Characterization of terrestrial organic carbon in the Amazon System : insights from biomarkers and their isotopic composition (Delta-C-13 and Delta 14C)

Abstract

Characterizing the origins and compositions of the terrestrial OC is critical for understanding the fate of terrestrial OC in marine sediments and constraining the terrestrial OC cycling. In this study, lignin and its isotope compositions (13C and 14C) are used as the major tools to provide a better understanding of the origins, pattern of distribution, processing, composition and fate of terrestrial OC in the Amazon system. In the Amazon system, Total organic carbon (TOC) and lignin content exhibit positive correlations with aluminium to silicon ratios (Al/Si, 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. Low I 13C values of bulk OC in the main tributaries consistently correspond with the dominance of C3 vegetation. Compositions of lignin, syringyl to vanillyl (S/V) and cinnamyl to vanillyl (C/V), suggest that non-woody angiosperm tissues are the dominant source of lignin in the Amazon basin. Based on the absolute content of bulk terrestrial OC, lignin, and long-chain n-alkanoic acids and their 14C ages, half-lives of bulk terrestrial OC, lignin and long-chain n-alkanoic acids during transport are estimated to be about 2310 years, 13860 and 470 years, respectively. This suggests that the preservation of terrestrial OC in the mud belt on the Amazon shelf is more efficient than previously assumed.