Organic carbon composition and preservation in macrotidal coastal wetland sediment: insights from biomarkers and isotopic signatures

Coastal wetlands store high amounts of organic carbon (OC) in their sediments, but long-term preservation of this carbon depends on habitat type, sediment depth, and the molecular characteristics of organic matter (OM). This study explores the dynamics of OC deposition and preservation across vertical profiles (0–30 cm) in two adjacent coastal habitats—mudflat, and salt-marsh—within the macrotidal system of the Aiguillon Bay (France). A multi-tracer approach was applied, combining stable isotopes δ 13 C, C/N ratios, lignin phenols, and fatty acids. Sediment OC content ranged from 13.4 to 23.2 mgC g −1 , with the highest concentrations found in the salt-marsh. δ 13 C and C/N signatures revealed dominant marine source in the mudflat, with a secondary contribution from microphytobenthos, and mixed marine–C₃ plant inputs in the salt-marsh. Fatty acids and lignin compositions supported this partitioning, with surface mudflat layers enriched in labile microbial and algal-derived compounds, whereas deeper salt-marsh sediments retained more resistant, C 3 plant-derived signatures resembling those of terrestrial OM source. OM degradation rates were closely linked to source composition and depth. Degradation was concentrated within the top 5 cm of salt-marsh and the top 10 cm of mudflat. Below these depths, biomarker profiles changed minimally, delineating a transition to longer-term preservation. First-order degradation constants were three times higher in mudflat (0.53 yr −1 ) than in salt-marsh (0.17 yr −1 ), despite similarly high sedimentation rates (1.8 and 2.2 cm yr −1 , respectively). This reflects differences in OM lability, with even minor contributions from microphytobenthos enhancing reactivity in mudflats. Salt-marshes, with their intermediate OM reactivity and high sedimentation rates, emerged as hotspots of carbon accumulation (366 gC m −2 yr −1 ), while mudflats also contributed substantially to coastal carbon sequestration (239 gC m −2 yr −1 ). These results highlight the value of depth-resolved, biomarker-based approaches to identify habitat-specific degradation dynamics; ultimately better understanding carbon accumulation in coastal ecosystems. • Depth-resolved multimarkers reveal habitat-specific organic carbon preservation. • Salt-marsh stores most organic carbon, but mudflats are also important. • Mudflat organic matter degrades 3 times faster with depth than salt-marsh OM. • Lignin is abundant in marshes, scarce yet persistent in mudflat sediments. • OM decay slows below 5–10 cm, marking habitat-specific preservation depths.