A review of methane and gas hydrates in the dynamic, stratified system of the Blake Ridge region, offshore southeastern North America

Abstract

The Blake Ridge region lies on the passive margin of southeastern North America and contains a large amount of methane gas hydrate. The methane and methane gas hydrate are predominantly biogenic, apparently produced by CO 2 reduction. Reflection seismics indicate that bottom-simulating reflectors (BSRs) enclose ∼55,000 km 2, with high-amplitude BSRs covering ∼26,000 km 2. Ocean Drilling Program (ODP) Leg 164 drilled three deep holes on a 10-km-long transect (Sites 994, 995, and 997; water depth 2770–2798 m). Based on sampling and geochemical, thermal, seismic, and borehole geophysical measurements, gas hydrates are most likely present between ∼190 and 450 m in sediment column. Gas hydrate is most often disseminated throughout the sediment column, although concentrations occur within specific sedimentary horizons, within supposed fault zones, and at the base of gas hydrate stability (BGHS) where methane recycling produces more pervasive concentrations of gas hydrate. Estimates of gas hydrate inventory are based on a variety of methods including geochemical proxies, vertical seismic profiling, electric logging, and measurements of in situ methane. Over the entire sediment column, at least ∼2–4% of pore space volume (1–2% sediment volume) is occupied by methane gas hydrates, but average and maximum estimates are 5.4% and 12%, respectively. Extrapolation of vertical gas hydrate and methane inventory over the area containing high-amplitude BSRs yields estimates of 67–406 Gt (gigatons, 10 15 g) of methane gas hydrate (or 9–52 Gt of methane) and 2.6–27 Gt of methane occurring as gas bubbles below the BGHS. Average values are 185 Gt of gas hydrate and 24 Gt of methane as gas hydrate. Any gas hydrate occurring outside the area underlain by BSRs (as suggested by geochemical evidence) or that associated with low-amplitude BSRs may increase these estimates by an unknown factor. Various data give conflicting pictures of mass transport with Blake Ridge sediments. The data can be reconciled by viewing the upper sedimentary section (<∼150 m) as dominated by diffusion, and the lower section characterized by buoyant advection (migration) of gaseous methane with both modes of transport overprinting generally low rates of pore-fluid movement (∼20 cm kyear −1). Methane migration seems necessary to produce observed gas hydrate distribution and inventory estimates. Accumulation of gas hydrate in the Blake Ridge sediments depends on the amount of methane leaving the system versus the amount of methane entering the gas hydrate stability zone (GHSZ) over geologic time. Although there are some point sources of methane loss from the sediments (e.g., seafloor seeps, ODP Site 996) of unknown magnitude, most of the documented methane loss occurs through diffusion and consumption at the sulfate–methane interface (SMI) by anaerobic methane oxidation (AMO; ∼2×10 8 mol year −1). Methane entering the GHSZ at a rate of ∼1.3×10 9 mol year −1 indicates a methane-trapping efficiency of ∼85%. 129I measurements suggest that the Blake Ridge system has accumulated gas hydrate over as much as 55 million years.