A multiproxy approach study of the inland Mississippi delta: paleoenvironment, marine transgression and subsidence during the Holocene and carbon burial potential

Abstract

Wetland environments are disappearing at an alarming rate, driven through rising sea levels, a warming climate and human-driven subsidence, a consequent from engineering and damming the natural flow and cycle of river systems. In addition to housing a vast biodiversity, attenuating storm surges and holding a large economic value, wetlands also act as the largest carbon sink per hectare. The Mississippi Delta has long undergone severe land and wetland loss with 5000 km2 of wetlands disappearing from the Louisiana coastlines. A multiproxy approach using bulk sediment, geochemical and palynological analyses has been conducted on a 38.7 m core taken during the installation of a subsidence monitoring superstation close to the Mississippi River, southeast of New Orleans. This new data, in addition to previous stratigraphic lithology description, has been used to interpret the depositional evolution of the Holocene Mississippi River Delta (MRD) and explore the carbon storage potential of the delta environment. An initial Holocene transgression between 38.7 and 28 m has been interpreted. Increased fluvial input into the coastal marine realm, but remaining within a pro delta environment between 27 and 19.5 m is inferred primarily through biomarkers. A lithology change between 19.5 and 9.5 m shows the transition from marine pro delta towards a more terrestrial environment with the deposition of a delta mouth bar. 9.5 to 2 m reflects a predominantly terrestrial environment. Here the influence of delta lobe switching on the depositional environment is clear with the St. Bernard, Lafouche and Plaquesmines-Modern sub-deltas redirecting and redistributing water and sediment across the Mississippi Delta landscape. Within the upper 2 m human influences such as damming the MRD are shown in the environment, reducing sediment load and therefore terrestrial signals to the study site. Two zones of optimal carbon storage have been identified, with the lithology, palynology, BIT Index and FC32 1,15 inferring a wetland environment. Carbon storage potential is inferred through total organic carbon and the n-alkane carbon preference indicator. The data supports initial lithological interpretation of the MRD and highlights the need for the protection and restoration these wetland environments with high carbon storage potential.