First-year land-fast Antarctic sea ice as an archive of ice shelf meltwater fluxes

Sampling beneath Antarctic ice shelves is sparse; therefore, tracking changes in ocean 6180 composition adjacent to ice shelves holds promise as an indicator of ice shelf basal melting. Sea ice archives of ice shelf-ocean interaction in particular could be important tools for future observational climate studies. Ocean delta O-18 values near the McMurdo Ice Shelf were reconstructed using observational data (sea ice delta O-18 snow depth, and ice formation dates) from McMurdo Sound, Antarctica, by combining a recently revised version of an isotope fractionation model with an established thermodynamic sea ice model, resulting in improvements compared to previous approaches. Growth rates from the thermodynamic sea ice model were validated using direct growth rate measurements. That validation and supporting analysis indicated that a change was needed in ocean heat flux assumption from 0 W m(-2) to around -13 W m(-2) part way through the sea ice growth season. A well-constrained range (+ 1.84 parts per thousand to + 2.21 parts per thousand) of effective fractionation coefficients for sea ice was derived, along with a mean of 1.99 parts per thousand. For the first time, reconstructed ocean delta O-18 values were validated using winter-long measurements of Antarctic near-surface water delta O-18. Taking uncertainties into account, the reconstructed ocean delta O-18 values generally agreed to within +/- 0.2 parts per thousand with the measured ocean delta O-18 mean values. Results indicated an overall decrease in measured ocean delta O-18 during the winter, but this was not statistically significant given the uncertainties in the measurements. Although the method works, it currently has limited utility for determining the presence and scale of any step-changes in ocean delta O-18 composition associated with present day ice shelf basal melting. This is because the uncertainty of the reconstructed values (+/- 0.2 parts per thousand) is of the same magnitude as the expected change. Also, the requirement to parameterise the ocean heat flux is a barrier to the method being an entirely retrospective method (i.e., one requiring only data from the end of the sea ice growth season). In a future Antarctic scenario of increased basal melting of the ice shelves, the method may become more valuable in an Antarctic context. The method developed in this paper will be useful currently in the Arctic, because Arctic waters exhibit much larger fresh water fluxes. (C) 2015 Elsevier B.V. All rights reserved.