Total organic carbon content, organic carbon isotopic composition, lignin phenol compositions and aluminium to silicon weight ratio and mean grain size of the riverbed and marine surface sediments from the lowland Amazon basin and offshore area

The Amazon River transports large amounts of terrestrial organic carbon (OCterr) from the Andean and Amazon neotropical forests to the Atlantic Ocean. In order to compare the biogeochemical characteristics of OCterr in the fluvial sediments from the Amazon drainage basin and in the adjacent marine sediments, we analysed riverbed sediments from the Amazon mainstream and its main tributaries as well as marine surface sediments from the Amazon shelf and fan for total organic carbon (TOC) content, organic carbon isotopic composition (d13CTOC) and lignin phenol compositions. TOC and lignin content exhibit positive correlations with Al/Si ratios (indicative of the sediment grain size) implying that the grain size of sediment discharged by the Amazon River plays an important role in the preservation of TOC and leads to preferential preservation of lignin phenols in fine particles. Depleted d13CTOC values (-26.1 per mil to -29.9 per mil) in the main tributaries consistently correspond with the dominance of C3 vegetation. Ratios of syringyl to vanillyl (S/V) and cinnamyl to vanillyl (C/V) lignin phenols suggest that non-woody angiosperm tissues are the dominant source of lignin in the Amazon basin. Although the Amazon basin hosts a rich diversity of vascular plant types, distinct regional lignin compositions are not observed. In marine sediments, the distribution of d13CTOC and Lambda8 (sum of eight lignin phenols in organic carbon (OC), expressed as mg/100mg OC) values implies that OCterr discharged by the Amazon River is transported north-westward by the North Brazil Current and mostly deposited on the inner shelf. The lignin compositions in offshore sediments under the influence of the Amazon plume are consistent with the riverbed samples suggesting that processing of OCterr during offshore transport does not change the encoded source information. Therefore, the lignin compositions preserved in these offshore sediments can reliably reflect the vegetation in the Amazon River catchment. In sediments from the Amazon Fan, low lignin content, relatively depleted d13CTOC values and high (Ad/Al)V ratios indicating highly degraded lignin imply that a significant fraction of the deposited OCterr is derived from petrogenic (sourced from ancient rocks) sources.

亚马孙河将大量陆地有机碳（terrestrial organic carbon, OCterr）从安第斯山脉及亚马孙新热带森林输送至大西洋。为对比亚马孙流域河流沉积物与邻近海洋沉积物中陆地有机碳的生物地球化学特征，我们分析了亚马孙干流、主要支流的河床沉积物，以及亚马孙陆架与陆扇的海洋表层沉积物，检测了其总有机碳（total organic carbon, TOC）含量、总有机碳碳同位素组成（δ¹³C_TOC）及木质素酚类组成。TOC与木质素含量与铝硅比（指示沉积物粒度）呈正相关，表明亚马孙河输送的沉积物粒度在TOC保存过程中发挥重要作用，并使得木质素酚类在细颗粒中优先得以保存。干流及主要支流中亏损的δ¹³C_TOC值（-26.1‰至-29.9‰）与C3植被占主导的特征一致。丁香基酚与香草基酚比值（S/V）及肉桂基酚与香草基酚比值（C/V）显示，亚马孙流域木质素的主要来源为非木本被子植物组织。尽管亚马孙盆地拥有丰富多样的维管植物类群，但并未观测到显著的区域木质素组成差异。在海洋沉积物中，δ¹³C_TOC与Λ8（有机碳中8种木质素酚类的总含量，单位为mg/100mg OC）的分布特征表明，亚马孙河输送的陆地有机碳通过北巴西海流向西北方向输送，并主要沉积在内陆架区域。受亚马孙冲淡水羽流影响的离岸沉积物中，木质素组成与河床沉积物样本一致，这表明离岸运输过程中陆地有机碳的转化并未改变其携带的源信息。因此，这些离岸沉积物中保存的木质素组成能够可靠反映亚马孙河流域的植被特征。在亚马孙陆扇沉积物中，较低的木质素含量、相对亏损的δ¹³C_TOC值，以及指示木质素高度降解的高香草基酚醛酸比值（(Ad/Al)_V），表明沉积的陆地有机碳中有相当一部分源自岩源碳（petrogenic sources，即来自古老岩石的碳）。