Thank you for your comments, Reviewer #1. We have responded below.

Q: It would be good to detail how you processed your data and how you combined the two datasets (Thermo and Nu). It looks like you used ETH standards, however, with the Thermo you didn't measure ETH.

A: As you point out, our early clumped isotope data was collected on one machine (MAT 253) and corrected using a gas-based reference frame and our later data was collected on another instrument (Nu Perspective) and corrected into the I-CDES carbonate-based reference frame. For some samples not analyzed for their clumped isotopic composition, we measured just d13C and d18O using a MAT253 + Kiel IV device. To ensure consistency, alongside samples we analyzed three to five in-house carbonate standards, whose isotopic compositions were tracked through time and across machines. We have compiled a table with summary statistics on these in-house standards through time and on both machines that can go into the supplementary material of our manuscript. This table shows that d13C, d18O, and D47 values of these standards have been constant (within error) through time and across machines, so all sample data can confidently be combined and treated as a single dataset.

Q: How reproducible is your data?

A: General reproducibility can be demonstrated by the long-term standard deviation of the above-mentioned in-house carbonate standards which are measured frequently yet not used in the absolute reference frame correction itself. For the MAT253 analyses, long-term 1sd on D47 is 0.020 permil. For the Nu Perspective, long-term 1sd on D47 is 0.018 permil. Long-term 1sd for d13C and d18O are generally better than 0.1 and 0.2 permil, respectively.  

Q: Are your ETH standards values comparable to Intercarb values?

A: Yes, when ETH standards are used for correction to the I-CDES, we assign values from Bernasconi et al 2021, so they are by definition extremely comparable to identical to Intercarb values. ETH standards were not measured on the MAT253 when a gas-based reference frame was used.

Q: Is this the first article published with data from your Nu-perspective? If no, please cite the article that describes the procedure, if yes, please give more details on the methodology of this MS.

A: Yes it is, although we did not know it at the time of original submission. Here are additional details on the method that can be added, along with citing Mackey et al (2020) which describes details of a similar instrument used in a slightly different mode (coldfinger mode).

The NuCarb device reacts sample powders in individual vials under vacuum at 70 °C by slowly injecting 150uL of 103% phosphoric acid. Vials are isolated for the first 5 minutes of the reaction to reduce excess bubbling, then opened to a variable temperature trap held at -160 ℃ for 15 additional minutes, continuously collecting evolved CO₂. Next, this variable-temperature trap is elevated to -60 °C and CO₂ is released to pass through a static trap filled with Porapak Q material held at -30 ℃. CO₂ is frozen on the far size in another variable temperature trap set at -160℃ for 800 seconds. CO₂ is warmed up to the gas phase and the yield is recorded with a transducer. The Nu Perspective and NuCarb can handle both larger (3-6 mg) and smaller (350-450 ug) samples. Depending on the transducer reading following purification, the automated sequence directs CO₂ from a larger sample into the bellows or freezes CO2 from a smaller sample into a cold finger. All samples in this study were between 3-5 mg in size and were therefore analysed in “bellows mode”. Previous studies using a Nu Perspective and NuCarb for clumped isotope analysis described “cold-finger mode” (Mackey et al., 2020). In bellows mode, bellows are compressed until a desired beam strength of 80 nA is achieved on the major (m/z 44) ion beam. Gas is analyzed for 4 blocks of 20 reference-sample cycles, with 20 seconds of integration on each half cycle. Bellows are continually adjusted between each cycle to maintain the initial beam strength at all times.

Q: How did you convert the data from the D47 into an absolute repository (software, codes)?

A: Between 2016-2020, a first set of fossil shell powders were measured for Δ₄₇ using a Thermo-Finnigan MAT 253 dual inlet isotope ratio mass spectrometer and a manual sample preparation device described in detail by Defliese et al. (2015). Raw Δ₄₇ values were placed in a gas-based absolute reference frame following methods introduced by Dennis et al. (2011), using theoretical equilibrium values for heated (1000 °C) and water-equilibrated (25 °C) standard gases (0.0266 and 0.9198, respectively) and the 75 °C acid fractionation factor of +0.072 ‰ from Petersen et al. (2019). Boundaries between each 1-3-week-long correction window were selected to account for drift in equilibrium gas line slopes and to maintain consistent values of in-house carbonate standards through time.

In 2021 and 2022, additional powders were analyzed in the University of Michigan SCIPP lab using a Nu Perspective isotope dual inlet ratio mass spectrometer connected to a NuCarb automated sample preparation device. Isotopic values were converted into the Intercarb Carbon Dioxide Equilibrium Scale (I-CDES25) absolute reference frame using Δ₄₇ values for four ETH standards defined by the Intercarb project (Bernasconi et al., 2021) and an acid fractionation factor of +0.066 ‰ for 70 °C from Petersen et al. (2019). First, a single slope was fitted through ETH 1 and ETH 2 (adjusted by 0.0033 ‰) in δ⁴⁷ vs. Δ₄₇ space, then all four ETH standards were used for the empirical transfer function step.

This description can be added to the methods section of the paper and all calculated equilibrium gas line slopes and transfer function slopes and intercepts will be included in the final data table archived in EarthChem ClumpDB.

Q: You can also specify the number of measurement sessions and how you calculate your temperature uncertainties. It must be written in the section method, not in the caption of a figure. In the legend, the definition of your uncertainties is not clear.

A: There is a blur between what counts as a “measurement session” (which might be a few months long between major maintenance or power outages) and a “correction window” (a period from which standards are selected to correct a subset of data). Multiple correction windows make up one measurement session. We describe the measurement sessions in the response above, which we can add to the methods section as written. For information about specific correction windows (which we feel is too granular for most readers), someone can easily find this information in a column labeled “window” in the EarthChem data template.

Long-term reproducibility (1sd) of Δ₄₇ in carbonate standards (0.020 ‰ for MAT253 and 0.018 ‰ for Nu) was used to calculate a long-term standard error in Δ₄₇ for each sample. For temperature and δ¹⁸O_sw values, a long-term standard error was calculated using the long-term standard error in Δ₄₇ and the pseudo-linear relationship between internal 1SE on Δ₄₇ and either temperature or δ¹⁸O_sw in all samples (for this study: long-term 1SE in Temperature = 306.7*(long-term 1SE in Δ₄₇) + 0.1778 and long-term 1SE in δ¹⁸O_sw = 66.33*(long-term 1SE in Δ₄₇) + 0.0461). The external 1SE for all three parameters was selected as the larger of the internal and long-term standard error. In all plots, we use the external 1SE for temperature or d18Osw.

Q: Would it be possible to replicate some samples to reduce the uncertainties on the temperatures?

A: Since receiving this review, we have added additional replicates to the few samples with the worst reproducibility. We have also done more thorough replicate screening (description of which can be added to methods), removing a single replicate if it falls more than 2.5 standard deviations from the mean of the remaining replicates. These combined efforts improved error bars on a number of samples. Additionally, we have now analyzed a few more new samples to improve the number of samples per horizon and to make our conclusions more robust. These new data reinforce prior conclusions, such as the timing and magnitude of warming during the Late Maastrichtian Warming Event.

Q: You have measured different carbonates (matrix and fossil bivalves; different species). However, you are not comparing this data. For example, how do you explain the temperature difference between Acutostrea and Agerostrea at 66.5 Ma? A discussion of this comparison would improve the manuscript.

A: We disagree with the reviewer that the temperature difference between Acutostrea and Agerostrea at 66.5 Ma is meaningful, as they are comparing two single samples from one horizon. Yes, that is the horizon with the largest variability and it happens to include multiple taxa, but we see a similar level if variability between samples of the same taxa from the same horizon. Fossils found at the same horizon were not necessarily living at exactly the same time, so may be accurately representing their living environment but the living environment is not exactly the same as for the other shell from that horizon. Also, for the most part, shells included in this study are small oysters, so it is possible that our drilled powder has aliased certain seasons in one shell vs. another.  

Q: You talk a lot about SEM images; however, no image is shown. Please put at least 3 with original material, traces of dissolution and secondary growth. You can include them in Figure 3.

A: We have created a figure showing SEM images of 9 shells showing different levels of preservation, along with their trace element concentrations and our overall preservation rating. This could be a main-text figure.

Q: I would suggest removing Figure 4 (as you have the same one in Figure 5) or skipping Figure S3 in the main text, which is much more detailed.

A: We think having a zoomed-in version of the temperature profile is beneficial to see interspecies differences that are not visible in the big composite figure (previously Figure 5), but we agree that Figure 4 and S3 are redundant. We now prefer to include Figure S3 in the main text which shows the removed data points, as well as the species-level differences.

Q: A table with the data, such as the last table in the material supplement, can be included in the main text.

A: We agree that a summary table of all isotopic compositions could be added to the main text. We think the trace element concentrations and SEM indices could remain in the supplement. Combining them into a huge table in the main text will just lead to it being harder to read.

Q: I would suggest adding a figure to illustrate the comparison between your data and previously published data.

A: We have come up with a figure that can be added to the main text, comparing our data to the composite dataset of Hull et al., 2020, which combines all published records over the Late Maastrichtian Warm Event. We would replace the latitudinal gradient figure with this new temperature timeseries comparison, as that is in greater alignment with the emphasis of this paper on the LMWE.

Q: Multiple other line-by-line comments suggesting slight rewording, addition of citations, or similarly small changes.

A: We are happy to carry out all these changes. They all seem warranted and identify areas needing clarification or additional detail.

Attached is the table of in-house carbonate standards mentioned in the above reply.

Attached is PDF including 2 figures:

First - a figure displaying SEM imagery of 9 different shells from our study displaying different preservation states. We have adopted a new rating criteria. Instead of rating with a single preservation index, we rate on a 0 to 3 scale for severity of dissolution and again for severity of secondary growth. Samples with a 2 or 3 for secondary growth are determined to be "bad". Samples with a 2 or 3 for dissolution but no/minor secondary growth (0 or 1) are labeled "okay". Samples with a 0 or 1 for both dissolution and secondary growth are determined to be "good" based on SEM imagery. This figure combines these SEM rating with trace element concentrations. If trace element thresholds of either 100 ppm Mn and/or 2050 ppm Fe are exceeded, the sample is labeled as "bad" for trace elements. If a single sample fails either trace or SEM screening, it is eliminated from interpretations of temperature.

Second - a figure comparing our time series (horizon means and 1sd) to the composite timeseries developed by Hull et al. 2020. This shows the timing of warming at our site and globally during the Late Maastrichtian Warm Event. Notably, our record

We are sorry to hear about your health struggles and appreciate you getting around to our review in the end.

Q: what are the true effects of this warming?  It is very short, basically a blip with a duration of no more than 100kyr, because the samples at ~66.3 Ma are indistinguishable from background.  In fact, it is possible it is much shorter than 100kyr because of the limits of the sampling resolution here.  However, what I see in the data is a longer-term _cooling_ from 66.37 to 66.07 Ma.  In fact, this is the most significant secular signal in the data – what does this mean?  It’s of the right timescale to be the predicted pCO₂/temperature decrease due to continental weathering following the emplacement of the Deccan traps (e.g., Desert et al 2001, 2003).  I think the authors should highlight this!

A: We appreciate this suggestion and will mention of silicate weathering feedbacks and timescales, with some of the citations you mention. However, the secular trend you picked up on does not hold up so well with the addition of a few additional samples analysed recently as part of this revision. In particular, in the Meerssen Member we previously only had one sample which was particularly cold and set a lot of the cooling trend you described. Now we have a second sample from that horizon which is closer to the “baseline” temperature of ~20C, so the sustained cooling trend has largely disappeared. It remains the case, however, that the clear warming shortly after the Deccan Traps onset is gone after ~100kyr or less, and this finding has only become stronger with the addition of more data points.

Q: There are always questions of timescale, so just so it’s clear (and I probably missed it), but has the ejecta horizon/dust/Ir level etc. been positively identified in these N. European sections? 

A: While an ejecta blanket was not preserved in the very shallow marine setting (i.e above fair weather wave base) of the type-Maastrichtian region, the K-Pg boundary is positively identified (see Smit & Brinkhuis 1996; Vellekoop et al. 2020, https://doi.org/10.1111/pala.12462). The typical planktonic foraminiferal ‘disaster’ assemblage (Smit & Zachariasse 1996), the presence of the earliest Paleocene dinocyst marker taxon Senoniasphaera inornata (see Brinkhuis & Schiøler 1996; Herngreen et al. 1998) and 87Sr/86Sr analyses of well-preserved foraminifera from the clay layers of the Geulhemmerberg underground galleries (Vonhof & Smit 1996) have all demonstrated that the K-Pg boundary occurs at the base of uning IVf-7 of the Meerssen Member, at the base of a sequence of shell hashes and clay layers.

Q: Can we be certain that the horizons identified as coincident with the initial phase of volcanism are indeed correctly correlated?  Bulk C-isotope data (Line 191) does not instill a lot of confidence…

A: The identification of the Deccan Traps interval is not only based on bulk d13C data. As highlighted in Vellekoop et al. 2022 (Newsletters on Stratigraphy), the age model for this succession is based on a combination of biostratigraphic markers (ammonites, belemnites, dinocyst) and the isotope record. Moreover, the presence of the acme of the dinocyst marker taxon Palynodium grallator, a marker for the LMWE (Vellekoop et al,. 2019, https://doi.org/10.5194/bg-16-4201-2019) in the type-Maastrichtian record (Schioler et al., 1997) is clear evidence for this phase of Deccan Traps volcanism.

Q: If I were to be critical of one point of this paper, it is the confidence with which the correlation is made between marine sections in Europe and the terrestrial record of Deccan volcanism that relies mostly on radiometric ages from India.  This should be expanded upon and the tie points made more explicit (is the age model published?).

A: We do not completely follow the argumentation of the reviewer here. Radiometric dating places the large outpouring phases of the Thakaurvadi to Puladpur lava deposits in the time interval between 66.3 and 66.05 Ma (e.g. Schoene et al. 2019, Science). At this point, it is well established that the Late Maastrichtian Warming Event is related to these Deccan outpouring phases, for example also highlighted in the recent paper by Nava et al. in PNAS (https://doi.org/10.1073/pnas.2007797118). Hence, no direct correlation with any terrestrial records in India is required. The age-model for type Maastrichtian (based on a combination of biostratigraphy and chemostratigraphy described above) clearly indicates that the Nekum & Meerssen members fall within the LMWE interval. There is even a specific marker for this warming event present in these records. We hope any concerns considering the age model of the type-Maastrichtian record are waylayed by the publication of the new age model for this section by Vellekoop et al (2022).

To be more transparent about the uncertainty in our age estimates and correlation, we have added uncertainty in the age model as a horizontal error bar in Figure 5 (Temp vs. Age), as well as uncertainty in the onset of Deccan volcanism (grey shading). The onset of Deccan volcanism here is pinpointed as the oldest date coming from Deccan lavas (Sprain et al., 2019).

Q: Can you add the timing (duration?) of Deccan volcanism to Fig. 5?  I need some frame of reference for where I should be looking for the increase in temperature.  The line at 66.4 is helpful but what is the duration?

A: We have thickened the bar showing the onset, now with error. This point is defined by the oldest dated volcanic rock from India (Sprain et al 2019). In terms of when to expect warming…CO2 driven warming does not happen instantaneously, and the CO2 addition would be expected to continue (perhaps nonlinearly) throughout the full period of eruptions, which extend well past the KPB.

Q: It may just be my general skepticism of absolute sea level estimates in ancient rocks, but can the authors expand upon the goals of the sea level reconstruction here?  To what end?

A: This is a sea level reconstruction from another paper (Schioler 1997). We put it here as a possible explanation for our co-evolving temperature and d18Osw records. We propose that changes in sea level correlate with our study site experiencing differing water masses due to the paleogeography.

Figure showing temperature through time, with multiple updates.