Geochemical controls on carbonate shell taphonomy in Northern Gulf of Mexico continental shelf and slope sediments

Abstract

We examine the linkage between the sediment geochemical milieu and the process of carbonate degradation over a wide range of continental shelf and slope sediments using molluscan shells deployed for 13 years by the Shelf and Slope Experimental Taphonomy Initiative (SSETI). Geochemical characterization of the environment of preservation included the breadth of the pore-water carbonate undersaturation window, a depth-integrated carbonate dissolution index, the depth of minimum pore-water saturation, diffusive fluxes of oxygen and calcium, average sulfate and chloride concentration in the upper 5 cm, and the carbonate and organic carbon fractions in the same sedimentary horizon. Taphonomic indices included the maximum degree of dissolution; average dissolution; the incidences of chalkiness, pitting, deep dissolution, and a soft shell surface; the maximum degree of discoloration; the incidences of fading, gray-to-black discoloration, brown discoloration, and orange discoloration; the presence of pyrite; and edge rounding. Geochemical variables characterize the extent of most taphonomic processes with high three-variable multiple regression coefficients (R2 > 0.85). Dissolution was most intense at petroleum seeps where enhanced sediment respiration fueled by petroleum carbon and oxidation of reduced species (e.g., H2S) resulted in acute pore-water carbonate undersaturation near the sediment-water interface and high diffusive oxygen flux. In contrast, discoloration occurred as often or more commonly in shelf and slope sediments that were not subject to seep influence. The tendency for correlations between many taphonomic metrics, including those relating to dissolution, pyritization, and discoloration, and the breadths of the calcite/aragonite undersaturation windows, calcite, and oxygen flux emphasize the importance of near-surface geochemical conditions relating to organic carbon decomposition in determining the degree and type of carbonate degradation occuring at SSETI sites. © 2012 SEPM (Society for Sedimentary Geology).