the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Apr 2022
Terrestrial carbon isotope stratigraphy and mammal turnover during post-PETM hyperthermals in the Bighorn Basin, Wyoming, USA
Sarah J. Widlansky
Ross Secord
Kathryn E. Snell
Amy E. Chew
William C. Clyde
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- Final revised paper (published on 08 Apr 2022)
- Preprint (discussion started on 13 Jul 2021)
Interactive discussion
Status: closed
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CC1: 'Comment on cp-2021-83', Hemmo Abels, 20 Jul 2021
Herewith my review of Widlanski et al. submitted to Climate of the Past.
The paper brings an important subject interesting for a relatively wide audience of paleontologist and paleoclimate workers, both terrestrial and marine. The early Eocene hyperthermals are well studied while little data are present concerning their impact on the continents both geochemically, as environmentally and faunally. The paper is a continuation of earlier work published in Climate of the Past (Chew 2015) where an attempt was made to correlate faunal records of the central Bighorn basin, Wyoming, to a carbon isotope record from more northern parts of the same basin. The faunal analysis of that previous paper is now supplemented by a series of carbon isotope sections much closer or directly at the mammal sites in the central parts of the basin and with that potentially making the correlations between fauna and isotope changes more straightforward. The authors face the problem that, in the central parts of the basin, outcrops and so carbon isotope samples and mammal sites are scattered over large areas due to low topography. A nearly 30-year old composite meter level system is used since then to anchor different data sets to the meter-level system over this entire area, while structural dips are reported to occassionally change within the area without much notice. The composite level stratigraphy (BCM named here) has thus a relative large uncertainty and mammal sites and carbon isotope records may erroneously overlap or be separated in stratigraphy while in reality that is not the case. Therefore, the current authors additionally use absolute dGPS levels within their sections to also tie sections together. Subsequently, a composite isotope stratigraphy is constructed where the mammal analysis of Chew 2015 is placed along and mammal and isotope changes are discussed and placed in a more global perspective.
Correct stratigraphy is thus the key to produce solid results in this paper as in many other papers. The difficult outcrops make that the stratigraphic approach and data should be produced and communicated with even more care. How do isotope and mammal finds stack together into stratigraphy? The authors must have spend considerable time to produce the current work, however, the written work under review here did not take away my concerns about the mammal and isotope composite stratigraphy produced. I think this is partly because the workflow and data are not sufficiently backed-up with maps and data in the paper or supplement. There are large stratigraphic thickness changes between steps within the workflow of up to 30% for individual sections. How is that possible? One would expect that dGPS data or Jacob-Staff data should be able to work at lower uncertainty? Were structural dips of the layering not determined sufficiently? The isotope records are at places of relatively high resolution, although there is no explanation of sampling strategy. Where all pedogenic nodules sampled? The carbon isotope records show some very good CIEs, but also some interval difficult to interpret. The resulting composite isotope record however is far from easy to interpret. Due to the stacking of series that do not have very similar carbon isotope results, the composite stratigraphy is blurry. Relating CIE1 to ETM2 and CIE2 to H2 seems logic with the additional stratigraphic constraints, but I wonder whether correlation to I1-I2 would be possible? CIE sizes have so-far been showing to be very stable, such as even similar PETM CIE body d13Cped values in the northern basin as in the southern basin. The paper has long discussion on what could have caused different CIE sizes within the basin, but while I am still doubting the stratigraphy, interpretations and composite stratigraphy, it is difficult to believe the remainder of the paper.
The seeming lack of documentation of stratigraphic data within the paper and appendix can be solved. Mammal sites, carbon isotope sections, and magnetostratigraphic sections should be plotted on a map(s) that is(are) detailed enough to see how different site (may) relate. Figure 1 is small and the background of it vague. In the supplement or paper, more detailed maps could help to show such relations. In these map figures, it is necessary to show topographic lines, the key mammal sites, the isotope sections and the structural dip of layering. This allows to understand how these result in stratigraphy. Besides, photographs of the sections sampled should be added such to see the outcrops and sampling trajectories. Are these far away even within single sections? It seems so, if I see the North Fork section on Figure 1 and try to find it using Google Earth, I only see very low hills with 5-10 meters of stratigraphy. What is the uncertainty in thickness of single sections when measured in the field by Jacob Staff? There misses a table with the dGPS data that are used and including GPS locations of the tops and bases of the (sub-)sections.
The Results section misses a detailed description of stratigraphy and stratigraphic results. The dGPS stratigraphies differ quite a bit from the BCM levels supplemented by Jacob Staff measurements. In Figure 4, both stratigraphies are provided, there is a description of the isotope stratigraphies of both methods to come to stratigraphy, but there lacks a description of the stratigraphic impact itself between the two methods. Basin Draw is shortened by 33% it seems in the dGPS method, Kraus Flats is shortened by 20%, while North Fork is expaned by 33%. One would expect that Jacob-Staff data are not that inaccurate? Why would this happen? Another argument could be that the approximate levels of the dGPS are indeed reliable, so still lining up the supposed ETM2 level, but for thicknesses the Jacob Staff data are quite good? What about structural dips of the bedding? In relation, there misses an impact on mammal site stratigraphy of the two stratigraphic compilations made. Part of these results are presented in the discussion, but these should really be in the Results section. Missing is a table with mammal site content in the new stratigraphic order listing the most important mammal species through stratigraphy, it must be different with these new approaches from Chew, 2015? Grey bars indicate BioB, B1 and B2, but the mammal sites where this is based upon should provided.
In addition, the authors do not include the notion of precession forcing of stratigraphy on the floodplain stratigraphy of the Bighorn Basin. Abdul Aziz et al. 2008 suggest precession forcing in what seems one of the currently studied sections, Red Butte. 20 kyr would correspond roughly to 8 meters of section according to the work of Abdul Aziz. For more northern sites, such numbers have been documented since. Also, Abels et al. 2016 discuss a ca 35m cycle in the carbon isotope record studied that would be in line with the precession forcing of the smaller scale cycles. Both these numbers give a fairly good control on sedimentation rates and from that point of view the isotope records could be analysed and interpreted. This is not used nor discussed. Line 353-355 are a little too easily stating that there are "overall differences in sediment thickness across the basin". Yes, there are clearly differences, and also hiatuses particularly at the basin margins, but sedimentation rates in the basin centres have been shown to be relatively constant. In the centres, sed rates depend on subsidence rates that controlled net accommodation space generation. One would not expect rapid changes in sedimentation rates at above 10^4 yr and below 10^6 yr time scales as are suggested by the correlation of CIE3 to I1. There is discussion on this in 5.2 and 5.3, but it is quite unstructured now also discussing the same things in different sections (line 360-366 in 5.3 and similar discussion with different arguments in 5.2). As a minimum, the authors should refer to the work on astronomical forcing of these series and why they think not to use these arguments in building stratigraphic framework for their series and for the interpretation of their carbon isotope records. The authors claim lower sedimentation rates by observing lower spacing between CIE1 and CIE2 in their series. If these are correlated to CIEs in marine records, it also imports 100-kyr eccentricity age control on the Fifteenmile Creek series that could be used to interpret the remainder of the series.
The second half of section 5.3 does not include the possibility that there may be remaining uncertainties in the study. There are at least some serious stratigraphic doubts to be placed along the composite stratigraphy with the dGPS stratigraphy so largely different from the Jacob's Staffed thicknesses. If the thicknesses of individual section can be different up to 30% between methods because of uncertainties in structural dip changing through the area and because of very low topography of outcrops, correlations between sections must contain serious uncertainties. The composite isotope record is far from clean likely because of these uncertainties. Thicknesses of intervals and sections are different also in the composite. How can the authors be certain about the composite? Should the composite actually be made or is it better to discuss the results from the individual sections as those are uncertain enough in themselves?
What could improve the carbon isotope data analysis is nailing down better the baseline carbon isotope values. This is around -10 per mille it seems. If a vertical line is placed at the -10 permille d13C value in all plots, it much better allows to identify excursions, also when records are relatively short.
The BioB, B1 and B2 'events' are used without much discussion on their reliability. There should be minimally some discussion on these data and interpretations, such as why sample size and different types of diversity go hand-in-hand in the records of Chew 2015. Plotting the average sample size plot on top of the diversity plots in the Figure 4 of Chew, 2015, lets them merge, which despite the statistics places some serious doubts along the B1 and B2 events. And, does the new stratigraphy not impact those previous results warranting a new analysis?
- the title is too general now and should be refined to cover the content of the paper. The paper is not improving carbon isotope stratigraphy of post-PETM time intervals. A reference to 'terrestrial' or 'continental' and the geographic location is needed.
- in Figure 1. Would there be the possibility to both show topographic lines and structural dip of bedding? It would give some more notion of the stratigraphic transects / sections measured
- in Figure 1. It would be good to add some trails / tracks / roads to provide the reader the opportunity to better orient
- in Figure 1. Legend is missing the light brown shading, from the caption that seems to be 'unshaded' Quaternary?
- section 3.1 is unnecessary, items can be discussed when applicable in the results interpretation or discussion sections
- Line 196. It is not clear what was the strategy to come to 'sampling sites'. Where all nodules encountered sampled, or was a certain stratigraphic resolution chosen?
- Line 212 refers to pedogenic carbonate sampling again, right? Should it be in 3.2 then?
- Line 276. I would not regard the whole range between -8 and -12 per mille d13C as background. There is clearly noise on these records, but not 4 per mille.
- Line 275-280. Plotting against Trimble positions suggests there is no structural dip in the area?
- Figure 3. 'colors represent outcrop color' is not a very clear statement, I suppose you mean after removing weathered surface material? Or you mean color of the weathered surface?
- Figure 1 and 3. How does the Red Butte section correlate to the Red butte section of Abdul Aziz et al. 2008 in Geology? As the same name is used here, and it seems to be in the same area, it should be indicated how these relate, or another name should be used for the section. There are workers around who can help identify the location of that previous Red Butte section.
- Figure 3: why is the level at 45m in North Fork not identified as a CIE? And 35m and above in Red Butte? It is clearly not baseline of -10 per mille
- Figure 3: in Basin Draw D1459 is given as 29 meters from D1460, but in the panel c of Basin Draw it is no more than 15m without visible uncertainty, how is that possible? The same occurs with D1822 and D1204top, where a separation is given of 18m while in the panel c it is no more than 4 meters
- Figure 3: it would be good to label the levels in panels c with the 1,2,3,4 numbers
- Figure 3: Basal/D1350 has only few dGPS points while it is 50 m thick, why are there so few points and is the whole section not covered by calibration points?
- Figure 4: for simplicity, delete -10 and -14 from all x-axis labels, it makes it easier to read
- Figure 4: the panel b misses biostratigraphic information, while these would just relate to the isotope data as they do in panel a, right?
- Line 330 is the PETM really an option? it seems a bit unnecessary to exclude the PETM as an option in the discussion I suppose? The PETM should be so much lower in stratigraphy? The early Eocene has a good bunch of CIEs to correlate to other than the PETM. Why are these Fifteenmile Creek CIEs not I1 and I2, their size would make that plausible at least.
- Figure 5: magnetostratigraphy should be plotted along Fifteenmile Creek Composition Section
- the distance between the centre of CIE2 and CIE3 in FCCSection, has decreased to just over 20m while nearly 40 m in North Fork section, the only section where the interval from CIE2 to CIE3 is measured in a continuous manner, how is the possible? It has a very big impact on the interpretation of the CIEs
- Figure 5: it would be good to place the marine isotope stratigraphy to the far left as a baseline, including labels for hyperthermals, the MCP record next it, and the FCCS to the right.
- Lines 360 to 366 is doubling of discussion with the 5.2 section.
- Line 429-430: this sentence is written as if it is needed to have every CIE requires faunal change. I would be happy to believe that though I doubt whether we have much at hand at this stage to confirm anything close to that.-
AC1: 'Reply on CC1', Sarah Widlansky, 03 Sep 2021
Thank you for your comment and for taking the time to review this work. We will incorporate your feedback in the final version. The next version will include an additional supplemental table of GPS coordinates as well as more detailed field maps showing the sampling locations and places where we traced beds between subsections. We agree that adding these maps will help clarify some of the uncertainty in the stratigraphy and strengthen our argument for choosing to correlate the sections as we did. When measuring sections with a Jacob Staff we usually assumed a dip of zero degrees. In local areas where strike and dip could be measured, we found that dip was very close to zero. When dip is close to zero, it is extremely difficult to accurately calculate the direction of strike. Thus, we do not have measurements of structural dip that can be placed on the maps. Most variation in dip in the Fifteenmile area is minor and localized and does not reflect broader structural patterns. If local differences in dip along the section transect were detected (by shooting a horizontal line between exposures of a marker bed along the transect), adjustments to dip were made on the Jacob Staff’s clinometer. It should be noted that in most cases, section thickness was measured on slopes of 30 degrees or more, where beds were well exposed, and small differences in dip would have had only a small effect on overall section thickness. These small, local subsections were tied together with bed traces, sometimes over long distances.
We appreciate that you pointed out the need to identify other potential correlations for the CIEs. While the magnitudes are more consistent with the lower CIEs being I1 and I2, this isn’t supported by the biostratigraphy – this interval occurs immediately above Biohorizon B and there is no evidence for a significant unconformity in the region that would prevent the preservation of ETM2 and H2.
There are differences between the thicknesses measured with a Jacob staff and using the differential GPS, but a better way to compare them would be to compare the “pure” Jacob staff method, where all samples are shown according to their measured local meter level (Figure 3), and the differential GPS method (lower panel in Figure 4) where the relative spacing of sample sites is maintained while adjusting for dGPS tie points. The BCM levels (upper panel in Figure 4) offer an alternative way to transform the original measurements where the relative spacing is again maintained, but the levels are instead adjusted using the measured stratigraphic levels for fossil localities from Bown et al. (1994). Both using dGPS data and using the Bown et al. (1994) measurements provide a framework for correlating between the five local sections we measured, but they each have their own uncertainty, as described in the text. When you compare the thicknesses measured in our local sections separately to each method, the differences are reduced significantly. We will incorporate more of this discussion into the final text.
We also appreciate your suggestion of incorporating precession forcing in our discussion of stratigraphic constraints and sedimentation rates as well as the nearby Red Butte section from Aziz et al. (2008). We agree that including these constraints will help us strengthen our discussion of basin deposition and comparisons to McCullough Peaks. We will also be sure to more clearly highlight places where significant uncertainty still remains.
We feel that re-doing the faunal analyses of Chew (2015), given this new chemostratigraphy, is outside the scope of the current study, however we do identify it as a promising area for future work. However, we will expand the discussion on the limits of interpreting the previous faunal work in the context of the new chemostratigraphy. A significant amount of additional field work is necessary to incorporate all of the > 400 fossil localities used by Chew (2015) into a detailed chemostratigraphy like what we provide for the 18 fossil localities in our study. An important note though – the basic premise of the Chew (2015) paper holds true: she proposed that two intervals of observed faunal turnover corresponded to the ETM2 and H2 CIEs, and our results generally support this (with some uncertainties that are discussed in the text). A more precise assessment of the faunal changes that took place during these hyperthermals can be revisited in a later study when the results presented here can be expanded to include the many additional fossil localities in the area that have yet to be directly correlated to a nearby chemostratigraphy.
Lastly, we appreciate the line-specific recommendations and suggestions for making the figures easier to interpret. We will address these specifically as we make the changes in the final version.
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AC1: 'Reply on CC1', Sarah Widlansky, 03 Sep 2021
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RC1: 'Comment on cp-2021-83', Clement Bataille, 13 Aug 2021
Widlansky et al. present much-needed chemostratigraphic information for the Fifteenmile Creek area covering some of the Eocene hyperthermals. There are two main objectives with this study: 1) compare the carbon isotope results with those of Abels et al. 2016 and assess the terrestrial amplification of the d13C signals and 2) provide a better chemostratigraphic context for some of the claims of Chew 2015 linking hyperthermal and faunal turnover. The authors produce an impressive isotope chemostratigraphic record with hundreds of samples. The analytical issues encountered were a bit surprising but they are well-explained and justified. I believe this is a good paper with a lot of potential to better explore the link between hyperthermals, climate and biostratigraphy at the scale of an entire sedimentary basin. However, at this point, I don’t think this study is fully reproducible mostly due to the lack of details relative to the stratigraphic work. I suggest major revisions to account for this limitation and make sure the findings can be compared to other locations across the Bighorn Basin
Major comments:
I recommend the authors to read for example “Lehman et al. Stratigraphy and depositional history of the Tornillo Group (Upper Cretaceous-Eocene) of West Texas”. When I worked there, I used that document and could easily correlate stratigraphic sections with each other and reproduce the work of the authors thanks to maps, tie-points and field photos. This was super helpful to build upon their work, particularly the maps with the different sections and the stratigraphic correlation between sub-sections. You can also read the paper Bataille et al. 2016 Chemostratigraphic age model for the Tornillo Group: A possible link between fluvial stratigraphy and climate”. I think your paper is more similar to Bataille et al. 2016 as it focuses mostly on chemostratigraphy and age model. I fully understand that this work is not a stratigraphic piece but I think that the study really needs to link with previous stratigraphic work in the area to strenghten the age model. So I am suggesting below some additional figures and material to help the readers use this new chemostratigraphic data.
So I suggest that:
- It is ok to focus on chemostratigraphic correlations (e.g. Fig. $) but this needs to be done with a stratigraphic context. The authors should add a figure linking sub-sections with the selected tie-points and the method to correlate each section (see for example Fig. 6 in Lehman et al. or also Fig. 2 in Bataille et al. 2016 or supplement in Bataille et al. 2016). For example, we use the marker bed XX to correlate between sub-section XX and XX …etc… Or we used elevation records to trace this bed… So that the reader can understand solid tie points and more uncertain ones… If available some field photos of these marker beds would be really useful so that they can easily be identified in the field. Giving a table of tie-points used and their stratigraphic level would also be useful.
- The authors should add some zoomed maps in the supplement for each sub-section measured showing where they were measured (see for example Fig. 5 in Lehman et al.). This is really helpful to go back on the field. I understand there is GPS but this is easier to look at a path on a map in my opinion.
- The author should add a full composite section of their subsections and compare it to Bown et al. 1994 with biostrat and magnetostatic data tie points.
- In Fig. 5 it might be good to change the symbol by sub-sections to check if some of the noise is related to stratigraphic mismatch between sub-sections or at least to add this figure in the supplement. A broader discussion of stratigraphic mistmatch or uncertainties is also needed.
- Once this is done it would be good to plot the chemostrat record using all the available age model information similar to Fig. 5 in Bataille et al. 2016… This is far from perfect but it helps the reader a lot in my opinion to link this record with broader record either from this basin or at the global scale.
- I think these figures will also help the authors to improve a bit some of the discussion relating chemostratigraphy with faunal turnover and comparing chemostratigraphy with the Abels et al. 2016 section.
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AC2: 'Reply on RC1', Sarah Widlansky, 03 Sep 2021
Thank you for your comment and for taking the time to review this work. We will incorporate your feedback in the final version. We appreciate the suggestions of Lehman et al. (2018) and Bataille et al. (2016) as examples of documenting the stratigraphy in more detail to make the results more reproducible. The next version will include an additional supplemental table of GPS coordinates as well as more detailed field maps showing the sampling locations and places where we traced beds between subsections. We will also be more specific about documenting specific marker beds that we used to correlate between sections. In a few places, we did not directly trace beds between different sections and the relative position of the sections was based on the previous stratigraphic framework for the area – referred to as “BCM levels” in the paper. This was expanded by using the differential GPS elevations and the isotope records. Bown et al. (1994) did not construct a chemostratigraphic section for the area, so it is not possible to show a chemostratigraphy that is entirely in that framework without incorporating some of our own stratigraphic measurements. Additionally, previous magnetostratigraphic work in the area (and for these rocks in general) is somewhat ambiguous and not possible to tie directly into our own sections so it is not possible to compare our sections with another existing “full composite section” from the same area. The different thicknesses measured between fossil localities in our sections and the Bown et al. (1994) section, however, do indicate some offset. Some of this may be the result of uncertainties in knowing which fossiliferous level was used in the Bown et al. sections to denote the level of a fossil locality, for localities with multiple fossiliferous levels.
We will also add a version of Figure 5 to the supplement in which we show the isotope results according to their section to help recognize and account for any stratigraphic mismatch in correlation. Due to the uncertainties, in the Fifteenmile Creek magnetostratigraphy described in the text, we would not feel comfortable placing it alongside our isotope results in Figure 5. This would introduce too much uncertainty as (1) the records were not measured concurrently and (2) previous work has identified a need for a revised magnetostratigraphy in the area (Clyde et al., 2007). Similarly, biostratigraphic correlation between Fifteenmile Creek and McCullough Peaks is complicated due to their significantly different fossil sample sizes and the relatively coarse biozonations relative to the resolution needed to differentiate the hyperthermals. Because the biostratigraphy is not useful for confirming most of the chemostratigraphic interpretations, we’ve left it out of Figure 5. The exception to this is Biohorizon B, which is the best basis for biostratigraphic correlation between the two regions. It has been included for reference.
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RC2: 'Comment on cp-2021-83', Gabriel Bowen, 27 Aug 2021
This manuscript reports new pedogenic carbonate isotope data from the Bighorn Basin in an attempt to link central/southern basin fossil localities to a global timescale and sequence of Eocene climate events. The substantial new dataset and goal of exploring within-basin variability in environmental proxy data and faunal change are strong aspects of this contribution. The manuscript also highlights challenging limitations to the correlation of local sections within their study area, which frankly leave some ambiguity with respect to the stratigraphic interpretations presented by the authors. I do think that the work has inherent value despite the uncertainties that remain, but given that the primary contributions of the manuscript derive from and rest on the correlations, I request that the authors try to further justify some of their reasoning and interpretations.
My biggest concern relates to the identification and interpretation of CIEs in the new records. In section 4.2 the authors identify a series of CIEs in their local study sections and implicitly propose and interpretation of those (that they are stratigraphically coherent and can be correlated between sections, and are therefore likely to be reflective of large-scale or global forcing). I think that both parts of this presentation need stronger support and justification.
First, the authors need to more clearly describe what criteria they identify excursions in their records. Like many continental records, the ones presented here are noisy, and their features are not always obvious. Some of the excursions are pretty obvious (e.g., the one in the “Basal” local section). Others less so (e.g., why is the feature at ~440 BCM in the Basin Draw section a CIE and the one at ~400 BCM not?). I understand that it may be challenging or impossible to offer a fully objective and quantitative set of metrics used to guide these interpretations, but I’d like to see the authors try to get as close as possible to this, and then elaborate and justify other information used to (more subjectively) guide their interpretation. Since the identification of the CIEs is the major contribution of the paper, this deserves more attention.
Second, the correlation of different CIEs between sections is implied in section 4.2, and then revisited in the discussion sections. The initial presentation implicitly accepts that the BCM levels are an accurate basis for correlation between sections (the identification of the CIEs and association between them is introduced in terms of their being associated with certain BCM intervals). This is then revisited and questioned, and a second basis for correlation (GPS elevations) introduced and discussed. Arguments are made for preferring the GPS data in some cases (e.g., correlation from the N to S sides of the wash). Given that bed tracing is difficult in the area, and that GPS does not account for structural and depositional surface features, we’re left with a pretty inconclusive case. I’m left wondering why two other potentially-useful sources of information, namely patterns in the CIE records and fossil evidence, aren’t used in the correlation exercise. For example, both the up-section trend and the relative amplitude of the excursions (IMO) strongly support the correlation of the two CIEs at Kraus Flats with the lower two features in the North Fork section (and would imply slight deviations from both the BCM and GPS-based correlations). Conversely, I continue to struggle with the proposed correlation between Basin Draw and Basal sections given the strong isotopic ‘structure’ exhibited in the lower part of Basin Draw (and not at Basal). The authors have discussed local effects on soil carbonate d13C at length, and it’s possible that’s what we’re seeing here, but I’m not so sure. In addition, the authors are drawn to work in this area because of the extensive history of fossil collecting, and there are several well-described faunal turnover events documented in their study sections. The text implies that there may be some noise or ambiguity in the pattern of turnover that might in part reflect incorrect correlation between sections (that could be resolved here)…why isn’t this information tapped and discussed/leveraged as part of building the correlation model here? Finally, what about lithostratigraphy? Are there any coherent patterns that might support the correlation model?
In summary, I think that this will be a nice contribution to our understanding of the stratigraphy of the BHB and advance our ability to link environmental and biotic events in the basin to global changes. I think some revision focused on shoring up interpretation of the new carbon isotope stratigraphy is need to accomplish that. As a final thought, I’m not sure that the discussion of the local controls on soil carbonate C isotope values adds much (it is important background, but doesn’t really support any clear or important conclusions from this work)…this could be cut substantially, IMHO, and some of the space allocated toward shoring up the stratigraphic interpretations.
Minor points:
Abstract: here 15 Mile Creek is described as being in the ‘central Bighorn Basin’, whereas throughout the introduction it is stated to be in the ‘southern Bighorn Basin’
Section 3.2: Were the nodules collected from/tied to individual paleosol B horizons? Was any attempt made to constrain depth below the paleo-soil surface?
L233: Cite these packages if you’re going to mention them here
L331-333: This makes it unclear why you prefer to associate the Basin Draw CIE with the lowest of the three events (as shown in Fig 4).
L409-421: These arguments are reasonable but not particularly strong…for example one could argue that the d13C values around the ~30m level in the Basin Draw local section represent the H1 CIE. What does the fossil evidence say? Is there anything there that provides evidence for the alignment of this section wrt the faunal events?
Paragraph starting on L422: Is there value in doing this work without recollecting the fossils with higher stratigraphic resolution? High-resolution isotope data combined with ambiguous fossil locality extents is still likely to yield confusion.
L476-479: This explanation isn’t quite right…atmospheric CO2 isn’t ‘penetrating deeper into the soil’, it’s always there at all depths. It’s just that it constitutes a larger fraction of the total CO2 in this situation. (Correct this in the subsequent paragraph, too.)
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AC3: 'Reply on RC2', Sarah Widlansky, 03 Sep 2021
Thank you for your comment and for taking the time to review this work. We will incorporate your feedback in the final version. We agree that the criteria used to define the excursions were somewhat ambiguous, partially due to variation in the ways the CIEs are preserved in the record. We feel that the Basin Draw section introduces the most uncertainty in our interpretations for several reasons (e.g., difficulty correlating to the other sections, ambiguous carbon isotope results, and complicated cut-and-fill deposition that likely obscures the stratigraphic patterns). Taking this into account, we will be revising our interpretation to not include the ETM2 excursion in this section. Instead, we will list this as a potential interpretation and clearly describe the uncertainty surrounding this section in the text. Keeping this section in will provide a target for future work in the area that may help resolve the Basin Draw section further.
We feel that the BCM levels, while not perfect, do provide a reasonable basis for correlating between the different sections. These levels were based on rigorous bed-tracing, measurement, and mapping that represents the best available stratigraphic framework that exists for the area (Bown et al, 1994). The primary uncertainties for this framework are: (1) bed tracing is inherently difficult in this area, and (2) knowing which stratigraphic level was used for fossil localities with multiple fossiliferous levels. Also, because sections were not measured concurrently with sampling for stable isotope work, there will be some inevitable offset. We did trace additional beds while measuring our chemostratigraphic sections to confirm the relative levels between sections, and the details of this work will be added to the next version to help strengthen our stratigraphic interpretations. We also agree that the isotope record itself can be used to correlate between sections. In the example you mentioned (correlation between Kraus Flats and North Fork) the isotope correlation is consistent with the correlations using BCM levels and dGPS. The primary difference between the three methods of correlation is the relative spacing between the excursions (which is expected given the different methods, and associated uncertainties, used for measuring stratigraphic thicknesses). Our composite section takes this into account by using only the minimum isotope excursion values for each excursion (and lining them up) and then using the thickness measured using a Jacob staff. This way, it does not include the uncertainty associated with BCM levels or elevation and uses only the uncertainty associated with Jacob staff measurements. The fossil record, while promising as an additional tool for correlation, is difficult to use due to considerable variation in numbers of specimens from each locality and low numbers of localities from each section. Additionally, one of the primary goals of the study is to develop a chemostratigraphic framework that does not rely on the fossil faunas so that the previously observed faunal turnover can be independently compared to the carbon cycle record.
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AC3: 'Reply on RC2', Sarah Widlansky, 03 Sep 2021
- EC1: 'Comment on cp-2021-83', Appy Sluijs, 30 Aug 2021
Peer review completion
