Abstract

Mg is an important constituent of silicates and a major nutrient and its stable isotopes have been shown to fractionate during geochemical and biological reactions. Mg isotope ratios therefore hold promise as a useful tracer of (bio)geochemical processes in the critical zone. We present preliminary results on the Mg isotopic composition of porewater and stream water (baseflow and stormflow), along with elemental data from depth profiles (1.0 - 9.3 m) in bulk regolith and exchangeable cations taken at 4 sites along a topo-sequence in a well-constrained catchment over volcaniclastic bedrock at the Luquillo Critical Zone Observatory, Puerto Rico..
Mg and K concentrations corrected for rain input and evapotranspiration indicate weathering contributions to pore water solutes. Pore water concentrations are relatively invariant with time below 0.5 m depth (2.0 m depth in the deepest profile). Above these depths, Mg concentrations vary in space and time, with all profiles showing overall higher concentrations in the surface layers, suggestive of biological influence. In contrast, 26Mg values in the deepest profile show a clear trend towards heavier 26Mg with increasing depth (-0.83‰ to -0.18‰) suggesting mixing between atmospheric Mg at the surface and a dissolving, isotopically heavier phase at depth. An excursion towards heavier 26Mg at the soil-saprolite interface (-0.7‰, ~1m depth) indicates a change in controlling processes. A similar heavy excursion is present at the same depth in 3 of the 4 profiles.
Stream water at baseflow is isotopically heavier than the pore waters and becomes progressively lighter with increasing stage.  These results may indicate that watershed export of Mg is dominated by deep-weathering processes during baseflow with contributions from rain and shallow soils during storms.

Citation

Chapela Lara, M., Buss, H.L., Pogge Von Strandman, P.A.E., Dessert, C. Gaillardet, J. (2013): Mg isotopes: insights into weathering in tropical volcanoclastic regolith. Goldschmidt2013 Conference Abstracts. Florence Italy. .

This Paper/Book acknowledges NSF CZO grant support.