The thermal structure of the Dvurechenskii mud volcano and its implications for gas hydrate stability and eruption dynamics
Introduction
Submarine mud volcanoes are geological structures related to the emission of gas, pore water seepage and mud expulsion at the sea floor (Dimitrov, 2002, Kopf, 2002). They are considered one of the most important natural sources of methane emission to the hydrosphere and atmosphere (e.g. Dimitrov, 2003, Kvenvolden and Rogers, 2005), but to quantify the role of mud volcanoes in global budgets, it is important to understand their activity (cf. Wallmann et al., 2006b). Since the ascent of warm mud and fluids creates temperature anomalies close to the sea floor, detection and quantification of these anomalies in turn provides information on the nature and strength of mud volcano activity (e.g. de Beer et al., 2006, Feseker et al., 2008).
The Dvurechenskii mud volcano (DMV) was in the focus of intense research during the cruise M52/1 (MARGASCH) of the German R/V Meteor in 2003. Measurements revealed elevated sediment temperatures of up to 16.5 °C at the center of the mud volcano against a background water temperature of around 9.1 °C (Bohrmann et al., 2003). Gas hydrates were found in all gravity cores recovered from an area covering the entire DMV plateau (Blinova et al., 2003, Bohrmann et al., 2003). During the same cruise, unusually high pore water salinity was observed at the center of the DMV (Bohrmann et al., 2003, Aloisi et al., 2004, Wallmann et al., 2006b), which causes a substantial shift of the gas hydrate stability limit towards lower temperatures and suggests that gas hydrates at the DMV are very close to the stability limit. With respect to variable intensities of gas bubble emissions into the water column, Greinert et al. (2006) suggested strong variability in the mud volcano activity, which point to dynamic fluid movement in the subsurface. Based on geochemical analyses of pore water samples, Aloisi et al. (2004) concluded that the high-salinity fluids that are expelled at DMV originate from source depths of around 3 km below the seabed and are subjected to alteration due to shallow gas hydrate formation. They presented simple transport models that pointed to flow rates between 0.25 m per year at the center of the mud volcano and 0.08 m per year at the edge.
More recently, the DMV was once more investigated in great detail during the cruises M72 and M72/3a of R/V Meteor and the ROV Quest 4000 m in February and March 2007. For the first time, an ROV was used for visual sea floor observations and precisely positioned in-situ measurements and sampling at this mud volcano. Winch-operated tools such as a gravity corer equipped with temperature sensors and a dynamic autoclave piston corer were used to obtain additional in-situ measurements and samples. Based on the results of these two cruises, this paper presents a detailed analysis of the temperature distribution in the sediments and bottom water at the DMV, its relation to the presence of gas hydrates, and its implications for the nature of the activity of this mud volcano.
Section snippets
Geological setting
The DMV is located in the Sorokin Trough on the northern margin of the Black Sea, SE of the Crimean Peninsula (Fig. 1). The Sorokin Trough is a 150 km long and 50 km wide NE–SW oriented depression. Towards the S, the depression is bounded by the Cretaceous–Eocene Tetyaev and Shatsky uplifts, gradually narrowing to the NE (Belousov et al., 1988). With water depths ranging between 600 and 2100 m, it is one of the deepest troughs in the eastern basin of the Black Sea.
The Sorokin Trough formed the
Temperature measurements
In-situ temperature measurements from the DMV were obtained during two cruises M72/2 and M72/3a (Bohrmann et al., 2007) of R/V Meteor in February and March 2007 using a gravity corer and a temperature lance operated by the ROV Quest 4000 m (MARUM, Bremen). The 6-m-long gravity corer was equipped with five to six autonomous temperature loggers mounted on outriggers that were welded to the corer barrel. Measuring at a resolution of 0.0006 °C, the accuracy of these sensors is better than 0.002 °C
In-situ temperature measurements using the gravity corer
In-situ sediment temperature profiles were measured during 20 deployments of the gravity corer at seven different targets on the DMV (Fig. 2). The stations are listed in Table 1, and the corresponding results of the measurements are listed in Table 2. All temperature measurements on the flanks of the mud volcano revealed linear profiles. The results from multiple measurements at the same targets agreed very well with each other. The lowest temperature gradients of around 0.037 °C/m were found on
Sediment temperature distribution
The in-situ measurements using the ROV lance and the gravity corer both revealed very high sediment temperatures at the summit of the DMV and showed a rapid decrease in near-surface temperature gradient away from the summit. Assuming a thermal conductivity of 1 W/(m°C) for the near-surface sediments, the highest temperature gradient at the summit corresponds to a heat flow of 11,020 mW/m2, which is more than four times greater than the maximum value reported previously by Kutas and Poort (2008).
Conclusions
The distribution of sediment temperatures at the DMV provides indications for both fluid seepage and mud eruptions. Increasing temperature gradients from the edges of the plateau towards the summit point to low rates of fluid seepage at the edges and higher rates at the summit. The high sediment temperatures close to the seabed at the summit, however, are probably the result of short, rapid mud expulsion.
Gas hydrate dissociation temperatures were calculated using (i) the HWHYD software and
Acknowledgements
The authors would like to acknowledge the Captain and crew of R/V Meteor as well as the team operating the ROV Quest 4000 m for their invaluable support at sea. Steffen Gauger (FIELAX GmbH, Bremerhaven) evaluated the positioning of all instruments at the sea floor and the navigation logs of the ROV dives. We are grateful to H.-J. Hohnberg for the development and the deployments of the DAPC, A.H. Mai (MARUM, Bremen) for support during maintenance of the DAPC, J. Rethemeyer and S. Kusch (AWI,
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