Letulle et al

This is an interesting paper that adds to the debate on the problematic nature of apparent polar warmth during at least some intervals during the Mesozoic greenhouse. In this context, the data from Siberia are particularly valuable, particularly as a number of workers are insisting on the presence of substantial Jurassic and Cretaceous ice to explain sea-level changes and cold-climate phenomena such as glendonites in high-latitude sites. The fact remains, of course, that the present work offers only a snapshot of geological time, in the Toarcian case during a well-established hyperthermal, and extreme extrapolation to much of the Mesozoic would probably be unwise. Given the importance of the Arctic data, I think it would be preferable in parts of the text (e.g. Results and Discussion) to separate out the Pliensbachian and Toarcian data sets in separate sub-sections rather than running them together, which can become confusing to the reader.

In terms of fidelity of the paleotemperature records, much depends on the preservational state of the aragonitic fossils, and the authors have made some obvious moves to determine the integrity of their material. I must say, however, that, from the photographs, the Arctic specimens have a white ‘powdery’ look to them, which is typical for partly degraded aragonite. As another test of alteration, strontium-isotope data would be useful, since the Toarcian global curve has particularly low values around the OAE interval and the presence of more radiogenic ⁸⁷Sr/⁸⁶Sr ratios would be a fingerprint for alteration. Ideally, of course, there would be some accompanying TEX86 data, which should be obtainable given the relative lack of maturity of the sediments and at least a modest amount of organic material in the sediment.

Line 24: change to ‘These data highlight the risk . . .’

Line 59: should be ‘as well as isolated’

Line 60: has undergone

Line 101: commonly flattened

Line 18: described by Daëron et al. (2016).

Line 120: CO₂

Line 132: need subscripts

Line 144: values, respectively (insert comma)

Line 157: well-preserved

Line 217: say where in the mid-latitudes

Line 227: high-latitude

Line 233: hence recording . . .

Line 241: near-shore

Line 243:should not have exceeded

Lines 247–248: relative to SMOW standard

Line 244: can ‘only a few degrees’ be more specific? Estimates of the temperature drop across the thermocline from some localities during the Jurassic and Cretaceous, based on belemnite delta-18O values and TEX 86, come in at about 14°C (Mutterlose et al., Earth and Planetary Science Letters, 298, 286-298 and Jenkyns et al., Climate of the Past, 8, 215-226). So, presumably the bivalves were living in the mixed layer? As noted above, it would be useful to have some TEX86 values for the accompanying sediments.

Figure 4, text figure explanation. Please explain what the different symbols mean and the shorthand for the zones. Should falciferum not now be serpentinum?

Line 276: mid-paleolatitude

Line 314: references required

Line 519: reference is incomplete

Fig 5, text-figure explanation is not comprehensive enough, making this diagram difficult to decipher. Make clear what grey bands signify. References should be given here, not in Supplementary data.

This manuscript describes a compelling dataset of carbon, oxygen, and clumped isotope ratios from late Jurassic fossil shells, one at northern polar latitudes in the Toarcian and another from mid-latitudes in the Pliensbacian. Their sub-period designation is about the only commonality between the samples and the data, yet the authors discuss them in nearly the same breath. For this and other reasons articulated below I am recommending that this manuscript could dramatically improve following careful, major revisions.

The major strengths of the paper are in the quality of the stable isotope data, which by all accounts is very high and uses the most up-to-date clumped isotope methods, correction schemes, and temperature calibration curves. However, I am routinely irked by the habitual reporting of clumped isotope temperatures to the tenth decimal place, which hardly seems to matter when the reported error is several degrees. Beyond this, I fear the manuscript falls into a common trope of being too quick to overlook the possibility of clumped isotope resetting during burial and too ready to extrapolate results across paleolatitudes and Phanerozoic timescales with grand paleoclimate ambition. In their revision I would encourage the authors take a more logical, considered, and even skeptical approach. What if the shells are not as pristinely aragonite as their SEM and Raman data imply? The chalky and fractured nature of some of their samples from the photographs in Fig. 2 calls into question the ubiquity of their SEM and Raman-based conclusions. Similarly, is it possible that the burial temperatures are slightly warmer than the best estimates from the literature? Aragonite clumped isotope bond reordering is complex (relative to calcite and dolomite), poorly understood, and seemingly faster for a given thermal history than calcite (see Chen et al. 2019, GCA). The authors hardly dwell on this fact and its associated uncertainty. Also, might the Polovinnaya River samples be estuarine, and not marine? Terrestrial fossils from the same shale exposures indicate that it might be a possibility, or at least one that needs detailed recognition even if it is not the preferred interpretation. An estuarine or non-marine origin might not impact the importance of their clumped isotope paleotemperatures, but it complicates the calculated water oxygen isotope ratios in ways that are interesting and not exclusive of comparisons in Fig. 5a.

Finally, in the text and in Figure 5 there is a casualness with comparing datasets over nearly 150 million years of the Mesozoic and Cenozoic, with dramatically different global paleoclimates and continental configurations, that makes the discussion hard to follow. For example, there is a large leap between the Early Jurassic and the Early Eocene on lines 254-259 that converts latitudinally ambiguous precipitation oxygen isotope ratios from Eocene proxy datasets to the calculation of paleosalinity during the Early Jurassic arctic. The leap is so large that it seems to obviate their point. In instances like this (see below for more line-specific commentary) I would encourage the authors to stick with data-model comparisons and well-reasoned hypotheticals. This reframing would still allow for the multi-period comparisons shown in Figure 5 with an edited discussion that better conforms to the study motivations outlined in the abstract and introduction.

Line-edits:

17-18 – The connection with the previous sentence is not very clear; why the distinction in time interval?

19 – “mildly buried” is a confusing term.

24 – Correct “highlight” to “highlights”

36-38 – The authors could elaborate on this statement for better effect, I think. It may not be obvious to all readers how clumped isotopes are sensitive to burial.

39 – “ante-Cretaceous” is an uncommon phrase.

43 – Regarding “marine carbonate shells”, there is some ambiguity on their marine origin in the discussion and I think the authors should specify that they are aragonitic fossils. This is important for two reasons: 1) aragonitic is exceptionally susceptible to geochemical alteration by conversion to calcite and 2) the bond reordering kinetics for aragonite are such that they are more prone to 'solid-state' clumped isotope change than calcite or dolomite.

60-61 – "Exceptionally low burial" might be hyperbole. A Tmax of 420 °C in indeed immature, but not exceptionally so.

61-66 – It would be useful for the authors to commit to a maximum burial temperature. Using models of bond reordering and a simple burial history curve, it would be possible to estimate the possible change in clumped isotopes due to burial heating alone.

Figure 2 – Samples (a), (b), (d), (i), (j), (k), and (l) all look too weathered or fractured to demonstrate that they retain primary shell material. Later it is revealed that (i) is not aragonitic (Fig. 3h). How are the authors able to admit that (i) is not aragonite, but call (e), (f), or (j) “pristine”? Each of these samples have the same coloration in these images.

102-103 – Regarding the white color, this chalky appearance can indicate shell alteration (mineralogical conversion or geochemically).

Section 3.2. – There are subscript and superscript formatting errors throughout this section.

158 – Was the prismatic layer avoided when microsampling the shells?

Figure 3 – Why are the Raman spectra truncated <200 cm-1 for (b) and (d) and not for (f) and (h)? Also, it is not clear here (or in the text) how the position of these images and spectra relate to the subsamples for isotope analysis.

194-196 – The temperature ranges cited are canonical values for calcite, not aragonite.

203-204 – Regarding the possibility that small shell-water interaction has been shown to change clumped isotope ratios with only modest change in oxygen isotopes, what might be specific, independent evidence that this sort of thing had occurred (or not) in these shells?

204-206 – I think the implication here, subtly, is that 31 °C is something like a maximum burial temperature. Given that this is a fracture-fill carbonate without any other paragenetic sequencing context, it is equally possible that it's an exhumation temperature (i.e., a temperature experienced during fluid infiltration after maximum burial was reached).

219 – I think it is notable that this range narrows considerably after removing the coolest temperature. The remaining 7 of 8 shells have an average of ~15 °C.

230-231 – Discussion of Jurassic food availability during the polar night, without any additional information, is entirely speculative and is too extrapolative from their isotope dataset.

243-237 – As mentioned above, this is an apples-to-oranges comparison. The similarity in SST between two shell populations separated in time by over a 100 million years might be entirely coincidental.

257-259 – Also as mentioned above, I don't understand the relevance of Eocene high latitude precipitation values here. Who knows what they were in the early Jurassic?! As the authors show in Fig. 5, the modeled Jurassic poles were warmer than modeled Eocene poles, yielding lower latitudinal gradients in precipitation oxygen isotopes (see dashed lines for Eocene and Cretaceous data).

266-268 – The authors should reframe this statement to consider an alternative scenario in which this locality and these shells are not marine at all. What if -4.9 to -2.5 are estuarine or mostly freshwater oxygen isotope values?

Figure 5 – Are the model results new or replotting of published results? If it’s the latter than proper attribution needs to be clear in the figure or the figure caption.