Geology Reference
In-Depth Information
along newly formed coastlines gobbled up much of the carbon dioxide present at the time.
Contemporaneously, the rise of oxygen spelled the demise of atmospheric methane, the
other important greenhouse gas. The faint Sun (perhaps 85 percent of modern levels) was
insufficient to maintain as effective a greenhouse, so a protracted cold period ensued.
For the next 1.4 billion years—almost a third of Earth history, including the boring bil-
lion—no trace of an ice age has been found. Earth's climate seems to have remained in re-
markable balance, not too hot and not too cold. To explain such a long time when changes
weresorestrained,wecaninvokealitanyofpossiblenegativefeedbacks,allofwhichmay
havecontributedtostasis,butit'shardtopinpointthecausewhenthere'snoobviouseffect.
What we can say for sure is that Earth reached a tipping point roughly 740 million years
ago, and the snowball-hothouse cycle ensued.
The Second Great Oxidation Event
Thelivingworldwasnotinsensitivetosuchextremeglobalchanges;foratleastthelast3.5
billion years, changes in the geosphere have profoundly affected the biosphere. As Earth
vacillated between its hot and cold extremes, exposed and weathered continental shores
contributed pulses of essential nutrients to coastal ecosystems. Manganese, required for
photosynthesis, was one such vital mineral. Molybdenum (used for processing nitrogen)
and iron (employed in varied metabolic roles) were also supplied in abundance. But of all
the chemical elements, phosphorus may have been the most important in the Neoprotero-
zoic seas. Phosphorus is essential for all life. It helps to form the backbone of the genetic
molecules DNA and RNA, it stabilizes many cell membranes, and it plays a key role in
storing and transferring chemical energy in every cell.
ThestoryofphosphorusfascinatesDominicPapineau,mycolleaguewhodidhispostdoc
work at the Geophysical Lab. Papineau's French Canadian roots are quickly revealed by
his gentle accent; his passion for Earth's oldest formations is evident in every corner of
hisrock-strewn office atBostonCollege. Polished chunksofstromatolites andbandediron
formations testify to his many field areas in remote lands.
Papineaurealizedthatinsomeecosystems,theextentofmicrobialgrowthisdirectlytied
to the amount of available phosphorus. He envisions a time when there was an unpreced-
ented amount of the nutrient flowing into the shallow coastal seas of the Neoproterozoic.
Someoftheworld'slargestphosphoritedeposits—sedimentslaiddownasphosphorus-rich
cells that died and settled to the bottom—are concentrated in the same time intervals as the
snowball-hothouse cycles. He has traveled the globe in search of these ancient phosphorite
strata—northern Canada, Finland, Africa, and India—to study their distinctive geological
settings and fascinating chemistries.
