Under the Seafloor

NOAA Teacher at Sea: Caroline Singler
Ship: USCGC Healy
Mission: Extended Continental Shelf Survey
Geographical area of cruise: Arctic Ocean
Date of Post: 31 August 2010

Location and Weather Data from the Bridge
Date: 29 August 2010 Time of Day: 23:15 (11:15 p.m. local time); 06:15 UTC
Latitude: 79º 40.2’ N Longitude: 130º 26.2’ W
Ship Speed: 9.4 knots Heading: 254º (SW)
Air Temperature: 0.6ºC / 33.0ºF
Barometric Pressure: 1008.2 mb Humidity: 92.8 %
Winds: 10.1 knots SSW Wind Chill: -6.3ºC/20.8ºF
Sea Temperature: -1.4ºC Salinity: 27.78 PSU
Water Depth: 3505.8 m

Date: 30 August 2010 Time of Day: 22:00 (10:00 p.m. local time); 05:00 UTC
Latitude: 76º 52.8’ N Longitude: 137º 35.8’ W
Ship Speed: 9.8 knots Heading: 200.9º (SW)
Air Temperature: -0.3ºC
Barometric Pressure: 1008.5 mb Humidity: 99%
Winds: 3.2 knots W
Sea Temperature: -0.5ºC Salinity: 25.8 PSU
Water Depth: 3675 m

Date: 31 August 2010 Time of Day: 22:25 (10:25 p.m. local time); 05:25 UTC
Latitude: 74º 43.9’ N Longitude: 137º 26.1’ W
Ship Speed: 8.5 knots Heading: 124.8º (SE)
Air Temperature: 1.35ºC / 34.42ºF
Barometric Pressure: 1009.2 mb Humidity: 91.7%
Winds: 10.8 knots NNW Wind Chill: -4.1ºC/25.1ºF
Sea Temperature: -0.5ºC Salinity: 24.33 PSU
Water Depth: 3418.4 m

Science and Technology Log
Most of the geology on this cruise is geophysics - we employ remote sensing techniques to generate computer images of the seafloor without direct observation. Bathymetric tools like the multibeam sonar system are valuable for oceanographers because it removes the veneer of the ocean water and reveals the shape of the underlying seafloor. It also makes a seafloor map look like a game of Candy Land – except when we are mapping in ice and it looks more like Pick Up Sticks. (One night on watch, my partner and I talked about how after a while you start to think of the seafloor as if it were colored like a rainbow!) Subbottom seismic profiles go even deeper and provide clues about the sediment and rock below the seafloor, and a trained geophysicist can read the signature reflections of different materials and make strong inferences about the subsurface. But for geologists like me, the highlight is sampling -- bringing pieces of the seafloor above sea level and directly observing what is there. One reason that I was excited to join this cruise was because I visited the core library at Woods Hole Oceanographic Institution (WHOI) with the Lincoln-Sudbury NOSB team two years ago. The realization of how important such samples are to our understanding of the geological and climatological history of the earth made me eager to be present when a core was taken from the seafloor.

On a bathymetric survey expedition like this, opportunities to stop the ship for an extended period of time are few and far between, but we have had a few windows of opportunity for seafloor sampling. USGS geologists Brian Edwards and Andy Stevenson, armed with bathymetric maps and subbottom profiles from previous surveys, came on the cruise with several potential sampling targets in mind. USGS engineering technicians Jenny White and Pete dal Ferro are ready at a moment’s notice to get to work assisted by Healy’s team of marine science technicians (MSTs).

Coring the seafloor is a lot different from coring on land. The work site is the fantail (stern) of ship in the Arctic Ocean. The target is a point on the seafloor thousands of meters below, guided only by bathymetry and the ship’s navigation system. It takes more than an hour on average to lower the coring equipment on cables to the seafloor, and the water around us is moving with the current, requiring great skill on the part of the Coast Guard crew to hold station – keep the ship in a steady position – for many hours during sampling operations. Add in some wind, cold temperatures, and sometimes ice floes moving around the ship, and it’s easy to see why everyone’s energy level is cranked up a notch when coring operations are the plan of the day.

So far, we have collected core samples at three locations. A core is a long cylindrical section of seafloor. A core provides a relatively undisturbed sample of a vertical section of seafloor, preserving sediments in their natural layers with internal structures more or less intact. This provides a vertical timeline of deposition on the seafloor – the sediment at the bottom of the core represents the oldest material and the sediment at the top is the youngest. Core samples provide “ground truth” that supports the findings of remote sensing techniques like subbottom profiling. They allow scientists to “read” the history of the area. Geologists analyze the size and composition of sediment and infer depositional processes and possible sediment sources. Oceanographers and climatologists use information from the sediment and the microfossils they may contain to learn how the ocean and atmosphere has changed over time with respect to physical parameters such as water temperature and salinity.

We have employed two coring techniques on this core – gravity coring and piston coring. A gravity core uses a 2,000 pound weight attached to a 10-foot section of pipe. The pipe is lowered by cables and winches to the seafloor and uses the force of gravity pulling on the weight to drive it into the subsurface. A piston core is a variation on the gravity core that allows for deeper sampling by stringing together multiple sections of pipe. The main core barrel is fitted with a retractable piston in the top of the tube and the same 2,000 pound weight attached. A separate smaller coring apparatus is connected to the top of the piston core barrel by cables and a trigger arm. It hangs beside the piston core barrel, and the entire apparatus is lowered together to the seafloor. The trigger core reaches the bottom first and penetrates the surface sediments. As it falls, it triggers the mechanism at the top of the piston core which freefalls into the sediment. As the piston retracts inside the core barrel, it creates suction inside the barrel that helps pull the sediment into the core barrel and allows for collection of a longer, deeper, and potentially less disturbed sample than a gravity core.

The steel pipes used for coring are lined with plastic liners. At the end of the core barrel is a core cutter and a core catcher with metal teeth that fits into the bottom of the core barrel and holds the core in the barrel. When the core is retrieved, grab samples are collected from the core cutter and core catcher. (In the photo on the right, USGS scientists Brian Edwards and Andy Stevenson collect samples from a gravity core.) The outside of the core barrel is scraped to provide a sample that can be examined for microfauna (remains of microscopic organisms) in the sediment. The plastic liner is removed from the core barrel, starting at the bottom of the core, and is cut into sections. In this case, the preferred section length is 150 centimeters because that is the size of the containers in which the core will be stored back in the laboratory. Each section is measured, capped, sealed, and carefully labeled to indicate the top of the section and the core location. (In the photo on the bottom right, USGS scientists Brian Edwards, Andy Stevenson, and Helen Gibbons measure and cut the core sleeve from a piston core.) All information is recorded on a log in the field. The core sections are then stored horizontally in a specially built box that is kept in a refrigerator on the ship. The cores will be transported back to the USGS laboratory in California after the cruise where they will be cut, examined and logged, and then carefully stored for future reference.

Sometimes a core contains a real surprise. When the piston core from our first location came up on deck, we saw a white crystalline substance in the core cutter and catcher. It was gas hydrate. (Photo courtesy of Helen Gibbons, USGS Scientist.) Water molecules under high pressure may start to solidify at temperatures above the normal freezing point of water, crystallizing into a solid form of water with an internal structure that contains larger open spaces than typical ice crystals. Normally, these crystals are very unstable and will continue to cool and form the more stable molecule we know as ice. However, gases present in the environment may become incorporated into the open spaces within the solid water molecules and form a gas hydrate. This is a physical combination – there is no chemical bonding between the two – but it allows the solid to remain stable as long as it remains in a high pressure and low temperature environment. Seafloor sediments on deep continental margins and buried continental sediments in polar regions (i.e. permafrost regions) are common places where these compounds form. They contain abundant organic matter. Over time, biogenic processes (bacterial action) or thermogenic processes (high pressure and temperature) act on the organic material and produce gases, most commonly methane. These may become trapped in the solid water and form gas hydrates.

There is a lot of scientific interest in gas hydrates. Some estimates suggest that methane hydrates in permafrost and marine sediments contain more organic carbon than all other known naturally occurring fossil fuel deposits combined. Thus, gas hydrates are considered to be a potential energy source. However, one concern is that hydrates are very unstable at conditions other than those under which they form – the solid water crystals dissociate (i.e. melt) and the gases escape. We saw this with the sample we brought up in the core which began fizzing and off-gassing as soon as it was exposed at the surface. Potential environmental changes that might destabilize naturally occurring hydrates could potentially result in the release of large quantities of methane, a greenhouse gas, to the atmosphere.

We have sampled at four locations to date, shown on the map below. One location was near the top of a small seamount that was first mapped during last year’s expedition. Another sample was from a submarine fan complex. All locations were selected based on some prior data followed by good inferences, a little luck and a lot of skill.

All coring attempts have been successful, with good core recovery each time. It is difficult to predict what we will get when aiming for a target that is so far beneath us. There is only so much that the monitors on the ship that track wire depth and tension can tell us. Given time constraints, there are no “do overs”, so we are happy whenever the core barrel comes up with something inside – it represents more information than we had before we sent it to the bottom. The moments before the barrel is back on deck are full of tense expectation, and one can tell from the look of satisfaction on a scientist’s face when there is a good sample inside. One person’s mud is another person’s treasure! Although I will not get to examine the cores myself, I look forward to hearing what they find when they cut and log the cores back in California. And I have a little bit of ocean floor mud of my own to take home as a souvenir.

Sources
National Energy Technology Laboratory: The National Methane Hydrates R&D Program – All about Hydrates
TDI-Brooks International: Piston Coring for Surface Geochemical Exploration.
USGS Fact Sheet: Gas (Methane) Hydrates – A New Frontier. 1992.
USGS Woods Hole Science Center
Woods Hole Ocean Instruments

Personal Log
This is the last week of the trip. After all the preparation that it took to get here, the time has passed rather quickly – even while I did not have a very clear perception of the passage of time. If I were home, I would have met my classes for the first time yesterday and today. I am sorry to miss school, but I am grateful to be among a relatively small group of people who have the opportunity to experience this part of the world. I am fortunate to have a strong support network of colleagues at Lincoln-Sudbury Regional High School who encouraged me to take advantage of this opportunity and did their best to assuage my feelings of guilt about not being at work. I am fortunate to have such caring friends and colleagues. Thank you, everyone who helped me prepare for the trip and to all those who are keeping things going for me while I am away. You gave me the peace of mind to do this.

The Arctic is a wilderness unlike any other. Whether in the icy desert at latitudes above 80ºN; in thin, patchy ice in the southern and western part of the basin; or in the open waters off the coast of Alaska, each day is something special. I look forward to my first trip out on deck each morning to enjoy the day’s views, and I have not been disappointed. And here in the last week of the trip, as the amount of darkness increases while the latitude decreases, it is actually snowing – enough to make a little snowman on the bow.

Caroline