Before and after riparian management: sediment and nutrient exports from a small agricultural catchment, Western Australia

Riparian vegetation can trap sediment and nutrients sourced from hillslopes and reduce stream bank erosion. This study presents results from a 10-year stream monitoring program (1991–2000), in a 6 km² agricultural catchment near Albany, Western Australia. After 6 years, a 1.7 km stream reach was fenced, planted with eucalyptus species and managed independently from the adjacent paddocks. Streamflow, nutrient and sediment concentration data were collected at the downstream end of the fenced riparian area, so there are data for before and after improved riparian management. Suspended sediment (SS) concentrations fell dramatically following improved riparian management; the median event mean concentration (EMC) dropped from 147 to 9.9 mg l⁻¹. Maximum SS concentrations dropped by an order of magnitude. As a result, sediment exports from the catchment decreased following improved riparian management, from over 100 to less than 10 kg ha⁻¹ yr⁻¹. Observations suggest that this was the result of reduced bank erosion and increased channel stability. Riparian management had limited impact on total phosphorus (TP) concentrations or loads, but contributed to a change in phosphorus (P) form. Before improved riparian management, around half of the P was transported attached to sediment, but after, the median filterable reactive P (FRP) to TP ratio increased to 0.75. In addition, the median FRP EMC increased by 60% and the raw median FRP concentration increased from 0.18 to 0.35 mg l⁻¹. These results suggest that there was a change in the dominant P form, from TP to FRP. Changes in total nitrogen (TN) following improved riparian management were less clear. There were reductions in TN concentrations at high flows, but little change in the loads or EMC. This study demonstrates the benefits of riparian management in reducing stream bank erosion, but suggests that in catchments with sandy, low P sorption soils, there may be limitations on the effectiveness of riparian buffers for reducing P and N exports.     

Quadratic jerk regulation and the seismic control of civil structures

Regulation of the total structural jerk is a means of managing the structural energy and enhancing the performance of civil structures undergoing large seismic events. A quadratic regulator is derived for the total structural jerk that produces a single algebraic Riccati equation to define the control gains. The resulting control method is tested using a realistic non‐linear structural control case study where the structural response is statistically quantified for large suites of scaled earthquakes. The control method developed is shown to be more effective than typical displacement‐focused active and semi‐active civil structural control methods. In particular, quadratic jerk regulation provides better performance than typical structural control methods for near‐field seismic events where the response is dominated by a large impulse, and relatively poorer results for far‐field seismic inputs where the response is vibratory. Hence, this type of control approach has strong potential for mitigating the damage for large impulse, near‐field events, where jerk regulation provides much more efficient response and damage management. Copyright © 2003 John Wiley & Sons, Ltd.     

Laser microprobe sulphur isotope analysis of arsenopyrite: experimental calibration and application to the Boliden Au–Cu–As massive sulphide deposit

Metamorphic remobilization of arsenopyrite-rich ores is a globally important process which can lead to significant concentrations of gold. In order to understand this and related processes, relations of sulphur isotopes can give a number of important clues. To resolve such relations in detail, we have successfully calibrated and applied a laser combustion system for in situ analysis of sulphur isotopic compositions of arsenopyrite. Experimental calibration of the laser fractionation factor (+0.4‰) was obtained by using compositionally and isotopically homogeneous natural samples from ore deposits at Boliden (Sweden) and Freiberg (Germany); subsequent to detailed microscopic study, the S isotope ratios of these samples were measured by conventional and laser combustion techniques.The present application to different types of arsenopyrite in the Palaeoproterozoic metamorphosed VHMS ores of the Boliden Au–Cu–As deposit, Skellefte district, northern Sweden, shows that the sulphur isotope composition of arsenopyrite is essentially unmodified during medium-grade metamorphic recrystallization and remobilization. Here, massive arsenopyrite ore is crosscut by later veins that carry a complex quartz–sulphosalt–sulphide assemblage. The latter ore type is markedly Au-rich compared to the host ore, and thus of significant economic interest. We find that both ore types exhibit very similar sulphur isotope compositions, ca. +2‰ to +3‰ (V-CDT), which is similar to most massive sulphide deposits in the Skellefte district. Thus, the crosscutting Au-rich vein ore has inherited the sulphur isotope composition from sulphur liberated by metamorphic reactions affecting the massive ore, and most likely also inherited the Au through this mechanism. The latter finding clearly has important implications both for the general discussion on, and the prospecting for, similar high-grade Au ores in this world-class mining region.     

Geology and diamond distribution of the 140/141 kimberlite, Fort à la Corne, central Saskatchewan, Canada

The Cretaceous age Fort à la Corne (FALC) kimberlite province comprises at least 70 bodies, which were emplaced near the edge of the Western Canadian Interior Seaway during cycles of marine transgression and regression. Many of the bodies were formed during a marine regression by a two-stage process, firstly the excavation of shallow, but wide, craters and then subsequent infilling by xenolith-poor, crater-facies, subaerial, primary pyroclastic kimberlite. The bodies range in size up to 2000 m in diameter but are mainly less than 200 m thick and thus comprise relatively thin, but high volume, pyroclastic kimberlite deposits. Each body is composed of contrasting types of kimberlite reflecting different volcanic histories and, therefore, are considered separately.

The 140/141 kimberlite is the largest delineated body in the province, estimated to have an areal extent below glacial Quaternary sediments in excess of 200 ha. The infilling of the 140/141 crater is complex, resulting from multiple phases of kimberlite. The central part of the infill is dominated by several contrasting phases of kimberlite. One of these phases is a primary pyroclastic airfall mega-graded bed up to 130 m in thickness. The constituents grade in size from very fine to coarse macrocrystic kimberlite, through to a basal breccia. The mega-graded bed is a widespread feature within parts of the body examined to date and at this current stage of evaluation appears to explain a variable diamond distribution within a tested portion of the pipe. A second different phase of kimberlite is interpreted as representing a younger nested crater within the mega-graded bed. Centrally located thicker intersections (>450 m) of this younger kimberlite may indicate a vent for the kimberlite crater. The thickness of the mega-graded bed increases with proximity to the younger kimberlite in the study area.

Macrodiamond minibulk sample grades from the mega-graded bed have been obtained from nine large diameter drill holes, located within the northwest part of the body from an area of 20 ha, which represents approximately 10% of the currently modeled kimberlite outline. Diamond grade increases with depth within the mega-graded bed and also increases, within the same unit, towards the centrally positioned younger kimberlite. Macrodiamond sample grades vary from low at the top of the mega-graded bed, to considerably higher grades near the base. Total sample grade per drill hole varies from moderate near the vent feature to lower grades 200–300 m from the vent feature. Macrodiamond stone frequency measured in stones per tonne shows a pronounced relationship with depth and proximity to the vent feature within the mega-graded bed. There is a strong correlation between depth and increased stones per tonne, and a similar correlation between stones per tonne and proximity to the vent feature. The data supports the emplacement model of the mega-graded bed and, in turn, this information is useful in understanding the macrodiamond distribution within this bed.     

Post-Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (Western Iberian margin)

Seismic reflection and well stratigraphic data are used to investigate the post‐Jurassic evolution of the Northern Lusitanian Basin, offshore west Iberia. Stratigraphic correlations between 11 exploration wells were attained in order to characterize the variations in depositional facies associated with salt tectonics. Latest Triassic–Hettangian salt, which generated multiple salt pillows during the Jurassic rifting, was reactivated after the early Aptian in two main phases. The first phase stretches from the late Turonian to the Maastrichtian. The second relates to Miocene tectonic inversion. The compression of the post‐salt overburden caused the amplification of Jurassic detachment folds, forming barriers to the westward progradation of sediment into distinct salt‐withdrawal sub‐basins. Particularly during the Miocene, thin‐skinned overburden shortening was accommodated by growing salt structures that suffered thrusting and extrusion. This structural style contrasts with that of salt‐scarce areas where a simple westerly tilted, fault‐bounded monocline was generated.     

Climatic influences on Holocene variations in soil erosion rates on a small hill in the Mojave Desert

A detailed study of a small hill in NE Mojave Desert in eastern California was conducted to elucidate the effect of climate on the variations in soil erosion rates through Holocene. Field surveys and sampling were carried out to obtain information on topography, geomorphology, soil and vegetation conditions, seismic refraction, sediment deposition, and hillslope processes. Integration of this information allowed reconstruction of the hill topography at the end of the Pleistocene, deduction of the evolution of the hill from the end of the Pleistocene to the present, and estimation of total soil losses resulting from various hillslope processes. The estimates are consistent with the premise that early Holocene climate change resulted in vegetation change, soil destabilization, and topographic roughening. Current, very slow, hillslope transport rates (e.g., 5 mm ky⁻¹ by rodent burrowing, a presently important transport form) appear inconsistent with the inferred total soil loss rate (31 mm ky⁻¹). Packrat midden studies imply that the NE Mojave Desert experienced enhanced monsoonal precipitation in the early Holocene, presumably accentuating soil loss. Water erosion on one slope of the hill was simulated using Water Erosion Prediction Project (WEPP), a process-based erosion model, using 4 and 6 ky of precipitation input compatible with an appropriate monsoonal climate and the present climate, respectively. The WEPP-predicted soil losses for the chosen slope were compatible with inferred soil losses. Identification of two time periods within the Holocene with distinct erosion characteristics may provide new insight into the current state of Mojave Desert landform evolution.     

Foliation development in serpentinites, Glenrock, New South Wales

At Glenrock, near the southern end of the Peel Fault System, two fault zones are delineated by mélanges in which serpentinite is the main rock type.Protogranular and mylonitic textures are present in relicts of the parent peridotite and in blocks of massive pseudomorphic serpentinite that are surrounded by schistose serpentinite. In schistose serpentinite, the earliest foliation (S₁) is defined, microscopically, by the parallel alignment of platy and fibrous serpentine minerals (lizardite and chrysotile) and by trains of magnetite and flattened serpentine pseudomorphs after olivine and pyroxene. It is considered that the schistosity formed perpendicular to the direction of maximum shortening, under conditions in which lizardite and chrysotile were ductile, but antigorite was not, by breakdown of pre-existing serpentine minerals and new growth of lizardite and chrysotile.Post-s₁ foliations (S₂andS₃) superficially resemble crenulation cleavages in the field but, microscopically, show evidence of shear displacement and are referred to as microshear sets. They probably originated in the ductile-brittle transitional field of serpentine behaviour (Raleigh and Paterson, 1965).