ASSESSING THE RELIABILITY OF MAGNESIUM IN FORAMINIFERAL CALCITE AS A PROXY FOR WATER MASS TEMPERATURES

Though many studies on the Mg contents in the calcitic tests of forami nifers exist, the processes controlling its uptake are still a matter of debate. Laboratory cultures offer an excellent opportunity to revea l these mechanisms. The Mg concentrations within single chambers of th e planktic foraminifer Globigerinoides sacculifer (BRADY) maintained u nder controlled laboratory conditions were measured (1) at variable te mperatures (19.5-29.5 degrees C) and constant salinity and (2) at vari able salinity (22-45 parts per thousand) and constant temperature. The experimental results suggest that under natural conditions, temperatu re is the leading mechanism controlling the Mg/Ca ratio. Temperature a nd magnesium are related proportionally. A temperature increase of ca. 10 degrees C gives rise to an increase of the magnesium concentration s of ca. 130%. Drastic (unnatural) salinity changes dominate the effec ts of temperature. A 110% change in the Mg/Ca ratio was observed when salinity was elevated or reduced by more than ca. 10 parts per thousan d. Specimens which underwent gametogenesis reveal significantly higher ME concentrations than specimens that did not release gametes. Partit ion coefficients for Mg in foraminiferal calcite are orders of magnitu de lower than values from inorganically precipitated calcite. When com paring observed Mg/Ca ratios of foraminiferal tests with predicted Mg/ Ca ratios calculated according to empirical equations, it becomes evid ent that foraminiferal tests are undersaturated with respect to Mg for the water temperature they have experienced. Apparently, foraminifers are capable of controlling their Mg concentration. The physiological processes presumably responsible for such depressed Mg/Ca ratios appea r to be temperature-controlled as deduced from the close relationship of the observed Mg/Ca ratios and water temperature. This study demonst rates that variations in temperature and salinity are definitely refle cted in the Mg content of foraminiferal tests. Magnesium may thus serv e as a paleo-proxy for past surface water temperatures, as long as pos tdepositional changes and salinity variations are of subordinate impor tance or can be excluded.