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

Abstract

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 (∼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.

Iron (hydr)oxide formation in Andosols under extreme climate conditions

“…It is noteworthy that, based on our Raman spectroscopy measurements (Fig. 3), we expect maghemitization to be more efficient for very fine-grained titanomagnetite, consistent with Yuan et al 57 and Qian et al 60 . Thus, the majority of nano-to micrometer-sized titanomagnetite crystals within MB 2 tephra should have already been transformed during the past ~ 4.2 kyrs.…”

“…Similar (trans)formation pathways, comparable to those shown in Fig. 2, include the development of maghemitization rims around partially oxidized (titano)magnetite 60 . Maghemitization has also been observed in soils with andic properties 56 , can be mediated by abiotic and biotic processes simultaneously and is suggested to be amplified under varying environmental conditions under a humid climate 55 .…”

“…Thus, the majority of nano-to micrometer-sized titanomagnetite crystals within MB 2 tephra should have already been transformed during the past ~ 4.2 kyrs. It follows from other sedimentary settings that intense maghemitization with subsequent Fe-(hydr)oxide precipitation can play an integral role in producing pronounced oxidation fronts in sapropels 60 , comparable to the process that resulted in elevated Fe 2 O 3 t contents at redox interfaces in these Ah-horizons described above (e.g., Table 1).…”