Intertidal salt-marsh foraminifera as sea-level indicators : lessons from the South African coastline.

Abstract

Microfossils of the group foraminifera are widely used as robust, high-precision sea-level proxy indicators associated with salt marshes. These microfossils exhibit vertical zones related to elevation across the intertidal zone, and are well preserved within salt marsh sediments, leaving a permanent record of sealevel change. This research explores the application of intertidal salt-marsh foraminifera as sea-level indicators along the southern African coastline. It further describes the development of a regional transfer function and assesses its performance in reconstructing sea-level change. Three permanently open estuarine environments were selected for this study, Kariega and Keiskamma along the eastern coastline, and Knysna along the southern coastline. Foraminiferal sea-level records depend on the accurate characterisation of modern foraminiferaenvironment relationships and salt-marsh zonation representative of a study site. Contemporary foraminiferal assemblages were surveyed across the intertidal zone, and corresponding vegetation and environmental data (pH, salinity, soil properties and elevation) were collected. Multivariate ordination was used to examine the correlation between living foraminiferal assemblages and environmental parameters. It was established that elevation was the key environmental variable governing the distribution of salt-marsh foraminifera at the Keiskamma and Knysna estuaries. Salinity had a significant but opposite influence to elevation at the Keiskamma Estuary. At Knysna Estuary, pH was the secondary driver of foraminiferal composition. The contemporary intertidal environments at Keiskamma and Knysna estuaries were described in relation to the zonation of foraminifera across each salt marsh. Cluster analysis was performed to separate foraminifera into salt-marsh zones. Whilst the composition and vertical ranges of assemblage zones vary between sites, we suggest that South African salt marshes can be classified according to four broad subdivisions. The mudflats have the most diverse assemblages, consisting predominately of calcareous species. Ammonia spp., dominates the lower marsh zone followed by Miliammina fusca, which dominates the middle- to lower marsh vegetated zone. The high marsh zone is characterized by the high abundance of Trochammina inflata. Modern training datasets from the selected study sites were used to investigate the suitability of local versus regional datasets for reconstructing recent sea-level trends. The results suggest that a regional transfer function using weighted averaging models is suitable for the analysis of fossil material, producing sea-level reconstructions with an error of ±0.22 m. As a validation exercise the regional transfer function was applied to a sediment core from Kariega, and compare with tidal gauge data. Sea-level records from far field sites offer important constraints on the timing and amplitude of global sea-level changes and improve our understanding of the driving mechanisms behind the late Holocene sea-level change. The regional transfer function has the potential to link short-term instrumental records with longer-term relative sea-level reconstructions, advancing research into past sea-level fluctuations along the South African coastline, and provide a baseline understanding of the nature and causes of sea-level variation. Intertidal salt-marsh foraminifera provide South African sea-level studies with an indicator that is reliable and can be used at multiple sites, allowing for comparisons between studies.