Apparent preservation of primary foraminiferal Mg/Ca ratios and Mg-banding in recrystallized foraminifera

Trace element and delta O-18 values of foraminifera are widely used to reconstruct oceanic temperatures throughout the Cenozoic and beyond. Previous work evaluating the geochemistry of foraminifera with differing degrees of physical preservation have shown that Mg/Ca and delta O-18 paleothermometers give discrepant values in recrystallized tests, with planktonic oxygen isotopes often yielding significantly lower temperatures than Mg/Ca ratios. To study the mobility of elements during diagenesis, we performed microspatial trace element analyses in Eocene Morozovella. Element maps show that trace element banding is readily identifiable and preserved, to an extent, in texturally recrystallized tests. A reaction-diffusion model was used to test whether the preservation of Mg-banding and the decoupling of delta O-18 and Mg/Ca values could be the result of diffusively limited closed-system recrystallization. Results show that, in a closed system, internal features (such as Mg-banding) will dissipate prior to changes in bulk Mg/Ca composition, while the bulk delta O-18 value will typically change faster than Mg/Ca. This is observed regardless of what partitioning coefficient is used for Mg and demonstrates that the planktonic Mg/Ca proxy is more diagenetically robust than the delta O-18 proxy. Thus, this model can explain the observed decoupling of these two proxies. Furthermore, the preservation of intra-test Mg-banding shows potential for use in evaluating the preservation of primary Mg/Ca values and hence the accuracy of paleotemperature reconstructions.

Automated summary

The Mg/Ca and δ18O paleothermometers give discrepant values in recrystallized tests, potentially due to diffusively limited closed-system recrystallization. The Mg/Ca ratio is more diagenetically robust and can be used as a more reliable paleotemperature proxy.