- Webb 1100
Title: A new terrestrial gross primary productivity estimate based on rare isotopologues of atmospheric CO2
Abstract: Terrestrial gross primary production (GPP) is the largest flux in the global carbon cycle and is one of the controls on land carbon sequestration. GPP fluxes are expected to be strongly sensitive to global climate change, however the magnitude of the total global GPP flux has been difficult to determine because there are no direct measures at scales greater than the leaf-level. Oxygen isotopes in CO2 have been previously used to estimate global GPP . Here, we expand this effort by measuring two new records of novel oxygen isotope measurements, mass-47 (D47) and 17O-excess (D17O), of atmospheric CO2 at La Jolla, California. We find that D17O (VSMOW-CO2) (l=.528) has a mean value of 0.021 and that D47 has a mean value of .95. Both of these isotopic tracers are Closely linked to the seasonal cycle of CO2 abundance and d18O of CO2,respectively. We use these two records as well as previous measurements of d13C and d18O of CO2 to construct a box model that constrains the gross mass fluxes of CO2 into and out of the atmosphere. We find that our box model and atmospheric records are consistent with a GPP of 139-181PgC/yr and a soil invasion flux of 133-141 PgC/yr during 2011 to 2013. Our GPP estimate is considerably larger than previous determinations indicating that our current estimates of terrestrial fluxes may be biased. This new estimate helps to refine an important benchmark by which to evaluate global biospheric models.