Showing papers on "Monterey Canyon published in 2005"

Trail of sand in upper Monterey Canyon: Offshore California

Abstract: Detailed sampling of the axis and flanks of upper Monterey Canyon (water depths of <1500 m) was undertaken using a remotely operated vehicle (ROV)-deployed vibracoring system. The objective was to document the characteristics and distribution of sediment within the canyon and to elucidate the sources and processes by which materials move into and through the canyon. Detailed multibeam bathymetric surveys guided the sampling. The combination of core data, multi-beam bathymetry, and multi-beam reflectivity reveals for the first time the facies and facies distribution patterns associated with the axial channel of an active submarine canyon. Coarse-grained deposits form a narrow trail of material that is tightly restricted to the axial channel floor, indicating that upper Monterey Canyon is an active submarine system closely coupled to sand transport along the shoreline. Sand from the modern beach and nearshore environment captured by the canyon head is moving down the axial channel of the canyon, while fine-grained sediments are accumulating on the flanks of the canyon. Our observations suggest that other submarine canyons in close proximity to the shoreline are presently active and that a modern supply of sand-sized material is moving down these canyon systems.

Semiannual patterns of erosion and deposition in upper Monterey Canyon from serial multibeam bathymetry

Abstract: Recently acquired 3-m-resolution 244 kHz multibeam seafloor bathymetry (0.5 m depth precision) reveals geomorphology at sufficient detail to interpret small-scale features and short-term processes in the upper 4 km of Monterey Canyon, California. The study area includes the continental shelf and canyon features from 10 m to 250 m depth. The canyon floor contains an axial channel laterally bounded by elevated complex terrace surfaces. Sand waves with 2 m height and 35 m average wavelength dominate the active part of the canyon floor. The sand waves are strongly asymmetrical, indicating net down-canyon sediment transport in this reach. Terraces, including a broad 25-m-tall terrace complex near the head of the canyon, bear evidence of recent degradation of the canyon floor. Slump scars and gullies having a variety of sizes and relative ages shape the canyon walls. Serial georeferenced digital elevation models were analyzed to detect net changes in bathymetry or morphology occurring during both a six month period (September 2002 to March 2003) and a 24-h period (24 March to 25 March). Significant changes over the six month period include: (1) complete reorganization of the sand waves on the channel floor, (2) local channel degradation creating new 2-m-tall erosional terraces on the channel margins, (3) local channel widening that laterally eroded older channel margin terraces, and (4) 60 m extension of one minor gully head on a steep canyon wall. There were no discernable changes in morphology during the 24-h study period. Raster subtraction of serial bathymetric grids provides estimates of sediment erosion and deposition that occurred between the canyon head and a point 2 km down canyon during the six month study. Erosion of 320,000 m 3 (±80,000 m 3 ) of sediment occurred mainly in the tributaries, along the margins of the axial channel, and in the lowest 700 m of the analyzed reach. This eroded volume was approximately balanced by 260,000 m 3 (±70,000 m 3 ) of sediment deposition that was concentrated in the nearshore region along the rim of the canyon head. There was no measurable sediment gain or loss during the 24-h study period.

Physical-biological coupling in Monterey Bay, California: topographic influences on phytoplankton ecology

Abstract: Physical-biological couplings impacting phytoplankton ecology are examined with synoptic, high-resolution observations of Monterey Bay, California. Influences of submarine canyon and shelf break topography on the physical-biological couplings are supported by 2 case studies. In the first case study, benthic-pelagic coupling was observed in southern shelf waters where a turbid plume extended from the bottom at ~60 m deep to the base of a phytoplankton layer centered at ~10 m deep. The alongshelf scale of the plume ranged from ~5 km near the bottom to ~1 km at its intersection with the phytoplankton layer. In situ and remote sensing data support the influence of Monterey Canyon on circulation forcing the benthic-pelagic coupling. In the second case study, a frontal zone and adjacent waters were rapidly surveyed over ~20 km 2 of the northern shelf. The front was associated with an isopycnal ridge/trough structure, surface slick, and frontal eddy <1 km in diameter. The magnitude and vertical location of a chlorophyll maximum layer were closely coupled with the physical environment through the frontal zone. The layer was dispersed by the isopycnal ridge and frontal eddy, and concentrated in the isopycnal trough and along the periphery of the eddy. Influence of an internal wave generated by interaction of tidal currents with the shelf break is sup- ported by the observed surface slick, measured water velocities, and the proximity and orientation of the shelf break. Significant and persistent influences of topography on phytoplankton ecology in Monterey Bay are indicated.

Bioerosion by chemosynthetic biological communities on Holocene submarine slide scars

Abstract: Geomorphic, stratigraphic, and faunal observations of submarine slide scars that occur along the flanks of Monterey Canyon in 2.0–2.5 km water depths were made to identify the processes that continue to alter the surface of a submarine landslide scar after the initial slope failure. Deep-sea chemosynthetic biological communities and small caves are common on the sediment-free surfaces of the slide scars, especially along the headwall. The chemosynthetic organisms observed on slide scars in Monterey Canyon undergo a faunal succession based in part on their ability to maintain their access to the redox boundaries in the sediment on which they depend on as an energy source. By burrowing into the seafloor, these organisms are able to follow the retreating redox boundaries as geochemical re-equilibration occurs on the sole of the slide. As these organisms dig into the seafloor on the footwall, they often generate small caves and weaken the remaining seafloor. While chemosynthetic biological communities are typically used as indicators of fluid flow, these communities may be supported by methane and hydrogen sulfide that are diffusing out of the fresh seafloor exposed at the sole of the slide by the slope failure event. If so, these chemosynthetic biological communities may simply mark sites of recent seafloor exhumation, and are not reliable fluid seepage indicators.