Award: EAR-2053163

Coastal blue carbon ecosystems store vast amounts of carbon and provide a wide range of ecosystem services to coastal communities. The purpose of this project was to better understand how changes in the geochemistry of coastal ecosystems affects carbon storage and turnover, specifically focusing on reactions between organic carbon and sulfide. We originally proposed three research goals:

1.     Characterize the distribution of organic matter (OM) sulfurization in coastal mangrove environments, comparing sites with either moderate or very low inputs of sedimentary iron.

2.     Identify the specific sources of OM that are relatively conducive to sulfurization and evaluate their contributions to OM storage.

3.     Investigate local mechanisms that affect OM sulfurization intensity, specifically oscillatory processes like tides.

During fieldwork, we unearthed an opportunity to compare our site with a nearby mangrove forest that had recently been the subject of a hydrological restoration project. This led us to add a fourth goal to the project:

4.     Evaluate the impacts of local hydrological restoration activities on OM sulfurization and storage.

Examples of key unpublished results from chapters 2 and 3 of Dr. Lena Capece’s PhD thesis are shown in the attached Figures.

To address goals #1 and #4, we collected sediment cores, litter, dissolved organic matter, and microbial mat materials and characterized their carbon, sulfur, and iron geochemistry. Figures “Fig 6” and “Fig7” show molar sulfur-to-carbon ratios and sulfur isotope compositions of organic matter from sediment cores at the three sites. Even in plant litter collected on the sediment surface, we find evidence for the accumulation of sulfur from microbial sulfate reduction. We also observe an overall increase in S:C ratios and decrease in d34S values with sediment depth, relative to primary biomass, that suggests ongoing concentration of sulfurization products over longer timescales. Sulfurization is remarkably widespread across even geochemically distinct mangrove sediments; iron availability and competition with pyrite formation does not appear to be a major control on its formation.

To identify the sources of sulfurization organic matter to the sediments (goal #2), we completed a series of paired lab and field experiments with six different representative biomass materials, including examples of algae, leaves, and marsh plants. The figures titled “FigS6” and “Fig9” show the results of a field experiment at Rookery Bay in which these biomass materials were deployed at 5–10 cm depth at restored and pristine locations and collected after 1, 2, 7, and 10 weeks of incubation. Nearly all algal biomass (Ulva and kelp) biomass was lost during field incubations, but the other four materials show strong evidence for gradual sulfurization. These sulfurization reactions vary only slightly with biomass type, and they occur on the timescale of weeks. A surprisingly wide range of plant materials appear conducive to sulfurization. Even though they are generally considered less reactive, leaves and coastal plants may be major contributors to the sulfurization signal in coastal sediments rather than algae, which are more readily lost.

Addressing goal #3, we find initial evidence than tidal cycles are particularly important for the production of dissolved rather than solid-phase organic sulfur. Figure “FigSI2” shows the sulfur content and S-isotope composition of dissolved organic sulfur (DOS) from restored and pristine sites at Rookery Bay. In the pristine sites, we observe plant-like materials in DOM (low d13C and high C:N) that also have S:C ratios greater than 2 and strongly 34S-depleted compositions (–10 to –20‰). The dissolved organic molecules released by healthy mangrove plants appear to be sulfurizable and may, through tidal pumping and other subsurface flows, contribute significantly to the volume and properties of marine DOM.

All finalized geochemical and spectroscopic results from this work are available on BCO-DMO under projects #938709 and # 983803.

In addition to training Dr. Lena Capece (graduation June 2025), who was immediately recruited into a professional position in the blue carbon management industry, this project provided intensive field research experiences to two highly engaged undergraduate researchers and involved several other undergraduate assistants.

This project supported the creation of a second season of the Ocean Solutions podcast, which interviews ocean professionals about their ongoing projects and career trajectories. Episodes are currently freely available on major podcast platforms (e.g., Spotify). Additionally, to evaluate the outcomes of using this type of podcast as an educational tool, we conducted an IRB-approved study comparing student perceptions of their own agency and career pathways in a large-format introductory class (Introduction to Oceanography) at UC Santa Barbara. We found that the podcast significantly decreased self-reported feelings of hopelessless and despair related to climate and ocean crises, although it did not immediate translate into changes in individual career considerations; these results are in prep for publication.

Last Modified: 09/17/2025
Modified by: Morgan Reed Raven