The Source of Magnetic Lineations Observed in Bottom Surveys of Lake IJssel (The Netherlands)

Lake IJssel (IJsselmeer) is the former 'Zuyderzee', a now closed off lagoon of the North Sea (in 1932) in the Central Netherlands. Portions of the lake are designated for sand mining, which requires clearance from unexploded ordnance, aircraft, and shipwrecks. Hereto, surveys are performed by ship-borne magnetic prospection to identify metal objects. Processing of the accumulated surveys has also unveiled the presence of winding linear structures reminiscent of natural channel features, such as those of small rivers, brooks, and creeks. The Holocene geology of the lake floor (marine transgression in the Middle Holocene, naturally freshened stages from before and after that, topped by renewed marine ingression in the last millennia) and genesis of magnetic minerals in brackish environments, enables making inferences on origin of the magnetic features. A series of vibrocores of up to 6 m long was collected to identify the magnetic anomaly sources and test geological interpretations. From three cores, magnetic properties were studied specifically: VC15 in a positive anomaly, VC17 in a negative anomaly, while core VC26 was sampled in a different area. After core screening with a hand-held susceptometer, discrete samples were subjected to AF demagnetization of the natural remanent magnetization (NRM). Also acquisition curves of the isothermal remanent magnetization (IRM) were measured supplemented with thermomagnetic analysis to determine the magnetic mineralogy. Greigite (Fe3S4) is shown to be the major magnetic mineral. VC17 features low susceptibility throughout the core (dominated by sandy lithologies) while VC15 and VC26 show intervals with notably higher susceptibility associated with peat and clay-rich lithologies. IRM trends follow those of the susceptibility. VC17 and the low susceptibility portion of VC15 are dominated by pyrite, while greigite occurs in the high susceptibility portion of VC15 and VC26. The greigite-bearing sediments in VC15 do indeed represent levee deposits from along a winding channel immediately post-transgression stage (end of Middle Holocene). As such the magnetic anomalies identified in bottom surveys of Lake IJssel may trace submerged prehistoric channel features enabling landscape reconstruction of early Neolithic habitation and archaeological potential.