| Abstract: Throughout the Cenozoic the Earth
experienced global cooling that led to the appearance of continental
glaciers in high northern latitudes around 3 Ma ago. At approximately
the same time, cold surface waters first appeared in regions that today
have intense oceanic upwelling: the eastern equatorial Pacific and the
coastal zones of southwestern Africa and California. There was
furthermore a significant change in the Earth's response to Milankovich
forcing: obliquity signals became large, but those associated with
precession and eccentricity remained the same. The latter change in the
Earth's response can be explained by hypothesizing that the global
cooling during the Cenozoic affected the thermal structure of the ocean;
it caused a gradual shoaling of the thermocline. Around 3 Ma the
thermocline was sufficiently shallow for the winds to bring cold water
from below the thermocline to the surface in certain upwelling regions.
This brought into play feedbacks involving ocean-atmosphere interactions
of the type associated with El Niņo and also mechanisms by which
high-latitude surface conditions can influence the depth of the tropical
thermocline. Those feedbacks and mechanisms can account for the
amplification of the Earth's response to periodic variations in
obliquity (at a period of 41K) without altering the response to
Milankovich forcing at periods of 100,000 and 23,000 years. This
hypothesis is testable. If correct, then in the tropics and subtropics
the response to obliquity variations is in phase with, and corresponds
to, El Niņo conditions when tilt is large and La Niņa conditions when
tilt is small. |