Manganese carbonate overgrowths on foraminifera tests

Below the zone where manganese is remobilized as Mn²⁺(aq), reductively cleaned foraminifera in deep sea sediments have much higher $\text{Mn}\text{Ca}$ than those in core tops and sediment traps. $\text{Mn}\text{Ca}$ ranges from less than 20 × 10^?6 in and above the MnO₂ maximum to as high as 700 × 10^?6 in reducing Panama Basin sediments. The most plausible explanation for this enrichment is that the tests are coated with Mn carbonate overgrowths. These coatings can account for a significant proportion of the Mn in reduced deep-sea sediments. Uptake of manganous ion by carbonate may explain the absence of Mn nodules in areas of high carbonate accumulation. Extreme degrees of overgrowth can alter foram trace element values, but this artifact can be avoided by avoiding foraminifera with high Mn/Ca.