Radiocarbon studies at ATTO

Tropical forests are among the most productive ecosystems on Earth. On an annual basis, one hectare of tropical forest can fix between 20 to 30 tons of carbon from the atmosphere through the process of photosynthesis. However, the fate of this amount of fixed carbon, as it moves across the forest, is highly uncertain and poorly quantified. Furthermore, there are no reliable estimates of the amount of time that a unit of carbon will remain in an Amazon forest after photosynthesis. Determining the fate and timescales of carbon storage in these Amazon forests is important to better assess their role in mitigating climate change. If the photosynthetically fixed carbon only stays for a short period of time inside the forest, the climate benefits would be lower than in other ecosystems where the carbon stays for a longer time. Therefore, it is crucial to develop methods and strategies to quantify the fate and timescales of carbon sequestration in tropical forests due to their large capacity to draw down carbon from the atmosphere.

The fate and timescale of carbon cycling in forests can be studied using radiocarbon produced by nuclear bomb tests performed in the 1950s and 1960s and which led to a steep increase of radiocarbon in global atmospheric carbon dioxide (CO₂). We are tracking this ‘bomb’ radiocarbon pulse in ecosystem carbon pools and in respired CO₂ using sampling profiles below the canopy at different heights of the 80 m walk-up tower, to determine the radiocarbon content of the CO₂ respired by the forest.

Radiocarbon of CO₂ measured below the forest canopy is a mix of the radiocarbon signature of the respired CO₂ and that in background air. Therefore, we are also monitoring the radiocarbon signature in air collected at 321 m at ATTO to better disentangle the signature of the respired CO₂ from the signature of the atmospheric background. These measurements are also of immense importance for the global monitoring of atmospheric CO₂ and its isotopic composition. The measurements at ATTO are being integrated into a globally distributed network of high-precision measurements of ¹⁴CO₂. Data from this network provide an independent constraint to evaluate the performance of mathematical models of the global carbon cycle. Up to now, measurements in tropical regions and in South America have been under-represented in these global networks – our measurements at ATTO are filling this gap. In addition, these measurements could be useful for a number of other applications related to the dating of organic materials that require information on the background radiocarbon signature of atmospheric CO₂.