Effects of seawater pH and calcification rate on test Mg/Ca and Sr/Ca in cultured individuals of the benthic, calcitic foraminifera Elphidium williamsoni

Abstract

We cultured the benthic, calcitic foraminifera, Elphidium williamsoni, over a range of pH NIST (7.89, 8.15 and 8.30) for 60 days. Secondary ion mass spectrometry was used to determine both the Mg/Ca and Sr/Ca of chambers deposited in the culture system. Both Mg/Ca and Sr/Ca vary significantly within individual test chambers and between specimens cultured under the same conditions. We do not typically observe significant correlations between Mg/Ca and Sr/Ca, either within single tests or between tests cultured at the same pH and we conclude that Rayleigh fractionation, indicated by a positive correlation between Sr and Mg in calcite, is not the predominant control on the chemistry of the tests. Although test morphology, particularly fossette and ponticuli appearance, varies substantially between individuals cultured at the same pH, we perceive no simple relationship between test morphology and geochemistry. Mean test Mg/Ca or Sr/Ca does not vary significantly between treatments. No significant correlation is observed between the amount of calcite deposited in each test and mean test Sr/Ca and Mg/Ca, in the foraminifera cultured at ambient pH (8.15), suggesting that calcification rate is not a significant influence on Sr and Mg test chemistry in this foraminifera species.