Title: The Spatial and Temporal Variability of Dissolved Inorganic Carbon and Total Alkalinity Production During Early Diagenesis in River Dominated Ocean Margin Sediments and Their Resultant Effect on Bottom Water Acidification

PhD Defense: Evan Magette

Committee: Dr. Martial Taillefert (Advisor), Dr. Christophe Rabouille, Dr. Yuanzhi Tang, Dr. Christopher Reinhard, Dr. Isaiah Bolden

Date and Time: Tuesday, May 12th, 2026 at 10:00 am

Location: Ford ES&T L1118 (Zoom link upon request)

Abstract: 

            Rapid anthropogenic release of CO2(g) to the atmosphere promotes global ocean acidification that is exacerbated in coastal environments by processes such as eutrophication and upwelling of low pH deep water. The decline in ocean pH through time poses a threat to marine ecosystems and the coastal communities that depend on them. Total alkalinity (TA), mainly supplied to the ocean by river discharge, allows seawater to buffer against future pH decline. Pore water DIC and TA generated in sediments during early diagenesis may be released to the overlying waters and either contribute to acidity or buffer against overlying water acidity, depending on the benthic TA to dissolved inorganic carbon (DIC) flux ratio (RTA:DIC), as RTA:DIC < 1 promotes acidification and RTA:DIC > 1 mitigates further acidification. The rapid accumulation of organic matter in river dominated ocean margin (RiOMar) sediments promotes the generation of relatively large benthic DIC and TA fluxes that may significantly influence overlying water acidification in these regions. A number of seasonally variable environmental factors such as river discharge and sediment resuspension may promote early diagenetic redox cycling in RiOMar sediments and induce seasonal RTA:DIC variability. This seasonal RTA:DIC variability is poorly characterized, however, despite their potential influence on overlying water acidification in these regions.

            The present dissertation investigated how early diagenetic processes in RiOMar sediments and resultant benthic solute fluxes and RTA:DIC respond to seasonally variable environmental factors. This was accomplished using a combination of in-situ benthic flux measurements on the northern Gulf of Mexico (nGoM) continental margin and ex-situ sediment core solid phase and pore water analyses from both nGoM margin and Rhône River prodelta sediments. Overall, the results presented in this dissertation revealed significant seasonal early diagenetic and RTA:DIC variability in RiOMars that was driven by seasonally variable environmental factors such as river discharge, primary production, sediment perturbations, and bottom water hypoxia development. The largest RTA:DIC from RiOMar sediments may be promoted either 1) proximal to river deltas and in eutrophic hypoxic systems, where anerobic TA generation is high and reoxidation of reduced metabolites is low, or 2) when CaCO3 dissolution generates significant TA in pore waters.