Department of Earth and Environmental Science

Quantifying holocene sea level change using intertidal foraminifera: Lessons from the British Isles

We have identified 84 foraminiferal species from fifteen study sites located on the east, south and west coasts of Great Britain, and west coast of Ireland.

Seasonal and sub-surface foraminiferal data suggest that the foraminiferal dead assemblages provide the most reliable dataset for studying patterns of foraminiferal distributions within the context of sea-level reconstruction. In contrast to the live or total assemblages, the dead assemblages are removed from most seasonal fluctuations and post depositional modifications. Sub-surface foraminiferal data also suggest that the foraminifera live primarily in epifaunal habitats and that the foraminifera of the 0-1 cm interval can serve as the model upon which fossil intertidal deposits can be related to past sea level change.

The modern foraminiferal dead assemblages from the fifteen study sites suggest a vertical zonation of foraminifera within British and Irish saltmarshes that is similar to those in other mid-latitude, cool temperate intertidal environments. Whilst the composition, vertical range and strength of zonation varies between sites, two general faunal zones are identified: first, an agglutinated assemblage that is restricted to the vegetated marsh; and second, a high diversity calcareous assemblage that occupies the mudflats and sandflats of the intertidal zone. Three of the fifteen study sites subdivide the agglutinated assemblage into a high and middle marsh zone (Ia) dominated by Jadammina macrescens with differing abundances of Trochammina inflata and Miliammina fusca, and a low marsh zone (Ib) dominated by M. fusca. The calcareous assemblage consists of species such as Ammonia spp., Elphidium williamsoni and Haynesina germanica.

The vertical zonations of the study areas suggest that the distribution of foraminifera in the intertidal zone is usually a direct function of elevation with the duration and frequency of intertidal exposure as the most important factors. This relationship is supported by canonical correspondence analyses of the foraminiferal data and a series of environmental variables (elevation, pH, salinity, substrate and vegetation cover). These modern foraminiferal data are used to develop predictive transfer functions capable of inferring the past elevation of a sediment sample relative to the tidal frame from its fossil foraminiferal content. The results indicate that transfer functions perform most reliably when they are based on modern data collected from a wide range of intertidal environments. The careful combination of foraminiferal estimates of paleomarsh-surface elevation with detailed lithostratigraphy and chronostratigraphy has the potential to produce high-resolution records of relative sea-level change.