THE RELATIVE CONTRIBUTION OF SOIL TILLAGE AND OVERLAND FLOW EROSION TO SOIL REDISTRIBUTION ON AGRICULTURAL LAND

This study uses evidence for the long-term (35 years) pattern of soil redistribution within two agricultural fields in the UK to identify the relative importance of tillage and overland flow erosion. Spatially distributed long-term total soil redistribution data for the fields (Dalicott Farm and Rufford Forest Farm) were obtained using the caesium-137 (¹³⁷Cs) technique. These data were compared with predicted patterns of soil redistribution. Recent studies have demonstrated that the redistribution of soil by tillage may be described as a diffusive process. A two-component model was, therefore, developed which accounts for soil redistribution by both overland flow and diffusive processes. Comparison of the predicted patterns of overland flow erosion alone with the observed (¹³⁷Cs-derived) data indicated a poor agreement (r² = 0.17 and 0.11). In contrast, a good agreement exists between the predicted pattern of diffusive redistribution and the observed data (r² = 0.43 and 0.41). These results give a clear indication that diffusive processes are dominant in soil redistribution within these fields. Possible diffusive processes include splash erosion, soil creep and tillage. However, the magnitude of the diffusion coefficients for the optimum predicted pattern (c. 350–400 kg m⁻¹ a⁻¹) demonstrates that tillage is the only process capable of explaining the very significant soil redistribution which is indicated by the ¹³⁷Cs data. Consideration is given to the implications of these results for both soil erosion prediction and landscape interpretation.     

The shelf sand-plume model: a critique

In recent years, a new model for deposition of sand bodies in a shelf environment has appeared. This model, known as the shelf sand-plume model, is hypothesized to result from storm-driven currents that are deflected around a deltaic headland, stripping sand from the headland and redepositing it in a downcurrent ‘plume’ on the inner shelf. The modern analogue for this model is considered to be an arcuate shelf sand body located off the Damietta branch of the Nile Delta. However, the distribution of older deltaic and shoreline sands probably controls the arcuate outline of the sand body. The present current system has certainly reworked these sands into ridges and large-scale bedforms but is not responsible for the overall outline of the sand body. Grain-size range and distribution of sand on the shelf demonstrate that the source of sand in the Nile shelf sand body is not the modern Damietta headland as postulated by the shelf sandplume model. In our view, the shelf sand-plume model is presently unsubstantiated and has orginated as a misapplication of the original Nile example. As a geological model, the shelf sand-plume model lacks a set of observable, consistently applicable criteria. The only common denominator to the model is the ‘plume’ geometry of a sand body located off a deltaic promontory. However, workers postulating the existence of shelf sand-plumes have neither clearly established a ‘plume’ geometry nor shown the juxtaposition of these bodies with respect to coeval deltaic headlands in their outcrop or subsurface examples. The model does not provide criteria to distinguish a ‘shelf sand-plume’ from other classes of shelf sand bodies, notably sand ridges and storm-generated sheet-like sands. Its application to the rock record should be re-evaluated.     

Dosage du Fluor dans les Etalons Géochimiques Du CRPG et de l'ANRT

La teneur en fluor, déterminée au moyen d'une électrode spécifique d'ions dans quelques standards géochimiques, est présentée.     

Modelling tidal current‐induced bed shear stress and palaeocirculation in an epicontinental seaway: the Bohemian Cretaceous Basin,Central Europe

Lower to Middle Turonian deposits within the Bohemian Cretaceous Basin (Central Europe) consist of coarse‐grained deltaic sandstones passing distally into fine‐grained offshore sediments. Dune‐scale cross‐beds superimposed on delta‐front clinoforms indicate a vigorous basinal palaeocirculation capable of transporting coarse‐grained sand across the entire depth range of the clinoforms (ca 35 m). Bi‐directional, alongshore‐oriented, trough cross‐set axes, silt drapes and reactivation surfaces indicate tidal activity. However, the Bohemian Cretaceous Basin at this time was over a thousand kilometres from the shelf break and separated from the open ocean by a series of small islands. The presence of tidally‐influenced deposits in a setting where co‐oscillating tides are likely to have been damped down by seabed friction and blocked by emergent land masses is problematic. The Imperial College Ocean Model, a fully hydrodynamic, unstructured mesh finite element model, is used to test the hypothesis that tidal circulation in this isolated region was capable of generating the observed grain‐size distributions, bedform types and palaeocurrent orientations. The model is first validated for the prediction of bed shear stress magnitudes and sediment transport pathways against the present‐day North European shelf seas that surround the British Isles. The model predicts a microtidal to mesotidal regime for the Bohemian Cretaceous Basin across a range of sensitivity tests with elevated tidal ranges in local embayments. Funnelling associated with straits increases tidal current velocities, generating bed shear stresses that were capable of forming the sedimentary structures observed in the field. The model also predicts instantaneous bi‐directional currents with orientations comparable with those measured in the field. Overall, the Imperial College Ocean Model predicts a vigorous tide‐driven palaeocirculation within the Bohemian Cretaceous Basin that would indisputably have influenced sediment dispersal and facies distributions. Palaeocurrent vectors and sediment transport pathways however vary markedly in the different sensitivity tests. Accurate modelling of these parameters, in this instance, requires greater palaeogeographic certainty than can be extracted from the available rock record.     

Bulk Composition of IRSID Iron Ore and Other Non- Metallic Reference Samples

A brief account of analytical information on thirteen IRSID non-metallic reference samples of direct interest to iron and steel industry is presented.     

Rapid Temporal Changes in Ocean Island Basalt Composition: Evidence from an 800 m Deep Drill Hole in Eiao Shield (Marquesas)

The Dominique drill hole has penetrated the volcanic shieldof Eiao island (Marquesas) down to a depth of 800 m below thesurface and 691•5 m below sea-level with a percentage ofrecovery close to 100%. All the lavas encountered were emplacedunder subaerial conditions. From the bottom to the top are distinguished:quartz and olivine tholeiites (800–686 m), hawaiites,mugearites and trachyte (686–415 m), picritic basalts,olivine tholeiites and alkali basalts (415–0 m). The coredvolcanic pile was emplaced between 5•560•07 Ma and5•220•06 Ma. Important chemical changes occurred during this rather shorttime span (0•34 0•13 Ma). In particular, the lowerbasalts differ from the upper ones in their lower concentrationsof incompatible trace elements and their Sr, Nd and Pb isotopicsignature being closer to the HIMU end-member, whereas the upperbasalts are EM II enriched. The chemical differences betweenthe two basalt groups are consistent with a time-related decreasein the degree of partial melting of isotopically heterogeneoussources. It seems unlikely that these isotopic differences reflectchanges in plume dynamics occurring in such a short time span,and we tentatively suggest that they result from a decreasingdegree of partial melting of a heterogeneous EM II–HIMUmantle plume. Some of the intermediate magmas (the uppermost hawaiites andmugearites) are likely to be derived from parent magmas similarto the associated upper basalts through simple fractionationprocesses. Hawaiites, mugearites and a trachyte from the middlepart of the volcanic sequence have Sr–Nd isotopic signaturessimilar to those of the lower basalts but they differ from themin their lower ²⁰⁶Pb/²⁰⁴Pb ratios, resulting in an increasedDMM signature. Some of the hawaiites-mugearites also displayspecific enrichments in P₂O₅, Sr and REE which are unlikelyto result from simple fractionation processes. The isotopicand incompatible element compositions of the intermediate rocksare consistent with the assimilation of MORB-derived wall rocksduring fractional crystallization. The likely contaminant correspondsto Pacific oceanic crust, locally containing apatite-rich veinsand hydrothermal sulphides. We conclude that a possible explanationfor the DMM signature in ocean island basalts is a chemicalcontribution from the underlying oceanic crust and that studiesof intermediate rocks may be important to document the originof the isotopic features of plume-derived magmas. KEY WORDS: alkali basalt; assimilation; mantle heterogeneity; Marquesas; tholeiile ^*Corresponding author     

SURFACE ROUGHNESS EVOLUTION OF SOILS CONTAINING ROCK FRAGMENTS

Soil surface roughness is a dynamic property which determines, to a large extent, erosion and infiltration rates. Although soils containing rock fragments are widespread in the Mediterranean region, the effect of the latter on surface roughness evolution is yet poorly understood. Therefore, laboratory experiments were conducted in order to investigate the effect of rock fragment content, rock fragment size and initial moisture content of the fine earth on the evolution of interrill surface roughness during simulated rainfall. Surface elevations of simulated plough layers along transects of 50 cm length were measured before and after simulated rainfall (totalling 192.5 mm, I = 70 mm h⁻¹) with a laser microreliefmeter. The results were used to investigate whether systematic variations in interrill surface roughness along stony hillslopes in southeastern Spain could be attributed to rock fragment cover and rock fragment size. Soil surface elevations were measured along the contour lines (50 cm long transects) with a contact microreliefmeter. Roughness was expressed by two parameters related to the height and frequency of roughness elements, respectively: standard deviation of de-trended surface elevations (random roughness: RR), and correlation length (L) derived from exponential fits of the autocorrelation functions. The frequently used assumption that surface roughness (RR) of cultivated topsoils decreases exponentially with cumulative rain is not valid for soil surfaces covered by rock fragments. The RR of soils containing small rock fragments (1.7–2.7 cm) increased with cumulative rainfall after an initial decrease during the first 17.5 mm of rainfall. For soils containing large rock fragments (7.7 cm), RR increased with rainfall above a threshold rock fragment content by mass of 52 per cent. For a given rainfall application, RR increased non-linearly with rock fragment content. The correlation length for soils containing small rock fragments decreases with rock fragment content and is significantly lower than for soils with large rock fragments. Soils covered with small rock fragments (large RR and small L) are thus well protected against raindrop impact by a water film in the depressions between the rock fragments. On abandoned agricultural fields along hillslopes in southeastern Spain, rock fragments cover increases non-linearly with slope owing to selective erosion of finer particles on steep slopes. The increase of surface cover by large rock fragments (>25 mm) is even more pronounced. The simultaneous increase of rock fragment cover and rock fragment size with slope explains the non-linear increase of RR with slope. These relationships differ for soils covered by platy misaschists and those covered with cubic andesites. The variations in correlation length along the hillslopes are not clear, probably owing to a simultaneous increase in rock fragment cover and rock fragment size. These findings may provide a better prediction of soil surface roughness of interrill areas covered by rock fragments using slope angle and lithology.     

An exploratory study on the use of enzyme activities as sediment tracers: biochemical fingerprints?

Little information exists on the potential of soil enzyme activities,which are sensitive to soil properties and management,for the characterization of sediment sources at the catchment scale.The objective of this study is to explore and evaluate enzyme activity as tracer for sediment fingerprinting in the Hiv catchment(55 km~2),Iran.Therefore,four enzymes were measured from 42 different sampling sites,covering three sediment source areas(rangeland/surface erosion,orchard/surface erosion,and streambank erosion),as well as from 12 sediment samples from reservoir check dams (sediment sinks).The results indicate that,based upon backward mode discriminant analysis,βglucosidase and dehydrogenase,allowed more than 95%of the samples to be correctly assigned to their source areas.These enzymes were selected as input data for a mixing-model to determine the relative contribution of the sampled sediment sources.The mean contributions from rangeland, orchard and streambank sources in the study area were estimated as 11.3%,15.1%and 73.7%, respectively.Using geochemical tracers,the mean contribution from rangeland,orchard and streambank sources was estimated as 14.1%,9.5%and 74.8%,respectively.Combined biochemical and geochemical tracers,similar values were obtained(18.7%,10.7%and 70.7%,respectively).Our results indicate that soil enzyme activity allows for a good characterization of sediment sources,and can provide a complementary tool to currently existing sediment fingerprinting approaches.However, the method should be also tested in other regions.