Methane occurrence in the Black Sea: From hydrates to the water column

Methane is widely found on continental margins. It originates from either microbial processes at shallow sedimentary depth or thermal cracking of organic matter at deep depth, and occurs as disolved or free gas, or hydrates. It is the main chemical compound found both in natural gas hydrate deposits and seafloor gas emissions at the cold-seep.

There are extensive methane manifestations both in the sedimentary and water columns of the Black Sea.  This stratified sea is characterized by large quantities of methane bubbles discharged at the seafloor from the very shallow coastal shelf to the deep basin (Riboulot et al., 2017), contributing to the high concentration level measurement in the water column. Hydrate-bearing sediments are also widely distributed within the sediment on the continental slope, and Riboulot et al. (2018) showed that the seawater infiltration make them vulnerable and prone to dissociation since the reconnection of the Atlantic Ocean via the Sea of Marmara.

The expeditions Ghass 2 in September 2021 allowed the investigation of several methane emission sites from the continental shelf to the deep basin in the Romanian sector of the Black Sea, including hydrate-bearing sites. The water column was probed to measure in situ dissolved methane concentration using a commercial methane sensor and the prototype laser spectrometer SubOcean and sampled from CTD-Rosette. A ~6m-length hydrate-bearing core was collected from a long Calypso piston corer from which a high-resolution sampling of hydrates was performed to estimate the influence of geological factors on their cage occupancy.

The presentation aims to provide further background on methane dynamics in the Black Sea.

 

References

Agnissan Constant Art-Clarie, Guimpier Charlène, Terzariol Marco, Fandino Olivia, Chéron Sandrine, Riboulot Vincent, Desmedt Arnaud, Ruffine Livio (2023). Influence of Clay-Containing Sediments on Methane Hydrate Formation: Impacts on Kinetic Behavior and Gas Storage Capacity . Journal Of Geophysical Research-solid Earth , 128(9).

Riboulot Vincent, Ker Stephan, Sultan Nabil, Thomas Yannick, Marsset Bruno, Scalabrin Carla, Ruffine Livio, Boulart Cedric, Ion Gabriel (2018). Freshwater lake to salt-water sea causing widespread hydrate dissociation in the Black Sea . Nature Communications , 9(117), 1-8

 

Acknowledgements

The authors thank the different projects and programs for their financial supports DOORS by the EU Project number 101000518, ENVRIPLUS by EC Project number 654182, Blame ANR-18-CE01-0007, ORAGGE by Interdisciplinary graduate School for the Blue planet (ANR-17-EURE-0015 and "Investissements d'Avenir"), SEAMLESS by INSU LEFE Programme 2022