Characterizing sulfurization in Carpinteria Salt Marsh Reserve: A potential mechanism for enhanced carbon storage in salt marshes in Southern California

Abstract:

Coastal vegetated habitats (CVHs), such as salt marshes, are ‘hot spots’ of organic matter preservation and are responsible for 50% of the total annual carbon burial in all marine sediments (Mcleod et al., 2010). One proposed mechanism for enhanced organic carbon (OC) preservation in coastal sediments is sulfurization—the abiotic addition of sulfur to reactive organic structures—and although this process has been observed in other ecosystems (e.g. mangroves, particles in the ocean, deep sea sediments), its significance in carbon sequestration in salt marshes has not yet been explored. Here, we investigate sulfurization in marsh sediments by looking at geochemical, sedimentological and biological depth gradients in five surface defined marsh sub-environments: vegetated, unvegetated, microbial mat, algae mat, salt pan (n=20). We ask: Where does sulfurization occur in sediments—horizontally between marsh sediment types and vertically with core depth? In order to characterize each sub-environment, we analyzed marsh sediments for grain size, abundance of bioturbators, concentration of highly reactive iron (Fe_HR), and the concentration and δ³⁴S of both inorganic and organic sulfur phases. We also characterized organic carbon along a spectrum of reactivity based on its susceptibility to acid hydrolysis and measured its concentration, S:C ratio, δ¹³C, and δ³⁴S. We established oxygen, temperature and pH profiles in each sub-environment and compared these data with solid-phase geochemical, sedimentological and biological data to define depths of active sulfurization. Results constrain the extent of organic S formation in the marsh and allow for an evaluation of the significance of these reactions for carbon storage. Sulfurization and carbon storage are evaluated against critical environmental parameters such as vegetation type, grain size distributions, extent of bioturbation, and iron content providing insight into the drivers of organic matter preservation. These data will provide the foundation for on-going and future research efforts to model these heterogeneous and dynamic environments in order to inform proper management strategies.