Enhanced Sediment Deposition Research Articles

Cynodon dactylon and Sediment Interaction in the Three Gorges Reservoir: Insights from a Three-Year Study

Sediment deposition is critical in maintaining riparian plant communities by providing essential nutrients and posing growth challenges. This study focuses on Cynodon dactylon, a dominant clonal species in the riparian zones of the Three Gorges Reservoir, and its interaction with sediment deposition over three years. Results indicated an average sediment deposition depth of 2.85 cm in the lower riparian regions. Observations revealed that C. dactylon coverage increased progressively at lower elevations despite its dominance diminishing with rising elevation levels. Additionally, positive linear correlations between C. dactylon coverage and sediment deposition depths were identified during flood periods, underscoring the species’ role in enhancing sediment deposition. These findings suggest that C. dactylon plays a significant role in sediment accumulation, which may bolster its growth and survival prospects during subsequent growing cycles. The study highlights the importance of riparian vegetation, mainly perennial clonal species like C. dactylon, in promoting sediment accumulation and contributing to the stability and functionality of riparian ecosystems.

Factors influencing microplastic abundances in the sediments of a seagrass-dominated tropical atoll

Seagrass meadows are one of the world's most diverse ecosystems offering habitats for an extensive array of species, as well as serving as protectors of coral reefs and vital carbon sinks. Furthermore, they modify hydrodynamics by diminishing water flow velocities and enhancing sediment deposition, indicating the potential for microplastic accumulation in their sediments. The build-up of microplastics could potentially have ecological impacts threatening to ecosystems, however little is known about microplastic abundance and controlling factors in seagrass sediments. Here we investigated microplastic characteristics and abundances within sediments underlying four seagrass meadow sites on the Turneffe Atoll, Belize. Sediment cores were collected and sub-sampled to include a range of replicate surface sediments (0–4 cm) and depth cores (sediment depths 0–2, 2–5, 5–10, 10–20 and 20–30 cm). These were analysed using 25 μm resolution μFTIR, with spectral maps processed using siMPle software. Microplastics were prevalent across the sites with an abundance range (limit of detection (LOD) blank-corrected) of < LOD to 17137 microplastics kg−1 dw found on the east side of the atoll. However, their abundances varied greatly between the replicate samples. Polyethylene and polypropylene were the most commonly detected polymers overall, although the dominant polymer type varied between sites. There were no differences in the abundance of microplastics between sites, nor could abundance distributions be explained by seagrass cover. However, abundances of microplastics were highest in sediments with lower proportions of fine grained particles (clay, <4 μm) suggesting that hydrodynamics override seagrass effects. Additionally, no patterns were seen between microplastic abundance and depth of sediment. This suggests that microplastic abundance and distribution in seagrass meadows may vary significantly depending on the specific geographical locations within those meadows, and that more complex hydrodynamic factors influence spatial variability at a localised scale.

Using a digital elevation model to place overland flow fields and uncleaned ditch sections for water protection in peatland forest management

Peatland forest management is an important source of human-induced additional nitrogen, phosphorus, and organic carbon loads to surface waters in Finland. Ditch network maintenance (DNM) is typically used to ensure sufficient soil drainage for profitable timber production on peatland sites, but nutrient mobilization, particularly of phosphorus, and suspended sediment loads from ditches can cause harmful ecological effects on the quality of water courses. Various water protection structures have been developed to capture the loads released after treatments and to decrease the management impacts in forestry. Overland flow fields are efficient mechanisms for water purification, but they are rarely used in practice due to difficulties in finding suitable locations. In addition, leaving breaks between cleaned ditch sections is recommended wherever possible, as uncleaned sections decrease flow velocity and enhance sediment deposition from runoff water. To plan the most adequate protection methods in DNM, new GIS-based tools are needed to propose e.g. appropriate locations for water protection structures. We developed a spatial analysis to suggest locations for overland flow fields and uncleaned ditch sections within the catchment of lake Kovesjärvi (Parkano, Western Finland). Airborne lidar data consisting of 5 pulses m−2 enabled detailed terrain analysis and finding strictly specified terrain properties that satisfied the conditions of the water protection structures. Half of the field-reviewed overland flow fields were evaluated as good suggestions by forest management professionals and researchers. The model for uncleaned ditch sections worked well on the relatively flat study area, as 77% of the field-reviewed suggestions were rated good by expert opinions. Overall, the approach was evaluated to be useful in peatland forest management. Model parameters can be modified for various topographies, but more studies are needed prior to wider use of the model.

Reconstructing the Fluvial History of the Lilas River (Euboea Island, Central West Aegean Sea) from the Mycenaean Times to the Ottoman Period

This paper aims to reconstruct the alluvial activity for the Lilas river, the second-largest catchment of Euboea Island (Central Western Aegean Sea), for approximately the last three and a half millennia. The middle reaches (Gides basin) exhibit several historical alluvial terraces that were first recognised in the 1980s but have remained poorly studied, resulting in uncertain chronological control of palaeofluvial activity. In order to reconstruct the past fluvial dynamics of the Lilas river, a ca. 2.5 m thick stratigraphic profile has been investigated for granulometry and magnetic parameters. Absolute dating of the sediments was possible by applying Optically Stimulated Luminescence (OSL). The results reveal: (i) two coarse-grained aggradational episodes dated from the Mycenaean/Early Iron Age and the Roman periods, respectively, (ii) a phase of rapid fine-grained vertical accretion corresponding to the Late Byzantine to early Venetian periods, (iii) potential evidence for final alluvial deposition from the Little Ice Age/Ottoman period, and (iv) two major incision episodes inferred from Ancient Greek times and most of the Byzantine period. Based on the published core material, the paper also evaluates the direct impacts of the Late Holocene alluviation recorded mid-stream on the fluvial system situated downstream in the deltaic area. Sediment sourcing is attempted based on the magnetic properties of the catchment lithology and of alluvium collected upstream along the main stream bed. Finally, the present paper discusses the possible links between Late Holocene hydroclimatic oscillations and the aggradational/incision phases revealed in the Gides basin. Correlations are attempted with regional palaeoclimate records obtained for the Aegean. In addition to climatic variability, anthropogenic factors are considered: specific land use for agricultural purposes, in particular during the Mycenaean period, the Roman and the Late Byzantine/Early Venetian periods, might have enhanced sediment deposition. Archaeological information and pollen records were also evaluated to reconstruct regional land-use patterns and possible impacts on soil accumulation over the last 3.5 millennia.

Soil conservation practices contribution in trapping sediment and soil organic carbon, Minizr watershed, northwest highlands of Ethiopia

Globally, land degradation and climate change are priority problems. Therefore, this study was conducted in Minizr catchment in the northwest highlands of Ethiopia to investigate the sediment and sediment organic carbon (SOC) trapping role of soil and water conservation practices (SWCPs) constructed inside fields, check dams (CDs) constructed inside gullies and micro-trenches constructed on degraded grazing lands, which would help to reduce land degradation and climate change. Our three research questions were: How much sediment was trapped by SWCPs? How much SOC was trapped by SWCPs? What was the role of SWCPs in mitigating climate change? Google Earth Imagery was used to map the SWCPs constructed inside fields; a Digital Elevation Model (ASTER DEM 20 m) was used to delineate the boundary of the study watershed and to evaluate elevation and slope characteristics, and GPS was used to collect watershed outlet coordinate points, CD locations and to accurately digitize SWCPs. Three different SWCPs, five different CDs and 30 micro-trenches were evaluated for their sediment and SOC trapping efficiency. The experiment was designed in three treatments with three replications, except for the CD studies. The volume of sediment trapped was quantified using field measurements of the deposited sediment and SOC was determined in a soil laboratory. The investigated SWCPs, CDs and micro-trenches trapped ~584,745 kg SOC together with 32,105 t of sediment. The percentage SOC was higher in the CD sediments (1.98%) than SWCPs (1.38%) and micro-trenches (1.49%). A large amount of SOC was deposited in vegetative-supported CDs than CDs constructed from structures alone, i.e. CDs supported with vegetative practices trapped more SOC. In general, SWCPs, CDs and micro-trenches were found to reduce soil erosion or land degradation by enhancing sediment deposition. They also trapped large amounts of SOC together with the sediment, which can reduce the greenhouse gases emission to the atmosphere. Therefore, governmental and non-governmental organizations and professionals should also consider SWCPs as climate change mitigation measures, like that of afforestation/plantation.

Sedimentation Patterns in Sorsogon Bay, Philippines Using 210Pb

210Pb has been used as a tracer to give an insight into the sedimentation process occurring in Sorsogon Bay. The sedimentation process can provide the possible sources, movement, pathways, and sinks of sediments and sediment-associated materials (e.g., chemicals, pollutants) – which are important for a better understanding of the changes happening in the bay and its watershed. The information that will be derived can be used as an input parameter by other researchers doing modeling work (material/water/pollutant flow, movement, dispersion, and residence time dynamics) at Sorsogon Bay. Seven sediment cores have been collected in Sorsogon Bay to determine the 210Pb-derived sedimentation rate estimates across the bay. A sedimentation rate of 1 cm/yr could be estimated for the eastern, central, and western areas of Sorsogon Bay. Areas near Sorsogon City (SO-07), Cadacan River (SO-03), and the area offshore of Buenavista and Rizal (SO-06) have enhanced sediment deposition, which could be due to an area where enhanced erosion from human activities is apparent, proximity to a river system that drains/carries volcanic material and debris from Mount Bulusan, and near an open dumpsite where possible materials (wastes, debris, leachates) could be carried offshore respectively. The sedimentation rates are shown to be higher in the shallower areas of the bay.

Seasonal variation in sediment transport and deposition on a muddy clinoform in the Yellow Sea

A calibrated, coupled model of the Bohai and Yellow Seas is developed to investigate sediment transport and deposition processes around a distinct clinoform east of the Shandong Peninsula. On the clinoform, deposition patterns vary seasonally with the regional hydrography and the East Asian Monsoon. A local maximum in sediment deposition is located offshore from spring to autumn, whereas sediment deposition is greater near the coast and decreases seaward in winter. From spring to autumn, a thermal front develops around the Yellow Sea Cold Water Mass and limits sediment transport seaward, enhancing sediment deposition and leading to high deposition rate in a focused region offshore. Sediment is transported to the region by the southward basin-scale cyclonic circulation, and the cross-shelf gradient in stratification, sediment resuspension, and vertical mixing at the front lead to a convergence in sediment transport and deposition. Sediment deposition over the clinoform is the greatest during the transition from summer to autumn when the thermal front persists but sediment supply increases. This results from weakened stratification and greater resuspension near the coast due to stronger winds, waves, and sea-surface cooling. A seasonal seaward shift of the frontal position toward deeper water is consistent with the decreasing heat flux, and the corresponding shift in location of maximum sediment deposition shifts seaward. In winter, the thermocline disappears and vertical mixing increases throughout the region. SSC over the clinoform increases due to active resuspension and net deposition decreases, and instead sediment is transported seaward and deposits in the South Yellow Sea. Current velocity, SSC and sediment deposition rates decrease seaward, in contrast to the focused trapping at the thermal front from spring to autumn. The deposition rates around the thermal front from spring to autumn are greater than in the winter, so the high net sediment accumulation over an annual cycle occurs in the same region as the longer-term accumulation preserved in the omega-shaped clinoform.

Iron mineralogy as a fingerprint of former steelmaking activities in river sediments

Submerged sediment cores were collected upstream of a dam in the Orne River, northeastern France. This dam was built in the context of steelmaking to constitute a water reservoir for blast furnace cooling and wet cleaning of furnace smokes. The dam also enhanced sediment deposition in the upstream zone. This study was performed to unravel the contamination status of sediments and to evidence possible contribution sources. The sediment layers were analyzed for water content, grain size, chemical composition, crystalline phases at a bulk scale and poorly crystalline and amorphous phases at a sub-micrometer scale. Visual aspect, texture, color, and chemical and mineralogical analyses showed that the settled sediments were mainly composed of fine black matter, certainly comprising steelmaking by-products. Those materials were highly enriched with Fe, Zn, Pb and other trace metals, except for a relatively thin layer of surficial sediments that had settled more recently. Bulk mineralogy revealed crystalline iron minerals, such as magnetite, goethite, wuestite and pyrite, in the deep layers of the sediment cores. Furthermore, microscopic investigations evidenced the presence of ferrospheres, goethite nanoparticles and newly formed Fe-aluminosilicates; all originating from the former steelmaking facilities. The variation of iron mineralogy, combined with specific chemical profiles and other sediment features, demonstrate the different contributions that constitute the sediment deposit. Furthermore, chemical and mineralogical features of goethite and Fe-aluminosilicates could be used as a fingerprint for such contaminated sediments.

Tidal-scale flow routing and sedimentation in mangrove forests: Combining field data and numerical modelling

Tidal-scale biophysical interactions establish particular flow routing and sedimentation patterns in coastal mangroves. Sluggish water flows through the mangrove vegetation and enhanced sediment deposition are essential to maintain these valuable ecosystems, thereby enabling their contribution to coastal protection and stabilization. Spatially explicit field observations of tidal-scale flow routing and sediment deposition were obtained in an elevated mangrove stand dissected by tidal creeks, located in the Trang river estuary at the Thai Andaman coast. An accurate and efficient depth-averaged process-based numerical model of this field site was developed in Delft3D to study the contributions of various biogeophysical mangrove settings to the observed tidal dynamics and to study the impacts of changes of these environmental conditions. The creeks are found to form the major pathway for the tidal inflow during the lower tides, while the sheltered interior of the forest is an effective sediment sink during the higher tides. A numerical sensitivity analysis of the initial response—or adaptive capacity—of the studied mangrove system to instantaneous environmental changes reveals the stable state of the study site: deposition rates are largely imposed by the topography and relative elevation, while they are rather independent of the vegetation density. Deeper inundations of the mangroves favor sheet flows through the forest and spatially averaged deposition rates decrease, particularly when this coincides with decreasing vegetation densities. Moreover, the sediment trapping efficiency is found to reduce significantly with diminishing sediment inputs and with mangrove area losses. These results clearly indicate the sensitivity of mangroves' ecosystem engineering ability—in terms of sedimentation—to climate change and anthropogenic threats.

Effects of long-term grazing on sediment deposition and salt-marsh accretion rates

Many studies have attempted to predict whether coastal marshes will be able to keep up with future acceleration of sea-level rise by estimating marsh accretion rates. However, there are few studies focussing on the long-term effects of herbivores on vegetation structure and subsequent effects on marsh accretion. Deposition of fine-grained, mineral sediment during tidal inundations, together with organic matter accumulation from the local vegetation, positively affects accretion rates of marsh surfaces. Tall vegetation can enhance sediment deposition by reducing current flow and wave action. Herbivores shorten vegetation height and this could potentially reduce sediment deposition. This study estimated the effects of herbivores on 1) vegetation height, 2) sediment deposition and 3) resulting marsh accretion after long-term (at least 16 years) herbivore exclusion of both small (i.e. hare and goose) and large grazers (i.e. cattle) for marshes of different ages. Our results firstly showed that both small and large herbivores can have a major impact on vegetation height. Secondly, grazing processes did not affect sediment deposition. Finally, trampling by large grazers affected marsh accretion rates by compacting the soil. In many European marshes, grazing is used as a tool in nature management as well as for agricultural purposes. Thus, we propose that soil compaction by large grazers should be taken in account when estimating the ability of coastal systems to cope with an accelerating sea-level rise.

Channel deposition induced by bank erosion in response to decreased flows in the sand-banked reach of the upstream Yellow River

Abstract Sand-banked rivers are characterized by severe lateral channel erosion due to weak bank resistance. High lateral channel erosion rates occur in most natural rivers in response to flood flows and contribute to increased suspended sediment loadings; however, evidence from our study shows that low flows also cause high lateral channel erosion rates in sand-banked rivers. Using the profile data of 113 channel cross-sections taken in 1962, 1982, 1991 and 2000 in the Kubq desert reach of the Yellow River, we show that the lateral channel erosion rate remained relatively constant of 0.6–0.67 ∗ 108 t yr− 1 despite flow decreases of 58% and led to the development of a unique bank-to-channel sediment transfer (BCST). This BCST process, in combination with the decreased flow which caused suspended sediment effluxes to decrease by 30%, enhanced sediment deposition in the channel and provided a mechanism of channel adjustment to decreased flow in sand-banked rivers. Our results also show that this high lateral channel erosion rate accounted for about 64% of the total measured sediment deposition rates, which showed, on average, to be as much as 0.90–1.15 ∗ 108 t yr− 1 in the channel shrinkage period of 1982–2000. Even in the channel widening period of 1962–1982, the lateral channel erosion was also found to cause about 0.41 ∗ 108 t of sediment deposition in the channel annually. As the lateral channel erosion is a substantial sediment source of river sediments and leads to lateral channel shifts for sand-banked rivers, channel bank stabilization should be a priority in river management and flood control in sand-banked rivers.

三峡水库蓄水初期鱼体汞含量及其水生食物链累积特征

PDF HTML阅读 XML下载 导出引用 引用提醒 三峡水库蓄水初期鱼体汞含量及其水生食物链累积特征 DOI: 10.5846/stxb201211091568 作者: 作者单位: 中国水利水电科学研究院水环境所,中国水利水电科学研究院水环境所,中国水利水电科学研究院水环境所,中国水利水电科学研究院水环境所,中国水利水电科学研究院水环境所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金资助项目(51179205);国家水体污染控制与治理科技专项三峡水库水环境演化与安全问题诊断研究课题(2009ZX07104-001) Mercury contents in fish and its biomagnification in the food web in Three Gorges Reservoir after 175m impoundment Author: Affiliation: Department of Water Environment,China Institute of Water Resources and Hydropower Research,Department of Water Environment,China Institute of Water Resources and Hydropower Research,Department of Water Environment,China Institute of Water Resources and Hydropower Research,Department of Water Environment,China Institute of Water Resources and Hydropower Research,Department of Water Environment,China Institute of Water Resources and Hydropower Research Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:于2011-2012年在三峡库区干流水域及7条典型支流采集了11种鱼类样品,以稳定同位素(δ13C和δ15N)营养等级分析为基础,研究了三峡水库蓄水后的鱼类汞含量水平及其食物链累积特征。结果表明,库区鱼类(以鲤为例)平均总汞含量为57.1 μg/kg,与蓄水前鱼体总汞水平相当,但在库区不同水域鱼体总汞含量差异显著:干流入库水域(洛碛断面)鱼体总汞含量平均值为88.0μg/kg,显著高于水库腹区水域(巫山断面)的平均值43.1μg/kg。稳定同位素分析表明,库区支流鱼体的δ13C范围为-22.08‰--28.92‰,大于干流鱼体的δ13C范围(-23.11‰--26.87‰),表明支流鱼类的食物来源具有更明显的湖沼型特征。同时,支流鱼类食物链上汞的累积放大效率显著高于干流水域。水文水动力差异可能是库区内汞累积等生态过程存在空间差异的重要原因。 Abstract:Three Gorges Reservoir (TGR) is a giant canyon-shaped reservoir located in the lower section of the upper reaches of the Yangtze River. After impoundment, hydrological signatures, chemical factors and species composition, had been greatly changed in TGR. The new environmental conditions could increase methylmercury (MeHg) production, and induce substantial increase of mercury concentration in the predator fish through food chain biomagnification. Concern about human consumption of fish contaminated with MeHg had been raised even before the construction of TGR. This study tried to investigate the total mercury concentrations in fish and biomagnification characters of mercury along food chains in TGR after 175m impoundment. The stable isotope analysis could be a useful method of checking and evaluating the trophic conditions of aquatic ecology. We collected eleven fish species from three main stem sections and seven typical tributaries of TGR from 2011 to 2012. Carp (Cyprinus carpio) was selected as a typical species to investigate the spatial distribution of mercury level in fish from TGR. Total mercury concentrations and stable isotope ratios (δ13C and δ15N) of all fish samples were determined. Result showed that mercury concentrations in fishes from TGR had not obviously increased after impoundment. The carp mercury concentrations in TGR were comparable to values reported for other areas of Yangtze River Basin. But spatial difference was found among different sections of main stem. Due to the enhanced sediment deposition along reservoir, there was a decreasing trend of mercury concentration in fish from upper stream to lower stream within TGR. Mercury concentrations in the fishes from Luoqi in up area of reservoir (88.0μg/kg, average) was significantly higher than those from Wushan in middle area of reservoir (43.1μg/kg, average) and tributaries (57.1μg/kg, average). The mercury concentrations in fish from tributaries were comparable to those from main stem, and there were no significant difference among most tributaries. Based on the carbon and nitrogen stable isotope ratios and total mercury contents in eleven species fish, the biomagnification power of mercury in two sections and one tributary were evaluated. While log-transformed mercury contents were consistently correlated with δ15N values for the fish food web in all sampling sites, the slope of the relationship with δ15N (biomagnification power value) was significant higher in Shennong River than that in Wanzhou and Wushan section. This indicated that biomagnification power of mercury is greater in tributary than in main stem of TGR. Carbon and nitrogen stable isotope analysis also indicate a difference in food source of fishes between tributary and main stem. In tributary (Shennong River), the values of fish samples were-22.08‰--28.92‰ for δ13C and 8.26‰-13.15‰ for δ15N; In main stem, the values of fish samples were-23.11‰--26.87‰ for δ13C and 8.95‰-16.98‰ for δ15N. This indicate fishes in tributary rely more on pelagic primary production while those in main stem tend to take allochthonous materials carried by runoff. In conclusion, the body type of tributary and main stem after impoundment could be different, and this may be an important reason of different mercury bioaccumulation between tributary and main stem within TGR. 参考文献 相似文献 引证文献

Sensitivity of the Eastern Mediterranean geomorphic system towards environmental change during the Late Holocene: a chronological perspective

AbstractAlluvial and colluvial sediment deposition provide a vital record of environmental change during the Holocene. Firm chronological control on these archives is necessary to enable us to relate sediment dynamics to human activity and climate variability. In the Eastern Mediterranean, such relationships are hard to establish due to the lack of spatially well‐distributed sediment archives with good chronological control. This scarcity is problematic with respect to regional‐scale reconstructions of the temporal variation of sediment dynamics. Here, we present a radiocarbon database (n = 178) of geomorphological activity collected from multiple distinct sediment archives within the territory of Sagalassos in south‐western Turkey. The data were grouped according to their sedimentary facies for analysis using cumulative probability distributions (CPDs) and sedimentation rate (SR) modelling. Two small‐scale colluvial valleys, where chronological information was abundant, were investigated in more detail. Results show that sedimentation chronology differs between individual, nearby cores, as it depends strongly on the local geomorphic situation. A generalizing approach combining multiple core results yields more widely valid conclusions. High sedimentation rates coincided with the initial major anthropogenic disturbance of the landscape and decreased afterwards, probably due to hillslope soil depletion. CPD and SR analysis indicates that in general colluvial sedimentation rates did not change much from 2000 BC onwards. River floodplain sedimentation, in contrast, increased markedly during the first millennium BC and during recent times, and a significant time lag in enhanced sediment deposition between the upper and lower reaches of the river valleys was observed. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Seismic interpretation of pelagic sedimentation regimes in the 18–53 Ma eastern equatorial Pacific: Basin‐scale sedimentation and infilling of abyssal valleys

Understanding how pelagic sediment has been eroded, transported, and deposited is critical to evaluating pelagic sediment records for paleoceanography. We use digital seismic reflection data from an Integrated Ocean Drilling Program site survey (AMAT03) to investigate pelagic sedimentation across the eastern‐central equatorial Pacific, which represents the first comprehensive record published covering the 18–53 Ma eastern equatorial Pacific. Our goals are to quantify (1) basin‐hill‐scale primary deposition regimes and (2) the extent to which seafloor topography has been subdued by abyssal valley‐filling sediments. The eastern Pacific seafloor consists of a series of abyssal hills and basins, with minor late stage faulting in the basement. Ocean crust rarely outcrops at the seafloor away from the rise crest; both hills and basins are sediment covered. The carbonate compensation depth is identified at 4440 m by the appearance of acoustically transparent clay intervals in the seismic data. Overall, we recognized three different sedimentation regimes: depositional (high sedimentation rate), transitional, and minimal sedimentation (low sedimentation rate) regimes. In all areas, the sedimented seafloor mimics the underlying basement topography, although the degree to which topography becomes subdued varies. Depositional regimes result in symmetric sedimentation within basins and subdued topography, whereas minimal sedimentation regimes have more asymmetric distribution of sediments within topographic lows and higher seafloor relief. Regardless of sedimentation regime, enhanced sediment deposition occurs within basins. However, we observe that basin infill is rarely more than twice as thick as sediment cover over abyssal hills. If this variation is due to sediment focusing, the focusing factor in the basins, as measured by 230Th, is no more than a factor of ∼1.3 of the total vertical particulate rain.

Turbidite system architecture and sedimentary processes along topographically complex slopes: the Makran convergent margin

AbstractThis study investigates the morphology and Late Quaternary sediment distribution of the Makran turbidite system (Makran subduction zone, north‐west Indian Ocean) from a nearly complete subsurface mapping of the Oman basin, two‐dimensional seismic and a large set of coring data in order to characterize turbidite system architecture across an active (fold and thrust belt) margin. The Makran turbidite system is composed of a dense network of canyons, which cut into high relief accreted ridges and intra‐slope piggyback basins, forming at some locations connected and variably tortuous paths down complex slopes. Turbidite activity and trench filling rates are high even during the Holocene sea‐level highstand conditions. In particular, basin‐wide, sheet‐like thick mud turbidites, probably related to major mass wasting events of low recurrence time, drape the flat and unchannellized Oman abyssal plain. Longitudinal depth profiles show that the Makran canyons are highly disrupted by numerous thrust‐related large‐scale knickpoints (with gradients up to 20° and walls up to 500 m high). At the deformation front, the strong break of slope can lead to the formation of canyon‐mouth ‘plunge pools’ of variable shapes and sizes. The plunge pools observed in the western Makran are considerably larger than those previously described in sub‐surface successions; the first insights into their internal architecture and sedimentary processes are presented here. Large plunge pools in the western Makran are associated with large scoured areas at the slope break and enhanced sediment deposition downstream: high‐amplitude reflectors are observed inside the plunge pools, while their flanks are composed of thin‐bedded, fine‐grained turbidites deposited by the uppermost part of the turbidity flows. Thus, these architectural elements are associated with strong sediment segregation leading to specific trench‐fill mechanisms, as only the finer‐grained component of the flows is transferred to the abyssal plain. However, the Makran accretionary prism is characterized by strong along‐strike variability in tectonics and fluvial input distribution that might directly influence the turbidite system architecture (i.e. canyon entrenchment, plunge pool formation or channel development at canyon mouths), the sedimentary dynamics and the resulting sediment distribution. Channel formation in the abyssal plain and trench‐fill characteristics depend on the theoretical ‘equilibrium’ conditions of the feeder system, which is related closely to the balance between erosion rates and tectonic regime. Thus, the Makran turbidite system constitutes an excellent modern analogue for deep‐water sedimentary systems with structurally complex depocentres, in convergent margin settings.

Floodplain sediment disconnectivity at a tributary junction and valley constriction site in the Fitzroy River basin, Queensland, Australia

Many catchment-scale sediment models assume connectivity from the top to the bottom of the catchment. The purpose of this study is to provide evidence that particular topographic features can affect catchment connectivity and the transfer of sediment through the catchment. This study investigates floodplain sediment deposition in a valley bottom constriction and tributary junction site within the Fitzroy River basin, a large semiarid–subtropical catchment in NE Australia. These sites can cause backwater effects during flooding that enhance sediment deposition. Isotope analysis of floodplain sediment cores for cesium-137 and plutonium-239 + 240 is used to determine deposition rates since the 1960s. Results show overall higher deposition rates across the lower tributary floodplain proximal to the confluence and within the valley constriction reach. Deposition rates in the upper constriction reach were less than in the constriction reach but similar to rates reported by other studies in the Fitzroy River basin. However, evidence suggests that the region of influence of the valley constriction on sediment deposition is dependent on flood size with large floods depositing sediment further upstream than the sampling sites of this study. Nevertheless, the study shows evidence that backwater-inducing features (such as confluences of disparate channels) can promote sediment deposition and cause a disconnection in the transfer of sediment from tributary to main channel. The effect of these processes on storing sediment needs to be considered in catchment-scale sediment budget models.

Numerical modeling of the effects of water flow, sediment transport and vegetation growth on the spatiotemporal patterning of the ridge and slough landscape of the Everglades wetland

A numerical model has been developed to simulate the spatiotemporal patterning of the ridge and slough landscape in wetlands, characterized by crests (ridges) and valleys (sloughs) that are elongated parallel to the direction of water flow. The model formulation consists of governing equations for integrated surface water and groundwater flow, sediment transport, and soil accretion, as well as litter production by vegetation growth. The model simulations show how the spatial pattern self-organizes over time with the generation of ridges and sloughs through sediment deposition and erosion driven by the water flow field. The spatial and temporal distributions of the water depth, flow rates and sediment transport processes are caused by differential flow due to vegetation and topography heterogeneities. The model was parameterized with values that are representative of the Everglades wetland in the southern portion of the Florida peninsula in the USA. Model simulation sensitivity was tested with respect to numerical grid size, lateral vegetation growth and the rate of litter production. The characteristic wavelengths of the pattern in the directions along and perpendicular to flow that are simulated with this model develop over time into ridge and slough shapes that resemble field observations. Also, the simulated elevation differences between the ridges and sloughs are of the same order of those typically found in the field. The width of ridges and sloughs was found to be controlled by a lateral vegetation growth distance parameter in a simplified formulation of vegetation growth, which complements earlier modeling results in which a differential peat accretion mechanism alone did not reproduce observations of ridge and slough lateral wavelengths. The results of this work suggest that ridge and slough patterning occurs as a result of vegetation's ability to grow laterally, enhancing sediment deposition in ridge areas, balanced by increased sediment erosion in slough areas to satisfy flow continuity. The interplay between sediment transport, water flow and vegetation and soil dynamic processes needs to be explored further through detailed field experiments, using a model formulation such as the one developed in this work to guide data collection and interpretation. This should be one of the focus areas of future investigations of pattern formation and stability in ridge and slough areas.

The Timing of Floods and Storms as a Controlling Mechanism for Shelf Deposit Morphology

Deltas and clinoforms are accretionary deposits with subaerial and subaqueous topsets, respectively. Various factors have been proposed that may control their geometry, especially the interaction of sediment supply and physical oceanographic energy. A conceptual model of how this interaction affects shelf sedimentation indicates that sediment from most rivers should form deltaic features in quiescent environments. In more energetic settings, most sediment from small rivers (sediment load of 106–107 t/y) should be dispersed, limiting significant shelf accumulation, but the increased supply from larger rivers (sediment load of 108–109 t/y) could allow for sediment retention on the shelf, leading to clinoform development. However, in the Adriatic Sea, a river delta and a shelf clinoform (i.e., adjacent to the Po River and the Apennine rivers, respectively) have developed from similar sediment supply and oceanographic energy. This suggests that other factors are likely important, particularly differences in the shelf gradient and the timing of floods and storms. The shelf gradient is lower near the Po River, favoring retention of sediment in shallower water as compared to the Apennine rivers. In addition, floods and storms are uncorrelated on the Po River shelf due to the large size of the drainage basin, enhancing sediment deposition in shallow water. For the Apennine rivers, the drainage basin of individual rivers is small, and floods and storms are generally correlated, facilitating offshore deposition of sediment and leading to the development of a shelf clinoform.

Sediment deposition and soil nutrient heterogeneity in two desert grassland ecosystems, southern New Mexico

The role of wind in changing the spatial heterogeneity of soil resources in erosion-dominated semiarid ecosystems is well known. Yet the effect of windblown sediment deposition on soil nutrient distribution and ecosystem dynamics at local and landscape scales has received little attention. We examined the effects of enhanced sediment deposition on the spatial distribution of soil nutrients at the Jornada Experimental Range, southern New Mexico. Enhanced sediment deposition was obtained as a result of grass cover reduction in the upwind portion of the experiment in two sites co-dominated by mesquite and one of two grass species with different morphologies. The spatial characteristics of soil available nitrogen (including ammonium, nitrite, and nitrate), phosphate, potassium, and calcium were quantified using a variety of traditional and geostatistical analyses. Our results showed that enhanced deposition led to considerable reduction in both mean soil nutrient concentrations and coefficients of variation over a two-year period (2004–2006). Given the observed increase in the scale of spatial dependence for available nitrogen, but not for potassium, phosphate, and calcium following enhanced sediment deposition, we suggest that soil available nitrogen may be particularly responsive to increased aeolian activities due to livestock grazing and other anthropogenic activities that remove vegetation. Our study further suggests that soil particles deposited in the downwind area may be “nutrient-imbalanced.” Specifically, the lower-than-normal available nitrogen concentrations in the wind-deposited soils may inhibit the growth of grasses and the germination of seeds. For wind-erodible ecosystems found in southern New Mexico, structures of Bouteloua-dominated communities may be particularly susceptible to change under enhanced soil erosion conditions.

Quantity, controls and functions of large woody debris in Midwestern USA streams

AbstractLarge woody debris (LWD) can increase stream habitat heterogeneity by providing structure, altering flow patterns, enhancing sediment deposition, forming pools and retaining organic matter. In North America, the role of LWD has been studied extensively in streams of mature forests (e.g. Pacific Northwest), but few studies have assessed LWD in streams of younger forests (e.g. Midwestern USA). Our objectives were to: (1) quantify the volume and abundance of LWD in a set of Midwestern streams; (2) evaluate possible factors influencing LWD quantity; (3) identify the functional roles of LWD; and (4) compare LWD levels in the upper Midwest to those elsewhere in North America. In 2002 and 2003, we measured LWD and geomorphological variables in 15 low‐gradient streams draining previously logged watersheds in the Upper Peninsula of Michigan. Mean (±SE) LWD volume (0.77 ± 0.12 m3 100 m−2) and abundance (33 ± 3 pieces 100 m−1) were 71% and 10% lesser, respectively, than in streams of similar gradient elsewhere in North America. Channel shape (width:depth ratio) explained 30% of the variation in LWD volume (multiple stepwise regression, P = 0.015) while LWD length and length:channel width combined, explained 72% of the variation in LWD density (multiple stepwise regression, P &lt; 0.0001). About 50% of the LWD either stored sediment or stabilized banks and 14% of the LWD formed pools, although pool density was not significantly related to LWD volume or density. LWD levels, overall, were low in upper Midwestern streams, but the relative importance of that LWD to ecosystem function may be magnified in these wood‐poor systems. Copyright © 2006 John Wiley &amp; Sons, Ltd.