Platinum Group Elements, Shocked Quartz, and Nanodiamonds in Greenland Ice Consistent with an Extraterrestrial Impact at the Younger Dryas Onset 12,800 Years Ago: Supporting Information

The cause of the Younger Dryas (YD) cooling event (~12,800 years ago) and its possible connection to a cosmic impact continue to be debated. Here, we present geochemical evidence from a late Pleistocene Greenland ice sheet record (Bölling-Ållerød through Holocene) that supports an extraterrestrial airburst/impact at the YD onset, previously identified in the sampled interval using dust stratigraphy, oxygen isotopes, and a nanodiamond peak. A platinum peak was also previously reported in the GISP2 ice core at the YD onset. These previous results are reinforced here by peaks in nine impact-related proxies: (1) nanodiamonds (cubic, n-diamond, i-carbon, and lonsdaleite); (2) shocked quartz grains, the first documented occurrence preserved within polar ice; (3) meltglass and microbreccia; (4) Fe- and Si-rich spherules; (5) carbon spherules; (6) glasslike carbon; (7) charcoal/soot; (8) a dark, dust-rich layer; and (9) PGEs (Pt, Ir, Pd, Os, Ru, Rh) with Ni and Co. The PGEs were revealed by the breakthrough use of single-particle inductively coupled plasma time-of-flight mass spectrometry to identify mono- and multi-element nanoparticles in a bimodal dust population that peaks just after the YD onset (~5.3 m along the sloping ice), suggesting continuous influx from terrestrial sources, combined with episodic influx from an ET impact event. Geochemical ratios, including elevated Ni/Fe, Ir/Al, and Pt/Al, also support an extraterrestrial origin. These materials collectively indicate exposure to extreme pressures and temperatures. We also conducted hydrocode modeling of a high-velocity cometary impact onto such an ice sheet, capable of producing shocked quartz, melt products, and widespread nanoparticle deposition. The potential formation by volcanism, biomass burning, authigenesis, or anthropogenesis is inconsistent with the evidence, which, alternatively, is consistent with investigations at more than 50 YDB sites across North America, South America, Europe, and western Asia. Together, the evidence supports an airburst/impact that triggered abrupt climate change at the Younger Dryas onset.