G05_02

Mean transit time of carbon estimated through ¹⁴CO₂ measurements in a vertical profile in the central Amazon.

Chanca I^1,2,  Levin I³, Sierra C^1,4, Hammer S³, Trumbore S¹, Macario K², Lavric J^1,5, Araújo A⁶

¹Max Planck Institute for Biogeochemistry, Jena, Germany, ²Radiocarbon Laboratory - Universidade Federal Fluminense, Niterói, Brazil, ³Institut für Umweltphysik - Universität Heidelberg, Heidelberg, Germany, ⁴Swedish University of Agricultural Sciences, Uppsala, Sweden, ⁵Acoem Australasia, Jena, Germany, ⁶Empresa Brasileira de Pesquisa Agropecuária (Embrapa), Belém, Brazil

The Amazon rainforest is important in the global carbon balance, but there is still a lack of information regarding the time scales of carbon cycling in these forests. One useful timescale metric is the transit time of carbon, defined as the age of carbon exiting the ecosystem, mostly as respiration. To estimate the mean transit time (TT) of ecosystem respiration (ER), we took advantage of the large variations in CO₂ in the atmosphere below the forest canopy to estimate the radiocarbon signature of mean ER (Δ¹⁴C{ER}) using the Miller-Tans model. We collected samples of air in a vertical profile in 2019 during the dry season at the ATTO (Amazon Tall Tower Observatory) site, in the central Amazon, ca. 150km NE of Manaus, Brazil. Air samples were collected in a diurnal cycle from two heights below the canopy (4m and 24m) and, for the background, above the canopy at 79m. The Miller-Tans model estimated Δ¹⁴C{ER} = (32.0 ± 7.4)‰. An estimate of the mean TT is derived from comparing this value with the atmospheric Δ¹⁴CO₂ records that show values of 32-34‰ in the years 2012-2013. Therefore, the mean TT for the ATTO site is estimated in 6 to 7 years with an uncertainty of 2 years. This result is consistent with other TT estimations obtained through simulations and compartmental models of tropical rainforest.