the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Apr 2026
Nascent Eocene Indian Monsoons recorded in oyster shells from the western Indian margin
Abstract. The origin, intensification and paleogeographic drivers of the South Asian Monsoon (SAM) since the Eocene, remain poorly understood mainly due to the paucity of suitable seasonal proxies coupled with climate simulations. This study evaluates the seasonal climate variability and potential presence of SAM-like conditions based on geochemical proxy-based sub-annual climatic reconstruction recovered from late-middle Eocene fossil oyster shell material from the Kutch basin (India), interpreted in the framework of Community Earth System Model (CESM) climate simulations. A thick-shelled honeycomb oyster species Pycnodonte kachchhensis is used as a natural palaeoclimate archive. Bulk sediment organic carbon isotope stratigraphy places the oyster horizon within the late Bartonian, after the Middle Eocene Climatic Optimum (MECO) from which a previous oyster fossil study provides comparison. Clumped isotope thermometry indicates low seasonal variability with sea surface temperatures of 31+2.2−2.1 to 36+2.8−2.7 °C and evaporative conditions (δ18Ow: 0.6+0.4−0.4 to 1.6+0.6−0.5 ‰), within a restricted late Bartonian lagoonal setting. Trace element profiles in oyster shells indicate a low-nutrient environment and capture around two years of the oysters’ lifespan, reflected in seasonal variations along the shell growth axis. Stable isotope data from the multiyear shell archive indicates annual temperature drops associated with periods of higher rainfall to evaporation. The observed pattern is consistent with CESM simulation showing a nascent SAM as well as precipitation outside the main monsoon months due to moisture transport from the open seaway between India and Eurasia. However, a modern-like monsoon with strong seasonal contrasts and pronounced winter temperature minima was not yet established in the study area, possibly owing to the tropical palaeolatitude of the region (~8° N) and palaeogeographic factors such as the open northern seaway of Greater India. Observed climatic and seasonal variability are comparable to the underlying MECO oyster record, despite the early Bartonian hyperthermal conditions and the gradual decline in pCO2 toward the late Bartonian, suggesting a limited sensitivity of equatorial climate to atmospheric CO2.
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- CC1: 'Comment on egusphere-2026-1393', Giacomo Medici, 16 Apr 2026 reply
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RC1: 'Comment on egusphere-2026-1393', Anonymous Referee #1, 15 Jun 2026
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Mitra et al studied oyster shells from the Bartonian and compared them to CESM model simulations, concluding that the provenance region experienced weaker monsoonal conditions relative to today. However, there are major flaws in their reasoning and discussions as outlined below.
Firstly, I don’t understand why you excluded the other two specimens from your analyses. You state that Fe concentrations are higher in one but the other elements have consistent trends. It is also absolutely not clear and seemingly subjective that the elements associated with dissolution and (re)precipitation (your Na in Figure 4a-b, 4e-f) or structural defects (Si in Figure 4a-c) or diagenesis (Fe in Figure 4a-b, 4d-e) are significantly different between each specimen, leading to the exclusion of two out of three specimens. Also, if the stable isotopes are almost the same for two specimens, why exclude one of them if there is no clear diagenetic signal? Ultimately what your discussion implies is that your entire story is based on only one specimen with four data points, which is not robust for publishing with the conclusions you write. I suggest adding a clear section in the methods on your exact definitions of what you consider to be “well-preserved” or what your preservation criteria are or adjusting your focus to also be that of preservation issues/analyses as well.
Secondly, the lack of a clear definition of what you define as seasons or the lack of a clear growth curve leaves the reader confused as to how robust your conclusions actually are. What if growth stop(s) occurred and you interpret your δ18O to be a different month than it is, such as your second datapoint being the winter and third your second year summer or the whole specimen being one year, your whole story would change and no concrete conclusions could be made on the monsoon in the Bartonian. Your element ratio profiles have three to four peaks, suggesting more seasons/years than you currently consider for example.
Thirdly, your entire conclusion and proxy-model comparison is based on four clumped isotope datapoints from one specimen. Even though it does take time and lots of material to measure clumped isotopes in carbonates, most studies still report at least more than the small handful of datapoints and come to less conclusions than this study does. While the temperature and inferred hydrology signal from clumped isotopes are the main focus of this study, there is much more data from the elemental ratios and on more specimens but almost no discussion of this. You need to either shift the focus to discuss these ratios more or make less grand conclusions from your limited dataset.
Overall, this manuscript shows potential but focusses on conclusions that lack data to support it and has limited explanations on their used assumptions.
Line-specific comments
Line 1: the title is misleading as you only consider one shell for your conclusions.
Line 13-14: I would change the comma to after “understood”
Line 20-22: you are misleading the reader here, since you only consider one specimen to be reliable, even though you measured three for trace elements. Clearly state that you only consider one specimen for your conclusions or readjust your focus to an exploration/analysis of Pycnodonte preservation.
Line 29: observed in what? The model or oysters? That is unclear.
Line 30: hyperthermals are defined as peak warmings above the normal background temperature, so this is misleading in naming an entire period of hyperthermal conditions. Overall warmth and frequent hyperthermals did occur but were not constant.
Line 47: I would change the word deciphering as you cannot decipher between multiple subjects. I imagine you mean decipher between the effects of/impact of these drivers.
Line 49-51: the order of this sentence is confusing, consider moving “the hydrological cycle” earlier in the sentence before the comma.
Line 51, 54: is there a reason you first mention oyster and then bivalves, since the former is a subgroup of the latter?
Line 59: is a very long sentence, split up after the “for example”.
Line 146: should be “one half was prepared” not halves.
Line 147: 05 mm doesn’t make sense, just 5 mm would be clearer.
Line 164: what is the resilifer region? Non-experts might not be familiar – explain or indicate on figure.
Line 200: you have four Δ47 datapoints, state that clearer here as the wording is confusing. How it is written now, you have a lot of equidistant datapoints along the entire shell that you measured for δ13C and δ18O but not for Δ47 following your Table 3. Did you pool together equidistant samples then state this, but this will significantly change your story of what months and seasons you define?
Line 210: why the ICDES90 scale, I-CDES is defined and reported to the 90°C reaction temperature so the specific indication of I-CDES90 is not needed. And you did not measure dual clumped, which is often denoted as CDES90.
Line 337: you use “which” twice, one needs to be “that”.
Line 340: were any clumped isotope measurements performed on the other two specimens to confirm your suspicions of diagenesis? Especially since you only have one specimen that you trust vs two you don’t i.e. more “diagenetic altered” specimens than not, it would make more sense to pivot part (or maybe even most) of your story to the potential effects of diagenetic alterations. Or at least have a more significant part exploring this, as this is still interesting to the clumped community (i.e. diagenetic effects, potential burial temperatures, or preservation analyses).
Line 346: “reflects” should be singular.
Lines 347-348: two things; one, you are comparing δ13C signals from foram carbonate vs organic carbon, how do you consider that to be comparable? Two, strong disagree that the MECO is not associated with a CIE, see (Spofforth et al., 2010 10.1029/2009PA001738; Sharma et al., 2024 10.5194/cp-20-935-2024; Mulch et al., 2015 10.2475/04.2015.02) for organic, benthic foraminifera, and bulk carbonate CIEs associated with the MECO.
Line 373: “on average”.
Line 375: “relatively much higher” sounds weird, either relatively higher or much higher.
Line 380: it is from one specimen that reflects local conditions, how variable is δ18Osw in the modern day, comparable enough to make such a statement?
Line 431: Your entire study depends on the definition of seasons within your one specimen. However, how you define your seasons is seemingly subjective and not robust. You compare your four datapoints to the CESM seasonal temperatures, but you automatically start in Jan, is there evidence that growth started in Jan and not in spring or summer as is the case with other molluscs? Especially as you have two May datapoints with completely opposite δ18O values, so they are seemingly from opposite seasons. Indeed, your δ13C has a much clearer sinusoidal pattern and suggests your second year should be shifted by several months, i.e. your 2nd year May/June has the same values as 1st year Jan/Feb.
Line 459: But your Ba/Ca is highest in the coldest CESM and your defined winter season, not spring. And the lowest values are not during your two warmest datapoints but the coldest.
Line 475: But this is circular reasoning as the δ18Osw is defined using the δ18Occ and temperature from the same shell and are thus not independent variables.
Line 509: are modern and Bartonian seasons comparable in how you defined them?
Line 512: what are the ITCZ dynamics in relation to what is observed for these CESM model simulations?
Line 531: although you just have four datapoints from one specimen.
Line 535: it just shows variability in one of your specimens though.
Line 535: do lagoons have strong δ18Osw fluctuations if the organism is always fully submerged and below the wavebase?
Line 545: the start of this sentence reads weird, we investigate how to explain – rewrite.
Line 551: here you should make clear that this precipitation signal you are describing comes from the river water in that region, if you reference to Figure 10.
Line 557: what are the Eocene SST and SAT estimates here?
Line 559: what do you define as summer and winter as there are two relative warm peaks and two relative colder periods?
Line 562: is this shown in a,b,c as your blue line? It should also be singular as it is only one specimen.
Line 572: is there a significant difference between modern and pre-industrial in this sense, since you only show data from 2010-2016 in Fig. 10 and model simulations in Fig. 8? Also, during the monsoon months of July-Sept in Fig. 8, the difference between PI and Eocene is only 50 mm (only 1/6th of the overall precipitation signal) but much larger in the preceding and following months. Consider rewording or adjusting what you mean with magnitude.
Line 580: consider adding “reduced temperature and inferred hydrology record”.
Lines 589-596: you only consider one shell with one datapoint in another year so I would give the opening sentence a lot more nuance. Further, you base this concluding paragraph on your one specimen, even though most of it is describing the model simulation results. Please rewrite this section and adjust the focus.
Figure and table comments
Table 3: Δ47 is classified as IDES and not ICDES and δ18Ow and not δ18Osw in the description.
Figure 5: you compare your δ13Corg record to that of Khanolkar et al. (2017) and both have a CIE close to each other at 5 m and similar trend around 25 m. Are these not the same? How did you correlate their record to yours if your y-axis in this case is m and not age?
Figure 7: The δ18Occ does show a sinusoidal variability but your definitions of which season/month are which are not clear. Is there (a lack of) evidence for growth cessation? The highest δ18Occ in the second year does not coincide with your coldest temperature rather with one of the warmest.
Figure 9: indicate where your study site is.
Figure 10: why are the y-axis gridlines not aligned in a,b,c? Are these not the same absolute values as this is confusing. Also, what do the red and blue lines in a,b,c mean, are they the modern SST and SAT as these are not the same colours as in the legend. And how did the Bartonian SST get calculated, is that from the CESM model simulation? The d,e,f panels are unclear with the stretched y-axis, please reconsider how to make this clearer. Consider adding the name of the region above the panels so the reader has a quick overview? The SAT are also distracting, not discussed, and should not be included.
General language comments
Throughout the manuscript you change from early Bartonian vs late Bartonian or MECO vs post-MECO – be consistent.
Also include a definite article when describing a time period; “the” or “a” “early/late Bartonian”.
You mostly use palaeo- so I assume you are using British spelling but throughout have used paleo- in Lines 13, 57, 81, 240, 362, 369, 403, 444, 448, 531, 533, 534.
Citation: https://doi.org/10.5194/egusphere-2026-1393-RC1
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General comments
Good research on paleoclimate. Please, follow my specific comments to improve your manuscript.
Specific comments
Line 32. I would not start an introduction from “Asia”. However, I would start with a global scale sentence. The introduction is very general at the beginning, and then it becomes progressively more specific.
Lines 32-91. Insert further and recent literature on the Cenozoic climate changes registered in the sedimentary record that incorporates the Eocene:
- Medici, G., Marianelli, D., Gori, F., Cornacchia, I., Brandano, M. 2026. Multi-disciplinary approach to paleokarst occurrence in the Eocene–Oligocene succession of the Apulia Carbonate Platform (Salento, Italy). Facies, doi: 10.1007/s10347-026-00729-5
- Jaramillo-Vogel D, Strasser A, Frijia G, Spezzaferri S 2013. Neritic isotope and sedimentary records of the Eocene-Oligocene greenhouse–icehouse transition: the Calcare di Nago Formation (northern Italy) in a global context. Palaeogeogr Palaeoclimatol Palaeoecol 369:361–376
Line 75. “The Fulra Limestone”. Is it a geological formation or a group from a lithostratigraphic point of view?
Line 91. Clarify the general goal of your research at the end of your introduction.
Line 91. Describe the 3 to 4 specific objectives of your research at the end of your introduction.
Lines 92-110. Insert information on tectonics and structural geology.
Lines 581-596. You can expand the conclusion adding a “take home message”.
Figures and tables
Figure 1. The outcrop images can go in a separate figure.
Figure 4. Increase the graphic resolution to make the graphs more visible.
Figure 6. The spatial scale is a bit unclear. Consider applying changes in the figure or in the caption.
Figure 9. Important figure. Please, increase the graphic resolution in terms of dpi.