Lake catchment based studies of erosion and denudation in the merevale catchment,Warwickshire, U.K.

Sediment yields estimated from contemporary stream monitoring of suspended sediment in the Merevale forested catchment, North Warwickshire, were compared with the sediment yield record obtained from analysis of lake sediment in the downstream reservoir. Total sediment volume and mass for nine periods since 1861 were calculated by extrapolation of synchronous levels in 54 cores, identified from magnetic susceptibility and other magnetic measurements, and by using ²¹⁰Pb and ¹³⁷Cs analysis to provide an absolute sediment chronology. Sediment yield estimated from the two methods show comparable but low (50-200 kg ha^?1 yr^?1) levels of sediment loss. While suggesting that lake sediments can be a useful means for extending the period over which contemporary monitoring exists, the study stresses the need for detailed historical records of land use and climate against which long term sediment yield records can be evaluated.     

A geochronological framework for orogenic gold mineralisation in central Victoria, Australia

New ⁴⁰Ar/³⁹Ar geochronological data support, and significantly expand upon, preliminary age data that were interpreted to suggest an episodic and diachronous emplacement of gold across the western Lachlan fold belt, Australia. These geochronological data indicate that mineralisation in the central Victorian gold province occurred in response to episodic, eastward progressing deformation, metamorphism and exhumation associated with the formation of the western Lachlan fold belt. Initial gold formation throughout the Stawell and the Bendigo structural zones can be constrained to a broad interval of time between 455 and 435 Ma, with remobilisation of metals into new structures and/or new pulses of mineralisation occurring between 420 and 400 Ma, and again between 380 and 370 Ma, linked to episodic variations in the regional stress-field and during intrusion of felsic dykes and plutons. This separation of ages is incompatible with the view that gold emplacement in the western Lachlan fold belt was the result of a single, orogen-wide event during the Devonian. A distinct phase of gold mineralisation, characterised by elevated Cu, Mo, Sb or W, is associated with both Late Silurian to Early Devonian (~420 to 400 Ma) and Middle to Late Devonian (~380 to 370 Ma) magmatism, when crustal thickening and shortening during the ongoing consolidation of the western Lachlan Fold Belt led to extensive melt development in the lower crust and resulted in widespread magmatism throughout central Victoria. These ~420 to 400 Ma and ~380 to 370 Ma occurrences, best exemplified by the Wonga deposit in the Stawell structural zone and many of the Woods Point deposits in the Melbourne structural zone, but also evidenced by occurrences at Fosterville and Maldon in the Bendigo structural zone, clearly formed synchronous with, or post-date, the emplacement of plutons and dykes, and thus are spatially (if not genetically) related to melt generation at depth. This later, magmatic-associated and polymetallic type of gold mineralisation is economically subordinate to the earlier, metamorphic-associated type of gold deposition in the Stawell and Bendigo structural zones, but tends to be the dominant style in the Melbourne Zone. These new geochronological constraints, together with zircon U-Pb data from felsic intrusive rocks of known relationship to gold mineralisation, demonstrate that initial hydrothermal alteration associated with gold emplacement in the western Lachlan fold belt was metamorphic-related, predating the emplacement of granite plutons by as much as 80 million years. This timing differs from other important orogenic gold districts where gold deposition is closely associated spatially with felsic magmatism. The early introduction of metamorphically derived fluids well before magmatism may reflect variations in the timing of peak metamorphic conditions at different crustal levels in an accretionary prism undergoing simultaneous deformation and erosion. Consequently, no genetic link exists between the main phase(s) of gold mineralisation and magmatism in the central Victorian gold province. With the exception of formation of a minor magmatism-related and geochemically-distinct mineralisation style at about 420 to 400 Ma, and again at about 380 to 370 Ma, the apparent spatial relationship between gold mineralisation and felsic intrusions is merely the result of melts and fluids being channelised along the same structures.     

Sediment delivery from agricultural land to rivers via subsurface drainage

Diffuse sources of sediment and sediment‐associated nutrients are of increasing environmental concern because of their impacts on receiving water courses. The aim of the research reported here was to monitor the outflow from four field (land) drains at two farms in the English Midlands in order to estimate the quantity of sediment delivered to the local rivers and the most likely sources and processes involved. A multiparameter sediment unmixing model was employed, using environmental magnetic, geochemical and radionuclide tracers in order to determine the most likely origin of sediments transported through the drains. Results demonstrated that there was a generally linear relationship between drainflow sediment loss and drainflow volume and that the majority (>70%) of the sediment exported from the drains was derived from topsoil. Macropore flow through heavily cracked soils is supported by the data to be the most likely means of sediment delivery to the drains. In one catchment, drains contributed over 50% of the annual sediment budget. Spatial and temporal variations in the sources of sediment reaching one drain outlet were investigated in detail. A link between soil moisture deficit (SMD) and the frequency of high‐intensity rainfall events was used to explain the appearance and persistence of a new sediment source in this drain after October 1998. It is concluded that field drains have the potential to be significant conduits of sediment and agrochemicals in a wide variety of environments in the UK. It is also suggested that this potential may increase if projected climate change leads to more intense rainfall events and increases in SMD across a greater area of the UK. Copyright © 2005 John Wiley & Sons, Ltd.     

Experiments to Study Radiation Transport in Clumpy Media

Clumpiness of the interstellar medium may play an important role in the transfer of infrared continuum radiation in star forming regions (Boisse, 1990). For example, in homogeneous models, C II emission should be confined to the cloud edge (Viala, 1986). However, in star formation regions (such as M17SW, M17 and W51), it is observed to extend deep into the molecular cloud (Stutzki et al., 1988; Keene et al., 1985). One plausible interpretation of these observations is that, due to their clumpiness, the clouds are penetrated by UV radiation far deeper than expected from simple homogeneous models. The interaction of H II regions around young massive stars with a clumpy medium is another area of interest. Molecular clouds are well established to be clumpy on length scales down to the limits of observational resolution. Clumps can act as localized reservoirs of gas which can be injected into the surroundings by photoionization and/or hydrodynamic ablation (Dyson et al., 1995; Mathis et al., 1998). The calculation of radiation transport in hot, clumpy materials is a challenging problem. Approximate, statistical treatments of this problem have been developed by several workers, but their application has not been tested in detail. We describe laboratory experiments, using the Omega laser to test modelling of radiation transport through clumpy media in the form of inhomogeneous plasmas.     

Clearing enhances dust devil formation

In the southwest of Western Australia, the large scale replacement of native perennial vegetation with agriculture based on winter growing annual species leads to a significant change in the surface albedo and roughness. Whilst the redistribution of the surface energy balance impacts on cloud climatology, the reduced roughness of the agricultural area has enhanced the injection of aerosols into the atmosphere through the increased potential for dust devils. Although the native vegetation experiences a higher sensible heat flux, it is the reduced frictional drag of the cleared agricultural lands that enhances the potential for dust devil formation and thus provides an additional source of atmospheric aerosols.