| 1. |
|
|
| 2. |
|
|
| 3. |
- Brüchert, Volker, et al.
(författare)
-
Hydrogen sulphide and methane emissions on the central Namibian shelf
- 2009
-
Ingår i: Progress in Oceanography. - : Elsevier. - 0079-6611 .- 1873-4472. ; 83:1-4, s. 169-179
-
Tidskriftsartikel (refereegranskat)abstract
- Hydrogen sulphide occurs frequently in the waters of the inner shelf coastal upwelling area off central Namibia. The area affected coincides with hatching grounds of commercially important pelagic fish, whose recruitment may be severely affected by recurring toxic sulphidic episodes. Both episodic biogenic methane gas-driven advective and molecular diffusive flux of hydrogen sulphide have been implicated as transport mechanisms from the underlying organic-matter-rich diatomaceous mud. To test hypotheses on the controls of hydrogen sulphide transport from the sediments on the inner Namibian shelf, water column and sediment data were acquired from four stations between 27 and 72 m water depth over a 3 year long period. On 14 cruises, temperature, salinity, dissolved oxygen, nitrate, methane, and total dissolved sulphide were determined from water column samples, and pore water dissolved methane, total dissolved sulphide, biomass of benthic sulphide-oxidising bacteria Beggiatoa and Thiomargarita, and bacterial sulphate reduction rates were determined from sediment cores. Superimposed on a trend of synchronous changes in water column oxygen and nutrient concentrations controlled by regional hydrographic conditions were asynchronous small-scale variations at the in-shore stations that attest to localized controls on water column chemistry. Small temporal variations in sulphate reduction rates determined with 35S-labeled sulphate do not support the interpretation that variable emissions of sulphide and methane from sediments are driven by temporal changes in the degradation rates of freshly deposited organic matter. The large temporal changes in the concentrations of hydrogen sulphide and the co-occurrence of pore water sulphate and methane support an interpretation of episodic advection of methane and hydrogen sulphide from deeper sediment depths – possibly due to gas bubble transport. Effective fluxes of hydrogen sulphide and methane to the water column, and methane and sulphide concentrations in the bottom waters were decoupled, likely due to the activity of sulphide-oxidising bacteria. While the causal mechanism for the episodic fluctuations in methane and dissolved sulphide concentrations remains unclear, this data set points to the importance of alternating advective and diffusive transport of methane and hydrogen sulphide to the water column.
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
- Brüchert, Volker, et al.
(författare)
-
The challenge of distinguishing anoxia and euxinia with the chemical and isotopic composition of sedimentary sulfides.
- 2007
-
Ingår i: Geophysical Research Abstracts.
-
Konferensbidrag (refereegranskat)abstract
- The chemical and isotopic composition of sedimentary sulfide and reactive iron are widely used parameters for the discrimination of oxic versus euxinic water column conditions in the geological past. Here we test the applicability of this proxy in a systematic study of organic-rich shelf and slope sediments from the Benguela upwelling system. These sediments deposit under (1) sulfidic, (2) anoxic, but non-sulfidic, and (3) fully oxic conditions, but below an oxygen minimum zone. Due to the high accumulation rates of organic matter, sulfate reduction and formation of sedimentary sulfides are observed at the sediment-water interface in all three environments. We find that the overall isotope difference between dissolved sulfate and pyrite increases from 25 ‰ in sulfidic to 45 ‰ in anoxic to 65 ‰ in fully oxic bottom waters. These net isotope fractionations do not reflect the isotopic fractionation by sulfate-reducing bacteria alone and thus cannot be used to extract information on sulfate reduction rates. Reaction transport modeling indicates that in the case of the anoxic, but non-sulfidic bottom waters, the isotope fractionation by sulfate-reducing bacteria substantially exceeds the net effect of 45 ‰, and may be as high as 85 ‰. The isotope effect by sulfate-reducing bacteria, however, is reduced by superposition of normal isotope effects during the anaerobic oxidation of sulfide with nitrate, by the large sulfur-storing bacteria Beggiatoa and Thiomargarita sp., which may be as high 45 ‰.The 34S-enrichment of dissolved sulfide in the zone of anaerobic methane oxidation varies between +14 and +19 ‰ vs. CDT and depends on the isotopic composition of the initial hydrogen sulfide formed at the sediment-water interface rather than the depth of the transition zone. Diagenetic overprinting by continuing sedimentary sulfide precipitation during burial is negligible in these sediments due to general iron limitation. Nevertheless, the isotopic composition of sulfides formed at the sediment-water interface is not preserved because of isotopic exchange of a fraction of the pyrite pool with coexisting dissolved sulfide. In these sediments, this exchange leads to 34S-enrichment in pyrite of about 15 ‰ relative to the initial isotope composition. Our observations indicate that buried sediments never preserve the initial isotope composition of sulfides formed at the sediment-water interface or in the water column. However, the isotopic imprints of oxic, anoxic, and sulfidic bottom water conditions are sufficiently distinct from each other that they remain preserved as relative isotopic differences in the isotopic composition of buried sedimentary sulfides.
|
|
| 8. |
- Currie, Bronwen, et al.
(författare)
-
Living dangerously: implications of hydrogen sulphide events for marine life along the Namibian coast : A.G.V. Salvanes6, R. Bahlo5
- 2008
-
Ingår i: International Symposium on Eastern Boundary Upwelling Ecosystems: Spain.
-
Konferensbidrag (populärvet., debatt m.m.)abstract
- Hypoxia, anoxia and free hydrogen sulphide in the water column are characteristic of the inner shelf coastal upwelling system off central Namibia. Biogeochemical monitoring of the diatomaceous sediments along the central coast over a 3-year period, coupled with acoustic studies and ROV video coverage, indicated a major role played by the seafloor in the generation and control of H2S into the water column. We determined the sedimentary processes resulting in the generation and dispersal of hydrogen sulphide, and its effect on some of the locally important fishes and invertebrates.Marked interannual variability with no obvious seasonal trends was apparent in methane contents of sediments even over the short three-year period, whilst sulphate reduction rates in the sediment showed little change. The large sulphur bacteria Thiomargarita namibiensis and Beggiatoa spp. form dense mats over extensive areas of mud and oxidize sulphide at the sediment-water interface, thereby regulating its flux into the overlying water. During episodic events however, hydrogen sulphide pervades the entire water column, followed by severe hypoxia. Under experimental conditions, the survival of juvenile horse mackerel Trachurus trachurus capensis was limited to less than two hours in water containing <0.7ml.l-1 dissolved oxygen. If sulphide was also present survival time was reduced to less than 30 minutes. In contrast, pelagic gobies Sufflogobius bibarbatus survive exposure to extended periods of anoxia and water containing sulphide. Gobies are found abundantly on the muddy seafloor where they evidently possess both physiological and behavioural strategies to survive sulphide and anoxia, accounting for the success of this species in Namibian waters and its importance as a key diet species for predatory fish, seabirds and marine mammals.
|
|
| 9. |
- Dale, Andrew, et al.
(författare)
-
An integrated sulfur isotope model for Namibian shelf sediments
- 2009
-
Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier. - 0016-7037 .- 1872-9533. ; 73:7, s. 1924-1944
-
Tidskriftsartikel (refereegranskat)abstract
- In this study the sulfur cycle in the organic-rich mud belt underlying the highly productive upwelling waters of the Namibian shelf is quantified using a 1D reaction-transport model. The model calculates vertical concentration and reaction rate profiles in the top 500 cm of sediment which are compared to a comprehensive dataset which includes carbon, sulfur, nitrogen and iron compounds as well as sulfate reduction (SR) rates and stable sulfur isotopes (32S, 34S). The sulfur dynamics in the well-mixed surface sediments are strongly influenced by the activity of the large sulfur bacteria Thiomargaritanamibiensis which oxidize sulfide (H2S) to sulfate () using sea water nitrate () as the terminal electron acceptor. Microbial sulfide oxidation (SOx) is highly efficient, and the model predicts intense cycling between and H2S driven by coupled SR and SOx at rates exceeding 6.0 mol S m−2 y−1. More than 96% of the SR is supported by SOx, and only 2–3% of the pool diffuses directly into the sediment from the sea water. A fraction of the produced by Thiomargarita is drawn down deeper into the sediment where it is used to oxidize methane anaerobically, thus preventing high methane concentrations close to the sediment surface. Only a small fraction of total H2S production is trapped as sedimentary sulfide, mainly pyrite (FeS2) and organic sulfur (Sorg) (∼0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (<1%). Yet, despite intense SR, FeS2 and Sorg show an isotope composition of ∼5 ‰ at 500 cm depth. These heavy values were simulated by assuming that a fraction of the solid phase sulfur exchanges isotopes with the dissolved sulfide pool. An enrichment in H2S of 34S towards the sediment-water interface suggests that Thiomargarita preferentially remove H232S from the pore water. A fractionation of 20–30‰ was estimated for SOx (εSOx) with the model, along with a maximum fractionation for SR (εSR–max) of 100‰. These values are far higher than previous laboratory-based estimates for these processes. Mass balance calculations indicate negligible disproportionation of autochthonous elemental sulfur; an explanation routinely cited in the literature to account for the large fractionations in SR. Instead, the model indicates that repeated multi-stepped sulfide oxidation and intracellular disproportionation by Thiomargarita could, in principle, allow the measured isotope data to be simulated using much lower fractionations for εSOx (5‰) and εSR (78‰).
|
|
| 10. |
|
|
