Description: 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.

Description: Compound-specific radiocarbon dating often requires working with small sample sizes (〈 100 µgC). This makes the radiocarbon dates of biomarker compounds very sensitive to biases caused by extraneous carbon of unknown composition, which is introduced to the samples during processing (i.e., isolating single compounds from a heterogeneous mixture) prior to AMS measurements (procedure blank). Contamination sources include solvents, column bleed (from preparative HPLC and GC), carry-over and atmospheric carbon during combustion and vacuum line handling. Reporting accurate radiocarbon dates thus requires a correction for the procedure blank. We present our approach to assess the F14C and the mass of the blanks introduced during the preparation procedures of lipid biomarkers and lignin phenols. The treatments of these compound classes differ significantly which may lead to different F14Cblank and mblank. In order to assess the procedure blanks, we isolated fatty acids and lignin phenols from differently sized aliquots (20-80 µgC) obtained from standard materials with known F14C. Each compound class was extracted from two standard materials (one fossil, one modern). The measured F14C of the processed aliquots were graphically correlated to 1/mass of the respective sample. For aliquots of both, fossil and modern standard materials, this yielded an inverse linear relationship between the F14C and the sample size. The intercept of the regression lines obtained from the aliquots of fossil and modern standards is used to infer F14Cblank and mblank. The uncertainties of F14Cblank (σF14Cblank) and mblank (σmblank) were determined by the regression coefficients R2 of the linear regressions. Assuming constant contamination during processing of individual samples, the F14Cblank and mblank can be used to correct AMS results of lignin and lipid samples by isotopic mass balance.

Description: 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 (δ13CTOC/, 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 δ13CTOC values (-26.1 to -29.9 ‰) 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 the marine sediments, the distribution of δ13CTOC and 38 (sum of eight lignin phenols in organic carbon (OC), expressed as mg/100 mg 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 δ13CTOC 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.