Controversies Concerning the End of the Last Ice Age
By Robert M. Schoch, Ph.D.
The end of the last ice age, nearly 12,000 years ago, was a pivotal period not only in the history of our planet, but also for early cultures and civilizations living during those tumultuous times. There were rapid climatic and environmental changes, as well as catastrophic geologic changes. Massive ice sheets (on the order of kilometers thick) quickly melted at high latitudes in the northern hemisphere; thus, pressure was released from the crust setting off increased earthquake and volcanic activity. Tremendous amounts of moisture in the atmosphere fell to the surface of Earth as torrential rains, causing widespread deluges and flooding. Huge quantities of fresh water flowed into the oceans, upsetting and changing ocean circulation patterns, which in turn had further effects on the climate, as well as raising sea levels around the globe on the order of 120 meters or more, inundating low-lying coastal areas.
The exact dating of the end of the last ice age has been subject to increasing refinements over the last few decades. In recent years, based on ice core data from Greenland and other detailed evidence, the end of the last ice age has been dated to “11700 calendar yr b2k (before AD 2000)... with a maximum counting error of 99 yr” (M. Walker et al. 2009, Jour. Quaternary Science). This translates to 9700 BCE. Studying a Greenland ice core utilizing ultra-high-resolution laser sampling techniques involving hundreds of samples per centimeter and year of ice time (measuring such markers as calcium, sodium, and iron concentrations), P. Mayewski et al. (2014, Jour. Quaternary Science) have pinpointed the abrupt end of the last ice age to within a year. Inspecting their published data, it is evident that they have refined information documenting the end of the last ice age at the level of months, weeks, and possibly even days. The ice age ended quite suddenly indeed!
The last ice age did not just steadily “warm” and then end, though. Rather, in the Northern Hemisphere there was a period of warming toward the end of the last ice age followed by a dramatic cold spell (cold even relative to those glacial times) known as the Younger Dryas before the final sudden warming at 9700 BCE. The onset of this cooling event was also quite abrupt, although perhaps not quite as abrupt as the dramatic warming that ended the Younger Dryas (thus ending the last ice age). Based on Greenland ice core data, this cooling event is dated to approximately 10,900 BCE (J. Steffensen et al. 2008, Science). The Younger Dryas cold spell lasted for 1200 years before Earth was suddenly snapped out of the last ice age circa 9700 BCE. What are the explanations for the beginning and end of the Younger Dryas? These are topics that have baffled geologists for decades. (As a side note, the cold period known as the Younger Dryas is named after the plant Dryas octopetala, an arctic-alpine member of the rose family that thrives under cold conditions.)
Perhaps one of the best known and most controversial theories to explain the onset of the Younger Dryas is that a comet, meteor, asteroid, or other extraterrestrial (ET) object (sometimes referred to as bolides) either hit Earth or exploded in the atmosphere 12,900 years ago, thus inducing the abrupt cooling event that marks the onset of the Younger Dryas. Although there were earlier theories along these lines, this idea gained widespread attention with the 2007 publication of an article by R. Firestone et al. that reputedly reported evidence for such an impact event. These authors (Proceedings of the National Academy of Sciences) proposed, “that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD [Younger Dryas] cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.”
The impact hypothesis has aroused heated controversy, with arguments and counterarguments flying back and forth in the scientific literature as well as in the popular press. Initially I found the arguments for such an impact at the beginning of the Younger Dryas quite intriguing. However, as more researchers have studied the issue, much of the data supporting the impact hypothesis has been questioned. For instance, D. Kennett et al. (2009, Proc. National Academy of Sciences) published an article apparently documenting the presence of “shock-synthesized hexagonal nanodiamonds (lonsdaleite)” from the base of the Younger Dryas on Santa Rosa Island, California. This, if true, would strongly support that an impact event occurred as lonsdaleite is known on Earth only from meteorites and impact craters. However, as pointed out by M. Boslough et al. (2012, Geophysical Monograph Series), it has since been determined that the supposed lonsdaleite is not lonsdaleite, but a misidentification of other material, and the same holds true of other supposed reported occurrences of lonsdaleite in sediments dating to approximately 12,900 years ago. With the invalidation of the supposed lonsdaleite finds, one of the strongest lines of evidence supporting the Younger Dryas impact hypothesis has evaporated. Furthermore, various studies have demonstrated that nanodiamonds per se (as opposed to lonsdaleite) “do not provide unique evidence for a Younger Dryas impact” event (H. Tian et al. 2012, Proc. National Academy of Sciences).
Other evidence put forth to support the Younger Dryas impact hypothesis has not fared any better, as discussed by Boslough et al. (2012). Here I mention a few pertinent points. Much has been made of “black mat” layers in the geological and archaeological record that, it is claimed, mark precisely the start of the Younger Dryas and contain evidence of an extraterrestrial impact (such as magnetic grains containing iridium, glass-like carbon containing nanodiamonds, carbon spherules, and so forth) and its associated effects, such as widespread wildfires (Firestone et al. 2007). In a study of black mats that ranged from 6,000 years old to 40,000 years old, various supposed impact markers were found, “suggesting that elevated concentrations of these markers arise from processes common to wetland systems, and not a catastrophic extraterrestrial impact event” (J. Pigati et al. 2012, Proc. National Academy of Sciences). Carbonaceous spherules were found to be the result of fungus, not a comet or other impactor (A. Scott et al. 2010, Geophysical Research Letters).
J. Wittke et al. (2013, Proc. National Academy of Sciences) argued that “10 million tonnes” of “impact spherules” were scattered across four continents by a fragmented comet or asteroid at the beginning of the Younger Dryas; however, it was subsequently demonstrated that these supposed “impact spherules” were of ancient human origin (for instance, produced when buildings were destroyed in fires) and furthermore do not all date to the start of the Younger Dryas (P. Thy et al. 2015, Jour. Archaeological Science). Indeed, dating is absolutely critical to an impact hypothesis – all of the evidence should date to the same point in time (it should be “isochronous”). The evidence from the geological and archaeological sites upon which the comet/asteroid proponents rely was put to the test by D. Meltzer et al. (2014, Proc. National Academy of Sciences) who concluded: “Only 3 of the 29 sites fall within the temporal window of the YD onset as defined by YDIH [Younger Dryas Impact Hypothesis] proponents. The YDIH fails the critical chronological test of an isochronous event at the YD onset, which, coupled with the many published concerns about the extraterrestrial origin of the purported impact markers, renders the YDIH unsupported. There is no reason or compelling evidence to accept the claim that a cosmic impact occurred . . . and caused the Younger Dryas.”
Another argument against the impact hypothesis is that no crater (or craters) has yet been definitively identified as dating to circa 12,900 years ago. Such a crater would be very recent geologically, and arguably should therefore remain relatively fresh and evident. It has been suggested that the impactor may have exploded in the atmosphere, breaking up into numerous fragments that cratered ice sheets, which subsequently melted (thus destroying the crater evidence). Possibly there is evidence of craters under the Great Lakes or Hudson Bay. Or perhaps the elliptical depressions known as the Carolina Bays found on the Atlantic Costal Plain of the United States were created by the impacts; but the dating of the Carolina Bays is questionable, and not firmly linked to the beginning of the Younger Dryas. The 1908 Tunguska explosion of an incoming extraterrestrial object over Siberia flattened a widespread area of forest without leaving a crater. Could a much larger explosion and fragmentation of an incoming object, an explosion that was a million or more times larger than the Tunguska event, have occurred at the beginning of the Younger Dryas? Boslough et al. (2012) analyzed this possibility and found it highly unlikely, concluding that “consideration of basic laws of physics indicate that such a fragmentation or high-altitude airburst event would not conserve momentum or energy, would lie outside any realistic range of probability, and therefore did not occur during the YD as described by Firestone et al.” (Boslough et al. 2012).
At this point, in my assessment, the evidence collectively points away from an extraterrestrial impact for the start of the Younger Dryas, but the issue of what exactly did trigger the beginning of the Younger Dryas is unresolved. The onset of the Younger Dryas remains somewhat of a mystery. As seen in the ice cores and sediment profiles, it was abrupt. But the changes that mark the beginning of the Younger Dryas “were not everywhere of the same severity, or in the same direction” (S. Fiedel 2011, Quaternary International). The Younger Dryas as a cooling event was primarily a Northern Hemisphere phenomenon. In other locations on the planet, due apparently to shifts in atmospheric and oceanic circulation patterns, the Younger Dryas was wet rather than dry (cold and dry being the predominant condition in the Northern Hemisphere). In the Southern Hemisphere, such as the southeast Atlantic, New Zealand, parts of South America, and Antarctica, the Younger Dryas was a period of warming. Still, there was greater cooling in high northern latitudes than the warming in the southern latitudes, resulting in an overall net global cooling during the Younger Dryas of approximately 0.6° C (A. Carlson 2013, Encyclopedia of Quaternary Science).
Suggestions as to the cause and/or triggering mechanism behind the onset of the Younger Dryas have generally fallen into several broad categories, including: 1) changes in oceanic circulation; 2) changes in atmospheric circulation; 3) changes in atmospheric carbon dioxide concentrations; 4) changes in surface albedo; 5) volcanic activity; 6) external “forcing events” such as an extraterrestrial impactor, a supernova that could generate an interstellar shock wave and wave of debris, causing perturbations to the atmosphere-Earth system, or various other forms of cosmic radiation or “cosmic dust”; and 7) changes in solar radiation. Of course, these various factors are not necessarily mutually exclusive, and indeed one triggering mechanism may have caused other events to take place, the combined results of which initiated the Younger Dryas.
Among geologists and paleoclimatologists the most widely accepted explanations have involved changes in oceanic circulation patterns with concomitant atmospheric and climatic changes. Key to most such theories is an influx of cold fresh water into the North Atlantic preventing the upwelling of warm salty water from the southern oceans, thus shutting down the overturning of ocean water with the result that the atmosphere over the ocean would remain cool rather than being warmed as it previously had been. What caused the influx of fresh water? Possibly the melting of the Laurentide ice sheet of eastern Canada and the release of fresh water from Lake Agassiz (a huge ancient lake, covering portions of Saskatchewan, Manitoba, and Ontario, that formed due to the retreating ice), spilling massive amounts of water into the North Atlantic. However, the timing of a major discharge of water from Lake Agassiz does not correspond to the onset of the Younger Dryas (Fiedel 2011); another mechanism needs to be found.
An important clue may be this: At the onset of the Younger Dryas a sudden increase in 14C concentrations is observed. A possible explanation is that the Sun (and possibly also Earth’s magnetic field) weakened, allowing the greater penetration of cosmic radiation into our atmosphere, thus creating more 14C (Fiedel 2011; cosmic rays can transform 14N to 14C). H. Renssen et al. (2000, Quaternary International) have argued that reduced solar activity could have initiated the onset of the Younger Dryas. They propose two mechanisms by which this could possibly occur. 1) A reduction in solar ultraviolet radiation would entail a reduction in stratospheric ozone production and attendant changes in atmospheric circulation patterns, ultimately resulting in a cooling effect on the surface of our planet in mid to high latitudes. 2) Reduced solar activity allowed more cosmic rays to penetrate our atmosphere, promoted aerosol formation and cloud nucleation, and resulted in more cloud cover globally with increased reflection of incoming solar radiation, thus cooling Earth.
While there may have been overall reduced solar activity during the Younger Dryas, P. LaViolette (2011, Radiocarbon) has presented evidence for a solar proton event (SPE), during which protons were accelerated by the Sun to incredibly high energy levels and penetrated our atmosphere, at the start of the Younger Dryas. An SPE may have ionized the atmosphere, increasing cloud cover and initiated a cooling spell. Then, I suspect, the Sun went into a period of reduced activity, essentially a partial shutdown, for 1200 years.
But then something happened to increase Earth temperatures dramatically, even catastrophically. The end of the last ice age can be located in the Greenland ice record within not just a year, but virtually to the month, week, and day. What could conceivably cause such a major and abrupt warming and climatic reorganization? Stars go through cycles of activity and inactivity; our Sun is simply another star. Circa 9700 BCE our Sun “woke up” and erupted with a mighty outburst – probably followed by a series of closely spaced outbursts (including CMEs [coronal mass ejections], major solar flares, and related phenomena) – thus terminating the Younger Dryas cold spell and literally snapping us out of the last ice age. In my assessment the isotope and geochemical data, sediment data, and archaeological evidence (such as ancient petroglyphs; see A. Peratt 2003, IEEE Trans. Plasma Science) all converge to corroborate the theory that a major solar outburst ended the last ice age (see my 2012 book, Forgotten Civilization). (One might suggest the alternative of an extraterrestrial object. I do not believe the evidence is compatible with a comet, meteor, asteroid, or other bolide hitting Earth at the end of the Younger Dryas. Furthermore, an impact on land or in shallow water [or even in deep water, depending on the size, speed, and trajectory of the impactor], or an atmospheric explosion, would most likely inject dust and debris into the atmosphere which would have a cooling effect on the climate rather than a dramatic, virtually overnight, warming effect.)
Based on the current evidence, a very strong case can be made that the last stages of the last ice age, the onset and termination of the Younger Dryas, are linked to solar activity. Reduced solar activity caused the initiation of a cold period 12,900 years ago. A major solar outburst ended the Younger Dryas, bringing the last ice age to a final close, circa 9700 BCE.
Since I wrote the above draft (in early July 2015), another article has appeared, “Bayesian chronological analyses consistent with synchronous age of 12,835–12,735 Cal B.P. for Younger Dryas boundary on four continents” (J. P. Kennett et al., Proc. Nat. Acad. Sci., accessed 31 July 2015), which claims to provide evidence supporting the impact hypothesis, but this is extremely problematic. For the sites analyzed, supposed evidence of an ET object has been questioned or discredited. Even the purported synchrony is questionable as one can take issue with the appropriateness of applying Bayesian analyses to the dataset (Bayesian analysis is a particular type of statistical analysis that can be misapplied, leading to incorrect conclusions). The authors attempt to link the start of the Younger Dryas to a platinum (Pt) anomaly in a Greenland ice core (M. I. Petaev et al., 2013, 44th Lunar and Planetary Science Conference) that might indicate an ET object. The Pt anomaly could have resulted from volcanic activity; the Laacher See super-volcano (Germany) exploded around 12,900 years ago (Petaev et al., 2013). If the Pt anomaly does indicate an ET object, it may have been localized (e.g., a relatively small meteorite), non-cataclysmic, and unconnected to the Younger Dryas triggering event (M. Boslough, PNAS, 24 December 2013).
I remain convinced that the known evidence fails to support a major ET object hitting Earth at the beginning of the Younger Dryas. The Younger Dryas was a cold period that prolonged the last Ice Age. The major catastrophic changes (including the extinction of large animals) occurred circa 9700 BCE when climates warmed dramatically, virtually instantaneously. In my assessment the evidence points to a major solar outburst (or series of closely spaced solar outbursts) at that time.