Deadly Voyager
Page 8
6. Pilauco is the first high-latitude, Southern Hemisphere site to be studied in detail, and in that hemisphere the climate change at the onset of the YD turns out to have been in the opposite direction to that in the Northern Hemisphere: from cold to warm. As we have seen, the return of colder and wetter conditions marked the onset of the YD in the Northern Hemisphere. But at the same time in Southern Chile, the climate turned warmer and drier, a change that also shows up plainly in the Antarctic ice cores. Broecker named this opposite response in the two hemispheres the “bipolar seesaw.”
The high resolution in the Pilauco section shows that the temperature increase took place much faster than during other Pleistocene temperature changes — in only a few years or less. Such a rapid rise cannot derive from changes in slow oceanic circulation, but must reflect speedier atmospheric processes. But major changes in the oceans were also taking place at the time: mean ocean temperature in both hemispheres began to rise at the start of the YD and continued rising for about 700 years.
The authors of the Pilauco article believe that all this evidence “appears consistent with the proposed effects of a YDB cosmic impact event that affected both the Northern and Southern Hemispheres.” We will explore later the implications of the seesaw temperature changes for the megafaunal extinction.
A VILLAGE ON THE EUPHRATES
The rise of agriculture was a pivotal event in human history, as a way of life based on hunting and gathering by nomadic tribes gave way to a more sedentary existence and farming. The advantages made the transition irreversible, allowing human population to grow far larger than it would otherwise have in a process that is still going on but whose days global warming may number. Archeologists believe the transition began first in Western Asia, in part as a response to the changed climate of the YD.
FIGURE 14:
Abu Hureyra
The oldest known archeological site to capture the shift to agriculture is at Tell Abu Hureyra on the Euphrates River in Northern Syria. Archeologist Andrew Moore excavated there in 1972 and 1973, on an emergency timetable because the site was soon to be drowned by Lake Assad (named for Hafez al-Assad, the father of Bashar al-Assad.) Although restricted to only two field seasons, Moore uncovered a rich trove of material. Radiocarbon dating would show that Abu Hureyra was occupied from what archeologists call the Epipaleolithic, 13,400 years ago, up to the early Neolithic at 7,500 years ago.
One of the first published reports came in 1992 from Moore and G. C. Hillman, titled: “The Pleistocene to Holocene Transition and Human Economy in Southwest Asia: The Impact of the Younger Dryas.” Pollen and plant remains showed how large grains indicative of domestication were rare or nonexistent in the stratigraphic record before the YD, but were increasingly abundant after it. This indicated that prior to the cool period, people mostly foraged for wild cereal grains, but later began to “select and cultivate domestic-type grains such as rye and einkorn wheat.” Thus the descendants of the foragers who founded Abu Hureyra became farmers raising cereals and legumes. Soon their villages grew larger and their innovation spread.
Moore and Hillman wrote at a time, remember, when the origin of the YD was regarded as “a mystery” and by some, likely to remain so. They did not try to solve the mystery, but rather showed that whatever caused the YD, the shift away from foraging at Abu Hureyra began at the same time. They concluded that although “The Younger Dryas episode was not the only factor, it was probably a significant catalyst in each of the areas within southwest Asia where cultivation is likely to have begun.”
In two articles that we have reviewed before, researchers reported microspherules and “scoria-like objects” (SLOs) at the YDB at Abu Hureyra. They also reported the amorphous silica glass called lechatelierite, found at Meteor Crater, in tektites, and at the sites of atomic explosions.
Then in March 2020 came an article on Abu Hureyra that places it with Pilauco at the top of the list of important YD sites, with Andrew Moore as the senior author. They reported that the YDB at Abu Hureyra shows peaks in all the event markers previously found at other sites, plus others.
FIGURE 15:
Abundance peaks at Abu Hureyra. The shaded band is the YDB.
In addition to the four markers shown, the researchers also found peaks in platinum, palladium, gold, nickel, cobalt, and chromium. The layer that features these abundance peaks dates by Bayesian analysis to ~12,825 ± 55 years ago, within the range from Kennett et al. There is no question that the researchers located the YDB and found it rich in markers diagnostic of impact.
Iron- and silica-rich microspherules were present at a concentration of about 600 per kg of sediment, comparable to other YD sites, but they tended to be much larger, suggesting that the impact had occurred close to the village. The chemical composition of the microspherules matched that of the sediment at the site except that they were richer in the element nickel, an extraterrestrial indicator. This is consistent with them being a mixture of the rock at ground zero and material from the impactor.
Using a panoply of techniques including SEM and XRS, the researchers found nanodiamonds at a concentration of about 450 parts per billion, compared to an average of about 200 ppb reported for other YD sites. Thus the nanodiamonds also prove reproducible at Abu Hureyra, as they have at dozens of other sites.
One of the most striking findings at Abu Hureyra was “meltglass,” equivalent to the SLOs. As shown in the chart above, meltglass also peaked at the YDB at Abu Hureyra, with the majority of the visible pieces ranging from one-half to one millimeter in size. The smaller ones had the aerodynamic shapes of spheroids or teardrops, resembling minuscule tektites. The authors estimated that there were likely to be thousands of even smaller pieces, ones that could not be counted because they were impossible to tell from the non-ET grains in the sediment. The meltglass also matches the composition of the bulk sediment, but has about 10 times the nickel content.
An unusual feature of the Abu Hureyra meltglass is that about 40 percent of the fragments bear the imprint of the common reeds that have grown in the Euphrates valley for millennia. Meltglass was also found splashed onto the surface of fragments of clay, evidently used by the settlers as plaster, and onto bits of vertebrate bone. These plant imprints would seem to be inconsistent with a high-temperature impact event, but it turns out that plants burned in fires leave behind “phytoliths,” silica particles deposited in the plant cells that serve as microfossils of the original. They have also been found in meltglass from several other impact sites.
Let us look more closely at the evidence for the switch to agriculture and the effect of the YD event on the people of Abu Hureyra. Toward the end of the last ice age, the climate of the Middle East was warm and moist, causing wooded areas to expand their range. The clement conditions allowed nomadic hunter-gatherers to find meat and edible plants close to home, making it possible at Abu Hureyra and other sites for them to become more sedentary. In addition to gazelle meat, they ate fruits, nuts, legumes such as lentils, and wild wheat, millet, and rye. Then came the YD, and everything changed.
Under the now cooler and drier conditions, the forest retreated some 200 km (125 mi) west of Abu Hureyra, almost to the shores of the Mediterranean Sea. Even over a millennium later when the warmer climate of the Holocene retuned, the area remained dry, as we find it today. As revealed by the seed archeology at Abu Hureyra, the proportion of plants on which the villagers had depended dropped quickly to about 65% of all plants, and by the middle of the YD, to 35%. As drought-susceptible plants declined, drought-resistant ones, including cereals, began to replace them, rising to about 50% of the villager’s diet by the mid-YD.
Several lines of evidence show that the villagers had gone beyond merely gathering wild grains, to cultivating them. Here is how the archeologists reached that conclusion. First, before the YD, discarded seeds were short and small, consistent with their wild origin. Afterwards, larger seeds became much more plentiful in the record, suggesting human selection. Second, lentil
s, which do not tolerate drought well, faded in abundance only to return in a few centuries, even though the climate had not improved, suggesting that they had been cultivated and watered by the villagers. A third and seemingly counter-intuitive indicator was the rise in the abundance of weed seeds. These tend to be smaller and inedible, but under cultivation are unavoidably harvested along with edible seeds and grains, and then show up in the record. Archeologists rarely find them before cultivation, since then they would have been mostly avoided during the gathering of wild seeds.
All this led Moore et al. to conclude that “The Abu Hureyra villagers were among the earlier if not the earliest peoples to practice cereal cultivation, ultimately leading to the development of full-scale agriculture across the Middle East.”
What effect did the YD event have on the population of Abu Hureyra? Archeologists have come up with clever ways to answer such questions, a topic we will revisit later. At Abu Hureyra, the abundance of charred material left by cooking fires dropped sharply at the YDB and did not recover for centuries. Another measure is the abundance of discarded food and “trash,” just as a larger family today may need a second trash barrel. By this indicator, the population at Abu Hureyra dropped by 80% after the YD. Not to zero though, showing that the settlers did not abandon the village. A third approach uses the raw number of radiocarbon dates as a proxy for population: “dates as data.” By this method, the population of Abu Hureyra rose until the beginning of the YD, after which it fell and did not recover for more than two millennia. All three findings corroborate a great decline in resources and population at the YD at Abu Hureyra. As we will see, there is evidence for a similar decline in the population of the Clovis people after the YD.
WHITE POND
In October 2019, a paper appeared on White Pond, South Carolina, which contains one of the oldest and most complete records of the paleoenvironment in the Southeast. Its layers date from at least 22,000 years ago upward through the YD. A group led by scientists from the South Carolina Institute of Archaeology and Anthropology reported on high-resolution analysis of a 1-meter (3-foot) section of a drill core from the lake. Within a 2-cm (3/4 of an inch) part of the section dated to 12,785 ± 58 years ago. They found a platinum peak five times the background level. A sharp peak in wildfire indicators coincides with the Pt peak, providing further evidence of the biomass burning reported by Wolbach and colleagues.
WONDERKRATER
In the Limpopo Province of South Africa lies an archeological site called Wonderkrater. Its peat deposits extend back to 30,000 years ago. One sample that dates to 12,744 years ago, and thus to the YD, shows a sharp spike in Pt roughly 20 times background. The rise and fall of the peak closely resembles that shown in Figure 7 for the Pt peak in the Greenland ice. At the same time, there were several megafaunal extinctions, including one horse species. A change in the archeological toolkit occurred that was “closely co-incident with the cultural change from Clovis to Folsom technologies in North America” (see Chapter 15). At the time of writing, the site had not been investigated for other impact proxies.
As shown by the map below, just as Pilauco revealed YD proxies deep down in the Southern Hemisphere, so Wonderkrater shows their presence almost at the southern tip of Africa. If this map shows the true extent of the YD event, then it covers roughly half the Earth’s surface. And as with our argument about absence of evidence, there are surely many other YD sites yet to be found and some may be even more far flung: literally.
The area suggested by the map is quite a bit larger than the “strewnfield” of the Australasian tektites, estimated at from 10% to 30% of the Earth’s surface. No impact specialist doubts that impact produced these tektites, nor that the corresponding crater must exist.
FIGURE 16:
Map location of Wonderkrater and 25 other YDB sites with Pt peaks (orange dots). Red dots = sites with other proxies but that have not been tested for Pt.
To sum up, each of the five studies described in this chapter offered an opportunity for the YDIH to fail. Instead, not only did each corroborate the impact hypothesis, they greatly expanded the range of its effects. One could ask no more of a hypothesis.
13
TRUE CLAIMS
The late Wallace Broecker ended his white paper, undated but written before early 2017, by saying, “I realize that [the YDIH] is distasteful to many because of the early false claims [italics added].” He continued, “But the new evidence [the Pt spike in the Greenland ice core] suggests that there was some kind of extraterrestrial impact. Hence it should be given further study.”
There is no doubt that in alleging “false claims,” Broecker spoke for the scientific community at large. By the mid-2010s, the YDIH had been as thoroughly rejected as had continental drift, meteorite cratering, and anthropogenic global warming at the nadir of their acceptance. But there was one difference: in science, the word “false,” as opposed to, say, “wrong” or “incorrect,” has a similar ring to “falsification” and may suggest intentionality. Though it was almost certainly not Broecker’s intention, to allege “false claims” is one small step from an accusation of scientific malpractice. The staunchest opponents of the three great theories never suggested that the evidence was false, only that what there was had been misinterpreted.
But if, as its opponents had claimed repeatedly, the evidence for the YDIH was not reproducible, then from where had that evidence come? The peaks in event markers could not have resulted from mistakes, so again, from where had they come? The implicit accusation, and indeed almost the only possible interpretation, was that the proponents had falsified their data to produce the peaks. No one ever made this accusation on the record, but it hung in the air, a dark cloud over the YDIH.
But was Broecker correct — were any of the claims of the YDIH proponents actually “false?” The two most critical kinds of evidence were the microspherules and nanodiamonds. LeCompte et al. sampled the very YDB sites where Surovell et al. could find not a single microspherule, and found them in abundance. No scientist who convincingly located the YDB and used SEM and XRS has failed to find ET microspherules. The allegations that the nanodiamonds are irreproducible came primarily from samples taken from the Channel Islands, where there is a strong likelihood that the researchers did not actually sample the YDB. Kinzie et al. found nanodiamonds in abundance at many sites.
Consider next the new kinds of evidence that came after the original FEA paper: the widespread Pt peak, the wildfire indicators, the geographic spread of the event markers over half the Earth’s surface, the synchronous age of the YDB, the meltglass, phytoliths, platinum, palladium, gold, nickel, cobalt, chromium, and native iron grains. The table below, updated from one in Wolbach I, summarizes the evidence. Note that a number of YDB sites have not been fully investigated for some of these proxies.
Table 1: Impact Proxies Found at YDB Sites
Marker
Number of YDB Sites
=YDB age
23
Charcoal
28
Carbon Spherules
24
Black Mat
27
Platinum
18
Other PGE
10
Fe/Si Spherules
30
Meltglass
12
Nanodiamonds
24
The early proponents of the YDIH did not make false claims and their evidence has been reproduced many times over. It was the absence of evidence reported by the opponents that has proved irreproducible.
THE IMPACTOR
If we have a plausible case that an ET event launched the YD and its ancillary effects, what was the nature of the impactor? A comet? An asteroid? An impact, an airburst, or both? A fact crucial to the answer is that scientists have now found YDB sites with impact indicators over four continents and both hemispheres. Even if Hiawatha is an impact crater of YD age, it likely would not have thrown its ejecta so far. This points toward multiple, simult
aneous impacts by fragments of a comet that burst apart in the air, or, in the case of Hiawatha, may have collided intact.
Airbursts may be more common than scientists have realized. John Wasson and Mark Boslough noted that “A sizeable fraction of asteroids and comets were never compacted, and have essentially no strength.” These “strengthless” objects are therefore likely to burst apart in the air and therefore may “comprise an important fraction of the accretionary events occurring on the Earth.”
Terrestrial analogs may give us clues. In June 1908, a large aerial explosion occurred near the Tunguska River in a remote and sparsely settled region of Siberia. The blast leveled 2,000 square km (700 sq mi) of Taiga forest, yet caused no casualties and left no crater. The largest ET event in recorded history, “Tunguska” prompted an estimated 1,000 scientific articles, mostly in Russian. Scientists have concluded that the object came from an airburst some 8 km (5 mi) above the Earth’s surface, and that the body most likely was a comet. Nevertheless, the explosion had enough energy to melt about 1 mm of dry soil over the blast area.
The Australasian tektites provide another analog. These glassy, dumbbell- or teardrop-shaped blebs range in size from millimeters up to centimeters. The streamlined shapes and wide distribution show that the Australasian tektites traveled through the air for hundreds of miles before settling to earth.