ALBERT

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: Extensive methane hydrate layers are formed in the near-surface sediments of the Cascadia margin. An undissociated section of such a layer was recovered at the base of a gravity core (i.e. at a sediment depth of 120 cm) at the southern summit of Hydrate Ridge. As a result of salt exclusion during methane hydrate formation, the associated pore waters show a highly elevated chloride concentration of 809 mM. In comparison, the average background value is 543 mM. A simple transport-reaction model was developed to reproduce the Cl- observations and quantify processes such as hydrate formation, methane demand, and fluid flow. From this first field observation of a positive Cl- anomaly, high hydrate formation rates (0.15–1.08 mol cm-2 a-1) were calculated. Our model results also suggest that the fluid flow rate at the Cascadia accretionary margin is constrained to 45–300 cm a-1. The amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water. Thus, most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from CH4 dissolved in the pore water.

Description: The usually high concentrations of Zn, Pb, Cd, and Cu in the most recently accreted portions of ferromanganese nodules from the western Baltic Sea are thought to reflect increased metal input due to anthropogenic mobilization. If so, the point of increase represents a time horizon within the structure of the nodule. Similar trace metal distributions of radiometrically dated sediments from the same area suggest that the ferromanganese nodules have grown in thickness between 0.02 and 0.16 mm yr−1. From this growth rate anthropogenic Zn flux to the nodule surface was calculated to be 80 mg m−2 yr−1.

Description: Microbial decomposition of organic matter in recent sediments of the Landsort Deep—an anoxic basin of the central Baltic Sea—resulted in the formation of a characteristic assemblage of authigenic mineral precipitates of carbonates, sulfides. phosphates and amorphous silica, The dominant crystalline phases are a mixed Mn-carbonate [(Mn0.85Ca0.10Mg0.05)CO3]. Mn-sulfide [MnS] and Fecarbonate [FeCO3]. Amorphous Fe-sulfide [FeS]. Mn-phosphate [Mn3(PO4)2] and a mixed Fe-Ca-phosphate [(Fe0.86Ca0.14)3(PO4)2] were identified by their chemical compositions only. The variability in composition of these solid phases and their mode of occurrence as a co-existing assemblage constrains the conditions and solution composition from which they precipitated. Estimates of activities for dissolved Fe. Mn. PO4, CO3 and S in equilibrium with such an assemblage are close to those found in recent anoxic interstitial water-sediment systems. It is important to have detailed knowledge of the composition and stability conditions of these solid precipitates in order to refine stoichiometric models of interstitial nutrient regeneration in anoxic sediments.

Description: Mixed methane–sulfide hydrates and carbonates are exposed as a pavement at the seafloor along the crest of one of the accretionary ridges of the Cascadia convergent margin. Vent fields from which methane-charged, low-salinity fluids containing sulfide, ammonia, 4He, and isotopically light CO2 escape are associated with these exposures. They characterize a newly recognized mechanism of dewatering at convergent margins, where freshening of pore waters from hydrate destabilization at depth and free gas drives fluids upward. This process augments the convergence-generated overpressure and leads to local dewatering rates that are much higher than at other margins in the absence of hydrate. Discharge of fluids stimulates benthic oxygen consumption which is orders of magnitude higher than is normally found at comparable ocean depths. The enhanced turnover results from the oxidation of methane, hydrogen sulfide, and ammonia by vent biota. The injection of hydrate methane from the ridge generates a plume hundreds of meters high and several kilometers wide. A large fraction of the methane is oxidized within the water column and generates δ13C anomalies of the dissolved inorganic carbon pool.

Description: Hydrate Ridge, Cascadia convergent margin, is characterized by abundant methane hydrates at and below the seafloor, active venting of fluids and gases, chemosynthetic communities composed of giant sulfide-oxidizing bacteria and clams, authigenic carbonates, and some of the highest methane oxidation rates ever found in the marine environment. Fluid flow rates vary over six orders of magnitude among closely spaced settings at the crest of Hydrate Ridge. The distribution of benthic communities is mainly related to the sulfide flux from the subsurface sediments produced by anaerobic oxidation of methane (AOM). Even at high flux rates, AOM removes most of the methane seeping from the subsurface. Less than 50% of the methane escapes from bacterial mats, from Calyptogena fields 〈15%, and from Acharax beds, no methane is emitted to the water column. The precipitation of carbonate derived from AOM is a significant and permanent carbon sink. Hence, compared to the amount of gas hydrates and methane-derived carbonates stored at the summit of Hydrate Ridge, the emission of methane is relatively low due to the efficient filtering capacity of methanotrophic microbial communities. This present review updates current knowledge of the geology of Hydrate Ridge, summarizes recently published data on geomicrobiology and biogeochemistry, and derives a local methane budget.

Description: Extensive deposits of methane hydrate characterize Hydrate Ridge in the Cascadia margin accretionary complex. The ridge has a northern peak at a depth of about 600 m, which is covered by extensive carbonate deposits, and an 800 m deep southern peak that is predominantly sediment covered. Samples collected with benthic instrumentation and from Alvin push cores reveal a complex hydrogeologic system where fluid and methane fluxes from the seafloor vary by several orders of magnitude at sites separated by distances of only a few meters. We identified three distinct active fluid regimes at Hydrate Ridge. The first province is represented by discrete sites of methane gas ebullition, where the bulk of the flow occurs through channels in which gas velocities reach 1 m s−1. At the northern summit of the ridge the gas discharge appears to be driven by pressure changes on a deep gas reservoir, and it is released episodically at a rate of ∼6×104 mol day−1 following tidal periodicity. Qualitative observations at the southern peak suggest that the gas discharge there is driven by more localized phenomena, possibly associated with destabilization of massive gas hydrate deposits at the seafloor. The second province is characterized by the presence of extensive bacterial mats that overlay sediments capped with methane hydrate crusts, both at the northern and southern summits. Here fluid typically flows out of the sediments at rates ranging from 30 to 100 cm yr−1. The third province is represented by sites colonized by vesicomyid clams, where bottom seawater flows into the sediments for at least some fraction of the time. Away from the active gas release sites, fluid flows calculated from pore water models are in agreement with estimates using published flowmeter data and numerical model calculations. Methane fluxes out of mat-covered sites range from 30 to 90 mmol m−2 day−1, whereas at clam sites the methane flux is less than 1 mmol m−2 day−1.

Description: Four mud extrusions were investigated along the erosive subduction zone off Costa Rica. Active fluid seepage from these structures is indicated by chemosynthetic communities, authigenic carbonates and methane plumes in the water column. We estimate the methane output from the individual mud extrusions using two independent approaches. The first is based on the amount of CH4 that becomes anaerobically oxidized in the sediment beneath areas covered by chemosynthetic communities, which ranges from 104 to 105 mol yr− 1. The remaining portion of CH4, which is released into the ocean, has been estimated to be 102–104 mol yr− 1 per mud extrusion. The second approach estimates the amount of CH4 discharging into the water column based on measurements of the near-bottom methane distribution and current velocities. This approach yields estimates between 104–105 mol yr−1. The discrepancy of the amount of CH4 emitted into the bottom water derived from the two approaches hints to methane seepage that cannot be accounted for by faunal growth, e.g. focused fluid emission through channels in sediments and fractures in carbonates. Extrapolated over the 48 mud extrusions discovered off Costa Rica, we estimate a CH4 output of 20·106 mol yr− 1 from mud extrusions along this 350 km long section of the continental margin. These estimates of methane emissions at an erosional continental margin are considerably lower than those reported from mud extrusion at accretionary and passive margins. Almost half of the continental margins are described as non-accretionary. Assuming that the moderate emission of methane at the mud extrusions off Costa Rica are typical for this kind of setting, then global estimates of methane emissions from submarine mud extrusions, which are based on data of mud extrusions located at accretionary and passive continental margins, appear to be significantly too high.

Description: In situ oxygen fluxes were measured at vent sites in the Aleutian trench at a water depth of almost 5000 m using a TV-guided benthic flux chamber. The flux was 2 orders of magnitude greater than benthic oxygen fluxes in areas unaffected by venting on the continental margin off Alaska. Porewater profiles taken from the surface sediment below a vent site showed high concentrations of sulfide, methane, and ammonia. The reduced carbon and nitrogen compounds are transported to the vent site by fluids expelled from deeper anoxic sediment layers by the forces of plate convergence. The tectonically driven fluid flow was determined from the biochemical turnover in vent communities and was found to be 3.4 ± 0.5 m yr−1. A model was used to quantify the transport of silica, Ca2+, and sulfate via diffusion, advection, and bioirrigation through the surface sediments of a vent site. A nonlocal mixing coefficient of 20–30 yr−1 was determined by fitting the model curves to the measured porewater profiles showing that the transport of solutes within the near-surface sediments and across the sediment-water interface is dominated by the activity of the vent fauna. Sulfate-containing oceanic bottom water and methane-rich vent fluids were mixed below the clam colony to produce sulfide and a CaCO3 precipitate. The vent biota shape their immediate environment and control the sediment-water exchange and the benthic fluxes at vent sites. The oxygen consumption at vent sites is a major sink for oxygen at the study area.

Description: Cold seeps in the Aleutian deep-sea trench support prolific benthic communities and generate carbonate precipitates which are dependent on carbon dioxide delivered from anaerobic methane oxidation. This process is active in the anaerobic sediments at the sulfate reduction-methane production boundary and is probably performed by archaea working in syntrophic co-operation with sulfate-reducing bacteria. Diagnostic lipid biomarkers of archaeal origin include irregular isoprenoids such as 2,6,11,15-tetramethylhexadecane (crocetane) and 2,6,10,15,19-pentamethylicosane (PMI) as well as the glycerol ether lipid archaeol (2,3-di-O-phytanyl-sn-glycerol). These biomarkers are prominent lipid constituents in the anaerobic sediments as well as in the carbonate precipitates. Carbon isotopic compositions of the biomarkers are strongly depleted in 13C with values of δ13C as low as −130.3‰ PDB. The process of anaerobic methane oxidation is also reflected in the carbon isotope composition of organic matter with δ13C-values of −39.2 and −41.8‰ and of the carbonate precipitates with values of −45.4 and −48.7‰. This suggests that methane-oxidizing archaea have accumulated within the microbial community, which is active at the cold seep sites. The dominance of crocetane in sediments at one station indicates that, probably due to decreased methane venting, archaea might no longer be growing, whereas high amounts of crocetenes found at other more active stations may indicate recent fluid venting and active archaea. Comparison with other biomarker studies suggests that various archaeal assemblages might be involved in the anaerobic consumption of methane. The assemblages are apparently dependent on specific conditions found at each cold seep environment. Selective conditions probably include water depth, temperature, degree of anoxia, and supply of free methane.