Low-Temperature Magnetic Properties of Marine Sediments-Quantifying Magnetofossils, Superparamagnetism, and Maghemitization: Eastern Mediterranean Examples

Periodic and marked redox changes in eastern Mediterranean marine sediments drive environmental and diagenetic changes to which magnetic minerals are sensitive. Magnetic property changes, therefore, provide useful indications of paleoceanographic conditions during and after periods of organic-rich sediment (sapropel) deposition. Magnetic properties of eastern Mediterranean sediments at room temperature have been studied for decades; however, few studies have considered low-temperature magnetic properties. Here, we investigate the low-temperature (10-300 K) magnetic properties of different eastern Mediterranean sediment types combined with room temperature (similar to 300 K) magnetic properties, transmission electron microscopy, and calibrated X-ray fluorescence elemental data to illustrate the valuable information that can be obtained from low-temperature magnetic analysis of sediments. Our low-temperature magnetic results suggest that magnetite magnetofossils and superparamagnetic (SP) particles occur widely in eastern Mediterranean sediments. SP particle contents are highest in diagenetically reduced intervals associated with sapropels. In contrast, magnetite magnetofossils are most abundant in oxidation fronts at the tops of sapropels, where strong redox gradients formed, but are also widespread throughout other sedimentary intervals that have not been subjected to extensive reductive diagenesis. Moreover, the surfaces of magnetite particles are maghemitized (i.e., partially oxidized) in oxidation fronts at the tops of sapropels, and in other oxic sediment intervals. Our results demonstrate the value of LT magnetic measurements for quantifying diverse sedimentary magnetic signals of interest in environmental magnetism when studying paleoceanographic and paleoenvironmental processes. Plain Language Summary Magnetic minerals are sensitive to environmental changes. Investigating their variations through sedimentary sequences can provide information about ancient environmental and climatic changes. Room temperature magnetic properties are widely measured in environmental magnetism, while low-temperature (LT) magnetic properties that are measured between room temperature and similar to 10 K are studied much less, even though they can provide useful information about the presence of particular magnetic minerals and their particle size distributions. We present extensive LT analyses of Mediterranean marine sediments, together with room temperature magnetic, transmission electron microscope, and calibrated X-ray fluorescence elemental analyses to discuss redox changes of magnetic minerals within organic-rich sediment intervals to quantify these diverse magnetic particle types. Our LT results suggest that the fossilized remains of magnetotactic bacteria and tiny superparamagnetic particles occur widely in eastern Mediterranean sediments, and that the surfaces of magnetite particles are widely oxidized and provide a measure of sedimentary oxidation variations. This work demonstrates that LT magnetic measurements can be used to quantify diverse sedimentary magnetic signals, and will be widely applicable in paleoenvironmental magnetic research.

Automated summary

Periodic redox changes in eastern Mediterranean marine sediments drive environmental and diagenetic changes to which magnetic minerals are sensitive, providing useful indications of paleoceanographic conditions. Low-temperature magnetic properties, along with room temperature properties, show the presence of magnetite magnetofossils and superparamagnetic particles in sediments, with magnetite particles partially oxidized in oxidation fronts, highlighting the value of LT magnetic measurements in quantifying diverse sedimentary magnetic signals.