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1.
早中新世以来南海北部陆坡古生产力的碳酸盐和生物成因Ba元素记录 总被引：2，自引：0，他引：2
通过对南海北部ODP1148站岩芯454．8mcd以上（约23．2Ma以来）的175个沉积物样品中CaCO3以及生物成因Ba含量变化的研究,并结合初期对ODP1148站岩芯分析的一些相关地球化学组成结果,探讨了碳酸盐、生物成因Ba作为海洋古生产力指标的影响因素,以及早中新世以来南海北部陆坡古生产力的变化特征。结果显示,除中新世早期（23～21．4Ma）以外,生物成因Ba与CaCO3含量和沉积速率变化有很好的相关性,反映南海北部古生产力变化波动较大。对钻孔中生物成因Ba的研究表明,在沉积埋葬以及埋葬后的保存过程中（特别是在320mcd以下）。生物成因Ba会在少氧或缺氧环境下发生还原反应而迁移,影响了它在古环境研究当中的作用。另外,结合钻孔中CaCO3含量变化的数据,在南海北部陆坡,CaCO3的溶解作用并不是很强,其沉积后保存良好,可作为研究南海北部古生产力的基础性指标。 相似文献
2.
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 总被引：2，自引：0，他引：2
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。 相似文献
3.
沉积物微量金属元素在重建水体环境变化中的意义 总被引：7，自引：0，他引：7
沉积物所记录的微量金属含量与形态的变化是指示人类活动影响下水体环境变化的有效指标,主要用于指示沉积物重金属污染、水体初级生产力变化和氧化还原条件等方面的水体环境状况。总体而言,沉积物中微量金属含量在近一个世纪以来显著上升,反映了采矿、冶金、污水排放、化肥使用、煤炭和石油燃烧等各种人类活动造成水体和沉积物重金属污染的记录作为浮游植物微量营养元素,Cu、Zn、Ni、Ba、Cd等在沉积物中的记录可以指示水体初级生产力水平。U、Mo、V、Cu、Cd、Mn等氧化还原敏感元素在沉积物中的富集或贫化,及其比值(如Re/Mo、Cd/U、Th/U和V/Sc)的变化,是指示水体和沉积物氧化还原环境的有效指标。但需要指出的是,在受人类活动影响的水体中,这些生产力和氧化还原指标很少能指示水体生产力或氧化还原状况,可能主要与人类活动同时造成这些金属元素大量污染输入而掩盖了其自生来源和内在变化的沉积记录有关。所以,对沉积物中微量金属元素来源的判别(陆源碎屑输入、人为输入和水体自生来源)是重建水体环境变化的重要前提。本文总结了多种化学和统计学方法(包括同位素示踪法、化学提取法、富集因子法和主成分分析法等)在沉积物金属来源判别中的应用另外,成岩作用等多种因素会干扰沉积物金属记录对环境变化的指示作用,所以构建多元素指标来综合判断沉积物记录所反映的环境信息是今后的研究所必须关注的 相似文献
4.
长江口及其邻近海域表层沉积物中氧化还原敏感性微量元素的环境指示意义 总被引：5，自引：2，他引：3
对长江口及其邻近海域表层沉积物和底层悬浮体中氧化还原敏感元素分布规律和富集特征进行了分析与研究。结果表明氧化还原敏感元素在研究区具有明显的“离岸富集”特征,去除粒度效应、陆源碎屑来源组分和有机质的吸附作用等因素的影响之后,氧化还原敏感元素仍显示出在缺氧区的富集。通过同一站位底层悬浮体和沉积物中氧化还原敏感元素含量的分析比较,发现底层水缺氧是导致氧化还原敏感元素Mo、Cd、V等在沉积物中富集的主要原因。Mo、Cd、V等元素的不同富集程度可用来反映缺氧区的缺氧程度。因此,Mo, Cd, V等RSE在长江口外缺氧区及其邻近海域具有氧化还原环境指示意义,可以指示长江口外缺氧区的存在与大体范围,并可在一定程度上用来衡量缺氧区的缺氧程度。U理论上虽然也对环境的氧化还原条件敏感,但由于受陆源碎屑来源组分的影响较大,在长江口外缺氧区的富集并不明显,因此U在研究区不具有氧化还原环境指示意义。 相似文献
5.
NS93 5钻孔沉积物高分辨率过渡金属元素变化及其古海洋记录 总被引：6，自引：0，他引：6
利用电感耦合等离子体质谱（ICP－MS）法分析了取自南沙南部陆坡的NS93－5钻孔沉积物的部分过渡金属元素含量的变化。结果表明，代表碎屑物质的量的Ti含量在不同气候时期变化明显，其中在氧同位素第4期碎屑比例最大，第2期和第3期末期，碎屑所占比例比较高，代表较大的碎屑输入通量。但与沉积速率相比较发现，NS93－5的沉积速率并不仅仅决定于碎屑的输入，自生沉积物的累积作用也极为明显。Mn、Mo、Cr、V、U、Cd、Co、Ni、Cu和Zn等元素含量变化表明，NS93－5钻孔沉积物中还原状态极低。反映出自过去15ka以来该区陆坡深度底层海水均富含氧气，并没有出现无氧状态，意味着南沙海域底层海水更新一直都比较快。 相似文献
6.
氧化还原敏感微量元素对古海洋沉积环境的指示意义 总被引：32，自引：1，他引：31
海洋的氧化还原条件控制着U、V和Mo等氧化还原敏感微量元素在沉积物或沉积岩中的富集程度,所以可以利用沉积物或沉积岩中这些微量元素的含量来重建古海洋的氧化还原状态和沉积环境。海洋沉积物或沉积岩中U、V和Mo的来源少,除了海水提供外,还有陆源碎屑,不过陆源碎屑提供的那部分含量能够被估计。沉积物埋藏之后这些元素（几乎）不发生迁移,保存了沉积时的组成和含量,能较好地反映沉积环境的特征,是古海洋环境的理想指示。在氧化—次氧化的海水环境中U、V和Mo不会富集,缺氧的条件下U和V富集,而在硫化（含溶解的硫化氢）的沉积环境中U、V和Mo在沉积物中都会强烈地富集。利用这种差异,可以根据沉积物或沉积岩中各种微量元素的含量变化来判别沉积时海水的氧化还原状态。但是,必须注意沉积环境的开放与否、成岩作用及重新氧化作用等对微量元素含量的影响。在利用氧化还原敏感元素进行古环境重建时,应首先评估沉积物或沉积岩中特定元素的来源和贡献,剔除非自生部分的影响,然后利用多元素指标综合判别古环境的氧化还原条件,才能获得可靠的结论。 相似文献
7.
氧化还原敏感微量元素 Re、Mo 和 U 主要依靠扩散作用通过沉积物—水界面,在不同氧化还原条件下的沉积物中自生富集,Re在轻度还原的次氧化沉积环境中富集,Mo在还原性更强的硫化环境中富集,而 U 具有较宽的富集沉积深度区间。Re、Mo和 U 独特的地球化学行为使其可用于指示海洋环境的氧化还原状态,其在沉积物中的自生富集程度与沉积时所处的氧化还原条件具有良好的相关性：Re、Mo 和 U 在氧化沉积环境(Re/Al<1.3×10-7,Mo/Al<0.4×10-4)和季节性缺氧区覆盖的沉积环境中富集程度较小,在常年性缺氧区覆盖的沉积环境(U/Al>5×10-4,Mo/Al>5×10-4)和硫化沉积环境(Mo/Al>5×10-4)中富集程度较大。除依据其地球化学行为特征和相对富集程度进行定性分析之外,还可以结合元素富集系数(TMEF<1 表示亏损,TMEF>1 表示富集,TMEF 相似文献
8.
安徽石台地区下寒武统黑色岩系微量元素地球化学特征 总被引：1，自引：1，他引：0
黑色岩系成因复杂,且与多金属元素成矿密切相关。对安徽石台地区下寒武统黑色岩系微量元素地球化学研究表明,其富集Ag、V、U、Mo、As、Sb等多种金属元素。且在这套黑色岩系地层剖面中,其中部碳质页岩的金属元素富集程度比其它岩性的富集程度明显偏高。微量元素组成及相关元素比值显示这套黑色岩系形成于海相热水沉积作用,代表了一种非正常海水沉积。沉积环境由早期缺氧的还原环境逐渐向后期的半还原-氧化环境演化,反映早寒武世安徽下扬子海由海侵向海退的逐渐过渡。 相似文献
9.
海相沉积氧化还原环境的地球化学识别指标 总被引：6，自引：0，他引：6
全球海洋在10-5.4亿年间演变成氧化环境,此后历经多次全球性的缺氧事件后演变到现在的氧化环境.海水和沉积物中多种无素的循环、分异和富集明显受氧化还原条件的影响.Mn、Mo、Cr、V和U等变价元素的溶解度随氧化还原条件的改变产生极大变化,导致沉积物中的元素含量分异:Ni、Co、Cu和Zn等在还原条件下形成硫化物沉淀,导致沉积物中对应元素的富集.这些元素的地球化学行为是古海洋氧化还原条件的灵敏指示剂,作为恢复古海洋氧化还原条件环境变化的地球化学指标.黄铁矿化程度(DOP)、生物樗化合物和Ce异常等也是沉积物氧化还原条件的常用判别指标.泥岩石研究中通常采用DOP、U/Th、自生U、V/Cr、Ni/Co和生物标志化合物等指标,碳酸盐岩则主要采用Ce异常指标.当前各种指标的定性分析都取得比较一致的结果,但是用一种或几种定量的地球化学指标来恢复整个古海洋的氧化还原环境目前还有很大的问题. 相似文献
10.
基于金属元素Mn、Co、Cd、Mo的海洋富有机质沉积岩沉积环境和有机质富集主控因素判别指标Al—Co·Mn和Co·Mn—Cd/Mo,能够有效地区分促进有机碳埋藏的两种海洋端元沉积环境:大陆边缘开放/上升洋流环境和滞留的边缘海盆环境。利用这种方法研究了四川盆地北缘城口县月亮坪剖面五峰组和龙马溪组页岩沉积时期的水体环境和有机质富集的主控因素。研究表明五峰组Mn、Co富集程度低,Mo富集程度自下而上明显增大;龙马溪组Mn整体亏损,Co富集程度低,Mo富集程度自下而上明显减小。Al—Co·Mn方法的使用发现滞留环境主要出现在五峰组早期和龙马溪组晚期,以及观音桥段附近,而五峰组中晚期和龙马溪组早—中期水体主要处于开放/上升洋流区。剖面[w(Co)/(μg/g)]·[w(Mn)/(%)]值均小于平均页岩值1.615;五峰组Cd/Mo值整体大于正常海水值0.006,全部小于平均页岩值0.308,龙马溪组整体小于正常海水值。Co·Mn—Cd/Mo联合图版的使用发现月亮坪剖面五峰组和龙马溪组有机质富集主要受到保存条件控制,与使用U/Th、V/Cr、P/Ti、Ba/Al与TOC相关性分析得出的结论一致。基于金属元素Mn、Co、Cd、Mo的判别指标对于判断古生代大陆边缘地区富有机质页岩沉积环境及有机质富集主控因素有较好的效果。 相似文献
11.
We analyzed Al, Ti, Fe, Mn, Cu, Ba, Cd, U, Mo, V, and Re in water column, settling particulate, and sediment (0 to 22 cm) samples from the intense oxygen minimum zone (OMZ) of the eastern tropical North Pacific near Mazatlán, Mexico. The goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique “suboxic” geochemical signature.The water column was characterized by a Mn maximum, reaching ∼8 nmol kg−1 at 400 m. Concentrations of Cu, Ba, Cd, Mo, Re, U, and V were unaffected by the low O2 conditions and were comparable to those of the open ocean. Sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin. Al, Ti, and Fe were mostly (at least 79%) lithogenic. About 75% of the Mn was nonlithogenic. Significant amounts (at least 58%) of Cu, Ba, Cd, and Mo were nonlithogenic.Sediment geochemistry varied across the continental shelf and slope. Cadmium, U, and Re have prominent maxima centered at 310 m, with 12.3 ppm, 10.9 ppm, and 68.3 ppb, respectively, at the core top. High values of Mo (averaging 6.8 ppm) and V (averaging 90 ppm) are seen in OMZ surface sediment. Additional down-core enrichment occurs for all redox-sensitive elements in the top 10 cm. For U, Mo, V, and Re, surface sediments are a poor indicator of metal enrichment. Comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition, especially for Cd, U, and Mo.We evaluated Re/Mo and Cd/U ratios as tracers for redox environments. Rhenium and Mo concentrations and Re/Mo ratios do not lead to consistent conclusions. Concurrent enrichments of Re and Mo are an indicator of an anoxic depositional environment. In contrast, high Re/Mo ratios are an indicator of suboxic conditions. Cadmium is enriched in surface sediments, while U has considerable down-core enrichment. The concentrations of Cd and U and the Cd/U ratio do not follow patterns predicted from thermodynamics. Though the water column is suboxic, these four redox-sensitive elements indicate that the sediments are anoxic. The implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column. 相似文献
12.
Geochemical and anthropogenic enrichments of Mo in sediments from perennially oxic and seasonally anoxic lakes in Eastern Canada 总被引：1，自引：0，他引：1
We measured the vertical distributions of Mo, Fe, Mn, sulfide, sulfate, organic carbon, major ions, and pH in sediment porewater from one perennially oxic and three seasonally anoxic lacustrine basins in Eastern Canada, as well as those of Mo, acid volatile sulfide, Fe, Mn, Al, organic C, 210Pb and 137Cs in sediment cores from the same sites. The only input of anthropogenic Mo to these lakes comes from atmospheric deposition.The relatively monotonous distribution of Mo in the porewater of three seasonally anoxic basins suggests that Mo is not redistributed in the sedimentary column during periods of anoxia. In contrast, porewater Mo profiles obtained at three sampling dates in a perennially oxic basin display sharp Mo peaks below the sediment-water interface, indicating redistribution subsequent to deposition. Modeling of these latter porewater Mo profiles with a diagenetic reaction-transport equation coupled to comparisons among the various porewater and solid phase profiles reveal that Mo is released at 1-2 cm depth as a consequence of the reductive dissolution of Fe oxyhydroxides and scavenged both at the vicinity of the sediment-water interface, by re-adsorption onto authigenic Fe oxyhydroxides, and deeper in the sediments where dissolved sulfide concentrations are higher. The estimated rate constant for the adsorption of Mo onto Fe oxyhydroxides is 36 ± 45 cm3 mol−1 s−1.Diagenetic modeling indicates that authigenic Mo in sediments of the perennially oxic basin represents about one-third of the total solid phase Mo in the first cm below the sediment-water interface and only one tenth below this horizon. If we assume that no authigenic Mo is accumulated in the seasonally anoxic lake sediments we conclude that the sediment Mo concentrations, which are up to 3-16 times higher than the average lithogenic composition, depending on the lake, are mainly due to atmospheric deposition of anthropogenic Mo and not to the formation of authigenic Mo phases. Reconstructed historical records of the atmospheric Mo deposition indicate maximum values in the 1970s and 1980s and significant decreases since then. Emissions to the atmosphere associated with the smelting of non-ferrous ores and coal combustion appear to be the most important sources of anthropogenic Mo. 相似文献
13.
G. N. Baturin 《Lithology and Mineral Resources》2002,37(5):412-428
The behavior of molybdenum and manganese is studied in phosphorite samples from shelves, seamounts, and islands of the ocean. In shelf phosphorites, molybdenum and manganese contents are 2–128 and 12–1915 ppm, respectively, while the Mo/Mn ratio ranges from 0.004 to 4.5. Phosphorites from oceanic seamounts impregnated with ferromanganese oxyhydroxides contain 0.84–14.5 ppm of Mo and 0.1–17% of Mn. The Mo/Mn ratio ranges within 0.0008–0.004. Phosphate-bearing ferromanganese crusts overlying the seamount phosphorites contain 54–798 ppm of Mo and 10–20% of Mn; Mo/Mn ratio varies within 0.002–0.005. Corresponding values for most island phosphorites are 0.44–11.2 ppm, 27–287 ppm, and 0.008–0.20, respectively. Phosphorites from reduced environments are characterized by a relative enrichment in Mo and depletion in Mn, whereas the Mo/Mn ratio reaches maximum values. The ratio decreases with transition to suboxic and oxic conditions. Molybdenum content in recent shelf sediments is commonly higher than that in authigenic phosphorites from these sediments. Recent phosphorite nodules from the Namibian shelf become depleted in Mo and Mn during their lithification, but Pliocene–Pleistocene nodules of similar composition and origin from the same region are enriched in Mo and characterized by a variable Mn content. The higher Mo content in phosphate-bearing ferromanganese crusts is a result of coprecipitation of Mo and Mn from seawater. Nonweathered phosphorites on continents and phosphorites from oceanic shelves are largely enriched in Mo with the Mo/Mn ratio ranging from 0.01 to 1.0. This is an evidence of their formation in reducing conditions. 相似文献
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15.
Alfonso Mucci Louis-Filip Richard Marc Lucotte Constance Guignard 《Aquatic Geochemistry》2000,6(3):293-324
The distribution and partitioning of dissolved andparticulate arsenic and phosphorus in the water columnand sediments of the Saguenay Fjord in Quebec, Canada,are compared. In addition, selective and/or sequentialextractions were carried out on the suspendedparticulate matter (SPM) and solid sediments tocontrast their geochemical behaviors in this naturalaquatic system.Results of our analyses show that both arsenic andsoluble reactive phosphate are actively scavenged fromthe water column by settling particles. Upon theiraccumulation at the sediment-water interface some Asand P may be released to porewaters following thedegradation of organic matter to which they areassociated. The porewater concentrations are, however,limited by their strong affinity for authigenic,amorphous iron oxyhydroxides which accumulate in theoxic sediments near the sediment-water interface.The geochemical behavior of arsenic and phosphorusdiverge most strikingly upon the development of anoxicconditions in the sediments. Following their burial inthe anoxic zone, amorphous iron oxyhydroxides arereduced and dissolved, releasing phosphate and arsenicto the porewaters. We observed, however, thatporewater arsenic concentrations increase at shallowerdepths than phosphate in the sediments. The reductionof arsenate, As(V), to arsenite, As(III), and itsdesorption prior to the reductive dissolution of thecarrier phase(s) may explain this observation.Driven by the strong concentration gradientestablished in the suboxic zone, phosphate diffuses uptowards the oxic layer where it is readsorbed byauthigenic iron oxyhydroxides. In the organic-rich andrapidly accumulating sediments at the head of theFjord, porewater sulfate depletion and the resultingabsence of a sulfide sink for Fe(II), may lead to theformation of vivianite in the fermentation zone, apotential sink for phosphate. Arsenite released to theporewaters in the suboxic and anoxic zones of thesediments diffuses either down, where it is adsorbedto or incorporated with authigenic iron sulfides, orup towards the oxic boundary. Arsenite appears tomigrate well into the oxic zone where it may beoxidized by authigenic manganese oxides before beingadsorbed by iron oxyhydroxides present at the samedepth. Whereas, in the absence of authigenic carbonatefluorapatite precipitation, the ability of oxicsediments to retain mineralized phosphate is afunction of their amorphous iron oxyhydroxide content,arsenic retention may depend on the availability ofmanganese oxides, the thickness of the oxic layer and,its co-precipitation with iron sulfides at depth. 相似文献
16.
Early diagenesis of redox-sensitive trace metals in the Peru upwelling area - response to ENSO-related oxygen fluctuations in the water column 总被引：1，自引：0，他引：1
Florian Scholz Christian Hensen Anna Noffke Anne Rohde Volker Liebetrau Klaus Wallmann 《Geochimica et cosmochimica acta》2011,75(22):7257-7276
Pore water and solid phase data for redox-sensitive metals (Mn, Fe, V, Mo and U) were collected on a transect across the Peru upwelling area (11°S) at water depths between 78 and 2025 m and bottom water oxygen concentrations ranging from ∼0 to 93 μM. By comparing authigenic mass accumulation rates and diffusive benthic fluxes, we evaluate the respective mechanisms of trace metal accumulation, retention and remobilization across the oxygen minimum zone (OMZ) and with respect to oxygen fluctuations in the water column related to the El Niño Southern Oscillation (ENSO).Sediments within the permanent OMZ are characterized by diffusive uptake and authigenic fixation of U, V and Mo as well as diffusive loss of Mn and Fe across the benthic boundary. Some of the dissolved Mn and Fe in the water column re-precipitate at the oxycline and shuttle particle-reactive trace metals to the sediment surface at the lower and upper boundary of the OMZ. At the lower boundary, pore waters are not sufficiently sulfidic as to enable an efficient authigenic V and Mo fixation. As a consequence, sediments below the OMZ are preferentially enriched in U which is delivered via both in situ precipitation and lateral supply of U-rich phosphorites from further upslope. Trace metal cycling on the Peruvian shelf is strongly affected by ENSO-related oxygen fluctuations in bottom water. During periods of shelf oxygenation, surface sediments receive particulate V and Mo with metal (oxyhydr)oxides that derive from both terrigenous sources and precipitation at the retreating oxycline. After the recurrence of anoxic conditions, metal (oxyhydr)oxides are reductively dissolved and the hereby liberated V and Mo are authigenically removed. This alternation between supply of particle-reactive trace metals during oxic periods and fixation during anoxic periods leads to a preferential accumulation of V and Mo compared to U on the Peruvian shelf. The decoupling of V, Mo and U accumulation is further accentuated by the varying susceptibility to re-oxidation of the different authigenic metal phases. While authigenic U and V are readily re-oxidized and recycled during periods of shelf oxygenation, the sequestration of Mo by authigenic pyrite is favored by the transient occurrence of oxidizing conditions.Our findings reveal that redox-sensitive trace metals respond in specific manner to short-term oxygen fluctuations in the water column. The relative enrichment patterns identified might be useful for the reconstruction of past OMZ extension and large-scale redox oscillations in the geological record. 相似文献
17.
《Geochimica et cosmochimica acta》1999,63(11-12):1735-1750
We analyzed the redox sensitive elements V, Mo, U, Re and Cd in surface sediments from the Northwest African margin, the U.S. Northwest margin and the Arabian Sea to determine their response under a range of redox conditions. Where oxygen penetrates 1 cm or less into the sediments, Mo and V diffuse to the overlying water as Mn is reduced and remobilized. Authigenic enrichments of U, Re and Cd are evident under these redox conditions. With the onset of sulfate reduction, all of the metals accumulate authigenically with Re being by far the most enriched. General trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes: oxic, reducing where oxygen penetrates ≤1 cm, and anoxic conditions. Using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations, we calculate the area of sediments below 1000 m water depth in which oxygen penetration is ≤1 cm to be 4% of the ocean floor. We conclude that sediments where oxygen penetrates ≤1 cm release Mn, V and Mo to seawater at rates of 140%–260%, 60%–150% and 5%–10% of their respective riverine fluxes, using the authigenic metal concentrations and accumulation rates from this work and other literature. These sediments are sinks for Re, Cd and U, with burial fluxes of 70%–140%, 30%–80% and 20%–40%, respectively, of their dissolved riverine inputs. We modeled the sensitivity of the response of seawater Re, Cd and V concentrations to changes in the area of reducing sediments where oxygen penetrates ≤1 cm. Our analysis suggests a negligible change in seawater Re concentration, whereas seawater concentrations of Cd and V could have decreased and increased, respectively, by 5%–10% over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10%–20% if the area increased by a factor of 3. The concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of Cd/Ca and V/Ca ratios observed in foraminifera shells over the last 40 kyr. This implies that the area of reducing sediments in the ocean deeper than 1000 m (4%) has not been greater than twice the present value in the recent past. 相似文献
18.
A. N. Derkachev N. A. Nikolaeva A. V. Mozherovsky T. N. Grigor’eva E. D. Ivanova S. P. Pletnev N. N. Barinov V. M. Chubarov 《Russian Journal of Pacific Geology》2007,1(3):203-229
This paper reports specific mineralogical and geochemical characteristics of deposits from the local depressions of the Derugin Basin. They were formed in an environment with periodic changes from oxic to anoxic conditions and show evidence for the presence of hydrogen sulfide in bottom waters. The deposits of this type can be considered as a modern model for ancient ore-bearing black shale associations. Compared with typical metalliferous black shale sequences, which are characterized by high contents of organic matter, the sediments described here are depleted in the elements of the organophilic association (Mo, Ni, Cu, Zn, V, and U) but have higher Mn contents. 相似文献