Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 119

Such isolated areas are rare outliers within today's well-ventilated
oceans. But such areas were once expanded enormously, to cover
large parts of the globe, at times when the oceans' circulatory system
worked less well than it does today. All it takes to visit them is the
time machine afforded by rock strata and then one can, almost liter-
ally, walk on those surreal sea floors of the deep past.
Hothouse Oceans
We are living today on a kind of world that represents a minority of
Earth time. Over most of our planet's history the Earth has had little
or nothing in the way of polar icecaps. Take, for instance, the Creta-
ceous Period, around 100 million years ago. The tropics then were
somewhat warmer than today—perhaps more than 10 degrees Cel-
sius warmer at times—but the high latitude regions could be as much
as 50 degrees warmer. Instead of ice and tundra, there were polar for-
ests inhabited by polar dinosaurs adapted to six months of continu-
ous daylight and six months of darkness. The temperature gradients
between pole and equator, therefore, were much lower than at present,
and it is still a little mysterious quite how the heat was transported so
effectively from low to high latitudes.
On such a world, there is little chance of forming large volumes of
cold deep waters by sea ice formation, or by cooling with freezing
winds to power the oceanic conveyor belt. These oceans, therefore,
must have been driven by forces different to the ones operating today.
Quite how did they work?
The great palaeo-oceanographer Bill Hay has spent a good deal of his
long and colourful career, which has oscillated between post-war
Europe and North America, puzzling over this problem. It was an
ocean system which, he observed, is utterly unfathomable if one fol-
lows Charles Lyell's dictum that the present is the key to the past. For so
much then, besides global ice volume and temperature, was different.