RECORD OF ARCTIC OSCILLATION DRIVEN SEA ICE VARIABILITY IN LANCASTER SOUND, CANADIAN ARCTIC, USING THE LONG-LIVED CORALLINE ALGA CLATHROMORPHUM COMPACTUM

Arctic sea ice cover has been rapidly declining since the beginning of satellite observations in the 1970s. However, limited length satellite records do not allow for an assessment of the long-term behaviour of sea ice. Recently, the shallow benthic coralline alga Clathromorphum compactum has been shown to archive annual resolution proxy information of sea ice cover. C. compactum is found throughout the Arctic and can exhibit a life-span of up to 650 years while depositing annual growth increments in a High-Mg calcite skeleton. Annual growth increment widths and Mg/Ca ratios in this photosynthesizing marine plant are strongly dependent on light availability on the shallow seafloor, where low growth rates and Mg/Ca ratios are related to long duration sea-ice cover. Here, we have analyzed Mg/Ca ratios in two specimens of C. compactum live-collected from Lancaster Sound, Nunavut, Canada in summer 2016. Mg/Ca ratios exhibit annual cyclicities, which were used to calculate annual growth increment widths and generate an age model extending back to 1863. Proxy strength was tested by comparison with satellite derived summer sea ice concentration data between 1979-2015 yielding significant negative relationships (western Lancaster Sound r=-0.8, p<0.0001). Spatial comparisons with satellite-derived sea-ice concentrations across the Canadian Arctic Archipelago exhibit highest correlations in the western-central Lancaster Sound and Barrow Strait region. The algal time series prior to satellite observations indicates a long-term sea-ice decline which was most pronounced between 1910-1950 followed by a period high sea-ice cover from the 1960s to 1980s. Lancaster Sound sea ice anomalies have previously been related to variability of the Arctic Oscillation (AO) - a measure of Northern Hemisphere sea-level pressure differences determining the exchange of atmospheric mass between the Arctic and mid-latitudes. The AO influences Lancaster Sound through wind patterns, where a positive AO results in strong winds driving ice out of the Sound. Variability of the algal sea ice reconstruction is closely related to an AO index (r_annual=0.67, p<0.0001, 1899-2000) demonstrating the long-term stability of the sea-ice – AO relationship in Lancaster Sound. However, from 2000 onwards algal anomalies indicate a steep decline in sea ice unrelated to the AO, suggesting that Lancaster Sound sea-ice variability has started to respond to other drivers, such as increasing Arctic temperatures.