FORAM FARMING: CALIBRATING BENTHIC FORAM MG/CA FOR SECULAR VARIATION IN SEAWATER MG/CA THROUGH CULTURE EXPERIMENTS

Foraminiferal Mg/Ca has been commonly used as a proxy for Quaternary seawater temperature. However, uncertainty about 1) the secular variation of seawater Mg/Ca and 2) how foraminiferal calcite responds to changing seawater Mg/Ca has precluded its widespread use across deeper time. This proxy has a second allure in that Mg/Ca-based paleotemperatures can be matched with contemporaneous foraminiferal δ¹⁸O to estimate paleo-seawater δ¹⁸O, which is a proxy for continental ice volume.

Four studies over the last decade has aimed to reduce this uncertainty by culturing foraminifera in waters of varying Mg/Ca and temperature. This study reports on success in establishing a long-term colony of the HMC benthic foraminifer Peneroplis planatus from Abu Dhabi and Qatar in Iowa, USA. Specimens are kept in independent colony reservoirs that are attached to a 650 L recirculating system utilizing artificial seawater. The system has been operational for 19 months.

Peneroplids from this system have been successfully grown under experimental conditions in water Mg/Ca from 1 to 8 at 25°C for 42 days (3-5 chambers of new growth) in sealed glass jars. Attempts to grow P. planatus below 25°C and in Mg/Ca less than 1 were not successful. Peneroplid calcite was analyzed via electron microprobe. Low-resolution, qualitative maps allowed quantitative spots to avoid areas of anomalously high or low Mg/Ca. Quantative analyses were performed with a 5 µm spots on experimentally grown calcite. Mg/Ca in new chambers gradually changed to reflect experimental conditions. Mg/Ca of the third chamber of new growth was taken to represent equilibrium with seawater Mg/Ca. Foram Mg/Ca vs. seawater Mg/Ca was best fit by the function: Foram Mg/Ca = 0.05 (Seawater Mg/Ca) ^ 0.56. This model is very similar to the only other model reported for a HMC benthic foraminifer, Operculina ammonoides.

This calibration can be used to model how the depositonal chemistry of HMC foraminifera changes with response to changing seawater Mg/Ca. For example, a peneroplid growing in the modern ocean (Mg/Ca = 5.2 mol/mol) precipitates a test with Mg/Ca = 132 mmol/mol. Growing in late Cretaceous seawater (Mg/Ca = 1 mol/mol), it would have a test Mg/Ca of 50 mmol/mol. In comparison, a LMC foram would only vary from 3 to 1 mol/mol from the modern ocean’s Mg/Ca to that of the Cretaceous.