Everyone is up early this morning, as our schedule starts at 0600 (we work on a twenty-four-hour clock here, military style). We have settled in about a nautical mile from the iceberg. (A nautical mile is 1.15 miles or 1.85 km).
The views of the iceberg in the shifting morning light are quite spectacular from this distance.
Waves roll in and crash against its side. Ice calves from its top with a tremendous splash.
Snow petrels are abundant here, perhaps using the iceberg as a handy resting place.
We all gather in the forward dry lab to observe the monitors as the CTD is lowered to a depth of 1,500 meters.
The CTD collects data related to the major properties of the water, including Conductivity, Temperature, and Depth, as well as other measures such as oxygen, salinity, and fluorescence (the amount of chlorophyll present). We use the CTD downcast as a sort of reconnaissance mission to decide where to fire the bottles to collect water. For example, we target the area where the fluorescence peaks, indicating an abundance of chlorophyll related to phytoplankton, including diatoms.
As the depth increases, fluorescence first spikes at about 70 meters down and soon tapers off to zero. Oxygen decreases, and salinity and density increase, just as I expected—the deeper the water, the saltier and denser it is and the fewer the photosynthesizing organisms. But the big surprise for me is that near the surface, as the depth increases, so too does the temperature. The really cold water at the very surface is also fresh, the product of large scale ice melt in the Southern Ocean. Once you get below this cold, fresh layer, the waters are coming from the deep ocean, where the average temperature is about 4°C.
During its return to the surface, the rosette of Niskin bottles captures seawater at various depths selected using the CTD downcast data.
Next, it’s time for the McLane pumps. Four pumps are clamped at different places on the line, so they end up at different depths. These pumps are designed to capture whatever particles are in the water including dead diatoms that are sinking through the water column. They will pump for 4 hours at a rate of 6 liters per minute. Any faster, and we risk damaging the material we are attempting to capture.
Anyone who ventures onto the aft deck must wear a full complement of safety gear, which includes a hard hat, a floatation jacket, and steel-toed boots. It’s wise to wear sturdy overalls both for protection and warmth, too. Anyone standing in the yellow zone, painted on the decks near to the gates where the ship is relatively open to the sea, must be clipped into a safety line, but everyone on the aft deck needs to be careful where they stand and remain aware of their surroundings at all times.
After lunch, when the pumps are retrieved, Pat recovers all the particles from the screens inside the pumps and distributes them for analysis. The samples get divided up so we can learn what organisms we captured and the overall chemical composition of the particles. Colin resumes his diatom isolation process. Diana measures the amount of chlorophyll in the water. Heather measures the biogenic silica (amount of particle silica that comes from diatoms, as opposed to silica from inorganic sources such as sand, dust, etc.). Ivia and Mark filter water as it comes out of the Niskin bottles. They will take some back to UCSB to measure silicon isotopes, and Becky will take some to measure nitrogen isotopes. (An isotope is any of two or more forms of a chemical element with the same number of protons in its nucleus but a different number of neutrons.)
It is well into the evening when we deploy the megacorer, a piece of equipment that, when lowered to the seafloor, sinks into the sediment, capturing mud cores. As with the sediment grab, we won’t know exactly what we have until it returns to the surface, but based on our position and what the “Chirp” and CTD have told us, we are hopeful the megacorer will return viable sediment cores.
While the megacorer takes less than a minute to sink into the sediment, the lowering and raising process takes a total of three hours, so the team is working late into the night. Yet again, I am impressed by how hard the scientists, technicians, and crew work and by what they put they put their bodies through. For me, this has been a day of fatigue and my first seasickness, so I’m off to bed long before the return of the megacorer. I can’t wait to see—tomorrow morning—what it has yielded!
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