Tropical small mountainous rivers (SMRs), characterized by extensive basin weathering, discharge high amounts of sediments and particulate terrestrial organic carbon (OCterr) into the ocean. Burial of OCterr 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 OCterr 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 (δ13COC) and OCterr biomarker proxies (BIT, #ringstetra, 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 OCterr contents, burial rates, and burial efficiencies along the river-estuary-ocean continuum. The substantial variation in δ13COC and OCterr biomarker proxies between riverbed and shelf sediments revealed a strong decrease in fractional OCterr contribution offshore and alongshore. Using a two-endmember mixing model based on δ13COC, we determined average OCterr contents of 1.40, 0.80, and 0.05 wt% in riverbed, river-mouth, and shelf sediments, respectively. The low OCterr 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 OCterr (99.2 ± 29.0 g m−2 yr−1) at river mouths. In comparison, burial rates of OCterr were significantly lower (0.1–0.5 g m−2 yr−1) offshore due to strong degradation of OCterr, resulting in a lower overall OCterr 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 OCterr during offshore transport is a potentially large CO2 source in the tropical coastal ocean.

中文翻译：

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河口是巽他大陆架陆地有机碳埋藏的热点地区：对热带沿海碳封存的影响


热带小型山地河流 （SMR） 的特点是广泛的盆地风化，向海洋排放大量沉积物和颗粒陆地有机碳 （OCterr）。OCterr 埋藏在海洋沉积物中对于大气 CO2 封存很重要，但其速率和效率仍未完全了解。以前的研究表明，在受 SMRs 影响的热带沿海海洋环境中，OCterr 的埋藏率和埋藏效率通常较低，但缺乏对特定过程的详细见解和埋藏效率的定量估计。本研究调查了马来半岛河流表面沉积物和邻近巽他大陆架海洋表层沉积物中体 OCterr 含量 （δ13COC） 和 OCterr 生物标志物代理 （BIT、#ringstetra、ΣIIIa/ΣIIa 和 %C32 1,15-二醇指数）的 δ13C 值，以量化河流-河口-海洋连续体沿线的沉积 OCterr 含量、埋藏率和埋藏效率。河床和陆架沉积物之间 δ13COC 和 OCterr 生物标志物代理的显着变化表明，近海和沿岸的 OCterr 分数贡献强烈减少。使用基于 δ13COC 的双端元混合模型，我们确定了河床、河口和陆架沉积物中的平均 OCterr 含量分别为 1.40、0.80 和 0.05 wt%。近海沉积物中 OCterr 含量低归因于影响沉积物扩散的物理机制（例如局部水动力状态和粘土矿物絮凝）。这个过程导致河口陆源沉积物的埋藏率（11.5 ± 2.1 kg m-2 yr-1）和 OCterr（99.2 ± 29.0 g m-2 yr-1）在河口的埋藏率非常高。相比之下，OCterr 的埋藏率显着降低 （0.1-0.5 g m-2 yr-1） 由于海上边缘海平均 （21%） 的严重降解，导致整个研究区域的整体 OCterr 埋藏效率 （18.2 %） 较低。这些发现强调了 CO2 汇时碳埋藏在 SMRs 口的重要性，而近海运输过程中 OCterr 的降解是热带沿海海洋潜在的大型 CO2 来源。