Dynamics of methane and hydrogen sulphide in the water column and sediment of the Namibian shelf

1. Dynamics of methane and hydrogen sulphide in the water column and sediment of the Namibian shelf Volker Brüchert 1, Bronwen Currie 2,Kay-Christian Emeis 3,Rudolf Endler 4, Thomas Leipe 4, Kathleen R. Peard 2, Thomas Vogt 5 1 Max-Planck Institute for Marine Microbiology and Research Center Ocean Margins, Bremen 2 Ministry of Fisheries and Marine Resources, Namibia 3 Institute of Biogeochemistry and Marine Chemistry, University of Hamburg 4 Institute for Baltic Sea Research, Warnemünde 5 Geoscience Department University of Bremen

2. The diatomaceous mud belt off central Namibia Porosity > 90% Diatom-rich Organic-C: > 12 % dry wt Accumulation rates: 50 – 1000 g/m2/a Coastal upwelling high primary productivity

3. Distribution of diatomaceous mud, free gas, and sediment craters Areal estimates: Diatomaceous mud: 17900 km2 Gas-filled sediments: 1357 km2 Sea floor with pockmarks and sediment craters: 380 km2 Emeis et al., 2004

4. Water column oxygen profiles and bottom water images No free gas, low hydrogen sulphide Shallow gas: Bacterial mats of Beggiatoa and Thiomargarita

5. 0 50 100 150 200 250 300 350 Hydrogen sulphide profile in the water column Concentration (mM) 0 Sulphide µM 20 Oxygen µM Oxygen 40 Depth (m) 60 80 Seawater 100 H2S Sediment Methane

6. Seismo acoustics • Parametric sediment echosounder Parasound – Paradigma (Frequency: 2-5 kHz) • Parametric sediment echosounderSES96 (Frequency: 4-12 kHz ) • Linear sediment echosounderSEL96 (Frequency: 5-20 kHz) • Sparker seismics

7. Outer shelf: Prograding mud and truncated coastal sands

8. Central shelf: Disappearance of horizontal beds and appearance of gas blankings

9. SEL96-Echogram: Transition from gas-free mud → partially gas-filled → crater structures → gas-saturated < 660m > Depth range 75 – 95 m (1m – contour lines) Ship direction 180° 14°20‘E, 22°51‘S

10. Efficient anaerobic oxidation of methane by sulfate in areas where the depth of free gas is greater than than one meter Emeis, Brüchert et al. (2004)

11. Evidence for gas escape from the sediment Bubbles tracked by echosounder Meteor M 57-3

12. Pore water methane, sulphate, and sulphide in sediment crater

13. Rising mud islands and coast-wide fish kills

14. Lobster kills Turquoise, elemental sulfur-containing surface water

15. MODIS satellite imagery: turquoise discolourations often coincide with reports of hydrogen sulphide smell; measurements indicate sulphur concentrations up to 30 mmoles/L. September 03, 2003, www.noaa.gov

16. Temporal variability Periods of whole-water column depletion Coincident hydrogen sulphide and methane pulses in the sediment

17. Brüchert V., Jørgensen B.B., Neumann K., Riechmann D., Schlösser M., and Schulz H. Regulation of bacterial sulfate reduction and hydrogen sulfide fluxes in the central Namibian coastal upwelling zone. Geochimica et Cosmochimica Acta, 2003; 67: 4505-4518. Brüchert V., Lass U., Endler R., Dübecke A., Julies E., Leipe T., and Zitzmann S. An integrated assessment of shelf anoxia and water column hydrogen sulphide in the Benguela coastal upwelling system off Namibia, In Past and Present Marine Water Column Anoxia, L.N. Neretin, B.B. Jorgensen, and J.W. Murray, eds.: Kluwer; in press. Emeis K.-C., Brüchert V., Currie B., Endler R., Ferdelman T.G., Kiessling A., Leipe T., Noli-Peard K., Struck U., and Vogt T. Shallow gas in shelf sediments of the Namibian coastal upwelling ecosystem. Continental Shelf Research, 2004; 24: 627-642. Weeks S.J., Currie B., Bakun A., and Peard K.R. Hydrogen sulphide eruptions in the Atlantic Ocean off southern Africa: implications of a new view based on SeaWIFS satellite imagery. Deep-Sea Research, 2004; 51: 153-172. Publications