2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 197-10
Presentation Time: 10:30 AM


OSTERMAN, Lisa E., SMITH, Christopher G. and ADAMS, C. Scott, St. Petersburg Coastal and Marine Science Center, U.S. Geological Survey, 600 Fourth St. South, St. Petersburg, FL 33701, osterman@usgs.gov

Uncertainty in coastal wetland response to storms and rising sea level is of great socio-economic concern in the Gulf of Mexico (GoM). Our goal is to examine historical hurricane deposits to develop a regional impact assessment. Storm impacts range from severe coastal degradation to wetland sediment addition offsetting sea-level rise. To get a true regional representation of hurricane impacts in the GoM, we analyzed sediment cores from sites in different geographic conditions across the storm track of Hurricane Fredric, a category 4 storm that made landfall over Dauphin Island, Alabama in 1979. A multi-proxy approach including foraminifera, bulk-sediment geochemistry, 210Pb dating, and sedimentology was used to aid in identifying this event and its subsurface characterization.

Hurricane Frederic deposits are fairly widespread across eastern Mississippi Sound and Mobile Bay marshes. The Mississippi Sound sites to the west of the storm tract and in more protected locations do not contain a sedimentary record of Frederic. In Mobile Bay, on the east side of the storm track, two marsh cores record Frederic as coeval shifts in bulk density and % OC (organic carbon). At other sites closer to Frederic’s landfall (Dauphin Island and Fowl River, AL), the storm impact coincides with changes in sediment regimes that reflect a shift to increased coastal deterioration. Because of the narrowing coastal zone, the faunal record at these marsh sites could be misinterpreted as sea-level rise. However, sustained changes in bulk density and % OC indicate an underlying change in depositional regime. We conclude that geomorphic variability across the region can be a significant determinant of event deposition and that changes in environmental conditions also need to be considered when identifying event layers.