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

a r t i c l e i n f o a b s t r a c t Keywords: dolomite authigenic carbonate magnesium isotopes calcium isotopes Monterey formation Authigenic carbonates in marine sediments frequently have carbon isotope ratios that reflect local organic carbon processing rather than the δ 13 C 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 δ 13 C ranging from −16 to +9h. Light δ 13 C values were likely acquired from the sedimentary zone of microbial sulfate reduction, while heavy δ 13 C values were acquired from the zone of methanogenesis. Mg and Ca isotopes are roughly anti-correlated, with intervals of negative δ 13 C associated with low δ 26 Mg and higher δ 44/40 Ca values. The variability is observed over a wide range of length-scales, from 10 −2 meters within individual authigenic beds/nodules, to 10 2 meters 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 δ 26 Mg and δ 44/40 Ca to the more common stable isotope measurements of δ 13 C and δ 18 O represents a new, diagenetically robust, geochemical fingerprint for identifying synsedimentary authigenic carbonates in the geological record.