Archive for November, 2018

New Impact Crater Discovered Under Greenland

November 17th, 2018

If you’re plugged into science news outlets, you’ve likely seen stories about a very large crater that has been detected underneath Hiawatha Glacier in northwest Greenland (e.g., at Science News).  Here’s the link to the peer-reviewed article in Science Advances, by Kurt Kjær and colleagues. This paper is being touted by some outlets as likely vindication for the “Younger Dryas Impact Hypothesis”, made famous by Firestone et al. (2007). This may sound exciting to science consumers, but for many climate scientists, this is cause for groans, not exhilaration. After seeing the headlines, a few questions that might arise are: 1) How much does Kjær et al. (2018) support Firestone et al. (2007)? 2) Wait, what was Firestone et al. (2007) all about? Actually, the first question might be: 3) What the heck is the Younger Dryas, anyway?

Figure 1 from Kjær et al. (2018), showing the location of the impact crater in northwest Greenland.

Let’s take these in opposite order:

Question 3: What’s the Younger Dryas? It’s the last gasp of the Pleistocene glacial Epoch.  Warming and retreat of the ice sheets didn’t always occur gradually.  Over the course of several thousand years (about 20,000 to 11,700 years ago), the ice retreated in fits and starts.  Often, the warming was abrupt, and often the warming was actually reversed for many years or decades as Earth’s atmosphere and ocean constantly adjusted to the shifting ice cover and long-term warming trend.  The Younger Dryas is the last big cold snap before the relative warmth and stability of the Holocene.  It was long — from 12,900 to 11,700 years ago. The transition both into and out of the Younger Dryas was was also abrupt — like decades or shorter. (Alley 2000)

Temperature and snow/ice accumulation in Greenland over the past 17,000 years (from Alley 2000, p. 9, fig. 12)

Question 2: In 2007, Firestone et al. proposed that a comet exploded over North America, leading to myriad devastations: Widespread wildfires across North America, collapse of the Clovis culture, the extinction of North American megafauna (e.g., woolly mammoths), and the abrupt onset of the Younger Dryas. Firestone et al. proposed a comet in part because there was no impact crater in North America and in part because the geochemical evidence they presented was “more consistent with an impactor that was carbon-rich, nickel–iron-poor”.

What followed was a contentious tear-down of the Firestone hypothesis.  I have 40 papers saved on my computer about this stuff, and it got nasty. Not only were the conclusions disputed, but also the results.  Some scientists presented contrary evidence using similar methods (Paquay et al. 2009; Daulton et al. 2010). Others questioned the validity of evidence presented by Firestone et al. (Buchanan et al. 2008Tian et al. 2011). Some scientists even tried to replicate the results at the same study sites but couldn’t ( Surovell et al. 2009; Haynes et al. 2010). By 2011, Pinter et al. published a paper called “The Younger Dryas impact hypothesis: A requiem”, declaring it dead. Of the original 12 lines of evidence provided by Firestone et al., 7 proved unreproducible, and the others were given alternate explanations, such as non-catastrophic mechanisms (e.g., an uptick in wildfires can be explained by drought) and/or terrestrial origins (e.g., magnetic grains occur many river sediments).

Question 1: So along comes this new paper that says there is an impact crater in North America. That’s big news, right?  Yeah, it’s cool. But are Firestone et al. are vindicated? Absolutely not — at least not yet.  Here’s a few problems that jump out to me about making the leap in logic from “there’s an impact crater in Greenland in the Pleistocene” to the conclusion that this impact caused the Younger Dryas:

  1. The timing.  The authors of the new paper state that the impact probably occurred during the Pleistocene.  That’s about 2,576,000 years of Earth history, and the Younger Dryas is dated down to decades.  Looking deeper at the paper, it seems most likely that the impact was in the later part of the Pleistocene, so it is absolutely possible that it hit at 12,900 years ago. However, even if we give error bars of ± 100 years on the Younger Dryas onset and say the impact had to be during the last 100,000 years of the Pleistocene (the last 3.8%), there’s still a 99.8% chance that the impact did not overlap with the Younger Dryas onset. So it’s too soon; we need to date this crater.
  2. Even if an impact occurred at 12,900 years ago, it doesn’t change the state of the evidence regarding mammoths or humans.  As summarized in several of the above papers, there’s no consensus of evidence for a catastrophe at the Younger Dryas for either.
  3. We still need an explanation for getting out of the Younger Dryas at 11,700 years ago.  And we still need an explanation for the various other abrupt climate shifts apparent in the Greenland ice cores. So the terrestrial mechanisms that caused other events (ice sheet and ocean dynamicscould still cause the Younger Dryas even if an asteroid could, too.
  4. The authors of this new paper are very clear that their geochemistry matches an iron meteorite. Firestone et al. were very clear that their geochemistry matched an iron-poor impactor like a comet.

To their credit, Kjær et al. are appropriately cautious in voicing implications. They never mention the Firestone hypothesis; they are conservative in their dating; and they do not speculate about broader implications beyond “this impact very likely had significant environmental consequences in the Northern Hemisphere and possibly globally”. (It does seem to be one of the top 25 largest in the world.) Now, if it turns out we later find this impact was at 12,900 years ago, that will get me excited.

A lonely Dryas plant in Kennecott, Alaska. (Photo: Alex Crawford)

 

p.s. If you’re wondering, yes, there’s also an Older Dryas period.  It’s similarly cold but much shorter and happened  around 14,000 years ago. Both periods are named after the Dryas genus, which is abundant in Scandinavian lake samples dating to these periods.

Petroleum Experts Limited Donates MOVE Suite to Wooster!!

November 12th, 2018

Wooster, Ohio — The Department of Earth Sciences is pleased to announce that Petroleum Experts Limited recently donated ten licenses of their MOVE suite software package to be used for educational and training purposes.  The MOVE suite, which has a market value of $2.18 million, is the industry standard for structural modelling, and its software modules include 2D/3D kinematic modelling, geomechanical modelling, fracture modelling, fault analysis, and stress analysis, to name a few.  When using the MOVE suite, Wooster faculty and students will be able to interpret data, build cross-sections, and kinematically and dynamically analyze structural histories.  More information about Petroleum Experts Limited and the MOVE suite can be found at http://www.petex.com/products/move-suite/.

Petroleum Experts Limited is based in Edinburgh, Scotland, with a satellite office in Houston, Texas.  The Department of Earth Sciences is appreciative of the efforts of all at Petroleum Experts Limited and The College of Wooster who worked to make this donation possible.  Our faculty and students will benefit enormously from using the integrative MOVE suite, especially those students in ESCI 340 (Structural Geology), ESCI 345 (Tectonics and Basin Analysis), and ESCI 401/154/452 (Independent Study).

(Image from: http://www.petex.com/products/move-suite/. Accessed 11/12/2018)

Wooster Earth Scientists at their annual “Mock GSA” — 2018 version

November 1st, 2018

Wooster, Ohio — Every Fall the students and faculty of Wooster’s Earth Sciences Department look forward to participating in the annual meeting of the Geological Society of America. Next week the meeting will be held in Indianapolis, and over a dozen Wooster Scots will be there. We’re bringing seven student and faculty posters. Today we had our usual practice presentation, which we call “Mock GSA”. I didn’t get images of all the participants, but at least you’ll see the enthusiasm of the group! Above are the posters displayed in Scovel Room 205.

Evan Shadbolt is above on the right discussing his poster (authored with our esteemed alumna Tricia Kelley) on modern shell boring patterns off Long Island, New York. Galen Schwartzberg is to the left in front of her poster on Jurassic sclerobionts in southwestern Utah.

Juwan Shabazz, Kendra Devereux, and Alexis Lanier are discussing their poster on tree-ring chronology patterns in Ohio.

Ben Sershen is pointing to a graph on his poster about Arctic sea ice.

Ethan Killian is presenting his poster on oyster balls of the Jurassic in southwestern Utah.

Josh Charlton and Victoria Race’s poster is on mass balance estimates and dynamics of Columbia Glacier in Alaska. Behind Josh are the hands of Michael Thomas on his poster on central Utah structural geology.

Galen Schwartzberg is here again with her poster on Jurassic sclerobionts in the Carmel Formation of southwestern Utah.

Good luck to everyone in Indianapolis! Our next posts will be from the GSA meeting.