| The Tibetan Plateau has been a hot issue for geographic studies due to its special characteristics. The wide and highly elevated plateau influences Asian monsoon and Westerlies and plays a very important role on global atmospheric circulation system. Previous studies on lacustrine sediments gave important hints for the Late Pleistocene to Holocene palaeoclimatic evolution. The results indicate spatially and temporally asynchronous climate impact. The validity of these results, however, is not sufficiently resolved and is therefore still under discussion.To improve the study, several field surveys to Hala lake, which located at the Northeastern Tibetan Plateau, were carried out. A 2.93 meter sediment core H12 was taken from 40 m water depth in February,2012. Multi-proxy data including water content, organic matter, geochemical elements, classification and statistic parameters of grain size and carbonates were applied by experiments including Loss on Ignition, XRF scanning, Grain size measurements. The dataset was used to infer sediment flux and geochemical characteristics of Hala Lake throughout the last 9000 cal. yr BP, in comparison to adjacent lake records from the same lake. The chronology sequence is based on consistent radiocarbon AMS ages.The data show that H12 core could be traced back to ca.8800 cal. yr BP, covering most parts of the Holocene period. The chronology model indicates a negligible reservoir error at H12 site, differently from the other cores H8 and H11. Oscillations of data which reflect sediments and paleoenvironment evolution in Hala lake could be divided into 6 units as:8800?7700 yr BP,7700?5800 yr BP,5800?2700 yr BP, 2700?2150 yr BP,2150-1560 yr BP and 1560-present (Unit 1?6 separately). From 8800 cal. yr BP to present, water content increases obviously, running parallel with organic matters, proving the issue that water content has a positive correlation with organic matters, except for some units where the data are stronger affected by grain size effects. Organic matter represents lake primary productivity, it fluctuates strongly with upwards trend from unit 4, indicating a warming tendency through late Holocene. Sr/Ca ratios are used reflecting salinity dynamics in lake water, indicating change of hydrological budget in lake. In this catchment, Ca/Si ratio could represent authigenic carbonates in unit 2 and 3. Grain size data imply the dominance of clay and silt in all samples, showing slightly different sorting. Especially at 7700,3100-2500 cal. yr BP and from 1560 cal. yr BP to present samples are poorly sorted due to increased proportions of sand. Variations in grain size composition through time are interpreted in terms of sediment flux from the catchment in relation to the distance (lake level) between core site and estuary. Clay percentage decreased while sand proportion get increased, especially from unit 5 to present, may denote water level decreased or influx increased.Carbonate decreasing trend does not correspond with other data, probably reflecting different sources of carbonates from catchment input throughout the Holocene. Layers at 270?280 cm (7700?8000 cal. yr BP) seemed to be transported from shallower part near paleoshoreline. Taking the disturbed structure and different sediment composition into consideration, this sequence is regarded to represent a mass flow layer most likely induced by an earthquake. Unconformity of calcium and carbonate content indicates that this layer is dominated by Mg-rich carbonates, such as dolomite.In order to avoid uncertainties in the interpretation of proxy data from core H12 with respect to climate variability, comparisons with cores H8 (20 m water depth) and H11 (63 m water depth) were facilitated. Silicon data in these 3 sites ran parallel, slight differences only occur at upper 3 units, denoting local effects at H12 site. Organic matter partly showed diverse data due to differences in bio-productivity at the three sites. The opposite trend in H8 with other cores might caused by unstable conditions in shallower water which was probably affected by lake-internal current processes and variable inflow characteristics of the near estuary, showing the complicated environment in H8 site. Carbonate data show the strongest differences between the three records. Owing to variations in catchment characteristics, inflow behavior and local conditions for carbonate formation, the percentage of carbonate content in the sediments varied considerably, testifying carbonates as an inappropriate proxy for climate change reconstructions in this catchment area. The mass flow layers were detected in H11 core but not H8 core, perhaps due to the limited range of influence.Most H12 core data are reliable after comparison with other cores. Thus regional climate and environment could be divided as follows:In Early Holocene from 8800?7700 cal. yr BP Hala lake suffered an earthquake, resulting in a mass flow that affected site H12 and H11, data here has no relation with climate; During the Middle Holocene from 7700?5800 cal. yr BP climate here experienced cool to warm shifts, while cool periods may have dominated for longer time.5800?3100 cal. yr BP was characterized by cooler and humid conditions as precipitation increased, lake expanded to a relatively high level and kept stable for a long period. Several severe changes may have happened during the Late Holocene, denoting variations happened in this region. But the entire dominated trend was warming and drying.Some data of H12 cores are conflicted with previous studies or other cores. This perhaps was caused by local effects, revealing spatial-temporal variability in lacustrine depositional conditions at Hala lake. This study again demonstrates the necessity of comparing proxy records from one site with other sites to enable separation of non-climatic processes from climate signals and to validate local versus catchment-wide effects that control sediment dynamics in Hala lake. |
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