Since the publication of Firestone et al. (1), numerous independent researchers have undertaken to replicate the results. Two groups were unable to confirm YDB peaks in spherules (6, 7), whereas seven other groups have confirmed them (*8–14), with most but not all agreeing that their evidence is consistent with a cosmic impact. Of these workers, Fayek et al. (8) initially observed nonspherulitic melted glass in the well-dated YDB layer at Murray Springs, Arizona, reporting “iron oxidespherules (framboids) in a glassy iron–silica matrix, which is one indicator of a possible meteorite impact…. Such a high formation temperature is only consistent with impact… conditions.”Similar materials were found in the YDB layer in Venezuela by Mahaney et al. (12), who observed “welded microspherules, brecciated/impacted quartz and feldspar grains, fused metallic Fe and Al, and…aluminosilicate glass,” all of which are consistent with a cosmic impact.
Some independent workers have been unable to reproduce earlier YDB results for MSp, CSp, and NDs (6–8), as summarized in a “News Focus” piece in Science (9), which claims that the YDB evidence is “not reproducible” by independent researchers. Refuting this view, multiple groups have confirmed the presence of abundant YDB markers, although sometimes proposing alternate hypotheses for their origin.
For example, Mahaney et al. (10–12) independently identified glassy spherules, CSps, high temperature melt-rocks, shocked quartz, and a YDB black mat analogue in the Venezuelan Andes. Those authors conclude the cause was “either an asteroid or comet event that reached far into South America” at 12.9 ka. At Murray Springs, Arizona, Haynes et al. (13) observed highly elevated concentrations of YDB MSp and iridium. Abundances of MSp were 340 × higher than reported by Firestone et al. (1) and iridium was 34 × higher, an extraordinary enrichment of 3,000 × crustal abundance. Those authors stated that their findings are “consistent with their (Firestone et al.’s) data.” In YDB sediments from North America and Europe, Andronikov et al. (2011) reported anomalous enrichments in rare earth elements (REE) and “overall higher concentrations of both Os and Ir [osmium and iridium]” that could “support the hypothesis that an impact occurred shortly before the beginning of the YD cooling 12.9 ka.”‡. Tian et al. (14)observed abundant cubic NDs at Lommel, Belgium, and concluded that “our findings confirm … the existence of diamond nanoparticles also in this European YDB layer.” The NDs occur within the same layer in which Firestone et al. (1) found impactrelated materials. Similarly, at a YDB site in the Netherlands, Van Hoesel et al. §observed “carbon aggregates [consistent with] nanodiamond.” Recently, Higgins et al. independently announced a 4- to 4.5-km-wide YDB candidate crater named Corossol in the Gulf of St. Lawrence, containing basal sedimentary fill dating to 12.9 ka. If confirmed, it will be the largest known crater in North and South America within the last 35 million years.
Because of the controversial nature of the YD impact debate, we have examined a diverse assemblage of YDB markers at Lake Cuitzeo using a more comprehensive array of analytical techniques than in previous investigations. In addition, different researchers at multiple institutions confirmed the key results.