17O18O and 18O18O in firn air O2 from East Greenland and Antarctica
Abstract
Abundances of 17O18O and 18O18O (called clumped isotopes, denoted by Δ35 and Δ36) of O2 in polar firn and ice core air can be useful to study past changes in atmospheric photochemistry. We present Δ35 and Δ36 values measured in firn air O2 from East Greenland (EGRIP: 75.63ºN, 35.99ºW) and EPICA Dome C, Antarctica (75.10 ºS, 123.33 ºE). Firn air samples were collected down to the bubble close-off depth. The mean age of the firn air increases with depth to ∼40 and ∼50 years at close-off at EGRIP and Dome C, respectively. Measurements of Δ35 and Δ36 were carried out using a high-resolution stable isotope ratio mass spectrometer Themro Fisher 253 ULTRA at medium mass resolution (mass resolving power ∼10000). We demonstrated that 253 ULTRA can resolve all the major isobaric interferences for O2 clumped isotope measurements such as the influence of 35Cl (mass 34.9688 u) on 17O18O (mass 34.9983) and H36Cl (mass 35.9767 u) and 36Ar (mass 35.9675 u) on 18O18O (mass 35.9983). Thus the two clumped isotope species of O2 can be measured without correction even if the O2 samples are not fully free from these potential isobars (Laskar et al., 2019). The isotopic effect due to gravitational settling is insignificant for the second order isotope signatures Δ35 and Δ36, although strongly affects the conventional isotope ratios. The average Δ35 and Δ36 values for the Dome C firn air O2 are 1.26±0.06 ‰ and 2.42±0.10 ‰ respectively. For the EGRIP, the values are 1.28±0.09 ‰ and 2.40±0.10 ‰ respectively. No significant difference in the Δ35 and Δ36 values with depth are observed indicating that potential temporal trends in the clumped isotope signatures of O2 are below the present measurement precision. In order to estimate the expected temporal variation, we simulated the temporal evolution of Δ36 from 1960 to 2010 using the European Centre for Medium-Range Weather Forecasts - Hamburg (ECHAM)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry (EMAC) model. The model results confirm that expected changes in Δ36 over this period are indeed smaller than the present analytical precision. The modeled Δ36 values agree well with the measurements. We plan to extend the measurements and model simulations to study glacial-interglacial variation in the atmospheric photochemistry and tropospheric temperature using Δ35 and Δ36 values in O2 trapped in polar ice core air with enhanced analytical precision. References Laskar A. H., Peethambaran, R., Adnew, G. A. and Röckmann, T. (2019) Measurement of 18O18O and 17O18O in atmospheric O2 using the 253 Ultra mass spectrometer and applications to stratospheric and tropospheric air samples. Rapid Comm. Mass Spec. (under review)
- Publication:
-
EGU General Assembly Conference Abstracts
- Pub Date:
- April 2019
- Bibcode:
- 2019EGUGA..2115928L