Mineralogical and stable isotope analyses of carbonates from the eastern Black Sea

DOI

Authigenic carbonates associated with cold seeps provide valuable archives of changes in the long-term seepage activity. To investigate the role of shallow-buried hydrates on the seepage strength and fluid composition we analysed methane-derived carbonate precipitates from a high-flux hydrocarbon seepage area ("Batumi seep area") located on the south-eastern Black Sea slope in ca. 850 m. In a novel approach, we combined computerized X-ray tomography (CT) with mineralogical and isotope geochemical methods to get additional insights into the three-dimensional internal structure of the carbonate build-ups.X-ray diffractometry revealed the presence of two different authigenic carbonate phases, i.e. pure aragonitic rims associated with vital microbial mats and high-Mg calcite cementing the hemipelagic sediment. As indicated by the CT images, the initial sediment has been strongly deformed, first plastic then brittle, leading to brecciation of the progressively cemented sediment. The aragonitic rims on the other hand, represent a presumably recent carbonate growth phase since they cover the already deformed sediment.The stable oxygen isotope signature indicates that the high-Mg calcite cement incorporated pore water mixed with substantial hydrate water amounts. This points at a dominant role of high gas/fluid flux from decomposing gas hydrates leading to the deformation and cementation of the overlying sediment. In contrast, the aragonitic rims do not show an influence of 18O-enriched hydrate water. The differences in d18O between the presumably recent aragonite precipitates and the older high-Mg cements suggest that periods of hydrate dissociation and vigorous fluid discharge alternated with times of hydrate stability and moderate fluid flow. These results indicate that shallow-buried gas hydrates are prone to episodic decomposition with associated vigorous fluid flow. This might have a profound impact on the seafloor morphology resulting e.g. in the formation of carbonate pavements and pockmark-like structures but might also affect the local carbon cycle.

Supplement to: Bahr, André; Pape, Thomas; Abegg, Friedrich; Bohrmann, Gerhard; van Weering, Tjeerd C E; Ivanov, M K (2010): Authigenic carbonates from the eastern Black Sea as an archive for shallow gas hydrate dynamics – results from the combination of CT imaging with mineralogical and stable isotope analyses. Marine and Petroleum Geology, 27, 1819-1829

Identifier
DOI https://doi.org/10.1594/PANGAEA.753970
Related Identifier https://doi.org/10.1016/j.marpetgeo.2010.08.005
Metadata Access https://ws.pangaea.de/oai/provider?verb=GetRecord&metadataPrefix=datacite4&identifier=oai:pangaea.de:doi:10.1594/PANGAEA.753970
Provenance
Creator Bahr, André; Pape, Thomas ORCID logo; Abegg, Friedrich; Bohrmann, Gerhard ORCID logo; van Weering, Tjeerd C E; Ivanov, M K
Publisher PANGAEA
Publication Year 2010
Rights Creative Commons Attribution 3.0 Unported; https://creativecommons.org/licenses/by/3.0/
OpenAccess true
Representation
Language English
Resource Type Supplementary Publication Series of Datasets; Collection
Format application/zip
Size 3 datasets
Discipline Earth System Research
Spatial Coverage (41.278W, 41.954S, 41.291E, 41.959N); Batumi seep area; Batumi Seep
Temporal Coverage Begin 2005-06-23T00:00:00Z
Temporal Coverage End 2007-04-07T09:12:00Z