Paul Aharon

Microbial sulfate reduction rates and sulfur and oxygen isotope fractionations at oil and gas seeps in deepwater Gulf of Mexico

Abstract Sulfate reduction and anaerobic methane oxidation are the dominant microbial processes occurring in hydrate-bearing sediments at bathyal depths in the Gulf of Mexico where crude oil and methane are advecting through fault conduits to the seafloor. The oil and gas seeps are typically overlain by chemosynthetic communities consisting of thiotrophic bacterial mats (Beggiatoa spp .) and methanotrophic mussels (Bathymodiolus spp .), respectively. Cores were recovered with a manned submersible from fine-grained sediments containing dispersed gas hydrates at the threshold of stability. Estimated sulfate reduction rates are variable but generally are substantially higher in crude oil seeps (up to 50 times) and methane seeps (up to 600 times) relative to a non-seep reference sediment (0.0043 μmol SO 4 2− cm −3 day −1 ). Sulfur and oxygen isotope fractionation factors are highest in the reference sediment (α S = 1.027; α O = 1.015) but substantially lower in the seep sediments (α S = 1.018 to 1.009; α O = 1.006 to 1.002) and are controlled primarily by kinetic factors related to sulfate reduction rates. Kinetic effects also control the δ 34 S/δ 18 O ratios such that slow microbial rates yield low ratios whereas faster rates yield progressively higher ratios. The seep data contradict previous claims that δ 34 S/δ 18 O ratios are diagnostic of either microbial sulfate reduction at a fixed δ 34 S/δ 18 O ratio of 4/1 or lower ratios caused by SO 4 –H 2 O equilibration at ambient temperatures. The new results offer a better understanding of methane removal via anaerobic oxidation in the sulfate reduction zone of hydrate-bearing sediments and have significant implications regarding the origin and geochemical history of sedimentary sulfate reconstructed on the basis of δ 34 S and δ 18 O compositions.

Hydrocarbon-derived carbonate buildups of the northern Gulf of Mexico continental slope: A review of submersible investigations

Hydrocarbon-derived and microbially mediated authigenic carbonates occur over the entire depth range of the northern Gulf of Mexico slope. These carbonates consist of nodules and incipient nodules in surface sediments, hardgrounds and isolated slabs, and moundlike buildups of up to 10–20 m relief above the surrounding seafloor. The authigenic carbonates are characterized byδ13C negative values in the range −18‰ to −55‰ (PDB) suggesting mixing of seawater carbon with13C-depleted carbon sources ranging from crude oil to biogenic methane. Near the shelf edge, carbonates are “diluted” with biogenic material produced by reefs—bioherms developed at low sea level stands. Fossil-poor carbonates over salt diapirs of the upper and middle slope formed in the shallow subsurface and have been exhumed by the combined processes of uplift and physical erosion. Middle and lower slope carbonates are generally rich in fossil shells of chemosynthetic organisms. Mg calcite pelloidal matrix and acicular to botryoidal aragonitic void-filling cements are common petrographic features of these hydrocarbonderived carbonates. At two sites carbonates are mixed with barite.

TIMS U-series dating and stable isotopes of the last interglacial event in Papua New Guinea

The extensive flight of uplifted reef terraces which occurs along the Vitiaz strait on the northern flank of the Huon Peninsula in PNG (Papua New Guinea) contains a particularly good record of sea level changes in the last 250 ky. The Huon terraces were the target of an international expedition which took place in July–August 1988. In particular, we searched for suitable samples for U-series dating in a reef complex designated as VII, which is correlated with the last interglacial episode and high sea level stand. This complex is composed of a barrier reef (VIIb), a lagoon, and a fringing reef (VIIa). Twelve corals from these terraces and two corals from the older reef complex VIII were selected for analysis. The petrography, oxygen and carbon isotope compositions, and magnesium and strontium concentrations were determined along with the concentrations and isotopic compositions of uranium and thorium. ^(230)Th-^(234)U ages of the corals with > 99% aragonite, having primary textures, and which show U/Sr ratios around 0.4 × 10^(−3) and initial δ^(234)U values close to that of present seawater, appear to be reliable. The “most reliable” ages from complex VII corals fall in two tight groups centered at 118 ky and 134 ky. Corals with δ^(234)U(T) values higher than 160 and U/SR ratios substantially lower than seawater are assumed to have undergone diagenetic alteration, which appears to be common in this area. The simplest model for sea level height for terrace VII is a continuous rise between 134 and 118 ky. Alternatively, there may have been two periods of rapid sea level rise. In contrast, in the Bahamas, there is evidence that sea level remained rather constant over the time interval 132 to 120 ky. The absence of ages between 132 and 120 ky in PNG could be the result of changes in the local tectonic uplift rates during that time, or erosion that disrupted the continuous record. In any event, we find no basis for accepting a single brief time for the age of the last interglacial and applying this age as a precise chronometer for worldwide correlation, or as a test of climatic models. The older ages reported here precede the Milankovitch solar insolation peak at 128 ky, and the younger ages are ~ 10 ky after this peak. If the present high-precision data are correct, then it will be necessary to reassess the validity of the Milankovitch theory of climatic changes. The fundamental issue which must now be resolved is a means of identifying coral samples that have not been disturbed by diagenetic processes.

Oxygen isotopes, sea level changes and the temperature history of a coral reef environment in New Guinea over the last 105 years

Abstract Coral reef terraces consisting of an upward succession of fringing and barrier reef types are preserved on land along the raising coast of Huon Peninsula, New Guinea. Seven coral reef units, I–VII from the coast landwards spaced at about 20-kyr interval, provide a rather complete record of sea levels, oxygen isotopes and temperature of teh surface tropical ocean over the duration of the high sea level events of the last glacial cycle. The systematic sampling of the coral reefs was restricted to the massive Tridacna gigas species (the giant clam) which are about 0.7%0 enriched in 18O relative to calcite equilibrium value and the presence of symbionts seems to have no effect on the δ18O of their aragonitic skeleton. Sea levels rose rapidly to 5–6.5 m above present during the last interglacial which on stratigraphic grounds and δ18O compositions can be separated into an early phase whose δ18O is similar to modern (reef VIIa dated at 138 kyr) and a late phase that is 0.6%0 heavier (reef VIIb dated at 118 kyr). Fringing reefs VI, V, IV, III and II record short-lived high sea levels stands at 107, 85, 60, 45 to 40 and 31 kyr respectively that stopped short of the present sea level (−12, −19, −28, −32 to −42 m). The general decline of the high sea level stands corresponds to an average 18O enrichment of 1.7%0 measured between the early interstadials of isotope stage 5 (VI and V) and the late interstadials of isotope stage 3 (IV, IIIa, IIIb, and II). The maximum 18O enrichment of 1.7%0 relative to present was registered during a regressive phase of reef IIIa. Stratigraphic evidence indicates that sea level lows in the range of −37 to −55 m below present have occurred in between the high sea level stands but those are unlikely to represent the lowest sea levels of the last glacial cycle. Sea levels and δ18O ddta pair offer important clues regarding the partitioning of ice volume and temperature in δ18O records. The results suggest that the ice volume effect accounts for the observed δ18O changes during the early interstadial culminations but only for 29–46% during the late interstadial culminations. Sea water cooling beyond the threshold limit of coral reef growth (∼ 18°C) did not occur during the last glacial cycle. The temperature of the surface tropical ocean was similar to present during the early interstadials of isotope stage 5 and cooler by 3°C below present during the later interstadials of isotope stage 3.

Blake Ridge methane seeps: characterization of a soft-sediment, chemosynthetically based ecosystem

Observations from the first submersible reconnaissance of the Blake Ridge Diapir provide the geological and ecological contexts for chemosynthetic communities established in close association with methane seeps. The seeps mark the loci of focused venting of methane from the gas hydrate reservoir, and, in one location (Hole 996D of the Ocean Drilling Program), methane emitted at the seafloor was observed forming gas hydrate on the underside of a carbonate overhang. Megafaunal elements of a chemosynthetically based community mapped onto dive tracks provide a preliminary overview of faunal distributions and habitat heterogeneity. Dense mussel beds were prominent and covered 20 � 20 m areas. The nearly non-overlapping distributions of mussels and clams indicate that there may be local (meter-scale) variations in fluid flux and chemistry within the seep site. Preliminary evidence suggests that the mussels are host to two symbiont types (sulfide-oxidizing thiotrophs and methanotrophs), while the clams derive their nutrition only from thiotrophic bacteria. Invertebrate biomass is dominated by mussels (Bathymodiolus heckerae) that reach lengths of up to 364 mm and, to a lesser extent, by small (22 mm length) vesicomyid clams (Vesicomya cf. venusta). Taking into account biomass distributions among taxa, symbiont characteristics of the bivalves, and stable-isotope analyses, the relative importance of methanotrophic vs thiotrophic bacteria in the overall nutrition of the invertebrate

Chemosynthetic bacterial mats at cold hydrocarbon seeps, Gulf of Mexico continental slope

Abstract White and pigmented filamentous bacterial mats dominated by several undescribed species of Beggiatoa were sampled during research submersible dives to cold hydrocarbon seep sites on the upper continental slope off Louisiana (130–550 m). Mats occur at the interface between reducing sediments and the oxygenated water column. They are localized at sea floor features related to seepage of biogenic methane and crude oil, but there is little evidence that the organisms utilize the hydrocarbons directly. Granules of elemental sulfur (S0) are visible within cells of Beggiatoa, and mat material is characterized by high contents of S0 (up to 193,940 ppm). The Beggiatoa biomass is isotopically light ( δ 13 C = −27.9‰ PDB ). Our geochemical data suggest that the Beggiatoa species are part of a complex bacterial assemblage in cold seep sediments. They oxidize H2S derived from the bacterial sulfate reduction that accompanies bacterial hydrocarbon oxidation when O2 is depleted in sediments, and fix isotopically light carbon from CO2 that is the result of bacterial hydrocarbon oxidation. Beggiatoa mats appear to retard loss of hydrocarbons to the water column by physically retaining fluids in sediments, a function that could enhance production by other bacteria of the H2S and CO2 needed by Beggiatoa.

Recorders of reef environment histories: stable isotopes in corals, giant clams, and calcareous algae

Time-series δ18O and δ13C records from cohabiting massive coralPorites australiensis and giant clamTridacna gigas from the Great Barrier Reed of Australia, and from calcareous green algae in a core through modernHalimeda bioherm accreting in the eastern Java Sea, provide insights into the complex links between environmental factors and stable isotopes imprinted in these reef skeletal materials. The aragonitic coral and giant clam offer 20 years and 15 years of growth history, respectively. The giant clam yields mean δ18O and δ13C values of-0.5±0.5‰ and 2.2±0.2‰ (n=67), which agree well with the predicted equilibrium values. The coral yields mean δ18O and δ13C values of-5.6±0.5‰ and-1.8±0.7‰ (n=84), offering a striking example of kinetic and metabolic fractionation effects. Although both the coral and giant clam harbor symbionts and were exposed to a uniform ambient environment during their growth histories, their distinct isotopic compositions demonstrate dissimilar calcification pathways. The δ18O records contain periodicities corresponding to the alternating annual density bands revealed by X-radiography and optical transmitted light. Attenuation of the δ18O seasonal amplitudes occurring in the giant clam record 8 years after skeletal growth commenced is attributed to a changeover from fast to slow growth rates. Extreme seasonal δ18O amplitudes of up to 2.2‰ discerned in both the coral and giant clam records exceed the equivalent seasonal temperature contrast in the reef environment, and are caused by the combined effect of rainfall and evaporation during the monsoon and dry seasons, respectively. Thus in addition of being useful temperature recorders, reef skeletal material of sufficient longevity, such asPorites andTridacna, may also indicate rainfall variations. Changing growth rates, determined from the annual growth bands, may exert a primary control on the coral δ13C record which shows a remarkable negative shift of 1.7‰ over its growth history, by comparison with only 0.15‰ negative shift in the contemporaneous giant clam record. Use of coral δ13C records as proxies of fossil fuel CO2 uptake by the ocean must be regarded with caution. The δ18O and δ13C records fromHalimeda are remarkably uniform over 1000 years of bioherm accretion history (δ18O=-1.7±0.2‰; δ13C=3.9±0.1‰,n=28), in spite of variable Mg-calcite cements present in the utricles. Most of the cement infilling is probably syndepositional, and both theHalimeda aragonite and the Mg-calcite cements containign 12.3 mole % MgCO3 are deposited in isotopic equilibrium. Therefore, in favorable circumstances these algal skeletal remains may act as the shallow water analogs of benthic foraminifera in deep sea sediments in recording ambient sea water isotopic composition and temperature.

Radiometric dating of submarine hydrocarbon seeps in the Gulf of Mexico

Massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid Bathymodiolus sp. containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and U-series isotopes. Stable carbon isotopes and 87 Sr/ 86 Sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon-rich fluids. Samples from five seepage sites on the northern Gulf of Mexico sea floor overlying subsurface salt diapirs and encompassing depths from 125 to >2000 m were selected as representative of the spectrum of active and extinct seeps examined from submersible dives. In general, paired 14 C and 230 Th dates of carbonate buildups consisting of aragonite, high-Mg calcite, and dolomite mineralogies are highly discordant. The cause of the discordance, established on the basis of paired Δ 14 C and δ 13 C values (Δ 14 C = −898‰ to −992‰ δ 13 C = −9.5‰ to −53.3‰, for n = 27), lies with the impairment of the radiocarbon dates resulting from dilution of the 14 C pool with fossil-hydrocarbon–derived carbon. The validity of the ionium dates based on U-rich (2.4–7.6 ppm) samples is demonstrated by the concordance between 234 U/ 238 U and 230 Th/ 234 U evolution in time, and by the ( 234 U/ 238 U) o activity ratios that are generally within the range of sea-water value of 1.14 ± 0.04 (2 sigma). Some dolomite-rich samples are exceptional because their ( 234 U/ 238 U) o ratios are significantly higher (1.22–1.38) than sea water, suggesting deposition from anoxic pore waters where the soluble U reached anomalous 234 U/ 238 U ratios. The formation of the carbonates from sea-water–derived fluids, rather than from formation fluids advecting from deep aquifers, is supported by the 87 Sr/ 86 Sr composition of the samples (mean 0.709145 ± 19 × 10 −6 , n = 14) that compares well with modern nonseep marine carbonates and the ambient Gulf of Mexico sea water (0.709171 ± 8 × 10 −6 ). Calcareous shells, the δ 13 C values of which indicate a carbon source in sea water (δ 13 C = −4.3‰ to −1.1‰), yield valid radiocarbon ages and show fair concordancy between their radiocarbon and ionium dates. Isotope migration attested by the observed U uptake in the fossil shells is likely to affect the accuracy of their ionium dates. Radiometric ages from extinct and senescent seep sites at upper bathyal depths indicate that hydrocarbon seepage occurred there during late Pleistocene time (195–13 ka). Ages derived from nascent seep sites at mid-bathyal and abyssal depths (12.3–0.0 ka) indicate that currently vigorous seepage was initiated at the end of the last deglaciation. These radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the sea floor.

Sulfur and oxygen isotopes of coeval sulfate-sulfide in pore fluids of cold seep sediments with sharp redox gradients

Abstract Leakage of crude oil and gas through fault conduits intersecting the seafloor gives rise to scores of point-source anoxic enclaves on the oxic northern Gulf of Mexico slope. A study of 13 short cores recovered with a manned submersible from these seepage-affected sediments reveals that microbial processes fueled by hydrocarbons cause extensive sulfur diagenesis. Sulfate reduction and sulfide release occurring in the pore fluids reach completion 10–25 cm below the sediment–water interface. Bacterial sulfate reduction (BSR) rates are highly variable between sites but maximum values (97 and 917 μmol SO4 cm−3 year−1) in a bacterial mat and mussel bed, respectively, are unusually high for cold, deepwater habitats. δ34S and δ18O values of the residual sulfate range from 20.7‰ to 70.8‰ (CDT) (n=45) and from 11.1‰ to 23.6‰ (SMOW) (n=33) compared to the overlying Gulf of Mexico bottom water values of 20.3‰ and 9.7‰, respectively. δ34S values of H2S yield a mean of 12.4±5.4‰ (CDT) (n=15). δ34S (SO4) data yield an integrated fractionation factor of αS=1.015 under a closed system assumption but a substantial higher fractionation of αS=1.023 under an open system assumption. Paired SO4–H2S inventory indicates that up to 28% of sulfide is removed from the system and supports the contention that seep sediments constitute an open system. The sulfur isotope fractionations reported here compare well with experimental data for cold-adapted sulfate-reducing bacteria but are substantially smaller than the “geological” fractionation of αS=1.055 derived from coeval sulfate-sulfide in Phanerozoic sediments. Isotope enrichments in the SO4 are 2.4 times greater in δ34S than in δ18O and the relations documented with f(SO4) are indicative of mixing between two end-member sulfate sources; seawater sulfate cycled through microbial dissimilatory sulfate reduction at depth and secondary sulfate produced by oxidation of H2S at near-surface through bacterial disproportionation (BDS) processes. The evidence for superimposed metabolic reactions in the reducing and oxidative parts of the anaerobic sulfur cycle in seep sediments has important implications regarding the proposed use of pore-water sulfate profiles as proxies of upward methane fluxes resulting from dissociation of marine-based gas hydrates.

Sea floor responses to hydrocarbon seeps, Louisiana continental slope

Observations and samples from research submersible dives confirm that brines, crude oil, fluid mud, and gases are common seep products. Through this mechanism a unique interplay of geochemical, geologic, and biological processes resulting in unusual sea floor features ranging from carbonate-rich nodular sediments to mounds with tens of meters relief. Stable carbon isotopes occluded in the carbonates provide a permanent imprint that links these authigenic carbonates to by-products of microbial breakdown of crude oil and gas. Recent DSV ALVIN dives confirm that hydrocarbon seeps and their accompanying chemosynthetic communities and authigenic carbonate mounds occur over the entire depth range of the slope.