Diatom Evidence of a Paleohurricane-Induced Coastal Flooding Event in Weeks Bay, Alabama, USA

Sand layers deposited in coastal backbarrier lakes and marshes have been taken as indicators of overwash processes generated by hurricane landfalls, but sand layers discovered in sediments from non-backbarrier estuarine settings have not been as thoroughly investigated. Sedimentological and diatom analyses of a 44-cm core (WB 08-04) taken from a seep forest on the north shore of Weeks Bay in southern Alabama contains an 8-cm-thick sand layer embedded in dark gray peat. As an aid to determining the provenance of the sand layer, diatoms were also analyzed from eight surface samples taken from five nearby environments representing different salinity conditions. The diatom assemblage in the sand layer is dominated by acidophilous (Eunotia spp., Frustulia krammeri) and freshwater (Pinnularia neomajor, Pinnularia latevittata) species, suggesting that they were transported by fluvial processes from the acidic and freshwater bogs above the banks of Fish River. The sand layer also contains small percentages of brackish and marine diatoms (Tryblionella marginulata, Planothidium delicatulum), suggesting that incursion of seawater and brackish water from Weeks Bay and Mobile Bay also played a role in its deposition. It is inferred that the sand layer was deposited by a hurricane that brought both intense precipitation to coastal Alabama and a storm surge into Weeks Bay. Heavy rainfall caused overbank flooding of Fish River, which transported sand from the river banks and diatoms from nearby bogs to the coring site. Concurrently, storm surge waters also deposited marine and brackish diatoms at the site. Historical records suggest that the Bernard Romans' Gulf Coast Hurricane in AD 1772 may have been responsible for the flooding that resulted in the deposition of the sand layer.