INTRODUCTION

Early diagenetic processes occurring in the pore waters of deep-water, continental-margin sediments are typically mediated by microbial populations. Microbes use various dissolved species to oxidize sedimentary organic matter resulting in modification of the concentration and isotopic composition of dissolved pore-water constituents by removing needed substrates, adding metabolic by-products, and creating a reactive chemical environment (e.g., Richards, 1965; Gieskes, 1981). Carbon transformations particularly reveal these fundamental processes, as organic carbon from sedimentary organic matter is ultimately transformed into CO₂ and methane, passing through a cascade of intermediate steps (generally called fermentation reactions).

Terminal carbon transformations of methane-rich diagenetic environments of the world's deep-water continental margins involve several diagenetic processes that are linked by common substrates and/or products. Sulfate reducers utilize interstitial sulfate to oxidize sedimentary organic matter and produce CO₂, which dominantly exists in marine pore waters as bicarbonate (HCO₃^-). Deeper in the sedimentary section, anaerobic methane oxidation and methanogenesis occur to further affect the concentration and isotopic composition of dissolved CO₂ (CO₂ or dissolved inorganic carbon) and methane.

Anaerobic Methane Oxidation

At the base of the sulfate reduction zone, microbially mediated anaerobic methane oxidation (AMO),

CH₄ + SO₄^2- HCO₃^- + HS^- + H₂O,

acting near the sulfate-methane interface (SMI), causes both sulfate and methane depletion (Reeburgh, 1976). Deep-water marine sediments on continental margins often show linear sulfate profiles, suggestive of large amounts of sulfate consumption at the SMI caused by AMO (Borowski et al., 1996). Borowski et al. (2000) estimate that at least 35% of interstitial sulfate is consumed by AMO in such settings, highlighting a significant role for AMO in deep-water diagenetic systems.

Carbon Cycling at the SMI

Carbon cycling is a demonstrably important process at the sulfate-methane interface and within the uppermost methanogenic zone of marine, deep-water continental margins. AMO strongly affects the carbon isotopic composition of the CO₂ and methane pools (Borowski et al., 1997) and also induces authigenic formation of carbonate minerals (e.g., Rodriguez et al., 2000). Borowski et al. (1997) showed that AMO, operating over long time spans in an open diagenetic system, induces extreme ¹³C depletions in both the CO₂ and methane pools. For example, at the Blake Ridge, pore water CO₂ can be as depleted in ¹³C as -38 PDB (Ocean Drilling Program [ODP] Site 995, Borowski et al., 2000), signaling a large contribution of methane carbon to the CO₂ pool. Complementary ¹³C depletions in the methane pool can be as large as -103 near the SMI (ODP Site 995, Hoehler et al., 2000). These large ¹³C depletions near the SMI ripple downward into the underlying sedimentary section. ¹³C depletions within CO₂ and methane occur within the upper methanogenic zone as a consequence of downward diffusion, and because ¹³C-depleted CO₂ is used as a substrate in methanogenesis (Claypool and Kaplan, 1974; Borowski et al., 1997; Paull et al., 2000).

Methanogenesis

Once pore-water sulfate nears depletion in marine sediments, microbially mediated methane production is favored so that dissolved methane concentrations rise in pore waters (Martens and Berner, 1974). Geochemical data from the Blake Ridge, gathered from Deep Sea Drilling Project (DSDP) and ODP drilling, show a connection between the CO₂ and methane pools as methanogenesis occurs (e.g., Claypool and Kaplan, 1974; Claypool and Threlkeld, 1983; Galimov and Kvenvolden, 1983). CO₂ reduction,

CO₂ + 4H₂ CH₄ + H₂O,

is probably the principal mechanism for biotic methane formation in marine sediments (Whiticar et al., 1986), and the process links these two carbon pools within the methanogenic zone. The isotopic composition of interstitial methane and CO₂ evolve in concert as ¹²C is preferentially utilized from the CO₂ pool for methane formation, causing the CO₂ pool to become depleted in ¹²C (yielding more positive, or heavy, ¹³C values), whereas the methane pool is immediately enriched in ¹²C (yielding very negative, or light, ¹³C values), but progressively accumulates more ¹³C with depth. However with increasing depth, the isotopic composition of CO₂ and methane begin to diverge with CO₂ becoming more enriched in ¹²C, and methane showing little change in its ¹³C values (e.g., ODP Sites 994, 995, and 997, Borowski et al., 2000; Paull et al., 2000). In other words, the two carbon pools seem to decouple, suggesting a drop in the rate of CO₂ reduction, even though sediments are warming along a geothermal gradient (Paull et al., 2000).

Methane and CO₂ Advection

It is likely that a significant proportion of the interstitial methane in the deeper sedimentary section is derived not from in situ production but from methane and/or pore-water transport (Paull et al., 2000). Paull et al. (2000) suggest that upward transport of methane and CO₂ affects the carbon isotopic composition of both pools at the Blake Ridge. A model that uses a typical fractionation factor for CO₂ reduction (CO₂ CH₄), that progressively decreases microbial methane production with depth, and that adds methane and CO₂ to the lower sedimentary section from deeper below, mimicked typical isotopic profiles seen at the Blake Ridge.

Blake Ridge

The Blake Ridge region has long been a favored area for geochemical investigations because of its relative geologic simplicity and the presence of gas hydrates (DSDP Leg 11, Sites 102, 103, and 104, Hollister, Ewing, et al., 1972; DSDP Leg 76, Site 533, Sheridan, Gradstein, et al., 1983; and ODP Leg 164, Sites 994, 995, and 997, Paull, Matsumoto, Wallace, et al., 1996) (Fig. F1). ODP Leg 172 continued investigations on the Blake Ridge (Sites 1054-1061) and included sites at the Bahama Ridge (Site 1062) and at the northeast Bermuda Rise (Site 1063) (Keigwin, Rio, Acton, et al., 1998).