HR: 0800h
AN: OS21A-1196    [Abstracts]
TI: Formation of carbonate concretions in deep-sea sediment below the CCD and above an active gas hydrate system
AU: * Dicus, C M
EM: cmd@rice.edu
AF: Rice University Earth Science, MS126 PO Box 1892, Houston, TX 77251 United States
AU: Snyder, G T
EM: gsnyder@rice.edu
AF: Rice University Earth Science, MS126 PO Box 1892, Houston, TX 77251 United States
AU: Dickens, G R
EM: jerry@rice.edu
AF: Rice University Earth Science, MS126 PO Box 1892, Houston, TX 77251 United States
AB: Site 1230 of the Ocean Drilling Program targeted the chemistry and microbiology of an active deep-water gas hydrate system in the Peru Trench. The site is noteworthy because, at nearly 6000 m water depth, it lies well below the carbonate compensation depth and the sediments comprise mostly terrigenous clays and biogenic silica. Shipboard work at this site delineated a prominent sulfate-methane transition (SMT) at 8-10 m below seafloor (mbsf) as well as some carbonate horizons. In this study, we present calcium and strontium data for pore waters and sediments at this site, including across the SMT. Concentration profiles show that dissolved Ca$^{2+}$ diffuses downward from the seafloor toward the SMT, where a sharp inflection indicates consumption of Ca$^{2+}$ into an authigenic phase. Dissolved Sr$^{2+}$, on the other hand, diffuses upward from depth toward the SMT. Again, however, a prominent inflection suggests removal of Sr$^{2+}$ to sediment. The inferences from pore water profiles are borne out by sediment chemistry. Large peaks in the calcium and strontium content of sediment mark the SMT. The calcium and strontium fronts reach $\sim$2700 and $\sim$5 mmol/kg, respectively, at 9 mbsf, which are much greater than average background values of $\sim$10 and $\sim$1 mmol/kg. These authigenic fronts are primarily composed of carbonate minerals, as determined by acetic acid extractions and x-ray diffraction. Because the calcium and strontium fronts coincide with both the SMT and changes in dissolved chemistry, it is proposed that the carbonates are currently forming as follows: methane rising from the underlying gas hydrate system reacts with dissolved sulfate through anaerobic oxidation of methane which releases HCO$^{3-}$ and alkalinity and causes carbonate precipitation. The overall process has been observed elsewhere; the Peru Trench is interesting, however, because the process leads to carbonate in sediments otherwise devoid of carbonate.
DE: 4806 Carbon cycling
DE: 4825 Geochemistry
DE: 4851 Oxidation/reduction reactions
DE: 4875 Trace elements
DE: 4802 Anoxic environments
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting