Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 121

would be more sluggish. Each of the polar regions, instead of being
permanent mirrors of ice to reflect the Sun's light and heat, would
have alternately warmed and cooled in their winters and summers,
destabilizing the Hadley cell wind patterns, and without the constant
tug of steady wind patterns there would not be a modern-day organ-
ized oceanic conveyor belt of deep and surface currents demarcating
central gyres.
Instead, there would have been a more sluggish and chaotic pattern
of circulation. Some movement of water downwards, to ventilate the
deep ocean, might have occurred through the modest formation of
dense sinking water masses generated by evaporation in arid climate
belts. Perhaps most ventilation was achieved through large storms,
which could generate 'mesoscale' eddies, perhaps of the order of a
hundred kilometres across or so (and so similar in scale to those that
we mentioned earlier in this chapter, spinning off from the bound-
aries of the oceanic currents to drift into the slowly moving gyres).
In the Cretaceous such eddies might have played a crucial role in
pumping water, here and there, through the oceanic water column.
This kind of mechanism is less effective as an oxygen provider than
is the present-day oceanic conveyor belt that loops through today's
oceans, and therefore in the Cretaceous hothouse the oceans were
more prone to anoxia than now. At times, this tendency spread widely
to suffocate much of the ocean floor. Many successions of chalk strata
contain, punctuating the usual dazzling white rocks, layers up
to several metres thick of dark grey, carbon-rich chalk. These mark
times when, over much of the sea floor, the faltering oxygen supply
mechanism broke down completely. Organic matter, drifting from
the surface, was preserved and buried (to give the strata their dark
colour). Multicellular life vanished from these sea floors. These are
called Oceanic Anoxic Events, and individual instances can be traced
across much of the world.