DISCUSSION

Several possible origins exist for the relatively abundant gases found in the black shale units of the five Leg 207 sites:

1. Hydrocarbon gases from deeper, more thermally mature sediments may have migrated into the gas-rich zone.
2. The gases can originate from some combination of in situ thermogenic and biogenic degradation of the organic matter in the black shales.
3. The gases could be a mixture of in situ and migrated gases.

Evidence for migration of presumably thermogenic methane into porous sediments from deeper sources has been found at other ODP sites. For example, a thermogenic source exists in the Jurassic rocks that underlie Sites 762 and 763 on the Exmouth Plateau (Meyers and Snowdon, 1993). Potential Mesozoic sources of thermogenic gas are indeed known on the South American margin between Trinidad and Surinam.

The C₁/C₂ ratio is particularly useful for distinguishing between biogenic and thermogenic gaseous hydrocarbons; biogenic gases commonly have ratios >1000, whereas values <200 imply gas generation related to increasing depth and temperature (cf. Claypool and Kvenvolden, 1983; Stein et al., 1995; Whiticar, 1999). High C₁/C₂ ratios and the absence of major contributions of higher molecular weight hydrocarbon gases (see the "Organic Geochemistry" sections in the site chapters) suggest that little of the gas in the sites drilled during Leg 207 is derived from the thermal degradation of organic matter.

The most likely source of most of the methane, the dominant hydrocarbon gas in these sediments, is in situ formation by methanogenic microorganisms. Methanogenesis becomes important in sediments devoid of interstitial sulfate but containing metabolizable organic matter. This process has two stages, in which a consortium of microorganisms first converts various components of organic matter into short-chain alcohols and acids, CO₂, and H₂. In the second stage, some of these products are converted to CH₄ by a metabolically limited group of obligate anaerobic microbes (Mechalas, 1974; Whiticar, 1999). The general reactions that represent the production of methane in the second stages are

CH₃COOH CH₄ + CO₂ (acetic acid fermentation) and

CO₂ + 4 H₂ CH₄ + 2 H₂O (CO₂ reduction).

The reduction of CO₂ to methane is the dominant process in marine sediments (Wiese and Kvenvolden, 1993; Whiticar, 1999).

Comparison of concentrations of methane and CO₂ in the black shale unit of Hole 1258A indicates that these two components of interstitial gases are indeed related. Although methane is usually more abundant than CO₂, higher concentrations of the two gases generally coincide in the Cenomanian–Turonian black shales (Fig. F6). However, this relation is not obvious in the underlying mid-Albian claystones of Unit V. One important difference between the two lithostratigraphic units is their TOC concentrations; the black shales average 7.9 wt%, whereas the claystones average 4.2 wt%. Another difference is that Rock-Eval pyrolysis identifies the organic matter as Type II in Unit IV and a mixture of Types II and III in Unit V (see "Organic Matter Source Characterization" in "Organic Geochemistry" in the "Site 1258" chapter). The organic matter in Unit V is evidently less suitable for gas generation.

At all five Leg 207 drill sites, the disappearance of interstitial sulfate coincides with the top of the black shales and the increase in methane concentrations (see the "Organic Geochemistry" sections in the site chapters). A microbial origin of the methane is implied by this observation, inasmuch as Claypool and Kvenvolden (1983) observe that the presence of interstitial sulfate inhibits microbial methanogenesis in marine sediments. Similar in situ microbial production of methane has been inferred from high microbial gas concentrations at numerous DSDP and ODP sites. Examples include Pliocene–Pleistocene sediments from under the Benguela Current (Meyers and Brassell, 1985; Meyers et al., 1998), at Sites 618 and 619 on the northern margin of the Gulf of Mexico (Burke et al., 1986; Pflaum et al., 1986), at Leg 112 sites on the Peru margin (Kvenvolden and Kastner, 1990; Kvenvolden et al., 1990), at Sites 897 and 898 on the Iberian Abyssal Plain (Meyers and Shaw, 1996), and in middle Miocene sediments at Site 767 in the Celebes Sea (Shipboard Scientific Party, 1990). The tops of the Leg 207 black shale units are between 176 and 565 mbsf (see the "Lithostratigraphy" sections in the site chapters), which places them within the range of microbial viability (surface to ~500 mbsf) reported by Cragg et al. (1992) in sediments from the Japan Sea.

The origin of the methane probably involves mainly reduction of interstitial CO₂ in the sediments and minor in situ microbial fermentation of marine organic matter. Because these two processes are carried out by distinct microbial populations, the production of methane ultimately depends on satisfying the metabolic requirements of the separate pathways. Comparison of the carbon isotopic contents of CO₂ and CH₄ in sediments at DSDP Site 533 on the Blake Plateau indicates that most of the CH₄ originates from reduction of CO₂ (Galimov and Kvenvolden, 1983). Inasmuch as the obligate anaerobes involved with CO₂ reduction are particularly metabolically limited (Mechalas, 1981; Whiticar, 1999), this pathway can become blocked despite an abundance of interstitial CO₂ and thereby limit CH₄ production.

The possible relation between interstitial methane concentrations and sediment organic matter contents was investigated by measuring the TOC concentrations of the headspace sediment samples from Holes 1259A, 1260A, and 1260B. A rough correspondence exists between larger gas concentrations and higher TOC values (Fig. F7). Marked excursions from a simple linear relation suggest that organic matter quality, and not simply quantity, affects gas generation from the black shales. Moreover, dramatic changes in methane concentrations at lithostratigraphic boundaries (e.g., Figs. F3, F4) suggest either that gas does not freely migrate from its origin in the black shales or that it migrates and is being quickly replenished from the organic matter in this unit so that elevated concentrations are maintained.

Inferential evidence for migration of methane out of the black shale units exists. Concentrations of interstitial sulfate start to increase and those of ammonium to decrease at the top of the black shale sequences at all five sites (see "Organic Geochemistry" in "Discussion and Conclusions" in the "Leg 207 Summary" chapter). These results indicate the existence of an important sulfate sink and ammonium source in the Cretaceous black shales, which in turn implies diffusion of methane from the black shales to support metabolic activity above the black shale unit, most likely as anaerobic methane oxidation. Because maximum concentrations of methane are found in the black shale units at each site, methanogenesis must be active to replace the methane that migrates out of these units.

The weak correspondence between concentrations of carbonate carbon and organic carbon and concentrations of CH₄ in sediments from the five sites implies that only a small percentage of organic matter is converted to gases. For example, even at saturation, methane represents only ~0.1% of the organic matter that is present in unconsolidated sediments (Whiticar, 1999). The limiting factor to microbial gas production is generally not the availability of organic matter but the availability of terminal electron receptors.