The duration and intensity of the early Holocene freshening events has been explained by subglacial outburst(s) from Lake Agassiz, the collapse of a Laurentide Ice Sheet ‘saddle’ overlying Hudson Bay, and/or a combination of the two events. In particular, the 8.2 ka cold event has been linked to final drainage of Lake Agassiz. Lake Agassiz drainage is often cited as 8.47 ka; an age that was based on just 14 radiocarbon ages (Barber et al., 1999). Here, we provide new field evidence and geomorphic observations to assess the deglacial history of this important region, allowing for revision of the sequence of events. We show that the collapse of the Hudson Bay Ice Saddle in southwestern Hudson Bay occurred between 8.57 ±0.28 ka BP and 8.11 ±0.19 ka BP. This event was preceded by at least one subglacial-drainage event through numerous newly-mapped subglacial channels onshore of southwestern Hudson Bay. The Tyrrell Sea entered these subglacial drainage channels, starting at 8.57 ka BP. Lake Agassiz may have experienced multiple subglacial drainage events prior to the final HBIS collapse at 8.11 ka BP, accounting for the timing discrepancies in freshwater cooling observed in the North Atlantic. Importantly, this new work links the chronology of events on the southwest (land-based) side of the HBIS to the northeast (marine-based) side of the ice sheet. Additionally, this work provides a potential analog for the behaviour of other ice sheets whose beds lie well below sea level, such as the West Antarctic Ice Sheet, during periods of warming climate.
Reconstruction of deglacial ice margins provides insights into the demise of past ice sheets and ice‐marginal lakes and helps to understand how former ice sheets responded to climate change. Here, we reconstruct deglacial Laurentide Ice Sheet margins across Manitoba (Canada), a dynamic region that in MIS 2 spanned from an inner core region of the Keewatin dome to the periphery of the ice sheet (~900 km north of the Last Glacial Maximum limit). The area was also overrun by ice flowing from both the Quebec‐Labrador dome and the Hudson Bay Ice Saddle. The surficial landscape of Manitoba contains inherited relict and palimpsest glacial landscapes, which need to be separated from deglacial features. Ice‐impounded glacial Lake Hind was present in southwest Manitoba at 13.0 cal. ka BP, meaning most of Manitoba was covered by ice at the start of the Younger Dryas. Northwest drainage of glacial Lake Agassiz in front of the Highrock Lake–Cree Lake moraine could have occurred near the end of the Younger Dryas, prior to 11.5 cal. ka BP, though the volume of the lake varies greatly depending on ice‐margin reconstructions. Our interpretation is incompatible with the hypothesis that Lake Agassiz drainage to the Arctic Ocean triggered the Younger Dryas climatic cooling. Numerous ice streams developed across central and southern Manitoba during deglaciation, including the Souris, Red River, The Pas, Hayes and Quinn Lake. The dominant ice source was from the north early in deglaciation, switching to the northeast with growth of the Hudson Bay Ice Saddle and then back to the north again with demise of the saddle. The ice‐margin ages are largely unconstrained, and thus we are unable to accurately assign climatic drivers to various ice stream events. Nonetheless, we record the development and demise of terrestrial ice streams over both hard‐bed and soft‐bed substrates.
The stratigraphic principle of superposition assumes that sediments at surface are youngest. And yet, patches of older preserved landscapes continue to be identified in the regions formerly covered by the Laurentide Ice Sheet. Two gravel pits, at the edge of the western Hudson Bay Lowland near the geographic centre of the North American Ice Sheet Complex, reveal additional patches where older glacial and nonglacial sediment is preserved. These sites expose 1–4 m of a darker, highly overconsolidated, clayier till, and 0–2 m of a lighter, less consolidated, sandier till over glaciofluvial and post-glacial gravels and sands; the waning stages of deposition occurred at 214 ± 22 ka (1σ minimum age model quartz grain optical age estimates, n = 2). This was during the latter end of the cool Marine Isotope Stage (MIS) 7-d (within 1σ range) or near the end of MIS-8 (within 2σ range). Next followed a warmer period similar to, or warmer than, present day with higher precipitation; inferred from pollen and macrofossils deposited in nonglacial low-energy floodplain or pond sediments. Our study highlights the need to accurately date Quaternary sediments, given that the shallowest nonglacial sediments at both gravel pits were deposited during an old (MIS 7) interglacial; there is likely no record of the youngest interglacial (MIS 5). Preservation of older sediment also means that in Quaternary stratigraphy, disjoint regional nonglacial “organic marker beds” should not automatically be considered correlative. Identification of similar “old” patches, together with till stratigraphy and composition, is essential to accurately model glacial sediment transport over time.
Manitoba (Canada) was a dynamic region during the last glaciation, spanning from an inner core region to the periphery of the Laurentide ice Sheet (~900 km north of the LGM max limit). Portions of the Manitoban landscape, especially in the north, are patchy and fragmented palimpsest or relict glacial terrains unrelated to deglaciation. These patches were partially to completely preserved during deglaciation meaning that the ‘youngest’ ice-flow indicators and/or landforms at a site may not have formed during the youngest (last) deglacial events. We utilize a diverse geologic dataset including glacial geomorphology, field-based ice flow indicators, stratigraphy, till geochemistry, till-clast lithology and detrital hornblende 40Ar/39Ar ages to identify these patches and reconstruct the glacial history of the region. Pre-deglacial landscapes can be demarcated by ribbed moraine, small patches of felsenmeer and regolith, fragmented linear and curvilinear streamlined landform flowsets, meltwater features cross-cutting flowsets (corridors and eskers), cross-cutting ice-flow indicators unrelated to the youngest ice-flow phase, and patches of locally-anomalous till composition. Manitoba was variably glaciated by ice flowing from the Keewatin dome, the Quebec-Labrador dome and the Hudson Bay Ice Saddle. Reconstructed ice-flow phases show that both pre-LGM and a penultimate glaciation followed similar growth patterns, where ice advanced into Manitoba from the east, followed by a switch in ice-flow direction indicating flow from the Keewatin ice centre to the northwest and north. The dominant ice source during MIS 2 deglaciation switched from the Keewatin sector (Souris Lobe and Red River Lobe) to the Hudson Bay Ice Saddle (Pas Lobe, Hayes Lobe, and Stephens Lake sublobe), and then back to the north again (Quinn Lake) with demise of the saddle. As such, hybrid tills produced by multiple cycles of entrainment, reworking, and deposition cover the majority of Manitoba. Deciphering the levels of relative inheritance and overprinting is essential for till stratigraphy studies and effective drift prospecting.
Abstract An understanding of the growth and demise of ice sheets over North America is essential to inform future climate models. One poorly studied subject is the glacial dynamics during interstadial Marine Isotope Stage (MIS) 3 (57–29 ka). To better constrain the southern margin of the Laurentide Ice Sheet during this time period, we re-examined a stratigraphic sequence in southeast Manitoba, Canada, and provide robust evidence for advance and retreat of ice. Around 46.6 ± 5.1 ka (1σ error), fluvial sands were deposited under similar precipitation and significantly cooler summer temperatures than present-day. Ice then advanced south over the area, before retreating once again and a return to boreal forest and grassland conditions. The area was then covered by proglacial Lake Vita, dammed by ice to the north. Geochronology constraints indicate Lake Vita existed from ca. 44.3 ± 3.6 to 30.4 ± 2.3 ka (1σ error), although gaps in the optical and finite radiocarbon ages suggest either a lack of data or plausible temporary ice-margin advances during this time period. Ice covering most of Manitoba during MIS 3 is in line with global δ 18 O records, and glacially influenced sediment deposition in the Mississippi River basin.
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Abstract Establishing the timing of glacial and nonglacial intervals in the core regions of the Laurentide Ice Sheet (LIS) is essential to constrain ice-sheet configuration at times of globally reduced ice volume, such as during Marine Isotope Stage (MIS) 3 (~57–29 ka). Hudson Bay Lowland (HBL) deglaciation, at the centre of the LIS, has been inferred at MIS 3 based on near-infinite wood radiocarbon ages and limited luminescence ages. To better constrain the age of the penultimate deglaciation of the western HBL, this study initially identified the youngest intertill nonglacial sediments, based on extensive fieldwork and till characterization. Next, vetted radiocarbon ages were combined with revised stratigraphy to show that five previously identified “MIS 3” sites were likely deposited during an earlier ice-free period. Finally, new optical ages targeting the youngest intertill nonglacial bed at three localities yielded ages ranging from 166 to 146 ka; all older than MIS 3. These ages indicate that the penultimate deglaciation of the western HBL likely occurred during MIS 5e. This interpretation better explains accompanying paleobotanic data sets that indicate vegetation similar to vegetation existing under present interglacial conditions. Currently no firm evidence exists in the terrestrial stratigraphic record for the deglaciation of the western HBL during MIS 3.
