Particle-size evidence of estuary evolution: A rapid and diagnostic tool for determining the nature of recent saltmarsh accretion

- A conceptual model of saltmarsh sedimentation is tested using PSDs.
- Two components of PSDs (particle-size distributions) are interpreted.
- Dated short cores are interpreted in the context of estuary infilling.
- This approach is a 'pre-filtering' method for reconstructing sea-level trends.

A conceptual model of saltmarsh sedimentation based on high-resolution particle-size analysis has been tested on short cores (c. 0.5 m) of known age from the Dee estuary, NW England, UK. Here, two components of the particle-size distribution (PSD) are interpreted as the traction load deposited by the faster tidal flow velocities ('fast tide') and the suspension load that settles during the turn of the tide ('slow tide'). The feasibility of this model for diagnosing the driving mechanism of estuary evolution in both time and space is tested with reference to historical evidence of saltmarsh accretion, up-core trends in dated saltmarsh cores, and the PSDs of present-day saltmarsh surface sediments. Cores that show an up-core progression from very fine-skewed to near symmetrical PSDs are interpreted in the context of estuary infilling due to a positive sediment budget (sediment surplus), whilst those that show a persistence of near-symmetrical, (very) poorly sorted, mesokurtic particle-size distributions in the fine to very fine silts size range are considered to be the result of 'slow tide' sedimentation. The influences of the two 'end-member' styles of saltmarsh accretion, i.e. (i) infilling due to sediment surplus and (ii) 'slow tide' settling linked to sea level, exhibit spatial and temporal trends as predicted, particularly in cores from mid- and lower saltmarsh locations. The upper saltmarsh cores also show evidence of estuary infilling due to 'slow tide' sedimentation at rates in excess of sea-level rise. The results confirm that the diagnostic approach can be applied as a 'pre-filtering' method for assessing the suitability of saltmarsh sediments for reconstructing sea-level trends, and for providing input data for improved estuary morphological modelling.