Search

The landscape hidden beneath the Greenland Ice Sheet remains one of the most sparsely mapped regions on Earth, but offers a unique record of environmental conditions prior to and during widespread glaciation, and of the ice sheet's response to changing climates. In particular, subglacial valleys observed across Greenland may preserve geomorphological information pertaining to landscape and ice sheet evolution. Here we analyze the morphology of a subglacial valley network in northern Greenland using bed elevation measurements acquired during multi‐year airborne radio‐echo sounding surveys. Channel profile morphologies are consistent with a primarily fluvial origin of the network, with evidence for localized modification by ice and/or meltwater. Gravity and magnetic anomalies suggest that the spatial organisation of the valley network is influenced by regional‐scale geological structure, implying a long‐lived and well‐established hydrological system. We also document two knickzones in the valley longitudinal profile and terraces above the channel floor in the lower course of the network. These observations, combined with stream power modeling, indicate that northern Greenland experienced two episodes of relative base level fall during the Neogene (∼150 m at ca. 12–3.7 Ma and ∼380 m at ca. 8.2–2.8 Ma) that resulted in channel profile adjustment via incision and knickzone retreat. The timing of the inferred base level fall correlates with other onshore and offshore records of uplift, denudation, and/or relative sea level change, and we suggest that tectonic and/or mantle‐driven uplift played an important role in the genesis of the modern landscape of northern Greenland. Ice currently covers more than 80% of the land surface of Greenland, but the region is currently losing ice at an increasing rate and is a major contributor to global sea level rise. The landscape hidden beneath the ice can yield valuable insights into environmental conditions in Greenland during the geological past, and in turn provide important analogs for future environmental change in Greenland in a warming world. In this study, we use radar data to image the topography beneath the ice and analyze the geomorphology of a subglacial valley network. Geomorphological analysis is combined with computer modeling of channel profile uplift and erosion. Our results indicate that the valley network was formed by an ancient river system that existed in Greenland prior to glaciation. The channel is well preserved beneath the ice sheet, although some areas show evidence for erosional modification by glacial ice and/or meltwater. Our modeling also indicates that the river experienced two phases of enhanced incision during the past 10 million years. This incision was likely triggered widespread uplift, the causes of which remain unclear, but may have been associated with regional tectonic activity and/or the mantle hotspot that currently resides beneath Iceland. Analysis of airborne radio‐echo sounding data reveals an extensive subglacial valley network in northern GreenlandThe network developed over multi‐million‐year timescales and is paleo‐fluvial in origin with localized modification by glacial processesThe channel profile records two phases of base level fall, which may have been induced by widespread Neogene uplift events in Greenland Analysis of airborne radio‐echo sounding data reveals an extensive subglacial valley network in northern Greenland The network developed over multi‐million‐year timescales and is paleo‐fluvial in origin with localized modification by glacial processes The channel profile records two phases of base level fall, which may have been induced by widespread Neogene uplift events in Greenland

The Getz Ice Shelf (GIS) produces major amounts of basal meltwater due to intrusions of warm modified Circumpolar Deep Water (mCDW) beneath the ice shelf. However, multiple cavity openings and complex geography mean that knowledge of bathymetry beneath the GIS is required to understand ice/ocean interactions. We invert NASA airborne gravity data to obtain bathymetry beneath the ice shelf. Our gravity/geology‐constrained bathymetry is a significant advance on Bedmap2 bathymetry. The sub‐ice shelf bathymetry consists of three cavities separated by topographic ridges extending from the ice shelf front to the grounding line. Passages allowing limited circulation of shallow (≲400 meters below sea level [mbsl]) water between cavities are present, but deeper water is confined to individual cavities. Within each cavity, bathymetric troughs (>900 mbsl) extend from the ice shelf front to subglacial valleys beneath the ice sheet. Our analysis of the gravity data also allows us to infer the presence of thick (>500 m) sediments near the grounding line through much of the GIS, as well as variations in the density and/or thickness of the crust underlying the ice shelf. Floating ice shelves surrounding Antarctica slow the flow of the Antarctic Ice Sheet toward the ocean. West Antarctic ice shelves, including the large Getz Ice Shelf, have thinned significantly over the past 20 years, due in part to increased flow of warming ocean water into the ocean cavities beneath them. This thinning results in acceleration of ice flowing into the ocean, so that more ice goes afloat, raising sea level. We mapped the shape of the ocean floor beneath the Getz Ice Shelf, which controls water circulation under the ice shelf. We found the ice shelf has three separate portions each with deep troughs that provide pathways for warm water to the edge of the ice sheet. Inversion of airborne gravity data for bathymetry shows the Getz Ice Shelf to consist of three separate cavities with only shallow (<400 mbsl) connections between themWithin each cavity, deep troughs extend from the ice front to the grounding line, allowing warm modified Circumpolar Deep Water access to the inner ice shelf

Basal melting of ice shelves is a major source of mass loss from the Antarctic Ice Sheet. In situ measurements of ice shelf basal melt rates are sparse, while the more extensive estimates from satellite altimetry require precise information about firn density and characteristics of near‐surface layers. We describe a novel method for estimating multidecadal basal melt rates using airborne ice penetrating radar data acquired during a 3‐year survey of the Ross Ice Shelf. These data revealed an ice column with distinct upper and lower units whose thicknesses change as ice flows from the grounding line toward the ice front. We interpret the lower unit as continental meteoric ice that has flowed across the grounding line and the upper unit as ice formed from snowfall onto the relatively flat ice shelf. We used the ice thickness difference and strain‐induced thickness change of the lower unit between the survey lines, combined with ice velocities, to derive basal melt rates averaged over one to six decades. Our results are similar to satellite laser altimetry estimates for the period 2003–2009, suggesting that the Ross Ice Shelf melt rates have been fairly stable for several decades. We identify five sites of elevated basal melt rates, in the range 0.5–2 m a−1, near the ice shelf front. These hot spots indicate pathways into the sub‐ice‐shelf ocean cavity for warm seawater, likely a combination of summer‐warmed Antarctic Surface Water and modified Circumpolar Deep Water, and are potential areas of ice shelf weakening if the ocean warms. Antarctic ice shelves are the floating extensions of the grounded ice sheet. The ice shelves provide forces that slow the flow of grounded ice into the ocean, thus helping to stabilize the ice sheet. Mass loss from ice shelves occurs by basal melting and calving of icebergs. Here, we present a novel method for estimating basal melting using airborne ice penetrating radars. We used data collected from an airborne survey of the Ross Ice Shelf to measure the thickness of a bottom unit of the ice shelf that is made of pure ice originating from the grounded ice sheet. The change in thickness of this bottom unit was used to estimate basal melting. These new melt rates are similar to satellite‐derived estimates, indicating that the Ross Ice Shelf has been fairly stable over the past several decades. Most of the melting is concentrated at the ice shelf front at five localized hotspots with melt rates of about half to two meters per year. The elevated melting in the hot spots is likely caused by inflows of warm ocean water through pathways provided by bathymetry and thinner ice, and indicates regions of potential future weakness of the ice shelf to ocean change. Airborne ice penetrating radar survey (2015–2017) reveals three‐dimensional internal structure of the Ross Ice ShelfChanging thickness of a basal ice unit along ice flowlines is used to estimate multidecadal basal melt ratesElevated melt rates at hot spots along the ice front indicate pathways for warm seawater into the sub‐ice‐shelf ocean cavity