VARIATION OF SEDIMENT GRAIN SIZE IN A LONG ISLAND MARSH: A DOWNCORE INTERPRETATION-POTENTIAL STORM EFFECTS OR DREDGE SPOIL?

Grain size analyses were performed on two vibracores taken from the low marsh (40° 35 42.03 N , 73° 35 56.63 W) in the Lido Beach Nature Preserve located on the South Shore of Long Island within the township of Hempstead, New York.. Cores were sampled at intervals where notable sedimentary changes were observed, treated with 3% H2O2 to remove organics, and washed several times in DI water to remove salts. Sediments were wet sieved through a 63 micron sieve to differentiate fine and coarse grained material. They were analyzed using a Ro-Tap (coarse grained material, >63 microns). The lithology of the low marsh cores (core 1-2.33 m and core 2-2.28m) consist of a series of fine as well as coarse grained lenses and some shell hash layers down core. The low marsh (core 1) core reveals a series of fining and coarsening upward sequences. Beginning at the bottom of the core, the sediments coarsen upward until 185 cm where they become fine again. They coarsen up to medium grain and then begin to fine upward until 89 cm. At 89 cm they begin to coarsen upward until the peat layer is reached. A change in the conditions, such as a large storm or a disturbance to the sediments natural setting, may have produced the two coarsening upward sequence found within the core. These could have been produced by two large storms that washed over the marsh and deposited coarse material before the fines. Core site 2, which is several meters away from Core site 1, also shows a similar trend but the top suspect sand lense is much thicker (98 cm vs. 76 cm). Both cores do show a coarse layer near the bottom with shell hash and mottling. These pronounced lithofacies changes can also be attributed to either a storm, migrating tidal channel or by the effects of a dredge spoil especially the top sand layer; a 1926 photograph of the area shows dredging activity in the area. Additional coring and C-14 of the shell and wood material of the sediment should aid in further interpretation.