Characterization of terrestrial organic carbon in the Amazon system: insights from biomarkers and their isotopic composition (δ13C and Δ14C)


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Gesine.Mollenhauer [ at ] awi.de

Abstract

The preservation of terrestrial organic carbon (OC) in marine sediments is a crucial component of global carbon cycle on geological timescale. 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. The amount of terrestrial OC discharged by the Amazon River to the Atlantic Ocean every year is about 8-10 % of the global annual input of terrestrial OC to oceans. Therefore, the terrestrial OC in the Amazon system has been extensively investigated. However, until now, many aspects regarding the origin and fate of terrestrial OC in the Amazon system are still ambiguous and need to be elucidated. Firstly, little is known about the disperse pattern of lignin (a major component of higher plants biomass and can serve as a biomarker of terrestrial OC) and the factors controlling the characteristics of terrestrial OC in the Amazon continental margin. Secondly, it is questionable whether the Amazon continental margin can efficiently store terrestrial OC or serve as a sedimentary OC incinerator. Thirdly, the response of vegetation to climate change during late Pleistocene is debated. To fill these knowledge gaps, 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 first part, the biogeochemical characteristics of terrestrial OC in the fluvial sediments from the Amazon drainage basin and in the adjacent marine sediments are compared. 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 δ13C 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. 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 δ13C values of bulk OC and lignin contents implies that terrestrial OC discharged by the Amazon River is transported northwestward 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 terrestrial OC 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 low δ13C values of bulk OC and high degradation degree of lignin demonstrate that a significant fraction of the deposited terrestrial OC is derived from petrogenic (sourced from ancient rocks) sources. In the second part, a new method of assessing procedural blank for compound-specific 14C analysis is developed with a Bayesian model. This method is successfully used in the third part of this thesis, which is about radiocarbon dating of source-specific biomarkers (n-alkanoic acids and lignin phenols) in riverbed sediments from the lowland Amazon basin and offshore sediments. The results show that Δ14C values of terrestrial OC on the Amazon continental margin are substantially influenced by matrix association effects, where terrestrial OC associated with the finer-grained particles is better preserved and more resistant to decomposition during residence in intermediate reservoirs. The compoundspecific Δ14C values imply that as expected short-chain n-alkanoic acids represent recently biosynthesized organic matter from riverine or marine primary production whereas both long-chain nalkanoic acids and lignin phenols used as markers for land vegetation have pre-aged in soils where they resided attached to mineral surfaces. By using a ternary mixing model, a well-constrained quantitative estimate of the composition of sedimentary OC in riverbed and marine sediments is obtained. Despite the variable composition of sedimentary OC in the Amazon system, the burial of fossil rock-derived OC is relatively constant. 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. In the fourth part, the δ13C analysis of lignin phenols are applied in marine surface sediments from the Amazon shelf and sediment core GeoB16224-1 recovered from the continental margin NW of the Amazon mouth. The weighted average δ13C values of lignin indicate that the modern terrestrial OC on the Amazon shelf is dominated by C3 plants and the vegetation source remained constant over the past 12.8-50 kyr, in agreement with previous studies. A general pattern of phenolic δ13C values is observed with the acid monomers of V and S phenols displaying lower δ13C values than their aldehyde counterparts, while C phenols are always more enriched in 13C than V and S phenols. The lignin content and composition paired with δ13C of lignin are used to reconstruct the characteristics of terrestrial OC deposited on the continental margin NW of the Amazon mouth over the period 12.8-50 kyr. Lignin composition, δ13C values of lignin, BIT index, δ13C and δD values plant-wax lipids show clear in-phase variation. Therefore, it can be proposed that next to vegetation change, the variation of δ13C values of lignin and plant-wax lipids during HS could reflect either enhanced discharge of more degraded terrestrial OC and/or more contributions of terrestrial from high altitude regions. These two possible scenarios suggest that the Amazon basin was still a stable ecological system dominated by C3 forest and the increases of δ13C values of lignin and plant-wax lipids were actually the consequence of changes of sources of terrestrial OC through the late Pleistocene.



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Thesis (PhD)
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Not peer-reviewed
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Eprint ID
51542
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Sun, S. (2018): Characterization of terrestrial organic carbon in the Amazon system: insights from biomarkers and their isotopic composition (δ13C and Δ14C) , PhD thesis, Universität Bremen.


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