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Post-depositional alteration of stable isotope signals by preferential degradation of algae-derived organic matter in reservoir sediments

Abstract

Post-depositional degradation of organic matter (OM) in freshwater sediments is crucial for driving the biogeochemical dynamics and influencing the carbon burial. This process also often causes diagenetic alteration on paleoenvironmental proxies. Yet, mechanisms behind degradation of sedimentary OM and depth-related variations in stable isotope ratios can so far only be explained in part. Degradation of sedimentary OM in two drinking water reservoirs with contrasting eutrophic and mesotrophic states and different catchment land use (agriculture versus forestry) was studied. A 4-step procedure was used to chemically separate sedimentary OM in terms of biochemical composition. Here we presented depth profiles of biochemical composition of sedimentary OM that helped to quantify preferential degradation of aquatic proteins and carbohydrates and the following removal of aquatic lipids. Sediment in the eutrophic reservoir, which reflected a larger contribution of algal-derived OM than the mesotrophic reservoir with a forest dominated catchment, was therefore subject to more intensive degradation of sedimentary OM along with δ13C and δ15N alterations. In addition, changes in the relative proportions of biochemical components in sedimentary OM had more pronounced impact on δ15N values relative to δ13C. Our findings suggest that the lability of algae-derived OM leads to uncertainties for the estimation of carbon burial in water bodies and obscures paleo-limnological information derived from isotopic proxies. Post-depositional modifications are more pronounced in eutrophic freshwaters that accumulated more readily degradable OM of algal origin in their sediments. Recognition of these modifications will help constrain carbon burial rates of productive lakes and reservoirs and assess the role of reservoirs in carbon cycling.

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Acknowledgements

We thank Michael Herzog, Karsten Rahn and Martin Wieprecht for field sampling. Xiaoqing Liu acknowledges support from the China Scholarship Council (No. 201406240130), the Guangdong Basic and Applied Basic Research Foundation (2021A1515011395), and the Key Laboratory of Global Change and Marine-Atmospheric Chemistry, MNR (GCMAC1904). Ines Locker is thanked for her assistance with the elemental and isotopic analysis. We acknowledge the help from Dr. Liana Hilfert for 13C-NMR analysis. Help with statistical analysis from Dr. Mick Wu is highly appreciated.

Funding

Funding was provided by china scholarship council (Grant No. 201406240130), the Guangdong Basic and Applied Basic Research Foundation (Grant No. 2021A1515011395).

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Correspondence to Xiaoqing Liu.

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Liu, X., Wendt-Potthoff, K., Barth, J.A.C. et al. Post-depositional alteration of stable isotope signals by preferential degradation of algae-derived organic matter in reservoir sediments. Biogeochemistry 159, 315–336 (2022). https://doi.org/10.1007/s10533-022-00930-y

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  • DOI: https://doi.org/10.1007/s10533-022-00930-y

Keywords

  • Carbon burial
  • Paleolimnology
  • Preferential degradation
  • Sedimentary organic matter
  • Stable isotopes