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

In rivers draining the Himalaya-Tibetan-Plateau region, the Mg-26/M-24 g ratio has a range of 2 parts per thousand and the Ca-44/Ca-42 ratio has a range of 0.6 parts per thousand. The average delta Mg-26 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 parts per thousand). The consistency of average riverine delta Mg-26 values is in contrast to the main rock types (limestone, dolostone and silicate) which range in their average (delta Mg-26 values by more than 2 parts per thousand, Tributaries draining the dolostones of the LHS differ in their delta Ca-44/42 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 delta Mg-26 (-1.31 parts per thousand) and delta Ca-44/42 (0.64 parts per thousand) values are within analytical uncertainty of the LHS dolostone. These are the most elevated delta Ca-44/42 values in rivers and rock reported so far demonstrating that both riverine and bedrock delta Ca-44/42 values may show greater variability than previously thought. Although rivers draining TSS limestone have the lowest delta Mg-26 and delta Ca-44/42 values at -1.41 and 0.42 parts per thousand, respectively, both are offset to higher values compared to bedrock TSS limestone. The average delta Mg-26 value of rivers draining mainly silicate rock of the HHCS is -1.25 parts per thousand, lower by 0.63 parts per thousand than the average silicate rock. These differences are consistent with a fractionation of delta Mg-26 values during silicate weathering. Given that the proportion of Mg exported from the Himalaya as solute Mg is small, the difference in Mg-26/Mg-24 ratios between silicate rock and solute Mg reflects the Mg-26/Mg-24 isotopic fractionation factor (alpha(Mg)(silicate-dissolved)) between silicate and dissolved Mg during incongruent silicate weathering. The value of alpha(Mg)(silicate-dissolved) of 0.99937 implies that in the TSS, solute Mg is primarily derived from silicate weathering, whereas the source of Ca is over-whelmingly derived from carbonate weathering. The average delta Ca-44/42 value in HHCS rivers is within uncertainty of silicate rock at 0.39 parts per thousand, The widespread hot springs of the High Himalaya have an average delta Mg-26 value of -0.46 parts per thousand and an average delta Ca-44/42 value of 0.5 parts per thousand, distinct from riverine values for Mg but similar to riverine delta Ca-44/42 values. Although rivers draining each 44 major rock type have delta Ca-44/42 and delta Mg-26 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 and 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. (c) 2007 Elsevier Ltd. All rights reserved.