Dynamics of methane and hydrogen sulphidein 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, Bremen2 Ministry of Fisheries and Marine Resources, Namibia3 Institute of Biogeochemistry and Marine Chemistry, University of Hamburg4 Institute for Baltic Sea Research, Warnemünde5 Geoscience Department University of Bremen

The diatomaceous mud belt off central Namibia

Porosity > 90%Diatom-richOrganic-C: > 12 % dry wtAccumulation rates: 50 – 1000 g/m2/a

Coastal upwellinghigh primary productivity

Distribution of diatomaceous mud, free gas, andsediment 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

Water column oxygen profiles and bottom water images

Shallow gas: Bacterial mats ofBeggiatoa and Thiomargarita

Dissolved oxygen (ml/l)

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Pre

ssu

re (

db

ar)

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0 2 4 6Dissolved oxygen (ml/l)

Pre

ssu

re (

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GPS: 26 °25.1S 014° 55.1E GPS: 23 °04.8S 014°16.2E

No free gas, low hydrogen sulphide

Hydrogen sulphide profile in the water column

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Concentration (M)

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De

pth

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Sulphide µM

H2S Methane

Oxygen µMOxygen

Sediment

Seawater

Seismo acoustics

• Parametric sediment echosounder Parasound – Paradigma (Frequency: 2-5 kHz)

• Parametric sediment echosounder SES96 (Frequency: 4-12 kHz )

• Linear sediment echosounder SEL96 (Frequency: 5-20 kHz)

• Sparker seismics

Outer shelf: Prograding mud and truncated coastal sands

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

SEL96-Echogram: Transition from gas-free mud → partially gas-filled → crater structures → gas-saturated

Depth range 75 – 95 m (1m – contour lines)Ship direction 180° 14°20‘E, 22°51‘S

< 660m >

Efficient anaerobic oxidation of methane by sulfate in areas where the depth of free gas is greater than than one meter

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CH4 (nmol/cm3)

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Sulphate (mM)

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Sulphide (M)

Emeis, Brüchert et al. (2004)

Evidence for gas escape from the sediment

Bubbles tracked by echosounder

Meteor M 57-3

Pore water methane, sulphate, and sulphide in sediment crater

181 gravity core

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Methane (M)

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th (

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Methane

Sulfate

181 Multicore

01

23

456

78

910

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Methane (M)

Dep

th (

cm)

0 5 10 15 20Hydrogen sulphide (mM)

Methane

Sulphide

Rising mud islands and coast-wide fish kills

Lobster kills

Turquoise, elemental sulfur-containing surface water

MODIS satellite imagery:turquoise discolourations often coincide with reportsof hydrogen sulphide smell;measurements indicatesulphur concentrations up to 30 moles/L.

September 03, 2003, www.noaa.gov

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De

pth

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Dissolved oxygen (ml/l)

Methane (nmol cm-3)

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pth

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)0

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0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

Ma

y 2

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ne

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er

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cem

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r 2

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Diffusive

sulfide flux

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mol

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-10

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Temporal variability

Periods of whole-water column depletion

Coincident hydrogen sulphide

and

methane pulses in the sediment

Publications

• 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.