Younger Dryas impact hypothesis
The Younger Dryas impact hypothesis proposes that the onset of the Younger Dryas cool period at the end of the Last Glacial Period, around 12,900 years ago was the result of some kind of cosmic event with specific details varying between publications. The hypothesis is widely rejected by relevant experts. It is influenced by creationism, and has been compared to cold fusion by its critics due to the lack of reproducibility of results. It is an alternative to the long-standing and widely accepted explanation that the Younger Dryas was caused by a significant reduction in, or shutdown of the North Atlantic Conveyor due to a sudden influx of freshwater from Lake Agassiz and deglaciation in North America.
In 2007, the first YDIH paper speculated that an air burst caused by a comet hitting the atmosphere over North America created a Younger Dryas boundary layer; however, inconsistencies have been identified in other published results. Authors have not yet responded to requests for clarification and have never made their raw data available. Some YDIH proponents have also proposed that this event triggered extensive biomass burning, a brief impact winter that destabilized the Atlantic Conveyor and triggered the Younger Dryas instance of abrupt climate change which contributed to extinctions of late Pleistocene megafauna, and resulted in the disappearance of the Clovis culture.
Comet research group
The Comet research group, dedicated to investigating the YDIH, was established in 2016 by Allen West. Their stated mission is to "find evidence about comet impacts and raise awareness about them before your city is next."The credibility and motivations of individual CRG researchers have been questioned by critics of the impact hypothesis, including their specific claims for evidence in support of the YDIH and the effects of meteor air bursts or impact events on ancient settlements, people, and environments. Doubts have been raised about several of the CRG's other claims.; for example a 2021 paper suggested that a Tunguska-sized or larger airburst destroyed Tall el-Hammam, a Middle Bronze Age city located in the Jordan Valley near the Dead Sea around 1650 BCE. Image forensics expert Elisabeth Bik discovered evidence for digital alteration of images used as evidence for the claim that the village of Tall el-Hammam was engulfed by an airburst. CRG members initially denied tampering with the photos but eventually published a correction in which they admitted to inappropriate image manipulation. Five of the paper's 53 images received retouching to remove labels and arrows present in other published versions of the photos, which Bik believed to be a possible conflict with Scientific Reports
Supposed evidence
Proponents believe that certain microscopic debris is evidence of impact and that "black mats" of sediment are evidence of widespread fires. They contend that extinction of megafauna was synchronous with associated effects on prehistoric human societies. They say that their observations and interpretations cannot be adequately explained by volcanic, anthropogenic, or other natural processes. They argue that there is a synchronous Younger Dryas boundary layer that should be used as a local, or even global stratigraphic marker. Archaeologist Stuart J Fiedel has remarked that "The bolide and its effects have been characterized inconsistently from one paper to the next, which makes this hypothesis difficult to refute." In 2011, a review of the evidence led researchers to state "The YD impact hypothesis provides a cautionary tale for researchers, the scientific community, the press, and the broader public" as "none of the original YD impact signatures have been subsequently corroborated by independent tests. Of the 12 original lines of evidence, seven have so far proven to be non-reproducible. The remaining signatures instead seem to represent either non-catastrophic mechanisms, or terrestrial rather than extraterrestrial or impact-related sources. In all of these cases, sparse but ubiquitous materials seem to have been misreported and misinterpreted as singular peaks at the onset of the YD. Throughout the arc of this hypothesis, recognized and expected impact markers were not found, leading to proposed YD impactors and impact processes that were novel, self-contradictory, rapidly changing, and sometimes defying the laws of physics." Additionally, a comprehensive refutation of the Younger Dryas Impact Hypothesis was published in 2023, stating "There is no support for the basic premise of the YDIH that human populations were diminished, and individual species of late Pleistocene megafauna became extinct or were diminished due to catastrophe." Another example is that of extensive wildfires claimed by some YDIH proponents that has been refuted by experts. "Evidence and arguments purported to support the YDIH involve flawed methodologies, inappropriate assumptions, questionable conclusions, misstatements of fact, misleading information, unsupported claims, irreproducible observations, logical fallacies, and selected omission of contrary information."Hypothetical impact markers
Proponents 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 has never been verified and is disputed.Some scientists have asserted that the carbon spherules originated as fungal structures or insect fecal pellets, and contained modern contaminants and that the claimed nanodiamonds are actually misidentified graphene and graphene/graphane oxide aggregates. A patent application by Allen West and James Kennett in 2009 for methods of forming nanodiamonds based on research in support of the impact hypothesis also likely misidentified copper and copper oxides and appears to have since been abandoned. 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 did not show unique evidence for a bolide impact. An independent group of researchers reported much lower concentrations of platinum group metals in the purported boundary layer. The original authors argued that these concentrations were still >300% above background in 2 of their samples. Another group was unable to confirm prior claims of magnetic particles and microspherules in 2009. Other studies involving YDIH proponents found concentrations of magnetic spherules but not all were associated with the YDB and not all were attributed to an ET event.
"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. 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.Marlon et al. suggest that wildfires were a consequence of rapid climate change. "The changes in woody biomass, fire frequency, and biomass burning are not coincident with changes in CO2, although increasing CO2 may have contributed to woody biomass production during the early part of the Bølling–Allerød. Clovis people appeared in North America between 13.4 and 12.8 ka, broadly coincident with the sharp increase in biomass burning at 13.2 ka, and then rapidly spread out across the continent."
Radiocarbon dating, microscopy of paleobotanical samples, and analytical pyrolysis of fluvial sediments in Arlington Canyon on Santa Rosa Island by another group found no evidence of lonsdaleite or impact-induced fires. Research published in 2012 has shown that the so-called "black mats" are easily explained by typical earth processes in wetland environments. This study of black mats, that are common in prehistorical wetland deposits which represent shallow marshlands, that were from 6000 to 40,000 years ago in the southwestern USA and Atacama Desert in Chile, showed elevated concentrations of iridium and magnetic sediments, magnetic spherules and titanomagnetite grains. It was suggested that because these markers are found within or at the base of black mats, irrespective of age or location, they likely arise from processes common to arid-climate wetland systems and not as a result of catastrophic bolide impacts.
Researchers have also criticized the conclusions of various studies for incorrect age-dating of the sediments, contamination by modern carbon, inconsistent hypothesis that made it difficult to predict the type and size of bolide, lack of proper identification of lonsdaleite, confusing an extraterrestrial impact with other causes such as fire, and for inconsistent use of the carbon spherule "proxy". Naturally occurring lonsdaleite has also been identified in non-bolide diamond placer deposits in the Sakha Republic.