Inorganic carbon isotope systematics in soil profiles undergoing silicate and carbonate weathering (Southern Michigan, USA) Lixin Jin a, ⁎, Nives Ogrinc b , Stephen K. Hamilton c , Kathryn Szramek a, 1 , Tjasa Kanduc b , Lynn M. Walter a a Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building,1100 N. University Ave., Ann Arbor, MI 48109-1005, USA b Department of Environmental Sciences, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia c Kellogg Biological Station and Department of Zoology, Michigan State University, 3700 E. Gull Lake Drive, Hickory Corners, MI 49060-9516, USA abstract article info Article history: Received 29 October 2008 Received in revised form 2 March 2009 Accepted 2 March 2009 Editor: J. Fein Keywords: δ13C Soil water Chemical weathering pCO 2 Global C cycle 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 13 C 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 13 C DIC and δ 13 C 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 13 C 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 13 C CaCO 3 =0‰) in reaction with soil CO 2 (δ 13 C CO 2 =-22‰). Soils beneath an agricultural site with a wheat/corn/soybean rotation (the Kalamazoo watershed) displayed a wide range in δ 13 C CO 2 values (-22 to -12‰), and the δ 13 C DIC of deeper soil waters in contact with carbonate minerals was controlled by seasonal variations of δ 13 C 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. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Rocks transform into soils at the Earth's surface, and the reactions of mineral weathering control inorganic carbon fluxes. The soils of the vadose zone are where large carbon reservoirs interact, including those of the lithosphere (carbonate rocks), atmosphere (CO 2 ), hydrosphere (dissolved and particulate organic and inorganic carbon), and biosphere (living and non-living organic matter). Interactions among these carbon reservoirs influence atmospheric CO 2 and are thus important to the energy budget of the Earth surface and in controlling the global surface temperature (Berner and Berner, 1996). The responses of the terrestrial biosphere to anticipated increases in atmospheric CO 2 will be complex (Zak et al., 1993; Richter et al., 1995; Schlesinger, 1997; Houghton et al., 1998, 1999; Zak et al., 2000; Williams et al., 2003), and an important feedback entails the consumption of CO 2 through chemical weathering of carbonate minerals and transport of dissolved inorganic carbon to the oceans, where most CO 2 is ultimately lost from the Earth surface by carbonate re-precipitation, but after long time delays (Holland, 1978; Berner and Berner, 1996; Williams et al., 2007; Szramek et al., 2007). Chemical weathering of the abundant carbonate minerals in the Northern Hemisphere represents a sink for atmospheric CO 2 of global importance (e.g., Berner and Berner, 1996; Williams et al., 2007; Szramek et al., 2007). Temperate mid-continents support the most Chemical Geology 264 (2009) 139–153 ⁎ Corresponding author. Current address: Center for Environmental Kinetics Analysis, Pennsylvania State University, State College, PA 16802, USA. Tel.: +1814 865 9384. E-mail address: luj10@psu.edu (L. Jin). 1 Current address: Department of Geology, Washington and Lee University, Lexington, VA 24450, USA. 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.03.002 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo