Ocean freshening near the end of the Mesozoic

Domain: earth_science
Published: 2025-08-06
Editorial outcome: major_revision

Abstract
Paleogeographic changes have significantly shaped ocean circulation and climate dynamics throughout Earth’s history. This study integrates geological proxies with climate simulations to assess how ocean gateway evolution influenced ocean salinity near the end of the Mesozoic (~66 Ma). Our modeling results demonstrate that 1) Central American Seaway shoaling reorganizes ocean currents, and 2) Arctic marine gateway restrictions, confining Arctic–Global Ocean exchange exclusively to the Greenland–Norwegian Seaway, drive Arctic Ocean surface freshening and southward outflow of buoyant, low-salinity waters. However, only the combined effect of these two factors lead to both Arctic freshening and increased water mass stratification in the Greenland–Norwegian Seaway, proto-North Atlantic, and the Western Tethys. This scenario aligns with Maastrichtian palynological, micropaleontological, and geochemical records from high- and low-latitude sites. Our findings highlight the profound impact of these latest Cretaceous paleogeographic reconfigurations in altering global salinity patterns, underscoring their role as key drivers of global climate dynamics.

Introduction
Disruptions in global marine environments have played a pivotal role in shaping the history of life on Earth. The reconfigurations of landmasses and marine gateway connections are fundamental drivers of changes in global circulation patterns, biogeochemical cycles, and marine ecosystems. These long-term transformations significantly impact the salinity of surface waters in the oceans, leading to the redistribution of water masses through salinity-driven stratification. While the substantial effects of marine gateways on oceanic systems have been well-documented1–4, and the role of global salinity distribution prior to the end of the Mesozoic is well-known2,5–7, the more direct impact of paleogeographic reconfigurations on the global salinity budget during the latest Cretaceous remains unexplored. This is particularly surprising, given the potential of the salinity to influence surface phytoplankton biomass and productivity, which in turn affects oxygen availability within the water column, alters the ocean-atmosphere carbon dioxide equilibrium, and ultimately shapes the ecological structure of marine environments, potentially driving changes on a planetary scale.

This study focuses on paleoceanographic changes in the Arctic Ocean (ArO), Atlantic, and Western Tethys (WT), driven by variations in marine connections around the ArO2,8,9 and shoaling in the Central American Seaway (CAS), which separated the American continents during the Late Cretaceous10–15. Some evidence suggests that a semi-emergent island chain between the Pacific and Atlantic oceans may have existed about 70 Ma12. The tectonic evolution of the Caribbean Plate, which led to the formation of the oceanic plateau and volcanic arc, started even earlier and occurred over a longer period, contributing to the shoaling of the CAS well before the end of the Cretaceous. These long-term processes likely played a crucial role in the formation of a marine threshold, reducing the exchange of water masses between the Atlantic and Pacific oceans. While the exact timing of this transformation remains uncertain, geological proxies indicate a significant reorganization of ocean circulation patterns in the Maastrichtian. This is supported by the emergence of volcanic promontories on the Caribbean oceanic plateau10,11, which helped create islands closely spaced between the Pacific and Atlantic oceans12. If the land connection was shallow enough to allow the exchange of terrestrial vertebrates between the Americas13, the water exchange between the Atlantic and Pacific oceans must have been significantly restricted. Interpreting the isolated palynological, micropaleontological, and geochemical data recovered from globally distributed sections, as illustrated in Fig. 1 and Supplementary Fig. 1(SF1), requires placing them within a broader global context, which can be achieved through numerical modeling. Therefore, we integrate the Earth System Model (ESM) simulations with geological proxies to enhance our understanding of the impact of paleogeographical changes on ocean circulation and climate transitions during the Maastrichtian. We examine contrasting scenarios, including CAS shoaling and variations in marine connections around the ArO, to identify the optimal fingerprint of global environmental shifts and compare their underlying effects with geological proxies. Building on this approach, we explore global patterns, providing valuable insights into how long-term disruptions in the salinity budget shape the chemical and physical properties of seawater, ultimately impacting marine primary producers that are particularly sensitive to such changes.

Results
Numerical model simulations
We utilize the fully coupled atmosphere-ocean ESM, COSMOS, to simulate key paleoceanographic scenarios for the Maastrichtian. These scenarios include hallowing in the CAS and restrictions in the marine connections between the ArO and the Global Ocean (GO). While these configurations have been previously proposed in the literature2,8–14, their precise timing and potential global impacts remain poorly understood. The shallowest water depth of the CAS is set to 10 m, ensuring that water masses flow through the CAS only from the Atlantic to the Pacific, preventing deeper Pacific waters from entering the Atlantic. This is essential, as it allows for the investigation of the effects of Arctic gateway opening and closure on salinity and temperature in the proto-North Atlantic (pNA), independent of Pacific water influence. Additional tested depths include 500 m, similar to the depth of the island arc in the western Pacific today. The deepest conditions tested range from 1700 to 2500 m, following reconstruction from the published literature16. Detailed information on the reasons for selecting these specific CAS depths can be found in the ‘Methods’ section. First, we examine changes in ocean current directions driven by various depths in the CAS and limitations in marine connections around the ArO. Our results show that water flow from the Atlantic to the Pacific oceans is restricted when the depth of the CAS is reduced to 10 m, regardless of constraints around the ArO (Fig. 2). Additionally, we investigate variations in global water salinity and temperature, both horizontally and vertically, driven by bathymetric changes in the CAS and around ArO (SF2). This experiment reveals that restrictions in the marine connections between the ArO and GO result in fresher surface waters outflow via the Greenland-Norwegian Seaway (GNS) (SF2I). It is also evident that paleogeographical changes have a stronger effect on sea surface salinity than on sea surface temperature distribution (SF2I), which is not the case for the deeper ocean, as shown for the depths 420 and 960 m (SF2II–III). Given that salinity variations can also result from shifts in the local precipitation-minus-evaporation (P-E) balance, we additionally simulate P-E in the North Atlantic–Arctic region under different gateway configurations between the Arctic and pNA oceans, as well as varying CAS depths (SF3). The results show that P-E differences between simulations are minimal and confined to localized areas. Therefore, it is highly unlikely that salinity changes in the North Atlantic region were driven by P-E variability.

To further explore the salinity trends, we analyze salinity and temperature transects within the ArO, pNA and WT, comparing various paleogeographic scenarios, including shallowing in the CAS and restrictions around the ArO (Fig. 3). Exclusive shallowing of the CAS increases sea-surface salinity in the ArO (Fig. 3.1A–C). Further limitation of marine connections around the ArO, restricting water-mass exchange between the ArO and GO to occur only through the GNS, yields a contrasting signal, resulting in decreasing sea-surface salinity in the ArO (Fig. 3.1D). Simulations with a CAS depth of 1700–2500 m show minimal differences in salinity in the Atlantic and WT (Fig. 3.2A), compared to simulations with a depth of 500 m (Fig. 3.2B). More pronounced changes in salinity stratification in the Atlantic occur when CAS depths are shallowest (Fig. 3.2C, D). Deep CAS and restricted ArO do not result in enhanced water column stratification. These findings suggest that the threshold for unidirectional flow from pNA to Pacific likely occurred between depths of 500 m and 10 m, within a paleodepth range supported by existing geological constraints (SN1).

Simulations of a shallow CAS with open marine connections around the ArO through the GNS, Western Interior Seaway (WIS), and Hudson Seaway (HS), compared to a shallow CAS with restricted marine connections around the ArO, provide important additional insights about the ArO freshening (Fig. 4). In the shallow and restricted configuration, water-mass exchange occurs solely through the GNS. The decrease in sea-surface salinity in the ArO, likely driven by riverine run-off, results in an outflow of buoyant, low-salinity waters southward toward the pNA. These results support the finding that restricted connections between the ArO and GO, cause freshening of the ArO surface. We continue our investigation on the Arctic-Atlantic-Tethys salinity gradient by examining water transects within the GNS and the pNA, with a particular focus on both salinity and temperature changes (Fig. 5). Establishing the influence of CAS depth on ocean freshening and water column stratification is also essential.

In the scenario with open marine connections and a CAS depth of 10 m (Fig. 5.1A, B), water masses flowing southward from the GNS sink in the pNA, resulting in well-mixed conditions within the uppermost ~500 m of the water column. However, under ArO restrictions, mixing in the GNS is confined to a surface layer ~100 m thick (Fig. 5.2A, B), as freshwater masses from the ArO are too light to sink. Deepening of the CAS leads to significant changes in the salinity and temperature stratifications of the pNA (Fig. 5.3A, B). With a shallow CAS, water masses flow only from the Atlantic into the Pacific. Thus, mixing between surface and intermediate waters in the pNA depends solely on the salinity of water masses exiting the Arctic. Deepening the CAS allows an inflow of Pacific waters into the Atlantic in the southern part of the CAS, reaching the intermediate and deeper layers of the water column. These water masses are colder and less saline than the near-surface waters, promoting the collapse of salinity stratification, particularly in the low latitudes (Fig. 5.3A). Therefore, only the combined effects of simultaneous Arctic gateway restrictions and CAS shoaling to depths that enforce unidirectional water flow from the pNA to the Pacific ensure increased water mass stratification in the pNA (Fig. 5.2). The 10 m CAS depth applied in our simulations serves as an experimental end-member, allowing us to isolate the effects of Arctic gateway restrictions on salinity and temperature in the pNA from Pacific water influence. While the specific CAS depth may vary, unidirectional flow–essential for pNA stratification in conjunction with Arctic restrictions–must be maintained.

Paleontological and geochemical proxies under refined age models
The Sidi Ziane section in northern Algeria provides a continuous, and expanded sedimentary record of the uppermost Maastrichtian (Figs. 6 and 7). This section offers an opportunity to examine changes in the biota and geochemistry that may represent responses to salinity variations, as investigated through our climate modeling. At Sidi Ziane, micropaleontological assemblages are mainly characterized by high relative abundances of the planktic foraminiferal genera Heterohelix, Planohedbergella and Guembelitria, as well as by a dominance of dinoflagellate cysts (dinocysts) from the family Areoligeraceae (Figs. 7–9). Within this section, we identify three significant foraminiferal events: 1) the last occurrence (LO) of Gansserina gansseri at 2750 cm below the Cretaceous/Paleogene boundary (KPB), marking the last 480 kyr of the Maastrichtian; 2) the first occurrence (FO) of Plummerita hantkeninoides, recorded at 1350 cm below the KPB, marking the last 100 kyr of the Maastrichtian; and 3) the LO of Archaeoglobigerina cretacea recorded at 100 cm below the KPB, marking the last 16 kyr of the Maastrichtian. Therefore, the Sidi Ziane section spans the last ~500 kyr of the Maastrichtian. This age estimate is supported by an acme of Manumiella seelandica (Fig. 7E), a dinocyst species typically occurring just below the KPB17. We refined this biochronological age model by establishing a chemostratigraphic correlation (Fig. 7) with the astronomically tuned section of Zumaia, Spain18. This approach enabled for (1) an independent validation of the age model and (2) an improved age resolution for Sidi Ziane section. Detailed explanations of the age model for Sidi Ziane and other studied localities are provided in the ‘Methods’ section.

For a more detailed comparison with the numerical model, bulk isotope stable analyses for complementary paleoclimatic and paleoenvironmental interpretations were carried out in the Sidi Ziane section. δ13C values show three short-term carbon isotopic excursions (CIEs) superimposed on an overall positive trend, with CIE-1 occurring between ~66.44 and 66.40 Ma, CIE-2 between 66.25 and 66.20 Ma, and CIE-3 between 66.17 and 66.11 Ma (Fig. 7A). The relationship of δ13C bulk with δ18O shows a nonlinear correlation, r=−0.159, ρ(α) > 0.05, bulk suggesting that the primary trend is preserved19. It yields a clear trend showing progressive warming from ~66.5 Ma to 66.058 Ma, followed by a noticeable cooling from ~66.051 to 66.021 Ma (Fig. 7B). The cooling is further supported by the acme of M. seelandica (66.047–66.040 Ma) (Fig. 7E), which is considered to have been particularly adapted to cooler surface waters20.

The latest Maastrichtian Sidi Ziane section is dominated by areoligeracean dinocysts (Fig. 7E), similar to the mid-Maastrichtian section in the Negev region (SF1, SD1 and SD4). These cysts were produced by marine, single-celled phototrophic algae capable of adapting to low-nutrient conditions. A different palynological signal is observed at higher latitudes, in two cores from the Norwegian Sea: 6711/4-U-1 and 6707/10-1 (SF1, SD5 and SD6). The pNA localities are rich in peridiniacean dinocysts, which are produced by algae that thrive in nutrient-enriched environments, such as those influenced by upwelling or river runoff. These observations are consistent with the dinocyst record recognized at Bajada del Jagüel (Argentina, western South Atlantic21) (SF1, SD7). Overall, these geological proxies highlight notable differences among the studied low- and high-latitude sections and record enhanced fluctuations preceding the end of the Mesozoic. Additional information on foraminifera, isotopes and palynology is available in the Supplementary Datasets.

Methods
Numerical model simulations
The ESM COSMOS used in the numerical simulations incorporates the ocean (MPI-OM) and atmosphere (ECHAM5) components. MPI-OM (Max Planck Institute Ocean Model) includes dynamic-thermodynamic sea ice model55 and is run in the GR30/L40 configuration56, with a formal horizontal resolution of 3.0°×1.8°. ECHAM5 operates in the T31/L19 resolution57 and has a horizontal resolution of ~3.75°. The high-resolution hydrological discharge model constitutes a part of the atmospheric component and simulates freshwater fluxes over the land surface58,59.

The setup for the spin-up simulation (C-1120) has been previously described in detail60. In this simulation, the CO2 level is set to 1120 ppm (4x pre-industrial (PI) levels of 280 ppm), while other greenhouse gases are maintained at PI levels. The solar constant is reduced by 1% relative to the PI level, and orbital parameters are set to 800 AD (beginning of a millennial run). No ice sheets are prescribed and the vegetation is fixed61. The simulation was run further to the year 14,800, with the depth of the CAS ranging from 1700 to 2500 m. Two sensitivity tests with different CAS depths and identical atmospheric land-sea masks as in C-1120, were performed. In the first experiment, the bathymetry around the CAS reflects recent constraints on the gateway depth, suggesting that the exchange of waters between the Atlantic and Pacific oceans was limited to a depth of 500 m. In the second experiment, the gateway depth was limited to 10 m, based on data indicating shallow connections between the Atlantic and Pacific oceans during the latest Cretaceous. All other boundary conditions, including depths of the marine gateways between the ArO and the GO were set identically to those in C-1120. Both simulations were branched off from C-1120 at year 10,800, and were run for 3000 years.

Additionally, we analyzed two more simulations with different atmospheric land-sea mask compared to C-1120. In both simulations, all gateways between the Arctic and the Atlantic oceans, except the GNS, were closed. The two simulations differ only in the CAS depth. In one, this depth is the same as in C-1120, while in the second, it is set to 10 m. All other boundary conditions remain identical to C-1120. The simulation with the deeper CAS (GNS-47_4x) was analyzed previously60, and here, we extend the simulation by additional 800 years. The simulation with the 10 m CAS depth was restarted from the deep CAS run and extended for 3000 years. The analyzed data shown in Fig. 3 include the ArO, pNA and WT (Region A), while in Fig. 5 depicts the GNS (Region B), as outlined in Fig. 1.

Simulated water depths of the CAS
The first assumption was that the depth of the Panama island arc could be comparable to that of an island arc in the western Pacific today (~500 m). This value seemed to be the most realistic estimate for the latest Cretaceous CAS. The selection of the deepest conditions (1700–2500 m) was supported by published paleogeographic data16. The shallowest depth was set to 10 m, ensuring that water masses flow through the CAS only from the Atlantic to the Pacific Ocean, preventing deeper Pacific waters from entering the Atlantic. This scenario assumed the presence of a shallow barrier, rather than fully emergent land, between the Atlantic and Pacific Oceans, enabling an assessment of its potential impact on the reorganization of oceanic circulation patterns. The estimated depth was inspired by the Maastrichtian dinoflagellate cyst record from Guatemala, CAS, which is characterized by areoligeracean morphotypes15, suggesting deposition in an inner-neritic environment, typically interpreted as ranging from 0 to 50 m.

Even shallower depths are suggested by the widespread lateral extent of the Angostura Formation, which during the Maastrichtian formed a carbonate platform extending across Guatemala and Mexico62. For comparison, the present water depths of the Bahama carbonate platform generally do not exceed 25 m, with approximately 60% (61,400 km²) of the platform lying at depths of 5 m or less63. Similar conditions in the CAS seem plausible, as the Angostura Formation typically contains large benthic foraminifera (including alveolinids), which inhabit the seafloor of shallow, tropical, and subtropical seas in carbonate-rich environments, such as reefs and lagoons62. However, the aim of this study was not to estimate the exact depth of the barrier, but rather to observe trends in paleoenvironmental changes driven by shoaling in the CAS, identify the threshold triggering these changes, and compare the results with the geological record.

Sections and samples studied
A total of 305 geological samples were analyzed for the purpose of this work. The last ~500 kyr of the Maastrichtian, preceding the Cretaceous/Paleogene boundary (KPB), were studied using 208 samples from the Sidi Ziane section in Algeria. These included marine palynology (54 samples), micropaleontology (41 samples), and geochemical signals (113 samples). Additional palynological analysis of 12 samples was conducted on the Maastrichtian Re-2 core from the Negev region, Israel. Cores 6711/4-U-1 and 6707/10-1, drilled in the Norwegian Sea, were re-analyzed from previous work64, with interpretations based on 85 palynological slides. Additional data were obtained by reviewing the published record, which included various Maastrichtian sections globally, such as the Barents Sea37 in the GNS, Bass River south of the HS65, Guatemala in the CAS15, and Bajada del Jagüel, Argentina, in the wSA21. The studied locations are illustrated in Fig. 1.

Marine palynology
Cysts of marine single-celled algae (dinoflagellates) are composed of organic material that is highly resistant to taphonomic processes and thus constitutes a powerful tool to reconstruct past environments. They were analyzed to identify differences between pre-Maastrichtian and Maastrichtian sediments, as well as to examine past oceanographic conditions in geographically dispersed sections in low- and high-latitude regions.

Sample preparation. Samples from the Sidi Ziane section, Algeria, and the Re-2 core, Israel, were processed, mounted, and analyzed at the Polish Academy of Sciences, Institute of Geological Sciences, Research Center in Kraków. ~20 grams of each sample were subjected to a rigorous cleaning process before being crushed into particles of about 2 mm in size. To eliminate carbonates and silicates, cold 38% hydrochloric (HCl) and 40% hydrofluoric acids were utilized. Heavy minerals were separated from light organic matter using a clock glass. Additional data from the Norwegian Sea cores 6711/4-U-1 and 6707/10-164, previously prepared using similar protocol, were re-analyzed for the purposes of this study.

Morphological associations used. The dinoflagellate cysts from the Algerian Sidi Ziane section were grouped into the following morphological categories: Chorate, dorso-ventrally compressed taxa including the Glaphyrocysta-Areoligera complex and other cysts with similar environmental preferences (SD1); Peridiniaceae group, including Isabelidinium spp. and specimens belonging to Palaeocystodinium australinum-bulliforme complex and Manumiella seelandica, both of which occur as influxes. Other cysts, such as Phelodinium spp., Thalassiphora spp., the Spiniferites ramosus group and cribroperidinioids, were treated separately. Additional groups were identified for the Re-2 core, including Achomosphaera spp., Pervo/Exochosphaeridium spp., Thalassiphora spp., and Florentinia spp.; and for the Norwegian Sea sections, including Impagidinium spp., and Microdinium cretaceum. For further details on the assemblage distributions in all studied profiles, see SF1, SD1, SD4, SD5, SD6, and SD7.

Micropaleontology
Foraminifera are single-celled protists commonly found in variety of marine environments. Many of these organisms construct tests by secreting calcium carbonate. This micropaleontological group is abundant in the marine sedimentary record, making it a valuable tool for exploring past climatic and environmental changes. They were analyzed to establish a detailed age model for the Sidi Ziane section and to provide insights into past oceanographic conditions in the WT region.

Sample preparation. The samples were processed at the University of Zaragoza, Spain. They were treated with 10% diluted H2O2 for 2 h each. The disaggregated samples were then washed through a 63 μm sieve and dried in an oven at temperatures below 50°C. Quantitative analysis was performed at the genus level, after obtaining a representative aliquot of ~300 specimens per sample using a microsplitter.

Age model. At Sidi Ziane, three planktic foraminiferal events have been identified: the LO of Archaeoglobigerina cretacea at 100 cm below the KPB, the FO of Plummerita hantkeninoides at 1350 cm33, and the LO of Gansserina gansseri at 2750 cm (this paper). The most recent age calibration for the LO of A. cretacea and the FO of P. hantkeninoides bioevents are 16 and 99 kyr, respectively18, and 490 kyr for the LO of G. gansseri66, before the KPB. These bioevents provide a comprehensive temporal framework when integrated with the δ13C record, enabling detailed chemostratigraphic correlations. The δ13C-based correlation is a reliable approach, as demonstrated in several Maastrichtian sections across different latitudes and environments67. By correlating the δ13C curve from Sidi Ziane with the astronomically calibrated δ13C curve from the Spanish section of Zumaia18,68, an age interpolation for each sample was performed, assuming a constant sedimentation rate between the anchored tie-points. The LO of G. gansseri, calibrated here at 480 kyr, aligns closely with the previously assigned value66, further supporting the validity of this age model. For detailed data on foraminiferal assemblages from Sidi Ziane, Algeria, see SD3.

Calcium carbonate content
Sample preparation. The samples were analyzed at the University of Zaragoza, Spain. The calcium carbonate content of each rock sample was estimated using a manocalcimeter, which measured the carbon dioxide pressure rise produced by the acid digestion on the 113 rock samples. The analysis involved adding 5 ml of 5 M HCl to one gram of powdered sample in the reaction cell, and was conducted independently of the atmospheric pressure.

Isotopes
Sample preparation. Measurements of δ13C and δ18O isotopes were performed on homogenized bulk powdered sediment from the 113 samples of the Sidi Ziane (SD2). The samples were analyzed at the Leibniz Laboratory for Radiometric Dating and Stable Isotope Research (Kiel University, Germany), using a Kiel IV carbonate preparation device connected to a MAT 253 mass spectrometer from Thermo Scientific. During preparation, the carbonates were reacted with 100% phosphoric acid (H3PO4) under vacuum conditions at 75°C, and the evolved carbon dioxide was analyzed eight times for each sample. All values are reported in Vienna Pee Dee Belemnite (VPDB) notation, compared to NBS19 and IAEA-603 standards. The precision of all different laboratory internal and international standards (NBS19 and IAEA-603) is <0.05‰ for δ13C and <0.09‰ for δ18O values.

Multivariate and compositional data analyses
PCA was conducted to reduce the dimensionality of the identified planktic foraminiferal genera (variables) and to identify the main components that explain the variance in planktic foraminiferal assemblages across the entire studied interval. The PCA was performed using PAST software69. Following the PCA, CoDA was carried out to explore the relationships between the main foraminiferal components and the predominant areoligeracean dinoflagellate cyst group from directly comparable samples (i.e., those from the same stratigraphic horizon; Fig. 8). To overcome the constraint of the relative abundance compositional data (percentages), a CLR transformation was applied to the compositional dataset. The CLR transformation ensures that the taxa abundance data are appropriately scaled, allowing for more robust statistical comparisons. Both CoDA and CLR transformation were calculated using the specific package (compositions) in the R Studio software. Additionally, the non-parametric Wilcoxon test was employed to assess the statistical significance of the relationship between the main foraminiferal genera and the areoligeracean dinocyst group. This test was selected due to its suitability for comparing distributions without assuming normality, especially given the potential deviations from normality and some small sample sizes present in this study.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The article by W. Radmacher et al. investigates the freshening of the oceans in the late Mesozoic era, a crucial process in\nocean dynamics with significant implications for marine biology. The study focuses on the period just before the K-Pg\n(Cretaceous–Paleogene) boundary. While previous research has primarily examined the effects of the Chicxulub impact and\nthe Deccan Traps, the role of ancient seaways has largely been overlooked, despite the fact that their evolution likely had\nsubstantial impacts on ocean circulation and water mass distribution.\nThis manuscript seeks to address this gap by clarifying the influence of seaways on oceanic changes during this period. The\nauthors employ climate modeling to analyze the seaways’ effects and examine shifts in marine organism populations as\nindicators of environmental change. The use of these two approaches to support the argument is promising. However, some\nconclusions lack precision or are not sufficiently supported by the data. In particular, I find that the analysis of the climate\nsimulations is somewhat superficial.\nIn its current form, this study cannot be published. It requires substantial revision, particularly in terms of clarity, precision,\nand support for its conclusions. The methodology itself is valid, but the Materials and Methods section lacks sufficient detail\nto allow for the reproduction of the digital experiments. The manuscript lacks precision and specific quantification in places.\nAdditionally, some of the figures do not clearly illustrate the changes discussed in the text.\nMy comments:\nThe palaeogeographic framework used in this study is based on Warwick and Valdes (2004), where the Caribbean plate is\nbordered to the west by an island arc associated with subduction—a chain of islands separated by narrow seaways—and to\nthe east by a second arc. The authors suggest that the shoaling of CAS may have restricted exchanges between the Pacific\nand Atlantic Oceans. According to studies by Burch et al. (2018), these low exchange rates potentially contributed to the\nOceanic Anoxic Event 3 (OAE3) around 90-84 Ma. The authors propose that the shallow Central American Seaway (CAS),\nalong with progressively restricted marine connections with the Arctic Ocean, limited water exchanges between the Pacific,\nAtlantic, and western Tethys.\nThe study focuses specifically on the late Maastrichtian period. However, the authors do not clearly specify whether these\nexchange restrictions, from a geological standpoint, were in effect during this particular time frame.\nThe initial results focus on depth sensitivity experiments for ocean passages, conducted using an Earth system model\ndescribed in the Materials and Methods section. However, the boundary conditions, such as pCO₂, solar constant, and\nvegetation, are not specified. Based on Niezgodzki et al. (2019), we can infer that a pCO₂ concentration of 1120 ppm was\nlikely used.\nThe authors test various configurations, including one with the Central American Seaway (CAS) at a depth of just 10 meters.\nThe choice of the 10-meter Central American Seaway (CAS) scenario appears to align with the available data, but the\nauthors do not explain whether such a scenario is realistic. Why was such a shallow depth chosen, and is this realistic given\nthe presence of an island arc? More discussion is needed on this point to assess its plausibility. The second experiment\ncombines a CAS depth of 10 meters with additional restrictions on Arctic Ocean exchanges. According to Niezgodzki et al.\n(2019), the Arctic passages were already quite shallow (<50 or 100 meters). However, the authors do not clarify the specific\nparameters they applied for this restricted Arctic configuration in their new experiment.\nThe authors note that experiments with a 10-meter-deep Central American Seaway (CAS) yield the most pronounced\npalaeoenvironmental changes, particularly in terms of water column stratification in the Arctic Ocean. However, it is unclear\nhow these changes are illustrated in Figure 2C4. In general, Figure 2 is challenging to interpret due to the small size and\nlimited contrast of the color scale, making it difficult to discern the changes described. The authors also mention that\nrestricted exchanges through the Greenland-Norwegian Seaway increase temperature gradients in the Atlantic Ocean and\nwestern Tethys. However, this effect is hard to see in Figure 2D2, as the data appear to be globally averaged. The authors\ntested additional scenarios, with and without the Western Interior Seaway (WIS) and Hudson Seaway (HUS). They argue\nthat the 10-meter CAS configuration best represents conditions at the end of the Cretaceous—specifically, the Maastrichtian.\nOn what basis is this depth chosen, and does this imply that these conditions were unique to the Maastrichtian? Additionally,\nwhat mechanisms could account for a change in CAS depth by the end of the Cretaceous?\nIn the second part of the study, the authors analyze multiple records, starting with the Sidi Ziane section in Algeria and\nboreholes in the Norwegian Sea. However, some sections presented in the appendix (e.g., Argentina, USA) are never\ndiscussed in the manuscript.\nLines 211-214: The abundance of certain organisms is interpreted as indicative of shifts in temperature or nutrient-rich\nconditions, potentially driven by upwelling or runoff. The authors conclude that significant environmental changes occurred\nprior to the K-Pg boundary (KPB). Borehole 6707/10-1, which spans approximately 3 million years (~70–67 Ma), shows\nrelatively minor variations (see Fig. S4B). In borehole 6711/4-U-1, there is a brief increase around 68 Ma; however, this\noccurs well before the KPB, and fluctuations between 68 Ma and the KPB do not appear markedly different from those\npreceding the 68 Ma event.\nLines 236-244: In the Discussion section, the authors suggest that a shallow Central American Seaway (CAS) (10 meters)\nincreases water mass stratification in the Atlantic Ocean and western Tethys. However, there is no geological evidence\nsupporting a continuous shallow barrier between North and South America; instead, geological data indicate the presence\nof an island arc associated with the subduction zone.\nLines 246-256: The authors propose that increased stratification occurred in the Atlantic Ocean and parts of the Tethys,\nyet they do not illustrate these changes in the figures. To support this claim, visual evidence of stratification changes should be\nprovided. Additionally, the mechanisms underlying these changes—particularly the interactions between the CAS and\nWestern Tethys—are not clearly presented or discussed.\nLines 261-266: The authors suggest comparing the scenario-induced changes (CAS uplift) with geological data and note\nthat these processes coincide with global cooling and sea-level fall. However, the timeframe remains unclear. What specific\nperiod are the authors addressing? Based on palaeontological data, this study focuses on the Maastrichtian, a time marked\nby warming followed by a brief cooling just before the K-Pg boundary. The Earth has experienced global cooling since the\nTuronian, but the late Maastrichtian includes a notable temperature rise prior to this cooling episode.\nLines 291-343: The authors suggest salinity changes in the Sidi Ziane section, possibly linked to increased continental\nrunoff (L310). For the Norwegian Sea sections, they similarly conclude that runoff may have increased, contributing to water\nmass stratification (L341).\nThe authors propose that these salinity changes result from shifts in ocean circulation driven by evolving ocean passages.\nWhile the Norwegian Sea sections indicate a salinity response (although the figure is difficult to interpret), the authors do not\nclearly present salinity changes in the Tethys from their sensitivity experiments. Additionally, changes in salinity can also\nstem from shifts in the local precipitation-minus-evaporation (P-E) balance. Modifying ocean circulation could potentially\nalter atmospheric circulation as well—especially given the possible removal of the Western Interior Seaway (WIS) and\nHudson Seaway (HUS)—which could impact the P-E balance. However, this point is not addressed.\nThe authors discuss the roles of the Deccan Traps and the Chicxulub impact, suggesting that these events may have\nmasked the effects of ocean passage changes. The establishment of the Deccan Traps and the associated CO₂ emissions\nlikely increased pCO₂ in the late Maastrichtian, leading to global warming, which could plausibly increase continental runoff.\nHowever, implicating the Chicxulub impact as a contributing factor seems more challenging, given that it occurred precisely\nat the K-Pg boundary, after the Maastrichtian.\nWhile the authors emphasize the impact of ocean passage changes, there is insufficient geological evidence and temporal\nconstraints to substantiate a causal link between these geological and oceanographic events. Why would the CAS have\nbeen shallow during or prior to the Maastrichtian? While some studies suggest a southern closure of the WIS, the northern\nportion of the WIS, along with much of the HUS, appears to remain open until the K-Pg boundary.", "reviewer_2": "Manuscript# NCOMMS-24-63460-T\nCorresponding Author: Wieslawa Marta Radmacher\nTitle: Freshening near the end of the Mesozoic?\nReviewer Assessment\nKey Results\nThe key results from the paper, as I understand them, are that the model experiments show that shallowing gateways result\nin an enhanced freshening of the oceans in the Maastrichtian. This is similar to what has been found in studies of the\nCenomanian and Early Eocene. This would suggest that such changes to the ocean system may be more common than\ncurrently apparent in the literature.\nValidity\nThe climate model experiments show ocean freshening (lower salinity) with increased isolation of ocean basins. This is\nneither new nor a surprise.\nNonetheless, I believe this study to be a useful addition, but I am concerned by the following:\n1. Setup of the paper – I re-read the manuscript many times and feel that the abstract and introduction would both benefit\nfrom being ‘tightened up”. By this I mean the following:\n(a) A more explicit statement of the aim of the study – is the aim to show how palaeogeography affects the potential for\nfreshening of the oceans in the geological past. That was my take.\n(b) What then confused me as a reader were the statements about the importance of understanding climate change, the\ncauses of extinction, palaeoecology, etc. Ok, yes, but these are not fully developed, and they feel like they have been added\nto the abstract and introduction to capture all the current buzz phrases. Yes, the potential for freshening of the oceans and its\nimpact on ocean ecology and circulation is important. However, this study is about longer-term factors (viz.\npalaeogeography), whereas today, we are concerned with short-term drivers, like CO2, land use changes and how these\nimpact the hydrological system.\n(c) Extinction – there is mention of the study interval preceding the mass extinction at the end of the Cretaceous\n(lines 33-35, 62-64). But a relationship between environmental changes (as described in this paper) and extinction is not then\ndeveloped. I cannot see that they are related except for certain marine species being affected by salinity changes, if these\nare global, which does not seem likely. Is the intention to state that there are environmental changes leading up to the end\nCretaceous which have not been examined enough?\n(d) Abstract – this needs to be more formulaic: what I would do is to start by (sentence 1) stating that the study is\naimed at understanding how common ocean freshening might be in the geological record (if that is indeed the aim);\n(sentence 2) How this might be important for future changes to the Earth ; (sentences 3 and 4) that the study does this by (1)\nrunning modelling experiments (long-term changes) and (2) outcrop sites to test model results in those areas; (5) That the\nresults indicate that freshening may have occurred in the Maastrichtian.\n2. Modelling experiments - The validity of using a 10m bathymetric experiment for the Central American Seaway and using\nthis to make conclusions. Regarding climate modelling, a 10m water depth is essentially land! It is also unlikely to be the\ncase for the entire length of the arcs that form the barriers. The authors could do no worse than look at the present-day\nbathymetries of the island and continental arcs in the western Pacific. Especially the discontinuous nature of some of these.\nDo I think that the Central American Seaway has barriers to deep water flow based on the geology I know? Certainly! I,\ntherefore, agree with the authors. But, I do not consider this new, although it is useful.\n3. The outcrop data – this section is fine and, to me, is a paper in its own right, especially the Algerian site. However, for the\ngeneral aim of the paper, if I understand this correctly, I am concerned that all the localities used in the study to indicate\nfreshening of the oceans are near continental areas and are therefore much more susceptible to changes in freshwater\nfluxes. They need not indicate global changes (though I think this is what the authors hope through the isotope data? If so,\nthis needs to be drawn out more clearly).\n4. The equation of ocean freshening and stratification (e.g. line 343). Although freshwater fluxes to the ocean can and do\nlead to water column stratification, in other circumstances, they can result in instability and over-turning.\n5. “…the effect of salinity is often speculated or even overlooked” (line 334). Is this really true? There are numerous\npapers on density stratification and the role of salinity on circulation, going back to at least 1982 and Brass and 1998 and\nHay et al. More recently, this has been discussed by Ladant et al. (2020), which the authors include in the reference list.\nSignificance\nThe conclusion that palaeogeography may be important for restricting ocean basins and enhancing the potential for\n‘freshening of the ocean in these basins” is not new, but it is useful. The important role of gateways on palaeoclimate has\nlong been known (see Berggren 1982 as a start). That the authors identify this for the Maastrichtian to add to previously\npublished work on the Cenomanian and Early Eocene is, to me, a useful addition.\nHowever, I am less clear on how the outcrop data fits into testing this, given my concern (see above) that the localities are all\nnear-shore and susceptible to local hydrological changes. This might be an area where the authors could strengthen the\narguments using the same data.\nData and Methodology\nIncluding observational data and model experiments is a common and powerful approach for examining the response of the\nEarth system to changes in boundary conditions. To that extent, the data and methodologies are valid. However, I am\nstruggling with the detailed analysis of the Algeria site data and the bigger-picture question being discussed. This may be\nme, but there feels like a disjunct here. It is possible that this could be addressed by rewriting the section or turning the\nwhole paper around, viz., “how the analysis of the Algeria data suggests changes to the ocean system and hydrology, that\nare then examined using the modelling”, rather than the modelling followed by the data. This is just a thought.\nI am less experienced with biostratigraphy and palynology, but from what I do understand, this all seems valid.\nThe modelling relies on one published Maastrichtian palaeogeography. What was the rationale for using this one?\nThere is an updated version to this map if the authors are interested.\nAnalytical approach\nThe analysis of the biostratigraphy data is outside of my principal area of expertise.\nSuggested improvements\nMy recommendation from reading this manuscript is that it would greatly benefit from a more explicit statement of the project\naims and objectives and a clearer link between the aims of the Algeria analysis and the big-picture modelling study.\nClarity and context\n1. Although the editorial guidelines say not to focus on grammar, I would urge the authors to avoid using words and\nstatements such as the following:\n(a) “favouring” (line 35) – I would be more direct, for example “indicate”, “show”.\n(b) “Seem to” (line 39) – instead use “…processes may have played…”\n(c) “We assume that” (line 152) – either your data shows this, or your assessment of the existing literature shows this.\n(d) “Reliable” (line 154) – what do you mean by “reliable”? Is this the view supported by data, or the most recent publication,\nor something else?\n(e) “Novel” (line 230; but used several times in the manuscript) – In this case, this is not a novel (meaning “new”) mechanism.\n“Novel” should only be used if the mechanism has not been recognized previously.\n2. Title: I would add the word “Ocean” at the beginning of the title to make the context of “freshening” clearer to readers.\n3. When you use “Open-marine” (Line 113), do you mean deep-water connectivity? The system is open to the global ocean\nsystem, just at shallow depths. For example, the Atlantic and Pacific were not separated in the Late Cretaceous as they are\ntoday by Central America.\nReferences\nThere is a comprehensive reference list, although there are a few papers I think the authors should have a look at,\nfor example, Laugié et al. (2021), https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020PA004202\nOverview\nThis manuscript describes some very interesting results, and there is much that I think would be useful to see published.\nHowever, as I have described above, I think the paper needs some reorganization and further thought. I encourage the\nauthors to make changes, and I wish them luck in their research.", "reviewer_3": "This study integrates modelling results with micropalaeontological and geochemical data from four geological sections in a\ntime slice near the end of the Mesozoic (around 66Ma), just preceding the K-T extinction event. Using the COSMOS Earth\nSystem Model, Radmacher and co-authors test five scenarios with varying depths of the Central American Seaway (CAS)\nand marine connectivity around the Arctic. By shoaling the CAS, the model shows an increased salinity stratification in the\nproto-Atlantic, an effect that is exacerbated when restricting seaway connections around the Arctic. The authors compare the\nmodelling results with previously published micropalaeontological and geochemical data (supplemented with additional\nanalyses) from four localities: the Sidi Ziane Section (Algeria), the Re-2 Core from the Negev region (Israel) and two\nNorwegian Sea cores. They report increased relative abundances of dinoflagellate cysts belonging to the Family\nAreoligeraceae which is surmised to be a result of increased stratification. Increased abundances of protoperidiniaceans,\nknown heterotrophs, are interpreted as evidence for the presence of nutrient-rich water prone to salinity fluctuations. Taken\ntogether, the authors conclude that their modelling and data point to a freshening at high latitudes with an increased outflow\nof low salinity waters through the Greenland-Norwegian Seaway.\nThe authors should be commended because they present results from a united effort of modelers and data driven scientists\nin a single study which will therefore be of interest to the diverse readership of Nature Communications. I find the manuscript\neasy to follow, but found a number of grammatical and formatting issues listed as comments in the annotated pdf (attached).\nThe text is accompanied by clear figures that effectively convey the authors’ message.\nMajor Comment\nRelative abundances are a form of compositional data (their total summing to 1) meaning that a rise in one species is\nautomatically at the expense of another (the constant-sum constraint). In figure 6, selected profiles of planktic foraminifera\nand dinoflagellate cysts are presented which are difficult to interpret without knowledge of the whole assemblage\ncomposition (which the authors nicely present in the figures of the supplementary data). This leads to interpretations that are\nnot necessarily backed up by the data. For example, on Line 294–295 the authors claim “when Guembelitria is more\nabundant than Planohedbergella, Areoligeraceae show higher abundances”. This is clearly not the case in CIE1 where\nareoligeraceans are close to 100% of the palynological assemblages while relative abundances of Planohedbergella\nexceed those of Guembelitra. Such covariance as suggested as suggested by the authors on lines 294–295 can only be\nexamined using rigorous statistics that obey the laws of compositional data analysis.\nIn addition, relying on changes in relative abundance, especially for dinoflagellate cysts, without knowledge of the burial flux\nor concentration of cysts can be challenging. Dinoflagellate cyst burial flux calculations are recommended (they can be\ncalculated if the marker grain method has been applied during preparation) as they can separate rises of number of cysts\nthat are result of increased incorporation of cysts in the sediment from those that are the result of changes in the\nsedimentation rate. If these data are available, they would be invaluable in interpreting the presented assemblage changes.\nOther comments\nAbstract: why is there no mention of any of the geochemical and micropalaeontological data in the abstract? The manuscript\ndiscusses an integration of modelling and data but the abstract only mentions modelling results. I believe this is a particular\nstrength of the manuscript and should be included here.\nLine 54: Reference 21 deals with the late Neogene and offers no evidence about the latest Cretaceous as the authors seem\nto suggest here.\nLine 196 and Figure 6A: CIE1 (high δ13C corresponding to low δ18O and peak in Areoligeraceae) behaves differently than\nthe other CIEs where lower than expected δ13C are recorded. This is not explained in the text. Later in the text these CIEs\nare ignored even though they were deemed important enough to highlight here and in Figure 5.\nLine 268: what about preservation of calcium carbonate in cooler waters? This seems like it could be an issue near the\nbottom of the Sidi Zaine section where %CaCO3 is very low. Will this not impact foraminifer preservation and the associated\nstable isotope signal?\nLine 287: “this group can be considered as indicative of stressed near shore marine environment, with notably fresh-water\ninput and changes in surface-water salinities followed by enhanced water stratification.” This seems like an unrealistically\nspecific condition for increased abundance of areoligeraceans. I agree the data shows raised abundances could be\nindicative of “stressed” near shore marine environments but “notably fresh-water input and changes in surface-water\nsalinities followed by enhanced water stratification” seems like an overreach. It may be the palaeoenvironment suggested in\nthe current study, but I think this must be rephrased as not to sound that an increase in areoligeraceans is automatically\nindicative of only this specific situation.\nLines 292–297: I find this very speculative, and this is definitively not the case for CIE1 were abundances of\nPlanohedbergella are higher than Guembelitria in the same sample when areoligeraceans form close to 100% of the\ndinoflagellate cyst assemblage (Figure 6 D–E). An inference like the one made by the authors in the sentence starting at line\n293 (Areoligeraceae showing an inverse paleoecological signal with respect to Planohedbergella…), should be backed up\nby multivariate statistics.\nLine 302–303: higher productivity could indeed result in higher CaCO3 but post-deposition dissolution is an important factor\nin this, especially in cooler high latitude water (e.g. Henrich et al., 2002). Again, calculating a dinoflagellate cyst burial flux\nwould give a much more reliable estimate of the organic matter burial rate and therefore productivity.\nLine 309: “Guembelitria bloom”. A high relative abundance is not evidence of a bloom which suggests a high burial flux\n(high productivity).\nLine 327–328: Reference needs replacing with reference number (Harris and Tocher, 2023).\nLine 336–337: sentence needs rephrasing.\nLine 359: I agree with the authors here. Only relative palaeo-salinity changes can be derived since the exact composition of\nthe Maastrichtian seawater is not known. These relative palaeo-salinities can be derived from planktonic foraminifer oxygen\nisotope records corrected for global ice volume using a normalised global compilation of benthic oxygen isotope records.\nThis has been done for the Pliocene–Quaternary but I am not aware of an equivalent for the Maastrichtian. If available,\nsuch a calculation could help solidify the inferred freshening. As the authors point out (e.g., line 396), the studied time interval is\nice-free and a conversion of the planktonic oxygen isotope signal using the Shackleton (1974) equation could yield a record\nthat is mainly influenced by salinity changes. Maybe this can be used to give a rough idea of salinity changes?\nFigure 2: check the layering of the red rectangle. Some of the differences discussed in the text are hard to discern at the\nscale of the individual maps. Add dimensions to the colour gradient variables along the X axis.\nFigure 2: “Caribbean Seaway” at the top of the figure is the same as “Central American Seaway” in the caption and text?\nFigure 3: add dimensions to the colour gradient variable displayed along the bottom of each figure. HS = Hudson Seaway\n(correct?). Note that in line 114 the Hudson Seaway was abbreviated HUS instead of HS. Make uniform.\nFigure 3A corresponds to the same model parameters employed in Figure 2A (Deep CAS with open Arctic marine\nconnections) while Figure 3B corresponds to Figure 2D (Shallow CAS with reduced Arctic marine connections).\nFigure 4A shows the difference between Open GNS/shallow CAS Vs. Open GNS, WIS and HUS/shallow CAS. Figure 4B\nshows the difference between Open GNS/shallow CAS Vs.Open GNS, WIS and HUS/Deep CAS.\nReferences\nHenrich, R., et al., 2002. Carbonate preservation records of the past 3 Myr in the Norwegian–Greenland Sea and the\nnorthern North Atlantic: implications for the history of NADW production. Deep-Sea Research Part I-Oceanographic\nResearch Papers 184, 17-39 10.1016/S0025-3227(01)00279-1.\nShackleton, N.J., 1974. Attainment of isotopic equilibrium between ocean water and the benthonic foraminifera genus\nUvigerina: isotopic changes in the ocean during the last glacial. Colloques internationaux du C.N.R.S. 219, 203-210"}, "author_response": "Response to the Editor and Reviewers\nWe sincerely appreciate the thoughtful and constructive feedback provided by the Reviewers on our manuscript,\nnow titled 'Ocean freshening near the end of the Mesozoic'. Each comment has been carefully considered, and we\nhave addressed all the points raised by incorporating the necessary revisions into the manuscript. Additionally, we\nhave submitted the improved and new figures as well as additional supplementary materials. We believe these\nrevisions have substantially enhanced the manuscript and, with the Reviewers' valuable insights, it is now in a\nmore refined state, ready for your consideration. We believe that the improvements made will meet the high stand-\nards of Nature Communications journal.\nRegarding the Editor's primary concern about the selection of parameters in the modelling experiments, such as\nthe choice of water depth, we would like to offer a detailed explanation:\nOne of the Reviewers noted that the water depth of the Panama island arc in the Maastrichtian could be comparable\nto that of an island arc in the western Pacific today. We agree that this scenario cannot be excluded. This is why\none of the water depth values selected for the modelling experiments was 500 m. However, as noted in the revised\nmanuscript and explained in the new Supplementary material S3, the Panama island arc was developed on top of\na volcanic (oceanic plateau) promontory and this is the rationale to favour a bathymetry shallower than 500 m. In\ncomparison, the modern Antilles island arc has an average water depth of approximately 200 m even though it did\nnot develop on top of a promontory (a more comprehensive comparison of water depths from various arcs is\npresented in the new Supplementary material S3).\nIn addition to the experiment testing the depth of 500 m, our objective was to test marginal ‘boundary conditions’.\nThe goal was not to determine the exact depths within the Central American Seaway (CAS), but to explore the\npotential ocean current reorganisations that could be triggered by changes in the depths of CAS. This was essential\nfor explaining the specific microfossil assemblage we found in the Maastrichtian sediments. Therefore, we have\nchosen additional CAS depths that render a wider phase space for exploring the limits of the dynamical imprint.\nThe selection of the deepest conditions (1700 to 2500 m) was supported by paleogeographic data from Markwick\nand Valdes (2004). The shallowest CAS condition was set to 10 m to ensure unidirectional water mass transport\n(exclusively from the North Atlantic to the Pacific) preventing the inflow of deeper Pacific waters into the Atlantic.\nThis controlled experimental isolation of the Central American Seaway served as a critical end-member scenario,\nallowing for a unique assessment of the impacts of Arctic gateway opening and closure on North Atlantic salinity\nand temperature, independent of Pacific water influence. This approach yielded to be crucial, as this way we\nestablished that only the combined effects of CAS shoaling and Arctic Ocean restrictions can drive global changes\nconsistent with our geological proxies.\nThough extreme, this depth of 10 m should not be considered as unrealistic. The palynological assemblage from\nMaastrichtian sediments in Guatemala (CAS) studied by the first author (Radmacher et al., 2021) support this\npossibility. Although the palynomorphs exhibit low diversity, they are represented by areoligeracean morphotypes\ntypically associated with shallow, inner-neritic zones and high energy regimes (Brinkhuis and Schiøler, 1996;\nPross and Brinkhusis, 2005). The inner neritic environment typically refers to the shallowest part of the continental\nshelf, extending from the shore to a depth of about 50 m. Even shallower depths in CAS are further supported by\nthe occurrence of the Angostura Formation, which represents a carbonate platform that during the Maastrichtian\nwas extended across Guatemala and Mexico (Fourcade et al., 1999). This formation is known of large benthic\nforaminifera (including alveolinids), which typically live on the seafloor of shallow, tropical and subtropical seas\nin carbonate-rich environments, such as reefs and lagoons. For comparison, the present water depths of the Bahama\ncarbonate platform are generally very shallow, typically not exceeding 25 meters, with 60 % of the platform\n(61,400 km2) lying at depths of 5 meters or less (Harris et al., 2014). All the mentioned above data indicate that\nthe general depth of the CAS during the Maastrichtian could be indeed that shallow, and even partially exposed,\nas shown by Buchs et al. (2010, fig. 9C) on his proposed model for the CAS arc development during Maastrichtian.\nSome authors even indicate the presence of “land bridge or closely located islands” between Americas that may\nhave been present as early as from the latest Campanian/earliest Maastrichtian (Iturralde-Vinent, 2006; fig. 5b).\nAccording to the author, these landforms were shallow enough to facilitate the interchange of terrestrial vertebrates\nbetween the both continents (Ortiz-Jaureguizar and Pascual, 2011).\nAs pointed out by Reviewer 1, none of the tested scenarios could be considered entirely realistic. Our overall goal\nwas not to estimate the exact depth of the CAS, but to observe the trends in paleoenvironmental change driven by\nshallowing in the CAS in combination with changes in the northern gateway configurations to deduce the optimal\nfingerprint by the underlying effects in comparison pattern of geological proxies. The resolution of global model-\nling cannot fully capture features such as islands or underwater volcanoes. However, additional conclusion from\nour methodological approach is that the average depth of the CAS, characterized by a general uplift and shallowing\ntrend towards the late Cretaceous (e.g. Buchs et al., 2018; Radmacher et al., 2021), during the Maastrichtian may\nhave ranged between 500 and 10 meters. This range is shallower than previously suggested (e.g. Valdes and\nMarckwick, 2004).\nThe conclusion aligns also with recognized trends regarding global ocean shallowing towards the end of Creta-\nceous, that would intensify the rate of land uplifting in the CAS. The global shallowing is also supported by our\npalynological record, where Maastrichtian intervals from various regions near the Atlantic Ocean are dominated\nby Areoligeracean dinoflagellate cysts. In contrast, the early-Late Cretaceous sediments are characterized by a\nricher and more diversified assemblage of ‘normal’ marine species (e.g. Radmacher et al., 2014a, 2014b, 2014c;\n2015; 2020). Areoligeraceae, which appear in exceptionally high-abundances during the Maastrichtian, apart of\nsignalling shallow, inner-neritic environments (Brinkhuis and Schiøler, 1996; Pross and Brinkhuis, 2005), also\nserve as proxies of increased water stratification (Frieling at al., 2018; Vellekoop et al., 2015).\nSubduction of the Pacific Farallon Plate beneath the Caribbean and South American plates, began around ∼75-73\nMa, in the Late Cretaceous (Buchs et al., 2010), and the subsequent collision between the Central American Pen-\ninsula and South America resulted in continuous uplift in the region, eventually leading to complete land exposure\nnear the beginning of the Quaternary. However, the oceanic changes commenced much earlier, long before the\nformation of the land bridge, and their precise timing had remained uncertain. What makes our work unique and\nnovel is that based on the model simulations we identified the threshold driving ocean current reorganizations in\nthe Central American Seaway, which influenced global Maastrichtian fossil assemblages long before significant\nexposure of land. In addition, we found that the combination of CAS shoaling and Arctic Ocean restrictions is\nnecessary to explain our geological proxies. These findings provide crucial insight into the oceanic dynamics that\npreceded the formation of the land bridge between the Americas.\nThanks to the insightful comments of the Reviewers we recognized that our methodological approaches and ex-\nperimental setup were not clearly explained in the original manuscript. In response, we have revised and improved\nall the sections of the manuscript and discussed the reasons for the specific depth selection (Materials and methods\nsection, starting from third paragraph). We believe these changes clarify the structure of our experiments and\nfindings.\nMain changes incorporated into the new version of the manuscript:\n The title has been improved by adding ‘Ocean’ at the beginning as suggested by the Reviewer, and by\nremoving the question mark.\n A revised version of the abstract, closely following Reviewer 2’s suggestions has been provided.\n All figures have been improved.\n Additional figures (now Figure 2, 5 and 8) have been provided.\n Figure 3 has been modified to more clearly demonstrate our findings.\n Figure 4 has been replaced (now Fig. 5).\n Additional Supplementary material (S2I to S2III) has been provided to illustrate variations in global water\nsalinity and temperature distribution at different depths, depending on the paleogeographic changes.\n A new Supplementary material (S3) has been added to explain in detail the depth characteristics of the\nCentral American Seaway during the Maastrichtian.\n Last subchapter ‘Maastrichtian Ocean freshening’ has been substantially rewritten and replaced by the\nnew text.\nBelow we address the Reviewer concerns individually. All these explanations are incorporated into the new version\nof the manuscript.\nSincerely,\nWiesława Radmacher and co-authors\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThe article by W. Radmacher et al. investigates the freshening of the oceans in the late Mesozoic era, a crucial\nprocess in ocean dynamics with significant implications for marine biology. The study focuses on the period just\nbefore the K-Pg (Cretaceous–Paleogene) boundary. While previous research has primarily examined the effects\nof the Chicxulub impact and the Deccan Traps, the role of ancient seaways has largely been overlooked, despite\nthe fact that their evolution likely had substantial impacts on ocean circulation and water mass distribution.\nThis manuscript seeks to address this gap by clarifying the influence of seaways on oceanic changes during this\nperiod. The authors employ climate modeling to analyze the seaways’ effects and examine shifts in marine\norganism populations as indicators of environmental change. The use of these two approaches to support the\nargument is promising. However, some conclusions lack precision or are not sufficiently supported by the data.\nIn particular, I find that the analysis of the climate simulations is somewhat superficial. In its current form, this\nstudy cannot be published. It requires substantial revision, particularly in terms of clarity, precision, and support\nfor its conclusions. The methodology itself is valid, but the Materials and Methods section lacks sufficient detail\nto allow for the reproduction of the digital experiments. The manuscript lacks precision and specific\nquantification in places. Additionally, some of the figures do not clearly illustrate the changes discussed in the\ntext.\n- We have thoroughly revised the manuscript in accordance with all Reviewer comments. Specifically, we have\naddressed the main concern raised, as detailed above, and have provided a clearer explanation of the issue of\nthe water depths selected for the experiments. The methodological chapter has been significantly improved to\nensure greater clarity and precision. Additionally, we have refined the expression of our findings to present them\nmore accurately. The figures have also been improved for better clarity and visual impact, and we have provided\nmore detailed explanations where necessary. We believe these revisions comprehensively address the feedback\nand significantly strengthen the manuscript.\nMy comments:\nThe paleogeographic framework used in this study is based on Markwick and Valdes (2004), where the\nCaribbean plate is bordered to the west by an island arc associated with subduction—a chain of islands separated\nby narrow seaways—and to the east by a second arc. The authors suggest that the shoaling of CAS may have\nrestricted exchanges between the Pacific and Atlantic Oceans. According to studies by Buchs et al. (2018), these\nlow exchange rates potentially contributed to the Oceanic Anoxic Event 3 (OAE3) around 90-84 Ma. The\nauthors propose that the shallow Central American Seaway (CAS), along with progressively restricted marine\nconnections with the Arctic Ocean, limited water exchanges between the Pacific, Atlantic, and western Tethys.\nThe study focuses specifically on the late Maastrichtian period. However, the authors do not clearly specify\nwhether these exchange restrictions, from a geological standpoint, were in effect during this particular time\nframe.\n-From a paleogeographic perspective, the early arc volcanoes began to develop during the Campanian and\ninfluenced the Maastrichtian paleogeography. From that time, the area became an island arc, developed on an\noceanic plateau basement, which could have shallowed more rapidly than a typical intra-oceanic volcanic arc.\nWe agree that we should refer to the Maastrichtian, since the paleontological and paleogeographical data used\nin this study covers this time interval. This is now changed and more clearly indicated throughout the\nmanuscript text.\nThe initial results focus on depth sensitivity experiments for ocean passages, conducted using an Earth system\nmodel described in the Materials and Methods section. However, the boundary conditions, such as pCO₂, solar\nconstant, and vegetation, are not specified. Based on Niezgodzki et al. (2019), we can infer that a pCO₂\nconcentration of 1120 ppm was likely used. The authors test various configurations, including one with the\nCentral American Seaway (CAS) at a depth of just 10 meters. The choice of the 10-meter Central American\nSeaway (CAS) scenario appears to align with the available data, but the authors do not explain whether such a\nscenario is realistic. Why was such a shallow depth chosen, and is this realistic given the presence of an island\narc? More discussion is needed on this point to assess its plausibility. The second experiment combines a CAS\ndepth of 10 meters with additional restrictions on Arctic Ocean exchanges. According to Niezgodzki et al.\n(2019), the Arctic passages were already quite shallow (<50 or 100 meters). However, the authors do not clarify\nthe specific parameters they applied for this restricted Arctic configuration in their new experiment.\n-These issues have now been addressed in the Materials and Methods chapter, Numerical model simulations\nsubchapter, starting from paragraph 2.\nThe authors note that experiments with a 10-meter-deep Central American Seaway (CAS) yield the most\npronounced paleoenvironmental changes, particularly in terms of water column stratification in the Arctic\nOcean. However, it is unclear how these changes are illustrated in Figure 2C4. In general, Figure 2 is\nchallenging to interpret due to the small size and limited contrast of the color scale, making it difficult to discern\nthe changes described. The authors also mention that restricted exchanges through the Greenland-Norwegian\nSeaway increase temperature gradients in the Atlantic Ocean and western Tethys. However, this effect is hard to\nsee in Figure 2D2, as the data appear to be globally averaged. The authors tested additional scenarios, with and\nwithout the Western Interior Seaway (WIS) and Hudson Seaway (HUS). They argue that the 10-meter CAS\nconfiguration best represents conditions at the end of the Cretaceous—specifically, the Maastrichtian.\n-The past Fig. 2 (now Fig. 3) is now improved to more clearly indicate our findings. We observe that the most\nintensive water column salinity stratification in the Atlantic Ocean occurs in the scenario where the CAS is 10 m\ndeep (Fig. 3.2C and 3.2D). However, in the case of open marine connections around the Arctic Ocean (AO)\ncombined with CAS shallowing, surface salinity in the AO increases (Fig. 3.1C), which we consider unrealistic,\nbased on our palynological record showing a dominance of peridinioids in that region, indicating freshening\nduring the latest Cretaceous. Therefore, we conclude that the scenario with the shallowest CAS and restricted\nmarine connections around AO (Fig. 3.D1-4), which leads to a decrease in sea-surface salinity in the Arctic\nOcean, is the most realistic. We further explore and illustrate additional findings in Figs 4 and 5.\nOn what basis is this depth chosen, and does this imply that these conditions were unique to the Maastrichtian?\n-The rationale behind the selection of depths used in the numerical simulations is explained in detail above, at\nthe beginning of the document. The conditions we interpreted are indeed specific to the Maastrichtian, as from\nthis interval come our Maastrichtian proxies. The manuscript has been revised to clarify this point.\nAdditionally, what mechanisms could account for a change in CAS depth by the end of the Cretaceous?\n-The shallowing in the CAS can be attributed to formation of an island arc on top of an oceanic plateau\npromontory, as explained in the new Supplementary materials S3.\nIn the second part of the study, the authors analyze multiple records, starting with the Sidi Ziane section in\nAlgeria and boreholes in the Norwegian Sea. However, some sections presented in the appendix (e.g., Argentina,\nUSA) are never discussed in the manuscript.\n-The Maastrichtian dinoflagellate cyst assemblages in all sections presented in the supplements, including those\nstudied herein and additional available data are dominated either by Areoligeraceae (in low latitudes) or\nPalaeoperidiniaceae dinoflagellate cyst (in high latitudes). We interpret these findings as indicating increased\nfreshening in the Arctic Ocean and heightened water stratification in the Atlantic. A detailed discussion on the\nillustrated data and the environmental preferences of the dinoflagellate cyst groups has been further developed\nin the main text of the manuscript, chapter ‘Global dinoflagellate cyst signals from Maastrichtian sections’.\nLines 211-214: The abundance of certain organisms is interpreted as indicative of shifts in temperature or\nnutrient-rich conditions, potentially driven by upwelling or runoff. The authors conclude that significant\nenvironmental changes occurred prior to the K-Pg boundary (KPB). Borehole 6707/10-1, which spans\napproximately 3 million years (~70–67 Ma), shows relatively minor variations (see Fig. S4B). In borehole\n6711/4-U-1, there is a brief increase around 68 Ma; however, this occurs well before the KPB, and fluctuations\nbetween 68 Ma and the KPB do not appear markedly different from those preceding the 68 Ma event.\nFigure from supplementary material (S1) illustrates the differences between low- and high-latitude assemblages,\nwith Areoligeraceae dominating the Maastrichtian interval in low-latitudes (Algeria, Israel, USA) and\nPeridiniaceae in high-latitudes (northern North Atlantic and Argentina). This record is important for\nunderstanding the latitudinal distribution of dinoflagellate cyst groups and supports the modelling results, which\nsuggests freshening in the Arctic Ocean and water column stratification in the Atlantic. Changes between the\nMaastrichtian and older assemblages is recognised in previous studies. Increase in Palaeoperidinium\npyrophorum and other peridinioids in the uppermost Cretaceous of the Barents Sea is (Radmacher et al., 2014a,\ntable 1 and 2, and 2014b, figure 3, cores 7119,12-1; 7119/9-1; 7120/7-3; 7120/5-1; 7121/5-1) and domination of\nIsabelidinium and Chatangeilla spp. in the Norwegian Sea (Radmacher et al., 2015, fig. 4, cores 6707/10-1 and\n6711/4-U-1 suggests an increase in freshening in the northern North Atlantic towards the end of the Cretaceous.\nLines 236-244: In the Discussion section, the authors suggest that a shallow Central American Seaway (CAS)\n(10 meters) increases water mass stratification in the Atlantic Ocean and western Tethys. However, there is no\ngeological evidence supporting a continuous shallow barrier between North and South America; instead,\ngeological data indicate the presence of an island arc associated with the subduction zone.\n-The shallowing of the CAS has been thoroughly addressed at the beginning of this document and in the Materi-\nals and Methods section of the main text. Broader regional considerations regarding shallow depths in the CAS\nare also provided in the newly added Supplementary Material S3.\nLines 246-256: The authors propose that increased stratification occurred in the Atlantic Ocean and parts of the\nTethys, yet they do not illustrate these changes in the figures. To support this claim, visual evidence of\nstratification changes should be provided. Additionally, the mechanisms underlying these changes—particularly\nthe interactions between the CAS and Western Tethys—are not clearly presented or discussed.\n-The visual evidence of stratification is provided in the new Figure 5. Concerning the simulated region, we have\nnow provided explanation stated in the text and shown in the Fig. 1 that the analysed region includes both the\nAtlantic Ocean and Western Tethys (Region A).\nLines 261-266: The authors suggest comparing the scenario-induced changes (CAS uplift) with geological data\nand note that these processes coincide with global cooling and sea-level fall. However, the timeframe remains\nunclear. What specific period are the authors addressing? Based on palaeontological data, this study focuses on\nthe Maastrichtian, a time marked by warming followed by a brief cooling just before the K-Pg boundary. The\nEarth has experienced global cooling since the Turonian, but the late Maastrichtian includes a notable\ntemperature rise prior to this cooling episode.\n-We agree with the comment. The micropaleontological data used in this study indeed cover the Maastrichtian\ninterval, and this has been clarified in the main text. The new version of the revised manuscript focuses primarily\non paleogeographically driven global salinity budget changes rather than temperature variations.\nLines 291-343: The authors suggest salinity changes in the Sidi Ziane section, possibly linked to increased\ncontinental runoff (L310). For the Norwegian Sea sections, they similarly conclude that runoff may have\nincreased, contributing to water mass stratification (L341). The authors propose that these salinity changes result\nfrom shifts in ocean circulation driven by evolving ocean passages. While the Norwegian Sea sections indicate a\nsalinity response (although the figure is difficult to interpret), the authors do not clearly present salinity changes\nin the Tethys from their sensitivity experiments. Additionally, changes in salinity can also stem from shifts in the\nlocal precipitation-minus-evaporation (P-E) balance. Modifying ocean circulation could potentially alter\natmospheric circulation as well—especially given the possible removal of the Western Interior Seaway (WIS)\nand Hudson Seaway (HUS)—which could impact the P-E balance. However, this point is not addressed.\n-As explained in the new version of the manuscript, the simulated region in Fig. 3 includes Atlantic Ocean +\nWestern Tethys (see the Region A in Fig. 1).\n-To address concerns regarding the P-E balance, we have prepared an additional figure (see below). This figure\nis now also added as Supplementary Data S2B. It presents simulated P-E in the North Atlantic-Arctic region\nunder different gateway configurations between the Arctic and proto-North Atlantic Oceans, as well as varying\nCAS depths. The results indicate that changes in P-E between simulations are minor and confined only to local\nareas. Therefore, it is very unlikely that salinity changes in the North Atlantic region were driven by P-E\nchanges.\nThe authors discuss the roles of the Deccan Traps and the Chicxulub impact, suggesting that these events may\nhave masked the effects of ocean passage changes. The establishment of the Deccan Traps and the associated\nCO₂ emissions likely increased pCO₂ in the late Maastrichtian, leading to global warming, which could plausibly\nincrease continental runoff. However, implicating the Chicxulub impact as a contributing factor seems more\nchallenging, given that it occurred precisely at the K-Pg boundary, after the Maastrichtian.\n-When we refer to how Chicxulub's impact could have masked the signal, we are thinking of the early Danian\nsince due to the catastrophic mass extinction event sudden global climatic and environmental perturbations\ncould have masked shifts in ocean circulation or more precise paleoceanographic reconstructions.\nWhile the authors emphasize the impact of ocean passage changes, there is insufficient geological evidence and\ntemporal constraints to substantiate a causal link between these geological and oceanographic events. Why\nwould the CAS have been shallow during or prior to the Maastrichtian? While some studies suggest a southern\nclosure of the WIS, the northern portion of the WIS, along with much of the HUS, appears to remain open until\nthe K-Pg boundary.\nThe geological evidence, as discussed above, indicates shallowing conditions in the Central American Seaway\nduring the Late Cretaceous, a conclusion also supported by Buchs et al. (2018) and Radmacher et al. (2021).\nThis information formed the basis for conducting numerical experiments to investigate paleoceangraphic\nchanges, including the reorganisation of oceanic circulation driven by restricted connections between the\nAtlantic and Pacific Oceans. Regarding the WIS and HUS, we simulated restrictions in the connections between\nthe Arctic Ocean and the rest of the Global Ocean, resulting in limited exchange of water masses. Even the\nsouthern closure of the WIS could have significant impact on water mass exchange and increased outflow\nthrough Greenland-Norwegian seaway. Given the uncertainty surrounding the paleogeographic conditions\nduring the latest Cretaceous, the goal of our experiments was to compare the results with available geological\nproxies. Following the manuscript revision, we now discuss these comparisons in greater detail, not only with\nthe presented Maastrichtian data but also in relation to boarder geological evidence, including the\npalynological record from the northern North Atlantic (Radmacher et al., 2014a, 2014b, 2015; 2021). We\nbelieve that this revision addresses the Reviewers’ comments and strengthens our interpretations. Variations in\nmarine connections around the ArO are based on published data, e.g. Ladant et al. (2020) and Schröder-Adams\n(2014).\nReviewer #2 (Remarks to the Author):\nManuscript# NCOMMS-24-63460-T\nCorresponding Author: Wieslawa Marta Radmacher\nTitle: Freshening near the end of the Mesozoic?\nReviewer Assessment\nKey Results\nThe key results from the paper, as I understand them, are that the model experiments show that shallowing\ngateways result in an enhanced freshening of the oceans in the Maastrichtian. This is similar to what has been\nfound in studies of the Cenomanian and Early Eocene. This would suggest that such changes to the ocean system\nmay be more common than currently apparent in the literature.\nValidity\nThe climate model experiments show ocean freshening (lower salinity) with increased isolation of ocean basins.\nThis is neither new nor a surprise. Nonetheless, I believe this study to be a useful addition, but I am concerned\nby the following:\n1. Setup of the paper – I re-read the manuscript many times and feel that the abstract and introduction would\nboth benefit from being ‘tightened up”. By this I mean the following:\n(a) A more explicit statement of the aim of the study – is the aim to show how paleogeography affects the\npotential for freshening of the oceans in the geological past. That was my take.\n(b) What then confused me as a reader were the statements about the importance of understanding climate\nchange, the causes of extinction, paleoecology, etc. Ok, yes, but these are not fully developed, and they feel like\nthey have been added to the abstract and introduction to capture all the current buzz phrases. Yes, the potential for\nfreshening of the oceans and its impact on ocean ecology and circulation is important. However, this study is about\nlonger-term factors (viz. paleogeography), whereas today, we are concerned with short-term drivers, like\nCO2, land use changes and how these impact the hydrological system.\n(c) Extinction – there is mention of the study interval preceding the mass extinction at the end of the Cretaceous\n(lines 33-35, 62-64). But a relationship between environmental changes (as described in this paper) and\nextinction is not then developed. I cannot see that they are related except for certain marine species being affected\nby salinity changes, if these are global, which does not seem likely. Is the intention to state that there are\nenvironmental changes leading up to the end Cretaceous which have not been examined enough?\n-We acknowledge these concerns. The abstract has been revised to clearly state the aim of the study. In the updated\nversion of the manuscript, we have addressed all the issues raised and implemented changes to better articulate\nour objectives and findings. As the study focuses on long-term changes, the previously mentioned statements have\nbeen either removed or modified accordingly. Overall, the manuscript has undergone substantial revisions, and\nwe have made every effort to eliminate the ambiguities highlighted above by the Reviewer.\n(d) Abstract – this needs to be more formulaic: what I would do is to start by (sentence 1) stating that the study is\naimed at understanding how common ocean freshening might be in the geological record (if that is indeed the\naim); (sentence 2) How this might be important for future changes to the Earth ; (sentences 3 and 4) that the study\ndoes this by (1) running modelling experiments (long-term changes) and (2) outcrop sites to test model results in\nthose areas; (5) That the results indicate that freshening may have occurred in the Maastrichtian.\n-We are grateful for these helpful suggestions. We have reorganised the abstract strictly following the\nrecommendations of the Reviewer.\n2. Modelling experiments - The validity of using a 10m bathymetric experiment for the Central American\nSeaway and using this to make conclusions. Regarding climate modelling, a 10m water depth is essentially land!\nIt is also unlikely to be the case for the entire length of the arcs that form the barriers. The authors could do no\nworse than look at the present-day bathymetries of the island and continental arcs in the western Pacific.\nEspecially the discontinuous nature of some of these. Do I think that the Central American Seaway has barriers\nto deep water flow based on the geology I know? Certainly! I, therefore, agree with the authors. But, I do not\nconsider this new, although it is useful.\nThe primary objective of this study was to determine the depth threshold of the Central American Seaway (CAS)\nthat would trigger a reorganization of water circulation, potentially influencing the latest Cretaceous micropale-\nontological assemblages based on the available geological record. Our findings indicate that this threshold falls\nbetween 500 and 10 meters, with geological evidence suggesting it was closer to 10 meters. To simulate a unidi-\nrectional flow of oceanic waters from the North Atlantic to the Pacific—preventing the intrusion of deeper Pacific\nwaters into the Atlantic—the shallowest CAS condition had to be set at 10 meters. This approach was crucial for\nisolating the effects of Arctic gateway opening and closure on North Atlantic salinity and temperature, independent\nof Pacific water influence. Although a 10 meter depth may appear extreme, it should not be considered inherently\nunrealistic, as geological evidence supports the occurrence of similarly shallow conditions. A detailed discussion\nof these evidences as well as of the reasons for selecting the specific CAS depths is provided on the first pages of\nthis document, as well as in the main text within the Materials and Methods section under Numerical Model Sim-\nulations (beginning with the third paragraph).\n3. The outcrop data – this section is fine and, to me, is a paper in its own right, especially the Algerian site.\nHowever, for the general aim of the paper, if I understand this correctly, I am concerned that all the localities\nused in the study to indicate freshening of the oceans are near continental areas and are therefore much more\nsusceptible to changes in freshwater fluxes. They need not indicate global changes (though I think this is what\nthe authors hope through the isotope data? If so, this needs to be drawn out more clearly).\n-Our purpose with the isotope data, rather than establishing global interpretations, was more focused on age\nmodel construction since we tie δ13C curves between Algeria and the astronomically tuned section of Zumaia,\nSpain. The δ13C based correlation is a reliable approach, as demonstrated in several Maastrichtian sections\nacross different latitudes and environments (Batenburg et al., 2018). This way we further refined the age model\ninstead of only extrapolating between the available biostratigraphic datums. Moreover, we used isotopic data in\ncombination with foraminiferal, dinocyst, and CaCO3 data for the paleoenvironmental interpretation of Sidi\nZiane. The palynological signal indicates general significant changes that occurred during latest Cretaceous.\nThis include both shallowing and freshening in selected regions. We learn from global modelling experiments\nthat shallowing contributing to oceanic restrictions could force significant environmental reorganisations. We\ndiscuss the modelled scenarios and compare the results to local geological proxies. We believe that the concern\nmentioned in the comment above is now widely addressed within the significantly improved new version of the\nmanuscript.\n4. The equation of ocean freshening and stratification (e.g. line 343). Although freshwater fluxes to the ocean\ncan and do lead to water column stratification, in other circumstances, they can result in instability and over-\nturning.\n-We agree with that statement.\n5. “…the effect of salinity is often speculated or even overlooked” (line 334). Is this really true? There are\nnumerous papers on density stratification and the role of salinity on circulation, going back to at least 1982 and\nBrass and 1998 and Hay et al. More recently, this has been discussed by Ladant et al. (2020), which the authors\ninclude in the reference list.\n-We have improved this part of the text accordingly.\nSignificance\nThe conclusion that paleogeography may be important for restricting ocean basins and enhancing the potential\nfor ‘freshening of the ocean in these basins” is not new, but it is useful. The important role of gateways on\npaleoclimate has long been known (see Berggren 1982 as a start). That the authors identify this for the\nMaastrichtian to add to previously published work on the Cenomanian and Early Eocene is, to me, a useful\naddition. However, I am less clear on how the outcrop data fits into testing this, given my concern (see above)\nthat the localities are all near-shore and susceptible to local hydrological changes. This might be an area where\nthe authors could strengthen the arguments using the same data.\n-This issue has been more clearly addressed in the text, following the advice of the Reviewer.\nData and Methodology\nIncluding observational data and model experiments is a common and powerful approach for examining the\nresponse of the Earth system to changes in boundary conditions. To that extent, the data and methodologies are\nvalid. However, I am struggling with the detailed analysis of the Algeria site data and the bigger-picture question\nbeing discussed. This may be me, but there feels like a disjunct here. It is possible that this could be addressed by\nrewriting the section or turning the whole paper around, viz., “how the analysis of the Algeria data suggests\nchanges to the ocean system and hydrology, that are then examined using the modelling”, rather than the\nmodelling followed by the data. This is just a thought. I am less experienced with biostratigraphy and\npalynology, but from what I do understand, this all seems valid.\n-Thank you very much for this valuable comment. It would indeed be preferable to use this approach, but only if\nwe had a complete geological record covering not only the Maastrichtian but also the Campanian and even\nSantonian. Unfortunately, the uppermost Cretaceous record is challenging, and we have determined that the\nmodelling results provide a more complete picture. This is why we discuss it at the outset. However, in the new\nversion of the text the Sidi Ziane section is discussed and compared with other sections worldwide, allowing for\na more detailed interpretation and comparison with the modelling results.\nThe modelling relies on one published Maastrichtian paleogeography. What was the rationale for using this one?\nThere is an updated version to this map if the authors are interested.\n-The detailed explanation of used paleogeographical constrain and selected water depths used in our\nsimulations is in detail explained at the beginning of this document, and within the updated manuscript text.\nAnalytical approach\nThe analysis of the biostratigraphy data is outside of my principal area of expertise.\nSuggested improvements\nMy recommendation from reading this manuscript is that it would greatly benefit from a more explicit statement\nof the project aims and objectives and a clearer link between the aims of the Algeria analysis and the big-picture\nmodelling study.\nClarity and context\n1. Although the editorial guidelines say not to focus on grammar, I would urge the authors to avoid using words\nand statements such as the following:\n(a) “favouring” (line 35) – I would be more direct, for example “indicate”, “show”.\n(b) “Seem to” (line 39) – instead use “…processes may have played…”\n(c) “We assume that” (line 152) – either your data shows this, or your assessment of the existing literature shows\nthis.\n(d) “Reliable” (line 154) – what do you mean by “reliable”? Is this the view supported by data, or the most\nrecent publication, or something else?\n(e) “Novel” (line 230; but used several times in the manuscript) – In this case, this is not a novel (meaning\n“new”) mechanism. “Novel” should only be used if the mechanism has not been recognized previously.\n-All the issues suggested above have been resolved.\n2. Title: I would add the word “Ocean” at the beginning of the title to make the context of “freshening” clearer to\nreaders.\n-We agree with this comment and we have changed the title accordingly.\n3. When you use “Open-marine” (Line 113), do you mean deep-water connectivity? The system is open to the\nglobal ocean system, just at shallow depths. For example, the Atlantic and Pacific were not separated in the Late\nCretaceous as they are today by Central America.\n-By ‘open-marine’ we refer to oceanic conditions in which the connections between the Arctic Ocean and the\nrest of the global ocean were not restricted, allowing for uninterrupted water flow.\nReferences\nThere is a comprehensive reference list, although there are a few papers I think the authors should have a look at,\nfor example, Laugié et al. (2021), https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020PA004202\n-This reference has been included in the new version of the manuscript. Laugié et al. (2021), by testing different\ndepths in the Central American Seaway (CAS) referring to the Cenomanian (4000, 2500, and 300 m), conclude\nthat shallower conditions lead to the development of complete anoxia in the Central Atlantic basin. This\nexperiment was conducted within a different time interval, and the applied water depths varied. However, the\noverall conclusion is that the depth of the CAS plays a crucial role in the reorganization of water circulation,\ninfluencing the distribution of oceanic conditions and potentially driving significant biogeochemical changes.\nThis corresponds well with our results.\nOverview\nThis manuscript describes some very interesting results, and there is much that I think would be useful to see\npublished. However, as I have described above, I think the paper needs some reorganization and further thought.\nI encourage the authors to make changes, and I wish them luck in their research.\nReviewer #3 (Remarks to the Author):\nThis study integrates modelling results with micropaleontological and geochemical data from four geological\nsections in a time slice near the end of the Mesozoic (around 66Ma), just preceding the K-T extinction event.\nUsing the COSMOS Earth System Model, Radmacher and co-authors test five scenarios with varying depths of\nthe Central American Seaway (CAS) and marine connectivity around the Arctic. By shoaling the CAS, the\nmodel shows an increased salinity stratification in the Atlantic, an effect that is exacerbated when restricting\nseaway connections around the Arctic. The authors compare the modelling results with previously published\nmicropaleontological and geochemical data (supplemented with additional analyses) from four localities: the\nSidi Ziane Section (Algeria), the Re-2 Core from the Negev region (Israel) and two Norwegian Sea cores. They\nreport increased relative abundances of dinoflagellate cysts belonging to the Family Areoligeraceae which is\nsurmised to be a result of increased stratification. Increased abundances of protoperidiniaceans, known\nheterotrophs, are interpreted as evidence for the presence of nutrient-rich water prone to salinity fluctuations.\nTaken together, the authors conclude that their modelling and data point to a freshening at high latitudes with an\nincreased outflow of low salinity waters through the Greenland-Norwegian Seaway.\nThe authors should be commended because they present results from a united effort of modelers and data driven\nscientists in a single study which will therefore be of interest to the diverse readership of Nature\nCommunications. I find the manuscript easy to follow, but found a number of grammatical and formatting issues\nlisted as comments in the annotated pdf (attached). The text is accompanied by clear figures that effectively\nconvey the authors’ message.\n-We thank the Reviewer for the comments. All the issues outlined in the attached pdf file have been addressed.\nMajor Comment\nRelative abundances are a form of compositional data (their total summing to 1) meaning that a rise in one\nspecies is automatically at the expense of another (the constant-sum constraint). In figure 6, selected profiles of\nplanktic foraminifera and dinoflagellate cysts are presented which are difficult to interpret without knowledge of\nthe whole assemblage composition (which the authors nicely present in the figures of the supplementary data).\nThis leads to interpretations that are not necessarily backed up by the data. For example, on Line 294–295 the\nauthors claim “when Guembelitria is more abundant than Planohedbergella, Areoligeraceae show higher\nabundances”. This is clearly not the case in CIE1 where areoligeraceans are close to 100% of the palynological\nassemblages while relative abundances of Planohedbergella exceed those of Guembelitra. Such covariance as\nsuggested as suggested by the authors on lines 294–295 can only be examined using rigorous statistics that obey\nthe laws of compositional data analysis.\nWhile the original relative abundance data allowed us to infer a direct relationship between Areoligeraceae,\nGuembelitria, and Planohedbergella based on their respective abundance curves, we agree with the Reviewer\nwho correctly pointed out that this relationship was not supported by statistical analyses or composition data\nanalyses. Therefore, we followed the suggestion and explored the relationship between Planohedbergella,\nGuembelitria, and Areoligeraceae in more detail. Before doing so, we performed multivariate analyses,\nspecifically Principal Component Analysis (PCA), to robustly establish the main groups we would later discuss.\nThe PCA results showed that PC1 explained 75.903% of the variance, while PC2 accounted for 17.02%,\ntogether explaining 93% of the total variance. The genera with the greatest loadings in PC1 were Heterohelix\n(0.756), Planohedbergella (-0.534), and Guembelitria (-0.364). In PC2, Guembelitria had a loading of -0.791,\nwhile Planohedbergella had a loading of 0.594, suggesting potentially inverse ecological preferences. Loading\nfor Heterohelix were irrelevant in PC2. The remaining taxa exhibited negligible loadings in both PC1 and PC2.\nGiven these results and the better-known ecological preferences of Guembelitria and Planohedbergella we\nfocused our discussion on these both taxa. Subsequently, we conducted a composition data analysis to accurately\ncompare them with the major dinocyst group, Areoligeraceae (See new Fig. 8). We applied centered log-ratio\n(CLR) transformations to the compositional data to avoid redundancy in relative abundance comparisons. This\nanalysis revealed that Areoligeraceae exhibited significantly higher values when Guembelitria was more\nabundant than Planohedbergella (Fig. 8C). Additionally, we performed a Wilcoxon test to assess this\nrelationship using the transformed compositional data, resulting in a p-value < 0.05 (0.02371), providing\nstatistical support for our interpretation. We also applied the same approach using the relative abundance data,\nwhich remained significant, yielding a p-value of less than 0.001.\nIn addition, relying on changes in relative abundance, especially for dinoflagellate cysts, without knowledge of\nthe burial flux or concentration of cysts can be challenging. Dinoflagellate cyst burial flux calculations are\nrecommended (they can be calculated if the marker grain method has been applied during preparation) as they\ncan separate rises of number of cysts that are result of increased incorporation of cysts in the sediment from\nthose that are the result of changes in the sedimentation rate. If these data are available, they would be invaluable\nin interpreting the presented assemblage changes.\n-This is an important suggestion, and we agree with the recommendations. However, the Lycopodium marker\nhas not been introduced for the new samples from the Sidi Ziane and Negev regions, and as such, we are unable\nto calculate the dinoflagellate burial fluxes. Nonetheless, the key observation from these two sections is that the\nMaastrichtian intervals are overwhelmingly dominated by Areoligeraceae. This signal is associated with\nenvironmental stress, such as increased freshwater input and changes in surface-water salinities, which likely\ncontributed to enhanced water stratification. We now also compare dinoflagellate cyst assemblages from older\nintervals, and this is based on published data. Furthermore, the lateral distribution of the studied sections and\nthe dominance of different groups at low and high latitudes are important factors, indicating different\nenvironmental conditions in these distal regions. In the new version of the manuscript, we discuss all of these\nfindings in detail, providing a clearer and more advanced discussion of the results and their significance.\nOther comments\nAbstract: why is there no mention of any of the geochemical and micropaleontological data in the abstract? The\nmanuscript discusses an integration of modelling and data but the abstract only mentions modelling results. I\nbelieve this is a particular strength of the manuscript and should be included here.\n-In the new version of the manuscript, the abstract has been completely reorganized in accordance with\nReviewer 2’s suggestions. We shortly highlight the use of palynological, micropaleontological, and geochemical\ndata to provide a more comprehensive interpretation of the modelling results. The length of the abstract is\nrestricted, which unfortunately hampers additional detailed explanations.\nLine 54: Reference 21 deals with the late Neogene and offers no evidence about the latest Cretaceous as the\nauthors seem to suggest here.\n-We cite this manuscript to highlight the differing views on the timing of the restriction of water mass exchange\nbetween the Atlantic and Pacific Oceans, with some authors suggesting ~70 Ma, while others indicate 6.2 to 1.8\nMa.\nLine 196 and Figure 6A: CIE1 (high δ13C corresponding to low δ18O and peak in Areoligeraceae) behaves\ndifferently than the other CIEs where lower than expected δ13C are recorded. This is not explained in the text.\nLater in the text these CIEs are ignored even though they were deemed important enough to highlight here and in\nFigure 5.\n-After careful examination, we found that CIE1 could be redefined. The actual CIE interval encompass a longer\ninterval while it should span from 66.44 to 66.40 Ma, with the peak of Areoligeraceae located in the center. As\nthe Reviewer pointed out, the relative abundance within CIE1 does not allow for a clear observation of the\nrelationship between Guembelitria, Planohedbergella, and Areoligeraceae. However, the compositional data\nanalyses indicate that this relationship is indeed well established. This anomaly within CIE1 may be related to\nother issues related to the limitations of the geological record affecting to paleontological content of the samples\nor could be influenced by other environmental variables that we do not fully catch on this study.\nLine 268: what about preservation of calcium carbonate in cooler waters? This seems like it could be an issue\nnear the bottom of the Sidi Zaine section where %CaCO3 is very low. Will this not impact foraminifer\npreservation and the associated stable isotope signal?\n-Regarding to the preservation state of the planktic foraminiferal tests in Sidi Ziane, it varies between moderate\nand good throughout the entire section. No clear relationship has been observed between %CaCO3 and state of\nforaminiferal preservation under microscope. In particular, no deterioration in the preservation of the\nforaminiferal tests of Sidi Ziane has been observed at samples with low %CaCO3 value. As stated within the\ntext, \"The relationship of δ13C bulk with δ18O bulk shows a nonlinear correlation, r = -0.159, p(a) >0.05 thus\nsuggests that the primary trend is preserved”.\nLine 287: “this group can be considered as indicative of stressed near shore marine environment, with notably\nfresh-water input and changes in surface-water salinities followed by enhanced water stratification.” This seems\nlike an unrealistically specific condition for increased abundance of areoligeraceans. I agree the data shows\nraised abundances could be indicative of “stressed” near shore marine environments but “notably fresh-water\ninput and changes in surface-water salinities followed by enhanced water stratification” seems like an overreach.\nIt may be the paleoenvironment suggested in the current study, but I think this must be rephrased as not to sound\nthat an increase in areoligeraceans is automatically indicative of only this specific situation.\n-We agree with the Reviewer that this part needed rephrasing. This interpretation was influenced by overall\nresults, including insights from the modelling. This sentence has been now removed, and the interpretations have\nbeen improved accordingly.\nLines 292–297: I find this very speculative, and this is definitively not the case for CIE1 were abundances of\nPlanohedbergella are higher than Guembelitria in the same sample when Areoligeraceans form close to 100% of\nthe dinoflagellate cyst assemblage (Figure 6 D–E). An inference like the one made by the authors in the sentence\nstarting at line 293 (Areoligeraceae showing an inverse paleoecological signal with respect to\nPlanohedbergella…), should be backed up by multivariate statistics.\nThis issue has already been addressed by the earlier mentioned comment.\nLine 302–303: higher productivity could indeed result in higher CaCO3 but post-deposition dissolution is an\nimportant factor in this, especially in cooler high latitude water (e.g. Henrich et al., 2002). Again, calculating a\ndinoflagellate cyst burial flux would give a much more reliable estimate of the organic matter burial rate and\ntherefore productivity. Line 309: “Guembelitria bloom”. A high relative abundance is not evidence of a bloom\nwhich suggests a high burial flux (high productivity).\n-We consider an increase in the relative abundance of opportunistic Guembelitria to more than 20 or 30% to be\nanomalous and actually evidence of a Guembelitria bloom, regardless of how long the bloom lasts. It would not\nbe evidence of a bloom only if this increase in abundance was due to the disappearance/extinction of the other\ngenera with which it is compared in the quantitative analysis (as occurs immediately after the KPB mass\nextinction, where Guembelitria comprises 100% of the planktic foraminiferal assemblages). On the other hand,\nwhen a eutrophic taxon as Guembelitria increases its abundance with respect to the other non-eutrophic ones, it\nis indicative of a bloom most likely caused by an increase in productivity.\nLine 327–328: Reference needs replacing with reference number (Harris and Tocher, 2023).\n-This reference has been replaced with the number, as suggested.\nLine 336–337: sentence needs rephrasing.\n-This part has been improved.\nLine 359: I agree with the authors here. Only relative paleo-salinity changes can be derived since the exact\ncomposition of the Maastrichtian seawater is not known. These relative paleo-salinities can be derived from\nplanktonic foraminifer oxygen isotope records corrected for global ice volume using a normalised global\ncompilation of benthic oxygen isotope records. This has been done for the Pliocene–Quaternary but I am not\naware of an equivalent for the Maastrichtian. If available, such a calculation could help solidify the inferred\nfreshening. As the authors point out (e.g., line 396), the studied time interval is ice-free and a conversion of the\nplanktonic oxygen isotope signal using the Shackleton (1974) equation could yield a record that is mainly\ninfluenced by salinity changes. Maybe this can be used to give a rough idea of salinity changes?\n-Although we find this Reviewer's suggestion very interesting, we lack isotopic analyses on individual species of\nplanktic foraminifera, so we cannot calculate it. Furthermore, we recognise that, despite all its virtues, the the\nSidi Ziane section has its limitations regarding planktic foraminiferal preservation, as they do not present pristine\ntests that are the best suited for single-species isotopic analyses.\nFigure 2: check the layering of the red rectangle. Some of the differences discussed in the text are hard to discern\nat the scale of the individual maps. Add dimensions to the colour gradient variables along the X axis.\nFigure 2: “Caribbean Seaway” at the top of the figure is the same as “Central American Seaway” in the caption\nand text?\nFigure 3: add dimensions to the colour gradient variable displayed along the bottom of each figure. HS = Hudson\nSeaway (correct?). Note that in line 114 the Hudson Seaway was abbreviated HUS instead of HS. Make uniform.\nFigure 3A corresponds to the same model parameters employed in Figure 2A (Deep CAS with open Arctic\nmarine connections) while Figure 3B corresponds to Figure 2D (Shallow CAS with reduced Arctic marine\nconnections).\nFigure 4A shows the difference between Open GNS/shallow CAS Vs. Open GNS, WIS and HUS/shallow CAS.\nFigure 4B shows the difference between Open GNS/shallow CAS Vs.Open GNS, WIS and HUS/Deep CAS.\n-We greatly appreciate all the valuable suggestions regarding the figures. All of them have been carefully\nconsidered, and the figures have been revised and improved accordingly. Additionally, we have incorporated\nfurther changes to enhance their clarity and appropriateness. We also added new figures to better explain our\nfindings. Figure 4 has been exchanged.\nAll the references cited above can be found in the main text of the manuscript.\nWhether the data provide unequivocal evidence supporting the posed hypotheses is another matter.\nHowever, the only way anyone could address this problem would be to generate planktonic biome\nmaps for the Atlantic using the deep-sea drilling data. However, I concede that correlating these\ndata would not be easy with the fine temporal resolution required. More could be done as a second\npaper, developing the modelling experiments and bringing in more data. But for the current\nhypothesis, this is enough.\nWe are grateful for this thoughtful suggestion. We fully agree that this approach could be highly\nvaluable in addressing additional questions and would indeed be a promising direction for future\nresearch. Given the scope and focus of the current manuscript, incorporating this analysis would\nrequire substantial time and effort beyond what is feasible here. We therefore consider it an\nexcellent idea to pursue in a dedicated future study.\nDiscussion. This is now a long section. There is much interest and importance here, but I admit I\ngot lost in it once or twice. There is also some repetition. I do not think that it would take much to\ntighten this section up. My suggestion would be to go through and highlight with a marker the key\ntakeaways - I find using the Read Aloud facility in MS Word quite useful (I am sure you already\ndo this).\nWe did our best to implement the suggested changes, while carefully considering the clarity and\nintent of the original sentences. Although further shortening proved challenging without\ncompromising important nuances, we have made several modifications where possible to improve\nconciseness and readability."}, {"round": 2, "reviewer_comments": {"reviewer_2": "Reviewer Assessment\nOverview\nHaving reviewed an earlier version of this manuscript, I must thank the authors for the changes they have made. These\nchanges have addressed many of the concerns I raised. Consequently, my review will be short and focus on any remaining\nor new points.\nThe title, abstract, and set-up are much stronger, and the balance and link between the modelling and observational data are\nstronger. I was particularly interested in the modelling result showing the need for changes in both the Arctic (North Atlantic)\nand Caribbean gateways to generate significant changes in salinity, which was much more clearly stated in this version.\nNoteworthy results\n1. The modelling suggests that to maximise the freshening of the Central Atlantic—western Tethys ocean, circulation\nchanges at the Arctic and Caribbean gateways need to be limited. Looking at the impact of multiple tectonic and\npalaeogeographic changes is important and, in my view, has long been overlooked. Too often, the focus has been on\ninvestigating a single factor, so this is good to see here.\n2. In the modelling, changing the sill depth of the Caribbean gateway only has local effects on precipitation-evaporation, P-E\n(lines 135-136). I find this interesting, because as the authors note, it is changes in freshwater runoff that are important in the\nArctic for salinity changes there. In contrast, regional changes in P-E have been shown to result from modelled changes in\natmospheric chemistry or orbital changes.\n3. There is a change in the planktonic assemblages during the Maastrichtian in the low-latitude sites, including the new site\nin Algeria that the authors discuss at length (lines 480-481). This may indicate increased stratification.\nSignificance\nThe climate model experiments show ocean freshening (lower salinity) with increased isolation of ocean basins. As I said in\nmy original review, this is neither new nor surprising. However, the authors have emphasised in this version the importance\nof changing both the Arctic and Caribbean gateways to generate freshening. This is interesting and certainly needs to be\npublished.\nDoes the work support the conclusions and claims, or is additional evidence needed\nThe observational data analysed and described in this paper are consistent with the hypotheses drawn from the modelling\nexperiments.\nWhether the data provide unequivocal evidence supporting the posed hypotheses is another matter. However, the only way\nanyone could address this problem would be to generate planktonic biome maps for the Atlantic using the deep-sea drilling\ndata. However, I concede that correlating these data would not be easy with the fine temporal resolution required.\nAre there any flaws in the data analysis, interpretation and conclusions? Do these prohibit publication or require\nrevision?\nMy only reservation is the degree to which the salinity changes in the biotic signal reflect local rather than regional salinity.\nHowever, in response to this comment in my original review, the authors pointed out the importance of the stable isotope\nsignal. Whilst I agree, much of this paper continues to focus on the biota.\nIs the methodology sound? Does the work meet the expected standards in your field?\nYes.\nMore could be done as a second paper, developing the modelling experiments and bringing in more data. But for the current\nhypothesis, this is enough.\nIs there enough detail provided in the methods for the work to be reproduced?\nYes.\nSuggested improvements\nThe changes to the previous version made by the authors have addressed most of my concerns.\nTwo minor observations/suggestions for this version:\n1. Lines 207-208: I suggest adding a connecting sentence explaining why this section in Algeria is important for this study\nand the modelling. For example, \"The Sidi Zidane section.... (Figs 6 and 7). This section provides an opportunity to examine\nchanges in the biota and geochemistry that may be responses to salinity variations investigated through the climate\nmodelling.\" (This is a suggestion only!)\n2. Discussion. This is now a long section. There is much interest and importance here, but I admit I got lost in it once or\ntwice. There is also some repetition. I do not think that it would take much to tighten this section up. My suggestion would be\nto go through and highlight with a marker the key takeaways - I find using the Read Aloud facility in MS Word quite useful (I\nam sure you already do this).\nClarity and context\nThis version is far clearer, and I thank the authors for making the effort to take my comments and those of my fellow\nreviewers on board.\nOne typo, (line 425), I think you mean \"statistically\"?", "reviewer_3": "The manuscript now reads a lot better than the initial draft and it is easier to follow the authors’ line of reasoning.\nIn the review of the revised manuscript, I focussed mostly on the implementation of my original comments. As such, I am\nsatisfied that my suggestions raised in the annotated pdf of the original manuscript have now been implemented. I attached\nanother annotated pdf with some additional minor suggestions, mostly editorial.\nThe major comment I had in the previous iteration of the manuscript referred to the handling of the relative abundance data,\nespecially that of the palynological analyses. The authors transferred the compositional diagrams from the supplementary\ndata to the main part of the manuscript (Figure 8A). This now allows the reader to assess the changes of the overall\nassemblages. The authors also conducted compositional data analysis using a centred log ratio which shows the\nrelationship between Areoligeraceae and the ratio of Guembelitria/Planohedbergella.\nOne major comment could not be addressed. No marker grains were added to assess whether dinoflagellate cyst\nassemblage changes were mainly due to changes in paleoproductivity or changes in sedimentation rate. Because the\nauthors integrate their new data with legacy data, which they have no control over, it is difficult to address this. Having said\nthis, the very significant abundance changes of Areoligeraceans combined with the ratio of Guembelitria/Planohedbergella\nalongside the geochemical analyses make it more likely that environmental changes are responsible. I am satisfied that this\npoint has been addressed as well as possible.\nI also note that CIE 1 now has been redefined and fits the criteria outlined in the manuscript better than in the original\nversion."}, "author_response": ""}]

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