Of Box Cores and Talwegs and Things that Go Splash in the Night
The night went by, but not too quickly for me, as the rocking of the boat generally kept me awake. I’m in the top bunk on the outside of the boat, on the lower deck, in the bow. The sea splashing against the hull is not quiet, and the smooth up and down rhythm of the boat was punctuated by the occasional lurch. I am told by the crew that this is a typical, smooth ride, so I hope I get used to it quickly. Fortunately, the Dramamine has not been needed yet.
Today we entered the Gulf Stream, and collected some cleaner water to be used in experiments later in the voyage. The crew ran some safety drills, including a fire drill, a man overboard drill, and an abandon ship drill. That’s when we all got to don these big orange rubber suits, one size fits all.
Studying the productivity of the ocean is important because of the amount of fish that we catch and eat every year and because humans have often added too many nutrients to ecosystems. At a basic level, the nutrients we utilize in the fish first have to enter the food chain. Nutrients are circulated into offshore waters and are eventually deposited in the deep areas. But deep sea waters are also often a source of nutrients which are upwelled onto the continental shelf to fuel the ecosystem. In many cases these upwelled deep waters are the major source of nutrients for the shelf, so the basic deep sea research conducted by this team is very important in understanding complex ecosystem dynamics. Just as on land, the primary productivity of the ocean is from small plants, known as phytoplankton. Most of this plankton grows near the surface and near the shore over the continental shelf, where dissolved nutrients are usually highest. As the plankton drift to the bottom, this “marine snow” (also a conglomerate of other things) nourishes sponges, gorgonians, corals and other inhabitants that in turn provide habitat for small invertebrates and fishes that serve as food for other crustaceans and fishes. It is not really a food chain, but more like a food web, with multiple often complex connections and interactions.
In addition to nutrients, sediments are also deposited on and moved around the ocean bottom. They can be re-suspended by currents, including tidal currents that can transport sediments up and down the canyons . When the sediments get thick and if they are on steep slopes, they sometimes break away, forming a turbidite, which is a flow that happens when sediments are liquefied rather than pushed along by water. These flows can go the length of the canyon and out onto the abyssal plain, which can be 100s of kilometers away. Down the middle of the canyon on the multi-beam maps there appears to be a channel, much like in a river. In many cases this is a sediment “river channel”, known as a talweg. Our knowledge of physical processes that transport sediments and what impact sediment and nutrient transport has on the deep ocean food web is an important part of the work of this project.
The first CTD was launched today, and a new sampling device was tested. Usually oceanographers use CTDs to sample conductivity, salinity, temperature, dissolved oxygen and depth. As part of the CTD are also attached 12 Niskin bottles, which are sent down open and can be closed by remote control whenever desired to get a larger water sample from a particular depth. Today for the first time, a small Box Core tube was suspended from the bottom of the CTD, and a bottom sample was collected. Success. That CTD will go up and down all night long over Baltimore Canyon, and now it will be multi-tasking.