Numerical simulation of comet nuclei I. Water-ice comets

A one-dimensional simulation of pure water-ice cometary nuclei is presented, and the effect of the nucleus as a heat reservoir is considered. The phase transition from amorphous to crystalline ice is studied for two cases: (1) where the released latent heat goes entirely into heating adjacent layers and (2) where the released latent heat goes entirely into sublimation. For a Halley-like orbit it was found that for case 1 the phase boundary penetrates about 15 m on the first orbit and does not advance until sublimation brings the surface to some 10 m from the phase boundary. For case 2 the phase boundary penetrates about 1 m below the surface and remains at this depth as the surface sublimates. For an orbit like that of Schwassmann-Wachmann 1 the phase boundary penetrates about 50 m initially for case 1 and about 1 m for case 2. There is no further transformation until the entire comet is heated slowly to near the transition temperature, after which the entire nucleus is converted to crystalline ice. For an Encke-type orbit case 1 gives a nearly continuous transition of the entire nucleus to crystalline ice, while for case 2 the initial penetration is about 8 m and remains at this depth relative to the surface as sublimation decreases the cometary radius. Thus the entire comet is converted to crystalline ice just before it is completely dissipated.     

Oil-weathering behavior in Arctic environments

Oil-weathering processes in ice-free subarctic and Arctic waters include spreading, evaporation, dissolution, dispersion of whole-oil droplets into the water column, photochemical oxidation, water-in-oil emulsification, microbial degradation, adsorption onto suspended particulate material, ingestion by organisms, sinking, and sedimentation. While many of these processes also are important factors in ice-covered waters, the various forms of sea ice (depending on the active state of ice growth, extent of coverage and/or decay) impart drastic, if not controlling, changes to the rates and relative importance of different oil-weathering mechanisms. Flow-through seawater wave-tank experiments in a cold room at −35°C and studies in the Chukchi Sea in late winter provide data on oil fate and effects for a variety of potential oil spill scenarios in the Arctic. Time-series chemical weathering data are presented for Prudhoe Bay crude oil released under and encapsulated in growing first-year columnar ice through spring breakup.     

Numerical simulations of comet nuclei: II. The effect of the dust mantle

The insulating effect of an evolving dust mantle is examined. The role of this mantle in determining the surface temperature of the ice core is studied as a function of the mass fraction of the dust in the ice-dust mixture and the thermal conductivity of the nucleus. Using the so-called “looselattice” model of D.A. Mendis and G.D. Brin (1977, Moon17, 359–372) (which was also extended to include cracks and pores in the mantle), it was found that both high dust to ice ratios and high core conductivities inhibit mantle blowoff. Indeed, it is often possible to build an essentially permanent dust mantle around an ice nucleus, so that the nucleus will take on an asteroidal appearance.     

Transient biouptake flux and accumulation by microorganisms: The case of two types of sites with Langmuir adsorption

The uptake of a chemical species by an aquatic microorganism is modelled considering two kinds of sites where Langmuirian adsorption is followed by first order internalisation kinetics. Simpler models, such as only one internalisation route (while most of the adsorption takes place on non-internalising sites) or a linear isotherm for adsorption on one or both sites, become limiting cases of this double-Langmuirian model. The model considers the sites located on the spherical (or semi-spherical) surface of the organism, and takes diffusion from the medium into explicit account. The numerical solution for the internalisation flux shows a maximum. We provide an estimate for the time needed to reach a certain proximity to steady state. The transient solution confirms that the analytical expressions for the steady-state flux are usually valid and that the accumulated amounts reflect the impact of the short-time uptake. The Instantaneous Steady-State Approximation (ISSA), where an intercept of the linear regression of accumulated amount as a function of time is interpreted as an adsorbed amount, can be critically assessed with the transient numerical code for two cases: (i) when the total burden of metal on the cell is the input data and (ii) when an extraction procedure provides further information on the adsorbed and internalised amount.     

HOLOCENE CARBONATE PRECIPITATES ON PRECAMBRIAN BEDROCK IN THE HIGH ARCTIC: AGE AND POTENTIAL FOR PALAEOCLIMATIC INFORMATION

ABSTRACT. Crusts of white carbonate precipitate occur commonly on the upper surfaces of glacially sculptured Precambrian granites and gneisses in east-central Ellesmere Island. Radiocarbon dating of 21 such carbonate precipitates, from elevations between 50 m and 1050 m a.s.l., has yielded only Holocene ages. Two samples from Ellesmere Island, plus one from Inglefield Land, Greenland, have calibrated ages over 5000 years, the rest are younger. The formation of these deposits, mainly calcite and characterized by unusually heavy δ¹³C ratios (+3.36 to +15.18‰), has apparently been aided in some cases by the presence of bacteria, and some crusts seem to have developed where Ca-bearing minerals are more prevalent. In the case of Bache Peninsula and Cape Herschel, where the carbonate crusts are particularly abundant, the presence of calcareous till may have played a role as well. The carbonate crusts may be related to the presence of small, thin carapace ice caps, when such features formed at lower elevations than those at which they exist today. The more extensive cover of ice and snow is postulated to have existed during the latter, cooler part of the Holocene, especially during the period from 2500 to 100 years ago, deduced as a period of low melt from ice core studies on the Agassiz Ice Cap, 200 km to the north. The existence of carapace ice caps at lower elevations also agrees with the radiocarbon evidence for outlet glacier advances during the last 2000 years on both east and west margins of the Prince of Wales Icefield. Alternatively, the white carbonate crusts may be, to a large degree, the result of weathering processes. In either case they provide minimum ages for the exposed, ice-sculptured rock surfaces on which they occur.     

The Dynamics of Small Postglacial Lake Basins and the Nature of Their Archaeological Record: A Case Study of the Middle Palaeolithic Site Neumark‐Nord 2, Germany

Due to favorable conditions of preservation, sedimentary basins provide rich records of human behavior and its environmental context. The conditions for the preservation of archaeological material however vary between basin types (large, river‐fed or small, closed basins), while conditions also differ within a particular basin environment. The goal of this paper is to understand how the dynamics of a small postglacial basin such as Neumark‐Nord 2, a context that dominates the Eemian archaeological record, affected the archaeology situated at its basin margin. The approach used here is to correlate the archaeological record with reconstructions of patterns of deposition and the water conditions within the basin, using lithology, micromorphology, pollen, and macroremains from a transect running from the basin center to the margin. The results show that (1) find levels were exposed to overland flow‐induced winnowing, which vertically concentrated finds but did not cause significant transport, (2) find levels correspond to phases of increased water presence in the basin, and (3) lateral shifts in hominin activity areas may reflect adjustments in the water level. The research shows the importance of large‐scale archaeological excavations and a multidisciplinary sampling strategy that covers both the basin center and the margins, when studying postglacial basin localities like Neumark‐Nord 2.