[1] The Ross Sea contains the most biologically productive continental shelf in Antarctica and is a region where the annual formation of sea ice drives substantial amounts of bottom water formation. We present snow pit chemistry data from Mt Erebus Saddle that provide a quantitative proxy to reconstruct summer sea ice conditions and rates of marine primary production. The methylsulphonate (MS) record is strongly correlated with changes in the area of open water (R2 = 0.903, p < 0.05) caused by differences in atmospheric circulation and the sea-ice-damming effect of large icebergs, B-15 and C-19, which calved from the Ross Ice Shelf in 2000 and 2002 respectively. Furthermore, MS and phytoplankton net primary production correlate significantly (R2 = 0.927, p < 0.01). Our results demonstrate the potential of the Mt Erebus Saddle ice core to reconstruct sea ice and primary productivity variability in the Ross Sea beyond the observational record.