In situ cosmogenic ¹⁰Be–¹⁴C–²⁶Al measurements from recently deglaciated bedrock as a new tool to decipher changes in Greenland Ice Sheet size

• Main Authors: N. E. Young, A. J. Lesnek, J. K. Cuzzone, J. P. Briner, J. A. Badgeley, A. Balter-Kennedy, B. L. Graham, A. Cluett, J. L. Lamp, R. Schwartz, T. Tuna, E. Bard, M. W. Caffee, S. R. H. Zimmerman, J. M. Schaefer
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-02-01
• Series: Climate of the Past
• Online Access: https://cp.copernicus.org/articles/17/419/2021/cp-17-419-2021.pdf

<p>Sometime during the middle to late Holocene (8.2 <span class="inline-formula">ka</span> to <span class="inline-formula">∼</span> 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 <span class="inline-formula">¹⁰Be</span>–<span class="inline-formula">¹⁴C</span>–<span class="inline-formula">²⁶Al</span> 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 (<span class="inline-formula"><i>n</i>=12</span> sites, 36 measurements; KNS), southwest Greenland. To maximize the utility of this approach, we refine the deglaciation history of the region with stand-alone <span class="inline-formula">¹⁰Be</span> measurements (<span class="inline-formula"><i>n</i>=49</span>) and traditional <span class="inline-formula">¹⁴C</span> 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 <span class="inline-formula">¹⁰Be</span>–<span class="inline-formula">¹⁴C</span>–<span class="inline-formula">²⁶Al</span> measurements reveal that (1) KNS retreated behind its modern margin just before 10 <span class="inline-formula">ka</span>, but it likely stabilized near the present GrIS margin for several thousand years before retreating farther inland, and (2) pre-Holocene <span class="inline-formula">¹⁰Be</span> 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 <span class="inline-formula">∼5</span> <span class="inline-formula">ka</span>, and the GrIS margin may have approached its eventual historical maximum extent as early as <span class="inline-formula">∼2</span> <span class="inline-formula">ka</span>. 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<span id="page420"/> 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 <span class="inline-formula">¹⁰Be</span>–<span class="inline-formula">¹⁴C</span>–<span class="inline-formula">²⁶Al</span> 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.</p>