As temperatures fell, the sea floor bloomed with soft-bodied echinoderms — invertebrates including starfish, brittlestars, sea lilies and sea cucumbers. At the same time, crush wounds caused by crabs or sharks on the arms of fossil starfish and sea lilies became rare — evidence that these predators were declining.
Crabs and lobsters were probably excluded by a physiological quirk. At temperatures below about 1 °C, they become unable to regulate magnesium in body fluids, leading to narcosis, clumsiness and paralysis of breathing. Most of the 100 or so fish species currently found on the Antarctic shelf belong to a single sub-order, whose members evolved antifreeze proteins to keep their blood flowing at subzero temperatures and then diversified to fill most niches in the frigid seas. They lack powerful jaws.
The result is an ecosystem reminiscent of that 350 million years ago, in which the top predators are slow-moving invertebrates such as starfish, sea spiders and ribbon worms. “All of this stuff has got a very Palaeozoic flavor to it,” says Aronson. The relaxation of natural selection allowed species to lose their natural armor, says James McClintock, a marine biologist at the University of Alabama at Birmingham. Animals on the Antarctic sea floor “are very weakly skeletonized”, he says. “You can pick up an Antarctic clam and crush it in your hand.”
By the mid 2000s, Aronson began to believe that if Antarctica's oceans warmed up, the ecological cascade that caused this blast-from-the-past ecosystem to flourish would run in reverse: crushing predators would return and wreak havoc. That prediction is now being tested.
Westerly winds are strengthening and the circumpolar current is intensifying, driven by atmospheric warming and a hole in the ozone layer over Antarctica. These changes are lifting warm, dense, salty water from 4,000 meters down in the Southern Ocean up over the lip of the continental shelf.
Douglas Martinson, an oceanographer at the Lamont–Doherty Earth Observatory in Palisades, New York, has documented this process on the western side of the Antarctic Peninsula, where crabs are invading. Martinson installed five temperature and current sensors around Marguerite Trough — a deep canyon carved into the sea floor by glaciers advancing to the edge of the continental shelf in past ice ages. The moorings captured an insidious process: as the circumpolar current skirts Antarctica's continental shelf, it runs head-on into the steep wall of the trough. About once a week, a swirling eddy containing 100 cubic kilometers of warm water wafts up from that collision, spilling onto the continental shelf. The same thing happens elsewhere, says Martinson: “It looks like this is what happens at all of the canyons that cut across the shelf.”
The temperature of this intruding water is only about 1.8 °C — but for an ocean region generally between 1 and −2 °C, the impact is substantial. And the incursion seems to have begun only recently, says Eugene Domack, a marine geologist at Hamilton College in Clinton, New York, who led the 2010 cruise to Palmer Deep.
Domack has managed to date the onset by measuring the amount of radioactive carbon-14 in deep-sea corals collected from the continental shelf — a process similar to reading tree rings. The corals had grown for 400 years before being dredged up. The carbon-14 content increased smoothly along the coral's growth axis for the first 350 years, and then dropped suddenly — indicating that the coral was being bathed in water with a reduced carbon-14 content. The water from the depths of the circumpolar current would fit the bill: it has been isolated from the carbon in the atmosphere for centuries. On the basis of these measurements, Domack has deduced that the warm-water incursion “kicked in somewhere around the turn of the last century, 1920 or 1930”.