Abstract
Abstract. The northern sector of the Greenland Ice Sheet is considered to be particularly susceptible to ice mass loss arising from increased glacier discharge in the coming decades. However, the past extent and dynamics of outlet glaciers in this region, and hence their vulnerability to climate change, are poorly documented. In the summer of 2019, the Swedish icebreaker Oden entered the previously unchartered waters of Sherard Osborn Fjord, where Ryder Glacier drains approximately 2 % of Greenland's ice sheet into the Lincoln Sea. Here we reconstruct the Holocene dynamics of Ryder Glacier and its ice tongue by combining radiocarbon dating with sedimentary facies analyses along a 45 km transect of marine sediment cores collected between the modern ice tongue margin and the mouth of the fjord. The results illustrate that Ryder Glacier retreated from a grounded position at the fjord mouth during the Early Holocene (> 10.7±0.4 ka cal BP) and receded more than 120 km to the end of Sherard Osborn Fjord by the Middle Holocene (6.3±0.3 ka cal BP), likely becoming completely land-based. A re-advance of Ryder Glacier occurred in the Late Holocene, becoming marine-based around 3.9±0.4 ka cal BP. An ice tongue, similar in extent to its current position was established in the Late Holocene (between 3.6±0.4 and 2.9±0.4 ka cal BP) and extended to its maximum historical position near the fjord mouth around 0.9±0.3 ka cal BP. Laminated, clast-poor sediments were deposited during the entire retreat and regrowth phases, suggesting the persistence of an ice tongue that only collapsed when the glacier retreated behind a prominent topographic high at the landward end of the fjord. Sherard Osborn Fjord narrows inland, is constrained by steep-sided cliffs, contains a number of bathymetric pinning points that also shield the modern ice tongue and grounding zone from warm Atlantic waters, and has a shallowing inland sub-ice topography. These features are conducive to glacier stability and can explain the persistence of Ryder's ice tongue while the glacier remained marine-based. However, the physiography of the fjord did not halt the dramatic retreat of Ryder Glacier under the relatively mild changes in climate forcing during the Holocene. Presently, Ryder Glacier is grounded more than 40 km seaward of its inferred position during the Middle Holocene, highlighting the potential for substantial retreat in response to ongoing climate change.
Highlights
Mass loss from the Greenland Ice Sheet (GrIS) occurs from surface ablation and through iceberg calving and subaqueous melt at marine-terminating glaciers
Data from the multi-sensor core logger (MSCL), X-ray fluorescence (XRF)-scanning data, and Computed tomography (CT) imaging are used to identify six major lithologic units that are correlated from the fjord mouth (10-gravity core (GC)) to the inner bathymetric sill lying seaward of the modern ice tongue (Figs. 5 and 6)
Ryder Glacier remained land-based until the Late Holocene (3.9±0.4 ka cal BP)
Summary
Mass loss from the Greenland Ice Sheet (GrIS) occurs from surface ablation (melting) and through iceberg calving (discharge) and subaqueous melt at marine-terminating glaciers. It has increased six-fold since the 1980s, contributing an estimated 13.7 mm to global sea level between 1972–2019 (Mouginot et al, 2019). In north and northeast Greenland (Fig. 1), ice discharge rates from marine-terminating glaciers are lower than those observed in the south and northwest (Mouginot et al, 2019). An understanding of how marine-terminating glaciers responded to past climate change and elucidating the geologic and environmental controls on their behavior are critical to reduce uncertainties in future sea-level predictions (Bamber et al, 2019)
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