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In Late Wisconsin sediments of Minnesota there occur fossil pollen and spores of certain taxa that are unlikely on ecological or phytogeographical grounds to have grown in Minnesota at that time. Long-distance dispersal is believed to be the cause. The occurrences of 21 such types in three sites in Minnesota are analyzed statistically. Contingency-table analysis suggests that some types occur together more commonly or more rarely than would be expected by chance. Principal co-ordinates analysis similarly reveals groups of taxa whose pollen commonly occur together in time. The patterns within the series of occurrences, when viewed as point processes, are characterized by the techniques of statistical analysis of series of events. These analyses show that the series of occurrences of Acer saccharum, Carya, Celtis occidentalis,Juglans cinerea, Platanus occidentalis, and probably A. saccharinum-type and Tilia americana pollen all have statistically significant trend and that their rates of occurrence are higher between 8300 and 10200 B.P. Athyrium fihix-fetnina spores show a higher rate of occurrence between 20500 and 12000 B.P. The series of occurrences of all the other pollen types accord with a trend-free renewal process. The series of Acer negundo, Iva ciliata-type, I. xanthzfolia-type, Morus rubra, and Sarcobatus vermiculatus pollen are long enough to permit more detailed analysis. All these series appear to reflect a homogeneous Poisson process with random occurrences in time. Hypotheses to explain these patterns are discussed in terms of migration and population expansion to the south and east of Minnesota and of changing atmospheric circulation patterns during the Late Wisconsin. The need for more detailed series of occurrences is emphasized. [ABSTRACT FROM AUTHOR]

The geographical and seasonal occurrences of dinoflagellate cysts at a depth of 10 m in the North Atlantic and North Sea are described from samples taken by the Continuous Plankton Recorder between June 1973 and December 1975. A special study was made of the large cyst of Polykrikos schwartzii Bütschli in the autumn of 1973 when CPR samples were supplemented by vertical sampling in the upper 500 m with a Longhurst-Hardy Plankton Recorder at Ocean Weather Station, INDIA. Only ten isolated records of cysts were made in the western Atlantic. All others occurred to the east of 30°W in both oceanic and neritic waters and especially in the shallow waters of the North Sea. Cysts were found in the spring plankton but were most abundant in the autumn. They have been recorded down to 500 m but occur most frequently in the top 30 m of the water column. Large aggregations of cysts in bottom sediments probably reflect concentration over many years since they cannot be explained by the low numbers found in the plankton. Generally, the distribution of cysts in the plankton is reflected in bottom sediments but concentrations in sediments may be outside the centres of production, implying movement by currents or sediment transport. [ABSTRACT FROM AUTHOR]

Summarizes works on the interaction of alpine snow conditions and processes of sediment movement and deposition in Green Lakes Valley, Colorado Front Range. Snow dynamics; Indirect influences at the ground surface.

Turbidity currents are a phenomenon of major geologic importance in marine sedimentation. Transport of shallow water sediments from near shore to the great depths of the sea has been abundantly demonstrated by several investigators. Depending upon nature, volume, composition, and velocity turbidity currents might affect the fauna in two principal ways; first as an immense destructive and burial force as evidenced by the Grand Banks current carrying 220 billion cubic meters of shallow water sediment over an area of 200,000 square kilometers of the abyss; and second, as a vehicle for the transport of nutritive matter from the shallow water. Transportation and sudden burial in the deep sea of organisms provides a mechanism for removing organic matter from the shallow water food cycle. These events are apparently necessary for the preservation of many fossil assemblages. It is commonly believed that petroleum formation occurred only in shallow-water sediments. The discovery of clearly translocated shallow water fossils in deep sea sediments suggests the possibility that petroleum formation may also have occurred in the deep sea. [ABSTRACT FROM AUTHOR]

Submarine flows carve canyons into continental shelves, yet the conditions and events responsible for canyon incision are incompletely understood. Coarse sediment flux has been shown to promote terrestrial bedrock incision via abrasion in rivers, but similar processes in submarine canyons have yet to be systematically documented. We use repeat bathymetry, provenance analysis, wave modeling, and channel network analysis to show that longshore sheltering and wave focusing by the Delgada submarine canyon induce sediment accumulation and elevated wave shear stresses in its headwall region that frequently mobilize coarse bed material. These mobilizations scour bedrock in the headwall and generate abrasive turbidity currents that work to carve the canyon's active channel into bedrock. These findings highlight an important positive feedback between submarine canyons, waves, and sediment supply and suggest that submarine canyons adjacent to wave‐dominated, coarse sediment‐rich coastlines seek the shoreline through headward incision. Plain Language Summary: The carving of submarine canyons into the continental shelf is poorly understood relative to river valleys on land. We studied a submarine canyon in Northern California that is connected through littoral transport to river sediment using a combination of seafloor mapping, tracking sediments using their chemical elements, and simulating waves and currents to better understand which processes are responsible for canyon erosion. Our findings suggest that canyon focusing of wave energy and an abundant supply of coarse sediment cause erosion at the canyon's head. This finding helps explain why submarine canyon channel networks erode toward shore and predicts that canyons near mountain ranges will preferentially remain connected the shoreline. Key Points: Repeat sonar surveys and sedimentary provenance document sediment transport and erosion in Delgada submarine canyon, Northern CaliforniaHydrodynamic modeling and littoral budgets suggest that coarse sediment is mobilized by waves in the canyon head, triggering turbidity currentsSediment supply and wave focusing promote erosion of submarine canyons along tectonic coasts, encouraging littoral to deep sea connectivity [ABSTRACT FROM AUTHOR]

Topographic relief and river bedforms generate nested streamline patterns, which drive the propagation and mixing at depth of changes in surface water concentration or temperature. While concentration distributions and biogeochemical reactions in such flow cells are often studied under steady state transport conditions, there is increasing evidence that transient mixing processes may have a significant contribution to effective mixing and reaction rates. Here we show that these streamline patterns act as shear flows, which significantly accelerate mixing dynamics within flow cells and can lead to the formation of transient mixing hot spots at depth. We provide analytical solutions that quantify the dynamics of mixing in a flow cell for a pulse and a front initial solute distribution, which represent two idealized end‐members of more complex solute distributions in natural systems. These results provide new insights into the patterns and dynamics of mixing at hyporheic zone, hillslope, and catchment scales. Plain Language Summary: As surface water infiltrates in the subsurface in riverbeds or hillslopes, it mixes with the resident groundwater, which leads to changes in the concentration of transported dissolved chemical species. Such transient mixing processes play a key role in contaminant transport and biogeochemical reactions. Here we demonstrate that flow patterns that are generated by topography gradients at different scales act effectively as shear flows due to the differential velocity of neighboring streamlines. We show that such shear flows strongly enhance transient mixing rates and may lead to the formation mixing hot spots where mixing rates are orders of magnitude larger than the background. These results will likely contribute to the understanding and modeling of solute transport and reaction in the context of surface water‐groundwater interactions. Key Points: Hydraulic head gradients induced by bedforms and hillslopes generate shear flows in the subsurfaceShear flows accelerate transient mixing rates in flow cells and trigger transient mixing hot spotsEstimates of shear rates for hyporheic zones, hillslopes, and catchments vary over orders of magnitude [ABSTRACT FROM AUTHOR]

Reading the sediment record in terms of past climates is challenging since linking climate change to the associated responses of sedimentary systems is not always straightforward. Here we analyze the erosional response of landscapes on the Tibetan Plateau to interglacial climate forcing. Using the theory of dynamical systems on Holocene time series of geochemical proxies, we derive a sedimentary response model that accurately simulates observed proxy variation in three lake records. The model suggests that millennial variations in sediment composition reflect a self‐organization of landscapes in response to abrupt climate change between 11.6 and 11.9 ka BP. The self‐organization is characterized by oscillations in sediment supply emerging from a feedback between physical and chemical erosion processes, with estimated response times between 3,000 to 18,000 years depending on catchment topography. The implications of our findings emphasize the need for landscape response models to decipher the paleoclimatic code in continental sediment records. Plain Language Summary: Lake sediments are an important source of information on past climates. Reading the information is not always straightforward. Complex interactions in landscapes can affect the transmission of climatic signals to the sediment record. However, the exact nature of such complex interactions remains unknown. This study compares sediment deposits of three lakes on the Tibetan Plateau. The deposits are continuous records of landscape responses to climate change during the last 12,000 years. We identified a mathematical model that accurately simulates changes in sediment composition at all sites. The model simulations suggest that an abrupt warming at the end of the last glacial period destabilized the landscapes. This caused fluctuations in the transport of sediments, which persisted for several thousand years. Our findings present evidence for a long‐lasting impact of abrupt climate change on fundamental Earth surface processes. Key Points: We analyzed the propagation of climatic signals to Holocene sediment records of three well‐studied Tibetan lakes in a dynamical system frameworkHolocene variation in geochemical proxies mainly reflect site‐specific landscape responses to abrupt climate changeOur results indicate a feedback between physical and chemical erosion processes on interglacial timescale [ABSTRACT FROM AUTHOR]

Records of fluvial suspended sediment fluxes are sensitive indicators of hydrometeorological and permafrost change. Here we document the watershed‐scale suspended sediment flux response to a period of hydrometeorological change and landscape disturbance in two High Arctic rivers. Net in‐channel and extra‐channel sediment storage and changing hydrometeorological conditions dampen the downstream transport of increased sediment delivery from localized permafrost slope disturbances. Our results show that the impact of permafrost disturbance is likely a smaller effect than a shift toward a pluvially (rainfall) dominated hydrological regime in these environments. Suspended sediment transport is energy limited under contemporary hydrometeorological conditions, and the transition from a nival to pluvial dominated flow and sediment transfer regime will likely accelerate landscape change in the High Arctic. Plain Language Summary: The long‐term impact and response of High Arctic river systems to climate and landscape change remain poorly constrained. We investigate the effects of climate and permafrost change on sediment transport in two High Arctic rivers in Canada (75°N) during a period of rapid climate change and landscape disturbance using a 15‐year record of suspended sediment export—the longest record of its kind in the High Arctic. We find that increased sediment erosion from permafrost landscape disturbances has not increased the downstream‐suspended sediment export beyond an immediate, short‐lived response due to (1) the rivers ability to store additional sediment and (2) offsetting hydroclimatic controls. Our findings further suggest that these river systems are potentially in a transition period, shifting from a snowmelt to a rainfall dominated flow regime, but current rainfall magnitudes are below a critical threshold to mobilize additional sediment in these systems. We conclude that increased rainfall magnitude is likely to be the tipping point to significant long‐term geomorphic change and downstream effects in the Canadian High Arctic. Key Points: High Arctic fluvial systems moderate the response of increased sediment delivery from localized disturbances through in‐channel storageNival generated geomorphic energy has decreased over time, coincident with the increased importance of sporadic pluvial eventsModern rainfall magnitudes are typically below the threshold to cause widespread mobilization of additional sediment in these systems [ABSTRACT FROM AUTHOR]

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. Key Points: To fully understand cohesive influences on delta evolution, vegetation influence should be considered in addition to sediment compositionSediment composition has a stronger influence on plan view shape, but vegetation affects channel dynamics and topography more stronglyVegetation may reduce total deltaic sediment retention and increase stratigraphic sorting by confining sediment‐laden flow in channels [ABSTRACT FROM AUTHOR]

Previous studies have shown that sediment transport models based on bed shear stress (τ) are not accurate for regions with vegetation. The present study demonstrated that the inaccuracy arises from the influence of vegetation‐generated turbulence. Bed‐load transport rate, Qs, and near‐bed velocity were measured in a sediment‐recirculating flume with model vegetation of different vegetation volume fractions (ϕ) and with bare sand beds. At the same τ, the measured Qs increased with increasing ϕ, suggesting that vegetation‐generated turbulence, which also increased with increasing ϕ, was augmenting the bed‐load transport. At the same near‐bed turbulent kinetic energy, kt, the Qs measured in both bare and vegetated channels agreed within uncertainty, suggesting that kt may be a more universal predictor of bed‐load transport than bed shear stress. A τ‐based bed‐load transport model was reinterpreted as a kt‐based model. The new kt‐based model predicted the Qs measurements for both bare and vegetated channels. Plain Language Summary: Aquatic vegetated habitats, including wetlands and mangroves, are disappearing at an annual rate of 1% to 7%. These ecosystems provide habitat important to fisheries, enhance water quality by filtering nutrients from runoff, and also protect coastal regions from storm surge and waves. To mitigate the loss of these habitats, restoration projects import sediment to eroded areas. The success of the restoration depends on its ability to retain sediment, so that restoration design requires a good understanding of sediment transport within vegetated landscapes. However, this understanding is currently lacking, and many restoration projects fail due to unanticipated sediment loss from the restored regions. The goal of our study was to develop a predictive model for sediment transport in regions with vegetation. Using laboratory experiments with a model vegetation, we first showed that existing sediment transport models, based on studies without vegetation, underestimate by orders of magnitude the sediment transport in vegetated regions. Second, we demonstrated that the enhanced sediment transport was caused by vegetation‐generated turbulence. Finally, we proposed and verified a new model for sediment transport that incorporates the role of vegetation‐generated turbulence, which we believe will be an important guide for future coastal and riverine restoration projects. Key Points: For the same channel velocity, sediment transport rate increases with increasing vegetation volume fractionBed shear stress models, which neglect vegetation‐generated turbulence, do not apply in vegetated regionsA turbulence‐based model predicted bed‐load transport in both bare and vegetated channels [ABSTRACT FROM AUTHOR]

Hydrological connectivity in coastal deltas is important for delivering flow, sediment, and nutrients to the island interiors. The roughness of island vegetation limits connectivity, but how important is the spatial distribution of vegetation? Using hydrodynamic modeling, we test the influence of vegetated percent cover, patch size, and stem density on the discharge‐fraction allocated to the islands of an idealized delta complex, modeled after Wax Lake Delta. We find that heterogeneity has negligible effects when vegetation is relatively sparse but is important when vegetation is relatively dense and covers less than a "disconnectivity" threshold of 40–50% of the islands, near the theoretical percolation limit. Below this threshold, preferential flow paths develop in the islands, which alter the hydraulics and residence time distribution of the delta complex and enhance potential sediment transport with respect to model runs with uniform vegetation. Patchiness has hydrogeomorphic and biogeochemical implications, which should be considered when modeling deltaic systems. Plain Language Summary: Coastal river deltas are vulnerable regions of tremendous economic and ecological importance. The long‐term sustainability of deltaic systems depends in part on the delivery of water, sediment, and nutrients to the interior of deltaic islands. The vegetated wetlands within these islands are typically hot spots of land growth and biological nutrient removal, which can counteract land loss and prevent the expansion of coastal oxygen‐depleted zones. Increasing the amount of island vegetation is known to decrease the fraction of flow that enters the islands, but it has yet to be shown how the spatial arrangement of vegetation patches affects this channel‐island "hydrological connectivity." In the present study, we numerically model flows in a simplified river delta to determine the influence of three vegetation characteristics on connectivity: the percentage of the island covered by patches, patch size, and patch density (how tightly packed the vegetation is within each patch). We find that all three characteristics can increase connectivity when compared to similar model runs with spatially uniform vegetation. More specifically, the spatial configuration becomes important when patches are dense and cover less than 40–50% of the deltaic islands. The results of this study have implications for our ability to model the success of restoration projects. Key Points: Channel‐island hydrological connectivity in river deltas is affected by the patchiness of vegetation within deltaic islandsIsland water and solute transport is dominated by preferential flow paths below a "disconnectivity" threshold of 40–50% vegetated coverPatchiness increases potential sediment transport and alters residence time distributions, with implications for modeling deltaic systems [ABSTRACT FROM AUTHOR]

Unique observations of the hydrodynamics across the sediment‐water interface are used to quantify the instantaneous response of the interstitial fluid to the passage of coherent turbulent motions in the overlying flow. Over a range of permeability Reynolds numbers ReK=Ku∗/ν (where K is the sediment permeability, u∗ is the shear velocity, and ν is the fluid viscosity), the passage of these turbulent motions create velocity fluctuations and momentum fluxes at the sediment‐water interface to greatly exceed their mean values. Sweep motions are observed to penetrate into the sediment bed and induce instantaneous momentum fluxes that can be an order of magnitude larger than the mean bed shear stress. By penetrating into the sediment bed, the turbulent motions increase the effective roughness experienced by the flow and therefore the flow resistance. The properties of the mean flow alone are thus insufficient to describe the interaction of the overlying flow with the sediments. Plain Language Summary: Biochemical and physical processes at the sediment‐water interface (SWI) are strongly affected by the magnitude of the instantaneous flow velocity and flow‐induced forces. As measurements of the flow across the SWI are complicated by the presence of the sediment bed, little is known about the influence of large turbulent motions on the flow at the interface. In this study, we use a rare experimental methodology of the flow across the interface to identify the response at the SWI to the passage of large turbulent motions in the overlying flow. We show that these large turbulent motions can penetrate across the SWI into the sediment bed and induce instantaneous interfacial velocities and forces that are significantly greater than their time‐averaged values. By penetrating into the sediment bed, these large turbulent motions enhance the flow resistance of the sediments to the overlying flow. Key Points: Sweep events can penetrate across the sediment‐water interface and induce periods of significant flow variability across the interfaceCoherent turbulent motions cause instantaneous momentum fluxes to greatly exceed the mean bed shear stressBy penetrating into the sediment bed, coherent turbulent motions increase the effective roughness experienced by the overlying flow [ABSTRACT FROM AUTHOR]

Autonomous underwater glider observations collected during and after 2017 Hurricanes Irma, Jose, and Maria show two types of transient response within the Gulf Stream. First, anomalously fresh water observed near the surface and within the core of the Gulf Stream offshore of the Carolinas likely resulted from Irma's rainfall being entrained into the Loop Current‐Gulf Stream system. Second, Gulf Stream volume transport was reduced by as much as 40% for about 2 weeks following Jose and Maria. The transport reduction had both barotropic and depth‐dependent characteristics. Correlations between transport through the Florida Straits and reanalysis winds suggest that both local winds in the Florida Straits and winds over the Gulf Stream farther downstream may have contributed to the transport reduction. To clarify the underlying dynamics, additional analyses using numerical models that capture the Gulf Stream's transient response to multiple tropical cyclones passing nearby in a short period are needed. Plain Language Summary: In September 2017, Hurricanes Irma, Jose, and Maria impacted the Gulf Stream in rapid succession. Observations collected by an autonomous underwater glider uniquely capture the subsurface structure of the Gulf Stream during this time period, revealing two distinct transient responses of the oceanic western boundary current in the weeks following the storms. Unusually, fresh water within the Gulf Stream off the coast of the Carolinas is attributable to rain from Irma that was entrained in the Loop Current‐Gulf Stream system. A reduction in Gulf Stream volume transport by as much as 40% over a period of about 2 weeks likely resulted from the hurricane‐related winds over the Gulf Stream. The success of the glider mission and a submarine cable in capturing the Gulf Stream's response to passing storms highlights the need for sustained deployment of ocean observing assets to measure before, during, and after the passage of tropical systems. To clarify the underlying dynamics of the Gulf Stream's transient response to these storms, additional analyses using realistic numerical models are needed. Key Points: A Spray glider collected multiple cross‐Gulf Stream transects during the 2017 Atlantic hurricane seasonAn upper ocean salinity anomaly in the Gulf Stream likely resulted from Irma‐induced precipitation over Florida and the west Florida shelfGulf Stream transport was reduced by as much as 40% for about 2 weeks after the passages of Hurricanes Jose and Maria [ABSTRACT FROM AUTHOR]

Large ripples with meter‐scale wavelengths are ubiquitous across Mars. Curiosity's traverse of the Bagnold Dune Field revealed a morphologic diversity of large Martian ripples that helps constrain their formative mechanism. Large ripples develop in isolated fields and on dunes. They form transversely and obliquely to longitudinally to the net sand‐flux direction in unimodally and bimodally distributed very fine to very coarse sand. They have either straight or sinuous crestlines. Inactive ripples are covered with dust, whereas migrating ripples are dust free. Here we present a unifying view of ripples that form in near‐bed sediment‐transport conditions (encompassing fluid‐drag and coarse‐grained ripples) to explain the range of large‐Martian‐ripple morphologies and expand the use of bedforms as environmental indicators. Plain Language Summary: Large sand ripples with meter‐scale crest‐to‐crest spacings abound on the Martian surface but are not found in terrestrial sandy deserts. Along its traverse through the Bagnold Dune Field, the Curiosity rover witnessed a rich diversity of such large ripples, both in terms of their shape and of their behavior. Rover observations help understand how these mysterious large Martian ripples form. Here we present a unifying view of ripples that form when grains are mobilized near the sediment bed by various fluids and across planets. Such an understanding of the formation mechanics of similar ripples will enable scientists to use the ripples' imprints on landscapes and in rocks to infer modern and past environmental conditions. Key Points: Martian ripples are diverse in morphology, dynamics, and activityLarge Martian ripples belong to a family of bedforms that forms when a significant fraction of sediment transport occurs near the bedNear‐bed‐transport bedforms are found across environments and planets and may prove useful quantitative paleoenvironment indicators [ABSTRACT FROM AUTHOR]

The patterns of underwater acoustic signals, called soundscapes, were characterized at the river segment scale. A hydrophone was mounted onto the frame of an inflatable multihull raft and held just below the water's surface while floating for 5-24 km to continuously record acoustic signals along the longitudinal axis of the main channel (thalweg) of five hydro-geomorphologically different river segments in Italy, Switzerland, and the United States of America. The river segments could be clearly distinguished based on sound pressure level (SPL), sound variability, and the spatial organization of the acoustic signal (10 octave bands: 0.0315-16 kHz). The spatial soundscape diversity between river segments was most likely related to the organization of turbulence along the segment length (distance between rapids) and to sound source (hydraulic turbulence or local sediment transport). Hydraulically and morphologically heterogeneous segments revealed more complex soundscapes than more homogeneous ones. Higher flow levels resulted in higher SPLs over most frequency bands. The acoustic variability of single octave bands increased from base to intermediate flow, while it decreased from intermediate to bankfull flow. The pulsating sound produced by breaking and reforming turbulent waves on flow obstacles was associated with SPL peaks and high acoustic variability in midrange frequency bands (0.063-0.5 kHz), whereas high-frequency SPLs (1-16 kHz) were related to particle collisions during streambed sediment transport. Underwater soundscapes provide an independent measure to quantify habitat heterogeneity and spatial habitat organization that can be applied to river corridors at the segment scale. [ABSTRACT FROM AUTHOR]