the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Jul 2025
Penultimate glacial sea surface temperature and hydrologic variability in the tropical South Pacific from 150 ka Tahiti corals
Abstract. Constraining climate models under extreme boundary conditions of the past on societally-relevant timescales is complicated by a common lack of high-resolution reconstructions of sea surface environmental variability for glacial periods. Here, we present subseasonally-resolved Sr/Ca and oxygen isotope (δ18O) records from well-preserved and precisely-dated fossil corals of the penultimate glacial and last glacial periods drilled by Integrated Ocean Drilling Program Expedition 310 "Tahiti Sea Level" in the central tropical South Pacific. The proxy records document the mean and seasonality of sea surface temperature (SST) and seawater δ18O (δ18Osw) at 153 ka and 148 ka, during Marine Isotope Stage (MIS) 6b, and around 30 ka during MIS 3a. Results show mean SST 2.8–4.0 °C lower than present for MIS 6b, and about 3.8 °C lower for MIS 3a. The MIS 6b SST differences are greater during the austral winter (3.7–4.4 °C lower) than during the austral summer (2.0–3.7 °C lower), indicating a greater SST seasonality relative to today during the penultimate glacial. A reconstructed higher mean δ18Osw for both MIS 6b and MIS 3a (+0.41‰ to +0.51‰ relative to today) suggest more saline surface waters in the central tropical South Pacific over the entire year. Our coral-based reconstructions of SST and hydrology may indicate a reduced mixed layer depth around Tahiti during the penultimate glacial and last glacial. A potential explanation is a westward-expanded South Pacific subtropical dry area relative to today, probably accompanied by lower activity and/or displacement of the South Pacific Convergence Zone.
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RC1: 'Comment on egusphere-2025-2996', Anonymous Referee #1, 27 Jul 2025
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2996/egusphere-2025-2996-RC1-supplement.pdf
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RC2: 'Comment on egusphere-2025-2996', Anonymous Referee #2, 05 Sep 2025
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General comments
This manuscript presents a highly significant study that reconstructs ocean environments during MIS 3 and MIS 6—periods for which paleoclimate records are particularly scarce—using high-precision geochemical analyses of fossil corals. The approach of employing well-preserved coral skeletons as archives of high-resolution climate information is appropriate and timely, and the dataset provides important insights into past oceanographic and climatic variability. I especially appreciate the careful attention the authors have paid to diagenetic screening and to the influence of coral growth rates on analytical resolution. This demonstrates that the study was conducted with great care and methodological rigor, which strengthens the reliability of the results.
However, there are several areas where the manuscript can be improved for clarity. First, descriptions of some correction methods are incomplete or missing, which makes it difficult for readers to fully evaluate the robustness of the results. Second, the interpretations of SST and salinity variations occasionally appear overstated given the relatively short duration of the records analyzed. These sections would benefit from a more cautious discussion that explicitly considers the uncertainties involved.
Another point concerns the discussion of seasonality. Since corals are among the very few archives that can resolve seasonal-scale variations, it would be highly valuable if the manuscript provided a more careful and detailed discussion of seasonality, including error estimates and an assessment of whether observed differences are statistically significant.
Finally, while the manuscript includes a discussion of SPCZ migration, it would be strengthened by explicitly considering ITCZ and/or ENSO, and by situating the findings within the broader context of paleoclimate records and model studies. This would allow the results to be more effectively placed in a global climatic framework and would broaden the potential impact of the study.
In summary, this is an important and promising manuscript that has the potential to make a substantial contribution to the field.
Major comments
Correction methods: Some descriptions of correction methods are omitted or unclear (e.g., P6 L138–140, L142; P7 L163–165). These should be explicitly stated in the Methods section.
Interpretation of short records: The discussion of SST and salinity variations occasionally appears overstated given the relatively short duration of the analyzed intervals (e.g., 30 ka, 153 ka). The associated uncertainties should be clearly acknowledged. The record of 30 ka has only for 2 years and a half.
Seasonality analysis: Coral skeletons are valuable archives for reconstructing seasonal variability. However, the discussion of seasonality requires more careful treatment, including error estimates and evaluation of whether differences are statistically significant (e.g., P13 L4–L6, discussion of 148 ka and 153 ka records).
The relationship between SPCZ and ENSO: The discussion on SPCZ migration would benefit from integration with previous work on ITCZ and ENSO. Relevant references should be added, and the relationship between salinity front, ITCZ position, and ENSO should be clarified (e.g., P11 L250, P13 L278).
Specific comments
In Figures and tables
Fig. 2: For 9B17R1 (50–55 cm), was age dating performed? If so, please indicate the ages in the figure; if not, provide a reason.
Fig. 3 and Supplement, Section 2.2 Mineral screening：
Based on the XRD and SEM observations, you state that only well-preserved skeletal portions were used for analysis. My understanding is that the analyzed areas correspond only to the red-lined segments in Fig. 3. However, other portions also appear well preserved. For example, in 9D25R1_65–75 cm, Table S1 suggests that segments 1–8 are all well preserved, yet only segments 2–5 were analyzed. Similarly, for 9D25R2_43–51 cm (segments 1–3) and 51–57 cm (segments 1–4), Table S1 indicates they are suitable for analysis. Beyond calcite content and the presence/absence of secondary aragonite cement noted in Table S1 and SEM observations, were there any additional criteria used to decide which portions were selected for analysis?
Fig. 4: Please clarify whether Sr/Ca data have been corrected for seawater Sr/Ca ratio (P10 L197–201). Also, in the caption, “Horizontal bars represent analytical errors” appears to be a typo; should this be “Vertical bars”?
In the manuscript
P6 L138–140：It is not clear how the averaging effects were specifically calculated.
P6 L142：Regarding “the offset in SST seasonality,” is this offset corrected for in the subsequent discussion?
P7 L155–156: δ¹³C is not discussed in the main text; either move it to the Supplement or add a brief summary in the main text.
P7 L163–165: Coral growth rates are relatively low (<5 mm/yr). Please clarify whether potential vital effects on δ¹⁸O were corrected, citing relevant studies (e.g., Hayashi et al., 2013, Hirabayashi et al., 2013).
Hayashi, E., Suzuki, A., Nakamura, T., Iwase, A., Ishimura, T., Iguchi, A., Sakai, K., Okai, T., Inoue, M., Araoka, D., Murayama, S. and Kawahata, H (2013) Growth rate influences on coral climate proxies tested by a multiple colony culture experiment. Earth Planet. Sci. Lett. 362, 198–206.
Hirabayashi, S., Yokoyama, Y., Suzuki, A., Kawakubo, Y., Miyairi, Y., Okai, T., and Nojima, S. (2013), Coral growth-rate insensitive Sr/Ca as a robust temperature recorder at the extreme latitudinal limits of Porites, GEOCHEMICAL JOURNAL , 47(3), e1-e5
P9 L193–195: This methodological description should be moved to the Methods section.
P10 L215–216: Since the 30 ka record covers only about 2.5 years, its treatment requires caution. For the 153 ka record, the Sr/Ca data suggest that during the 20–30 mm interval, summer SSTs were comparable to those of 2000–2008, indicating that temperatures were not necessarily 3–4 °C cooler than today. In particular, for the 153 ka Sr/Ca record, both the amplitude (i.e., seasonality) and the mean values appear to differ between 0–15 mm and 20–30 mm. I would encourage the authors to take special care in discussing this aspect.
P13 L4-L6：Regarding seasonality, what are the associated uncertainties for each record? Do the differences remain significant when errors are taken into account? Both the 153 ka and 148 ka records seem to include years where the seasonality is nearly identical to that of the modern record. For example, in the 148 ka record, the 0–20 mm interval appears to show reduced cyclicity compared to the 20–45 mm interval. Similarly, in the 153 ka record, especially within the 0–20 mm interval, the seasonality appears comparable to that of the modern record.
P7 L163–165：The coral skeletal extension rates are relatively low (<5 mm/yr). Could the influence of δ¹⁸O vital effects be an issue in this case (e.g., Gagan et al., Inoue et al., Hayashi et al., Hirabayashi et al.)? If a correction has been applied, it would be helpful to briefly describe the method in the Materials and Methods section.
L200–201 & L205–206: Slopes of Sr/Ca–SST and δ¹⁸O–SST conversion equations are given, but the associated uncertainties are not. Please estimate how these propagate into reconstructed SST and SSS.
The Discussion focuses on SPCZ displacement — is it possible to also discuss contemporaneous changes in the ITCZ and ENSO? In the Conclusion the authors note that longer coral records would be needed to discuss ENSO; however, are there existing paleoclimate records or model studies that currently allow discussion of ENSO or ITCZ behavior for these time intervals? If so, please cite and discuss those prior findings (from proxy records and/or model simulations) and clarify whether they support or contradict the SPCZ-centered interpretation.
P11 L250 — The salinity front: how is this feature related to ITCZ position and to ENSO variability?
P13 L278 — The “stronger zonal SST gradient”: can this be interpreted as being related to ENSO variability?
Citation: https://doi.org/10.5194/egusphere-2025-2996-RC2
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