Charred wood reveals maximum age of newly discovered Hiawatha impact
crater in Greenland
study using a set of unusual methods shows that the Hiawatha crater
discovered in 2018 is the youngest of the 25 large impact structures
known on Earth.
The news went around the world, when a Danish and international research
team led by Kurt Kjær at the Globe institute at University of Copenhagen
published their discovery of a new, large impact crater in northwest
Greenland in November 2018. The 31-km wide structure is buried under the
Greenland Ice Sheet behind a glacier named Hiawatha. Therefore, this also
became the name of the crater. The scientists found that the crater must be
very young, seen from a geological perspective, but not precisely how young.
Now, a new study from the group, published in The Geological Society of
America’s journal Geology, reveals that the crater must almost
certainly be younger than 3 million years old, which makes it the youngest
of Earth’s 25 large impact craters.
Adam Garde, the first author of the new study and emeritus scientist at the
Geological Survey of Denmark and Greenland (GEUS), explains that “the
cellular structures of the charred wood that we recovered in outwash from
the hidden crater contained an unexpected clue to the young age of the
Exploded wood cells
According to Adam Garde, “The maximum age of the Hiawatha crater was
determined in a rather untraditional way,” but this was also quite elegant.
“One of my coauthors Jette Dahl-Møller and I investigated the cell
structures in the charred wood under the microscope. We could see that some
of the bark cells were full of large, spherical voids and had been greatly
expanded, as if the fatty material in these cells had been vaporized by
extreme and rapid heating.”
“We also found many small grains of shock-melted glass, which both
contained fragments of shocked minerals and a lot of finely dispersed
organic material. This could not have been incorporated into the glass
unless is also itself had been shocked, disintegrated and mixed with the
molten rock from the impact.”
With support from other types of analysis of the organic material, the
scientists were eventually convinced that the charred wood stemmed from the
impact and not from natural wildfires, which could be an alternative
explanation for the charring.
Identification of the wood
The researchers also found out that from preserved cell structures in the
charred wood that it stems from conifer trees such as pine, spruce and
“Today, no conifer trees can grow in the northernmost parts of Greenland at
about 80 degrees north, but remnants of thin conifer forests are known from
two warmer periods at 2.4 and 3 million years ago, and the meteorite impact
therefore took place at or after this time,” Adam Garde explains.
He elaborates that actually, there has also been conifer vegetation in
North Greenland during still older warm periods several million years ago,
but any organic remains from those much older forests would have been much
more downgraded than the wood recovered from the crater.
Searching for exact age
In the first paper from 2018 that described the discovery of the Hiawatha
crater, the authors suggested a very young age of the impact, not least
because the internal layering of the Greenland Ice Sheet is strongly
disturbed precisely where it covers the crater. However, it was difficult
to argue for a precise age.
According to the authors, “Now, with the identification of conifer
charcoal, we can at least determine a maximum age with confidence, although
an exact age determination is still missing.”
“Perhaps we now have some new means of obtaining not only a maximum age but
also an absolute age. In the coming months we will try to analyze the trace
uranium and lead contents of some highly shocked minerals, which we have
discovered in material collected last summer. This may yield an absolute,
so-called radiometric age of the impact,” says the first author.
Adam Garde further explains that if the impact crater is in fact very
young, it can be used in climate research.
“If the age of the crater overlaps with the age of ice cores from the
Greenland Ice Sheet, it will be possible to study how such a large
impact might affect the Earth’s climate. Such information is
potentially hidden in the ice cores, which cover a time span up to
about 100,000 years back in time. But this requires much more precise
information of the age of the crater than we have at present.”
The study is published in the journal Geology and is based on
collaboration between scientists at the Geological Survey of Denmark and
Greenland, the Globe Institute (University of Copenhagen), the Institute of
Geoscience (Aarhus University) and the Alfred Wegener Institute, Germany.
Pleistocene organic matter modified by the Hiawatha impact, northwest
Adam A. Garde et al.
The discovery of the Hiawatha impact crater was published in ScienceAdvances in 2018 and was nominated the same year by Science magazine
for the most important scientific discovery of the year. Read the 2018 article here:
Emeritus, Geological Survey of Denmark and Greenland (GEUS)
Telephone (+45) 50551135
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