Storage diagenesis versus sulphide authigenesis: possible implications in environmental magnetism

Abstract

Experiments involving the gradual drying out of controlled mixtures of soil and organic lake sediment during storage at room temperature show that this leads to\a loss of magnetic susceptibility and isothermal remanence greatly in excess of the initial values for the sediment components of the mixtures. We conclude that during storage in the moist state, soil-derived, fine-grained, ferrimagnetic iron oxides (magnetite and/or maghemite) are transformed to paramagnetic and/or imperfect antiferrimagnetic minerals. The imperfect anti-ferromagnetic component of the initial mixtures, which probably includes goethite, appears to survive and may even increase during storage. The experimental results compare well with the previously documented effects of storing wet sediment from the site, Peckforton Mere, Cheshire, U.K., over a comparable time interval. We conclude that transformation of fine grained ferrimagnets during 'storage diagenesis' may be responsible for many of the examples of loss of magnetic susceptibility and remanence attributed by other authors solely to the oxidation of an iron sulphide such as greigite. Only where greigite is positively identified is it valid to infer a contribution from it to the magnetic properties of lake sediments: loss of susceptibility or remanence during storage is not alone a sufficient basis for such an inference. Early drying of samples will normally avoid the effects of storage diagenesis; and recent sediment samples so treated will, where greigite formation, bacterial magnetite and magnetite dissolution are insignificant, provide a valid basis for source identification on the basis of magnetic properties.

Magnetic properties of surficial sediments in Lake Ogawara on the Pacific coast of northeastern Japan: spatial variability and correlation with brackish water stratification

“…Such modifications of magnetic properties are known as "storage diagenesis" (Oldfield et al 1992). As proposed by the studies cited above, the loss of magnetic material in our samples might reflect dissolution of fine-grained magnetite or decay of a metastable iron-sulfide such as greigite.…”

“…Rapid loss of magnetization during storage has been reported for organic-rich lake sediment cores (Oldfield et al 1992) and for anoxic marine sediments Roberts et al 1999;Yamazaki et al 2000). Such modifications of magnetic properties are known as "storage diagenesis" (Oldfield et al 1992).…”

Rock magnetism of sediments in the Angola-Namibia upwelling system with special reference to loss of magnetization after core recovery

“…In the latter case, it was found that magnetization of only about 10% or less remained several months after core recovery. Oldfield et al (1992) also reported loss of magnetization during storage of lake sediments. Fidelity of paleo-and rock-magnetic records on marine sediments could seriously be impaired if this phenomenon prevails and has been overlooked.…”

Magnetic susceptibility of Gleysolic and Chernozemic soils in Saskatchewan

“…All cores were segmented by soil horizons, air dried, and ground to pass through a 2-mm sieve. The ground samples had been stored air-dry, which should not affect their χ (Oldfield et al 1992). The topography, stratigraphy, salinity and typical groundwater levels for the Miller transect are shown in Miller et al (1985).…”