Collaborative Resarch: A Comparative Study of Carbonate Weathering Mechanisms and Fluxes in Carbonate-Rich, Mid-Latitude Watersheds Across Landscape and Landuse Types

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EAR-0518965WALTERMineral weathering rates are likely to increase in the future under the influence of global warming, increasing atmospheric CO2, changing land use, and acidification. Because carbonate mineral solubility is CO2-dependent and reaction rates are rapid, terrestrial carbonate dissolution fluxes will change and may feed back to the climate system as well as influence the geochemistry of surface waters. Our previous work on the hydrogeochemistry of carbonate-rich watersheds in the Upper Midwest and in Slovenia shows that there is close linkage between soil zone PCO2 and that of shallow groundwaters. Both of these regions have among the largest area normalized dissolved inorganic carbon fluxes in the world. Riverine export of dissolved inorganic carbon could increase if weathering of carbonates accelerates, and may also be influenced by agricultural liming, a common practice in the Midwest and other humid agricultural regions of the world. The Mississippi River already shows evidence of increasing inorganic carbon export. The net effect of liming on atmospheric CO2 is uncertain because the dissolution of lime minerals in surface soils may sequester or generate CO2 depending on pH. Carbonate mineral dissolution also impacts weathering of silicate minerals, especially by controlling soil pH, which further influences weathering feedbacks to atmospheric CO2. In fact, the Na fluxes from plagioclase feldspar weathering appear to be significantly higher in catchment soils with both silicates and carbonates than in granitic watersheds in warmer settings.Intellectual Merit: The present proposal is a new cross-disciplinary collaboration that brings together a Principal Investigator and a Slovenian collaborator to pursue several distinct, but highly interrelated, lines of research on carbonate geochemistry. Combining efforts in a synergistic way will broaden and integrate the understanding of the roles of carbonate minerals in landscape-scale solute fluxes and soil-atmospheric CO2 exchanges. Walter has extensive experience working on the hydrogeochemical aspects mineral weathering in diverse landscapes. Part of the proposed research is an extension of projects initiated under previous funding, including comparisons of carbonate geochemistry of streams and groundwaters in glaciated Michigan watersheds with those in alpine and karst regions of Slovenia. Hamilton has recently begun to investigate carbonate fluxes associated with liming in connection with an LTER project on agricultural row-crop ecosystems. Through a new collaboration with Stephen Hamilton at the Kellog Biological Station at Michigan of Michigan State University, Walter will extend her work to Michigan agricultural soils, bringing geochemical expertise to bear on the questions of how intensive agriculture affects the weathering of native minerals as well as the effects and fate of lime added to soils. Approaches will include the study of carbonate and silicate geochemistry in soil profiles beneath forests and limed and unlimed row crops, examination of riverine export of chemical weathering products, and experimental lime additions to cropping systems equipped with soil monoliths that permit sampling of soil solutions and gases at various depths. Particular attention is devoted to the near-surface soils and spatially discrete catchments of different land use types where reaction rates are intense and anthropogenic activities exert the most influence.Broader Impacts: Educational impacts include training of several graduate students as well as scientific exchanges with Slovenia. The association with an LTER site brings outreach and K12 educational opportunities. Societal impacts involve understanding global change effects on weathering rates and potentially significant feedbacks. Investigation of net effect of liming on atmospheric CO2 is critical to the integrated assessment of the contribution of agriculture to greenhouse gas fluxes and may reveal opportunities to reduce the net global warming potential of row crop systems.