Single-species dinoflagellate cyst carbon isotope fractionation in core-top sediments: environmental controls, CO.sub.2 dependency and proxy potential

Sedimentary bulk organic matter and various molecular organic components exhibit strong CO.sub.2 -dependent carbon isotope fractionation relative to dissolved inorganic carbon sources. This fractionation (ε.sub.p) has been employed as a proxy for paleo-pCO.sub.2 . Yet, culture experiments indicate that CO.sub.2 -dependent ε.sub.p 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 CO.sub.2 proxy using controlled growth experiments with dinoflagellate species, also showing highly species-specific ε.sub.p 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 and 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 appreciably more .sup.13 C-depleted compared to the cultured motile dinoflagellate cells, implying higher overall ε.sub.p values, and, moreover, exhibit large isotope variability. Where several species could be analyzed from a single location, we often record significant differences in isotopic variance and offsets in mean [delta].sup.13 C values between species, highlighting the importance of single-species carbon isotope analyses. The most geographically expanded dataset, based on O. centrocarpum, shows that ε.sub.p correlates significantly with various environmental parameters. Importantly, O. centrocarpum shows a CO.sub.2 -dependent ε.sub.p above ⼠240 µatm pCO.sub.2 . Similar to other marine autotrophs, relative insensitivity at low pCO.sub.2 is in line with active carbon-concentrating mechanisms at low pCO.sub.2, although we here cannot fully exclude that we partly underestimated ε.sub.p sensitivity at low pCO.sub.2 values due to the relatively sparse sampling in that range. Finally, we use the relation between ε.sub.p and pCO.sub.2 in O. centrocarpum to propose a first pCO.