Subtill nonglacial deposits and their climatic implications for the Last Interglacial (MIS 5e), Hudson Bay Lowlands, Canada

Abstract

In Hudson Bay Lowlands (HBL), the Wisconsinan tills are often underlain by organic-bearing nonglacial deposits formed during episodes of significant reduction of ice volumes or complete ice-free conditions in this region. The subtill nonglacial deposits, often correlated with the Missinaibi Formation, are beyond the method limit of radiocarbon dating in general. As such, they have been a subject of debates for their stratigraphy and associated climatic conditions owing to poor age constraints. Extensive studies were undertaken on a subtill argillaceous lacustrine deposit uncovered recently on the Winisk River. This 9 m thick organic-rich deposit, referred to as the Webequie Beds, was dated at 121 ± 9 ka, 126 ± 24 ka and 105 ± 11 ka using optically stimulated luminescence (OSL) dating method and correlated with the Last Interglacial (LIG) or Marine Isotope Stage (MIS) 5e. Pollen and plant macrofossils suggest spruce boreal forests along with extensive wetlands, not unlike what exists in the HBL today. Of particular note is the sporadic rise of birch in the upper part of the pollen sequence, interpreted as a response to the availability of frequent forest clearance by fires under a warmer climate. The birch zone is noteworthy because it corresponds to the development of conspicuous S-rich, ferromagnetic nodules (0.125–1.5 mm diameter) in the lake sediment. The nodules comprise native sulphur and greigite (Fe3S4), along with iron oxyhydroxide goethite and lepidocrocite. They likely resulted from significant warming during this interval that enhanced microbial sulphate reduction and increased the release of H2S, an essential component for the formation of the S minerals in the lake. Besides the strengthened microbial metabolization, a prolonged summer lake stratification and development of an anoxic bottom water also likely promoted the sulphate reduction. Although confirmation is needed, the warming probably linked to the LIG thermal maximum (∼120 ka) when a climate with a higher summer temperature than today prevailed. A similar subtill nonglacial deposit on the Little Current, a tributary of the Albany River, was also examined for comparison. Apart from a similar pollen flora, S-rich nodules were also recovered near a horizon OSL dated at 118 ± 13 ka comparable to the LIG thermal maximum. Results of this study provide new insights into the stratigraphy and climatic conditions of the LIG in the HBL but also help better understand the behavior and dynamics of the Laurentide Ice Sheet during the Late Pleistocene.