Yilong

Although Holocene-scale trends in Indian Summer Monsoon (ISM) variability have been well-established, manifestations and drivers of centennial- and multi-decadal-scale variability are still debated. Additionally, the extent to which proxies reflect abrupt climatic changes rather than proxy-specific thresholds is unclear. To address these questions, here we summarize a 9200-year record from Yilong Lake in Yunnan China using multiple proxies including oxygen and carbon isotopes of authigenic calcite as well as Ti/Al ratio to characterize lake hydrologic balance and catchment erosion. We compare these results to two other well-studied nearby lakes on the Yunnan Plateau. At all three lakes, the Holocene-scale waning of the ISM is evident, but punctuated by abrupt shifts indicative of lower precipitation and/or greater evaporation at 7900, 5500, and 4000 cal yr B.P. We suggest that a shift to a positive mean-state of the Indian Ocean Dipole lasting multiple centuries can account for these events, with possible moderating influence from El Niño Southern Oscillation. After 1500 cal yr B.P., Yilong oxygen isotopes shift to more positive values as a result of human manipulation of hydrologic balance, coincident with the deposition of a red clay layer from catchment erosion and high concentrations of mercury. These results are similar to other Yunnan lakes although the intensity of anthropogenic management of Yilong’s hydrologic balance is substantially smaller than at the other regional lakes. These results underscore the diversity of anthropogenic impacts to lakes, even ones that are only a few kilometers apart, and demonstrate spatiotemporal differences in freshwater resource use.

尽管全新世尺度的印度夏季风（Indian Summer Monsoon, ISM）变率的整体趋势已得到广泛证实，但百年至年代际尺度变率的具体表现与驱动机制仍存在争议。此外，代用指标究竟在多大程度上反映了气候突变，而非指标自身的临界阈值，目前仍不明确。为解答上述科学问题，本研究对中国云南异龙湖的9200年沉积记录进行综合分析，采用自生方解石（authigenic calcite）氧碳同位素、Ti/Al比值等多类代用指标，以刻画湖泊水文平衡与流域侵蚀特征。我们将该结果与云南高原另外两处研究较为充分的邻近湖泊的相关记录进行对比。三处湖泊均呈现出印度夏季风全新世尺度的减弱趋势，但在公元前7900、5500及4000校准年（cal yr B.P.）出现了指示降水减少或蒸发加剧的突变事件。我们认为，持续数百年的印度洋偶极子（Indian Ocean Dipole, IOD）平均位相转为正位相的过程可以解释上述突变事件，厄尔尼诺南方涛动（El Niño Southern Oscillation, ENSO）或起到了调节作用。在公元前1500校准年之后，异龙湖的氧同位素值显著偏正，这一变化源于人类活动对湖泊水文平衡的干预，同期沉积记录中出现了流域侵蚀带来的红土层以及高浓度汞元素。尽管异龙湖水文平衡的人为调控强度远低于云南高原其他区域湖泊，但其研究结果与其他云南湖泊的相关记录具有相似性。本研究结果凸显了即使相距仅数公里的湖泊，其受人类活动的影响也存在显著差异，同时揭示了淡水资源利用的时空分异特征。