New 10Be exposure ages improve Holocene ice sheet thinning history near the grounding line of Pope Glacier, Antarctica

Abstract. Evidence for the timing and pace of past grounding line
retreat of the Thwaites Glacier system in the Amundsen Sea embayment (ASE)
of Antarctica provides constraints for models that are used to predict the
future trajectory of the West Antarctic Ice Sheet (WAIS). Existing
cosmogenic nuclide surface exposure ages suggest that Pope Glacier, a former
tributary of Thwaites Glacier, experienced rapid thinning in the early to
mid-Holocene. There are relatively few exposure ages from the lower ice-free
sections of Mt. Murphy (<300 m a.s.l.; metres above sea level) that are uncomplicated by
either nuclide inheritance or scatter due to localised topographic
complexities; this makes the trajectory for the latter stages of
deglaciation uncertain. This paper presents 12 new 10Be exposure ages
from erratic cobbles collected from the western flank of Mt. Murphy, within
160 m of the modern ice surface and 1 km from the present grounding line.
The ages comprise two tightly clustered populations with mean deglaciation
ages of 7.1 ± 0.1 and 6.4 ± 0.1 ka (1 SE). Linear regression
analysis applied to the age–elevation array of all available exposure ages
from Mt. Murphy indicates that the median rate of thinning of Pope Glacier
was 0.27 m yr−1 between 8.1–6.3 ka, occurring 1.5 times faster than
previously thought. Furthermore, this analysis better constrains the
uncertainty (95 % confidence interval) in the timing of deglaciation at
the base of the Mt. Murphy vertical profile (∼ 80 m above the
modern ice surface), shifting it to earlier in the Holocene (from 5.2 ± 0.7 to 6.3 ± 0.4 ka). Taken together, the results presented
here suggest that early- to mid-Holocene thinning of Pope Glacier occurred
over a shorter interval than previously assumed and permit a longer duration
over which subsequent late Holocene re-thickening could have occurred.
