纳木错高湖面时期的溢流：基于萝卜螺壳体氧同位素及地貌证据

OVERFLOW OF THE LAKE NAM CO DURING ITS HIGH LEVEL PERIOD： INFERENCES FROM OXYGEN ISOTOPE OF RADIX SHELLS AND GEOMORPHOLOGICAL EVIDENCES

湖泊的古水位及古水文状态的重建是湖泊研究领域最具挑战性的工作之一。本文以纳木错高湖面沉积物为研究对象，利用高湖面沉积中的萝卜螺（Radix）壳体化石，定量重建了纳木错高湖面时期（约4．4—2．2ka）湖水古电导率（Paleo—EC）和古水化学特征（δ18OPaleo-water和Sr／CaPaleo-water）；进而利用萝卜螺壳体化石δ18Oshell，并结合古湖岸线地貌证据，讨论了古湖水水文状态。重建结果指示纳木错在高湖面时期湖水的Paleo—EC、δ18OPaleo-water和Sr／CaPaleo-water的变化范围分别是514～1063μs／cm、-12．0％0-～7．8‰和0．0002—0．0095。本文证实萝卜螺壳体δ18Oshell与其宿生湖泊水文状态存在一定的关系。在纳木错高湖面沉积剖面中，萝卜螺化石壳体的δ18Oshell值指示，纳木错在约距今4．4—2．2ka的高湖面期间可能处于一种间歇性外流状态。此外，基于纳木错古湖岸线分布高程，推测高湖面时期古水位可能高出现今湖面约30m左右；而这一湖面高度高于纳木错流域与仁错流域之间分水岭的高度。因此，萝卜螺壳体化石δ18Oshell和古湖岸线地貌证据指示，纳木错在该高湖面期间可能存在间歇性外流；外溢湖水注入毗邻的仁错湖。

The paleo-lake level and paleohydrological reconstruction is one of the most challenging issues in lake investigation. On the Tibetan Plateau, many lakes are surrounded by multiple paleo-shorelines and exposed high level lake sediments. Nevertheless, the research on the high level lake sediments on the Tibetan Plateau is not sufficient. The Lake Nam Co （30°30′-30°55′N, 90°16′-91°03′E; 4718m a.s.1.） is located at the south-central part of the Tibetan Plateau. It is characterized by the semi-arid monsoon climate. Here, the exposed high level lake sediments （at 30°52′N and 90°53′E with 2.96m in thickness） and paleo-shoreline from the Lake Nam Co have been investigated. The top of the profile is 2.76m above the modern lake level. We found that the fossil shells of Radix are widely distributed in the exposed sediment profile from 0.76m to 1.87m depth. Most of these fossil shells are gastropoda Radix auricularia （Pulmonata, Lymnaeidae） , while a few of the broken shells cannot be identified to species level. In this study, we refer to the Radix species as ＇ Radix sp. ＇. Based on the geochemical relationship between Radix sp. shells and its ambient water derived from lakes of south central Tibetan Plateau or the specific Lake Nam Co, the fossil shells of Radix sp. were used to reconstruct the paleo-electrical conductivity （Paleo-EC） and paleohydrochemical features （δ18Opaleo-water and Sr/Capaleo-water ） of the paleo-lake water during the high level period （ca.4.4-2.2ka） of the Lake Nam Co. The reconstructed Paleo-EC values of the paleo-lake water during the high lake level period of Nam Co were between 514μs/cm and 1063μs/cm. However, the results indicate the reconstructed Paleo-EC values of the paleo-lake water depended on the specific relationship between the Sr/Ca ratio of the Radix sp. shells and the EC of the lake water in Nam Co, rather than relationship built based on many various lakes in south central Tibetan Plateau. The reconstructed δ18Opaleo-water and Sr/Capaleo-water values of the paleo-lake water during the high level period of the Lake Nam Co were between -12.0‰ and -7.8‰, 0. 0002 and 0. 0095, respectively. Moreover, we confirm that the δ18Oshell values of Radix sp. significantly depend on hydrological status of lakes. Based on the δ18Oshell values of fossil Radix sp., we infer that the Lake Nam Co existed as a semi-closed lake during its high level period. On the other hand, the summit top of watershed between the Lake Nam Co basin and its neighboring Lake Ring Co basin is about 4738m a.s.1. The present lake water level of both lakes is 4718m a.s.1, and 4648m a.s.l., respectively. Near the study site, we found that the paleo-shoreline is ca.30m above the modern lake level, while paleo-water level of the Lake Nam Co had exceeded the altitude of watershed. In summary, based on the δ18Oshell values of fossil Radix sp., the reconstructed δ18Opaleo-water of the paleo-water body and the geomorphological evidence of paleo-shoreline, we infer that the Lake Nam Co may be intermittent overflow during its high level period, while the outflow water spilled into the neighboring Lake Ring Co basin.