Abstract
We test a hypothesis relating large pore water sulfate gradients to upward methane flux and the presence of underlying methane gas hydrate on continental
rises by examining: (1) pore water geochemical data available from the global data set of Deep Sea Drilling Project–Ocean Drilling Program (DSDP–ODP)
sites; (2) sulfate data from 51 coring sites located at the Carolina Rise and Blake Ridge (offshore southeastern United States); and (3) the relationship between
the distribution of bottom-simulating reflectors (BSRs) and sulfate depletion patterns at the Carolina Rise–Blake Ridge (CR–BR) area. Within continental
rise sediments, large sulfate gradients are correlative with marine methane gas hydrate settings (recognized by gas hydrate recovery and the presence of BSRs).
This correlation is in part due to the rapid consumption of sedimentary organic matter by sulfate reduction and early microbial production of methane during
burial and early diagenesis. However, detailed interstitial geochemical evidence from sediments of the CR–BR area strongly suggests that sulfate and methane
co-consumption (anaerobic methane oxidation) at the sulfate–methane interface (SMI) is an important additional process in depleting interstitial sulfate,
producing steep (and perhaps linear) sulfate gradients and shallow depths to the SMI. The presence of BSRs is currently the only routine technique used to
identify gas hydrate localities. However, BSRs seem to represent an abrupt interface at the base of gas hydrate stability (BGHS) where methane gas bubbles
occur, rather than being a direct indicator of gas hydrates in overlying sediments. Detailed comparisons between BSR distribution and geochemical data at the
CR–BR show that BSRs are patchy in their occurrence, consistent with BSRs representing accumulations of free methane gas that pool within structural and
stratigraphic traps near the crest and on the flanks of the Blake Ridge. In contrast, steep sulfate gradients (and proxy indicators of gas hydrate) are pervasive
components of the CR–BR area, suggesting that steep sulfate gradients may be a better general indicator of gas hydrate potential. Steep sulfate gradients
apparently identify large upward fluxes of methane, indicating conditions conducive to the formation of gas hydrates, given favorable pressure and temperature
conditions. Global DSDP–ODP geochemical data identify many additional deep-water marine sites with large sulfate gradients that lack BSRs, perhaps
suggesting the occurrence of previously unrecognized gas hydrate localities.
Reprinted with permission from Elsevier.
Abstract
We analysed the bottom simulating reflector (BSR) of the Blake Ridge, by using the 'multi-attribute' analysis. This technique allows prediction of
petrophysical/geological parameters along seismic lines (velocity, porosity, density, resistivity etc.) starting from log data. Seismic attributes can be thought of as all
parameters derived from seismics, by assuming that a seismic trace is the real component of a complex one. The first attributes that can be calculated are the
instantaneous amplitude, the instantaneous phase and the instantaneous frequency. All the other attributes that will be considered are derived from the previous ones.
The algorithm that allows the correlation between logs and seismics is the generalised multiple linear regression. This equation is solved by a least squares
approach, called 'training phase', which effectively consists of training the seismics to predict the reservoir parameter of interest at the tie locations. The resulting
function is then applied to the seismic profile, generating a target log predicted section. We obtained five sections (VSP and P-wave velocity, density, porosity and
resistivity), each of them representing the distribution of the corresponding property along the profile; they permitted better characterisation of the nature of the BSR
present in the Blake Ridge area. The obtained-velocity section was finally translated in terms of gas hydrate and free gas distribution by using theoretical approach.
Reprinted with permission from Elsevier.
Abstract
Ocean Drilling Program (ODP) Sites 994, 995, and 997 were drilled into a large gas hydrate
deposit on the crest of the Blake Ridge (southeast U.S. margin) where upward CH4 fluxes
(FOut) are related to depths of pore water SO42- depletion. High-resolution pore water SO42-
and sediment Ba profiles have been constructed at these sites to assess present and past FOut.
Pore water SO42- profiles are linear with zero SO42- concentration occurring at 21.4, 21.6,
and 22.8 mbsf at holes 994A, 995A and 997A, respectively. Using steady state solutions to
diffusion equations with appropriate parameters, the steep SO42- gradients support upward
CH4 fluxes between 7.2 and 8.6 mol/m2ky at present-day, with the range primarily reflecting
different approaches for incorporating porosity (phi). Taking into account the generally
decreasing phi with depth and the high clay content of the sediment, the best estimates for FOut
are 7.9, 7.6 and 7.2 mol CH4/m2ky at sites 994, 995 and 997, respectively. However,
non-steady state solutions to diffusion equations show that the SO42- gradients do not imply
steady state conditions. Elevated Ba concentrations (530-1410 ppm) exist in sediment between
18.23 and 20.65, between 17.31 and 20.31, between 19.40 and 21.80, and between 19.58 and
21.91 mbsf at holes 994A, 994C, 995A, and 997A, respectively. These Ba fronts coincide with
highs in bulk sediment Ba/Al (to 2.5 x 102) and are caused by Ba cycling just above time
averaged depths of SO42- depletion. Because the Ba fronts lie immediately above the
present-day depths of pore water SO42- depletion, and because no other Ba fronts are found in
the upper 25 m at the three sites, the depth of SO42- depletion beneath the seafloor has been
nearly constant for considerable time (>18,000 years). Thus, CH4 fluxes can be determined
through SO42- gradients and steady state solutions to diffusion equations. More importantly,
FOut through the crest of the Blake Ridge has not varied significantly across major changes in sea level and hydrostatic pressure.
Reprinted with permission from Elsevier Science and ScienceDirect.
Egeberg, P.K., and Barth, T., 1998. Contribution of dissolved organic species to the carbon and energy budgets of hydrate bearing deep sea sediments, ODP Site 997 Blake Ridge. Chem. Geol., 149:25-35.
Abstract
Pore water extracted from sediments penetrated on Leg 164 of the Ocean Drilling Program at the Blake Ridge (West Atlantic) were analysed for acetate, total dissolved organic carbon, bromide and iodide, to help explain the occurrence of subsurface maxima in bacteria biomass and activity reported previously from a nearby site. The high concentrations of these organic matter decomposition by-products in the pore waters from sediments with moderate concentrations of sedimentary organic matter can convincingly be modelled as resulting from upward migration of pore water. The amount of acetate and unidentified DOC transported by the pore water contribute significantly to the pool of metabolizable carbon, and may be the most important substances in energetic terms.
Reprinted with permission from Elsevier Science, Chemical Geology.
Egeberg, P.K., and Dickens, G.R., 1999. Thermodynamic and pore water halogen constraints on gas hydrate distribution at ODP Site 997 (Blake Ridge). Chem. Geol., 153:53-79.
Abstract
Marine sediment sequences with CH4 hydrate are characterized by an atypical depth profile in dissolved Cl- squeezed from pore space: a shallow subsurface Cl- maximum overlies a lengthy and pronounced Cl- minimum. This pore water Cl- profile represents a combination of multiple processes including glacial–interglacial variations in ocean salinity, advection and diffusion of ions that are excluded during gas hydrate formation at depth, and release of fresh water from dissociation of hydrate during core recovery. In situ quantities of gas hydrate can be determined from a measured pore water Cl- profile provided the in situ pore water signature prior to core recovery can be separated. Ocean Drilling Program (ODP) Site 997 was drilled into a large CH4 hydrate reservoir on the Blake Ridge in the western Atlantic Ocean. Previously we have constructed a high-resolution pore water Cl- profile at this location; here we present a "coupled chloride-hydrate" numerical model to explain basic trends in the Cl- profile and to isolate in situ Cl- concentrations. The model is based on thermodynamic and ecological considerations, and uses established equations for describing chemical behavior in marine sediment–pore water systems. The model incorporates four key concepts: (1) most gas hydrate is formed immediately below the SO42- reduction zone; (2) fluid, dissolved ions and gas advect upward through the sediment column; (3) CH4 hydrate dissociates at the base of hydrate stability conditions; and (4) seawater salinity fluctuates during glacial–interglacial cycles of the late Pliocene and Quaternary. Rates of upward advection in the model are sufficient to account for measured Br- and I- concentrations as well as CH4 oxidation at the base of the SO42- reduction zone. In situ pore water Cl- inferred from the model is similar to that determined by limited direct sampling; in situ CH4 hydrate amounts inferred from the model (an average of about 4% of porosity) are broadly consistent with those determined by direct gas sampling and indirect geophysical techniques. The model also predicts production of substantial quantities of free CH4 gas bubbles (>2.5% of porosity) at a depth immediately below the lowest accumulation of CH4 hydrate in the sediment column. Our explanation for the pore water Cl- profile at Site 997 is important because it provides a theoretical mechanism for understanding the distribution of interstitial water Cl-, gas hydrate, and free gas in a marine sediment column.
Reprinted with permission from Elsevier Science, Chemical Geology.
Flemings, P.B., Liu, X., and Winters, W.J., 2003. Critical pressure and multiphase flow in Blake Ridge gas hydrates. Geology, 31:1057-1060.
Abstract
We use core porosity, consolidation experiments, pressure core sampler data, and capillary pressure measurements to predict water pressures that are 70% of the lithostatic stress, and gas pressures that equal the lithostatic stress beneath the methane hydrate layer at Ocean Drilling Program Site 997, Blake Ridge, offshore North Carolina. A 29-m-thick interconnected free-gas column is trapped beneath the low-permeability hydrate layer. We propose that lithostatic gas pressure is dilating fractures and gas is migrating through the methane hydrate layer. Overpressured gas and water within methane hydrate reservoirs limit the amount of free gas trapped and may rapidly export methane to the seafloor.
Reprinted with permission from the Geological Society of America.
Holbrook, W.S., Hoskins, H., Wood, W.T., Stephen, R.A., Lizarralde, D. and the Leg 164 Science Party, 1996. Methane hydrate and free gas on the Blake Ridge from vertical seismic profiling. Science, 273:1840-1843.
Abstract
Seismic velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that substantial free gas exists to at
least 250 meters beneath the bottom-simulating reflection (BSR). Both methane hydrate and free gas exist even where a clear BSR is absent. The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity
of the sediments rather than by hydrate cementation. The average methane hydrate saturation above the BSR is
relatively low (5 to 7 percent of porosity), which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3.
Reprinted with permission* from Science.
Holbrook, W.S., Lizarralde, D., Pecher, I.A., Gorman, A.R., Hackwith, K.L., Hornbach, M., and Saffer, D., 2002. Escape of methane gas through sediment waves in a large methane hydrate province. Geology, 30:467-470.
Abstract
Despite paleoceanographic evidence that large quantities of methane have escaped from marine gas hydrates into the oceans, the sites and mechanisms of methane release remain largely speculative. New seismic data from the Blake Ridge, a hydrate-bearing drift deposit in the western Atlantic, show clear evidence for methane release and suggest a new mechanism by which methane gas can escape, without thermal or mechanical disruption of the hydrate-bearing layer. Rapid, post-2.5 Ma formation of large sediment waves and associated seafloor erosion created permeable pathways connecting free gas to the seafloor, allowing methane gas expulsion. The amount of missing methane, 0.6 Gt, is equivalent to ~12% of total present-day atmospheric methane. Our results imply that significant amounts of methane gas can bypass the hydrate stability zone and escape into the ocean. Mechanisms of tapping methane directly from the free-gas zone, such as widespread seafloor erosion, should be considered when seeking the causes of large negative carbon isotope excursions in the geological record.
Reprinted with permission from the Geological Society of America.
Ruppel, C., 1997. Anomalously cold temperatures observed at the base of the gas hydrate stability zone on the U.S. Atlantic passive margin. Geology, 25:699-702.
Abstract
In situ measurements to depths of ~415 m below sea floor in methane
hydrate-bearing sediments on the U.S. Atlantic passive margin indicate
that temperatures at the bottom simulating reflector (BSR) are
anomalously low by 0.5-2.9 ¯C if the BSR marks the base of gas hydrate
stability (BGHS). Several hypotheses may explain the occurrence of
the BSR at inappropriate pressure-temperature (P-T) conditions. (1)
If the BSR does not mark the BGHS, then P-T conditions need not be
sufficient to dissociate gas hydrate at this depth. (2) The BSR may
lie at nonequilibrium P-T conditions due to incomplete readjustment in
response to upper Pleistocene-Holocene climate change. However, the
occurrence of the Blake Ridge BSR at an overly shallow depth cannot be
easily explained by realistic combinations of pressure-driven
deepening (sea-level rise) and temperature-driven shoaling (bottom
water temperature changes). (3) The properties of sediments or pore
fluids may inhibit the stability of gas hydrate. In particular,
capillary forces arising in the fine-grained, montmorillonite-rich
sediments of the Blake Ridge may lead to shoaling of the BSR in this
setting.
Reprinted with permission from the Geological Society of America.
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