Earthquakes drive large-scale submarine canyon development and sediment supply to deep-ocean basins

Microbial extracellular polymeric substances in the environment, technology and medicine

Microbial biofilms exhibit a self-produced matrix of extracellular polymeric substances (EPS), including polysaccharides, proteins, extracellular DNA and lipids. EPS promote interactions of the biofilm with other cells and sorption of organics, metals and chemical pollutants, and they facilitate cell adhesion at interfaces and ensure matrix cohesion. EPS have roles in various natural environments, such as soils, sediments and marine habitats. In addition, EPS are relevant in technical environments, such as wastewater and drinking water treatment facilities, and water distribution systems, and they contribute to biofouling and microbially influenced corrosion. In medicine, EPS protect pathogens within the biofilm against the host immune system and antimicrobials, and emerging evidence suggests that EPS can represent potential virulence factors. By contrast, EPS yield a wide range of valuable products that include their role in self-repairing concrete. In this Review, we aim to explore EPS as a functional unit of biofilms in the environment, in technology and in medicine.

Transport and sedimentation of microplastics by turbidity currents: Dependence on suspended sediment concentration and grain size

Using lock-exchange experiments, this study investigates the transport and sedimentation of microplastics (MPs) via turbidity currents. Two hypotheses were tested: MP sedimentation is influenced by suspended sediment concentration and grain size. Utilizing flows with different sediment concentrations and grain sizes in combination with three different MPs (PET fibers, melamine, and PVC fragments), the experiments revealed distinct sedimentation patterns: higher sediment concentrations enhance MP transport, and turbidity currents with finer sediments transported MPs over greater distances, highlighting the importance of sediment characteristics to predict MP distribution by such flows. Further, MP sedimentation patterns varied in dependence on MP-particle shape, size, and density, highlighting the crucial role of MP particle properties in determining MP distribution in turbidites. These findings enhance our understanding of the mechanisms controlling the spatial distribution of MPs in marine sedimentary-environments and underscores the importance of considering both hydrodynamic and particle-specific factors when addressing the complex behaviour of MPs.

Extreme erosion and bulking in a giant submarine gravity flow

Sediment gravity flows are ubiquitous agents of transport, erosion, and deposition across Earth's surface, including terrestrial debris flows, snow avalanches, and submarine turbidity currents. Sediment gravity flows typically erode material along their path (bulking), which can dramatically increase their size, speed, and run-out distance. Hence, flow bulking is a first-order control on flow evolution and underpins predictive modeling approaches and geohazard assessments. Quantifying bulking in submarine systems is problematic because of their large-scale and inaccessible nature, complex stratigraphy, and poorly understood source areas. Here, we map the deposits and erosive destruction of a giant submarine gravity flow from source to sink. The small initial failure (~1.5 cubic kilometers) entrained over 100 times its starting volume, catastrophically evolving into a giant flow with a total volume of ~162 cubic kilometers and a run-out distance of ~2000 kilometers. Entrainment of mud was the critical fuel, which promoted run-away flow growth and extreme levels of erosion.

Recovery of deep-sea meiofauna community in Kaikōura Canyon following an earthquake-triggered turbidity flow

Turbidity flows can transport massive amounts of sediment across large distances with dramatic, long-lasting impacts on deep-sea benthic communities. The 2016 Mw 7.8 Kaikōura Earthquake triggered a canyon-flushing event in Kaikōura Canyon, New Zealand, which included significant submarine mass wasting, debris, and turbidity flows. This event provided an excellent opportunity to investigate the effects of large-scale natural disturbance on benthic ecosystems. Benthic meiofauna community structure before and after the event was analysed from a time series of sediment cores collected 10 years and 6 years before, and 10 weeks, 10 months, and 4 years after the disturbance. Immediately after the 2016 event abundances of all meiofauna dramatically decreased. Four years later the meiofauna community had recovered and was no longer distinguishable from the pre-event community. However, the nematode component of the community was similar, but not fully comparable to the pre-event community by 4 years after the disturbance. Community recovery was systematically correlated to changes in the physical characteristics of the habitat caused by the disturbance, using physical and biochemical variables derived from sediment cores, namely: sediment texture, organic matter, and pigment content. While these environmental variables explained relatively little of the overall variability in meiofauna community structure, particle size, food availability and quality were significant components. The minimum threshold time for the meiofauna community to fully recover was estimated to be between 3.9 and 4.7 years, although the predicted recovery time for the nematode community was longer, between 4.6 and 5 years. We consider the management implications of this study in comparison to the few studies of large-scale disturbances in the deep sea, in terms of their relevance to the efficacy of the marine reserve that encompasses Kaikōura Canyon, along with potential implications for our understanding of the impacts of anthropogenic seafloor disturbances, such as seabed mining.

The Global Turbidity Current Pump and Its Implications for Organic Carbon Cycling

Submarine turbidity currents form the largest sediment accumulations on Earth, raising the question of their role in global carbon cycles. It was previously inferred that terrestrial organic carbon was primarily incinerated on shelves and that most turbidity current systems are presently inactive. Turbidity currents were thus not considered in global carbon cycles, and the burial efficiency of global terrestrial organic carbon was considered low to moderate (∼10-44%). However, recent work has shown that burial of terrestrial organic carbon by turbidity currents is highly efficient (>60-100%) in a range of settings and that flows occur more frequently than once thought, although they were far more active at sea-level lowstands. This leads to revised global estimates for mass flux (∼62-90 Mt C/year) and burial efficiency (∼31-45%) of terrestrial organic carbon in marine sediments. Greatly increased burial fluxes during sea-level lowstands are also likely underestimated; thus, organic carbon cycling by turbidity currents could play a role in long-term changes in atmospheric CO2 and climate.

Volcaniclastic density currents explain widespread and diverse seafloor impacts of the 2022 Hunga Volcano eruption

The impacts of large terrestrial volcanic eruptions are apparent from satellite monitoring and direct observations. However, more than three quarters of all volcanic outputs worldwide lie submerged beneath the ocean, and the risks they pose to people, infrastructure, and benthic ecosystems remain poorly understood due to inaccessibility and a lack of detailed observations before and after eruptions. Here, comparing data acquired between 2015 - 2017 and 3 months after the January 2022 eruption of Hunga Volcano, we document the far-reaching and diverse impacts of one of the most explosive volcanic eruptions ever recorded. Almost 10 km3 of seafloor material was removed during the eruption, most of which we conclude was redeposited within 20 km of the caldera by long run-out seafloor density currents. These powerful currents damaged seafloor cables over a length of >100 km, reshaped the seafloor, and caused mass-mortality of seafloor life. Biological (mega-epifaunal invertebrate) seafloor communities only survived the eruption where local topography provided a physical barrier to density currents (e.g., on nearby seamounts). While the longer-term consequences of such a large eruption for human, ecological and climatic systems are emerging, we expect that these previously-undocumented refugia will play a key role in longer-term ecosystem recovery.

Assessing the impact of the global subsea telecommunications network on sedimentary organic carbon stocks

The sequestration of organic carbon in seafloor sediments plays a key role in regulating global climate; however, human activities can disturb previously-sequestered carbon stocks, potentially reducing the capacity of the ocean to store CO₂. Recent studies revealed profound seafloor impacts and sedimentary carbon loss due to fishing and shipping, yet most other human activities in the ocean have been overlooked. Here, we present an assessment of organic carbon disturbance related to the globally-extensive subsea telecommunications cable network. Up to 2.82-11.26 Mt of organic carbon worldwide has been disturbed as a result of cable burial, in water depths of up to 2000 m. While orders of magnitude lower than that disturbed by bottom fishing, it is a non-trivial amount that is absent from global budgets. Future offshore developments that disturb the seafloor should consider the safeguarding of carbon stocks, across the full spectrum of Blue Economy industries.

Longest sediment flows yet measured show how major rivers connect efficiently to deep sea

Here we show how major rivers can efficiently connect to the deep-sea, by analysing the longest runout sediment flows (of any type) yet measured in action on Earth. These seafloor turbidity currents originated from the Congo River-mouth, with one flow travelling >1,130 km whilst accelerating from 5.2 to 8.0 m/s. In one year, these turbidity currents eroded 1,338-2,675 [>535-1,070] Mt of sediment from one submarine canyon, equivalent to 19–37 [>7–15] % of annual suspended sediment flux from present-day rivers. It was known earthquakes trigger canyon-flushing flows. We show river-floods also generate canyon-flushing flows, primed by rapid sediment-accumulation at the river-mouth, and sometimes triggered by spring tides weeks to months post-flood. It is demonstrated that strongly erosional turbidity currents self-accelerate, thereby travelling much further, validating a long-proposed theory. These observations explain highly-efficient organic carbon transfer, and have important implications for hazards to seabed cables, or deep-sea impacts of terrestrial climate change.

This paper analyses the longest sediment flows measured in action on Earth. These seabed flows were caused by floods and spring tides, and flushed prodigious sediment and carbon volumes into the deep sea, as they accelerated for a thousand kilometres.

Numerical Investigation of the Sediment Hyperpycnal Flow in the Yellow River Estuary

Sediment hyperpycnal flow is one of the most important processes for mass transport, which is essential to coastal morphodynamics. Herein, we studied the generation and maintenance of the sediment hyperpycnal flow in the Yellow River Estuary (YRE) using a three-dimensional finite volume coastal ocean model (FVCOM). The model considered the effect of sediment-laden water on density stratification, and was validated by field hydrodynamic and sediment data. Numerical results revealed that the hyperpycnal flow shows periodic characteristics with tidal cycles where the flow is weakened during flood tides and enhanced during ebb tides. A high suspended sediment concentration (SSC) of about 30–40 kg/m3 constitutes an important factor in the formation of hyperpycnal flows. High river discharge with high SSC is essential for maintaining the hyperpycnal flow in the YRE. The Simpson potential energy theory was applied to study the processes of estuarine circulation, tidal straining, and tidal stirring in the YRE. The tidal straining is the main control factor of the periodic stratification-mixing process of hyperpycnal flows in the YRE. Along the axis of the river mouth, the momentum balance is mainly dominated by the pressure gradient and advection.

Controls of Submarine Canyons Connected to Shore during the LGM Sea-Level Rise: Examples from Taiwan

During the Last Glacial Maximum (LGM) (~20,000 years ago), the sea level was lower than that during the present day by 120 m and the heads of most submarine canyons were close or connected to the coastline or shore, with enhanced terrestrial sediment input due to direct connections with river mouths. This was followed by a relative sea-level rise of 120 m and the migration of coastlines landward. As a result, the heads of some paleo-submarine canyons were no longer near river mouths or connected to the shore. Such canyons became inactive due to the lack of terrestrial sediment input. Only 4% of the world’s submarine canyons reach the coastline and remain active today. Among 13 submarine canyons off the shore of Taiwan, we identified seven (n = 7, 54%) that remain connected to the shore and are active during the present-day highstand. The purpose of this study is to determine the key controls of canyon heads that remain connected to the shore with terrestrial sediment input during the Holocene sea-level rise. As a result of high uplift rates, narrow coastal range, steep gradients, frequent earthquakes, and typhoon development in the Taiwan mountain belt, Taiwan has the highest-yield river and sediment supply. This has led to the transportation of large volumes of sediment to the surrounding deep seas. Narrow steep shelves and large sediment volumes associated with small mountain rivers are the main controls involved in the development of shore-connected canyons on the active Taiwan margin. Shore-connected canyons are present in greater numbers in the major earthquake zone on the eastern Taiwan margin. Frequent earthquake events are another significant factor in the occurrence of shore-connected canyons in the Taiwan region.

Integrated Morpho-Bathymetric, Seismic-Stratigraphic, and Sedimentological Data on the Dohrn Canyon (Naples Bay, Southern Tyrrhenian Sea): Relationships with Volcanism and Tectonics

Submarine canyons are geomorphologic lineaments engraving the slope/outer shelf of continental margins. These features are often associated with significant geologic hazard when they develop close to densely populated coastal zones. The seafloor of Naples Bay is deeply cut by two incisions characterized by a dense network of gullies, namely the Dohrn and Magnaghi canyons, which develop from the shelf break of the Campania margin, down to the peripheral rise of the Eastern Tyrrhenian bathyal plain. Seismic-stratigraphic interpretation of multichannel seismic reflection profiles has shown that quaternary tectonics and recent to active volcanism have exerted a significant control on the morphological evolution and source-to sink depositional processes of the Dohrn and Magnaghi submarine canyons. The Dohrn canyon is characterized by relatively steep walls hundreds of meters high, which incise a Middle-Late Pleistocene prograding wedge, formed by clastic and volcaniclastic deposits associated with the paleo-Sarno river system during the Mid-Late Pleistocene. The formation of the Dohrn canyon predates the onset of the volcanic eruption of the Neapolitan Yellow Tuff (NYT), an ignimbrite deposit of ca. 15 ka that represents the bedrock on which the town of Napoli is built. Integrated stratigraphic analysis of high-resolution seismic profiles and marine gravity core data (C74_12) collected along the flanks of the eastern bifurcation of the head of Dohrn Canyon suggests that depositional processes along the canyon flanks are dominated by gravity flows (e.g., fine-grained turbidites, debris flows) and sediment mass transport associated with slope instability and failure.

Seafloor microplastic hotspots controlled by deep-sea circulation

Although microplastics are known to pervade the global seafloor, the processes that control their dispersal and concentration in the deep sea remain largely unknown. Here, we show that thermohaline-driven currents, which build extensive seafloor sediment accumulations, can control the distribution of microplastics and create hotspots with the highest concentrations reported for any seafloor setting (190 pieces per 50 grams). Previous studies propose that microplastics are transported to the seafloor by vertical settling from surface accumulations; here, we demonstrate that the spatial distribution and ultimate fate of microplastics are strongly controlled by near-bed thermohaline currents (bottom currents). These currents are known to supply oxygen and nutrients to deep-sea benthos, suggesting that deep-sea biodiversity hotspots are also likely to be microplastic hotspots.

Transport and Burial of Microplastics in Deep-Marine Sediments by Turbidity Currents

The threat posed by plastic pollution to marine ecosystems and human health is under increasing scrutiny. Much of the macro- and microplastic in the ocean ends up on the seafloor, with some of the highest concentrations reported in submarine canyons that intersect the continental shelf and directly connect to terrestrial plastic sources. Gravity-driven avalanches, known as turbidity currents, are the primary process for delivering terrestrial sediment and organic carbon to the deep sea through submarine canyons. However, the ability of turbidity currents to transport and bury plastics is essentially unstudied. Using flume experiments, we investigate how turbidity currents transport microplastics, and their role in differential burial of microplastic fragments and fibers. We show that microplastic fragments become relatively concentrated within the base of turbidity currents, whereas fibers are more homogeneously distributed throughout the flow. Surprisingly, the resultant deposits show an opposing trend, as they are enriched with fibers, rather than fragments. We explain this apparent contradiction by a depositional mechanism whereby fibers are preferentially removed from suspension and buried in the deposits as they are trapped between settling sand-grains. Our results suggest that turbidity currents potentially distribute and bury large quantities of microplastics in seafloor sediments.

Earthquake Impact on Active Margins: Tracing Surficial Remobilization and Seismic Strengthening in a Slope Sedimentary Sequence

Strong earthquakes at active ocean margins can remobilize vast amounts of surficial slope sediments and dynamically strengthen the margin sequences. Current process understanding is obtained from resulting event deposits and low-resolution shear strength data, respectively. Here we directly target a site offshore Japan where both processes are expected to initiate, that is, at the uppermost part (15 cm) of a sedimentary slope sequence. Based on a novel application of short-lived radionuclide data, we identified, dated, and quantified centimeter-scale gaps related to surficial remobilization. Temporal correlation to the three largest regional earthquakes attest triggering by strong earthquakes (M w >8). Also, extremely elevated shear strength values suggest a strong influence of seismic strengthening on shallow sediments. We show that despite enhanced slope stability by seismic strengthening, earthquake-induced sediment transport can occur through surficial remobilization, which has large implications for the assessment of turbidite paleoseismology and carbon cycling at active margins.

Megathrust earthquake drives drastic organic carbon supply to the hadal trench

The giant 2011 Tohoku-oki earthquake has been inferred to remobilise fine-grained, young surface sediment enriched in organic matter from the slope into the >7 km deep Japan Trench. Yet, this hypothesis and assessment of its significance for the carbon cycle has been hindered by limited data density and resolution in the hadal zone. Here we combine new high-resolution bathymetry data with sub-bottom profiler images and sediment cores taken during 2012–2016 in order to map for the first time the spatial extent of the earthquake-triggered event deposit along the hadal Japan Trench. We quantify a sediment volume of ~0.2 km³ deposited from spatially-widespread remobilisation of young surficial seafloor slope sediments triggered by the 2011 earthquake and its aftershock sequence. The mapped volume and organic carbon content in sediment cores encompassing the 2011 event reveals that this single tectonic event delivered >1 Tg of organic carbon to the hadal trench. This carbon supply is comparable to high carbon fluxes described for other Earth system processes, shedding new light on the impact of large earthquakes on long-term carbon cycling in the deep-sea.