MOSS KILL-DATES AND MODELED SUMMER TEMPERATURE TRACK EPISODIC ICE-CAP EXPANSION IN ARCTIC CANADA THROUGH THE COMMON ERA

Most extant ice caps mantling low-relief Arctic Canada landscapes remained cold-based throughout the late Holocene, preserving in situ bryophytes killed as ice expanded across vegetated landscapes. As Arctic summers warmed after 1900 CE, ice caps receded, exposing entombed vegetation. The calibrated radiocarbon ages of dead moss collected near ice-cap margins (kill-dates) define when ice advanced across the site, killing the moss, and remained over the site until the year of their collection. In 2005 we obtained 38 Last Millennium kill-dates from two ice complexes on Baffin Island, Arctic Canada. Tight clustering of those ages indicated an abrupt onset of the Little Ice Age ~1240 CE, and further expansion ~1480 CE. In 2018/19 we tested the confidence in kill dates as reliable predictors of expanding ice caps by re-sampling those previously sampled ice complexes, after ~250 m of ice recession. The probability density functions (PDF) of the more recent series of ages matches PDFs of the earlier series, but with a larger fraction of early CE ages. Post 2005 CE ice recession exposed relict ice caps that grew during earlier Common Era advances, and were preserved beneath later ice-cap growth. The PDF of 107 kill-dates from the two ice complexes and 79 kill-dates from 62 other ice caps within 250 km of the two ice complexes document episodes of widespread ice expansion restricted almost exclusively to 250-450 CE, 850-1000 CE and a dense early Little Ice Age cluster with peaks at ~1240 and ~1480 CE. Ice continued to expand after 1480 CE, reaching maximum dimensions ~1880 CE. Intervals of ice-cap expansion coincide with persistent decreases in mean summer surface air temperature for the region in a fully coupled CESM Common Era simulation, suggesting primary forcing of the observed snowline lowerings were both modest declines in summer insolation, and cooling resulting from explosive volcanism, most likely intensified by positive feedbacks from sea-ice expansion and reduced northward heat transport by the oceans. During the coldest century of the Common Era, 1780-1880 CE, ice caps mantled >11,000 km² of north-central Baffin Island, whereas <100 km² is glaciated at present. The peak Little Ice Age state approached conditions expected during the inception phase of an ice age, and was only reversed after 1880 CE by anthropogenic alterations of the planetary energy balance.