Seasonal and latitudinal variability of troposphere Δ14CO2: Post bomb contributions from fossil fuels, oceans, the stratosphere, and the terrestrial biosphere

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Abstract

[1] During the mid-1960s, large seasonal amplitudes were observed in surface measurements of Δ14C in the Northern Hemisphere. These seasonal oscillations were initially caused by stratosphere-troposphere exchange, with the injection of bomb 14C into the troposphere during winter and spring mixing. Here we show how fossil, ocean, and terrestrial biosphere fluxes modified the stratospheric signal during the 1960s, and the evolution of each of these components in the post bomb era. In our analysis, we used the Goddard Institute for Space Studies (GISS) atmospheric tracer model, gross ocean CO2 fluxes from the Lawrence Livermore National Laboratory (LLNL) ocean model, and terrestrial CO2 fluxes from a biosphere-atmosphere model driven by normalized difference vegetation index and surface air temperatures. We found that 14C-depeleted respiration from the terrestrial biosphere partially canceled the 14C-enriched stratosphere flux in the Northern Hemisphere in the mid and late 1960s. In more recent decades, our analysis suggested that the terrestrial biosphere contribution to the Δ14C seasonal cycle reversed phase, with the terrestrial biosphere currently releasing relatively 14C-enriched CO2 that mixes with relatively depleted troposphere CO2. The timing of this reversal depended on the residence times of carbon within the footprint of the observation station. Measurements of Δ14C in respiration from tundra and boreal ecosystems in Alaska provide evidence that some boreal forests have undergone this transition, while some tundra ecosystems have not. We predict that over the next century, several features of the latitudinal profile of Δ14C will substantially change because of continued fossil fuel emissions in the Northern Hemisphere, and the partial release of bomb 14C that has accumulated in Southern Hemisphere oceans.

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