Oxygen isotopes

As a parallel project to ANDREX, we obtained funding from the NERC Small Grants round to investigate the freshwater composition of water masses exiting the Weddell Gyre, specifically by making comprehensive measurements of the stable isotopes of oxygen (O18) in seawater on ANDREX and I6S. O18 is of great utility in high latitude freshwater studies, since it enables quantification of freshwater from separate sources. Specifically, when measured alongside salinity, O18 allows the determination of inputs of freshwater from meteoric sources (glacial ice melt, precipitation) separately from those due to oceanic sources (sea ice melt) (e.g., Weiss et al., 1979; Meredith et al., 2007). Such partitioning is needed since these separate components will vary differently on seasonal, interannual and longer timescales, and to fully understand the nature and impact of the changing freshwater budget of the Weddell Gyre, we need to understand how its individual components vary. The Weddell Gyre is a region where the O18 of the pure glacial and non-glacial freshwater components are sufficiently well-known to enable quantification of their exports by this method (e.g., Weiss et al., 1979; Weppernig et al., 1996).

The measurements of O18 will be combined with salinity and velocity fields derived from the cruises, and will enable us to quantify for the first time the individual exports of different forms of freshwater (sea ice melt, precipitation and glacial melt) from the Gyre into the global oceanic THC. Comparison with (sparse) historical data will reveal any changes in the freshwater composition of individual water masses, and place our measurements in the context of interannual and decadal change. The implications for controls on ocean circulation and climate against a background of a changing hydrological cycle will be assessed.