Mn/Ca and Fe/Ca data from multiple species of planktonic foraminifers from the equatorial Pacific and subtropical South Atlantic: Indicators of surface ocean productivity, diagenesis of the shells, or both?

Abstract:

Mn/Ca ratios in planktonic foraminifers have been purported to be indicators of terrestrial input, marine oxidation-reduction reactions and sea surface productivity in the eastern tropical Pacific. We investigated this hypothesis using Mn/Ca and Fe/Ca data from five species of planktonic foraminifers in 32 core tops from the eastern equatorial Pacific (EEP), five species from 12 core tops in the western equatorial Pacific, and two species from 15 core tops on the Rio Grande Rise. We present Mn/Ca and Fe/Ca data from Neogloboquadrina dutertrei, Globorotalia menardii, Globigerina bulloides, Pulleniatina obliquiloculata and Globorotalia tumida from the eastern and western equatorial Pacific and from Globorotalia truncatulinoides and Globorotalia inflata on the Rio Grande Rise. We find that the environmental parameter depicted by Mn/Ca and Fe/Ca ratios depends on the species of foraminifer from which the data was derived. Fe/Ca from N. dutertrei in the EEP have statistically significant and quantifiable relationships with apparent oxygen utilization, dissolved phosphate concentration and dissolved oxygen concentration in the habitat depths of this species. The Mn/Ca ratio from P. obliquiloculata and G. tumida are highest in regions of greatest productivity in the EEP. On the Rio Grande Rise, we explored the effect of dissolution in the sediments on Mn/Ca and Fe/Ca ratios from planktonic foraminifers. We used the G. menardii fragmentation index as our sedimentary dissolution indicator, and found that dissolution does not have a strong effect on the Mn/Ca and Fe/Ca ratios measured from foraminifers tests.