Floodplain Deposits Research Articles

Conditions of sedimentation and distribution of organic matter in the Lower-Middle Jurassic complex of the South-Western part of the West Siberian oil and gas basin

The article is devoted to the variation of generation potential of the Lower-Middle Jurassic organic matter in the Karabash area (South-Western part of the West Siberian basin) depending on facies and sedimentary cyclisity. Based on geochemical and lithological core studies, specific geochemical features of organic matter established for main facies groups of Early-Middle Jurassic in the study area. The best generation potential inherent in deposits of swamps, swampy floodplains and tidal plains. These facies groups characterized by predominance of the continental organic matter and considered to gas source rocks. However, macerals composition, kinetic spectra and biomarker analysis show the presence also of marine organic matter. Some facies groups, such as flooded swamps and swampy floodplains has potential to generate hydrocarbons due to preservation of liptinites with high hydrogen index. Cyclic analysis with basics of sequence stratigraphy of the continental deposits allow tracing the correlation between generation potential and relative sea level changes.

Hydroclimate change and its controlling factors during the middle to late Holocene and possible 3.7-ka climatic shift over East Asia

AbstractThe history of past hydroclimatic extreme events, essential information for predicting future changes, is preserved in fluvial sediments. Here, we reconstruct changes in decadal-scale extreme flooding events over the period 7700–1700 cal yr BP from floodplain sediments in the middle reach of the Nakdong River, Korea, based on lithogenic elemental ratios (e.g., Zr/Ti and Sr/Si). For example, Nakdong extreme flooding (NEF) events frequently occurred at 7700, 7200, 6000, 5000, 3800, 3200, 2900, 2600, 2300, 2000, and 1700 cal yr BP, and were associated with higher sea-surface temperatures and strong El Niño-Southern Oscillation (ENSO) activity. Notably, we found a significant change in the frequency of extreme events ca. 3700 cal yr BP over East Asia. The hydroclimate fluctuated with dominant periodicities of 950 and 540 years in 7700–3700 cal yr BP and shorter centennial to decadal cycles (320, 110–120, and 60–75 years) in 3700–1700 cal yr BP. This 3.7-ka climatic shift is consistent with a marked southward shift of the intertropical convergence zone, intensified ENSO activity, increased frequency of recurving typhoons, and deep-ocean circulation changes in both the northern and southern hemispheres, demonstrating the urgent need for investigating the critical role of past deep-water circulation in hydroclimate changes.

Organic carbon burial by river meandering partially offsets bank erosion carbon fluxes in a discontinuous permafrost floodplain

Abstract. Arctic river systems erode permafrost in their banks and mobilize particulate organic carbon (OC). Meandering rivers can entrain particulate OC from permafrost many meters below the depth of annual thaw, potentially enabling the production of greenhouse gases. However, the amount and fate of permafrost OC that is mobilized by river erosion is uncertain. To constrain OC fluxes due to riverbank erosion and deposition, we collected riverbank and floodplain sediment samples along the Koyukuk River, which meanders through discontinuous permafrost in the Yukon River watershed, Alaska, USA, with an average migration rate of 0.52 m yr−1. We measured sediment total OC (TOC) content, radiocarbon activity, water content, bulk density, grain size, and floodplain stratigraphy. Radiocarbon activity and TOC content were higher in samples dominated by silt as compared to sand, which we used to map OC content onto floodplain stratigraphy and estimate carbon fluxes due to river meandering. Results showed that the Koyukuk River erodes and re-deposits a substantial flux of OC each year due to its depth and high migration rate, generating a combined OC flux of a similar magnitude to the floodplain net ecological productivity. However, sediment being eroded from cutbanks and deposited as point bars had similar OC stocks (mean ± 1 SD of 125.3±13.1 kg OC m−2 in cutbanks versus 114.0±15.7 kg OC m−2 in point bars) whether or not the banks contained permafrost. We also observed radiocarbon-depleted biospheric OC in both cutbanks and permafrost-free point bars. These results indicate that a substantial fraction of aged biospheric OC that is liberated from floodplains by bank erosion is subsequently re-deposited in point bars rather than being oxidized. The process of aging, erosion, and re-deposition of floodplain organic material may be intrinsic to river–floodplain dynamics, regardless of permafrost content.

Thaw Settlement Monitoring and Active Layer Thickness Retrieval Using Time Series COSMO-SkyMed Imagery in Iqaluit Airport

Thaw consolidation of degrading permafrost is a serious hazard to the safety and operation of infrastructure. Monitoring thermal changes in the active layer (AL), the proportion of the soil above permafrost that thaws and freezes periodically, is critical to understanding the conditions of the top layer above the permafrost and regulating the construction, operation, and maintenance of facilities. However, this is a very challenging task using ground-based methods such as ground-penetrating radar (GPR) or temperature sensors. This study explores the integration of interferometric measurements from high-resolution X-band Synthetic Aperture Radar (SAR) images and volumetric water content (VWC) data from SoilGrids to quantify detailed spatial variations in active layer thickness (ALT) in Iqaluit, the territorial capital of Nunavut in Canada. A total of 21 SAR images from COSMO Sky-Med (CSK) were first analyzed using the freely connected network interferometric synthetic aperture radar (FCNInSAR) method to map spatial and temporal variations in ground surface subsidence in the study area. Subsequently, we built an ALT retrieval model by introducing the thaw settlement coefficient, which takes soil properties and saturation state into account. The subsidence measurements from InSAR were then integrated with VWC extracted from the SoilGrids database to estimate changes in ALT. For validation, we conducted a comparison between estimated ALTs and in situ measurements in the airport sector. The InSAR survey identifies several sites of ground deformation at Iqaluit, subsiding at rates exceeding 80 mm/year. The subsidence rate changes along the runway coincide with frost cracks and ice-wedge furrows. The obtained ALTs, ranging from 0 to 5 m, vary significantly in different sediments. Maximum ALTs are found for rock areas, while shallow ALTs are distributed in the till blanket (Tb), the intertidal (Mi) sediments, and the alluvial flood plain (Afp) sediment units. The intersection of taxiway and runway has an AL thicker than other parts in the glaciomarine deltaic (GMd) sediments. Our study suggests that combining high-resolution SAR imagery with VWC data can provide more comprehensive ALT knowledge for hazard prevention and infrastructure operation in the permafrost zone.

Palaeoenvironmental changes and anthropogenic impact recorded in floodplain sediments: A case study from the lower Morava River Basin (Czech Republic)

The Morava River catchment in the Czech Republic serves as sedimentary archive reflecting various natural and man-made processes, and was studied by several authors from the point of view of flood frequency, anthropogenic contamination, or alluvial history. However, more consistent and detailed information on the effect of human activities, namely in terms of land use and river regulation, is missing. The aim of this study is to reconstruct the palaeoenvironmental and anthropogenic processes affecting the floodplain sediments in this area, focusing on the last few hundreds of years. Alluvial sediments from river banks and drilled cores were collected in the Strážnické pomoraví area from sites both inside and outside dams constructed to avoid flooding. Magnetic parameters, reflecting the composition and grain-size distribution of iron oxides, which serve as fingerprints of lithogenic vs. pedogenic vs. anthropogenic origin, are complemented by the radiocarbon and dendrochronological dating. In addition, deposition age was estimated using the 137Cs activity and persistent organic pollutant content. Our results reveal continuous increase of ferrimagnetic minerals (magnetite) input to the floodplain, suggesting increased soil erosion in the catchment. Significantly different pattern was observed inside and outside the flood dykes. The erosion accelerated since the 1950s due to incorrect land use and introduction of modern agriculture techniques. Finally, the industrial pollution significantly contributes to the magnetic enhancement of the topmost 50 cm of the floodplain sequences. Although the results represent local case study, they have more general validity in the sense that records of paleoenvironmental changes and human activities in floodplain sediments may not be well preserved in all the strata, and also their spatial distribution of individual markers may show significant variability.

Cretaceous (Maastrichtian) chelonian burrows preserved in floodplain deposits in the Bauru Basin of Brazil: Evidence for the fossorial origin of turtle shells

Burrowing behavior is an important adaptation of animals that live in arid and semi-arid conditions. In this paper, we describe examples of vertebrate burrows from the Upper Cretaceous (Maastrichtian) Adamantina Formation of the Bauru Basin, Brazil, most likely produced by turtles. The Adamantina Formation preserves abundant and diverse turtle body fossils such as Bauruemys elegans (Testudines: Pleurodira); however turtle burrows have not been previously documented. The newly reported burrows are preserved in fluvial sandstone facies and exposed in sections that partially preserve their three-dimensional geometry. Burrows are simple J-shaped tunnels with a cross-sectional shape that is semicircular (half-dome) with a flattened floor. Such burrows show a partially preserved entrance with an inclined ramp angle (22°), and grooves and ridges up to 1 cm in width preserved along the burrows walls and floor. The architecture and sedimentary facies of the host sandstone body, together with the occurrence of Taenidium barretti and the absence of rhizoliths, suggest that the burrow was excavated by scratch digging into an exposed point bar of a meandering river channel. Based on burrow morphology, dimensions, as well as ridges,and grooves in the walls and floor, we propose that burrows were formed by a chelonian (such as a freshwater turtle) during aestivation. We highlight that these are first examples of turtle burrows reported from the Cretaceous, and their occurrence reinforces the hypothesis that the original function of turtle shells was as an adaptation to fossorial behavior.

Thanatosdrakon amaru, gen. et sp. nov., a giant azhdarchid pterosaur from the Upper Cretaceous of Argentina

The largest pterosaur discovered in South America and one of the largest flying vertebrates in the world is described. Thanatosdrakon amaru gen. et sp. nov. is a new azhdarchid found in the upper-most levels of the Plottier Formation (upper Coniacian–lower Santonian, Neuquén Basin), Mendoza, Argentina. Two specimens were identified, the holotype (UNCUYO-LD 307) and the paratype, with an estimated wingspan of ∼7 m and ∼9 m, respectively. The cladistic analysis, based on 216 characters and 97 taxa, recovered Thanatosdrakon deeply nested within Azhdarchidae. Relationships within this clade are well resolved and the clade Quetzalcoatlinae presents two well-defined sister-groups: (Arambourgiania, Mistralazhdarcho, Aerotitan, Hatzegopteryx, Albadraco) + (Cryodrakon, Thanatosdrakon, Quetzalcoatlus ssp.). Thanatosdrakon is the oldest taxon of the clade Quetzalcoatlinae so far. It is represented by several well-preserved axial and appendicular bones in three dimensions. Some of these elements have never been described in giant azhdarchids (e.g. complete norarium, dorsosacral vertebrae and caudal vertebra) and this allows to expand the knowledge about the anatomy of this diverse group of pterosaurs. Finally, from a paleoecological point of view, Thanatosdrakon was found in floodplain deposits of ephemeral meandering systems indicating that this large flying species inhabited continental environments.

Evaluation of Climatic and Tectonic Imprints In Fluvial Successions of An Early Permian Depositional System (Asselian VrchlabÍ Formation, Krkonoše Piedmont Basin, Czech Republic)

ABSTRACT The Krkonoše Piedmont Basin, an early post-Variscan basin (c. 310–280 Ma) located in the NE Bohemian Massif (Czech Republic, central Europe), contains up to 300-m-thick non-marine Lower Permian (Asselian) deposits of the Vrchlabí Formation that crop out in its western part. The Early Permian Krkonoše Piedmont Basin exhibits striking similarities—in terms of tectonostratigraphic evolution, depositional patterns, and volcanism—to other early post-Variscan, near-equatorial intermontane basins. This work focuses on sedimentological analysis of the Vrchlabí Formation using outcrop data, combined with borehole and newly acquired field gamma spectrometry data. The formation consists predominantly of fluvial deposits in the southern part of the basin that pass laterally into deltaic and lacustrine deposits of the Rudník Member, located in the central part of the basin. Fluvial deposits are formed by sandstone and conglomeratic bodies, decimeters to several meters thick, interpreted as single-story and multi-story channel fills as well as various macroforms (e.g., bars) deposited by a low-sinuosity fluvial system. Channel fills intercalate with floodplain deposits centimeters to few meters thick. Vertically, the studied fluvial succession is divided into five intervals based on i) variable preservation ratio between channel fill and floodplain deposits and ii) external and internal geometries of sandstone and conglomerate channel bodies. In order to explain the observed vertical changes in fluvial style and to determine the main controlling factors, the interaction of the fluvial system with the contemporaneous lake was investigated by interpreting base-level changes of the fluvial system and their correlation with lake level changes. In the central part of the basin the transition from fluvio-deltaic to lacustrine deposits is represented by alternating beige to gray sandstones centimeters to few meters thick and by dark gray mudstones up to several meters thick that pass northwards into shales. At the outcrop scale, the new sedimentological data in combination with newly acquired gamma-ray logs enabled tracing the lateral extent of individual fluvial bodies. The integration of the new data with older borehole data leads to a substantially better understanding of lateral and vertical relationship between the fluvio-deltaic and the lacustrine facies at the basin scale. Tectonic subsidence played a substantial role during the initial deposition of the Vrchlabí Formation. However, the external geometries, the internal architecture of fluvial channel bodies, and the variable degree of floodplain preservation throughout the entire Vrchlabí Formation cannot be explained by tectonics only, but requires the consideration of climatic controls. The Early Permian fluvial system is interpreted to reflect seasonal discharge variability with dry and wet periods under long-term subhumid climatic conditions that shifted towards more arid conditions during the latest depositional stage of the Vrchlabí Formation.

The structure of the nonmarine fossil record: predictions from a coupled stratigraphic–paleoecological model of a coastal basin

AbstractPresented here is a coupled model of the nonmarine fossil record, based on a geometric model of deposition, a random-branching model of evolution, and an ecological model based on an elevation gradient. This model provides testable predictions about the stratigraphy and fossil occurrences in coastal nonmarine settings under three scenarios of sea-level change. A slow relative rise in sea level causes a declining ratio of channel to floodplain deposits, plus changes in community composition that reflect an upward increase in elevation relative to sea level. A rapid relative rise in sea level drives increasing aggradation rates, decreases the ratio of channel to floodplain deposits, and triggers a shift from higher-elevation (more inland) to lower-elevation (more coastal) communities. A fall in sea level produces an unconformity, manifested by valleys separated by interfluves. The resumption of deposition following the sea-level fall causes an abrupt shift in community composition across the unconformity, reflecting the duration of the hiatus and the increased elevation relative to sea level. This produces a cluster of first and last occurrences at the unconformity, and it is the only sequence-stratigraphic source of such clusters in a nonmarine system, in contrast to the multiple mechanisms for generating these clusters in marine systems. A central prediction of these models is that the nonmarine fossil record preserves systematic changes in community composition that reflect elevation (or equivalently, distance from shore). Diagnosing these gradients in ancient systems is a promising avenue of future research.

Illuminating past river incision, sediment source and pathways using luminescence signals of individual feldspar grains (Rangitikei River, New Zealand)

AbstractSingle‐grain post‐infrared infrared stimulated luminescence (SG‐pIRIR) of feldspar has recently been introduced as a method to date Quaternary deposits. The method is particularly appropriate for fluvial deposits that cannot be dated by more conventional quartz optically stimulated luminescence dating and that are heterogeneously bleached (i.e. where only part of the grains is exposed to sufficient light to remove the full luminescence signal). Besides age estimation, single grain equivalent dose (De) distributions also reflect the variable bleaching degree and origins of grains. Thereby, the SG‐pIRIR signal offers a valuable tool to reconstruct sediment pathways. This study builds upon these ideas and develops a dual aspect that combined river terrace dating and SG‐pIRIR sediment pathway reconstruction for fluvial deposits in terraces and modern river floodplain along the Rangitikei River (RR), New Zealand. We found that the RR last aggrading phase (17.4 ± 1.9 ka to 11.6 ± 1.5 ka) was followed by a first phase of fast incision related to knickpoint retreat followed by a steady incision and widening of the RR canyon. TheDedistribution of fluvial deposits varies accordingly, due in particular to variable input of bedrock particles with saturated pIRIR signal from landsliding of valley flanks. Our study illustrates that the SG‐pIRIR approach is well suited to date terraces and shows how SG‐pIRIRDedistributions provide proxies to reconstruct sediment sources and pathways.

Fingerprinting historical tributary contributions to floodplain sediment using bulk geochemistry

Sediment deposition on floodplains is essential for the development and maintenance of riparian ecosystems. Upstream erosion is known to influence downstream floodplain construction, but linking these disparate processes is challenging, especially over large spatial and temporal scales. Sediment fingerprinting is thus a robust tool to establish process linkages between downstream floodplain development and sediment production in distal headwater basins. Here we use sediment geochemistry to connect historical erosion in several tributaries of the Yampa River in Colorado and Wyoming, USA, to the construction of downstream floodplains on which extensive cottonwood forests established. Using a combination of conventional techniques and the relatively novel machine-learning random forest algorithm, we build multiple fingerprints of diagnostic geochemical tracers that are then input into a Bayesian mixing model to apportion provenance of floodplain sediment. Sediment samples for provenance analysis were collected from an excavated floodplain in Deerlodge Park on the Yampa River at the rooting surface of the surrounding cottonwood forest and dominantly comprised of very fine (4Φ) sand. Fingerprinting analysis of the 4Φ fraction of collected floodplain sink (n = 38) and tributary source (n = 218) samples revealed floodplain sediment to be dominantly sourced from the tributaries of Muddy Creek (45 ± 4%) and Sand Wash (42 ± 6%). Dendrochronology results moreover indicate the Deerlodge floodplain sediment was deposited in ∼1912, which falls squarely within the time (1880–1940) these tributaries were actively eroding. Taken together, study results indicate a demonstrable link between historical tributary erosion and downstream floodplain construction and concomitant forest establishment. Our findings suggest processes operating in tributary watersheds play an important role in the dynamics of large rivers and emphasize both the need for holistic, collaborative management of sediment as an essential resource and the potential to utilize sediment fingerprinting to inform and direct river ecosystem management.

Application of Hierarchical Clustering Endmember Modeling Analysis for Identification of Sedimentary Environment in the Houtao Section of the Upper Yellow River

The unmixing of grain-size distribution (GSD) with multivariate statistical analysis provides insight into sediment provenance, transport processes and environment conditions. In this article, we performed hierarchical clustering endmember modeling analysis (CEMMA) to identify the sedimentary environment of fluvial deposits at core HDZ04 drilled in the paleofloodplain on the north bank of the upper Yellow River. The CEMMA results show that four end members can effectively explain the variance in the dataset. End-Member 1 and End-Member 2 are polymodal and dominated by silty clay, and they are associated with the suspended load. End-Member 3 is composed of fine sand and silt, and medium-coarse sand makes up the majority of End-Member 4, corresponding to a mixed saltation load and bed load, respectively. Combined with the end-member scores, we constructed energy values to further divide the core samples into different depositional environments. Unit 2 and unit 5 have a high proportion of coarser end-member components, presenting a shallow channel and a high-energy channel environment, respectively. Unit 1 and unit 3 are composed of fine-grained silt and clay and are dominated by finer end-member components, which can be interpreted as a floodplain situation. Unit 4 is characterized by frequent fluctuations in grain-size composition and energy values, indicating the transition from a high-energy river channel to floodplain deposits. For the channel sedimentary environment, the accumulation rate was relatively low (0.32 mm/yr) due to the frequency migration of the channel. A high accumulation rate of the fluvial deposits had occurred in unit 1 during 1.6 Ka (4.35 mm/yr), which was a response to the influence of increased fluvial instability and human activity during the late Holocene.

Swift Weathering Response on Floodplains During the Paleocene‐Eocene Thermal Maximum

AbstractSilicate weathering is thought to increase and offset the rapid, massive input of CO2 into the atmosphere and ocean during the Paleocene‐Eocene Thermal Maximum (PETM), but few nonmarine records have been used to quantify this. We probe changes in silicate weathering intensity by measuring Li isotope ratios of bedrock and ancient floodplain deposits spanning the PETM in the Bighorn Basin, Wyoming (USA). Our results reveal a rapid increase in silicate weathering intensity during the PETM that remained high during at least the initial stage of climate recovery. Additionally, we determine that soils that formed farthest from ancient river channels underwent larger weathering changes than near‐channel soils. Alongside increased temperatures and pCO2, the simplest explanation for this response relates to increased seasonal fluctuations in water table height in the floodplain that promote dissolution and precipitation reactions. These findings newly demonstrate that weathering on floodplains, like mountain hillslopes, responds to climate change.

Sustainability of the coastal zone of the Ganges-Brahmaputra-Meghna delta under climatic and anthropogenic stresses

The Ganges-Brahmaputra-Meghna (GBM) delta is one of the world's largest deltas. It is currently experiencing high rates of relative sea-level rise of about 5 mm/year, reflecting anthropogenic climate change and land subsidence. This is expected to accelerate further through the 21st Century, so there are concerns that the GBM delta will be progressively submerged. In this context, a core question is: can sedimentation on the delta surface maintain its elevation relative to sea level? This research seeks to answer this question by applying a two-dimensional flow and morphological model which is capable of handling dynamic interactions between the river and floodplain systems and simulating floodplain sedimentation under different flow-sediment regimes and anthropogenic interventions. We find that across a range of flood frequencies and adaptation scenarios (including the natural polder-free state), the retained volume of sediment varies between 22% and 50% of the corresponding sediment input. This translates to average rates of sedimentation on the delta surface of 5.5 mm/yr to 7.5 mm/yr. Hence, under present conditions, sedimentation associated with quasi-natural conditions can exceed current rates of relative sea-level rise and potentially create new land mass. These findings highlight that encouraging quasi-natural conditions through the widespread application of active sediment management measures has the potential to promote more sustainable outcomes for the GBM delta. Practical measures to promote include tidal river management, and appropriate combinations of cross-dams, bandal-like structures, and dredging.

Variations in water discharge at different temporal scales in a mud-prone alluvial succession: The Paleocene-Eocene of the Tremp-Graus Basin, Spain

The late Paleocene to earliest Eocene sedimentary record in the eastern Tremp-Graus Basin (southern Pyrenees, Spain) consists of a mud-prone fluvial succession informally known as ‘Upper Red Garumnian’, which is dominantly composed of non-channelized deposits interspersed with fluvial channel bodies. The succession records evidence of variations in water discharge at multiple temporal scales. One-hundred-and-eighty sand-prone and conglomeratic bodies present within otherwise fine-grained deposits have been examined by facies and architectural-element analyses. These have been assigned to four distinctive types: (i) simple and (ii) compound channelized deposits, (iii) multilateral and multistorey elements, and (iv) sediment gravity-flow deposits. Simple and compound channel bodies represent deposits of channelized high-energy flows associated respectively with single floods or multiple events occurring in the same channel belt. Multilateral and multistorey elements are associated with sedimentation in longer-lived channel belts. An increase in the occurrence of these latter elements through the stratigraphy can be related to changes in tectonic and/or climatic drivers, heralding a change in facies architecture that signifies a shift to perennial discharge conditions and incised-valley backfilling. This is itself followed by a marked change in architectural style in a part of stratigraphy recording the Paleocene-Eocene Thermal Maximum (PETM), when variations in precipitation regime and/or intrabasinal characteristics (e.g., vegetation) related to this event favoured the lateral migration of channels, causing the development of laterally extensive coarse-grained bodies. Floodplain sediments comprise (i) heterolithic deposits accumulated in the vicinities of the fluvial channels where riparian ecosystems were developed, and (ii) overbank mudstones with variable degree of pedogenesis, especially associated with wetting and drying cycles in areas of the floodplain located distally away from fluvial channels. This succession records the importance of water-discharge variations in the evolution of alluvial systems: it displays facies arrangements recording the effects of highly variable discharge in ephemeral to perennial channels, as well as variations in stratigraphic architecture testifying to longer-term hydrological changes in the late Paleocene and earliest Eocene.

Aeolian sediment transport rates in the middle reaches of the Yarlung Zangbo River, Tibet Plateau

Aeolian sediment emission from surfaces and subsequent transport are important geological processes. The Tibet Plateau experiences strong aeolian activity in areas such as the Yarlung Zangbo River basin. The dust storms have caused grounding of aircraft, highway closures, and other consequences for the region's residents. However, few researchers have studied this activity, which means that little knowledge is available on aeolian activity to support efforts to mitigate or prevent aeolian disasters. We measured aeolian sediment transport in the middle reaches of the Yarlung Zangbo River from 2020 to 2021. Field observations showed spatial and temporal variation of the sediment transport rate, with the greatest aeolian sediment transport in spring and winter. The largest total aeolian sediment transport rate occurred over sandy desert, with the smallest emission by a floodplain grassland. The change in sediment transport rate with height followed an exponential function, but the coefficients differed among landscapes. The mean sediment transport rate was greatest above shifting sand near riverbanks (0.21 kg m−1 d−1), where the sand is exposed in the winter and spring, followed by shifting floodplain sands (0.13 kg m−1 d−1), and was lowest above a floodplain grassland (0.03 kg m−1 d−1). Mean grain size also decreased with increasing height above 0.25 m, with a minimum mean grain size (about 52.6 μm) at 3.0 m above a floodplain grassland, and maximum mean grain size (about 100.2 μm) at 3.0 m above a floodplain shifting sand surface. The spatial variation in sediment transport rates and grain size related to the proportion of fine particles in the surface material. By comparing the aeolian sediment transport over different landscapes, we found that river banks and floodplains, which had rich deposits of very fine sand, silt, and clay, were the major sources of dust in this region. Our results indicate that efforts to mitigate or prevent aeolian disasters require a focus on riverbank and floodplain deposits.

Using a fluvial archive to place extreme flood sediment (dis)connectivity dynamics in context of a longer-term record

The aim of this study was to establish the source and provenance of sediments deposited in a large floodplain sink during extreme floods in the Lockyer Creek catchment, Australia, in 2011 and 2013. We place the sediment source patterns in context of the longer-term record to determine whether coarse-grained sediment sources (i.e., very fine sand to very coarse sand) and the spatio-temporal pattern of (dis)connectivity have changed over time. We do this by matching the geochemical properties and age structure of a sediment profile located in a downstream floodplain sink to the elemental composition of source sediments. One hundred and fifty-seven sediment samples from 20 sites across the catchment are analysed using X-ray fluorescence (XRF) spectrometry to compare the elemental ratio composition of the downstream floodplain sink to its source materials. We use Optically Stimulated Luminescence (OSL) dating to determine the age structure of the sediments in the floodplain sink. The northern tributaries and parts of the Lockyer River trunk stream are the primary sources of coarse sediment. These areas are connected (coupled) to the lower trunk stream and floodplain sediment sink. Southern tributaries are largely disconnected (decoupled) and supply little sediment to the floodplain sediment sink. This pattern of sediment source contribution has remained similar over the last 6.8 ka at least. Sediment sources as observed in the 2011 flood have predominated over the mid-late Holocene whilst those in the 2013 flood are rare.

Paleosols record dry and humid paleoenvironments during the Upper Pleistocene in the Brazilian Pantanal

Information on Quaternary landscapes and paleoenvironmental changes in the interior of Brazil are scarce. One of such areas, particularly sensitive to these variations, is the modern tropical Pantanal wetland, whose alluvial deposits record paleoenvironmental and paleoclimatic changes during the late Quaternary. Based on detailed pedological and sedimentological descriptions, stimulated optical luminescence dating and analytical data we described a well-exposed sedimentary succession that crops out in slope terraces of the Aquidauana River, located in the southern area of the Pantanal. We interpret environmental changes that have occurred since the beginning of the Upper Pleistocene, and which affected sedimentation and biogeochemical processes. Basal deposits comprise cross-stratified sands, which record active fluvial sedimentation from ∼106 to ∼70 ka, a period of probable increased river discharge. Fluvial channel facies are overlaid by pedogeneized floodplain deposits, in which we recognized a pedocomplex composed of a sequence of three late Pleistocene truncated paleosol profiles and a Holocene soil profile. The two lower paleosols are Vertisols with mottling, slickensides, rhizolites and septarian glaebules of carbonate. These characteristics indicate prolonged drought periods under semi-arid to arid climates ca. ∼70 and ∼30 ka. The upper paleosol profile is a Planosol (Ultisol) with block structure, waxiness, iron glaebules, and manganese films, pointing to prevailing humid climates during the terminal Pleistocene (since ∼30 ka). Covering this truncated Planosol is a layer of fine to medium-grained Holocene sands, with development of a horizon A of an Arenosol (Entisol). These paleoenvironmental changes correlate with regional changes in precipitation deduced from speleothems in nearby plateaus and in southeastern Brazilian sites.

Compositional, micromorphological and geotechnical characterization of Holocene Tiber floodplain deposits (Rome, Italy) and sequence stratigraphic implications

AbstractThis study reports a high‐resolution micromorphological characterization of floodplain deposits to investigate the relationships among compositional, textural and geotechnical data, and integrate soil micromorphology with sequence stratigraphy. Compositional and textural characterization of facies associations and soil features are calibrated against geotechnical parameters. The latter, obtained from cone penetration and pocket penetrometer tests from a borehole advanced 60 m into the Tiber channel belt and floodplain, show that depositional features and post‐depositional modifications are intrinsically associated with cone penetration test parameters: cone resistance; sleeve friction; and friction ratio. Petrographic and micromorphological features document pedogenetic modifications across stratigraphic markers evidenced by faunal and plant activity, accumulation of peat, and typified by precipitation of heavy metals, iron oxides and secondary carbonates. All of these features developed in correspondence with alluvial flooding surfaces that are correlated with non‐marine and marine flooding surfaces recognized in the transgressive and highstand coastal and lagoonal deposits of the Tiber Depositional Sequence. These observations may serve as a model to reconstruct the sequence‐stratigraphic evolution of ancient relict soils. Nevertheless, additional criteria (for example, their stratigraphic position and correlative surfaces) are necessary to adequately interpret the genesis of such low‐rank stratigraphic surfaces. This work demonstrates that a combination between sedimentological and stratigraphic observations and soil micromorphology can be critical to supplement field observations and determine the relative effect of pedogenic and depositional processes on the organization, composition and texture, and geotechnical properties of floodplain in urban areas.

Yedoma Permafrost Genesis: Over 150 Years of Mystery and Controversy

Since the discovery of frozen megafauna carcasses in Northern Siberia and Alaska in the early 1800s, the Yedoma phenomenon has attracted many Arctic explorers and scientists. Exposed along coastal and riverbank bluffs, Yedoma often appears as large masses of ice with some inclusions of sediment. The ground ice particularly mystified geologists and geographers, and they considered sediment within Yedoma exposures to be a secondary and unimportant component. Numerous scientists around the world tried to explain the origin of Yedoma for decades, even though some of them had never seen Yedoma in the field. The origin of massive ice in Yedoma has been attributed to buried surface ice (glaciers, snow, lake ice, and icings), intrusive ice (open system pingo), and finally to ice wedges. Proponents of the last hypothesis found it difficult to explain a vertical extent of ice wedges, which in some cases exceeds 40 m. It took over 150 years of intense debates to understand the process of ice-wedge formation occurring simultaneously (syngenetically) with soil deposition and permafrost aggregation. This understanding was based on observations of the contemporary formation of syngenetic permafrost with ice wedges on the floodplains of Arctic rivers. It initially was concluded that Yedoma was a floodplain deposit, and it took several decades of debates to understand that Yedoma is of polygenetic origin. In this paper, we discuss the history of Yedoma studies from the early 19th century until the 1980s—the period when the main hypotheses of Yedoma origin were debated and developed.