河口离子交换过程对流域化学风化通量估算的影响  

Influence of estuarine cation exchange on the estimation of catchment weathering flux

作　　者：刘楚青 杨承帆 杨守业[1,2] LIU Chuqing;YANG Chengfan;YANG Shouye(State Key Laboratory of Marine Geology,Tongji University,Shanghai 200092,China;School of Ocean and Earth Science,Tongji University,Shanghai 200092,China)

机构地区：[1]同济大学,海洋地质国家重点实验室,上海200092 [2]同济大学海洋与地球科学学院,上海200092

出　　处：《地球化学》2024年第2期234-245,共12页Geochimica

基　　金：自然科学基金项目(42103050、41991324);上海市科委项目(18230750600);中国博士后科学基金项目(2021M692416)联合资助。

摘　　要：大陆风化是构造尺度上维持地球系统气候稳定的重要机制,河水主要离子组成分析是约束流域风化类型和速率的主要方法,然而悬浮物与水体间阳离子置换对风化通量评估的重要性并未得到足够重视。本研究使用NH4Cl及Mg(NO_(3))_(2)溶液提取长江入海口悬浮物可交换态组分,探讨离子交换过程对元素循环及流域风化通量评估的影响。结果表明,悬浮物向口门外搬运过程中,可交换态Ca^(2+)含量降低了9.3×10^(-2) meq/g,约占河流端元悬浮物可交换态Ca^(2+)含量的53.8%,Na^(+)、Mg^(2+)、K^(+)含量分别增加了19.6×10^(-2) meq/g(约3266.7%),8.9×10^(-2) meq/g(约89.9%)和1.1×10^(-2)meq/g(约220.0%);离子交换量与颗粒粒径、矿物组成以及有机碳含量(TOC)等参数无明显相关性,主要受控于水体阳离子组成;根据可交换态阳离子含量变化以及长江入海泥沙通量,估算长江悬浮物入海过程中可释放Ca^(2+)通量约为7.9×10^(9)mol/a,吸附的Na^(+)、Mg^(2+)和K^(+)通量分别为26.3×10^(9)mol/a、6.0×10^(9)mol/a和1.4×10^(9) mol/a,相当于长江每年输入这些阳离子通量的2.5%、9.2%、4.4%和2.3%;大河河口离子交换过程是海陆间元素地球化学循环的重要环节,流域内海相沉积岩或沉积物吸附的Na^(+)和K^(+)再释放到河水中会导致硅酸岩风化通量的高估,初步估算现在长江流域的硅酸岩风化通量被高估15%,考虑到近几十年流域内人类活动导致入海泥沙量显著下降,地质历史时期该占比可能显著增大。因此,未来流域风化与河口元素循环研究应充分考虑水体和沉积物阳离子交换和人类活动的可能影响。Continental weathering is an important mechanism for maintaining the climate stability of the earth system over a tectonic scale.Analyzing the major ion compositions of river water is the primary method of constraining the weathering type and rate within drainage basins.However,the significance of cation exchange between sediments and water for weathering flux assessment has received less attentions.In this study,suspended particulate matter(SPM)and water samples were collected along a salinity profile in the Changjiang(Yangtze)Estuary.We use NH4Cl and Mg(NO_(3))_(2) solutions to extract the SPM exchangeable fraction,aiming to disclose the influence of cation exchange on the element cycle and weathering flux evolution.Our results show that the content of SPM exchangeable Ca^(2+)decreased by 9.3×10^(-2) meq/g during transportation,accounting for 53.8%of exchangeable Ca^(2+)in the river SPM.In comparison,an increase of 19.6×10^(-2) meq/g(ca.3266.7%)for Na^(+),8.9×10^(-2) meq/g(ca.89.9%)for Mg^(2+),and 1.1×10^(-2) meq/g(ca.220.0%)for K^(+)can be observed at the Changjiang Estuary.There is no significant correlation among exchangeable cation content,SPM grain size,mineral composition,and organic carbon content.We further infer that the variation of exchangeable cation concentration is mainly controlled by the chemical compositions of estuarine water.Based on the changes of cation content and annual sediment flux,the Ca^(2+)flux released by the Changjiang SPM into the sea is estimated to be 7.9×10^(9) mol/a,while the adsorbed fluxes of Na^(+),Mg^(2+),and K^(+)are 26.3×10^(9) mol/a,6.0×10^(9) mol/a,and 1.4×10^(9) mol/a,respectively.These values account for 2.5%,9.2%,4.4%,and 2.3%of the corresponding Changjiang-derived dissolved flux.Cation exchange in large river estuaries is an important part of geochemical cycles,and the re-release of Na^(+)and K^(+)absorbed by marine sedimentary rocks/sediments into river water may result in an overestimation of silicate weathering flux.In the case of the Changjiang River,an

关 键 词：长江口 悬浮物 阳离子交换 风化通量 元素循环 

分 类 号：P342[天文地球—水文科学] P512.1[天文地球—地球物理学]