Eberhard Sauter

Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles

The assessment of climate change factors includes a constraint of methane sources and sinks. Although marine geological sources are recognized as significant, unfortunately, most submarine sources remain poorly quantified. Beside cold vents and coastal anoxic sediments, the large number of submarine mud volcanoes (SMV) may contribute significantly to the oceanic methane pool. Recent research suggests that methane primarily released diffusively from deep-sea SMVs is immediately oxidized and, thus, has little climatic impact. New hydro-acoustic, visual, and geochemical observations performed at the deep-sea mud volcano Hakon Mosby reveal the discharge of gas hydrate-coated methane bubbles and gas hydrate flakes forming huge methane plumes extending from the seabed in 1250 m depth up to 750 m high into the water column. This depth coincides with the upper limit of the temperature-pressure field of gas hydrate stability. Hydrographic evidence suggests bubble-induced upwelling within the plume and extending above the hydrate stability zone. Thus, we propose that a significant portion of the methane from discharged methane bubbles can reach the upper water column, which may be explained due to the formation of hydrate skins. As the water mass of the plume rises to shallow water depths, methane dissolved from hydrated bubbles may be transported towards the surface and released to the atmosphere. Repeated acoustic surveys performed in 2002 and 2003 suggest continuous methane emission to the ocean. From seafloor visual observations we estimated a gas flux of 0.2 (0.08-0.36) mol s−1 which translates to several hundred tons yr−1 under the assumption of a steady discharge. Besides, methane was observed to be released by diffusion from sediments as well as by focused outflow of methane-rich water. In contrast to the bubble discharge, emission rates of these two pathways are estimated to be in the range of several tons yr−1 and, thus, to be of minor importance. Very low water column methane oxidation rates derived from incubation experiments with tritiated methane suggest that methane is distributed by currents rather than oxidized rapidly.

Deep carbon export from a Southern Ocean iron-fertilized diatom bloom

Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence—although each with important uncertainties—lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments.

Organic carbon flux and remineralization in surface sediments from the northern North Atlantic derived from pore-water oxygen microprofiles

Abstract:Organic carbon fluxes through thesediment/water interface in the high-latitude NorthAtlantic have been calculated from oxygenmicroprofiles. A wire-operated in situ oxygen bottomprofiler was deployed, and oxygen profiles were alsomeasured on board (ex situ). Diffusive oxygen fluxes,obtained by fitting exponential functions to the oxygenprofiles, were translated into organic carbon fluxes andorganic carbon degradation rates. The mean Corg inputto the abyssal plain sediments of the Norwegian andGreenland Seas was found to be 1.9 mgCm-2d-1.Typical values at the seasonally ice-covered EastGreenland continental margin lay between 1.3 and 10.9mgCm-2d-1 (mean 3.7 mgCm-2d-1), whereas fluxes onthe East Greenland shelf are considerably higher, with9.1 to 22.5 mgCm-2d-1. On the Norwegian continentalslope Corg fluxes of 3.3 to 13.9 mgCm-2d-1 (mean 6.5mgCm-2d-1) were found. Fluxes are considerably higherhere compared to stations on the East Greenland slopeat similar water depths. By repeatedly occupying threesites off southern Norway in 1997 the temporalvariability of diffusive O2 fluxes was found to be quitelow. The seasonal signal of primary and exportproduction from the upper water column appears to bestrongly dampened at the seafloor. Degradation ratesof 0.004 to 1.1 mgCcm-3a-1 at the sediment surfacewere calculated from oxygen profiles. First-orderdegradation constants, obtained from Corg degradationrates and sediment organic carbon content, are in therange of 0.03 to 0.6 a-1. Thus, the corresponding meanlifetime of organic carbon lies between 1.7 and 33.2years, which also suggests that seasonal variations inCorg flux are small. The data presented herecharacterize the Norwegian and Greenland Seas asoligotrophic and relatively low organic carbon deep-seaenvironments.Keywords: Diffusive oxygen uptake,benthic fluxes, organic carbon, in situ oxygenmicroelectrodes, North Atlantic, high latitudes.

Spatial budget of organic carbon flux to the seafloor of the northern North Atlantic (60°N-80°N)

The transfer of organic carbon from surface waters to the seafloor was calculated for the northern North Atlantic east of Greenland. This calculation is based on an empirically derived relationship between the rain rate of remineralizable organic carbon, derived by in situ O2 profiles, water depth, and primary production. The reliability of this attempt is supported by the good correspondence of calculated rain rates with an independent data set of particle trap studies and shipboard measurements of O2 profiles. For water depths of > 500 m the total seafloor remineralization rate is 2.7 × 106 tC yr−1 for the northern North Atlantic. Low and nearly similar average rain rates of 0.60 and 0.65 gC m−2 yr−1 have been derived for the deep basins of the Norwegian and Greenland Seas. Therefore, 1.7–1.8% of the primary production is transferred to the seafloor of the basins. A considerably higher average flux of 3.8 gC m−2 yr−1 was calculated for the Iceland Plateau, where ∼3.3% of primary produced organic carbon reaches the seafloor. The sediments of the Iceland Plateau receive 1.0 × 106 tC yr−1 or ∼37% of the organic carbon rain rate to the seafloor derived for the entire northern North Atlantic. The transfer of primary produced organic carbon below water depths of 500 and 1000 m suggests that 10.3 × 106 tC yr−1 and 4.5 × 106 tC yr−1are exported from surface waters. This is 2-4.4% of the organic carbon produced in the photic zone of the northern North Atlantic east of Greenland.

Seasonal variations of bioirrigation in coastal sediments: Modelling of field data

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.

Biogenic silica cycle in surface sediments of the Greenland Sea.

Abstract -In contrast to several investigations of biogenic silica (BSi) content and recycling in surface sediments of the Southern Ocean, little isknown about the benthic cycle of BSi in high northern latitudes. Therefore, we investigated the silicic acid concentration of pore waterand BSi content of surface sediments from the Greenland Sea. Low BSi contents of less than 2% were observed. High-resolution (2-5mm) BSi profiles and comparisons to trap studies suggest that only relatively dissolution-resistant siliceous components reach the seafloor.Pore water investigations reveal BSi fluxes of more than 300 mmol m-2 a-1 only for a few sites on the shelf. A statistically significantrelationship between water depth and BSi rain rate reaching the seafloor was not observed. Sampling along a transect perpendicular tothe marginal ice zone (MIZ) revealed no enhanced rain rate of BSi reaching the seafloor in the vicinity of the ice edge. Although the MIZof the Greenland Sea is characterized by the enhanced export of biogenic particles from surface waters, this feature is not reflected in thebenthic cycle of biogenic silica. The lack of such a relationship, which is in contrast to observations of shelf and continental marginsediments in the southern South Atlantic, is probably caused by the enhanced dissolution of BSi in the water column and highly dynamicice conditions in the Greenland Sea.Keywords: biogenic silica; Greenland Sea; North Atlantic; surface sediment

Effects of dropstone-induced habitat heterogeneity on Arctic deep-sea benthos with special reference to nematode communities

Abstract During an expedition to the deep-sea long-term observatory HAUSGARTEN in the eastern Fram Strait in summer 2003, the availability of a Remotely Operated Vehicle allowed a targeted sampling of surface sediments around a relatively large dropstone (0.9 m2) to determine suspected differences in community structure and dynamics of nematode assemblages in relation to the confined flow regime and patchy food availability in the immediate vicinity of the stone. The almost rectangular dropstone was about 150 cm in length, 60 cm in width, and up to 15 cm in height. Small-scale current measurements around the dropstone using a MAVS-3 acoustic current meter exhibited a rather complex pattern. A computational fluid dynamics simulation revealed areas of constantly flowing near-bottom currents as well as the generation of vortices in certain areas around the dropstone. Concentrations of biogenic compounds in the sediments surrounding the dropstone generally followed the complex flow pattern. The differences in physical and biochemical conditions around the dropstone were reflected in species composition and diversity, trophic structure and life-history traits of the nematode communities, and to a lesser extent in their total abundance and biomass.

The hydroacoustic method for the quantification of the gas flux from a submersed bubble plume

This article presents an inverse hydroacoustic method for the remote quantification of the total gas flux transported from an underwater bubble plume. The method includes the surveying of the bubble plume by a vertically looking echo sounder and the calculation of the flux using the spatial distribution of the ultrasound backscattering at a fixed depth. A simplified parameterization containing only a few parameters is introduced to describe the empirical bubble size distribution. The linear correlation between the backscattering cross section of the bubble stream and the vertical gas flux is found. The calculation procedure takes into account the occurrence of a gas hydrate film at the bubble’s surface. The influence of different parameters on the accuracy of the method is investigated. The resolution volume of the echo sounder corresponding to the fixed distance is considered as a two-dimensional spatial window. The method was applied to quantify the total convective methane flux at the Haakon-Mosby mud volcano (HMMV) depth 1280 m. The calculated values of the total flux near the bottom (100–400 t/year) are in good agreement with the independently estimated flux for the single bubble jet observed from the ROV (70 t/year). These calculations also show significant temporal variability of the flux at the HMMV. The total flux was found to vary by about a factor of 2–3 within time scales of days.

Sampling of sub-seafloor aquifers by a temporary well for CFC age dating and natural tracer investigations

Abstract:Based on a sediment vibro corer, a tool for the sampling of sub-seafloor groundwater aquifershas been developed and successfully deployed in a coastal area of the western Baltic. Thedevice was designed to obtain pure groundwater samples from coarse sediments to be used fortracer investigations and CFC age dating.Operated from a medium size research vessel, a well pipe tipped with a filter segment isvibrated into the sediment down to the aquifer. Groundwater entering the filter is pumped tothe ship by a conventional submersible pump situated in the wells filter tip. Groundwater iscontinuously analysed on board for O2, salinity, pH, Eh and temperature, prior to sampling forCFC and radioisotope analysis. All parametres indicate that pure groundwater had beenobtained.CFC concentrations are very low suggesting that the groundwater of this shallow sub-seaflooraquifer recharged prior to 1950. This finding is in accordance with other hydrogeologicalevidence that this aquifer, located only 4-5 metres below the seafloor, is connected to fairlydeep confined sandy aquifers on land of Pleistocene or Miocene age.Applying the method described, it is possible to obtain sufficient sample volumes foranalyses of natural groundwater tracers such as radon-222 and CFCs which can beused to trace submarine groundwater discharge as well as the origin ofgroundwater in such environments.Keywords: submarine springs, groundwater, pockmark, submarine well, CFC, radon

Numerical Simulation of Freshwater, Salt Water and Methane Interaction Processes in a Coastal Aquifer

A two-phase (water, gas)/three-component (water, salt, methane) model concept for flow and transport processes in a porous medium was developed. It accounts for density-driven flow as well as for mass transfer between the phases.