Time‐Lapse Seafloor Surveys Reveal How Turbidity Currents and Internal Tides in Monterey Canyon Interact With the Seabed at Centimeter‐Scale.

Here we show how ultra‐high resolution seabed mapping using new technology can help to understand processes that sculpt submarine canyons. Time‐lapse seafloor surveys were conducted in the axis of Monterey Canyon, ∼50 km from the canyon head (∼1,840 m water depth) over an 18‐month period. These surveys comprised 5‐cm resolution multibeam bathymetry, 1‐cm resolution lidar bathymetry, and 2‐mm resolution stereophotographic imagery. Bathymetry data reveal centimeter‐scale textures that would be undetectable by more traditional survey methods. Upward‐looking Acoustic Doppler Current Profilers at the site recorded the flow character of internal tides and the passage of three turbidity currents, while sediment cores collected from the site record flow deposits. Combined with flow and core data, the bathymetry shows how turbidity currents and internal tides modify the seabed. The turbidity currents drape sediment across the site, infilling bedform troughs and smoothing erosional features carved by the internal tides (e.g., rippled scours). Turbidity currents with speeds of 0.9–3.3 m/s failed to cause notable bedform movement, which is surprising given that flows with similar speeds produced rapid bedform migration elsewhere, including the upper Monterey Canyon. The lack of migration may be related to the character of the underlying substrate or indicate that turbidity currents at the site lack dense, near‐bed layers. The scale of scours produced by the internal tides (≤0.7 m/s) approaches the scale of features recorded in the ancient rock record. Thus, these results illustrate how the scale gap between seabed mapping technology and the rock record may eventually be bridged. Plain Language Summary: This study presents the highest resolution repeat seafloor mapping surveys to date in a submarine canyon. Centimeter‐scale seafloor surveys were conducted over an 18‐month period at a location ∼50 km from the head of Monterey Canyon. Turbidity currents (rapid, down‐slope density‐driven flows of water) and internal tides (tidal currents that flow along the seabed) were recorded by a seafloor instrument node deployed in the study area. Turbidity currents drape sediment across the bedforms, infill troughs, and smooth out erosional features carved by internal tides. While the turbidity currents caused considerable migration of bedforms in the upper canyon, there was surprisingly no notable migration at the study site. The lack of migration may be related to the relatively coarse nature of the seafloor underlying the bedforms and suggests that the turbidity currents at the site lacked a dense near‐bed layer characteristic of flows farther up‐canyon. The internal tides carved centimeter‐scale scours in the seabed, a scale that approaches that of features preserved in the rock record. This study, therefore, shows how new mapping technology may eventually bridge the scale gap between modern seafloor surveys and the ancient rock record. Key Points: Ultra‐high resolution time‐lapse mapping is combined with flow monitoring and sediment cores to document submarine canyon activityFast turbidity currents fail to cause notable bedform migration, though flows with similar speeds cause significant migration up‐canyonSub‐meter to meter‐scale scours, likely carved by internal tides, approach the scale of features preserved in the rock record [ABSTRACT FROM AUTHOR]