Seasonal diversity & behaviour of internal tidal bores revealed by year-long direct monitoring within a submarine canyon: Implications for particulate transport

Submarine canyons are the primary conduits for particulate transport from the continents to the deep sea, including sediments, organic carbon, and pollutants. Previously, focus has primarily been on transport due to downslope gravity currents. There is increasing recognition of the role internal tides may play, however, a lack of long-term seabed monitoring limits understanding of their significance. Here, we present novel near-seafloor monitoring data of breaking internal tides (‘bores’), spanning one year within Whittard Canyon, a land-detached submarine canyon on the NE Atlantic margin. Throughout the year, these bores are energetic, with flow speeds regularly exceeding 0.5 m/s, and capable of resuspending seafloor sediments. Bores were observed for ∼80 % of tidal cycles during the deployment. The shape and structure of bores varies throughout the year, in relation to spring-neap tidal cycles and longer-term seasonal changes. During the period of November-March, these bores generally attain higher speeds, show more abrupt changes in flow speed, and hold the greatest potential for up-canyon sediment transport. While fine-grained material may be moved throughout the year, this stronger winter activity has the greatest capacity to mobilize larger grains up-canyon. We show that bores, driven by internal tides, are an important, but previously poorly-observed agent for particulate transport. Bores occur frequently, yet exhibit variability at both seasonal and shorter time scales that may shape particulate transport within submarine canyons. These findings change our view of the general significance of internal tides in submarine canyons, with wide implications for hazards, tempo and fluxes of particulate transport.