Origin of natural waters and gases within the Upper Carboniferous coal-bearing and autochthonous Miocene strata in South-Western part of the Upper Silesian Coal Basin,Poland

The molecular and stable isotope compositions of coalbed gases from the Upper Carboniferous strata and natural gases accumulated within the autochthonous Upper Miocene Skawina Formation of the D?bowiec-Simoradz gas deposit were determined, as well as the chemical and stable isotope compositions of waters from the Skawina Formation and waters at the top of the Upper Carboniferous strata of the Kaczyce Ridge (the abandoned “Morcinek” coal mine) in the South-Western part of the Upper Silesian Coal Basin. Two genetic types of natural gases within the Upper Carboniferous coal-bearing strata were identified: thermogenic (CH₄, small amounts of higher gaseous hydrocarbons, and CO₂) and microbial (CH₄, very small amounts of ethane, and CO₂). Thermogenic gases were generated during the bituminous stage of coalification and completed at the end of the Variscan orogeny. Degassing (desorption) of thermogenic gases began at the end of late Carboniferous until the late Miocene time-period and extended to the present-day. This process took place in the Upper Carboniferous strata up to a depth of about 550 m under the sealing Upper Miocene cover. A primary accumulation zone of indigenous, thermogenic gases is present below the degassing zone. Up to 200 m depth from the top of the Upper Carboniferous strata, within the weathered complex, an accumulation zone of secondary, microbial gas occurs. Waters within these strata are mainly of meteoric origin of the infiltration period just before the last sea transgression in the late Miocene and partly of marine origin having migrated from the Upper Miocene strata. Then, both methanogenic archaebacteria and their nutrients were transported by meteoric water into the near-surface Carboniferous strata where the generated microbial CH₄ saturated coal seams. Waters within the Miocene strata of the D?bowiec-Simoradz and Zab?ocie are of marine origin, and natural gases accumulated within autochthonous Miocene strata of the D?bowiec-Simoradz gas deposit were most probably generated by microbial processes of on organic matter dispersed within the strata, though some contribution of gases migrating from the Carboniferous coal-bearing strata cannot be excluded.     

Geochemical and stable carbon isotopic composition of coal-related gases from the SW Upper Silesian Coal Basin,Czech Republic

Gas associated with Carboniferous coal seams and younger Miocene sediments in the SW Upper Silesian Basin in Czech Republic shows wide compositional variation. Coal-related gas extracted from cross-measure degassing boreholes, as well as gas released during canister desorption of coal samples from three different mines was analyzed to evaluate the genetic origin and the influence of sorption/desorption processes on the gas composition. Analyses comprised the compositional and stable carbon and isotope composition of coal-related gases. The isotopic composition of gas from cross-measure boreholes indicates thermogenic origin in the southern part of the basin and microbial CO₂ reduction and mixed type origin in the northern part. Gas from canister desorption shows similar origins, but larger compositional and isotopic variation. No consistent isotopic fractionation due to desorption could be observed. Differences in geochemical composition of thermogenic gases could indicate a contribution of gas migrated from deeper formations below the Carpathian overthrust.     

Production of gas from coal seams in the Upper Silesian Coal Basin in Poland in the post-injection period of an ECBM pilot site

A pilot site for CO₂ storage in coal seams was set-up in the Upper Silesian Coal Basin in Poland in the scope of the RECOPOL project, funded by the European Commission. About 760 tons CO₂ were injected into the reservoir from August 2004 to June 2005. Breakthrough of the injected CO₂ was established, which resulted in the production of about 10% of the injected CO₂ in this period. This paper reports on activities performed under the European Commission project MOVECBM that aimed at the assessment of the storage performance of the reservoir in the follow-up period, i.e. whether the injected CO₂ was adsorbed onto the coal or whether it was still present as free gas in the pore space. The injection well was used for this purpose, as the production well had to be abandoned for permitting reasons. Several operational periods can be defined between the last injection in June 2005 and the abandonment of the well in October 2007. In the first period the well was shut-in to observe the pressure fall-off, from about 15.0 MPa at the wellhead after the last injection until about 4.5 MPa at the end of 2005. This pressure fall-off curve showed that the reservoir permeability was very low. This seemed to confirm the observed swelling of the coal during the injection period. In the first months of 2006 the pressure at the wellhead was decreased by releasing gas in a controlled way. The amount and composition of the gas were measured. As a result of the pressure reduction, the well flooded with water. A production pump was placed on the former injection well, enabling active production from the coal from March to September 2007. Results of these operations showed that whereas the gas production rates were as expected based on the experience with the production well, the water production was remarkably low. This could be related to permeability issues or, alternatively, indicate a drying effect of the CO₂ in the reservoir. Further, the gas composition showed a predominance of CO₂ over CH₄ during the gas release that changed gradually into a predominance of CH₄ over CO₂ during the production phase. Although stabilization was not reached within the given production period, the composition approached a 60% methane, 40% CO₂ ratio. This indicates that the exchange of these gases is more complex than often envisaged. After removal of the pump the well was filled with water, which ceased the gas release. This indicates that the pressure in the reservoir was back to its original, hydrostatic, state. As the total volume of CO₂ produced was only a fraction of the amount that was injected, it can be concluded that the CO₂ was taken up by the coal and is currently adsorbed. This gives confidence in the long-term stability of the injected CO₂.     

The occurrence of anthracites in an area characterized by lower rank coals in the Upper Silesian Coal Basin of Poland

The results of petrographical-geological and chemical examinations on anthracites, semianthracites and medium-low volatile bituminous coals from Jastrzebie in the Upper Silesian Coal Basin of Poland are presented. The coking coals mined in this region exhibit volatile matter V^daf = 18–26%, free swelling index FSI = 3–8 and reflectance R_m = 1.10–1.35% and are inertiniterich coals (I = 25–63%).Coal Seam 504 of the Anticlinal beds (Namurian B) has been affected by thermal metamorphism and contains both coking coals and coals of higher rank. According to the criterion of Polish Standards this coal seam varies from anthracite (V^daf <10%) to semianthracite (V^daf = 10–14%) in rank. The carbon content is slightly lower and the hydrogen content a little higher than those of typical anthracites and semianthracites. The reflectance values (R_m = 1.56–2.62%) are generally lower than the R_m values proposed by the International Committee for Coal Petrology as boundary values for anthracites and bituminous coal. The magnitude of anisotropy and microhardness were also examined. Examinations of optical properties prove that the metamorphism exhibited by the coals is the result of elevated temperature and variable pressure. The analyses of the maceral composition indicate that there is a decrease in the inertinite content in anthracites. Vitrinite exhibits the features of thermally altered coal. The micrinite content shows a little variation. In coking coals, a strongly fluorescing bituminous substance with the optical features of exsudatinite was found. The constructed geological section of Coal Seam 504 shows distinct regular changes in chemical and physical properties as well as the petrographic composition which may be caused by the heat flux of a magma intrusion, not localized so far.     

Distribution of Selected Critical Elements in the Carboniferous Coal-bearing Series of the Upper Silesian and Lublin Coal Basins (Poland)

The distribution of selected critical elements in the sedimentary rocks of the Carboniferous coal-bearing series within the Polish Coal Basins is presented.Critical elements such as Be,Mg,Si,P,Sc,V,Co,Y,Nb,In,Sb,La,Ce,Hf,Ta,W,Bi were analysed using inductively-coupled plasma mass spectrometry(ICP/MS).Concentrations of elements such as Sb,Bi,In and,to a slightly lesser extent,Nb,as well as Sc,show average concentrations higher than those from the upper continental crust.The average concentrations of elements like Hf,Mg,P,Y,La,and Ce are slightly lower than in the upper continental crust.Other elements,such as Be,Co,Si,Ta,W and V have average concentrations that are similar,but slightly enriched or slightly depleted,relative to the upper continental crust.The research showed enrichment of some critical elements in the analysed samples,but not high enough that extraction would be economically viable.Statistical methods,which include correlation coefficients between elements and cluster analysis,reveal a strong positive correlation between elements like Be,Bi,Nb,Sc,Ta,W and V.Very high,almost total,positive correlation is also noted between La and Ce.     

Peat-forming environments of Westphalian A coal seams from the Lower Silesian Coal Basin of SW Poland based on petrographic and palynologic data

The petrographic and palynologic compositions of coal seams of the acler formation (Upper Carboniferous, Westphalian A) from northwestern and southeastern part of the Lower Silesian Coal Basin (LSCB) were examined. Coals studied are highly volatile bituminous coal, where R_o ranges from 0.91% to 1.09%. Seam 430 from the northwestern part of the basin contains high vitrinite percentage with rather low inertinite and liptinite contents, while percentage of mineral matter is variable. This petrographic composition is associated either with a predominance of Lycospora in miospore assemblage, or with a miospore assemblage of mixed character. The abundance of Lycospora reflects vegetation composed of the arborescent lycopsids while the mixed miospore assemblage is connected with diverse palaeoplant communities, namely, arborescent lycopsids, calamites and ferns. Seams 409 and 412/413 from the southeastern part of the LSCB are rich in inertinite and liptinite, while the vitrinite content is moderate. Their characteristic feature is the occurrence of a diagnostic crassisporinite (densosporinite). Amount of the mineral components in these coals is very low. Densosporites and related crassicingulate genera are main components of these miospore assemblages and were produced by herbaceous and/or sub-arborescent lycopsids. These petrographic and palynologic features were the basis for distinguishing three maceral–miospore associations: an arborescent lycopsid and mixed associations, occurring in the seam 430 and a herbaceous and/or sub-arborescent lycopsid association which was recorded in seams 409 and 412/413. The first two assemblages are interpreted as having been deposited in a planar rheotrophic mire, whereas the herbaceous and/or sub-arborescent lycopsid association is thought to have developed in an ombrotrophic, domed mire.     

A Study of Spatial Distribution of Induced Seismicity in the Upper Silesian Coal Basin

The Upper Silesian Coal Basin (USCB) is a region where high seismic activity is observed. Most of these seismic events have a local magnitude not exceeding 2.3. Their location and type of focal mechanism suggest that they are connected with new discontinuities in the rock mass caused by mining activity. Shocks with higher magnitude are also observed but their frequency is much lower. The strong events have a different focal mechanism than the low energetic events. The spatial distribution of shock epicentres is not uniform in the USCB area and shows the fractal character of seismic phenomena. Despite extensive mining activity throughout the area, the epicentres of strong events are concentrated in four regions belonging to different geological units. The temporal variability of epicentre co-ordinates and the general decrease of seismic activity points to the migration of seismicity across the investigated area. Analysing the spatial and temporal distributions of the strongest shocks, one can suppose this area to be critically self-organised. These facts suggest that induced seismicity in the USCB is triggered by a nonlinear dynamic process, having regional character and connected with recent tectonic activity.     

The role of sulphide and carbonate minerals in the concentration of chalcophile elements in the bituminous coal seams of a paralic series (Upper Carboniferous) in the Upper Silesian Coal Basin (USCB), Poland

Sulphide and carbonate minerals from nine bituminous coal seams of a Paralic Series were investigated by means of polished-section microscopy, scanning electron microscopy and absorption spectral analyses. In addition to syngenetic accumulations of kaolinite, illite and quartz, diagenetic veinlets of subhedral pyrite and marcasite most often occur in vitrinite clast fissures and in post-tectonic fissures, nests and lenses with fusinite. Epigenetic anhedral and subhedral grains of ankerite, dolomite, siderite and calcite are also frequently found in post-tectonic veins. Pyrite replaced some of the marcasite grains and it dominates in older coal seams in the Flora Beds as compared with the Grodziec Beds. Occasionally there are anhedral and subhedral galena, sphalerite and chalcopyrite grains among coal macerals as well as cerussite among post-tectonic carbonate veins. They all represent the only minerals that are abundant in definite chalcophile elements (Cd, Co, Cu, Ni, Pb, Zn). In addition to the minerals just mentioned, the elements occurred in pyrite and ankerite grains, which contained inclusions of fusinite and other minerals (among others, clay and carbonate minerals in pyrite, pyrite in carbonates). Although there is a low content of minerals accumulating Cd, Co, Cu, Ni, Pb and Zn, the minerals significantly influence the average concentration of elements in the coal seams. In the Grodziec Beds, mineral matter, especially carbonates and sulphides, determines (>50%) the concentration of Cd, Cu, Pb and Zn in coal. The basic part of Cd, Co and Ni in the coal seams of the Grodziec Beds and of Co, Cu, Ni, Pb and Zn in coal seams of the Flora Beds originates from organic matter. These regularities can be important, from an ecological perspective, in stating whether the coals investigated are useful for combustion and in chemical processing.     

Modeling and monitoring of mine water rebound in an abandoned coal mine complex: Siersza Mine, Upper Silesian Coal Basin, Poland

A variable-volume, head-dependent mine water filling model (MIFIM) has been utilized to simulate the post-abandonment flooding of the Siersza coal mine in the Upper Silesian Coal Basin of southern Poland. It is demonstrated that desaturated pore space in the aquifer adjacent to the mine comprises a significant component of the resaturable mine-related void. The model results are very sensitive to the value of this poorly constrained parameter. Nevertheless, the model successfully predicted the first appearance of mine water in an observation well and its subsequent rise. Despite this apparent success, it is concluded that such modeling approaches generally lack predictive power for mines in permeable, porous host rocks. As real monitoring data accumulate, however, such models can be calibrated and their utility increased.     

Fluid regimes in the deformation of the Helvetic nappes,Switzerland, as inferred from stable isotope data

The stable isotope composition of veins, pressure shadows, mylonites and fault breccias in allochthonous Mesozoic carbonate cover units of the Helvetic zone show evidence for concurrent closed and open system of fluid advection at different scales in the tectonic development of the Swiss Alps. Marine carbonates are isotopically uniform, independent of metamorphic grade, where ¹³C=1.5±1.5 (1 ) and ¹⁸O=25.4±2.2 (1 ). Total variations of up to 2 in ¹³C and 1.5 in ¹⁸O occur over a cm scale. Calcite in pre- (Type I) and syntectonic (Type II) vein arrays and pressure shadows are mostly in close isotopic compliance with the matrix calcite, to within ±0.5, signifying isotopic buffering of pore fluids by host rocks during deformation, and closed system redistribution of carbonate over a cm to m scale. This is consistent with microstructural evidence for pressure solution — precipitation deformation.Type III post-tectonic veins occur throughout 5 km of structural section, extend several km to the basement, and accommodate up to 15% extension. Whereas the main population of Type III veins is isotopically undistinguishable from matrix carbonates, calcite in the largest of these veins is depleted in ¹⁸O by up to 23 but acquired comparable ¹³C values. This generation of veins involved geopressurized hydrothermal fluids at 200 to 350° C where ¹⁸O H₂O=–8 to +20, representing variable mixtures of ¹⁸O enriched pore and metamorphic fluids, with ¹⁸O depleted meteoric water. Calc-mylonites ( ¹⁸O=25 to 11) at the base of the Helvetic units, and syntectonic veins from the frontal Pennine thrust are characterized by a trend of ¹⁸O depletion relative to carbonate protoliths, due to exchange with an isotopically variable reservoir ( ¹⁸O H₂O=20 to 4). The upper limiting value corresponds to carbonate-buffered pore fluid, whereas the lower value is interpreted as ¹⁸O-depleted formation brines tectonically expelled at lithostatic pressure from the crystalline basement. Carbonate breccias in one of the large scale late normal faults exchanged with infiltrating ¹⁸O-depleted meteoric surface waters ( ¹⁸O=–8 to –10).During the main ductile Alpine deformation, individual lithological units and associated tectonic vein arrays behaved as closed systems, whereas mylonites along thrust faults acted as conduits for tectonically expelled lithostatically pressured reservoirs driven over tens of km. At the latest stages, marked by 5 to 15 km uplift and brittle deformation, low ¹⁸O meteoric surface waters penetrated to depths of several km under hydrostatic gradients.     

Origin of polyhalite deposits in the Zechstein (Upper Permian) Zdrada platform (northern Poland)

Polyhalite deposits in the Zechstein (Upper Permian) of northern Poland occur in the Lower Werra Anhydrite. In the Zdrada Sulphate Platform, the polyhalite appears to be a very early replacement of anhydrite. The replacement was caused by the halite-precipitating brines which contained potassium and magnesium ions. The formation of polyhalite was preceded by the syndepositional anhydritization of the original gypsum deposit which has often preserved its primary textures. This anhydritization on the platform and its slopes was a reaction of the precipitated gypsum in a hydrologically open evaporite basin, with brines of salt basins adjacent to the sulphate platform. These brines, when nearly saturated with respect to halite, and potassium and magnesium rich, reacted with anhydrite to precipitate polyhalite along the slopes of the Zdrada Platform. The oxygen and sulphur isotopic compositions of sulphate evaporites indicate that marine solutions were the only source of sulphate ions supplied to the Zechstein basin, and that anhydrite was transformed to polyhalite by reaction with marine brines more concentrated than those that precipitated precursor calcium sulphate minerals.     

Properties of altered coals associated with Carboniferous red beds in the Upper Silesian Coal Basin and their tentative classification

In the Czech and Polish parts of the Upper Silesian Coal Basin, red beds associated with the pre-Neoide tectonic structures are found. The occurrence of these beds is connected with changes in the development of the coal seams, in the macroscopic and microscopic character of coal and with its chemical and technological properties. These changes are due to the thermal and oxidation alteration of coal as a result of geological processes which, up to now, have not been clarified unambiguously. These altered coals were divided into groups according to their chemical and physical properties and vitrinite reflectance using statistical multicomponent methods. Coal samples classified as belonging to one of these groups correspond to a certain type of coal, characterized by a particular grade of thermal and oxidation alteration and petrographical composition.     

Facies analysis of coal seams from the Cracow Sandstone Series of the Upper Silesia Coal Basin, Poland

The main purpose of this study was to recognise the variability of petrographical structure of two coal seams occurring in the Cracow Sandstone Series (Upper Carboniferous/Pennsylvanian, Upper Westphalian), being exploited in the Siersza mine. This mine is located in the eastern part of the Upper Silesia Coal Basin (USCB). The chemical analyses and petrographical features allow the inclusion of these coals to the group of hard brown coals belonging to subbituminous class.Two coal seams (207 and 209/210) of a considerable thickness (7.44 and 6.54 m, respectively), representative of the Cracow Sandstone Series (CSS), were chosen for the petrographic studies. Dominant macroscopic constituents of both seams are banded bright coal and banded coal.The coal seams were sampled in 284 intervals using a channel profile sampling strategy. The microscopical examinations revealed the majority of macerals from the vitrinite group (55%), followed by inertinite (21%), liptinite (11%), and mineral matter (13%). Low values of the vitrinite reflectance (R_o=0.46%) confirm very low coalification of the coal in both seams. Facies analysis indicates that in the course of a mire development, in which the studied coal seams originated, wet forest swamp conditions dominated characterized by a high degree of flooding and gelification as well as by a prevalence of arborescent plants. In such conditions, lithotypes with a large content of bright coal were mainly formed. Petrographic and facies data point to the rheotrophic character of these peatbogs. Frequent changes of the conditions in the peatbog, as it is shown by the variability of petrographic structure of the studied profiles, as well as by lateral changes of the phytogenic sedimentary environment within the coal seams, indicate a strong influence of a river channel on the adjoining peatbogs. An accretion of clastic sediments within the wide river channel belts was balanced mainly by the peatbog growth on the areas outside channels. Frequency and rate of avulsion of the river channels influenced the size, continuity and variability of the peatbogs.     

Palaeobathymetric model of the Upper Senonian Flysch sediments of a part of the Silesian Basin (Polish Carpathians)

The occurrence of redeposited sediments and foraminiferids within the Upper Senonian deposits of the Silesian Unit (Northern Carpathians) have allowed a reconstruction of a part of the southern slope of the Silesian Basin. Three main zones have been established with specific sediments and associations of foraminiferids; basin plain, slope and upper slope-shelf which correspond to part of the “Recurvoides” and “Marssonella” associations sensu Haig (1979). Nine species of Foraminiferida from the uppermost associations are described.     

Mammoth tooth enamel growth rates inferred from stable isotope analysis and histology

Mammoth (Mammuthus sp.) teeth are relatively abundant in Quaternary deposits from Eurasia and North America, and their isotopic compositions can be used to reconstruct past seasonal patterns in precipitation, diet, and migration. Strategies for collecting and interpreting such data, however, are strongly dependent on growth rates, which can vary among species, individuals, and within teeth. In this study, we use histological and isotopic measurements to determine enamel growth rates for a Columbian mammoth (Mammuthus columbi) tooth in two directions. Using histology, the growth rate through the enamel thickness (ET; perpendicular to the height of the tooth) is estimated at 0.8 to 1.5 mm/yr. Isotopic sampling through the innermost 0.36 mm of the ET recovered less than half a period of variation (i.e., half an inferred year of growth), which is consistent with the histological estimate for ET growth rate. A combination of histological and isotopic measurements suggests that the enamel extension rate (growth in the height of the tooth) is 13–14 mm/yr. Knowledge of enamel growth rates should improve the design and interpretation of future isotopic studies of mammoth teeth. The combination of histological and isotopic measurements may also prove useful in determining growth rates for other extinct taxa.     

The Main Ostrava Whetstone: composition, sedimentary processes, palaeogeography and geochronology of a major Mississippian volcaniclastic unit of the Upper Silesian Basin (Poland and Czech Republic)

The Main Ostrava Whetstone (MOW) is an important lithostratigraphic horizon of the Late Carboniferous sedimentary fill of the late Palaeozoic foreland Upper Silesian Basin. It is the largest and best-identified volcanogenic horizon in the basin, reaching thicknesses of 15.3 m and occupying an area of ca 2,973 km² and a volume after lithification of 9.24 km³. It consists of volcanic materials transported to the basin probably by an aeolian process. Just after sedimentation, these materials were redeposited a short distance away in a shallow water environment. Granularity corresponds to a range from argillaceous siltstones to fine-grained sandstones. The components are dominated by glass shards replaced by clay minerals (mixed illite–smectite structures) in addition to quartz of volcanogenic and terrigenous origins. Sanidine and a plagioclase close to albite are also present. The sedimentary structures, micro-structures and composition of the MOW indicate variable and dynamic hydrodynamic conditions. The MOW represents a series of flooding events, which could be connected with unusual rainfall. Such major flooding events were most likely induced by volcanic eruptions. The available drill-core log data were used to construct a digital model of the whetstone, which showed an east–west zonality in the thicknesses, with the majority being synsedimentary. CA-TIMS U–Pb dating the volcanogenic zircons yields an age of 327.35 ± 0.15 Ma. The source location of the volcanogenic material is not clear; however, it is presumed to have been located in the west of the Upper Silesian Basin.     

Geochemistry and isotope chemistry of Michigan Basin brines: Devonian formations

Detailed chemical and isotope analysis of 87 formation waters collected from six Devonian-aged units in the Michigan Basin are presented and discussed in terms of the origin of the dissolved components and the water. Total dissolved solids in these waters range from 200,000 to >400,000mg/1. Upper Devonian formations produce dominantly NaCaCl brine, while deeper formations produce CaNaCl water. Ratios of Cl/Br and Na/Br along with divalent cation content (MCl₂), indicate that these brines are derived from evapo-concentrated seawater. Other ion concentrations appear to be extensively modified from seawater values by water-rock reactions. The most important reactions are dolomitization, which explains the Ca content of the brines, and reactions involving aluminosilicate minerals. Stable isotope (δ¹⁸O and δD) compositions indicate that water molecules in the deeper formations are derived from primary concentrated seawater. Isotope enrichment by exchange with carbonates and perhaps gypsum cannot be discounted. Isotope values indicate water in the Upper Devonian formations is a mixture of seawater brine diluted with meteoric-derived water. Dilution has predominantly occurred in basin margins. Two scenarios are presented for the origin of the brines in the Devonian formations: (1) they originated when the Devonian sediments and evaporites were first deposited; or (2) they are residual brine liberated from the deeper Devonian and possibly Silurian salt deposits.     

Short-lived vegetational and environmental change during the Preboreal in the Biebrza Upper Basin (NE Poland)

A pollen diagram from the upper Biebrza basin (NE Poland) shows a conspicuous pollen fluctuation around 9650 BP, which is superimposed on a broader pollen fluctuation correlated with a more open vegetation phase during the Preboreal generally found in Europe. High-resolution pollen analyses indicate a duration of only a few years in which Pinus forests became more open. The resulting niches were partly occupied by Betula trees and partly by Betula shrubs, Artemisia, and Chenopodiaceae. Shortly before the increased opening of the upland vegetation, reed vegetation developed in the valley, in which Sparganium and later Typha became prominent. At the end of the short fluctuation, this reed vegetation diminished. The wetland vegetation development might be partly related to hydrological changes caused by a reduced evapotranspiration of the opened upland forest. The data presented here show that the open vegetation phase of the Preboreal was not of uniform character, but that within its short time span of ca 150 years an even shorter fluctuation of a few years to a few decades occurred. It is unknown whether this is a regionally restricted phenomenon or whether it has an extra-regional character.