notation relative to the Peedee belemnite (PDB) reference (Craig, 1957).About 40 samples of diagenetic carbonates from the Cape Fear Diapir Sites 991, 992, and 993, and from the Blake Ridge Sites 994, 995, and 997, have been selected for the combined mineralogical and stable isotope studies (Table 1). In addition, the sampling of about 90 levels was conducted in Hole 994C in order to follow the general background of the pelagic carbonate sedimentation (Table 2). For most samples, the mineralogy was determined by X-ray diffraction (XRD) during the cruise (Paull, Matsumoto, Wallace, et al., 1996). To characterize the crystal morphologies, scanning electron microscope (SEM) observations have been made on a few samples. The elemental analyses of the diagenetic carbonates were also made using a transmission electron microscope (TEM) to verify the chemical composition of individual crystals.
The oxygen and carbon
isotopic compositions of the different carbonate phases were measured using a
specific procedure necessary because the sediments contain mixtures of calcite
and either dolomite or siderite. From these mixtures, the CO2
released after 20 min of reaction at 25ºC with 100% phosphoric acid was
considered to come solely from calcite (Clayton et al., 1968). Dolomite or
siderite was isolated from the carbonate mixture after 20 min of reaction in 1N
acetic acid at room temperature, which eliminates the calcite component; the
elimination of calcite from the mixture was systematically checked by XRD. The
remaining dolomite or siderite was then treated at 25ºC with 100% phosphoric
acid for one week, and the CO2 gas released was analyzed using a
Isogas-Optima triple collector mass spectrometer. The isotopic compositions are
expressed in the conventional
notation relative to the Peedee belemnite (PDB) reference (Craig, 1957).
The analytical precision
is 0.01
for
both 18O
and 13C
values; the reproducibility of the carbonate reference run on separate days is
± 0.05 for 18O
and ± 0.02
for 13C.
Similar tests on samples containing carbonate mixtures give a reproducibility of
± 0.15 for 18O
and ± 0.05
for 13C.
The 18O
values measured for dolomite and siderite have been corrected by -0.8
and -1.55,
respectively, for the fractionation factor at 25ºC, which occurs during the
reaction of the carbonate with the phosphoric acid (Sharma and Clayton, 1965;
Carothers et al., 1988).
Four pyrite samples from
Sites 994 and 997 have been selected for sulfur isotopic measurements (Table
3). Pyrite was oxidized as SO2 by burning in pure oxygen
(Filly et al., 1975). The 34S
values are expressed relative to the Cañon Diablo Troilite (CDT) reference
(Jensen and Nakai, 1962). The SO2 gas was analyzed on a triple
collector mass spectrometer (Isogas-Sira 9) with an analytical precision close
to 0.03, and
a reproducibility within the range ± 0.1.
