Time-Varying Rates of Organic and Inorganic Mass Accumulation in Louisiana Marshes and Relation to Sea-Level Change
Semester of Graduation
Master of Science (MS)
Louisiana wetlands are complex systems that require a continuous input of organic and inorganic material to keep pace with relative sea-level rise (RSLR). Marshes, specifically, rely on organic material to build soil volume and maintain surface elevation. Interannual-to-decadal sea-level anomalies induced by Earth's internal climate variability and atmospheric disturbances such as hurricanes can affect marsh accretion, mineral deposition, and plant productivity. Coastal restoration projects such as sediment diversions are in progress to mitigate land loss and increase availability of inorganic sediment to coastal wetlands. The goal of this project is to determine if organic mass accumulation rates in Louisiana marshes are related to interannual sea-level fluctuations in Louisiana. One-meter cores were extracted from six saltmarsh locations in Barataria basin near the proposed Mid-Barataria Sediment Diversion and dated using 210Pb and 137Cs chronology to quantify organic and inorganic mass accumulation rates over the past ~100 years. Accumulation rates are compared to local surface water elevations in Grand Isle, Louisiana over the last ~60 years. Palynological analyses were conducted as well to investigate environmental and vegetation changes over the past ~100 years.
Results using 137Cs and the 210Pb Constant Rate of Supply (CRS) model show vertical accretion rates (VAR) range from 0.63 cm/yr to 1.69 cm/yr and total mass accumulation rates (MAR) range from 0.11 to 0.43 g/cm./yr. Spatially, average VAR and MAR increase with decreased distance from the Louisiana coast. Temporally, VAR and MAR (total, inorganic, and organic) are characterized by gradual increases in rates with decreasing age along with episodic peaks in VAR/MAR. Our findings show there is no relationship between sea-level anomalies and long-term VAR or organic/inorganic MAR. Rather, the results of this study highlight the importance of hurricanes to VAR and MAR contributions. Furthermore, the high water content (81 ± 8%) and organic-rich soils in the sediment cores highlight the importance of belowground biomass and water in maintaining marsh volume and elevation in the study area. Palynology results reveal Poaceae (grasses), Chenopodiaceae-Amaranthaceae (amaranth), Asteraceae (asters and daisies), and Cyperaceae (aquatic sedges) has been the dominant vegetation families during the last ~100 years.
Vincent, Sophie, "Time-Varying Rates of Organic and Inorganic Mass Accumulation in Louisiana Marshes and Relation to Sea-Level Change" (2022). LSU Master's Theses. 5674.
Available for download on Friday, October 31, 2025