Nannostructure and significance of fish coprolites in phosphorites

Phosphatic coprolites (0.5–2 cm long) occurring in Cretaceous-Eocene phosphorites of Tunisia, Morocco, Mauritania, Senegal, and Egypt were investigated for their petrology by using SEM. They exhibit a homogeneous porous apatite structure with a few fish remains. The nannostructures of the coprolites consist of inframicron-sized and botryoid-type apatite microparticles. Spherical cavities and cavity-infill cavity-infilled inframicron-sized apatite globules are typical in the coprolites and are apparently formed by extracellular precipitation of phosphate around microbial organisms. We presume that the coprolites studied here may belong to fishes, whose excrements contain abundant organic matter and phosphate. Phosphatization of excrements appears to be a microbial process controlled by the microenvironment.

The nannostructures observed in the coprolites investigated here differ significantly from the nannostructures of other studied phosphatic grains (pellets and coated grains) in the same samples which are mostly heterogeneous and consist of ovoid-type apatite particles. In the case they are not reworked, the centimetre to decimetre thick coprolite beds in phosphorites indicate a lack of detrital input and strong bottom water currents at the sediment-water interface during deposition and subsequent phosphatization of the excrements.     

Petrology and major element geochemistry of Peru margin phosphorites and associated diagenetic minerals: Authigenesis in modern organic-rich sediments

New petrographic and major element geochemical data from modern Peru margin upper slope-outer shelf phosphorites are presented, which provide insight into their origin and paragenetic relationship with other authigenic minerals (glauconite, pyrite and dolomite) occurring in organic-rich sediments. Glauconites are precipitated relatively early following the partial reduction of ferric iron and, following this process, phosphate, pyrite, and then dolomite precipitation take place at progressively deeper levels in the sediment in association with microbial reduction of sulfate. As in many ancient economic phosphorite deposits, the phosphatic facies here consist of nodules, crusts, coatings and strata composed of phosphatic pelletal grains (ooids, structureless grains, intraclasts, clumps and biogenic grains) in association with organic-rich biosiliceous sediments. All are considered to have formed within a few centimeters or within a few tens of centimeters below the sediment-water interface. Important factors that influence which morphology will tend to develop include the amount of available pore space, the presence of suitable nucleation sites, the amount and size of siliciclastic detritus incorporated as inclusions and the relative solution chemistries of the precipitating solutions. Bacterial mediation may play an important, but as yet unspecified role in the precipitation process. Textural data and factor analysis of chemical data suggest that structureless pellets are relatively inclusion-free Na-F-Mg-CO₃-substituted pore-water precipitates whereas ooids are inclusion-rich pore-water precipitates poor in lattice-substituted components. Variations in nodular cement birefringence and crystallinity are suggested to have been produced by similar lattice substitutions that directly reflect pore-water carbonate ion concentrations and thus relative degrees of organic-matter degradation. Phosphate and dolomite are intimately mixed, yet mineralogically distinct phases in phosphatized dolomicrites.

Depth-stratified threshold carbonate ion concentrations may control the lower limit at which phosphatic minerals may precipitate. Below depths of a few centimeters, excessive carbonate ion concentrations and diminished reactive iron and sulfate concentrations favor the development of dolomite while precluding further development of phosphatic minerals and pyrite. Periodic sediment reorganization (bioturbation, current winnowing and erosion, mass wasting, etc.) plays an important role in both concentrating pelletal grains and maintaining nodules and crusts at critical depth levels in the sediment, as well as mixing ordered mineral parageneses into complicated sequences.     

Apatite and associated minerals in ferromanganese crusts from the Magellan Seamounts

The phosphate fraction extracted from a combined technological sample of ferromanganese crusts obtained from the Magellan Seamounts (Northwest Pacific) was studied with analytical electron microscopic methods. The material examined is represented by small rock fragments composed of well crystallized calcium phosphate with pores filled in with Mn oxides consisting largely of vernadite. The rock hosts crystallized barite aggregates, disseminated ultramicroscopic cubic pyrite crystals, filamentous halloysite aggregates, and assemblages of finely dispersed REE minerals represented by cerianite and parisite, which incrust the surface of apatite or are dispersed in the phosphatic matrix. The presence of the latter minerals shows that some REE related to the phosphatic phase do not enter the apatite lattice, which might influence the general pattern of the REE distribution in bulk samples     

海洋沉积物不同相态中Sr、Nd同位素提取方法研究

海洋沉积物中Fe-Mn氧化物相和残渣态的Sr、Nd同位素组成能够敏感地指示洋流循环及物质来源,但实验室对沉积物中上述相态的Sr和Nd同位素的提取易产生过量或提取不完全,进而影响同位素测定结果的准确性,因此制定有效的提取流程显得非常重要。本文采用不同浓度盐酸羟胺(Hydroxylamine Hydrochloride,HH)与醋酸(Acetic Acid,HAc)混合溶液对中印度洋海盆深海沸石黏土、北极半深海沉积物以及安达曼海近海沉积物的Fe-Mn氧化物相进行提取,残渣态用HNO3-HF高压密闭消解法溶融,测定了各相态的主微量元素含量及Sr、Nd同位素组成,通过分析不同实验条件下得到的Fe-Mn氧化物相与残渣态的稀土元素(REE)配分模式、Al/Nd含量比值及Sr、Nd同位素组成,建立了3种不同成因类型海洋沉积物不同相态的化学提取方法。提取深海沸石黏土中Fe-Mn氧化物相的理想试剂条件为0.25 mol/L HH和15%HAc,北极半深海沉积物和安达曼海近海沉积物的试剂条件为0.5 mol/L HH和15%HAc。该方法可以准确获得沉积物中Fe-Mn氧化物相与残渣态的Sr、Nd同位素组成信息,为古海洋学的研究提供方法支持。     

Ferromanganese Nodules and their Associated Sediments from the Central Indian Ocean Basin: Rare Earth Element Geochemistry

The rare earth element (REE) distribution in nine deep-sea ferromanganese nodules and their associated siliceous sediments from the Central Indian Ocean Basin (CIOB) have been studied to elucidate the REE relationship among them. Total REE concentration varies from 398-928 ppm in the nodules and 137-235 ppm in the associated sediments, suggesting two- to four-fold enrichment in the nodules compared to associated sediments. REE of nodules and their associated sediments show a positive correlation, suggesting REE are supplied from a common source such as seawater. The positive correlation between REE of nodules and sediments from the CIOB is contrary to the competitive scavenging of REE between nodules and sediment in the equatorial Pacific Ocean. REEs in the nodules are carried by Fe, P, and Ti, whereas in the sediment they are carried by P and Mn phases. A similar REE fractionation pattern with middle REE enrichment over heavy and light REE in both the nodules and their associated sediment suggest fractionation is independent of REE abundance and their carrier phases.     

Ferromanganese Nodules and their Associated Sediments from the Central Indian Ocean Basin: Rare Earth Element Geochemistry

Abstract

The rare earth element (REE) distribution in nine deep-sea ferromanganese nodules and their associated siliceous sediments from the Central Indian Ocean Basin (CIOB) have been studied to elucidate the REE relationship among them. Total REE concentration varies from 398–928 ppm in the nodules and 137–235 ppm in the associated sediments, suggesting two- to four-fold enrichment in the nodules compared to associated sediments. REE of nodules and their associated sediments show a positive correlation, suggesting REE are supplied from a common source such as seawater. The positive correlation between REE of nodules and sediments from the CIOB is contrary to the competitive scavenging of REE between nodules and sediment in the equatorial Pacific Ocean. REEs in the nodules are carried by Fe, P, and Ti, whereas in the sediment they are carried by P and Mn phases. A similar REE fractionation pattern with middle REE enrichment over heavy and light REE in both the nodules and their associated sediment suggest fractionation is independent of REE abundance and their carrier phases.     

Major,Trace, and Rare Earth Elements in the Sediments of the Central Indian Ocean Basin: Their Source and Distribution

Abstract

A large number of surface sediments as well as short sediment cores collected in the Central Indian Ocean Basin have been subjected to various geochemical investigations during the last one and half decade. The studies varied, covering different aspects of sediments and resulting in a number of publications. In the present article, we have put together the data from 82 surface sediments and 14 short sediment cores, including 25 new analyses, to study the trend of their distribution and source at large. The distribution maps of elements show that highest concentrations of Mn, Cu, Ni, Zn, Co, and biogenic opal in the surface sediment occurs between 10°S and 16°S latitude, where diagenetic ferromanganese nodules rich in Mn, Cu, Ni, and Zn are present. The studies highlight that the excess element concentration (detrital unsupported) such as Mn, Cu, Ba, Ni, Co, Pb, and Zn have contributed >80% of their respective bulk composition. These excess elements exhibit strong positive correlation with each other suggesting their association with a single authigenic phase such as Mn oxide. Biogenic opal contributes 30–50% of the total silica in the siliceous sediment. Aluminum, Fe, and K have contributed >60% from terrigenous detrital source compared to their bulk composition. In calcareous ooze, Ca, and Sr excess contribute >95% while, in siliceous ooze it is only 50% of their bulk composition. Nearly 35% of structurally unsupported Al in the sediment raises doubt of using Al as a terrigenous index element to normalize the trace and minor elements. Biogenic apatite is evident by the positive correlation between Ca (<1%) and P. Calcium, Sr, and P depict a common source such as biogenic. Bulk element concentration such as Li, V, Cr, Sc, and Zr are positively correlated with Ti indicating their terrigenous detrital source. Rare earth element (REE) concentration increases from calcareous ooze to siliceous ooze and reaches a maximum in the red clay. Presence of positive Eu-anomaly in these sediments has been attributed to aeolian input. REE in these sediments are mostly carried by authigenic phases such as manganese oxide and biogenic apatite. Based on the distribution of transition elements in the sediment cores, three distinct zones—oxic at top, suboxic at intermediate depth, and a subsurface maxima—have been identified. Oxic and suboxic zones are incidentally associated with high and low micronodule abundance in the coarse fraction (>63 μm) respectively. Ash layers encountered at intermediate depth between 10 to 35 cm are correlative with the Youngest Toba eruption of ～74ka from Northern Sumatra. This ash is mainly responsible for the high bulk Al/Ti ratio up to 48.5 (three times higher than Post Archean Australian Shale), other than scavenging of dissolved Al by biogenic components.     

印度洋钙质软泥和硅质软泥稀土元素组成和富集机制

本文针对采自印度洋深海中最常见的两类生物成因沉积物——钙质软泥和硅质软泥,开展了全岩样和不同粒级组分中常微量元素、稀土元素和Y（REY）含量的系统分析,探讨了两类沉积物中REY的组成特征、物质来源和富集机制。研究表明,钙质软泥以富含CaO和Sr为主要特征,硅质软泥则富集SiO₂、Al₂O₃、Fe₂O₃等。钙质软泥中∑REY平均含量为40.56×10-6,轻稀土元素（LREE）略有富集,REY有向细粒沉积物中富集的特征,PAAS标准化后全岩样和不同粒级组分均表现为Ce负异常、Eu和Y正异常;REY以自生来源为主,继承了海水的组成特征,同时也受到了热液流体物质和洋底玄武岩风化产物的影响。硅质软泥中∑REY的含量为248.54×10-6,LREE相对富集,REY在4φ以细的沉积物中富集;研究站位沉积物中∑REY含量处于边界品位附近,但在细粒级沉积物中重稀土元素（HREY）含量则达到了工业品位;该类沉积物细粒组分中REY主要来自陆源或火山碎屑组分中黏土矿物和铁锰氧化物吸附作用,粗粒组分中REY来源则主要与生物作用相关;硅质软泥中REY的富集与沉积物中磷灰石等矿物相关,部分不同来源的REY可能是在沉积之后的成岩过程中再次分配向磷灰石、钙十字沸石等矿物中富集。     

南海尖峰海山多金属结壳地球化学

南海尖峰海山多金属结壳富含30多种元素,其锰铁矿物主要由钡镁锰矿,δ-MnO_2和FeOOH·xH_2O组成。与其它海区的结壳相比,尖峰海山结壳富含Cu、Ni、Ba、Zn、Pb等元素,而Co、Ti、稀土元素(REE)、Sr等元素相对较贫。研究表明,HREE亏损,具明显的Ce正异常,较明显的Tb正异常和Yb负异常。这是氧化弱碱性海洋环境所致。结壳是水成作用的产物,它的形成受南海独特的古海洋环境所控制,海底火山热液作用,可能也是影响因素之一。     

Sediment geochemistry in coastal maritime Antarctica (Admiralty Bay, King George Island): Evidence from rare earths and other elements

We report the concentrations of 22 elements in short coastal sediment cores of Admiralty Bay (King George Island, Antarctica) determined by Neutron Activation Analysis. We focus the discussion on rare earth elements (REE) because their fractionation patterns in sediments relative to local sources may offer insights about weathering, transportation, deposition, and diagenetic processes. We found strong correlations between REE and Th, indicating detrital origin. Despite strong Eh gradients, the Th-normalized redox-sensitive elements (e.g., Eu, Ce, Fe, As, and others) showed little variability within the sedimentary environment. The exceptions were U (depleted in the upper few centimeters) and Br (enriched in the upper few centimeters). While U appears to be removed from seawater via uptake across the boundary of reducing sediments, the mechanisms driving Br accumulation are unclear, but perhaps related to increasing diatom production driven by regional warming. A comparison with published concentrations from rocks representing the regional eroding units showed that the characteristics of source rock could be recognized in the REE fractionation patterns in our sediments. These results imply no significant alteration during weathering and sediment transport in the coastal region of Admiralty Bay and the prevalence of strong periglacial erosion in ice-free areas of maritime Antarctica in spite of the relatively mild regional environmental setting (e.g., high moisture and high temperatures).     

Uranium-series and AMS C studies of modern phosphatic pellets from Peru shelf muds

Recent sediments and separated phosphate pellets ( 125–500 μm in diameter) from the Peru shelf have been analyzed for uranium decay-series isotopes and ¹⁴C in order to determine age relationships and mineralization rates. Uranium-series ages of pellets separated from one box core are significantly higher than AMS radiocarbon ages determined for the same pellets. These differences appear to be a consequence of mixing of an older generation of pellets with ones which are more recently formed. Postdepositional adsorption of reactive elements such as thorium and protactinium onto pellet surfaces may also contribute to the observed discordancy with radiocarbon ages.

Sediment radiocarbon and ²¹⁰Pb sediment results, as well as some trends in the uranium-series data, suggest that high concentrations of phosphate pellets have accumulated in some Peru shelf sediments without extensive reworking. Individual pellets apparently form very quickly, on time scales of a few years. Estimated authigenic uptake rates of phosphorus into pellets ranges from 0.5 to 9.40 μmol-P cm^{− 2} yr^{− 1}, somewhat higher than rates measured for nodules from the same area. This is consistent with observations that pelletal morphologies predominate over nodular forms within ancient phosphorite deposits.     

Early diagenesis of organic matter in Peru continental margin sediments: Phosphorite precipitation

Pore water chemistry (total dissolved CO₂, NH₄, NO₃, NO₂, PO₄, Si(OH)₄, Ca, Mg, Fe, Mn, SO₄, H₂S and F, and titration alkalinity), solid phase chemistry (C_org, P_org, C_TOT, N_TOT, F, Si_OPAL and S_II), and sediment characteristics (porosity, dry bulk density and formation factors) were determined on a centimeter-scale spacing in the upper 20–40 cm of sediments under intense upwelling areas on the Peru continental shelf. These data demonstrate that carbonate fluorapatite (CFA) is precipitating from pore waters in the upper few centimeters of a gelatinous mud with high organic carbon content (up to 20% C_org), very high porosity ( > 0.96 ml cm⁻³) and very low dry bulk density (< 0.1 g cm⁻³). Dissolved phosphate concentrations at the sediment-water interface range from 20 to 100 μM, orders of magnitude higher than bottom-water concentrations, and much higher than predicted from regeneration of organic matter. The mechanism of this interfacial phosphate release is unclear, but is apparently uncoupled from carbon and nitrogen metabolism and thus may be linked either to dissolution of fish debris or to the presence of a microbial mat in surficial sediments. Fluoride is incorporated into CFA by diffusion from the overlying seawater, and carbonate ions are provided from pore-water alkalinity. Magnesium concentrations in this reaction zone are not significantly different from those of seawater, suggesting that magnesium depletion is not a necessary prerequisite for CFA precipitation.

The environment of precipitation is interface-linked rather than driven by organic diagenesis of phosphorus deeper in the sediment. Most of the cores display a wide range of diagenetic characteristics below the immediate interfacial region, but almost all show the precipitation signature near the interface. This interface-linked early diagenetic porewater environment for the precipitation of CFA explains many of the geochemical characteristics of phosphorites and provides a “testable” model to compare the modern phosphogenic analog with ancient phosphorite deposits. Two of the cores display very high solid phase phosphorus and fluoride contents reflecting the presence of apparently modern pelletal apatites.     

Guest editorial

Surficial sediments rich in apatite pellets were recovered from an 80‐m‐deep area that runs parallel to the eastern coast of Vizcaino Bay, Baja California. This area is subject to intense seasonal upwelling, the sediments presenting elevated phosphate (P₂O₅), organic carbon (C_org), and carbonate concentrations. The pellets are moderately well sorted, concentrated mainly in the 3ø size class within a poorly sorted sediment matrix. Abundant nodular apatite and phosphatized bone fragments were found in only one of the pellet‐bearing samples. This sample has a P₂O₅concentration of over 20%, the ≥ — 2.5ø to 0.5ø grain size classes alone being formed of up to 32% P₂O₅. The high proportion of fragmented nodules and the presence of volcanic rock pebbles indicate that at least part of this deposit is allochthonous. The apatite pellets have a flat to concave‐downward heavy rare earth (HREE) shale normalized pattern and, except for La, are depleted in light rare earth elements (LREE). The close resemblance between the HREE patterns and the relatively high La concentration in the pellets of Vizcaino Bay, and the onland phosphorites of the southern Baja California Peninsula, suggest similar depositional histories for these deposits. No significant Ce anomalies were observed, but a negative Eu deviation is common in all of the pellet‐bearing sediments. The depletion of LREE in the pellets of Vizcaino Bay may indicate the preferential removal of these elements by weathering processes.     

Cerium anomaly variations in ferromanganese nodules and crusts from the Indian Ocean

Fifty analyses of rare earth elements as well as mineralogical studies have been carried out on a suite of manganese nodules and crusts from the Central Indian Basin and the Western Indian Ocean. The aim was to identify the processes controlling the REE patterns of the phases hosting the REE in the manganese nodules, with an emphasis on an understanding of the Ce anomaly. This has involved separating the encrusting layers and nuclei physically as well as Fe-Mn oxides from the aluminosilicate phase chemically (using a 2 M HCl leach) prior to analysis.

The presence of nodule nuclei seems to have little influence (mostly <5% to a maximum of 30%) on the overall magnitude of the Ce anomalies in these nodules. The ratios of concentrations of elements in the acid leachates and the corresponding bulk values yield flat REE patterns indicating that the aluminosilicate phase contributes very little to the Ce anomalies. Interelement relations indicate that the Ce anomalies are largely controlled by the amorphous mineral phase FeOOH.xH₂O. The relationship of Fe, Ce anomaly and δ-MnO₂ further suggests that Ce is chemisorbed onto iron oxyhydroxides which are epitaxially intergrown with δ-MnO₂.

The regional distribution of the Ce anomaly values appears to depend on many of the factors responsible for the uptake of other minor metals in nodules and crusts.     

Distribution des terres rares dans les fractions de sediments et nodules de Fe et Mn associes en l'ocean Indien

The REE distribution in size fractions of sediments and associated ferromanganese nodules from the Indian Ocean was studied. Bulk-sample patterns of sediments result from the combination of coarsest fractions depleted in Ce and fine fractions enriched in Ce. Ce depletion of the coarsest fractions is related to biogenic silica; on the contrary, REE distribution patterns of fine fractions are closely similar to those of associated ferromanganese nodules. The Ce excess in fine fractions is probably of continental origin, but it could also be derived from submarine weathering of volcanic glass or related to Ce oxydation in the marine environment. In fine fractions Ce is probably in its tetravalent state, hence it can easily be scavenged by fine clays and oxyhydroxydes. Trivalent REE can be incorporated in nodules, partly by occlusion of fine clays or oxyhydroxydes and partly by surface to surface transfer as was proposed by Ehrlich (1968). Comparison between REE patterns of fine-sized fractions, nodules and seawater also supported adsorption as a possible mechanism governing the incorporation of REE of sediments and nodules from seawater. Light REE are probably incorporated as oxyhydroxyde complexes, whereas heavy REE are markedly fractionated with increasing atomic number. This fractionation reflects the complexed form of the heavy REE in seawater.     

东海盆地丽水凹陷古新统锆石示踪作用分析

锆石具有较强的稳定性，是地质研究中一种强有力的工具。通过对其形态学研究，再结合其地球化学特征，可讨论其母岩的成因；同时，结合颗粒锆石的微区定年，可以讨论重要的地质事件成因和演化，如锆石的成因和微区离子探针技术在研究造山带构造年代学中发挥着重要作用。利用激光探针等离子体质谱(LAM-ICPMS)技术对丽水凹陷古新统沉积岩中锆石进行微区定年和微量元素测定，并对稀土元素配分模式和微量元素特征进行分析，结果表明，对于复杂来源的锆石，利用其地球化学特征进行区分效果不理想，锆石地球化学特征并不是有利的沉积和锆石来源显示。     

Notes on a sediment map for the South Otago continental shelf

Modern sand and silty sand cover the inner shelf, with silty sand accumulating off the mouth of the Clutha River. Palimpsest sediment dominated by shell debris and polymodal detrital gravel and sand covers the outer shelf. The detrital sediment is largely or wholly of Clutha River origin.     

陆架环流作用下的北黄海中北部细颗粒物质输运

海洋泥质沉积是流域、古气候及海平面变化信息的重要载体,对该粒级物质的系统研究,是获取环境信息的重要手段。为此,采集北黄海中北部表层沉积物80件,经室内筛分处理,并对细颗粒组分（<63 μm）进行了稀土元素（REE）测试。结果显示REE的含量及分布模式在长山列岛东西两侧差异显著:东侧物质明显富集REE,尤其轻稀土元素（LREE）,Eu元素则明显缺失,指示了受鸭绿江的显著控制;长山列岛以西,REE除含量降低外,分布模式也变平缓。造影剂马根维显（Gd-DTPA）在磁共振成像（MRI）中的大量使用,可能是造成Gd明显富集的主要原因。此外,辽南沿岸物质的来源及分布与以往认识存在差异,以往认为辽南沿岸泥质沉积为鸭绿江为主的辽东半岛河流的远端泥沉积,而本文的研究发现:辽东半岛东侧长山列岛至大连湾外海域以黄河来源物质为主,鸭绿江物质对北黄海西部泥质区的贡献在西侧大于东侧,其中陆架环流对物质输运发挥着重要作用。     

稀土元素在成岩型海洋铁锰结核中的富集特征及机制

借助相分析中的偏提取方法对取自东太平洋深海平原上的成岩型铁锰结核进行了选择性提取实验以研究稀土元素在其中各矿物或氧化物相中的分布模式以及铁氧化物和锰氧化物对稀土元素的吸附机制.结果显示,尽管1 nm-水锰矿相对无定形铁的氧化物/氢氧化物而言是成岩型结核中的优势矿物,但是稀土元素主要富集在无定形铁的氧化物/氢氧化物中.虽然稀土元素在海水中主要是以碳酸盐络合物的形式存在,但是无定形铁的氧化物/氢氧化物则是从海水中获得自由稀土元素离子来络合,而1 nm-水锰矿则直接吸附稀土元素的碳酸盐络合物.因此,无定形铁的氧化物/氢氧化物对稀土元素来说具有比1 nm-水锰矿更强的络合能力.成岩型结核的Ce负异常是由于成岩型结核形成于缺氧微环境中,该环境不能把可溶性的Ce~(3+)氧化成不溶性的Ce~(4+)发生沉淀.     

大型海藻龙须菜凋落物分解对水质的影响

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