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 delta C-13 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, delta C-13 ranging from -16 to +9 parts per thousand. Light delta C-13 values were likely acquired from the sedimentary zone of microbial sulfate reduction, while heavy delta C-13 values were acquired from the zone of methanogenesis. Mg and Ca isotopes are roughly anti-correlated, with intervals of negative delta C-13 associated with low delta Mg-26 and higher delta Ca-44/46 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 delta(26)mg and delta Ca-44/40 to the more common stable isotope measurements of delta C-13 and delta O-18 represents a new, diagenetically robust, geochemical fingerprint for identifying synsedimentary authigenic carbonates in the geological record. (C) 2015 Elsevier B.V. All rights reserved.