| 29 Oct 2025
Late Holocene cooling and increased zonal asymmetry in the mid-latitude North Atlantic
Abstract. Sea Surface Temperature reconstructions derived from alkenone biomarker (SST-alk) reveal a cooling trend in the North Atlantic during the late Holocene (the last 5,000 years), contrary to the warming simulated by transient climate models driven by 20 ppm increase in greenhouse gas concentrations. It has been suggested that the apparent cooling in paleo-records may reflect the evolution of summer temperatures, a seasonal signal biased by the preferential growth of haptophyte algae during warm months. Here, we investigate the spatial pattern of SST-alk changes and show that late Holocene cooling is characterized by an increased zonal SST gradient in the mid-latitude North Atlantic, with greater cooling in the west than in the east. Multiple proxies indicate that this increase in zonal asymmetry is associated with reorganizations of the inter-gyre ocean circulation. We find that transient simulations, such as TraCE-21k, do not reproduce the zonally asymmetric cooling and the inferred changes in inter-gyre circulation from the mid- to late Holocene. This misrepresentation of spatial and temporal variability likely explains the data-model discrepancy in the mid-latitude North Atlantic.
This manuscript by Si et al., titled "Late Holocene cooling and increased zonal asymmetry in the mid-latitude North Atlantic," presents new records of alkenone-based sea surface temperature (SST) estimates and planktonic foraminiferal analyses from two sediment cores in the North Atlantic. The authors integrated these new data with a suite of paleoreconstructions to examine the spatiotemporal trends of SSTs in the North Atlantic inter-gyre zone from the early to late Holocene. The study addresses an important topic related to the discrepancy between model simulations and proxy data, an important and still-debated “Holocene conundrum”, and it has the potential to make a useful contribution to the field. However, in its current form, the manuscript requires revision before it can be considered for publication, specifically regarding the overall structure and flow of the manuscript.
Below I outline several concerns and suggestions that require the authors' attention.
Specific comments
Title: I think “Middle to late Holocene cooling” will be more appropriate.
Abstract: The authors should first present the results from their own cores (as it currently reads more like a synthesis of previously published data) and then demonstrate how these new data are integrated at the regional scale (mid-latitude North Atlantic) to support their findings regarding an increased zonal SST gradient in relation to inter-gyre circulation.
Introduction: The introduction is well written; however, in my opinion, the authors should better summarize previous studies in the North Atlantic and further discuss the current understanding of the mid- to late-Holocene cooling trend recorded by proxies in this region in order to more clearly identify the existing gaps.
Sections 2 and 3: While Section 2 can be accepted as a regional setting, except for the last paragraph, which should be moved to the Materials and Methods section, Section 3 is problematic and, in its current form, makes the manuscript more difficult to read. This section describes some of the methods adopted by the authors, namely the calculation of productivity-weighted SSTs and their comparison with (i) annual-mean SSTs to compute the differences and (ii) core-top SSTalk, and it also presents the results of these calculations. To improve the overall structure and flow of the manuscript, I suggest that the authors split this section between the Materials and Methods and Results sections, after first presenting their own reconstructions from the two IODP cores.
Materials and methods section:
As highlighted above, a large part of the content currently included in the previous two sections should be moved here. I also recommend organizing this section into subsections to improve clarity and readability.
Age models: It is unclear why the authors used the Marine13 calibration curve rather than Marine20. The authors should also explain why the 14C dates of the other cores included in their compilation were not recalibrated. Given that some of these records have low temporal resolution, it is not clear whether this choice could influence the calculation of mean SSTalk over the two selected time intervals.
Heaton, T. J., Köhler, P., Butzin, M., Bard, E., Reimer, R. W., Austin, W. E., ... & Skinner, L. C. (2020). Marine20—the marine radiocarbon age calibration curve (0–55,000 cal BP). Radiocarbon, 62(4), 779-820.
Results: Please present your new reconstructions first and then the result of your compilation as I suggested before. Why only SST-alk from core U1304 are presented here and in figure 2? I couldn’t find those from U1308! Also, for paragraph 165, it’s not clear from which core the alkenone concentrations are. This section should be substantially revised to make it clearer for the readers.
Discussion and conclusions: Overall, I found these two sections well written, except for some typos, which I have listed below.
Figures: The figures are of high quality; however, in general, their captions lack sufficient detail.
Figure 1: Please mark your two cores with different color. I also suggest to move Figure 1b to a new Figure 3.
Figure 2: Please add the name of the core for 2d and move the legend for foraminiferal assemblage to be in parallel.
Technical corrections
Lines 36-37: Please rephrase and further explain this sentence. How can variations in Earth's precession simultaneously cause both cooling and warming at lower latitudes?
First paragraph of the Materials and Methods section: Please provide more information about the two cores (e.g. water depth, core length, and geographical location). In addition, please indicate here that core ODP U1304 was also used for planktonic foraminiferal analyses.
Line 217: Please add references.
Line 218: What do you mean by “Starting ~11 ka”?
Line 225: Please rephrase this sentences for more clarity.
Lines 233-234: Please add the core names used by Colin et al., 2010 or add "Rockall Trough" on Figure 1.
Lines 236 and 238: Please be consistent when you write "subpolar mode water".
Line 245: Please explain how “the termination of deglaciation after ~8 ka” has strengthened the subpolar gyre.
Line 247: “Nevertheless, the effects of stronger subpolar convection appear to have been limited to subsurface layers or winter conditions.” Please further clarify the underlying assumption and support it with appropriate references from the existing literature.
Line 262: “a weaker subpolar gyre over the last 6 kyrs” are you sure? According to Tornalley et al., 2009, the subpolar gyre oscillated between weak and strong phases during the last 6 ka.
Line 267: please change “millennials” to “millennia”.
Line 279: Maybe I am missing something here, but I can't see any Red arrow in Figure 1a.
Lines 280-283: We can’t see this information from Figure 2e!
Line 284: “Enhanced summer heating” when and where?
Line 287: “during this period” please indicate the time-interval here.
Line 290: Please change “In the late Holocene” to “During the late Holocene”.
Line 294: "Consequently, the east-west SST gradient reversed" from what to what? Please explain further.
Line 395: Please change “The late-Holocene summertime cooling” to “The mid- to late Holocene …”.