Data report: bulk mineralogical composition of Cascadia margin sediments, IODP Expedition 311

The bulk mineralogical compositions of sediment samples from Sites U1325–U1329, northern Cascadia margin, were determined by X-ray diffraction analyses. The results document a highly variable mineralogy dominated by detrital components, mainly quartz, feldspar, mica, and clay minerals. Carbonate minerals are present only in minor amounts (<5 wt%). The long-term variations in the relative abundance of each mineral correspond to the particular lithostratigraphic units

Pliocene-Quaternary mass wasting along the Ionian Calabrian margin, offshore southern Italy

The Ionian Calabrian margin, offshore southern Italy, is a tectonically active area, located above a subduction zone dominated by the rollback of the African plate. A variety of mass wasting features are known to occur along the inner continental slope, based on seafloor mapping during the Italian project MaGIC (Marine Geohazards Along the Italian Coasts). New high-resolution geophysical data are available from a wider area following two surveys, in 2014 of the German RV Meteor, which acquired multibeam bathymetry (50 m DTM) and Parasound sub-bottom profiles, and in 2015 of the Italian RV OGS Explora, which acquired Chirp sub-bottom and multichannel seismic reflection profiles. Here we integrate these data with existing geophysical datasets and published exploration wells to map submarine slope failures and mass wasting deposits within the Pliocene-Quaternary succession. The results show that features of mass failures are widespread along the steep (higher than 10\ub0) slopes of the Ionian margin south of Calabria and within the intra-slope basins of the margin east of Calabria. Seafloor features range from small-scale features (hundreds of meters in extent), mainly located on the canyon headwalls and sidewalls, to larger slides ( up to 10 km in extent) on open slopes. Subsurface profiles across open slopes and intra-slope basins provide evidence of repeated failures, particularly in the upper Quaternary. The stratigraphic distribution of failures suggests that widespread mass wasting features occur above an unconformity tentatively dated to the Middle Pleistocene (<1 Ma). This unconformity also provides a lower bound for the onset of canyon formation. We infer that the onset of both mass wasting and canyon formation could be a response to the rapid km-scale differential uplift of Calabria over last 1 Ma, which has driven a seaward tilting of the Ionian Calabrian margin

Target mass number dependence of subthreshold antiproton production in proton-, deuteron- and alpha-particle-induced reactions

Data from KEK on subthreshold \bar{\mrm{p}} as well as on $\pi^\pm$ and \mrm{K}^\pm production in proton-, deuteron- and $\alpha$-induced reactions at energies between 2.0 and 12.0 A GeV for C, Cu and Pb targets are described within a unified approach. We use a model which considers a nuclear reaction as an incoherent sum over collisions of varying numbers of projectile and target nucleons. It samples complete events and thus allows for the simultaneous consideration of all final particles including the decay products of the nuclear residues. The enormous enhancement of the \bar{\mrm{p}} cross section, as well as the moderate increase of meson production in deuteron and $\alpha$ induced compared to proton-induced reactions, is well reproduced for all target nuclei. In our approach, the observed enhancement near the production threshold is mainly due to the contributions from the interactions of few-nucleon clusters by simultaneously considering fragmentation processes of the nuclear residues. The ability of the model to reproduce the target mass dependence may be considered as a further proof of the validity of the cluster concept.Comment: 9 pages, 4 figure

Shallow Gas Hydrate Accumulations at a Nigerian Deepwater Pockmark—Quantities and Dynamics

The evolution of submarine pockmarks is often related to the ascent of fluid from the subsurface. For pockmarks located within the gas hydrate stability zone, methane oversaturation can result in the formation of gas hydrates in the sediment. An ~600 m‐wide sea floor depression in deep waters offshore Nigeria, Pockmark A, was investigated for distributions and quantities of shallow gas hydrates, origins of hydrocarbons, and time elapsed since the last major fluid ascent event. For the first time, pressure coring of shallow sediments and drilling of more than 50 m‐long cores with the sea floor drill rig MARUM‐MeBo70 were conducted in this pockmark. Unusually, high hydrate saturations of up to 51% of pore volume in the uppermost 2.5 m of sediment in the pockmark center substantiate that deepwater pockmarks are a relevant methane reservoir. Molecular and stable C and H isotopic compositions suggest that thermogenic hydrocarbons and secondary microbial methane resulting from petroleum biodegradation are injected into shallower sediments and mixed with primary microbial hydrocarbons. Two independent pore water chloride and sulfate modeling approaches suggest that a major methane migration event occurred during the past one to three centuries. A rough sea floor topography within the pockmark most likely results from combined sediment removal through ascending gas bubbles, hydrate clogging and deflection of migration pathways, gas pressure build‐up, and hydrate sea floor detachment. This study shows for the first time the chronological interrelationship between gas migration events, hydrate formation, and sea floor shaping in a deep sea pockmark

Origin and transformation of light hydrocarbons ascending at an active pockmark on Vestnesa Ridge, Arctic Ocean

We report on the geochemistry of hydrocarbons and pore waters down to 62.5 mbsf, collected by drilling with the MARUM‐MeBo70 and by gravity coring at the Lunde pockmark in the Vestnesa Ridge. Our data document the origin and transformations of volatiles feeding gas emissions previously documented in this region. Gas hydrates are present where a fracture network beneath the pockmark focusses migration of thermogenic hydrocarbons characterized by their C1/C2+ and stable isotopic compositions (δ2H‐CH4, δ13C‐CH4). Measured geothermal gradients (~80°C km‐1) and known formation temperatures (>70°C) suggest that those hydrocarbons are formed at depths >800 mbsf. A combined analytical/modeling approach, including concentration and isotopic mass balances, reveals that pockmark sediments experience diffuse migration of thermogenic hydrocarbons. However, at sites without channeled flow this appears to be limited to depths > ~50 mbsf. At all sites we document a contribution of microbial methanogenesis to the overall carbon cycle that includes a component of secondary carbonate reduction (CR) – i.e. reduction of dissolved inorganic carbon (DIC) generated by anaerobic oxidation of methane (AOM) in the uppermost methanogenic zone. AOM and CR rates are spatially variable within the pockmark and are highest at high‐flux sites. These reactions are revealed by δ13C‐DIC depletions at the sulfate‐methane interface at all sites. However, δ13C‐CH4 depletions are only observed at the low methane flux sites because changes in the isotopic composition of the overall methane pool are masked at high‐flux sites. 13C‐depletions of TOC suggest that at seeps sites, methane‐derived carbon is incorporated into de novo synthesized biomass

Textural variations in Neogene pelagic carbonate ooze at DSDP Site 593, southern Tasman Sea, and their paleoceanographic implications

Changes in Neogene sediment texture in pelagic carbonate-rich oozes on the Challenger Plateau, southern Tasman Sea, are used to infer changes in depositional paleocurrent velocities. The most obvious record of textural change is in the mud:sand ratio. Increases in the sand content are inferred to indicate a general up-core trend towards increasing winnowing of sediments resulting from increasing flow velocity of Southern Component Intermediate Water (SCIW), the forerunner of Antarctic Intermediate Water. In particular, the intervals c. 19-14.5 Ma, c. 9.5-8 Ma, and after 5 Ma are suggested to be times of increased SCIW velocity and strong sediment winnowing. Within the mud fraction, the fine silt to coarse clay sizes from 15.6 to 2 µm make the greatest contribution to the sediments and are composed of nannofossil plates. During extreme winnowing events it is the fine silt to very coarse clay material (13-3 µm) within this range that is preferentially removed, suggesting the 10 µm cohesive silt boundary reported for siliciclastic sediments does not apply to calcitic skeletal grains. The winnowed sediment comprises coccolithophore placoliths and spheres, represented by a mode at 4-7 µm. Further support for seafloor winnowing is gained from the presence in Hole 593 of a condensed sedimentary section from c. 18 to 14 Ma where the sand content increases to c. 20% of the bulk sample. Associated with the condensed section is a 6 m thick orange unit representing sediments subjected to particularly oxygen-rich, late early to early middle Miocene SCIW. Together these are inferred to indicate increased SCIW velocity resulting in winnowed sediment associated with faster arrival of oxygen-rich surface water subducted to form SCIW. Glacial development of Antarctica has been recorded from many deep-sea sites, with extreme glacials providing the mechanism to increase watermass flow. Miocene glacial zones Mi1b-Mi6 are identified in an associated oxygen isotope record from Hole 593, and correspond with times of particularly invigorated paleocirculation, bottom winnowing, and sediment textural changes