Fjord network in Namibia: A snapshot into the dynamics of the late Paleozoic glaciation

• Published in: Geology
• Main Authors: Dietrich, Pierre, Griffis, Neil P., Le Heron, Daniel P., Montañez, Isabel P., Kettler, Christoph, Robin, Cécile, Guillocheau, François
• Format: Article in Journal/Newspaper
• Language: English
• Published: GSA (Geological Society of America) 2021
• Subjects: Ice Sheet
• Online Access: https://oceanrep.geomar.de/id/eprint/58884/; https://oceanrep.geomar.de/id/eprint/58884/1/Dietrich%20et%20al.pdf; https://doi.org/10.1130/G49067.1

Fjords are glacially carved estuaries that profoundly influence ice-sheet stability by draining and ablating ice. Although abundant on modern high-latitude continental shelves, fjord-network morphologies have never been identified in Earth's pre-Cenozoic glacial epochs, hindering our ability to constrain ancient ice-sheet dynamics. We show that U-shaped valleys in northwestern Namibia cut during the late Paleozoic ice age (LPIA, ca. 300 Ma), Earth's penultimate icehouse, represent intact fjord-network morphologies. This preserved glacial morphology and its sedimentary fill permit a reconstruction of paleo-ice thicknesses, glacial dynamics, and resulting glacio-isostatic adjustment. Glaciation in this region was initially characterized by an acme phase, which saw an extensive ice sheet (1.7 km thick) covering the region, followed by a waning phase characterized by 100-m-thick, topographically constrained outlet glaciers that shrank, leading to glacial demise. Our findings demonstrate that both a large ice sheet and highland glaciers existed over northwestern Namibia at different times during the LPIA. The fjords likely played a pivotal role in glacier dynamics and climate regulation, serving as hotspots for organic carbon sequestration. Aside from the present-day arid climate, northwestern Namibia exhibits a geomorphology virtually unchanged since the LPIA, permitting unique insight into this icehouse.