USE OF RADIOISOTOPES TO CONSTRAIN PRESERVATION POTENTIAL OF PALEOSTORM DEPOSITS IN GULF OF MEXICO COASTAL SEDIMENTS

Tropical cyclones affect the Gulf of Mexico coastline annually; however, the historical record of these events is limited to about 400 years. To find reliable patterns in the history of cyclone landfalls along the Gulf of Mexico coastline, this record needs to be extended. By looking for storm bedding in coastal sediments in regions of pronounced storm activity it may be possible to establish patterns in cyclone activity. St. Vincent Island National Wildlife Refuge is located in Apalachicola Bay, FL, and provides a research area with minimal human impact and frequent occurrence of tropical cyclone landfall, including 3 large (Category 3+) hurricanes in the past 100 years. Costal ponds and salt marshes have fine grain and organic bedding which if interrupted or altered by storm deposition may be detected by changes in bulk density, magnetic susceptibility, or lithology. Cores were collected in two coastal ponds and two salt marshes on St. Vincent Island to establish their potential as storm bedding depocenters. The preservation potential of a storm event in coastal sediments is related to three factors: biologic mixing depth and intensity, storm layer thickness, and sediment accumulation rate in the coastal environment. A number of radioisotopes were used to quantify these various processes including: mixing depth and intensity by Th-234 and Be-7; accumulation rates by Pb-210 and Cs-137; and U-238, K-40, and Ra-226 were used to identify storm beds. Mixed depths in ponds was <1 cm but >5 cm in coastal marshes and sedimentation rates are <3 mm/y, suggesting strong biological mixing and dissipation of storm beds. Storm bedding is preserved, though, and is due to variability in the thickness of the deposited bed.