the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Aug 2026
Quantifying trends and transitions in a Miocene benthic δ18O stack
Abstract. Marine isotopic records from benthic foraminifera are central to understanding Miocene climate, but developing reliable Miocene age models that are consistent across sites has been challenging due ambiguities in biostratigraphy and magnetostratigraphy. Detailed chemostratigraphic alignment of globally distributed marine records can assist in comparing paleoclimate proxy records and their proposed forcing mechanisms. We present a continuous benthic δ18O stack (average) across the Miocene based on alignment of published records from 24 sites. Stack estimates of global mean benthic δ18O changes may improve reconstructions of Miocene ice volume change and deep-sea cooling and help distinguish between causal mechanisms. We estimate the rate of change in the global stack’s benthic δ18O throughout the Miocene as well as the amplitude of abrupt transitions associated with the onset of the Miocene Climatic Optimum (MCO) and the Middle Miocene Climatic Transition (MMCT). We find that the global stack’s change in benthic δ18O across the abrupt onset of the MCO at ~16.9 Ma is 0.27 ‰, which is about half the amplitude recorded in the deep Eastern Equatorial Pacific (EEP) and in the CENOGRID splice (Westerhold et al., 2020). We additionally find the total change in stack benthic δ18O across the Miocene is 0.82 ‰, which is 15 % less than in the CENOGRID splice.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Climate of the Past.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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- CC1: 'Comment on egusphere-2026-4648', Giacomo Medici, 22 Aug 2026 reply
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RC1: 'Comment on egusphere-2026-4648', Anonymous Referee #1, 04 Nov 2026
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Hobart and coauthors have used high-resolution δ¹⁸O records from 24 globally distributed published sites to construct a new Miocene stack record. They then compare this stack with the East Equatorial Pacific (EEP) stack and the CENOGRID record. They also seek to quantify the magnitude of δ¹⁸O changes at the onset of the MCO and the end of the MMCT, as well as long-term trends across different stages of the Miocene. In principle, if there are enough records of high quality and their age models are accurate, a stack should better capture global mean change than a record assembled by splicing together individual curves. However, the authors must also recognize that constructing a record that perfectly represents global change remains very difficult. I appreciate the time and effort the authors have devoted to this work. To help them improve the study, I offer the following comments.
It is difficult to conclude that this stack represents the global signal, particularly during the key intervals of the MCO and MMCT. With the exception of Site 1443, the data for these intervals come entirely from the Pacific; no Atlantic records are included. Because the authors discuss in detail the magnitude of δ¹⁸O changes at the onset of the MCO and the end of the MMCT, they should carefully select appropriate sites and discuss the effects of uneven basin, water-depth, and temporal coverage. For example, I question whether Sites 751, 1090, 1171, and 1335 genuinely capture the onset of the MCO. The authors should check the original stratigraphic depths for these sites and, at a minimum, provide enlarged plots of the data for these intervals in the appendix, showing the age models before and after applying BIGMACS. Some sites should likewise be checked for the MMCT. I am also concerned about the resolution at Site 751. The authors explicitly state that they selected only records with resolution higher than 10 ka; I recommend reporting this information in the table as well. In addition, why are the data from Site 1264 between 23 and 17 Ma shown as discontinuous in Figure 2, and why is this interval absent from the table? My understanding is that the Site 1264 record is generally of very high quality. The authors should check whether similar issues affect any other sites.
The authors should not simply construct an EEP stack and compare it with the global stack. It may be more informative to construct separate stacks for the eastern and western Pacific, Indian Ocean, Atlantic, and Southern Ocean, and to further subdivide them by water depth. This would help show how basin and depth differences contribute to the stack at different times.
Figure 3 clearly shows that the stack in this study smooths out many of the high-frequency variations. In the comparison with CENOGRID, are the records therefore being compared at the same timescale? This is particularly relevant to the repeated comparisons among the curves at the onset of the MCO and during the MMCT. Throughout the manuscript, the authors do not clearly state the timescale over which their data processing and comparisons are reliable.
For Table 2, I would like to know whether the authors calculated the δ¹⁸O change at the MCO onset and MMCT end for every individual site. I would particularly like to see how these values vary among sites, the approximate resolution at each site, and whether the reported changes are truly being compared over the same timescale.
Figure 6 shows that Liu et al. (2026) have updated the Site 1505 record to cover the entire Miocene. The record also clearly differs from those of other sites during the early Miocene. The authors suggest that this may reflect changes in water depth and the water mass at the site. This raises a broader question: how did the authors account for differences in water depth when determining the contributions of individual sites to the stack? The authors constructed a separate EEP stack; did they also consider constructing stacks for other regions, in addition to accounting for water depth? Such comparisons might better illustrate the complexity of the Miocene record.
I understand that the authors aimed to include only records with a resolution finer than 10 ka. However, when a site record is highly continuous, data should not be excluded simply because part of the record falls below this threshold—for example, at 12 ka. Stratigraphic continuity should also be considered when selecting data. How did the authors account for this? Also, when applying their method, how did they bring records with different resolutions to a common resolution, and what was that resolution? This is essential for determining the timescale over which the results can be discussed.
I would also like the authors to show the age changes at each site before and after applying BIGMACS, and explain why they consider the resulting ages to be accurate. For example, the authors use CENOGRID as the initial alignment target for BIGMACS, while also noting that the U1337 age model in CENOGRID may contain a tuning error around 18–17 Ma, with a maximum offset of approximately 300 kyr. Although alignment may improve relative synchrony among sites, if the target record’s age scale contains a systematic error, that error may still be propagated to all aligned records and the resulting stack. Therefore, good alignment among sites alone is not sufficient to establish that the absolute ages in the current results are more accurate. The manuscript also does not clearly state whether the revised age model for Site 1337 was adopted as the final tuning target.
The Introduction reads more like a slightly expanded version of the abstract. Would it be clearer to begin directly with Section 2.1 and provide a general overview of the Miocene before introducing the MCO? This is just a suggestion.
The Results and Abstract report that the global Miocene stack δ¹⁸O increased by 0.823‰, whereas the Conclusion states that it “decreases by 0.823‰.”
The Results report a full-Miocene change of 0.967 ± 0.018‰ for CENOGRID, whereas the Conclusion gives 0.967 ± 0.025‰.
Citation: https://doi.org/10.5194/egusphere-2026-4648-RC1
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General comments
Very good research on paleoclimate. Please, follow my suggestion to improve the manuscript.
Specific comments
Lines 21-39. The introduction is very short. You need to expand starting more general.
Line 21. Insert a general introduction on oxygen isotope ratio δ18O to study the paleoclimate of the overall Cenozoic period referring to recent literature, see below:
- Rohling, E. J., T. M. Gernon, D. Heslop, G. J. Reichart, A. P. Roberts, J. Yu 2024. Reconciling the apparent discrepancy between Cenozoic deep‐sea temperatures from proxies and from benthic oxygen isotope deconvolution. Paleoceanography and Paleoclimatology 39, 11, e2024PA004872.
- Medici, G., Marianelli, D., Cornacchia, I., Gori, F. Brandano, M. 2026. Multi-disciplinary approach to paleokarst occurrence in the Eocene–Oligocene succession of the Apulia Carbonate Platform (Salento, Italy). Facies, 72(2).
- Mudelsee, Manfred, Torsten Bickert, Caroline H. Lear, Gerrit Lohmann 2014. Cenozoic climate changes: A review based on time series analysis of marine benthic δ18O records. Reviews of Geophysics 52, 333-374.
Line 39. Disclose the specific objectives of your research by using numbers (e.g., i, ii, and iii)
Lines 46-84. You should mention somewhere in the manuscript the importance of orogenesis (e.g., Alps, Pyrenees, Himalaya) on oxygen isotope ratio δ18O in the stratigraphic record.
Line 57. I agree on the importance of the volcanism in your manuscript, and I have studied the Columbia Plateau between Washington and Idaho! However, the Miocene was affected by intense volcanisim also in other areas of the world (e.g., Mediterranean region) due to the subduction zones. Can the activities from other volcanic districts be relevant to your research?
Lines 311-324. Highlight better the potential large impact of your research. The Conclusion section is very short.
Figures and tables
Figure 1. There is the chance to increase the graphic resolution as well as enlarge the figure.
Figure 2. Increase the font size for the axes.
Figure 3. Explain the meaning of the vertical and coloured bands in the caption.
Figure 6. Do you need to insert formal internal boundaries for the Miocene age on this figure?
Figure 6. Key figure of large impact. Please, pay attention to the graphic resolution and the information in the caption.