Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control

Tipper, E. T. and Galy, A. and Bickle, M. J. (2008) Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control. Geochimica Et Cosmochimica Acta, 72 (4). pp. 1057-1075. ISSN 0016-7037 DOI 10.1016/j.gca.2007.11.029

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Abstract

In rivers draining the Himalaya-Tibetan-Plateau region, the 26Mg/24Mg ratio has a range of 2‰ and the 44Ca/42Ca ratio has a range of 0.6‰. The average δ26Mgδ26Mg values of tributaries from each of the main lithotectonic units (Tethyan Sedimentary Series (TSS), High Himalayan Crystalline Series (HHCS) and Lesser Himalayan Series (LHS)) are within 2 standard deviation analytical uncertainty (0.14‰). The consistency of average riverine δ26Mgδ26Mg values is in contrast to the main rock types (limestone, dolostone and silicate) which range in their average δ26Mgδ26Mg values by more than 2‰. Tributaries draining the dolostones of the LHS differ in their View the MathML sourceδ4442Ca values compared to tributaries from the TSS and HHCS. The chemistry of these river waters is strongly influenced by dolostone (solute Mg/Ca close to unity) and both δ26Mgδ26Mg (−1.31‰) and View the MathML sourceδ4442Ca (0.64‰) values are within analytical uncertainty of the LHS dolostone. These are the most elevated View the MathML sourceδ4442Ca values in rivers and rock reported so far demonstrating that both riverine and bedrock View the MathML sourceδ4442Ca values may show greater variability than previously thought. Although rivers draining TSS limestone have the lowest View the MathML sourceδ26Mgandδ4442Ca values at −1.41 and 0.42‰, respectively, both are offset to higher values compared to bedrock TSS limestone. The average δ26Mgδ26Mg value of rivers draining mainly silicate rock of the HHCS is −1.25‰, lower by 0.63‰ than the average silicate rock. These differences are consistent with a fractionation of δ26Mgδ26Mg values during silicate weathering. Given that the proportion of Mg exported from the Himalaya as solute Mg is small, the difference in 26Mg/24Mg ratios between silicate rock and solute Mg reflects the 26Mg/24Mg isotopic fractionation factor (View the MathML sourceαsilicate–dissolvedMg) between silicate and dissolved Mg during incongruent silicate weathering. The value of View the MathML sourceαsilicate–dissolvedMg of 0.99937 implies that in the TSS, solute Mg is primarily derived from silicate weathering, whereas the source of Ca is overwhelmingly derived from carbonate weathering. The average View the MathML sourceδ4442Ca value in HHCS rivers is within uncertainty of silicate rock at 0.39‰. The widespread hot springs of the High Himalaya have an average δ26Mgδ26Mg value of −0.46‰ and an average View the MathML sourceδ4442Ca value of 0.5‰, distinct from riverine values for δ26δ26Mg but similar to riverine View the MathML sourceδ4442Ca values. Although rivers draining each major rock type have View the MathML sourceδ4442Ca and δ26Mgδ26Mg values in part inherited from bedrock, there is no correlation with proxies for carbonate or silicate lithology such as Na/Ca ratios, suggesting that Ca and Mg are in part recycled. However, in spite of the vast contrast in vegetation density between the arid Tibetan Plateau and the tropical Lesser Himalaya, the isotopic fractionation factor for Ca and Mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of Ca and Mg in these catchments. The discrepancy between solute and solid Ca and Mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of Ca and Mg, bound as minerals in rock, and released as cations in rivers. The fractionation of Ca and Mg isotope ratios may prove useful for tracing mechanisms of chemical alteration. Ca isotope ratios of solute riverine Ca show a greater variability than previously acknowledged. The variability of Ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global Ca cycling, if past variations in oceanic Ca isotope ratios are to be of use in constraining the past carbon cycle.

Item Type: Article
Uncontrolled Keywords: 2007 AREP IA55 2008 P
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
05 - Petrology - Igneous, Metamorphic and Volcanic Studies
02 - Geodynamics, Geophysics and Tectonics
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
02 - Geodynamics, Geophysics and Tectonics
05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: Geochimica Et Cosmochimica Acta
Volume: 72
Page Range: pp. 1057-1075
Identification Number: 10.1016/j.gca.2007.11.029
Depositing User: Sarah Humbert
Date Deposited: 16 Feb 2009 13:03
Last Modified: 31 Jul 2016 21:39
URI: http://eprints.esc.cam.ac.uk/id/eprint/734

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