Semester of Graduation

Spring 2026

Degree

Master of Science (MS)

Department

Department of Oceanography and Coastal Sciences

Document Type

Thesis

Abstract

Coastal marshes globally are increasingly vulnerable to accelerating relative sea-level rise (RSLR), which threatens their capacity to maintain elevation through feedbacks among sediment supply, vegetation productivity, hydrology, and soil processes. Although many marshes can persist under moderate rates of sea-level rise through vertical accretion and belowground biomass production, this resilience is strongly constrained by sediment availability and subsurface processes such as autocompaction and organic matter decomposition. High accretion is often assumed to confer marsh resilience; however, subsurface processes, particularly autocompaction driven by surface loading, can substantially offset elevation gains. Coastal Louisiana experiences among the highest rates of RSLR worldwide due to the combined effects of eustatic sea-level rise, river–floodplain disconnection, shallow subsidence of unconsolidated deltaic sediments, and geologic subsidence of former delta lobes, which are compounded by a variety of localized human impacts to hydrology. Additionally, coastal Louisiana hosts a diversity of wetland types and geomorphic settings including saline to freshwater marshes in swamps in settings of active deltas, transgressive deltas and chenier plain regions. In general, uncertainty remains regarding how elevation dynamics and particularly subsurface processes vary between geomorphic settings and wetland types in this region. Yet this information is essential for distinguishing resilient marshes from those most vulnerable to RSLR and for prioritizing restoration sites and strategies.

This thesis examines spatial and mechanistic drivers of wetland elevation change across coastal Louisiana by integrating long-term monitoring data with targeted field experiments. Using 12 years (2008–2020) of data from 291 Coastwide Reference Monitoring System (CRMS) sites, surface accretion averaged 1.14 ± 0.04 cm yr⁻¹ for all environments coastwide, while surface elevation change averaged only 0.53 ± 0.01 cm yr⁻¹, indicating that more than half of accreted material was offset by shallow subsidence (0.61 ± 0.04 cm yr⁻¹). Shallow subsidence occurred at 92% of sites, and subsurface change was strongly positively related to surface accretion, demonstrating the dominant role of autocompaction. Elevation trajectories differed systematically by geomorphic setting. Wetlands of Active delta lobes exhibited the highest surface accretion and elevation change, whereas sediment-starved transgressive deltas and the Chenier Plain had substantially lower elevation gains and stronger sensitivity to inundation. Freshwater marsh and swamp wetlands in active deltas showed the greatest elevation gains, while intermediate and brackish marshes across settings experienced lower accretion combined with moderate to high subsidence, resulting in reduced elevation change.

Hydrology affected elevation dynamics particularly outside active deltas. Increasing inundation and percent time flooded were associated with reduced surface elevation change and surface accretion, and with enhanced shallow subsidence in transgressive deltas and the Chenier Plain. Soil properties varied across the coast, with higher bulk density and lower organic matter content in active deltas corresponding to greater elevation gain, while organic-rich soils in the Chenier Plain were more vulnerable to subsurface collapse under increasing inundation.

A ten-month field study in paired intermediate-salinity marshes undergoing long-term shallow subsidence or shallow expansion revealed that subsiding soil types in both Big Branch and Sabine had higher salinity than the expanding soil type. Soil shear strength was substantially greater at both soil types of Sabine, which are minerogenic (organic matter < 25%), compared to the soil types of Big Branch, which are organic-rich marshes. This indicates that soil composition and geomorphic context strongly influence subsurface stability.

Together, these results demonstrate that coastal wetland vulnerability to RSLR in Louisiana is governed by geomorphically mediated interactions among sediment supply, hydrology, and subsurface processes. Active deltas have greater net elevation gains, while sediment-limited transgressive deltas and the Chenier Plain are increasingly vulnerable to inundation-driven subsidence, emphasizing the need for restoration strategies that explicitly account for subsurface dynamics and geomorphic setting.

Date

4-16-2026

Committee Chair

Quirk, Tracy

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

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