23 Jun 2022
23 Jun 2022
Status: this preprint is currently under review for the journal BG.

Single-species dinoflagellate cyst carbon isotope fractionation in from coretop sediments: environmental controls, CO2-dependency and proxy potential

Joost Frieling1,a, Linda van Roij1, Iris Kleij1, Gert-Jan Reichart1,2, and Appy Sluijs1 Joost Frieling et al.
  • 1Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Princetonlaan 8, 3584CB Utrecht, The Netherlands
  • 2NIOZ Royal Netherlands Institute for Sea Research and Utrecht University, Texel, The Netherlands
  • anow at: Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, Oxford, United Kingdom

Abstract. Sedimentary bulk organic matter and various molecular organic components exhibit strong CO2-dependent carbon isotope fractionation relative to dissolved inorganic carbon sources. This fractionation (εp) has been employed as proxy for paleo-pCO2. Yet, culture experiments indicate this CO2-dependent εp is highly specific at genus and even species level, potentially hampering the use of bulk organic matter and non-species specific organic compounds. In recent years, significant progress has been made towards a CO2 proxy using controlled growth experiments with dinoflagellate species, also showing highly species-specific εp values. These values were, however, based on motile specimens and it remains unknown whether these relations also hold for the organic-walled resting cysts (dinocysts) produced by these dinoflagellate species in their natural environment. We here analyze dinocysts isolated from core-tops from the Atlantic Ocean and Mediterranean Sea, representing several species (Spiniferites elongatus, S. (cf.) ramosus, S. mirabilis, Operculodinium centrocarpum sensu Wall & Dale (1966) (hereafter referred to as O. centrocarpum) and Impagidinium aculeatum) using Laser ablation – nano Combustion – Gas Chromatography – Isotope Ratio Mass Spectrometry (LA/nC/GC-IRMS). We find that the dinocysts produced in the natural environment are all significantly more 13C-depleted compared to the cultured motile dinoflagellate cells, implying higher overall εp values and, moreover, exhibit large isotope variability. Where several species could be analysed from a single location, we often record significant differences in isotopic variance and offsets in mean δ13C values between species, highlighting the importance of single-species carbon isotope analyses. The most geographically expanded dataset, based on O. centrocarpum, shows that εp correlates significantly with various environmental parameters. Importantly, O. centrocarpum shows a CO2-dependent εp above ~240 μatm pCO2. Similar to other marine autotrophs, relative insensitivity at low pCO2 is in line with a carbon concentrating mechanism being active at low pCO2, although we here cannot fully exclude that we partly underestimated εp sensitivity at low pCO2 values due to the relatively sparse sampling in that range. Finally, we use the relation between εp and pCO2 in O. centrocarpum to propose a first pCO2 proxy based on a single dinocyst species.

Joost Frieling et al.

Status: open (until 04 Aug 2022)

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Joost Frieling et al.

Joost Frieling et al.


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Short summary
We present a first species specific evaluation of marine core-top dinoflagellate cyst carbon isotope fractionation (εp) in order to establish natural pCO2 dependency and explore its geological deep-time paleo-pCO2 proxy potential. We build a first version of the paleo-CO2 proxy based on Operculodinium centrocarpum. εp differs between species and is controlled by pCO2 and nutrients. Our results illustrate the value of δ13C analyses on single micrometer-scale sedimentary organic carbon particles.