Description: For the investigation of organic carbon fluxes reaching the seafloor, oxygen microprofiles were measured at 145 sites in different sub-regions of the Southern Ocean. At 11 sites, an in situ oxygen microprofiler was deployed for the measurement of oxygen profiles and the calculation of organic carbon fluxes. At four sites, both in situ and ex situ data were determined for high latitudes. Based on this data set as well as on previous published data, a relationship was established for the estimation of fluxes derived by ex situ measured O2 profiles. The fluxes of labile organic matter range from 0.5 to 37.1 mg C m−2 d−1. The high values determined by in situ measurements were observed in the Polar Front region (water depth of more than 4290 m) and are comparable to organic matter fluxes observed for high-productivity, upwelling areas like off West Africa. The oxygen penetration depth, which reflects the long-term organic matter flux to the sediment, was correlated with assemblages of key diatom species. In the Scotia Sea (not, vert, similar3000 m water depth), oxygen penetration depths of less than 15 cm were observed, indicating high benthic organic carbon fluxes. In contrast, the oxic zone extends down to several decimeters in abyssal sediments of the Weddell Sea and the southeastern South Atlantic. The regional pattern of organic carbon fluxes derived from microsensor data suggests that episodic and seasonal sedimentation pulses are important for the carbon supply to the seafloor of the deep Southern Ocean.

Description: The role of lakes in the global methane budget seems to be higher than previously thought. Numerous pockmarks have been described for the marine environment, but at freshwater fluid seeps the geological, chemical and biological processes operating are largely unknown. Based on different observations of intense gas flow through the water column at Lake Constance two joint research cruises were conducted in winter 2005/06 to systematically search and study these structures. Several large fields of pockmark like structures were found in the eastern part of the lake (side scan sonar, echo sounder). Water and gas was selectively sampled and based on these preliminary data a joint DFG-funded research project was arranged. The objectives are to (1) locate, describe and map pockmark areas at Lake Constance, (2) identify the pockmark formation mechanism, (3) quantify the methane flux and the temporal variability and (4) identify the source of methane.So far more than 450 pockmarks larger 〉 2 m have been identified by side-scan sonar. The pockmarks vary largely in size (dm-range to maximum diameters of 15m) and spatial distribution. At some places, they are irregular spaced, but now and then smaller decimetre sized pockmarks are evenly spaced along small lineaments. Often, large pockmarks are located at morphological highs, such as channel rims or little hills at the lake floor. There is no morphological evidence for a catastrophic gas release (solid discharge at the rim, or irregular sediment patterns near the pockmarks). The observed structures point to a constant gas release from a deeper reservoir.Two representative locations (water depth 12m and 85m) close to the former Rhine estuary have been selected for further geochemical examinations. After deploying a digital horizontal sonar system at the lake floor to allow for exact positioning, water and sediment samples were taken within the pockmark with niskin bottles from a rosette and multicorer. In addition, sediment samples and gas samples were taken by divers across the shallow pockmark. Gas concentration and isotope ratios of dissolved gas in water were measured using standard GC-FID and GC-irMS techniques. Methane concentration in sediment samples was measured directly after coring by means of by a diffusion-based methane sensor.Preliminary results of the isotope analysis of methane (δ13C, δD) and CO2 (δ13C) as well as the absence of higher molecular weight alkanes strongly indicate a bacterial formation of the gases rather than a thermogenic origin of the methane. The results of the analysis of free gas of the sediments indicate the methyl formation as dominant pathway. However, up to now available data suggest a certain difference in the gases located at the deep pockmark, the corresponding reference site and gas sampled at the shallow pockmark. In the sediments of the deep pockmark high methane concentrations were recorded, however with no significant differences between the pockmark and control cores. At the shallow pockmark sediment cores could be positioned more precisely. Here methane concentrations vary within meters in order of magnitude. Several autonomous devices for gas flow measurements using different approaches were deployed in March 2008 at selected gas emanating pockmarks. Data are not yet available but will hopefully presented

Description: Marine mud volcanoes are significant source locations contributing to the marine methane cycle. Enhanced heat flow, unique chemoautotrophic communities, occurrence of massive gas hydrates and large gas plumes are direct evidences of elevated methane concentrations and the dynamic environment of mud volcanoes. Related to the high concentrations and large inventories of CH4 in surface sediments only a fraction of the methane is exported to the bottom water. This is mainly due to chemoautotrophic communities oxidizing methane and proving a microbial filter reducing CH4 fluxes. Although these processes were studied for several mud volcanoes still little is known about the spatial pattern and the areas covered by chemoautotrophic communities or by present mud flows.For this purposes the Håkon Mosby Mud Volcano (HMMV), which is located at the continental slope of the Barents Sea, was studied by several dives with the Remotely Operated Vehicle Victor6000. During these dives a high resolution microbathymetric map, with a footprint of 25 x 25 cm and a vertical resolution of better than 10 cm was derived. Furthermore, video streams of the bottom camera was converted into georeferenced mosaics, providing a detailed image about the spatial distribution of seafloor features as bacterial mats, pogonophorans, both indicating methane oxidation, or mud flows. Based on visual inspection of 2310 georeferenced mosaics covering an area of 46160 m², different biogeochemical habitats were identified and quantified on a m²-basis and stored within a geodatabase. By application of GIS based and geostatistical techniques as indicator kriging the distribution of different biogeochemical habitats were quantified and mapped for the entire HMMV.Considering the flat and hummocky area of HMMV, approximately 16% of the flat centre is nearly void of any benthic communities. This area is considered as a region of high methane discharge into bottom water. An area of 5% located in the south-eastern part, is densely inhabited by Beggiatoa. The hummocky outer part is colonised dominantly by pogonophoran tube worms (37.3%) and only occasional by Beggiatoa. Source locations and drainage directions for current mud flows were identified by computation of trend surfaces and consideration of temperature data.

Description: The role of lakes in the global methane budget seems to be higher than previously thought. Numerous pockmarks have been described for the marine environment, but at freshwater fluid seeps the geological, chemical and biological processes operating are largely unknown. In Lake Constance, so far more than 450 pockmarks larger 〉 2m have been identified by side-scan sonar. The pockmarks vary largely in size (dm-range to maximum diameters of 15m) and spatial distribution. Two representative locations (water depth 12m and 85m) close to the former Rhine estuary have been selected for further geochemical examinations. Under-water videos demonstrate continuous release of gas bubbles. Preliminary results of the gas analysis indicate 99% pure methane formed by bacterial acitivtiy rather than a thermogenic origin of the methane. In the sediment the escaping methane gas results in small scale heterogenity of most geochemical parameters. In the water column increased methane concentrations are observed in large areas. The further impact of this massive methane flux from deep reservoir(s) on sediments and water remains to be investigated.