泸沽湖近代沉积环境时空变化特征及原因分析

Temporal and spatial variations in sedimentary characteristics of Lake Lugu during the last hundred years and the influence factors analysis

通过对泸沽湖沉积岩芯粒度、磁化率(χ_(lf))、化学蚀变指数(CIA)等沉积指标的分析,结合^(210)Pb和^(137)Cs定年结果以及区域降水和人类活动等资料,研究了近150年来泸沽湖沉积环境的时空变化特征与主要影响因素.泸沽湖沉积物粒度组成以黏土与细粉砂为主(>80%),细颗粒组分(如黏土)含量与χ_(lf)、CIA之间具有显著相关性.各岩芯沉积指标垂向变化规律相似,1920s之前,沉积指标较为稳定,为人类活动影响较弱的准自然沉积阶段;1920s以来,χ_(lf)、CIA值与黏土含量逐渐升高,反映了风化与成壤作用较强的细颗粒表土物质侵蚀开始加强,可能与流域农业发展及森林砍伐等人类活动影响有关;约1970年以来,χ_(lf)、CIA值与黏土含量进一步升高,指示了流域内表土侵蚀与上述人类活动影响的进一步增强,与文献记录的1970s—1980s两次大规模的森林砍伐吻合;约2002年以来,χ_(lf)、CIA值与黏土含量降低,反映了表土侵蚀减弱,与近年来流域植被逐渐恢复及降水减少有关.与器测资料对比研究表明,降水等气候因素对近代泸沽湖沉积环境演变的影响相对较弱.空间上,各沉积指标表现出一定的异质性.黏土含量在南部和北部湖区两侧靠近洪积扇及冲积平原的湖区沉积物中较高;近50年以来,各沉积岩芯所反映的平均沉积通量为0.020~0.043 g/(cm^2·a),南部湖区高于北部湖区,主要受入湖水系分布及流域南部地区高强度人类活动导致的土壤侵蚀的影响.

Twelve short sediment cores were collected from Lake Lugu to analyze the particle size composition, magnetic susceptibility （ χIf） , metal content and chemical index of alteration （CIA） in this study. Referencing to the 210Pb and 137Cs dating results, natural factor such as precipitation on the sedimentary characteristics, and human activity records such as agricultural expansion and deforestation, the temporal and spatial variations in sedimentary characteristics of Lake Lugu in recent 150 years and were discussed. The results indicated that the sediments of Lake Lugu were mainly （ 〉 80% ） composed of clay and fine silt. The fine fractions （ such as clay） showed typical positive correlations with χIf and CIA. The sedimentary proxies showed similar vertical changes in the cores from different lake areas. Before 1920s, χIf and CIA remained low values and the grain size compositions were stable, suggesting weak human disturbance and stable particle input from the catchment. The clay content, χIf. and CIA increased gradually since 1920s, which were more remarkable change since ca. 1970, indicating enhanced erosion of well-weathered and fine-grainedtopsoil probably relating to intensive agricuhural development and deforestation in Lake Lugu Catchment. The χIf and CIA tended to decreasing and median grain size increased since around 2002, which suggested decreasing topsoil erosion attributing to the implement of environmental protection measures such as vegetation recovery in the catchment. Study between the sedimentary proxies and instrumental data showed that climate factors such as precipitation had a weaker effect on the sedimentary evolution of Lake Lugu when compared to human perturbation. Spatially, the sedimentary proxies showed obvious heterogeneity. There was higher clay con- tent in the surface sediments in north and south lake areas where they were near the pluvial-alluvial plains. The mean mass accumulation rates were 0.020-0.043 g/（ cm2 . a） since 1964, which showed higher values in the south Lake Lugu than in the north Lake Lugu. The high accumulation rate in the south lake should be ascribed to a fact that are influenced by inflow and outflow rivers and the strong soil erosion from the south catchment caused by intensive human activities.