Implications of High Ba/Ca and Mn/Ca Ratios in the Non-Spinose Planktonic Foraminifer Neogloboquadrina dutertrei for Water-Column Microenvironments and Cycling of Barite

The Ba/Ca ratio in spinose planktonic foraminiferal calcite is a promising tracer of river inflow into coastal areas, because it depends only on the Ba/Ca of seawater and is independent of changes in temperature, salinity, alkalinity, pH, calcification rate, or the presence/absence of algal symbionts. A Ba/Ca of ~1 μmol/mol is typical for spinose planktonic foraminifera from the open ocean. However, intrashell Ba/Ca ratios from the non-spinose species N. dutertrei, and other open ocean non-spinose species that inhabit depths in or below the thermocline, are often in the range of 10-100 μmol/mol, or higher. These elevated Ba/Ca ratios are often accompanied by high Mn/Ca ratios. Coupled with the observation that foraminiferal Ba/Ca depends only on seawater Ba/Ca, high Ba/Ca ratios suggest that N. dutertrei precipitates its shell within a microenvironment that is highly enriched in Ba. Using Ba/Ca ratios from empty gametogenic N. dutertrei shells collected from plankton tows off Catalina Island in 2014, we show that the seawater Ba concentration within the calcifying microenvironment is often ~10x higher than that in the surface ocean, and that the seawater within this microenvironment is highly oversaturated with respect to barite (Ω ~ 2-50). Such high Ω values imply that sulfate reduction must occur within the microenvironment, removing 50-100% of seawater sulfate. We hypothesize that N. dutertrei attaches to marine snow containing particulate Mn and barite crystals. As the marine snow decays, particulate Mn and dissolved sulfate are reduced, ultimately dissolving barite, and elevating Mn and Ba concentrations in the calcifying fluid near the attached foraminifers. This model suggests that Ba/Ca and Mn/Ca ratios in N. dutertrei may contain useful information about the mechanisms of Ba and barite cycling in the water column. The Ba/Ca ratio may also serve as a proxy for primary productivity, because marine snow is most abundant in productive environments.