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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-373
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-373
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 28 Oct 2019

Submitted as: research article | 28 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Better molecular preservation of organic matter in an oxic than in a sulphidic depositional environment: evidence from of Thalassiphora pelagica (Dinoflagellata, Eocene) cysts

Gerard J. M. Versteegh1,2, Alexander J. P. Houben3,4, and Karin A. F. Zonneveld2 Gerard J. M. Versteegh et al.
  • 1Heisenberg Group Marine Kerogen, Marum Research Faculty, Universität Bremen, Bremen, D-28359, Germany
  • 2Micropaleontology Group, Division Marine Palynology, Marum Research Faculty, Universität Bremen, Bremen, D-28359, Germany
  • 3Geological Survey of the Netherlands, TNO, Utrecht, 3548 CB, the Netherlands
  • 4Marine Palynology and Palaeoceanograhy, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, the Netherlands

Abstract. Anoxic sediments as compared to oxic settings encompass a much higher proportion of relatively labile and thus more reactive organic matter, naturally giving rise to condensation reactions (such as vulcanisation) transforming the original biomolecules into geomolecules. For the oxic environment where the labile, reactive, component is rapidly removed, such transformation and condensation is much less likely so that one would expect a structurally much better preservation of the more refractory initial biomolecules. To test this hypothesis, initially identical biomolecules need to be compared between different preservational environments. Here, we use the species specific morphology of organic microfossils to assure a single initial biosynthetic product (the cysts of the fossil dinoflagellate species Thallasiphora pelagica) for comparison. We assess the macromolecular structures of cysts from the Eocene (~ 40 Ma) sulphidic Rhine Graben and the oxic Kerguelen Plateau and compare them with each other and the structures of recent cysts. While between the sites the T. pelagica cysts are morphologically identical, pyrolysis gas chromatography mass spectroscopy and micro Fourier transform infra red analyses show that their macromolecular characteristics are strongly different. The cysts deposited in the sulphidic Rhine Graben show a strong contribution of long-chain aliphatic moieties and organic sulphur, absent in the material deposited on the oxic Kerguelen Plateau. Comparison with recent cyst walls suggests a much better molecular preservation for the oxic depositional environment, confirming our initial hypothesis. This leads to the conclusion that the best preservation of molecular structure is not necessarily where most organic matter gets preserved, which, in turn, is important for understanding the nature and fate of sedimentary organic matter.

Gerard J. M. Versteegh et al.
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Gerard J. M. Versteegh et al.
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Micro-FTIR and py-GC-MS data on Thalassiphora pelagica of ODP sample 120-748B-18H-17W,55-57 G. J. M. Versteegh, A. J. P. Houben, and K. A. F. Zonneveld https://doi.pangaea.de/10.1594/PANGAEA.905696

Gerard J. M. Versteegh et al.
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Short summary
Anoxic sediments contain mostly much more organic matter than oxic ones and therefore organic matter in anoxic settings is often considered to be preserved better than in oxic settings. However, through the analysis of the same fossil dinoflagellate cyst species from both oxic and anoxic settings we show that at a molecular level the preservation in the oxic sediments may be better since int the anoxic setting the cyst macromolecule has been altered by post depositional modification.
Anoxic sediments contain mostly much more organic matter than oxic ones and therefore organic...
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