Threshold in North Atlantic-Arctic Ocean circulation controlled by the subsidence of the Greenland-Scotland Ridge

Climate transition at the Eocene-Oligocene influenced by bathymetric changes to the Atlantic-Arctic oceanic gateways

The results show that dynamic variations in the Earth’s interior could have played a key role in the Eocene–Oligocene climatic transition (∼33.9 Ma) and the inception of glaciations. Pulsations in the Iceland mantle plume modified the bathymetry of the Greenland–Scotland Ridge, which affected deep water formation in the North Atlantic. Our model simulations show that the changes in the Atlantic–Arctic oceanic gateways cooled the Southern Hemisphere, and later the Northern Hemisphere, paving the way for the growth of major land-based ice sheets. This supplements the current view that decreasing atmospheric CO₂ concentrations and/or changes to the Southern Ocean gateways or the Tethys Seaway dominated climate changes and the inception of glaciations at the time.

The Eocene–Oligocene Transition (∼33.9 Ma) marks the largest step transformation within the Cenozoic cooling trend and is characterized by a sudden growth of the Antarctic ice sheets, cooling of the interior ocean, and the establishment of strong meridional temperature gradients. Here we examine the climatic impact of oceanic gateway changes at the Eocene–Oligocene Transition by implementing detailed paleogeographic reconstructions with realistic paleobathymetric models for the Atlantic–Arctic basins in a state-of-the-art earth system model (the Norwegian Earth System Model [NorESM-F]). We demonstrate that the warm Eocene climate is highly sensitive to depth variations of the Greenland–Scotland Ridge and the proto–Fram Strait as they control the freshwater leakage from the Arctic to the North Atlantic. Our results, and proxy evidence, suggest that changes in these gateways controlled the ocean circulation and played a critical role in the growth of land-based ice sheets, alongside CO₂-driven global cooling. Specifically, we suggest that a shallow connection between the Arctic and North Atlantic restricted the southward flow of fresh surface waters during the Late Eocene allowing for a North Atlantic overturning circulation. Consequently, the Southern Hemisphere cooled by several degrees paving the way for the glaciation of Antarctica. Shortly after, the connection to the Arctic deepened due to weakening dynamic support from the Iceland Mantle Plume. This weakened the North Atlantic overturning and cooled the Northern Hemisphere, thereby promoting glaciations there. Our study points to a controlling role of the Northeast Atlantic gateways and decreasing atmospheric CO₂ in the onset of glaciations in both hemispheres.

Glacial episodes of a freshwater Arctic Ocean covered by a thick ice shelf

Following early hypotheses about the possible existence of Arctic ice shelves in the past^1-3, the observation of specific erosional features as deep as 1,000 metres below the current sea level confirmed the presence of a thick layer of ice on the Lomonosov Ridge in the central Arctic Ocean and elsewhere^4-6. Recent modelling studies have addressed how an ice shelf may have built up in glacial periods, covering most of the Arctic Ocean^7,8. So far, however, there is no irrefutable marine-sediment characterization of such an extensive ice shelf in the Arctic, raising doubt about the impact of glacial conditions on the Arctic Ocean. Here we provide evidence for at least two episodes during which the Arctic Ocean and the adjacent Nordic seas were not only covered by an extensive ice shelf, but also filled entirely with fresh water, causing a widespread absence of thorium-230 in marine sediments. We propose that these Arctic freshwater intervals occurred 70,000-62,000 years before present and approximately 150,000-131,000 years before present, corresponding to portions of marine isotope stages 4 and 6. Alternative interpretations of the first occurrence of the calcareous nannofossil Emiliania huxleyi in Arctic sedimentary records would suggest younger ages for the older interval. Our approach explains the unexpected minima in Arctic thorium-230 records⁹ that have led to divergent interpretations of sedimentation rates^10,11 and hampered their use for dating purposes. About nine million cubic kilometres of fresh water is required to explain our isotopic interpretation, a calculation that we support with estimates of hydrological fluxes and altered boundary conditions. A freshwater mass of this size-stored in oceans, rather than land-suggests that a revision of sea-level reconstructions based on freshwater-sensitive stable oxygen isotopes may be required, and that large masses of fresh water could be delivered to the north Atlantic Ocean on very short timescales.

Antarctic icebergs reorganize ocean circulation during Pleistocene glacials

The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today¹; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled². An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian-Atlantic Southern Ocean (0-50° E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to 'escape' into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the 'southern escape' and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the '100-kyr world', in which glacial-interglacial cycles occur at roughly 100,000-year periods.

A Warm, Stratified, and Restricted Labrador Sea Across the Middle Eocene and Its Climatic Optimum

Several studies indicate that North Atlantic Deep Water (NADW) formation might have initiated during the globally warm Eocene (56-34 Ma). However, constraints on Eocene surface ocean conditions in source regions presently conducive to deep water formation are sparse. Here we test whether ocean conditions of the middle Eocene Labrador Sea might have allowed for deep water formation by applying (organic) geochemical and palynological techniques, on sediments from Ocean Drilling Program (ODP) Site 647. We reconstruct a long-term sea surface temperature (SST) drop from ~30°C to ~27°C between 41.5 to 38.5 Ma, based on TEX86. Superimposed on this trend, we record ~2°C warming in SST associated with the Middle Eocene Climatic Optimum (MECO; ~40 Ma), which is the northernmost MECO record as yet, and another, likely regional, warming phase at ~41.1 Ma, associated with low-latitude planktic foraminifera and dinoflagellate cyst incursions. Dinoflagellate cyst assemblages together with planktonic foraminiferal stable oxygen isotope ratios overall indicate low surface water salinities and strong stratification. Benthic foraminifer stable carbon and oxygen isotope ratios differ from global deep ocean values by 1-2‰ and 2-4‰, respectively, indicating geographic basin isolation. Our multiproxy reconstructions depict a consistent picture of relatively warm and fresh but also highly variable surface ocean conditions in the middle Eocene Labrador Sea. These conditions were unlikely conducive to deep water formation. This implies either NADW did not yet form during the middle Eocene or it formed in a different source region and subsequently bypassed the southern Labrador Sea.

Arctic closure as a trigger for Atlantic overturning at the Eocene-Oligocene Transition

The Eocene-Oligocene Transition (EOT), approximately 34 Ma ago, marks a period of major global cooling and inception of the Antarctic ice sheet. Proxies of deep circulation suggest a contemporaneous onset or strengthening of the Atlantic meridional overturning circulation (AMOC). Proxy evidence of gradual salinification of the North Atlantic and tectonically driven isolation of the Arctic suggest that closing the Arctic-Atlantic gateway could have triggered the AMOC at the EOT. We demonstrate this trigger of the AMOC using a new paleoclimate model with late Eocene boundary conditions. The control simulation reproduces Eocene observations of low Arctic salinities. Subsequent closure of the Arctic-Atlantic gateway triggers the AMOC by blocking freshwater inflow from the Arctic. Salt advection feedbacks then lead to cessation of overturning in the North Pacific. These circulation changes imply major warming of the North Atlantic Ocean, and simultaneous cooling of the North Pacific, but no interhemispheric change in temperatures.

Proxies of deep circulation suggest that the onset or strengthening of the Atlantic meridional overturning circulation occurred at the Eocene-Oligocene Transition. The authors show, using a paleoclimate model of the late Eocene, that a shift from Pacific to Atlantic overturning can be triggered at this time by closing the Arctic–Atlantic gateway.

Climate- and gateway-driven cooling of Late Eocene to earliest Oligocene sea surface temperatures in the North Sea Basin

During the late Eocene, the Earth’s climate experienced several transient temperature fluctuations including the Vonhof cooling event (C16n.1n; ~35.8 Ma) hitherto known mainly from the southern oceans. Here we reconstruct sea-surface temperatures (SST) and provide δ¹⁸O and δ¹³C foraminiferal records for the late Eocene and earliest Oligocene in the North Sea Basin. Our data reveal two main perturbations: (1), an abrupt brief cooling of ~4.5 °C dated to ~35.8 Ma and synchronous with the Vonhof cooling, which thus may be a global event, and (2) a gradual nearly 10 °C temperature fall starting at 36.1 Ma and culminating near the Eocene-Oligocene transition at ~33.9 Ma. The late Priabonian temperature trend in the North Sea shows some resemblance IODP Site U1404 from the North Atlantic, offshore Newfoundland; and is in contrast to the more abrupt change observed in the deep-sea δ¹⁸O records from the southern oceans. The cooling in the North Sea is large compared to the pattern seen in the North Atlantic record. This difference may be influenced by a late Eocene closure of the warm gateways connecting the North Sea with the Atlantic and Tethys oceans.