Michael A. Clare

Distal turbidites reveal a common distribution for large (>0.1 km3) submarine landslide recurrence

Submarine landslides can be far larger than those on land, and are one of the most important processes for moving sediment across our planet. Landslides that are fast enough to disintegrate can generate potentially very hazardous tsunamis and produce long run-out turbidity currents that break strategically important cable networks. It is important to understand their frequency and triggers. We document the distribution of recurrence intervals for turbidity currents triggered by large landslides (>0.1 km3) in three basin plains. A common distribution of recurrence intervals is observed, despite variable ages and disparate locations, suggesting similar underlying controls on slide triggers and frequency. This common distribution closely approximates a temporally random Poisson distribution, such that the probability of a large disintegrating slide occurring along the basin margin is independent of the time since the last slide. This distribution suggests that non-random processes such as sea level are not a dominant control on frequency of these slides. Recurrence intervals of major (>M 7.3) earthquakes have an approximately Poissonian distribution, suggesting they could be implicated as triggers. However, not all major earthquakes appear to generate widespread turbidites, and other as yet unknown triggers or sequential combinations of processes could produce the same distribution. This is the first study to show that large slide-triggered turbidites have a common frequency distribution in distal basin plains, and that this distribution is temporally random. This result has important implications for assessing hazards from landslide-tsunamis and seafloor cable breaks, and the long-term tempo of global sediment fluxes.

Long-term (17 Ma) turbidite record of the timing and frequency of large flank collapses of the Canary Islands

Volcaniclastic turbidites on the Madeira Abyssal Plain provide a record of large-volume volcanic island flank collapses from the Canary Islands. This long-term record spans 17 Ma, and comprises 125 volcaniclastic beds. Determining the timing, provenance and volumes of these turbidites provides key information about the occurrence of mass wasting from the Canary Islands, especially the western islands of Tenerife, La Palma and El Hierro. These turbidite records demonstrate that landslides often coincide with protracted periods of volcanic edifice growth, suggesting that loading of the volcanic edifices may be a key preconditioning factor for landslide triggers. Furthermore, the last large-volume failures from Tenerife coincide with explosive volcanism at the end of eruptive cycles. Many large-volume Canary Island landslides also occurred during periods of warmer and wetter climates associated with sea-level rise and subsequent highstand. However, these turbidites are not serially dependent and any association with climate or sea level change is not statistically significant.

Newly recognized turbidity current structure can explain prolonged flushing of submarine canyons

Runaway turbidity currents stretch into the deep ocean to form the largest sediment accumulations on Earth. Seabed-hugging flows called turbidity currents are the volumetrically most important process transporting sediment across our planet and form its largest sediment accumulations. We seek to understand the internal structure and behavior of turbidity currents by reanalyzing the most detailed direct measurements yet of velocities and densities within oceanic turbidity currents, obtained from weeklong flows in the Congo Canyon. We provide a new model for turbidity current structure that can explain why these are far more prolonged than all previously monitored oceanic turbidity currents, which lasted for only hours or minutes at other locations. The observed Congo Canyon flows consist of a short-lived zone of fast and dense fluid at their front, which outruns the slower moving body of the flow. We propose that the sustained duration of these turbidity currents results from flow stretching and that this stretching is characteristic of mud-rich turbidity current systems. The lack of stretching in previously monitored flows is attributed to coarser sediment that settles out from the body more rapidly. These prolonged seafloor flows rival the discharge of the Congo River and carry ~2% of the terrestrial organic carbon buried globally in the oceans each year through a single submarine canyon. Thus, this new structure explains sustained flushing of globally important amounts of sediment, organic carbon, nutrients, and fresh water into the deep ocean.

The relationship between eruptive activity, flank collapse, and sea level at volcanic islands: a long-term (>1 Ma) record offshore Montserrat, Lesser Antilles

Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass-wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (?930 ka to ?900 ka, ?810 ka to ?760 ka, and ?190 ka to ?120 ka) that coincide with periods of increased volcano instability and mass-wasting. The youngest of these periods marks the peak in activity at the Soufriere Hills volcano. The largest flank collapse of this volcano (?130 ka) occurred towards the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufriere Hills edifice, and their timing also coincides with periods of rapid sea-level rise (>5 m/ka). Available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea-level rise, but this is not observed for volcanoes in intra-plate ocean settings. We thus infer that rapid sea-level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean-island settings.

Constraining Geohazards to the Past: Impact Assessment of Submarine Mass Movements on Seabed Developments

This paper presents a conceptual review of submarine mass movements and associated hazards such as run-out and turbidity currents, and their implications for the planning of exploration and development activities. Submarine mass movements are placed in the context of ground models and geohazard scenarios from which conditioning factors and trigger mechanisms can be assessed and event magnitudes estimated. The approach to dating these events in order to derive a frequency is discussed with reference to radiometric, biostratigraphic and Optically Stimulated Luminescence techniques that have been successfully deployed in recent projects. The need for good stratigraphic control, both seismostratigraphic and lithological, is emphasized. Based on the ground model and geohazard scenarios, ground movement at different locations within the mass movement system can be inferred. This understanding along with predicted soil conditions derived from the ground model and evolutionary history of the site allows the potential consequences on well conductors, facilities and flowlines to be assessed and the need for geohazard design and mitigation evaluated. These findings indicate the constraints on possible exploration and development activities.

Sea-level - induced seismicity and submarine landslide occurrence: Comment

Brothers et al. (2013) present modeling showing how rapid sea-level rise (SLR) could generate increases in stresses experienced by fault systems, which may lead to increased seismicity. This potential for linkage between rapid SLR and increased seismicity is an important result of widespread interest. However, a key assertion by Brothers et al. is that such increases in seismicity can then explain the “temporal coincidence between rapid late Pleistocene sea-level rise and large-scale slope failures.” The primary purpose of this Comment is to note that available age-dating of large submarine landslides (hereafter: slides) is not consistent with such a view. Once realistic uncertainties in slide ages are considered (Urlaub et al., 2013), there is either (1) no statistical relationship between rapid SLR and large slide frequency, or (2) the uncertainties in age-dates are too large for a correlation between rapid SLR and landslide timing to be recognized, even if a correlation existed. It is important to have a clear understanding of whether the frequency of large slides is (or is not) dependent on sea level. First, submarine slides can potentially generate damaging tsunamis. We need to understand whether there is an increased landslide-tsunami risk in the future as sea level rises. Second, these large slides are one of the major processes for moving sediment across Earth, and factors that precondition and trigger these prodigious failures are still poorly understood. Urlaub et al. (2013) provide the most recent and largest (n = 68) collection of ages for large (>1 km) submarine slides. Importantly, this study considered the uncertainties in these ages, which are mainly due to the position of samples relative to the slide deposit, and to vertical mixing of sediment by organisms (Urlaub et al., 2013, their figure 1). Urlaub et al. show that large slide ages can be described by a Poisson distribution that characterizes a temporally random process, and that peaks in slide frequency can be reproduced when ages are drawn from a temporally random distribution. The frequency distributions of ancient slides surrounding a basin margin, which disintegrate to form large turbidity currents, also appear to have a temporally random Poisson distribution (Clare et al., 2014).

Eustatic sea level controls on the flushing of a shelf-incising submarine canyon

Turbidity currents are the principal processes responsible for carving submarine canyons and maintaining them over geological time scales. The turbidity currents that maintain or “flush” submarine canyons are some of the most voluminous sediment transport events on Earth. Long-term controls on the frequency and triggers of canyon-flushing events are poorly understood in most canyon systems due to a paucity of long sedimentary records. Here, we analyzed a 160-m-long Ocean Drilling Program (ODP) core to determine the recurrence intervals of canyon-flushing events in the Nazare Canyon over the last 1.8 m.y. We then investigated the role of global eustatic sea level in controlling the frequency and magnitude of these canyon-flushing events. Canyon-flushing turbidity currents that reach the Iberian Abyssal Plain had an average recurrence interval of 2770 yr over the last 1.8 m.y. Previous research has documented no effect of global eustatic sea level on the recurrence rate of canyon flushing. However, we find that sharp changes in global eustatic sea level during the mid-Pleistocene transition (1.2–0.9 Ma) were associated with more frequent canyon-flushing events. The change into high-amplitude, long-periodicity sea-level variability during the mid-Pleistocene transition may have remobilized large volumes of shelf sediment via subaerial weathering, and temporarily increased the frequency and magnitude of canyon-flushing turbidity currents. Turbidite recurrence intervals in the Iberian Abyssal Plain have a lognormal distribution, which is fundamentally different from the exponential distribution of recurrence intervals observed in other basin turbidite records. The lognormal distribution of turbidite recurrence intervals seen in the Iberian Abyssal Plain is demonstrated to result from the variable runout distance of turbidity currents, such that distal records are less complete, with possible influence from diverse sources or triggering mechanisms. The changing form of turbidite recurrence intervals at different locations down the depositional system is important because it ultimately determines the probability of turbidity current–related geohazards.

How to recognize crescentic bedforms formed by supercritical turbidity currents in the geologic record: Insights from active submarine channels

Submarine channels have been important throughout geologic time for feeding globally significant volumes of sediment from land to the deep sea. Modern observations show that submarine channels can be sculpted by supercritical turbidity currents (seafloor sediment flows) that can generate upstream-migrating bedforms with a crescentic planform. In order to accurately interpret supercritical flows and depositional environments in the geologic record, it is important to be able to recognize the depositional signature of crescentic bedforms. Field geologists commonly link scour fills containing massive sands to crescentic bedforms, whereas models of turbidity currents produce deposits dominated by back-stepping beds. Here we reconcile this apparent contradiction by presenting the most detailed study yet that combines direct flow observations, time-lapse seabed mapping, and sediment cores, thus providing the link from flow process to depositional product. These data were collected within the proximal part of a submarine channel on the Squamish Delta, Canada. We demonstrate that bedform migration initially produces back-stepping beds of sand. However, these back-stepping beds are partially eroded by further bedform migration during subsequent flows, resulting in scour fills containing massive sand. As a result, our observations better match the depositional architecture of upstream-migrating bedforms produced by fluvial models, despite the fact that they formed beneath turbidity currents.

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 flood water (hyperpycnal flow) dominated the triggering of turbidity currents on delta-fronts. Here we re-analyse 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.

Understanding Engineering Challenges Posed by Natural Hydrocarbon Infiltration and the Development of Authigenic Carbonate

Infiltration of shallow soils by naturally occurring hydrocarbons has been documented in several deepwater environments worldwide. The potential for significant soil modification, such as the development of authigenic carbonates and alteration of the generally expected background geotechnical properties may provide constraints to flowline routing, foundation installation and engineering lifetime performance. This paper presents a review of the current state of knowledge of the authors with special reference to recent investigations in deepwater offshore Angola, and a suggested method for identification, characterization and prediction. Interpretation of 3D exploration seismic, enhanced by AUV (Chirp) data facilitates an initial identification of areas prone to hydrocarbon infiltration and the vertical and spatial extent of potential soil modification. A first pass geotechnical characterisation is developed by targeted seabed CPTs and soil sampling. The generation of an integrated predictive model requires a multidisciplinary advanced testing programme, including geophysical, geotechnical, geochemical and geological analyses. Once the extent, nature, and formative processes of hydrocarbon-related soil modification are understood, an assessment can be made of the challenges posed to a field development. This provides the necessary input to foundation and /or routing feasibility and determines if there is requirement to mitigate, through avoidance or design.