Constraining the age of the last interglacial–glacial transition in the Hudson Bay lowlands (Canada) using U–Th dating of buried wood

Paleoecological investigations of organic-rich sediments and radiometric dating of fossil wood fragments were undertaken to constrain the age of an extensive pre-Holocene nonglacial sediment sequence exposed along the Nottaway River in the southeastern sector of the Hudson Bay Lowlands, a key region for the study of the dynamics of the Laurentide ice sheet (LIS). Pollen and macrofossil analyses indicate that these sediments were deposited under climate conditions similar to those of today, whereas radiocarbon dating of a wood log yielded an age beyond the effective limit of this method. U and Th isotopic analysis on 9 fossil and 1 modern wood samples showed that the fossil wood fragments were subject to a significant episode of U uptake during burial. Despite mechanical cleaning of the wood outer surfaces, non-authigenic 230Th was found in most wood samples. Correction for this detrital contamination was made using 232Th as an index and assuming typical crustal Th/U ratio. Plotting of the data within a 3-D isochron indicates that the presence of non-authigenic 230Th is not the only factor affecting the U–Th system, thereby showing evidence for a complex pattern of U uptake and/or intermittent open system behavior. This U mobility, however, appears to have been limited, as indicated by the relatively narrow range of corrected 230Th/U ages and the cluster of U–Th data between the 100–120ka isochrons in the U-series evolution diagram. Taken together, our paleoecological and U–Th results tend to suggest that the apparent calculated corrected 230Th/U ages may be considered as a reliable geochronological constraint that assigns the Nottaway River sediments to the end of the last interglacial (marine isotopic stage 5e). The high degree of compaction of the fine-grained deposits enclosing the fossil wood appear to be one of the main factor that limited hydraulic gradients and associated isotopic fluxes through the unit. The stratigraphic context indicates that the overconsolidated nature of the wood-bearing deposit is associated with ice advance in the region. Accordingly, the minimum age limit provided by the corrected 230Th/U ages (i.e., ∼100ka) may also be interpreted as a coarse constraint on the timing and extent of the eastern sector of the LIS at the onset of the last glacial cycle.