The sulfate D¹⁷O record (black diamonds) from the Vostok, Antarctica ice core (figure on the right) revealed an interesting climate variability, with lower D¹⁷O values during the cold interglacial period (dD isotopes, indicative of temperature, are shown by the red line) [Alexander et al., 2002].  Oxidant concentrations, particularly OH, O₃ and H₂O₂, play a large role in determining the oxygen isotopic composition of sulfate aerosols, suggesting that this tracer could provide a valuable proxy for paleo oxidant concentrations.  Higher OH concentrations lead to lower sulfate D¹⁷O values, suggesting that OH concentrations were higher during the cold glacial period.  However other factors, such as cloud cover or pH of aerosols and clouds, can also influence the oxygen isotopic composition that is preserved in the ice.  We are performing sensitivity studies using a coupled global chemistry-climate model, to determine the sensitivity of the sulfate D¹⁷O proxy to oxidant concentrations and climate change over a full glacial-interglacial cycle, to calibrate the D¹⁷O proxy on these time scales.

We use the GEOS-CHEM model [Park et al., 2004], a global three-dimensional model of atmospheric composition and chemistry.  The figure to the left shows summer mean sulfate D¹⁷O (= d¹⁷O - 0.5xd¹⁸O) values.  We are coupling the GEOS-Chem model to the NASA GISS ModelE (GCM) [Schmidt et al., 2006].  This coupled model will be used calibrate the D¹⁷O proxy, and will eventually be used to quantitatively interpret our ice core sulfate and nitrate D¹⁷O measurements.

People:

Loretta J. Mickley and Daniel J. Jacob, Harvard University

Gavin Schmidt ,NASA GISS

Jed O. Kaplan, University of Bern

Sofen, E.D., B. Alexander (2008), "The sensitivity of the oxygen isotopes of ice core sulfate to changes in climate and chemistry: Implications for quantitative interpretation of the oxidizing capacity of the paleo-atmosphere", EOS Trans, AGU, Fall Meet. Suppl., Abstract PP51C-1511. (Poster)

Alexander, B., Savarino, J., Thiemens, M.H., and Delmas, R., “Climate driven changes of the oxidation pathways of atmospheric sulfur,” Geophys. Res. Lett., 29(14), 30 (2002).

Park, R.J., D.J. Jacob, B.D. Field, R.M. Yantosca, M. Chin, Natural and transboundary pollution influences on sulfate-nitrate-ammonium aerosols in the United States: implications for policy, J. Geophys. Res., D15204 (2004).

Schmidt, G.A. et al., Present day atmospheric simulations using GISS ModelE: Comparison to in-situ, satellite and reanalysis data, J. Clim, 19, 153-192 (2006).