Northern Hemisphere Summer Insolation Research Articles

Centennial- to millennial-scale Asian summer monsoon changes during the MIS 5/4 transition revealed by high-resolution stalagmite records from southwestern China

During the Marine Isotope Stage (MIS) 5/4 transition, the global climate changed from an interglacial to a glacial state, and hence it was characterized by high-amplitude climate changes. High-resolution stalagmite records can potentially improve our understanding of climate change during this transition. Here we use a high-precision 230Th-dated, 50-yr-resolution stalagmite δ18O record from Yangkou Cave, in Chongqing, southwestern China, to reveal centennial- to millennial-scale changes in Asian summer monsoon (ASM) intensity during the interval of 98.8–59.3 kyr B.P. (thousands of years before 1950 CE). The record reveals five Chinese Interstadial events, namely CIS 18–22, and pronounced centennial-scale oscillations are evident and verified within these millennial-scale events. There are four centennial-scale events of monsoon strengthening in both CIS 21 and CIS 22, corresponding to Greenland Interstadial events GI 21 and GI 22. By contrast, CIS 18, CIS 19 and CIS 20 differ in both structure and onset time relative to the corresponding GI events during MIS 4. During MIS 5, reduced ice sheet and sea-ice cover and strong Atlantic Meridional Overturning Circulation (AMOC) forced the Intertropical Convergence Zone (ITCZ) towards its northern limit, which enhanced the teleconnection between the ASM and climate change in northern high latitudes. During MIS 5, the millennial-scale events (CIS 21 and 22) show a rapid atmospheric teleconnection between the ASM and the climate of northern high latitudes, but this coupling did not exist during MIS 4. The weakening of Northern Hemisphere summer insolation, the expansion of ice sheets and sea ice, combined with the increased influence of Antarctica, may have led to the decoupling of the Asian summer monsoon and climate change in high northern latitudes.

Late Pleistocene hydroclimatic variabilities in arid north‐west China: geochemical evidence from Balikun Lake, eastern Tienshan, China

ABSTRACTReconstructing the environmental and hydrological response to millennial‐scale fluctuations of climate‐sensitive areas of mid‐latitudes is crucial to understanding the Late Pleistocene climate in terrestrial inland regions. We use sedimentological and geochemical proxies (e.g. lithology, gain size, loss‐on‐ignition, soluble salts, X‐ray fluorescence elements) from Balikun Lake in the eastern Tienshan Mountains, to elucidate variations in sedimentation, water chemistry and watershed weathering intensity of the Late Pleistocene interval (~20−60 ka). The record documents high‐frequency oscillations and alternating hydrological patterns on (multi‐) millennial timescales. Balikun Lake changes from nearly fresh to brackish and finally to hypersaline conditions during MIS3. The warm and wet climate during the early MIS3 facilitated regional vegetation and bioproductivity. Halite sedimentation took place from the middle to late MIS3, indicating a gradual drying trend. This reduced the lake area and vegetation cover, as well as weakened the chemical weathering rates of the watershed. These new interpretations challenge the idea of climate amelioration with the highest lake level and wettest conditions prevailing in the arid areas of north‐west China during the late MIS3, indicating a possible westerlies‐dominated Late Pleistocene climate in these areas. The evolution of the palaeohydrological regime and climate change in the Balikun Basin correlated well with the millennial‐scale high‐latitude Atlantic climate superimposed on the Northern Hemisphere summer insolation.

Interstadial conditions over the Southern Alps during the early penultimate glacial (MIS 6): a multiproxy record from Rio Martino Cave (Italy)

Identifying the hydrological and environmental response of the European Alpine region to different combinations of climate boundary conditions is crucial to advance the reliability of predictive climate models and thus shape climate adaptation policies that will impact millions of people in seven countries. Here we present a high-resolution multiproxy speleothem record (stable oxygen and carbon isotope ratios, petrography and magnetic properties) from Rio Martino Cave (Piedmont, Southern Alps, Italy), which covers the first part of the Penultimate Glacial (early MIS 6, 182–157 ka). During early MIS 6, the combination of high climatic precession and obliquity amplified the peak in Northern Hemisphere (NH) summer insolation intensity at ca. 174 ka to almost interglacial levels, leading to northward migration of the Intertropical Convergence Zone and the enhancement of the boreal monsoon system. At orbital scale, the hydroclimatic record from Rio Martino closely follows the precession pattern, and shows a wet interstadial phase between 180 and 170 ka, peaking at the precession minimum, characterized by glacial retreat and by the likely development of soils and vegetation up to 1900–2000 m a.s.l. in this alpine sector. This phase can be traced across the Southern Alps, and corresponds to pluvial conditions inferred from Western Mediterranean records, and to the interval of deposition of the cold Sapropel S6 in the eastern Mediterranean. We suggest that the interaction between an intensified northwesterly cold flow (relating to increased ice volume under glacial conditions), and the relatively warm waters of the NW Mediterranean (due to the peculiar atmospheric configuration occurring at the precession minimum) strongly enhanced the autumn cyclogenesis in the Northern Tyrrhenian Sea, fuelling intense precipitation to reach the Southern Alps. The Rio Martino record also shows a prominent sub-orbital variability, the overall structure of which is coherent with hemispheric changes in climate driven by cyclic perturbations of North Atlantic conditions related to the operation of the bipolar seesaw.

Analysis of the surface mass balance for deglacial climate simulations

Abstract. A realistic simulation of the surface mass balance (SMB) is essential for simulating past and future ice-sheet changes. As most state-of-the-art Earth system models (ESMs) are not capable of realistically representing processes determining the SMB, most studies of the SMB are limited to observations and regional climate models and cover the last century and near future only. Using transient simulations with the Max Planck Institute ESM in combination with an energy balance model (EBM), we extend previous research and study changes in the SMB and equilibrium line altitude (ELA) for the Northern Hemisphere ice sheets throughout the last deglaciation. The EBM is used to calculate and downscale the SMB onto a higher spatial resolution than the native ESM grid and allows for the resolution of SMB variations due to topographic gradients not resolved by the ESM. An evaluation for historical climate conditions (1980–2010) shows that derived SMBs compare well with SMBs from regional modeling. Throughout the deglaciation, changes in insolation dominate the Greenland SMB. The increase in insolation and associated warming early in the deglaciation result in an ELA and SMB increase. The SMB increase is caused by compensating effects of melt and accumulation: the warming of the atmosphere leads to an increase in melt at low elevations along the ice-sheet margins, while it results in an increase in accumulation at higher levels as a warmer atmosphere precipitates more. After 13 ka, the increase in melt begins to dominate, and the SMB decreases. The decline in Northern Hemisphere summer insolation after 9 ka leads to an increasing SMB and decreasing ELA. Superimposed on these long-term changes are centennial-scale episodes of abrupt SMB and ELA decreases related to slowdowns of the Atlantic meridional overturning circulation (AMOC) that lead to a cooling over most of the Northern Hemisphere.

Holocene Moisture Variability and Its Lagged Response to Summer Insolation Revealed by Eolian Deposits in the Lake Qinghai Basin, NE Tibetan Plateau

AbstractThe evolution of the East Asian Summer Monsoon (EASM) and its mechanism since the Holocene remains controversial. Here, we investigate four eolian sediment sequences in the Lake Qinghai Basin (LQB), Northeastern Tibetan Plateau (NETP), in the marginal zone of the modern EASM. The results indicate that paleoclimatological interpretation of the color parameters and CaCO3 in eolian sand‐paleosol sequence from LQB region are different from the loess‐paleosol sequence in Loess Plateau, and a comprehensive analysis that combines lithostratigraphic and other proxy data is required. Changes in the EASM dominated climate change in the NETP, with a gradual increase in moisture from the beginning of the early‐Holocene, however the environment was relatively cold and dry with strong regional eolian sand activity. During 8.5–3.3 ka, the intensity of the EASM and effective moisture levels in the LQB reached a peak. Eolian activity was weak and dunes were effectively fixed in place, while pedogenesis processes were enhanced and paleosols developed widely across the region. Moisture levels decreased after 3.3 ka and the dunes were reactivated, eolian activity was enhanced, and there was extensive accumulation of eolian sand and sandy loess. These records show that moisture variability has been controlled by the EASM system in east Asia since at least the Holocene, and has generally responded to changes in northern hemisphere summer insolation with a lag of 3–5 ka, through mechanisms that are modulated by high‐latitude ice‐volume forcing and large‐scale ocean‐atmospheric circulation patterns.

Orbital-scale Asian summer monsoon variations: Paradox and exploration

The Asian summer monsoon (ASM) is a vast climate system, whose variability is critical to the livelihoods of billions of people across the Asian continent. During the past half-century, much progress has been made in understanding variations on a wide range of timescales, yet several significant issues remain unresolved. Of note are two long-standing problems concerning orbital-scale variations of the ASM. (1) Chinese loess magnetic susceptibility records show a persistent glacial-interglacial dominated ~100 kyr (thousand years) periodicity, while the cave oxygen-isotope (δ18O) records reveal periodicity in an almost pure precession band (~20 kyr periodicity)—the “Chinese 100 kyr problem”. (2) ASM records from the Arabian Sea and other oceans surrounding the Asian continent show a significant lag of 8–10 kyr to Northern Hemisphere summer insolation (NHSI), whereas the Asian cave δ18O records follow NHSI without a significant lag—a discrepancy termed the “sea-land precession-phase paradox”. How can we reconcile these differences? Recent and more refined model simulations now provide spatial patterns of rainfall and wind across the precession cycle, revealing distinct regional divergences in the ASM domain, which can well explain a large portion of the disparities between the loess, marine, and cave proxy records. Overall, we also find that the loess, marine, and cave records are indeed complementary rather than incompatible, with each record preferentially describing a certain aspect of ASM dynamics. Our study provides new insight into the understanding of different hydroclimatic proxies and largely reconciles the “Chinese 100 kyr problem“ and “sea-land precession-phase paradox”.

Greenhouse Gas and Ice Volume Drive Pleistocene Indian Summer Monsoon Precipitation Isotope Variability

AbstractOrbital‐scale Indian Summer Monsoon variability is often interpreted as a direct response to northern hemisphere summer insolation. Here we present a continuous (0–640 kyr) orbital scale precipitation isotope (δDprecip) record using leaf wax δD from the core monsoon zone of India. The δDprecip record is quantitatively coherent with, and δDprecip minima in phase with, greenhouses gas maxima, and ice volume minima across all orbital bands. The δDprecip record is also coherent and in phase with the two existing orbital‐scale Indian speleothem δ18O records, demonstrating a consistent regional response among independent proxies. These findings preclude interpretation of Indian precipitation isotope records as a direct response to northern hemisphere summer insolation. Rather, they dominantly reflect changes in moisture source and transport paths associated with changes in greenhouse gases and ice volume. The orbital‐scale precipitation isotope responses of the Indian and East Asian monsoon systems are uncoupled and are driven by different forcings.

Atmospheric rivers in high-resolution simulations of the Paleocene Eocene Thermal Maximum (PETM)

Atmospheric rivers (ARs) and related precipitation are analyzed in high resolution climate simulations during the Eocene using a sophisticated Earth System Model framework. Simulations are conducted across different forcing scenarios spanning the transition from the cooler Late Paleocene period to the Paleocene Eocene Thermal Maximum (PETM), testing both greenhouse gas and orbital sensitivities. AR metrics are compiled and compared across the various simulations and shown to reflect underlying storm tracks where changes in the low-level jet are the primary forcing mechanism for landfall location. Precipitation attributable to ARs is characterized by intensity and amount and compared to total precipitation. Generally, AR precipitation intensity is predominantly moderate, however, this varies by forcing scenario, region, and latitudinal location depending on location of the storm tracks. Extreme rates exist particularly for elevated greenhouse gases and an orbital configuration that maximizes Northern Hemisphere summer insolation. Three regions are diagnosed for landfalling ARs: Western North America, Europe, and Australia. Proxy evidence from the Pyrenees is directly compared to modelled European streamflow and suggests that precipitation produced by ARs can potentially explain the proxy hydrological signal.

Holocene variability of East Asian summer monsoon as viewed from the speleothem [formula omitted]O records in central China

Monsoon precipitation in East China shows distinct spatial distribution and its variability is closely linked with the changes of the East Asian summer monsoon (EASM). Located in the transition zone between the southern subtropical humid climate and the northern warm temperate semi-humid climate, central China is a core region for recognizing and understanding the spatio-temporal variability of the EASM. Using U-series dating and stable isotope analysis on five stalagmites (MG-1, MG-2, MG-7, MG-40 and MG-64) from Magou Cave, Henan Province, Central China, we construct a high-resolution and precisely dated composite stalagmite δ18O time series covering most of the Holocene. This composite record reveals variations in precipitation δ18O between 11.7 and 1.1 ka BP with average resolution of ∼4 yrs. The Magou composite record demonstrates that EASM intensity dominates long-term changes in precipitation δ18O, which generally follows the northern hemisphere summer insolation (NHSI) trend. Both, Ensemble Empirical Mode Decomposition (EEMD) and wavelet filtering analyses real that the amplitudes of long-term (100-500 and 500-3000 yrs) components were slightly reduced between 8.5 and 4.9 ka BP, implying a weakened influence of climatic forcings on centennial and even millennial timescales during this warm period. Variance on 1-30-yr timescales is relatively low and ascribed to sampling resolution. Fourteen weak EASM intervals, including the 8.2 ka event, were identified within the period corresponding broadly with the Holocene Megathermal. Since no cold excursions other than the 8.2 ka event are found in the Greenland ice core records, we tentatively propose that oscillations in tropical sea surface temperature (SST) likely play an important role in steering other weak monsoon events. Aligning the Magou composite record and other moisture records with archaeological records from the study region, it seems that climate change influenced both the spatial distribution and agricultural practices of ancient cultures. However, overall moderate climatic changes in this region, most likely characterized by shifts between subtropical humid climate and warm temperate semi-humid climate, supported a generally consecutive development of ancient cultures without major hiatuses.

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.

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.

Revised chronology of the ICDP Dead Sea deep drill core relates drier-wetter-drier climate cycles to insolation over the past 220 kyr

The Dead Sea Deep Drilling Project drilled 456 meters into the deepest floor of the Dead Sea and recovered a record of the past ∼220 kyr of the Levant hydroclimate history, that is, Marine Isotope Stages 1–7, including the last three interglacials and the last two glacials. We present an updated chronology of the core from DSDDP Hole 5017-1-A, from the Dead Sea’s deepest basin, that refines our previous chronology (Torfstein et al. 2015) based on new information. The updated chronology uses the following approaches: (1) radiocarbon ages of Kitagawa et al. (2017); (2) correlation of specific layers in the core with U–Th-dated sediments on the Dead Sea margin, particularly during the interval of glacial Lake Lisan (MIS 2,3,4); (3) tuning of the δ18O data of DSDDP core aragonite to the U–Th dated oxygen isotopes of regional cave speleothems; and (4) tuning of the DSDDP aragonite δ18O data to summer insolation curves when the cave δ18O chronology is less clear. The updated chronology reveals a strong relationship between the sedimentary facies comprising the core and Northern Hemisphere summer insolation variations. It shows that sequences of sediments marking drier/wetter/drier climate conditions in the lake’s watershed (e.g., salt/muds/salt, respectively) appear across the flank/peak/flank segments of several summer insolation peaks. In particular, the transition from drier to wetter sedimentary facies during mid-latitude insolation peaks coincides with the intervals of sapropel conditions in the Mediterranean, reflecting enhanced Nile flow due to intense African monsoonal conditions, and marking the impact of the tropical precession cycles on Eastern Mediterranean hydroclimate. This pattern was lost during MIS 2,3,4, when mostly sequences of primary aragonite are punctuated by gypsum precipitation during Heinrich events, marking the strong impact of the North Atlantic on the last glacial Levant hydroclimate.

Centennial-scale climatic changes in Central China during the Holocene climatic optimum

Our knowledge about climatic changes at centennial-scale and their causes during the Holocene climatic optimum (HCO) remains poorly constrained. Here, we present δ18O and δ13C records from a precisely dated stalagmite (13.2 to 1.9 kyr BP) from Xiniu Cave, central China. The δ18O record shows a gradual weakening in the strength of the East Asian summer monsoon (EASM) from early to middle Holocene, corresponding with a decrease in Northern Hemisphere summer insolation (NHSI). A similar pattern is absent in our δ13C record, and is replaced by persistent centennial-scale oscillations throughout the Holocene. Variations of δ13C reflect changes in vegetation density, soil CO2 production, CO2 degassing, prior calcite precipitation (PCP), and cave ventilation, associated with temperature and rainfall variations at local scale. Based on the δ13C record, three dry/cold events at approximately 6.0, 8.2, and 10.5 kyr BP, can be identified during the HCO (11.1 ± 0.1 to 5.4 ± 0.1 kyr BP), coinciding with the well-known Holocene Bond events. The power spectrum analysis shows that the significant periodicity (843 yr) of the δ13C record within the HCO may be related to the North Atlantic-cycle. Therefore, we speculate that the centennial-scale climatic changes during the HCO probably resulted from cooling in the North Atlantic, and that the intensified and southward Westerly jet played a role in transmitting this signal from high latitudes to central China.

Holocene East Asian Summer Monsoon Rainfall Variability in Taiwan

Characterizing the spatiotemporal variability of the East Asian summer monsoon (EASM) advances our understanding of its rhythm, dynamics, and future impacts. EASM variations during the Holocene have been reconstructed from a variety of geological archives and proxies. However, the spatiotemporal heterogeneity of EASM rainfall during the Holocene remains controversial. Taiwan is geographically suitable for studying the EASM history through its geological archives. Herein, we synthesize the reported lake and peat sedimentary records of the entire Holocene, in addition to the records of mass-wasting and on-land deposition from cores collected from Taiwan to illustrate the EASM induced hydroclimate changes in Taiwan throughout the Holocene. Records from Taiwan indicate that the EASM rainfall maximum occurred during the early Holocene, concurring with other EASM records from monsoon region of southern China. We suggest that the early Holocene EASM rainfall maximum in southern China was mainly forced by the higher Northern Hemisphere summer insolation and sea surface temperatures in the Western Pacific Warm Pool (WPWP). A synthesis of EASM rainfall records from across China shows that the timing of Holocene EASM rainfall maximum occurred progressively later from southern to northern China. This time-transgressive EASM rainfall maximum may be due to the latitudinal shift of the westerlies and Western Pacific subtropical high (WPSH) that induced by changes of interhemispheric temperature gradients (ΔTN‐S) and northern high latitude ice volume. Moreover, records from Taiwan suggest a significant collapse of the EASM in Taiwan at ~4–2 ka BP. Based on the records from Taiwan, coastal East Asian and Tropical Pacific, we propose that the sea surface temperatures (SSTs) of the WPWP and/or El Niño -Southern Oscillation (ENSO) activity may have exerted a strong influence on the EASM rainfall changes during the late Holocene. Moreover, increased EASM rainfall in southern China during the last 2 ka was likely caused by a southward shift of WPSH which was associated with gradual decreases in ΔTN‐S during the late Holocene.

Insolation and greenhouse gases drove Holocene winter and spring warming in Arctic Alaska

Global surface temperature changes and their drivers during the Holocene Epoch remain controversial. Syntheses of proxy data indicate that global mean annual temperature declined from the mid-Holocene until the Pre-industrial Era, a trend linked to decreasing Northern Hemisphere summer insolation. In contrast, global climate models simulate increasing mean annual temperatures driven by retreating ice sheets and increasing greenhouse gas concentrations. This proxy-model disagreement may originate from a warm season bias in Northern Hemisphere proxy reconstructions, highlighting the need for new proxies that quantify cold season temperature, especially in Arctic regions that were devoid of continental ice sheets during the Holocene. Here, we present a new 16,000-year winter-spring temperature reconstruction derived from the unsaturation ratio of alkenones (U37K) in a continuous sedimentary sequence from Lake E5, northern Alaska. We employ a thermodynamic lake model to convert alkenone-inferred lake temperatures into winter-spring air temperature anomalies and we contextualize our proxy reconstruction with climate model output from the region. Our reconstruction shows that winter-spring temperatures warmed rapidly during the deglaciation at 16 and 14 thousand years before present and continued to warm gradually throughout the middle and late Holocene (0.12–0.28 °C/thousand years) in concert with regional sea surface temperature and sea ice records. Our results are consistent with climate model simulations and we attribute Holocene warming to rising winter-spring insolation, radiative forcing from rising greenhouse gas concentrations and regional feedbacks. Our reconstructed cold season warming equaled or exceeded summer cooling according to a regional synthesis of temperature records, suggesting that seasonal biases in temperature reconstructions may account for proxy-model disagreements in Holocene temperature trends from Eastern Beringia.

Great Basin Paleoclimate and Aridity Linked to Arctic Warming and Tropical Pacific Sea Surface Temperatures

AbstractThe arid southwestern United States is susceptible to sustained droughts that impact water resources and economic activity for millions of residents. Previous work has not systematically investigated the structure, timing, and possible forcings of Holocene Great Basin sub‐orbital hydroclimate changes, impeding our ability to understand the potential future controls on Southwestern aridity. The objective of this paper is to constrain the potential forcings on Holocene aridity and temperature, via comparison of new high‐resolution speleothem data, an Aridity Index synthesizing hydroclimate records, and linkages of Southwestern paleoclimate to other regions. The high‐resolution data from Leviathan Cave provide a paleoclimate record since 13,400 yr ago: A cool Younger Dryas was followed by two pronounced Middle Holocene aridity intervals between 9,850 and 5,310 yr B2k characterized by low growth rates and high δ18O and δ13C values. Subsequently, stalagmite δ18O values show near‐modern levels for the last four millennia during which time growth rates were high and δ13C values were low in response to wetter conditions. The regional Aridity Index documents that Middle Holocene drying coincided with a warm Arctic and decreased sea ice extent, a warm western tropical Pacific, and a large sea surface temperature gradient across the tropical Pacific, all of which likely responded to northern hemisphere summer insolation forcing. Our data suggest that extreme Middle Holocene aridity is more severe than the short medieval droughts evident in the tree ring record, and such extreme aridity may represent a worst‐case analog for future climate.

Lacustrine record from the eastern Tibetan Plateau associated with Asian summer monsoon changes over the past ~ 6 ka and its links with solar and ENSO activity

A multi-parameter, absolute-dated lacustrine record from the eastern Tibetan Plateau provides a continuous history of the Asian summer monsoon (ASM) over the past ~ 6000 years. The record broadly follows the trend in northern hemisphere summer insolation. However, it diverges during a period of increased moisture and thermal conditions during the past ~ 1300 years, broadly corresponding to the ASM “2-kyr shift”. This shift as referred by Cheng et al. (Nature 534:640–646, 10.1038/nature18591, 2016) may be related to Atlantic Meridional Overturning Circulation variations, and potentially to increased forcing by greenhouse gases coincident with reduced ENSO activity. The record is punctuated by several weak monsoon intervals. Cross-correlation of the sub-millennial-scale monsoon record with solar activity and ENSO records shows that most of the monsoon variability potentially results from changes in solar irradiation, and has a strong teleconnection with ENSO activity.

Quantitative precipitation reconstructions from Chagan Nur revealed lag response of East Asian summer monsoon precipitation to summer insolation during the Holocene in arid northern China

East Asian summer monsoon (EASM) variability is primarily responsible for climate change in East Asia, influencing over a billion lives. The pattern and mechanisms of EASM change during the Holocene are intensely debated. In this study, we produce a high-quality age-depth model for a 11.7 m lacustrine record (CG18B core) from central Chagan Nur (a closed-basin lake located on the southern Mongolia plateau at the fringe of the modern EASM region) using 27 radiocarbon dates on organic materials and 10 quartz optically stimulated luminescence (OSL) ages on the lake sediments. Multiple-proxy analyses of pollen, grain size, total organic content, and CaCO3 content are used to reconstruct Holocene monsoonal precipitation change in the region. Reconstructed Chagan Nur level changes during the Holocene are based on 18 quartz OSL ages from well-preserved paleolake shorelines at different elevations on the northwestern basin margin. A combination of the lacustrine pollen records and a paleohydrology lake level change model is used to define quantitative precipitation change at the fringe of the EASM dominated region during the Holocene. Our results show that a lake highstand of 15-12 m higher than present occurred at 11–5 ka, followed by a decrease in lake levels to 8-7 m higher than present at 4–3 ka, and a general lake lowering during the late Holocene. EASM dominated annual precipitation varied between 420 mm and 280 mm during the Holocene period in the basin, indicating a generally semi-arid environment on the southern Mongolian Plateau, and corresponding to prevailing open steppe vegetation in the basin throughout the Holocene. EASM precipitation intensity increased from the early Holocene to the beginning of the mid-Holocene, with a maximum precipitation 30–50% higher than present at 8–6 ka, followed by a decrease in precipitation during the late Holocene. EASM precipitation shown in these records lags northern Hemisphere solar insolation shifts by ∼3 ka, indicating that variation in EASM intensity did not respond directly to northern Hemisphere summer insolation, but rather was likely modulated by high latitude forcing of ice volume and greenhouse gases.

Heavy mineral assemblages and sedimentation rates of eastern Central Asian loess: Paleoenvironmental implications

Central Asian loess deposits record the evolution of aridification across the Asian interior. However, paleoclimatic proxies are strongly influenced by the variation of the dust source and accumulation process. In order to decipher the paleoenvironmental changes, a sensitive paleoclimatic proxy with clear source and accumulation process will be needed. In this study, heavy mineral assemblages and sedimentation rates were investigated in a 30-kyr loess section in the Ili Basin, Xinjiang, northwestern China. During Marine Isotope Stage (MIS) 2, trends in dust sedimentation rate were similar to those of the Chinese Loess Plateau (CLP), in anti-phase with Northern Hemisphere Summer Insolation (NHSI) at 65°N. Amphibole/epidote ratios (contents and mean grain size) were low during cold phases, such as the late Last Glacial Maximum (LGM) and Heinrich Event (H) 1; their ratios were higher during adjacent warm periods; this was attributed to aeolian sorting during transportation. Namely, greater sorting between amphibole and epidote occurred in warmer phases, and weaker sorting occurred in colder phases. The amphibole/epidote ratio could serve as a proxy for wind intensity, and thus the Siberian High (SH) intensity. Amphibole/epidote ratio was a more appropriate proxy to record wind intensity than dust sedimentation rate which was also influenced by sediment availability. A higher amphibole/epidote ratio corresponded to a more negative East Asian stalagmite δ18O, indicating an anti-phase relationship between the SH and East Asian Summer Monsoon (EASM) during the MIS 2.