[Show abstract][Hide abstract] ABSTRACT: Thick Cenozoic deposits in the northwestern Qaidam Basin record erosion of the Altyn Tagh and high terrain west of that basin and presumably the concurrent growth of the northeastern Tibetan Plateau. A detailed magnetostratigraphic study of the Huatugou section, northwestern Qaidam basin, reveals that this section spans the period from ∼30 to ∼11 Ma. Magnetostratigraphic and sedimentological studies indicate that the accumulation rate abruptly increased near ∼15 Ma. The acceleration in sedimentation rate suggests enhanced tectonic deformation in the Qaidam basin since 15 Ma that may have begun simultaneously with accelerated deformation along the Altyn Tagh, Kunlun, and Haiyuan faults, which contributed to the growth history of the Qaidam basin and its surroundings since ∼15 Ma.
[Show abstract][Hide abstract] ABSTRACT: Geochemistry of basin sediments from semi-arid regions is valuable to understand past hydroclimatic changes. Here, we investigate the links of sedimentary geochemistry (Rb, Sr, Ca/Zr, TOC, and %CaCO3), carbonate mineralogy and ostracod shell δ18O of Lake Qinghai, a basin proximal to major dust production centers at mid-latitudes of the Northern Hemisphere, to changes in depositional conditions and hydroclimate during the past 32 ka. Surface lacustrine sediments are characterized by low-Rb, high-Sr, low-Rb/Sr, high-%CaCO3 and high-Ca/Zr values, in contrast to the chemical compositions of eolian loess (high-Rb, low-Sr, high-Rb/Sr, low-%CaCO3, and low-Ca/Zr). A direct comparison of soluble Ca and Sr in two short cores with instrumental water discharge data suggests that lacustrine precipitates in Lake Qinghai are dominated by authigenic aragonite formed under Ca2+-limited water conditions, and that the accumulation rate of aragonite dominantly depends on solute fluxes into the lake during the rainy seasons (late May to September). Our high-resolution down-core records show that sediments during the last glacial (∼32–19.8 ka) had high-Rb, low-Sr, low-%CaCO3, and low-Ca/Zr, indicating eolian dust (loess) accumulation in a desiccated basin under dry glacial conditions, further supported by grain size and pollen results. This type of sedimentation was maintained during the last deglacial (∼19.8–11.5 ka), but interrupted by episodic lacustrine precipitates with high-Sr, high-%CaCO3, high-Ca/Zr, and low-Rb. At ∼11.5 ka, sedimentary Rb/Sr, Ca/Zr, %CaCO3 and TOC show dramatic and permanent changes, implying an abrupt shift in the atmospheric circulation at the onset of the Holocene in the Lake Qinghai region. Lacustrine precipitates have persisted throughout the Holocene with a maximum during the early to mid-Holocene (∼10.5–8.0 ka). Since ∼8.0 ka, the gradual and significant decreases in aragonite and Sr accumulations in tandem with increasing dust deposit and more positive ostracod δ18O may be linked to a weakening of Asian summer monsoons during the mid-to-late Holocene. Overall, our records appear to show a high sensitivity of sediment development and geochemistry in Lake Qinghai to the regional hydroclimate changes since the last glacial.
[Show abstract][Hide abstract] ABSTRACT: The collapse of some pre-historical and historical cultures, including Chinese dynasties were presumably linked to widespread droughts, on the basis of synchronicities of societal crises and proxy-based climate events. Here, we present a comparison of ancient inscriptions in Dayu Cave from Qinling Mountains, central China, which described accurate times and detailed impacts of seven drought events during the period of 1520–1920 CE, with high-resolution speleothem records from the same cave. The comparable results provide unique and robust tests on relationships among speleothem δ18O changes, drought events, and societal unrest. With direct historical evidences, our results suggest that droughts and even modest events interrupting otherwise wet intervals can cause serious social crises. Modeling results of speleothem δ18O series suggest that future precipitation in central China may be below the average of the past 500 years. As Qinling Mountain is the main recharge area of two large water transfer projects and habitats of many endangered species, it is imperative to explore an adaptive strategy for the decline in precipitation and/or drought events.
[Show abstract][Hide abstract] ABSTRACT: The extent of unsaturation in long chain alkenones (LCKs) in lakes has potential for quantitatively indicating paleotemperature in terrigenous environments. However, the traditional temperature proxies derived from marine LCKs may not be able to address the problem related to the prevalent species effect, which restricts the application of LCKs in lacustrine studies. In our LCK record from the northeastern Tibetan Plateau, C37/C38 differences revealed that the LCK producer species in Lake Qinghai may have changed during the Holocene. This possible species effect may have caused inconsistency between the UK'37 and UK37 records. Therefore, we formulated a new proxy, UKs37, with a parameter "s" to indicate the different exponential response in LCK unsaturation to temperature change in multiple species. Our UKs37-derived temperature time series resembles the general temperature trends of the Northern Hemisphere on a millennial scale, while exhibiting several apparent cold events, especially at ca. 9ka. The result shows that the Holocene optimum in Lake Qinghai was interrupted by cold events in the early Holocene. This could be a result of a weak Asia summer monsoon and cold air transmission in the North Atlantic.
[Show abstract][Hide abstract] ABSTRACT: The evolution of the Asian monsoon-arid environmental system during the Cenozoic was closely related to the growth of the Himalayan-Tibetan Plateau and global climate change. However, due to inconsistencies in paleoclimatic reconstructions and to various constraints on the timing of the growth of the Tibetan Plateau, the relative impacts of regional uplift and global cooling on Asian climate change remain controversial. Here we investigate the mineralogical composition of a Miocene Red Clay deposit on the western Chinese Loess Plateau in order to infer changes in chemical weathering and monsoon intensity. Variations of four mineralogical ratios (chlorite/quartz, illite/quartz, calcite/quartz, and protodolomite/quartz) reveal that the summer monsoon intensity was relatively strong during the early Miocene (23.5–18.5 Ma), weakened radually until �9.5 Ma, and strengthened again in the late Miocene. We synthesized previously published thermochronological data from the northeastern Tibetan Plateau and surrounding mountains, and the results suggest that two phases of the rapid growth of northern Tibet occurred around 24–17 and 13–7 Ma. Comparison of paleoclimatic proxies and thermochronological data suggests that the gradual weakening of the summer monsoon intensity from 18.5 to 9.5 Ma paralleled global cooling, whereas two intervals of strengthened monsoon in the early and late Miocene were possibly related to the rapid growth of northern Tibet. Our combination of paleoenvironmental proxies and thermochronological data reveals possible links between Miocene Asian monsoon evolution, phased growth of the Tibetan Plateau, and global climate change, and confirms the interconnection of geodynamic and atmospheric processes in the geological past.
[Show abstract][Hide abstract] ABSTRACT: Sensitivity-corrected quartz optically stimulated luminescence (OSL) dating methods have been widely accepted as a promising tool for the construction of late Pleistocene chronology and mass or dust accumulation rates (MARs or DARs) on the Chinese Loess Plateau (CLP). Many quartz OSL ages covering marine isotope stage (MIS) 2 (equal to L1-1 in Chinese loess) have been determined for individual sites within the CLP in the past decade. However, there is still a lack of detailed MAR or DAR reconstruction during MIS 2 across the whole of the CLP. Here, we present detailed MARs determined for eight sites with closely-spaced quartz OSL ages covering ∼MIS 2, and relative MARs suggested by a probability density analysis of 159 quartz OSL ages ranging from ∼30 to 10 ka ago, from 15 sites on the CLP. The results show enhanced dust accumulation during the Last Glacial Maximum (LGM), with particularly rapid dust accumulation from ∼23 to 19 ka ago (the late LGM). In contrast, MARs determined for the last deglaciation (from ∼19 to 12 ka ago) are low. The MAR changes during MIS 2 in Chinese loess are mainly controlled by the East Asian winter monsoon (EAWM) intensity, which is forced by Northern Hemisphere ice volume. The MAR changes also indicate that dust accumulation during MIS 2 is generally continuous at millennial time scales on the CLP. Comparison of Asian-sourced aeolian dust MARs in Chinese loess with those preserved in Greenland ice cores and North Pacific Ocean sediments indicates that rapid dust accumulation occurred from ∼26 to 23 ka ago (the early LGM) in Greenland ice cores and North Pacific Ocean sediments, suggesting a several kilo-year difference in timing when compared with the rapid dust accumulation during the late LGM in Chinese loess. This asynchronous timing in enhanced dust accumulation is probably related to both changes in the EAWM intensity and changes in the mean position of zone axis of the Westerly jet, both of which are greatly influenced by Northern Hemisphere ice volume. This study highlights the possible influence of changes in the mean position of zone axis of the Westerly jet on long-range transport of Asian-sourced dust.
Aeolian Research 06/2015; DOI:10.1016/j.aeolia.2015.05.005 · 2.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The dynamics of glacialeinterglacial monsoon variability can be attributed to orbitally induced changes in
summer insolation and internal boundary conditions. However, the relative impacts of astronomical and
internal factors on East Asian summer monsoon variability remain controversial. Here we combine proxy
data and model results to evaluate the response of East Asian summer monsoon change to these forcings.
d13C of loess carbonate, a sensitive summer monsoon proxy from the western Chinese Loess Plateau,
demonstrates coexistence of distinct 100-, 41- and 23-ka periods, in contrast to precession-dominated
speleothem d18O records in South China. Model results indicate that insolation, ice and CO2 have distinct
impacts on summer precipitation changes in East Asia, whereas their relative impacts are spatially
different, with a relatively stronger insolation effect in south China and a more dominant ice/CO2 influence
in north China. Combined proxy data and model results indicate that East Asian summer monsoon variability
was induced by integrated effects of summer insolation and changing boundary conditions (e.g., ice
sheets and CO2 concentration). Our proxy-model comparison further suggests that gradual weakening of
the summer monsoon related to slowly decreasing summer insolation at astronomical timescales will be
likely overwhelmed by the projected ongoing anthropogenic CO2 emissions.
[Show abstract][Hide abstract] ABSTRACT: The Mongolian Plateau (MP), which is relatively lower in altitude and smaller in extent than the Tibetan Plateau (TP), has received little attention about its climate effect. Building upon previous work in which we highlighted the role of the MP on the high-level westerly jet stream, the response of surface-level features of the Asian climate is examined in this study. The results show that the Indian and East Asian summer monsoonal and inland precipitation are mainly enhanced by the uplift of the TP. The precipitation during the onset of the summer monsoon is also intensified over India and eastern China. In addition, the East Asian monsoon domain is significantly expanded with the uplift of the TP, while the Indian summer monsoon domain does not change obviously. The MP plays a significant role in the strengthening of the East Asian winter monsoon, which is larger than the TP. With the uplift of the MP, the cold northerly wind in winter intensifies significantly in East Asia from higher latitudes to the South China Sea. The Siberian high is also enhanced and moves remarkably northward to its modern location. The strengthening of the Asian winter monsoon is related to the MP-induced diversion of westerly wind. The bypassing flows around the plateau modify the temperature advections over middle latitudes and the atmosphere thermal structure in winter, which leads to the strengthening of the East Asian winter monsoon.
Journal of Geophysical Research Atmospheres 05/2015; 120(10):n/a-n/a. DOI:10.1002/2014JD022880 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The carbon isotope composition (δ13C values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C3 and C4 photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting δ13C values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C27 and C29) in nearshore sediments of the Lake Qinghai catchment was contributed from submerged aquatic plants, which displayed relatively positive carbon isotope composition (e.g. −26.7‰ to −15.7‰ for C29) similar to that of terrestrial C4 plants. Thus, the use of δ13C values of sedimentary C27 and C29 n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimate/underestimate of the proportion of terrestrial C4 plants. For sedimentary C31, however, the contribution from submerged plants was minor, so that the δ13C values for C31 n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the δ13C values of sedimentary C27 and C29 n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C4 vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the δ13C values of sedimentary long chain n-alkanes (C27, C29 and C31) may carry different environmental signals. While the δ13C values of C31 were a reliable proxy for C4/C3 terrestrial vegetation composition, the δ13C values of C27 and C29 n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth.
[Show abstract][Hide abstract] ABSTRACT: Lake Qinghai in North China is the largest interior plateau lake in Central Asia and is climatically sensitive. An almost continuous 626-m long sediment core was drilled in an in-filled part of the southern lake basin of Lake Qinghai. The magnetic susceptibility record reveals the presence of two distinct peaks within an interval of fine-grained lacustrine sediments of Lower Pliocene age. We selected a depth interval of approximately 40 m spanning the magnetic susceptibility peaks for detailed rock magnetic and geochemical analyses in order to identify the magnetic mineralogy responsible and to assess its possible paleoenvironmental and paleomagnetic implications. Rock magnetic, X-ray diffraction analysis, scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX) analyses reveal that the main magnetic carrier is greigite (Fe3S4). The greigite is of early diagenetic origin and formed in an interval of high lake level and inferred relatively warm, humid climate. The greigite-enriched zones are separated by an interval of relatively high total sulfur and organic carbon content and we infer that in the adjacent greigite-bearing zones the lower concentrations of sulfur and organic carbon, and high levels of reactive iron, arrested the process of pyritization resulting in the preservation of the greigite on a timescale of several million years. The greigite zones contain narrow intervals of normally-magnetized sediments which may be previously unrecognized cryptochrons within the Gilbert Chron, or alternatively they may reflect the continued formation of greigite long after the age of deposition of the surrounding sediment matrix. This article is protected by copyright. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: A speleothem δ(18)O record from Xiaobailong cave in southwest China characterizes changes in summer monsoon precipitation in Northeastern India, the Himalayan foothills, Bangladesh, and northern Indochina over the last 252 kyr. This record is dominated by 23-kyr precessional cycles punctuated by prominent millennial-scale oscillations that are synchronous with Heinrich events in the North Atlantic. It also shows clear glacial-interglacial variations that are consistent with marine and other terrestrial proxies but are different from the cave records in East China. Corroborated by isotope-enabled global circulation modeling, we hypothesize that this disparity reflects differing changes in atmospheric circulation and moisture trajectories associated with climate forcing as well as with associated topographic changes during glacial periods, in particular redistribution of air mass above the growing ice sheets and the exposure of the "land bridge" in the Maritime continents in the western equatorial Pacific.
Proceedings of the National Academy of Sciences 03/2015; 112(10):201424035. DOI:10.1073/pnas.1424035112 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Precipitation in low latitudes is primarily controlled by the position of the intertropical convergence zone, which migrates from south to north seasonally. The Little Ice Age (defined as AD 1400–1850) was associated with low solar irradiance and high atmospheric aerosol concentrations as a result of several large volcanic eruptions. The mean position of the intertropical convergence zone over thewestern Pacific has been proposed to have shifted southwards during this interval, whichwould lead to relatively dry Little Ice Age conditions in the northern extent of the intertropical convergence zone and wet conditions around its southern limit. However, here we present a synthesis of palaeo-hydrology records from the Asian–Australian monsoon area that documents a rainfall distribution that distinctly violates the expected pattern. Our synthesis instead documents a synchronous retreat of the East Asian Summer Monsoon and the Australian Summer Monsoon into the tropics during the Little Ice Age, a pattern supported by the results of our climate model simulation of tropical precipitation over the past millennium. We suggest that this pattern over the western Pacific is best explained by a contraction in the latitudinal range over which the intertropical convergence zone seasonally migrates during the Little Ice Age. We therefore propose that rather than a strict north–south migration, the intertropical convergence zone in this region may instead expand and contract over decadal to centennial timescales in response to external forcing.
[Show abstract][Hide abstract] ABSTRACT: Speleothem-based oxygen isotopic sequences have been widely used to reveal regional climatic changes worldwide. However, the climatic significance of speleothem δ18O variations in monsoonal China on decadal- to annual timescales remains in debate, which limits its application to paleoclimate reconstruction in the world’s largest country by population. In this study, we analyzed a seasonally resolved δ18O record of an annually layered stalagmite, XL21 whose variability covers a period of 98 years (1912–2009 AD), from Xianglong Cave, central China. The annual δ18O trend shows an inverse relationship with local monsoon precipitation, suggesting speleothem δ18O variations in this area can reflect monsoon precipitation at least on decadal timescale. Changes in moisture sources and transport pathways have little effect on the δ18O variations in speleothems/monsoon precipitation on decadal timescale, which may be ascribed to similar distances from this region to the two main moisture sources, the Bay of Bengal and West Pacific. There is an anti-phase relationship between speleothem δ18O records from central China and India during the last 98 years, which indicates the relationship between Indian monsoon intensity and speleothem δ18O from central China on short timescales requires further investigation.
Journal of Asian Earth Sciences 03/2015; 106:150-155. DOI:10.1016/j.jseaes.2015.03.008 · 2.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Elemental carbon (EC) and polycyclic aromatic compounds (PACs) are potential proxies for the reconstruction of change in human activities and the origin of air masses in historic times. In this study, the historic deposition of char and soot (the two subtypes of EC) and PACs in a 150-yr sediment core from different topographic sub-basins of Lake Qinghai on the Qinghai Tibetan Plateau (QTP) were reconstructed. The objective was to explore how the variations in the concentrations of EC and PACs, the ratios of char to soot and of oxygenated-PAHs to parent-PAHs and of the composition of the PACs mixtures reflect historical changes in climate and human activity and the origin of air masses arriving at QTP. The deposition fluxes of soot in the different sub-basins were similar, averaging 0.18 (range of 0.15-0.25) and 0.16 (0.13-0.23) g m-2 yr-1, respectively, but varied for char (averaging 0.11 and 0.22 g m-2 yr-1, respectively), suggesting ubiquitous atmospheric deposition of soot and local river inputs of char. The different vertical distributions of the char/soot ratios in the different sub-basins can be interpreted by the different transport mechanisms of char and soot. An abrupt increase in soot concentrations since 1980 coincides with results from the QTP ice cores that were interpreted to be indicative of soot transport from South Asia. Similar concentration pattern of PAHs with soot and the 9,10-anthraquinone/anthracene (9,10-AQ/ANT) ratios all >2.0 suggest regional PAC sources. Increasing PAHs/soot ratios and decreasing 9,10-AQ/ANT ratios since the beginning of the 1970s indicate increasing local emissions. The historical trends of diagnostic ratios indicate an increase in fossil fuel contribution since the beginning of the 1970s. The increase of perylene concentrations with increasing core depth and the ratio of perylene to its penta-aromatic isomers indicate that perylene originates mainly from in-situ biogenic diagenesis. We demonstrate that the concentrations of EC, char, soot, and PACs in sediments can be used to reconstruct local, regional, and remote sources and transport pathways of pollutants to the QTP.
[Show abstract][Hide abstract] ABSTRACT: The North Atlantic Oscillation (NAO), although a local atmospheric mode over the North Atlantic Ocean, plays an important remote role in the Eurasian climate. However, the link between the NAO and Eurasian climate might be unstable. Here, we present a study on the relationship between the winter NAO and mid-latitude Eurasian climate, especially the mid-latitude East Asian precipitation, mainly based on meteorological data and output from an 1155-year-long coupled ocean-atmosphere model simulation. The results show that the winter NAO exerts a remarkable effect on the changes in mid-latitude Eurasian climate; however, the impact of the NAO is nonstationary. According to the model output, the impact of the NAO varies synchronously with the NAO variance with a period of around 150 years. During the high NAO variance period, the NAO has significant correlation with mid-latitude East Asian precipitation; low NAO variance periods do not. The variation of the NAO-precipitation teleconnection may arise from the changing influence of NAO on the local temperature. The NAO signal moves eastward by a zonally oriented wave train, where it modulates the atmospheric circulation structure, and thus results in the nonstationary relationship between the NAO and mid-latitude East Asian precipitation.
Theoretical and Applied Climatology 02/2015; DOI:10.1007/s00704-015-1396-z · 2.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: High-resolution sedimentary charcoal and black carbon (BC, including char and soot) records from a loess–soil profile, combined with magnetic susceptibility, δ13C of soil organic matter of analyses, pollen counts and other paleoenvironmental proxies reveal past fire patterns and landscape evolution over the past 12,000 years. Results from the analyses of charcoal and BC influx show that regional fire activity was high in the early and late Holocene, whereas fire was less frequent and pervasive in the middle Holocene. Locally, fires were infrequent near the study site until the Late Holocene. Soot and char analyses do not parallel changes in charcoal variability, and thus appear to reflect either a different aspect of fire activity or else these data are registering aspects of particle transportation and deposition in addition to fire characteristics.
[Show abstract][Hide abstract] ABSTRACT: Variations in speleothem δ13C can reflect changes in overlying surface vegetation, which, over historical time scales, may represent the influence of human activities. Here, we examined δ13C variations in two stalagmites growing for the last 2200 years in Shennong Cave, Jiangxi Province, SE China. The two δ13C records corroborate well one another and show a prominent 6‰ enrichment of the δ13C values from AD 700 to 1100. The isotopic equilibrium for modern calcite and negative correlation between δ18O and δ13C along the growth axis suggest that the influences of kinetic fractionation are negligible. Varied correlations between Mg/Ca and Sr/Ca ratios and divergent changes between δ13C and Mg/Ca and Sr/Ca ratios from AD 700 to 1100 reveal that the prior calcite precipitation (PCP) and water–rock interaction did not dominate the increase of δ13C values. It is plausible that the obvious δ13C variation was largely influenced by the changes in vegetation cover overlying the cave. Our δ13C results, together with the records of climate and human activity from historical documentary records, suggest that: (i) prior to AD 700, small fluctuations in relatively light δ13C values reflect the presence of lush forest coverage above the cave, which was minimally disturbed by human activities; (ii) during AD 700–1100, the drastic increase in δ13C values indicates persistent and massive deforestation associated with large-scale immigration into northern Jiangxi after the Rebellion of An & Shi (AD 755–763) in the Tang Dynasty and the subsequent development of agriculture and economic activity; and (iii) since AD 1100, fluctuations in relatively high δ13C values suggest that local vegetation during the last millennium has been sparse. Since the Rebellion of An & Shi, SE China was progressively developed, coincident with deforestation and vegetation deterioration caused by human disturbance in the form of deforestation and cultivation.
[Show abstract][Hide abstract] ABSTRACT: Many studies have observed that leaf wax δD
n-alkanes values differed significantly between woods and grasses in modern plants, with grasses D-depleted by 40 ‰–70 ‰. The reasons for the differences in leaf wax δD
n-alkane values between woods and grasses, however, remain unclear. In this study, we measured the δD values of soil water (δDsw), leaf water (δDlw), and leaf wax n-alkane (δD
n-alkane) for woods and grasses. We found no significant differences in the δD values of soil water (P = 0.82) and leaf water (P = 0.74) between the two life forms of plants. Therefore, the differences in leaf wax δD
n-alkane values between woods and grasses may correlate with inherent properties of different plant life forms, such as leaf structures, biosynthetic processes, and leaf morphologies. Moreover, it is also possible that soil water with different δDsw at different depths utilized by woods and grasses may be responsible for some of the differences in leaf wax δD
n-alkane values between the two life forms of plants, if woods mainly use soil water from the >100 cm depth, whereas grasses mainly use soil water from the <100 cm depth. The results of this work allow us to better understand the leaf wax δD
n-alkane values of different plant life forms in a region.