Inorganic carbon isotope systematics in soil profiles undergoing silicate and carbonate weathering (Southern Michigan, USA)

• Published in: Chemical geology 2009-06, Vol.264 (1), p.139-153
• Main Authors: Jin, Lixin, Ogrinc, Nives, Hamilton, Stephen K., Szramek, Kathryn, Kanduc, Tjasa, Walter, Lynn M.
• Format: Article
• Language: English
• Subjects: Chemical weathering; Global C cycle; pCO 2; Soil water; δ13C
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2009.03.002

The upper Midwest USA features glacial-derived till materials enriched in carbonate minerals, but with the uppermost soil layer progressively leached of carbonates in the interval since glaciation. Groundwaters and groundwater-fed surface waters are profoundly influenced by carbonate mineral dissolution. Stable carbon isotope compositions of soil waters and groundwaters in two southern Michigan watersheds (Huron and Kalamazoo) were studied as a function of pH, δ 13C CO 2 , types of weathering reactions (silicate vs. carbonate), and degree of isotope equilibration. This comprehensive study of carbon isotope biogeochemistry in the vadose zone, including soil gas, soil water/groundwater, and soils (organic matter/carbonate phases), elucidates relations between the chemical weathering rates and CO 2 fluxes in the soil zone. Such information is important to evaluate responses of terrestrial ecosystems to global climate change. In shallow soil zones where only silicate weathering was occurring, respiratory CO 2 was the major source of soil water DIC with little addition from the atmospheric CO 2. Isotopic equilibration between δ 13C DIC and δ 13C CO 2 occurred in an open system with respect to soil CO 2. In the deeper soil horizons carbonate dissolution dominated soil water chemistry and saturation with respect to calcite and dolomite was attained rapidly. Mass balance calculation showed that large amounts of soil CO 2 were consumed by carbonate dissolution, such that the deeper soil zone may not have been an open system with respect to CO 2. Constant δ 13C DIC values (∼ − 11‰) were observed in these deep soil waters and also in shallow groundwaters of the Huron watershed. Thus, isotopic equilibrium might not be reached between DIC and CO 2, possibly due to a rapid kinetics of carbonate dissolution and limited gas–water exchange in the soils. If so, DIC was equally contributed by carbonate minerals ( δ 13C CaCO 3 = 0‰) in reaction with soil CO 2 ( δ 13C CO 2 = − 22‰). Soils beneath an agricultural site with a wheat/corn/soybean rotation (the Kalamazoo watershed) displayed a wide range in δ 13C CO 2 values (− 22 to − 12‰), and the δ 13C DIC of deeper soil waters in contact with carbonate minerals was controlled by seasonal variations of δ 13C CO 2 as well as by strong acids produced by nitrification and to a lesser degree by pyrite oxidation, both of which could react to dissolve carbonate minerals, in addition to carbonic acid dissolution.