Seasonal sensitivity of weathering processes: Hints from magnesium isotopes in a glacial stream
Seasonal sensitivity of weathering processes: Hints from magnesium isotopes in a glacial stream
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DOI：
10.1016/j.chemgeo.2012.04.002
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发表时间：
2012-06
期刊：
影响因子：
3.9
通讯作者：
E. Tipper;E. Lemarchand;R. Hindshaw;B. Reynolds;B. Bourdon
中科院分区：
文献类型：
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作者：
E. Tipper;E. Lemarchand;R. Hindshaw;B. Reynolds;B. Bourdon
河流化学的季节性变化提供了评估风化过程如何响应不断变化的气象参数以及最终化学风化如何响应气候参数的可能性。来自瑞士阿尔卑斯山（达玛冰川）的单一岩性花岗岩、受气候影响有限的一级流域的溪流沃茨中镁同位素比值（26 Mg/24 Mg比值表示为δ 26 Mg/mil单位）的系统性季节变化。除了植物、岩石、矿物分离物和土壤外，还报告了雨水、地面和孔隙水。与流量的大幅变化相比，河流沃茨的浓度响应减弱。然而，在同位素数据的系统趋势意味着，无论是在一个系统的方式的Mg源的变化，或Mg被释放到解决方案的过程中的变化作为一个功能的放电。需要解释的数据中的两个一级观测结果是：1）与它们排出的花岗岩相比，溪流沃茨中24 Mg的系统富集; 2）夏季融化季节期间沃茨中δ 26 Mg的系统增加。这两种观测结果（与许多其他河流排放硅酸盐岩相似）可以通过以下方式解释：1）至少两种不同Mg源（除降水输入外）之间的保守混合，或2）与过程相关的分馏。如果可以通过多组分混合来合理化河流水的组成，则至少有一种δ 26 Mg <−1.2‰的未识别组分。这被认为是不可能的。多种物理化学过程可使Mg同位素比值发生变化，如1）24 Mg的优先淋溶，2）Mg在矿物表面和粘土层间交换，3）植物吸收，4）26 Mg在粘土、无定形相或氧化物等次生相形成过程中优先保留。这些过程并不相互排斥，在田间规模上区分它们并不容易，但在该部位不太可能发生显著的生物吸收。除非有一个未确定的外部输入的镁，26镁必须积累在集水区的固相残留物，因为至少有一个物理化学过程。此类过程可能已被Rayleigh蒸馏模型很好地描述，至少在一级。简单的计算可以说明单位时间内26 Mg在集水区的累积量。在第一顺序中，固体中的同位素富集是如此之小，以至于它们在与该现场相关的时间尺度上是不可检测的，尽管对水化学有显著的影响。Mg同位素比值检测到的季节性信号是有希望的，使用它们（更好地了解分馏机制），以量化具体的风化过程如何影响两个出口通量，并保留集水区内的元素。
Seasonal changes in river chemistry offer the potential to assess how weathering processes respond to changing meteorological parameters and ultimately how chemical weathering might respond to climatic parameters. Systematic seasonal variations in magnesium isotope ratios (the26Mg/24Mg ratio expressed as δ26Mg in per mil units) are reported in stream waters from a mono-lithological granitic, weathering-limited, first order catchment from the Swiss Alps (Damma glacier). Rain, ground, and pore-waters, in addition to plants, rocks, mineral separates and soil are also reported. The concentration response of the river waters is attenuated compared to the large changes in discharge. However, the systematic trends in the isotope data imply that either the source of the Mg changes in a systematic manner, or that the process by which Mg is released into solution changes as a function of discharge. The two first order observations in the data that need to be explained are 1) the systematic enrichment in24Mg in the stream waters compared to the granitic rocks they drain and 2) a systematic increase in δ26Mg in the waters during the summer melt season. Both observations (which are similar to many other rivers draining silicate rock) can either be accounted for by 1) conservative mixing between at least two different sources of Mg (in addition to precipitation inputs), or 2) process related fractionation. If the stream water compositions can be rationalised by multi-component mixing, there is at least one unidentified component with a δ26Mg<−1.2‰. This is considered unlikely. Multiple physicochemical processes could fractionate Mg isotope ratios such as 1) preferential leaching of24Mg, 2) exchange of Mg onto (or from) mineral surfaces and into interlayer sites of clays, 3) uptake by plants, and 4)26Mg could be preferentially retained during the formation of secondary phases, such as clays, amorphous phases or oxides. These processes are not mutually exclusive and distinguishing between them at a field scale is not trivial, but significant biological uptake is improbable at this site. Unless there is a non-identified external input of Mg,26Mg must be accumulating in solid phase residues in the catchment because of at least one physicochemical process. Such processes are likely well described, at least in the first order by a Rayleigh distillation model. Simple calculations illustrate how much26Mg would accumulate in the catchment per unit time. In the first order, the isotopic enrichments in the solids are so small that they would not be detectable for the time-scales that are relevant to this field site, in spite of the marked impact on the water chemistry. The seasonal signal detected by Mg isotope ratios is promising for using them (with a better understanding of fractionation mechanisms) to quantify how specific weathering processes impact upon both export fluxes, and retention of elements within catchments.