Magnetic fingerprinting of fluvial suspended particles in the context of soil erosion: example of the Canche River watershed (Northern France)

In Northern France, land use is dominated by agriculture. Erosion by runoff results in heavy loss of fertile soil into surface waters. There is a need for cost-effective tools to trace the sediment flux in catchments. This study highlights the potential of environmental magnetism to provide rapid non-destructive parameters to characterize the spatiotemporal runoff versus bedload signal in a watershed. Between 2015 and 2017, within the Canche River watershed, several spatiotemporal sampling campaigns of suspended particulate matter (SPM) were undertaken and two distinct "snapshot campaigns" of flood events were conducted at key locations acquiring, in addition, hydrodynamical observations (discharge, turbidity, sediment load, etc.). Agricultural soils and SPM within the watershed have similar values for bulk magnetic concentration (isothermal remanent magnetization, IRM). The magnetic mineral assemblage of the SPM is dominated by soft ferromagnetic (sensu lato) minerals during low water conditions and is enriched in high-coercivity phases during high water stages associated with rainfall events. Low-temperature remanence experiments identified goethite and hematite as the high-coercivity phases. IRM acquisition component analysis of the room temperature results quantify the relative input of high- versus low-coercivity components focused on a given confluence site. Results from snapshot campaigns highlight the link between the enhanced runoff input of high-coercivity particles during flood events and show that the S ratio parameter is an effective tool to trace erosion of topsoils.