By Sid Perkins
Cave formations along the coast of an island in the Mediterranean Sea hold evidence that sea level can rise and fall abruptly during an ice age, a finding that casts some doubt on current notions about how those lengthy cold spells develop and progress.
At the height of an ice age, immense volumes of water are locked up in land-based ice sheets, and ocean levels can be as much as 130 meters below where they are today. By contrast, when that ice melts during warm periods, sea level can be a few meters higher than the modern-day standard, says Jeffrey Dorale, a paleoclimatologist at the University of Iowa in Iowa City. Now, Dorale and his colleagues report in the Feb. 12 Science that during a brief interval well within the most recent ice age, sea level suddenly and inexplicably rose to a height more than one meter above today’s.
Evidence supporting that conclusion comes from cave formations on the Spanish island of Majorca, the researchers say. As sea levels rose and fell, waters sloshing into coastal caves left crusts of minerals on their walls and floors as well as on existing cave formations, Dorale says.
Radioisotope dating of mineral crusts in one cave along Majorca’s southern coast indicates that sea level sat about 2.6 meters higher than today between 121,000 and 116,000 years ago, during the last warm spell between ice ages. That level is consistent with sea level data gathered at other sites worldwide, Dorale notes.
But three samples from other crusts in the same cave — samples deposited about 1.5 meters above modern-day sea level — yielded surprising results. Those crusts formed around 81,000 years ago, well after the most recent ice age — which lasted from roughly 110,000 until 10,000 years ago — had begun, Dorale says. Similar analyses of samples from nearby caves show that between 80,000 and 82,000 years ago, sea level ranged between 1.25 and 1.6 meters above today’s standard.
“The [team’s] results are strong but not absolutely watertight,” comments R. Lawrence Edwards, a paleoclimatologist at the University of Minnesota in Minneapolis. One possible confounding factor, for instance, could be the rebound of Earth’s crust in the region since the end of the most recent ice age. After the ice mass smothering Northern Europe melted and ran to the sea, pressure from viscous material at the top of Earth’s mantle would have lifted the area, thereby influencing apparent sea level.
But Dorale and his colleagues contend that tectonic uplift hasn’t affected their data. He cites similar analyses of now-submerged mineral crusts in Majorcan caves indicating sea level was about 20 meters below today’s level about 85,000 years ago and about 15 meters below the modern standard about 79,000 years ago — readings that match most data gleaned elsewhere at those times.
Studies at a handful of sites worldwide have noted that sea level reached an exceedingly brief and similarly enigmatic high point around 81,000 years ago, says Dorale. Those results have been controversial and, for the most part, have been “politely ignored because they don’t fit the presumed pattern” of how ice ages develop and progress, he says.
Scientists have long noted erratic dips and jumps in sea level during Earth’s ice ages, but debate has typically focused on the magnitude of those swings, says Dorale. The new findings are somewhat disturbing because they suggest that at some points during an ice age, sea level can rise as much as 2 meters over the course of a century. “It’s tough to explain how to melt that much ice that fast,” he admits.