Ninth international conference on - Marum

Abstracts of oral presentations 13
Hydrodynamically constrained flux of in-situ generated methane hydrate
dissolving into undersaturated seawater
N. Bigalke 1 , G. Gust 2 , G. Rehder 3
1 Leibniz Institute of Marine Sciences, Wischhofstr. 1-3, 24148 Kiel, Germany
2 Technical University Hamburg-Harburg, Schwarzenbergstr. 95, 21073 Hamburg, Germany
3 Baltic Sea Research Institute Warnemuende, Seestr. 15, 18119 Rostock, Germany
Clathrate hydrates of natural gases (“gas hydrates“ in the following) are widely distributed within sediments
along active and passive continental margins. Gas hydrates and gas hydrate stability specifically have become an
important field of study in the past decades. The stability of methane hydrate depends on the physicochemical
equilibrium between all coexisting phases. In terms of P and T, the stability of methane hydrate is well
constrained. Dissolution due to an undersaturation of dissolved methane (hydrate) in a coexisting liquid phase
has, however, not been given sufficient attention. Specifically, the flux of methane dissolving into undersaturated
seawater has yet to be quantified. This is especially true for hydrates that are exposed to flow whether they are
buried in the marine sediment or whether they are exposed at the immediate sea-floor surface. Assessment of the
stability of gas hydrates in these environments requires the test if incorporation of hydrodynamic as well as
thermodynamic variables into numerical models is required to quantify the flux of methane into the ocean. To
the best of our knowledge, the role of flow on the dissolution of gas hydrates has not yet been determined for
lack of concise hydrodynamic data, though qualitatively, a response of hydrate dissolution on current velocity
has been described from an open field experiment (Rehder et al., 2004) Here we report on mass transfer rates of
CH4 from decomposing in-situ generated flat methane hydrate surfaces exposed to precisely adjusted, spatially
homogeneous wall shearing stresses at selected P-/T-conditions within the hydrate stability field (30 MPa, 4°C).
The data reveal that hydrate decomposition is an entirely diffusion-controlled process under the conditions
investigated. The experiments were carried out in an autoclaved interfacial flux chamber with calibrated,
spatially homogeneous wall shearing stress characterized as ‘microcosm’ in Tengberg et al. (2004).
References
Rehder, G.; Kirby S. H.; Durham, W. B.; Stern, L. A.; Peltzer, E. T.; Pinkston, J.; Brewer, P. G. Dissolution rates
of pure methane hydrate and carbon-dioxide hydrate in undersaturated seawater at 1000-m depth. Geochim.
Cosmochim. Acta 2004, 68, 285-292.
Tengberg, A.; Stahl, H.; Gust, G.; Müller, V.; Arning, U.; Andersson, H.; Hall, P. O. J. Intercalibration of
benthic flux chambers I. Accuracy of flux measurements and influence of chamber hydrodynamics. Prog.
Oceanogr. 2004, 60, 1-28.
Highly variable seep systems along the Makran subduction zone – influence from the
accretionary wedge structure and the oxygen minimum zone
G. Bohrmann 1 , A. Bahr 1 , M. Brüning 1 , A. Gassner 1 , S. Kasten 2 , S. Klapp 1 , M. Nasir 3 , T. Pape 1 , V. Spiess 1 ,
J. Rethemeyer 2 , P. Rossel 1 , H. Sahling 1 , K. Thomanek 1 , M. Yoshinaga 1 , K. Zonnefeld 4
1 Research Center Ocean Margins, University Bremen, Germany
2 Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremerhaven, Germany
3 National Institute of Oceanography, Karachi, Pakistan
4 Department of Geosciences at University of Bremen, Germany
Up to 7 km sediments from the oceanic part of the Arabic Plate are folded, sheared and repeatedly thrust within
the accretionary wedge of the Makran subduction zone. Although these sediments form a huge reservoir for fluid
and gas seepage just minor amounts of cold seeps have been reported up to recent from this collision zone. We
therefore conducted a research cruise (R/V METEOR M74/3; 30 October – 28 November, 2007) to the area in
order to explore the margin for further sea floor seep structures. Indications for potential sites of seepage came
from TOBI backscatter sea floor mapping and from acoustic plume imaging measured by the onboard
PARASOUND echosounder during Cruise M74/2. Fifteen acoustic plumes distributed over the entire Makran
slope showed gas emission sites from the sea floor. We could investigate 9 sites between 500 m and 3000 m
water depth during 18 dives using ROV QUEST4000. Gas seeps between 575 m and 1020 m water depth are
highly variable and the occurrence of chemosynthetic fauna shows clear relationship with the oxygen
concentration within the oxygen minimum zone. Four seeps between 1460 m and 1820 m are characterised by
large communities of bivalves (Mytilidae, Vesicomyidae), tube worms (Vestimentifera) and authigenic
carbonates which cover hundreds of square meters of the sea floor. I all cases free gas bubbles have been
observed to be released at the sea floor. Seeps deeper as 2000 m water depth do not show the carbonate
pavement and seem to represent recently developed cold seeps. In one case fluid outflow could be observed and
documented, where no chemosynthetic fauna was found. The distribution of various seep systems from the
Makran margin will be presented during the talk.