JR211: Gas hydrates and climate change in the Arctic
The RRS James Clark Ross is working in Arctic waters west of Svalbard (80ºN) to investigate the presence of gas hydrate (methane gas trapped in water ice) in the seafloor sediments, and the occurrence of methane gas venting as a result of these hydrates breaking down. This cruise is part of the IPY project 'Dynamics of gas hydrates in polar environments', funded by the UK's Natural Envieronment Research Countcil and involving scientists from NOCS, University of Birmigham and Royal Holloway University of London. For more information aobut the project, please go to the project website.
PhD students Anya Crocker, Clara Bolton and Anne Osborne report from the ship:
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During the first leg (23 August-02 September) of JR211 working hours are divided into two 4-hour shifts per day, as the CTD and TOBI (towed ocean bottom instrument) deployments as well as data acquisition by acoustic instruments (TOPAS, EK60 (echosounder) and SIMRAD EM120 (multibeam)) happen round the clock. During the entire cruise, we’re collecting data round the clock so we can get as much information as possible in a short time, which means everyone has to work two 4-hour shifts each day. At the moment, this is facilitated by it being arctic summer as night time is barely dimmer than daytime, but it does make it difficult to get a full night’s sleep!
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Waves are reflected (and refracted) when they hit a change in the material they’re travelling through, and so by measuring how long it takes the waves to bounce back, we can calculate how deep these reflectors are, both within the water column, as well as below the seafloor. Different wavelengths of energy can penetrate to different depths through the layers of water and rock below us before too much of the energy is lost to get a good signal back. The various data sets we’re collecting all use different wavelengths of energy to produce images of the water column, especially the objects within it (EK60), the seafloor (multibeam, TOBI), and the sediments just below the surface (TOPAS). Together, this provides a range of evidence for the venting of trapped methane from the seabed in certain areas off the west coast of Svalbard. Bubble plumes, rising a couple of hundred metres from the seafloor and tens of metres across have so far been imaged in several different locations where methane is escaping. Pockmarking of the seabed is relatively widespread, and is thought to indicate sites of gas release, which is typically associated with high methane concentrations with a light carbon signature in the water (we plan to test this by analysing the samples from the CTD depth profiles). There are also occasional ‘chimney’ structures, where the trapped gases escape through the sediment, distorting the visible layers (made visible using TOPAS which can “see” beneath the sea floor). |
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The CTD (Conductivity, Temperature and Depth sensor) is an instrument supported on a large circular frame surrounded by a rosette of 24 bottles. As it is lowered to the deep ocean, this instrument sends back depth profiles of pressure, salinity, fluorescence and temperature. As it is slowly recovered, the bottles are ‘fired’ shut at selected depths between the sea surface and the sea floor, collecting water samples for chemical analysis. Typically the water sampling resolution is highest in the surface layer and near the seabed, as changes can occur most rapidly in these parts of the water column. Surface solar heating and the rapid attenuation of sunlight in the surface layer leads to a steep temperature gradient termed the thermocline, below which temperature remains more constant around –0.6 degrees C. In stratified waters, the thermocline can constitute a physical barrier to the mixing of nutrients from the deep ocean into the surface (photic) zone, where phytoplankton (microscopic algae) have sufficient light to grow. Analyses carried out on these water samples on board will include oxygen and methane concentrations and nutrient concentrations (phosphate, nitrate and silicate) whilst the oxygen isotopic signature of the water and methane carbon isotopes will be measured after returning to shore. The CTD has now been deployed at two stations, the first to give background levels of seawater chemistry in an area not thought to be affected by the release of gases from the seafloor, the second in a potentially active area.
As for day-to-day life, mealtimes become the focus of one’s day! Breakfast is served at 7.30am; lunch at 12.00pm and dinner (for which there is a smart dress code) is served at 6.30pm. I was surprised by how much fresh food (salad, fruit etc.) is available, and the menus are varied and tasty. It is easy to get carried away and eat all three courses… luckily there is an onboard gym to relieve guilt! At present we still have contact with the outside world via a satellite link, although as the satellite is so low on the horizon (0.6 degrees!) that the signal is very weak so the internet is rather slow! No polar bear sightings yet because we are too far from the ice edge where they feed, but whales, dolphins and puffins have been sighted near the ship
Monday 1st September
It’s early morning and another sun ‘event’ is colouring the sky with pinks and reds. Sun ‘event’ as no one is really sure when sunset stops and sunrise starts. The sea is calm and we can clearly see the corries and glaciers of Karlbanken to the east. It has been a bit of a challenge getting accustomed to the watchkeeping hours. There are three teams – 12-4, 4-8 and 8-12, both am and pm. Trying to work out when is best to sleep and eat is the tricky part. Those with the 8-12 shift have it the easiest, definitely!
Watch keeping duties are mainly keeping a log of any interesting features that show up on any of the instruments. These will help us to decide where to revisit for taking water samples with the CTD, where to take sediment cores, and where to acquire seismic data to image what is deeper below the seafloor. If nothing of interest happens for half an hour, we still have to make an entry on the log sheet – noting our position, the direction we are heading in, the depth of the sea floor and the speed of the ship. This also serves to keep us awake, particularly on the night watches!
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This morning, however, there is no danger of falling asleep as the underway lab is a hive of activity. The CTD team are poised for another sampling session and the Ocean Bottom Seismometer is due to be tested soon afterwards. The OBS will be used on the second leg of the cruise ??and the higher energy of the seismic sources means that the waves penetrate deeper into the seafloor than the sonar instruments. This will help us build up a picture of the sediments and rock layers beneath the surface and help us to choose suitable sites for coring. Just before picking up the rest of the team from Longyearbyen on the 2nd September, the entire science party (and ship’s crew!) were excited by the discovery of frequent columns that appear within the water column, and we think that they are bubbles rising from the seafloor in the shape of plumes --- Very exciting as this is the main aim of our cruise! |
