Identifying weathering sources and processes in an outlet glacier of the Greenland Ice Sheet using Ca and Sr isotope ratios
Identifying weathering sources and processes in an outlet glacier of the Greenland Ice Sheet using Ca and Sr isotope ratios
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DOI：
10.1016/j.gca.2014.09.016
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发表时间：
2014-11
影响因子：
5
通讯作者：
R. Hindshaw;J. Rickli;J. Leuthold;J. Wadham;B. Bourdon
中科院分区：
文献类型：
--
作者：
R. Hindshaw;J. Rickli;J. Leuthold;J. Wadham;B. Bourdon
2009年，在西格陵兰岛莱弗莱特冰川出口河段连续28天，每天2次采集河水的化学和同位素数据(ε40Ca，δ44/42Ca，87Sr/86Sr，δ18O)。水化学数据与大量岩石、矿物分离和沉积物样品的详细地球化学分析和岩石学分析相结合，以约束矿物风化源到河流。河流中测得的平均同位素组成(2SD)为：ε~(40)Ca~(2+)=0.40±0.14，δ~(44/42)Ca~(2+)=‰+0.60±0.10~(-1)，~(87)Sr/~(86)Sr~(3+)=0.74243±0.00327。根据融水流量的变化，将水化学数据划分为三个水文期。第一个阶段以暴发事件的尾部为标志，以水为特征，悬浮泥沙浓度(SSC)、离子浓度和pH值都在下降。在第二次水文期间，流量增加，而~(87)Sr/~(86)Sr由0.74550降至0.74164。根据87Sr/86Sr与Na/Sr、Ca/Sr和ε40Ca的二元混合图，这被解释为反映了随着水停留时间的减少，反应矿物风化作用的增加，特别是绿帘石。水停留时间的减少是从分布式(长水停留时间)向沟道化(短水停留时间)冰下排水网络演变的结果。第三个水文期被定义为总流量减少的时期。这一水文期的特点是流量有明显的日循环。在此期间，δ44/42Ca与SSc和δ18O之间存在显著的相关性，表明吸附过程中存在分馏现象。这项研究表明，放射性成因的钙既有可能识别随时间变化的矿物源，也有可能在δ44/42Ca的值中分离来源和分馏效应。
Chemical and isotope data (ε40Ca,δ44/42Ca,87Sr/86Sr,δ18O) of river water samples were collected twice daily for 28 days in 2009 from the outlet river of Leverett Glacier, West Greenland. The water chemistry data was combined with detailed geochemical analysis and petrography of bulk rock, mineral separates and sediment samples in order to constrain the mineral weathering sources to the river. The average isotopic compositions measured in the river, with 2SD of all the values measured, wereε40Ca = +4.0 ± 1.4,δ44/42Ca = +0.60 ± 0.10‰ and87Sr/86Sr = 0.74243 ± 0.00327. Based on changes in bulk meltwater discharge, the hydrochemical data was divided into three hydrological periods. The first period was marked by the tail-end of an outburst event and was characterised by water with decreasing suspended sediment concentrations (SSC), ion concentrations and pH. During the second hydrological period, discharge increased whilst87Sr/86Sr decreased from 0.74550 to 0.74164. Based on binary mixing diagrams using87Sr/86Sr with Na/Sr, Ca/Sr andε40Ca, this is interpreted to reflect an increase in reactive mineral weathering, in particular epidote, as the water residence time decreases. The decrease in water residence time is a result of the evolution from a distributed (long water residence time) to a channelised (short water residence time) subglacial drainage network. The third hydrological period was defined as the period when overall discharge was decreasing. This hydrological period was marked by prominent diurnal cycles in discharge. During this period, significant correlations betweenδ44/42Ca and SSC andδ18O were observed which are suggestive of fractionation during adsorption. This study demonstrates the potential of radiogenic Ca to both identify temporally changing mineral sources in conjunction with87Sr/86Sr values and to separate source and fractionation effects inδ44/42Ca values.