Johan Rydberg

Climate driven release of carbon and mercury from permafrost mires increases mercury loading to sub-arctic lakes

In sub-arctic and arctic regions mercury is an element of concern for both wildlife and humans. Over thousands of years large amounts of atmospherically deposited mercury, both from natural and anthropogenic sources, have been sequestered together with carbon in northern peatlands. Many of these peatlands are currently underlain by permafrost, which controls mire stability and hydrology. With the ongoing climate change there is concern that permafrost thawing will turn large areas of these northern peatlands from carbon/mercury-sinks into much wetter carbon/mercury-sources. Here we can show that such a change in mire structure in the sub-arctic Stordalen mire in northern Sweden actually is responsible for an increased export of mercury to the adjacent lake Inre Harrsjön. We also show that sediment mercury accumulation rates during a warm period in the pre-industrial past were higher than in the 1970s when atmospheric input peaked, indicating that in areas with permafrost, climate can have an effect on mercury loading to lakes as large as anthropogenic emissions. Thawing of permafrost and the subsequent export of carbon is a widespread phenomenon, and the projection is that it will increase even more in the near future. Together with our observations from Stordalen, this makes northern peatlands into a substantial source of mercury, at risk of being released into sensitive arctic freshwater and marine systems.

An explorative study of mercury export from a thawing palsa mire

Thawing of permafrost and a subsequent accelerated loss of mercury from the soil constitute a possible threat to the quality of high-latitude surface waters. In this paper we estimate the export of ...

Widespread waterborne pollution in central Swedish lakes and the Baltic Sea from pre-industrial mining and metallurgy

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--Swedens 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 16th-18th centuries with improvements occurring over recent centuries, i.e., historical pollution > modern industrial pollution.

Environmental history: a piece in the puzzle for establishing plans for environmental management.

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.

Evaluating paleoproxies for peat decomposition and their relationship to peat geochemistry

The past decade has seen a rapid increase in interest in the biogeochemical record preserved in peat, particularly as it relates to carbon dynamics and environmental change. Importantly, recent studies show that carbon dynamics, that is, organic matter decomposition, can influence the record of atmospherically derived elements such as halogens and mercury. Most commonly, bulk density, light transmission, or carbon/nitrogen (C/N) ratios are used as a proxy to qualitatively infer the degree of decomposition in peat, but do these three proxies reflect the same patterns? Furthermore, how do each of these proxies relate to other geochemical data? To address these questions, we analyzed bulk density, light transmission, and C/N ratios, as well as multielement geochemistry (wavelength-dispersive x-ray fluorescence (WD-XRF)), in three hummock cores (70 cm in length, c. 500 years) from an ombrotrophic Swedish bog. To compare the proxies, we applied principal component analysis (PCA) to identify how the proxies relate to and interact with the geochemical matrix. This was coupled with changepoint modeling to identify and compare statistically significant changes for each proxy. Our results show differences between the proxies within and between cores, indicating each responds to a different part of the decomposition process. This is supported by the PCA, where the three proxies fall on different principal components. Changepoint analysis also showed that the inferred number of changepoints and their depths vary for each proxy and core. This suggests that decomposition is not fully captured by any one of these commonly used proxies, and thus, more than one proxy should be included.

Compaction of recent varved lake sediments

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 Nylandssjön (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 5 cm). Thereafter, the compaction rate declined, but after 33 years (the study period) there was still a weak trend of continued compaction (sediment depth 12 cm). 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.

Assessment of the spatial distributions of total- and methyl-mercury and their relationship to sediment geochemistry from a whole-lake perspective

The aim of this study was to determine the spatial variability for total-and methylmercury in surface sediments (0-2 cm) across a single whole-lake basin, and to relate this variability to the sedi ...

The Importance of Eolian Input on Lake-Sediment Geochemical Composition in the Dry Proglacial Landscape of Western Greenland

ABSTRACT In proglacial landscapes, such as western Greenland, eolian transport plays an important role for the influx of particulate material to lakes. On the basis of an analysis of a sediment profile and surface sediments from several lakes, we show that eolian activity has a strong influence on sediment deposition in time and space. Principal component analysis revealed that sediments that accumulated during periods with high eolian activity were enriched in zirconium—originating from coarse silt and sand fractions preferentially transported by wind—and depleted in rubidium. In addition, zirconium to rubidium ratios in the surface sediment of four additional lakes decreased with distance from the ice sheet. Finally, previously published data show that pH and alkalinity tend to be higher in lakes close to the front of the ice sheet, which we speculate is coupled to a larger supply of fresh eolian material. These findings demonstrate that lakes in proglacial landscapes may receive a substantial part of their sediment load through eolian deposition, and that this is especially true close to the glacial outwash plains along the ice margin.

Influence of catchment vegetation on mercury accumulation in lake sediments from a long-term perspective

Organic matter (OM) cycling has a large impact on the cycling of mercury (Hg) in the environment. Hence, it is important to have a thorough understanding on how changes in, e.g., catchment vegetation - through its effect on OM cycling - affect the behavior of Hg. To test whether shifts in vegetation had an effect on Hg-transport to lakes we investigated a sediment record from Herrenwieser See (Southern Germany). This lake has a well-defined Holocene vegetation history: at ~8700years BP Corylus avellana (hazel) was replaced by Quercus robur (oak), which was replaced by Abies alba (fir) and Fagus sylvatica (beech) ~5700years BP). We were particularly interested in testing if coniferous vegetation leads to a larger export of Hg to aquatic systems than deciduous vegetation. When hazel was replaced by oak, reduced soil erosion and increased transport of DOM-bound mercury from the catchment resulted in increases in both Hg-concentrations and accumulation rates (61ngg(-1) and 5.5ngcm(-2)yr.(-)(1) to 118ngg(-1) and 8.5ngcm(-2)yr.(-)(1)). However, even if Hg-concentrations increased also in association with the introduction of fir and beech (173ngg(-1)), as a result of higher Hg:C, there was no increase in Hg-accumulation rates (7.6ngcm(-2)yr.(-)(1)), because of a decreased input of OM. At around 2500years BP Hg-accumulation rates and Hg-concentration indicated an additional input of Hg to the sediment (316ngg(-1) and 10.3ngcm(-2)yr.(-)(1)), which might be due to increased human activities in the area, e.g., forest burning or mining. Our results contrast those of several paired-catchment studies that suggest a higher release of Hg from coniferous than deciduous forest, and there is a need for studies with a long-term perspective to increase our understanding of the effects of slow and gradual processes on mercury cycling.

Late-Holocene climate and vegetation dynamics in eastern Lesotho highlands

The eastern Lesotho highlands are of considerable hydrological importance to southern Africa as a so-called ‘water tower’ for the surrounding region. Here, we contribute proxy-data inferring climate and vegetation changes over the past 1600 years, assessing in parallel inorganic and organic chemical analyses on a sediment core from Ladybird wetland, eastern Lesotho. Several proxies were used to determine changes in local vegetation dynamics, productivity, hydrology (δ13 C, δ15 N, C/N, TOC) and the input and source of the detrital components (Ca/Ti, CIA). The first part of the multi-proxy record (AD 400–800) shows stable terrestrial conditions and low detrital input, followed by higher variability in almost all proxies between ca. AD 900 and 1200. The δ13 C record infers a higher proportion of C4 vegetation, tentatively associated with higher temperatures during this phase, coeval with the Medieval Climate Anomaly (MCA). After AD 1200, local conditions change gradually from purely terrestrial, towards the typical wetland environment prevailing today. A higher proportion of C3 plants and possibly an increase in aquatic organisms within the organic matrix corresponds with decreasing detrital input, suggesting locally high available moisture in this part of Lesotho during the Little Ice Age (LIA). Although age-model constraints impedes a robust regional comparison, the inferred climate variability is discussed as a tentative response to enhanced mid-latitude cyclonic activity during LIA, and the variable MCA climate conditions as indirectly dictated by changes in solar activity.