Silicate weathering in anoxic marine sediments

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm⁻³) may promote the dissolution of silicate phases through complexation of Al³⁺ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO₂ m⁻² year⁻¹) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO₂ m⁻² year⁻¹) suggesting a decrease in marine weathering over time. The production of CO₂ through reverse weathering in surface sediments (4.22-15.0 mmol CO₂ m⁻² year⁻¹) is about one order of magnitude smaller than the weathering-induced CO₂ consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO₂ consumption through marine silicate weathering estimated here as 5-20 Tmol CO₂ year⁻¹ is as high as the global rate of continental silicate weathering.     

Plutonium in sediments—apparent half-lives

The sediment surface is the boundary over which exposure from contaminated sediments takes place. It is therefore important to find simple and generalizable tools for the prediction of how long discharged hazardous substances remain at this boundary before being buried by sedimentation and mixing processes. In this article, a risk-assessment model is presented. The model is based on the hypothesis that the reduction of plutonium activities from sediment surfaces can be approximated as first-order reactions. From this approximation, apparent half-lives in the range 7–20 years, for three different types of Baltic sediment are estimated.     

CaCO3 solubility in marine sediments: Evidence for equilibrium and non-equilibrium behavior

Studies of the ion concentration product (ICP) [Ca²⁺] × [CO₃^2?]in the pore waters of marine sediments have been carried out using in situ sampling techniques. For sediments near the sedimentwater interface the ICP of the pore waters is indistinguishable from that predicted from laboratory studies of calcite solubility at 1 atm. The pressure dependence of calcite solubility is also consistent with laboratory studies.At depths greater than ten to fifteen cm enhanced solubility is almost always seen. In some instances ICP gradients occur in the upper 15 cm. The ICP increases indicate that calcite solubility cannot be adequately described by a single thermodynamic equilibrium constant.     

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。     

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:

     

Polyphosphates as a source of enhanced P fluxes in marine sediments overlain by anoxic waters: Evidence from <Superscript>31</Superscript>P NMR

Sedimentary phosphorus (P) composition was investigated in Effingham Inlet, a fjord located on the west coast of Vancouver Island in Barkley Sound. Solid-state ³¹P nuclear magnetic resonance (NMR) spectroscopy was applied to demineralized sediment samples from sites overlain by oxic and anoxic bottom waters. The two sites were similar in terms of key diagenetic parameters, including the mass accumulation rate, integrated sulfate reduction rate, and bulk sediment organic carbon content. In contrast, P benthic fluxes were much higher at the anoxic site. ³¹P NMR results show that P esters and phosphonates are the major organic P species present at the surface and at depth in sediments at both sites. Polyphosphates were only found in the surface sediment of the site overlain by oxic waters. The varying stability of polyphosphates in microorganisms under different redox conditions may, in part, explain their distribution as well as differences in P flux between the two sites.     

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.     

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.     

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.     

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.     

Methane production from bicarbonate and acetate in an anoxic marine sediment

Methane production from ¹⁴C-labelled bicarbonate and acetate was measured over the top 28 cm of anoxic Cape Lookout Bight sediments during the summer of 1983. The depth distribution and magnitude of summed radioisotopically determined rates compare well with previous measurements of total methane production and the sediment-water methane flux. Methane production from CO₂ reduction and acetate fermentation accounts for greater than 80% of the total production rate and sediment-water flux.Methane production from bicarbonate was found to occur in all depth intervals sampled except those in the top 2 cm, whereas significant methane production from acetate only occurred at depths below 10 cm where sulfate was exhausted. Acetate provided 20 to 29% of the measured methane production integrated over the top 30 cm of the sediments.     

藏南白垩纪缺氧与富氧沉积的稀土元素地球化学特征

研究了藏南Cenomanian-Turonian界线缺氧沉积和Santonian-Campanian富氧沉积的稀土元素特征,重点探讨古海洋氧化还原条件变化与稀土元素地球化学之间的关系。结果表明,缺氧事件层ΣREE增加30%,Ce和Eu含量分别增加40%和114%;富氧沉积层ΣREE明显增加,其中以Eu含量增加107%最为明显,Ce增幅最小,表现为相对亏损。缺氧沉积稀土元素分布型式为Ce和Eu富集的右倾型,富氧沉积分布型式以Ce亏损和Eu富集为特征。Ce异常显示缺氧事件期间为稳定而持续的低溶解氧还原条件,而富氧事件由古海洋瞬间的氧化事件和逐渐趋于正常状态的氧化条件两个阶段组成。     

New constraints on the provenance of hopanoids in the marine geologic record: Bacteriohopanepolyols in marine suboxic and anoxic environments

The abundance and structural diversity of bacteriohopanepolyols (BHPs) was examined in three marine pelagic environments that are characterized by strong vertical redox gradients and water column suboxia or anoxia. The abundance and, in most instances, structural diversity of BHPs was highest at depths where conditions were suboxic or anoxic. However, the majority of the BHP structures that were identified are environmentally cosmopolitan and their biological sources are presently not well constrained. An isomer of bacteriohopanetetrol (denoted BHT II) was observed at all three study sites in association with anoxic and suboxic conditions within the water column. Based on the absence of BHT II from terrigenous and oxic marine environments studied to date, and its strong association with suboxic and anoxic marine pelagic environments, we propose that BHT II is a promising candidate biomarker for water column suboxia and anoxia in the marine geologic record. The molecular fingerprint of BHPs in suspended and sinking particles and core-top sediments indicates that hopanoids produced within the water column are exported to marine sediments and that their biological source is most likely associated with settling particles and not the free-water phase. Based on our observations, BHPs likely represent an important input to the sedimentary hopanoid inventory, particularly in upwelling environments characterized by pelagic oxygen minimum zones (OMZ) and anoxic marine basins.     

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.