U23D-0075
Gas hydrates and climate evolution: I-129 chronology in active margins
Continental margins are the locations of large quantities of gas hydrates, containing a substantial amount of carbon, mostly in the form of methane. Although this methane is of organic origin, the specific sources and the history of carbon deposition and transport are not well understood. We have investigated gas hydrate fields in active margins from the Pacific Ocean, using the I-129 system as a proxy for the determination of organic-rich source formations responsible for the release of methane. The I-129 isotopic system is well suited for this approach, given the strong affiliation of iodine with organic material and the presence of the long-lived cosmogenic isotope I-129 (T½ = 15.7 My). We determined iodine ages in more than 200 pore water samples from gas hydrate fields collected from seven active margins with slab ages ranging from less than 10 My to more than 140 My. In the vast majority of cases, iodine ages were considerably older than the host sediments for the gas hydrates, but did not show correlation with the ages of subducting marine sediments. These observations point to sources of iodine and methane in the upper plates of subduction zones. A statistical analysis of all the data shows that the distribution for the iodine ages starts around 55 Ma, with a broad peak around 30 Ma. The distribution follows closely the changes in atmospheric oxygen concentration, which in turn is related to the evolution of the global climate and the deposition of carbon. The data suggest that, along active margins, large amounts of carbon (and iodine) were deposited in the Early Eocene, which are slowly released through fractures in the upper plates leading to the accumulation of methane in gas hydrate fields. Gas hydrate fields at active continental margins are thus the surface expression of the presence of large amounts of carbon deposited there following changes in global climate patterns.
U23D-0076
Stability of Gas Hydrates on Continental Margins: Implications of Subsurface Fluid Flow
Gas hydrates are found at or just below the sediment-ocean interface in continental margins settings throughout the world. They are also found on land in high latitude regions such as the north slope of Alaska. While gas hydrate occurrence is common, gas hydrates are stable under a fairly restricted range of temperatures and pressures. In a purely conductive thermal regime, near surface temperatures depend on basal heat flow, thermal conductivity of sediments, and temperature at the sediment-water or sediment-air interface. Thermal conductivity depends on porosity and sediment composition. Gas hydrates are most stable in areas of low heat flow and high thermal conductivity which produce low temperature gradients. Older margins with thin continental crust and coarse grained sediments would tend to be colder. Another potentially important control on subsurface temperatures is advective heat transport by recharge/discharge of groundwater. Upward fluid flow depresses temperature gradients over a purely conductive regime with the same heat flow which would make gas hydrates more stable. Downward fluid flow would have the opposite effect. However, regional scale fluid flow may substantially increase heat flow in discharge areas which would destabilize gas hydrates. For example, discharge of topographically driven groundwater along the coast in the Central North Slope of Alaska has increased surface heat flow in some areas by more than 50% over a purely conductive thermal regime. Fluid flow also alters the pressure regime which can affect gas hydrate stability. Modeling results suggest a positive feedback between gas hydrate formation/disassociation and fluid flow. Disassociation of gas hydrates or permafrost due to global warming could increase permeability. This could enhance fluid flow and associated heat transport causing a more rapid and/or more spatially extensive gas hydrate disassociation than predicted solely from conductive propagation of temporal changes in surface or water bottom temperature. Model results from both the North Slope of Alaska and the Gulf of Mexico are compared.
U23D-0077
Understanding Laboratory-scale Methane Hydrate Dissociation in Porous Media: A Model for Marine Hydrate Occurrences
Methane hydrates are inclusion compounds in which water molecules form a well-defined cage to encapsulate a methane molecule. The seismic data and recovery of hydrate cores from several worldwide locations show that gas hydrates are a common occurrence, both in permafrost and marine settings where high-pressure and low-temperature conditions coexist. There is growing concern of kinetic instability of methane hydrate with respect to slight temperature changes that could impact seafloor stability and methane gas arising from massive hydrate dissociation in the ocean may contribute to global warming. In sediment- hosted methane hydrates, the sediment-hydrate interaction governs the mechanical strength as well as other geophysical properties of formations or well bores in the event of a rapid release of methane. We are investigating the laboratory-scale hydrate growth and decomposition habits of hydrates formed in natural depleted sediments. The data collection is being carried out in the BNL Flexible Integrated Study of Hydrates (BNL-FISH) unit fitted with a 300 mL customized Temco cell, in which methane input/output is precisely measured. Our present focus is on the formation of consolidated cores and the first data set utilized Ottawa sand as baseline host sediment. Methane hydrate cores were repeatedly formed at various temperature/pressure combinations and subsequent gas output responses were measured during dissociation by setting non-equilibrium conditions. Several P/T curves were generated as the hydrate formation kinetics was monitored over time until the pore pressure asymptoted at the hydrate equilibrium pressure. The decomposition was introduced by incremental depressurization (100 and 200 psi) from the equilibrium pressure. Three thermocouples, placed at various locations within the cell, measured temperatures to provide a glimpse of the decomposition front. The talk will show comparative decomposition data, methane hydrate dissociation profile and the calculated hydrate saturation and enthalpy of dissociation.
U23D-0078
Oceanic gas hydrate dissociation in response to climate change and the fate of hydrate- derived methane
Paleooceanographic evidence has been used to postulate that methane from oceanic hydrates may have had a significant role in regulating past global climate. However, the behavior of contemporary oceanic methane hydrate deposits subjected to rapid temperature changes, like those predicted under future climate change scenarios, has only recently been investigated, and the fate of the hydrate-derived methane is still unclear. The release of methane from oceanic deposits is controlled and constrained by coupled heat transfer, fluid flow, and thermodynamic processes, and this methane may interact with benthic biogeochemical systems before reaching the seafloor. In this work, computational simulations of hydrate dissociation in oceanic sediments have been coupled to models of sediment biogeochemistry. Important considerations include dynamic exchange of methane in the aqueous and gas phases within the chemically active zone, transport of reaction products through the sediment, the effects of depth and temperature, the properties of the hydrate-bearing sediments, and the one- and two-dimensional configuration of the hydrate deposit itself. This coupled model establishes bounds for benthic methane oxidation (and possibly, sequestration of the carbon as solid carbonate) for potential hydrate release scenarios and provides the first assessment of the fate of hydrate-derived methane released due to climate change.
U23D-0079
Gas hydrate reservoir degassing: thermodynamic and kinetic data as basis for predictions
Natural gas hydrates contain predominantly methane but sometimes also other hydrocarbon- and non- hydrocarbon gases such as CO2 or H2S. The amount of other gases beside methane depends on the source of the gas: in case of a microbial origin the gas is almost pure methane whereas gases from thermal origin may contain a high percentage of higher-molecular weight compounds, such as ethane, propane and larger hydrocarbons. Calculated compositions of gas leaking from an oil reservoir also show a significant amount of nitrogen beside the other components. All components in addition to methane have a strong influence on the stability field of the resulting hydrate phase. In the presence of higher hydrocarbons the stability of the resulting gas hydrate is shifted to higher temperatures and lower pressures whereas the enclathration of nitrogen induces a shift of the hydrate stability to higher pressures and lower temperatures in comparison to pure methane hydrate. Furthermore, hydrate formation kinetics also depend on the composition of the gas phase: recent studies have shown the rapid formation of hydrates containing H2S in addition to methane, whereas the formation of hydrates containing small amounts of ethane and propane seemed to be kinetically inhibited. Due to the significant changes in hydrate stability and formation kinetics depending on gas composition thermodynamic and kinetic data for gas mixtures is crucial for all calculations and predictions regarding gas hydrate reservoir degassing as a consequence of climate change. In this study we will present thermodynamic and kinetic data from in-situ measurements (X-ray diffraction and Raman spectroscopy) on gas hydrates that had been synthesized under natural conditions.
U23D-0080
Pockmarks, Western Ross Sea, Antarctica and Mendeleev Ridge, Central Arctic Ocean: Recent and/or Prevalent?
In 2004, the NBP-0401 cruise to the western Ross Sea, found a large field of pockmarks to the north and west of Franklin Island. The pockmarks ranged in size up to 300 m or more in diameter and are up to 30 m deep. The pockmarks are generally circular and are found in a partially surveyed 3,000 km2 region at water depths ranging from 450 m to 510 m. The pockmarks were most concentrated in an area of approximately 400 km2 where they cover as much as 20% of the seafloor. About 50 km to the west of the heavily pockmarked area, a series of seafloor constructions, up to 5 km in diameter and 120 m high were found in water depths of 490 m to 520 m. Again, ice conditions precluded a complete survey but it is believed the circular features may be carbonate mounds very similar in size and water depth to the ones found by Shannon et al. (2007) in the Porcupine Bight region, offshore Ireland. In 2006, the HLY-0602 cruise undertook a seismic refraction experiment along the Mendeleev Ridge in the Arctic ocean. In the course of the experiment, two to three multibeam lines were run approximately along the crest of the ridge from 76° 40'N to 78° 50'N. On HLY-0503, pockmarks were found in the vicinity of 78° 15'N including one extraordinary cluster of pockmarks at 78° 20'N which were cored on HLY-0602. Three gravity cores taken within pockmarks recovered a significant shell hash in the upper 1 cm but carbon analysis on the shells did not reveal any evidence of chemosynthetic origin for the mollusks. Cores taken along the ridge but away from pockmarks had at most a single shell in the upper 1 cm. Shells were not found below the surface of the cores. Pockmarks along the Mendeleev Ridge are found at depths from 820 m to an extraordinary feature at a depth of ~1420 M. This collapse? feature is 10 km by 5 km with a series of pockmarks in its deepest part. The major feature itself has a central depth of 1480 m but pockmarks withing the feature are as deep as 1520 m at their individual centers. There are additional concentrations of pockmarks along the spine of the Mendeleev Ridge at depths between the two mentioned above, with clusters at 889-996 m, 1030-1182 m, and 1260 m. There are regions of the ridge without significant concentrations of pockmarks but it is unknown if this is a result of our very sparse survey or some other factor. An attempt will be made to estimate possible methane outgassing if it can be assumed that the pockmarks observed on HLY-0602 represent recent phenomena. Shannon, P.M., McDonnell, A., and Bailey, W.R., 2007. The evolution of the Porcupine and Rockall basins, offshore Ireland: the geological template for carbonate mound development, Int J Earth Sci (Geol Rundsch) 96: 21-35.
U23D-0081 INVITED
Why should the East Siberian Shelf be considered a new focal point for methane studies in terms of Global Climate Change?
The inner East Siberian Arctic Shelf (ESAS) has not been considered to be a methane (CH4) source to hydrosphere or atmosphere because submarine permafrost, which underlies most of the ESAS, was believed, first, not to be conducive to methanogenesis, and, second, to be acting as an impermeable lid, preventing CH4 escape through the seabed. However, recent experimental data obtained during summer expeditions (2005-2008) on dissolved CH4 and atmospheric CH4 mixing ratio, and data on dissolved CH4 collected during a winter expedition (2007) show that extremely high concentrations of dissolved CH4 and an impulsively (non-gradually) increasing atmospheric mixing ratio of CH4 in some areas of the ESAS can be attributed to methanogenesis in taliks of different origins (exogenous, endogenous), and to year-round release through these taliks of old CH4 from seabed CH4 reservoirs. The occurrence of taliks that are not located above fault zones and/or river paleo-valleys (which mostly coincide) is not consistent with currently existing theoretical notions and modeling results on submarine permafrost thermal state, spatial extent, and the exclusive role of endogenous taliks as pathway providers for CH4 release from seabed reservoirs. It is highly likely that the role of exogenous taliks and taliks of combined origin have been significantly underestimated. It is also highly likely that the CH4 released from seabed reservoirs could be emitted by different types of decaying gas hydrates, one of which is unique relic hydrates, which occur as shallowly as 20 m below the seabed and could be disturbed by the warming of as much as 12-17°C that has occurred as the sea intruded during the last 10-15 Kyr. If submarine permafrost fails to impede CH4 emission from seabed reservoirs the modern biogeochemical cycle could be significantly altered because the extreme shallowness of the inner ESAS allows the majority of CH4 to pass through the water column and escape to the atmosphere.
U23D-0082
Past and Future Methane Loss From Arctic Gas Hydrate Based on Pore-Fluid Overpressure
Observations from the shallow shelf region of the Beaufort Sea indicate several sites with known or inferred gas hydrate. This gas hydrate was formed on land beneath permafrost during glacial periods when sea level was lower and has been dissociating since submergence initiated warming several thousand years ago. Current anthropogenic warming is substantially smaller, and shorter in duration, than the ~ 17°C warming due to submergence of the shelves. Thus, we do not expect the total methane output from the Arctic shelves to be dramatically altered by global warming (at the present time). We instead characterize the past long-term effect of warming in the Arctic by modeling the gas hydrate response to sea level change. Insights gained from past warming offer clues to the future response of gas hydrate to anthropogenic warming. Furthermore, the model establishes a baseline for gas hydrate release, necessary to identify the contribution from anthropogenic warming. A key parameter for assessing the long-term response to warming is the overpressure of the sediments, both within and below the hydrate stability zone. We attribute this overpressure to the phase change from hydrate to free gas bubbles and interpret this to mean that the rate of conversion is faster than the rate of pressure dissipation due to fluid flow. The low permeability of the frozen permafrost sediments above the hydrate stability zone may be an important factor in setting these rates. We use measured profiles of overpressure from several wells in the Beaufort Sea to place limits on the gas hydrate volume and the rates of dissociation. By utilizing these limits, we can improve the prediction of the hydrate response to future warming.
U23D-0083
Degrading gas hydrates as a possible source for gas release and the formation of pockmark features, Mackenzie Delta, N.W.T., Canada
Arctic gas hydrates occurring within and beneath permafrost in N.W. Canada have experienced significant post glacial warming with millennial time scales sufficient to cause gas hydrate dissociation, migration and release of free gas at the surface. Forcing processes imposing changes in ground surface temperatures include warming post-glacial air temperature and large-scale geologic processes such as marine transgression, lake development and fluvial processes including delta progradation. The Geological Survey of Canada has conducted field investigations to characterize more than twenty methane-venting pockmark features in the outer Mackenzie Delta to assess if they are formed by degradation of gas hydrates at depth. The pockmarks occur in a small pond and channel of the Mackenzie River where mean annual bottom temperature is presently above 0°C. As terrestrial permafrost in the area is only ~60 m thick, it is anticipated that a through going talik exists beneath the water bodies and conditions at depth do not allow for stable methane hydrate. However, the permafrost and gas hydrate setting changes substantially approximately 5 km to the east of the study area where permafrost is more than 400 m thick, gas hydrates have been identified in exploration well logs, and the Niglintgak gas field has been delineated. Geochemical analyses of gas released from the pockmarks have confirmed that the source of the gas is thermogenic and similar to the gas at the Niglintgak field. The flux of methane to the atmosphere is estimated to be approximately 5 x105 m3yr-1 and several of these features have been actively venting gas for more than 40 years confirming a substantial source of gas feeding them. Degrading gas hydrates are thought to be a candidate source of the gas with significant lateral migration controlled by both structural and permafrost characteristics.
U23D-0084
Gas Hydrates and Perturbed Permafrost: Can Thermokarst Lakes Leak Hydrate-Derived Methane?
Thermokarst lakes are common features in the continuous permafrost of Siberia, the Alaskan North Slope, and the Canadian Arctic and have been intensely studied as the loci of rapid and substantial methane flux to the atmosphere. Previous numerical modeling has constrained the conditions under which deep thermokarst lakes can develop organic-rich thaw bulbs (talik) tens of meters thick, and seismic surveys have imaged thaw bulbs more than 75 m thick beneath some thermokarst lakes. Microbial processes active in talik organic material are likely the predominant source for thermokarst methane emissions, although coalbed methane and methane associated with conventional hydrocarbons may contribute in some geologic settings. Here we evaluate the possibility that another source--methane released from dissociating gas hydrate--could contribute to methane emissions from these lakes. Temperatures within and beneath thermokarst lakes are significantly warmer than those in surrounding permafrost, and these relatively warm conditions can persist to depths several times greater than the thickness of the thaw bulb. For a 95-m-thick thaw bulb and a geothermal gradient consistent with the regional top of gas hydrate stability at ~200 m depth, the warmer temperatures beneath a thermokarst lake could lead to destabilization of up to 75 m of gas hydrate. Arguably, the presence of gas hydrate near the top of the stability zone in permafrost regions has not yet been observed. Nonetheless, the potential dissociation of such relatively shallow gas hydrate and the widespread availability in terrestrial settings of high permeability conduits (e.g., faults, sandy strata) that could facilitate the migration of hydrate-derived methane to the surface render this an important topic for future investigation. The susceptibility of permafrost gas hydrate zones to thermal perturbations is in sharp contrast to the situation in conventional marine hydrate provinces. There, gas hydrate first dissociates at the base of the stability zone, and upward migration of released methane may result in refreezing of the methane as gas hydrate. The shallow Arctic Ocean offers the opportunity to examine the interplay between both permafrost and marine gas hydrate stability processes in an area where thermokarst thaw bulbs flooded by Holocene sea level rise pierce relict permafrost. Large-scale climate change processes have already affected methane emissions from these now-offshore thermokarst systems. Onshore, the challenges will be determining the contribution (if any) of hydrate-derived methane in contemporary methane emissions using sophisticated analytical techniques and documenting past gas hydrate dissociation events, which are most likely to have coincided with past warming trends in the Arctic.
U23D-0085
Microbial Communities in Methane Hydrate-Bearing Sediments from the Alaskan North Slope
High latitude soils and sediments often contain large quantities of methane as well as microbial communities capable of producing and consuming the methane. We studied the microbial communities collected from hydrate-bearing sediments on the Alaskan North Slope to determine how abiotic variables (e.g., grain size, hydrate presence, original depositional environment) may control the type and distribution of microbes in the sediments. The cores were acquired from sub-permafrost, Eocene (35-36 million years ago [MYA]) sediments laid down as a marine transgressive series within which hydrates are believed to have formed 1.5 MYA. Forty samples, eight of which originally contained hydrates, were acquired from depths of ca. 606–666 meters below land surface. Five samples from drilling fluids acquired from the same depth range were included in the analysis as a control for contamination during the drilling and handling of cores. DNA was extracted from the samples (typically <1 ng DNA/g sediment was recovered) and then amplified using polymerase chain reaction with primers specific for bacterial and archaeal 16S rDNA. Only bacterial DNA amplicons were detected. Terminal-restriction fragment length polymorphism (t-RFLP) was used to measure bacterial diversity in the respective samples. Non-metric multidimensional scaling (NMDS) was then used to determine the abiotic variables that may have influenced bacterial diversity. NMDS analysis revealed that sediment samples were distinct from those obtained from drilling fluids suggesting that the samples were not contaminated by the drilling fluids. All samples had evidence of microbial communities and sample depth, temperature, and hydrate presence appeared to have some influence on community diversity. Samples sharing these environmental parameters often shared common t-RFLP profiles. Further examination of selected samples using clone libraries should help to identify the key taxa present in these unique sediments and yield a better understanding of the biogeochemistry of these gas-bearing systems.
U23D-0086
Methane Seepage From the Arctic Shelf; 20 Years of Research on the Beaufort Sea Margin
The U. S. Geological Survey has lead or played major roles in several efforts over the past 20 years to find geochemical evidence for gas hydrate dissociation on the Beaufort Sea shelf, a region of complex and varied geologic features that include: 1) several river deltas entering the Arctic Ocean, the largest of which is the Mackenzie River, 2) submerged continental shelf underlain by permafrost, 3) known petroleum systems of northern Alaska and the Mackenzie River Delta - Canada, 4), submerged pingo-like features (PLF's ) and, 5) pockmark fields. The results of these studies show that gas hydrate is present and that methane source can be both microbial and thermogenic. In light of our rapidly changing climate, the instability and potential methane release from Arctic gas hydrate deposits are reemerging as pivotal uncertainties. On the Alaskan Beaufort Shelf in water depths or about 10 m or less, methane concentrations in seawater are elevated relative to atmosphere. This methane likely originates from microbial degradation of organic matter deposited by rivers or coastal currents, and may be associated with organics in destabilized from recently thawed submerged shelf permafrost. In deeper water, north and west of the Prudhoe Bay area, some exceptionally high bottom water methane concentrations were measured with carbon isotopic signatures very similar (about -46 to -48‰) to gas hydrate sampled from the Mount Elbert 01 gas hydrate test well drilled in 2007 in the same area. This methane is presumably associated with the Prudhoe Bay gas hydrate and petroleum system, and likely from either gas hydrate dissociation or simple gas migration. Gas venting in and around the Mackenzie River delta is associated with offshore Pingo-like features (PLF's) and pockmarks. These PLF's resemble onshore pingos, but with an unknown origin. The region is underlain by an active petroleum system, submerged shelf permaforst, and gas hydrate. Methane concentrations are elevated in cores from the PLF's and remotely operated vehicle (ROV) surveys revealed streams of microbially-sourced methane bubbles (-76 to -80‰, 14C dead) emanating from at least two PLF's. These PLF's are associated with an exceptionally thick area of submerged permafrost and may be produced by upward-migrating, overpressured gas and water. The gas and water may originate from decomposing, intra-permafrost gas hydrate, or non hydrate-bound methane from decomposition of deeply buried organic matter sequestered in permafrost. Microbial methane (-72 to -99‰) from the Kugmallit pockmark field, near the mouth of the Mackenzie delta is isotopically similar to methane emanating from sampled PLF's. Methane in deeper gas hydrates of the Mackenzie delta is thermogenic (mean value -42.7‰,). Inland, gas seeps occur in shallow ponds and streams, resulting in steep-sided pockmarks. This methane is thermogenic, 14C dead, has been venting vigorously for at least 45 years, and is very similar in composition to deeper gas hydrate and thermogenic gas in nearby gas fields.
U23D-0087
High-Resolution 3D Seismic Data Characterize Pockmark and Chimney Structures: Methane Flow Through Hydrated Sediments at the Vestnesa Ridge off the W-Svalbard Margin in the Arctic
The P-Cable 3D seismic system of the University of Tromsoe deployed onboard RV Jan Mayen allowed to acquire high-resolution 3D seismic data at the Vestnesa Ridge, a bottom current-controlled sediment drift off the West Svalbard margin. The main aim was to investigate the active fluid flow through the gas hydrate stability zone and to understand where and how fluids escape in a hydrated sediment drift. The survey area located at the north-western part of the Vestnesa Ridge concentrates on the ridge crest in a water depth of 1250-1320 m. The seafloor morphology is characterized by an aligned band of pockmarks that show diameters of up to 600 m and indicate recent fluid flow activity. Detailed acoustic imaging of the seabed pockmarks that are connected to sub-seabed chimneys down to about 600 m bsf (below seafloor) show an unprecedented resolution with full 3D spatial coverage. The chimneys pierce through stratified sediments where a bottom simulating reflector (BSR) indicates the base of the gas hydrate stability (BGHS) zone at about 200 ms TWT (two-way time) bsf corresponding to 160-180 m bsf. Gas hydrate occurrence is inferred from increased velocities above the BSR. Decreased P- wave velocities and high-amplitude reflections terminating at and beneath the BSR indicate free gas zones. The P-wave velocities are obtained from ocean bottom hydrophone (OBH) modelling. The acoustic chimney structures act as conduits for focused upward flow of methane-rich fluids and gases from depth > 600 m bsf. They show complex internal structures. High-amplitude anomalies in the upper 50 m of a chimney structure point to the formation of near-surface gas hydrates and/or authigenic carbonate precipitation. However, the 3D seismic data analysis also reveals irregularly distributed patches of high amplitudes and amplitude blanking throughout the deeper parts of the chimney. This may indicate a variable formation of hydrate and/or authigenic carbonate within the chimneys in time and space. High-amplitude reflections might also be caused by free gas accumulating beneath hydrated, impermeable sediments. These features may control the plumbing systems of the Vestnesa sediment drift.
U23D-0088
Persistence of methane seepage from pockmarks in the Storegga Slide region
The Storegga Slide complex is a three-stage slope failure where the most recent stage occurred 8.1 kya. The mechanisms and consequences of these slope failures are likely linked to deglaciation, sediment overpressurization, and/or methane hydrate instability. The northern flanks of the Storegga Slide complex on the Norwegian continental margin contain pockmark features that are inferred to be related to shallow methane deposits and gas venting, based on bottom simulating reflectors and chimney structures present in seismic reflection profiles. Three jumbo piston cores (JPC), two taken from separate pockmarks and one core taken near the pockmarks on the northern flank of the Storegga Slide (800m water depth), were sampled at 10 cm resolution. Radiocarbon ages indicate typical sedimentation rates of 40-70 cm/ka at these sites. Concentrated layers of Bathymodiolus mussels, an indicator of methane-rich environments, are dated to 18.2 and 22.2 ka (calendar years before present; 670-710 cmbsf and 780-790 cmbsf, respectively) in one core. Oxygen and carbon stable isotope analysis on planktonic foraminifera, Neogloboquadrina pachyderma sinistral, and benthic foraminifera, Melonis barleeanum and Islandiella norcrossi, from pockmark region sediment provide a high-resolution record of temperature change and methane activity in the Storegga Slide area. Oxygen isotopic results show a clear glacial/deglacial transition (-1.95 ‰ δ18O), including Heinrich Event 1 and Terminations 1 with the Younger Dryas. Multiple negative excursions in the planktonic carbon isotope record (as much as -7.59 ‰ δ13C) indicate methane-derived carbon is episodically available within the pockmark sediments. However, these isotopic shifts do not occur during the Storegga slide event and are instead concurrent with the timing of deglacial warming. Carbon isotope values from benthic foraminifera suggest frequent exposure to methane seepage. Together the planktonic and benthic data from these cores show that these pockmarks have been sites of methane seepage since at least the last glacial maximum, and that seepage rate has varied.
U23D-0089
Methane Emission From the Congo Deep Sea Fan and Subsequent Aerobic Oxidation in the Quaternary Tropical Atlantic
The Congo Fan is a well-documented region of important methane (CH4) storage and gas seepage: gas hydrates abound at and just below the sediment surface as do large deeply-buried reservoirs of thermogenic methane linked with hydrocarbon source rocks. In the Congo Fan, both sources of methane are intimately connected through a complex network of faults, structuring this massive sediment wedge in a unique way. Methane release from both reservoirs has the potential to drive or respond to changes in local and global climate, thus causing changes in ocean chemical properties and biotic responses. Understanding these poorly-constrained mechanisms of methane emission and reconstructing the history of past emissions in the ocean is the main focus of our study. The ultimate fate of CH4 is, typically, its oxidation to CO2; this process can occur aerobically and anaerobically. Compared to anaerobic processes, aerobic methane oxidation, and its underlying mechanisms and possible feedbacks for the ocean-climate system, has received little attention. Here we present molecular evidence from Congo Fan sediments for aerobic methane oxidation and highlight how the process may play a previously unrecognised role in carbon cycling and oxygen availability in the water column. Bacteriohopanepolyols (BHPs) are lipid membrane constituents and occur with a wide range of structural and functional variability in many bacteria. Amino-BHPs are produced in large abundances by methane-oxidising bacteria and the 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) is a highly specific biomarker for aerobic methane oxidation. The Congo Fan record (ODP Leg 175, Site 1075; 2996 m depth) spans the last 1 Myr and reveals remarkable organic biomarker preservation, with a suite of 13 different BHPs identified in most sediment horizons, including aminopentol. Aminopentol abundance varies widely throughout the section and appears to do so cyclically, with markedly greater concentrations between ca. 500 and 600 ka and compound-specific stable carbon isotope analyses confirm that the amino-BHPs are of methanotrophic origin. Although suspected to be primarily biogenic in origin, δ13C values of ca. -42‰ further suggest a potential contribution from deep thermogenic sources to the emitted methane. Ongoing sea surface temperature reconstruction, using the TEX86 proxy, seeks to investigate potential perturbations in local climate with relation to these previously unrecognized emission events. The hopanoid record pushes direct evidence for aerobic microbial oxidation of methane far back into the geological record. This process is believed to be intrinsically linked with methane gas hydrate dissolution. Thus, the variability in amino-BHP abundance could provide an indicator for methane emission events, directly linking key aspects of structural geology with gas hydrate stability, deep ocean processes, and methane cycling.
U23D-0090
Effect of climate variations on gas hydrate stability conditions at fluid escape structures in the Gulf of Cadiz
In the Gulf of Cadiz several occurrences of mud volcanoes, diapiric ridges, pockmarks and methane seepages (both active and fossil, as interpreted from occurrences of methane-derived authigenic carbonates) are characterized by high methane contents in shallow sediments and the presence of gas hydrates on the most active structures. These features indicate preferential areas for the escape of deep fluids enriched in hydrocarbons, mainly methane. Numerous fluid escape structures occur along the upper and mid-continental slope, where the Mediterranean Outflow water is in direct contact with the seafloor. This area is especially sensitive to paleoceanographic changes and the estimated ages of some of the methane- derived authigenic carbonates indicate formation over discrete episodes that correspond to periods of rapid paleoceanographic changes (such as the onsets of glacial stages terminations). Calculations for the depth of the gas hydrate stability zone were done using gas compositions based on measurements from active mud volcanoes, which reflect a mixture of biogenic and thermogenic sources. The depths of the gas hydrate stability zone were calculated for different paleoceanographic scenarios, both present day conditions and at the Last Glacial Maximum, using variable intensities of the Mediterranean Outflow. Modeling results indicate that pressure variations have negligible effects on gas hydrate stability, but temperature variations can have significant impacts. The geological significance of the delay effect of the temperature perturbations on the gas hydrate stability zone was also evaluated. Seabottom warming by 2° C, as resulting from changes of the Mediterranean Outflow pathway under present-day similar conditions, can destabilize potential shallow gas hydrates occurrences in the northern margin of the Gulf of Cadiz. The transition from glacial to interglacial conditions (corresponding to a 4-6° C seabottom warming) significantly reduces the depth of the gas hydrate stability zone, and at several sites the stability zone disappears entirely for both gas compositions. Therefore, increases in the seafloor temperature associated with glacial to interglacial transitions and changes of the position of the Mediterranean Outflow as a bottom current, are processes that can efficiently trigger episodes of dissociation of potential gas hydrates that would result in intense flux of methane rich fluids to shallow sediments or even into the seabottom.