Calcium isotopes in planktonic foraminifera - core tops and sediment trap samples

For paleoceanographic studies, it is important to understand the processes that influence the calcium (Ca) isotopic composition of foraminiferal calcite tests preserved in the sediment record. Seven species of planktonic foraminifera from coretop sediments collectively exhibited a Ca temperature dependent fractionation of 0.013 per mil per °C. This is in agreement with previously published estimates for most species of planktonic foraminifera as well as biogenic and inorganic calcite and aragonite. Four species of planktonic foraminifera collected from a sediment trap showed a considerable amount of scatter and no consistent temperature dependent fractionation. Analyzed size fractions of coretop samples show no significant relationship with d44/40Ca. However, preliminary results suggest that the symbiotic and spinose foraminifera G. sacculifer might exhibit a relationship between test size and d44/40Ca. A one-box model in which Ca isotopes are allowed to fractionate by Rayleigh distillation from a biomineralization reservoir (internal pool) was used to constrain the isotopic composition of the original biomineralization Ca reservoir, assuming around 85% of the Ca reservoir is precipitated and the fractionation factor during precipitation is 0.9985 + 0.00002(T ºC). To explain the foraminiferal Ca isotope data, this model indicates that the Ca isotopic composition of the biomineralization reservoir is offset from seawater (approximately -0.8per mil).

在古海洋学研究中，明确影响保存在沉积记录中的有孔虫方解石壳体（foraminiferal calcite tests）的钙（Ca）同位素组成的过程具有重要意义。本研究针对从表层沉积物中获取的7种浮游有孔虫开展分析，结果显示其钙同位素分馏与温度呈相关性：每1℃的温度变化对应0.013‰的分馏值改变。该结果与此前已发表的多数浮游有孔虫、生物成因及无机成因方解石和文石的相关估算结果一致。从沉积物捕获器中采集的4种浮游有孔虫则呈现出显著的离散性，未表现出一致的温度依赖分馏效应。对表层沉积物样品的不同粒级组分的分析表明，其钙同位素组成（δ44/40Ca）与粒级无显著相关性。不过初步结果显示，具共生藻且具棘的有孔虫G. sacculifer的壳体尺寸与δ44/40Ca可能存在相关性。本研究采用单箱模型，设定约85%的钙储库发生沉淀，且沉淀过程中的分馏系数为0.9985 + 0.00002(T/℃)，允许钙同位素通过瑞利分馏从生物矿化储库（内部池）中发生分馏，以此约束原始生物矿化钙储库的同位素组成。为解释有孔虫钙同位素数据，该模型显示生物矿化储库的钙同位素组成与海水存在偏移，偏移量约为-0.8‰。