Variations in Mg and Ca stable isotope ratios provide constraints on a variety of geological and biogeochemical processes. Recent improvements in thermal and plasma mass spectrometry have allowed the precise measurement of small differences in Ca and Mg isotopic compositions in CaCO_3, and consequently, their isotopic fractionation has been considered to have complementary information regarding marine temperature reconstructed from conventional proxies. These new tools can provide an opportunity to refine a biological control during biomineralization processes, which is critical for improve...