Iron oxide formation in the active oxidation front above sapropel S1 in the eastern Mediterranean Sea as derived from low-temperature magnetism

Abstract

Low-temperature magnetic properties of eastern Mediterranean sediments from a box-core have been investigated. This box-core contains the present-day oxic-suboxic boundary that is situated at the top of the relic of the youngest sapropel (Sl). The upper half of sapropel Sl has been oxidized, and Fe oxides have precipitated in the oxidized sapropel. Zero-field-cooling (ZFC) and field-cooling (FC) saturation remanent magnetization (Mr; induced in a field of 2.5 T at 20 K after cooling from 300 K with or without the 2.5 T field) was measured during warming to 300 K. An Mr imparted at room temperature (RTSIRM) was cycled to 20 K as well. The difference between ZFC and FC curves around the Verwey transition, as estimated by the parameter δFC/δZFC, suggests the presence of magnetosomes in the oxidized part of the sapropel. We propose a new parameter D = (M̄r, FC150-300K - M̄r, ZFC150-300K)/ Mr, FC300K, where M̄r,(Z)FC150-300K represents the average remanence between 150 and 300 K, to estimate the difference between ZFC and FC curves above the Verway transition. This is interpreted as being indicative of the number of defects and the extent of partial maghemitization. At the oxic-suboxic boundary where Fe oxides actively precipitate, the magnetite appears to be least maghemitized as inferred from low values for D. Further upward in the oxidized sapropel, in older precipitates, the magnetite is slightly more maghemitized. Just below the sapropel, maghemitization is most pronounced. In addition, the initial slope of ZFC and FC curves indicates that small grains or grain coatings that are superparamagnetic at room temperature (SP), are enriched at the oxic-suboxic boundary. Higher in the oxidized sapropel, the relative contribution of SP grains decreases, presumably because they age to larger grains.