HURRICANE-INDUCED DEPOSITION AND PALYNOLOGICAL ASSESSMENT OF A RAPIDLY SUBSIDING DELTAIC ENVIRONMENT IN COASTAL LOUISIANA

Coastal backbarrier lagoons located in regions susceptible to hurricane strikes have been shown to serve as active repositories for recording geologic changes caused by storm events and environmental stresses. To evaluate the geological impacts of recent hurricanes and the long-term vegetation changes in a rapidly subsiding coastal zone, X-ray Fluorescence (XRF) and palynological analyses were performed on two sediment cores (BCM and BC53) extracted from mangrove-covered wetlands around Bay Champagne, a backbarrier lagoon situated along Louisiana’s Gulf coast. This study aims at extending the records of previous studies by Naquin et al. (2014) and Liu et al. (2011) by analyzing the sedimentary sequence of a new core retrieved from the backside of the lagoon.

Core BC53 is roughly 2.2 m long and was extracted in September 2012, two weeks following the landfall of Hurricane Isaac. The pollen record is marked by significant shifts in dominance between Amaranthaceae and Poaceae, both common taxa in brackish and salt marshes around the site. Loss-on-Ignition (LOI) data and visual inspection suggest three distinct stratigraphic zones. Zone I (222-142 cm) contains light grey clay interspersed with reddish brown laminations occurring at 0.5-1.0 cm intervals. Zone II (141-102 cm) shows evidence suggesting hurricane disturbance from 125-130 cm as noted by a 25% decrease in water content, the presence of foraminifers, and a large spike in the Cl/Br ratio. Zone III (101-0 cm) is marked by fairly constant LOI results with the exception of two event layers positioned at 20-12 cm and 2-0 cm, the latter probably representing Hurricane Isaac. These event layers are characterized by increases in Cl/Br, decreases in water content, and the presence of dinoflagellates. Zone III also features the first appearance of Avicennia germinans (black mangrove) at 60 cm. In view of the detection of the ¹³⁷Cs peak (33 cm), this migration likely took place in the last 150 years. A dark band at 5-10 cm marked by a 15% increase in LOI values is attributed to the 2010 Deepwater Horizon oil spill incident, as this layer was tested positive for hydrocarbons linked to the MC252 site.