Executive summary: During the last glacial maximum, much of North America, extending into the present-day United States, was covered in glaciers, ice and snow, and deciduous forests stretched much farther south than they do today. These "ice age" conditions were reversed when the climate suddenly began to warm an estimated 12,000 or so years ago, at which time mass extinctions of large animals such as mammoths and saber-toothed tigers began to occur. The reasons for this change in climatic conditions have long been debated, and in this paper (published in the University of Chicago's Journal of Geology but with contributions from various other institutions), a number of collaborating researchers provide geological evidence in the form of a layer of nano-diamonds (created by immense heat and pressure) found across a wide swathe of the northern hemisphere (see image below), that the Earth was impacted by a celestial body which catastrophically altered the global climate.
A major cosmic-impact event has been proposed at the onset of the Younger Dryas (YD) cooling episode at ≈12,800 ± 150 years before present, forming the YD Boundary (YDB) layer, distributed over >50 million km2 on four continents. In 24 dated stratigraphic sections in 10 countries of the Northern Hemisphere, the YDB layer contains a clearly defined abundance peak in nanodiamonds (NDs), a major cosmic-impact proxy. Observed ND polytypes include cubic diamonds, lonsdaleite-like crystals, and diamond-like carbon nanoparticles, called n-diamond and i-carbon. The ND abundances in bulk YDB sediments ranged up to ≈500 ppb (mean: 200 ppb) and that in carbon spherules up to ≈3700 ppb (mean: ≈750 ppb); 138 of 205 sediment samples (67%) contained no detectable NDs. Isotopic evidence indicates that YDB NDs were produced from terrestrial carbon, as with other impact diamonds, and were not derived from the impactor itself. The YDB layer is also marked by abundance peaks in other impact-related proxies, including cosmic-impact spherules, carbon spherules (some containing NDs), iridium, osmium, platinum, charcoal, aciniform carbon (soot), and high-temperature melt-glass. This contribution reviews the debate about the presence, abundance, and origin of the concentration peak in YDB NDs. We describe an updated protocol for the extraction and concentration of NDs from sediment, carbon spherules, and ice, and we describe the basis for identification and classification of YDB ND polytypes, using nine analytical approaches. The large body of evidence now obtained about YDB NDs is strongly consistent with an origin by cosmic impact at ≈12,800 cal BP and is inconsistent with formation of YDB NDs by natural terrestrial processes, including wildfires, anthropogenesis, and/or influx of cosmic dust.
The paper was published in the Journal of Geology, 122, no. 5 (September 2014):475-506, with the following researchers contributing:
Charles R. Kinzie, Shane S. Que Hee, Adrienne Stich, Kevin A. Tague, Chris Mercer, Joshua J. Razink, Douglas J. Kennett, Paul S. DeCarli, Ted E. Bunch, James H. Wittke, Isabel Israde-Alcántara, James L. Bischoff, Albert C. Goodyear, Kenneth B. Tankersley, David R. Kimbel, Brendan J. Culleton, Jon M. Erlandson, Thomas W. Stafford, Johan B. Kloosterman, Andrew M. T. Moore, Richard B. Firestone, J. E. Aura Tortosa, J. F. Jordá Pardo, Allen West, James P. Kennett, and Wendy S. Wolbach.
The full paper can be downloaded as a PDF from this website.