Environmental signals from chemical measurements on Law Dome ice cores

This study presents a 695-year record of the trace ion chemistry preserved in ice cores from the summit of Law Dome, East Antarctica. The record has seasonal resolution. The record was dated by counting the annual cycles in the oxygen isotope ratio. Ambiguous years were resolved by examination of the seasonality of the time-series of hydrogen peroxide, sodium, and non-sea-salt sulphate, as well as by comparison with other ice cores from the Law Dome region. This allowed unambiguous dating during the most recent period 1807-1995, with a maximum uncertainty of plus or minus 1 year at the beginning of the record in 1301. The time-series of sodium provides an excellent record of past storm activity in the Law Dome region. Extreme storm events appear as positive deviations from the mean annual cycle, which reaches maximum values in the winter months. The time-series also shows evidence for the chemical fractionation of sea-salt aerosol, with an average 60% depletion of sodium sulphate relative to sodium chloride. The depletion is greatest during the winter months. The nitrate time-series preserves evidence of short-duration high-energy solar events during the period 1840-1995 when instrumental records of solar events are available. This is also true for the period back to 1561 during which 68 solar events have been identified using the GISP2 Greenland ice-core. However, the Law Dome nitrate record does not preserve strong evidence of long-term solar variations such as the Schwabe (11-year), Hale (22-year) and Gleissberg (80-90 year) periodicities. The non-sea-salt sulphate record is sensitive to modulations in local marine biological productivity, with a large seasonal variation peaking in the austral summer. On longer time-scales the record is modulated by a combination of the natural variability of marine biota and the input of volcanic dust. Twenty-one volcanic events, characterised by elevated summer and winter sulphate levels for up to four years post-eruption, were identified. Volcanic sulphate from the Tambora eruption was reliably dated for the first time to reach Antarctica in the winter of 1816. Further study of the mean volcanic signature for 10 well-dated events gave an average stratospheric aerosol transport time of 1.5 years with a range of 0.6-2.6 years. Application of this estimate to the Kuwae volcanic signature gave an eruption date between 1455.9-1459.9, some three years after the date estimated from historical and tree-ring records.