• NEANIAS Underwater Research Community

[Example of collonade and entablature cooling joints within a ponded lava of the Rosebank Field Upper Volcanics revealed by formation microimager log data alongside a field analogue example from the Isle of Staffa. , Abstract The Rosebank Field is located in the Faroe‐Shetland Basin and hosts hydrocarbons within siliciclastic sediments interlayered with volcanic packages of the Late Paleocene to Early Eocene aged Flett Formation. Within this study the volcanic sequences are investigated based on an integrated appraisal of available drill cuttings, sidewall cores, core and wireline logs including image log and geochemical logs from eight wells supported by 3D seismic data. The Rosebank lower (RLV), middle (RMV) and upper (RUV) volcanic sequences are inter‐layered with Colsay Member (C1–C4) fluvial to shallow marine siliciclastic intervals. A comprehensive cross‐field borehole based lithofacies interpretation is presented characterising simple, compound and ponded effusive lava flow facies along with pillow lavas, invasive lava flows, volcaniclastic sediments and complex lava–sediment interactions. Geochemical analyses of core, sidewall core, and hand‐picked cuttings spanning the field reveal separate high‐titanium (RHT) and relatively lower‐titanium (RLT) basaltic magma suites. These compositions can be identified and correlated across much of the field utilising geochemical logging data which, in combination with the geochemical analyses, reveals a two‐part stratigraphic sub‐division of each of the RLV, RMV and RUV. Geochemical logging data is also used to define a volcanic proxy (Fe/10+Ti) which utilises the elevated iron (Fe) and titanium (Ti) within all effusive and volcaniclastic basaltic lithologies to differentiate siliciclastic from volcaniclastic sediments where other logging parameters overlap. By comparing the borehole analyses with seismic data, a localised eruptive vent is interpreted within the north of the field. Finally, a cross‐field volcanic model is presented and compared with relevant global field analogues, providing a constrained spatial framework for sub‐surface modelling of inter‐volcanic sequences.]

Surface-ship and submarine pendulum gravity measurements have been compiled in a new free air gravity anomaly map of the central Pacific Ocean in the region of Hawaii. The main features of the map are large amplitude positive anomalies (up to +700 mGal) over the Hawaiian ridge, large amplitude negative anomalies (up to −136 mGal) flanking the ridge, and a broad belt (>250 km) of positive anomalies (+25 to +50 mGal) bordering the negative anomalies. The map has been used to construct 1°×1°, 5°×5°, and 10°×10° free air anomaly averages. The main feature of the 5°×5° average map is a long-wavelength (∼2200 km) positive anomaly (up to +15 mGal) over the southeastern end of the Hawaiian ridge. A long-wavelength positive anomaly is also seen on the 10°×10° average map, which agrees well with the Gem 6 satellite-derived solution to harmonic degree 16. Computations suggest that crustal structure of the Hawaiian ridge is unlikely to contribute significantly to these long-wavelength positive anomalies. The positive anomalies correlate closely with the Hawaiian swell upon which the Hawaiian ridge is superimposed. The regression lines representing 1°×1° and 5°×5° averages of gravity against topography slope at 21 mGal/km and 22 mGal/km, respectively. These slopes are smaller than those over other regions where the lithosphere is warped for large distances, suggesting that if the swell is warped, it must be compensated. The form which the compensation takes is uncertain, but it may be related to some pattern of flow beneath the lithosphere which maintains both the swell and the associated long-wavelength gravity anomalies.

Abstract The topographic steering of the baroclinic western branch of the Norwegian Atlantic Current (NwAC) determines the extent of Atlantic Water and location of the Arctic Front in the Nordic Seas. In this paper the geographical spread of hydrographic measurements at the Ocean Weather Station M (OWSM, 66 ∘ N 2 ∘ E ) is utilized to create mean sections across the Voring Plateau Escarpment in the Norwegian Sea. In concert with a theoretical framework involving the impact of low pressure systems on frontal jets over steep bathymetry, the behaviour of the front-current system at this location is described. It is shown that the halocline and thermocline are sloped from about 200 m in the west and down to 400 m in the east over 40 km centred on the station, indicating that the western branch of the NwAC is located here. The horizontal gradients introduced by this slope are 2 ∘ C and 0.1 for salinity. The frontal slope is not seen to change its inclination on seasonal, multi-annual, nor decadal timescales, indicating that the dynamic control of this frontal slope does not change appreciably. Further supported by the theoretical framework it is shown that the subsurface part of this front and the associated western branch of the NwAC is strongly locked by topography along the Voring Plateau also on short timescales. From large scale bathymetry it is also shown how this kind of frontal locking can be expected over most of the ridges and continental slopes in the Nordic Seas.

Arctic Ocean primary productivity is limited by light and inorganic nutrients. With sea ice cover declining in recent decades, nitrate limitation has been speculated to become more prominent. Although much has been learned about nitrate supply from general patterns of ocean circulation and water column stability, a quantitative analysis requires dedicated turbulence measurements that have only started to accumulate in the last dozen years. Here we present new observations of the turbulent vertical nitrate flux in the Laptev Sea, Baffin Bay, and Young Sound (North-East Greenland), supplementing a compilation of 13 published estimates throughout the Arctic Ocean. Combining all flux estimates with a Pan-Arctic database of in situ measurements of nitrate concentration and density, we found the annual nitrate inventory to be largely determined by the strength of stratification and by bathymetry. Nitrate fluxes explained the observed regional patterns and magnitudes of both new primary production and particle export on annual scales. We argue that with few regional exceptions, vertical turbulent nitrate fluxes can be a reliable proxy of Arctic primary production accessible through autonomous and large-scale measurements. They may also provide a framework to assess nutrient limitation scenarios based on clear energetic and mass budget constraints resulting from turbulent mixing and freshwater flows.

As ice sheets grow or decay, the net flux of freshwater into the ocean changes and the bedrock adjusts due to isostatic adjustments, leading to variations in the bottom topography and the oceanic boundaries. This process was particularly intense during the last deglaciation due to the high rates of ice-sheet melting. It is, therefore, necessary to consider transient ocean bathymetry and coastlines when attempting to simulate the last deglaciation with Earth system models (ESMs). However, in most standard ESMs the land-sea mask is fixed throughout simulations because the generation of a new ocean model bathymetry implies several levels of manual corrections, a procedure that is hardly doable very often for long runs. This is one of the main technical problems towards simulating a complete glacial cycle with general circulation models. For the first time, we present a tool allowing for an automatic computation of bathymetry and land-sea mask changes in the Max Planck Institute Earth System Model (MPI-ESM). The algorithms developed in this paper can easily be adapted to any free-surface ocean model that uses the Arakawa-C grid in the horizontal and z-grid in the vertical including partial bottom cells. The strategy applied is described in detail and the algorithms are tested in a long-term simulation demonstrating the reliable behaviour. Our approach guarantees the conservation of mass and tracers at global and regional scales; that is, changes in a single grid point are only propagated regionally. The procedures presented here are an important contribution to the development of a fully coupled ice sheet–solid Earth–climate model system with time-varying topography and will allow for transient simulations of the last deglaciation considering interactive bathymetry and land-sea mask.

Abstract Shallow rivers provide important habitat for various aquatic and terrestrial species. The bathymetry of such environments is, however, difficult to measure as devices and approaches have been traditionally developed mainly for deeper waters. This study addresses the mapping of shallow water bathymetry with high spatial resolution and accuracy by comparing three remote sensing (RS) approaches: one based on echo sounding (active RS) and two on photogrammetry (passive RS): bathymetric Structure from Motion (SfM) and optical modelling. The tests were conducted on a 500 m long and ~30 m wide reach of sand-bedded meandering river: (1) during a rising spring flood (Q = 10–15 m3/s) with medium turbidity and high water color and; (2) during autumn low discharge (Q = 4 m3/s) with low turbidity and color. Each method was used to create bathymetric models. The models were compared with high precision field measurements with a mean point spacing of 0.86 m. Echo sounding provided the most accurate (ME~−0.02 m) and precise (SDE = ± 0.08 m) bathymetric models despite the high degree of interpolation needed. However, the echo sounding-based models were spatially restricted to areas deeper than 0.2 m and no small scale bathymetric variability was captured. The quality of the bathymetric SfM was highly sensitive to flow turbidity and color and therefore depth. However, bathymetric SfM suffers less from substrate variability, turbulent flow or large stones and cobbles on the river bed than optical modelling. Color and depth did affect optical model performance, but clearly less than the bathymetric SfM. The optical model accuracy improved in autumn with lower water color and turbidity (ME = −0.05) compared to spring (ME = −0.12). Correlations between the measured and modelled depth values (r = 0.96) and the models precision (SDE = 0.09–0.11) were close to those achieved with echo sounding. Shadows caused by riparian vegetation restricted the spatial extent of the optical models.

Abstract An autonomous underwater vehicle (AUV) carrying a multibeam sonar and a chirp profiler was used to map sections of the seafloor within the La Jolla Canyon, offshore southern California, at sub-meter scales. Close-up observations and sampling were conducted during remotely operated vehicle (ROV) dives. Minisparker seismic-reflection profiles from a surface ship help to define the overall geometry of the La Jolla Canyon especially with respect to the pre-canyon host sediments. The floor of the axial channel is covered with unconsolidated sand similar to the sand on the shelf near the canyon head, lacks outcrops of the pre-canyon host strata, has an almost constant slope of 1.0° and is covered with trains of crescent shaped bedforms. The presence of modern plant material entombed within these sands confirms that the axial channel is presently active. The sand on the canyon floor liquefied during vibracore collection and flowed downslope, illustrating that the sediment filling the channel can easily fail even on this gentle slope. Data from the canyon walls help constrain the age of the canyon and extent of incision. Horizontal beds of moderately cohesive fine-grained sediments exposed on the steep canyon walls are consistently less than 1.232 million years old. The lateral continuity of seismic reflectors in minisparker profiles indicate that pre-canyon host strata extend uninterrupted from outside the canyon underneath some terraces within the canyon. Evidence of abandoned channels and point bar-like deposits are noticeably absent on the inside bend of channel meanders and in the subsurface of the terraces. While vibracores from the surface of terraces contain thin (

We conclude this book by considering three challenges and opportunities that are shared by all fields of submarine geomorphological research: (i) big data at multiple spatio-temporal scales, (ii) direct observation, and (iii) interaction with subaerial geomorphologists.

A radar scatterometer operates by transmitting a pulse of microwave energy toward the ocean’s surface and measuring the normalized (per-unit-surface) radar backscatter coefficient (r8). The primary application of scatterometry is the measurement of near-surface ocean winds. By combining r 8 measurements from different azimuth angles, the 10 m vector wind can be determined through a Geophys- ical Model Function (GMF), which relates wind and backscatter. This paper proposes a mission concept for the measurement of both oceanic winds and surface currents, which makes full use of earlier C-band radar remote sensing experience. For the determination of ocean currents, in particular, the novel idea of using two chirps of opposite slope is introduced. The fundamental processing steps required to retrieve surface currents are given together with their associated accuracies. A detailed description of the mission proposal and comparisons between real and retrieved surface currents are presented. The proposed ocean Doppler scatterometer can be used to generate global surface ocean current maps with accuracies better than 0.2 m/s at a spatial resolution better than 25 km (i.e., 12.5 km spatial sampling) on a daily basis. These maps will allow gaining some insights on the upper ocean mesoscale dynamics. The work lies at a frontier, given that the present inability to measure ocean currents from space in a consistent and synoptic manner repre- sents one of the greatest weaknesses in ocean remote sensing.