The response of Li and Mg isotopes to rain events in a highly-weathered catchment

Abstract

Storms are responsible for up to ~50 % of total annual rainfall on tropical islands and result in rapid increases in discharge from rivers. Storm events are, however, notoriously under-sampled and their effects on weathering rates and processes are poorly constrained. To address this, we have undertaken high-frequency sampling of Quiock Creek catchment, a Critical Zone Observatory located in Guadeloupe, over a period of 21 days, encompassing several storm events. Chemical and isotopic (Li and Mg) analyses of different critical zone reservoirs (throughfall, soil pore water, groundwater and river water) were used to assess the interactions between rock, water and secondary minerals. The Li concentrations and δ7Li values of these different reservoirs range from 14 to 95 nmol/kg and 1.8 to 16.8‰, respectively. After several rain events, the average δ7Li value (13.3‰) of soil solutions from the lower part of the soil profile (> ~150 cm below the surface) was unchanged, whereas in the upper part of the profile δ7Li values increased by ~2 - 4‰ due to increased contribution from throughfall. By contrast, the δ26Mg value of soil waters in the upper part of the soil profile were not significantly affected by the rain events with an average value of - 0.90‰. The δ26Mg values of the different fluid reservoirs were generally close to the value of throughfall (~ -0.90‰), but higher δ26Mg values (up to -0.58‰) were measured in the deeper parts of the soil profile, whereas groundwaters that have a long residence 28 time had lower δ26Mg values (down to -1.48‰). These higher and lower values are attributed to, respectively, adsorption/desorption of light Mg isotopes on/from the surface of clay minerals. The δ7Li value of the river waters was ~9.3‰, with a Li concentration of 60 μmol/kg, but during a storm these values decreased to, respectively, 7.8‰ and 40 μmol/kg. This change in δ7Li is consistent with an increased contribution of Li from the soil solution. Thus, even in highly weathered catchments, changes in hydrological conditions can have a significant impact on weathering processes and therefore the composition of river waters delivered to the ocean.

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ORCID for Rachael James: orcid.org/0000-0001-7402-2315

ORCID for Aurelien Beaumais: orcid.org/0000-0001-8693-4872

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