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 being open."
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 models.
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, see: ftp://220.127.116.11/pub/geology/28-551.pdf.
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