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.