Sulfur biogeochemistry of pore waters and sediments from IODP Expedition 307
Final Report Abstract
Over the last decade, thousands of mound structures have been discovered in muddy sediments along continental margins at water depths of 400 to 900 meters. These structures, which can form conical bodies reaching the width over several km and the height over 100 to 200 meters height, are covered with living and/or dead cold-water corals. To establish a depositional model for cold-water carbonate mounds, lODP Expedition 307 drilled the Challenger Mound and adjacent continental slope sites in the Porcupine Seabight off the coast of Ireland in May 2005. Within the coral-bearing mound sediment succession interstitial water profiles of sulfate, alkalinity, Mg, and Sr indicated a tight coupling between carbonate diagenesis and micrrobial sulfate reduction. In this project we examined the diagenetic processes in the deep biogeochemical sulfur-iron cycle of sediments recovered from the coral-bearing Challenger Mound and surrounding sedimentary sites (lODP Expediton 307). To characterize these geomicrobial sulfur transformations, inorganic and stable isotope geochemical compositions of pore water sulfate and solid phase reduced sulfur compounds were performed. The oxygen isotopic composition of residual dissolved sulfate indicated increasing equilibration between extracellular pore water and sulfate and indicates that intracellular isotope exchange processes within the cells of sulfate reducing bacteria occurs in throughout the sediments. However, present-day rates of sulfate reducing activity in the mound sediments (lODP Site U 1317) appear to be very low and changes in pore water S distributions in the mound may reflect non-steady state deposition/erosion features. Challenger Mound rests on eroded methane-bearing sediments from the Miocene. It appears that the ongoing microbial reactions (sulfate dependent oxidation of methane) in these old (>9 Ma) sediments are driving the distributions of sulfate concentrations and isotopic signatures in the overlying mound sediments. The sulfur isotopic composition of pyrite sulfur generally became increasing enriched with increasing depth reflecting a light diagenetic overprint, both within the mound and through the Miocene sediments underlying the mound succession. Surprisingly, several excursions to more 34S sulfur enriched pyrite to values >0 per mil were observed in the deeper sections of the mound sequence. These excursions may be relict signals of paleo-(greigite-)pyritization (sulfides) fronts arising from higher rates of sulfate-dependent methane oxidation in the underlying Miocene sediments during the early stages of mound growth. Thus, microbial processes occurring in the "Miocene bioreactor" underlying the Challenger Mound play a significant and interesting role in the overall geochemistry of the Challenger Mound - both presentday and in the paleo-record.
Publications
- (2007) Isotope biogeochemistry of sulfur cycling by the deep biosphere of Porcubine Seabight coral mountains (lODP Leg 307). Geophys. Res. Abs. 9, #01381
Böttcher M.E. & Ferdelman T.G.
- (2007) Isotope biogeochemistry of sulfur cycling by the deep biosphere of Porcubtne Seabight sediments (lODP Leg 307). Kolloquium des DFG-SPP IODP, Potsdam, 19.-21.3.2007
Böttcher M.E., Ferdelman T.G. & Schipper A.
- (2007) Sulfur cycling by the deep biosphere of Porcupine Seabight sediments (lODP Leg 307): Evidence from 34S and 18O partitioning. TERRA NOSTRA 2007/1-2:49-50
Böttcher M.E. & Ferdelman T.G.
- (2007), Isotope Biogeochemistry of Sulfur in a Cold-Water Carbonate Mound (lODP Site 1317), Eos Trans. AGU, 88{52), Fall Meet. Suppl., Abstract B22A-07
Ferdelman, T.G. and Böttcher M.E.
- 2007. Age constraints on the origin on growth history of a deep-water coral mound in the northeast Atlantic drilled during Integrated Ocean Drilling Program Expedition 307. Geology, 35, 1051-1054
Kano, A., Ferdelman, T.G., Williams, T., Henriet, J.P., Ishikawa, T., Kawagoe N., Takashima, C , Kakizaki, Y., Abe. K, Sakai, S, Browning, E.L, Li, X., and lODP Expedition 307 Scientists
- 2009. Subsurface microbiology and biogeochemistry of a deep, cold-water carbonate mound from the Porcupine Seabight (lODP Expedition 307) Env. Microbiol. 11, 239-257
Webster, G., Blazejak, A., Cragg, B.A., Schippers, A., Sass, H., Rinna, J., Tang, X., Mathes, F., Ferdelman, T.G., Fry, J.G., Weightman, A.J., Parkes, R.J.
(See online at https://doi.org/10.1111/j.1462-2920.2008.01759.x)
