the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Speleothem evidence for late Miocene extreme Arctic amplification – an analogue for near future anthropogenic climate change?
Abstract. The Miocene provides an excellent climatic analogue for near future anthropogenic warming, with atmospheric CO2 concentrations and global average temperatures similar to those projected for the coming century. However, the magnitude of Miocene Arctic warming remains unclear due to the scarcity of reliable proxy data. Here we use stable oxygen isotope and trace element analyses, alongside clumped isotope and fluid inclusion palaeothermometry of speleothems to reconstruct palaeo-environmental conditions near the Siberian Arctic coast during the late Tortonian (8.68 ± 0.09 Ma). Stable oxygen isotope records suggest warmer than present temperatures. This is supported by temperature estimates based on clumped isotopes and fluid inclusions giving mean annual air temperatures between +6.6 and +11.1 °C, compared with -12.3 °C today. Trace elements records reveal a highly seasonal hydrological environment.
Our estimate of >18 °C of Arctic warming supports the wider consensus of a warmer-than-present Miocene and provides a rare paleo-analogue for future Arctic amplification under high emissions scenarios. The reconstructed increase in mean surface temperature far exceeds those projected in fully coupled global climate models, even under extreme emissions scenarios. Given that climate models have consistently underestimated the extent of recent Arctic amplification, our proxy data suggest Arctic warming may exceed current projections. If Arctic warming by 2100 matches our late Miocene estimates, it would have large-scale impacts on global climate, including extensive thawing of Siberian permafrost – a vast fossil carbon store.
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CC1: 'Comment on egusphere-2024-1691', Arthur Oldeman, 19 Jun 2024
Dear authors,
Your manuscript is interesting and exploring the Miocene climate and its possible similarity to future climate is a relevant endeavour. I have one specific comment on your Miocene future comparison.
You state (L470-475) that the Tortonian saw a similar climate as predicted under a high emission future scenario with atmospheric CO2 levels of 600 ppm. Based on similarities in atmospheric CO2 levels and global temperature anomalies, you proceed to conclude that your ‘findings provides estimates for end-of-century Arctic temperature amplification and precipitation’.
I am not convinced that you can safely make this statement. It assumes to things: 1. CO2 is the dominant driver of (regional) warming, both in the future and Miocene, and 2. Global temperature anomalies are a good proxy for regional temperature patterns and polar amplification. I don’t think that those are a priori assumptions that you can make, and I would like to suggest the authors to explore these assumptions, or find references that support it, before making claims that your findings provide estimates for future Arctic temperatures and precipitation.
That CO2 drives present warming is clear. Next, it is more and more clear that global temperature and CO2 are also connected in the Miocene, something that was not always clear. However, the Tortonian - in fact, the whole Miocene - did not see Northern hemisphere ice sheets. This will have a large effect on the planetary albedo, and with that the global energy budget, resulting in a likely redistribution of heat over the latitudes. This could imply that Tortonian Arctic temperatures and precipitation patterns are in fact not similar to what is projected for the future, regardless of similarity in atmospheric CO2 levels. I am speculating here because I also havent made this analysis, but I find the absence of Northern Hemisphere ice sheets a compelling argument to not automatically assume similarity - or, analogy - between Tortonian and future temperature and precipitation patterns. Hence, I would advise caution with making statements as in L470-475.
Good luck with finishing the manuscript!
Best, Arthur Oldeman
Citation: https://doi.org/10.5194/egusphere-2024-1691-CC1 -
AC3: 'Reply on CC1', Stuart Umbo, 10 Sep 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1691/egusphere-2024-1691-AC3-supplement.pdf
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AC3: 'Reply on CC1', Stuart Umbo, 10 Sep 2024
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RC1: 'Comment on egusphere-2024-1691', Anonymous Referee #1, 09 Jul 2024
This paper deals with clumped isotope and oxygen isotope thermometric analyses as well as trace element concentration measurements of Miocene speleothem pieces from an area which is rarely investigated. The paper is well written, easy to follow, and concise enough. In addition to the paleoclimatic interpretations of thermometric and geochemical results, the data are compared with results of local paleoclimate studies and are evaluated with respect to global climate change processes. This would also mean that reviewing the paper requires different expertise, and I’ll concentrate on the speleothem geochemistry part. In summary, I recommend acceptance after minor revision. Comments suggesting minor changes are given below.
Anderson et al. (2021) use the D47crunch algorithm whereas the present paper uses the Easotop algorithm. In addition to the different regression calculation of the present study and that of Anderson et al., the two algorithms may also yield different temperatures. I suggest to recalculate the clumped isotope data with the D47crunch algorithm and compare the temperatures obtained with the different regressions in Table3.
Compared with Table 1, Table 3 contains only the temperatures yielded by the original Anderson et al. regression as a plus. I think the two tables can be combined.
The temperatures obtained using the two regressions differ by 3.5 °C, but due to the small number of data it is not certain which calibration is better. Using the regression based on data similar to the present study makes sense, but the narrower temperature range means also increased uncertainty in the regression slope.
Using the temperatures of the Anderson et al. regression the δ18Odw values are shifted only by +0.7 ‰, which is still close to the fluid inclusion composition. It would be interesting to use the D47crunch algorithm and the related temperatures, and include all these into the uncertainty calculations.
line 371: Sr-Mg correlation may also be produced by changing precipitation amount and dilution effect. The precipitation amount effect is also indicated by the P/Ca changes and the anticorrelation with Sr and Mg concentrations. Wetter climate would mean improved soil activity, higher P concentration, but dilution of Sr and Mg in the karstic water. Of course arid climate would also mean evaporation along the water migration route and related PCP, so the Sr and Mg concentrations can be affected by dilution and PCP changes together.
Citation: https://doi.org/10.5194/egusphere-2024-1691-RC1 - AC1: 'Reply on RC1', Stuart Umbo, 10 Sep 2024
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RC2: 'Comment on egusphere-2024-1691', Julian Murton, 18 Jul 2024
- AC2: 'Reply on RC2', Stuart Umbo, 10 Sep 2024
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