Isotopic Investigations of Carbonate Growth on Concrete Structures


Stable C and O isotope ratios were measured in carbonate minerals, growing under concrete structures from two locations in the United States. These locations were under a bridge in Michigan and under an overpass in New York. The δ13C of the carbonate samples ranged from -21.6 to -31.4‰ (with respect to V-PDB) and clearly indicated precipitation under non-equilibrium conditions. Indeed, the values in some cases were more negative than could be accounted for by existing models that invoke 4 stages of kinetic fractionation. There have been suggestions that microbial activity involving C from gasoline and other fossil fuel sources might be responsible for the relatively low C isotope ratios measured in these carbonates. To explore this possibility, 14C measurements were made in some of the samples. All samples measured for 14C contained bomb C. The range of 14C concentrations suggested a non-uniform growth rate, although possible fossil fuel-derived carbon in the system needs future investigation. The δ18O values of the carbonates analyzed from Michigan range from 12.5 to 15.7‰ (with respect to V-SMOW), with a mean value of 13.7‰. The δ18O values of the NY samples range from 11.8 to 15.2‰, with a mean value of 13.1‰. The nearly identical mean values at both locations favors incorporation of O from atmospheric CO2 in carbonate precipitation. Additionally, the 210Pb radiometric technique was also attempted to explore the applicability of this technique in dating concrete derived carbonates as well as recent carbonates forming in a wide variety of environments. The results gave ages between 64 and 3.8 a and are consistent when compared with the date the bridge was constructed.


Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Bridges; Carbonate Minerals; Fossil Fuels; Precipitation (chemical); Kinetic Fractionations; Isotopes; Carbon Isotope; Carbonate; Concrete Structure; Mineral; Oxygen Isotope

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Article - Journal

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