Report series in Geophysics

 

Recent Submissions

  • Järvinen, Onni (Helsingin yliopisto, 2013)
    Antarctica is a major component in the climate system of the earth, acting as a large heat sink in the energy balance. The climatic conditions of Antarctica maintain the snow and ice cover that blankets almost completely the surface area of the continent. Physical properties of snow readily respond to changing environmental conditions and remote sensing signals are sensitive to these properties. The annual changes in the physical properties of the snow cover, especially in the coastal area, must be taken into account when snow cover and climate models are produced. In situ observations are needed for calibration and validation of these models. The aim of the present study was to examine the annual cycle of the active 10-m surface layer in western Dronning Maud Land, Antarctica. The data were collected along a 300-km-long transect from the coast to the edge of the high plateau during the field campaigns in austral summers 2004-2005, 2009-2010 and 2010-2011 as a part of the Finnish Antarctic Research Programme (FINNARP). The studies were focused on the uppermost part of the ice sheet covering the most recent annual accumulation in the coastal area. The results showed that the present study lakes froze completely during winter and showed similar evolution but the exact timing depended on the location. In January, the general structure of lake Suvivesi was following: two layers, each about 1 m thick, an upper layer with a thin ice layer on top and main body of liquid water, and a lower layer containing slush and hard ice sub-layers. The formation and the depth scale of the present study lakes are determined by the light extinction distance and thermal diffusion coefficient, limiting the growth to less than ~1.5 m in one summer. In Antarctica, the mean spectral diffuse extinction coefficient varied between 0.04 and 0.31 cm-1 (10-20-cm snow layer) and varied only slightly between locations when the grain type was the same. The theoretically calculated average depth where broadband irradiance (400-700-nm band) was 1 % of the downwelling irradiance at the surface, was 50 cm. On the continental ice sheet, the compaction rate of the snowpack was 0.0201 ± 0.02 y-1 and the power spectra revealed a daily cycle, synoptic scale variability (~10 days), and variability in a low-frequency band of 60-120 days at a depth of 54 cm. The investigations of snow patches in Basen nunatak revealed that much more snow was lost in summer 2010-2011 (6.3 mm d-1 water equivalent (w.e.)) than in 2004-2005 (4 mm d-1 w.e.).
  • Kitaigorodskii, Sergey (2013)
    Report series in Geophysics 72
    The several conclusions of similarity theory for wind driven waves are discussed together with experimental data being revisited by the author (Kitaigorodskii, 1983, 1998). First of them is so-called wind speed scaling, often being used in literature on wind waves as Kitaigorodskii scaling. It is stressed that existing data cannot be served for rejection of concept of fully developed (mature) wind waves. Second of them is the derivation of so-called law omega–4 (Kitaigorodskii, 1962, 1983) for replacement of Phillips law (Phillips, 1958). The main question related to this problem is about existence of the transition from direct cascade regime to wind wave breaking dissipation scales. It is established "a bridge" between description of wave spectra used in Kitaigorodskii (1962) and weakly nonlinear theory of waves. This was the third question discussed here.
  • Uusikivi, Jari (Helsingin yliopisto, 2013)
    Sea ice has been recognized as one of the key elements of polar and sub-polar seas, including Baltic Sea. The existence of sea ice cover and its properties have influence to many aspects of marine biology, climate and seafaring. This thesis is concentrated on describing physical and optical properties of landfast ice, and also pack ice, in the Baltic Sea. The aim of the thesis is to use measurements to study the interactions between optical and physical properties of sea ice and how these can affect the biology in sea ice. Decade long observations of ice properties were used to construct a statistical model of properties of landfast ice. Temperature was found to be the most important factor determining ice thickness and contribution of snow ice to the ice thickness was determined by the amount of winter time precipitation. Stratigraphy of ice and growth history had influence to the vertical distribution of organisms in the ice cover as snow ice layers and columnar ice layers were found to favor different types of organisms. Thickness of meteoric ice layer, including snow ice and superimposed ice, controlled the albedo of ice cover when no snow cover was on the ice. Based on the observations of fast ice conditions and albedo, the effects of snow thickness and meteoric ice thickness to the albedo of sea ice were formulated as albedo parameterization equations. The optical properties of sea ice with spectral resolution were studied on the landfast sea ice. Emphasis in these studies was given to optical properties in the ultraviolet and visible wavelengths. Organic matter, dissolved and particulate, was the most important factor determining the ultraviolet properties of sea ice cover. The optical properties in the ultraviolet were also actively modified by the living organisms in the ice cover by producing mycosporine like amino acids (MAAs) in relatively high amounts. MAAs are a family of photoprotective compounds that absorb UV radiation efficiently. At the visible part of spectrum the ice by itself and the thickness of meteoric ice layer were the most important determinants. Salinity and the initial salt entrapment during ice growth in the Baltic Sea were measured to be less than in the oceans with equal ice growth rates. The turbulent fluxes of heat and salinity under the landfast sea ice were measured to be small.
  • Koistinen, Esa (2007)
    Report series in Geophysics 54
  • Oikkonen, Annu (2008)
    Report series in Geophysics 56
  • Leppäranta, Matti (2009)
    Report series in Geophysics 61
  • Mäkiranta, Eeva (2009)
    Report series in Geophysics 58
  • Rasmus, Kai; Granberg, Harry; Kanto, Kimmo; Kärkäs, Eija; Lavoie, C.; Leppäranta, Matti (2003)
    47
    The project ’Seasonal Snow in Antarctica’ was set up to measure the physical properties of Antarctic snow cover in 1999-2001. The present document is the project data report. An introduction is given of the project, descriptions of the experiments are made, and the data obtained by the scientists in the different working groups during the two expeditions to Antarctica are described
  • Leppäranta, Matti (1998)
  • Leppäranta, Matti (1998)
  • Järvinen, Onni; Leppäranta, Matti; Vehviläinen, Juho (University of Helsinki, Division of Geophysics and Astronomy, 2012)
    Report series in Geophysics No 71
    Field programme on the surface layer of the ice sheet in the Dronning Maud Land, Antarctica has been performed in 2009-2011. The objectives were to examine the annual accumulation and sublimation history, snow melting, chemistry of snow impurities, and life history of supraglacial and epiglacial lakes in blue ice regions. Fieldwork was done during FINNARP 2009 and 2010 expeditions. The sites were at the Finnish research station Aboa (73 02.5'S, 013 24.4'W), a snow line from Rampen at the edge of the ice shelf pass Aboa to the station Svea, and blue ice at Basen and neighboring nunataks. Snow measurements were made using classical snow pit method, ice and snow sampling, and with automatic observation stations (surface radiation balance, penetration of sunlight into snow and ice, and temperature within the surface layer of snow and ice). Life history, physics, and ecological state of lakes were mapped. This document is the data report including a brief project introduction, descriptions of the experiments, and the data obtained.
  • Raiskila, Selen (Helsingin yliopisto, 2013)
    Meteorite impact cratering has played a key role in Earth's geological past and has left a dramatic effect on biological and geological records forming large volumes of igneous rocks and important mineral deposits. By studying terrestrial impact craters we can have valuable information of impact generated changes in rocks and minerals. Integrated geophysical study in this dissertation combines different methods to distinguish the meteorite impact related features from features caused by endogenic processes. Optical microscopy provides diagnostic evidence of shock produced deformations in minerals.Shocked and un-shocked rocks have contrasts in petrophysical properties, which cause anomalies to regional magnetic and gravity fields over meteorite impact structures.Magnetic minerals may re-magnetize during impact and thus provide information of the past geomagnetic field and the ancient paleoposition of impact site. This dissertation focuses to the Finnish impact structure, Keurusselkä, which was discovered in 2003. The structure is situated in central Finland (62°08 N, 24°37 E) within the Central Finland Granitoid Complex, which formed 1890 1860 million years ago during the Svecofennian orogeny. Keurusselkä is deeply eroded remain of a complex crater, named after the Lake Keurusselkä, which is the dominant present day feature of the crater area. For this study rock and drill core samples were collected from Keurusselkä region. The samples were chosen according to a sampling strategy, where samples were taken in and outside the impact region to investigate the impact related features and their radial distance from the centre of the impact. The main focus was to prove or disprove the impact origin of Keurusselkä. First indication of impact was shatter cones, which are conical features in rocks formed by an impact shock and pressure. Shatter cones were found in a circular area interpreted as the central uplift of the original crater. In this dissertation petrographic analysis of thin sections was done to find evidence of deformational features in minerals. Diagnostic evidence for Keurusselkä was achieved when planar deformation features(PDFs) were found from quartz grains in shatter cones indicating impact pressures up to 20 GPa. Samples from 3 shallow drillings in southwest part of the Keurusselkä structure were also studied. Impact like monomictic breccia was found from one of the drill cores. Petrographic analysis revealed clay minerals (illite, smectite-group), which most likely have altered from impact glass. Magnetic minerals were obtained for paleomagnetic purposes, i.e. to examine the ancient position and drift of Baltica. Paleomagnetic directions obtained from shatter cones indicate re-magnetization. The virtual geomagnetic pole implies that the impact event took place ~1120 million years ago during the formation of Rodinia supercontinent. Petrophysical properties (density, susceptibility) of rocks were measured for differences between shocked and un-shocked rocks. The exposed bedrock in Keurusselkä was noticed to be fractured and damaged, which causes anomalies to the regional magnetic and gravity field. The geophysical signals were modelled along two profiles using measured physical properties of Keurusselkä rocks. The highly magnetized centre of the structure forms an eroded circular central uplift with diameter of 6 km. Based on the gravity minimum around the structure an area of ~16 km in diameter and depth of ~1200 meters was modelled to explain the observed anomalies. The original size of the crater is estimated to be 24-27 km in diameter.
  • Bućko, Michał (Helsingin yliopisto, 2012)
    Road traffic is at present one of the major sources of environmental pollution in urban areas. Magnetic particles, heavy metals and others compounds generated by traffic can greatly affect ambient air quality and have direct implications for human health. The general aim of this research was to identify and characterize magnetic vehicle-derived particulates using magnetic, geochemical and micro-morphological methods. A combination of three different methods was used to discriminate sources of particular anthropogenic particles. Special emphasis was placed on the application of various collectors (roadside soil, snow, lichens and moss bags) to monitor spatial and temporal distribution of traffic pollution on roadsides. The spatial distribution of magnetic parameters of road dust accumulated in roadside soil, snow, lichens and moss bags indicates that the highest concentration of magnetic particles is in the sampling points situated closest to the road edge. The concentration of magnetic particles decreases with increasing distance from the road indicating vehicle traffic as a major source of emission. Significant differences in horizontal distribution of magnetic susceptibility were observed between soil and snow. Magnetic particles derived from road traffic deposit on soil within a few meters from the road, but on snow up to 60 m from the road. The values of magnetic susceptibility of road dust deposited near busy urban motorway are significantly higher than in the case of low traffic road. These differences are attributed to traffic volume, which is 30 times higher on motorway than on local road. Moss bags placed at the edge of urban parks situated near major roads show higher values of magnetic susceptibility than moss bags from parks located near minor routes. Enhanced concentrations of heavy metals (e.g. Fe, Mn, Zn, Cu, Cr, Ni and Co) were observed in the studied samples. This may be associated with specific sources of vehicle emissions (e.g. exhaust and non-exhaust emissions) and/or grain size of the accumulated particles (large active surface of ultrafine particles). Significant correlations were found between magnetic susceptibility and the concentration of selected heavy metals in the case of moss bags exposed to road traffic. Low-coercivity magnetite was identified as a major magnetic phase in all studied roadside collectors (soil, snow, moss bags and lichens). However, magnetic minerals such as titanomagnetite, ilmenite, pyrite and pyrrhotite were also observed in the studied samples. The identified magnetite particles are mostly pseudo-single-domain (PSD) with a predominant MD fraction (>10 μm). The ultrafine iron oxides (>10 nm) were found in road dust extracted from roadside snow. Large magnetic particles mostly originate from non-exhaust emissions, while ultrafine particles originate from exhaust emissions. The examined road dust contains two types of anthropogenic particles: (1) angular/aggregate particles composed of various elements (diameter ~1-300 µm); (2) spherules (~1-100 µm) mostly composed of iron. The first type of particles originates from non-exhaust emissions such as the abrasion of vehicle components, road surface and winter road maintenance. The spherule-shaped particles are products of combustion processes e.g. combustion of coal in nearby power plants and/or fuel in vehicle engines. This thesis demonstrates that snow is an efficient collector of anthropogenic particles, since it can accumulate and preserve the pollutants for several months (until the late stages of melting). Furthermore, it provides more information about spatial and temporal distribution of traffic-generated magnetic particles than soil. Since the interpretation of data obtained from magnetic measurements of soil is problematic (due to its complexity), this suggests the application of alternative collectors of anthropogenic magnetic particulates (e.g. snow and moss bags). Moss bags and lichens are well suited for magnetic biomonitoring studies, since they effectively accumulate atmospheric pollution and can thus be applied to monitor the spatio-temporal distribution of pollution effects.
  • Airo, Meri-Liisa; Kiuru, Risto (University of Helsinki, Division of Geophysics and Astronomy, 2012)
    Report series in Geophysics No 68
  • Wang, Caixin (Helsingin yliopisto, 2011)
    Earth s ice shelves are mainly located in Antarctica. They cover about 44% of the Antarctic coastline and are a salient feature of the continent. Antarctic ice shelf melting (AISM) removes heat from and inputs freshwater into the adjacent Southern Ocean. Although playing an important role in the global climate, AISM is one of the most important components currently absent in the IPCC climate model. In this study, AISM is introduced into a global sea ice-ocean climate model ORCA2-LIM, following the approach of Beckmann and Goosse (2003; BG03) for the thermodynamic interaction between the ice shelf and ocean. This forms the model ORCA2-LIM-ISP (ISP: ice shelf parameterization), in which not only all the major Antarctic ice shelves but also a number of minor ice shelves are included. Using these two models, ORCA2-LIM and ORCA2-LIM-ISP, the impact of addition of AISM and increasing AISM have been investigated. Using the ORCA2-LIM model, numerical experiments are performed to investigate the sensitivity of the polar sea ice cover and the Antarctic Circumpolar Current (ACC) transport through Drake Passage (DP) to the variations of three sea ice parameters, namely the thickness of newly formed ice in leads (h0), the compressive strength of ice (P*), and the turning angle in the oceanic boundary layer beneath sea ice (θ). It is found that the magnitudes of h0 and P* have little impact on the seasonal sea ice extent, but lead to large changes in the seasonal sea ice volume. The variation in turning angle has little impact on the sea ice extent and volume in the Arctic but tends to reduce them in the Antarctica when ignored. The magnitude of P* has the least impact on the DP transport, while the other two parameters have much larger influences. Numerical results from ORCA2-LIM and ORCA2-LIM-ISP are analyzed to investigate how the inclusion of AISM affects the representation of the Southern Ocean hydrography. Comparisons with data from the World Ocean Circulation Experiment (WOCE) show that the addition of AISM significantly improves the simulated hydrography. It not only warms and freshens the originally too cold and too saline bottom water (AABW), but also warms and enriches the salinity of the originally too cold and too fresh warm deep water (WDW). Addition of AISM also improves the simulated stratification. The close agreement between the simulation with AISM and the observations suggests that the applied parameterization is an adequate way to include the effect of AISM in a global sea ice-ocean climate model. We also investigate the models capability to represent the sea ice-ocean system in the North Atlantic Ocean and the Arctic regions. Our study shows both models (with and without AISM) can successfully reproduce the main features of the sea ice-ocean system. However, both tend to overestimate the ice flux through the Nares Strait, produce a lower temperature and salinity in the Hudson Bay, Baffin Bay and Davis Strait, and miss the deep convection in the Labrador Sea. These deficiencies are mainly attributed to the artificial enlargement of the Nares Strait in the model. In this study, the impact of increasing AISM on the global sea ice-ocean system is thoroughly investigated. This provides a first idea regarding changes induced by increasing AISM. It is shown that the impact of increasing AISM is global and most significant in the Southern Ocean. There, increasing AISM tends to freshen the surface water, to warm the intermediate and deep waters, and to freshen and warm the bottom water. In addition, increasing AISM also leads to changes in the mixed layer depths (MLD) in the deep convection sites in the Southern Ocean, deepening in the Antarctic continental shelf while shoaling in the ACC region. Furthermore, increasing AISM influences the current system in the Southern Ocean. It tends to weaken the ACC, and strengthen the Antarctic coastal current (ACoC) as well as the Weddell Gyre and the Ross Gyre. In addition to the ocean system, increasing AISM also has a notable impact on the Antarctic sea ice cover. Due to the cooling of seawater, sea ice concentration and thickness generally become higher. In austral winter, noticeable increases in sea ice concentration mainly take place near the ice edge. In regards with sea ice thickness, large increases are mainly found along the coast of the Weddell Sea, the Bellingshausen and Amundsen Seas, and the Ross Sea. The overall thickening of sea ice leads to a larger volume of sea ice in Antarctica. In the North Atlantic, increasing AISM leads to remarkable changes in temperature, salinity and density. The water generally becomes warmer, more saline and denser. The most significant warming occurs in the subsurface layer. In contrast, the maximum salinity increase is found at the surface. In addition, the MLD becomes larger along the Greenland-Scotland-Iceland ridge. Global teleconnections due to AISM are studied. The AISM signal is transported with the surface current: the additional freshwater from AISM tends to enhance the northward spreading of the surface water. As a result, more warm and saline water is transported from the tropical region to the North Atlantic Ocean, resulting in warming and salt enrichment there. It would take about 30 40 years to establish a systematic noticeable change in temperature, salinity and MLD in the North Atlantic Ocean according to this study. The changes in hydrography due to increasing AISM are compared with observations. Consistency suggests that increasing AISM is highly likely a major contributor to the recent observed changes in the Southern Ocean. In addition, the AISM might contribute to the salinity contrast between the North Atlantic and North Pacific, which is important for the global thermohaline circulation.
  • Elbra, Tiiu (Helsingin yliopisto, 2011)
    Physical properties provide valuable information about the nature and behavior of rocks and minerals. The changes in rock physical properties generate petrophysical contrasts between various lithologies, for example, between shocked and unshocked rocks in meteorite impact structures or between various lithologies in the crust. These contrasts may cause distinct geophysical anomalies, which are often diagnostic to their primary cause (impact, tectonism, etc). This information is vital to understand the fundamental Earth processes, such as impact cratering and associated crustal deformations. However, most of the present day knowledge of changes in rock physical properties is limited due to a lack of petrophysical data of subsurface samples, especially for meteorite impact structures, since they are often buried under post-impact lithologies or eroded. In order to explore the uppermost crust, deep drillings are required. This dissertation is based on the deep drill core data from three impact structures: (i) the Bosumtwi impact structure (diameter 10.5 km, 1.07 Ma age; Ghana), (ii) the Chesapeake Bay impact structure (85 km, 35 Ma; Virginia, U.S.A.), and (iii) the Chicxulub impact structure (180 km, 65 Ma; Mexico). These drill cores have yielded all basic lithologies associated with impact craters such as post-impact lithologies, impact rocks including suevites and breccias, as well as fractured and unfractured target rocks. The fourth study case of this dissertation deals with the data of the Paleoproterozoic Outokumpu area (Finland), as a non-impact crustal case, where a deep drilling through an economically important ophiolite complex was carried out. The focus in all four cases was to combine results of basic petrophysical studies of relevant rocks of these crustal structures in order to identify and characterize various lithologies by their physical properties and, in this way, to provide new input data for geophysical modellings. Furthermore, the rock magnetic and paleomagnetic properties of three impact structures, combined with basic petrophysics, were used to acquire insight into the impact generated changes in rocks and their magnetic minerals, in order to better understand the influence of impact. The obtained petrophysical data outline the various lithologies and divide rocks into four domains. Based on target lithology the physical properties of the unshocked target rocks are controlled by mineral composition or fabric, particularly porosity in sedimentary rocks, while sediments result from diverse sedimentation and diagenesis processes. The impact rocks, such as breccias and suevites, strongly reflect the impact formation mechanism and are distinguishable from the other lithologies by their density, porosity and magnetic properties. The numerous shock features resulting from melting, brecciation and fracturing of the target rocks, can be seen in the changes of physical properties. These features include an increase in porosity and subsequent decrease in density in impact derived units, either an increase or a decrease in magnetic properties (depending on a specific case), as well as large heterogeneity in physical properties. In few cases a slight gradual downward decrease in porosity, as a shock-induced fracturing, was observed. Coupled with rock magnetic studies, the impact generated changes in magnetic fraction the shock-induced magnetic grain size reduction, hydrothermal- or melting-related magnetic mineral alteration, shock demagnetization and shock- or temperature-related remagnetization can be seen. The Outokumpu drill core shows varying velocities throughout the drill core depending on the microcracking and sample conditions. This is similar to observations by Kern et al., (2009), who also reported the velocity dependence on anisotropy. The physical properties are also used to explain the distinct crustal reflectors as observed in seismic reflection studies in the Outokumpu area. According to the seismic velocity data, the interfaces between the diopside-tremolite skarn layer and either serpentinite, mica schist or black schist are causing the strong seismic reflectivities.
  • Zhang, Zhanhai (Helsingin yliopisto, 2000)
  • Elo, Aija-Riitta (Helsingin yliopisto, 2007)
    To a large extent, lakes can be described with a one-dimensional approach, as their main features can be characterized by the vertical temperature profile of the water. The development of the profiles during the year follows the seasonal climate variations. Depending on conditions, lakes become stratified during the warm summer. After cooling, overturn occurs, water cools and an ice cover forms. Typically, water is inversely stratified under the ice, and another overturn occurs in spring after the ice has melted. Features of this circulation have been used in studies to distinguish between lakes in different areas, as basis for observation systems and even as climate indicators. Numerical models can be used to calculate temperature in the lake, on the basis of the meteorological input at the surface. The simple form is to solve the surface temperature. The depth of the lake affects heat transfer, together with other morphological features, the shape and size of the lake. Also the surrounding landscape affects the formation of the meteorological fields over the lake and the energy input. For small lakes the shading by the shores affects both over the lake and inside the water body bringing limitations for the one-dimensional approach. A two-layer model gives an approximation for the basic stratification in the lake. A turbulence model can simulate vertical temperature profile in a more detailed way. If the shape of the temperature profile is very abrupt, vertical transfer is hindered, having many important consequences for lake biology. One-dimensional modelling approach was successfully studied comparing a one-layer model, a two-layer model and a turbulence model. The turbulence model was applied to lakes with different sizes, shapes and locations. Lake models need data from the lakes for model adjustment. The use of the meteorological input data on different scales was analysed, ranging from momentary turbulent changes over the lake to the use of the synoptical data with three hour intervals. Data over about 100 past years were used on the mesoscale at the range of about 100 km and climate change scenarios for future changes. Increasing air temperature typically increases water temperature in epilimnion and decreases ice cover. Lake ice data were used for modelling different kinds of lakes. They were also analyzed statistically in global context. The results were also compared with results of a hydrological watershed model and data from very small lakes for seasonal development.

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