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Life in the Mud

September 7, 2017

When you think of canyons on land, you picture deep, steep sides with a river at the bottom. Rockslides and mudslides bring debris crashing down, and evidence of them can be seen strewn across the canyon floor. But what happens in deepwater canyons? What evidence can we find of what is raining down from above?

Launching the CTD.

Launching the CTD.

During this mission we are using a Conductivity, Temperature, and Depth (CTD) instrument to sample the water column. Attached to that is a monocore, which plunges into the mud and brings back a 2-inch-diameter core sample. These two tools are helping us gain an understanding of how nutrients and minerals cycle in the canyons by sampling “what is raining down from above.”

The CTD consists of both sensors to collect water chemistry data and Niskin bottles, which sample water at specific depths. We use charts to determine the best locations for sampling based on depth and the geography of the canyon. The sampling sites cover a variety of areas in the canyon – some near the mouth of the canyon, others in its deepest recesses. Using a winch, we lower the CTD and monocore over the side, letting the apparatus sink until the monocore hits bottom. As the CTD returns to the surface, we stop at the predetermined depths and fire the Niskin bottles. The bottles seal tightly, allowing no additional water in. The number of samples you take is dependent upon the number of bottles your CTD can hold. We usually took eight water samples, and always sampled at the bottom and surface.

Rob filtering water from the CTD.

Rob filtering water from the CTD.

Once the CTD is back on deck, the water from each Niskin bottle is collected in clean containers. This water is run through a 0.7-micron glass fiber filter, concentrating the particulate matter. The filters are kept frozen until they are brought back to the lab. Once back on shore, the researchers scrape off the material captured on the filters, and analyze it for organic content. Comparing what we find at the top to what is found near the seafloor helps us understand what “food” is making its way down into the canyons. The surface of the ocean is highly productive, with phytoplankton converting the sun’s energy into fuel useful for animals. Examining these filters tells us the quantity, and chemical signature, of the food that is available at the top. We can then compare it to the quantity and chemical signature of what we collect close to the bottom.

The core sample of mud.

The core sample of mud.

The monocore also returns to the deck, bringing a cylindrical core sample of the bottom. While the sample depends upon the make-up of the seafloor (for example, we do not get a sample if the bottom is rocky), we usually bring up a tube of mud that is about 10 centimeters high. Precise slices are taken from the core and preserved for future examination. Formalin is added to some samples in order to preserve the animals that might be present, such as small worms and tiny crustaceans. Most of these animals are found in the top two centimeters. Other samples are used to look at the physical and chemical make-up of the mud, such as particle size (is it more sand or more silt?) and the percentage of organic matter.

Amanda is ready to preserve samples.

Amanda is ready to preserve specimens.

While these tiny animals that live in the deep dark ocean might not appear to be relevant to our life on land, they perform important functions, helping cycle nutrients and sequestering carbon. They are part of the food chain and are eaten by many things including bottom-dwelling fish and crabs that sift through the mud hunting for these prey.

Some of the jars of preserved mud.

Some of the jars of preserved mud.

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