Investigations of the environmental and conditions of formation of carbonate crusts and chimneys

The Sea of Marmara is intercepted by active fault segments of the North Anatolian Fault System. It consists of ca. 1250 m-deep basins (Tekirdağ, Central and Çınarcık), 600-350 m deep NE-tranding pressure highs (Western and Central), and less than 100 m deep shelf areas. The Sea of Marmara is a gateway between the saline Aegean Sea (̴ 38.5 psu) and the brackish Black Sea (̴ 18 psu). It is connected to these adjacent basins via the Çanakkale (Dardanelles) straits and İstanbul (Bosporus) having present day sill depths of -65 m and -35 m, respectively. The Sea of Marmara is characterized by a two-way water exchange between Mediterranean and Black Sea with a permanent halocline located at depth of -25 m. The Mediterranean water forming the subhalocline waters enters the Sea of Marmara through the Çanakkale Strait as an undercurrent. It flows eastwards slowly over the deep basins and highs, gradually consuming its oxygen content in the bottom waters from 50 µmol/kg in the Tekirdağ Basin to 8 µmol/kg in the Çınarcık Basin. Because of the shallowness of the Çanakkale Strait, the Sea of Marmara lost its connection with the global ocean during the glacial periods and became a lacustrine environment. The last marine reconnection was fully established at 12.55 ka BP. Soon after the full connection sapropel deposition took place under suboxic-dysoxic conditions. Widespread methane-related seafloor authigenic carbonate crusts occur as pavements, mounds and chimneys along the the North Anatolian Fault System in the Sea of Marmara, which are often associated with patches of reduced (Fe-sulphide-rich) sediments. These sites were observed and sampled during the Nautile submersible and Victor 6000 Remotely Operated Vehicle (ROV) dives carried out during MARNAUT and MARSITE cruises in 2007 and 2014, respectively. In the present study we analysed seafloor carbonate crust, mound and chimney samples collected during the recent Marsite cruise with some additional samples from the Marnaut cruise. The methods used includes petrographic analyses by binocular, thin-section and scanning electron microscopy (SEM), mineralogical analysis by X-ray diffraction (XRD), elemental analysis by mass spectrometry and inductively coupled plasma-mass spectrometry and emission spectroscopy (ICP-MS-ES), and stable isotope analysis by mass spectrometry. The results are discussed in terms of environmental conditions, tectonic setting, origin of fluids and mechanisms of formation of the seafloor authigenic carbonates and black sulphidic sediments along the active faults in the Sea of Marmara. In the deep basins, the carbonate crusts and the black reduced sediments are located on the hydrocarbon-rich fluid emissions from the active faults, whereas on the compressional Western and Central highs they are observed on the mud volcanoes and anticlines nearby the active fault. Considering the average sedimentation rates of 1-2 m/ kyr in the seep basins and 0.3-0.5 m/kyr over the pressure highs over the last 12 kyr, the authigenic carbonates exposed on the seafloor in the Sea of Marmara today are likely to be no older than about 1-2 kyrs, and hence, were precipitated under bottom water conditions similar to those of the present day. The textures and structures of the carbonate crusts are variable with sinter-like porous, botryoidal and sugary-granular textures, and consists mainly of carbonate cemented bivalve shells and shell fragments, serpulid tubes, fibrous and filamentous microbial organic matter and rarely pebbles. The authigenic cements is composed mainly of aragonite in most sites, except for some samples from Tekirdağ and Central basins, in which major amounts of high Mg-calcite are present. High Mg-calcite occurs as microsrytalline, rice-like grains and aragonite as early microcrystalline and late cavity filling acicular crystals. The δ13C values of the seafloor authigenic carbonates range between -47.62‰ and -13.65‰ V-PDB, which together with their close association with reduced sediments, indicate a dissolved inorganic carbon (DIC) pool supplied mainly by the anaerobic oxidation of biogenic or thermogenic methane. Relatively heavy carbon isotope values (-24.9‰ to -13.7‰) of authigenic carbonates from the compressional Western and Central highs suggest a DIC source mainly of thermogenic methane with some contribution from the biodegradation of heavy hydrocarbons and gas hydrate dissociation. These conclusions are in agreement with the presence thermogenic gas and oil seeps and shallow gas hydrate sampled in the compressional highs, all having compositional and isotope signatures similar to those of the Thrace Basin. Formation of the carbonate crusts and the black reduced sediments is explained by high methane flux that results in the anaerobic oxidation of methane by sulphate reduction (AMO) reaction taking place at or near the seafloor: CH4 + SO42- → HCO3- + HS- + H2O The AMO reaction provides the high alkalinity (HCO3-) and HS- ions, necessary for the formation of the authigenic carbonates and Fe-sulphides of the black reduced sediments. The aragonite-rich cements in the Tekirdağ and Central basins are due to the low Mg and SO4-2 and low salinity brackish Marmara "Lake" water emerging from these sites. On the orther hand high Mg-calcite cements on the compressional highs were depositd under the influence high salinity formation waters. The buoyant emittance of brackish waters in the deep Marmara basins and deep formation waters of Eocene-Miocene Thrace basin in the compressional highs are supported by relatively low δ18O values (+0.5‰ to +3.8‰ V-PDB, average= +2.1‰, n=24) of carbonates in the former and high values (+2.6‰ to +3.4‰, average= +3.0‰, n=9) in the latter areas. Low δ13C values (-47.6‰ to -13.7‰ V-PDB, average: -34.9‰, n=33) and close association of the seafloor authigenic carbonates with black reduced (Fe-sulphide-rich) sediments indicate that they are formed over the last about 1-2 k years by the anaerobic methane oxidation (AMO) at or near the seafloor, as result of high methane flux, possibly during periods of high seismic activity. Authigenic carbonates from the Western and Central highs are relatively more enriched in 13C (-36.6‰ to -13.7‰, average= -23.3 ‰) than the those of the deep basin sites (-47.6‰ to - 29.6‰, average= -39.3 ‰). These results suggest both biogenic and thermal methane source for the deep basins carbonates and mainly thermogenic hydrocarbon, with some contribution from the biodegradation of heavy hydrocarbons and gas hydrate dissociation, for carbonates from the compressional highs. Many carbonate crust samples contain pyrite grains and framboids and Fe-oxyhydroxide veins, and are characterized by higher than 0.5 wt% S, 1 wt% Fe and 20 µg/g Mo. The presence of pyrite in the carbonate crusts suggests deposition under reducing conditions. Barium in the Western High barite precipitates and the carbonate crust samples from the eastern edge of the Central Basin with higher than 400 µg/g Ba are sourced from dissolution of biobarite below the sulphate/methane interface (SMI), which diffused and/or advected upwards to deposit at or near seafloor by reacting with the seawater SO4-2. Fracturing and fine fracture filling by carbonate and pyrite suggest later fault activity and/or associated fluid activity. However, black to dark brown Fe-Mn staining on the surface of some crusts together with common presence of solution pores indicate that these carbonate crusts were later oxidized and dissolved by the acidic conditions created by the pyrite oxidation. Minor amount of well-developed gypsum observed in such carbonate crusts are formed by reaction of Ca2+ with the SO4-2 produced by pyrite oxidation. The fact that there were gas flares with fire balls during the 1999 İzmit earthquake and the intense gas emissions in the water column following the event, which progressively decreased about ten years after the earthquake, strongly suggest a relation between seismic and fluid activities in the Sea of Marmara. Methane-related seafloor and buried authigenic carbonates could therefore provide a valuable archive for paleo-seismic activity along the individual fault segments of the North Anatolian Fault system. Therefore, a systematic study involving U/Th dating and stable isotope analysis of the seafloor and buried carbonate crusts and nodules along the various segments of NAFS is recommended. Such a study would contribute to the discussion of the relations between seismic and fluid activities and the temporal evolution of the processes leading to the formation of the carbonate crusts, chimneys and mounds.