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Located in the northwestern edge of the modern Asian summer monsoon (ASM), the northeastern Tibetan Plateau (NETP) is sensitive to changes of the ASM climate. However, variations of climate and vegetation during the Holocene remain unclear in this marginal region of the monsoon climate. Here we present a Holocene high-resolution pollen record from Lake Gahai in the NETP since 11.4 (±0.3) ka BP to reconstruct regional vegetation history. A quantitative reconstruction of precipitation is also tried using fossil pollen assemblages. Results suggested that before 10.2 (±0.4) ka BP in the early Holocene, the vegetation cover was low and the climate was arid. A relatively wet climate indicated by high A/C ratio values occurred between 10.2 (±0.4) and 7.4 (±0.2) ka BP. After 7.4 (±0.2) ka BP, the A/C ratio decreased, indicating that the climate was getting drier. The overall environment of the basin has become similar to modern conditions since 5.4 (±0.1) ka BP. The reconstructed precipitation is not comparable with the tree-ring-based reconstruction from the Delingha region, although the reconstruction processes passed significance tests statistically. In addition to abundant surface pollen data and gradually improved quantitative reconstruction techniques, other factors such as the environmental settings and vegetation dynamics also affect the reliability of the pollen-based quantitative reconstruction of regional precipitation. Therefore, the applicability of pollen data for quantitative precipitation reconstruction in arid regions should be assessed and the results should be treated cautiously. • Pollen from NE Tibetan Plateau suggested increased humidity during 10.2 (±0.4) −7.4 (±0.2) ka BP. • Vegetation and climate became similar to modern situation since 5.4 (±0.1) ka BP. • Pollen data similar to Lake Gahai may not be suitable for a quantitative reconstruction. [ABSTRACT FROM AUTHOR]

Abstract: There has been much recent debate about Holocene climate variation in the monsoon region of China, especially the temporal pattern of variations in precipitation, the time-transgressive nature of the Holocene precipitation maximum, and the extent to which variations in regions influenced by the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM) have been synchronous. We summarize and compare carbonate oxygen-isotope records (δ18Ocarb) from ten lakes within the present-day ISM region. We discuss their paleoclimate significance considering the present-day moisture source, isotopic composition of precipitation and the hydrological setting. The δ18Ocarb records are controlled mainly by the isotopic composition of lake water, which in turn is a function of regional Precipitation/Evaporation (P/E) balance and the proportion of precipitation that is monsoon-derived. We normalized the δ18Ocarb data and used these records to generate an integrated moisture index. This index, along with oxygen-isotope records from speleothems and carbon-isotope records (δ13Corg) from peats within the monsoon region, suggests that Holocene climate was broadly synchronous across the monsoon region and, within the limits of accuracy of the existing age models, provides no strong evidence for previously-proposed anti-phasing of the ISM and the EASM. Stable-isotope records from lake sediments and peat bogs have excellent potential for providing high-quality paleoclimate data for monsoon Asia, and complement high-resolution speleothem sequences, which are only found in certain localities. [Copyright &y& Elsevier]

The green alga Pediastrum is an ecologically significant component of freshwater ecosystems, but its potential as an independent proxy for quantitative paleoenvironmental reconstruction is relatively unexplored. Here we use multivariate statistical analysis to explore the relationship between the Pediastrum composition of lake surface sediments and environmental variables. The findings highlight the mean growing season air temperature between March and October (T Mar-Oct) as the most important factor determining the Pediastrum composition. We then establish the first transfer function between the Pediastrum taxa and T Mar-Oct , which enables us to provide a quantitative temperature reconstruction for the past ∼8.2 kyr based on sedimentary subfossil Pediastrum records from Bosten Lake in Xinjiang, China. Our findings reveal an exceptionally warm climate during ∼4.8–3.6 kyr, with temperatures ∼4 °C higher compared to those during ∼8.2–4.8 kyr. This unusually warm climate is verified by the carbonate clumped isotopes record of the same core and other paleoclimate records. These findings contribute to our understanding of past temperature changes in Central Asia and provide a basis for future research on long-term lake ecosystem dynamics under past climatic warming. • We developed the first Pediastrum -temperature transfer function. • Paleotemperature was reconstructed based on the subfossil Pediastrum record from Bosten Lake (China). • An unusually warm climate occurred in Central Asia during 4.8–3.6 kyr. [ABSTRACT FROM AUTHOR]

This paper aims to achieve an improved understanding of the long-term change trends of precipitation δD and δ 18 O values (δD p and δ 18 O p ) in the Asian monsoonal region and their relationship with the corresponding humidity trends during the Holocene. To do this we first review the observed modern spatial pattern of summer precipitation distribution in the East Asian summer monsoon (EASM) region under different EASM intensities, and the relationship between modern observed δ 18 O p values and corresponding precipitation amounts on monthly and inter-annual timescales in the EASM and Indian summer monsoon (ISM) regions. Second, we compare Holocene lacustrine and marine compound-specific hydrogen isotopic records of n -alkanes/ n -alkanoic acid (δD n ), lacustrine authigenic carbonate and cave stalagmite oxygen isotopic records (δ 18 O c and δ 18 O s ) from the Asian monsoonal region, all of which are closely related to δD p and δ 18 O p variations. The results demonstrate that in both the ISM and EASM regions, all of these isotopic records exhibit roughly similar long-term characteristics, i.e. they were all more negative during the early-Holocene and early mid-Holocene (ca. 11–6 ka B.P.; B.P. means before present, present = 1950 AD), and then became more positive towards the late-Holocene. Third, we compare representative paleo-humidity records from the Asian monsoonal region; the results confirm that, in the ISM region, a humid interval occurred in the early-Holocene and early mid-Holocene (ca. 11–6 ka B.P.) and subsequently the climate became more arid towards the late-Holocene. This indicates an enhanced ISM during the early-Holocene and early mid-Holocene (ca. 11–6 ka B.P.), and an ISM of decreasing intensity towards the late-Holocene. On a Holocene orbital scale, both δ 18 O p and δD p appear to be controlled by an “amount effect” in the ISM region, consistent with the region's inter-annual modern δ 18 O p data. This evidence indicates that both δ 18 O p and δD p paleo-records are significantly related to paleo-humidity in the ISM region. In contrast, Holocene humidity variations in the EASM region exhibit clear spatial differences: a humid mid-Holocene interval (ca. 8–3 ka B.P.) occurred in southern and northern China, but an arid interval from ca. 7–3 ka B.P. occurred in central China, in the middle reaches of the Yangtze River. Based on precipitation distribution patterns under different EASM intensities in the EASM region over the past few decades, we conclude that EASM intensity was enhanced during the mid-Holocene (ca. 8–3 ka B.P.). Relative to the ISM intensity, the response of EASM intensity to summer insolation was relatively slow. In the EASM region the relationship between climate and δ 18 O p and δD p is more complex, consistent with analyses of regional inter-annual modern δ 18 O p data. This evidence demonstrates that both δ 18 O p and δD p paleo-records cannot be used directly as paleo-humidity (i.e. precipitation amount or EASM intensity) indicators in the EASM region. Further comparison and analyses demonstrate that the coupled variations in west–east Equatorial Pacific temperature gradients and the West Pacific subtropical high (WPSH) played an important role in determining EASM intensity during the Holocene. [ABSTRACT FROM AUTHOR]

Ongoing global warming will have a significant impact on terrestrial water resources, especially in arid and semi-arid regions. Assessing changes in the hydrology and water resources in inland basins under a warm climate in the recent geological past can provide a valuable context for assessing their current and possible future status. Here we present a record from a rapidly-accumulating sediment core from Lake Dalianhai in the Gonghe Basin in the northeastern Tibetan Plateau. The chronology of the uppermost 51.63 m of the core is based on AMS 14C dates from 11 samples of terrestrial plant microfossils and 14 bulk samples. Hydroclimatic variations within the basin during the last 23,000 yr were reconstructed from sedimentological and geochemical parameters and paleoshoreline records. The results indicate that the Gonghe Basin experienced relatively arid conditions during 23,000-14,700 cal yr BP. Lake Dalianhai was refilled at 14,700 cal yr BP and a high lake level was sustained until the beginning of the Holocene, as indicated by changes in the concentration of OH-GDGTs. However, the lake level was low during the early to middle Holocene (11,700-4700 cal yr BP) and increased during the late Holocene. Based on a comparison with lake level records and aeolian sand deposits from the region, we conclude that the climate was extremely dry in basins in the northeastern Tibetan Plateau during the early to middle Holocene. We propose that strong evaporation combined with high summer temperatures during the early to middle Holocene were responsible for the dry climate at that time, rather than changes in mid-latitude precipitation. Given the situation occurred during the early to middle Holocene, ongoing global warming may seriously threaten water resources in inland China, even if precipitation increases substantially. • A 23,000-year hydroclimatic record was reconstructed from a rapidly-accumulating sediment core from Lake Dalianhai in the northeastern Tibetan Plateau. • Geochemical indices and paleoshorelines reveal that the lake level of Lake Dalianhai was low during the early to middle Holocene. • The climate was extremely dry in basins in the northeastern Tibetan Plateau during the early to middle Holocene resulted from high summer temperatures. • Ongoing global warming may seriously threaten water resources in inland China, even if precipitation increases substantially. [ABSTRACT FROM AUTHOR]

The 'westerlies-dominated climatic regime' (WDCR) in the present interglacial period was proposed because precipitation/moisture variations between arid central Asia and mid-latitude monsoonal Asia are out-of-phase or anti-phased on different timescales during the Holocene. In this study. we first review the development of the theoretical framework of the WDCR, and then outline the boundary of its core area: from the Caspian Sea in the west to the western Hexi Corridor in the east, with the northern and southern limits coinciding with the domain of arid central Asia. Next, we present a synthesis of multiple lines of evidence for the occurrence of the WDCR on multi-millennial ('sub-orbital' herein) to decadal timescales during the Holocene. Finally, we examine the possible physical mechanisms responsible for the WDCR. We find that external factors (insolation changes induced by orbital factors) generated the WDCR on a sub-orbital timescale, whereas a circum-global teleconnection/Silk Road pattern was the most significant factor responsible for the WDCR on centennial and decadal timescales. The study provides a comprehensive summary of the development of our knowledge of the WDCR over the past several decades, together with a tentative theoretical framework for understanding climatic and environmental changes within its region of influence. In addition, it forms a scientific basis for environmental management and ecological restoration in this arid region in the context of global warming. [ABSTRACT FROM AUTHOR]

There is much debate regarding the temporal and spatial pattern of hydroclimate variations on the Tibetan Plateau during the Holocene, focusing especially on the dipolar pattern of precipitation and moisture between the southern and northern regions, on multiple timescales. Modern observations reveal that the δ18O in precipitation is an integrated tracer of the atmospheric processes of the Indian summer monsoon (ISM), the East Asian summer monsoon (EASM), and the westerlies, which dominate the climate of the Tibetan Plateau. Here, we summarize and compare 20 Holocene lacustrine authigenic carbonate δ18O records from lakes distributed across the Tibetan Plateau. After carefully considering the potential effects of temperature, evaporation, site elevation, meltwater supply, and moisture sources, we eliminate their influences on the isotopic composition of Tibetan lakes, and suggest that the long-term variations of the lacustrine δ18O records were dominated by the moisture sources. The δ18O values of the lake sediments in the central and western Tibetan Plateau increased since the early Holocene, which agrees with the trend of δ18O in speleothem records from the plateau, as well as with the variations of monsoon precipitation reconstructions for the region. Overall, this evidence indicates the substantial influence of the monsoon in the abovementioned regions. However, the δ18O records from lakes in the northeastern Tibetan Plateau show a decreasing trend during the second half of the Holocene (after ∼5 ka), highlighting the operation of a different hydrological process, with enhanced moisture availability, in the regions. It has been suggested that the increased contribution from westerlies-derived moisture in the non-monsoon season, with depleted δ18O (e.g., from melting snow or frozen soil in spring), is responsible for such variations. We propose that the interplay between the Asian summer monsoon and the westerlies was the major control of the hydroclimatic variations on the Tibetan Plateau during the Holocene, which led to the dipolar pattern of moisture variation between the northeastern Tibetan Plateau and other parts of the plateau, on the multi-millennial timescales during the Holocene. [ABSTRACT FROM AUTHOR]