Stable isotopic composition of bottom water and benthic foraminifera from Hakon Mosby Mud Vulcano

To investigate the use of benthic foraminifera as a means to document ancient methane release, we determined the stable isotopic composition of tests of live (Rose Bengal stained) and dead specimens of epibenthic Fontbotia wuellerstorfi, preferentially used in paleoceanographic reconstructions, and of endobenthic high-latitude Cassidulina neoteretis and Cassidulina reniforme from a cold methane-venting seep off northern Norway. We collected foraminiferal tests from three push cores and nine multiple cores obtained with a remotely operated vehicle and a video-guided multiple corer, respectively. All sampled sites except one control site are situated at the Håkon Mosby mud volcano (HMMV) on the Barents Sea continental slope in 1250 m water depth. At the HMMV in areas densely populated by pogonophoran tube worms, d13C values of cytoplasm-containing epibenthic F. wuellerstorfi are by up to 4.4 per mil lower than at control site, thus representing the lowest values hitherto reported for this species. Live C. neoteretis and C. reniforme reach d13C values of -7.5 and -5.5 per mil Vienna Pee Dee Belemnite (VPDB), respectively, whereas d13C values of their empty tests are higher by 4 per mil and 3 per mil. However, d13C values of empty tests are never lower than those of stained specimens, although they are still lower than empty tests from the control site. This indicates that authigenic calcite precipitates at or below the sediment surface are not significantly influencing the stable isotopic composition of foraminiferal shells. The comparatively high d13C results rather from upward convection of pore water and fluid mud during active methane venting phases at these sites. These processes mingle tests just recently calcified with older ones secreted at intermittent times of less or no methane discharge. Since cytoplasm-containing specimens of suspension feeder F. wuellerstorfi are almost exclusively found attached to pogonophores, which protrude up to 3 cm above the sediment, and d13C values of bottom-water-dissolved inorganic carbon (DIC) are not significantly depleted, we conclude that low test d13C values of F. wuellerstorfi are the result of incorporation of heavily 13C-depleted methanotrophic biomass that these specimens feed on rather than because of low bottom water d13CDIC. Alternatively, the pogonophores, which are rooted at depth in the upper sediment column, may serve as a conduit for depleted d13CDIC that ultimately influences the calcification process of F. wuellerstorfi attached to the pogonophoran tube well above the sediment/water interface. The lowest d13C of live specimens of the endobenthic C. neoteretis and C. reniforme are within the range of pore water d13CDIC values, which exceed those that could be due to organic matter decomposition, and thus, in fact, document active methane release in the sediment.

Seep communities from two mud volcanoes in the deep eastern Mediterranean Sea: faunal composition, spatial patterns and environmental control

The Mediterranean Sea constitutes a unique environment to study cold-seep ecosystems due to the presence of different geodynamic settings, from an active margin along the Mediterranean Ridge (MR) to a passive margin in the Nile Deep-Sea Fan (NDSF). We attempted to identify the structure of benthic communities associated with the Napoli and Amsterdam mud volcanoes (MVs) located on the MR and to establish the links between faunal distribution and environmental conditions at different spatial scales. Comparison between the 2 MVs revealed that the faunal distribution seemed to be mainly controlled by the characteristics of the microhabitats. On both geological structures, the variability between the different microhabitats was higher than the variability observed between replicates of the same microhabitat, and the distribution of macro-fauna was apparently linked to gradients in physico-chemical conditions. The peripheral sites from Napoli were generally more oxygenated and harboured lower species richness than the active sites. The reduced sediment microhabitat from Amsterdam presented the highest methane concentrations and was mainly colonised by symbiont-bearing vesicomyid bivalves and heterotrophic dorvilleid polychaetes. Overall, a higher taxonomic diversity was observed on Napoli. Sub-stratum type was hypothesised to be the second factor influencing faunal distribution. The results of this study highlight the high heterogeneity of faunal communities associated with seep ecosystems within this region and the need to pursue investigations at various spatial and temporal scales.

Geochemistry of mafic clasts of ODP Holes 125-778A and 125-779A

Clasts of metamorphosed mafic igneous rock of diverse composition were recovered in two drill sites on a serpentine mud volcano in the outer Mariana forearc during Ocean Drilling Program Leg 125. These clasts are xenolithic fragments that have been entrained in the rising serpentine mud, and make up less that 9% of the total rock recovered at Sites 778 and 779. Most samples are metabasalt or metadiabase, although one clast of possible boninite and one cumulate gabbro were recovered. On the basis of trace element signatures, samples are interpreted to represent both arc-derived and mid-ocean ridge-derived compositions. Rocks with extremely low TiO2 (<0.3 wt%) and Zr (<30 ppm) are similar to boninite series rocks. Samples with low TiO2 (<0.9 wt%) and Zr (<50 ppm) and extreme potassium enrichment (K2O/Na2O >3.9) may represent island arc rocks similar to shoshonites. However, the K2O/Na2O ratios are much higher than those reported for shoshonites from modem or ancient arcs and may be the result of metamorphism. Samples with moderate TiO2 (1.4 to 1.5 wt%) and Zr (72 to 85 ppm) are similar to rocks from mid-ocean ridges. A few samples have TiO2 and Zr intermediate between island arc and mid-ocean ridge basalt-like rocks. Two samples have high iron (Fe2O3* = >12.8 to 18.5 wt%) (Fe2O3* = total iron calculated as Fe2O3) and TiO2 (>2.3 wt%) and resemble FeTi basalt recovered from mid-ocean ridges. Metamorphism in most samples ranges from low-temperature zeolite, typical of ocean floor weathering, to prehnite-pumpellyite facies and perhaps lower greenschist. Blue amphibole and lawsonite minerals are present in several samples. One diabase clast (Sample 9) exhibits Ca enrichment, similar to rodingite metamorphism, typical of mafic blocks in serpentinized masses. The presence of both low-grade (clays and zeolites) and higher grade (lawsonite) metamorphism indicates retrograde processes in these clasts. These clasts are fragments of the forearc crust and possibly of the subducting plate that have been entrained in the rising serpentine and may represent the deepest mafic rocks ever recovered from the Mariana forearc. The variable compositions and degree of metamorphism of these clasts requires at least two tectonic origins. The recovery of clasts with mid-ocean ridge and arc chemical affinities in a single drill hole requires these clasts to have been "mixed" on a small scale either (1) in the forearc crustal sequence, or (2) after inclusion in the rising serpentine mud. The source of the MORB-like samples and an explanation for the presence of both MORB-like and arc-like rocks in close proximity is critical to any model of the evolution of the Mariana forearc. The source of the MORB-like samples likely will be one (or more) of the following: (1) accretion of Pacific plate lithosphere, (2) remnants of original forearc crust (trapped plate), (3) volcanism in the supra-subduction zone (arc or forearc) environment, or (4) derivation from the subducting slab by faulting along the dÈcollement.

Mats of psychrophilic thiotrophic bacteria associated with cold seeps of the Barents Sea

This study focused on the bacterial diversity associated with microbial mats of deep-sea cold seeps at the Norwegian continental margin. Study sites included the Storegga and Nyegga areas as well as the Håkon Mosby mud volcano, where the mats occurred at temperatures permanently close to the freezing point of seawater. Two visually different mat types, i.e. small gray mats and extensive white mats, were studied with the aim to determine the identity of the mat-forming sulfide oxidizers, and to investigate which environmental factors (e.g. sulfate reduction and methane oxidation rates) shown here could explain the observed diversity. Sequence data have been submitted to the EMBL database under accession No. FR847864-FR847887 (giant sulfur bacteria), No. FR827864 (Menez Gwen filament; see Supplementary Material) and No. FR875365-FR877509 (except FR875905; remaining partial sequences).