Organic Geochemical Proxies as Indicators of Paleoenvironmental Conditions and Source Sediment Provenance in the Chenier Plain, Vermilion and Cameron Parishes, Louisiana, USA

摘要

Estuarine environments along Louisiana's coast provide sheltered depositional environments where sediment accumulates at or near relative mean sea level rise in a consistent horizontally-layered sequence. This depositional environment provides a sequential record of sediment and environmental changes. Geochemical and organic geochemical proxies are increasingly being used to study Holocene sedimentation to reconstruct relative sea level rise and environmental changes (Lamb et al., 2006). The carbon isotope composition of organic matter (delta13C) and the ratio of total organic carbon to total nitrogen (C/N) indicate source, providing a tracer of the carbon pathway and storage in estuarine environments. Carbon isotope ratios of preserved organic matter have been proven as useful environmental proxies; thus, changes can be used to interpret an estuary's position relative to sea level and to paleoriver discharge (Lamb et al., 2006). Additionally, using 137 Cs, as an indicator of accretion rate, aids in constraining the data to a timeline. Cesium-137 is a useful timestamp and has been used extensively in the Northern Gulf of Mexico and coastal Louisiana as an indicator of accretion rate (Delaune et al., 1989). This thesis applies these geochemical proxies to a sequentially sampled one-meter-long soil core from the Chenier Plain in Southwest Louisiana. The paleo-environmental record of accretion rate and organic carbon sources to the Chenier Plain estuarine environment are compared to known environmental changes in sea level, plant community, and hydrology over the past century. The resulting data indicates that human influences on hydrology in the Chenier Plain have served to alter organic carbon sources, namely increasing marine influence to previously freshwater habitats and restricting freshwater sheet flow from the Mermentau River basin. These changes and new hydrologic projects that aim to restore historic conditions have increased intermediate marsh habitat by facilitating increased movement of fresh and marine water throughout the Chenier Plain.