Coercivity analysis of magnetic phases in sapropel S1 related to variations in redox conditions, including an investigation of the S ratio

Abstract

The most recent sapropel (S1) in the eastern Mediterranean has been extensively investigated with geochemical and rock-magnetic techniques. Different redox conditions prevailed in different zones of the sediment through time. The oxidized sapropel zone is particularly interesting, because earlier studies indicated that new magnetic material was formed, including possible magnetosomes. Here we utilize component analyses of isothermal remanent magnetization (IRM) acquisition curves and the analysis of first-order reversal curves (FORC) to further investigate the magnetic mineralogy. In the entire box core ABC26, the original input of eolian dust consisted of both magnetite and hematite. In the oxidized sapropel and in the active oxidation zone, an additional magnetite component is present. This magnetite component has a higher coercivity than the eolian magnetite and a very small coercivity dispersion, suggesting a narrow grain size distribution. This is a strong indication that magnetosomes, which are formed in the active oxidation zone, are the magnetic carriers of this coercivity fraction. FORC diagrams support these findings. The S ratio is forwardly modeled for mixed magnetic mineralogies with varying coercivity distributions to explain the down core S ratio behavior: close to 1 in the top sediment and in the oxidized sapropel, a drop in values in the active oxidation zone, and again close to 1 in the (syn)sapropel. The observed S ratio pattern in the oxidized sapropel and in the active oxidation zone can be explained by the recovered IRM components. Whereas both oxidized sapropel and active oxidation zone contain the extra magnetite component, their S ratios are different because of the differences in coercivity characteristics (coercivity and dispersion) of the two magnetites in these zones. Thus, in these zones the S ratio does not reflect variations in the relative contributions of hematite to magnetite but variations in the characteristics of the individual magnetite assemblages. Also in the rest of the core, the S ratio depends on the coercivity characteristics of the magnetite component rather than on the relative contributions of high-versus low-coercivity minerals. The S ratio thus appears to be an unsuitable parameter to describe variations in the magnetic mineralogy, especially when more than two components are present. Therefore the classical interpretation of the S ratio should be treated with caution.

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

“…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).…”

“…These values indicate subtle magnetite concentration variations in background sediments (Figure 3). Magnetic data indicate that hematite is also ubiquitous in Eastern Mediterranean Sea sediments (Kruiver and Passier, 2001;Larrasoaña et al, 2003bLarrasoaña et al, , 2006Larrasoaña et al, , 2008Balsam et al, 2007;Köhler et al, 2008), as has also been shown by diffuse reflectance spectroscopy (DRS) data (Heslop et al, 2007;Liu et al, 2012b). Hematite abundances are typically inferred using HIRM or other properties aimed at isolating high-coercivity mineral signals (e.g., an alternating field-demagnetized IRM or IRM@AF) (Larrasoaña et al, 2003b(Larrasoaña et al, , 2006(Larrasoaña et al, , 2008Liu et al, 2012b).…”

The chert from the Castelltallat Formation (south-central Pyrenees): archaeometric characterisation and archaeological implications

“…The forward S ratio, defined as IRM@0.3T/IRM@1.2T (Kruiver and Passier 2001), has been used to constrain the relative ratio between high-and low-coercivity minerals. All measurements were normalized by the weight of the samples.…”

The Gediz River fluvial archive: A benchmark for Quaternary research in Western Anatolia

“…In particular, magnetotactic biogenic particles have a very low DP (log values of Page 55 of 61 ~0.15, e.g. Kruiver and Passier, 2001;Egli, 2004) because they are almost identical. However, in the present volcanic rocks they do not occur.…”