Cycling of trace metals (Mn, Fe, Mo, U, V, Cr) in deep pore waters of intertidal flat sediments

Trace metals (Mn, Fe, Mo, U, Cr, V) were studied in pore waters of an intertidal flat located in the German Wadden Sea. The study system is an example of a permeable tidal flat system where pore water exchange is affected by tidal driven pressure gradients besides diffusion. Permanently installed in situ samplers were used to extract pore waters down to 5 m depth throughout one year. The samplers were either located close to the tidal flat margin or in central parts of the tidal flat. Despite dynamic sedimentological and hydrological conditions, the general trends with depth in deep tidal flat pore waters are remarkably similar to those observed in deep sea environments. Rates of trace metal cycling must be comparably large in order to maintain the observed pore water profiles. Trace metals further show similar general trends with depth close to the margin and in central parts of the tidal flat. Seasonal sampling revealed that V and Cr vary concurrent with seasonal changes in dissolved organic carbon (DOC) concentration. This effect is most notable close to the tidal flat margin where sulphate, DOC, and nutrients vary with season down to some metres depth. Seasonal variations of Mn, Fe, Mo, and U are by contrast limited to the upper decimetres of the sediment. Their seasonal patterns depend on organic matter supply, redox stratification, and particulate matter deposited on sediment surfaces. Pore water sampling within one tidal cycle provides evidence for pore water advection in margin sediments. During low tide pore water flow towards the creekbank is generated by a hydraulic gradient suggesting that deep pore waters may be seeping out of creekbank sediments. Owing to the enrichment of specific elements like Mn in pore water compared to sea water, seeping pore waters may have an impact on the chemistry of the open water column. Mass balance calculations reveal that the impact of deep pore waters on the Mn budget in the open water column is below 4%. Mn deep pore water discharge of the whole Wadden Sea is estimated to be about 9% of the total dissolved riverine Mn input into the Southern North Sea.     

Fluvial and hydrothermal input of manganese into the Arctic Ocean

A total of 773 samples were analysed for dissolved manganese (Mn) in the Arctic Ocean aboard R.V. Polarstern during expedition ARK XXII/2 from 28 July until 07 October 2007 from Tromsø (Norway) to Bremerhaven. Concentrations of Mn were elevated in the surface layer with concentrations of up to 6 nM over the deep Basins and over 20 nM in the Laptev Sea. The general distribution of Mn through the water column is consistent with previous studies, but there are differences in the absolute concentrations that are most likely related to differences in sample area, sampling and filtration.The elevated concentrations of Mn in the surface layer are related to fresh water input. This was visible in the strong negative correlations observed between dissolved Mn and salinity. The correlation between Mn and salinity and the correlation between Mn and the quasi conservative trace water mass tracer PO₄^∗, showed fluvial and melt water input and the Pacific and Atlantic origin of the surface waters. A large portion of the Mn delivered by the Arctic rivers is removed in the shelf seas and does not pass into the central basins. Most likely a benthic flux is at the origin of the elevated concentrations of Mn near the sediments in the Barents and Kara Seas. These elevated concentrations of Mn apparently affected the deep basins as well, as maxima in the concentrations of Mn were observed that corresponded with lowered transmission over the continental slope.A maximum in the concentration of Mn in the deep basin corresponded with anomalies in light transmission, potential temperature and dissolved iron, confirming the hydrothermal origin. The hydrothermal plume was observed throughout the Nansen Basin and over the deep Gakkel Ridge around 2500 m depth and a smaller plume was observed around 3200 m. The concentration of Mn at the Mn maximum around 2500 m depth decreased exponentially, consistent with a first order scavenging model. The concentrations of Mn were extremely low in the deep Makarov Basin (∼0.05 nM) and slightly higher in the Eurasian Basin (∼0.1 nM) outside the influence of the hydrothermal activity.     

The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland

Molybdenum (Mo) isotope studies in black shales can provide information about the redox evolution of the Earth’s oceans, provided the isotopic consequences of Mo burial into its major sinks are well understood. Previous applications of the Mo isotope paleo-ocean redox proxy assumed quantitative scavenging of Mo when buried into sulfidic sediments. This paper contains the first complete suite of Mo isotope fractionation observations in a sulfidic water column and sediment system, the meromictic Lake Cadagno, Switzerland, a small alpine lake with a pronounced oxygen-sulfide transition reaching up to H₂S ∼ 200 μM in the bottom waters (or about 300 μM total sulfide: ΣS²⁻ = H₂S + HS⁻ + S²⁻). We find that Mo behaves conservatively in the oxic zone and non-conservatively in the sulfidic zone, where dissolved Mo concentrations decrease from 14 nM to 2-8 nM across this transition. Dissolved Mo in the upper oxic waters has a δ⁹⁸Mo_oxic = 0.9 ± 0.1‰, which matches that of the riverine input, δ⁹⁸Mo_river = 0.9 ± 0.1‰. In the deeper sulfidic waters, a subaquatic source delivers Mo at 1.55 ± 0.1‰, but the dissolved Mo is even heavier at δ⁹⁸Mo_sulfidic = 1.8‰. Sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in Mo with depth, with δ⁹⁸Mo values significantly fractionated at −0.8‰ to −1.2‰ both near the chemocline and in the deepest trap. Suspended particulates in the sulfidic waters carry lighter Mo than the ambient dissolved Mo pool by ∼0.3-1.5‰. Sedimentary Mo concentrations correlate with total organic carbon and yield Mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area. Solid-phase Mo in the sediment shows a slightly positive δ⁹⁸Mo trend with depth, from δ⁹⁸Mo = 1.2‰ to 1.4‰ while the pore waters show dramatic enrichments of Mo (>2000 nM) with a relatively light isotope signature of δ⁹⁸Mo = 0.9-1.0‰.These data are explained if Mo is converted to particle-reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles. Isotope fractionation is expressed in the water column, despite the high sulfide concentrations, because the rate of Mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation. However, elemental and isotopic mass balances show that Mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water. This efficient Mo drawdown is expected to occur in settings where H₂S is very much in excess over Mo or in a restricted setting where the water renewal rate is slow compared to the Mo burial rate. We present a model for the Mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems.     

Distribution and reactivity of oxyanions (Sb, As, V, Mo) in the surface freshwater reaches of the Gironde Estuary (France)

The present work reports on the temporal variations of Sb, As, V and Mo concentrations in the surface freshwater reaches of the Gironde Estuary (SW France). Dissolved (<0.2 and <0.02 μm) and particulate Sb, As, V and Mo concentrations were measured in samples collected with high temporal resolution (every 30 min) during two consecutive tides at a fixed station located upstream of the city of Bordeaux and the maximum turbidity zone (MTZ). In addition, measurements of suspended particulate matter concentration, physico-chemical parameters, Cl⁻ concentration, water depth, current velocity and particulate Th concentration were performed either at the same time resolution or continuously. The data obtained suggested that variations in particulate As, V and Mo concentrations were most probably related to tidal cycling near the sampling site of fine grained particles from the MTZ located downstream. Significant differences in the behavior of the dissolved target oxyanions, mostly present in the <0.02 μm fraction, occurred. The behavior of Sb was conservative. Variations of the dissolved As and V concentrations showed similar cycling trends, strongly related to tidal cycles. These As and V cycles were interpreted as a mixing between upstream freshwater and downstream water enriched in dissolved As and V by desorption from the MTZ particles. The observed trend in the variation of the dissolved Mo was more complex than that of As and Sb and attributed to the mixing of the water bodies mentioned above coupled to a point source input of dissolved Mo from an intra-estuarine source. The contribution of this suspected Mo source to the dissolved Mo concentrations measured at the sampling site was estimated from the ratio of dissolved Mo to dissolved As concentrations. The additional dissolved Mo signal, coupled to the monitored ebb and flood length and associated current velocities, suggested an anthropogenic input which may derive from industrial activity near the city of Bordeaux. Extrapolating these results to the annual scale suggested that this dissolved Mo may be equivalent to 45-90% of the annual dissolved Mo flux into the Garonne Branch, highlighting the importance to further investigate the origin and behavior of Mo in the fluvial estuary near Bordeaux.     

Radium-based pore water fluxes of silica, alkalinity, manganese, DOC, and uranium: A decade of studies in the German Wadden Sea

A decade of studies of metal and nutrient inputs to the back-barrier area of Spiekeroog Island, NW German Wadden Sea, have concluded that pore water discharge provides a significant source of the enrichments of many components measured in the tidal channels during low tide. In this paper we add studies of radium isotopes to help quantify fluxes into and out of this system. Activities of radium isotopes in surface water from tidal channels in the back-barrier area exhibit pronounced changes in concert with the tide, with highest activities occurring near low tide. Other dissolved components: silica, total alkalinity (TA), manganese, and dissolved organic carbon (DOC) exhibit similar changes, with patterns matching the Ra isotopes. Uranium follows a reverse pattern with highest concentrations at high tide. Here we use radium isotope measurements in water column and pore water samples to estimate the fluxes of pore waters that enter the tidal channels during low tide. Using a flushing time of 4 days and the average activities of ²²⁴Ra, ²²³Ra, and ²²⁸Ra measured in the back-barrier surface and pore waters, we construct a balance of these isotopes, which is sustained by a deep pore water flux of (2-4) × 10⁸ L per tidal cycle. This flux transports Ra and the other enriched components to the tidal channels and causes the observed low tide enrichments. An independent estimate of pore water recharge is based on the depletion of U in the tidal channels. The U-based recharge is about two times greater than the Ra-based discharge; however, other sinks of U could reduce the recharge estimate. The pore waters have wide ranges of enrichment in silica, alkalinity, manganese, DOC, and depletion of U with depth. We estimate concentrations of these components in pore water from the depth expected to contribute the majority of the pore water flux, 3.5 m, to determine fluxes of these components to the tidal channels. Samples from this depth have minimum concentrations of silica, alkalinity, manganese, and DOC. We also estimate the exports of these components (and import of U) due to mixing based on average measured concentrations in the tidal creeks and the 4-day flushing time. A comparison of these estimates reveals that the exports (negative in the case of U) equal or exceed the pore water fluxes. By using values slightly higher than the minimum concentrations at 3.5 m to calculate inputs, the two estimates could be forced to match. We conclude that pore water drainage is the major factor regulating fluxes of Ra isotopes, silica, alkalinity, manganese, DOC, and uranium in this system.     

Early diagenesis of redox-sensitive trace metals in the Peru upwelling area - response to ENSO-related oxygen fluctuations in the water column

Pore water and solid phase data for redox-sensitive metals (Mn, Fe, V, Mo and U) were collected on a transect across the Peru upwelling area (11°S) at water depths between 78 and 2025 m and bottom water oxygen concentrations ranging from ∼0 to 93 μM. By comparing authigenic mass accumulation rates and diffusive benthic fluxes, we evaluate the respective mechanisms of trace metal accumulation, retention and remobilization across the oxygen minimum zone (OMZ) and with respect to oxygen fluctuations in the water column related to the El Niño Southern Oscillation (ENSO).Sediments within the permanent OMZ are characterized by diffusive uptake and authigenic fixation of U, V and Mo as well as diffusive loss of Mn and Fe across the benthic boundary. Some of the dissolved Mn and Fe in the water column re-precipitate at the oxycline and shuttle particle-reactive trace metals to the sediment surface at the lower and upper boundary of the OMZ. At the lower boundary, pore waters are not sufficiently sulfidic as to enable an efficient authigenic V and Mo fixation. As a consequence, sediments below the OMZ are preferentially enriched in U which is delivered via both in situ precipitation and lateral supply of U-rich phosphorites from further upslope. Trace metal cycling on the Peruvian shelf is strongly affected by ENSO-related oxygen fluctuations in bottom water. During periods of shelf oxygenation, surface sediments receive particulate V and Mo with metal (oxyhydr)oxides that derive from both terrigenous sources and precipitation at the retreating oxycline. After the recurrence of anoxic conditions, metal (oxyhydr)oxides are reductively dissolved and the hereby liberated V and Mo are authigenically removed. This alternation between supply of particle-reactive trace metals during oxic periods and fixation during anoxic periods leads to a preferential accumulation of V and Mo compared to U on the Peruvian shelf. The decoupling of V, Mo and U accumulation is further accentuated by the varying susceptibility to re-oxidation of the different authigenic metal phases. While authigenic U and V are readily re-oxidized and recycled during periods of shelf oxygenation, the sequestration of Mo by authigenic pyrite is favored by the transient occurrence of oxidizing conditions.Our findings reveal that redox-sensitive trace metals respond in specific manner to short-term oxygen fluctuations in the water column. The relative enrichment patterns identified might be useful for the reconstruction of past OMZ extension and large-scale redox oscillations in the geological record.     

Trace element cycling in a subterranean estuary: Part 2. Geochemistry of the pore water

Submarine groundwater discharge (SGD) is an important source of dissolved elements to the ocean, yet little is known regarding the chemical reactions that control their flux from sandy coastal aquifers. The net flux of elements from SGD to the coastal ocean is dependent on biogeochemical reactions in the groundwater-seawater mixing zone, recently termed the “subterranean estuary.” This paper is the second in a two part series on the biogeochemistry of the Waquoit Bay coastal aquifer/subterranean estuary. The first paper addressed the biogeochemistry of Fe, Mn, P, Ba, U, and Th from the perspective of the sediment composition of cores Charette et al. [Charette, M.A., Sholkovitz, E.R., Hansell, C.M., 2005. Trace element cycling in a subterranean estuary: Part 1. Geochemistry of the permeable sediments. Geochim. Cosmochim. Acta, 69, 2095-2109]. This paper uses pore water data from the subterranean estuary, along with Bay surface water data, to establish a more detailed view into the estuarine chemistry and the chemical diagenesis of Fe, Mn, U, Ba and Sr in coastal aquifers. Nine high-resolution pore water (groundwater) profiles were collected from the head of the Bay during July 2002. There were non-conservative additions of both Ba and Sr in the salinity transition zone of the subterranean estuary. However, the extent of Sr release was significantly less than that of its alkaline earth neighbor Ba. Pore water Ba concentrations approached 3000 nM compared with 25-50 nM in the surface waters of the Bay; the pore water Sr-salinity distribution suggests a 26% elevation in the amount of Sr added to the subterranean estuary. The release of dissolved Ba to the mixing zone of surface estuaries is frequently attributed to an ion-exchange process whereby seawater cations react with Ba from river suspended clay mineral particles at low to intermediate salinity. Results presented here suggest that reductive dissolution of Mn oxides, in conjunction with changes in salinity, may also be an important process in maintaining high concentrations of Ba in the pore water of subterranean estuaries. In contrast, pore water U was significantly depleted in the subterranean estuary, a result of SGD-driven circulation of seawater through reducing permeable sediments. This finding is supported by surface water concentrations of U in the Bay, which were significantly depleted in U compared with adjacent coastal waters. Using a global estimate of SGD, we calculate U removal in subterranean estuaries at 20 × 10⁶ mol U y⁻¹, which is the same order of magnitude as the other major U sinks for the ocean. Our results suggest a need to revisit and reevaluate the oceanic budgets for elements that are likely influenced by SGD-associated processes.     

Sources and flux of trace elements in river water collected from the Lake Qinghai catchment,NE Tibetan Plateau

River waters play a significant role in supplying naturally- and anthropogenically-derived materials to Lake Qinghai, northeastern Tibetan Plateau. To define the sources and controlling processes for river water chemistry within the Lake Qinghai catchment, high precision ICP-MS trace element concentrations were measured in water samples collected from the Buha River weekly in 2007, and from other major rivers in the post-monsoon (late October 2006) and monsoon (late July 2007) seasons. The distributions of trace elements vary in time and space with distinct seasonal patterns. The primary flux in the Buha River is higher TDS and dissolved Al, B, Cr, Li, Mo, Rb, Sr and U during springtime than those during other seasons and is attributed to the inputs derived from both rock weathering and atmospheric processes. Among these elements, the fluxes of dissolved Cr, B and Rb are strongly influenced by eolian dust input. The fluxes of dissolved Li, Mo, Sr and U are also influenced by weathering processes, reflecting the sensitivity of chemical weathering to monsoon conditions. The anthropogenic sources appear to be the dominant contribution to potentially harmful metals (Ni, Cu, Co, Zn and Pb), with high fluxes at onset of the main discharge pulses due, at least partially, to a runoff washout effect. For other major rivers, except for Ba, concentrations of trace elements are higher in the monsoon than in the post-monsoon season. A total of 38.5 ± 3.1 tons of potentially harmful elements are transported into the lake annually, despite human activities within the catchment being limited. Nearly all river water samples contain dissolved trace elements below the World Health Organization guidelines for drinking water, with the exception of As and B in the Daotang River water samples collected in late July probably mobilized from underlying lacustrine sediments.     

Major and trace element geochemistry of Lake Bogoria and Lake Nakuru,Kenya, during extreme draught

The physico-chemical properties of water samples from the two athalassic endorheic lakes Bogoria and Nakuru in Kenya were analysed. Surface water samples were taken between July 2008 and October 2009 in weekly intervals from each lake. The following parameters were determined: pH, salinity, electric conductivity, dissolved organic carbon (DOC), the major cations (FAAS and ICP-OES) and the major anions (IC), as well as certain trace elements (ICP-OES). Samples of superficial sediments were taken in October 2009 and examined using Instrumental Neutron Activation Analysis (INAA) for their major and trace element content including rare earth elements (REE). Both lakes are highly alkaline with a dominance of Na > K > Si > Ca in cations and HCO₃ > CO₃ > Cl > F > SO₄ in anions. Both lakes also exhibited high concentrations of Mo, As and fluoride. Due to an extreme draught from March to October 2009, the water level of Lake Nakuru dropped significantly. This created drastic evapoconcentration, with the total salinity rising from about 20‰ up to 63‰. Most parameters (DOC, Na, K, Ca, F, Mo and As) increased with falling water levels. A clear change in the quality of DOC was observed, followed by an almost complete depletion of dissolved Fe from the water phase. In Lake Bogoria the evapoconcentration effects were less pronounced (total salinity changed from about 40‰ to 48‰). The distributions of REE in the superficial sediments of Lake Nakuru and Lake Bogoria are presented here for the first time. The results show a high abundance of the REE and a very distinct Eu depletion of Eu/Eu* = 0.33–0.45.     

Geochemical and anthropogenic enrichments of Mo in sediments from perennially oxic and seasonally anoxic lakes in Eastern Canada

We measured the vertical distributions of Mo, Fe, Mn, sulfide, sulfate, organic carbon, major ions, and pH in sediment porewater from one perennially oxic and three seasonally anoxic lacustrine basins in Eastern Canada, as well as those of Mo, acid volatile sulfide, Fe, Mn, Al, organic C, ²¹⁰Pb and ¹³⁷Cs in sediment cores from the same sites. The only input of anthropogenic Mo to these lakes comes from atmospheric deposition.The relatively monotonous distribution of Mo in the porewater of three seasonally anoxic basins suggests that Mo is not redistributed in the sedimentary column during periods of anoxia. In contrast, porewater Mo profiles obtained at three sampling dates in a perennially oxic basin display sharp Mo peaks below the sediment-water interface, indicating redistribution subsequent to deposition. Modeling of these latter porewater Mo profiles with a diagenetic reaction-transport equation coupled to comparisons among the various porewater and solid phase profiles reveal that Mo is released at 1-2 cm depth as a consequence of the reductive dissolution of Fe oxyhydroxides and scavenged both at the vicinity of the sediment-water interface, by re-adsorption onto authigenic Fe oxyhydroxides, and deeper in the sediments where dissolved sulfide concentrations are higher. The estimated rate constant for the adsorption of Mo onto Fe oxyhydroxides is 36 ± 45 cm³ mol⁻¹ s⁻¹.Diagenetic modeling indicates that authigenic Mo in sediments of the perennially oxic basin represents about one-third of the total solid phase Mo in the first cm below the sediment-water interface and only one tenth below this horizon. If we assume that no authigenic Mo is accumulated in the seasonally anoxic lake sediments we conclude that the sediment Mo concentrations, which are up to 3-16 times higher than the average lithogenic composition, depending on the lake, are mainly due to atmospheric deposition of anthropogenic Mo and not to the formation of authigenic Mo phases. Reconstructed historical records of the atmospheric Mo deposition indicate maximum values in the 1970s and 1980s and significant decreases since then. Emissions to the atmosphere associated with the smelting of non-ferrous ores and coal combustion appear to be the most important sources of anthropogenic Mo.     

Insights on geochemical cycling of U, Re and Mo from seasonal sampling in Boston Harbor, Massachusetts, USA

This study examined the removal of U, Mo, and Re from seawater by sedimentary processes at a shallow-water site with near-saturation bottom water O₂ levels (240-380 μmol O₂/L), very high organic matter oxidation rates (annually averaged rate is 880 μmol C/cm²/y), and shallow oxygen penetration depths (4 mm or less throughout the year). Under these conditions, U, Mo, and Re were removed rapidly to asymptotic pore water concentrations of 2.2-3.3 nmol/kg (U), 7-13 nmol/kg (Mo), and 11-14 pmol/kg (Re). The depth order in which the three metals were removed, determined by fitting a diffusion-reaction model to measured profiles, was Re < U < Mo. Model fits also suggest that the Mo profiles clearly showed the presence of a near-interface layer in which Mo was added to pore waters by remineralization of a solid phase. The importance of this solid phase source of pore water Mo increased from January to October as the organic matter oxidation rate increased, bottom water O₂ decreased, and the O₂ penetration depth decreased. Experiments with in situ benthic flux chambers generally showed fluxes of U and Mo into the sediments. However, when the overlying water O₂ concentration in the chambers was allowed to drop to very low levels, Mn and Fe were released to the overlying water along with the simultaneous release of Mo and U. These experiments suggest that remineralization of Mn and/or Fe oxides may be a source of Mo and perhaps U to pore waters, and may complicate the accumulation of U and Mo in bioturbated sediments with high organic matter oxidation rates and shallow O₂ penetration depths.Benthic chamber experiments including the nonreactive solute tracer, Br⁻, indicated that sediment irrigation was very important to solute exchange at the study site. The enhancement of sediment-seawater exchange due to irrigation was determined for the nonreactive tracer (Br⁻), TCO₂, , U and Mo. The comparisons between these solutes showed that reactions within and around the burrows were very important for modulating the Mo flux, but less important for U. The effect of these reactions on Mo exchange was highly variable, enhancing Mo (and, to a lesser extent, U) uptake at times of relatively modest irrigation, but inhibiting exchange when irrigation rates were faster. These results reinforce the observation that Mo can be released to and removed from pore waters via sedimentary reactions.The removal rate of U and Mo from seawater by sedimentary reactions was found to agree with the rate of accumulation of authigenic U and Mo in the solid phase. The fluxes of U and Mo determined by in situ benthic flux chamber measurements were the largest that have been measured to date. These results confirm that removal of redox-sensitive metals from continental margin sediments underlying oxic bottom water is important, and suggest that continental margin sediments play a key role in the marine budgets of these metals.     

Mo isotope fractionation during adsorption to Fe (oxyhydr)oxides

Molybdenum (Mo) isotopes have great potential as a paleoredox indicator, but this potential is currently restricted by an incomplete understanding of isotope fractionations occurring during key (bio)geochemical processes. To address one such uncertainty we have investigated the isotopic fractionation of Mo during adsorption to a range of Fe (oxyhydr)oxides, under variable Mo/Fe-mineral ratios and pH. Our data confirm that Fe (oxyhydr)oxides can readily adsorb Mo, highlighting the potential importance of this removal pathway for the global Mo cycle. Furthermore, adsorption of Mo to Fe (oxyhydr)oxides is associated with preferential uptake of the lighter Mo isotopes. Fractionations between the solid and dissolved phase (Δ⁹⁸Mo) increase at higher pH, and also vary with mineralogy, increasing in the order magnetite (Δ⁹⁸Mo = 0.83 ± 0.60‰) < ferrihydrite (Δ⁹⁸Mo = 1.11 ± 0.15‰) < goethite (Δ⁹⁸Mo = 1.40 ± 0.48‰) < hematite (Δ⁹⁸Mo = 2.19 ± 0.54‰). Small differences in isotopic fractionation are also seen at varying Mo/Fe-mineral ratios for individual minerals. The observed isotopic behaviour is consistent with both fractionation during adsorption to the mineral surface (a function of vibrational energy) and adsorption of different Mo species/structures from solution. The different fractionation factors determined for different Fe (oxyhydr)oxides suggests that these minerals likely exert a major control on observed natural Mo isotope compositions during sediment deposition beneath suboxic through to anoxic (but non-sulfidic) bottom waters. Our results confirm that Mo isotopes can provide important information on the spatial extent of different paleoredox conditions, providing they are used in combination with other techniques for evaluating the local redox environment and the mineralogy of the depositing sediments.     

Early diagenesis of trace metals (Cd, Cu, Co, Ni, U, Mo, and V) in the freshwater reaches of a macrotidal estuary

Vertical profiles from the water column, including the maximum turbidity zone (MTZ) to the consolidated sediment were sampled in September 2000 in the freshwater reaches of the Gironde Estuary during a complete neap tide-spring tide cycle. The vertical distributions of dissolved major redox parameters and metals (Mn, Fe, Cd, Cu, V, Co, Ni, Mo, and U) were determined. Reactive particulate metal fractions were also determined from selective leaching. The studied system is characterized by density layers functioning at different time-scales, consisting of two mobile layers, i.e., the liquid (LM) and the soft mud (SM), overlying consolidated sediments (CS). This results in a three-zone diagenetic regime where (1) O₂ dynamics are fast enough to show depletion in the rapidly mixed LM sequence (tidal time-scale), (2) denitrification occurs on the weekly time-scale mixing SM sequence, and (3) the Mn, Fe, and sulfate cycling occurs in the CS layer (annual time-scale). The studied trace metals show differential behavior during early diagenesis: (1) Cd, Cu, and V are released into pore water preferentially from organic matter in the SM, (2) Co, Ni, and U are released in the CS from Mn and Fe oxides during reductive dissolution, and (3) Mo from both processes. Transient conditions (i.e., oscillations of redox fronts and reoxidation processes), due to the dynamics of the mobile layers, strongly influence the trace metal distributions as inducing resolubilization (Cd, Cu, and Mo). In the CS, authigenic metal phases accumulate, either by direct precipitation with sulfides (Cu, Cd) or co-precipitation with Fe-sulfides (Mo). Microbially mediated reduction of Fe oxides is proposed to control U removal from pore water by reduction of U(VI) to U(IV) at depth. However, a significant fraction of the trace metals is trapped in the sediment in exchangeable forms, and therefore is susceptible to be mobilized due to resuspension of estuarine sediment during strong river flood periods and/or dredging activities.     

Distinction between sortable silts and aggregated particles in muddy intertidal sediments of the East Frisian Wadden Sea, southern North Sea

In muddy sediments, the distinction between sortable silt and aggregated silty clay is important for the understanding of fine particle dynamics because both have different hydraulic properties. The Wadden Sea of the southern North Sea is severely depleted in fine-grained sediments mainly due to high energy levels along the diked coastline. As a result, muddy sediments are restricted to a narrow belt along the diked mainland shore. In the present study, the mechanism by which this mud is deposited and how floc deposition and break-up are reflected in the size distribution, has been investigated. For this purpose, surficial sediments from four intertidal nearshore transects were monitored and repeatedly sampled in the course of two years. High-resolution grain-size analyses were performed by an automated settling tube and a Sedigraph particle analyser for the sand and mud fractions, respectively.The grain size frequency distributions of the fine fractions demonstrate that the Wadden Sea muds are composed of two subpopulations, a well-sorted coarse silt and an unsorted silty clay population. A depletion of grain size around 8 μm (7 phi) demarcates the grain-size boundary between the two populations, suggesting that the finer mud population (< 8 μm) is deposited in the form of flocs and aggregates which are hydraulically equivalent to the local sands and coarser silts. Floc break-up and reconstitution in response to seasonally changing energy regimes lead to apparent seasonal sedimentation patterns in the back-barrier tidal basins. Furthermore, in the course of sample preparation, the flocs and aggregates are broken down into their constituent particles. This mechanical artefact in the size distributions produces an artificial seasonal fining/coarsening pattern. It was found that the comparison of clay/silt and < 8 μm/63 − 8 μm particle ratios are good indicators of floc behaviour. Higher ratios are found in mixed flats which are relatively protected from wave action, thereby promoting deposition of flocs. In addition, progressive size sorting and mixing processes along the transects are recognized in the frequency distributions of sands. The skewness pattern shows a landward decrease in positive values, which is exactly opposite to previously reported patterns, suggesting progressive winnowing of fine particles caused by increased wave action over the last decade. This effect of climate change further promotes depletion of fine-grained sediments in the basin.     

Distribution of trace elements in filtered and non filtered aqueous fractions: Insights from rivers and streams of Sardinia (Italy)

The distribution of several trace elements in different aqueous fractions has been studied in running waters from Sardinia (Italy). Trace elements and major components were determined in water samples collected at high- and low-discharge from rivers (90 samples) and streams (70 samples). At selected sites, total (non filtered samples) and dissolved (0.4 μm and 0.015 μm pore-size filtered samples) amounts of trace elements were determined, and the composition of the solid matter retained on the filters was investigated for estimating the eventual interrelationship. The elements B, Li, Rb, Sr, Ba, As, Sb, Mo, Tl and U in the studied waters showed small differences between total and dissolved amounts; dissolved concentrations were higher under low flow conditions, when the contribution of rainwater to the rivers was minimum; their concentrations were often correlated with total dissolved solids (TDS), and appeared to be related to the intensity of water-rock interaction processes. The elements Al, Fe, Mn, Pb, Zn, Cd, Cu, Co, Ni, Cs, Y, REE and Th were not related to TDS and/or major ions; they showed higher concentrations under high flow conditions; marked differences occurred between total and dissolved amounts; much lower concentrations were generally observed in the water filtered through 0.015 μm than in the water filtered through 0.4 μm, especially when sampling was carried out after heavy rain events that enhanced the load of solid matter in the water. These observations indicate an aqueous transport via sorption processes on very fine particles, such as Fe-oxide/hydroxide and clay mineral particles, which have been inferred by SEM-EDX analyses of the matter retained on the filters.     

Trace metal speciation in sea and pore water of the Gotland Deep,Baltic Sea, 1994

Studies on the speciation (particulate, colloidal, anionic and cationic forms) of trace metals (Cd, Co, Cu, Fe, Mn, Mo, Ni, Pb, Zn) in the water column and in pore waters of the Gotland Deep following the 1993/94 salt-water inflows showed dramatic changes in the total “dissolved” metal concentrations and in the ratios between different metal species in the freshly re-oxygenated waters below 125 m. Changes in concentrations were greatest for those metals for which the solubility differs with the redox state (Fe, Mn, Co) but were also noted for those metals which form insoluble sulphides (Cd, Pb, Cu, Zn) and/or stable complexes with natural ligands (Cu). Pore water data from segmented surface muds (0–200 mm) indicated that significant redox and related metal speciation changes took place in the surface sediments only a few weeks after the inflow of the oxygenated sea water into the Gotland Deep.     

Temporal trends of dissolved weathering products released from a high Arctic coal mine waste rock pile in Svalbard (78°N)

It is well known that oxidation of sulphide-containing coal mine waste has considerable environmental impacts due to generation of acid mine drainage (AMD) containing high dissolved metal concentrations. This study is the first to evaluate seasonal trends in the release of AMD from high arctic coal mine waste rock. Runoff from an abandoned coal mine waste pile in Svalbard (78°N) was studied during the entire 3–4 month period with running water in 2005. Temporal variation in concentrations and fluxes of dissolved elements were quantified based on daily water sampling and used to evaluate weathering processes and estimate element budgets on a daily, seasonal and annual basis. Apart from alkali- and alkaline earth metals; Fe, Al, Mn, Zn and Ni were found to be the most abundant metals in the runoff. Element concentrations were highly correlated and suggest that the processes of sulphide oxidation, ion exchange and silicate weathering occurring within the waste pile were linked throughout the measuring period. Observed pH values varied from 2.8 to 5.2 and SO₄ concentrations from 21 to 1463 mg L⁻¹. Manganese and Al concentrations were observed above phytotoxic levels (up to 4 and 23 mg L⁻¹, respectively) and were considered the most critical elements in terms of environmental impact. Throughout the summer a total dissolved quantity of 58 kg Mn, 238 kg Al and 13,700 kg SO₄ was released from the pile containing approximately 200,000 m³ of pyritic waste material (<1% FeS₂). The highest concentrations of metals, lowest pH values and a very high daily release of H₂SO₄ (up to twice as high as the following month) were observed during the first week of thaw. This is considered a result of an accumulation of weathering products, generated within the waste pile during winter and released as a pollution-flush during early spring. Similar accumulation/flush sequences were observed later in the summer where rain events following relatively long dry periods caused high daily metal fluxes and on some occasions also elevated dissolved metal concentrations. Despite highly variable weather/climate conditions during the rest of the summer the investigated waste rock pile acted like a relative constant pollution-source during this period. Future investigations regarding the environmental impact of mine waste in the region should include measurements of bioavailable metals in order to provide further details on the seasonal trends in environmental impact.     

Source/process apportionment of major and trace elements in sinking particles in the Sargasso sea

Elemental composition of the particle flux at the Oceanic Flux Program (OFP) time-series site off Bermuda was measured from January 2002 to March 2005. Eighteen elements (Mg, Al, Si, P, Ca, Sc, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Sr, Cd, Ba and Pb) in sediment trap material from 500, 1500 and 3200 m depths were quantified using fusion-HR-ICPMS. Positive Matrix Factorization (PMF) was used to elucidate sources, elemental associations and processes that affect geochemical behavior in the water column.Results provide evidence for intense elemental cycling between the sinking flux material and the dissolved and suspended pools within mesopelagic and bathypelagic waters. Biological processing and remineralization rapidly deplete the sinking flux material in organic matter and associated elements (N, P, Cd, Zn) between 500 and 1500 m depth. Suspended particle aggregation, authigenic mineral precipitation, and chemical scavenging enriches the flux material in lithogenic minerals, barite and redox sensitive elements (Mn, Co, V, Fe). A large increase in the flux of lithogenic elements is observed with depth and confirms that the northeast Sargasso is a significant sink for advected continental materials, likely supplied via Gulf Stream circulation.PMF resolved major sources that contribute to sinking flux at all depths (carbonate, high-Mg carbonate, opal, organic matter, lithogenic material, and barite) as well as additional depth-specific elemental associations that contribute about half of the compositional variability in the flux. PMF solutions indicate close geochemical associations of barite-opal, Cd-P, Zn-Co, Zn-Pb and redox sensitive elements in the sinking flux material at 500 m depth. Major reorganizations of element associations occur as labile carrier phases break down and elements redistribute among new carrier phases deeper in the water column.Factor scores show strong covariation and similar temporal phasing among the three trap depths and indicate a tight coupling in particle flux compositional variability throughout the water column. Seasonality in flux composition is primarily driven by dilution of the lithogenic component with freshly-produced biogenic material during the late winter primary production maximum. Temporal trends in scores reveal subtle non-seasonal changes in flux composition occurring on month long timescales. This non-seasonal variability may be driven by changes in the biogeochemical properties of intermediate water masses that pass through the region and which affect rates of chemical scavenging and/or aggregation within the water column.     

Assessing the concentration,speciation, and toxicity of dissolved metals during mixing of acid-mine drainage and ambient river water downstream of the Elizabeth Copper Mine,Vermont, USA

The authors determine the composition of a river that is impacted by acid-mine drainage, evaluate dominant physical and geochemical processes controlling the composition, and assess dissolved metal speciation and toxicity using a combination of laboratory, field and modeling studies. Values of pH increase from 3.3 to 7.6 and the sum of dissolved base metal (Cd + Co + Cu + Ni + Pb + Zn) concentrations decreases from 6270 to 100 μg/L in the dynamic mixing and reaction zone that is downstream of the river’s confluence with acid-mine drainage. Mixing diagrams and PHREEQC calculations indicate that mixing and dilution affect the concentrations of all dissolved elements in the reach, and are the dominant processes controlling dissolved Ca, K, Li, Mn and SO₄ concentrations. Additionally, dissolved Al and Fe concentrations decrease due to mineral precipitation (gibbsite, schwertmannite and ferrihydrite), whereas dissolved concentrations of Cd, Co, Cu, Ni, Pb and Zn decrease due to adsorption onto newly formed Fe precipitates.     

Effects of slow recovery rates on water column geochemistry in aquitard wells

Monitoring wells are often installed in aquitards to verify effectiveness for preventing migration of surface contaminants to underlying aquifers. However, water sampling of aquitard wells presents a challenge due to the slow recovery times for water recharging the wells, which can take as long as weeks, months or years to recharge depending on the sample volume needed. In this study, downhole profiling and sampling of aquitard wells was used to assess geochemical changes that occur in aquitard wells during water level recovery. Wells were sampled on three occasions spanning 11 years, 1 year and 1 week after they were purged and casing water showed substantial water chemistry variations. Temperature decreased with depth, whereas pH and specific conductance increased with depth in the water column after 11 years of water level recovery. Less stable parameters such as dissolved O₂ (DO) and Eh showed strong zonation in the well column, with DO stratification occurring as the groundwater slowly entered the well. Oxidation of reduced till groundwater along with degassing of CO₂ from till pore water affects mineral solubility and dissolved solid concentrations. Recommendations for sampling slowly recovering aquitard wells include identifying the zone of DO and Eh stratification in the well column and collecting water samples from below the boundary to better measure unstable geochemical parameters.