Abstract
This paper reconstructs the deglaciation of the Laurentide Ice Sheet (LIS; including the Innuitian Ice Sheet) from the Last Glacial Maximum (LGM), with a particular focus on the spatial and temporal variations in ice streaming and the associated changes in flow patterns and ice divides. We build on a recent inventory of Laurentide ice streams and use an existing ice margin chronology to produce the first detailed transient reconstruction of the ice stream drainage network in the LIS, which we depict in a series of palaeogeographic maps. Results show that the drainage network at the LGM was similar to modern-day Antarctica. The majority of the ice streams were marine terminating and topographically-controlled and many of these continued to function late into the deglaciation, until the ice sheet lost its marine margin. Ice streams with a terrestrial ice margin in the west and south were more transient and ice flow directions changed with the build-up, peak-phase and collapse of the Cordilleran-Laurentide ice saddle. The south-eastern marine margin in Atlantic Canada started to retreat relatively early and some of the ice streams in this region switched off at or shortly after the LGM. In contrast, the ice streams draining towards the north-western and north-eastern marine margins in the Beaufort Sea and in Baffin Bay appear to have remained stable throughout most of the Late Glacial, and some of them continued to function until after the Younger Dryas (YD). The YD influenced the dynamics of the deglaciation, but there remains uncertainty about the response of the ice sheet in several sectors. We tentatively ascribe the switching-on of some major ice streams during this period (e.g. M'Clintock Channel Ice Stream at the north-west margin), but for other large ice streams whose timing partially overlaps with the YD, the drivers are less clear and ice-dynamical processes, rather than effects of climate and surface mass balance are viewed as more likely drivers. Retreat rates markedly increased after the YD and the ice sheet became limited to the Canadian Shield. This hard-bed substrate brought a change in the character of ice streaming, which became less frequent but generated much broader terrestrial ice streams. The final collapse of the ice sheet saw a series of small ephemeral ice streams that resulted from the rapidly changing ice sheet geometry in and around Hudson Bay. Our reconstruction indicates that the LIS underwent a transition from a topographically-controlled ice drainage network at the LGM to an ice drainage network characterised by less frequent, broad ice streams during the later stages of deglaciation. These deglacial ice streams are mostly interpreted as a reaction to localised ice-dynamical forcing (flotation and calving of the ice front in glacial lakes and transgressing sea; basal de-coupling due to large amount of meltwater reaching the bed, debuttressing due to rapid changes in ice sheet geometry) rather than as conveyors of excess mass from the accumulation area of the ice sheet. At an ice sheet scale, the ice stream drainage network became less widespread and less efficient with the decreasing size of the deglaciating ice sheet, the final elimination of which was mostly driven by surface melt.
Highlights
IntroductionIce streams have long been recognised for the Pleistocene ice sheets of the NorthernHemisphere (Løken and Hodgson, 1971; Hughes et al, 1977; Denton and Hughes, 1981; Dyke and Prest, 1987a, b; Dyke and Morris, 1988; Mathews, 1991; Patterson, 1998; Stokes and Clark, 2001; Ottesen et al, 2005; Kleman and Glasser, 2007; Winsborrow et al, 2012).Most attention has been given to the largest of these ice sheets, the Laurentide Ice Sheet (LIS), where some of the first investigations of palaeo-ice streams were undertaken (Løken and Hodgson, 1971; Dyke and Morris, 1988) and where an ice-discharge pattern broadly similar to the pattern of ice flow in modern ice sheets has gradually emerged (Dyke and Prest, 1987a, b; Patterson, 1998; De Angelis and Kleman, 2005; Stokes et al, 2009; Margold et al., 2015a, b).A large number of ice streams have been identified for the LIS and ice streams are inferred to have operated during the build-up to the Last Glacial Maximum (LGM), at the LGM, and most commonly during its deglaciation (Denton and Hughes, 1981; Dyke and Prest, 1987a, b; Patterson, 1998; Stokes and Clark, 2003a, b; Winsborrow et al, 2004; De Angelis and Kleman, 2005, 2007; Stokes et al, 2009; Stokes and Tarasov, 2010; Stokes et al, 2012; Margold et al, 2015a, b)
1468 This paper represents the first attempt to reconstruct the transient evolution of ice streams in 1469 the Laurentide Ice Sheet (LIS) from the Last Glacial Maximum (LGM) throughout the deglaciation, together with a series of 1470 palaeogeographic maps
The LIS had an ice drainage 1471 network that resembled the organisation of ice streams in the modern ice sheets of Antarctica 1472 and Greenland
Summary
Ice streams have long been recognised for the Pleistocene ice sheets of the NorthernHemisphere (Løken and Hodgson, 1971; Hughes et al, 1977; Denton and Hughes, 1981; Dyke and Prest, 1987a, b; Dyke and Morris, 1988; Mathews, 1991; Patterson, 1998; Stokes and Clark, 2001; Ottesen et al, 2005; Kleman and Glasser, 2007; Winsborrow et al, 2012).Most attention has been given to the largest of these ice sheets, the Laurentide Ice Sheet (LIS), where some of the first investigations of palaeo-ice streams were undertaken (Løken and Hodgson, 1971; Dyke and Morris, 1988) and where an ice-discharge pattern broadly similar to the pattern of ice flow in modern ice sheets has gradually emerged (Dyke and Prest, 1987a, b; Patterson, 1998; De Angelis and Kleman, 2005; Stokes et al, 2009; Margold et al., 2015a, b).A large number of ice streams have been identified for the LIS and ice streams are inferred to have operated during the build-up to the Last Glacial Maximum (LGM), at the LGM, and most commonly during its deglaciation (Denton and Hughes, 1981; Dyke and Prest, 1987a, b; Patterson, 1998; Stokes and Clark, 2003a, b; Winsborrow et al, 2004; De Angelis and Kleman, 2005, 2007; Stokes et al, 2009; Stokes and Tarasov, 2010; Stokes et al, 2012; Margold et al, 2015a, b). Dyke and Prest (1987a, b) included some of the largest ice streams, most importantly the Hudson Strait Ice Stream, and they recognised several of the smaller ice streams in the Canadian Arctic that are characterised by distinct sediment dispersal trains Their reconstruction lacked many of the ice streams on the continental shelf due to what is known to be their overly restricted ice extent at the LGM (see review in Stokes, 2017). The development of objective criteria for palaeo-ice stream identification (Stokes and Clark, 1999; Stokes and Clark, 2001), their application to the research of the LIS (see e.g., Clark and Stokes, 2001; De Angelis and Kleman, 2005; Kehew et al, 2005; Ross et al, 2006; Shaw et al, 2006), together with updated LGM ice extents on the continental shelf (England, 1999; Dyke et al, 2003; Dyke, 2004; England et al, 2006; Shaw et al, 2006), has resulted in a rapid increase in the number of ice streams that have been recognised (e.g., ~10 in Stokes and Clark, 2001; ~50 in Winsborrow et al, 111 2004; ~120 in Margold et al, 2015a, b)
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