Mg isotope constraints on soil pore-fluid chemistry: Evidence from Santa Cruz, California

Edward T. Tipper, Jerome Gaillardet, Pascale Louvat, Francoise Capmas, Art F. White

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    Mg isotope ratios (Mg-26/Mg-24) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant Mg-26/Mg-24 m ratio (expressed as delta Mg-26) at -0.79 +/- 0.05 parts per thousand, identical to seawater delta Mg-26. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a delta Mg-26 value of 0.11 parts per thousand, potentially enriched in Mg-26 by up to 0.3 parts per thousand to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for delta Mg-26, ranging from -0.99 parts per thousand near the surface to -0.43 parts per thousand at the base of the profile. Shallow pore-waters (<1 m) have delta Mg-26 values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as <32%, calculated using the average Mg isotope enrichment factor between grass and rain (delta Mg-26(grass) - delta Mg-26(rain)) of 0.21 parts per thousand. The deep pore-waters (1-15 m deep) have delta Mg-26 values that are intermediate between the smectite and rain, ranging from -0.76 parts per thousand to -0.43 parts per thousand, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation inputs. A consistent interpretation is only possible if Mg isotope ratios are fractionated either by the precipitation of a secondary Mg bearing phase, not detected by conventional methods, or selective leaching of Mg-24 from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering. (C) 2010 Elsevier Ltd. All rights reserved.

    Original languageEnglish
    Pages (from-to)3883-3896
    Number of pages14
    JournalGeochimica et Cosmochimica Acta
    Issue number14
    Publication statusPublished - 15 Jul 2010


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