The Younger Dryas climate change: was it caused by an extraterrestrial impact?

Abstract

The Younger Dryas is an abrupt cooling event at the end of the last Glacial associated to a change in ocean circulation. According to the Younger Dryas impact hypothesis, however, one or more extraterrestrial airbursts or impacts occuring around 12.8 ka caused the Younger Dryas cooling, extensive wildfires, megafaunal extinctions and changes in human population. Evidence presented for this hypothesis consisted of peak concentrations of various markers found in sedimentological profiles taken across the Allerød- Younger Dryas boundary at several sites in North America and Europe. However, most reported Younger Dryas impact markers are not considered diagnostic evidence for impacts and subsequent studies failed to reproduce some of the results, leaving nanodiamonds as the most promising line of evidence two years after the Younger Dryas impact hypothesis had first been proposed. No unambiguous evidence for diagnostic shock indicators, such as shocked quartz, were reported. The main objective of the research presented in this thesis is to test the hypothesis that abrupt climate change at the Allerød-Younger Dryas transition was related to an extraterrestrial impact. Existing literature has been reviewed and multiple known Allerød-Younger Dryas boundary layers (mainly in Europe) were dated and sampled for the occurrence of nanodiamonds and shocked quartz. Most sites contained a palaeosol (either the Usselo or Finow palaeosol) or a thin peat layer. Although multiple sites have been investigated, both nanodiamonds and shocked quartz were found only in the Usselo horizon at Geldrop Aalsterhut, the Netherlands. These nanodiamonds were found in glass-like carbon, which is a known wildfire product. Transmission electron microscopy analsysis of charcoal and glass-like carbon of different ages, however, suggests that wildfires however, cannot serve as an alternative explanation for the presence of the nanodiamonds. Assuming that the glass-like carbon was formed during the same fire that created the radiocarbon dated charcoal in the layer, the nanodiamonds post-date the Allerød-Younger Dryas boundary. Only one shocked quartz grain was found. However, based on the current evidence it is not possible to determine whether this shocked quartz grain is related to the Younger Dryas impact event. In order to prove that a single impact event (multiple airbursts or just one impactor) caused the Younger Dryas, all markers found must be deposited synchronous and date to the onset of the Younger Dryas. This synchronous-site requirement presents several challenges to the Younger Dryas impact hypothesis. Most critically, there is an age discrepancy of up to two centuries between sites where Younger Dryas impact markers have been found. If the markers are not of the same age, they cannot be related to the same event. In conclusion, there is no convincing evidence for the occurrence of one or more climate changing extraterrestrial impacts or airbursts at the onset of the Younger Dryas at this point in time. The change in ocean circulation that has generally been held responsible for the Younger Dryas climate cooling thus probably had a more common trigger, possible inherent to the Earth’s system.