Geology Reference
In-Depth Information
Linda Kah jokes, “The Mesoproterozoic was the smelliest time on Earth,” in a riff on
Roger Buick's “dullest” line.
“When was it smelly?” I ask.
“I think it was smelly the whole time,” she replies.
The bad news for life was its reliance on nitrogen. Nitrogen gas (N
2
) is abundant, con-
stituting 80 percent of today's atmosphere. The problem is that life's biochemistry can't
usenitrogengas;instead, liferequiresnitrogeninitsreducedform,called ammonia (NH
3
).
Consequently, life has evolved a clever protein, an enzyme called nitrogenase, that con-
verts nitrogen to ammonia. But there's a catch. The nitrogenase enzyme relies on a cluster
of atoms containing sulfur plus a metal, either iron or molybdenum, but neither metal was
presentintheintermediateocean.TheironhadbeenremovedduringtheformationofBIFs,
so that wasn't an option. Molybdenum, on the other hand, is soluble only in oxygen-rich
water,asintoday'soceans.Duringtheanoxictimeoftheintermediateocean,molybdenum
wastobefoundonlynearweatheringcoastlinesinrelativelyshallowwater—environments
where those sulfur bacteria are suspected to have thrived.
So it was that Canfield's seminal paper was followed by a stream of publications linking
Mesoproterozoic geochemistry andpaleontology—two disciplines whosepractitioners sel-
dom spoke to one another twenty years ago. The conclusion: the intermediate ocean har-
boredmicrobial life,butsuchlifecouldthriveonlynearthecoasts.Sulfur-reducingbacter-
ia coexisted with oxygen-producing algae. For a billion years, life hung on, but there were
few biological innovations.
The Mineral Explosion
Enter mineralogy, another field that has long been taught in a manner strangely divorced
from the grand story of Earth—as separate from geochemistry and paleontology as those
fieldshadbeenfromeachother.It'saninexplicablebias,foreverythingweknowofEarth's
distant past comes from evidence locked in minerals. Yet the majority of mineralogists
rarely talk about the ages or evolution of their samples. Rather, for more than two cen-
turies, mineralogical research has focused on their static physical and chemical properties.
Investigations of hardness and color, chemical elements and isotopes, crystal structure and
external form have dominated the literature of my livelihood.
I, too, was once firmly committed to this two-hundred-year tradition. For the first two
decades of my research career, I isolated perfect tiny crystals of common rock-forming
minerals, squeezed them to unimaginable pressures between two brilliant-cut diamond an-
vils, zapped the compressed samples with X-rays, and measured subtle changes in their
atomicarrangements.MycolleaguesandIignoredgeologicaltimeandgeographicalplace,
aswecaredlittleabouttheageorlocationofourmicroscopicsamples.Wecalledourselves
