The importance of appropriate isotope reference standards for determination of the isotopic composition of C and O in atmospheric CO2

      Our process-based understanding of stable isotope signals, as well as technological developments, has progressed significantly, opening new frontiers in marine interdisciplinary research. This has promoted the broad utilisation of carbon and oxygen isotope applications to gain insight into carbon cycling in marine ecosystems and their interaction with the atmosphere.

Our study was performed in the Gulf of Trieste in the N Adriatic where the influence of biological processes, riverine loads and local climate conditions on the atmosphere-water CO₂ exchange and on the carbonate system equilibrium was investigated, in order to elucidate what drives the CO₂ exchange and to estimate the vulnerability of the Gulf of Trieste to acidification processes. On an annual scale, the Gulf of Trieste clearly acts as a sink of CO₂, strongly controlled by the seasonal variability of water temperature, biological processes, wind speed and riverine inputs. The calculated air-sea CO₂ flux was estimated to be -1.47 ± 1.41 mol C m^-2 yr^-1. The sink was generally stronger during the winter months, whereas during early summer and autumn the CO₂ fluxes were lower. It was interesting to note that the atmospheric CO₂ concentrations exhibited large fluctuations on a daily basis, as well as on a seasonal time scale. The average atmospheric CO₂ concentration during our study in 2013 was 438 ± 16 ppm. This is significantly higher than the global average, which, at the time was around 400 ppm. Further The isotopic composition of carbon in atmospheric carbon dioxide (δ¹³C_CO2,air) values panned from -12.7‰ to -9.1‰ with an average value of -10.8 ± 0.9‰. This is considerably different to the “background” value of -8‰ (from NOAA/ESRL) and can most probably be attributed to the presence of fossil fuel emissions. The results are comparable with the data obtained in the Adriatic between Ravenna and Otranto.

The presented study indicate that the quality and comparability of datasets is critical to improve the estimation of processes that influence the carbon dynamics in marine environment. Thus, the implementation of the principle in our laboratory, the monitoring of our measurement quality, validation and status of newly developed gas CO₂ reference materials (SIRS1, SIRS2 and SIRS3) as a part of SIRS project will be also presented.