Research paper Impacts of glacial/interglacial cycles on continental rock weathering inferred using Sr/Ca and 87 Sr/ 86 Sr ratios in Michigan watersheds Lixin Jin a, b, ⁎, Samuel B. Mukasa b, 1 , Stephen K. Hamilton c , Lynn M. Walter b a Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, United States b Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, United States c Kellogg Biological Station and Department of Zoology, Michigan State University, Hickory Corners, MI 49060, United States abstract article info Article history: Received 6 August 2011 Received in revised form 12 January 2012 Accepted 13 January 2012 Available online 28 January 2012 Editedor: J. Fein Keywords: Chemical weathering Soil mineralogy Soil water Glaciation Michigan soils have developed on thick glacial-drift deposits that include different proportions of ground granite and gneiss from the Canadian Shield region (with radiogenic Sr) and carbonate sedimentary rocks from within the Michigan Basin (with non-radiogenic Sr). This study of the Cheboygan, Huron and Kalama- zoo watersheds shows how Sr-isotope and Sr/Ca ratios in soil waters, ground waters, and soils reflect relative weathering intensities of the dominant minerals in Michigan soils, including carbonates from the Michigan Basin, and amphibole, plagioclase and K-feldspar derived from the Canadian Shield. Soil water 87 Sr/ 86 Sr ratios evolve quickly to the carbonate weathering end-member (0.709–0.711) once a cal- cite and dolomite layer is reached at depth (~ 100–200 cm) in the Huron and the Kalamazoo watersheds. Dis- solution of plagioclase and amphibole controls shallow soil water 87 Sr/ 86 Sr ratios (0.711–0.713), with minor contributions from K-feldspar weathering. In contrast, soils in the previously studied Cheboygan watershed are completely depleted in carbonate minerals and contain little plagioclase and amphibole in the top 300 cm of the profile. As a result, soil waters in this watershed are ionically dilute with high 87 Sr/ 86 Sr ratios (0.72 and 0.74), dominantly contributed by K-feldspar dissolution. Subsequent dissolution of plagioclase and amphibole at greater depths sharply increases soil water and ground water Mg 2+ , Ca 2+ , and Sr 2+ concentra- tions, and lowers the Sr-isotopic ratios to ~ 0.709 for the Cheboygan watershed. Similarly, along hydrologic flow paths, soil water Sr/Ca ratios move from the silicate end-member (defined by amphibole and plagioclase) towards the carbonate end-member. The Sr-isotopic compositions and Sr/Ca ra- tios of soil waters thus reveal the types, directions and extent of chemical weathering processes in Michigan soils, augmenting information from previous soil water chemistry and soil mineralogy studies. This work also highlights the two-fold impacts of glacial/interglacial cycles on the riverine and oceanic Sr iso- topes: due to the great extent of continental glaciation, Paleozoic carbonate minerals from the Michigan Basin were redistributed widely within the interior of the North American continent, leading to elevated Sr fluxes with lower Sr-isotopic ratios in natural waters after glacial retreat. The glacial ice also ground up the ancient Precambrian Canadian Shield, accelerating mineral weathering rates and releasing highly radiogenic Sr from K-feldspar. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The residence time of strontium (Sr) in the oceans is on the order of a few million years, much longer than the ocean mixing time of only about 1600 years (e.g., Goldberg, 1963; Holland, 1978; Frank, 2002). Therefore, the Sr seawater budget reflects the balance of Sr addition (riverine radiogenic Sr fluxes and mantle-derived hydrother- mal non-radiogenic Sr fluxes) and removal processes (precipitation of carbonate minerals) on geological timescales (Veizer and Compston, 1974; Palmer and Edmond, 1989). Seawater 87 Sr/ 86 Sr ratios have fluc- tuated over the Phanerozoic eon, reflecting the change in Sr fluxes and/or isotopic compositions of the riverine and hydrothermal inputs (Burke et al., 1982). The possible mechanisms to generate such fluctu- ations have to be globally significant, such as breakup of super- continents, major mountain-building events, and glacial/interglacial cy- cles (e.g., Zachos et al., 1999; Frank, 2002; Sharp et al., 2002; Vance et al., 2009). Seawater 87 Sr/ 86 Sr has been used to understand continental weathering fluxes at global scales. For example, an increase in seawa- ter 87 Sr/ 86 Sr ratio from less than 0.708 to a current value of 0.709 dur- ing Cenozoic has been attributed to uplift of the Himalayas and subsequently elevated continental weathering (Burke et al., 1982; Chemical Geology 300-301 (2012) 97–108 ⁎ Corresponding author at: Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, United States. Tel.: + 1 915 747 5559; fax: + 1 915 747 5073. E-mail address: ljin2@psu.edu (L. Jin). 1 Now at: College of Engineering and Physical Sciences, University of New Hampshire, Durham, NH 03824-3591, United States. 0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2012.01.017 Contents lists available at SciVerse ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo