SGER: Experimental Determination of Partition Coefficients in Carbonates

对海洋无脊椎动物碳酸盐骨骼进行的地球化学测量是用于了解和模拟地球气候系统的古海洋表面温度(SST)数据的重要组成部分。事实上，大气环流模型预测未来全球变暖的能力与上一次冰河时代的代理温度估计值进行了对比，其中许多估计值是基于海洋化石骨架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示器，但控制微量元素在海水和不断生长的骨骼之间分布或分配的因素尚不清楚。因此，在地球气候历史的关键时期，SST的代理估计几乎没有达成一致，许多仍然存在争议。有了这笔探索性研究的小额拨款，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的一个关键缺口。他们打算通过实验和分析方法来实验确定微量元素在碳酸盐矿物和海水之间的分配，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，以便根据形成低温碳酸盐沉淀物的环境条件，适当地解释它们的微量元素含量。他们希望通过对实验室条件下沉淀的方解石和文石晶体中重要阳离子(钠、镁、锶、硼、钡)的碳酸盐/海水分配系数的实验测定来实现这一点。实验将在~100℃的温度范围内进行，以便准确地确定分配的温度依赖关系。晶体生长速度将被最小化，以获得热力学控制的分配系数，而不是动力学控制的分配系数。实验期间生长的方解石和文石的单个颗粒将使用UV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仪技术(离子微探针)确定单个颗粒的元素组成。SIMS离子微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有了很大的进步，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的聚集体进行整体分析来确定的。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钙、硼和镁同位素的基线分馏系数。

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.