Controls on Mg/Ca and Sr/Ca in Planktonic Foraminifera: Results From Culturing Experiments

Culturing experiments were carried out on Globigerinoides ruber (white) and Globigerinella siphonifera, collected from the Gulf of Eilat and grown in seawater (i) at constant salinity but varying temperatures of 18 to 31 degC, (ii) at constant temperature but varying salinities of 32 to 44 psu, (iii) at constant salinity, temperature and dissolved inorganic carbon but varying pH of 7.9 to 8.4. Temperature response of Mg/Ca in cultured G. ruber (with an exponential constant of 11 +/- 2% per degC) agrees well with previous calibrations from core tops and net catches. Mg/Ca in G. ruber shows secondary dependence on salinity (6 +/- 2% per salinity unit). This compares well with the salinity response of Mg/Ca in cultured G. sacculifer (approx. 7% per salinity unit; Nuernberg et al., 1996), implying a similar salinity effect for shallow dwelling species. Mg/Ca in G. ruber shows no dependence on pH at ambient seawater range (8.1 to 8.3; NBS scale), but demonstrates considerable variation below and above the normal range of seawater pH. We observe no significant control of seawater temperature or pH on Sr/Ca in G. ruber. However, there is a significant correlation between Sr/Ca ratios and salinity (approx. 0.016 mmol/mol per salinity unit). Multiple regression analysis on Mg/Ca data of cultured G. ruber gives a calibration equation as a function of temperature and salinity [Mg/Ca = 0.29(+/-0.13)Sal + 0.46(+/- 0.13)Temp - 17.0(+/-5.9), R2=0.96, p=0.05]. Salinity can be independently derived from the regression for Sr/Ca [Sr/Ca = 0.016(+/-0.015)Sal + 0.72(+/-0.55), R2=0.91, p<0.05]. Nuernberg, D. et al., 1996. Assessing the reliability of magnesium in foraminiferal calcite as a proxy for water mass temperatures. Geochimica et Cosmochimica Acta, 60: 803-814.