NOAA/WDS Paleoclimatology - Late Pleistocene Baldcypress (Taxodium distichum) Forest Microfossil Assemblage

Approximately 13 km south of Gulf Shores, Alabama, US local divers found in situ baldcypress (Taxodium distichum) stumps 18 m below the ocean surface in a seafloor depression. These trees could have only lived when sea level fell during the Pleistocene exposing the continental shelf as new land. Here we investigate the geophysical properties of this site along with microfossil and stratigraphic analyses of sediment cores recovered from the site to understand the geomorphological properties that lead to the wood preservation on these time scales. Radiocarbon dates of 11 wood and 9 woody debris samples from sediment cores were mostly infinite (i.e. radiocarbon dead) with two dates (45,210 (±889), 41,830 (±330) cal BP) thus optically stimulated luminescence (OSL) dating was used to constrain site age. This site contains Holocene sands that vary in depth (~0.1–4 m) from the modern ridge and trough sand sheet. Below the Holocene sands are late Pleistocene mud-peat (72 ±8 ka OSL), mud-sand (63 ±5, 73 ±6 ka OSL), and paleosol (56 ±5 ka OSL) facies, which grade laterally from west to east, respectively, separated by an unconformity. Foraminiferal analysis reveals the location of the marine-terrestrial transitional layer in the interbedded sand and mud facies, above the Pleistocene facies, dated to 3,920 (±30) cal BP and is part of a lower shoreface or marine-dominated estuarine environment. The occurrence of paleosol and swamp facies of broadly similar ages and elevation suggests the glacial landscape possessed topographic relief that allowed baldcypress wood and swamp mud and peats to be preserved for ~50 ka of exposure before transitioning to the modern marine environment. We hypothesize that rapid sea level rise occurring ~60 ka and ~40 ka provided opportunities for local floodplain aggradation to bury the swamp thus preserving the stumps and we suggest other sites may exist nearby.