Marine outlet glaciers play a crucial role in the mass balance of the Greenland Ice Sheet (GrIS), and their dynamics significantly influence global sea-level rise. Understanding the processes governing their advance and retreat is therefore paramount, yet remains a significant challenge. The GrIS has experienced a six-fold increase in mass loss over the past four decades, with outlet glacier discharge and melting contributing substantially to this loss. These glaciers are particularly sensitive to the inflow of warmer subsurface ocean water, which causes basal melting when in contact with the ice. Increased basal melting leads to thinner ice tongues, making them more susceptible to calving and potentially destabilizing the grounding line, further accelerating ice flow and contributing to sea-level rise. The role of bathymetry and warm-water inflow in these dynamics is not fully understood and represents a major uncertainty in future sea-level rise projections. Northwestern Greenland hosts four fjords with large marine outlet glaciers discharging into the Arctic Ocean. This study focuses on Ryder Glacier, one of only two remaining glaciers in this region with a substantial floating ice tongue, located in Sherard Osborn Fjord. This previously uncharted fjord offered an ideal location to investigate the impact of warm Atlantic water inflow and bathymetry on glacier dynamics, particularly in comparison with the well-studied Petermann Glacier. By acquiring comprehensive oceanographic data and high-resolution multibeam bathymetry, this research aims to quantify the influence of the fjord's bathymetric features on the under-ice melting of Ryder Glacier and contribute to improved predictions of future sea-level rise.

Prior research has highlighted the significant role of ocean-induced basal melting in the retreat of Greenland's outlet glaciers. Studies such as Straneo & Heimbach (2013) and Christoffersen et al. (2011) have demonstrated the connection between North Atlantic warming and glacier retreat. The mechanisms of calving and the dynamics of calving glaciers have also been extensively investigated (Benn et al., 2007), with research indicating that calving can lead to a significant acceleration of ice flow (Rückamp et al., 2019; Mouginot et al., 2015). Recent work has focused on quantifying the contribution of Greenland's outlet glaciers to overall ice sheet mass loss (Mouginot et al., 2019; van den Broeke et al., 2017), emphasizing the large uncertainty in predicting future sea-level rise due to the complex interplay of factors affecting these glaciers. The influence of bathymetry on ice-ocean interactions has also been explored (Fenty et al., 2016; Rignot et al., 2015), but further research is needed to understand the specific role of fjord circulation and subglacial topography in determining glacier vulnerability. Previous work on Petermann Glacier (Jakobsson et al., 2018; Münchow et al., 2014; Johannessen et al., 2013) has provided valuable insights into the impact of ocean warming on a glacier with a floating ice tongue, offering a useful comparison for this study.

The research employed a multi-faceted approach combining high-resolution bathymetric mapping and comprehensive oceanographic data collection. The bathymetric mapping was conducted during the Ryder 2019 Expedition using the Swedish icebreaker Oden, equipped with a Kongsberg EM122 multibeam echosounder. This system, supported by advanced navigation and motion-sensing technology, provided detailed seafloor topography data across Sherard Osborn Fjord. The data were processed using QPS Qimera software, applying corrections for acoustic refraction based on sound speed profiles derived from the concurrent oceanographic program. Rigorous quality assurance checks were implemented to ensure data accuracy. Oceanographic data were collected using a Seabird 911 CTD (Conductivity, Temperature, Depth) system, which was equipped with a rosette sampler. This allowed for the acquisition of vertical profiles of temperature, salinity, dissolved oxygen, turbidity, and fluorescence at multiple locations within the fjord and the adjacent Lincoln Sea. Two RDI Workhorse ADCPs (Acoustic Doppler Current Profilers) were also deployed to measure current velocities. In situ conductivity and temperature data were converted to potential temperature and absolute salinity using the TEOS-10 equation of state. A one-dimensional buoyant-plume model, based on the conservation of mass, momentum, heat, and salt, was employed to analyze the impact of differing water column properties on subglacial melt rates. The model utilized two sets of boundary conditions: one representing colder water near the ice-tongue terminus and another reflecting warmer water from the main fjord basin. Data from Petermann Fjord, collected during previous expeditions, were used for comparison. The various datasets were analyzed using appropriate software and statistical methods to assess the relationships between bathymetry, water properties, and glacier dynamics.

The multibeam bathymetry revealed a previously unknown double-sill system in Sherard Osborn Fjord. The outer sill, located at the fjord mouth, has a depth of 375-475 m, while the inner sill, located closer to the glacier, has a maximum depth of 390 m, confined to a narrow channel. The oceanographic data revealed a stratified water column, with a cold, relatively fresh surface layer overlying warmer, saltier Atlantic water at depths greater than 150 m. A marked temperature difference was observed between waters landward and seaward of the inner sill. Landward of the inner sill, water temperatures remained near 0°C, significantly colder than the warmer Atlantic Water (>0.3°C) found in the main fjord basin and Lincoln Sea. This temperature contrast indicates that the inner sill acts as a barrier, restricting the inflow of warmer Atlantic water to the glacier. The cross-sectional area of the inner sill through which water warmer than 0°C can pass is significantly smaller compared to the sill in Petermann Fjord. The area of the inner sill is estimated to be approximately 7.5 times smaller than the equivalent area in Petermann Fjord. The buoyant-plume model simulations indicated that a 15% reduction in subglacial melt rate could be attributed to the colder water temperatures landward of the inner sill. Analysis of temperature-salinity relationships suggested enhanced vertical mixing in the narrow channel of the inner sill, cooling the inflowing warmer water before it reaches Ryder Glacier's grounding line. The ADCP measurements supported this, showing a near-bottom inflow with speeds exceeding 0.2 m/s, indicating a Richardson number of 0.6, suggesting significant shear-driven vertical mixing. The difference in sill geometries between Sherard Osborn and Petermann Fjords helps explain the contrasting behaviours of Ryder and Petermann Glaciers. The protective effect of the inner sill likely contributes to the relative stability of Ryder Glacier's ice tongue, while Petermann Glacier, with its more open sill configuration, is more directly exposed to warmer Atlantic water, leading to increased melting and retreat.

The findings of this study significantly advance our understanding of the complex interactions between oceanographic conditions, bathymetry, and glacier dynamics. The double-sill configuration in Sherard Osborn Fjord plays a crucial role in determining the extent of warm water inflow to Ryder Glacier. The inner sill acts as an effective barrier, reducing the under-ice melting and potentially contributing to the observed stability of the ice tongue. This contrasts sharply with Petermann Glacier, where the lack of a similar protective sill contributes to its more significant retreat. The study emphasizes the importance of considering bathymetric features in assessing the vulnerability of marine-terminating glaciers to ocean warming. The results suggest that even small changes in sill depth or configuration can have a disproportionate impact on subglacial melt rates, highlighting the necessity of high-resolution bathymetric data and coupled ice-ocean models to accurately predict future glacier behavior and sea-level rise. The combination of observational data and numerical modeling provides a robust framework for understanding the processes controlling these dynamics.

This study provides compelling evidence for the significant influence of bathymetry on the sensitivity of marine-terminating glaciers to changes in ocean temperature. The double-sill system in Sherard Osborn Fjord acts as a natural buffer, protecting Ryder Glacier from the full impact of warm Atlantic water inflow. This contrasts with the more exposed Petermann Glacier, illustrating the critical role of fjord morphology in shaping glacier dynamics. The findings emphasize the necessity of incorporating detailed bathymetric data and sophisticated ice-ocean interaction models into assessments of future sea-level rise. Future research should focus on deploying long-term oceanographic moorings within Sherard Osborn Fjord to track seasonal and interannual variations in water temperature and salinity near the grounding line, and to better understand the influences of fjord geometry and side-wall stresses on glacial flow. This would enhance our predictive capabilities and further refine projections of Greenland Ice Sheet mass balance.

The oceanographic data collected during this study were limited to a two-week period in late summer. This may not fully represent the annual range of oceanographic conditions in Sherard Osborn Fjord. The study did not directly measure conditions at the grounding line, which is approximately 26 km from the ice-tongue margin. The numerical plume model employed was a one-dimensional approximation, and could not fully account for the complexities of three-dimensional fjord circulation and mixing processes. The relative position of Ryder Glacier's more northerly location compared to Petermann Glacier might also play a role in differing access to warm Atlantic Water, although this was not the focus of this study.