Dust Flux Research Articles

Periodic input of dust over the Eastern Carpathians during the Holocene linked with Saharan desertification and human impact

Abstract. Reconstructions of dust flux have been used to produce valuable global records of changes in atmospheric circulation and aridity. These studies have highlighted the importance of atmospheric dust in marine and terrestrial biogeochemistry and nutrient cycling. By investigating a 10 800-year-long paleoclimate archive from the Eastern Carpathians (Romania) we present the first peat record of changing dust deposition over the Holocene for the Carpathian–Balkan region. Using qualitative (X-ray fluorescence (XRF) core scanning) and quantitative inductively coupled plasma optical emission spectrometer(ICP-OES) measurements of lithogenic (K, Si, Ti) elements, we identify 10 periods of major dust deposition between 9500–9200, 8400–8100, 7720–7250, 6350–5950, 5450–5050, 4130–3770, 3450–2850, 2000–1450, 800–620, and 60 cal yr BP to present. In addition, we used testate amoeba assemblages preserved within the peat to infer local palaeohydroclimatic conditions. Our record highlights several discrepancies between eastern and western European dust depositional records and the impact of highly complex hydrological regimes in the Carpathian region. Since 6100 cal yr BP, we find that the geochemical indicators of dust flux have become uncoupled from the local hydrology. This coincides with the appearance of millennial-scale cycles in the dust input and changes in geochemical composition of dust. We suggest that this is indicative of a shift in dust provenance from local–regional (likely loess-related) to distal (Saharan) sources, which coincide with the end of the African Humid Period and the onset of Saharan desertification.

Late Pleistocene – Holocene surface processes and landscape evolution in the central Swiss Alps

The European Alps are a geomorphologically active region and experience a number of gravity-driven hillslope processes. Soil and landscape formation in the Alps has consequently undergone several minor and major traceable changes of developmental trajectories during the Holocene. Soil development is hypothesised to be often non-linear with time and characterised by stages of progressive and regressive evolution caused by upbuilding (formation, profile deepening) and erosion (profile shallowing). Several cold and warm climate phases are identified during the Holocene but it is largely unknown which effects these might have had on slope processes. By using datable moraines (10Be) and mires (14C), we have constructed a temporal framework for these processes. Using the geochemical imprint of mires in the Alpine setting of the Göschener-valley of the Central Swiss Alps, we reconstructed general (mostly erosional) landscape processes for the last ca. 10ka. As this is the type locality for the Göschener cold phase, we assumed that this phase (Göschener cold phase I and II ~1.5 and ~2.5kaBP) should have left easily recognizable traits. After deglaciation (11–12kaBP), soil evolution was progressive. Beginning around 8kaBP, we detect a distinct increase in erosion here, together with a vegetation change (towards tundra vegetation) and the highest measured rates of carbon sequestration. Other phases of high geomorphic activity were recognised ca. 5–6kaBP, 4kaBP and, to a lesser extent, 1–3ka ago. The cold phase at 5–6kaBP corresponds to a less distinct change in vegetation and lessened erosion. Human impact is increasingly obvious since about 2.4kaBP which overlaps with the Göschener cold phase. Nonetheless, erosion processes were not extraordinarily high during this period and a climate effect cannot be distinguished. We detect evidence of increasing human disturbance (regressive soil evolution) for about the last 1ka. We also detect an increase in dust flux during the last ca. 4–5ka, presumably due to the landscape change(s) in the Sahara during this time.

Dust ablation on the giant planets: Consequences for stratospheric photochemistry

Ablation of interplanetary dust supplies oxygen to the upper atmospheres of Jupiter, Saturn, Uranus, and Neptune. Using recent dynamical model predictions for the dust influx rates to the giant planets (Poppe et al., 2016), we calculate the ablation profiles and investigate the subsequent coupled oxygen–hydrocarbon neutral photochemistry in the stratospheres of these planets. We find that dust grains from the Edgeworth–Kuiper Belt, Jupiter-family comets, and Oort-cloud comets supply an effective oxygen influx rate of 1.0−0.7+2.2×107 O atoms cm−2−1 to Jupiter, 7.4−5.1+16×104 cm−2 s−1 to Saturn, 8.9−6.1+19×104 cm−2 s−1 to Uranus, and 7.5−5.1+16×105 cm−2 s−1 to Neptune. The fate of the ablated oxygen depends in part on the molecular/atomic form of the initially delivered products, and on the altitude at which it was deposited. The dominant stratospheric products are CO, H2O, and CO2, which are relatively stable photochemically. Model-data comparisons suggest that interplanetary dust grains deliver an important component of the external oxygen to Jupiter and Uranus but fall far short of the amount needed to explain the CO abundance currently seen in the middle stratospheres of Saturn and Neptune. Our results are consistent with the theory that all of the giant planets have experienced large cometary impacts within the last few hundred years. Our results also suggest that the low background H2O abundance in Jupiter’s stratosphere is indicative of effective conversion of meteoric oxygen to CO during or immediately after the ablation process — photochemistry alone cannot efficiently convert the H2O into CO on the giant planets.

Detection of a persistent meteoric metal layer in the Martian atmosphere

Interplanetary dust particles sporadically enter planetary atmospheres at orbital velocities and ablate as collisions occur with ambient gases to produce a persistent layer of metallic atoms (for example, Fe, Mg, Na) in their upper atmospheres. Such layers are well studied at Earth, but have not been directly detected elsewhere in the Solar System. Here we report the detection of a meteoric layer consisting of Mg+ ions near an altitude of 90 km in the Martian atmosphere from ultraviolet remote sensing observations by NASA’s MAVEN spacecraft. We observe temporal variability in the Mg+ layer over the course of a Martian year, moving up and down in altitude seasonally and in response to dust storms, and displaying diurnal fluctuations in density. We also find that most meteor showers do not significantly perturb this layer, which constrains the fluence of eleven observed Martian meteor showers to less than our estimated global dust flux. The persistence and variability of the Mg+ layer are difficult to explain with existing models and reconcile with other transient layers of ions observed in the Martian ionosphere. We suggest that the transient layers are not sourced from the persistent Mg+ layer and thus not derived from meteoric material, but are ambient ions produced by some unknown mechanism.

Downward particle fluxes of biogenic matter and Saharan dust across the equatorial North Atlantic

Abstract. Massive amounts of Saharan dust are blown from the coast of northern Africa across the Atlantic Ocean towards the Americas each year. This dust has, depending on its chemistry, direct and indirect effects on global climate which include reflection and absorption of solar radiation as well as transport and deposition of nutrients and metals fertilizing both ocean and land. To determine the temporal and spatial variability of Saharan dust transport and deposition and their marine environmental effects across the equatorial North Atlantic Ocean, we have set up a monitoring experiment using deep-ocean sediment traps as well as land-based dust collectors. The sediment traps were deployed at five ocean sites along a transatlantic transect between north-west Africa and the Caribbean along 12° N, in a downwind extension of the land-based dust collectors placed at 19° N on the Mauritanian coast in Iouîk. In this paper, we lay out the setup of the monitoring experiment and present the particle fluxes from sediment trap sampling over 24 continuous and synchronized intervals from October 2012 through to November 2013. We establish the temporal distribution of the particle fluxes deposited in the Atlantic and compare chemical compositions with the land-based dust collectors propagating to the downwind sediment trap sites, and with satellite observations of Saharan dust outbreaks. First-year results show that the total mass fluxes in the ocean are highest at the sampling sites in the east and west, closest to the African continent and the Caribbean, respectively. Element ratios reveal that the lithogenic particles deposited nearest to Africa are most similar in composition to the Saharan dust collected in Iouîk. Downwind increasing Al, Fe and K contents suggest a downwind change in the mineralogical composition of Saharan dust and indicate an increasing contribution of clay minerals towards the west. In the westernmost Atlantic Ocean, admixture of re-suspended clay-sized sediments advected towards the deep sediment trap cannot be excluded. Seasonality is most prominent near both continents but generally weak, with mass fluxes dominated by calcium carbonate and clear seasonal maxima of biogenic silica towards the west. The monitoring experiment is now extended, with autonomous dust sampling buoys for better quantification of Saharan dust transport and deposition from source to sink and their impact on fertilization and carbon export to the deep ocean.

Holocene fluvial valley fill sources of atmospheric mineral dust in the Skeleton Coast, Namibia

AbstractWestern Namibia is a significant global source of atmospheric mineral dust. We investigate the relationship between dust and source sediments, assessing the sustainability of dust flux. Remote sensing studies have highlighted specific ephemeral fluvial systems as important contributors to dust flux, including highlighting sections of valleys that are the origins of dust plumes in the period 2005–2008. Little is known however about the specific within‐valley dust sediment sources, particularly whether dust is derived from modern ephemeral channel floors or older valley fill sediments, many of which have been reported in the region. As part of a region‐wide analysis of aeolian dust flux, we investigate the sediment properties of atmospheric dust samples and valley sediments from the Huab valley, one of the principal regional dust sources. Trapped dust samples contain up to 88% very fine sand and silt when collected samples are disaggregated prior to analysis. Valley fill surface samples comprise 80% very fine sand and silt, and the surface of the modern ephemeral channel 30%. Valley fill sediments were sampled at depths up to 3.6 m below the present surface and reveal Holocene depositional ages from 0.6 ± 0.03 ka back to 9.79 ± 0.73 ka. These sediments contain 30% to 6% very fine sand and silt, with levels decreasing with depth and age. Aeolian bedforms in the valley system (nebkhas on the fill surface and climbing dunes on valley margins) indicate that aeolian processes under the influence of strong seasonal easterly winds likely result in dust being winnowed out of the valley fill surfaces, with sandy bedforms being constructed from the coarser component of the fill sediments. The volume of valley fill sediment suggests dust sourced from Holocene sediments is likely to continue into the future regardless of flow conditions in the modern channel system. Copyright © 2017 John Wiley & Sons, Ltd.

Change in dust seasonality as the primary driver for orbital‐scale dust storm variability in East Asia

AbstractGlacial periods are recognized to be dustier than interglacials, but the conditions leading to greater dust mobilization are poorly defined. Here we present a new high‐resolution dust record based on 230Th‐normalized 4He flux from Ocean Drilling Program site 882 in the Subarctic North Pacific covering the last 170,000 years. By analogy with modern relationships, we infer the mechanisms controlling orbital‐scale dust storm variability in East Asia. We propose that orbital‐scale dust flux variability is the result of an expansion of the dust season into summer, in addition to more intense dust storms during spring and fall. The primary drivers influencing dust flux include summer insolation at subarctic latitudes and variable Siberian alpine glaciation, which together control the cold air reservoir in Siberia. Changes in the extent of the Northern Hemisphere ice sheets may be a secondary control.

The hybrid disks: a search and study to better understand evolution of disks

Context. The increased sensitivity of millimeter-wave facilities now makes possible the detection of low amounts of gas in debris disks. Some of the gas-rich debris disks harbor peculiar properties, with possible pristine gas and secondary generated dust. The origin of the gas in these hybrid disks is strongly debated and the current sample is too sparse to understand this phenomenon. Aims. More detections are necessary to increase the statistics on this population. Lying at the final stages of evolution of proto-planetary disks and at the beginning of the debris disk phase, these objects could provide new insight into the processes involved in the making of planetary systems. Methods. We carried out a deep survey of the CO J = 2 → 1 and CO J = 3 → 2 lines with the APEX and IRAM radiotelescopes in young debris disks selected according to hybrid disk properties. The survey is complemented with a bibliographic study of the ratio between the emission of the gas and the continuum (SCO/Fcont) in CTTS, Herbig Ae, WTTS, hybrid, and debris disks. Results. Our sub-mm survey comprises 25 stars, including 17 new targets, and we increase the sensitivity limit by a factor 2 on eight sources compared to similar published studies. We report a 4σ tentative detection of a double-peaked CO J = 2 → 1 line around HD 23642; an eclipsing binary located in the Pleiades. We also reveal a correlation between the emission of the CO gas and the dust continuum from CTTS, Herbig Ae and few debris disks. The observed trend of the gas to dust flux ratio suggests a concurrent dissipation of the dust and gas components. Hybrid disks systematically lie above this trend, suggesting that these systems may witness a transient phase, when the dust has evolved more rapidly than the gas, with a flux ratio SCO/Fcont enhanced by a factor of between 10 and 100 compared to standard (proto-)planetary disks.

Dust outpaces bedrock in nutrient supply to montane forest ecosystems

Dust provides ecosystem-sustaining nutrients to landscapes underlain by intensively weathered soils. Here we show that dust may also be crucial in montane forest ecosystems, dominating nutrient budgets despite continuous replacement of depleted soils with fresh bedrock via erosion. Strontium and neodymium isotopes in modern dust show that Asian sources contribute 18–45% of dust deposition across our Sierra Nevada, California study sites. The remaining dust originates regionally from the nearby Central Valley. Measured dust fluxes are greater than or equal to modern erosional outputs from hillslopes to channels, and account for 10–20% of estimated millennial-average inputs of bedrock P. Our results demonstrate that exogenic dust can drive the evolution of nutrient budgets in montane ecosystems, with implications for predicting forest response to changes in climate and land use.

Meteorological constraints on characteristics of daily dustfall in Xi'an

Dust deposition is a crucial link of dust cycle that is less constrained in model studies. This study acquired profiles of flux and size distribution of daily dustfall from 2012 to 2013 in Xi'an on the southern Chinese Loess Plateau. On this basis, we made quantitative estimates of dust contribution from particular sources and processes, which provides important boundary conditions for model studies. Regrouping the data into transport- and source-limited deposition scenarios, we revealed that besides precipitation extreme wind speed and average relative humidity are the primary meteorological constraints in the transport- and source-limited scenarios, respectively. Stronger extreme wind speed promotes higher flux and deposition of dust >16 μm, corroborating previous interpretations of variation of flux and grain size of aeolian deposits. However, higher average relative humidity favors lower flux and deposition of dust <16 μm, which is a deposition process not recognized before, and is possibly due to hygroscopicity of mineral dust or the influence of water vapor on air convection. Elucidating this process in future studies might substantially improve model performance on dust deposition.

The entry heating and abundances of basaltic micrometeorites

AbstractBasaltic micrometeorites (MMs) derived from HED‐like parent bodies have been found among particles collected from the Antarctic and from Arctic glaciers and are to date the only achondritic particles reported among cosmic dust. The majority of Antarctic basaltic particles are completely melted cosmic spherules with only one unmelted particle recognized from the region. This paper investigates the entry heating of basaltic MMs in order to predict the relative abundances of unmelted to melted basaltic particles and to evaluate how mineralogical differences in precursor materials influence the final products of atmospheric entry collected on the Earth's surface. Thermodynamic modeling is used to simulate the melting behavior of particles with compositions corresponding to eucrites, diogenites, and ordinary chondrites in order to evaluate degree of partial melting and to make a comparison between the behavior of chondritic particles that dominate the terrestrial dust flux and basaltic micrometeroids. The results of 120,000 simulations were compiled to predict relative abundances and indicate that the phase relations of precursor materials are crucial in determining the relative abundances of particle types. Diogenite and ordinary chondrite materials exhibit similar behavior, although diogenite precursors are more likely to form cosmic spherules under similar entry parameters. Eucrite particles, however, are much more likely to melt due to their lower liquidus temperatures and small temperature interval of partial melting. Eucrite MMs, therefore, usually form completely molten cosmic spherules except at particle diameters &lt;100 μm. The low abundance of unmelted basaltic MMs compared with spherules, if statistically valid, is also shown to be inconsistent with a low velocity population (12 km s−1) and is more compatible with higher velocities which may suggest a near‐Earth asteroid source dominates the current dust production of basaltic MMs.

The thermal decomposition of fine-grained micrometeorites, observations from mid-IR spectroscopy

We analysed 44 fine-grained and scoriaceous micrometeorites. A bulk mid-IR spectrum (8–13μm) for each grain was collected and the entire micrometeorite population classified into 5 spectral groups, based on the positions of their absorption bands. Corresponding carbonaceous Raman spectra, textural observations from SEM-BSE and bulk geochemical data via EMPA were collected to aid in the interpretation of mid-IR spectra. The 5 spectral groups identified correspond to progressive thermal decomposition. Unheated hydrated chondritic matrix, composed predominantly of phyllosilicates, exhibit smooth, asymmetric spectra with a peak at ∼10μm. Thermal decomposition of sheet silicates evolves through dehydration, dehydroxylation, annealing and finally by the onset of partial melting. Both CI-like and CM-like micrometeorites are shown to pass through the same decomposition stages and produce similar mid-IR spectra. Using known temperature thresholds for each decomposition stage it is possible to assign a peak temperature range to a given micrometeorite. Since the temperature thresholds for decomposition reactions are defined by the phyllosilicate species and the cation composition and that these variables are markedly different between CM and CI classes, atmospheric entry should bias the dust flux to favour the survival of CI-like grains, whilst preferentially melting most CM-like dust. However, this hypothesis is inconsistent with empirical observations and instead requires that the source ratio of CI:CM dust is heavily skewed in favour of CM material. In addition, a small population of anomalous grains are identified whose carbonaceous and petrographic characteristics suggest in-space heating and dehydroxylation have occurred. These grains may therefore represent regolith micrometeorites derived from the surface of C-type asteroids. Since the spectroscopic signatures of dehydroxylates are distinctive, i.e. characterised by a reflectance peak at 9.0–9.5μm, and since the surfaces of C-type asteroids are expected to be heated via impact gardening, we suggest that future spectroscopic investigations should attempt to identify dehydroxylate signatures in the reflectance spectra of young carbonaceous asteroid families.

ATLASGAL-selected massive clumps in the inner Galaxy

The ATLASGAL survey provides an ideal basis for detailed studies of large numbers of massive star forming clumps covering the whole range of evolutionary stages. The ATLASGAL Top100 is a sample of clumps selected from their infrared and radio properties to be representative for the whole range of evolutionary stages. The ATLASGAL Top100 sources are the focus of a number of detailed follow-up studies that will be presented in a series of papers. In the present work we use the dust continuum emission to constrain the physical properties of this sample and identify trends as a function of source evolution. We determine flux densities from mid-infrared to submm wavelength (8-870 micron) images and use these values to fit their spectral energy distributions (SEDs) and determine their dust temperature and flux. Combining these with recent distances from the literature including maser parallax measurements we determine clump masses, luminosities and column densities. We find trends for increasing temperature, luminosity and column density with the proposed evolution sequence, confirming that this sample is representative of different evolutionary stages of massive star formation. We show that most of the sample has the ability to form massive stars (including the most massive O-type stars) and that the majority is gravitationally unstable and hence likely to be collapsing. The highest column density ATLASGAL sources presented cover the whole range of evolutionary stages from the youngest to the most evolved high-mass star forming clumps. Their study provides a unique starting point for more in-depth research on massive star formation in four distinct evolutionary stages whose well defined physical parameters afford more detailed studies. As most of the sample is closer than 5 kpc, these sources are also ideal for follow-up observations with high spatial resolution.

An increased abundance of micrometeorites on Earth owing to vesicular parachutes

AbstractMicrometeorites (MMs) are extraterrestrial dust particles that survive atmospheric entry and can be recovered from sedimentary rocks. Fossil MMs allow events beyond the Earth, such as the collisional breakup of asteroids, to be identified. Here the effects of vesicle formation during melting of dust are investigated through numerical modeling and observations of Antarctic MMs. Vesicle formation is shown to cause a parachute effect that causes rapid deceleration, decreasing peak temperature. Vesicular parachuting enhances the abundance of melted MMs formed from phyllosilicate‐bearing C‐type asteroid dust on the Earth surface by a factor of 2. Micrometeorites recovered from the geological record, therefore, are biased toward breakup events involving hydrated C‐type asteroids, whilst those involving phyllosilicate‐poor particles are diluted by the enhanced background flux of hydrous dust. The parachute effect is also likely to increase the delivery of 3He to ocean sediments by C‐type asteroid dust.

Long Term Flux of Saharan Dust to the Aegean Sea around the Attica Region, Greece

In this study, Particulate Matter (PM) samples, collected during two summer and two winter months over a long-term period (1984-2012) at a suburban site in Athens (Greece), were used in order to examine the connection between Sahara dust long range transport events and mass concentrations of the aerosol mineral component, as well as the relative abundance of specific crustal components. As a result, the average deposition flux of dust to the Aegean Sea around the Attica Region, during days with Sahara dust transport events, was calculated. The elemental concentration of aerosol samples was determined by means of ET-AAS, ED-XRF. Mineral dust was chemically reconstructed by using the elemental concentrations of the crustal species based on their common oxides. Two different air mass transport models (HYSPLIT and FLEXTRA) were used for the identification of the days with dust transport events. Τhe dust deposition velocity of the particles was calculated by using Stokes drag law, while the dust deposition flux was calculated taking into account the mean particle size of the aerosol coarse size fraction, which is dominated by the transported crustal component during Sahara dust intrusions. The mineral dust contribution was higher in summer, when dry weather conditions prevail. The Ca/Fe ratio was examined for all years, since this ratio is often used for the identification of Saharan dust events. For the years 1996 and 1998 the Ca/Fe ratio indicates an influence by local urban generated dust. The dust deposition flux per day of a Sahara intrusion event varied from 61 –199 μg/m2 with an average value of 131 ± 41 μg/m2. The total dust deposition over the 4-month measuring period ranged from 237 to 2935 μg/m2.

U-Pb ages of detrital zircon and provenances of Red Clay in the Chinese Loess Plateau

The Chinese Loess Plateau (CLP), which is located in the central part of China, mainly contains two sequences: the Quaternary loess-paleosol sequence and the late Miocene-Pliocene Red Clay sequence. U-Pb ages of detrital zircon from the Quaternary loess-paleosol sequences indicate that the current spatial pattern of central Asian aridification was largely established during the early Quaternary. U-Pb ages of detrital zircon from the underlying Red Clay sequences demonstrate that the northern source regions were important contributors for the dust since the late Miocene, except for the periods of ∼8Ma and ∼5.5 to ∼4Ma. These two periods correspond to increased and decreased dust deposition in the North Pacific, respectively, ascribed to the late Miocene aridification of the Qaidam Basin (in the west of the CLP) and the early Pliocene aridification of the Tarim Basin (in the west of the CLP). We attribute the increase of the Qaidam Basin-derived North Pacific dust fluxes to the uplift of the NE Tibetan Plateau. Such uplift also controlled the climatic and environmental changes in central Asian during the late Miocene. This study highlights the dynamical process of the central Asian aridification during the late Miocene-Pliocene.

Late Eocene 3He and Ir anomalies associated with ordinary chondritic spinels

During the late Eocene there was an enigmatic enhancement in the flux of extraterrestrial material to Earth. Evidence comes from sedimentary 3He records indicating an increased flux of interplanetary dust during ca. 2Myr, as well as two very large impact structures, Popigai (100km diameter) and Chesapeake Bay (40–85km), that formed within 10–20kyr at the peak of the 3He delivery. The Massignano section in Italy has one of the best sedimentary records of these events, including a well-defined 3He record, an Ir-rich ejecta bed related to the Popigai impact event, and two smaller Ir anomalies. Recently we showed that the Popigai ejecta is associated with a significant enrichment of chromite grains (>63μm) with an H-chondritic elemental composition (17 grains in 100kg of rock). Most likely these grains are unmelted fragments from the impactor. Slightly higher up (ca. 20cm) in the section, where a small Ir anomaly possibly related to the Chesapeake Bay impact has been measured, we found a weak enrichment in L-chondritic grains (8 grains in 208kg of rock). Here we report an extended data set increasing the total amount of sediment dissolved in acid and searched for extraterrestrial chromite grains from 658 to 1168kg. In altogether 760kg of background sediment from 17 levels over 14m of strata outside the interval corresponding to the Popigai and Chesapeake Bay impacts, we only found 2 extraterrestrial chromite grains. Both grains have L-chondritic compositions and were found in a 100kg sample from the ca. 10.25m level in the section where the second of the smaller Ir anomalies has been reported. A correlation appears to exist between Ir, 3He and chromite from ordinary chondrites. We also report oxygen three-isotope measurements of the extraterrestrial chromite grains associated with the Popigai ejecta and confirm an H-chondritic composition.The new results strengthen our scenario that the upper Eocene 3He and Ir enrichments originate from the asteroid belt rather than the Oort cloud as originally proposed when the 3He anomaly was discovered. The generally low background concentrations of extraterrestrial chromite through the section speak against any major single asteroid breakup event such as in the mid-Ordovician after the break-up of the L-chondrite parent body. Instead the data reconcile with a small, possibly a factor of 2–3, increase in the flux of extraterrestrial material to Earth, but of both H- and L-chondritic composition. We also report the composition of all the 2310 terrestrial chrome spinel grains recovered, and show that their chemical composition indicates a dominantly regional ophiolitic source. Four anomalous chrome spinel grains with high Ti and V concentrations were found in the Popigai ejecta. These grains originate from Siberian Traps basalts in the Popigai crater at the time of impact.

An urban collection of modern-day large micrometeorites: Evidence for variations in the extraterrestrial dust flux through the Quaternary

Abstract We report the discovery of significant numbers (500) of large micrometeorites (&amp;gt;100 μm) from rooftops in urban areas. The identification of particles as micrometeorites is achieved on the basis of their compositions, mineralogies, and textures. All particles are silicate-dominated (S type) cosmic spherules with subspherical shapes that form by melting during atmospheric entry and consist of quench crystals of magnesian olivine, relict crystals of forsterite, and iron-bearing olivine within glass. Four particles also contain Ni-rich metal-sulfide beads. Bulk compositions are chondritic apart from depletions in the volatile, moderately volatile, and siderophile elements, as observed in micrometeorites from other sources. The reported particles are likely to have fallen on Earth in the past 6 yr and thus represent the youngest large micrometeorites collected to date. The relative abundance ratio of barred olivine to cryptocrystalline spherule types in the urban particles of 1.45 is shown to be higher than a Quaternary average of ∼0.9, suggesting variations in the extraterrestrial dust flux over the past 800 k.y. Changes in the entry velocities of dust caused by quasi-periodic gravitational perturbation during transport to Earth are suggested to be responsible. Variations in cosmic spherule abundance within the geologic column are thus unavoidable and can be a consequence of dust transport as well as major dust production events.

An Investigation into the Processes and Quantity of Dust Emissions over Gravel and Sand Deserts in North-Western China

Year-long field observations have shown that there are spatial and temporal variations in the quantity of dust emissions for particulate matter $${<}$$ 10 $$\upmu \hbox {m}$$ (PM10), particulate matter $${<}$$ 63 $$\upmu \hbox {m}$$ (PM63) and vertical dust flux over different gravel surfaces (with loose sand, without loose sand, with a crust, and without a crust), with the greatest emissions occurring in the spring. The largest quantity of PM10 and PM63 emissions occurred over gravel with a loose sand surface ( $$1.1 \times 10^{-3}$$ and $$10.2 \times 10^{-3 }\hbox { kg m}^{-1}\hbox { day}^{-1}$$ , respectively). The gravel surface without loose sand and without a crust presents the lowest values of PM63 ( $$1.6 \times 10^{-3 }\hbox { kg m}^{-1}\hbox { day}^{-1}$$ ) and PM10 ( $$3.3 \times 10^{-4}\hbox { kg m}^{-1}\hbox { day}^{-1}$$ ). However, the vertical dust flux was largest at over sandy surface ( $$373 \times 10^{-3 }\hbox { kg m}^{-2}\hbox { day}^{-1}$$ ). Multivariate correlation analysis indicates that the quantity of PM10 is strongly negatively correlated to gravel coverage ( $$R^{2 }= 0.55$$ ). The quantity of PM10 dust emissions over a gravel surface with loose sand is approximately three times greater than that of a gravel surface with a crust. The mean quantity of PM10, PM63 and vertical dust flux over a gravel surface decreased with increasing gravel coverage. By comparing the quantity of PM10 dust emissions over gravel and sandy deserts, we found that gravel deserts and sandy deserts are both major sources of dust for dust storms in this region.

Estimation of micrometeorites and satellite dust flux surrounding Mars in the light of MAVEN results

Recently, MAVEN observed dust around Mars from ∼150km to ∼1000km and it is a puzzling question to the space scientists about the presence of dust at orbital altitudes and about its source.A continuous supply of dust from various sources could cause existence of dust around Mars and it is expected that the dust could mainly be from either the interplanetary source or the Phobos/Deimos. We have studied incident projectiles or micrometeorites at Mars using the existing model, in this article. Comparison of results with the MAVEN results gives a new value of the population index S, which is reported here. The index S has been referred in a power law model used to describe the number of impacting particles on Mars.In addition, the secondary ejecta from natural satellites of Mars can cause a dust ring or torus around Mars and remain present for its lifetime. The dust particles whose paths are altered by the solar wind over its lifetime, could present a second plausible source of dust around Mars. We have investigated escaping particles from natural satellites of Mars and compared with the interplanetary dust flux estimation. It has been found that flux rate at Mars is dominated (∼2 orders of magnitude higher) by interplanetary particles in comparison with the satellite originated dust. It is inferred that the dust at high altitudes of Mars could be interplanetary in nature and our expectation is in agreement with the MAVEN observation. As a corollary, the mass loss from Martian natural satellites is computed based on the surface erosion by incident projectiles.