Ba/Ca ratios in non-spinose
planktic foraminifera
Ba/Ca ratios in non-spinose
planktic foraminifera
Forty years ago, researchers exploring foraminifera geochemistry were perplexed that species that lacked spines had elevated barium (Ba) within their shell calcite. They speculated various mechanisms were responsible, from contamination, to prey preference, to a greater affinity for Ba incorporation into their shell during growth.
In Fehrenbacher et al. (2018), we conducted a barium partition coefficient (DBa) experiment for a non-spinose species, demonstrating that they incorporate Ba solely as a function of the seawater they grow in. Yet specimens that grow in the open ocean have highly elevated and variable Ba that can't be explained by seawater Ba composition alone (Figure 1). We hypothesize the elevated Ba is due to a unique microhabitat. That is, these species live in particulates (marine snow, particulate organic matter or POM) during their lifecycle. Ba is elevated in POM and this can explain the high Ba concentrations and high specimen-to-specimen Ba variability. This hypothesis is supported by microbiome analyses of living specimens that found cercozoa, a bacterium associated with particulates (Bird et al., 2018) in their microbiome, and recent experiments demonstrating their affinity for attaching to particulates and elevated Ba in other non-spinose species. Our findings suggest non-spinose foraminifera occupy a profoundly different ecological niche compared to spinose-species and this fundamentally changes how they can be used in paleoclimate reconstructions. NanoSIMS images of two cultured N. dutertrei. Mg/Ca data for these shells was published in Fehrenbacher et al., 2017. Ocean grown calcite (marked with an 'o') has elevated Ba compared to the cultured calcite (marked with a 'c'). Ratios are as high as 200 micromol/mol in the cultured calcite and far exceed expected Ba/Ca values for foraminifera. The cultured calcite has low Ba/Ca ratios, as expected given the Ba/Ca ratios of the culture seawater (see Fehrenbacher et al., 2018).
This research led to a successfully funded NSF proposal that supported OSU graduate student Theresa Fritz-Endres PhD research and several undergraduate student projects. Theresa establish that Ba/Ca ratios may serve as a proxy for paleoproductivity (Fritz-endres et al., 2022). She also demonstrated that intrashell Ba/Ca ratios can be heavily altered using standard foraminifera cleaning protocols (Fritz-Endres et al., 2020). Theresa's last thesis proposal, investigating MicroCT images of fossil foraminifera as a proxy for bottom water carbonate ion concentration.
Additional grants to expand the Ba partition coefficient experiments have been awarded. We've grown other non-spinose species in elevated Ba and confirm that Ba/Ca is incorporated as a function of seawater Ba/Ca ratios. We've also conducted feeding experiments and have confirmed that non-spinose species readily envelope their shells in POM (results in prep). |
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