Arctic regions are changing rapidly as permafrost thaws and sea ice retreats. These changes directly affect Arctic river deltas, but how permafrost and ice alter delta hydrology and sediment transport are not well researched. This knowledge gap limits our ability to forecast how these systems will respond to continued warming. We adapt the reduced complexity model of delta morphodynamics DeltaRCM to investigate the influences of permafrost and landfast ice on delta morphology and channel dynamics. We find that ice cover and permafrost decrease channel mobility, increase shoreline roughness, and route and deposit more sediment offshore. Ice cover also enhances overbank deposition, increasing subaerial delta elevations. Our modeling suggests that permafrost and ice loss in a warming climate could lead to less overbank and offshore deposition and more dynamic and spatially distributed fluxes of water and sediment across Arctic river deltas.

Cohesive sediment exerts a significant influence on delta evolution, increasing shoreline rugosity and decreasing channel mobility. Vegetation has been assumed to play a similar role in delta evolution, but its cohesive effects have not been explicitly studied. We use the model DeltaRCM to directly explore two effects of vegetation: decreasing lateral transport of sediment and increasing flow resistance. We find that vegetation and cohesive sediment do alter delta morphology and channel dynamics in similar ways (e.g., more rugose shorelines and deeper, narrower, less mobile channels) but that vegetation may have additional implications for deltaic sediment retention and stratigraphy, by confining flow and sand in channels. Our results suggest that sediment composition is a first-order control on delta morphology, but vegetation has a stronger influence on channel mobility time scales. To fully understand the cohesive influences acting on a delta, vegetation influence should be considered in addition to fine sediment.Plain Language Summary Human use of deltaic land relies on the existence of stable wetland habitat. Both vegetation and mud influence delta stability by making sediment more difficult to erode. Both introduce cohesion-mud because individual grains stick together, and vegetation because roots hold the sediment together. Vegetation also introduces friction, which has a similar effect to cohesion (decreasing erosion). When it is more difficult to erode channel banks, channels remain in the same location for longer, influencing where water and sediment are transported on the delta. Most studies consider only mud, but we compare the effects of these two sources of cohesion using a simple model that routes water and sediment on a delta. We find that mud and vegetation can affect deltas in similar ways. However, vegetation also changes the patterns in which sand and mud are deposited on deltas and can cause more sediment to be transported to the ocean instead of deposited on the delta. Additionally, we find that cohesive sediment is more important in determining delta shape but that vegetation is more important in controlling how much channels move. To fully understand how cohesion influences deltas, we should consider the effects of both mud and vegetation.

Volcanic ash has long been recognized in marine sediment, and given the prevalence of oceanic and continental arc volcanism around the globe in regard to widespread transport of ash, its presence is nearly ubiquitous. However, the presence/absence of very fine-grained ash material, and identification of its composition in particular, is challenging given its broad classification as an "aluminosilicate" component in sediment. Given this challenge, many studies of ash have focused on discrete layers (that is, layers of ash that are of millimeter-to-centimeter or greater thickness, and their respective glass shards) found in sequences at a variety of locations and timescales and how to link their presence with a number of Earth processes. The ash that has been mixed into the bulk sediment, known as dispersed ash, has been relatively unstudied, yet represents a large fraction of the total ash in a given sequence. The application of a combined geochemical and statistical technique has allowed identification of this dispersed ash as part of the original ash contribution to the sediment. In this paper, we summarize the development of these geochemical/statistical techniques and provide case studies from the quantification of dispersed ash in the Caribbean Sea, equatorial Pacific Ocean, and northwest Pacific Ocean. These geochemical studies (and their sedimentological precursors of smear slides) collectively demonstrate that local and regional arc-related ash can be an important component of sedimentary sequences throughout large regions of the ocean.

Deltas are vulnerable landscapes that are highly susceptible to climate change. High-latitude deltas face additional unique challenges arising from the thaw of permafrost (