The carbon-14 spike in the 8th century was not caused by a cometary impact on Earth

A mysterious increase of radiocarbon C-14 ca. 775 AD in the Earth's atmosphere has been recently found by Miyake et al. (Miyake, F., Nagaya, K., Masuda, K., Nakamura, T. [2012]. Nature, 486, 240). A possible source of this event has been discussed widely, the most likely being an extreme solar energetic particle event. A new exotic hypothesis has been presented recently by Liu et al. (Liu, Y. [2014]. Sc!. Rep., 4, 3728) who proposed that the event was caused by a cometary impact on Earth bringing additional C-14 to the atmosphere. Here we calculated a realistic mass and size of such a comet to show that it would have been huge (approximate to 100 km across and 10(17)-10(20) g of mass) and would have produced a disastrous geological/biological impact on Earth. The absence of an evidence for such a dramatic event makes this hypothesis invalid. (C) 2014 Elsevier Inc. All rights reserved.