TOWARD THE DISCRIMINATION OF FINE-GRAINED FERRIMAGNETS BY MAGNETIC MEASUREMENTS IN LAKE AND NEAR-SHORE MARINE-SEDIMENTS

The magnetic properties of low field susceptibility (chi), frequency d ependent susceptibility (chi(fd)), and susceptibility of anhysteretic remanent magnetisation (chi(ARM)) are used to define sets of natural s oil and sediment samples within which, by the criteria of King et al. (1982) and Maher (1988), the modal ferrimagnetic grain diameter is les s than approximately 0.07 - 0.1 mum. Within this sample set, two disti nct groups with low and high values, respectively, for the quotients c hi(ARM)/chi and chi(arm)/chifd are defined. The first group includes s ediment samples from sites where published studies propose a detrital origin for the fine-grained ferrimagnetic content, Where catchment sam ples are available for comparison, they fall within the same envelope, as do clays from palaeosol samples within Chinese loess. This envelop e of low quotient values also overlaps with the values for the fine gr ained synthetic magnetite samples within Maher's (1988) New MT series. The high quotient envelope of values includes sediments from the Adri atic Sea and clays from saltmarsh and shallow water marine sediments i n NW Britain. In these cases, no catchment source is postulated for th e fine-grained magnetite. The size range of magnetite in this set appe ars to be almost exclusively stable single domain (SSD), and a bacteri al origin seems likely. Sample sets from estuarine environments betwee n river inflow sites and the open sea show intermediate values which, in the case of the Potomac, are ordered by distance down river. Althou gh at this stage, magnetic measurements alone cannot discriminate betw een bacterial and fine-grained detrital ferrimagnets in sediments, the y hold out some promise of doing so provided the distinctions proposed here can be substantiated by transmission electron microscopy.