Punctuated sediment record resulting from channel migration in a shallow sand-dominated micro-tidal lagoon, Northern Wadden Sea, Denmark

Facies analysis of five sediment cores and a detailed absolute chronology, consisting of 35 optically stimulated luminescence (OSL) ages, form the basis for reconstruction of the paleoenvironmental evolution of the coastal lagoon behind the barrier island of Fanø, in the Northern Wadden Sea, SW Denmark. The lagoonal sedimentary succession was deposited within the last 5500yr, and 5 principal depositional environments are identified in the lagoonal fill. These are: 1) tidal channel, 2) sand flat, 3) mouth bar, 4) reed swamp, and 5) mudflat/salt marsh. Sand flats are the dominant present day depositional environment, but tidal channel sediments dominate in the five sediment cores, making up 56% of the 15m of sediment core. Sedimentation in the lagoon alternated between slow vertical aggradation of sand flats (1.5-2mmyr ^-1) and very fast lateral progradation of point bars in tidal channels, which caused the formation of a punctuated lagoonal fill. Frequent and comprehensive reworking of the sand flat sediments by tidal channel migration entails loss of sedimentary structures and bioturbation related to sand flat deposits, and old sand flat sediments are only very sparsely preserved. We further conclude that long-term (millennial timescale) sediment accumulation in the lagoon was controlled by rising sea-level, whereas short-term (centurial timescale) sediment accumulation was controlled by local erosion and depositional events caused by lateral migration of channels. Records of short-term sea-level fluctuations are not preserved, due to continuous reworking of the lagoonal sediment within the study area. The paper provides significant inputs to a conceptual model describing the Holocene sedimentation in the sand-dominated tidal lagoon. Furthermore, the study successfully demonstrates the role of OSL dating in deciphering complicated sedimentology and stratigraphy of tidal lagoonal environments.