the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jun 2026
Middle to Late Miocene paleoceanographic evolution of the south-eastern Indian Ocean (ODP Site 752) inferred from nannofossil assemblages
Abstract. Understanding how the Earth’s system behaves under climate forcing conditions is critical for predicting how future climate change scenarios may affect the planet. In this regard, the Miocene, with an atmospheric temperature ~4 °C higher than modern and a near-modern ocean configuration, stands out as a potential analogue for future climate projections. Our current understanding of the Miocene derives principally from the Atlantic and Pacific Oceans, whereas the Indian Ocean remains understudied. Nevertheless, different studies performed in the equatorial Indian Ocean have revealed that the multiple climate shifts that occurred during the Miocene modified both the atmospheric and oceanic processes in this basin (e.g., changes in westerlies position, nutrient flux and productivity). In the present study, we investigate how changes in the climate conditions affected the surface ocean system at Ocean Drilling Program (ODP) Site 752, located in the southeastern Indian Ocean, across the Middle to Late Miocene. For this purpose, we present a new quantitative dataset of calcareous nannofossil assemblages, in conjunction with already existing multi-proxy data.
Our results indicate that the warm atmospheric conditions reached during the Miocene Climatic Optimum (MCO), starting at ~16.9 Ma, caused an intensification of the seasonal signal at ODP Site 752 that lasted until 15.32 Ma. After this period, we detected a shift in the calcareous nannofossil assemblage towards species characteristic of low-nutrient and warm-water conditions (e.g., Reticulofenestra pseudoumbilicus, Reticulofenestra haqii). Nevertheless, with the progression towards the Late Miocene Cooling, which started at ~7 Ma, and the increased influence of the cooler and more productive Southern Ocean waters, the calcareous nannofossil assemblage evolved into a community characterised by relatively high-nutrient and cold-water taxa (e.g., Coccolithus pelagicus, Calcidiscus leptoporus). By 10.67 Ma, we recorded a reversion in the nannoplankton assemblage to species typical of low-nutrient and warm-water conditions that lasted until 9.88 Ma. The comparison with available eNd(t) records for Site 752 and 707 (located in the equatorial Indian Ocean) revealed that this change in the assemblage composition responded to a strengthening of the Pacific Ocean water influx occurring at this time. High-nutrient and cold-water conditions were reestablished after 9.88 Ma, lasting until the end of the studied record at ~7.34 Ma. Our evaluation of the primary producer community from a multi-proxy-based perspective revealed a progressive evolution of the surface ocean conditions in the southeastern Indian Ocean region across the Middle to Late Miocene for the first time. Furthermore, the strong correlation with existing proxy data for global climate records suggests that large-scale climate events are the main factors driving the changes in the primary producer community and surface ocean conditions at the Broken Ridge during the Middle to Late Miocene. However, certain regional processes (e.g., strengthening of the Pacific Ocean influx) may also occasionally influence surface ocean conditions. Moreover, this multi-proxy approach allowed us to better understand the Indian Ocean–Southern Ocean interaction, and more specifically, the nutrient transport efficiency between these two ocean basins during a climate change scenario. Specifically, our multi-proxy-based dataset revealed that the Antarctic divergence region experienced a northward migration after the MCO, leading to a deep reorganisation of nutrient transport across the surface southeastern Indian Ocean.
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RC1: 'Comment on egusphere-2026-3191', Anonymous Referee #1, 12 Jul 2026
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AC1: 'Reply on RC1', Xabier Puentes Jorge, 12 Aug 2026
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Answer to reviewer:
Major Comments
1.
We appreciate the reviewer´s feedback and suggestions. We agree that phytoplankton ecology encompasses multiple parameters and that changes in the whole assemblage composition are influenced by all these parameters simultaneously. However, some of these parameters have a low influence in an open-ocean context and within the studied time interval. For example, light availability in an open ocean setting tends to remain stable, in contrast to coastal areas where higher productivity (e.g., bloom episodes) or sediment discharge from riverine inputs can lead to a decrease in light intensity in the water column. On the other hand, the geochemical data provided in this study (e.g., the δ13C isotope data in Figure 6C) provide the necessary information to rule out changes in carbonate geochemistry as a driver of nannoplankton assemblage composition. To address this comment, we will add a statement regarding the influence of sample preservation (see answer to comment 3).
In regard to the evolutionary changes, the observed variations in the abundance of the calcareous nannofossil assemblage do not correspond to any of the recognised extinctions or evolutionary events of the species used to infer the different taphogroups and are, therefore, not considered a driver. Furthermore, it has been proven that evolutionary changes are commonly related to major variations in environmental conditions (Lowery et al., 2020), and they should not be considered a problem for environmental reconstruction as long as the ecological preference of the species can be inferred. Nevertheless, we agree that other parameters may either bias the assemblage distribution (e.g., preservation state) or displace certain nannoplankton species (e.g., nutrient availability, temperature or water-column stratification). To address this comment, which we fully agree needs to be better clarified within the present manuscript, we will improve the clarity of the text when referring to the parameters or processes that influence the nannoplankton.
Regarding the reviewer's concerns about the interpretation based on the increased abundance of R. minuta, we would like to point out that a system limited by N need not be oligotrophic in the typical sense. In fact, there are multiple examples of nutrient-enriched systems where productivity is limited because the abundance of available N is insufficient to sustain even higher productivity (e.g., estuarine areas or coastal regions). Additionally, R. minuta is commonly regarded as an r-selected species based on the ecological preference of its modern analogues belonging to the small Gephyrocapsa spp. group, which thrives in these high-nutrient systems with low available N. Therefore, we infer that our assemblage composition represents the same environmental conditions at ODP Site 752. Nevertheless, we agree that the current phrasing may be somewhat ambiguous in its presentation of our reasoning. We consequently intend to modify the related section to improve clarity regarding the taphogroup interpretation. With respect to the ecology of large-sized reticulofenestrids, lines 355 – 360 present the ecological affinities of large reticulofenestrids according to different authors; meanwhile, lines 360 – 362 provide further support for an ecological preference of large reticulofenestrids for higher-nutrient environments. These same higher nutrient conditions are noted for TG 5a and 5b, where large reticulofenestrids are dominant (See lines 388–403).
2.
We thank the reviewer for this valuable observation, and we agree that further explanation about what we considered a taphogroup would improve the clarity and readability of the manuscript. Therefore, we added a statement on lines 292 – 296 with the corresponding definition: “a group of species clustered together based on their ecological characteristics while accounting for taphonomic filtering”.
Regarding the validation of the taphogroups as discrete ecological states, we acknowledge that the statistical values do not fall perfectly within the range of “strong dissimilarities”. Nevertheless, an increased number of samples can introduce greater variability in the results and, therefore, lower cophenetic correlation values. To provide more clarity, we will introduce this information into Section 3.3. Concerning the treatment of the taphogroups as discrete ecological states, we treated them as a gradient with an arbitrary cut-off to allow sensible inferences about the observed changes within the assemblage. The differentiation between the methodology used for cluster distribution and for the determination of environmental aspects is outlined in lines 215 – 220 “2-D and 3-D non-metric Multidimensional Scaling (nMDS) was applied to the assemblage record to correlate each cluster with a set of available paleoproxy data from ODP Site 752, and nearby locations (see subchapter 2.4 for details on proxy data). The paleoenvironmental proxy data used in the nMDS method were previously interpolated to ensure a proper fit with the calcareous nannofossil assemblage record”. Nevertheless, we will modify lines 316–325 to clarify this aspect in the results section as well.
3.
We thank the reviewer for the comment. We agree that changes in calcareous nannofossil preservation, or poor preservation, can influence assemblage distribution. Nevertheless, prominent dissolution and recrystallisation effects were exclusively found in very delicate specimens (e.g., the genus Discoaster), and therefore, identification at species level was problematic for this group whereas it was possible for the remaining species in the assemblage. For example, light and scanning electron microscopy for the genus Reticulofenestra did not reveal major alterations indicative of dissolution or recrystallisation. Furthermore, despite the impossibility to separate specimens of the genus Discoaster at the species level, all of their representatives share same ecological affinities. Therefore, it can be assumed that the observed variations in the assemblage composition reflect environmental changes, rather than a preservation effect. However, we fully agree that this information is necessary for the clarity of the manuscript and will add an expanded statement on the preservation effect in section 3.1.
4.
The decision to cut off taxa with an abundance below 0.5% was made to identify the major species influencing the overall assemblage distribution. Adding species below the 0.5% cut-off introduces noise into the clustering data, making it more difficult to detect robust ecological patterns. Regarding the weak bootstrapping support for individual clusters, this term refers to the formation of a cluster based on a single sample. Nevertheless, the bootstrapping applied in this study supports the clustering ordination from Section 3.3. After the reviewers' comment, we realised that this term can lead to a misleading interpretation of clustering robustness. Therefore, we will modify this sentence to clearly express its meaning.
5.
We thank the reviewer for the insightful comment. We agree that precise age control is needed in order to ascertain the correspondence with climatic or oceanographic events identified in other records. In the case of the mentioned transitions (10.67 and 9.88 Ma) and their relation with the increase in eNd from De Vleeschower et al. (2025), the neodymium data were acquired specifically for ODP Site 752, and therefore share the same age-depth model. The remaining proxies were also recorded at site locations with an astronomically tuned age-depth model, limiting the age error and allowing for a feasible intercomparison with Site 752. The only data without an astronomically tuned age-depth model available correspond to the data from Site 707 (Gourlan et al., 2008). Nevertheless, the age-depth model for this site was based on both micropaleontological and Sr-isotope data and is therefore highly accurate. Moreover, we compared the data against the GTS20 Sr-isotope and micropaleontological ages to verify that the data from Gourlan et al. (2008) are accurate. We consider it necessary to clarify the suitability of these datasets and will add the above-mentioned details about the age models used in the methodology in section 2.4.
6.
We are grateful to the reviewer for this helpful suggestion. We agree that the presented dataset does not reveal per se the movement of the Antarctic Divergence Zone. Therefore, we modified lines 50-52 as follows: “Specifically, our multi-proxy-based dataset revealed how the surface ocean of the Broken Ridge region and the southeastern Indian Ocean nutrient transport evolved in relation to the northward migration of the Antarctic divergence region after the MCO.”.
Regarding the multiple processes involved in shaping the environmental conditions at Site 752 and, more specifically, the calcareous nannoplankton assemblage distribution, we are aware that multiple processes may occur concomitantly, enhancing or weakening the general effect on the planktonic community. Nevertheless, we consider that the regional oceanographic processes taking place at ODP Site 752 are determined by the intensity and position of the Antarctic Circumpolar Current and, therefore, by the position of the Antarctic divergence region. In this regard, we state in Lines 537 – 551 that we hypothesise that 2 main factors have a higher influence in shaping the environmental conditions at Site 752:
“Substantial variations in the nutricline depth and surface ocean nutrient content can be inferred through the 5 intervals defined at ODP Site 752. According to the calcareous nannofossil data, we can hypothesise that these changes in the nutrient cycle are linked to two main factors: 1) changes in the nutrient availability due to variations in the nutrient transport (William and Follows 2003; Sarmiento et al., 2004; Moore et al., 2013); 2) changes in the nutrient composition via intensification of the organic matter degradation in the water column, also known as remineralisation (Lefèvre et al., 1996; Feely et al., 2004). Here, we explore in detail how factor 1 could potentially affect the nutrient availability in the surface ocean during the Middle to Late Miocene. Nutrients are transported into the Broken Ridge area via the Tasman Leakage or the Antarctic divergence region in the modern ocean (Koch-Larrouy et al., 2010; Lyu et al., 2023; Bali et al., 2025). However, Lyu et al. (2023) established the onset of the Tasman Leakage during the Late Miocene, at 7 Ma, although some sporadic occurrences of the Tasman Leakage could occur between 10.8-7.5 Ma. Therefore, we consider the nutrient supply in the Broken Ridge region intrinsically connected to the dynamics of Antarctic divergence during our study interval.”.
Here we clearly state that nutrients are sourced to the Site 752 region through two main pathways, which are the Tasman Leakage and the Antarctic divergence region. Furthermore, we clearly explain that the Tasman Leakage was not an active pathway for most of the studied time interval, leaving only the Antarctic divergence region as a potential source.
Minor comments:
1.
We thank the reviewer for the insightful comment. The Indian Ocean reached a near-modern oceanographic configuration at ~15 Ma, whereas for the Atlantic and Pacific Oceans the Central American Seaway was still open. This makes the Indian Ocean a comparable ocean basin between the Miocene time interval and the modern ocean. In relation to the suitability of ODP Site 752 in comparison to other sites located in the Indian Ocean, it needs to be emphasized that this location was at a perfectly suitable water depth to trace intermediate water redistribution across the Indian Ocean. This is of great relevance, as these intermediate waters, which originated at higher latitudes, are responsible for the nutrient transport from the Southern Ocean into the Indian Ocean and can provide important insights about nutrient transport changes across climate change scenarios. We acknowledge that providing this information to the readers is important to better understand the relevance of ODP Site 752 in this study and, thus, we will add it to the introduction section.
2.
We are grateful to the reviewer for this helpful suggestion. We will specify that transects were always finalised and that the 400 counted specimens were stablished as a stopping criterion.
3.
We thank the reviewer for the comment. The definition ofthe taphogroups will be added to the manuscript (see answer to major comment 2). Regarding the use of cluster and taphogroup throughout the manuscript, we want to clarify that if the reviewer's concern is related to the mention of clusters in Figure 6-A, we intended this term to remain as a "cluster". As the figure only displays the distribution of the nannoplankton assemblage groups (clusters) across the studied time interval, and not its ecological significance. Apart from that, we will check the manuscript to ensure the consistent use of the terms Taphogroup and Cluster when needed.
Additional corrections
Line 418: 4 subintervals was changed for 5 subintervals.
Citation: https://doi.org/10.5194/egusphere-2026-3191-AC1
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AC1: 'Reply on RC1', Xabier Puentes Jorge, 12 Aug 2026
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- 1
This manuscript presents a new quantitative calcareous nannofossil record from ODP Site 752 spanning the middle to late Miocene and integrates these data with previously published geochemical proxies to reconstruct the evolution of the southeastern Indian Ocean. The dataset itself is valuable because this region remains considerably less studied than the Atlantic and Pacific Oceans, and the new assemblage record represents a useful addition to the Miocene palaeoceanographic literature. The manuscript is generally well written, the figures are of high quality, and the stratigraphic dataset will likely become an important reference for future work.
However, I have substantial concerns regarding the interpretation of the ecological data. Throughout the manuscript, the authors repeatedly infer specific environmental conditions (nitrogen limitation, phosphorus excess, thermocline depth, nutrient availability, seasonality and frontal migration) from changes in calcareous nannofossil assemblages with a degree of certainty that is not supported by either the statistical analyses or the current understanding of Miocene nannoplankton ecology. In many cases, the discussion extends considerably beyond what can be demonstrated from the available evidence. Overall, I believe the dataset is worthy of publication, but the manuscript requires substantial revision before the conclusions can be considered robust. I therefore recommend major revision.
Major comments
1. The ecological and nutrient-regime interpretations are more specific than the available evidence supports (Lines 221–237; 326–528)
The discussion repeatedly interprets the identified assemblages as evidence for relatively specific environmental conditions, including changes in nutrient availability, nitrogen limitation, phosphorus excess, thermocline depth, mixed-layer dynamics, and seasonality. While many of these interpretations are plausible, they are not necessarily uniquely constrained by the assemblage data.
Calcareous nannoplankton ecology is multivariate, and assemblage composition may reflect the combined influence of temperature, nutrient availability, water-column stratification, light availability, carbonate chemistry, evolutionary change, and preservation. I therefore encourage the authors to distinguish more clearly between the observed assemblage changes and the environmental mechanisms proposed to explain them.
For example, R. minuta is interpreted as reflecting nitrogen-limited conditions and, through the proposed relationship between nitrogen limitation and phosphorus excess, TG1 is ultimately characterised as representing “high-nutrient (N-limited) cold waters” (Lines 328–346). This interpretation appears more specific than the available evidence permits. The cited literature provides a plausible mechanistic framework, but the Site 752 data presented here do not directly reconstruct nitrogen or phosphorus availability or their relative balance. Moreover, the description of these waters as simultaneously “high-nutrient” and “N-limited” requires clearer explanation of which nutrient pool is inferred to be elevated and which is limiting.
Similarly, the ecological significance of Reticulofenestra size variation remains complex. The manuscript itself acknowledges contrasting interpretations of large reticulofenestrids, which have been associated with both oligotrophic/deep-nutricline conditions and enhanced nutrient availability (Lines 355–363). The authors should therefore explain more clearly why particular ecological interpretations are preferred for Site 752 and acknowledge where alternative explanations remain possible.
The comparison with independent geochemical proxy records is a strength of the study and can provide support for broader changes in surface-ocean conditions. However, the authors should distinguish between nutrient availability, primary productivity, and export productivity, which are related but not equivalent. Agreement with a productivity or export-productivity proxy may support a broader environmental change but does not necessarily independently validate a specific mechanism such as nitrogen limitation, phosphorus excess, or a particular N:P balance. I therefore suggest moderating the most mechanistically specific interpretations unless they can be supported by additional independent evidence.
2. Definition and interpretation of the “taphogroups” (Lines 198–220; 279–325)
The manuscript uses hierarchical clustering to identify five statistically defined assemblage groups, which are subsequently termed “taphogroups” and assigned specific environmental interpretations. I suggest that the authors clarify their use of the term taphogroup. The groups are defined primarily from variations in taxonomic relative abundance rather than from taphonomic characteristics, yet preservation at Site 752 is described as moderate to moderately poor (Lines 240–242). It is therefore important to clarify whether “taphogroup” is intended to refer to (i) assemblages shaped primarily by taphonomic processes, (ii) fossil assemblages used as palaeoecological groupings while acknowledging taphonomic filtering, or (iii) simply statistically defined assemblage clusters. If the latter, “assemblage group” or “assemblage cluster” may be clearer. If the authors retain “taphogroup”, the term should be clearly defined and its use justified.
I also encourage the authors to be more cautious in treating these groups as discrete ecological states. The reported cophenetic correlation (0.76) indicates a reasonable but not exceptionally strong representation of the original dissimilarities by the dendrogram, while the authors acknowledge weak bootstrap support for individual clusters and relatively high nMDS stress values (0.2565 for two dimensions and 0.2018 for three dimensions). Importantly, these different statistics address different aspects of the analysis: the nMDS stress does not directly test the validity of the hierarchical clusters, and ANOSIM significance indicates differences among the predefined groups but does not, by itself, demonstrate that they represent naturally discrete ecological states. I therefore suggest that the authors better distinguish between statistically defined assemblage groupings and the subsequent environmental interpretation of those groups.
3. Potential influence of preservation on assemblage composition and morphotype identification (Lines 240–242)
The authors state that calcareous nannofossil preservation ranges from moderate to moderately poor, based on observations using light and scanning electron microscopy. Given that the principal interpretations of this study rely on changes in relative abundance, diversity, and Reticulofenestra morphotype distributions, I think the potential influence of preservation warrants more consideration.
Preservation does not necessarily affect all taxa or morphotypes equally, and dissolution, fragmentation, and overgrowth can alter relative assemblage composition. This issue may be particularly relevant to the Reticulofenestra classification used here, because several morphotypes are distinguished not only by size but also by whether the central area is interpreted as open or closed (Lines 183–197). The authors should therefore discuss whether preservation could affect the identification or relative abundance of these morphotypes and whether any systematic changes in preservation coincide with the major assemblage transitions identified in the study.
I am not necessarily suggesting that preservation drives the observed patterns, but the manuscript should provide sufficient evidence to demonstrate that the major assemblage and morphotype changes are primarily ecological rather than taphonomic. If preservation was assessed systematically, the authors should describe the criteria used and, if possible, show whether preservation varies through the studied interval.
4. The definition and robustness of the assemblage clusters require further clarification (Lines 198–220; 279–325)
The overall multivariate approach is broadly appropriate for ecological assemblage data, but several analytical decisions require clearer justification. In particular, the authors should explain the rationale for excluding taxa with an average abundance below 0.5% and discuss whether this filtering materially affects the resulting assemblage structure. More importantly, the basis for selecting the similarity cut-off used to define the five clusters (and the subsequent subdivision of cluster 5) should be explained. At present, it is unclear whether these thresholds were determined using an objective criterion or selected primarily to obtain ecologically interpretable groups.
This is particularly relevant because the authors report weak bootstrap support for individual clusters. I therefore encourage the authors to provide a clearer assessment of cluster robustness. This does not necessarily require repeating the entire analysis using multiple clustering methods, but the authors should demonstrate that the principal assemblage groupings are not highly sensitive to the chosen cut-off or explain the limitations associated with the weaker-supported groups. The methodological rationale for the final number of assemblage groups should be made transparent.
5. Chronological uncertainty should be considered when discussing the timing and apparent synchronicity of events (Lines 159–168 and throughout Section 4.2)
The study uses the astronomically tuned age model of Lyu et al. (2023), with additional biostratigraphic support from Puentes-Jorge et al. (2025). Given that several interpretations in Section 4.2 rely on the apparent temporal correspondence between changes in the Site 752 nannofossil assemblage and climatic or oceanographic events identified in other records, I suggest that the authors briefly discuss the chronological uncertainty associated with these comparisons.
This is particularly relevant where relatively precise assemblage transitions (e.g. 10.67, 9.88 and 9.22 Ma) are interpreted as coincident with changes in independently dated records. The authors do not necessarily need to provide a new quantitative age-model uncertainty analysis, but they should clarify which proxy records share the Site 752 chronology and which are based on independent age models. Where records are independently dated, apparent synchronicity should be interpreted with appropriate caution unless the respective chronological uncertainties are sufficiently small to resolve the proposed temporal relationship.
6. Spatial scale and certainty of some palaeoceanographic interpretations (Lines 47–52; 404–528)
The manuscript uses the Site 752 nannofossil assemblage together with previously published regional proxy records to develop a broader reconstruction of Southern Ocean–Indian Ocean interactions. This is a potentially valuable synthesis. However, some conclusions appear more spatially and mechanistically specific than can be directly constrained by the available evidence. In particular, the statement in the Abstract that the dataset “revealed that the Antarctic divergence region experienced a northward migration after the MCO” (Lines 50–52) appears stronger than the evidence presented. The conceptual model itself appropriately describes the migration of the polar front system as hypothesised (Lines 522–527), and I suggest maintaining a similar level of caution throughout the manuscript.
A change in assemblage composition at Site 752 that is consistent with increased influence of cooler and/or nutrient-rich southern-sourced waters does not, by itself, uniquely demonstrate a northward migration of the Antarctic Divergence. Similar changes at the site could potentially arise from changes in the strength of water-mass advection, frontal intensity, mixing, nutrient supply, or circulation pathways without requiring a simple latitudinal displacement of the frontal system. The authors should therefore distinguish more clearly between what is directly recorded at Site 752 and the broader circulation mechanism proposed to explain that signal. Where the latter is not independently constrained by spatially distributed records, it should be presented as a hypothesis or conceptual interpretation.
Minor comments
Lines 101-114
The introduction would benefit from explaining more clearly the specific scientific significance of Site 752. The authors establish that the Indian Ocean is comparatively understudied, but the rationale for studying this particular site could be developed further. What does the location of Site 752 at Broken Ridge allow the authors to address that cannot be resolved from existing Indian Ocean records, or from better-studied records in the Atlantic and Pacific? Clarifying the site's particular sensitivity to Southern Ocean–Indian Ocean exchange and/or regional circulation changes would provide a stronger motivation for the study and better define its broader palaeoclimatic significance.
Lines 170–177
Please clarify the counting protocol. The authors state that additional transects were counted when the initial count was below 400 specimens, whereas the average count was approximately 600 specimens per sample. Was a minimum count of 400 specimens therefore used as the stopping criterion, with higher counts resulting from completion of the final transect? Clarifying this would make the quantitative methodology more reproducible.
Section 3.3 and corresponding figures
I miss a clear explanation of the point at which the statistically defined clusters are subsequently termed “taphogroups” (TG1–TG5). Please define what is meant by a taphogroup in this study and explain the basis for this transition in terminology. The use of “cluster” and “taphogroup” should also be consistent across the text and corresponding figures.