Plutonium and americium in anoxic marine sediments: Evidence against remobitization

^{239 + 240}Pu activities of 100–450dpm/kg are found down to 15–18 cm in anoxic Saanich Inlet sediments, with a subsurface maximum in undisturbed deposits. Integrated ^{239 + 240}Pu inventories which overlap delivery estimates are present both in two cores of anoxic sediments from Saanich Inlet and in one core of oxic sediments 65 km away in Dabob Bay, Washington. ${}^{241}\text{Am}{}^{\mathrm{239\; +\; 240}}\text{Pu}$ ratios in Saanich Inlet sediments overlap ratios in unfractionated midnorthern latitude fallout, in oxic sediments from the Washington continental shelf, and in anoxic sediments from two basins off southern California and Mexico. The ${}^{\mathrm{239\; +\; 240}}\text{Pu}{}^{137}\text{Cs}$ ratios in three intervals of Saanich Inlet sediments are also in agreement with ratios previously reported for oxic coastal marine sediments. The Pu inventories, the $\text{Am}\text{Pu}$ and $\text{Pu}\text{Cs}$ ratios, and the Saanich Inlet Dabob Bay comparison all argue that Pu is not rapidly remobilized in anoxic sediments.The subsurface ^{239 + 240}Pu activity maximum is not in agreement with the historical record of peak Pu fallout in 1963–1964 unless our ²¹⁰Pb-derived sedimentation rates are incorrectly high. However, they are in good agreement with previous ²¹⁰Pb and varve chronologies in Saanich Inlet, and also give reasonable dates for times when ^{239 + 240}Pu and SNAP-9A supplied ²³⁸Pu first appear in the sediments. We conclude they properly date the maximum in sedimentary ^{239 + 240}Pu activity at 1970–1973, and seek explanations for the 7–10yr time lag after peak fallout.^{239 + 240}Pu inventories in one core from the eastern basin of the Cariaco Trench and in two cores from Golfo Dulce. an anoxic basin off the Pacific coast of Costa Rica, are also in reasonable agreement with fallout delivery to these latitudes when excess ²¹⁰Pb inventories and fluxes are used to verify recovery of at least a major fraction of the most recently deposited sediments.     

海洋沉积物中氧化还原敏感元素对水体环境缺氧状况的指示作用

近年来由于人为污染水体富营养化加剧,缺氧区面积不断增大,利用沉积物中氧化还原敏感元素反演水体缺氧情况已经发展成为海洋化学领域的热点研究方向。本文详细阐述了氧化还原敏感元素的富集机制,并总结了利用沉积物中氧化还原敏感元素在不同氧化还原条件下的富集程度反映海水缺氧程度和底质氧化还原状况的一系列指标,如Re/Mo、Cd/U、Th/U、V/Sc、V/(V+Ni)值,U—Mo共变模型,δ~(98/95)Mo,多指标微/痕量元素模型以及氧化还原敏感元素-有机质共变模型等。沉积物中Re、Mo含量、Re/Mo值、自生Mo/U值、Th/U值对上层水体缺氧和氧化条件区分良好,可定量指征上层水体的缺氧情况。沉积物中Re含量近似于1 ng/g(地壳值),Mo含量1μg/g(地壳值),Re/Mo值接近0.3×10~(-3),Mo—U富集系数比为(0.1～0.3)×现代海水,Th/U值2,可指示氧化环境;Re含量在10～30 ng/g,Mo含量近似于1μg/g,Re/Mo值10×10~(-3)～30×10~(-3),Mo—U富集系数比1×现代海水,Th/U值在0～2范围内,可指示缺氧环境;Re含量30 ng/g,Mo富集达到20～40μg/g,Re/Mo值接近0.7×10~(-3)(海水中Re/Mo值),Mo—U富集系数比为(3～10)×现代海水,可指示极度缺氧的硫化环境。Mo_(EF)—U_(EF)交会对数坐标图、氧化还原敏感元素-有机质共变模型指标可定性分析上层水体的缺氧情况;V/(V+Ni)值对于次氧化沉积物指示效果不佳;Cd/U值在次缺氧条件下的变化机制复杂,还需进一步研究。生物扰动、成岩作用、人为污染、水体局限、高有机碳通量、Fe、Mn氧化物循环等因素通过影响氧化还原敏感元素在沉积物中的富集与迁移,从而影响氧化还原敏感元素指标的应用,应剔除有机质吸附与陆源输入等非自生部分的影响,结合各种指标互相印证,综合判别水体氧化还原状态。     

Heavy-mineral variability in miocene marine sediments in Denmark: a combined effect of weathering and reworking

Heavy-mineral analyses have been performed on marine sandy and silty deposits of Miocene age from the western part of Denmark. The heavy-mineral associations are dominated by epidote and amphibole, but the mutual distribution of these minerals is variable. Studies of surface textures show a large, but variable, proportion of the grains to be corroded. In a few cases the surface textures are indicative of post-depositional processes; however, most of these may have existed prior to deposition. It is suggested that the variations in heavy-mineral distribution and surface textures result from mixing of freshly supplied, unweathered detritus from Fennoscandia, with weathered detritus from the same provenance. This weathering took place during a temporary deposition in fluvial systems between the source area and the North Sea Basin. The relation to transgressive and regressive phases is discussed.     

Silicate weathering of soil-mantled slopes in an active Alpine landscape

Despite being located on high, steep, actively uplifting, and formerly glaciated slopes of the Swiss Central Alps, soils in the upper Rhone Valley are depleted by up to 50% in cations relative to their parent bedrock. This depletion was determined by a mass loss balance based on Zr as a refractory element. Both Holocene weathering rates and physical erosion rates of these slopes are unexpectedly low, as measured by cosmogenic ¹⁰Be-derived denudation rates. Chemical depletion fractions, CDF, range from 0.12 to 0.48, while the average soil chemical weathering rate is 33 ± 15 t km⁻² yr⁻¹. Both the cosmogenic nuclide-derived denudation rates and model calculations suggest that these soils have reached a weathering steady-state since deglaciation 15 ky ago. The weathering signal varies with elevation and hillslope morphology. In addition, the chemical weathering rates decrease with elevation indicating that temperature may be a dominant controlling factor on weathering in these high Alpine basins. Model calculations suggest that chemical weathering rates are limited by reaction kinetics and not the supply rate of fresh material. We compare hillslope and catchment-wide weathering fluxes with modern stream cation flux, and show that high relief, bare-rock slopes exhibit much lower chemical weathering rates despite higher physical erosion rates. The low weathering fluxes from rocky, rapidly eroding slopes allow for the broader implication that mountain building, while elevating overall denudation rates, may not cause increased chemical weathering rates on hillslopes. In order for this sediment to be weathered, intermediate storage, for instance in floodplains, is required.     

Kinetics of organic matter degradation, microbial methane generation, and gas hydrate formation in anoxic marine sediments

Seven sediment cores were taken in the Sea of Okhotsk in a south-north transect along the slope of Sakhalin Island. The retrieved anoxic sediments and pore fluids were analyzed for particulate organic carbon (POC), total nitrogen, total sulfur, dissolved sulfate, sulfide, methane, ammonium, iodide, bromide, calcium, and total alkalinity. A novel method was developed to derive sedimentation rates from a steady-state nitrogen mass balance. Rates of organic matter degradation, sulfate reduction, methane turnover, and carbonate precipitation were derived from the data applying a steady-state transport-reaction model. A good fit to the data set was obtained using the following new rate law for organic matter degradation in anoxic sediments:

     

The marine chemistry of iodine in anoxic basins

The distribution of dissolved iodate and iodide has been determined in two anoxic basins, the Black Sea and the Cariaco Trench; and the oxic Venezuela Basin which serves as a comparison for normal oceanic conditions. In normal oceanic waters, iodate is the predominant species; its concentration is lowest at the surface (ca. 0.3 μM) and increases with depth to ca. 0.5 μM. In contrast, the iodide concentration shows maximum values in surface waters and rapidly decreases to <0.01 μM below the euphotic zone. In anoxic basins, the reduced pE reverses this trend. The concentration of iodide increases rapidly in the oxygen-sulfide mixing zone from 0.02 to 0.46 μM, and 0.01 to 0.43 μM, in the Cariaco Trench and the Black Sea, respectively. The iodate concentration, meanwhile, decreases to zero.The total iodine to salinity ratio is lower in the surface waters with a range of 7.3–12.1 nmoles/g suggesting a possible depletion by organisms. In the anoxic basins, a maximum in this ratio is observed just above the oxygen-sulfide boundary (15–17 nmoles/g) and is indicative of particle dissolution in a strong pycnocline. In the anoxic zone of the Cariaco Trench, the ratio is constant at 12.3 nmoles/g, whereas in the Black Sea, it increases with depth from 10.0 to 19.3 nmoles/g, suggesting a possible flux of iodide from the bottom sediments.By considering the distribution of iodate and iodide in oxic and anoxic basins, the lower limit of the pE of the oxic ocean is estimated to be 10.7, given our present analytical capability. Thermodynamic considerations further suggest that the iodide-iodate couple is a poor indicator for the pE of the oceans with a limited usable range of 10.0–10.7.     

Denitrification in anoxic sediments supported by biological nitrate transport

Biologically available nitrogen (fixed N) is removed from the oceans by metabolic conversion of inorganic N forms (nitrate and ammonium) to N₂ gas. Much of this removal occurs in marine sediments, where reaction rates are thought to be limited by diffusion. We measured the concentration and isotopic composition of major dissolved nitrogen species in anoxic sediments off the coast of California. At depths below the diffusive penetration of nitrate, we found evidence of a large nitrate pool transported into the sediments by motile microorganisms. A ∼20‰ enrichment in ¹⁵N and ¹⁸O of this biologically transported nitrate over bottom water values and elevated [N₂] and δ¹⁵N-N₂ at depth indicate that this nitrate is consumed by enzymatic redox reactions with the production of N₂ as the end product. Elevated N₂O concentrations in pore waters below the nitrate diffusion depth confirm that these reactions include the denitrification pathway. A data-constrained model shows that at least 31% of the total N₂ production in anoxic sediments is linked to nitrate bio-transport. Under suboxic/anoxic regimes, this nitrate bio-transport augments diffusive transport, thus increasing benthic fixed nitrogen losses and the reducing burial efficiency of sedimentary organic matter.     

Chemical influences on trace metal-sulfide interactions in anoxic sediments

Interactions of trace metals with sulfide in anoxic environments are important in determining their chemical form and potential toxicity to organisms. In recent years, a considerable body of observational data has accumulated that indicates very different behavior for various trace metals in sulfidic sediments. These differences in behavior cannot be entirely attributed to thermodynamic relationships, but also reflect differences in ligand exchange reaction kinetics, and redox reaction pathways.Pb, Zn, and Cd, which are generally pyritized to only a few percent of the “reactive” fraction, have faster water exchange reaction kinetics than Fe²⁺, resulting in MeS phases precipitating prior to FeS formation and subsequent pyrite formation, whereas, Co and Ni, which have slower H₂O exchange kinetics than Fe²⁺, are incorporated into pyrite. Although Hg and Cu have faster reaction kinetics than Fe²⁺, both are incorporated into pyrite or leached from the pyrite fraction with nitric acid. Hg undergoes significant chloride complexation, which can retard reaction with sulfide, but can also replace Fe in FeS to form HgS, which can only be dissolved in the pyrite fraction. Cu²⁺ is reduced by sulfide and forms a variety of sulfides with and without Fe that can only be dissolved with nitric acid. Mn²⁺ does not form a MnS phase easily and is incorporated into pyrite at high iron degrees of pyritization (DOP).Oxyanions of Mo and As are first reduced by sulfide. These reduced forms may then react with sulfides resulting in incorporation into pyrite. However, the oxyanion of Cr is reduced to Cr³⁺, which is kinetically inert to reaction with sulfide and, therefore, not incorporated into pyrite.     

Quaternary sediments and their weathering history in northcentral Missouri

Quaternary strata in northcentral Missouri were deposited prior to, during, and subsequent to glaciation of the area. Major depositional units are separated by buried soils. The lower Quaternary Whippoorwill formation is derived from local bedrock and has the intensely weathered New Florence soil developed in it. The pre-Illinoian McCredie formation, primarily till, is derived from distant shield and local cratonic rocks. The moderately weathered Kingdom City soil within the formation suggests that an intcrglacial separated two ice advances. The lllinoian Lovcland and Wisconsinan Peoria Loesses are derived from Missouri River outwash. Since glaciation, the welded Yarmouth-Sangamon Soil has developed in till and in lllinoian loess on stable sites but middle and late Sangamon Soils have developed on erosional surfaces. Holocene soils are developed in Wisconsinan loess and alluvium-colluvium.     

Aerobic respiration in pelagic marine sediments

Analyses for dissolved oxygen, nitrate and total CO₂ in the interstitial water have been combined with solid phase sediment analyses of carbon and nitrogen to calculate the rates of reaction and stoichiometry of decomposing organic matter in central Equatorial Pacific pelagic sediments. The diagenesis is dominated by aerobic respiration and nitrification.Organic carbon and total nitrogen decrease exponentially with depth in both red clay and carbonate ooze sediments. In addition, there is a correlation between surface organic carbon and total nitrogen with distance from the equator. Fixed NH₄ is relatively constant with depth and constitutes 12 to 64% of the total nitrogen. The remainder is considered to be organic nitrogen.The $\text{C}\text{N}$ ratio of the decomposing organic matter was obtained using three approaches. Using the correlations of organic carbon with total nitrogen or organic nitrogen the molar ratios varied from 3.4 to 18.1. The average of all stations was 12.6 using total nitrogen and 13.7 using organic nitrogen. The Redfield ratio is 6.6. Approaches using interstitial water chemistry gave lower ratios. The average value using correlations between dissolved oxygen and nitrate was 8.1. The same approach using total CO₂ and nitrate gave an average of 9.1. Due to difficulties in unambiguously interpreting the solid phase data we favor the ratios obtained from the pore water analyses.The rate of organic matter decomposition can be obtained from model calculations using the dissolved oxygen and solid organic carbon data. Most gradients occur in the upper 10 to 20 cm of the sediments. Assuming that bioturbation is more important than sedimentation we have calculated first order rate constants. The average values using organic carbon and dissolved oxygen was 3.9 kyr^? and 4.2 kyr^? respectively using a biological mixing coefficient of 100 cm² kyr^?1. These rate constants decrease in direct proportions to the mixing coefficient.     

Biogeochemistry of acetate in anoxic sediments of Skan Bay,Alaska

The role of acetate in the biogeochemical cycling of organic matter in contemporary marine anoxic sediments of Skan Bay, Alaska was investigated with inhibition and quasi in situ turnover experiments. The turnover time for acetate oxidation in the upper 30 cm of the sediment column is ca. 1 hr. A molybdate inhibition experiment indicated that sulfate reducing bacteria were responsible for more than 95% of acetate oxidation. However, measured acetate oxidation rates exceeded sulfate reduction rates indicating that acetate oxidation rates are overestimated. Values for acetate concentration calculated from sulfate reduction rates (0.3–3.4 μM) were considerably lower than directly measured acetate concentrations (3.1–10.8 μM). Much of the chemically measured acetate may be microbially unavailable, perhaps in the form of a soluble or colloidal complex. A sorption experiment indicates that 10% to 40% of added acetate associates with Skan Bay sediment particles. Production of methane from acetate was detected only at 2 m depth.     

Organic matter in anoxic marine pore water: oxidation effects

A series of cores were obtained from the Great Bay Estuary, New Hampshire during 1980 and 1981 in order to investigate the effects on the dissolved organic matter of exposure of anoxic marine pore water to atmospheric oxygen during laboratory handling. Sediment sections from these cores were homogenized and split into two equal parts, one of which was handled in a glove bag under a nitrogen atmosphere, and the other exposed to atmospheric oxygen during laboratory processing. Analysis of the pore water for dissolved organic carbon (DOC) indicated two effects: (1) in the top 15 cm, DOC was lost from the pore water of the oxidized subsections, probably as a consequence of co-precipitation with iron (III) oxi-hydroxides, whereas (2) deeper in the cores, the pore water from the oxidized subsections showed a net increase in DOC, possibly due to oxidation of sedimentary organic matter. In addition, structural changes in the dissolved organic matter in anoxic pore water following oxidation was indicated by high pressure liquid chromatography and ultrafiltration analysis.     

Predictive isotopic biogeochemistry: hydrocarbons from anoxic marine basins

Carbon isotopic compositions were determined for individual hydrocarbons in water column and sediment samples from the Cariaco Trench and Black Sea. In order to identify hydrocarbons derived from phytoplankton, the isotopic compositions expected for biomass of autotrophic organisms living in surface waters of both localities were calculated based on the concentrations of CO2(aq) and the isotopic compositions of dissolved inorganic carbon. These calculated values are compared to measured delta values for particulate organic carbon and for individual hydrocarbon compounds. Specifically, we find that lycopane is probably derived from phytoplankton and that diploptene is derived from the lipids of chemoautotrophs living above the oxic/anoxic boundary. Three acyclic isoprenoids that have been considered markers for methanogens, pentamethyleicosane and two hydrogenated squalenes, have different delta values and apparently do not derive from a common source. Based on the concentration profiles and isotopic compositions, the C31 and C33 n-alkanes and n-alkenes have a similar source, and both may have a planktonic origin. If so, previously assigned terrestrial origins of organic matter in some Black Sea sediments may be erroneous.     

Microscopic gold in marine and oceanic sediments

The present-day contribution of coastal-marine placers into the bulk gold production is insignificant. As usual, only gold of coarse- and medium-grained classes is recovered while the fine-grained and dispersed gold are disposed into tailings. During the sedimentation, such a floating gold is removed far from the wave-surf zone. Despite the common belief about poor prospects of the Black Sea shelf for modern gold placers, we have proved the expediency to study the distribution of floating microscopic gold in Holocene marine sediments and carried out the respective works. Using the special concentrating methods, we enabled to detect the gold in most of the 830 samples collected. Geomorphological, lithological, hydrodynamic, and other factors controlling the gold potential were determined. In some cases, the gold content exceeds the minimal economic grade in continental placers. The prospective sites for further investigation were outlined. It was established that the polygenic microscopic gold can be divided into at least clastic, authigenic, and clastic-authigenic types. According to our data, the alluvial, lagoonal-marine, liman, and other sediments at the adjacent land also contain substantial amounts of microscopic gold. The pre10987nary study of oceanic bottom sediments near the Antarctic Peninsula and within the Argentine Basin proved the possibility of microscopic gold to accumulate under various facies conditions. The microscopic gold, mainly of clastic type, was detected here in 82% of samples. The obtained results testify to the global-scale deposition of floating microscopic gold in sedimentation basins of various age and may serve as a basis for the further comprehensive tackling of the problem in different regions.     

Sterols in interstitial waters of marine sediments

In order to attempt to elucidate the nature of biogeochemical processes occurring at the water-sediment interface, sterols have been analysed in near bottom sea and interstitial waters collected in the eastern and western intertropical Atlantic ocean. Free and esterified sterol concentrations range from 0.2 to 82 μg l^?1 and are much higher than those found in overlying sea water, which range from 0.2 to 1.7 μg l^?1 for the dissolved fraction and from 0.01 to 0.07 μg l^?1 for the particulate fraction. Cholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol are the dominant sterols in sea and interstitial waters. The variability encountered for the relative importance of minor sterols such as 24-methylcholesta-5,24(28)-dien-3β-ol and stanols, 5α-cholest-22(E)-en-3β-ol, 5α-cholestan-3β-ol and 24-ethyl-5α-cholestan-3β-ol in interstitial water and their variation with depth is discussed in terms of diversity of inputs and bacterial activity. For sediments cored off the Mauritanian coast, a productive area characterized by an intense upwelling, the chemical signatures observed in interstitial water through stanol/stenol ratios occur at levels of very high heterotrophic aerobic bacterial biomass estimations. The study of the sterol composition of interstitial water could constitute a valuable tool in appreciating the intensity of chemical and biological processes occurring in the first few metres of recent marine sediments.     

Vanadyl ions in ancient marine carbonaceous sediments

Vanadyl ions in ancient shaly-type sedimentary rocks of marine origin from a variety of world sources and geological periods have been investigated using electron spin resonance. These and other results provide evidence that there are two types of vanadyl ions. The first is inorganically bound in the clay/ silicate fraction of these rocks and the second type is associated with vanadyl porphyrin compounds.     

Sulfate reduction and methanogenesis in marine sediments

Methanogenesis and sulfate-reduction were followed in laboratory incubations of sediments taken from tropical seagrass beds. Methanogenesis and sulfate-reduction occurred simultaneously in sediments incubated under N₂, thereby indicating that the two processes are not mutually exclusive. Sediments incubated under an atmosphere of H₂ developed negative pressures due to the oxidation of H₂ by sulfate-respiring bacteria. H₂ also stimulated methanogenesis, but methanogenic bacteria could not compete for H₂ with the sulfate-respiring bacteria.     

Factors affecting dissolved organic matter dynamics in mixed-redox to anoxic coastal sediments

Mixed-redox (suboxic, or oscillating between oxidizing and reducing conditions) to anoxic marine sediments from the Raritan -New York Bay complex and the Inner New York Bight of the eastern U.S. were studied to investigate the factors controlling the accumulation of pore-water dissolved organic carbon (DOC). DOC increased with depth at each of four study sites, but accumulation was generally limited in the mixed-redox zone relative to the anoxic zone. Humic-like fluorescence intensity also differed between mixed-redox and anoxic zones of the sediment, such that anoxic pore waters were relatively enriched in fluorescent, humic-like compounds.A pore-water DOC model was tested for its capacity to explain these geochemical patterns. Model results for a heavily irrigated and a non-bioturbated site both suggest that, excluding the uppermost few cm of the sediment column, pore-water DOC is predominantly comprised of poorly-reactive material with estimated degradation rate constants on the order of 10⁻³ yr⁻¹. Model results are also consistent with the suggestion that DOC accumulation is suppressed in the mixed-redox compared to the anoxic zones of the sediment due to rapid oxidation of high-molecular-weight DOC, and limited production and enhanced oxidation of the less reactive polymeric low-molecular-weight component of the DOC pool. An assessment of sorptive behavior of DOC in the surface sediments of the study area suggests that sorption can influence pore-water DOC distributions under conditions of high sedimentation and low bioirrigation.