Collaborative Research: Effects of sulfate on boron incorporation into calcite: Relevance and implications for foraminiferal boron isotope and B/Ca paleo-proxies.

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Microscopic fossil shells can provide records of ocean chemistry in the past. The boron to calcium ratio (B/Ca) and boron isotopic composition (d11B) of such fossils are widely used to record seawater pH and carbon chemistry. These records provide valuable insights into past changes in atmospheric carbon dioxide and the global carbon cycle. Recent experiments show that the addition of sulfate ions increases the amount of B incorporated into carbonate minerals. The proposed study will explore how and why B incorporation is enhanced by sulfate. It also will examine if sulfate influences the d11B of synthetic calcite. And the study will use culture experiments to assess whether the boron chemistry of biogenic calcite is impacted by seawater sulfate. This will be a crucial test of the reliability of d11B and B/Ca fossil records, since seawater sulfate has varied substantially over the last 60 million years. The project also will provide training for graduate and undergraduate students and educational outreach activities to local communities. This study integrates inorganic calcite precipitation experiments and foraminiferal culturing to investigate the mechanistic link between B incorporation and sulfate (SO42−) and its relevance to biological systems using benthic foraminifers as the model organism. First, we will comprehensively analyze δ11B, B/Ca and other trace elements (S/Ca and Na/Ca) of synthetic calcite precipitated in solutions added with aqueous B and variable amounts of SO42−. Second, we will grow sub-mm scale calcite crystals in growth solutions added with aqueous B and/or SO42−, and compare the intra-crystal distribution/zonation of B and S using a secondary ion mass spectrometer (SIMS). Finally, we will culture a well-studied species of benthic foraminifera under various seawater SO42− concentrations. If foraminiferal δ11B and B/Ca are indeed sensitive to changes in seawater SO42−, we will provide the equations to account for the SO42− effect.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.