Debra A. DeFreitas

NOAA gulf of Mexico status and trends program: Trace organic contaminant distribution in sediments and oysters

Polynuclear aromatic hydrocarbons (PAH), chlorinated pesticides, and polychlorinated biphenyls (PCB) concentrations were determined in sediment and oysters to provide information on the current status of the concentration of these contaminants in Gulf of Mexico coastal areas removed from point sources of input. Coprostanol analyses of sediments showed that anthropogenic materials are associated with the sediments at all 153 stations sampled. The levels of contaminants encountered are low compared with areas of known contamination. Average PAH concentrations are nearly the same in oysters and sediments, although the molecular weight distribution is different. Average DDT and PCB concentrations are higher by a factor of 10 to 130 in oysters as compared to sediments. Continued sampling and analyses will allow for long-term trends in the concentrations of these contaminants to be determined.

Bacterial methane oxidation in sea-floor gas hydrate: Significance to life in extreme environments

Samples of thermogenic hydrocarbon gases, from vents and gas hydrate mounds within a sea-floor chemosynthetic community on the Gulf of Mexico continental slope at about 540 m depth, were collected by research submersible. The study area is characterized by low water temperature (mean = 7 C), high pressure (about 5,400 kPa), and abundant structure II gas hydrate. Bacterial oxidation of hydrate-bound methane (CH{sub 4}) is indicated by three isotopic properties of gas hydrate samples. Relative to the vent gas from which the gas hydrate formed, (1) methane-bound methane is enriched in {sup 13}C by as much as 3.8% PDB (Peedee belemnite), (2) hydrate-bound methane is enriched in deuterium (D) by as much as 37% SMOW (standard mean ocean water), and (3) hydrate-bound carbon dioxide (CO{sub 2}) is depleted in {sup 13}C by as much as 22.4% PDB. Hydrate-associated authigenic carbonate rock is also depleted in {sup 13}C. Bacterial oxidation of methane is a driving force in chemosynthetic communities, and in the concomitant precipitation of authigenic carbonate rock that modifies sea-floor geology. Bacterial oxidation of hydrate-bound methane expands the potential boundaries of life in extreme environments.

Distribution of phytoplankton pigments in the North Pacific Ocean in relation to physical and optical variability

Abstract To investigate phytoplankton distributions in the North Pacific Ocean, samples of suspended particulate material were collected from the upper 300 m during two cruises in 1985 for detailed analysis of algal pigments by high-performance liquid chromatography (HPLC). Transpacific Leg I along 24°N in April and May, crossed three prominent hydrographic features: the California Coastal Current, the North Pacific Central Gyre and Kuroshio Current. Transpacific Leg II, along 47°N in August and September, crossed the Kuroshio extension, the Subarctic Gyre and the North Pacific Current. Individual pigments were partitioned vertically in the water column, showing distinct spatial patterns across the Pacific Ocean which reflected the large-scale circulation. Vertical distributions of phytoplankton pigments displayed consistent patterns over spatial scales of thousands of kilometers. In near-surface, nitrate-rich waters, fucoxanthin was the dominant carotenoid. In nitrate-poor surface waters, zeaxanthin was the dominant carotenoid at the surface, and 19′-hexanoyloxyfucoxanthin and chlorophyll b concentrations were elevated near the base of the euphotic zone. Phacopigment concentrations greater than a few tens of nanograms per liter were never encountered. Based on Principal Component Analysis, station clustered into three general pigment categories which followed specific hydrographic characteristics of oligotrophic, highly productive and transitional regions.

Thermogenic vent gas and gas hydrate in the Gulf of Mexico slope: Is gas hydrate decomposition significant?

Samples of vent gas and gas hydrate on the Gulf of Mexico slope were collected by research submersible (∼540 m water depth) and by piston coring (∼1060–1070 m water depth). Although gas hydrate that crops out is transiently unstable, the larger volume of structure II gas hydrate in the gulf is stable or increasing in volume because gas from the subsurface petroleum system is venting prolifically within the gas hydrate stability zone. Vent gas from gas hydrate shows no meaningful molecular evidence of gas hydrate decomposition. Gas hydrate fabrics, mainly vein fillings, are typical of ongoing crystallization. Once crystallized, most hydrocarbons are protected from bacteria within the crystal lattice of gas hydrate. A leaky petroleum system is proposed to be the main source of thermogenic greenhouse gases in the central gulf. Stable gas hydrate sequesters large volumes of greenhouse gases, suggesting that gas hydrate may not be a significant factor in models of climate change at present.

Selected organic matter source indicators in the Orinoco, Nile and Changjiang deltas

Carbon isotopic composition, carbon to nitrogen ratios, carbon preference (n-C23 to n-C32), and microscopic examination of selected samples were compared and contrasted as organic matter source indicators. The Orinoco, Nile, and Changjiang deltas were sampled and analyzed. The δ 13C of organic matter increased with distance offshore at all three locations. Organic carbon content decreased seaward on the Orinoco and Changjiang deltas but increased on the Nile delta. C/N ratios decreased regularly with distance offshore on the Orinoco delta, ranged from 10 to 25, and correlated positively with δ 13COM. Estimations of the relative input of marine versus terrestrial organic matter to the sediments was dependent on the parameter analyzed. These discrepancies were primarily due to the definition of end-member compositions and the fraction of the total organic matter that is measured by a given parameter. Multiple-source indicators must be measured and integrated and end-members must be properly defined in order to obtain realistic input estimates.

Gas venting and subsurface charge in the Green Canyon area, Gulf of Mexico continental slope: evidence of a deep bacterial methane source?

Abstract Questions as to the role of modern carbon in methanogenesis and the maximum depth of methane sources in the Gulf of Mexico continental slope remain unanswered. A research submersible was used to sample mixed bacterial and thermal gas ( δ 13 C of methane=−62.8‰, δD =−176‰) venting to the water column from the Gulf slope in Green Canyon (GC) 286. The Δ 14 C value of the methane (−998‰) is consistent with fossil carbon. Another gas vent on GC 185 is 100% methane ( δ 13 C =−62.9‰, δD =−155‰) and may be from a bacterial source. The Δ 14 C (−997‰) of this bacterial methane is also consistent with fossil carbon. Fossil bacterial methane and thermal hydrocarbons are present in Pliocene to Pleistocene reservoirs (∼3509–4184 m) of Genesis Field (GC 205, 161, 160). Oil in these reservoirs is biodegraded but gas is not, suggesting that gas charge to reservoirs continues presently at 3–4 km depth. Mixed thermal and bacterial methane may charge the deep reservoirs, and fossil methane from depth may ultimately vent on the sea floor at GC 286 and GC 185. Results of this study of Green Canyon suggest that bacterial methane in gas vents and in reservoirs is from deep fossil sources.

Gas hydrate and crude oil from the Mississippi Fan Foldbelt, downdip Gulf of Mexico Salt Basin: significance to petroleum system

Abstract Structure II gas hydrate, methane–ethane hydrate, and crude oil occur on the sea floor at ∼1920–1930 m water depth in Atwater Valley (AT) Block 425, near the juncture of the lower slope of the Gulf of Mexico Salt Basin and the abyssal plain. The site is in the eastern Mississippi Fan Foldbelt (MFF), a distinct structural province at the downdip limit of the Gulf of Mexico Salt Basin. The presence of thermogenic hydrocarbons confirms an active petroleum system in the deep eastern MFF. The hydrate-bound C 2 –C 5 hydrocarbon gases of the MFF are isotopically distinct when compared to other gases from the upper and middle slope, being strongly depleted in 13 C. The biomarkers ( m / z =191 and 217) of oil inclusions from AT 425 gas hydrate are also distinct when compared to oils of the upper Gulf slope and the Smackover Trend. Biomarkers of AT 425 oil are consistent with a marine source rock deposited in an area of strong siliciclastic influx, with possible higher plant organic matter. The presence of a shale or mudstone source rock at the downdip limit of the Gulf raises new questions as to paleogeography during source rock deposition. Emergent highlands immediately to the south and east of the downdip limit of the Gulf of Mexico Salt Basin during the Mesozoic may explain the occurrence of the shale or mudstone source rock that gave rise to the gas and oil at AT 425.

Geochemical evidence of rapid hydrocarbon venting from a seafloor-piercing mud diapir, Gulf of Mexico continental shelf

Abstract A research submersible was employed to collect sediments from a previously undescribed diapiric mud mound on the Gulf of Mexico continental shelf. The sediments contain high concentrations of C1–C6+ hydrocarbon gases and crude oil. The mud mound hydrocarbons are relatively unaffected by biodegradation, in contrast to the heavily biodegraded hydrocarbons that characterize the sediments of some other Gulf of Mexico seep sites, including those colonized by chemosynthetic communities. The molecular and isotopic properties of the gas and oil suggest rapid hydrocarbon transport from the mound sediments to the water column. The mud mound is an episodic point source of an oil slick on the sea surface. Gas venting is observed on the seafloor, and bubble trains recorded close to the sea surface suggest that greenhouse thermogenic gases (mainly methane) may escape to the atmosphere. Improved understanding of the fate of C1–C6+ gases and crude oil in shallow marine sediments will contribute to better assessment of the impact of seep hydrocarbons on the global inventory of atmospheric sources.

Exclusion of 2-methylbutane (isopentane) during crystallization of structure II gas hydrate in sea-floor sediment, Gulf of Mexico

Structure II gas hydrate is abundant across the central Gulf of Mexico continental slope. Sediment that directly overlies nodular gas hydrate was collected with a piston core at1920 m water depth in the Atwater Valley (AT) 425 area of the lower continental slope. The gas hydrate has C1‐C5 molecular and isotopic properties consistent with structure II gas hydrate that crystallized from relatively unaltered thermogenic vent gas. The gas hydrate contains mainly methane, ethane, propane and butanes, with 2-methylbutane (isopentane ) as a minor component (< 0.2%). Sediment that closely overlies the gas hydrate (within < 1 m) is characterized by an anomalous abundance of 2methylbutane (as much as 9.6%). Because the molecular diameter of 2-methylbutane is too large for structure II gas hydrate, the 2-methylbutane appears to accumulate preferentially in adjacent sediment as a direct consequence of massive gas hydrate crystallization. The 2-methylbutane is interpreted to be a molecular marker of recent or ongoing net accumulation of structure II gas hydrate. Abundant 2-methylbutane in sediment also could be a precursor to the natural occurrence of structure H gas hydrate, and other new gas hydrate structures not yet discovered in the geologic environment. # 2000 Elsevier Science Ltd. All rights reserved.

ABSTRACT: Molecular and Isotopic Properties of High Flux Gas Seeps, Northwestern Gulf of Mexico

Study of the molecular distributions and carbon isotopic properties of methane from 160 high flux seep sites across the Gulf of Mexico from the outer shelf to the lower slope reveals new insight to the complex origin of gas at the sea floor. Each site represents the intersection of a major migration conduit from the deep subsurface to the sea floor related to salt or faults. Sites were selected to minimize bias from known gas and oil fields. Mean concentration of methane is 99,407 ppm. Means of ethane (1,837 ppm), iso-butane (825 ppm), and normal butane (607 ppm) are anomalous. To provide perspective, typical Gulf sediment has methane < 10 ppm and higher hydrocarbons may be in low abundance or below detection limits. The mean carbon isotopic composition (per mil, PDB standard) of methane is -74.0, whereas the range is from -30.1 to -116.6. The occurrence of isotopically heavy methane probably represents bacterially oxidized thermogenic methane, but the occurrence of extremely light methane implies that carbon is recycled through bacterial hydrocarbon oxidation and reduction of carbon dioxide in natural bioreactors. Molecular and isotopic properties of gas suggest that nearly all high-flux gas seeps across the Gulf are a mixture of bacterial and thermogenic hydrocarbons. Contrary to previous generalizations, much bacterial methane may originate from several kilometers depth. The carbon isotopic threshold between bacterial and thermogenic methane often cannot be defined because considerable overlap exists and bacterial alteration is common.