Transport- reaction modeling of marine gas hydrate deposits- global results

We have developed a multi-1D numerical model of gas hydrate formation and dissolution processes in anoxic marine sediments and, by this model, we have estimated the new global gas hydrate inventory (BURWICZ E. B. et al., 2011). The reaction-transport model contains various chemical compounds (solid organic carbon, dissolved methane, inorganic carbon, and sulfates, gas hydrates, and free methane gas). The rates of POC degradation, anaerobic methane oxidation, sulfate reduction, and methanogenesis are kinetically controlled. Gas hydrate stability zone (GHSZ) is defined as a combination of pressure, temperature, and (to a smaller degree) salinity conditions. The lower boundary of the GHSZ is defined as the intersection of gas hydrate and methane gas solubilities. The diffusion equations are solved using a fully-implicit finite-differences method, while all transport processes are resolved by a Semi-Lagrangian scheme. Global input data sets (1°x1° resolution) were compiled from various oceanographic, geological and geophysical sources. The entire model was implemented in Matlab

Simulating the onset and spread of anoxic conditions during Cretaceous OAE2

A new model of the global atmosphere-ocean-continent-mantle system was set-up to investigate the triggering of the Oceanic Anoxic Event OAE2 through volcanic degassing processes at large igneous provinces (LIPs). The model simulates the changes in oceanic dissolved oxygen, phosphate, and carbon and the evolution of atmospheric pCO2 values under mid-Cretaceous boundary conditions. It considers the effects of pCO2 on element ratios in marine plankton (C : P) and includes new parameterizations for phosphorus and carbon burial at the seafloor based on modern observations. Independent isotopic and chemical time-series of ocean and atmosphere change over OAE2 are applied to evaluate the model results. The model results support the hypothesis that OAE2 was triggered by massive CO2 emissions at LIPs. According to the model, the phosphorus weathering flux into the ocean and the C : P ratio in marine plankton were enhanced by the rise in surface temperature and atmosphere pCO2 caused by mantle degassing. Marine export production and oxygen consumption in intermediate and deep water masses increased in response to the expansion of the dissolved phosphate inventory of the ocean and the change in plankton element ratios. The spread of anoxic conditions in bottom waters -induced by enhanced carbon export and respiration- was further amplified by the oxygen-dependent burial of phosphorus in marine sediments in a positive feedback loop. The modeling implies that enhanced CO2 emissions favor the spread of low-oxygen conditions also in modern oceans

Cool episodes in the Late Cretaceous - exploring the effects of physical forcing on Antarctic snow accumulation

Until recently it was assumed that the major modern ice sheets on Antarctica became established around the Eocene-Oligocene boundary about 34 Ma ago. But new evidence (e.g. Miller et al., 2008) indicates that continental ice may have been present much earlier, some of it probably even since the greenhouse times of the Late Cretaceous. Deep sea drilling data suggest changes in sea-level during the Late Cretaceous that could have been caused by the melting and freezing of vast ice sheets on Antarctica. Using a GCM approach to test the whether it would be possible to generate the described high-amplitude sealevel falls is one additional way to test this vigorously discussed issue. As shown above, our numerical approach indicates the possibility of a substantial Antarctic glaciation by changing the physical boundary conditions, eccentricity, pCO2, and elevation within reasonable Late Cretaceous ranges. Our simulations suggest that simulated snowfall and consecutive ice formation on Antarctica might yield sufficient volumes to account for the documented rapid, low-amplitude Cretaceous sea-level fluctuations. Based on cautious assumptions and possible errors the model results show that ice build-up could take place in realistic time spans and in accordance with the proxy records. Thus, the possibility of an Antarctic ice shield build-up large enough to drive sea level fluctuations on the order of tens of meters within 20,000-220,000 years is supported. The initial snow accumulation and following growth of Antarctic ice-sheets in the Cretaceous can be attributed to changes in southern hemisphere summer insolation due to reduced orbital eccentricity. Alternatively and/or additionally, declining atmospheric CO2 values caused further coolin

A new global gas hydrate budget based on numerical reaction-transport modeling and a novel parameterization of Holocene and Quaternary sedimentation

This study provides new estimates for the global methane hydrate inventory based on reaction-transport modeling [1]. A multi-1D model for POC degradation, gas hydrate formation and dissolution is presented. The model contains an open three-phase system of two solid (organic carbon, gas hydrates), three dissolved (methane, sulfates, inorganic carbon) and one gaseous (free methane) compounds. The reaction module builds upon the kinetic model of POC degradation [2] which considers a down-core decrease in reactivity of organic matter and the inhibition of methane production via accumulation of metabolites in sediment pore fluids. Global input grids have been compiled from a variety of oceanographic, geological and geophysical data sets including a parameterization of sedimentation rates in terms of water depth (Holocene) and distance to continents (Quaternary).The world's total gas hydrate inventory is estimated at 1.74 x 1013 m3 – ~2 x 1015 m3 CH4 (STP) or, equivalently, 8.3 – ~900 Gt of methane carbon. The first value refers to the present day conditions using the relatively low Holocene sedimentation rates; the second value corresponds to a scenario of higher Quaternary sedimentation rates along continental margins. This increase in the POC input could be explained by re-deposition process at the continental rise and slope due to erosion of continental shelf sediments during glacial times. Our results show that in-situ POC degradation is at present not an efficient hydrate forming process. Significant hydrate deposits are more likely to have formed at times of higher sedimentation during the Quaternary or/and as a consequence of active upward fluid transport

In situ benthic fluxes from an intermittently active mud volcano at the Costa Rica convergent margin

Along the erosive convergent margin off Costa Rica a large number of mound-shaped structures exist built by mud diapirism or mud volcanism. One of these, Mound 12, an intermittently active mud volcano, currently emits large amounts of aqueous dissolved species and water. Chemosynthetic vent communities, authigenic carbonates, and methane plumes in the water column are manifestations of that activity. Benthic flux measurements were obtained by a video-guided Benthic Chamber Lander (BCL) deployed at a vent site located in the most active part of Mound 12. The lander was equipped with 4 independent chambers covering adjacent areas of the seafloor. Benthic fluxes were recorded by repeated sampling of the enclosed bottom waters while the underlying surface sediments were recovered with the lander after a deployment time of one day. One of the chambers was placed directly in the centre of an active vent marked by the occurrence of a bacterial mat while the other chambers were located at the fringe of the same vent system at a lateral distance of only 40 cm. A transport-reaction model was developed and applied to describe the concentration profiles in the pore water of the recovered surface sediments and the temporal evolution of the enclosed bottom water. Repeated model runs revealed that the best fit to the pore water and benthic chamber data is obtained with a flow velocity of 10 cm yr− 1 at the centre of the vent. The flux rates to the bottom water are strongly modified by the benthic turnover (benthic filter). The methane flux from below at the bacterial mat site is as high as 1032 μmol cm− 2 yr− 1, out of which 588 μmol cm− 2 yr− 1 is oxidised in the surface sediments by microbial consortia using sulphate as terminal electron acceptor and 440 μmol cm− 2 yr− 1 are seeping into the overlaying bottom water. Sulphide is transported to the surface by ascending fluids (238 μmol cm− 2 yr− 1) and is formed within the surface sediment by the anaerobic oxidation of methane (AOM, 588 μmol cm− 2 yr− 1). However, sulphide is not released into the bottom water but completely oxidized by oxygen and nitrate at the sediment/water interface. The oxygen and nitrate fluxes into the sediment are high (781 and 700 μmol cm− 2 yr− 1, respectively) and are mainly driven by the microbial oxidation of sulphide. Benthic fluxes were much lower in the other chambers placed in the fringe of the vent system. Thus, methane and oxygen fluxes of only 28 and 89 μmol cm− 2 yr− 1, respectively were recorded in one of these chambers. Our study shows that the aerobic oxidation of methane is much less efficient than the anaerobic oxidation of methane so that methane which is not oxidized within the sediment by AOM is almost completely released into the bottom water. Hence, anaerobic rather than aerobic methane oxidation plays the major role in the regulation of benthic methane fluxes. Moreover, we demonstrate that methane and oxygen fluxes at cold vent sites may vary up to 3 orders of magnitude over a lateral distance of only 40 cm indicating an extreme focussing of fluid flow and methane release at the seafloor

Report on range of long-term scenarios to be simulated

In order to proceed with speculative modelling of the impacts of potential leakage of geologically stored carbon, it is necessary to develop plausible scenarios. Here a range of such scenarios are developed based on a consensus of the possible geological mechanisms of leakage, namely abandoned wells, geological faults and operational blowouts. Whilst the resulting scenarios remain highly speculative, they do enable short term progress in modelling and provide a basis for further debate and refinement

3-D numerical modeling of methane hydrate deposits

Within the German gas hydrate initiative SUGAR, we have developed a new tool for predicting the formation of sub-seafloor gas hydrate deposits. For this purpose, a new 2D/3D module simulating the biogenic generation of methane from organic material and the formation of gas hydrates has been added to the petroleum systems modeling software package PetroMod®. T ypically, PetroMod® simulates the thermogenic generation of multiple hydrocarbon components including oil and gas, their migration through geological strata, and finally predicts the oil and gas accumulation in suitable reservoir formations. We have extended PetroMod® to simulate gas hydrate accumulations in marine and permafrost environments by the implementation of algorithms describing (1) the physical, thermodynamic, and kinetic properties of gas hydrates; and (2) a kinetic continuum model for the microbially mediated, low temperature degradation of particulate organic carbon in sediments. Additionally, the temporal and spatial resolutions of PetroMod® were increased in order to simulate processes on time scales of hundreds of years and within decimeters of spatial extension. As a first test case for validating and improving the abilities of the new hydrate module, the petroleum systems model of the Alaska North Slope developed by IES (currently Shlumberger) and the USGS has been chosen. In this area, gas hydrates have been drilled in several wells, and a field test for hydrate production is planned for 2011/2012. The results of the simulation runs in PetroMod® predicting the thickness of the gas hydrate stability field, the generation and migration of biogenic and thermogenic methane gas, and its accumulation as gas hydrates will be shown during the conference. The predicted distribution of gas hydrates will be discussed in comparison to recent gas hydrate findings in the Alaska North Slope region