Impacts of recurring ice jams on channel geometry and geomorphology in a small high-boreal watershed

River ice jams are generally perceived as significant erosive events and are well known to impact both channel morphology and geometry. However, the extent of these impacts and the frequency of events required to maintain erosion-induced morphologies remain unexplored in most cold region watersheds. In this study, we investigated downstream variations in channel width, cross-sectional area, depth, and geomorphological characteristics in a small high-boreal basin. We coupled these observations to dendrochronological data on ice jam frequency. Our results show that channels affected by ice erosion appear enlarged and present an important retreat of the upper bank. Such enlarged channels present a typical two-level, ice-scoured morphology when ice jams recur more often than once every 5 years. By contrast, channels appear unaffected when ice jams are less frequent. These results suggest that ice jams maintain ice-scoured and enlarged morphologies once a minimal frequency-of-occurrence threshold is exceeded. We therefore conclude that ice jam frequencies should be taken into account in order to better define the role of ice as a geomorphological agent in cold environments.     

Lessons of Comet Hale-Bopp for Coma Chemistry: Observations and Theory

Many new cometary molecules — both parents and daughters — were detected in the exceptionally productive comet C/1995 O1 (Hale-Bopp).The space distribution of several of these species could be investigated from radio interferometry or from long-slit spectroscopy in the infrared. The distinction between parent species — directly sublimated from nucleus ices — and secondary species — resulting from chemical processing in the coma or produced by a secondary source — is not always clear. It is important to assess whether or not observed minor species (HCOOCH₃, HCOOH...) could be synthesized by chemical reactions favoured by the high density of the coma of comet Hale-Bopp. Chemical modelling by Rodgers and Charnley suggests that this is notthe case. CO and H₂CO are abundant cometary species which partly come from distributed sources. The nature of these sources is still a mystery. A special case, now well documented, is that of HNC, for which the abundance evolution with heliocentric distance could be observed in comet Hale-Bopp and which was observed in several much less productive comets.     

Lithospheric mantle beneath the south-eastern Siberian craton: petrology of peridotite xenoliths in basalts from the Tokinsky Stanovik

We provide petrographic, major and trace element data for over 30 spinel peridotite xenoliths from the Tokinsky Stanovik (Tok) volcanic field on the Aldan shield to characterize the lithospheric mantle beneath the south-eastern margin of the Siberian craton, which formed in the Mesoproterozoic. High equilibration temperatures (870–1,010°C) of the xenoliths and the absence of garnet-bearing peridotites indicate a much thinner lithosphere than in the central craton. Most common among the xenoliths are clinopyroxene-poor lherzolites and harzburgites with Al₂O₃ and CaO contents nearly as low as in refractory xenoliths from kimberlite pipes (Mir, Udachnaya) in the central and northern Siberian craton. By contrast, the Tok peridotites have higher FeO, lower Mg-numbers and lower modal orthopyroxene and are apparently formed by shallow partial melting (3 GPa). Nearly all Tok xenoliths yield petrographic and chemical evidence for metasomatism: accessory phlogopite, amphibole, phosphates, feldspar and Ti-rich oxides, very high Na₂O (2–3.1%) in clinopyroxene, LREE enrichments in whole-rocks.Electronic Supplementary Material Supplementary material is available for this article at     

The transition of the electricity system towards decarbonization: the need for change in the market regime

The mainstream community of energy experts is not aware of the long-term impacts that carbon policies directly concerned with promoting the development of low-carbon technologies produce on the electricity market regime. Long-term market coordination should be replaced by public coordination with long-term arrangements. The current market coordination makes carbon pricing ineffective in orienting investors towards capital-intensive low-carbon technologies. Fossil fuel generation technologies are preferred because their investment risks are much lower in the market regime, even with a high but unstable carbon price. Thus, in order to avoid delaying investment that is aimed at the decarbonization of the electricity system, a number of new market arrangements that lower the investment risk of low-carbon technologies and provide output-based subsidization have or are being selected by governments. As the use of low-carbon equipment to produce electricity develops, long-term market coordination for other technologies (e.g. peaking units, combined cycle gas turbine) will fade away because they alter the market price setting. Thus it is likely that, in the future, public coordination and planning will replace the decisions of market players not only for low-carbon technologies but also for every other type of capacity development.

Policy relevance

The development of renewables as promoted by both feed-in tariffs and green certificate obligations, which answer to different market failures, is well known. Similar long-term arrangements, which both subsidize and de-risk low-carbon investments for every small-sized and large-sized technology, shift learning costs and risks onto consumers. Energy experts and regulators have ignored that the expansion and generalization of these arrangements are changing the coordination function of the electricity markets. Apart from those in the UK, they are still unaware of the impacts that such technology-focused policies produce on the electricity market regime. The transition from market coordination to public coordination, which is inconsistent with the market principles of European electricity legislation, and long-term contracting is inevitable and should be anticipated.     

The Key Tuffite,Matagami Camp,Abitibi Greenstone Belt,Canada: petrogenesis and implications for VMS formation and exploration

The Key Tuffite is a stratigraphic marker unit for most of the zinc-rich volcanogenic massive sulfide deposits of the Matagami Camp in the Abitibi Greenstone Belt. This 2- to 6-m-thick unit was previously interpreted as a mixture of ash fall (andesitic to rhyolitic tuffaceous components) and volcanogenic massive sulfide (VMS)-related chemical seafloor precipitate (exhalative component). Previous attempts to develop geochemical exploration vectoring tools using metal content within the Key Tuffite were mostly inconclusive due to the complex nature of the Key Tuffite unit and a poor understanding of its composition, origin and relationship with the VMS-forming hydrothermal systems. Detailed mapping and thorough lithogeochemistry of the Key Tuffite in the vicinity of the Perseverance and Bracemac-McLeod deposits indicate that the Key Tuffite is a homogeneous calc-alkaline, andesitic tuff that was deposited before the VMS deposits were formed. The unit is mostly devoid of exhalative component, but it is strongly hydrothermally altered close to orebodies. This is characterized by a strong proximal chloritization and a distal sericitization, which grades laterally into the unaltered Key Tuffite. Neither the Key Tuffite nor the ore was formed by seafloor exhalative processes for the two studied deposits. This probably explains why previously proposed exploration models based on metal scavenging proved unsuccessful and suggests that a re-evaluation of the exhalative model should be done at the scale of the mining camp. However, as shown in this study, hydrothermal alteration can be used to vector towards ore along the Key Tuffite.     

The sodium exosphere of Mercury: Comparison between observations during Mercury's transit and model results

In this study we compare the sodium exosphere observations made by Schleicher et al. [Schleicher, H., and 4 colleagues, 2004. Astron. Astrophys. 425, 1119-1124] with the result of a detailed numerical simulation. The observations, made during the transit of Mercury across the solar disk on 7 May 2003, show a maximum of sodium emission near the polar regions, with north prevalence, and the presence of a dawn-dusk asymmetry. We interpret this distribution as the resulting effect of two combined processes: the solar wind proton precipitation causing chemical alteration of the surface, freeing the sodium atoms from their bounds in the crystalline structure on the surface, and the subsequent photon-stimulated and thermal desorption of the sodium atoms. While we find that the velocity distribution of photon desorbed sodium can explain the observed exosphere population, thermal desorption seems to play a minor role only causing a smearing at the locations where Na atoms are released on the dayside. The observed and simulated distributions agree very well with this hypothesis and indicate that the combination of the proposed processes is able to explain the observed features.     

Boron isotopes as pH proxy: A new look at boron speciation in deep-sea corals using B MAS NMR and EELS

Dissolved boron in modern seawater occurs in the form of two species, trigonal boric acid B(OH)₃ and tetrahedral borate ion . One of the key assumption in the use of boron isotopic compositions of carbonates as pH proxy is that only borate ions, , are incorporated into the carbonate. Here, we investigate the speciation of boron in deep-sea coral microstructures (Lophelia pertusa specimen) by using high field magic angle spinning nuclear magnetic resonance (¹¹B MAS NMR) and electron energy-loss spectroscopy (EELS). We observe both boron coordination species, but in different proportions depending on the coral microstructure, i.e. centres of calcification versus fibres. These results suggest that careful sampling is necessary before performing boron isotopic measurements in deep-sea corals. By combining the proportions of B(OH)₃ and determined by NMR and our previous ion microprobe boron isotope measurements, we propose a new equation for the relation between seawater pH and boron isotopic composition in deep-sea corals.