Investigations on sediments from Lake Nam Co (Tibetan Plateau)@en

This study focuses on the analysis of lake sediments retrieved from the deepest part of Lake Nam Co (Tibetan Plateau). One gravity core of 115 cm length, covering the last ~ 4000 cal BP, was analyzed for geochemical and biological parameters. High organic content at ~ 4000 cal BP and the coinciding presence of pyrite framboids until ~ 2000 cal BP point to hampered decomposition of organic material due to anoxic conditions within the lake sediments. At the same time sedimentological and biological proxies suggest a rather high lake level, but still ~ 5 m below the recent one, with less saline lake water due to enhanced monsoonal activity. During this time a change in the source of organic matter to lowered input of terrestrial components is observed. A rather quick shift to a dry environment with less monsoonal influence and a lake level ~ 15 m lower than today at ~ 2000 cal BP lead to the oxygenation of sediment, the degradation of organic matter and the absence of pyrite. Oscillations of the lake level thereafter were of minor amplitude and not able to establish anoxia at the lake bottom again. A wet spell between ~ 1500 cal BP and ~ 1150 cal BP is visible in proxies referring to catchment hydrology and the ostracod-based water depth transfer function gives only a slightly elevated lake level. The last ~ 300 years are characterized by low TOC and rising TN values reflecting enhanced nutrient supply and hence an advancing influence of human activity in the catchment. Decreasing TOC/TN values point to a complete shift to almost solely aquatic biomass production. These results show that hydrological variations in terms of lake level change based on monsoonal strength can be linked to redox conditions at the lake bottom of Nam Co. Comparison with other archives over larger parts of the Tibetan Plateau and beyond exhibits a rather homogeneous climatic pattern throughout the late Holocene.本研究针对青藏高原纳木错最深区域获取的湖泊沉积物展开分析。研究采集了一根长115 cm的重力岩芯，其沉积序列覆盖约4000校准距今年（cal BP）以来的时段，并对其地球化学与生物学参数开展了分析。距今约4000 cal BP时沉积物有机碳含量较高，且直至约2000 cal BP期间均伴随黄铁矿莓状体（pyrite framboids）产出，这表明湖泊沉积物内部处于缺氧环境，抑制了有机质的降解。与此同时，沉积学与生物学代用指标显示当时湖泊水位较高，但仍较现代水位低约5 m；且因季风活动增强，湖水盐度偏低。此阶段还观测到有机质来源发生变化，陆源输入占比降低。在约2000 cal BP时，环境快速向干旱化转变，季风影响减弱，湖泊水位较现代低约15 m，这使得沉积物处于氧化环境，有机质发生降解，且黄铁矿不再产出。此后湖泊水位波动幅度较小，未能再次使湖底形成缺氧环境。在约1500 cal BP至1150 cal BP期间，流域水文代用指标显示存在一段湿润期，但基于介形类（ostracod）的水深转换函数结果仅显示湖泊水位略有上升。近约300年以来，沉积物以低总有机碳（Total Organic Carbon, TOC）含量与持续上升的总氮（Total Nitrogen, TN）含量为特征，这反映出流域内营养盐输入增加，人类活动影响不断加剧。总有机碳与总氮的比值（TOC/TN）持续下降，表明有机质来源完全转变为几乎仅依赖水生生物生产。上述研究结果表明，基于季风强度变化的湖泊水位波动所反映的水文变异，可与纳木错湖底的氧化还原环境状态建立关联。将本研究结果与青藏高原及更广区域内的其他气候记录进行对比后发现，全新世晚期的气候格局整体较为均一。