Unraveling the Effects of Ice and Permafrost on Arctic Deltas
Arctic delta channel dynamics and coastal depositional patterns are uniquely affected by ice and permafrost. Building on previous experiments that documented the individual effects of ice and permafrost, we conducted new numerical modeling experiments to examine the combined influences of ice thickness and permafrost erodibility. We quantified the effects of ice thickness and permafrost erodibility on channel dynamics, overbank deposition, offshore transport, and overall landscape morphology and use these results to infer future changes to Arctic deltas in a warmer climate.
Arctic deltas are covered with ice and underlain by permafrost, but no previous studies have examined how these effects (both individually and in combination) may alter the rates and patterns of water and sediment transport in the coastal zone. We showed that ice and permafrost both stabilize channels, such that channel mobility is likely to increase in the future, leading to less carbon storage in deltas and more carbon delivery to the ocean. Ice also increases overbank deposition that may help deltas keep up with sea-level rise and increases deposition in the coastal ocean due to under-ice transport that funnels water, sediments, and nutrients farther from shore. The amount of offshore transport, however, depends on the erodibility of permafrost that affects sediment supply, and the extent of overbank flooding is reduced by permafrost due to higher levee formation around permafrost-stabilized channels. The loss of ice and permafrost may therefore make Arctic deltas more susceptible to drowning by sea-level rise and may also change the onshore and offshore patterns of deposition and nutrient delivery.
Arctic delta channel dynamics and coastal deposition patterns are uniquely affected by ice and permafrost. Building on previous experiments that documented the individual effects of ice and permafrost, we conducted new numerical modeling experiments to examine the combined influences of ice thickness and permafrost erodibility. We quantified the effects of ice, permafrost, and their combination on channel mobility; onshore and nearshore deposition; and large-scale landscape morphology. Both ice and permafrost limited channel mobility and enhanced overbank deposition on the delta landscape. Thick ice increased offshore channel incision and sediment delivery. Permafrost stabilized channels and focused deposition that led to enhanced channel levee development. The increased levee deposition limited widespread deposition, leading to the preservation of abandoned channels. Levee growth was enhanced by thick ice that drives overbank flooding. This flooding led to more extensive levees on deltas with both ice and permafrost compared to deltas with only permafrost. Ice also preserved small channels, as the presence of ice in the channels reduced the fraction of time small channels remained active relative to larger channels, but this effect was damped by resistant permafrost. Preserving small channels creates a higher channel density that creates more transport pathways for riverine fluxes to the coast and also makes it easier for deltas to grow vertically to keep up with sea-level rise. Finally, our results suggest that the loss of both ice and permafrost will increase channel mobility, decrease overbank flooding, increase lateral growth rates of the deltas, and decrease offshore transport of riverine fluxes.