Geoscience Reference
In-Depth Information
zero lay in that part of the world. Alan Hildebrand, then completing his Ph.D. at
the University of Arizona, scoured the literature for reports of unusual K-T depos-
its and descriptions of the circular geophysical patterns that might reveal a buried
impact crater. In May 1990, Hildebrand and William Boynton reported in
Science
that the only candidate their literature survey had turned up was a vaguely circular
structure lying beneath two to three kilometers of younger sediment in the Carib-
floor site probably could not have provided the continental grains and rock frag-
ments found in the boundary clay. In a throwaway final sentence, they noted that
in 1981 the geologists Glen Penfield and Antonio Camargo had reported circu-
lar magnetic and gravity anomalies from the northern Yucatan Peninsula and had
speculated that there, beneath younger sedimentary rocks, might lie a buried im-
pact crater.
Penfield's discovery of the K-T crater is another example of a young scientist
being in the right place at the right time. But as we have seen, that may not be
enough. The scientist's mind must be prepared to think the previously unthinkable
and recognize the previously unrecognizable. Even though still only in his twen-
ties, Penfield, a 1975 graduate of Oberlin College, had already gained experience
from magnetic surveys of volcanoes and volcanic features in Alaska and Mexico.
In the spring of 1978, as he was transferring the results of magnetic surveys over
the Yucatán to paper, he spotted an unusual set of concentric magnetic highs and
lows. The deeply buried structure had been accepted as volcanic by authorities on
the geology of the region, but Penfield knew that he was not looking at the mag-
nicus crater,” as he later described his revelation. He and Camargo presented the
idea at the 1981 meeting of the Society of Exploration Geophysicists, but until the
mention by Hildebrand and Boynton, no one seems to have noticed.
In a 1991 article in
Natural History
titled “Cretaceous Ground Zero,”
Hildebrand and Boynton took partial credit for the discovery: “In 1990, we, to-
gether with geophysicist Glen Penfield and other coworkers, identified a second
candidate for the crater. It lies on the northern coast of Mexico's Yucatán Penin-
sula, north of the town of Merida. The structure, which we named Chicxulub for
the small village at its center, is buried by a half mile of sediments.”
11
In a reply a few months later, Penfield took exception, noting that he had “iden-
tified” the structure in 1978 and reminding readers of the words with which he and
Camargo had closed their 1981 presentation: “We would like to note the proxim-
ity of this feature in time to the hypothetical Cretaceous-Tertiary boundary event
responsible for the emplacement of iridium-enriched clays on a global scale and
