Archive for the 'Uncategorized' Category

Team Jurassic Utah 2019 begins its adventure

March 11th, 2019

Hurricane, Utah — A new crew of Wooster Geologists traveled today to southwestern Utah to explore the marvelous Carmel Formation (Middle Jurassic). We are continuing the work of last year’s team and expanding into new areas. We are five people this time: Dr. Shelley Judge, Evan Shadbolt (’20), Anna Cooke (’20), Nick Wiesenberg (our ace departmental technician), and me. Three of us flew today from Salt Lake City to St. George, Utah, enjoying the spectacular geological scenery from the plane. Above is a view of the Wasatch Range just south of Salt Lake City. Very snowy.

Here is the view as we approach the St. George airport. It is the spectacular Hurricane Monocline of the Moenkopi Formation. No snow down here.

This was the first time any of us had used the new St. George Regional Airport. So, so much better than flying into chaotic Las Vegas and making the long drive to Utah.

Fieldwork begins tomorrow!

Science Olympiad training, fossil events

February 18th, 2019

Wooster, Ohio — I had the pleasure today of assisting with the training of Olympians! The above three students are from Orrville High and Middle Schools, along with their coach the ace geology high school teacher Jim Duxbury. They came to Scovel Hall to see our paleontology collections and learn how to identify dozens of fossil types for competition next month in the Science Olympiad. They were a lot of fun in the lab, asking excellent questions and showing their skills in taxonomy. I hope I helped.

I helped train another Science Olympiad team in the summer of 2013, this one in Israel (above). Hanan Ginat was the coach, and these students were also enthusiastic and knowledgeable.

There are few activities more fun than talking about fossils!

Advice from a Grad Student

February 11th, 2019

I caught up with Wooster Geologist Clara Deck (’17) at the AGU meeting in December. Clara is a grad student who has been doing exciting research on glaciers. I asked her to tell us a little about her research and how her experience at Wooster helped her prepare for grad school. She discovered that her research has called on concepts and skills from Structural Geology and Computer Science. Stories like Clara’s motivated us to design our Geology and Environmental Geoscience majors to be flexible, allowing students to explore their interdisciplinary interests and pursue their passions. We’ve also added computational methods to our curriculum. Thanks, Clara, for the inspiration!

Guest blogger: Clara Deck (’17)

Clara Deck on Jarvis Glacier in the East Alaska Range, August 2018.

As a master’s student at the University of Maine’s School of Earth and Climate Sciences, my research has focused on dynamic processes of Antarctic Ice Shelves. Glacial ice in Antarctica flows outward to sea and creates tabular masses called ice shelves, which are tethered to about 75% of the continent’s coastline. These floating ice shelves are critical to the stability of land-based ice because they keep glaciers from flowing freely into the ocean and contributing to global sea level rise. The Ross Ice Shelf (RIS) is the largest in Antarctica and supports the flow of the West Antarctic Ice Sheet, which contains an ice volume of over three meters of global sea level equivalence if it were to enter the ocean. At the western coastal margin of RIS, ice is continuously dragged along protruding bedrock features and becomes deformed, exhibiting large cracks or rifts over a kilometer in length which can be seen in satellite imagery. If a rift were to propagate across RIS, the ice shelf could break up and allow land ice to flow seaward much more quickly. My thesis work aims to characterize the origin and evolution of rifts on RIS using three-dimensional modeling.

Mapping a crevasse on Jarvis Glacier.

I never expected this, but Structural Geology was one of the most helpful classes at Wooster to help me prepare for my graduate work on glaciers and ice shelves. The same stresses at play in tectonics are important within ice, just on a different temporal and spatial scale. I wish I had known that these processes, ductile and brittle, from the micro- to macro-scale, were also applicable to ice!

Evolution of rifts on the Ross Ice Shelf with ice flow, shown through optical imagery from Landsat 1, Landsat 4, Landsat 7, and Sentinel 2, respectively.

At Wooster I took CSCI 100 and 110, classes in Python and C computing languages. These were helpful to learn basics in computing syntax, but it was a steep learning curve in graduate when using computing for glaciological applications. It would have been really useful to have had some background in modeling and writing code related to geological physical processes. I’ve been excited to hear that some modeling is now incorporated into some courses in the Earth Sciences department. Another surprise when starting my master’s was how much calculus is involved in glaciology. I took calc 1 and 2 at Wooster, but I still had quite a bit to catch up on. For students going into climate change or cryospheric studies, I think learning quantitative analysis is just as important as the qualitative side.

Model of a floating ice tongue showing the differential stress field due to buoyancy force from underlying marine water. Solved using a particle approximation of Navier-Stokes for forces associated with water and finite element analysis for high viscosity ice. This model explores the connection of ice shelf deformation with upstream grounding line processes.

Overall, I feel that Wooster Geology prepared me very well for my graduate studies. Because of Sophomore Research and I.S., I had a lot of experience in independent research, seeking out my own resources, and laying out my own goals and timeline. All those skills have helped me be successful so far, because I’ve found that much more responsibility is put on the student at the graduate level. I’m glad I pursued grad school and I hope to defend my master’s thesis this spring!

Bringing three new Silurian bryozoan species into the world

February 10th, 2019

I love being part of the scientific process of naming new organisms and placing them into the grand narrative that is the history of life. It is a kind of rescue — retrieving species from oblivion by giving them identities. Carolus Linnaeus, the father of taxonomy, said it well:

The first step in wisdom is to know the things themselves; this notion consists in having a true idea of the objects; objects are distinguished and known by classifying them methodically and giving them appropriate names. Therefore, classification and name-giving will be the foundation of our science.

The bryozoans described in this post are from a project led by my very accomplished bryozoologist friend Andrej Ernst at the University of Hamburg, Germany (above). In the summer of 2015, Andrej and I met up with our colleague Carl Brett (University of Cincinnati) to collect bryozoans from the Lower Silurian (Aeronian) of western New York. My fieldwork was supported by a grant from the Luce Fund at The College of Wooster. We had a very productive time and saw much geology and paleontology, as you can see from these August 2015 blog posts. That fieldwork was followed by Andrej’s prodigious lab work with the bryozoans. The results have now appeared in the Journal of Paleontology.

The abstract: Thirteen bryozoan species are described from the Brewer Dock (Hickory Corners) Member of the Reynales Formation (lower Silurian, Aeronian) at the locality Hickory Corners in western New York, USA. Three species are new: trepostomes Homotrypa niagarensis n. sp. and Leioclema adsuetum n. sp. and the rhabdomesine cryptostome Moyerella parva n. sp. Only one species, Hennigopora apta Perry and Hattin, 1960, developed obligatory encrusting colonies whereas the others produced erect ramose colonies of various thicknesses and shapes: cylindrical, branched, and lenticular. Bryozoans display high abundance and richness within the rock. This fauna is characteristic of a moderately agitated environment with a stable substrate. The identified species reveal paleobiogeographic connections to other Silurian localities of New York as well as Ohio and Indiana (USA) and Anticosti (Canada).

The top photo in this post is one of the new bryozoans, the trepostome Homotrypa niagarensis. The images are from Figure 8, with the caption: (2) branch oblique section, holotype SMF 23.470; (3) rock thin section with transverse and oblique sections of branches, holotype SMF 23.472; scale bars are 3 mm and 5 mm respectively.

Above is the new trepostome Leioclema adsuetum. The image is from Figure 10, with the caption: (1) longitudinal section of exozone showing autozooecia, mesozooecia, and acanthostyles, paratype SMF 23.553; scale bar is 0.5 mm.

This is the third new species, the cryptostome Moyerella parva. The images are from Figure 11, with the caption: (3) longitudinal section of a colony segment with a pointed base and widened proximal part showing medial axis and autozooecia, holotype SMF 23.559; (4) tangential section showing autozooecial apertures, tubules, and tectitozooecia, holotype SMF 23.559; scale bars are 0.5 mm and 0.2 mm respectively.

The paper is about more than these new species, of course. There are other bryozoans assessed, and Carl Brett’s stratigraphy section is magnificent and a new resource for the area. The new taxa, though, are worth celebrating by themselves.

Thank you to Andrej and Carl for being such good colleagues. I hope we return to the Silurian of western New York for more work.

Reference:

Ernst, A., Brett, C.E. and Wilson, M.A., 2019. Bryozoan fauna from the Reynales Formation (lower Silurian, Aeronian) of New York, USA. Journal of Paleontology, doi.org/10.1017/jpa.2018.101.

Sometimes a Cold Snap is Just a Cold Snap

February 7th, 2019

On Wednesday, January 30, 2019, The College Wooster closed due to cold. This cold snap was felt across much of the central and eastern USA. The message Wooster staff and faculty received included this statement:
“The National Weather Service is forecasting daytime temperatures tomorrow between -3 and -7 degrees Fahrenheit, and wind chills of -25 to -30. In these weather conditions, exposed skin can begin to show signs of frostbite in as little as 10 to 15 minutes outside.”

However, in the past week, I’ve heard diverging narratives from people across the eastern USA about their experience. Some lament that this was horrible weather, the worst ever — how can climate change be real? Others lament that it used to get much colder — we have it easy today because of global warming. So what’s the truth? 1) Was Wooster’s cold spell out of the ordinary? 2) Is winter not as harsh as (or harsher than) it used to be? 3) Is climate change to blame?

1) Last week’s cold was not out of the ordinary.

There’s two ways to think about how cold it was.  One is that the daily high was only 9.6°F, which is frigid — it never topped 10°F. Another is that the daily low was -6.0°F, and that occurred while students would have been walking to classes in the morning. I can’t speak much to the wind chill because the OARDC station is too far from campus to give an accurate assessment.  Wind varies a lot more temperature from place to place, so it’s hard to know exactly how bad the wind chill was for any random person walking with exposed skin outside.  For temperature, though, the weather forecast was spot on.

Figure 1: The distribution of the annual low temperature in Wooster (the lowest daily low) from 1900 to 2018, with 2019 marked.

That temperature, however, was not exceptional.  Funny enough, January 22, 2019 actually had a lower low of -7.2°F — it just wasn’t as windy. In Wooster, the average annual low temperature since 1900 is -7.6°F. The average lowest daily high is 11.0°F. Our 2019 is currently right in line with those numbers (although the winter is not yet over).

2) The coldest days might be getting less severe.

This is actually a tricky one to answer. If you look at the coldest temperature recorded each year at the OARDC, nearly every year before 1930 had at least one day in which the temperature fell below 0°F — but not so from 1930 to 1960.  The coldest cold was above 0°F about in about 25% of the years in that second period.  From 1960 to 1990, the reliable sub-zero temperatures returned.  Since 1990, the annual coldest day has been less severe again on average.  In other words, if you only look back to 1960, yes, the worst days have been getting less severe.  But if you look back to 1900, the last 120 years suggest that Wooster still gets plenty of cold.  So if you were born in the 1950s, no, the new generation doesn’t have it easier, but they may be more sensible about preventing frostbite.

Figure 2: Time series of the the lowest daily low temperature (the coldest temperature each year) in Wooster from 1900 to 2019 (so far).

3) There’s not a clear climate change signal here.

The problem with evoking climate change is that weather extremes are by definition rare, so it’s hard to pinpoint the immediate cause of local-scale weather extremes to long-term, global-scale warming.  There is some evidence out there that the polar jet stream (a.k.a. the “polar vortex”) is becoming more erratic as the world warms, leading to more days like January 30 when the Arctic Ocean is warmer than Minnesota, but that is not settled science. Plus, there’s no clear trend with this particular measure. In other words, it’s premature to blame climate change for every weather event you don’t like.

Conulariid and trepostome bryozoan symbiosis in the Upper Ordovician of Estonia

January 22nd, 2019

A new paper is just out in which all the characters have been covered previously in this blog, but not as parts of a single story. It describes an interprets the relationship between the mysterious conulariids and trepostome bryozoans in the Katian and Sandbian (Upper Ordovician) of northern Estonia. The authors have all made appearance here, including lead author Olev Vinn (Institute of Ecology and Earth Sciences, University of Tartu, Estonia). Andrej Ernst (Institut für Geologie, Universität Hamburg, Germany), myself, and Ursula Toom (Institute of Geology, Tallinn University of Technology, Estonia). It was a fun team to work on, and Olev led it masterfully.

There are numerous trepostome bryozoans in the Upper Ordovician of Estonia that grew up and around the bases of conulariids, which are extinct cnidarians. This is, in fact, an example of bryoimmuration as covered in my last post. The puzzle is what was the relationship between these two groups. Were the conulariids parasites on the bryozoans? Did they gain protection from predators by embedment in the bryozoan calcitic skeleton? Were the bryozoans prime real estate for the conulariids because they were hard substrate islands on a muddy seafloor? We think the answers are probably yes to all these questions.

The top composite of images is Figure 3 in the paper. The caption: A, Two conulariids Climacoconus bottnicus (Holm, 1893) in Diplotrypa bicornis (Eichwald, 1829) from Haljala Regional Stage, northern Estonia, note the slightly elevated apertures of conulariids (GIT 720-4). B, Longitudinal section of Diplotrypa abnormis (Modzalevskaya, 1953) with conulariid Climacoconus bottnicus (Holm, 1893) (GIT 537-1822) from Haljala Regional Stage, northern Estonia. C, Longitudinal section of completely embedded Climacoconus bottnicus (Holm, 1893) in Esthoniopora communis (GIT 537-1656) from Haljala Regional Stage, northern Estonia. D, Conulariid in Mesotrypa expressa Bassler, 1911 from Oandu Regional Stage, northern Estonia; note the depression around the conulariid’s aperture (GIT 770-7). E, Conulariid in Mesotrypa expressa Bassler, 1911 from Oandu regional Stage, northern Estonia; note the malformation of a zooid near the aperture of the conulariid (GIT 770-92). F, Conulariid in Esthoniopora subsphaerica from Rakvere Regional Stage, northern Estonia; note the strongly elevated aperture of the conulariid (GIT 537-1760).

This work is another product of Wooster’s generous research leaves program that has supported many trips to Estonia.

Reference:

Vinn, O., Ernst, A., Wilson, M.A., and Toom, U. 2019. Symbiosis of conulariids with trepostome bryozoans in the Upper Ordovician of Estonia (Baltica). Palaeogeography, Palaeoclimatology, Palaeoecology 518: 89-96.

The Everglades Are All About Geology

January 16th, 2019

If you’ve ever been to the Everglades or even heard of them, you probably are picturing something like this:

Taylor Slough

Or maybe this:

American alligator

In other words, Everglades National Park exists because it is “important habitat for numerous rare and endangered species”  (from the Everglades NP homepage).  I visited the Everglades recently, and thankfully all four major National Park Service sites in South Florida are still open despite the government shutdown thanks to non-profit partners like the Florida National Parks Association. (Seriously, South Florida’s private groups have done an admirable job preventing the sort of issues experienced at other National Parks.) One thing I learned, starting with a diagram in the Big Cypress National Preserve welcome center, then while driving and walking around the Everglades, was that the great diversity of habitats found in the Everglades is strongly dependent on the geology.  More specifically, the habitats you in the Everglades are dependent on two things: 1) the seasonal rise and fall of the water table between the rainy and dry seasons and 2) minute changes in elevation.

Ignoring for a moment the low-lying areas along the coast that are impacted by seawater, even the freshwater ecosystems of the Everglades are sensitive to very small changes in elevation.  The sensitivity comes from an interaction between precipitation cycles and elevation.  During the rainy season, so rain water enters the Everglades that the water table (the level below which the ground is saturated) rises, and water seeps out of the ground to flood low-lying areas.  Lower areas get flooded for a longer period of the year.

In the heart of Big Cypress National Preserve, for example, the high points are about 13 ft in elevation.  This high ground is dominated by “hardwood hammocks”, evergreen broadleaf trees live gumbo limbo, live oak, and mahogany.  It looks almost like a tropical rainforest, but with fewer vines and ferns.  These hammocks are the only reliably dry ground in the wet season.

The edge of a hardwood hammock; the light at the far end of the photo is from the adjacent pinelands.

About two feet lower (11 ft elevation), you’ll transition into the pinelands.  Having lived out West, I’d call this “parkland” — lots of grass, with tall slash pines dispersed throughout. It’s the only habitat in the Everglades that looks appealing for a stroll, and it only floods about 2-3 months during the year. The pinelands have regular fires, just like the ponderosa pine forests of the Rockies.

Pinelands; this photo is taken a few horizontal feet from the hammock photo above!

Go another two feet lower (9 ft elevation), and there is “open” prairie.  “Open” is deceptive here because 4-6 months of the year it’s basically a giant pond.  This is the habitat that wading birds like egrets, storks, and spoonbills love. Still, they also burn regularly enough to prevent trees from growing.

Sawgrass Prairie (hardwood hammock in the background)

Another foot lower (8 ft) elevation, and you’re into the cypress swamps. These swamps are flooded more than half the year, so it’s too wet for hardwoods or pine trees.  They’re even too wet for prairie because they rarely burn. Bald cypress trees, though, are well-suited for this life. They thrive in freshwater — so much so, in fact, that they shed their leaves in the dry season (boreal winter).  That’s right, in South Florida, the broadleaf trees are evergreen and the needle-leaf trees are deciduous! The madness!

A bald cypress “strand” in Big Cypress National Preserve

The lowest inland elevations are the “gator holes” and “sloughs”; permanent pools of water that are wet all year round.  This is where all of the fish, the turtles, and the water birds congregate (with the alligators) in the dry season. (That’s also why the dry season is the best time to see wildlife — it’s confined!)

Closer to the coast, the entire system is shifted down and compressed to an even tighter elevation range.  The water table slopes toward the coast, so the top of the system (hardwood hammock) is 7 feet elevation, the cypress are down at about 3.5 feet, and anything lower is probably brackish water. Depending on salinity of the water and soil, you might see mangroves, grasses, pickleweed, or even cactus. But suffice it to say, the interplay of the wet and dry seasons and subtle changes in elevation dictate the balance of four distinct freshwater habitats and several brackish habitats, as well.  This is a very sensitive system, and even slight changes to elevation (or sea level) can make a world of difference.

Red mangroves in Biscayne National Park

New paper on bryoimmuration and taphonomic engineering

January 12th, 2019

I’m pleased to link to a new paper that has just appeared in the journal Lethaia. My wonderful coauthors are Caroline Buttler (National Museum Wales) and Paul Taylor (Natural History Museum, London). The paper explores the role calcitic bryozoans play in preserving molds of aragonitic shells, a process we call bryoimmuration. In the image above we have two views of a single specimen from the Cincinnatian (Upper Ordovician) of the Cincinnati, Ohio, region. It is a trepostome bryozoan that encrusted the exterior of a bivalve shell. The bivalve shell was aragonitic and thus dissolved away during diagenesis. The bryozoan skeleton is calcite, a mineral that does not dissolve as easily as its cousin aragonite. The surviving bryozoan skeleton thus preserved our only record of the now-dissolved bivalve shell. The larger concept of one group of organisms affecting the preservation of another we call taphonomic engineering. Below are cross-sections of these bryoimmuring bryozoans, with the original caption.

Fig. 3. Acetate peels of bryoimmuring bryozoans cut perpendicular to basal growth surface (longitudinal); all from the Upper Whitewater Formation (Katian) near Richmond, Indiana (locality as in Fig. 2). A, heterotrypid bryozoan that grew across the ribs of an ambonychid bivalve. Note the thin zooecial walls in the early fast-growing stage, later thickening upwards (CW‐148‐92). B, very thin sheet of a trepostome bryozoan that encrusted an ambonychid bivalve (CW‐148‐93). This bryozoan did not develop an exozone and is thus impossible to identify. C, heterotrypid bryozoan that developed a thick exozone while growing on an ambonychid bivalve shell (CW‐148‐94). The shell later dissolved and sediment took its place. D, multilaminar growth of a heterotrypid bryozoan on an ambonychid bivalve (CW‐148‐95). The bryozoan colony overgrew itself.

This research was supported by an award from the Henry Luce III Fund for Distinguished Scholarship at The College of Wooster. Nick Wiesenberg helped with the fieldwork. It was a fun project.

Geomorphology – Fall 2018

December 7th, 2018

Post, photos, and illustrations by Victoria Race

This semester, the Geomorphology class from the College of Wooster Earth Sciences department went to several local field sites to study geomorphological features, soil catenas, groundwater flow, and other investigations.

One of these sites was Browns Lake Bog, a preserve located near Shreve in Wayne County, Ohio and run by the Nature Conservancy that was established in the 1960’s. College of Wooster students have been involved in several past projects at Browns Lake Bog including tree coring, sediment coring and lake drilling.

It is one of a handful of sites in Ohio which contain an open kettle lake surrounded by a floating sphagnum moss mat (ODNR, 2018). The bog, lake, and hill features here are glacial relicts formed after the last ice age (~20,000 years ago). The preserve is known for its unique boreal plant community which is supported by the special acidic properties sustained by the presence of the sphagnum moss and its insulative capabilities that protect the community from drastic air temperature changes. More than twenty rare plant species can be found here including round-leaved sundew, large cranberry, and grass-pink orchid.

The 2018 Geomorphology class at the entrance to Browns Lake Bog.

The carnivorous Northern Pitcher Plant (Sarracenia purpurea) receives nutrients by trapping insects in its fluid-filled cavities.

The geomorphology class went to Browns Lake Bog to study the glacial features and soil profiles across the preserve. The knolls in the northeast corner and southern part of the property are glacially formed hills called kames. These features form from the collection of sediment on top of the ice during glaciation and are left behind when the ice melts. The open lake at the preserve is known as a kettle lake, a depression also left behind from the glacier. The geomorphology class focused on creating a sediment profile from the top of the kame to the bottom, taking several soil cores to form a soil catena.

Diagram showing the formation of kettle lake and kame features similar to those at Browns Lake Bog.

Soil from sample location at the top of kame.

A sediment core taken at the bottom of the kame showing a dark peat layer and ancient lake sediments.

The cores revealed a high organics content at the bottom of the kame and a deep layer of loess on the top of the kame. Loess is a term used to describe wind-blown sediment that has accumulated over time since the last ice age. The loess cap is thickest at the top of the kame and thins as you move down towards the lake.

Claire Wineman expanded on this soil catena study for her Geomorph project.

Claire at the entrance to the preserve.

Cross section showing general sample locations used in Claire’s study.

Looking up at the kame from the trail.

Soil profile at the top of the kame. Deep loess layer (silty light brown sediment).

Claire took samples from three locations along the kame. One at the top, middle and bottom. The soil pit at Site One (top of kame) was 10 inches wide and 12 inches deep; the soil pit at Site Two (middle of kame) was 13 inches wide and 12 inches deep; and the soil pit at Site Three (bottom of kame) was 16 inches wide and 12 inches deep.

The top of the kame has much lighter soil and a thick loess layer. Sediment at the bottom of the kame was much darker and rich in organics. It had gray lake clays instead of loess. The top of the kame’s loess layer was thicker than the middle showing that that loess cap thins as you move down the hill. Claire reports that the soils at Site One and Site Two at the top and middle of the kame, respectively, largely consist of alfisols and are significantly less acidic and contain far less organic material than Site Three.

Sediment core taken from same location at the top of the kame showing soil profile. Note the thickness of the loess layer which is lighter in color. Claire reported two inches of organic O horizon and ten inches of clay loess B horizon.

Hole dug at the base of the kame. Note the soil darkness which indicates a richness in organics due to the dominance of histosols in the bog and at the base of the kame.

Sediment core taken from same location. Note the dark peat layer at the surface and lake clays towards the bottom. Claire reported a dominant O horizon and portion of lacustrine clay lens.

Her conclusion states, “The soil catena at Brown’s Lake Bog provides another portrait of the geomorphic events that have shaped the landscape over the last 15,000 years. The alfisols and clay loess found on the upper levels of the kame are representative of the weathered sediments once deposited atop the glacier that eventually formed the kame itself; the histosols and lacustrine clay lenses of the lower levels of the kame depict the legacy of kettle lakes and the resulting bog environment. While the Bog’s landscape alone is enough to show us its geomorphological history, its soil catena gives us a perspective on its internal processes, the events that have shaped it since glacial activity, and the events that will continue to shape it into the future.”

 

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.

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