Brian D. Bornhold

Processes of marine dispersal and deposition of suspended silts off the modern mouth of the Huanghe (Yellow River)

The processes responsible for the transport and deposition of concentrated suspended silts over the delta front of the Huanghe were observed during three cruises and have been modeled numerically. Suspended sediment concentrations in the lower Huanghe average about 25 kg m−3 and exceed 200 kg m−3 during flood stage. Cruises were conducted during normal discharge conditions in spring 1985 and summer 1986, and during low-discharge storm-dominated conditions in autumn 1987. During the first two cruises, the shallow delta-front top (depth≤ 5m) was covered by a turbid water mass with suspended sediment concentrations of 1–10 kg m−3. Strong (∼1m s−1) parabathic tidal currents resuspended newly deposited muds and advected them alongshore. Near a break in slope, the turbid layers plunged beneath the ambient water and descended the delta-front slope as gravity-driven hyperpycnal underflows. In 1987 the hyperpycnal underflows occurred only during an intense strom that resuspended delta-front sediments to produce underflows with concentrations on the order of 100 kg m−3. We infer that gravity-driven underflows constitute the most important mode of suspended sediment transport across isobaths. Concentrated and channelized “point source” underflows, apparently associated with flood conditions, were not observed but were inferred from morphological evidence and were modeled numerically. Modeling results show that the Coriolis force and ambient momentum should cause appreciable curvature to the paths of underflows, while entrainment of ambient mass contributes to underflow decay. Early extinction of all underflow types is suggested by field and modeling results, and is considered to be responsible for extremely rapid delta-front deposition and for the fact that most of the sediments discharged by the Huanghe remain close to the mouth.

Morphology of a submarine slide, Kitimat Arm, British Columbia

A digitally acquired, scale-corrected side-scan sonar survey yielded high-resolution imagery of a submarine landslide in British Columbia. The landslide, in a fjord-head setting at Kitimat, was last active in 1975 and created a wide area of deformed sea floor. The sediment failure involved shallow rotational movements on the slopes of a fjord-head delta, marginal tearing, translational sliding, compressional folding, and block gliding of fjord-bottom marine clays. The slide is shallow and elongate and appears to have been produced by failure in mobile, low-strength sediments.

Hyperpycnal plumes and plume fronts over the Huanghe (Yellow River) delta front

The Huanghe (Yellow River) discharges extremely high suspended sediment concentrations (25 to 220 g/l) which favor sustained hyperpycnal plumes (underflows). Observations of weakly hyperpycnal unchannelized plumes and indirect evidence of strongly hyperpycnal channelized underflows over the delta front indicate the importance of these modes of sediment dispersal. The weakly hyperpycnal plumes occupy the entire water column over the shallow (<5 m) delta top. From a pronounced front near the break in slope at about 5 m depth, they descend over the delta-front slope as wide-spread underflows. Evidence of strongly hyperpycnal underflows was shown from subaqueous valleys partly filled with low-density mud.

Sedimentary framework of the modern Huanghe (Yellow River) delta

The geometry, stratigraphy, and structure of recently deposited Huanghe (Yellow River) Delta sediments were examined by high resolution subbottom profiles and medium-penetration boomer profiles. The results indicate that the active (post-1976) subaqueous delta advances as a single thin localized lobe with a maximum thickness of only 15 m. Calculations of sediment volumes indicate that 90% or more of the sediment supplied by the Huanghe remains within 30 km of the mouth. Sediment on the delta platform near the mouth is fine sand; elsewhere silts and clays prevail.[/p]

Surficial slides and slumps on the continental slope and rise of South West Africa: A reconnaissance study☆

Abstract Off South West Africa, a large slide and a large slump have displaced about 250 km 3 of surficial sediment from the lower continental slope onto the upper continental rise. The slide and slump scars together cover an area of about 6000 km 2 ; they affect only the upper few tens of meters of the sediment column. A debris flow extends about 250 km downslope from the slide scar, and deformed sediments extend a similar distance downslope from the slump scar. Apparently the slide and the slump were triggered during the late Pleistocene; neither of them gave rise to turbidity currents. Slides and slumps like these appear to be common on both active and passive continental margins. Their presence in these environments merits careful study in view of deep water exploration and production drilling for oil and gas.

Water Mass Conversion in the Glacial Subarctic Pacific (54°N, 148°W): Physical Constraints and the Benthic‐Planktonic Stable Isotope Record

Benthic (Uvigerina spp., Cibicidoides spp., Gyroidinoides spp.) and planktonic (N. pachyderma sinistral, G. bulloides) stable isotope records from three core sites in the central Gulf of Alaska are used to infer mixed-layer and deepwater properties of the late glacial Subarctic Pacific. Glacial-interglacial amplitudes of the planktonic δ18O records are 1.1–1.3‰, less than half the amplitude observed at core sites at similar latitudes in the North Atlantic; these data imply that a strong, negative δw anomaly existed in the glacial Subarctic mixed layer during the summer, which points to a much stronger low-salinity anomaly than exists today. If true, the upper water column in the North Pacific would have been statically more stable than today, thus suppressing convection even more efficiently. This scenario is further supported by vertical (i.e., planktic versus benthic) δ18O and δ13C gradients of >1‰, which suggest that a thermohaline link between Pacific deep waters and the Subarctic Pacific mixed layer did not exist during the late glacial. Epibenthic δ13C in the Subarctic Pacific is more negative than at tropical-subtropical Pacific sites but similar to that recorded at Southern Ocean sites, suggesting ventilation of the deep central Pacific from mid-latitude sources, e.g., from the Sea of Japan and Sea of Okhotsk. Still, convection to intermediate depths could have occurred in the Subarctic during the winter months when heat loss to the atmosphere, sea ice formation, and wind-driven upwelling of saline deep waters would have been most intense. This would be beyond the grasp of our planktonic records which only document mixed-layer temperature-salinity fields extant during the warmer seasons. Also we do not have benthic isotope records from true intermediate water depths of the Subarctic Pacific.

Massive sulfides in a sedimented rift valley, northern Juan de Fuca Ridge

A number of mounds each several hundred meters across and up to sixty meters high have been observed with SeaMARC II acoustic imagery and Seabeam bathymetry in the sediment-filled axial valley at the northern end of the Juan de Fuca Ridge. The mounds are located a few kilometers west of the eastern valley-bounding normal fault scarp where the local sediment fill is approximately 300 m thick. All of the mounds are believed to be of hydrothermal origin, and one is associated with anomalously high heat flow in excess of 1 W m−2. A piston core collected from that mound comprises coarse clastic sulfide units interbedded with sulfidic muds. Hydrothermal minerals present in the 2.3 m section include pyrrhotite, pyrite, marcasite, sphalerite, chalcopyrite, iss (intermediate solid solution in the CuFeZnS system), chalcopyrrhotite, galena, talc, barite, and amorphous silica. Mineral fabrics of the clasts indicate that the material was precipitated at or near the sea floor by mixing of hot hydrothermal fluids with cold seawater. Low concentrations of Zn, Cu, Cd, and Ag relative to those found in unsedimented ridge hydrothermal deposits, and the presence of pyrrhotite as an early phase mineral indicates that the vent fluids have been modified by reaction with sediments beneath the mound. Rapid sedimentation in a rift valley is clearly conducive to the formation of large hydrothermal mineral deposits like those believed to be present within and beneath these mounds. The relatively impermeable sediment cover insulates the crust, inhibits groundwater recharge, promotes long-lived discharge at a restricted number of sites, provides a substrate for the efficient subsurface precipitation of minerals, and through continued sedimentation, protects surficial deposits from the corrosive effects of seawater. No reliable estimate of the bulk composition of the mounds can be made with existing data, but their size is comparable to major hydrothermal mineral deposits found on land; ancient settings in which many land deposits formed are in many ways similar to the one in which the features described here are currently forming.

Active sand transport along a fjord-bottom channel, Bute Inlet, British Columbia

An underwater channel system is incised in the Holocene fjord basin sediments of Bute Inlet, British Columbia. High-resolution side-scan sonar swaths and seismic profiles reveal two channels within a zone of extensive rotational sliding on the slopes of a fjord-head delta. The channels coalesce into a single sinuous channel that extends 14 km downfjord on a 0.9° slope where it splits into two distributaries, which continue another 18 km, ending within stacked depositional lobes. Piston cores show sands in the channels and lobes in contrast to deep-water silty clays in the basin floor. The sea-floor features and sediment distributions result from active, highly mobile slide-generated sediment flows, which transport sands via the channels from the delta to the fjord basin.

Active slope failure, sediment collapse, and silt flows on the modern subaqueous Huanghe (Yellow River) delta

Post-depositional slope instability and bottom mass-movement processes strongly modify the progradational subaqueous slopes of the modern Huanghe (Yellow River) Delta. Wide, shallow gullies dissect the submarine slopes with gradients of 0.3 to 0.4°. Lower delta-front sediments experiencein situ subsidence, forming numerous collapse depressions. These processes are pronounced over much of the delta, incising and redistributing the most recently deposited silt-rich sediment. Principal causative factors include low sediment strengths created by rapid deposition in the delta during annual peak discharges from the river and severe bottom perturbations by surface storm-generated waves.

Suspended sediment advection by tidal currents off the Huanghe (Yellow River) delta

Studies to date indirectly indicate that only a small percentage of the sediment discharged by the Huanghe (Yellow River) is presently transported from the Gulf of Bohai to the Huanghai (Yellow Sea). Direct measurements in early summer 1985 show low concentrations of suspended sediment east of 119°45′E but high concentrations in Bohai Bay. Stokes drift associated with an amphidrome of the M2 tide may contribute to a northwestward transport of Huanghe sediment.