Species‐Specific Offsets in Manganese Incorporation in Hyaline Foraminiferal Calcite Across a Gradient of Seawater [Mn]

Abstract Mn/Ca of foraminiferal calcite has been proposed as a tool to reconstruct past oxygen conditions, but the impact of the concentration of Mn ([Mn]) in seawater on partitioning of Mn in foraminiferal calcite remains unclear. Here, we explore Mn incorporation of different species of foraminifera across a gradient of seawater [Mn] by culturing small and large benthic foraminifera in hypoxic conditions using a controlled laboratory set‐up. Our observations confirm previous results that Mn incorporation varies greatly between species and underline the need for species‐specific calibrations and mono‐species application of Mn‐based proxies. We explore whether the observed species‐specific incorporation of Mn could be due to an interaction between Mn incorporation and Mg content, or other processes related to the co‐uptake, ‐transport and ‐precipitation of Mn and Mg to the calcification site and ultimately foraminiferal calcite. Furthermore, results show that for several species ( Ammonia confertitesta , Bulimina marginata , Cassidulina laevigata and Amphistegina lessonii ) partitioning of Mn increases below a species‐specific threshold of seawater [Mn]. For application of the Mn‐proxy, this slightly higher Mn partitioning can lead to a slight overestimation (up to 5‐fold) of reconstructed seawater Mn/Ca at very low concentrations close to 0 (e.g., well oxygenated conditions), when assuming a constant distribution coefficient for this proxy. However, this only occurs at extremely low seawater Mn/Ca, below 1.06 mmol/mol. This trend is less clear for the larger benthic foraminifera ( Amphistegina lessonii and Operculina ammonoides ), where it is potentially masked by the larger range of Mn/Ca data and additional impacts on Mn incorporation, such as ontogeny.