Identifying forcing agents of environmental change and ecological response on the Mississippi River Delta, Southeastern Louisiana

Abstract

Freshwater wetlands on the Mississippi River delta plain, containing extensive swamps and marshes, have experienced land loss of 5197 km² since the 1930s as the ocean has transgressed landward, resulting in landward retreat of bottomland forest, and transition of fresh to intermediate marsh. The timing and rapidity of these ecotonal transitions, and the impacts of natural and anthropogenic forces on this deltaic environment are uncertain. This study details a 448 cm sediment core retrieved from the intermediate marsh on the northern edge of Lake Salvador in southeastern Louisiana. Multi-proxy data identify six ecological shifts since 3500 BP. The ecosystem has shifted from interdistributary environment with high concentrations of such terrestrial and marine elements as Ca, Zr (3.5-3.0 cal kyr BP), to a freshwater deltaic-plain with an increase in freshwater herbs and trees (3.0-2.6 cal kyr BP), to a lacustrine environment marked by high Mn, Fe concentrations (2.6-2.2 cal kyr BP), to a swamp ecosystem with high concentrations of Zn, Br (2.2-1.4 cal kyr BP), to freshwater marsh with an increase in marsh plants (1.4-0.3 cal kyr BP), and to an intermediate marsh marked by Typha and Baccharis with elevated marine elements (since 0.3 cal kyr BP). The study identified the external forcing agents driving each deltaic environmental transition using multivariable analyses. Ecosystem dynamics are highly associated with the St. Bernard deltaic cycles, with dominant fluvial processes introducing freshwater ecosystems while forming geomorphological features such as levees, oxbow lakes, and back swamp and marsh during delta progradation. Thereafter, reduced sediment supply and decreased freshwater flow during delta transgression caused land subsidence and uneven topography. As a result, the swamp converted to marsh. Eighteenth century logging and canal development by Activities from French and Spanish settlements reduced the cypress forests and enlarged the coastal lakes.

Keywords: Ecological succession; Human activities; Pollen; The Mississippi Delta; Wetland; X-ray fluorescence.