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

Magnetic properties (IRM, ARM, chi (in), 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 (AB C26 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 consi sts predominantly of single-domain (SD) to pseudo-single-domain (PSD) magne tite in the entire cores. The original input of magnetic grains comes from two sources: aeolian dust (both cores) and volcanic ash from the Minoan eru ption of Santorini (core BC19 only). Non-steady-state diagenesis has change d the magnetic mineralogy considerably in these alternating organic-rich /o rganic-poor sediments. During deposition of sapropel S1, reductive diagenes is and pyritization in and just below the sapropel caused lower magnetic in tensities, coarser magnetic grain sizes and partial maghemitization. In the rmomagnetic curves two types of pyrite can be identified: one oxidizes belo w 450 degreesC and the other above 450 degreesC. The higher oxidation tempe rature is predominantly found below the sapropel. This may be related to th e microtexture of pyrite, which is euhedral below sapropels and mainly fram boidal within sapropels. Since the end of sapropel deposition a downward mo ving 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 o xides with relatively high coercivity and ARM. The maximum coercivity is fo und 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 fr eshly precipitated Fe oxides have aged into generally slightly lower-coerci vity SD grains, with relatively high ARM. In addition to the diagenetic for mation of Fe oxides at the top of the sapropel, formation of a ferrimagneti c 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.