Re‐examination of Bagnold's empirical bedload ­formulae

Bagnold developed his formula for bedload transport over several decades, with the final form of the relation given in his 1980 paper. In this formula, bedload transport rate is a function of stream power above some threshold value, depth and grain size. In 1986, he presented a graph which illustrated the strength of his relation. A double‐log graph of bedload transport rate, adjusted for depth and grain size, versus excess stream power was shown to collapse along a line having a slope of 1·5. However, Bagnold based his analyses on limited data. In this paper, the formula is re‐examined using a large data set in order to define the most consistent empirical representation, and dimensional analysis is performed to seek a rationalization of the formula. Functional analysis is performed for the final version of the equation defined by Bagnold to determine if the slope of 1·5 is preserved and to assess the strength of the relation. Finally, relations between excess stream power and bedload transport are examined for a fixed slope of 1·5 to assess the performance of various depth and grain size adjustment factors. The rational scaling is found to provide the best result. Copyright © 2000 John Wiley & Sons, Ltd.     

Diffusion in Landscape Development Models: On The Nature of Basic Transport Relations

In constructing large-scale landscape development models, processes must be appropriately represented. The diffusion equation is rewritten with separate coefficients for slow, continuous mass movements and rapid, episodic mass movements. Using available transport data, transport/gradient relations are assessed and estimates of diffusivities are given. Diffusivities estimated in this study are compared with values derived in scarp studies and values adopted in landscape development models. The relation between transport and gradient may be non-linear, which would require modification of the simple diffusion equation normally accepted in modelling. New approaches are required to place these essential relations on a firmer foundation. © 1997 by John Wiley & Sons, Ltd.     

An epeiric ramp: low-energy, cool-water carbonate facies in a Tertiary inland sea, Murray Basin, South Australia

The Murray Supergroup records temperate‐water carbonate deposition within a shallow, mesotrophic, Oligo‐Miocene inland sea protected from high‐energy waves and swells of the open ocean by a granitic archipelago at its southern margin. Rocks are very well preserved and exposed in nearly continuous outcrop along the River Murray in South Australia. Most facies are rich in carbonate silt, contain a background assemblage of gastropods (especially turritellids) and infaunal bivalves, and are packaged on a decimetre‐scale defined by firmground and hardground omission surfaces. Bioturbation is pervasive and overprinted, resulting in rare preservation of physical sedimentary structures. Facies are grouped into four associations (large foraminiferan–bryozoan, echinoid–bryozoan, mollusc and clay facies) interpreted to represent shallow‐water (<50 m) deposition under progressively higher trophic resource levels (from low mesotrophy to eutrophy), and restricted marine conditions from relatively offshore to nearshore regions. A large‐scale shift from high‐ to low‐mesotrophic conditions within lower Miocene strata reflects a change in climate from wet to seasonally dry conditions and highlights the influence terrestrially derived nutrients had upon this shallow, land‐locked sea. Overall, low trophic resource levels during periods of seasonally dry climate resulted in a deepening of the euphotic zone, a widespread proliferation of foraminiferan photozoan fauna and a relatively high carbonate productivity. Inshore, heterozoan facies became progressively muddier and restricted towards the shoreline. In contrast, periods of wet climate led to rising trophic resource levels, resulting in a shallowing of the euphotic zone, a decrease in epifaunal and seagrass cover and widespread development of a mostly heterozoan biota dominated by infaunal echinoids. Rates of carbonate production and accumulation were relatively low. The Murray Basin is best described as an epeiric ramp. Wide facies belts developed in a shallow sea on a low‐angled slope reaching many hundreds of kilometres in length. Grainy shoal and back‐barrier facies were absent. Internally generated waves impinged the sea floor in offshore regions and, because of friction along a wide and shallow sea floor, created a low‐energy expanse of waters across the proximal ramp. Storms were the dominating depositional process capable of disrupting the entire sea floor.     

Trace-element patterns at the Cretaceous-Tertiary boundary—Consequences of a large impact

The earlier discovery of high concentrations of iridium and osmium at the very base of the Tertiary in marine pelagic sequences, has led to the hypothesis that the impact of a large projectile on Earth terminated the Mesozoic era.This paper describes the distribution of other trace elements in samples closely spaced around the Cretaceous-Tertiary boundary. We found an enrichment of Co, Cr, Ni, As, Sb and Se, and a depletion of the rare earth elements (REE), coupled with the anomalously high concentration of Ir and Os. Also, in the enrichment layer, problematic sanidine spherules occur which may be spheres, ablated from the impacting projectile or microtektites.The REE depletion could not only be demonstrated in the basal millimetres of the relatively thick (～ 10 cm) boundary marl bed of the Barranco del Gredero section, Caravaca, Spain, but also in the boundary clay of the Stevns Klint section—the “Fiskeler”—Denmark. In these boundary clay beds are also the highest concentrations of Ir recorded [44 and 86.7 ppb (1 ppb = 10^?9 g/g), respectively].This REE depletion is tentatively connected with the low REE concentration in common meteorites. In that case the basal millimetres are dominated by a meteoritic component and represent a “fall-out” layer of the impact event. This in turn would imply low terrestrial dilution at impact on Earth, suggesting that the projectile might have fallen into the ocean.Identification of the projectile remains speculative; especially the high As, Sb and Se anomalies are problematic. These latter may point to a projectile of uncommon composition, possibly a comet.An integrated scenario of a possible sequence of events is presented, chiefly based on the interpretation of micropaleontological, lithological, geochemical and stable isotope data of the studied sections.The direct and indirect effects of an impact leading to mass extinction constitute a complicated problem and may comprise a sudden temperature rise, a poisoning by cyanide or arsenic, and a few years suppression of sunlight, or preferably, a combination of these effects.     

Determination of humidity and temperature fluctuations based on MOZAIC data and parametrisation of persistent contrail coverage for general circulation models

Humidity and temperature fluctuations at pressure levels between 166 and 290 hPa on the grid scale of general circulation models for a region coveredn by the routes of airliners, mainly over the Atlantic, have been determined by evaluation of the data obtained with almost 2000 flights within the MOZAIC programme. It is found that the distributions of the fluctuations cannot be modelled by Gaussian distributions, because large fluctuations appear with a relatively high frequency. Lorentz distributions were used for the analytical representation of the fluctuation distributions. From these a joint probability distribution has been derived for simultaneous temperature and humidity fluctuations. This function together with the criteria for the formation and persistence of contrails are used to derive the maximum possible fractional coverage of persistent contrails in a grid cell of a GCM. This can be employed in a statistical formulation of contrail appearance in a climate model.     

On the Exchange of NO3 Radicals with Aqueous Solutions: Solubility and Sticking Coefficient

The exchange of NO₃ radicals with the aqueous-phase was investigated at room temperature (293 K) in a series of wetted denuders. From these experiments, the uptake coefficient of NO₃ was determined on 0.1 M NaCl solutions and was found to be (NO₃) 2 × 10^-3 in good agreement with recent studies. The Henry coefficient of NO₃ was estimated to be K_H(NO₃) = 1.8 M · atm^-1, with a (2) uncertainty of ±3 M · atm^-1. From the upper limit for the Henry coefficient (K_H = 5 M · atm^-1) and available thermodynamic data, the redox potential of dissolved NO₃/NO ₃ ^– is estimated to be in the range of 2.3 to 2.5 V. This range is at the lower boundary of earlier estimates. The results are discussed in the light of a recent publication. Based on our data and a model of the transport and chemistry in the liquid film, an upper limit is derived for the product of the Henry coefficient K_H and the rate coefficient k ₁₀ of the potential reaction NO₃ + H₂O HNO₃ + OH. For K_H = 0.6 M · atm^-1, we find k ₁₀ < 0.05 s^-1 · atm^-1, i.e., about 100 times smaller than what was suggested by Rudich and co-workers. Because of its small solubility, heterogeneous removal of NO₃ is only important under conditions where the dissolved NO₃ is removed quickly from equilibrium, for example by reactions with Cl^– or HSO ₃ ^– ions in the liquid-phase. Otherwise, heterogenous removal should mainly proceed via N₂O₅.     

Proximal impact deposits at the Cretaceous-Tertiary boundary in the Gulf of Mexico: a restudy of DSDP Leg 77 Sites 536 and 540

Restudy of Deep Sea Drilling Project Sites 536 and 540 in the southeast Gulf of Mexico gives evidence for a giant wave at Cretaceous-Tertiary boundary time. Five units are recognized: (1) Cenomanian limestone underlies a hiatus in which the five highest Cretaceous stages are missing, possibly because of catastrophic K-T erosion. (2) Pebbly mudstone, 45 m thick, represents a submarine landslide possibly of K-T age. (3) Current-bedded sandstone, more than 2.5 m thick, contains anomalous iridium, tektite glass, and shocked quartz; it is interpreted as ejecta from a nearby impact crater, reworked on the deep-sea floor by the resulting tsunami. (4) A 50-cm interval of calcareous mudstone containing small Cretaceous planktic foraminifera and the Ir peak is interpreted as the silt-size fraction of the Cretaceous material suspended by the impact-generated wave. (5) Calcareous mudstone with basal Tertiary forams and the uppermost tail of the Ir anomaly overlies the disturbed interval, dating the impact and wave event as K-T boundary age. Like Beloc in Haiti and Mimbral in Mexico, Sites 536 and 540 are consistent with a large K-T age impact at the nearby Chicxulub crater.     

Role of granite intrusions for the formation of ring structures in granulite complexes: Examples from the Limpopo belt,South Africa

Ring structures are cylindrical or sheath folds with concentrically distributed beds, often with granites in the cores. This paper reports structural, petrographic, and petrological evidence for four such structures from the Central Zone of the Limpopo complex, which were formed during granulite exhumation in Neoarchean time (event D2/M2). It was demonstrated that the orientation of linear and planar elements in the rocks are practically identical and independent of the position within the Central Zone. All existing measurements (hundreds for the four structures) project within the same fields in stereograms. The fold axes plunge SW at an angle of ∼40° within the whole area of the Central Zone of the Limpopo complex. This implies that the rocks were metamorphosed and deformed during event D2/M2, which is typical of the Neoarchean stage of the development of the Limpopo granulite complex. Local mineral equilibria and fluid inclusions were studied in a series of key rocks, and P-T paths were derived for them. A gravitation mechanism was substantiated for the ascent of granulites and accompanying granite bodies. The structure of ring complexes was evaluated on the basis of various erosion sections. It was shown that stocklike granite bodies occur at the base of each ring structure. Petrochemical and structural data were used to demonstrate that the granites (2627 Ma) had been derived by the complete or partial melting of the lower parts of the section of Neoarchean (2651 Ma) country rocks. The upwelling of a less dense granite magma synchronously with the exhumation resulted in the helical squeezing of the overlying gneisses. This led to the concentric arrangement of beds and development of a ring structure, a sheath fold containing a granite core in some sections. A preliminary numeric 2D model is considered for the ascent of a granite diapir accompanied by the downwelling of colder and denser country rocks. A better understanding of this process can be gained by 3D numerical simulation.