Table_1_Planktic foraminifera iodine/calcium ratios from plankton tows.xlsx

Planktic foraminifera test iodine to calcium ratios represent an emerging proxy method to assess subsurface seawater oxygenation states. Several core-top studies show lower planktic foraminifera I/Ca in locations with oxygen depleted subsurface waters compared to well oxygenated environments. The reasoning behind this trend is that only the oxidized species of iodine, iodate, is incorporated in foraminiferal calcite. The I/Ca of foraminiferal calcite is thought to reflect iodate contents in seawater. To test this hypothesis, we compare planktic foraminifera I/Ca ratios, obtained from plankton tows, with published and new seawater iodate concentrations from 1) the Eastern North Pacific with extensive oxygen depletion, 2) the Benguela Current System with moderately depleted oxygen concentrations, and 3) the well oxygenated North and South Atlantic. We find the lowest I/Ca ratios (0.07 µmol/mol) in planktic foraminifera retrieved from the Eastern North Pacific, and higher values for samples (up to 0.72 µmol/mol) obtained from the Benguela Current System and North and South Atlantic. The I/Ca ratios of plankton tow foraminifera from environments with well oxygenated subsurface waters, however, are an order of magnitude lower compared to core-tops from similarly well-oxygenated regions. This would suggest that planktic foraminifera gain iodine post-mortem, either when sinking through the water column, or during burial.

浮游有孔虫壳体（planktic foraminifera test）碘钙比（iodine to calcium ratios）是一种用于评估海水下层含氧状态的新兴代用指标。多项岩芯表层（core-top）研究表明，相较于氧气充足的环境，下层海水缺氧区域的浮游有孔虫碘钙比更低。该趋势的核心原理为：仅有碘的氧化态形式——碘酸盐——会被纳入有孔虫方解石（calcite）壳体中。学界普遍认为，有孔虫方解石的碘钙比可反映海水中碘酸盐的含量。为验证这一假说，我们将浮游生物拖网（plankton tows）获取的浮游有孔虫碘钙比，与已发表及新测得的三处海域的海水碘酸盐浓度展开对比：1）存在大范围下层海水缺氧的东太平洋海域；2）氧浓度适度降低的本格拉洋流系统（Benguela Current System）海域；3）氧含量充足的北大西洋与南大西洋海域。研究发现，采自东太平洋海域的浮游有孔虫壳体碘钙比最低，为0.07 μmol/mol；采自本格拉洋流系统及南北大西洋的样品碘钙比则更高，最高可达0.72 μmol/mol。然而，下层海水含氧充足环境中浮游生物拖网获取的有孔虫壳体碘钙比，相较于同含氧水平区域的岩芯表层样品，低了一个数量级。这一结果提示，浮游有孔虫可能在死亡后，即在沉降穿过水层（water column）过程中或埋藏（burial）阶段，额外获取了碘元素。