Moss kill dates and modeled summer temperature track episodic snowline lowering and 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. After reaching peak late Holocene dimensions similar to 1900 CE, ice caps receded as Arctic summers warmed, exposing entombed vegetation. The calibrated radiocarbon ages of entombed moss collected near icecap 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 an earlier study, we reported 94 last millennium radiocarbon dates on in situ dead moss collected at ice cap margins across Baffin Island, Arctic Canada. Tight clustering of those ages indicated an abrupt onset of the Little Ice Age at similar to 1240 CE and further expansion at similar to 1480 CE coincident with episodes of major explosive volcanism. Here we test the confidence in kill dates as reliable predictors of expanding ice caps by resampling two previously denselysampled ice complexes similar to 15 years later after similar to 250 m of icerecession. The probability density functions (PDFs) of themore recent series of ages match PDFs of the earlier series but with a larger fraction of early Common Era ages. Post2005 CE ice recession has exposed relict ice caps that grew during earlier Common Era advances and were preserved beneath later ice cap growth. We compare the 106 kill dates from the two ice complexes with 80 kill dates from 62 other ice caps within 250 km of the two densely sampled ice complexes. The PDFs of kill dates from the 62 other ice caps cluster in the same time windows as those from the two ice complexes alone, with the PDF of all 186 kill dates documenting 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 similar to 1240 and similar to 1480 CE.Ice continued to expand after 1480 CE, reaching maximum dimensions at similar to 1880 CE that are still visible as zones of sparse vegetation cover in remotely sensed imagery. Intervals of widespread ice cap expansion coincide with persistent decreases in mean summer surface air temperature for the region in a Community Earth System Model (CESM)fully coupled Common Era simulation, suggesting the primary forcings of the observed snowline lowering were both modest declines in summer insolation and cooling resultingfrom explosive volcanism, most likely intensified by posi-tive feedbacks from increased snow cover and sea ice and re-duced northward heat transport by the oceans. The clusters ofice cap expansion defined by moss kill dates are mirrored inan annually resolved Common Era record of ice cap dimen-sions in Iceland, suggesting this is a circum-North-Atlantic-Arctic climate signal for the Common Era. During the coldestcentury of the Common Era, 1780-1880 CE, ice caps man-tled>11 000 km(2)of north-central Baffin Island, whereas<100 km(2)is glaciated at present. The peak Little Ice Agestate approached conditions expected during the inceptionphase of an ice age and was only reversed after 1880 CE byanthropogenic alterations of the planetary energy balance.