Geochemical measurements made on the carbonate skeletons of marine invertebrates comprise a significant portion of the paleo sea surface temperature (SST) data used to understand and model Earth's climate system. Indeed, the capability of general circulation models to predict future global warming is tested against proxy temperature estimates for the Last Ice Age, many of which are based on the concentrations of trace elements such as Sr and Mg in fossil marine skeletons. Despite the widespread use of skeletal chemistry as an indicator of paleoenvironments, the factors that control how trace elements are distributed, or partitioned, between seawater and a growing skeleton are not well understood. As a result, few of the proxy SST estimates for crucial periods of Earth's climate history are in agreement and many remain controversial.
With this Small Grant for Exploratory Research, researchers at the Woods Hole Oceanographic Institution will address a critical gap in knowledge about how paleotemperature proxies work. They intend to experimentally determine the partitioning of trace elements between carbonate minerals and seawater using experimental and analytical approaches that have been successfully used to study partitioning in high-temperature, magmatic systems. The objective is to provide the framework within which the trace element content of low temperature carbonate precipitates can be properly interpreted in terms of the environmental conditions under which they formed. They expect to achieve this through an experimental determination of carbonate/seawater partition coefficients for the important cations (Na, Mg, Sr, B, Ba) in calcite and aragonite crystals precipitated under laboratory conditions. Experiments will be carried out over a temperature range of ~100 .'C in order to accurately determine the temperature dependence of partitioning. Crystal growth rates will be minimized in order to obtain thermodynamically, as opposed to kinetically, controlled partition coefficients. Individual grains of calcite and aragonite grown during an experiment will be identified using UV fluorescence microscopy and electron back scatter diffraction techniques The elemental composition of individual grains will then be determined using Secondary Ion Mass Spectrometric techniques (ion microprobe). The high spatial resolution provided by SIMS ion microprobe allows for the selective analysis of aragonite and calcite crystals several microns in diameter. This analytical approach represents a significant advance over previous studies in which partition coefficients were determined through bulk analyses of aggregates of coexisting calcite and aragonite crystals. Once the research team has demonstrated that the approach provides accurate and unambiguous results for these cations, the same techniques will be applied to determine baseline fractionation factors for Ca, B, and Mg isotopes.
