J.E. Hughes Clarke

Dense biological communities at 3850 m on the Laurentian Fan and their relationship to the deposits of the 1929 Grand Banks earthquake

During Alvin dives on the Laurentian Fan aimed at exploring the nature of the deposit of the 1929 Grand Banks earthquake and turbidity current, large, dense communities of living vesicomyid and thyasirid clams, gastropods, and other epifaunal taxa similar to those found in hydrothermal and cold seep environments were unexpectedly discovered. The communities are at 3800–3900 m in a passive margin setting, with no apparent mechanism for enhanced fluid flow. The communities occur near the crests of ‘gravel waves’, depositional bedforms created during the passage of the turbidity current, and on the slope and crest of a steep (20–30°) scarp of outcropping valley floor material. We speculate that these communities have established themselves since 1929 and that they are sustained by chemosynthetic processes. The reduced compounds fueling the chemosynthesis presumably are derived from older, organic-rich fan valley floor sediments that were exposed by the 1929 event.

Sonography of a glaciated continental shelf

Sonographs of the inner continental shelf off Halifax, Nova Scotia, show a large part of the inner shelf area consisting of exposed bedrock in which the inferred sequence of structural events is clearer than the record on land. Ribbed moraines overlie eroded drumlins and provide new information on the deglaciation history. Morphologic features allow a precise definition of the maximum postglacial lowstand of sealevel at -65 to -70 m. A 1000 km 2 area in eastern Canada was surveyed with a multibeam sounder EM100, manufactured by Simrad of Norway. A 100% coverage of the sea floor was achieved with 50 to 100 m track spacing, and position control was achieved using the differential global positioning system (DGPS) with an accuracy of 1-2 m. Data were corrected for ship motion, gridded at a 10 m spacing, and displayed as obliquely illuminated relief image. The sonograph revealed characteristics of the sea bed in detail for the interpretation of bedrock and surficial structures and processes. The bathymetric images may be more useful for geologic mapping than air photographs and satellite images on nearby land, where vegetation commonly obscures the geologic features.

Processing and interpretation of 95 kHz backscatter data from shallow-water multibeam sonars

95 kHz backscatter data are routinely collected by the Canadian Hydrographic Service using shallow water multibeam sonars (Simrad EM100 and EM1000 systems). This data is co-registered with high resolution bathymetry and provides a means of generating sidescan images of the seabed which can be correlated with the bottom topography. Changes in amplifier gains are corrected so that the authors can compare the acoustic signature of distinct seabed lithologies for use as part of a remote sediment classification scheme. In order to groundtruth the acoustic signatures, an experiment is underway in the Bay of Fundy (using the intertidal zone where tidal ranges locally exceed 15 m) using terrestrial survey and sampling methods. As part of the experiment, interactive graphical tools have been developed that allow the data to be processed as it is acquired and enable near-real time data quality assessment.<<ETX>>

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.

Glacial lineations in Navy Board Inlet, Nunavut, Canada

All inlets and fjords on northern Baffin Island record glacial erosion by outlet glaciers (Shepard 1931; Pelletier 1966). However, their positions were probably influenced by prior fluvial erosion (Fortier & Morley 1956; Pelletier 1966; Gilbert 1982) or by graben-style faulting (Andrews & Miller 1979; Gilbert 1982; Dowdeswell & Andrews 1985; England 1987). Navy Board Inlet is a narrow waterway (about 10 km wide) located south of Lancaster Sound between Baffin Island and Bylot Island in Arctic Canada (Fig. 1). Lineations formed by the movement of glacial ice are preserved on the seafloor of this inlet. Fig. 1. Multibeam bathymetry, cross-sections and sub-bottom profiler data over crag-and-tail features in Navy Board Inlet, Nunavut, Canada. ( a ) Sun-illuminated multibeam-bathymetric data showing crag-and-tail features. Acquisition system Kongsberg EM300. Frequency 30 kHz. Grid-cell size 10 m. ( b ) Location of study area (red box; map from IBCAO v. 3.0). ( c ) Detail of crag-and-tail features located on the margin of a bedrock high. Locations of cross-sections indicated by labelled white lines. ( d …

Quantification of near-bed dense layers and implications for seafloor structures: new insights into the most hazardous aspects of turbidity currents

Turbidity currents pose a serious hazard to expensive oil and gas seafloor installations, especially in deep-water where mitigation, re-routing or repair is costly and logistically challenging. These sediment-laden flows are hazardous because they can be exceptionally powerful (up to 20 m/s), and can flow for long distances (>100s km) over several days duration, causing damage over vast areas of seafloor. Even less powerful flows (~1-2 m/s) can damage seafloor equipment, or break strategically important submarine telecommunication cables. The consequences of turbidity currents impacting seafloor structures depends on the velocity, duration, direction of impact and, perhaps most crucially, the sediment concentration (or density) of the flow. While some recent studies have successfully monitored turbidity currents in deep-water, imaging flow properties close to the seafloor has proven problematic. We present innovative approaches to the quantification of the velocity and sediment concentration of dense near-bed layers that provide new insights into this important aspect of turbidity current flow. Firstly, we describe a novel experimental setup that is capable of measuring near-bed sediment concentration in dense (>10% volume by concentration) flows. Density contrasts are measured using Electrical Resistivity Tomography – a technique initially developed for geophysical characterisation of subsurface reservoirs. Velocity is measured using Ultrasonic Doppler Velocity Profiling and concentration is characterized using an Ultra High Concentration Meter. Secondly, we outline some recently developed geophysical approaches for the quantification of sediment concentration and velocity for real-world flows based on recent work in fjords, estuaries and deep-sea canyons. This includes integrated moored deployments of Acoustic Doppler Current Profilers, Multibeam Sonars, and a novel Chirp array. We outline some limitations and advantages of these methods. Finally, we underline the value and importance of establishing multiple field-scale test sites in a variety of settings, including deep-water, that will enhance the industrys understanding of turbidity current hazards. Our results demonstrate the importance of near-bed dense layers for turbidity current interaction with seafloor structures. Density contrasts and pressure build up at the base of a flow may lead to uplift, undermining and loss of support, dragging, or pipeline rupture; hence quantification of this layer is crucial for hazard assessment. Measurements of sediment concentration within turbidity currents are incredibly rare, and yet are a vital input for any numerical model that aims to predict sediment transport by turbidity currents in deep-water settings. Currently it is necessary to infer densities and velocities; however, such inferences are poorly calibrated against experimental or real world data. Our measurements underline the importance of understanding near-bed dense layers.

Mega-scale glacial lineations, Peel Sound, Canadian Arctic Archipelago

The presence of streamlined glacial landforms in the submarine geological record from high-latitude fjords and channels provides evidence with which to identify the manner and direction of glacier flow and to infer subglacial processes. Numerous linear and curvilinear elongate features extending up to 15 km in length have been recognized in Peel Sound, Canadian Arctic Archipelago. These well-preserved sedimentary landforms reflect the former presence of a fast-flowing ice stream that flowed through Franklin Strait and Peel Sound. Multibeam swath-bathymetric imagery from an overdeepened region of Peel Sound near its junction with Franklin Strait, where the trend of the channel changes from NE to north (Fig. 1b, c), is shown in Figure 1a. Water depths here range from 390 to 460 m. Parallel ridge and groove bedforms form prominent seabed features on all the survey transects that …

Pockmarks in Passamaquoddy Bay, New Brunswick, Canada

Pockmarks are seafloor depressions associated with fluid escape (Judd & Hovland 2007). They proliferate in the muddy seafloors of coastal Gulf of Maine and Bay of Fundy, where they are associated with shallow natural gas likely of biogenic origin (Ussler et al. 2003; Rogers et al. 2006; Wildish et al. 2008). In North America, shallow-water pockmark fields are not reported south of Long Island Sound, despite the abundance of gassy, muddy estuaries. The absence of pockmarks south of the limit of North American glaciation suggests that local and regional heterogeneities, possibly related to glacial or sea-level history or bedrock geology, influence pockmark field distribution. In shallow-water embayments, such as Passamaquoddy Bay, New Brunswick, pockmarks can be large (>200 m diameter) and number in the thousands. Over 4500 pockmarks litter the seafloor of Passamaquoddy Bay. These pockmarks are within 5 km of shore in water depths ranging from 7 to 80 m (Fig. 1a). Occurring either as discrete depressions or in linear chains, pockmarks are generally circular with concave sidewall …

Moat features, Amundsen Gulf, Canadian Arctic Archipelago

Multibeam sonar imagery provides evidence of depositional and erosional seafloor features emplaced by a glacial ice stream that flowed through Amundsen Gulf into the Beaufort Sea during the last glaciation. Figure 1a, c illustrate seafloor features that occur SW of Banks Island in the northwestern part of Amundsen Gulf (Fig. 1b). Water depths in this part of the axial trough range from 350 to 435 m. Prominent seafloor landforms include parallel ridges and grooves that trend northwesterly across the region. Depressions up to 135 m wide and 45 m deep occur as irregular linear and arcuate-shaped interruptions in many of the ridges. The linear interruptions extend across adjoining ridges along trends that lie approximately normal to the trend of the ridges (Fig. 1a, c). Arcuate-shaped depressions (moats) connect with wider inter-ridge valleys trending to the NW. In many instances the linear ridges, apparently undiminished in height adjacent …

Crag-and-tail features, Amundsen Gulf, Canadian Arctic Archipelago

Palaeo-ice streams existed in many marine channels of the Canadian Arctic Archipelago (e.g. Clark & Stokes 2001; Stokes et al. 2006; MacLean et al. 2010, 2015). These include Amundsen Gulf at the southwestern end of the Northwest Passage, where multibeam imagery has revealed a variety of subglacial features (Stokes et al. 2006; MacLean et al. 2012, 2015) (Fig. 1a, b). Six or more stacked ice-contact deposits in NW Amundsen Gulf indicate successive advances of a grounded ice stream from a pinning point on the rocky shallow seabed south of Banks Island. Stokes et al. (2006) also considered this to be a pinning point for the ice stream. Further evidence of the dynamic nature of glacial events in Amundsen Gulf is provided by Batchelor et al. (2014), who recognized sediment sequences deposited by eight individual Amundsen Gulf ice streams or readvances of the same ice stream in the outer gulf and on the Beaufort Shelf. Fig. 1. Multibeam sonar imagery and profile of crag-and-tail features formed subglacially in the lee of bedrock outcrops in northwestern Amundsen Gulf, Canadian Arctic Archipelago. ( a ) Location of study area (red box; map from IBCAO v. 3.0). …