Evidence for the Dissolution of Magnetite in Recent Scottish Peats

Abstract

AbstractMagnetic, hydrological, and geochemical data for 11 Scottish peat cores confirm previous reports that anthropogenic pollutants constitute the principal source of ferrimagnets to such environments, but indicate that postdepositional Fe3O4 persistence may be more limited than formerly envisaged. In all cores, the lower stratigraphic limit of magnetic enrichment coincides with the water table, below which negative mV values and enhanced ratios of Fe/Mn and Cu/Zn depict an abrupt transition to a strongly anoxic regime. Dissolution of Fe3O4 in the reducing, inundated zone is also signified by the more rapid down-core loss of magnetic properties in cores from topographically depressed “Sphagnum lawns” than in those from more elevated “Calluna hummocks”. As a consequence, the peak and cumulative SIRM values for cores from the former settings appear disproportionately low, and the magnetostratigraphic records from adjacent, but topographically variant sites are frequently asynchronous. In contrast to most marine and lacustrine sediments, dissolution of Fe3O4 in peat induces a refinement of magnetic mineral assemblages, highlighted by declining SIRM/ARM ratios with depth and a “hardening” of remanence near the base of the magnetically enhanced strata. SEM micrographs of magnetic extracts from one Scottish core indicate that this refinement results from preferential dissolution along crystallographic weaknesses within multidomain phases, ultimately causing their disintegration and the adoption of “single domain-type” magnetic behavior. Rates of dissolution are considerably greater than those reported for marine sediments, probably due to the unusually high dissolved sulfide concentrations in mire waters.