Geology Reference
In-Depth Information
Thephosphorus-drivenalgalbloomspumpedatmosphericoxygentonewlevels,perhaps
breathable concentrations of 15 percent. But paradoxically, rotting clumps of algae settling
to the ocean floor would have reacted rapidly with oxygen in the water column, returning
thedeepoceanstoadeadlyanoxicstate.Thustheresurgenceoflifefollowingthesnowball
Earth may well have led to a stratified ocean with an oxygen-rich layer near the surface,
anoxic waters below. Dominic Papineau also sees strong parallels to today's coastal areas,
where large fluxes of phosphate from fertilizer runoff may stimulate similar algal blooms
and deep-water anoxic dead zones.
Which returns us to one of the central tenets of mineral evolution: the coevolution of
the geosphere and biosphere. Minerals change life, even as life changes minerals. When I
began my graduate studies in Earth science four decades ago, biology seemed all but ir-
relevant to geology. The grand rock cycle was viewed as separate from the cycles of life.
When I asked my thesis adviser whether I should take a biology course as my final elect-
ive, he persuaded me to take quantum mechanics instead. “You'll never use biology,” he
assured me.
Dubious advice, considering that at every phase of Earth's evolution, from the origins of
lifeonward,lifehasinfluencedgeologyandgeologyhasinfluencedlife.In2006geochem-
ist Martin Kennedy of the University of California's Riverside campus and four coauthors
proposed a particularly novel, if speculative, example of this codependence. Their article,
“The Inception of the Clay Mineral Factory,” appeared in the March 10 issue of
Science
.
According to their clever thesis, the rise of atmospheric oxygen from a few percent to its
present level was accelerated by positive feedbacks between microbes and clay minerals.
Clay consists primarily of ultra-fine-grained microscopic mineral bits that soak up water
and form sticky, gooey masses. If you've ever gotten your foot or your car stuck in deep,
wet clay, you won't soon forget. A principal mode of clay mineral formation is weather-
ing, especially weathering by chemical alteration under the wet, acidic conditions of the
late Neoproterozoic. Kennedy and his coworkers suggest that the rapid postglacial weath-
ering of continents produced significantly more clay minerals than before the three great
snowball-hothouse cycles. What's more, there is growing evidence that microbial colonies
began to colonize the coastal landscape about this time, and microbes can be especially ef-
ficient at turning hard rock into soft clay.
Oneofthemoststrikingpropertiesofclaymineralsistheirabilitytobindtoorganicbio-
molecules. An increased production of clay minerals would have sequestered carbon-rich
biomass,andastheclaymineralswashedintotheoceans,theywouldhavesequesteredthat
carbon in thick piles of fine-grained sediments. According to the Kennedy scenario, burial
of carbon led to the rise of oxygen, which further accelerated the chemical production of
claymineralsonland,whichledtoevenmorecarbonburial.Hence,the“claymineralfact-
