Geologist Charlie Paull's research team will use high resolution maps collected by MBARI’s autonomous underwater vehicle (AUV) to aid in remotely operated vehicle (ROV) dives to continue their ongoing investigations of processes associated with seafloor gas venting and modern submarine canyons. They plan to focus on one geographically restricted area surrounding Eel Canyon, offshore of Eureka, California, where both these themes can be addressed simultaneously.
Eel River Margin—Submarine Gas Venting Plumes and Seeps
Several gas vent sites exist along the Eel River margin (see Figure 1 below). A group of persistent water-column acoustic anomalies (referred to as plumes) were discovered during NOAA Ocean Exploration mapping cruises (Gardner et al., 2009). Methane gas bubbles rising from the seafloor are believed to be the cause of these acoustic anomalies. The first discovered plume can be traced using multibeam sonar data from the seafloor (at a depth of approximately 1,800 meters) into the water column for about 1,400 meters (Gardner et al., 2009). A second mapping cruise passed over this area, confirmed the persistence of the original plume and identified four similar plumes. The tops of all five plumes are at about 400 meters water depth, which is above the top of pure methane hydrate stability in this area (~520 meters water depth), suggesting that thermogenic gases may be present, which enhance gas hydrate stability. Four of these plumes emanate from Eel Canyon, and one is from a slide scar just south of the canyon. The existing surface ship multibeam bathymetry indicates that some of these plumes originate from seafloor topographic features similar to the Santa Monica and Barkley Canyon mounds, which are topics of ongoing research by several groups at MBARI.
Other methane seep sites in shallower water depths (520 to 600 meters) also occur in the Eel River area. A fortuitous discovery of gas hydrate in a piston core during the 1980s resulted in the Eel River area becoming a classic seep research site (Brooks et al., 1999). Since then, many research groups have visited the same three gas vents largely because their locations are known. Very little is still known about the geologic context of these features and even a detailed basemap of the classic Eel River seep area does not exist. Existing seafloor images in this area reveal very rough, fine-scale topography. These sites are especially curious because they are at or near the top of methane-hydrate stability in this area, and the present seafloor here could have been impacted and altered by multiple cycles of gas hydrate formation and decomposition.
Understanding the processes associated with seafloor gas venting and the impact of associated phenomena on seafloor morphology has been a long-term goal of the Paull group. These efforts include detailed work looking at the local biogeochemistry, sources of energy to support chemosynthetic biological communities, and the formation of authigenic carbonates. Recently collected AUV multibeam data, complemented by ROV observations and sampling have also revealed a remarkable amount of fine-scale geomorphology and seafloor structures associated with gas venting and/or near subsurface gas hydrate accumulations. These include previously unknown karst-like seafloor textures, seafloor depressions, and local topographic highs. These features clearly indicate dramatic modification of the seafloor, involving both localized excavation in some areas and the elevation of the seafloor in others. Apparently, these features are somehow related to gas venting, gas hydrate development, and related phenomena. However, the processes that generate such features are poorly understood. MBARI’s AUV high-resolution seafloor surveys provide a unique and new dimension in seafloor vent studies. Application of MBARI’s combined assets now enable imaging and sampling the seafloor associated with these features in enough detail to begin to understand the geologic evolution and geochemical history of these morphologic features. Efforts will also be taken to utilize sensors on our AUV and ROV to image these water column hydrocarbon plumes.
Eel Canyon—Sediment Transport and Hyperpycnal Flows
Sediments on the floor of Eel Canyon and its fan (Figure 1) are likely to reflect processes that are substantially different from those responsible for the most recent sediment deposition on the floor of Monterey Canyon or any of the other canyon systems we have investigated to date. A supply of sand appears to be necessary for generating the types of sediment transport "events" we now know occur regularly within Monterey Canyon. Repeated AUV multibeam mapping has shown that these transport events alter the shape of the axial channel and leave behind the crescent-shaped bedforms that have attracted so much interest from the geologic community. Because Eel Canyon’s head is not connected to the shoreline, its does not receive a regular supply of sand. However, Eel Canyon may have experienced significant activity of a different kind. More specifically, floods from the Eel River are thought to be capable of generating periodic hyperpycnal flows—water washing out of the river laden with sediments and more dense than the ocean water, so flows along the seafloor—which presumably descends into the Eel Canyon carrying huge amounts of fine sediment (Mulder and Syvitski, 1995).
Figure 1: Part A shows bathymetry at 100-meter contour intervals of the continental margin offshore of Eureka, California, and locations of Eel River, Mendocino Fracture Zone (MFZ) and Eel Canyon. The location of the classic Eel River seeps are indicated with an X. The area indicated with the red box is shown in more detail in Part B. Part B shows multibeam bathymetry of the area where six huge water column plumes were discovered (black dots; Gardner et al., 2009). Two emanate from slide scars to the south of Eel Canyon, and four emanate from the seafloor and northern flanks of Eel Canyon. Arrows indicate steps within the canyon floor. Part C is a detailed map of the slide scar area where a feature similar to the Santa Monica mounds occurs (black arrow).
The continental shelf off the Eel River was the site of the STRATAFORM project, an intensive study of sediment transport from a river onto a continental shelf (e.g., Nittouer, 1999). This project benefited from the fortuitous occurrence of huge floods on the Eel River in 1995 and 1997. These were 25- to 100-year flood events and had sediment loads capable of generating hyperpycnal flows. These flooding events temporarily dumped an appreciable blanket of sediment on the continental shelf. However, only 25 percent of the discharged sediment could be accounted for and the fate of the majority of the sediment carried in these flooding events has never been established (Imran and Syvitski, 2000). Whether or not any of the historical floods generated hyperpycnal flows that descended into Eel Canyon, the axis of Eel Canyon is a place where periodic hyperpycnal flows are likely to have occurred in recent times.
Lamb et al. (2008) have shown using 10-meter-resolution surface-ship multibeam data that huge repetitive bedforms with wavelengths of approximately one kilometer and amplitudes over 10 meters occur on the flanks of Eel Canyon. They suggest these bedforms were formed by gigantic hyperpycnal flows, which is further evidence for the occurrence of hyperpycnal flows in Eel Canyon. Moreover, the existing surface ship multibeam data indicate that a series of curious topographic steps occur along the canyon axis, which are not clearly resolved (Figure 1). Our previous experience has shown that the much more detailed data provided by AUV multibeam surveys are critical for conducting an intelligent sampling program and provide dramatically more understanding and constraints on the nature of the processes that occur within submarine canyons.
While hyperpycnal flows are believed to be among the major sediment transport processes on earth, surprisingly little is know about them, especially as suspended sediments move away from the river mouths and flow downslope. For example, no substantial documentation exists about the nature of the sedimentary deposits within Eel Canyon below 600 meters water depth. In fact, remarkably little is known about the deposits that hyperpycnal flows produce. Most of the putative hyperpycnal flow deposits have been identified within terrestrial outcrops. However, it is simply an inference that any of the deposits in Eel Canyon were actually deposited by hyperpycnal flows (Mulder and Syvitski, 1995). Obviously, this is an area that needs calibration with a modern process study of the type we have pioneered using our combined AUV multibeam mapping and ROV-deployed vibracoring systems and propose to conduct in Eel Canyon.
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