Roger A. Burke

Stable isotope partitioning in seep and vent organisms: chemical and ecological significance

Abstract Several hundred stable isotopic ratios (C, N and S) acquired over seven years of investigations at both seep and vent locations are compiled and interpreted. The stable isotopic compositions of tissues derived from the chemosynthetic fixation of carbon reflect a complex interaction between chemical and biological processes. The stable isotopic composition of bivalves that utilize reduced sulfur suggests that seawater-and/or vent water-dissolved inorganic carbon (DIC) is their primary source of carbon during chemosynthesis. All thiotrophic bivalves studied appear to possess a similar sulfide oxidation metabolism. The δ13C-values of tissues from methanotrophic mussels are close to the δ13C of the methane utilized. Apparently, little of the kinetic isotope fractionation associated with methanotrophy is expressed in the hosts tissue. Vestimentiferan carbon isotopic composition reflects both carbon limitation and the isotopic composition of the substrate utilized. The δ13C-values of vent vestimentiferans tend to be affected by carbon limitation, whereas those of seep vestimentiferans reflect the variable isotopic composition of pore-water DIC. Stable nitrogen isotopic compositions are consistent with nitrogen (N2) fixation, but the presence of the enzyme responsible for nitrogen fixation, nitrogenase, has not been conclusively demonstrated. A variety of nitrogen sources [N2, NH4+, PON (particulate organic nitrogen), DON (dissolved organic nitrogen) and NO4−] may be utilized by vent and seep organisms. However, the δ15N data suggest that the mechanism of nitrogen metabolism is not a function of the species or the symbiont type. Sulfur is a key element in vent and seep environments and thiotrophy is the major chemosynthetic activity. The sources of sulfur are highly variable in quantity and isotopic composition but are almost always linked to bacterial activity,l either free-living and/or symbiont. Nitrogen and sulfur nutritional requirements appear to be derived from a wide variety of sources. The relative importance of nutrition derived from heterotrophy and chemoautotrophy depends on the chemical environment and animal physiology. Stable isotope compositions provide insight into these diverse metabolic strategies; however, a complete inventory of the concentration and isotopic composition of inorganic and organic substrates, as well as supporting biochemical, enzymatic and observational studies, are needed to resolve fundamental ecological questions.

Light hydrocarbons in Red Sea brines and sediments

Abstract Light hydrocarbon (C1-C3) concentrations in the water from four Red Sea brine basins (Atlantis II, Suakin, Nereus and Valdivia Deeps) and in sediment pore waters from two of these areas (Atlantis II and Suakin Deeps) are reported. The hydrocarbon gases in the Suakin Deep brine (T = ~ 25°C, Cl − = ~ 85‰ , CH 4 = ~ 71 1 ) are apparently of biogenic origin as evidenced by C 1 (C 2 + C 3 ) ratios of ~ 1000. Methane concentrations (6–8 μl/l) in Suakin Deep sediments are nearly equal to those in the brine, suggesting sedimentary interstitial waters may be the source of the brine and associated methane. The Atlantis II Deep has two brine layers with significantly different light hydrocarbon concentrations indicating separate sources. The upper brine (T = ~ 50°C, Cl − = ~ 73‰ , CH4 = ~ 155 μl/l) gas seems to be of biogenic origin [ C 1 (C 2 + C 3 ) = ~1100 ], whereas the lower brine (T = ~ 61°C, Cl − = ~ 155‰ , CH4 = ~ 120μl/l) gas is apparently of thermogenic origin [ C 1 (C 2 + C 3 ) = ~ 50 ]. The thermogenic gas resulting from thermal cracking of organic matter in the sedimentary column apparently migrates into the basin with the brine, whereas the biogenic gas is produced in situ or at the seawater-brine interface. Methane concentrations in Atlantis II interstitial waters underlying the lower brine are about one half brine concentrations; this difference possibly reflects the known temporal variations of hydrothermal activity in the basin.

Hydrocarbon Seepage, Gas Hydrates, and Authigenic Carbonate in the Northwestern Gulf of Mexico

Rapid deposition of organic rich sediments onto thick Jurassic salt deposits created conditions conducive to the formation and entrapment of large volumes of oil and gas as well as active salt diapirism. Resulting hydrocarbon seepage, brine seepage, hydrate formation and decomposition, methane oxidation, oil degradation and authigenic carbonate precipitation have significantly affected the geology, geochemistry and morphology of the continental slope of the northern Gulf of Mexico. Regional topography is largely controlled by salt diapirism and slumping, and is characterised by pock marks, mud volcanoes, disrupted sediments and large accumulations of carbonate. Sediment chemistry is driven by the influx of oil, gas and brine seepage and secondarily altered by the enhanced microbial and benthic biology. See also Abstract No. 89051216.

Gaseous and volatile hydrocarbon inputs from a subsurface oil spill in the Gulf of Mexico

This paper is the result of a collaborative study. The authors express their gratitude to the technical staff of the Bedford Institute of Oceanography and to the officers and men of the research vessel CSS H u d s o n . We thank Mr. R. T. Rantala of the Marine Ecology Laboratory, who carried out the Hg and organic carbon matter analyses in the laboratory of one of the authors (D.H.L.), and Dr. P. Yeats and Dr. D. Buckley for their critical reviews of this manuscript. We especially thank Dr. J. M. Bewers for his many thoughtful discussions during the course of this work.

Gaseous hydrocarbons around an active offshore gas and oil field

Low molecular weight hydrocarbons (LMWHs, C/sub 1/-C/sub 4/) were measured from the water column and sediments around an oil and gas field. No significant differences in mean methane levels were observed between platforms that were and were not discharging brine. However, in the 20-station grid, the relative standard deviations were greater and the highest individual methane and ethane concentrations were found in surface waters near the platform discharging brine. Higher methane values at all depths observed during summer coinciding with decreased ethane/ethene ratios in a near-bottom nepheloid layer provided direct evidence of in situ biological production associated with increases in zooplankton and bacterial biomass in the water column. The sediment LMWHs are predominantly of thermogenic origin probably due to seepage from the subsurface, as evidenced by high levels of methane and elevated ethane/ethene ratios. The LMWH input from brine discharge in the field is estimated at 283 g/day.