Consistency and environmental variability in mineral–organic interactions: The role of illite in organic carbon preservation in the northern South China Sea

Abstract

Mineral–organic matter interactions are globally recognized as a primary control on the long-term preservation of sedimentary organic carbon (OC). By integrating new ramped pyrolysis/oxidation (RPO) data from the northern South China Sea (SCS) with datasets from global fjords, estuaries, and marine systems, we confirm a consistent inverse relationship between OC content and both mean activation energy (μ_E) and bond diversity (σ_E), reinforcing the thermodynamic signature of mineral-associated OC preservation. However, SCS sediments reveal a key deviation: σ_E in the SCS positively correlates with low-E OC fractions, in contrast to global patterns, suggesting that mineral interactions in this setting preferentially stabilize thermally labile components. We further show that illite, the dominant clay mineral in the SCS, correlates with broader σ_E and lower μ_E, consistent with its role in retaining OC across a thermally diverse spectrum. While smectite is often cited as more protective, it is scarce in this region and shows no significant correlation with RPO parameters. Spatial trends also highlight the importance of OC source: a unique terrestrial-influenced site (BC15) exhibits elevated thermal stability and low low-E OC fractions, likely due to the intrinsic recalcitrance of vascular plant-derived material rather than mineral protection. Our findings demonstrate that while mineral–OC interactions are globally consistent in their role, their expression is modulated by sediment mineralogy, OC sources, and depositional settings. This underscores the importance of combining thermal reactivity metrics with mineralogical context to resolve the mechanisms underpinning OC preservation.