Atmospheric measurements of Δ17O in CO2 in Göttingen, Germany reveal a seasonal cycle driven by biospheric uptake

Abstract

The triple oxygen isotope composition of tropospheric CO2 might be a promising new tracer for terrestrial gross carbon fluxes. This notion is based on global box modeling of its abundance, and on highly challenging and therefore very sparse measurements of 16O, 17O and 18O in CO2 in the lower atmosphere. Here, we present additional high-precision triple oxygen isotope measurements of ambient air CO2 sampled in Göttingen (NW Germany) over the course of 2years and of two air samples taken on top of the Brocken Mountain (1140m, NW Germany). Göttingen differs from other locations where Δ17O was measured by its proximity to both urban sources of CO2, and to extensive uptake of CO2 by vegetation. In our analysis, we specifically try to discern this latter influence on our measurements, and to distinguish it from other known sources of variation in Δ17O.Our triple oxygen isotope data are reported as Δ17O values relative to a CO2-water equilibration line with Δ17O=ln (δ17O+1)−0.5229×ln (δ18O+1). We report an average of -0.02±0.05‰ (SD) in the first year and -0.12±0.04‰ (SD) in the second year of our measurements. This year-to-year difference is higher than expected based on other available Δ17O records, but careful scrutiny of our measurement approach did not reveal obvious analytical biases, leaving this aspect of our record unexplained. After removing the year-to-year trend, our time series shows a statistically robust seasonal cycle with maximum values in June/July and an amplitude (peak-to-trough) of 0.13±0.02‰. We compare our observational data to a revised triple oxygen isotope mass balance “box” model of tropospheric CO2 where we reconcile both 18O/16O and 17O/16O fractionation processes. We also compare them to Göttingen-specific output from a three-dimensional transport model simulation of Δ17O in CO2 performed with the Tracer Model 5 (TM5). Both the modeled isofluxes at the surface, and the modeled stratospheric, fossil, and biospheric Δ17O components in the atmosphere at Göttingen confirm that the observed seasonal cycle in Δ17O is driven primarily by the seasonal cycle of gross primary productivity (GPP), and that the seasonal variations in both stratospheric transport and fossil fuel emissions play a minor role at our location. Our results therefore strengthen earlier suggestions that GPP is reflected in Δ17O, and call for more seasonally resolved measurements at continental locations like Göttingen.