The End of Earthly Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 202

pursued, and most species eventually succumb (for contrary to their
fearsome reputation, they are slow to grow and reproduce, and so
now ecologically vulnerable) there will surely be a niche there—just
as the dolphins are body doubles for the ichthyosaurs of the Jurassic.
A radiation of shark-shaped bony fish seems most likely, but perhaps
some mammal—a hyper-evolved sea otter, for instance—might step
into the breach. One wonders, though, if something like the hammer-
head shark will evolve again. It is having a thin time of it right now,
and some creatures—as one can see in the fossil record—are essen-
tially one-offs.
The Leaking Oceans
The oceans, as we saw in Chapter 3, have not simply been a constant
mass of water sitting within the ocean basins that they rest in. The
ocean basins are continually changing their shape, as sea floor is sub-
ducted down ocean trenches and created at mid-ocean ridges. The
Earth is in effect cracked, and water can move through these cracks,
into the depths of the Earth and back again. The evidence from the
distant past of the Precambrian is that there are hints—and that is all
there can be given the scrappiness of the rock evidence—that the
oceans were more voluminous then.
So what is happening now? There have been several attempts to
measure how much water is currently being forced down into the
mantle along subduction zones ( by looking, for instance, at the chem-
istry and water contents of the rocks along subduction zones), versus
how much is coming back via the gas-charged volcanic eruptions that
overlie these subduction zones and also much later (after a long jour-
ney through the mantle) at the mid-ocean ridges ( by, say, measuring
the amount of steam released during volcanic eruptions).
The amount of water going into the Earth seems to be something
over a cubic kilometre of seawater each year (one estimate puts it at