Geology Reference
In-Depth Information
red megacontinent. The inhospitable atmosphere contained only a small fraction of today's
oxygenbudget,toolittletoformmuchofanultravioletprotectiveozonelayer.Atimetrav-
eler with a reliable supply of oxygen and sunblock might have survived along the coast on
a bland diet of algae, but life would have been no picnic on that desolate Neoproterozoic
world.
Rodinia's unbalanced juxtaposition of land and sea was not destined to last. For most
of Earth's history, climate had been moderated by negative feedbacks. It had varied
throughout Earth history, to be sure, but the fluctuations seldom reached life-threatening
extremes. Beginning about 850 million years ago, however, several changes disrupted the
former equilibrium and pushed Earth to a climatic tipping point. The most important was
the gradual breakup of equatorial Rodinia. The first rift 850 million years ago was modest,
as the Congo and Kalahari cratons (what are now parts of southern Africa) began to separ-
ate to the southwest of the otherwise intact supercontinent. About 800 million years ago, a
second small rift zone isolated the West African craton, which moved south from the main
landmass.Rodinia'sfragmentationwasinfullswingby750millionyearsago,atimewhen
extensive chains of volcanoes and basaltic lava flows reveal major cracks in the crust. The
supercontinent split in half, as a great north-south rift zone separated Ur to the west and a
continental cluster of Laurentia, Baltica, Amazonia, and other smaller cratons to the east.
With rifting came thousands of miles of new coastline and associated pulses of rapid
coastal erosion. Dynamic sedimentary basins formed in the intercratonic seas and marked
an end to the long hiatus in Earth's rock record—that virtual cessation of sedimentary rock
deposition that had begun in the Mesoproterozoic Era and lasted for almost a quarter of
a billion years. Microbial life flourished in this shifting, fragmenting world. Eroded lands
contributed mineral nutrients to photosynthetic algae, which had long been limited by the
ocean'smeagerstoreofphosphate,molybdenum,manganese,andotheressentialelements.
Paleontologistsimagineatimeofshallow,sandytidalzoneswiththickmatsofslimygreen
filaments and offshore waters choked with smelly algal rafts.
Tectonic events further conspired to alter Earth's oceans, atmosphere, and climate. At-
mospheric oxygen rose in part because of the profligate coastal algal blooms, but also
because increased production of algal biomass led to the rapid burial of organic carbon.
Throughout Earth's history, carbon-rich biomass has been a major consumer of oxygen.
The more biomass that decays, the faster oxygen is consumed. (Forest fires represent an
unusually rapid enactment of this ongoing oxygen-depleting phenomenon.) By the same
token, the faster carbon-rich biomass is buried, the faster oxygen levels rise. But how can
we know if biomass was buried? It turns out that limestone, which precipitates carbon-rich
mineral layers on the shallow ocean floor, preserves a subtle telltale record.
Carbon isotopes in limestone point to changes in the production rate of algae. Life's es-
sential chemical reactions—the conversion of water and carbon dioxide to sugar in photo-
