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High precision measurements of Δ 14 C were performed on CO 2 sampled at La Jolla, California, USA over 1992–2007. A decreasing trend in Δ 14 C was observed, which averaged −5.5 ‰ yr −1 yet showed significant interannual variability. Contributions to the trend in global tropospheric Δ 14 C by exchanges with the ocean, terrestrial biosphere and stratosphere, by natural and anthropogenic 14 C production and by 14 C‐free fossil fuel CO 2 emissions were estimated using simple models. Dilution by fossil fuel emissions made the strongest contribution to the Δ 14 C trend while oceanic 14 C uptake showed the most significant change between 1992 and 2007, weakening by 70%. Relatively steady positive influences from the stratosphere, terrestrial biosphere and 14 C production moderated the decreasing trend. The most prominent excursion from the average trend occurred when Δ 14 C decreased rapidly in 2000. The rapid decline in Δ 14 C was concurrent with a rapid decline in atmospheric O 2 , suggesting a possible cause may be the anomalous ventilation of deep 14 C‐poor water in the North Pacific Ocean. We additionally find the presence of a 28‐month period of oscillation in the Δ 14 C record at La Jolla.
Graven et al. (Thu,) studied this question.