13C)
of one sample from each of Sites 991 and 993 measured -72.4 and -77.7
PDB,
respectively (Paull et al., Chap.
7, this volume). These isotopically light values suggest that the
methane is microbial in origin (Bernard et al., 1976).The sources of methane and
other hydrocarbon gases in residual gas in sediments can be inferred from
molecular ratios and from isotopic compositions. At the three Cape Fear Diapir
sites (Sites 991, 992, and 993), the residual methane apparently is controlled
by the availability of sulfate, as discussed earlier. The implication is that
methane is microbial in origin, resulting from CO2 reduction
following the microbial depletion of sulfate (Claypool and Kaplan, 1974).
However, the C1/(C2 + C3) ratios are
<100 in many cases (Table 2),
suggesting the presence of thermogenically formed hydrocarbons, according to
criteria of Bernard et al. (1976). Methane carbon isotopic compositions (13C)
of one sample from each of Sites 991 and 993 measured -72.4 and -77.7PDB,
respectively (Paull et al., Chap.
7, this volume). These isotopically light values suggest that the
methane is microbial in origin (Bernard et al., 1976).
The molecular and isotopic
evidence are thus in conflict. One possible explanation is that the gas mixture
is microbial but has incurred preferential loss of methane through methane
oxidation, leading to diminished C1/(C2 + C3)
ratios. For this idea to be correct, the 13C
values of the pre-oxidized methane should have been lighter than about -72.
Unfortunately, no 13C
determinations were made on methane occurring immediately below the zone of
sulfate reduction at these sites. However, at sites on the nearby Blake Ridge
transect, the 13C
values of methane in this zone are lighter, ranging from about -80 to -101
(Paull et al., Chap. 7,
this volume). The pre-oxidized methane at the diapir sites may have had similar
carbon isotopic compositions. If this interpretation is correct, then methane
oxidation explains the molecular and isotopic compositions observed at these
diapir sites.
Concentrations of methane
relative to other hydrocarbon gases are higher at the Blake Ridge Diapir site
(Site 996) as reflected in C1/(C2 + C3)
ratios ranging from 1100 to 1600 (Table
2), indicating a microbial origin for the gas mixture (Bernard et al.,
1976). The 13C
values of 13 samples of methane measured -68.0 ± 2.4,
a range suggesting microbial diagenesis (Paull et al., Chap.
7, this volume). Thus both molecular and isotopic compositions indicate
that these hydrocarbon gases are likely the product of very early microbial
diagenesis, and the methane is likely the product of intense microbial
methanogenesis, leading to sufficient methane for gas-hydrate formation.
At the three sites (Sites 994, 995, and 997) on the Blake Ridge transect, methane dominates the hydrocarbon gas mixtures, and C1/(C2 + C3) ratios are large, ~10,000 below ~30 mbsf, and decrease with depth to ~500 (Table 3). The diminishing C1/(C2 + C3) ratios with depth can be explained, in part, as the result of (1) diagenesis wherein amounts of C2 and C3 increase with depth, and/or (2) preferential loss of methane during outgassing. For example, during core recovery, methane saturates the pore fluid and is preferentially lost while the other hydrocarbon gases, which are present in much lower concentrations, do not reach saturation and thus are retained in the pore waters. Preferential methane outgassing affects methane concentrations and the C1/(C2 + C3) ratios, but not the methane carbon isotopic compositions, which are determined for the most part by the isotopic fractionation that occurs during microbial methane formation from CO2 reduction as suggested by Galimov and Kvenvolden (1983).
The 13C
values of methane fall in the range -68.4 ± 7.0
with values becoming uniform (- 64.0 ± 0.9)
below 300 mbsf (Paull et al., this volume). The molecular ratios C1/(C2
+ C3) > 500) and the isotopic results suggest that methane is the
product of microbial methanogenesis, following the criteria of Bernard et al.
(1976). Although C2 concentrations tend to increase with depth,
amounts never reach levels suggestive of a thermal origin, and the 13C
values of ethane (-65.7 ± 2.1)
in 12 samples from Site 997 (Paull et al., this volume) are consistent with the
ethane also being microbial as proposed by Waseda and Didyk (1995) for Leg 141
results, and are supported by arguments given by Vogel et al. (1982) and
Oremland et al. (1988).
The low C1/(C2
+ C3) ratios in residual gas samples collected within the thin zone
of sulfate reduction (most shallow sample at each site in Table
3) likely result from methane oxidation, leaving behind less methane and
microbially produced C2 and C3. The 13C
values of this residual methane should be isotopically heavy, but,
unfortunately, samples of this methane were not measured for carbon isotopic
compositions.
All of the hydrocarbon gas results from Sites 994, 995, and 997 extend and do not contradict results obtained earlier from Site 533 (Kvenvolden and Barnard, 1983; Galimov and Kvenvolden, 1983; Kvenvolden et al., 1990). The molecular and isotopic compositions of the residual hydrocarbon gases found in coring the Blake Ridge thus indicate that these gases are a product of microbial processes including microbial diagenesis. Sufficient methane was generated and recycled in sediment of the Blake Ridge to cause gas-hydrate formation. There is no evidence for a significant contribution of thermogenic methane, although thermogenesis may be responsible for some of the hydrocarbon gases larger than methane, for example, isopentane, which often occurs in anomalous concentrations (Table 3).
