Hydrate, ice and quartz content of eight holes of Leg 204

The state of preservation of natural gas hydrate samples, recovered from 6 sites drilled during ODP Leg 204 at southern summit of Hydrate Ridge, Oregon Margin, has been investigated by X-ray diffraction (XRD) and cryo-scanning-electron-microscopy (cryo-SEM) techniques. A detailed characterization of the state of decomposition of gas hydrates is necessary since no pressurized autoclave tools were used for sampling and partial dissociation must have occurred during recovery prior to the quench and storage in liquid nitrogen. Samples from 16 distinct horizons have been investigated by synchrotron X-ray diffraction measurements at HASYLAB/ Hamburg. A full profile fitting analysis ("Rietveld method") of synchrotron XRD data provides quantitative phase determinations of the major sample constituents such as gas hydrate structure I (sI), hexagonal ice (Ih) and quartz. The ice content (Ih) in each sample is related to frozen water composed of both original existing pore water and the water from decomposed hydrates. Hydrate contents as measured by diffraction vary between 0 and 68 wt.% in the samples we measured. Samples with low hydrate content usually show micro-structural features in cryo-SEM ascribed to extensive decomposition. Comparing the appearance of hydrates at different scales, the grade of preservation seems to be primarily correlated with the contiguous volume of the original existing hydrate; the dissociation front appears to be indicated by micrometer-sized pores in a dense ice matrix.

Carbonat-free sediment components of ODP Leg 115 holes

From the equatorial Indian Ocean, carbonate-free portions of sediment samples of Paleocene to Miocene calcareous oozes and chalks from Sites 707, 709, and 711 were studied using X-ray diffraction measurements and the scanning electron microscope. Downhole variations in biogenic opal, quartz, barite, and clinoptilolite were investigated. The abundance patterns of these major mineral phases show several similarities and may be used for additional lithologic correlations. Variations in biogenic opal contents reflect biogenic silica productivity. Beside the general pattern, a succession in biogenic silica decrease through time is generally recorded since the Oligocene. This succession started earliest at northernmost Site 711 and latest at southernmost Site 707, including Site 709 within these two. Opal-A variations as well as the barite distribution may be influenced by the paleoposition of the sites in relation to the high-productivity zone, which today lies south of the equator. Authigenic clinoptilolite apparently formed in two different modes. In deeper sediment intervals, clinoptilolite was the last mineral phase formed associated with enhanced silica diagenesis. In late Oligocene to middle Miocene sediments, clinoptilolite was the only authigenic silica phase encountered where otherwise strong opal dissolution was observed. The sponge spicules showed special dissolution features probably related to microbiological activity. Silica concretions mainly composed of opal-CT and authigenic quartz occur in carbonate-rich environments and are formed during later diagenesis when burial depth causes the sediments to reach higher temperatures. Opal-CT concretions in carbonate-free siliceous oozes were found at Site 711 and are probably formed during an early stage of silica diagenesis.

(Table 1, page 376), Composition of manganese deposits from the Gulf of Aden and the Carlsberg Ridge

Iron-manganese nodules from the ocean floor have been extensively studied. But, because of the fine grain size of the particles of the nodules, structural identification by X-ray and electron diffraction techniques is difficult and the mineralogy of the iron oxide phase has not been well characterized. The observation of the Mössbauer spectrum-in which each nucleus absorbs gamma-rays independently-is not limited by particle size in the same way as is the observation of Bragg peaks in diffraction measurements, in which radiation must be scattered coherently from a large number of atoms. The magnetic hyperfine splitting in the Mössbauer spectrum of magnetic materials is affected, however, when the particles are so small that they become superparamagnetic. We describe here an investigation using the 57Fe Mössbauer effect of two iron-manganese nodules in which the iron oxide phase could not be detected by X-ray or electron diffraction.

Diurnal and seasonal measurements of seawater chemistry, temperature and PAR in Hurricane Hole, U.S. Virgin Islands - 2010-2012

Risk analyses indicate that more than 90% of the world's reefs will be threatened by climate change and local anthropogenic impacts by the year 2030 under "business-as-usual" climate scenarios. Increasing temperatures and solar radiation cause coral bleaching that has resulted in extensive coral mortality. Increasing carbon dioxide reduces seawater pH, slows coral growth, and may cause loss of reef structure. Management strategies include establishment of marine protected areas with environmental conditions that promote reef resiliency. However, few resilient reefs have been identified, and resiliency factors are poorly defined. Here we characterize the first natural, non-reef coral refuge from thermal stress and ocean acidification and identify resiliency factors for mangrove-coral habitats. We measured diurnal and seasonal variations in temperature, salinity, photosynthetically active radiation (PAR), and seawater chemistry; characterized substrate parameters; and examined water circulation patterns in mangrove communities where scleractinian corals are growing attached to and under mangrove prop roots in Hurricane Hole, St. John, US Virgin Islands. Additionally, we inventoried the coral species and quantified incidences of coral bleaching, mortality, and recovery for two major reef-building corals, Colpophyllia natans and Diploria labyrinthiformis, growing in mangrove-shaded and exposed (unshaded) areas. Over 30 species of scleractinian corals were growing in association with mangroves. Corals were thriving in low-light (more than 70% attenuation of incident PAR) from mangrove shading and at higher temperatures than nearby reef tract corals. A higher percentage of C. natans colonies were living shaded by mangroves, and no shaded colonies were bleached. Fewer D. labyrinthiformis colonies were shaded by mangroves, however more unshaded colonies were bleached. A combination of substrate and habitat heterogeneity, proximity of different habitat types, hydrographic conditions, and biological influences on seawater chemistry generate chemical conditions that buffer against ocean acidification. This previously undocumented refuge for corals provides evidence for adaptation of coastal organisms and ecosystem transition due to recent climate change. Identifying and protecting other natural, non-reef coral refuges is critical for sustaining corals and other reef species into the future.

Thermomagnetic measurements on sediment cores GeoB1523-1, GeoB2910-1 and Geob4313-2

Low-temperature rock magnetic measurements have distinct diagnostic value. However, in most bulk marine sediments the concentration of ferrimagnetic and antiferromagnetic minerals is extremely low, so even sensitive instrumentation often responds to the paramagnetic contribution of the silicate matrix in the residual field of the magnetometer. Analysis of magnetic extracts is usually performed to solve the problems raised by low magnetic concentrations. Additionally magnetic extracts can be used for several other analyses, for example electron microscopy or X-ray diffraction. The magnetic extraction technique is generally sufficient for sediments dominated by magnetite. In this study however, we show that high-coercivity components are rather underrepresented in magnetic extracts of sediments with a more complex magnetic mineralogy. We test heavy liquid separation, using hydrophilic sodium polytungstenate solution Na6[H2W12O40], to demonstrate the efficiencies of both concentration techniques. Low-temperature cycling of zero-field-cooled, field-cooled and saturation isothermal remanent magnetization acquired at room temperature was performed on dry bulk sediments, magnetic extracts, and heavy liquid separates of clay-rich pelagic sediments originating from the Equatorial Atlantic. The results of the thermomagnetic measurements clarify that magnetic extraction favours components with high spontaneous magnetization, such as magnetite and titanomagnetite. The heavy liquid separation is unbiased with respect to high- and low-coercive minerals, thus it represents the entire magnetic assemblage.

Age model, isotope analyses, geochemical measurements and grain size analyses of sediment core GeoB9508-5 from the Senegal River Mouth

The influence of the large-scale ocean circulation on Sahel rainfall is elusive because of the shortness of the observational record. We reconstructed the history of eolian and fluvial sedimentation on the continental slope off Senegal during the past 57,000 years. Our data show that abrupt onsets of arid conditions in the West African Sahel were linked to cold North Atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with Heinrich Stadials. Climate modeling suggests that this drying is induced by a southward shift of the West African monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric African Easterly Jet.

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.

Pollen analysis, particle size analysis, end-member modelling and x-ray fluorescence measurements from sediment cores off Cape Ghir, Morocco

Understanding past human-climate-environment interactions is essential for assessing the vulnerability of landscapes and ecosystems to future climate change. This is particularly important in southern Morocco where the current vegetation is impacted by pastoralism, and the region is highly sensitive to climate variability. Here, we present a 2000-year record of vegetation, sedimentation rate, XRF chemical element intensities, and particle size from two decadal-resolved, marine sediment cores, raised from offshore Cape Ghir, southern Morocco. The results show that between 650 and 850 AD the sedimentation rate increased dramatically from 100 cm/1000 years to 300 cm/1000 years, and the Fe/Ca and pollen flux doubled, together indicating higher inputs of terrestrial sediment. Particle size measurements and end-member modelling suggest increased fluvial transport of the sediment. Beginning at 650 AD pollen levels from Cichorioideae species show a sharp rise from 10% to 20%. Pollen from Atemisia and Plantago, also increase from this time. Deciduous oak pollen percentages show a decline, whereas those of evergreen oak barely change. The abrupt increase in terrestrial/fluvial input from 650 to 850 AD occurs, within the age uncertainty, of the arrival of Islam (Islamisation) in Morocco at around 700 AD. Historical evidence suggests Islamisation led to population increase and development of southern Morocco, including expanded pastoralism, deforestation and agriculture. Livestock pressure may have changed the vegetation structure, accounting for the increase in pollen from Cichorioideae, Plantago, and Artemisia, which include many weedy species. Goats in particular may have played a dominant role as agents of erosion, and intense browsing may have led to the decline in deciduous oak; evergreen oak is more likely to survive as it re-sprouts more vigorously after browsing. From 850 AD to present sedimentation rates, Fe/Ca ratios and fluvial discharge remain stable, whereas pollen results suggest continued degradation. Pollen results from the past 150 years suggest expanded cultivation of olives and the native argan tree, and the introduction of Australian eucalyptus trees. The rapidly increasing population in southern Morocco is causing continued pressure to expand pastoralism and agriculture. The history of land degradation presented here suggests that the vegetation in southern Morocco may have been degraded for a longer period than previously thought and may be particularly sensitive to further land use changes. These results should be included in land management strategies for southern Morocco.