A new study casts doubt on whether asteroid impacts led to extinctions.  It’s based on re-interpreting geological evidence used to identify impacts.  This finding, if sustained, would undermine the theory that an impact killed off the dinosaurs and a later impact led to the extinction of many large mammals.  Even more significant, an overturn of the impact hypothesis would illustrate that scientists are capable of going off on wrong tangents for decades.

The study by the U.S. Geological Survey, reported by PhysOrg, found that “impact markers,” such as “elevated levels of iridium, magnetic spherules, and titanomagnetite grains,” can form in wetlands and marshes called black mats.  The impact markers had been used to support the Cretaceous-Tertiary (K-T) extinction and a more “hotly contested” Younger Dryas Impact theory that led to extinction of the Clovis culture allegedly 12,900 years ago.  The new study published by Pigati et al in PNAS1 casts doubt on the uses of these markers to infer asteroid impacts.  The abstract states,

In this study, we investigated black mats ranging in age from approximately 6 to more than 40 ka in the southwestern United States and the Atacama Desert of northern Chile. At 10 of 13 sites, we found elevated concentrations of iridium in bulk and magnetic sediments, magnetic spherules, and/or titanomagnetite grains within or at the base of black mats, regardless of their age or location, suggesting that elevated concentrations of these markers arise from processes common to wetland systems, and not a catastrophic extraterrestrial impact event.

The “impact” of this re-interpretation goes beyond extinction theories:

“Luis and Walter Alvarez’s proposal that an extraterrestrial impact was responsible for extinctions at the Cretaceous-Tertiary boundary eventually moved from unlikely hypothesis to accepted theory, and with its acceptance came the temptation to apply this explanation to any rapid change in Earth’s conditions,” said USGS Director Marcia McNutt. “The results of this study demonstrate the importance of maintaining a healthy skepticism and multiple working hypotheses.”….

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