During Ocean Drilling Program (ODP) Leg 201, gas hydrates were recovered at Site 1230 on the Peru... more During Ocean Drilling Program (ODP) Leg 201, gas hydrates were recovered at Site 1230 on the Peruvian outer continental margin. Previous work at ODP Site 685, which is located in close proximity to Site 1230, suggested that gas hydrates occurred below 40 meter below sea floor (mbsf) in this region, but were concentrated in two intervals at about 107 and 165 mbsf. A combination of infrared thermal imaging data, observations of core disturbance patterns, high-resolution profiles of interstitial water chemistry, physical properties, and wireline logging data collected at Site 1230 supplements and expands these findings. Distinct horizons of gas hydrate at Site 1230 appear to be offset from those of Site 685 by approximately 20 m. This offset may be caused by a concentration of gas hydrate along oblique fault planes. One of the gas hydrate pieces recovered at Site 1230 was composed of several-millimeter thick layers oriented at a high angle to the bedding surfaces. We suspect that this ...
Numerous Gulf of Mexico hydrocarbon seeps are well documented through studies of chemosynthetic c... more Numerous Gulf of Mexico hydrocarbon seeps are well documented through studies of chemosynthetic communities, seafloor deposits, and gas hydrates associated with seeps. Furthermore, sea surface oil slicks can be identified on satellite images, and subsurface geology can be determined through geophysical recordings. Initial reviews of satellite images indicate that certain areas with recurring surface slicks are associated with known seafloor chemosynthetic communities. However some areas with documented seafloor chemosynthetic communities produce no sea surface hydrocarbon slicks. The relationship of sea surface slicks to underlying seafloor seeps is influenced by several geologic characteristics and these characteristics may determine which seeps develop into surface slicks. Site specific studies have investigated the relationship of hydrocarbon seeps to seismic character, carbonate character, geothermal patterns, source and pathway, chemosynthetic communities, mineralogy and geoche...
A high-resolution elemental, isotopic, and molecular record of or-ganic matter was established fo... more A high-resolution elemental, isotopic, and molecular record of or-ganic matter was established for the uppermost sediments of Hole 1229E (0–700 cm below seafloor), drilled during Ocean Drilling Pro-gram (ODP) Leg 201 on the Peruvian margin. Diagnostic parameters of organic matter (OM) such as the atomic C org over total nitrogen ratio (C/N), the organic carbon isotopic signatures (δ 13 C org), and lignin-derived biomarker concentrations (Σ8) all suggest that terrigenous OM influx was more important during glacial than interglacial periods on the Peruvian shelf. In contrast to other shelf environments, however, fluctuations in lignin ratios ([Ad/Al]v, S/V, and C/V) across the Pleis-tocene–Holocene transition point to shifting composition in terrige-nous OM sources from coarse and undegraded woody materials during the Last Glacial Maximum (LGM) to much more altered and finer soft tissue materials during the Holocene. In addition, the total OM content preserved in the Peruvian shelf se...
ABSTRACT During October and November 2003, the German ship R/V Sonne visited the Campeche Escarpm... more ABSTRACT During October and November 2003, the German ship R/V Sonne visited the Campeche Escarpment in the southern Gulf of Mexico. The primary objectives of the cruise were the following: 1) extend our knowledge of gas hydrates buried just below the seafloor in the deep ocean; 2) determine whether natural hydrocarbon seeps in the southern Gulf of Mexico support chemosynthetic fauna, and 3) pinpoint sites for future exploration with deep-diving submersibles. Recent findings indicate that marine deposits of gas hydrate represent an important contribution to the global carbon cycle. Methane released from the gas hydrate reservoir may interact with other components of the Earth system influencing climate and is a potential source of energy. Although most gas hydrate is buried under hundreds of meters of marine sediment, the Gulf of Mexico is one place where it exists at or near the seafloor, where it can be sampled and studied. Gas hydrate is one component of the natural hydrocarbon system of the Gulf of Mexico; other components - known primarily from the northern Gulf - include oil seeps that produce natural oil "spills" visible from space and lush communities of chemosynthetic tubeworms and mussels. Natural oil seeps also occur in the southern Gulf of Mexico on the Campeche Escarpment and were expected to harbor chemosynthetic fauna and possibly gas hydrates. Recently, satellite remote sensing has been successfully used to pinpoint active seeps based on the unusual synthetic aperture radar signature generated by oil slicks on the sea surface in the northern Gulf of Mexico. Using a combination of satellite image analyses, acoustic profiling techniques, detailed bathymetric mapping, and interpretation of seafloor reflectivity data, we were able to identify and study such areas of active seafloor seepage of hydrocarbons in the previously unexplored southern Gulf of Mexico.
ABSTRACT The DOE/NETL funded HyFlux project combines sea-air, water column, and subsurface observ... more ABSTRACT The DOE/NETL funded HyFlux project combines sea-air, water column, and subsurface observations and measurements to better quantify the amount of methane released from seeps and gas hydrate deposits in the Gulf of Mexico. A recently completed research cruise investigated Gulf of Mexico seeps in 540, 900, and 1200 m water depths. Piston, gravity, and push cores were collected for pore water and sediment analysis. Analyses of major pore water constituents were conducted using an ICP-OES. Preliminary pore water analyses included SO42-, alkalinity, HS-, and Ca2+. Cores taken at Mississippi Canyon site MC 118 showed sulfate concentrations decreasing from 25.2 mM at 1 cmbsf to 1.91 mM at 36.5 cmbsf. HS- measurements show a decrease from 3.02 mM at 1.5 cmbsf to 0 mM at 12 cmbsf, followed by an increase in HS- due to SO42- reduction in 12 cmbsf. Alkalinity concentrations show an increase with depth from 3.63 mM at 1.5 cmbsf to 27.8 mM at 36.5 cmbsf. Calcium concentrations decrease from 10.55 mM at 1.5 cmbsf to 4.87 mM at 36.5 cmbsf due to the precipitation of calcium carbonate. Data from a 10 cm-long push core taken at site MC 118 showed the same trends in SO42- reduction, a decrease in Ca2+ concentrations, along with an alkalinity increase and HS- production. Two piston cores were taken at site GC (Green Canyon) 600. The first core showed SO42- concentrations decreasing from 21.34 mM at 0 cmbsf to 0 mM at 20 cmbsf. HS- concentrations increased to 32.11 mM 20 cmbsf, then decreased to 3.05 mM at 282.5 cmbsf. Alkalinity and Ca2+ concentrations both increased with depth. Alkalinity concentrations were 10.27 mM at 1.5 cmbsf and increased to 35.59 mM at 282.5 cmbsf. Ca2+ also increased from 8.96 mM at 1.5 cmbsf to 12.27 mM at 282.5 cmbsf, which may be the result of carbonate dissolution at depth. The second piston core showed a more typical concentration profile for seep areas. The sulfate concentration at 3 cmbsf decreased from 21.93 mM to 3 mM at 44.5 cmbsf, coupled with the typical inverse relationship with HS-, which increased from 1.04 mM at 3 cmbsf to 27.26 mM at of 44.5 cmbsf. Alkalinity concentrations yielded a steady increase from 5.34 mM at 3 cmbsf to 26.44 mM at 44.5 cmbsf and Ca2+ depletion was observed in the same depth range from 8.28 mM to 4.80 mM. One piston core was taken at site GC 185 (Bush Hill). Sulfate concentrations did not vary much with depth and ranged from 22.99 mM at 3 cmbsf to 23.56 mM at 44.5 cmbsf. HS- concentrations similarly showed small changes with an initial increase from 1.91 mM at 3 cmbsf to a concentration of 3.45 mM at 44.5 cmbsf. Alkalinity concentrations at 3 cmbsf were 4.26 mM and increased slightly to 4.47 mM at 44.5 cmbsf. Similarly, Ca2+ concentrations varied only slightly throughout the sampled interval. Results from pore water analysis in combination with air-sea measurements of methane and water column data will allow for a more accurate model for methane flux from seeps and gas hydrate deposits to the water column and atmosphere.
ABSTRACT In the northern Gulf of Mexico (GOM), numerous seafloor hydrocarbon seeps are well docum... more ABSTRACT In the northern Gulf of Mexico (GOM), numerous seafloor hydrocarbon seeps are well documented through seismic, seafloor, and chemical studies. In some areas, the seafloor seeps expend sufficient hydrocarbons to pass through the water column and present as sea-surface oil slicks, which with careful review and certain conditions can be identified on satellite images. Some areas with recurring potential sea-surface oil slicks as identified by oil-criteria screened satellite images are located above or near proven subsurface hydrocarbon-producing areas. However, other areas with established production have no sea-surface hydrocarbon slicks, and several areas with potential sea-surface slicks currently have no subsurface production. A publicly available data set of reservoir information for the northern GOM was correlated to a data set of sea-surface slick sites as identified on satellite data. Thirteen variables were selected from the database to evaluate depth relationships, fluid properties, and reservoir properties in relation to slick presence. Because initial multivariate analysis indicated the slick-to-reservoir-variables relationships were nonlinear, a random forest (RF) classification-tree analysis was performed to identify which variables are more important for slick development. The RF analysis used a collection of binary decision trees to determine the relative importance of the reservoir variables in identifying sea-surface slicks. The RF model was able to correctly classify or predict the presence of a slick and identified water depth, gas-oil ratio, and reservoir chronozone as the three most important variables when predicting the formation of a sea-surface slick.
... 1. Introduction. Zeolites are among the most common authigenic silicate minerals in marine se... more ... 1. Introduction. Zeolites are among the most common authigenic silicate minerals in marine sediments, consisting of a hydrated framework of aluminosilicates with varying amounts of alkali and alkaline earth metals (Kastner and Stonecipher, 1978; Stonecipher, 1978). ...
The recently funded DOE/NETL study "HyFlux: Remote sensing and sea-truth mea... more The recently funded DOE/NETL study "HyFlux: Remote sensing and sea-truth measurements of methane flux to the atmosphere" (see MacDonald et al.: HyFlux - Part I) will combine sea surface, water column and shallow subsurface observations to improve our estimates of methane flux from submarine seeps and associated gas hydrate deposits to the water column and atmosphere along the Gulf of
HyFlux - Part I: Regional modeling of methane flux from near-seafloor gas hydrate deposits on con... more HyFlux - Part I: Regional modeling of methane flux from near-seafloor gas hydrate deposits on continental margins MacDonald, I.R., Asper, V., Garcia, O., Kastner, M., Leifer, I., Naehr, T.H., Solomon, E., Yvon-Lewis, S., and Zimmer, B. The Dept. of Energy National Energy Technology Laboratory (DOE/NETL) has recently awarded a project entitled HyFlux: "Remote sensing and sea-truth measurements of methane flux to the atmosphere." The project will address this problem with a combined effort of satellite remote sensing and data collection at proven sites in the Gulf of Mexico where gas hydrate releases gas to the water column. Submarine gas hydrate is a large pool of greenhouse gas that may interact with the atmosphere over geologic time to affect climate cycles. In the near term, the magnitude of methane reaching the atmosphere from gas hydrate on continental margins is poorly known because 1) gas hydrate is exposed to metastable oceanic conditions in shallow, dispersed deposits that are poorly imaged by standard geophysical techniques and 2) the consumption of methane in marine sediments and in the water column is subject to uncertainty. The northern GOM is a prolific hydrocarbon province where rapid migration of oil, gases, and brines from deep subsurface petroleum reservoirs occurs through faults generated by salt tectonics. Focused expulsion of hydrocarbons is manifested at the seafloor by gas vents, gas hydrates, oil seeps, chemosynthetic biological communities, and mud volcanoes. Where hydrocarbon seeps occur in depths below the hydrate stability zone (~500m), rapid flux of gas will feed shallow deposits of gas hydrate that potentially interact with water column temperature changes; oil released from seeps forms sea-surface features that can be detected in remote-sensing images. The regional phase of the project will quantify verifiable sources of methane (and oil) the Gulf of Mexico continental margin and selected margins (e.g. Pakistan Margin, South China Sea, and West Africa Margin) world-wide by using the substantial archive of satellite synthetic aperture radar (SAR) images. An automated system for satellite image interpretation will make it possible to process hundreds of SAR images to increase the geographic and temporal coverage. Field programs will quantify the flux and fate of hydrate methane in sediments and the water column.
ABSTRACT Authigenic carbonates were recovered from several horizons between 0 and 52 mbsf in sedi... more ABSTRACT Authigenic carbonates were recovered from several horizons between 0 and 52 mbsf in sediments that overlay the Blake Ridge Diapir on the Carolina Rise (Ocean Drilling Program [ODP] Site 996). Active chemosynthetic communities at this site are apparently fed by fluid conduits extending beneath a bottom-simulating reflector (BSR). Gas hydrates occur at several depth intervals in these near-surface sediments. The carbonate nodules are composed of rounded to subangular intraclasts and carbonate cemented mussel shell fragments. Electron microprobe and X-ray diffraction (XRD) investigations show that aragonite is the dominant authigenic carbonate. Authigenic aragonite occurs both as microcrystalline, interstitial cement, and as cavity-filling radial fibrous crystals. The δ13C values of the authigenic aragonite vary between -48.4‰ and -30.5‰ (Peedee belemnite [PDB]), indicating that carbon derived from 13C-depleted methane is incorporated into these carbonates. The δ13C of pore water ΣCO2 values are most negative in the upper 10 mbsf, near the sediment/water interface (-38‰ ± 5‰), but noticeably more positive below 25 mbsf (+5‰ ± 6‰). Because carbonates derive their carbon from HCO3-, dissimilarities between the δ13C values of carbonate precipitates recovered from greater than 10 mbsf and δ13C values of the associated pore fluids suggests that these carbonates formed near the seafloor. Differences of about 1‰ in the oxygen isotopic composition of carbonate precipitates from different depths are possibly related to changes in bottom-water conditions during glacial and interglacial time periods. Measurements of the strontium isotopic composition on 13 carbonate samples show 87Sr/86Sr values between 0.709125 and 0.709206 with a mean of 0.709165, consistent with the approximate age of their host sediment. Furthermore, the 87Sr/86Sr values of six pore-water samples from Site 996 vary between 0.709130 and 0.709204. The similarity of these values to seawater (87Sr/86Sr = 0.709175), and to 87Sr/86Sr values of pore water from similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997), indicates a shallow Sr source. The 87Sr/86Sr values of the authigenic carbonates at Site 996 are not consistent with the Sr isotopic values predicted for carbonates precipitated from fluids transported upward along fault conduits extending through the base of the gas hydrate-stability zone. Based on our data, we see no evidence of continuing carbonate diagenesis with depth. Therefore, with the exception of their seafloor expression as carbonate crusts, fossil vent sites will not be preserved. Because these authigenic features apparently form only at the seafloor, their vertical distribution and sediment age imply that seepage has been going on in this area for at least 600,000 yr.
Anaerobic oxidation of methane (AOM) impacts carbon cycling by acting as a methane sink and by se... more Anaerobic oxidation of methane (AOM) impacts carbon cycling by acting as a methane sink and by sequestering inorganic carbon via AOM-induced carbonate precipitation. These precipitates commonly take the form of carbonate nodules that form within methane seep sediments. The timing and sequence of nodule formation within methane seep sediments are not well understood. Further, the microbial diversity associated with sediment-hosted nodules has not been well characterized and the degree to which nodules reflect the microbial assemblage in surrounding sediments is unknown. Here, we conducted a comparative study of microbial assemblages in methane-derived authigenic carbonate nodules and their host sediments using molecular, mineralogical, and geochemical methods. Analysis of 16S rRNA gene diversity from paired carbonate nodules and sediments revealed that both sample types contained methanotrophic archaea (ANME-1 and ANME-2) and syntrophic sulfate-reducing bacteria (Desulfobacteraceae and Desulfobulbaceae), as well as other microbial community members. The combination of geochemical and molecular data from Eel River Basin and Hydrate Ridge suggested that some nodules formed in situ and captured the local sediment-hosted microbial community, while other nodules may have been translocated or may represent a record of conditions prior to the contemporary environment. Taken together, this comparative analysis offers clues to the formation regimes and mechanisms of sediment-hosted carbonate nodules.
The Campeche Knolls region of the southern Gulf of Mexico contains numerous diapiric mounds and r... more The Campeche Knolls region of the southern Gulf of Mexico contains numerous diapiric mounds and ridges that are associated with persistent oil slicks and extensive flows of solidified asphalt. Previous investigations1 have documented chemosynthetic communities, gas hydrates, and bubble streams that rise hundreds of meters into the water column. However, only a few of the potential features had previously been
During Ocean Drilling Program (ODP) Leg 201, gas hydrates were recovered at Site 1230 on the Peru... more During Ocean Drilling Program (ODP) Leg 201, gas hydrates were recovered at Site 1230 on the Peruvian outer continental margin. Previous work at ODP Site 685, which is located in close proximity to Site 1230, suggested that gas hydrates occurred below 40 meter below sea floor (mbsf) in this region, but were concentrated in two intervals at about 107 and 165 mbsf. A combination of infrared thermal imaging data, observations of core disturbance patterns, high-resolution profiles of interstitial water chemistry, physical properties, and wireline logging data collected at Site 1230 supplements and expands these findings. Distinct horizons of gas hydrate at Site 1230 appear to be offset from those of Site 685 by approximately 20 m. This offset may be caused by a concentration of gas hydrate along oblique fault planes. One of the gas hydrate pieces recovered at Site 1230 was composed of several-millimeter thick layers oriented at a high angle to the bedding surfaces. We suspect that this ...
Numerous Gulf of Mexico hydrocarbon seeps are well documented through studies of chemosynthetic c... more Numerous Gulf of Mexico hydrocarbon seeps are well documented through studies of chemosynthetic communities, seafloor deposits, and gas hydrates associated with seeps. Furthermore, sea surface oil slicks can be identified on satellite images, and subsurface geology can be determined through geophysical recordings. Initial reviews of satellite images indicate that certain areas with recurring surface slicks are associated with known seafloor chemosynthetic communities. However some areas with documented seafloor chemosynthetic communities produce no sea surface hydrocarbon slicks. The relationship of sea surface slicks to underlying seafloor seeps is influenced by several geologic characteristics and these characteristics may determine which seeps develop into surface slicks. Site specific studies have investigated the relationship of hydrocarbon seeps to seismic character, carbonate character, geothermal patterns, source and pathway, chemosynthetic communities, mineralogy and geoche...
A high-resolution elemental, isotopic, and molecular record of or-ganic matter was established fo... more A high-resolution elemental, isotopic, and molecular record of or-ganic matter was established for the uppermost sediments of Hole 1229E (0–700 cm below seafloor), drilled during Ocean Drilling Pro-gram (ODP) Leg 201 on the Peruvian margin. Diagnostic parameters of organic matter (OM) such as the atomic C org over total nitrogen ratio (C/N), the organic carbon isotopic signatures (δ 13 C org), and lignin-derived biomarker concentrations (Σ8) all suggest that terrigenous OM influx was more important during glacial than interglacial periods on the Peruvian shelf. In contrast to other shelf environments, however, fluctuations in lignin ratios ([Ad/Al]v, S/V, and C/V) across the Pleis-tocene–Holocene transition point to shifting composition in terrige-nous OM sources from coarse and undegraded woody materials during the Last Glacial Maximum (LGM) to much more altered and finer soft tissue materials during the Holocene. In addition, the total OM content preserved in the Peruvian shelf se...
ABSTRACT During October and November 2003, the German ship R/V Sonne visited the Campeche Escarpm... more ABSTRACT During October and November 2003, the German ship R/V Sonne visited the Campeche Escarpment in the southern Gulf of Mexico. The primary objectives of the cruise were the following: 1) extend our knowledge of gas hydrates buried just below the seafloor in the deep ocean; 2) determine whether natural hydrocarbon seeps in the southern Gulf of Mexico support chemosynthetic fauna, and 3) pinpoint sites for future exploration with deep-diving submersibles. Recent findings indicate that marine deposits of gas hydrate represent an important contribution to the global carbon cycle. Methane released from the gas hydrate reservoir may interact with other components of the Earth system influencing climate and is a potential source of energy. Although most gas hydrate is buried under hundreds of meters of marine sediment, the Gulf of Mexico is one place where it exists at or near the seafloor, where it can be sampled and studied. Gas hydrate is one component of the natural hydrocarbon system of the Gulf of Mexico; other components - known primarily from the northern Gulf - include oil seeps that produce natural oil "spills" visible from space and lush communities of chemosynthetic tubeworms and mussels. Natural oil seeps also occur in the southern Gulf of Mexico on the Campeche Escarpment and were expected to harbor chemosynthetic fauna and possibly gas hydrates. Recently, satellite remote sensing has been successfully used to pinpoint active seeps based on the unusual synthetic aperture radar signature generated by oil slicks on the sea surface in the northern Gulf of Mexico. Using a combination of satellite image analyses, acoustic profiling techniques, detailed bathymetric mapping, and interpretation of seafloor reflectivity data, we were able to identify and study such areas of active seafloor seepage of hydrocarbons in the previously unexplored southern Gulf of Mexico.
ABSTRACT The DOE/NETL funded HyFlux project combines sea-air, water column, and subsurface observ... more ABSTRACT The DOE/NETL funded HyFlux project combines sea-air, water column, and subsurface observations and measurements to better quantify the amount of methane released from seeps and gas hydrate deposits in the Gulf of Mexico. A recently completed research cruise investigated Gulf of Mexico seeps in 540, 900, and 1200 m water depths. Piston, gravity, and push cores were collected for pore water and sediment analysis. Analyses of major pore water constituents were conducted using an ICP-OES. Preliminary pore water analyses included SO42-, alkalinity, HS-, and Ca2+. Cores taken at Mississippi Canyon site MC 118 showed sulfate concentrations decreasing from 25.2 mM at 1 cmbsf to 1.91 mM at 36.5 cmbsf. HS- measurements show a decrease from 3.02 mM at 1.5 cmbsf to 0 mM at 12 cmbsf, followed by an increase in HS- due to SO42- reduction in 12 cmbsf. Alkalinity concentrations show an increase with depth from 3.63 mM at 1.5 cmbsf to 27.8 mM at 36.5 cmbsf. Calcium concentrations decrease from 10.55 mM at 1.5 cmbsf to 4.87 mM at 36.5 cmbsf due to the precipitation of calcium carbonate. Data from a 10 cm-long push core taken at site MC 118 showed the same trends in SO42- reduction, a decrease in Ca2+ concentrations, along with an alkalinity increase and HS- production. Two piston cores were taken at site GC (Green Canyon) 600. The first core showed SO42- concentrations decreasing from 21.34 mM at 0 cmbsf to 0 mM at 20 cmbsf. HS- concentrations increased to 32.11 mM 20 cmbsf, then decreased to 3.05 mM at 282.5 cmbsf. Alkalinity and Ca2+ concentrations both increased with depth. Alkalinity concentrations were 10.27 mM at 1.5 cmbsf and increased to 35.59 mM at 282.5 cmbsf. Ca2+ also increased from 8.96 mM at 1.5 cmbsf to 12.27 mM at 282.5 cmbsf, which may be the result of carbonate dissolution at depth. The second piston core showed a more typical concentration profile for seep areas. The sulfate concentration at 3 cmbsf decreased from 21.93 mM to 3 mM at 44.5 cmbsf, coupled with the typical inverse relationship with HS-, which increased from 1.04 mM at 3 cmbsf to 27.26 mM at of 44.5 cmbsf. Alkalinity concentrations yielded a steady increase from 5.34 mM at 3 cmbsf to 26.44 mM at 44.5 cmbsf and Ca2+ depletion was observed in the same depth range from 8.28 mM to 4.80 mM. One piston core was taken at site GC 185 (Bush Hill). Sulfate concentrations did not vary much with depth and ranged from 22.99 mM at 3 cmbsf to 23.56 mM at 44.5 cmbsf. HS- concentrations similarly showed small changes with an initial increase from 1.91 mM at 3 cmbsf to a concentration of 3.45 mM at 44.5 cmbsf. Alkalinity concentrations at 3 cmbsf were 4.26 mM and increased slightly to 4.47 mM at 44.5 cmbsf. Similarly, Ca2+ concentrations varied only slightly throughout the sampled interval. Results from pore water analysis in combination with air-sea measurements of methane and water column data will allow for a more accurate model for methane flux from seeps and gas hydrate deposits to the water column and atmosphere.
ABSTRACT In the northern Gulf of Mexico (GOM), numerous seafloor hydrocarbon seeps are well docum... more ABSTRACT In the northern Gulf of Mexico (GOM), numerous seafloor hydrocarbon seeps are well documented through seismic, seafloor, and chemical studies. In some areas, the seafloor seeps expend sufficient hydrocarbons to pass through the water column and present as sea-surface oil slicks, which with careful review and certain conditions can be identified on satellite images. Some areas with recurring potential sea-surface oil slicks as identified by oil-criteria screened satellite images are located above or near proven subsurface hydrocarbon-producing areas. However, other areas with established production have no sea-surface hydrocarbon slicks, and several areas with potential sea-surface slicks currently have no subsurface production. A publicly available data set of reservoir information for the northern GOM was correlated to a data set of sea-surface slick sites as identified on satellite data. Thirteen variables were selected from the database to evaluate depth relationships, fluid properties, and reservoir properties in relation to slick presence. Because initial multivariate analysis indicated the slick-to-reservoir-variables relationships were nonlinear, a random forest (RF) classification-tree analysis was performed to identify which variables are more important for slick development. The RF analysis used a collection of binary decision trees to determine the relative importance of the reservoir variables in identifying sea-surface slicks. The RF model was able to correctly classify or predict the presence of a slick and identified water depth, gas-oil ratio, and reservoir chronozone as the three most important variables when predicting the formation of a sea-surface slick.
... 1. Introduction. Zeolites are among the most common authigenic silicate minerals in marine se... more ... 1. Introduction. Zeolites are among the most common authigenic silicate minerals in marine sediments, consisting of a hydrated framework of aluminosilicates with varying amounts of alkali and alkaline earth metals (Kastner and Stonecipher, 1978; Stonecipher, 1978). ...
The recently funded DOE/NETL study "HyFlux: Remote sensing and sea-truth mea... more The recently funded DOE/NETL study "HyFlux: Remote sensing and sea-truth measurements of methane flux to the atmosphere" (see MacDonald et al.: HyFlux - Part I) will combine sea surface, water column and shallow subsurface observations to improve our estimates of methane flux from submarine seeps and associated gas hydrate deposits to the water column and atmosphere along the Gulf of
HyFlux - Part I: Regional modeling of methane flux from near-seafloor gas hydrate deposits on con... more HyFlux - Part I: Regional modeling of methane flux from near-seafloor gas hydrate deposits on continental margins MacDonald, I.R., Asper, V., Garcia, O., Kastner, M., Leifer, I., Naehr, T.H., Solomon, E., Yvon-Lewis, S., and Zimmer, B. The Dept. of Energy National Energy Technology Laboratory (DOE/NETL) has recently awarded a project entitled HyFlux: "Remote sensing and sea-truth measurements of methane flux to the atmosphere." The project will address this problem with a combined effort of satellite remote sensing and data collection at proven sites in the Gulf of Mexico where gas hydrate releases gas to the water column. Submarine gas hydrate is a large pool of greenhouse gas that may interact with the atmosphere over geologic time to affect climate cycles. In the near term, the magnitude of methane reaching the atmosphere from gas hydrate on continental margins is poorly known because 1) gas hydrate is exposed to metastable oceanic conditions in shallow, dispersed deposits that are poorly imaged by standard geophysical techniques and 2) the consumption of methane in marine sediments and in the water column is subject to uncertainty. The northern GOM is a prolific hydrocarbon province where rapid migration of oil, gases, and brines from deep subsurface petroleum reservoirs occurs through faults generated by salt tectonics. Focused expulsion of hydrocarbons is manifested at the seafloor by gas vents, gas hydrates, oil seeps, chemosynthetic biological communities, and mud volcanoes. Where hydrocarbon seeps occur in depths below the hydrate stability zone (~500m), rapid flux of gas will feed shallow deposits of gas hydrate that potentially interact with water column temperature changes; oil released from seeps forms sea-surface features that can be detected in remote-sensing images. The regional phase of the project will quantify verifiable sources of methane (and oil) the Gulf of Mexico continental margin and selected margins (e.g. Pakistan Margin, South China Sea, and West Africa Margin) world-wide by using the substantial archive of satellite synthetic aperture radar (SAR) images. An automated system for satellite image interpretation will make it possible to process hundreds of SAR images to increase the geographic and temporal coverage. Field programs will quantify the flux and fate of hydrate methane in sediments and the water column.
ABSTRACT Authigenic carbonates were recovered from several horizons between 0 and 52 mbsf in sedi... more ABSTRACT Authigenic carbonates were recovered from several horizons between 0 and 52 mbsf in sediments that overlay the Blake Ridge Diapir on the Carolina Rise (Ocean Drilling Program [ODP] Site 996). Active chemosynthetic communities at this site are apparently fed by fluid conduits extending beneath a bottom-simulating reflector (BSR). Gas hydrates occur at several depth intervals in these near-surface sediments. The carbonate nodules are composed of rounded to subangular intraclasts and carbonate cemented mussel shell fragments. Electron microprobe and X-ray diffraction (XRD) investigations show that aragonite is the dominant authigenic carbonate. Authigenic aragonite occurs both as microcrystalline, interstitial cement, and as cavity-filling radial fibrous crystals. The δ13C values of the authigenic aragonite vary between -48.4‰ and -30.5‰ (Peedee belemnite [PDB]), indicating that carbon derived from 13C-depleted methane is incorporated into these carbonates. The δ13C of pore water ΣCO2 values are most negative in the upper 10 mbsf, near the sediment/water interface (-38‰ ± 5‰), but noticeably more positive below 25 mbsf (+5‰ ± 6‰). Because carbonates derive their carbon from HCO3-, dissimilarities between the δ13C values of carbonate precipitates recovered from greater than 10 mbsf and δ13C values of the associated pore fluids suggests that these carbonates formed near the seafloor. Differences of about 1‰ in the oxygen isotopic composition of carbonate precipitates from different depths are possibly related to changes in bottom-water conditions during glacial and interglacial time periods. Measurements of the strontium isotopic composition on 13 carbonate samples show 87Sr/86Sr values between 0.709125 and 0.709206 with a mean of 0.709165, consistent with the approximate age of their host sediment. Furthermore, the 87Sr/86Sr values of six pore-water samples from Site 996 vary between 0.709130 and 0.709204. The similarity of these values to seawater (87Sr/86Sr = 0.709175), and to 87Sr/86Sr values of pore water from similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997), indicates a shallow Sr source. The 87Sr/86Sr values of the authigenic carbonates at Site 996 are not consistent with the Sr isotopic values predicted for carbonates precipitated from fluids transported upward along fault conduits extending through the base of the gas hydrate-stability zone. Based on our data, we see no evidence of continuing carbonate diagenesis with depth. Therefore, with the exception of their seafloor expression as carbonate crusts, fossil vent sites will not be preserved. Because these authigenic features apparently form only at the seafloor, their vertical distribution and sediment age imply that seepage has been going on in this area for at least 600,000 yr.
Anaerobic oxidation of methane (AOM) impacts carbon cycling by acting as a methane sink and by se... more Anaerobic oxidation of methane (AOM) impacts carbon cycling by acting as a methane sink and by sequestering inorganic carbon via AOM-induced carbonate precipitation. These precipitates commonly take the form of carbonate nodules that form within methane seep sediments. The timing and sequence of nodule formation within methane seep sediments are not well understood. Further, the microbial diversity associated with sediment-hosted nodules has not been well characterized and the degree to which nodules reflect the microbial assemblage in surrounding sediments is unknown. Here, we conducted a comparative study of microbial assemblages in methane-derived authigenic carbonate nodules and their host sediments using molecular, mineralogical, and geochemical methods. Analysis of 16S rRNA gene diversity from paired carbonate nodules and sediments revealed that both sample types contained methanotrophic archaea (ANME-1 and ANME-2) and syntrophic sulfate-reducing bacteria (Desulfobacteraceae and Desulfobulbaceae), as well as other microbial community members. The combination of geochemical and molecular data from Eel River Basin and Hydrate Ridge suggested that some nodules formed in situ and captured the local sediment-hosted microbial community, while other nodules may have been translocated or may represent a record of conditions prior to the contemporary environment. Taken together, this comparative analysis offers clues to the formation regimes and mechanisms of sediment-hosted carbonate nodules.
The Campeche Knolls region of the southern Gulf of Mexico contains numerous diapiric mounds and r... more The Campeche Knolls region of the southern Gulf of Mexico contains numerous diapiric mounds and ridges that are associated with persistent oil slicks and extensive flows of solidified asphalt. Previous investigations1 have documented chemosynthetic communities, gas hydrates, and bubble streams that rise hundreds of meters into the water column. However, only a few of the potential features had previously been
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