摘要:青藏高原东南部高寒山区广泛发育冰川湖泊，湖泊沉积过程同时受控于区域气候、流域水文、地质条件及湖泊形态特征。基于放射性²¹⁰Pb/¹³⁷Cs和¹⁴C定年法，对巴松错沉积物理（粒度、分选系数）和化学指标（TOC、TN、C/N）进行分析，发现18世纪末到19世纪末湖泊沉积过程显著变化，表现为迅速变缓趋势。通过分析区域树轮重建的气候序列（温度、降水及相对湿度）及冰川地貌调查资料，认为气候变化及流域冰川分布位置是影响该湖泊沉积过程的重要因素。小冰期末期冰川前缘靠近湖区，随后温度上升导致冰川融水激增、水动力加强，从而引起湖泊沉积粒度的粗化。随着冰川前缘不断后退，径流输送距离增长、沉积分选变好、粒度细化。此外，该地区活跃的地质活动也可能是湖泊沉积过程明显变化的重要诱因。湖泊沉积硅藻是研究气候环境变化的有力指标。过去200多年巴松错硅藻组合变化不明显（DCCA=0.47 SD），说明该地区气候环境变化未超过其生态阈值。通过与其他沉积指标进行对比分析发现，巴松错硅藻记录受到流域水文和湖泊沉积过程影响，主要表现为外源输入和/或湖岸浅水区来源的底栖属种在湖心沉积物中的相对丰度增加。1770-1901年总体上具有较低的Procruste残差，说明期间沉积硅藻和粒度具有较一致的波动过程，也说明了巴松错沉积硅藻记录对沉积过程的响应较为敏感。藏东南地区很多湖区均受到冰川和地质作用的强烈影响，因此在利用微体古生物手段对该地区湖泊进行气候变化研究时，建议考虑沉积过程的影响并进行多指标对比分析。



Abstract:Glacial lakes distribute widely in the alpine mountains on the southeastern Tibet Plateau. Lake sedimentary process in this region is synthetically controlled by regional climate change, hydrological and geological conditions in the catchment, and lake morphological feature. To gain a more comprehensive understanding of lake sedimentary process, a 480 cm long core BSCW-1 was collected from the deepest part (120 m in depth) of the Lake Basomtso on October, 2012. The top 90 cm was used in this study to reveal the sedimentary process over the past 200 years. The chronology was constructed based on ²¹⁰Pb/¹³⁷Cs measurements from the upper 60 cm and one AMS ¹⁴C result at 90 cm depth. Physical (grain size, sorting coefficient) and chemical data (TOC, TN, C/N) were measured and analyzed. These data reveal a remarkable transition in sedimentary process between the late 18th and 19th centuries, showing abrupt decrease in sedimentation rate. By comparing tree-ring-based climate reconstructions (temperature, precipitation and relative humidity) and records of palaeoglacial investigation, we suggested that regional climate change and glacier distribution in the catchment together influenced the sedimentary process in Lake Basomtso. Given that glacier had extended to lower altitude during the late 18th century, the following rise in temperature may have intensified meltwater which can transport more coarse particles into the lake basin. Along with glacier recession, the headwater moved gradually away from the lake, which caused finer particles depositing in the lake. In addition, the impact of geological event on sedimentary process cannot be excluded because this region is located in the orogenic belt. Fossil diatom has been a practical approach for climate and environment research. Diatom record from Lake Basomtso reflects moderate species turnover (DCCA=0.47 SD), indicating that diatom assemblage had not passed the ecological threshold over the past two centuries. This phenomenon is consistent with other diatom records from lakes on the southeastern and eastern Tibet Plateau. The ratio of planktonic to benthic species abundance (P/B) is relatively low between the 1770s to the early 1900s, which corresponded to the distinct coarse-grained sediments in core BSCW-1, implying some connection between sedimentary process and fossil diatoms. Additionally, although the nMDS, Procrustes and Protest analysis reveal modest similarity between diatom and grain size datasets, the lower Procrustes residuals between 1770-1901 AD support the argument that sedimentary process has some impacts on fossil diatom record. High-energy fluid with high velocity and strong sand-carrying ability can wash and carry more lotic and littoral diatoms into the open water (or the profundal zone) then deposit in the lake center. Since benthic diatoms are dominated in rivers and shallow waters, decline in P/B ratio is a reflection of enhanced hydrological and sedimentary conditions given that lake water level varied little during the past two centuries. As most lakes on the southeastern Tibet Plateau are strongly influenced by glacier and geologic activities, we suggested to consider the effect of sedimentary process and recommend multi-proxy analysis when using biostratigraphic data to interpret climate and environment change in this region.





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