Sediment Redistribution Processes Research Articles

Seasonal hydrological and suspended sediment transport dynamics and their future modelling in the Orwell Glacier proglacial stream, Signy Island, Antarctica

AbstractClimate change in the Antarctic over the past 50+ years has caused contraction of ice and snow cover, longer melt seasons and intensified glacier melting. These changes affect erosion and sediment redistribution processes that are vital to our understanding of terrestrial and freshwater ecosystems and sediment input to oceans. This 79 day study of the Orwell Glacier meltwater stream on Signy Island (5 December 2019–21 February 2020) used 5 min recordings of turbidity, stream discharge (Q) and air temperature (AT), supplemented by 454 water samples from which suspended sediment concentration (SSC) was gravimetrically determined, to calculate daily suspended sediment loads (SSLs). Qmean was 47.8 ± 3.5 l s-1, SSCmean was 71.0 ± 15.9 mg l-1 and daily SSLmean was 75 ± 8 kg day-1 with a suspended sediment yield of 43.6 t km-2 yr-1. A multiple regression model predicted SSLs reliably (multiple r = 0.95, r2 = 0.91, n = 79) and, when run with ATmean + 1°C (expected on Signy Island by 2060) and ATmean + 2°C (expected by 2100) scenarios, the model predicted 7% and 13% increases in SSLs, respectively. The SSLs estimated in this study are low when compared with others from around the world.

Oligo-Miocene carbonate platform evolution in the northern margin of the Asmari intra-shelf basin, SW Iran

Abstract This study is based on seven outcrops along a regional transect in the central subdivision of the Zagros Foreland Basin. This NE-SW trending transect extends for 140 km and covers the northern margin of the Asmari intra-shelf basin in dip direction (from proximal to distal). Results of facies analysis led to the recognition of eight facies associations (Planktonic Wkt, Nummulitic Eu F/R, Alg LR Pkt, Coralgal F-R/Bdt, LR LPBF P-G, Epy.R PBF P-G, Dol M, and TRGS) along the depositional dip direction. They are dominated by large and flat rotalids (e.g., Nummulites, Lepidocyclina and Spiroclypeus), coral and coralline red algae (e.g., build-up features), porcellaneous benthic foraminifera (e.g., Archaias, Peneroplis, and Borelis), and fossiliferous lime-mud. Spatial and temporal distribution of these facies associations conform five platform types with different depositional profiles: a) a Nummulites-bearing mixed terrigenous/carbonate distally steepened ramp, lower Rupelian, b) a Nummulitidae-Lepidocyclina bearing carbonate distally steepened ramp, upper Rupelian-lower Chattian, c) a Coral bearing flat-topped open shelf, middle-upper Chattian, d) an Aquitanian homoclinal ramp, and e) a very shallow-water Burdigalian carbonate shelf. Amongst these platform types, the Late Rupelian-Early Chattian carbonate ramp additionally consists of three successive phases and the Middle-Late Chattian flatted top open shelf was developed in two phases. The changes in platform type resulted from changes in the carbonate producing biotas, sediment redistribution processes and significant impact of relative sea level changes.

Nannofossils in upper quaternary bottom sediments of back-arc basins in the southwestern Pacific

The analysis of calcareous nannoplankton assemblages in bottom sediments sampled during Cruise 21 of the R/V Akademik Mstislav Keldysh in three areas located in back-arc basins of the southwestern Pacific (western Woodlark in the Solomon Sea, Manus in the Bismarck Sea, Central Lau) reveal that they belong to the Emiliania huxleyi Acme Zone, the most detailed one in the Gartner’s scale of 1977. The content of coccoliths and their taxonomic composition indicate warm subtropical-tropical conditions. Long cores demonstrate a decrease in species diversity reflecting the transition from the cold late Pleistocene to the Holocene. The changes in species diversity and presence/absence of thermophilic representatives indicate transformation of depositional environments with unstable conditions in the water column and bottom layer, seismic activity, and widely developed processes of sediment redistribution and reworking.

Particle sorting during sediment redistribution processes and the effect on 230Th‐normalized mass accumulation rates

AbstractThe 230Th method of determining mass accumulation rates (MARs) assumes that little to no fractionation occurs during sediment redistribution processes at the seafloor. We examine 230Th inventories in radiocarbon‐dated multicore sediments from paired winnowed and focused sites at Cocos and Carnegie Ridges, Panama Basin. Radiocarbon‐derived sand MARs, which likely represent the vertical rain of particles poorly transported by bottom currents, are similar at each of the paired sites but are different using 230Th normalization. 230Th‐normalized MARs are about 60% lower at focused sites and likely underestimate vertical MARs, while the reverse is true for winnowed sites. We hypothesize that size fractionation occurs most frequently at lower current velocities, resulting in the coarse fraction being left behind and primarily the fine 230Th‐rich grains being transported downslope. 230Th‐normalization works well for recording fine‐grained (detrital and opal), but not coarse‐grained (carbonate), fluxes in regions that have undergone sediment redistribution.

Impact of the spatial and thematic resolution of Holocene anthropogenic land-cover scenarios on modeled soil erosion and sediment delivery rates

Recent reconstructions of global Holocene vegetation may provide a powerful means to quantify long-term cumulative sediment fluxes induced by anthropogenic land-cover changes. However, the former’s low spatial resolutions question their potential for use in geomorphic models, since sediment redistribution processes operate at much smaller scales. Furthermore, current land-cover reconstructions often do not differentiate the typology of human impact, although the susceptibility of different anthropogenic land uses towards erosion varies greatly. Hence, the present study investigates the sensitivity of a spatially distributed erosion and sediment redistribution model to the spatial and thematic resolution of input land-cover information. The geomorphic model was applied to the Scheldt Basin (~19,000 km2) with low-resolution and high-resolution land-cover input, and subsequently compared to a field-based reference sediment budget of the Dijle subcatchment. Results indicate that low-resolution land-cover information, expressed as proportions of different land-cover types within each grid cell, leads to largely overestimated sediment fluxes. In contrast, spatial allocation of individual land-cover types to a 100-m grid yields more accurate results. These variations in model outcomes are related to differences in landscape connectivity between high- and low-resolution land cover. Moreover, geomorphic model results are non-linearly related to the area under cropland. This indicates that there is not only a need for land-cover reconstructions at high spatial resolution but also that differentiation of anthropogenic land-cover types is essential for accurate geomorphic modeling.

Spatiotemporal distribution of 137Cs in the sea surrounding Japanese Islands in the decades before the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011

The historic spatiotemporal distribution of 137Cs in the seawaters and sea-floor sediments adjacent to nuclear power plants in Japan are summarized, using data obtained over a period of time more than 20years prior to the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011. Relatively uniform distributions of 137Cs were observed both in the surface seawaters (1m in depth) and in deeper seawaters (10 to 30m above the seabed and ranging from tens to hundreds of meters in depth) independent of the geographical position, although lower concentrations were observed in significantly deeper bottom seawaters. Conversely, there were wide variations in 137Cs levels between sediments, such that higher 137Cs concentrations were observed in the deeper sampling locations.A mathematical model describing the successive transfer of 137Cs from surface waters through deeper waters to sediments suggested that the transfer rate of 137Cs from deep water to the sediments, and the loss rate from bottom sediments, were both greater than the transfer rate from surface water to deeper water. It was found that the calculated regression lines for 137Cs depletion rates over time for surface waters, deeper waters, and sediments were approximately parallel when plotted on a semi-logarithmic coordinate system, regardless of the sampling location. A radionuclide depletion half-life was calculated to be 4months to 16years with the geometric mean of 2.22y for the sediments in the Fukushima region, suggesting that nuclear contamination will be remediated over time through sediment redistribution processes such as remobilization, bioturbation, and migration due to sea currents.

Geomorphology and community structure of Middle Reef, central Great Barrier Reef, Australia: an inner-shelf turbid zone reef subject to episodic mortality events

Middle Reef is an inshore turbid zone reef located 4 km offshore from Townsville, Queensland, Australia. The reef consists of four current-aligned, interconnected reef patches that have reached sea level and formed reef flats. It is regularly exposed to high turbidity (up to 50 mg l−1) generated by wave-driven sediment resuspension or by episodic flood plumes. Middle Reef has a high mean hard coral cover (>39%), relatively low mean macro-algal cover (<15%) and a coral community comprising at least 81 hard coral species. Cluster analysis differentiated six benthic communities which were mapped onto the geomorphological structure of the reef to reveal a spatially patchy community mosaic that reflects hydrodynamic and sediment redistribution processes. Coral cover data collected annually from windward slope transects since 1993 show that coral cover has increased over the last ~15 years despite a history of episodic mortality events. Although episodic mortality may be interpreted as an indication of marginality, over decadal timescales, Middle Reef has recovered rapidly following mortality events and is clearly a resilient coral reef.

3D initial sediment distribution and quantification of mass balances of an artificially-created hydrological catchment based on DEMs from aerial photographs using GOCAD

The spatial distribution and properties of parent material components form the starting point for any soil and ecosystem. Initial phases of ecosystem development are predominantly characterized by the redistribution of sediment components. To improve the understanding of soil-landscape development, the initial sediments need to be quantified in space and time. This study aims at developing and testing methods for the quantification of initial sediment mass balances in the three dimensions of space and in time. The initial mass balance and composition of parent material in an artificially-created hydrological catchment were quantified from a 3D-model of the catchment’s water storage layer. Multi-date digital elevation models (DEMs) were constructed from photogrammetrically derived elevation data after the assessment and improvement of elevation data quality. Three-dimensional models of volume change were constructed from the digital elevation models. Regions of significant volume change were identified. Mass changes were calculated from the volume changes in combination with bulk density information. Based on information about the catchment’s construction, mass changes were separately analyzed in subregions of the models. Terrain attributes were computed to characterize surface structure and to examine correlations with mass change. From the 3D-model, an initial sediment volume of 122,608 m 3 was quantified. A variation of about ±12,300 m 3, due to uncertainty in DEMs from aerial photographs, was calculated. The 3D-model indicates differences in sediment properties between the western and eastern part of the catchment, which are most probably resulting from the dumping of two separate material deliveries during the construction. Models of volume and mass change are constructed for three time periods of catchment evolution. Spatial variations in volume and mass change are observed. The total mass balance reveals a considerable mismatch between the detected amounts of erosion and sedimentation, which gives reason to closely examine the quality of the DEMs. Terrain attributes of four elevation models reflect the diversification of surface structures. Correlations between volume change and surface structures show that erosion processes are dependent on initial surface structures and that these structures, in turn, are enhanced by the processes of sediment redistribution. Although there is a considerable uncertainty in the observed mass changes, the 3D-modelling approach allows a first approximation of the initial, mostly erosion-affected, surface structural dynamics of the artificial catchment. The comparison of multi-date elevation data allows a critical evaluation of the quality of models of change obtained from repeated topographical surveys.

A method for developing a large-scale sediment yield index for European river basins

Background, aim, and scope Sediment fluxes within continental areas play a major role in biogeochemical cycles and are often the cause of soil surface degradation as well as water and ecosystem pollution. In a situation where a high proportion of the land surface is experiencing significant global land use and climate changes, it appears important to establish sediment budgets considering the major processes forcing sediment redistribution within drainage areas. In this context, the aim of this study is to test a methodology to estimate a sediment yield index at a large spatial resolution for European river basins. Data and methods Four indicators representing processes respectively considered as sources (mass movement and hillslope erosion), sinks (deposits), and transfers of sediments (drainage density) are defined using distributed data. Using these indicators we propose a basic conceptual approach to test the possibility of explaining sediment yield observed at the outlet of 29 selected European river basins. We propose an index which adds the two sources and transfers, and subsequently subtracts the sink term. This index is then compared to observed sediment yield data. Results With this approach, variability between river basins is observed and the evolution of each indicator analyzed. A linear regression shows a correlation coefficient of 0.83 linking observed specific sediment yield (SSY) with the SSY index. Discussion To improve this approach at this large river basin scale, basin classification is further refined using the relation between the observed SSY and the index obtained from the four indicators. It allows a refinement of the results. Conclusions This study presents a conceptual approach offering the advantages of using spatially distributed data combined with major sediment redistribution processes to estimate the sediment yield observed at the outlet of river basins. Recommendations and perspectives Inclusion of better information on spatial variability should refine the approach. In this respect, basin classification and partition can be useful when applying the model to homogeneous areas. Moreover, to assess the relative effect of each sediment redistribution process, indicators could be weighted for each basin typology.

Debris flow-dominated and rockfall-dominated talus slopes: Genetic models derived from GPR measurements

Stratified talus deposits are reported from many different mountain environments. Numerous possible explanations are discussed in the literature; however, the sediment stores are rarely accessible as exposures are sparse. We applied ground-penetrating radar (25, 50 and 100 MHz antennae) to gain insight into the internal sediment structures of 23 alpine scree slopes; ten examples are presented in this paper. The study areas are spread over the Eastern European Alps at altitudes ranging from 1500 to 2900 m. The bedrock type is primarily limestone and dolostone; one area is composed of gneiss and mica–schist. GPR turned out to be highly suitable for investigating sediment structures of dry talus debris. The results showed that almost all of the deposits investigated are characterized by pronounced stratification. Several different types of layering were identified. Discordant layers which are restricted to confined parts of the talus are probably related to sediment redistribution processes like surficial debris flows or dry grain flows. These features frequently occur at the uppermost part of the slope caused by overland flow from the adjacent rock face, but may also develop in the downhill part of a talus. One talus in the Reintal area showed surface-parallel, persistent layers of different grain sizes which cannot be explained by any known models. We suggest a novel model of talus development which is driven by climatic fluctuations. In periods of enhanced freeze–thaw activity like the Little Ice Age, the delivery of coarse debris prevails. In warmer climate with a higher frequency of rainstorms, the depletion of finer-grained intermediate stores in less inclined rockwall positions leads to delivery of clasts smaller than 2 cm. The type of layering found within a talus is determined by rockwall parameters like height, steepness, topography and dissection of the rock face. The “storage depletion” model applies to high rockwalls with a considerable volume of intermediate storage.

Coccolith evidence for instabilities in surface circulation south of Iceland during Holocene times

The coccolith-based micropaleontological investigation of an exceptionally thick deep-sea Holocene sediment core was conducted in order to document the pattern and timing of surface circulation changes south of Iceland, over the Gardar contour drift, during the last 12 000 years. Fluctuations in bulk carbonate content at the core location are primarily driven by the coccolith fraction. The observed overall correlation between bottom flow speed, as given by the ‘sortable silt’ mean size index, and the bulk coccolith concentration (abundance/g dry sediment) suggests that the accumulation of this fine carbonate fraction is, as a first order, controlled by processes of sediment redistribution by bottom current. The down-core variations in coccolith assemblage structure (species %) indicate that changes in properties of surface waters south of Iceland occurred as two distinct steps at ca. 11.2 and 6 ka. The onset of North Atlantic Drift water influence over the Gardar Drift after 11.2 ka was associated with excess export flux of coccolith carbonate in the vicinity of an active frontal system. The period from 10 to 6 ka saw the progressive warming of the study area, which culminated between 6 and 7 ka. Cooling of the surface waters after 6 ka, as indicated changes in the relative abundances of the dominant coccolith species, took place in two phases, the present hydrological regime being only established after a last cooling step between 3.5 and 2.8 ka. These long-term reorganisations of the surface hydrology are interpreted as the response of the North Atlantic to the combined force of the solar insolation and the waning Laurentide ice sheet. Millennial-scale perturbations of the surface hydrology are documented by changes in accumulation of the species Emiliania huxleyi. These successive decreases in the export fluxes of E. huxleyi exhibit a distinct millennial pacing, in phase with previously recorded Holocene advection of cool, ice-bearing waters from the Greenland–Iceland seas to the British islands. From a clear analogy with the most recent extreme variation in global ocean climate, we argue that mechanisms responsible for these observed millennial-scale perturbations might be found in long-term modulations of the atmospheric processes associated with the North Atlantic Oscillation (NAO).

Late Quaternary fluctuations of biogenic component fluxes on the continental slope of the Ross Sea, Antarctica

A sediment core, collected from the western part of the continental slope of the Ross Sea at 2380 m water depth, records events of the last two climatic cycles (250 kyr). A 230 Th ex -based chronology was obtained and boundaries of the isotope stages were set assuming that biological productivity was enhanced during periods of less ice cover. Then, 230 Th ex 0 , organic carbon, biogenic silica and biogenic Ba distributions were compared to the glacial–interglacial stage boundaries and corresponding ages of the δ 18 O record of Martinson et al. [Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Jr., Shackleton, N.J, 1987. Age dating and the orbital theory of the ice ages: development of a high-resolution 0 to 300,000-year chronostratigraphy. Quaternary Research, 27: 1–29]. Sediment accumulation rates ranged between 1.2 cm kyr −1 in the isotope stage 6 and 3.8 cm kyr −1 during the Holocene. Variations in the concentrations and fluxes of organic carbon, biogenic Ba, biogenic silica and Mn gave information on palaeoclimate changes. Processes of sediment redistribution in the Ross Sea margin were enlightened from a comparison of the measured and expected fluxes of 230 Th ex . Calculation of the focusing-corrected accumulation rates of biogenic Ba enabled us to evaluate the export palaeoproductivity. Corrected accumulation rates of biogenic components and calculated palaeoproductivities were low, compared to the Antarctic Polar Front in the Atlantic sector, throughout the last two climatic cycles. Glacial–interglacial changes of sea ice cover and ventilation of the Ross Sea were probably major causes of variations in biogenic particle flux and distribution of redox-sensitive elements within the sediment column.

Sediment redistribution versus paleoproductivity change: Weddell Sea margin sediment stratigraphy and biogenic particle flux of the last 250,000 years deduced from 230Th ex, 10Be and biogenic barium profiles

High resolution 230Th ex and 10Be and biogenic barium profiles were measured at three sediment gravity cores (length 605–850 cm) from the Weddell Sea continental margin. Applying the 230Th ex dating method, average sedimentation rates of 3 cm/kyr for the two cores from the South Orkney Slope and of 2.4 cm/kyr for the core from the eastern Weddell Sea were determined and compared to δ 18O and lithostratigraphic results. Strong variations in the radionuclide concentrations in the sediments resembling the glacial/interglacial pattern of the δ 18O stratigraphy and the 10Be stratigraphy of high northern latitudes were used for establishing a chronostratigraphy. Biogenic Ba shows a pattern similar to the radionuclide profiles, suggesting that both records were influenced by increased paleoproductivity at the beginning of the interglacials. However, 230Th ex 0 fluxes (0 stands for initial) exceeding production by up to a factor of 4 suggest that sediment redistribution processes, linked to variations in bottom water current velocity, played the major role in controlling the radionuclide and biogenic barium deposition during isotope stages 5e and 1. The correction for sediment focusing makes the ‘true’ vertical paleoproductivity rates, deduced from the fluxes of proxy tracers like biogenic barium, much lower than previously estimated. Very low 230Th ex 0 concentrations and fluxes during isotope stage 6 were probably caused by rapid deposition of older, resedimented material, delivered to the Weddell Sea continental slopes by the grounded ice shelves and contemporaneous erosion of particles originating from the water column.

Solubility behavior of atmospheric 7Be in the marine environment

The solubility of natural 7Be from air particulate matter in seawater was determined as a function of time. Adsorption of the thus solubilized 7Be on a variety of substrates was investigated. 7Be appears to be strongly adsorbed on suspended matter and inorganic materials at high suspended loads (> 20 mg 1 −1), but at natural levels (∼1 mg 1 −1) is only partially adsorbed. Further, at natural concentrations of suspended matter, adsorption of 7Be appears to be directly proportional to suspended load. Because a non-steady-state flux of 7Be to the bottom could be expected over its 53-day half-life, 7Be may be a poor tracer of sedimentation rates and sediment redistribution processes.

Sediment accumulation rates and geochronologies measured in the Saguenay Fjord using the Pb-210 dating method

Sediment accumulation rates were estimated from-the vertical distribution of excess Pb-210 measured in sediment cores collected at seven stations in the Saguenay Fjord, Quebec. These rates decrease with increasing water depth and distance from the mouth of the Saguenay River, ranging from 4.0 g cm −2 yr −1 (∼- 7 cm yr −1) near the head of the fjord to 0.07 g cm −2 yr −1 (∼- 0.1 cm yr −1) in the deep inner basin of the fjord. In one core from the head of the fjord, layered sediment structures, having different physical characteristics and composition, appear related to recent, pulsed inputs of older raised marine deposits displaced by a landslide in 1971. Synchronous depositional anomalies in several cores provide evidence of other large scale sediment redistribution processes in the fjord. Pb-210 geochronologies are generally in good agreement with time-stratigraphic horizons inferred both from Cs-137 activity profiles and from the analysis of pollen assemblages in one core.

The dissolution of siliceous microfossils in surface sediments of the eastern tropical Pacific

Three assemblages of Quaternary siliceous microfossils were dissolved for varying lengths of time in natural seawater. Changes in the relative abundances of certain radiolarians, diatoms, and sponge spicules were measured, and a dissolution index scheme was developed. This scheme allows an objective assessment of the preservation of tropical siliceous microfossil assemblages of Quaternary age. Its application to a study of near-surface sediments in the eastern tropical Pacific shows that the siliceous microfossil preservation pattern roughly reflects surface water productivity. Two second-order features are superimposed upon this pattern. First, preservation is usually better at deeper depositional sites at most latitudes, and this is attributed primarily to sediment redistribution processes. Second, the best preservation is found not in equatorial sediments, but in those obtained from an area near 10°N, 117°W. This is attributed to the unusually high abundance of fragments of the diatom genus Ethmodiscus found in those sediments.