Environmental Magnetism

The scientist will be forced, in the unenthusiastic words of one of my scientific colleagues, 'to slosh about in the primordial ooze known as inter-disciplinary studies'. John Passmore Man's responsibility for nature The present text has arisen from some thirteen years advances in our perception, appraisal and creative use of collaboration between the two authors. During that of order in natural systems. Out of this can come period, upwards of a dozen postgraduates in enhanced insight into processes, structures and Edinburgh, the New University of Ulster and Liver­ systems interactions on all temporal and spatial scales pool have been closely involved in exploring many of and at all integrative levels from subatomic to cosmic. the applications of magnetic measurements described In the environment, elements of order are often in the second half of the book. Much of the text is difficult to appraise and analyse, not only because of based on their work, both published and unpublished. intrinsic complexity, but as a consequence of our lack A great deal of the work summarised reflects extensive of techniques, instrumentation and suitable co-operation not only between the authors and among methodologies. Magnetic properties, whether natural their postgraduate groups, but also involving or induced, reflect forms of order which, in recent colleagues in geology, geography, ecology, hydrology, years, have become dramatically more accessible to a meteorology, glaciology, archaeology, limnology, growing range of instruments and techniques.

1 Introduction.- 2 Magnetic properties of solids.- 2.1 Introduction.- 2.2 Basic magnetic properties.- 2.2.1 Diamagnetism.- 2.2.2 Paramagnetism.- 2.2.3 Ferromagnetism.- 2.2.4 Ferrimagnetism and antiferromagnetism.- 2.3 Hysteresis.- 2.4 Effects of crystal size, shape and structure.- 2.4.1 Anisotropy.- 2.4.2 Domains and domain walls.- 2.4.3 Multidomain behaviour.- 2.4.4 Single-domain behaviour.- 2.4.5 Superparamagnetism.- 2.4.6 Critical grain sizes.- 2.5 Time dependence of magnetisation.- 2.6 Grain interactions.- 2.7 Summary.- Further reading.- 3 Natural magnetic minerals.- 3.1 Iron and its abundance.- 3.2 Iron oxides.- 3.2.1 The spinel group.- 3.2.2 The corundum group.- 3.3 Pyrrhotite and the iron sulphides.- 3.4 Iron hydroxides and oxyhydroxides.- 3.5 Other magnetic minerals.- 3.6 Formation of natural magnetic minerals.- 3.6.1 Igneous rocks.- 3.6.2 Metamorphic rocks.- 3.6.3 Sediments.- 3.7 Summary.- Further reading.- 4 Magnetic properties of natural materials.- 4.1 Introduction.- 4.2 Units.- 4.3 Magnetic remanence.- 4.3.1 Natural magnetic remanences.- 4.3.2 Laboratory-imparted remanences.- 4.4 Magnetic susceptibility.- 4.5 Anisotropy of susceptibility.- 4.6 Magnetic hysteresis.- 4.7 General magnetic properties of natural materials.- 4.8 Temperature dependence of magnetic properties.- 4.9 Summary.- Further reading.- 5 The Earth’s magnetic field.- 5.1 Geomagnetism.- 5.1.1 Description of the geomagnetic field.- 5.1.2 Secular variation.- 5.1.3 Origin of the geomagnetic field.- 5.2 Palaeomagnetism.- 5.2.1 Archaeomagnetism.- 5.2.2 Palaeosecular variation.- 5.2.3 Polarity timescale.- 5.2.4 Apparent polar wandering.- 5.3 Summary.- Further reading.- 6 Techniques of magnetic measurements.- 6.1 Introduction.- 6.2 Measurement of remanent magnetisation.- 6.2.1 Generator magnetometer.- 6.2.2 Astatic and parastatic magnetometers.- 6.2.3 Fluxgate magnetometer.- 6.2.4 Superconducting magnetometer.- 6.3 Measurement of initial susceptibility.- 6.3.1 The a.c. method.- 6.3.2 The direct method.- 6.3.3 Anisotropy of initial susceptibility.- 6.3.4 Quadrature and frequency-dependent susceptibility.- 6.4 Measurement of induced magnetisation.- 6.4.1 The force method.- 6.4.2 Ballistic magnetometer.- 6.4.3 Vibrating sample magnetometer.- 6.4.4 Alternating field method.- 6.5 Magnetic cleaning techniques.- 6.5.1 Alternating field demagnetisation.- 6.5.2 Thermal demagnetisation.- 6.6 Magnetic fields.- 6.6.1 Measurement.- 6.6.2 Generation.- 6.6.3 Shielding.- 6.7 Portable instruments.- 6.7.1 Magnetometers.- 6.7.2 Gradiometers.- 6.7.3 Pulsed induction.- 6.7.4 Induction balance.- 6.8 A basic environmental magnetic kit.- 6.9 Summary.- Further reading.- 7 Magnetic minerals and environmental systems.- 7.1 Surface processes and magnetic minerals.- 7.2 Primary and secondary magnetic minerals.- 7.3 Magnetic minerals and material flux.- 7.4 Natural remanence and mineral magnetic properties.- 7.5 Sampling and measurement.- 7.6 Summary.- 8 Soil magnetism.- 8.1 Introduction.- 8.2 Magnetic properties of soil minerals.- 8.3 Weathering and magnetic properties.- 8.4 The magnetic enhancement of surface soils.- 8.5 Particle size relationships.- 8.6 Some representative soil profiles.- 8.7 The effects of gleying on magnetic properties.- 8.8 Soil magnetism and slope processes.- 8.9 The persistence of magnetic oxides in the soil.- 8.10 Soil magnetism and archaeology.- 8.11 Conclusions.- 9 Magnetic minerals and fluvial processes.- 9.1 Introduction.- 9.2 Suspended sediment sources.- 9.2.1 Suspended sediments in the Jackmoor Brook.- 9.2.2 Other studies of suspended sediment sources.- 9.2.3 Prospects and problems.- 9.3 Magnetic tagging and tracing of stream bedload.- 9.3.1 The Plynlimon case study.- 9.3.2 The prospects for magnetic tracing.- 9.4 Magnetic measurements of stormwatersuspended solids.- 9.5 Conclusions.- 10 Mineral magnetic studies of lake sediments.- 10.1 Lake sediments and environmental reconstruction.- 10.2 The origin of magnetic minerals in lake sediments.- 10.3 Sampling and measurement.- 10.4 Prospecting, core correlation and sediment accumulation rates.- 10.5 Sediment resuspension and focusing.- 10.6 Sediment sources and ecological change.- 10.7 Magnetic measurements and fire.- 10.8 Lake sediment magnetism and climatic change.- 10.9 Summary and conclusions.- 11 Magnetic minerals in the atmosphere.- 11.1 Introduction.- 11.2 Sources of magnetic minerals in the atmosphere.- 11.3 Magnetic properties and aerosol modes.- 11.4 Magnetic-heavy metal linkages.- 11.5 Peat magnetism and the history of atmospheric particulate deposition.- 11.6 Contemporary particulate pollution monitoring.- 11.7 Magnetic particulates in ice and snow.- 11.8 Global dust studies.- 11.9 Summary and conclusions.- 12 Mineral magnetism in marine sediments.- 12.1 Introduction.- 12.2 The origin and flux of marine magnetic minerals.- 12.3 Core correlation in marine sediments.- 12.4 Mineral magnetism and palaeoclimate in deep-sea sediments.- 12.5 Particulate pollution monitoring in coastal waters.- 12.6 Summary and conclusions.- 13 Reversal magnetostratigraphy.- 13.1 Introduction.- 13.2 Geomagnetic signatures.- 13.2.1 Origin of remanent magnetisation in igneous rocks.- 13.2.2 Origin of magnetisation in sediments.- 13.3 The geomagnetic polarity timescale.- 13.4 Polarity transitions.- 13.5 Summary.- 14 Secular variation magnetostratigraphy.- 14.1 Introduction.- 14.2 Experimental methods.- 14.3 Magnetic dating and magnetostratigraphy.- 14.4 Origin of palaeolimnomagnetic secular variation.- 14.5 Palaeomagnetic pitfalls.- 14.6 Excursions and the reinforcement syndrome.- 14.7 Summary.- 15 Biomagnetism.- 15.1 Introduction.- 15.2 Magnetic navigation.- 15.3 Pneumomagnetism.- 15.4 Cardiomagnetism.- 15.5 Neuromagnetism.- 15.6 Summary.- 16 The Rhode River, Chesapeake Bay, an integrated catchment study.- 16.1 Physical setting.- 16.2 Sediment sources.- 16.3 Study aims.- 16.4 Methods.- 16.5 The magnetic mineralogy of the Rhode River catchment.- 16.6 Suspended sediment samples.- 16.7 Estuarine sediment cores: mineral magnetic characteristics.- 16.8 Chronology and links with land-use change.- 16.9 Summary and implications.- 17 Prospects.- 17.1 Palaeomagnetism of recent sediments.- 17.2 The mineral magnetic approach.- Glossary of magnetic terms.- References.