World's End | Illumination

World's End. Story by Charles E. Reineke.

And so it was. The demarcation of the boundary was obvious even before researchers began preparing the sample for analysis, MacLeod says. "It was just gorgeous."

Thanks to the lack of K/T boundary specialists on board, the first opportunity to analyze the samples fell to MacLeod, the researcher whose area of expertise most closely matched the K/T boundary time frame.

"Getting the boundary was, well, I wouldn't say luck, it was always there, and when you drill through exciting intervals you might get exciting samples," he says. "But it was unexpected."

Because of their remarkable clarity, the cores provide an unusually clear picture of the events at the time of the mass extinction. "With our samples, there just aren't many complications to confuse interpretation," MacLeod says, holding up for view the key sample itself. It's a narrow strip of sedimentary rock clearly delineated by distinct fields of color, now encased in protective plastic.

"The key fossil taxa, those that are used to identify the youngest Cretaceous and the oldest Tertiary, are present in the section right here, bracketing on either side a single layer that can be convincingly tied back to an impact."

MacLeod next explains how, at a point some two-and-a-half meters below this line, one can see the first appearance of a specific type of foraminifera, tiny organisms that he describes as "essentially an amoeba with a shell." The leader of the multi-impact hypothesis camp, he says, has argued that this species of foraminifera appeared around 300,000 years before the boundary event.

"We've now got a continuous record, from before this foraminifera appeared all the way up to the impact," MacLeod says. "The multi-impacters say that there was a first impact, one that they tie to the Chicxulub crater in the Yucatan, some 300,000 years before the boundary event. We can say, 'Look, we've got the fossil that tells us this two and half meters of core was deposited over the last 300,000 years of the Cretaceous. And you can see there is absolutely no evidence of impact until you get to this [boundary] level. And then there is a wholesale extinction.'"

Published by the Office of Research.