Phase Two: JC114

On Thursday morning the second stage of the OSCAR cruises began, with the start of cruise JC114. The James Cook set sail once again, this time with an (almost) completely different set of crew, technicians, scientists and equipment. We're spending another 6 weeks in the Panama Basin, this time to acquire our seismic data over the Costa Rica Rift, and the ODP borehole 504B, a section drilled ~1350 m into the oceanic crust.

The Cook alongside in Caldera, Costa Rica

Active seismology uses acoustic (sound) pulses to try and acquire an idea of the properties and structure of the Earth's crust and interior. We will be letting off these pulses using equipment towed behind the ship, which will travel down to and through the seabed, reflecting and refracting off layers of different materials as they go. We then record the pulses that come back to the ship after going through the crust, using receiver sensors called 'hydrophones' which detect these acoustic vibrations. Our sensors are deployed in two different ways: towed behind the ship will be a 1.5 km hydrophone streamer, mostly for recording the reflection data; and then we will also deploy 75 ocean-bottom seismographs (OBSs) to sit on the seafloor, predominantly for wide-angle refraction data.

Up to now, we have deployed four OBSs on the Sandra Ridge, in collaboration with Colombia, to passively monitor seismic activity (earthquakes!) that may occur in that area. We have then steamed to the Costa Rica Rift, and have deployed our OBSs in this area in a grid format over the rift. After some more equipment deployment this afternoon and through the night, we will hopefully begin sending our acoustic pulses tomorrow morning and the data will begin streaming in!

Dolphins seen whilst still on the continental shelf

Wildlife update
We've been lucky enough to have a few sightings of dolphins already on this trip, and even a possible whale seen in the distance from its blow. A group of dolphins saw the ship out of the Gulf of Nicoya when leaving Caldera, and a pod of bottle-nosed dolphins were riding the bow wave yesterday afternoon. More unexpectedly, we've found some terrestrial stowaways on board, including several large crickets which are providing some tropical background noise in the hangar at night.

JC112: A brief review

This morning we awoke to the sight of land on the horizon for the first time in six weeks. The cruise JC112 is over and we’ve arrived in Caldera, Costa Rica. It’s been an excellent experience for us all, and not one we’ll soon forget! We’ve travelled thousands of nautical miles in our efforts to collect a cornucopia of data, which will be used to learn more about the oceanography of the Panama basin. A nautical mile is 1.852 km, about 1.15 miles, the approximate distance of one minute of arc measured along a meridian.

Sunrise over the first land we've seen since leaving Balboa at the beginning of December.

Since leaving Panama at the start of December we’ve done 63 CTD and VMP casts, collecting 15120 litres of seawater in the niskin bottles from which we’ve analysed 1512 samples for salinity and 1644 for oxygen content, and stored 406 samples to be analysed for helium back on land. We’ve deployed 12 magnetotelluric landers and 3 moorings, performed 85 penetrations of the seabed with a heat probe and processed 1696 swath bathymetry data files.

The onboard map showing where the ship has travelled.

Our wildlife spotting has gone fairly well too, with reported sightings of dolphins, sharks, turtles, pilot whales, flying fish, sea snakes, pyrosomes, squid and many different seabird species. This morning saw rays added to that list, as well as dolphins riding the ship’s bow waves and flocks of pelicans flying alongside us.

A dolphin accompanying us on the way into Caldera.

And this was only the first cruise. Next week the RRS James Cook will once again be sailing out into the Panama basin. With an (almost) entirely different crew and a new set of scientific instruments, this second cruise will be focusing on geophysics, measuring seismic activity within the Earth under the basin. At the beginning of February a third cruise will depart from Panama aboard the German RV Sonne. The two ships will be out in the basin simultaneously and will be working on joint operations for a portion of that time.

So while JC112 is now over, there’s still plenty of exciting science to come from the OSCAR project. Watch this space!

The science team of JC112 on the aft deck of RRS James Cook.

Mapping the ocean floor: swath bathymetry

As well as data collected at stations by the instruments we deploy, we also take measurements underway as the ship is steaming. This is done by instruments mounted on the hull of the ship, and include water temperature, salinity, and sound speed, but perhaps most importantly, seabed depth.

The instrument, called a multi-beam echo sounder, uses a fan of beams of sound to map the ocean floor beneath the ship. This works under similar principles to sonar: the instrument measures the time it takes for the beams to reflect off the seafloor and return to the receiver on the ship, and this time is then converted to distance, giving seabed depth. This maps a swath of the ocean floor, thus is known as swath bathymetry data (with bathymetry meaning the topography of the seafloor). To enable an accurate conversion of time to distance, it is important to have an accurate velocity model of the ocean water column. This is measured as often as possible using a sound velocity probe attached to the CTD.

Multibeam swath acquisition (credit: Fisheries and Oceans Canada)

After acquisition, the swath data must be 'cleaned', to remove false reflections that we know are not the true seafloor. After processing, we then have a usable map of the ocean floor, revealing canyons, seamounts, vast plains and even underwater rivers.

A potential underwater river system, formed by streams of denser ocean water.

This map is then used to help aid the placement of our instruments: we may want to deploy a CTD over a trough, for example, or try and find a flatter area that will have accumulated enough sediment for heat probe measurements. It is also interesting to see the changes in the shape of the seafloor from the Costa Rica Rift outwards. Close to the rift depths are shallower and topography rougher, full of ridges and troughs formed by the basalt lavas at the rift, with no sediment cover. Older oceanic crust further from the rift is starting to develop into flatter abyssal plains, with thicker sediment cover.

Sediment ponds between ridges in the Ecuador Fracture Zone

Swath bathymetry data is also used for non-scientific purposes, perhaps to map out harbours or sunken ships. The shallower the water, the greater the resolution of the resulting map. Our map will also eventually be added to a global database, which allows anyone to view all swath data currently collected around the world.

Vehicle for Marine Perlustration

In a previous post we focused on one of our main instruments, the CTD. But that isn’t the only device we’re frequently sending on dives into the ocean. Now it is time for our other scientific submersible to have some time in the spotlight!

The vertical microstructure profiler (VMP) is designed to take measurements of temperature, conductivity (from which salinity can be calculated) and velocity over small length scales. Dissipation of turbulent flows typically occurs over distances of a few millimetres to tens of centimetres, so in order to observe their effects the instrument must take measurements within this range. The microstructure measurements can be used to infer information about mixing in the ocean due to turbulence and convection.

The VMP ready for an early morning deployment.

In contrast to the CTD, which is attached to a cable and lowered into the sea by winches on the ship, the VMP is a freefalling instrument. Its torpedo-like shape minimises the resistance it will encounter on its journey to the deep ocean. Before deployment, weights are attached to the end of the VMP. Without these weights, the device is buoyant enough to float to the surface.

The weights attached to the end of the VMP before deployment.

The VMP is an expensive and important instrument, so great care is taken to minimise any risk of losing it to the abyss. The weight release mechanism receives instructions from the instrument’s main computer, and can be triggered by several different conditions. The first release trigger, the one we ideally want, is the instrument reaching a pressure which is calculated and programmed in for each individual dive. This pressure will relate to the depth we want the VMP to reach, usually around 50m above the seabed in order to get as much data as possible without risking a collision with the ocean floor. A weight release can also be triggered by reaching the time limit calculated for each dive, the fall rate dropping below 0.4ms^-1 or the main computer in the instrument shutting down. The weight release mechanism, which runs from a separate computer, will also be triggered if no signal is received from the main computer for over 4 hours. The final failsafe, in case of complete electronic failure, is the ‘burn wire’ used to attach the weights to the VMP. The wire will completely dissolve after being submerged for around 24 hours.

After the weights have been released, the VMP returns to the surface.

When the VMP returns to the surface, it must be recovered. Its position is tracked by the ship using a USBL beacon, and when at the surface it can be identified by the flag and flashing lights attached to the top. However, it can be surprisingly difficult to spot a bright orange flag in the ocean!

A little game we like to call 'spot the VMP'!

Once all the data collected from the VMP dives is processed, we will have a better idea of the level of mixing which occurs across the Panama basin. When used in conjunction with other data, this could tell us a lot about how hydrothermal vents are contributing to circulation in the ocean.

Wildlife update

Over the last couple of weeks the CTD has been picking up some very strange objects. After a little Wikipedia research, we have decided they are probably a type of pyrosome. These bioluminescent tubular structures are actually colonies of small creatures called zooids. An individual zooid is only a few millimetres long, but they are embedded in a gelatinous substance that holds together colonies which can be several metres long! The ones we’ve found haven’t quite been that large, but it’s still very difficult to imagine them being made up of thousands of tiny creatures.

One of the pyrosomes found on the CTD, before being thrown back to the ocean.

Crossing the line

A couple of weeks into the cruise, we crossed into the southern hemisphere to take measurements and samples at the southern boundary of the Panama Basin. This was the first time crossing the line at sea (or indeed at all) for many on board, and as is tradition, King Neptune conducted a ceremony to mark the occasion. Pollywogs had to pay their respects to the ruler of the oceans and after a small penance had officially joined the ranks of shellbacks. 

The new recruits after crossing the line

By far the most widely believed falsehood about crossing the equator is that water in your bathtub or sink will drain rotating in the opposite direction. This common misconception is due to a misunderstanding of what is known as the Coriolis Force (not actually a real force, but a ‘pseudo-force’)- an effect caused by the rotation of the Earth on objects (e.g. the ocean and atmosphere) not fixed to the Earth’s surface. You can see the effect in the rotation direction of hurricanes and other weather systems, and in the movement of large bodies of water. 

Hurricane rotating counter-clockwise in the Northern Hemisphere

A simplified explanation for the effect is because the Earth is spherical, it rotates more quickly at the equator than at the poles, or the ‘linear velocity’ of the Earth’s rotation is larger at the equator than the poles. A point at the equator such as Quito in Ecuador must travel 40,000 km in one day to return to the same position, but Durham at 54 degrees N, for example, must only travel 23,000 km. One outcome of this effect is that if you fired a cannonball north from Quito, it would land to the right of immediately north even though the object was traveling on a straight course, as the cannonball would retain the faster linear velocity of Quito. Likewise, if you were at the North Pole, and fired a cannonball towards Durham, it would land to the right, as Durham has a faster linear velocity and would have moved further around. You can see the effect of this in the formation of a hurricane. Air moves from high to low pressure, so air moving from nearer the equator towards low pressure in the north would also look like it was moving to the right, thus storm systems tend to spin anti-clockwise in the northern hemisphere, and as a mirror-image, they spin clockwise in the southern hemisphere. 

Cannonball fired north from the equator has a faster linear velocity than the ground beneath it, so lands to the right of directly north. A cannonball fired south lands to the left of directly south, looking from the equator.

An analogy closer to home would be if you and a friend were standing on a merry-go-round, throwing a ball to each other. The ball would appear to have a curved path to you, as your friend would have already been moved round by the movement of the merry-go-round, but in fact the ball is travelling in a straight line to an outside observer. 

Winds appear to bend to the right in the Northern Hemisphere, creating counter-clockwise rotation about a low pressure zone

The myth about the water draining in your sink rotating in the opposite direction in each hemisphere is scientific misconception; the effect is real but the scale is too small to show a noticeable impact in this scenario. Other factors have a much greater effect on the direction of spinning: the geometry of the bowl, or the direction of input of water are much more important. A study done in Boston used a tank filled with over 1000 litres of water to investigate the Coriolis Effect. If left to stand after filling the tank, to let any movement disperse, and then emptied with a plug designed not to effect the motion too much, the average direction of rotation around the plug after many repeats of the experiment was indeed counter-clockwise.

 (For more information on this study, please see Ascher Shapiro's letter to Nature journal, December 1962, or for the equivalent experiment in the southern hemisphere, Lloyd Trefethen et al's letter to Nature, September 1965.)

Please feel free to ask any questions you may have in the comments section below.

Happy Holidays!

We’re just reaching the end of the holiday season now, and from all on the Cook, a belated Merry Christmas and Happy New Year!

One of our three Christmas trees, guarding the Secret Santa presents

Although collecting as much data for the scientific purpose of this cruise is the priority on the ship, activities were allowed to stop for a period of 18 hours to celebrate Christmas Day. 

The morning was generally a busy time with scientists rushing to finish sampling the Christmas morning CTD and the catering and stewarding team working their socks off to create a special meal for the whole ship. After an aperitif in the bar, along with the opening of Secret Santa presents, we all sat down to an incredible lunch, complete with a toast to the Queen by the youngest officer, Paul, and speeches from the Captain and the Principal Scientist. After being thoroughly stuffed, we all retired to the bar or had a quick nap before the afternoon quiz, and partook in some terrible dancing in the evening.

The mess decorated and ready for the Christmas meal, complete with crackers with terrible jokes

For New Years Eve, we were steaming to the next station, so everyone was able to gather in the bar to welcome in 2015. As is tradition at sea, the oldest person on board, Bob our chief engineer, rang the ship’s bell to ring out the new year, and the youngest person, Emma, than rang in the new year. 

The Captain's speech

Despite being many miles from home, friends and family, the team on the ship put a lot of effort in to create a sense of holiday spirit, and we are all very thankful.

We hope you all have a fantastic 2015, and your year didn’t begin with too much of a headache.