attributable to global geological events (e.g., wildfire events, eustatic sea-level oscillations, and the transient development of glaciations) in the Middle Jurassic.

Application of a Topsoil Phytolith Dataset for Quantitative Paleoclimate Reconstruction in Northeast China

Application of a Topsoil Phytolith Dataset for Quantitative Paleoclimate Reconstruction in Northeast China

Seasonal climate signals preserved in biochemical varves: insights from novel high-resolution sediment scanning techniques

Abstract. Varved lake sediments are exceptional archives of paleoclimatic information due to their precise chronological control and annual resolution. However, quantitative paleoclimate reconstructions based on the biogeochemical composition of biochemical varves are extremely rare, mainly because the climate–proxy relationships are complex and obtaining biogeochemical proxy data at very high (annual) resolution is difficult. Recent developments in high-resolution hyperspectral imaging (HSI) of sedimentary pigment biomarkers combined with micro X-ray fluorescence (µXRF) elemental mapping make it possible to measure the structure and composition of varves at unprecedented resolution. This provides opportunities to explore seasonal climate signals preserved in biochemical varves and, thus, assess the potential for annual-resolution climate reconstruction from biochemical varves. Here, we present a geochemical dataset including HSI-inferred sedimentary pigments and µXRF-inferred elements at very high spatial resolution (60 µm, i.e. &gt; 100 data points per varve year) in varved sediments of Lake Żabińskie, Poland, over the period 1966–2019 CE. We compare these data with local meteorological observations to explore and quantify how changing seasonal meteorological conditions influenced sediment composition and varve formation processes. Based on the dissimilarity of within-varve multivariate geochemical time series, we classified varves into four types. Multivariate analysis of variance shows that these four varve types were formed in years with significantly different seasonal meteorological conditions. Generalized additive models (GAMs) were used to infer seasonal climate conditions based on sedimentary variables. Spring and summer (MAMJJA) temperatures were predicted using Ti and total C (Radj2=0.55; cross-validated root mean square error (CV-RMSE) = 0.7 ∘C, 14.4 %). Windy days from March to December (mean daily wind speed &gt; 7 m s−1) were predicted using mass accumulation rate (MAR) and Si (Radj2=0.48; CV-RMSE = 19.0 %). This study demonstrates that high-resolution scanning techniques are promising tools to improve our understanding of varve formation processes and climate–proxy relationships in biochemical varves. This knowledge is the basis for quantitative high-resolution paleoclimate reconstructions, and here we provide examples of calibration and validation of annual-resolution seasonal weather inference from varve biogeochemical data.

Late Quaternary Climate Reconstruction and Lead-Lag Relationships of Biotic and Sediment-Geochemical Indicators at Lake Bolshoe Toko, Siberia

Millennial-scale climate change history in eastern Siberia and relationships between diatom diversity, paleoclimate, and sediment-geochemical lake system trajectories are still poorly understood. This study investigates multi-proxy time series reaching back to the Late Pleistocene derived from radiocarbon dated Lake Bolshoe Toko sediment cores, southeastern Yakutia, Russia. We analyzed diatoms, elements (XRF), minerals (XRD), grain-size, organic carbon, and included chironomid analyses and published pollen-data for quantitative paleoclimate reconstruction. Changes in diatom species abundances reveal repeated episodes of thermal stratification indicated by shifts from euplanktonicAulacoseiratoCyclotellaspecies. Chironomid and pollen-inferred temperature reconstruction reveal that the main shift between these diatom species is related to the onset of Holocene Thermal Maximum (HTM) at 7.1 cal ka BP. Comparison to other paleoclimate records along a north-south transect through Yakutia shows that the HTM was delayed as far south as the Stanovoy mountains. Relationships between sediment-geochemistry, paleoclimate variability and diatom species richness (alpha diversity) was tested in a moving temporal offset approach to detect lead-lag relationships. Sediment-geochemical data, mainly uniform during the Holocene, revealed strongest positive or negative correlations ahead of species richness changes. Mean July air temperature (TJuly) reconstructions correlate with both Hill numbers and relative assemblage changes indicated by sample scores of multidimensional scaling analysis (MDS) over the entire time series. We found that sediment organic carbon revealed distinct positive correlations, i.e., centennial-scale delay to increases in diatom effective richness (Hill numbers N0 and N2). We conclude that a lag of deposited organic carbon concentrations behind changes in diatom alpha diversity reveals that species richness can augment the production and thus sequestration of organic matter in comparable lake systems.