Rates of aerodynamic entrainment in a developing boundary layer

Despite its significance for inception of grain transport by wind, the initial dislodgement of grains from a static surface by aerodynamic forces of drag and lift in the absence of grain collision has received little attention. This paper describes a series of wind-tunnel experiments in which the erosion of narrow strips of loose grains from the roughened surface of a flat plate exposed to a range of wind speeds was examined. The progressive downwind development of the boundary layer over the plate provided a range of airflow conditions which permitted systematic evaluation of grain entrainment rates arising from purely aerodynamic forces. Use of closely graded size fractions in flat, single grain layers resting on identical, fixed grain support eliminated the effects of surface irregularities and impacts from saltation. Results show that erosion of strips of loose grains develops with time according to an inverse exponential function in which the entrainment rate time constant relates to Shields dimensionless shear stress function. An empirical expression defining aerodynamic entrainment rate in terms of rate of strip erosion is derived and comparisons are made between present and published data. The need for additional data to resolve several questions raised by the present investigation is stressed. In addition, a simple, objective technique for accurate determination of the aerodynamic entrainment threshold of any loose, granular sediment is proposed.     

Tectonic and sedimentary history of a late Cenozoic intramontane basin (The Pitalito Basin, Colombia)

Abstract The Pitaiito Basin is an intramontane basin (15 × 20 km²) situated at the junction of the Central and Eastern Cordillera in the southern part of the Colombian Andes. Tectonic structures, evolution of the basin and distribution of the sediments suggest that the basin was formed adjacent to an active dextral strike-slip fault. Based on sedimentation rates it is estimated that subsidence started around 4.5 Ma. The basin can be divided into a relatively shallow western part (c. 300 m deep) and a deep eastern part (c. 1200 m deep). The transition between both areas is sharp and is delineated by a NW/SE-oriented fault. The position of this fault is reflected by the areal distribution of the deep non-exposed sediments as well as sediments at the surface: west of this fault the basin infill consists of coarse to medium elastics (conglomerates and sand) whereas in the eastern part fine elastics (clay and peat) are present. The lateral transition between both types of sediment is abrupt and its position is stable in time. The surface and near surface sediments in the Pitalito Basin reflect the last stage of sedimentary infill which came to a halt between 17,000 and 7500 years bp . These sediments were deposited by an eastward prograding fluvial system. The western upstream part of this system differs significantly from that of the eastern part which forms the downstream continuation. The western part exhibits unstable, shallow fluvial channels that wandered freely over the surface which predominantly consists of clayey overbank sediments. The alluvial architecture in the eastern half is characterized by stable channels and thick accumulations of organic-rich flood basin sediments and resembles an anastomosing river. The transition between both alluvial systems also coincides with the N/S-oriented normal fault. Palaeoclimatic conditions over the last c. 61,500 years were determined by means of a pollen record. From c. 61,500 to 20,000 years BP the mean annual temperature fluctuated considerably and decreased by 2–3^oC during the relatively warm periods (interstadials) and by 6–8^oC during the cold periods (stadials) in comparison with modern temperatures. These changes led to a displacement of the zonal vegetation belts from c. 200 m during the stadials to c. 1500 m in interstadial times without significant effects on the fluvial system present in the Pitaiito Basin until c. 20,000 years BP. Around this period the organic-rich eastern flood basins were choked with sediments and peat growth came to an end. Palynological and sedimentological data suggest that around that period the climate was cold (Δ 6–8^oC) and very dry and that a sparse vegetation cover was present around the basin. In these semi-arid climatic conditions the river system changed from an anastomosing pattern to one with ephemeral stream characteristics. This may have lasted until at least 17,000 years BP. Somewhere between 17,000 and 7500 years BP the eastward-flowing infilling river system changed into a NW-flowing erosive river system due to climatic or tectonic control and the present state was reached.     

Freshwater and Atlantic water inflows to the deep northern Barents and Kara seas since ca 13 C ka: : foraminifera and stable isotopes

Foraminiferal stable isotopes and assemblages from Franz Victoria and St. Anna troughs provide a valuable record of freshwater and Atlantic Water flows to the northern Barents and Kara seas from deglaciation to present. The δ¹⁸O and δ¹³C of planktonic Neogloboquadrina pachyderma (s) and benthic Elphidium excavatum were up to 1.4‰ lower than present at ca 13, 11.5, and 10 ¹⁴C ka (global sea-level corrected), mostly reflecting substantial freshwater inputs coincident with glacial–marine sediment deposition. Cassidulina teretis exceeded 40% of benthic foraminifera ca 13 and 10 ¹⁴C ka, indicating subsurface penetrations of Atlantic Water. The transition to postglacial marine conditions is marked by a 1‰ rise in foraminiferal δ¹⁸O and a sharp fall in % C. teretis soon after 10 ¹⁴C ka. These changes imply reduced inputs of freshwater and Atlantic Water. Subsequent isotopic and foraminiferal assemblage variations reflect changing Atlantic Water conditions “upstream” in the Nordic Seas and shifts between the warm Fram Strait and cold Barents Sea branches of Atlantic Water. We hypothesize that glacial-isostatically induced deepening by up to 150 m influenced Atlantic Water inflows to the northern Barents Sea during deglaciation and the Holocene. Thus, effects of isostatic recovery have to be factored into paleoceanographic reconstructions.     

Toxicity of No. 2 fuel oil to coon stripe shrimp

Bioassay of a No. 2 fuel oil dispersion with shrimp in a continuous flow system using measured waterborne oil as the indicator of oil concentrations reveals a treatment more definable than those previously described in terms of volume ratios and produces lower lethal concentrations. Shrimp 96-h LC₅₀ was 0.8 mg/l in this study as compared to values from 1.5 to 50 mg/l reported for other methods. Mean concentrations in tests do not give significant differences in concentration with respect to day of the test or spatial distribution in the exposure tanks.     

Mixing,hypersalinity and gradients in Hervey Bay,Australia

Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia, is a shallow tidal area (average depth = 15 m), close to the continental shelf. It shows features of an inverse estuary, due to the high evaporation rate (approx. 2 m/year), low precipitation (less than 1 m/year) and on average almost no freshwater input from rivers that drain into the bay. The hydro- and thermodynamical structures of Hervey Bay and their variability are presented here for the first time, using a combination of four-dimensional modelling and observations from field studies. The numerical studies are performed with the Coupled Hydrodynamical Ecological Model for Regional Shelf Seas (COHERENS). Due to the high tidal range (>3.5 m), the bay is considered as a vertically well-mixed system, and therefore, only horizontal fronts are likely. Recent field measurements, but also the numerical simulations, indicate characteristic features of an inverse/hypersaline estuary with low salinity (35.5 psu) in the open ocean and peak values (>39.0 psu) in the head water of the bay. The model further predicts a nearly persistent mean salinity gradient of 0.5 psu across the bay (with higher salinities close to the shore). The investigation further shows that air temperature, wind direction and tidal regime are mainly responsible for the stability of the inverse circulation and the strength of the salinity gradient across the bay. Due to an ongoing drying trend, the occurrence of severe droughts at the central east coast of Australia and, therefore, a reduction in freshwater supply, the salinity flux out of the bay has increased, and the inverse circulation has also strengthened.     

Solar soft X-rays and solar activity

Peak fluxes of flare-associated 8–12 Å X-ray bursts occur at or near the time of the maximum energy content of the soft X-ray source volume. The amplitudes of flare-associated bursts may thus be used as a measure of the energy deposited in the source volume by non-thermal electrons and other processes. In the mean, the soft X-ray burst amplitude is apparently independent of the occurrence of a type III event. This is interpreted to indicate that electrons accelerated by the type III process do not directly participate in establishing the soft X-ray source volume.     

The ECOMAN project: A novel approach to defining sustainable ecosystem function

The ECOMAN was initiated in 2001 by the University of Plymouth, UK, Plymouth Marine Laboratory and the Department of the Environment, Fisheries and Rural Affairs (DEFRA) to address the need for more pragmatic assessment techniques linking environmental degradation with its causes. The primary aim of the project was to develop an evidence-based approach in which suites of easy-to-use, cost-effective and environmentally valid biological responses (biomarkers) could be used together to assess the health of coastal systems through the general condition of individuals. A range of sub-lethal endpoints, chosen to reflect successive levels of biological organisation (molecular, cellular, physiological), was evaluated in common coastal organisms showing different feeding types (filter feeding, grazing, predation) and habitat requirements (estuary, rocky shore). Initially, the suite of biomarkers was used in laboratory studies to determine the relative sensitivities of key species within different functional groups to common contaminants. These results were then validated in field studies performed in a range of ecosystems exhibiting different degrees/signatures of contamination. Here, an example is provided of a field study in the Humber Estuary, UK, which illustrates how multivariate statistical analysis can be used to identify patterns of response to discriminate between contaminated and clean sites. The use of a holistic, integrated approach of this kind is advocated as a practical means of assessing the impact of chemical contamination on organismal health and of ranking the status of marine ecosystems.