Harbor sediment is often subjected to physical, chemical, toxicological, and bioaccumulation analyses as part of dredging projects that keep our harbors navigable; however, the fossils in the sediment often go unnoticed and unappreciated during such testing. What can the fossil remains—the critters that once inhabited the area—tell us about the age and history of the sediment? An ANAMAR biologist set out to learn just that, by sieving about 5 gallons of extra material from a recent sediment sampling event at Charleston Harbor, South Carolina.
The sample was collected with a vibracore sampler at the area of Mount Pleasant Range and Benice Reach. The matrix was a mixture of shells, sand, silt, and clay. The particles smaller than the 2-mm mesh size screen were washed away during the sieving process. The remaining shells and other hard objects were left to dry over a few week’s time. The remains were then identified and photographed.
So what was found?
The sample contained:
Estimating the age of the sediment based on fossil remains
The snaggletooth shark Hemipristis serra occurred during the Pleistocene (12,000 to 2.6 million years ago) to the Eocene (34 million to 56 million years ago) and its fossil remains indicate warm, well-oxygenated marine conditions back then (Cappetta 1987). The extinct tiger shark Physogaleus (Galeocerdo) contortus occurred during the Miocene (5 million to 23 million years ago) (possibly also during the Eocene) and is a characteristic fossil of the Southeastern Coastal Plain (Cappetta 1987). The devil ray teeth were damaged but appeared to resemble the extinct species Paramobula fragilis of the Oligocene epoch (23 to 34 million years ago) and Plinthicus stenodon of the Miocene (Cappetta 1987, www.elasmo.com).
The tiger shark Galeocerdo cuvier and many other requiem shark species are still around today. The stingray genera Dasyatis and Himantura, the eagle rays, and the cownose rays are also still around today, although the stingray genus Himantura is now absent from the western North Atlantic. Similarly, the barracuda genus Sphyraena is still around today including four modern species inhabiting the western North Atlantic (Robins and Ray 1986).
Identification of the invertebrate remains was not yet attempted to the lowest practical taxonomic level, but they appeared to range in age from the Holocene (recent) to the Pleistocene (12,000 to 2.6 million years ago) based on initial observations.
Based on the shark and ray fauna represented by fossils, the sediment taken from Charleston Harbor probably dates back to the Miocene. Given that some fish and invertebrate remains may be modern, the sediment probably also includes relatively recently deposited material. The fact that multiple geological epochs appear to be represented in the sediment is typical for fluvial (riverine) deposits such as in Charleston Harbor. In addition to the fossil remains, the mineral apatite, containing phosphate, was found as smooth spherical black stones. Mineral identification was made by W.G. Harris (pers. comm.).
Sources Cited:
Cappetta, H. 1987. Handbook of Paleoichthyology, Chondricththyes II. Mesozoic and Cenozoic Elasmobranchii. Gustav Fischer Verlag, Stuttgart, Germany.
Harris, W.G. 2013. Professor of Soil Mineralogy, Soil and Water Science Department, University of Florida, Gainesville, FL. Pers. Comm.
Robins, C.R. and G.C. Ray. 1986. A Field Guide to Atlantic Coast Fishes. North America. Houghton Mifflin Co., New York, NY.
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