13.
DIAGENESIS IN THE GAS HYDRATE SEDIMENTS OF THE BLAKE RIDGE: MINERALOGY AND
STABLE ISOTOPE COMPOSITIONS OF THE CARBONATE AND SULFIDE MINERALS1
Catherine
Pierre,2 Jean Marie Rouchy,3
and Annie Gaudichet4
ABSTRACT
During Ocean Drilling
Program Leg 164, gas hydrates were recovered in the Blake Ridge where the top of
the gas hydrate zone lies at about 200 meters below seafloor (mbsf) and the
bottom-simulating reflector (BSR) is located at about 450 mbsf. There is no
sedimentological discontinuity crossing the BSR. The BSR is disrupted by the
salt piercement of the Cape Fear Diapir. The authigenic carbonates (dolomite and
siderite) are always present in small amounts (a few weight percent) in the
sediments; they are also concentrated in millimeter- to centimeter-sized nodules
and layers composed of dolomite above the top of the gas hydrate reservoir, and
of siderite below the BSR. In the Blake Ridge, the dolomite/siderite boundary is
located near 140 mbsf. The distribution with depth of the 
18O
values of dolomite and siderite shows a sharp decrease from high values (maximum
7.5
) in the
topmost 50 m, to very low values (minimum -2.7)
at 140 mbsf, and at greater depth increase to positive values within the range
of 1.8 to 5.0.
The 13C
distribution is marked by the rapid increase with greater depth from low values
(-31.3 to
-11.4) near
50 mbsf to positive values at 110 mbsf, which remain in the range of 1.7
to 5.4 down
to 700 mbsf. Diagenetic carbonates were precipitated in pore waters in which 18O
and 13C
values were highly modified by strong fractionation effects, both in the water
and in the CO2-CH4 systems associated with the formation
and dissociation of gas hydrates.
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).
2LODYC, Université Pierre
et Marie Curie, UMR 7617, 4 Place Jussieu, 75252 Paris Cedex 05, France. cat@lodyc.jussieu.fr
3Laboratoire
de Géologie du Muséum, ESA 7073, 43 rue Buffon, 75005 Paris, France.
4LISA, UMR 7583, Université
de Créteil, 61 avenue du Général de Gaulle, 94010 Créteil Cedex, France.
Date of
initial receipt: 15 April 1998
Date of acceptance: 12 January 1999
Ms 164SR-226
