S U M M A R YThe theory, measurement and interpretation of frequency-dependent susceptibility ( xFD) are examined. A new model is proposed which explains xFD in terms of the behaviour of all superparamagnetic grains (SP) with diameters between 0 and ~0 . 0 3 pm. The model predicts maximum xFD percentage values of 14-17 per cent for spherical SP ferrimagnetic grains in the grain-size range 0.01-0.025 pm, and a maximum value of 10-12 per cent for grain assemblages spanning a wider range of grain sizes (0-0.03 pm). Synthetic and experimental data support the model predictions in terms of both maximum xFD percentage values and the relationship between xFD percentage and mass specific xFD, which exhibits an envelope of data points partly related to grain-size distributions within the SP range. When the xFD percentage is at a maximum, the mass specific xFD term can be used to estimate the concentration of SP grains in a sample.Lower values of xFD percentage in soils are caused by the presence of narrow distributions of ultrafine SP grains, frequency-independent stable single and multidomain ferrimagnetic grains. Some soils with low susceptibilities may have low xFD percentages because of an appreciable content of paramagnetic and canted antiferromagnetic minerals. A simple mixing model predicts proportions of SP grains in mixed grain assemblages, but model validation requiring further characterization of grain interaction and grain-size distributions is needed before it can be applied to environmental data.

Abstract:The relative significance of primary and secondary suspended sediment sources has been identified in a small (15 km 2 agricultural catchment through the application of a linear unmixing model using mineral magnetic, geochemical and radionuclide signatures of fine sediments. Roads appear to be important as a secondary source of suspended sediment and in the conveyance of topsoils to the river channel. Roads also alter the source signatures of sediment in transport, especially that derived from agricultural topsoils. Modelling suggests that c. 30% of the suspended sediment collected from suspended sediment traps in the River Leadon were derived from, or transported via, roads. Relative contributions from primary sources of c. 43% and c. 27% were estimated from a combined (subsoil and channel bank) subsurface source, and a combined (arable and grassland) topsoil source, respectively.

S U M M A R YMineral magnetic properties have been used recently to classify and to attempt to quantify the sources of sediments through environmental systems. Linear modelling techniques could be used with a wide range of concentration-dependent magnetic measurements to quantify the sources of sediments. To investigate wider application of linear modelling techniques using mineral magnetic properties, research has been conducted using laboratory mixtures of up to six source materials, including both natural environmental materials and synthetic compounds. While six sources may seem ambitious, this figure was used as an absolute upper limit rather than giving a real prospect of mathematically unmixing six sources. It has been found that even with the most magnetically differentiable materials, large errors are encountered when modelling the sources of the mixtures. This paper investigates the causes of 'non-additivity' of certain magnetic measurements and the failure of the linear modelling of the sources of the mixtures. Possible reasons for this failure include source homogeneity, calibration and linearity of equipment, magnetic viscosity of materials and/or the changing physical characteristics of the source materials once mixed together (interaction effects). In testing linear additivity, low-frequency susceptibility is the most reliable mineral magnetic measurement, while remanence measurements suffer from a systematic error in the expected results. Results have shown that in the best controlled conditions where the sources are identified and are artificially mixed together, the results of linear modelling are quite poor and at best four sources can be 'unmixed' with reasonable success. It is suggested that interaction within the mixtures, especially when containing highly ferrimagnetic burnt environmental materials, causes some of the non-additivity phenomena.

Abstract:This paper describes the methods used to reconstruct suspended sediment yields over the last century, based on 210 Pb and 137 Cs dated bottom sediments in nine lake and/or reservoir catchments. Magnetic susceptibility and dry bulk density are used to correlate between multiple cores taken from each lake/reservoir basin in order to estimate sediment in¯ux through time and reconstruct sediment yield histories. The catchments represent a range of morphological and land use systems, including upland moorland, upland forestry and upland and lowland agricultural systems. The reconstruction provides some background information on pre-20th century sediment yields and permits an evaluation of the eect of 20th century land management systems, especially upland aorestation and agriculture, on changing suspended sediment yields. Reconstructed sediment yields over the last century range from c. 7 to 86 t km À2 yr À1. Average post-1953 yields from pasture, arable, moorland and forested catchments are 13, 31, 29 and 13 t km À2 yr À1 , respectively. The history of human disturbance in individual catchments is clearly recorded in the reconstructed sediment yields. Short-lived disturbance, such as aorestation, produces high yields (over 40 t km À2 yr À1 ) for up to a decade after planting, but yields subsequently decline to pre-planting levels (c. 12 t km À2 yr À1 ). Sustained increases in sediment yield are associated with arable, mixed agricultural and, to a lesser extent, grazing systems. While the strong controls exerted on sediment yield by human disturbance mask the potential eect of regional contrasts in runo and topography between catchments, there is some evidence to suggest that a long-term climate change signal is recorded in the temporal trends in sediment yield from an analysis of Lamb weather types.

Research on suspended sediment transport in the catchments of the Old Mill reservoir and Slapton Lower Ley, South Devon, has attempted to discriminate changing catchment sources on the basis of downcore variations in the mineral magnetic properties of lake, reservoir and floodplain sediments. Here, we examine these downcore variations and also explore the variability in catchment sources and the influence of topographic controls on mineral magnetic signatures of topsoils and subsoils. Particle size controls on the mineral magnetic signatures are explored by an analysis of a fractionated sediment sample, whilst the possible impact of diagenesis is assessed by an examination of the Mn profiles in the lake and reservoir sediments. From this analysis it is evident that the mineral magnetic signatures of well sorted floodplain deposits are more likely to reflect the particle size composition of the transported material. By contrast, the mineral magnetic record in the sediment of Slapton Ley appears to be most strongly influenced by dissolution of magnetic minerals. The sediment of the Old Mill reservoir provides the only suitable record for the application of a simple mixing model which is developed in order to quantify changes in the relative contribution of topsoil and subsoil through time. The research has important implications for attempting to reconstruct sediment sources in highly eutrophic lakes and emphasizes the uncertainty in the application of simple mixing models.