Younger Dryas impact hypothesis

The Younger Dryas impact hypothesis (YDIH) or Clovis comet hypothesis is a speculative attempt to explain the onset of the Younger Dryas (YD) as an alternative to the long standing and widely accepted cause due to a significant reduction or shutdown of the North Atlantic "Conveyor" in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. The YDIH posits that fragments of a large (more than 4 kilometers in diameter), disintegrating asteroid or comet struck North America, South America, Europe, and western Asia around 12,850 years ago, coinciding with the beginning of the Younger Dryas cooling event. Multiple meteor air bursts and/or impacts are said to have produced the Younger Dryas boundary layer (YDB), depositing peak concentrations of platinum, high-temperature spherules, meltglass, and nanodiamonds, forming an isochronous datum at more than 50 sites across about 50 million km² of Earth's surface. Some scientists have proposed that this event triggered extensive biomass burning, a brief impact winter and the Younger Dryas abrupt climate change, contributed to extinctions of late Pleistocene megafauna, and resulted in the end of the Clovis culture. A group known as the Comet Research Group are the primary advocates for the impact hypothesis, though other groups have published supporting evidence. The YDIH remains a minority and disputed view and has failed to gain acceptance by the mainstream planetary impact and paleoclimate communities.

Background

Template:MainA number of theories have been put forward about the cause of the Younger Dryas climate event. The most widely accepted is that it began because of a significant reduction or shutdown of the North Atlantic "Conveyor" – which circulates warm tropical waters northward – as the consequence of deglaciation in North America. Geological evidence for such an event is not fully secure, but recent work has identified a pathway along the Mackenzie River that would have spilled fresh water from Lake Agassiz into the Arctic and thence into the Atlantic. The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the North Atlantic.

An alternative theory suggests instead that the jet stream shifted northward in response to the melting of the North American ice sheet, which brought more rain to the North Atlantic, which freshened the ocean surface enough to slow the thermohaline circulation. Another proposed cause has been volcanic activity.However, this has been challenged recently due to improved dating of the most likely suspect, the Laacher See volcano. In 2021, research by Frederick Reinig et al. precisely dated the eruption to 200 ± 21 years before the onset of the Younger Dryas, therefore ruling it out as a culprit. firmly date the [Laacher See eruption] to 13,006 ± 9 calibrated years before present (BP; taken as AD 1950), which is more than a century earlier than previously accepted. ...thereby dating the onset of the Younger Dryas to 12,807 ± 12 calibrated years BP, which is around 130 years earlier than thought. The same study also concluded that the onset took place synchronously over the entire North Atlantic and Central European region. A press release from the University of Mainz stated, "Due to the new dating, the European archives now have to be temporally adapted. At the same time, a previously existing temporal difference to the data from the Greenland ice cores was closed."That is 126 years earlier than the generally accepted dating based on sediments in the Meerfelder Maar from the Eifel region in Germany. ... This difference has far-reaching consequences for the synchronization of European climate archives and the understanding of North Atlantic and European climate history. ... This means that the [onset of the Younger Dryas] also occurred in Central Europe 130 years earlier, around 12,870 years ago respectively. This is in line with the onset of the cooling in the North Atlantic region identified in ice cores from Greenland. ... 'This strong cooling did not take place time transgressively, as previously thought, but rather synchronously over the entire North Atlantic and Central European region,' said Frederick Reinig.

In contrast, proponents of the impact hypothesis posit that fragments of a large disintegrating asteroid or comet struck the earth around 12,850 years ago, causing the Younger Dryas cooling event. They also hypothesize that the impact event triggered extensive biomass burning, a brief impact winter, and an abrupt climate changeTemplate:Sfnp which, they contend, directly brought about the extinction of many species of North American Pleistocene megafaunaTemplate:Sfnp including camels, mammoths, and the giant short-faced bear. They also say the event contributed to the transition from Clovis culture to subsequent traditions.

Evidence

Meltglass from Abu Hureyra

Examples of meltglass from Tell Abu HureyraTemplate:Hair space

Proponents have offered as evidence for the impact event microscopic structures (spherules), "black mats" of sediment they contend are evidence of widespread fires, their proposed dates for the Hiawatha crater in Greenland in the range of 12,800 years agoTemplate:Sfnp (though later dates indicated an event from 55 million years ago), and the synchronous extinction of megafauna and associated impacts on prehistoric human societies. Proponents of the hypothesis say that these data cannot be adequately explained by volcanic, anthropogenic, or other natural processes. They argue that the Younger Dryas boundary layer should be used as a local, or even global stratigraphic marker.

Impact debris

Proponents, most of whom are scientists, have reported materials including nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules, iridium, platinum, platinum/palladium ratios, charcoal, soot, and fullerenes enriched with helium-3 that they interpret as evidence for an impact event that marks the beginning of the Younger Dryas. One of the most widely publicized discoveries (nanodiamonds in Greenland) has never been verified and is disputed.

Some scientists have asserted that the carbon spherules originated as fungal structures and/or insect fecal pellets, and contained modern contaminants and that the claimed nanodiamonds are actually misidentified graphene and graphene/graphane oxide aggregates. Iridium, magnetic minerals, microspherules, carbon, and nanodiamonds are all subject to differing interpretations as to their nature and origin, and may be explained in many cases by purely terrestrial or non-catastrophic factors. An analysis of a similar Younger Dryas boundary layer in Belgium yielded carbon crystalline structures such as nanodiamonds, but the authors concluded that they also did not show unique evidence for a bolide impact. One group of researchers also reported they were unable to replicate platinum group metals in the boundary layer,Template:Efn despite reporting enhanced Iridium concentrations up to >300% of background in 2 of their samples. Another group was unable to confirm prior claims of magnetic particles and microspherules in 2009, though all subsequent studies successfully replicated them, including multiple independent teams.

"Black mats"

The evidence given by proponents of a bolide or meteorite impact event includes "black mats", or strata of organic-rich soil that have been identified at about 50 archaeological sites across North America.Template:Efn Using statistical analysis and modeling, James P. Kennett and others concluded that widely separated organic-rich layers, including black mats, were deposited synchronously across multiple continents as an identifiable Younger Dryas boundary layer. In 2019, Jorgeson and others tested this conclusion with the simulation of radiocarbon ages. They accounted for measurement error, calibration uncertainty, "old wood" effects, and laboratory measurement biases, and compared against the dataset of radiocarbon ages for the Laacher See eruption. They found the Laacher See 14C dataset to be consistent with expectations of synchroneity. They found the Younger Dryas boundary layer 14C dataset to be inconsistent with the expectations for its synchroneity, and the synchronous global deposition of the hypothesized Younger Dryas boundary layer to be extremely unlikely.