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the Creative Commons Attribution 4.0 License.
| 04 Mar 2021
Tectonics, Climate and Topography: Oxygen stable isotopes and the early Eocene growth of the Pyrenees
Abstract. The topographic history of an orogen, a key element to study the interactions of the climate and tectonic conditions that drove it, can be reconstructed by inverting the sedimentary record of its adjacent basins. Previous tectono-stratigraphic studies, including flexural models, and sparse stable oxygen and carbon isotope data from the South-Pyrenean foreland basin suggest a major topographic rise occurred in the late Paleocene-early Eocene. To further test this hypothesis, we present a stack of 658 stable isotope measurements on whole-rock marine carbonate mudstone from a 4800-m-thick composite sedimentary succession which provides a 12 Ma continuous record of environmental conditions during the early to middle Eocene (54 to 42 Ma). From the base of this record (at 54 Ma), oxygen isotopes (δ18O values) show a faster decrease rate than the coeval global negative excursion associated with the Early Eocene Climatic Optimum (EECO). This local alteration of the global δ18O signal indicates that topographic growth during this period, associated with significant tectonic activity, perturbed the oxygen isotopic composition of foreland waters. Thus, our data suggest that significant topographic uplift of the Pyrenean orogen started from at least 54 Ma and continued until ca. 49 Ma, reaching the maximum elevations of 2000 ± 500 m in this phase from previous isotope and flexural studies. In addition, our record shows that the long-term carbon stable isotope composition during this period remained relatively stable with no similarity to the global bell-shaped long-term trend of the EECO. This is consistent with the restricted physiography of the South-Pyrenean foreland basin, mainly influenced by local sedimentary and water inputs. Overall, the Pyrenean topographic growth from the late Cretaceous to the Miocene displays several growth stages that seem to be primarily determined by episodes of increased rate of tectonic plate convergence. The duration of these growth stages (several millions of years) is a possible documentation of the response time of mountain ranges to tectonic perturbations. The results of this work, therefore, demonstrate that stable isotope measurements on whole-rock sediments in foreland basins can provide key information for tectono-climatic and topographic reconstructions of mountain ranges.
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Interactive discussion
Status: closed
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RC1: 'Comment on se-2021-12', Anonymous Referee #1, 24 Jun 2021
I read with great interest the paper by Honegger et al. The introduction is well written, and I agree that the topic of orogenic evolution is important. The authors have framed the problem as a Raileigh distillation exercise, whereby the d18O of carbonates can be used as a proxy for elevation. This is commonly done in continental tectonic using soil nodules (see for instance Huntington et al, and Jay Quads et al). This paper is however using marine carbonates previously shown to contain a record of orogenic effect on d18O, but they do this at a much higher temporal resolution than previously published.
Overall this is an easy to read, paper, and a nice narrative between oxygen and carbon isotopes and tectonics. I like the fact that the authors have independent magnetostratigraphy and that they use these as the only tie points in Analyseries: it gives me confidence that their age model is probably correct (lines 219-220)and that comparison with the global stack of Cramers et al is not a chicken and egg problem where isotope trends are matched first, and then their correlation discussed.
But as an isotope geochemist, I am not fully convinced with the fact that the data actually supports the authors story. The story *might* be correct, but it might not. And many details were omitted, or simply not fully discussed in this manuscript. Therefore I recommend some major revisions on the stable isotope side..
The first block to me being fully convinced is the choice of sample, and the implications that are not discussed. The analytical targets are marine mudstones (Lines 170-175) deposited between a few tens to hundreds of meters. This raises a number of issues.
Perhaps one of the most difficult one is mineralogy. There is no indication that systematic quantitative x-ray analysis was conducted, and that the amount of difference carbonate species has been established. If not, then the authors are blind to what they are measuring. This is highly relevant, because the acid fractionation factor and temperature relationship of different carbonate minerals vary. So if a mix in mineralogy existed in the fine-mud matrix (as is not uncommon), then some of the discrepancies in the magnitude of the signal could be explained by changes in mineralogy. I also note (line 187) that neither the acid temperature nor the acid correction factor are indicated in the methods, something that needs to be remediated.
For me, the second problem is of course diagenesis. The authors did discuss it somewhat, but I don’t think that the question was fully addressed. For instance, on line 240-243 a statement is made that a lack of correlation between d13C and d18O can be used as evidence of the lack of meteoric diagenesis. The problem with this statement is that it ignores the work from Lohman in the late 80’s, and the concept of meteoric water line. It is well-know that oxygen is more likely to be reset by meteoric processes whilst carbon remains constant unless you reach very high fluid/rock ratios. Thus, the absence of a correlation between d13C and d18O is not necessarily a good way to prove the absence of meteoric diagenesis, especially given the low d18O values.
The large spread in d13C that the authors mentioned is suspicious, i.e. could this tell us something about diagenesis of the host rock under variable water/rock ratios? Note that the oxygen isotope values are also very scattered, i.e. about 2 permil variation which is significant for marine values. As presented, I am not convinced that I am not looking at a marine trend (the overall trend) but with diagenetic overprinting on the absolute values and the scatter. Petrography, cathodoluminescence and perhaps some trace metal analysis would go a long way convincing me rather than the isotopic values alone.
The third problem in my opinion is that if these mudstones were deposited at water depth differing by hundred(s) of meter(s), then would you not expect a difference in recorded water temperature, and thus d18O of the carbonates? At the very least, if one counts on the input of freshwater from the Pyrenees to explain the low d18O, then one problem is how well mixed is the low-salinity runoff waters were with respect to the much higher salinity, epeiric platform water mass at 100-200 meters? I would think the chances of a fresh water lense forming would be high, and that the freshwater signal would be lost as you go deeper in the water column. This could also have implications on oxygenation of the basin. Water depth is a very important factor in the interpretation of the isotopic values, in my opinion.
The authors could also try to support their freshwater mixing hypothesis (320-330) with additional evidence, for instance a change in the faunal assemblage of the microfossils that indicates a decrease in salinity. Are there any studies showing this? Can they see this in their assemblages?
Overall the oxygen isotope curve does look similar in shape to the global curve, which indicates that the global ocean and the epeiric sea were well-mixed (Fig 5). But then if the epeiric sea is connected to the global ocean, I would have expected regional rain effects to create deviations from the global trend (irrespective of the absolute d18O value) not just the same trend but slightly different (more negative) minimum values. Is it not a strange coincidence that the global trend is captured, just with a lower d18O minima and a offset in timing?
The difference in absolute value from Cramer et al will also be due in part to the fact that we are looking at epicontinental seas, with much warmer (essentially surface) waters, rather than deep-sea sediments (line 270). How much could this temperature effect explain the difference in d18O?
Overall, my problem I think is that the authors actually correctly identify the complexity of using d18O and d13C alone to disentangle processes at this scale, but yet in line 295 they still propose what they call the alternative hypothesis (that the trend is driven by the orogeny) as their chosen one. This hypothesis does appear more strongly supported by the data than the alternative hypotheses.
I believe that the points I raise above need to be addressed convincingly to warrant publication of the model. This will most likely require to either generate or use existing data on the mineralogy of the samples, and ideally their petrography. But I do hope the authors can address these concerns, because their paper is really interesting and if they convincingly demonstrate the validity of their interpretation then this will be a very significant contribution to the field.
Small additional comment:
Line 226: in this context, the term ‘fresh’ does not apply. Surely you don’t mean benthic forams fresh out of the see? I think you want to say well-preserved?
Citation: https://doi.org/10.5194/se-2021-12-RC1 -
RC2: 'Comment on se-2021-12', Anonymous Referee #2, 19 Apr 2022
This paper aims to utilize stable isotope geochemistry from marine strata in the South Pyrenean foreland to test the hypothesis, originally based primarily on flexural modelling, that a major topographic rise occurred in the late Paleocene-early Eocene. The authors present stable isotope measurements on whole-rock marine carbonate mudstone succession proving a 12 Ma continuous record during the early to middle Eocene (54 to 42 Ma) to provide additional constraints on a possible significant topographic growth during the early Eocene and place this important tectonic and basin reconfiguration period within the evolution of the Pyrenees, from its initiation in the upper Cretaceous to the end of convergence in the Miocene. Finally, the authors state that results of this study might allow discussing the respective roles of climate and tectonics on the topographic evolution of a mountain range. Oxygen isotopes of this record (δ18O values) show a faster decrease rate than the coeval global negative excursion associated with the Early Eocene Climatic Optimum (EECO). The authors claim that this local alteration of the global δ18O signal provided evidence of topographic growth during this period.
The approach of this paper should be praised for its interdisciplinary nature of how a research question identified and the methods to address it are identified. The paper reads well and the figures are clear and well designed. Especially the introduction is very well written. As the paper progresses, the structure is not always clear as data, interpretation and discussion are not always clearly separated.
There are several points which require discussion. As the stable isotope geochemistry of marine carbonates provides the basis for the conclusions of this study, this is the domain that should receive the most careful evaluation.
The isotopic composition of oxygen (δ18O) preserved in continental sedimentary rocks has been used to reconstruct paleotopography and paleoelevation based the precipitation and “continentality” effects generated through Rayleigh distillation process. However, the strata being investigated here are marine. The processes affecting the isotopic composition of marine carbonates are more complex and need to be addressed specifically. If formed in situ, the oxygen stable isotope composition could reflect changes in water depth or in water mass composition.
Furthermore, the authors use a bulk sample approach which brings an additional set of considerations which need to be made before reaching a reliable interpretation. There is no information on the mineralogy nor on the petrographic components present in the samples. This is essential to reach any conclusion. Is the carbonate land-derived? From which sources? It is marine and precipitated in the basin? Are these ratios constant throughout the studied interval? These questions are fundamental and need to be addressed with more data. Without this information the interpretations are just speculative.
The choice of samples also needs to be arued for and discussed better. Muds can form at any depth and may carry very different signatures.
Diagenesis is always a key aspect of any stable isotope discussion and has to be related to mineralogical and petrographic composition, and possibly new diagenetic phases, which record the rock fluid interaction through time. The authors use organic geochemistry proxies from organic matter present in some samples. These are not adequately discussed, but are used to make implication on T and pressure conditions. I feel the authors are not discussing this sufficiently.
These points need be better illustrated and clarified by additional data before any further discussion on the respective roles of climate and tectonics on the topographic evolution of a mountain range.
Citation: https://doi.org/10.5194/se-2021-12-RC2
Interactive discussion
Status: closed
-
RC1: 'Comment on se-2021-12', Anonymous Referee #1, 24 Jun 2021
I read with great interest the paper by Honegger et al. The introduction is well written, and I agree that the topic of orogenic evolution is important. The authors have framed the problem as a Raileigh distillation exercise, whereby the d18O of carbonates can be used as a proxy for elevation. This is commonly done in continental tectonic using soil nodules (see for instance Huntington et al, and Jay Quads et al). This paper is however using marine carbonates previously shown to contain a record of orogenic effect on d18O, but they do this at a much higher temporal resolution than previously published.
Overall this is an easy to read, paper, and a nice narrative between oxygen and carbon isotopes and tectonics. I like the fact that the authors have independent magnetostratigraphy and that they use these as the only tie points in Analyseries: it gives me confidence that their age model is probably correct (lines 219-220)and that comparison with the global stack of Cramers et al is not a chicken and egg problem where isotope trends are matched first, and then their correlation discussed.
But as an isotope geochemist, I am not fully convinced with the fact that the data actually supports the authors story. The story *might* be correct, but it might not. And many details were omitted, or simply not fully discussed in this manuscript. Therefore I recommend some major revisions on the stable isotope side..
The first block to me being fully convinced is the choice of sample, and the implications that are not discussed. The analytical targets are marine mudstones (Lines 170-175) deposited between a few tens to hundreds of meters. This raises a number of issues.
Perhaps one of the most difficult one is mineralogy. There is no indication that systematic quantitative x-ray analysis was conducted, and that the amount of difference carbonate species has been established. If not, then the authors are blind to what they are measuring. This is highly relevant, because the acid fractionation factor and temperature relationship of different carbonate minerals vary. So if a mix in mineralogy existed in the fine-mud matrix (as is not uncommon), then some of the discrepancies in the magnitude of the signal could be explained by changes in mineralogy. I also note (line 187) that neither the acid temperature nor the acid correction factor are indicated in the methods, something that needs to be remediated.
For me, the second problem is of course diagenesis. The authors did discuss it somewhat, but I don’t think that the question was fully addressed. For instance, on line 240-243 a statement is made that a lack of correlation between d13C and d18O can be used as evidence of the lack of meteoric diagenesis. The problem with this statement is that it ignores the work from Lohman in the late 80’s, and the concept of meteoric water line. It is well-know that oxygen is more likely to be reset by meteoric processes whilst carbon remains constant unless you reach very high fluid/rock ratios. Thus, the absence of a correlation between d13C and d18O is not necessarily a good way to prove the absence of meteoric diagenesis, especially given the low d18O values.
The large spread in d13C that the authors mentioned is suspicious, i.e. could this tell us something about diagenesis of the host rock under variable water/rock ratios? Note that the oxygen isotope values are also very scattered, i.e. about 2 permil variation which is significant for marine values. As presented, I am not convinced that I am not looking at a marine trend (the overall trend) but with diagenetic overprinting on the absolute values and the scatter. Petrography, cathodoluminescence and perhaps some trace metal analysis would go a long way convincing me rather than the isotopic values alone.
The third problem in my opinion is that if these mudstones were deposited at water depth differing by hundred(s) of meter(s), then would you not expect a difference in recorded water temperature, and thus d18O of the carbonates? At the very least, if one counts on the input of freshwater from the Pyrenees to explain the low d18O, then one problem is how well mixed is the low-salinity runoff waters were with respect to the much higher salinity, epeiric platform water mass at 100-200 meters? I would think the chances of a fresh water lense forming would be high, and that the freshwater signal would be lost as you go deeper in the water column. This could also have implications on oxygenation of the basin. Water depth is a very important factor in the interpretation of the isotopic values, in my opinion.
The authors could also try to support their freshwater mixing hypothesis (320-330) with additional evidence, for instance a change in the faunal assemblage of the microfossils that indicates a decrease in salinity. Are there any studies showing this? Can they see this in their assemblages?
Overall the oxygen isotope curve does look similar in shape to the global curve, which indicates that the global ocean and the epeiric sea were well-mixed (Fig 5). But then if the epeiric sea is connected to the global ocean, I would have expected regional rain effects to create deviations from the global trend (irrespective of the absolute d18O value) not just the same trend but slightly different (more negative) minimum values. Is it not a strange coincidence that the global trend is captured, just with a lower d18O minima and a offset in timing?
The difference in absolute value from Cramer et al will also be due in part to the fact that we are looking at epicontinental seas, with much warmer (essentially surface) waters, rather than deep-sea sediments (line 270). How much could this temperature effect explain the difference in d18O?
Overall, my problem I think is that the authors actually correctly identify the complexity of using d18O and d13C alone to disentangle processes at this scale, but yet in line 295 they still propose what they call the alternative hypothesis (that the trend is driven by the orogeny) as their chosen one. This hypothesis does appear more strongly supported by the data than the alternative hypotheses.
I believe that the points I raise above need to be addressed convincingly to warrant publication of the model. This will most likely require to either generate or use existing data on the mineralogy of the samples, and ideally their petrography. But I do hope the authors can address these concerns, because their paper is really interesting and if they convincingly demonstrate the validity of their interpretation then this will be a very significant contribution to the field.
Small additional comment:
Line 226: in this context, the term ‘fresh’ does not apply. Surely you don’t mean benthic forams fresh out of the see? I think you want to say well-preserved?
Citation: https://doi.org/10.5194/se-2021-12-RC1 -
RC2: 'Comment on se-2021-12', Anonymous Referee #2, 19 Apr 2022
This paper aims to utilize stable isotope geochemistry from marine strata in the South Pyrenean foreland to test the hypothesis, originally based primarily on flexural modelling, that a major topographic rise occurred in the late Paleocene-early Eocene. The authors present stable isotope measurements on whole-rock marine carbonate mudstone succession proving a 12 Ma continuous record during the early to middle Eocene (54 to 42 Ma) to provide additional constraints on a possible significant topographic growth during the early Eocene and place this important tectonic and basin reconfiguration period within the evolution of the Pyrenees, from its initiation in the upper Cretaceous to the end of convergence in the Miocene. Finally, the authors state that results of this study might allow discussing the respective roles of climate and tectonics on the topographic evolution of a mountain range. Oxygen isotopes of this record (δ18O values) show a faster decrease rate than the coeval global negative excursion associated with the Early Eocene Climatic Optimum (EECO). The authors claim that this local alteration of the global δ18O signal provided evidence of topographic growth during this period.
The approach of this paper should be praised for its interdisciplinary nature of how a research question identified and the methods to address it are identified. The paper reads well and the figures are clear and well designed. Especially the introduction is very well written. As the paper progresses, the structure is not always clear as data, interpretation and discussion are not always clearly separated.
There are several points which require discussion. As the stable isotope geochemistry of marine carbonates provides the basis for the conclusions of this study, this is the domain that should receive the most careful evaluation.
The isotopic composition of oxygen (δ18O) preserved in continental sedimentary rocks has been used to reconstruct paleotopography and paleoelevation based the precipitation and “continentality” effects generated through Rayleigh distillation process. However, the strata being investigated here are marine. The processes affecting the isotopic composition of marine carbonates are more complex and need to be addressed specifically. If formed in situ, the oxygen stable isotope composition could reflect changes in water depth or in water mass composition.
Furthermore, the authors use a bulk sample approach which brings an additional set of considerations which need to be made before reaching a reliable interpretation. There is no information on the mineralogy nor on the petrographic components present in the samples. This is essential to reach any conclusion. Is the carbonate land-derived? From which sources? It is marine and precipitated in the basin? Are these ratios constant throughout the studied interval? These questions are fundamental and need to be addressed with more data. Without this information the interpretations are just speculative.
The choice of samples also needs to be arued for and discussed better. Muds can form at any depth and may carry very different signatures.
Diagenesis is always a key aspect of any stable isotope discussion and has to be related to mineralogical and petrographic composition, and possibly new diagenetic phases, which record the rock fluid interaction through time. The authors use organic geochemistry proxies from organic matter present in some samples. These are not adequately discussed, but are used to make implication on T and pressure conditions. I feel the authors are not discussing this sufficiently.
These points need be better illustrated and clarified by additional data before any further discussion on the respective roles of climate and tectonics on the topographic evolution of a mountain range.
Citation: https://doi.org/10.5194/se-2021-12-RC2
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Louis Honegger
Thierry Adatte
Jorge E. Spangenberg
Miquel Poyatos-Moré
Alexandre Ortiz
Magdalena Curry
Damien Huyghe
Cai Puigdefàbregas
Miguel Garcés
Andreu Vinyoles
Luis Valero
Charlotte Läuchli
Andrés Nowak
Andrea Fildani
Julian D. Clark
Sébastien Castelltort
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