Date:12/28/2012

crust production rates, higher frequency of large igneous provinces, or increases in pelagic carbonate deposition, the last leading to enhanced carbonate subduction into the mantle source regions of arc volcanoes. However, these are not the only volcanic sources of CO2 during this time interval. Cin-Ty A. Lee and colleagues show that ocean-continent subduction zones, manifested as a global chain of continental arc volcanoes, were as much as 200% longer in the Cretaceous and early Paleogene than in the late Paleogene to present, when a cooler climate prevailed. They suggest that greenhouse-icehouse oscillations are a natural consequence of plate tectonics operating in the presence of continental masses, serving as a large capacitor of carbonates that can be episodically purged during global flare-ups in continental arcs. Importantly, they note that if the global crustal carbonate reservoir has grown with time, as might be expected because platform carbonates on continents do not generally subduct, the greenhouse-driving potential of continental arcs would have been small during the Archean, but would have increased in the Neoproterozoic and Phanerozoic after a significant reservoir of crustal carbonates had formed in response to the evolution of life and the growth of continents.

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Rejection of the lake spillover model for initial incision of the Grand Canyon, and discussion of alternatives
William R. Dickinson, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/GES00839.1.

Almost 150 years after John Wesley Powell first ran its rapids, geologists still cannot agree about the origin of the Grand Canyon of Arizona. All agree that the canyon was cut by the Colorado River, but why the river cut the canyon exactly where it did and when it did is fiercely debated. This paper by William R. Di
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Contact: Kea Giles kgiles@geosociety.org Geological Society of America Source:Eurekalert

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