Eve Lundsten

Origins of large crescent-shaped bedforms within the axial channel of Monterey Canyon, offshore California

Crescent-shaped bedforms with wavelengths from 20 to 80 m, amplitudes to 2.5 m, and concave down-canyon crests occur in the axial channel of Monterey Canyon (offshore California, USA) in water depths from 11 to more than 350 m. The existence of these features may be an important new clue as to how sediment moves through submarine canyons. Three complementary studies were initiated in 2007 to understand the origin and evolution of these bedforms. (1) Vibracoring. Three transects of closely spaced remotely operated vehicle–collected vibracores were obtained across these bedforms. The seafloor underneath these features is composed of gravity-flow deposits. (2) Acoustic array. Three boulder-sized concrete monuments containing acoustic beacons were buried just below the surface of the canyon floor in ∼290 m water depth and their locations were redetermined on 17 subsequent occasions. Although the beacons became more deeply buried >0.6 m below the seafloor, they still could be tracked acoustically. Over a 26-month period the position of 1 or more of the beacons moved down-canyon during at least 6 discrete transport events for a total displacement of 994–1676 m. The movement and burial of the monuments suggest that the seabed was mobilized to >1 m depth during gravity-flow events. (3) Autonomous underwater vehicle (AUV) repeat mapping. AUV-acquired high-resolution multibeam mapping, and CHIRP (compressed high-intensity radar pulse) subbottom profiling surveys of the seafloor in the active channel were repeated four times in the first half of 2007. In addition, the movement of large instrument frames deployed in 2001–2003 within the axis of Monterey Canyon in the area now known to be associated with the crescent-shaped bedforms is documented. The fate of the frames has helped elucidate the frequency, transport potential, and processes occurring within the axis of Monterey Canyon associated with these bedforms. The crescent-shaped bedforms appear to be produced during brief gravity-flow events that occur multiple times each year, commonly coincident with times of large significant wave heights. Whether the bedforms are generated by erosion associated with cyclic steps in turbidity flows or internal deformation associated with slumping during gravity-flow events remains unclear.

High-resolution bathymetry of the axial channels within Monterey and Soquel submarine canyons, offshore central California

High-resolution multibeam bathymetry and chirp (compressed high-density radar pulse) seismic data acquired from an autonomous underwater vehicle outline in unprecedented detail the shape and near subbottom character of the axial channels within upper Monterey and Soquel Canyons (offshore California, USA). In Monterey Canyon, the bathymetric data span water depths from 100 m to >2100 m, and include the confluence with Carmel Canyon at ∼1900 m water depth. The bathymetric data for Soquel Canyon begin close to the canyon head at 100 m water depth and extend down to the intersection with Monterey Canyon. The seafloor within the axis of Monterey Canyon is covered with sediment fill out to 910 m water depth. Below this water depth exposures of underlying strata are common, presumably because of decreasing sediment drape and generally increased erosional resistance of the pre-canyon host strata. The seafloor within the axial channel of upper Soquel Canyon is smooth and contains horizontally layered sediment fill. In contrast, the sediment fill within the incised portions of the axial channel of Monterey Canyon is characterized by distinctive crescent-shaped bedforms down to the limit of the surveys. These differences in morphology and texture correspond with the contrasting cohesive strength of the sediments filling these canyons and the increased propensity for weakly cohesive sands and gravels in Monterey Canyon to fail. Episodic movement of coarse-grained sediments down Monterey Canyon maintains a longitudinal gradient of ∼1.6°. The more cohesive fine-grained sediments in Soquel Canyon stabilize the seafloor and maintain a substantially higher longitudinal gradient (3°–6°) than that measured in Monterey Canyon. The textural and lithologic data, plus previously published observations, indicate that upper Monterey Canyon is currently active, whereas upper Soquel Canyon appears to be inactive as a coarse sediment transport conduit. Episodic seabed sediment failures in active submarine canyons are hypothesized to control the gradient of the axial channel. The propensity for sediment failure in weakly cohesive coarse-grained sediments results in shallower horizontal gradients compared to submarine canyons stabilized by more cohesive fine-grained sediments.

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.

Seafloor geomorphic manifestations of gas venting and shallow subbottom gas hydrate occurrences

High-resolution multibeam bathymetry data collected with an autonomous underwater vehicle (AUV) complemented by compressed high-intensity radar pulse (Chirp) profiles and remotely operated vehicle (ROV) observations and sediment sampling reveal a distinctive rough topography associated with seafloor gas venting and/or near-subsurface gas hydrate accumulations. The surveys provide 1 m bathymetric grids of deep-water gas venting sites along the best-known gas venting areas along the Pacific margin of North America, which is an unprecedented level of resolution. Patches of conspicuously rough seafloor that are tens of meters to hundreds of meters across and occur on larger seafloor topographic highs characterize seepage areas. Some patches are composed of multiple depressions that range from 1 to 100 m in diameter and are commonly up to 10 m deeper than the adjacent seafloor. Elevated mounds with relief of >10 m and fractured surfaces suggest that seafloor expansion also occurs. Ground truth observations show that these areas contain broken pavements of methane-derived authigenic carbonates with intervening topographic lows. Patterns seen in Chirp profiles, ROV observations, and core data suggest that the rough topography is produced by a combination of diagenetic alteration, focused erosion, and inflation of the seafloor. This characteristic texture allows previously unknown gas venting areas to be identified within these surveys. A conceptual model for the evolution of these features suggests that these morphologies develop slowly over protracted periods of slow seepage and shows the impact of gas venting and gas hydrate development on the seafloor morphology.

The timing of sediment transport down Monterey Submarine Canyon, offshore California

While submarine canyons are the major conduits through which sediments are transported from the continents out into the deep sea, the time it takes for sediment to pass down through a submarine canyon system is poorly constrained. Here we report on the first study to couple optically stimulated luminescence (OSL) ages of quartz sand deposits and accelerator mass spectrometry 14 C ages measured on benthic foraminifera to examine the timing of sediment transport through the axial channel of Monterey Submarine Canyon and Fan, offshore California. The OSL ages date the timing of sediment entry into the canyon head while the 14 C ages of benthic foraminifera record the deposition of hemipelagic sediments that bound the sand horizons. We use both single-grain and small (∼2 mm area) single-aliquot regeneration approaches on vibracore samples from fining-upward sequences at various water depths to demonstrate relatively rapid, decadal-scale sand transport to at least 1.1 km depth and more variable decadal- to millennial-scale transport to a least 3.5 km depth on the fan. Significant differences between the time sand was last exposed at the canyon head (OSL age) and the timing of deposition of the sand (from 14 C ages of benthic foraminifera in bracketing hemipelagic sediments) are interpreted as indicating that the sand does not pass through the entire canyon instantly in large individual events, but rather moves multiple times before emerging onto the fan. The increased spread in single-grain OSL dates with water depth provides evidence of mixing and temporary storage of sediment as it moves through the canyon system. The ages also indicate that the frequency of sediment transport events decreases with distance down the canyon channel system. The amalgamated sands near the canyon head yield OSL ages that are consistent with a sub-decadal recurrence frequency while the fining-upward sand sequences on the fan indicate that the channel is still experiencing events with a 150–250 year recurrence frequency out to 3.5 km water depths.

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.