Dissolution of silica and the development of concentration profiles in freshwater sediments

The dissolution of silica and diffusion of reactive dissolved Si in the porewaters of river sediments are investigated using sediments of different physical and chemical properties. Three sediments are considered: (a) from sectioned cores taken from a river-bed, (b) fine organic-rich surface sediment (<5 cm depth) installed in a fluvarium channel and, (c) coarse river sediment of low organic matter content also installed in a fluvarium channel. Dissolution rates of silica are measured at 10°C using batches of suspended material. The derived dissolution rate constants show large differences between the sediments. The river bed-sediment cores had vertical concentration profiles of dissolved Si that are consistent with the diffusion and dissolution of biogenic silica. Experiments in a fluvarium channel enabled Si fluxes to be calculated from a mass-balance of the overlying solution. The results are consistent with the attainment of a steady-state concentration profile of dissolved Si in the sediment. There are no discernible effects of water velocity over the sediment between 5 and 11 cm s⁻¹. However, at 20 cm s⁻¹, the flux increases as a result of either entrainment of fine particles at the surface or advective effects in the surface sediment. A fluvarium experiment with the fine sediment (<125 μm) over 61 days, produced a concentration profile with the highest concentration of 1025 μmol dm⁻³ at a depth of 4–5 cm in the sediment. A FORTRAN program is used to model the results of the increase in dissolved Si in the overlying water and development of a concentration profile in the porewater. This leads to a sediment diffusion coefficient of 1.21×10⁻⁹ m² s⁻¹ at 8.8°C at the beginning of the experiment and rate constant k=13.1×10⁻⁷ s⁻¹ at pH=7.82 and average temperature of 7.6°C for the entire experiment. Fluxes measured at the sediment–surface interface and calculated assuming steady-state profiles had developed are typically 0.01–0.04 μmol m⁻² (of river bed) s⁻¹. The approach enables the efflux of dissolved Si from bottom-sediments to be estimated from dissolution rates measured using suspensions of bed-sediment.     

Inhibition of dissolution within shallow water carbonate sediments: impacts of terrigenous sediment input on syn-depositional carbonate diagenesis

The early diagenetic chemical dissolution of skeletal carbonates has previously been documented as taking place within bioturbated, shallow water, tropical carbonate sediments. The diagenetic reactions operating within carbonate sediments that fall under the influence of iron‐rich (terrigenous) sediment input are less clearly understood. Such inputs should modify carbonate diagenetic reactions both by minimizing bacterial sulphate reduction in favour of bacterial iron reduction, and by the reaction of any pore‐water sulphide with iron oxides, thereby minimizing sulphide oxidation and associated acidity. To test this hypothesis sediment cores were taken from sites within Discovery Bay (north Jamaica), which exhibit varying levels of Fe‐rich bauxite sediment contamination. At non‐impacted sites sediments are dominated by CaCO₃ (up to 99% by weight). Pore waters from the upper few centimetres of cores show evidence for active sulphate reduction (reduced SO₄/Cl^? ratios) and minor CaCO₃ dissolution (increased Ca²⁺/Cl^? ratios). Petrographic observations of carbonate grains (specifically Halimeda and Amphiroa) show clear morphological evidence for dissolution throughout the sediment column. In contrast, at bauxite‐impacted sites, the sediment is composed of up to 15% non‐carbonate and contains up to 6000 μg g^?1 Fe. Pore waters show no evidence for sulphate reduction, but marked levels of Fe(II), suggesting that bacterial Fe(III) reduction is active. Carbonate grains show little evidence for dissolution, often exhibiting pristine surface morphologies. Samples from the deeper sections of these cores, which pre‐date bauxite influence, commonly exhibit morphological evidence for dissolution implying that this was a significant process prior to bauxite input. Previous studies have suggested that dissolution, driven by sulphate reduction and sulphide oxidation, can account for the loss of as much as 50% of primary carbonate production in localized platform environments. The finding that chemical dissolution is minor in a terrigenous‐impacted carbonate environment, therefore, has significant implications for carbonate budgets and cycling, and the preservation of carbonate grains in such sediment systems.     

Chesapeake Bay nutrient and plankton dynamics: III. The annual cycle of dissolved silicon

Silicic acid (H₄SiO₄) flux from the sediment, H₄SiO₄ concentration and river flow were used to obtain an annual dissolved silicon budget for Chesapeake Bay. H₄SiO₄ concentrations vary seasonally in the estuary: for a 12-year period, mean H₄SiO₄ concentrations in the mesohaline region were high both in spring and in late summer to early fall, and were low in late spring—occasionally approaching levels potentially limiting to diatom growth. Most of the annual allochthonous H₄SiO₄ supply to the estuary derives from the three major rivers, but regenerative H₄SiO₄ flux from the sediment to the water column exceeds the total riverine input by a factor of at least five. Sediment H₄SiO₄ efflux exhibits seasonality and averages approximately 2–3 mol Si m^?2 yr^?1. The high rates of sediment dissolution and efflux appear to maintain high levels of H₄SiO₄ in the mesohaline region, and Si-limitation of diatom growth there seems unlikely. The relative rates of biogenic silica formation and dissolution do not vary synchronously: seasonal variations in diatom productivity, sedimentary release of H₄SiO₄ and river flow all contribute to the observed late winter and late summer seasonal maxima and late spring minimum in water column H₄SiO₄ concentrations. If the only source of Si to support sedimentary H₄SiO₄ efflux is biogenic particulate silica recently deposited from the water column and this silica in turn was produced by diatoms in a ratio of 8C:1 Si, the minimum annual primary production by diatoms is at least 260 g C m^?2, approximately half of annual total plankton primary production. This estimate would be revised upwards according to the amount of particulate biogenic silica dissolving in the water column. Burial of biogenic silica amounts to from 2 to 84% of the sediment efflux of H₄SiO₄, depending on location in the bay. On an annual basis, burial represents from 60 to 100% of fluvial H₄SiO₄-Si inputs.     

Sediment oxygen consumption and benthic nutrient fluxes on the Louisiana continental shelf: A methodological comparison

There has been considerable discussion but little experimental evidence regarding the comparability of in-situ and remote (shipboard or laboratory) incubations for the determination of sediment oxygen consumption and benthic nutrient flux rates. This paper presents the results of such a comparison, using in situ chamber and shipboard chemostatic systems, for a shallow station on the Louisiana, continental shelf during April 1992. Results indicated no methodological differences between rates of sediment oxygen consumption and nutrient flux (NH₄ ⁺, NO₅ ^?, NO₂ ^?, PO₄ ^3?, and SiO₂/Si(OH)₂) that could be attributed to the removal of cores from shelf sediments. This conclusion implies that subcoring from box cores is no more destructive of sediment structure and salient environmental characteristics than chamber emplacement. Differences between the methods occurred when ambient oxygen concentrations were low (<2 ml l^?1). These differences were caused by initial reaeration of bottom water in the shipboard system and reflect the sensitivity of heterotrophic metabolism, dissolution kinetics, and diffusive fluxes to low oxygen concentrations. The differences in exchange rates observed in this study reiterate the importance in maintaining ambient conditions in the experimental apparatus. The results of this study corroborate the small body of, data that addresses this issue and extends methodological similarities to include nutrient exchanges. Given the comparability of rates, use of remote chemostatic systems is more advantageous for work in shelf environments than in-situ batch methods due to increased statistical rigor, logistical convenience, and the ability to minimize changes in experimental conditions during incubations.     

Calcite dissolution in two deep eutrophic lakes

The calcium cycle, in particular carbonate dissolution, was analyzed in two deep eutrophic lakes, Lago di Lugano (288 m maximum depth) and Sempachersee (87 m) located in Switzerland. A box model approach was used to calculate calcite dissolution in the water column and at the sediment-water interface based on various lake monitoring data such as sediment traps, sediment cores, water and pore-water analysis. A model for stationary conditions allowing the calculation of calcite dissolution in the water column for a given particle size distribution was developed. The relative values of the simulated flux were consistent with sediment trap observations. The best fit of the dissolution rate constant of sinking calcite in Lago di Lugano was on the same order of magnitude (3 · 10⁻¹⁰ kg^1/3 s⁻¹) as published laboratory values for this surface controlled process.Both lakes show a similar specific calcite precipitation rate of 170 g Ca m⁻² a⁻¹. The diffusive flux across the sediment-water interface amounts to about 15 and 10% of total calcite precipitation in Sempachersee and Lago di Lugano, respectively. However, 61% of the precipitated calcite is dissolved in the water column of Lago di Lugano compared to only 13% in Sempachersee. These results point towards the importance of grain size distributions and settling times in stratified deep waters as the two most important factors determining calcite retention in sediments of hard water lakes.     

Production and pelagic dissolution of biogenic silica in the Southern Ocean

Vertical distributions of particulate silica, and of production and dissolution rates of biogenic silica, were determined on two N-S transects across the Pacific sector of the Antarctic Circumpolar Current during the austral spring of 1978. Particulate silica profiles showed elevated levels in surface water and near the bottom, with low (35–110 nmol Si · 1^?1) and vertically uniform values through the intervening water column. Both the particulate silica content of the upper 200 m and the production rate of biogenic silica in the photic zone increased from north to south, reaching their highest values near the edge of the receding pack ice. A significant, but variable, fraction (18–58%) of the biogenic silica produced in the surface layer was redissolving in the upper 90–98 m. Net production of biogenic silica in the surface layer (production minus dissolution) was proceeding at a mean rate of ca. 2 mmol Si · m^?2 · day^?1. This is ca. 4 times greater than the most recent estimate of the mean accumulation rate of siliceous sediments beneath the ACC. We estimate, based on mass balance, that the mean dissolution rate of biogenic silica in subsurface water column in the Southern Ocean is 1.2–2.9 mmol Si · m^?2 · day^?1.     

Denitrification and N2O production in near-shore marine sediments

Methods were developed for determining rates of denitrification in coastal marine sediments by measuring the production of N₂ from undisturbed cores incubated in gas-tight chambers. Denitrification rates at summer temperatures (23°C) in sediment cores from Narragansett Bay, Rhode Island, were about 50μmol N₂m^?2 hr^?1. This nitrogen flux is equal to approximately one-half of the NH⁺₄flux from the sediments at this temperature and is of the magnitude necessary to account for the anomalously low N/P and anomalously high O/N ratios often reported for benthic nutrient fluxes. The loss of fixed nitrogen as N₂ during the benthic remineralization of organic matter, coupled with the importance of benthic remineralization processes in shallow coastal waters may help to explain why the availability of fixed nitrogen is a major factor limiting primary production in these areas. Narragansett Bay sediments are also a source of N₂O, but the amount of nitrogen involved was only about 0.2 μmol m^?2 hr^?1 at 23°C.     

Sedimentation rate variations and anthropogenic metal fluxes into Lake Constance sediments

Sedimentation rates were determined by²¹⁰Pb in three sediment cores from the main basin of Lake Constance. Rates vary from 0.094 to 0.133 g cm^?2y^?1, in agreement with previous determinations. A constant radionuclide flux (CR) model reveals long-term, quasi-synchronous fluctuations of sedimentation rate on the basin-wide scale. On average, the rate of sedimentation remains much the same from the beginning of this century indicating little effect of human activity in the watershed on fine-silt sediment supply. The anthropogenic fluxes of Zn, Pb, and Cd in the three cores are compared and the application of²¹⁰Pb as a heavy metal tracer in Lake Constance is examined. It seems to be a very good tracer for Pb, moderately good for Zn and Cu, and not useful for Cd.     

Diagenetic processes in Holocene carbonate sediments: Florida Bay mudbanks and islands

The sedimentology, mineralogy and pore fluid chemistry of seven cores from the Holocene sediments of Florida Bay were studied to determine the physical processes and diagenetic reactions affecting the sediments. The cores were taken in a transect from a shallow mudbank onto a small adjacent island, Jimmy Key. Steady state models of pore fluid chemistry are used to estimate the rates of various reactions. In the mudbank sediments, little carbonate mineral diagenesis is taking place. No change in sediment mineralogy is detectable and pore water profiles of Ca²⁺, Mg²⁺ and Sr²⁺ show only minor variation. Chloride concentrations indicate substantial biological mixing of seawater from the bay into the sediments in one of the cores. Pore water analyses of sulphate and alkalinity show only a low degree of sulphate depletion and a decreasing extent of sulphate reduction downcore. Models of sulphate reduction in the mudbank show that there is substantial chemical exchange between the sediment pore fluids and water from the bay probably as a result of bio-irrigation. The sulphate and alkalinity data also suggest that the underlying Pleistocene rocks contain water of near normal seawater composition. Stratigraphic analysis and δ¹³C analyses of the organic carbon in the sediments of the island cores show that the sediments were primarily deposited in a subtidal mudbank setting; only the upper 20–30 cm is supratidal in origin. Nevertheless, island formation had a significant effect on pore fluid chemistry and the types of diagenetic reactions throughout the sediment column. Chloride in the sediment pore fluids is more than twice the normal seawater concentrations over most of the depth of the cores. The constant, elevated chloride concentrations indicate that hypersaline fluids which formed in ponds on the island are advected downward through the sediments. Models of the chloride profiles yield an estimate of 2·5 cm yr^?1 as a minimum advective velocity. Changes in pore water chemistry with depth are interpreted as indicating the following sequence of reactions: (1) minor high-Mg calcite dissolution and low-Mg calcite precipitation, from 0 to 35 cm; (2) Ca- or Mg-sulphate dissolution and low-Mg calcite precipitation, from 5 to 35 cm; (3) dolomite or magnesite precipitation together with sulphate reduction, from 35 to 55 cm; and (4) little reaction below 55 cm. In addition, one or more as yet unidentified reactions must be taking place from 5 to 55 cm depth as an imbalance in possible sources and sinks of alkalinity is observed. The imbalance could be explained if chloride is not completely conservative. Despite the pore fluid chemical evidence for diagenetic reactions involving carbonate minerals, no changes in sediment mineralogy were detected in X-ray diffraction analyses, probably because of the comparatively young age of the island.     

Accretion rates and sediment accumulation in Rhode Island salt marshes

In order to test the assumption that accretion rates of intertidal salt marshes are approximately equal to rates of sea-level rise along the Rhode Island coast,²¹⁰Pb analyses were carried out and accretion rates calculated using constant flux and constant activity models applied to sediment cores collected from lowSpartina alterniflora marshes at four sites from the head to the mouth of Narragansett Bay. A core was also collected from a highSpartina patens marsh at one site. Additional low marsh cores from a tidal river entering the bay and a coastal lagoon on Block Island Sound were also analyzed. Accretion rates for all cores were also calculated from copper concentration data assuming that anthropogenic copper increases began at all sites between 1865 and 1885. Bulk density and weight-loss-on-ignition of the sediments were measured in order to assess the relative importance of inorganic and organic accumulation. During the past 60 yr, accretion rates at the eight low marsh sites averaged 0.43±0.13 cm yr^?1 (0.25 to 0.60 cm yr^?1) based on the constant flux model, 0.40±0.15 cm yr^?1 (0.15 to 0.58 cm yr^?1) based on the constant activity model, and 0.44±0.11 cm yr^?1 (0.30 to 0.59 cm yr^?1) based on copper concentration data, with no apparent trend down-bay. High marsh rates were 0.24±0.02 (constant flux), 0.25±0.01 (constant activity), and 0.47±0.04 (copper concentration data). The cores showing closest agreement between the three methods are those for which the excess²¹⁰Pb inventories are consistent with atmospheric inputs. These rates compare to a tide gauge record from the mouth of the bay that shows an average sea-level rise of 0.26±0.02 cm yr^?1 from 1931 to 1986. Low marshes in this area appear to accrete at rates 1.5–1.7 times greater than local relative sea-level rise, while the high marsh accretion rate is equal to the rise in sea level. The variability among the low marsh sites suggests that marshes may not be poised at mean water level to within better than ±several cm on time scales of decades. Inorganic and organic dry solids each contributed about 9% by volume to low marsh accretion, while organic dry solids contributed 11% and inorganic 4% to high marsh accretion. Water/pore space accounted for the majority of accretion in both low and high marshes. If water associated with the organic component is considered, organic matter accounts for an average of 91% of low marsh and 96% of high marsh accretion. A dramatic increase in the organic content at a depth of 60 to 90 cm in the cores from Narragansett Bay appears to mark the start of marsh development on prograding sand flats.     

Sediment flux distribution in the Southern Brazil Basin during the late Quaternary: the role of deep-sea currents

Detailed sedimentological and stratigraphic analyses were carried out on seven Kullenberg cores collected across the Brazilian continental margin during the French cruises Byblos and Apsara III, in order to highlight the factors controlling the sediment flux distribution in the Southern Brazil Basin during the late Quaternary. On the continental slope and upper continental rise above 3000 m depth, sediment fluxes are important and highly variable (4·2–14·2 g cm^?2 10^?3 yr). The sediments show a pelagic or turbiditic character, depending on the width of the shelf and proximity of canyons. The material is characterized by high kaolinite contents, and originates from the coastal rivers draining the South American continent north of Rio de Janeiro. On the middle continental rise between 3000 and 4000 m depth, sediment fluxes are the lowest observed in the area (0·9 g cm^?2 10^?3 yr), because terrigenous input is trapped at shallower depths on the São Paulo Plateau. Pelagic settling is the dominant process. In the deep domains, below 4000 m depth, contouritic accumulations are developed on the path of the northwards moving Antarctic bottom water (AABW) currents. The deposits consist of fine-grained silty-clayey muds with very low carbonate contents. The sediment fluxes (1·45 g cm^?2 10^?3 yr) are higher than on the middle continental rise, as a consequence of fine-grained terrigenous supply derived from higher latitudes (Argentine Basin and Southern Ocean), and transported in the basin through the Vema Channel by the AABW currents. This material is characterized by high smectite and chlorite contents. These data reveal large sediment flux variations which are linked to distinct depth-related domains. Such a distribution is the consequence of the presence of two available sources of terrigenous sediments: (1) the Brazilian continental areas with a downslope material transport and a sediment distribution controlled by the morphology of the margin, and (2) the Argentine Basin with an alongslope material transport by deep-sea currents which dominate the sedimentation in the abyssal domains.     

Rate of mercury loss from contaminated estuarine sediments

The concentration of mercury in contaminated estuarine sediments of Bellingham Bay, Washington was found to decrease with a half-time of about 1.3 yr after the primary anthropogenic source of mercury was removed. In situ measurements of the mercury flux from sediments, in both dissolved and volatile forms, could not account for this decrease. This result suggests that the removal of mercury is associated with sediment particles transported out of the study area. This decrease was modeled using a steady-state mixing model.Mercury concentrations in anoxic interstitial waters reached 3.5 μg/l, 126 times higher than observed in the overlying seawater. Mercury fluxes from these sediments ranged from 1.2 to 2.8 × 10^?5 ng/cm²/sec, all in a soluble form. In general, higher Hg fluxes were associated with low oxygen or reducing conditions in the overlying seawater. In contrast, no flux was measurable from oxidizing interstitial water having mercury concentrations of 0.01-0.06 μ/l.     

Geochronology and sedimentology of the lower Passaic River,New Jersey

In an attempt to characterize localized rates of sediment accretion, 10 sediment cores were collected from the lower reach of the Passaic River, a major tributary of Newark Bay, New Jersey. Sediments were assayed for ²¹⁰Pb activity at predetermined depths and the rate of sediment accretion (cm yr^?1) was estimated from the least squares regression of the log of unsupported activity versus depth. Sediment accretion rates, derived from ²¹⁰Pb measurements (R_Pb) were used to predict the depth interval within the core containing sediments deposited around 1954; subsequent ¹³⁷Cs analyses were focused on this depth interval. Sediment accretion rates derived from ¹³⁷Cs measurements (R_Cs) were extrapolated from the depth of the 1954 horizon. Lead-210 derived sediment accretion rates in cores collected from a sediment bench extending along the inside bend on the southern shore of a meander in the river, ranged from 4.1 cm yr^?1 to 10.2 cm yr^?1 and averaged 6.8 cm yr^?1. The R_Cs estimates for cores from this area ranged from 3.8 cm yr^?1 to 8.9 cm yr^?1 and averaged 6.6 cm yr^?1. The R_Cs for cores collected in a more hydrologically dynamic reach of the river upstream of the sediment bench, were only 0.41 cm yr^?1 and 0.66 cm yr^?1. The results of this investigation indicate that this reach of the lower Passaic River is an area of high sediment accumulation, retaining much of the sediment load deposited from upstream and downstream sources. The rates of sediment accretion in the lower Passaic River are among the highest reported anywhere in the Newark Bay estuary.     

Determination of recent deposition rates in Lake Constance with radioisotopic methods

Sedimentation rates were determined with the ²¹⁰Pb method in eight sediment cores from Lake Constance. The rate of deposition in the main basin (Obersee) varies from about 0.06 g cm^?2 y^?1 in the central part to 0.13 g cm^?2 y^?1 in the eastern part of the lake and then increases rapidly towards the Rhine delta. In the central lake area the rate of deposition has been approximately constant since 1900, and dating with the ²¹⁰Pb method is in good agreement with sedimentological observations. In the Konstanzer Trichter area, the deposition rate has been increasing since about 1955 as a result of eutrophication and subsequent high carbonate production. Dating with ¹³⁷Cs is fairly accurate for sediments deposited at a high rate, but is questionable for slowly accumulating ones. A positive correlation of ²¹⁰Pb fluxes and sedimentation rates indicates that ²¹⁰Pb flux into sediments follows the distribution pattern of solids. ²¹⁰Pb profiles in four sediment cores interpreted in terms of a constant flux model display synchronous fluctuations of the sedimentation rate; however, their relation to long-range particulate input variations remains to be proved. Sedimentation rates determined with the ²¹⁰Pb method were used to calculate recent nutrient and heavy metal fluxes. Anthropogenic fluxes of Zn and Pb are in the same range of magnitude as in other polluted areas in Europe and America.     

A potential nitrogen sink discovered in the oxygenated Chukchi Shelf waters of the Arctic

The western Arctic Shelf has long been considered as an important sink of nitrogen because high primary productivity of the shelf water fuels active denitrification within the sediments, which has been recognized to account for all the nitrogen (N) removal of the Pacific water inflow. However, potentially high denitrifying activity was discovered within the oxygenated Chukchi Shelf water during our summer expedition. Based on ¹⁵N-isotope pairing incubations, we estimated denitrification rates ranging from 1.8 ± 0.4 to 75.9 ± 8.7 nmol N₂ L^?1 h^?1. We find that the spatial pattern of denitrifying activity follows well with primary productivity, which supplies plentiful fresh organic matter, and there was a strong correlation between integrated denitrification and integrated primary productivity. Considering the active hydrodynamics over the Chukchi Shelf during summer, resuspension of benthic sediment coupled with particle-associated bacteria induces an active denitrification process in the oxic water column. We further extrapolate to the whole Chukchi Shelf and estimate an N removal flux from this cold Arctic shelf water to be 12.2 Tg-N year^?1, which compensates for the difference between sediment cores incubation (~ 3 Tg-N year^?1) and geochemical estimation based on N deficit relative to phosphorous (~ 16 Tg-N year^?1). We infer that dynamic sediment resuspension combined with high biological productivity stimulates intensive denitrification in the water column, potentially creating a nitrogen sink over the shallow Arctic shelves that have previously been unrecognized.     

Pore water fluoride in Peru continental margin sediments: Uptake from seawater

Fluoride analyses display downward decreasing pore water gradients in Peru shelf phosphatic muds that require diffusion from the overlying seawater into the sediment column and removal by reaction within the upper few tens of centimeters, presumably by incorporation into carbonate fluorapatite. The profiles can be modeled as first-order F-removal with rate constants of ~3 yr^?1 and asymptotic F-concentrations deep in the cores of 35–45 μM, almost one-half the seawater value. The integrated flux of fluoride from seawater into organic-rich shelf sediments in coastal-upwelling zones (phosphatic muds) yields a contemporaneous global F-burial of 0.54 × 10¹⁰ mol-F yr^?1, about one-fifth the burial in other sinks (mostly carbonates and opal). The associated burial flux of phosphorus in shelf phosphorites is about 1.6 × 10¹⁰ mol-P yr^?1, comparable to P-burial in the deep sea with organic matter (~1.4 × 10¹⁰ mol yr^?1) and biogenic carbonates (~1.4 × 10¹⁰ mol yr^?1). Thus phosphorite formation on the Peru shelf is a significant contemporaneous process.     

Sediment accumulation rates and geochronologies measured in the Saguenay Fjord using the Pb-210 dating method

Sediment accumulation rates were estimated from-the vertical distribution of excess Pb-210 measured in sediment cores collected at seven stations in the Saguenay Fjord, Quebec. These rates decrease with increasing water depth and distance from the mouth of the Saguenay River, ranging from 4.0 g cm^?2 yr^?1 (～- 7 cm yr^?1) near the head of the fjord to 0.07 g cm^?2 yr^?1 (～- 0.1 cm yr^?1) in the deep inner basin of the fjord. In one core from the head of the fjord, layered sediment structures, having different physical characteristics and composition, appear related to recent, pulsed inputs of older raised marine deposits displaced by a landslide in 1971. Synchronous depositional anomalies in several cores provide evidence of other large scale sediment redistribution processes in the fjord. Pb-210 geochronologies are generally in good agreement with time-stratigraphic horizons inferred both from Cs-137 activity profiles and from the analysis of pollen assemblages in one core.     

Iodine-129 enrichment in sediment of the Baltic Sea

Sediments are an excellent archive for evaluation of time-series environmental contamination of water systems. Measurements of ultra-trace radioactive species, such as ¹²⁹I, provide information for both chronologic calibration and anthropogenic emissions during the nuclear era. Here data are presented on ¹²⁹I and other chemical parameters from two sediment cores collected in the Baltic Sea during 1997. The sediment sections have a relatively uniform grain size (clay–mud) and cover a period of about 50 a. Distribution of ¹²⁹I in the sediment strongly relates to the liquid release records from the nuclear reprocessing facilities at Sellafield and La Hague. However, syn- and post-depositional alteration of organic matter at the sediment–water interface and within the sediment column may have contributed to slightly obliterating the anthropogenic ¹²⁹I signals. Indication of Chernobyl-derived ¹²⁹I occurs in the sediment profile, but is apparently overridden by the overwhelming flux from the nuclear reprocessing facilities. Although the record did not cover the pre-nuclear era (before 1945) sections, the ultra sensitive ¹²⁹I profile provides a potential tool for relative dating and monitoring sources of water and sediment to the region.     

Hydrogeochemistry of Lake Turkana,Kenya: Mass balance and mineral reactions in an alkaline lake

Lake Turkana, in northwestern Kenya, is a closed-basin, alkaline (pH = 9.2) lake of moderate salinity (TDS = 2500 ppm). Principal ions are Na⁺, HCO^?₃ and CI^?. The lake is essentially polymictic in the northern basin and little compositional variation occurs in surface waters. The Omo River is the principal influent, providing some 80–90% of water input to the lake. Chloride has an apparent accumulation time of about 2500 years after accounting for burial of interstitial water.The bottom sediments are predominantly detrital and fine-grained, yet mineral-water reactions are very important for the geochcmical budget. Ca²⁺ is precipitated as calcite; Na⁺ is removed as an exchangeable cation on smectite; Mg²⁺ is probably incorporated into a Mg-silicate phase, most likely poorlycrystalline smectite, as it enters the lake water; K⁺ may be used in illite regradation. Cation exchange is a very important process in the mass balance of this lake. Over 40% of incoming Na is removed as an exchangeable cation. After cation exchange and interstitial water burial, Na has a response time of 2650 years, which compares favorably with that of chloride. These processes seem to occur rapidly within the water mass of the lake: other reactions may be important in regulating interstitial water compositions.Several changes occur in the upper 3m of sediment: interstitial-water pH drops to 8.3 and alkalinity increases slightly with depth, SO^2?₄ decreases slightly, and amorphous silica saturation is approached. These changes are a response to organic matter oxidation and the dissolution of unstable silicates rather than a reversal of reactions occurring in the lake water. High rates of sedimentation (up to 1 cm per year) may minimize the effects of diffusion between the interstitial waters and the lake water, although burial of interstitial water assumes considerable importance.