Last Days of the Dinosaurs

Last Days of the Dinosaurs

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Abstract

This essay explores Earth’s climatory and atmospheric conditions during the late-Cretaceous Period (i.e., when dinosaurs still ruled the Earth), as well as the rapid changes to those conditions thought to have been brought about by the Yucatán bolide (i.e., asteroid) event.  Many of the familiar, cosmetic features of Earth’s biosphere that we take for granted today were different 65 million years ago, including: the absence of the polar ice caps; different continental and oceanic layouts; higher concentrations of Greenhouse Gases (GHGs) such as carbon dioxide (CO2) and methane (CH4) in the atmosphere.  These prehistoric conditions created an environment that was favorable for reptilian species to thrive in, thereby relegating the various mammalian species to a subordinate role.  However, dramatic changes to the planet’s biosphere which contributed to the mass extinctions of over fifty percent of flora and fauna (due to the 9.5 km long bolide that impacted our planet), turned the tables on Earth’s dino-overlords, thus paving the way for the rise of mammals.

Introduction

The most recent estimate from the scientific community is that our planet is around 4.6 billion years old.1 To put that in perspective, current theory also holds that Homo sapiens sapiens (i.e., Latin for “wise person”, the scientific designation for humanity) has only been around for the last 195,000 – 276,000 of those years.2 That means, if you could somehow travel back in time and start at Day 1 on Earth’s planetary calendar, you would have to play out the entirety of human history more than 15,000 times — from start to finish — before reaching today’s date!

If that makes you feel a bit more humble about mankind’s role in the grand scheme of things, then brace yourself — there’s more:

To the best of our knowledge, dinosaurs roamed the planet for 165 million years before they were devastated by extinction level event(s).  And while these giant, scaly leviathans may not have produced a rich and immersive cultural legacy during their lengthy tenure, they still have humanity beat in terms of sheer longevity by an impressive margin.  In fact, you would have to play out the entirety of human history over 600 times to cover the complete saga of dino-history!

Illustrated timeline of the history of life on Earth, by Mariana Ruiz Villarreal
Illustrated timeline of the evolution of life on Earth.* Note that homo sapiens don’t appear until the end– i.e., We humans are relatively new in town!

Dinosaurs walked the Earth between 230 – 65 million years ago, and that includes a period of time that is known to paleontologists as the Cretaceous Period (i.e., 145 – 65 million years ago).  At the end of this period, a six mile long (9.7 km) bolide crashed into the Yucatán Peninsula of modern day Mexico, resulting in an explosion roughly a billion times more powerful than the atomic bomb dropped on Hiroshima in August of 1945.3 This, in turn, triggered a sequence of events that made the Earth a very inhospitable place.

First, massive quantities of particulate matter (e.g., rock, soot, dust) were heaved into the atmosphere post-impact, forming clouds dense enough to block out much of the Sun’s life-giving radiation.  During this time, the Earth’s average surface temperature plunged dramatically, making it too cold for many plants and animals to survive.  Then, as these clouds began to settle after a few months, a runaway Greenhouse Effect caused by gases created in the impact explosion forced average global temperatures to the opposite extreme, and the Earth became even hotter than it was before the bolide hit.  In just a few years, the extremely cold then hot fluctuations in our planet’s climate wreaked havoc on the Earth’s biosphere, and more than half of all plants and animals – including many of the dinosaurs – were killed.4

The impact event described above quickly destroyed a rich and vibrant ecological diversity that had existed and evolved, under comparatively stable conditions, over the course of millions of years.  This is certainly unsettling when you take into account humanity’s relative “newcomer” status in the history of our planet.  Indeed, it would seem that our species’ rise to power was made possible only after a fluke occurrence (or, perhaps divine intent) altered the evolutionary destiny of life on Earth.

Artist Don Davis' rendering of an asteroid impacting the Earth -- like the Yucatán bolite event that decimated the dinosaurs, 65 million years ago.
When a 9.2 km (approx. 6 mi) long asteroid impacted Earth in the Yucatán peninsula of modern day Mexico 65 million years ago, the evolutionary destiny of life on our planet was changed forever.

But, what were the conditions on our planet like before the asteroid hit, allowing dinosaurs to thrive for millions of years while simultaneously condemning mammals (like our forbears) to eek out a meager existence under their terrifying, pitiless reign?  We have seen that changes in climate can have a profound impact on life here at the surface of our planet.  So, what were the characteristics of Earth’s climate during the last years of the Cretaceous Period, before they were drastically changed in such violent fashion?

Last Days of the Dinosaurs: the Late-Cretaceous Period

A long diagram of the geologic time scale by USGS (United States Geological Survey), including eons, eras, periods, epochs, etc.
Earth’s geologic time scale in millions of years. Note: the Cretaceous Period is abbreviated with the letter “K,” for “kriede” (the German word for chalk).

Among scientists, the letter K is used to abbreviate the Cretaceous Period.  This might seem odd at first, because creta is derived from the Latin word for chalk.  However, kriede is the German word for chalk, and — as with other perplexing nomenclatures that are sprinkled throughout humanity’s scientific journey of discovery — for some reason that Teutonic abbreviation stuck.  Even more disconcerting is that the period was named after rock formations with high concentrations of chalk within them, which were found in Europe and dated to this time-frame.  But, rock formations in other parts of the world that are comparable in age are rich in other substances — limestone, for example — so the Cretaceous Period could just as well have been arbitrarily christened the Calxaceous Period! (i.e., Calx is the Latin word for limestone.)

The youngest period of the Mesozoic Era and the longest period in the Phanerozoic Eon — i.e., the eon we are currently living in, dated from 545 million years ago (at the end of the Proterozoic Eon) through to this very moment — the Cretaceous Period was first designated in 1882 by Jean d’Omalius, a Belgian scientist, during his work in the chalky rock beds of White Cliffs of Dover, Great Britain.  As mentioned in the preceding paragraph, the high concentrations of chalk in these rock beds inspired d’Omalius when it came time to name the period.

A six-mile (9.5 km) long asteroid ended the Cretaceous Period 65 million years ago when it crashed into the Yucatán Peninsula, and that event also marked the end of the Mesozoic Era, as well as the beginning of the Cenozoic Era.  But during the eighty million years comprising the entirety of the Cretaceous Period (i.e., 145 – 65 million years ago) Earth was a drastically different place from the world that our species evolved in.5,6 However, in order to more keenly appreciate those differences, we should first take a moment to review the major characteristics of Earth’s surface and atmosphere today:

Earth Today

Pie chart of Earth's atmospheric composition.
There are only trace amounts of gases like CO2 and CH4 in our atmosphere. However, these (and other) GHGs play a major role in keeping the surface of our planet warm.

The dry air (i.e., ignoring water vapor) that we are used to breathing is composed of seventy-eight percent diatomic nitrogen molecules (N2), twenty-one percent diatomic oxygen molecules (O2) and one percent gaseous argon (Ar).  Furthermore, less than one percent of our breathable air is composed of trace elements: carbon dioxide (CO2) at .0387% of total atmospheric mass (and rising, due to anthropogenic activity); methane (CH4) at about two-parts per million; nitrous oxide (N2O) at around three-hundred parts per billion; ozone (O3) at only forty parts per billion; chlorofluorocarbons (CFCs) at one or two parts per ten billion.7 However, even though trace elements make up only a small, seemingly insignificant part of total atmospheric mass, they nevertheless play a very significant role in our lives as greenhouse gases (GHGs).  In fact, without trace gases in the atmosphere, life as we know it on Earth would be quite different.

A graph showing the Sun's radiation output. We see that the output falls largely within the visible spectrum (400-700 nm).
The lion’s share of the electromagnetic radiation that we receive from the Sun falls primarily within the visible spectrum– i.e., approx. 400-700 nanometers (nm).

Consider this: air is largely transparent to much of the radiation that we receive from the Sun (particularly at visual wavelengths, from about 390-700 nm).  As a result, sun-rays penetrate Earth’s atmosphere virtually unimpeded and fly straight through to the ground where they are absorbed due to the surface’s near perfect black body characteristics (i.e., the ability to absorb all wavelengths of electromagnetic radiation).  Afterwards, the surface spits this radiation back out into the atmosphere at longer wavelengths, where it would fly right back out into space (in the same manner that it arrived), were it not for the GHGs described in the preceding paragraph.

Unlike other gases in the atmosphere, GHGs are able to react with longer waveform radiation coming up from the ground by reflecting it.  Moreover, some of that radiation is reflected back down to the surface (where it is absorbed and subsequently re-emitted by the ground again, etc.).  This feedback loop of electromagnetic radiation is what makes comparatively warm, temperate climates at our planet’s surface a reality.  Were it not for GHGs, we would find life on Earth to be a frigid, Hoth-like, largely unpleasant experience.**

A diagram of the Greenhouse Effect, showing how sunlight is absorbed by the ground and reflected by GHGs.
The Greenhouse Effect: Most of the Sun’s radiation passes unimpeded through Earth’s atmosphere before being absorbed by the ground. GHGs in the atmosphere then reflect this radiation (after the ground spits it back out at longer wavelengths). In this way, our planet’s surface is able to maintain relatively warm average temperatures.

However, it is worth noting that there are some exceptions to the atmosphere’s indifference to radiation from the Sun.  For example, ozone molecules in the Stratosphere intercept harmful, short-wavelength radiation (e.g., gamma rays, ultra-violet rays) from our local star before it can reach us down on the ground.  Also, one slight hiccup to the greenhouse effect is that it is possible to have “too much of a good thing.” Recall the plight of dinosaurs after the dust from the Yucatán bolide impact settled, when a barrage of GHGs made the Earth unbearably hot.  This sweltering heat was detrimental to the monstrous lizards just as the unbearable cold that came before, due to the post-impact, atmospheric dust blanket that blocked out the Sun (almost) entirely.8,9

Earth 65 Million Years Ago

In any case, even before the Yucatán impact, Earth was different from what we are used to.  For one thing, the Cretaceous Period was warmer, thought to be a consequence of larger concentrations of carbon dioxide in the atmosphere owing to volcanic emissions.  Also, evidence suggests that average temperature differences from Pole to Equator were probably much less dramatic during the Cretaceous Period.

In fact, in the time before the bolide struck, Earth didn’t even have polar ice caps!  While there may have been snowfall during the beginning of the period, glaciation was confined to higher altitudes – e.g., alpine mountains – and by the late-Cretaceous period Earth’s temperature had already been undergoing a slow climb for years.  This net upward trend in (average) temperatures continued until the events of the Yucatán impact set in motion a domino effect that would lead to our planet’s most recent ice age.  Scientists believe that the average global surface temperature during the Cretaceous Period was approximately thirty-seven degrees Celsius.  Similarly, ocean water temperatures (including deep water) may have been as much as fifteen degrees Celsius higher than in modern times.

Nevertheless, as a result of the relatively gentle transitions in temperatures from high to low latitudes, there weren’t such extreme differences in air pressure that drive the winds today.  Moreover, an added complication to reduced wind activity worldwide is that ocean waters don’t mix as well, and this leads to stagnation on a greater scale.  Scientists have found evidence of this reduced “upswelling” during the Cretaceous Period by noting that oceanic rock formations from that time contain large amounts of black shale.  Furthermore, it has been determined that more anoxic events took place in Earth’s oceans during this period, which occur when water below a body’s surface has been starved of oxygen.

A series of five pictures showing the breakup of super-continent Pangaea, from 225 million years ago to the present.
Earth’s continental plates are constantly in motion. 65 million years ago, during the (late) Cretaceous period, continental layout more closely resembled the super-continent Pangaea than what we have grown accustomed to.

Continental plates were in flux at the time of the Cretaceous Period even as they are today, but the relative locations of the continents were more akin to the supercontinent, Pangaea, during this time.  Indeed, the Atlantic Ocean was much smaller, as North America and Europe had by now only moved away from each other by a (comparatively) small margin. Furthermore, India was closer to Antarctica than Asia.  Global plate allocations such as these played a part in determining Earth’s weather and climate characteristics, as well.6

Summary

The Cretaceous Period began approximately 145 million years ago, and continued on until 65 million years ago when a large, nearly 10 km long asteroid collided with the Earth in the Yucatán Peninsula near modern day Mexico.  This triggered a domino effect of catastrophic natural phenomena that largely contributed to the eventual extinction of over half of the world’s plants and animals (including many of the dinosaurs).  But afterwards – with the dinosaurs all but vanquished – mammals were free to begin their slow climb up the food chain, eventually resulting in humanity’s dominance of our world.

Before the Yucatán bolide struck, conditions on Earth were somewhat different than those we enjoy today.  First, average global temperatures were warmer, thought to be due to the higher concentrations of carbon dioxide in the atmosphere resulting from heightened volcanic activity.  Second, since temperature gradients from Pole-to-Equator were not extreme, the air pressure differences that drive winds in the world we know today were less severe back then.  In fact, the Earth did not even have polar ice caps during the Cretaceous Period.

Furthermore, continental plate locations at this time were more akin to the supercontinent Pangaea than the continental arrangements that we are used to today.  For example, the Atlantic Ocean was much smaller and India was closer to Antarctica than Asia.  The relatively close-knit distribution of continental and oceanic plates also influenced climate characteristics during the Cretaceous Period.

*On the Ruiz Villarreal timeline of the evolution of life on Earth: “aracnids” should be “arachnids”; “moluscs” should be “mollusca” (and the creatures are mollusks); “Anfibians” should be “Amphibians.”  

**The planet “Hoth” is a fictional ice world from George Lucas’ “Star Wars” series.  For more information, see: http://www.starwars.com/databank/hoth     

Resources

  1. Braterman, P.S. How science figured out the age of the Earth [Internet]. NYC, NY; Scientific American: 2013 Oct 20 [cited 2015 Apr 27]. Available from:   http://www.scientificamerican.com/article/how-science-figured-out-the-age-of-the-earth/
  2. Ravilious, Kate. Humans 80,000 Years Older Than Previously Thought [Internet]. Washington, D.C.; National Geographic News: 2008 Dec 3 [cited 2015 Apr 29]. Available from: http://news.nationalgeographic.com/news/2008/12/081203-homo-sapien-missions.html
  3. Stephen L. Brusatte, Richard J. Butler, Paul M. Barrett, Matthew T. Carrano, David C. Evans, Graeme T. Lloyd, Philip D., Mannion, Mark A. Norell, Daniel J. Peppe, Paul Upchurch, and Thomas E. Williamson. The Extinction of the Dinosaurs [Internet]. Hoboken, NJ; Wiley Online Library: 2014 Jul 28 [cited 2015 Apr 29]. Available from: http://onlinelibrary.wiley.com/doi/10.1111/brv.12128/full
  4. Strobel, N. Effects of an asteroid impact on Earth [Internet]. Bakersfield, CA; Bakersfield College: 2013 Apr 25 [cited 2015 Apr 29]. Available from: http://www.astronomynotes.com/solfluf/s5.htm
  5. “Extinction of the dinosaurs.” Washington, D.C.; Smithsonian National Museum of Natural History: Date unknown [cited 2015 Apr 29]. Available from: http://paleobiology.si.edu/geotime/main/htmlversion/cretaceous4.html
  6. Koch, F. Cretaceous period [Internet]. Chicago, IL; Encyclopædia Britannica: Date unknown [cited 2015 Apr 29]. Available from: http://www.britannica.com/EBchecked/topic/142729/Cretaceous-Period
  7. Composition of the atmosphere [Internet]. Raleigh, NC; NC State University: 2013 Aug 9 [cited 2015 Apr 29]. Available from: https://www.nc-climate.ncsu.edu/edu/k12/.AtmComposition
  8. Visible light [Internet]. Atlanta, GA; Georgia State University, Physics and Astronomy Department: Date unknown [cited 2015 Apr 29]. Available from:
    http://hyperphysics.phy-astr.gsu.edu/hbase/ems3.html#c2
  9. The Greenhouse Effect [Internet]. Atlanta, GA; Georgia State University, Physics and Astronomy Department: Date unknown [cited 2015 Apr 29]. Available from:
    http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/grnhse.html

Photo Credits

  1. “Europasaurus holgeri Scene 2.” Source: Boeggemann, G., CC-BY-SA 2.5, via Wikimedia Commons. Date unknown [cited 2015 Apr 29]. Available from: http://commons.wikimedia.org/wiki/File:Europasaurus_holgeri_Scene_2.jpg.
    (Description: Jurassic scene set on one of the large islands in the Lower Saxony basin in northern Germany, showing an adult and a juvenile specimen of the sauropod Europasaurus holgeri, passing Iguanodons, two Compsognathus in the foreground and an Archaeopteryx at right.)
  2.  “Timeline evolution of life.” Source: Ruiz Villarreal, M., Public Domain, via Wikimedia Commons. 2012 Jul 24 [cited 2015 Apr 29]. Available from: http://commons.wikimedia.org/wiki/File:Timeline_evolution_of_life.svg
  3. “Coastline Remodeling.” Source: Davis, D., PD-Gov, via NASA website. Date unknown (cited 2015 Apr 29). Available from: http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=2306
  4. “Divisions of Geologic Time.” Source: United States Geological Survey, PD-Gov, via USGS website. Circa 2010 Jul [cited 2015 Apr 29]. Available from: http://pubs.usgs.gov/fs/2010/3059/pdf/FS10-3059.pdf
  5. “Composition of Earth’s atmosphere.” Source: Dbc334, Public Domain, via Wikimedia Commons. 2009 Jan 9 [cited 2015 Apr 29]. Available from: http://commons.wikimedia.org/wiki/File:Composition_of_Earth’s_atmosphere_en.svg
  6. “Solar AM0 (Air Mass Zero) spectrum.” Source: Dan Michael O, Public Domain, via Wikimedia Commons. 2012 May 16 [cited 2015 Apr 29]. Available from: http://commons.wikimedia.org/wiki/File:Solar_AM0_spectrum_with_visible_spectrum_background_(en).png
  7. “Earth’s Energy Budget.” Source: Wong, T., Lee, S., Marvel, T., Hopson, V., PD-Gov, via My NASA Data. Date unknown [cited 2015 Apr 29]. Available from: http://science-edu.larc.nasa.gov/energy_budget/
  8. “Pangaea to present.” Source: Kious, J., Tilling, R.I., Kiger, M., Russel, J., PD-Gov, via USGS website. Circa 1996 [cited 2015 Apr 29]. Available from: http://pubs.usgs.gov/gip/dynamic/historical.html
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