Late Miocene to Early Pliocene radiolarians from ODP Hole 178-1095B

Early Pliocene to middle late Miocene hemipelagic and distal turbidite sediments from Hole 1095B, near the Antarctic Peninsula, yield moderately abundant, moderately well preserved radiolarian faunas and other biosiliceous material (diatoms, silicoflagellates, and sponge spicules). Preservation characteristics, however, vary strongly even between closely related samples, and there are many intervals of poor preservation. In the 140- to 460-meters below seafloor interval studied, it was possible to identify the following standard Southern Ocean radiolarian zones: Upsilon, Tau, Amphymenium challengerae, Acrosphaera? labrata, Siphonosphaera vesuvius, and upper Acrosphaera australis (total age range ~4-10 Ma). Some normally common radiolarian groups, such as actinommids, are unusually rare in the studied material, and the relative ranges of several individual species, such as Acrosphaera labrata vs. A. australis, appear to be somewhat anomalous. These observations imply that the ranges of taxa in this section may be somewhat diachronous, due to either local ecologic factors and/or the highly variable preservation of the faunas. Thus, the ages of events reported are probably only approximate, although they are still useful for constraining the age of sediments in this section.

Gridded bathymetry from multibeam echosounder EM12S data of the cruise TTR6/2 (1996)

Bathymetry based on data recorded during TTR6 between 05.07.1996 and 20.08.1996 in the Black Sea. In the central Black Sea, the aim of TTR-6 the investigation was a bathymetric map of a field of mud volcanoes known from the previous TTR cruises, during transit. In the Sorokin Trough, where gas hydrates were earlier obtained from sea bottom sediments the primary goals concerned the elucidation of the structure of clay diapiric folds and the searching for mud volcanoes and other evidence for fluid flux through the seafloor. The task of looking for the seafloor manifestation of deep fluid emanation was set in the Pallas Uplift area. The EM12s surveying on the Caucasian margin was aimed at the construction of the first detailed bathymetric map of this area.

Carbon and carbonate concentrations of recent and Pleistocene sediments of the Norwegian-Greenland Sea

Surface sediment samples from the Norwegian-Greenland Sea were investigated to reconstruct the spatial distribution of recent carbonate dissolution on the seafloor. Additionally, carbonate dissolution records of Ocean Drilling Program sites 985 and 987 are presented to outline the development of Pleistocene carbonate preservation. Today, well-preserved carbonate tests can be observed along the inflow of warm Atlantic surface water, extending as far as into the northernmost Norwegian-Greenland Sea. Increased dissolution is indicated along the continental margins and in the deepest parts of the Greenland Basin. Factors favoring carbonate preservation were found to be supersaturation of the water column with respect to calcium carbonate, high carbonate rain and probably excess alkalinity of bottom waters supplied by the arctic river discharge. Supralysoklinal dissolution is most important for recent carbonate dissolution in the Norwegian-Greenland Sea, whereas the deepest parts of the Greenland Basin reaches the calcite saturation horizon. Pleistocene dissolution records show some prominent peaks of extreme carbonate dissolution. During the Brunhes chron, carbonate dissolution maxima can be related to meltwater pulses, which probably inhibited deep-water formation in the Norwegian-Greenland Sea during deglaciation events. Long-term severe carbonate dissolution is evident during the late Matuyama chron. This can be probably related to low carbonate rain, due to a more eastwards located East Greenland Current and the nearly absence of the not yet polar adapted Neogloboquadrina pachyderma sin. during that period. Extreme dissolution events during the late Matuyama indicate strongly reduced deep-water formation.

Age determination of sediment cores from the Northeast Atlantic

Radiocarbon dating series, bulk sediment, and organic carbon flux from various Atlantic deep-sea regions reveal that the thickness of the bioturbated zone increases by 2 cm if food supply increases by 1 gC/m**2/yr (r = 0.8). Bulk sediment accumulation rates do not influence the depth of bioturbational mixing under normal pelagic sedimentary conditions. We believe that this relationship between nutrient supply and benthic mixing can be used for a quantitative and time-variable unmixing procedure to improve high-resolution stratigraphic correlations and paleoclimatic interpretations of deep-sea records.

Profile of sediment echo sounding with links to ParaSound data files and swath sonar multibeam bathymetry during METEOR cruise M70/2b

Two highly active mud volcanoes located in 990-1,265 m water depths were mapped on the northern Egyptian continental slope during the BIONIL expedition of R/V Meteor in October 2006. High-resolution swath bathymetry and backscatter imagery were acquired with an autonomous underwater vehicle (AUV)-mounted multibeam echosounder, operating at a frequency of 200 kHz. Data allowed for the construction of ~1 m pixel bathymetry and backscatter maps. The newly produced maps provide details of the seabed morphology and texture, and insights into the formation of the two mud volcanoes. They also contain key indicators on the distribution of seepage and its tectonic control. The acquisition of high-resolution seafloor bathymetry and acoustic imagery maps with an AUV-mounted multibeam echosounder fills the gap in spatial scale between conventional multibeam data collected from a surface vessel and in situ video observations made from a manned submersible or a remotely operating vehicle.

(Table 1) Heat flow data in the western Black Sea

Results of heat flow measurements are presented. On the basis of new data on structure of the sedimentary sequence, corrections are introduced that take account of effect of sedimentation. Diagrammatic maps of distribution of observed and deep-seated heat flow have been constructed. A hypothesis is offered that the regional zone of anomalously high heat-flow values on the northern continental slope has been controlled by processes of subsidence of an oceanic plate beneath its continental counterpart.

Lithology of sediments from the Arabian Sea

The role of hotter than ambient plume mantle in the formation of a rifted volcanic margin in the northern Arabian Sea is investigated using subsidence analysis of a drill site located on the seismically defined Somnath volcanic ridge. The ridge has experienced >4 km of subsidence since 65 Ma and lies within oceanic lithosphere. We estimate crustal thickness to be 9.5-11.5 km. Curiously <400 m of the thermal subsidence occurred prior to 37 Ma, when subsidence rates would normally be at a maximum. We reject the hypothesis that this was caused by increasing plume dynamic support after continental break-up because the size of the thermal anomalies required are unrealistic (>600°C), especially considering the rapid northward drift of India relative to the Deccan-Réunion hotspot. We suggest that this reflects very slow lithospheric growth, possibly caused by vigorous asthenospheric convection lasting >28 m.y., and induced by the steep continent-ocean boundary. Post-rift slow subsidence is also recognized on volcanic margins in the NE Atlantic and SE Newfoundland and cannot be used as a unique indicator of plume mantle involvement in continental break-up.

Biogenic silica and silicic acid benthic fluxes of a North-eastern Atlantic Abyssal Locality

Within the framework of the EU-funded BENGAL programme, the effects of seasonality on biogenic silica early diagenesis have been studied at the Porcupine Abyssal Plain (PAP), an abyssal locality located in the northeast Atlantic Ocean. Nine cruises were carried out between August 1996 and August 1998. Silicic acid (DSi) increased downward from 46.2 to 213 µM (mean of 27 profiles). Biogenic silica (BSi) decreased from ca. 2% near the sediment-water interface to <1% at depth. Benthic silicic acid fluxes as measured from benthic chambers were close to those estimated from non-linear DSi porewater gradients. Some 90% of the dissolution occurred within the top 5.5 cm of the sediment column, rather than at the sediment-water interface and the annual DSi efflux was close to 0.057 mol Si/m**2/yr. Biogenic silica accumulation was close to 0.008 mol Si/m**2/yr and the annual opal delivery reconstructed from sedimentary fluxes, assuming steady state, was 0.065 mol Si/m**2/yr. This is in good agreement with the mean annual opal flux determined from sediment trap samples, averaged over the last decade (0.062 mol Si/m**2/yr). Thus ca. 12% of the opal flux delivered to the seafloor get preserved in the sediments. A simple comparison between the sedimentation rate and the dissolution rate in the uppermost 5.5 cm of the sediment column suggests that there should be no accumulation of opal in PAP sediments. However, by combining the BENGAL high sampling frequency with our experimental results on BSi dissolution, we conclude that non-steady state processes associated with the seasonal deposition of fresh biogenic particles may well play a fundamental role in the preservation of BSi in these sediments. This comes about though the way seasonal variability affects the quality of the biogenic matter reaching the seafloor. Hence it influences the intrinsic dissolution properties of the opal at the seafloor and also the part played by non-local mixing events by ensuring the rapid transport of BSi particles deep into the sediment to where saturation is reached.

Contents and stable carbon isotope compositions of biomarkers indicative for AOM-communities dominated by different ANME groups in two carbonate samples from Hydrate Hole pockmark

Different types of seep carbonates were recovered from the 'Kouilou pockmarks' on the Congo deep-sea fan in approximately 3100 m water depth. The carbonate aggregates are represented by pyritiferous nodules, crusts and slabs, tubes, and filled molds. The latter are interpreted to represent casts of former burrows of bivalves and holothurians. The nodules consisting of high-Mg-calcite apparently formed deeper within the sediments than the predominantly aragonitic crusts and slabs. Nodule formation was caused by anaerobic oxidation of methane dominantly involving archaea of the phylogenetic ANME-1 group, whereas aragonitic crusts resulted from the activity of archaea of the ANME-2 cluster. Evidence for this correlation is based on the distribution of specific biomarkers in the two types of carbonate aggregates, showing higher hydroxyarchaeol to archaeol ratios in the crusts as opposed to nodules. Formation of crusts closer to the seafloor than nodules is indicated by higher carbonate contents of crusts, probably reflecting higher porosities of the host sediment during carbonate formation. This finding is supported by lower d18O values of crusts, agreeing with precipitation from pore waters similar in composition to seawater. The aragonitic mineralogy of the crusts is also in accord with precipitation from sulfate-rich pore waters similar to seawater. Moreover, the interpretation regarding the relative depth of formation of crusts and nodules agrees with the commonly observed pattern that ANME-1 archaea tend to occur deeper in the sediment than members of the ANME-2 group. Methane represents the predominant carbon source of all carbonates (d13C values as low as -58.9 per mil V-PDB) and the encrusted archaeal biomarkers (d13C values as low as -140 per mil V-PDB). Oxygen isotope values of some nodular carbonates, ranging from + 3.9 to + 5.1per mil V-PDB, are too high for precipitation in equilibrium with seawater, probably reflecting the destabilization of gas hydrates, which are particularly abundant at the Kouilou pockmarks.

(Table T1) Geochemical composition of xenoliths and host rocks from ODP Hole 193-1188A and dredge haul MD-138, PACMANUS field

A number of intensely altered, dark xenoliths with palimpsest quench textures were recorded within altered dacitic host rocks at Site 1189 (Roman Ruins, PACMANUS) during Ocean Drilling Program (ODP) Leg 193. Several of these displayed puzzling marginal fringes, apparently of altered plagioclase with variolitic texture, protruding into adjacent host rocks. Despite their alteration, the xenoliths were interpreted as fragments of rapidly chilled, possibly olivine-bearing basalts incorporated into the dacitic magmas either within the crustal plumbing system or during eruption at the seafloor (figures F15, F16, F17, F42, and F43 in Shipboard Scientific Party, 2002, doi:10.2973/odp.proc.ir.193.104.2002). An additional example of formerly spinifex-textured xenolith, the first from Site 1188 (Snowcap) and the first from the upper cristobalite-bearing zone of alteration, has been revealed by postcruise studies. Furthermore, a pristine sample of the parent lithology has been found within a dredge haul (MD-138, Binatang-2000 Cruise of Franklin; 3°43.60'S, 151°40.35'E, 1688 meters below sea level) from the Satanic Mills hydrothermal field at PACMANUS, near ODP Site 1191. The purpose of this report is to document these discoveries and thereby to confirm and build on shipboard interpretations. To my knowledge, similar xenoliths have never before been found in modern island arc or backarc volcanic sequences. Spinifex textures are most common in Archean komatiites, some of which are bimodally associated with calc-alkaline felsic volcanic rocks.

Uranium chemistry of oceanic basalts

Measurements of uranium concentration and the 234U/238 U activity ratio in oceanic basalts which have undergone low-temperature seafloor alteration indicate that uranium uptake is a pervasive occurrence but that the various phases involved behave differently with respect to this process. Palagonite exhibits uranium contents 8-20 times higher than unaltered glass coupled with low 234U/238U, suggesting ongoing preferential leaching of 234U. Altered crystalline interiors of several old basalts have 234U/238U > 1, indicative of recent uranium exchange with seawater. The data also provide evidence for uranium sources with 234U/238U higher than the seawater value of 1.14. Manganese crusts on basalts of a variety of ages have isotopic ratios indicating that they either are recent deposits or also have experienced continuing uranium exchange with seawater.

Magnetostratigraphy of ODP Leg 138 sites

The aims of this study are twofold. First, the study tries to provide the most reliable chronology possible for two critical sections by correlating the magnetic polarity stratigraphy measured in these sediments with a newly revised geomagnetic polarity time scale. Second, this study attempts to examine in detail the nature of seven short events not included in the shipboard standard time scale, but for which abundant magnetostratigraphic evidence was obtained during the Leg. Data presented here force some modifications of the shipboard interpretations of the magnetostratigraphy of Sites 845 and 844 on the basis of new data generated using discrete samples and from a greater appreciation of the magnetostratigraphic signature of Miocene-age short events. Those short events can be classified into two groups: those that probably reflect short, full-polarity intervals and those that more likely represent an interval of diminished geomagnetic intensity. Three of the seven events documented here correspond well with three subtle features, as seen in marine magnetic profiles, that have been newly included in the geomagnetic polarity time scale as short, full-polarity chrons. One of the seven events corresponds to a poorly defined feature of the marine magnetic record that has also been newly included in the geomagnetic polarity time scale, but which was considered of enigmatic origin. The three remaining events investigated here, although they have not been identified with features in the seafloor magnetic record, are suggested to be events of a similar nature, most likely times of anomalously low geomagnetic intensity. In addition to the Miocene magnetostratigraphic results given, several sets of averaged paleomagnetic inclinations are presented. Although these results clearly show the effects of a residual coring overprint, they demonstrate that paleomagnetic estimates of paleolatitudes can be made which are in good general agreement with ancient site positions calculated using hot spot-based plate reconstructions.

Ocean bottom seismometer sgy-files of refraction seismic profiles from Professor Logachev cruise TTR16/3

Over the last decade pockmarks have proven to be important seabed features that provide information about fluid flow on continental margins. Their formation and dynamics are still poorly constrained due to the lack of proper three dimensional imaging of their internal structure. Numerous fluid escape features provide evidence for an active fluid-flow system on the Norwegian margin, specifically in the Nyegga region. In June-July 2006 a high-resolution seismic experiment using Ocean Bottom Seismometers (OBS) was carried out to investigate the detailed 3D structure of a pockmark named G11 in the region. An array of 14 OBS was deployed across the pockmark with 1 m location accuracy. Shots fired from surface towed mini GI guns were also recorded on a near surface hydrophone streamer. Several reflectors of high amplitude and reverse polarity are observed on the profiles indicating the presence of gas. Gas hydrates were recovered with gravity cores from less than a meter below the seafloor during the cruise. Indications of gas at shallow depths in the hydrate stability field show that methane is able to escape through the water-saturated sediments in the chimney without being entirely converted into gas hydrate. An initial 2D raytraced forward model of some of the P wave data along a line running NE-SW across the G11 pockmark shows, a gradual increase in velocity between the seafloor and a gas charged zone lying at ~300 m depth below the seabed. The traveltime fit is improved if the pockmark is underlain by velocities higher than in the surrounding layer corresponding to a pipe which ascends from the gas zone, to where it terminates in the pockmark as seen in the reflection profiles. This could be due to the presence of hydrates or carbonates within the sediments.