What happened at the end of the Eocene?

 

Southern margin of La Garita Caldera, Colorado, from USGS
http://pubs.usgs.gov/imap/i2799/photos/8-S_LG_caldera_margin.html

I have a long standing interest in large discontinuous events in history – essentially things that go “boom” – and their impact on geology, living things, and the course of history – how these events happen, what makes them happen, and why they happen.  This interest has led me to volcanoes and their eruptions.  It has also led me to look into impact events that wipe out types of life and significantly rearrange the landscape.

A lot of the causes overlap in time or take place so far in the distant past that it is difficult to figure out which caused which.  We also need to remember that volcanic activity is for most intents and purposes continuous, though it does wax and wane in intensity over the millions of years as driven by plate movements, rifting, collisions and mantle plume / hot spot activity.

Fish Canyon Tuff – La Garita Caldera, Colorado

To me, the most important questions are what causes the discontinuity in climate?  What causes multiple extinction events?  What causes sufficiently large insults to the environment that lead to mass extinctions of entire species and significant long-lived change in climate?  Usually, the Flying Finger of Fate is pointed at a single cause which may be volcanic eruptions, impact events, close supernovae explosions, methane blooms, massive global heating, massive global cooling, aliens, etc.; none of which we have ever see in real time.

What I am wondering is that while volcanic eruptions place significant stress on the environment, cooling things down for years to decades or longer, perhaps they need something else happening at nearly the same time, something big to trigger an extinction event.  A few examples follow.

Graphic showing extinction events, large igneous province outbreaks and massive climate shifts. From Large Igneous Provinces Commission, August 2013 LIP of the Month http://www.largeigneousprovinces.org/13aug

A Few Extinction Events

The Cretaceous – Tertiary boundary 65 million years ago marking the end of the dinosaurs is thought by some to be linked to the Deccan Traps in India.  There is a roaring argument about which caused which, as there are a pair of impact structures identified dating to around that time – the Chixculub crater in the Yucatan and Shiva off the west coast of India.  Regardless of the resolution of the argument, it appears both volcanism and impact events were involved.  http://news.nationalgeographic.com/news/2009/10/091016-asteroid-impact-india-dinosaurs.html

Graphic showing location of proposed Shiva impact structure. Unable to find original paper.

The Permian – Triassic extinction event which wiped out over 90% of marine species and over 70% of all vertebrate life on dry land some 250 million years ago is another.  It is mostly blamed on the eruptions from the Siberian Traps that caused a massive rise in global temperatures.  Some have suggested an impact component with a couple candidate large crater structures suggested.  But if the Traps erupted over an extended period of time, several millions of years, as have been suggested here, then the impact events punctuate and immediately push an already stressed environment into failure.  Causation on this is even less well known, but volcanism is clearly involved.  Impact events may also be involved but several other mechanisms have been proposed like mass release of methane (greenhouse gas) into the atmosphere as the planet heated up.  http://www.space.com/2452-giant-crater-tied-worst-mass-extinction.html

Photo of Siberian Traps – Copyright the Daily Galaxy
http://www.sott.net/article/231128-Did-a-Massive-Volcanic-Event-Cause-the-Great-Dying-250-Million-Years-Ago

There also seems to be an antipodal effect where volcanic activity or mantle hot spot is found on the other side of the globe from an impact structure that Geolurking wrote about last year or so.  Hot spots may end up being fossil evidence of impact events into the ocean.  http://www.mantleplumes.org/WebDocuments/Antip_hot.pdf

The gradualist geologic view of the world has changed over the last century, with grudging acceptance that large, relatively instantaneous catastrophic events have driven the history of life on this planet.  Not having seen one actually happen (and I don’t want to), we are left with the scientific method of comparing observations and theory against models while building them up and tearing them down.

Younger Dryas impact field – from Dartmouth paper
http://wattsupwiththat.com/2013/05/21/widespread-evidence-of-cosmic-impact-documented-likely-cause-of-the-younger-dryas-cool-climate-episode/

Impact events, even relatively recent ones in the last few thousand years, are difficult to pin down unless they are really, really large.  70% of the surface of the planet is ocean bottom, crustal plates that have a finite lifetime and are either subducted into oblivion or crunched up into mountains when trapped between two impacting continents (Himalayas).  We can extrapolate the flux of inbound bodies based on the number of craters known on the continents, but this is at best a low estimate as it does not consider airburst events (Tunguska, Chelyabinsk) which apparently regularly take place.  And sometimes those airburst events get very, very big.

There is a group of people investigating the possibility that a comet storm caused a continental level extinction event over North America some 12,000 years ago – think tens of thousands of Tunguska sized airbursts raining down over North America in the course of a few hours – plunging global temperatures back to ice age levels for over a thousand years (Younger Dryas).  There has been no volcanic component yet proposed for the event outside a suggestions by a Mexican geologist that “trap-door” calderas in Mexico were responsible for the presence of ignimbrites in Mexico and the Southwest US.  An alternate view based in experience with what happens under nuclear fireballs believes the ignimbrites were formed by multiple airbursts and the melted, glazed rock is the remains of airburst created density currents.  http://cosmictusk.com/first-harvard-now-dartmouth-evidence-identified-for-younger-dryas-impact/

With this background in mind, I would like to explore for a bit the minor extinction event at the end of the Eocene, some 33 million years ago.  The event is marked by large impact craters, most notably one centered on the mouth of the Chesapeake Bay in North America, and an outburst of caldera activity in what is now Colorado.  It also marked a downward shift in global temperatures toward the ice ages we now endure.  This is in no way a comprehensive exploration, but more a way to publicly scratch my head.  Something clearly happened.  The question is what and how?

Calderas

Fish Canyon Tuff – From USGS Geologic Map of the Central San Juan Caldera Cluster, Southwestern Colorado.

There was a caldera outbreak in Colorado some 40 – 25 million years ago.  This outbreak spanned the Eocene extinction event.  Whatever impact on climate due to the eruptions was exacerbated by impact events around the 33 million year point that were sufficient insult to change climate and wipe out species.

http://books.google.com/books?id=KLtUAAAAYAAJ&pg=RA1-PA93#v=onepage&q&f=false

Activity in the San Juan Volcanic Field started some 40 – 35 million years ago and for the first 5 million years was dominated by andesite, rhyolite and mafic quartz latities.  Height of the activity was some 35 – 30 million years ago, at the time of the Eocene event.  Around 30 million years ago, the activity shifted to a predominantly ash-rich, pyroclastic output which tailed off over the next several million years.

http://pubs.usgs.gov/imap/i2799/

There is a paper or two that suggests the caldera outbreak was related to a chunk of the subducted Farallon plate that either broke off or traveled parallel under the North American continent rather than diving into the mantle to melt.  As the continent stretched a little bit with the uplift of the Colorado Plateau and the failed Rio Grande rift, this allowed the melt to rise to the surface and eventually erupt.

http://www.colorado.edu/geolsci/Resources/WUSTectonics/CzIgnimbrite/ignimbrite_intro.html

Snowshoe Mountain (resurgent dome) within Creede Caldera, Colorado (image behind paywall)
http://elements.geoscienceworld.org/cgi/content-nw/full/4/1/17/FIG4

Fish Canyon Tuff – La Garita Caldera
http://www.greendiary.com/biggest-volcanic-eruptions-that-changed-the-face-of-earth.html

And erupt it did, climaxing in the eruption of the Fish Canyon Tuff out of the La Garita caldera some 28 million years ago.

This eruption deposited some 1,200 cubic miles (5,000 cubic kilometers) of ignimbrites across North America in the largest known single eruption in the last half billion years.

The San Juan Volcanic Field in southern Colorado contains up to 18 calderas and as many identified flow deposits and represents the center of this activity.

The La Garita caldera measures some 22 by 47 miles (35 by 75 kilometers) and was active for a couple million years after the Fish Canyon eruption.

Its output was prodigious.

http://gsabulletin.gsapubs.org/content/120/7-8/771.abstract

Map of San Juan Volcanic Field from Geological Society of America
http://gsabulletin.gsapubs.org/content/120/7-8/771.abstract

A related volcanic field in central Colorado buried a stand of redwood trees west of Pike’s Peak near Florissant, Colorado.  The fossilized stumps have been partly excavated and can be seen Florissant Fossil Beds National Monument.  They date from around 35 million years ago.

http://www.nps.gov/flfo/index.htm

Fossilized redwood stump – Florissant Fossil Bed National Monument, Colorado. Buried by volcanic eruption from 39-mile volcanic area, Colorado
http://www.conifers.org/cu/Sequoia.php

As the disconnected piece of the Farallon plate melted, it is thought to have provided a source of eruptible material in Nevada, Utah and Northern New Mexico until around some 19 million years ago when the source was depleted.

As large and as active as this area was, it does not formally comprise a Large Igneous Province (LIP).  Interestingly enough, during this time there was a LIP erupting in the Afar – Ethiopia region that was most active around 30 million years ago.

http://www.largeigneousprovinces.org/0events

Impact Events.

There are three large craters identified that date at around the 33 million year point when the climate changed.  One formed the mouth of Chesapeake Bay south of Washington DC.  The crater is 53 miles (85 kilometers) across and is thought to be formed by a body a mile or two (3 – 5 kilometers) in diameter.

http://pubs.usgs.gov/fs/fs49-98/

Map / drawing of Chesapeake Impact Crater – from US Impact Craters
http://www.impactcraters.us/chesapeake_bay_virginia

A second impact crater has been identified about 200 miles (320 kilometers) northeast of this crater.  It is offshore New Jersey and called Toms Canyon.  It has also been dated around the same time as Chesapeake Bay.  Toms Canyon measures 9 – 12 miles (15 – 20 kilometers) across and was formed by a much smaller body.

http://www.sciencedirect.com/science/article/pii/S0025322797001138

The third large impact crater is Popigai in Siberia.  It dates around the same time as Chesapeake and Tom’s Canyon, but was formed by a larger body.  It measures some 62 miles (100 kilometers) across and was thought to be formed by a 3 – 5 mile (5 – 8 kilometer) diameter body.  This crater is little studied as the Soviets found shock-created diamonds at the site and used residents of the Gulag to set up mines for them.

http://www.passc.net/EarthImpactDatabase/popigai.html

Popigai Impact Crater, Russia (Siberia). From Elite Daily

There are numerous smaller impact craters measuring in the few miles to kilometers across that date sometime in the Eocene, and it appears that Earth was in the midst of a shower of inbound bodies for several million years.  Eventually the serial insults to the climate built up to the point where natural processes were unable to keep up and the climate changed.

http://en.wikipedia.org/wiki/Category:Eocene_impact_craters

Discussion and Conclusions

Where to go with all of this?  There are several observations to sort out and place into some sort of framework.

The first observation would be that massive volcanic eruptions appear to be insufficient to place the planet into a new Ice Age or by themselves cause an extinction event.  This is not to say that they don’t do enormous damage, kill a lot of plants, animals and people.  But an extinction event means you wipe out all members of a species worldwide.  Some actual study of reported global temperatures for the last century compared with volcanic eruptions by Willis Eschenbach over at “Watts up With That” tend to show less impact than we would think.  I find both the article and the comments significant food for thought.

http://wattsupwiththat.com/2012/03/16/volcanic-disruptions/

From Western Geo Hikes – reference map showing all calderas in San Juan Volcanid Field and their dates of origin.
http://www.coloradogeohikes.0catch.com/Colorado/SJVF/SJVF.html

The second observation would be that massive release of volcanic material into the environment (Large Igneous Province outbreak) does not necessarily lead to a massive global temperature change or extinction event.

http://www.largeigneousprovinces.org/13aug

A third observation would be that existing volcanic activity needs to be supplemented with some other massive insult to the planet to trigger a long-term change in climate or an extinction event.  And that additional insult, whether impact event(s), close supernovae or other interstellar event, must be sufficiently large so as to push an already stressed system over the edge.

A final observation is that we know that this planet gets hit by impacting bodies at some currently unknown rate.  That flux of bodies is not limited to asteroids whose orbits are (mostly) known.  They also comprise comets and the remains of fragmented comets whose orbits we find out about the first time we see them days to months before they arrive in the inner solar system.  They can be significantly faster than asteroids and are not limited to orbits near the plane of the ecliptic.  And these bodies can be very large.  Comet 1993 Hale Bopp was some 25 miles (40 kilometers) across.  We have yet to characterize either the population or the danger posed by these bodies.  Note finally that the majority of meteor showers take place when the orbit of the earth crosses that of known comets.

My conclusion would be that as large as possible volcanic eruptions may be, even they need help from one to many somethings equally or more powerful to push this planet over the edge into massive climate change or mass extinctions.  All of which makes me a lot more comfortable about ignoring the incessant hectoring from the environmentalist community.  This does not mean we shouldn’t be Good Stewards of our home.  We absolutely should.  But our home is a whole lot more stable and resilient than any of us suspects.

Thanks for reading and I look forward to your comments.

AGIMARC

Additional reading:

http://gsabulletin.gsapubs.org/content/120/7-8/771.abstract
http://petrology.oxfordjournals.org/content/43/8/1469.abstract
http://petrology.oxfordjournals.org/content/46/5/859.abstract
http://gsabulletin.gsapubs.org/content/81/8/2329.short