Potential and actual trace gas fluxes in Arctic terrestrial ecosystems

This paper provides an overview of results obtained through a number of studies of actual and potential trace gas exchanges in Eurasian and Greenlandic tundra ecosystems. The chief findings include:
i) Long-term accumulation rates of carbon in organic tundra soils, i.e. net uptake of atmospheric CO₂, are strongly controlled by simple climatic parameters (mean July temperature, annual precipitation). Warmer and wetter conditions stimulate carbon sequestration rates in Arctic terrestrial ecosystems.
ii) The release of carbon through ecosystem respiration is also heavily influenced by climate. However, the release of dead organic soil carbon as CO₂ is constraind by the lability of the stored organic compounds. This lability decreases significantly with depth (i.e. age) of the soils; moreover, this in turn decreases the temperature sensitivity of the decomposition process.
iii) Methane emissions from typical tundra habitats in northern Eurasia are slightly lower than from seemingly similar habitats in North America although this difference probably can be attributed to the colder climatic setting of the studied sites compared with the general climatic conditions at the North American sites. There is a strong linkage between CO₂ exchange, CH₄ formation and emission rates in some wet tundra ecosystems.
iv) Atmospheric uptake of CH₄ occurs in some dry and mesic tundra habitats and there are indications that these uptake rates could be affected negatively by atmospheric nitrogen deposition. Emissions of N₂O are rarely seen fromArctic soils but there appear to be a strong potential for denitrification and, hence, N₂O release. This might be due to high rates of denitrification during the spring thaw and possibly associated significant releases of N₂O in this period.     

Sveconorwegian igneous complexes beneath the Norwegian–Danish Basin

Gravity and magnetic anomalies have previously been interpreted to indicate strongly magnetic Permian or even Tertiary intrusive bodies beneath the Skagerrak waterway (such as the ‘Skagerrak volcano’) and beneath Silkeborg in Denmark. Our combined modelling of the magnetic and gravity anomalies over these rock bodies indicates that a steep upward magnetisation is required to explain the magnetic anomalies at the surface, reminiscent of the magnetic direction in the Sveconorwegian rocks of the Rogaland Igneous Province in southern Norway. The younger rocks of the Permian Oslo Rift region have intermediate and flat magnetisation that is inadequate to explain the observed magnetic field. The positive part of the Skagerrak aeromagnetic anomaly is continuous with the induced anomalies associated with the eastward extension of the Rogaland Igneous Province. This relation also suggests that rocks of the Rogaland Igneous Province and its offshore extension are responsible for the Skagerrak anomalies. Both the negative, remanence-dominated aeromagnetic anomaly and the positive gravity anomaly can be modelled using constraints from seismic reflection lines and available density data and rock-magnetic properties. A 7 km thick complex of ultramafic/mafic intrusions is located below a southward dipping 1–4 km thick section of Mesozoic sediments and 1–2 km of Palaeozoic sediments. The enormous body of dense, ultramafic/mafic rocks implied by the modelling could be the residue of the parental magma that produced the voluminous Rogaland anorthosites. The application of similar petrophysical properties in the forward modelling of the Silkeborg source body provides an improved explanation of the observed gravity and magnetic anomalies compared with earlier studies. The new model is constrained by magnetic depth estimates (from the Located Euler method) ranging between 6 and 8 km. Forward modelling shows that a model with a reverse magnetic body (anorthosite?) situated above a dense, mafic/ultramafic body may account for the Silkeborg anomalies. The anorthosites may have formed by differentiation of the underlying mafic intrusion, similar to the intrusive relations in the Rogaland Igneous Province. We conclude that there is strong evidence for a Sveconorwegian age for both the Skagerrak and the Silkeborg anomalous rock bodies.     

Lidar measurements of plume statistics

Surface-layer aerosol diffusion experiments have been conducted using artificial smoke plume releases at ground level over flat and homogeneously vegetated terrain at the Meppen proving grounds in the Federal Republic of Germany (1989). At fixed downwind locations in the range out to 800 m from the source, instantaneous crosswind plume profiles were detected repetitively at high spatial (1.5 m) and temporal (3 sec) intervals by use of a mini LIDAR system. The experiments were accompanied by measurement of the surface-layer mean wind and turbulence quantities by sonic anemometers. On the basis of measured crosswind concentration profiles, the following statistics were obtained: 1) Mean profile, 2) Root mean square profile, 3) Fluctuation intensities, and 4) Intermittency factors. Furthermore, some experimentally determined probability density functions (pdf's) of the fluctuations are presented. All the measured statistics are referred to a fixed and a moving frame of reference, the latter being defined as a frame of reference from which the (low frequency) plume meander is removed. Finally, the measured statistics are compared with statistics on concentration fluctuations obtained with a simple puff diffusion model (RIMPUFF) developed at Risø.     

Regional Arctic sea ice variations as predictor for winter climate conditions

Seasonal prediction skill of winter mid and high northern latitudes climate from sea ice variations in eight different Arctic regions is analyzed using detrended ERA-interim data and satellite sea ice data for the period 1980–2013. We find significant correlations between ice areas in both September and November and winter sea level pressure, air temperature and precipitation. The prediction skill is improved when using November sea ice conditions as predictor compared to September. This is particularly true for predicting winter NAO-like patterns and blocking situations in the Euro-Atlantic area. We find that sea ice variations in Barents Sea seem to be most important for the sign of the following winter NAO—negative after low ice—but amplitude and extension of the patterns are modulated by Greenland and Labrador Seas ice areas. November ice variability in the Greenland Sea provides the best prediction skill for central and western European temperature and ice variations in the Laptev/East Siberian Seas have the largest impact on the blocking number in the Euro-Atlantic region. Over North America, prediction skill is largest using September ice areas from the Pacific Arctic sector as predictor. Composite analyses of high and low regional autumn ice conditions reveal that the atmospheric response is not entirely linear suggesting changing predictive skill dependent on sign and amplitude of the anomaly. The results confirm the importance of realistic sea ice initial conditions for seasonal forecasts. However, correlations do seldom exceed 0.6 indicating that Arctic sea ice variations can only explain a part of winter climate variations in northern mid and high latitudes.     

Mapping of glaciotectonic deformation in an ice marginal environment with ground penetrating radar

A series of closely spaced parallel ground penetrating radar (GPR) profiles of glaciotectonic deformed glacio-fluvial sediments have been obtained in an ice marginal environment in Northwest Zealand, Denmark. The radar profiles can be differentiated into several radar facies with distinct reflection characteristics. The lithology and depositional environment of the radar facies is interpreted by correlation with information from profiles in gravel pits, geological maps and drill hole data. The radar facies include glaciotectonically disturbed glacio-fluvial sediments, sediments deposited penecontemporaneously with the deformations and sediments deposited post-tectonically. Several thrust planes with dip angles between 25° and 30° as well as major folds and minor faults have been interpreted from the GPR data. The deformation style of the deformed glacio-fluvial sediments is a thin-skinned pro-glacial thrust complex, with associated folding. The deformations have resulted in the present ridge morphology seen in the rim of the composite ridge. Syn- and post-tectonic sediments are deposited on top and in front of the deformed sediments, smoothing the ridge relief created by the thin-skinned thrust complex. A structural geological map constructed from the ground penetrating radar data reveals the extension of the individual radar facies in the thrust complex. Tectonic features such as thrust planes and folds can be followed throughout the mapped area.     

Sectoral approaches to improve regional carbon budgets

Humans utilise about 40% of the earth’s net primary production (NPP) but the products of this NPP are often managed by different sectors, with timber and forest products managed by the forestry sector and food and fibre products from croplands and grasslands managed by the agricultural sector. Other significant anthropogenic impacts on the global carbon cycle include human utilization of fossil fuels and impacts on less intensively managed systems such as peatlands, wetlands and permafrost. A great deal of knowledge, expertise and data is available within each sector. We describe the contribution of sectoral carbon budgets to our understanding of the global carbon cycle. Whilst many sectors exhibit similarities for carbon budgeting, some key differences arise due to differences in goods and services provided, ecology, management practices used, land-management personnel responsible, policies affecting land management, data types and availability, and the drivers of change. We review the methods and data sources available for assessing sectoral carbon budgets, and describe some of key data limitations and uncertainties for each sector in different regions of the world. We identify the main gaps in our knowledge/data, show that coverage is better for the developed world for most sectors, and suggest how sectoral carbon budgets could be improved in the future. Research priorities include the development of shared protocols through site networks, a move to full carbon accounting within sectors, and the assessment of full greenhouse gas budgets.     

Precise position determination using a Galileo E5 single-frequency receiver

The European Galileo system offers one dedicated signal that is superior to all other signals currently available in space, namely the broadband signal E5. This signal has a bandwidth of at least 51 MHz using an AltBOC modulation. It features a code range noise at centimeter level. Additionally, the impact of multipath effects on this signal is significantly lower compared to all other available GNSS signals. These unique features of Galileo E5 drastically improve the precision of code range measurements and hence enable precise single-frequency positioning. Certain scientific and non-scientific applications in the positioning domain could likely benefit from the exploitation of E5 measurements. A positioning approach based on an additive combination of code range and carrier phase measurements (CPC—“code-plus-carrier”) to eliminate the ionospheric delay could be used to perform precise positioning over long distances. Unfortunately, this derived observable contains the ambiguity term as an additional unknown what normally requires longer observation windows in order to allow sufficient convergence of the ambiguity parameters. For this reason, a rapid convergence algorithm based on Kalman filtering was implemented in addition to the conventional CPC approach that is also discussed. The CPC-based results yield a positioning precision of 2–5 cm after a convergence time of about 3 h. The rapid convergence filter allows fixing the ambiguity terms within a few minutes, and the obtained position results are at the sub-decimeter level. Regarding one selected test, real data from Galileo experimental satellite GIOVE A were used in order to confirm our assumptions. However, since the current Galileo constellation is not sufficient for real-world positioning trials yet, all major results are based on simulated data.     

The TropGrid2 standard tropospheric correction model

TropGrid2 is a new version of a tropospheric model that is based on climatology and provides tropospheric propagation delay corrections for standard positioning users without temperature, pressure and humidity measurements. Zenith hydrostatic and wet delays are modeled as special harmonic functions taking seasonal and diurnal variations into consideration. The grid-point values are height-reduced and can be interpolated horizontally to the user position. The database used to derive this model consists of more than 9 years of 3D numerical weather fields of the NOAA NCEP GDAS weather model. We validated this standard model using 10 years of GPS-derived zenith path delays at 290 International GPS service reference stations. The gridded version is accurate at a level of 3.8 cm (root mean square in zenith direction) on global average; the average long-term bias is ?0.3 cm. The standard deviations computed by the model turn out to be slightly too pessimistic for almost all stations under investigation, in contrast to the site-specific version, which is only marginally (1 mm) more accurate on global scale.     

Modelling a real-world buried valley system with vertical non-stationarity using multiple-point statistics

Stationarity has traditionally been a requirement of geostatistical simulations. A common way to deal with non-stationarity is to divide the system into stationary sub-regions and subsequently merge the realizations for each region. Recently, the so-called partition approach that has the flexibility to model non-stationary systems directly was developed for multiple-point statistics simulation (MPS). The objective of this study is to apply the MPS partition method with conventional borehole logs and high-resolution airborne electromagnetic (AEM) data, for simulation of a real-world non-stationary geological system characterized by a network of connected buried valleys that incise deeply into layered Miocene sediments (case study in Denmark). The results show that, based on fragmented information of the formation boundaries, the MPS partition method is able to simulate a non-stationary system including valley structures embedded in a layered Miocene sequence in a single run. Besides, statistical information retrieved from the AEM data improved the simulation of the geology significantly, especially for the deep-seated buried valley sediments where borehole information is sparse.