This proposal seeks to understand how a prodigious 1,250 km runout submarine sediment avalanche (turbidity current) was triggered on 14th January 2020, by the largest flood in 50 years along the Congo River. This submarine flow broke two seabed telecommunication cables that underpin data traffic to West Africa causing the internet to slow from Nigeria to South Africa. These submarine cables had not previously broken in the last 20 years. This flow also caused a series of oceanographic moorings to surface, placed along Congo Submarine Canyon by a NERC project (NE/R001952). Cable breaks and surfaced moorings show that this remarkable flow ran out for over 1,200 km, as measured along the canyon axis. Moreover, the flow continuously self-accelerated, such that it reached front speeds of >8 m/s, some 1,150 to 1,250 km from its source at the mouth of the Congo River. This is the longest runout turbidity current yet monitored in action, and the only monitored flow to continuously self-accelerate for over a thousand kilometres. It is important to understand how such powerful and very long runout turbidity currents are triggered, especially for hazards to strategic seabed cables, including cable routes that are planned for 2020-21 off West Africa. The January 14-16th submarine flow is not associated with an earthquake, and it occurred during a period of low wave heights. However, it does coincide with an extreme flood of 80,000 m3s-1 observed in December 2019 along the Congo River. It is thus also important to determine how the frequency of submarine flows will be effected by future climate and hydrological changes in the Congo Basin. Here we seek to understand how this exceptional river flood triggered a thousand kilometre submarine flow, by conducting a detailed survey of the Congo River mouth. We will use the geomorphology of that river-to-submarine-canyon transition to understand how the offshore flow was triggered by the river flood, for example by mapping landslide scars, or testing a hypothesis that river bedload was driven over a single steep avalanche face. This is an urgency grant because evidence of how the Jan 2020 flow was triggered (e.g. seabed failure scarps) will be buried or wiped-out by the next peak discharge of the Congo River in Oct 2020. There are extremely few direct measurements of the most powerful turbidity currents that run out for hundreds to thousands kilometres to the deep ocean, and the few measurements available previously produced step changes in understanding. Indeed, there has only been one previously directly-measured turbidity current on this scale, which is the Grand Banks event in 1929 that broke all ~20 cables across the N. Atlantic. The Grand Banks event ran out for over 800km, but decelerated from 19 m/s to 3 m/s, rather than continuously accelerating as in the Jan 2020 event. Moreover, the Jan 2020 event already has much more detailed measurements from the timing of offshore moorings, with further data to come via recovery of these moorings and 12 OBS (with hydrophones and geophones) on a NERC cruise. This Jan 2020 event is thus a rare and extremely valuable opportunity to understand how far large-scale flows operate, linked to exceptional river floods with much longer (50-100 year) recurrence intervals. The main gap in our understanding of the Jan 2020 event is what happened at the river mouth, and this is key for predicting flow frequency and links to climate change. The geomorphology of the river to canyon-head transition is currently unknown. For example, UKHO bathymetric charts mainly use data collected in the 1890s. Here we will use swath multibeam echosounder systems to survey the river to canyon transition at much higher resolution and in three dimensions, thereby documenting its geomorphology in unprecedented detail. Past work shows how a single multibeam bathymetric survey can produce major insights into turbidity currents triggering at river mouths.

Changes in Ocean Heat Transport (OHT) have been associated with changes in climate over the oceans and continents. Signals such as the Atlantic Multidecadal Oscillation (an oscillation of the averaged surface temperature in the North Atlantic ocean), which drives anomalously dry/wet conditions over European, are thought to be primarily generated by basin wide fluctuations in the Atlantic Overturning Circulation and associated OHT. At the core of the link between OHT and anomalous climate conditions is a chain of events: changes in OHT result in changes in the Ocean Heat Content (OHC) over a broad range of depths, which in turn modifies the Sea Surface Temperature (SST). It is ultimately through this SST modification that the OHT changes can influence the atmospheric circulation. It has to be emphasized, however, that the OHT-OHC-SST chain is not systematic. In fact, it is likely that a large fraction of the OHT variability does only result in weak SST changes and thus little climate impact. Our current understanding of the link between OHT changes and climate variability is primarily based on statistical analyses of climate model simulations. Our proposal is that we need to understand better the mechanisms behind this link to be able to understand what the limited available observations can tell us about the ocean's impact on climate in real world, and whether or not our climate models are good approximations of the latter. Specifically, here are a few key questions that we wish to address: What makes an OHT change climatically important and potentially relevant to society? Is the OHT-OHC-SST chain properly represented in ocean and climate models? What are the implications for predictions of future climate change on the 1 to 10 year timescale of most relevance to many environmental policies of governments and businesses? To address this, we propose to explore the ocean interior dynamics that constrains the OHT-OHC-SST chain and its representation in climate prediction systems. An important innovation of the project is a methodology to evaluate the potential climate impact of OHT changes, to be used to interpret decade-long observations of the Meridional Overturning Circulation and associated OHT changes by the UK funded RAPID project and its successors. We will combine an analysis of subsurface ocean observations (Argo dataset) and ocean reanalysis products, a modeling approach with idealized models (to study physical processes) and realistic configurations (to link most readily with climate models and observations), and an evaluation of a global climate model and decadal prediction products. Besides contributing to the success of UK-funded projects (like RAPID mentioned above, but also the upcoming OSNAP), this research will also benefit the UK community in terms of evaluation and potential improvements of the UK climate models and prediction systems (UKESM1/NEMO, Met Office DePreSys).

Our overall aim is to make fundamental step-changes in understanding of seafloor processes and hazards by developing and demonstrating novel sensor systems, which can form widespread and long-term listening networks. These low-cost and energy-efficient sensors comprise hydrophones (acoustic noise in water column) and geophones (ground shaking). Data will be returned via pop-up floats and satellite links, as has been pioneered by the highly successful Argo Project for water-column profile. This type of low-cost network could have unusually widespread applications for warning against threats to valuable seabed infrastructure, monitoring leaks from CCS facilities or gas pipelines, or for tsunami warning systems. Here we aim to answer fundamental questions about how submarine mass-flows (turbidity currents and landslides) are triggered, and then behave. These hazardous and often powerful (2-20 m/s) submarine events form the largest sediment accumulations, deepest canyons, and longest channel systems on our planet. Turbidity currents can runout for hundreds to thousands of kilometres, to break seabed cable networks that carry >95% of global data traffic, including the internet and financial markets, or strategic oil and gas pipelines. These flows play a globally important role in organic carbon and nutrient transfer to the deep ocean, and geochemical cycles; whilst their deposits host valuable oil and gas reserves worldwide. Submarine mass flows are notoriously difficult to measure in action, and there are very few measurements compared to their subaerial cousins. This means there are fundamental gaps in basic understanding about how submarine mass flows are triggered, their frequency and runout, and how they behave. Recent monitoring has made advances using power-hungry (active source) sensors, such as acoustic Doppler current profilers (ADCPs). But active-source sensors have major disadvantages, and cannot be deployed globally. They can only measure for short periods, are located on moorings anchored inside these powerful flows (which often carry the expensive mooring and sensors away), and they need multiple periods of expensive research vessels to be both deployed and recovered. We will therefore design, build and test passive sensors that can be deployed over widespread areas at far lower cost. These novel sensors will record mass-flow timing and triggers; and changes in front speed (from transit times), and flow power (via strength of acoustic or vibration signal). We will first determine how submarine mass flows are best recorded by hydrophones and geophones, and how that record varies with flow speed and type, or distance to sensor. Our preliminary work at three sites already shows that hydrophone and geophones do record mass-flows. Here we will determine the best way to capture that mass-flow signal, and to distinguish it from other processes. This work will form the basis for designing a new generation of low-cost (< £5k) smart sensors that return data without expensive surface vessels; via pop-up floats and satellite links. Advances in technology make this project timely, as they allow on-board data processing by smart hydrophones or geophones to reduce data volumes, which can be triggered to record for short periods at much higher frequency. We will field-test the new smart sensors, and thus demonstrate how they can answer major science questions. We seek to understand what triggers submarine flows, and how this initial trigger mechanism affects flow behaviour. In particular, how are submarine flows linked to hazardous river floods, storms or earthquakes, and hence how do they record those hazards? Do submarine flows in diverse settings show consistent modes of behaviour, and if not, what causes those differences? To do this, we will deploy these new sensors along the Congo Canyon (dilute river, passive margin, no cyclones) offshore Taiwan.

The balance between the production of organic carbon during phytoplankton photosynthesis and its consumption by bacterial, zooplankton and phytoplankton respiration determines how much carbon can be stored in the ocean and how much remains in the atmosphere as carbon dioxide. The amount of organic carbon stored in the ocean is as large as the amount of carbon dioxide in the atmosphere, and so is a key component in two global carbon cycle calculations needed to avoid a global temperature rise of more than 1.5 degrees C: the calculation of the technological and societal efforts required to achieve net zero carbon emissions and the calculation of the efficiency of ocean-based engineering approaches to directly remove carbon dioxide from the atmosphere. Yet, despite its vital role, our ability to predict how ocean carbon storage will change in the future is severely limited by our lack of understanding of how plankton respiration varies in time and space, how it is apportioned between bacteria and zooplankton and how sensitive it is to climate change-induced shifts in environmental conditions such as increasing temperature and decreasing oxygen. This woeful situation is due to the significant challenge of measuring respiration in the deep-sea and the uncoordinated way in which these respiration data are archived. This project will directly address these two problems. We will take advantage of our leadership and participation in an international programme which deploys thousands of oceanic floats measuring temperature, oxygen and organic carbon in the global ocean, in an international team of experts focused on quantifying deep-sea microbial respiration, and our experience of collating international datasets, to produce an unprecedented dataset of bacterial and zooplankton respiration. We will derive estimates of respiration based on data from floats, so that together with estimates derived from recently developed methods including underwater gliders, the new database will include respiration measurements calculated over a range of time and space scales. Crucially, respiration rates will be coupled with concurrent environmental data such as temperature, oxygen and organic carbon. This dataset will enable us to quantify the seasonal and spatial variability of respiration and derive equations describing how respiration changes with the proportion of bacteria and zooplankton present and with the chemical and physical properties of the water. These equations can then be used in climate models to better predict how respiration and therefore ocean carbon storage will change in the future with climate-change induced shifts in temperature, oxygen, organic carbon and plankton community. We will take part in a hybrid hands-on and online international training course on observations and models of deep-water respiration targeted to early career researchers from developing and developed countries to showcase the useability of the respiration database and the global array of oceanic floats. We will also prepare Science Festival exhibits on observing life in the deep ocean for schoolchildren. The deliverables of the project - a unique global open-access database of respiration measurements, new equations describing the sensitivity of respiration to changing temperature and oxygen suitable for climate models and online training materials for early career researchers - are of benefit to scientists who aim to predict how a changing climate will affect the storage of carbon in the ocean, educators who train the next generation of ocean scientists and practitioners, policy makers who need to quantify nationally determined contributions to actions limiting global warming, and scientists, engineers, lawyers, governing bodies and commercial companies designing, evaluating and implementing ocean-based carbon dioxide removal technologies.

Seafloor flows called turbidity currents form the largest sediment accumulations on Earth (submarine fans). They flush globally significant amounts of sediment, organic carbon, nutrients and fresher-water into the deep ocean, and affect its oxygen levels. Only rivers transport comparable volumes of sediment across such large expanses of our planet, although a single turbidity current can transport more sediment than the combined annual flux from all of the World's rivers combined. Here we will make a step change in understanding of turbidity currents, and their wider impacts, by making the first detailed measurements of turbidity current that runout into the deep (2-5 km) ocean. Such direct monitoring of turbidity currents that form major submarine fan systems has been a 'holy grail' for sedimentology, oceanography, and marine geology for decades. It would be broadly comparable to the first detailed measurements of major river systems or other first-order processes for moving sediment across the planet. This project is especially timely due to recent successful tests of new methods and technology for measuring turbidity currents in shallower (less than 2 km) water, which can now be used for deep-water, large-scale submarine fan settings. We choose to study the Congo Canyon off West Africa due to an exceptional set of initial measurements collected in 2010 and 2013. These measurements at 2 km water depth are the deepest yet for turbidity currents. Surprisingly, they showed that individual turbidity currents lasted for almost a week, and occupied 20% of the time. This was surprising because all previously measured oceanic turbidity currents lasted for just a few hours or minutes, and occurred for < 0.1% of the total time. It suggests that turbidity currents that runout into the deep ocean to form major submarine fans may differ from their shallow water cousins in key regards. These preliminary measurements show how monitoring is feasible for the Congo Canyon. They help us to design a project that will now show how these flows runout into the deeper ocean. We will deploy 8 moorings along the Congo Canyon at water depths of 2 to 5 km that will measure frequency, duration, and run-out distance of multiple flows; together with their velocity, turbulence and sediment concentration structures; as well as changes in water, sediment and organic carbon discharge. Our overall aim is to show how deep-sea turbidity current behave using the first direct measurements, and understand causes and wider implications of this behaviour. We will answer the following key questions about flow behaviour: (1) What controls flow duration, and does flow stretching cause near-continuous canyon flushing? We will test a new hypothesis that predicts flows will stretch dramatically as a 'hot spot' of faster moving fluid runs away from the rest of the event, thereby producing near-continuous flushing of submarine canyons. (2) What controls runout and whether flows become more powerful? We will test whether turbidity currents tend towards one of two distinct modes of behaviour, in which they erode and accelerate (a process termed ignition), or deposit sediment and dissipate. (3) How is flow behaviour and character recorded by deposits? This is important because deposits are the only record of most turbidity currents. (4) How does flow behaviour affect the transfer and burial of terrestrial organic carbon in the deep-sea? It was proposed recently that burial of terrestrial organic carbon in the deep sea is very efficient, and an important control on long-term atmospheric CO2 levels. This hypothesis implies little fractionation of terrestrial organic carbon occurs during submarine transport. Composition of organic carbon buried by the offshore flows is similar to that supplied by the river. We will test this hypothesis by analysing amounts and types of organic carbon along the offshore pathway in both flows and deposits.