Independent investigators have failed to confirm the reports of enhanced concentrations of spherules and platinum-group elements in YD boundary sediments.
Boslough, Surovell et al., January 30, 2013, Arguments and Evidence Against a Younger Dryas Impact Event
Interhemispheric evidence of a cosmic impact 12.9 ka is known now from North to South America, Europe and Eurasia, all data supporting a cosmic event derived from cores and from geological sections. Most databases supporting the impact hypothesis at the Younger Dryas Boundary (YDB) rely on high-temperature microspherules, melted minerals, cracked clasts, presence of nanodiamond, high-temperature scoria, high Fe/Ni ratios, pdf’s, shock melted quartz, high 10 Be/9 Be ratios and occasional presence of platinum metals. Controversy over the impact, the so-called Black Mat enigma, and its relation to the Younger Dryas re-advance at the end of the last ice age, is fueled by arguments over whether a single extraterrestrial impact might sustain a 1 kyr-long downturn in insolation engendering a substantial increase in worldwide icevolume. New evidence in the form of impact microfeatures extreme breccia, high crack propagation,thick carbon encrustations and partial to full shock-melted/contorted grains in weathering rinds from the Western Alps, France, as documented here, adds to the growing body of evidence that the event was truly widespread, if not worldwide in effect. Whereas evidence of cosmic impacts may be erased by glacial and ﬂuvial erosion in high alpine areas, such events as demonstrated herein are recorded as punctuated time-stratigraphic events in microcosm, preserved in weathered clasts.
I was pleased to see the piece concerning the continuing controversy over the black mat. I tend to support Napier’s contention [See Napier: Not So Fast Bos; Tusk, Cosmic] that the radial spread of samples across North and South America, parts of Europe and Central Asia suggest fragmental airbursts of variable magnitude such that records in aquifers, glacial deposits, outlet glaciers, paleosols and weathering rinds are showing up in a variety of places.
At other investigated sites similar to those already reported for the MUM7B site in Venezuela, there is a wide range of well-documented evidence, consisting of a variable mix of magnetic spherules, microtektite-like glassy spherules, partly devitrified glass shards, melted sedimentary and igneous clasts, native metal, grape-clustered glassy spheroids, glass-like carbon, and/or amorphous carbon spherules (Ge et al., 2009; Bunch et al., 2012; Wittke et al., 2013). This evidence is all part of dated YD-aged sedimentary beds in the U.S., Canada, Mexico, Peru, France, Syria, and the Caspian Sea of Central Asia that appear to be consistent with the breakup of a cosmic impactor and subsequent ejecta dispersal of interhemispheric proportions. The known YDB sites in North America, South America, Western Europe, and Central Asia display a thin bed of ejecta-rich debris, which forms a well-dated microfacies that typically includes charcoal, ash, and carbonized local vegetation resulting from high-temperature wildfires, suggestive of the high-temperature ionizing cloud that would accompany an impact. Whether the YDB actually initiated the YD is still speculative but the relationship continues to firm up in various parts of the world.
I think Dr. Scott’s comments about the YD hypothesis reaching a ‘zombie’ status does little to assist in interpreting the available evidence as the databank is growing. New evidence of undisputed impact origin for aerodynamically melted/quenched Fe spherules from the MUM7B Andean site (in press, J.Geology) are a case in point as are new data from the Alps that show impact grains in rock rinds and associated paleosols (pdf attached). I hope these comments will help others to carefully weigh new evidence as it comes on stream. In the end I think the YDB will go down as the initiator of the YD and the longevity of the YD may well be shown to result from Earth’s continued encounter with Taurid fragments over a 1 kyr interval or an unusual sustenance of a negative radiation balance over a similar span of time. In the end the naysayers may well say they believed the impact theory was tenable from the beginning. Agassiz went through a similar turbulent time with his glacial theory nearly two centuries ago.