Dune Grains Research Articles

Sand source and formation mechanism of riverine sand dunes: a case study in Xiangshui River, China

Riverine sand dunes develop as a result of fluvial-aeolian interactions. The primarily barchan dune chains along the Xiangshui River (a branch of the Xar Moron River in the western part of the Horqin Sandy Land of China) form a typical riverine dune field. We collected a series of samples from the riverine sand dunes parallel to the direction of the prevailing wind and investigated the sand sources and formation mechanisms of these dunes by determining the grain size, heavy mineral content and optically stimulated luminescence (OSL) of the samples. The sand of the near-river dunes was coarser than the sand of the dunes distant from the river, indicating that coarse sand of the valley mainly deposited on near-river dunes. The heavy mineral analysis suggested that wind-sand activity levels were intense on the upwind dunes, but relatively weak on the downwind dunes. This indicated that the sand sources for the near-river dunes were more abundant than those of the distant dunes. Our OSL analysis of samples suggested that the deposition rates on dunes near the river were greater than the deposition rates on dunes distant from the river. The development of dunes along the river indicated that the river played an important role in dune formation and development. In addition, airflow fluctuation and the formation of the waveform dunes had a type of feedback relationship. Grain size, heavy mineral and OSL analyses are widely used methods in wind-sand research. Sand dune grain size characteristics reflect the effects of airflow on the transport and separation of sand materials, as well as the physical characteristics of the sand sources. Heavy mineral characteristics are often used to investigate the relationships between sediments and sand sources. OSL indicates dune age, revealing formation of dunes. Therefore, it is useful to explore dune sand sources, as well as the mechanisms underlying dune formation, by determining grain size, heavy mineral content and OSL. This study investigated the sand sources of riverine dunes and provided new information about riverine dune formation and development.

Geotechnical performance of sandy bricks made with fine aggregates of sand dunes, Saudi Arabia

The utilization of eolian sands as construction fine aggregates in bricks and mortar production has been achieved on dune sands collected from Jeddah-Al-Lith area (Saudi Arabia). These dune sands had been mixed with well-graded crushed fine aggregates at a variety mixing ratio fluctuated between 10 and 100% of the total volume of sand at constant ratio 1:2:3 (cement/water/sand). By comparing the physical characteristics (grain size, specific gravity, texture) mineral composition, workability and compressive strength features of the study materials, it is found that similarity with the most of the dune sands characteristics which reported in pervious literatures. The obtained results show that except for the gradational and textural features, these materials are fitting as construction fine aggregates when the mixing ratio not exceed than 50% of the total quantity of sands. Up to 50%, the compressive strength will be abruptly decreased owing to internal failure as a result of bleeding and segregation of raw mixtures before consolidation processes that due to increasing of surface area of well-rounded and high sphericity of dune grains.

Eolianite Grain Size Distributions as a Proxy for Large Changes in Planetary Atmospheric Density

AbstractAtmospheres are dynamic over geologic timescales, making large changes in planetary air density possible. For the Earth, geological proxies suggest that air density in the Neoarchean was similar to or lower than today. This air density variation possibly affected eolian dune grain sizes by controlling the trajectories of grains though the air. Balancing the fall velocity and threshold friction velocity, a metric separating saltation and suspension transport, suggests that a lower air density could increase the mean grain size of dunes because decreased air drag extends the size range of grains in modified saltation and incipient suspension regimes. Consequently, the dune‐forming sand left behind in pure saltation, the dominant dune‐forming transport mode, could have coarser grains. We analyzed size distributions of two eolianites from 2.64 and 1.5 Ga (billion years ago) for deviations from modern sand dunes emplaced at sea level, which globally exhibit similar mean grain sizes. Both aeolianites have mean grain sizes within one standard deviation of the modern mean and are not statistically separable at 95% confidence. Overall, this suggests that while air density is important in eolian physics, a factor of 2 to 4 change in density is insufficient to produce an unambiguous grain size signal. This suggests that while eolian dune grain sizes have not significantly changed over the range of Earth's atmospheric conditions, they could be useful when investigating the order of magnitude changes thought to have occurred on Mars.

Indications of brine related local seepage phenomena on the northern hemisphere of Mars

Springtime low albedo features, called Dark Dune Spots, on the seasonal frost covered dunes on Mars between 77°N and 84°N latitude have been analyzed. Two groups of these spots have been identified: “small” and “large” ones, where large spots have diameters above 4 m, and complex internal structure. From these “large” spots branching seepage-like features emanate and grow on the steep slopes. They show a characteristic sequence of changes: first only wind-blown features emanate from them, while later a bright circular and elevated ring forms, and dark seepage-features start from the spots. These streaks grow with a speed between 0.3 m/day and 7 m/day respectively, first only from the spots, later from all along the dune crest. During this “seepage period” the temperature is between 150 K and 180 K at a 3–9 km spatial resolution scale, indicating that CO 2 ice-free parts must be present at the observed dark spots. Around the receding northern seasonal CO 2 cap, an annulus of water ice lags behind, which is probably present in the spots too where the CO 2 frost has sublimated. Our model estimates show in the present work and in Kereszturi et al. (Kereszturi, A., Möhlmann, D., Berczi, Sz., Ganti, T., Kuti, A., Sik, A., Horvath, A. [2009b]. Icarus 201, 492–503) that the warming driven by solar insolation may produce not only interfacial water, but also bulk brines around the dune grains. The brine can support the movement of liquids and dune grains, enhances the sublimation of CO 2 frost, and produce the dark features, as well as liquid modifies the optical properties of the surface. Signs of movement of dune material after the total defrosting of the terrain is also visible but it is uncertain because of the limit of resolution. In our previous work (Kereszturi et al., 2009b) we showed that resembling seepage-like streaks at the southern hemisphere might have been formed by ephemeral interfacial water, as well as these northern features. Such wet environments may have astrobiological importance too.

Recent rheologic processes on dark polar dunes of Mars: Driven by interfacial water?

In springtime on HiRISE images of the Southern polar terrain of Mars flow-like or rheologic features were observed. Their dark color is interpreted as partly defrosted surface where the temperature is too high for CO 2 but low enough for H 2O ice to be present there. These branching streaks grow in size and can move by an average velocity of up to about 1 m/day and could terminate in pond-like accumulation features. The phenomenon may be the result of interfacial water driven rheologic processes. Liquid interfacial water can in the presence of water ice exist well below the melting point of bulk water, by melting in course of interfacial attractive pressure by intermolecular forces (van der Waals forces e.g.), curvature of water film surfaces, and e.g. by macroscopic weight, acting upon ice. This melting phenomenon can be described in terms of “premelting of ice”. It is a challenging consequence, that liquid interfacial water unavoidably must in form of nanometric layers be present in water ice containing soil in the subsurface of Mars. It is the aim of this paper to study possible rheologic consequences in relation to observations, which seem to happen at sites of dark polar dunes on Mars at present. The model in this work assumes that interfacial water accumulates at the bottom of a translucent water-ice layer above a dark and insolated ground. This is warmed up towards the melting point of water. The evolving layer of liquid interfacial water between the covering ice sheet and the heated ground is assumed to drive downward directed flow-like features on slopes, and it can, at least partially, infiltrate (seep) into a porous ground. There, in at least temporarily cooler subsurface layers, the infiltrated liquid water refreezes and forms ice. The related stress built-up is shown to be sufficient to cause destructive erosive processes. The above-mentioned processes may cause change in the structure and thickness of the covering ice and/or may cause the movement of dune grains. All these processes may explain the observed springtime growing and downward extension of the slope streaks analyzed here.

The particle size of Martian aeolian dunes

The effective particle size of unconsolidated materials on the Martian surface can be determined from thermal inertia, due to a pore size dependence of thermal conductivity at Martian atmospheric pressures. Because dunes consist of a narrow range of well‐sorted, unconsolidated particles, they provide for a test of the relationship between particle size and thermal inertia calculated from midinfrared emission data for the Martian surface. We use two independent approaches. First, thermal inertia data indicate that Martian dunes have an average particle size of about 500±100 μm, or medium to coarse sand. Second, we determine expected dune particle sizes from grain trajectory calculations and the particle size transition from suspension to saltation. On Earth, the transition occurs for a grain when the ratio of the terminal fall velocity to the wind friction speed (u*t) is near unity; for grains at u*t, this occurs at about 52 μm. Terrestrial dune sands have a mean of 250 μm and are composed entirely of grains >52 μm. The corresponding Martian transition grain size is about 210 μm, suggesting that Martian dunes should be significantly coarser than terrestrial dunes. Grain saltation path length as a function of particle size also shows that under Martian conditions, larger grains than on Earth will become suspended. Both approaches indicate that Martian dune sand should be coarser than terrestrial dune sand. Thus, while terrestrial dune grains are in the fine to medium sand range, the average Martian dune sediments are probably medium to coarse sands. These results closely match the grain sizes determined from thermal inertia models, providing the first direct test of the validity of these models for actual Martian surface materials.

Chlorite Vermiculitization and Pyroxene Etching in an Aeolian Periglacial Sand Dune, Allen County, Indiana

AbstractWeathering has etched and deeply-denticulated the constituent orthopyroxenes, and chlorite has been transformed to vermiculite in the upper 3 m of an aeolian, periglacial sand dune formed in northeastern Indiana about 13,000 b.p. Pyroxene weathering begins with the development of cleavage-parallel etch pits on {010} and {100} surfaces. These pits coalesce and eventually crop out on basal surfaces as denticulations. The mean denticulation size increases logarithmically toward the surface, and the denticulation size of the orthopyroxenes is a quantifiable feature of the weathering process. Ferruginous pendants, microboxworks of iron oxides, and other indications of iron redeposition within the ortho-pyroxene microenvironments were not observed.Chlorite in the dune has been weathered to vermiculite. The parent chlorite is a high-Fe variety, and the transformation to vermiculate does not involve the development of a chlorite/vermiculite intermediary phase. Fe2+ is oxidized as part of the transformation process and this iron is retained in the sediment as discrete goethite and as crystalline and noncrystalline coatings on the dune grains. The vermiculite from depths shallower than 64 cm is only partly expandable and is completely collapsed by K-saturation or heat treatment. This is a hydroxy-Al vermiculite and its formation is typical of intense weathering under the acid conditions prevalent at the dune surface.

Analysis of barrier island surface sediments by scanning electron microscopy

The analysis of quartz grain surface microtextures from a sedimentary environment where grain transport and depositional pathways were closely interconnected, allowed insight into subtle microfeature alteration during movement between subenvironments. Four Fire Island (New York) and one Montauk Point (beach) sample were analysed, principally through use of a checklist approach, using 36 common surface features. Beach, overwash, floodtidal delta, dune and inlet grains displayed similar surface features, only the latter two samples displaying distinctive microtextural patterns. The pattern of development of inlet grains was related to the application of higher shear stresses on the grains during transport in the shallow channel. The results from dune grains testify to the effectiveness of aeolian transport mechanisms in rapidly altering existing features. Glacially derived microtextures altered by subsequent subaqueous action were prevalent but whether these glacial microfeatures are derived from the headland region, Montauk Point, or from a closer offshore “glacial lobe” source is not determined. Comparison of offshore lobe and adjacent sediments revealed contrasting surface and morphological differences. Statistical treatment of checklist data in binary form (1,0) using stepwise discriminant analysis proved workable, but more rigorous testing is required to prove its relevance and use in deciphering raw textural data.

The shape of desert sand dune grains

Examination of the shape characteristics of 21,600 individual sand grains from a variety of world dune areas indicates that true roundness is relatively rare, that different ergs have different grain roundness characteristics, that smaller grains tend to be more angular, and that position on a dune seems to be an unimportant control of grain shape.

Hydrodynamic Implications of Beach, Beach Ridge and Dune Grain Size Studies

ABSTRACT Stepwise linear discriminant and canonical analyses of beach, beach ridge and coastal dune (slip-face) sands from localities around the Gulf of Mexico show that beach ridges have textural affinities with both beaches and coastal dunes. However, the internal structure shows that they were deposited largely by swash action. Washover and slip-yface bedding are, in general, not present in beach ridges. Skewness is the most important moment measure entering the discriminant functions and the cannonical plots; standard deviation is the next most important. These two measures for beach ridge sands show both swash-zone and dune affinities. In these samples kurtosis reflects shell content. Mean grain size, an indicator of local availability, is of minimum importance for determining hydrodynamic conditions of transport and deposition in such a regional study. A progression of moment measures was determined for the surf zone, the upper wash zone, and the coastal dune. Changes from zone to zone are largely confined to the coarse tail; however, individual sample grain size distributions (plotted on probability paper) have important but subtle characteristics for each zone. Differences in skewness values bear this out. Our beach ridge data generally fall between the upper swash zone and the coastal dune (in most instances closer to the upper swash zone). The statistical process, from surf to beach ridge to coastal dune, is one of filtering (but not mixing). Each filter is truncated. The total result is to improve the internal sorting from zone to zone, but the terminal distribution is nevertheless still not simple. The lack of simplicity aids in making an hydrodynamic interpretation.

Distinguishing between Beach and Dune Sands

ABSTRACT The comparison of adjacent pairs of beach (largely foreshore) and dune sands from 74 localities of world-wide distribution has shown that it is generally possible to distinguish between the two, particularly if there are predominant onshore winds. The dune sands are usually rounder than the adjacent beach sands, have a larger silt content, and this silt has a higher content of heavy minerals. The beach foreshores, on the other hand, usually contain more shells and other calcareous organisms than the dunes. There is also some reason to believe that mica content is greater in beach foreshores than in adjacent dunes. The sand of beach berms appears to be intermediate in most respects between that of the foreshores and dunes. The areas where beaches and dunes are most difficult to distinguish are those where there are longshore, extremely variable, or offshore winds. In such localities it is thought that the dune sands are blown back onto the beaches causing intermixture and that in some cases the dunes are derived from sources other than from the adjacent beach. The best results in separating the two environments have come from beaches of the west coast of the United States and Baja California where onshore winds predominate. It is suggested that the chief reason for the differences between dune and beach sands is that the wind picks up from the beaches more of the rounder grains than of the flat and angular grains. There is no reason to believe that the grains are rounded appreciably by the wind in transit to the dunes. Nor does there appear to be any evidence from these comparisons that the dune grains become frosted. The greater quantity of heavy minerals in the dune silt fraction may be caused by removal by the wind of the light minerals.