Cosmogenic isotope measurements from recently deglaciated bedrock as a new tool to decipher changes in Greenland Ice Sheet size

Abstract

Abstract. During the middle to late Holocene (8.2 ka BP to present), 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 on Greenland that utilizes in situ cosmogenic 10Be-14C-26Al in bedrock samples that have become ice free only in the last few decades by the retreating ice-sheet margin at Kangiata Nunaata Sermia (n = 12 sites; 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, the magnitude of Holocene inland GrIS retreat, and 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 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 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 has yet to tap into a landscape that remained ice covered throughout the entire Holocene.

Centennial‐ and Orbital‐Scale Erosion Beneath the Greenland Ice Sheet Near Jakobshavn Isbræ

“…If so, the GrIS margin likely stabilized near its current position during mid‐Holocene warmth and the magnitude of ongoing retreat is nearly unprecedented during the Holocene. Yet, similar work in the Kangiata Nunaata Sermia region in southwest Greenland suggests that the ice margin has yet to retreat behind its minimum Holocene extent (Young et al., 2021). In sum, the low 10 Be concentrations of 18JAK‐23 and 18JAK‐24 could tentatively represent the ice margin revealing unprecedented terrain, or could indicate significant sediment cover, but we cannot distinguish between the two scenarios with our current data set.…”

“…We first compared the amount of modeled exposure during the last interglacial period (Marine Isotope Stage [MIS] 5e) in each of our model runs to what is known about the likely duration of MIS 5e exposure along the western GrIS. Triple cosmogenic‐nuclide measurements ( 14 C‐ 26 Al‐ 10 Be) from the Nuuk region indicate that ∼10–15 kyr of inheritance is present in the surficial bedrock at several ice‐marginal locations that also deglaciated during historical times; based on the 26 Al/ 10 Be concentrations at these locations, this excess exposure most likely comes from MIS 5e (Young et al., 2021). The exposure histories for our core site derived using δ 18 O thresholds of 3.3‰ (zero exposure during the Last Interglacial) and 3.4‰ (3 kyr exposure during the Last Interglacial from 131–129 ka), likely have too little exposure during the Last Interglacial, while the history associated with the δ 18 O threshold of 4.0‰ (53 kyr exposure during the Last Interglacial and into the last glacial period), likely has too much, although not impossible, exposure during the last glacial cycle (Table 3).…”

Simulating the Holocene deglaciation across a marine-terminating portion of southwestern Greenland in response to marine and atmospheric forcings

“…Decades of radiocarbon dating and, more recently, cosmogenic 10 Be dating track the retreat of the GrIS in the KNS region through the Holocene (Weidick et al, 2012, and references therein;Larsen et al, 2014;Young et al, 2021).…”

“…The current interglacial, the Holocene (the last 11.7 kyr), is characterized by prolonged warmth with proxy records suggesting that temperatures during the Early to Middle Holocene were 3 ± 1 • C warmer than the pre-industrial period (Briner et al, 2016;Lecavalier et al, 2017), which drove widespread retreat of the GrIS margin at a rate of ice mass loss exceeding 20th century values (1900( -2000Young and Briner, 2015;Briner et al, 2020). Across southwestern Greenland, a detailed geologic record of Holocene ice-margin retreat encompassing both terrestrial and marineterminating environments exists, providing an ideal test bed for ice sheet models to test the sensitivity of past ice-margin migration to atmospheric and marine forcings (Larsen et al, 2014;Young et al, 2020Young et al, , 2021. Where land-based ice existed, well-dated moraine sequences constrain ∼ 120 km of ice retreat from the present-day coastline to just outboard of the present-day ice margin Young et al, 2020) and have been shown by ice sheet models to be driven by negative SMB in response to Early Holocene warming (Cuzzone et al, 2019;Downs et al, 2020;Briner et al, 2020).…”

“…Unlike the land-based portions of southwestern Greenland, however, across the marine-based region covering the forefield around Kangiata Nunaata Sermia (KNS; Fig. 1), it remains unknown what drove this rapid ice-margin retreat during the Early Holocene (Young et al, 2021). While links between atmospheric warming and runoff-induced terminus retreat have been implicated as reasons for the most recent historical retreat across the KNS region (Lea et al, 2014a, b), the longer-term triggers of rapid Holocene ice retreat are not constrained by the geologic data alone.…”