Sediment Grain Size Research Articles

Grain size and shape analysis of recent and paleo sediments along Poyang Lake with insight into its environmental significance

Grain shape is a crucial sedimentological attribute commonly employed for reconstructing depositional environments and transport history. The new dynamic image analysis (DIA) techniques enable testing a large number of individual mineral grains simultaneously and overcomes many shortcomings of traditional visual evaluation. However, there is a relative dearth of comprehensive analysis on large quantities of sediments in recent and geological period to accurately document their grain shape properties and further evaluate their applicability in distinguishing sedimentary setting and its environmental significance. In this study, we measured the grain shape and size of 77 samples collected from modern surface sand (28 beach sand and 17 dune sand samples) and stratigraphic sediment (16 paleo-dunes sand and 16 paleo-lacustrine sand samples) along the Poyang Lake in southeastern China using the DIA method, and optically stimulated luminescence (OSL) dating was used to constrain the age of stratigraphic sediments from two sand hill sections. Also 217 published grain shape data from various sedimentary environments (coastal beach, desert dune and fluvial sand) were compiled as comprehensively assess the applicability of grain shape discrimination for sedimentary setting. Our results show the significant difference on grain shape feature among fluvial, coastal beach and dune sand. Dune sand exhibit the highest values for circularity (Ci), symmetry (Sy), aspect ratio (AR), and convexity (Cx), while fluvial sand displays high Ci, Sy, Cx value, and lowest AR value. This indicate that the grain shape can be a powerful tool for discriminating sedimentary setting. Principal component analysis shows that circularity and aspect ratio effectively describe the shape of the analyzed sediment and are decisive indicators for identification depositional environments, whereas the convexity has limited sensitivity in distinguishing grain edge roundness. The paleo-lacustrine sand (PLS) intercalated within the sand hill profiles returns OSL age of age of ∼ 26.5 ± 1.4 ka, which indicate the emergence of interdune lake rather than high lake level of Poyang Lake. PLS and modern beach sand (BS) exhibiting identical grain shape characteristics to those of modern aeolian dune sand (DS) suggested the occurrence of aeolian-fluvial interaction processes in our study area during the Last Glacial Maximum and present day. The paleo-dune sand (PDS) samples, yielding OSL age of 1.2 ± 0.1 ka, are characterized by higher Ci, Sy, and AR, indicating strong aeolian activity and dune reworking during the past two millennia along the Poyang Lake.

Grain size of Quaternary sediments in the continental shelf-margin: implications for paleo-environment in the Northwestern South China Sea

Grain size of Quaternary sediments in the continental shelf-margin: implications for paleo-environment in the Northwestern South China Sea

Model Sensitivity Analysis for Coastal Morphodynamics: Investigating Sediment Parameters and Bed Composition in Delft3D

Numerical simulation of sediment transport and subsequent morphological evolution rely on accurate parameterizations of sediment characteristics. However, these data are often not available or are spatially and/or temporally limited. This study approaches the problem of limited sediment grain-size data with a series of simulations assessing model sensitivity to sediment parameters and initial bed composition configurations in Delft3D, leading to improved modeling practices. A previously validated Delft3D sediment transport and morphology model for Dauphin Island, Alabama, USA, is used as the benchmark case. A method for the generation of representative sediment grain sizes and their spatially varying distributions is presented via end-member analysis of in situ surficial sediment samples. Derived sediment classes and their spatial distributions are applied to two sensitivity case simulations with increasing bed composition complexity. First, multiple sediment classes are applied in a single fully mixed layer, regardless of sediment type. Second, multiple sediment classes are applied in a thin, fully mixed transport layer with underlayers containing only the non-cohesive sediment classes below. Simulations were carried out in a probabilistic, Delft3D MorMerge configuration to capture long-term morphology change for 10 years. We found there is sensitivity to the inclusion of additional sediment classes and sediment distribution made evident in bed level and morphology change. Inclusion of highly mobile fine sediments altered model results in each sensitivity case. The model was also found to be sensitive to initial bed composition in terms of bed level and morphology change, with notable differences between sensitivity cases on decadal timescales, indicating an armoring effect in the second sensitivity case, which used the transport and underlayer bed configuration. The results of this study offer guidance for numerical modelers concerned with sediment behavior in coastal and estuarine environments.

Hydrodynamic mechanisms of topographic evolution in straight sandy beach: a case study of Wanpingkou beach, China

Stricter controls on destructive human activities in recent years have improved the protection and management of sandy coasts in China. Marine-driven geomorphic changes have become the predominant process influencing future beach evolution. However, in the complex geomorphic conditions of mixed artificial and natural coastlines, the mechanisms and contributions of various marine-driven factors to beach changes remain unclear. Using methods of field observations and numerical simulations by FVCOM model, this study reveals the sedimentary dynamic mechanism of straight sandy beach in the southern Shandong Peninsula (Wanpingkou beach, WPK beach). From 2005 to 2016, the northern section of WPK Beach eroded at a rate of approximately 2-3 m/year, the middle section at about 0.5-1.5 m/year, and the southern section accrued at an average rate of about 1-2 m/year, demonstrating an overall pattern of “northern erosion and southern accretion.” During winter, the average grain size of beach sediments is about 0.5 mm with minimal variations, indicating an onshore transport trend; in summer, the average grain size is about 0.7 mm with larger variations, indicating an offshore transport trend. Offshore sediment transport correlates mainly with seasonal changes in wind direction, while alongshore sediment transport is influenced by tidal currents, wind-induced currents, and wave-induced currents. Wave-induced currents are the primary force in alongshore sediment transport with a velocity of 0.1-0.3 m/s toward southwest. Followed by tidal currents with a velocity of 0.05-0.1 m/s, and wind-induced currents with a velocity of 0-0.1 m/s, which have a relatively minor impact. Therefore, seasonal changes in beach morphology are primarily controlled by waves, while interannual variations is mainly influenced by a combination of wave-induced currents and tidal currents. With increasing efforts in beach maintenance and coastal ecological restoration in recent years, understanding the sedimentary dynamics of beaches remains of vital theoretical and practical value.

Early diagenesis, sedimentary dynamics and metal enrichment reveal deep-sea ventilation in Magellan Seamounts during the middle Pleistocene

Seamounts are ubiquitous topographic units in the global oceans, and the Caiwei Guyot in the Magellan Seamounts of the western Pacific is a prime example. In this study, we analyzed a well-dated sediment core using magnetic properties, sediment grain size, and metal enrichment to uncover regional ventilation history during the middle Pleistocene and explore potential linkages to global climate changes. Our principal findings are as follows: (1) The median grain size is 3.3 ± 0.2 μm, and clay and silt particles exhibit minimal variation, with average values of 52.8 ± 1.8% and 38.2 ± 1.6%, respectively, indicating a low-dynamic process; (2) Three grain-size components are identified, characterized by modal patterns of ~3 μm (major one), ~40 μm, and 400–500 μm, respectively; (3) Magnetic coercivity of the deep-sea sediments can be classified into three subgroups, and their coercivity values are 6.1 ± 0.5 mT, 25.7 ± 1.0 mT, and 65.2 ± 2.1 mT. Based on these results, we propose a close linkage between magnetic coercivity and metal enrichment, correlating with changes in deep-sea circulation intensity. Conversely, sediment grain-size changes seem to be more strongly influenced by eolian inputs. Consequently, we suggest that regional ventilation has weakened since ~430 ka, likely linked to a reduction in Antarctic bottom water formation.

Study on Grain Size, Physical Properties and Organic Matter Characteristics of Tidal Flat Surface Sediments: May 2022 Hwangdo Tidal Flat Dataset, Cheonsu Bay

This study analyzes the geological and geochemical features of surface sediments in the Hwangdo Tidal Flat, located on Korea’s West Coast. The tidal flat experiences semi-diurnal tides, impacting organic matter decomposition and nutrient cycling. Ninety one sediment samples were collected and analyzed for physical and chemical properties including grain size, density, water content, organic carbon, and nitrogen. Sediments consist mainly of sand and silt, with coarser sediments near the main channel and finer sediments towards the west. Sediment grain size averages 4.12 Φ with a sorting coefficient of 1.96 Φ, indicating diverse energy environments. Total organic carbon and nitrogen correlate positively with grain size and density, reflecting sediment origin and environment. Kriging maps sediment grain size distribution, while correlation and linear regression analyses show relationships between variables. High correlations exist between various parameters, aligning with tidal flat characteristics and aiding understanding of sediment transport and deposition. The study provides baseline data for understanding the tidal flat’s geological, geochemical, and physical aspects, valuable for remote sensing validation and environmental monitoring. The dataset is freely available for research and management purposes.

CORE-VTRCC Cruise Report: Geophysical Acquisition and Seafloor Sampling along the Vitória-Trindade Ridge

This work reports the data obtained by the multidisciplinary cruise CORE-VTRCC surveying on the region around the Vitória-Trindade Ridge (VTR) aboard the vessel NHo Cruzeiro do Sul (October 18th to November 6th, 2021). The main objectives of the VTRCC Cruise were (1) to assess the role of the VTR as an oceanographic barrier for bottom currents along the Brazilian margin; and (2) to characterize the morphology of the volcanic seamounts and its relationships with the carbonate sediments that cover it. For that, we performed multibeam bathymetry, magnetometry, high-resolution multichannel seismic survey, subbottom profiling survey, and seafloor sampling along the VTR and along the Columbia Deepwater Channel. The raw dataset was processed for quality control and organized for public access. When required, additional geophysical processing occurred to improve data quality. Seafloor samples are characterized based on the concentration of its biofacies, the sediment grain-sizes and the morphology of rhodoliths sampled. The dataset reveals comprehensively the geological and oceanographic heterogeneity of the region around the VTR.

Precipitation variation in the northern South China Sea of the last 700 years reconstructed by lagoon sediments

Precipitation changes in the East Asia are closely linked to the monsoonal climate in this region and the hydrothermal processes in the western tropical Pacific. However, trends of reconstructed precipitation records for the past millennium are inconsistent and the influencing factors are in dispute. Here we reconstruct a 700-year precipitation record for the northern South China Sea (SCS) using grain size of lagoon sediments. Our data revealed that precipitation increased in the early to middle Little Ice Age, possibly modulated by tropical cyclones and the Walker Circulation. The East Asian Summer Monsoon and Pacific Decadal Oscillation had the major influence on the precipitation changes in the northern SCS. This study provides new insight into the processes and the underlying mechanisms of climate changes in the SCS.

1500-Year Sedimentary Records of the East Asian Summer Monsoon and Yellow Sea Warm Current from the Muddy Area of the North Yellow Sea, China

Advances in reconstructing the East Asian monsoon have provided important insights into the natural climate variability in Asia during the pre-instrumental period. However, there are still unresolved paleoclimate issues that necessitate the use of geological proxy data to further our understanding of past climate changes. This study focused on core B13, located in the muddy area of the North Yellow Sea (NYS), to investigate the evolutionary history over the past 1500 years and reconstruct the records of the East Asian summer monsoon (EASM) and Yellow Sea warm current (YSWC). The mean grain size of sediment ranged from 4.2 Φ to 5.6 Φ, with the sorting coefficient ranging from 1.9 to 2.2, indicating poor sorting. The C–M pattern showed a limited range of values, with the M values being between 33 and 83 μm and the C values being between 165 and 287 μm, suggesting uniform-suspension transport. The L* index ranged from 40.41 to 44.12, while the a* and b* indexes ranged from 0.55 to 1.78 and 2.86 to 5.94, respectively. A stable and relatively strong sedimentary environment is indicated through a comprehensive analysis of the C–M plot, triangular plot, the relationship between the mean grain size and sorting, and the changes in grain-size and color parameters. The sedimentary evolution in the muddy area of the NYS over the past 1500 years can be categorized into three distinct stages. In this study, proxies for the EASM and YSWC were extracted using the VPCA method from the sediment grain size and diffuse spectral reflectance (DSR) data, respectively. The reliability of these proxies has been confirmed through comparison with other validated proxies. The results indicated that the strength of the EASM and YSWC also exhibited three stages, corresponding to the Dark Ages Cold Period (DACP), Medieval Warm Period (MWP), and Little Ice Age (LIA), respectively. On a centennial scale, the correlation between the EASM and YSWC was predominantly negative. This research validates the reliability of the VPCA method for paleoclimate reconstruction, contributes important climate records in a special muddy area, and provides a new perspective on how to eliminate temporal errors in verifying the correlation between the two climate systems.

Grain Size in Landscapes

Earth's terrestrial topography evolves in response to the interaction of tectonics, climate, and lithology. Recent discoveries suggest that the grain size of sediments produced on hillslopes and transported through river networks is key to understanding these interactions. Hillslope grain size varies systematically with erosion rate and residence time, the degree of chemical and physical weathering, and the fracture density and susceptibility to weathering of rock. Variations in initial grain size strongly influence the spatial evolution of grain size distributions as particles mix and wear during downstream transport through channel networks. In rivers, the size and flux of the coarse fraction of the sediment load control the rate of incision into bedrock and thus govern channel slope and ultimately the relief of actively eroding landscapes. These relationships suggest that a primary way that tectonics, climate, and lithology influence landscape evolution is through their controls on sediment grain size. ▪Recent research reveals the central role of sediment grain size in controlling bedrock river morphodynamics, linking grain size to channel slope and topographic relief.▪Tectonics, climate, and lithology govern the size of sediments produced on hillslopes; hence, grain size mediates their influence on landscape evolution.▪Feedbacks linking sediment grain size, topography, weathering, erosion, and sediment transport provide new opportunities for advances in Earth surface science.

Exploring historical changes in mountain river hydrodynamics induced by human impact

During the 20th-century many mountain rivers in Europe were subjected to intensive human impacts which substantially modified their channel morphology. How these changes affected river hydrodynamics and response to floods remains uncertain. In this work, we perform hydraulic modelling using data from archival aerial photos to explore relations between hydraulic parameters of floods and human-induced channel incision occurring on the Czarny Dunajec River (Polish Carpathians) between 1964 and 2012. Data on vertical position of the channel used for two-dimensional modelling of flood flows were extracted (as Digital Elevation Models DEMs) from archival aerial photos from 1964 and 1983 and ALS (Airborne Laser Skanning)-derived DEM from 2012. Water depth, flow velocity, bed shear stress, and sediment critical diameter were modelled for four flood scenarios (2-year, 5-year, 20-year, and 50-year floods) as well as the extent of flooded area and additionally the grain size of channel sediment was calculated. The values of water depth, flow velocity, bed shear stress and sediment critical diameter increased significantly between 1964 and 1983, especially for 20-year and 50-year floods. Only the flow velocity within the floodplain zone did not increase for the two largest flood scenarios due to the expansion of riparian forest in the second half of the twentieth century. The increase in flow rate was accompanied by a progressive reduction of the extent of flooded area, especially between 1964 and 1983, as well as by increase in mean grain size of channel sediment. Between 1983 and 2012 changes in hydraulic parameters were less pronounced, and coarser and well packed channel sediment dominated on the river bed. Our work demonstrates that reconstruction of past river hydrodynamics, rather than river state at time horizons, can give essential insights into functioning of the river channel and floodplain during the intensification of human impacts after 1950s.

Reconciling Coulter Counter and laser diffraction particle size analysis for aquaculture monitoring

The disaggregated inorganic grain size (DIGS) of bottom sediment analyzed with a Coulter Counter (CC) has been used to show that the fraction of sediment deposited in flocs (floc fraction) increased in both the near and far field after the introduction of open cage salmon aquaculture, altering benthic habitat and species composition. As a result, DIGS was identified as a potential indicator of regional environmental changes due to aquaculture. Laser diffraction is an attractive alternative to the CC because of its greater efficiency and larger size range. To determine if a laser diffraction instrument, Beckman-Coulter LS 13 320 (LS), could replace the CC within a Canadian national aquaculture monitoring program, the DIGS of 581 samples from five different regions in eastern Canada were analyzed with an LS and a CC. Results show that the LS could not be used to calculate floc fraction. Instead, % sortable silt and the volume % of inorganic particles < 16 µm were evaluated as alternative proxies for fine sediment properties. LS and CC values for these parameters were correlated, but they were significantly different and the relationship between the instruments was dependent on the area sampled. The LS did not capture variations between areas seen in the CC DIGS data. Where the DIGS from the CC found no sorting in the finest size classes, all the LS samples had similar size distributions characterized by smooth modal peaks. The LS and CC both return values that can be used to monitor changes in the deposition of fine-grained sediment, but the LS cannot determine changes in floc deposition and caution is required if comparing different sedimentary environments.

Quantifying the impact of anthropogenic emissions and aquatic environmental impacts on sedimentary mercury variations in a typical urban river

In urban and industrial regions, sedimentary mercury (Hg) serves as the crucial indicator for Hg pollution, posing potential risks to ecology and human health. The physicochemical processes of Hg in aquatic environments are influenced by various factors such as anthropogenic emissions and aquatic environmental impacts, making it challenging to quantify the drivers of total mercury (THg) variations. Here, we analyzed the spatiotemporal variations, quantified driving factors, and assessed accumulation risks of sedimentary THg from the mainstream of a typical urban river (Haihe River). THg in the urban region (37−3237 ng g−1) was significantly higher (t-test, p < 0.01) than in suburban (71−2317 ng g−1) and developing regions (156−916 ng g−1). The sedimentary THg in suburban and developing regions increased from 2003 to 2018, indicating the elevated atmospheric deposition of Hg. Together with the temperature, grain size of sediments, total organic carbon (TOC), the pH and salinity of water, 40 components of parent and substituted polycyclic aromatic hydrocarbons (PAHs) were first introduced to quantify the driver of sedimentary THg based on generalized additive model. Results showed that anthropogenic emissions, including three PAHs components (31%) and TOC (63%), accounted for 94% of sedimentary THg variations. The aquatic environmental impacts accounted for 5% of sedimentary THg variations. The geo-accumulation index of THg indicated moderate to heavy accumulation in the urban region. This study demonstrates that homologous pollutants such as PAHs can be used to trace sources and variations of Hg pollution, supporting their co-regulation as international conventions regulate pollutants.

Role of macroalgal forests within Mediterranean shallow bays in blue carbon storage

Although seaweeds rank among the most productive vegetated habitats globally, their inclusion within Blue Carbon frameworks is at its onset, partially because they usually grow in rocky substrates and their organic carbon (Corg) is mostly exported and stored beyond their habitat and thus, demonstrating its long-term storage is challenging. Here, we studied the sedimentary Corg storage in macroalgal forests dominated by Gongolaria barbata and in adjacent seagrass Cymodocea nodosa mixed with Caulerpa prolifera algae meadows, and bare sand habitats in Mediterranean shallow coastal embayments. We characterized the biogeochemistry of top 30 cm sedimentary deposits, including sediment grain-size, organic matter and Corg contents, Corg burial rates and the provenance of sedimentary Corg throughout stable carbon isotopes (δ13Corg) and pyrolysis analyses. Sediment Corg stocks and burial rates (since 1950) in G. barbata forests (mean ± SE, 3.5 ± 0.2 kg Corg m−2 accumulated at 15.5 ± 1.6 g Corg m−2 y−1) fall within the range of those reported for traditional Blue Carbon Ecosystems. Although the main species contributing to sedimentary Corg stocks in all vegetated habitats examined was C. nodosa (36 ± 2 %), macroalgae contributed 49 % (19 ± 2 % by G. barbata and 30 ± 3 % by C. prolifera) based on isotope mixing model results. Analytical pyrolysis confirmed the presence of macroalgae-derived compounds in the sediments, including N-compounds and α-tocopherol linked to G. barbata and C. prolifera, respectively. The sedimentary Corg burial rate linked to macroalgae within the macroalgal forests examined ranged from 5.4 to 9.5 g Corg m−2 y−1 (7.4 ± 2 g Corg m−2 y−1). This study provides empirical evidence for the long-term (∼70 years) sequestration of macroalgae-derived Corg within and beyond seaweed forests in Mediterranean shallow coastal embayments and thereby, supports the inclusion of macroalgae in Blue Carbon frameworks.

Deformation of vegetated channel bed under ice-covered flow conditions

A common feature of cold regions is the presence of ice cover on water surface. In the presence of vegetation in the channel bed which is normally leafless in winter, interaction between river ice and vegetated channel bed becomes very complex. In the present study, the impacts of ice cover and submerged leafless vegetation on channel bed deformation and flow resistance have been investigated based on 144 experiments conducted in a large-scale outdoor flume. The independent variables associated with the maximum scour depth around vegetation elements have been assessed and equations have been developed. Results indicate that the most important variable affecting the maximum scour depth around vegetation under ice-covered flow conditions is the ratio of the ice cover roughness to the bed roughness (ni/nb). However, under open channel flow conditions, the flow Froude number is the most important variable influencing the maximum scour depth. The methods based on the law of the wall provide reliable Manning's coefficient for both smooth and rough ice covers in the presence of submerged vegetation in the channel bed. As the vegetation density increases, the size of scour holes becomes smaller. With the increase in vegetation density, the median grain size of the armour layer in scour holes decreases correspondingly. When vegetation elements are placed in a staggered configuration, the median grain size of the armour layer in scour holes is less than that of squared configuration of vegetation elements. Regardless of the roughness of ice cover on water surface and the grain size of sediment in the bed, the maximum depth of scour holes always occurs at the upstream front face of each vegetation element. Under the rough ice-covered flow condition, the maximum depth of scour holes is more than that under the smooth ice-covered and open flow conditions.

Characteristics and Environmental Indications of Grain Size and Magnetic Susceptibility of the Late Quaternary Sediments from the Xiyang Tidal Channel, Western South Yellow Sea

To reveal the characteristics and environmental indications for the combination of the grain size and magnetic susceptibility of coastal sediments, we provided a necessary basis for further study on their genetic mechanisms. Based on the data of grain size and magnetic susceptibility of the 36.10 m long core 07SR01 sediments in the Xiyang tidal channel of western South Yellow Sea, we analyzed their variations and correlations and further revealed their environmental indications and corresponding regional sedimentary evolution via the combination of the aforementioned analysis results, the reinterpretation results of the sedimentary sequence and the age of core 07SR01 and shallow seismic profiles, and the findings of climate and glacial–eustatic cycles during Late Quaternary. The three stages of the sedimentary evolution of the Xiyang tidal channel between marine isotope stage (MIS) 7 and MIS 5 were summarized as follows: First is the stage of marginal bank and riverbed developments in the tidal estuary under a relatively high sea level and strong hydrodynamic conditions during MIS 7 (core section: 36.10–26.65 m). The sediments deposited in this stage were mainly affected by the paleo-Changjiang River and characterized by a coarse grain size (mean: 4.02 Φ) and relatively high magnetic susceptibilities (mean: 27.06 × 10−8 m3·kg−1), with small fluctuations which were strongly and positively correlated with the sand component. Second is the stage dominated by fluviolacustrine and littoral environments with the weak hydrodynamics during MIS 6–5, in which the climate changed from cold and dry to warm and humid as the sea level rose after a drop (core section: 26.65–15.77 m). The sediments deposited in this stage were characterized by a fine grain size (mean: 5.27 Φ) and low magnetic susceptibilities with minor variations (mean: 10.83 × 10−8 m3·kg−1) which were weakly and positively correlated with the coarse silt component. Third is the stage of delta front in the tidal estuary with a relatively high sea level and strong hydrodynamics during MIS 5 (core section: 15.77–0 m). The sediments deposited in this stage were strongly influenced by the paleo-Yellow River and characterized by a relatively coarse grain size (mean: 4.86 Φ), and high magnetic susceptibilities (mean: 37.15 × 10−8 m3·kg−1) with large fluctuations which were weakly and positively correlated with the sand and coarse silt components.

Effect of turbulence-induced sediment resuspension and sedimentation on underwater wireless optical communication

In real-life marine environments, the composition and grain size of suspended sediments and the resuspension and sedimentation of sediments caused by turbulence may have a significant impact on underwater wireless optical communication (UWOC). However, to date, researchers have not conducted quantitative research on this issue. To this end, we innovatively study the effects of different compositions and grain sizes of suspended sediments on UWOC and the effects of turbulence-induced sediment resuspension and sedimentation on UWOC in this paper. Quartz and kaolin with different grain sizes are used to simulate sediments in seawater. An oscillating grid that can vary frequency and stroke is used to generate turbulence of different intensities. By comparing the turbidity and optical power density of different simulated sediments with different grain sizes, we find that the smaller the grain size of the simulated sediments, the higher the bit error rate (BER) under the same turbidity. But different simulated sediments with different grain sizes have similar effects on BER performance under the same optical power density. Therefore, turbidity can be used to characterize the changes of underwater channels, and optical power density can be used to evaluate the attenuation of light at the receiving end after transmission through the underwater channel. By continuously changing the frequency of the grid to cause the sediments to resuspend and sink, we prove that the process of turbulence-induced sediment resuspension and sedimentation can seriously affect the BER performance. The larger the frequency of the grid, the greater the turbulence intensity and the worse the BER performance. This study lays a foundation for the practical application of UWOC in mobile ocean observation networks.

Morphological characteristics and conditions of drainage basins contributing to the formation of debris flow fans: an examination of regions with different rock strength using decision tree analysis

Abstract. Debris flows cause severe disasters that can result in human casualties and the collapse of houses. The establishment of early warning systems in basins with high debris flow risks is needed to reduce the negative impacts of debris flow disasters. Because debris flows often form debris flow fans near the mouths of valleys, debris flow fans are regarded as important topographical elements that indicate the occurrence of debris flows. The presence or absence of a debris flow fan makes it possible to clarify the morphological conditions of the contributing area that has generated debris flows. These morphological conditions may depend on rock strength, which controls the weathering activity and grain size of sediments. In this study, we investigated the morphological conditions of a drainage basin that contribute to the formation of debris flow fans using decision tree analysis. The analysis was conducted at two sites with clear differences in rock strength due to geological processes: Neogene sedimentary rock and Paleogene accretionary complex sites. As a result of decision tree analysis using data sets containing a total of 158 basins, the thresholds of morphological parameters needed for forming debris flow fans differed depending on the geological features. When the relief ratio was less than 0.29 at the Paleogene accretionary complex site, coarse-grained sediments were less likely to pass out of the valley, resulting in the absence of debris flow fans. On the other hand, at Neogene sedimentary rock sites, short basins were determined to form debris flow fans, even if the relief ratio was less than 0.36, because the sediments were fine-grained and tended to flow downstream. In contrast, morphological factors that influence the presence or absence of debris flow fans were common at both sites. The first, second, and third most important morphological factors were the relief ratio, most frequent slope gradient, and basin length, respectively. Therefore, these morphological factors are considered important in evaluating debris flow risks. This study demonstrates that the decision tree analysis is an effective tool for determining the hierarchy and threshold of morphological factors that classify the presence or absence of debris flows that reach valley mouths.

The grain size of sediments delivered to steep debris‐flow prone channels prior to and following wildfire

AbstractDebris flows are powered by sediment supplied from steep hillslopes where soils are often patchy and interrupted by bare‐bedrock cliffs. The role of patchy soils and cliffs in supplying sediment to channels remains unclear, particularly surrounding wildfire disturbances that heighten debris‐flow hazards by increasing sediment supply to channels. Here, we examine how variation in soil cover on hillslopes affects sediment sizes in channels surrounding the 2020 El Dorado wildfire, which burned debris‐flow prone slopes in the San Bernardino Mountains, California. We focus on six headwater catchments (&lt;0.1 km2) where hillslope sources ranged from a continuous soil mantle to 95% bare‐bedrock cliffs. At each site, we measured sediment grain size distributions at the same channel locations before and immediately following the wildfire. We compared results to a mixing model that accounts for three distinct hillslope sediment sources distinguished by local slope thresholds. We find that channel sediment in fully soil‐mantled catchments reflects hillslope soils (D50 = 0.1–0.2 cm) both before and after the wildfire. In steeper catchments with cliffs, channel sediment is consistently coarse prior to fire (D50 = 6–32 cm) and reflects bedrock fracture spacing, despite cliffs representing anywhere from 5% to 95% of the sediment source area. Following the fire, channel sediment size reduces most (5‐ to 20‐fold) in catchments where hillslope sources are predominantly soil covered but with patches of cliffs. The abrupt fining of channel sediment is thought to facilitate postfire debris‐flow initiation, and our results imply that this effect is greatest where bare‐bedrock cliffs are present but not dominant. A patchwork of bare‐bedrock cliffs is common in steeplands where hillslopes respond to channel incision by landsliding. We show how local slope thresholds applied to such terrain aid in estimating sediment supply conditions before two destructive debris flows that eventually nucleated in these study catchments in 2022.

Organic Carbon Sources in Surface Sediments on the Northern South China Sea

AbstractThe burial of organic carbon (OC) in marine sediments regulates CO2 content in the atmosphere. However, the OC sources and their effect on the OC preservation in sediments of the continental marginal sea remain elusive. Here, we survey the abundance, stable carbon and radiocarbon isotopes of OC, as well as mineral surface area and grain size in surface sediments from the shelf to the abyssal plain of the South China Sea. We found that the marine OC comprises the 63 ± 9% and 78 ± 14% of sedimentary OC off South China and Luzon, respectively. The petrogenic OC contributing to sedimentary OC in these sites is generally higher than that of soil OC (22 ± 10% vs. 12 ± 6%). The sedimentary OC off Taiwan is predominantly derived from petrogenic OC, accounting for 57 ± 10%, with remaining consisting of marine OC (36 ± 10%) and soil OC (6 ± 1%). High soil OC contents are found in fine sediments on the inner shelf off South China, and high petrogenic OC contents occur in fine sediments off Taiwan. Marine OC contents are high in fine sediments on the middle shelf and upper slope off South China, but low in coarse sediments on the outer shelf and upper slope, and fine sediments on the lower slope and abyssal plain. The OC sources, mineral surface area, and oxygen exposure time of OC together control the preservation of OC in sediments with their relative importance differing on varying depositional settings of this sea.