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

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.

Chemical and isotopic compositions of altered MORB from ODP Holes 187-1154A, 187-1155B, and 187-1160B

Boron and Pb isotopic compositions together with B-U-Th-Pb concentrations were determined for Pacific and Indian mantle-type mid-ocean ridge basalts (MORB) obtained from shallow drill holes near the Australian Antarctic Discordance (AAD). Boron contents in the altered samples range from 29.7 to 69.6 ppm and are extremely enriched relative to fresh MORB glass with 0.4-0.6 ppm B. Similarly the d11B values range from 5.5? to 15.9? in the altered basalts and require interaction with a d11B enriched fluid similar to seawater ~39.5? and/or boron isotope fractionation during the formation of secondary clays. Positive correlations between B concentrations and other chemical indices of alteration such as H2O CO2, K2O, P2O5, U and 87Sr/86Sr indicate that B is progressively enriched in the basalts as they become more altered. Interestingly, d11B shows the largest isotopic shift to +16? in the least altered basalts, followed by a continual decrease to +5-6? in the most altered basalts. These observations may indicate a change from an early seawater dominated fluid towards a sediment-dominated fluid as a result of an increase in sediment cover with increasing age of the seafloor. The progression from heavy d11B towards lighter values with increasing degrees of alteration may also reflect increased formation of clay minerals (e.g., saponite). A comparison of 238U/204Pb and 206Pb/204Pb in fresh glass and variably altered basalt from Site 1160B shows extreme variations that are caused by secondary U enrichment during low temperature alteration. Modeling of the U-Pb isotope system confirms that some alteration events occurred early in the 21.5 Ma history of these rocks, even though a significant second pulse of alteration happened at ~12 Ma after formation of the crust. The U-Pb systematics of co-genetic basaltic glass and variably low temperature altered basaltic whole rocks are thus a potential tool to place age constraints on the timing of alteration and fluid flow in the ocean crust.

Sulphate concentrations and sulphur isotopic composition in pore fluids from ODP Leg 168 sites

A numerical model of sulfate reduction and isotopic fractionation has been applied to pore fluid SO4**2- and d34S data from four sites drilled during Ocean Drilling Program (ODP) Leg 168 in the Cascadia Basin at 48°N, where basement temperatures reach up to 62°C. There is a source of sulfate both at the top and the bottom of the sediment column due to the presence of basement fluid flow, which promotes bacterial sulfate reduction below the sulfate minimum zone at elevated temperatures. Pore fluid d34S data show the highest values (135 per mil) yet found in the marine environment. The bacterial sulfur isotopic fractionation factor, a, is severely underestimated if the pore fluids of anoxic marine sediments are assumed to be closed systems and Rayleigh fractionation plots yield erroneous values for a by as much as 15 per mil in diffusive and advective pore fluid regimes. Model results are consistent with a = 1.077+/-0.007 with no temperature effect over the range 1.8 to 62°C and no effect of sulfate reduction rate over the range 2 to 10 pmol/ccm/day. The reason for this large isotopic fractionation is unknown, but one difference with previous studies is the very low sulfate reduction rates recorded, about two orders of magnitude lower than literature values that are in the range of µmol/ccm/day to tens of nmol/ccm/day. In general, the greatest 34S depletions are associated with the lowest sulfate reduction rates and vice versa, and it is possible that such extreme fractionation is a characteristic of open systems with low sulfate reduction rates.

Boron concentrations and isotope ratios of authigenic carbonates from the Oregon margin

Boron contents and boron, carbon and oxygen stable isotopes were determined for authigenic carbonates recovered from Ocean Drilling Program Leg 146, Oregon margin. Carbonate precipitates are the most widespread authigenic phase in the shallow accretionary wedge and carry chemical information about long-term variations in pore fluid origin and flow paths in the Cascadia subduction zone. Drilling the first ridge (toe area including the frontal thrust) and the second ridge (or Hydrate Ridge) of the prism demonstrated different fluid regimes, with higher B contents in the authigenic precipitates at the toe. The delta11B of 18 authigenic precipitates analysed ranges from 13.9 per mil to as high as 39.8 per mil, extending the upper range of previously reported carbonate delta11B values considerably. When related to the delta11B ratio of their parent solutions, these data are characteristic of fluid-related processes in accretionary prisms. Together with delta13C and delta18O, delta11B ratios of the carbonate concretions, nodules and crusts allow one to distinguish between precipitation influenced by (i) seawater, (ii) fluid reservoirs at different depth levels within the accretionary prism and (iii) cage water from dissociated gas hydrates, the latter possibly indicating a fluctuation of the bottom simulating reflector during most recent Earth's history. From this first systematic boron study on authigenic precipitates from an accretionary prism it is suggested that B contents of such carbonate crusts and concretions exceed those reported for other marine carbonates. Given the abundance of such precipitates at convergent margins, they represent a significant B sink in geochemical cycling. Isotopic compositions of the parent fluids to the carbonates mirror B chemistry of modern pore waters from convergent margins. The precipitates carry information of different subduction-related fluid processes over a certain period of time, and hence are a crucial tracer in the investigation of palaeo-fluid flow.

Gridded bathymetry from multibeam echosounder EM122 data of the cruise M84/2, off Eregli (2011)

We investigated gas bubble emissions at the Don-Kuban paleo-fan in the northeastern Black Sea regarding their geological setting, quantities as well as spatial and temporal variabilities during three ship expeditions between 2007 and 2011. About 600 bubble-induced hydroacoustic anomalies in the water column (flares) originating from the seafloor above the gas hydrate stability zone (GHSZ) at ~700 m water depth were found. At about 890 m water depth a hydrocarbon seep area named "Kerch seep area" was newly discovered within the GHSZ. We propose locally domed sediments ('mounds') discovered during ultra-high resolution bathymetric mapping with an autonomous underwater vehicle (AUV) to result from gas hydrate accumulation at shallow depths. In situ measurements indicated spatially limited temperature elevations in the shallow sediment likely induced by upward fluid flow which may confine the local GHSZ to a few meters below the seafloor. As a result, gas bubbles are suspected to migrate into near-surface sediments and to escape the seafloor through small-scale faults. Hydroacoustic surveys revealed that several flares originated from a seafloor area of about 1 km**2 in size. The highest flare disappeared in about 350 m water depth, suggesting that the released methane remains in the water column. A methane flux estimate, combining data from visual quantifications during dives with a remotely operated vehicle (ROV) with results from ship-based hydroacoustic surveys and gas analysis revealed that between 2 and 87 x 10**6 mol CH4 yr-1 escaped into the water column above the Kerch seep area. Our results show that the finding of the Kerch seep area represents a so far underestimated type of hydrocarbon seep, which has to be considered in methane budget calculations.

In-situ sediment temperature and pore water chloride concentration at pockmarks of the Nigerian continental margin

A joint research expedition between the French IFREMER and the German MARUM was conducted in 2011 using the R/V 'Pourquoi pas?' to study gas hydrate distributions in a pockmark field (1141-1199 m below sea surface) at the continental margin of Nigeria. The seafloor drill rig MeBo of MARUM was used to recover sediments as deep as 56.74 m below seafloor. The presence of gas hydrates in specific core sections was deduced from temperature anomalies recorded during continuous records of infrared thermal scanning and anomalies in pore water chloride concentrations. In situ sediment temperature measurements showed elevated geothermal gradients of up to 258 °C/km in the center of the so-called pockmark A which is up to 4.6 times higher than that in the background sediment (72 °C/km). The gas hydrate distribution and thermal regime in the pockmark are largely controlled by the intensity, periodicity and direction of fluid flow. The joint interaction between fluid flow, gas hydrate formation and dissolution, and the thermal regime governs pockmark formation and evolution on the Nigerian continental margin.

Gridded bathymetry from multibeam echosounder EM120 data of the cruise M72/2 (2007)

Bathymetry based on data recorded during M72-2 between 23.02.2007 and 13.03.2007 in the Black Sea. The main focus of the cruise was to study the fluxes and turnover of methane and sulphur in the Black Sea hydrocarbon seep systems and investigating the microbial diversity in two contrasting permanently anoxic settings associated with fluid flow and gas seepage: the methane seeps at the shelf break of the Palaeo-Dnepr area and the hydrocarbon seeps of the mud volcanoes in the 2000 m deep Sorokin trough east of Crimea.