https://www.lehigh.edu/~ziy2/pubs/Chen2008QSR.pdf

Holocene moisture evolution in arid central Asia and its out-of-phase relationship with Asian monsoon history

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.

/pdf/interplay-between-the-westerlies-and-asian-monsoon-recorded-5awlp9an81.pdf

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

The lack of a precisely-dated, unequivocal climate proxy from northern China, where precipitation variability is traditionally considered as an East Asian summer monsoon (EASM) indicator, impedes our understanding of the behaviour and dynamics of the EASM. Here we present a well-dated, pollen-based, ~20-yr-resolution quantitative precipitation reconstruction (derived using a transfer function) from an alpine lake in North China, which provides for the first time a direct record of EASM evolution since 14.7 ka (ka = thousands of years before present, where the "present" is defined as the year AD 1950). Our record reveals a gradually intensifying monsoon from 14.7-7.0 ka, a maximum monsoon (30% higher precipitation than present) from ~7.8-5.3 ka, and a rapid decline since ~3.3 ka. These insolation-driven EASM trends were punctuated by two millennial-scale weakening events which occurred synchronously to the cold Younger Dryas and at ~9.5-8.5 ka, and by two centennial-scale intervals of enhanced (weakened) monsoon during the Medieval Warm Period (Little Ice Age). Our precipitation reconstruction, consistent with temperature changes but quite different from the prevailing view of EASM evolution, points to strong internal feedback processes driving the EASM, and may aid our understanding of future monsoon behaviour under ongoing anthropogenic climate change.

/pdf/east-asian-summer-monsoon-precipitation-variability-since-1qsvcpqx5u.pdf

East Asian summer monsoon precipitation variability since the last deglaciation

The modern Indian summer monsoon (ISM) is characterized by exceptionally strong interhemispheric transport, indicating the importance of both Northern and Southern Hemisphere processes driving monsoon variability. Here, we present a high-resolution continental record from southwestern China that demonstrates the importance of interhemispheric forcing in driving ISM variability at the glacial-interglacial time scale as well. Interglacial ISM maxima are dominated by an enhanced Indian low associated with global ice volume minima. In contrast, the glacial ISM reaches a minimum, and actually begins to increase, before global ice volume reaches a maximum. We attribute this early strengthening to an increased cross-equatorial pressure gradient derived from Southern Hemisphere high-latitude cooling. This mechanism explains much of the nonorbital scale variance in the Pleistocene ISM record.

http://www.niglas.ac.cn/xwdt_1/zhxw/201308/P020130823631355013912.pdf

Glacial-Interglacial Indian Summer Monsoon Dynamics

Palaeoclimatic changes in the Qinghai Lake area during the last 18,000 years

Changes in the vegetation and climate of the westerly-dominated areas in Central Asia during the Holocene were interpreted using pollen-assemblages and charcoal data from a 300-cm-long sediment core of the Sayram Lake, northern Xinjiang. Accele-rator Mass Spectrometry (AMS) radiocarbon dating methods were applied to bulk organic matter of the samples. Artemisia spp./Chenopodiaceae ratios and results from principal component analysis were used to infer that the lake basin was dominated by desert vegetation before ca. 9.6 cal. ka BP, which suggests a warm and dry climate in the early Holocene. Desert steppe/steppe expanded during 9.6-5.5 cal. ka BP, indicating a remarkable increase both in the precipitation and temperature during the mid-Holocene. Desert vegetation dominated between 6.5 and 5.5 cal. ka BP, marking an extreme warmer and drier interval. The steppe/meadow steppe recovered, and temperatures decreased from 5.5 cal. ka BP in the late Holocene, as indicated by the increased abundance of Artemisia and the development of meadows. Holocene temperatures and moisture variations in the Sayram Lake areas were similar to those of adjacent areas. This consistency implies that solar radiation was the main driving factor for regional temperature changes, and that the effect of temperature variations was significant on regional changes in humidity. The evolution of climate and environment in the Sayram Lake areas, which were characterized as dry in the early Holocene and relatively humid in the middle-late Holocene, are clearly different from those in monsoonal areas. Dry conditions in the early Holocene in the Sayram Lake areas were closely related to decreased water vapor advection. These conditions were a result of reduced westerly wind speeds and less evaporation upstream, which in turn were caused by seasonal changes in solar radiation superimposed by strong evaporation following warming and drying local climate.

Holocene vegetational and climatic variation in westerly-dominated areas of Central Asia inferred from the Sayram Lake in northern Xinjiang, China

The evolutions of diatom floras and the total phosphorous (TP) concentrations in the historical period were reconstructed for two lakes, Longgan and Taibai in the middle Yangtze River, based on high resolutional fossil diatom study from two sediment cores and an established regional diatom-TP transfer function. The TP concentration in Longgan Lake changed slightly in the range of 36–62 μg/L and kept its middle trophic level in the past 200 years. The changes of diatom assemblages reflect a macrophyte-dominated history of the lake. During the nineteenth century, the lake TP concentration increased comparatively, corresponding to the increase in abundance of benthic diatoms. The progressive increase of epiphytic diatoms since the onset of the twentieth century indicates the development of aquatic plants, coinciding with the twice drops of water TP level. The TP concentration in Taibai Lake kept a stable status about 50 μg/L before 1953 AD, while diatoms dominated by facultative planktonic Aulacoseira granulata shifted quickly to epiphytic diatom species, indicating a rapid expansion of aquatic vegetation. During 1953–1970 AD, the coverage of aquatic plants decreased greatly inferred by the low abundance of epiphytic diatoms as well as declined planktonic types, and the reconstructed TP concentration shows an obvious rising trend firstly, suggesting the beginning of the lake eutrophication. The lake was in the eutrophic condition after 1970, coinciding with the successive increase of planktonic diatoms. The comparison of the two lakes suggests the internal adjustment and purification function of aquatic plants for nutrients in water. The discrepancy of TP trends in the two lakes after 1960 reflects two different patterns of lake environmental response to human disturbance. Sediments in Taibai Lake clearly recorded the process of lake ecological transformation from the macrophyte-dominated stage to the algae-dominated stage. The limits of TP concentration (68–118 μg/L) in the transitional state can be considered as the critical value between the two stable ecosystems. Further work will be necessary to provide more evidence from the sediments in more eutrophic lakes for the primary inference. The reconstructive TP level and the inference of aquatic plants from fossil diatoms in different lakes, as well as their comparison provide a scientific basis for ecological restoration of eutrophic lakes in research regions.

Historical trophic evolutions and their ecological responses from shallow lakes in the middle and lower reaches of the Yangtze River: Case studies on Longgan Lake and Taibai Lake

Climate proxies, such as total organic carbon and nitrogen (TOC, TN), carbonate content (CaCO3), grain-size and pollen of the sediment core retrieved from enclosed Lake Daihai which lies in the north China environment sensitive zone are analyzed to reconstruct the environment evolution of the area based on high-resolution radiocarbon chronology. The results reveal that the TOC and TN contents of the sediments correlate well with pollen percentage and pollen flux variations during the Holocene, and both reach their peak values simultaneously at 6.7–3.5 ka BP (calendar age, 7.6–3.6 ka BP). Since 6.7 ka BP, both the CaCO3 and organic matter contents of the core have simultaneous variations, and their high values also occur during 6.7–3.5 ka BP. While during 9.0–6.7 ka BP (calendar age, 10–7.6 ka BP) relatively lower level of organic matter content and pollen flux correspond to the higher level of carbonate content. The above relations suggest that during 6.7–3.5 ka BP, the productivity and effective precipitation were greatly improved in the lake drainage area, and this would probably strengthen the hydrodynamic conditions, enhancing organic matter, pollen and carbonate inputs from terrestrial sources. Such processes would account for the enrichment of both organic matter and carbonate in the sediments. While during 9.0–6.7 ka BP, the lower level of organic matter, pollen flux but high carbonate content show depressed productivity and declined vegetation coverage. The higher carbonate content at this stage would have probably resulted from the higher evaporation ratio of the lake water under relatively drier climate conditions. Therefore, it is inferred that during 6.7–3.5 ka BP, the climate was more humid with abundant rainfalls and vegetation was more flourishing in the Lake Daihai area. This can be seen as Holocene Climate Optimum (HCO). As a result, this has evident discrepancies with the traditional notion that the HCO occurs at Early Holocene or early Mid-Holocene.

Environmental characteristics of Mid-Holocene recorded by lacustrine sediments from Lake Daihai, north environment sensitive zone, China

Heavy metals of natural and anthropogenic sources are mixed in lake sediments, and it is dif-
ficult to discern Based on linear regression analysis results and element chemical properties, Al
and V are selected as the concentration normalizing elements The natural and anthropogenic
sources contributing to the elemental concentrations in core MS of Lake Taihu are analyzed in terms
of element--to--Al ratio versus V--to--Al ratio Results show that heavy metals were from natural
sources before mid 1920s The heavy metals, such as Al, Fe, Mn, V, Cr, Pb, Cu, Zn and As,
were from changed natural sources, and Hg originated from human pollution partly between mid
1920s and 1970s The heavy metals, such as Pb, Cu, Zn, Hg, As, are contaminated by human
pollution since mid 1970s The source characters of heavy metals are correspondent with the hu-
man activities and economic development in Lake Taihu drainage basin

Shen Ji

Papers

Palaeoclimatic changes in the Qinghai Lake area during the last 18,000 years

Holocene vegetational and climatic variation in westerly-dominated areas of Central Asia inferred from the Sayram Lake in northern Xinjiang, China

Historical trophic evolutions and their ecological responses from shallow lakes in the middle and lower reaches of the Yangtze River: Case studies on Longgan Lake and Taibai Lake

Environmental characteristics of Mid-Holocene recorded by lacustrine sediments from Lake Daihai, north environment sensitive zone, China

Source Analysis of Heavy Metals in Surface Sediments of Lake Taihu