J Cameron Thrash
300 tons of scientific equipment, a dozen research groups and nine days: welcome to the Western Atlantic Climate Survey cruise.
Photos courtesy J Cameron Thrash.
Sunset in the Sargasso Sea, 200 miles north of Bermuda.
Our first sampling station on the Georges Bank, around 150 miles off the coast of Boston, was a region of extremely high productivity, meaning that the phytoplankton discussed in previous posts were in extremely high abundance, fueling the food chain up to fish and larger animals, of which we saw many. It also meant the water was a characteristically green color from the chlorophyll in the phytoplankton. Now we are in the deeper ocean around 200 miles north of Bermuda, what oceanographers refer to as “blue water” because it is so much more blue due to the greater depth and lower productivity. Many of the analyses on the Brown, including those I am participating in, aim to understand what is different about the transfer of compounds between the sea and the atmosphere between such contrasting locations.
The types of activities that go on aboard a working research vessel can vary widely, and this cruise on the Brown exhibits a remarkable diversity of projects. Each of these requires specific equipment to collect, process, and analyze data from the air and sea around us. The atmospheric chemists use large snorkles to suck in air, which gets passed to extremely sensitive equipment like mass spectrometers or captured for analysis by fourier transform infrared spectroscopy, techniques which measure particle sizes and chemical composition. See a groundbreaking study on the chemical composition of aerosol particles by Lynn Russell’s team from Scripps Institute of Oceanography (some of which are on our cruise) here.
Left: Atmospheric sampling takes place from inlets on the jackmast (black, forward) and the inlet snorkels on top of the containers. Right: Each inlet has a variety of ports which send samples to different pieces of equipment for analysis.
Several groups also make use of bubbling mechanisms to simulate breaking waves and measure how this facilitates chemical exchange with the atmosphere. Trish Quinn, our Chief Scientist, Tim Bates, and their team use a machine called the “sea sweep,” which sits in the water on pontoons and generates bubbles the way wave action would over large swaths of water. For more information about the sea sweep, check out the results of a recent study off the coast of California here. The aerosol particles created by this process are collected in a large tube and sent to the same instruments as those collected from the air. Bill Keene (also on the California study, above) and his group use a similar method that controls the bubble capture more precisely than the sea sweep, but at the expense of the volume of particles it can collect.
Left: Deployment of the sea sweep over the side. Center: The sea sweep in operation. The bubbles trailing behind are what create the aerosol release under the hood. Collection hoses can be seen snaking out of the top. Right: the bubbler machine inside another container.
Collectively, these measurements of aerosol particles at the sea surface and in the atmosphere determine the composition, size distribution, and flux of compounds between the water and air. These fluxes, or exchanges, of particles between the air and the sea, are critical for building accurate models of how the atmosphere is influenced by the chemical and biological components of the sea in different places. This information all contributes to better understanding of how the atmosphere retains/reflects heat, and helps refine models of global climate.
A classic tool of oceanographers is the CTD, which stands for Conductivity, Temperature, and Depth, because it measures these things, but most CTDs also measure a lot more. Ours also has a rosette of collection bottles that are used for sampling water at discrete depths. They can be electronically closed at a given depth and brought to the surface for analysis. Water collected from CTDs can be processed in a number of ways. For example, Dave Kieber and his group are using CTD water to determine chlorophyll concentrations and dissolved organic carbon in the water column.
The CTD in three different environments. Left: Secure on the deck. Center: Mid deployment over the side in green, productive water at Station 1. Right: Into the blue, low productivity water at Station 2.
In my case, Kim Halsey and I are looking at the microorganisms in the seawater by concentrating them through a series of filtration steps. We then test these for utilization and production of compounds in the surface water, using an advanced technique called proton-transfer mass spectrometry (PTR-MS). Our collaborator Martin Graus operates the PTR-MS, which we use because it offers extraordinary sensitivity and resolution in detecting compounds that are present in very small quantities. In addition, I am collecting the DNA from microorganisms at different depths, which I will have sequenced to determine what kind of microorganisms are in the water and what their genes tell us about what they do there.
Left: The Tangental Flow Filtration system I have been using to concentrate microbes in seawater. Right: Clay holding an example XBT and storage/release canister.
Anywhere the Brown goes it operates sophisticated sonar systems to collect mapping data about the ocean bottom. I spoke with Lead Electronics Technician Clay Norfleet about sonar operations on the Brown. He was a sonar technician in the Navy, and now oversees all the electronics onboard, including the Wi-Fi system that lets me place these posts while at sea. He described how accurate sonar mapping requires four hour drops of Expendable Bathyothermograph probes, or XBTs, to measure water temperature and density in order to predict sound velocity from the ship to the ocean floor, in some cases, 12,000 meters deep. We’re not dropping them on this trip, since it is not a mapping cruise, but the Brown’s sonar is running all the same to provide additional data to seafloor mapping efforts.
There is also time for naturalism, the most classic version of biology- observing the natural world as is. From our mobile research island, we have seen a variety of marine life- seabirds, turtles, sharks, a pod of pilot whales, a school of dolphins, and I even had the remarkable treat of viewing a whale in full breach a mile off the bow a few days ago.
This afternoon we will be leaving station and heading towards Bermuda, which will take a day of sailing. There, cranes will lift the containers full of gear back off the Brown for transport back to research labs or other ships. A new group of scientists will board after us, and the Brown will continue its mission to glean important information about our oceans.
See another blog about our journey by scientist Gina Henderson here.
Keene, W. C., et al. (2007), Chemical and physical characteristics of nascent aerosols produced by bursting bubbles at a model air-sea interface, J. Geophys. Res., 112, D21202, doi:10.1029/2007JD008464.