Thirteen of 18 piston cores recovered from 'Robinson Lake' in the mid-Arctic vegetation zone of Baffin Island, Arctic Canada, penetrated stratified lacustrine sediment beneath a thin over-consolidated diamict (till) and postglacial lacustrine sediment. The sub-till lacustrine units are up to 120 cm thick, of which the upper several decimeters frequently contain thick, layered mats of aquatic moss; pollen and diatoms are common throughout both lacustrine units. A series of 23 AMS 14C dates defines the chronology of the postglacial sequence, which records a succession from a pioneer grass- and Oxyria-dominated tundra between 10.4 and 8 ka BP, to a sedge-dominated tundra after 8 ka BP. Limiting 14C dates place the sub-till lacustrine sediments more than 40 ka BP; 10 luminescence dates centered on 85 ka indicate they were deposited late in oxygen isotope stage (OIS) 5. The dominance of shrub and tree pollen, especially shrub birch and alder, in sub-till lacustrine samples, indicates low-arctic tundra farther north than at any time during the Holocene. Pollen concentrations are comparable to or higher than in the Holocene units. Cooling late in the interglacial is indicated by declining birch and alder pollen percentages in the upper part of the section. Diatom floras in both the sub-till and postglacial lacustrine sequences indicate similar development of lake-water chemistry, but input of silica and weathering products was greater in the older lake cycle, suggesting more vigorous catchment processes. Macrofossils in the sub-till units are characteristic of lakes ice-free in summer. Based on pollen assemblages indicating local and regional vegetation diagnostic of summer temperatures higher than the Holocene, we interpret the sub-till lacustrine units to be of interglacial character. By analogy with Holocene plant succession in central and eastern Canada, all of Keewatin and Labrador/Ungava must have been ice free throughout this interval, suggesting essentially complete deglaciation of the Laurentide Ice Sheet at the time.
- Programområde 5: Natur og klima