Kinzie et al (2014) claim that nanodiamonds are indicators of cosmic impacts and look at the evidence of nanodiamonds at the start of the Younger Dryas at two dozen sites across three continents. They find them, and argue that a cosmic impact is the only explanation.
The paper is dependent on the quality of the chronologies used to date the nanodiamond containing layers – if they are not synchronous the hypothesis of a global catastrophe fails. Some of the chronologies are woeful, for example, the age-depth model from Lake Cuitzeo. This age-depth model is based on only six radiocarbon dates over the last 30,000 years (that would be a low number for a Holocene-length core); the dates are on bulk sediment, so are potentially affected by radiologically-dead volcanic-carbon; many outliers are rejected; and one of these dates is not from the lake core, but from a nearby trench that has been stratigraphically linked to the lake core. This is not the most robust age depth model.
Rather than examining all the age-depth models, I want to look at something that caught my attention in the abstract.
Isotopic evidence indicates that YDB NDs [nanodiamonds] were produced from terrestrial carbon, as with other impact diamonds, and were not derived from the impactor itself.
Isotopic evidence from nanodiamonds! Someone must either have extracted and purified a lot of nanodiamonds or have a very sensitive mass-spectrometer. I want to know more.
More details in the body of the paper
YDB NDs were most likely formed from terrestrial carbon, based on their carbon isotopic composition (Tian et al. 2011; Israde-Alcántara et al. 2012b)
This line of evidence is important as Kinzie et al (2014) use the terrestrial origin to exclude the possibility that the nanodiamonds are the product of cosmic dust, and so must be the result of a high energy event on Earth.
Tian et al. (2011) examine a site in Belgium and show that the black layer supposed to mark the start of the Younger Dryas has carbon isotopic values consistent with a terrestrial source. They explicitly state that they did not measure the isotopic composition of the nanodiamonds.
Also carbon isotope measurements and C/N values were determined from the black material of the Lommel YDB layer. The nanodiamond particles in the present material could not be analyzed separately because of their small size.
Israde-Alcántara et al. (2012) do not measure the isotopic composition of nanodiamonds either. Instead they measure the isotopic composition of bulk sediment.
Sediment samples of approximately 1 cm thickness were taken every 5 cm across the critical section between 2.80 and 2.65 m and at 10 cm intervals above and below this section. These samples were quantitatively analyzed for diatoms and pollen assemblages, carbonate (%TIC), organic carbon (%TOC), bulk major-element composition, stable carbon isotopes (both organic and inorganic), organic nitrogen, MSp, NDs, CSp, charcoal, and aciniform soot.
They do, however, mis-cite Tian et al.
Cosmic NDs occur in meteorites and cosmic dust, but Tian et al. concluded that YDB NDs are not cosmic because they display δ13C abundances (−28.1 to −26.3‰) that are terrestrial.