Anchor ice formation in streams: a field study

In northern steep streams anchor ice is commonly observed during winter, and plays a key role when considering in‐stream conditions. The understanding, however, of the nature of anchor ice formation is less understood, in particular, under natural conditions. In the following, observations of anchor ice formation in three stream environments with different physical characteristics are presented. Results demonstrate that anchor ice not only form in riffle areas, but also in shallow and slow running stream sections. No linkage between spatial distribution of anchor ice and calculated dimensionless numbers (Froude and Reynolds number) was found. Furthermore, analyses on growth and density showed that anchor ice may be distinguished by two main types. (1) Type I: Lower density forming on top of substrata. (2) Type II: Higher density forming between the substrata filling interstitial spaces. Distribution of anchor ice Types I and II suggests a relation between intensity of turbulence expressed by the Reynolds number, growth pattern and density. As anchor ice has both physical and biological implications on in‐stream environments, findings from the present study may be of particular interest to cold region freshwater stream management. Copyright © 2009 John Wiley & Sons, Ltd.     

Snow research in Svalbard–an overview

This paper summarizes the most significant snow-related research that has been conducted in Svalbard. Most of the research has been performed during the 1990s and includes investigations of snow distribution, snow-melt, snow pack characteristics, remote sensing of snow and biological studies where snow conditions play an important role. For example, studies have shown regional trends with about 50% higher amounts of snow accumulation at the east coast of Spitsbergen compared to the west coast. Further, the accumulation rates are about twice as high in the south compared to the north. On average, the increase in accumulation with elevation is 97 mm water equivalents per 100 m increase in elevation. Several researchers reported melt rates, which are primarily driven by incoming short-wave radiation, in the range of 10-20 mm/day during spring. Maximum melt rates close to 70 mm/day have been measured. In addition to presenting an overview of research activities, we discuss new, unpublished results in areas where considerable progress is being made. These are i) modelling of snow distribution, ii) modelling of snowmelt runoff and iii) monitoring of snow coverage by satellite imagery. We also identify some weaknesses in current research activities. They are lacks of i) integration between various studies, ii) comparative studies with other Arctic regions, iii) applying local field studies in models that can be used to study larger areas of Svalbard and, finally, iv) using satellite remote sensing data for operational monitoring purposes.     

Ultrarapid chondrite formation by hot chondrule accretion? Evidence from unequilibrated ordinary chondrites

Abstract– Unequilibrated ordinary chondrites (UOCs) of all groups (H, L, LL) contain unique chondrite clasts, which are characterized by a close‐fit texture of deformed and indented chondrules. These clasts, termed “cluster chondrites,” occur in 41% of the investigated samples with modal abundances between 5 and 90 vol% and size variations between <1 mm and 10 cm. They show the highest chondrule abundances compared with all chondrite classes (82–92 vol%) and only low amounts of fine‐grained interchondrule matrix and rims (3–9 vol%). The mean degree of chondrule deformation varies between 11% and 17%, compared to 5% in the clastic portions of their host breccias and to values of 3–5% found in UOC literature, respectively. The maximum deformation of individual chondrules is about 50%, a value which seemingly cannot be exceeded due to geometric limitations. Both viscous and brittle chondrule deformation is observed. A model for cluster chondrite formation is proposed where hot and deformable chondrules together with only small amounts of co‐accreting matrix formed a planetesimal or reached the surface of an already existing body within hours to a few days after chondrule formation. They deformed in a hot stage, possibly due to collisional compression by accreting material. Later, the resulting rocks were brecciated by impact processes. Thus, cluster chondrite clasts are interpreted as relicts of primary accretionary rocks of unknown original dimensions. If correct, this places a severe constraint on chondrule‐forming conditions. Cluster chondrites would document local chondrule formation, where chondrule‐forming heating events and the accretion of chondritic bodies were closely linked in time and space.     

Correlation of late precambrian and early Palaeozoic sequences by eustatic sea-level changes and the selection of the Precambrian-Cambrian boundary

The author argues for a pragmatic approach to the problem of defining the Precambrian-Cambrian boundary. A boundary at first occurrence of shelly fauna as recommended by the Precambrian-Cambrian working group will create severe difficulties in worldwide correlation. By placing the boundary stratigraphically somewhat higher (i.e. the Fallotaspis zone) with a more abundant fauna, there is a much better basis for biostratigraphic correlation.Eustatic sea-level changes may be another important tool for late Precambrian-early Palaeozoic correlation, and more detailed facies interpretation of such sequences should be carried out in order to recognise these eustatic sea-level changes.     

A reindeer herder's perspective on caribou, weather and socio-economic change on the Seward Peninsula, Alaska

Non-climate variables shape vulnerability and adaptive capacity to climate change. Here, we describe how recent environmental and socio-economic developments have transformed reindeer herding and perceptions of weather on the Seward Peninsula, Alaska. The reindeer industry has shrunk considerably since the early 1990s, when the winter range of the Western Arctic Caribou Herd expanded, and over 17 000 reindeer mixed with migrating caribou and left the region. Socio-economic and environmental repercussions make the continuation of herding tenuous, and erode the ability of herders to cope with weather variability, among other perturbations. We present a case study of one herder's annual cycle, and juxtapose physical drivers of herding activities, including weather-station and herder observations of local weather variability, with socio-economic factors. There is an increased urgency to access and monitor reindeer with caribou present, but herding plans are constrained by lower economic returns and the need to spend more time in non-herding jobs. Although weather is a greater concern now for immediate herd access, standard weather data are largely irrelevant to the mechanics of herding, whereas variables pertaining to the timing of biotic events (e.g., synchrony of spring break-up and calving) and visibility are attributed to lost herding opportunities. Short-term responses to weather conditions stem from more long-term vulnerability associated with caribou presence, reduced herd size, difficulties affording snowmobile maintenance or crew assistance, and dwindling market opportunities. We emphasize the environmental and socio-economic interactions that affect vulnerability and adaptive capacity for modern herding.     

Late Pleistocene and Holocene terrestrial geomorphodynamics and soil formation in northeastern Germany: a review of geochronological data

ABSTRACT

This study is based on 616 geochronological ages from aeolian and colluvial sediments as well as paleosols, representing the largest database of geochronological data from northeastern Germany available to date. Cumulative probability density functions for radiocarbon data and kernel density estimates for luminescence data were created covering the last 15 ka. The data analysis aimed at the identification of changes and their drivers in geomorphodynamics and soil formation. The ages representing aeolian activity cluster in the Late Glacial, the Early Holocene, and the Late Holocene, where the first two clusters are assumed to result mostly from climatic impact with only a minor share of human impact triggering the mobilization of aeolian sediments. The third cluster is considered to result mainly from human impact. The Late Glacial to Early Holocene activity phase is interrupted by a phase of surface stability around 11.5–12.7 ka, which is indicated by the occurrence of initial soil formations of Finow and Usselo types. Colluvial sedimentation predominantly occurred during the last 7 ka and clearly accelerated since the last 1000 a. According to the ages of specific paleosol types, related soil-forming processes started already in the Late Glacial and were completed in the Holocene.     

Resolving frontal structures: on the payoff using a less diffusive but computationally more expensive advection scheme

This article presents some advantages using a shape-preserving total variation diminishing (TVD) advection scheme in an ecosystem model. The superbee flux-limiter has been used to the second-order Lax–Wendroff advection scheme to make it TVD. We performed simulations for three shelf sea regions with different characteristic time scales, namely, the North Sea, the Barents Sea, and the Baltic Sea. To explore the advantages, we also performed reference runs with the much simpler and computationally cheaper upwind advection scheme. Frontal structures are much better resolved with the TVD scheme. The bottom salinity in the Baltic Sea stays at realistic values throughout the 10-year simulation with the TVD scheme, while with the upwind scheme, it drifts towards lower values and the permanent haline stratification in the Baltic is almost completely eroded within one seasonal cycle. Only when applying TVD for both the vertical and horizontal advections the model succeeded to preserve haline stratification in the decadal simulation. Lower trophic level patterns are far better reproduced with the TVD scheme, and for the estimated cod larval survival, the advantages seem to be even stronger. Simulations using the TVD-derived prey fields identified distinct regions such as Dogger Bank to favor potential larvae survival (PLS), which did not appear as particularly favorable in the upstream simulations. The TVD scheme needs about 25 % more time on the central processing unit (CPU) in case of a pure hydrodynamic setup with only two scalar state variables (Barents Sea application). The additional CPU time cost increases for a coupled physical–biological model application with a large number of state variables. However, this is more than compensated by all the advantages found, and, hence, we conclude that it is worthwhile to use the TVD scheme in our ecosystem model.

     

Quartz Cement in Middle Jurassic Reservoir Sandstones in North Sea. A Review. Part Ⅰ: Occurrence and Character

Quartz cementation is a very important diagenetic event and main porosity-occluding factor in the reservoir sandstones in middle Jurassic i n North Sea. Petrological studies indicate the quartz cementation is limited in shallower burial depth and increases significantly at depth around 3.5 - 4 km. T he amount of quartz cement increases with burial depth from an average of approx imately 4%-5 vol% at 2.5 km to 17%-20 vol% at 4 km. Highest abundances of quartz cement exceeds 28% which has been found in the Ness Formation. Variations in qu artz cementation between different sandstone facies is minimal for any one forma tion, with exception of the generally lower quantities present in the heterolith ic sandstone. Homogenisation temperatures of fluid inclusion in quartz overgrowt h from the North Sea reservoirs indicate that few quartz cement occur below the temperature 70 ℃～80 ℃, the onset of large volume quartz cement starts at the temperatures higher than 90 ℃ which suggests the quartz cementation is temperat ure - dependent during progressing diagenesis of sandstones.