Greigite formation in Dutch Holocene levee systems

Lake IJssel has a complex Holocene history. The region shifted several times between a terrestrial, riverine, subtidal, intertidal, lacustrine, and lagoonal environment. Extensive magnetometer surveys of the lake bottom revealed curvy magnetic lineations evocative of river/tidal channels. Vibro-cores were recovered to investigate the magnetic signal’s source. Previous core samples (from 2022) showed the magnetic mineral greigite (Fe3S4). One core, VC26, was resampled in 2024 in high resolution (~2.5 cm spacing). Focus was on whether the greigite formed biogenically or early diagenetically with implications for interpreting the NRM (natural remanent magnetization) record. By comparing 2024 data to similar 2022 data the effect of cold storage on the magnetic signal can be assessed.
After alternating field demagnetization of the NRM acquisition curves of the anhysteretic and isothermal remanent magnetization were measured. First order reversal curve (FORC) analyses can distinguish between diagenetic and magnetotactic greigite. A paleosecular variation and a relative paleointensity record (with the pseudo-Thellier method) were established for core VC26. Comparing the 2022 and 2024 data show that the amount of magnetic material went down, yet the NRM directions were consistent. FORC-data made it clear that while secondary authigenic greigite is present, the main source is magnetotactic greigite. Thus, the NRM signal is only slightly younger than the sediments themselves and the record can be used for paleosecular variation analysis. Interpretation of the relative paleointensity record is less straightforward. Comparison to master curves and integrating with 14C dates showed that VC26 sediment was formed between 7.2 and 4.5 ka BP.