The different positions of large continents in the Southern Hemisphere influenced the oceanography of the Eocene and thus the distribution of heat. The position of Australia and South America in the Eocene was much closer to Antarctica than today see, e.g.,. This arguably prevented circumpolar surface water flow , i.e. the Antarctic Circumpolar Current (ACC), that thermally isolates Antarctica today . Instead, it was thought that low-latitude currents bathed Antarctic coastlines. The subsequent opening of a circumpolar passage, through deepening of the Drake Passage (DP) and Tasman Gateway (TG), would then allow the ACC to develop, leading to the isolation of Antarctica and progressive cooling, ultimately leading to full glaciation of Antarctica . Support for this hypothesis came when it was discovered that the deep opening of these gateways appeared to be roughly coeval with the onset of continental-scale Antarctic glaciation around the Eocene–Oligocene transition EOT; about 34 million years ago, Ma;.

Objections to the gateway hypothesis came from two fields of research. First of all, drill cores obtained from the Tasman Gateway region showed that accelerated widening and deepening of the passage likely started 2 Myr prior to the glaciation, indicating that a direct causal link between the two is unlikely . Unlike the quite precise estimates for the opening of the Tasman Gateway, the timing of the opening of the Drake Passage is debated and less well constrained . Several structural geologic investigations in the region have inferred crustal stretching from the early Eocene onwards , but it remains unclear whether the vertical displacement of that tectonism caused a shallow passageway through the Drake Passage. We cannot, however, rule out the possibility that the Drake Passage was already open, but shallow, in the early Eocene.

Secondly, numerical modelling studies with general circulation models (GCMs) configured for the Eocene emerged that suggested an ocean circulation pattern with clockwise circulating gyres , where western boundary currents near Antarctica (as part of a subpolar gyre) still prevented low-latitude currents from reaching and warming Antarctica (a general feature of subpolar gyres). Microfossil biogeography studies (notably, but not exclusively, done on organic-walled dinoflagellate cysts) clearly support the gyre configuration as suggested by numerical modelling experiments rather than the strong influence of low-latitude-derived ocean currents. It should be noted here that these considerations of the gyre circulation alone are centred on the surface circulation and do not include influences of thermohaline heat transport, which has been shown to have the potential to significantly enhance oceanic heat transport to Antarctica in a closed-gateway configuration .

The existing proxy records for atmospheric CO2 during the Paleogene and ice sheet modelling have steered opinion in the direction of declining atmospheric greenhouse gas concentrations as the primary forcing factor in explaining Eocene cooling in general and, ultimately, the onset of continental-scale glaciation.

Although gateway changes appear not to be the direct cause of EOT glaciation, unlike the hypothesis first proposed by and , their role in the general long-term Cenozoic cooling trend remains plausible but poorly reconciled with data. Indeed, glaciation (34 Ma) took place against the backdrop of a long and gradual cooling trend in the Southern Ocean , whereas detailed geological reconstructions of the Tasman Gateway have, until recently , largely ignored the potential climatic effects of earlier stages of its opening. Immediately preceding this slow climatic deterioration is the time with the warmest global temperatures of the past 85 million years, the protracted greenhouse episode known as the Early Eocene Climatic Optimum EECO; 52–50 Ma;. Whereas it is generally believed that a decline in atmospheric greenhouse gas concentrations terminated the EECO, compelling direct and unequivocal proxy evidence for this is generally lacking.

As a contrasting hypothesis, find evidence for the first signs of throughflow of south-west (SW) Pacific surface waters into the Australo-Antarctic gulf (AAG) across the Tasman Gateway coinciding with the onset of regional surface water and continental Antarctic cooling of 2–4∘. infer from the distinct difference in dinocyst assemblages between the AAG and the SW Pacific that the Tasman Gateway served as an effective barrier to surface water exchange before 50 Ma. From 49–50 Ma onwards, endemic dinocyst species originating in the SW Pacific began to dominate sediments of the Antarctic margin of the AAG. They propose that the timing is consistent with the drowning of continental blocks in the southern part of the Tasman Gateway inferred from accelerated rifting during the early-to-middle Eocene transition 52–48 Ma;. Surface water throughflow of SW Pacific surface waters into the AAG would bring the endemic Antarctic species into the AAG. A crucial observation here is that the south Australian margin remains isolated from the SW Pacific influence but remains exclusively inhabited by cosmopolitan and low-latitude-derived dinocyst species. Furthermore, the influence of surface water throughflow from the AAG into the south-west Pacific remains restricted as well: no low-latitude-derived species appear in the SW Pacific Ocean. Eastward flow through the Tasman Gateway would be expected to bring the low-latitude species from the Australian margin into the SW Pacific Ocean. This is contrary to their findings; therefore, they hypothesize that the throughflow must have occurred to the south, perhaps within the reach of the easterlies, allowing the throughflow of the taxa present in the Ross Sea that consistently dominate the westward flow along Antarctica, the Eocene Antarctic Counter Current.

Organic biomarker proxy records for the paleotemperature of the sea surface and for the air temperature were derived from the same sedimentary archives as the oceanographic reconstructions. They show that the opening of the Tasmanian Gateway coincided with surface water and air temperature cooling of several degrees (2–4 ∘C), with the Antarctic hinterland cooling the most (4 ∘C). Simultaneously, benthic foraminiferal oxygen isotope records show the onset of gradual cooling as well. Although this study could not prove causality between oceanographic changes and cooling, the closeness in time is intriguing and requires a follow-up to prove causality.

The uncertainty related to the role of gateways in long-term climate evolution, as well as the increasing need for more detailed knowledge of Cenozoic ocean current development in general, stimulates modelling studies on the impact of gateway changes on ocean circulation through time. Here, we will use a coupled climate model of intermediate complexity to numerically simulate the effect of an initial opening of the Tasman Gateway at the early-to-middle Eocene transition. We show that this can lead to the inferred patterns of dinoflagellate biogeography: a westward current emerges, but only when the Drake Passage remains closed. Numerical modelling provides a physical basis to explain micropalaeontological observations.

We use a modified version of the intermediate-complexity coupled model described in detail in , the so-called UVic (University of Victoria) model. The model consists of an ocean general circulation model GFDL MOM, Geophysical Fluid Dynamics Laboratory Modular Ocean Model, Version 2.2; coupled to a simplified one-layer energy–moisture balance model for the atmosphere and a dynamic–thermodynamic sea-ice model. Air–sea heat and freshwater fluxes evolve freely in the model, while a non-interactive wind field is employed for reasons of computational speed. The turbulent kinetic energy scheme of based on models vertical mixing due to wind and vertical velocity shear. The model is identical to , with a modification to the Eocene geography in the primary experiments where Australia is located further south by 6∘ in latitude and the Antarctic margin facing it is also shifted south.

The original geography will also be discussed as a secondary set of experiments in the sensitivity study below, where both geography is altered and the Southern Hemisphere westerlies are shifted. Motivation for these sensitivity studies of the true paleolatitude of the Tasman Gateway arises as comparisons of Eocene palaeogeographies may vary by about 6∘ in the SW Pacific region dependent on which reference frame is used: a hotspot versus a paleomagnetic reference frame .

We have run the model to equilibrium for a period of 9000 years in four configurations in which the Drake Passage is open to 517 m depth and in simulations where it is closed; in each case the Tasman Gateway is open, to a shallow depth of around 350 m, and closed. This choice is informed by the expectation that the state of the Drake Passage exerts a strong influence on the result of the opening of the Tasman Gateway: the uncertainty surrounding the timing of the opening of the Drake Passage see, e.g., makes it necessary for us to consider an open and a closed case.

Limitations arise from the absence of a fully coupled ocean and atmosphere in the UVic model. However, this model allows high enough spatial resolution to represent the shallow, southern opening of the Tasmanian Gateway during the early Eocene combined with long enough integration times to equilibrate the deep ocean to examine a possible global temperature signal. Furthermore, it provides the flexibility to conduct many simulations at once, each representing different scenarios. To provide backup for the realistic set-up of our model and cover both uncertainty in the location of the wind fields and their possible changes, our study prominently features experiments where the winds have been shifted, precisely to test the robustness of the results and cover the changes a fully coupled system might exhibit. Furthermore, a simulation with an alternative wind field is explored in the Appendix. Nonetheless, there could potentially be benefits to examining first-order geostrophic feedbacks on the wind, an option that is present in the UVic model.

The latitudinal section of the zonal velocity through the TG gap for the DP-closed case is shown in Fig. a and for the DP-open case in Fig. a. In the simulation with the southern Tasman Gateway open, a closed DP leads to a westward flow throughout the water column near Antarctica and a shallower eastward flow to the north (Fig. a). In contrast, the flow is eastward throughout the gap when the DP is open (Fig. a). In this simulation, the eastward flow is weak throughout most of the gap but strong at its northern margin.

To examine the sensitivity of these results to boundary conditions, we conducted two additional simulations where the core of the Southern Hemisphere (SH) westerlies is shifted 6∘ north, the TG is open, and the DP is open and closed (thus yielding two simulations). For reference, we show the zonal wind stress overlaid on the horizontal stream function in Fig. : the average latitude of maximum westerly wind stress (zero wind stress curl) is 5∘ S in the standard case and 44∘ S latitude in the wind-shifted case. This wind shifting procedure is essentially equivalent to , and we refer to this work for reference. We will refer to the original simulations without the modification as the “standard” case. In this modified model configuration, the flow pattern (Fig. b) is very similar to the standard case with the DP closed (Fig. a). Similarly, in the DP-open case, the flow pattern remains easterly throughout the TG gap, again with weak flow through most of the gap, with the exception of the northern margin (Fig. b). Note that, in contrast to the corresponding standard simulation (Fig. a), there is weak westward flow in a very narrow band below 250 m depth; nonetheless, the general flow is overwhelmingly eastward.

In addition, we also conducted two simulations, again with the TG open and the DP open and closed but now with altered geography. Here, the Australian continent is shifted north by 6∘ latitude with respect to our standard simulation, along with some northward shift of the Antarctic margin of the AAG thus obtaining the original geography used in. A southward-extending peninsula is also attached at the location of Tasmania, in agreement with . Again, a similar flow pattern emerges, where flow is eastward throughout the TG gap in the DP-open case (Fig. c), while there is a westward current near Antarctica in the DP-closed case (Fig. c). We conclude from our four additional simulations that our results are robust with respect to the location of the major wind circulation patterns and geography.

From here on, we will focus on the standard simulations. First, we examine the oceanic horizontal circulation. With the DP closed, the circulation of the Southern Ocean is split into two subpolar clockwise gyres, the Ross Sea gyre to the east of Australia and the Weddell gyre spanning the (south) Indian and Atlantic oceans to the east of the DP. To the north lie the subtropical gyres, where a super-gyre spans the Indian and Pacific oceans. In agreement with the Sverdrup balance, eastward flow takes place at the latitudes of positive wind stress curl (approximately minus the latitudinal derivative of the wind stress shown in Fig. ). A proto-ACC is absent due to the closure of the DP. Practically all eastward flow that passes Australia does so to the north of the continent and is part of the southern branch of the subtropical super-gyre.

Eastward flow in the northern branches of the subpolar gyres generally ends by returning south along the western margins of Australia and South America when the DP is closed (Fig. a). A discussion of the possibility of a linkage between the two subpolar gyres, using additional simulations with another model, can be found in Appendix A. Although the barotropic stream function shows no flow at the selected contour interval, the velocity field indicates a westward flow in the Tasman Gateway closely restricted to the Antarctic margin, naturally consistent with Fig. a. This westward flow through the TG is consistent with the generally negative wind stress curl at these latitudes.

An open DP under otherwise early Eocene boundary conditions yields around 35 Sv flow through this gap, as read from the 5 Sv interval contours immediately west of the DP (Fig a) and as determined by closer analysis on the computer of the numerical model output (figure not shown). The 35 Sv throughflow at the DP leads to eastward flow through the TG of around 8 Sv as determined from the model output file by computer analysis and shown less precisely in Fig a. The remaining 27 Sv must pass north of Australia. This total of 35 Sv of flow constitutes a circumpolar flow component that is not part of the gyre structure and is reminiscent of the ACC, despite the lack of latitudinal alignment of the gateways. We stress that this flow is much weaker than today and not comparable, even qualitatively. Nonetheless, we will refer to this current as the “proto-ACC”. As such, the uniformly eastward flow through the TG can be viewed as a consequence of the flow through the DP gap that joins the eastward flow along the northern branch of the Weddell gyre that must split to flow around Australia, yielding a southern component. In addition, when opening the Drake Passage, there is less necessity for significant westward return flow in the subpolar gyres. Indeed, high-resolution ocean model simulations have shown that closing the DP forces the ACC water to mainly turn southwards and join the subpolar gyres (unpublished results). The circumpolar flow is in line with idealized ocean modelling studies suggesting that strong circumpolar flow is possible even without a fully unrestricted latitude band . This eastward flow occurs entirely within the latitudes of the polar easterlies in the model (Fig. ) and within the latitudes of negative wind stress curl (with a small exception in the wind-shifted case). This shows that the direction of the flow is not only determined by the wind field but also by the continental geometry.

Our model results provide a numerical underpinning of the recent field observations and interpretations of that a southerly opening of the Tasman Gateway causes throughflow of SW Pacific surface waters into the AAG. For the first time, we reproduce a westward propagation of oceanic properties and species originating east of the Tasman Gateway upon its early opening, leading to the introduction of endemic dinocyst species originating in the SW Pacific into the southern margin of the AAG but not the northern margin. This is consistent with our finding that the westward current is restricted to the southern margin of the AAG. Also consistent with micropalaeontological observations, no low-latitude-derived species (released west of Australia in our simulations) could be routed via the TG to appear in the SW Pacific Ocean upon its opening.

Importantly, our model results indicate that the waters of the DP, or upstream or downstream areas close to it, are likely to have been obstructed by large-scale geostrophic flow during the early Eocene, as the passive tracer experiments in a scenario with a closed DP are much more consistent with microfossil evidence compared to a scenario with an open DP. , , , , , and infer a progressive opening of the Drake Passage, through continental extensional tectonics and oceanic spreading , after about 50 Ma from the analysis of seafloor magnetic anomalies in the Scotia Sea and adjoining oceanic areas. This timing suggests that the DP may have been sufficiently obstructed to prevent an ACC.

Our results, and those of , indicate a later opening of the DP. Alternative to the DP closure, we raise the question (for future research) of whether a similar effect could have been achieved from severe obstructions to a wide and deep flow through the DP and nearby areas at similar latitudes, a possibility not explored in our model experiments. For instance, model results by indicate that, regardless of the state of the DP and TG, a coherent ACC was not possible during, for instance, the Oligocene (a period much later than that under study here) due to the Australasian palaeogeography, although no inferences are made for the Eocene. Future work on these obstructions is therefore important. Finally, our numerical study is not consistent with the idea that such an oceanographic change can cause a significant and uniform Antarctic cooling. Nonetheless, our result of a westward current made possible by a restriction of the ACC away from the TG region provides an ocean dynamics background to the finding of and other studies that point to this current.