A better-ventilated ocean triggered by late cretaceous changes in continental configuration

ABSTRACT Oceanic anoxic events (OAEs) are large-scale events of oxygen depletion in the deep ocean that happened during pre-Cenozoic periods of extreme warmth. Here, to assess the role of

major continental configuration changes occurring during the Late Cretaceous on oceanic circulation modes, which in turn influence the oxygenation level of the deep ocean, we use a coupled

ocean atmosphere climate model. We simulate ocean dynamics during two different time slices and compare these with existing neodymium isotope data (_ɛ_Nd). Although deep-water production in

the North Pacific is continuous, the simulations at 94 and 71 Ma show a shift in southern deep-water production sites from South Pacific to South Atlantic and Indian Ocean locations. Our

modelling results support the hypothesis that an intensification of southern Atlantic deep-water production and a reversal of deep-water fluxes through the Caribbean Seaway were the main

causes of the decrease in _ɛ_Nd values recorded in the Atlantic and Indian deep waters during the Late Cretaceous. SIMILAR CONTENT BEING VIEWED BY OTHERS VERTICAL DECOUPLING IN LATE

ORDOVICIAN ANOXIA DUE TO REORGANIZATION OF OCEAN CIRCULATION Article 01 November 2021 EXPANDED SUBSURFACE OCEAN ANOXIA IN THE ATLANTIC DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM Article

Open access 20 October 2024 SOUTHERN OCEAN GLACIAL CONDITIONS AND THEIR INFLUENCE ON DEGLACIAL EVENTS Article 09 June 2023 INTRODUCTION In the context of recent warming, modern ocean

de-oxygenation that occurs not only on continental margins but also in the tropical oceans worldwide1 resembles the model initially invoked for the onset of oceanic anoxic events (OAEs)2.

Recent studies point to a major role of increased nutrient inputs as a trigger for OAEs3,4,5. Nevertherless, ocean circulation, through its impact on oxygen concentration in deep waters, may

have affected the thresholds required to trigger an OAE. It has been suggested that Late Cretaceous changes in climate and continental configuration6,7,8, namely the widening of the

Atlantic Ocean and the deepening of the Central Atlantic (CA) gateway9, could have induced major changes in oceanic circulation that may have had an impact on the general oxygenation state

of the oceanic basins and contributed to the conclusion of these large-scale anoxic events in the deep ocean10,11. Nevertheless, no consensus exists on oceanic circulation modes and their

possible evolution during the Cretaceous, despite recent improvements of the spatial and temporal coverage of neodymium isotopic data (ɛNd), a proxy for oceanic circulation11,12,13,14,15.

For instance, a decrease in bottom water ɛNd values during the Late Cretaceous in the Atlantic and Southern Oceans has been interpreted either as reflecting the initiation or intensification

of deep-water production in the northern Atlantic15 or in the South Atlantic (SA) and in the Indian Ocean10,11,13. Additional sources for deep waters have been suggested in the North or

South Pacific, or even at low latitudes, based on ɛNd or oxygen isotope data16,17,18. General circulation models have also been used to study oceanic circulation during the

Cretaceous7,19,20. To our knowledge, no modelling studies have reconstructed the evolution of ocean dynamics resulting from the widening of the South and CA Ocean occurring between the

Cenomanian and the Maastrichtian. Published simulations either focus on a specific time period20,21 or are devoted to the impact of the CA gateway opening between the Albian and the

Cenomanian22. Here we use the fast ocean atmosphere model (FOAM), to explore the evolution of oceanic circulation occurring during the Late Cretaceous (see Methods). Our simulations

highlight an evolution from a sluggish circulation in the South and CA using a Cenomanian/Turonian land–sea mask towards a much more active circulation in these basins with an early

Maastrichtian land-sea mask. RESULTS CHANGES IN OCEANIC CIRCULATION AT 1,120 P.P.M The 94 Ma simulation at a CO2 concentration of 1,120 p.p.m. (four times the pre-industrial atmospheric

level) displays a bipolar oceanic circulation characterized by large areas of deep-water formation located both in the north and in the south of the Pacific Ocean (Fig. 1). The production of

deep waters along the northwest boundary of the Pacific Ocean is common to many numerical Cretaceous simulations4,20,22 but not all23. The sinking of water masses occurs over the cyclonic

subpolar gyres and results from the winter cooling of warm and salty surface waters located within this large region20. The use of the early Maastrichtian land–sea mask for the same

atmospheric CO2 level induces substantial changes in the location of deep-water production in the model. The north Pacific area of deep-water formation is reduced to the northwestern Pacific

area only and sinking in the south Pacific area completely disappears. Conversely, the Atlantic and the Indian sectors of the Austral Ocean become larger contributors of deep waters and a

small area of intermediate-water formation appears along the northeast American margin in the North Atlantic (Fig. 1). SENSITIVITY TO ATMOSPHERIC CO2 LEVELS Changes in atmospheric CO2 level

from 8 to 2 pre-industrial atmospheric level have a small effect on the distribution of deep-water formation areas in our study (Supplementary Figs 1 and 2). The main features of oceanic

circulation modes remain quite similar, with a disappearance of deep-water convection in the southern Pacific and a larger area of deep-water formation appearing in the southern Atlantic

when using early Maastrichtian palaeogeography. Nevertheless, the sensitivity of overturning to CO2 level remains an open debate, as some models24,25 found that ocean dynamics vary with CO2

while others did not4,26. We emphasize that all simulations presented here represent an equilibrated climate. The ocean can go through a transient stratified state resulting from a faster

warming of the oceanic upper layers when confronted with a CO2 increase imposed on a short time scale (a few hundred to a few thousand years). Winter convection at high latitudes would then

stop, owing to the decrease in vertical density gradient, and the classical thermohaline circulation may stop as well, but only transiently. We thus recognize that there is room for

improvement in this area of research and await a modelling inter-comparison project for warm climates such as the one that has been initiated on the warm Eocene climate27; however, such a

comparison is beyond the scope of this study. The results of our model suggest at this point that the changes in ocean dynamics simulated between 94 and 71 Ma are driven by changes in

palaeogeography rather than by the cooling recorded between the Cenomanian and the Maastrichtian. As only minor changes occur with varying CO2 levels, only the simulations at 1,120 p.p.m.

will be further described and compared for each palaeogeography in the remainder of the study. Our main aim in this study is to describe and understand the changes in ocean dynamics that

occur. It is however interesting to note that the atmospheric CO2 concentration range used in our simulations produces sea-surface and deep-water temperatures that are comparable to oceanic

latitudinal thermal gradients reconstructed from palaeoceanographic data for the Cenomanian/Turonian boundary and for the early Maastrichtian (Supplementary Fig. 7). PATHWAYS OF WATER MASSES

BETWEEN THE OCEANIC BASINS To go a step beyond this first-order description of ocean dynamics, we have computed zonal and/or meridional water transport across several key sections of the

ocean basins (Figs 2 and 3). In most previous modelling studies, ocean dynamics have been described through a global overturning function. However, a global overturning function may be

dominated by the largest oceanic basin, that is, the Pacific Ocean, and does not allow for the identification of ocean dynamics occurring in the still relatively narrow Atlantic Ocean, in

the Tethys or in the Indian Ocean. Our aim here is to provide a better three-dimensional picture of water flow within and between the different oceanic basins. In the CTRL94Ma run, 7 Sv are

transported from the South Pacific to the SA across the D section (Fig. 3c and Table 1). The circulation simulated within the SA Ocean appears quite sluggish at depth, with a very limited

northward flow of 4.2 Sv throughout the SA section between 1,200 and 2,750 m (Fig. 3c and Table 1). Conversely, across the East India section, water flow is directed northward down to a

depth of 3,000 m and transports 25 Sv to the North (Fig. 3c and Table 1). In the CTRL71Ma run, deep-water production intensifies in the southern Atlantic, extends eastward to the Indian

sector and does not occur in the southern Pacific sector any more (Figs 1 and 3). Figure 3d shows a drastic intensification of the northern flow across the SA section for depth below 800 m.

These northward-flowing deep waters reach the North Atlantic, as testified by the large flux of water across the CA section (Fig. 3b). Surface-to-intermediate waters across the Central to

the Southern Atlantic are also flowing more intensively from the North to the South. In the CTRL94Ma run and in the North Atlantic, the vertical flux profiles computed for the Caribbean

Seaway section, CA section and Mediterranean (Med) section indicate that the surface and intermediate waters down to about 1,000 m flow from the Tethys to the North Atlantic, and then

southward across the CA Gateway and westward into the equatorial Pacific via the Caribbean Seaway. Across the Caribbean Seaway below 1,000 m, waters flow from the Pacific to the Atlantic as

shown by the eastward water flux from 1,000 to ∼2,500 m (Fig. 3). The existence of an estuarine circulation pattern, with the surface layers flowing from the Tethys to the Pacific and the

deeper layers flowing in the opposite direction, has been noticed several times in previous modelling studies4,19,21. In the CTRL71Ma runs, the model simulates an increase in

circum-equatorial transport across the Caribbean Seaway and the Med gateway. Deep waters are now flowing from the Atlantic to the Pacific at all depths across the Caribbean Seaway, which

marks the end of estuarine circulation in the Atlantic. COMPARISON WITH AVAILABLE _Ɛ_ND Nd in seawater ultimately derives from continents and the rocks eroded around an area of deep-water

production imprint the surface waters with a distinct isotopic composition that depends on the age and lithology of the rock28. This signature is then exported to depth as the water sinks.

The residence time of Nd in the ocean is shorter than the oceanic mixing time, which prevents complete homogenization but is long enough for Nd to be transported by deep-water masses along

their pathways28. The Nd isotope composition of deep waters has thus been used to track oceanic circulation patterns in both modern and ancient oceans10,28,29,30. Available data for the Late

Cretaceous highlight a major decrease in deep-water _ɛ_Nd recorded both in the North and South Atlantic and in the Indian Ocean10,14,31, with more unradiogenic (lower) _ɛ_Nd values recorded

during the Maastrichtian (on average in the −8.5 to −11 _ɛ_-units range) compared with the Turonian (on average in the −5 to −8.5 _ɛ_-units; Fig. 2). This decrease has been interpreted to

reflect initiation of deep-water production in an area receiving unradiogenic Nd inputs from the nearby continents, with the northern Atlantic15, or the Atlantic or Indian sector of the

Southern Ocean proposed as possible sources10,11. Indeed at present, _ɛ_Nd values as low as −14 to −26 _ɛ_-units recorded in Baffin Bay and Labrador Sea waters reflect the unradiogenic Nd

continental supply from nearby Archean terranes32,33,34 that were already present during the Cretaceous. In the southern Ocean, unradiogenic detrital inputs occur at present (Fig. 2,

Supplementary Fig. 5 and Supplementary Data 1). The presence of similar unradiogenic inputs during the Late Cretaceous is supported by Campanian–Maastrichtian _ɛ_Nd values of detrital

material of around −10 _ɛ_-units at ocean drilling program (ODP) site 690 (ref. 12). Because of these unradiogenic detrital inputs from the Antarctic continent, the more radiogenic _ɛ_Nd

values (higher) recorded in the SA and Indian deep waters during the mid-Cretaceous have been interpreted10 to reflect at that time additional inputs of radiogenic Nd from abundant volcanic

dust in the context of a sluggish circulation within a restricted basin. It has also been suggested that the subsidence of Rio Grande Rise and other large igneous provinces (for example,

Kerguelen or the Madagascar Plateau) during the Late Cretaceous could have diminished such inputs of radiogenic Nd to the southern Atlantic and Indian surface waters, contributing to

decrease the _ɛ_Nd values of the deep waters sourced in this region31. The oceanic circulation modelled here is in agreement with existing _ɛ_Nd and part of the proposed scenarios. In the

CTRL94Ma runs, the model simulates deep-water formation in the northern and southern Pacific. In the modern ocean, the southern Pacific receives inputs of quite radiogenic Nd eroded from the

nearby Antarctic continent (Fig. 2, Supplementary Fig. 5 and Supplementary Data 1). 40Ar/39Ar ages of detrital hornblende grains from West Antarctica35 support the presence of intrusive and

volcanic rocks during the Cretaceous (and as early as the Jurassic) that probably provided radiogenic material to the nearby surface waters36, imprinting the deep water produced there with

a radiogenic signature. Deep water produced in the North Pacific region should also have a radiogenic composition, because weathering of radiogenic young circum-Pacific volcanic arcs linked

to the subduction of the Kula and Farallon plates was already active during the mid-Cretaceous37. This is supported by the high _ɛ_Nd values of neritic seawater inferred from Late Cretaceous

fish remains recovered from the northwestern Pacific, (typically in the −3 to +1 _ɛ_-units range)38. In the North Atlantic, deep waters below 1,000 m are conveyed in our simulation from the

equatorial eastern Pacific through the Caribbean Seaway and result from the mixing of radiogenic deep waters formed both in the North and in the South Pacific. Therefore, the ocean dynamics

simulated by the model are expected to result in quite radiogenic _ɛ_Nd values of deep waters in the North Atlantic, in agreement with existing _ɛ_Nd (Fig. 2 and Supplementary Data 2). In

the CTRL94Ma run, the deep waters bathing the southern Atlantic and Indian Oceans originate in the area located near the modern Weddell Sea, a region that most probably received unradiogenic

continental supply from Antarctica as discussed above (Fig. 2). It has been suggested10,31 that the southern Atlantic may have received during the mid-Cretaceous additional inputs of

radiogenic Nd from subaerially exposed continental volcanic provinces, exposure of submarine Large Igneous Provinces and hot spots, and from associated volcanic activity. The sluggish

circulation simulated in this work in the SA in the CTRL94Ma run (Fig. 3 and Table 1) would have favoured seawater-particle exchange processes with such radiogenic volcanic particles10,

resulting in a quite radiogenic composition of SA and Indian deep waters as depicted in the available Nd data set (Fig. 2). In the CTRL71Ma run, convection in the southern Pacific Ocean

completely disappears and is reduced in the North Pacific to the northwest area, whereas convection in the SA intensifies and extends eastward to Australia (Figs 2 and 3). The development of

sites of deep-water formation near areas of probable unradiogenic continental supply in the southern Atlantic is expected to result in a relatively unradiogenic composition of SA and Indian

Ocean waters. This is in agreement with available _ɛ_Nd that display quite negative _ɛ_Nd values of deep waters during the latest Cretaceous in the southern Atlantic and Indian Oceans, in

the range of about −9.5 to −10.5 _ɛ_-units at ODP site 690 for the Late Campanian–Maastrichtian interval12 and of −8.4 to −11 _ɛ_-units on average for the Maastrichtian at several Indian

Ocean ODP sites10,12,13 (Fig. 2). Following the simulated circulation patterns, this unradiogenic deep-water composition would then be exported to the North Atlantic through the deeper CA

gateway. In addition, with the inversion of the deep-water flux through the Caribbean Seaway in the model, the radiogenic Pacific deep waters would no longer enter the North Atlantic. A

westward flux of deep waters throughout the Caribbean Seaway during the Campanian and Maastrichtian is supported by _ɛ_Nd values of the detrital fraction higher than that of the bottom

waters at site 152 (Nicaragua Rise)39. The oceanic circulation simulated for the Maastrichtian should thus generate a much less radiogenic Nd isotope signature of North and SA deep waters,

as well as in the Indian Ocean down to 3,000 m of depth, in agreement with _ɛ_Nd data (Fig. 2). Our work thus supports an intensification of deep-water production in the southern Atlantic

and Indian Oceans along with a reversal of the deep-water flux through the Caribbean Seaway as a driver for the decrease in deep-water _ɛ_Nd depicted during the Late Cretaceous. The

subsidence of large igneous provinces such as the Kerguelen plateau could also have contributed to reducing the inputs of radiogenic Nd to the surface waters of the southern Atlantic and

Indian Oceans31. As Nd is not incorporated as a tracer into our model, the impact of reduced inputs of radiogenic Nd linked to this mechanism on the decrease recorded by deep-water _ɛ_Nd

values cannot be further discussed here. The simulated circulation during the Maastrichtian remains difficult to reconcile with the highly non-radiogenic values recorded at Cape Verde and

Demerara Rise14,16 in the low-latitude Atlantic Ocean. It has been suggested that intermediate to deep waters could have been generated in the area of Demerara Rise (‘Demerara Rise Bottom

Waters’)16. Nevertheless, the Late Cretaceous _ɛ_Nd from Demerara Rise stands in marked contrast to the data from other North Atlantic sites11. Demerara Rise bottom waters may have been

restricted to intermediate depths, similar to Med outflow water, and thus would not have greatly influenced deep-water masses in the North Atlantic11. Our results show no evidence for water

sinking in this area (Fig. 1). One way to reconcile the _ɛ_Nd of Demerara Rise and Cape Verde with the modelled circulation could be to invoke the impact of boundary exchange processes that

are known to occur along continental margins and can modify the Nd isotope composition of the local bottom waters32,40. Considering the proximity of the two sites to very old, unradiogenic

terranes (Guyana and African Shields), such a process may have locally lowered the Nd isotopic composition of the water masses. The increase in Nd isotope values recorded at this site at the

end of the Maastrichtian when they reach values similar to the other Atlantic sites may then reflect the progressive opening of the Demerara Rise region to the remaining of the Atlantic.

MECHANISMS DRIVING THE CHANGES IN OCEANIC CIRCULATION In the model, the shift in the location of deep-water formation from the Pacific to the Atlantic/Indian sector is clearly due to

modifications of the land–sea mask. In detail, it is the hydrological cycle over the SA basin that seems to be responsible for the shift in the deep-water formation area between the

Cenomanian and the early Maastrichtian. During the early Maastrichtian, the fall in sea level and the retreat of the sea from the tropical continents induce more tropical monsoons and less

vertical advection of moist air, which result in a more negative P−E (precipitation minus evaporation) budget in the CA Ocean (Fig. 4). When integrated over the latitudes 40°S–10°N,

corresponding to the Central and SA, the P−E budget is more negative in the 71 Ma run with a value around—1.5 mm per day (−0.14 mm per day for the 94 Ma run). These changes will favour

convection in the SA during the early Maastrichtian, because the saltier the surface waters are, the denser and heavier they will be when they reach the southern high latitudes (Fig. 5).

SENSITIVITY TO THE DRAKE PASSAGE AND TO THE CARIBBEAN SEAWAY The results presented here are based on the two palaeogeographies published in Sewall _et al._9, in which the depth of the Drake

Passage has been reduced to 145 m. Major uncertainties exist on the configuration of the Drake Passage during the Late Cretaceous, with reported depths ranging from over 1,000 m9 to a

shallow41 or completely closed passage42. Similarly, the depth and configuration of the Caribbean gateway is not well constrained. Island arc volcanism was already present across the

Caribbean Seaway during the Late Cretaceous, possibly partly impeding deep or intermediate water communications across the seaway43,44. Nevertheless, the configuration of these two seaways

have been shown to have important implications for oceanic circulation in more recent periods45,46. To test the impact of the configuration of the Drake Passage and of the Caribbean Seaway

on oceanic circulation, we performed a series of sensitivity experiments. In a first set of simulations, we modified the Cenomanian/Turonian and the early Maastrichtian palaeogeographies of

Sewall _et al._9 using the same horizontal configuration but varying the depth of the Drake Passage, using the depth originally specified by Sewall _et al._9, which is around 1,000 m (SEWALL

simulations), a depth of 408 m (DRAKE408m simulations), a depth of 145 m (CTRL simulations) and a closed Drake Passage (DRAKE CLOSED simulation). In a second set of simulations, two runs

for each palaeogeography were performed with a very shallow Caribbean Gateway, fixed at 560 m depth (CAS560m simulations) and a small continental plate across the Caribbean Seaway

(Supplementary Fig. 8) to simulate the presence of islands (CAS-Islands simulation). The results of these experiments are presented in Figs 6, 7, 8, 9 and in Table 1. Most features described

above for the two land–sea masks in the CTRL runs remain the same independently of the depth of the Drake Passage (Figs 6, 7, 8, 9). These features are the inversion of the direction of

deep-water flow across the Caribbean Seaway for depths >1,000 m and the increase in northward water fluxes at depth across the SA basin. Summed over the whole water column, water flows

globally from the North Atlantic to the SA during the Cenomanian–Turonian and in the opposite direction during the early Maastrichtian. However, the depth of the Drake Passage substantially

affects the intensity of these flows. The main changes occur when going from a closed passage to a depth of 408 m. Most notably, the deeper branch of the thermohaline circulation coming from

the Weddell Sea decreases its volume transport from 5.8 to 3.7 Sv for the 94 Ma experiments and from 35.4 to 16.2 Sv for the 71 Ma experiments (Table 1). Nevertheless, the trend from a

sluggish circulation to a more active circulation at depth in the South and CA with the early Maastrichtian palaeogeography holds independently of the configuration of the Drake Passage.

Sensitivity experiments assessing the effect of Caribbean Seaway depth on circulation yield contrasting results, especially for the early Maastrichtian simulations. A shallow gateway across

the Caribbean Seaway (CAS560m simulations) completely modifies deep circulation and results in a shut down of thermohaline circulation in the SA (Figs 8 and 9). By contrast, the same

configuration but for the 94 Ma experiments has a minor impact on the general pattern of oceanic circulation (Figs 6 and 7). Finally, experiments with a small continental plate across the

Caribbean Seaway result in only minor changes in the modelled oceanic circulation for both palaeogeographies. The circulation modelled with a shallow Caribbean Seaway (CAS560 m) for the

Cenomanian/Turonian is less consistent with the _ɛ_Nd , because a deep-water influx of radiogenic Pacific waters through the Caribbean Seaway as observed in the other simulations is in

better agreement with the radiogenic composition of North Atlantic deep waters. In the early Maastrichtian, the weaker fluxes of surface and intermediate waters across the Tethys, Med and

Caribbean Seaway modelled in the CAS560m experiment are also less consistent with an intense equatorial circumglobal current that has been suggested based on the large-scale deposition of

phosphorites along the southern Tethyan margin47. These additional sensitivity experiments reveal a major role of the circulation pattern across the Caribbean Seaway on the overall modelled

thermohaline circulation and shed new light on the mechanisms driving the change in oceanic circulation observed between the simulations using the Cenomanian/Turonian versus early

Maastrichtian palaeogeographies. The hydrological cycle changes described above contribute to the intensification of deep-water production in the southern Atlantic and Indian Oceans. Changes

in land–sea configuration and the associated atmospheric feedbacks induce a saltier subtropical SA Ocean during the early Maastrichtian, which in turn results in a larger area of deep-water

formation around the eastern Antarctic (Fig. 1). Indeed, the saltier the surface waters are, the denser and heavier they will be when they reach the southern high latitudes. An additional

important factor appears to be the strong flow of Atlantic water into the Pacific at all depths during the Maastrichtian, observed in all experiments, except the CAS 560 m simulation. This

flow creates a large divergence area in the CA, drawing in waters from the Med gateway and from the SA (Table 1). The transition from an estuarine circulation during the Cenonamian–Turonian

to the one characterizing the early Maastrichtian as simulated here is probably the main mechanism explaining the shift from a sluggish to an active deep circulation. This transition occurs

in response to continental drift and the subsequent modification of the geometry of the Caribbean Seaways. The sensitivity of oceanic circulation across the Caribbean Seaways has already

been investigated many times and results from a complex interplay between local processes, such as wind-driven circulation and salinity contrast45, and remote processes such as the geometry

of other gateways46,48. DISCUSSION Our simulations highlight an evolution from a sluggish circulation in the SA and CA using the Cenomanian/Turonian land–sea mask towards a much more active

circulation in these basins with the early Maastrichtian land–sea mask. Alongside changes in the location of deep-water production areas at high latitudes, these features may have paved the

way for a well-oxygenated deep ocean. The absence of large-scale anoxia in deep waters during abrupt warming events after the Late Cretaceous, such as the Paleocene–Eocene transition49 (∼55 

Ma), supports this hypothesis. The experiments conducted here emphasize the potentially important role of land–sea configuration as a pre-conditioning factor that affects the ease with which

OAEs can develop. In that sense, palaeogeography represents one of the required conditions for the occurrence of OAEs but would not be sufficient on its own to trigger these events,

implying additional causal factors50,51. As a result of the palaeogeographic changes occurring during the Late Cretaceous, our results suggest that worldwide anoxic conditions in deep waters

were more likely to develop as a result of the same triggering factor (for example, increased nutrient input into the oceans2,3,4) during the Cenomanian/Turonian compared with the early

Maastrichtian. Two recent geochemical modelling studies have emphasized the need for a sluggish circulation in the Atlantic basin, to explain the large spatial extent of OAE-2 (ref. 52) and

a substantial sensitivity of the onset of anoxia/euxinia to oceanic overturning53. Based on the latter study, we predict that in a sluggish state such as the one simulated at the

Cenomanian/Turonian boundary with <5 Sv of deep water flowing northward in the SA, a 20–40% increase in nutrient input to the ocean could generate deep-water euxinia. In contrast, a 225%

increase in nutrient input is required to provoke euxinia with the early Maastrichtian palaeogeography in which 25.8 Sv feed the deep-water flowing northward (Fig. 3c,d). Our results imply

that thresholds for the ocean-climate system to shift towards a state of global anoxia in deep waters are likely to be much higher at present than during the Cenomanian/Turonian because of

the strength of the modern thermohaline circulation. In summary, this study provides a clear description of oceanic circulation changes occurring during the Late Cretaceous and is able to

broadly explain available published _ɛ_Nd data. Our work points to changes in continental configuration as the major driver for the depicted ocean circulation change, through a modification

of the hydrological cycle over the SA basin and the development of a strong westward flow of water at all depths throughout the Caribbean Seaway, that both favour an intensification of

deep-water production in the southern Atlantic and Indian region. We note that our knowledge of the geometry of Cretaceous oceanic gateways is contentious and a definitive history of ocean

circulation during this time must await further constraints on the state of the various oceanic connections. We also speculate on the consequences of long-term oceanic circulation changes on

the likelihood that the Earth System will be affected by an OAE. More explicit modelling including complex climate models and biogeochemistry will help reveal the extent to which the

tectonic and climatic background state can influence the development of oceanic anoxic conditions. METHODS CODE AVAILABILITY The code of the model FOAM is available on request by e-mail to

the first author. DESCRIPTION OF THE MODEL The model experiments were completed using the FOAM, developed by Jacob54. This model combines a low spectral resolution R15 (48 × 40 grid)

atmosphere model counting 18 altimetric levels with a highly efficient medium resolution (128 × 128 grid) ocean module composed by 24 bathymetric levels. FOAM successfully simulates many

aspects of the present-day climate and compares well with other contemporary medium-resolution climate models. It was previously used to investigate numerous past climate changes, ranging

from the Neoproterozoic glaciations to the onset of the ACC at the Eocene–Oligocene boundary55,56. DESCRIPTION OF THE EXPERIMENTS The two sets of simulations (called hereafter CTRL94Ma and

CTRL71Ma) developed for this study are based on existing palaeogeographic reconstructions for the Cenomanian/Turonian boundary and for the early Maastrichtian9 (Fig. 2). For the sake of

simplicity, we choose to use 94 Ma though the accurate age of the boundary is 93.9 Ma57. Based on existing palaeogeographies41,58,59, the depth of the Drake Passage in the CTRL simulations

has been restricted to a shallower depth (that is, 145 m) than in the original bathymetry in which the depth reached more than a thousand metres. Indeed, first evidences for the deepening of

the Drake Passage correspond to the creation of oceanic crust and are generally dated between the Eocene and the Oligocene60,61. During the Late Cretaceous, pull-apart basins were formed

between the Antarctica Peninsula and southern South America62, which represent the onset of the opening of the Scotia Sea. Therefore, the Drake Passage most probably formed a narrow and

shallow marine domain at that time63. To account for existing uncertainties on the configuration of the Drake Passage and of the Caribbean Seaway, sensitivity experiments assessing the

effect of the depth specified for these two marine gateways are also presented in the discussion. Data and model-based atmospheric CO2 level estimates for the Late Cretaceous are between two

and eight times the pre-industrial atmospheric CO2 concentration (280 p.p.m.)64,65,66,67,68,69. Accordingly, three atmospheric CO2 concentrations were tested for the two palaeogeographies:

560, 1,120 and 2,240 p.p.m. The solar luminosity was specified as 99% of modern. Orbital parameters and other greenhouse gases were set to present-day values. River routing is specified

using the Sewall _et al._ reconstruction. The experiments were integrated for 2,000 years without flux corrections or deep ocean acceleration. During the last 100 years of model integration,

there is no apparent drift in the upper ocean (between the surface and 300 m depth) and <0.1 °C per century change in globally averaged ocean temperature. All model results have been

averaged over the last 50 model years. ADDITIONAL INFORMATION HOW TO CITE THIS ARTICLE: Donnadieu, Y. _et al._ A better-ventilated ocean triggered by Late Cretaceous changes in continental

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Scholar  Download references ACKNOWLEDGEMENTS This work was supported by a funding from the ANR project Anox-Sea. We thank three anonymous reviewers for their constructive comments that

greatly contributed to improve this manuscript. We thank the CEA/CCRT for providing access to the HPC resources of TGCC under the allocation 2014–012212 made by GENCI. AUTHOR INFORMATION

AUTHORS AND AFFILIATIONS * Laboratoire des Sciences du Climat et de l’Environnement, LSCE-IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Gif sur Yvette, 91191, France Yannick Donnadieu *

Biogéosciences Dijon, Université Bourgogne-Franche-Comté, UMR CNRS 6282, Dijon, 21000, France Emmanuelle Pucéat, Mathieu Moiroud & Jean- François Deconinck * Géosciences Rennes,

Université de Rennes, UMR CNRS 6118, Rennes, 35042, France François Guillocheau Authors * Yannick Donnadieu View author publications You can also search for this author inPubMed Google

Scholar * Emmanuelle Pucéat View author publications You can also search for this author inPubMed Google Scholar * Mathieu Moiroud View author publications You can also search for this

author inPubMed Google Scholar * François Guillocheau View author publications You can also search for this author inPubMed Google Scholar * Jean- François Deconinck View author publications

You can also search for this author inPubMed Google Scholar CONTRIBUTIONS Y.D. and E.P. conceived the project. E.P., M.M. and J.-F.D. built the neodymium database and performed the

analysis. Y.D. conducted all numerical climate modelling. Y.D. and E.P. wrote the manuscript with contributions from all authors. CORRESPONDING AUTHOR Correspondence to Yannick Donnadieu.

ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests. SUPPLEMENTARY INFORMATION SUPPLEMENTARY FIGURES AND REFERENCES Supplementary Figures 1-8 and

Supplementary References (PDF 2141 kb) SUPPLEMENTARY DATA 1 Neodymium isotope composition of modern detrital material on continental margins. Database from Jeandel et al., (2007) with

additional references included (XLS 93 kb) SUPPLEMENTARY DATA 2 Compilation of eNd(t) of intermediate to deep waters at ODP sites for the Cenomanian/Turonian boundary and for the early

Maastrichtian (XLS 162 kb) RIGHTS AND PERMISSIONS This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this

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THIS ARTICLE CITE THIS ARTICLE Donnadieu, Y., Pucéat, E., Moiroud, M. _et al._ A better-ventilated ocean triggered by Late Cretaceous changes in continental configuration. _Nat Commun_ 7,

10316 (2016). https://doi.org/10.1038/ncomms10316 Download citation * Received: 18 March 2015 * Accepted: 27 November 2015 * Published: 18 January 2016 * DOI:

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