The Younger Dryas impact hypothesis (YDIH) proposes that at around 12,900 cal a BP (calibrated date), North America was subject to an extraterrestrial impact event. This event is hypothesised to be responsible for the end Pleistocene environmental changes such as the Younger Dryas cooling, huge wildfires, the extinction of late Pleistocene megafauna, and the end of the Clovis culture (Holliday et al., 2014).
[Younger Dryas cooling: a period of rapid cooling of the North Atlantic and a weakening of the Northern Hemisphere monsoon. The reduction in heat moving north resulted in a warmer Southern Hemisphere. The cooling is widely thought to be a result of a slowing of the Atlantic meridional overturning circulation, AMOC. However, the forcing behind this is debated] (Carlson, 2010)
The YDIH is argued to overcome many of the shortfalls of the overkill and abrupt environmental change hypotheses - see my previous posts! (Firestone et al., 2007) (Holliday et al., 2014). For example, the lack of kill sites for 33 genera of now extinct megafauna, including camels and ground sloths, has been identified as a problem with the overkill hypothesis. The fact that similar environmental shifts occurred throughout the Pleistocene but the extinctions only occur at the end of this period has likewise been highlighted as a flaw in the climate shift theory (Firestone et al., 2007).
- The discovery of markers, including nanodiamonds, aciniform soot, high-temperature melt-glass, and magnetic microspherules; all attributed to cosmic impacts/ mid-air explosions.
- A carbon rich deposit overlying Clovis-age sites in North America.
- The impact's apparent coincidence with the onset of Younger Dryas cooling, caused by the destabilisation of the Laurentide ice sheet as a result of the impact.
- Bones of megafauna and Clovis tool assemblages have been reported to occur below this layer only.
Despite this, the YDIH has fallen out of favour more recently. Here I'll outline some arguments against the markers and carbon deposit lines of evidence.
LeCompte et al. (2012) state that morphological and geochemical analyses of the microspherules (see this post on the Earth Science Eratics blog for more about spherules) suggest they are not cosmic, volcanic, authigenic (a deposit formed where it's found), or anthropogenic in origin. The spherules were found to be similar in composition to terrestrial metamorphic rocks and very different from those formed by cosmic or authigenic processes.
They appear to have formed by the rapid melting, then quenching of terrestrial materials. LeCompte et al. (2012) describe spherules occurring above the Clovis artifacts with a significant drop below, implying that they were deposited on top of the artifacts at the surface. This implies an impact/ explosion event left behind a spherule layer, after which the humans and megafauna are absent.
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Spherules from the Younger Dryas boundary, at three sites sampled. Topper and BWD (Blackwater Draw) sites are discussed further below. White numbers indicate the diameter of the spherules in microns
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Holliday et al., (2014) pose a strong argument against the YDIH. They point out that the stratigraphic, depositional and pedogenic (soil related) contexts of the YDIH have rarely been addressed. But they probably have a significant effect on the record of indicators of impacts (Holliday et al., 2014)
The sedimentological and geochemical data used in support of the YDIH are:
- Changes in the rates of sedimentation (magnetic microspherules, nanodiamonds and other features of cosmic dust regularly fall on Earth) (Holliday et al., 2014)
- The nature of the depositional environments
- Discontinuities/ breaks in the rock record created by erosion. The carbon rich layer, according to Holliday et al., (2014) represents stability following more rapid/ energetic sedimentation.
Translocation is a common soil process, where water moving through a soil moves particulates and solutes. These can then accumulate, the accumulation increasing with depth (Holliday et al., 2014, see p.523 for an explanation of this).
Spherules were present in all samples collected from Blackwater Draw and Topper (LeCompte et al., 2012, see figure above). Samples collected below and above the highest concentration of spherules still contained high numbers of spherules (Holliday et al., 2014) (see also Firestone et al., 2007, figure 1).
Magnetic microspherules and magnetic grains are <500mm to <2mm (Firestone et al., 2007) and nanodiamonds are 2–300 nm (typical of translocated materials). These particulates increase in frequency with depth in the carbon rich layer at Blackwater Draw and Topper (LeCompte et al., 2012, figures 3 and 4)
The spherules, magnetic grains and nanodiamonds may therefore be affected by translocation and accumulation in the soil. If these markers used to identify the YDB (Younger Dryas Boundary) are present in other sediments as well, then they probably can't be used as reliable indicators of an impact event. They may have accumulated in a layer due to translocation rather than deposition at the surface after an impact/ explosion.
Also, the layer of spheres was 4 cm thick and buried by only 50 cm of sand. This suggests that either:
- All the sand from just below the Clovis artifacts to near the surface was deposited with spheres and the amount of spheres depends on the rate of sand deposition, or
- The spheres were translocated downward and accumulated at the lithologic break created by the artifacts.
Therefore, the Younger Dryas boundary zones used to support the YDIH are in depositional environments that either A) select for the microscopic indicators by being low energy environments (lakes and marshes) compared with immediately underlying high-energy alluvium (riverine) or, B) the indicators are from soils that represent landscape stability over a long time, therefore concentrating those materials
Next post: the arguments against the ice sheet destabilisation, human cultural shifts/ population decline and the link to megafaunal extinctions
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