the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Feb 2025
The Eocene-Oligocene Transition in the Paratethys: Boreal Water Ingression and its Paleoceanographic Implications
Abstract. The Eocene-Oligocene Transition (EOT) represents a pivotal period in Earth's climatic history, marked by the onset of Antarctic glaciation and global cooling. While deep-sea records have extensively documented this transition, its impacts on marginal and epicontinental seas remain less understood. This study investigates the impacts of the EOT in the Karaburun composite section, located in the eastern Paratethys. Using a multidisciplinary approach that integrates biostratigraphy, geochemistry, geochronology and sequence stratigraphy, a robust chronostratigraphic framework for the latest Eocene to early Oligocene was established. The isotopic shifts observed in benthic and planktic foraminifera δ¹⁸O and δ¹³C records at Karaburun align with global patterns but also reveal localized effects, such as freshwater influx and basin restriction, specific to the semi-restricted Paratethys. The abrupt negative δ¹⁸O shift across the EOB in the Paratethys reflects boreal water ingressions driven by the onset of anti-estuarine circulation between the Nordic seas and Atlantic and the closure of the Arctic-Atlantic gateway, which redirected cold, low-salinity boreal waters through interconnected basins towards the Paratethys. These findings highlight the interplay between global climate drivers and regional hydrological dynamics, providing critical insights into the evolution of marginal marine environments during the EOT. Our results underscore the significance of the Paratethys as a unique archive for studying the onset of global icehouse climate conditions and regional responses.
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Status: open (until 21 Apr 2025)
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RC1: 'Comment on egusphere-2025-479', Noel Vandenberghe, 10 Mar 2025
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paper addresses relevant scientific question, presents novel data, conclusions are acceptable, methods are sufficiently outlined and references can be traced, references title and abstract are OK, presentation is well structured and language is fluent
The main merit of the paper is to present a future reference section for the E/O transitional time in the Eastern Paratethys based on its field description, sedimentology, bio-content, stable isotopes, a new radiometric date,.. and the integration of these multiple data sets. The results are discussed in the broad paleogeogeographic context and refers to key references in the extensive literature about the E/O transitional time. The data set is very valuable although some of the detailed derived conclusions may be amended in the future as common in scientific progress.
Some minor remarks that the authors could take into consideration :
1. Even in the North Sea area microfossil biostratigraphy is suffering from endemism making correlations with the international zonation sometimes difficult. Is the not a fortiori an issue in the Paratethys area ?
2. use consistent sedimentologic terminology : hemipelagic ( line 115) and also bathyal (line 288) together with so many indications of shallow water and lagoonal microfossils, alluvial fans...
3. would a word of explanation about the ratio of 32 examined zircons versus only 11 usable zircons ( line 183) not helping to confirm the value of the new tuff age ? ( the additional table S3 is not helping in this respect).
4. the absence of microfossils on itself is generally not a good argument for a biostrat zonation ( line 206) and the term 'bizarre' dinoflagellate ( line 280) is not helping to understand its biostrat value.
5. Regarding the sequence stratigraphic interpretations (section 5.3) : I prefer the term 'relative sea-level evolution' as is used in the figures. The comparison with global SL is hard to do in this case as 10 cycles are identified in only less than 2 million years...Also it can be expected that vertical tectonic influence plays an important role in the delivery of detrital sediments (a.o. the alluvial fans) to the basin seen the Paratethys context ...I do not understand the reference to Westerhold et al. 2020 relating thickness to orbital changes . Therefore I would consider the detailed interpreted Relative Sea Level curve derived in the section as a well documented RSL to compare in the future to other such RSL curves to be constructed in the area rather than already pretending now its general character in the area.
Citation: https://doi.org/10.5194/egusphere-2025-479-RC1 -
AC1: 'Reply on RC1', Mustafa Yücel Kaya, 17 Mar 2025
reply
Paper addresses relevant scientific question, presents novel data, conclusions are acceptable, methods are sufficiently outlined and references can be traced, references title and abstract are OK, presentation is well structured and language is fluent
The main merit of the paper is to present a future reference section for the E/O transitional time in the Eastern Paratethys based on its field description, sedimentology, bio-content, stable isotopes, a new radiometric date,.. and the integration of these multiple data sets. The results are discussed in the broad paleogeogeographic context and refers to key references in the extensive literature about the E/O transitional time. The data set is very valuable although some of the detailed derived conclusions may be amended in the future as common in scientific progress.
Some minor remarks that the authors could take into consideration :
- Even in the North Sea area microfossil biostratigraphy is suffering from endemism making correlations with the international zonation sometimes difficult. Is the not a fortiori an issue in the Paratethys area ?
Thank you for your comment. We acknowledge that endemism can pose challenges for biostratigraphic correlations, particularly in semi-isolated basins like the Paratethys. However, in the context of our study, this issue becomes more prominent starting from the NP23 Calcareous Nannofossil zonation, which was not covered here. During NP21 and (early part of) NP22, the Paratethys remained connected to the world oceans, as also evidenced by our findings. This connection is reflected in the presence of microfossils—including dinoflagellates, calcareous nannofossils, and both benthic and planktic foraminifera—that were instrumental in constructing the international biozonation schemes.
Moreover, the regional biozonations can still be effectively correlated with international schemes through integrated stratigraphic approaches, such as combining biostratigraphy with chemostratigraphy. In our study, we have carefully considered these factors and have utilized the most robust correlations available to ensure the reliability of our stratigraphic framework.
- use consistent sedimentologic terminology : hemipelagic ( line 115) and also bathyal (line 288) together with so many indications of shallow water and lagoonal microfossils, alluvial fans...
Thank you for highlighting the importance of consistency. We will carefully review and improve the consistency of sedimentological terminology.
We use the term hemipelagic to describe sediments deposited on a continental shelf, consisting primarily of clay- and silt-sized terrigenous grains, with additional biogenic material derived either from the nearest landmass or from organisms living in the water column. Given this definition, we prefer to retain the term as it is (Line 115).
Regarding the depositional environment interpretation, we acknowledge that the original sentence was not comprehensive in addressing shallow-water indicators. To improve clarity, we will revise it as follows:
Original:
"Based on lithological, sedimentological, and paleontological characteristics, we interpret the depositional environment as a bathyal outer-shelf setting."Revised:
"Based on lithological, sedimentological, and paleontological characteristics, we interpret the depositional environment as a shelf setting exhibiting littoral and neritic characteristics, depending on fluctuating sea levels."- would a word of explanation about the ratio of 32 examined zircons versus only 11 usable zircons ( line 183) not helping to confirm the value of the new tuff age ? ( the additional table S3 is not helping in this respect).
Thank you for your suggestion. We acknowledge that the discrepancy between the 35 examined zircons and the 11 usable zircons may require further clarification. We will add a brief explanation in the text to clarify the selection criteria for usable zircons. Specifically, we applied rigorous filtering based on zircon morphology, U-Pb concordance, and common Pb content to ensure the reliability of the final age determination. While Table S3 provides detailed data, we will also explicitly discuss these criteria in the main text to enhance transparency and support the robustness of the new tuff age.
- the absence of microfossils on itself is generally not a good argument for a biostrat zonation ( line 206) and the term 'bizarre' dinoflagellate ( line 280) is not helping to understand its biostrat value.
We respectfully disagree with Referee 1’s comment. The extinction event of the rosette-shaped Discoaster species is a well-established marker for defining the base of Zone NP21. Furthermore, this bioevent is not an isolated criterion but is corroborated by its correlation with dinoflagellate cyst zonations and geochemical data (chemostratigraphy), further reinforcing its reliability.
We will remove the word “bizarre” as suggested by the Referee 1.
- Regarding the sequence stratigraphic interpretations (section 5.3) :
I prefer the term 'relative sea-level evolution' as is used in the figures. The comparison with global SL is hard to do in this case as 10 cycles are identified in only less than 2 million years...Also it can be expected that vertical tectonic influence plays an important role in the delivery of detrital sediments (a.o. the alluvial fans) to the basin seen the Paratethys context ...
We agree with the comment and will use “relative sea-level evolution” instead of “relative sea-level fluctuations” throughout the manuscript. The necessary changes will be made accordingly.
The comparison with global sea-level reconstructions can be effectively conducted by comparing the Eastern Paratethys relative sea-level evolution with the smoothed global curve of Miller et al. (2020). While tectonic influence is certainly expected, the parallel trends observed between the Eastern Paratethys, the Mediterranean (Italian sections), and the North Sea (see Section 6.3) further support the dominance of an eustatic control.
I do not understand the reference to Westerhold et al. 2020 relating thickness to orbital changes .
Original:
“This variation in depositional thickness may be linked to orbital forcing across the late Eocene to early Oligocene (e.g., Westerhold et al., 2020).”
We agree that the original sentence was unclear and will revise it for better clarity. Specifically, we will elaborate on the role of obliquity forcing in controlling depositional thickness during the latest Eocene. This orbital influence has been recognized in both terrestrial and marine records, as supported by previous studies (e.g., Abels et al., 2011; Jovane et al., 2006; Boulila et al., 2021). The revised sentence will be as below:
Revised:
"This variation in depositional thickness could be related to obliquity forcing during the latest Eocene, a process that has been identified in several terrestrial and marine records (e.g., Abels et al., 2011; Jovane et al., 2006; Boulila et al., 2021)."
Therefore I would consider the detailed interpreted Relative Sea Level curve derived in the section as a well documented RSL to compare in the future to other such RSL curves to be constructed in the area rather than already pretending now its general character in the area.
Original:
“These high resolution sea-level fluctuations provide a refined reconstruction of Eastern Paratethys sea-level changes, improving upon previous studies (e.g., Popov et al., 2010).”
The original sentence will be changed as below as suggested by the Referee 1.
Revised:
"These high-resolution sea-level fluctuations offer a refined reconstruction of Eastern Paratethys sea-level changes (e.g., Popov et al., 2010) and provide a valuable framework for future studies to further reveal the history of Paratethys sea-level evolution."
Citation: https://doi.org/10.5194/egusphere-2025-479-AC1 -
RC2: 'Reply on AC1', Noel Vandenberghe, 17 Mar 2025
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Thanks for your replies. Accept the replies.
Citation: https://doi.org/10.5194/egusphere-2025-479-RC2
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AC1: 'Reply on RC1', Mustafa Yücel Kaya, 17 Mar 2025
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