THE INFLUENCE OF PARTICLE-SIZE VARIATIONS ON THE MAGNETIC-PROPERTIES OF SEDIMENTS FROM THE NORTH-EASTERN IRISH SEA

This paper considers the magnetic properties of a range of recent and contemporary sediments from the north eastern part of the Irish Sea. p rincipal component ordinations of the results show a close link betwee n magnetic property variations and particle size. The magnetic propert ies of a subset of samples, particles sized by a combination of sievin g and pipette analysis, confirm that variations in ferrimagnetic ('mag netite') grain size parallel those in particle size, despite the fact that the magnetic grains in the fine grades have diameters 1-2 orders of magnitude smaller than those of the particle size fraction in which they occur. This is best explained by postulating that the fine magne tic gains occur in the clay fraction but are present in declining conc entrations in the coarser grades up to 4 phi as an artefact of the pip ette method. Most samples have a biomodal distribution of magnetic min erals, with a coarse mode associated with heavy minerals in the sands or coarse silts, and a fine mode in the clays. Magnetic susceptibility (chi) and saturation isothermal remanent magnetization (SIRM) largely pick out the coarse mode where present; anhysteretic remanent magneti zation (ARM) largely picks out the fine mode. The results open up the possibility of normalizing samples from these environments for particl e size and, more specifically, clay content, by means of ARM or ARM/(c hi) values. The most likely source for the uniform and almost exclusiv e stable single domain magnetite, which dominates the magnetic propert ies of the clays, is thought to be bacterial magnetosomes. The measure ments as a whole do not appear to hold much promise for discriminating sediment source types.