Wolbach II again used the YDB Pt peak at continental sites as a marker to which they could compare possible fire indicator peaks. At ten sites they found that Pt or microspherules and fire indicators like carbon and charcoal peaked at the time of the YDB as identified by the Bayesian study.
Next, they reported the results of analysis of charcoal from 125 lake deposits. This allowed them to confirm “a major, widespread peak in biomass burning on at least four continents at the warm-to-cold transition marking the YD onset.” Again, the fire-indicator peak coincided with ones for Pt, microspherules, and meltglass.
As we noted early on, the YD was one of a series of rapid, short-term temperature changes during the waning of the ice ages. This has perennially raised the question of whether there is anything special about it, whether high amounts of charcoal, indicating wildfires, were common in other, non-YD, warm-to-cold climate changes. Wolbach and colleagues found that to the contrary, those other warm-to-cold events had low wildfire activity, showing that the YD climate change was highly anomalous and possibly unique.
IMPACT WINTER
Wolbach and colleagues estimated the area covered by the YD wildfires and the amount of vegetable material consumed. She had made similar calculations for the KT, where she showed that the amount of aciniform carbon is directly proportional to the percentage of biomass burned. Thus, if one knows approximately the amount of aciniform carbon at multiple sites at some geological boundary, one can make a rough estimate of the areal extent of wildfire burning at that time. After performing the calculation, the Wolbach II authors concluded that “Wildfires at the YD onset rapidly consumed ∼10 million km2 of Earth’s surface, or ∼9% of Earth’s biomass, considerably more than for the [KT] impact.”
The fires would have injected a vast amount of soot and dust into the atmosphere, where it would have remained for several weeks before settling out. Earlier, scientists had come to believe that the KT impact had a similar but even greater effect, causing an “impact winter” that would have blocked sunlight, shut down photosynthesis, and contributed to the great mass extinction. Now the Wolbach II authors invoke another impact winter that began with an extraterrestrial event at the onset of the YD. But this one happened in the presence of modern humans.
The Wolbach II authors conclude that “a cosmic impact is the only known event capable of simultaneously producing all this evidence.”
11
HIAWATHA
Two of our four predictions from Chapter 3 have been met:
• Within the precision of Bayesian analysis, the YDB has the same age on several different continents, strongly corroborating an ET event.
• The internal composition and the external features of the event markers, when scientists study them using SEM and XRS, show that they are extraterrestrial.
We also predicted that newly discovered or newly investigated YDB sites would show the same age and event marker peaks as the established sites. This prediction is important enough that we will devote the next chapter to it.
Then we come to the critical fourth prediction: scientists may find a crater of YDB age. Before the Alvarez team introduced their theory in Science Magazine in 1980, in spite of 150 years of effort, the search for the cause of dinosaur extinction had reached a dead end. Luis Alvarez thought that this would persuade vertebrate paleontologists to quickly accept the impact theory. But just the opposite happened: opponents dug in their heels and many have them still dug in 40 years later — roughly the length of a scientific career.
Although the Chicxulub crater had actually been discovered by Glen Penfield soon after the Science paper appeared, for reasons that no one has fully explained, the impact specialists did not become aware of his report until some 10 years later.
During that period, opponents of the Alvarez theory would frequently point out that the “smoking gun”: the crater, was missing. But over the decade of the 1980s, new evidence knocked down one objection after another. Then in 1990 came the discovery of the crater, which scientists are still studying today through drill cores pulled back to the surface.
Logically, the failure to find the KT crater was an absence of evidence argument that did not disprove the Alvarez Theory, but certainly it had a negative psychological effect. Looking back, it seems likely that the theory would have prevailed even had the crater never been found, which was a distinct possibility, even a likely one given the vicissitudes of our mobile planet’s behavior.
As of 2018, the YDIH stood in roughly the same position as the Alvarez theory during the 1980s: a considerable amount of evidence had been produced in favor of impact, but the telltale crater remained to be discovered.
Then in early 2018, at the Nordic Geological Winter Meeting in Copenhagen, two little-noticed abstracts appeared. The first had the lengthy and intriguing title: “A bag of sand from the ice-concealed, 31-km-large (18 mi) Hiawatha crater in north-west Greenland: samples of the impact plume, ejecta blanket and crater floor from a recent meteorite impact through the Greenland ice sheet.” The more succinct title of the second read: “A large young impact crater beneath the ice in northwest Greenland.” Its text included these lines:
Beneath Hiawatha Glacier in northwest Greenland [under] nearly a kilometer of ice lies a 31-kilometre-wide, circular bedrock depression with an elevated rim and a subdued central peak. Outwash sediments contain shocked quartz and other unweathered impact grains. Geochemical analysis shows that the impactor was a Pt-rich iron asteroid that would have had a diameter of [about] 1.5 km in order to produce a crater of the observed size. Radiostratigraphic, morphological and geochemical evidence suggest that the Hiawatha impact took place between 38 and 11.7 kyr ago, which would make it the largest known impact experienced by modern humans. Our discovery has potentially wide-ranging implications for our understanding of the recent history of Earth.
Then followed in November 2018 a full article describing the finding, which received instant and widespread media attention. Just as Penfield had used geophysical methods to penetrate a half-mile of sedimentary rock in the Yucatan and discover the Chicxulub crater, so the Greenland scientists used radar to peer through thick glacial ice to find what just may be the missing YDB crater. Hiawatha Crater is the largest known from the last 5 million years and the second largest in the last 35 million.
FIGURE 12:
Hiawatha Crater as detected by radar. The white line is the present ice sheet margin.
The image shows a circular feature with an elevated rim and a flat, depressed floor. Its width to depth ratio is similar to those of known impact craters and it has the rebounded central peaks (small circles) diagnostic of large impact craters on the Moon and Earth. There is little doubt that Hiawatha is an impact crater, but is it of YDB age?
Abstracts at professional meetings do not undergo formal peer review and scientists have more freedom to say whatever they like. One of the two abstracts above used “Young” in the title and the other “Recent.” In the text of the second abstract, as shown above, the scientists said that the “impact took place between 38 and 11.7 kyr ago,” a range that includes the YDB. Senior author Kurt Kjaer says that in drafts of the published paper, “The team explicitly called out a possible connection between the Hiawatha impact and the Younger Dryas,” elsewhere saying, “I think it is a possibility.”
But published articles have gone through peer review. With a hypothesis as controversial as the YDIH, the journal editors would likely have chosen at least one opponent as a reviewer. Not surprisingly then, the published article on the Hiawatha crater took an unusually long time from receipt by the journal to publication and the explicit connection to the YD disappeared, replaced by:
The sum of these tentative age constraints suggests that the Hiawatha impact crater formed during the Pleistocene. Regardless of its exact age, based on the size of the Hiawatha impact crater, this impact very likely had significant environmental consequences in the Northern Hemisphere and possibly globally.
The titl
e of the first abstract contained the odd phrase, “bag of sand.” This was apt because, since the crater is buried underneath a half-mile of ice, scientists could not sample it directly. Instead they collected sediment from deposits left by an under-ice outwash river that drains the crater area. In other words, they had no samples from the crater itself, which would be necessary to establish a convincing age.
By now the astute reader will have wondered whether they found ejecta from the impact that formed Hiawatha Crater in the Greenland ice cores. Surprisingly, the answer is no, but there is a possible explanation. The impactor may have landed not on bare land, but on the Greenland ice sheet that likely covered the area at the time of impact. Spurred by the finding of the Hiawatha crater, scientists have recently made computer models of the impact of an asteroid onto thick ice. When the thickness of the ice sheet is set at 1.5-2 km (1-1.5 mi), the model produces a crater the size of Hiawatha, as well as the central peaks. The model also showed that impact onto an ice sheet inhibits the blasting of impact material out of the crater, which might explain why no ejecta are found in the ice cores. If the ejecta were mainly ice, they would not be recognizable in the cores.
This is all we have to go on for the time being. Obviously, to date the Hiawatha Crater directly using radiometric methods, scientists would have to lay their hands on actual samples of the impact melt rock. That would require drilling into the structure from atop the ice. But certainly it is possible that Hiawatha is the missing YDB impact crater.
In early responses, some impact specialists were dubious, one saying that “Impacts the size of Hiawatha occur only every few million years and so the chance of one just 13,000 years ago is small.” Another made the same point, saying it is “quite unlikely” that a crater formed within the last couple of million years.” One veteran was “intrigued [but] not wholly convinced that this is an impact crater.” One of the discoverers of the crater had a different view: “When you think about it, the bed below the ice sheets has to have impact craters that have not been explored yet, and there may even be some in Antarctica as well, but more radar measurements are necessary to locate them, and dating them is extremely challenging.”
Time will tell whether the Hiawatha crater is of YDB age. Its discoverers clearly believe it could be, but evidently could not get a statement to that effect through peer review. As science reporter Paul Voosen summed up, “A drilling expedition to the hole at the top of the world would be costly. But an understanding of recent climate history — and what a giant impact can do to the planet — is at stake. “Somebody’s got to go drill in there,” said paleoclimatologist Lloyd Keigwin of the Woods Hole Oceanographic Institution. “That’s all there is to it.”
There may be another, albeit indirect, way of assessing whether Hiawatha is a YD impact crater. In 2018, a group from the Geological Survey of Canada described a set of 12 drill cores collected from the seafloor of Baffin Bay to the west and south of Greenland. One section of the cores captures the period from about 25,000 to about 11,000 years ago, and thus includes the YDB. Some of the cores were taken only about 1,000 km (600 mi) from the Hiawatha crater. They have been well-dated and the location of the YDB identified. If Hiawatha is indeed a YDB impact crater of YD age, these cores should be full of the characteristic event markers. Not surprisingly, a group is at work testing this possibility. Stay tuned.
12
OVER HALF THE EARTH’S SURFACE
Now to our prediction that when new YDB sites are found, they too will show peaks in the distinctive event markers. A corollary of our first prediction is that they will date to the YDB as defined by the Bayesian study. During 2019 and early 2020, scientists reported on five such studies. If even one site were to fail to meet these predictions, the YDIH itself would be called into question.
STARA JIMKA
In September 2019, a group led by Gunther Kletetschka of the Czech Academy of Sciences reported on their study of a now dried-up ice-age lake called Stara Jimka, located in the Bohemian Forest of the Czech Republic. The scientists drilled and extracted 50-cm (20-in) cores of sediment laid down on the floor of this “paleolake,” then dated them using radiocarbon and Bayesian analysis.
One issue that has come up several times is whether the YD cooling, if not due to cosmic impact, could have been caused by a volcanic eruption. These are the only two processes that could deposit a thin layer of ejecta over an area the size of a continent. The most likely candidate for the volcanic hypothesis is the eruption that created the Laacher See, a large water-filled caldera in the Rhineland of Germany known to date from close to the beginning of the YD. If volcanic ejecta from the Laacher See eruption were to occur at the YDB, cause and effect would become a distinct possibility.
The oldest Stara Jimka core dated to 13,299 years ago and the youngest to 11,602, thus capturing the time of the YDB and Laacher See eruptions. The unusually rapid rate of sedimentation at Stara Jimka widens the separation between layers of different ages and allows them to be told apart. To illustrate the point, consider the converse: If the rate at which sediment accumulated on the floor of the lake was so slow that 1 cm of sediment would have taken 500 years to form, say, then events within that period would lie right on top of each other in the core and could not be distinguished. Fortunately, the opposite was true and to accumulate that 1 centimeter at Stara Jimka took only 30-60 years.
The authors dated the YDB at Stara Jimka at 12,755 ± 92 years ago, overlapping the range from the Bayesian study of Kennett et al. (2015). The Laacher See deposits dated to ∼12,820 ± 20 years ago, and thus the two age ranges overlap and could be the same. But in the core, the fast sedimentation rate clearly separates the two levels and shows the YDB to be distinctly younger than the Laacher See layer.
This result confirms an earlier study of the location of the Laacher See tephra in the Greenland ice cores, which concluded that it took place 200 years before YD. Thus, although it did cause brief cooling on its own, the Laacher See eruption did not cause the YD.
The Czech team used SEM and XRS to measure the highest concentration of ET microspherules ever found at any YDB site: 17,000 per kg compared to the average from other YDB sites of about 400 spherules per kg, a difference of 425 times. Some melted microspherules were enriched in nickel and contained unmelted “framboids,” the raspberry-like mineral clusters that have been found at other YDB sites and attributed to impact. They also detected significant changes in the lake’s weathering proxies, fauna, and flora, reflecting a shift to cooler YD conditions. “This evidence,” they said, “is consistent with the YD impact hypothesis and evidence of one or more cosmic airburst events occurring at this time.”
PILAUCO
As we will learn in a later chapter, one of the most important archeological sites in the Western Hemisphere is at Monte Verde, Chile. Located far down at 41.5°S, it is among the oldest habitations in the hemisphere and is key to establishing when humans first arrived in the Americas. In 1986, a building company excavating in Pilauco, a suburb of Osorno, Chile, about an hour’s drive north of Monte Verde, uncovered the bones of what were judged to be extinct mastodon and horse. The bones were housed in the local history museum, where they became the subject of two undergraduate theses. The students found soft remains in the collection that they thought might be the skin of a Gomphothere, an elephant-like mammal that was abundant in South America but that went extinct at the YD. Further study of the Pilauco site unearthed a rich assemblage of Pleistocene bones from seven mammal families. Pilauco turned out to be the same age as the habitation at Monte Verde, but has some additional features that make it as important to geology as Monte Verde is to archeology. The Chilean Council of National Monuments recognized its importance to paleontology and archeology by declaring Pilauco a protected site.
Pilauco’s YDB age, location far down in the Southern Hemisphere, ease of excavation, and a sedimentation rate faster even than that of Stara Jimka, made it a natural for further study. In mid-March 2019, a group of 16 aut
hors published, “Sedimentary record from Patagonia, southern Chile supports cosmic-impact triggering of biomass burning, climate change, and megafaunal extinctions at 12.8 ka.” Their intent was “to determine whether the evidence at Pilauco is consistent or inconsistent with the YDB impact hypothesis and to explore the potential consequences of the proposed impact event.” Here is a summary of the findings:
1. Pilauco shows peak concentrations of YDB event markers, not only microspherules and platinum but also peaks in other proxies not previously seen at the YDB and in some cases, rarely found in nature. These include palladium, gold, high-temperature iron-rich and chromium-rich spherules, and grains of native iron. Some of the microspherules are a rare form of iron that almost never occurs on Earth but is common in meteorites and impact melts.
FIGURE 13:
Palladium and gold peak at the YDB at Pilauco, Chile
2. Pilauco contains a charcoal-rich, peaty deposit interpreted to be analogous to the black mat, thus also greatly extending its range.
3. The charcoal record of biomass burning at Pilauco is the same age as the one for the Northern Hemisphere described in the Wolbach articles, confirming that YDB wildfires burned far south into the high latitudes of the Southern Hemisphere.
4. Abundant human artifacts and megafaunal bones are found only below the YDB at Pilauco. The bones represent at least 8 species that appear to have gone extinct at the YDB, including one horse species.
5. The most important fact about Pilauco may simply be its location. First, as shown on the map in Figure 16, the discovery extends the range of YD proxies ~6,000 km (3,600 mi) south of the nearest site in Northern Venezuela, suggesting that the effects of impact spread across not only North America but South America as well. Pilauco is ~12,000 km (7,500 mi) from the northernmost YD site in Canada, a distance of about one-third of the Earth’s circumference, comparable to the wide range of the glassy impact ejecta known as the Australasian tektites.
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