Preprints
https://doi.org/10.5194/bg-2021-172
https://doi.org/10.5194/bg-2021-172

  07 Jul 2021

07 Jul 2021

Review status: this preprint is currently under review for the journal BG.

Origin, transport, and retention of fluvial sedimentary organic matter in South Africa's largest freshwater wetland, Mkhuze Wetland System

Julia Gensel1, Marc Steven Humphries2, Matthias Zabel1, David Sebag3,4, Annette Hahn1, and Enno Schefuß1 Julia Gensel et al.
  • 1MARUM - Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany
  • 2School of Chemistry, University of the Witwatersrand, Johannesburg, South Africa
  • 3IFP Energies Nouvelles, Direction Sciences de la Terre et Technologies de l’Environnement, France
  • 4Normandie Univ, UNIROUEN, UNICAEN, CNRS, M2C, 76000 Rouen, France

Abstract. Sedimentary organic matter (OM) analyses along a 130 km-long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St. Lucia, Africa’s most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. A combination of organic bulk parameters, thermal analyses, and determination of plant waxes, and their corresponding stable carbon (δ13C) and hydrogen (δD) isotopic signatures in surface sediments and local plant species enabled characterization and comparison of sedimentary OM in terms of stability, degradation status, sources, and sinks within and among the respective sub-environments of the Mkhuze Wetland System. This approach showed that fluvial sedimentary OM originating from inland areas is mainly deposited on the floodplain and Mkhuze Swamps. In contrast to samples from upstream areas, a distinctly less degraded signature characterizes the sedimentary OM in the northern section of Lake St. Lucia. Although lake sedimentary plant waxes are similar in the observed wax distribution pattern and δ13C values, they exhibit considerably higher δD values. This offset in δD indicates that lakeshore vegetation dominates plant-derived sedimentary OM in the lake, elucidating the effective capturing of OM and its fate in a sub-tropical coastal freshwater wetland. These findings raise important constraints for environmental studies assuming watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore.

Julia Gensel et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-172', Anonymous Referee #1, 23 Jul 2021
    • AC1: 'Reply on RC1', Julia Gensel, 19 Sep 2021
  • RC2: 'Comment on bg-2021-172', Anonymous Referee #2, 11 Aug 2021
    • AC2: 'Reply on RC2', Julia Gensel, 19 Sep 2021

Julia Gensel et al.

Julia Gensel et al.

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Short summary
We investigated organic matter (OM) and plant wax-derived biomarkers in sediments and plants along the Mkhuze River to constrain OM's origin and transport pathways within South Africa's largest freshwater wetland. Presently, it efficiently captures OM, so neither transport from upstream areas nor export from the swamp occurs. Thus, we emphasize that such geomorphological features can alter OM provenance, questioning the assumption of watershed-integrated information in downstream sediments.
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