Mg and Ca isotope signatures of authigenic dolomite in siliceous deep-sea sediments

Authigenic carbonates in marine sediments frequently have carbon isotope ratios that reflect local organic carbon processing rather than the δC13 of the global DIC (dissolved inorganic carbon) reservoir, but their contributions to ancient sedimentary sections are difficult to assess. In this study of authigenic dolomite from the Miocene-age Monterey Formation of offshore California, Mg and Ca isotopes are shown to vary with stratigraphic depth as a result of early diagenetic processes. The dolomite is a pre-compaction authigenic phase that occurs as beds and nodules with δC13 ranging from -16 to +9‰. Light δC13 values were likely acquired from the sedimentary zone of microbial sulfate reduction, while heavy δC13 values were acquired from the zone of methanogenesis. Mg and Ca isotopes are roughly anti-correlated, with intervals of negative δC13 associated with low δMg26 and higher δCa44/40 values. The variability is observed over a wide range of length-scales, from 10^-2meters within individual authigenic beds/nodules, to 10²meters over the entire stratigraphic column, and can be understood as the consequence of dolomite precipitation in pore fluids where Mg supply is limited by diffusive transport. The relationship of δMg26 and δCa44/40 to the more common stable isotope measurements of δC13 and δO18 represents a new, diagenetically robust, geochemical fingerprint for identifying synsedimentary authigenic carbonates in the geological record.