Lisa Marie Mertz ' , Marylea Hart

Tropical cyclones affect the Gulf of Mexico coastline annually; however, the historical record of these events is limited to about 400 years. To establish the occurrence frequency and the importance of cyclones in creating strata, this record needs to be validated by examining coastal sediments for preservation potential of storm bedding. 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 making it ideal to study the natural processes controlling bed preservation. Cores were collected in two coastal ponds and salt marshes on to establish preservation potential of storm bedding. 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. Storm deposition can be detected by changes in bulk density, magnetic susceptibility, or lithology. Radioisotopes were used to quantify these various processes including: mixing depth and intensity by a'Th and 'Be; accumulation rates by 210Pb and 13'Cs. Mixed depths in ponds were <1 cm but >5 cm in coastal marshes, and sedimentation rates are <3 mm y' in both. A comparison of transit time of a stonn layer through the surface mixed layer to the dissipation time of the bed through mixing reveals that beds zl cm thick are likely preserved. The most important control on preservation appears to be the initial thickness of the storm bed, which is expected to be highly variable for each storm. Tropical cyclones affect the Gulf of Mexico coastline annually, however, the historical record of these events is limited to human accounts (400 years). This record can be extended by examining coastal sedimentary strata for evidence of storm bedding (i.e., paleocyclone deposits). However, it has been noted that preservation of known storm beds in a range of coastal environments is not 53rd Annual Convention + Baton Rouge, Louisiana 537 constant (Collins et al., 1999; Davis et al., 1989). This observation results irom the fact that these environments are dynamic, reflecting the combined influences of Holocene sea-level rise, sediment input, bioturbation, wave and tidal reworking (Harris et al., 2002; Frey & Howard, 1969). In order to better resolve the most important characteristics that favor storm bed preservation, it is possible to establish a set of criteria favoring preservation based on paleocyclone studies that have been performed on a number of different coastal depositional environments (e.g. subtidal, intertidal, and supratidal) (e.g. Davis et al., 1989; Donnelly et al., 2001; Liu & Fearn, 2000). An ideal setting would fit the following criteria: (1) high sedimentation rates that quickly bury event beds and prevent subsequent sediment mixing; (2) experience regular cyclone activity resulting in likely production of paleocyclone indicators; and (3) minimal post-depositional erosionlmixing by tides or waves (Wheatcroft & Drake, in press). To evaluate preservation of historic storm (<I00 ybp) bedding, it is also necessary to perform the study in an area of minimal human impact, which is known to severely alter sedimentation processes in coastal settings. Two environments that fit these criteria are coastal ponds and salt marshes along the northern Gulf of Mexico. Coastal ponds are typically located very near the beach but are not regularly affected by tidal influences with the exception of possible ovenvash flooding during cyclone activity (Liu & Fearn, 2000). Bottom sediments in coastal ponds generally are organic rich muds, therefore sand and associated marine microfossils from dunes, beaches, and offshore that are transported landward during a cyclone should be easy to differentiate from the resident sediments. Although inundated daily from tides, salt marshes trap sediments within the root plants making them theoretically a good environment to preserve tropical cyclone sediments. In addition, salt marshes typically are organic-rich sandy muds, which if interrupted or altered by sandy storm deposition, may be detected by changes in bulk density, magnetic susceptibility, or lithology (e.g., Donnelly et al., 2001). Because the preservation potential of any event bed is a function of how fast sedimentation buries it through the rapid-mixed surface layer (Wheatcroft, 1990), the most important variables to measure would include: 1) sediment accumulation rate, 2) mixing layer thickness, 3) bioturbation intensity, and 4) event layer thickness. Hypothetically, sandy, marine-influenced storm beds should be easier to detect and would be more likely preserved in freshwater coastal ponds than marshes due to potentially higher sedimentation rates (not limited by sea-level rise like marshes) and less bioturbation, as marshes are intensely mixed (Frey & Bassan, 1984). To evaluate this hypothesis, sediment cores were collected on St. Vincent Island along the Florida panhandle in both freshwater ponds and marshes (Fig. 1). Multiple analytical techniques were used to identify possible storm deposition, sedimentation, and bioturbation were characterized using a number of particle reactive radioisotopes (mixing depth and intensity by 2"Th and 'Be; accumulation rates by 2101'b and I3'Cs). The preservation potential of storm deposits was evaluated with Wheatcroft's (1990) model of event bed preservation potential that is dependent on the resident time of the event layer in the nearsurface zone of rapid sediment mixing (Fig. 1). Tropical storms and hurricanes regularly impact the Gulf Coast of the United States. While it is uncommon for hurricanes to hit the west coast of the Florida peninsula, the panhandle is frequently affected by such storms. Fifty-six percent of the hurricanes to hit the Florida panhandle from 1885 to 1984 occurred in the Apalachicola Bay area (Davis et al., 1989) (Figure 1). In the Apalachicola Bay area one region that has been severely impacted by hurricane landfall is the Holocene barrier island St. Vincent Island, which has been part of the St. Vincent National Wildlife Refuge since 1968 but was minimally impacted prior to then (Fig. 1). The morphology of the island is characterized by a system of twelve beach ridge sets, formed 800-6000 ybp (Cambell, 1986). The relatively young age of the ponds and marshes make them poor environments for Holocene paleocyclone studies, but the lack of human disturbance and frequency and good documentation of storm events makes this an ideal environment to examine sedimentary processes controlling event bed preservation. 538 GCAGSIGCSSEPM Trarisacfions * Volume 53 + 2003 Marsh A I ' 1 I ,, 1