River mouths are hotspots for terrestrial organic carbon burial on the Sunda Shelf: Implications for tropical coastal carbon sequestration

Tropical small mountainous rivers (SMRs), characterized by extensive basin weathering, discharge high amounts of sediments and particulate terrestrial organic carbon (OC_terr) into the ocean. Burial of OC_terr in marine sediments is important for atmospheric CO₂ sequestration, yet its rate and efficiency remain not fully understood. Previous studies indicated generally low burial rates and burial efficiencies of OC_terr in tropical coastal ocean settings influenced by SMRs but lacked detailed insights into specific processes and quantitative estimates of burial efficiencies. This study investigated δ¹³C values of bulk OC (δ¹³C_OC) and OC_terr biomarker proxies (BIT, #rings_tetra, ΣIIIa/ΣIIa and %C₃₂ 1,15-diol indices) in river surface sediments on the Malay Peninsula and marine surface sediments on the adjacent Sunda Shelf to quantify sedimentary OC_terr contents, burial rates, and burial efficiencies along the river-estuary-ocean continuum. The substantial variation in δ¹³C_OC and OC_terr biomarker proxies between riverbed and shelf sediments revealed a strong decrease in fractional OC_terr contribution offshore and alongshore. Using a two-endmember mixing model based on δ¹³C_OC, we determined average OC_terr contents of 1.40, 0.80, and 0.05 wt% in riverbed, river-mouth, and shelf sediments, respectively. The low OC_terr contents in offshore sediments are attributed to physical mechanisms (such as local hydrodynamic regime and clay mineral flocculation) that affect sediment dispersal. This process leads to notably high burial rates of terrigenous sediment (11.5 ± 2.1 kg m⁻² yr⁻¹) and OC_terr (99.2 ± 29.0 g m⁻² yr⁻¹) at river mouths. In comparison, burial rates of OC_terr were significantly lower (0.1–0.5 g m⁻² yr⁻¹) offshore due to strong degradation of OC_terr, resulting in a lower overall OC_terr burial efficiency across the study area (18.2 %) compared to the global marginal sea average (21 %). These findings highlight the significance of carbon burial at the mouths of SMRs as CO₂ sinks, whereas degradation of OC_terr during offshore transport is a potentially large CO₂ source in the tropical coastal ocean.