DISCUSSION

Anaerobic methane oxidation is a key process that controls the isotopic composition of CO₂ and methane near the sulfate-methane interface. Cycling between these two carbon pools within an open system causes large ¹³C depletions (Borowski et al., 1997). Because CO₂ is so depleted in ¹³C as it enters shallow methanogenic zone, when it serves as the substrate for CO₂ reduction the resultant methane is also strongly depleted in ¹³C.

CO₂ reduction is the dominant methane-forming process within the shallow methanogenic zone of many marine deep-water localities (Whiticar et al., 1986), and the Blake Ridge region is no exception (e.g., Claypool and Kaplan, 1974; Claypool and Threlkeld, 1983; Galimov and Kvenvolden, 1983). Like other methane-rich regions, the carbon isotopic composition of each pool changes concomitantly with depth as ¹²C is preferentially sequestered in methane. However, at some sites (1061 and 1063) the carbon isotopic composition of the pools begins to diverge with increasing depth as the CO₂ pool becomes progressively enriched with ¹²C.

Paull et al. (2000) suggest that lower rates of microbial methane production coupled with delivery of CO₂ and methane from below combine to cause this divergence in isotopic trends. Advection perhaps occurs at Leg 172 sites as well, although its effects are usually not observed, probably because of inadequate hole depths. At Sites 1061 and 1063 (the deepest holes), ¹³C CO values become slowly but progressively lighter with increasing depth, moving toward typical ¹³C values of Blake Ridge sedimentary organic matter (-21; Borowski et al., 1998).

Data from ODP Leg 172 sites at the Blake-Bahama Ridge and Bermuda Rise are consistent with carbon isotopic data for CO₂ and methane at other Blake Ridge sites (i.e., Sites 533, 994, 995, and 997; see references above) and at other continental margin sites worldwide (Borowski et al., 1997). The processes that create these isotopic trends are pervasive within the world's continental-rise sediments, although many factors combine to control the absolute carbon isotopic composition of CO₂ and methane.