GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 378-18
Presentation Time: 9:00 AM-6:30 PM


RICHEY, Julie N.1, REYNOLDS, Caitlin E.1 and FEHRENBACHER, Jennifer S.2, (1)US Geological Survey, St. Petersburg Coastal and Marine Science Center, 600 4th Street S, Saint Petersburg, FL 33701, (2)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR 97331,

The Ba/Ca of planktic foraminifera in marine sediment cores has been used primarily to track changes in riverine input over time, and thus may be a potentially powerful proxy for reconstructing past changes in the terrestrial hydrologic cycle. Using Ba/Ca as a proxy for riverine freshwater input requires the assumption that Ba/Ca in foraminiferal calcite reflects the Ba/Ca of seawater, and that the partition coefficient for barium between seawater and foraminiferal calcite is independent of the influence of temperature, salinity, pH, alkalinity and light availability. Although it has been shown that this partition coefficient is nearly identical for common species of spinose planktic foraminifera (e.g., Globigerinoides ruber, Orbulina universa, Globigerinoides sacculifer), some non-spinose species have been demonstrated to have Ba/Ca ratios that are much higher than that of co-occurring spinose specimens.

We investigate environmental controls on Ba/Ca in the tests of Globorotalia truncatulinoides, a planktic species of foraminifera with a unique life history in the Gulf of Mexico (GoM). G. truncatulinoides experiences 92% of its annual flux to the sediment trap during winter (JFM) in the GoM. The Mg/Ca and ∂18O of the ontogenetic calcite suggests that primary calcification occurs within the surface mixed layer (0-150 meters), and a thick secondary crust is added at depths below the thermocline. We use LA-ICP-MS to analyze the Ba/Ca of both encrusted and non-encrusted G. truncatulinoides from a sediment trap time series in the GoM and find that the Ba/Ca in ontogenetic calcite of non-encrusted specimens varies between 10 and 200 mmol/mol, while the Ba/Ca of the secondary crust varies between 0 and 3 mmol/mol. The Ba/Ca of the non-encrusted G. truncatulinoides specimens is two to three orders of magnitude higher than that of co-occurring spinose planktic foraminifera (O. universa and G. ruber) in the GoM sediment trap, while the secondary crust has Ba/Ca similar to the spinose species. This suggests that the Ba/Ca of ontogenetic calcite in G. truncatulinoides is governed by fundamentally different processes, and does not necessarily reflect the Ba/Ca of seawater.