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Innovative Monitoring Captures the Passage and Impact of Submarine Canyon Turbidity Currents

Abstract

While sediment density flows (i.e., turbidity currents) are the most significant mechanism for depositing thick sand-rich beds in deep-water environments, rarely can beds be linked to specific flows with measured properties. Yet, this was achieved in a recently completed study that documented the passage of 15 flows along the axis of Monterey Canyon, offshore California using multiple sensors over an 18-month period. The flows were recorded by six downward looking acoustic Doppler current profilers (ADCP) moored between 285 and 1850 meters water depth (mwd) and upward looking ADCPs on a seabed frame in 1830 mwd. These flows had propagation velocities up to 7.2 m/sec, the fastest yet recorded by instruments. Heavy (800 kg) objects were carried for kilometers and left entombed in meters-thick sediment cover. Three of the flows ran for >50 km and continued past the deepest sensor (1,850 mwd). Repeated seafloor mapping and sediment coring documented the impact of these flows on the canyon floor morphology and deposits. During the experiment the canyon floor was mapped with multibeam sonars 6 times between 190-540 mwd and 1500-1900 mwd using Autonomous Underwater Vehicles (AUV) at 1-m lateral grid resolution and 4 times using a Remotely Operated Vehicle (ROV) at 5-cm lateral grid resolution in 1830-1840 mwd. The shallower AUV surveys (190 to 540 mwd) revealed flow-associated ±3 m bathymetric changes, interpreted as migrations of crescent shaped bedforms along the canyon axis. Below 1500 mwd the bathymetric changes are more-subtle. Bathymetric differences between surveys bracketing the largest flow (January 15th, 2016) show up to 87 cm of net deposition and up to 39 cm of net erosion where the slowing flow passed over the upward looking ADCPs at a velocity of approximately 3.3 m/s. To characterize the deposits left by this flow and to track its runout, 40 ROV-collected vibracores (<1.5 m) of bottom sediments were taken at water depths from 1840 to 2870 mwd along the canyon axis. The freshly accreted material around the 1840 depth site is predominantly disorganized gravel containing <7 cm cobbles. Sediment cores farther down-canyon are sand-rich, lack significant gravel beds and have a greater relative abundance of mud. There is no evidence that the January 15, 2016 flow or other flows detected in this study reached water depths >2100 m. This novel dataset relates the character and composition of turbidite deposits to the properties of specific sediment density flows.