ABSTRACT: Ice-cap history from northern Baffin Island: forcing mechanisms and comparison to the modern state.

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Abstract

The static, cold-based ice caps of the interior of Baffin Island are simple, yet sensitive reflectors of climate. The ice caps are less than 100 m thick and are frozen to the ground beneath them, therefore they do not flow, but instead preserve intact the landscape on which they first grew. The ice caps overlie a flat plateau where 70% of the topography is between 500-800 m above sea level. Due to the flat nature of the landscape, small vertical changes in the equilibrium line altitude (ELA) correspond to large changes in the area covered by permanent snow and ice. For ice bodies on Baffin Island, the ELA is largely controlled by summer temperatures; changes in precipitation have a significantly smaller effect (Koerner, 2005). The absence of lichen cover on rock surfaces in the Canadian Arctic has been well documented to be the result of past ice cover (Andrews et al., 1976; Wolken, 2006). 48% of the plateau is devoid of lichen cover, equivalent to the area covered by permanent snow and ice during the Little Ice Age. At present only ~4% of the plateau is covered by ice, representing a loss of 91% of Little Ice Age ice cover. Observational records of the ice caps exist in the form of aerial photographs and satellite imagery beginning in 1958. Since that time, more than half of the total area of ice has disappeared. Projecting this average melt rate over the last 50 years into the future provides an estimated disappearance date for all ice on the plateau of 2070. Conventional 14C-dating of dead moss preserved beneath the ice caps dates the duration of the most recent period of ice cap cover, while cosmogenic 14C exposure dating provides an integrated history of the total duration of exposure and ice cover since deglaciation by the Laurentide Ice Sheet. Deglaciation occurred ~6 ka, as determined by 2 14C dates on macrofossil aquatic moss in basal sediment from a lake core in the center of the plateau of 5.6 ka and by Dyke et al. (2003). 50 moss samples from the modern ice cap margin provide a record of periods of ice-cap expansion that lasted until 2005. Two sites preserve moss from ~350 AD, indicating that ice cover had existed continuously on the plateau for the last 1600 years. The second oldest dates record a period of ice-cap growth at ~930 AD. Some of the ice that grew at that time remained until 2005, indicating thatthe warm period that followed between 1000-1250 AD, correlative with the Medieval Warm Period and represented by only three 14C dates, was of insufficient magnitude or duration to melt ice that is melting today. 39 of the 50 14C dates occur between 1280-1550 AD, representing a period of variable but gradually cooling climate, associated with an early onset of the Little Ice Age around 1280 AD. The largest ice-cap growth event occurred at 1450 AD, as seen by 13 dates from three ice caps. Ice expansion at this time coincides with the 1453 tropical eruption of Kuwae and with a time of reduced solar output, suggesting that these forcing mechanisms may have triggered ice-cap growth. A similar, but smaller ice cap growth event at ~1280 AD also coincides with both volcanic and solar forcings. No moss was dated that was younger than 1650 AD, indicating that the ELA depression that occurred with the 1450 ice-cap growth event and afterward blanketed the plateau in permanent snow and ice cover continuously until the twentieth century. The concentration of cosmogenic 14C in rocks on the plateau is a product of exposure during times of no ice cover since deglaciation 6 ka. Two samples have exposure histories of at most 4000 years, requiring a total of at least 2000 years of ice cover since deglaciation 6 ka. These samples are located next to moss that has been buried beneath an ice cap for the last 1000 years, indicating that there has been at least an additional 1000 years of ice on the plateau prior to the last 1000 years. Two other samples, located near 500-850 year-old moss, have been exposed for 4700 of the last 6000 years, requiring 1300 total years of ice cover, 500-800 of which occurred prior to Little Ice Age ice cover. Organic material was deposited continuously in a nearby lake from deglaciation until2800 cal years BP, thus limiting prior periods of ice cover to sometime between 2.8 ka and the most recent period of ice cover.

Bibliography of Canadian Geomorphology