Subaqueous pyroclastic flows and ignimbrites: an assessment

An assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary, hot gas-rich pyroclastic flows, mass-flows which resulted from the transformation of gassupported flows into water-supported ones, and secondary mass-flows carrying redeposited pyroclastic debris. Based on subaerial pyroclastic flows, the term pyroclastic flow should be restricted to demonstrably hot, gas-rich mass-flows of pyroclastic debris. Using this definition, very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described. In the majority of these cases, the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate. The only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the Ordovician nearshore, shallow marine ignimbrites of Ireland and Wales, and Miocene ignimbrites of southwest Japan, resulting from the passage of subaerially erupted pyroclastic flows into shallow water. Other possible examples are near-vent dense clast deposits in the Donzurobo Formation of Japan, possible submarine intra-caldera ponded ignimbrite successions in California and Wales, and near-vent pumiceous deposits of Ramsay Island, Wales. All other purported cases are either clearly the result of water-supported mass-flow transportation and deposition (debris avalanches, debris flows, turbidity currents), or lack adequate supporting evidence regarding the heat state or the palaeoenvironment. Only the shallow marine ignimbrites of Ireland and Wales show adequate evidence of welding, but even these could have been nearly wholly exposed above sea-level when welding occurred. We conclude that when pyroclastic flows enter water they are generally disrupted explosively and/or ingest water and transform into water-supported mass-flows, and we suggest the various scenarios in which this occurs. There is no evidence to suggest that welding in wholly subaqueous environments is common.     

Response of the suspended sediment transport system to continental shelf dynamics

Surface currents influenced by a wind-driven upwelling event in San Pedro Bay moved total suspended matter (TSM) confined to the inner shelf on 19 April 1978 seaward, so that by 27 April surface TSM had increased over the outer shelf. Near-bottom concentrations of TSM also increased across the shelf during this time. This is explained by sediment resuspended by large surface waves being advected from the inner shelf seaward by the mean flow after this flow had turned from southeasterly to southerly when upwelling ceased on 26 April. These complex shelf dynamics contribute to the off-shelf transport of mud to the slope and deep basins.     

Nepheloid layers and internal waves over continental shelves and slopes

Theoretical and laboratory results indicate that bottom velocities within shoaling internal gravity waves intensify upslope approximately inversely proportional to the water depth. The elevated velocities (and bottom stresses) caused by shoaling and, possibly, breaking internal waves might explain the generation and maintenance of near-bottom nepheloid zones and attached turbid plumes that have been observed over certain continental shelves and slopes. This process is proposed as an explanation of zones of relatively low transmissibility that emanate from the upper continental slope near Newport submarine canyon off southern California.     

Spatial and temporal variation of the nekton and hyperbenthos from a temperate European estuary with regulated freshwater inflow

The aquatic macrofauna of the Guadalquivir estuary were sampled (1 mm mesh persiana net) at 5 sampling sites located along the entire (except the tidal freshwater region) estuarine gradient of salinity (outer 50 km). A total of 134 fish and macroinvertebrate species was collected but only 62 were considered common or regularly present in the estuary. Univariate measures of the community structure showed statistically significant differences among sampling sites: species richness, abundance, and biomass decreased in the upstream direction, being positively correlated with the salinity. Temporal differences of these three variables were also statistically significant. While a clear seasonal pattern (minimum densities in winter and maximum in spring-summer) was observed for abundance and biomass, no such pattern existed for the number of species. Mysids was the most dominant group throughout the estuary (96% to 99% of abundance; 49% to 85% of biomass), although fish biomass was also important at the outer estuary (36% to 38%). Multivariate analyses indicated highly significant spatial variation in the macrofaunal communities observed along the salinity gradient. These analyses suggest that the underlying structure was a continuum with more or less overlapping distributions of the species dependent on their ability to tolerate different physicochemical conditions. There were also significant temporal (intermonthly + interannual) variation of the estuarine community; the relative multivariate dispersion indicated that monthly variation was more considerable (relative multivariate dispersion >1) at the outer part of the estuary during the wet year (last 20 km) and was higher in the inner stations during the dry year (32 to 50 km from the river mouth). Since a clear negative exponential relationship was observed between the freshwater input (from a dam located 110 km upstream) and water salinity at all sampling stations, it is concluded that the human freshwater management is probably affecting the studied estuarine communities. While the higher seasonal (long-term) stability of the salinity gradient, due to the human control of the freshwater input, may facilitate the recruitment of marine species juveniles during the meteorologically unstable early-spring, the additional (short-term) salinity fluctuations during the warm period may negatively affect species that complete their lifecycle within the estuary.     

Middle Paleolithic occupation on a Marine Isotope Stage 5 lakeshore in the Nefud Desert,Saudi Arabia

Major hydrological variations associated with glacial and interglacial climates in North Africa and the Levant have been related to Middle Paleolithic occupations and dispersals, but suitable archaeological sites to explore such relationships are rare on the Arabian Peninsula. Here we report the discovery of Middle Paleolithic assemblages in the Nefud Desert of northern Arabia associated with stratified deposits dated to 75,000 years ago. The site is located in close proximity to a substantial relict lake and indicates that Middle Paleolithic hominins penetrated deeply into the Arabian Peninsula to inhabit landscapes vegetated by grasses and some trees. Our discovery supports the hypothesis of range expansion by Middle Paleolithic populations into Arabia during the final humid phase of Marine Isotope Stage 5, when environmental conditions were still favorable.     

Feeding preferences of estuarine mysids Neomysis integer and Rhopalophthalmus tartessicus in a temperate estuary (Guadalquivir Estuary,SW Spain)

Mysid shrimps are an important component of estuarine food webs because they play a key role in energy transfer as intermediate prey. We investigated the seasonal, tidal and depth specific variation in the diet of the estuarine mysids Neomysis integer and Rhopalophthalmus tartessicus and explored its implications for the planktonic community structure of a temperate estuary (Guadalquivir Estuary, SW Spain). Neomysis integer is an opportunistic omnivore feeding mainly on mesozooplankton and on members of the detrital–microbial loop, shifting prey seasonally according to availability. In contrast, R. tartessicus showed a more carnivorous diet and shifted its target prey during seasons of low resource availability. Despite statistically significant differences in diet composition, both species shared prey of similar size, particularly juvenile Mesopodopsis slabberi, the most abundant mysid species in this estuary, and copepods. Although these similarities imply inter-specific resource competition, their co-existence is achieved by niche partitioning and spatial segregation: the higher osmoregulatory capacity and foraging plasticity of N. integer confers a broader niche breadth for this species allowing N. integer to inhabit the more stressful oligohaline region of the estuary where R. tartessicus cannot survive. We propose that this mechanism relaxes the potential for competition between N. integer and R. tartessicus.