A Holocene trace chemistry record from law dome ice cores

The short instrumental period from the Southern Hemisphere presents challenges to assessing long term variability in the climate system. Proxy records developed from ice core data provide an opportunity to address this issue. Following new analysis of ice cores from the high snowfall Dome Summit South (DSS), Law Dome, East Antarctica, the full Holocene chemistry records of sea salts, sulphate and methanesulphonic acid (MSA) from this site are presented for the first time. The records have been annually layer counted to 333 BCE +13/-7 years; comparable or better than the presently available, sub-annual resolution Antarctic ice core records covering 2,000 years. The annually dated record has been used to construct proxy records of El Nino-Southern Oscillation, Australian drought, and improve the accumulation record for the site. The detailed volcanic history from DSS has been used to refine the timing of the Kuwae (Vanuatu) eruption to between 1456 and 1458 CE. This eruption is one of the largest of the past 2,000 years and constraining its timing and separating it from a 1453 CE Northern Hemisphere event improves volcanic forcing estimates. Beyond the layer counting ages were based on an ice flow model with dating ties to other Antarctic ice core records. This study indicates aerosols at DSS were primarily dry deposited at the Last Glacial Maximum (LGM) through to 12,000 years before 2000 CE (b2k). From 12,000 b2k wet deposition of aerosols increased in dominance, with the current wet deposited regime reached by approximately 7,500 b2k. Sea salt concentrations at DSS were lowest at 12,000 b2k. From 12-8,000 b2k sea salt concentrations increased markedly, corresponding with the timing of ice sheet retreat, bringing the DSS site relatively closer to the coast. This increase is also observed at inland Antarctic sites, although is more extreme at DSS. Sulphate concentrations at DSS remained largely unchanged through this period suggesting more of the larger sea salt particles were being deposited at DSS as the ice sheet retreated. However sea salt concentrations continued to gradually increase after the ice sheet retreat had occurred, only stabilising by 5,000 b2k. The increase in sea salt concentrations following the ice sheet retreat are consistent with an increase in wind speed across the Southern Ocean and a southward shift of the westerly wind belt bringing greater aerosol loads. This is supported by other proxy data and modelling studies which suggest increases in cyclonic activity in the circumpolar trough, a contraction of the Antarctic High and possible strengthening or shift in position of the zonal westerly wind belt around Antarctica. However the evidence from the Law Dome sea salt record cannot rule out a northward shift of the mean wind belt over open waters. The more gradual change in sea salt concentrations from 8-5,000 b2k is not clearly observed in other inland Antarctic ice cores. It is possible that small changes in the westerly winds around Antarctica would be more strongly observed in the coastal DSS core than inland Antarctic sites. Agreement between solar insolation and sea salt concentrations suggest insolation is an important driver of changes to processes affecting sea salt deposition at DSS in the Holocene. An inverse relationship between the sea salt record and sea surface temperature (SST) reconstructed from marine sediment cores is observed, potentially driven by increased wind speed or cyclogenesis over the Southern Ocean releasing greater latent heat from the sea surface, increasing atmospheric moisture and sea salt aerosol loads. A positive correlation between sea salt concentrations and sea ice presence reconstructions off the Dronning Maud Land coast suggest either regional coherence in sea ice formation or another control such as SST or wind advection of ice floes is affecting sea ice formation. No consistent relationship between SST, sea ice and MSA over millennial timescales is evident in this study, possibly reflecting the complex relationship between MSA production and deposition.