Of course, such broad claims will be difficult to assess and establish. Furthermore, they expect the largest impacts took place over the Laurentide ice sheet, reducing further the possibility that any large crater will be found. This should be interpreted to mean one or more large comet fragments, and therefore they envisage a scenario where an impact crater might be absent. It should be emphasized also that they claim the impact was caused by one or more large, low density extraterrestrial (ET) objects. (2007) are not claiming that the impact event itself wiped out many genera of megafauna or an entire human culture in an instant. Equally, it should be clear that Firestone et al. The proposal is that this impact triggered the Younger Dryas (YD) cooling and contributed to the end-Pleistocene megafaunal extinctions and human cultural changes. It is important to note that the main claim is for a major cosmic impact near 12.9 ka. , extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America. The shock wave, thermal pulse, and event-related environmental effects (e.g. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. Intense debate in this context is therefore expected.īefore describing this research debate in detail, it is necessary to first state the main claims of the hypothesis.
The impact theory also has far-reaching consequences for other cultural transitions, such as the end of the Clovis culture in North America, and for our understanding of Earth's local cosmic environment, for late Pleistocene megafaunal extinctions and the related issue of Earth's climate system. Indeed, its apparent occurrence at the dawn of the Palaeolithic - Neolithic transition within the Fertile Crescent of south west Asia is intriguing in the light of recent archaeological discoveries like Gobekli Tepe (Notroff et al., 2017 Notroff et al., 2016 Sweatman, 2019 Sweatman and Tsikritsis, 2017a Sweatman and Tsikritsis, 2017b). However, the possibility of such a large cosmic event on the timescale of human civilisation, just the last 13,000 years, is perhaps surprising. Now, globally important cosmic impacts on Earth are expected on the timescale of millions of years (Harris and D'Abramo, 2015). But in recent decades, with the discovery of many large impact craters on terrestrial planets and moons, including Earth and our own moon, and with the discovery of over 1000 large (>1 km) asteroids in near-Earth space, the situation has reversed. It was thought for many years to be resolved by Darwinian evolution and Hutton's uniformitarian geological principles, at least within the general scientific community. The debate surrounding catastrophism versus gradualism goes back at least as far as the great classical philosophers (Palmer, 2003). The occurrence of a global cosmic catastrophe, which the impact hypothesis suggests only slightly preceded the onset of human civilisation in the Fertile Crescent of south west Asia (as revealed by excavation of remarkable sites like Gobekli Tepe in this region), represents a paradigm-shift in understanding with profound consequences (Dietrich et al., 2012 Dietrich et al., 2017 Schmidt, 2012). Since its introduction in 2007, the Younger Dryas impact hypothesis (YDIH) has received considerable attention, and sparked heated debate (Firestone et al., 2007). Notably, arguments by a small cohort of researchers against their claims of a major impact are, in general, poorly constructed, and under close scrutiny most of their evidence can actually be interpreted as supporting the impact hypothesis. Their other claims are not yet confirmed, but the scale of the event, including extensive wildfires, and its very close timing with the onset of dramatic YD cooling suggest they are plausible and should be researched further. Evidence is mainly in the form of geochemical signals at what is known as the YD boundary found across at least four continents, especially North America and Greenland, such as excess platinum, quench-melted materials, and nanodiamonds. Fourteen years after this initial work the overwhelming consensus of research undertaken by many independent groups, reviewed here, suggests their claims of a major cosmic impact at this time should be accepted. BCE, triggered the Younger Dryas (YD) climate shift along with changes in human cultures and megafaunal extinctions.
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