South-central Finland Research Articles

Correlation between mammal track abundance and Forest Landscape Integrity Index validates actual forest ecological integrity

Human disturbance compromises the ecological integrity of forests, negatively affecting associated species. Assessing the impact of forest integrity on biodiversity is complex due to the interplay of various human activities, ecological factors, and their interactions. Current large-scale indices assess forest integrity but often lack a direct connection to the biotic environment. We tested the effectiveness of the global Forest Landscape Integrity Index (FLII) in evaluating aspects of anthropogenic forest degradation on the biotic community. We analyzed the relationship between changes in the ecological integrity of Finnish forests and variations in mammal species abundance, using the number of tracks from 17 different species collected during the winter seasons between 2016 and 2020 in south-central Finland. Beyond the FLII, we analyzed forest and canopy cover to enhance the accuracy of habitat preference assessments. We found that the FLII captures the varying degrees of forest integrity, as reflected by the correlation between the abundance of winter tracks and the FLII for most mammals. Species that were positively associated with forest integrity were all native to the boreal forest, while mammals that adapt well to human-disturbed environments including two invasive species were more common in lower FLII forests. Significant differences in habitat preferences were also observed in relation to forest and canopy cover, revealing additional nuances that the FLII alone did not capture. This study demonstrates that the FLII, when combined with a comprehensive dataset and supplemented with region-specific factors, can assess species' adaptability to human-modified forests, aiding in the development of conservation strategies.

Age and structural constraints on the tectonic evolution of the Paleoproterozoic Saimaa orocline in Fennoscandia

The Paleoproterozoic oroclines of Fennoscandia played a vital role in the making of continental equidimensional crust during the Svecofennian orogeny. We propose that the 1.90–1.88 Ga arc rocks in south-central Finland originally formed a single linear arc system that is now represented by the coupled Bothnian oroclines and their southern continuation, the Saimaa orocline, as defined in this study. A regional large-scale approach with robust geochronological age determinations obtained from metasedimentary rocks and granitoids, and field-based structural studies are used to produce a coherent view on the tectonic evolution of the Saimaa orocline.The pre-orocline tectonic evolution started with a NE-SW collision between the Karelia continent (lower plate) and Proto-Svecofennia (upper plate) at ca. 1.91 Ga (D1). This was followed by subduction reversal and arc magmatism at ca. 1.90–1.88 Ga. Collision or flat subduction at 1.885 Ga caused a widespread collisional event (D2). Our results indicate that the presently E-W oriented D2 structures are not, as is widely proposed, products of N-S contraction but instead originated as a N-S to NNW-SSE oriented belt that was subject to E-W contraction. Dorocline, recorded as the first deformation stage in post-D2 sediments, folded the pre-existing structures into their E-W and ENE-WSW orientation. Following orocline formation, the main shortening direction switched back to the NE-SW (D3) resulting in type-1 and type-2 refolded folds, long wavelength regional folds and bending of the hinge zone of the Saimaa orocline. D4-D5 deformation stages are localized intra-continental tectonic events at 1.83–1.79 Ga. The formation of the Saimaa orocline is bracketed at 1875–1865 Ma and formed by margin-parallel shortening. The Bothnian and Saimaa oroclines form continuous buckles, but whereas the Bothnian coupled oroclines are crustal-scale parallel folds due to more rigid crustal core, Keitele microcontinent, the Saimaa orocline is a crustal-scale isoclinal similar fold.

Browsing hinders the regeneration of broadleaved trees in uneven-aged forest management in southern Finland

ABSTRACT Uneven-aged forest management has attained increasing popularity. It may benefit biodiversity, buffer against pests and abiotic damages, decrease runoff from clearcuts and improve recreational values. We studied how gap cuttings and single-tree selection cuttings influence browsing damage in spruce-dominated forests in south-central Finland. Overall, browsing damage was especially frequent in aspen and rowan, but also in birch. The larger the basal area left in harvesting, the lower the browsing pressure on birch and rowan; neither the harvesting method nor the seedling density had an independent effect on browsing damage. Although larger tree retention levels may reduce browsing damage, the regeneration and recruitment of shade-intolerant broadleaved trees may be compromised.

Soil respiration after slash and stump removal in three clear-cut areas with patch scarification and mounding

ABSTRACTWe studied the soil carbon dioxide respiration (Rs) at three clear-cut mesic forest sites in south, central and north central Finland, which had been treated with different intensities of stump lifting and slash removal and then patch mounded and planted with spruce. The follow-up period after the initial calibration lasted for five consecutive years. Throughout the study the Rs remained at fairly steady levels according to the study site and soil disturbance level. Based on a split-plot test using the general linear model there were no significant differences in the Rs between the different stump and slash removal treatments at the three study sites, but unaffected and moderately affected soil surfaces had significantly higher Rs than mounds and wheel ruts. We conclude that the removal of stumps and slash have minor direct effects on Rs, but large indirect effects through soil disturbance.

Changes in dissolved organic matter and microbial activity in runoff waters of boreal mires after restoration

A considerable proportion of boreal mires have been drained for soil amelioration purposes. In response to drainage-induced degradation, restoration practices have been implemented in recent decades. Restoration by raising the water level is often followed by changes in the quality of runoff waters, especially in concentrations of dissolved organic carbon (DOC), nitrogen (N) and phosphorus (total P, PO4-P). We studied how mire restoration affected bacterial production (BP), bacterial growth efficiency (BGE%) and respiration (R) in mire runoff waters from spruce swamps and Sphagnum pine bogs in south-central Finland. The quality of runoff water was monitored for 8 years (2008–2015) and bacterial activity was measured during 3 years (2010–2012) at runoff weir sites, including two pristine controls, one drained control and four treatment sites. The concentrations of DOC, N and P increased for 3–5 years after restoration. The increased availability of nutrients was followed by doubled BP (from ca. 0.34 to 0.88 µmol C L−1 d−1, averages of restored sites) and BGE% (from ca. 2.7 to 9.2%), whereas microbial respiration was only slightly increased. However, bacterial activity in mire waters was low compared with those generally measured in river and lake waters. This was presumably related to the recalcitrant quality of the mire-originated DOC, which was not clearly influenced by restoration. Dissolved organic matter (DOM) of low bioavailability contributes to browning of headwaters. As our study was focused only on short-term (1–5 years) effects, more research is needed for evaluating long-term impacts of peatland origin DOM on carbon fluxes, microbial activity and food webs of recipient aquatic ecosystems.

Establishing a high-resolution surface sediment chronology with multiple dating methods – Testing 137Cs determination with Nurmijärvi clastic-biogenic varves

Abstract The uppermost 50 cm of sediments from the 23 m deep lacustrine basin of Nurmijarvi, a lake in south-central Finland, are composed of the clastic-biogenic type of varves. We use multiple dating methods to render a sediment chronology that is as precise and accurate as possible for future paleoenvironmental research on this high-fidelity sedimentary archive. The 250-year-long varve chronology contains a chronological error of ±2%, as estimated based on repeated varve counts. The varve-based chronology was verified with the vertical distribution of 137Cs by comparing variations in the paleomagnetic relative declination of the surface sediments with instrumental observations and a calculated representation of declination features in Finland. A comparison of the sediment cesium content using two gamma counters, different sediment pretreatments, and normalization of the 137Cs concentration with masses and accumulation rates of different sediment components revealed that the AD 1986 fallout peak from the Chernobyl accident is the dominant feature in the studied sediments, regardless of the sample pretreatment or normalizing procedure. The vertical distribution of the AD 1986 peak in sediments with clastic-biogenic varves is fairly narrow, thus providing an accurate age-depth marker for the recent deposits. However, the Nurmijarvi section also reveals a ca. 10-year-long period of catchment-to-basin 137Cs mobilization and redeposition after the AD 1986 fallout, as well as significant downwards diffusion of the Chernobyl-derived cesium in the sediment. This study highlights the ways in which the quality of surface sediment chronology needs to be considered and secured prior to comparisons between paleoproxy records and instrumental observations.

Impact of Re-wetting of Forestry-Drained Peatlands on Water Quality—a Laboratory Approach Assessing the Release of P, N, Fe, and Dissolved Organic Carbon

A laboratory column study with peats from four sites from south-central Finland and two sites from blanket peats in the west of Ireland was established to assess the factors contributing to P, N, Fe, and dissolved organic carbon (DOC) transfer to receiving water courses from restored forestry-drained peatlands. The study indicated that the DOC and Fe release from re-wetted peats are likely governed by the amount of Fe in peat and the degree of Fe reduction upon re-wetting. In contrast to our other hypothesis concerning DOC, high degradability of organic matter was not related to high DOC release. Nitrate release was found to largely cease along with oxygen depletion, but ammonium release was considerable from a site with high nitrification potential before wetting. The release of P from anoxic peat was complicated in the sense that it appeared to be controlled by many factors simultaneously. In the nutrient-poor sites, the P release increased following re-wetting, probably because of their high easily soluble peat P content and low Al and Fe content, resulting in high anoxic P mobilization, but limited re-sorption of the mobilized P. Among the three nutrient-rich sites, there was either no P release upon re-wetting or higher P release than from the nutrient-poor sites. Low risk for P release following re-wetting in nutrient-rich sites was associated with low content in peat of easily soluble P and a high molar Fe/P ratio.

A proxy record of winter temperatures since 1836 from ice freeze-up/breakup in lake Näsijärvi, Finland

One obstacle on the way to a comprehensive spatial reconstruction of regional temperature changes over the past centuries is the sparseness of long winter temperature records. This paper reconstructs a proxy record of April and November–December temperatures in south-central Finland for the interval from 1836 to 1872 from breakup and freeze-up dates and ice-cover duration of a lake. Emphasis is on detecting the suitable winter months and quantifying the calibrations with measured temperatures (1873–2002). The calibration slope for the breakup date (0.158°C/day) is larger than for freeze-up date (0.119°C/day) or duration (0.090°C/day). A comparison with results from other proxy records shows that the slope may depend also on the geographical site. Trend analyses of the full temperature records (1836–2002) indicate the existence of minor change-points at around 1867 (April temperature) and 1874 (November–December temperature), with warming rates thereafter of 1.67°C per century (April) and 1.16°C per century (November–December). Spectral analyses reveal peaks in the band between 2 and 5 year period, which may point to influences of the North Atlantic Oscillation, and less power in the decadal band (up to 42 year period).

Export of suspended solids and dissolved elements from peatland areas after ditch network maintenance in south-central Finland

In Finland nearly 6 million hectares of peatlands are drained for forestry purposes. Ditch network maintenance in the drained peatlands, i.e. cleaning old ditches or digging complementary ditches, deteriorates surface water quality by increasing the export of dissolved elements and suspended solids (SS). Effect of ditch network maintenance on the export of SS, dissolved organic carbon (DOC), and dissolved nitrogen (N), phosphorous (P), iron (Fe), aluminum (Al) and manganese (Mn) was studied in nine pairs of treated and control (no maintenance) catchments located in southern and central Finland. In this study we extended the paired catchment approach by combining data from several catchments and identifying the treatment effect on SS and element loads from the entire dataset. Following the method of Laurén et al. (2009) we identified how uncertainty in correlation between treatment and control catchments during pre-treatment period is reflected in the estimated treatment effect on SS and element loads. In the experiment, the export of SS increased significantly for the four year study period following the ditch network maintenance and Al export increased for one year. The export of N, P and Fe was not significantly changed and DOC and Mn export decreased after the ditch maintenance operation.

A comparison of cratonic roots through consistent analysis of seismic surface waves

Four cratonic regions are compared using existing dispersion curves and inverting these curves with an identical inversion method and parametrization. The four areas are the Archaean Kaapvaal Craton (southern Africa), Slave Province (central Canada) and Yilgarn Craton (western Australia), and the predominately Proterozoic region of South-Central Finland. The aim of this study is to identify the reliably resolved differences, and similarities, between the four study areas rather than obtaining a ‘best’ model for each area. Alongside the inversion, we also compare the observed dispersion curves with predicted ones using models with either constant shear velocities or constant composition in the lithosphere. The three main conclusions of this study are that 1) models of constant composition within the lithosphere do not explain the dispersion curves, in fact the less physically-based models of constant shear velocity provide a better fit to the data for all of the areas; 2) a low velocity zone in the deep lithosphere or below the lithosphere is resolved beneath the Kaapvaal while the three other areas have no such low velocity zone; 3) in spite of age differences, the shear velocities in the Kaapvaal, Slave and South-Central Finland are similar while the Yilgarn has significantly faster velocities. We also suggest that in order to understand the creation and evolution of the lithosphere a model driven approach to the analysis of surface wave dispersion is a valuable complement to classical inversion of dispersion curves, which suffer from the problem of non-unique solutions.

Shear wave splitting as a diagnostic of variable anisotropic structure of the upper mantle beneath central Fennoscandia

We analyze splitting of shear waves recorded during the SVEKALAPKO passive seismic experiment in south-central Finland to study fabrics of the mantle lithosphere of the Precambrian region and thus to bring information into a debate on existence of plate tectonics or its forms in the early stage of continent formation. Geographical variations of the splitting parameters and their distinct dependence on direction of wave propagation through the upper mantle allow us to identify six domains of the central Fennoscandian mantle lithosphere, including the Proterozoic–Archean transition, and to model their fabrics by joint inversion of body wave anisotropic parameters. Fabrics of the Archean mantle lithosphere can be approximated by a peridotite aggregate with lineation a dipping to the NE. On the other hand, anisotropy of the Proterozoic mantle lithosphere is weaker and we model its fabric by the (a, c) foliations dipping to the SE. We present a 3D self-consistent anisotropic model of the Proterozoic and Archean upper mantle along the SW-NE profile in the south-central Finland. Boundaries of inter-growing wedges of the Proterozoic and Archean mantle lithospheres explain the longitudinal and shear wave propagation and polarization, mantle xenolith ages, surface wave tomography and location of the upper mantle reflectors. We interpret the six anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric which was created before these micro-continents assembled.

Seasonal activity patterns and movements of the raccoon dog, a vector of diseases and parasites, in southern Finland

Activity patterns and movements of raccoon dogs ( Nyctereutes procyonoides) were studied in Virolahti, southeast Finland, in 2000–2003. Activity data were compared to those collected from Evo, south-central Finland, in 1990–1993. Activity in winter was compared to weather (temperature and snow depth), day length and moon. Also circadian activity rhythm was studied in Evo. Raccoon dogs moved fastest in late winter after winter dormancy and slowest in autumn before settling in their winter dens. In March, males were moving more often than females. Raccoon dogs stayed usually in their dens in mid-winter (December–February) but were sometimes wandering around also during the harshest months of the year and changed their winter den on average three times. Both day length and weather affected the activity of raccoon dogs in winter. Animals usually stayed in their dens, when temperature was below –10 °C, snow depth >35 cm and day length <7 h and were moving around, when temperature was >0 °C, there was no snow and day length was >10 h. Day length and snow depth together predicted rather well the probability of animals being active during winter. Although raccoon dogs were more often active at night than during the light hours, they also showed rather much diurnal activity.

Is there an association ofPneumocystisinfection with the presence of arena-, hanta-, and poxvirus antibodies in wild mice and shrews in Finland?

As part of studies on the nature of the endemic virus infections in natural rodent hosts, the possible association of cyst forms of Pneumocystis spp. with the presence of hanta-, cowpox-, and arenavirus antibodies in wild mice (Apodemus flavicollis, N=105; Apodemus agrarius, N=63; Micromys minutus, N=50) and the common shrew (Sorex araneus, N=101) was studied in south-central Finland. One hantavirus (Saaremaa virus, SAAV) seropositive A. agrarius, and 2 cowpoxvirus (CPXV) seropositive S. araneus were detected, and antibodies against an arenavirus (Lymphocytic choriomeningitis virus, LCMV) were found in all 3 mouse species but not in shrews. Cyst forms of Pneumocystis spp. were detected in all species except A. agrarius. There was no significant association between virus antibodies (LCMV in mice, and CPXV in shrews) and cyst forms of Pneumocystis in any of the species. Concurrent presence of virus antibodies (LCMV) and cyst forms of Pneumocystis were detected only in 1 M. minutus. In conclusion, we found no evidence of any association between Pneumocystis and antibodies to any of the viruses tested.

Capacity of riparian buffer zones to reduce sediment concentrations in discharge from peatlands drained for forestry

In 1995–2001, the efficiency of riparian buffer zone areas to reduce the concentrations of suspended solids in discharge from peatlands drained for forestry purposes was studied at 7 locations in south-central Finland. The two largest buffer zones reduced the concentrations of suspended solids by &gt; 70%. The efficiency of the three medium-sized buffer zones to reduce through-flow sediment concentrations was 50–60%, but no reduction occurred at the smallest two buffer areas. Thus, the capacity of buffer zones to reduce sediment concentrations was strongly related to their size. However, significant correlations were also found between reduction capacity and inflow water sediment concentrations, although the correlations at the two smallest buffer zones were low. The use of buffer zones in reducing sediment load from peatlands drained for forestry purposes is recommended, but relatively large areas for efficient removal capacity are needed.

Spatial clustering of amyotrophic lateral sclerosis in Finland at place of birth and place of death.

Previous evidence for spatial clustering of amyotrophic lateral sclerosis is inconclusive. Studies that have identified apparent clusters have often been based on a small number of cases, which means the results may have occurred by chance processes. Also, most studies have used the geographic location at the time of death as the basis for cluster detection, rather than exploring clusters at other points in the life cycle. In this study, the authors examine 1,000 cases of amyotrophic lateral sclerosis distributed throughout Finland who died between June 1985 and December 1995. Using a spatial-scan statistic, the authors examine whether there are significant clusters of the disease at both time of birth and time of death. Two significant, neighboring clusters were identified in southeast and south-central Finland at the time of death. A single significant cluster was identified in southeast Finland at the time of birth, closely matching one of the clusters identified at the time of death. These results are based on a large sample of cases, and they provide convincing evidence of spatial clustering of this condition. The results demonstrate also that, if the cluster analysis is conducted at different stages of the cases' life cycle, different conclusions about where potential risk factors may exist might result.

Hygroscopic and CCN properties of aerosol particles in boreal forests

The measurements of the hygroscopic and cloud condensation nuclei (CCN) properties of sub-micrometer atmospheric aerosol particles were performed with two tandem differential mobility analysers (TDMA) and a CCN counter at the Hyytiälä forest field station in south-central Finland during the BIOFOR campaign. The TDMAs were used to measure hygroscopic diameter growth factors of individual aerosol particles in the dry particle diameter range 10–365 nm when taken from the dry state (relative humidity RH <5%) to RH=90%. The CCN counter was used to study the activation of aerosol particles when exposed to supersaturated conditions. The measurements show clear diurnal pattern of particle solubility. The pattern was strongest for particles in nucleation and Aitken modes. The lowest growth factor (soluble fraction) values were detected during late evening and early morning and the maximum was observed during noon-afternoon. The highest soluble fractions were determined for nucleation mode particles. The response of hygroscopic growth to changes of relative humidity suggests that the soluble compounds are either fully soluble or deliquescent well before 70% RH. The hygroscopic growth was investigated additionally by a detailed model using the size-resolved composition from the impactor samples. The comparison between different instruments shows good consistency. We found good agreement for the 20 nm growth factors measured with two TDMAs, not only on average but also regarding the temporal variation. The similar conclusion was drawn for comparison of TDMA with CCNC for Aitken mode particles with dry sizes 50 and 73 nm. Differences between wet and dry spectra measured using APS and CSASP spectrometer probes were used to derive growth factors for coarse mode particles. Growth factors for coarse mode particles (Dp ca. 2 μm) ranged between 1.0 and 1.6. Agreement between the evolution of growth factors with time for both accumulation and coarse modes was observed regularly. However, similar portions of the data set also indicated clear differences and consequently different chemical compositions between both modes. When the differences between both modes were observed, the coarse mode always behaved in a less hygroscopic manner, with growth factors near one.

Composite model with fractional flow dimensions for well test analysis in fractured rocks

The represented analytical model is addressed to fracture-zone aquifers in which well test responses can be commonly matched with models involving fractional flow dimensions. The model assumes a composite structure representing the fractures opening into the pumping well and the fracture system comprising the actual aquifer. The aim is to assess the hydraulic properties for the near-well zone and the aquifer in terms of concrete parameters even if flow dimensions have fractional values. Dimensionless drawdown in pumping and observation wells was solved by applying Laplace transforms. The early-time approximation for the pumping well can be used to assess the transmissivity ( T 1) if the flow dimension in the inner zone ( n 1) is greater than about 1.7. The long-time solutions indicate that the inner zone induces a pseudo-skin effect that can be estimated by comparing the pumping well and observation well data. If K 1 is known, the extent of the inner zone ( r′) can be estimated from the pseudo-skin factor. The responses tend to form log–log straight lines with slopes of 1− n 2. By plotting drawdown vs. t (1− n 2) on the double-logarithmic scale, the product of the hydraulic conductivity and the through-flow area at the wellbore radius ( KA 2) and the hydraulic diffusivity ( D 2) of the aquifer can be estimated from the slope and the intercept. The hydraulic conductivity of the aquifer ( K 2) is estimated by considering the through-flow area as a projection of an n 2-dimensional sphere with a radius of r wb through three-dimensional space by an amount r′ (3− n 2) . The analyzed field example comprises a pumping test of a fracture zone aquifer in south–central Finland. For the pumping well, simultaneous matching of drawdown and derivative type-curves with observed responses can be used to verify the results of the early-time analysis and the proposed straight-line procedure. In addition, type-curve matching yields estimates of the hydraulic diffusivity ratio and a value of the dimensionless wellbore storage. The analyzed observation wells are considered to be sufficiently far from the inner zone. Consequently, type-curves are produced based on the dimensionless drawdown solution that is identical to generalized radial flow model. Type-curve matching of the observation well responses produces estimates for KA 2, K 2 and D 2. The obtained values are similar to previous studies that included models relying on integer flow dimensions. However, models relying on the fractional flow dimensions are capable to simulate the observed responses more accurately.

Disease patterns in field and bank vole populations during a cyclic decline in central Finland

Declining field vole ( Microtus agrestis) and bank vole ( Clethrionomys glareolus) populations were sampled (117 field voles and 34 bank voles) in south-central Finland during the winter of 1988–89. The last surviving field voles were caught in April and bank voles in February. A subsample (16) of the April field voles were taken live to the laboratory for immunosuppression. The histopathology of the main internal organs and the presence of aerobic bacteria and certain parasites were studied. In the lungs, an increase in lymphoid tissue, probably caused by infections, was the most common finding (52% of all individuals). The prevalences in the voles, in the whole material, of Chrysosporium sp. and Pneumocystis carinii in lungs were 13 and 10% in field voles, and 9 and 0% in bank voles, respectively. Cysts of Taenia mustelae (9 and 27%) were the most common pathological changes in the liver. Enteritis was also rather common (14 and 34%). In field voles the prevalences of Frenkelia sp. in the brain and Sarcocystis sp. in leg muscles were low (both 6%). Bordetella bronchiseptica was commonly (31%) isolated from field vole lungs and Listeria monocytogenes from the intestines (34%). Salmonella spp. could not be found. The dynamics and abundance of inflammations in the lungs and intestines, as well as B. bronchiseptica isolations from the lungs, indicate that obvious epidemics took place in declining vole populations. Of the Luhanka subsample of 16 field voles brought to the laboratory in April, one died of listeriosis, two of Bordetella, and five died for unknown reasons. Even if small mustelids are the driving force in microtine cycles, it is possible that diseases also contribute to the decline.

Mineral chemistry constraints on the evolution of the 1.88–1.87 Ga post-kinematic granite plutons in the Central Finland Granitoid Complex

A suite of post-kinematic, 1.88–1.87 Ga, silicic plutons crosscut 1.89–1.88 Ga synkinematic granitoids in the Central Finland Granitoid Complex (CFGC) in south-central Finland. The plutons range from biotite±hornblende quartz monzonite to syenogranite and include pyroxene- and olivine-bearing varieties. Mineral chemical data on feldspars, biotite, amphibole, pyroxenes, olivine, and oxides of the post-kinematic plutons are presented. The data are interpreted to show that these plutons register (1) a considerable range in pressure from 2–4 kbar (amphibole barometry) to 5–7 kbar (olivine–pyroxene barometry), (2) temperatures mostly reflecting resetting during cooling (450–800°C; QUIlF thermometry), and (3) low fO 2 (log fO 2 ΔFMQ −0.3 to −1.5; QUIlF equilibria). In particular, plutons with olivine- and pyroxene-bearing margins and amphibole-dominated central parts record progressive oxidation and hydration upon cooling, shifting from the QUIlF equilibrium toward KUIlB. The post-kinematic granites can be considered post-collisional in regard to compressional events in the CFGC and display many of the characteristics of the anorogenic 1.6 Ga rapakivi granites further south. They were presumably derived from a deep and dry crustal source, like the rapakivi granites.

Origins and ages of Proterozoic granitoids in the Bothnian Basin, central Sweden; isotopic and geochemical constraints

The Paleoproterozoic Bothnian Basin in central Sweden and south-central Finland is dominated by a 10 km thick sequence of metasediments, mainly metaturbidites. These have been intruded by granitoids, which have traditionally been classified as early, late and postorogenic relative to the c. 1.9-1.8 Ga S vecofennian Orogeny. The early orogenic granitoids are part of a calcalkaline suite ranging in composition from ultramafic to granitic, while the late orogenic granitoids mainly are purely granitic and associated with abundant pegmatites. The postorogenic group forms large massifs of mainly granitic composition, and associated pegmatites are rare. An early orogenic tonalité yields a U-Pb zircon age of 1877 ± 6 Ma, which is within the age range for S vecofennian granitoids of this tectonic position elsewhere in Sweden and Finland. A late orogenic granite belonging to the Härnö granites from the southern part of the Bothnian Basin gives a U-Pb monazite age of 1822 ± 5 Ma. This provides a minimum age for the main phases of Svecofennian deformation in the region, and a correlation with the 1.84-1.83 Ga old granite-migmatite zone of southern Finland is suggested. A postorogenic granite belonging to the Revsund granitoids gives a U-Pb zircon age of 1798 ± 8 Ma. Together with previously published age data, this indicates that the Revsund granitoids were emplaced successively during an age interval of at least between 1.80 and 1.77 Ga. REE patterns and trace element model calculations indicate that late orogenic granites could have been produced by partial melting of metasediments similar to those exposed at the surface. A contribution from the early orogenic granitoids is possible but not discerned by these calculations. The postorogenic granites, on the other hand, have too high concentrations of Sr, Ba, Y and HREE to be derived from such a metasedimentary source. The initial ε Nd values for the early orogenic granitoids range between + 2 and −1. Corresponding values for both late and postorogenic granites range between −1 and −3, which is on average slightly higher than those for surrounding metasediments. The initial Sr isotope ratios for the Härnö granites are intermediate between those for the metasediments and the early orogenic granitoids, while the initial Sr isotope ratios for the Revsund granites are distinctly lower. The geochemical and isotopic data support a model in which the Härnö granites were derived mainly from Bothnian Basin metasediments, with a contribution from early orogenic granitoids. The composition of the Revsund granitoids is, however, inconsistent with such a derivation. For these rocks a deeper, as yet unknown crustal source with a significant pre-1.9 Ga crustal component is indicated.