Sediment transport processes at the head of Halibut Canyon, eastern Canada margin: An interplay between internal tides and dense shelf-water cascading Pere Puig a, ⁎, Blair J.W. Greenan b , Michael Z. Li c , Robert H. Prescott d , David J.W. Piper c a Institut de Ciències del Mar (CSIC), Passeig Marítim de la Barceloneta, 37-49, 08003 Barcelona, Spain b Fisheries and Oceans Canada, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada c Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada d Prescott and Zou Consulting, 6 Glenn Dr., Halifax, Nova Scotia B3M 2B9, Canada abstract article info Article history: Received 1 December 2012 Received in revised form 6 May 2013 Accepted 12 May 2013 Available online 19 May 2013 Communicated by: Dr. J.T. Wells Keywords: submarine canyon sediment transport internal tide dense shelf water cascade gravity flow To investigate the processes by which sediment is transported through a submarine canyon incised in a glaciated margin, the bottom boundary layer quadrapod RALPH was deployed at 276-m depth in the West Halibut Canyon (off Newfoundland) during winter 2008–2009. Two main sediment transport processes were identified throughout the deployment. Firstly, periodic increases of near-bottom suspended-sediment concentrations (SSC) were recorded associated with the up-canyon propagation of the semidiurnal internal tidal bore along the canyon axis, carrying fine sediment particles resuspended from deeper canyon regions. The recorded SSC peaks, lasting less than 1 h, were observed sporadically and were linked to bottom inten- sified up-canyon flows (~40 cm s -1 ) concomitant with sharp drops in temperature. Secondly, sediment transport was also observed during events of intensified down-canyon current velocities that occurred dur- ing periods of sustained heat loss from surface waters, but were not associated with large storm waves. High-resolution velocity profiles throughout the water column during these events revealed that the highest current speeds (~1 m s -1 ) were centered several meters above the sea floor and corresponded to the region of maximum velocities of a gravity flow. Such flows had associated low SSC and cold water temperatures and are interpreted as dense shelf water cascading events channelized along the canyon axis. Sediment transport during these events was largely restricted to bedload and saltation, producing winnowing of sands and fine sediments around larger gravel particles. Analysis of historical hydrographic data suggests that such gravity flows are not related to the formation of coastal dense waters advected towards the outer shelf that reached the canyon head. Rather, the dense shelf waters appear to be generated around the outer shelf, where convection during winter is able to reach the sea floor and generate a pool of near-bottom dense water that cascades into the canyon during one or two tidal cycles. A similar transport mechanism is likely to occur in other submarine canyons along the eastern Canadian margin, as well in other canyoned margins where winter convection can reach the shelf-edge. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Submarine canyons are morphological features that are found on many continental margins, acting as preferential conduits for transport of sediment from continental shelves towards deep-sea environments. During Plio-Pleistocene lowstands of sea level, sediment–gravity mecha- nisms (i.e. turbidity currents, debris flows) dominated transport through submarine canyons, funneling large volumes of terrigenous sediment to deeper parts of the continental margins (Shanmugam et al., 1985; Piper and Normark, 2009). Although Holocene sea-level rise has reduced drastically the supply of sediments to submarine canyons, it is widely recognized that canyons at present continue to be preferential conduits for the transfer of sediments from the shelf to the deep ocean. During the last decades, several studies have provided information about contemporary sediment-transport processes acting within submarine canyons by means of analysis of combined currents and suspended-sediment concentration data. Most of them have been conducted using moored instruments placed at several heights above the sea floor, although few studies also have involved bottom- boundary-layer measurements. Storm-induced sediment gravity flows (Xu et al., 2002; Puig et al., 2003, 2004a; Xu et al., 2010; Martín et al., 2011; Masson et al., 2011; Mulder et al., 2012), enhanced off-shelf ad- vection during storms (Carson et al., 1986; Martín et al., 2006), hyper- pycnal flows and failures from recently deposited fluvial sediments (Khripounoff et al., 2012), dense shelf water cascading (Canals et al., Marine Geology 341 (2013) 14–28 ⁎ Corresponding author. Tel.: +34 93 230 9518; fax: 34 93 230 9555. E-mail address: ppuig@icm.csic.es (P. Puig). 0025-3227/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2013.05.004 Contents lists available at SciVerse ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo