Geology Reference
In-Depth Information
(and thus most of its water). The little bit of oxygen produced by hydrogen loss makes the
surface red, like a thin coating of paint a fraction of an inch thick, but that oxygen can't
penetrate very deeply into the Martian crust.
Our new framing of Earth's mineralogical past challenged some prior views. In a 2007
Science
article, provocatively titled “A Whiff of Oxygen Before the Great Oxidation
Event?,” geochemist Ariel Anbar and his coworkers meticulously documented trace ele-
ments that occur in a sequence of 2.5-billion-year-old black shales from Mount McRae in
Western Australia. These finely layered sediments, deposited in the offshore environment
of an ancient ocean, appear monotonous to the eye but contain chemical surprises when
subjectedtoclosescrutiny.Mostnotably,athirty-foot-thicksectionnearthetopoftheshale
is significantly enriched in molybdenum and rhenium—chemical elements that don't gen-
erally appear in sedimentary rocks unless they are oxidized. In their more oxidized forms,
molybdenumandrheniumdissolveeasilyfromigneoushostrock,flowingdownriversand
into the ocean, where they can be incorporated into black shale on the ocean floor.
Everyone agrees that these enrichments of molybdenum and rhenium are telling us
something about erosion 2.5 billion years ago. Molybdenite, the commonest mineral of
molybdenum (and one that often incorporates rhenium, as well), is exceptionally soft and
easily abraded. Perhaps molybdenite-bearing granite was exposed on an ancient mountain
slope. Perhaps mechanical weathering produced microscopic bits ofmolybdenite that were
carried to the sea and settled to the black muddy bottom—sediments that would become
buried and solidified to form the Mount McRae Shale.
Anbar and his team reached a different conclusion; they proposed that a “whiff of oxy-
gen” from early photosynthetic cells was the agent that did the trick. Perhaps a local con-
centration of slimy green cells created a microenvironment with enough oxygen to mobil-
ize molybdenum andrhenium. Afterall, wehaveunambiguous evidence fortheglobal rise
of oxygen 2.4 billion years ago, so why not locally 100 million years earlier?
Sverjensky and I counter that there are plenty of ways besides oxygen to move molyb-
denum,rhenium,andotherelements.Commonatmosphericmoleculescontainingsulfuror
nitrogen or carbon could have done the electron-accepting trick just as well in the absence
of any O
2
. Such is the nature of scientific debate as new ideas and arguments are met with
alternate claims and counterarguments.
Whatever the exact timing of the rise of oxygen, by Earth's two-and-a-half-billionth birth-
day, its surface had changed once again. The first dramatic changes occurred on land, as
Earth rusted. Oxygen-driven surface weathering began to break down iron-bearing gran-
ite and basalt into brick-red soils. As the land aged, it subtly shifted hue from predomin-
antly gray and black to the ruddy color of rust. From space, Earth's continents two bil-
lionyearsago—thoughstillsignificantlysmallerthantoday'slandmasses—mighthaveap-
