Seasonal controls on the retreat of lake-terminating paleo ice streams in central Canada revealed by LiDAR mapping of De Geer moraines and iceberg pits

Examination of high-resolution (0.5 m) LiDAR imagery across some 100,000 km 2 of Late Pleistocene proglacial lake floors in Southern Ontario, Canada reveals a record of the changing rate and style of retreat of lake-terminating paleo ice streams during regional deglaciation of the Laurentide Ice Sheet between 15,500 and 13,000 Cal yr BP. This geomorphic record occurs in the form of extensive swaths of repetitively-spaced minor moraine ridges up to 5 m high. Ridges occur either as broadly linear forms (Type A 1 ) with a ridge spacing of approximately 30-60 m or are more sinuous and often conjoined (Type A 2 ). Type A 1 and A 2 ridges are interpreted as De Geer moraines formed by pushing and squeezing of soft glaciolacustrine sediment during winter readvances of the grounding lines of actively retreating ice margins in near equilibrium. Regional climate reconstructions support a strong seasonal control on ice margin retreat. Conjoined type A 2 ridges were constructed by ice margins that advanced to abut the preceding ridge suggesting phases of reduced retreat rate and temporary stabilization of ice margins. De Geer ridges are locally pitted by craters with enclosing elevated rim ridges that resemble donuts (Type B ridges). These are interpreted as ‘iceberg pits’ formed where large ice masses calved from unstable thermally undercut ice margins. These sank through the water column to impact the lake floor, forming craters with enclosing ridges of displaced sediment. Localized concentrations of iceberg pits within swaths of De Geer moraines suggest phases of grounding line instability and accelerated calving, possibly arising from fluctuations in water depth, or minor surges. Ridges and associated iceberg pits comprise a distinct proglacial lacustrine landform assemblage left by the seasonally-controlled retreat of paleo ice streams standing in water. • LiDAR reveals ice stream retreat recorded by minor (De Geer) moraine ridges on proglacial lake floors. • Ridge morphology suggests a strong seasonal control on the style of retreat. • Linear to sinuous ridges record steady ice retreat in near equilibrium. • Crater-like “iceberg pits” with raised rims record episodic calving and instability. • This distinct proglacial lacustrine landform assemblage aids paleoglaciological and paleoclimatic studies.