In situ cosmogenic 10Be–14C–26Al measurements from recently deglaciated bedrock as a new tool to decipher changes in Greenland Ice Sheet size
Authors: Young Nicolás E. , Lesnek Alia J. , Cuzzone Josh K. , Briner Jason P. , Badgeley Jessica A. , Balter-Kennedy Alexandra , Graham Brandon L. , Cluett Allison , Lamp Jennifer L. , Schwartz Roseanne , Tuna Thibaut , Bard Edouard , Caffee Marc W. , Zimmerman Susan R. H. , Schaefer Joerg M. 
Published: February 2021

Abstract. Sometime during the middle to late Holocene (8.2 ka to ∼ 1850–1900 CE), the Greenland Ice Sheet (GrIS) was smaller than its current
configuration. Determining the exact dimensions of the Holocene ice-sheet
minimum and the duration that the ice margin rested inboard of its current
position remains challenging. Contemporary retreat of the GrIS from its
historical maximum extent in southwestern Greenland is exposing a landscape
that holds clues regarding the configuration and timing of past ice-sheet
minima. To quantify the duration of the time the GrIS margin was near its
modern extent we develop a new technique for Greenland that utilizes in situ
cosmogenic 10Be–14C–26Al in bedrock samples that have become
ice-free only in the last few decades due to the retreating ice-sheet margin at
Kangiata Nunaata Sermia (n=12 sites, 36 measurements; KNS), southwest Greenland. To maximize
the utility of this approach, we refine the deglaciation history of the region
with stand-alone 10Be measurements (n=49) and traditional 14C ages
from sedimentary deposits contained in proglacial–threshold lakes. We combine
our reconstructed ice-margin history in the KNS region with additional
geologic records from southwestern Greenland and recent model simulations of
GrIS change to constrain the timing of the GrIS minimum in southwest
Greenland and the magnitude of Holocene inland GrIS retreat, as well as to explore the
regional climate history influencing Holocene ice-sheet behavior. Our
10Be–14C–26Al measurements reveal that (1) KNS retreated behind
its modern margin just before 10 ka, but it likely stabilized near the
present GrIS margin for several thousand years before retreating farther
inland, and (2) pre-Holocene 10Be detected in several of our sample sites
is most easily explained by several thousand years of surface exposure during
the last interglaciation. Moreover, our new results indicate that the minimum
extent of the GrIS likely occurred after ∼5 ka, and the GrIS
margin may have approached its eventual historical maximum extent as early as
∼2 ka. Recent simulations of GrIS change are able to match the
geologic record of ice-sheet change in regions dominated by surface mass
balance, but they produce a poorer model–data fit in areas influenced by oceanic
and dynamic processes. Simulations that achieve the best model–data fit
suggest that inland retreat of the ice margin driven by early to middle
Holocene warmth may have been mitigated by increased precipitation. Triple
10Be–14C–26Al measurements in recently deglaciated bedrock
provide a new tool to help decipher the duration of smaller-than-present ice
over multiple timescales. Modern retreat of the GrIS margin in southwest
Greenland is revealing a bedrock landscape that was also exposed during the
migration of the GrIS margin towards its Holocene minimum extent, but it has yet
to tap into a landscape that remained ice-covered throughout the entire
Holocene.