Implications of diverse sedimentation patterns in Hala Lake, Qinghai Province, China for reconstructing Late Quaternary climate

Hala Lake is located in the Qilian Mountains, Qinghai Province, China, at 4,078 m a.s.l. Its sediments contain an archive of climate and hydrologic changes during the Late Quaternary, as it is located close to the area influenced by the East-Asian summer monsoon and westerly-driven air masses. Sedimentation patterns and depositional conditions within the lake were investigated using eight sediment cores from different water depths, and this information was used to evaluate the feasibility of using a single core to reconstruct past climate and hydrological conditions. Long core H7, from the center of the lake (65 m water depth) and core H8 from a western, near-shore location (20 m water depth), were compared in detail using sediment composition and geochemical data (X-ray fluorescence, loss-on-ignition and CNS analysis). Age models were constructed using 17 AMS radiocarbon dates and indicate negligible reservoir error for sediments from the lake center and similar to 1,000 year errors for the near-shore sediment core. Cores H1-H5 and HHLS21-1 revealed a sediment succession from sand and silty clay to laminated clay on the southern side of the lake. Undisturbed, finely laminated sediments were found at water depths a parts per thousand yen15 m. Core H5 (2.5 m long), from 31 m water depth, yielded abundant green algal mats mixed with clayey lake deposits and was difficult to interpret. Algae occurred between 25 and 32 m water depth and influenced the dissolved oxygen content of the stratified lake. Comparison of cores H7 and H8 yielded prominent mismatches for different time periods, which may, in part, be attributed to internal lacustrine processes, independent of climate influence. We thus conclude that data from a single sediment core may lead to different climate inferences. Common shifts among proxy data, however, showed that major climate shifts, of regional to global significance, can be tracked and allow reconstruction of lake level changes over the last 24,000 years. Results indicate advance of glaciers into the lake basin during the LGM, at which time the lake experienced lowest levels, 25-50 m below present stage. Stepwise refilling began at ca. 16 kyr BP and reached the -25 m level during the Bolling/Allerod warm phase, ca. 13.5 kyr BP. A desiccation episode falls within the Younger Dryas, followed by a substantial lake level rise during the first millennium of the Holocene, a result of climate warming, which promoted glacier melt. By ca. 7.6 kyr BP, the lake reached a stable high stand similar to the present level, which persisted until ca. 6 kyr BP. Disturbed sediments in core H7 indicate a single mass flow that was most likely triggered by a major seismic event similar to 8.5 kyr BP. Subsequent lake development remains unclear as a consequence of data mismatches, but may indicate a general trend to deteriorating conditions and lake level lowstands at ca. 5.0-4.2, 2.0 and 0.5 kyr BP.