Gas emissions and ROV survey tracks of the Regab pockmark (Northern Congo Fan)

Pockmarks are seafloor depressions commonly associated with fluid escape from the seabed and are believed to contribute noticeably to the transfer of methane into the ocean and ultimately into the atmosphere. They occur in many different areas and geological contexts, and vary greatly in size and shape. Nevertheless, the mechanisms of pockmark growth are still largely unclear. Still, seabed methane emissions contribute to the global carbon budget, and understanding such processes is critical to constrain future quantifications of seabed methane release at local and global scales. The giant Regab pockmark (9°42.6' E, 5°47.8' S), located at 3160 m water depth near the Congo deep-sea channel (offshore southwestern Africa), was investigated with state-of-the-art mapping devices mounted on IFREMER's (French Research Institute for Exploitation of the Sea) remotely operated vehicle (ROV) Victor 6000. ROV-borne micro-bathymetry and backscatter data of the entire structure, a high-resolution photo-mosaic covering 105,000 m2 of the most active area, sidescan mapping of gas emissions, and maps of faunal distribution as well as of carbonate crust occurrence are combined to provide an unprecedented detailed view of a giant pockmark. All data sets suggest that the pockmark is composed of two very distinctive zones in terms of seepage intensity. We postulate that these zones are the surface expression of two fluid flow regimes in the subsurface: focused flow through a fractured medium and diffuse flow through a porous medium. We conclude that the growth of giant pockmarks is controlled by self-sealing processes and lateral spreading of rising fluids. In particular, partial redirection of fluids through fractures in the sediments can drive the pockmark growth in preferential directions.

Gridded bathymetry from multibeam echosounder EM122 data of the cruise MSM34/2 (2013)

Bathymetry based on data recorded during MSM34-2 between 27.12.2013 and 18.01.2014 in the Black Sea. The main objective of this cruise was the mapping and imaging of the gas hydrate distribution and gas accumulations as well as possible gas migration pathways. Objectives of Cruise: Gas hydrates have been the focus of scientific and economic interest for the past 15-20 years, mainly because the amount of carbon stored in gas hydrates is much greater than in other carbon reservoirs. Several countries including Japan, Korea and India have launched vast reasearch programmes dedicated to the exploration for gas hydrate resources and ultimately the exploitation of the gas hydrates for methane. The German SUGAR project that is financed the the Ministry of Education and Research (BmBF) and the Ministry of Economics (BmWi) aims at developing technology to exploit gas hydrate resources by injecting and storing CO2 instead of methane in the hydrates. This approach includes techniques to locate and quantify hydrate reservoirs, drill into the reservoir, extract methane from the hydrates by replacing it with CO2, and monitor the thus formed CO2-hydrate reservoir. Numerical modeling has shown that any exploitation of the gas hydrates can only be succesful, if sufficient hydrate resources are present within permeable reservoirs such as sandy or gravelly deposits. The ultimate goal of the SUGAR project being a field test of the technology developed within the project, knowledge of a suitable test site becomes crucial. Within European waters only the Norwegian margin and the Danube deep-sea fan show clear geophysical evidence for large gas hydrate accumulations, but only the Danube deep-sea fan most likely contains gas hydrates within sandy deposits. The main objective of cruise MSM34 therefore is locating and characterising suitable gas hydrate deposits on the Danube deep-sea fan.

Seawater carbonate chemistry and physiological and mechanical properties of the starfish Asterias rubens in a laboratory experiment

The increase in atmospheric CO2 due to anthropogenic activity results in an acidification of the surface waters of the oceans. Its impact will depend on the considered organisms and ecosystems. The intertidal may harbor organisms pre-adapted to the upcoming changes as they face tidal pH and temperature fluctuations. However, these environments will be more affected as shallow waters will face the highest decrease in seawater pH. In this context, the effects of reduced environmental pH on the physiology and tube feet mechanical properties of the intertidal starfish Asterias rubens, a top predator, were investigated during 15 and 27 days. A. rubens showed a respiratory acidosis with its coelomic fluid pH always lower than that of seawater. This acidosis was most pronounced at pH 7.4. Notwithstanding, the starfish showed no significant variations in RNA/DNA ratio of different tissues and in tube feet strength. However, respiration rates were significantly lower for individuals maintained at reduced seawater pH. Within the ocean acidification context, the present results suggest that A. rubens withstands the effects of reduced seawater pH, at least for medium term exposures.