Development of a non-cloggable subsea data logger for harsh turbidity current monitoring

Large submarine flows of sediment (sand and mud), known as turbidity currents, transfer and bury significant amounts of organic carbon and pollutants to the deep sea via submarine canyons. They are also significant geohazards, regularly breaking networks of seabed telecommunications cables that carry > 99% of global data that underpin the internet. Despite this, key parameters (notably their sediment concentration) in these flows are yet to be directly measured in real-time due to their inherently harsh environment that is unsuitable for commercial conductivity sensors. To address this issue, a subsea datalogger (SSDL) is developed with a planar conductivity sensor head that can measure the sediment concentration within dense turbidity currents. Unlike conventional sensors, the planar design of the SSDL’s sensor prevents clogging at high sediment concentrations, allowing for continuous measurements within turbidity currents. The conductivity sensor is developed with a temperature sensor which is measured using an external 16-Bit ADC which is controlled with a SAMD21 32-Bit ARM microcontroller. The SSDL measures the temperature and the conductivity of the seawater once every 4 seconds for over a year. In an initial device test, the SSDL can record a turbidity current within the Bute Inlet, Canada. It is found that the seawater’s conductivity increases with salinity concentration and decreases with sediment concentration. The SSDL developed here can thus be used for both conventional subsea datalogging applications and high turbidity current applications

The submarine Congo Canyon as a conduit for microplastics to the deep sea

The increasing plastic pollution of the world’s oceans represents a serious threat to marine ecosystems and has become a well-known topic garnering growing public attention. The global input of plastic waste into the oceans is estimated to be approximately 10 million tons per year and predicted to rise by one order of magnitude by 2025. More than 90% of the plastic that enters the oceans is thought to end up on the seafloor, and seafloor sediment samples show that plastics are concentrated in confined morphologies and sedimentary environments such as submarine canyons. These canyons are occasionally flushed by powerful gravity-driven sediment flows called turbidity currents, which transport vast volumes of sediment to the deep sea and deposit sediment in deep-sea fans. As such, turbidity currents may also transport plastics present in the canyon and bury plastics in deep-sea fans. These fans may therefore act as sinks for seafloor plastics. Here we present a comprehensive dataset showing the spatial distribution of microplastics in seafloor sediments from the Congo Canyon, offshore West Africa. Multicores taken from 16 locations along the canyon, sampled different sedimentary sub-environments including the canyon thalweg, canyon terraces, and distal lobe. Microplastics were extracted from the sediments by density separation and the polymer type, size, and shape of all individual microplastic particles were analysed using laser-direct infrared-spectroscopy (LDIR). Microplastic number concentrations in the sediments of the distal lobe are significantly higher than in the canyon, indicating that the Congo Canyon system is a highly efficient conduit for microplastic transport to the deep sea. Moreover, microplastic concentrations of >20,000 particles per kg of dry sediment were recorded in the lobe, which represent some of the highest ever recorded microplastic number concentrations in seafloor sediments. This shows that deep-sea fans can serve as hotspots and potential terminal sinks for seafloor microplastics

Permeability heterogeneity of sandstone intrusion fluid-escape systems, Panoche Hills, California: Implications for sedimentary basins globally

Natural surface gas seeps provide a significant input of greenhouse gas emissions into the Earth’s atmosphere and hydrosphere. The gas flux is controlled by the properties of underlying fluid-escape conduits, which are present within sedimentary basins globally. These conduits permit pressure-driven fluid flow, hydraulically connecting deeper strata with the Earth’s surface; however they can only be fully resolved at sub-seismic scale. Here, a novel minus-cement-and-matrix permeability method using 3D X-ray micro-CT imaging enables the improved petrophysical linkage of outcrop and sub-surface data. The methodology is applied to the largest known outcrop of an inactive fluid-escape system, the Panoche Giant Intrusion Complex in Central California, where samples were collected along transects of the 600-800 m stratigraphic depth range to constrain porosity and permeability spatial heterogeneity. The presence of silica cement and clay matrix within the intergranular pores of sand intrusions are the primary control of porosity (17-27 %) and permeability (≤ 1 to ~500 mD) spatial heterogeneity within the outcrop analogue system. Following the digital removal of clay matrix and silica (opal-CT and quartz) cement derived from the mudstone host strata, the sand intrusions have porosity-permeability ranges of ~30-40 % and 103-104 mD. These calculations are closely comparable to active sub-surface systems in sedimentary basins. Field observations revealed at decreasing depth, the connected sand intrusion network reduces in thickness and becomes carbonate cemented, terminating at carbonate mounds formed from methane escape at the seafloor. A new conceptual model integrates the pore-scale calculations and field-scale observations to highlight the key processes that control sand intrusion permeability, spatially and temporally. The study demonstrates the control of matrix and cement addition on the physical properties of fluid-escape conduits, which has significance for hydrocarbon reservoir characterisation and modelling, as well as subsurface CO2 and energy storage containment assessment

Permeability heterogeneity of sandstone intrusion fluid‐escape systems, Panoche Hills, California: Implications for sedimentary basins globally

Natural surface gas seeps provide a significant input of greenhouse gas emissions into the Earth’s atmosphere and hydrosphere. The gas flux is controlled by the properties of underlying fluid-escape conduits, which are present within sedimentary basins globally. These conduits permit pressure-driven fluid flow, hydraulically connecting deeper strata with the Earth’s surface; however they can only be fully resolved at sub-seismic scale. Here, a novel ‘minus cement and matrix permeability’ method using three-dimensional X-ray micro-computed tomography imaging enables the improved petrophysical linkage of outcrop and sub-surface data. The methodology is applied to the largest known outcrop of an inactive fluid-escape system, the Panoche Giant Intrusion Complex in Central California, where samples were collected along transects of the 600 to 800 m stratigraphic depth range to constrain porosity and permeability spatial heterogeneity. The presence of silica cement and clay matrix within the intergranular pores of sand intrusions are the primary control of porosity (17 to 27%) and permeability (≤1 to ca 500 mD) spatial heterogeneity within the outcrop analogue system. Following the digital removal of clay matrix and silica (opal-CT and quartz) cement derived from the mudstone host strata, the sand intrusions have porosity−permeability ranges of ca 30 to 40% and 103 to 104 mD. These calculations are closely comparable to active sub-surface systems in sedimentary basins. Field observations revealed that, at decreasing depth, the connected sand intrusion network reduces in thickness and becomes carbonate cemented, terminating at carbonate mounds formed from methane escape at the seafloor. A new conceptual model integrates the pore-scale calculations and field-scale observations to highlight the key processes that control sand intrusion permeability, spatially and temporally. The study demonstrates the control of matrix and cement addition on the physical properties of fluid-escape conduits, which has significance for hydrocarbon reservoir characterization and modelling, as well as subsurface CO2 and energy storage containment assessment

Seismic and Acoustic Monitoring of Submarine Landslides: Ongoing Challenges, Recent Successes and Future Opportunities

Submarine landslides pose a hazard to coastal communities due to the tsunamis they can generate, and can damage critical seafloor infrastructure, such as the network of cables that underpin global data transfer and communications. These mass movements can be orders of magnitude larger than their onshore equivalents and are found on all of the world’s continental margins; from coastal zones to hadal trenches. Despite their prevalence, and importance to society, offshore monitoring studies have been limited by the largely unpredictable occurrence of submarine landslide and the need to cover large regions of extensive continental margins. Recent subsea monitoring has provided new insights into the preconditioning and run-out of submarine landslides using active geophysical techniques, but these tools only measure a very small spatial footprint, and are power and memory intensive, thus limiting long duration monitoring campaigns. Most landslide events therefore remain entirely unrecorded. Here we first show how passive acoustic and seismologic techniques can record acoustic emissions and ground motions created by terrestrial landslides. We then show how this terrestrial-focused research has catalysed advances in the detection and characterisation of submarine landslides, using both onshore and offshore networks of broadband seismometers, hydrophones and geophones. We then discuss some of the new insights into submarine landslide preconditioning, timing, location, velocity and their down-slope evolution that is arising from these advances. We finally outline some of the outstanding challenges, in particular emphasising the need for calibration of seismic and acoustic signals generated by submarine landslides and their run-out. Once confidence can be enhanced in submarine landslide signal detection and interpretation, passive seismic and acoustic sensing has strong potential to enable more complete hazard catalogues to be built, and opens the door to emerging techniques (such as fibre-optic sensing), to fill key, but outstanding, knowledge gaps concerning these important underwater phenomena

Chronology and stratigraphy of the valley systems

This project has received funding from the European Research Council (ERC) under the European Union’s Seventh Framework Programme (FP7-2007-2013) (Grant agreement No. 323727)

Morphometric fingerprints and downslope evolution in bathymetric surveys: insights into morphodynamics of the Congo canyon-channel

Submarine canyons and channels are globally important pathways for sediment, organic carbon, nutrients and pollutants to the deep sea, and they form the largest sediment accumulations on Earth. However, studying these remote submarine systems comprehensively remains a challenge. In this study, we used the only complete-coverage and repeated bathymetric surveys yet for a very large submarine system, which is the Congo Fan off West Africa. Our aim is to understand channel-modifying features such as subaqueous landslides, meander-bend evolution, knickpoints and avulsions by analyzing their morphometric characteristics. We used a new approach to identify these channel-modifying features via morphometric fingerprints, which allows a systematic and efficient search in low-resolution bathymetry data. These observations have led us to identify three morphodynamic reaches within the Congo Canyon-Channel. The upper reach of the system is characterized by landslides that can locally block the channel, storing material for extended periods and re-excavating material through a new incised channel. The middle reach of the system is dominated by the sweep and swing of meander bends, although their importance depends on the channel’s age, and the time since the last up-channel avulsion. In the distal and youngest part of the system, an upstream migrating knickpoint is present, which causes multi-stage sediment transport and overspill through an underdeveloped channel with shallow depths. These findings complement previous less-detailed morphometric analyses of the Congo Canyon-Channel, offering a clearer understanding of how submarine canyon-channels can store sediment (due to channel-damming landslides, meander point bars, levee building due to overspill), re-excavate that sediment (via thalweg incision, meander propagation, knickpoint migration) and finally transport it to the deep sea. This improved understanding of the morphodynamics of the Congo Canyon-Channel may help to understand the evolution of other submarine canyon-channels, and assessment of hazards faced by seabed infrastructure such as telecommunication cables

Carbon and sediment fluxes inhibited in the submarine Congo Canyon by landslide-damming

Landslide-dams, which are often transient, can strongly affect the geomorphology, and sediment and geochemical fluxes, within subaerial fluvial systems. The potential occurrence and impact of analogous landslide-dams in submarine canyons has, however, been difficult to determine due to a scarcity of sufficiently time-resolved observations. Here we present repeat bathymetric surveys of a major submarine canyon, the Congo Canyon, offshore West Africa, from 2005 and 2019. We show how an ~0.09 km3 canyon-flank landslide dammed the canyon, causing temporary storage of a further ~0.4 km3 of sediment, containing ~5 Mt of primarily terrestrial organic carbon. The trapped sediment was up to 150 m thick and extended >26 km up-canyon of the landslide-dam. This sediment has been transported by turbidity currents whose sediment load is trapped by the landslide-dam. Our results suggest canyon-flank collapses can be important controls on canyon morphology as they can generate or contribute to the formation of meander cut-offs, knickpoints and terraces. Flank collapses have the potential to modulate sediment and geochemical fluxes to the deep sea and may impact efficiency of major submarine canyons as transport conduits and locations of organic carbon sequestration. This has potential consequences for deep-sea ecosystems that rely on organic carbon transported through submarine canyons