Long-term peat organic matter stability influenced by botanical composition and oceanic bromide inputs

• All Br in peat cores from four Patagonian sites is bonded to aromatic carbon. • Sphagnum macrofossils in peat are positively correlated with carbohydrate content. • Herbaceous macrofossils in peat are positively correlated with aromatic content. • Organo-Br and aromatic content increase with age up to 7,500 years before present. • In older peat, organo-Br content correlates with early Holocene sea level rise. The geochemical recalcitrance of peat organic matter (OM) is often indexed by its proportion of aromatic organic carbon. The factors leading to the enrichment and stabilization of aromatic moieties in peat are poorly understood. Hypothesizing that source vegetation and natural halogenation contribute to aromatic recalcitrance, we paired plant macrofossil analysis with infrared and Br K -edge X-ray absorption spectroscopy of four Patagonian peat cores from different ecohydrological environments that have undergone a fen-to-bog transition. Aromatic abundance is positively correlated with herbaceous materials and anticorrelated with Sphagnum moss. All Br in the peat is bonded to aromatic carbon. This organobromine, which is not solvent-extractable, becomes enriched alongside aromatic and herbaceous content for several thousand years after burial, suggesting that brominated aromatic polymers confer stability on millennial scales. In the cores with the longest records, peat organobromine concentration increases from 14,000 to ∼7500 years before present, contemporaneous with early-Holocene sea level rise. We use spectroscopic evidence to rule out downcore debromination in favor of the implication that lower bromide deposition yielded less organobromine during the early Holocene. Br content in coastal peat may therefore serve as a geochemical marker for post-glacial increases in sea level. These results demonstrate that both marine-derived Br and botanical factors affect the relative quantity and stabilization of aromatic structures in peat OM. Rising sea levels could stimulate further organobromine production in coastal peatlands, enhancing long-term preservation of aromatic carbon in peat OM.