Authigenic carbonates from the eastern Black Sea as an archive for shallow gas hydrate dynamics e Results from the combination of CT imaging with mineralogical and stable isotope analyses A. Bahr a, b, * , T. Pape b , F. Abegg b, c , G. Bohrmann b , T. van Weering d, e , M.K. Ivanov f a Leibniz-Institut für Meereswissenschaften IFM-GEOMAR, FB 1 Ocean Circulation and Climate Dynamics, Wischhofstr. 1-3, D-24148 Kiel, Germany b MARUM e Center for Marine Environmental Sciences and Department of Geosciences, University of Bremen, University of Bremen, Klagenfurter Strasse, D-28334 Bremen, Germany c Leibniz-Institut für Meereswissenschaften IFM-GEOMAR, TLZ, Wischhofstr. 1-3, D-24148 Kiel, Germany d Royal Netherlands Institute for Sea Research, Department of Marine Chemistry and Geology, P.O. Box 59,1790 AB Den Burg, The Netherlands e Vrije Universiteit, Faculty of Earth and Life Sciences, Department of Paleoclimatology and Geomorphology, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands f UNESCO MSU Center for Marine Geology and Geophysics, Faculty of Geology, Moscow State University, Vorobjevy Gory, Moscow 119899, Russia article info Article history: Received 2 January 2010 Received in revised form 20 August 2010 Accepted 23 August 2010 Available online 27 August 2010 Keywords: Methane-derived carbonates Black sea Gas hydrate Computerized X-ray tomography Stable oxygen isotopes Batumi seep area abstract 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 arago- nitic 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 18 O-enriched hydrate water. The differences in d 18 O 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. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Vast amounts of methane are stored in gas hydrates in the submarine seafloor. The Black Sea is an important marine gas hydrate reservoir as it is estimated to contain ca. 10e50  10 3 km 3 of hydrate-bound methane (Vassilev and Dimitrov, 2002). Espe- cially shallow-buried methane hydrates are sensitive to changes in the environmental conditions controlling hydrate stability (i.e. temperature, salinity, hydrocarbon availability, hydrostatic pres- sure) compared to their deeply buried counterparts. In case one or more of these factors change, submarine hydrates might dissociate quickly and release significant methane amounts to the hydro- sphere with consequences for the seafloor topography (e.g. by creating pockmarks; Limonov et al., 1997; MacDonald et al., 1994), biogeochemical carbon cycling, and global climate (Dickens, 2003; Kennett et al., 2000). Despite the relevance of shallow-buried marine gas hydrates for these processes, relatively few is known about their dynamic behaviour. An aspect of particular interest is e.g., whether hydrate-derived methane is released constantly over * Corresponding author. Goethe-Universität Frankfurt, Institut für Geowissenschaften, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany. Tel.: þ49 69 798 40209; fax: þ49 60 798 40185. E-mail address: a.bahr@em.uni-frankfurt.de (A. Bahr). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2010.08.005 Marine and Petroleum Geology 27 (2010) 1819e1829