Seawater carbonate chemistry and biological processes of foraminifera, Globigerinoides ruber and Globigerinella siphonifera during experiments, 2011

Specimens of two species of planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera, were grown under controlled laboratory conditions at a range of temperatures (18-31 °C), salinities (32-44 psu) and pH levels (7.9-8.4). The shells were examined for their calcium isotope compositions (d44/40Ca) and strontium to calcium ratios (Sr/Ca) using Thermal Ionization Mass Spectrometry and Inductively Coupled Plasma Mass Spectrometry. Although the total variation in d44/40Ca (~0.3 per mill) in the studied species is on the same order as the external reproducibility, the data set reveals some apparent trends that are controlled by more than one environmental parameter. There is a well-defined inverse linear relationship between d44/40Ca and Sr/Ca in all experiments, suggesting similar controls on these proxies in foraminiferal calcite independent of species. Analogous to recent results from inorganically precipitated calcite, we suggest that Ca isotope fractionation and Sr partitioning in planktic foraminifera are mainly controlled by precipitation kinetics. This postulation provides us with a unique tool to calculate precipitation rates and draws support from the observation that Sr/Ca ratios are positively correlated with average growth rates. At 25 °C water temperature, precipitation rates in G. siphonifera and G. ruber are calculated to be on the order of 2000 and 3000 µmol/m**2/h, respectively. The lower d44/40Ca observed at 29 °C in both species is consistent with increased precipitation rates at high water temperatures. Salinity response of d44/40Ca (and Sr/Ca) in G. siphonifera implies that this species has the highest precipitation rates at the salinity of its natural habitat, whereas increasing salinities appear to trigger higher precipitation rates in G. ruber. Isotope effects that cannot be explained by precipitation rate in planktic foraminifera can be explained by a biological control, related to a vacuolar pathway for supply of ions during biomineralization and a pH regulation mechanism in these vacuoles. In case of an additional pathway via cross-membrane transport, supplying light Ca for calcification, the d44/40Ca of the reservoir is constrained as -0.2 per mill relative to seawater. Using a Rayleigh distillation model, we calculate that calcification occurs in a semi-open system, where less than half of the Ca supplied by vacuolization is utilized for calcite precipitation. Our findings are relevant for interpreting paleo-proxy data on d44/40Ca and Sr/Ca in foraminifera as well as understanding their biomineralization processes.

本数据集包含两种浮游有孔虫（planktic foraminifera）物种：红拟抱球虫（Globigerinoides ruber）与管抱球虫（Globigerinella siphonifera）的实验室培养样本。实验于可控实验室条件下开展，设置了温度（18~31 ℃）、盐度（32~44 psu）及pH（7.9~8.4）的梯度变量。研究人员采用热电离质谱（Thermal Ionization Mass Spectrometry）与电感耦合等离子体质谱（Inductively Coupled Plasma Mass Spectrometry），对壳体的钙同位素组成（d⁴⁴/⁴⁰Ca）与锶钙比（Sr/Ca）进行了检测。尽管所研究物种的d⁴⁴/⁴⁰Ca总变化量（约0.3‰）与外部重现性处于同一数量级，但本数据集仍揭示出若干受多环境参数共同调控的显著趋势。所有实验中均观测到d⁴⁴/⁴⁰Ca与Sr/Ca之间存在明确的负线性相关关系，这表明浮游有孔虫方解石中的这两类代用指标受相似机制调控，且与物种种类无关。与无机沉淀方解石的近期研究结果类似，本研究认为浮游有孔虫中的钙同位素分馏与锶分配主要受沉淀动力学控制。这一假说为我们提供了一种计算沉淀速率的独特工具，且得到了Sr/Ca比值与平均生长速率呈正相关这一观测结果的支撑。在水温25 ℃条件下，管抱球虫与红拟抱球虫的沉淀速率分别约为2000 μmol/m²/h与3000 μmol/m²/h。两个物种在29 ℃时观测到的更低d⁴⁴/⁴⁰Ca值，与高温下沉淀速率升高的现象一致。管抱球虫的d⁴⁴/⁴⁰Ca（及Sr/Ca）对盐度的响应规律表明，该物种在其自然栖息地的盐度条件下沉淀速率最高；而红拟抱球虫的沉淀速率则随盐度升高而增大。浮游有孔虫中无法用沉淀速率解释的同位素效应，可通过生物调控机制阐释：该机制与生物矿化过程中离子供应的液泡途径，以及这些液泡中的pH调控机制相关。若存在一条通过跨膜运输为钙化作用供应轻钙的额外途径，则储库的d⁴⁴/⁴⁰Ca值相较于海水被限定为-0.2‰。借助瑞利蒸馏模型（Rayleigh distillation model），本研究计算得出钙化作用发生于半开放系统中：通过液泡作用供应的钙中，仅有不到一半被用于方解石沉淀。本研究结果既可用于解释有孔虫中d⁴⁴/⁴⁰Ca与Sr/Ca的古代用指标数据，也有助于理解其生物矿化过程。