River–aquifer interfaces are essential for ecosystem functioning in terms of nutrient exchange and biological habitat, but are greatly threatened world-wide. This study examined geochemical aspects ...of river–aquifer interaction in one regulated and one unregulated boreal river in Northern Sweden to determine whether the geochemical functioning of the hyporheic zone is affected by hydrological alterations, e.g. regulated river discharge and river–aquifer connectivity. In the unregulated Kalix River, the hyporheic pore water was well-oxygenated with orthogonal fluxes (≈0.6–0.7md−1) and acted as a sink for Fe, Mn, Al, NH4, and Ca, with fractional losses of 95%, 92%, 45%, 31%, and 15%, respectively. A corresponding elevation in the concentrations of these elements in the hyporheic sediment was observed, with higher saturation indices of Fe-, Mn-, and Al-bearing secondary minerals in hyporheic waters. In the regulated Lule River, hydraulic connectivity at the river–aquifer interface was altered by the presence of a clogging layer (0.04md−1). In addition, the river discharge oscillated daily, severely reducing exchange flows across the riverbed (<0.01md−1). As a result, the hyporheic pore water was suboxic, with elevated concentrations of filtered Fe and Mn (fractional increases of ≈3700% and ≈2500%, respectively) and other solutes (NH4, Si, S, Ca). A conceptual model revealed functional differences between geochemical features of the hyporheic zone of regulated and unregulated rivers. Overall, the results showed that hyporheic processes are altered along regulated rivers, with resulting impacts on the geochemistry of riverine, riparian and related marine ecosystems.
•Hyporheic hydrogeochemistry is driven by river–aquifer connectivity and discharge.•Regulation favours riverbed colmation, impairing hyporheic exchange.•The hyporheic zone in the unregulated river acts as a geochemical barrier to Fe and Mn.•The hyporheic zone in the regulated river acts as a source of Fe, Mn, Al, DOC and NH4.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Results are presented for the first in-depth investigation of Tl isotope variations in marine materials. The Tl isotopic measurements were conducted by multiple collector-inductively coupled plasma ...mass spectrometry for a comprehensive suite of hydrogenetic ferromanganese crusts, diagenetic Fe–Mn nodules, hydrothermal manganese deposits and seawater samples. The natural variability of Tl isotope compositions in these samples exceeds the analytical reproducibility (±0.05‰) by more than a factor of 40. Hydrogenetic Fe–Mn crusts have
ϵ
205Tl of +10 to +14, whereas seawater is characterized by values as low as −8 (
ϵ
205Tl represents the deviation of the
205Tl/
203Tl ratio of a sample from the NIST SRM 997 Tl isotope standard in parts per 10
4). This ∼2‰ difference in isotope composition is thought to result from the isotope fractionation that accompanies the adsorption of Tl onto ferromanganese particles. An equilibrium fractionation factor of
α∼1.0021 is calculated for this process. Ferromanganese nodules and hydrothermal manganese deposits have variable Tl isotope compositions that range between the values obtained for seawater and hydrogenetic Fe–Mn crusts. The variability in
ϵ
205Tl in diagenetic nodules appears to be caused by the adsorption of Tl from pore fluids, which act as a closed-system reservoir with a Tl isotope composition that is inferred to be similar to seawater. Nodules with
ϵ
205Tl values similar to seawater are found if the scavenging of Tl is nearly quantitative. Hydrothermal manganese deposits display a positive correlation between
ϵ
205Tl and Mn/Fe. This trend is thought to be due to the derivation of Tl from distinct hydrothermal sources. Deposits with low Mn/Fe ratios and low
ϵ
205Tl are produced by the adsorption of Tl from fluids that are sampled close to hydrothermal sources. Such fluids have low Mn/Fe ratios and relatively high temperatures, such that only minor isotope fractionation occurs during adsorption. Hydrothermal manganese deposits with high Mn/Fe and high
ϵ
205Tl are generated by scavenging of Tl from colder, more distal hydrothermal fluids. Under such conditions, adsorption is associated with significant isotope fractionation, and this produces deposits with higher
ϵ
205Tl values coupled with high Mn/Fe.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Iron chemistry measurements were conducted during summer 2007 at two distinct locations in the Baltic Sea (Gotland Deep and Landsort Deep) to evaluate the role of iron for cyanobacterial bloom ...development in these estuarine waters. Depth profiles of Fe(II) were measured by chemiluminescent flow injection analysis (CL-FIA). Up to 0.9 nmol Fe(II) L−1 were detected in light penetrated surface waters, which constitutes up to 20% to the dissolved Fe pool. This bioavailable iron source is a major contributor to the Fe requirements of Baltic Sea phytoplankton and apparently plays a major role for cyanobacterial bloom development during our study. Measured Fe(II) half life times in oxygenated water exceed predicted values and indicate organic Fe(II) complexation. Potential sources for Fe(II) ligands, including rainwater, are discussed. Fe(II) concentrations of up to 1.44 nmol L−1 were detected at water depths below the euphotic zone, but above the oxic anoxic interface. Mixed layer depths after strong wind events are not deep enough in summer time to penetrate the oxic-anoxic boundary layer. However, Fe(II) from anoxic bottom water may enter the sub-oxic zone via diapycnal mixing and diffusion.
To indentify sources and transport mechanisms of iron in a coastal marine environment, we conducted measurements of the physiochemical speciation of Fe in the euphotic zone at three different ...locations in the Baltic Sea. In addition to sampling across a salinity gradient, we conducted this study over the spring and summer season. Moving from the riverine input characterized low salinity Bothnian Sea, via the Landsort Deep near Stockholm, towards the Gotland Deep in the Baltic Proper, total Fe concentrations averaged 114, 44, and 15 nM, respectively. At all three locations, a decrease in total Fe of 80–90% from early spring to summer was observed. Particulate Fe (PFe) was the dominating phase at all stations and accounted for 75–85% of the total Fe pool on average. The Fe isotope composition (δ 56Fe) of the PFe showed constant positive values in the Bothnian Sea surface waters (+0.08 to +0.20‰). Enrichment of heavy Fe in the Bothnian Sea PFe is possibly associated to input of aggregated land derived Fe-oxyhydroxides and oxidation of dissolved Fe(II). At the Landsort Deep the isotopic fractionation of PFe changed between −0.08‰ to +0.28‰ over the sampling period. The negative values in early spring indicate transport of PFe from the oxic-anoxic boundary at ∼80 m depth. The average colloidal iron fraction (CFe) showed decreasing concentrations along the salinity gradient; Bothnian Sea 15 nM; Landsort Deep 1 nM, and Gotland Deep 0.5 nM. Field Flow Fractionation data indicate that the main colloidal carrier phase for Fe in the Baltic Sea is a carbon-rich fulvic acid associated compound, likely of riverine origin. A strong positive correlation between PFe and chl-a indicates that cycling of suspended Fe is at least partially controlled by primary production. However, this relationship may not be dominated by active uptake of Fe into phytoplankton, but instead may reflect scavenging and removal of PFe during phytoplankton sedimentation.
Based on an international workshop (Gothenburg, 14–16 May 2008), this review article aims to combine interdisciplinary knowledge from coastal and open ocean research on iron biogeochemistry. The ...major scientific findings of the past decade are structured into sections on natural and artificial iron fertilization, iron inputs into coastal and estuarine systems, colloidal iron and organic matter, and biological processes. Potential effects of global climate change, particularly ocean acidification, on iron biogeochemistry are discussed. The findings are synthesized into recommendations for future research areas.
Thorough characterization of the Si isotopic composition of the terrestrial biogenic pool could potentially allow Si isotope information to be used when assessing the relative contributions from ...biogenic and mineral sources to soil water, plants and surface waters. In the present study, the Si isotopic compositions of major biomass components in a boreal forest in Northern Sweden were investigated, along with the relative contributions from exogenous Si incorporated in the plant structure. This was achieved using chemical purification and high-resolution multi-collector inductively coupled mass spectrometry (MC-ICP-MS) for the precise and accurate determination of the Si isotopic composition of plants. The technique, which relies on multi-elemental analysis of plant ashes and sample-specific HF-dissolution followed by strong-anion exchange chromatography, allows efficient separation of Si from matrix and interfering elements, while recovering in excess of 99% Si. The long-term instrumental reproducibility, expressed as two standard deviations (2
σ), for the isotopic reference material NBS28 (
n
=
12) was 0.06‰ for
δ
29Si and 0.12‰ for
δ
30Si.
Results for the analyses of composite plant samples for the eight most prolific species in the boreal forest yielded a surprisingly homogenous Si isotopic composition, expressed as
δ
29Si (±
expanded combined uncertainty) and
δ
30Si, ranging from (−
0.15
±
0.11)‰ to (0.13
±
0.06)‰ and (−
0.31
±
0.08)‰ to (0.22
±
0.13)‰, respectively. Isotopic and elemental analyses of local airborne particulate material suggest that the exogenous Si contribution varies between <
1% and >
70%, indicating that the potential surface contribution must be considered during Si uptake studies. The present study thus provides evidence that thorough appreciation of the forms of Si in plants is an absolute requirement when assessing the plant impact on the Si cycle via the difference in dissolution kinetics for phytoliths and lithogenic Si.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Once or twice weekly, water sampling was undertaken for a two and a half year period in the Kalix River, northern Sweden. Soil water, groundwater, water in tributaries and mire water were also ...sampled at several occasions. Samples were filtered and analysed for major dissolved elements and TOC. Although only 5 of the bedrock in the Kalix River drainage basin is situated in the Caledonian mountains (mostly schist, with some outcrops of dolomite and limestone), the chemical composition of the river, at the river mouth, is clearly influenced by water from the mountain areas. High dissolved Ca/Mg ratios in June and July indicate a large influence of water from the mountain areas during summer. The dissolved Si/Mg ratio increases when water from the woodland (bedrock consisting of Precambrian granitoids) predominates during snowmelt in May, but the ratio is low during summer when water from the mountains is increased. However, the low Si concentrations in the mountain areas are probably not primarily the result of the different rocks but more a reflection of the less intense weathering of silicate minerals in the mountains. High Si/Mg ratios are closely related to high TOC. All the major dissolved elements, except TOC, are diluted by snowmelt in May. However, the dilution varies for different elements. Based on the interpretations of major element ratios the melt water discharge in May reflects two major compartments in the woodland; peatland areas and the upper section of the soil. During summer and autumn storm events in the woodland most of the storm water originated from peatland. High K/Mg ratios in the river in May are related to water discharge from the upper section of the till. Low S/Mg ratios in the river indicate an influence of mire water from the woodland both during melt water discharge in May and during increased water discharge in autumn. The Ca/Mg ratios in tributaries in the woodland are consistently lower during melt water discharge compared with values in August. The lower Ca/Mg ratio in May probably reflects water that has been in contact with the B-horizon in the till during spring flood. Data show that the TOC discharged during spring flood originates from two major compartments in the landscape, the upper soil profile and peatland. Storm discharge of TOC during the rest of the year originates mostly from peatland.PUBLICATION ABSTRACT
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A considerable amount of colloidally bound Ca has been detected in water samples from Amazonian rivers and the Kalix River, a sub-arctic boreal river. Fractionation experiments using several ...analytical techniques and processing tools were conducted in order to elucidate the matter. Results show that on average 84% of the total Ca concentration is present as free Ca. Particulate, colloidal and complexed Ca constitute the remaining 16%, of which the colloidal fraction is significant. Ultrafiltration experiments show that the colloidal fraction in the sampled Amazonian rivers and the Kalix River range between 1% and 25%.
In both the Amazonian and the Kalix rivers the technique of cross-flow ultrafiltration was used to isolate particles and colloids. The difference in concentration measured with ICP-AES and a Ca ion-selective electrode in identical samples was used to define the free Ca concentration and thus indirectly the magnitude of the particulate, colloidal and complexed fractions. Results from the Kalix and Amazonian rivers are in excellent agreement. Furthermore, the results show that the colloidal concentrations of Ca can be greatly overestimated (up to 227%) when conventional analysis and calculation of ultrafiltration data is used due to retention of free Ca ions during the ultrafiltration process. Calculation methods for colloidal matter are presented in this work, using complementary data from ISE analysis.
In the Kalix River temporal changes in the fractionation of Ca were studied before, during and after a spring-flood event. Changes in the size distribution of colloidally associated Ca was studied using FlFFF (Flow Field-Flow Fractionation) coupled on-line to a HR ICP-MS. The FlFFF–HR ICP-MS fractograms clearly show the colloidal component of Ca, supporting the ultrafiltration findings. During winter conditions the size distribution of colloidally associated Ca has a concentration maximum at ∼5 to 10 nm in diameter, shifting to smaller sizes (<5 nm) during and after the spring flood. This shift in size distribution follows a change in the river during this period from ironoxyhydroxy colloids being the most important colloidal carrier phase to humic substances during and after the spring flood.
WHAM and NICA-Donnan models were used to calculate the amount of colloidally bound Ca. The results similar for both models, show that on average 16% of the Ca may be associated to a colloidal phase, which is in broad agreement with the measurements.
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IJS, IMTLJ, KILJ, KISLJ, NUK, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Particle-mediated removal processes of U isotopes were investigated during spring flood discharge in the low-salinity zone (LSZ, up to 3 practical salinity units psu) of a stable estuary. A shipboard ...ultrafiltration cross-flow filtration (CFF) technique was used to separate particles (>0.2 μm) and colloids (between 3000 daltons (3 kD) and 0.2 μm) from ultrafiltered water (<3 kD) containing “dissolved” species. Sediment traps were used to collect sinking material. Concentration of Fe and organic C, which are indicators of the major U carrier phases, were used to interpret the behavior of
234U-
238U during estuarine mixing.
Colloids dominated the river water transport of U, carrying ≈90% of the U. On entering the estuary, colloids accounted for the dominant fraction of U to about a salinity of 1 psu, but only a minor fraction (<5%) at 3 psu. A substantial fraction of the total U is removed at <1 psu by Fe-organic rich colloids that aggregate and sink during initial estuarine mixing in the Kalix River estuary. In contrast, at salinities >1 psu, there is a general correlation between U and salinity in all filtered fractions. The
234U/
238U ratios in different filtered fractions and sinking particles were generally indistinguishable at each station and showed enrichment in
234U, compared with secular equilibrium (δ
234U = 266–567). This clearly shows that all size fractions are dominated by nondetrital U. Consideration of U isotope systematics across the estuary reveals that substantial U exchange must occur involving larger particles at least to 1 psu and involving colloids at least to ≈1.5 psu. Further exchange at higher salinities may also occur, as the proportion of U on colloids decreases with increasing salinity. This may be due to decreasing colloid concentration and increasing stabilization of uranyl carbonate complexes during mixing in the estuary.
The results show that although U is a soluble element that shows generally conservative mixing in estuaries, removal occurs in the very low salinity zone, and this zone represents a significant sink of U. Variation in composition and concentration of colloidal particles between different estuaries might thus be an important factor for determining the varying behavior of U between estuaries.
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IJS, IMTLJ, KILJ, KISLJ, NUK, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The importance of colloids and organic deposits for the transport of uranium isotopes from continental source regions and through the estuarine environment was investigated in the mire-rich Kalix ...River drainage basin in northern Sweden and the Baltic Sea. Ultrafiltration techniques were used to separate uranium and other elements associated with colloids > 10 kD and >3 kD from “solute” uranium and provided consistent results and high recovery rates for uranium as well as for other elements from large volume samples.
Uranium concentrations in 0.45 μm-filtered Kalix River water samples increased by a factor of 3 from near the headwaters in the Caledonides to the river mouth while major cation concentrations were relatively constant.
234
U
238
U
ratios were high (
δ
234U = 770−1500) throughout the basin, without showing any simple pattern, and required a supply of
234U-rich water. Throughout the Kalix River, a large fraction (30–90%) of the uranium is carried by >10 kD colloids, which is compatible with uranium complexation with humic acids. No isotopic differences were found between colloid-associated and solute uranium.
Within the Baltic Sea, about half of the uranium is removed at low salinities. The proportion that is lost is equivalent to that of river-derived colloid-bound uranium, suggesting that while solute uranium behaves conservatively during estuarine mixing, colloid-bound uranium is lost due to rapid flocculation of colloidal material. The association of uranium with colloids therefore may be an important parameter in determining uranium estuarine behavior.
Mire peats in the Kalix River highly concentrate uranium and are potentially a significant source of recoil
234U to the mirewaters and river waters. However, mirewater data clearly demonstrate that only small
234
U
238
U
shifts are generated relative to inflowing groundwater. A simple box model of uranium accumulation in peat and transport through the mire that is compatible with the mire data demonstrates that with efficient removal of uranium from solution, only small shifts in
234
U
238
U
ratios can be generated in mirewater uranium. The measurements and model calculations show that mirewaters are not the primary source of the uranium in the river. Bedrock groundwaters with high
234
U
238
U
ratios and uranium concentrations must be the dominant source of riverine uranium.
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IJS, IMTLJ, KILJ, KISLJ, NUK, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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