Experimental refinement of the boron isotope proxy in planktic foraminifera - temperature, asymbiotic sensitivity, and seawater elemental composition

Culture experiments will be used to study the shell composition of planktonic foraminifera. Planktonic foraminifera are single-celled organisms that live in the surface ocean. Scientists use the chemistry of fossil foraminifera shells to record seawater conditions in the past. Those data reveal the ocean's response to natural climate change. That knowledge improves scientists' ability to predict future climate change. The boron isotopic composition of planktonic foraminifera records the acidity of the surface ocean. However, boron isotopes may be influenced by other factors including temperature and seawater calcium concentration. The proposed experiments will quantify those impacts. That will improve scientists' ability to know how ocean acidity has changed in the past. The project broader impacts include support for a graduate student and two undergraduate helpers. The PI will attempt to recruit undergraduates from groups that are underrepresented in the earth sciences.

The boron isotope proxy in planktic foraminifera shells is one of the most successful tools for reconstructing surface seawater acidity, and by inference atmospheric CO2. A strong empirical framework has already been established for this proxy, however, some unknowns still remain that affect the certainty with which paleo-pH and paleo-CO2 can be reconstructed from this proxy. Specifically, the effect of temperature on the aqueous boron isotope fractionation between dissolved borate ion and boric acid has been suggested by thermodynamic principles and modeling and several lines of evidence suggest that this effect is manifested in the boron isotopic composition of planktic foraminifera, but solid experimental proof is missing. If such an effect exists, it may explain differences in boron proxy sensitivity between surface- and deep-dwelling foraminifera species. An alternative hypothesis to explain species differences is based on the observation that laboratory culture experiments are performed over a much wider range of pH and calcite saturation states than present in the natural ocean, which could affect foraminiferal growth rates. The following experiments are proposed to resolve these uncertainties: (1) establish the first laboratory calibration of a symbiont-barren foraminifera species across a wide range of experimental pH, (2) grow planktic foraminifera over a wide range of temperatures but constant pH, (3) grow planktic foraminifera across a wide range of calcium concentrations and saturation states at constant pH. (4) Establish a pH-calibration under simulated Paleocene seawater conditions, when seawater calcium concentrations were higher than in the modern ocean, but magnesium and boron concentrations significantly lower. This study will provide fundamental constraints on the application of the boron isotope proxy and improve reconstructions of surface ocean pH and atmospheric CO2, from the Pleistocene to the early Cenozoic.

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.