ROOT AND RHIZOME MORPHOLOGY OF THE SMOOTH CORDGRASS SPARTINA ALTERNIFLORA AS A MODEL FOR TRACE FOSSIL EVIDENCE OF ANCIENT SALT MARSH DEPOSITS

Ancient salt marsh deposits are rarely interpreted, despite well-described modern analogues. We suggest that because salt marshes are defined as well-vegetated tidal flats, the most diagnostic traits of modern marshes are their plant communities, which are dominated by a few species of distinctive halophytes. Accordingly, geologists should look for evidence of similar species in suspected ancient marsh deposits. However, because the preservation potential is low for above-ground portions of these plants, we propose that trace fossils caused by growth and movement of their underground portions might be more likely encountered, and if distinctive enough could aid in interpreting ancient examples. Furthermore, halophyte trace fossils would be macroscopic, in situ, and easily identifiable in outcrops or cores, as opposed to other fossil criteria normally used for interpreting marsh deposits, such as foraminifera or pollen. For our study, the root and rhizome morphology of the smooth cordgrass Spartina alterniflora, the dominant halophyte in salt marshes of Sapelo Island, Georgia, was described in detail. S. alterniflora spreads primarily by clonal growth, which in the subsurface can link two or more plants seen on a marsh surface. As a result, an extensive system of rhizomes connect individual culms; adventitious roots and foliage shoots rise from the culms and rhizome nodes. Rhizomes typically are 20 cm below the marsh surface, horizontal, and about 20 cm long. Roots emerge from rhizomes at nearly 90° angles, have fine root hairs, and penetrate to maximum depths of about a meter, although the shallowest are only about 8 cm below the surface. These characteristics were then compared to roots and rhizomes of Spartina sp. in a relict marsh on Sapelo. This comparison revealed similarities in qualitative and quantitative characteristics but also important differences, such as meter-long rhizomes. However, these preliminary descriptions represent a good start toward better identification of trace fossils formed by halophytes in ancient marsh deposits.