UNDERSTANDING THE EFFECTS OF DISSOLUTION ON MG/CA RATIOS IN PLANKTIC FORAMINIFERA: EVIDENCE FROM A NOVEL INDIVIDUAL FORAMINIFERA METHOD

It has been well documented that partial dissolution of planktic foraminiferal tests results in a reduction of Mg/Ca ratios; however, traditional analysis techniques have made it difficult to identify the exact mechanism through which Mg is lost. Three hypotheses have been proposed as models for Mg loss: (1) a percent loss of Mg in individuals per degree of dissolution; (2) a molar loss of Mg in individuals per degree of dissolution; and (3) a loss of the highest-Mg (warmest) individuals in a population. It is vital to better constrain these models as they have vastly different implications for Mg/Ca-based paleotemperature dissolution corrections.

In this study we examine the effects of dissolution on Mg/Ca ratios in three species of planktic foraminifera, N. dutertrei, G. ruber, and P. obliquiloculata, from a depth transect of core tops on the Ontong Java Plateau in the western equatorial Pacific. Mg/Ca is measured in individual foraminifera that are chemically cleaned following standard protocols. We compare the results with the three models for Mg loss by constructing inferred temperature histograms for each species along the depth transect, observing the changes in the populations as dissolution proceeds: a shift to colder temperatures with no change in the shape of the distribution would indicate a percent loss of Mg; a shift to colder temperatures with a broadening of the distribution would indicate a molar loss of Mg; and a shift to colder temperatures with a narrowing of the distribution would indicate a loss of the warmest individuals. Finally, we discuss how the observed patterns can be applied to dissolution corrections in Mg/Ca-based temperature reconstructions.