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The easternmost Gulf of Finland is the recipient of the Neva River, the largest river draining to the Baltic Sea. The depositional history and endozoobenthic activity in the area have not been studied before based long sediment cores, although such studies are desirable to provide a paleoenvironmental context for the recent environmental changes and the ecosystem projections for the future Baltic Sea. Two replicate long sediment cores from the easternmost Gulf of Finland record brackish-water mud accumulation beginning at 5900±60 calendar years before present (cal. BP). The brackish-water muds are characterized by thin beds with erosional bases, fining-upward grain sizes and increasing-upward organic contents. A storm-induced flow origin is inferred for the thin mud beds, contrasting with previous interpretations that these muds accumulate by quiescent suspension settling. The bed primary microstructures are overprinted by biodeformation and burrow mottling by incipient small Planolites and Arenicolites/Polykladichnus ichnofossils of the continental Skolithos Ichnofacies, left behind by endobenthic oligochaetes and chironomid larvae. The depositional succession is cut by a strong erosional hyperpycnal flow event at 3320±40cal. BP, caused by the birth of the Neva River, which is now documented offshore for the first time. Mineral magnetic properties show pseudosingle-domain magnetite as the main magnetic mineral present. Increased magnetic grain sizes and reduced LOI values indicate a strong lithic influx immediately after the Neva River birth. The dominance of riverborne material in the accumulating sediments ever since highlights the strong influence of reworking in the shallow sea area. Increased diameters and the deeper penetration of biogenic sedimentary structures demonstrate improved seafloor oxygenation after the river birth. During the Medieval Warm Period (ca. 1000–700cal. BP) and the last century, increased sedimentary lithic contents and magnetic grain sizes indicate an elevated riverine influx due to increased precipitation, and/or the increased reworking of sediments by storms. The results provide a paleoenvironmental context for the study area, and deepen the understanding of storm-induced sediment dispersal and the formation of sedimentary fabric in shallow, wave-dominated sea areas. [ABSTRACT FROM AUTHOR]

Abstract: Channels are relatively common on river-mouth deltas, but the process by which they arise from river sediment discharge is unclear because they can potentially be explained either by negatively buoyant (hyperpycnal) flows produced directly from the river outflow or by flows generated by repeated failure and mobilisation of sediment rapidly deposited at the delta front. Channels eroded through a dump site of dredge spoils are described here from multibeam and older sonar data collected in Commencement Bay, at the mouth of the Puyallup River. Shallow channels on the seaward upper surface of the dump site, away from any flows that could have been produced by delta front failures, suggest that at least some hyperpycnal flows were produced directly from the positively buoyant river outflow up to 200 m from the edge of the river mouth platform. The form of channel bed erosion is revealed by the longitudinal shape of the main eroded channel compared with the adjacent dump site profile. It suggests that the channel evolved by its steep front retreating, rather than by simple vertical entrenchment or diffusive-like evolution of the profile, a geometry interpreted as evidence that repeated failure of the bed occurred in response to shear stress imposed by bottom-travelling flows. Model calculations based on shear strengths back-calculated from the geometry of channel wall failures suggest that, if the main channel were eroded solely by hyperpycnal flows, their generation was remarkably efficient in order to create flows vigorous enough to cause channel bed failure. Besides the sediment concentration and discharge characteristics that have been considered to dictate the ability of rivers to produce hyperpycnal flows, it is suggested that the timing of floods with respect to the tidal cycle should also be important because extreme low tides may be needed to ensure that coarse sediment is transferred vigorously to the edge of river mouth platforms. [Copyright &y& Elsevier]

Hyperpycnal flows may be important mechanisms for transporting particles from continents to oceans given their frequency of generation at many modern river mouths. Their deposits are probably very common in the rock record, although they are not frequently recorded in the literature. These currents may last longer and be steadier than surge-like turbidity flows and consequently their deposits are likely to be significantly different. In this paper simple laboratory experiments are used to investigate how flows and deposits may vary. The thickness of the experimental flows near the flow front varies with distance from the source depending on slope, effluent discharge, grain size and sorting. Runout distance varies with the proportion of fine-grained sediment rather than with mean grain size. Flow front velocity varies with effluent discharge and sediment concentration and in a complex manner with mean grain size and sorting. The variation of deposit mass with runout distance depends on proximal slope angle and the maximum mass per unit area (approximating to bed thickness) is inversely dependent on slope angle. Variation in initial suspended particle concentration results in deposits of similar shape but different volume. Although the total mass of the deposit depends on the discharge the maximum mass per unit area (approximating to maximum bed thickness) does not. The distance over which particles are deposited increases and the deposit flattens as particles are deposited progressively further downstream at higher discharges, perhaps relating to more efficient turbulent suspension at higher Re (more complete energy cascade), in addition to longer horizontal component of settling trajectory at higher velocity. The amplitude of mass variation depends on the grain size. As with laboratory surges [Gladstone et al., Sedimentology (1998) 833–844], the finer the sediment the more efficient is the sediment transport. [Copyright &y& Elsevier]

The Holocene morpho-stratigraphic evolution of a compound submarine deltaic system linked with the Antas and Almanzora Rivers on the narrow (< 5 km) shelf along the Palomares margin (southeastern Iberia) has been reconstructed from the integrated analysis of geomorphology, seismo-stratigraphy and sedimentology. The shelf morphology is characterized by a 3–4 km wide deltaic body off the Almanzora River, where seafloor undulations and gullies are present. Larger and more incised gullies, crescent-shaped bedforms and pockmarks are recognizable at or close to the head of shelf-indenting canyons associated with the Almanzora-Garrucha canyon system. The seismo-stratigraphy displays a strong and irregular basal reflector overlain by a sedimentary wedge, showing variable thicknesses (1–25 m) and seismic characteristics along and across the shelf. The main depocenter of this wedge is not located off the Almanzora River mouth (i.e., the main sedimentary input in the study area) but along the southern shelf encompassing the Almanzora and Antas Rivers. Grain-size and sand compositional analyses show that silty sand with high terrigenous content (mostly quartz and mica) is mainly found on the submarine deltaic body. In contrast, sandy silt with variable ratios between terrigenous and biogenic contents is retrieved from the canyon head and associated gullies as well as from the surrounding mid-outer shelf. The evolution of the Antas-Almanzora compound delta has been divided into three evolutionary stages, mainly driven by the interplay between paleo-topography and riverine and marine processes and modulated by sea level changes during the Holocene. The origin of the present-day morphological features is also discussed, again highlighting a complex interaction between sedimentary gravity flows (mainly flash flood-generated hyperpycnal flows), storm-waves and shelf currents, fluid seepage and retrogressive slope failures. [ABSTRACT FROM AUTHOR]