AbstractUltra-high-resolution firn-core records covering four annual cycles of oxygen isotope ratios (Δ¹⁸O) and trace-ion species were generated from a high-accumulation site on Law Dome, East Antarctica. Event-scale dating of the records was established using hourly snow accumulation measurements from a co-located automatic weather station (AWS). These net accumulation events were used to examine the seasonal timing of Δ¹⁸O and a suite of trace-ion species including marine biogenic sulphur compounds (methanesulphonic acid (MSA), non-sea salt sulphate), nitrate and major sea-salt species (sodium, chloride, magnesium). The ultra-high-resolution nature of this study and independent dating scale provide an opportunity to examine exact timings in the seasonality of each species. The traditional summer-maximum species of Δ¹⁸O and MSA show consistent relative phasing during midsummer over the four annual cycles. Nitrate shows an erratic seasonal cycle with a general trend characterized by narrow peaks during spring and early summer, preceding the mid-summer peaks in Δ¹⁸O and MSA. Non-sea-salt sulphate cycles indicate similar characteristics to MSA signals during summer, but are more comparable to nitrate signals during spring, autumn and winter. This suggests the summer non-sea-salt sulphate signal is driven by biological activity, although this species appears to be linked with nitrate signals outside the summer season. Finally, the sea-salt species indicate a seasonal cycle characterized by maximum concentrations during spring, winter and autumn. Event-scale dating of the firn-core records allows direct comparisons between the seasonal cycles and meteorological conditions. Contemporaneous local air-temperature measurements are compared with the high-resolution Δ¹⁸O record. This allows a detailed investigation of the relationship between site temperature and Δ¹⁸O signals in the ice core.