Geochemical and radiochronological evidence of North Sea storm surges in salt marsh cores from The Wash embayment (UK)

Abstract

The accumulation history of intertidal silty salt marsh sediments, from the northern shore of The Wash embayment (eastern UK), has been investigated along two cores, using a multiproxy approach involving visual logging, grain size measurements, as well as determination of particulate elements concentrations and 137Cs, 210Pb activities. The cores were collected in the high marsh area; one core was collected on the marsh platform, whilst the other was collected 5 m from the edge of a creek. Grain size distribution reveals two distinct silty sand layers within the lower parts of both cores. One of these is thought to represent the well-documented 1953 storm surge catastrophic event. A second silty sand horizon, found in the core located closer to the creek, is interpreted as being a later storm surge deposit produced in 1978. The redox states of both cores are similar and oxic conditions throughout are inferred from geochemical variations. The absence of a redox interface along the sediment column implies regular oxygen penetration during tidal cycles, thereby limiting the post-depositional mobility of 137Cs and 210Pb. The Caesium-137 distributions, in both of the cores, show a broad hump shape; these activities are interpreted as being largely affected by marine inputs from Sellafield. 210Pb dating is used as an independent chronometer that confirms 137Cs dating. Following the 1978 storm event, the mean accumulation rates, considering 137Cs and 210Pb radionuclide data and the discrete coarse-grained marker horizons, are 0.75 ± 0.09 g/cm2 per yr (nominally 1.00 ± 0.11 cm/yr) close to the creek edge and 0.46 ± 0.04 g/cm 2 per yr (nominally 0.74 ± 0.10 cm/yr) on the marsh platform. Storm erosion is followed by moderately high accumulation rates.