Single-species dinoflagellate cyst carbon isotope fractionation in from coretop sediments: environmental controls, CO2-dependency and proxy potential
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: final response (author comments only)
RC1: 'Comment on bg-2022-118', Anonymous Referee #1, 11 Jul 2022
- AC1: 'Reply on RC1', Joost Frieling, 06 Dec 2022
RC2: 'Comment on bg-2022-118', Anonymous Referee #3, 20 Oct 2022
- AC2: 'Reply on RC2', Joost Frieling, 06 Dec 2022
Joost Frieling et al.
Joost Frieling et al.
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Frieling et al., present records of carbon isotope fractionation from the resting cysts of dinoflagellates to investigate their utility in reconstructing ancient atmospheric CO2. This record of core-top material advances earlier work based on laboratory cultures (and based on sound theoretical basis) and so brings the community closer to confidence that this proxy may work in environmental settings. They show there are differences in carbon isotope fractionation between different species, emphasising the importance of single-species records, and show greater 13-C depletion in their core-top samples compared to cultured, motile organisms. The paper is interesting and makes an important contribution, but some of the analysis is unsatisfactory due to uncertainty about the age of the individual cysts in the “core-top” samples (detailed below). Therefore without a thorough treatment of that uncertainty (which is currently lacking) it’s difficult to know whether this proxy has utility. There are certainly hints that it does, but unfortunately this paper does not yet demonstrate that compellingly.
The problem with using core-top samples is the substantial increase in atmospheric CO2 since the industrial revolution. As the authors note, it is highly uncertain whether the cysts are from the last week, the last year, decades or even centuries ago. The uncertainty around the contemporaneous CO2 is potentially very large. The “rough correction” to 1850 isn’t really a correction at all, but an assumption which is not well supported, at best highly uncertain, and not really dealt with satisfactorily in the later analysis. The best approach (although expensive) would be to 14-C date some of these samples to see when this material actually dates to. The cheaper, and for this present study, more plausible approach would be to propagate through what is a really quite large uncertainty and see whether the conclusions still hold. Lines 147-8 state that “With the exception of pCO2, we hence assume all parameters (SST, SSS, nutrients) to be constant over the period the core top samples represent.” A fundamental problem here is that the authors have little information (or at least present little data) about how long a period of time the core top samples do in fact represent. I’m not sure that the approach taken to this, systematically removing the most 13-C depleted samples is appropriate. Whilst it is certainly plausible that these individuals represent modern samples, the evidence is fairly circumstantial, and they could represent another confounding variable. What is the impact on the analysis if these samples are not removed?
Ie L 213-4 “We assume these assemblages are representative of ocean conditions prior to the massive increase in anthropogenic carbon emissions.” This is very sizable assumption which whilst plausible is not currently supported by very much evidence.
It appears from Figure 5a that no uncertainty at all has been applied to the assumed CO2 value – is this correct? This is not a fair assumption given the uncertain age of each cyst. In fact a plain reading of Figure 5a suggests that, rather than supporting a function between CO2 and ep apart from at <240 uatm CO2, ep is effectively constant, and only slightly higher above 310. Why therefore has 240 uatm been emphasised?
Whilst the data presented here are interesting and important, the analysis at present is not sufficient to support the conclusions drawn robustly.
L104-5 is the 0.3-0.4 permil number precision or accuracy? How has accuracy been determined.
L343-4 “Badger, 2003, 2021;” These are two difference Badgers. Check BG style but likely need to include initials (lots, because they share first first name initial too).
L 385. I’m not sure this is sufficient to meet the journal data policy. Pangaea doi should be available at publication.