The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 80

Saltiness through Time
As we go back through Earth's history, other salt-depositing events are
encountered, each one an accident of geography, tectonics, and cli-
mate. The Wieliczka deposits are about 14 million years old and
represent a smaller event—the blocking off of one arm of the Medi-
terranean. As the Atlantic Ocean was first forming, its narrowness led
to intermittent isolation and drying—and more salt deposits. In the
Permian Period, over a quarter of a million years ago, a desiccating sea
called the Zechstein Sea covered much of Europe, from eastern
England into Germany—and another one stretched across Texas and
New Mexico. Each of these events drew enormous amounts of salt
out of the sea as it formed, and each significantly reduced the ocean's
salt content.
The salt layers now lie at depth, and they make strange strata. Rock
salt for instance, when put under pressure, slowly flows—like an
enormous underground salt glacier. It flows upwards, because the salt
is less dense than silicate rock, into pipe-like structures termed dia-
pirs that can be hundreds of metres across and many kilometres high.
They punch through the overlying strata, fracturing and dislocating
the rock layers. Oil geologists love such salt diapirs, because oil often
becomes trapped at the contact between layers of permeable rock and
the impermeable salt.
The sea's overall saltiness, then, is governed by a balance between
import by rivers into the sea and export of salt minerals in rock strata.
The former is a more or less steady, incremental process globally; the
latter is fitful, and dependent upon those tectonic accidents. Which
process, over geological timescales, is winning?
It used to be thought that the world's oceans have been getting
slowly saltier through geological time. Indeed, the saltiness of the
oceans was once used as one of many ingenious attempts, before the