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 13C
values of the authigenic aragonite vary between -48.4
and -30.5
(Peedee
belemnite [PDB]), indicating that carbon derived from 13C-depleted
methane is incorporated into these carbonates. The
13C
of pore water
CO2
values are most negative in the upper 10 mbsf, near the sediment/water interface
(-38
± 5
),
but noticeably more positive below 25 mbsf (+5
± 6
). Because
carbonates derive their carbon from HCO3-, dissimilarities
between the
13C
values of carbonate precipitates recovered from greater than 10 mbsf and
13C
values of the associated pore fluids suggests that these carbonates formed near
the seafloor. Differences of about 1
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 87Sr/86Sr values
between 0.709125 and 0.709206 with a mean of 0.709165, consistent with the
approximate age of their host sediment. Furthermore, the 87Sr/86Sr
values of six pore-water samples from Site 996 vary between 0.709130 and
0.709204. The similarity of these values to seawater (87Sr/86Sr
= 0.709175), and to 87Sr/86Sr values of pore water from
similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997),
indicates a shallow Sr source. The 87Sr/86Sr 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.
1Paull,
C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc. ODP,
Sci. Results, 164: College Station, TX (Ocean Drilling Program).
2GEOMAR
Research Center for Marine Geosciences, 24148 Kiel, Federal Republic of Germany.
3Present address: Monterey Bay Aquarium
Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, U.S.A.
Correspondence author: tnaehr@mbari.org
4Department of
Geology, University of North Carolina, Chapel Hill, NC 27599, U.S.A.
5Present address: Exxon
Exploration Company, P.O. Box 4778, Houston, TX 77210-4778, U.S.A.
6Department of
Geology and Paleontology, University of Kiel, 24118 Kiel, Federal Republic of
Germany.
Date of
initial receipt: 20 April 1998
Date of acceptance: 12 January 1999
Ms 164SR-228