|The manuscript by Martin-Garcia et al. is improved, but I believe that it needs additional revisions. The authors appear to have addressed the majority of my comments, but not all. It is a bit difficult to determine because the cover letter does not contain refences to the specific line numbers where the changes have been implemented so that one can easily cross-check. In addition, the manuscript text is still unclear at times due to the awkward use of the English language. |
It occurred to me that the title is a bit misleading. If a study examines the role of North Atlantic circulation in the mid-Pleistocene transition, then I would expect that it spans the entire interval of time. Here the records do not begin until after the mid-point (MIS 20). So only half of the event is captured.
I still think that a mention of the closed sum problem is important. In my mind changes in a dominant assemblage can drive the percentages of a less dominant species. True, this approach is often used. But that does not mean that it does not have uncertainties.
Regarding my comment to Line 251 of the original manuscript (now section 5.3): The authors maintain that there is an increase in NADW formation rate during glacial intervals begin with MIS 22 and ending with MIS 14. They cite Wright and Flower, 2002; Hodell et al., 2008, Poirier and Billups 2014, Hodell et al., 2015 for this statement (in the text or rebuttal). I have looked at all of these articles and cannot find a single statement to this effect. In fact, Wright and Flower cite Raymo et al., 1997 saying that there is greater suppression of NADW …. from 950 to 350 Ka. Hodell et al 2015 is about the age model for Site U1385, and I don’t see a discussion of deep water circulation in that paper, the d13C record is not shown. Hodell and Channel (2016) note that d13C minima increase from MIS 22 to MIS 14, and they cite Raymo for this observation. However, to my reading of this article, they do not say that this is due to an increase in NADW formation.
The rest of my comments are in order of occurrence in the manuscript text:
Line 47: add a citation?
Line 68/69: related to the
Line 70: no comma after which
Line 71/72: the climate system did not switch, the 100 kyr cycle started to appear. The 41 and 23 kyr cycles continue to be present
Line 80: why glacial? The manuscript seems to address both, glacial and interglacial intervals
Line 89: I would replace the word ‘proven’ with something like ‘shown’
Line 100: of the Quaternary
Line 102 at the surface
Line 155: No single sentence paragraphs. What is the difference between a calcareous mud and a calcareous clay? Calcareous means biogenic carbonate as well, right? This paragraph needs a bit rewording to make it more clear.
Lines 121-124: What about the age models of the other sites? For calculation of the thermal gradient it is really important that the age models are comparable.
Line 131: More detail? I think that it would be appropriate to include a brief list of warm versus cold assemblages etc., then refer to the Appendix with more details
Line135-141: How are the SSTs subtracted? Are the records interpolated to the same ages? The method needs more detail.
Line 142: SSTS from Site 607 and 980 have been published. Not all SSTs records have been published as you are presenting those from Site 1385 here for the first time?
Line 150: replace the two ‘to’ with ‘with’
Line 156: replace ‘keeps’ with ‘stays’
Line 157: specify which ones, ‘some’ is vague
Lines 167-174: To my eye the variations are subtle. It seems very qualitative
Line 177: instead of referring to a particular climate cycle, just say which MIS, or which Termination? Those are labeled in the figure and it is thus easier to find.
Line 177: ‘since’ is not correct as the records end at MIS 14.
Lines 176-180: I am not sure I can follow the sentence at all. MIS 20 is not an interglacial. And, the variations are subtle, what do you mean with’ fairly’? I would suggest providing the changes of the percentages to give the reader a sense of how much of a change is actually occurring.
Line 244: is a 0.2 per mil difference in d13C values really significant in terms of NADW? There are other factors that determine the d13C values of benthic forams.
Line 283: associated with
Lines 285-292: see comment above. I cannot find any statements to this effect in the literature cited in this section. The authors seem to base this on a slight increase in benthic foram d13C values at their site? I think the interpretation of the d13C record is a lot more complex as presented in this study. Do these studies really say that there is an increase in NADW during glacial intervals specifically? Or are they referring to a more general increase in NADW over time, which includes interglacial intervals? As noted above, I cannot find d13C records in the Hodell et al 2015 paper, do you mean Hodell and Channel 2016? They describe an increase in d13C values, but I don’t think they discuss deep water circulation.
Line 313: southern-more or more southern?
Line 314: I cannot find any reference to d13C in the Hodell et al. 2015 paper.
Line 315: How can you tell from your data that the overturning cell was deepening? Hodell and Channel describe that d13C minima get higher between MIS 22 and MIS14 citing Raymo. But I don't think that they say that this is due to more NADW formation. In any case, there could be other factors.