Alvarez team read an iridium layer as the fingerprint of the dinosaur-killing asteroid

By 1980 science knew roughly when the dinosaurs died — around 65 million years ago, at the boundary between the Cretaceous and Tertiary — but not why. The question had been argued for a century in prose: gradual decline versus sudden catastrophe, with almost no physical evidence to referee. Fossils said that something happened; the rocks would not yet say what.

A physicist and a geologist decided to interrogate the clay. Luis Alvarez, Nobel laureate at Berkeley, teamed with his son Walter, a geologist, and nuclear chemists Frank Asaro and Helen Michel, whose neutron activation analysis could count vanishingly small amounts of platinum metals. Deep-sea limestones exposed in Italy, Denmark and New Zealand span the extinction layer, and in all three the iridium concentration jumped to about 30, 160 and 20 times the background level, exactly at the extinction horizon, 65 million years ago.

The interpretation made the measurement speak. Platinum metals are scarce in Earth's crust but abundant in meteorites, so the only way to lay down that much iridium at once is a burst of extraterrestrial material; the paper also showed it could not have come from a nearby supernova. Their hypothesis: a large earth-crossing asteroid struck the Earth, vaporizing into a cloud that injected about 60 times the object's mass as pulverized rock; a fraction stayed in the stratosphere for years, darkening the sky worldwide — and the extinctions followed.

The work is a model of how to convert a hunch into science: the team weighed alternatives and excluded them quantitatively, and the hypothesis came with numbers attached — impactor size, global dust layer, and an impact crater on the order of 100 kilometres that geologists should be able to find.

Read meteoritic iridium as a crime-scene fingerprint

The tracer was chosen so that only an extraterrestrial source could explain a spike: crustal processes could not concentrate iridium like that.

One measurement did two jobs — the layer marked the extinction instantaneously (a clock) and betrayed the projectile's origin (a fingerprint).

Borrowed instrument capability, neutron activation analysis, let the team see parts-per-trillion concentrations that no previous extinction study could measure.

Every claim carried a number — 30, 160, 20 times background; 60 times the impactor's mass in dust — and each number could be checked at new sites.

When a debate has run on narrative for decades, look for the quantitative trace only your instrument can see. One geochemical fingerprint turned a century of speculation into a testable cause.

The hypothesis split the field for a decade, but its predictions kept landing: shocked quartz, glassy impact ejecta and a global soot layer turned up in the boundary clay, and researchers later identified the Chicxulub crater on Mexico's Yucatán Peninsula as the likely crater the paper had implicitly promised. The impact scenario moved from heresy to the mainstream explanation of the end-Cretaceous extinctions, and geochemical fingerprints of this kind became standard tools for reading catastrophes in the rock record.