Trace metal dynamics in a seasonally anoxic lake

Selected results are presented from a detailed 12-month study of trace metals in a seasonally anoxic lake. Dissolved concentrations of Fe, Mn, organic carbon, Cd, Cu, Pb, Zn, and pH were determined in the water column and the interstitial waters on 39 occasions. Trace metal concentrations remained low throughout the year in both water column and pore waters. There was evidence for some remobilization at the sediment-water interface but sediments deeper than 3 cm acted as a sink throughout the year. Variations in the water concentrations were largely associated with increased loading during periods of heavy rainfall. During the summer, concentrations of Cu and Zn in the waters overlying the sediments were enhanced by release from decomposing algal material. Similarly, enhanced concentrations of Cd, Cu, Pb, and Zn were observed during periods of much reduced mixing during ice-cover. Although there were large seasonal variations in the concentrations of dissolved and particulate Fe and Mn, there were no comparable changes in the concentrations of trace metals.     

南海颗粒物质的通量、组成及其与沉积物积累率的关系初探

通过大孔径时间系列沉积物捕获器的多年测量及对样品的多学科综合分析表明:南海北部与中部深海区1000m左右水深颗粒通量大约为90mg·m-2·d-1,在多数情况下,季风期间的颗粒通量有比较明显的增高。颗粒物主要组成为钙质生物来源的CaCO₃、生物硅、岩源物质及海洋生物来源的有机质。颗粒通量与组成在水柱中的垂向变化表明,生源组分中CaCO₃及有机质随深度具有较为明显的减少。颗粒物侧向运动可能是造成某些时段南海中部深层颗粒通量增加的主要原因。颗粒物质在进入深海沉积物之前,CaCO₃、生物硅均在深层水与沉积物界面之间发生大量的溶解作用。有机质在沉降过程中的减少,一方面是由于硅质与钙质壳体的溶解而使结合在壳体内部的有机质随之溶解造成;另一方面可能与生物及生物地球化学作用有关。岩源物质除水柱沉降之外,还可以通过浊流等底层搬运机制进入南海北部及中部海盆,其中在南海北部这种搬运作用更为明显。     

Lipid composition of the pelagic crab Pleuroncodes planipes, its feces, and sinking particulate organic matter in the Equatorial North Pacific Ocean

Fecal pellets from the pelagic crab Pleuroncodes planipes were a substantial portion of the particulate organic matter in a sediment trap deployed at the bottom of the mixed layer in the eastern tropical North Pacific Ocean. The lipids of fresh P. planipes feces were compared to lipids of the sediment trap material, of mixed zooplankton which may comprise part of the diet of the crab, and of the crab itself in order to elucidate the source of organic compounds found in the trap. Hydrocarbons, wax esters, triacylglycerols, steroidal alcohols, steroidal ketones, and fatty acids were determined by capillary gas chromatography and gas chromatography/mass spectrometry. Significant input of lipids via sedimentation of crab fecal material is indicated, and modification of dietary lipid within the gut of the crab is inferred. Labile dietary fatty acids are depleted and sterols enriched in the fecal pellets and trap material relative to the zooplankton and crab. Nuclear saturated and unsaturated 3-ketosteroids and unsaturated steroidal hydrocarbons were detected in the crab, its feces, and in the sediment trap particulate material.     

Remobilization of transition metals in surficial pelagic sediments from the eastern Pacific

The top thirty centimeters of sediment at two sites in the eastern equatorial Pacific contain evidence of post-depositional remobilization of Mn, Fe, Co, Ni, Cu, and Zn. Remobilization takes place as Mn and Fe oxyhydroxides are released to the pore water during the microbially-mediated decomposition of organic matter. Precipitation of the dissolved metals in near-surface more oxic strata controls the solid-phase distribution of Mn, Fe, and Zn. The solid-phase redistribution of Co and Ni requires only suitable material for readsorption. Comparison of pore water fluxes with solid-phase metal distributions in the solid sediment indicates no loss of dissolved metal to the overlying water column at the present time. Loss of Mn during the Quaternary is indicated by the composition of the sediments, however. Leaching experiments suggest that portions of the mobile Mn, Fe, Co, Ni, and Cu are fixed by incorporation in authigenic smectite in the surficial sediments.     

Composition and vertical flux of organic matter in a large Alaskan estuary

Observations of the composition and rate of input of organic matter to the sea floor were made at three locations in lower Cook Inlet, Alaska, during five cruises taken in the spring and summer of 1978. Total particulate, plant pigment, carbon, nitrogen, fecal pellet, and phytoplankton cell fluxes, inferred from sediment trap samples, were related to algal biomass and production in overlying waters. A daily average of 7.5% of the phytoplankton biomass was lost to the bottom. Of this loss, 83% was attributable to zooplankton grazing and fecal pellet production. At the three sampling sites, an average of 39 g C m^?2 (range of 17–60 g C m^?2, was sedimented to the bottom between May and August. This carbon flux represented an average of 12% of the total primary production measured for that time period. Kachemak Bay eastern arm of the inlet, is identified as an extremely productive embayment in which large amounts of organic matter were transferred to the sea floor.     

Particulate trace metal speciation in stream sediments and relationships with grain size: Implications for geochemical exploration

Sediment samples were collected from streambeds in an undisturbed watershed in eastern Quebec (Gaspé Peninsula). Two sampling sites were located on a stream draining an area of known mineralization (Cu, Pb, Zn) and two on a control stream. The sediment samples were separated into 8 distinct size classes in the 850 μm to <1 μm size range by wet sieving, gravity sedimentation or centrifugation. Each sediment subsample was then subjected to a sequential extraction procedure designed to partition the particulate heavy metals into five fractions: (1) exchangeable; (2) specifically adsorbed or bound to carbonates; (3) bound to Fe-Mn oxides; (4) bound to organic matter; (5) residual. The following metals were analyzed in each extract: Cu, Pb, Zn; Fe, Mn.Comparison of samples from the mineralized area with control samples revealed the expected increase in total concentrations for Cu, Pb and Zn. Non-detrital metals were mainly associated with Fe oxides (specifically adsorbed; occluded) and with organic matter or resistant sulfides. For a given sample, variation of trace metal levels in fractions 2 and 3 with grain size reflected changes in the available quantities of the inorganic scavenging phase (FeOx/MnOx); normalization with respect to Fe and Mn content in fraction 3 greatly reduced the apparent dependency on grain size.The results of this study suggest that a single reducing extraction (NH₂OH.HCl) could be used advantageously to detect anomalies in routine geochemical surveys. A second leaching step with acidified H₂O₂ could also be included, as the trace metal concentrations in fraction 4, normalized with respect to organic carbon content, also showed high {anomaly/background} ratios.     

The role of zooplankton fecal pellets in the sedimentation of polycyclic aromatic hydrocarbons in Dabob Bay,Washington

Polycyclic aromatic hydrocarbon (PAH) compositions were determined in plankton, sediment-trap-collected particulate material and sediment cores from Dabob Bay using a high performance liquid Chromatographie (HPLC)/fluorescence technique. The annual flux of individual PAH measured in a series of sediment traps was compared with the flux of corresponding compounds determined from ²¹⁰Pb dated bottom sediments. Systematic seasonal variations in the fluxes and concentrations of PAH, Al and organic carbon in the trap-collected particulates and seasonally collected plankton were also investigated to determine whether or not PAH are associated with either terrestrial or marine-derived materials.Concentrations of all PAH studied increased tenfold within the last 80–100 yr of sediment deposition, except for perylene which displayed a reasonably constant concentration profile. This suggests at least two sources contribute to the observed sedimentary PAH compositions in Dabob Bay, i.e., anthropogenic combustion and a natural source. Plankton and sediment trap-collected particulates contained PAH mixtures qualitatively similar to underlying surface sediments. Microscopic examination indicated fecal pellets were the major form of particulate material in the sediment traps. The fecal pellets collected in the sediment trap time series quantitatively account for essentially 100% of the PAH fluxes measured in the ²¹⁰Pb dated sediments, implying Zooplankton fecal pellets control the removal of PAH to Dabob Bay sediments. These measurements provide clear evidence that the PAH studied are not produced after sediment deposition. The observed seasonal covariations of PAH and Al in both sediment trap and plankton samples further indicate that PAH originate from terrestrially-based sources, are introduced into the marine environment by runoff and erosion or atmospheric deposition and are not produced by marine plankton.     

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.     

Trace metals in suspended particulate matter and in sediment trap material from a permanently anoxic fjord — Framvaren, South Norway

Geochemical studies of the trace metal concentrations in suspended particulate matter (SPM) and sediment trap material from a permanently anoxic fjord, Framvaren, South Norway in 1989 and 1993 indicate that extremely high concentrations of zinc (max = 183920 mg/kg), copper (max = 4130 mg/kg), lead (max = 2752 mg/kg), and cadmium (max= 8.1 mg/kg) sometimes (1993) occur in the SPM collected in the anoxic water layer. The highest concentrations of Zn occur just below the redoxcline at 22 m water depth (in 1993), and copper, lead and cadmium have maximum concentrations between 30 and 80 m depth, where the amount of total SPM is at a minimum (about 0.3 mg/L). On a mass per volume (g/L) basis, the maximum concentrations of Cd, Cu and Fe occur at the interface (21m) and those of Zn occur just below the redoxcline (22 m depth). The SPM and sediment trap data suggest that the metals are precipitated as sulfide minerals in the anoxic water. The presence of particulate sulfides was confirmed by SEM studies that show the occurrence of discrete metal (Cu, Fe, Pb, and Zn) sulfide particles in size from 10–20 m as well as framboidal pyrites (1–5 m in size). Higher levels of metal sulfides at intermediate depths rather than in the deep water of Framvaren (> 100 m), may be due to input of trace metals by water exchange over the sill in the upper part of the water column. In the deep water, less metal sulfide precipitation takes place due to depletion of trace metals, and the dilution of particulate metal concentrations by organic matter and by the chemogenic formation of calcite.     

Global Radiation and Onset of Stratification as Forcing Factors of Seasonal Carbonate and Organic Matter Flux Dynamics in a Hypertrophic Hardwater Lake (Sacrower See,Northeastern Germany)

A 2-year (October 2003–October 2005) high-resolution sediment trap study was conducted in Sacrower See, a dimictic hardwater lake in northeastern Germany. Geochemical and diatom data from sediment trap samples were compared with a broad range of limnological and meteorological parameters to quantify the impact of single parameters on biochemical calcite precipitation and organic matter production. Our goals were to disentangle how carbonaceous varves and their sublaminae form during the annual cycle to better understand the palaeorecords and to detect influences of dissolution, resuspension as well as of global radiation and stratification on lake internal particle formation. Total particle fluxes in both investigated years were highest during spring and summer. Sedimentation was dominated by autochthonous organic matter and biochemically precipitated calcite. Main calcite precipitation occurred between April and July and was preceded and followed by smaller flux peaks caused by resuspension during winter and blooms of the calcified green algae Phacotus lenticularis during summer. In some of the trap intervals during summer up to 100% of the precipitated calcite was dissolved in the hypolimnion. High primary production due to stable insolation conditions in epilimnic waters began with stratification of the water column. Start and development of stratification is closely related to air and water surface temperatures. It is assumed that global radiation influences the onset and stability of water column stratification and thereby determining the intensity of primary production and consequently of timing and amount of calcite precipitation which is triggered by phytoplanktonic CO₂ consumption. Sediment fluxes of organic matter and calcite are also related to the winter NAO-Index. Therefore these fluxes will be used as a proxy for ongoing reconstruction of Holocene climate conditions.     

Hydrological and biogeochemical dynamics of the minor and trace elements in the St. Lawrence River

Surface water samples from the St. Lawrence River were collected in order to study the processes controlling minor and trace elements concentrations (Al, Fe, Mn, Cd, Co, Cu, Ni and Zn), and to construct mass balances allowing estimates of the relative importance of their natural and anthropogenic sources. The two major water inputs, the upper St. Lawrence River, which drains waters originating from the Lake Ontario, and the Ottawa River were collected fortnightly over 18 months. In addition, other tributaries were sampled during the spring floods. The output was monitored near Quebec City at the river mouth weekly between 1995 and 1999. Dissolved metal concentrations in the upper St. Lawrence River carbonated waters were lower than in the acidic waters of the tributaries draining the crystalline rocks of the Canadian shield and the forest cover. Biogeochemical and hydrodynamic processes occurring in Lake Ontario drive the seasonal variations observed in the upper St. Lawrence River. Biogeochemical processes relate to biological uptake, regeneration of organic matter (for Cd and Zn) and oxyhydroxide formation (for Mn and Fe), while hydrodynamic processes mainly concern the seasonal change in vertical stratification (for Cd, Mn, and Zn). In the Ottawa River, the main tributary, oxyhydroxide formation in summer governs seasonal patterns of Al, Fe, Mn, Cd, Co and Zn. The downstream section of the St. Lawrence River is a transit zone in which seasonal variations are mainly driven by the mixing of the different water masses and the large input of suspended particulate matter from erosion. The budget of all dissolved elements, except Fe and Zn, was balanced, as the budget of particulate elements (except Cd and Zn). The main sources of metals to the St. Lawrence River are erosion and inputs from tributaries and Lake Ontario. Direct anthropogenic discharges into the river accounted for less than 5% of the load, except for Cd (10%) and Zn (21%). The fluxes in transfer of dissolved Cd, Co, Cu and Zn species from the river to the lower St. Lawrence estuary were equal to corresponding fluxes calculated for Quebec City since the distributions of dissolved concentrations of these metals versus salinity were conservative. For Fe, the curvature of the dilution line obtained suggests that dissolved species were removed during early mixing.     

Dissolved and particulate trace metals and their partitioning in a hypoxic estuary: The Tanshui Estuary in Northern Taiwan

The distribution and partitioning of trace metals (Co, Cu, Fe, Mn, Ni, and Zn) between dissolved and particulate phases were studied in the Tanshui Estuary. The upper reach of the estuary is hypoxic and heavily polluted due to domestic and industrial discharges. The concentration ranges of dissolved and leachable particulate trace metals in the Tanshui Estuary were: Co: 0.3–6.1 nM, 1.8–18.6 mg kg⁻¹; Cu: 5–53 nM, 22–500 mg kg⁻¹; Fe: 388–3,364 nM, 1.08–6.67%; Mn: 57–2,914 nM, 209–1,169 mg kg⁻¹; Ni: 7–310 nM, 6–108 mg kg⁻¹; and Zn: 12–176 nM, 62–1,316 mg kg⁻¹; respectively. The dissolved concentrations of the metals were 2–35 times higher than the average values of the world river water. The distributions of dissolved and particulate studied metals, except Mn, in the estuary showed scattering, which could be attributed to the discharges from many industrial wastewater disposal works located in the upper tributaries. The daily input of dissolved metals from the disposal works to the Tanshui Estuary ranged from 0.1–0.4 tons. Dissolved Mn was nearly conservative in the region with salinity higher than 10 psu, while particulate Mn decreased in the region with salinity of 10–15 psu. The concentration increased significantly seawards, corresponding with the distribution of dissolved oxygen. The distribution coefficient (K_D) for Mn in the lower estuary was nearly three orders of magnitude higher than in the upper estuary. This phenomenon may be attributed to the diffusion of Mn from the anoxic sediment in the upper estuary and gradual oxidation into particulate Mn in the middle and lower estuary as the estuarine water became more oxygenated. The distribution coefficient for Cu decreased with increasing salinity. The percentages of trace metals bound by suspended particulate matter decreased in the following order: Fe>Zn, Cu>Co>Mn>Ni.     

Biogeochemical characteristics of settling particulate organic matter in Lake Superior: A seasonal comparison

To assess settling particulate organic matter (POM) seasonality and its availability to the benthic community, settling particulate matter was studied in terms of mass fluxes and main biogeochemical characteristics (including organic carbon (OC), nitrogen, and stable carbon and nitrogen isotopic values) at two Lake Superior offshore sites over the course of a year. Fourier transform infrared spectroscopy (FTIR) and hydrolysis, extraction, and derivatization were used to provide further compositional information. Carbon and nitrogen content, isotopic and wet chemical data, and FTIR spectra show that summer particulate material is mainly autochthonous, with higher proportions of amide and carbohydrate. FTIR shows that spring particulate material contains relatively high proportions of clay minerals, indicating major sources from sediment resuspension and/or spring runoff. Distinct amino acid distributions at the two sites, revealed by principal component analysis (PCA) based on amino acid mol% composition, possibly result from differences in OM sources and the degree of degradation occurring at the two sites. Carbohydrate (PCHO), total hydrolyzable amino acid (THAA) and FTIR data suggest that the nutritional value of bulk POM to benthic heterotrophs should be lower in spring than summer-fall, although both periods exhibited high sinking fluxes of total mass and OC. Due to sediment resuspension events and an oxic water column, organic matter eventually buried in Lake Superior’s sediments has probably experienced extensive alteration due to several cycles through the water column and the bacterially-active sediment-water interface.     

The distribution of trace metals in the western North Atlantic off Nova Scotia

The distributions of Fe, Mn, Zn, Cu, Ni, Cd and Co have been determined in a section across the Scotian Shelf into the Atlantic Slope water. Significant differences in concentration exist for most of the trace metals between the four water masses in the section. Depletions of trace metal concentration in the highly productive Atlantic Slope water relative to the underlying Central Atlantic water are thought to be due to biological activity.The distributions of Fe and Mn are strongly related to the distribution of suspended particulate matter. The concentrations of Fe and Mn, extracted from the suspended matter on the Scotian Shelf, are considerably higher than those in the non-detrital fraction of the underlying sediments. This suggests that post-depositional changes cause the loss of both elements from the non-detrital fraction of the particles. Whereas Mn shows major nearshore increases in concentration related to continental runoff, nearshore Fe concentrations are largely controlled by particulate matter distribution. Continental runoff does not appear to have much influence upon the distributions of the other trace metals.     

Regional geochemical reconnaissance in the Belgian Ardennes: Secondary dispersion patterns in stream sediments

Two geochemical stream sediment surveys were conducted in the Belgian Ardennes for Fe, Cu, Pb, Zn, Co, Ni and Mn. The geological setting is of quartzo-pelitic rocks of Early Devonian and Cambro-Ordovician age. One survey covers the Ourthe and Salm valleys, the other the Lesse and Lomme drainage basins. Results of these surveys show the geochemical similarity of the two regions. Anomalies have been ascribed to four factors: human activity, Fe-Mn enrichment, known mineralization and potential mineralization. Mean values of metal concentration and statistical dispersion are identical. This results from similar physico-chemical conditions which induce the same types of surficial metallic association. The stream environment is characterized by two main phases, an oxidizing phase where Fe-Mn oxides trap trace metals, and a reducing phase where organic matter and secondary sulphides are bound with Pb, Zn and Cu. In oxidizing conditions, Cu and Ni show a greater affinity for the Fe-oxides while Zn and especially Co are related to the Mn-oxides. Pb has only a weak relationship with the Fe-Mn oxides but is also associated with the organic matter and the secondary sulphides in the reducing environment. Consequently, one may propose the following scale of mobility: Zn > Ni-Mn > Cu-Co > Pb > Fe.     

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.     

Metal speciation and attenuation in stream waters and sediments contaminated by landfill leachate

The degree of metal contamination (Zn, Pb, Cu, Ni, Cd) has been investigated in the vicinity of an old unmonitored municipal landfill in Prague, Czech Republic, where the leachate is directly drained into a surface stream. The water chemistry was coupled with investigation of the stream sediment (aqua regia extract, sequential extraction, voltammetry of microparticles) and newly formed products (SEM/EDS, XRD). The MINTEQA2 speciation-solubility calculation showed that the metals (Zn, Pb, Cu, Ni) are mainly present as carbonate complexes in leachate-polluted surface waters. These waters were oversaturated with respect to Fe(III) oxyhydroxides, calcite (CaCO₃) and other carbonate phases. Three metal attenuation mechanisms were identified in leachate-polluted surface waters: (i) spontaneous precipitation of metal-bearing calcite exhibiting significant concentrations of trace elements (Fe, Mn, Mg, Sr, Ba, Pb, Zn, Ni); (ii) binding to Fe(III) oxyhydroxides (mainly goethite, FeOOH) (Pb, Zn, Cu, Ni); and (iii) preferential bonding to sediment organic matter (Cu). These processes act as the key scavenging mechanisms and significantly decrease the metal concentrations in leachate-polluted water within 200 m from the direct leachate outflow into the stream. Under the near-neutral conditions governing the sediment/water interface in the landfill environment, metals are strongly bound in the stream sediment and remain relatively immobile.     

Manganese-rich brown layers in Arctic Ocean sediments: Composition, formation mechanisms, and diagenetic overprint

We present inorganic geochemical analyses of pore waters and sediments of two Late Quaternary sediment cores from the western Arctic Ocean (southern Mendeleev Ridge, RV Polarstern Expedition ARK-XXIII/3), focussing on the composition and origin of distinct, brown-colored, Mn-rich sediment layers. Carbonate enrichments occur in association with these layers as peaks in Ca/Al, Mg/Al, Sr/Al and Sr/Mg, suggesting enhanced input of both ice-rafted and biogenic carbonate. For the first time, we show that the Mn-rich layers layers are also consistently enriched in the scavenged trace metals Co, Cu, Mo and Ni. Distinct bioturbation patterns, specifically well-defined brown burrows into the underlying sediments, suggest these metal enrichments formed close to the sediment-water interface. The geochemical signature of these metal- and carbonate-rich layers most probably documents formation under warmer climate conditions with an intensified continental hydrological cycle and only seasonal sea ice cover. Both rivers and sea ice delivered trace metals to the Arctic Ocean, while enhanced seasonal productivity exported reactive organic matter to the sea floor. The coeval deposition of organic matter, Mn (oxyhydr)oxides and trace metals triggered intense diagenetic Mn cycling at the sediment-water interface. These processes resulted in the formation of Mn and trace metal enrichments, and the degradation of labile organic matter. With the onset of cooler conditions, reduced riverine runoff and/or a solid sea ice cover terminated the input of riverine trace metal and fresh organic matter, resulting in deposition of grayish-yellowish, metal-poor sediments. Oxygen depletion of Arctic bottom waters under these cooler conditions is not supported by our data, and did not cause the sedimentary Mn distribution. While the original composition and texture of the brown layers resulted from specific climatic conditions and corresponding diagenetic processes, pore water data show that diagenetic Mn redistribution is still affecting the organic-poor deeper sediments. Given persistent steady state conditions, purely authigenic Mn-rich brown layers may form, while others may be partly or completely dissolved. The degree of diagenetic Mn redistribution largely depends on the depositional environment, the Mn and organic matter availability, and apparently affected the Co/Mo ratios of Mn-rich layers. Thus, brown Arctic layers are not necessarily synchronous features, and should not be correlated across the Arctic Ocean without additional age control.     

Partitioning of copper and zinc in the sediments and porewaters of a high-elevation alkaline lake,east-central Arizona,U.S.A.

In Pacheta Lake, a high-elevation alkaline lake proximal to the smelting region of southern Arizona-New Mexico, concentrations of transition metal ions in pore waters and co-existing sediments were compared. Copper, Fe, Mn and Zn have been partitioned among operationally defined sediment solid phases (exchangeable sites, organic complexes, amorphous oxides, crystalline oxides, sulfides and residual silicates) and their concentrations in interstitial waters were measured. Concentrations are reported as a function of depth in the sediment column. The diagenetic environment is described and cycling mechanisms postulated for the above metals.Selective, sequential extraction of metals from lake sediments showed different binding mechanisms for Cu and Zn, the former most strongly associated with organic complexes, and the latter with iron oxyhydroxides. This difference has strong implications for selective metal remobilization under variable environmental conditions, both naturally and anthropogenically induced. Copper and zinc in porewaters were estimated to diffuse to overlying waters at 12.8 and 21.9 μg/cm²/a, respectively. These fluxes are large enough to account for observed concentrations of Cu and Zn in overlying waters. No sediment metal contamination was directly attributable to smelting activity. However, this study does document a flux from sediments, which have accumulated Cu and Zn, to overlying waters no longer receiving trace metal deposition from now inactive smelters.