Theater aficionados know that noise and commotion behind the curtain during intermission of a theatrical performance intensifies expectation. Then, when the curtain rises after intermission, the audience sees that the scene has been radically altered and their “wow” factor is heightened. Earth’s scene (or ‘cene) has also changed dramatically in history to bring about surprising changes for humanity’s benefit.

The ‘cene that we and the rest of Earth’s inhabitants are in right now is the Pleistocene, the epoch during which an ice age cycle operates, which was preceded by the Pliocene. During the very brief intermission between the Pliocene and the Pleistocene, some of the greatest commotion Earth has ever experienced took place. Scientists now recognize, though, that this commotion had to be extremely fine-tuned and perfectly timed for our present-day civilization to be possible.

The Pliocene is the geologic epoch that extends from 5.333 to 2.58 million years ago. The Pleistocene extends from 2.58 million years ago to the present.

The Pliocene was characterized by gradual cooling and drying of the global climate. However, the global mean temperature was about 3°C (5°F) warmer than it is today. Sea levels were about 30 meters (100 feet) higher. While ice sheets were beginning to form over Antarctica and Greenland, there was no ice age cycle.

What Started the Pleistocene?
The beginning of the Pleistocene marks the onset of the ice age cycle. In my book, Improbable Planet, I describe five simultaneous, unprecedented tectonic plate movement events that played a crucial role in cooling our planet sufficiently to allow for the initiation of our current ice age cycle.1 The most dramatic and most significant initiation event, however, was the Eltanin impactor, named after the research vessel responsible for its discovery. The Eltanin impactor ranks as the only known asteroid impact in a deep ocean (see figure 1).

Figure 1: The Eltanin asteroid impact site. The red dot below the southernmost tip of South America in the Bellingshausen Sea marks the site of the impact. The size of the dot represents the crater diameter if the impact had occurred on land. Background image credit: NASA

The Eltanin impactor was first discovered in 1981 as an iridium anomaly in sediments from deep-sea core E13-3 (depth 5,090 meters) that was recovered by the USNS Eltanin, an oceanographic research vessel, in 1964.2 Two geophysicists and a chemist at the University of California, Los Angeles discovered in core E13-3 a gold to iridium abundance ratio that is known to be unique to chondritic meteorites.

The same three scientists published a follow-up study in 1988.3 They reported that debris from the asteroid impact had been found across at least 600 kilometers of ocean floor. Based on iridium concentrations in sediments from six deep-sea cores, they concluded that the asteroid was at least 0.5 kilometers (0.3 miles) in diameter.

In 1995, an expedition by the research vessel Polarstern returned to the Eltanin impact site. This expedition successfully collected three cores containing deposits from the impact. Analysis of the geological record within the region of impact yielded an approximate estimate of the impact date of 2.15 million years ago.4 The same analysis yielded a size for the impacting asteroid of 1–4 kilometers (0.6–2.5 miles) in diameter. The calculated explosive force from such an asteroid colliding into a deep ocean of Earth is equivalent to the explosion of 100 billion to 10 trillion tons of TNT. For comparison, the global nuclear arsenal is equal to 6.4 billion tons of TNT.

In 2001, Polastern returned to the impact site, explored a region of 80,000 square kilometers (about 31,000 square miles), and collected 17 new sediment cores containing meteoritic ejecta. At the 2005 American Geophysical Union Fall Meeting, geophysicists Frank Kyte, Rainer Gersonde, and Gerhard Kuhn presented a summary of results from the 2001 expedition.5

The three geophysicists reported that the known field of strewn meteorite fragments extends over a region 660 by 200 kilometers (51,000 square miles). They found “no evidence that the impactor had penetrated the ocean floor or formed a crater.”6 However, they found extensive evidence of the mixing of melted ejecta with seawater salts. Their analysis of all the data from the 2001 expedition led them and others to conclude that the impactor was at least 1 but could not be larger than 2 kilometers in diameter.7

Analysis of over 20 deep-sea sediment cores in the impact region yielded a different and much more reliable date for the impact. Chronostratigraphic (dating the ages of layers) data from these cores constrain the Eltanin impact date sometime after the Gauss Chron date of 2,581,000 years ago and sometime before the C2r.2r event date of 2,441,000 years ago.8 That is, the impact date = 2,511,000 ±70,000 years ago. This date is consistent with the date for the beginning of the ice age cycle. The date of 2,580,000 years ago was established by decree by the International Commission on Stratigraphy on June 30, 2009.9

Climate-Changing Effects of the Eltanin Impactor 
James Goff and a team of five Australian researchers from University of New South Wales School of Biological, Earth, and Environmental Sciences performed detailed computer modeling of the effects of a 1–2-kilometer diameter asteroid crashing into a 5,000-meter deep ocean basin.10 To say the least, the asteroid caused a big splash. Waves hundreds of meters high (a thousand plus feet high) radiated out from the impact site.

Goff’s team reviewed all the existing evidence for megatsunami deposits in Antarctica, Chile, Australia, and New Zealand. They found that the Eltanin impactor consistently explained this evidence. The team’s computer modeling established that the Eltanin impactor, in addition to generating megatsunamis throughout the South Pacific Ocean, would have ejected enormous amounts of water vapor, sea salts, sulfur, and dust up into the stratosphere. That material would have remained in the stratosphere for at least two years.

The researchers showed that the material thrown up into the stratosphere by the Eltanin impactor would cause two climate-changing consequences. First, the albedo or reflectivity of Earth’s atmosphere would be greatly enhanced. Therefore, much more of the Sun’s heat and light would be reflected away from Earth. Second, the opacity of Earth’s atmosphere would also be enhanced. That is, much less of the Sun’s light and heat that was not reflected away would be able to penetrate Earth’s atmosphere to illuminate Earth’s surface.

Both consequences would have drastically and quickly reduced Earth’s surface temperatures. Thanks to five simultaneous major tectonic plate movements that I described in Improbable Planet,11 Earth’s surface, even before the Eltanin impact event, was experiencing a gradual cooling phase. The Eltanin impact event radically and rapidly accelerated the global cooling brought on by the five tectonic plate movements.

Goff’s team showed that the Eltanin impactor so accelerated global cooling that it hurtled Earth into the cycle of glaciations that has persisted for the past 2.58 million years. Without the Eltanin impactor there would have been no ice age cycle. If there was no ice age cycle, several benefits12 essential for the launch of global civilization would have been lacking. In other words, without Earth’s great scene change of an Eltanin impactor being of the just-right size and composition, being perfectly timed, and its impact site being perfectly placed, there would be no global human civilization.

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