Use of magnetic and radiometric measurements to investigate erosion and sedimentation in a British upland catchment

In response to the potential shortcomings of single-technique strategies in the investigation of erosion and sedimentation, a combined magnetic and radiometric (¹³Cs and ²¹⁰Pb) approach has been undertaken in the upland, watershed-lake system of Howden Reservoir, Derbyshire. By combining these techniques, some assessment of sediment sources and the erosion status of the catchment has been achieved. Alone, each approach would have been unable to determine unequivocally sediment provenance. Furthermore, the parallel use of these measurements has highlighted limitations and/or uncertainties in both the magnetic and ¹³⁷Cs techniques. These problems reflect the particular soil characteristics and drainage conditions of this upland catchment. Despite the documented severity of peat erosion in the region, Howden Reservoir has a mixture of sediment sources and a relatively moderate rate of sedimentation. Sediment yields (total 127·7 t km^?2 yr^?1 including organic fraction 31·3 t km^?2 yr^?1) are, however, higher than in other British upland areas.     

A partial explanation of the dependency of hydraulic conductivity on positive pore water pressure in peat soils

Part of the relationship between positive pore water pressures and hydraulic conductivity in peat soils may be explained by accumulations of methane bubbles. We show how compression and expansion of gas bubbles with changes in pore water pressure could cause changes in hydraulic conductivity and thus help to explain some observations of dependency of hydraulic conductivity in peats on pore water pressure. Consideration is also given to the effect on hydraulic conductivities of methane gas going into solution with increase in pore water pressure.     

Spontaneous strain at the structural phase transition in NaNO3

The temperature dependence of the hexagonal c unit cell parameter of high-purity NaNO₃ shows an anomaly at 553 K corresponding to the orientational ordering transition. The a unit cell parameter is barely influenced by the transition. The single component spontaneous strain for this zone boundary instability is large (55×10^–3 at 295 K), and couples quadratically with the order parameter. The critical exponent is found to have the value 0.22 ± 0.01, which differs from that expected in the classical case. Below ca 450 K, crossover to tricritical behaviour is observed (=1/4). The temperature evolution of the macroscopic order parameter as revealed by the temperature dependence of the spontaneous strain follows a tricritical behaviour between 70 K and 450 K. At temperatures below 70 K order parameter saturation is observed. Combined with recent data from specific heat measurements, the critical exponents suggest that the three-dimensional, three-states Potts model may describe the transition.Precursor spontaneous strain above T _c is consistent with local ordering and may result from fluctuations associated with an antiordered NO₃ group pair configuration.     

Oxygen isotope geochemistry of the Mesozoic volcanics of the Etendeka Formation,Namibia

The Etendeka Formation volcanics consist of a bimodal association of basalts and quartz latites. Forty three new whole rock oxygen isotope analyses are reported for all the major magma types. All the rocks except a minor suite of dolerites have higher ¹⁸O values than normal mantle. The basic rocks (average of 29=8.8) have significantly different ¹⁸O to the acid rocks (average of 10=14.4) These data are apparently consistent with previously published petrogenetic models, which propose that the basalts were affected by crustal contamination and that the quartz latites are crustally derived. However, mineral oxygen data show that there is significant oxygen isotopic disequilibrium between phenocryst and whole rock, the latter being significantly higher in most cases. One of the basic magma types (the Tafelberg basalts) shows mutual positive correlations between ¹⁸O, SiO₂ and _Sr. If these correlations are due to crustal contamination, then as much as 45% contamination is required by material having a ¹⁸O value of 15 which is the maximum observed value in the Damaran basement rocks. In the absence of pyroxene phenocryst ¹⁸O data for the high _Sr Tafelberg basalts (they are aphyric), it is not possible to confirm that contamination has taken place. An alternative explanation is that the correlation between _Sr and SiO₂ resulted from assimilation coupled with fractional crystallization (AFC) (before emplacement). Post-eruption alteration resulted in a correlation between SiO₂ ¹⁸O because the material with the most Si-O bonds was able to concentrate ¹⁸O more effectively. The limited mineral data for the quartz latites suggests that there is some source heterogeneity. A pyroxene ¹⁸O value of 10% for a southern Etendeka quartz latite is consistent with a crustal source.     

A model of channel response in disturbed alluvial channels

Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic changes along modified reaches and tributary streams. Degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed-levels by as much as 6·1 m. Following degradation, reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction. Aggradation downstream of the area of maximum disturbance reached 0·12 m per year with the greatest rates occurring near the stream mouths. The adjustment of channel geometry and phases of channel evolution are characterized by six process-oriented stages of morphologic development—premodified, constructed, degradation, threshold, aggradation, and restabilization. Down-cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded (threshold stage). Channel widening continues through the aggradation stage as the ‘slough line’ develops as an initial site of lower-bank stability. The bank profile develops three dynamic elements (1) vertical face (70° to 90°), (2) upper bank (25° to 50°), and (3) slough line (20° to 25°). Alternate channel bars form during the restabilization stage and represent incipient meandering of the channel.     

Hydraulic borehole characterization through the application of moment methods to fluid conductivity logs

In any type of groundwater transport problem (contaminant solutes, heat, etc.), knowledge of the location and properties of pathways of increased hydraulic conductivity is essential. However, answering such questions in strongly heterogeneous media, such as fractured rock, can be very difficult and budget-intensive with standard geophysical or hydrogeological field investigations. We present a new testing concept and analysis procedure based on a time sequence of wellbore electric conductivity logs, which provides the exact location and the outflow parameters (transmissivity, formation fluid conductivity) of flowing features (fractures, faults, layers) intercepted by the borehole.Previously the quantitative analysis of this time sequence of electrical conductivity logs was based on a code, called BORE, used to simulate borehole fluid conductivity profiles given these parameters. The present report describes a new direct (not iterative) method for analyzing a short time series of electric conductivity logs which is based on moment quantities of the individual outflow peaks, and applies it to synthetic as well as to field data. The results of the method are promising and are discussed in terms of the method's advantages and limitations. In particular it is shown that the method is capable of reproducing hydraulic properties derived from packer tests well within a factor of three, which is below the range of what is recognized as the accuracy of packer tests themselves. Furthermore the new method is much quicker than the previously used iterative fitting procedure and is even capable of handling transient fracture outflow conditions.     

The influence of rock strength and abrasive mineral content on the Cerchar Abrasive Index

The Cerchar abrasive index is based on a simple test and is widely used to assess the abrasiveness of rocks. Detailed examination of the test has shown that the index is largely influenced by the rock strength, and only partially by the rock abrasiveness. The standard test was found to be incapable of providing accurate indices for rocks of low relative abrasiveness and a modification to the test is presented to overcome this problem. The interpretation of the test results has been extended to enable calculation of the specific wear rate which is a more precise measure of the abrasiveness of materials.     

Chernobyl radiocaesium in upland watershed-lake systems,Galloway, south west Scotland

Levels of Chernobyl radiocaesium found in the upland region of Galloway, south west Scotland were relatively high e.g. catchment mean¹³⁴Cs inventories ranged from 5.72±2.64 kBq m² to 8.72±1.46 kBq m². The nuclear weapons test¹³⁷Cs content of soils and peats was significantly augmented. Accurate assessment of spatial variations and temporal changes in the levels of contamination, however, are inhibited by its marked, short-range heterogeneity. No significant change in mean catchment inventories was detected over a twelve month period. Lake sediment inventories, however, were consistently enhanced from initially lower levels of contamination than their catchments' surfaces. Pre Chernobyl catchment and lake sediment¹³⁷Cs inventories, however, show no such contrast. Both are substantially lower than rainfall based predictions. These observations of the relative levels of Chernobyl and pre-existing radiocaesium in watersheds and their waterbodies have important implications for both the assessment of the catchment and lake residence times of particle-associated contaminants and the use of lake sediment records as monitors of such pollutants.     

Serpentinization and infiltration metasomatism in the Trinity peridotite,Klamath province,northern California: implications for subduction zones

The Trinity peridotite was emplaced over metabasalts and metasedimentary rocks of the central metamorphic belt along the Devonian Trinity thrust zone. Three metamorphic events can be recognized in the Trinity peridotite: (1) antigorite (D= –63 to –65%.) formation related to regional underthrusting of the central metamorphic belt; (2) contact metamorphism associated with Mesozoic dioritic plutons; and (3) late-stage formation of lizardite ± brucite and chrysotile (D= –127 to –175%.) due to infiltration of meteoric waters. Abundant relict phases indicate incomplete reactions and strongly suggest that the availability of H₂O was a controlling factor during serpentinization.Antigorite (event 1) formed as a result of infiltration into the Trinity peridotite of mixed H₂O-CO₂ fluids derived from the underlying central metamorphic belt. Foliation defined by magnetite veins and shear zones within antigorite serpentinites are subparallel to the Trinity thrust. The assemblage Fo + Atg + Chl + Mag ± Tr ± Carb reflects partial hydration of peridotite at 425–570° C. Talc-rich serpentinite formed along the thrust as a result of the infiltration of silica-bearing fluids. Metasomatic mass-balance calculations based on silica solubilities and the extent of antigorite serpentinization suggest that 80–175 volumes of fluid have passed through a given volume of original peridotite at the Trinity thrust.The Trinity thrust probably represents a Devonian subduction zone. Thermodynamic calculations suggest that hydration reactions account for 30–35% of the total heat released by the cooling Trinity peridotite. By analogy, similar hydration reactions are to be expected in the overlying mantle wedge of a subduction zone which act to retard cooling of the hanging wall, just as dehydration reactions delay heating of the downgoing slab. Metasomatic zones formed in peridotite at the Trinity thrust may reflect similar metasomatic processes to those proposed to occur in the mantle wedge above a subducting slab.