Search

Holocene hydrological changes in regions dominated by the westerlies significantly differ from those by the Asian summer monsoon. The high-elevated northeastern Tibetan Plateau, located in between, is likely influenced by the interactions of both circulation systems. Here, we attempt to use biomarkers, n-alkanes and alkenones, to reconstruct Holocene lake-level changes at Lake Gahai in the Qaidam Basin. We choose a sediment core drilled at the lake shore, where biomarkers would be sensitive to lake-level changes. The n-alkane records show high average chain length (ACL), high carbon preference index (CPI), and low proportion of aquatic macrophyte (Paq) values at 7–2 kyr (thousand calibrated years ago) with peaked values around 6 kyr, whereas low ACL, low CPI, and high Paq values occurred after 2 kyr and before 7 kyr. No alkenones were detected at 7–2 kyr, suggesting that lake level at this period was incapable of constantly reaching to the coring site. Therefore, combined results provide unambiguous evidence of relatively low lake level at 7–2 kyr, probably lowest at ~6 kyr. Holocene lake-level changes in this marginal region thus display a different pattern from either of the core regions dominated by the westerlies (anti-phase) and the Asian summer monsoon (out-of-phase). We suggest that in the arid marginal region, temperature-induced evaporation could significantly affect regional hydrological balance, resulting in the discrepancy with the Holocene long-term precipitation decreasing trend in Asian monsoon-dominated regions. [ABSTRACT FROM AUTHOR]

Abstract: The northern Tibetan Plateau involves complex interactions between the mid-latitude westerly circulation and the subtropical Asia monsoon circulation, acting as a bridge communicating high and low latitude climatic processes. Previous studies from the region suggest relatively wet conditions in cold periods during the late Holocene, for instance, the Little Ice Age (LIA). However, the inference of such temperature-moisture association is subject to the large uncertainty in lacustrine 14C chronology, due to the particularly large lake reservoir effect in the region. Here we take a different approach by reconstructing paired temperature and moisture records from the same sediment cores to assess the temperature-moisture association, independent of chronology uncertainty. We use alkenone indices UK′ 37 and %C37:4 to reconstruct high resolution temperature and moisture changes simultaneously from two lakes in the Qaidam Basin, northern Tibetan Plateau, over the last ∼2500 years. Characterized by marked climatic variability, our paired records confirm the warm-dry and cold-wet association in arid northwestern China during the late Holocene, opposite to the warm–wet and cold–dry association in subtropical Asian monsoonal regions. Our moisture records further suggest substantially drier conditions during the Medieval Warm Period (MWP) than the current warm period. Lastly, the temperature and moisture changes inferred from our records can be well correlated with solar irradiance changes, suggesting a possible link between solar forcing and natural climate variability during the late Holocene on the northern Tibetan Plateau. [Copyright &y& Elsevier]

We present a hydrogen isotopic record of long-chain n-alkanes in Lake Sugan to reconstruct regional moisture changes in the last 1700 years at a sampling resolution of 25 years. The δD values decreased by over 30‰ from the ‘Medieval Warm Period’ (MWP; c. ad 600–1500) to the ‘Little Ice Age’ (LIA; c. ad 1500–1850), and indicated that the moisture pattern included a relatively humid climate before the MWP, becoming drier overall in the MWP, wetter in the LIA, and then tending towards drought in the post-industrial era. In the study region, Chenopodiceae shrubs were more abundant in dry climates than in wet. Meanwhile, δD values of shrubs were more positive than those of grasses. Therefore, we suggest that the vegetation type (shrub or grass), which depends on moisture changes, is the controlling factor for δD variations in Lake Sugan. [ABSTRACT FROM AUTHOR]

Abstract: Arid Central Asia (ACA) lies on a major climatic boundary between the mid-latitude westerlies and the northwestern limit of the Asian summer monsoon, yet only a few high-quality reconstructions exist for its climate history. Here we calibrate a new organic geochemical proxy for lake temperature, and present a 45-yr-resolution temperature record from Hurleg Lake at the eastern margin of the ACA in the northern Tibetan Plateau. Combination with other proxy data from the same samples reveals a distinct warm–dry climate association throughout the record, which contrasts with the warm–wet association found in the Asian monsoon region. This indicates that the climatic boundary between the westerly and the monsoon regimes has remained roughly in the same place throughout the Holocene, at least near our study site. Six millennial-scale cold events are found within the past 9000yr, which approximately coincide with previously documented events of northern high-latitude cooling and tropical drought. This suggests a connection between the North Atlantic and tropical monsoon climate systems, via the westerly circulation. Finally, we also observe an increase in regional climate variability after the mid-Holocene, which we relate to changes in vegetation (forest) cover in the monsoon region through a land-surface albedo feedback. [Copyright &y& Elsevier]

Considerable efforts have been made to extend temperature records beyond the instrumental period through proxy reconstructions, in order to further understand the mechanisms of past climate variability. Yet, the global coverage of existing temperature records is still limited, especially for some key regions like the Tibetan Plateau and for earlier times including the Medieval Warm Period (MWP). Here we present decadally-resolved, alkenone-based, temperature records from two lakes on the northern Tibetan Plateau. Characterized by marked temperature variability, our records provide evidence that temperatures during the MWP were slightly higher than the modern period in this region. Further, our temperature reconstructions, within age uncertainty, can be well correlated with solar irradiance changes, suggesting a possible link between solar forcing and natural climate variability, at least on the northern Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Holocene climatic and environmental changes on the northeastern Tibetan Plateau (TP) have been widely discussed based on the climatic records from sedimentary cores. However, differences in the reconstructed climatic history from various studies in this region still exist, probably due to influence of climatic proxies from multiple factors and the chronological uncertainties in lacustrine sediments. Here we present records of terrestrial plant δ 13 C, soil color and total organic carbon content over the past 8400 years from a well-dated meadow section on the northeastern TP. The terrestrial plant δ 13 C value serves as a good summer precipitation/moisture indicator in the studied region. Soil color property and TOC content are also able to disentangle the moisture evolution history. All the data show much wet climates at 8400–7400 cal yr BP, dry climates at 7400–6000 cal yr BP and then wet conditions with fluctuation at 6000–3200 cal yr BP. Late Holocene moisture appears to be comparable with moist conditions from 6000 to 3200 cal yr BP. By further comparing the climatic variations in the Lake Qinghai area with records of the reconstructed summer temperature and the Asian Monsoon precipitation, we believe that the pattern of moisture/precipitation evolution in the Lake Qinghai area was not completely consistent with regions around Lake Qinghai, probably due to complicated interaction between the East Asian Summer Monsoon and the Indian Summer Monsoon. [ABSTRACT FROM AUTHOR]

Abstract: The distribution of glycerol dialkyl glycerol tetraethers (GDGTs) can reflect continental environmental changes. Recently, the distribution of branched GDGTs (bGDGTs) has been proposed as a novel tool for paleoelevation reconstructions. Here we report the variation in TEX86 (tetraether index of 86 carbon atoms) of isoprenoidal GDGTs (iGDGTs) and MBT (methylation of branched tetraether index) of bGDGTs along an altitudinal transect on Mt. Xiangpi, NE Qinghai-Tibetan Plateau. Both TEX86 and MBT values of surface soils showed significant linear decreases with altitude (TEX86: R 2 =0.65; n =50; MBT: R 2 =0.69; n =24). We suggest that the apparent relationships between the two indices and altitude may be related to temperature. Our preliminary investigation suggests that the TEX86 index can potentially be applied as a paleoelevation indicator in addition to the MBT index on the Qinghai-Tibetan Plateau. [Copyright &y& Elsevier]