The Nepal Himalayas is the source of many glacial and spring-fed river systems crisscrossing the mountainous terrain. There is an increasing recognition of small mountain rivers (SMRs) to have a significant combined export of dissolved and particulate organic carbon to the global carbon flux. We analyzed fluvial sediments from two SMRs and compared the results with two large mountain rivers (LMRs) in Nepal. We investigated the organic matter (OM), its compositional variability, and seasonal export using a suite of lipid biomarkers, namely n-alkanes, n-alkanoic acids, n-alkanols, and sterols. The SMRs indicated a similarity in lipid distribution and were affected by a strong seasonal variability. The LMRs showed a distinct contrast in the distribution of lipids in suspended sediments. Bedload sediments in SMRs were derived from diverse sources with weak terrigenous dominance all-year-round compared to the suspended load. Functional lipids (n-alkanoic acids and n-alkanols) were the major constituents in SMR sediments, indicating better preservation. In contrast, n-alkane concentration dominated over other fractions in suspended sediments retrieved from LMRs. The biomarker trends differentiate SMRs from LMRs with lower transformed/degraded OM in SMRs. A common observation was the strong presence of even carbon compounds in short-chain n-alkanes in SMR bedload sediments and their predominance in suspended sediments in LMRs. Such an unusual trend is attributed to specific biomarker sources from the catchment and ongoing processes in fluvial systems. Topsoil colonized by fungal species under moist acidic conditions and autochthonous bacteria contributes to the organic matter pool in shallow SMRs. In LMRs, the contribution from thermally mature sedimentary hydrocarbons and the diagenetic reduction of n-alkanoic acids to n-alkanes are additional contributors to the allochthonous carbon pool. The differences in lipid concentrations, their distribution, seasonality, and the size of rivers suggest differential preservation/degradation of the organic matter pool and their importance in contributing to the carbon budget.

尼泊尔喜马拉雅山是许多冰川和泉水河流系统的发源地，这些河流系统纵横交错在山区。人们越来越认识到，小山河 (SMR) 对全球碳通量有显着的溶解性和颗粒状有机碳的综合输出。我们分析了来自两个 SMR 的河流沉积物，并将结果与​​尼泊尔的两条大型山区河流 (LMR) 进行了比较。我们使用一套脂质生物标志物（即正构烷烃、正构烷酸、n-链烷醇和甾醇。SMR 表明脂质分布的相似性，并受到强烈季节性变化的影响。LMR 显示出悬浮沉积物中脂质分布的明显对比。与悬浮荷载相比，SMR 中的床荷载沉积物来自多种来源，全年陆源优势较弱。功能性脂质（正链烷酸和正链烷醇）是 SMR 沉积物中的主要成分，表明保存较好。相比之下，在从 LMR 回收的悬浮沉积物中，正烷烃浓度占主导地位。生物标志物趋势将 SMR 与 LMR 区分开来，在 SMR 中具有较低的转化/降解 OM。一个常见的观察结果是在短链n 中强烈存在偶数碳化合物-SMR 床载沉积物中的烷烃及其在 LMR 中悬浮沉积物中的优势。这种不寻常的趋势归因于来自流域和河流系统中正在进行的过程的特定生物标志物来源。在潮湿的酸性条件下被真菌物种定殖的表土和本地细菌有助于浅层 SMR 中的有机物质库。在LMRS，从热的贡献成熟沉积烃和成岩减少Ñ链烷酸，以ñ -烷烃是附加的贡献者异地碳库。脂质浓度，它们的分布，季节和河流大小的差异表明有机物库的保存/降解程度不同，以及它们在碳预算中的重要性。