the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Development of longitudinal dunes under Pangaean atmospheric circulation
- 1Deptartment of Earth and Planetary Science, Tokyo Institute of Technology, Tokyo, 152-8550, Japan
- 2Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, 152-8550, Japan
- 3Faculty of Science and Technology, Kochi University, Kochi 780-8520, Japan
- These authors contributed equally to this work.
- 1Deptartment of Earth and Planetary Science, Tokyo Institute of Technology, Tokyo, 152-8550, Japan
- 2Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, 152-8550, Japan
- 3Faculty of Science and Technology, Kochi University, Kochi 780-8520, Japan
- These authors contributed equally to this work.
Abstract. As a result of the large difference in heat capacity between land and ocean, global climate and atmospheric circulation patterns in the supercontinent Pangaea were significantly different from today. Modelling experiments have suggested the seasonal overturning of cross-equatorial Hadley circulation; however, there are large discrepancies between model-generated surface wind patterns and the reported palaeo-wind directions from aeolian dune records. Here, we present the results of measurements of spatial distribution of dune slip-face azimuths recorded in Lower Jurassic aeolian sandstones over a wide area of the western United States (palaeolatitude: ~19°–27° N). The azimuth data of dune slip-faces reveal a bi-directional and oblique angular pattern that resembles the internal structures of modern longitudinal dunes. Based on the spatial pattern of slip-face directions and outcrop evidences, we suggest most of Lower Jurassic aeolian sandstones to be NNE–SSW- to NNW–SSE- oriented longitudinal dunes, which likely formed as the result of a combination of westerly, northwesterly, and northeasterly palaeo-winds. The reconstructed palaeo-wind pattern at ~19°–27° N appears to be consistent with the model-generated surface wind pattern and its seasonal turnover. The reconstructed palaeo-wind patterns also suggest an influence of orbitally induced changes in atmospheric pressure configuration in Pangaea.
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Hiroki Shozaki and Hitoshi Hasegawa
Status: closed
-
RC1: 'Comment on cp-2021-181', Anonymous Referee #1, 28 Jan 2022
This paper is to interpret the geological evidence of slip-face azimuths of dunes recorded in Lower Jurassic aeolian sandstones in the western US. The authors provided substantial evidence of the orientation of dunes in this area, and suggested that the presence of longitudinal dunes in
the Early Jurassic desert is a result of tri-directional palaeo-wind patterns. They argued that the interpretation is consistent with model simulated atmospheric circulations. Finally, they concluded that they have solved the enigma between Pangaean atmospheric circulation patterns and aeolian dune records, and that the paleo-Colorado Plateau should be located near the NH subtropics.Overall, the geological records are solid, and their interpretation is reasonable. Therefore, I would recommend publication after minor revision. I do have some specific comments for the authors' revision.
1. A major comment is the teminology of atmospheric circulations. I would suggest authors use monsoonal circulation, instead of the Hadley circulation. The former is regional, while the latter is zonal-mean circulation. Please read Parrish (1993).
2. In some places, "palaeo" is used, while "paleo" is used in other places. It is better to integrate them.
3. Line 227: "evidences" --> evdience,
4. Line 302: the equatorial Pangea
5. Line 316: evidence .. exhibits
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CC1: 'Reply on RC1', Hiroki Shozaki, 31 Jan 2022
1. A major comment is the teminology of atmospheric circulations. I would suggest authors use monsoonal circulation, instead of the Hadley circulation. The former is regional, while the latter is zonal-mean circulation. Please read Parrish (1993).
According to the referee’s suggestion, we revised the term “cross-equatorial Hadley circulation” to “cross-equatorial flow induced by monsoonal circulation” in Line 11 and 27-28 in the revised manuscript.
2. In some places, "palaeo" is used, while "paleo" is used in other places. It is better to integrate them.
We unified into “palaeo” in Line 52, 53, 56, 126, 127, 222, 230, and 234 in the revised manuscript.
3. Line 227: "evidences" --> evdience,
We have corrected it as suggested.
4. Line 302: the equatorial Pangea
We have corrected it as suggested.
5. Line 316: evidence .. exhibits
We have corrected it as suggested.
The attachment is a potential revised manuscript, and Red characters indicate revised parts.
-
RC2: 'Reply on CC1', Anonymous Referee #1, 07 Feb 2022
The authors have addressed my comments and made changes. I have no further comments. The paper can be accepted.
- AC3: 'Reply on RC2', Hitoshi Hasegawa, 19 Apr 2022
-
RC2: 'Reply on CC1', Anonymous Referee #1, 07 Feb 2022
-
AC1: 'Reply on RC1', Hitoshi Hasegawa, 07 Feb 2022
Response to Anonymous Referee #1’s comments
1. A major comment is the teminology of atmospheric circulations. I would suggest authors use monsoonal circulation, instead of the Hadley circulation. The former is regional, while the latter is zonal-mean circulation. Please read Parrish (1993).
According to the referee’s suggestion, we revised the term “cross-equatorial Hadley circulation” to “cross-equatorial flow induced by monsoonal circulation” in Line 11 and 27-28 in the revised manuscript.
2. In some places, "palaeo" is used, while "paleo" is used in other places. It is better to integrate them.
We unified into “palaeo” in Line 52, 53, 56, 126, 127, 222, 230, and 234 in the revised manuscript.
3. Line 227: "evidences" --> evdience,
We have corrected it as suggested.
4. Line 302: the equatorial Pangea
We have corrected it as suggested.
5. Line 316: evidence .. exhibits
We have corrected it as suggested.
The attachment is a potential revised manuscript, and Red characters indicate revised parts.
-
CC1: 'Reply on RC1', Hiroki Shozaki, 31 Jan 2022
-
RC3: 'Comment on cp-2021-181', Anonymous Referee #2, 01 Mar 2022
2202 Shozaki and Hasegawa CP2021-181
Development of longitudinal dunes under Pangaean atmospheric circulation.
I found this an interesting paper dealing with a topic (dunes and climate) with which I’m familiar in a time-frame that I am not (beyond the Quaternary).
In large part the paper is well-written and structured, the evidence appears sound and the main conclusions seem to me to be well founded.
The aspects of the paper which deal with palaeo-wind directions, establishment of the aeolian sands as being deposited by longitudinal dunes, and the palaeogeographic interpretation I thought were unproblematic.
However, the paper also made some highly speculative forays into the periodicity of past aeolian events, timing and association with orbital forcing and then to reconciliation with orbitally-forced climate simulations which I thought were unsupported. Principally, the authors did not recognise that any sediment sequence (and perhaps particularly sand dunes) is incomplete. They suffer erosion almost synchronously with deposition and for long periods afterwards until deeply buried. This is quite well modelled in papers cited by the authors (Thomas and Bailey, 2017). With only very broad dating available (millions of years) it is impossible to establish if the preserved cross-bed sets had any temporal or causal association with orbital cycles. Some description of the bounding surfaces may have gone part way to answering this, but see Leighton et al, 2013, QSR; 2014 ESR, for cautionary tales. I think that this section, specifically any claims for orbital forcing, should be removed from the paper.
They detract from what is otherwise quite a clear story with a strong conclusion in which the apparent conflict between climate models and field data is resolved. This is worthy of publication. However, some parts should be rewritten (as indicated on PDF) to make clear that the model predictions are indeed being tested.
Another weakness is the current arguments suggesting a role for vegetation in stabilising the dunes and causing sand accumulation seem ill-founded. There is no parallel in the Sahara today between sand thickness and vegetation cover. Indeed, globally, where dunes are vegetated or partly vegetated sand cover is thin.
Appendix A should be elevated to the main paper. It is interesting and valuable support for the interpretation of the dunes as being longitudinal in origin. Conversely, I question the interpretation of some sites as being barchans dune deposits. I think it is impossible today to find a site where barchans dunes form in a thick sediment sequence with preservation potential. It is quite well documented that they occur where sand supply is very low and quite often on hard surfaces which aid sand transport. Furthermore, there is no modern parallel for contemporaneous and nearby barchans and longitudinal dunes to have divergent orientations (figure 1). I think the explanation for your single slip-face orientations is most likely to be a sampling issue.
Nevertheless, the overall conclusions of the paper regarding palaeogeography and circulation patterns are interesting, well documented and well written.
I have noted minor issues of grammar, spelling and word usage on the manuscript.
-
AC2: 'Reply on RC3', Hitoshi Hasegawa, 02 Apr 2022
In response to referee’s comments, we extensively revised the discussion of orbital cycles, and deleted almost all of the sentences related to orbital-scale changes of dune-fields based on Quaternary records, as well as Appendix B and Fig. B1. Instead, we focus on comparison of the observed surface wind pattern with model results. As part of this change in focus, we altered the title of section 3.2 to “Comparison of modelled and observed surface wind patterns”.
In the last part of section 3.2, we retained some discussion of the possibility of orbital-scale climatic change being recorded in aeolian depositional sequences in subtropical Pangaea, using evidence from previous studies. We disagree slightly with the referee’s claim, and believe that our finding of periodicity in boundary surfaces can be linked to several cyclic climate forcings, one of which is orbital cycles. Nevertheless, we agree that aeolian sequences are influenced by both erosion and deposition, and so the preservation of orbital-scale climatic changes is not straightforward. Thus, we added such discussion to the end of the section. For more details, please see attached response letter and revised manuscript.
-
AC2: 'Reply on RC3', Hitoshi Hasegawa, 02 Apr 2022
Status: closed
-
RC1: 'Comment on cp-2021-181', Anonymous Referee #1, 28 Jan 2022
This paper is to interpret the geological evidence of slip-face azimuths of dunes recorded in Lower Jurassic aeolian sandstones in the western US. The authors provided substantial evidence of the orientation of dunes in this area, and suggested that the presence of longitudinal dunes in
the Early Jurassic desert is a result of tri-directional palaeo-wind patterns. They argued that the interpretation is consistent with model simulated atmospheric circulations. Finally, they concluded that they have solved the enigma between Pangaean atmospheric circulation patterns and aeolian dune records, and that the paleo-Colorado Plateau should be located near the NH subtropics.Overall, the geological records are solid, and their interpretation is reasonable. Therefore, I would recommend publication after minor revision. I do have some specific comments for the authors' revision.
1. A major comment is the teminology of atmospheric circulations. I would suggest authors use monsoonal circulation, instead of the Hadley circulation. The former is regional, while the latter is zonal-mean circulation. Please read Parrish (1993).
2. In some places, "palaeo" is used, while "paleo" is used in other places. It is better to integrate them.
3. Line 227: "evidences" --> evdience,
4. Line 302: the equatorial Pangea
5. Line 316: evidence .. exhibits
-
CC1: 'Reply on RC1', Hiroki Shozaki, 31 Jan 2022
1. A major comment is the teminology of atmospheric circulations. I would suggest authors use monsoonal circulation, instead of the Hadley circulation. The former is regional, while the latter is zonal-mean circulation. Please read Parrish (1993).
According to the referee’s suggestion, we revised the term “cross-equatorial Hadley circulation” to “cross-equatorial flow induced by monsoonal circulation” in Line 11 and 27-28 in the revised manuscript.
2. In some places, "palaeo" is used, while "paleo" is used in other places. It is better to integrate them.
We unified into “palaeo” in Line 52, 53, 56, 126, 127, 222, 230, and 234 in the revised manuscript.
3. Line 227: "evidences" --> evdience,
We have corrected it as suggested.
4. Line 302: the equatorial Pangea
We have corrected it as suggested.
5. Line 316: evidence .. exhibits
We have corrected it as suggested.
The attachment is a potential revised manuscript, and Red characters indicate revised parts.
-
RC2: 'Reply on CC1', Anonymous Referee #1, 07 Feb 2022
The authors have addressed my comments and made changes. I have no further comments. The paper can be accepted.
- AC3: 'Reply on RC2', Hitoshi Hasegawa, 19 Apr 2022
-
RC2: 'Reply on CC1', Anonymous Referee #1, 07 Feb 2022
-
AC1: 'Reply on RC1', Hitoshi Hasegawa, 07 Feb 2022
Response to Anonymous Referee #1’s comments
1. A major comment is the teminology of atmospheric circulations. I would suggest authors use monsoonal circulation, instead of the Hadley circulation. The former is regional, while the latter is zonal-mean circulation. Please read Parrish (1993).
According to the referee’s suggestion, we revised the term “cross-equatorial Hadley circulation” to “cross-equatorial flow induced by monsoonal circulation” in Line 11 and 27-28 in the revised manuscript.
2. In some places, "palaeo" is used, while "paleo" is used in other places. It is better to integrate them.
We unified into “palaeo” in Line 52, 53, 56, 126, 127, 222, 230, and 234 in the revised manuscript.
3. Line 227: "evidences" --> evdience,
We have corrected it as suggested.
4. Line 302: the equatorial Pangea
We have corrected it as suggested.
5. Line 316: evidence .. exhibits
We have corrected it as suggested.
The attachment is a potential revised manuscript, and Red characters indicate revised parts.
-
CC1: 'Reply on RC1', Hiroki Shozaki, 31 Jan 2022
-
RC3: 'Comment on cp-2021-181', Anonymous Referee #2, 01 Mar 2022
2202 Shozaki and Hasegawa CP2021-181
Development of longitudinal dunes under Pangaean atmospheric circulation.
I found this an interesting paper dealing with a topic (dunes and climate) with which I’m familiar in a time-frame that I am not (beyond the Quaternary).
In large part the paper is well-written and structured, the evidence appears sound and the main conclusions seem to me to be well founded.
The aspects of the paper which deal with palaeo-wind directions, establishment of the aeolian sands as being deposited by longitudinal dunes, and the palaeogeographic interpretation I thought were unproblematic.
However, the paper also made some highly speculative forays into the periodicity of past aeolian events, timing and association with orbital forcing and then to reconciliation with orbitally-forced climate simulations which I thought were unsupported. Principally, the authors did not recognise that any sediment sequence (and perhaps particularly sand dunes) is incomplete. They suffer erosion almost synchronously with deposition and for long periods afterwards until deeply buried. This is quite well modelled in papers cited by the authors (Thomas and Bailey, 2017). With only very broad dating available (millions of years) it is impossible to establish if the preserved cross-bed sets had any temporal or causal association with orbital cycles. Some description of the bounding surfaces may have gone part way to answering this, but see Leighton et al, 2013, QSR; 2014 ESR, for cautionary tales. I think that this section, specifically any claims for orbital forcing, should be removed from the paper.
They detract from what is otherwise quite a clear story with a strong conclusion in which the apparent conflict between climate models and field data is resolved. This is worthy of publication. However, some parts should be rewritten (as indicated on PDF) to make clear that the model predictions are indeed being tested.
Another weakness is the current arguments suggesting a role for vegetation in stabilising the dunes and causing sand accumulation seem ill-founded. There is no parallel in the Sahara today between sand thickness and vegetation cover. Indeed, globally, where dunes are vegetated or partly vegetated sand cover is thin.
Appendix A should be elevated to the main paper. It is interesting and valuable support for the interpretation of the dunes as being longitudinal in origin. Conversely, I question the interpretation of some sites as being barchans dune deposits. I think it is impossible today to find a site where barchans dunes form in a thick sediment sequence with preservation potential. It is quite well documented that they occur where sand supply is very low and quite often on hard surfaces which aid sand transport. Furthermore, there is no modern parallel for contemporaneous and nearby barchans and longitudinal dunes to have divergent orientations (figure 1). I think the explanation for your single slip-face orientations is most likely to be a sampling issue.
Nevertheless, the overall conclusions of the paper regarding palaeogeography and circulation patterns are interesting, well documented and well written.
I have noted minor issues of grammar, spelling and word usage on the manuscript.
-
AC2: 'Reply on RC3', Hitoshi Hasegawa, 02 Apr 2022
In response to referee’s comments, we extensively revised the discussion of orbital cycles, and deleted almost all of the sentences related to orbital-scale changes of dune-fields based on Quaternary records, as well as Appendix B and Fig. B1. Instead, we focus on comparison of the observed surface wind pattern with model results. As part of this change in focus, we altered the title of section 3.2 to “Comparison of modelled and observed surface wind patterns”.
In the last part of section 3.2, we retained some discussion of the possibility of orbital-scale climatic change being recorded in aeolian depositional sequences in subtropical Pangaea, using evidence from previous studies. We disagree slightly with the referee’s claim, and believe that our finding of periodicity in boundary surfaces can be linked to several cyclic climate forcings, one of which is orbital cycles. Nevertheless, we agree that aeolian sequences are influenced by both erosion and deposition, and so the preservation of orbital-scale climatic changes is not straightforward. Thus, we added such discussion to the end of the section. For more details, please see attached response letter and revised manuscript.
-
AC2: 'Reply on RC3', Hitoshi Hasegawa, 02 Apr 2022
Hiroki Shozaki and Hitoshi Hasegawa
Hiroki Shozaki and Hitoshi Hasegawa
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