U34A-01 INVITED
Towards century-timescale simulation of the response of ocean sediment methane hydrates to climate change
We show preliminary results from SpongeBOB, a new two-dimensional model of the accumulation, compaction (Sponge), and geochemistry (Beneath-Ocean Biosphere) of a deep continental margin sediment lens from the coast to the abyss, applied to the passive margin on the East coast of the United States. The model tracks the 200 million-year life history of the sediment mass from initial rifting to the present, with specified rates of subsidence and sediment accumulation to match inferences from seismic sections and deep drilling data. Gas migration is controlled by the gas volume, which allows the gas phase to become interconnected at a critical value. The present-day distribution of methane as hydrate and gas in sediments, and pore fluid tracers such as delta-13C of methane and DIC, are used to ground truth the rates of processes such as methanogenesis and gas and pore fluid migration. We subject the model to transient forcing of the glacial / interglacial cycles and of anthropogenic climate change. Most of the methane is in the stratigraphic type deposits, distributed widely at low concentrations, tending also to be found near the base of the stability zone hundreds of meters below the sea floor. The time scale for atmospheric warming to reach and melt these deposits would generally be thousands of years. The structural deposits could melt more quickly. These are controlled by subsurface gas migration through geological structures such as faults and permeable layers. Hydrate concentrations can be much higher in structural deposits, and much closer to the sea floor. We make a preliminary attempt to resolve these two parts of the hydrate methane reservoir by explicitly incorporating representative geological features into the two-dimensional model, such as extensional faulting in the crust and overlying sediments and the effects of permeable layers associated with turbidites.
U34A-02
Onset and Stability of Gas Hydrates under Permafrost in an Environment of Surface Climatic Change in the Beaufort-Mackenzie basin – Past and Future
Modeling of the onset of permafrost formation and succeeding gas hydrate formation in the changing surface temperature environment has been done for the Beaufort-Mackenzie Basin (BMB). Numerical modeling is constrained by deep heat flow from deep well bottom hole temperatures, deep conductivity, present permafrost thickness and thickness of Type I gas hydrates. Latent heat effects were applied to the model for the entire ice bearing permafrost and Type I hydrate intervals. Modeling for a set of surface temperature forcing during the glacial-interglacial history including the last 14 Myr, the detailed Holocene temperature history and a consideration of future warming due to a doubling of atmospheric CO2 was performed. Two scenarios of gas formation were considered; case 1: formation of gas hydrate from gas entrapped under deep geological seals and case 2: formation of gas hydrate from gas in a free pore space simultaneously with permafrost formation. In case 1, gas hydrates could have formed at a depth of about 0.9 km only some 1 Myr ago. In case 2, the first gas hydrate formed in the depth range of 290 – 300 m shortly after 6 Myr ago when the GST dropped from -4.5° C to -5.5° C. The gas hydrate layer started to expand both downward and upward subsequently. More detailed modeling of the more recent glacial–interglacial history and extending into the future was done for both BMB onshore and offshore models. These models show that the gas hydrate zone, while thinning will persist under the thick body of BMB permafrost through the current interglacial warming and into the future even with a doubling of atmospheric CO2.
U34A-03
Potential High Latitude Hydrocarbon Venting From the Barents Sea and Gas Hydrate Response to Climate Change
During the last decade an increasing focus has been on gas hydrates and fluid flow in continental margins and their impact on the environment and climate. Our research concentrates on high latitudes in a region where the on average 300 m deep Barents Sea covers an area of about 1.3 million km2. Since the greatest gas discovery in the Norwegian economic zone so far (Snohvit in 1984), the petroleum industry requested several 3D seismic surveys to be carried out in the area in addition to 2D seismic lines. The data allow an assessment of fluid flow from a gas reservoir to the gas hydrate stability zone. The two main objectives are: a) detecting where and how fluids migrate from greater depth to the seabed and 2) identifying shallow acoustic anomalies and their relationship to fluid migration pathways. We will show that fluids are migrating through the whole stratigraphic column to the seafloor, but that they are also trapped in specific horizons as for example the gas hydrate stability zone. The geophysically inferred fluid migration occurs over a vertical distance of ~ 1700 m where the time involved remains unknown. At the seafloor, pockmarks or seabed craters exists depending on the dynamics of the involved processes. Fluids and gas that reach the seafloor can rapidly escape to the hydrosphere and, because of the shallow water depth, may contribute to greenhouse gas concentrations in the atmosphere. The total volumes of gas that may leave the chimneys are unknown but may be of importance in terms of glacial-interglacial methane cycles. The Barents Sea area may have experienced significant cycles of fluid expulsion and natural hydrocarbon leakage due to major episodes of sediment erosion and pressure changes driven by ice ages.
U34A-04 INVITED
Do Isotope Data From Ice Help Us Identify Any Relationships Between Palaeo- Atmospheric Methane and Gas Hydrates?
Considerable debate surrounds the potential of gas hydrates to substantially, and possibly catastrophically, influence the methane budgets in water columns and/or the atmosphere. To some degree the carbon and hydrogen isotope ratios of methane can test this clathrate gun hypothesis. In the Holocene and late Pleistocene, isotope measurements of methane in ice by others and us (Schaefer et al., 2006, Fischer et al., 2008) have produced unexpected results, i.e., d13CH4 ca. -46 vs. -47 permil present day. Of particular interest are the constant 12-C enriched methane isotope signatures across the age transitions displaying rapid methane increases, which is not consistent with clathrate release. During the methane rise at the Younger Dryas-Preboreal transition (YD-PB), for example, there is no support from the ice data, given our current understanding of the size and composition of the global clathrate reservoir, for a major clathrate input. In addition, ice records of methane concentration rule out methane bursts of sufficient magnitude to drive greenhouse warming (Chappelaz et al., 1997). Temperature-methane lead-lag relationships show that the methane rise followed and did not trigger Greenland warming (Severinghaus et al., 1998). Stable hydrogen isotopes rule out sizeable contributions of marine clathrates (both biogenic and thermogenic) to the rapid methane increases at the YD-PB (Sowers, 2005). Also, 14-C methane measurements show no additional release of fossil clathrate methane at critical times (Petrenko et al., 2007). Apart from possible minor contribution of clathrate methane to (late) glacial concentration variations, none of the ice datasets published to date support the clathrate gun. Alternatively, we suggest that permafrost and varying onshore thermogenic gas seepages may play a more critical role in the palaeo-atmospheric methane sources and budgets.
U34A-05
Measurements of Carbon-14 of Methane in Greenland Ice: Investigating Methane Sources During the Last Glacial Termination
We present the first measurements of 14C of methane (14CH4) in ancient glacial ice. 14CH4 should distinguish unambiguously between wetland and fossil (clathrate or other geologic CH4) contributions to abrupt atmospheric CH4 increases observed at times of rapid warming in Greenland ice cores. 1000-kg-sized ice samples, dating to the Younger Dryas - Preboreal (around 11,600 yr BP) and Oldest Dryas - Bølling (around 14,700 yr BP) abrupt climatic transitions, were obtained from an ablation site in West Greenland. Measured 14CH4 values (28 - 35 pMC) were higher than predicted under any scenario based on sample age. Sample 14CH4 appears to be elevated by in- situ CH4 production in the ice for some samples as well as by a second process that is likely direct cosmogenic production of 14CH4 molecules in the ice. 14C of CO and CO2 was measured to better understand these processes and corrections were applied to sample 14CH4. Although the corrected results have substantial uncertainties, they suggest that wetland sources were responsible for the majority of the Younger Dryas - Preboreal CH4 rise. The uncertainties in the corrected results for the Oldest Dryas - Bølling transition are too large to draw conclusions about 14CH4 changes during that transition.
U34A-06 INVITED
Impact of Variations in Seafloor Temperature and Sea-level on Gas Hydrate Stability
We have developed a one-dimensional numerical computer model (simulator) to describe methane hydrate formation, decomposition, reformation, and distribution with depth below the seafloor in the marine environment. The simulator was used to model hydrate distributions at Blake Ridge (Site 997) and Hydrate Ridge (Site 1249). The numerical models for the two sites were conditioned by matching the sulfate, chlorinity, and hydrate distribution measurements. The constrained models were then used to investigate the impact of variations in seafloor temperature and sea-level on gas hydrate stability. For Blake Ridge (site 997), changes in hydrate concentration are small. Both the variations in seafloor temperature and sea-level lead to a substantial increase in gas venting at the seafloor for Hydrate Ridge (site 1249).
U34A-07
Methane Index (MI): a tetraether archeal lipid biomarker index for monitoring the instability of marine gas hydrates
Gas hydrates are one of the largest pools of readily exchangeable carbon on Earth's surface. Perturbations in this reservoir can induce significant changes in global carbon budget, and hence the global climate. Releases of methane from gas hydrates have been increasingly recognized to be responsible for a number of drastic climate changes. A large portion of the inferred events of hydrate dissociation were based on the d13C values of marine carbonates, which have been challenged lately. Here we propose a molecular fossil proxy called "Methane Index (MI)" to better reveal the destabilization of marine gas hydrates. MI is constructed by the relative distribution of certain glycerol dialkyl glycerol tetraethers (GDGTs), which are the core membranes of Archaea. Examination of the MI with all available GDGT data from the world's oceans shows that MI is greater than 1 in gas hydrate-impacted environments and less than 1 for normal marine sediments. This distinction in combination with compound-specific d13C analysis of biphytanes shows clearly methane imprints in samples with higher MI but not with lower MI. Application of MI in a Gulf of Mexico sediment core reveals drastic methane release events, where gas hydrates abundantly occur. Thus, the Methane Index may provide us a precise and easy way to evaluate the gas hydrate instability in Earth's geological history.
U34A-08
Oceanographic and Microbial Control on Methane Oxidation along the California Margin
Microbially-mediated methane oxidation in the marine water column is an important but under-characterized sink for methane released from the seafloor in hydrate and seep environments. Our present understanding of the magnitude of this process is based on limited measurements of methane oxidation rates and concentration distributions. In situ methanotrophic activity is generally thought to be controlled primarily by substrate availability, with higher potential for methane consumption in environments with elevated methane concentrations and sufficient oxygen availability. However, while substrate levels provide a first-order control on the metabolic state and sustainability of the methanotrophic community, here we show that basin- and regional- scale circulation patterns strongly affect the methanotrophic potential of the waters of the California margin environment. During a 2007 research cruise, 220 oxidation rate measurements and 540 methane concentration measurements were performed with samples collected from throughout the water column at sites in the Santa Barbara (SBB) and Santa Monica (SMB) Basins. When combined with previous measurements from the Eel River Basin (ERB) the dataset illustrates a striking relationship between degree of basin restriction and methane turnover time, with turnover times on the order of weeks to months below sill depth in the SBB and SMB and on the order of years to decades in ERB bottom waters with comparable methane concentrations. While methane concentrations show little variability through the SMB water column above sill depth, depth distributions of methane oxidation rate show that mixing of water masses can either decrease or increase methanotrophic potential, depending on water mass origin. Furthermore, a continuing survey of methane concentrations and methane oxidation rates indicates that distributions up-current from the Coal Oil Point Seep Field in the SBB (Mau et al., 2007; Mau et al., unpublished) are strongly controlled by surface circulation patterns. These observations have important implications for understanding the role and response of the marine methane cycle in positive climate feedback mechanisms associated with the ocean- atmosphere balance of both methane and carbon dioxide.