Page 80 - AlertDiver_Winter2013

66 | WINTER 2012
W eddell seals range farther
south than any other
mammal in the world.
Adélie penguins, whales
and petrels all migrate
north in the dark winter.
But the Weddells stay,
braving the fierce winter storms, temperatures of -40°F
and below, and more than four months of night. They
are, in fact, the only air-breathing vertebrates that live in
the Ross Sea year-round, and their ability to do so is
entirely dependent on the cracks. On the coldest days of
winter, the water can be almost 100°F warmer than the air,
and the seals spend all of their time under the ice, surfacing
only to breathe. But even the predictable cracks freeze over,
so the seals must continually maintain their portals, raking
their teeth across the edges to keep the holes from freezing
shut. They survive by literally eating their way through
the ice.
A dagger of light cut down through one of the holes,
piercing the black water and illuminating a patch of the
seafloor 230 feet below. As we approached the crack, a seal
slipped smoothly through another hole, pausing for a curious
look at us before turning head-down and gliding into the
black depths and out of sight.
There was no telling when the seal would return. Weddells
are superb divers, having been known to dive to more than
2,300 feet, where the pressure is in excess of 1,000 psi. Besides
dealing with that pressure, the seal has had to solve two basic
problems to maximize its diving potential. First, it had to
manage decompression stress. Under high pressures like those
encountered when diving deep, more nitrogen dissolves into the
bloodstream from air in the lungs. When pressure returns to
normal, as it does on ascent, the excess dissolved nitrogen can
come out of solution, forming gas bubbles in the blood. These
bubbles can damage vessels, and, in severe cases, cause a stroke or
heart attack. Neither seal nor diver wants the bends. To solve this
issue, the seal actually exhales right before a dive. But Weddells
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