Stratigraphic Reconstruction of a Late Pleistocene Bald Cypress Forest Discovered on the Northern Gulf of Mexico Continental Shelf

Abstract

A previously buried bald cypress forest (Taxodium distichum) was discovered on the continental shelf, offshore of Orange Beach, Alabama, USA, in ~20 m water depth. The forest was exhumed by Hurricane Ivan in 2004, and is now exposed as stumps in life position in a trough located in the northern Gulf of Mexico continental shelf seafloor. We are investigating the local stratigraphy, paleo-landscape, and mode of forest preservation of this unique site. In August 2015 and July 2016, submersible vibracores (18 in total) were collected. Core analysis included: bulk density and imaging via Geotek multi sensor core logger, sediment grain size, structure, and organic content via loss-on-ignition. Selected samples have been dated using 14C and optically stimulated luminescence (OSL) methods. Multibeam and CHIRP subbottom bathymetry provide context for litho- and chrono-stratigraphy of the site. Integration of core lithostratigraphy and modern shelf bathymetry reveal Holocene transgressive sands blanketing diverse sedimentary facies that are truncated by the late Pleistocene-early Holocene ravinement. Deposits below the ravinement surface include a Holocene interbedded sand and mud, overlying a swamp facies of woody debris, peat, and mud (dated by OSL to 72±8 ka). These units grade laterally into paleosols that appear to be 10-16 ka younger, based on OSL dates. Occurrence of paleosols and swamp deposits of broadly similar age and elevation suggests that the ancient landscape possessed topographic relief that allowed wetland and upland habitats to develop in close proximity. Floodplain aggradation in the area was a key factor that allowed forest preservation. Two temporary sea-level rises of 10-15 m occurred ca. 40 ka and another one of 30-35 m occurred ca. 60 ka, produced two pulses of local floodplain aggradation that buried the swamp and forest sediments. During the subsequent lowstand, sediments that comprise the floodplain were eroded. Subsequently, paleosols were formed in nearby areas. Some swamp sediments located in paleo-topographic lows were preserved and buried due to the deep coverage of the eastern-trending channel infill sediments. Coastal wave erosion during transgression eroded high ground but enough sediment remained to keep the cypress forest blanketed and therefore allowed preservation until recent marine exposure.