March 19, 2014
Reimagining How Earth Emerged Out Of Ice Age
Lee Rannals for redOrbit.com - Your Universe Online
Researchers, publishing a paper in the journal Nature, say rapid erosion in mountain regions could explain why the Earth isn’t essentially still a snowball.
Scientists have long believed that rocks pushed up to the surface by plate tectonics absorbed atmospheric carbon dioxide, and volcanic processes are what helps to emit this CO2. However, despite this theory and a variety of other hypotheses being put forward to balance the Cenozoic C cycle, none of them have adequately described observations witnessed.
The team from the University of Southern California, which includes Joshua West and Mark Torres, say the Cenozoic uplift increased rates of sulfide oxidation along with carbonate dissolution, providing the CO2 needed to Earth’s atmosphere to contribute to the stability of Cenozoic atmospheric partial pressure of carbon dioxide (pCO2).
Previous models have suggested that global changes in organic matter burial during the Cenozoic were insufficient to balance excess CO2 consumption. The release of CO2 from the thermal decomposition of carbonate minerals during metamorphism has been suggested to be the missing source of CO2 required to balance the cycle, according to the study.
“Petrologic and geochronologic data on the timing and potential magnitude of associated fluxes call this mechanism into question,” the authors wrote in the journal. “Additionally, any estimate of CO2 release during metamorphism is subject to considerable uncertainty because it is not known what fraction of the CO2 makes it into the atmosphere after passing through kilometers of variably permeable and reactive rock.”
A new model suggests that the acceleration of continental weathering by Cenozoic uplift compensated by a decrease in the weather of ocean island basalts in response to changing climate conditions. The researchers say that this is not implausible, but requires significant changes and there is currently no direct evidence to test this prediction.
The researchers calculated the change in the net CO2 drawdown from mountain uplift throughout the Cenozoic. After this, they found the net CO2 drawdown from mountain uplift is relatively content and consistent with proxy-based pCO2.
“The general consistency between the proxy-based pCO2 record and the model calculations supports the hypothesis that sulfide oxidation played an important role in in [sic] the Cenozoic C cycle and may help, at least in part, to reconcile radiogenic isotope records with atmospheric pCO2 proxies and the requirement of mass balance in the long-term carbon cycle,” the authors wrote.
The team said it is unlikely the evolution of the carbon cycle during the Cenozoic depended on changes in silicate weathering balanced by sulfide-carbonate weathering. They said that additional CO2 being released from metamorphic decarbonation, decreasing rates of ocean island basalt weathering and changes in the organic carbon cycle may have also been important.
“Although many questions remain, including about the coincident variation in pO2, sulfide oxidation coupled to carbonate dissolution does provide an attractive and thus far under-recognized source of CO2 because it is driven by the same forcing mechanism as enhanced CO2 consumption by silicate weathering, operates on a sufficiently long time scale, and is consistent with proxy records of weathering fluxes and atmospheric pCO2 over Cenozoic timescales,” the authors wrote.