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Groundwater solute mass flux discharged from the continents to the oceans is between 56% and 63% of particulate sediment transport mass flux. Herein we utilized newly developed continental geospatial groundwater concentration estimates that were multiplied by groundwater volumetric recharge flux to provide a continental-scale discharge mass flux to the oceans of 7.3 Pg DS/yr (petagrams dissolved solutes per year). This mass flux was evaluated from six continental ecosystems: direct ocean discharge (0.28 Pg DS/yr), endorheic basins (0.59 Pg DS/yr), cold-wet exorheic basins (0.55 Pg DS/yr), cold-dry exorheic basins (1.1 Pg DS/yr), warm-dry exorheic basins (0.82 Pg DS/yr), and warm-wet exorheic basins (4.0 Pg DS/yr), thus providing insight into the role of rainfall and temperature on continental weathering and denudation. A new, robust molar silicate/carbonate ratio of 0.42 was calculated for weathering of continental rocks, which is important in the Urey model of climate change. We estimate that rock weathering accounts for ~50% of the total solute mass flux discharged from the continents, the remainder being from externally derived marine aerosols and organic-derived bicarbonate. [ABSTRACT FROM AUTHOR]

Submarine channels parallel river channels in their ability to transport sediment. However, in contrast to rivers, sediment transport and bed-form development in submarine channels are less well understood. Many steep (>1°), sandy submarine channels are dominated by upstream-migrating bed forms. The flow conditions required to form these upstream-migrating bed forms remain debated because the interactions between turbidity currents and active bed forms are difficult to measure directly. Consequently, we used a depth-resolved numerical model to test the role of flow parameters that are hypothesized to control the formation of upstream-migrating bed forms in submarine channels. While our modeling results confirmed the importance of previously identified flow parameters (e.g., densiometric Froude number), we found that basal sediment concentration in turbidity currents is the strongest predictor of upstream-migrating bed-form formation. Our model shows how locally steep gradients enable high sediment concentrations (average >5 vol%) in the basal parts of flows, which allow the development of cyclic step instabilities and their associated bed forms. This new insight explains the previously puzzling observation that upstream-migrating bed forms are abundant in proximal, steep, sandy reaches of submarine channels, while their occurrence becomes more intermittent downslope. [ABSTRACT FROM AUTHOR]

Fluvial fans are large, low-gradient depositional systems that occur in sedimentary basins worldwide. Fluvial fans can represent much of the geologic record of foreland basins, create hazards, and record paleoclimate and tectonic signals. However, we lack an understanding of how fluvial fans grow into the variety of shapes observed around the world. We explored this aspect using a cellular model of foreland basin landscape evolution with rules for sediment transport, river avulsion, and floodplain processes. We tested the hypothesis that avulsion dynamics, namely, avulsion trigger period and abandoned channel dynamics, are a primary control on fluvial fan development. We found that shorter trigger periods lead to rounder planform fluvial fan shapes because, between avulsions, channel aggradation (and thus avulsion setup) propagates shorter distances from the upstream boundary along channel pathways. This prioritizes lateral sediment dispersion, creating shorter, rounder fans, over sediment delivery further into the basin, which would create elongated fans. Modeled fans with abandoned channel attraction (but not repulsion) generated a commonly observed abrupt fan boundary marked by a transition from distributary to tributary channel patterns. While fluvial fans are thought to be linked to climate, they can occur anywhere that rivers aggrade, lose lateral confinement, and preserve alluvial topography. Instead, fluvial fans might be more recognizable in environments that frequently trigger avulsions and preserve abandoned channels that capture future avulsions. [ABSTRACT FROM AUTHOR]

The geological processes that occurred during the deposition of the Mediterranean salt giant are poorly constrained, limiting our understanding of the earliest phase of the Messinian Salinity Crisis (MSC). Using three-dimensional seismic reflection data from the northern Levant Basin in the eastern Mediterranean Sea, we investigated a previously unrecognized deposit at the base of the Messinian halite that we interpret as an extensive mass-transport complex (MTC). While MTCs have been described within stratigraphically equivalent deposits in the western Mediterranean Sea, this is the first such MTC documented at the base of the halite in the eastern Mediterranean. The spatial distribution and seismic facies of the MTC suggest a slope failure origin, likely in response to multiple processes including tectonic activity and sea-level fall. Our study indicates that margin instability was diffuse across the entire Mediterranean during the early stages of Messinian halite deposition, with implications for the chronology of erosional events and sediment transport to the deep basin. [ABSTRACT FROM AUTHOR]

Bedrock river-gorge incision represents a fundamental landscape-shaping process, but a dearth of observational data at >10 yr timescales impedes understanding of gorge formation. I quantify 102 yr rates and processes of gorge incision using historical records, field observations, and topographic and image analysis of a human-caused bedrock meander cutoff along the North Fork Fortymile River in Alaska (USA). Miners cut off the meander in 1900 CE, abruptly lowering local base level by 6 m and forcing narrowing and steepening of the channel across a knickpoint that rapidly incised upstream. Tectonic quiescence, consistent rock erosivity, and low millennial erosion rates provide ideal boundary conditions for this 102 yr gorge-formation experiment. Initial fast knickpoint propagation (23 m/yr; 1900-1903 CE) slowed (4 m/yr; 1903-1981 CE) to diffusion (1981-2019 CE) as knickpoint slope decreased, yielding an ~350-m-long, 6-m-deep gorge within the pre-1900 CE channel. Today, diffusion dominates incision of a 500-m-long knickzone upstream of the gorge, where sediment transport likely limits ongoing adjustments to the anthropogenic cutoff. Results elucidate channel width, slope, discharge, and sediment dynamics consistent with a gradual transition from detachment- to transport-limited incision in fluvial adjustment to local base-level lowering. [ABSTRACT FROM AUTHOR]

Carboniferous sediment dispersal from the Appalachian orogenic system (eastern United States) has become a topic of widespread interest. However, the actual pathways for continental-scale, east-to-west sediment transfer have not been documented. This study presents detrital zircon (DZ) U-Pb ages and Hf isotopic values from the Lower Pennsylvanian (Morrowan) Jackfork Group and Johns Valley Shale of the synorogenic Ouachita deepwater basin of Arkansas to document provenance and delineate the likely sediment-routing systems within the broader context of sediment dispersal across Laurentia. Twelve (12) DZ U-Pb age distributions are interpreted to indicate that sediments were derived from the Appalachians to the east and northeast, as well as the midcontinent region to the north. All samples display prominent ca. 500-400 Ma, 1250-950 Ma, 1550-1300 Ma, and 1800-1600 Ma grains, consistent with ultimate derivation from the Appalachian, Grenville, Midcontinent, and Yavapai-Mazatzal provinces. DZ Hf values obtained from the Ouachita Basin are similar to published Hf values from Pennsylvanian samples in the Appalachian and Illinois Basins. Age distributions are generally consistent for seven samples collected from the Jackfork Group and Johns Valley Shale in the southern Ouachita Mountains through ~2400 m of stratigraphic section and are interpreted to indicate little change in provenance during the Morrowan in this part of the system. However, samples from the most northern and most source-proximal site in Little Rock, Arkansas, exhibit modest percentages of Appalachian ages and elevated contributions of Yavapai-Mazatzal ages when compared with samples collected farther to the south and west. We interpret differences between DZ signatures to indicate distinct sediment-routing pathways to the Ouachita Basin. We infer the strong Appalachian and Grenville signals to represent an axial system flowing through the Appalachian foredeep, whereas the more diverse signals represent a confluence of rivers from the northeast through the backbulge of southern Illinois and western Kentucky and from the north across the Arkoma shelf. Collectively, the Ouachita Basin represents a terminal sink for sediments derived from much of the eastern and central United States. [ABSTRACT FROM AUTHOR]

The Paleocene-Eocene thermal maximum (PETM) was the most extreme example of an abrupt global warming event in the Cenozoic, and it is widely discussed as a past analog for contemporary climate change. Anomalous accumulation of terrigenous mud in marginal shelf environments and concentration of sand in terrestrial deposits during the PETM have both been inferred to represent an increase in fluvial sediment flux. A corresponding increase in water discharge or river slope would have been required to transport this additional sediment. However, in many locations, evidence for changes in fluvial slope is weak, and geochemical proxies and climate models indicate that while runoff variability may have increased, mean annual precipitation was unaffected or potentially decreased. Here, we explored whether changes in river morphodynamics under variable-discharge conditions could have contributed to increased fluvial sand concentration during the PETM. Using field observations, we reconstructed channel paleohydraulics, mobility, and avulsion behavior for the Wasatch Formation (Piceance Basin, Colorado, USA). Our data provide no evidence for changes in fluvial slope during the PETM, and thus no evidence for enhanced sediment discharge. However, our data do show evidence of increased fluvial bar reworking and advection of sediment to floodplains during channel avulsion, consistent with experimental studies of alluvial systems subjected to variable discharge. High discharge variability increases channel mobility and floodplain reworking, which retains coarse sediment while remobilizing and exporting fine sediment through the alluvial system. This mechanism can explain anomalous fine sediment accumulation on continental shelves without invoking sustained increases in fluvial sediment and water discharge. [ABSTRACT FROM AUTHOR]

Middle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; southwestern United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposition with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527-519 Ma) grains at this provenance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local basement uplifts were largely buried by Middle to Late Jurassic time. This study aims to delineate spatial and temporal patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrography, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth elemental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in western Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53-1.3 Ga, 1.3-0.9 Ga, and 500-300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540-500 Ma zircon grains are anomalously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochemistry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540-500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identification of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wanakah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin suggests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT. [ABSTRACT FROM AUTHOR]

Waterfall plunge pools experience cycles of sediment aggradation and scour that modulate bedrock erosion, habitat availability, and hazard potential. We calculate sediment flux divergence to evaluate the conditions under which pools deposit and scour sediment by comparing the sediment transport capacities of waterfall plunge pools (Qsc_pool) and their adjacent river reaches (Qsc_river). Results show that pools fill with sediment at low river discharge because the waterfall jet is not strong enough to transport the supplied sediment load out of the pool. As discharge increases, the waterfall jet strengthens, allowing pools to transport sediment at greater rates than in adjacent river reaches. This causes sediment scour from pools and bar building at the downstream pool boundary. While pools may be partially emptied of sediment at modest discharge, floods with recurrence intervals >10 yr are typically required for pools to scour to bedrock. These results allow new constraints on paleodischarge estimates made from sediment deposited in plunge pool bars and suggest that bedrock erosion at waterfalls with plunge pools occurs during larger floods than in river reaches lacking waterfalls. [ABSTRACT FROM AUTHOR]

The amount of silt and clay available to rivers reflects source-terrain composition and weathering and can be a primary control on the form and dynamics of channel networks. Fine sediment also affects the permeability of buried fluvial reservoirs. Despite this significance, there is currently a lack of methods for reconstructing how much fine sediment was transported by ancient rivers. Mud accumulations in sandy river deposits are often interpreted as indicators of variable flow conditions; however, these deposits may present an opportunity to constrain how much fine sediment was transported through ancient rivers. We report results from a series of experiments designed to evaluate how much clay and silt are preserved in sandy riverbed deposits under constant and variable discharge conditions. Our results demonstrate that (1) mud deposits, including drapes and lenses, form readily under constant, high-discharge conditions, (2) the amount of fine sediment recovered from bed-material deposits increases as fine-sediment supply increases, and (3) fine-sediment retention is higher during bed aggradation than during bypass conditions. These results indicate that the net retention of clay and silt in sandy riverbed deposits may be a simple but powerful proxy for comparing the overall amount of fine sediment supplied to ancient rivers. [ABSTRACT FROM AUTHOR]

The active Pacific margin of the Haida Gwaii and southeast Alaska has been subject to vigorous storm activity, dramatic sea-level change, and active tectonism since glacial times. Glaciation was minimal along the western shelf margin, except for large ice streams that formed glacial valleys to the shelf break between the major islands of southeast Alaska and Haida Gwaii. Upon deglaciation, sediment discharge was extensive, but it terminated quickly due to rapid glacial retreat and sea-level lowering with the development of a glacio-isostatic forebulge, coupled with eustatic lowering. Glacial sedimentation offshore ended soon after 15.0 ka. The shelf became emergent, with sea level lowering by, and possibly greater than, 175 m. The rapid transgression that followed began sometime before 12.7 ka off Haida Gwaii and 12.0 ka off southeast Alaska, and with the extreme wave-dominated environment, the unconsolidated sediment that was left on the shelf was effectively removed. Temperate carbonate sands make up the few sediment deposits presently found on the shelf. The Queen Charlotte fault, which lies just below the shelf break for most of its length, was extensively gullied during this short period of significant sediment discharge, when sediment was transported though the glacial valleys and across the narrow shelf through fluvial and submarine channels and was deposited offshore as sea level dropped. The Queen Charlotte fault became the western terminus of the glacio-isostatic forebulge, with the fault acting as a hinged flap taking up the uplift and collapse along the fault of 70+ m. This may have resulted in the development of the distinctive fault valley that presently acts as a very linear channel pathway for sediment throughout the fault system. [ABSTRACT FROM AUTHOR]

Submarine channels are the main conduits and intermediate stores for sediment transport into the deep sea, including organics, pollutants, and microplastics. Key drivers of morphological change in channels are upstream-migrating knickpoints whose initiation has typically been linked to episodic processes such as avulsion, bend cutoff, and tectonics. The initiation of knickpoints in submarine channels has never been described, and questions remain about their evolution. Sedimentary and flow processes enabling the maintenance of such features in non-lithified substrates are also poorly documented. Repeated high-resolution multibeam bathymetry between 2012 and 2018 in the Capbreton submarine canyon (southeastern Bay of Biscay, offshore France) demonstrates that knickpoints can initiate autogenically at meander bends over annual to multi-annual time scales. Partial channel clogging at tight bends is shown to predate the development of new knickpoints. We describe this initiation process and show a detailed morphological evolution of knickpoints over time. The gradients of knickpoint headwalls are sustained and can grow over time as they migrate through headward erosion. This morphology, associated plunge pools, and/or development of enhanced downstream erosion are linked herein to the formation and maintenance of hydraulic jumps. These insights of autogenically driven, temporally high-frequency knickpoints reveal that cut-and-fill cycles with depths of multiple meters can be the norm in submarine systems. [ABSTRACT FROM AUTHOR]

The Jurassic-Cretaceous Nutzotin, Wrangell Mountains, and Wellesly basins provide an archive of subduction and collisional processes along the southern Alaska convergent margin. This study presents U-Pb ages from each of the three basins, and Hf isotope compositions of detrital zircons from the Nutzotin and Wellesly basins. U-Pb detrital zircon ages from the Upper Jurassic- Lower Cretaceous Nutzotin Mountains sequence in the Nutzotin basin have unimodal populations between 155 and 133 Ma and primarily juvenile Hf isotope compositions. Detrital zircon ages from the Wrangell Mountains basin document unimodal peak ages between 159 and 152 Ma in Upper Jurassic-Lower Cretaceous strata and multimodal peak ages between 196 and 76 Ma for Upper Cretaceous strata. Detrital zircon ages from the Wellesly basin display multimodal peak ages between 216 and 124 Ma and juvenile to evolved Hf compositions. Detrital zircon data from the Wellesly basin are inconsistent with a previous interpretation that suggested the Wellesly and Nutzotin basins are proximal-to-distal equivalents. Our results suggest that Wellesly basin strata are more akin to the Kahiltna basin, which requires that these basins may have been offset ~380 km along the Denali fault. Our findings from the Wrangell Mountains and Nutzotin basins are consistent with previous stratigraphic interpretations that suggest the two basins formed as a connected retroarc basin system. Integration of our data with previously published data documents a strong provenance and temporal link between depocenters along the southern Alaska convergent margin. Results of our study also have implications for the ongoing discussion concerning the polarity of subduction along the Mesozoic margin of western North America. [ABSTRACT FROM AUTHOR]

A long-standing goal of sedimentary geoscience is to understand how tectonic and climatic changes are reflected in basin fill. Here, we use 14 numerical models of continental-scale sediment-routing systems spanning millions of years to investigate the responses of sediment supply and basin sedimentation to changes in uplift and precipitation in the source area. We also investigate the extent to which these signals can be altered by relative sea level (the sum of subsidence and eustasy). In cases of constant relative sea level, sediment supply and margin progradation have similar responses because nearly all of the sediment is transported beyond the coastal plain and continental shelf to the basin margin. Thus, margin progradation can be used as a proxy for sediment supply. However, changes in uplift and precipitation result in different erosional patterns in the source area and different basin-margin depositional patterns. Changes in uplift result in gradual (over several million years) adjustment to new steady states of source-area erosion and margin progradation, whereas changes in precipitation result in abrupt changes in erosion and progradation followed by a return to the initial steady states. In cases of changing relative sea level, sediment storage on the shelf attenuates signals of uplift, but signals of precipitation change can be interpreted in the basin-margin record because climate-induced sediment supply changes are large enough to influence margin progradation. Understanding the relationship between sediment supply and basin-margin progradation, and their linked responses to forcings, improves our ability to interpret signals of environmental change in the stratigraphic record. [ABSTRACT FROM AUTHOR]

Sandy trench-fill sediments at accretionary margins are commonly scraped off at the frontal wedge and rarely subducted to the depth of high-pressure (HP) metamorphism. However, some ancient exhumed accretionary complexes are associated with high-pressure--low-temperature (HP-LT) metamorphic rocks, such as psammitic schists, which are derived from sandy trench-fill sediments. This study used sandbox analogue experiments to investigate the role of seafloor topography in the transport of trench-fill sediments to depth during subduction. We conducted two different types of experiments, with or without a rigid topographic high (representing a seamount). We used an undeformable backstop that was unfixed to the side wall of the apparatus to allow a seamount to be subducted beneath the overriding plate. In experiments without a seamount, progressive thickening of the accretionary wedge pushed the backstop down, leading to a stepping down of the décollement, narrowing of the subduction channel, and underplating of the wedge with subducting sediment. In contrast, in experiments with a topographic high, the subduction of the topographic high raised the backstop, leading to a stepping up of the décollement and widening of the subduction channel. These results suggest that the subduction of stiff topographic relief beneath an inflexible overriding plate might enable trench-fill sediments to be deeply subducted and to become the protoliths of HP-LT metamorphic rocks. [ABSTRACT FROM AUTHOR]

We describe a late Miocene to early Pliocene axial drainage system in the East Carpathian foreland, which was an important sediment supplier to the Black Sea and the Dacian Basin. Its existence explains the striking progradation of the northwest Black Sea shelf prior to the onset of sediment supply from the continental-scale Danube River in the late Pliocene to Pleistocene. This axial drainage system evolved due to the diachronous along-strike evolution of the Carpathians and their foreland; continental collision, overfilling, slab breakoff, and subsequent exhumation of the foreland occurred earlier in the West Carpathians than in the East Carpathians. After overfilling of the western foreland, excess sediment was transferred along the basin axis, giving rise to a 300-km-wide by 800-km-long, southeast-prograding river-shelf-slope system with a sediment flux of ~12 × 103 km3/m.y. Such late-stage axial sediment systems often develop in foreland basins, in particular, where orogenesis is diachronous along strike. Substantial lateral sediment transport thus needs to be taken into account, even though evidence of these axial systems is often eroded following slab breakoff and inversion of their foreland basins. [ABSTRACT FROM AUTHOR]

Erosion related to glacial activity produces enormous amounts of sediment. However, sediment mobilization in glacial systems is extremely complex. Sediment is derived from headwalls, slopes along the margins of glaciers, and basal erosion; however, the rates and relative contributions of each are unknown. To test and quantify conceptual models for sediment generation and transport in a simple valley glacier system, we collected samples for 10Be analysis from the Kahiltna Glacier, which flows off Denali, the tallest mountain in North America. We collected angular quartz clasts on bedrock ledges from a high mountainside above the equilibrium line altitude (ELA), amalgamated clast samples from medial moraines, and sand samples from the river below the glacier. We also collected sand from nine other rivers along the south flank of the Alaska Range. In the upper catchment of the Kahiltna drainage system, toppling, rockfall, and slab collapse are significant erosional processes. Erosion rates of hundreds of millimeters per thousand years were calculated from 10Be concentrations. The 10Be concentrations in amalgamated samples from medial moraines showed concentrations much lower than those measured from the high mountainside, a result of the incorporation of thick, and effectively unexposed, blocks into the moraine, as well as the incorporation of material from lower-elevation nearby slopes above the moraines. The 10Be sediment samples from downstream of the Kahiltna Glacier terminus showed decreasing concentrations with increasing distance from the moraine, indicating the incorporation of material that was less exposed to cosmic rays, most likely from the glacier base as well as from slopes downstream of the glacier. Taken together, 10Be concentrations in various samples from the Kahiltna drainage system indicated erosion rates of hundreds of millimeters per thousand years, which is typical of tectonically active terrains. We also measured 10Be concentrations from river sediment samples collected from across the south flank of the Alaska Range. Calculation of basinwide weighted erosion rates that incorporated hypsometric curves produced unrealistically high erosion rates, which indicates that the major source of sediment was not exposed to cosmic rays and was primarily derived from the base of glaciers. Moreover, the apparently high erosion rates suggest that parts of each drainage system are not in erosional steady state with respect to cosmogenic isotope accumulation. [ABSTRACT FROM AUTHOR]

The interaction of deep-marine bottom currents with episodic, unsteady sediment gravity flows affects global sediment transport, forms climate archives, and controls the evolution of continental slopes. Despite their importance, contradictory hypotheses for reconstructing past flow regimes have arisen from a paucity of studies and the lack of direct monitoring of such hybrid systems. Here, we address this controversy by analyzing deposits, high-resolution seafloor data, and near-bed current measurements from two sites where eastward-flowing gravity flows interact(ed) with northward-flowing bottom currents. Extensive seismic and core data from offshore Tanzania reveal a 1650-m-thick asymmetric hybrid channel levee-drift system, deposited over a period of ~20 m.y. (Upper Cretaceous to Paleocene). High-resolution modern seafloor data from offshore Mozambique reveal similar asymmetric channel geometries, which are related to northward-flowing near-bed currents with measured velocities of up to 1.4 m/s. Higher sediment accumulation occurs on the downstream flank of channel margins (with respect to bottom currents), with inhibited deposition or scouring on the upstream flank (where velocities are highest). Toes of the drift deposits, consisting of thick laminated muddy siltstone, which progressively step back into the channel axis over time, result in an interfingering relationship with the sandstone-dominated channel fill. Bottom-current flow directions contrast with those of previous models, which lacked direct current measurements or paleoflow indicators. We finally show how large-scale depositional architecture is built through the temporally variable coupling of these two globally important sediment transport processes. Our findings enable more-robust reconstructions of past oceanic circulation and diagnosis of ancient hybrid turbidite-drift systems. [ABSTRACT FROM AUTHOR]

Landscapes following wildfire commonly have significant increases in sediment yield and debris flows that pose major hazards and are difficult to predict. Ultimately, post-wildfire sediment yield is governed by processes that deliver sediment from hillslopes to channels, but it is commonly unclear the degree to which hillslope sediment delivery is driven by wet versus dry processes, which limits the ability to predict debris-flow occurrence and response to climate change. Here we use repeat airborne lidar topography to track sediment movement following the 2009 CE Station Fire in southern California, USA, and show that post-wildfire debris flows initiated in channels filled by dry sediment transport, rather than on hillsides during rainfall as typically assumed. We found widespread patterns of 1-3 m of dry sediment loading in headwater channels immediately following wildfire and before rainfall, followed by sediment excavation during subsequent storms. In catchments where post-wildfire dry sediment loading was absent, possibly due to differences in lithology, channel scour during storms did not occur. Our results support a fire-flood model in bedrock landscapes whereby debris-flow occurrence depends on dry sediment loading rather than hillslope-runoff erosion, shallow landslides, or burn severity, indicating that sediment supply can limit debris-flow occurrence in bedrock landscapes with more-frequent fires. [ABSTRACT FROM AUTHOR]

Sediment gravity flows are the primary process by which sediment and organic carbon are transported from the continental margin to the deep ocean. Up to 40% of the total marine organic carbon pool is represented by cohesive extracellular polymeric substances (EPS) produced by microorganisms. The effect of these polymers on sediment gravity flows has not been investigated, despite the economic and societal importance of these flows. We present the first EPS concentrations measured in deep-sea sediment, combined with novel laboratory data that offer insights into the modulation of the dynamics of clay-laden, physically cohesive sediment gravity flows by biological cohesion. We show that EPS can profoundly affect the character, evolution, and runout of sediment gravity flows and are as prevalent in deep oceans as in shallow seas. Transitional and laminar plug flows are more susceptible to EPS-induced changes in flow properties than turbulent flows. At relatively low concentrations, EPS markedly decrease the head velocity and runout distance of transitional flows. This biological cohesion is greater, per unit weight, than the physical cohesion of cohesive clay and may exert a stronger control on flow behavior. These results significantly improve our understanding of the effects of an unrealized biological component of sediment gravity flows. The implications are wide ranging and may influence predictive models of sediment gravity flows and advance our understanding about the ways in which these flows transport and bury organic carbon globally. [ABSTRACT FROM AUTHOR]

Late Cretaceous trench basin strata were deposited in the subduction zone that consumed Neo-Tethyan oceanic lithosphere along the southern margin of the proto-Tibetan Plateau. We conducted detrital zircon (DZ) U-Pb geochronology on six trench basin samples (n = 1716) collected near Dênggar, Tibet (~500 km west of Lhasa), to assess the provenance of these rocks and reconstruct Late Cretaceous sediment transport pathways. They contained DZ ages that point to a unique source around Lhasa city, north of the Late Cretaceous Gangdese magmatic arc. The modern Lhasa River catchment contains the requisite sources, and its main trunk transects the Gangdese magmatic arc, joining with the Yarlung River at a barbed junction at the India-Asia suture. We infer that the Lhasa River is an ancient feature that transported sediment to the subduction zone in Late Cretaceous time and persisted during India-Asia collision. [ABSTRACT FROM AUTHOR]

Sediment supplies are a fundamental component of alluvial river systems, but the importance of sustained supplies of externally derived sediments for the evolution of meandering planforms remains unclear. Here we demonstrate the importance of sediment supply in enhancing the growth of point bars that influence the rate of sinuosity increase through flow deflections in meander bends. We use an archive of Landsat images of 16 meandering reaches from across the Amazon Basin to show that rivers transporting larger sediment loads increase their sinuosity more rapidly than those carrying smaller loads. Sediment-rich rivers are dominated by downstream-rotating meanders that increase their sinuosity more rapidly than both extensional and upstream-rotating meanders. Downstream-rotating meanders appear to establish larger point bars that expand throughout the meander, in contrast to extensional meanders, which have smaller bars, and upstream rotating meanders, which are characterized by deposition over the bar head. These observations demonstrate that the size and position of point bars within meander bends influences flow routing and thus controls the dominant direction of meander growth. Rivers with low sediment supplies build smaller point bars, which reduces their capacity to increase meander curvature and the resulting sinuosity. [ABSTRACT FROM AUTHOR]

Geologists frequently debate the origin of iconic river canyons, as well as the extent to which they record climatic and tectonic signals. Fluvial and hillslope processes work in concert to control canyon evolution; rivers both set the boundary conditions for adjoining hillslopes and respond to delivery of hillslope-derived sediment. But, what happens when canyon walls deliver boulders that are too large for a river to carry? River canyons commonly host large blocks of rock derived from resistant hillslope lithologies. Blocks have recently been shown to control the shapes of hillslopes and channels by inhibiting sediment transport and bedrock erosion. Here, we developed the first process-based model for canyon evolution that incorporates the roles of blocks in both hillslope and channel processes. Our model reveals that two-way negative channel-hillslope feedbacks driven by block delivery to the river result in characteristic plan-view and cross-sectional river canyon forms. Internal negative feedbacks strongly reduce the rate at which erosional signals pass through landscapes, leading to persistent local unsteadiness even under steady tectonic and climatic forcing. Surprisingly, while the presence of blocks in the channel initially slows incision rates, the subsequent removal of blocks from the oversteepened channel substantially increases incision rates. This interplay between channel and hillslope dynamics results in highly variable long-term erosion rates. These autogenic channel-hillslope dynamics can mask external signals, such as changes in rock uplift rate, complicating the interpretation of landscape morphology and erosion histories. [ABSTRACT FROM AUTHOR]

Few modern sediment dispersal pathways predate the breakup of Pangea. This suggests that river lifespan can be controlled by continental assembly and dispersal cycles, with the longest-lived river systems present during supercontinent regimes. Based on the strikingly similar age spectra and Hf isotopic array extracted from Paleozoic to early Mesozoic sedimentary sequences from the Paleo-Tethyan margin basins, we argue that a long-lived supercontinental- scale system, with headwaters originating in Antarctica, flowed northward to finally debouch on the margin with the Paleo-Tethys Ocean. Channel-belt thickness scaling relationships, which provide an estimate of drainage area, support the notion that this was a supercontinental-scale system. Sediments were eroded from Proterozoic orogenic belts and flanked resistant kernels of Archean cratons. Remnants of this system, which can still be traced today as topographic lows, controlled post-breakup drainage patterns in Gondwanan fragments in Western Australia. We conclude that supercontinental regimes allow sediment dispersal systems to be long-lived, as they provide both an abundant sediment supply, due to erosion of large-scale, collision-related internal mountain systems, and a stable, large-scale configuration that lasts until breakup. [ABSTRACT FROM AUTHOR]

Long-lived, multi-event bedforms hundreds of meters in wavelength in outcrop are interpreted as deepwater antidunes, the first recognized of this nature. Deepwater sediment gravity flows commonly reach a Froude supercritical flow state, but interpretation of their deposits largely excludes antidunes, which are commonly assumed to be ephemeral. Well-exposed, extensive slope turbidites of the Fish Creek-Vallecito Basin of Southern California (USA) are organized into 3-10-m-thick bedsets of 20-30 distinguishable lenticular backset beds that build low-angle (<10°), undulating geometries and accrete opposite to paleoflow. Bedsets lack high-angle geometries, deep scour surfaces, and structureless facies intrinsic to cyclic steps. Instead, bedsets are differentiated by rhythmic down-dip transitions from thin, subparallel fine-grained beds into thicker, inclined coarser-grained beds and back into thinner, flattening, and, in cases, downflow-dipping finer-grained beds. Within bedsets, compensationally stacked waveforms have ~3-7 m amplitudes and ~75-215 m wavelengths that increase upsection and are comparable to modern upstream-migrating sediment waves. Bioturbated fine-grained caps of each sand bed indicate that antidune bedforms evolved across multiple flow events. Recognition of antidunes in deep water can have important implications for paleoflow reconstruction. [ABSTRACT FROM AUTHOR]

Hillslopes constitute the majority of Earth's land surface area and dominate the supply of sediment to rivers. Hillslope sediment transport is commonly modeled with a rate law that depends on slope and a rate coefficient, D, that is understood to represent the intensity of transport mechanisms. Although many transport mechanisms are related to water and biota, it is unclear whether D varies predictably with climate and life. We compiled previous estimates of D from around the world and also made new estimates for additional sites. The compilation reveals an overall trend in which D increases strongly with increasing moisture among relatively dry sites and less strongly with increasing moisture among relatively wet sites. Vegetation type has a secondary effect on D among drier sites, with D increasing from deserts to grasslands to forests, but not among wetter sites. These trends suggest that the establishment of life in a landscape substantially accelerates soil creep, whereas differences in biological communities among sites with abundant moisture have a relatively small effect on creep. [ABSTRACT FROM AUTHOR]

Several decades of studies on shelf-margin evolution have led to recognition that both accommodation-dominated and supply-dominated sediment-delivery systems are capable of transporting sediments from the shelf down into deep-water basins. The former case relies on falling sea level and lowstands to move deltas to the shelf edge, whereas the latter depends on well-supplied deltas reaching the shelf edge regardless of sea-level rise. However, it remains unclear how to distinguish between the two sediment-dispersal alternatives, and which of these is more efficient in delivering sediments to deep water. We explore sediment-volume partitioning into deep-water areas by analyzing >1600 runs of a geometric delta model with varying eustatic, shelf-morphologic, and sediment-supply conditions. Previous studies suggest that greenhouse eustatic (low amplitude and frequency) conditions generate lower shelf accommodation, and permit the shoreline to arrive at the shelf edge quickly. Further investigation reveals that (1) this argument works only for the supply-dominated system, and (2) the proportion of total sediment that reaches deep water is not correlated to the frequency of sea-level change, but depends strongly on the shelf width and the amplitude of sea-level change. We suggest a ratio between (1) the product of shelf width and the amplitude of sea-level change and (2) total sediment supply to quantitatively characterize the sediment dispersal system. A ratio of 0.4 forms a good boundary between accommodation- and supply-dominated systems in the modeling results, and in three well-studied ancient systems (the Maastrichtian Washakie Basin, Wyoming, USA; the Pliocene paleo--Orinoco margin, Trinidad and Tobago; and the Miocene New Jersey margin, northeastern USA). This work also suggests that the sediment mass balance becomes more important for continental margin building regardless of sea-level scenarios over the longer term. [ABSTRACT FROM AUTHOR]

We present the first spatiotemporal surface suspended sediment concentration (SSSC) analysis of the lower Amazon River, by combining geomorphological mapping, hydrosedimentological field investigations, and 15 years (2001-2015) of weekly averaged wash-load (fine sediment) fluxes at gauging stations estimated from field-calibrated remote-sensing models of SSSC. We found a downstream increase in the magnitude of sediment sinks, where the floodplain geomorphic style controls sediment trapping, and we conclude that the sediment net loss in the floodplain happens during the rising phase through seasonal hydrosedimentological connectivity. On average, 120 million tons per year of fine sediments are accumulated in the floodplain along 765 km of the lower Amazon River (from the confluence with the Madeira River to Monte Alegre city), making the lower Amazon River one of the most important fluvial sediment sinks among the world's axial rivers. [ABSTRACT FROM AUTHOR]

Radiogenic isotopes of strontium (87Sr/86Sr) and neodymium (144Nd/143Nd) are widely used to trace sediment across source-to-sink networks, with samples typically collected from outcrops at basin headwaters and from sediments along the channel margin, floodplain, and/or seafloor. Here, we established the Sr-Nd isotope systematics of recent (in the past 1 k.y.) Mississippi River (USA) basin alluvial sediments, evaluated the sensitivity of these isotope systems to the presence of artificial impoundments that trap sediments behind them, and tested their ability to provenance mixed sedimentary records. Sediment cores collected from floodplain depressions and oxbow lakes along the Mississippi River and its major tributaries, where an extensive lock and dam system was constructed during the mid-twentieth century, show that the isotopic signatures of major tributaries are distinct, and that some of these signatures shift following dam closure. We then used mixing models to demonstrate that, near the confluence of major tributaries, Sr and Nd isotope signatures can be used to ascertain provenance of sediments deposited in floodplain lakes during overbank floods. Further downstream, where sediments are well mixed, the provenance of overbank deposits is more challenging to evaluate using Sr-Nd isotope systematics. Given the global pervasiveness of artificial impoundments on rivers, our findings imply that widely employed sediment fingerprinting techniques based on modern conditions may not be representative of conditions from as recently as a century ago. [ABSTRACT FROM AUTHOR]

Sediment routing systems (SRSs) are a critical element of the global response to ongoing climate change. However SRS response to climate forcing is complex, fragmentary, and obscured when viewed over short, human time scales (10-1-10² yr). Over long time scales (>10²-10³ yr), the aggregated, system-wide response of SRSs to climate forcing can be gleaned with more confidence from the sedimentary record, but the nature and time scales of this aggregated response to abrupt climate change are still poorly understood. Here, we investigate the aggregated temporal response of a SRS in northern Spain to abrupt climate warming at the Paleocene-Eocene thermal maximum (PETM). Our results show that terrestrial sites in northern Spain record a temporal lag of 16.5 ± 7.5 k.y. between the onset of the PETM, defined by an abrupt negative excursion in the δ13C profile, and the onset of coarse-grained deposition. Within the same SRS at a deep marine site 500 km to the west, we observe a temporal lag of 16.5 ± 1.5 k.y. using an age model that is independent of that used for the terrestrial sites. These results suggest that the aggregated, system-wide response of SRSs to present-day global warming--if we take the PETM as an appropriate modern-day analogue--may persist for many millennia into the future. [ABSTRACT FROM AUTHOR]

Advancements in mass spectrometry methods over the past two decades have allowed for rapid measurement of (U-Th)/Pb isotopes for geochronologic applications, a tool that has deeply influenced the way sediment provenance and paleo-tectonic reconstructions are approached. Geochronology-based studies of sediment provenance typically rely on dating n ≈ 100-150 single detrital zircon crystals from individual samples, where the sample age distributions are used to make inferences about the parent age distributions, make qualitative geologic interpretations, and/or perform quantitative intersample comparisons. Most efforts to quantitatively compare detrital zircon age spectra make use of non-parametric dissimilarity statistics. Here, we use laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb detrital zircon moderate-n (n ≈ 100) and large-n (n ≈ 1000) results from unconsolidated fluvial sediments of the Río Orinoco delta, eastern Venezuela, to highlight the concealed pitfalls of making geological interpretations based on quantitative comparisons of U-Pb age distributions alone. Three samples analyzed at large n, selected from contrastingly different mean sediment grain sizes along the active channel of the Río Orinoco, yield large dissimilarities amongst their age spectra, resulting in the misleading conclusion that these were likely not sourced from the same parent distribution. We demonstrate that statistically significant differences amongst detrital zircon samples derived from the same (integrated) source region can be introduced by the dynamics of sediment transport, which may in turn lead to erroneous geologic interpretations arising from the inaccurate assumptions that, at present, condition the quantitative treatment of detrital zircon data. [ABSTRACT FROM AUTHOR]

Although the shifting of deltaic river branches (avulsion) is a natural process that has become increasingly influenced by humans, the impact of early human activities as a driver of avulsion success has remained poorly explored. This study demonstrates how two important avulsions in the downstream part of the Rhine-Meuse delta, The Netherlands, were stimulated by human activities in the first millennium CE, before historic embankment constrained the river courses. Peatland reclamation induced land subsidence in the lower delta. This effect, together with a human-induced increase in suspended fluvial sediments and tidal backwater effects, allowed for a gradual ingression of tidal creek channels and progradation of fluvial crevasse channels into human-occupied and drained peatlands, where they eventually connected. We reconstructed the initial situation and identified the feedback loops among overbank sedimentation, tidal incursion, and land drainage subsidence that led to avulsion success. The processes and feedbacks resulting from human activities are generic and hence relevant to many other deltas today where humaninduced subsidence results in tidal ingression, potentially connecting to rivers and causing unexpected avulsions. [ABSTRACT FROM AUTHOR]

Sequence stratigraphy relies on the identification of unconformity-bound sedimentary packages in order to understand variations in sediment supply, subsidence, and eustasy, which are themselves controlled by external (allogenic) drivers such as climate and tectonics. However, intrinsic (autogenic) river dynamics can also create a rich stratigraphic architecture in the absence of allogenic changes. Here, we outline scaling relationships for the expected depth and length scales of autogenic scour resulting from non-uniform flows in coastal rivers, and apply these relationships to the Upper Cretaceous Castlegate Sandstone--a classic fluvial sandstone unit in the Book Cliffs of Utah (USA). Theoretical and experimental work suggests that hydrodynamics within the backwater reach of coastal rivers--the zone of non-uniform flow that extends upstream of the river mouth--causes spatially extensive erosion during floods; this in turn creates erosional surfaces within fluvio-deltaic stratigraphy that may appear similar to sequence boundaries. Results demonstrate that scour patterns within the Castlegate Sandstone are consistent with the predictions of backwater-induced scours, and show how allogenic versus autogenic erosional surfaces can be parsed within fluvio-deltaic stratigraphy. [ABSTRACT FROM AUTHOR]

Humid tropical and subtropical regions commonly host thick soils that supply sediment to adjacent streams. These soils are depleted in Ca and Na and enriched in Al and Si, yielding high chemical weathering indices, including chemical index of alteration (CIA) values >90, indicative of intense chemical weathering. Here, we investigated chemical weathering in granitoid-sourced fluvial sediments of Puerto Rico--a region with one of the highest documented rates of chemical weathering on Earth. Although thick soils occur throughout the study area, weathering signatures from fluvial muds (CIA values of 58-69) are significantly lower than expected. Indeed, these weathering indices are similar to values expected in climates 10-20 °C cooler. These remarkably low values likely reflect significant input from partially weathered bedrock mobilized during mass wasting. Furthermore, future climate warming may increase torrential precipitation and accompanying mass wasting, producing a counterintuitive decrease in weathering intensity recorded in fluvial sediments. [ABSTRACT FROM AUTHOR]

Turbidity currents regulate the transport of terrigenous sediment, abundant in carbon and nutrients, from the shelf to the deep sea. However, triggers of deep-sea turbidity currents are diverse and remain debatable in individual cases due to few direct measurements and unpredictable occurrence. Here we present long-term monitoring of turbidity currents at a water depth of 2104 m on the margin of the Gaoping Submarine Canyon off Taiwan, which has the world's highest erosion rates and wettest typhoons. The unique 3.5 year record of in situ observations demonstrates the frequent occurrence of deep-sea turbidity currents (an average of six times per year from May 2013 to October 2016), most of which show enhanced sediment flux, raised temperature, and lowered salinity. They are attributed to elevated discharge of the Gaoping River due to typhoons traversing Taiwan. The total duration of these prolonged turbidity currents amounts to 30% of the entire monitoring period, contributing to ~72% of total sediment transport in the lower canyon. Our study demonstrates for the first time that typhoons are the most important triggers, in the long term, of frequent turbidity currents and enhanced sediment delivery into the deep sea in the typhoon-rivercanyon environment. [ABSTRACT FROM AUTHOR]

Bedload sediment transport is a fundamental parameter controlling geomorphic processes in alluvial rivers. Direct measurement in modern rivers is labor intensive, typically cost prohibitive, and often dangerous. In addition, few reliable methods exist for estimating bedload fluxes from ancient fluvial strata. In this paper, we present a model for estimating bedload transport rates from modern sand-bed fluvial systems and ancient fluvial sandstones using substitutions into the bedform-bedload equation. This is enabled by a newly defined empirical relationship between characteristic bedform migration rate and reach slope in normalflow reaches. For modern rivers, this slope-velocity relationship is combined with published relationships for bedform scales to allow remote estimation of bedload flux. In stratigraphic applications, the slope-velocity model is combined with relationships for bedform scale from cross-set thickness and for paleoslope from barform height and grain size to enable estimation of ancient bedload. The modern application is evaluated using bedload and bedform data from a single survey in the North Loup River near Taylor, Nebraska, USA. The stratigraphic application is demonstrated using outcrop data from the Jurassic Kayenta Formation in Colorado National Monument, Colorado, USA. These two applications yield approximately order-of-magnitude total uncertainty (95% interval) in mean bedload flux estimates. [ABSTRACT FROM AUTHOR]

Fluvial systems represent a key component in source-to-sink analysis of ancient sediment-dispersal systems. Modern river channels and channel- related deposits possess a range of scaling relationships that reflect drainage-basin controls on water and sediment flux. For example, channel-belt sand-body thicknesses scale to bankfull discharge, and represent a reliable first-order proxy for contributing drainage-basin area, a proxy that is more robust if climatic regimes can be independently constrained. A database of morphometrics from Quaternary channel belts provides key modern fluvial system scaling relationships, which are applied to Cretaceous- to Paleocene-age fluvial deposits. This study documents the scales of channel-belt sand bodies within fluvial successions from the northern Gulf of Mexico passive-margin basin fill from well logs, and uses scaling relationships developed from modern systems to reconstruct the scale of associated sediment-routing systems and changes in scale through time. We measured thicknesses of 986 channel-belt sand bodies from 248 well logs so as to estimate the scales of the Cretaceous (Cenomanian) Tuscaloosa- Woodbine, Paleocene-early Eocene Wilcox, and Oligocene Vicksburg-Frio fluvial systems. These data indicate that Cenozoic fluvial systems were significantly larger than their Cenomanian counterparts, which is consistent with Cretaceous to Paleocene continental-scale drainage reorganization that routed water discharge and sediment from much of the continental United States to the Gulf of Mexico. At a more detailed level, Paleocene-early Eocene Wilcox fluvial systems were larger than their Oligocene counterparts, which could reflect decreases in drainage-basin size and/or climatic change within the continental interior toward drier climates with less runoff. Additionally, these data suggest that the paleo-Tennessee River, which now joins the Ohio River in the northernmost Mississippi embayment of the central United States, was an independent fluvial system, flowing southwest to the southern Mississippi embayment, or directly to the Gulf of Mexico, through the early Eocene. Changes in scaling relationships through time, and interpreted changes in the scales of contributing drainage basins, are generally consistent with previously published regional paleogeographic maps, as well as with newly published maps of paleodrainage from detrital-zircon provenance and geochronological studies. As part of a suite of metrics derived from modern systems, scaling relationships make it possible to more fully understand and constrain the scale of ancient source-to-sink systems and their changes through time, or cross-check interpretations made by other means. [ABSTRACT FROM AUTHOR]

Submarine mass failures triggered by energetic forcing events such as hurricanes and earthquakes are relatively well studied due to the potential for infrastructure damage and tsunami generation; such failures are common on heavily sedimented margins where underconsolidated deposits are preconditioned to fail. However, studies of seafloor sediment movement between large events remain scarce. Using repeat bathymetric surveys of the Mississippi River Delta Front (MRDF), we document substantial seafloor movement in absence of major hurricanes. About 1 m/yr of deepening was observed within preexisting failures, with downslope sediment transport on the order of 105 m³/yr. Outside failure features, seafloor depths remained stable or showed minor (<20 cm/yr) accretion. MRDF volumetric sediment flux during hurricanedriven mass failures is an order of magnitude greater than the annual flux during a quiescent interval. When normalized by time, however, sediment flux during the quiescent interval (5.5 × 105 m³/yr) was half that of hurricane-driven mass failures (1.1 × 106 m³/yr). These observations corroborate our wave modeling results, which infer that even waves of 1 yr recurrence interval can generate differential seafloor pressures sufficient to trigger submarine landslides; this does not exclude the possibility of river floods also being agents of failure. These findings indicate that sub-decadal submarine landslides are important to MRDF dynamics, comparable to the role of major hurricanes, and observation during seemingly quiescent periods is necessary to holistically assess sediment flux. The periodicity and prevalence of moderate-scale mass transport documented here corroborates similar recent studies offshore other deltas globally, indicating that highstand mass and time budgets of shelf to deep-sea sediment flux, in addition to organic carbon and bioreactive particles, may need to be revised. [ABSTRACT FROM AUTHOR]

The extent to which Aboriginal Australians used fire to modify their environment has been debated for decades and is generally based on charcoal and pollen records rather than landscape responses to land-use change. Here we investigate the sensitivity of in-situ-produced 10Be, an isotope commonly used in geomorphological contexts, to anthropogenic perturbations in the southeastern Australian Tablelands. Comparing 10Be-derived erosion rates from fluvial sediment (8.7 ± 0.9 mm k.y.-1; 1 standard error, SE; n = 11) and rock outcrops (5.3 ± 1.4 mm k.y.-1; 1 SE; n = 6) confirms that landscape lowering rates integrating over 104-105 yr are consistent with rates previously derived from studies integrating over 104 to >107 yr. We then model an expected 10Be inventory in fluvial sediment if background erosion rates were perturbed by a low-intensity, high-frequency Aboriginal burning regime. When we run the model using the average erosion rate derived from 10Be in fluvial sediment (8.7 mm k.y.-1), measured and modeled 10Be concentrations overlap between ca. 3 ka and 1 ka. Our modeling is consistent with intensified Aboriginal use of fire in the late Holocene, a time when Aboriginal population growth is widely recognized. [ABSTRACT FROM AUTHOR]

Storm-driven overwash is a sediment-transport process fundamental to the evolution of low-lying coastal environments. Physical insight into overwash morphodynamics is crucial for improved risk assessment and hazard forecasting in vulnerable coastal zones. Spatially extended observations of washover deposits have shown that back-barrier shoreline planforms can be quasi-periodic. These rhythmic patterns have been attributed to the influence of a forcing template in bathymetry or topography, or as inherent in the forcing itself. With an alternative to this prevailing explanation, we present results of a physical experiment and numerical model in which quasi-periodic patterns in washover deposits are self-organized, arising from interactions between barrier topography, routing of overwash flow, and sediment flux. [ABSTRACT FROM AUTHOR]

A quantitative comparison of 20 literature case studies of fluvial sedimentary successions tests common assumptions made in published models of alluvial architecture concerning (1) inverse proportionality between channel-deposit density and floodplain aggradation rates, and (2) resulting characteristics of channel-body geometries and connectedness. Our results do not support the relationships predicted by established stratigraphy models: the data suggest that channel-body density, geometry, and stacking pattern are not reliable diagnostic indicators of rates of accommodation creation. Hence, these architectural characteristics alone do not permit the definition of accommodation-based "systems tracts" and "settings", and this calls into question current sequence stratigraphic practice in application to fluvial successions. [ABSTRACT FROM AUTHOR]

Establishing background (geologic) rates of erosion is prerequisite to quantifying the impact of human activities on Earth's surface. Here, we present 10Be estimates of background erosion rates for ten large (10,000-100,000 km²) river basins in the southeastern United States, an area that was cleared of native forest and used intensively for agriculture. These 10Be-based rates are indicative of the pace at which the North American passive-margin landscape eroded before European settlement (~8 m/m.y.). Comparing these background rates to both rates of post-settlement hillslope erosion and to river sediment yields for the same basins, we find that following peak disturbance (late 1800s and early 1900s), rates of hillslope erosion (~950 m/m.y.) exceeded 10Be-determined background rates more than one-hundred fold. Although large-basin sediment yields during peak disturbance increased 5-10x above pre-settlement norms, rivers at the time were transporting only ~6% of the eroded material; work by others suggests that the bulk of historically eroded material remained and still remains as legacy sediment stored at the base of hillslopes and along valley bottoms. Because background erosion rates, such as we present here, reflect the rate at which soil is generated over millennial time scales, they can inform and enhance landscape-management strategies. [ABSTRACT FROM AUTHOR]

Submarine canyons are prevalent in the world's oceans and are instrumental in transporting sediment from coastal regions to deep-sea fans. Conventional sediment parameters such as mean grain size, sorting, and provenance have typically been used to characterize these deposits, but they provide little information on sediment source or the delivery processes involved. Fortunately, transported along with the mineral grains are the remains of organisms living within the sediment. Biological constituents have unique environmental signatures that are more precise proxies for source areas than are mineral grains alone. They may be used to identify a single biofacies deposit (SBD) due to local sediment transport or a displaced, multiple biofacies deposit (MBD) resulting from staged sediment transport or a full-canyon flushing event. This Ascension-Monterey Canyon system study demonstrates that by using the biological constituents in marine deposits, the source areas and transport mechanisms may be identified. The 19,000 year record of core S3-15G captured hemipelagic mud interspersed with individual turbidites of sand and silt transported to the core site at lower bathyal depths (3491 m). The relative abundance of displaced benthic foraminifera was found to correlate positively with grain size, with 75% of the fauna being displaced in the cross-bedded turbiditic sand (Tc) units, 39% in the laminated turbiditic sand (Td) units, and 15% in the turbiditic mud (Tet) units. Nineteen samples were SBDs representing local canyon wall sloughing, bioerosion, or hemipelagic deposition at or near the core site. Sixty-five were MBDs, 31 of which were turbiditic sands originating in the estuarine to inner shelf, outer shelf, upper slope, or upper middle slope, and the remaining 34 were turbiditic muds with displacement initiated in the estuarine to inner shelf, outer shelf, and the upper slope. Commonly, the biological remains of several biofacies characterize these MBDs, reflecting staged sediment transport with storage occurring behind slumps that act as barriers to movement until finally released. Sediment bypassing, typically of the deeper biofacies, and full-canyon flushing, were also evident. Identifying and interpreting the distribution of allochthonous biological sediment constituents in marine deposits is a powerful tool in the investigation of sediment transport that can be applied to other submarine canyon systems. [ABSTRACT FROM AUTHOR]

Hydroacoustic and sedimentological data from the western leeward flank of the Great Bahama Bank document the interplay of off-bank sediment export, along-slope transport, and erosion, which together shape facies and thickness distribution of slope carbonates. The integrated data set depicts the combined product of these processes and allows formulation of a comprehensive model of a periplatform drift that significantly amends established models of carbonate platform slope facies distribution and geometry. The basinward-thinning wedge of the periplatform drift at the foot of the bank escarpment displays along-slope and downslope variations in sedimentary architecture. Sediments are muddy carbonate sands that coarsen basinward. The drift wedge has a pervasive cover of cyclic steps. In zones of lower contour current speed, depth-related facies belts develop, whereas strike-discontinuous sediment lobes, scarps, and gullies characterize areas with higher current speed. This understanding of the impact of currents on carbonate-slope sedimentation has wider implications for seismic and sequence stratigraphic interpretation of carbonate platforms and for applied aspects such as hydrocarbon exploration. [ABSTRACT FROM AUTHOR]

New laboratory experiments reveal that cohesionless turbidity currents are able to enter cohesive soft muddy substrates without losing their shape. These intrabed currents are driven by bed shear stress exceeding bed cohesive strength, and by flow density exceeding bed density. The flows produce unique turbidites with internal mud layers, mixed cohesive-noncohesive sediment layers, and flame and load structures. A depositional model for intrabed (I) turbidites is proposed, comprising, from base to top: I1--sand-bearing mud, with a scoured base, dispersed mud, and mud clasts; I2--muddy sand from the intrabed portion of the turbidity current; I3--sandy mud with a speckled appearance; and I4--mud-poor sand from the suprabed portion of the flow. Complete I1-I4 turbidites are inferred to dominate locations in nature where the currents mix with the bed and deep erosional scours form, filled with deformed or chaotic sand-mud mixtures. Further downflow, base-missing I2-I4 and I4 sequences signify gradual deceleration, loss of erosivity, and termination of intrabed flow. [ABSTRACT FROM AUTHOR]

We reconstruct the behavior of a catastrophic sustained radial pyroclastic density current as it waxed then waned during its brief lifespan. By subdividing the deposit into 8 time slices using a chemical tracer, we show that the sustained current initially was topographically restricted, but that its leading edge advanced in all directions, encroaching upon and gradually ascending hills. During peak flow the current reached its maximum extent and overtopped all topographic highs. After this, and while the current direction from source was maintained, the leading edge gradually retreated sourceward. High-resolution analysis of the depositional architecture reveals how the flow dynamics evolved and runout distance of the sustained density current rapidly increased then decreased, reflecting the dominant influence of changing mass flux, as demonstrated in numerical models but not previously distinguished in a natural deposit. [ABSTRACT FROM AUTHOR]

Analyses of river suspended sediment response to record-breaking regional rainfall in southern Taiwan during typhoon Morakot, 7-9 August 2009, reveal systematic changes in the regional sediment transport regime as characterized by rating curve parameters, with decreasing b (typically considered to be a constant for a given gauging station) and increasing intercept K values in concentration-discharge (C-Q) relations (C = KQb). These changes result in much greater sediment concentration, and thus sediment transport, in subsequent lowflow events after the typhoon, an effect that amplifies and extends the influence of such extreme events through increased low-fl ow sediment transport. Our fi nding that the exponent b is not constant contrasts with the conventional assumption that large events influence sediment yields through increased intercept (K) values, thereby supporting the interpretation that basin sediment delivery influences both rating parameters, and increases post-event low-fl ow sediment transport. Surveys of landslide density and riverbed grain sizes before and after typhoon Morakot support the interpretation that the observed decrease in b values and the synchronous shift from an inverse correlation between K and mean catchment slope before the typhoon to a positive correlation afterward reflect an altered sediment transport regime and a change from channel migration and bank erosion to reworking of landslide debris and enhanced bed mobility as the dominant processes supplying fl uvial sediment. [ABSTRACT FROM AUTHOR]

Two stacked parasequences exposed continuously along a 35-km dip section in the Permian lower Waterford Formation, Karoo Basin, South Africa, form basin margin- scale clinothems, and their internal facies distributions have been mapped out from shelf to upper submarine-slope settings. Sedimentary facies changes have been determined by walking out key surfaces between measured sections. The two parasequences (Waterford clinothems WfC 3 and WfC 4) share progradational profiles, but WfC 3 is characterized by a strong fluvial influence, whereas overlying WfC 4 is a wave-storm- dominated delta front system. When correlated basinward, the two clinothems exhibit stratigraphic thickening as well as differing process responses to the increased gradient at the shelf-edge rollover. WfC 3 exhibits synsedimentary, wedge-shaped rotational growth faults. These growth faults trapped sand at the shelf-edge rollover, so minimal sand was delivered to the upper slope; therefore, the clinothem downlaps into the slope mudstones within 7 km of the shelf-edge rollover. In contrast, the top of WfC 4 is marked by closely spaced gullies cut into deformed delta front deposits. The delta front deposits pass into sand-prone slope turbidites 3 km downdip. Locally, these turbidites are truncated by a 60-m-thick turbidite-sandstone- filled slope channel fill. In this case, most of the slope delivery is associated with a wavedominated process regime. It is important to consider the sequence stratigraphic setting of clinothems in such analyses; WfC 4 represents a minimum accommodation point in a depositional sequence and is overlain by a Type 2 sequence boundary. Despite wave and storm dominance, the low accommodation and high sediment supply at that time is interpreted to have driven sand beyond the gullied shelf-edge rollover. Therefore, the delivery of sediment to deep-water settings is governed by parameters other than the presence and proximity of a fluvial point source, which is heavily advocated in current models for shelf construction. [ABSTRACT FROM AUTHOR]

Large alluvial rivers transport globally significant quantities of water, sediment, and nutrients to the oceans, temporarily storing and cycling this material within the bars, islands, and floodplains that define their morphology. The world's largest rivers display a remarkable variety of morphologies. However, existing theory and numerical modeling fail to explain this diversity, which remains poorly understood. This study applies a new numerical model of water flow and sediment transport to show how the morphology of large sand-bed rivers is influenced by bed sediment mobility, bank erodibility, and rate of floodplain development. Simulations demonstrate that a wide range of river styles, including meandering, anabranching, and braiding, can occur over a relatively narrow range of environmental conditions. Results highlight the suspension of bed material, which limits the gravitational deflection of sediment in the direction of the local bed slope, as a key control on sediment transport direction and hence river morphology. Moreover, high mobility of bed and bank sediments are hypothesized to favor contrasting river styles, although both may be promoted by increasing stream power. These results explain the inability of existing stream power theory to predict the morphology of the world's largest rivers, and highlight the potential for investigating river-floodplain co-evolution using physics-based simulation models. [ABSTRACT FROM AUTHOR]