| 1. |
- Cuss, Chad, et al.
(författare)
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Advanced residuals analysis for determining the number of PARAFAC components in dissolved organic matter
- 2016
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Ingår i: Applied Spectroscopy. - Thosand Oaks : Sage Publications. - 0003-7028 .- 1943-3530. ; 70:2, s. 334-346
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Tidskriftsartikel (refereegranskat)abstract
- Parallel factor analysis (PARAFAC) has facilitated an explosion in research connecting the fluorescence properties of dissolved organic matter (DOM) to its functions and biogeochemical cycling in natural and engineered systems. However, the validation of robust PARAFAC models using split-half analysis requires an oft unrealistically large number (hundreds to thousands) of excitation–emission matrices (EEMs), and models with too few components may not adequately describe differences between DOM. This study used self-organizing maps (SOM) and comparing changes in residuals with the effects of adding components to estimate the number of PARAFAC components in DOM from two data sets: MS (110 EEMs from nine leaf leachates and headwaters) and LR (64 EEMs from the Lena River). Clustering by SOM demonstrated that peaks clearly persisted in model residuals after validation by split-half analysis. Plotting the changes to residuals was an effective method for visualizing the removal of fluorophore-like fluorescence caused by increasing the number of PARAFAC components. Extracting additional PARAFAC components via residuals analysis increased the proportion of correctly identified size-fractionated leaf leachates from 56.0 ± 0.8 to 75.2 ± 0.9%, and from 51.7 ± 1.4 to 92.9 ± 0.0% for whole leachates. Model overfitting was assessed by considering the correlations between components, and their distributions amongst samples. Advanced residuals analysis improved the ability of PARAFAC to resolve the variation in DOM fluorescence, and presents an enhanced validation approach for assessing the number of components that can be used to supplement the potentially misleading results of split-half analysis.
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| 2. |
- Hirst, Catherine, et al.
(författare)
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Characterisation of Fe-bearing particles and colloids in the Lena River basin, NE Russia
- 2017
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Ingår i: Geochimica et Cosmochimica Acta. - Cambridge : Elsevier. - 0016-7037 .- 1872-9533. ; 213, s. 553-573
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Tidskriftsartikel (refereegranskat)abstract
- Rivers are significant contributors of Fe to theocean. However, the characteristics of chemically reactive Fe remain poorly constrained, especially in large Arctic rivers, which drain landscapes highly susceptible to climate change and carbon cycle alteration. The aim of this study was a detailed characterisation (size, mineralogy, and speciation) of riverine Fe-bearing particles (> 0.22 µm) and colloids (1 kDa – 0.22 µm) and their association with organic carbon (OC), in the Lena River and tributaries, which drain a catchment almost entirely underlain by permafrost. Samples fromthe main channel and tributaries representing watersheds that span a wide rangein topography and lithology were taken after the spring flood in June 2013 and summer baseflow in July 2012. Fe-bearing particles were identified, usingTransmission Electron Microscopy, as large (200 nm – 1 µm) aggregates of smaller (20 nm - 30 nm) spherical colloids of chemically-reactive ferrihydrite.In contrast, there were also large (500 nm – 1 µm) aggregates of clay (illite) particles and smaller (100 - 200 nm) iron oxide particles (dominantly hematite) that contain poorly reactive Fe. TEM imaging and Scanning Transmission X-raymicroscopy (STXM) indicated that the ferrihydrite is present as discrete particles within networks of amorphous particulate organic carbon (POC) and attached to the surface of primary produced organic matter and clay particles.Together, these larger particles act as the main carriers of nanoscale ferrihydrite in the Lena River basin. The chemically reactive ferrihydrite accounts for on average 70 ± 15 % of the total suspended Fe in the Lena River and tributaries. These observations place important constraints on Fe and OC cycling in the Lena River catchment area and Fe-bearing particle transport to the Arctic Ocean.
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| 3. |
- Hirst, Catherine, 1989-, et al.
(författare)
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Iron isotopes reveal seasonal variations in the mechanisms for iron-bearing particle and colloid formation in the Lena River catchment, NE Siberia
- 2023
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Ingår i: Geochimica et Cosmochimica Acta. - 0016-7037 .- 1872-9533. ; 363, s. 77-93
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Tidskriftsartikel (refereegranskat)abstract
- Large Arctic rivers are an important source of iron (Fe) to the Arctic Ocean, though seasonal variations in the terrestrial source and supply of Fe to the ocean are unknown. To constrain the seasonal variability, we present Fe concentrations and isotopic compositions (δ56Fe) for particulate (>0.22 µm) and colloidal (<0.22 µm–1 kDa) Fe from the Lena River, NE Russia. Samples were collected every month during winter baseflow (September 2012–March 2013) and every 2–3 days before, during and after river ice break-up (May 2015).Iron in particles have isotope ratios lower than crustal values during winter (e.g., δ56FePart = −0.37 ± 0.16‰), and crustal-like values during river ice break-up and spring flood (e.g., δ56FePart = 0.07 ± 0.08‰), indicating a change in the source of particulate Fe between winter and spring flood. Low isotope values are indicative of mineral dissolution, transport of reduced Fe in sub-oxic, ice-covered sub-permafrost groundwaters and near-quantitative precipitation of Fe as particles. Crustal-like isotopic compositions result from the increased supply of detrital particles from riverbank and soil erosion during river ice break-up and flooding. Iron colloids (<0.22 μm) have δ56Fe values that are comparable to or lower than crustal values during winter (e.g., δ56FeCol = −0.08 ± 0.05‰) but similar to or higher than crustal values during spring flood (e.g., δ56FeCol = +0.24 ± 0.11‰). Low δ56Fe ratios for colloidal Fe during winter are consistent with precipitation from isotopically light Fe(II)aq transported in sub-permafrost groundwaters. Higher colloidal δ56Fe ratios during the spring flood indicate that these colloids are supplied from surface soils, where Fe is fractionated via oxidation or organic carbon complexation, similar to during summer. Approximately half of the annual colloidal Fe flux occurs during spring flood while most of the remaining colloidal Fe is supplied during summer months. The total amount of colloidal Fe transported during winter was relatively low. The seasonal variation in colloidal Fe isotope values may be a useful tool to trace the source of colloidal Fe to the Arctic Ocean and monitor future changes in the sources and supply of Fe from the permafrost landscape to the Lena River basin.
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| 4. |
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| 5. |
- Hirst, Catherine, et al.
(författare)
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Iron isotopes reveal the sources of Fe-bearing particles and colloids in the Lena River basin
- 2020
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 269, s. 678-692
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Tidskriftsartikel (refereegranskat)abstract
- Large Arctic rivers are important suppliers of iron to the Arctic Ocean. However, the sources of Fe-bearing particles in permafrost-dominated systems and the mechanisms driving this supply of Fe are poorly resolved. Here, Fe isotope ratios were used to determine the sources of Fe-bearing particles and colloids in the Lena River and tributaries. In samples collected after the spring floods, Fe-bearing particles (>0.22 mu m) carried similar to 70% of the Fe and have isotope ratios that are lower than, or similar to that of the continental crust. These particles are composed of a leachable Fe fraction of largely ferrihydrite, with isotope values of -1.40 parts per thousand to -0.12 parts per thousand, and a fraction of clays and Fe oxides with continental crust values. Co-existing Fe-bearing colloids (<0.22 mu m), composed mainly of ferrihydrite, have higher isotope values, of -0.22 parts per thousand to +1.83 parts per thousand. A model is proposed in which soil mineral weathering generates aqueous Fe with lower delta Fe-56 values. During transport, a small fraction of the dissolved Fe is precipitated as colloidal ferrihydrite with higher delta Fe-56 values. Most of the Fe is precipitated onto mineral grains in oxic riparian zones, with the delta Fe-56 values largely generated during weathering. Groundwater discharge and riparian erosion supply the colloids and coated particles to the rivers. The differences between delta Fe-56 values in leachates and detrital grains in Fe-bearing particles agree with values determined in mineral dissolution experiments and in Fe accumulation horizons in soils. The difference in delta Fe-56 values between leachates and colloids reflects isotope fractionation during incremental Fe(III)(aq) precipitation and Fe-OC complexation during transport towards the riparian zone. Overall, the Fe isotope values of riverine particles and colloids reflect processes that occur during mineral dissolution, transport, and secondary mineral formation in permafrost soils.
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| 6. |
- Kutscher, Liselott, et al.
(författare)
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Spatial variation in concentration and sources of organic carbon in the Lena River, Siberia
- 2017
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Ingår i: Journal of Geophysical Research - Biogeosciences. - New York. - 2169-8953 .- 2169-8961. ; 122
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Tidskriftsartikel (refereegranskat)abstract
- Global warming in permafrost areas is expected to change fluxes of riverine organic carbon (OC) 22 to the Arctic Ocean. Here OC concentrations, stable carbon isotope signatures (δ13C) and carbon-23 nitrogen ratios (C/N) are presented from 22 sampling stations in the Lena River and 40 of its 24 tributaries. Sampling was conducted during two expeditions: the first in July 2012 in the south 25 and southeastern region and the second in June 2013 in the northern region of the Lena basin. 26 The data showed significant spatial differences in concentrations and major sources of OC. Mean 27 sub-catchment slopes were correlated with OC concentrations, implying that mountainous areas 28 in general had lower concentrations than lowland areas. δ13C and C/N data from tributaries 29 originating in mountainous areas indicated that both dissolved and particulate OC (DOC and 30 POC) were mainly derived from soil organic matter (SOM). In contrast, tributaries originating in 31 lowland areas had larger contributions from fresh vegetation to DOC, while aquatically produced 32 OC was the major source of POC. We suggest that these differences in dominant sources 33 indicated differences in dominant flow pathways. Tributaries with larger influence of fresh 34 vegetation probably had surficial flow pathways, while tributaries with more SOM influence had 35 deeper water flow pathways. Thus, the future export of OC to the Arctic Ocean will likely be 36 controlled by changes in spatial patterns in hydroclimatology and the depth of the active layers 37 influencing the dominant water flow pathways in Arctic river basins.
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| 7. |
- Li, Bingxi, et al.
(författare)
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Thaw pond development and initial vegetation succession in experimental plots at a Siberian lowland tundra site
- 2017
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Ingår i: Plant and Soil. - : Springer Science and Business Media LLC. - 1573-5036 .- 0032-079X. ; 420:1-2, s. 147-162
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Tidskriftsartikel (refereegranskat)abstract
- Permafrost degradation has the potential to change the Arctic tundra landscape. We observed rapid local thawing of ice-rich permafrost resulting in thaw pond formation, which was triggered by removal of the shrub cover in a field experiment. This study aimed to examine the rate of permafrost thaw and the initial vegetation succession after the permafrost collapse. Methods In the experiment, we measured changes in soil thaw depth, plant species cover and soil subsidence over nine years (2007–2015). Results After abrupt initial thaw, soil subsidence in the removal plots continued indicating further thawing ofpermafrost albeit at a much slower pace: 1 cm y−1 over 2012–2015 vs. 5 cm y−1 over 2007–2012. Grass cover strongly increased after the initial shrub removal, but later declined with ponding of water in the subsiding removal plots. Sedges established and expanded in the wetter removal plots. Thereby, the removal plots have become increasingly similar to nearby ‘natural’ thawponds. Conclusions The nine years of field observations in a unique shrub removal experiment at a Siberian tundra site document possible trajectories of small-scale permafrost collapse and the initial stage of vegetation recovery,which is essential knowledge for assessing future tundra landscape changes.
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| 8. |
- Magnússon, Rúna Í., et al.
(författare)
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Extremely wet summer events enhance permafrost thaw for multiple years in Siberian tundra
- 2022
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- Permafrost thaw can accelerate climate warming by releasing carbon from previously frozen soil in the form of greenhouse gases. Rainfall extremes have been proposed to increase permafrost thaw, but the magnitude and duration of this effect are poorly understood. Here we present empirical evidence showing that one extremely wet summer (+100 mm; 120% increase relative to average June-August rainfall) enhanced thaw depth by up to 35% in a controlled irrigation experiment in an ice-rich Siberian tundra site. The effect persisted over two subsequent summers, demonstrating a carry-over effect of extremely wet summers. Using soil thermal hydrological modelling, we show that rainfall extremes delayed autumn freeze-up and rainfall-induced increases in thaw were most pronounced for warm summers with mid-summer precipitation rainfall extremes. Our results suggest that, with rainfall and temperature both increasing in the Arctic, permafrost will likely degrade and disappear faster than is currently anticipated based on rising air temperatures alone. Thawing permafrost releases carbon that serves as a positive feedback on climate warming. Here the authors experimentally demonstrate that rainfall extremes in the Siberian tundra increase permafrost thaw for multiple years, especially if rainfall coincides with warm periods.
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| 9. |
- Murphy, Melissa, et al.
(författare)
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Tracingsilicate weathering processes in the permafrost-dominated Lena River watershedusing lithium isotopes
- 2019
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Ingår i: Geochimica et Cosmochimica Acta. - : Saunders Elsevier. - 0016-7037 .- 1872-9533. ; 245, s. 154-171
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Tidskriftsartikel (refereegranskat)abstract
- Increasing global temperatures are causing widespread changes in the Arctic, including permafrost thawing and altered freshwater inputs and trace metal and carbon fluxes into the ocean and atmosphere. Changes in the permafrost active layer thickness can affect subsurface water flow paths and water-rock interaction times, and hence weathering processes. Riverine lithium isotope ratios (reported as δ7Li) are tracers of silicate weathering that are unaffected by biological uptake, redox, carbonate weathering and primary lithology. Here we use Li isotopes to examine silicate weathering processes in one of the largest Russian Arctic rivers: the Lena River in eastern Siberia. The Lena River watershed is a large multi-lithological catchment, underlain by continuous permafrost. An extensive dataset of dissolved Li isotopic compositions of waters from the Lena River main channel, two main tributaries (the Aldan and Viliui Rivers) and a range of smaller sub tributaries are presented from the post-spring flood/early-summer period at the onset of active layer development and enhanced water-rock interactions. The Lena River main channel (average δ7Lidiss ~19‰) has a slightly lower isotopic composition than the mean global average of 23‰ (Huhet al., 1998a). The greatest range of [Li] and δ7Lidiss are observed in catchments draining the south facing slopes of the Verkhoyansk Mountain Range. South-facing slopes in high-latitude, permafrost dominated regions are typically characterised by increased summer insolation and higher daytime temperatures relative to other slope aspects. The increased solar radiation on south-facing catchments promotes repeated freeze-thaw cycles, and contributes to more rapid melting of snow cover, warmer soils, and increased active layer thaw depths. The greater variability in δ7Li and [Li] in the south-facing rivers likely reflect the greater infiltration of melt water and enhanced water rock interactions within the active layer. A similar magnitude of isotopic fractionation is observed between the low-lying regions of the Central Siberian Plateau (and catchments draining into the Viliui River), and catchments draining the Verkhoyansk Mountain Range into the Aldan River. This is in contrast to global rivers in non permafrost terrains that drain high elevations or areas of rapid uplift, where high degrees of physical erosion promote dissolution of freshly exposed primary rock typically yielding low δ7Lidiss, and low lying regions exhibit high riverine δ7Li values resulting from greater water-rock interaction and formation of secondary mineral that fractionates Li isotopes. Overall, the range of Li concentrations and δ7Lidiss observed within the Lena River catchment are comparable to global rivers located in temperate and tropical regions. This suggests that cryogenic weathering features specific to permafrost regions (such as the continual exposure of fresh primary minerals due to seasonal freeze-thaw cycles, frost shattering and salt weathering), and climate (temperature and runoff), are not a dominant control on δ7Li variations. Despite vastly different climatic and weathering regimes, the same range of riverine δ7Li values globally suggests that the same processes govern Li geochemistry – that is, the balance between primary silicate mineral dissolution and the formation (or exchange with) secondary minerals. This has implications for the use of δ7Li as a palaeo weathering tracer for interpreting changes in past weathering regimes.
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| 10. |
- Siewert, Matthias B., et al.
(författare)
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Comparing carbon storage of Siberian tundra and taiga permafrost ecosystems at very high spatial resolution
- 2015
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Ingår i: Journal of Geophysical Research - Biogeosciences. - 2169-8953 .- 2169-8961. ; 120:10, s. 1973-1994
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Tidskriftsartikel (refereegranskat)abstract
- Permafrost-affected ecosystems are important components in the global carbon (C) cycle that, despite being vulnerable to disturbances under climate change, remain poorly understood. This study investigates ecosystem carbon storage in two contrasting continuous permafrost areas of NE and East Siberia. Detailed partitioning of soil organic carbon (SOC) and phytomass carbon (PC) is analyzed for one tundra (Kytalyk) and one taiga (Spasskaya Pad/Neleger) study area. In total, 57 individual field sites (24 and 33 in the respective areas) have been sampled for PC and SOC, including the upper permafrost. Landscape partitioning of ecosystem C storage was derived from thematic upscaling of field observations using a land cover classification from very high resolution (2x2m) satellite imagery. Nonmetric multidimensional scaling was used to explore patterns in C distribution. In both environments the ecosystem C is mostly stored in the soil (86%). At the landscape scale C stocks are primarily controlled by the presence of thermokarst depressions (alases). In the tundra landscape, site-scale variability of C is controlled by periglacial geomorphological features, while in the taiga, local differences in catenary position, soil texture, and forest successions are more important. Very high resolution remote sensing is highly beneficial to the quantification of C storage. Detailed knowledge of ecosystem C storage and ground ice distribution is needed to predict permafrost landscape vulnerability to projected climatic changes. We argue that vegetation dynamics are unlikely to offset mineralization of thawed permafrost C and that landscape-scale reworking of SOC represents the largest potential changes to C cycling.
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| 11. |
- Sun, Xiaole, et al.
(författare)
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Stable silicon isotopic compositions of the Lena River and its tributaries : Implications for silicon delivery to the Arctic Ocean
- 2018
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 241, s. 120-133
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Tidskriftsartikel (refereegranskat)abstract
- Silicon isotope values (delta Si-30(DSi)) of dissolved silicon (DSi) have been analyzed in the Lena River and its tributaries, one of the largest Arctic watersheds in the world. The geographical and temporal variations of delta Si-30(DSi) range from +0.39 to +1.86% with DSi concentrations from 34 to 121 mu M. No obvious patterns of DSi concentrations and delta Si-30(DSi) values were observed along over 200 km of the two major tributaries, the Viliui and Aldan Rivers. In summer, the variations of DSi concentrations and delta Si-30(DSi) values in the water are either caused by biological uptake by higher plants and phytoplankton or by mixing of water masses carrying different DSi concentrations and delta Si-30(DSi) values. DSi in tributaries from the Verkhoyansk Mountain Range seems to be associated with secondary clay formation that increased the delta Si-30(DSi) values, while terrestrial biological production is likely more prevalent in controlling delta Si-30(DSi) values in Central Siberian Plateau and Lena Amganski Inter-River Area. In winter, when soils were frozen, the delta Si-30(DSi) values in the river appeared to be controlled by weathering and clay formation in deep intrapermafrost groundwater. During the spring flood, dissolved silicate materials and phytoliths were flushed from the upper thawed soils into rivers, which reset delta Si-30(DSi) values to the values observed prior to the biological bloom in summer. The results indicate that the Si isotope values reflect the changing processes controlling Si outputs to the Lena River and to the Arctic Ocean between seasons. The annual average delta Si-30(DSi) value of the Lena Si flux is calculated to be +0.86 +/- 0.3 parts per thousand using measured delta Si-30(DSi) values from each season. Combined with the estimate of + 1.6 +/- 0.25 parts per thousand for the Yenisey River, an updated delta Si-30(DSi) value of the major river Si inputs to the Arctic Ocean is estimated to be + 1.3 +/- 0.3 parts per thousand. This value is expected to shift towards higher values in the future because of the impacts from a variety of biological and geochemical processes and sources under global warming.
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| 12. |
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