Assessing the influence of seawater sulfate, pH, and Mg/Ca on the sulfate concentration of foraminiferal calcite.

Abstract:

The sulfur cycle is linked to the carbon and oxygen cycles by the anoxygenic respiration of organic carbon during bacterial sulfate reduction and the oxidative weathering of reduced sulfur on land. Understanding carbon and oxygen cycles over geological time requires an understanding of the evolution of the sulfur cycle, which can be accomplished by reconstructing the concentration and isotopic composition of sulfate in seawater. Sulfate concentration records are problematic because they are predominantly based on the composition of fluid inclusions found in evaporites, which cannot provide a continuous record through time [1]. Carbonate Associated Sulfate (CAS) and barite are routinely used to reconstruct the isotopic composition of seawater sulfate. The sulfate content of the foraminifer Orbulina universa grown in culture has previously been shown to correlate with the concentration of sulfate in seawater [2]. CAS in foraminifera could potentially be used to build a continuous record of seawater sulfate concentrations that could be directly correlated to the sulfur isotopic record.

To understand the influence of seawater chemistry variation on the CAS content of foraminifers during the Cenozoic, we have conducted experiments to quantify the influence of seawater sulfate concentration, pH and Mg/Ca ratios on the CAS in Orbulina universa grown in the laboratory. Juvenile specimens were collected from the San Pedro Basin off Santa Catalina Island Southern California and grown through gametogenesis in artificial seawater at constant temperature (22°C) in a controlled 12h:12h light:dark cycle. We changed the sulfate concentration or the Mg/Ca ratio of seawater by mixing in different proportions ambient seawater with either a sulfate free artificial seawater or a seawater with Mg/Ca ~0.5. The pH was changed by adding HCl or NaOH to ambient seawater, covering a range from 7.85 to 8.6. Additionally, we compare these experimental results to data from shells collected in plankton tows at the same location and core-tops from close locations to document how early diagenesis affects the CAS content of the shells.

[1] Horita et al., 2002, GCA 66-21, 3733-3756

[2] Paris et al., 2014, Geochem. Geophys. Geosyst. 15, 1452–1461