Ocean Drilling Program, United States

Authigenic carbonates were recovered from several horizons between 0 and 52 mbsf in sediments that overlay the Blake Ridge Diapir on the Carolina Rise (Ocean Drilling Program [ODP] Site 996). Active chemosynthetic communities at this site are apparently fed by fluid conduits extending beneath a bottom-simulating reflector (BSR). Gas hydrates occur at several depth intervals in these near-surface sediments. The carbonate nodules are composed of rounded to subangular intraclasts and carbonate cemented mussel shell fragments. Electron microprobe and X-ray diffraction (XRD) investigations show that aragonite is the dominant authigenic carbonate. Authigenic aragonite occurs both as microcrystalline, interstitial cement, and as cavity-filling radial fibrous crystals. The delta (super 13) C values of the authigenic aragonite vary between -48.4 per mil and -30.5 per mil (Peedee belemnite [PDB]), indicating that carbon derived from (super 13) C-depleted methane is incorporated into these carbonates. The delta (super 13) C of pore water Sigma CO (sub 2) values are most negative in the upper 10 mbsf, near the sediment/water interface (-38 per mil+ or -5 per mil), but noticeably more positive below 25 mbsf (+5 per mil+ or -6 per mil). Because carbonates derive their carbon from HCO (sub 3) (super -) , dissimilarities between the delta (super 13) C values of carbonate precipitates recovered from greater than 10 mbsf and delta (super 13) C values of the associated pore fluids suggests that these carbonates formed near the seafloor. Differences of about 1 per mil in the oxygen isotopic composition of carbonate precipitates from different depths are possibly related to changes in bottom-water conditions during glacial and interglacial time periods. Measurements of the strontium isotopic composition on 13 carbonate samples show (super 87) Sr/ (super 86) Sr values between 0.709125 and 0.709206 with a mean of 0.709175), consistent with the approximate age of their host sediment. Furthermore, the (super 87) Sr/ (super 86) Sr values of six pore-water samples from Site 996 vary between 0.709130 and 0.709204. The similarity of these values to seawater ( (super 87) Sr/ (super 86) Sr = 0.709175), and to (super 87) Sr/ (super 86) Sr values of pore water from similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997), indicates a shallow Sr source. The (super 87) Sr/ (super 86) Sr values of the authigenic carbonates at Site 996 are not consistent with the Sr isotopic values predicted for carbonates precipitated from fluids transported upward along fault conduits extending through the base of the gas hydrate-stability zone. Based on our data, we see no evidence of continuing carbonate diagenesis with depth. Therefore, with the exception of their seafloor expression as carbonate crusts, fossil vent sites will not be preserved. Because these authigenic features apparently form only at the seafloor, their vertical distribution and sediment age imply that seepage has been going on in this area for at least 600,000 yr.