It's a bit like the iridium spike at the K-T boundary in that the use of nuclear weapons is an event that will have a worldwide signal, in fact it wouldn't surprise me if they got the idea from the asteroid impact. This would be a bit ironic because Alvarez, the guy who discovered the impact, was a Manhattan project alum who actually worked on the explosive lenses and triggers used in the Trinity implosion bomb.
The issue with using Trinity is that from a biological/evolutionary standpoint its not that meaningful an event. The Chicxulub impact is a huge deal, it's the driver of the biggest mass extinction in 250 million years. The Trinity test has the advantage of being easy to measure but nuclear weapons have had pretty much zero effect on the biosphere. In fact, primitive hunter-gatherers running around with fire and spears have a vastly larger effect than nuclear bombs. After Homo sapiens moves out of Africa into Australia, Europe, and the Americas, we see massive dieoffs of the megafauna which, combined with the use of fire to alter the landscape, dramatically alter the fauna and vegetation on a continental scale. From an evolutionary standpoint, these migrations are important; they mark the first time the species began to alter the world on the level of entire ecosystems. So I'd argue that the migration of Homo sapiens out of Africa would be the defining event, but obviously that's kind of hard to date.
