A 16-ka oxygen-isotope record from Genggahai Lake on the northeastern Qinghai-Tibetan Plateau: Hydroclimatic evolution and changes in atmospheric circulation

Moisture source history and changes in atmospheric circulation are the most important attributes for portraying past climate changes and for estimating possible future trends. However, few climate records reflecting these attributes are available from the marginal zones of the Asian summer monsoon. Here, we present a record of the oxygen isotopic composition of authigenic carbonates (delta(18)Ocarb) of sequential sediments from Genggahai Lake in the northeastern Qinghai-Tibetan Plateau (QTP). Isotopic analyses were performed on the fine-grained carbonate fraction (<38 mu m), mollusc shells, and stem encrustations from submerged plants. The stratigraphic variations of the delta O-18 record from the different carbonate components exhibit a remarkably similar pattern, probably reflecting the fact that delta(18)Ocarb variability was controlled primarily by changes in the oxygen isotopic composition of the lake water (delta(18)OLw). Disequilibrium effects and water temperature are precluded as major factors affecting the delta(18)Ocarb variations. Genggahai Lake is hydrologically open and characterized by a rapid discharge rate, as indicated by analysis of the hydrological setting of the lake system and by the observed significant positive correlation between delta O-18(LW) and the oxygen isotopic composition of the inflowing water (delta O-18(1)). Under such hydrological conditions, we argue that the isotopic signals of different moisture sources should be reflected in the carbonate isotopic composition. Furthermore, placing the delta(18)Ocarb record in the context of regional palaeoclimate archives, we found that the isotopic signals, particularly the negative shifts from the average values, cannot be interpreted consistently, despite a process of evaporative enrichment at the lake surface. During the early- to mid-Holocene, low delta(18)Ocarb values during 10.6-9.4 and 7.4-63 ka were associated with higher lake levels, and thus the record may have been significantly affected by a strengthened Asian summer monsoon at those times and hence by a positive moisture balance. Isotopic depletions with similar magnitudes occurred at 15-14.5, 13.8-13.3, 12.5-11.4, 5.3-4.8, 3.7-3.4, 2.8-2.3, 1.7, 1.3, and 0.6 1<a, i.e., within the Lateglacial and the late Holocene. These negative shifts in delta(18)Ocarb corresponded to lower lake-level phases, and most likely were a response to intensification of the mid latitude westerly circulation that may have transported significantly more O-18-depleted moisture to the region, compared to that from the Asian summer monsoon. Overall, our results suggest that the alternating influence of the Asian summer monsoon and the westerlies played a significant role in determining the pattern of atmospheric circulation on the northeastern QTP. (C) 2017 Elsevier Ltd. All rights reserved.