Contributions of Sulfur, Iron, and Carbon Biogeochemistry to Alkalinity Production in a Northern California Zostera marina Meadow

Vegetated coastal marine systems, also known as blue carbon ecosystems, are traditionally recognized for carbon capture and long-term storage both within their biomass and via organic carbon accumulation in their underlying sediments. Recently, alkalinity generation within the sediments of blue carbon ecosystems, such as seagrasses, has been proposed as an additional long-term sink for atmospheric carbon dioxide. Seagrasses can capture and provide the organic matter required to fuel biogeochemical processes that increase alkalinity in underlying sediments, such as bacterial sulfate reduction (BSR). BSR produces alkalinity by reducing sulfate in seawater to sulfide (e.g., H₂S). This sulfide can then react with iron to form the mineral pyrite (FeS₂) which, once buried, prevents sulfide reoxidation and frees the alkalinity produced to buffer overlying waters. Although seagrasses and pyrite formation are each well studied, there is still much to uncover about how they are related, how their interrelation impacts alkalinity production, and how that varies across a seagrass meadow. This study evaluates the intra-meadow variability of iron, sulfur, and carbon pools in a northern California Zostera marina meadow to better understand how sediment and seagrass characteristics influence alkalinity production at a higher spatial resolution. Initial results indicate significant organic matter variability across this seagrass meadow. Since BSR and subsequent pyrite formation are in part limited by organic matter availability, this variability suggests that alkalinity generation is also likely to be largely heterogeneous across a single seagrass meadow.  

In addition to helping resolve gaps in local and global alkalinity budgets, this study also has implications for proposed marine carbon dioxide removal (mCDR) technologies. Deepening our understanding of alkalinity and carbon cycling in coastal environments, such as seagrasses, is imperative to assess the efficacy of many proposed mCDR interventions which served as additional motivation for this study.