NSF PR 00-41 - June 7, 2000
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Discovery of Fossil Mollusks in Alaska Links Histories
of Arctic Ocean and Isthmus of Panama
Finding two fossil mollusks in a California collection
led a researcher funded by the National Science Foundation
(NSF) to undertake field work in Alaska that he says
links the formation of the Isthmus of Panama approximately
3.6 million years ago to a reversal of water flow
through the Bering Strait.
Louie Marincovich, of the California Academy of Sciences,
is the first to produce fossil evidence that the flow
of water through the strait, which separates Russia
and Alaska, was reversed from southward to northward
by the uplifting of the Isthmus. He also is the first
to date the flow shift.
Marincovich's findings also validate computer models
of Northern Hemisphere oceanography for that time
period, at least as they affected the Arctic Ocean.
"This discovery was only possible because someone picked
up two fossils in Alaska in the 1970's, not knowing
what they were and donated them to the California
Academy of Sciences, where I recognized them 25 years
later," Marincovich said. "I was going through the
collections with another topic in mind when I saw
them and had my 'Eureka moment,' when I knew they
were the first datable evidence of the Bering Strait's
Astarte, the fossil mollusk, lived only in the Arctic
and North Atlantic oceans until prior to the opening
of the strait.
The discovery of an Astarte in southern Alaska in rocks
almost 5.5 million years old led Marincovich to conclude
that the Bering Strait must have first opened at that
time. In order to be found in southern Alaska, Astarte
must have migrated southward through Bering Strait.
What was puzzling about his find is that nearly two
million years passed before mollusks from the Pacific
began migrating northward through the open Bering
Strait to the Arctic and North Atlantic oceans. Pacific
mollusks first appear in the fossil record there only
3.6 million years ago.
Marincovich's research on fossil mollusks in the North
Pacific, Arctic and North Atlantic oceans led him
to conclude that the direction of seawater flow through
the Bering Strait gateway must have changed from a
southerly flow to a northerly one around 3.6 million
years ago. This reversal in flow direction had been
theorized by computer models of past ocean flow, and
was thought to have been caused by formation of the
Isthmus of Panama as a land barrier where a broad
tropical seaway between North and South America had
existed for millions of years.
The formation of this tropical isthmus caused drastic
shifts in Northern Hemisphere ocean currents, and
initiated the flow of the Gulf Stream. However, just
when these changes took place and affected the Arctic
Ocean was a mystery not predicted by the computer
Marincovich's work was funded by the Arctic natural
sciences section of NSF's Office of Polar Programs.
An article about his findings may be found in the
June issue of Geology, a publication of the
Geological Society of America.
Editors: For a PDF file of the research article,