Description: In the Arctic Seas, the West Spitsbergen continental margin represents a prominent methane seep area. In this area, free gas formation and gas ebullition as a consequence of hydrate dissociation due to global warming are currently under debate. Recent studies revealed shallow gas accumulation and ebullition of methane into the water column at more than 250 sites in an area of 665 km2. We conducted a detailed study of a subregion of this area, which covers an active gas ebullition area of 175 km2 characterized by 10 gas flares reaching from the seafloor at~245 m up to 50 m water depth to identify the fate of the released gas due to dissolution of methane from gas bubbles and subsequent mixing, transport and microbial oxidation. The oceanographic data indicated a salinity-controlled pycnocline situated ~20 m above the seafloor. A high resolution sampling program at the pycnocline at the active gas ebullition flare area revealed that the methane concentration gradient is strongly controlled by the pycnocline. While high methane concentrations of up to 524 nmol L−1 were measured below the pycnocline, low methane concentrations of less than 20 nmol L−1 were observed in the water column above. Variations in the δ13CCH4 values point to a 13C depleted methane source (~−60‰ VPDB) being mainly mixed with a background values of the ambient water (~−37.5‰ VPDB). A gas bubble dissolution model indicates that ~80% of the methane released from gas bubbles into the ambient water takes place below the pycnocline. This dissolved methane will be laterally transported with the current northwards and most likely microbially oxidized in between 50 and 100 days, since microbial CH4 oxidation rates of 0.78 nmol d−1 were measured. Above the pycnocline, methane concentrations decrease to local background concentration of ~10 nmol L−1. Our results suggest that the methane dissolved from gas bubbles is efficiently trapped below the pycnocline and thus limits the methane concentration in surface water and the air–sea exchange during summer stratification. During winter the lateral stratification breaks down and fractions of the bottom water enriched in methane may be vertically mixed and thus be potentially an additional source for atmospheric methane.

Description: Based on a sediment vibro corer, a tool for the sampling of sub-seafloor groundwater aquifers has been developed and successfully deployed in a coastal area of the western Baltic. The device was designed to obtain pure groundwater samples from coarse sediments to be used for tracer investigations and CFC age dating. Operated from a medium size research vessel, a well pipe tipped with a filter segment is vibrated into the sediment down to the aquifer. Groundwater entering the filter is pumped to the ship by a conventional submersible pump situated in the well's filter tip. Groundwater is continuously analysed on board for O2, salinity, pH, Eh and temperature, prior to sampling for CFC and radioisotope analysis. All parameters indicate that pure groundwater had been obtained. CFC concentrations are very low suggesting that the groundwater of this shallow sub-seafloor aquifer recharged prior to 1950. This finding is in accordance with other hydrogeological evidence that this aquifer, located only 4–5 m below the seafloor, is connected to fairly deep confined sandy aquifers on land of Pleistocene or Miocene age. Applying the method described, it is possible to obtain sufficient sample volumes for analyses of natural groundwater tracers such as radon-222 and CFCs which can be used to trace submarine groundwater discharge as well as the origin of groundwater in such environments.

Description: The deep-sea borehole seal CORK was deployed for the first time on a modern accretionary prism during ODP Leg 146 to the Cascadia Margin. Ten months after the deployment the fluid flow and geochemistry of the borehole fluids was investigated during several dives by DSRV Alvin. The chemical analysis of the borehole fluids revealed methane concentrations of more than 3.5 mM, whereas oxygen and dissolved ions as Cl, NO3, or PO4 are still close to the ambient seawater composition. The exceedingly high methane content measured at the top of the sealed borehole and the observed degassing during the ascent of the submersible indicates that the sampled fluid was initially saturated or close to saturation with respect to CH4. The hydrocarbons are characterized by ratios of 170–200 and δ13C values of − 59.5 to − 62.4%o which indicates a considerable admixture of thermogenic hydrocarbon gases. The occurrence of methane of partly thermogenic origin demonstrates that CH4 enters the sealed borehole in the lower, perforated section (94–178 mbsf) and accumulates at the top of the borehole. This suggests the occurrence of free gas within the encapsulated borehole. Considering the stability field of CH4-hydrates, the formation of these ice-like structures may take place and potentially results in a clogging of the top of the borehole. Such precipitates could result in a decoupling of the top of the borehole from the hydraulic and geochemical regime of the accretionary complex, an important aspect for future plans of CORK deployments.

Description: In near-shore and coastal margin sediments remineralization of organic carbon is significantly affected by biologically mediated solute exchange caused by burrow-dwelling infauna. Although irrigation rates have been determined for various environments, little is known about their seasonal variations and their coupling to the food-supply or the oxygen level in bottom waters. These aspects have been investigated at two sites in the Kiel Bight by modelling pore water concentrations of Cl, which is a suitable tracer for transport processes. A very similar temporal pattern of irrigation was determined at both sites. In spring and fall the effect of bioirrigation on the pore water concentration of Cl is important at both sites, and a more than two to five fold enhancement of solute exchange, relative to diffusional transport, was calculated. The temporal pattern of bioirrigation correlates with that of the Chl.-a (eq) inventory of the surface sediments. Enhanced irrigation rates follow the settling of plankton blooms in this region. During the summer, when low oxygen levels were observed in bottom waters, overall irrigation rates are low. Furthermore, the relative importance of irrigation processes operating close to the sediment surface increases suggesting an upward movement and migration of burrow-dwelling organisms in response to low O2-concentrations. Because bioirrigation is an important transport process coupling organic carbon flux, remineralization at the seafloor, and redox zonation in the sediment quantifying the seasonal cycle of the irrigation intensity represents a step forward in the dynamic understanding of benthic processes.

Description: Production of biogenic silica and dissolution processes in the water column and surface sediment are important aspects for the investigation and reconstruction of present and past productivity of the ocean. Although the geological record of biogenic silica is often used as a proxy for paleoceanographic processes in the Southern Ocean, little is known about the present regional distribution of biogenic silica flux and accumulation and their relation to primary production in surface waters. Based on more than 130 sediment and pore water samples, the regional differences of the biogenic silica flux to the sea floor of the southern South Atlantic were investigated. In contrast to biogenic silica content, the dissolved Si-flux through the sediment/water interface, caused by intense dissolution of BSi in surface sediments, reflects biogenic production in surface waters. This was inferred by observed increases of Si-fluxes in regions of recurrent polynya formation or in the vicinity of Marginal Ice Zones as at the Weddell-Scotia Sea boundary. In the Scotia Sea, where no benthic fluxes were reported before, we found a considerable burial of biogenic silica and biogenic silica fluxes to the sea floor of ∼800–1300 mmol m-2 a-1. This is a significantly higher flux than derived for the known opal accumulation area in the SE Atlantic, further to the east in the Antarctic Circumpolar Current, where a flux of ∼600–767 mmol m-2 a-1 was observed. This shows that the Scotia Sea is not a gap within the Circumpolar Antarctic Opal Belt as previously assumed. The geochemical budget for different sub-regions of the South Atlantic was considered by a Geographic Information System. In contrast to most previous attempts, this ensures the accurate consideration of the spatial distribution of sampling sites, a crucial aspect for the accuracy of geochemical budgets. For the South Atlantic we calculated the flux of biogenic silica to the sea floor as ∼5.1×1012 mol a-1. Only ∼0.84×1012 mol a-1 is buried in these sediments, which is considerably less than previous estimates.

Description: The flux of reactive organic carbon (C(org)) into sediments of the southern and eastern Weddell Sea was estimated by modelling measured oxygen and nitrate pore-water profiles. Highest flux of reactive organic carbon into the sediment was calculated for the shelf region (500 and 600 mmol C m-2 year-1), whereas for pelagic and continental slope sediments C(org) fluxes of less than 60 mmol C m-2 year-1 and 100-200 mmol C m-2 year-1 respectively were calculated. The oxygen penetration depth (OPD) ranged from less than 2 cm in shelf sediments to much greater than 40 cm in pelagic sediments. For the first time, sediments covered by the Filchner Ice Shelf (probably cut off from a source of primary production for a few decades) were sampled. In this area a restricted vertical flux of reactive organic carbon was expected. However, the C(org), content of these sediments was as high as that of Antarctic shelf sediments, which suggests lateral transport of organic matter. In contrast, pore-water profiles and calculated reactive organic carbon fluxes off Filchner Ice Shelf are similar to those of much deeper depositional environments (3000-4000 m water depth).

Description: Stirred flow-through experiments were conducted for the first time with planktonic biogenic silica (BSi). We investigated the dissolution kinetics of uncleaned and chemically cleaned BSi collected in ocean surface water, sediment traps, and sediments from the Norwegian Sea, the Southern Ocean, and the Arabian Sea. The solubility at 2°C is rather constant (1000 to 1200 μM). The dissolution rates are, however, highly variable, declining with water depth, and phytoplankton reactivity is two to three orders of magnitude higher than pure siliceous oozes. The reactivity decrease correlates well with an increase in the integrated peak intensity ratios of Si-O-Si/Si-OH measured by Fourier transform infrared (FTIR) spectroscopy. The removal of organic or inorganic coatings enhance the reactivity by at least an order of magnitude. Atomic Al/Si ratios of 0.03 to 0.08 in sedimentary diatom frustules decrease significantly to 0.02 as a result of removal of inorganic coatings and detritals present. Near equilibrium, the dissolution rates exhibit a linear dependence on the degree of undersaturation. At higher degrees of undersaturation—that is, at low concentrations of dissolved silica—the dissolution rates of uncleaned samples define a nonlinear trend. The nonlinear kinetics imply that the dissolution of natural BSi is strongly accelerated in silica-depleted surface waters. The FTIR results suggest that internal condensation reactions reduce the amount of surface reaction sites and are partly responsible for the reactivity decrease with depth. The high content of Al in sedimentary BSi is likely caused by precipitation of dissolved silica with Al dissolved from minerals in sediment. Nonbiogenic silica as coatings or detritals are partly responsible for the solubility and reactivity decrease of BSi in sediments. One order of magnitude different rate constants measured in Norwegian Sea and Southern Ocean sediment trap material support the so-called opal paradox—that is, high BSi accumulation rates in sediments in spite of low BSi production rates in surface waters of the Southern Ocean.

Description: Organic carbon fluxes through the sediment/water interface in the high-latitude North Atlantic were calculated from oxygen microprofiles. A wire-operated in situ oxygen bottom profiler was deployed, and oxygen profiles were also measured onboard (ex situ). Diffusive oxygen fluxes, obtained by fitting exponential functions to the oxygen profiles, were translated into organic carbon fluxes and organic carbon degradation rates. The mean Corg input to the abyssal plain sediments of the Norwegian and Greenland Seas was found to be 1.9 mg C m−2 d−1. Typical values at the seasonally ice-covered East Greenland continental margin are between 1.3 and 10.9 mg C m−2 d−1 (mean 3.7 mg C m−2 d−1), whereas fluxes on the East Greenland shelf are considerably higher, 9.1–22.5 mg C m−2 d−1. On the Norwegian continental slope Corg fluxes of 3.3–13.9 mg C m−2 d−1 (mean 6.5 mg C m−2 d−1) were found. Fluxes are considerably higher here compared to stations on the East Greenland slope at similar water depths. By repeated occupation of three sites off southern Norway in 1997 the temporal variability of diffusive O2 fluxes was found to be quite low. The seasonal signal of primary and export production from the upper water column appears to be strongly damped at the seafloor. Degradation rates of 0.004–1.1 mg C cm−3 a−1 at the sediment surface were calculated from the oxygen profiles. First-order degradation constants, obtained from Corg degradation rates and sediment organic carbon content, are in the range 0.03–0.6 a−1. Thus, the corresponding mean lifetime of organic carbon lies between 1.7 and 33.2 years, which also suggests that seasonal variations in Corg flux are small. The data presented here characterize the Norwegian and Greenland Seas as oligotrophic and relatively low organic carbon deep-sea environments.