Jon Martin¹, Peter Eichhubl², H. Gary Greene^2
1 University of Florida, Gainsville, FL 32611
² Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039

Crusts and nodules of authigenic carbonate were observed and sampled during recent MBARI ROV dives on the coastal shelf and along the shelf break of the Santa Barbara Basin. These carbonate crusts or nodules are typically arkosic silt- and sandstones cemented with micritic high-Mg calcite and pore-filling aragonite and less common calcite and dolomite. Although sampled from the sediment surface their agglutinated texture indicates formation within the sediment and later exhumation. The strontium isotopic composition of dilute acetic acid carbonate leachate ranges between 0.70899 and 0.71028. The radiogenic strontium isotopic composition of some of these samples may be due to partial dissolution of clays and feldspar during the acetic acid leach or by water-sediment interaction in the sediment. Sr isotopic values lower than modern seawater of 0.70916 are explained by carbonate precipitation from pore water containing a significant component of formation water seeping from underlying Neogene units. The ⁸⁷Sr/⁸⁶Sr composition of Monterey formation water sampled from producing oil wells in the Santa Barbara Basin ranges between 0.70827 and 0.70863. Authigenic carbonate precipitation from seeping formation water is also consistent with carbonate d¹⁸O values of up to +7 permil (PDB). Assuming bottom water temperatures of +7 degr. C, the carbonate d¹⁸O composition corresponds to a pore water composition of up to +4 permil (SMOW), or +2 permil assuming a most conservative water temperature of 0 degr. C. In comparison, the d18O composition of produced Monterey formation water ranges from +2 to +6 permil (SMOW).

The isotopic composition of authigenic carbonate is somewhat surprising considering the general lack of evidence for active aqueous seepage in the basin. Several carbonate samples are associated with active methane venting, but with the exception of one sampling location, pore fluid analyses of push and piston cores lack evidence for modern aqueous seepage. We therefore conclude that authigenic carbonates in the Santa Barbara Basin are largely fossil occurrences.