EXAMINING LONG-TERM RESPONSE OF A ESTUARINE MARSH ENVIRONMENTS IN SOUTHWEST LOUISIANA TO SEA-LEVEL AND STORM IMPACTS

Tropical cyclones are a major driver of change in marsh-dominated estuarine environments often redistributing sediment among various sub-environments. The fate of this redistributed material is of great importance to the overall sediment and carbon budgets of both the marsh and estuary. As such, the relative role of long-term shoreline migration and discrete shot-term storm events remains unclear.

To further understand the role that sea-level and storms have had on the Vermillion Bay (Louisiana) estuarine system over the last 65 years, sediment cores were collected and analyzed for vertical accumulation rates and remote sensing data evaluated for shoreline change. Three cores were collected along a shore-normal transect and sectioned at 1-cm intervals; each section was analyzed for bulk sediment parameters, microfossils, and geochronometers (excess lead-210 and cesium-137). Six sets of aerial images dating back to 1950 were georectified and shorelines digitized along a 1-km stretch of the coast.

Despite numerous hurricanes that were bracketed by the remote sensing data, long-term shoreline erosion rates remained constant at 1.1 m y^-1. The effect of erosion is evident at the most seaward site S1, which based on geochronometers, is largely comprised of reworked sediment. The evolution of the two inland sites (S2 and S3) displays more temporal variability. Between 1950 and 1990s, the average mass accumulation for the two sites was 21±15 kg m^-2 y^-1 corresponding to a vertical accretion rate of ~5 x 10^-3 m y^-1. Slight increases in accumulation rate around the 1960s occurred but are within the error of the long-term average for the two sites. Starting in the 1990s, accumulation rate at S2 nearly doubles while accumulation at S3 continues at the pre-1990 rates. Both sites experience a pulse of sediment (40 - 60 kg m^-2) between the modeled years of 2002 and 2006, which we interpret as material redistributed by Hurricane Katrina and/or Rita (2005). Based on these observations, it appears that hurricane-induced shoreline erosion at this site is fairly benign, yet hurricane-related deposition is quantitatively important. Further examination of hurricane-induced erosion and subsequent material deposition onto the marsh, is required to understand the role that hurricanes play in the evolution of estuarine systems.