Geology Reference
In-Depth Information
solvedchemicals, andonlygradually achieved thesaltiness wefindtoday.Onthecontrary,
recent evidence suggests that the hot early ocean quickly became far saltier than today.
Common table salt, sodium chloride (NaCl), readily dissolves in hot water. Today about
half of Earth's salt is tied up in landlocked salt domes and other evaporite deposits related
to dried-up bodies of salt water. Most of this salt is sequestered in thick layers deep under-
ground,butduringEarth'sfirsthalf-billionyears,therewerenocontinentsonwhichtohar-
borsalt.Consequently,thesalinityofthefirstoceanmayhavebeenasmuchastwicethatof
themodernworld.Moreover,otherelementsdissolvedinthewarmoceanwater—primarily
iron, magnesium, and calcium that dominate basalt—would have been present in higher
concentrations as well.
Scientists also wonder if the Hadean ocean was acidic or basic. The most critical single
factor controlling the ocean's pH and salinity is atmospheric carbon dioxide. By most ac-
counts,theCO
2
contentoftheearlyatmospherewasthousandsoftimeshigherthantoday's
value of a bit less than four hundred parts per million (though year by year we are rapidly
approachingandsoonwillsurpassthatlevel).AlotmoreCO
2
intheHadeanairmeantalot
more CO
2
in the water as well, and that must have had significant consequences for both
pH and salinity. Carbon dioxide combines with rainwater to form carbonic acid, H
2
CO
3
.
In the ocean, this carbonate partly dissociates to hydrogen ions, which form hydronium
ions (the H
3
O
+
of acids) plus bicarbonate (or HCO
3
−
). That net addition of H
+
turns the
oceans more acidic, perhaps to a pH as low as 5.5. Such acidic ocean conditions in turn
probably accelerated the weathering of basalt and other rocks, adding even more solutes to
an already salty ocean.
The Faint Sun Paradox
Asifthedetailed,sometimesconflictingstoriesofEarth'sfirstoceanweren'tcontroversial
enough, there's one additional big wrinkle to contend with: according to increasingly sens-
itive astronomical observations and astrophysical calculations, stars like our Sun undergo
a slow, inexorable brightening over their lifetimes. By these estimates, the youthful Sun of
4.4 billion years ago was 25 to 30 percent less bright than it is today. What's more, the Sun
would have remained uncomfortably faint for at least another 1.5 billion years. If today's
Sunweresuddenlytodimbythatextremeamount,Earthwouldquicklyenteradevastating
icebox phase; the oceans would freeze solid from the poles to the Equator, and most life
on Earth would die. Only the hardiest organisms, deep-subsurface microbial life and anim-
als living in protected hydrothermal zones associated with volcanoes, could survive such a
catastrophic climate change.
GivensuchacoolerearlySun,Earthmustsurelyhavequicklyfrozenover.Andyetgeo-
logical evidence for abundant surface water at least as far back as four billion years is un-
