September Geology and GSA Today media ...(onate minerals. Identification of these ...)

Date:8/28/2007

onate minerals. Identification of these calcified sheaths in the geologic record thus place an upper limit on pCO2. Atmospheric concentrations of CO2 <10 times PAL in the Mesoproterozoic are significantly lower than previous estimates, and indicate the requirement of additional greenhouse gasses, such as methane, in the atmosphere to keep the oceans from freezing.

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Cryptic meteoric diagenesis in freshwater bivalves: Implications for radiocarbon dating
Gregory E. Webb, Queensland University of Technology, School of Natural Resource Sciences, Brisbane, QLD 4001, Australia; et al. Pages 803-806.

Webb et al. present some surprising data that have implications for carbon dating of rocks. Such dating techniques are critical for understanding recent climate change, the extinction of Pleistocene mammals, and early archaeology. The technique works by measuring the amount of radioactive decay in some carbon atoms that are taken into an organism or a skeleton during the life of the organism. As carbon is no longer taken in after the organism dies, the amount of decay can indicate how much time has passed since the organic matter was alive - thereby dating the material. However, the technique requires that no additional carbon be added to the material after the death of the organism. For clam shells, carbon is commonly added in the form of subsequent cements, but the cements are easy to recognize because they are a different mineral than the original clam (i.e., calcite instead of aragonite). Webb et al. have found clams that contain cements that were formed in unusual groundwater with high magnesium contents, which caused the cements to form as aragonite, like in the original shell. Hence, they are very difficult to recognize by traditional methods. Thus, although the cements cause the radiocarbon ages to be erroneous, the presence of the cements could escape detection, allowing the erroneous dates to be accepted as genuine. Such clams should b
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Contact: Ann Cairns acairns@geosociety.org 303-357-1056 Geological Society of America Source:Eurekalert

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