Beschreibung: Geochemical data (CH4, SO42−, I−, Cl−, particulate organic carbon (POC), δ13C-CH4, and δ13C-CO2) are presented from the upper 30 m of marine sediment on a tectonic submarine accretionary wedge offshore southwest Taiwan. The sampling stations covered three ridges (Tai-Nan, Yung-An, and Good Weather), each characterized by bottom simulating reflectors, acoustic turbidity, and different types of faulting and anticlines. Sulfate and iodide concentrations varied little from seawater-like values in the upper 1–3 m of sediment at all stations; a feature that is consistent with irrigation of seawater by gas bubbles rising through the soft surface sediments. Below this depth, sulfate was rapidly consumed within 5–10 m by anaerobic oxidation of methane (AOM) at the sulfate-methane transition. Carbon isotopic data imply a mainly biogenic methane source. A numerical transport-reaction model was used to identify the supply pathways of methane and estimate depth-integrated turnover rates at the three ridges. Methane gas ascending from deep layers, facilitated by thrusts and faults, was by far the dominant term in the methane budget at all sites. Differences in the proximity of the sampling sites to the faults and anticlines mainly accounted for the variability in gas fluxes and depth-integrated AOM rates. By comparison, methane produced in situ by POC degradation within the modeled sediment column was unimportant. This study demonstrates that the geochemical trends in the continental margins offshore SW Taiwan are closely related to the different geological settings.

Beschreibung: 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.

Beschreibung: The accumulation of methane hydrate in marine sediments is basically controlled by the accumulation of particulate organic carbon at the seafloor, the kinetics of microbial organic matter degradation and methane generation in marine sediments, the thickness of the gas hydrate stability zone (GHSZ), the solubility of methane in pore fluids within the GHSZ and the ascent of deepseated pore fluids and methane gas into the GHSZ. Our present knowledge on these controlling factors is discussed and new estimates of global sediment and methane fluxes are presented. A new transport-reaction model is applied at a global grid defined by these up- dated parameter values. The model yields an improved and better constrained estimate of the global inventory of methane gas hydrates in marine sediments (3000 ± 2000 Gt of methane carbon).

Beschreibung: 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.

Beschreibung: Formed under low temperature – high pressure conditions vast amounts of methane hydrates are considered to be locked up in sediments of continental margins including the Arctic shelf regions[1-3]. Because the Arctic has warmed considerably during the recent decades and because climate models predict accelerated warming if global greenhouse gas emissions continue to rise [3], it is debated whether shallow Arctic hydrate deposits could be destabilized in the near future[4, 5]. Methane (CH4), a greenhouse gas with a global warming potential about 25 times higher than CO2, could be released from the melting hydrates and enter the water column and atmosphere with uncertain consequences for the environment. In a recent study, we explored Arctic bottom water temperatures and their future evolution projected by a climate model [1]. Predicted bottom water warming is spatially inhomogeneous, with strongest impact on shallow regions affected by Atlantic inflow. Within the next 100 years, the warming affects 25% of shallow and mid- depth regions (water depth 〈 600 m) containing methane hydrates. We have quantified methane release from melting hydrates using transient models resolving the change in stability zone thickness. Due to slow heat diffusion rates, the change in stability zone thickness over the next 100 years is small and methane release limited. Even if these methane emissions were to reach the atmosphere, their climatic impact would be negligible as a climate model run confirms. However, the released methane, if dissolved into the water column, may contribute to ocean acidification and oxygen depletion in the water column.