Ancient driftwood tracks 500 years of Arctic warming and sea ice
Fallen trees kept afloat in sea ice reflect accelerated ice loss in the
last 30 years

Date:
October 19, 2021
Source:
American Geophysical Union
Summary:
A new study reconstructs the path of frozen trees as they made
their way across the Arctic Ocean over 500 years, giving scientists
a unique look into changes in sea ice and currents over the last
half millennium. By dating and tracing pieces of driftwood on
beaches in Svalbard, Norway's archipelago in the Arctic Circle,
scientists have determined where these fallen trees floated.



FULL STORY ==========================================================================
A new study reconstructs the path of frozen trees as they made their way
across the Arctic Ocean over 500 years, giving scientists a unique look
into changes in sea ice and currents over the last half millennium.


==========================================================================
By dating and tracing pieces of driftwood on beaches in Svalbard,
Norway's archipelago in the Arctic Circle, scientists have determined
where these fallen trees floated. Retracing the driftwood's journey let
the researchers reconstruct, for the first time, both the level of sea
ice over time and the currents that propelled the driftwood-laden ice.

Borne by rivers to the ocean, fallen trees from the north's expansive
boreal forests can be frozen in sea ice and float far, but the new
research showsfewer trees are making the long journey as the sea ice
that carries them shrinks away.

The new study found a distinct drop in new driftwood arrivals over the
last 30 years, reflecting the steep decline in sea ice coverage in a
warming Arctic and provides a higher-resolution picture of past Arctic
Ocean conditions than other methods allow. The study is published in
the Journal of Geophysical Research: Oceans, which publishes research
that advances our understanding of the ocean and its processes.

Sea ice is sensitive to climate change and is an important part of
Arctic ecosystems, so understanding how ice, ocean temperatures and
currents have varied together over time is necessary for predicting
coming changes in the Arctic. But doing so can be elusive: Ice melts,
after all. The oldest sea ice is only about four years old (and getting younger), so scientists need to turn to other records.

"This is the first time driftwood has been used to look at large-scale
changes in Arctic sea ice dynamics and circulation patterns," said
geoscientist Georgia Hole at the University of Oxford, who led the study.



========================================================================== "They're taking the analysis one step further to connect changes in
driftwood to changes in sea ice, and that's where we want to go. It's
really exciting," said Hans Linderholm, a paleoclimatologist at the
University of Gothenburg in Sweden who was not involved in the research.

Important ice cubes The Arctic Ocean collects trees that naturally fall
into high-latitude rivers in North America and Eurasia. When it was
cold enough, some of the trees were frozen into the sea ice. The ice
then floated across the ocean, swept along by ocean currents and winds,
until beaching on Svalbard's shores. There they sat, some for hundreds
of years, until researchers like Hole and Linderholm came along.

Researchers have used driftwood for climate-change studies before,
but the new study is the first to test how useful Arctic driftwood is
for peering into past currents and ice coverage. To check their work,
the study directly compared driftwood-inferred sea ice coverage to the observational record of sea ice.

"This is a fantastic resource to say something about ocean currents and
sea ice conditions," said Linderholm. "I think they do have a case for
matching [tree] provenance changes to changes in sea ice conditions,
which is what we're looking for: to have sea ice information prior to observations." Tracing trees


==========================================================================
In the summer months of 2016 and 2018, Hole and her collaborators combed several beaches in northern Svalbard for driftwood. Back in the lab,
they analyzed the tree rings to determine what kind of tree it was and
compared the tree ring patterns of each driftwood slice to a database
of measured rings from trees across the boreal forests. Hole could then
trace trees to individual countries, watersheds and even rivers and see
how driftwood sources varied over time.

Hole paired her driftwood data with early sea ice observations, from
1600 to 1850, thanks to records from Icelandic fishers, seal hunters
and passing ships.

More recent sea ice data came from airplane and satellite
imagery. Finally, she compared driftwood-tracking data with sea ice
conditions and currents to see how well they correlated.

Her data revealed a slow and steady northward migration of the
lowest-latitude sea ice, reflecting warming, along with swings in
driftwood arrivals between North America and Eurasia.

"We also saw an increase in variability in the driftwood record from
1700 to 1850, which we interpret as increased variability in sea ice,"
said Hole.

Colder conditions tend to have more sea ice, so earlier driftwood
reflected a wider range of sources. As the Arctic warmed up and sea ice
melted, less driftwood could make the long journey.

The unique method provides nuanced insights that other techniques can't
offer, and this study is just the beginning -- until the Arctic loses
its sea ice altogether, that is.

"It's such a fragile system," Hole said. "If the sea ice does
decline as predicted, then this will kind of be a dying field." ========================================================================== Story Source: Materials provided by American_Geophysical_Union. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Georgia M. Hole, Thomas Rawson, Wesley R. Farnsworth, Anders
Schomacker,
O'lafur Ingo'lfsson, Marc Macias‐Fauria. A
Driftwood‐Based Record of Arctic Sea Ice During the Last
500 Years From Northern Svalbard Reveals Sea Ice Dynamics in the
Arctic Ocean and Arctic Peripheral Seas.

Journal of Geophysical Research: Oceans, 2021; 126 (10) DOI:
10.1029/ 2021JC017563 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211019120150.htm

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