Mineral composition, rare earth elements, and trace elements from samples of the northern Congo deep-sea fan

Authigenic carbonates were collected from methane seeps at Hydrate Hole at 3113 m water depth and Diapir Field at 2417 m water depth on the northern Congo deep-sea fan during RV Meteor cruise M56. The carbonate samples analyzed here are nodules, mainly composed of aragonite and high-Mg calcite. Abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix. The d13C values of the Hydrate Hole carbonates range from -62.5 permil to -46.3 permil PDB, while the d13C values of the Diapir Field carbonate are somewhat higher, ranging from -40.7 permil to -30.7 permil PDB, indicating that methane is the predominant carbon source at both locations. Relative enrichment of 18O (d18O values as high as 5.2 permil PDB) are probably related to localized destabilization of gas hydrate. The total content of rare earth elements (REE) of 5% HNO3-treated solutions derived from carbonate samples varies from 1.6 ppm to 42.5 ppm. The shale-normalized REE patterns all display positive Ce anomalies (Ce/Ce* > 1.3), revealing that the carbonates precipitated under anoxic conditions. A sample from Hydrate Hole shows a concentric lamination, corresponding to fluctuations in d13C values as well as trace elements contents. These fluctuations are presumed to reflect changes of seepage flux.

Pore water (SO4, CH4, alkalinity, HS-, Ca, Sr, Mg, Ba) and solid phase (Ca, Sr, Mg, Ba) data measured in pockmark sediments of the Northern Congo Fan

This study focuses on the vertical distribution of authigenic carbonates (aragonite and high Mg-calcite) in the form of finely disseminated precipitates as well as massive carbonate concretions present in and above gas hydrate bearing sediments of the Northern Congo Fan. Analyses of Ca, Mg, Sr and Ba in pore water, bulk sediments and authigenic carbonates were carried out on gravity cores taken from three pockmark structures (Hydrate Hole, Black Hole and Worm Hole). In addition, a background core was retrieved from an area not influenced by fluid seepage. Pore water Sr/Ca and Mg/Ca ratios are used to reveal the current depths of carbonate formation as well as the mineralogy of the authigenic precipitates. The Sr/Ca and Mg/Ca ratios of bulk sediments and massive carbonate concretions were applied to infer the presence and depth distribution of authigenic aragonite and high Mg-calcite, based on the approach presented by Bayon et al. [Bayon et al. (2007). Sr/Ca and Mg/Ca ratios in Niger Delta sediments: Implications for authigenic carbonate genesis in cold seep environments. Marine Geology 241(1-4), 93-109, doi:10.1016/j.margeo.2007.03.007]. We show that the approach developed by Bayon et al. (2007) for sediments of cold seeps of the Niger Delta is also suitable to identify the mineralogy of authigenic carbonates in pockmark sediments of the Congo Deep-Sea Fan. We expand this approach by combining interstitial with solid phase Sr/Ca and Mg/Ca ratios, which demonstrate that high Mg-calcite is the predominant authigenic carbonate that currently forms at the sulfate/methane reaction zone (SMRZ). This is the first study which investigates both solid phase and pore water signatures typical for either aragonite or high Mg-calcite precipitation for the same sediment cores and thus is able to identify active and fossil carbonate precipitation events. At all investigated pockmark sites fossil horizons of the SMRZ were deduced from high Mg-calcite located above and below the current depths of the SMRZ. Additionally, aragonite enrichments typical for high seepage rates were detected close to the sediment surface at these sites. However, active precipitation of aragonite as indicated by pore water characteristics only occurs at the Black Hole site. Dissolved and solid phase Ba concentrations were used to estimate the time the SMRZ was fixed at the current depths of the diagenetic barite fronts. The combined pore water and solid phase elemental ratios (Mg/Ca, Sr/Ca) and Ba concentrations allow the reconstruction of past changes in methane seepage at the investigated pockmark sites. At the Hydrate Hole and Worm Hole sites the time of high methane seepage was estimated to have ceased at least 600 yr BP. In contrast, a more recent change from a high flux to a more dormant stage must have occurred at the Black Hole site as evidenced by active aragonite precipitation at the sediment surface and a lack of diagenetic Ba enrichments.

Osmium isotope composition of leachates and bulk sediment from ODP Holes in the Bengal Fan

Os isotopic compositions and OS and Re concentrations were measured in H2O2-H2SO4 leachates and bulk sediment samples from Holes 717C and 718C of ODP Leg 116 in the Bengal Fan. Os isotopic results indicate that, at the sediment surface, the leachable Os fraction is derived from seawater. In contrast, leachable Os from Ganges River sediments has 187Os/188Os ratios (Pegram et al., 1994, doi:10.1016/0012-821X(94)90172-4) much higher than the marine value. This difference suggests that the leachable radiogenic Os carried by the river sediments is completely released to the oceans prior to sediment deposition in the Fan. A simple calculation, assuming these sediments to be typical of those delivered by the Ganges-Brahmaputra river system, suggests that this process can account for a substantial part of the rise in the seawater Os isotopic ratio observed over the past 16 m.y. Bengal Fan leachate 187Os/188Os ratios increase with increasing depositional age, in contrast to the seawater Os isotopic ratio, which decreases with increasing age. Several lines of evidence suggest that, at the time of sediment burial, the leachate Os compositions most likely reflected the seawater values. Thus, the current divergence is probably the result of post-depositional processes. One such process, in situ radiogenic ingrowth of 187Os, can be excluded because the measured Re concentrations of these sediments are too low. Similarly, since most of the bulk rock Os isotopic ratios were lower than those of the associated leachates, the high leachate 187Os/188Os values cannot be explained by in situ sediment alteration. Instead, it is proposed that the increase with age results from radiogenic OS brought in by thermoconvective circulation from further upslope in the Fan. The ultimate source of this 187Os would then be alteration of radiogenic sediments or post-depositional radioactive decay of Re in sediments rich in organic carbon. Finally, the divergence between the results obtained on Bengal Fan sediments and those obtained in the open ocean (Pegram et al., 1992, doi:10.1016/0012-821X(92)90132-F) by the same leaching technique suggest that Os sediment leachate data must be interpreted with caution.

Solid phase analysis of sediment core GeoB9309-1 from the Zambezi (or Mozambique) deep-sea fan

The Zambezi deep-sea fan, the largest of its kind along the east African continental margin, is poorly studied to date, despite its potential to record marine and terrestrial climate signals in the southwest Indian Ocean. Therefore, gravity core GeoB 9309-1, retrieved from 1219 m water depth, was investigated for various geophysical (magnetic susceptibility, porosity, colour reflectance) and geochemical (pore water and sediment geochemistry, Fe and P speciation) properties. Onboard and onshore data documented a sulphate/methane transition (SMT) zone at ~ 450-530 cm sediment depth, where the simultaneous consumption of pore water sulphate and methane liberates hydrogen sulphide and bi-carbonate into the pore space. This leads to characteristic changes in the sediment and pore water chemistry, as the reduction of primary Fe (oxyhydr)oxides, the precipitation of Fe sulphides, and the mobilization of Fe (oxyhydr)oxide-bound P. These chemical processes also lead to a marked decrease in magnetic susceptibility. Below the SMT, we find a reduction of porosity, possibly due to pore space cementation by authigenic minerals. Formation of the observed geochemical, magnetic and mineralogical patterns requires a fixation of the SMT at this distinct sediment depth for a considerable time-which we calculated to be ~ 10 000 years assuming steady-state conditions-following a period of rapid upward migration towards this interval. We postulate that the worldwide sea-level rise at the last glacial/interglacial transition (~ 10 000 years B.P.) most probably caused the fixation of the SMT at its present position, through drastically reduced sediment delivery to the deep-sea fan. In addition, we report an internal redistribution of P occurring around the SMT, closely linked to the (de)coupling of sedimentary Fe and P, and leaving a characteristic pattern in the solid P record. By phosphate re-adsorption onto Fe (oxyhydr)oxides above, and formation of authigenic P minerals (e.g. vivianite) below the SMT, deep-sea fan deposits may potentially act as long-term sinks for P.

Bulk rock magnetic measurements and thermomagnetic analyses of sediment core GeoB4901-8 from the southeastern flank of the Niger deep-sea fan

Diagenesis has extensively affected the magnetic mineral inventory of organic-rich late Quaternary sediments in the Niger deep-sea fan. Changes in concentration, grain size, and coercivity document modifications of the primary magnetic mineral assemblages at two horizons. The first front, the modern iron redox boundary, is characterized by a drastic decline in magnetic mineral content, coarsening of the grain size spectrum, and reduction in coercivity. Beneath a second front, the transition from the suboxic to the sulfidic anoxic domain, a further but less pronounced decrease in concentration and bulk grain size occurs. Finer grains and higher coercive magnetic constituents substantially increase in the anoxic environment. Low- and high-temperature experiments were performed on bulk sediments and on extracts which have also been examined by X-ray diffraction. Thermomagnetic analyses proved ferrimagnetic titanomagnetites of terrigenous provenance as the principal primary magnetic mineral components. Their broad range of titanium contents reflects the volcanogenic traits of the Niger River drainage areas. Diagenetic alteration is not only a grain size selective process but also critically depends on titanomagnetite composition. Low-titanium compounds are less resistant to diagenetic dissolution. Intermediate titanium content titanomagnetite thus persists as the predominant magnetic mineral fraction in the sulfidic anoxic sediments. At the Fe redox boundary, precipitation of authigenic, possibly bacterial, magnetite is documented. The presence of hydrogen sulfide in the pore water suggests a formation of secondary magnetic iron sulfides in the anoxic domain. Grain size-specific data argue for a gradual development of a superparamagnetic and single-domain iron sulfide phase in this milieu, most likely greigite.

Shapefile of the survey tracks of 'Mosaic 02' at the Regab pockmark (Northern Congo Fan)

The Regab pockmark is a large cold seep area located 10 km north of the Congo deep sea channel at about 3160 m water depth. The associated ecosystem hosts abundant fauna, dominated by chemosynthetic species such as the mussel Bathymodiolus aff. boomerang, vestimentiferan tubeworm Escarpia southwardae, and vesicomyid clams Laubiericoncha chuni and Christineconcha regab. The pockmark was visited during the West African Cold Seeps (WACS) cruise with RV Pourquoi Pas? in February 2011, and a 14,000-m**2 high-resolution videomosaic was constructed to map the most populated area and to describe the distribution of the dominant megafauna (mussels, tubeworms and clams). The results are compared with previous published works, which also included a videomosaic in the same area of the pockmark, based on images of the BIOZAIRE cruise in 2001. The 10-year variation of the faunal distribution is described and reveals that the visible abundance and distribution of the dominant megafaunal populations at Regab have not changed significantly, suggesting that the overall methane and sulfide fluxes that reach the faunal communities have been stable. Nevertheless, small and localized distribution changes in the clam community indicate that it is exposed to more transient fluxes than the other communities. Observations suggest that the main megafaunal aggregations at Regab are distributed around focused zones of high flux of methane-enriched fluids likely related to distinct smaller pockmark structures that compose the larger Regab pockmark. Although most results are consistent with the existing successional models for seep communities, some observations in the distribution of the Regab mussel population do not entirely fit into these models. This is likely due to the high heterogeneity of this site formed by the coalescence of several pockmarks. We hypothesize that the mussel distribution at Regab could also be controlled by the occurrence of zones of both intense methane fluxes and reduced efficiency of the anaerobic oxidation of methane possibly limiting tubeworm colonization.

Low-temperature magnetic remanence and hysteresis measurements on sediment core GeoB4901-8 from the Niger deep-sea fan

Low-temperature (LT) magnetic remanence and hysteresis measurements, in the range 300-5 K, were combined with energy dispersive spectroscopy (EDS) in order to characterize the magnetic inventory of strongly diagenetically altered sediments originating from the Niger deep-sea fan. We demonstrate the possibility of distinguishing between different compositions of members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series on a set of five representative samples, two from the upper suboxic and three from the lower sulfidic anoxic zone of gravity core GeoB 4901. Highly sensitive LT magnetic measurements were performed on magnetic extracts resulting in large differences in the magnetic behavior between samples from the different layers. This emphasizes that both Fe-Ti oxide phases occur in different proportions in the two geochemical environments. Most prominent are variations in the coercivity sensitive parameter coercive field (BC). At room-temperature (RT) hysteresis loops for all extracts are narrow and yield low coercivities (6-13 mT). With decreasing temperature the loops become more pronounced and wider. At 5 K an approximately 5-fold increase in BC for the suboxic samples contrasts a 20-25-fold increase for the samples from the anoxic zone. We demonstrate that this distinct increase in BC at LT corresponds to the increasing proportion of the Ti-rich hemoilmenite phase, while Fe-rich (titano-)magnetite dominates the magnetic signal at RT. This trend is also seen in the room-temperature saturation isothermal remanent magnetization (RT-SIRM) cycles: suboxic samples show remanence curves dominated by Fe-rich mineral phases while anoxic samples display curves clearly dominated by Ti-rich particles. We show that the EDS intensity ratios of the characteristic Fe Kalpha and Ti Kalpha lines of the Fe-Ti oxides may be used to differentiate between members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series. Furthermore it is possible to calculate an approximate composition for each grain if the intensity ratios of natural particles are linked to well-known standards. Thus, element spectra with high Fe/Ti intensity ratios were found to be rather typical of titanomagnetite while low Fe/Ti ratios are indicative of hemoilmenite. The EDS analyses confirm the LT magnetic results, Fe-rich magnetic phases dominate in the upper suboxic environment whereas Ti-rich magnetic phases comprise the majority of particles in the lower anoxic domain: The mineral assemblage of the upper suboxic environments is composed of magnetite (~19%), titanomagnetite (~62%), hemoilmenite (~17%) and ~2% other particles. In the lower anoxic sediments, reductive diagenetic alteration has resulted in more extensive depletion of the (titano-)magnetite phase, resulting in a relative enrichment of the hemoilmenite phase (~66%). In these strongly anoxic sediments stoichiometric magnetite is barely preserved and only ~5% titanomagnetite was detected. The remaining ~28% comprises Ti-rich particles such as pseudobrookite or rutile.

The effects of elevated CO2 on the growth and toxicity of field populations and cultures of the saxitoxin-producing dinoflagellate, Alexandrium fundyense

The effects of coastal acidification on the growth and toxicity of the saxitoxin-producing dinoflagellate Alexandrium fundyense were examined in culture and ecosystem studies. In culture experiments, Alexandrium strains isolated from Northport Bay, New York, and the Bay of Fundy, Canada, grew significantly faster (16-190%; p < 0.05) when exposed to elevated levels of PCO2 ( 90-190 Pa=900-1900 µatm) compared to lower levels ( 40 Pa=400 µatm). Exposure to higher levels of PCO2 also resulted in significant increases (71-81%) in total cellular toxicity (fg saxitoxin equivalents/cell) in the Northport Bay strain, while no changes in toxicity were detected in the Bay of Fundy strain. The positive relationship between PCO2 enrichment and elevated growth was reproducible in natural populations from New York waters. Alexandrium densities were significantly and consistently enhanced when natural populations were incubated at 150 Pa PCO2 compared to 39 Pa. During natural Alexandrium blooms in Northport Bay, PCO2 concentrations increased over the course of a bloom to more than 170 Pa and were highest in regions with the greatest Alexandrium abundances, suggesting Alexandrium may further exacerbate acidification and/or be especially adapted to these acidi-fied conditions. The co-occurrence of Alexandrium blooms and elevated PCO2 represents a previously unrecognized, compounding environmental threat to coastal ecosystems. The ability of elevated PCO2 to enhance the growth and toxicity of Alexandrium indicates that acidification promoted by eutrophication or climate change can intensify these, and perhaps other, harmful algal blooms.

Mineralogy of silts from the Bengal Fan

Early Miocene to Quaternary sediments drilled from the Bengal Fan are divided into six zones by modal proportions of heavy minerals. The sediments were mostly derived from the Himalayas. Detritus from the Indian subcontinent is found sporadically in clay-rich sediments that were deposited during periods of slow sedimentation, when the deep-sea channel migrated away from the drilled sites. The oldest sediments, ranging from 17 to about 15 Ma, were derived mostly from the Precambrian and Paleozoic sedimentary rocks of the lower Himalayas. At about 15 Ma, metamorphic terrains were eroded in the source area. Further large-scale unroofing of metamorphic rocks occurred around 11 Ma. After 10 Ma, the major constituents in the drainage basin or the drainage pattern changed a few times. Between 3.5 and 0.5 Ma, a large peridotite body was unroofed by uplift and successive erosion of the central Himalayas. At this time, the single large river that had supplied detritus to the early Bengal Fan was divided into the Indus and Ganges rivers.