Stratified interglacial lacustrine sediments from Baffin Island, Arctic Canada: chronology and paleoenvironmental implications

Thirteen of 18 piston cores recovered from 'Robinson Lake' in the mid-Arcti c vegetation zone of Baffin Island, Arctic Canada, penetrated stratified la custrine sediment beneath a thin over-consolidated diamict (till) and postg lacial lacustrine sediment. The sub-till lacustrine units are up to 120 cm thick, of which the upper several decimeters frequently contain thick, laye red mats of aquatic moss; pollen and diatoms are common throughout both lac ustrine units. A series of 23 AMS C-14 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 tund ra after 8 ka BP. Limiting C-14 dates place the sub-till lacustrine sedimen ts more than 40 ka BP; 10 luminescence dates centered on 85 ka indicate the y were deposited late in oxygen isotope stage (OIS) 5. The dominance of shr ub and tree pollen, especially shrub birch and alder, in sub-till lacustrin e samples, indicates low-arctic tundra farther north than at any time durin g the Holocene. Pollen concentrations are comparable to or higher than in t he Holocene units. Cooling late in the interglacial is indicated by declini ng birch and alder pollen percentages in the upper part of the section. Dia tom floras in both the sub-till and postglacial lacustrine sequences indica te similar development of lake-water chemistry, but input of silica and wea thering products was greater in the older lake cycle, suggesting more vigor ous catchment processes. Macrofossils in the sub-till units are characteris tic of lakes ice-free in summer. Based on pollen assemblages indicating loc al and regional vegetation diagnostic of summer temperatures higher than th e Holocene, we interpret the sub-till lacustrine units to be of interglacia l character. By analogy with Holocene plant succession in central and easte rn Canada, all of Keewatin and Labrador/Ungava must have been ice free thro ughout this interval, suggesting essentially complete deglaciation of the L aurentide Ice Sheet at the time. (C) 1999 Elsevier Science Ltd. All rights reserved.