The geochemistry of Orca Basin sediments

• Main Author: Sheu, Der-Due
• Other Authors: Presley, B. J., Gartner, Stefan, Hajash, Andrew, Jr., Hossner, L. R., Reid, Robert O.
• Format: Thesis
• Language: English
• Published: Texas A&M University. Libraries 1983
• Subjects: Oceanography; 1983 Dissertation S554; Geochemistry; Orca Basin; Marine sediments; Orca
• Online Access: https://hdl.handle.net/1969.1/DISSERTATIONS-529667

Typescript (photocopy). The Orca Basin is a depression on the continental slope of the northern Gulf of Mexico which is partially filled with anoxic hypersaline brine. Steady-state diagenetic modelling of sulfate reduction in the basin sediments shows that sulfate reduction by sulfate-reducing bacteria is not inhibited by the high salt content of the basin water. The rate constants of sulfate reduction estimated range from 3.61 to 17.67 x 10('-12) sec('-1), if a simplifying assumption of a constant rate of sedimentation is used in the calculation. The rate of sulfate reduction is thus between 0.042 exp (-0.0019)x and 0.203 exp (-0.0093)x n moles S/ml brine/day, where x is the depth. The diffusion coefficients of sulfate ion in sediment interstitial water are estimated to range between 1.99 and 9.83 x 10('-6) cm('2)/sec. The extent of sulfate reduction in the sediment is complicated and limited by reactions in the brine which affect the nature of organic matter available to the sulfate-reducing bacterial community near and at the brine-sediment interface. The measured amount of solid-phase sulfide buried in the sediments shows that there is an appreciable amount of additional sulfide other than that which can be accounted for by sulfate reduction in sediment interstitial water alone. This suggests that sulfate reduction occurs in the brine overlying the sediments and at the brine-sediment interface. Sediments accumulated in this basin are intricately laminated on the mm to cm scale into visually highly color-contrasted layers. Distinctively bright red layers enriched in hematite are found which probably form during isolated incidences of enhanced mixing and oxygen supply across the normally stable brine-seawater interface. In contrast, the light gray, dark gray and black layers are enriched in sulfide, organic carbon and carbonate relative to the red ones, and are deposited under strictly anoxic conditions. The distribution and partitioning of Fe and Mn into various components of the sediment units show that Fe ...