Magnetic properties and geochemistry of the active oxidation front and the youngest sapropel in the eastern Mediterranean Sea

Abstract

Summary Magnetic properties (IRM, ARM, χin, S‐ratio at 0.3 T, room temperature (RT) hysteresis and thermomagnetic curves) and geochemical data (Fe, S, Mn, Al, Ti, organic C) were studied in two eastern Mediterranean boxcores (ABC26 and BC19) at a resolution of 3–5 mm. The boxcores contain sapropel S1 (9–6 kyr BP) at a few decimetres below seafloor. The magnetic fraction consists predominantly of single‐domain (SD) to pseudo‐single‐domain (PSD) magnetite in the entire cores. The original input of magnetic grains comes from two sources: aeolian dust (both cores) and volcanic ash from the Minoan eruption of Santorini (core BC19 only). Non‐steady‐state diagenesis has changed the magnetic mineralogy considerably in these alternating organic‐rich/organic‐poor sediments. During deposition of sapropel S1, reductive diagenesis and pyritization in and just below the sapropel caused lower magnetic intensities, coarser magnetic grain sizes and partial maghemitization. In thermomagnetic curves two types of pyrite can be identified: one oxidizes below 450 °C and the other above 450 °C. The higher oxidation temperature is predominantly found below the sapropel. This may be related to the microtexture of pyrite, which is euhedral below sapropels and mainly framboidal within sapropels. Since the end of sapropel deposition a downward moving oxidation front has oxidized the upper half (c. 5 cm) of the sapropel. The oxidized part of the sapropel is enriched in diagenetically formed Fe oxides with relatively high coercivity and ARM. The maximum coercivity is found in a distinct layer between the present‐day Mn‐ and Fe‐redox boundaries at the top of the unoxidized sapropel. The freshly precipitated Fe oxides in this centimetre‐thick layer contain a mixture of superparamagnetic (SP) grains and high‐coercivity SD magnetite. Higher in the oxidized zone the freshly precipitated Fe oxides have aged into generally slightly lower‐coercivity SD grains, with relatively high ARM. In addition to the diagenetic formation of Fe oxides at the top of the sapropel, formation of a ferrimagnetic Fe monosulphide may have occurred within the sapropel during later stages of diagenesis, which may have enhanced the ARM signal in the organic‐rich interval in particular.

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

“…It is, therefore, expected that both fine-grained pedogenic magnetite/maghemite and relatively coarser lithogenic (possibly maghemitized) magnetite grains will accumulate in Eastern Mediterranean marine sediments as constituents of eolian dust, as has been also reported for eolian sediments on the Chinese Loess Plateau (Liu et al, 2007a). The eolian origin of magnetite/maghemite is further demonstrated on the basis of comparisons between ARM and geochemical and mineralogical data from different terrigenous sources, including eolian dust (Kruiver and Passier, 2001;Passier et al, 2001;Larrasoaña et al, 2008). For example, the midPleistocene transition at about 0.95 Ma witnessed a sharp increase in eolian dust concentration as recorded by both hematite and magnetite/maghemite abundances (Figure 3).…”

“…Thus, high organic carbon contents and prevailing sulphidic conditions during sapropel formation are responsible for the pervasive reductive dissolution of fine-grained magnetite/maghemite within and below sapropels, which explains the lowest ARM values observed at these positions (Figure 3) (Van Santvoort et al, 1997;Kruiver and Passier, 2001;Larrasoaña et al, 2003aLarrasoaña et al, , 2006Larrasoaña et al, , 2008Liu et al, 2012b). Above sapropels, high ARM values have been interpreted to result from authigenic magnetite growth, mostly biogenic, at paleo-oxidation fronts that developed above sapropels when oxic conditions were reestablished after their formation (Figure 3) (Kruiver and Passier, 2001;Passier et al, 2001;Liu et al, 2012b). In contrast to magnetite/maghemite, the striking correlation between hematite abundances and Ti/Al values, even in the vicinity of sapropels (Figure 3), indicates that hematite is largely unaffected by short-term periods of sulphidic diagenesis associated with accumulation and degradation of organic matter in sapropels.…”

“…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.…”

“…These studies have also proposed a dominantly eolian origin for magnetite in background sediments (Kruiver and Passier, 2001;Passier et al, 2001;Larrasoaña et al, 2008), with a minor contribution from other terrigenous (e.g., fluvial and volcanic) or biogenic sources at some stratigraphic intervals (Dinarès-Turell et al, 2003;Larrasoaña et al, 2008;Roberts et al, 2012). An eolian origin has also been proposed for goethite (Heslop et al, 2007;Köhler et al, 2008;Liu et al, 2012b), and a diagenetic (Roberts et al, 1999;Larrasoaña et al, 2003bLarrasoaña et al, , 2006 or biogenic origin (Roberts, 2015, submitted) has been proposed for greigite.…”

“…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).…”