Science Spin September 2007

Ireland's 'Grand Canyon'

Ireland's underwater seabed is vast, about nine times the island's area, and contains many surprises. At its southern limit, near the border with the UK seabed territory, and about 400 km due south of Mizen Head, lies the Whittard Canyon. The canyon runs down the continental margin, connecting the shallower continental shelf edge (200 metres depth) to the deep sea Porcupine Abyssal Plain (4,300 metres depth). The canyon is just about deep enough to be able swallow up the Alpine giant, the Matterhorn, which stands 14,693 feet tall on the border of Italy and Switzerland.

The first ever underwater survey of the Whittard Canyon, undertaken last summer, reveals a fascinating world, teeming with all kinds of sea-life, even thousands of metres down, at the ocean floor. This life is fed by nutrients which arrive with new water that is regularly flushed downwards from the seas above. Scientists believe a greater understanding of life here can help with our understanding of sea pollution, nutrient circulation, biological richness, carbon balance, and climate change.

Background

At the start of July this year, the new British research vessel, the RRS James Cook, equipped with Europe's leading robotic submersible, the ISIS remotely-operated vehicle (ROV), set out to explore the Whittard Canyon, a little understood submarine canyon on the southern limit of Ireland's seabed. ISIS is a remarkable ROV, equipped with cameras, robotic arms, as well as sampling and surveying devices. Dr Andy Wheeler, based at the UCC Department of Geology and the Environmental Research Institute, UCC, was the resident Irish-based scientist on board.

The branching canyon is 200 km long and 4,000 km deep. That's about half the length of the 'Grand Canyon', but two-and-a-half times as deep. It has been mapped by the Geological Survey of Ireland (GSI) using high-tech echo sounders, but was never really explored. Dr Wheeler and the EU-HERMES project international team onboard the RRS James Cook, led by Professor Doug Mason of the National Oceanography Centre, UK, set out to explore the canyon at a number of locations.

Scientists want to understand just what the Whittard Canyon is like, how it functions, and what aspects of its behaviour are worth following up in subsequent surveys.

Hermes project

This fits in with the goals of the EU-Hermes Project, which is defined as "an integrated project designed to gain new insights into the biodiversity, structure, function and dynamics of ecosystems along Europe's deep-ocean margin".

The Hermes Project website further explains its reason to exist:

"Europe's deep-ocean margin stretches over a distance of 15,000 km along the Atlantic Ocean from the Arctic to the Iberian margin and from western to eastern Mediterranean, through to the Black Sea. The margin extends from the shelf edge at about 200m depth until around 4000m depth where the abyssal plain or oceanic basins begin, and covers 3 million km2, an area about one third of that covered by Europe's landmass. Most of this deep-ocean frontier lies within Europe's Exclusive Economic Zone (EEZ) and is, therefore, of direct interest for the exploitation of biological, energy and mineral resources."

Biological hot-spots

Submarine canyons cut into the continental margin connecting the shallow continental shelf areas with the deep-sea abyssal plain. The steep rocky sides of these canyons and their constrained tidal flows offer a wide range of habitats for a variety of organisms. For this reason such canyons have long been recognised as "biological hotspots".

Submarine cables

Perhaps more importantly, canyons offer a direct and rapid connection from the shelf sea to the abyssal plains. Here, submarine slides and sediment avalanches are common, and this is a constant hazard for the many submarine cables that criss-cross the Atlantic from North America to Europe, and pass through Irish waters. In fact, it is these sediment avalanches, or 'turbidity flows' as marine scientists would call them, that can come tumbling down from the shelf to the deep sea and cut off the canyons.

Pollution

There is also evidence that, at least periodically, canyons are the sites for "cold water cascading events", where shelf water gets pumped directly into the deep sea in large volumes. This rapid transfer of surface water directly to the bottom of the ocean has implications for pollution levels at the ocean depths. Scientists now understand that the coast pollution which spreads to the shelf seas has, in turn, a rapid transfer route via submarine canyons to the deep sea, where the water may not have been at the surface for 4,000 years.

This effect could be even more profound in other settings where canyons have a direct connection to river systems. Such a scenario exists, for example, in the Setubal Canyon, which lies in Portuguese waters. This canyon was studied by the team onboard the RRS James Cook, before the vessel moved up into Irish waters last summer.

Carbon and nutrients

The pumping of water up and down canyons has implications for regional nutrient circulation, which affects biological richness. This pumping is a mechanism for transferring carbon from surface waters rapidly to the deep sea. This process usually takes thousands of years so understanding the scale of this process in submarine canyons can help scientific understanding of the carbon balance in the oceans, which, in turn, is crucial to formulating a better understanding of climate change.

Survey begins

The disastrous summer weather in late June meant heavy rain and huge seas in the area around Whittard made life difficult. The team decided to stay longer than they had anticipated studying the canyon systems off Portugal, with a shorter stay than had been planned at Whittard. Nonetheless, following a break in the bad weather, the RRS James Cook, with the ISIS ROV onboard, steamed north, and began, finally, to survey in Irish waters. This was the first chance humans ever had to see what it was really like deep down inside this huge canyon.

Lower reaches

"Firstly, we concentrated on the lower reaches of the canyon at about 4,000 metres below the sea surface. An ISIS dive crossed the main axis of the channel and showed active sediment movement with sand dunes and ripples covering a boulder-choked channel. Obviously the Whittard Canyon has witnessed some pretty strong sediment flows in the past. Some of the boulders in the channels were rounded: typical of erosive sand-blasting from suspended sands in strong currents. The sides of the channel were heavily eroded, with a vertical 30 metre cliff section on steep flanks rising a few hundred metres. Exposed in the side of this cliff were a series of cross-beds, suggesting that we were seeing a channel that was incised into basal (fan) deposits of the canyon."

"A 10.5 metre sediment core retrieved from the terrace above the channel at 3,800 metres depth revealed a beautiful sequence of layered turbidites (sediment avalanche deposits) that had spilled out of the channel. These were glacial in age, showing that the canyon was even more active thousands of years ago. At that time, it was fed by sediment laden glacial melt-waters from a coastline that was much nearer to the canyon heads (when sea-level was lower)."

"Despite the episodic hostile conditions in the lower canyon, life also thrived with numerous types of deep-sea fish, sponges, sea fans and sea pens, sea urchins, starfish, octopus, as well as numerous Xenophyophores (single-celled protozoans, the size of a fist)."

Middle reaches

"For the next ISIS dive, we moved further up the canyon to its middle reaches, exploring a long transect 1,000 metres up the side of the canyon from the channel floor, at 3,360 metres water depth. The water was very turbid, suggesting active background sediment transport down the system. This impression was also supported by extensive areas of rippled sand covering the sediment drifts."

"At the edge of the channel, the canyon walls were near vertical in places covered by large, red branching anemones. Sediment drapes sometimes covered the exposed rock, with erosion furrows suggesting small vertical sediment flows cascading into the canyon on a regular basis."

"The sediment drapes were highly bioturbated (full of burrows) despite the fact that they were at such a precipitous angle. Who made the burrows was not clear, but presumably they were free swimming and scared of our lights. As we moved further up the canyon wall the amount of biology decreased, as did the amount of 'marine snow' - caused by the constant fall of particulate material in the water mainly comprising plankton, larvae and dead organic matter. This was interesting as it suggested that the flanks were effectively starved of food in comparison with the canyon floor. Paradoxically, therefore, the flushing of the canyon floor, far from being a problem for marine life, was, in fact, providing a fresh and readily available supply of food."

Higher reaches

"Concluding our explorations in the limited weather window, we completed a further ISIS dive higher up the canyon system, starting in the channel again, but this time at 2,500 metres water depth and continuing up the side of the canyon to the top at 460 metres water depth. The base of the canyon was again turbid with a layer of silt covering boulders and rippled sands deposited during previous more violent canyon activity. Life was evident especially underneath protected overhangs with anemones, sea lilies and fish present. Moving up the sides of the canyon we again saw sediment drapes covering some of the exposed and eroded rock sides. This time we also saw small metre-deep channels cut into the sediment running vertically down -- a small turbidity flow (sediment avalanche) running down on the channels, something rarely captured on film."

"Once out of the channel zone, signs of life again became sparse corresponding to an increase in water clarity. This situation continued up-slope, until we reached about 1,000 metres water depth where life again became more abundant. This water depth corresponds to the base of the 'Mediterranean Outflow Water', which, as the name suggests, is a water mass that has exited the Straits of Gibraltar and flowed north."

"Being more saline than the Atlantic surface water, but warmer than the Atlantic deep water, it finds an appropriate depth usually between 1,000 and 600 metres water depth. Life here consists of tulip-shaped sponges, sea fans and sea pens, brittle stars and starfish, sea urchins, crabs, sea lilies, numerous fish species and a white octopus who found ISIS rather intriguing."

"We also found the remains of a cold-water coral reef, now badly eroded. Similar reefs have been found by towing a video camera near the seabed in an adjacent canyon. This was undertaken in May 2007 as part of a joint Irish Marine Institute and UK JNCC cruise, and as part of the habitat mapping project."

"Whether the coral reef's fate was due to natural causes or because of fishing activity is not clear. Numerous trawl-marks caused by the dragging of fishing nets across the seabed, were identified on the seabed near by. The coral reef did support some live coral, with the reef edge, although now mainly composed of dead coral, still clearly defined. Above 600 metres water depth we went from a zone dominated by sea fans to one dominated by crabs, possibly related to a change in water mass from Mediterranean Outflow Water to Northeast Atlantic Surface Water."

"At the end of the dive we found further evidence of sediment avalanching into the canyon with a 10 metre wide shelly gravel rise, or avalanche chute, running straight down the slope. We followed this raised deposit for several hundred metres up-slope where it petered out at a general area of erosion at the base of cliff. With more bad weather coming it was time to take some quick sediment cores, as we had done throughout the dives, and bring the ISIS safely back onboard. Until next time."