Neogene-Quaternary Uplift and Landscape Evolution in Northern Greenland Recorded by Subglacial Valley Morphology

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 (similar to 150 m at ca. 12-3.7 Ma and similar to 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. Plain Language Summary 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. Key Points 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

Automated summary

Analysis of airborne radio-echo sounding data reveals an extensive subglacial valley network in northern Greenland that developed over multi-million-year timescales. The channel network has a paleo-fluvial origin with localized modification by glacial processes. The study shows that the valley network experienced two episodes of relative base level fall during the Neogene, which may have been induced by widespread uplift events in Greenland.