Convection clashes: Plume splitting beneath eastern Australia

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Geosciences

Abstract

Convection of the Earth's mantle is a fundamental dynamic process that profoundly influences the surface of our planet, affecting processes as diverse as plate tectonics, long-term sea level change, and climate. Convection requires a balance between material sinking deep into the mantle and rising towards the surface. Whilst we know that downwelling material is dominated by subducting slabs that eventually sink thousands of km into the mantle, the locations, durations, and dynamics of the required deep upwelling material are much more ambiguous. The best-known surface indication of such upwelling is intraplate volcanism (volcanism located far from plate boundaries), classically associated with plumes (or 'hotspots') of hot material rising from the lower mantle. Particularly voluminous examples of intraplate volcanism occur when the broad heads of these plumes reach the surface to produce large igneous provinces (LIPs). These LIPs affect the world's atmosphere via the release of massive amounts of gases like sulphuric acid, changing the climate with damaging effects on the ocean, atmosphere, and biology (i.e., mass extinctions). The trails of hotspot volcanoes that come after the LIP have also proved a powerful tool in discovering the past motions of tectonic plates. For these reasons, understanding the origins and evolution of intraplate volcanism is an important part of Earth science.

The classic example of hotspot intraplate volcanism is Hawaii on the Pacific plate: a series of volcanic islands and submerged undersea mountains ('seamounts') that stretches away to the northwest, becoming progressively older the further they are from the actively erupting island of Hawaii. However, intraplate volcanism on Earth is very diverse. Many localities do not fit the classic model of a hot plume rising from the deep mantle, but instead appear to have been caused by processes in the upper mantle or have a mix of deep and shallow characteristics.

For this project, the seas off Eastern Australia are an ideal region for studying the processes involved in the formation of intraplate volcanism. This region is crossed by not one, but three sub-parallel chains of intraplate volcanoes, which erupted simultaneously between 35 and 6 million years ago. These volcanoes are up to 5 km high and 100 km across, and are almost entirely submerged beneath the ocean. The long life and exceptional age progression of the chains are strong indicators of a classic deep upwelling source, but the configuration of the three chains challenges our understanding of this fundamental driving force of our planet. Neither three closely spaced plumes (~500 km apart) nor an upwelling sheet fit well with our understanding of the underlying physics: they are either unstable or are not observed in models of Earth's mantle convection. Instead, these observations suggest a deep upwelling splitting as it nears the surface, perhaps due to obstacles in the mantle, or eddies in the mantle convection.

This proposal builds on a collaboration with Australia, who has already funded a 28 day voyage (worth ~£1.8 million) to collect rock samples and carry out geophysical studies. The voyage will target the two marine chains, as well as the Louisiade Plateau (a 100,000 square km area of raised seafloor that could be a LIP) north of the Tasmantids. We will study these volcanoes using a multi-faceted approach combining chronology (to determine their ages), chemistry (to determine what type of mantle melted), and geodynamic modelling (to examine the processes in the mantle that formed the volcanoes). The geodynamic models will also be applied to the Canary and Comoros Islands (west and east of Africa, respectively) to examine the mechanisms behind intraplate volcanoes elsewhere on the planet. This project will give us significant insight into the formation of enigmatic intraplate volcanism and how material flowing from deep in the Earth's mantle interacts with obstacles as it rises.

Planned Impact

Public Engagement Impact
Our proposal has strong potential impact via outreach to schools and the general public. Marine science has a powerful natural draw in public interest: it is adventure and exploration in a sense that is rare in the modern, ultra-connected world. In an era where we travel more widely than ever before and can instantly share pictures, video, and conversation with people around the world, the ocean remains unknown, a vastness and mystery familiar to anyone who has been to the seaside. Mapping the usually unseen landscape of the seafloor and recovering rock and biological specimens from deep underwater - which we will disseminate via press releases, a public website, and videoconferences - provides an excellent opportunity to engage with the public's natural curiosity about the ocean and ocean exploration.

Having two educators sail on the 2019 voyage, and our planned outreach activities at UK schools and museums, will provide opportunities for all ages to interact with active scientists, technicians, and ship's crew and be exposed to a part of science not often taught in school. This contributes to both UK's Industrial Strategy and the Scottish Government's STEM Strategy on inspiring young people and adults to study STEM. It also contributes to the Australian government's 'Vision for a Science Nation', including closer links between schools and STEM research/professionals, and to the 'Inspiring Australia' initiative, which highlighted marine science as a key area for increased communication and engagement.

Marine Reserve Management
The bathymetry and geophysical mapping programme focuses on seamounts, a key habitat for open-ocean ecosystems, and may help identify areas of important conservation value. This is particularly significant for the internationally important Coral Sea Commonwealth Marine Reserve and will contribute to the evidence base for monitoring its geology, geomorphology, and biodiversity. The study area is almost completely unmapped, so this work will provide crucial new high-resolution bathymetry data in the Coral Sea. A small portion of the survey also falls in the Great Barrier Reef Marine Park, a World Heritage Site, for which we will also collect high-resolution bathymetric data. Our results will be disseminated to the Coral Sea and Barrier Reef agencies via our Australian project partners. This mapping will also contribute to commitments under the UN Convention on the Law of the Sea on 'the conservation and sustainable use of marine biological diversity of areas beyond national jurisdiction (BBNJ)', as well as areas under the jurisdiction of Australia and neighbouring countries. The UN is currently strengthening its commitment to preserving BBNJ, working toward a treaty to protect marine biodiversity (Preparatory Committee phase complete, Intergovernmental Conference beginning Sept 2018).

Marine Safety and Charting
Limited availability of high-resolution bathymetric mapping means that our data can also contribute to valuable information for maritime regulations and policies in areas such as updating charts, designating shipping routes, and planning search and rescue operations in remote areas. A representative from the Australian Hydrographic Office will sail on the 2019 voyage, providing that agency with direct access to the data.

Resource/Economic Impact
Hydrocarbon Maturation and Basin Evolution
Identification of a plume head impacting the lithosphere along Australia's northeastern margin could provide a critical new constraint on thermal maturation and basin evolution of central and east Australian onshore sedimentary basins, such as the Cooper, Eromanga, and East Australian Coal-Seam Gas Area. Data will be disseminated to industry via the Australian Research Council's 'Industry Transformation Research Hub' on basin genesis via our Australian Project Partners Seton and Williams.

Publications

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