Effect of supra-lysoclinal dissolution on oxygen isotopes and Mg/Ca in planktonic foraminifera

Abstract

Mg/Ca in planktonic foraminiferal tests is now well established as a temperature proxy for surface and near-surface waters. The advantage of using Mg/Ca coupled with oxygen isotope ratio measured on the same foraminiferal sample provides the possibility to determine seawater d18O. There has been concern, however, about the post-mortem processes (e.g., dissolution) affecting foraminiferal Mg/Ca and d18O. We document the effect of dissolution on both Mg/Ca and d18O of several planktonic foraminiferal species along two depth transects (a) ~450-4000 m off Somalia and (b) ~1500-4500 m from Ontong Java Plateau. The planktonic foraminiferal species, Globigerinoides ruber, Globigerinoides sacculifer, Pulleniatina obliquiloculata, Neogloboquadrina dutertrei and Globorotalia tumida were analysed for Mg/Ca and d18O from core-top samples along the depth transects within a narrow size range (e.g., 250-300, 355-425 um). Mg/Ca and d18O in shells of surface dwelling planktonic foraminiferal species (e.g., G. ruber and G. sacculifer) show a small decrease while Mg/Ca and d18O of the deeper dwelling species (e.g., N. dutertrei and G. tumid) show more pronounced effect of supralysoclinal dissolution. The effect of supralysoclinal dissolution has an insignificant effect on the reconstruction of seawater d18O based on surface dwelling planktonic species compared to the deeper dwelling species