Saturday, December 08, 2018

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.
  

1 comment:

Isotopic said...

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...