General Comments:

Mega et al present multiproxy records to reconstruct sea surface temperature changes in the eastern reaches of the subtropical gyre (southern Portuguese margin, U1387) during the mid Pleistocene (MPT) climate transition. Sea surface temperature reconstructions are based on faunal assemblages and the Uk’ 37 index. A record of the coiling direction of Globorotalia truncatulinoides at the site is used to infer changes in the relative strength of subtropical gyre waters reaching the site. The age model is based on the stable isotope record of Globigerinoides bulloides correlated with an equivalent record from nearby Site U1385, which itself has ages reported with respect to the Lisicki and Raymo (2005) stack.

One of the significant findings of the study is the incursions of subpolar waters beginning at about 800-900 ka recorded by increases in subpolar foraminiferal species. This provides evidence for the ‘900 ka’ event of the MPT at lower latitudes. The authors suggest that cooling was communicated to the study region via shifts in atmospheric circulation related to a relatively weak meridional overturning circulation.

The manuscript is thorough, the text is well written, and the conclusions are supported by a multiproxy data set and a robust age model. The discussion is thorough as well and places the results into the context of relevant prior work. Figures and tables are clear.

I would only have a couple of very minor suggestions/edits:

1-Shorten the title

2-First sentence of the abstract: Clarify that this is change in the dominant cyclicity exhibited by proxy records

3-Line 22: should it read: planktic foraminiferal ecosystems?

Review of Subtropical gyre persistence in the Gulf of Cadiz, southern Iberian margin, interrupted by extremely cold surface water incursions during the Early – Middle Pleistocene Transition by Mega et al.

This study presents new foraminiferal faunal and alkenone measurements, from a site off the Iberian Margin, to identify changes in surface ocean temperature and hydrography across the MPT. The data presented add a valuable resource for improving our understanding of an important climatic transition and ultimately should be published in a venue such as this. Overall the data are well presented and much of the discussion is sensible. However, I am not so confident about the use of coiling direction in G. truncatulinoides as a gyre indicator and I think more or different discussion on surface versus deep circulation changes needs to be included.

More specifically, the authors make use of the trunc coiling direction proxy to support their arguments yet this approach appears quite speculative and gives inconsistent results (e.g. MIS 1 versus MIS 11c according to Billups et al., 2020). Furthermore the proxy has not been demonstrated in the region under study here and it is not at all clear that the %GTS record presented here can be interpreted in terms of gyre strength. I note that the absolute counts of trunc are generally very low (Fig. 4C, typically <10). Kaiser et al., 2019 suggested that low counts should not be relied on and considered only those samples with >20 trunc counts. The record of %GTS presented here also shows an inconsistent link with glacial/interglacial climate, with high values observed during both warm and cold periods (although more dominantly during cold periods). And high numbers of shells are only observed when GTS is high, which could mean that the thermocline is deep (gyre is healthy?) only during those times.

Yet the authors seem to suggest that the dominance of both right and left coiling varieties can be taken as evidence for a strong /healthy subtropical gyre. For example, (L570) “the presence of G. truncatulinoides, especially in its right coiling form, supports subtropical gyre influence during much of the studied interval” and later, (L613) “When the subarctic front receded northward after the terminal stadial event of Termination X, the subtropical gyre expanded again as evidenced by the % GTS maxima at Site 607 and U1387 (Fig. 7), facilitating subtropical water transport to the north and deep-water convection in the North Atlantic (Fig. 8G) (Hodell and Channell, 2016; Hodell et al., 2023a; Kaiser et al., 2019) and the establishment of interglacial conditions.” I think I am not mistaken in assuming that some sort of the subtropical gyre will always exist but with variations in position and intensity. Yet I cannot see how the results presented here help with understanding those variations and the authors’ discussion offers little illumination. I am sorry that my words seem harsh but I have read the Billups and Kaiser papers and read through the text more than once and still am left wondering what I have learned from the record %GTS.

Some of the results section is overly descriptive and potentially verbose. Consider condensing to save space and improve readability.

Below I list the major items for consideration, followed by more technical comments.

Line 40/41: What is the evidence for this sequence of events? You imply that extreme AMOC weakening caused the migration of the subarctic front but could it not be the other way around?

L53: We don’t yet know what caused the transition to more intense and longer-lasting glacials although CO₂ and ocean circulation changes were intimately involved. Perhaps tone down the implied certainty of cause and effect here.

L56: A recent paper by Hines et al. [Hines et al., 2024] suggests that no substantial change in glacial deep ocean circulation occurred across the MPT. This requires us to rethink some of our assumptions about ocean dynamical changes across the MPT and should be cited (and discussed – see later).

L93: Again, there is emphasis placed on changes CAUSED by AMOC variations across the MPT but this order of cause and effect needs to be reconsidered.

L140: The study relies heavily on the use of trunc coiling direction as a proxy for gyre circulation as described by Billups et al., 2016 (which I think implies a weaker gyre circulation during the LGM). A recent study by Wharton et al., [Wharton et al., 2024], using sites in a similar region as the Billups study, suggests that the glacial gyre was stronger and deep than today. This paper needs to be considered in the light of how reliable the trunc proxy can be.

L218: Please give details of how the age model used here differs from the Bajo study. The Bajo study was defined by it's age model (tied to U1385 and speleothem records) and changing it here would seem to contradict the Bajo study.

L241: “the amount of sinistral coiling direction of this species increases when the subtropical gyre circulation is more intense.” Specifically, a higher concentration of left coiling truncs supposedly reflects a deeper permanent thermocline and hence healthy gyre circulation. But this depends very much on the site selected. Are the authors confident that their site will work in the same way?

L465: I wouldn't disagree with cooling bit but I don't think we know what the climate sensitivity would be.

L476: so are the waters at the site actually warmer or is it just an illusion? And if so, then should you plot the faunal SST at all?

L492: But Uk37 temp was as high during MIS 27 as e.g. MIS 21.

L507: earlier (L458) you stated that interglacials were associated with stable SSTs

L512: the difference between faunal and Alk SST is important and should be further discussed (is it to do with the gyre again?). Also the comparison with MIS 11 is interesting but needs to be filled out a lot.

L515-519: this is interesting, since MIS 28b was only missed at more northerly sites and was warm at the southern margin of the Atlantic Inflow and at the site described here. Could the authors comment on the conclusions of Barker et al. (2021) on how surface ocean circulation might have changed across the MPT?

L688: Benthic d¹³C cannot be interpreted as a quantitative proxy for the AMOC.

L711: this was speculation in the Elderfield paper

Technical:

L19: Cooler SSTs in general or during glacials?

L110: Never proven – replace with demonstrated?

L202 etc: Use of ‘weighted’ instead of ‘weighed’

L322: “In total, 16 species were identified (Table 1; Fig. 3), with the diversity of the subtropical fauna appears to be diminished” – please check sense (or lack of)

Para beginning L352: This is very descriptive and wordy. Please condense.

L389: do you mean extremely short or extremely cold?

L465 etc: use of ‘conform’ instead of ‘conforming with’?

L480: Begin new paragraph before ‘Site U1387…’

L532: MIS 21e is not between MIS 25 and 22

refs

Hines, S. K., C. D. Charles, A. Starr, S. L. Goldstein, S. R. Hemming, I. R. Hall, N. Lathika, M. Passacantando, and L. Bolge (2024), Revisiting the mid-Pleistocene transition ocean circulation crisis, Science, 386(6722), 681-686.

Wharton, J. H., M. Renoult, G. Gebbie, L. D. Keigwin, T. M. Marchitto, M. A. Maslin, D. W. Oppo, and D. J. Thornalley (2024), Deeper and stronger North Atlantic Gyre during the Last Glacial Maximum, Nature, 632(8023), 95-100.