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

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 O-16, O-17 and O-18 in CO2 in the lower atmosphere. Here, we present additional high-precision triple oxygen isotope measurements of ambient air CO2 sampled in Gottingen (NW Germany) over the course of 2 years and of two air samples taken on top of the Brocken Mountain (1140 m, NW Germany). Gottingen differs from other locations where Delta O-17 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 Delta O-17. Our triple oxygen isotope data are reported as Delta O-17 values relative to a CO2-water equilibration line with Delta O-17 = ln (delta O-17 + 1) - 0.5229 x ln (delta O-18 + 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 Delta O-17 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 O-18/O-16 and O-17/O-16 fractionation processes. We also compare them to Gottingen-specific output from a three-dimensional transport model simulation of Delta O-17 in CO2 performed with the Tracer Model 5 (TM5). Both the modeled isofluxes at the surface, and the modeled stratospheric, fossil, and biospheric Delta O-17 components in the atmosphere at Gottingen confirm that the observed seasonal cycle in Delta O-17 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 Delta O-17, and call for more seasonally resolved measurements at continental locations like Gottingen. (C) 2016 Elsevier Ltd. All rights reserved.