Life at cold seeps: a synthesis of biogeochemical and ecological data from Kazan mud volcano, eastern Mediterranean Sea

Recent field observations have identified the widespread occurrence of fluid seepage through the eastern Mediterranean Sea floor in association with mud volcanism or along deep faults. Gas hydrates and methane seeps are frequently found in cold seep areas and were anticipated targets of the MEDINAUT/MEDINETH initiatives. The study presented herein has utilized a multidisciplinary approach incorporating observations and sampling of visually selected sites by the manned submersible Nautile and by ship-based sediment coring and geophysical surveys. The study focuses on the biogeochemical and ecological processes and conditions related to methane seepage, especially the anaerobic oxidation of methane (AOM), associated with ascending fluids on Kazan mud volcano in the eastern Mediterranean. Sampling of adjacent box cores for studies on the microbiology, biomarkers, pore water and solid phase geochemistry allowed us to integrate different biogeochemical data within a spatially 0009-2541/$ see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.12.031 * Corresponding author. Current address: Large Lakes Observatory and Department of Chemistry, University of Minnesota Duluth, 10 University Dr. 109 RLB, Duluth, MN 55812, USA. Tel.: +1-218-726-7435; fax: +1-218-726-6979. E-mail addresses: [email protected] (J.P. Werne), [email protected] (R.R. Haese). 1 Address as of October 2003: Geoscience Australia, Canberra. 2 Current address: Department of Marine Environmental Geology, GEOMAR, Kiel, Germany. 3 Current address: Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol, UK. 4 Current address: Department of Biological Sciences, University of Idaho, Moscow, ID, USA. 5 S. Asjes, K. Bakker, M. Bakker, J.L. Charlou, J.P. Donval, R. Haanstra, P. Henry, C. Huguen, B. Jelsma, S. de Lint, M. van der Maarel, S. Muzet, G. Nobbe, H. Pelle, C. Pierre, W. Polman, L. de Senerpont, M. Sibuet. J.P. Werne et al. / Chemical Geology 205 (2004) 367–390 368 highly heterogeneous system. Geophysical results clearly indicate the spatial heterogeneity of mud volcano environments. Results from pore water geochemistry and modeling efforts indicate that the rate of AOM is f6 mol m 2 year , which is lower than at active seep sites associated with conditions of focused flow, but greater than diffusion-dominated sites. Furthermore, under the non-focused flow conditions at Kazan mud volcano advective flow velocities are of the order of a few centimeters per year and gas hydrate formation is predicted to occur at a sediment depth of about 2 m and below. The methane flux through these sediments supports a large and diverse community of microand macrobiota, as demonstrated by carbon isotopic measurements on bulk organic matter, authigenic carbonates, specific biomarker compounds, and macrofaunal tissues. Because the AOM community appears to be able to completely oxidize methane at the rate it is seeping through the sediments, ultimate sinks for methane in this environment are authigenic carbonates and biomass. Significant differences in organic geochemical data between this site and those of other cold seep environments, even within the eastern Mediterranean mud volcanoes, indicate that the microbiological communities carrying out AOM varies depending on specific conditions such as methane flux and salinity. D 2004 Elsevier B.V. All rights reserved.