Storm-Driven Tree Exposure and Geomorphic Change: Predicting the Distribution of Preserved Late Pleistocene Tree Stumps on the Outer Alabama Continental Shelf

The Alabama Underwater or Drowned Forest is a well-preserved Late Pleistocene (dated to 72–56 ± 8 ka, 2σ) terrestrial landform on the northern Gulf of Mexico continental shelf that provides geomorphic and ecosystem information rarely preserved during the glacial intervals. Stumps of bald cypress (Taxodium distichum (L.) Rich.) trees were exposed in ∼18 m of water following Hurricane Ivan in 2004. This research investigates geomorphic changes to the Mississippi-Alabama-Florida (MAFLA) sand sheet, which presents as shore-oblique Holocene sand ridges, and the exposure and burial of tree stumps following the passage of Hurricane Sally in 2020 using repeat sidescan and bathymetric surveys (2015–2016 and 2021). Using two newly identified tree exposure areas and their geological properties, this research also hypothesized a new location where tree stumps may be outcropping. The bathymetry indicates regions with up to ∼1 m of erosion and deposition over the five years between the two surveys. Similarly, the sidescan sonar indicates changes in the location and numbers of exposed tree stumps as well as between 47,000 and 62,500 tons of sediment erosion within the study area following Hurricane Sally. The 2015 and 2016 data found 25 tree contacts whereas the 2021 survey found 76 tree contacts and only 5 of them occurred in both surveys suggesting the tree exposures are dynamic and presumably changing with the passing of large tropical cyclones. Additionally, the hypothesized exposure location had 26 newly identified tree stump contacts within a mixed texture unit along the sand-mud boundary, confirming our understanding of the geomorphic characteristics leading to the exposure of the buried forest. This research will expand the potential areas for investigations into Late Pleistocene ecosystems and landforms and their associated climatic and ecologic conditions in the Gulf of Mexico as well as in other passive continental margins.