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.”
Paul Baker of Duke University tried to put a positive spin on the paradigm-shifting study: “This is a great object lesson for how scientific hypotheses are done and undone.” But unless hypotheses get better and more accurate over time, this instance could be a tragic story of a waste of scientific energy that went on for decades. It also implies that those seeking to understand the causes of extinctions will have to start over at square one.
1. Pigati et al, “Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis,” PNAS April 23, 2012, doi: 10.1073/pnas.1200296109.
How many documentaries have been made about the Alvarez impact scenario? It was the grand finale of the BBC’s popular Walking with Dinosaurs, for sure. The impact theory may not be dead yet; undoubtedly critics will have a rebuttal to the USGS study. Even so, the “healthy skepticism and multiple working hypotheses” advice has been sadly lacking in the news media and on science TV as many jumped on the impact-theory bandwagon without considering the impact on science’s credibility should the popular hypothesis be overturned. What do the popular-media producers care? They made their money and some retired. The losers are students who grow up believing whatever scientists tell them and what animators “show” them, but don’t get to hear “the rest of the story” till too late if at all.