Fossil Discovery Indicates Bering Strait Opened Millions of Years Earlier
Rummaging through the fossil-filled basement of the California Academy of Sciences’ National History Museum two years ago, curator Louie Marincovich Jr. made one of those serendipitous finds that mark the annals of science--a mixture of luck, intuition and persistence.
In a stash of fossils excavated from a ridge of ancient ocean rocks rising high above the Alaska Peninsula, he came across the carefully labeled fossils of a North Atlantic clam called Astarte.
But the clam shouldn’t have been there. It was several million years old, much older than any similar clam found in the North Pacific.
Marincovich had stumbled across evidence that the Bering Strait land bridge--that famous strip of land that humans and a parade of other animals migrated across--had been breached by ocean waters far earlier than scientists had thought. And that meant Arctic-Atlantic and Pacific marine animals were commingling at a much earlier date.
“Here I am in the basement of the California Academy of Sciences in San Francisco and I have in my hand evidence for the early opening of the Bering Strait. I’m thinking, ‘Gee, the Bering Strait is open,’ ” said Marincovich, an adjunct curator of paleontology at the academy’s department of invertebrate zoology and geology.
Fifteen years earlier, Marincovich had visited the same wind-swept site in southwest Alaska where the fossils had been collected, a brutally raw and beautiful area known as Sandy Ridge. As he climbed the 800-foot ridge, he had noticed that at a certain point, warm-water clams appeared to have given way to a cold-water species.
It was just a thought, but it stuck in his mind until he happened across those carefully labeled Astarte fossils.
After a return to Sandy Ridge last summer to collect and date more fossils, Marincovich and a Russian coauthor published their findings in the Jan. 14 issue of the journal Nature.
They presented convincing evidence that the Bering Strait had opened 1 million to 2 million years earlier than previously thought.
Scientists had estimated that the land bridge submerged 3 million to 4 million years ago, allowing the mixing of Pacific and Arctic-Atlantic ocean waters for the first time in about 100 million years and halting land migration of animals between Eurasia and North America.
But the clam finds push the opening back to about 5 million years--between 4.8 million and 5.5 million years ago. And there’s tantalizing evidence the clams were present as early as 7.4 million years ago.
The findings have ramifications for land and marine animal migration and the climate of the North Pacific, which became colder as the Arctic’s chillier waters were free to spill southward.
“This is very significant,” said Julie Brigham-Grette, an associate professor of geology at the University of Massachusetts at Amherst. “When you open the Bering Strait it means putting in touch two bodies of water that haven’t flowed together for a long period of time.
“If we know the timing of the Bering Strait’s submergence we can place that in the right order of climate changes--when did we get the modern oceanic system and when did the climate allow the Northern Hemisphere to begin to go through the modern period of glaciation?”
Scientists think the icecap that covers much of the Arctic Ocean formed about 2.6 million years ago, she said.
Brigham-Grette said Marincovich’s findings support research published a year ago by Russian scientists and Michigan State University researchers who found that the ocean floor beneath the Bering Sea is actually a huge tectonic plate rotating clockwise relative to the North American plate.
Brigham-Grette said research suggests that the plate’s motion would have opened the Bering Strait between 5 million and 6 million years ago--about the same time as suggested by Marincovich’s work.
“That’s pretty exciting to find two different disciplines discovering the same thing from two different points of view,” she said.
After his basement find in 1997, Marincovich applied for and received funding from the National Science Foundation for a three-year project to confirm those findings.
In August, he returned to the remote peninsula about 600 miles southwest of Anchorage with two Russian colleagues. They stayed at a hunting camp 15 miles from the fossilized layers of ancient ocean sediment.
Each morning the weather cooperated, a helicopter dropped them off along the rocky ridge for a 10-hour day of chipping out the grayish Astarte clams and other specimens from the blackish bedrock.
Sandy Ridge, at an elevation of 1,000 to 2,000 feet above sea level, is known for its tempestuous weather. Though the midsummer temperatures were in the relatively balmy 30s and 40s, it could be clear and calm one minute, then a few minutes later the wind might be howling at 100 mph, accompanied by the dense, smoke-like fog that often rolls in without warning from the Bering Sea.
Marincovich and his colleagues would take shelter in the downwind quiet of boulders until the winds subsided and their work could resume. By day’s end, their backpacks might tip the scales at 80 pounds.
But it wasn’t all work.
Some days, when the skies cleared, they stood in awe of a huge glacier-topped volcano 10 miles away venting gases that tinged the air with a taste of sulfur.
“On a quiet day you could hear the glaciers moving. They kind of make a distant boom like artillery. You might see a caribou or a moose foraging. It was really strange; you can’t believe you’re there. It’s so remote and so beautiful,” Marincovich said.
In July, the American-Russian trio will return to Sandy Ridge. They are hopeful they can pinpoint the date of the land bridge’s opening and possibly when tectonic forces or falling sea levels closed it up again.
Samples of volcanic dust taken from the layers where they found the clams may pin down the mollusks’ ages to within 10,000 years, Marincovich said.
The project has given him an opportunity to work with three Russians--Konstantin Barinov and Andrey Y. Gladenkov, both paleontologists with Moscow’s Russian Academy of Sciences--and Anton Oleinik, a paleontologist at Florida Atlantic University in Boca Raton, who is now a naturalized American.
Barinov and Oleinik, who added fossils they had collected from the Russian side of the Bering Sea to the research, accompanied Marincovich to Sandy Ridge. Gladenkov stayed behind in Moscow awaiting the return of the specimens. An expert in tiny ocean algae calleddiatoms, he dated the clams by studying the micro-fossils in the sediment around the clams.
Little more than a decade ago, such Russian-American scientific cooperation would have been almost impossible. The two countries were isolated by the Cold War, much as the Bering Strait now separates Siberia and Alaska with a 50-mile-wide strip of shallow ocean.
Marincovich recalls a 1976 international conference on rocks and fossils he attended in Tokyo. During the conference, he spoke with Gladenkov’s father, Yuri Gladenkov, also a paleontologist.
“We talked wistfully about how we wished we could work together, but we couldn’t because there was always the Cold War,” he said.
In the early 1990s, the collapse of the Soviet Union broke down the political barriers that had kept Russian and American scientists from sharing their research and ideas.
“It’s sort of one of those subplots of the end of the Cold War. And it’s appropriate that the age of the Bering Strait is being determined by scientists from the two nations separated by the Bering Strait,” Marincovich said.