by Bill Johns
Today marks a week at sea on our cruise aboard the R/V Knorr, and things have been going quite well up to this point. We have settled into a routine of deploying two moorings per day as we lay out the combined Dutch/U.S. mooring array across the Reykjanes Ridge. Each day begins with a mooring deployment just after breakfast, then a steam to the next site (usually 20-30 miles away) and an afternoon deployment, and then we spend the night running ahead to the next two sites to perform bottom topography surveys so we can pick spots for the moorings to be deployed the next day.
Fortunately, the R/V Knorr has a multi-beam echo sounding system called Seabeam that measures the bottom topography along swaths perpendicular to the ship’s motion. These swaths have a width about 3 times the water depth, so this allows us to paint a picture of the bottom topography for several miles to either side of the ship. Without it, we would have to steam a grid pattern with the ship to get a two-dimensional view of the bottom, a very time consuming task. (Think of it as running order klonopin a paint roller over the bottom instead of drawing a single line with a pencil.)
Even with the Seabeam system, choosing mooring spots is not trivial. The bottom topography here is very rough, full of deep fractures, ridges and pinnacles that are characteristic of the newly-formed earth’s crust near mid-ocean spreading centers, which is what the Reykjanes ridge is; it is the northern arm of the great mid-ocean ridge system in the Atlantic Ocean. Finding a good spot for a mooring involves several factors, but one of the most important things is that it needs to have reasonably constant depth over an area that we can be sure to land the anchor in.
For the uninitiated, deploying a mooring consists of first streaming the whole mooring out behind the ship, starting from the top of the mooring that will be nearest the surface. Instruments and floats to support them are then added at various lengths along the mooring wire as the ship slowly steams toward the deployment site from some distance away. The last thing to be attached is the anchor, which is then lifted over the side and let go when the target spot for the mooring is reached. Actually the ship usually steams a bit past the target site before the anchor is released, because as the anchor sinks it drags all the mooring components laying on the surface toward it, causing the anchor to literally swing backward as it sinks rather than falling straight down. This is called “fall back”, and especially for very tall moorings it needs to be factored in. (And we are talking heavy anchors here, usually a few thousand pounds, that are made up of cast iron, or scrap railroad wheels, or leftover heavy anchor chain from large ships; each group onboard has their own favorite style of anchor.) Also, ocean currents can move the whole mooring horizontally as it falls, and since it can take up to a half an hour or so for an anchor to reach the bottom, this can also affect the landing spot. So, there is always some uncertainty in exactly where the anchor will wind up on the bottom, and even those with lots of experience can seldom place an anchor within a tenth of a mile of the target site in full ocean depths. That is why we try to find flat spots to land the anchors in. And these can be very hard to find along the Reykjanes Ridge!
Why is hitting the target depth so important? Mainly it is because the moorings are designed to measure currents or water properties at specific depths, and the instruments will miss those depths if the mooring winds up where it is deeper or shallower than the mooring was designed for. Also, some of our moorings have instruments very near the surface, up to 50 m from the surface. If we land the anchor at a depth that is 50 m shallower than planned, then those instruments will be laying at the surface and can be damaged by surface waves or be run over by ships. If we miss too deep, then the instruments are not measuring the near-surface properties we want to observe. (Note that all the moorings we are deploying are “subsurface” moorings, meaning that they have no surface buoy and lie completely below the surface. We retrieve them by sending coded sonar commands to a device called an “acoustic release”that releases the rest of the mooring just above the anchor, and it is then recovered when it floats to the surface.)
So far we have been hitting our target spots pretty well, and this is something that requires excellent coordination between the scientists, the deck crew, and the bridge officers driving the ship. How do we know if we hit our targets? By transmitting sonar signals to the acoustic releases on the bottom, we can triangulate on them and determine precisely where they landed.
By now we have deployed 10 moorings, exactly half of the total number of moorings we will deploy in this cruise. On these moorings are countless instruments measuring currents, temperature, and salinity at depths from near the surface to the bottom.
While we are out on deck adding components to the moorings as they are streamed out (which can take several hours for each one), there is sometimes an opportunity to look around at the sea and take in the ocean vista. This is especially true for me, since I am usually just standing back and taking notes as others do all the hard work on deck. For the last few days I have been noticing especially the birds. First of all, there are an amazing number of sea birds out here. Ever since we have left Reykjavik, one can see birds from horizon to horizon, darting about just above the waves. I am used to seeing sea birds, but in the tropics and subtropics, where I have done most of my field work, they seem to be much more scattered and are seen only occasionally. Here they are everywhere, and it boggles the mind to think of the vast number of birds that must be out here. They are mostly Northern (or Arctic) Fulmars, which look like a very stout seagull with a trimmer tail.
Whenever we stop the ship to work, they flock to us in the hundreds and set down on the water behind the ship, waiting expectantly. Of course, they think we are a fishing vessel, and they are hoping for some morsels of by-catch. I can almost hear what they are saying to each other: “What poor fisherman these people are!” “I have been following this ship for 4 days now and haven’t yet gotten a single scrap to eat!”. It is amazing to think how we have impacted their existence, and how they have learned to follow ships for an easy meal in the years since we humans began setting out to sea (or is it even in their genes now?). Perhaps in their next stage of evolution they will learn to recognize a research vessel from a fishing boat!
Tonight is the World Cup final, and all of us who are not on watch will be huddled around the radio listening to it. Unfortunately the Dutch team did not make the final, but they had a brilliant cup, culminating in a third place victory over Brazil, and our Dutch colleagues onboard certainly have much to be proud of.
[portfolio_slideshow id=369]