The Greenland impact crater
This is a brief note on the recent Science Advances paper on
the Hiawatha Impact crater, a large, recent crater which lies under the
Hiawatha glacier in extreme northern Greenland.
In the past, I have bagged out impact crater scientists and
being alarmist and even dangerous. However, this discovery is the real deal. Similarly,
I have occasionally criticized the “glamour-mag” approach to scientific
publication, but in this instance, a big splash is appropriate, because it is a
big deal, and the evidence is overwhelming.
The short version: Ice penetrating radar and analysis of
glacial outwash sand show a large (31km diameter), recent impact crater under a
Greenland glacier, complete with central peak.
The outwash shows shocked quartz, probable melt glass, and PGE anomalies
consistent with an iron (or stony iron) impactor. This is not one of those
ancient, deformed, maybe-if-you-squint-you-can-see-a-circle crators, this is in
your face and completely obvious to anyone who has studied even a little
geology.
Like many short format papers, a lot of the details are in
the supplementary materials. For
example:
This is a recent discovery because of global warming! Prior to
2012, the outlet glacer emptied into a lake. It is only ni the last 6 years
that it has retreated onto land, so that the sediment they sampled and found
the shocked quartz, impact glass, etc in was only exposed from beneath the
melting ice sheet a few years ago.
They are planning on running conventional gravity surveys to
look for rebound, but because all the ice is melting, the melt signal dominates
the GRACE gravity signature.
No known impact ejecta is known from any of the North
Greenland ice cores, making the crater likely to be older than the oldest of
them (about 100ka). Ice cores are regularly checked for volcanic debris, and it
is unlikely that they would miss something this large and close (quick math
suggests the ejecta volume should be about 200-600 km3, making it a medium to
large VEI-7 equivalent).
The crater overprints pre-glacial river valleys, and this is
(as the authors state) probably Pleistocene in age (10ka-2.5Ma).
The melt glass should be datable via Ar/Ar dating, but it is
not clear if they have recovered a large enough volume of the material to date
at this stage.
I would expect a tektite field from an impact this size, but
it isn’t clear where those tektites would end up. If they fell on ice (By
definition, the Arctic was mostly ice-covered during most of the Pleistocene),
then they would get carried to a moraine (on land), or float around until the
ice was exported through the Fram Strait and melted somewhere in the NE
Atlantic Ocean.
There is a controversial Younger Dryas impact hypothesis,
which basically calls for an Arctic impactor as a trigger for the Younger Dryas
cooling and extinction of the Clovis culture in North
America. I would be careful connecting this crater to that event, as
the NEEM ice core, less than 400km away, doesn’t have any reported ejecta, as
known tephra are mostly basaltic.
Finally, they report carbon in the silicate impact melt. That seems odd to me, as neither crustal gneisses nor iron meteorites have much carbon. they should do ion probe d13C to get the isotopic composition. Who knows, maybe the impactor hit a peat bog.
2 comments:
In an interview, they mention having done some Ar/Ar work, but not recovering useful data from it. They also imply they are also attacking a U-daughter product chronometer...
Were they doing Ar/Ar on melt or shocked feldspar?
Post a Comment