Anthropogenic disruptions of the sedimentary record in coastal marshes: Examples from the southern Bay of Biscay (N. Spain)

Abstract

Understanding sedimentological discontinuities is essential to accurately reconstruct former climatic and sea-level changes from coastal sediments. Four short cores (<50cm) from two estuaries located in the Bay of Biscay, Northern Spain, have been collected to provide analogues of recent anthropogenic sedimentary disturbance in salt marsh environments. Accurate maps and aerial photography can identify previously reclaimed areas but anthropogenic disruptions are more difficult to detect in regions abandoned before the 1950s and in areas with poor historical records. Our study aimed to provide the tools to identify former land reclamation horizons and apply an established environmental template to interpret a new core record from a previously unsampled estuary. Cores were analyzed using a multidisciplinary approach that includes sedimentology, micropaleontology, geochemistry, and bulk magnetic susceptibility (BMS). Results suggest that areas used for agricultural purposes are characterized in the sediment record by the absence of foraminifera that, after abandonment, are rapidly colonized by salt marsh plants in response to rapid vertical accretion. The initial colonization is marked by the presence of low foraminiferal numbers and a sharp increase in magnetic susceptibility, which proved helpful to improve the timing of the sedimentological boundaries when combined with other proxies. Our results indicate that anthropogenic disruptions to the sedimentary record are common over the recent past (100 years) and likely extend over the historic period in this part of southwestern Europe. These disruptions reflect historic land use changes. In the case of land clearance, proximal sites reflect sudden grain size changes, foraminiferal data and BMS. Distal sites affected by land clearance and the transition from agricultural soils to salt marsh occur in the mud fraction and require a multidisciplinary approach to pin them down. BMS has proven to be very useful when combined with other proxies. This study demonstrates the need for multiple proxies when reconstructing an environmental history through sedimentary strata in order to identify correctly event layers, their source and time of emplacement.