Polymodal Grain-size Distributions Research Articles

Validation of appropriate estimation criteria for the number of components for separating a polymodal grain-size distribution into lognormal distributions

Polymodal particle size distributions are generally analyzed by separating them into lognormal distributions, but estimating the precise number of lognormal components required remains a considerable problem. In the present study, appropriate evaluation criteria for the estimation of the number of components were examined by using artificial data for which the true number of components was known. The characteristics of estimations of the number of components by four evaluation criteria, the mean square error (MSE), Akaike information criterion (AIC), Bayesian information criterion (BIC), and adjusted R-squared (ARS), were investigated. The results showed that the MSE and ARS were less sensitive to the true number of components and tended to overestimate the number of components. By contrast, the AIC and BIC tended to underestimate the number of components, and their correct answer rates decreased as the true number of components increased. The BIC tended to include the true number of components among its higher ranked models. The present evaluation results suggest that the MSE, although frequently used, is not necessarily the most appropriate evaluation criterion, and that the AIC and ARS may be more appropriate criteria. Furthermore, checking whether the number of components estimated by the AIC or ARS is included among higher ranked BIC models might prevent overestimation and thereby allow for more valid estimation of the number of components. When the criteria were applied to grain-size distributions of lacustrine sediments, it was possible to estimate the number of components that reflected differences in grain-size distribution characteristics.

Texture and mineralogical composition of Quaternary terrestrial and marine sediments from Macedonia and Thrace, Greece

Clastic sediments collected from land and gulfs of Macedonia and Thrace in Greece are examined on the basis of their texture and mineral composition. All terrestrial samples are coarse-grained and poorly sorted, with angular to sub-angular grains. These are mainly composed of quartz and feldspars, followed by micas, calcite and Fe-Mg minerals. Among the clay minerals illite predominates over smectite and smectite over vermiculite (+chlorite+kaolinite). In addition, the interstratified phases illite/smectite, chlorite/vermiculite, and smectite/vermiculite are present in significant amounts in the clay fraction (<2 μm), signifying the incomplete weathering of the primary minerals. Mixing during transportation, flocculation, differential settling processes and organic matter are the main mechanisms for the distribution of the discharged terrigenous load into the North Aegean Gulfs. All gulf bottom samples are coarse to fine grained and medium sorted, and their grains are angular to sub-angular. Quartz and feldspars predominate. In addition, biogenic calcite, micas and various Fe-Mg minerals exist as primary and/or accessory minerals. Among clay minerals, illite predominates over smectite and smectite over kaolinite (+chlorite+vermiculite). Almost in all gulf sediments the interstratified phase illite/smectite is apparent. The presence of feldspars and Fe-Mg minerals, as well as the high content of clay minerals and the polymodal grain-size distribution with angular to sub-angular grains, signify mineralogical and textural immaturity of all the examined sediments, as well as predominance of mild climatic conditions and thus mild weathering processes. The quartz content in these sediments is usually <70%. Therefore, a sedimentation cycle of these materials has not been completed.

Millennial- and centennial-scale droughts at the northern margin of the East Asian summer monsoon during the last deglaciation: Sedimentological evidence from Dali Lake

The semi-arid areas at the northern margin of the East Asian summer monsoon (EASM) have experienced an increased frequency of drought in recent decades. However, it is unclear whether past monsoonal precipitation in the region were determined mainly by El Niño-Southern Oscillation (ENSO) variability or high-latitude cooling. In this study, we use a high-resolution grain-size dataset combined with a new Bayesian age–depth model of a sediment core from Dali Lake to analyze the occurrence of millennial- and centennial-scale droughts at the EASM margin during the interval of climatic warming during the last deglaciation. The polymodal grain-size distributions are partitioned using a log-normal distribution function fitting method. High percentages of the coarse components (C4+C5+C6) are recognized as proxy indicators of strong aeolian activities and low lake levels, and thus droughts in the region. The drought events indicated thereby are generally accompanied by decreases in regional temperature, catchment surface runoff and bio-productivity, on both millennial and centennial timescales. These results imply that the droughts at the EASM margin were caused by a significantly weakened EASM intensity. In addition, the droughts may be linked, within the age uncertainties, to the Heinrich 1 (H1) and Younger Dryas (YD) events on millennial timescales, and to the Older Dryas (OD) cold event on centennial timescales, implying strong high-latitude forcing. Given that these cold reversals over northern high latitudes were induced by the rapid input of glacial melt-water to the North Atlantic during the last deglaciation, it is possible that ongoing climatic warming, and the resulting high northern-latitude ice-sheet melting, may cause an increased incidence of drought in northern China.

Characteristic grain-size component - A useful process-related parameter for grain-size analysis of lacustrine clastics?

Lacustrine sediments are important archives for paleoclimate reconstructions. The application of grain-size analysis as palaeoclimatic proxy in lacustrine clastics is valuable but also difficult because the typical polymodal grain-size distribution in these clastics. To better understand the grain-size distribution of lacustrine clastics and to promote the application of grain size in paleoenvironmental interpretation, this study investigates lacustrine clastics from northern and southern China. The grain-size distribution of these sediments was decomposed by log-normal distribution function fitting method. Based on the results, and drawing upon the concept of paleomagnetic demagnetization and “Characteristic Remnant Magnetization” from paleomagnetism, a conceptual system has been established and defined for grain-size distribution analysis. The system is composed of four components: (I) Characteristic Grain Size Component (ChGSC), (II) Affiliated Grain Size Component, (III) Meaningful Grain Size Component, and (IV) Combination Feature of Grain Size Components (CFGSCs). Based on the proposed system, ChGSC and CFGSCs were used to detect the grain-size distribution of clastics from the different lake zones investigated. Our results show the number, modal size, and percentage of ChGSC(s) in grain-size distributions are sensitive to changes in the lacustrine environment. The ChGSC(s) mirrors the dominant depositional process and hydrodynamic conditions. The modal size of ChGSC(s) is more sensitive to hydrological conditions than the widely used mean grain-size approach. Thus, the ChGSC(s) provide a useful process-related parameter for paleoenvironmental reconstructions. To test this promising application, we applied this approach to a deep drill core from the Qaidam Basin in the northeastern Tibetan Plateau.

An R-Based Function for Modeling of End Member Compositions

Clastic sediment grains have been exposed to numerous physical and chemical processes during erosion, transport and accumulation. It is widely accepted that weathering and sorting processes are strongly controlled by climate, making grain-size distributions a potential recorder for paleoenvironmental conditions. Polymodal grain-size distributions of sediments are in general a result of mixing of unimodal grain-size distributions (end members) from different sources. Thus, unmixing of polymodal grain-size distributions into their unimodal end member compositions (end member modeling) provides a powerful method for revealing sediment sources and environmental conditions of the past. Since the early 1960s, several algorithms for unmixing of such compositional data were published. However no source code is available in a modern open access programming language. This study provides an algorithm and some additional functions for the decomposition of compositional data into end-member compositions based on a free available statistical programming language. The source code is provided as supplementary materials. To prove the functionality of the algorithm, end member models of sandstone compositions and grain-size distributions were calculated.

Perils in distinguishing phreatic from phreatomagmatic ash; insights into the eruption mechanisms of the 6 August 2012 Mt. Tongariro eruption, New Zealand

The weak geophysical precursors of the 6 August 2012 Te Maari eruption of Mt. Tongariro and a lack of obvious juvenile components in its proximal ballistic deposits imply that the eruption was caused by the sudden decompression of a sealed, hot hydrothermal system. Strong magmatic signals in pre- and post-eruption gas emissions indicate that fresh magma had intruded to shallow levels shortly before this eruption. Here we examine the volcanic ash produced during the August eruption with the aim of determining whether juvenile magma was erupted or not. The widely applied criteria for identifying fresh juvenile pyroclasts provided inconclusive results. The Te Maari ash sorting and trend towards a unimodal grain-size distribution increase with distance along the dispersal axis. Proximal to intermediate sites showing polymodal grain-size distributions can be related to the re-fragmentation of different pre-existing lithologies, overlapped erupted pulses and transport mechanisms, and to particle aggregation. Between 69 and 100vol.% of particles coarser than 3 ϕ and 45–75vol.% of grains finer than 3 ϕ were sourced from the pre-existing, commonly hydrothermally altered, vent-area lavas and pyroclasts. Free crystals (pyroxene>plagioclase>magnetite>pyrite) make up 0–23vol.% of particles coarser than 3 ϕ, and 22–41vol.% of grains finer than 3 ϕ. Brown to black fragments of fresh glass are a small (1–15vol.%), but notable, component. Under SEM, these blocky, glassy particles are poorly vesicular, and irregularly shaped, some with fluidal or bubble-wall surfaces, and others with fragmented stepped surfaces and fine adhering ash. In thin section, they contain variable amounts of microlites within an isotropic groundmass. The range in silica content of the microprobe-analysed glass is very wide (56–77wt.%) and cannot be correlated to any specific particle textural type. These chemically and texturally diverse glassy fragments are identical to mechanically broken pieces of country rock lavas and pyroclasts; both their diversity, and their match with vent country rocks, argue strongly against a “juvenile” origin for the glassy fragments. We conclude that rising magma provided only heat and gas into the overlying, sealed vapour-dominated hydrothermal system. A landslide from this area led to a rapid decompression and ash was produced by top-down hydrothermal explosions. Careful attention must be paid to the combination of compositions and textures of fine ash particles in such situations, as well as to the context of their source vent, in order to be confident that new magma has reached the surface.

Differentiation of Heavy Minerals in the Alongshore Debris Flow and Modeling of Processes of Coastal-Marine Placer Formation

The mode of material transportation and differentiation in the coastal zone of shelf is analyzed with the Val'kumei coastal-marine tin placer (Chaun Gulf, East Siberian Sea) as example. The placer incorporates two zones of cassiterite concentration. The first zone with coarse-grained cassiterite is located within the beach and adjacent coastal water area. The second zone with relatively fine-grained cassiterite is located on the submarine coastal slope in the distal area. This type of placer structure reflects the polymodal grain-size distribution of cassiterite in the bedrock source and peculiarities of material distribution in the alongshore debris flow. The proposed mathematical diffusive–convective model of coastal-marine placer formation adequately reflects the real placer structure and makes it possible to predict placer parameters in unilateral (alongshore) debris flow areas based on the composition of placer-forming source and lithodynamic parameters of the coastal water area.

Hemipelagites: processes, facies and model

Abstract Detailed sedimentological studies have been carried out on Pleistocene to Recent sediments from the Makran margin and on Miocene to Recent sediments from the Oman margin of the NW Indian Ocean. In both area, hemipelagites make up c. 50% of the succession, being closely interbedded with turbidites (Makran) and pelagites (Oman). Their sedimentary features (structure, texture, fabric and composition), bedding style and rates of sedimentation are reported and compared with previous detailed studies of both modern and ancient hemipelagites. A composite facies model is presented. Hemipelagites are finegrained sediments typically occurring in deep-water settings. They generally comprise an admixture of >10% biogenic pelagic material and >10% terrigenous or volcanigenic material, in which >40% of the terrigenous (volcanigenic) fraction is silt size or greater (i.e. >4 µm). They are deposited by a combination of vertical settling and slow lateral advection. Hemipelagites are mostly thoroughly bioturbated with ichnofossils including Zoophycos, Planolites, Chondrites, Phycosiphon and others. They are generally devoid of primary sedimentary structures, except for an organic-rich facies deposited in anoxic conditions which has a distinctive fissile lamination. The submicroscopic fabric is closely packed with subparallel alignment of clays. The grain size is strongly influenced by the composition, although in general the deposits are fine grained (mean size 5–35 µm), and poorly sorted, in some cases with bimodal, trimodal or polymodal grain-size distributions. The composition includes a biogenic fraction that is calcareous and/or siliceous, and a terrigenous fraction dependent on the nature of the supply pathway, i.e. river plumes, aeolian dust, glacigenic input or volcaniclastic fallout. Chemogenic components (phosphorites, glauconite, ferromanganese nodules) are common in some settings, whereas high organic carbon contents characterize others. Cyclic bedding, typical of climatic forcing, is common although bed boundaries are generally very gradational. Sedimentation rates typically vary from <5 cm/ka to >20 cm/ka and greatly influence development of the different characteristics outlined above. Extremely high rates (>100 cm/ka) occur in zones of hemipelagic sediment focusing, such as slope canyon systems and sediment pathways.

The use of the textural parameters of sand in studying the characteristics and depositional processes of coastal sediments south of Jeddah, Saudi Arabia

The subtidal sediments of four coastal environments (beach, swash zone, lagoon and nearshore bar) south of Jeddah have been studied in terms of their textural characteristics and depositional process. The skeletal grains are composed mainly of fragments of corals, benthic foraminifera, and different types of shells together with sand-sized rock fragments and quartz and feldspar grains. The skeletal material of the sediments has been derived from the mechanical breakdown of the adjacent reefs, while the small clastic portion was provided from the hinterland by rivers. Bimodal and polymodal grain-size distributions are common and can be attributed to heterogeneity in the types of skeletal debris. The size distribution of such sediments suggests that the various processes that are responsible for their transportation and deposition are wave, current and wind action. The different energy conditions and the morphology of each coastal environment seem to be reflected in the characteristics of the sediments in each environment.

Young pumice deposits on Nisyros, Greece

The island of Nisyros (Aegean Sea) consists of a silicic volcanic sequence upon a base of mafic-andesitic hyaloclastites, lava flows, and breccias. We distinguish two young silicic eruptive cycles each consisting of an explosive phase followed by effusions, and an older silicic complex with major pyroclastic deposits. The caldera that formed after the last plinian eruption is partially filled with dacitic domes. Each of the two youngest plinian pumice falls has an approximate DRE volume of 2–3 km3 and calculated eruption column heights of about 15–20 km. The youngest pumice unit is a fall-surge-flow-surge sequence. Laterally transitional fall and surge facies, as well as distinct polymodal grainsize distributions in the basal fall layer, indicate coeval deposition from a maintained plume and surges. Planar-bedded pumice units on top of the fall layer were deposited from high-energy, dry-steam propelled surges and grade laterally into cross-bedded, finegrained surge deposits. The change from a fall-to a surge/flow-dominated depositional regime coincided with a trend from low-temperature argillitic lithics to high-temperature, epidote-and diopside-bearing lithic clasts, indicating the break-up of a high-temperature geothermal reservoir after the plinian phase. The transition from a maintained plume to a surge/ash flow depositional regime occurred most likely during break-up of the high-temperature geothermal reservoir during chaotic caldera collapse. The upper surge units were possibly erupted through the newly formed ringfracture.

Charge measurements on particle fallout from a volcanic plume

THE aggregation of fine ash particles has an important role in controlling the deposition of widely dispersed volcanic ash. Here we report measurements of electrical charge on ash particles falling from the eruption columns of Sakurajima volcano in Japan. Absolute charge to mass (q/m) ratios ranged from +3 to +6x10-4C kg-1 and from -2 to -5 x 10-4 C kg-1. The average q/m ratio ranged from +2 to +5x10-5 C kg-1. The generation of electrostatic charge may result from triboelectric effects in the plume, or from fracture-induced charging. Charge on ash particles provides attractive forces large enough to cause the aggregation of smaller particles and the adhesion of dust to larger particles. Particle aggregation may explain the polymodal grain-size distributions commonly found in ash-fall deposits, and the proximal deposition of fine ash, as well as the distal deposition of coarse particles in these deposits. Our data suggest that electrostatic effects greatly influence the dispersal and deposition of ash during explosive volcanic eruptions.

Intertidal conglomerate south of Yanbu — An episodic clastic deposit in the eastern Red Sea

A thin sheet of clastic conglomerate has been located overlying the Pleistocene coral limestone in the intertidal zone, south of Yanbu on the west coast of Saudi Arabia. The conglomerate exhibits a polymodal grain-size distribution with gravel to cobble size clasts and sand-silt matrix. Grain and fabric characters of the conglomerate are consistent with a flash flood deposit rather than with a wave-processed nearshore sediment. Wadi Wasit draining the adjacent highlands and the coastal plain, covered with alluvial deposits derived from the Tertiary volcanic and metamorphic rocks, provided the coarse-grained sediments to the shelf. The carbonate cement in the conglomerate is composed of low Mg-calcite and appears to have been precipitated in non-marine conditions. The void filling low Mg-calcite exhibits a coarsening of crystal size away from the grains towards pore interiors. The 14C dating of the carbonate material gave an age of 3580±80 yr B.P.. All evidences suggest that the conglomerate was deposited on the emergent reef flat during an episodic flood stage of the Wadi during the Middle Holocene pluvial period on the west coast of Saudi Arabia.

The 1982 eruptions of El Chichón Volcano (Chiapas, Mexico): Character of the eruptions, ash-fall deposits, and gasphase

Three modest plinian eruptions occurred at El Chichón Volcano during March—April 1982, producing an ENE-trending lobe of ash covering 45,000 km 2 within the 1 mm isopach, and ash-flow and surge deposits in proximal areas. The total volume of juvenile magma erupted was 0.38 km 3 DRE. Ash-fall samples show polymodal grain-size distributions with median grain size decreasing away from the volcano. The ash-fall deposit is relatively fine-grained; more than 80% of the erupted material is in the sub-mm fraction. Larger ash particles were commonly armored with fines, and fines clustered together to form tightly cemented aggregates. Both processes contributed to efficient and rapid removal of fines from the eruption plume. The bulk chemical composition of ash samples can be closely modeled by fall-out fractionation of glass and minerals from the eruptive plume combined with adsorption of sulfur, sodium, and potassium. Leachate analyses of fresh ashes indicate large amounts of adsorbed sulfate and chlorine, and moderate quantities of adsorbed fluorine. During the eruption, nearly all sulfur that was not present as microphenocrystic anhydrite was released as vapor; at least 35% of the chlorine was degassed, and only 10% of the fluorine. Vapor transport of metals was limited in the eruption plume. Nearly 30% of the vaporous sulfur in the plume was adsorbed onto ash particles, probably after oxidation to sulfate, and thereby carried to the ground. The rest of the vaporous sulfur was injected into the stratosphere, presumably as SO 2. The exceptionally high sulfur content of the El Chichón magma, estimated at about 2.6 wt.% SO 3, resulted in a large stratospheric cloud of sulfuric acid aerosols which may have an impact on the earth's climate.

The geochemistry of sediments from the northern Reykjanes Ridge and the Iceland—Faroes Ridge

Samples from the active Reykjanes Ridge and the inactive Iceland—Faroes Ridge have been investigated sedimentologically, mineralogically, and geochemically. The sediments display polymodal grain-size distributions and are poorly sorted, indicating deposition by various mechanisms and contributions from numerous sources. The mineralogy is fairly typical for the region and strongly reflects the large input of volcanic ash and ice-rafted material. Bulk chemical analyses indicate that the Reykjanes Ridge sediments appear to be enriched in Fe, Mn, Cu, Cr, and Zn as has been reported for other active ridges while the inactive Iceland—Faroes Ridge does not display such enrichments. The enriched metals in the ridge sediments do not show a particular affinity for any one size class. Partition studies indicate that the enriched Fe and Mn are held in separate phases while the other metals are present in all phases. Adsorption is not a major concentrating mechanism for the enhanced elements. Li distributions are apparently unaffected by active ridges and Pb seems to be partially concentrated biologically. There are indications that other criteria must be used in conjunction with bulk chemical analyses, in order to establish the presence of active ridge metal contributions.

Darton Ridge, Pennsylvanian Feature of Wyoming Shelf

Despite the apparent uniform nature of the Tensleep Sandstone and its genetically related time equivalents, petrologic analysis of sections measured in the Big Horn basin of Wyoming yields evidence of statistically significant variability in texture and composition. This variability provides a basis for interpretation of the nature and circulation pattern of shallow-marine currents which transported and deposited the well-sorted, very fine-grained orthoquartzite as a thin blanket sand on the Pennsylvanian cratonic shelf in Wyoming and adjoining areas. Approximately half of the sediment comprising the Tensleep Sandstone was accumulated under the influence of low-velocity longshore currents that moved from northwest to southwest along the shoreline. These currents produced thinly and evenly bedded deposits that display essentially unimodal grain-size distribution, but contain a small silt The other half of the detritus may have been transported by high-energy, storm-related bottom currents whose mean flow direction was southwest. Deposits produced by these currents display planar cross-lamination and are characterized by a polymodal grain-size distribution curve with a coarse tail. Although cross-bedded strata are approximately equal in volume to the evenly laminated beds with which they are intercalated, bottom-current trans ort probably dominated the depositional regime only 10 per cent of the time, and longshore movement was most important during the remaining 90 per cent. An elongate bathymetric prominence, here named the Darton Ridge, significantly affected the distribution pattern of Tensleep thickness, texture, and composition in the Big Horn basin-Big Horn Mountains area by its interaction with current flow. Oriented essentially northeast-southwest across north-central Wyoming, the approximately 45-by-90+-mile Darton Ridge created throughout the Pennsylvanian Period a shallow-water area of reverse by-passing, swept by bottom currents and characterized by a thin accumulation of evenly bedded, unimodal deposits. Strata surrounding the Darton Ridge are thicker and contain a much higher percentage of coarsely bimodal, cross-laminated beds. Feldspar and metamorphic grains, derived from a northeastern source, show characteristic concentration along the north-facing slope of the ridge.