Natural Methane Degassing, Mud Volcanoes And Chemosynthetic Life; Exploring A Complex Environment In The Gulf of Cadiz, Southern Iberian Peninsula.
Ireland's largest survey vessel the RV Celtic Explorer has taken a full complement of Irish and international scientists to survey unique seascapes and habitats where the Mediterranean waters spill into the Atlantic sea at the Gulf of Cadiz, south of Spain. The research cruise called DEEP-LINKS, under the guidance of lead scientist Dr. Jens Carlsson from University College Dublin’s genetics lab, includes twelve scientists from five Irish and four international marine research institutes. DEEP-LINKS got its name from the mission’s research objective: to explore mud volcano sites in order to classify their unique ecosystem that thrives without sunlight, and define how this ecosystem is linked to deep sea fauna such as corals, sponges and fish.
All hands have been on deck!
On-board the Celtic Explorer life has been hectic for the ship’s crew, the ROV team and the scientists since the vessel departed Galway on Oct 23rd. With unified focus on the task at hand, the mission has succeeded in completing all tasks and has exceeded expectation! The team aboard have successful mapped four target locations using the new RV Celtic Explorer’s high resolution multibeam echosounder. The resulting maps have been used by the geologists to interpret the environment and geological events at each location; these interpretations provide an essential component when establishing target locations for the biologists to visually survey and sample each location's ecosystem, using the Marine Institutes Holland I Remotely Operated Vehicle (ROV).
Site 1: Multibeam image of the Hesperides mud volcano, Gulf of Cadiz. Hesperides is a complex structure of three mounds, ~200m high, 2 km x 4km in size and with east-west oriented ridges extending to the east.
The geology-biology-geology interaction
Geologically unique, in that these volcanoes extrude mud and methane as opposed to magma; they form when mud buried deep within the stratigraphic column becomes subject to pressure from the Earths tectonic plates and follows a path of escape to the seabed. In the Gulf of Cadiz, pressure caused by the northward movement of the African plate with respect to the Eurasian plate has resulted in diaperic upwelling of mobile sediment through fractures in the rock above, to result in the formation of a series of mud volcanoes.
Along with mobile sediment, methane and hydrogen sulphide gas seep from several kilometres beneath the seafloor. This degassing is particularly interesting to the biologists on-board, as microorganisms that acquire their energy from methane and hydrogen sulphide, in a process known as chemosynthesis, thrive here. Following the survey, further research will be carried out on the 300+ samples collected during the survey, to establish the ‘deep links’ between microorganisms and the deep sea fauna endemic to this area. It is hoped we will be able to establish how chemosynthetic energy is dispersed through the non-chemosynthetic deep sea life such as corals, sponges, deep-water crustaceans and fish, so we can assess the importance of chemosynthetic derived energy to the wider ecosystem.
Photograph of some coral and two species of crab (including a sponge wielding crab!), just some of the deep sea fauna we have sampled, that could be influences by the chemosynthetic ecosystem.
Most of life we are used to on earth derives its life energy from photosynthesis, some microorganisms thriving in deep dark environments, such as in the Gulf of Cadiz, are often thought of as usual because their life energy is derived from deep within the Earth. But chemosynthetic life is far from unusual, chemosynthetic populations continue to be found as we continue to explore the deep oceans. Many scientists now believe energy from within the Earth could have been the initial trigger for life on Earth, billions of years ago. Similar microorganisms are now also targeted by space missions on the hunt for life in space.
The picture to the left is a multibeam image of Anastasia, the second site we mapped and surveyed with the ROV. It is a perfect cone volcano 3km wide, with deep motes to north and south west.
The pictures to the right show some of the team (myself included) working away in the ROV shack, a dense growth of large deep see corals and some of the sample preparation that happens back on board in the wet lab.
Geologists have also established a link with chemosynthetic life and a geological process that results in the formation of carbonate chimneys at mud volcanoes. During the ROV survey of the volcanoes, the geologists on board began to notice carbonate chimneys up to 1m in length occasionally scattered in areas on the volcanoes flanks, elongated down slope. Further investigation resulted in the discovery of a forest of upright carbonate chimneys, each up to 2m tall and 50cm in diameter, embedded in the mud at the summit of an active site at Hesperides mud volcano. The summit of the volcano is also where we see bacterial mats and methane gas actively bubbling from the surface of the youngest mud flows.
The origin of these carbonate chimneys is thought to be a result of methane and hydrogen sulphide degassing and its interaction with bacteria and seawater. Bacteria live off the methane and increase the alkalinity of the sediment pore fluids. This results in: anaerobically oxidisation of the methane to bicarbonate and the transformation of the sulphate to sulphide, which in turn reacts with calcium ions in the seawater to produce the precipitation of carbonate and the generation of hydrogen sulphide.
Lead team geologist Dr Bramley Murton proposes that these carbonate chimneys grow as cylindrical structures around the methane streams, both in the sediment and in the water column immediately above the seafloor at the summit of the mud volcano, where methane flow is focused. These chimneys are then pushed down slope after being incorporated into subsequent eruptions of mud and gas on the summit as the mud volcano edifice grows, which explains why the fallen chimneys are found on the volcano flanks. The abundance of carbonate chimneys found at Hesperides mud volcano show that there has been a long history of intense methane gas escape and mud eruptions that support chemosynthetic life here. It appears as though this site has been active for many thousands of years!
Extinct chimneys up to 2km in height litter the summit of a dormant mud volcano, in the Gulf of Cadiz.
Sediment temperature and core samples were taken with instrumentation attached to the ROV, where active degassing and thick white fluffy microbial mats occupy the seabed. These samples will be analysed to establish gas composition and temperature flow that will in turn assist with further geological characterisation. Physical samples of the chimneys will be analysed to establish a robust understanding of their formation and development.
Gravity cores were attempted by launching a 3m long tube over the side of the vessel, so sediment profiles at the summit of volcanoes could be analysed and compared to cores taken in the moats on the flanks of the volcanos. We target the deep moat areas to profile historical records of mudflows. While it was evident the Gravity corer penetrated the sediment fully, sadly no complete records were retrieved. A set of samples up to 1m were achieved and will be studied at the National Oceanography Centre, UK.
RV Celtic Explorer deploying the Holland I ROV
The photograph on the top left shows the ROV’s arm taking a core sample beside white microbial mats and degassing.
The photograph to the top right shoes Dr. Bramley Murton (NOC) taking fluid samples, each cm along the core for further analysis.
Above is a photograph of the geochemist on-board Dr. Kate Peel (NOC) and I taking samples of each cm of the core.
Water chemistry, the mixing of the oceans and the transport of life
During the initial mapping, measurements of seawater chemistry recorded during sound velocity profiles (SVP), essential for multibeam mapping, were noticed to be producing interesting deviations. Subsequent data were captured to profile the conductivity, temperature and density (CTD) of the 1.2Km water column that concurred with what we were seeing in the SVP data. At many sites, strong warm saline currents were flowing east to west. We believe these currents represent Mediterranean seawater flowing out into the Atlantic and comprise a deep dense layer, without mixing with Atlantic seawater above. Our observations concur with previous studies that indicate a layer of Mediterranean seawater can be traced as far out into the Atlantic as the Mid-Atlantic Ridge. Unequivocal evidence in the geological record shows the Mediterranean was once closed off to the Atlantic; the DEEP-LINKS team are now interested in mapping historic and present day interaction of seawater and the sediments and life it transports between both seas.
Conclusion
Further scrutiny of the high-resolution sonar mapping, ROV footage, physical rock and biological samples will enrich Irish marine scientific research and international collaborations designed to understand the secrets hidden within the darkness of this complex deep-sea environment. Exciting and progressive research will include: the identification and description of new species, genetic studies of the chemosynthetic ecosystem and its links with deep-sea fauna, the characterisation of degassing and fluid rock interaction at mud volcanoes. Societal benefits will include interrogation of biological samples for potential drug development, aimed at detecting chemicals that may be useful in the treatment of infectious and invasive diseases.
This type of multidisciplinary study is critical to our holistic understanding of the ocean. Here we see how tectonic movement, instigated deep within the earth, influences seabed morphology and degassing processes, that in turn host chemosynthetic organisms which feed mobile deep sea life.
The RV Celtic Explorer returned to Galway on the 12th November with a full complement, comprising crew, ROV pilots and scientists.
The team thank the vessels crew for all their help and support over the duration of the survey. DEEP-LINKS was supported by the Marine Institute ship time grant under the National Development Plan.
Some of the team look tense in the ROV shack; where the ROV is operated by the pilots and live feeds of the footage is interpreted by the scientist.
Maria Judge
