Assessing the viability of using foraminifera from Mersey Estuary saltmarsh sediments to reconstruct former sea level

The viability of using a foraminifera-based transfer function method to reconstruct the local relative sea-level for the Mersey Estuary was assessed in this study, which has not been previously investigated in the UK in the context of application in a strongly macrotidal setting. A total of 105 surface samples were collected across two saltmarshes. Foraminiferal analysis was carried out, along with several environmental variables (organic matter content, salinity, pH, and grain size) to establish the species distribution of foraminifera and their relationship with elevation. Two main zonations were found: a high-to-middle marsh zone occupied by Haplophragmoides spp., J. macrescens and M. fusca; and a low marsh zone composed of increasing numbers of calcareous species including Elphidium spp.; and Haynesina spp. Foraminiferal distributions along each transect were found to be controlled predominantly by elevation and distance from tidal influence, whilst combined datasets reflected intra- and inter-site variability in the assemblages. Elevation was still found to have an important control over the distributions, with a strong relationship between the species zonations and elevation (r2 = 0.8). Therefore, the dataset (82 samples) formed a local training set in which a transfer function for the relationship between foraminifera species and elevation was developed. WAPLS was used as it produced the highest predictive ability (r2 jack = 0.85) and lowest prediction errors (RMSEPjack = 0.11 m). Regional and combined (local plus regional) transfer functions were also developed but the local transfer function produced the most accurate and reliable reconstruction. Reconstructions were carried out for both saltmarshes with reference to a sediment chronology which was established using radionuclides and pollution indicators. The reconstructions demonstrated the vast difference in the saltmarsh development and record of sea level between the sites. Oglet Bay developed as a result of increased accommodation space arising from changing estuary morphology, resulting in rapid accretion (2.34 cm year-1) and was found to be strongly influenced by tidal channel migration. Decoy Marsh accreted at a slower pace (0.32 cm year-1) and was less affected by tidal or morphological changes. Both reconstructions were affected by decalcification resulting in the reconstructions dating back to 1978 at the most. The reconstructed rates of sea-level change were 1.8 cm year-1 for Oglet Bay and 1.1 cm year-1 for Decoy Marsh, both of which over-estimate the trend from the monthly instrumental record (1.04 cm year-1) over the same period. The study highlights the problems which may arise when conducting research in an inner estuary which is strongly macrotidal, including tidal range changes, tidal asymmetry, and decalcification, but also demonstrates that a relatively precise and reliable reconstruction is achievable.