Day 4: Eel Canyon scour
July 30, 2013
The Eel River is California’s third largest watershed. Approximately 320 kilometers (200 miles) long, it drains a rugged area in the California Coast Ranges. For most of its course, the river flows northwest, parallel to the coast, originating in Bald Mountain in Mendocino County, winding through part of Trinity County and entering the Pacific Ocean 24 kilometers (15 miles) south of Eureka in Humboldt County. The river has both State and Federal Wild and Scenic River status and the river’s estuary is protected by the California Bays and Estuaries Policy.
Because the Eel River drains an area of high relief and high rainfall, it is prone to generate hyperpycnal events, where the water washing out of the river is more dense than the ocean water where it discharges. The resulting cyclic layers in the sediment in Eel Canyon probably represent individual river flood events. Stratigraphic sections in the canyon promise to contain an unprecedented climate record spanning the Holocene and late Pleistocene epochs.
There has been extensive research at the head of Eel Canyon, but very little is known about the canyon dynamics deep in the canyon. Therefore, we know a lot about events bringing sediment into the canyon, but very little about flow of sediment out of the canyon. Today, we sampled sediments in an interesting scoured area at about 2,600 meters in the canyon using the remotely operated vehicle (ROV) Doc Ricketts. The scour has dramatic bedding that is exposed on large and small ledges that span about 60 vertical meters. Two years ago, Charlie Paull’s team sampled at a similar scour that spanned about 90 vertical meters. These features give Charlie the opportunity to count the layers up the scour and sample individual layers within the bedding.
The bathymetric map of the scour shows the dramatic ledges. The red shape is the ship’s location and the blue rectangle is the ROV’s locations. The red dots show the ROV’s track over a few hours today. We sampled along an 800 meter transect from below the scour, up the ledges, and above and beyond the scour.
In the top image, you can see the dramatic ledges with bedding that dominates this scour. In the bottom image, the ROV manipulator prepares to take a horizontal push core in one of the layers of a ledge.
There were patches of vesicomyid clams along the ROV transect. Vesicomyid clams are chemosynthetic, meaning they use energy released from inorganic chemical reactions for food. The presence of these clams means that there is hydrogen sulfide available in the sediment. Studying the geochemistry and knowing what gases are present in the sediment in all of the areas sampled today can help the team understand how and where gas is reaching the seafloor.
Vesicomyid clams have symbiotic bacteria that use hydrogen sulfide and oxygen to create energy.
We sampled the smaller clams by inserting the pushcore into the clam bed. Here you can see the clams poking out of the sediment from the pushcore. These clams were collected and frozen for later molecular analysis by our colleague Bob Vrijenhoek. Bob and his lab study the evolution of chemosynthetic communities.
Roberto Gwiazda will analyze the pore water from push cores for the presence of methane, a chemical that can be converted to hydrogen sulfide by the free-living or symbiotic bacteria. The pore water from the vibracores will be analyzed for chloride and sulfate.
Roberto squeezes the pore water out of push-core-sampled sediment in the clam bed.
Pete Dartnell (left) inserts tubing attached to a syringe to extract pore water from a vibracore while Tom Lorenson (right) drills a hole in the core so the pore water can be extracted from that core.
We were treated to a beautiful sunset tonight. This is the first time we’ve seen the sun since before we left Moss Landing (the summer fog is determined out here). We will, however, take calm seas and little wind over sun any day!
— Susan von Thun