The collapse of the Cordilleran–Laurentide ice saddle and early opening of the Mackenzie Valley, Northwest Territories, Canada, constrained by 10Be exposure dating

Abstract. Deglaciation of the northwestern Laurentide Ice Sheet in
the central Mackenzie Valley opened the northern portion of the deglacial
Ice-Free Corridor between the Laurentide and Cordilleran ice sheets and a
drainage route to the Arctic Ocean. In addition, ice sheet saddle collapse
in this section of the Laurentide Ice Sheet has been implicated as a
mechanism for delivering substantial freshwater influx into the Arctic Ocean
on centennial timescales. However, there is little empirical data to
constrain the deglaciation chronology in the central Mackenzie Valley where
the northern slopes of the ice saddle were located. Here, we present 30 new
10Be cosmogenic nuclide exposure dates across six sites, including two
elevation transects, which constrain the timing and rate of thinning and
retreat of the Laurentide Ice Sheet in the area. Our new 10Be dates
indicate that the initial deglaciation of the eastern summits of the central
Mackenzie Mountains began at ∼15.8 ka (17.1–14.6 ka),
∼1000 years earlier than in previous reconstructions. The
main phase of ice saddle collapse occurred between ∼14.9 and
13.6 ka, consistent with numerical modelling simulations, placing this event
within the Bølling–Allerød interval (14.6–12.9 ka). Our new dates
require a revision of ice margin retreat dynamics, with ice retreating more
easterly rather than southward along the Mackenzie Valley. In addition, we
quantify a total sea level rise contribution from the Cordilleran–Laurentide
ice saddle region of ∼11.2 m between 16 and 13 ka.
