15. Authigenic Carbonates, Methane Generation, and Oxidation in Continental Rise and Shelf Sediments, ODP Leg 188 Sites 1165 and 1166, Offshore Antarctica (Prydz Bay)¹

George E. Claypool,² Thomas D. Lorenson,³ and Craig A. Johnson⁴

ABSTRACT

Carbon and oxygen isotopic compositions of authigenic carbonate nodules or layers reflect the diagenetic conditions at the time of nodule growth. The shallowest samples of carbonate nodules and dissolved inorganic carbon of pore water samples beneath the sulfate reduction zone (0-160 meters below seafloor [mbsf]) at Site 1165 have extremely negative ¹³C values (-50 and -62, respectively). These negative ¹³C values indicate nodule formation in association with anaerobic methane oxidation coupled with sulfate reduction. The ³⁴S of residual sulfate at Site 1165 shows only minor ³⁴S enrichment (+6), even with complete sulfate reduction. This small degree of apparent ³⁴S enrichment is due to extreme "open-system" sulfate reduction, with sulfate abundantly resupplied by diffusion from overlying seawater.

Ten calcite nodules from Site 1165 contain minor quartz and feldspar and have ¹³C values ranging from -49.7 to -8.2. The nodules with the most negative ¹³C values currently are at depths of 273 to 350 mbsf and must have precipitated from carbonate largely derived from subsurface anaerobic methane oxidation. The processes of sulfate reduction coupled with methane oxidation in sediments of Hole 1165B are indicated by characteristic concentration and isotopic (³⁴S and ¹³C) profiles of dissolved sulfate and bicarbonate.

Three siderite nodules from Site 1166 contain feldspar and mica and one has significant carbonate-apatite. The siderite has ¹³C values ranging from -15.3 to -7.6 These siderite nodules probably represent early diagenetic carbonate precipitation during microbial methanogenesis.

¹Claypool, G.F., Lorenson, T.D., and Johnson, C.A., 2003. Authigenic carbonates, methane generation, and oxidation in continental rise and shelf sediments, ODP Leg 188 Sites 1165 and 1166, offshore Antarctica (Prydz Bay). In Cooper, A.K., O'Brien, P.E., and Richter, C. (Eds.), Proc. ODP, Sci. Results, 188 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/188_SR/004/004.htm>. [Cited YYYY-MM-DD]

²8910 West Second Avenue, Lakewood CO 80226, USA. geclaypool@aol.com

³U.S. Geological Survey, 345 Middlefield Road, MS 999, Menlo Park CA 94025, USA.

⁴U.S. Geological Survey, Federal Center, Box 25046, MS 963, Denver CO 80225, USA.

Initial receipt: 12 July 2002
Acceptance: 12 December 2002
Web publication: 11 March 2003
Ms 188SR-004