Geology Reference
In-Depth Information
interval from 1.85 billion to 850 million years ago witnessed many notable changes as a
consequence of the cratonic dance. Two supercontinents were assembled during the boring
billion, each producing a dozen mountain ranges by cratonic collisions. In between those
two gatherings of the lands, as the Columbian supercontinent rifted apart, some of Earth's
most impressive sedimentary sequences were laid down. Much of Earth's land was sub-
merged and then became dry again. The rates of sedimentation varied by orders of mag-
nitude. Ice caps disappeared and reappeared. That's a lot of change for a “boring” aeon.
But there's another side to the story.
The Intermediate Ocean
Whatevertheexactgeometryoftheglobe,everyoneagreesthattheRodiniasupercontinent
must have been surrounded by an even larger superocean, a body that has been named
Mirovia (after the Russian word for “global”). Geochemists who study Earth's past have
reached the conclusion that if the Mesoproterozoic Era was boring, then Mirovia is the
principal reason why.
The Great Oxidation Event, which sets the dynamic period from 2.4 to 1.8 billion years
ago apart from all others in Earth history, was primarily a time of changes in atmospheric
chemistry. Earth's atmosphere transformed from having essentially no oxygen to having a
percent or two. That's a monumental change as far as the near-surface environment goes,
but to Earth's oceans, such a change was insignificant.
The key lies in relative masses. The oceans contain more than 250 times the mass of the
atmosphere. Any small change in atmospheric chemistry, even a 1 percent increase in oxy-
gen, takes a very long time to be reflected in the oceans—perhaps about a billion years.
Geochemistswhowanttounderstandthehistoryoftheoceansscrutinizeahostofchem-
ical elements and their isotopes. Prior to 2.4 billion years ago the oceans were rich in dis-
solved iron, a state that could be maintained only if the water column was utterly devoid
of oxidants (which would have caused iron oxides to precipitate) and also low in sulfur
(which would quickly lead to the formation of pyrite and other iron sulfide minerals). With
the atmospheric changes of the Great Oxidation Event, some of that iron was removed as
iron oxides in shallow water, either directly by oxygen or indirectly by reaction with ox-
idized weathering products from the land. Oxygen in the atmosphere also led to the rapid
weathering and erosion of sulfur-bearing minerals, which flowed into the oceans and con-
sumed more iron. These chemical changes triggered a massive deposition of banded iron
formations,orBIFs—thethickocean-floorsedimentswithlayeruponcolorfullayerofiron
mineralsthatnowconstitutemostoftheworld'sironorereserves.TheBIF-makingprocess
was gradual, and the oceans held a lot of iron, so BIF deposition continued for another six
