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Abstract. The Greenland Ice Sheet's negative mass balance is driven by a sensitivity to both a warming atmosphere and ocean. The fidelity of ice-sheet models in accounting for ice-ocean interaction is inherently uncertain and often constrained against recent fluctuations in the ice-sheet margin from the previous decades. The geological record can be utilised to contextualise ice-sheet mass loss and understand the drivers of changes at the marine margin across climatic shifts and previous extended warm periods, aiding our understanding of future ice-sheet behaviour. Here, we use the Ice-sheet and Sea-level System Model (ISSM) to explore the Holocene evolution of Ryder Glacier draining into Sherard Osborn Fjord, Northern Greenland. Our modelling results are constrained with terrestrial reconstructions of the paleo-ice sheet margin and an extensive marine sediment record from Sherard Osborn Fjord that details ice dynamics over the past 12.5 ka years. By employing a consistent mesh resolution of <1 km at the ice-ocean boundary, we assess the importance of atmospheric and oceanic changes to Ryder Glacier's Holocene behaviour. Our simulations show that the initial retreat of the ice margin after the Younger Dryas cold period was driven by a warming climate and the resulting fluctuations in Surface Mass Balance. Changing atmospheric conditions remain the first order control in the timing of ice retreat during the Holocene. We find ice-ocean interactions become increasingly fundamental to Ryder's retreat in the mid-Holocene; with higher than contemporary melt rates required to force grounding line retreat and capture the collapse of the ice tongue during the Holocene Thermal Maximum. Regrowth of the tongue during the neo-glacial cooling of the late Holocene is necessary to advance both the terrestrial and marine margins of the glacier. Our results stress the importance of accurately resolving the ice-ocean interface in modelling efforts over centennial and millennial time scales, in particular the role of floating ice tongues and submarine melt, and provide vital analogous for the future evolution of Ryder in a warming climate. 

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.