Composition of bottom sediments from Core SO201-2-101, Shirshov Ridge, Bering Sea

Analysis of lithology, grain-size composition, clay minerals, and geochemistry of Upper Pleistocene bottom sediments from the submarine Shirshov Ridge (Bering Sea) showed that the Yukon-Tanana terrane of the Central Alaska was main source area of the sediments. Sedimentary material was transported by the Yukon River through Beringia up to the shelf break, where they were entrained by a strong north-west sea current. Lithological data revealed several pulses of ice-rafted debris deposition roughly synchronous with Heinrich events and periods of weaker bottom current intensity. Based on geochemical results we distinguished intervals of an increase in paleoproductivity and extension of the oxygen minimum zone. Our results suggest that there were three stages of deposition driven by glacioeustatic sea-level fluctuations and glacial cycles in Alaska.

Magnetic and gravity measurements on North-South profiles across the Grosse Meteor Bank, North Atlantic

During the first section of the "Meteor" cruise No. 2 a profile was run from the Azores to the south across the flanks of the Mid-Atlantic Ridge with a chain of seamounts. The profile extended between the Cruiser (living) and the Hyeres seamounts, which, according to our soundings, form a connected massif, and across the centre of the Grosse Meteor Bank (30°N, 28.5 °W). These seamounts rise from a depth of more than 4000 m up till close to the surface of the sea forming there a large almost flat plateau. In the case of the Grosse Meteor Bank, this plateau has a N-S extension of approx. 30 nautical miles and an E-W extension of approx. 20 nautical miles and reaches a height of 275 m in water depth. The gravity measurements yielded a density of the topographic masses of 2.6 g/cm**3 for the Grosse Meteor Bank. Magnitude and shape of the measured free-air anomaly are very well shown in a model computation with this density. The theoretical gravity effects of the seismically detected swell of cristalline rock and of the Moho depression (mountain root) are not indicated by the observational data. It can, therefore, be assumed that the latter two neutralize each other. It seems, accordingly, that there is no local isostatic compensation of the topographic masses. Hence, the density of 2.6 g/cm**3 obtained would be about the true density of rock. In connection with the mean velocity of P waves (Aric et al., 1968) obtained by seismic refraction methods it must be concluded that the material of the 1200-4000 m thick surface layer of the Grosse Meteor Bank consists of consolidated sediments. This finding is supported by the total intensity of the Earth's magnetic field over the Grosse Meteor Bank. On the assumption of a homogeneous magnetization in the direction of the present Earth's field, the computed anomaly of the massif deviates considerably from the measured anomaly while the magnetic field of the seismically detected crystalline body is capable of interpreting the observed data. Deviating from the prevailing interpretation of the seamounts' plateau as a volcanic cone with submarine abrasion, the Grosse Meteor Bank and the seamounts in the vicinity are assumed to be of continental origin. The questions whether these seamounts submerged later on or whether the sealevel has risen subsequently are, therefore, largely nonexistent.

Layer-by-layer correlation of ODP Leg 117 sites

One hundred thirty-one marker horizons relating to the distinct and traceable layers were described for the Owen Ridge and Oman Margin sites. The correlations incorporated the calculations of true depth, corrected for coring disturbance and gas expansion. Intersite correlation of marker horizons has been improved based on color density data, measured with video densitometer, and oxygen isotope stratigraphic data. Distinct hiatuses were detected by the intersite correlation of the marker horizons in the Owen Ridge. The hiatuses are related to submarine slides induced by increasing gravitational instability for the accumulation of the pelagic sediments on the top of the Owen Ridge. The large amount of sediment supply with variable lithofacies during the glacial stages is represented by layer-bylayer correlation in the Oman Margin. The color density patterns with glacial-interglacial cycles are controlled by the balance of organic carbon content, increasing in the interglacial stages with strong upwelling induced by the southwest monsoon, and flux of terrigenous matter, increasing in the glacial stages. The present distinct climatic cycle relating to the southwest monsoon has been developed since Stage 8, 250 ka. The large amount of sediment supply in the glacial stages can be assumed as fluvial in origin from the humid Arabian Peninsula, relating to the weakened Tropical Easterly Jet, which is induced by the counter-current of the southwest monsoon and maintains the present arid climate in the north Africa and Arabian Peninsula.

Eruption of the Håkon Mosby mud volcano recorded by the long-term observatory on mud-volcano eruptions (LOOME) between 2009 and 2010

Submarine mud volcanoes are considered an important source of methane to the water column. However, the temporal variability of their fluid transport including mud and methane emissions is largely unknown. Assuming that this transport was continuous and at steady state, methane emissions were previously proposed to result from a dynamic equilibrium between upward migration and consumption at the seabed by methane-consuming microbes. Here we have investigated non-steady state situations of vigorous mud movements and their reflection in fluid flow, seabed temperature and bathymetry. Time series of pressure, temperature, pH and seafloor photography were collected by a benthic observatory (LOOME) for 431 days at the active Håkon Mosby mud volcano. These new data document eruptions, which were accompanied by pulses of hot subsurface fluids and triggered rapid sediment uplift and lateral movement, as well as emissions of free gas.

Turbidite layers in sediments of the Dakar Canyon

The relationship of sea-level changes and short-term climatic changes with turbidite deposition is poorly documented, although the mechanisms of gravity-driven sediment transport in submarine canyons during sea-level changes have been reported from many regions. This study focuses on the activity of the Dakar Canyon off southern Senegal in response to major glacial/interglacial sea-level shifts and variability in the NW-African continental climate. The sedimentary record from the canyon allows us to determine the timing of turbidite events and, on the basis of XRF-scanning element data, we have identified the climate signal at a sub-millennial time scale from the surrounding hemipelagic sediments. Over the late Quaternary the highest frequency in turbidite activity in the Dakar Canyon is confined to major climatic terminations when remobilisation of sediments from the shelf was triggered by the eustatic sea-level rise. However, episodic turbidite events coincide with the timing of Heinrich events in the North Atlantic. During these times continental climate has changed rapidly, with evidence for higher dust supply over NW Africa which has fed turbidity currents. Increased aridity and enhanced wind strength in the southern Saharan-Sahelian zone may have provided a source for this dust.

Pore water chemistry of sediment core GeoB15101-7 and water chemistry of CTD casts GeoB15101-1 and GeoB15101-6

Submarine brine lakes feature sharp and persistent concentration gradients between seawater and brine, though these should be smoothed out by free diffusion in open ocean settings. The anoxic Urania basin of the Eastern Mediterranean contains an ultra sulfidic, hypersaline brine of Messinian origin above a thick layer of suspended sediments. With a dual modeling approach we reconstruct its contemporary stratification by geochemical solute transport fundamentals, and show that thermal convection is required to maintain mixing in the brine and mud layer. The origin of the Urania basin stratification was dated to 1650 years before present, which may be linked to a major earthquake in the region. The persistence of the chemoclines may be key to the development of diverse and specialized microbial communities. Ongoing thermal convection in the fluid mud layer may have important, yet unresolved consequences for sedimentological and geochemical processes, also in similar environments.

K-Ar dating of altered deep-sea igneous rocks from DSDP Legs 2, 34, and 35

In an attempt to establish criteria for obtaining reliable K-Ar dates, conventional K-Ar studies of several Deep Sea Drilling Project sites were undertaken. K-Ar dates of these rocks may be subject to inaccuracies as the result of sea-water alteration. Inaccuracies may also result from the presence of excess radiogenic 40Ar trapped in rapidly cooled rocks at the time of their formation. The results obtained for DSDP Leg 34 basalts indicate that lowering of K-Ar dates, which is related to potassium addition by weathering, is a major cause of uncertainty in obtaining reliable K-Ar dates for deep-sea rocks. It could not be determined if the potassium addition to the basalts occurred at the time of formation, t_o, or continuously from t_o to the present. Calculations show that sediment cover is not a significant barrier to the diffusion of potassium into the basalt. 40Ar loss contributes, at least in part, to the lowering of the K-Ar date in rocks that have added potassium. The meaning of the K-Ar results obtained for DSDP Legs 35 and 2 basalts could not be unambiguously established. Because of the problems involved, caution must be used in interpreting the meaning of conventional K-Ar dates for deep-sea rocks.

Pore water concentrations and solid phase concentrations of sediment cores from the Ligurian continental slope

In October 1979, a period of heavy rainfall along the French Riviera was followed by the collapse of the Ligurian continental slope adjacent to the airport of Nice, France. A body of slope sediments, which was shortly beforehand affected by construction work south of the airport, was mobilized and traveled hundreds of kilometers downslope into the Var submarine canyon and, eventually, into the deep Ligurian basin. As a direct consequence, the construction was destroyed, seafloor cables were torn, and a small tsunami hit Antibes shortly after the failure. Hypotheses regarding the trigger mechanism include (i) vertical loading by construction of an embankment south of the airport, (ii) failure of a layer of sensitive clay within the slope sequence, and (iii) excess pore fluid pressures from charged aquifers in the underground. Over the previous decades, both the sensitive clay layers and the permeable sand and gravel layers were sampled to detect freshened waters. In 2007, the landslide scar and adjacent slopes were revisited for high-resolution seafloor mapping and systematic sampling. Results from half a dozen gravity and push cores in the shallow slope area reveal a limited zone of freshening (i.e. groundwater influence). A 100-250 m wide zone of the margin shows pore water salinities of 5-50% SW concentration and depletion in Cl, SO4, but Cr enrichment, while cores east or west of the landslide scar show regular SW profiles. Most interestingly, the three cores inside the landslide scar hint towards a complex hydrological system with at least two sources for groundwater. The aquifer system also showed strong freshening after a period of several months without significant precipitation. This freshening implies that charged coarse-grained layers represent a permanent threat to the slope's stability, not just after periods of major rainfall such as in October 1979.