Submarine turbidity currents are arguably the volumetrically most important process for moving sediment across our planet. They form the largest sediment accumulations (submarine fans) on earth, and single flows can transport ten times the annual flux from all of the world's rivers. However, the most remarkable feature of turbidity currents is how few direct measurements are available from these flows, as they are notoriously difficult to monitor in action. This is a stark contrast to other major sediment transport processes, such as rivers for which we have many thousands of direct measurements. Powerful long run-out turbidity currents are especially difficult to monitor, yet it is these flows that build submarine fans. Such flows are important because they break sea-floor cables that carry > 95% of global data traffic, including internet and financial markets that underpin daily lives. The velocity of turbidity currents that reach beyond the continental slope had previously been measured in just five locations, primarily from cable breaks that only record averaged front velocities. Their sediment concentration had never been measured directly. This globally important sediment transport process is therefore poorly understood, and laboratory or numerical models for such flows are poorly validated. This PhD student will analyse a remarkable dataset comprising the first synchronous velocity and concentration profiles for turbidity currents beyond the continental slope, collected at a cost of > $1M by CASE partner Chevron and co-workers in the Congo Canyon (from 2009-2013). This is the first time that high temporal resolution (>1/min) synchronous profiles of both velocity and concentration have been measured for turbidity currents beyond the continental slope. They are also the fastest (2.5 m/s) turbidity currents yet measured by instruments. The data were collected for a major oil and gas pipeline that will need to cross the Congo Canyon. This is a challenging project as previous cable breaks show the canyon is regularly swept by powerful flows. The data comes from moorings with downward pointing Acoustic Doppler Velocity Profilers (ADCPs) that measure velocity and acoustic backscatter. Backscatter is partly dependent on grain size, but also records changes in sediment concentration. Initial results were surprising for two reasons. First, flows had surprising durations of several days, with average speeds of ~1 m/s. Interestingly, it was observed that the larger flows always had a similar duration of ~6 days. This seems to indicate the establishment of an equilibrium flow configuration over the first 150 km of the canyon. Several hypothesis have been put forward to explain this behaviour, however, none of them have yet been validated. Second, the measured turbulence intensity decreased as flow speeds increased, this counter-intuitive relation suggests damping of turbulence by elevated suspended sediment concentrations. Although it has been speculated previously that turbulence damping by sediment may be of fundamental importance, such damping has never previously been documented in direct observations from full scale flows in the field. Thus, the relation between turbulence damping and sediment concentration remains to be validated in real submarine flows. A numerical model is needed to test hypotheses that equilibrium flow configurations produce multi-day flows, and to explore how turbulence dampening may affect submarine flows. Such numerical model will benefit from the well-mapped bathymetry of the Congo canyon (from Chevron and past publications by IFREMER). In this proposal we will use a state-ofthe-art fully three-dimensional numerical model that has been developed over the last eight years by project partner Complex Flow Design AS (CFD), Norway. This model is unique due to its 3D approach and its capability to introduce sediment concentration effects into its turbulence model.

Plankton in the ocean, microscopic plants (phytoplankton) and tiny animals (zooplankton) that eat the plants, are vital to marine life and to Earth's climate. They form the base of food chains that support ocean ecosystems, and remove carbon from the atmosphere and bury it in (or export it to) the ocean depths. It is currently thought that plankton are responsible for removing 6 billion tonnes of carbon from the atmosphere each year; fossil fuel burning releases about 10 billion tonnes of carbon into the atmosphere annually. Without this export of carbon in the ocean, atmospheric CO2 would be twice the current concentration. The importance of plankton to food chains and carbon export depends on the species of plankton. Larger phytoplankton are better at supporting food chains and at exporting carbon because (1) larger phytoplankton sink quicker, removing carbon away from the sea surface and contact with the atmosphere, and (2) larger phytoplankton support larger zooplankton, which are eaten by fish and which also excrete large, fast-sinking faecal pellets which quickly transfer carbon away from the atmosphere. We have discovered a new link between which types of plankton can grow and the tides flowing over a mid-ocean ridge. The ocean is layered, with warmer, less dense layers at the surface and colder, denser layers deeper in the ocean. When tidal currents flow up and down the flanks of a mid-ocean ridge, these layers are pushed up and down, causing waves on the layers called "internal tidal waves". These internal tidal waves reach up to the sun-lit upper ocean, where photosynthesis by the phytoplankton takes place. We think these waves have two important effects. (1) The waves cause mixing between the layers of ocean, bringing nutrients from deep in the ocean up to the phytoplankton; this will help extra phytoplankton growth, but crucially it is also known that extra nutrient supplies allow larger species of phytoplankton to grow. (2) The waves move the phytoplankton up and down; this provides more light to the phytoplankton, because as they are moved upward they get closer to the light at the sea surface and are able to grow more. Thus, we think that the internal tidal waves create more growth of larger plankton over a mid-ocean ridge, which means better food for marine food chains and more carbon exported away from the atmosphere. This new link may explain why ridges support such diverse ecosystems, and it also means that the ocean over ridges is far better at exporting carbon than we previously thought. We have calculated that, for the whole Atlantic Ocean, including the tidal effect of the mid-Atlantic ridge adds about 50% to current estimates of how much carbon the plankton export. This means that current understanding of the ocean's role in Earth's climate, which ignores the ridge-tide effect, significantly underestimates how much CO2 plankton remove from the atmosphere. We need to fix this because our predictions of our future climate depend on having correct descriptions of the processes that govern atmospheric CO2. We will conduct an expedition to the mid-ocean ridge in the S. Atlantic. We will measure the internal tidal waves and the upward mixing of nutrients, and the effect the waves have on light received by phytoplankton. We will measure how fast the phytoplankton and zooplankton grow in response to these waves, how the species of plankton change over the ridge, and how much carbon is exported downward over the ridge compared to the adjacent ocean basin. This will be the first time that internal tidal waves are linked to patterns of carbon export in the ocean: internal tidal waves occur wherever there are ridges or seamounts in the ocean and our results will have important global implications for our understanding of ocean food webs and Earth's climate.

Doctoral Training Partnerships: a range of postgraduate training is funded by the Research Councils. For information on current funding routes, see the common terminology at https://www.ukri.org/apply-for-funding/how-we-fund-studentships/. Training grants may be to one organisation or to a consortia of research organisations. This portal will show the lead organisation only.

The need for the UK to shift to NetZero was highlighted at COP26 in Glasgow, and there is a clear need for UK energy security. UK policy to achieving these is based on massive expansion of off-shore wind. In 2022 Crown Estate Scotland "ScotWind" auctioned 9,000 km2 of sea space in the northern North Sea, with potential to provide almost 25 GW of offshore wind. Further developments are planned elsewhere, for example, the 300 MW Gwynt Glas Offshore Wind Farm in the Celtic Sea. These developments mark a shift in off-shore wind generation, away from shallow, well mixed coastal waters to deeper, seasonally stratified shelf seas This shift offers both challenges and opportunities which this proposal will explore. Large areas of the NW European shelf undergo seasonal thermal stratification. This annual development of a thermocline, separating warm surface water from cold deep water, is fundamental to biological productivity. Spring stratification drives a bloom of growth of the microscopic phytoplankton that are the base of marine food chains. During summer the surface layer is denuded of nutrients and primary production continues in a layer inside the thermocline, where weak turbulent mixing supplies nutrients from the deeper water and mixes oxygen and organic material downward. Tidal flows generate turbulence; the strength of turbulence controls the timing of the spring bloom, mixing at the thermocline, and the timing of remixing of the water in autumn/winter. Determining the interplay between mixing and stratification is fundamental to understanding how shelf sea biological production is supported. Arrays of large, floating wind turbines are now being deployed over large areas of seasonally-stratifying seas. These structures will inject extra turbulence into the water, as tidal flows move through and past them. This extra turbulence will alter the balance between mixing and stratification: spring stratification and the bloom could occur later, biological growth inside the thermocline could be increased, and more oxygen could be supplied into the deep water. There could be significant benefits of this extra mixing, but we need to understand the whole suite of effects caused by this mixing to aid large-scale roll-out of deep-water renewable energy. eSWEETS will conduct observations at an existing floating wind farm in the NW North Sea to determine how the extra mixing generated by tides passing through the farm affect the physics, biology and chemistry of the water. We will measure the mixing of nutrients, organic material and oxygen within the farm, and track the down-stream impacts of the mixing as the water moves away from the wind farm and the phytoplankton respond to the new supply of nutrients. We will use autonomous gliders to observe the up-stream and down-stream contrasts in stratification and biology all the way through the stratified part of the year. We will use our observations to formulate the extra mixing in a computer model of the NW European shelf, so that we can then use the model to predict how planned renewable energy developments over the next decades might affect our shelf seas and how those effects might help counter some of the changes we expect in a warming climate. Stratification is so fundamental to how our seas support biological production that we will develop a new, cost-effective way of monitoring it. We will work with the renewables industry and modellers at the UK Met Office on a technique that allows temperature measurements to be made along the power cables that lie on the seabed between wind farms and the coast. Our vision is that large-scale roll-out of windfarms will lead to the ability to measure stratification across the entire shelf. This monitoring will help the industry (knowledge of operating conditions), government regulators (environment responses to climate change) and to operational scientists at the UK Met Office (constraining models for better predictions).

At 2 petagrams of carbon per year, the downward export of dissolved organic carbon (DOC) from the ocean surface represents a large fraction (>15%) of the total carbon sequestration by the biological carbon pump. The Atlantic Meridional Overturning Circulation (AMOC) is the main conduit by which DOC makes its way down to the deep ocean, sequestering carbon for centuries. This important carbon sink may be critically endangered if model predictions of future decline of the AMOC are correct. However, DOC measurements in the high-latitude North Atlantic are at present too scarce to understand how the AMOC-driven DOC carbon sink works, and how it may evolve in the future. Recent research by our team has further evidenced our limited understanding by suggesting that (1) the AMOC is impacted by small-scale processes occurring near the East Greenland margin, which are not captured by climate models; (2) these processes are responsible for the bulk of DOC sequestration; and (3) they respond in unexpected ways to climate change. The overarching goal of DOGMA is to prepare the ground for a future collaborative proposal to address this pressing problem. Specifically, DOGMA will (1) build an interdisciplinary team of world-leading experts to develop an ambitious research programme on the topic, and (2) leverage existing cruise opportunities in the Nordic Seas to perform a pilot study to reinforce and magnify our previous findings, allowing identification of critical areas of focus for the future proposal.