杏吧原创

Boston

THE idea that wobbles in the Earth鈥檚 axis caused the ice ages is on a
roll again. Strong evidence to support this 鈥渁stronomical theory鈥� has emerged,
despite the counter-argument that interglacial warming began before changes in
the orbit could have triggered it.

In last week鈥檚 Science (vol 276, p 782), Lawrence Edwards of the
University of Minnesota in Minneapolis reports that fossilised coral reefs on
Barbados marking peaks in sea level, which indicate the warmest spells between
ice ages, are 82 000, 102 000 and 126 000 years old.

These dates match the times when the northern hemisphere was warmer because
it was tilting more towards the Sun than usual. The tilt changes because of a
small periodic wobble of the Earth鈥檚 axis known as the Milankovitch cycle. 鈥淚n
all three instances we dated, the record was consistent with the astronomical
theory,鈥� says Edwards.

The new dates are fairly close to those obtained in earlier studies. But the
dating methods used in these studies did not have a high degree of accuracy.
Edwards鈥檚 group, however, used a new technique鈥攎easuring concentrations of
protactinium-231鈥攖hat cut down the margin of error by a factor of 10.

Researchers challenged the astronomical theory in 1992 when evidence from a
flooded fault system in Nevada, called the Devil鈥檚 Hole, suggested that the ice
age before last ended 140 000 years ago鈥攁 date that measurements by
Edwards have confirmed. This is much earlier than the astronomical theory
suggests, and Isaac Winograd of the US Geological Survey in Reston, Virginia,
says that this discrepancy has not been resolved.

But Edwards argues that the Nevada data may reflect local climate change,
caused by some unknown process. 鈥淚t seems to be out of step with the global
record,鈥� he says.