Incorporation and preservation of Mg in Globigerinoides sacculifer: Implications for reconstructing the temperature and¹⁸O/¹⁶O of seawater

Using bathymetric transects of surface sediments underlying similar sea surface temperatures but exposed to increasing dissolution, we examined the processes which affect the relationship between foraminiferal Mg/Ca and δ¹⁸O. We found that Globigerinoides saccculifer calcifies over a relatively large range of water depth and that this is apparent in their Mg content. On the seafloor, foraminiferal Mg/Ca is substantially altered by dissolution with the degree of alteration increasing with water depth. Selective dissolution of the chamber calcite, formed in surface waters, shifts the shell's bulk Mg/Ca and δ¹⁸O toward the chemistries of the secondary crust acquired in colder thermocline waters. The magnitude of this shift depends on both the range of temperatures over which the shell calcined and the degree to which it is subsequently dissolved. In spite of this shift the initial relationship between Mg/Ca and δ¹⁸O, determined by their temperature dependence, is maintained. We conclude that paired measurements of δ¹⁸O and Mg/Ca can be used for reconstructing δ¹⁸O_water, though care must be taken to determine where in the water column the reconstruction applies.