September Geology and GSA Today media ...(tric tons gold) to be 287.5 1.7 Ma (mil...)

Date:8/28/2007

tric tons gold) to be 287.5 1.7 Ma (millions of years before present). These results suggest a possible link between gold mineralization and intrusion into the crust of ca. 290 Ma regional granitoid magmas. This temporal association was previously clouded because commonly used geochronometers are unable to accurately and/or precisely date the gold mineralizing event directly. Isotopic compositions of Os and helium provide important new constraints on potential metal and fluid sources to the deposit, including a small but detectable mantle contribution to the ore system. These new results provide a more comprehensive understanding of one of Earths giant gold accumulations, and rule out several current hypotheses for its origin.

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Mesoproterozoic carbon dioxide levels inferred from calcified cyanobacteria
Linda C. Kah, University of Tennessee, Department of Geological Sciences, Knoxville, TN 37996, USA; and and Robert Riding, School of Earth, Ocean, and Planetary Sciences, Cardiff University, Cardiff CF10 3YE, UK. Pages 799-802.

Determining the concentration of carbon dioxide in the ancient atmosphere remains a critical hurdle to understanding Earth surface temperatures, compositional changes in atmospheric chemistry, and the evolution of Earths earliest biosphere. Kah and Riding report the finding of petrographic fabrics in 1.2-billion-year-old carbonate strata that suggest the process of cyanobacterial calcification. When ambient atmospheric partial pressure of CO2 (pCO2) concentrations fall below roughly 10 times present atmospheric levels (PAL), cyanobacteria begin to use a combination of dissolved carbonate species (CO32- and HCO3-) in the photosynthetic production of organic matter. The cyanobacteria, however, must perform a series of biochemical gymnastics to utilize HCO3- in this process. As a byproduct of these biochemical changes, the pH of the microbial sheath rises dramatically and induces the precipitation of calcium carb
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Contact: Ann Cairns acairns@geosociety.org 303-357-1056 Geological Society of America Source:Eurekalert

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