Post-depositional Redistribution Research Articles

Temporal and spatial variabilities in surface mass balance at the EGRIP site, Greenland from 2009 to 2017

Temporal variability in surface mass balance (SMB) on the Greenland ice sheet is important for understanding the mass balance of the ice sheet. Additionally, knowledge of the spatial variability in SMB at ice core drilling sites helps to interpret the spatial representativeness of SMB data obtained from a single ice core. In this study, to investigate the spatiotemporal variability in recent SMB in the East Greenland Ice Core Project (EGRIP) area in the northeastern Greenland ice sheet, pit observations were made at six sites in the summers of 2016–2018. In all pits, depth profiles of water isotope ratios showed clear seasonal variations. The annual SMB differed from site to site, which is probably due to post-depositional redistribution of snow caused by wind erosion and snowdrift. However, the multiple-site averages of annual SMBs, which ranged from 134 to 157 mm w. e. yr−1 (average 146 mm w. e. yr−1) during 2009–2017, were very similar. This indicates that annual SMBs in the EGRIP area were nearly constant in this period. The seasonal SMBs in the EGRIP area tended to be larger in the summer–winter period than in the winter–summer period.

Spatial variability of tephra and carbon accumulation in a Holocene peatland

Microscopic tephra layers (‘cryptotephras’) represent important age-equivalent stratigraphic markers utilised in many palaeoenvironmental reconstructions. When used in conjunction with proximal records of volcanic activity they can also provide information about volcanic ash cloud fallout and frequency. However, the spatial distributions of tephra layers can be discontinuous even within the same region. Understanding the deposition and post-depositional redistribution of tephra is vital if we are to use cryptotephras as records of ash cloud occurrence and chronostratigraphic markers. The discrete nature of tephra layers also allows for detailed study into processes of deposition and reworking which affect many palaeoenvironmental proxy records.We undertook a multi-core study in order to examine the historical tephrostratigraphy of a raised peatland in Northern Ireland. Three tephra layers originating from Iceland (Hekla 1947, Hekla 1845 and Hekla 1510) are present in 14 of the 15 cores analysed. This suggests that in areas not influenced by snowfall or anthropogenic disturbance at the time of tephra delivery, the presence or absence of a tephra layer is generally consistent across a peatland of this type. However, tephra shard counts (per unit area) vary by an order of magnitude between cores. These intra-site differences may confound the interpretation of shard counts from single cores as records of regional ash cloud mass/density. Bootstrap resampling analysis suggests that total shard counts from multiple cores are required in order to make a reliable estimate of median shard counts for a site. The presence of three historical tephras in 14 cores enables a spatio-temporal analysis of the long-term apparent rate of carbon accumulation (LARCA) in the peatland. Substantial spatial and temporal variations in LARCA are identified over the last ∼450 years. This high variability needs to be taken into account when designing studies of peatland carbon accumulation.

Why would we use the Sediment Isotope Tomography (SIT) model to establish a 210Pb-based chronology in recent-sediment cores?

After half a century, the use of unsupported 210Pb (210Pbexc) is still far off from being a well established dating tool for recent sediments with widespread applicability. Recent results from the statistical analysis of time series of fluxes, mass sediment accumulation rates (SAR), and initial activities, derived from varved sediments, place serious constraints to the assumption of constant fluxes, which is widely used in dating models. The Sediment Isotope Tomography (SIT) model, under the assumption of non post-depositional redistribution, is used for dating recent sediments in scenarios in that fluxes and SAR are uncorrelated and both vary with time. By using a simple graphical analysis, this paper shows that under the above assumptions, any given 210Pbexc profile, even with the restriction of a discrete set of reference points, is compatible with an infinite number of chronological lines, and thus generating an infinite number of mathematically exact solutions for histories of initial activity concentrations, SAR and fluxes onto the SWI, with these two last ranging from zero up to infinity. Particularly, SIT results, without additional assumptions, cannot contain any statistically significant difference with respect to the exact solutions consisting in intervals of constant SAR or constant fluxes (both being consistent with the reference points). Therefore, there is not any benefit in its use as a dating tool without the explicit introduction of additional restrictive assumptions about fluxes, SAR and/or their interrelationship.

Reaction rates, depositional history and sources of indium in sediments from Appalachian and Canadian Shield lakes

Sediment cores were collected at the deepest site of twelve headwater lakes from the Province of Québec, Canada that receive contaminants only from atmospheric deposition, either directly to the lake surface or indirectly from the watershed. Several of the lakes are located within relatively short distance (<40km) and others at more than 200km from potential sources of contamination. The sediments were dated and analyzed for In and other elements including Fe, Mn, Al and organic C. Fe-rich authigenic material was collected on Teflon sheets inserted vertically into the sediments at the only study site whose hypolimnion remains perennially oxic. Porewater samples collected at the coring site of four of the lakes were also analyzed for In and other solutes including sulfide, sulfate, Fe, Mn, inorganic and organic C and major ions. The porewater In profiles display concentration gradients at or below the sediment–water interface. Modeling these profiles with a one-dimensional transport-reaction equation assuming steady state allows definition of depth intervals (zones) where In is either released to or removed from porewater and quantification of net In reactions rates in each zone.The position of the In consumption zones, the shape of the vertical profiles of dissolved In, sulfide and iron, as well as thermodynamic calculations of saturation states collectively suggest that In(OH)3(s) and In2S3(s) do not precipitate in the sediments and that adsorption of In onto sedimentary FeS(s) does not occur. However, similarities in the In and Fe porewater profiles, and the presence of In in the authigenic Fe-rich solids, reveal that part of the In becomes associated with authigenic Fe oxyhydroxides in the perennially oxic lake and is coupled to the Fe redox cycling. Comparison of the In/Corg and In/Fe molar ratios in the authigenic Fe-rich material and in surface sediments (0–0.5cm) of this lake suggests that most non-lithogenic In was bound to humic substances. From the magnitude of the net In reaction rates, we infer that the post-depositional redistribution of this element is quantitatively not important and that the In sedimentary record represents accurately In deposition at the sampling sites. Reconstructed chronologies of the anthropogenic In deposition and comparison of In inventories among lakes point to non-ferrous metal smelters as a past significant source of atmospheric In contamination and to a significant reduction of industrial In emissions into the North American atmosphere in recent decades.

Post-depositional redistribution processes and their effects on middle rare earth element precipitation and the cerium anomaly in sediments in the South Korea Plateau, East Sea

We sampled two box-core sediments from the slope of the eastern South Korea Plateau (SKP) in the East Sea (Sea of Japan) at water depths of 1400 and 1700m. Two chemical fractions of extractable (hydroxylamine/acetic acid) and residual rare earth elements (REEs) together with Al, Ca, Fe, Mg, Mn, P, S, As, Mo, and U were analyzed to assess the post-depositional redistribution of REEs. Extractable Fe and Mn are noticeably abundant in the oxic topmost sediment layer (<3cm). However, some trace elements (e.g., S, As, Mo, U) are more abundant at depth, where redox conditions are different. Analysis of upper continental crust (UCC)-normalized (La/Gd)UCC, (La/Yb)UCC, and (Ce/Ce*)UCC revealed that the extractable REE is characterized by middle REE (MREE) enrichment and a positive cerium (Ce) anomaly, different from the case of the residual fraction which shows slight enrichment in light REEs (LREEs) with no Ce anomaly. The extractable MREEs seem to have been incorporated into high-Mg calcite during reductive dissolution of Fe oxyhydroxides. In the top sediment layer, the positive Ce anomaly is attributed to Ce oxide, which can be mobilized in deeper oxygen-poor environments and redistributed in the sediment column. In addition, differential concentrations of Ce and other LREEs in pore water appear to result in variable (Ce/Ce*)UCC ratios in the extractable fraction at depth.

Control of early diagenesis processes on trace metal (Cu, Zn, Cd, Pb and U) and metalloid (As, Sb) behaviors in mining- and smelting-impacted lacustrine environments of the Bolivian Altiplano

A combination of mineralogical (SEM–EDS, EMPA) and geochemical (redox dynamics, selective extractions) approaches was applied to mining- and smelting-impacted sediments from Lake Uru Uru and from the Cala Cala Lagoon, a non-impacted reference site, in the Bolivian Altiplano. The purpose was to assess the factors controlling the post-depositional redistribution (mobilization/sequestration) of trace metals and metalloids. As expected, trace metals and metalloids are less reactive at the reference site than in Lake Uru Uru. In the latter, trace metals and metalloids are principally hosted by Fe- and Mn-oxyhydroxides, authigenic sulfides and silicate particulates. Post-depositional redistribution is ascribed to early diagenetic processes driven by organic matter (OM) mineralization, including dissolution of trace metal-bearing phases and precipitation of authigenic sulfide and carbonate phases. Seasonal climate variability exerts a strong influence on these processes. Evaporation of surface water during the dry season in the northern part of Lake Uru Uru promotes large redox front oscillations in the sediments and, therefore, transient redox conditions, likely leading to the weakening of anoxia in near-surface sediments and to a ‘compressed’ redox zonation compared to the southern part of the lake. Seasonal disappearance of the water column in the Northern Lake Uru Uru entails an alternation of: (i) low trace element mobility in the dry season due to elemental precipitation; and (ii) an increase of trace element mobility via diffusive transport during the wet season due to release from OM, Fe- and Mn-oxyhydroxides and carbonates upon mineralization, reductive dissolution and destabilization processes, respectively. Reoxidation of authigenic Fe-sulfides likely following the re-instalment of the water column above the sediment at the end of the dry season and prior to the return of anoxia probably favors transport of trace elements to the water column. As a consequence of this intricate web of redox- and climate-related processes, both northern and southern sediments of Lake Uru Uru are a source of dissolved trace elements, particularly As and Cd, for the overlying water column via diffusive transport. However, much of the diffused As and Cd is likely to be removed from the dissolved phase by the redox loop through Fe-oxyhydroxide precipitation. Finally, it is suggested that long term sequestration of trace metals and metalloids in the sediments could be controlled by inter-annual climate variability such as ENSO events.

Organic matter mineralization and trace element post-depositional redistribution in Western Siberia thermokarst lake sediments

Abstract. This study reports the very first results on high-resolution sampling of sediments and their porewaters from three thermokarst (thaw) lakes representing different stages of ecosystem development located within the Nadym-Pur interfluve of the Western Siberia plain. Up to present time, the lake sediments of this and other permafrost-affected regions remain unexplored regarding their biogeochemical behavior. The aim of this study was to (i) document the early diagenesic processes in order to assess their impact on the organic carbon stored in the underlying permafrost, and (ii) characterize the post-depositional redistribution of trace elements and their impact on the water column. The estimated organic carbon (OC) stock in thermokarst lake sediments of 14 ± 2 kg m−2 is low compared to that reported for peat soils from the same region and denotes intense organic matter (OM) mineralization. Mineralization of OM in the thermokarst lake sediments proceeds under anoxic conditions in all the three lakes. In the course of the lake development, a shift in mineralization pathways from nitrate and sulfate to Fe- and Mn-oxyhydroxides as the main terminal electron acceptors in the early diagenetic reactions was suggested. This shift was likely promoted by the diagenetic consumption of nitrate and sulfate and their gradual depletion in the water column due to progressively decreasing frozen peat lixiviation occurring at the lake's borders. Trace elements were mobilized from host phases (OM and Fe- and Mn-oxyhydroxides) and partly sequestered in the sediment in the form of authigenic Fe-sulfides. Arsenic and Sb cycling was also closely linked to that of OM and Fe- and Mn-oxyhydroxides. Shallow diagenetic enrichment of particulate Sb was observed in the less mature stages. As a result of authigenic sulfide precipitation, the sediments of the early stage of ecosystem development were a sink for water column Cu, Zn, Cd, Pb and Sb. In contrast, at all stages of ecosystem development, the sediments were a source of dissolved Co, Ni and As to the water column. However, the concentrations of these trace elements remained low in the bottom waters, indicating that sorption processes on Fe-bounding particles and/or large-size organo-mineral colloids could mitigate the impact of post-depositional redistribution of toxic elements on the water column.

Natural and anthropogenic trace metals in sediments of the Ligurian Sea (Northwestern Mediterranean)

The magnitude and the chronology of anthropogenic impregnation by Hg and other trace metals of environmental concern (V, Cr, Ni, Cu, Zn, Ag, Cd and Pb, including its stable isotopes) in the sediments are determined at the DYFAMED station, a site in the Ligurian Sea (Northwestern Mediterranean) chosen for its supposed open-sea characteristics. The DYFAMED site (VD) is located on the right levee of the Var Canyon turbidite system, at the end of the Middle Valley. In order to trace the influence of the gravity current coming from the canyon on trace metal distribution in the sediment, we studied an additional sediment core (VA) from a terrace of the Var Canyon, and material collected in sediment traps at the both sites at 20 m above sea bottom. The patterns of Hg and other trace element distribution profiles are interpreted using stable Pb isotope ratios as proxies for its sources, taking into account the sedimentary context (turbidites, redox conditions, and sedimentation rates). Major element distributions, coupled with the stratigraphic examination of the sediment cores point out the high heterogeneity of the deposits at VA, and major turbiditic events at both sites. At the DYFAMED site, we observed direct anthropogenic influence in the upper sediment layer (< 2 cm), while on the Var Canyon site (VA), the anthropization concerns the whole sedimentary column sampled (19 cm). Turbiditic events superimpose their specific signature on trace metal distributions. According to the 210Pb xs-derived sedimentation rate at the DYFAMED site (0.4 mm yr − 1 ), the Hg-enriched layer of the top core corresponds to the sediment accumulation of the last 50 years, which is the period of the highest increase in Hg deposition on a global scale. With the hypothesis of the absence of significant post-depositional redistribution of Hg, the Hg/C org ratio changes between the surface and below are used to estimate the anthropogenic contribution to the Hg flux accumulated in the sediment. The Hg enrichment, from pre-industrial to the present time is calculated to be around 60%, consistent with estimations of global Hg models. However, based on the chemical composition of the trapped material collected in sediment traps, we calculated that epibenthic mobilization of Hg would reach 73%. Conversely, the Cd/C org ratio decreases in the upper 5 cm, which may reflect the recent decrease of atmospheric Cd inputs or losses due to diagenetic processes.

Mercury dynamics in lake sediments

Triplicate porewater depth-profiles of pH and concentrations of total Hg (HgT), methylmercury (MeHg), Fe, Mn, sulfate, total sulfide, total zero-valent sulfur, organic C and major ions were determined at two sampling dates in a perennially oxygenated basin and a seasonally anoxic basin from Lake Tantaré, a Canadian Shield lake. The vertical distribution of HgT, MeHg, acid volatile sulfide, total S, Fe, Mn, Al and organic C were also determined in dated sediment cores from the same lake basins and from the deepest site of two other lakes, one also located in the Canadian Shield and the other in the Northeastern part of the Appalachian Mountains. Application of a one-dimensional transport-reaction equation to the dissolved HgT and MeHg profiles constrains the depth intervals (zones) where these species are produced or consumed in the sedimentary column and yields estimates of net reaction rates of HgT or MeHg in each of the zones as well as their fluxes at the sediment–water interface.Dissolved HgT and MeHg diffused from the overlying water into the sediments, except for MeHg at one of the sampling dates in the perennially oxygenated basin. About 97% and 50% of the MeHg flux to the sediments is presently deposited with settling particles in the perennially oxygenated and seasonally anoxic basins, respectively. Removal of porewater HgT and MeHg occurred at all dates and sampling sites. Comparison of the consumption zones of porewater HgT and MeHg with the profiles of ancillary parameters, coupled with thermodynamic calculations, suggest that pure Hg mineral phases do not form in the sediments, that HgT and MeHg adsorption onto authigenic Fe oxyhydroxides occurs in minor proportions, and that the association of HgT and MeHg to Fe sulfide phases or sulfidized organic matter is possible. Assuming that the net consumption of MeHg in the porewaters was essentially due to demethylation, an apparent first-order rate constant for MeHg demethylation of 0.04–0.8d−1 was estimated. Production of porewater MeHg occurred only in the perennially oxygenated basin, at sediment depths where SO4 was consumed. Assuming that the net production of porewater MeHg was essentially due to methylation, an apparent first-order rate constant for Hg methylation ranging between 0.006d−1 and 0.1d−1 was calculated. These field-derived Hg methylation and MeHg demethylation rate constant values are within the range of those derived from Hg-spiked experiments. We also show that the post-depositional redistribution of total Hg during the early stages of sediment diagenesis is minor and that the solid-phase HgT record can be used to reconstruct the evolution of the anthropogenic HgT deposition.

Multiple site study of recent atmospheric metal (Pb, Zn and Cu) deposition in the NW Iberian Peninsula using peat cores

In order to estimate atmospheric metal deposition in Southern Europe since the beginning of the Industrial Period (~ 1850 AD), concentration profiles of Pb, Zn and Cu were determined in four 210Pb-dated peat cores from ombrotrophic bogs in Serra do Xistral (Galicia, NW Iberian Peninsula). Maximum metal concentrations varied by a factor of 1.8 for Pb and Zn (70 to 128 μg g −1 and 128 to 231 μg g −1, respectively) and 3.5 for Cu (11 to 37 μg g −1). The cumulative metal inventories of each core varied by a factor of 3 for all analysed metals (132 to 329 μg cm −2 for Pb, 198 to 625 μg cm −2 for Zn and 22 to 69 μg cm −2 for Cu), suggesting differences in net accumulation rates among peatlands. Although results suggest that mean deposition rates vary within the studied area, the enhanced 210Pb accumulation and the interpretation of the inventory ratios ( 210Pb/Pb, Zn/Pb and Cu/Pb) in two bogs indicated that either a record perturbation or post-depositional redistribution effects must be considered. After correction, Pb, Zn and Cu profiles showed increasing concentrations and atmospheric fluxes since the mid-XX th century to maximum values in the second half of the XX th century. For Pb, maximum fluxes were observed in 1955–1962 and ranged from 16 to 22 mg m −2 yr −1 (mean of 18 ± 1 mg m −2 yr −1), two orders of magnitude higher than in the pre-industrial period. Peaks in Pb fluxes in Serra do Xistral before the period of maximum consumption of leaded petrol in Europe (1970s–1980s) suggest the dominance of local pollutant sources in the area (i.e. coal mining and burning). More recent peaks were observed for Zn and Cu, with fluxes ranging from 32 to 52 mg m −2 yr −1 in 1989–1996, and from 4 to 9 mg m −2 yr −1 in 1994–2001, respectively. Our results underline the importance of multi-core studies to assess both the integrity and reliability of peat records, and the degree of homogeneity in bog accumulation. We show the usefulness of using the excess 210Pb inventory to distinguish between differential metal deposition, accumulation or anomalous peat records.

Post-depositional redistribution of trace metals in reservoir sediments of a mining/smelting-impacted watershed (the Lot River, SW France)

Abstract Mining/smelting wastes and reservoir sediment cores from the Lot River watershed were studied using mineralogical (XRD, SEM–EDS, EMPA) and geochemical (redox dynamics, selective extractions) approaches to characterize the main carrier phases of trace metals. These two approaches permitted determining the role of post-depositional redistribution processes in sediments and their effects on the fate and mobility of trace metals. The mining/smelting wastes showed heterogeneous mineral compositions with highly variable contents of trace metals. The main trace metal-bearing phases include spinels affected by secondary processes, silicates and sulfates. The results indicate a clear change in the chemical partitioning of trace metals between the reservoir sediments upstream and downstream of the mining/smelting activities, with the downstream sediments showing a 2-fold to 5-fold greater contribution of the oxidizable fraction. This increase was ascribed to stronger post-depositional redistribution of trace metals related to intense early diagenetic processes, including dissolution of trace metal-bearing phases and precipitation of authigenic sulfide phases through organic matter (OM) mineralization. This redistribution is due to high inputs (derived from mining/smelting waste weathering) at the water–sediment interface of (i) dissolved SO4 promoting more efficient OM mineralization, and (ii) highly reactive trace metal-bearing particles. As a result, the main trace metal-bearing phases in the downstream sediments are represented by Zn- and Fe-sulfides, with minor occurrence of detrital zincian spinels, sulfates and Fe-oxyhydroxides. Sequestration of trace metals in sulfides at depth in reservoir sediments does not represent long term sequestration owing to possible resuspension of anoxic sediments by natural (floods) and/or anthropogenic (dredging, dam flush) events that might promote trace metal mobilization through sulfide oxidation. It is estimated that, during a major flood event, about 870 t of Zn, 18 t of Cd, 25 t of Pb and 17 t of Cu could be mobilized from the downstream reservoir sediments along the Lot River by resuspension-induced oxidation of sulfide phases. These amounts are equivalent to 13-fold (Cd), ∼6-fold (Zn), 4-fold (Pb) the mean annual inputs of the respective dissolved trace metals into the Gironde estuary.

Platinum element group variations at the Permo–Triassic boundary in Kashmir and British Columbia and their significance

The end Permian marks the greatest extinction in the geological record, but there is no consensus on whether it was caused by terrestrial or extraterrestrial factors. Platinum group elements (PGE) can readily separate extraterrestrial from terrestrial sources and the few studies so far (from China and Europe) indicate terrestrial sources for the PGE. We report here detailed PGE analyses of two expanded sections from the late Permian northern and southern hemispheres that allow separate events to be distinguished and confirm a terrestrial source for the PGE. These sources are either contemporaneous seawater or older basaltic volcanics associated with the sections and the PGE were precipitated by a possible combination of development of anoxia in the oceans and post-depositional redistribution. The PGE content in sections at the Permo–Triassic boundary is the result of geochemical and lithological control on metal precipitation that is found in many other black shales throughout geological time.

Selenium distribution and mode of occurrence in the Kanawha Formation, southern West Virginia, U.S.A.

Abstract Selenium concentrations were measured in five rock cores from a mine in Boone County, West Virginia to determine their relationship to sulfur, rock type and stratigraphic location. The samples spanned the Winifrede and Coalburg coal beds of the Middle Pennsylvania upper Kanawha Formation and included coals and clastic lithologies. The coals generally contained the highest concentrations of Se and the sandstones the lowest. No correlation was observed between total Se and sulfur concentrations for either the entire data set or for individual lithologies. Better correlations were identified for log concentrations between total Se and total organic carbon. Principal component analysis identified a strong sulfur–acid component, interpreted to be due to sulfide mineral presence, which correlates well with S but not with Se. Coal sample chemistry is more likely to load onto this component while clastic rock chemistry is more likely to load onto the second, non-sulfur component. Se concentrations load onto both components indicating they are not controlled by a single overriding chemistry. Selective extractions indicate that Se is distributed between both sulfide and organically-bound fractions, as defined by the test. Overall, the lack of correlation between Se and S, the distribution of Se among extracted fractions, and the loading of Se onto both sulfur and non-sulfur components support that the Se in these rock layers is distributed in more than one chemically-bound form. Non-coal rock units with the highest concentrations of Se were found adjacent to coal beds, suggesting that redistribution of Se may have occurred post-deposition. The acidic domed swamps that formed these coals were low in sulfur and a reasonable chemical setting to accumulate Se if it were introduced. The preservation of Se in the domed swamp peat coupled with post-depositional redistribution may partly explain the data presented.

Sources and mobility of thallium and indium in Canadian lakes

Chemical and biological reactions occurring in lake sediments (early diagenesis) can influence the vertical distribution of trace metals and compromise the use of sediment profiles as historical records of anthropogenic metal pollution. There is thus a need to evaluate the importance of post-depositional redistribution of metals in sediments. Large uncertainties also exist concerning sources of metal emissions. In this study, we use porewater and solid phase measurements to determine the post-depositional mobility of thallium and indium in Canadian Shield lakes and to establish atmospheric T1 and In sources and deposition chronology. Two lakes exposed to different emission sources of trace metal pollution were chosen for the study. One, L. Tantar6, is located in an ecological reserve about 40 km from Qu6bec City (about 520000 inhabitants); according to prevalent wind trajectories, this lake is under the influence of atmospheric emissions from the Midwest-Great Lakes and Eastern regions of the USA, as well as from cities located along the St. Lawrence River. The other one, L. Vose, is located 25 km downwind from an important smelter in the mining city of Rouyn-Noranda. The uninhabited drainage basins of these two lakes have not been affected by wildfires or wood harvesting for at least 100 years. Thus, atmospheric deposition has been the only input of atmospheric T1 and In to these basins. Sediment cores were obtained from the deepest part of each lake and analyzed for total T1 and In. Porewaters were collected at the coring sites by in-situ dialysis and analyzed for T1, In, as well as for major ions, sulfide, organic carbon and pH which are needed for speciation calculations. The porewater and sediment geochemical data were used, together with information on infaunal benthos, in the diagenetic reaction-transport equation which was solved numerically for net reaction rates as a function of sediment depth with the code PROFILE. This information was used to estimate the importance of diagenetic processes in shaping the solid phase T1 and In profiles, to estimate the deposition chronology of these metals and to identify the diagenetic reactions involving these metals. Comparison of T1 and In deposition chronology with those of other contaminants (e.g. PAHs, stable lead isotopes) measured at the same sites allowed identification of the main atmospheric sources of TI and In. Our results suggest that T1 and In are useful tracers for atmospheric emissions from coal combustion and metal smelters, respectively.

Determination of Holocene sedimentation rates from a carbonate lake using excess 226Ra profiles

Research on the development of excess 226Ra decay as a novel technique for dating lake sediments has been undertaken on Hawes Water, NW England. Investigation of the behaviour 226Ra and its precursors in the 238U-series in the lake environment supports the theoretical justification for dating carbonate sediments based on the decay of excess 226Ra activity that is transferred from the water column at the time of deposition via calcite coprecipitation. Depth profiles of excess 226Ra activity in bulk and biogenic carbonate phases of the sediment are exponential in form below a surface layer of lake sediment in which enhanced 226Ra influx can be identified. Application of a dating model that assumes a constant rate of supply and no significant post-depositional mobility of 226Ra yields sedimentation rates for the lower parts of two cores of the order of 0.02 g cm − 2 y − 1 that compare well with those obtained from 210Pb dating. Whilst post-depositional redistribution of 226Ra activity is shown to have negligible impact on the down-core profile, the approach is sensitive to changes in the rate of supply and the initial 226Ra activity — both of which can be identified and are confirmed by comparison with the excess 210Pb activity profile.

Post-depositional redistribution and gradual accumulation of 137Cs in a riparian wetland ecosystem in Sweden

After the Chernobyl accident, high activity concentrations of 137Cs (>1 MBq m −2) were detected in a riparian swamp in the central-eastern part of Sweden. The objective of this study was to clarify the redistribution processes behind the accumulation of 137Cs in the wetland. A mass balance budget of 137Cs was calculated based on soil and sediment samples and reports in the literature. Results showed that accumulation occurred over several years. Of all the 137Cs activity discharged between 1986 and 2002 from the upstream lake, 29% was estimated to be retained in the wetland. In 2003, measurements showed that 17 kBq m −2 sedimented on the stream banks of the wetland. Continuing overbank sedimentation by spring flooding prolongs the time that the wetland will contain high activity concentrations of 137Cs. Consequently, organisms living in wetlands serving as sinks for 137Cs may become exposed over long time periods to high activity concentrations.

Phosphorus burial as a function of paleoproductivity and redox conditions in Arabian Sea sediments

In this study the response of sedimentary phosphorus (P) burial to changes in primary productivity and bottom water oxygen concentrations during the Late Quaternary is investigated, using two sediment cores from the Arabian Sea, one recovered from the continental slope and the other from the deep basin. The average solid-phase P speciation in both cores is similar, authigenic and biogenic (fish debris) apatite make up the bulk of the P inventory (ca. 70%);whereas P adsorbed to iron oxides, organic P, and detrital apatite constitute minor fractions. Postdepositional redistribution has not significantly altered the downcore distribution of total solid-phase P. Phosphorus burial efficiencies are generally lower during periods of increased paleoproductivity. This is caused by (a) partial decoupling of the P export flux, consisting primarily of particulate organic P, and the P burial flux, consisting primarily of biogenic and authigenic apatite; and (b) the lack of increased rates of authigenic CFA formation during periods of higher P deposition. In addition, fluctuations in bottom water oxygen concentrations may have affected P burial in continental slope sediments. The results of this study indicate that higher primary productivity induces more efficient P cycling. On time scales exceeding the oceanic P residence time, this process may induce higher surface water productivity, thus creating a positive feedback loop. In the Arabian Sea, this feedback mechanism may have contributed to changes in sea surface productivity on sub-Milankovitch time scales because P, regenerated on the continental slopes of the Oman and Somalian coastal upwelling zones, is reintroduced into the photic zone relatively fast.

Use of fallout 137Cs for documenting the chronology of overbank sediments from the river Sava, Croatia, and interpreting their geochemical patterns

This paper attempts to determine the impact of former Pb-Zn-Ba mining activities in Slovenian drainage basin on overbank sediments of the Sava river (West Croatia). Among the four studied sequences (S7, S8, S9, S10), the most upstream profile S7 showed the most pronounced impact of ore mining. Mercury was enriched more than 100-fold; Pb and Zn had a strong positive correlation (r > 0.99). Two factor models clearly revealed the Pb-Ba-Hg association in the lower-lying carbonate (CA) population of all four profiles, but only in the overlying silicate (SI) population of profile S7. Factor analysis confirmed this association in the total sediment (90 samples) as well as separately in the CA (33 samples) and SI (57 samples) populations. Based on a comparison with an undisturbed soil profile, the vertical distribution of 137Cs in profile S7 was interpreted to be the combination of caesium deposition caused by a flood in 1964 and by post-depositional redistribution.

Comprehensive 1000 year climatic history from an intermediate-depth ice core from the south dome of Berkner Island, Antarctica: methods, dating and first results

AbstractA 181 m deep ice core drilled in 1994/95 on the south dome of Berkner Island, Antarctica, was analyzed for stable isotopes, major ions and microparticle concentrations. Samples for ion chromatography were prepared by using a novel technique of filling decontaminated sample from a device for continuous ice-core melting directly into the sample vials. The core was dated through identification of volcanic horizons and interpolative layer counting. The core, together with a similar core from the north dome, reveals a 1000 year history of relatively stable climate. Temporal variations in the two cores deviate from each other owing to changing patterns of regional-scale circulation; the best correspondence between them is found for MSA–. δ18O, accumulation rate and a sea-salt proxy show only negligible correlation, which suggests a complex meteorological setting. Increasing annual accumulation is observed for the last 100 years. A period of increased sea-salt concentrations started around AD 1405, as has also been observed in other cores. Microparticle concentrations are on average 1220 particles (&gt;1.0 μm diameter) mL–1; they are enhanced from AD 1200 to 1350, possibly because of a higher atmospheric mineral dust load or because local volcanic activity was stronger than previously thought. Microparticles and NH4+ show marked but multiple and very irregular sub-annual peaks; long-term stacking of 1 year data intervals yields seasonal maxima in austral spring or mid-summer, respectively. Post-depositional redistribution was observed for MSA, NO3– and F– at volcanic horizons.

Quantifying Postdepositional Redistribution of the Archaeological Record Produced by Freeze–Thaw and Other Mechanisms: An Experimental Approach

Archaeologists regularly cite frost-related mechanisms as potential transport agents capable of redistributing the archaeological record. Specific data quantifying the scale of frost-induced disturbances, however, are regrettably few. In this paper I present several sets of empirical data obtained from the initial results of a long-term experiment. Test objects positioned in a control plot configured to minimize forces extraneous to frost activity (e.g., wind) demonstrated an average lateral movement of 7.8 cm over a 3-year period. In a second plot that lacked restraints on other variables, test objects migrated an average of 31.8 cm over the same period. Extrapolated over prolonged periods, these data have profound implications for the interpretation of intrasite spatial patterning.