| 18 Nov 2025
South Atlantic lipid biomarkers support synchronous Plio-Pleistocene global cooling: Revising the ODP Site 1090 sea surface temperature record
Abstract. The Pliocene epoch, 5.33–2.58 Ma, is considered a key analogue for near-future climate scenarios, as it had atmospheric CO2 levels (>400 ppm) comparable to today and similar continental positioning. Understanding Pliocene climate evolution is also critical to establishing the conditions that enabled large ice sheets to form in the Arctic region during the intensification of Northern Hemisphere glaciation (iNHG) around 2.7 Ma. The causes of iNHG remain unclear, with hypotheses ranging from tectonic changes to CO2 reductions. Based on anomalous, pre-iNGH cooling signals recorded in a sea surface temperature (SST) record from Ocean Drilling Program (ODP) Site 1090, located in the southeastern Atlantic Ocean, studies have posited that early cooling of the already-glaciated Southern Hemisphere could have driven Earth’s climatic descent into the Pleistocene. Here, we provide an orbitally resolved alkenone-based SST record of ODP Site 1090 spanning the time interval ~4.3–2.6 Ma with improved laboratory protocols that significantly revises conclusions based on prior work. Our revised record of SSTs from ODP 1090 shows similar cooling trends to those found of equatorial and high latitude Northern Hemisphere sites, suggesting that a global forcing, such as a reduction in atmospheric CO2, prompted iNHG, as opposed to an early cooling of the Southern Hemisphere.
Hoegler et al. presented a revised alkenone-based sea surface temperature (SST) record from ODP Site 1090, spanning the critical Pliocene-Pleistocene transition (~4.3 - 2.6 Ma), and challenging the earlier reconstruction by Martínez-Garcia et al. (2010) and the hypothesis that regional cooling in the Southern Hemisphere drove the intensification of Northern Hemisphere glaciation (iNHG). The authors have made robust methodological improvements, including sample purification, the use of HPLC-MS to verify the absence of C37:4, and an updated age-depth model. These methodological advances yield a SST record that is systematically warmer (~1.27°C on average) than the previous estimate by Martínez-Garcia et al. (2010). Also, the revised ODP 1090 record exhibits a cooling pattern synchronous with, and even milder than, those observed in the equatorial Pacific and North Atlantic, which supports a globally synchronous cooling mechanism. Overall, the manuscript is important for understanding the climatic fluctuations and the hidden mechanisms of the Cenozoic. Here, I am providing some concerns, questions, and suggestions that may help improve the manuscript.
Major questions:
1) The authors rigorously addressed the potential bias introduced by silver-nitrate purification, as raised by Martin et al. (2024), by conducting control experiments using both an in-house alkenone standard and a Greenland standard. They concluded that their cleanup methods showed "little to no evidence of biases to the results." However, this conclusion may be overly reliant on the behavior of standards and not fully transferable to the actual ODP 1090 sediment samples. Martin et al. (2024) specifically demonstrated that the impact of silver-nitrate purification is not linked to initial alkenone concentration but is likely driven by lake-specific matrix effects. The complex, open-ocean sediment matrix of ODP Site 1090 is fundamentally different from the chemical environment of a purified standard. Therefore, while the standard experiments are valuable for assessing general method performance, they do not conclusively prove that the alkenone distributions in the geologically complex sediments of ODP 1090 were unaffected by the silver-nitrate purification process.
2) While the data presented in Figure 1 is fundamental to the study, I would like to suggest several visual enhancements to improve its clarity and professional presentation, which would ultimately help the reader better interpret the geographical context. First of all, the current landmass polygons appear somewhat pixelated and exhibit a jagged, low-resolution outline. I would recommend using a higher-resolution coastline dataset to create smoother, more accurate continental outlines. Secondly, the chosen color bar for the SST data could be significantly optimized. The green-dominated palette is non-standard for representing temperature. It is highly advisable to adopt a more intuitive, red-to-blue diverging color scheme (e.g., similar to RdYlBu or other common palettes) where reds indicate warmer temperatures and blues indicate cooler ones. Besides, using white to represent the lowest temperatures causes the cold waters to visually blend with the white landmasses. I suggest ensuring the coolest tones in the new color scheme are distinctly non-white or changing the color of landmasses to maintain a clear distinction between ocean and land. Finally, the current y-axis labels (e.g., -20°, -30°) are clear to domain experts but do not explicitly state that they represent South latitude. For absolute clarity and adherence to cartographic standards, I strongly recommend appending an "S" to the labels (e.g., 20°S, 30°S).
3) The authors used linear regression to compare the long-term trends between their revised record and that of Martínez-Garcia et al. (2010). To more effectively visualize and contrast the underlying low-frequency trends embedded within the high-amplitude orbital cycles, I strongly recommend supplementing the linear fits with a moving average (or Loess smoothing). Applying a consistent smoothing window (e.g., 100-kyr or 400-kyr) to both records would provide an intuitive visual representation of the long-term climate evolution and test the robustness of the trend without imposing a presupposed linear shape.
4) The authors highlight a critical finding: the mildest late Pliocene cooling trend is observed at the sub-Antarctic ODP Site 1090, compared to the ODP Site 846 (eastern equatorial Pacific) and IODP Site U1313 (north Atlantic). However, the manuscript lacks critical metadata for the latter two sites, which is essential for assessing the validity of such comparisons. I recommend that the authors provide a summary table or explicitly state the temporal resolution, age model construction principles, and estimated analytical errors for the ODP 846 and IODP U1313 SST records used in this study. Besides, the discussion lacks a mechanistic explanation for this counter-intuitive result. From a climatology perspective, one would expect polar amplification to drive stronger cooling at high latitudes under a decreasing CO₂ forcing (e.g., Snyder, 2016). The fact that the opposite is observed here is, in itself, a significant result that points to the operation of important regional climate feedbacks. I strongly recommend that the authors expand the discussion to address this point.
Minor comments:
Line 7-11: The background information in the abstract is somewhat lengthy. I would recommend to condense it to quickly lead into this study’s own findings and contributions.
Line 11: "iNGH" is a typographical error and should be corrected to "iNHG" for consistency with the standard abbreviation used throughout the manuscript.
Line 11-14: The sentence is overly long and complex. Consider to break it into two shorter sentences for improved readability.
Line 13-14: The phrase "Earth’s climatic descent into the Pleistocene" is somewhat poetic but could be perceived as subjective. Recommend rephrasing for a more neutral, scientific tone.
Line 63: "iNGH" should be corrected to "iNHG".
Line 67: The sentence "We were motivated to re-examine... by two aspects..." uses first-person and has a slightly awkward structure.
Line 79: "iNGH" should be corrected to "iNHG".
Line 70-71: The clause "which would be unusual given the moderate temperatures..." is a key rationale but is presented almost as an aside. Its logical importance should be emphasized.
Line 105: This is the first instance where "GC-FID" appears. Add the full name: "...using gas chromatography with flame-ionization detection (GC-FID)."
Line 279: It is recommended to provide the limit of detection.
Line 308: "iNGH" should be corrected to "iNHG".
Line 333: The phrase "does not exhibit a cooling trend that is distinct from" is wordy. Replace with "...does not show a cooling trend indistinguishable from...".
Line 339: The description of the SST record "sharing orbital-scale features" is clear, but the subsequent sentence about spectral analyses feels like a non-sequitur.
Table 1: It is recommended to reformat the table using the standard three-line table format. This will clearly separate the table content from the notes below, improving its professionalism and readability.
Fig 2: I recommend that the authors add error envelopes to their own data.
Fig 3: The letter “C” in the figure is positioned too high; it should be moved down slightly.