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T
he Arctic Ocean hasn’t always
been as salty as other oceans.
In the Eocene, between
56 million and 34 million
years ago, the water surrounding the
North Pole — freshened by melt from sea
ice and river runoff — was cut off from
the Atlantic and Pacific oceans by land
bridges. At some point, plate tectonic
processes opened the North Atlantic,
submerging the land bridges and allow-
ing saltwater to pour into the Arctic, but
the timing and details of these events are
largely unknown. New research suggests
a tipping point may have been reached as
one particular land bridge submerged far
enough below the ocean surface.
In 2004, deep-ocean drilling by the
International Ocean Discovery Program
near the North Pole unearthed sediments
containing fossils of freshwater algae
dating to the Eocene. This was the first
hard evidence that the Arctic Ocean was
once filled with freshwater. Establishing
the timeline of events that led to the
Arctic’s modern connections with the
North Atlantic has long proved elusive,
however, due in part to a lack of indicator
fossils, says Michael Stärz, a climate sci-
entist at the Helmholtz Center for Polar
and Marine Research in Germany and
lead author of the new study in Nature
Communications. “We have almost no
direct evidence from this time period.
As far as we know, [the Arctic’s switch
to saltwater] could have occurred any-
time between 55 million and 10 million
years ago.”
To study the salinization process, Stärz
and his colleagues developed a climate
model to test how submersion of a formerland bridge known as the Greenland
Scotland Ridge would have affected cir-
culation between the North Atlantic and
Arctic oceans. The Greenland Scotland
Ridge is a wall of basalt that stretches
across the North Atlantic between the
namesake landmasses, with Iceland in
the middle. In modern times, only Iceland
and a few smaller islands remain above
the ocean surface while the rest of the
ridge is submerged at least 500 meters.
By incrementally submerging this
ridge in their model, the team found
that the greatest changes in circulation
patterns between the North Atlantic and
the Arctic would have occurred once the
Greenland Scotland Ridge was submerged
to a depth of 50 meters. This threshold
matches the thickness of the wind-mixed
layer, in which surface waters are read-
ily mixed by wind currents, facilitating
mixing between the basins, Stärz says.
Once this gateway between the North
Atlantic and Arctic was deep enough,
an unrestricted flow of denser saltwater
would have displaced the freshwater,
turning the Arctic Ocean saline in a geo-
logic blink. The event would have had
far-reaching consequences, Stärz says,
affecting oceanic and atmospheric circu-
lation patterns, heat transport from the
mid-latitudes to the poles, as well as the
freshwater-adapted organisms living in
the Arctic.
When this switch happened is an
open question, Stärz says. This study
provides the “critical gateway depths for
the paleoceanographic events, which
helps geologists focus further research
on the depth regime,” he says. “Ongoing
efforts to retrieve better geophysical andgeological data should help provide a
better timeline still,” he adds.
“Whether this switch took decades
or a century or a thousand years, we
still don’t know. That may be a ques-
tion to be addressed by the next drilling
project,” says James Wright, a paleocean-
ographer at Rutgers University who was
not involved in the new research. “The
study does a nice job of establishing the
boundary conditions for where we need
to collect more data. The next drilling
phase can use this critical depth to target
drilling in places that might tell us more
about when this mixing took place.”
Mary Caperton MortonAlternative metals
show promise for
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