Variations of the carbon isotopic ratio in atmospheric CO_2 over the last 250 years recorded in an ice core from H15,Antarctica

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Variations of δ^<13>C in atmospheric CO_2 were reconstructed for the period of 1724 to 1961 by analyzing a high-accumulation ice core drilled at H15,Antarctica. Measured values of δ^<13>C were corrected for the gravitational enrichment which occurred in the firn, on the basis of the analytical results of δ^<15>N in N_2 in the core. From the vertical profile of δ^<15>N calculated using a diffusion model for the firn, it was suggested that the air in the layer shallower than 17m depth was mixed well with the atmosphere over the firn surface. The average value of δ^<13>C during the period 1724-1775 was found to be -6.67±0.11‰. δ^<13>C decreased secularly from this value to -7.33‰ in 1961,and its trend was enhanced especially after the late 19th century, corresponding to the long-term increase of the CO_2 concentration. CO_2 fluxes estimated by deconvolution analysis of the reconstructed CO_2 concentrations with the box-diffusion model accounted for the observed variations of δ^<13>C fairly well. It was also suggested that a dip of δ^<13>C found at the end of the 18th century was attributable to terrestrial biospheric CO_2 emissions enhanced under high air temperatures.
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