Grain‐size variability within a mega‐scale point‐bar system,False River,Louisiana

Point bars formed by meandering river systems are an important class of sedimentary deposit and are of significant economic interest as hydrocarbon reservoirs. Standard point‐bar models of how the internal sedimentology varies are based on the structure of small‐scale systems with little information about the largest complexes and how these might differ. Here a very large point bar (>25·0 m thick and 7·5 × 13·0 km across) on the Mississippi River (USA) was examined. The lithology and grain‐size characteristics at different parts of the point bar were determined by using a combination of coring and electrical conductivity logging. The data confirm that there is a general fining up‐section along most parts of the point bar, with a well‐defined transition from massive medium‐grained sands below about 9 to 11 m depth up into interbedded silts and fine–medium sand sediment (inclined heterolithic strata). There is also a poorly defined increase in sorting quality at the transition level. Massive medium sands are especially common in the region of the channel bend apex and regions upstream of that point. Downstream of the meander apex, there is much less evidence for fining up‐section. Finer sediment accumulated more readily after the establishment of a compound bar in the later stages of construction, at the terminal apex and in the bar tail. This work implies that the best reservoir sands are likely to be located in the centre of the point bar, deposited in a simple bar system. Reservoir quality decreases towards the bar edge. The early‐stage channel plug is largely composed of coarsening‐upward cycles of silt to clay and is dominated by clay and clayey silt material with poor reservoir characteristics.     

Strontium in coral aragonite: 3. Sr coordination and geochemistry in relation to skeletal architecture

Use of the coral Sr palaeothermometer assumes that the Sr in coral skeletons is substituted randomly for Ca in the aragonite structure. The presence of Sr in additional phases e.g., strontianite, or the non random distribution of Sr across metal sites in aragonite, would complicate the Sr/Ca-sea surface temperature relationship. We have used Sr K-edge microEXAFS (extended X-ray absorption fine structure) to determine the structural state of Sr across selected microvolumes of four coral skeletons (Porites lobata, Acropora palmata, Pavona clavus, and Montastrea annularis). We used a 5 × 3 μm beam to analyse specific areas of the coral skeletal architecture, i.e., centres of calcification, fasciculi, and dissepiments. All EXAFS analyses refine, within error, to an ideally substituted Sr in aragonite, and we found no evidence of strontianite or partly ordered structural states. Anisotropy in the first shell responses results from the fact that the analysed microvolumes are not necessarily averaged for the responses of all crystal orientations in the aragonite. Although secondary ion mass spectrometry confirmed that Sr/Ca composition can vary substantially between skeletal components, we find no evidence for any contrast in Sr structural state. Sr heterogeneity may result from kinetic effects, reflecting complex disequilibrium processes during crystal precipitation, or biological effects, resulting from variations in the composition of the calcifying fluid which are biologically mediated.     

A Technology Fusion Approach for Augmented Reality Applications

Technology fusion through the integration of multiple sensors and data sources is a common feature of current spatial information applications. Augmented reality is one such example. Augmented reality is a technology that overlays views of the real world with digital information. It provides an innovative application of spatial databases and can be applied in both real‐time and non‐real‐time applications. In order to correctly align digital (augmented) data with views of the real world, position and attitude determination is required. This paper describes the development of an augmented reality prototype, which includes the design and implementation of a position and attitude determination component. The prototype relies on spatial information to improve the accuracy of position and attitude determination. Successful field tests of the prototype are carried out in Melbourne, Australia. The field tests are focused around a case study that uses the prototype to enable drivers to ‘see’ the road and surrounding vehicles despite heavy fog, rain, snow or other low visibility situations.     

Field evidence for the control of grain size and sediment supply on steady‐state bedrock river channel slopes in a tectonically active setting

We exploit a natural experiment in Boulder Creek, a ~ 30 km² drainage in the Santa Cruz mountains, CA, USA to explore how an abrupt increase in the caliber of bedload sediment along a bedrock channel influences channel morphology in an actively uplifting landscape. Boulder Creek's bedrock channel, which is entirely developed on weak sedimentary rock, has a high flow shear stress that is about 3.5 times greater where it transports coarse (~ 22 cm D₅₀) diorite in the lower reaches in comparison with the upstream section of the creek that transports only relatively finer bedload (~2 cm D₅₀) derived from weak sedimentary rocks. In addition, Boulder Creek's channel abruptly widens and shallows downstream and transitions from partial to nearly continuous alluvial cover where it begins transporting coarse diorite. Boulder Creek's tributary channels are also about three times steeper where they transport diorite bedload, and within the Santa Cruz mountains channels in sedimentary bedrock are systematically steeper when >50% of their catchment area is within crystalline basement rocks. Despite this clear control of coarse sediment size on channel slopes, the threshold of motion stress for bedload, alone, does not appear to control channel profile slopes here. Upper Boulder Creek, which is starved of coarse sediment, maintains high flow shear stresses well in excess of the threshold for motion. In contrast, lower Boulder Creek, with a greater coarse sediment supply, exerts high flow stresses much closer to the threshold for motion. We speculate that upper Boulder Creek has evolved to sustain partial alluvial cover and transfer greater energy to the bed via bedload impacts to compensate for its low coarse sediment supply. Thus bedload supply, bedrock erosion efficiency, and grain size all appear to influence channel slopes here. Copyright © 2017 John Wiley & Sons, Ltd.     

Clast exposure on boulder-covered desert slopes

Variation in the degree of clast exposure on stony desert slopes is examined in an area of northeast Jordan. Geological influences on the characteristics of stone mantles are modified by relative slope position. Observations that the nature of the surface stone cover changes downslope because of the transport and accumulation of the underlying substrate are confirmed by quantitative analysis. An elliptic function is described as a means of estimating relative clast exposure from simple field measurements of clast dimensions. There are no significant differences in mean clast exposure between the four main basalt lithologies of the study area, but a distinctive pattern of clast exposure catenas can be identified. Convex slopes maintain relatively high levels of clast exposure from crest to toe. About two-thirds of the variation in clast exposure is accounted for by lithology and four slope variables: relative relief, slope length, gradient and curvature. Copyright © 1999 John Wiley & Sons, Ltd.     

Convective Self-Aggregation in Numerical Simulations: A Review

Organized convection in the tropics occurs across a range of spatial and temporal scales and strongly influences cloud cover and humidity. One mode of organization found is “self-aggregation,” in which moist convection spontaneously organizes into one or several isolated clusters despite spatially homogeneous boundary conditions and forcing. Self-aggregation is driven by interactions between clouds, moisture, radiation, surface fluxes, and circulation, and occurs in a wide variety of idealized simulations of radiative–convective equilibrium. Here we provide a review of convective self-aggregation in numerical simulations, including its character, causes, and effects. We describe the evolution of self-aggregation including its time and length scales and the physical mechanisms leading to its triggering and maintenance, and we also discuss possible links to climate and climate change.     

The distribution of deuterium and O in dry soils : 1. Theory

A mathematical model is developed describing the shape of H₂¹⁸O and HDO depth profiles which result from evaporation of water from dry soil under quasi-steady state conditions. Typically, isotope concentrations rise from a minimum at the soil surface to a maximum a short distance beneath the surface, and then decrease approximately exponentially to constant concentrations at depth. The model predicts that for isothermal conditions, the slope of the relationship between ¹⁸O and deuterium δ-values of samples of the soil water will be 30% lower for a dry soil than for a wet soil evaporating under the same conditions. It is concluded that low slopes should be indicative of soil water or groundwater recharged under arid or semi-arid conditions. Using the shape of the ¹⁸O and deuterium profiles, three independent methods for estimating evaporation for dry soils are developed. When water loss occurs by both transpiration and evaporation, the slope of the ¹⁸O-D relationship should be slightly lower than that for a site where water loss occurs by evaporation alone.     

The use of natural tracers as indicators of soil-water movement in a temperate semi-arid region

In a semi-arid area of southern Australia a change in land use from Eucalyptus scrub to cropping with wheat is shown to have caused considerable change in the mechanism of the movement of soil water and the amount of deep drainage. Chloride concentrations of soil water have been used to show the mean annual amount of deep drainage increases from less than 0.1 to 3 mm yr.⁻¹ following clearing of the native vegetation.

The concentration of environmental tritium in soil water beneath the native vegetation is consistent with the hypothesis that some relatively recent water (post 1960) has penetrated to depths of at least 12m along channels occupied by living roots. Where the native vegetation has been cleared, no water which fell as rain since 1960 was found at depths greater than 2.5 m.

¹⁸O and deuterium concentrations suggest that some water movement to the roots of the native vegetation is in the vapour phase.     

Compression behaviour of minerogenic low energy intertidal sediments

Existing geotechnical approaches that describe volumetric changes in intertidal sediments in response to applied vertical effective stresses are limited by a lack of empirical research into their one-dimensional compression behaviour. In this paper we address this deficiency by presenting the results of an investigation into the compression behaviour of minerogenic low marsh and tidal flat sediments. We have tested samples of these sediment types obtained from Greatham Creek (Cowpen Marsh, Tees Estuary, UK). Analysis of physical properties and oedometer compression tests demonstrates that, contrary to the implicit assumptions of existing models, the surface sediments studied are overconsolidated. Structural variability between samples arises due to sedimentological factors, notably variations in organic content. We attribute overconsolidation to tidal exposure and falls in groundwater level that permit desiccation and cause capillary suction stresses. Greater rates of compression with respect to effective stress occur in sediments with higher initial voids ratios and more open, unstable initial structures. Variability in structure decreases with application of higher effective stresses due to the destructuration of the sediments, which also creates increased homogeneity of compression behaviour under higher effective stresses. We subsequently develop a new conceptual framework to describe compression behaviour in minerogenic intertidal sediments that incorporates overconsolidation. We advocate a statistical approach that accounts for structural variability and variations in compression behaviour at effective stresses less than and greater than the yield stress. We argue that our conceptual framework is broadly applicable to minerogenic intertidal sediments at different locations and burial depths within Holocene stratigraphic sequences providing site-specific compression data are collected. Inter-site transfer and application of measured material properties should not be undertaken due to local variations in compression behaviour resulting from varying ecological, sedimentological, geochemical, climatic, geomorphic and hydrographic conditions. The individual characteristics of different field locations should be carefully considered before the suggested framework is routinely applied.     

The water balance of a small lake using stable isotopes and tritium

A small maar lake, known as the Blue Lake, set in a karstic region of southeastern Australia, provides the municipal water supply for a population of 20,000. The lake has a volume of 3.6·10⁷m³, of which 10–15% is pumped from it each year. The lake is recharged almost entirely from groundwater and the main objective of this study was to estimate the total groundwater inflow and outflow rates. Estimation of groundwater throughflow in a lake is difficult to assess using classical hydrological techniques and an alternative method involving measurement of the concentrations of the environmental isotopes ³H, ¹⁸O, ²H and ¹⁴C in the lake water and in the recharging groundwater was used to establish the lake-water balance. The water-balance calculations indicated a total groundwater inflow to Blue Lake of between 5.0·10⁶ and 6.5·10⁶m³yr.⁻¹, corresponding to a residence time of water in the lake of 6 yr. It was not possible to derive the relative proportions of inflow to the lake from the two possible source aquifers, using these isotopes, because their concentrations did not show a sufficiently large contrast to distinguish the two water sources.