Implications of reduced turbidity current and landslide activity for the Initial Eocene Thermal Maximum - evidence from two distal, deep-water sites

Previous studies propose that submarine landslides and turbidity currents may become more likely due to future rapid global warming. Determining whether global warming increases likelihood assists in assessment of landslide-triggered tsunami hazards and risk to seafloor structures. Other studies propose that landslides helped to trigger past rapid climate change due to sudden release of gas hydrates. Two deep-water turbidite records show prolonged hiatuses in turbidity current activity during the Initial Eocene Thermal Maximum (IETM) at ∼55 Ma. The IETM represents a possible proxy for future anthropogenically-induced climate change. It is likely that our records mainly represent large and fast moving disintegrative submarine landslides. Statistical analysis of long term (>2.3 Myr) records shows that turbidity current frequency significantly decreased after the IETM. Our results indicate that rapid climate change does not necessarily cause increased turbidity current activity, and do not provide evidence for landslides as a primary trigger for the IETM

Which Triggers Produce the Most Erosive, Frequent, and Longest Runout Turbidity Currents on Deltas?

Subaerial rivers and turbidity currents are the two most voluminous sediment transport processes on our planet, and it is important to understand how they are linked offshore from river mouths. Previously, it was thought that slope failures or direct plunging of river floodwater (hyperpycnal flow) dominated the triggering of turbidity currents on delta fronts. Here we reanalyze the most detailed time‐lapse monitoring yet of a submerged delta; comprising 93 surveys of the Squamish Delta in British Columbia, Canada. We show that most turbidity currents are triggered by settling of sediment from dilute surface river plumes, rather than landslides or hyperpycnal flows. Turbidity currents triggered by settling plumes occur frequently, run out as far as landslide‐triggered events, and cause the greatest changes to delta and lobe morphology. For the first time, we show that settling from surface plumes can dominate the triggering of hazardous submarine flows and offshore sediment fluxes

Preconditioning and triggering of offshore slope failures and turbidity currents revealed by most detailed monitoring yet at a fjord-head delta

Rivers and turbidity currents are the two most important sediment transport processes by volume on Earth. Various hypotheses have been proposed for triggering of turbidity currents offshore from river mouths, including direct plunging of river discharge, delta mouth bar flushing or slope failure caused by low tides and gas expansion, earthquakes and rapid sedimentation. During 2011, 106 turbidity currents were monitored at Squamish Delta, British Columbia. This enables statistical analysis of timing, frequency and triggers. The largest peaks in river discharge did not create hyperpycnal flows. Instead, delayed delta-lip failures occurred 8–11 h after flood peaks, due to cumulative delta top sedimentation and tidally-induced pore pressure changes. Elevated river discharge is thus a significant control on the timing and rate of turbidity currents but not directly due to plunging river water. Elevated river discharge and focusing of river discharge at low tides cause increased sediment transport across the delta-lip, which is the most significant of all controls on flow timing in this setting

Are large submarine landslides in Polar Regions temporally random, or do current observations and age constraint make it impossible to tell?

Submarine landslides are one of the major mechanisms through which sediment is transported across our planet, and it has been proposed that they can generate exceptionally damaging tsunamis. Polar margins represent one of the environmental settings where these events have been identified. A large number of triggers and preconditioning factors have been proposed as possible causes for these events; including earthquakes, rapid sedimentation and gas hydrate dissociation. Rapid climate change in the Arctic has the potential to impact on these preconditioning and triggering factors. First, crustal rebound associated with ice melting is likely to produce larger and more frequent earthquakes. Second, Arctic Ocean warming over the next few decades may lead to dissociation of methane hydrates in marine sediments, thereby weakening sediment. In order to better understand whether landslide frequency will increase in the future, we need to determine whether landslide frequency has been affected by previous episodes of rapid climate or eustatic sea level change. Previous working whether landslide frequency is affected strongly by climatic change has been based predominantly on qualitative analysis, and has concluded that event clustering has occurred under specific environmental conditions. In contrast, two recent statistical investigations of submarine landslides have found events frequencies to follow a Poissonian distribution and thus are temporally random (Urlaub et al, 2013, QSR; Clare et al., Geology, Vol 42 (3)). However, these recent studies acknowledge the significant uncertainties in most landslide dates, and that these uncertainties could mask underlying relationships with climate or sea level. This presentation extends previous statistical work to assess whether landslide frequency is most likely temporally random, or whether the dating is just too uncertain to tell. Chi-Squared statistics are used to explore the extent to which we can be statistically sure that submarine landslides do indeed follow a Poissonian distribution. This is achieved by analysing the ease with which ordered frequency data can appear Poissonian according to the Chi-Squared statistic and the number of events needed before a certain distribution can be guaranteed. From this we are able comment on the extent to which we can use event frequency as a means with which to analyse triggers and preconditioning factors. We can also assess the implications for future submarine landslide risk analysis

Current-aligned dewatering sheets and ‘enhanced’ primary current lineation in turbidite sandstones of the Marnoso-arenacea Formation

Turbidite sandstones of the Miocene Marnoso-arenacea Formation (northern Apennines, Italy) display centimetre to decimetre long, straight to gently curved, 0.5 to 2.0 cm regularly spaced lineations on depositional (stratification) planes. Sometimes these lineations are the planform expression of sheet structures seen as millimetre to centimetre long vertical ‘pillars’ in profile. Both occur in the middle and upper parts of medium-grained and fine-grained sandstone beds composed of crude to well-defined stratified facies (including corrugated, hummocky-like, convolute, dish-structured and dune stratification) and are aligned sub-parallel to palaeoflow direction as determined from sole marks often in the same beds. Outcrops lack a tectonic-related fabric and therefore these structures may be confidently interpreted to be sedimentary in origin. Lineations resemble primary current lineation formed by the action of turbulence during bedload transport under upper stage plane bed conditions. However, they typically display a larger spacing and micro-topography compared to classic primary current lineation and are not associated with planar-parallel, finely-laminated sandstones. This type of ‘enhanced lineation’ is interpreted to develop by the same process as primary current lineation, but under relatively high near-bed sediment concentrations and suspended load fallout rates, as supported by laboratory experiments and host facies characteristics. Sheets are interpreted to be dewatering structures and their alignment to palaeoflow (only noted in several other outcrops previously) inferred to be a function of vertical water-escape following the primary depositional grain fabric. For the Marnoso-arenacea beds, sheet orientation may be genetically linked to the enhanced primary current lineation structures. Current-aligned lineation and sheet structures can be used as palaeoflow indicators, although the directional significance of sheets needs to be independently confirmed. These indicators also aid the interpretation of dewatered sandstones, suggesting sedimentation under a traction-dominated depositional flow – with a discrete interface between the aggrading deposit and the flow – as opposed to under higher-concentration grain or hindered settling dominated regimes

Composition, geometry, and emplacement dynamics of a large volcanic island landslide offshore Martinique: From volcano flank-collapse to seafloor sediment failure

Landslides are common features in the vicinity of volcanic islands. In this contribution, we investigate landslides emplacement and dynamics around the volcanic island of Martinique based on the first scientific drilling of such deposits. The evolution of the active Montagne Pelée volcano on this island has been marked by three major flank-collapses that removed much of the western flank of the volcano. Subaerial collapse volumes vary from 2 to 25 km3 and debris avalanches flowed into the Grenada Basin. High-resolution seismic data (AGUADOMAR – 1999, CARAVAL – 2002 and GWADASEIS – 2009) is combined with new drill cores that penetrate up to 430 m through the three submarine landslide deposits previously associated to the aerial flank-collapses (Site U1399, Site U1400, Site U1401, IODP Expedition 340, Joides Resolution, March-April 2012). This combined geophysical and core data provide an improved understanding of landslide processes offshore a volcanic island. The integrated analysis shows a large submarine landslide deposit, without debris avalanche deposits coming from the volcano, comprising up to 300 km3 of remobilized seafloor sediment that extends for 70 km away from the coast and covers an area of 2100 km2. Our new data suggest that the aerial debris avalanche deposit enter the sea but stop at the base of submarine flank. We propose a new model dealing with seafloor sediment failures and landslide propagation mechanisms, triggered by volcanic flank-collapse events affecting Montagne Pelée volcano. Newly recognized landslide deposits occur deeper in the stratigraphy, suggesting the recurrence of large-scale mass-wasting processes offshore the island and thus, the necessity to better assess the associated tsunami hazards in the region. This article is protected by copyright. All rights reserved

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

Different frequencies and triggers of canyon filling and flushing events in Nazaré Canyon, offshore Portugal

Submarine canyons are one of the most important pathways for sediment transport into ocean basins. For this reason, understanding canyon architecture and sedimentary processes has importance for sediment budgets, carbon cycling, and geohazard assessment. Despite increasing knowledge of turbidity current triggers, the down-canyon variability in turbidity current frequency within most canyon systems is not well constrained. New AMS radiocarbon chronologies from canyon sediment cores illustrate significant variability in turbidity current frequency within Nazaré Canyon through time. Generalised linear models and Cox proportional hazards models indicate a strong influence of global sea level on the frequency of turbidity currents that fill the canyon. Radiocarbon ages from basin sediment cores indicate that larger, canyon-flushing turbidity currents reaching the Iberian Abyssal Plain have a significantly longer average recurrence interval than turbidity currents that fill the canyon. The recurrence intervals of these canyon-flushing turbidity currents also appear to be unaffected by long-term changes in global sea level. Furthermore, canyon-flushing and canyon-filling have very different statistical distributions of recurrence intervals. This indicates that the factors triggering, and thus controlling the frequency of canyon-flushing and canyon-filling events are very different. Canyon-filling appears to be predominantly triggered by sediment instability during sea level lowstand, and by storm and nepheloid transport during the present day highstand. Canyon-flushing exhibits time-independent behaviour. This indicates that a temporally random process, signal shredding, or summation of non-random processes that cannot be discerned from a random signal, are triggering canyon flushing events

Detailed monitoring reveals the nature of submarine turbidity currents

Seafloor sediment flows, called turbidity currents, form the largest sediment accumulations, deepest canyons, and longest channels on Earth. It was once thought that turbidity currents were impractical to measure in action, especially due to their ability to damage sensors in their path, but direct monitoring since the mid 2010s has measured them in detail. In this Review, we summarise knowledge of turbidity currents gleaned from this direct monitoring. Monitoring identifies triggering mechanisms from dilute river-plumes, and shows how rapid sediment accumulation can precondition slope failure, but the final triggers can be delayed and subtle. Turbidity currents are consistently more frequent than predicted by past sequence stratigraphic models, including at sites >300 km from any coast. Faster (>~1.5 m s–1) flows are driven by a dense near-bed layer at their front, whereas slower flows are entirely dilute. This frontal layer sometimes erodes large (>2.5 km3) volumes of sediment, yet maintains a near-uniform speed, leading to a travelling wave model. Monitoring shows that flows sculpt canyons and channels through fast-moving knickpoints, and how deposits originate. Emerging technologies with reduced cost and risk can lead to widespread monitoring of turbidity currents, so their sediment and carbon fluxes can be compared with other major global transport processes

A multi-disciplinary investigation of the AFEN Slide: The relationship between contourites and submarine landslides

Contourite drifts are sediment deposits formed by ocean bottom currents on continental slopes worldwide. Although it has become increasingly apparent that contourites are often prone to slope failure, the physical controls on slope instability remain unclear. This study presents high-resolution sedimentological, geochemical and geotechnical analyses of sediments to better understand the physical controls on slope failure that occurred within a sheeted contourite drift within the Faroe-Shetland Channel. We aim to identify and characterize the failure plane of the late Quaternary landslide (the AFEN Slide), and explain its location within the sheeted drift stratigraphy. The analyses reveal abrupt lithological contrasts characterized by distinct changes in physical, geochemical and geotechnical properties. Our findings indicate that the AFEN Slide likely initiated along a distinct lithological interface, between overlying sandy contouritic sediments and softer underlying mud-rich sediments. These lithological contrasts are interpreted to relate to climatically-controlled variations in sediment input and bottom current intensity. Similar lithological contrasts are likely to be common within contourite drifts at many other oceanic gateways worldwide; hence our findings are likely to apply more widely. As we demonstrate here, recognition of such contrasts requires multi-disciplinary data over the depth range of stratigraphy that is potentially prone to slope failure