Past ice core work has focused on the recent ice age, a time when the climate fluctuated
rapidly between mild and cool periods.
The researchers found that these spreading rates have fluctuated
a number of times over the last 20 million years, causing the tectonic plates to speed up and slow down as they move away from the ridges.
During the 1990s enplanement levels have fluctuated
, with overall growth of just 0.