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

Abstract

Summary 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 (S1). The upper half of sapropel S1 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 , where represents the average remanence between 150 and 300 K, to estimate the difference between ZFC and FC curves above the Verwey 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.

Source-to-sink magnetic properties of NE Saharan dust in Eastern Mediterranean marine sediments: review and paleoenvironmental implications

“…This pedogenic pathway is supported by some paleomagnetic studies in northern Sudan and the Red Sea coast that reported maghemite and hematite formation (Bachtadse and Briden, 1991;Lean et al, 1998), but not magnetite formation, under present-day hyper-arid weathering conditions (Lean et al, 1998). Further support for this pedogenic pathway comes from the suggestion that magnetite in Eastern Mediterranean marine sediments appears to be invariably maghemitized to some extent, with the exception of paleooxidation fronts that developed above sapropels where relatively fresher biogenic magnetite formed (Passier and Dekkers, 2002) (Figure 5). In this case, it is also likely that airborne lithogenic magnetite grains would have undergone oxidation and maghemitization on their outer surfaces.…”

“…Greigite has been reported within some sapropels with exceptionally high organic carbon contents (Roberts et al, 1999;Larrasoaña et al, 2003bLarrasoaña et al, , 2006, which are typically found around eccentricity maxima. Maghemite has been reported in some studies, with the degree of maghemitization appearing to be larger within and immediately below sapropels, lowest above sapropels, and intermediate in background sediments Passier and Dekkers, 2002). Partially maghemitized magnetite is, therefore, the dominant magnetic mineral in sediments studied by Passier and Dekkers (2002), with the exception of positions immediately above sapropels (Figure 5).…”

“…Maghemite has been reported in some studies, with the degree of maghemitization appearing to be larger within and immediately below sapropels, lowest above sapropels, and intermediate in background sediments Passier and Dekkers, 2002). Partially maghemitized magnetite is, therefore, the dominant magnetic mineral in sediments studied by Passier and Dekkers (2002), with the exception of positions immediately above sapropels (Figure 5). …”

“…ARM values, and hence magnetite abundances, are lowest (<0.05 Am −1 ) within the lower parts of sapropels and down to 50 cm below them (Figure 3). ARM values are highest (0.2-0.4 Am −1 ) up to 20 cm above sapropels, thereby indicating that magnetite concentration peaks at those positions (Van Santvoort et al, 1997;Kruiver and Passier, 2001;Passier et al, 2001;Passier and Dekkers, 2002;Larrasoaña et al, 2003bLarrasoaña et al, , 2006Larrasoaña et al, , 2008Liu et al, 2012b). ARM values that range between 0.1 and 0.2 Am −1 characterize most background sediments, whether intercalated between sapropels around eccentricity maxima or within thick sapropel-barren intervals near eccentricity minima.…”

“…Magnetic data indicate that a common magnetic mineral in Eastern Mediterranean marine sediments is magnetite, the abundance of which is best assessed using ARM data (Van Santvoort et al, 1997;Roberts et al, 1999;Kruiver and Passier, 2001;Passier et al, 2001;Passier and Dekkers, 2002;Larrasoaña et al, 2003bLarrasoaña et al, , 2006Larrasoaña et al, , 2008Liu et al, 2012b). ARM values, and hence magnetite abundances, are lowest (<0.05 Am −1 ) within the lower parts of sapropels and down to 50 cm below them (Figure 3).…”