ALBERT

Description: Cruise M86/5 aimed at collecting data from potential fluid dewatering sites located in the deep-sea region of the western part of the Gulf of Cadiz and the adjacent deep sea plain. Previous work on mud volcanoes (cruises SO175 and MSM1/3) located on the accretionary wedge in the Gulf of Cadiz showed that mud volcano fluids are typically sourced at several km depth below the seafloor. In addition, the geochemical composition of fluids from the deepest mud volcano in this area which is located on a west-east trending transform lineament (SWIM1) indicated that these fluids are typical for having been altered by the reaction with oceanic crust. This implies that there is active flow connecting the oceanic basement and the seafloor. To date, such kind of hydrothermal circulation is only known for relatively young oceanic crust (〈60 Ma). Hence, the existence of a hydrological connection between old, sedimented oceanic crust and the seafloor is a phenomenon, which essentially has not been investigated in the past, and may represent a (missing) link between hot vents at mid-ocean ridges and cold seeps at continental margins. On the cruise, we followed an interdisciplinary approach characterized by extensive geochemical sampling in the water column and the sediment, heat flow measurements, and detailed mapping with AUV and ship-based hydroacoustic systems. We mainly investigated selected sites with conspicuous backscatter anomalies recorded on previous cruises along the SWIM1 lineament: (i) within the transition between the accretionary prism and the Horseshoe Abyssal Plain and (ii) on seafloor highs within the Horseshoe Abyssal Plain itself. The geotectonic environment in both sections is completely different from the situation on the accretionary wedge, where numerous mud volcanoes were detected on previous cruises. Within the transition zone three new mud volcanoes were discovered and extensively sampled during M86/5. Typal chemoautotrophic organisms were found here together with conspicuous methane anomalies in der water column. Preliminary pore water analyses show that the fluids indicate a deep origin, but show distinctive differences to those found on the mud volcanoes on the wedge. Future analyses will show, if this phenomenon is - as hypothesized - due to decreasing sediment thickness with increasing westward distance from the wedge, and hence decreasing sediment-water interaction overprinting the original fluid composition. On the seafloor highs along the SWIM1 lineament in the Horseshoe Abyssal Plain no seeps or mud volcanoes were discovered. However, the lineament seems to be active in terms of fluid flow as indicated by pore water and heat flow anomalies as compared to values measured off the lineament. Overall, the major goal of the cruise could be fulfilled: fluid seeps related to active faults were discovered in a new geotectonic environment. The overall significance in terms of fluid transport in old oceanic crust and/or the role of deeply-rooted fault systems needs to be further addressed in subsequent studies.