Carbon Isotope Fractionation in Magnesian Calcite: Effects of Precipitation Rate and Mg Content

这是第一个实验研究镁的影响 含量和沉淀速率对镁方解石中~（13）C的影响。 镁方解石（Mg-calcite），方解石与粗 超过10摩尔%的MgCO 3是重要的且通常占主导地位的 组成的浅海沉积物，并作为镁和二氧化碳 源，用于后续处理。 令人惊讶的是，13 C 镁方解石和HCO 3之间的分馏从未被 测定 实验程序已经开发用于 方解石和文石的沉淀在恒定的速度， 恒温、化学和同位素溶液 混合物. 先前的85个实验都产生了方解石- HCO 3和文石-HCO 3 13 C富集系数平均为1.0， 2.5%。 方解石-HCO 3富集因子（cl-HCO 3-） 不随温度（10 ° C至40 ° C）而显著变化，或 沉淀速率（103.0 ~ 105.2 μ mol·m ~（2·hr）~（-1））。 文石-HCO 3 富集因子（ar-HCO 3）显示与 温度，表明ar-HCO 3降低0.020%oC-1。 只 在10 o和25 oC的实验中，ar-HCO_3表现出弱的 与降水率呈负相关。 项目 包括大约30个实验，在25 oC的范围内， 溶液的沉淀速率在2和5 umol m2 hr 1之间 Mg/Ca比为5 - 20，产生Mg-方解石， 20摩尔% MgCO 3。 这些Mg-方解石-HCO 3 13 C的测定 分馏关系将允许研究人员更好地 量化海洋沉积物的原始同位素组成 和沃茨，研究全球变化和 碳酸盐成岩作用

This is the first experimental study of the effect of magnesium content and precipitation rate on the 13C of magnesian calcite. Magnesian calcite (Mg-calcite), calcite with roughtly greater than 10 mole percent MgCO3, is an important and often dominant component of shallow marine sediments, and serves as a Mg and CO3 source for subsequent dolomitization. Surprisingly, the 13C fractionation between Mg-calcite and HCO3 has never been determined. Experimental procedures have been developed for the precipitation of calcite and aragonite at constant rates in solutions of constant temperature and chemical and isotopic composition. Eighty-five previous experiments yield calcite- HCO3 and aragonite-HCO3 13C enrichment factors averaging 1.0 and 2.5% respectively. Calcite-HCO3 enrichment factors ( cl-HCO3-) do not vary significantly with temperature (10o to 40oC) or precipitation rate (103.0 to 105.2 umol m2hr1). Aragonite-HCO3 enrichment factors ( ar-HCO3) show a weak correlation with temperature, suggesting that ar-HCO3 decreases 0.020%oC-1. Only in experiments at 10o and 25oC does ar-HCO3 exhibits a weak inverse correlation with precipitation rate. The project includes roughly 30 experiments at 25oC over a range of precipitation rates between 2 and 5 umol m2 hr1 for solutions having Mg/Ca ratios of 5 to 20, producing Mg-calcites with 7 to 20 mole % MgCO3. Determination of these Mg-calcite-HCO3 13C fractionation relations will permit researchers to better quantify the original isotopic composition of marine sediments and waters, vital information for studies of global change and carbonate diagenesis.