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| 13 May 2024
Southern Ocean control on atmospheric CO2 changes across late-Pliocene Marine Isotope Stage M2
Abstract. During the Pliocene, atmospheric CO2 concentrations (pCO2) were similar to today’s and global average temperature was ~3 °C higher. However, the relationships and phasing between variability in climate and pCO2 on orbital time scales are not well understood. Specifically, questions remain about the nature of a lag of pCO2 relative to benthic foraminiferal δ18O in the late-Pliocene Marine Isotope Stage M2 (3300 kiloannum ago, ka), which was longer than during the Pleistocene. Here, we present a multi-proxy paleoceanographic reconstruction of the late-Pliocene subantarctic zone, which is today one of the major ocean sinks of atmospheric CO2. New dinoflagellate cyst assemblage data is combined with previously published sea surface temperature reconstructions, to reveal past surface conditions, including latitudinal migrations of the subtropical front (STF) over the late-Pliocene at ODP Site 1168, offshore west Tasmania. We observe strong oceanographic variability at the STF over glacial-interglacial timescales, especially across the M2 (3320–3260 ka). By providing tight and independent age constraints from benthic foraminiferal δ18O, we find that, much more than benthic δ18O or local SST, latitudinal migrations of the STF are tightly coupled to pCO2 variations across the M2. Specifically, a northerly position of the STF during M2 deglaciation coincides with generally low pCO2. We postulate that the efficiency of the Southern Ocean carbon outgassing varied strongly with migrations of the STF, and that is in part accounted for the variability in pCO2 across M2.
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CC1: 'Comment on cp-2024-33', Ian Bailey, 15 May 2024
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Great study and fascinating work, and I look forward to seeing your work published in CP. Full disclosure; this is a biased comment from a retired academic, but I noticed that you did not cite Kirby et al. (2020; https://doi.org/10.1016/j.quascirev.2020.106644). I thought I'd draw your attention to this work, though, since it was the first study to provide a data-driven discussion and hypothesis on the location and mechanisms involved in marine carbon storage during MIS M2. The ideas we presented in Kirby et al. on this subject are certainly consistent with the conclusions you have drawn.
Citation: https://doi.org/10.5194/cp-2024-33-CC1 -
RC1: 'Comment on cp-2024-33', Anonymous Referee #1, 12 Jun 2024
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This paper presents dinoflagellate cyst assemblages from one sediment core (ODP 1168) off the coast of Tasmania across the M2 glacial interval in the Pliocene. By using modern analogs and available SST and CO2 reconstructions it it then concluded how not only SST but also salinity might have changed and how changes relate to CO2. The information is used to suggest that changes in the polar fronts were probably responsible for the observed CO2 changes which are delayed to SST / d18O anomalies across M2.
I am not a data person, so I cannot judge the quality of the data. However, assuming they are ok I try to understand to given story (support for the conclusions) and while I can follow it in principle I have problems in the details. So, therfore, please find my suggestions to improve the text below, which hopefully helps all readers, not only the non-experts, to better follow and understand the content of the paper. Comments are in chronological order.
1. First sentence of abstract: Todays CO2 are above 420 ppm, while for the Pliocene data are still uncertain, ranging from 150 to 450 ppm, as you also show in Fig 4c. So this needs adjustment, or maybe use a different opening.
2. line 11: You show new data from one core. One conclusions is, that the subtropical front migrated several times across the core site, placing the site at some times in the subtropical zone, in other times in the subantarctic zone. So your sentence …of the late-Pliocence subantarctic zone… ist not completely correct.
3. line 11-12: The fact that the subantarctic zone is nowadays a major ocean sink for CO2 comes from the fact that our anthropogenic emissions have reversed the natural atmosphere-ocean CO2 fluxes. However, it makes little sense to compare fluxes of today (fossil fuel driven, so surface ocean CO2 is a slave to atmospheric CO2) with a natural state (atmospheric CO2 is the gas-phase of the ocean, so dominately oceanic-driven atmopsheirc CO2). More interestingly here would be a comparison with the natural role of the Southern Ocean to CO2 (which is a source in the south, see your Fig 1, process 1, and a sink in the north, Fig 1, process 3). Maybe cite for details in the introduction / main text on this subject also Gruber, N., et al. (2009), Oceanic sources, sinks, and transport of atmospheric CO2, Global Biogeochem. Cycles, 23, GB1005, doi:10.1029/2008GB003349.
4. lines 10 and 15: Give the same dates for M2, or better, state it only once at line 10. I do not think ka has to be introduced to the reader of this journal.
5. lines 49ff and Fig 4c: On CO2. I would expect a little more critical discussion of CO2. If I look at the CenCO2PIP 2023 data I also find d11B-based CO2 as low as 220 ppm around M2, something which was also highlighted in Köhler (2023, https://doi.org/10.1029/2022PA004439), who only plotted and analysed d11B-based CO2. This together with the even lower alkenone (here called phytoplankton-based) gives me some difficulties in accepting the statement that Pliocene CO2 was ~400 ppm. Maybe yes, maybe not. I can agree, that the de la Vega et al (2020) data are the highest resolved CO2 data across that interval, so the temporal changes are probably real, but the absolute values to some extend also depends on the calibration (and assumptions made for a 2nd variable of the carbonate system, see Köhler 2023 for discussions, if interested). PLIOMIP1 initally used 405ppm, stating explictly that this should also cover changes in non-CO2 greenhouse gases, such as CH4 and N2O. While I think this statement is somehow lost in PLIOMIP2, it is nevertheless contained there, since CH4 and N2O are fixed at preindustrial values. Furthermore, PLIOMIP3 (Haywood et al., 2024, https://doi.org/10.1016/j.gloplacha.2023.104316) now also added a scenario with 490ppm to account for the finding of Hopcroft et al (2020, https://doi.org/10.1073/pnas.2002320117) of higher CH4 in the Pliocene, adding a radiative forcing of 0.9 W/m^2, or a warming of 0.6-1.0 K. Maybe discuss this more widely in your introduction.
6. line 66: … one record… Please added details, which record this is and where.
7. Fig 1: Arrowheads of the CO2 fluxes in Fig 1a are too small. Captions: STZ not explained, while SAZ should be both subtropical and subantarctic zone? I believe the details of the data generation might be better placed in the methods and/or data availability section. What is MODIS-Aqua? I believe there is reference missing for the SST data plotted in Fig 1b.
8. line 99: Reference missing for modern SST and BWT at site 1168.
9. line 111: The definition of the STF index is crucial for understanding the paper. Please be more specific how this is calculated. How do the dinocysts enter the index. At best state the used equation.
10. line 129: what is the sh_655 data set?
11. line 147: The sentence „3 carbonate standards (ETH-1,2,3)“ is incomplete.
12. line 150: „0.06 permil and 0.06 permil, respectively“ is better described as „0.06 permil in both variables“.
13. The age model:
1. On fig 3 you only show d18Obf from CENOGRID, but you mentioned also LR04 in line 178. Is the later really used? If so, maybe include in Fig 3, but consider using the update (Prob-stack, Ahn et al., 2017, https://doi.org/10.1093/climsys/dzx002).
2. line 179: „4 solid age points“: well, which ones are they? there are 7 in Fig 2. The reasoning for chosing the age points is not clear to me, e.g. #7 a minima in CENOGRID d18O is tuned to a maximum in d18O from the site, similarly in #4, or is color reflectance more important here (would make sense for #4, but not for #7), but this is not mentioned in Table 1 as such. Please add details. What about the age model previously used for the published SST data of the site (Hou et al., 2023a)? Is it different, why is this not taken here? Maybe add a depth-age plot.
3. In summary, I have the feeling that the available tie points do not allow for a perfect matching of records. It would be good if you could also state an age uncertainty to the tie points, which would allow to judge how certain the discussed phasing with CO2 is.
14. Fig 3c: Something is wrong with obliquity (right y-axis), should be around 23°, not around 0.4? Or define precisely, what „obliquity insolation curve“ means. Whatever, a unit in the y-axis label would help.
15. SST, lines 189ff: I have problems to bring the text together with Fig 4a. What I see: there is a SST minima in M2 in BAYSPAR (whatever that is, is not discussed) of 9-10°C, mabye after a 5°C cooling, Pre- and post-M2 SST seem to be on average the same, if you write differently you might need to state more clearly the age window you are refering to.
16. Fig 4: Please also mark the mPWP in the figure with another vertical bar (as for M2), since you refer to it a lot in the text. You can simplify the legend to Fig 4a: there is only 1 SST from site 959, and that is TEXT86-HL86, so merge green with triangle. All SST records from site 1168 are NOT from „this study“ as said in the legend, but if I understood correctly from Hou et al. (2023a). Fig 4c: Maybe highlight the CO2 data set, that is purely coming from de la Vega et al. (2020), since this was consistently measured on the same site in the same lab. Fig 4a: Why is summer insolation at 80°S plotted, why not at the core site (40°S)?
17. lines 223:ff: SST, again what is described here seems to disagree with Fig 4a, see #15 above. Maybe more details help.
18. lines 238ff: I cannot follow why you deduce that salinity changed. Maybe if the STF index is somehow related to SST or SSS I might be able to follow. Or is it simply: SST is already back to preM2 values after the M2, but the assembly distribution is not, therefore it has to be salinity? I can follow that, but how do you get a precise number of 1.5 psu?
19. line 247: What is the „recovery of M2“? After M2 comes the mPWP, no time lag between both.
20. line 251: I see that front and SST have a phase lag, since both have been measured on the same core. For the lag with CO2 a statement of age uncertainty is in my view helpful.
21. line 263: „influence the ocean uptake effiency of atmospheric carbon“. I am not sure this is the right wording. The fronts change the air-sea gas exchange of CO2 and eventually the net CO2 flux. What has efficiency to do with it?
22. line 270ff: „.. enhanced stratification … preventing respired CO2 from outgassing“. I am not sure you can go that far. Your frontal movement showed, that site 1168 was sometimes north/south of the STF. From that and Fig 1a you can say that the oceanic CO2 uptake (process 3) might be changed, so maybe a smaller marine sink. I do not think you can say a lot more on how the other fronts moved and therefore it is hard to see that the conclusion on stratification and the preventing of outgassing respired CO2 is supported, which would be a smaller marine CO2 source. The net effect on CO2 would be the same but for a different reason.
23. line 305ff: I am sorry, but I do not see any cooling in BWT in Fig 4e. The data give me more or less a straight line. Furthermore the full citation to Braaten et al (2023) is missing in the reference list, so I cannot check on the details.Citation: https://doi.org/10.5194/cp-2024-33-RC1 -
RC2: 'Comment on cp-2024-33', Jan Hennissen, 25 Jun 2024
reply
Hou and co-authors present dinoflagellate cyst census counts, benthic δ18O and bulk carbonate stable isotope results obtained from 56 samples covering an interval containing the Pliocene MIS M2 in a single borehole, ODP Site 1168, just west of Tasmania. They complement these results with previously published SST data from the same borehole. They compare their dinoflagellate cyst assemblage compositions to a Southern Hemisphere database detailing the modern distribution pattern of some of the species they encountered in the Pliocene samples. The changes in the composition of the dinoflagellate cyst assemblages are compelling proof of the changing paleoceanographic conditions at ODP Site 1168 during the studied interval. Furthermore, the authors discuss a change in palaeosalinity (“~1.5 psu freshening during the M2 deglaciation”), and a lag pCO2 relative to the benthic foraminiferal δ18O. I find these two findings speculative and I could not find clear evidence for either of those two important claims (see major comments below).
Major Comments
I believe this is a valuable study as it is located in an area with a notable data shortage in Pliocene paleoceanography in general and MIS M2 in specific. However, I believe there are five points in the current iteration of the manuscript that require addressing:
1) Line 239: 1.5 psu? How did the authors come to that exact number? I was always under the impression only relative palaeo-salinity changes can be derived and not absolute values, like the one stated by the authors. This is because the exact composition of the seawater at the time, the Pliocene, is unknown (e.g., Rohling, 2007). The authors refer to the presence of Nematosphaeropsis labyrinthus, a species which is interpreted as an opportunistic taxon which thrives in rapidly changing oceanic conditions (e.g., Eynaud et al., 2004; Penaud et al., 2008). Presumably, it tolerates a wide range of salinities and may be better adjusted to changing salinities than other taxa. However, is it possible to put an exact number in psu based on the relative abundance changes of a single taxon?
2) The authors discuss a lag in pCO2 relative to benthic foraminiferal δ18O, but this is difficult to judge accurately given the unconvincing tie points for the age model presented in Figure 3 and Table 1:
- The authors match peaks and troughs which is a less objective approach to match sinusoidal curves than using the intervals of highest flux (Martinson et al., 1987; Paillard, 1996) which seems to cause mismatches between their bulk carbonate and their δ18O from mundulus (tie points 3, 5 and 6) where peaks in one curve seem to correlate with troughs in the other and vice versa.
- Some of the datapoints for δ18O from mundulus during MIS M2 seems to be outliers (unusually low value), are these a true value and are the authors confident it fits the age model? Because of the lack of any error bars this is hard to judge.
3) The authors directly compare their dinoflagellate cyst assemblages to the modern distribution of some of these taxa. At the very least, some of the caveats in doing this should be acknowledged to inform the reader:
- This assumes the optimum ecological conditions for the considered taxa has not changed, which we know for at least one taxon, pallidum, to be contentious (e.g., De Schepper et al., 2011; Hennissen et al., 2017).
- Lateral transport and reworking of dinoflagellate cysts has been minimal in the studied samples. Is this so for the studied samples in ODP 1168?
- When comparing SST records to changes in dinoflagellate cyst assemblages, it is assumed the biotic carrier at the basis of the SST reconstruction shares the same habitat with the dinoflagellates and both the SST biotic carrier and dinoflagellates have the same seasonality.
4) Line 102: “spiking samples with Lycopodium clavatum”. Why spike the samples if then dinoflagellate cyst concentrations or dinoflagellate cyst burial fluxes are not calculated (I could not find any further results based on the addition of L. clavatum , have I missed this?) ? This could be very enlightening as a proxy for palaeoproductivity, especially when the paleoceanography changes as profoundly as the authors suggest. For most IODP/ODP projects the salt free bulk density is measured which in combination with counts of L. clavatum and dinoflagellate cyst census counts gives you a dinoflagellate cyst burial flux, a rough indicator for palaeoproductivity corrected for sedimentation rate (e.g., Hennissen et al., 2014; Versteegh et al., 1996)
5) This study by Hou et al. shows many parallels to De Schepper et al. (2009) and De Schepper et al. (2013) where a dinoflagellate cyst record is combined with SST measurements and salinity estimates to document paleoceanographic changes over MIS M2. The only difference is the oceanic domain: the current authors focus on the Southern Ocean while De Schepper and co-authors studied the North Atlantic, although they also synthesised their findings in a review paper which includes the Antarctic domain (De Schepper et al., 2014). I find it surprising that the current authors do not compare or contrast their results with those studies given the use of the exact same proxies over the same interval. De Schepper et al. (2013) is only mentioned in passing (line 226) even though those authors clearly articulate potential scenarios for the origins of MIS M2, favouring “a specific forcing, unique within this time period” over astronomical forcing alone.
Minor comments
Line 7: It should be noted that the reason why the Pliocene is an excellent paradigm for end-of-21st century climate is that not only CO2 concentrations and temperatures are comparable to those projected for the end of the century by the IPCC, but certainly as important, especially for a paleoceanographic study, is that the continental configuration was roughly comparable to today’s with some important differences in the Panamanian Isthmus that may have regulated ocean currents (e.g., Driscoll and Haug, 1998; Lunt et al., 2008).
Line 15: I suggest replacing “the M2” with MIS M2 throughout. Also, note the inconsistency here with the date in line 10.
Line 32: “past decades”. Sabine et al. (2004) looked at the period 1800–1994 with some references to the period up to 1999. Since then, nearly three decades passed, invalidating the statement “the past decades”. Maybe refer to Gruber et al. (2023)?
Line 70: I would add ‘current’ or ‘extant’ ahead of ocean carbon sink, because how sure are we it was a major carbon sink during MIS M2?
Line 76–77: is it plausible to fix the latitudinal position of an oceanographic front using a single location/borehole?
Lines 169–173: I think this is great to mention why Hole 1168A is preferred over the composite while warning against this practice when unfamiliar with the study material.
Table 1: Should the source of the first line not be Exon et al., 2001. I understand the tiepoint was chosen by the authors, but they did not generate the data behind it.
Lines 189–195: why is there such a spread in SST in Figure 4?
Line 224: indicate mPWP on Figure 4.
Line 228: SST from the deep ocean? Rephrase.
Lines 228–232: I am lost here. The authors state that M2 does not appear to coincide with a major cooling, which is contradicted by their own study (Hou et al., 2023), the results in Figure 4 and what they write in line 229 (“the extreme SST response to M2…”).
Lines 240–242: is this substantiated by for example an increase in IRD in the core?
Lines 246–247: these appear to be statements of fact not backed up by either observations and calculations or citations. Please amend.
Line 258: which species are they? Discuss (with citations) why they are considered to be warm water taxa.
Line 313: freshened the sea water, not the STF, right? Rephrase.
Just as an aside. When discussing the distribution patterns of dinoflagellates, it would be nice (but not necessary if there are size restrictions) to see a plate depicting the most important dinoflagellate cyst taxa.
References
De Schepper, S., et al., 2011. Deciphering the palaeoecology of Late Pliocene and Early Pleistocene dinoflagellate cysts. Palaeogeog. Palaeoclimatol. Palaeoecol. 309, 17-32 10.1016/j.palaeo.2011.04.020.
De Schepper, S., et al., 2014. A global synthesis of the marine and terrestrial evidence for glaciation during the Pliocene Epoch. Earth Sci. Rev. 135, 83-102 http://dx.doi.org/10.1016/j.earscirev.2014.04.003.
De Schepper, S., et al., 2013. Northern Hemisphere Glaciation during the Globally Warm Early Late Pliocene. PLoS ONE 8, e81508 10.1371/journal.pone.0081508.
De Schepper, S., et al., 2009. North Atlantic Current variability through marine isotope stage M2 (circa 3.3 Ma) during the mid-Pliocene. Paleoceanogr. 24, PA4206 10.1029/2008PA001725.
Driscoll, N.W., Haug, G.H., 1998. A Short Circuit in Thermohaline Circulation: A Cause for Northern Hemisphere Glaciation? Science 282, 436-438 10.1126/science.282.5388.436.
Eynaud, F., et al., 2004. Comparison of the Holocene and Eemian palaeoenvironments in the South Icelandic Basin: dinoflagellate cysts as proxies for the North Atlantic surface circulation. Review Of Palaeobotany And Palynology 128, 55-79 10.1016/S0034-6667(03)00112-X.
Gruber, N., et al., 2023. Trends and variability in the ocean carbon sink. Nature Reviews Earth & Environment 4, 119-134 10.1038/s43017-022-00381-x.
Hennissen, J.A.I., et al., 2014. Palynological evidence for a southward shift of the North Atlantic Current at ~2.6 Ma during the intensification of late Cenozoic Northern Hemisphere glaciation. Paleoceanogr. 29, 564-580 10.1002/2013pa002543.
Hennissen, J.A.I., et al., 2017. Dinoflagellate cyst paleoecology during the Pliocene–Pleistocene climatic transition in the North Atlantic. Palaeogeog. Palaeoclimatol. Palaeoecol. 470, 81-108 http://dx.doi.org/10.1016/j.palaeo.2016.12.023.
Hou, S., et al., 2023. Lipid-biomarker-based sea surface temperature record offshore Tasmania over the last 23 million years. Clim. Past 19, 787-802 10.5194/cp-19-787-2023.
Lunt, D.J., et al., 2008. Closure of the Panama Seaway during the Pliocene: implications for climate and Northern Hemisphere glaciation. Climate Dynamics 30, 1-18 10.1007/s00382-007-0265-6.
Martinson, D.G., et al., 1987. Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300,000-year chronostratigraphy. Quaternary research 27, 1-29
Paillard, D., 1996. Macintosh Program performs time-series analysis. Eos Trans. AGU. 77, 379 10.1029/96EO00259.
Penaud, A., et al., 2008. Interglacial variability (MIS 5 and MIS 7) and dinoflagellate cyst assemblages in the Bay of Biscay (North Atlantic). Mar. Micropaleontol. 68, 136-155 10.1016/j.marmicro.2008.01.007.
Rohling, E.J., 2007. Progress in paleosalinity: Overview and presentation of a new approach. Paleoceanogr. 22, PA3215 10.1029/2007PA001437.
Sabine, C.L., et al., 2004. The Oceanic Sink for Anthropogenic CO2. Science 305, 367-371 doi:10.1126/science.1097403.
Versteegh, G.J.M., et al., 1996. The relation between productivity and temperature in the Pliocene North Atlantic at the onset of northern hemisphere glaciation: a palynological study. Global and Planetary Change 11, 155-165 10.1016/0921-8181(95)00054-2.
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