Magnetic lineations in bottom surveys of Lake IJssel (The Netherlands)

Ship-borne magnetic surveys in Lake IJssel (IJsselmeer), the former ’Zuyderzee’, a now closed off lagoon of the North Sea (since 1932) in the Central Netherlands, showed winding linear magnetic structures reminiscent of natural channel features, such as those of small rivers, brooks, and creeks, next to numerous point sources (small metal objects). The Holocene geology of the lake floor involves marine transgression in the Middle Holocene, with naturally freshened stages before and after that; the sediments are topped by renewed marine ingression in the last millennia. This enables reconstruction of magnetic features. A series of vibrocores of up to 6 m long was collected to this end. We studied three cores: 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. Low susceptibility and IRM core sections are dominated by sandy lithologies and pyrite while portions with notably higher susceptibility and IRM are associated with peat and clay-rich lithologies and greigite. In VC15 greigite-bearing levels do indeed represent levee deposits from a winding channel of immediate post-transgression age, i.e., the end of Middle Holocene. The identified magnetic anomalies may trace submerged prehistoric channel features enabling landscape reconstruction of early Neolithic habitation