 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.Using statistical analysis and modeling, Kennett and others concluded that widely separated organic-rich layers, including "black mats", were deposited synchronously across multiple continents as an indentifiable "Younger Dryas boundary layer. "