Geology Reference
In-Depth Information
synthesis, for example—always concentrate lighter carbon-12 relative to carbon-13. Con-
sequently,thecarboninbiomass(thatis,algae,deadoralive)isalways“isotopically light”
compared with inorganic carbon in limestone. During normal times, when microbial life
flourishes and light carbon is depleted from the oceans, limestone displays a correspond-
ingly heavy isotopic signature. And at times of unusually rapid biomass burial, as even
more of the lighter carbon isotope is systematically removed from the oceans, the leftover
carbon that makes limestone is even heavier on average. Sure enough, limestone deposited
along the shores of Rodinia 790 to 740 million years ago is unusually heavy. During that
interval, algae must have spread and been buried at an unprecedented rate.
ThisprofligatebloomingoflifemayhavehadasignificanteffectonEarth'sclimate.Mi-
crobial life consumes the greenhouse gas carbon dioxide, which is constantly pumped into
the atmosphere by volcanoes. In normal times, CO
2
inputs and outputs are balanced, so at-
mospheric concentrations remain relatively constant, but during the Neoproterozoic pulses
of rapid algal growth, carbon dioxide levels may have dropped, thus reducing greenhouse
warming.
AnotherratherconvolutedfeedbacklooprelatedtoCO
2
mayhavealsoenhancedEarth's
cooling. The rifting of Rodinia resulted in thousands of miles of new seafloor volcanoes,
which manufactured hot, lower-density ocean crust. Such hot, buoyant crust tended to sup-
port shallower oceans than before, so average sea levels rose. It follows that the period
after 750 million years ago was probably one of many inland seas. Inland seas mean more
evaporation and increased rainfall, which leads to more rapid weathering of exposed rock.
But rock weathering rapidly consumes the greenhouse gas carbon dioxide, and lower CO
2
levels can in turn lead to global cooling.
The distinctive positions of continents and oceans just prior to and during Rodinia's
breakup may have played an additional role in altering the global climate. Oceans and land
contrastsharplyinalbedo—theabilitytoreflectorabsorbsunlight.Thedarkeroceanshave
a correspondingly low albedo; they soak up most of the Sun's energy and grow warmer in
the process. Dry barren land, by contrast, is much more reflective. A desiccated, desolate
supercontinent like Rodinia would have bounced much of the incident sunlight back into
space. Such a juxtaposition of polar oceans and equatorial continents would have exagger-
ated any global cooling event, since the Equator receives much more solar energy than the
poles.
Details of such global-scale processes and complex feedback loops are still being re-
solved,butit'sclearthattheNeoproterozoicEarth,afteritslongperiodofrelativestability,
was poised for big changes.
Snowball Earth-Hothouse Earth
