Geo-Marine Letters

This study compares bathymetry extracted from 3D seismic data at two Australian study sites of differing morphological complexities to two sources of collocated multibeam data: 50-m and 5-m multibeam digital bathymetric models (DBMs). Seafloor horizons are extracted from the 3D seismic data and converted to depth using sound velocity profiles collected during seismic acquisition. The resulting seismic-derived DBMs are independent of the multibeam DBMs and are shown to be highly comparable. For the morphologically simple site, the seismic-derived DBM was within ± 2% of the multibeam DBMs and, at 2σ, 95% of differences are in the range − 1.22 to 0.10% (− 1.02 to 0.48%) for the 50-m (5-m) multibeam DBM. For the morphologically complex site, > 80% (> 99%) of seismic-derived depths were within ± 2% (± 5%) of multibeam DBMs. At 2σ, 94% of differences are in the range − 3.48 to 1.69% (− 2.73% to 2.44%) for the 50-m (5-m) multibeam DBM. Increasing morphological complexity and slope angle were the most important factors affecting DBM comparisons, with seismic-derived depths typically underestimated in canyon thalwegs. Despite these differences, the higher data density, multichannel stacking and migration of the 3D seismic data resulted in seismic-derived DBMs with high resolution and improved feature relief and clarity when compared to multibeam DBMs for the conditions in this study (depths of 120–1900 m), particularly for morphological features such as individual rills and gullies. This method has the potential to expand the spatial coverage of high-resolution DBMs, for example, in Australia, by over 150,000 km2.

The Mississippi Fan is a large, mud-dominated submarine fan over 4 km thick, deposited in the deep Gulf of Mexico during the late Pliocene and Pleistocene. Analysis of 19,000 km of multifold seismic data defined 17 seismic sequences, each characterized by channel, levee, and associated overbank deposits, as well as mass transport deposits. At the base of nine sequences are a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections, which constitute 10–20% of the sediments in each the sequences. These facies are externally mounded and occur in two general regions of the fan: (1) in the upper and middle fan they are elongate in shape and mimic the channel's distribution; (2) in the middle fan to lower fan they are characterized by a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows, and turbidites (informally called “mass transport complexes”). Overlying this basal interval, characteristic of all sequences, are well-developed channel-levee systems that constitute 80–90% of the fan's sediments. Channels consist of high amplitude, subparallel reflections, whereas the flanking levee sediments appear as subparallel reflections that have high amplitudes at the base changing upward to low amplitude. The vertical change in amplitude may reflect a decrease in grain size and bed thicknesses. Overbank sediments are characterized by interbedded subparallel to hummocky and mounded reflections, suggesting both turbidites from the channel, as well as slides and debris flows derived both locally and from the slope updip.

X-ray diffraction analyses show that the clay mineralogies of near-surface muds in the Gulf of Alaska (mostly illite and chlorite) are consistent with detrital sources in southern Alaska. Expandable clay minerals are derived from the Aleutian volcanic arc, and their percentages increase progressively toward the west. Smectite values are lower than expected, however, particularly in the central forearc, and there is less smectite on the insular trench slope than farther seaward. The regional clay-mineral distribution is controlled by two opposed contour currents and by the influx of suspended sediment via both transverse and trench-axis turbidity currents.

Authigenic carbonates from outcrops of the northern Apennines consist of small and irregular lenses and exhibit numerous features indicative of cold-seep settings. Detailed petrographic, mineralogical and geochemical studies from two Miocene deposits are presented. The first carbonate outcrop, named Fosso Riconi, is located in the foredeep basin of the Apenninic chain, whereas the second deposit represents a satellite basin called Sarsetta. The stable isotope data from specific carbonate minerals show a wide range of values well known from other palaeoseeps of the Apennine Mountains. The majority of seep carbonates are formed by low-Mg calcite and ankerite. Those minerals have δ13C values between −7 and −23‰ V-PDB, suggesting variable amounts of carbonate derived from oxidized methane, seawater (dissolved inorganic carbon) and sedimentary organic matter. Dolomite samples have the lowest δ13C values (−30.8 to −39.0‰ V-PDB), indicating methane as the main carbon source. The findings suggest an evolutionary formation of the seeps and development of authigenic carbonates influenced by the activity of chemosynthetic organisms, of which large lucinid clams are preserved. Bioirrigation by the clams controlled the sediment–water exchange, and is here considered as an explanation for the anomalous Mg content of the calcite. We hypothesize that the seep carbonates were formed during periods of active methane-rich seepage, whereas during periods of slow seepage carbonate formation was reduced. Despite different geological settings, the two examined deposits of Sarsetta and Fosso Riconi show similar features, suggesting that a common pattern of fluid circulation played a major role in carbonate formation at both seep sites.

Mercury, cadmium, arsenic, and antimony were analyzed in cores sampled on the Azores-Iceland Ridge. High values of 780 μg · kg−1 for Hg, 1.7 μg · g−1 for Cd, 87 μg · g−1 for As, and 8.1 μg · g−1 for Sb occur in the rift valley and transform faults. These enrichments, strictly linked to the ridge, could not have an allochtonous origin. A local hydrogenous flux may explain this phenomenon. These metallic enrichments may be connected to a hydrothermal activity extended between 43° N and 47° N.

Intermittently laminated shale of the Jemtland Formation in Maine is characterized by thin lenticular silt segregations interlaced with argillaceous and organic material (including graptolites). This shale is thinly interbedded with nonlaminated shale, siltstone, and thicker turbidite graywacke beds. Experiments suggest that the intermittently laminated shale was deposited by silt/clay-laden currents and may have been part of an upward turbidite progression from parallel-laminated silt (>60% silt), through intermittently laminated mud (40 to 60% silt), to nonlaminated mud (20 to 40% silt). Intermittently laminated mud may be produced from silt/clay flows that are: decelerating at a constant silt content, losing silt at constant velocity; or both decelerating and losing silt.

The semi-arid shelf of Northeastern Brazil, adjacent to the State of Ceará, was analyzed using high-resolution seismic imaging, side-scan sonar imaging, sedimentary patterns, remotely operated vehicle (ROV), scuba dive surveys, and satellite images. Three geomorphic sectors were found according to physiographic, sedimentological, and morphological aspects: Coreaú, Mundaú, and Jaguaribe. The Ceará shelf is characterized by a mixed sedimentological pattern including a significant modern carbonate supply, relict siliciclastic grains, and a mixing of carbonates and siliciclastics. The modern siliciclastic supply is concentrated near the coast (shoreface/shoaling zone) and solid discharge from rivers is low due to a semi-arid climate. The subaqueous features (dunes, ripples, reefs, beachrocks, and incised valleys), both relict and modern, formed both as a result of sea-level changes (Wisconsin Glacial and Holocene transgression) and modern processes (longshore and wind-driven currents, waves, and tides).

A spectacular hummocky topography was discovered offshore of the south-eastern slope of the Nisyros island volcano in the eastern sector of the Aegean volcanic arc in 2000–2001, using multibeam bathymetric mapping and seismic profiling, and interpreted as part of a volcanic debris avalanche originating onland. During E/V Nautilus cruise NA011 in 2010, a detailed side-scan sonar and ROV exploration aimed at evaluating the surface morphology of this avalanche field. Combining the new data with selected older datasets reveals that the debris avalanche is characterized by numerous (at least 78) variously sized and shaped hummocks. Some of these are distinctly round, either scattered or aligned in groups, whereas others are elongated in the form of ridges. This is consistent with existing models accounting for variations in the longitudinal and lateral velocity ratio of landslides. Maximum dimensions reach 60 m in height above the sea bottom, 220 m in length and 230 m in width. The structures outline a large tongue-shaped, submarine hummock field of about 22.2 km2, approx. 4.8 km wide and 4.6 km long and with an estimated volume of 0.277 km3. Due to its characteristic shape, the collapsed volcanic flank is interpreted to represent a singular failing event, involving a rapid and virtually instantaneous downslope movement of the slide mass into the sea. Indeed, the H/L (height of 280 m vs. run-out of 7 km) ratio for the Nisyros slide is 0.04; plotted against volume, this falls within the theoretical bounds as well as measured values typical of submarine landslides. The timing of the event is probably related to the extrusion of Nikia lavas and their subsequent failure and formation of a main scarp observed at about 120 m depth on an 8-km-long seismic profile and a map of slope angle distribution, at the depth where the palaeo-coastline was located 40 ka ago. An inferred age of ca. 40 ka for the avalanche awaits confirmation based on dating of core material.