Southern Ocean biogenic blooms freezing-in Oligocene colder climates.

Crossing a key atmospheric CO2 threshold triggered a fundamental global climate reorganisation ~34 million years ago (Ma) establishing permanent Antarctic ice sheets. Curiously, a more dramatic CO2 decline (~800-400 ppm by the Early Oligocene(~27 Ma)), postdates initial ice sheet expansion but the mechanisms driving this later, rapid drop in atmospheric carbon during the early Oligocene remains elusive and controversial. Here we use marine seismic reflection and borehole data to reveal an unprecedented accumulation of early Oligocene strata (up to 2.2 km thick over 1500 × 500 km) with a major biogenic component in the Australian Southern Ocean. High-resolution ocean simulations demonstrate that a tectonically-driven, one-off reorganisation of ocean currents, caused a unique period where current instability coincided with high nutrient input from the Antarctic continent. This unrepeated and short-lived environment favoured extreme bioproductivity and enhanced sediment burial. The size and rapid accumulation of this sediment package potentially holds ~1.067 × 1015 kg of the 'missing carbon' sequestered during the decline from an Eocene high CO2-world to a mid-Oligocene medium CO2-world, highlighting the exceptional role of the Southern Ocean in modulating long-term climate.

Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling.

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2-4 °C, up to 5 °C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.

Fingerprinting Proterozoic Bedrock in Interior Wilkes Land, East Antarctica.

Wilkes Land in East Antarctica remains one of the last geological exploration frontiers on Earth. Hidden beneath kilometres of ice, its bedrock preserves a poorly-understood tectonic history that mirrors that of southern Australia and holds critical insights into past supercontinent cycles. Here, we use new and recently published Australian and Antarctic geological and geophysical data to present a novel interpretation of the age and character of crystalline basement and sedimentary cover of interior Wilkes Land. We combine new zircon U-Pb and Hf isotopic data from remote Antarctic outcrops with aeromagnetic data observations from the conjugate Australian-Antarctic margins to identify two new Antarctic Mesoproterozoic basement provinces corresponding to the continuation of the Coompana and Madura provinces of southern Australia into Wilkes Land. Using both detrital zircon U-Pb-Hf and authigenic monazite U-Th-Pb isotopic data from glacial erratic sandstone samples, we identify the presence of Neoproterozoic sedimentary rocks covering Mesoproterozoic basement. Together, these new geological insights into the ice-covered bedrock of Wilkes Land substantially improve correlations of Antarctic and Australian geological elements and provide key constraints on the tectonic architecture of this sector of the East Antarctic Shield and its role in supercontinent reconstructions.

A cryptic Gondwana-forming orogen located in Antarctica.

The most poorly exposed and least understood Gondwana-forming orogen lies largely hidden beneath ice in East Antarctica. Called the Kuunga orogen, its interpolation between scattered outcrops is speculative with differing and often contradictory trends proposed, and no consensus on the location of any sutures. While some discount a suture altogether, paleomagnetic data from Indo-Antarctica and Australo-Antarctica do require 3000-5000 km relative displacement during Ediacaran-Cambrian Gondwana amalgamation, suggesting that the Kuunga orogen sutured provinces of broadly Indian versus Australian affinity. Here we use compiled data from detrital zircons offshore of East Antarctica that fingerprint two coastal subglacial basement provinces between 60 and 130°E, one of Indian affinity with dominant ca. 980-900 Ma ages (Indo-Antarctica) and one of Australian affinity with dominant ca. 1190-1140 and ca. 1560 Ma ages (Australo-Antarctica). We combine this offshore compilation with existing and new onshore U-Pb geochronology and previous geophysical interpretations to delimit the Indo-Australo-Antarctic boundary at a prominent geophysical lineament which intersects the coast east of Mirny at ~94°E.

Onset of Antarctic Circumpolar Current 30 million years ago as Tasmanian Gateway aligned with westerlies.

Earth's mightiest ocean current, the Antarctic Circumpolar Current (ACC), regulates the exchange of heat and carbon between the ocean and the atmosphere, and influences vertical ocean structure, deep-water production and the global distribution of nutrients and chemical tracers. The eastward-flowing ACC occupies a unique circumglobal pathway in the Southern Ocean that was enabled by the tectonic opening of key oceanic gateways during the break-up of Gondwana (for example, by the opening of the Tasmanian Gateway, which connects the Indian and Pacific oceans). Although the ACC is a key component of Earth's present and past climate system, the timing of the appearance of diagnostic features of the ACC (for example, low zonal gradients in water-mass tracer fields) is poorly known and represents a fundamental gap in our understanding of Earth history. Here we show, using geophysically determined positions of continent-ocean boundaries, that the deep Tasmanian Gateway opened 33.5 ± 1.5 million years ago (the errors indicate uncertainty in the boundary positions). Following this opening, sediments from Indian and Pacific cores recorded Pacific-type neodymium isotope ratios, revealing deep westward flow equivalent to the present-day Antarctic Slope Current. We observe onset of the ACC at around 30 million years ago, when Southern Ocean neodymium isotopes record a permanent shift to modern Indian-Atlantic ratios. Our reconstructions of ocean circulation show that massive reorganization and homogenization of Southern Ocean water masses coincided with migration of the northern margin of the Tasmanian Gateway into the mid-latitude westerly wind band, which we reconstruct at 64° S, near to the northern margin. Onset of the ACC about 30 million years ago coincided with major changes in global ocean circulation and probably contributed to the lower atmospheric carbon dioxide levels that appear after this time.

How supercontinents and superoceans affect seafloor roughness.

Seafloor roughness varies considerably across the world's ocean basins and is fundamental to controlling the circulation and mixing of heat in the ocean and dissipating eddy kinetic energy. Models derived from analyses of active mid-ocean ridges suggest that ocean floor roughness depends on seafloor spreading rates, with rougher basement forming below a half-spreading rate threshold of 30-35 mm yr(-1) (refs 4, 5), as well as on the local interaction of mid-ocean ridges with mantle plumes or cold-spots. Here we present a global analysis of marine gravity-derived roughness, sediment thickness, seafloor isochrons and palaeo-spreading rates of Cretaceous to Cenozoic ridge flanks. Our analysis reveals that, after eliminating effects related to spreading rate and sediment thickness, residual roughness anomalies of 5-20 mGal remain over large swaths of ocean floor. We found that the roughness as a function of palaeo-spreading directions and isochron orientations indicates that most of the observed excess roughness is not related to spreading obliquity, as this effect is restricted to relatively rare occurrences of very high obliquity angles (>45 degrees ). Cretaceous Atlantic ocean floor, formed over mantle previously overlain by the Pangaea supercontinent, displays anomalously low roughness away from mantle plumes and is independent of spreading rates. We attribute this observation to a sub-Pangaean supercontinental mantle temperature anomaly leading to slightly thicker than normal Late Jurassic and Cretaceous Atlantic crust, reduced brittle fracturing and smoother basement relief. In contrast, ocean crust formed above Pacific superswells, probably reflecting metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, is not associated with basement roughness anomalies. These results highlight a fundamental difference in the nature of large-scale mantle upwellings below supercontinents and superoceans, and their impact on oceanic crustal accretion.