A combination of ‘bomb spike’ calibration and conventional calibration of AMS 14C dating has been used to determine a detailed age-depth model for a 1-m sediment section collected from a salt marsh in Poole Harbour, southern England. These data were compared with the chronology obtained from 210Pb analysis and 137Cs age markers. We report post bomb values of over 1.46 F14C (> 146% modern 14C), and both the rising and falling limbs of the atmospheric ‘bomb spike’ are identified. Five pre-bomb samples were analysed using multi-target high-precision 2‰ AMS analysis, and after the replicates were combined the one-sigma uncertainty was as low as ± 9 14C yr on some ages. These data, and an additional three normal-precision pre-bomb 14C samples, were calibrated using CALIB 5.0 and the chronology constrained using the ‘prior knowledge’ of independent age markers obtained from the analysis of pollen and spheroidal carbonaceous particle (SCPs). No agreement was found between the 14C ‘bomb spike’ dates and the CRS 210Pb chronology modelled for this sequence. In addition, poor agreement was found between the signal of the 1960s weapons test fallout indicated by the 14C ‘bomb spike’ dates and the timing suggested by the 137Cs data. This disagreement is attributed to the influence of the local discharge of 137Cs from the former UKAEA site at Winfrith. We use our new chronology to confirm the existence of an acceleration in sedimentation rates in Poole Harbour during the last 100 yr previously reported for this site by Long et al. (Long, A.J., Scaife, R.G., Edwards, R.J. 1999. Pine Pollen in intertidal sediments from Poole Harbour, UK; implications for late-Holocene sediment accretion rates and sea-level rise. Quaternary International, 55, 3–16.), and conclude that ‘bomb spike’ 14C calibration dating may offer a more robust alternative to the use of 210Pb chronologies for dating sediment deposition in salt-marsh environments. In addition, we demonstrate how the use of high-precision AMS analysis has the potential for reducing some of the uncertainties involved in the high-resolution dating of recent salt-marsh sediments.
