Methane concentration profiles and isotopic composition of sediment cores from the Black Sea

We present high resolution profiles for the methane concentration and the carbon isotope composition of methane from surface sediments and from the sediment-water transition in the Black Sea. At shallow water sites methane migrates from the sediment into the water column, and the magnitude of this upward migrating flux depends on the depth of the sulfate-methane transition (SMT) in the sediment. The isotope data reveal that the sediments at shallow water sites are a source for methane depleted in 13C relative to the isotope composition of methane in the water column. At deep water sites the methane concentration first decreases with depth in the sediment to reach lowest values at the Unit I to Unit II transition. Below this transition the concentration increases again. Numerical modeling of methane concentration and isotope data shows that high methane oxidation rates occur in the surface sediment layer, indicating that the removal of methane in the surface sediments is not related to the anaerobic oxidation of methane coupled to sulfate reduction that occurs a few meters deep in the sediment, at the SMT. Instead, near-surface methane consumption in the euxinic Black Sea sediments appears to be related to lithological stratification. Furthermore, a map of the diffusive methane fluxes in the Black Sea surface sediments indicates that approximately half of the Black Sea seafloor acts as a sink for methane and thus limits the flux of methane to the atmosphere.

Geochemical data and magnetic measurements of IODP Expedition 308, Hole U1319A from the Gulf of Mexico

A 160 m mostly turbiditic late Pleistocene sediment sequence (IODP Expedition 308, Hole U1319A) from the Brazos-Trinity intraslope basin system off Texas was investigated with paleo- and rock magnetic methods. Numerous layers depleted in iron oxides and enriched by the ferrimagnetic iron-sulfide mineral greigite (Fe3S4) were detected by diagnostic magnetic properties. From the distribution of these layers, their stratigraphic context and the present geochemical zonation, we develop two conceptual reaction models of greigite formation in non-steady depositional environments. The "sulfidization model" predicts single or twin greigite layers by incomplete transformation of iron monosulfides with polysulfides around the sulfate methane transition (SMT). The "oxidation model" explains greigite formation by partial oxidation of iron monosulfides near the iron redox boundary during periods of downward shifting oxidation fronts. The stratigraphic record provides evidence that both these greigite formation processes act here at typical depths of about 12-14 mbsf and 3-4 mbsf. Numerous "fossil" greigite layers most likely preserved by rapid upward shifts of the redox zonation denote past SMT and sea floor positions characterized by stagnant hemipelagic sedimentation conditions. Six diagenetic stages from a pristine magnetite-dominated to a fully greigite-dominated magnetic mineralogy were differentiated by combination of various hysteresis and remanence parameters.

Geochemical and rock magnetic investigation of sediment cores of the western South Atlantic

Geochemical and rock magnetic investigations of sediments from three sites on the continental margin off Argentina and Uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane (AOM). The western Argentine Basin represents a suitable sedimentary environment to study nonsteady-state processes because it is characterized by highly dynamic depositional conditions. Mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals. As a typical feature of these deposits, distinct minima in magnetic susceptibility (k) are observed. Pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate/methane transition (SMT) where HS- is generated by the process of AOM. The released HS- reacts with the abundant iron (oxyhydr)oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility. Modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate (SR) which occurred during the Pleistocene/Holocene transition. We assume that the strong decrease in mean SR encountered during this glacial/interglacial transition induced a fixation of the SMT at a specific depth. The stagnation has obviously enhanced diagenetic dissolution of iron (oxyhydr)oxides within a distinct sediment interval. This assumption was further substantiated by numerical modeling in which the mean SR was decreased from 100 cm/kyr during glacial times to 5 cm/kyr in the Holocene and the methane flux from below was fixed to a constant value. To obtain the observed geochemical and magnetic patterns, the SMT must remain at a fixed position for ~9000 yrs. This calculated value closely correlates to the timing of the Pleistocene/Holocene transition. The results of the model show additionally that a constant high mean SR would cause a concave-up profile of pore water sulfate under steady state conditions.

Geochemistry of sediments from the Eastern Cape Basin

Sediment cores retrieved in the Benguela coastal upwelling system off Namibia show very distinct enrichments of solid phase barium at the sulfate/methane transition (SMT). These barium peaks represent diagenetic barite (BaSO4) fronts which form by the reaction of upwardly diffusing barium with interstitial sulfate. Calculated times needed to produce these barium enrichments indicate a formation time of about 14,000 yr. Barium spikes a few meters below the SMT were observed at one of the investigated sites (GeoB 8455). Although this sulfate-depleted zone is undersaturated with respect to barite, the dominant mineral phase of these buried barium enrichments was identified as barite by scanning electron microscopy (SEM). This is the first study which reports the occurrence/preservation of pronounced barite enrichments in sulfate-depleted sediments buried a few meters below the SMT. At site GeoB 8455 high concentrations of dissolved barium in pore water as well as barium in the solid phase were observed. Modeling the measured barium concentrations at site GeoB 8455 applying the numerical model CoTReM reveals that the dissolution rate of barite directly below the SMT is about one order of magnitude higher than at the barium enrichments deeper in the sediment core. This indicates that the dissolution of barite at these deeper buried fronts must be retarded. Thus, the occurrence of the enrichments in solid phase barium at site GeoB 8455 could be explained by decreased dissolution rates of barite due to the changes in the concentration of barite in the sediment, as well as changes in the saturation state of fluids. Furthermore, the alteration of barite into witherite (BaCO3) via the transient phase barium sulfide could lead to the preservation of a former barite front as BaCO3. The calculations and modeling indicate that a relocation of the barite front to a shallower depth occurred between the last glacial maxium (LGM) and the Pleistocene/Holocene transition. We suggest that an upward shift of the SMT occurred at that time, most likely as a result of an increase in the methanogenesis rates due to the burial of high amounts of organic matter below the SMT.

Solid phase analysis of sediment core GeoB9309-1 from the Zambezi (or Mozambique) deep-sea fan

The Zambezi deep-sea fan, the largest of its kind along the east African continental margin, is poorly studied to date, despite its potential to record marine and terrestrial climate signals in the southwest Indian Ocean. Therefore, gravity core GeoB 9309-1, retrieved from 1219 m water depth, was investigated for various geophysical (magnetic susceptibility, porosity, colour reflectance) and geochemical (pore water and sediment geochemistry, Fe and P speciation) properties. Onboard and onshore data documented a sulphate/methane transition (SMT) zone at ~ 450-530 cm sediment depth, where the simultaneous consumption of pore water sulphate and methane liberates hydrogen sulphide and bi-carbonate into the pore space. This leads to characteristic changes in the sediment and pore water chemistry, as the reduction of primary Fe (oxyhydr)oxides, the precipitation of Fe sulphides, and the mobilization of Fe (oxyhydr)oxide-bound P. These chemical processes also lead to a marked decrease in magnetic susceptibility. Below the SMT, we find a reduction of porosity, possibly due to pore space cementation by authigenic minerals. Formation of the observed geochemical, magnetic and mineralogical patterns requires a fixation of the SMT at this distinct sediment depth for a considerable time-which we calculated to be ~ 10 000 years assuming steady-state conditions-following a period of rapid upward migration towards this interval. We postulate that the worldwide sea-level rise at the last glacial/interglacial transition (~ 10 000 years B.P.) most probably caused the fixation of the SMT at its present position, through drastically reduced sediment delivery to the deep-sea fan. In addition, we report an internal redistribution of P occurring around the SMT, closely linked to the (de)coupling of sedimentary Fe and P, and leaving a characteristic pattern in the solid P record. By phosphate re-adsorption onto Fe (oxyhydr)oxides above, and formation of authigenic P minerals (e.g. vivianite) below the SMT, deep-sea fan deposits may potentially act as long-term sinks for P.

Low-temperature experiments of sediment core GeoB6229-6 at the South American continental margin off the Rio de la Plata estuary

With various low-temperature experiments performed on magnetic mineral extracts of marine sedimentary deposits from the Argentine continental slope near the Rio de la Plata estuary, a so far unreported style of partial magnetic self-reversal has been detected. In these sediments the sulphate-methane transition (SMT) zone is situated at depths between 4 and 8 m, where reductive diagenesis severely alters the magnetic mineral assemblage. Throughout the sediment column magnetite and ilmenite are present together with titanomagnetite and titanohematite of varying compositions. In the SMT zone (titano-)magnetite only occurs as inclusions in a siliceous matrix and as intergrowths with lamellar ilmenite and titanium-rich titanohematite, originating from high temperature deuteric oxidation within the volcanic host rocks. These abundant structures were visualized by scanning electron microscopy and analysed by energy dispersive spectroscopy. Warming of field-cooled and zero-field-cooled low-temperature saturation remanence displays magnetic phase transitions of titanium-rich titanohematite below 50 K and the Verwey transition of magnetite. A prominent irreversible decline characterizes zero-field cooling of room temperature saturation remanence. It typically sets out at ~210 K and is most clearly developed in the lower part of the SMT zone, where low-temperature hysteresis measurements identified ~210 K as the blocking temperature range of a titanohematite phase with a Curie temperature of around 240 K. The mechanism responsible for the marked loss of remanence is, therefore, sought in partial magnetic self-reversal by magnetostatic interaction of (titano-)magnetite and titanohematite. When titanohematite becomes ferrimagnetic upon cooling, its spontaneous magnetic moments order antiparallel to the (titano-)magnetite remanence causing an drastic initial decrease of global magnetization. The loss of remanence during subsequent further cooling appears to result from two combined effects (1) magnetic interaction between the two phases by which the (titano-)magnetite domain structure is substantially modified and (2) low-temperature demagnetization of (titano-)magnetite due to decreasing magnetocrystalline anisotropy. The depletion of titanomagnetite and superior preservation of titanohematite is characteristic for strongly reducing sedimentary environments. Typical residuals of magnetic mineral assemblages derived from basaltic volcanics will be intergrowths of titanohematite lamellae with titanomagnetite relics. Low-temperature remanence cycling is, therefore, proposed as a diagnostic method to magnetically characterize such alteration (palaeo-)environments.

Depth profiles of dissolved sulfide and sulfate in the pore water of hydrocarbon seep sediments offshore Pakistan

The interaction between fluid seepage, bottom water redox, and chemosynthetic communities was studied at cold seeps across one of the world's largest oxygen minimum zones (OMZ) located at the Makran convergent continental margin. Push cores were obtained from seeps within and below the core-OMZ with a remotely operated vehicle. Extracted sediment pore water was analyzed for sulfide and sulfate concentrations. Depending on oxygen availability in the bottom water, seeps were either colonized by microbial mats or by mats and macrofauna. The latter, including ampharetid polychaetes and vesicomyid clams, occurred in distinct benthic habitats, which were arranged in a concentric fashion around gas orifices. At most sites colonized by microbial mats, hydrogen sulfide was exported into the bottom water. Where macrofauna was widely abundant, hydrogen sulfide was retained within the sediment. Numerical modeling of pore water profiles was performed in order to assess rates of fluid advection and bioirrigation. While the magnitude of upward fluid flow decreased from 11 cm yr**-1 to <1 cm yr**-1 and the sulfate/methane transition (SMT) deepened with increasing distance from the central gas orifice, the fluxes of sulfate into the SMT did not significantly differ (6.6-9.3 mol m**-2 yr**-1). Depth-integrated rates of bioirrigation increased from 120 cm yr**-1 in the central habitat, characterized by microbial mats and sparse macrofauna, to 297 cm yr**-1 in the habitat of large and few small vesicomyid clams. These results reveal that chemosynthetic macrofauna inhabiting the outer seep habitats below the core-OMZ efficiently bioirrigate and thus transport sulfate down into the upper 10 to 15 cm of the sediment. In this way the animals deal with the lower upward flux of methane in outer habitats by stimulating rates of anaerobic oxidation of methane (AOM) with sulfate high enough to provide hydrogen sulfide for chemosynthesis. Through bioirrigation, macrofauna engineer their geochemical environment and fuel upward sulfide flux via AOM. Furthermore, due to the introduction of oxygenated bottom water into the sediment via bioirrigation, the depth of the sulfide sink gradually deepens towards outer habitats. We therefore suggest that - in addition to the oxygen levels in the water column, which determine whether macrofaunal communities can develop or not - it is the depth of the SMT and thus of sulfide production that determines which chemosynthetic communities are able to exploit the sulfide at depth. We hypothesize that large vesicomyid clams, by efficiently expanding the sulfate zone down into the sediment, could cut off smaller or less mobile organisms, as e.g. small clams and sulfur bacteria, from the sulfide source.

Sediment and pore water chemistry of cores 214 and 755 recovered during Meteor M72/1 and POSEIDON cruise POS317/2, northwestern Black Sea

High-resolution sedimentary records of major and minor elements (Al, Ba, Ca, Sr, Ti), total organic carbon (TOC), and profiles of pore water constituents (SO42-, CH4, Ca2+, Ba2+, Mg2+, alkalinity) were obtained for two gravity cores (core 755, 501 m water depth and core 214, 1686 m water depth) from the northwestern Black Sea. The records were examined in order to gain insight into the cycling of Ba in anoxic marine sediments characterized by a shallow sulfate-methane transition (SMT) as well as the applicability of barite as a primary productivity proxy in such a setting. The Ba records are strongly overprinted by diagenetic barite (BaSO4) precipitation and remobilization; authigenic Ba enrichments were found at both sites at and slightly above the current SMT. Transport reaction modeling was applied to simulate the migration of the SMT during the changing geochemical conditions after the Holocene seawater intrusion into the Black Sea. Based on this, sediment intervals affected by diagenetic Ba redistribution were identified. Results reveal that the intense overprint of Ba and Baxs (Ba excess above detrital average) strongly limits its correlation to primary productivity. These findings have implications for other modern and ancient anoxic basins, such as sections covering the Oceanic Anoxic Events for which Ba is frequently used as a primary productivity indicator. Our study also demonstrates the limitations concerning the use of Baxs as a tracer for downward migrations of the SMT: due to high sedimentation rates at the investigated sites, diagenetic barite fronts are buried below the SMT within a relatively short period. Thus, 'relict' barite fronts would only be preserved for a few thousands of years, if at all.

Physical oceanography, pore water chemistry and water column methane turover rates during POLARSTERN cruise ANT-XXIX/4

Recent studies have suggested that the marine contribution of methane from shallow regions and melting marine terminating glaciers may have been underestimated. Here we report on methane sources and potential sinks associated with methane seeps in Cumberland Bay, South Georgia's largest fjord system. The average organic carbon content in the upper 8 meters of the sediment is around 0.65 wt.%; this observation combined with Parasound data suggest that the methane gas accumulations probably originate from peat-bearing sediments currently located several tens of meters below the seafloor. Only one of our cores indicates upward advection; instead most of the methane is transported via diffusion. Sulfate and methane flux estimates indicate that a large fraction of methane is consumed by anaerobic oxidation of methane (AOM). Carbon cycling at the sulfate-methane transition (SMT) results in a marked fractionation of the d13C-CH4 from an estimated source value of -65 per mil to a value as low as -96 per mil just below the SMT. Methane concentrations in sediments are high, especially close to the seepage sites (~40 mM); however, concentrations in the water column are relatively low (max. 58 nM) and can be observed only close to the seafloor. Methane is trapped in the lowermost water mass, however, measured microbial oxidation rates reveal very low activity with an average turnover of 3.1 years. We therefore infer that methane must be transported out of the bay in the bottom water layer. A mean sea-air flux of only 0.005 nM/m²/s confirms that almost no methane reaches the atmosphere.

High-resolution pore-water and solid-phase analyses of sediment cores GeoB6520-3 (Hydrate Hole) and GeoB6521-2 (Worm Hole) of the northern Congo Fan

The geochemical cycling of barium was investigated in sediments of pockmarks of the northern Congo Fan, characterized by surface and subsurface gas hydrates, chemosynthetic fauna, and authigenic carbonates. Two gravity cores retrieved from the so-called Hydrate Hole and Worm Hole pockmarks were examined using high-resolution pore-water and solid-phase analyses. The results indicate that, although gas hydrates in the study area are stable with respect to pressure and temperature, they are and have been subject to dissolution due to methane-undersaturated pore waters. The process significantly driving dissolution is the anaerobic oxidation of methane (AOM) above the shallowest hydrate-bearing sediment layer. It is suggested that episodic seep events temporarily increase the upward flux of methane, and induce hydrate formation close to the sediment surface. AOM establishes at a sediment depth where the upward flux of methane from the uppermost hydrate layer counterbalances the downward flux of seawater sulfate. After seepage ceases, AOM continues to consume methane at the sulfate/methane transition (SMT) above the hydrates, thereby driving the progressive dissolution of the hydrates "from above". As a result the SMT migrates downward, leaving behind enrichments of authigenic barite and carbonates that typically precipitate at this biogeochemical reaction front. Calculation of the time needed to produce the observed solid-phase barium enrichments above the present-day depths of the SMT served to track the net downward migration of the SMT and to estimate the total time of hydrate dissolution in the recovered sediments. Methane fluxes were higher, and the SMT was located closer to the sediment surface in the past at both sites. Active seepage and hydrate formation are inferred to have occurred only a few thousands of years ago at the Hydrate Hole site. By contrast, AOM-driven hydrate dissolution as a consequence of an overall net decrease in upward methane flux seems to have persisted for a considerably longer time at the Worm Hole site, amounting to a few tens of thousands of years.