Photophoretic Structuring of Circumstellar Dust Disks

We study dust accumulation by photophoresis in optically thin gas disks. Using formulae of the photophoretic force that are applicable for the free molecular regime and for the slip-flow regime, we calculate dust accumulation distances as a function of the particle size. It is found that photophoresis pushes particles (smaller than 10 cm) outward. For a Sun-like star, these particles are transported to 0.1-100 AU, depending on the particle size, and forms an inner disk. Radiation pressure pushes out small particles (< 1 mm) further and forms an extended outer disk. Consequently, an inner hole opens inside ~0.1 AU. The radius of the inner hole is determined by the condition that the mean free path of the gas molecules equals the maximum size of the particles that photophoresis effectively works on (100 micron - 10 cm, depending on the dust property). The dust disk structure formed by photophoresis can be distinguished from the structure of gas-free dust disk models, because the particle sizes of the outer disks are larger, and the inner hole radius depends on the gas density.Comment: 15 pages, 9 figures, Accepted by ApJ; corrected a typo in the author nam

Influence of primary particle density in the morphology of agglomerates

Agglomeration processes occur in many different realms of science such as colloid and aerosol formation or formation of bacterial colonies. We study the influence of primary particle density in agglomerate structure using diffusion-controlled Monte Carlo simulations with realistic space scales through different regimes (DLA and DLCA). The equivalence of Monte Carlo time steps to real time scales is given by Hirsch's hydrodynamical theory of Brownian motion. Agglomerate behavior at different time stages of the simulations suggests that three indices (fractal exponent, coordination number and eccentricity index) characterize agglomerate geometry. Using these indices, we have found that the initial density of primary particles greatly influences the final structure of the agglomerate as observed in recent experimental works.Comment: 11 pages, 13 figures, PRE, to appea

Radiometric force in dusty plasmas

A radiofrequency glow discharge plasma, which is polluted with a certain number of dusty grains, is studied. In addition to various dusty plasma phenomena, several specific colloidal effects should be considered. We focus on radiometric forces, which are caused by inhomogeneous temperature distribution. Aside from thermophoresis, the role of temperature distribution in dusty plasmas is an open question. It is shown that inhomogeneous heating of the grain by ion flows results in a new photophoresis like force, which is specific for dusty discharges. This radiometric force can be observable under conditions of recent microgravity experiments.Comment: 4 pages, amsmat

Stratigraphic control of landscape response to base-level fall, Young Womans Creek, Pennsylvania, USA

Landscapes are thought to respond to changes in relative base level through the upstream propagation of a boundary that delineates relict from adjusting topography. However, spatially-variable rock strength can influence the topographic expression of such transient landscapes, especially in layered rocks, where strength variations can mask topographic signals expected due to changes in climate or tectonics. Here, we analyze the landscape response to base-level fall in Young Womans Creek, a 220 km2 catchment on the Appalachian Plateau, USA underlain by gently folded Paleozoic sedimentary rocks. We measured in situ 10Be concentrations in stream sands from 17 nested watersheds, and used a spatially-distributed model of sediment and 10Be production to constrain a threefold increase in the rate of base-level fall propagating upstream from the catchment outlet. Using lidar topography and a nearby detailed stratigraphic section, we map the extent of continuous, blocky, resistant sandstone strata that act as a caprock overlying more easily erodible sandstones and siltstones. The caprock influences landscape response in two ways. First, it serves as a boundary between slowly eroding (11.5 m Myr−1), low-sloping (3–5°) areas of relict topography and lower, steeper portions of the landscape adjusting to base-level fall. Second, hillslopes supported by the overlying caprock are armored with coarse sediment and are significantly steeper (20–30°) than hillslopes where the caprock has been eroded (10°), despite having similar erosion rates (36 m Myr−1) and bedrock substrate. Our results illustrate how gently dipping, layered rocks engender complicated relationships between lithology, topography and erosion rate, highlighting the importance of understanding how rock material properties influence surface processes and landscape evolution

Antarctic permafrost processes and antiphase dynamics of cold-based glaciers in the McMurdo Dry Valleys inferred from 10Be and 26Al cosmogenic nuclides

Soil and sediment mixing and associated permafrost processes are not widely studied or understood in the McMurdo Dry Valleys of Antarctica. In this study, we investigate the stability and depositional history of near-surface permafrost sediments to ∼ 3 m depth in the Pearse and lower Wright valleys using measured cosmogenic 10Be and 26Al depth profiles. In Pearse Valley, we estimate a minimum depositional age of ∼ 74 ka for the active layer and paleoactive-layer sediments (&lt; 0.65 m). Combined depth profile modelling of 10Be and 26Al gives a depositional age for near-surface (&lt; 1.65 m) permafrost in Pearse Valley of 180 +20/-40 ka, implying that the deposition of permafrost sediments predates MIS 5 advances of Taylor Glacier. Deeper permafrost sediments (&gt; 2.09 m) in Pearse Valley are thus inferred to have a depositional age of &gt; 180 ka. At a coastal, lower-elevation site in neighbouring lower Wright Valley, 10Be and 26Al depth profiles from a second permafrost core exhibit near-constant concentrations with depth and indicate the sediments are either vertically mixed after deposition or sufficiently young so that post-depositional nuclide production is negligible relative to inheritance. 26Al/10Be concentration ratios for both depth profiles range between 4.0 and 5.2 and are all lower than the nominal surface production rate ratio of 6.75, indicating that prior to deposition, these sediments experienced complex, yet similar, exposure–burial histories. Assuming a single-cycle exposure–burial scenario, the observed 26Al/10Be ratios are equivalent to a total minimum exposure–burial history of ∼ 1.2 Myr. In proximity to the depth profile core site, we measured cosmogenic 10Be and 26Al in three granite cobbles from thin, patchy drift (Taylor 2 Drift) in Pearse Valley to constrain the timing of retreat of Taylor Glacier. Assuming simple continuous exposure, our minimum, zero-erosion exposure ages suggest Taylor Glacier partially retreated from Pearse Valley no later than 65–74 ka. The timing of retreat after 65 ka and until the Last Glacial Maximum (LGM) when Taylor Glacier was at a minimum position remains unresolved. The surface cobble ages and permafrost processes reveal Taylor Glacier advances during MIS 5 were non-erosive or mildly erosive, preserving the underlying permafrost sediments and peppering boulders and cobbles upon an older, relict surface. Our results are consistent with U/Th ages from central Taylor Valley and suggest changes in moisture delivery over Taylor Dome during MIS 5e, 5c, and 5a appear to be associated with the extent of the Ross Ice Shelf and sea ice in the Ross Sea. These data provide further evidence of antiphase behaviour through retreat of a peripheral lobe of Taylor Glacier in Pearse Valley, a region that was glaciated during MIS 5. We suggest a causal relationship of cold-based glacier advance and retreat that is controlled by an increase in moisture availability during retreat of sea ice and perhaps the Ross Ice Shelf, as well as, conversely, a decrease during times of sea ice and Ross Ice Shelf expansion in the Ross Sea.</p

Short communication: Cosmogenic noble gas depletion in soils by wildfire heating

Measurements of cosmic-ray-produced beryllium-10, neon-21, and helium-3 in quartz in a soil profile from a forested landscape in the Oregon Coast Range show that the cosmogenic noble gases 21Ne and 3He are depleted relative to 10Be in the shallow subsurface. The noble gases are mobile in mineral grains via thermally activated diffusion and 10Be is not, implying that noble gas depletion is the result of surface heating by wildfires and subsequent mixing of partially degassed quartz downward into the soil. Cosmogenic noble gas depletion by wildfire heating of soils is a potential means of estimating wildfire intensity and/or frequency over pre-observational timescales.</p

Observations of aerosols over Southern California coastal waters

To characterize the background aerosol in air off the California coast, observations of suspended particles were made in the summer and fall of 1970 on San Nicolas Island (SNI), located about 130 km west-south-west of Los Angeles. Measurements of the physical and chemical properties of aerosols showed that the particles are a complex mixture of material from marine and continental origins. The Aitken nuclei population averaged 2400 cm−3, while the particles >0.5 μm diameter averaged 20–100 cm−3. The average total mass concentration of aerosol evaluated from 22 samples was 29.8 μg m−3, but the refined fraction defined as particles ~< 3.5 μm diameter was 40% of this level. The averages of the chemical analysis of 13 samples revealed that 11% of the aerosol sampled at a 200 m height above the ocean was sea salt, while approximately 20% evidently was soil dust, as indicated by silicates. Over 25% of the suspended material was found to be sulfate, nitrate or ammonium, which are constituents believed to be produced primarily from gaseous transformation reactions in the atmosphere. More than 20% of the sampled material was volatile in nature and is believed to be partly water. The ratio for the average chlorine to sodium concentrations from the analysis of 13 different samples was 2.4 compared with 1.8 for sea water. This high ratio supports other limited results for unpolluted air from offshore sources taken in this geographical region. The anomaly is believed to be linked with chemical reactions transforming gaseous chlorine compounds into condensed material. Comparison was made between the aerosol sampled on San Nicolas Island and the “natural” background contribution of the Los Angeles smog aerosol, as estimated elsewhere assuming contributions solely from sea salt and soil dust. The calculated background and the composition of the SNI aerosol were qualitatively similar, with the principal differences showing in Ca, Zn, SO4− and NO3−. Further comparison with aerosol analyzed from Pasadena illustrated the major contribution of localized pollution to the chemical properties of the particles, particularly in the concentrations of Mu, Ca, Br, Fe, Pb, SO4− and NO3−

Sensitivity of simulated soil water content, evapotranspiration, gross primary production and biomass to climate change factors in Euro-Mediterranean grasslands

Grassland models often yield more uncertain outputs than arable crop models due to more complex interactions and the largely undocumented sensitivity of grassland models to environmental factors. The aim of the present study was to assess the impact of single-factor changes in temperature, precipitation, and atmospheric [CO2] on simulated soil water content (SWC), actual evapotranspiration (ET), gross primary production (GPP) and yield biomass, and also to link the sensitivity analysis with experimental results. We employed an unprecedented multi-model framework consisting of seven grassland models at nine sites with different environmental characteristics in Europe and Israel, with two management options at three sites. For warming/cooling and wetting/drying, models showed general consistency in the direction of SWC and ET changes, but less agreement regarding GPP and biomass changes. The simulated responses consistently revealed an overall positive effect of CO2 enrichment on GPP and biomass, while the direction of change differed for SWC and ET. Comparing with single-factor experimental manipulations, SWC simulations slightly underestimated the observed effect of warming, while the overall mean model sensitivity for biomass (+7.5%) closely matched the mean response observed with 1–2 °C warming (+6.6%). The models exhibited lower sensitivity of SWC to wetting or drying compared to the experiments. The overall mean sensitivity of biomass to drying was -4.3%, contrasting with the mean experimental effect size of -9.6%, which proved to be more realistic than the mean wetting effect (+3.2%, against +38.9% in the field trials). The simulated sensitivity of SWC to CO2 enrichment was markedly underestimated, while the biomass response (+12.0%) closely matched the observations (+17.5%). Although the multi-model averaging did not manifestly improve the realism of the simulations, it ensured a realistic response in the direction of change to varying conditions. The results suggest a paradigm shift in grassland modelling meaning that the usual practice of model optimisation/validation needs to be complemented by a sensitivity analysis following the approach presented. The results also highlight the importance of model improvements, especially in terms of soil hydrology representation, a key environmental driver of grassland functioning

Measuring multiple cosmogenic nuclides in glacial cobbles sheds light on Greenland Ice Sheet processes

The behavior of the Greenland Ice Sheet during the Pleistocene remains uncertain due to the paucity of evidence predating the Last Glacial Maximum. Here, we employ a novel approach, cosmogenic nuclide analysis of individual subglacially-derived cobbles, which allows us to make inferences about ice sheet processes and subglacial erosion. From three locations in western Greenland, we collected 86 cobbles from the current ice sheet margin and nine cobbles exposed on the modern proglacial land surface. We measured the concentration of in situ 10Be in all cobbles (n = 95) and 26Al and 14C in a subset (n = 14). Cobbles deposited during Holocene retreat have 10Be exposure ages generally consistent with the timing of ice retreat determined by other methods. Conversely, most of the 86 subglacial cobbles contain very low concentrations of 10Be (median 1.0×10 3 atoms g −1), although several have ∼10 4 and one has ∼10 5 atoms g −1. The low concentrations of 10Be in most subglacial cobbles imply that their source areas under the Greenland Ice Sheet are deeply eroded, preserving minimal evidence of surface or near-surface exposure. The presence of measurable 14C in ten of the cobbles requires that they experienced cosmogenic nuclide production within the past ∼30 ka; however, 14C/ 10Be ratios of ∼6 suggest that nuclide production occurred during shielding by overlying material. Only two of the 86 subglacial cobbles definitively have cosmogenic nuclide concentrations consistent with prior surface exposure. Overall, isotopic analysis of subglacial cobbles indicates that much of western Greenland's subglacial landscape is characterized by deep erosion and minimal subaerial exposure

Cosmogenic nuclide and solute flux data from central Cuban rivers emphasize the importance of both physical and chemical mass loss from tropical landscapes

We use 25 new measurements of in situ produced cosmogenic 26Al and 10Be in river sand, paired with estimates of dissolved load flux in river water, to characterize the processes and pace of landscape change in central Cuba. Long-term erosion rates inferred from 10Be concentrations in quartz extracted from central Cuban river sand range from 3.4–189 Mg km−2 yr−1 (mean 59, median 45). Dissolved loads (10–176 Mg km−2 yr−1; mean 92, median 97), calculated from stream solute concentrations and modeled runoff, exceed measured cosmogenic-10Be-derived erosion rates in 18 of 23 basins. This disparity mandates that in this environment landscape-scale mass loss is not fully represented by the cosmogenic nuclide measurements. The 26Al / 10Be ratios are lower than expected for steady-state exposure or erosion in 16 of 24 samples. Depressed 26Al / 10Be ratios occur in many of the basins that have the greatest disparity between dissolved loads (high) and erosion rates inferred from cosmogenic nuclide concentrations (low). Depressed 26Al / 10Be ratios are consistent with the presence of a deep, mixed, regolith layer providing extended storage times on slopes and/or burial and extended storage during fluvial transport. River water chemical analyses indicate that many basins with lower 26Al / 10Be ratios and high 10Be concentrations are underlain at least in part by evaporitic rocks that rapidly dissolve. Our data show that when assessing mass loss in humid tropical landscapes, accounting for the contribution of rock dissolution at depth is particularly important. In such warm, wet climates, mineral dissolution can occur many meters below the surface, beyond the penetration depth of most cosmic rays and thus the production of most cosmogenic nuclides. Our data suggest the importance of estimating solute fluxes and measuring paired cosmogenic nuclides to better understand the processes and rates of mass transfer at a basin scale.</p