Kill dates from re-exposed black mosses constrain past glacier advances along the western Antarctic Peninsula

Glaciers retreating along the western Antarctic Peninsula (AP) reveal previously entombed soils and plants. We collected black (dead) mosses to constrain the timing of late Holocene glacier advances at four sites along the AP from ice-free terrain and from rapidly retreating ice margins. The results of radiocarbon measurements from 39 black mosses were used to infer glacier activity over the past 1500 years along with established criteria for sample collection. The criteria ensure robust estimates of when plant growth ended, referred to hereafter as “kill date”. From these kill dates we report distinct periods of ice advance during ca. 1300, 800, and 200 calibrated calendar years before 1950 (cal yr BP) and the first estimates of glacier rate of advance around 800 cal yr BP of 2.0 and 0.3 meters per year from Gamage and Bonaparte Points (southern Anvers Island), respectively. Kill dates reveal a narrow range of ages within a region, suggesting that multiple glacier termini advanced together, and that the rate of local advances may have varied by an order of magnitude. Other evidence for glacier advances in the northern AP ca. 200 cal yr BP and ages of penguin remains (a proxy for penguin colony abandonment) centered ca. 800 cal yr BP from several sites across the AP coincide with our kill dates. Combining several lines of terrestrial evidence for past glacier activity is critical to improving our understanding of the regional synchroneity of glacial dynamics and cryosphere-biosphere connections.