Slope streaks on Mars: A new “wet” mechanism

Slope steaks are one of the most intriguing modern phenomena observed on Mars. They have been mostly interpreted as some specific type of granular flow. We propose another mechanism for slope streak formation on Mars. It involves natural seasonal formation of a modest amount of highly concentrated chloride brines within a seasonal thermal skin, and runaway propagation of percolation fronts. Given the current state of knowledge of temperature regimes and the composition and structure of the surface layer in the slope streak regions, this mechanism is consistent with the observational constraints; it requires an assumption that a significant part of the observed chlorine to be in form of calcium and ferric chloride, and a small part of the observed hydrogen to be in form of water ice. This “wet” mechanism has a number of appealing advantages in comparison to the widely accepted “dry” granular flow mechanism. Potential tests for the “wet” mechanism include better modeling of the temperature regime and observations of the seasonality of streak formation.     

Impact of non-outbreak insect damage on vegetation in northern Europe will be greater than expected during a changing climate

Background insect herbivory, in addition to insect outbreaks, can have an important long term influence on the performance of tree species. Since a projected warmer climate may favour insect herbivores, we use a dynamic ecosystem model to investigate the impacts of background herbivory on vegetation growth and productivity, as well as distribution and associated changes in terrestrial ecosystems of northern Europe. We used the GUESS ecosystem modelling framework and a simple linear model for including the leaf area loss of Betula pubescens in relation to mean July temperature. We tested the sensitivity of the responses of the simulated ecosystems to different, but realistic, degrees of insect damage. Predicted temperature increases are likely to enhance the potential insect impacts on vegetation. The impacts are strongest in the eastern areas, where potential insect damage to B. pubescens can increase by 4–5%. The increase in insect damage to B. pubescens results in a reduction of total birch leaf area (LAI), total birch biomass and birch productivity (Net Primary Production). This effect is stronger than the insect damage to leaf area alone would suggest, due to its second order effect on the competition between tree species. The model's demonstration that background herbivory may cause changes in vegetation structure suggests that insect damage, generally neglected by vegetation models, can change predictions of future forest composition. Carbon fluxes and albedo are only slightly influenced by background insect herbivory, indicating that background insect damage is of minor importance for estimating the feedback of terrestrial ecosystems to climate change.     

Climate signals in sediment mineralogy of Lake Baikal and Lake Hovsgol during the LGM-Holocene transition and the 1-Ma carbonate record from the HDP-04 drill core

Modeling the bulk sediment XRD patterns allows insight into the environmental and depositional histories of two neighboring rift lake basins within the Baikal watershed. Parallel ¹⁴C-dated LGM-Holocene records in Lakes Baikal and Hovsgol are used to discuss the mineralogical signatures of regional climate change. In both basins, it is possible to distinguish ‘glacial’ and ‘interglacial’ mineral associations. Clay minerals comprise in excess of 50% of layered silicates in bulk sediment.The abundance of smectite (expandable) layers in mixed-layer illite–smectites and the total illite abundance are the main paleoclimatic indices in the clay mineral assemblage. Both indices exhibit coherent responses to the Bølling–Allerød and the Younger Dryas. The smectite layer index is not equivalent to the abundance of illite–smectite, because illite–smectite tends to transform into illite. Repeated wetting–drying cycles in soils and high abundance of expandable layers in illite–smectites (>42%) favor the process of illitization. This relationship is clearly shown in both Baikal and Hovsgol records for the first time. The opposite late Holocene trends in illite abundance in Lake Baikal and Lake Hovsgol records suggest that a sensitive optimal regime may exist for illite formation in the Baikal watershed with regard to warmth and effective moisture.The Lake Hovsgol sediments of the last glacial contain carbonates, suggesting a positive trend in the lake's water budget. A progressive change towards lower Mg content in carbonates indicates lowering mineralization of lake waters. This trend is consistent with the lithologic evidence for lake-level rise in the Hovsgol basin.The pattern of mineralogical changes during the past 20 ka is used to interpret bulk sediment and carbonate mineralogy of the long 81-m Lake Hovsgol drill core (HDP-04) with a basal age of 1 Ma. The interglacial-type silicate mineral associations are confined to several thin intervals; most of the sediment record is calcareous. Carbonates are represented by six main mineral phases: calcite, low-Mg calcite, intermediate/high-Mg calcite, dolomite, excess-Ca dolomite and metastable monohydrocalcite. These mineral phases tend to form stratigraphic successions indicative of progressive changes in lake water chemistry. Five sediment layers with abundant Mg-calcites in the HDP-04 section suggest deposition in a low standing lake with high mineralization (salinity) and high Mg/Ca ratios of lake waters. Lake Hovsgol sediments contain the oldest known monohydrocalcite, found tens of meters below lake bottom in sediments as old as 800 ka. This unusual find is likely due to the conditions favorable to preservation of this metastable carbonate.     

Improving the crescent visibility limits due to factors causing decrease in the sky twilight brightness

The measured sky twilight brightness of a site is believed to be connected with main factors such as geographical latitude, elevation of the site above sea level, the season of observations and the aerosol pollution. These factors may decrease the sky twilight brightness and thus improve the crescent visibility limits. The effect of these factors on the visibility conditions to see the new Moon are investigated in the present work. The results show that the visibility conditions has improved at sites situated at higher northern geographical latitudes and higher elevation above sea level. The conditions to see the new Moon in winter season is better than the summer season. The aerosol pollution has a great effect on the visibility of the new Moon at sun's depression 5°, while for sun's depression greater than 5° the aerosol pollution has a small effect.     

Decline in Length of the Summer Season on the Kola Peninsula, Russia

By analysing records made in the northern taiga forests of the Lapland Reserve (Kola Peninsula, Russia) during 1930–1998, we unexpectedly discovered a decline in the length of the snow-free and ice-free periods by 15–20 days due to both delayed spring and advanced autumn/winter. Respective seasonal temperatures best explained the dates of all phenological phases: 1 °C shift in temperature was approximately equal to 2–5 day shift in phenology. However the phenological shiftsduring the observation period are much larger than could be expected from the slight (0.56 °C) drop in temperatures during August–September, suggesting that the biotic effects of a very slight cooling have been enhanced by one or more unknown factors. Although emissions of sulphur dioxide from the nickel-copper smelter at Monchegorsk may have contributed to the observed trend (via changes in regional radiative budget), we found no evidence of direct pollution impact on dates of birch autumnal coloration or birch leaf fall, which exhibited the largest (22 days) shift between 1930 and 1998. The detected phenological trends agree with an increase in winter (snow) precipitation in the study area by 44%; however, effects of precipitation on any of the investigated phenological phases were far from significant. Our results highlight the importance of phenological records for the assessment of past regional environmental changes, and demonstrates that the prediction of even the simplest biotic responses to the Global Changes requires a profound understanding of the interactive impact of abiotic factors on the ecosystem.     

Effect of soil structure on soil fauna in a desert wadi in Southern Egypt

This study investigates the soil fauna in a part of the rainless desert of southern Egypt. Three sites were examined: two downstream of Wadi Allaqi, both of which have been inundated by the water of Lake Nasser for several years. The third site is in Wadi Quleib, a principal tributary of Wadi Allaqi which was not inundated. Soil fauna were collected by pitfall traps during April-November 1989. Data were treated by multivariate analysis techniques, including correspondence analysis and ascending hierarchic classification. Most species were Arthropoda, particularly Coleoptera (7 species). Wadi Quleib was richer than Wadi Allaqi both in number of species and density. In Wadi Allaqi, the first site was richer than the second, both in species and density. These differences are partially related to differences in soft texture between Wadi Quleib and Wadi Allaqi, and to the presence of dense groves of Tamarix nilotica in the first site of Wadi Allaqi.     

Experimental investigation of single carbon compounds under hydrothermal conditions

The speciation of carbon in subseafloor hydrothermal systems has direct implications for the maintenance of life in present-day vent ecosystems and possibly the origin of life on early Earth. Carbon monoxide is of particular interest because it represents a key reactant during the abiotic synthesis of reduced carbon compounds via Fischer-Tropsch-type processes. Laboratory experiments were conducted to constrain reactions that regulate the speciation of aqueous single carbon species under hydrothermal conditions and determine kinetic parameters for the oxidation of CO according to the water water-gas shift reaction (CO₂ + H₂ = CO + H₂O). Aqueous fluids containing added CO₂, CO, HCOOH, NaHCO₃, NaHCOO, and H₂ were heated at 150, 200, and 300 °C and 350 bar in flexible-cell hydrothermal apparatus, and the abundances of carbon compounds was monitored as a function of time. Variations in fluid chemistry suggest that the reduction of CO₂ to CH₃OH under aqueous conditions occurs via a stepwise process that involves the formation of HCOOH, CO, and possibly CH₂O, as reaction intermediaries. Kinetic barriers that inhibit the reduction of CH₃OH to CH₄ allow the accumulation of reaction intermediaries in solution at high concentrations regulated by metastable thermodynamic equilibrium. Reaction of CO₂ to CO involves a two-step process in which CO₂ initially undergoes a reduction step to HCOOH which subsequently dehydrates to form CO. Both reactions proceed readily in either direction. A preexponential factor of 1.35 × 10⁶ s⁻¹ and an activation energy of 102 kJ/mol were retrieved from the experimental results for the oxidation of CO to CO₂. Reaction rates amongst single carbon compounds during the experiments suggest that ΣCO₂ (CO₂ + HCO₃⁻ + CO₃²⁻), CO, ΣHCOOH (HCOOH + HCOO⁻), and CH₃OH may reach states of redox-dependent metastable thermodynamic equilibrium in subseafloor and other hydrothermal systems. The abundance of CO under equilibrium conditions is strongly dependent on temperature, the total carbon content of the fluid, and host-rock lithology. If crustal residence times following the mixing of high-temperature hydrothermal fluids with cool seawater are sufficiently long, reequilibration of aqueous carbon can result in the generation of additional reduced carbon species such as HCOOH and CH₃OH, and the consumption of H₂. The present study suggests that abiotic reactions involving aqueous carbon compounds in hydrothermal systems are sufficiently rapid to influence metabolic pathways utilized by organisms that inhabit vent environments.     

Mercury distribution and transport in a contaminated river system in Kazakhstan and associated impacts on aquatic biota

The River Nura in Central Kazakhstan has been heavily polluted by Hg originating from an acetaldehyde plant. A number of studies were undertaken to investigate the transport, fate and bioavailability of Hg in this river system. The sediments within a 20 km section of the river downstream of the effluent outfall canal are highly polluted and are acting as a strong source of surface water contamination. Mercury transport in the river is dominated by the remobilization of contaminated bed sediments and river bank erosion during the annual spring flood. Peak Hg concentrations in unfiltered surface water samples during a larger than usual flood event in 2004 were in the order of 1600–4300 ng L⁻¹. The majority of the particulate-bound Hg appears to be sedimented in the shallow Intumak reservoir 75 km downstream of the source of the pollution, leading to a drop in aqueous Hg concentrations by an order of magnitude. Nevertheless, background concentrations of Hg in surface water are not reached until at least 200 km downstream, and during the flood period Hg is also detected in the terminal wetlands of the river.Mercury concentrations in sediment cores taken from the river bed in the most contaminated section of the Nura ranged from 9.95 to 306 mg kg⁻¹. Methylmercury (MeHg) levels in shallow sediment cores were highest in surface sediments and ranged between 4.9 and 39 μg kg⁻¹, but were generally less than 0.1% of total Hg (THg). A significant inverse relationship was found between THg concentrations and the percentage of MeHg formed in the sediments, irrespective of the sampling depth. The observed relationship was confirmed by comparison with results from a different river system, indicating that it may be true also for other highly contaminated aquatic systems. It is hypothesized that at high THg levels in severely contaminated sediments, the accumulation of MeHg may be limited by increasingly efficient demethylation processes, and that this underlying trend in sediments is the reason why MeHg levels in surface water are often found to be higher at less contaminated sites compared to upstream sites.Mercury concentrations in biota in the most contaminated section of the river were 15–20 times higher than background levels. Fish were found to be impacted for more than 125 km downstream from the source, indicating significant transport of dissolved MeHg to downstream areas and/or in-situ MeHg production in less contaminated downstream reaches. There were also indications that impoundments may increase the bioavailability of Hg.     

Erratum to: Neutron Star Mergers and Gamma-Ray Bursts: Stripping Model

Astronomy Reports - An Erratum to this paper has been published: https://doi.org/10.1134/S1063772921340011