Over the Mid-Atlantic Ridge

A post by Lola Pérez Hernández

As Eleanor said in the last post we have been crossing the Mid-Atlantic Ridge (MAR). The MAR is a subsea mountain range that divides the Atlantic into eastern and western basins.  The MAR rises in some points up to 1000 m from the surface. From a geological point of view the MAR is where the oceanic crust gets renewed on a long time basis. The RAPID project has several moorings over the MAR. Can you imagine why? Apart form being in the middle of the ocean, having this mountain chain through the whole Atlantic Ocean affects the deep-flow. This is the case for the Antarctic Bottom Water (AABW).  The AABW is formed in the Southern Ocean around Antarctica and flows northwards along off the east coast of South America and then along the west side of the MAR in the North Atlantic.    The AABW is cold and we can trace the path that it follows by looking at the temperature of the water

Topography of the Atlantic Ocean,
colour indicates the depth in metres

Temperature of the Atlantic water at 4,500m depth, 

the arrows indicate the flow of Antarctic Bottom Water.

Shrinking cups

Recovering the CTD after a cast.  The CTD itself is the horizontal silver instrument at the level of the guys' hands.  The vertical cylinders are a mix of instruments and bottles.  (Photo by Ben Moat)

Today we were working at the Mid-Atlantic Ridge.  This underwater mountain range runs from north to south in the Atlantic.  While some of the work today was on landers, we also did a CTD cast, where we send an instrument which senses the temperature and salinity of the ocean to the seabed and then pull it back up on a long wire.  This wire is about 6 kilometers long, so when the instrument is down near the seabed, there is nearly 6 kilometers of water sitting on top of it.  The weight of this water creates a unique environment for creatures living at the bottom of the sea.  It also presents an engineering challenge to instruments and sensors that we use to measure the ocean.  They need to be able to withstand incredible pressures.

Adding the cups to the CTD frame.  (Photo by Lola)

One of the fun ways to observe the effect of the pressure at the bottom of the ocean is to see their effect on polystyrene (styrofoam--for Americans) cups.  Before leaving Southampton, I took 50 cups to my daughters' nursery and school.  The children decorated their cups, and then gave them back to me.  These cups flew with me to Trinidad, and then sailed with us to the Bahamas and across the Atlantic to the Mid-Atlantic Ridge.  Today, our CTD cast was finally in the daytime (usually they start or end at night).  So Lola and I stuffed the cups with a bit of paper, and then put them into some tights.  These tights were then attached to the CTD frame with cable ties (many cable ties) and they went on an adventure to the bottom of the sea.

The CTD frame going out to sea, with the cups attached.  For the oceanograhers out there, if you notice there's a bit more on the frame that usual, the large silver cylinders are acoustic releases.  These are tested at the depths they'll be operating at before they're used for the moorings.  The smaller silver cylinders are MicroCATS (the one with the 5 circles cutout of it).  These are compared against the wired CTD sensor to improve their accuracy.

If we were to send a balloon to the bottom of the ocean, it would shrink as the weight of the water squeezed the air inside it.  When it came back up and the weight of the ocean is no longer squeezing the balloon, the air would expand again, and the balloon would look like nothing happened to it.  Polystyrene is a little bit like a balloon.  It has lots of little air bubbles in it.  Under high pressure, like 6 kilometers of water, these air bubbles collapse.  But unlike a balloon, when the cups come back up to the surface and the pressure is released, the cups don't expand again.  They are more rigid, and the keep their collapsed, squeezed out shape, permanently.   The air bubbles have popped and the cups have shrunk.


You can see the results here--teeny tiny cups where the drawings and writing of the children are now miniature.

Lots of teeny tiny cups.  The big white one is what they started out as.

Tiny cups can be used for thimbles, or tea parties.

Tomorrow, we continue with the Mid-Atlantic Ridge west side, and then a few days later will travel to the east side of the Mid-Atlantic Ridge.

You can read more about ocean pressure on Deep Sea News: http://deepseanews.com/2014/05/how-to-shrink-a-styrofoam-cup-and-other-side-effects-of-deep-ocean-pressure/ (leaves this website)

Moorings work is well underway

A view of the back deck of the RRS James Cook.  (Photo by Ben Moat)

We’ve been busy with gathering our sensors from the moorings which were deployed 18 months ago, and then left recording data until now.  Since Wednesday, when we started out with the PIES recovery, there hasn’t been much of a break.  You can see some of the work below.

So, what is a day's work on a research cruise like this one?  This is a moorings cruise, which is a little different than some cruises.  Here, when we're talking about a mooring, we are referring to a large anchor or weight, with wire and flotation attached.  The flotation is like underwater balloons, but because of the weight of water, they are made of strong material like steel or glass.  The anchor holds it to the seabed, while the flotation helps keep the wire upright.  Along the wire, we attach different instruments or sensors to record information about the ocean.  I'll see if I can find a picture of one of these--a diagram really, to show what it looks like.  Since some of the tall moorings are in water which is 5 kilometers deep, the mooring itself is 5 kilometers long.  This would be difficult to take a picture of!

Looking for a mooring which has been released.  Since we are sometimes a good distance away, we need binoculars to spot the flotation on the surface of the water.

When we're working on the moorings, we typically start by "listening" to the mooring using some acoustics from the ship.  We “ping” a particular sequence of sounds that mooring is listening for, and it responds.  From the response, we can tell how far away it is.  When the ship is in a good position, we then send a command to tell the mooring to drop its anchor.  Depending on how tall it is, and how far from the surface the top instrument is, we can see the floatation almost immediately.

Some of the wildlife that can be found on a mooring.  Here is a worm or "polychaete" from one of the floats.

We then wait to make sure the whole thing is on the surface and nicely laid out.  We don’t want to come in too quickly, because some of the mooring could come up underneath the ship, which would be a problem.  Once it’s all up, the ship gets into position and the top float is hooked using something that looks like a grappling hook with a line (rope) attached.  The mooring is then slowly pulled in by the double-barrel winch, while the guys on the back deck (the technicians) take instruments off it one by one.  Sometimes the instruments come up with some animals or algae growing on them (see the green stuff in the picture).

This is an RCM11 current meter--it measures how fast the water is moving past it using acoustics.  You can see it also became a nice home for some algae (the green, furry stuff).

One of the tall moorings—5 km tall, since it’s in 5 km of water, can take a good 6 hours to recover.  During this time, we’re out on deck in the heat (27 deg C), humidity (80%) and sun, which can be quite tiring.  We have plenty of sunscreen, but somehow, I think everyone will end up with a tan before the trip is over.

After the mooring is redeployed (sent back out again with fresh instruments and batteries and wire), the night's work begins.  The mooring position on the seabed is triangulated using the same acoustics to range from the ship.  The ship is moved to three different locations so that we can figure out where the mooring landed on the bottom.  Then we do some ship-based work---sending down a sensor on a wire and back up again---to calibrate the sensors which are left out.  In the morning, we start again with the moorings.

Martin, Lola, Darren and Paul, looking at the bathymetry around WBAL.

Yesterday, we were planning to release and recover a mooring which we call WBAL.  The L stands for lander, since it’s a small mooring with just a bottom pressure and temperature recorder on it.  We ranged it, and then released it, then went up to the bridge to wait for it.  It was in just 500 m of water, but after an hour, and more ranging that confirmed it hadn’t moved, we left it to complete some other work.  We tried, briefly, to see it in the swath bathymetry system, but with no luck.

Today, Darren got a message from his Argos alerts that the WBAL beacon—a satellite receiver on the top float of WBAL—that it was on the surface and a few miles south of us.  So instead of the work we had planned for this afternoon, we’re setting out to recover the mooring.  We don’t know yet why it didn’t come up as it should’ve yesterday, but we’ll hopefully know soon.  Perhaps there was some fouling (what we call the overgrowth by plants or animals) on the releases and it couldn’t drop it’s anchor immediately.

We have a few more days at this intense pace before we head out for the Mid-Atlantic Ridge.  It’ll take a few days to get there, where we can regroup, check data further, calibrate some more instruments, and get ready for the next bit of work.

Floating PIES at 5:30 am!

A post by Lola Pérez Hernández

On Wednesday morning we made our first recovery. Some of us woke up at 4:30 to activate the release that freed the instrument WBP1. It took about 45 minutes to reach the surface. So at 5:30 a group of us stood on the bridge of the boat, searching the horizon for a tiny flashing white light. It finally made its appearance ahead of the starboard bow.  We moved the boat towards that tiny flashing light and there it was. I’m sorry to tell there were no pies but there was a big floating white buoy making tics every seconds. WBP1 is a PIES is a Pressure and Inverted Echo-Sounder that sends noise to the surface and listens for it to came back, and with that procedure we can know things about the water column such us the dynamic height, the depth of the mixing layer or the stratification

  

First a small light in the distance

Recovering the PIES

Retrieving the data

Back on depth after 18 months
on the ocean floor