As climate change progresses, glaciers continue to retreat worldwide. In the Arctic, glacial meltwater delivers sediments and nutrients to fjords and, ultimately, to the ocean. New research by Normandeau et al. reveals how glacial retreat impacts downstream sediment delivery and associated currents in Arctic fjords.
Heavier flows resulting from glacial retreat can boost the amount of sediment delivered to the mouths of rivers that empty into fjords, rapidly increasing the size of the river deltas. However, the links between glacial retreat and underwater delta dynamics are complex and have been poorly understood, limiting predictions of how, exactly, glacier retreat will reshape Arctic nearshore fjords.
To get a better picture of this system, the authors of the new study mapped the underwater features of 31 river mouths in fjords along the eastern coast of Canada’s Baffin Island. The maps incorporated high-resolution bathymetric data collected over several years from aboard the R/V Nuliajuk and the CCGS Amundsen as part of the ArcticNet program.
These mapping efforts revealed which deltas contained sediment waves, large-scale patterns in deposited sediment that are formed by the fast, downhill flow of sediment-laden water. These fast flows are known as turbidity currents, and their presence or absence depends on upstream glacial and watershed dynamics.
Statistical analysis of links between the mapping data and watershed data compiled for each river mouth showed that the presence of turbidity currents depends on the presence and size of upstream glaciers, which erode material that becomes transported as sediment. However, if lakes form upstream from fjords during glacial retreat, they may trap sediment, keeping it from flowing downstream and halting turbidity currents.
The researchers used these findings to create a model of evolving delta dynamics over the course of upstream glacial retreat. The model accounts for the formation of lakes that halt downstream turbidity currents, as well as reactivation of turbidity currents that may occur if lakes later fill up with sediment. The scientists applied their model to 644 rivers emptying into fjords along Baffin Island, predicting which are likely to contain turbidity currents.
This work could help improve predictions of future coastal changes worldwide, including effects on marine ecosystems that rely on nutrients transported in sediments. It could also help refine understanding of past glacial retreat. (Journal of Geophysical Research: Earth Surface, https://doi.org/10.1029/2018JF004970, 2019)
—Sarah Stanley, Freelance Writer