An Alaskan Trigger for the Last Ice Age
Alaskans sometimes seems to be perversely proud of the local capacity for catastrophe. We brag about the bad stuff, from Southcentral's great earthquakes to the Aleutian's great winds, from the North Slope's mosquitoes to Southeastern's devil's club. That enjoyment of bad news may serve us in good stead if some preliminary research findings prove true: Alaska's volcanoes may have triggered the last great ice age, with a little help from the 30 volcanoes on Russia's Kamchatka Peninsula.
The suggestion comes from work based neither on volcanoes nor glaciers. Instead, the evidence is written in the muddy bottom of the North Pacific Ocean. It was gathered by scientists aboard the JOIDES Resolution, a drill ship that worked in a broad area between Japan and Alaska last summer. The researchers were able to collect extensive core samples of the sea-bottom sediments from 25 holes at seven different sites along the cruise track.
Just as in an archaeological dig, deeper holes offer older evidence. The core samples of soupy sub-Pacific mud totalled more than four kilometers long, so they offered the researchers a glimpse of the far past. They found some interesting juxtapositions of debris in the layers near 2.6 million years old, about the time when the Pleistocene ice age began.
At four of the sites near the northwest edge of their voyage, the scientists encountered a high number of layers of volcanic ash. They believe volcanoes on the Kamchatka Peninsula were the ultimate source of these layers; some ash also occurs in layers earlier in the record---but only a tenth as often as it does beginning about 50,000 years before the great ice sheets covered the land.
Another kind of evidence in the sea-bottom sediments helped check the timing of the glaciers' triumph. As is occurring at Antarctica's shores now, ice sheets reaching the sea calve off prodigious quantities of icebergs. Rock, gravel, and whatever has been frozen into the ice during its travels downslope to the ocean all gets carried off to sea with the bergs. As they melt, the icebergs drop their stony load. The sediments show layers of this ice-worn stone debris, and thus leave clues for the scientists to find upon microscopic examination.
In the northwestern Pacific, on the Japanese side of their cruise, the team found the ice-carried debris appearing about 50,000 years after the erupted ash increased. In the northeastern Pacific, where the Aleutian volcanoes began a tenfold increase in activity at about the same time their Kamchatka counterparts did, the ice took longer to follow the eruptions. Nearly 300,000 years passed before the bergy detritus appears in the muds.
In geological terms, the difference between 50,000 years and 300,000 is a mere twitch of the calendar (though I'd certainly appreciate the extra time to move out of the glaciers' way). Why the difference exists is only one of the numerous questions the research has raised. Nobody knows why the volcanoes turned up the heat when they did, and nobody understands how their comparatively few years of eruptions could have led to more than two and a half million years of ice age.
Probably most of the researchers involved would go along with one of the chief scientists on the project, paleoceanographer David Rea. He thinks the climate was deteriorating over time, and the volcanoes were the straw that broke the climate-camel's back---"just a threshold phenomenon, the little bit that makes all the difference."
Nevertheless, it's enough to make one hope our volcanoes snooze peacefully for a long, long time.
