Composition and sources of sediment organic matter in a western Iberian salt marsh: Developing a novel prediction model of the bromine sedimentary pool.

Salt marshes are sensitive highly productive habitats crucial for carbon cycling. This study presents a comprehensive analysis of organic geochemical indicators and geochronology in the Mira salt marsh (SW Portugal) over eight centuries. The closely intertwined carbon and bromine (Br) biogeochemical cycles in these environments can influence the fluxes of volatile compounds such as ozone-depleting methyl bromide, emphasizing the importance of understanding sediment organic matter (OM) origin, budget, and composition in salt marshes. To characterize the strong Br-OM relationship, we used n-alkane signatures, bulk elemental data (total carbon, total nitrogen, C/N ratio), and stable isotopes (δN, δC) from a sediment core. Findings revealed a mixed composition of terrestrial and marine OM, posing challenges in distinguishing ex situ higher plant sources from in situ production by marsh vegetation. n-Alkanes (C to C) were found in all the sediment samples, predominantly C-C. Changes in their presence were linked to marsh succession, evolving from a vegetation-free tidal flat to a C halophyte-dominated high marsh ecosystem. Despite the area's low industrial and population impact, regulation of water flow through the dam affected the balance between continental and marine waters. This study aimed to create a cost-effective predictive model for total Br, enhancing paleoclimatic studies using sedimentary samples. The n-alkane model had limited resolution, but an alternative infrared (IR) spectroscopy-based model, requiring less time and smaller sample sizes, was developed. Combining FT-IR spectra with statistical analysis enabled the creation of a reliable total Br concentration prediction model (mean absolute error = 14.39). These findings have implications for controlling Br enrichment in marsh environments and can be applied in various coastal wetlands with different mineralogical and organic characteristics.