In one of my earlier posts I described the scheduling challenge of stopping to take samples while still moving the ship eastward toward Cape Town. We are now at station 22 out of a total of 120 stations. This puts us about one day behind schedule and Barney has decided to eliminate six stations to put us back on track and to allow for at least one day of bad weather. The crew doesn’t like to deploy gear over the side when the wind is blowing over forty knots, and any wind over thirty makes sampling VERY interesting. It has been blowing between twenty to thirty knots for most of the trip and we are getting pretty good at collecting samples in rough weather.
The scientists on the ship use at least five different sampling systems. As a bit of background, it is important to know that the ocean is not uniform top to bottom. The surface water is warmer than the deep water and can have very different concentrations of oxygen, nutrients, and dissolved metals. On this cruise we use different sampling gear to collect samples from just a few centimeters below the surface to over 5000 meters in depth.
Lanyard hooks at top of rosette
Most of our samples are taken with Niskin bottles. Niskin bottles are heavy PVC tubes with top and bottom caps connected with a strong piece of surgical tubing. The top and bottom are held open with a lanyard that can be “triggered” or released at depth. When the lanyard is released the end caps snap closed capturing a water sample. For most of our samples we use a large steel rosette that holds 12 Niskin bottles. The center of the rosette has a lanyard release mechanism that is controlled by the scientists on the ship. Using the rosette system we can collect samples from 12 different depths. The rosette also has a sensor package to measure depth, temperature, salinity, dissolved oxygen, and a number of optical properties. The scientist in charge of the water cast records sensor data as the rosette is being lowered into the ocean. Using the sensor data, interesting features of the water column are identified and the sample depth for each of the 12 bottles is determined. The water samples are collected as the rosette is raised to the surface.
Profile of oxygen, temperature and density as a function of depth
We are using twelve, twenty liter Niskin bottles on the rosette. That means that we must lift over 500 pounds of water plus 300 pounds of rosette out of the ocean and onto the deck of a rolling ship at the end of each cast. The short video below shows a rosette being deployed and then recovered on the Melville. It takes six people to collect samples using the rosette; the Captain to keep the ship stationary, the winch operator to raise and lower the rosette, and four people on the deck to hook the rosette and guide it to the deck.
Once the rosette is back on the ship the sampling begins. One member of the science party acts as bottle cop. You can’t sample until the cop gives you permission. Samples are taken in a specific order to keep from missing samples and to reduce contamination. Samples for dissolved oxygen and total CO2 and alkalinity go first since gases from the atmosphere can contaminate these samples. Next in line come samples for thorium, primary production, photosynthetic pigments, nutrients, salinity, and finally hydrogen peroxide. One to two hours after the rosette leaves the surface we have fresh samples and the analysis begins. Twenty-two stations down and about a hundred more to go.
Check back in a few days for more sampling details and some results from the early samples.
– Whitney
