Summer Insolation Research Articles

Seasonality in Holocene Temperature Reconstructions in Southwestern China

AbstractReconstructions of global surface temperature, dominated by records from the northern extratropics, show an apparent Holocene cooling trend after the early Holocene climatic optimum. However, model simulations suggest a global warming Holocene tendency. This “Holocene temperature conundrum” may be caused by the seasonal bias of paleotemperature proxies. Here we report a quantitative Holocene record with ∼100‐year resolution based on branched glycerol dialkyl glycerol tetraethers from an alpine lake in southwestern China. Our reconstructed Holocene temperature record displays a steady long‐term trend without distinct cooling or warming changes. Based on the temperature values and their evolution over time, our reconstruction is interpreted to present temperature changes in ice‐free seasons from March to November. Unlike the often‐documented Holocene cooling of regional summer temperatures driven by boreal summer insolation, this observed trend in our reconstructed temperatures is probably caused by slightly decreasing local ice‐free season insolation and somewhat compensated by increasing atmospheric greenhouse gas concentrations. Our results demonstrate that the climatic drivers of ice‐free season and summer temperature changes could be different and highlight the significance of elucidating the seasonality of proxies before using them for paleoclimate reconstructions.

Holocene sedimentary evolution of the Mekong River floodplain, Cambodia

The Mekong River, which has high economic and ecological importance, has an extensive floodplain and delta plain that extends from Cambodia to Vietnam. Understanding the Holocene evolution of the fluvial system is needed in order to predict the future development of the system and develop a strategy to guide the sustainable management. This paper reconstructs the Holocene sedimentary evolution of the Mekong River floodplain, Cambodia, from 12 auger cores and 69 optically stimulated luminescence ages, and compares the reconstructed floodplain history with that of other large river systems. Rapid deposition of thick crevasse splays implies floodplain aggradation in response to sea-level rise before 6.5 ka. After 6.5 ka, when the sea-level reached a stillstand, the rapid aggradation ceased and levee and crevasse-splay deposition were localized near the anastomosing river channels. The rate of levee accretion was very low in the anastomosing river system (<1 m/kyr), which implies that the riverbed aggradation caused by sea-level rise before 6.5 ka outpaced levee accretion and led to avulsion by diversion into the floodbasin. Some distributary channels were abandoned around 4 ka, corresponding to a shift from an anastomosing to a single-channel river system. This shift might be attributed to an abrupt decrease in precipitation caused by weakening of the East Asian and Indian summer monsoons. Downstream of Phnom Penh, rapid levee deposition started on the uppermost delta plain after 1 ka while rapid lateral migration of the river channel occurred upstream. These increased fluvial activities correspond to accelerated mud accretion of the Mekong delta coast and potentially reflect an increase in sediment supply caused by human activities. The evolution of the Mekong River floodplain might be comparable with other large river systems in the EASM, ISM, and African monsoon regions, where a substantial weakening of the summer monsoon and hence a decrease in precipitation occurred in response to declining summer insolation in the middle to late Holocene.

Past Warmth and Its Impacts During the Holocene Thermal Maximum in Greenland

Higher boreal summer insolation in the early to middle Holocene drove thousands of years of summer warming across the Arctic. Modern-day warming has distinctly different causes, but geologic data from this past warm period hold lessons for the future. We compile Holocene temperature reconstructions from ice, lake, and marine cores around Greenland, where summer temperatures are globally important due to their influence on ice sheet mass balance, ocean circulation, and sea ice. Highlighting and accounting for some key issues with proxy interpretation, we find that much of Greenland experienced summers 3 to 5°C warmer than the mid-twentieth century in the early Holocene—earlier and stronger warming than often presumed. Warmth had dramatic consequences: Many glaciers disappeared, perennial sea ice retreated, plants and animals migrated northward, the Greenland Ice Sheet shrank rapidly, and increased meltwater discharge led to strong marine water stratification and enhanced winter sea ice in some areas. ▪ Summer air temperatures and open ocean temperatures around much of Greenland peaked in the early Holocene in response to elevated summer insolation. ▪ Peak summer air temperatures ranged from 3 to 5°C warmer than the mid-twentieth century in northwest and central Greenland to perhaps 1 to 2°C warmer in south Greenland. ▪ Many differences between records can be explained by proxy seasonality, ice sheet elevation changes, vegetation analogs and lags, and the nearshore effects of ice sheet meltwater. ▪ Early Holocene warmth dramatically affected glaciers and the Greenland Ice Sheet; meltwater discharge, nearshore ocean salinity, and sea ice; and diverse flora and fauna.

Late Quaternary dynamics of the Lambert Glacier-Amery Ice Shelf system, East Antarctica

The Lambert Glacier-Amery Ice Shelf system (LGAISS) is the largest outlet glacier system in East Antarctica but its response to past climate variability is poorly constrained. In this study, we explore its dynamics over the last ∼520 thousand years using new high-resolution sedimentary records retrieved off Prydz Bay. Episodic occurrences of iceberg rafted debris indicate a dynamic ice sheet throughout this interval, while changes in clay mineral compositions provide detailed evidence on the waxing and waning of the LGAISS. Our data indicate that advance and retreat of the LGAISS was sensitive to oceanic forcing, but also responded to local summer insolation when insolation peaks combined with interglacial sea level high stands. Subglacial bed topography may have played an additional role in modulating the LGAISS behaviour, which could explain sectoral differences in the response of the East Antarctic Ice Sheet to climate change. Overall, our records indicate that the LGAISS advanced more extensively during previous late Quaternary glacial periods than during Marine Isotope Stages 2 and 4. Furthermore, the LGAISS retreated more significantly than present during Marine Isotope Stage 13, and only moderately during Marine Isotope Stage 7, which suggests that the duration of warm climatic states could be a key factor affecting the dynamics of the East Antarctic Ice Sheet.

Profiling interactions between the Westerlies and Asian summer monsoons since 45 ka: Insights from biomarker, isotope, and numerical modeling studies in the Qaidam Basin

AbstractThe Qaidam Basin marks a crucial boundary between the Westerlies and the Asian summer monsoons. Previous studies in the Qaidam Basin have advanced our knowledge of the paleoclimate over glacial to interglacial cycles. However, our understanding of the paleoclimatic sensitivity of the Qaidam Basin to the relative strength of these two climatic driving forces remains limited due to the lack of regional paleoclimatic reconstructions. The Qaidam Basin is proposed as a regional and global eolian dust source during the glacial periods, during which a cold, dry climate is associated with the equatorward shift of the jet stream. On the contrary, paleoshoreline records suggest that a highstand lake stage prevailed in late Marine Isotope Stage 3 (MIS 3) and lasted until 15 ka. To address this conundrum, we have applied an integrated approach to reconstructing the regional paleoclimatic history by combining compound-specific isotope analysis, lake temperature reconstruction, and numerical modeling. Our results show varying paleoclimate associated with the dynamic climate boundary since 45 ka: (1) a wet climate during late MIS 3, when the Asian summer monsoons are strengthened under high summer insolation and penetrate further into Central Asia; (2) a general cold, dry but wetter than at present climate in the Last Glacial Maximum (LGM), when the Asian summer monsoons retreat and the Westerlies become dominant; and (3) three short periods of extreme aridity corresponding to the Younger Dryas and Heinrich 2 and 4 events, when the normal moisture transport via the Westerlies and Asian summer monsoons is interrupted. The numerical modeling supports an increase in the effective precipitation during the LGM due to reduced evaporation under low summer insolation. These results suggest that the Westerlies and Asian summer monsoons alternately controlled the climate in the Qaidam Basin in response to precessional forcing during the late Pleistocene.

A New Theory of Change in the Insolation of the Earth over Millions of Years against Marine Isotope Stages

Results from a new astronomical theory of paleoclimate for the last 1, 5, and 20 Myr are presented. Evolutions of obliquity e and insolation at 65°N $$Q_{s}^{{65\,{\text{N}}}}$$ are considered for the summer half year and insolation I at equivalent latitudes over the last 1 Myr; the results are compared with the evolution of these parameters obtained from the former theory. The oscillations in the obliquity e range from 14.7° to 32.1° according to the new theory and from 22.08° to 24.45° according to the former theory; i.e., the amplitude of oscillations have increased 7–8 times. The insolation oscillations have increased to the same extent. The summer insolation $$Q_{s}^{{65\,{\text{N}}}}$$ was used to introduce six (from very cold to very warm) climate gradations. Oscillations in the parameters of orbital and rotational motions of the Earth and various insolation components, as well as their periods and amplitudes, are considered for the last 5 Myr. Orbit eccentricity e varies with periods of 94.6 kyr, 413 kyr, and 2.31 Myr, while the periods of perihelion rotation relative to the equator are nonuniform and vary from 13.8 to 41.8 kyr. These periods can also be found in changes in the obliquity and insolation. The distribution of insolation over the Earth’s latitude for the summer and winter caloric half-years and for the year in three different (modern, coldest, and warmest) epochs is considered. The maximum changes in summer and annual insolation occur at high latitudes. The statistics of coldest and warmest periods are considered for the last 20 Myr. On average, there can be six such periods per 1 Myr; their number can reach 10 in unquiet time intervals and can decrease to 2 in quiet time intervals. The results of measurements of the oxygen isotope 18O content in marine sediments are analyzed. The content of 18O is shown to be inconsistent with the change in the Earth’s insolation according to both the former theory and the new one; in addition, the variation in 18O is inconsistent with known paleoclimate changes. An analysis of short-period variations in radioactive isotopes, as well as 18O in stalagmites and glacial cores, indicates that variations in 18O in marine sediments can be caused by a change in the 18O content in the Earth’s atmosphere, rather than by climate fluctuations.

The elemental enrichments at Dajiuhu Peatland in the Middle Yangtze Valley in response to changes in East Asian monsoon and human activity since 20,000 cal yr BP

Here we present multiproxy inorganic geochemical records from a peat core (ZK5) from the Dajiuhu Basin in central China to investigate peatland deposition processes and atmospheric metal pollution and to explore their relationships with East Asian monsoon change and human activities in the Middle Yangtze Valley since 20,000 cal yr BP. The peat physicochemical data including total organic carbon (TOC), trace elements, and grain-size show that the site has changed from a lake during the cold-wet Last Glacial Maximum (LGM; 20,000–18,000 cal yr BP), to a marshy wetland during the mild last deglaciation (18,000–11,500 cal yr BP) and a peatland during the mostly warm and dry Holocene (11,500 cal yr BP–present). This general sequence corresponds with changes in East Asian monsoon indicated by stalagmites δ18O records and boreal summer insolation. Marked decreases in trace element concentrations correspond to two periods of peatland expansion during the abrupt hydroclimatic transitions from the LGM to the last deglaciation and from the last deglaciation to the early Holocene. Warm-dry mid-Holocene might induce high organic matter decomposition in peat sediments. Increasing natural element concentrations since the late Holocene are correlated with the weakening of the summer monsoon and elevated atmospheric dust deposition. Increasing Cu and Pb concentrations in peat record indicate large-scale Cu smelting during the Bronze Age and excessive coal burning during the 10th century or so. The anthropogenic heavy metals were transported by prevailing East Asian summer monsoon and deposited in the Dajiuhu Basin during periods of heightened human activities. Our compilation of heavy metals records across China confirmed the noticeable impacts of the historical human activity on deposition environments during the late Holocene. Consequently, trace elements from the Dajiuhu Basin are reliable proxies for capturing monsoon climate-induced peatland deposition response and present important evidence for a historical atmospheric heavy metal pollution in the Middle Yangtze Valley. Our results offer useful references for peatland evolution and protection under the background of global change.

Sedimentary grain-size record of Holocene runoff fluctuations in the Lake Lugu watershed, SE Tibetan Plateau

Changes in moisture conditions or precipitation in the SE Tibetan Plateau during the Holocene have been studied using various environmental archives and proxies. However, due to different interpretations of the proxies and records, the pattern of Holocene precipitation/moisture variations in the region remains unclear. A lake-sediment-based reconstruction of runoff variations, which can directly and sensitively reflect changes in precipitation, provides the opportunity to reconstruct the evolution of moisture conditions in the SE Tibetan Plateau during the Holocene. In this study, we used a well-dated sediment core (LGH2) from Lake Lugu, a deep alpine lake charged mainly by precipitation on the lake surface and by runoff from the watershed, to reconstruct variations in runoff during the Holocene. In addition, 70 lake surface sediment samples were collected to examine the spatial variation of grain size. Endmember modeling analysis of the grain-size data was used to characterize the processes of sediment transport and runoff fluctuations. The carbonate content of core LGH2 shows that the lake level was generally high during 11,600–3100 cal years BP, and that the lake basin was closed after 3100 cal years BP and semi-closed since 90 cal years BP. Grain-size endmember EM 3, which represents the runoff input clastic materials, is used to reconstruct runoff fluctuations in the Lake Lugu watershed. The record indicates a gradual increase in runoff during 11,600–9000 cal years BP, stable and high runoff during 9000–2000 cal years BP, and weak runoff and a low lake level since 2000 cal years BP. Our reconstruction of runoff fluctuations tracks changes in regional temperature and tropical SSTs rather than in boreal summer insolation. This finding supports the hypothesis that increasing tropical SSTs strengthened ITCZ convection which enhanced the flux of water vapour from the ocean to the air, and hence the moisture supplies to SW China.

Climatic and environmental changes in the Yana Highlands of north‐eastern Siberia over the last c. 57 000 years, derived from a sediment core from Lake Emanda

The sediment succession of Lake Emanda in the Yana Highlands was investigated to reconstruct the regional late Quaternary climate and environmental history. Hydro‐acoustic data obtained during a field campaign in 2017 show laminated sediments in the north‐western and deepest (up to ̃15 m) part of the lake, where a ̃6‐m‐long sediment core (Co1412) was retrieved. The sediment core was studied with a multi‐proxy approach including sedimentological and geochemical analyses. The chronology of Co1412 is based on 14C AMS dating on plant fragments from the upper 4.65 m and by extrapolation suggests a basal age of c. 57 cal. ka BP. Pronounced changes in the proxy data indicate that early Marine Isotope Stage (MIS) 3 was characterized by unstable environmental conditions associated with short‐term temperature and/or precipitation variations. This interval was followed by progressively colder and likely drier conditions during mid‐MIS 3. A lake‐level decline between 32.0 and 19.1 cal. ka BP was presumably related to increased continentality and dry conditions peaking during the Last Glacial Maximum (LGM). A subsequent rise in lake level could accordingly have been the result of increased rainfall, probably in combination with seasonally high meltwater input. A milder or wetter Lateglacial climate increased lake productivity and vegetation growth, the latter stabilizing the catchment and reducing clastic input into the lake. The Bølling‐Allerød warming, Younger Dryas cooling and Holocene Thermal Maximum (HTM) are indicated by distinct changes in the environment around Lake Emanda. Unstable, but similar‐to‐present‐day climatic and environmental conditions have persisted since c. 5 cal. ka BP. The results emphasize the highly continental setting of the study site and therefore suggest that the climate at Lake Emanda was predominantly controlled by changes in summer insolation, global sea level, and the extent of ice sheets over Eurasia, which influenced atmospheric circulation patterns.

Seasonal rainfall patterns in stable carbon isotopes in the Mu Us Desert, northern China during the early and middle Holocene

Intense Asian summer monsoon rainfall responds to high boreal summer insolation and environmental feedbacks in the early and middle Holocene. However, it is unclear what role of the Asian summer monsoon strength might have played for the heterogeneous wetness pattern in northern China. Here, we report two wet periods in the early and middle Holocene by paleosol, lacustrine, and peat stratigraphies in the southeast Mu Us Desert, and that early-Holocene wetness records are common and seen in valleys and missing from valley walls while mid-Holocene wetness records are found everywhere in the region. The preliminary analysis of the stable carbon isotopic composition (δ13C) of soil carbonate and organic carbon from a riverine dune–paleosol sequence in a river valley reveals that wet springs/falls with dry summers frequently prevailed in the early Holocene, while wet summers with dry springs/falls became predominant in the middle Holocene. Strong-wind and low-temperature springs and falls on valley walls and uplands cause eolian sedimentation faster than soil formation but weak-wind and warmer-soil temperature in valleys facilitate vegetation metabolism and thus soil formation. Warm summer rainfall with no strong northwesterly wind causes paleosol, lacustrine and peat formation either in valleys or on uplands. The early-Holocene heterogeneous and middle-Holocene uniform wetness records are indeed controlled by seasonal hydroclimate conditions but not a single factor of summer monsoon rainfall strength in the southeast Mu Us Desert.

Reconstruction of temperature and precipitation spanning the past 28 kyr based on branched tetraether lipids from Qionghai Lake, southwestern China

The Indian summer monsoon (ISM) significantly influences the climate of southwestern China; however, continuous quantitative temperature reconstructions since the Last Glacial Maximum are rare, which limits our understanding of ISM evolution. In this study, we analyzed the branched glycerol dialkyl glycerol tetraethers (brGDGTs) from a lacustrine sediment core from Qionghai Lake to reconstruct the paleoclimatology of southwestern subtropical China over the past 27.7 kyr. Our results suggest that the brGDGTs in Qionghai Lake sediments were probably sourced from catchment soils, with some contributions from in situ lacustrine production. We found that the temperature of the Last Glacial Maximum and Holocene Climatic Optimum was ~3 °C cooler and ~ 2.5 °C warmer than present-day conditions, respectively, based on brGDGTs-reconstructed mean annual air temperatures (MATs). Generally, temperature and precipitation showed synchronous paleovariability in ISM regions. Moreover, Northern Hemisphere summer insolation was the dominant control on climate variability in ISM regions on the precessional scale, while Atlantic Meridional Overturning Circulation activity, Intertropical Convergence Zone positioning, and glacial boundary conditions (ice sheet extent and sea surface temperature) contributed to climate shifts on millennial timescales.

Pollen records of orbitally modulated variation in East Asian winter monsoon intensity and freshwater inflow to the Ulleung Basin of East Sea, South Korea, during the last glacial period

Most pollen analysis in Korea has focused on the Holocene Epoch, because pre-last glacial maximum terrestrial archives suitable for palynological study in the Korean Peninsula are limited. Thus, to verify the responses of vegetation to climate changes over the orbital timescales, analysis of pollen in marine sediments is needed. Here, we present the pollen record of hemipelagic sediments of core DD09-ST21 obtained from the northern Ulleung Basin in the East Sea. The palynofloral assemblages were dominated by warm temperate broadleaved trees and boreal conifers during interglacial and glacial periods, respectively. We also demonstrated that bisaccate pollen and herbaceous pollen in the palynological sequence can be used as indicators of the variation in the East Asian winter monsoon (EAWM) intensity during the last glacial period and freshwater inflow into the East Sea, respectively. During glacial periods, the EAWM intensified in response to weakening of the summer insolation at 60° N, while freshwater influx into the East Sea decreased due to a decrease in precipitation.

Mid to late-Holocene sea-surface temperature variability off north-eastern Newfoundland and its linkage to the North Atlantic Oscillation

In recent decades the surface water temperature and salinity in the Labrador Sea have been influenced by atmospheric circulation patterns, such as the North Atlantic Oscillation (NAO), as well as a trend to increasingly warm atmospheric temperatures in recent years. These changes are concerning, given the important role that temperature and salinity have on deep convection in the Labrador Sea. Yet, due to the shortness of available records, the long-term patterns of climate variability in the region are not clear. Here, a diatom-based reconstruction of summer sea-surface temperature (SST) developed from Trinity Bay, Newfoundland, provides insight into variations of SST since 7.2 cal ka BP in the southwestern Labrador Sea. The results show that the Holocene Thermal Maximum (HTM) lasted until c. 5.2 cal ka BP, which was followed by a gradual cooling trend overprinted by centennial temperature fluctuations of 1–2°C. Long-term cooling was likely the result of declining Northern Hemisphere orbital summer insolation, potentially amplified by long-term changes in surface and bottom water salinity, which led to a gradual reduction in the stratification of the water column. Centennial fluctuations in temperature vary in-phase with reconstructed variations in the NAO, supporting a consistent relationship between atmospheric circulation and SST over centennial-millennial timescales. Other factors influencing the SST variability may have been solar forcing during the mid-Holocene and variations in the strength of the subpolar gyre during the late-Holocene. The most prolonged cool period at 5.2–4.1 cal ka BP coincides with sharply reduced salinity in the Labrador Sea and a weakening of deep ventilation in the northeast Atlantic, highlighting a period with altered ocean surface conditions and circulation across the northern North Atlantic.

The application of autonomous systems based on renewable energy sources in rural settlements of different regions of Russian Federation

The aim of the research is to analyze and determine the most perspective renewable energy source (RES) for autonomous off-grid electrification in small settlements (less than 500 people) of Russia. Three climate zones were investigated in the research – South region, Far East region, Central region where average summer insolation is 5,76 kWh/m2; 5,88 kWh/m2; 5,35 kWh/m2 and average winter insolation is 1,93 kWh/m2; 0,75 kWh/m2; 1,03 kWh/m2, respectively. Output power values were similar in all investigated regions, however the main economic impact comes from social-economic factors – regional fuel price, transporting costs, assumed to be 75$US/month and size of photovoltaic system – higher solar activity needs fewer PV cells to satisfy required load. Using simulating software, hybrid photovoltaic-diesel systems demonstrated various costs of energy (LCOE): 0.257$/kWh in Krasnodar region, 0,378$/kWh in Orenburg region and 0,4$/kWh in Irkutsk region. Higher solar activity during winter season in Krasnodar region that allows to reduce fuel consumption as well as LCOE. Increased solar activity also allows to reduce the number of panels required to satisfy the demand. These results was compared to diesel only generators. Krasnodar region generators LCOE was 1,19$US/kWh, Orenburg region generators LCOE was 1,13$US/kWh, Irkutsk region generators LCOE was 1,31$US/kWh. The investigation also shows the influence of the ecological factor on the utilization of renewable energy sources – decrease in fuel consumption from 6000 liters per year to about 250 liters per year as well as reducing the carbon dioxide emissions.

Modeling climate-vegetation interactions during the last interglacial: The impact of biogeophysical feedbacks in North Africa

Previous climate model simulations of the early Last Interglacial (LIG) underestimated the temperatures compared to proxy-based reconstructions. One possible reason for this underestimation is that in these simulations the vegetation was prescribed to a pre-industrial state. To study the impact of climate-vegetation interactions, we performed a series of model experiments using the iLOVECLIM climate model, in which either VECODE or LPJ-GUESS was coupled as vegetation component. We specifically assessed the evolution of vegetation during the LIG and the magnitude of dynamical vegetation feedbacks. Our results show a relatively high vegetation cover (>70%) in the Sahara during the early LIG when the summer insolation at 20°N was high. This early stage is followed by an accelerated desertification phase after 123 ka BP. The rates of desertification in the Sahara peak at 122 ka BP, responding to the fast decline in 20°N July insolation. This desertification is accelerated when the magnitude of positive vegetation feedbacks on precipitation cannot offset the moisture deficit due to decreased summer insolation.Simulations including the LIG vegetation feedback suggest warmer conditions than simulations with prescribed pre-industrial vegetation, but they still slightly underestimate the temperature suggested by proxy records. This is particularly the case in the high latitude regions and the tropics, where the 125 ka BP vegetation cover is significantly higher than the pre-industrial state. The magnitude of vegetation feedbacks in North Africa to local climate peaks in the early LIG and decreases during the LIG, corresponding to temporal vegetation changes. During the early LIG, experiments with dynamical vegetation suggested a doubling in the amount of precipitation (∼60 cm/yr) in comparison to experiments with fixed pre-industrial vegetation. In addition, adding dynamical vegetation causes higher surface temperatures by about 2.5 °C in North Africa. At a global scale, a vegetated Sahara during the early LIG leads to an increase in surface temperature and a decline in surface air pressure due to local feedbacks, thereby enhancing mid-latitude westerlies as a result of increased latitudinal temperature and pressure gradient, leading to an increase in the amount of heat transported by the atmosphere from tropical regions to the Arctic. This green Sahara feedback provides 30% of the total contribution of global vegetation feedbacks to high latitudes warming.

Response of heterogeneous rainfall variability in East Asia to Hadley circulation reorganization during the late Quaternary

The Quaternary East Asian summer rainfall evolution reconstructed with the Chinese speleothem δ18O records shows the consistency throughout the region of East Asia, and has long been considered equivalent to the monsoon intensity. Its variation and teleconnection with global climate were usually interpreted by the meridional shift of the intertropical convergence zone (ITCZ). However, many other proxy records, climate simulations and meteorological observations suggest the inconsistent rainfall patterns in the different regions of East Asia on multi-timescales. Such spatial heterogeneity in Quaternary East Asian summer rainfall indicates that the hydroclimate in this region is not fully paced by the migration of the ITCZ. Here, we report a sediment record of rainfall evolution during the last 400 ka in the northern East China Sea, and this record, in combination with a transient climate model simulation, indicates an out-of-phase relationship between rainfall over middle-southeastern East Asia and northern and southwestern East Asia on the precession band, with high boreal summer insolation corresponding to the increased rainfall intensity in the northern and southwestern East Asia, however, decreased rainfall intensity in the middle East Asia. We attribute this regional heterogeneity in East Asian rainfall to the reorganization of the Hadley circulation, including shifts in the ascending branch (ITCZ) and descending branch (subtropical westerly jet), in response to changes in the hemispheric meridional temperature gradient. Our results highlight the crucial role of the Hadley circulation in the East Asian hydroclimate and have important implications for future climate projections.

Geochemistry of Holocene sediments from Chilika Lagoon, India: inferences on the sources of organic matter and variability of the Indian summer monsoon

We present bulk organic carbon isotopic composition (δ13C), total organic carbon (TOC) content, and total organic carbon to total nitrogen (C/N) ratio along with major and trace element geochemistry of an AMS radiocarbon-dated Holocene sediment core collected from Chilika Lagoon, India. The data are used to understand changes in the organic matter (OM) sources and the strength of the Indian summer monsoon (ISM). Our results indicate that during early to middle Holocene (10.4–4.9 ka), terrestrial (23–65%) and macrophyte (24–32%) sources were significant contributors to the sediment OM; marine component contributed ≤23%. An abrupt shift in the OM sources was observed at ~4.0 ka, marked by a significant decrease in OM contributions from the terrestrial (5%) and macrophyte (14%) end-members, and an increase in the marine end-member source (70%) into the lagoon. The period from ~4.0 to 2.1 ka recorded the lowest and highest contribution from the terrestrial (5–11%) and marine (50–70%) sources, respectively. This was followed by a gradual increase in the terrestrial and decrease in the marine contribution to 21% and 25% till the present, respectively. During this period, phytoplankton became the major source (31–40%) of OM to the lagoon. This period also experienced increasing anthropogenic activities in the catchment. In this study, proxy records show a strong ISM during early Holocene (10.4–9.0 ka), which was followed by a gradual weakening until 4.9 ka. At ~4.0 ka, an abrupt and pronounced weakening of ISM occurred that was followed by an amelioration of ISM precipitation towards 1.1 ka. Later, the ISM gradually weakened to the present-day condition. Overall, Chilika Lagoon core records exhibit that changes in the Northern Hemisphere summer insolation regulate the long-term trend of ISM during the Holocene.

Increasing effective moisture during the Holocene in the semiarid regions of the Yili Basin, Central Asia: Evidence from loess sections

The evolution of Holocene moisture in semiarid and arid regions of Central Asia is key to understanding changes in current and future water resources, and has been a hotly debated topic in the past few decades. Central Asian loess deposits have shown potential as records of past moisture changes, particularly during the Holocene. Here, the variations in effective moisture during the Holocene in the semiarid regions of the Yili Basin, Central Asia, are obtained from three loess sections. Reliable chronologies were constructed using 26 quartz optically stimulated luminescence (OSL) ages. The strengthening pedogenesis, steady increases in magnetic susceptibility, and continuous decreases in mean grain size in the three loess sections all suggest that effective moisture continuously increased throughout the Holocene, indicating a dry early Holocene, a moderately humid middle Holocene, and a wet late Holocene. Rainfall carried by the Asian summer monsoon and the Westerlies contributes to the moisture changes in Central Asia. However, considering the dominance of evaporation over rainfall in controlling aridity-humidity shifts in this region, the continuous weakening of evaporation caused by a decrease in Northern Hemisphere summer insolation and associated atmospheric temperatures is primarily responsible for the wetting trend throughout the Holocene in the semiarid regions of the Yili Basin.

Evolution of the meridional shift of the subtropical and subpolar westerly jet over the Southern Hemisphere during the past 21,000 years

Deciphering the variations in the subtropical westerly jet (STJ) and the subpolar westerly jet (SPJ) over the Southern Hemisphere in Earth’s history will help us to better understand how the westerlies may respond to various external forcings in the future. Using Simulation of Transient Climate Evolution over the past 21,000 years simulations, we demonstrate that the STJ migrated equatorward in the Last Glacial Maximum relative to the present, which followed by abrupt poleward, equatorward, and poleward shifts during the Heinrich Stadial I, Antarctic Cold Reversal, and Younger Dryas, respectively. These large-amplitude fluctuations are controlled by changes in meltwater fluxes and polar ice-sheet topography. During the Holocene, the STJ shows no obvious long-term trend, due to the offsetting effect between increased orbital insolation in austral summer and decreased Antarctic ice-sheet topography. The meridional shift of the SPJ shows an in-phase change with the STJ during the past 21 kyr. In terms of the dynamic mechanisms, the meridional shift of the STJ and SPJ is induced by changes in the meridional temperature gradient and the maximum eddy growth rate, respectively; and is also linked to changes in the mean meridional circulations (that is, the Hadley and Ferrel cells). The modeled meridional shift of the southern westerlies is supported by the results from the Paleoclimate Modelling Intercomparison Project Phase Ⅲ and is consistent with several reconstructions, but it is difficult to constrain robustly by geological evidence as the reconstructed westerlies vary across sites and proxies. Nevertheless, our study provides a possible scenario for the evolution of the southern westerlies during the past 21 kyr and identifies the dominant forcing and the associated mechanisms, which may advance our knowledge on the future behavior of the southern westerlies.

Abrupt climate change and its influences on hominin evolution during the early Pleistocene in the Turkana Basin, Kenya

Rapid climate variability has been hypothesized to play an important role in hominin evolution, yet our knowledge of Plio-Pleistocene climate change on short timescales is poor. Here, we developed centennial-scale reconstructions of precipitation from leaf wax biomarker hydrogen isotope ratios (δDwax) using lacustrine sediment from West Turkana, Kenya. We analyzed two time intervals (∼1.72 and ∼1.60 Ma) with different orbital configurations (0.043 and 0.025 eccentricity, respectively) to examine the influence of seasonal insolation forcing on high-frequency climate variability and the rates of climate transitions. Our data indicate that under low summer insolation, which should induce high latitude glaciation and tropical African aridity, millennial-scale climate variability was stronger. This suggests that hominins may have been forced to contend with increased climate variability during already extreme environmental conditions. Additionally, we observe a rapid shift from arid to humid conditions occurring in less than 200 years under high-amplitude precessional-scale insolation change. The rate of this transition is similar to that observed in some proxy records of the onset of the African Humid Period, indicating high sensitivity to gradual insolation forcing in the Turkana Basin. Such abrupt climate changes could induce evolutionary selection for generalist behavioral traits in hominins.