Geology Reference
In-Depth Information
not all of which produce oxygen. So the details of how those three-billion-year-old mat-
forming organisms made their livings will remain a hot topic for some time to come.
The Mineralogical Explosion
The grand arc of the story of Earth's oxygenation is widely accepted. Prior to 2.5 billion
yearsago,Earth'satmospherewasessentially lackinginO
2
.Theriseofphotosyntheticmi-
crobes caused dramatic cumulative changes between about 2.4 and 2.2 billion years ago,
whenatmosphericoxygenrosetogreaterthan1percentoftoday'sconcentration.Thisirre-
versible change transformed Earth's near-surface environment and paved the way for even
more dramatic changes.
Asthepreviousaccountsdemonstrate,thedetailsofthistransitionhavebecomethefocal
points of many scientists' careers. In recent years, my longtime colleague Dimitri Sver-
jenskyandIhavejumpedintothefraywithastrikingandsomewhatcounterintuitiveclaim:
mostkindsofmineralsonEartharetheconsequenceoflife.Forcenturies,thetacitassump-
tion has been that the mineral kingdom operates independently of life. Our new “mineral
evolution” approach, by contrast, stresses the coevolution of the geosphere and biosphere.
We suggest that fully two-thirds of the approximately forty-five hundred known mineral
species could not have formed prior to the Great Oxidation Event, and that most of Earth's
rich mineral diversity probably could not occur on a nonliving world. In this view, such
mineralfavoritesassemipreciousturquoise,deepblueazurite,andbrilliantgreenmalachite
are unambiguous signs of life.
The reasons for this mineralogical dependence on the living world are simple. These
beautiful minerals, along with thousands of other species, are formed in the shallow crust
by the interactions of oxygen-rich waters and preexisting minerals. Subsurface waters dis-
solve, transport, chemically alter, and otherwise modify the upper few thousand feet of
rock. In the process, new chemical reactions occur for the first time, producing new suites
of minerals. Sverjensky and I have cataloged long lists of the minerals that are gener-
ated this way, deriving from copper, uranium, iron, manganese, nickel, mercury, molyb-
denum, and many other elements. Prior to the rise of oxygen, such mineral-forming reac-
tions simply could not have occurred.
“What of the red planet Mars?” our colleagues ask. Isn't the rusted surface of our plan-
etary neighbor evidence that Mars has been oxidized and could possess a deep mineral
diversity similar to Earth's? No, we argue. The crucial difference is that Mars, and pre-
sumably other small planets like it, didn't experience the dynamic circulation of oxygen-
rich
subsurface
waters that produces Earth's astounding mineral diversity. There may be
stores of groundwater on Mars, as recent data have tantalizingly suggested, but that water
is frozen. The only reason Mars is red is that it has lost most of its near-surface hydrogen
