Evaluating the Mn/Ca Ratio of Foraminiferal Calcite Determined by Flow-Through ICP-MS as a Proxy for Terrigenous Input, Upwelling, and Carbon Rain Rate

The Mn/Ca ratio of the biogenic calcite preserved in deep-sea sediments has potential as a tracer of terrestrial input, upwelling, and carbon rain rate over geologic time scales. The basis for this potential lies in features of the Mn cycle in the oceans, which are well known. Manganese is a biogeochemically reactive element, but has a lower affinity for dissolved oxygen and organic matter than iron, making it more stable over short time scales, and less affected by speciation. Depth profiles of Mn in oligotrophic ocean waters show a sharp contrast between low concentrations in deep water (0.20 nM) and relatively high concentrations in the mixed layer (2-5 nM). Mn oxides are stable in high oxygen environments but reduced in the suboxic conditions found in the oxygen minimum zone (OMZ). This behavior makes the intermediate water to surface water concentration ratio of Mn sensitive to the intensity of the OMZ, an artifact of the carbon rain rate, and dust/river input. In sediments, suboxic dissolution is balanced by the formation of carbonate making Mn highly reactive during early diagenesis. These features of the Mn cycle in seawater make the Mn/Ca ratio of foraminifera an attractive paleoproxy, but only if the primary signature can be recovered after diagenetic alteration. Recently our laboratory developed a flow-through extraction system that gives us fresh insight into this problem by making it possible to separate mineral phases associated with the foraminiferal fraction by differences in their solubilities. This paper examines foraminiferal Mn/Ca ratios in core tops and down core records from the eastern equatorial Pacific determined with this new technique. We access the potential of flow-through Mn/Ca by comparing its record to those of Mg/Ca and stable isotopes.