Grey Sand Research Articles

New Stratigraphic Subdivision, Depositional Environment, and Age Estimate for the Upper Cretaceous Djadokhta Formation, Southern Ulan Nur Basin, Mongolia

Studies of key and newly discovered sections of the Upper Cretaceous Djadokhta Formation along the southern margin of the Ulan Nur Basin allow a new subdivision based on lithology. The formation and its members were mapped at both Bayn Dzak, an area that includes the Flaming Cliffs, and Tugrugyin Shireh, an area about 50 km to the northwest of Bayn Dzak. Stratigraphic sections at both localities were remeasured. The considerably enlarged formation comprises a lower Bayn Dzak Member, dominated by moderate reddish orange sands with subordinate mudstone units, and an upper Tugrugyin Member, composed of pale orange to light gray sands. Investigations of key sections at Tsonzh and Alag Teer demonstrate the presence of transitional mudstone lenses between these members within the Djadokhta Formation. Two distinct, sandy, sedimentologic facies are recognized in both members. Crossbedded intervals, occasionally exhibiting wind-ripple cross lamination, document the presence 1 The Geological Institute of the Mongolian Academy of Sciences, Peace Street 63, Ulaanbaatar-51, Mongolia

Morphologic and stratigraphic evolution of muddy ebb‐tidal deltas along a subsiding coast: Barataria Bay, Mississippi River delta

AbstractThe Barataria barrier coast formed between two major distributaries of the Mississippi River delta: the Plaquemines deltaic headland to the east and the Lafourche deltaic headland to the west. Rapid relative sea‐level rise (1·03 cm year−1) and other erosional processes within Barataria Bay have led to substantial increases in the area of open water (> 775 km2 since 1956) and the attendant bay tidal prism. Historically, the increase in tidal discharge at inlets has produced larger channel cross‐sections and prograding ebb‐tidal deltas. For example, the ebb delta at Barataria Pass has built seaward > 2·2 km since the 1880s. Shoreline erosion and an increasing bay tidal prism also facilitated the formation of new inlets. Four major lithofacies characterize the Barataria coast ebb‐tidal deltas and associated sedimentary environments. These include a proximal delta facies composed of massive to laminated, fine grey‐brown to pale yellow sand and a distal delta facies consisting of thinly laminated, grey to pale yellow sand and silty sand with mud layers. The higher energy proximal delta deposits contain a greater percentage of sand (75–100%) compared with the distal delta sediments (60–80%). Associated sedimentary units include a nearshore facies consisting of horizontally laminated, fine to very fine grey sand with mud layers and an offshore facies that is composed of grey to dark grey, laminated sandy silt to silty clay. All facies coarsen upwards except the offshore facies, which fines upwards. An evolutionary model is presented for the stratigraphic development of the ebb‐tidal deltas in a regime of increasing tidal energy resulting from coastal land loss and tidal prism growth. Ebb‐tidal delta facies prograde over nearshore sediments, which interfinger with offshore facies. The seaward decrease in tidal current velocity of the ebb discharge produces a gradational contact between proximal and distal tidal delta facies. As the tidal discharge increases and the inlet grows in dimensions, the proximal and distal tidal delta facies prograde seawards. Owing to the relatively low gradient of the inner continental shelf, the ebb‐tidal delta lithosome is presently no more than 5 m thick and is generally only 2–3 m in thickness. The ebb delta sediment is sourced from deepening of the inlet and the associated channels and from the longshore sediment transport system. The final stage in the model envisages erosion and segmentation of the barrier chain, leading to a decrease in tidal discharge through the former major inlets. This process ultimately results in fine‐grained sedimentation seaward of the inlets and the encasement of the ebb‐tidal delta lithosome in mud. The ebb‐tidal deltas along the Barataria coast are distinguished from most other ebb deltas along sand‐rich coasts by their muddy content and lack of large‐scale stratification produced by channel cut‐and‐fills and bar migration.

Hydraulic head in a clayey sand till over multiple timescales

This paper presents an analysis of hydraulic head data measured every 2 h for 2 years in the leading edge of a drumlin that consists of weathered, brown clayey sand till and underlying unweathered gray clayey sand till. The brown till responds as an unconfined aquifer and acts as a boundary condition for the confined gray till aquifer. Analytical models are presented for interpretation of the hydraulic head data using annual, monthly, and daily timescales, for estimation of drumlin (landform) hydraulics and till (formation) properties. The annual average head data from open-standpipe piezometers calibrate drumlin hydraulics that feature steady radial flow in the brown till, because of its higher permeability (1.1 × 10–13 m2) and the finite radius of the drumlin. The lower permeability (1.4 × 10–15 m2) gray till has steady cylindrical flow with an appreciable downward gradient. Open-standpipe piezometer data are modeled at the monthly timescale to confirm the landform hydraulics and calibrate a brown till macroporosity of 0.016. The amplitude of periodic monthly head fluctuations increases towards the edge of the drumlin. Hydraulic head response to aperiodic storm events is significantly magnified by this macroporosity as measured by precipitation gages and buried pressure transducers. The storm data are modeled on a daily timescale to confirm the brown till macroporosity value estimated from the monthly data and to further elucidate gray till hydraulics. Compressibility delays and attenuates the groundwater mound, with a calibrated gray till compressibility of 3 × 10–9 Pa–1. The calibrated models accurately recover drumlin flow fields with plausible property values of the clayey sand till, and accordingly should describe other sites with similar landforms and deposits.Key words: drumlins, till, hydraulics, compressibility, permeability, macroporosity, piezometer.

Stratigraphy of a site with Eemian interglacial deposits in north Estonia (Juminda Peninsula)

Sediment lithology, lithofacies relationships, palynological and diatom analysis indicate that two drill cores extracted from ancient valleys of the Juminda Peninsula, north Estonia, represent a previously undetected interglacial sedimentary sequence sandwiched between the Middle and Late Pleistocene glacigenic deposits. Four different coloured major lithofacies associations (units), representing two glacigenic cycles, and deposits with the Late Saalian pollen assemblages zone (LS1), the uppermost Eemian (E9), and Early Weichselian pollen zone (EW) were distinguished at the section. The bottommost depositional unit consists of brownish coloured clay, diamicton and gravel beds. The second depositional unit, correlative to the Eemian Interglacial, consists of a black organic-rich palaeosoil and stratified sand. A thick grey coloured bed of massive and partly stratified Weichselian diamicton, sand, clay and silt comprises the third unit, and the fourth, the topmost depositional unit, consists of yellowish grey sands and silts of the Baltic Sea.

Impact of harvest residue management on soil nitrogen dynamics in Eucalyptus globulus plantations in south western Australia

More than 200,000 ha of short rotation Eucalyptus globulus plantations have been established in south-western Australia to supply wood for the pulp and paper industries. Sustaining the productivity of these tree crops over successive rotations will depend in part on maintenance of soil fertility, especially soil nitrogen (N) supply. We investigated the impact of four alternative strategies for management of harvest residues on soil N dynamics in recently logged first rotation plantations. The experiments were conducted over 5 years following harvesting at two sites with contrasting soils—a coarse textured grey sand over laterite (Podzol) with low natural fertility and a relatively fertile red earth soil (Ferralsol). At the grey sand site, 31 t ha −1 of residues containing 219 kg N ha −1 were deposited following harvest while at the red earth site the equivalent figures were 51 t ha −1 of residues and 347 kg N ha −1. Experimental treatments applied included residues burned, removed, retained and retained with double the amount of residues. The impact of treatments on soil nitrogen supply was investigated by incubating intact soil cores in the field to determine rates of net N mineralization. Additionally, the effect of treatments on soil moisture and temperature, the resident pool of soil mineral N and the amount of N potentially available for mineralization was assessed. The mulching effect of retained residues resulted in higher soil moisture where residues had been retained and a trend for soil on these treatments to dry out more slowly with the onset of the dry summer season, especially in the first year following harvest. Diurnal variations in soil temperature were moderated and average soil temperatures were reduced during summer where residues were retained. Concentrations of mineral N in soil were high in the 2 years following harvest at both sites and declined as newly established seedlings developed. At the more fertile site, where mineral N occurred predominantly as nitrate, retention of residues resulted in lower pools of soil mineral N following harvest. The effect of residue treatments on soil mineral N pools was less marked at the grey sand site. Concentrations of potentially mineralizable soil N and the amounts of N mineralized annually were greater where residues were retained at both sites. The results indicate that retention of harvest residues will favour the conservation of N following logging. However, accumulation of soil mineral N following harvesting due to reduced plant uptake will result in leaching of N early in the rotation that is largely independent of residue management. Retaining harvest residues will contribute to enhanced N supply for the next tree crop through mineralization in the long term. However, on some sites, additions of nitrogenous fertilizers will still be required to maximise the rate of tree growth.

Residue management effects on soil carbon and nutrient contents and growth of second rotation eucalypts

We examined tree growth and dynamics of organic matter and soil nutrient pools annually for 7 years under contrasting harvest residue management treatments in south-western Australia. Two second rotation Eucalyptus globulus sites were established on soils of contrasting fertility and productivity. Harvest residues were either (i) burnt, (ii) removed, (iii) retained, or (iv) retained at double the normal quantity. More than 31 and 51 Mg ha −1 of harvest residues resulted from harvesting of 8-year-old first rotation stands at a low fertility Grey Sand site, and a higher fertility Red Earth site, respectively. Harvest residue retention increased tree growth at the lower fertility Grey Sand site, but had no effect on plantation productivity at the Red Earth site up to 7 years. Burning resulted in a direct loss of most of the organic material, and up to 200, and 350 kg ha −1 of N at the Grey Sand and Red Earth sites, respectively. Significant quantities of organic material in harvest residues (>50 Mg ha −1 C in the double residues treatment at the Red Earth site) had a limited effect on soil C pools during the 7 years of this study. Retention of residues limited immediate losses of nutrients, and resulted in higher quantities of soil exchangeable K, Ca and Mg during the 7 years after establishment. However, the content of soil exchangeable cations, especially K, decreased during the first 4 years of establishment in all treatments, including those where residues were retained. After 4 years, cation quantities in soil started to increase again, probably due to the decomposition of leaves and twigs from litterfall.

Spatial variability in Sediment Oxygen Consumption under winter conditions in a lagoonal system in New Caledonia (South Pacific)

Sediment Oxygen Consumption (SOC) was investigated during a winter (Southern Hemisphere) cruise in the southwest lagoon of New Caledonia. Oxygen fluxes were measured at 11 sampling stations distributed along two coast to reef transects. Three different methods of flux measurements were used: diver-operated benthic chambers, laboratory incubation of sediment cores and oxygen microprofiles determinations. SOC values varied between 450 and 2250 μmol O 2 m −2 h −1. The level of agreement between the three techniques strongly varied as a function of sediment type. Most of the SOC values from the grey sand zone in the middle part of the lagoon and the muddy bottoms of the bays did not show significant differences. A central station presenting a dense seagrass bed gave lower SOC determined by oxygen microprofiles compared to the two other methods. In coarse carbonated sands from the back reef area, SOC measured by in situ benthic chambers were higher than SOC measured by incubation techniques. This discrepancy could be explained by physical disturbance of the sediments, macroscale variability in benthic communities or technical efficiency of the sediment sampling device and probably by a combination of all three processes. Nevertheless, for the other sediment types that represented 85% of the lagoon bottoms, the results from the three techniques used for SOC determination were strongly convergent. Based on this assumption, it could be stated that the oxygen fluxes were essentially driven by microbial activity compared to biologically mediated vertical transport of solutes. The SOC values determined during this study were in agreement with budgets previously calculated for the lagoon. Regardless of the back reef area, spatial variability in SOC can be explained by the organic matter content of sediments which clearly showed a coast to reef gradient with higher organic carbon and nitrogen contents in the coastal sediments. The C/N ratios demonstrate the higher rate of freshly deposited organic matter near the coast compared to more central stations in the lagoon.

Nitrogen release from eucalypt leaves and legume residues as influenced by their biochemical quality and degree of contact with soil

Large areas of short-rotation eucalypt plantations are being established in south-western Australia on land previously used for agriculture. Options for maintaining soil N supply include retention of harvest residues and legume inter-cropping. We evaluated the effects of adding the residues of five legume species and Eucalyptus globulusleaves on inorganic N dynamics in two soils (a Rhodic Ferralsol or red earth and a Haplic Podzol or grey sand) using two modes of residue application in a laboratory incubation experiment (519 days). The time course of net N immobilisation and mineralisation in both soils was strongly influenced by the type and mode of application. Eucalypt leaves caused strong N immobilisation (−7 mg N g−1 residue-C) over the entire 519-day incubation, whereas for the legume species, N that was eventually immobilised at the start of the incubation, remineralised later to different degrees. Amongst the legumes, largest amounts of N were released from lupin residues (18 mg N g−1 residue-C) and lowest amounts from field pea (2 mg N g−1 residue-C). However, initial residue quality parameters were not significantly (P > 0.05) correlated with N release from the residues. Grinding and incorporating of the residues caused a much greater immobilisation of N than when residues were cut and surface applied. When ground residues were incorporated, immobilisation of N was more severe and endured for longer in the finer textured red earth than in the coarse textured grey sand. Where residues were surface applied, N dynamics were similar for both soil types. The results of this study suggest that legumes used as a mulch in eucalypt plantations are a readily available source of N for trees, and that the benefits from retention of harvest residues are more likely in maintaining soil N fertility on the long-term.

Erosion and Sedimentation from the 17 July, 1998 Papua New Guinea Tsunami

— This paper describes erosion and sedimentation associated with the 17 July 1998 Papua New Guinea tsunami. Observed within two months of the tsunami, distinct deposits of a layer averaging 8-cm thick of gray sand rested on a brown muddy soil. In most cases the sand is normally graded, with more coarse sand near the base and fine sand at the top. In some cases the deposit contains rip-up clasts of muddy soil and in some locations it has a mud cap. Detailed measurements of coastal topography, tsunami flow height and direction indicators, and deposit thickness were made in the field, and samples of the deposit were collected for grain-size analysis in the laboratory. Four shore-normal transects were examined in detail to assess the shore-normal and along shore distribution of the tsunami deposit. Near the shoreline, the tsunami eroded approximately 10–25 cm of sand from the beach and berm. The sandy layer deposited by the tsunami began 50–150 m inland from the shoreline and extended across the coastal plain to within about 40 m of the limit of inundation; a total distance of up to 750 m from the beach. As much as 2/3 of the sand in the deposit originated from offshore. Across most of the coastal plain the deposit thickness and mean grain size varied little. In the along-shore direction the deposit thickness varied with the tsunami wave height; both largest near the entrance to Sissano Lagoon.

Eucalyptus globulus harvest residue management effects on soil carbon and microbial biomass at 1 and 5 years after plantation establishment

Residues from harvest of eucalypt plantations represent a significant proportion of site organic matter and nutrients, and their management may influence longer term plantation productivity through changes in soil organic matter and nutrient supply. Experiments at two sites in south-western Australia were established to investigate the effects of harvest residue management on soil fertility and plantation productivity. The sites were a relatively fertile red earth, and a poorer-fertility grey sand. Harvest residue treatments were burning, complete removal of the above-ground material, retention and uniform distribution on site, and double the normal quantity of residues retained and distributed on site. Soil microbial biomass, an indicator of longer term (potentially several rotations) impacts of management on soil organic matter, was measured 1 and 5 years after planting. Retention of residues had minimal influence on total soil C and N, or C oxidized by 333 mM permanganate, but it significantly increased surface soil (0–5 cm) microbial biomass carbon (MBC) and nitrogen (MBN), as well as increasing the MBC and MBN quotients (ie. MBC and MBN as a proportion of the total soil C and N). Effects of residue management on MBC and MBN were generally highly significant 1 year after establishment, but differences between treatments had decreased after 5 years. Residue retention significantly increased MBC at both sites down to 20 cm depth, and significantly increased MBN at the red earth site at 1 year after establishment, but effects at 5 years were only evident in the surface (0–5 cm) soil. At 1 year, MBN in the surface (0–5 cm) soil was more responsive to residue retention than MBC, as the MBN quotient increased more than 2-fold at the red earth site, compared with only a 30% increase in MBC quotient. Higher MBC and MBN levels are due to greater availability of substrate derived from residues early in the rotation. The effect of residue management on microbial biomass was reduced at 5 years, and confined to the surface soil (0–5 cm), suggesting that availability of the organic material from the residues had declined. The decline was mainly due to utilization by the microbial biomass (respiration) or conversion to less labile forms.

Geotechnical classification and distribution of the quaternary deposites in basrah city south of iraq

as due to the strategic importance of basrah city, south of iraq, the architectural development of the city requires a great deal of studies of the geo technical properties .engineering behaviors and classification of the soil bearing strata represented by the quaternary deposites for such purposes a number of (121) sites distibuted randomly all around the city through (491) boreholes of depths (10-48) M below mean sea level are studied. the data are obtained from the test results of (SPT), grain size distribution and the atterberg's limits the quaternary deposites are classified into two main groups : firstly is the cohesive deposites represented by the recent clay and silty clay as well as alhammar formation deposits. secondly is the cohesionless deposits represented by sands of dibdiba formation according to the consistency of cohensive deposits and the compactness of the cohesionless deposits, ten strata can be identified starting from the ground surface , as follows. hard brown silty clay, very stiff brownish-grey silty clay or clayey slit, stiff grey clayey slit, medium stiff grey clayey slit laminated with slit, soft grey clayey slit laminated with slit, medium stiff grey clayey slit, stiff grey clayey slit, very stiff grey clayey slit & sandy clayey slit, hard brown clayey slit and very dense grey sand with slity sand

Efficacy Of Balneotherapy For Rheumatoid Arthritis: A Meta-analysis

Balneotherapy is an accepted therapeutic approach in Europe. However, its efficacy for treating rheumatoid arthritis (RA) remains controversial in North America. This is a meta-analysis of randomized (RCT ) and controlled clinical trials (CCT ), case-control and cohort studies of balneotherapy compared to control (no treatment) or active therapy (head to head studies). All the included studies (n =7) in this review were identified as RCTs. This meta-analysis examines different balneotherapy modalities: Radon-carbon dioxide baths, mud packs, hot sulphur baths, Dead Sea baths, Red Sea baths, and grey sand. Several types of balneotherapy, including radon-carbon dioxide baths, mud packs, hot sulphur baths, combination of hot sulphur baths and mud packs, Dead Sea baths, combination of Dead Sea baths and sulphur baths, and Grey sand and Red Sea baths compared to control were found to be beneficial for RA. These modalities were found to be especially beneficial for pain, grip strength, tender/swollen joints, patient and physician, and global function. These improvements ranged from 5-93% greater improvement relative to the control group. However, methodological considerations including the poor quality of trials impact the conclusions of this review.

Linear contraction of grey iron sand castings

A study has been made on the linear contraction of grey iron castings in greensand and silicate-bonded sands. Square cross sections were used to establish the effect of size, for free, as well as constrained contraction, while circular and triangular sections were used to study the effects of shape. A range of sizes of end flange was used to study the effect of constraint.It was found that free linear contraction for both greensand and silicate-bonded sands is inversely related to modulus. For thin section castings the contraction is close to 1.20%. For greensand the contraction decreased to 0.70% and for silicate-bonded sand it decreased to about 0.9% when the modulus was increased to 12.5 mm. For any given values of modulus the silicate-bonded sand had higher contraction values. The effect of shape appeared to relate to the difference in modulus values.An evaluation of constrained contraction of bars indicated that contraction was a function of mechanical constraint, Cm, defined as a product of (flange area/casting cross section area) and the third power of the ratio (casting modulus/constraining flange modulus). The latter ratio is a measure of the difference in cooling rates of the two sections. However, a limit was observed where further increase in Cm did not further reduce contraction values in either of the two moulding media.The study indicates a pattern of contraction values for simple shaped castings. This is the first step towards the prediction at the design stage of contraction values for complex castings.

Problem of Arsenic in Groundwater in West Bengal, India

From 2002 to 2010 inclusive we monitored concentrations of arsenic (As) and major ions (Ca, Mg, Sr, Na, K, Fe, Mn, Cl, and SO4) in groundwater from 14 domestic wells and three piezometer nests in a shallow aquifer (<60 m depth), and 3 wells in a deep aquifer (>70 m depth), in southern West Bengal, India. In the deep aquifer, concentrations of As did not change over time despite increases in the concentration of Fe in two wells. The shallow aquifer occurs in two sedimentological settings: palaeo-channel and palaeo-interfluve. At the top of the shallow aquifer of the palaeo-channel, decreases in all constituent concentrations with time, and an 3H/3He age of 1.4 years, proves that the aquifer is beginning to be flushed of pollutants. In As-polluted groundwater (>50 μg/L As) tapped from deeper grey sands of the shallow, palaeo-channel, aquifer, concentrations of As were mostly stable over time, but both increases and decreases occurred with time in response to downward migration of the chemically-stratified water column. In groundwater tapped from Pleistocene brown sands, the concentration of As remained either low and stable (<2 μg/L As), or increased at rates up to 34 μg/L per year. The increases were caused by the flow of As-rich groundwater either downward into brown sand at the base of palaeo-channels, or laterally into a confined, unpolluted, palaeo-interfluvial, aquifer of brown sand that lies regionally beneath a palaeosol.Under the present pumping regime, the prognosis for As-pollution in the shallow aquifer is complex. Wells in brown sand may become polluted over timescales of as little as 2 years, whilst some wells tapping As-polluted groundwater from grey sand will become fit for potable use (<50 μg/L) within a few decades. The evidence of flushing, and of declining As in some of the groundwater from palaeo-channels, which are conduits for recharge of the confined, As-free, palaeo-interfluve aquifer, and probably also the deeper aquifer, offers hopes that the spread of As-pollution will be limited.

Provenance of beach gray sands from western México

Twenty-nine surficial beach dark colored sand samples from about 6500 km of the western littoral of Mexico were chemically and petrologically analyzed to search for the relationship between their heavy mineral content and metal and rare earth element variations. Sand samples were dried and grouped according to color in three classes of gray. In order to search for a possible influence on petrological and chemical variations, the sands were also grouped according to grain size, sorting, composition, main sources, wave energy, and climate. It was observed that quartz, feldspars, and rock fragments averaged low values and, as the sand darkened, the heavy mineral content increased. Also, the higher content of heavy minerals in dark gray and very dark gray sands correlated with a humid rather than arid climate. Rare earth element and other trace element contents seemed to be related more to the composition of the source area than to heavy mineral content. However, some elements, such as Zn, Hf, Th, Ni, and Cr, did relate more to heavy mineral content. Geochemical and petrological analyses of these sands suggest that most are derived from juvenile sources. Sedimentary processes such as wave energy appear to play a significant role locally in heavy mineral content, making the energy conditions of the beach environment an important local factor. Concentrations of rare earth and other elements are not of economic interest.

A field method for determing percentage of coated sand grains

Several USDA‐NRCS hydric soil field indicators require estimation of the percentage of organic‐coated sand grains (black grains). For example, to meet the Dark Surface field indicator the soil layer must contain at least 70% coated (black) grains. Field experience has shown that the estimation of the percentage of coated sand grains is often subjective and highly variable from one soil scientist to another. In order to overcome this variability a set of standards was created using a mixture of black and light gray (representing uncoated grains) sand grains. Weighing out each component for the desired ratio and mixing them in a 47‐mm‐diam. petri dish we made a set of three standards consisting of 50, 70, and 90% black grains. To test the effectiveness of these standards, soil scientists estimated the percentage of coated grains from similarly prepared samples first without the use of the standards and then with the use of the standards for comparison. Individuals improved the accuracy of their estimates by 10 to 60% and their hydric soil identification by 16%. The standards are easily prepared, easy to use, and portable.

Effects of an Oil Spill on Soil Physico-Chemical Properties of a Spill Site in a Typic Udipsamment of the Niger Delta Basin of Nigeria

Physico-chemical analysis of soil samples at an oil spill site in a Typic Udipsamment of the Niger Delta Basin of Nigeria showed that the total hydrocarbon content of top soil layers ranged from 18.6 to 23.6 ppm in the heavy impact zone and the oil had penetrated to a depth op 8.4 m. The concentration of hydrocarbons in the medium impact zone ranged from 10.04 to 10.38 ppm while hydrocarbons were not detected in 85% of samples from the unimpacted reference zone. Heavy metal concentration measurements in the soil revealed a significant build-up (P < 0.05) of lead, copper and zinc in the heavy impact zone. Other quality parameters including electrical conductity, exchangeable cations, total nitrogen and available phosphorus in impacted soils were relatively low, while the total organic carbon was high compared with the reference site. Textural class of soil from the different depths showed a predominantly brown sand at the topsoil, loamy sand and grey coarse sand at medium depths, and grey coarse sand and greyish sandy clay at greater depths.

Nature of sedimentary deposits in the western Makgadikgadi basin, Botswana

Quaternary sedimentation in the western Makgadikgadi basin of north central Botswana is evaluated on the basis of new evidence from satellite imagery and sedimentological analyses. Thematic Mapper imagery interpretation, combined with field evidence, has led to the identification of geomorphological features which are mainly composed of light grey calcareous sandstones (formerly calcretes) overlain by dark grey sands. The literature suggests that palaeolake Makgadikgadi I formed and developed intermittently after initial downwarping in the early–mid Pleistocene. The calcareous sandstones were formed when calcium carbonate precipitation took place in pre-existing Kalahari sands along the western shoreline of Makgadikgadi I. Field evidence, supported by X-ray diffraction and SEM analyses, indicates that CaCO3precipitated mainly in marshy conditions around plant roots and stems and in association with bacteria in embayments along the lakeshore. The sandstones thickened and became partially indurated as a result of increasing palaeolake levels. Deposition was terminated by renewed tectonism which uplifted the shoreline zone relative to the lake basin, leading to falling palaeolake levels. Post-uplift reworking led to case hardening and pedogenic calcrete formation in the upper sections of the calcareous sandstones. Sedimentary conditions altered during the late Pleistocene. Extensive distributaries from the proto-Okavango system incised the shoreline ridge contributing to the filling of Makgadikgadi II. Satellite data suggest that the proto-Okavango rivers formed a series of fan deltas at this time along the western Makgadikgadi basin. Widespread dispersal of fluvial grey sands took place as a result of basin tilting which led to anastomosing channels flowing southward possibly around 18,000 B.P. These results, although preliminary in nature, augment previous geomorphological analyses by adding some detail in terms of depositional environments and by providing a tentative age and origin for the ubiquitous grey sands.

Seasonal variation in abundance and sex ratio of Grey Nurse (Sand Tiger) Sharks Carcharias taurus in northern New South Wales, Australia: a survey based on observations of recreational scuba divers

The Grey Nurse or Sand Tiger Shark Carcharias taurus is a protected species in Australian waters. In order to gain an insight into this shark's migratory habits and relative abundance at popular recreational diving sites, a survey was conducted using the observations of recreational divers' in northern New South Wales coastal waters over 15 months from August 1996 to October 1997. The bulk of shark sightings were reported during seasons of low diver activity and when sea surface temperatures were around 20?21?C. The number of reported sightings in each month was adjusted for variations in diver activity (i.e., sampling effort) to give an index of shark abundance. More southerly sites experienced peak shark abundance from August to November 1996, whereas sharks were most common at more northerly sites from April to June 1997, suggesting either that the sharks were migrating northwards, or that seasonal movement into shallower waters was occurring later at the northern sites. The sex ratio of the population shifted from a majority of females in spring to a majority of males in autumn/winter at the northern sites, indicating that the movements of the sexes may differ. Management strategies for this species, such as providing adequate protection of habitat at critical localities and times, require more detailed knowledge of this shark's migratory pattern, and the timing of reproductive events.

Aeolian deposition and its effect on soil and vegetation changes on stabilised desert dunes in northern China

Spatial and temporal patterns of aeolian deposition at Shapotou, northern China, were studied on a chronosequence (0, 12, 29 and 37 years) of stabilised desert dunes. Aeolian deposition markedly influenced soil and vegetation changes on the dunes. The spatial pattern of aeolian deposition was studied using dust traps and measurements of the depth of aeolian accumulation. Mean deposition from August 1993 to July 1994 was 372 g/m 2, and showed a complex spatial pattern, determined by prevailing wind direction, topography and shrub distribution. Contrasts between the topographic pattern of aeolian deposition (trap data) and accumulation (thickness of aeolian deposits) indicate that the measured pattern of aeolian deposition alone is insufficient to explain the pattern of accumulation. An accretionary surface soil or `grey sand' (containing much aeolian-derived particles) is developing on the stabilised dunes at a mean rate of 1.6 mm/year. Rapid accumulation of aeolian particles significantly changed the surface environment. Increased moisture retention by the finer `grey sand' resulted in decreased moisture penetration and subsequent desiccation of the deeper dune sands. This has led to vegetation changes, with the deep-rooted, planted xerophytic shrubs declining from 12 to only 3% cover after 37 years of stabilisation. The shrubs were replaced by a widespread microphytic crust and later by shallow-rooted annual species, which increased from 0 to approximately 12% cover over the same period. The implications of projected increases in the dustiness of arid regions on the stability of desert dunes and vegetated desert margins are discussed. Stabilised dunes may become increasingly vulnerable to deflation with the progressive decline in shrub cover.