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The Amazon River transports large amounts of terrestrial organiccarbon (OCterr) from the Andean and Amazon neotropical forests to theAtlantic Ocean. In order to compare the biogeochemical characteristics ofOCterr in the fluvial sediments from the Amazon drainage basin and inthe adjacent marine sediments, we analysed riverbed sediments from the Amazonmainstream and its main tributaries as well as marine surface sediments fromthe Amazon shelf and fan for total organic carbon (TOC) content, organiccarbon isotopic composition (δ13CTOC), and lignin phenolcompositions. TOC and lignin content exhibit positive correlations with Al ∕ Siratios (indicative of the sediment grain size) implying that the grain sizeof sediment discharged by the Amazon River plays an important role in thepreservation of TOC and leads to preferential preservation of lignin phenolsin fine particles. Depleted δ13CTOC values (−26.1to −29.9 ‰) in the main tributariesconsistently correspond with the dominance of C3 vegetation. Ratios ofsyringyl to vanillyl (S ∕ V) and cinnamyl to vanillyl (C ∕ V) ligninphenols suggest that non-woody angiosperm tissues are the dominant source oflignin in the Amazon basin. Although the Amazon basin hosts a rich diversityof vascular plant types, distinct regional lignin compositions are notobserved. In the marine sediments, the distribution ofδ13CTOC and Λ8 (sum of eight lignin phenols inorganic carbon (OC), expressed as mg/100 mg OC) values implies thatOCterr discharged by the Amazon River is transportednorth-westward by the North Brazil Current and mostly deposited on the innershelf. The lignin compositions in offshore sediments under the influence ofthe Amazon plume are consistent with the riverbed samples suggesting thatprocessing of OCterr during offshore transport does not changethe encoded source information. Therefore, the lignin compositions preservedin these offshore sediments can reliably reflect the vegetation in the AmazonRiver catchment. In sediments from the Amazon fan, low lignin content,relatively depleted δ13CTOC values and high(Ad ∕ Al)V ratios indicating highly degraded lignin implythat a significant fraction of the deposited OCterr is derivedfrom petrogenic (sourced from ancient rocks) sources

Origin and processing of terrestrial organic carbon in the Amazon system: lignin phenols in river, shelf, and fan sediments