Noemi Fekete

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5xa0m length indicates a maximum age of about 60,000–80,000xa0years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0u2009<u2009δ13C ‰ V-PDBu2009<u2009–40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4u2009<u2009δ18O ‰ V-PDBu2009<u2009+6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ18O values might reflect the contribution of 18O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca2+ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ18O–δ13C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.

A conceptual model for hydrocarbon accumulation and seepage processes around Chapopote asphalt site, southern Gulf of Mexico: From high resolution seismic point of view

[1]xa0As part of the German R/V Meteor M67/2 expedition in 2006 to the southern Gulf of Mexico, a set of 2D high resolution seismic profiles was acquired across the Chapopote knoll to study seafloor asphalt occurrences. On the basis of regional seismostratigraphic studies and DSDP drilling, a more highly reflective, coarse-grained sediment unit of late Miocene age is identified as a potential shallow hydrocarbon reservoir. Although a unit of that age would typically be buried by Pliocene and Pleistocene sediment cover, at Chapopote, local salt tectonism has highly elevated the structure since the late Miocene, and the Miocene reservoir is locally above present-day regional datum. The elevation resulted in a thin (100–200 m), fine grained sediment cover on the crest of the knoll above the reservoir. Because oil and gas production can be expected at depth in Jurassic, Cretaceous and Tertiary source rocks, the presence of high-amplitude reflector packages within the reservoir unit is interpreted as an evidence of hydrocarbons. This is variously supported by observations of crosscutting reflectors, polarity reversal, and drops in instantaneous frequency. The thin seal above the reservoir unit facilitates leakage of trapped hydrocarbons. Hydrocarbons apparently invaded into the seal sediments in the wider vicinity around the crest of the knoll, even extending beyond the area where seafloor asphalt is known. The asphalt site thus may be a currently active spot, while the rest of the crest may be temporarily sealed by solid phase hydrocarbons. We propose that a shallow, large reservoir with deeply sourced, relatively heavy petroleum is principally responsible for the formation of asphalts on the seafloor.

Interaction between accretionary thrust faulting and slope sedimentation at the frontal Makran accretionary prism and its implications for hydrocarbon fluid seepage

[1]xa0Using high-resolution seismic profiles and other geophysical data, collected during R/V Meteor Cruise M74/2, we investigate the distribution patterns of shallow sediments, their structure and deformation processes, and their role in the migration, accumulation and seepage of hydrocarbon-rich fluids. Here, we show that rapid syn-kinematic sedimentation at the frontal Makran accretionary prism provides a mechanism by which emerging imbricated thrust packets override the footwall at the seafloor without significant mass-wasting and destruction of fault-related anticline in the hanging wall. These anticlines may rise high above the seafloor to form plate-boundary-parallel ridges, and distinguish from simple thrust blocks seen at convergent margins elsewhere. With the fast burial of many thrust faults by the syn-kinematic sediments, anticlinal growth structures form in these syn-kinematic sediments by continuous thrust activity. The anticlinal structures preserved within the cores of the ridges or formed from these syn-kinematic sediments act as structural traps for migrating hydrocarbon-rich fluids, above which fluid escape structures are generated leading to seafloor seeps. Most of the discovered hydrocarbon seeps around Sixth Ridge are sourced from these traps. Despite the compressional environment and the rapid syn-kinematic sedimentation rates, shallow subsurface of the frontal Makran is a normally pressured regime, in which the buoyancy of hydrocarbons may account for the fluid migration. In this important respect, the Makran accretionary prism differs from many other convergent margins and accretionary prisms, where fluid flow is largely driven by tectonically induced overpressure.

Strike-slip tectonics in the Pannonian basin based on seismic surveys at Lake Balaton

Strike-slip tectonics has been the dominant style of deformation during the neotectonic (Pliocene and Quaternary) evolution of the Pannonian basin. Main faults are exposed in the “island mountains” of the basin, but strike-slip tectonic features can be best studied in the basin fill by seismic data. Lake Balaton offers the opportunity to carry out high to ultra-high-resolution seismo-acoustic surveys to image stratigraphic and tectonic features in the central part of the Pannonian basin. Several campaigns in the lake using different acquisition techniques have resulted in more than 2000-km seismo-acoustic profiles with a range of resolutions and penetration depths. Interpretation of faults and folds shows a few kilometers wide shear zone below the lake in Late Miocene–Pliocene strata. This zone can be identified as the continuation of the Balatonfő line known onshore to the east of the lake. Mapping revealed a set of duplex structures and highlighted the importance of this shear zone in the formation of Lake Balaton. Comparison of our results to analogue clay models suggests that the observed shear zone is sinistral and the horizontal displacement is on the order of hundreds of meters. Looking at 3D industrial seismic data to the south of the lake, we suggest that the first-order Balaton line, which represents the continuation of Periadriatic line, is also sinistral and characterized by small horizontal displacement of about 1.0–1.5xa0km during Pliocene and Quaternary times. This indicates a 0.2–0.3xa0mm/year average slip rate, which is compatible with recent GPS measurements.