The influence of weathering processes on riverine magnesium isotopes in a basaltic terrain
The influence of weathering processes on riverine magnesium isotopes in a basaltic terrain
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DOI：
10.1016/j.epsl.2008.09.020
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发表时间：
2006-03
影响因子：
5.3
通讯作者：
P. V. Strandmann;K. Burton;R. James;P. Calsteren;S. Gíslason;B. Sigfússon
中科院分区：
文献类型：
--
作者：
P. V. Strandmann;K. Burton;R. James;P. Calsteren;S. Gíslason;B. Sigfússon
这项研究提出了主要的，微量元素和镁同位素数据的溶解负荷和悬浮颗粒物的冰岛河流排水主要是玄武岩集水区，包括冰川和直接径流的河流。这些样品提供了了解化学风化过程中镁同位素行为的机会，其中由于岩性的变化是不存在的。鉴于镁在碳循环中的重要作用，这种变化可能提供有关地球气候调节的重要信息。还分析了热液沃茨、地下水、降水（冰川冰）、玄武岩玻璃、橄榄石和代表性土壤。溶解负荷的δ 26 Mg组成范围很广，与母玄武玻璃（δ 26 Mg =−0.29‰）相比，从−0.96到+0.64‰，而沉淀和热液沃茨的δ 26 Mg值分别为−0.83‰和+0.85‰，Mg浓度低于溶解负荷。植被中的生物量活动和土壤和河流中的有机物质（胶体）似乎对镁同位素组成的影响不大。相反，这些数据表明，镁元素和同位素的变化在很大程度上控制的形成和稳定性的第二相在不同的水文条件。在一些样品中，海水，以直接降水或冰川径流的形式，似乎也是镁的重要来源。冰川河流、地下水和一些直接径流河流的pH值较高，δ 26 Mg含量高于玄武岩，这很可能是由于次生矿物中含有轻Mg同位素。相比之下，那些具有相对较低pH值的直接径流河流具有较低的δ 26 Mg（相对于玄武岩），这与优先将重Mg同位素掺入次生相一致，尽管不可能排除降水的一些贡献。河流悬浮颗粒物中移动的元素含量较低，δ 26 Mg组成值既高于未风化玄武岩，也低于未风化玄武岩。在冰川补给和直接径流的河流中，溶解相δ 26 Mg重，由于次生相的形成，悬浮物轻，因为它含有更多的次生相。相反的是真实的直接径流的河流具有低pH值的其余部分。这可能是由于溶解的次生矿物，富含轻镁，这是不稳定的低pH值，或形成新的第二相。
This study presents major-, trace-element and Mg isotope data for the dissolved load and suspended particulates of Icelandic rivers draining dominantly basaltic catchments, including both glacier-fed and direct-runoff rivers. These samples provide the opportunity to understand the behaviour of Mg isotopes during chemical weathering, where variations due to lithology are not extant. Given the significant role of Mg in the carbon cycle, such variations may provide important information on the regulation of Earth's climate. Hydrothermal waters, groundwater, precipitation (glacial ice), basalt glass, olivine and representative soils have also been analysed. The dissolved load shows a wide range of δ26Mg compositions, compared to the parent basaltic glass (δ26Mg=−0.29‰), ranging from −0.96 to +0.64‰, while precipitation and hydrothermal waters possess δ26Mg values of −0.83‰ and +0.85‰, respectively, with lower Mg concentrations than the dissolved load. Biomass activity in vegetation and organic material in soils and rivers (colloids) appear to have little effect on the Mg isotope compositions. Rather, the data suggest that Mg elemental and isotopic variations are largely controlled by the formation and stability of secondary phases in response to differing hydrological conditions. In some samples seawater, in the form of direct precipitation or glacial runoff, also appears to be an important source of Mg. Glacier-fed rivers, groundwaters, and some direct-runoff rivers, with a high pH, have higher δ26Mg than basalt, which is most likely due to the incorporation of light Mg isotopes in secondary minerals. In contrast, those direct-runoff rivers which have a relatively low pH, have low δ26Mg (relative to basalt), consistent with preferential incorporation of heavy Mg isotopes into secondary phases, although it is not possible to rule out some contribution from precipitation. Riverine suspended particulates are depleted in mobile elements, and have δ26Mg compositions values both higher and lower than unweathered basalt. In the glacier-fed and direct-runoff rivers where the δ26Mg of the dissolved phase is heavy, due to the formation of secondary phases, the suspended load is light, because it contains more of those phases. The opposite is true for the remainder of the direct-runoff rivers which have low pH. This could be due to dissolution of secondary minerals, enriched in light Mg, which are unstable at low pH, or the formation of new secondary phases.