Origin, age, and paleoenvironmental significance of carbonate precipitates from a granitic environment, Akshayuk Pass, southern Baffin Island, Canada
This study documents the discovery of calcite crusts on the upper surface of clasts within morainic complexes in Akshayuk Pass, southern Cumberland Peninsula (Baffin Island), a region underlain by granitic and gneissic rocks of the Precambrian Canadian Shield. Strontium isotope ratios ( 87 Sr/ 86 Sr) indicate that the major source of calcium is derived from the local dissolution of fracture-filling calcite and a minor source is derived from silicate weathering. The calcite crusts have δ 13 C values between 1.6‰ and 12.0‰ and δ 18 O values in the -13.0‰ and -7.9‰ range. These values are highly enriched over the predicted isotopic values based on the δ 13 CDIC and δ 18 O of the local water and temperature at which calcite precipitation occurred. The isotopic enrichments are attributed to a combination of both equilibrium and kinetic evaporation and were verified experimentally. The series of evaporative experiments indicate that kinetic evapo - ration produces a far-from-equilibrium isotope effect on both the δ 13 C and δ 18 O composition of the precipitating calcite ( 13 CKIE CaCO3-CO2 between 20.2‰ and 40.5‰ VPDB; 18 OKIE averaging 34.9% ± 3.7% VSMOW). Based on these re- sults, the formation of the calcite crusts is ascribed to the evaporation of stagnating ephemeral lakes and streams fol- lowing the retreat of valley glaciers. Given the >1500 m difference in heights between the highland and the valley, the katabatic winds originating from the Penny Ice Cap can act as a catalyst to increase the rate of evaporation. In ad- dition, radiocarbon dating of ten of these crusts yielded Holocene ages (7640 cal (calibrated) BP to modern). These re- sults suggest that the maximum expansion of Holocene valley glacier in Akshayuk Pass, previously thought to have occurred during the Little Ice Age, is probably much older.