Monterey Canyon: Sediment Super-Highway to the Deep Sea?
The Monterey Bay National Marine Sanctuary encompasses some of the world’s most spectacular physiography. One of the deepest and largest submarine canyons on the coast of North America is the Monterey Canyon, which is 470 kilometers long, approximately twelve kilometers wide (at its widest point), and has a maximum rim to floor relief of 1,700 meters. Imagine the Grand Canyon covered by ocean.
Monterey Canyon begins within 100 meters of the beach at the mouth of Moss Landing Harbor and can be traced down slope into more than four kilometers of water. Like most canyons on land, the shapes of submarine canyons indicate that erosional processes that are focused within their axial channels have carved them. The occurrence of huge volumes of sediment within the adjacent deep-sea fans documents that these canyons are major conduits that also funnel sediment from the continent into the deep sea. Similar submarine canyons scar the continental margins of the world. Unfortunately, little is known about the processes and rates of sediment transport and erosion within submarine canyons.
|Figure 1. Contour and slope map showing Monterey Canyon. Contours interval is 500 meters, and the intensity of the shading is proportional to the slope of the seafloor. Note that a well-developed axial channel exists at the base of Monterey Canyon that has comparatively gentle slopes with respect to the slopes on the canyon’s side walls. This channel is believed to be a major sediment transport conduit.
Our collective ignorance about the dynamics of submarine canyons has persisted into the twenty-first century because the oceanographic community has lacked adequate technologies to study submarine canyons effectively. Until recently, most of what was known about the morphology of submarine canyons was based on scattered echo sounder survey lines and rock samples from the sides of various canyons. Developments in multi-beam bathymetry have greatly improved the ability to image canyons and have made it clear that there is a well-defined axial channel between the steep sidewalls of Monterey Canyon. This axial channel is relatively flat-floored (≥2°) and has a series of bars and meanders that look like those in a terrestrial riverbed.
While the morphologic similarity between the axial channel in Monterey Canyon and a terrestrial river channel is striking, the canyon lacks the equivalent of the river. Unlike rivers, a regular down-slope flow of water is not known to occur within submarine canyons. Moreover, the water in the ocean is usually well stratified, which makes it difficult to sustain down-slope flows. Most of the existing measurements of the currents within submarine canyons suggest that the strongest daily flows travel up, rather than down, the canyon. Thus, the assumption is that periodically there must be energetic “events” that move material down slope. One analogy might be with avalanches in terrestrial mountains. The deposits that occur in the deep-sea fan at the base of the canyon also suggest that sand and even coarser materials only periodically come out of the canyon, because these deposits consist of isolated sand layers, interspersed within fine sediments. Still, only a few samples and limited environmental data have been collected within axial channels of submarine canyons.
In 2000 the Monterey Bay Aquarium Research Institute (MBARI) began an effort to study the active processes within Monterey Canyon, using the same techniques used to study a river system on land. This involves sampling the materials within the channel, monitoring how the channel shape changes with time, and measuring the physical conditions that occur within this environment. While these operations are relatively straightforward on land, access to the channel systems within submarine canyons is much more difficult. However, MBARI’s remotely operated vehicles (ROVs) Ventana and Tiburon have made it possible to study the channels within the base of Monterey Canyon systematically. During the first two years of this project we have made a number of basic measurements and important observations that provide us with an unprecedented view of how Monterey Canyon operates.
A vibracoring system was built for use off ROVs. These corers work by inducing high-frequency vibrations in the core liner that in turn liquefy the sediment immediately around the core cutter, greatly reducing the sediment resistance and allowing long cores to be collected from coarse-grained sediments. ROV-based vibracoring operations have revealed that an essentially continuous tongue of sand extends from the beach down the canyon floor to a depth of about 1,400 meters. This tongue of sand, indistinguishable from the beach sands of Monterey Bay, is tightly restricted to the very axis of the canyon. The sediments on the flanks of the canyon, more than about five to ten meters above its axial channel, are predominantly fine grained. Thus, the processes that carry sand into the canyon are very narrowly focused within the canyon’s axis. Moreover, carbon-14 dating has shown that the materials exposed on the floor of the upper canyon are relatively young, which suggests the canyon is filling rather than eroding. One sample was collected at thirty-two centimeters below the canyon floor from a sediment core taken in 1,400 meters of water in the axis of the canyon. This sample contained algae so fresh that it was still green; it had apparently been washed down the canyon recently.
Pesticide residue studies demonstrate that fine-grained sediments have penetrated through the entire canyon and out onto the fan. Surprisingly, these fine sediments have passed through the canyon without substantial dilution. Thus, while the canyon is clearly an active sediment transport conduit, volumetrically significant quantities of old materials are not being eroded from the canyon sides, which would dilute the DDT-bearing sediments moving down through the canyon axis.
Four energetic sediment transport events were documented in the upper reaches of Monterey Canyon between December 2002 and March 2003. These events have been documented because robust instrument platforms were recovered after they had been washed considerable distances down canyon, damaged, and buried in up to two meters of sand. The frequency of these energetic events makes it obvious that the axis of the upper canyon is a very active sand and coarse sediment transport conduit. Developing instrument packages that can be deployed within the canyon axis to monitor the conditions that occur during these sediment transport events is an ongoing research objective.
Monterey Bay Aquarium Research Institute