CHANNEL TO UPLAND TRANSITION: MODERN SALT-MARSH FORAMINIFERA FROM GRAND BAY, MISSISSIPPI SOUND

Studies of Holocene salt-marsh foraminiferal assemblages are providing essential data for reconstructing both sea-level and frequency of major storm events. We sampled surface sediments at 17 stations along three transects to investigate the similarity between live (rose Bengal stained) and dead (unstained) assemblages, and their small-scale spatial variations (with replicates) in the Grand Bay salt marsh (Gulf of Mexico, Alabama/Mississippi). We compared data from the three transects: a long transect from seashore to near-upland (GB-long: 6 stations, 2720m), a channel-to-inland transect (GB18: 5 stations, 100 m), and a channel-to-upland transect (GB285: 6 stations, 377m). Open-coast salt marshes in the Mississippi Sound are very wide (~3300m) and flat; elevation increases only near the upland. Cluster analysis shows that dead assemblages occur in distinct zones in the tidal frame, an obligatory requirement for applying salt-marsh foraminifera as sea-level indicators. However, low-marsh samples do not contain exclusive species but are characterized by variable species abundances. The GB-long transect and the GB18 transect are set entirely in the Juncus roemerianus-covered vascular-plant zone, while the GB285 transect covers channel-side to upland transition with trees. Dead assemblages of the Grand Bay low marshes are dominated by Miliammina fusca and Ammotium salsum, the middle marsh is dominated by Tiphotrocha comprimata and Arenoparella mexicana, and the dominant species in the high marsh and upland–marsh transition zone are Entzia macrescens and Trochamminita irregularis. Univariate, two-sample paired tests on replicates showed that patchiness did not statistically influence live (stained) or dead assemblages. However, dissimilarity between live and dead assemblages increased with gain of elevation in the tidal frame. Higher proportions of calcareous foraminifera occurred in the live fraction than in the dead assemblages, suggesting dissolution as a process of taphonomic loss. Thus, the dead surface assemblages provide the best comparison with downcore observations for paleoenvironmental reconstructions.