Mediterranean isolation preconditioning the Earth System for late Miocene climate cooling

A global Neogene cooling trend culminated ~7 million years ago with the onset of Greenland glaciation. Increased ocean-atmosphere interaction and low- to high-latitude circulation are thought to be key factors in reorganizing late Miocene global temperature and precipitation patterns, but the drivers of this reorganization have yet to be identified. Here, we present new information about the evolution of the Atlantic-Mediterranean gateway that generated Mediterranean overflow. We use sedimentary and palaeogeographic evidence to constrain the timing and dimensions of this gateway and document the initiation of a saline plume of water within the North Atlantic. Today, this saline jet entrains and transports Eastern North Atlantic water and its dissolved inorganic carbon into the interior of the ocean, contributing to the drawdown of CO2 and the sensitivity of the ocean to atmospheric changes. We show that during the Miocene this transport emerged simultaneously with gateway restriction and propose that the resulting interaction of ocean-surface and ocean-interior carbon inventories would have greatly enhanced ocean-atmosphere exchange, preconditioning the Earth System for late Miocene cooling.

. Bold lines indicate the preferred theory to compute velocity for a given state of the gateway: rotational control theory is more accurate for larger gateways, whereas hydraulic control becomes preferable once the two lines intersect. (e) Proportion of the water from the upper layers of the North Atlantic gyre (NACW) that is entrained into the MOW-generated plume, based on the calculations and references shown in Supplementary Table 1 (Supplementary Material). Evidence of strengthening arctic overflows are based on refs 60,61 . Pockets of glacial activity in Greenland are from refs 62,63 .

Atlantic-Mediterranean exchange, properties and processes
In the Atlantic, several marine overflows (Denmark Strait, Mediterranean, Weddell Sea) supply the dense water that collectively feeds the thermohaline circulation system 16 . Of these, the transportation of hypersaline water from the Mediterranean into the interior of the Atlantic is amongst the largest and densest in the global ocean 9 and the exchange also provides a key exit point for Atlantic buoyancy, which is the underlying driver behind Atlantic deep convection 17 .
The Mediterranean's dense outflow is generated as a consequence of its mid-latitude setting where evaporation exceeds precipitation 18 forming a warm, but salty water mass. This negative hydrologic budget amplifies the climate signal transmitted principally through the Mediterranean's southern catchments from North African monsoon rainfall 19 . The sub-tropical monsoonal climate signal with its strong precessional pulse, is then propagated into the Atlantic by density-driven exchange 20 through the Gibraltar Strait. Water flowing out of the Mediterranean at depth entrains ambient Atlantic water as it goes 21 , generating Atlantic-Mediterranean Water 22 . This distinctive water mass forms large depositional and erosional features including extensive sandy contouritic drifts 23 , which allow the reconstruction of Mediterranean overflow activity in the past 24,25 . Atlantic-Mediterranean Water flows north, fuelling the Norwegian Seas with higher density water that helps to sustain the formation and southward flow of North Atlantic Deep Water 22 .
Despite the challenges of modelling the gateway, the exchange that occurs through the Gibraltar Strait today is a sufficiently influential component of the Earth System for general circulation models to capture at least part of its impact 26,27 . Experiments without Atlantic-Mediterranean exchange show that its presence makes Greenland warmer and Antarctica cooler 27 . This in turn, is sufficient to shift the position of the Intertropical www.nature.com/scientificreports www.nature.com/scientificreports/ Convergence Zone, and hence the location of monsoons, storm tracks and the hyper-arid zones between them. Atlantic-Mediterranean exchange is also a critical component of Atlantic Meridional Overturning Circulation (AMOC) particularly at times of weak North Atlantic Deep Water formation 26,[28][29][30][31][32] , as a consequence of dense, salty water being transported from Gibraltar into the high latitudes. Given the weaker AMOC during the late Miocene 33,34 , the relative contribution of Mediterranean density to late Miocene deep-water formation is likely to have been greater than today 10 . Furthermore, in entraining ambient Atlantic water, the transport of dense Mediterranean Overflow into the Atlantic interior also transfers 0.06 GtC yr 1 of anthropogenic carbon from the ocean surface to intermediate depths 35 , contributing a 2-5% of today's total net ocean carbon sink [36][37][38] .
Exchange through Gibraltar, however, is a relatively recent phenomenon (Fig. 2). Progressive isolation of the Mediterranean from the Atlantic occurred throughout the Miocene, driven by Africa-Eurasia convergence coupled with the westward drift of the Alboran Plate 39 . This restriction replaced a wide gateway (Fig. 2) floored with oceanic crust 40 with narrower, shallower connections: the Rifian and Betic corridors 41,42 , through which Atlantic-Mediterranean exchange was funnelled (Fig. 2).
The onset of episodic organic-rich sedimentation (sapropels) in the Middle Miocene 43 is the earliest evidence of Mediterranean oceanography distinct from the Atlantic, although this alone does not require the existence of Mediterranean overflow. Only at some point during the late Miocene did on-going restriction of the marine corridors permit Mediterranean salinity to rise forming a dense water mass that overspilled into the Atlantic for the first time 13 . Progressive narrowing and closure of these connections resulted in extreme salinity fluctuations in the Mediterranean (Fig. 2), leading to the precipitation of more than 1 million km 3 of salt, equivalent to ~6% of the total dissolved oceanic NaCl in the latest Miocene ocean 44,45 . This event, known as the Messinian Salinity Crisis, lasted between 5.97-5.33 Ma 46 . Ultimately, tectonic convergence 39 coupled with slab-dragging 47 and isostatic rebound related to slab-tear 48 , not only severed these marine connections, but also uplifted and exposed them on land.
Field evidence from northern Morocco and Southern Spain suggests that the Rifian and Betic Corridor closed in the early Messinian, at about 7 Ma 42,49 while two-way exchange continued through a proto-Gibraltar Strait 50 .

New constraints on Late Miocene flow through the Atlantic-Mediterranean gateways
The late Miocene Betic 41 and Rifian 13 corridors both contain Miocene contourites (i.e. sediments deposited by bottom-currents) formed by Mediterranean water overflowing into the Atlantic, at flow-velocities greater than 0.5-1 m/s. Here we focus on the Rifian Corridor contourites, which preserve a complete sequence of sediments and sedimentary facies representative of the bottom-current behaviour 13 .
Contourites in the Rifian Corridor are composed mainly of bigradational, sandy-muddy beds and cross-stratified sandstone that formed as smooth sand sheets and cross-bedded dunes (Fig. 3). These bedforms are known from deep contourite-dominated environments and for fine to coarse sand represent flow velocities fluctuating between 0.15-1.0 m/s 51 . The occurrence of these sandy contourite bedforms, above non-contouritic mudstone 13 indicates that bottom-current flow through the Rifian Corridor increased above a critical threshold of 0.13-0.15 m/s 51 .
Field evidence of contourites from the Rifian Corridor allows us to cover the initiation of Mediterranean overflow between 7-8 Ma. Postdating the Betic and Rifian corridors closure at around 7 Ma 42 , we assume that the Strait of Gibraltar took over, accommodating all the exchange 50 . Therefore, to explore the effect of a restricting gateway from its wide oceanic seaway to its modern-like configuration (Fig. 2), we have adopted published paleogeographic and dimensional constraints 12,42,50 , to reconstruct the evolution of a simplified, linearly reducing gateway which narrowed by approximately 500 km and shallowed by about 1000 m during the middle to late Miocene (Fig. 1a).

The link between sedimentary evidence and overflow behaviour
To find a quantitative link between the field evidence of overflow formation 13 and the palaeogeographic evolution of the Atlantic-Mediterranean gateway (Figs 1a and 2), we combined the theory of sea straits dynamics with an empirical relationship between flow velocities at the seafloor and the resulting bedforms for a given grain size 51 . The theory (Fig. 3) generates an average overflow velocity as a function of Mediterranean basin evaporation, which in turn influences the water density and thus the vigour of the basin outflow, and of gateway width at the point of greatest constriction, which reflects the process of gradual restriction and approximates the condition and location where the observed contourite bedforms developed (see also methods). Figure 3 shows the resulting relationship between Atlantic-Mediterranean gateway width (x-axis), which varied during the Miocene (Fig. 2), and the mean velocities of Mediterranean overflow at the sill (y-axis). The model suggests that a reduction in width of the Atlantic-Mediterranean gateway focusses the flow and leads to an increase in overflow velocity. Furthermore, the graph shows three different net evaporation values, covering wetter(dotted line) and dryer (dashed line) periods, representing the possible fluctuations controlled by climate. The theory shows that tectonics is driving the trend towards a narrower gateway, but at any one time, the detail of the regime at the sill itself can be influenced by the Mediterranean climate.
The model results support the notion that only after a certain threshold value of width is passed are velocities high enough to produce erosional and depositional bedforms on the gateway floor, and that the late Miocene, 7-8 Ma gateway reconfiguration (Fig. 2) corresponded to the increase in velocity that deposited coarse and cross-stratified sands in the gateway, corroborating field evidence 13 . Dating the first occurrence of the sandy contourites above finer-grained marlstone 13 therefore suggests that flow velocity exceeded the critical threshold for contourites formation at around ca. 7.8 Ma and that this is the time at which saline Mediterranean water first started to contribute to North Atlantic circulation. (2019) 9:3795 | https://doi.org/10.1038/s41598-019-40208-2 www.nature.com/scientificreports www.nature.com/scientificreports/

Mediterranean overflow velocity and Atlantic water entrainment
The initiation of Atlantic-Mediterranean Water with its characteristic properties and distribution 22 can therefore be inferred from first sedimentary expressions of restriction in the Betic and Rifian corridors. Since the settling depth of the plume depends predominantly on Atlantic salinity 52 , the effect on surface water entrainment will also have been initiated at this time. The Atlantic-Mediterranean Water plume entrains North Atlantic Central Water (NACW) from within the upper layers of the North Atlantic gyre, taking with it dissolved CO 2 35 . Increasing overflow velocity leads to an increase in Atlantic entrainment and associated carbon transport 35 .
In Fig. 1e we show the variations of the entrainment parameter 14 , which represents the proportion of NACW entrained by Mediterranean overflow as a function of varying velocity values (Fig. 1d). Velocity values are computed from the dimensions of the Atlantic-Mediterranean gateway at the point of greatest constriction (Fig. 1a) using a simplification for strait exchange 53 (see methods). NACW becomes entrained within the plume when the gateway restricts beyond a threshold value which here occurred between 9 and 8.5 Ma (Fig. 1e) and increases to a peak value at the beginning of the Messinian Salinity Crisis (Fig. 1a,e). Data from this study therefore suggests that late Miocene restriction of the Mediterranean-Atlantic gateway initiated a new ocean pump that increased the interaction between deep and upper ocean reservoirs 35 and that this occurred synchronously with the onset of a long-term surface water cooling trend, (Fig. 1c). www.nature.com/scientificreports www.nature.com/scientificreports/

Rapid-moving Mediterranean overflow plume and late Miocene cooling
Global cooling that affected the mid-and high-latitude of both hemispheres between ~7.5 and 5.5 Ma occurred in concert with strengthening of the biological pump 2 and changes in terrestrial vegetation due to a reduction in late Miocene CO 2 concentration 1 . We suggest that the synchronous narrowing of the Atlantic-Mediterranean gateway favoured ocean -atmospheric CO 2 decoupling in two ways: (i) injection of Mediterranean hypersaline waters enhanced northern hemisphere overturning, causing a deepening of the global carbonate compensation depth, shorter residence times of bottom water, and greater sensitivity of the ocean-atmosphere system. At the same time, (ii) by enhanced entrainment of surface water into the Atlantic-Mediterranean Water plume, the new Mediterranean Outflow altered storage of CO 2 in the Atlantic interior, promoting global changes in the distribution of carbon at the onset of northern hemisphere cooling and during the switch to modern, C 4 -dominated ecosystems.

Methods
The theory behind Fig. 3 describes the condition of maximal flow in a wide, rotationally controlled gateway with two-way exchange and negligible net flow 54 . For narrower gateways than those considered in this figure, rotational control would give way to hydraulic control 15 , as exemplified in Fig. 1d when width decreases below a certain value. The residence time of the Mediterranean Sea is of the order of 100 s of years, for larger gateway exchange than that presented in this study 55 . However, as we are considering gateway dimension changes on much greater time-scales (Fig. 2), we can assume that the exchange and Mediterranean salinity are close to its equilibrium, allowing us to ignore their time-dependent effect. By having the rotationally-controlled strait connect to a basin subject to a specified net evaporation and ignoring the role of temperature, we can express the flow velocity in terms of gateway dimensions and the value chosen for evaporation. This step assumes that the Rifian corridor exerted the dominant control on basin salinity. While the volume transport of water through the gateway is dependent on strait depth, the mean velocity is not, and therefore we can represent velocity (y axis) as being in function of width (x axis).
How would the results be affected if a significant second gateway was present? If this additional gateway also accommodated outflow then, intuitively, one would expect the outflow through the Rifian corridor to have been less vigorous. This can easily be shown to be consistent with theory as long as the volume transport of the outflow through both gateways is proportional to the salinity (density) excess of the basin. In Fig. 3, a given evaporation and gateway width would thus correspond to lower flow rates than now found. In order to reach the flow velocity required for an observed bedform, a greater restriction would be needed than the one we now infer in the width-range for contourite formation (Fig. 3). The effect of a second gateway is less straightforward in a scenario that it only accommodated inflow. This is expected to happen when the second gateway is relatively shallow 56 . In, perhaps the most likely, situation that the second gateway takes over part of the inflow from the main gateway, the volume transport of outflow would be unaffected but the average outflow velocity may again decrease because the outflow occupies a greater depth range.
Net evaporation is varied around the value of 0.5 m/yr, which is within the correct range for the present-day 57 and the late Miocene 15 . The velocity at the sill is linked to expected bedform for a grain size of fine (0.125-0.25 mm) to coarse sand (0.5-1 mm) following Stow et al. 51 . This range of grain-size reflects the dominant sand composition in the first sandy-muddy beds with bigradational sequences occurring in the Rifian Corridor 13 . In the bedform-velocity matrix of Stow et al. 51 we started with a given grain size (x-axis) and increased flow velocity (y-axis) until we intersected a threshold value representing the minimum velocity to have depositional bedforms on the seafloor (smooth sand sheets and straight ripples).
In the context of the Rifian Corridor, our correlation assumes that the velocity calculated at the sill (i.e. outcome of the computation) and the velocity of the overflow within ~50 km down its path (i.e. where the gateway contourites form) do not differ significantly, as observed in present day monitoring at the exit of the Strait of Gibraltar 58,59 .
The theory behind the entrainment parameter (Fig. 1e) is based on the representation showed in Baringer and Price 14 and Whitehead 53 and employs cumulative values of width and depth (Fig. 1a) and resulting velocities (Fig. 1d) for a simplified, linearly-reducing Atlantic-Mediterranean gateway from middle to late Miocene. Supplementary Table 1