The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 81

discovery of radioactivity, to estimate the age of the Earth. This was
done by simply estimating the rate of salt inflow from rivers and
assuming that the greater part of that salt simply stayed in the sea
(this was before the enormous scale of the 'giant salt' deposits such as
those of the Mediterranean was appreciated).
However, to create giant salt deposits needs, above all, landscape—
or, more precisely, a good balance between land, shallow sea, and
deep sea, to give maximum opportunity for the kind of tectonic acci-
dents that can dry out a good-sized sea. In the depths of Precambrian
time, in the Hadean and early Archaean eons, there seems to have
been less dry land and more ocean, partly because there may have been
more water (before part of the ocean got dragged into the mantle
along with subducting tectonic plates) and partly because continents
were fewer and smaller.
If that was the case, then salt import into the oceans may have
dominated over salt export in the early part of the Earth's history, to
allow the oceans to become saltier than today—perhaps up to twice
as salty in some estimates. Then, when a different continent-ocean
balance was established, salt could begin to be buried in large enough
amounts to begin to reduce the Earth's salinity to current levels. There
is a hint of this kind of scenario, in that giant salt deposits seem to be
rare until relatively late in Precambrian times. 55 Few rock strata sur-
vive from those early days, though, so the evidence is slender.
Even without an ocean drying up completely, Messinian-style,
reduced flow from one part of the world ocean to another might lead
to large differences in salinity from one part of the world to another.
Offshore from eastern North America beneath Chesapeake Bay, there
is a buried crater some 50 kilometres across from a meteorite that, 35
million years ago, ploughed into strata of Early Cretaceous age (about
100 million years old). Boreholes put down into the crater showed
that the strata within it are bathed in very strong brine, about twice as