FORAMINIFER AND ARCELLACEAN ASSEMBLAGES OF MISSISSIPPI MARSHES

Trends in surficial foraminifer and arcellacean assemblages were related to the environment of low-lying, microtidal marshes of coastal Mississippi in order to use them as analogues of late Holocene marshes in the Mississippi region. Five sites were selected along a salinity gradient in the Mississippi Sound and Pearl River delta at ~25, 20, 5, and 2 psu. These are typical late-summer values, and it is noted that salinities at each locale vary substantially with local river flow, rainfall, and inflow of Gulf water. At each site, five samples of the top 10 cm were taken from distinct sub-environments in a transect perpendicular to the channel, including subtidal channel floor, algal mat of the low intertidal zone, the grassy border of the middle marsh, the levee crest, and the marsh interior ~50 ft beyond the levee. Samples were split by settling until they contained 300 or more foraminifers, and a census was made of all foraminifers and arcellaceans >45 µm in diameter while the fossils remained suspended in water.

Assemblages vary with salinity. The two high-salinity locales bear assemblages dominated by Miliammina fusca with important Arenoparrella mexicana and Ammoastuta salsa. The highest salinity site also hosts significant numbers of Tiphotrocha comprimata and Trochammina inflata. The assemblages are similar to those of many mesohaline to polyhaline clastic marshes of the northern Gulf of Mexico. In contrast, the low-salinity sites host abundant arcellaceans with common Jadammina macrescens, M. fusca, Trochamminita irregularis, T. salsa, and Polysaccammina ipohalina. The assemblage resembles those reported from oligohaline, intermediate marshes of Louisiana.

Assemblages also vary with subenvironment. Channel sands bear a distinct assemblage regardless of salinity, and are dominated by a superabundance of M. fusca with significant numbers of Ammotium spp. and often with a trace of calcareous species and reworked marsh taxa. Samples from the algal mat subenvironment (lower marsh) are generally most different from other intertidal samples at the same site. Based on analogy with surficial samples, assemblages from a vibracore from the lower Pearl River delta show a clear succession from a channel or estuarine assemblage to an intermediate, middle-marsh assemblage despite taphonomic modifications.