Evolution of sediment grain-size profiles on a sheltered, continental shelf in response to punctuated flood deposition

The evolution and preservation of flood deposits on continental shelves offshore river mouths is influenced by the dynamic relationship between punctuated discharge events and post-deposition processes. These short-term dynamic changes at the seafloor that ultimately shape the sedimentary record are difficult to monitor and not well understood. The aim of this study was to examine how punctuated deposition of flood sediments from a coastal river on the continental shelf, biological mixing and winnowing interplay in shaping grain size profiles and their transient evolution, and the preservation of flood layers in the sedimentary record. For this end an observations-based model was devised for the northern shelf of the sheltered, hyper-arid Gulf of Aqaba-Eilat, where infrequent floods from ephemeral rivers occur ∼ yr−1 and there is low physical perturbation by currents and waves in between, conditions that facilitate the examination of grain size profiles' evolution in time. The model shows that in between floods mixing and winnowing shape grain size profiles in the seafloor, causing the sediment to coarsen from the top down and shape a subsurface peak of the fine flood-sediments. Over longer time periods, without additional floods, mixing and winnowing gradually erase the peak of fines, coarsening the entire mixed layer thus nulling the flood layers' chances of preservation. This important role of winnowing near the sediment surface is missing in previous studies. Concurrently, our modelling suggests that long-term positive sedimentation, usually estimated on the years' time-scale, is comprised of a series of short-term, steep sediment deposition events brought by the punctuated floods (lasting hours to days) and followed by a gradual (months to years' time-scale) winnowing of finer sediments (erosion). These high-resolution dynamics that were captured by the model and included short-term flood depositions and their reoccurrence, could not be deduced from the long-term sedimentary records.