Experimental Determination of Temperature and Precipitation Rate Effects on Element Partitioning Between Aragonite and Seawater

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The elemental composition of the aragonitic skeletons of marine invertebrates contribute an increasingly significant proportion of the climate proxy data used to understand and model Earth's climate system. However, the factors that control how elements are partitioned between aragonite and seawater are not well understood. To date only three experimental studies have determined element distribution between aragonite and seawater, all focused exclusively on the behavior of Sr2+ and two were carried out at a single temperature. In this project, researchers at the Woods Hole Oceanographic Institution will address this critical gap in knowledge through an experimental determination of aragonite- seawater partition coefficients for a range of divalent cations that show promise as environmental proxies (e.g., Mg2+, Sr2+, Ba2+, Cd2+, and Mn2+) as functions of temperature and precipitation rate. The experimental data will be used to calibrate a lattice strain partitioning model that will provide a quantitative basis for interpreting the compositions of aragonitic skeletons. In addition, a comparison of experimental results with data obtained from selected, naturally occurring aragonites will permit identification of combinations of the proxies and aragonite accretions that most faithfully record environmental variables. The proposed work builds on results from an experimental pilot study to determine the partitioning of Mg2+, Ca2+, Sr2+, and Ba2+ between abiogenic aragonite and seawater at 15 to 45 C. This approach differs from previous studies in that the SIMS ion microprobe will be used to analyze the concentrations of a full range of divalent cations in individual aragonite grains, rather than focusing exclusively on Sr partitioning using the bulk precipitate. The high spatial resolution afforded by SIMS analyses allows recognition of the presence of compositional gradients across individual grains, and the presence of aragonite that may have been epitaxially converted from high-Mg-calciteThis research promises a number of broader impacts. There is reason to believe that this research will provide the paleoclimate, geochemical, biological and environmental conservation communities with a quantitative basis for interpreting the compositional record preserved in living and ancient aragonitic accretions, which is presently unavailable. It will bridge traditional disciplinary boundaries and bring together scientists with very different but complementary expertise. Moreover, this study will contribute significantly toward the PhD thesis of NSF graduate student fellow. This project will also provide for the education and training of graduate and summer student fellows. The results from this study will be made publicly available in a timely manner on the investigators' website.