Ninth international conference on - Marum

Abstracts of posters 75
Mapping of vent-related structures with multi-frequency seismo-acoustic profiling
at the Makran Continental Margin, offshore Pakistan
N. Fekete 1 , M. Bruening 1 , F. Ding 1 , T. Schwenk 1 , V. Spiess 2
1 Research Centre Ocean Margins (MARUM), Bremen, Germany
2 Department of Earth Sciences, University of Bremen, Germany
A multi-frequency seismo-acoustic dataset was collected offshore Pakistan in October 2007. The margin wedge,
which is covered by a uniquely thick pile of sediments, was surveyed from the shelf edge to the abyssal plain by
various geophysical profiling tools including sediment echosounder, side scan sonar, and a high-resolution
multichannel seismic system. On our poster we present first results with special focus on subsurface structures
and gas accumulation and their relevance for margin dewatering.
The primary aim of investigations was to identify, and map, recently active fluid and gas vent locations. While
such are well-known to occur at tectonically active areas such as the Black Sea or the Middle America
Subduction Zone, the influence of such a thick sedimentary cover on the fate of fluids and the nature of
expulsion has been a matter of speculation.
Another intriguing scientific goal concerns the structure and dynamics of Makran vent sites. Their variation as a
consequence of local seafloor features such as nearby faults, canyons, and slumps, is studied in detail.
The data indicate the presence of over a dozen active gas and fluid seep sites on the Makran accretionary margin.
They are manifested in form of gas flares in the water column, seafloor locations of high backscatter, shallow
gas accumulations within the sediments, and active pathways along tectonic boundaries, often in connection with
one another. Seeps occur along the entire surveyed area from the shelf edge to the proto deformation zone in as
much as 3000 m water depth. Investigations show that intense deformation allows upward gas migration
especially near the lower-slope ridges, where gas enters the gas hydrate stability field locally.
The unique dataset of simultaneously acquired side scan and seismic profiles allows joint geological
interpretation and thus contributes significantly to our present understanding of this tectonically unique setting.
Mature hydrocarbons in Fe-Mn nodules from the Gulf of Cadiz: deep-seated fluids migration,
bacterial activity and biomineralization products
F.J. González 1 , L. Somoza 1 , T. Torres 2 , J.E. Ortiz 2 , R. Lunar 3 and J. Martínez-Frías 4
1 Geological Survey of Spain (IGME). Madrid, Spain
2 Laboratorio de Estratigrafía Biomolecular (ETSIM/UPM). C/ Ríos Rosas 21, Madrid, Spain
3 Facultad de Ciencias Geológicas (UCM). Madrid, Spain
4 Centro de Astrobiología (CAB/CSIC/INTA). Madrid, Spain
During the Tasyo Project cruises extensive fields of ferromanganese nodules were discovered at the base and
flanks of carbonate-mud mounds and mud volcanoes along the Guadalquivir Diapiric Ridge (Central eastern
Atlantic). The presence of biomarkers has been detected based on semi-quantitative analyses, over powdered
samples of the oxide layers and nuclei from ten nodules. Extracts from Fe-Mn nodules were analyzed using Gas
Chromatography (GC) and carbon isotopic analyses were done using Gas Chromatography-Combustion-Isotope
Ratio Mass Spectrometry (GC-C-IRMS).
The ferromanganese nodules are tabular to irregular in shape, and show an internal structure characterised by a
layered disposition of Fe-Mn oxides surrounding a carbonated nucleus. Only in some samples is possible
distinguish clearly their cores and layers. Fe-Mn layers and carbonated nucleus display the same microtextural
features, composed by zoned rhombohedral crystals (authigenic Fe-Mn oxides in layers, and siderite to
rhodochrosite in the nuclei) surrounded by detrital grains (silicates) and framboidal associations (fresh pyrite or
totally pseudomorphised by goethite). The centers of the rhombohedral crystals are enriched in manganese and
their exterior edges are iron enriched for both, Fe-Mn oxides and Fe-Mn carbonates. Fe-Mn oxides exhibit
filamentous and bulbous textures that could have been generated by microorganisms mediation.
The nodules hold in their oxide layers hydrocarbons (n-alkanes) derived from marine bacterial activity, also with
presence of aromatic hydrocarbons as phenanthrene and antracenes, characteristic of mature petroleum. The
mean value for TOC in bulk samples is 1.12% with contents ranging between 0.33 and 3.85%. Both, nucleus and
oxide layers present presence of the same biomarkers. Gas chromatograms of the total hydrocarbon fraction
show similar pattern in all the studied samples, comprising a modal n-alkane distribution with a first
concentration maximum at n–C18 and an important presence at n-C16 and n-C20. Pristane and phytane and/or
crocetane (2, 6, 11, 15-tetrametilhexadecano) are present in all the samples analysed. The chromatograms show a
convex morphology known as unresolved complex mixture (UCM), characteristic of samples which have
underwent an intense degree of marine microbial degradation. The lack of n-alkanes with large chains (>n-C25)