Greenland Pt Anomaly May Point to Noncataclysmic Cape York Meteorite Entry
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LETTER LETTER Greenland Pt anomaly may point to noncataclysmic Cape York meteorite entry Petaev et al. (1) tested the suite of hypotheses velocity and entry angle yields a crater of simplest explanation is to associate the anom- (collectively known as the “impact hypothe- between 15 and 20 km in diameter, unlikely aly with a local noncataclysmic event that is sis”) that a swarm of impacts or airbursts to have eluded discovery. Petaev et al. (1) also independently known to have taken place, from comets, chondritic, or stony asteroids suggest that the anomaly could be explained and that would have produced Pt-rich fallout: caused an abrupt climate change, continen- by multiple impacts of smaller iron meteor- the Cape York meteorite fall. tal-scale wildfires, mass extinctions, and col- ites, like Sikhote-Alin, a crater-forming clus- lapse of the Clovis culture at or near the ter in 1947 with a total recovered mass of ACKNOWLEDGMENTS. This work was supported in ∼ part by Sandia National Laboratories. Sandia is a multi- Younger Dryas Boundary (YDB). The authors 23 tons (3). A global Pt anomaly would program laboratory operated by Sandia Corporation, a identify a large Pt anomaly in the Green- require on the order of 105 such impacts. Lockheed Martin Company, for the US Department of land Ice Sheet Project 2 (GISP2) core and Widely dispersed impact swarms of this scale Energy, under Contract DE-AC04-94AL85000. suggest that it hints at an extraterrestrial have never been observed and no physical 1 source. Because there is no corresponding model has been proposed for their formation. Mark Boslough Sandia National Laboratories, Albuquerque, Ir spike, Petaev et al. challenge the impact A hypothetical cluster from a highly improb- hypothesis by proposing a highly fraction- able iron asteroid encounter is not the sim- NM 87185 ated iron meteorite. The Pt anomaly pre- plest explanation, and there are many other dates ammonimum and nitrate peaks in probabilistic and physical problems with 1 Petaev MI, Huang S, Jacobsen SB, Zindler A (2013) Large Pt the GISP2 core by decades, eliminating the a globally catastrophic cluster impact (4). anomaly in the Greenland ice core points to a cataclysm at the possibility that the source event triggered Relaxing the global assumption leads to onset of Younger Dryas. Proc Natl Acad Sci USA 110(32): wildfires, as proposed by the impact hypoth- a much more likely hypothesis. Cape York is 12917–12920. 2 NASA Near-Earth Object Science Definition Team (2003) Study to esis. The authors rightly argue that it would an iron meteorite from western Greenland Determine the Feasibility of Extending the Search for Near-Earth not generate a wildfire-producing airburst with a total recovered weight of 58.2 tons (3). Objects to Smaller Limiting Diameters, Office of Space Science (Solar System Exploration Division, Washington, DC). anyway because iron meteorites do not de- A terrestrial age of 12.9 ka cannot be ruled 3 Meteoritical Society (2005) Meteoritical Bulletin Database. posit enough energy in the atmosphere for out (5). Whereas it is true that the Pt/Ir ratio Available at www.lpi.usra.edu/meteor/metbull.php?code=5262. a cataclysmic airburst over a wide area. in the GISP2 anomaly is inconsistent with Accessed November 26, 2013. 4 Boslough M, et al. (2012) Arguments and evidence against By assuming a global anomaly, Petaev et al. Cape York, there are no data on hetero- a Younger Dryas impact event. Climates, Past Landscapes, and (1) calculate an iron meteorite ∼0.8 km in geneity of the pre-entry body, nor on Pt/Ir Civilizations, Geophysical Monograph Series, eds Giosan L, et al. diameter. This is an extremely rare event, re- fractionation because of ablation, conden- (American Geophysical Union, Washington, DC), Vol 198, pp 13–26. 5 Nishiizumi K, et al. (1987) Terrestrial ages of Antarctic and curringontimescalesoftensofmillionsof sation, and precipitation. The YDB is sur- Greenland meteorites. Meteoritics 22:473. years based on observed asteroid populations roundedbymanyotherspikes,including (2) and the fraction of meteorites that are the Laacher See eruption, an ammonium iron. The authors also suggest that an impact spike, and three large volcanogenic sulfate Author contributions: M.B. designed research, performed research, of that size would be expected to form a crater spikes. There is no reason to favor the Pt and wrote the paper. of a few kilometers in diameter. However, event as the trigger of the YDB climate The author declares no conflict of interest. a representative range of assumptions for change or its putative consequences. The 1E-mail: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1320328111 PNAS | December 24, 2013 | vol. 110 | no. 52 | E5035 Downloaded by guest on September 29, 2021.