Annually laminated (varved) lake sediments constitutes excellent environmental archives, and have the potential to play an important role for understanding past seasonal climate with their inherent annual time resolution and within-year seasonal patterns. We propose to use functional data analysis methods to extract the relevant information with respect to climate reconstruction from the rich but complex information in the varves, including the shapes of the seasonal patterns, the varying varve thickness, and the non-linear sediment accumulation rates. In particular we analyze varved sediment from lake Kassjon in northern Sweden, covering the past 6400 years. The properties of each varve reflect to a large extent weather conditions and internal biological processes in the lake the year that the varve was deposited. Functional clustering is used to group the seasonal patterns into different types, that can be associated with different weather conditions. The seasonal patterns were described by penalized splines and clustered by the k-means algorithm, after alignment. The observed (within-year) variability in the data was used to determine the degree of smoothing for the penalized spline approximations. The resulting clusters and their time dynamics show great potential for seasonal climate interpretation, in particular for winter climate changes.
Lake acidification became an environmental issue of international significance in the late 1960s and early 1970s when Scandinavian scientists claimed that ‘acid rain’ was the principal reason why fish populations had declined dramatically in Swedish and Norwegian lakes (Odén, 1968; Jensen & Snekvik, 1972; Almer et al., 1974). Similar claims were being made at about the same time in Canada (Beamish & Harvey, 1972). However, these claims were not immediately accepted by all scientists. It was argued instead that acidification was due to natural factors or to changes in catchment land-use and management (Rosenqvist 1977, 1978; Pennington 1984; Krug & Frink, 1983).
In the scientific debate that followed, diatom analysis played a pivotal role. It enabled the timing and extent of lake acidification to be reconstructed (Charles et al., 1989; Battarbee et al., 1990; Dixit et al., 1992a) and allowed the various competing hypotheses concerning the causes of lake acidification to be evaluated (Battarbee et al., 1985; Battarbee & Charles 1994; Emmett et al., 1994). However, diatoms had been recognized and used as indicators of water pH well before the beginning of this controversy. The ‘acid rain’ issue served to highlight the importance of diatoms and stimulated the advance of more robust and sophisticated techniques, especially the development of transfer functions for reconstructing lakewater pH and related hydrochemical variables.
This chapter outlines the history of diatoms as pH indicators, and describes how diatoms are currently used in studies of acid and acidified waters.
Implementation of the European Union Water Framework Directive and associated national guidelines has emphasized the value of using biota, such as epilithic diatoms in streams, as indicators of water quality. However, guidelines for evaluating diatom samples have been established without explicitly evaluating their statistical robustness. We used epilithic diatom samples from 73 streams in northern Sweden and simulated the effects of variations in the counting sum size and taxonomic resolution of classifications for two indices indicating pollution (Indice de Polluo-sensibilité Spécifique, IPS) and acidity (acidity index for diatoms, ACID). Instead of the stipulated 400, we found that a count sum of 40 diatom valves for 50 streams, and 80 valves for 60 streams, would have been sufficient to obtain the same IPS index classification. The ACID index is more sensitive to count sum reductions, since the same classification would only have been obtained for 12 streams with 40 counted diatom valves or 24 streams with a count of 80 valves. Excluding rare taxa had negligible effects on the IPS and ACID indices. Excluding taxa occurring with less than 1.0% frequency affected the IPS classification of only one stream, and excluding taxa with less than 2.5% and 5.0% frequencies affected those of just one and no streams, respectively. The ACID index was affected for none, five, and 12 streams, respectively. At least in relatively unpolluted regions such as northern Sweden, our simulations suggest that a simplified methodological approach with site-specific counting sum sizes and reduced taxonomical resolution could be adopted, taking into account the way sites are classified in relation to established class boundaries. The simplified method is a step forward in improving the cost efficiency for stream monitoring, as costs of diatom analysis to obtain identical IPS and ACID classifications of our streams could be reduced considerably. Before the simplified method can be widely adopted, further simulations including regions with a higher proportion of polluted streams are required.
Metal pollution is viewed as a modern problem that began in the 19th century and accelerated through the 20th century; however, in many parts of the globe this view is wrong. Here, we studied past waterborne metal pollution in lake sediments from the Bergslagen region in central Sweden, one of many historically important mining regions in Europe. With a focus on lead (including isotopes), we trace mining impacts from a local scale, through a 120-km-long river system draining into Mälaren Sweden's third largest lake, and finally also the Baltic Sea. Comparison of sediment and peat records shows that pollution from Swedish mining was largely waterborne and that atmospheric deposition was dominated by long-range transport from other regions. Swedish ore lead is detectable from the 10th century, but the greatest impact occurred during the 16th18th centuries with improvements occurring over recent centuries, i.e., historical pollution > modern industrial pollution.
Paleolimnology provides long-term data that are often essential for understanding the current state of the environment. Even though there is great potential, paleolimnology is rarely used together with process-related studies to solve issues regarding cycling of elements in the environment. Clearly, this is a drawback because the cycling of many elements, which cause great concern in the present-day environment, was altered long before the advent of monitoring programs. The pioneering work of C.C. Patterson and his colleagues emphasized the importance of a long-term perspective for understanding the current cycling of metals, with a focus on lead, and in particular for estimating background concentrations and human-related impacts in the environment. In Sweden the first traces of atmospheric lead pollution are found in lake sediments dated to about 3500 years ago. The long-term changes in the pollution lead record in lake sediments led us to consider how lead biogeochemistry has changed over time in response to this historical deposition‹where has this lead gone, and how much does this lead contribute to the present-day biogeochemical cycling of lead? How was lead distributed in Œpre-industrial¹ soils or more properly in natural soils not impacted by atmospheric pollution? There are many studies that have examined the effects of increased metal concentrations on soil biota, but what are the appropriate background conditions for comparison? Using lake sediments as our foundation we have analyzed lead, including its stable isotopes, in other environmental compartments, including peat, soil, and a range of boreal forest plant species, to develop a better understanding of the fate of lead derived from long-term pollution. Three important conclusions from our studies in Sweden are: (1) atmospheric lead deposition rates during the 20th century were 100 to as much as 1000 times higher than natural deposition rates a few thousand years ago. Even with stricter emission standards during the past three decades and the resultant reductions in deposition, lead deposition rates today are still 10100 times greater than natural rates. This increase in deposition rates modeled from sediment and peat records is of a similar scale to estimated changes in body burdens of lead in modern versus ancient humans. (2) In Europe about half of the cumulative burden of atmospherically deposited lead was deposited before industrialization. In southern Sweden the cumulative burden of pollution lead during the past 3500 years is 25 g Pb m-2 and in the Œpristine¹ northern parts of the country there is about 1 g Pb m-2. (3) Predicted recovery rates for soils are slow; in the cold climate of Scandinavia, we find that the soil surface (O horizon), where most soil biota reside, retains lead deposited over the past 150500 years. Therefore, although lead deposition rates in Europe, as well as N. America, are only 10% of those a few decades ago, it will take several decades or longer for lead concentrations in soils to respond appreciably. The slow turnover rates for lead in the environment and gradual immobilization of lead in deeper soil mineral horizons also inhibits a loss of lead to surface waters in areas removed from point sources.
The intention of the European Water Framework Directive (WFD) and the national guidelines that implement the WFD is that present-day conditions and future management strategies are to be based on an understanding of reference conditions for the particular water body of interest. In the context of non-synthetic pollutants such as lead, mercury and cadmium, the criteria for a high ecological status are that "concentrations [are] within the range normally associated with undisturbed conditions". How this normal range is to be defined is open to interpretation; for example, in Sweden reference conditions based on sediment records are defined as the conditions prior to modern industrialization, i.e. prior to the mid-1800's. These pre-industrial reference conditions would correspond to sediments 15-30 cm depth. However, 'reference conditions' are not always synonymous with 'natural background conditions'. Analyses of long sediment profiles from Swedish lakes and from a few other areas, however, have shown that pre-industrial pollution-at least with regard to lead-was extensive. Atmospheric lead pollution has its origin in antiquity, with a small, well-defined peak already during the Greek-Roman period 2,000 years ago. Sediments deposited 300-500 years in Sweden and Scotland, for example, show a dominance of pollution lead, and in some sediment records also cadmium and copper pollution was extensive. Thus, in order to characterize natural background concentrations of metals, long sediment profiles are needed to reach sediments unaffected by pollution (> 3,000 years BP); this can correspond to sediments below 50 cm in some lakes, but in others sediments below 300 cm or more.
The historical Norberg mining district in central Sweden with its shallow, easily accessible iron ores figures prominently in the earliest documents from the 14th century concerning mining or metallurgy. This 1000-km2 district is considered to be one of the first areas in Sweden exploited for iron ores and, in fact, Europe’s oldest known blast furnace, Lapphyttan, is located in the Norberg district about 10 km from the mines in the village of Norberg (Norbergsby). Earlier archaeological excavations suggest the furnace was in operation as early as the 11th or 12th century (870 and 930 14C yr BP), and a number of other sites in the district have been dated to the 13th–15th centuries. Here, we have analyzed two lake sediment records (Kalven and Noren) from the village of Norberg and a peat record from Lapphyttan. The Lapphyttan peat record was radiocarbon dated, whereas the sediment from Kalven is annually laminated, which provides a fairly precise chronology. Our pollen data indicate that land use in the area began gradually as forest grazing by at least c. AD 1050, with indications of more widespread forest disturbance and cultivation from c. 1180 at Lapphyttan and 1250 at Kalven. Based on 206Pb/207Pb isotope ratios in Kalven’s varved sediment record, there is an indication of mining or metallurgy in the area c. 960, but likely not in immediate connection to our sites. Evidence of mining and metallurgy increases gradually from c. 1180 when there is a decline in 206Pb/207Pb ratios and an increase in charcoal particles at Lapphyttan, followed by increasing inputs of lithogenic elements in Noren’s sediment record indicating soil disturbance, which we attribute to the onset of mining the iron ore bodies surrounding Noren. From AD 1295 onwards evidence of mining and metallurgy are ubiquitous, and activities accelerate especially during the late 15th century; the maximum influence of Bergslagen ore lead (i.e., the minimum in 206Pb/207Pb isotope ratios) in both Kalven and Noren occurs c. 1490–1500, when also varve properties change in Kalven and in Noren sharp increases occur in the concentrations of a range of other ore-related metals (e.g., arsenic, cadmium, copper, iron, lead, mercury and zinc). From the 15th century onwards mining and metallurgy are the dominant feature of the sediment records.
Sulfur has played a central role in the acidification of many lakes in Scandinavia and other regions. As part of the research into sulfur cycling, numerous studies have analyzed the sediment record in order to develop insights into past in-lake cycling of sulfur, particularly in the context of reconstructing past deposition rates. Although many of these studies have shown that it is not easy to interpret the sediment record in terms of past sulfur deposition rates, analyses of sulfur in sediment still provide valuable information on the response of lakes to anthropogenic sulfur deposition. Here, we have analyzed sulfur in top and bottom samples from short surface cores (25-35 cm, representing >= 250 years) as well as bulk cores from similar to 110 lakes located throughout Sweden, which were collected during 1986, as well as in more-detailed profiles from six lakes. The lakes with the highest surface sediment concentrations (9-24 mg S g(-1) dry mass) and the highest calculated inventories of 'excess' sulfur (20-180 g S m(-2)) are found in southern Sweden and around one industrial area along the northeastern coast where sulfur deposition rates and lake-water concentrations have been highest. For many lakes in the central and northern inland region it is common that the sediment cores exhibit either no enrichment or even a decline in sulfur concentrations in near-surface sediments, which we suggest was the pre-pollution norm for lakes. Although interpreting sulfur sediment profiles is problematic for reconstructing deposition, a more-comprehensive spatial sampling approach shows that there is a good geographic agreement between sulfur deposition, lake-water chemistry and sediment sulfur accumulation.
In lake sediment investigations of heavy metal pollution history, it has become a common approach to calculate enrichment factors (EFs) by normalizing elemental distributions to a reference lithogenic element. However, this approach requires that the reference element remains stable once it has been deposited to the sediment (it is not affected by diagenetic processes). This is rarely studied in well-controlled field experiments. Here, we test the commonly used reference elements titanium (Ti), zirconium (Zr), aluminum (Al), and rubidium (Rb). We use a unique series of freeze cores collected in different years since 1979 in Lake Nylandssjon in northern Sweden. This lake has sediment with distinct varves (annually laminated deposit). Element concentrations in individual varves were analyzed using X-ray fluorescence (XRF) spectroscopy. By tracking the newly formed surface varve from different cores across this core series, i.e., following the element concentration in a specific varve as it becomes progressively aged, it was possible to assess the potential impact of diagenetic processes on geochemical signatures. Results confirm the conservative character of the studied elements; there was neither an increasing nor a decreasing concentration trend with time during sediment ageing for any of these elements. Secondly, we addressed the question 'which of them is the most appropriate for EFs estimates with the aim of distinguishing anthropogenic from geogenic inputs, for example in pollution studies'. To assess the reliability of the EFs we used lead (Pb) as an example, because anthropogenic Pb in the sediment could be independently calculated using stable Pb isotopes. When anthropogenic Pb concentrations calculated with Pb-EFs were compared to the anthropogenic Pb concentrations derived from stable Pb isotopes, the differences found were 20% for Ti, 10% for Zr, 11% for Al, and 27% for Rb when upper continental crust concentrations were used for the background ratio. Based on the results from Nylandssjon our suggestions are that (1) when using EFs on a single core, which is the normal case in paleolimnology, multiple reference elements should be used together and (2) the results from those should be critically evaluated.
Over recent decades, palaeolimnological records from remote sites have provided convincing evidence for the onset and development of several facets of global environmental change. Remote lakes, defined here as those occurring in high latitude or high altitude regions, have the advantage of not being overprinted by local anthropogenic processes. As such, many of these sites record broad-scale environmental changes, frequently driven by regime shifts in the Earth system. Here, we review a selection of studies from North America and Europe and discuss their broader implications. The history of investigation has evolved synchronously with the scope and awareness of environmental problems. An initial focus on acid deposition switched to metal and other types of pollutants, then climate change and eventually to atmospheric deposition-fertilising effects. However, none of these topics is independent of the other, and all of them affect ecosystem function and biodiversity in profound ways. Currently, remote lake palaeolimnology is developing unique datasets for each region investigated that benchmark current trends with respect to past, purely natural variability in lake systems. Fostering conceptual and methodological bridges with other environmental disciplines will upturn contribution of remote lake palaeolimnology in solving existing and emerging questions in global change science and planetary stewardship.
Near-infrared spectrometry (NIR) is a rapid, inexpensive and reagent-free technique, widely used in industry in areas such as quality control and process management. The technique has great potential for environmental monitoring of aqueous systems. This study assesses relationships, using PLS regression, between NIR spectra of seston collected on glass fibre filters and the following measured lake water parameters: total organic carbon (TOC), total phosphorus (TP), Abs420 and pH. Water samples were collected from 271 oligotrophic lakes during autumn 1995. The predictive model for TOC explained 68% of the variance (SEP=2.1 mg L-1, range 14.9 mg L-1), and that for colour 71% (SEP=0.04 A, range 0.36 A), while the explained variances for pH and TP were 72% (SEP=0.36 μg L-1, range 3.13 μg L-1) and 45% (SEP=4 μg L-1, range 41 μg L-1), respectively. A model correlating NIR spectra and the actual amount of phosphorus in the seston captured on filters explained 86% of the variance (SEP=0.044 μg/filter, range 0.47). Several pretreatments and regression techniques were used in an attempt to enhance modeling performance. However, straightforward PLS on raw data performed best in all cases.
Detta är den tredje rapporten i en serie paleolimnologiska undersökningar av kalkade referenssjöar.
Despite the relatively well-recognized emission rates of the anthropogenic 1291, there is little knowledge about the temporal fallout patterns and magnitude of fluxes since the start of the atomic era at the early 1940s. We here present measurements of annual 1291 concentrations in sediment archives from Sweden and Finland covering the period 1942-2006. The results revealed impression of 1291 emissions from the nuclear reprocessing facility at Sellafield and La Hague and a clear Chernobyl fallout enhancement during 1986. In order to estimate relative contributions from the different sources, a numerical model approach was used taking into account the emission rates/estimated fallout, transport pathways, and the sediment system. The model outcomes suggest a relatively dominating marine source of 1291 to north Europe compared to direct gaseous releases. A transfer rate of 1291 from sea to atmosphere is derived for pertinent sea areas (English Channel, Irish Sea, and North Sea), which is estimated at 0.04 to 0.21 y(-1).
Since the 1970s liming has been practised on a large scale in Sweden to mitigate acidification and several thousand lakes and streams are limed repeatedly. The Swedish monitoring program ISELAW (Integrated Studies of the Effects of Liming Acidified Waters) studies the long-term effects of liming. This paper summarizes the paleolimnological part of the program. Sediment cores from 12 lakes were analysed to study the development of the lakes from pre-industrial time until the present, and address questions about natural conditions and the effects of early human impact, acidification, and liming. Diatoms were used to reconstruct the pH history and to study shifts in species composition due to acidification and liming. Analyses of lead and spheroidal carbonaceous particles were applied for indirect dating and as indicators of the atmospheric deposition of pollutants associated with acid rain. Pollen analysis was performed in eight of the lakes to study the vegetation and agricultural history. The natural pH (prior to human disturbance) was between 5.3 and 6.5 in the eight lakes where the complete post-glacial sediment sequence was recovered. Pollen from anthropochores and apophytes indicated early agricultural land use in the vicinity of the lakes from 1000 to 2000 years ago, and pH increased with land use in six of these lakes. Five of the lakes have been acidified during recent decades, and in all 12 lakes some effects of liming were recorded in the diatom assemblage. The lakes show different responses to liming, including a return to a pre-acidification diatom composition or a shift to a state previously not recorded in the lake’s histories. This study accentuates the complexity of biological response to acidification and liming, and highlights the importance of historical perspectives to assess the current state of a lake’s ecosystem and to establish adequate restoration goals.
Detta är den fjärde rapporten i en serie paleolimnologiska undersökningar av kalkade referenssjöar. Undersökningen har utförts inom ramen för IKEU-projektet på uppdrag av Naturvårdsverket.
Detta är den andra rapporten i en serie paleolimnologiska undersökningar av kalkade referenssjöar. Dessa undersökningar har utförts inom IKEU-projektet på uppdrag av Naturvårdsverket.
Analys av kiselalger från stenar i 73 bäckar i Västerbotten har använts för att bestämma bäckvattnets status vad gäller surhet och näringsgrad/organisk förorening. Provtagningen gjordes under sensommaren/hösten 2007 med en provtagare som kallas ”Stone Brusher”. Kiselalgsanalyserna utfördes enligt metod SS-EN 14407 (SIS 2005) och Naturvårdsverkets handbok för miljöövervakning. Surhetsindex ACID resp IPS index för näringsgrad/organisk förorening bestämdes enligt anvisningarna, och utvärderingen gjordes enligt de senaste bedömningsgrunderna (Naturvårdsverket 2007). Av de 73 bäckarna fick 6 bäckar bedömningen Alkaliskt vatten, 27 bäckar Nära neutralt vatten, 27 bäckar Måttligt surt vatten, 10 bäckar Surt vatten och 3 bäckar Mycket surt vatten. Således dominerar bäckar med nära neutralt eller måttligt surt vatten. De alkaliska bäckarna är belägna i kustens jordbruksbygd eller är kalkade gruvbäckar. Några av de sura bäckarna flyter genom sulfidrika jordar vid kusten och andra är sura av organiska syror (bäckar med mycket brunt vatten). De tre bäckarna som kiselalgerna klassificerar som mycket sura ligger i samma område i Nordmaling, och dessa bäckar är inte särskilt bruna. När det gäller statusbedömningen enligt näringsgrad/organisk förorening (IPS index) fick 62 bäckar bedömningen Hög status, 10 bäckar God status, och 1 bäck Måttlig status. Överlag är således de undersökta bäckarna inte påverkade eller ganska lite påverkade av näringsämnen. De bäckar som inte uppnår högsta status är främst jordbruksbäckar. För ett par bäckar i Lycksele kan påverkan av näringsämnen ha en naturlig geologisk förklaring. Undersökningen visar att miljöövervakning med kiselalger är en väl fungerande metod. Samtidigt visar undersökningen att miljöövervakning med hjälp av kiselalger enligt Naturvårdsverkets anvisningar är en metod för specialister. I andra delen av rapporten visar vi med hjälp av simuleringstester att metoden kan förenklas. Det är i de allra flesta bäckar inte nödvändigt att räkna mera än en bråkdel av det antal skal (400 st) som föreskrivs i anvisningarna. Det är inte heller meningsfullt att ägna tid åt att bestämma kiselalger som bara uppträder med någon procent i preparaten eftersom dessa taxa inte spelar någon avgörande roll för statusbedömningarna. Man behöver inte heller splittra upp arter i varieteter och former i den grad som anvisningarna förespråkar. Faktum är att ett begränsat antal dominerande arter bestämmer vilken status en bäck får. Vi förespråkar att en enklare modell för kiselalgsbaserad miljöövervakning utarbetas. Det finns redan för många exempel på att alltför specialiserade och ambitiösa miljöövervakningsmetoder inte blir långsiktigt hållbara – de läggs ned med sina skapare innan tidsserierna egentligen blir utvärderingsbara. I sista delen av rapporten pekar vi på en kompletterande, eller om man så vill, alternativ modell för miljöövervakning i rinnande vatten, nämligen närinfraröd spektroskopi (NIRS) på det epilitiska materialet. NIRS är en sedan länge etablerad, enkel och billig kvalitetskontrollmetod inom industrin. Allt talar för att den också skulle kunna bli det inom miljöövervakningen.
Koltjärnen and Nylandssjön are two closely situated lakes (<2 km apart) in northern Sweden. During the past century, distinct varved sediments have formed in these lakes. Nylandssjön has two varved, deep basins. Since lake and catchment characteristics superficially appear very similar for the two lakes and they are exposed to the same climate, one would expect the sediment varves to be similar. This investigation compares the varves in the two deep basins of Nylandssjön, and in the two lakes. The comparison of basins of Nylandssjön shows that varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1970–2003). The grey-scale curves are only clearly similar in about 50% of the varves. In the between-lake comparison varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1950–1996). However, the annual accumulation rates of dry mass, minerogenic matter and biogenic silica differ between the lakes, as well as within-varve successions in grey-scale. A general explanation to the differences is that the prerequisites for varve formation are not totally similar because of differences in catchment size, catchment- to-lake material fluxes, lake productivity and land-use influence. This study illustrates the complex relationships that exist between a lake, its catchment, in-lake productivity and formation of sediment varves. As a consequence, we must not apply a too simplistic view of the potential of varves as past climatic indicators, especially if the lakes are affected by land-use.
Easily discernible sediment varves (annual laminations) may be formed in temperate zone lakes, and reflect seasonal changes in the composition of the accumulating material derived from the lake and its catchment (minerogenic and organic material). The appearance of varves may also be influenced by chemical processes. We assessed the role of iron (Fe) and sulfur (S) in the appearance of varves in sediments from Lake Nylandssjön in northern Sweden. We surveyed Fe in the lake water and established whether there is internal transport of Fe within the sediment. We used a unique collection of seven stored freeze cores of varved sediment from the lake, collected from 1979 to 2004. This suite of cores made it possible to follow long-term changes in Fe and S in the sediment caused by processes that occur in the lake bottom when the sediment is ageing. We compared Fe and S concentrations using X-ray fluorescence spectroscopy (XRF) in specific years in the different cores. No diagenetic front was found in the sediment and the data do not suggest that there is substantial vertical transport of Fe and S in the sediment. We also modeled Fe and S based on thermodynamic, limnological, and sediment data from the lake. The model was limited to the five components H+, e-, Fe3+, SO42-, H2CO3 and included the formation of solid phases such as Fe(OH)3 (amorphous), FeOOH (aged, microcrystalline), FeS and FeCO3. Modeling showed that there are pe (redox) ranges within which either FeS or Fe(OH)3/FeOOH is the only solid phase present and there are pe ranges within which the two solid phases co-exist, which supports the hypothesis that blackish and grey-brownish layers that occur in the varves were formed at the time of deposition. This creates new possibilities for deciphering high-temporal-resolution environmental information from varves.
We used a collection of ten freeze cores of annually laminated (varved) lake sediment from Nylandssjön in northern Sweden collected from 1979 to 2007 to follow the long-term loss of carbon (C) and nitrogen (N) due to processes that occur in the lake bottom as sediment ages. We compared specific years in the different cores. For example, the loss of C from the surface varve of the 1979 core (sediment deposited during 1978) was followed in the cores from 1980, 1985, 1989, and so on until 2006. The C concentration of the sediment decreased by 20% and N decreased by 30% within the first five years after deposition, and after 27 yr in the sediment, there was a 23% loss of C and 35% loss of N. Because the relative loss of C with time was smaller than loss of N, the C:N ratio increased with increasing age of the sediment; the surface varves start with a ratio of ~10, which then increases to ~12.
Peak activities of radiocaesium (137Cs) in lake sediments have frequently been used to infer the ages of sediments deposited in the 1960s (137Cs derived from nuclear bomb testing) or in 1986 (Chernobyl derived 137Cs). Records of the vertical distribution of 137Cs in sediments can thus be used to provide accurate dates for a critical period in which palaeoecological reconstructions often overlap contemporary monitoring data. However, knowledge regarding how the distribution of 137Cs in sediments is affected by post-depositional processes is limited to interpretations based on the 137Cs distribution in sediments sampled at a single given date. This study assesses the extent to which the 137Cs record in annually laminated (varved) lake sediments is affected by post-depositional diffusion, using 11 archived sediment cores sampled between 1986 and 2007. The sediment record reveals how Chernobyl 137Cs incorporated into the 1986 varve diffused downwards in the core at a decreasing rate over time, whereas the surface sediments continued to receive inputs of 137Cs mobilized from the catchment soils or lake margin. In spite of these processes, all cores post-dating the Chernobyl accident had a clear and well-resolved peak in the 1986 varve, justifying the use of this feature as a fixed chronostratigraphic feature. Because of the very high levels of Chernobyl fallout at this site, downwards migration of Chernobyl 137Cs has, however, completely masked the nuclear weapons 137Cs fallout peak that had been clearly preserved in the 1964 varve of a pre-Chernobyl core sampled just three weeks before the Chernobyl accident. In consequence, the weapons fallout marker is likely to be of little use for determining 137Cs dates in areas strongly affected by high levels of Chernobyl fallout.
It is not well-known how the accumulated pool of atmospheric lead pollution in the boreal forest soil will affect the groundwater and surface water chemistry in the future as this lead migrates through the soil profile. This study uses stable lead isotopes (206Pb/207Pb and 208Pb/ 207Pb ratios) to trace the transport of atmospheric lead pollution within the soil of a small catchment and predict future lead level changes in a stream draining the catchment. Low 206Pb/207Pb and 208Pb/207Pb ratios for the lead in the soil water (1.16 ± 0.02; 2.43 ± 0.03) and streamwater (1.18 ± 0.03; 2.42 ± 0.03) in comparison to that of the mineral soil (>1.4; >2.5) suggest that atmospheric pollution contributes by about 90% (65−100%) to the lead pool found in these matrixes. Calculated transport rates of atmospheric lead along a soil transect indicate that the mean residence time of lead in organic and mineral soil layers is at a centennial to millennial time scale. A maximum release of the present pool of lead pollution in the soil to the stream is predicted to occur within 200−800 years. Even though the uncertainty of the prediction is large, it emphasizes the magnitude of the time lag between the accumulation of atmospheric lead pollution in soils and the subsequent response in streamwater quality.
The use of stable Pb isotopes for tracing Pb contamination within the environment has strongly increased our understanding of the fate of airborne Ph contaminants within the boreal forest. This paper presents new stable Pb isotope (Pb-206/Pb-207 ratio) measurements of solid soil samples, stream water (from a mire outlet and a stream draining a forest dominated catchment) and components of Picea abies (roots, needles and stemwood), and synthesizes some of the authors' recent findings regarding the biogeochemistry of Ph within the boreal forest. The data clearly indicate that the biogeochemical cycling of Pb in the present-day boreal forest ecosystem is dominated by pollution Pb from atmospheric deposition. The Pb-206/Pb-207 ratios of the mor layer (O-horizon), forest plants and stream water (mainly between 1.14 and 1.20) are similar to atmospheric Pb pollution (1.14-1.19), while the local geogenic Pb of the mineral soil (C-horizon) has high ratios (>1.30). Roots and basal stemwood of the analyzed forest trees have higher Pb-206/Pb-207 ratios (1.15-1.30) than needles and apical stemwood (1.14-1.18), which indicate that the latter components are more dominated by pollution derived Pb. The low Pb-206/Pb-207 ratios of the mor layer suggest that the upward transport of Ph as a result of plant uptake is small (<0.04 mg m(-2) a(-1)) in comparison to atmospheric inputs (similar to 0.5 mg m(-2) a(-1)) and annual losses with percolating soil-water (similar to 2 mg m(-2) a(-1)); consequently, the Ph levels in the mor layer are now decreasing while the pool of Pb in the mineral soil is increasing. Streams draining mires appear more strongly affected by pollution Ph than streams from forested catchments, as indicated by Pb concentrations about three times higher and lower Pb-206/Pb-207 ratios (1.16 +/- 0.01 in comparison to 1.18 +/- 0.02). To what extent stream water Pb levels will respond to the build-up of Ph in deeper mineral soil layers remains uncertain. (c) 2008 Elsevier Ltd. All rights reserved.
The organic horizon (the mor layer) of podzolized boreal forest soils has accumulated atmospheric fallout of mercury and lead over centuries, resulting in current concentrations close to levels where negative effects on soil biota are thought to occur. To what extent the pollution history is preserved in the stratigraphy of this horizon is not well known. In this study we asses whether the chronology of a large historic pulse of atmospheric pollution emitted from the Ronnskar smelter in northern Sweden, particularly between 1950 and 1980, is preserved within the stratigraphy of the mor layer, which is typically 5-cm thick. Vertical sub-sampling (<= 5 mm) of five mor profiles sampled along a 100-km pollution gradient away from the smelter are analyzed for mercury and lead concentrations, spheroidal carbonaceous particles from fossil fuel combustion (SCPs) and stable lead isotopes (Pb-206/Pb-207 and Pb-208/Pb-207). Their vertical distribution is compared with the temporal variations in atmospheric inputs reconstructed for the last similar to 100 years from analyses of an ombrotrophic peat core and a varved take sediment core sampled within a distance of 50 km of the smelter. The mor profiles situated <= 12 km from the smelter record the pollution history of the smelter. There is a 20 to 40-times enrichment of Hg, Pb and SCP at the transition in the O-horizon from the F- to H-layer compared to the basal part and a distinct peak in the Pb-206/Pb-207 ratio (similar to 1.22) in the F-layer. The mor profiles situated outside the historical contamination range of the smelter (80 and 100 km away) record no obvious influence from the Ronnskar smelter, instead their vertical Pb-206/Pb-207 profiles follow the general regional pollution history in northern Sweden. We conclude that the mor layer preserves a record of atmospheric Hg, Pb and SCP inputs and due to low leaching rates this organic horizon serves as a semi-archive of atmospheric Hg and Pb pollution. We stress the need of including this property in the existing 'black-box' models predicting the fate of Hg and Pb within contaminated boreal forest soils.
Detta är första rapporten i en serie paleolimnologiska undersökningar av kalkade referenssjöar. Undersökningarna har gjorts inom ramen för IKEU-projektet på uppdrag av Naturvårdsverket.
Given the current scenario of increasing global temperatures, it is valuable to assess the potential influence of changing climate on pollution distribution and deposition. In this study we use long-term sediment records from three lakes (spanning ca. 1000, 4800, and 8000 years, respectively) from the Greenland west coast to assess recent and long-term variations in mercury (Hg) and lead (Pb), including stable Pb isotopes (206Pb and 207Pb), in terms of pollution and climate influences. The temporal trends in sediment deposited from about the mid-19th century and forward are in general agreement with the history of industrial emissions at lower latitudes. Therefore, in recent sediment a possible influence from changing climate is difficult to assess. However, by using deeper sediment layers we show that changes in Greenland climate caused changes in the lake influx of material from regional aeolian activity, which resulted in large fluctuations in Hg and Pb concentrations and206Pb/207Pb ratios. The aeolian material is primarily derived from glacio-fluvial material with low Hg and Pb concentrations and a different isotopic composition. For one of the lakes, the fluctuations in Hg concentrations (10 to 70 ng g-1) prior to the 19th century are equal to the anthropogenic increase in the uppermost layers, suggesting that when studying recent concentrations and time trends of pollution in relatively low-contaminated areas such as the Arctic, the early natural fluctuations must be considered.
To assess the rates of compaction in recent, varved (annually laminated) lake sediments, we used a collection of 13 freeze cores sampled from 1979 to 2012 in Nylandssjon (northern Sweden). This unique series of stored freeze cores allowed us to measure how the thickness of individual varves changed when they were overlain by new varves. The compaction rate was greatest during the first few years after deposition; varve thickness decreased by 60% after 5 years (sediment depth 5cm). Thereafter, the compaction rate declined, but after 33 years (the study period) there was still a weak trend of continued compaction (sediment depth 12cm). The rate of compaction is clearly linked to loss of water along with an increase in dry-bulk density of the sediment. Despite compaction causing considerable varve thickness changes over time, the year-to-year variation in varve thickness is preserved while sediment ages. Understanding compaction processes is of fundamental importance to paleolimnology, for example in studies involving calibration of varve thicknesses of recent varves against environmental parameters.
Sediment trap studies and high frequency monitoring are of great importance to develop a deeper understanding of how seasonal environmental processes are imprinted in sediment signal formation. We collected whole year diatom assemblages from 2002 to 2014 with a sequential sediment trap from a varved boreal lake (Nylandssjon, Sweden) together with environmental and limnological parameters, and compared them with the corresponding diatom record of the annual laminated sediment. Our data set indicates a large year-to-year variability of diatom succession and abundance patterns, which is well reflected in the varved sediments. Specifically, Cyclotella glomerata dominated the annual sediment trap record (as well as in the corresponding sediment varves) in years with warmer air temperatures in March/April, and Asterionella formosa dominated the annual sediment assemblages as a consequence of years characterized by higher runoff before lake over-turn. Years succeeding forest clearance in the lake catchment showed marked increase in diatom and sediment flux. The DCA scores of the yearly diatom trap assemblages clearly resemble the lake's thermal structure, which indicates that the relative abundance of major taxa seems primarily controlled by the timing of seasonal environmental events, such as above-average winter air temperature and/or autumn runoff and the current thermal structure of the lake. The high seasonal variability between environmental drivers in combination with the physical limnology leaves us with several possible scenarios leading to either an A. formosa versus C. glomerata dominated annual diatom sediment signal. With this study we highlight that short-term environmental events and seasonal limnological conditions are of major importance for interpreting annual sediment signals.
Sediments from seven lakes in West Greenland were used as natural archives to study past and present levels of PCBs (polychlorinated biphenyls, tri- to decachlorinated), tetra-BDE #47 (2,2',4,4'-bromodiphenyl ether), chlordane (cis- and trans-octachlordane) and HxCBz (hexachlorobenzene). The concentrations found are lower than or comparable to concentrations found in sediments from other Arctic regions and one to 2 orders of magnitude lower than concentrations typically found in sediments at lower latitudes. The observed temporal trends (direct and indirect dating) show a decreasing total PCB concentration. Even though local contamination sources exist, the POP deposition in the studied area is most likely a result from long-range transport. The hypothesis about "cold condensation" suggests a latitudinal fractionation to occur between different volatile compounds during the transport toward the pole. In this study a time delay in the deposition for the low-chlorinated PCBs (tri- and tetrachlorinated), compared to their emission histories and compared to higher chlorinated PCBs, was indicated. Although very low tetra-BDE #47 concentrations are observed in this study, there are indications for an increasing concentration in recent sediment layers that may reflect increasing environmental concentrations at lower latitudes. The investigated pesticides are still in use at lower latitudes, however neither chlordane nor HxCBz show any distinct temporal trend of increasing or decreasing concentration toward the sediment surface.
In the 1960s and 1970s, acidification was identified as a major environmental problem in Scandinavia, Great Britain and North America. In Sweden, a liming program was launched in order to counteract the effects of acidification on surface waters. More than 30 years after large-scale liming began, there is still debate about whether liming actually achieves its goals, i.e., to prevent acidification in acid-sensitive surface waters and to restore natural conditions in acidified waters. We used Detrended Correspondence Analysis (DCA) and analogue matching of diatom assemblages in surface sediment samples (recent conditions) from 31 limed lakes and pre-industrial samples from 291 reference lakes to help answer the question as to whether the Swedish liming program achieves its goals. Diatoms are important primary producers in lakes and established indicator organisms for lake-water quality. First we compared pre-industrial with post-limed diatom communities to address the question whether liming causes unnatural conditions, i.e., diatom communities that have not previously occurred in Swedish lakes. Second, we addressed the issue of what is a realistic condition to use as a reference (natural condition) or a target in management programs. We found that the diatom communities in limed lakes were not different from the communities in the reference lakes. Most of the limed lakes had one or more analogues within the reference data set and many of them had at least one within-lake analogue. Hence, liming does not create unique diatom communities in lakes. Based on this and previous paleolimnological studies in Swedish lakes we suggest a conceptual model integrating the natural lake condition, the historical human impact, and the recent and contemporary human impact, when defining realistic targets in management programs.
Surface water acidification was identified as a major environmental problem in the 1960s. Consequently, a liming program was launched in Sweden in the 1970s. The primary purpose of liming is to restore conditions that existed prior to acidification. To reach this goal, as well as achieve ‘good status' (i.e. low levels of distortion resulting from human activity) in European freshwaters until 2016 under the European Union Water Framework Directive, lake data are required to define reference conditions. Here, we compare data from chemical/biological monitoring of 12 limed lakes with results of
paleolimnological investigations, to address questions of reference conditions, acidification, and restoration by liming. Using diatom-based lake-water pH inferences, we found clear evidence of acidification in only five of the 12 lakes, which had all originally been classified as acidified according to monitoring data. After liming, measured and diatom-inferred pH agree well in seven lakes. The sediment record of three of the five remaining lakes gave ambiguous results, presumably due to sediment mixing or low sediment accumulation rates. It is difficult to determine whether liming restored the lakes to a good status, especially as some of the lakes were not acidified during the twentieth century. In addition to acid deposition, other factors, such as natural lake and catchment ontogeny or human impact through agricultural activity, influence lake acidity. This study shows that monitoring series are usually too short to define reference conditions for lakes, and that paleolimnological studies are useful to set appropriate goals for restoration and for evaluation of counter measures.
The catechol O-methyltransferase (COMT) gene—encoding an enzyme that is essential for the degradation of dopamine (DA) in prefrontal cortex (PFC)—contains a single nucleotide polymorphism (val/met) important for cognition. According to the tonic–phasic hypothesis, individuals carrying the low-enzyme-activity allele (met) are characterized by enhanced tonic DA activity in PFC, promoting sustained cognitive representations in working memory. Val carriers have reduced tonic but enhanced phasic dopaminergic activity in subcortical regions, enhancing cognitive flexibility. We tested the tonic–phasic DA hypothesis by dissociating sustained and transient brain activity during performance on a 2-back working memory test using mixed blocked/event-related functional magnetic resonance imaging. Participants were men recruited from a random sample of the population (the Betula study) and consisted of 11 met/met and 11 val/val carriers aged 50 to 65 years, matched on age, education, and cognitive performance. There were no differences in 2-back performance between genotype groups. Met carriers displayed a greater transient medial temporal lobe response in the updating phase of working memory, whereas val carriers showed a greater sustained PFC activation in the maintenance phase. These results support the tonic–phasic theory of DA function in elucidating the specific phenotypic influence of the COMT val158met polymorphism on different components of working memory.
Processes controlling sediment yield vary over a range of timescales, although most process-based observations are extremely short. Lake sediments, however, can be used to extend the observational timescale and are particularly useful when annually laminated (varved) sediment is present. The sediment record at Kassjön (N. Sweden) consists of ∼6400 varves, each 0·5–1 mm thick. Image analysis was used to determine grey-scale variation and varve thickness from which annual minerogenic accumulation rate (MinAR) (mg cm−2 year−1) was inferred for the period 4486 BC – AD 1900. MinAR varies on annual to centennial scales and mainly reflects channel bank erosion by the inflow streams. The mineral input reflects the intensity of the spring run-off, which is dependent on the amount of snow accumulated during the winter, and hence MinAR is a long-term record of variability in past winter climate; other factors will be a variable response to catchment uplift, vegetation succession and pedogenesis. A major shift from low to high MinAR occurred ∼250 BC, and peaks occurred around AD 250, 600, 1000, 1350 and 1650. Wavelet power spectrum analysis (confirmed by Fourier analyses) indicated significantly different periodicities throughout the period 4000 BC – AD 1700, including 275 years for the period 4000 BC – 2900 BC, 567 years for the period 2901 BC – 1201 BC, and 350 and 725 years for the period 1200 BC – AD 1700. The long-term, centennial scale variability (∼350 years) may reflect solar forcing (cf the 385-year peak in tree-ring calibrated 14C activity) but interestingly, there is no obvious link to high frequency forcing, such as the North Atlantic Oscillation. The high resolution component of the record highlights the relevance of varved lake sediment records for understanding erosion dynamics in undisturbed forested catchments and their link to long-term climate dynamics and future climate change.
Establishment of plans for environmental planning and management requires that a number of natural and societal factors must be taken into consideration. Insights into the inherent dynamics of nature as well as the role that past human activities have played for establishing the current condition of the landscape and the natural environment in general are essential. Many natural and man-made changes occur over time scales of decades or centuries, and these are difficult to comprehend without a historical perspective. Such a perspective can be obtained using palaeoecological studies, i.e. by geochemical and biological analyses of lake sediment and peat deposits. To illustrate the long-term dynamics of nature and particularly the role of man, we present here five case studies from Sweden concerning pollution, lake acidification, lake eutrophication, biodiversity, and landscape dynamics and conservation – topics of broad interests – and discuss benefits of including a longer time perspective in environmental management.
We present a design for a piston corer that can take a sediment core up to 1 m long, with an undisturbed sediment-water interface. The coring device possesses a tripod unit and a core-tube unit, the latter comprised of a core tube, a piston with a wire, a drive weight in which the core tube is mounted and a steel rod. The tripod stands on the sediment surface during coring, stabilizing the system and serving as an anchor point for the piston wire. A ball clamp, the critical component of the design, is mounted on top of the tripod. The steel rod runs through the ball clamp and at its lower end, holds the drive weight and the core tube. The ball clamp allows the core tube to slide downward, while the piston is held in a fixed vertical position by the wire connected to the tripod. When the corer is lifted, however, the ball clamp locks. This makes it possible to operate the corer with a single cable, because the piston is not subject to any lifting force when the corer is retrieved. The piston remains in position in the core tube even if the collected sediment core is very short. The piston corer can be deployed from a raft or a boat.
Freezing of sediment in situ at the lake bottom using a freeze corer has become an important method for taking samples of soft sediments aimed for detailed stratigraphic analyses. Over the past decades a variety of freeze corers have been designed, from very simple metal tubes filled with dry ice that are dropped into the sediment to high-tech samplers using electric pumps, hydraulics and liquid nitrogen. The freeze corer described here is a compromise of getting good samples without a too complicated technique. It is designed for coring from lake ice, using a steel wire with a stopper on the ice to keep the corer in fixed position during freezing. It consists of a thermos connected to a thin freeze wedge (width 15 cm, length 100 cm, max thickness 6 cm). The dry ice is kept in the thermos until the freeze wedge has been lowered into the sediment to avoid water freezing on the wedge during descent through the water column; a wet steel-wedge surface disturbs the sediment stratigraphy less than a wedge on which rapid freezing occurs. The dry ice is kept in the thermos by a trapdoor that is opened by a messenger. When the trapdoor is opened the dry ice begins to drop into the wedge, which is filled with ethanol. This process continues concurrently with the consumption of ice in the wedge until the thermos is empty. Using 6–8 kg of dry ice and 4 l of ethanol (95%), a 3-cm-thick and up to 80-cm-long crust of frozen sediment is obtained in 20–25 min on each side of the wedge. The crust is easily detached intact from the wedge using a small volume of hot water, poured inside the wedge.
Gravity corers have been in use for a long time and in many different configurations. There are, however, reasons for improvements since new manufacturing materials have become available, and demands for accurate coring and sub-sampling in both research and environmental monitoring have increased. The HTH-coring equipment, which has been tested for > 10 years by several users, is a further development of the Kajak-corer concept and has some unique features which are described here. To avoid contamination in pollution studies, the equipment is made of stainless steel and plastic (polyoxymethylene and polyethylene), and the use of these materials also makes the equipment more durable. The extruding device is the main improvement over previous versions. It consists of a piston that seals perfectly to the core tube wall, a threaded rod that can be mounted on a foot plate, and an extruder head with a stationary upper-half and a rotatable lower-half that is screwed along the rod. One 360 degrees-turn gives a 5-mm thick sediment increment that is scraped off using the sectioning tray. Sub-sampling is quick and accurate and can be performed by one person. The equipment is constructed to allow coring and sub-sampling both in summer and winter.
The accuracy of three methods used to estimate the whole-lake accumulation rate of chemicals in sediment, the simple mean accumulation rate, regression against water depth and the mean accumulation rate in sedimentation zones, was assessed. The concentrations and accumulation rates of organic carbon, four major cations, phosphorus and four heavy metals in the 1902 to 1988 sediment layer at 43 sites in Kassjon were used and the three methods gave identical results. The accuracy of the simple mean accumulation rate method using 5 or 10 sites was investigated using Monte Carlo simulation and both versions gave accurate results. If the method used the dry sediment accumulation rate at 5 or 10 sites, along with chemical concentration at one central site, then the errors were less than +/- 10% for five of the chemicals and larger for the others (-20 or 20-40%), although the direction of the bias was predictable. If chemical accumulation rate at the central site only was used to estimate the whole-lake value, the bias increased to 25% for the major cations, 50% for the heavy metals and P, and 85% for organic carbon. Having 43 sites in Kassjon meant that the true whole-lake chemical accumulation rate was known and so the accuracy of the methods and their variants could be assessed for the first time. These findings allow the particulars of the simple mean accumulation rate method to be chosen to suit the accuracy needed for a palaeolimnological investigation.
This study tests the hypothesis that lake sediments contain climate-related information that can be detected by near-infrared spectrometry (NIRS), and that NIRS can be used to infer past climatic changes from analysis of sediment cores. NIRS is a rapid and non-destructive technique that measures attributes of the chemical composition of organic materials. A training set of 76 lakes from northern Sweden, spanning a broad altitudinal gradient, was used to assess whether lake altitude and vegetation zones can be modelled from NIR spectra of surface sediments (0-1 cm) using partial least squares (PLS) regression and soft independent modelling of class analogies (SIMCA) classification. Lake altitude served as a surrogate variable reflecting differences in climatic conditions among sites. After spectral filtering using orthogonal signal correction (OSC), cross-validated predictions explained 86% of the variance in altitude and the prediction error (root mean square error) was 78 m, corresponding to 8.3% of the gradient (390-1250 m above sea level). To evaluate the significance of NIR spectral differences between surface sediments of lakes in different vegetation zones (mountain-birch forest, dwarf shrub and alpine heath), principal component analysis (PCA) models were developed separately for lakes in each vegetation zone. Multivariate classification analysis demonstrated that NIR spectra of surficial sediments differed between lakes located in different vegetation zones. A separate sediment data set from 56 lakes was used to assess sediment ageing effects on NIR signals. Marked similarities between NLR spectra in surface sediments (0-1 cm) and sediments from 1-2 cm depth indicated that degradation of organic material following sediment consolidation resulted in little loss or change of climate-related information. Finally, to assess the ability of NIRS methods to reconstruct past climatic changes over Holocene timescales, we applied the NIRS-altitude model to sediments in a core from a small mountain lake. Estimates of mean July air temperature based on the NIRS-altitude transfer function showed similar trends compared with inferences from chironomids, diatoms and pollen from the same core. Overall, the results indicate that changes in NIR spectra from lake sediments reflect differences in climate, and that NIRS models based on surface-sediment samples can be applied to sediment cores for retrospective analysis.
The relationships between diatoms (Bacillariophyceae) in surface sediments of lakes and summer air temperature, pH and total organic carbon concentration (TOC) were explored along a steep climatic gradient in northern Sweden to provide a tool to infer past climate conditions from sediment cores. The study sites are in an area with low human impact and range from boreal forest to alpine tundra. Canonical correspondence analysis (CCA) constrained to mean July air temperature and pH clearly showed that diatom community composition was different between lakes situated in conifer-, mountain birch- and alpine-vegetation zones. As a consequence, diatoms and multivariate ordination methods can be used to infer past changes in treeline position and dominant forest type. Quantitative inference models were developed to estimate mean July air temperature, pH and TOC from sedimentary diatom assemblages using weighted averaging (WA) and weighted averaging partial least squares (WA-PLS) regression. Relationships between diatoms and mean July air temperature were independent of lake-water pH, TOC, alkalinity and maximum depth. The results demonstrated that diatoms in lake sediments can provide useful and independent quantitative information for estimating past changes in mean July air temperature (R-jack(2) = 0.62, RMSEP = 0.86 degrees C; R-2 and root mean squared error of prediction (RMSEP) based on jack-knifing), pH (R(j)(2)ack = 0.61, RMSEP = 0.30) and TOC (R-jack(2) = 0.49, RMSEP = 1.33 mg l(-1)). The paper focuses mainly on the relationship between diatom community composition and mean July air temperature, but the relationships to pH and TOC are also discussed.
The results of a multiproxy study reconstructing the climate history of the last 9300 years in northern Sweden are presented. It is based on diatom, chironomid and pollen analyses, as well as near-infrared spectroscopy (NIRS), of a radiocarbon dated sediment core from Sjuodjijaure (67 degrees 22'N, 18 degrees 04'E), situated 100 m above tree-line in the Scandes mountains. Mean July air temperature was reconstructed using transfer functions established for the region. The biological proxies show significant changes in composition during the Holocene and the inferred temperatures all follow the same general trend. For the period between about 9300 to 7300 cal. BP the reconstructions should be interpreted with caution due to the lack of convincing modem analogues in the training set. However the reconstruction suggest that July temperature was on average about the same as today, with several rapid short-term cold and warm periods. Cold periods were dated to about 8500, 8200 and 7600 cal. years BP and a warm period to about 7700 cal. BP. About 7300 cal. BP, a major shift to a warmer climate occurred. Pine migrated into the area, which was previously covered with birch forest. From the mid-Holocene until today the sediment record suggests a descending tree-limit and a gradual lowering of July temperature.
Using lake sediments to infer past total mercury and methylmercury loading to the environment requires that diagenetic processes within the sediment do not significantly affect the concentrations or net accumulation rates of the mercury species. Because carbon is lost during early sediment diagenesis, the close link between carbon and mercury raises the question of how reliable lake sediments are as archives of total mercury and methylmercury loading. In this study we used a series of freeze cores taken in a lake with varved (annually laminated) sediment to assess the stability of total mercury and methylmercury over time. By tracking material deposited in specific years in cores collected in different years, we found that despite a 20–25% loss of carbon in the first 10–15 years, there was no apparent loss of total mercury over time; hence, lake sediments can be considered as reliable archives. However, over the first 5–8 years after sedimentation, about 30–40% of the methylmercury was lost (a decrease of 0.025–0.030 μg MeHg m−2 yr−1), suggesting that sediment profiles showing increasing methylmercury concentrations toward the sediment surface are in large part an artifact of diagenetic processes (net demethylation), rather than a record of changes in methylmercury loading.