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

Abstract

Magnetic properties (IRM, ARM, xin, 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-singledomain (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 uC and the other above 450 uC. 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.