Date of Award


Document Type



Sediment cores from the Atchafalaya Marsh in southern Louisiana were intensively studied by means of pollen and loss-on-ignition analyses to reconstruct the history of late-Holocene environmental changes and hurricane strikes in the area. The data indicate that the coring site was occupied by a fresh marsh about 4000 years ago. Afterwards, the fresh marsh was replaced by a brackish marsh in the following 400 years. The marsh became a salt marsh in response to sea level rise beginning 3250 yr BP. During 2800-1200 yr BP, the study site became an estuarine environment inundated by sea water as a result of sea level rise. With the development of the delta plain in the study area, brackish marsh resumed and existed for about 500 years from ca. 1200-750 BP. During the last 750 years, the coring site has been occupied by a salt marsh indicating a consistent sea level rise. The vegetation history reveals two cycles of relative sea level change in the Atchafalaya Marsh area. The Atchafalaya marsh was struck by intense hurricanes at least ten times in the past 1200 years and three times during 3100-2800 yr BP as indicated by both pollen and sediment stratigraphies. Two intriguing phenomena are found in the proxy record of hurricane strikes in the Atchafalaya Marsh. First, fewer sand layers occur between the 15th and 19th century. This coincides with the Little Ice Age. This may reflect fewer intense hurricane strikes due to a cooler ocean surface. Alternatively, the coring site may have been farther away from the coastline so that only the most intense hurricane strikes were recorded. Second, no hurricane strikes were recorded during 3100-4000 yr BP. These results are consistent with the hypothesis that the Bermuda High was situated in a more northeasterly position during the mid-Holocene.