More Like this

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

   https://doi.org/10.5194/cp-17-419-2021  

   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; et al (January 2021, Climate of the Past)

   Abstract. Sometime during the middle to late Holocene (8.2 ka to ∼ 1850–1900 CE), the Greenland Ice Sheet (GrIS) was smaller than its currentconfiguration. Determining the exact dimensions of the Holocene ice-sheetminimum and the duration that the ice margin rested inboard of its currentposition remains challenging. Contemporary retreat of the GrIS from itshistorical maximum extent in southwestern Greenland is exposing a landscapethat holds clues regarding the configuration and timing of past ice-sheetminima. To quantify the duration of the time the GrIS margin was near itsmodern extent we develop a new technique for Greenland that utilizes in situcosmogenic 10Be–14C–26Al in bedrock samples that have becomeice-free only in the last few decades due to the retreating ice-sheet margin atKangiata Nunaata Sermia (n=12 sites, 36 measurements; KNS), southwest Greenland. To maximizethe utility of this approach, we refine the deglaciation history of the regionwith stand-alone 10Be measurements (n=49) and traditional 14C agesfrom sedimentary deposits contained in proglacial–threshold lakes. We combineour reconstructed ice-margin history in the KNS region with additionalgeologic records from southwestern Greenland and recent model simulations ofGrIS change to constrain the timing of the GrIS minimum in southwestGreenland and the magnitude of Holocene inland GrIS retreat, as well as to explore theregional climate history influencing Holocene ice-sheet behavior. Our10Be–14C–26Al measurements reveal that (1) KNS retreated behindits modern margin just before 10 ka, but it likely stabilized near thepresent GrIS margin for several thousand years before retreating fartherinland, and (2) pre-Holocene 10Be detected in several of our sample sitesis most easily explained by several thousand years of surface exposure duringthe last interglaciation. Moreover, our new results indicate that the minimumextent of the GrIS likely occurred after ∼5 ka, and the GrISmargin may have approached its eventual historical maximum extent as early as∼2 ka. Recent simulations of GrIS change are able to match thegeologic record of ice-sheet change in regions dominated by surface massbalance, but they produce a poorer model–data fit in areas influenced by oceanicand dynamic processes. Simulations that achieve the best model–data fitsuggest that inland retreat of the ice margin driven by early to middleHolocene warmth may have been mitigated by increased precipitation. Triple10Be–14C–26Al measurements in recently deglaciated bedrockprovide a new tool to help decipher the duration of smaller-than-present iceover multiple timescales. Modern retreat of the GrIS margin in southwestGreenland is revealing a bedrock landscape that was also exposed during themigration of the GrIS margin towards its Holocene minimum extent, but it has yetto tap into a landscape that remained ice-covered throughout the entireHolocene. 

   more » « less
2. Contrasting regional variability of buried meltwater extent over 2 years across the Greenland Ice Sheet

   https://doi.org/10.5194/tc-15-2983-2021  

   Dunmire, Devon; Banwell, Alison F.; Wever, Nander; Lenaerts, Jan T.; Datta, Rajashree Tri (January 2021, The Cryosphere)

   Abstract. The Greenland Ice Sheet (GrIS) rapid mass loss is primarily driven by an increase in meltwater runoff, which highlights the importance of understanding the formation, evolution, and impact of meltwater features on the ice sheet. Buried lakes are meltwater features that contain liquid water and exist under layers of snow, firn, and/or ice. These lakes are invisible in optical imagery, challenging the analysis of their evolution and implication for larger GrIS dynamics and mass change. Here, we present a method that uses a convolutional neural network, a deep learning method, to automatically detect buried lakes across the GrIS. For the years 2018 and 2019 (which represent low- and high-melt years, respectively), we compare total areal extent of both buried and surface lakes across six regions, and we use a regional climate model to explain the spatial and temporal differences. We find that the total buried lake extent after the 2019 melt season is 56 % larger than after the 2018 melt season across the entire ice sheet. Northern Greenland has the largest increase in buried lake extent after the 2019 melt season, which we attribute to late-summer surface melt and high autumn temperatures. We also provide evidence that different processes are responsible for buried lake formation in different regions of the ice sheet. For example, in southwest Greenland, buried lakes often appear on the surface during the previous melt season, indicating that these meltwater features form when surface lakes partially freeze and become insulated as snowfall buries them. Conversely, in southeast Greenland, most buried lakes never appear on the surface, indicating that these features may form due to downward percolation of meltwater and/or subsurface penetration of shortwave radiation. We provide support for these processes via the use of a physics-based snow model. This study provides additional perspective on the potential role of meltwater on GrIS dynamics and mass loss. 

   more » « less
3. A multimillion-year-old record of Greenland vegetation and glacial history preserved in sediment beneath 1.4 km of ice at Camp Century

   https://doi.org/10.1073/pnas.2021442118  

   Christ, Andrew J.; Bierman, Paul R.; Schaefer, Joerg M.; Dahl-Jensen, Dorthe; Steffensen, Jørgen P.; Corbett, Lee B.; Peteet, Dorothy M.; Thomas, Elizabeth K.; Steig, Eric J.; Rittenour, Tammy M.; et al (March 2021, Proceedings of the National Academy of Sciences)

   Understanding the history of the Greenland Ice Sheet (GrIS) is critical for determining its sensitivity to warming and contribution to sea level; however, that history is poorly known before the last interglacial. Most knowledge comes from interpretation of marine sediment, an indirect record of past ice-sheet extent and behavior. Subglacial sediment and rock, retrieved at the base of ice cores, provide terrestrial evidence for GrIS behavior during the Pleistocene. Here, we use multiple methods to determine GrIS history from subglacial sediment at the base of the Camp Century ice core collected in 1966. This material contains a stratigraphic record of glaciation and vegetation in northwestern Greenland spanning the Pleistocene. Enriched stable isotopes of pore-ice suggest precipitation at lower elevations implying ice-sheet absence. Plant macrofossils and biomarkers in the sediment indicate that paleo-ecosystems from previous interglacial periods are preserved beneath the GrIS. Cosmogenic²⁶Al/¹⁰Be and luminescence data bracket the burial of the lower-most sediment between <3.2 ± 0.4 Ma and >0.7 to 1.4 Ma. In the upper-most sediment, cosmogenic²⁶Al/¹⁰Be data require exposure within the last 1.0 ± 0.1 My. The unique subglacial sedimentary record from Camp Century documents at least two episodes of ice-free, vegetated conditions, each followed by glaciation. The lower sediment derives from an Early Pleistocene GrIS advance.²⁶Al/¹⁰Be ratios in the upper-most sediment match those in subglacial bedrock from central Greenland, suggesting similar ice-cover histories across the GrIS. We conclude that the GrIS persisted through much of the Pleistocene but melted and reformed at least once since 1.1 Ma. 

   more » « less
4. Plant, insect, and fungi fossils under the center of Greenland’s ice sheet are evidence of ice-free times

   https://doi.org/10.1073/pnas.2407465121  

   Bierman, Paul R; Mastro, Halley M; Peteet, Dorothy M; Corbett, Lee B; Steig, Eric J; Halsted, Chris T; Caffee, Marc M; Hidy, Alan J; Balco, Greg; Bennike, Ole; et al (August 2024, Proceedings of the National Academy of Sciences)

   The persistence and size of the Greenland Ice Sheet (GrIS) through the Pleistocene is uncertain. This is important because reconstructing changes in the GrIS determines its contribution to sea level rise during prior warm climate periods and informs future projections. To understand better the history of Greenland’s ice, we analyzed glacial till collected in 1993 from below 3 km of ice at Summit, Greenland. The till contains plant fragments, wood, insect parts, fungi, and cosmogenic nuclides showing that the bed of the GrIS at Summit is a long-lived, stable land surface preserving a record of deposition, exposure, and interglacial ecosystems. Knowing that central Greenland was tundra-covered during the Pleistocene informs the understanding of Arctic biosphere response to deglaciation. 

   more » « less
5. Simulating the Holocene deglaciation across a marine-terminating portion of southwestern Greenland in response to marine and atmospheric forcings

   https://doi.org/10.5194/tc-16-2355-2022  

   Cuzzone, Joshua K.; Young, Nicolás E.; Morlighem, Mathieu; Briner, Jason P.; Schlegel, Nicole-Jeanne (January 2022, The Cryosphere)

   Abstract. Numerical simulations of the Greenland Ice Sheet (GrIS) over geologictimescales can greatly improve our knowledge of the critical factors drivingGrIS demise during climatically warm periods, which has clear relevance forbetter predicting GrIS behavior over the upcoming centuries. To assess thefidelity of these modeling efforts, however, observational constraints ofpast ice sheet change are needed. Across southwestern Greenland, geologicrecords detail Holocene ice retreat across both terrestrial-based and marine-terminating environments, providing an ideal opportunity to rigorouslybenchmark model simulations against geologic reconstructions of ice sheetchange. Here, we present regional ice sheet modeling results using theIce-sheet and Sea-level System Model (ISSM) of Holocene ice sheet historyacross an extensive fjord region in southwestern Greenland covering thelandscape around the Kangiata Nunaata Sermia (KNS) glacier and extendingoutward along the 200 km Nuup Kangerula (Godthåbsfjord). Oursimulations, forced by reconstructions of Holocene climate and recentlyimplemented calving laws, assess the sensitivity of ice retreat across theKNS region to atmospheric and oceanic forcing. Our simulations reveal thatthe geologically reconstructed ice retreat across the terrestrial landscapein the study area was likely driven by fluctuations in surface mass balancein response to Early Holocene warming – and was likely not influencedsignificantly by the response of adjacent outlet glaciers to calving andocean-induced melting. The impact of ice calving within fjords, however,plays a significant role by enhancing ice discharge at the terminus, leadingto interior thinning up to the ice divide that is consistent withreconstructed magnitudes of Early Holocene ice thinning. Our results,benchmarked against geologic constraints of past ice-margin change, suggestthat while calving did not strongly influence Holocene ice-margin migrationacross terrestrial portions of the KNS forefield, it strongly impactedregional mass loss. While these results imply that the implementation andresolution of ice calving in paleo-ice-flow models is important towardsmaking more robust estimations of past ice mass change, they also illustratethe importance these processes have on contemporary and future long-term icemass change across similar fjord-dominated regions of the GrIS. 

   more » « less