Estimates Of Lake Level Changes In Hala Lake, China Since The LGM:Evidences From Ostracod Assemblages And Distribution Patterns In Multiple Sediment Records

Hala Lake, a close basin at 4078 m a.s.l, is located in the Qilian Mountains (97°24’-97°47’E, 38°12’-38°25’N), an ideal site to do analysis on palaeo-evironment and climate reconstruction to understand more about the system in Tibetan Plateau.12 sediment cores were taken from different water depths in Hala Lake and three cores of them were used to analyze ostracods distribution and transfer function within the lake. Long core H7, from the center of the lake (65 m water depth), core H12 from western of H7 site (39 m water depth) and core H8 from a western, near-shore location (19.8 m water depth), have different sediment composition in lithology and were sampled to do analysis on ostracods with 5-6 cm resolution, meanwhile AMS dating, XRF scanning and stable isotope analysis on Leucocythere sp. shells have been done.A potential reservoir error of 240 years for core H7 and 1200 years for core H8 was subtracted, as new data from H12 reveals an almost negligible reservoir error at that site. Core age H7 has the longest sediment history of about 24 kyr. Core H7 spans the time from about 15 kyr BP to present, whereas core H12 only comprises the last ca 8.3 kyr.Three species were found in Hala Lake:Leucocythere sp., Limnocythere inopinata and Eucypris mareotica. Uneven ostracods distribution patterns of three sediment cores were compared with age model and lithology layer.Based on the ecological information from ostracods, the present limnological setting with seasonal anoxic conditions in the profundal zone of the lake, and the sediment composition of the lake records we can draw a palaeo-lake level change curve and even quantify certain water depth during the last 23 kyr BP.Hala Lake stayed at a very low lake level (<10m) from the global LGM until-15 ka BP and was partly filled by a glacier. After ca.15 kyr BP lake level increased strongly by>45 m to a highstand until～13 ka BP and reached the site H8 already at around 13.2 kyr BP. This first filling phase is a result of strong glacier melt, most likely due to climate warming rather than monsoon-derived moisture supply.A second filling phase started at the beginning of the Holocene (11.5-7.8 kyr BP). The lake reached a similar level as today and fluctuated afterwards, explained by the strong climate warming at the beginning of the Holocene and increased moisture availability. However, relative lake level lowstands (declines by about 10-14 m) peaking at around 10.5,9.0, and 8.3-8.1 kyr BP maybe caused by variable effective moisture supply by the EASM. The identified low lake level in Hala Lake between 8.4 and 8.1 kyr BP coincides with a mass flow layer observed in two records (H7 and H12).The mid-Holocene is characterized by a third filling phase, which started after a lake level lowstand between 7.2 and 6.2 kyr BP. Highest lake levels occurred between 6 and 5 kyr BP, peaking at 5.8 kyr BP (ca.10-12 m higher than present), while the reasons for it remain unclear, but it can be assumed that during this period both wetter climate conditions (reduced P/E ratio) and perhaps increased melt water supply supported maximum lake level rise. A strong lake level decline after 4.8 kyr BP towards a lowstand at around 4.3 kyr BP could be identified by the re-occurrence of abundant shallow water ostracods. That could be a result of reduced moisture supply and probably also cooler conditions.A short-term refilling of Hala Lake peaked at around 3 kyr BP, followed by a decline towards a lowstand, culminating at around 2.4 kyr BP, explained by a short-term warm spell with enhanced water supply and a rapid return to previous climatic conditions. The last recognizable lake level lowstand in Hala Lake falls into the period between 1.8 and 0.8 kyr BP, as revealed by a thick algae layer in core H12 and the occurrence of shallow water ostracods in core H8. This lowstand may be explained by a reduction in melt water supply and reduced local precipitation, due to cooler and drier climate conditions. The final filling towards the present lake level is not well recorded, because ostracods remained absent.