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Great Shearwaters
Two Great Shearwaters flying around the Odyssey.
Photo - Chris Johnson

May 30, 2005
Shearwaters and the Mid-Atlantic Ridge


Log Transcript

This is Genevieve Johnson speaking to you from the Odyssey in the mid-Atlantic Ocean.

As the crew crosses their third and final ocean, the Voyage of the Odyssey is sailing closer to its end. However, there is really only one true, interconnected ocean covering 71% of our planet. On the floor of this one, enormous ocean, forces generated by underwater volcanic activity continually expand and shrink the length and width of the ocean floor, and even move continents.

Broadly defined ocean basins (Atlantic, Pacific and Indian) continue to evolve as the sea floor pulls apart - a phenomenon known as "sea floor spreading". As tectonic plates in the Earth's crust separate, resulting in enormous cracks and rifts, new crust (molten rock from Earth's interior) is continuously being spewed forth from volcanoes that form continuous peaks between these plates called the Mid-Ocean Ridge. The Mid-Ocean Ridge is an underwater series of mountain ranges that snakes a remarkable 40,000 miles (64,000 kilometers) around the globe, from the Arctic, through the Atlantic Ocean, the Indian Ocean and across the Pacific Ocean. At four times the length of the Andes, Himalayas and Rocky mountains combined, the mid-ocean ridge is the largest single geological feature on the Earth's surface.

Map of the mid-Atlantic Ridge
A relief map of the Mid-Atlantic ridge stretching south of Iceland to the southern Atlantic.

In the mid-twentieth century, knowledge of tectonic plates and the idea of continental drift through sea floor spreading, helped to solve the mystery behind Earth's ongoing re-arrangements. Prior to this, the jigsaw like way landmasses appeared to have once fit together, baffled scientists. The infant Atlantic Ocean began spreading out from the central Mid-Atlantic ridge (the Atlantic section of the Mid-Ocean Ridge), forming along lines now recognizable as the coastlines of Africa, Europe and the Americas. Geological similarities on either side of the Atlantic, not to mention fossilized remains of plants and animals, supported and reinforced the notion that the sea floor was spreading. For example, fossils characteristic of North America, have turned up in Norway, Scotland and Ireland.

Throughout the Earth's surface, the formation of plates occurs at the Mid-Ocean Ridge, and are consumed at "subduction trenches". Most deep trenches are long, narrow gashes in the sea floor close to continental land-masses. It is in these "subduction zones" that plates collide and one slides under the edge of another.

The second largest ocean, the Atlantic, stretches Pole to Pole and is dissected almost down the centre (north to south) by the jagged spine of the mid-Atlantic ridge. The tectonic plates continue to separate due to underwater volcanic activity, causing the Atlantic Ocean to widen by about 1 inch per year.

When we first began to cross the Mid-Atlantic Ridge, the highest peaks rise 10,000 feet (3,000 meters) above the sea floor. As the Odyssey sailed on the sea surface a further 6,600 feet (2,000 meters) above their summit, we were joined by almost a dozen Great Shearwaters (Puffinus gravis). These seabirds have stayed with us for almost a week, soaring high above the unseen mountain range.

Great Shearwaters are a large, powerful, brown and white seabird. They are an unmistakable species at sea, distinguished by a dark brown crown, a white collar around the neck and across the tail, and a dark belly patch. Their wings are long, broad and slightly bowed, designed specifically to generate large amounts of lift with minimal need for flapping. We watch from deck with envy and appreciation as they glide and bank between swells.

A group of great shearwaters behind Odyssey
As the Odyssey stopped to listen for sperm whales through our acoustic array, the Great Shearwaters landed and gathered in our wake.
Photo - Chris Johnson

This species congregates around and follows fishing vessels. Perhaps they flew beside us in the hope of an easy meal. Regardless of the fact that no fish materialized from Odyssey, they choose to remain in our company, seizing small fish at the sea surface. When we stopped the Odyssey to drift and make acoustic recordings, the birds glided in for a landing skidding across the water on pinkish grey legs and webbed feet. Some dove, flying beneath the sea surface, perhaps searching for and pursuing small fish around Odyssey's hull. Others bobbed along in the swell, content to use the time to preen.

When the engine fired up, the shearwaters were airborne once again. They traveled almost a thousand miles with us - a comparatively short distance for a seabird equipped to range far and wide in search of food. Some of the greatest concentrations of seabirds are found where rich, cold water currents flow northward from the Antarctic. The Great Shearwater is believed to number somewhere between 2-4 million and breeds in the southern Atlantic Ocean on the Falkland Islands, Tristan da Cunha and Gough Island, thousands of miles from Odyssey's current position. Monogamous pairs arrive abruptly in September to lay a single white egg in a burrow. Juveniles fledge and disperse in May, when the species begins its trans-equatorial migration flying as far north as the Arctic Circle. Migration is a common characteristic of seabirds nesting at higher latitudes. As with many Antarctic species, perhaps we are encountering the Great Shearwater avoiding the rigors of winter by moving north.

For now, we too are moving north, tracking the distinct echolocation clicks of a lone bull sperm whale. After seeing his impressive, broad fluke, we estimated his length to be over 50 feet - a large, old animal, perhaps over 50 or 60 years of age. We expected him to stay submerged for at least and hour, perhaps as much as two by simply holding his breath. Having reached the western side of the Mid-Atlantic Ridge, the ocean floor is over 5,000 meters (5 kilometers) below.

With a sperm whale far beneath our keel, and shearwaters above our rigging, we are anything but starved for company after three weeks at sea crossing the Atlantic Ocean.

A group of great shearwaters behind Odyssey
A Great Shearwater puts its head underwater to look for fish beneath Odyssey's hull.
Photo - Chris Johnson

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Log written by Genevieve Johnson.

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