Taphofacies Implications of Infaunal Foraminiferal Assemblages in a Georgia Salt Marsh, Sapelo Island

Abstract

The occurrence of living infaunal foraminifera and selective preservation significantly alter the composition of assemblages that accumulate in subsurface marsh sediments of Sapelo Island, Georgia. Rose bengal staining and observations on live foraminifers from surface and subsurface marsh sediments identified live infaunal populations of Arenoparrella mexicana (Komfeld 1931) and Haplophragmoides wilberti Andersen 1953 to a depth of 30cm below the marsh surface. Other foraminifera are predominantly surface-dwellers, although some (e.g., Ammonia beccarii (Linne 1758), Miliamminafusca (Brady 1870) and Trochammina inflata (Montagu 1808)) also have shallow infaunal occurrences. INTRODUCTION Salt marsh foraminifera have been widely reported from many localities along U.S. coastlines and selected marshes worldwide (e.g., Murray 1971; Scott and Medioli 1980a; Scott and Leckie 1990). Most previous studies traditionally have emphasized live and/or assemblages present in surface sediments of the marsh. Distributional patterns observed in surface sediments have been correlated to environmental factors such as elevation (Scott and Medioli 1978, 1980a), salinity (Murray 1971, 1973; Scott and Medioli 1980a), pH (Phleger and Bradshaw 1966; Scott and Medioli 1980b) and geographic region (Phleger 1967, 1970; Scott et al. 1990), and constitute the basis for many paleoenvironmental interpretations. Infaunal foraminifera, however, reportedly occur in a variety of marine environments ranging from salt marshes (Akers 1971; Matera and Lee 1972; Steineck and Bergstein 1979; Goldstein 1988) to the deep sea (Corliss 1985; Gooday 1986; Mackensen and Douglas 1989; Corliss and Emerson 1990; Loubere and Gary 1990). In salt marshes, infaunal occurrences may ultimately change the composition of death assemblages that accumulate in subsurface sediments. Foraminifera with infaunal occurrences may become enriched (i.e., have higher relative abundances) with depth in sediments compared to those species that are primarily epifaunal. This process, in addition to selective preservation, may produce subsurface remnant or death assemblages that differ from their counterparts at the marsh surface. Subsurface death assemblages should therefore be evaluated to enhance the accuracy of paleoenvironmental interpretations. This study documents the occurrence of foraminifera living infaunally in selected high, transitional, and low marsh habitats of Sapelo Island, Georgia, and evaluates their effects on death assemblages that accumulate in subsurface sediments, particularly with regard to distributional patterns, diversity and stratification. PREVIOUS WORK Although distributional patterns of salt marsh foraminifera have been widely studied, foraminifera of the extensive salt marshes of the southeastern U.S. have received little attention (Goldstein and Frey 1986; Goldstein 1988). Similarly, few studies on taphonomy, infaunal occurrences and comparisons between surface and subsurface assemblages have been conducted. In an earlier report on foraminiferal taphonomy in salt marshes, Scott and Medioli (1980b) studied the long-term accumulation of foraminiferal assemblages in surface sediments and concluded that (live + dead) counts of foraminifera in surface sediments better reflect distributional patterns with regard to paleoenvironmental studies than those of their live or dead counterparts alone. The composition of total assemblages diminishes seasonal effects. However, Jonasson and Patterson (1992) reported that selective preservation in subsurface salt marsh sediments reduces the number of biofacies or assemblages that can be recognized compared to those present at the marsh surface. Several previous studies on infaunal salt marsh foraminifera (Akers 1971; Matera and Lee 1972; Steineck and Bergstein 1979; Goldstein 1988) found that representatives of some species live infaunally. Furthermore, live surface and subsurface assemblages may differ in density and species composition (Steineck and Bergstein 1979). Akers (1971) reported rose bengal-stained specimens of several agglutinated species to a depth of 30-35cm in salt marshes near Beaufort, North Carolina. In studies on foraminifera of selected marshes and adjacent habitats of Long Island, New York, Matera and Lee (1972) also reported rose bengal-stained foraminifera in subsurface sediments. Abundances of Trochammina inflata (Montagu 1808), in particular, increased with depth (to about 8cm) below the marsh surface, and stratification of foraminifera over shallow sediment depths was noted. Steineck and Bergstein (1979), applying cluster analysis to data on living (rose bengal-stained) surface and subsurface foraminifera, identified two biotopes: a surface assemblage and an impoverished subsurface assemblage. Total (live + dead) assemblages and taphonomy, however, were not evaluated. ENVIRONMENTAL SETTING Sapelo Island, one of several Pleistocene barrier islands of the central coast of Georgia, lies approximately seven kilometers micropaleontology, vol. 39, no. 1, pp. 53-62, text-figures 1-6, tables, 1-2, 1993 53 This content downloaded from 157.55.39.211 on Mon, 08 Aug 2016 06:24:55 UTC All use subject to http://about.jstor.org/terms S. T Goldstein and E. B. Harben: Taphofacies implications of infaunalforaminiferal assemblages in a Georgia salt marsh, Sapelo Island TABLE 1 Classification of salt marsh habitats and subenvironments HABITAT