New constraint on the global CO2 oxygen isotope recycling time and implications for gross primary productivity

The newly utilized isotopic tracer, Δ¹⁷O = ln(1+δ¹⁷O) - 0.516×ln(1+δ¹⁸O), provides an new constraint on the recycling flux of CO₂ between the atmosphere and biosphere/hydrosphere. Unlike δ¹⁸O that is largely affected by kinetic and equilibrium processes between CO₂ and substrate water and also source water isotopic inhomogeneity and dynamics, Δ¹⁷O variation is much smaller and is better defined. This is because conventional bio-geo-chemical processes that modify δ¹⁷O and δ¹⁸O follow a well-recognized fractionation slope. Consequently, the formulation of CO₂ budget using Δ¹⁷O is a lot simplified, compared to that using δ¹⁸O. The currently new estimate of utilizing Δ¹⁷O for tropospheric CO₂ gives a global recycling time of 1.5±0.2 yrs. The implied gross primary productivity (GPP) is 170±20 PgC/yr. With an additional constraint from atmospheric O₂, we deduce an oceanic GPP value of 120±30 PgC/yr. Comparison with other models places stronger constraint for biogeochemical models, essential components for assessing global carbon cycles under changing climatic and environmental conditions. Details and implications of the results will be presented and discussed, along with an assessment with a state-of-the-art community carbon-nitrogen cycling climate model.