Magnetic Lineations in the Lake IJssel

Abstract

In 2020, Periplus Archeomare was commissioned by de Rijksdienst voor het Cultureel Erfgoed (Cultural Heritage Agency of the Netherlands) to magnetically map the Lake IJssel (The Netherlands). Subsequent processing of this data resulted in the discovery of curvy linear magnetic structures, reminiscent of gullies. It is our aim to discover the origin and elucidate the genesis of the positive/negative magnetic anomalies using paleomagnetic and rock magnetic methods. If the strata are paleomagnetically well behaved they could be dated paleomagnetically. Cores were sampled from a cross section of both positive as well as negative anomalies in a presumed river structure. The three sampled cores, VC15 of 5 meters, VC17 of 5 meters, and VC26 of 7 meters, contain the geologic history from the late Pleistocene to present of the Lake IJssel. Its geological make up consists, from the bottom up, of terrestrial – riverine – lacustrine – deltaic – lacustrine deposits. Magnetic susceptibility measurements indicate a stronger magnetic signal in the core segments containing clays. Acquisition curves of the isothermal remanent magnetization (IRM) and thermomagnetic analysis of the same strata show that the magnetic carrier is greigite (Fe3S4), diagentic in origin. The stratigraphic units in which greigite was primarily found are the Velsen Bed and Wormer Member. These are coastal deposits, or lacustrine/estuarine, allowing for an anoxic, sulfate reducing environment in which greigite can be formed. The influx of riverine freshwater of the Oer-Vecht and Oer-IJssel lowers the sulfate concentration in the water, and limits the reaction, allowing the preservation of greigite. The natural remanent magnetization (NRM) yielded interpretable trends from considerable portions of the cores. These trends however cannot be correlated faithfully to master curves of the paleosecular variation. Additional data points and age markers would allow to compare the Lake IJssel record to the paleomagnetic data sets of Western Europe