VERTICAL ZONATION OF FORAMINIFERA ASSEMBLAGES IN GALPINS SALT MARSH, SOUTH AFRICA

Abstract

ABSTRACT Salt-marsh foraminifera are used as precise sea-levelchange indicators as surface assemblages vary in relation totheir position in the tidal frame. Surface-sediment sampleswere collected across an elevation gradient at Galpins saltmarsh, South Africa, to study the vertical distribution offoraminifera and their potential use for sea-level studies. Themarsh is divided into three vertical zones (high marsh, middlemarsh, and mud flats) represented by three assemblagegroups, with agglutinated species restricted to the upperreaches of the marsh and calcareous species more dominanttowards the intertidal channel. The high marsh area isdominated by Jadammina macrescens with a presence ofTrochammina inflata. The middle marsh is characterised byboth T. inflata and Miliammina fusca. Calcareous speciesfound in the mud flats consist of Haynesina germanica,Ammonia batava, and Quinqueloculina sp. This paperdescribes how marsh foraminifera can be used to definesmall-scale vertical zones along modern marsh surfaces andhow these zones correspond to floral zones. We demonstratethat marsh foraminifera have potential to be used as preciseindicators for sea-level reconstructions in South Africa.INTRODUCTIONModern foraminiferal assemblages form discrete verticalzones in salt marshes, strongly correlated with tidal levels(Scott et al., 2001; Gehrels & Newman, 2004). Therelationship between modern foraminiferal distributionand a range of environmental variables, including elevation,vegetation cover, pH, and salinity, can be preciselyquantified (e.g., using multivariate statistics), representinga modern analogue against which to compare fossilassemblages. Salt marshes experience daily and seasonalvariations in salinity and frequency of flooding linked totidal overflow. Several authors suggest that verticalzonation of foraminifera is strongly related to elevation,especially in temperate environments (Scott & Medioli,1980; Scott & Leckie, 1990; Horton & Edwards, 2003, 2006;Leorri et al., 2010).It is well known that a variety of ecological controls exertan influence on the distribution of surface foraminifera(e.g., Murray, 1971; Scott et al., 1998; Horton, 1999),although assemblages are consistently shown to be verti-cally zoned in accordance with tidal frames, either directlyor indirectly (Berkeley et al., 2007). A study conducted inthe Great Marshes of Massachusetts by De Rijk T Horton & Edwards, 2006).Modern analogues of marsh foraminifera are traditionallycollected at the location where a sediment core is extractedfor palaeontological analysis (Gehrels, 1994; Kemp et al.,2009). Where assemblages are identified solely at a singlesite, some fossil samples may lack a modern analogue,thereby compromising the accuracy of the reconstruction(Murray, 2006). This lack of modern analogues could beaddressed by sampling a broader range of contemporaryenvironments (Hayward et al., 2004; Horton & Edwards,2006; Kemp et al., 2009).Researchers have debated the best assemblage make-upfor foraminiferal population studies. Some advocate totalassemblage use, indicating environmental conditions forboth seasonal and temporal fluctuations (Scott et al., 2001;Horton et al., 2005). According to Murray (1971), livingpopulations best represent the modern environment as theuse of dead assemblages alone fails to account for post-mortem changes. However, it can be argued that thedistribution of living assemblages will be dependent on theconditions at time of sampling and not an average overtime. In temperate environments some suggest it isimportant to exclusively use dead assemblages as they arenot susceptible to seasonal variations, thus accuratelyreflecting subsurface assemblages (Murray, 1979; Hortonet al., 2005; Horton & Murray, 2007). On occasion, trainingsets comprised of total assemblages (live + dead) are stillused based on the assumption that live assemblages in timewill contribute to the fossil record (Booth et al., 2010). Astudy conducted on foraminiferal assemblages in surfacesediments from a marsh in Nova Scotia concluded thattotal foraminiferal assemblages provide a good basis forpalaeoenvironmental studies (Scott & Medioli, 1980).Previous studies have described the distribution of salt-marsh foraminifera in temperate environments (e.g. Leorriet al., 2010). The highest intertidal zones are oftendominated by Jadammina macrescens and Trochamminainflata, which are replaced by species such as Haplophrag-moides sp. and Miliammina fusca as elevation decreases(Hawkes et al., 2010). Agglutinated species are prevalent inthe upper and middle marsh while calcareous species arescarce. This relationship depends on the accessibility of