Reconciled: Secular Variation in Ocean Mg/Ca and Mg/Ca Paleothermometry in Foraminifera

The δ¹⁸O of foraminiferal calcite has long been the standard proxy of ancient ocean temperatures. However, it is confounded by the fact that foraminiferal calcite δ¹⁸O, in addition to temperature, is also a function of the δ¹⁸O of the ambient seawater. Therefore, it has become common recently to use the Mg/Ca ratio of foraminiferal calcite as a paleothermometer independent of ice volume. Paleotemperatures calculated thus can be applied to the δ¹⁸O -paleothermometer to calculate the δ¹⁸O of ancient seawater. Unfortunately, in a similar way that the δ¹⁸O paleothermometer is impacted by changing water chemistry and temperature, foraminiferal calcite Mg/Ca is a function of ambient seawater Mg/Ca as well as temperature. Attempts to date to employ the Mg/Ca paleothermometer to calculate temperatures and seawater δ¹⁸O over long time scales have been hampered by an inaccurate treatment of the effect of seawater Mg/Ca on calcite Mg/Ca. By using a power function, the Mg/Ca paleothermometer can be more accurately applied to deep geologic time (i.e., ages older than 5 Ma). While previous methods assumed little to no change in seawater Mg/Ca over the last 60 My to produce realistic paleocean temperatures and δ¹⁸O, the updated methodology presented here reduces the sensitivity of paleotemperature estimates to seawater Mg/Ca. This reconciles the results of Mg/Ca paleothermometry studies with geochemical evidence that propose a large change (from ~1 to ~5) in seawater Mg/Ca since 60 Ma.