长江流域河流沉积物宇宙成因核素^(10)Be特征与侵蚀速率估算

CHARACTERISTICS OF COSMOGENIC NUCLIDE ^(10)Be IN THE YANGTZE RIVERINE SEDIMENTS AND ESTIMATIONS OF EROSION RATE

地球表面每时每刻都在经历各式各样的侵蚀作用,了解侵蚀过程及其速率大小有助于人们认识许多重要的地质作用和过程。本文介绍的内容是长江流域河流沉积物宇宙成因核素^(10)Be的研究工作,目的是在于定量估算长江流域及其子流域的平均侵蚀速率,更好地理解沉积物的由源到汇过程以及评价人类活动对水土流失的影响提供自然背景,分析样品来自于长江主要干流和支流的表层现代沉积。研究表明,长江干流^(10)Be含量从金沙江流域到长江口呈现出由高到低的趋势,不同的是支流^(10)Be含量值偏低而且比较稳定。这很可能受核素产生率和侵蚀速率两个因素的共同影响。在长江上游干流沉积物中^(10)Be含量最高,随着低含量物质的不断从支流汇入,产生"西高东低"的现象。^(10)Be侵蚀速率估算表明长江干流金沙江上游段平均侵蚀速率较低,在44.7~48.1m/Ma之间;长江中游段(枝江至彭泽)长江干流侵蚀速率数据变化较大,在65.7~175m/Ma的范围内波动;到长江口平均侵蚀速率比较稳定,在50~60m/Ma之间变化。与干流相比,长江支流侵蚀速率显著偏高。侵蚀速率最高的地区在大渡河-岷江流域,平均侵蚀速率在300m/Ma之上;侵蚀速率最低的区域发生在乌江流域,平均的侵蚀速率在10~30m/Ma之间。比较长江流域^(10)Be和水文估算的侵蚀速率可以看出,水文估算总体上反映的侵蚀速率要普遍高于^(10)Be反映的侵蚀速率。大渡河-岷江流域地表侵蚀速率高主要与构造活动、地貌发育、岩石特征以及气候条件等自然因素有关。嘉陵江、汉江等流域水文数据估算侵蚀速率明显超过^(10)Be估算的结果,可能与地形地貌等地质因素对侵蚀作用的影响显著下降,以及人类长期活动导致水土流失加剧有关。

The surface of the Earth is always subjected to erosion.Understanding the erosion processes and rates is helpful to recognize many important geological processes and evolution.This paper presents a research on cosmogenic nuclide 10 Be in the Yangtze Riverine sediments,with scientific objectives to quantitatively estimate average erosion rates in the Yangtze River catchment and its sub-catchments,and to provide a natural baseline that is helpful in understanding the source to sink processes of sediments and in assessing humans impacts on soil erosion.The analyzed samples were taken from surface sediments in the Yangtze River main-stream and major tributaries.The results show that 10 Be concentration decreases downstream in the mainstream from Jinshajiang River to the Yangtze River estuary,but the 10 Be concentrations in tributaries are relatively low and stable.This characteristic might be attributed to 10 Be production and erosion rate.The highest concentration of 10 Be occurs in the upper reaches,with inputs of lower-concentration sediments,causing a phenomenon of high in the west but low in the east reaches.The estimated erosion rates based on 10 Be show that the erosion rates are low in the upper reaches of Jinshajiang River（44.7~48.1m/Ma）,relatively variable in the middle reaches fluctuating in the range of 65.7~175.0m/Ma,and rather stable in the Yangtze River estuary（50~60m/Ma）.The erosion rates are considerably higher in the tributaries than in the mainstream.The region with lowest erosion rate is Wujiang River catchment,which average erosion rate is in the range of 10~30m/Ma.The comparison of erosion rate between 10 Be and hydrological gauge data illustrate that the erosion rates estimated by hydrological gauge are generally higher than those by 10 Be.The high erosion rates in the Daduhe River-Minjiang River catchment are related to natural factors,such as tectonic,relief,lithology and climate.However,the erosion conditions of the rest sub-catchments,particularly those in the east of Daduhe River-Mingjiang River,are mainly impacted by human activities that have caused erosion rates estimated by hydrological gauge exceeding those estimated by 10 Be.