Sr, S, and O Isotope Compositions of Evaporites in the Lanping–Simao Basin, China
Round 1
Reviewer 1 Report
In its actual shape the manuscript is not suitable for publication. I think that to achieve the objectives of this study, aimed to i) determinate the origin of the parent brine and to ii) interpret the environmental changes during evaporite deposition, a throughout revision is required and focused to solve the main critical points listed here below.
Origin of the evaporites
The evaporites are very interesting rocks but also very difficult ones. In particular the sulphates are characterized by diagenetic processes that can completely obliterate the original rocks. For this reason, it is very important to provide a detailed sedimentological and petrographic description of these rocks and of the associated non-evaporitic deposits. In particular it is important to know if gypsum is macro or micro-crystalline, if it is of primary origin, clastic, diagenetic after anhydrite, if it is found in nodules, in veins, and so on. In the latter case it would be important to know which kind of digenetic process So, I suggest to add this kind of information and to clearly indicate the different kind of rocks in the discussion of the geochemical signature.
Stratigraphy and sedimentology
The description of the sampled sections in both the evaporitic and the non-evaporitic components is completely lacking. I suggest to include detailed lithological column for each section. Moreover, it is not clear if the rocks sampled in the different sites belong to the same unit and how the section can be correlated each other.
BSR
The authors suggest the occurrence of BSR, but it is not clear if sulphur deposits after gypsum is found somewhere associated to the original rocks. In this case a petrographic study would be useful to highlight the occurrence of microscopic sulphur
Sr isotope ratio
The values of the gypsum are quite high with respect to those expected for the late Mesozoic. However, the values are also characterized by an elevate dispersion. I agree that an important contribution of high radiogenic sources is needed. However, I suggest to check if traces of the supposed sources are found in the non-evaporitic deposits intercalated to the evaporites; this could be useful to confirm the contribution of these rocks in the modification of the Sr isotope value. I noticed that in the Mengyejing section the deposits are mainly halite, however from the table 2 it seems that only the sulphates where geochemically characterized. It would be interesting to see if the 87Sr/86Sr in the halite is different from that of the gypsum. Data in table 2 must include the errors.
The great part of the data is from Baozang; it seems that they show a bi- or trimodal distribution. Is there any relationship with the evaporitic facies?
Figures
1) please include the age on the units. What the white color is for?
2) this table should include a general lithostratigraphic column and detailed lithostratigraphic columns for each sampled section where the position of samples must be highlighted.
3) The description supporting these pictures in insufficient. They should be located in the detailed columns of the single sections different facies must be differentiated.
4) I suggest a different plot where the single values are visible and where the different evaporite types can be identified. Sr isotope; please add values for the coeval global ocean and for the granites suggested as the main contributors for the increase values.
5) these data should be shown with the indication of the different lithologies
7) the shaded box should be limited to coeval rocks
8) this figure should include a lithological column and evaporite rock types should be indicated.
More: I would suggest to add a picture of S vs. Sr to show if the different gypsum facies show similar anomalies in both the proxies.
Author Response
Origin of the evaporites
The evaporites are very interesting rocks but also very difficult ones. In particular the sulphates are characterized by diagenetic processes that can completely obliterate the original rocks. For this reason, it is very important to provide a detailed sedimentological and petrographic description of these rocks and of the associated non-evaporitic deposits. In particular it is important to know if gypsum is macro or micro-crystalline, if it is of primary origin, clastic, diagenetic after anhydrite, if it is found in nodules, in veins, and so on. In the latter case it would be important to know which kind of digenetic process So, I suggest to add this kind of information and to clearly indicate the different kind of rocks in the discussion of the geochemical signature.
Response: we have added the descriptions of micro-features of those evaporite minerals in the discussion section.
Stratigraphy and sedimentology
The description of the sampled sections in both the evaporitic and the non-evaporitic components is completely lacking. I suggest to include detailed lithological column for each section. Moreover, it is not clear if the rocks sampled in the different sites belong to the same unit and how the section can be correlated each other.
Response: The detailed lithological column for each section have been added and described in the Manuscript.
BSR
The authors suggest the occurrence of BSR, but it is not clear if sulphur deposits after gypsum is found somewhere associated to the original rocks. In this case a petrographic study would be useful to highlight the occurrence of microscopic sulphur
Response: Sulfide minerals (pyrite) was found and described in gypsums
Sr isotope ratio
The values of the gypsum are quite high with respect to those expected for the late Mesozoic. However, the values are also characterized by an elevate dispersion. I agree that an important contribution of high radiogenic sources is needed. However, I suggest to check if traces of the supposed sources are found in the non-evaporitic deposits intercalated to the evaporites; this could be useful to confirm the contribution of these rocks in the modification of the Sr isotope value. I noticed that in the Mengyejing section the deposits are mainly halite, however from the table 2 it seems that only the sulphates where geochemically characterized. It would be interesting to see if the 87Sr/86Sr in the halite is different from that of the gypsum. Data in table 2 must include the errors.
Response: the contribution of Mesozoic sedimentary rocks has been discussed in the manuscript. We compared the Sr isotope composition of sulphates with that of halite from previous study. (see manuscript in detail)
The great part of the data is from Baozang; it seems that they show a bi- or trimodal distribution. Is there any relationship with the evaporitic facies?
Response: There is no notable correlation between lithology and Sr isotope composition. Nearly all samples consist of gypsum laminae. Some samples contain a trace amount of carbonates. However, there is no apparent relationship between carbonate contents and S isotope compositions.
Figures
1) please include the age on the units. What the white color is for?
Response: Ages of the legends have been added, the white color represent blocks encompassing the Lanping-Simao Basin.
2) this table should include a general lithostratigraphic column and detailed lithostratigraphic columns for each sampled section where the position of samples must be highlighted.
Response: The column of each sampled section is added in the manuscript as Figure 4
3) The description supporting these pictures in insufficient. They should be located in the detailed columns of the single sections different facies must be differentiated.
Response: The sampling sites are described in the manuscript.
4) I suggest a different plot where the single values are visible and where the different evaporite types can be identified. Sr isotope; please add values for the coeval global ocean and for the granites suggested as the main contributors for the increase values.
Response: The single values of Sr, S and O isotopes are depicted in Figure 5.
5) these data should be shown with the indication of the different lithologies
Response: The 87Sr/86Sr ratios of different lithologies have been differentiated.
7) the shaded box should be limited to coeval rocks
Response: The shaded box has been limited to an area representing the Late Cretaceous marine evaporites.
8) this figure should include a lithological column and evaporite rock types should be indicated.
Response: Figure 4 contains all lithological columns of each sampled section. The lithological column of this figure could be referred to Figure 4.
More: I would suggest to add a picture of S vs. Sr to show if the different gypsum facies show similar anomalies in both the proxies.
Response: The S vs. Sr pattern has been added and discussed in the manuscript.
The attached word file is the corrected version of the manuscript.
thank you for your comments
Author Response File: 
Author Response.docx
Reviewer 2 Report
This is a well written paper on a series of Triassic evaporitic deposits in Simao Basin of China. Amount of data is good and appropriate for such an effort.
Paper has practical significance for potash deposits in China. There is a difference between continental and marine evaporites and isotopic parameters employed are called to resolve these differences. Authors found non-trivial isotope systematics that require various effects related to postdepositional oxidation, bacterial sulphate reduction etc. One formation is least altered
Fig 1 is good,
Fig 2 no radiometric ages?
Fig 3 indicate by an asterisk what was sampled
Line i128 Did you use TCEA carbon furnace?
134 indicate offset (instrumental fractionation for which you converted the dataset)
XRD examination is good.
Fig 4 Do you want to present all your isotope data vs stratigraphic height?
Fig 4 a indicate by a star Sr isotopic composition of seawater of that time from a global curve
Section 5.1 can you compute the initial Sr values?
Fig 5 are these initial Sr values?
235-156 I agree that high and low S values can be explained this way.
259 “dissolved sulphates re-oxidized by sulphides” you mean reduced?
I accept explanation by authors of Longinelli and Flora’s process type.
321 spelling Turchin et al
After reading the section 5.2 I wish you had better textural documentation (thin section type) what you sampled for analysis
Fig 8 this looks like least altered/modified formation
It would be beneficial to perhaps measure D17O in sulfate; Martin and Bindeman (epsl, 2009) found that sulfates in lakes record excesses of D17O, while marine samples do not.
Author Response
Fig 1 is good
Fig 2 no radiometric ages?
Response: Radiometric ages are added
Fig 3 indicate by an asterisk what was sampled
Response: All sampling site are indicated by asterisks
Line i128 Did you use TCEA carbon furnace?
Response: Yes, we have added the TCEA carbon furnace
134 indicate offset (instrumental fractionation for which you converted the dataset)
Response: The offset calibration was conducted by the two-point linear normalization based on two standard samples, as described in the manuscript.
XRD examination is good.
Fig 4 Do you want to present all your isotope data vs stratigraphic height?
Response: In Nuodeng, all samples are collected from vein-shaped gypsums. In Mengyejing, the sampled section is only a small part of the salt profile. The measurement of evaporite height is not practical in Nuodeng and Mengyejing. Thus, we prefer not to compare the isotopes values to stratigraphic height.
Fig 4 a indicate by a star Sr isotopic composition of seawater of that time from a global curve
Response: The range of Sr isotopic composition of the Late Cretaceous seawater has been added in this figure.
Section 5.1 can you compute the initial Sr values?
Fig 5 are these initial Sr values?
Response: Although the continental evaporite were thought to be formed during the Late Cretaceous, apparently those evaporites were altered by post-depositional process, including dissolution, recrystallization, dehydration and re-hydration etc. We could not ascertain the “initial formation age” for calculation.
235-156 I agree that high and low S values can be explained this way.
259 “dissolved sulphates re-oxidized by sulphides” you mean reduced?
Response: Yes, the re-oxidation of reduced sulphides with low δ34S values would generate low δ34S values sulphates.
I accept explanation by authors of Longinelli and Flora’s process type.
321 spelling Turchin et al
Response: The spelling mistake is corrected.
After reading the section 5.2 I wish you had better textural documentation (thin section type) what you sampled for analysis
Response: the micro-features of evaporite minerals are added and discussed in the manuscript.
Fig 8 this looks like least altered/modified formation
Response: Yes
It would be beneficial to perhaps measure D17O in sulfate; Martin and Bindeman (epsl, 2009) found that sulfates in lakes record excesses of D17O, while marine samples do not.
Response: We are grateful for this suggestion, and planning to conduct triple oxygen isotopes research in the future.
The attached word file is the corrected version of the manuscript.
Thank you for your comments
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
I appreciated the effort produced by the authors to improve the manuscript as requested in my previous revision.
The Authors provided some new information about the different lithologies, but I think that more details should be still added here and that the different evaporitic facies should be kept distinct in the geochemical analyses to show if there are some relationships between the evaporites origin (primary, diagenetic…) and the geochemical signature.
I think that after some further changes aimed to improve the lithological description of the studied rocks, the manuscript could be suitable for publication. Here my suggestions:
Define the lithological/sedimentary facies and provide pictures for each one, both at outcrop/hand sample and microscopic scale (not only SEM images, these are too small, an intermediate scale is needed). Describe the gypsum laminae, which kind of crystals are they formed by? Acicular? Granular? Is the crystal size changing vertically within each lamina or not? Are crystals entire or broken? Clearly indicate the existence or absence of sedimentary structures, gradations etc.
It should be better shown if the gypsum is microcrystalline or not. This is to shown possible diagenetic features. Example, from line 280: swallowtail gypsum is commonly formed by macrocrystals, did you find somewhere this facies? I can’t’ see in the text or pictures. Fig. 8E is microcrystalline, not macrocrystalline. Moreover, did you find overgrowth nodules?
All these data should be included in paragraph 3 and the lithological columns should include the described facies.
The indication “gypsum” in figure 4 is too general, for example. The patterns adopted in the lithostratigraphic column should be more descriptive of the lithologies; For example, laminated gypsum alternated to mud should not be represented by vertical lines.
Please differentiate were you find gypsum and where anhydrite, and if they are associated or not.
Separates these different sulphates in the plots to allow the readers to understand if some geochemical changes are related to i) gypsum-anhydrite-gypsum transformations or ii) if primary gypsum vs. primary anhydrite.
Veins should be described too: are they limpid crystals or fibrous ones? Different crystals indicated different origin for the veins, this could have a response in the different geochemical signatures found here.
Facies information should also be included in table 1.
Fig. 2. It is still not simple for the readers. I suggest to transform the table into a general lithostratigraphic column
Fig. 4. This is a welcome new figure. It would be better to include the position of each sample, not to highlight the sampled interval only.
Fig. 8 some details are very small ones, I suggest to add some intermediate-scale (this sections) picture to better show the evaporite facies from the hand samples-scale to the SEM-scale
Fig. 11 are these samples from the same facies? I would like to see a detailed lithological column too.
Author Response
We very much appreciate the suggestions and comments given by the reviewer. After reading the comments carefully, we have modified and corrected all the mentioned points as best as we can. Here are some detailed correction works:
The Authors provided some new information about the different lithologies, but I think that more details should be still added here and that the different evaporitic facies should be kept distinct in the geochemical analyses to show if there are some relationships between the evaporites origin (primary, diagenetic…) and the geochemical signature.
I think that after some further changes aimed to improve the lithological description of the studied rocks, the manuscript could be suitable for publication. Here my suggestions:
Define the lithological/sedimentary facies and provide pictures for each one, both at outcrop/hand sample and microscopic scale (not only SEM images, these are too small, an intermediate scale is needed). Describe the gypsum laminae, which kind of crystals are they formed by? Acicular? Granular? Is the crystal size changing vertically within each lamina or not? Are crystals entire or broken? Clearly indicate the existence or absence of sedimentary structures, gradations etc.
It should be better shown if the gypsum is microcrystalline or not. This is to shown possible diagenetic features. Example, from line 280: swallowtail gypsum is commonly formed by macrocrystals, did you find somewhere this facies? I can’t’ see in the text or pictures. Fig. 8E is microcrystalline, not macrocrystalline. Moreover, did you find overgrowth nodules?
All these data should be included in paragraph 3 and the lithological columns should include the described facies.
The indication “gypsum” in figure 4 is too general, for example. The patterns adopted in the lithostratigraphic column should be more descriptive of the lithologies; For example, laminated gypsum alternated to mud should not be represented by vertical lines.
Please differentiate were you find gypsum and where anhydrite, and if they are associated or not.
Separates these different sulphates in the plots to allow the readers to understand if some geochemical changes are related to i) gypsum-anhydrite-gypsum transformations or ii) if primary gypsum vs. primary anhydrite.
Veins should be described too: are they limpid crystals or fibrous ones? Different crystals indicated different origin for the veins, this could have a response in the different geochemical signatures found here.
Response:
Based on the reviewer’s suggestions, we have described the detailed lithologies, petrology and mineralogy of collected samples, and the sampling section and location accordingly in section 3. We only used gypsum laminae and veined gypsum for chemical analyses. Other types of samples, for instance, selenite, is not included for Sr, S and O isotoes analyses in this study. Some specific modifications are as follows:
In figure 3, the alternating gypsum and mudstone layers are differentiated by different legends; The description part is included in section 3; The XRD results indicate which samples contains anhydrite or gypsum, or both; In Figure 7, 8, 9, 10, the layered and veined samples are indicated by different symbols; All veined gypsum crystals are in fibre shape, etc.
Facies information should also be included in table 1.
Respomse: Facies information has been added into Table 1
Fig. 2. It is still not simple for the readers. I suggest to transform the table into a general lithostratigraphic column
Response: A schematic general lithostratigraphic column has been included into the manuscript.
Fig. 4. This is a welcome new figure. It would be better to include the position of each sample, not to highlight the sampled interval only.
Response: The sampling sites of all collected samples are marked in each columns.
Fig. 8 some details are very small ones, I suggest to add some intermediate-scale (this sections) picture to better show the evaporite facies from the hand samples-scale to the SEM-scale
Response: Some immediate scale images are added and depicted in the manuscript.
Fig. 11 are these samples from the same facies? I would like to see a detailed lithological column too.
Response: The detailed lithological column is added.
Author Response File: Author Response.docx
