Krystle Anderson

Sub-decadal turbidite frequency during the early Holocene: Eel Fan, offshore northern California

Remotely operated and autonomous underwater vehicle technologies were used to image and sample exceptional deep sea outcrops where an ~100-m-thick section of turbidite beds is exposed on the headwalls of two giant submarine scours on Eel submarine fan, offshore northern California (USA). These outcrops provide a rare opportunity to connect young deep-sea turbidites with their feeder system. 14 C measurements reveal that from 12.8 ka to 7.9 ka, one turbidite was being emplaced on average every 7 yr. This emplacement rate is two to three orders of magnitude higher than observed for turbidites elsewhere along the Pacific margin of North America. The turbidites contain abundant wood and shallow-dwelling foraminifera, demonstrating an efficient connection between the Eel River source and the Eel Fan sink. Tur bidite recurrence intervals diminish fivefold to ~36 yr from 7.9 ka onward, reflecting sea-level rise and re-routing of Eel River sediments.

Powerful turbidity currents driven by dense basal layers

Seafloor sediment flows (turbidity currents) are among the volumetrically most important yet least documented sediment transport processes on Earth. A scarcity of direct observations means that basic characteristics, such as whether flows are entirely dilute or driven by a dense basal layer, remain equivocal. Here we present the most detailed direct observations yet from oceanic turbidity currents. These powerful events in Monterey Canyon have frontal speeds of up to 7.2 m s−1, and carry heavy (800 kg) objects at speeds of ≥4 m s−1. We infer they consist of fast and dense near-bed layers, caused by remobilization of the seafloor, overlain by dilute clouds that outrun the dense layer. Seabed remobilization probably results from disturbance and liquefaction of loose-packed canyon-floor sand. Surprisingly, not all flows correlate with major perturbations such as storms, floods or earthquakes. We therefore provide a new view of sediment transport through submarine canyons into the deep-sea.The structure of turbidity currents has remained unresolved mainly due to lack of observations. Here the authors present data from a high-resolution monitoring array deployed for 18 months over Monterey Bay, that suggests turbidity currents are driven by dense near-bed layers.

Fine-Scale Morphology of Tubeworm Slump, Monterey Canyon

Multibeam bathymetry and chirp seismic reflection profiles collected using an autonomous underwater vehicle reveal the morphology and shallow seafloor structure of Tubeworm Slump on the flank of Monterey Canyon at an unprecedented resolution. The data show smaller subsidiary deformation above the headwall, on the headwall, within the sediment drape that covers the sole of the slide, and on the sidewall of Monterey Canyon below Tubeworm Slump. The AUV data indicate that the existing slump scar represents a composite of gravity-driven deformation generated by multiple failure events.

Eel Canyon Slump Scar and Associated Fluid Venting

Autonomous underwater vehicles have been used to characterize Eel Slump, a slide scar located south of Eel Canyon, California. The presence of a well developed dendritic network on the headwall with gullies tens of meters deep, thick sediment drape cover on the slide scar sole, and the absence of fresh surfaces on the scarp suggest that the mass failure(s) that produced this feature did not take place in the recent past. Thermogenic oil and gas emanating from a large mound in the sole of the slide scar were sampled with a remotely operated vehicle. Other distinctive morphologies observed from the seafloor of the slide scar indicate fluid seep has occurred at multiple sites within the slide scar sole.