Contribution of high-intensity gas seeps in the Black Sea to methane emission to the atmosphere (CRIMEA)
CONTRACT EVK2-CT-2002-00162

The innovative aspects of the project
Graphical presentation of the project's components

The CRIMEA project aims at establishing a budget of methane release to the atmosphere related to high-intensity outbursts of methane at the seafloor of the Black Sea, and at modelling the effects of the methane release to the atmosphere. A large number of data sets related to physical and chemical processes connected to the outburst and ascent of methane will be generated. All datasets will be integrated in a geographically-related database that will be distributed on the Internet.
The project proposes an integrated, multidisciplinary study, addressing the following topics:
  • THE ORIGIN OF HIGH-INTENSITY METHANE SEEPS AND OUTBURSTS: as the destabilisation of gas hydrates has been considered as a major source of submarine methane, the relation between gas hydrate destabilisation and methane outbursts will be investigated. In the project two sites where high-intensity seeps occur will be studied, one shallow site that is located outside the stability field of gas hydrates, another deep site that is located within the gas hydrate stability field.
  • THE METHANE FLUX FROM THE SEAFLOOR, IN THE WATER COLUMN AND AT THE SEA SURFACE TO THE ATMOSPHERE: the quantity of methane released at the seafloor will be measured in two seep areas. The physical characteristics of injected methane clouds will be analysed: bubble characteristics, concentration of dissolved methane and transformation during vertical transport in the water column and at the anoxic/oxic interface.
    The chemical characteristics of the injected gasses will be determined and the concentrations at several levels throughout the water column and at the sea/atmosphere interface will be measured. Also the role of the bacteria will be investigated and their role as a methane sink will be quantified. The results will be combined to quantify the methane losses and transformation during vertical transport and to quantify the flux resulting from methane injections to the atmosphere.Physical and chemical changes in the water mass caused by ascending methane clouds will be quantified.
  • THE SIGNIFICANCE OF SUBMARINE OUTBURSTS AS A CAUSE OF ATMOSPHERIC CHANGES: the changes in the oxidizing capacity of the atmosphere above the Black Sea will be calculated in box models on basis of the established methane flux from submarine high-intensity seeps at the sea surface.
In the margin of this study, the project will generate important data for environmental and climatic issues in the Black Sea region:
  • Submarine gas release causes extreme eutrophication of the water mass. The boundary between anoxic and oxic water layers oscillates between 150 m and 180 m below the sea surface. The maximum depth of the Black Sea being 2212 m, it represents the largest anoxic water mass on Earth. Measurements suggest that the oxic-anoxic interface is shallowing, which has catastrophic effects for fisheries, etc.
  • The Black Sea is a key region for South European climate, the source of South European rainfall. Changes in climate in the Black Sea region have immediate effects on climate conditions in southern Europe.

The global climate change is steered by pulses of increased concentration of greenhouse gases, one of the most important natural gases is methane:

  • Methane is a powerful greenhouse gas, 25 times more efficient than carbon dioxide at trapping heat but has a shorter residence time in the atmosphere (10 years compared to 100 years for CO2).
  • Its release in large quantities to the atmosphere may cause a sudden warming of the global climate and trigger rapid global climate change
  • Evidence of increased atmospheric methane levels has been found in ice cores and ocean-bottom sediment cores coincident with periods of rapid climate change, e.g. Late Palaeocene Thermal Maximum, etc
  • In case of the LPTM, large amounts of methane -assumed to be derived from massive destabilisation of marine hydrate deposits- were injected from the seafloor into the ocean and the atmosphere, causing warming of ocean waters and atmosphere, changes in the ocean currents and extinction of many benthic species.
Extended information already is known about methane in the Black Sea. Several studies have been dealing with the methane content in the seawater. Methane fluxes at the sea surface have been measured and evaluations of the methane release have been made for several areas. Outbursts of methane at the seafloor have been well documented. The presence of gas hydrates in the sub-bottom sediments is becoming a popular topic for investigation: several on-going projects or ready to start are dealing with the processes of destabilization of gas hydrates and their effects on the methane content in the seawater.

The CRIMEA project will focus on the transfer of methane from the seafloor through the water column and into the atmosphere from submarine high-intensity methane seeps and outbursts in the Black Sea. In high-intensity seeps, methane escapes as free gas in large volumes. Such seeps are more likely to have a pronounced effect on atmosphere and climate than the "normal" steady seeps where the larger part of the methane is dissolved and oxidised, consumed by bacteria or fixed in seafloor carbonate crusts.

  • Bubble density, size and ascending velocity are much larger then in steady, slow seeps.
  • Dense bubble curtains, or plumes (also flares), can reach up to 200 m high in the water column or even reach the sea surface where the methane may inflame. A "burning sea" is not unusual during thunderstorms over the Black Sea.
  • Such methane outbursts emit methane directly from the ocean floor into the atmosphere.
  • In deeper water (>200 m) the flares may not reach the sea surface but the methane travelled through a large part of the water column before the methane is dissolved, diluted and eventually oxidized and its contribution to methane or carbon dioxide emission at the ocean-atmosphere interface may be significant
  1. It will attempt to quantify the influence on the atmospheric composition of large outbursts of methane at the seafloor. Indeed, methane from steady seafloor seeps is partly consumed by bacteria at the emanation sites, and is further for a large part oxidized at the anoxic/oxic water interface. Only the methane abruptly erupting at the seafloor and ascending in plumes in the water column (which is well documented in the proposed study areas) is able to reach in huge quantities the upper water layers and the atmosphere. In the Black Sea enormous amounts of methane are continuously injected into the water at several places. It would be nearly impossible to perform this study in other areas where outbursts are significant but occur episodically.
  2. In the research for submarine sources of external methane destabilization of gas hydrates is often considered the main methane source; it is believed to be capable of rapidly liberating large amounts of methane. The CRIMEA project will be able to test this hypothesis by:
    - quantifying the real contribution of submarine methane outbursts to changes in atmospheric composition, and
    - studying the role of gas hydrates as a buffer for, or originator of methane outbursts.
  3. It builds an interactive and interdisciplinary program to define and to quantify all the processes involved, from the origin of the methane in the sub-bottom sediments to its release to the atmosphere:
    - methane generation in the sea sediments, and particularly the role of gas hydrate dissociation as a buffer for, or originator of methane outbursting in high-energy events
    - the physical, chemical and biological processes occurring at the venting sites, in the water column and at density barriers (e.g. oxic/anoxic interface)
    - the methane emanation at the sea surface caused by methane outbursts compared to other undisturbed areas.
    - the influence of this contribution on the atmospheric composition.
    To measure and model all involved processes the CRIMEA project makes use of state-of-the-art technology and mathematical models from different disciplines and for various applications, combined in an efficient work program.

To adequately study and quantify the methane release from the seafloor and its contribution to atmospheric methane emission, the project comprises four main experimental components:

  1. Characterization of methane sources and analysis of migration pathways. The work to be done will be limited to selected venting areas and seeps, relying further on the results of nationally funded on-going research projects dealing with gas hydrate destabilisation and methane seepages;
  2. Characterization and mapping of surface seeps. This component is mainly based on geophysical techniques;
  3. Characterization and quantification of fluids, gasses and microbiological activity in the shallow subsurface, at the seafloor in high-intensity active seeps. This component is mainly based on gas, water and sediment sampling and analysis combined with remote and in-situ visualisation techniques;
  4. Characterization of the physical, chemical and biological processes related to the fate of methane in the water column. Particular attention will be paid to the processes acting at the different interfaces: oxic/anoxic and water/atmosphere. This component relies on in-situ physical and chemical measurements, and water sampling and analysis.

The experiments will be performed in two test areas: at the limit of the shelf in the NW Ukrainian part of the Black Sea, at depths varying between 100 and 2000 m; in the abyssal plain of the Black Sea, at a water depth of about 2000 m.

The experimental work will be completed by modelling the physical and chemical processes in the water column connected to methane ascent in plumes, and by modelling the effects on the atmosphere of methane release related to these outbursts.