Effects of paleogeographic changes and CO_2 variability on northern mid-latitudinal temperature gradients in the Cretaceous

Domain: earth_science
Published: 2023-08-25
Editorial outcome: accept

Abstract
The Cretaceous ‘greenhouse’ period (~145 to ~66 million years ago, Ma) in Earth’s history is relatively well documented by multiple paleoproxy records, which indicate that the meridional sea surface temperature (SST) gradient increased (non-monotonically) from the Valanginian (~135 Ma) to the Maastrichtian (~68 Ma). Changes in atmospheric CO2 concentration, solar constant, and paleogeography are the primary drivers of variations in the spatio-temporal distribution of SST. However, the particular contribution of each of these drivers (and their underlying mechanisms) to changes in the SST distribution remains poorly understood. Here we use data from a suite of paleoclimate simulations to compare the relative effects of atmospheric CO2 variability and paleogeographic changes on mid-latitudinal SST gradient through the Cretaceous. Further, we use a fundamental model of wind-driven ocean gyres to quantify how changes in the Northern Hemisphere paleogeography weaken the circulation in subtropical ocean gyres, leading to an increase in extratropical SSTs.

Introduction
Analysis of oxygen isotopes in foraminifera tests (δ18O) and the index of tetraethers that quantifies the archaeal lipid distributions in marine sediments (TEX86) indicate that the sea surface temperature (SST) during the Cretaceous period (~145 to ~66 million years ago, Ma) ranged between 28–35°C in low latitudes to 11–22°C in high latitudes (first and third quartiles of LOESS smoothened data)1. A similar trend is observed during the Paleocene (~66 to ~56 Ma) and Eocene (~56 to ~34 Ma) epochs of the Paleogene period (~66 to ~23 Ma) when the tropical and high latitude temperatures were between 31–33°C and 11–14°C, respectively2. The persistence of such high globally averaged SST and low pole-to-tropics temperature gradient during the Cretaceous–Paleocene–Eocene (CPE) is attributed predominantly to the strong greenhouse effect ensuing from the higher partial pressure of CO2 (pCO2) in the atmosphere3,4. However, in addition to changes in pCO2, the spatiotemporal variability in SST can also stem from tectonically driven changes in paleogeography, including surface topography, ocean bathymetry, and the arrangement of continents5–7. Specifically, paleogeographical alterations during the Cretaceous have been demonstrated to dampen seasonal variations and trigger widespread flooding, ultimately leading to moderation of the pole-to-tropics temperature gradient8,9. The rearrangement of continents during the period has also been associated with shifts in the deep-water formation sites7,10, alternations in the oceanic overturning circulation11, and variations in mid-Cretaceous deep-sea ventilation12. Furthermore, the paleogeographic features associated with certain ages in the Early and Late Cretaceous have been found to be more conducive to glacial events than the mid-Cretaceous13. Moreover, changes in Antarctic orography14 and the opening of the Drake passage15 could have both contributed to the variability in meridional SST gradients during the Paleocene–Eocene (PE).

Although previous studies have investigated the effect of different paleogeographies corresponding to multiple ages in the CPE on variations in the pole-to-tropics temperature gradient and the ocean circulation10–12,16, the effect of paleogeographic changes on the surface ocean circulation specifically has not been examined before. Surface ocean gyres contained between two meridional continental boundaries are prominent features of the present-day ocean. The meridional volume (mass) transport (ψ) associated with these gyres redistributes heat in the ocean and is critical to regulating the climate on Earth’s surface.

Henry Stommel was the first to formulate a simple mathematical model comprising five parameters that capture the fundamental characteristics of the circulation in such gyres17. While the linear dependence of ψ on the amplitude of zonally averaged wind stress (τ) is evident from the analytical solution of Stommel’s model, discerning its dependence on the zonal and meridional extents of the ocean basin (Lx and Ly, respectively) is rather non-trivial. It should be noted that in Stommel’s model, the meridional boundaries of the basin are co-located with the latitudes of zero wind-stress curl. However, in the real ocean, the two do not necessarily coincide in general, and the extent of the surface gyres is constrained only by the latitudes corresponding to the vanishing wind-stress curl. Previous studies established that both ψ and the poleward heat transport (H) associated with a surface gyre increase with the horizontal aspect ratio Ly/Lx of the ocean basin containing the gyre18–20 (hereafter referred to as the ‘gyral basin’). Thus, the aspect ratio of any gyral basin is governed by the curl of the overlying wind stress and the relative position of the zonal boundaries set by the continents. Based on these first-order constraints, it was recently postulated that the large meridional SST gradients observed during some geologic periods may have resulted from the smaller ψ (and consequently a smaller H) associated with the surface gyres contained in basins with small aspect ratios20.

Here we use data generated from two ensembles of paleoclimate simulations carried out using the HadCM3L (specifically, HadCM3LB-M2.1aD) model6,7 to better constrain the relative contribution of changes in pCO2 and paleogeographic effects on the SST distribution during the CPE. In particular, we develop a simple model that isolates the role of paleogeographically driven changes in extratropical gyral circulation and the corresponding effect on the evolution of pole-to-tropics SST gradient. The model is then applied to the HadCM3L data to show that paleogeography-driven reduction in ψ (and consequently H) associated with the surface ocean gyres may have been a key factor contributing to the variability in meridional SST gradients observed during the CPE1,2.

Results
Numerical simulations
All simulations necessary for this work were carried out using the UK Met Office coupled atmosphere-ocean model, HadCM3L (resolution: 3.75° in longitude × 2.5° in latitude), which includes multiple climate feedbacks, including vegetation feedbacks6,21. The atmospheric CO2 concentration in the two ensembles is kept constant at 560 ppmv and 1120 ppmv (i.e., ×2 and ×4 preindustrial atmospheric CO2 concentration) while the geography and the solar constant are varied. The chosen values of atmospheric CO2 provide an idealized framework that allows us to isolate the effect of CO2 versus paleogeography. In addition, this range of CO2 values captures the maximum variation (530 ppmv to 840 ppmv) in the long-term trend of LOESS smoothened data on estimated atmospheric CO2 concentration during the CPE22. The range does not capture the very lowest values of CO2 seen in some proxies in the latest Cretaceous23, but these low values are inconsistent with relatively high reconstructed global mean temperatures at this time24. The geographies corresponding to different geological ages are constrained by data aggregated from lithological, tectonic, fossil, and deep-sea studies6. The solar constant in both ensembles increases monotonically from the Berriasian age (~143 Ma) to the Priabonian age (~36 Ma) by 0.9% (see Methods). In practice, the relatively small magnitude of the forcing associated with the increase in solar constant is dwarfed by changes in pCO2 and paleogeography between simulations and their relative impact on SST distributions.

Wind-driven gyral circulation in the CPE
The mid-latitudinal gyral basin in the northern paleo-Pacific (hereafter referred to as the North Pacific) retains an approximately rectangular geometry—similar to Stommel’s idealized ocean—during the CPE, despite considerable plate tectonic activity25,26. Variations in the North Pacific’s circulation are expected to strongly affect the climate during the CPE, since for these ~110 million years much of the planet’s seawater was in the large paleo-Pacific27,28.

The Lx of the mid-latitudinal gyral basin in the North Pacific, i.e., the typical longitudinal distance between continental margins (see Methods), remained nearly constant during the CPE (~13,000 km, Supplementary Fig. 1). On the other hand, Ly of this basin, which is defined as the distance between the two latitudes where the wind-stress curl over the surface vanishes, decreased (non-monotonically) from ~3900 km to ~2800 km and then increased to ~3600 km (Fig. 1a). These variations in Ly determine the variations in the aspect ratio and hence the value of ψ in this gyral basin. This is illustrated by comparing ψ in the longer Valanginian gyral basin (~135 Ma; Fig. 1b), which is double that of the ~30% narrower Maastrichtian gyral basin (~68 Ma; Fig. 1c). The ocean during the Valanginian is also characterized by a much weaker SST gradient, which is consistent with a stronger H compared to the Maastrichtian.

Variability in meridional SST gradients and its dependence on atmospheric CO2 and paleogeography
To examine the variability in meridional SST gradients during the CPE, we first consider the SST gradients from the Early to the Late Cretaceous and subsequently focus on the trend from the Paleocene to the Eocene. Specifically, we compare the Maastrichtian to the Valanginian (rather than the Berriasian, given the paucity of the proxy-derived SST data in this age1) in the Cretaceous and the Priabonian to the Danian age (~63 Ma) in the PE.

In the HadCM3L simulations carried out for an atmospheric CO2 concentration of 560 ppmv, the meridional gradient of the zonally averaged SST (over all longitudes, ΔSSTy) calculated between 20°N and 50°N increases from 12.5°C in the Valanginian to 15.7°C in the Maastrichtian, whereas, ΔSSTy decreases from 16.5°C in the Danian to 15.5°C in the Priabonian. The same trend persists for an atmospheric CO2 concentration of 1120 ppmv, ΔSSTy increases from 11.1°C to 13.9°C during the Cretaceous and decreases from 14.3°C to 13.7°C during the PE. Figure 2a illustrates the temporal trend in ΔSSTy for the two atmospheric CO2 concentrations. In both simulation ensembles, the temporal variation in ΔSSTy from the Valanginian to the Maastrichtian is ~3°C even though the global mean SST varies only slightly (standard deviation of less than 1°C about the ensemble averages, see Supplementary Fig. 3). Alternatively, the temporal variation in ΔSSTy from the Danian to the Priabonian (~0.8°C) is comparable to the standard deviation about the global mean SST for the ensembles. Furthermore, comparing the two simulation ensembles shows that during the CPE, a doubling of pCO2 decreases ΔSSTy between 0.7°C (observed in the Albian age, ~106 Ma) and 2.6°C (observed in the Selandian age, ~60 Ma), resulting in an average decrease of 1.8°C in ΔSSTy.

The long-term trend of LOESS smoothened data on estimated atmospheric CO2 concentration suggests that a decrease in atmospheric CO2 from ~700 ppmv in the Valanginian to ~570 ppmv in the Maastrichtian22 can cause a decrease in global mean SST by ~1.5°C (based on a typical climate sensitivity parameter of ~4.5°C per CO2 doubling7) and an increase in ΔSSTy by ~0.6°C (since a doubling of atmospheric CO2 decreases ΔSSTy by 1.8°C a decrease in CO2 from ~700 ppmv to ~570 ppmv, i.e., a reduction by a factor of 0.8, is expected to increase ΔSSTy by log2(0.8) × −1.8 = 0.6°C). Similarly, a decrease in atmospheric CO2 from ~590 ppmv in the Danian to ~530 ppmv in the Priabonian22 is expected to cause a 0.7°C decrease in global mean SST and an 0.3°C increase in ΔSSTy. These estimates are consistent with the inverse relation between ΔSSTy and global mean SST observed across multiple climate states in Earth’s history with varied paleogeographies2. It should be noted that changes in atmospheric CO2 may have had a more pronounced effect on ΔSSTy since this study does not account for the short-term fluctuations in pCO2 trend or the lowest values of CO2 inferred from some proxies23.

Comparing the typical variation in ΔSSTy from the change in pCO2 with paleogeography-driven variation in ΔSSTy indicates that paleogeographic changes had a greater effect on the mid-latitudinal temperature gradient during the Cretaceous than the long-term changes in pCO2. This is also true for the PE. However, since the overall ΔSSTy variation during the PE is comparable to the standard deviation in the ensemble-averaged global mean SST, the analysis that follows only focuses on the Cretaceous.

The differences between the long-term SST gradients in the simulations and estimates based on paleotemperature reconstructions may be attributed to the limited spatial distribution of available proxies. Paleotemperature reconstructions based on δ18O and TEX86 proxies suggest that meridional SST gradients first decreased from ~10–17°C in the Valanginian to ~3–5°C in the Aptian (~119 Ma) and subsequently increased through the middle—Late Cretaceous to reach a maximum of ~19–21°C in the Maastrichtian1. However, the trend in this proxy-derived meridional SST gradient is only expected to capture the long-term trends in Equator-to-pole temperature difference. It does not capture the mid-latitudinal temperature gradient in the Northern Hemisphere (the focus of this study) since no data exists between 30°N and 80°N and the data for the high latitude paleo-temperature originates from sites located between 48°S and 54°S paleolatitudes. Nevertheless, the estimated ΔSSTy variation resulting from the combined effect of reduction in atmospheric CO2 concentration and paleogeographic changes from the Valanginian to the Maastrichtian (~3.6°C) is within 55% of the variation estimated from proxies (~6.5°C).

Role of gyral circulation in determining the meridional SST gradients
The decrease in ΔSSTy resulting from an increase in pCO2 of the model atmosphere can be attributed to polar amplification22,29–32, whereas the increase in ΔSSTy from the Early to Late Cretaceous in the two ensembles of simulations is related to paleogeography-driven changes. Paleogeography can influence many factors that could in turn affect polar amplification (e.g., cloud distribution and other local feedbacks); here, we focus on its influence on oceanic heat transport (H). Figure 2b shows a strong anti-correlation between ΔSSTy and the maximal volumetric (mass) transport (ψ) in mid-latitudes, which indicates that Cretaceous basins with smaller Ly contain surface gyres with smaller ψ. Thus, the weakening of mid-latitudinal H (Supplementary Fig. 4), caused by the weakened mid-latitude ocean gyres contributes substantially to the increase of ΔSSTy in the middle—Late Cretaceous. To quantify this effect, we assume that:
1. To first order, ΔSSTy decreases linearly with H and with the poleward heat transport associated with the atmospheric meridional overturning circulation (MOC).
2. The heat transport associated with the atmospheric MOC is proportional to H33.
3. The ocean circulation, to first order, is similar to the circulation in a ‘Stommel-like’ gyre. As such, due to rapid advection, there is negligible loss of heat by warm water along the western boundary. In contrast, the water mass loses much of its heat while it traverses the northern and eastern edges of the basin at a much lower speed. Thus, SST along the southern and western boundaries is high, whereas SST along the northern and eastern boundaries is low. Further, the East–West SST gradient is proportional to ΔSSTy.

Based on these assumptions, we find (see Methods):
ΔSSTy = ΔSSTy_ad / (1 + κ × ψ_max) (1)
where ΔSSTy_ad is the meridional gradient of SST at radiative equilibrium (H=0), ψ_max is the maximal gyral volume (mass) transport in the mid-latitudinal North Pacific, and κ is an empirical constant. The above equation is developed based on the paleogeographic considerations relevant to the Cretaceous, but we expect it to be also applicable to the PE since the majority (>60%) of the volumetric (mass) transport in the ocean’s surface is concentrated in the North Pacific until the Priabonian. Performing a regression analysis, based on this equation, on ΔSSTy and ψ_max estimated from the HadCM3L model data for the 19 ages considered here yields ΔSSTy_ad = 22.6 ± 1.4 °C and κ = (3.9 ± 0.5) × 10−4 Sv−1 °C−1 (1 Sv = 106 m3 s−1) for an atmospheric CO2 of 560 ppmv, and ΔSSTy_ad = 18.0 ± 0.9 °C and κ = (3.7 ± 0.6) × 10−4 Sv−1 °C−1 for an atmospheric CO2 of 1120 ppmv. The best-fit curves (Fig. 2a, dashed curves) illustrate that the reduction in ψ_max explains ~80% and ~75% of the variance in ΔSSTy from the Berriasian to the Priabonian in the simulation ensemble with an atmospheric CO2 of 560 ppmv and 1120 ppmv, respectively. This underscores the role of Ly in the increase in ΔSSTy during the Cretaceous and in the subsequent decrease in ΔSSTy from the Paleocene to the Eocene.

Though our regression analysis capture the general trend in the evolution of ΔSSTy during the CPE for both simulation ensembles, we observe a steep increase in ΔSSTy between ~130 Ma and ~100 Ma for the simulations with an atmospheric CO2 of 1120 ppmv (Fig. 2a, solid red curve). This steep increase lies outside the range in which the best-fit curve is expected to vary based on one standard deviation variation in κ and ΔSSTy_ad. It stems from an abrupt decrease in the SST at 50°N between the Barremian age (~128 Ma) and the Albian age (Supplementary Fig. 5). However, examining the precise climatic variations and feedback associated with paleogeographic changes that led to this abrupt cooling between the Barremian and the Albian are beyond the scope of this study.

The HadCM3L simulations show that geologic ages in the Cretaceous with a large Ly (e.g., the Valanginian) are characterized by a wide Equator-to-pole extent of the ocean basin (Ley) (Fig. 3a, b), while geologic ages with a small Ly (e.g., the Maastrichtian) are marked by a narrower Ley (Fig. 3c, d). Moreover, like Ly, Ley also decreased from the Early to Late Cretaceous (Supplementary Fig. 6). The correlation between Ly and Ley can be attributed to the increase in land area in the polar North Pacific since the mid-Cretaceous (Supplementary Fig. 7), because the increased land area amplifies the damping effect (friction) over the surface, which reduces the meridional distance between the zonal wind extremes (thus reducing the distance between the latitudes where the wind-stress curl over the surface vanishes). However, multiple other features of the atmosphere-ocean system not considered here can affect Ly, e.g., meridional extent and/or intensity of the Hadley cell, the intensity of the Walker circulation, and exchange of water masses between ocean basins34. For instance, Ly and Ley do not coevolve during the PE (Supplementary Fig. 6), which could be related to one or more of these features. Exploring a causal relation between changes in these climatic features and Ly on geologic timescales is left for future study.

Methods
Identifying the meridional and zonal extents of the mid-latitudinal gyral basin in the North Pacific
To determine the typical meridional and zonal extents of the gyral basin in the mid-latitudinal North Pacific, we construct a ‘trapezoidal’ domain in the ocean. The meridional margins of the gyral basin are given by the latitudes, ϕ1 and ϕ2, where the zonally averaged wind calculated by the HadCM3L model reaches an extremum. These extrema correspond to the points where the curl of the zonally averaged wind stress vanishes. Subsequently, we determine four typical longitudes, λ1–λ4, based on the continental boundaries between the latitudes ϕ1 and ϕ2 such that the region enclosed between the vertices P(ϕ1, λ1), Q(ϕ1, λ2), R(ϕ2, λ3) and S(ϕ2, λ4) denotes the ‘gyral basin’. The meridional extent of the gyral basin (Ly) is defined as the latitudinal distance between the edges PQ and RS and the zonal extent (Lx) is defined as the cosine (latitude) weighted average of the lengths of PQ and RS. Supplementary Fig. 1 illustrates the gyral basins during different geologic ages in the Cretaceous and the Paleogene. We note that both Ly and Lx depend on the grid resolution. Despite the rather stringent definition of Ly and Lx and their dependence on the grid resolution, the correlation between Ly and ΔSSTy is very strong, which further demonstrates the robustness of our analysis.

Relation between meridional sea surface temperature gradients and the volume (mass) transport in the ocean
Poleward volume (mass) and heat transport associated with the atmospheric meridional overturning circulation and the gyral circulation in the ocean’s surface lower the meridional sea surface temperature gradient (ΔSSTy), which is driven by differential solar heating. We assume that ΔSSTy is given by (see assumption 1, main text):
ΔSSTy = ΔSSTy_ad [1 − cO HO − cA HA] (2)
where ΔSSTy_ad is the meridional gradient of SST at radiative equilibrium when ocean and atmosphere heat transport are zero (HO = HA = 0), and cO and cA are the appropriate empirical constants. Assuming that atmosphere and ocean heat transport are proportional to each other (on average, and not at every latitude)33, we may write cA HA = (γ − 1)cO HO, where γ > 1 is an arbitrary constant (see assumption 2, main text). Equation (2) can then be rewritten as:
ΔSSTy = ΔSSTy_ad [1 − γcO HO] (3)
We further assume that the gyral heat transport can be written as HO = cp ρ ψ_max ΔSSTx, where ΔSSTx is the East-West SST gradient38, yielding:
ΔSSTy = ΔSSTy_ad [1 − γcO cp ρ ψ_max ΔSSTx] (4)
Given the well-constrained geometry of a Stommel-like gyre, we substitute ΔSSTx = cxy ΔSSTy (see assumption 3, main text) to obtain:
ΔSSTy = ΔSSTy_ad / [1 + γcO cp ρ cxy ψ_max ΔSSTy_ad] (5)
where 0 < cxy ≤ 1 is another constant. Substituting κ = γcO cp ρ cxy, yields Eq. (1).

Coupled atmosphere–ocean model description
The data analyzed in this study was generated from two ensembles of simulations carried out using the UK Met Office coupled atmosphere-ocean model, HadCM3L, which includes multiple climate feedbacks, including vegetation feedbacks6,21. The model has a horizontal resolution of 3.75° in longitude × 2.5° in latitude. The atmosphere and the ocean are divided into 19 and 20 vertical levels, respectively. The CO2 in the model atmosphere is kept fixed at 560 ppmv and 1120 ppmv for the two ensembles of simulations, while the paleogeography and solar constant are varied. The chosen range of atmospheric CO2 captures the maximum variation (530 ppmv to 840 ppmv) in the long-term trend of LOESS smoothened data on estimated atmospheric CO2 concentration during the CPE22. The geographies corresponding to different geological ages are developed by Getech Plc using methods described in ref. 39, and are constrained by geological data aggregated from lithological, tectonic, fossil, and deep-sea studies. The geographies were originally produced at a resolution of 0.5° in both latitude and longitude, which were subsequently used to generate geographies at model resolution. Additionally, some smoothening was applied to ensure model stability. A more detailed description is available in Section 2.1 of ref. 6. Given that the variation in solar constant was quite small (a monotonic increase of ~0.9% from the Berriasian age in the Cretaceous to the Priabonian age in the Paleogene), we assume that for a given pCO2 in the model atmosphere, any changes in the spatial distribution of SSTs between simulations arise primarily from the changing paleogeography during the CPE and the atmosphere–ocean feedbacks associated with it.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of \"Both tectonic activity and CO2 variability affect temperature gradients in the\nCretaceous\" by Gianchandani et al.\nThis paper uses a suite of climate model simulations to investigate the Cretaceous greenhouse\nperiod. In particular, the authors study modelled mid-latitude sea-surface temperature (SST)\ngradients in the Northern hemisphere and the respective roles of changes in continental\nconfiguration, atmospheric carbon dioxide (CO2), and meridional heat transport in the wind-driven\nocean gyres. The novel aspect of this work consists of an analysis of oceanic heat transport\nrelating to the changing aspect ration of the paleo-Pacific ocean basin.\nThis is certainly an interesting paper which will be useful to the paleoclimate community. My main\nrecommendation would be not to focus on the Hadley model alone, but to include existing (and\npublicly available) ensembles of paleoclimate model simulations, in particular Li et al. (2022,\nScientific Data) covering the Phanerozoic. There is also a set of Mesozoic simulations published by\nLandwehrs et al. (2021) using an intermediate-complexity model to disentangle the effects of\npaleogeography and CO2 which is relevant in the context of this work. Finally (and just a very\nminor issue), the end of the Cretaceous is now usually set at 66 Ma rather than 65 Ma.", "reviewer_2": "Summary: Gianchandani et al. use an ensemble of HadCM3L simulations to investigate and\nquantify the relative contributions of atmospheric CO2 and tectonic activity to variations in mid-\nlatitude SSTs in the North Pacific Basin during the Cretaceous. The simulations suggest that\nchanges in CO2 and paleogeography had comparable impacts on mid-latitude SST gradients.\nGianchandani et al. also highlight the importance of ocean basin aspect ratio in determining gyral\ncirculation, heat transport, and mid-latitude SST patterns in the North Pacific Basin by developing\na fundamental model of wind-driven ocean gyres.\n• What are the noteworthy results?\nThe main finding of this study is that changes in CO2 and paleogeography contribute similarly to\nincreasing mid-latitude SST gradients over the Cretaceous. Although it is well established in the\nliterature that changes in CO2 and paleogeography contributed to SST variability over the\nCretaceous, this finding is noteworthy because the relative contributions of CO2 and tectonic\nactivity to SST variability are not established. Another noteworthy result is that a decrease in gyral\nvolume transport, and by extension ocean basin aspect ratio, can account for ~80% variability in\nthe meridional SST gradient in the mid-latitude North Pacific over the Cretaceous.\n• Will the work be of significance to the field and related fields? How does it compare to the\nestablished literature? If the work is not original, please provide relevant references.\nYes, this work is significant for the understanding of ocean circulation and temperature during the\nCretaceous. The study outlines a simple model of wind-driven gyres based on the horizontal aspect\nratio of the ocean basin that may be applied to other deep-time intervals to better understand the\ncontribution of tectonic activity to changes in ocean circulation and temperature.\n• Does the work support the conclusions and claims, or is additional evidence needed?\nOverall, yes. However, my main concern is that the range of CO2 captured in these simulations\ndoes not capture the inferred paleo-CO2 range of the Cretaceous based on Foster et al. (2017),\nwhich could have reached decreased to values of ~280 ppmv during the Late Cretaceous (Lines\n116-120). The justification for the CO2 levels of the simulations needs to be explained, particularly\nbecause the relationship between CO2 and meridional SST gradient supported by the simulations\n(at 560-1120 ppmv) is being extended beyond this range of CO2 (down to ~280 ppmv). This\ndirectly impacts the how the relationship between CO2 and meridional SST gradient is calculated\nfor the Cretaceous, which is one of the main findings of the study.\n• Are there any flaws in the data analysis, interpretation, and conclusions? Do these prohibit\npublication or require revision?\nThe statement that atmospheric CO2 fluctuations and paleogeographic changes have a comparable\neffect on mid-latitudinal SST gradient through the Cretaceous may be too strong of a statement\ngiven that the study is restricted to the North Pacific Basin. The authors state that the fundamental\nmodel of wind-driven ocean gyres does not apply to the Southern Hemisphere (Lines 193-197).\nSome revisions would help broaden the conclusions of the study by investigating the relative\nimportance of CO2 and tectonic activity on SST variability in other ocean basins during the\nCretaceous and/or demonstrating how the fundamental model of gyral circulation could also be\napplied to other time intervals.\n• Is the methodology sound? Does the work meet the expected standards in your field?\nOverall, yes. However, the methodology requires more justification and explanation CO2 and\npaleogeographic reconstructions used for the simulations. Both CO2 level and paleogeographic\nreconstruction are subject to uncertainty that should be more explicitly described.\n• Is there enough detail provided in the methods for the work to be reproduced? Yes.\nMajor comments:\n• The title reiterates knowledge that is already well established about Cretaceous SSTs and does\nnot highlight the novelty or impact of the study. This title could instead highlight the relative\nimportance of CO2/tectonic activity in the northern mid-latitudes\n• Main/Introduction: The gap in knowledge motivating this study and broader scientific context\nfrom previous literature needs to be explained in the introduction. The first paragraph is already\ndiving into methodology of the study, which has only been motivated by a few sentences. As a\nresult, I am unsure if this study is motivated primarily by a desire to better understand SST\nvariability during the Cretaceous or develop a more general framework for understanding SST\nvariability based on gyral circulation/basin geometry in which the Cretaceous is a case study. After\nthe sentence that motivates looking at other environmental factors (beyond just CO2) (Lines 38-\n41), there should be a brief discussion of the work that has investigated the impacts of CO2 and\npaleogeography on ocean temperature and circulation during the Cretaceous to explicitly outline\nthe novelty of the current study (e.g., Ladant et al, 2020 in Climate of the Past; Donnadieu et al.,\n2016 in Nature Comm).\n• Lines 44-45: Why did you choose 560 and 1120 ppmv CO2? This should be justified in the\nmethods section. Although 560 and 1120 ppmv capture most of the inferred paleo-CO2 variability\nduring the Cretaceous, CO2 levels could have been much lower than 560 ppmv during the Late\nCretaceous, as you state on Lines 116-120. Regarding Lines 116-120, why do you expect that the\nrelationship between meridional SST gradient and CO2 between two relatively high CO2 levels\n(560 and 1120 ppmv) would extend to lower CO2?\n• Lines 177-179: Although the simple model explains a large proportion of the variance (Fig. 2A),\nit shows a roughly linear trend of increasing SST gradient at both CO2 concentrations, and thus\ndoes not capture a threshold-like behavior due to paleogeographic changes between ~130-110\nMya, which is very interesting. Before and after this interval, the changes in SST gradient are\nmuch smaller. What changes/feedbacks associated with those paleogeographic changes are\ncreating such a dramatic increase in SST gradient during that specific interval?\nMinor comments:\n• Figure 1: I would suggest making red/blue lines and markers thicker to differentiate from black\narrows and making black contour labels larger/more visible (I cannot read them)\n• Line 189-191: Two “results” in the sentence, perhaps break up this sentence with commas or\nchoose a different combination of words here.\n• Line 197-200: Citations from previous work should be added here.\n• Line 206: Add period to end of sentence.", "reviewer_3": "Gianchandani et al. present a study that uses general circulation modeling to investigate the driving mechanisms of changes in the Cretaceous meridional temperature. The authors conclude that both paleogeographic changes and decreasing pCO contributed equally to a decrease in the midlatitude temperature gradient of about 6 °C from the Early to Late Cretaceous (i.e., from the Valanginian to the Maastrichtian). This change in temperature gradient was caused by a decrease in oceanic heat transport resulting from a weakening of the wind-driven gyre circulation in the northern Paleo-Pacific in response to a decreased latitudinal extent of the Pacific basin.\nAs someone who has worked a lot on Cretaceous paleoceanography and paleoclimatology, but is not an expert in general circulation modeling, I find the manuscript to be well written, well structured, and easy to follow. The science is sound and the interpretations are supported by the data. The conclusions are presented in a clear manner and are relevant to the broader geoscientific community. As such, I consider the manuscript to be well suited for publication in Nature Communications and have only a few minor comments.\n1.) Paleogeography: I recommend including some background information on the\npaleogeography implemented in the model in the Methods section. Because\npaleogeographic restorations of the Paleo-Pacific are complicated by extensive subduction of\noceanic crust since the Cretaceous, it would be particularly helpful to include a brief\ncomparison between the implemented paleogeography and other published plate tectonic\nmodels to show that the general trends in L and L are consistent across the various\nX Y\navailable models.\n2.) Role of sea-level change: Do the changes in land-sea distribution shown in Fig. S1 and Fig. S6\nreflect solely plate tectonic processes (i.e., changes in the distribution of oceanic and\ncontinental crust) or also global sea-level changes (i.e., flooding of continents)? If both\nprocesses are involved, the authors may consider replacing \"tectonic activity\" with\n\"paleogeographic changes\" in the title of the manuscript.\n3.) Paleo-SST data: I suggest to include a graphical comparison of proxy-based SST estimates and\nmodel-derived latitudinal temperature gradients, at least for the Valanginian and\nMaastrichtian, to demonstrate that the model is capable of capturing the general patterns\nevident in the proxy data.\n4.) Figure 1: The labeling of the contour lines in Figures 1A and 1B is barely visible.\n5.) Figure 3: Correct “Maastrrichtian” in the caption of the Figure.\n6.) Line 95: Please indicate whether #SST were averaged over the Pacific basin or globally.\n7.) Line 192: Please provide add a short sentence or a reference to explain what tectonic\nprocesses drove the changes in the northern hemisphere paleogeography, specifically the\nclosure of the “proto-Bering Strait” connecting the northernmost Pacific and the Arctic\nOcean."}, "author_response": "Re: Point-by-_^X]c aTb_^]bT c^ aTeXTfTabs R^\\T]cb\nReviewer #1:\nThis paper uses a suite of climate model simulations to investigate the\nCretaceous greenhouse period. In particular, the authors study modelled mid-\nlatitude sea-surface temperature (SST) gradients in the Northern hemisphere\nand the respective roles of changes in continental configuration, atmospheric\ncarbon dioxide (CO2), and meridional heat transport in the wind-driven ocean\ngyres. The novel aspect of this work consists of an analysis of oceanic heat\ntransport relating to the changing aspect ratio of the paleo-Pacific ocean basin.\nWe thank the reviewer for her/his positive remarks on our manuscript and appreciate\nthe suggestion to analyze the datasets provided by Li et al. (2022) and Landwehrs et\nal. (2021). We summarize the findings from our examination of the two datasets below.\nThis is certainly an interesting paper which will be useful to the paleoclimate\ncommunity. My main recommendation would be not to focus on the Hadley\nmodel alone, but to include existing (and publicly available) ensembles of\npaleoclimate model simulations, in particular Li et al. (2022, Scientific Data)\ncovering the Phanerozoic. There is also a set of Mesozoic simulations published\nby Landwehrs et al. (2021) using an intermediate-complexity model to\ndisentangle the effects of paleogeography and CO2 which is relevant in the\ncontext of this work. Finally (and just a very minor issue), the end of the\nCretaceous is now usually set at 66 Ma rather than 65 Ma.\nThe high-resolution data provided in Li et al. (2022) does not contain the ocean\nvariables relevant to our analysis, in particular, the sea surface temperature (SST) and\nthe volumetric (mass) transport corresponding to the surface layer (stream function,\n{&) C^aT^eTa' X] cWT bX\\d[PcX^]b SXbRdbbTS X] BX Tc P[) %-+--&' Q^cW cWT Pc\\^b_WTaXR\nCO concentration and the paleogeography change simultaneously, which makes it\n2\ndifficult to disentangle the respective contribution of each of these components to the\nchanges in the climate.\nThe ensemble of simulations discussed in Landwehrs et al. (2021) shows a decrease\nin mid-latitudinal SST gradient during the Cretaceous period, which is in contrast with\nwhat the paleotemperature proxies suggest %Es8aXT] Tc P[)' -+,2&. Furthermore, the\nfast statistical-dynamical atmospheric model in CLIMBER-3z employed for these\nsimulations has a rather coarse resolution of 7.5° in latitude (compared with 2.5° in the\nHadCM3L model). Thus, it does not capture the subtle variations in the curl of the\nsurface wind-stress induced by paleogeographic changes and the corresponding\nchanges in the distance between wind-stress extrema (L ) on geological timescales,\ny\nwhich underpin our analysis.\nGiven these constraints, we are unable to extend our analysis to other ensembles of\npaleo-climate simulations at this stage. Nonetheless, we are grateful for the reviewerss\nsuggestion.\nWe have modified the end of the Cretaceous from 66 Ma to 65 Ma as per the\nrevieweasb bdVVTbcX^], thank you.\nReviewer #2 (Remarks to the Author):\nSummary: Gianchandani et al. use an ensemble of HadCM3L simulations to\ninvestigate and quantify the relative contributions of atmospheric CO2 and\ntectonic activity to variations in mid-latitude SSTs in the North Pacific Basin\nduring the Cretaceous. The simulations suggest that changes in CO2 and\npaleogeography had comparable impacts on mid-latitude SST gradients.\nGianchandani et al. also highlight the importance of ocean basin aspect ratio in\ndetermining gyral circulation, heat transport, and mid-latitude SST patterns in\nthe North Pacific Basin by developing a fundamental model of wind-driven\nocean gyres.\no NYRe RcV eYV _`eVh`ceYj cVdf]ed8\nThe main finding of this study is that changes in CO2 and paleogeography\ncontribute similarly to increasing mid-latitude SST gradients over the\nCretaceous. Although it is well established in the literature that changes in CO2\nand paleogeography contributed to SST variability over the Cretaceous, this\nfinding is noteworthy because the relative contributions of CO2 and tectonic\nactivity to SST variability are not established. Another noteworthy result is that\na decrease in gyral volume transport, and by extension ocean basin aspect ratio,\ncan account for ~80% variability in the meridional SST gradient in the mid-\nlatitude North Pacific over the Cretaceous.\no NZ]] eYV h`c\\ SV `W dZX_ZWZTR_TV e` eYV WZV]U R_U cV]ReVU WZV]Ud8 @`h U`Vd Ze\ncompare to the established literature? If the work is not original, please provide\nrelevant references.\nYes, this work is significant for the understanding of ocean circulation and\ntemperature during the Cretaceous. The study outlines a simple model of wind-\ndriven gyres based on the horizontal aspect ratio of the ocean basin that may\nbe applied to other deep-time intervals to better understand the contribution of\ntectonic activity to changes in ocean circulation and temperature.\no <`Vd eYV h`c\\ dfaa`ce eYV T`_T]fdZ`_d R_U T]RZ^d( `c Zd RUUZeZ`_R] VgZUV_TV\nneeded?\nOverall, yes. However, my main concern is that the range of CO2 captured in\nthese simulations does not capture the inferred paleo-CO2 range of the\nCretaceous based on Foster et al. (2017), which could have reached decreased\nto values of ~280 ppmv during the Late Cretaceous (Lines 116-120). The\njustification for the CO2 levels of the simulations needs to be explained,\nparticularly because the relationship between CO2 and meridional SST gradient\nsupported by the simulations (at 560-1120 ppmv) is being extended beyond this\nrange of CO2 (down to ~280 ppmv). This directly impacts the how the\nrelationship between CO2 and meridional SST gradient is calculated for the\nCretaceous, which is one of the main findings of the study.\no 9cV eYVcV R_j W]Rhd Z_ eYV UReR R_R]jdZd( Z_eVcacVeReZ`_( R_U T`_T]fdZ`_d8 <`\nthese prohibit publication or require revision?\nThe statement that atmospheric CO2 fluctuations and paleogeographic changes\nhave a comparable effect on mid-latitudinal SST gradient through the\nCretaceous may be too strong of a statement given that the study is restricted\nto the North Pacific Basin. The authors state that the fundamental model of wind-\ndriven ocean gyres does not apply to the Southern Hemisphere (Lines 193-197).\nSome revisions would help broaden the conclusions of the study by\ninvestigating the relative importance of CO2 and tectonic activity on SST\nvariability in other ocean basins during the Cretaceous and/or demonstrating\nhow the fundamental model of gyral circulation could also be applied to other\ntime intervals.\noAdeYV^VeY`U`]`Xjd`f_U8<`VdeYVh`c\\^VVeeYVViaVTeVUdeR_dards in your\nfield?\nOverall, yes. However, the methodology requires more justification and\nexplanation of CO2 and paleogeographic reconstructions used for the\nsimulations. Both CO2 level and paleogeographic reconstruction are subject to\nuncertainty that should be more explicitly described.\no Ad eYVcV V_`fXY UVeRZ] ac`gZUVU Z_eYV ^VeY`Ud W`c eYV h`c\\ e` SV cVac`UfTVU8\nYes.\nWe thank the reviewer for the overall positive comments and for providing us some\ncritical feedback which has helped us improve the manuscript. We provide a point-by-\npoint response to her/his comments below.\nMajor comments:\noLYVeZe]VcVZeVcReVd\\_`h]VUXe that is already well established about Cretaceous\nSSTs and does not highlight the novelty or impact of the study. This title could\ninstead highlight the relative importance of CO2/tectonic activity in the northern\nmid-latitudes\nWe have changed the manusRaX_csb cXc[T c^ pEffects of paleogeographic changes and\nCO variability on northern mid-latitudinal temperature gradients in the 9aTcPRT^dbq.\n2\no ERZ_+A_ec`UfTeZ`_6 LYV XRa Z_ \\_`h]VUXV ^`eZgReZ_X eYZd defUj R_U Sc`RUVc\nscientific context from previous literature needs to be explained in the\nintroduction. The first paragraph is already diving into methodology of the\nstudy, which has only been motivated by a few sentences. As a result, I am\nunsure if this study is motivated primarily by a desire to better understand SST\nvariability during the Cretaceous or develop a more general framework for\nunderstanding SST variability based on gyral circulation/basin geometry in\nwhich the Cretaceous is a case study. After the sentence that motivates looking\nat other environmental factors (beyond just CO2) (Lines 38-41), there should be\na brief discussion of the work that has investigated the impacts of CO2 and\npaleogeography on ocean temperature and circulation during the Cretaceous to\nexplicitly outline the novelty of the current study (e.g., Ladant et al, 2020 in\nClimate of the Past; Donnadieu et al., 2016 in Nature Comm).\nThe focus of our study is to better understand how the interplay between fluctuations\nin atmospheric CO P]S _P[T^VT^VaP_Wh SaXeT] RWP]VTb X] cWT ^RTP]sb RXaRd[PcX^]\n2\naffected the meridional SST gradients during the Cretaceous. Given that surface\nocean gyres are a prominent feature of the ocea]sb RXaRd[PcX^] PRa^bb VT^[^VXRP[\ntimescales, we develop a general framework to quantify the effect of paleogeography\ndriven changes in surface ocean circulation which is applicable to well constrained\nStommel like ocean basins which persist in climate model simulations of multiple\ngeologic periods including the Cretaceous.\nWe thank the reviewer for suggesting that we clarify this further. We have incorporated\nmultiple relevant references regarding how paleogeographic alterations during the\nCretaceous affTRc cWT R[X\\PcT P]S cWT ^RTP]sb RXaRd[PcX^] SdaX]V cWT _TaX^S (Lines 45-\n52). Additionally, we explicitly state the novelty of our study in Lines 64-70 and specify\nour aim in Lines 90-93. Furthermore, we have extended our analysis to include 7 ages\nfrom the Paleogene period as well to show the applicability of our model to geologic\ntimes other than the Cretaceous (see Fig. 2 and Lines 225-229).\no DZ_Vd 00-45: Why did you choose 560 and 1120 ppmv CO2? This should be\njustified in the methods section. Although 560 and 1120 ppmv capture most of\nthe inferred paleo-CO2 variability during the Cretaceous, CO2 levels could have\nbeen much lower than 560 ppmv during the Late Cretaceous, as you state on\nLines 116-120. Regarding Lines 116-120, why do you expect that the\nrelationship between meridional SST gradient and CO2 between two relatively high CO2\nlevels (560 and 1120 ppmv) would extend to lower CO2?\nWe elaborate on our choice of CO in Lines 96 o 105 of the revised manuscript.\nestimated atmospheric CO levels have on the temperature gradient (Lines 163 o 180).\n2\nvariation in the long-term trend in estimated atmospheric CO during the Cretaceous\n2\nperiod (Valdes et al., 2021) and this is discussed in the revised manuscript as well\n(Lines 96 o 102). We also acknowledge that there is an uncertainty in the estimated\natmospheric CO and by extension in the magnitude of variation in temperature\n2\ngradient we attribute to fluctuation in atmospheric CO during the Cretaceous (Lines\n2\n270 o 274). Note also that the apparently low values of CO as inferred solely from\n2\nat this time inferred from multiple SST proxies (no less than 20 °C global annual mean;\nScotese et al., 2021)\no DZ_Vd -33-179: Although the simple model explains a large proportion of the\nvariance (Fig. 2A), it shows a roughly linear trend of increasing SST gradient at\nboth CO2 concentrations, and thus does not capture a threshold-like behavior\ndue to paleogeographic changes between ~130-110 Mya, which is very\ninteresting. Before and after this interval, the changes in SST gradient are much\nsmaller. What changes/feedbacks associated with those paleogeographic\nchanges are creating such a dramatic increase in SST gradient during that\nspecific interval?\nWe agree that the steep increase in temperature gradients between 130 and 100 Ma\nis an interesting feature. In the revised version, we highlight this and acknowledge that\nit lies outside the range in which our best-fit curve is expected to vary. The steep\nincrease stems from the abrupt decrease in SST at 50°N (Fig. S5, Revised\nManuscript). This can potentially be related to vanishing of the sub-polar gyre from the\nBarremian to the Albian and the entrainment of high-latitude cold water by the mid-\nlatitudinal gyre (Fig. R1A-C). In the present-day Atlantic, there is clear demarcation\nbetween the mid-latitudinal and the subpolar gyres, which limits the mixing of water at\nthe poleward edge (Equatorward edge) of the mid-latitudinal (subpolar) gyre. The\nvanishing of the subpolar gyres from the Berremian to the Albian age could lead to\nrQ[daaX]Vs ^U cWXb ST\\PaRPcX^]) JWT ]^acWTa] QaP]RW ^U \\XS-latitudinal gyre could then\nentrain some of the cold high-latitudinal water, which can potentially explain the abrupt\ncooling from the Barremian to the Albian age.\nHowever, other factors including paleogeography driven polar amplification could also\ncontribute to the cooling effect. Examining the precise climatic variations/feedbacks\nassociated with paleogeographic changes that lead to this abrupt cooling is beyond\nthe scope of the present short manuscript. This is discussed in Lines 240 - 248.\nReviewer #3:\nGianchandani et al. present a study that uses general circulation modeling to\ninvestigate the driving mechanisms of changes in the Cretaceous meridional\ntemperature. The authors conclude that both paleogeographic changes and\ndecreasing pCO2 contributed equally to a decrease in the midlatitude\ntemperature gradient of about 6 °C from the Early to Late Cretaceous (i.e., from\nthe Valanginian to the Maastrichtian). This change in temperature gradient was\ncaused by a decrease in oceanic heat transport resulting from a weakening of\nthe wind-driven gyre circulation in the northern Paleo-Pacific in response to a\ndecreased latitudinal extent of the Pacific basin. As someone who has worked\na lot on Cretaceous paleoceanography and paleoclimatology, but is not an\nexpert in general circulation modeling, I find the manuscript to be well written,\nwell structured, and easy to follow. The science is sound and the interpretations\nare supported by the data. The conclusions are presented in a clear manner and\nare relevant to the broader geoscientific community. As such, I consider the\nmanuscript to be well suited for publication in Nature Communications and have\nonly a few minor comments.\nWe thank the reviewer for the encouraging remarks and for providing us critical\nfeedback which has helped us improve the manuscript. We provide a point-by-point\nresponse below.\n1.) Paleogeography: I recommend including some background information on\nthe paleogeography implemented in the model in the Methods section. Because\npaleogeographic restorations of the Paleo-Pacific are complicated by extensive\nsubduction of oceanic crust since the Cretaceous, it would be particularly\nhelpful to include a brief comparison between the implemented paleogeography\nand other published plate tectonic models to show that the general trends in L\nX\nand L are consistent across the various available models.\nY\n?] PRR^aSP]RT fXcW cWT aTeXTfTasb bdVVTbcX^], we have further elaborated on the\npaleogeography implemented in the HadCM3L model in the Data Availability section\nof the revised manuscript (Lines 441-449). We agree that a comparison between L\nx\nand L as obtained from multiple models will strengthen our sermon and we attempt\ny\nto do the same by examining the publicly available data provided by Li et al. (2022)\nand Landwehrs et al. (2021). However, we are unable to present a comparison at this\nstage for the following reasons:\na) In the simulations discussed in Li et al. (2022), both the atmospheric CO\n2\nconcentration and the paleogeography change simultaneously which makes it\ndifficult to disentangle the respective contribution of each of these components\nto the changes in the curl of the surface wind-stress and the corresponding\nchanges in the distance between wind-stress extrema (L ).\ny\nb) The atmospheric component of the climate model (CLIMBER-3z) employed for\nthe simulations discussed in Landwehrs et al. (2021) has a rather coarse\nresolution of 7.5° in latitude. Thus, it does not resolve the paleogeography\ndriven changes in the meridional extent of the Hadley cell or the subtle\nvariations in the surface winds which is critical for calculating the changes in\nthe gyral circulation and therefore the oceanic heat transport.\nWe note that our approach for quantifying the effect of aspect ratio Ly on the intensity\n( )\nLx\nof gyral circulation was applied in a previous study to the present-day ocean gyres in\ndifferent ocean basins (Gianchandani et al., 2021). We showed that the small\nvolumetric (mass) transport of the East Australian Current compared to other Western\nBoundary Currents can be attributed to the geometry of the South Pacific Basin, i.e.,\n7dbcaP[XPsb;PbcTa]coastline is not long enough to support a strong WBC in the zonally\nwide South Pacific Ocean.\n2.) Role of sea-level change: Do the changes in land-sea distribution shown in\nFig. S1 and Fig. S6 reflect solely plate tectonic processes (i.e., changes in the\ndistribution of oceanic and continental crust) or also global sea-level changes\n(i.e., flooding of continents)? If both processes are involved, the authors may\nconsider replacing \"tectonic activity\" with \"paleogeographic changes\" in the\ntitle of the manuscript.\nThe change in land-sea distribution does include a sea level component and in general\nour chosen paleogeographic represent the high-stands in sea level. Thus, we thank\nthe reviewer for pointing this out accept the suggestion to replace tectonic activity with\npaleogeographic changes in the title and. The modified title now is:\npEffects of paleogeographic changes and CO variability on northern mid-latitudinal\n2\ntemperature gradients in the Cretaceous)q\n3.) Paleo-SST data: I suggest to include a graphical comparison of proxy-based\nSST estimates and model-derived latitudinal temperature gradients, at least for\nthe Valanginian and Maastrichtian, to demonstrate that the model is capable of\ncapturing the general patterns evident in the proxy data.\nWe do not compare the model-derived temperature gradients with proxy data since\nthe atmospheric CO and continental arrangement do not coevolve in the current set\n2\nof simulations. These simulations are not designed to represent actual changes in\nclimate, but instead are idealized with constant CO2 concentrations through time.\n>^fTeTa' fT PVaTT fXcW cWT aTeXTfTasb bdVVTbcX^] cWPc Xc Xb X\\_^acP]c c^ QT]RW\\PaZ\nmodel data against geochemical proxies and plan to address it in a future work using\ndata from multiple climate models in which atmospheric CO and geography are varied\n2\nsimultaneously (Lines 279-286).\n4.) Figure 1: The labeling of the contour lines in Figures 1A and 1B is barely\nvisible.\nThe suggestion was incorporated and the figure was modified accordingly.\n1*' >ZXfcV /6 ;`ccVTe pERRdeccZTYeZR_q Z_ eYV TRaeZ`_ `W eYV >ZXfcV*\nCorrected.\n2*' DZ_V 516 H]VRdV Z_UZTReV hYVeYVc xKKL hVcV RgVcRXVU `gVc eYV HRTZWZT SRdZ_\nor globally.\nSpecified.\n7.) Line 192: Please provide add a short sentence or a reference to explain what\ntectonic processes drove the changes in the northern hemisphere\naR]V`XV`XcRaYj(daVTZWZTR]]jeYVT]`dfcV`WeYVpac`e`-:VcZ_XKecRZeqT`__VTeZ_X\nthe northernmost Pacific and the Arctic Ocean.\nRelevant references added.\nReferences:\nGianchandani, K., Gildor, H., & Paldor, N. (2021). On the role of domain aspect ratio\nin the westward intensification of wind-driven surface ocean circulation. Ocean\nScience, 17(1), 351o363. https://doi.org/10.5194/os-17-351-2021\nLandwehrs, J., Feulner, G., Petri, S., Sames, B., & Wagreich, M. (2021). Investigating\nMesozoic Climate Trends and Sensitivities With a Large Ensemble of Climate\nModel Simulations. Paleoceanography and Paleoclimatology, 36(6),\ne2020PA004134. https://doi.org/https://doi.org/10.1029/2020PA004134\nLi, X., Hu, Y., Guo, J., Lan, J., Lin, Q., Bao, X., Yuan, S., Wei, M., Li, Z., Man, K., Yin,\nZ., Han, J., Zhang, J., Zhu, C., Zhao, Z., Liu, Y., Yang, J., & Nie, J. (2022). A high-\nresolution climate simulation dataset for the past 540 million years. Scientific\nData, 9(1), 371. https://doi.org/10.1038/s41597-022-01490-4\nEs8aXT]' 9) B)' H^QX]b^]' I) 7)' FP]R^bc' H) :)' IX]]X]VWT :P\bck' @) I)' IRW^dcT]',\nS., Lunt, D. J., Alsenz, H., Bornemann, A., Bottini, C., Brassell, S. C., Farnsworth,\nA., Forster, A., Huber, B. T., Inglis, G. N., Jenkyns, H. C., Linnert, C., Littler, K.,\nCPaZfXRZ' F)' CR7]T]P' 7)' n La^QT[' D) ;) %-+,2&) 9aTcPRT^db bTP-surface\ntemperature evolution: Constraints from TEX86 and planktonic foraminiferal\noxygen isotopes. Earth-Science Reviews, 172(July), 224o247.\nhttps://doi.org/10.1016/j.earscirev.2017.07.012\nScotese, C. R., Song, H., Mills, B. J. W., & van der Meer, D. G. (2021). Phanerozoic\npaleocT\\_TaPcdaTb5JWTTPacWsbRWP]VX]VR[X\\PcTSdaX]VcWT[Pbc0/+\\X[[X^]hTPab)\nEarth-Science Reviews, 215, 103503.\nhttps://doi.org/10.1016/J.EARSCIREV.2021.103503\nValdes, P. J., Scotese, C. R., & Lunt, D. J. (2021). Deep ocean temperatures through\ntime. Climate of the Past, 17(4), 1483o1506. https://doi.org/10.5194/cp-17-1483-\nAfter reading the revised manuscript as well as the point-by-point response to the reviewers'\ncomments, I have only a few very minor comments:\nLines 17, 38, 227 (twice): In contrast to the statement in the authors' response, the age for the\nend of the Cretaceous has in fact not been corrected from 65 Ma to 66 Ma during revision... ;)\nCaption Figure 3: Please correct \"~65 Ma\" to \"~68 Ma\".\nLine 138: I am aware that this change is due to a suggestion of Reviewer 3, but \"global zonally\naveraged SST\" sounds weird. Maybe rather specify that the average was computed \"over all\nlongitudes\" or similar?\nReviewer #2 (Remarks to the Author):\nI appreciate the work that the Gianchandani et al. have accomplished to better motivate and\nexpand the impact of the study. Specifically, the extension of the gyral circulation model from the\nCretaceous to the Paleogene is a great addition. However, much of the manuscript (title, abstract,\nintroduction, methods, some results, and discussion) have not all been equally updated to reflect\nthe inclusion of the Paleogene and how this alters/broadens the impact and conclusions of the\nstudy. I have included more detailed suggestions in the comments below.\nMajor Comments\n• Introduction: You have provided the background to explain how your study advances our\nunderstanding of Cretaceous SSTs and ocean circulation, but you have expanded the scope to\ninclude the Paleogene which is not mentioned in the introduction. Additionally, you have not fully\nincorporated the paleoclimate and gyre circulation components of the introduction. One suggestion\nto incorporate Cretaceous-Paleogene climate with your gyre model in the introduction is to\nintroduce (1) description of meridional SST patterns in the Cretaceous-Paleogene, (2) problem:\nquantifying the impact of paleogeography on these SST patterns is important but complicated as\nprevious work has shown, (3) gap: surface gyres are an important driver of SST patterns and have\nnot been directly linked to paleogeographic changes in previous work, (4) solution: Stommel’s\nsimple model of gyral circulation can be used to quantify the influence of paleogeography on past\nSST patterns, and thus better understand the relative importance of other climate forcings like\nCO2. I would also introduce all of these general ideas before including any details about your\nsimulations (lines 53-56).\n• The Results section, including “Wind-driven gyral circulation in the Cretaceous” should include a\njustification of why Stommel’s model also works for the Paleogene\n• The Results section, “Variability in meridional SST gradients and its dependence on atmospheric\nCO2 and paleogeography”, does not touch on the differences in long-term trends of meridional\nSST gradients during the Paleogene compared to the Cretaceous and this is a new interesting\ncomponent of the study.\n• Line 188: I think this should be modified to “compared to long-term trends in pCO2”. I think you\nmust be careful with these statements because you are not capturing the entire inferred range of\nCO2 during this Cretaceous-Paleogene (you are capturing the range of the smoothed long-term\ntrend). You may even add a sentence that states that CO2 may have a larger impact if shorter-\nterm fluctuations in atmospheric CO2 and/or if low CO2 estimates from some proxies\n(line 102-103) are accounted for.\n• Lines 253-258: How does this logic extend from the Cretaceous through the Paleogene? My\nunderstanding is that Figure S7 demonstrates that land area fraction continues to increase through\nthe Paleocene and Eocene, but in Figure S6 there is not a concurrent decrease in Ly with a\ndecrease geographical extent of the ocean basin during this time. The relationship between Ly and\nmaximal volume transport with SST gradient seems to hold from the Cretaceous-Paleogene, but\nthis does not track with the geographical extent of the ocean basin during the Paleogene. If I\nunderstand that correctly, is there something other than paleogeography that is driving an\nincrease in Ly during the Paleogene?\n• Lines 435-454: I think much of these simulation details should be moved out of the Data\nAvailability statement into the the first paragraph of the Results “Numerical Simulations” to outline\nbasic structure/motivation of simulations used and details should be placed in a Methods section at\nthe end. I’m thinking of an organization like Sauermilch et al. (2021) in Nature Comms\nhttps://doi.org/10.1038/s41467-021-26658-1\nMinor Comments\n• Line 189: I would add a topic sentence to this paragraph saying that differences in long-term\nSST gradients between the simulations and paleotemperature reconstructions may be attributed to\nthe limited spatial distribution of available proxies (the topic of this paragraph is unclear from the\nfirst sentence so the transition from the last paragraph to this one is difficult to understand)\n• Lines 234-239: Split this into two sentences at “which”\nReviewer #3 (Remarks to the Author):\nThe authors have done a thorough job of addressing my concerns. I especially welcome the\nexpanded discussion on the model's paleogeography and recommend the publication of the\nmanuscript.\nRe: Point-by-dc]bh fYgdcbgY hc fYj]YkYfgs WcaaYbhg\nO\\Y fYj]YkYfgs WcaaYbhg UfY \\][\\`][\\hYX ]b blue and h\\Y Uih\\cfgs response is in\nblack.\nReviewer #1 (Remarks to the Author):\nAfter reading the revised manuscript as well as the point-by-point response to the\nreviewers' comments, I have only a few very minor comments:\n1. Lines 17, 38, 227 (twice): In contrast to the statement in the authors' response,\nthe age for the end of the Cretaceous has in fact not been corrected from 65\nMa to 66 Ma during revision... ;)\nWe thank the reviewer for pointing this oversight on our part, the end of the\nCretaceous has now been corrected to 66 Ma throughout the manuscript.\n2. Caption Figure 3: Please correct \"~65 Ma\" to \"~68 Ma\".\nDone.\n3. Line 138: I am aware that this change is due to a suggestion of Reviewer 3, but\n\"global zonally averaged SST\" sounds weird. Maybe rather specify that the\naverage was computed \"over all longitudes\" or similar?\nDone. See line 145-146 in the revised version.\nReviewer #2 (Remarks to the Author):\nI appreciate the work that the Gianchandani et al. have accomplished to better\nmotivate and expand the impact of the study. Specifically, the extension of the gyral\ncirculation model from the Cretaceous to the Paleogene is a great addition. However,\nmuch of the manuscript (title, abstract, introduction, methods, some results, and\ndiscussion) have not all been equally updated to reflect the inclusion of the Paleogene\nand how this alters/broadens the impact and conclusions of the study. I have included\nmore detailed suggestions in the comments below.\nWe thank the reviewer for the kind remarks about the work we have put in revising the\nmanuscript. We have taken all the comments that s/he has made into consideration\nwhile preparing the revised version of the manuscript. These comments have definitely\nhelped us broadening the impact of our research. Please note that while we discuss\nthe novel findings regarding the Paleogene in the qMesultsr section, we refrain from\nmodifying the title and the abstract for reasons discussed in the response to major\ncomment # 3.\nMajor Comments\n1. Introduction: You have provided the background to explain how your study\nadvances our understanding of Cretaceous SSTs and ocean circulation, but\nyou have expanded the scope to include the Paleogene which is not mentioned\nin the introduction. Additionally, you have not fully incorporated the paleoclimate\nand gyre circulation components of the introduction. One suggestion to\nincorporate Cretaceous-Paleogene climate with your gyre model in the\nintroduction is to introduce (1) description of meridional SST patterns in the\nCretaceous-Paleogene, (2) problem: quantifying the impact of paleogeography\non these SST patterns is important but complicated as previous work has\nshown, (3) gap: surface gyres are an important driver of SST patterns and have\nnot been directly linked to paleogeographic changes in previous work, (4)\ngc`ih]cb8 NhcaaY`sg g]ad`Y acXY` cZ [mfU` W]fWi`Uh]cb WUb VY igYX hc eiUbh]Zm\nthe influence of paleogeography on past SST patterns, and thus better\nunderstand the relative importance of other climate forcings like CO2. I would\nalso introduce all of these general ideas before including any details about your\nsimulations (lines 53-56).\nWe restructured the introduction along the four pointers highlighted by the\nreviewer as can be seen in the Lines marked by blue/red in the track changes\nversion (Lines 33-37, 49-56, 59-61, 77-88, 90-91). The modifications also\ninclude a discussion the Paleogene SST gradients and the two new references\nthat underscores the effect of paleogeographic changes on the gradients.\n2. O\\Y MYgi`hggYWh]cb* ]bW`iX]b[qQ]bX-Xf]jYb[mfU`W]fWi`Uh]cb]bh\\Y>fYhUWYcigr\ng\\ci`X ]bW`iXY U ^igh]Z]WUh]cb cZ k\\m NhcaaY`sg acXY` U`gc kcf_g Zcf h\\Y\nPaleogene\nNhcaaY`sg acXY` kcf_g Zcf Vch\\ h\\Y >fYhUWYcig UbX h\\Y KU`Yc[YbY VYWUigY\ndespite considerable tectonic activity the zonal dimensions of the ocean basin\nremained ~13000 km for the span of 110 million years (the Cretaceous-\nPaleocene-Eocene, PCE, epochs). We underscore this in the revised\nmanuscript, see Lines 119-121. Furthermore, this point is reiterated in the\nqMYgi`hgr gYWh]cb k\\YfY kY ghUhY h\\Uh >60% of the oceanic volumetric transport\nwas in the large paleo pacific during the PCE. See Lines 244-245.\n3. O\\Y MYgi`hg gYWh]cb* qPUf]UV]`]hm ]b aYf]X]cbU` NNO [fUX]Ybhg\nUbX ]hg XYdYbXYbWYcb Uhacgd\\Yf]W>J0 UbX dU`Yc[Yc[fUd\\mr* XcYgbch hciW\\ cb h\\Y\ndifferences in long-term trends of meridional SST gradients during the\nPaleogene compared to the Cretaceous and this is a new interesting\ncomponent of the study.\nWe expanded the subsection entitled qPUf]UV]`]hm ]b aYf]X]cbU` NNO [fUX]Ybhg\nUbX ]hg XYdYbXYbWY cb Uhacgd\\Yf]W >J0 UbX dU`Yc[Yc[fUd\\mr, which now\nincludes a discussion of the Paleogene. See Lines 136-137, 139-140, 149-151,\n161-162, 172-176, 189-191, 200-202. However, we still keep the discussion in\nthe paper focused around the Cretaceous in particular since zNNOy variation\nduring the Paleogene is of the order of the standard deviation around the\nensemble averaged global mean SST.\n4. G]bY /668 D h\\]b_ h\\]g g\\ci`X VY acX]Z]YX hc qWcadUfYX hc `cb[-term trends in\nd>J0r, D h\\]b_ mci aigh VY WUfYZi` k]h\\ h\\YgY ghUhYaYbhg VYWUigY mci UfY bch\ncapturing the entire inferred range of CO2 during this Cretaceous-Paleogene\n(you are capturing the range of the smoothed long-term trend). You may even\nadd a sentence that states that CO2 may have a larger impact if shorter-term\nfluctuations in atmospheric CO2 and/or if low CO2 estimates from some proxies\n(line 102-103) are accounted for.\nWe thank the reviewer for highlighting this subtle issue. We have now modified\nthe text to further emphasize the sensitive dependence of our results and\ninferences on the choice of atmospheric CO concentration. See Lines 193-196.\n2\n5. Lines 253-258: How does this logic extend from the Cretaceous through the\nPaleogene? My understanding is that Figure S7 demonstrates that land area\nfraction continues to increase through the Paleocene and Eocene, but in Figure\nS6 there is not a concurrent decrease in Ly with a decrease geographical extent\nof the ocean basin during this time. The relationship between Ly and\nmaximal volume transport with SST gradient seems to hold from the Cretaceous-\nPaleogene, but this does not track with the geographical extent of the ocean\nbasin during the Paleogene. If I understand that correctly, is there something\nother than paleogeography that is driving an increase in Ly during the\nPaleogene?\nThe reviewer is correct in pointing out that factors other than the Equator-to-\npole extent of the ocean basin can affect L on geological timescales. A\ny\nprevious paper by Adam et al., 2022 discusses the correlations between L and\ny\nseveral other climatic features such as the meridional extent/intensity of the\nHadley cell, the intensity of the Walker circulation, zonal asymmetries in the\nSST fields. Changes in one or more of these features can potentially be a result\nof the opening of Atlantic in the Paleogene, which can in turn affect L .\nHowever,\ny\na causal link between changes in climate features and the corresponding effect\non L has not yet been established and should be explored in a future work. We\ny\nthank the reviewer for bringing this issue up and we acknowledge this in Lines\n272-277.\n6. Lines 435-454: I think much of these simulation details should be moved out of\nthe Data Availability statement into the the first paragraph of the Results\nqIiaYf]WU` N]ai`Uh]cbgr hc cih`]bY VUg]W ghfiWhifY-ach]jUh]cb cZ g]ai`Uh]cbg\nused and details should VY d`UWYX ]b U HYh\\cXg gYWh]cb Uh h\\YYbX, Dsa h\\]b_]b[\nof an organization like Sauermilch et al. (2021) in Nature Comms\nhttps://doi.org/10.1038/s41467-021-26658-1\nThe details regarding the model were moved from Data Availability statement\nhc h\\Y qIiaYf]WU` N]ai`Uh]cbgr subsection in the Results section and the\nMethods section.\nMinor Comments\n1. Line 189: I would add a topic sentence to this paragraph saying that differences\nin long-term SST gradients between the simulations and paleotemperature\nreconstructions may be attributed to the limited spatial distribution of available\nproxies (the topic of this paragraph is unclear from the first sentence so the\ntransition from the last paragraph to this one is difficult to understand)\nAn introductory sentence to bridge the paragraphs was added. See Lines 203-\n205\n2. Lines 234-0178 Nd`]h h\\]g ]bhc hkc gYbhYbWYg Uh qk\\]W\r\nDone.\nReviewer #3 (Remarks to the Author):\nThe authors have done a thorough job of addressing my concerns. I especially\nwelcome the expanded discussion on the model's paleogeography and recommend\nthe publication of the manuscript.\nWe thank the reviewer for the gracious comments about the manuscript and are glad\nto hear that s/he has recommended the manuscript for publication in Nature\nCommunications.\nReferences\nAdam, O., Farnsworth, A., & Lunt, D. J. (2022). Modality of the Tropical Rain Belt Across\nModels and Simulated Climates. Journal of Climate, 1p35. https://doi.org/10.1175/JCLI-\nD-22-0521.1"}]

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