The application of saltmarsh foraminifera in the reconstruction of sea level along the southern African coastline.

Abstract

Accelerations in global sea-level rise are a major concern for coastal areas, but the geographical expression of sea-level variability is poorly constrained, especially in data-scarce far-field locations. Southern Africa is a case in point, where the tide gauge record is limited, and longterm sea level data points are spatially and temporally discontinuous. One technique which has shown promise for producing continuous sea-level records, is the application of intertidal saltmarsh foraminifera, which are widely used as a robust, high precision sea-level proxy in temperate regions. This research uses saltmarsh foraminifera from the Kromme Estuary, to investigate late Holocene sea level on the southern coastline of South Africa. The first paper presents a review of recent sea-level research from southern Africa, with a focus on the Common Era (past 2000 years), to contextualize the second, data-based paper from the Kromme Estuary. Modern saltmarsh foraminifera were sampled across intertidal zone, to establish vertical zonation of foraminiferal assemblages relative to the tidal frame, and ultimately used to develop a transfer function for quantitative sea-level reconstruction. Subsurface marsh sediment was surveyed and used to inform coring locations. The master sediment core was processed for sedimentological and foraminiferal analysis. Six samples of picked plant macrofossils were sent for Accelerator Mass Spectrometry radiocarbon dating, and used to produce a Bayesian age-depth model. The Kromme record extends ~1 000 cal yrs with a hiatus observed from ~400 - 600 cal yrs BP. Calcareous species dominate the basal part of the record, preventing quantitative sea-level reconstruction, however, species assemblages and sedimentology data consisting of low organic content and medium to coarse sand occurring from ~340 cal yrs BP to present, suggest lower than present sea level. Agglutinated species from the upper portion of the core with associated high organic matter and medium sand from ~180 cal yrs BP were used to reconstruct sea level. The quantitative reconstructions spans ~200 years, with the lowest sea-level estimated at 180 cal yrs BP which broadly coincides with the Little Ice Age (~650 - 100 cal yrs BP) and Maunder Minimum (~305 - 235 cal yrs BP), followed by rising sea level till present day level. The sea-level reconstruction from the Kromme Estaury is supported by previously published sea-level studies in southern Africa and provides a detailed reconstruction of sea level that can inform regional sea-level trends, contributing to coastal planning, and provides an opportunity to explore the possible anthropogenic effect on sea-level variability.