Palynological and Geochemical Records of Environmental Changes in a Taxodium Swamp near Lake Pontchartrain in Southern Louisiana (USA) during the Last 150 Years

The hydrological and environmental histories of estuarine wetlands in southern Louisiana are not well-documented. To better understand their local developmental processes, a 59 cm sediment core (WMA-1) was extracted from a bald cypress (Taxodium distichum) swamp located approximately 800 m west of Lake Pontchartrain. This area has been heavily influenced by human activities, and is susceptible to inundation from storm surges from Lake Pontchartrain as well as fluvial and lacustrine processes. Pb-210 analysis indicates that WMA-1 has a sedimentation rate of 0.39 cm/year, consistent with the detection of a Cs-137 peak at 18 cm depth. Sedimentological and geochemical analyses reveal two distinct sedimentological facies: an organic-rich dark brown peat unit from 0 to 29 cm containing low concentrations of terrestrial elements (e.g., Ti, and Fe), and a clay unit from 29 to 59 cm with elevated concentrations of most elements (e.g., Sr, Cl, Ti, Mn, and Fe). The pollen assemblage in the clay section is dominated by TCT (mainly Taxodium), but it is replaced by Salix and wetland herbaceous taxa in the overlying peat. The Taxodium swamp was present at the site at least 150 years ago, and was replaced by Salix and other bottomland hardwood trees and wetland herbs due to a decrease in water level around AD 1940. While climate may have played a role, this hydrological change was most likely caused by the significant regional human activities during the last 100 years. Two thin clay layers (at 3-5 cm, 14-19 cm) embedded in the upper peat section were likely deposited from hurricanes Isaac (2012) and Betsy (1969): two events responsible for heavy rain and significant storm-surge flooding. Our findings suggest that human activities are capable of altering local wetland hydrology and ecology, while the clay layers represent hurricane-induced floods at the site during the past century.