SGER: Experimental Determination of Partition Coefficients in Carbonates

海洋无脊椎动物碳酸盐骨架的地球化学测量是理解和模拟地球气候系统的重要组成部分。事实上，大气环流模型预测未来全球变暖的能力，是根据末次冰河期的代用温度估算来测试的，其中许多估算是基于海洋骨骼化石中锶和镁等微量元素的浓度。尽管骨骼化学被广泛用作古环境的指标，但控制微量元素如何在海水和生长中的骨骼之间分布或分配的因素还没有得到很好的理解。因此，对地球气候历史关键时期的代理海温估计很少是一致的，许多仍然存在争议。有了这笔探索性研究的小额赠款，伍兹霍尔海洋研究所的研究人员将解决关于古温度代理如何工作的知识方面的一个关键空白。他们打算用实验和分析方法来实验确定碳酸盐矿物和海水之间微量元素的分配，这些方法已经成功地用于研究高温岩浆系统中的分配。目的是提供一个框架，在这个框架内，低温碳酸盐沉淀物的微量元素含量可以根据它们形成的环境条件进行适当的解释。他们希望通过实验确定在实验室条件下沉淀的方解石和文石晶体中重要阳离子（Na, Mg, Sr， B, Ba）的碳酸盐/海水分配系数来实现这一目标。实验将在~100℃的温度范围内进行。C才能准确地确定配分的温度依赖性。晶体生长速率将被最小化，以获得热力学，而不是动力学，控制分配系数。在实验中生长的方解石和文石的单个颗粒将使用紫外荧光显微镜和电子背散射衍射技术进行鉴定，然后使用二次离子质谱技术（离子微探针）确定单个颗粒的元素组成。SIMS离子探针提供的高空间分辨率允许对直径几微米的文石和方解石晶体进行选择性分析。这种分析方法比以前的研究取得了重大进展，在以前的研究中，分配系数是通过对共存的方解石和文石晶体聚集物的总体分析来确定的。一旦研究小组证明该方法为这些阳离子提供了准确和明确的结果，同样的技术将被应用于确定Ca， B和Mg同位素的基线分馏因子。

ABSTRACTOCE 0347328Geochemical 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.