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

Highlights

• First documentation of subtill nonglacial deposits for the Winisk River, Hudson Bay Lowlands (HBL).

• Only site of the HBL where multiple luminescence dates (OSL) indicate a Last Interglacial or MIS 5e affinity.

• First report on S-rich ferromagnetic nodules and their development interpreted as a result of significant warming.

• Climatic conditions inferred from multiple lines of evidence including palaeobotany, sedimentology and geochemistry.

• Provide new insights into the stratigraphy and climatic conditions associated with the Missinaibi Formation of the HBL.

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.

Introduction

Hudson Bay Lowlands (HBL) constitute a poorly drained, low-relief area along Hudson and James bays (Fig. 1). During the recent geological past, the HBL was located near the geographic centre of the Pleistocene Laurentide Ice Sheet (LIS). The Quaternary stratigraphic records of this region can thus provide key information for better understanding of the behavior and dynamics of the LIS. In the HBL, the Wisconsinan tills are often underlain by a suite of organic-bearing nonglacial deposits that have been used as the marker beds in regional stratigraphic correlations (Fig. 1). Because these subtill nonglacial deposits are, in general, beyond the method limit of radiocarbon dating, dating of them has been proved very challenging. Consequently, these deposits have been a subject of debates for their age and climatic conditions, i.e., interglacial vs. interstadial as highlighted recently by Miller and Andrews (2019) and Dalton et al. (2016).

The subtill nonglacial deposits accumulated in lakes, rivers and inland seas during episodes of significant reduction of ice volumes or complete ice-free conditions in this region. They occur primarily as a single lithostratigraphic unit in the sedimentary succession and are commonly correlated with the Missinaibi Formation of the Last Interglacial as defined by Skinner (1973) in the Moose River basin in the southern part of the HBL. The pollen from these deposits indicates a vegetation very similar to today in this region (Terasmae and Hughes, 1960; Netterville, 1974; Mott and DiLabio, 1990). Because the Last Interglacial was warmer than today such as seen in the Don Formation in southern Ontario (Terasmae, 1960; Karrow, 1990; Occhietti et al., 2016), the lack of significant compositional change in the pollen assemblages has led some to question its interglacial status (e.g., Terasmae and Hughes, 1960). Furthermore, radiometric dating, which includes radiocarbon, luminescence and U/Th as well as amino acid epimerization and racemization (AAR) studies, has returned inconsistent results, with ages assigned to various Marine Isotope Stages (MIS), including MIS 3, MIS 5e (the Last Interglacial), MIS 5, and MIS 7 (Andrews et al., 1983; Nielsen et al., 1986; Forman et al., 1987; Berger and Nielsen, 1990; Wyatt, 1990; Thorleifson et al., 1993; Roy, 1998; Allard et al., 2012; Dubé-Loubert et al., 2013; Dalton et al., 2016, 2017, 2018; Miller and Andrews, 2019). Lastly, the Missinaibi Formation at its type site is a composite derived from fragmentary records of marine, fluvial, peat and glaciolacustrine deposits (Skinner, 1973). While the sedimentary model derived from this formation has been widely used in the study of the subtill nonglacial deposits in the HBL interior, it may not be applicable for the margin of the HBL where different deposits and sedimentary processes, e.g., lacustrine, may have predominated.

Recent Quaternary geological mapping by the Ontario Geological Survey in the HBL has resulted in the finding of additional subtill nonglacial deposits on the Winisk, Attawapiskat and Albany rivers and their tributaries (Fig. 1) (Gao, 2011; Gao et al., 2015; Bajc et al., 2014). Of the major rivers in the HBL, the Winisk River is the only one where deposits of this type have not been previously documented. Therefore, their discovery on this river provides a unique opportunity with which to better understand the stratigraphy and climatic conditions associated with the nonglacial deposits. The objectives of this study are 1) to constrain the age of the deposit, and 2) to discuss the associated climatic conditions using multiple proxies derived from palaeobotany, sedimentology and geochemistry. Previous work in the adjacent lower stream of the Winisk River provides important information on glacial lithostratigraphy for correlation (Thorleifson et al., 1993). In addition, for comparison, a subtill nonglacial deposit of lacustrine origin on the Little Current, a tributary of the Albany River, was also examined (Fig. 1). To avoid confusion, unless otherwise stated in the following discussions, the Last Interglacial (LIG) is used in a strict sense, roughly correlating with MIS 5e (132-116 ka) (Shackleton et al., 2003; Otvos, 2015).