the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Mar 2023
Dynamic environment but no temperature change since the late Paleogene at Lühe Basin (Yunnan, China)
Abstract. The complex tectonic evolution in the Tibetan region has impacted climate, the Asian monsoon system, and the development of major biodiversity hotspots, especially since the onset of the India-Eurasia continental collision during the early Paleogene. Untangling the links between the geologic, climatic, and ecological history of the broader region can provide insights into these Earth system mechanisms, relevant for the future of our rapidly changing planet. To better understand environmental conditions across this critical time and place, we reconstruct the climatic and environmental history from a key sedimentary repository within the Lühe Basin, Yunnan, China, uniquely located between high elevation Tibet and low elevation coastal China. We investigate a 340-m long section using a multi-proxy organic geochemistry approach, complemented with sedimentological interpretations and climate model simulations. The complementary organic geochemical proxies, including n-alkanes, terpenoids, and hopanes, suggest that these thermally immature sediments were deposited in a dynamic environment that fluctuated between low energy floodplains and high energy fluvial systems. Our branched glycerol diakyl glycerol tetraether-based proxies indicate terrestrial temperatures of around 17 °C ± 3 SD and our model-based temperatures indicate terrestrial temperatures for Chattian of around 19 °C, consistent with the literature palaeobotany-based temperatures from the nearby Lühe town section. These combined palaeotemperatures match present-day values, suggesting that this area has not undergone significant temperature change since the late Paleogene.
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RC1: 'Comment on egusphere-2023-373', Anonymous Referee #1, 03 May 2023
Witkowski et al. reported multiple biomarker records from a late Paleogene section from the Luhe Basin on the southeastern Tibetan Plateau. The biomarker indicators include TOC, n-alkanes, hopanes, terpenoids, and brGDGTs. The section may or may not include the Eocene-Oligocene Transition (EOT) around 33.9 Ma. Based on these biomarker records, authors infer that regional vegetation did not change significantly over the study period (likely the Rupelian), while the depositional environment changed dramatically from a lower energy flood basin, wetland/peatland and/or shallow lake to a higher energy flood plain and/or channel. The brGDGT-based MBT’5me temperature record shows high variability but does not show particular long-term trending. The authors conclude that the regional temperature has not changed much since the late Paleogene (with the support of model simulations), despite the dynamic changes in depositional environment.
The tectonic evolution in the Tibetan region has affected the Asian monsoon system and global climate, as well as biodiversity. However, the links among them is far from being understood. This study, reporting climatic and environmental changes during the Rupelian period from the southeastern Tibetan Plateau, would definitely contribute to our knowledge on this topic, and should be of broad interest to scientific community. This manuscript is worth to be considered by the journal. I provide a few comments below for authors’ consideration.
- Authors stated that the regional temperature during the Rupelian period was close to present-day value, which is also reflected in the title. Although authors cited paleobotany- and model-based results to support their claim, it is quite difficult to accept this claim As in authors’ simulations, regional temperature cooled by ~6C from 4x to 2x preindustrial pCO2 (with topographic effect contributing little), it is hard to imagine the regional temperature would have remained the same when pCO2 dropped from 2X to the preindustrial value. I would suggest that authors might check their 2x simulation to find out why temperature did not change significantly from the preindustrial value and provide such an explanation in their text. If such an explanation is not available, then authors might consider weakening this statement a bit.
- Authors provided two possibilities that the EOT may or may not be present in their studied section. However, I note that at the bottom of their stratigraphic column, Chron C15n, which is around 35 Ma, appears to be well defined. Authors do not have much confidence in this chron? Also, the study period covers the most of Rupelian period, not Chattian period authors claimed (although simulation results for the Chattian period should be OK for comparison with authors’ results).
- Authors attributed the large variability in MBT’5me, ~0.3 unit which is equivalent to 10-15 C temperature changes to the mixture of in situ vs. allochthonous brGDGTs. Could authors compare the MBT’5me with one of the environmental indicators (for instance, Paq?) to see if a possible relationship could be found? This is quite important as authors stated that depositional environmental changed from earlier to later time. That is, such depositional environment changes could have contributed to/resulted in the no overall trending. Similarly, authors in one place (around Line 360) mentioned the increase in 6-methyl brGDGTs, which could also be verified by authors’ CBT/pH record. Authors might be aware that the increase in 6-methyl brGDGTs, i.e., the change of IR6me value, could significantly affect MBT’5me value, as reported in recent studies, for instance, Wu et al., 2021, CG, https://doi.org/10.1016/j.chemgeo.2021.120348 and Wang et al., 2021, GCA, https://doi.org/10.1016/j.gca.2021.05.004. Authors are strongly suggested to evaluate this effect. My sense is that MBT’5me value could be lowered a bit if this effect is corrected, based on the information authors provided. If correct, authors may not need to present two possibilities whether EOT is present or not (my Point #2), and authors might see relatively high temperature at the lower part, whihc could represent the late Eocene interval. Authors might also want to look at the MBT’ index.
- A recent study He et al., 2022, Science Bulletin https://doi.org/10.1016/j.scib.2022.10.006 might be added to support the view that this region might have reached present elevation since the late Eocene.
- Some explanation should be provided for the lithologic column in Fig. 2. pH reconstruction is not plotted in Fig 6 but stated so in figure caption.
Citation: https://doi.org/10.5194/egusphere-2023-373-RC1 - AC1: 'Reply on RC1', Caitlyn Witkowski, 05 Oct 2023
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RC2: 'Comment on egusphere-2023-373', Anonymous Referee #2, 10 May 2023
Witkowski et al. reconstructed the temperature and deposition environment at a site in Yunnan, China, using multiple biomarkers. Their reconstruction indicates that the temperature at this location was similar to today at ~33 Ma, which may imply that its altitude might have reached similar height to today. Because this site is in the southeastern margin of the Tibetan Plateau, the results may be of value to people who study the evolution of the Tibetan Plateau.
The part of manuscript that is related to biomarkers is well written but the part related to climate modeling is quite poor. Lots of information are missing. I could not find even a single relevant figure which made it difficult to understand how the experiments were setup. The results of the climate modeling were not used in a meaningful way either, making me wonder why bother carrying out that many model simulations. Detailed questions and comments are listed below.
- A figure is certainly needed in order for the readers to understand how the model experiments were set up. For example, what do they mean by a constant valley or plateau (near line 195)? What do the corresponding results tell us?
- Since the model results show that there should be a large change of annual mean temperature (~6 °C) when CO2 is changed from 4x to 2x, shouldn’t they indicate that the section does not include the Eocene-Oligocene Transition (EOT)?
- In my opinion, the results are insufficient to claim that the altitude of the site had reached present-day value based on only the surface temperature. Many other factors could impact on the temperature of a local region. For example, did the latitude of the site change much between the Early Oligocene and PI? Was there a large change in the temperature of South China Sea? Was the region more cloudy during the Early Oligocene than in PI?
- Some of the co-authors had published climate evolution for the past 100 million years, it should be convenient to look at how the temperature at this site had changed during the past 30 million years. Similar data were also published by Li et al. (2022, Scientific Data, https://doi.org/10.1038/s41597-022-01490-4).
- The authors may also look at how the precipitation changed at the site from the early to middle Oligocene from the two datasets mentioned above since it is directly related to their reconstruction regarding the deposition environment.
Minor comments
WMMT and CMMT are not defined, are they the temperatures for the warmest and coldest month or the average of a few months. While at lines 445-446, the dry or wet months are defined
L190: downscaled -> upscaled?
L397: increases -> varies? Otherwise, I do not understand why the precipitation always increases.
L445: what are the modeled precipitation?
L449: where -> were
Figs. 4-6, in all three figures, the Chrons C12n are located above C11n, inconsistent with Fig. 2.
Citation: https://doi.org/10.5194/egusphere-2023-373-RC2 - AC2: 'Reply on RC2', Caitlyn Witkowski, 05 Oct 2023
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RC3: 'Comment on egusphere-2023-373', Anonymous Referee #3, 22 Aug 2023
In this manuscript, the authors investigated a 340-m long section in the Lühe Basin in Yunnan, China, located along the SE margin of the Tibetan Plateau. Based on analyses of organic geochemical proxies and model simulations, the authors reached several conclusions: (1) the sediments in the Lühe section were deposited in a highly variable and dynamic environment; (2) there was no significant temperature change (no cooling trend) at the study site between 35 Myr and 27 Myr; and (3) their reconstructed temperatures were similar to the present-day, suggesting that the study area has reached its present elevation since at least the early Oligocene. The topic of this study is interesting and important. The investigation of the sediments from the SE margin of the Tibetan Plateau could contribute to a better understanding of the complex tectonic and environmental history of the Plateau. However, there seem some major problems in the manuscript, which makes the conclusions (2) and (3) unconvincing.
In general, there is lack of discussion and information on the limitation and uncertainty of various temperature reconstructions used in this study. It is difficult to judge whether different reconstructions are comparable, whether the past and the present are comparable, and whether the conclusions are reliable. From some information given in the main text and Fig.6, it is difficult to conclude that there is no trend in temperature change over the study period.
Some model results are hard to understand (see specific comments below), so using the model results to support the GDGTs-estimated temperature is unconvincing.
Given the many uncertainties and doubts detailed in my comments below, conclusions (2) and (3) are too strong. Moreover, regarding the elevation of the Tibetan Plateau, many other studies suggest that rapid elevation increase in SE Tibet had happened during the Miocene (e.g. Clark et al 2005, doi: 10.1130/G21265.1; Royden et al 2008, 10.1126/science.1155371), which contradicts the conclusion (3).
My specific comments and questions are listed below:
1. About proxy problems:
- Regarding temperature estimation from brGDGTs, to which extent could the global calibration be applied at the study site and for such an old time period, and what could be the uncertainty and limitation?
- It is written that “the GDGTs in our section could have been produced in mineral soils, peats, or a shallow lake environment based on our environmental reconstructions”. Then what is the exact meaning of MAAT_soil, MAAT_peat and MAAT_lake? They are assumed to reflect the temperature of the soil, peat and lake water where the GDGTs were produced, instead of air temperature. If the present-day MAAT and the simulated MAAT are air temperature, they would not be comparable with the GDGTs-based temperatures.
- The authors conclude that “our observations indicate a dynamic but evolving fluvial-lacustrine environment throughout the entire section”. So to which extent the reconstructed temperature reflects in-situ temperature or temperature at remote source region is uncertain. The comparison with the present-day temperature at the study site is therefore questionable.
- Given the highly variable depositional environment at the study site, it is difficult to understand why only the lake-calibrated temperature is considered. Given the marked difference between the lower (0-73 m) and upper part (73-340 m), should different calibration methods be used at least for the upper and lower parts? Could this affect the conclusion?
- I read from Fig.6 that the uncertainty of the lake MAF is ~8 degree C (shown by blue horizontal bars). Given such a large uncertainty, it is impossible to conclude whether there is a trend or not over the study period. Many studies cited in the paragraph of line 411 suggest a cooling across EOT of only a few degree C. If there was also a cooling trend of a few degree C at the study site, it could not be reflected in the GDGTs temperature reconstruction because of the large uncertainty and large variability.
- The authors state that “The temperature trends throughout the section show variability, possibly due to mixing of in situ and allochthonous sources within the rapidly changing and dynamic depositional environment (line 382)”. So the lack of temperature trend over the 8 Myr at the study site could be due to mixing and large variability but not really related to climate change.
- As the brGDGT indices exhibit large variability throughout the section and the MAF_lake has a range from 11.8 to 22.2°C, which covers a wide range of possible temperatures, considering only the mean temperature of 17.3°C in the comparison with the present-day temperature and the simulated one does not make sense.
- In Fig.6, there are only five points for peat brGDGT MAAT and no information on MAAT_soil, so it is impossible to get trend information from these two reconstructions. The BA and CLAMP MAAT are only an averaged estimate for a thick portion (70-130m), so they can not indicate trend.2. About model results
- Line 400-402: It is difficult to understand that the simulated temperature at the study site is not sensitive to a large change of the Tibetan topography from 2.5-km valley to 4.5-km plateau. This shows that the temperature at the study site is not sensitive to the change of the Tibetan topography. Then it would not make sense to use the reconstructed or simulated temperature at the study site to indicate the Tibetan elevation.
- Line 397: “In all model simulations, mean annual precipitation (MAP) increases between 150-200 mm/yr but does not significantly vary amongst different pCO2 values nor topographic configurations.”: It is difficult to understand that the large boundary condition changes have no significant influence on MAP. It means that in the model, the MAP in the Asian region is not sensitive to these condition changes. Then what could have caused the changes in hydrology and overall energy of the system in the study region (line 458-459).
- The simulated CMMT is 12.1°C, and the CLAMP reconstructed CMMT is 4.5°C. Given such a big difference, which result is more reliable, the model or the CLAMP?
- Line 390: What is the purpose to analyze the results for such a broad Asian region? Its relevance for the current study is unclear. In Table 1, it would be more relevant to show the model results for the Lühe basin instead of the very broad Asian region.
- Line 186-189: It would be helpful to show a figure with the present-day climate simulation for the Lühe basin and the comparison with observation.
- Line 190: It would be helpful to show the boundary conditions on the model resolution to show to which extent the model could resolve the regional orography condition at the Lühe basin and the SE Tibetan Plateau region.
- It would be helpful to give more explanation on experiment setup and provide significance test for the simulated changes of temperature and precipitation. What does it mean "valley, plateau, v-to-PI", please give more information and illustrate with figures.3. About temperature response to CO2 decrease:
The CO2 concentration decreased from 1120 ppm for the Priabonian to 560 ppm for the Rupelian, and the model simulates a cooling of ~6C for the broad Asian region (Table 1) in response to the CO2 decrease. However, based on the GDGTs reconstructed temperature, the authors conclude that there was no significant temperature change (no cooling trend) over the 8Myr period at the study site. It is difficult to understand that such a large CO2 decrease has no significant impact on the temperature at the Lühe basin. With a high elevation, the temperature of this region would be more sensitive to the large CO2 decrease than low-elevated region. Does the model simulate a cooling at the Lühe basin in response to the large CO2 decrease?Other comments:
- As the sediments at the Lühe coalmine section were deposited in a highly dynamic and variable environment that fluctuated between low energy floodplains and high energy fluvial systems, it would be important to prove the continuality of the deposition in the section.
- Line 435: What would be the local factors?Citation: https://doi.org/10.5194/egusphere-2023-373-RC3 - AC3: 'Reply on RC3', Caitlyn Witkowski, 05 Oct 2023
Status: closed
-
RC1: 'Comment on egusphere-2023-373', Anonymous Referee #1, 03 May 2023
Witkowski et al. reported multiple biomarker records from a late Paleogene section from the Luhe Basin on the southeastern Tibetan Plateau. The biomarker indicators include TOC, n-alkanes, hopanes, terpenoids, and brGDGTs. The section may or may not include the Eocene-Oligocene Transition (EOT) around 33.9 Ma. Based on these biomarker records, authors infer that regional vegetation did not change significantly over the study period (likely the Rupelian), while the depositional environment changed dramatically from a lower energy flood basin, wetland/peatland and/or shallow lake to a higher energy flood plain and/or channel. The brGDGT-based MBT’5me temperature record shows high variability but does not show particular long-term trending. The authors conclude that the regional temperature has not changed much since the late Paleogene (with the support of model simulations), despite the dynamic changes in depositional environment.
The tectonic evolution in the Tibetan region has affected the Asian monsoon system and global climate, as well as biodiversity. However, the links among them is far from being understood. This study, reporting climatic and environmental changes during the Rupelian period from the southeastern Tibetan Plateau, would definitely contribute to our knowledge on this topic, and should be of broad interest to scientific community. This manuscript is worth to be considered by the journal. I provide a few comments below for authors’ consideration.
- Authors stated that the regional temperature during the Rupelian period was close to present-day value, which is also reflected in the title. Although authors cited paleobotany- and model-based results to support their claim, it is quite difficult to accept this claim As in authors’ simulations, regional temperature cooled by ~6C from 4x to 2x preindustrial pCO2 (with topographic effect contributing little), it is hard to imagine the regional temperature would have remained the same when pCO2 dropped from 2X to the preindustrial value. I would suggest that authors might check their 2x simulation to find out why temperature did not change significantly from the preindustrial value and provide such an explanation in their text. If such an explanation is not available, then authors might consider weakening this statement a bit.
- Authors provided two possibilities that the EOT may or may not be present in their studied section. However, I note that at the bottom of their stratigraphic column, Chron C15n, which is around 35 Ma, appears to be well defined. Authors do not have much confidence in this chron? Also, the study period covers the most of Rupelian period, not Chattian period authors claimed (although simulation results for the Chattian period should be OK for comparison with authors’ results).
- Authors attributed the large variability in MBT’5me, ~0.3 unit which is equivalent to 10-15 C temperature changes to the mixture of in situ vs. allochthonous brGDGTs. Could authors compare the MBT’5me with one of the environmental indicators (for instance, Paq?) to see if a possible relationship could be found? This is quite important as authors stated that depositional environmental changed from earlier to later time. That is, such depositional environment changes could have contributed to/resulted in the no overall trending. Similarly, authors in one place (around Line 360) mentioned the increase in 6-methyl brGDGTs, which could also be verified by authors’ CBT/pH record. Authors might be aware that the increase in 6-methyl brGDGTs, i.e., the change of IR6me value, could significantly affect MBT’5me value, as reported in recent studies, for instance, Wu et al., 2021, CG, https://doi.org/10.1016/j.chemgeo.2021.120348 and Wang et al., 2021, GCA, https://doi.org/10.1016/j.gca.2021.05.004. Authors are strongly suggested to evaluate this effect. My sense is that MBT’5me value could be lowered a bit if this effect is corrected, based on the information authors provided. If correct, authors may not need to present two possibilities whether EOT is present or not (my Point #2), and authors might see relatively high temperature at the lower part, whihc could represent the late Eocene interval. Authors might also want to look at the MBT’ index.
- A recent study He et al., 2022, Science Bulletin https://doi.org/10.1016/j.scib.2022.10.006 might be added to support the view that this region might have reached present elevation since the late Eocene.
- Some explanation should be provided for the lithologic column in Fig. 2. pH reconstruction is not plotted in Fig 6 but stated so in figure caption.
Citation: https://doi.org/10.5194/egusphere-2023-373-RC1 - AC1: 'Reply on RC1', Caitlyn Witkowski, 05 Oct 2023
-
RC2: 'Comment on egusphere-2023-373', Anonymous Referee #2, 10 May 2023
Witkowski et al. reconstructed the temperature and deposition environment at a site in Yunnan, China, using multiple biomarkers. Their reconstruction indicates that the temperature at this location was similar to today at ~33 Ma, which may imply that its altitude might have reached similar height to today. Because this site is in the southeastern margin of the Tibetan Plateau, the results may be of value to people who study the evolution of the Tibetan Plateau.
The part of manuscript that is related to biomarkers is well written but the part related to climate modeling is quite poor. Lots of information are missing. I could not find even a single relevant figure which made it difficult to understand how the experiments were setup. The results of the climate modeling were not used in a meaningful way either, making me wonder why bother carrying out that many model simulations. Detailed questions and comments are listed below.
- A figure is certainly needed in order for the readers to understand how the model experiments were set up. For example, what do they mean by a constant valley or plateau (near line 195)? What do the corresponding results tell us?
- Since the model results show that there should be a large change of annual mean temperature (~6 °C) when CO2 is changed from 4x to 2x, shouldn’t they indicate that the section does not include the Eocene-Oligocene Transition (EOT)?
- In my opinion, the results are insufficient to claim that the altitude of the site had reached present-day value based on only the surface temperature. Many other factors could impact on the temperature of a local region. For example, did the latitude of the site change much between the Early Oligocene and PI? Was there a large change in the temperature of South China Sea? Was the region more cloudy during the Early Oligocene than in PI?
- Some of the co-authors had published climate evolution for the past 100 million years, it should be convenient to look at how the temperature at this site had changed during the past 30 million years. Similar data were also published by Li et al. (2022, Scientific Data, https://doi.org/10.1038/s41597-022-01490-4).
- The authors may also look at how the precipitation changed at the site from the early to middle Oligocene from the two datasets mentioned above since it is directly related to their reconstruction regarding the deposition environment.
Minor comments
WMMT and CMMT are not defined, are they the temperatures for the warmest and coldest month or the average of a few months. While at lines 445-446, the dry or wet months are defined
L190: downscaled -> upscaled?
L397: increases -> varies? Otherwise, I do not understand why the precipitation always increases.
L445: what are the modeled precipitation?
L449: where -> were
Figs. 4-6, in all three figures, the Chrons C12n are located above C11n, inconsistent with Fig. 2.
Citation: https://doi.org/10.5194/egusphere-2023-373-RC2 - AC2: 'Reply on RC2', Caitlyn Witkowski, 05 Oct 2023
-
RC3: 'Comment on egusphere-2023-373', Anonymous Referee #3, 22 Aug 2023
In this manuscript, the authors investigated a 340-m long section in the Lühe Basin in Yunnan, China, located along the SE margin of the Tibetan Plateau. Based on analyses of organic geochemical proxies and model simulations, the authors reached several conclusions: (1) the sediments in the Lühe section were deposited in a highly variable and dynamic environment; (2) there was no significant temperature change (no cooling trend) at the study site between 35 Myr and 27 Myr; and (3) their reconstructed temperatures were similar to the present-day, suggesting that the study area has reached its present elevation since at least the early Oligocene. The topic of this study is interesting and important. The investigation of the sediments from the SE margin of the Tibetan Plateau could contribute to a better understanding of the complex tectonic and environmental history of the Plateau. However, there seem some major problems in the manuscript, which makes the conclusions (2) and (3) unconvincing.
In general, there is lack of discussion and information on the limitation and uncertainty of various temperature reconstructions used in this study. It is difficult to judge whether different reconstructions are comparable, whether the past and the present are comparable, and whether the conclusions are reliable. From some information given in the main text and Fig.6, it is difficult to conclude that there is no trend in temperature change over the study period.
Some model results are hard to understand (see specific comments below), so using the model results to support the GDGTs-estimated temperature is unconvincing.
Given the many uncertainties and doubts detailed in my comments below, conclusions (2) and (3) are too strong. Moreover, regarding the elevation of the Tibetan Plateau, many other studies suggest that rapid elevation increase in SE Tibet had happened during the Miocene (e.g. Clark et al 2005, doi: 10.1130/G21265.1; Royden et al 2008, 10.1126/science.1155371), which contradicts the conclusion (3).
My specific comments and questions are listed below:
1. About proxy problems:
- Regarding temperature estimation from brGDGTs, to which extent could the global calibration be applied at the study site and for such an old time period, and what could be the uncertainty and limitation?
- It is written that “the GDGTs in our section could have been produced in mineral soils, peats, or a shallow lake environment based on our environmental reconstructions”. Then what is the exact meaning of MAAT_soil, MAAT_peat and MAAT_lake? They are assumed to reflect the temperature of the soil, peat and lake water where the GDGTs were produced, instead of air temperature. If the present-day MAAT and the simulated MAAT are air temperature, they would not be comparable with the GDGTs-based temperatures.
- The authors conclude that “our observations indicate a dynamic but evolving fluvial-lacustrine environment throughout the entire section”. So to which extent the reconstructed temperature reflects in-situ temperature or temperature at remote source region is uncertain. The comparison with the present-day temperature at the study site is therefore questionable.
- Given the highly variable depositional environment at the study site, it is difficult to understand why only the lake-calibrated temperature is considered. Given the marked difference between the lower (0-73 m) and upper part (73-340 m), should different calibration methods be used at least for the upper and lower parts? Could this affect the conclusion?
- I read from Fig.6 that the uncertainty of the lake MAF is ~8 degree C (shown by blue horizontal bars). Given such a large uncertainty, it is impossible to conclude whether there is a trend or not over the study period. Many studies cited in the paragraph of line 411 suggest a cooling across EOT of only a few degree C. If there was also a cooling trend of a few degree C at the study site, it could not be reflected in the GDGTs temperature reconstruction because of the large uncertainty and large variability.
- The authors state that “The temperature trends throughout the section show variability, possibly due to mixing of in situ and allochthonous sources within the rapidly changing and dynamic depositional environment (line 382)”. So the lack of temperature trend over the 8 Myr at the study site could be due to mixing and large variability but not really related to climate change.
- As the brGDGT indices exhibit large variability throughout the section and the MAF_lake has a range from 11.8 to 22.2°C, which covers a wide range of possible temperatures, considering only the mean temperature of 17.3°C in the comparison with the present-day temperature and the simulated one does not make sense.
- In Fig.6, there are only five points for peat brGDGT MAAT and no information on MAAT_soil, so it is impossible to get trend information from these two reconstructions. The BA and CLAMP MAAT are only an averaged estimate for a thick portion (70-130m), so they can not indicate trend.2. About model results
- Line 400-402: It is difficult to understand that the simulated temperature at the study site is not sensitive to a large change of the Tibetan topography from 2.5-km valley to 4.5-km plateau. This shows that the temperature at the study site is not sensitive to the change of the Tibetan topography. Then it would not make sense to use the reconstructed or simulated temperature at the study site to indicate the Tibetan elevation.
- Line 397: “In all model simulations, mean annual precipitation (MAP) increases between 150-200 mm/yr but does not significantly vary amongst different pCO2 values nor topographic configurations.”: It is difficult to understand that the large boundary condition changes have no significant influence on MAP. It means that in the model, the MAP in the Asian region is not sensitive to these condition changes. Then what could have caused the changes in hydrology and overall energy of the system in the study region (line 458-459).
- The simulated CMMT is 12.1°C, and the CLAMP reconstructed CMMT is 4.5°C. Given such a big difference, which result is more reliable, the model or the CLAMP?
- Line 390: What is the purpose to analyze the results for such a broad Asian region? Its relevance for the current study is unclear. In Table 1, it would be more relevant to show the model results for the Lühe basin instead of the very broad Asian region.
- Line 186-189: It would be helpful to show a figure with the present-day climate simulation for the Lühe basin and the comparison with observation.
- Line 190: It would be helpful to show the boundary conditions on the model resolution to show to which extent the model could resolve the regional orography condition at the Lühe basin and the SE Tibetan Plateau region.
- It would be helpful to give more explanation on experiment setup and provide significance test for the simulated changes of temperature and precipitation. What does it mean "valley, plateau, v-to-PI", please give more information and illustrate with figures.3. About temperature response to CO2 decrease:
The CO2 concentration decreased from 1120 ppm for the Priabonian to 560 ppm for the Rupelian, and the model simulates a cooling of ~6C for the broad Asian region (Table 1) in response to the CO2 decrease. However, based on the GDGTs reconstructed temperature, the authors conclude that there was no significant temperature change (no cooling trend) over the 8Myr period at the study site. It is difficult to understand that such a large CO2 decrease has no significant impact on the temperature at the Lühe basin. With a high elevation, the temperature of this region would be more sensitive to the large CO2 decrease than low-elevated region. Does the model simulate a cooling at the Lühe basin in response to the large CO2 decrease?Other comments:
- As the sediments at the Lühe coalmine section were deposited in a highly dynamic and variable environment that fluctuated between low energy floodplains and high energy fluvial systems, it would be important to prove the continuality of the deposition in the section.
- Line 435: What would be the local factors?Citation: https://doi.org/10.5194/egusphere-2023-373-RC3 - AC3: 'Reply on RC3', Caitlyn Witkowski, 05 Oct 2023
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