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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 17 Jun 2019

Research article | 17 Jun 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Age and origin of leaf wax n-alkanes in fluvial sediment-paleosol sequences, and implications for paleoenvironmental reconstructions

Marcel Bliedtner1,2, Hans von Suchodoletz3,4, Imke Schäfer2, Caroline Welte5, Gary Salazar6, Sönke Szidat6, Mischa Haas7, Nathalie Dubois8, and Roland Zech1 Marcel Bliedtner et al.
  • 1Institute of Geography, University of Jena, Löbdergraben 32, 07743 Jena, Germany
  • 2Institute of Geography and Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland
  • 3Institute of Geography, University of Technology Dresden, Dresden, Germany
  • 4Institute of Geography, University of Leipzig, 04103 Leipzig, Germany
  • 5Laboratory of Ion Beam Physics, ETH Zurich, 8093 Zurich, Switzerland
  • 6Department of Chemistry and Biochemistry and Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland
  • 7Department of Surface Waters Research and Management, Eawag, 8600 Dübendorf, Switzerland
  • 8Department of Earth Sciences, ETH Zürich, 8006 Zürich, Switzerland

Abstract. Leaf wax n-alkanes are increasingly used for quantitative reconstructions of past environmental changes. However, this is complicated in sediment archives with associated hydrological catchments since the stored n-alkanes can have different ages and origins. Direct 14C-dating of the n-alkanes has a great potential to derive independent age information for these proxies, allowing their correct paleoenvironmental interpretation. This holds also true for fluvial sediment-paleosols sequences (FSPS), where the n-alkane signal is divided into (i) a catchment signal that is deposited with fluvial sediments, and (ii) an in-situ signal from local biomass that increasingly dominates (paleo)soils with time. Therefore, age and origin of n-alkanes in FSPS are complex: In fluvial sediment layers they can be pre-aged and reworked when originating from eroded catchment soils, or originate from organic-rich sediment rocks in the catchment. In (paleo)soils, besides an inherited contribution from the catchment they were formed in-situ during pedogenesis and originate from local biomass. Depending on the different relative contributions from these sources, the n-alkane signal from an FSPS can show variable age offsets between its formation and final deposition.

During this study, we applied compound-class 14C-dating to n-alkanes from a fluvial sediment-paleosol sequence (FSPS) along the upper Alazani River in eastern Georgia with Jurassic black clay shales in its upper catchment. Our results show that pre-heating the n-alkanes with 120 °C for 8 h before 14C-dating effectively removed the shorter chains (< C25), including a part of the catchment-derived n-alkanes from Jurassic black clay shales. The remaining petrogenic contributions on the longer chains (≥ C25) were estimated and corrected for using a constant correction factor. This was based on n-alkane concentrations in a Jurassic black clay shale sample from the upper catchment. Due to different degrees of pre-aging and reworking, the corrected leaf wax n-alkane ages still indicate relatively large age offsets between n-alkane formation and deposition: While due to a dominance of in-situ produced leaf wax n-alkanes no age offsets existed in intensively developed (paleo)soils, much larger age offsets in less intensively developed paleosols indicate larger proportions of inherited leaf wax n-alkanes from the fluvial parent material. Accordingly, age offsets in non-pedogenic fluvial sediments were largest, and strongly increase after ~ 4 ka cal. BP due to a greater proportion of pre-aged and reworked leaf wax n-alkanes. The leaf wax n-alkanes from intensively developed (paleo)soils indicate a local dominance of grasses/herbs throughout the Holocene, which was most likely caused by anthropogenic activity. The leaf wax n-alkanes from fluvial sediments show a dominance of both deciduous trees/shrubs and grasses/herbs in different parts of the FSPS between ~ 8 and ~ 5.6 cal. ka BP. Since no older deciduous trees/shrub-derived n-alkanes from the catchment were dated, this seems to confirm the formerly proposed delayed regional post-glacial reforestation compared with western and central Europe.

Marcel Bliedtner et al.
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Marcel Bliedtner et al.
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Short summary
This study investigates the age and origin of leaf wax n-alkanes from a fluvial sediment-paleosol sequence (FSPS) by compound-class 14C-dating. Our results show varying age offsets between the formation and sedimentation of leaf wax n-alkanes from well developed (paleo)soils and fluvial sediments that is mostly due to their complex origin in such sequences. Directly dating the leaf wax n-alkanes is thus an important step for more robust leaf wax-based paleoenvironmental reconstructions in FSPS.
This study investigates the age and origin of leaf wax n-alkanes from a fluvial...