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CRIMEA - Contribution of high-intensity gas seeps in the Black Sea to methane emission to the atmosphere

Summary information

Funding:FP5 - Research project
Total cost:1497155
Ec contribution:1020802
Start date:2003-02-01
End date:2006-01-31
Duration:48 months
Coordinator:Marc de Batist (
Organisation:University of Gent– Belgium
Regio:Black Sea
Project name:CRIMEA - Contribution of high-intensity gas seeps in the Black Sea to methane emission to the atmosphere
Project summary:The CRIMEA Project studied and quantified the transfer of methane that is released at submarine high-intensity methane seeps and methane outbursts in the Black Sea as it ascends through the water column and reaches the atmosphere. This has been done by focussing on the following aspects: (i) characterisation and mapping of the active high-intensity seeps at the sea floor; (ii) characterisation of sub-sea-floor methane sources and migration pathways; (iii) quantification and physical, chemical and microbiological characterisation of fluids and gasses in the shallow sub-sea floor and at the sea floor in active high-intensity seeps; (iv) characterisation of the physical, chemical and microbiological processes controlling the fate of methane in the water column; and (v) establishment of a budget of methane release to the atmosphere from submarine outbursts and assessment of the possible impact of this methane on atmospheric composition.

The research activities of the CRIMEA Project were carried out in two main study areas:
1. The Dnepr paleo-delta area: located at the shelf edge in the NW Ukrainian part of the Black Sea and characterised by the presence of thousands of active seeps. In this area, three specific venting sites are studied in more detail: a site at 100 m, a site at 250 m and a site at 600 m water depth.
2. The Sorokin Trough area: in the deep parts of the Black Sea, at a water depth of about 2.000 m, characterised by the presence of active mud volcanoes. Focus was on the area of the active gasreleasing Dvurechenskiy and Vodyanitskiy mud volcanoes.
Project outputs:Scientific achievements
Methane is produced in vast amounts by microbial activity in the sediments all across the Black Sea. Transfer of this methane from the sea floor into the overlying water column is strongly controlled by the process of anaerobic methane oxidation and associated sulphate reduction that occur commonly in the shallow sub-sea floor and that form an effective barrier minimising (or even eliminating) the transfer of dissolved pore-water methane into the sea water. Methane transfer takes therefore mainly place by means of gas bubbles, i.e. in bubble-releasing seeps.

The Dnepr paleo-delta area is probably one of the most prolific bubble-releasing seep areas in the World Ocean. Over 2.500 seeps in water depths between 100 and 725 m release gas bubbles, consisting mainly (> 80 %) of methane, into the water column. The methane is produced microbially at burial depths of 30-300 m in this lowstand paleo-delta depocentre. Upward migration of methane and the location of the seeps are controlled by sedimentary factors (i.e. stratigraphy and sediment properties) and by morphology, although locally deep-rooted faults may act as an additional migration pathway and cause advection of minor portions of deep-sourced gas. Below ~725 m water depth, the presence of hydrates forms an efficient barrier against upward methane migration. Bubble-release from these seeps is highly variable and not continuous in time, but taking into account the observed variability, the total flux of methane released from the sea floor in the Dnepr paleo-delta area can be estimated as 1-2 x 107 m3/yr (at STP conditions).

In the Sorokin Trough area, the distribution of seeps is mainly controlled by the deep structural-geological context and by the presence of migration pathways under the form of mud volcanoes. The high temperature of the entrained fluids allows methane to by-pass the hydrate-stability zone and be released at the sea floor. Gas released at mud volcanoes is a mixture of biogenic and deeper thermogenic methane with source depths of > 6 km. Upward migration of the fluids is focused along the feeder channels of the mud volcanoes, possible involving an intermediate mud and fluid chamber at about 300-600 m depth. Gas outbursts at the Dvurechenskiy and neighbouring mud volcanoes, involving > 1.300 m high reaching gas bubbles, have been shown to be able to last for several years. The release intensity gradually builds up in the beginning and decreases at the end of the outburst period. No short-term variations (within days or hours) have been observed.

Upon their release at the sea floor, gas bubbles will start to dissolve as they rise through the water column. The released methane will therefore be transported through the water column either as dissolved methane or as bubble methane. Bubbles can migrate upwards through the water column by themselves or as part of a bubble plume, which involves also the upward movement of water.

Single-bubble methane transport: The bubble and site characteristics determine the bubble rise velocity, the rates of methane dissolution into the surrounding water and, thus, the height of methane transfer. Modelling shows that single-bubble transport is, in fact, highly ineffective. Significant methane transfer to the atmosphere by single-bubble transport is only possible from very shallow water depths, i.e. less than 100 m. For deeper waters, unrealistically large bubbles would have to be released at the sea floor to still contain methane up to the sea surface. This is supported by surface-water methane concentrations, which show elevated concentrations only above seep areas at the shelf (< 100 m), even though the amount of sediment-released methane from the deeper (i.e. 250 m) seeps is about 100 times larger.

Bubble-plume methane transport: Plume transport allows for the gas bubbles to rise much higher in the water column than what would be possible by single-bubble transport. The formation of a plume can be driven by various mechanisms, such as a mud-volcano “eruption”, water entrainment via the release of a very large number of bubbles, or the release of pore water together with the bubbles that has a higher temperature or lower salinity than the surrounding sea water. In the Dnepr paleo-delta area, no evidence was found for the existence of plumes. In the Sorokin Trough there are indications that plumes exist that could be driven by the intermittent release of slightly warmer water from the mud volcanoes. A plume could explain the anomalous rising height of the bubbles (> 1.300 m) observed above the mud volcanoes; alternatively, this could also be explained by the formation of a hydrate rim on the bubble surface while rising within the hydrate-stability zone. Nevertheless, there are no indications that the observed plumes are indeed capable of transporting methane up to the upper water column above the Sorokin Trough mud volcanoes.

Dissolved methane transport: Dissolved methane is gradually consumed by microbial processes (involving Archaea and Bacteria), but is also transported through the water column by diffusion and by water mixing processes. The Black Sea is a strongly stratified system with various interfaces (e.g. surface mixed layer, oxycline, thermocline, pycnocline, as well as other less well-delineated boundaries), which all have the potential to influence migration of dissolved methane. For example, the base of the oxycline is a level of strong lateral currents, hereby forming also a physical boundary to the upward migration of methane. Transport across these boundaries takes place via mixing but is slow, so that the methane concentration in the water column can be regarded as in steady state. The entire Black Sea water column represents a very important pool of dissolved methane. There are indications that there is potential for overloading of the steady-state system during a large-scale bubble release episode.

All in all, the Black Sea water column acts as a very efficient buffer against methane transport from the sea floor to the sea surface.

The final transfer of methane from the water column to the atmosphere can take place by diffusion of dissolved methane across the water-air interface or by the direct migration of bubbles (if still containing methane) to the water surface. For the first mechanism, only seeps located in shallow water (i.e. < 100 m water depth) have a measurable impact with slightly higher flux densities than the background value. Modelling of direct bubble transfer has shown the potential for effective transfer even from seeps located in water depths up to 250 m (but not from deeper waters). A direct comparison of the two mechanisms –diffusion versus bubble-transfer– shows that the bubble-transfer mechanism may contribute up to 4 orders of magnitude more methane to the atmosphere above the shallow (< 250 m) seep areas, provided that these bubbles had the right properties when released to still contain sufficient methane at the sea surface. There appears to be no transfer to the atmosphere at all above the observed gas outburst at the Sorokin Trough mud volcanoes.

The methane fluxes from the Black Sea to the atmosphere above the investigated high-intensity seeps have only a negligible impact on the regional atmospheric background concentration of methane. On the other hand, calculations performed for hypothetic episodic events (i.e. a catastrophic mud-volcano eruption, or a sudden exposure of the methane-charged deeper water column) suggest that in those cases the dispersion of methane gas that reaches the atmosphere may result in very large enhancements in the methane concentrations near the sea surface downwind the release point. Photochemical and radiative transfer modeling over a representative mesoscale (10 km < length < 1.000 km) spatial and temporal window pointed that episodically raised methane concentrations can in some cases have a significant impact on the atmospheric composition and radiative forcing. However, mainly due to the limited temporal duration of such catastrophic eruptions, their influence is considered unimportant in the atmospheric system.

Socio-economic relevance and policy implications
CRIMEA addresses a problem that is of wide European interest and that can only be approached by a large, multidisciplinary team at the European level. It contributes to the formation of young scientists and to the transfer of knowledge and of scientific skills throughout Europe, towards pre-accession states and NIS partner countries, and produces data that can be directly used in global climate prediction models to better understand natural versus human influence on global warming. The project thus aids in the European Community’s policy implementation on greenhouse gas emissions and pollution abatement.