A history of Western Arctic sea ice extent: A tale of two ice cores

Paleo-proxy data have provided the critical foundation from which Earth's climate history has been reconstructed, often in great detail. It is natural for skepticism to arise regarding the veracity of paleorecords when proximal records are seemingly unrelated, and yet discordant regional histories can provide valuable information regarding differential local sensitivity to large-scale forcings. For example, the Bona-Churchill (61.4°N, 141.7°W; 4420 masl) and Mount Logan (60.595°N, 140.5°W; 5340 masl) ice cores from southeast Alaska and southwest Yukon Territory, respectively, are located ~110 km apart and would be expected to record similar climate histories. However, the proxy records from these ice cores differ substantially. Large-scale atmospheric circulation, specifically the Aleutian Low, greatly influences moisture transport to this region, yet the interannual variability in this system is preserved quite differently in the Bona-Churchill and Mount Logan ice cores. These differences are explored in detail and demonstrate that both ice core records are climatologically very relevant although they capture quite different climate mechanisms. Previous literature indicates that the Mount Logan site is very sensitive to multi-decadal to multi-centennial climate shifts in the tropical Pacific. Low frequency variability at the Bona-Churchill site, on the other hand, shows a strong connection to Western Arctic sea ice extent.

In response to the strong and rapid warming in the Arctic, sea ice extent has been rapidly declining in the recent decades, but the brevity of the satellite records makes it impossible to assess this decline within a longer historical context. A high resolution oxygen isotope (d18O) record from the Bona-Churchill ice core offers an excellent opportunity to investigate the history of Western Arctic sea ice extent over the last millennium. Surprisingly, the Bona-Churchill ice core, along with other paleorecords, reveals enriched d18O and reduced Western Arctic sea ice during the most recent neoglacial or “Little Ice Age.” This observation highlights the presence of large regional variations. Possible mechanisms are explored to explain the connection between the Western Arctic and d18O in southeast Alaska using meteorological observations, reanalysis data, and other paleo-proxy data. This investigation also highlights the uniqueness of the Bona-Churchill ice core record relative to ice core-derived proxy histories from other sites in close proximity.