Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 118

too much organic matter and, as in an overfertilized lake or ditch, the
oxygen is used up, the waters turn anoxic, and they can no longer
support multicellular life.
The supply is not quite even. The Atlantic Ocean is a little better
ventilated than is the Pacific, and there is a widespread layer at inter-
mediate depths, in-between the most rapidly moving parts of the
conveyor belt, that is known as the Oxygen Minimum Zone. But the
parts of the ocean floor that, today, have no oxygen are so few that
they have become geological classics. Offshore from Santa Barbara in
California there is in effect a hole in the sea floor, about 30 kilometres
across, called the Santa Barbara Basin. It contains a puddle of sub-
marine water which, protected by the surrounding basin sides, is
excluded from the surrounding ocean circulatory system. The water
there contains no free oxygen, and so there are no worms, no crusta-
ceans, no starfish to disturb the layers of sediment that drift down,
year by year. These layers are a treasure-store for marine scientists,
who pull up cores of the finely laminated strata from the sea floor.
They can measure within them, for instance, the scales of fish that
lived in the waters above, and compare these with fisheries records to
pinpoint good years and bad years for fish stocks. They can then go
down to layers of sediment from well before the times when humans
made records, and reconstruct the climate and oceanography of the
sea in fine detail, deep into prehistoric times.
There is another such small basin, the Carioco Basin, offshore from
Venezuela. And there is also that much larger but completely land-
locked sea, the Black Sea. Below its sunlit, productive surface layers,
the great, still bulk of the 2-kilometre-deep waters are so depleted in
oxygen that they are not just anoxic but euxinic—they contain dis-
solved hydrogen sulphide, a gas with the humorous connotations of
rotten eggs and stink bombs, but one that is as deadly to most living
creatures as is hydrogen cyanide.