Cruise JR269A, west of Svalbard: Understanding gas escape from the ocean floor
Methane hydrate is formed from methane and water at high pressures and low temperatures, both of which are found at the bottom of the deep ocean.
It is very widespread in the parts of the deep ocean nearest to the continents. If the ocean warms, the hydrate can become unstable and methane gas can make its way into the ocean, forming plumes of bubbles.
In 2008 James Clark Ross cruise JR211 found numerous such plumes exactly where we would expect to see them. Evidence was found for the presence of gas and movement of fluids beneath the seabed, but not of exactly how the gas is getting out into the ocean. The main aim of cruise JR269A is to image the pathways taken by the gas and thereby to establish the link between the sub-surface fluid flow system and the surface gas seepage. We will also collect further images of the bubble plumes to contribute to an international effort monitor changes in these plumes over time, and test scientific equipment that will be used in 2012 to better determine how much gas and hydrate are present beneath the seabed in the region of the bubble plumes.
We will image the gas pathways using two different sound sources. Firstly we will use SYSIF, a ‘Chirp’ sonar system developed by IFREMER, which is towed close to the seafloor and sends out a variable-pitch ‘hum’ which allows us to detect features up to a few tens of metres below the seafloor in great detail. Secondly we will use a small ‘gun’ that creates an oscillating bubble of compressed air and sends out sounds of lower frequency that penetrate deeper beneath the seafloor. The sound of this gun will be detected by a 60-metre cable of hydrophones (like microphones) towed just beneath the sea surface, which will allow us to see features several hundred metres or more beneath the seafloor, but with less detail. We will also record the signals from both sound sources on instruments placed on the ocean floor, and use measurements of the time it takes for the sounds to reach these instruments to work out the speed of sound beneath the seafloor and therefore the depths of the features that we see. We will use a high-frequency echosounder system to detect the bubble plumes and we will measure salinity and temperature variations in the ocean to work out the effects of the ocean on our other observations. Finally, we will test an electromagnetic source towed close to the seabed and record signals from this source on seabed instruments.
National Oceanography Centre Southampton
Tim Minshull (Chief Scientist), Graham Westbrook, Martin Sinha, Angus Best, Simon Dean, Veit Huehnerbach, John Davis, Mark Vardy, Indika Samarakoon, Sudipta Sarkar, Hector Moreno, Neil Sloan, Andy Webb
Henri Martinossi, Bruno Marsset, Pierre Leon, Stephan Ker, Laurent Artzner, Jean-Pierre Regnault, Yannick Thomas, Jean Luc Le Philippe (Genavir)
University of Durham
University of Tromsø