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.

Wooster Records its Third Wettest Year on Record

October 4th, 2018

If you live in Ohio and have felt wet and miserable the past year, you now have vindication. Based on the long-term record from the OARDC weather station, Wooster has just completed it’s third wettest year on record (i.e., since continuous record-keeping began at the OARDC in 1900).  I know, it’s the first week of October, but in the hydrology world, the “water year” typically begins on Oct 1.  This makes sense if you think about agriculture — water falling in Oct-Dec of 2018 isn’t all that helpful for most crops growing in 2018, but it can replenish surface reservoirs and/or  groundwater for 2019.  Therefore, the 2018 water year just ended Sunday.

Figure 1: Annual precipitation at the OARDC station in Wooster, Ohio by water year beginning Oct 1, scaled to a 365-day year. The record extends 118 years: 1901-2018. Linear (red) and cubic (green) fits to the dataset are also included. 

The total precipitation in 2018 was 50.17 in., which fell about an inch short of the record, set in 2004 (51.18 in.).  The only other year with over 50 in. was 1996 (50.81 in.). Both 2004 and 1996 were leap years, but even if you adjust precipitation to 365 days per year, 2018 still ranks third. Another interesting thing to note is that the annual precipitation in Wooster has been increasing over the past 118 years.   On average, the change is about 0.07 in./year. That might not sound like a lot, but over 118 years, it adds up.  In the past decade (2009-2018), Wooster has received 102.6 in. more precipitation than the period 1901-1910. In other words, we’re getting about 32% more precipitation in the 2010s than we did in the 1900s.  A linear trend is pretty good for explaining this long-term change, but you might notice that most of the change in annual precipitation takes place in the periods 1900 to 1925 and 1980 to today.  The green curve is a little better at fitting the data and better captures the mid-century stability and the rapid increase in wetness over the past few decades.

Figure 2: Histogram of Aug-Sep precipitation in Wooster with the extreme years of 2017 and 2018 indicated.

One more thing that’s interesting to note is just how different late summer was in Wooster this year compared to last.  The Aug-Sep period of 2017 was one of the driest on record in Wooster (even though the year overall was unexceptional). Only 2.42 in. fell in Wooster then. On the other hand, 2018 was one of the wettest years on record, with 11.36 in. falling. Only 3% of all Aug-Sep periods were drier than 2017, and only 4% were wetter than 2018. What a difference a year makes!

Wooster’s Invertebrate Paleontology class at work

September 18th, 2018

Wooster, Ohio — The Invertebrate Paleontology class at Wooster set to work this afternoon on the excellent fossils they collected at the beginning of last week. They had already washed them carefully, using soft brushes and soap, and now were learning how to trim them down with our faithful basement rock saw. Grant Holter is seen above doing his very first cut. All the specimens are from a single outcrop of the Upper Whitewater Formation (Upper Ordovician, Katian) just south of Richmond, Indiana.

The spinning steel blade has industrial diamonds embedded in its periphery, which grind quickly through our soft limestones. The blade and rock are continually sprayed with water to keep the blade from overheating, lubricate the cut, and to capture the dust. The newbies to our saw learned fast.

Each student has two trays of specimens, which are right now in their raw, unprepared and unlabelled state. Julia Pearson examines her very full trays. Juwan Shabazz is behind her.

A closer look at Julia’s treasures.

An even closer view. We can easily now identify abundant brachiopods, bryozoans, and rugose corals — the big three groups.

Finally for today the paleo students learned how to label their specimens using water-soluble white glue and printed paper tags, a technique I learned at the University of California Museum of Paleontology.

Next week the class will use the saws, grinders, polishing plates and hydrochloric acid to make acetate peels. This is my favorite paleo process!

2018 Invertebrate Paleontology field trip — with the Ghost of Gordon

September 9th, 2018

The Invertebrate Paleontology class at Wooster had its annual field trip today to the Upper Ordovician (Katian) Cincinnati Group (Upper Whitewater Formation) in eastern Indiana. The weather looked terrible as the remnant of Tropical Storm Gordon worked its way into the Great Lakes region. Three to five inches of rain were forecast for our field area just south of Richmond, Indiana (locality C/W-148). For all I know, that massive amount of rain actually fell today — but not while we were there! As you can see above, we collected treasures in the dry. In fact, the specimens were nicely washed for us, with the fossils standing out better than I’ve ever seen.

Here’s a random image of the rubbly limestone we examined. Count the bryoimmurations! This is perfect material for beginning paleontology students. Each one made a representative collection to clean, prepare and interpret in our cozy Wooster lab the rest of the semester.

We’ve certainly had better weather here in past years, but I’m not complaining about today. We slipped by a ghost.

Using Snow to Predict Sea Ice

August 24th, 2018

One of my active areas of research is trying to find physical links in the Arctic climate system that may help us better predict when seasonal sea ice cover will disappear each summer. Good sea ice predictions are important because shipping, tourism, resource extraction, and any other human activity in the Arctic Ocean is much more dangerous when sea ice is present.  As the open water season gets longer (thanks to global warming), more shipping companies (like Maersk) are using the Northern Sea Route through the Arctic Ocean. The earlier we know when the sea ice will be gone and the waters open, the earlier we can plan shipping schedules.

The Northern Sea Route through the Arctic Ocean and the day sea ice concentration falls below 50% (left) or 15% (right).

A recently accepted article at the Journal of Geophysical Research: Atmospheres by myself and colleagues in Colorado and the UK describes how one physical link that can help predictions is when snow cover retreats in Siberia.  More specifically, the paper focuses on how snow retreat in the West Siberian Plain (WSP) can help predictions of sea ice retreat over 1,200 km (over 700 miles) away in the southern Laptev Sea (SLS).  It’s a complicated system of interactions, but here’s the short version:

1. When snow disappears from the West Siberian Plain (WSP), the land surface warms up quickly and releases substantial energy up to the atmosphere.

2. That energy generates waves in the atmospheres. Unlike waves in the ocean, which make swimmers and boats bob up and down, these waves oscillate north and south.  When they first initiate, these waves look like a northward bulge or ridge on a map.  The arrows below show the way winds blow when a wave occurs. Warm air moves north (red arrows) on the west side of the ridge and cold air moves south (blue arrows) on the east side.  (This phenomenon of waves in the atmosphere is a big reason why temperatures vary so much in the Midwest, by the way.)

3. The geography of Siberia is special in being a huge swath of land without major impediments like the Rockies, Alps, or Greenland ice sheet. This allows the waves to easily migrate without breaking down.  Therefore, as the waves build in late spring, they also shift eastward.

4. By June, the wave setup is fully formed, with the main ridge not over the initiation point, but rather  the southern Laptev Sea.  This means winds that blow from south to north over the Laptev Sea, carrying warm, moist air — air that is ideal for melting sea ice.

5. In this way, earlier snow retreat from the WSP means earlier wave generation in the atmosphere and earlier sea ice melt in the southern Laptev Sea.

This link isn’t the only thing that matters — it only explains around 1/3 of the variation of sea ice retreat in the Laptev Sea.  However, for one variable in a complicated system like this, 1/3 is actually really helpful.  Moreover, the snow typically disappears in the WSP in late April, and the sea ice doesn’t retreat from the southern Laptev Sea until late July — on average, there’s about 90 days in between.  That’s a lot of planning time. For the interested parties, here’s a more detailed flow chart of the relationships being described in the paper:

Full Citation:

Crawford, A. D., Horvath, S., Stroeve, J., Balaji, R., & Serreze, M. C. (2018). Modulation of Sea Ice Melt Onset and Retreat in the Laptev Sea by the Timing of Snow Retreat in the West Siberian Plain. Journal of Geophysical Research: Atmospheres, 123. https://doi.org/10.1029/2018JD028697

2018 Expedition to Estonia

August 10th, 2018

Bill Ausich (Academy Professor, Ohio State University) and I just finished an excellent research trip to Estonia. As is the custom on this blog, here are the relevant posts in chronological order:

July 27: Wooster and Ohio State paleontologists return to Estonia
July 29: First full day in Estonia for the intrepid paleontologists
July 30: Starting work in Estonia
July 31: Fieldwork in Estonia, with a bonus visit to Narva
August 1: Back to the paleontology lab in Tartu, Estonia
August 2: Starting work in the University of Tartu Natural History Museum
August 3: Back to work in the University of Tartu Geology Department
August 4: Saturday at the Estonian National Museum (plus a street festival)
August 5: Sunday at the University of Tartu Natural History Museum — this time as tourists
August 6: Last day in the University of Tartu Geology Department — and a great garden party
August 7: Last day at the Tartu Natural History Museum, and a visit to a grim museum
August 8: Wooster and Ohio State Paleontologists in Tallinn, Estonia

One of the gorgeous Estonian crinoids from the Silurian we studied. See posts for details!

Wooster and Ohio State Paleontologists in Tallinn, Estonia

August 8th, 2018

Tallinn, Estonia — This morning Bill Ausich (Ohio State University) took the bus from Tartu to Tallinn to finish one more research task and then prepare for the long journey home. Above is the view from my hotel room towards the Old City section of Tallinn.

After getting settled, we visited Ursula Toom at the Department of Geology, Tallinn University of Technology. She and Bill (above) exchanged crinoids, and then Ursula discussed with me a wide variety of Ordovician borings as part of her dissertation work.

This is a small part of the various mystery specimens Ursula shared with me. There are some fantastic undescribed borings in this lot.

Afterwards Bill and I had an early evening dinner in the Old City, beautiful in the setting sun.

Our research in Estonia is done! Tomorrow we pack up and then walk around Tallinn taking in the sights and culture. On Friday we fly home. I hope to describe the results of our work soon in this blog.

Can Heat Flow in Ocean Models Predict Seasonal Arctic Sea Ice Retreat?

August 8th, 2018

Note: The following blog post is by Ben Sershen (’19), who worked with Dr. Crawford on a summer research project.

Source: https://csmphotos.wordpress.com/2013/01/17/bering-sea-opies-and-the-reality-of-the-deadliest-catch/

Intro: My summer research work aimed to further my Junior I.S. in the fields of oceanography and climatology. My research question was: “How well do computer ocean models work for predicting the melt of Arctic sea ice?” This is an important question to ask because many companies are looking to use the Arctic as a shipping passage. To answer this question, I analyzed the data from two models: The Simple Ocean Data reanalysis (SODA) and the Ocean ReAnalysis System 4 (ORAS4), which was produced by the European Center for Medium-Range Weather Forecasts (ECMWF). SODA and ORAS4 are programs that uses physics as well as real-world data, such as from underwater moorings, automated Argo floats, and satellites, to estimate aspects of the oceans such as temperature, water velocity, and salinity. These data could then, in theory, be used to determine when the enough heat has entered the Arctic Ocean to melt sea ice each year.

 

Figure 1: (Left) A map of the Bering Strait with the A3 mooring location labeled. The Chukchi Sea is the area to the north (above) the strait. The magenta line represents the location of my study. (Right) A similar map of the Bering Strait with the Alaskan Coastal Current (ACC) and the Siberian Costal Current (SCC) represented by the red and blue arrows, respectively (Woodgate et al., 2015).

Methods: I focused on the Bering Strait/Chukchi Sea region. This area is important as it is the strait where warm water enters the Arctic, as seen in the second map in figure 1. I calculated how much heat was passing through the Bering Strait, into the Arctic using data from SODA and ORAS4. Once I had done this, I had to compare my results to the data from the A3 mooring. This was done by first performing simple correlations (when the heat flow increases do the mooring temperatures increase?). I then correlated the heat flow values to sea ice data – more specifically, the date of the year when the sea ice concentration in the Chukchi Sea dropped below 30%. As heat flow increases, the ice melts faster, retreating earlier. To be a good predictor of sea ice retreat, the heat flow from the models must show this relationship (a positive correlation between the amount of heat and the rate of ice melt).

 

Figure 2: An example of a cross-section contour plot through the Bering Strait. The darker red represents more heat traveling through the cross-section at that location. The location of the cross-section is marked by the magenta line on the left map in figure 1.

Results: I found that SODA was only accurate when there were mooring data being fed into it. That meant that SODA would not do a very good job at predicting future ocean heat because it relied heavily on real-world data. I performed the same data analysis of ORAS4 that I had performed on SODA and found that ORAS4 produced data that was closer to the mooring data even when mooring data was not available. The ORAS4 data also had a stronger correlation with the sea ice data. I would guess that this is because the ORAS4 is simply better than the SODA model generating realistic data. The ORAS4 model focused on tropical data to generate data and produced better data than SODA, which had a bit more of an Arctic focus. I think that result goes to show how interconnected the world’s oceans really are if you can make accurate predictions of the temperature of the Arctic Ocean from data produced by a tropical model. Using the monthly heat flow values from ORAS4, I created a heat map to visualize the data (figure 3).

Figure 3: A heat map of heat flow data from ORAS4. The y-axis represents months of the year. Note the warmer values occurring in June-October. This represents the seasonal cycle. The x-axis represents years 1990-2017 which illustrates substantial interannual variability.

 

References

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