CARBONATE DISSOLUTION/PRECIPITATION AND REDOX CHEMISTRY AT A PETROLEUM SEEP SITE IN GULF OF MEXICO CONTINENTAL MARGIN SEDIMENTS: EVIDENCE FROM FINE-SCALE PORE WATER STUDY

During the SSETI 2001 summer cruise, we examined redox reactions and calcium carbonate dissolution/preservation in microbial mat-associated sediments (site-2¢), in sediments near microbial mats (site-2), and in sediments without microbial mats (site-4) at station 234, Green Canyon (27°45¢N, 91°13¢W), a petroleum seep site on the Gulf of Mexico continental slope. A suite of geochemical parameters and stable isotopic ratios in porewaters were determined and O₂, pH, and pCO₂ microelectrode measurements were made in sediment cores.

There are great differences in porewater chemistry between site-2 and site-4. Total dissolved inorganic carbon (DIC) and total alkalinity (TA) profiles show a much higher microbial respiration rate at site-2 than at site-4, which is consistent with the local biological environment. High H₂S and lower sulfate concentrations at depth indicate sulfate reduction at site-2, but not at site-4. In the top 2-cm depth, the pH at site-2 decreased much more dramatically than at site-4. The minimum pH at site-2 is lower than that at site-4, suggesting that the oxidation of reduced species (sulfide, ammonium etc.) is much stronger at site-2. The lower pH also caused more CaCO₃ dissolution, resulting in higher pCO₂ and [Ca²⁺] in porewaters. Taphonomic studies also show that shell dissolution at site-2 is higher than at site-4. Microelectrode works confirm that carbonate dissolution at the petroleum seepage site only occurs in a very thin near-surface layer of sediments where a pH minimum exists, and the driving force for dissolution is the oxidation of reduced species such as sulfide. Once carbonate shells survive this thin and corrosive layer, they can be preserved.

Based on porewater [Ca²⁺] changes, pH profiles and measured CaCO₃ saturation states, it is concluded that high TA introduced by microbial sulfate reduction also promoted authigenic carbonate precipitation at deep depths (> 6cm) at site-2. No such change was observed away from microbial mats.