Beschreibung: Bottom-simulating reflections (BSRs) are probably the most commonly used indicators for gas hydrates in marine sediments. It is now widely accepted that BSRs are primarily caused by free gas beneath gas-hydrate-bearing sediments. However, our insight into BSR formation to date is mostly limited to theoretical studies. Two endmember processes have been suggested to supply free gas for BSR formation: (i) dissociation of gas hydrates and (ii) migration of methane from below. During a recent campaign of the German Research Vessel Sonne off the shore of Peru, we detected BSRs at locations undergoing both tectonic subsidence and non-sedimentation or seafloor erosion. Tectonic subsidence (and additionally perhaps seafloor erosion) causes the base of gas hydrate stability to migrate downward with respect to gas-hydrate-bearing sediments. This process rules out dissociation of gas hydrates as a source of free gas for BSRs at these locations. Instead, free gas at BSRs is predicted to be absorbed into the gas hydrate stability zone. BSRs appear to be confined to locations where the subsurface structure suggests focusing of fluid flow. We investigated the seafloor at one of these locations with a TV sled and observed fields of rounded boulders and slab-like rocks, which we interpreted as authigenic carbonates. Authigenic carbonates are precipitations typically found at cold vents with methane expulsion. We retrieved a small carbonate-cemented sediment sample from the seafloor above a BSR about 20 km away. This supported our interpretation that the observed slabs and boulders were carbonates. All these observations suggest that BSRs in Lima Basin are maintained predominantly by gas that is supplied from below, demonstrating that this endmember process for BSR formation exists in nature. Results from Ocean Drilling Program Leg 112 showed that methane for gas hydrate formation on the Peru lower slope and the methane in hydrocarbon gases on the upper slope is mostly of biogenic origin. The δ13C composition of the recovered carbonate cement was consistent with biologic methane production below the seafloor (although possibly above the BSR). We speculate that the gas for BSR formation in Lima Basin also is mainly biogenic methane. This would suggest the biologic productivity beneath the gas hydrate zone in Lima Basin to be relatively high in order to supply enough methane to maintain BSRs.

Beschreibung: Marine seismic wide-angle data acquisition and earthquake seismology observations are at the verge of a quantum leap in data quality and density. Advances in micro-electronic technology facilitates the construction of instrumcnts that enable large data volumes to be collected and that are small and cheap enough so that large numbers can be built and operated economically. The main improvements are a dramatic decrease of power consumption ( 〈 250 m W) and increase in clock stability ( 〈 0.05 ppm}. Several scenarios for future experiments arc discussed in this contrihution

Beschreibung: Seismic velocities obtained from ocean-bottom hydrophone, expanding spread profile and multi-channel seismic data were used to compile a velocity model for the Mediterranean Ridge along a 220-km-long transect extending from the Sirte Abyssal Plain to the Cleft region near the Hellenic Trough. A 200–300-m-thin layer of Plio–Quaternary sediments with velocities of 1800–2200 m s−1 covers the whole Ridge. The Messinian evaporites (4000–4500 m s−1) occur in the southwest as a tectonically thickened layer and in a basin just northeast of the crest of the Ridge. In the intervening region however, the evaporites appear absent and the seismic velocities are generally lower. Arched reflectors, imaged in the depth-migrated section, suggest that the sediments beneath the Ridge crest belong to a Pre-Messinian accretionary wedge. Beneath the Messinian evaporites a northeastward-thinning layer of probable Tertiary sediments shows laterally increasing velocities from 3300 m s−1 to 4600 m s−1. Assuming that the layer thinning is caused by compaction due to increased overburden alone, we have calculated a porosity reduction from 15% to 4% and an associated fluid expulsion of 10 km3 km−1 along the trench axis. This corresponds to c. 60% of the initial fluid volume of an undeformed sediment column from the abyssal plain. The almost impermeable evaporitic cap over these sediments leads to high fluid pressures at the base of the evaporites, likely to make this horizon the basal décollement of the modern accretionary system. A 2.5-km-thick unit of probable Mesozoic carbonates with velocities of 4500–4600 m s−1 is inferred at c. 8 km depth. The top of the oceanic crust occurs at a depth of about 10 km. The results from this study have widespread implications for the understanding of the regional geological history.