The history of mining and smelting and the associated pollution have been documented using lake sediments for decades, but the broader ecological implications are not well studied. We analyzed sediment profiles covering the past similar to 10,000 years from three lakes associated with an iron blast furnace in central Sweden, as an example of the many small-scale furnaces with historical roots in the medieval period. With a focus on long-term lake-water quality, we analyzed multiple proxies including geochemistry, pollen and charcoal, diatom composition and inferred pH, biogenic silica (bSi), visible near-infrared spectroscopy (VNIRS)-inferred lake-water total organic carbon (LW-TOC), and VNIRS-inferred sediment chlorophyll (sed-Chl). All three lakes had stable conditions during the middle Holocene (similar to 5000 BCE to 1110 CE) typical of oligo-dystrophic lakes: pH 5.4-5.6, LW-TOC 15-18 mg L-1. The most important diatom taxa include, for example, Aulacoseira scalaris, Brachysira neoexilis, and Frustulia saxonica. From similar to 1150 CE, decreases in LW-TOC, bSi, and sed-Chl in all three lakes coincide with a suite of proxies indicating disturbance associated with local, small-scale agriculture, and the more widespread use of the landscape in the past (e.g. forest grazing, charcoal production). Most important was a decline in LW-TOC by 30-50% in the three lakes prior to the 20th century. In addition, the one lake (Fickeln) downstream of the smelter and main areas of cultivation experienced a shift in diatom composition (mainly increasing Asterionella formosa) and a 0.6 pH increase coinciding with increasing cereal pollen and signs of blast furnace activity. The pH did not change in the other two lakes in response to disturbance; however, these lakes show a slight increase (0.3-0.5 pH units) because of modern liming. LW-TOC has returned to background levels in the downstream lake and remains lower in the other two.

To assess the sensitivity of lakes to anthropogenically-driven environmental changes (e.g., nutrient supply, climate change), it is necessary to first isolate the effects of between-year variability in weather conditions. This variability can strongly impact a lake's biological community especially in boreal and arctic areas where snow phenology play an important role in controlling the input of terrestrial matter to the lake. Identifying the importance of this inherent variability is difficult without time series that span at least several decades. Here, we applied a molecular approach (metabarcoding on eukaryotic 18S rRNA genes and qPCR on cyanobacterial 16S rRNA genes) to sedimentary DNA (sed-DNA) to unravel the annual variability of microbial community in 40 years' sediment record from the boreal lake Nylandssjon which preserve annually-laminated sediments. Our comparison between seasonal meteorological data, sediment inorganic geochemistry (X-ray fluorescence analyses) and organic biomarkers (pyrolysis-gas chromatography/mass spectrometry analyses), demonstrated that inter-annual variability strongly influence the sediment composition in Nylandssjon. Spring temperature, snow and ice phenology (e.g., the percentage of snow loss in spring, the timing of lake ice-off) were identified as important drivers for the inputs of terrestrial material to the lake, and were therefore also important for shaping the aquatic biological community. Main changes were detected in the late-80s/mid-90s and mid-2000s associated with increases in algal productivity, in total richness of the protistan community and in relative abundances of Chlorophyta, Dinophyceae as well as Cyanobacteria abundance. These changes could be linked to a decline in terrestrial inputs to the lake during the snow melt and run-off period, which in turn was driven by warmer winter temperatures. Even if our data shows that meteorological factors do affect the sediment composition and microbial communities, they only explain part of the variability. This is most likely a consequence of the high inter-annual variability in abiotic and biotic parameters highlighting the difficulty to draw firm conclusions concerning drivers of biological changes at an annual or sub-annual resolution even with the 40-year varved sediment record from Nylandssjon. Hence, it is necessary to have an even longer time perspective in order to reveal the full implications of climate change.

Volcanismis one of the major natural processes emitting mercury (Hg) to the atmosphere, representing a significant component of the global Hg budget. The importance of volcanic eruptions for local-scale Hg deposition was investigated using analyses of Hg, inorganic elemental tracers, and organic biomarkers in a sediment sequence from Lake Chungara (4520 m a.s.l.). Environmental change and Hg deposition in the immediate vicinity of the Parinacota volcano were reconstructed over the last 2700 years, encompassing the pre-anthropogenic and anthropogenic periods. Twenty eruptions delivering large amounts of Hg (1 to 457 mu g Hg m(-2) yr(-1) deposited at the timescale of the event) were locally recorded. Peaks of Hg concentration recorded after most of the eruptions were attributed to a decrease in sedimentation rate together with the rapid re-oxidation of gaseous elemental Hg and deposition with fine particles and incorporation into lake primary producers. Over the study period, the contribution of volcanic emissions has been estimated as 32% of the total Hg input to the lake. Sharp depletions in primary production occurred at each eruption, likely resulting from massive volcaniclastic inputs and changes in the lake-water physico-chemistry. Excluding the volcanic deposition periods, Hg accumulation rates rose from natural background values (1.9 +/- 0.5 mu g m(-2) yr(-1)) by a factor of 2.3 during the pre-colonial mining period (1400-900 yr cal. BP), and by a factor of 6 and 7.6, respectively, during the Hispanic colonial epoch (400-150 yr cal. BP) and the industrial era (similar to 140 yr cal. BP to present). Altogether, the dataset indicates that lake primary production has been the main, but not limiting, carrier for Hg to the sediment. Volcanic activity and climate change are only secondary drivers of local Hg deposition relative to the magnitude of regional and global anthropogenic emissions.

Ombrotrophic peatlands are recognized archives of past atmospheric mineral dust deposition. Net dust deposition rates, grain size, mineral hosts and source areas are typically inferred from down-core elemental data. Although elemental analysis can be time efficient and data rich, there are some inherent limitations. X-ray diffraction (XRD) analysis allowsdirect identification of mineral phases in environmental samples but few studies have applied this method to peat samples and a well-developed protocol for extracting the inorganic fraction of highly organic samples (>95%) is lacking. We tested and compared different levels of pre-treatment: no pre-treatment, thermal combustion (300, 350, 400, 450, 500 and 550 degrees C) and chemical oxidation (H2O2 and Na2S2O8) using a homogenised highly organic (>98%) composite peat sample. Subsequently, minerals were identified by XRD. The results show that combustion is preferred to chemical oxidation because it most efficiently removes organic matter (OM), an important pre-requisite for identifying mineral phases by XRD analysis. Thermally induced phase transitions can be anticipated when temperature is the only factor to take into consideration. Based on the data required in this studythe recommended combustion temperature is 500 degrees C which efficiently removes OM while preserving a majority of common dust minerals.

1. Detection of DNA in lake sediments holds promise as a tool to study processes like extinction, colonization, adaptation and evolutionary divergence. However, low concentrations make sediment DNA difficult to detect, leading to high false negative rates. Additionally, contamination could potentially lead to high false positive rates. Careful laboratory procedures can reduce false positive and negative rates, but should not be assumed to completely eliminate them. Therefore, methods are needed that identify potential false positive and negative results, and use this information to judge the plausibility of different interpretations of DNA data from natural archives.
2. We developed a Bayesian algorithm to infer the colonization history of a species using records of DNA from lake-sediment cores, explicitly labelling some observations as false positive or false negative. We illustrate the method by analysing DNA of whitefish (Coregonus lavaretus L.) from sediment cores covering the past 10,000 years from two central Swedish lakes. We provide the algorithm as an R-script, and the data from this study as example input files.
3. In one lake, Stora Lögdasjön, where connectivity with the proto-Baltic Sea and the degree of whitefish ecotype differentiation suggested colonization immediately after deglaciation, DNA was indeed successfully recovered and amplified throughout the post-glacial sediment. For this lake, we found no loss of detection probability over time, but a high false negative rate. In the other lake, Hotagen, where connectivity and ecotype differentiation suggested colonization long after deglaciation, DNA was amplified only in the upper part of the sediment, and colonization was estimated at 2,200 bp based on the assumption that successful amplicons represent whitefish presence. Here the earliest amplification represents a false positive with a posterior probability of 41%, which increases the uncertainty in the estimated time of colonization.
4. Complementing careful laboratory procedures aimed at preventing contamination, our method estimates contamination rates from the data. By combining these results with estimates of false negative rates, our models facilitate unbiased interpretation of data from natural DNA archives.

To study the long-range transport of atmospheric pollutants from lower latitude industrial areas to the Arctic, we analysed a peat core spanning the last similar to 700 cal. yr (similar to 1300-2000 CE) from southern Greenland, an area sensitive to atmospheric pollution from North American and Eurasian sources. A previous investigation conducted in the same location recorded atmospheric lead (Pb) pollution after similar to 1845, with peak values recorded in the 1970s, and concluded that a North American source was most likely. To confirm the origin of the lead, we present new Pb isotope data from Sandhavn, together with a high-resolution record for mercury (Hg) deposition. Results demonstrate that the mercury accumulation rate has steadily increased since the beginning of the 19th century, with maximum values of 9.3 mu g m(-2) yr(-1) recorded similar to 1940. Lead isotopic ratios show two mixing lines: one which represents inputs from local and regional geogenic sources, and another that comprises regional geogenic and pollution sources. Detrending the Pb isotopic ratio record (thereby extracting the effect of the geogenic mixing) has enabled us to reconstruct a detailed chronology of metal pollution. The first sustained decrease in Pb isotope signals is recorded as beginning similar to 1740-1780 with the lowest values (indicating the highest pollution signature) dated to similar to 1960-1970. The 206Pb/207Pb ratio of excess Pb (measuring 1.222, and reflecting pollution generated Pb), when compared with the Pb isotopic composition of the Sandhavn peat record since the 19th century and the timing of Pb enrichments, clearly points to the dominance of pollution sources fromNorth America, although it did not prove possible to further differentiate the emissions sources geographically.

Peatlands in northern latitudes sequester one third of the world's soil organic carbon. Mineral dusts can affect the primary productivity of terrestrial systems through nutrient transport but this process has not yet been documented in these peat-rich regions. Here we analysed organic and inorganic fractions of an 8900-year-old sequence from Store Mosse (the "Great Bog") in southern Sweden. Between 5420 and 4550 cal yr BP, we observe a seven-fold increase in net peat-accumulation rates corresponding to a maximum carbon-burial rate of 150 g C m^-2 yr^-1 - more than six times the global average. This high peat accumulation event occurs in parallel with a distinct change in the character of the dust deposited on the bog, which moves from being dominated by clay minerals to less weathered, phosphate and feldspar minerals. We hypothesize that this shift boosted nutrient input to the bog and stimulated ecosystem productivity. This study shows that diffuse sources and dust dynamics in northern temperate latitudes, often overlooked by the dust community in favour of arid and semi-arid regions, can be important drivers of peatland carbon accumulation and by extension, global climate, warranting further consideration in predictions of future climate variability.

Alterations in fire activity due to climate change and fire suppression may have profound effects on the balance between storage and release of carbon (C) and associated volatile elements. Stored soil mercury (Hg) is known to volatilize due to wildfires and this could substantially affect the land air exchange of Hg; conversely the absence of fires and human disturbance may increase the time period over which Hg is sequestered. Here we show for a wildfire chronosequence spanning over more than 5000 years in boreal forest in northern Sweden that belowground inventories of total Hg are strongly related to soil humus C accumulation (R-2 = 0.94, p < 0.001). Our data clearly show that northern boreal forest soils have a strong sink capacity for Hg, and indicate that the sequestered Hg is bound in soil organic matter pools accumulating over millennia. Our results also suggest that more than half of the Hg stock in the sites with the longest time since fire originates from deposition predating the onset of large-scale anthropogenic emissions. This study emphasizes the importance of boreal forest humus soils for Hg storage and reveals that this pool is likely to persist over millennial time scales in the prolonged absence of fire.

Historical documents, archaeological evidence and lake-sediment records indicate thus far that significant mining of iron and copper ores in the Berglsagen mining region in central Sweden did not begin until the late 12th century -first with iron in Norberg - and thereafter spreading rapidly throughout the region during the 13th century when also copper was included (e.g. Falun). Prior to this, iron was produced domestically from secondary sources such as bog iron, while geochemical analyses of bronze artefacts indicate copper was imported. The parish of Garpenberg was at the intersection between historical iron-and copper-mining districts, and consequently we expected our sediment record from the lake Gruvsjon ('mine lake') to follow the established 13th century development. However, a 2-3-fold enrichment in copper and lead occurred already during 375-175 BCE (pre-Roman Iron Age), together with small increases in zinc, magnesium and charcoal particles, and changes in pollen. Together these indicate a clear pattern of human disturbance connected with the ore body bordering the lake. A second distinct phase occurred 115-275 CE, but with an 8-9-fold increase in copper and lead along with other indicators. From 400 CE a permanent increase in copper and lead occurred, which then accelerated from the 13th century as seen elsewhere in the region. Our results push back the evidence for early ore mining in Sweden from the Middle Ages to the pre-Roman Iron Age. 

Boreal mire ecosystems are predicted to experience warmer air temperatures as well as changed deposition loads of nitrogen and sulfur during the coming century. In this study, we hypothesized that vegetation changes that accompany these new environmental conditions alter the chemical composition of peat. To test this hypothesis, we quantified changes in peat geochemistry (Al, Ca, Fe, Mg, Na, P, Pb, and Zn) that have occurred in field manipulation plots exposed to 12 years of warming and nitrogen and sulfur additions in a nutrient-poor boreal mire. In Contrast to non-nutrients with a mainly atmospheric origin (i.e. Pb), Al-normalized inventories of micronutrients (Zn and Fe) and macronutrients (P and Ca) were significantly (P < 0.05) higher as a result of warming. For P and Ca, enrichments were also induced by nitrogen additions alone. These results suggest that mires evolving under increasing temperatures and availability of nitrogen are around two times More effective in storing nutrients in the accumulating peat. Our study provides the first empirical evidence that predicted changes in climate and nitrogen deposition scenarios will increase the retention of Ca, Fe, P, and Zn in surface peat of boreal mires in the near future, which may cause a depletion of nutrients released to inland waters dependent on mire inputs.