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1.
This paper describes the detrital mineralogy, early diagenetic reactions and authigenic mineral precipitates for freshwater contaminated sediments deposited in an urban water body (the Salford Quays of the Manchester Ship Canal, Greater Manchester, UK). These sediments contain a mix of natural and anthropogenic detrital grains. Detrital grains are dominated by quartz and clay grains, whilst anthropogenic grains are dominated by metal-rich glass grains, concentrated at a depth of 12–17 cm in the sediment as a result of historical inputs. Sediment porewaters contain significant concentrations of Fe, Mn, Zn and phosphate. Bacterial Fe(III) and Mn(IV) reduction are hypothesised to supply Fe2+ and Mn2+ to porewaters, with phosphate released from Fe oxide reduction or organic matter oxidation. Petrographic observations indicate that the metal-rich glass grains are undergoing chemical dissolution during early diagenesis, supplying Fe and Zn to porewaters.  相似文献   

2.
Many (bio)geochemical processes that bring about changes in sediment chemistry normally begin at the sediment-water interface, continue at depth within the sediment column and may persist throughout the lifetime of sediments. Because of the differential reactivity of sedimentary phosphate phases in response to diagenesis, dissolution/precipitation and biological cycling, the oxygen isotope ratios of phosphate (δ18OP) can carry a distinct signature of these processes, as well as inform on the origin of specific P phases. Here, we present results of sequential sediment extraction (SEDEX) analyses combined with δ18OP measurements, aimed at characterizing authigenic and detrital phosphate phases in continental margin sediments from three sites (Sites 1227, 1228 and 1229) along the Peru Margin collected during ODP Leg 201. Our results show that the amount of P in different reservoirs varies significantly in the upper 50 m of the sediment column, but with a consistent pattern, for example, detrital P is highest in siliciclastic-rich layers. The δ18OP values of authigenic phosphate vary between 20.2‰ and 24.8‰ and can be classified into at least two major groups: authigenic phosphate precipitated at/near the sediment-water interface in equilibrium with paleo-water oxygen isotope ratios (δ18Ow) and temperature, and phosphate derived from hydrolysis of organic matter (Porg) with subsequent incomplete to complete re-equlibration and precipitated deeper in the sediments column. The δ18OP values of detrital phosphate, which vary from 7.7-15.4‰, suggest two possible terrigenous sources and their mixtures in different proportions: phosphate from igneous/metamorphic rocks and phosphate precipitated in source regions in equilibrium with δ18Ow of meteoric water. More importantly, original isotopic compositions of at least one phase of authigenic phosphates and all detrital phosphates are not altered by diagenesis and other biogeochemical changes within the sediment column. These findings help to understand the origin and provenance of P phases and paleoenvironmental conditions at/near the sediment-water interface, and to infer post-depositional activities within the sediment column.  相似文献   

3.
We report solid phase sulfur speciation of six cores from sediments underlying oxic, suboxic and anoxic-sulfidic waters of the Black Sea. Our dataset includes the five sulfur species [pyrite-sulfur, acid volatile sulfides (AVS), zerovalent sulfur (S(0)), organic polysulfides (RSx), humic sulfur] together with reactive iron and manganese, as quantified by dithionite extraction, and total organic carbon. Pyrite – sulfur was the major phase in all cores [200-400 µmol (g dry wt)- 1] except for the suboxic core. However, zerovalent sulfur and humic sulfur also reached very significant levels: up to about 109 and 80 µmol (g dry wt)- 1, respectively. Humic sulfur enrichment was observed in the surface fluff layers of the eastern central basin sediments where Unit-1 type depositional conditions prevail. Elemental sulfur accumulated as a result of porewater sulfide oxidation by reactive iron oxides in turbidities from the anoxic basin margin and western central basin sediments. The accumulation of elemental sulfur to a level close to that of pyrite-S in any part of central Black Sea sediments has never been reported before and our finding indicates deep basin turbidites prevent the build-up of dissolved sulfide in the sediment. This process also contributes to diagenetic pyrite formation whereas in the non-turbiditic parts of the deep basin water column formed (syngenetic) pyrite dominates the sulfur inventory. In slope sediments under suboxic waters, organic sulfur (humic sulfur + organic polysulfides) account for 33-42% of total solid phase S, indicating that the suboxic conditions favor organosulfur formation. Our study shows that the interactions between depositional patterns (Unit 1 vs. turbidite), redox state of overlying waters (oxic-suboxic-sulfidic) and organic matter content determine sulfur speciation and enable the accumulation of elemental sulfur and organic sulfur species close to a level of pyrite-S.  相似文献   

4.
Nineteen sediment cores from the Madeira, Seine, Tagus and Nares Abyssal Plains and the Alboran Sea have been used to evaluate the speciation, fluxes and diagenesis of iodine in the deep sea. The sediments have surficial molar I/C ratios of 10–30 × 10−4 in excess of previous reported values for planktonic material (~1 × 10−4). Solid phase I contents decrease exponentially with depth corresponding to decomposition rate constants of 5–260 × 10−6 yr−1 which vary with the carbon accumulation rate.Iodine species in the pore waters follow a vertical sequence of four zones: 1. a zone of I− production where total dissolved iodine (∑I) concentrations initially increase at the seawater-sediment interface; 2. a zone of I− oxidation where interconversion of I− to IO−3 occurs; 3. a zone of IO−3 reduction where interconversion of IO−3 back to I− occurs which corresponds to the suboxic part of the sediment column; and 4. a further zone of I− production which is confined to the lower anoxic part of the sediment column. Benthic ∑I fluxes in the Madeira Abyssal Plain measured from shipboard incubation experiments and calculated from porewater gradients are similar, averaging 0.55 and 0.36 × 10−8 μmol cm−2 sec−, respectively.In the surface sediment the observed I enrichment results from a quasi-closed cycle for iodine initially involving release of I− from decomposing marine organic matter followed by rapid removal onto organic matter at the sediment-seawater interface where I/C regeneration ratios of up to 200 × 10−4 are found, lodate reduction occurs during suboxic diagenesis, after denitrification and before MnO2 reduction, consistent with the sequence of reactions predicted from the free energy yields for organic matter oxidation. There is some further I− production in the anoxic section of sediments but at much smaller rates than occur during the interfacial diagenetic cycling.  相似文献   

5.
Organogenic sediments (sapropels) in lakes are characterized by a reduced type of diagenesis, during which organic compounds are decomposed, the chemical composition of the pore waters is modified, and authigenic minerals (first of all, pyrite) are formed. Pyrolysis data indicate that organic matter undergoes radical transformatons already in the uppermost sapropel layers, and the composition of this organic matter is principally different from the composition of the organic matter of the its producers. The sapropels contain kerogen, whose macromolecular structure starts to develop during the very early stages of diagenesis, in the horizon of unconsolidated sediment (0–5 cm). The main role in the diagenetic transformations of organic matter in sediments is played by various physiological groups of microorganisms, first of all, heterotrophic, which amonifying, and sulfate-reducing bacteria. SO42? and Fe2+ concentrations in the pore waters of the sediments are determined to decrease (because of bacterial sulfate reduction), while concentrations of reduced Fe and S species (pyrite) in the solid phase of the sediment, conversely, increase. Comparative analysis shows that, unlike sapropels in lakes in the Baikal area, sapropels in southern West Siberia are affected by more active sulfate reduction, which can depend on both the composition of the organic matter and the SO42? concentration in the pore waters.  相似文献   

6.
Sediment cores from the middle to lower slope of the southern continental margin of Australia between the Great Australian Bight and western Tasmania are compared in terms of marine and terrigenous input signals during the Holocene. The mass accumulation rates of carbonate, organic carbon, biogenic Ba, and Al are corrected for lateral sediment input (focusing), using the inventory of excess 230Th in the sediment normalised to its known production rate in the water column above each site. The biogenic signal is generally higher in the eastern part of the southern margin probably due to enhanced productivity associated with seasonal upwelling off southeastern South Australia and the proximity of the Subtropical Front, which passes just south of Tasmania. The input of Al, representing the terrigenous signal, is also higher in this region reflecting the close proximity of river runoff from the mountainous catchment of southeastern Australia. The distribution pattern of Mn and authigenic U, together with pore‐water profiles of Mn++, indicate diagenetic reactions driven by the oxidation of buried organic carbon in an oxic to suboxic environment. Whereas Mn is reduced at depth and diffuses upwards to become immobilised in a Mn‐rich surface layer, U is derived from seawater and diffuses downward into the sediment, driven by reduction and precipitation at a depth below the reduction zone of Mn. The estimated removal rate of U from seawater by this process is within the range of U removal measured in hemipelagic sediments from other areas, and supports the proposition that hemipelagic sediments are a major sink of U in the global ocean. Unlike Mn, the depth profile of sedimentary Fe appears to be little affected by diagenesis, suggesting that little of the total Fe inventory in the sediment is remobilised and redistributed as soluble Fe.  相似文献   

7.
Enrichments in reactive iron occur under euxinic marine conditions, that is, where dissolved sulfide is present in the water column. These enrichments result primarily from the export of remobilized iron from the oxic shelf, which is scavenged from the euxinic water column during syngenetic pyrite formation and deposited in the underlying sediments. Strongly elevated ratios of highly reactive iron to total iron (FeHR/FeT) and total iron to aluminum (FeT/Al) and high degrees of pyritization (DOP) are each products of this enrichment process. These paleoredox proxies are among the most faithful recorders of ancient euxinia.Contrary to previous arguments, iron enrichment is decoupled from biogenic sediment inputs, but it does appear to be a uniquely euxinic phenomenon. In other words, we can rule out a major contribution from preferential physical transport of FeHR-rich detrital sediment to the deep basin, which could also operate under oxic conditions. Furthermore, enrichment via the shuttling of iron remobilized from oxic shelves appears to be limited by inefficient transport and trapping processes in deep oxic basins. Elevated FeT/Al ratios in the euxinic sediments also cannot be a product of internal enhancement of the reactivity of the detrital iron pool without net FeHR addition. These conclusions are supported by observations in the modern Black Sea, Orca Basin, and Effingham Inlet.FeT/Al ratios are unambiguous recorders of paleoredox even in sediments that have experienced high degrees of metamorphic alteration. However, this study suggests that high siliciclastic accumulation rates can swamp the enrichment mechanism, resulting in only intermediate DOP values for euxinic sediments and FeT/Al ratios that mimic the oxic shelf. Such dilution effects are well expressed in Black Sea basinal turbidites and rapidly accumulating muds on euxinic basin margins. Under conditions of persistent euxinia, varying extents of FeHR enrichment can illuminate spatial and temporal gradients in siliciclastic sedimentation. The magnitude of enrichment is a function of the source (shelf) to sink (ocean basin) areal ratio, suggesting that iron proxies can also record ocean-scale paleoenvironmental properties through muted enrichments at times of very widespread euxinia. For the first time, manganese data are interpreted in light of the redox shuttle model. As for the iron data, the Black Sea, Orca Basin, and Effingham Inlet show enrichments in total manganese in the deep euxinic basin, suggesting export from the suboxic porewaters of the oxic shelf and scavenging and burial in the basin. The Black Sea data reveal iron and manganese enrichment across the broad, deep euxinic basin, suggesting efficient lateral transport and deep-water mixing tied to the physical properties of the water column.  相似文献   

8.
以长江口崇明岛北支潮滩的3个潮滩为对象,在粒度测量和磁性分析的基础上,揭示了现代潮滩表层沉积物的磁性变化,探讨了磁性矿物、尤其是自生亚铁磁性矿物—硫复铁矿（Fe3S4）在潮滩沉积微相的分布规律及机制。研究结果表明：长江口潮滩表层沉积物的磁性特征由亚铁磁性矿物及超顺磁颗粒(SP)主导。淤积型中高潮滩磁铁矿以单畴(SD)和SP为主,侵蚀型潮滩的中低潮滩则更多假单畴/多畴(PSD/MD)晶粒,反映了水动力的分选及以细颗粒矿物溶解为主的早期成岩作用。此外,最东侧的北四滧港剖面潮上带—高潮滩芦苇带普遍存在自生亚铁磁性硫复铁矿,北堡港和新卫剖面的高潮滩也局部存在硫复铁矿,反映了本区高潮滩—潮上带丰富的有机质及细颗粒沉积物、加上有限的海水淹没时间,是早期成岩作用过程中生成硫复铁矿的主要机制。  相似文献   

9.
ABSTRACT Detrital alkali feldspars currently at burial depths of 3·2–3·5 km in the Upper Jurassic Humber Group of the Fulmar oilfield, UK North Sea, are overgrown and have been partially replaced by authigenic Or‐rich feldspar. Intracrystal microtextures suggest several different provenances for the detrital grains. The overgrowths are uniformly non‐cathodoluminescent and have occasional celsian‐rich zones. Transmission electron microscopy shows that they are composed of a microporous mosaic of subµm‐ to µm‐sized sub‐grains associated with barite, illite and pyrite. The subgrains are somewhat rounded but have an approximate {110} Adularia habit and display a faint modulated microtexture on the nanometre scale. They have triclinic symmetry, but the lattice angles depart only slightly from monoclinic symmetry. These features are characteristic of K‐feldspar precipitated relatively rapidly and at low temperature. Authigenic Or‐rich feldspar has also partially replaced microcline and perthitic albite within the detrital grains, often at a suboptical scale. Although, like diagenetic albitization, replacement by K‐feldspar is probably a very common diagenetic reaction, it has rarely been reported owing to difficulties in imaging the diagnostic textures with the scanning electron microscopy techniques used by most workers. The permeability of the subgrain microtexture may significantly hinder the use of feldspar overgrowths for K/Ar and 40Ar/39Ar dating of diagenesis, and the existence of suboptical, replacive authigenic K‐feldspar within detrital grains may significantly modify the apparent Ar ages of detrital grains. Similar subgrain microtextures in optically featureless quartz overgrowths are also illustrated.  相似文献   

10.
Studies of the δ13C of pore water dissolved inorganic carbon (δ13C-DIC) were carried out in shallow water carbonate sediments of the Great Bahamas Bank (GBB) to further examine sediment-seagrass relationships and to more quantitatively describe the couplings between organic matter remineralization and sediment carbonate diagenesis. At all sites studied δ13C-DIC provided evidence for the dissolution of sediment carbonate mediated by metabolic CO2 (i.e., CO2 produced during sediment organic matter remineralization); these observations are also consistent with pore water profiles of alkalinity, total DIC and Ca2+ at these sites. In bare oolitic sands, isotope mass balance further indicates that the sediment organic matter undergoing remineralization is a mixture of water column detritus and seagrass material; in sediments with intermediate seagrass densities, seagrass derived material appears to be the predominant source of organic matter undergoing remineralization. However, in sediments with high seagrass densities, the pore water δ13C-DIC data cannot be simply explained by dissolution of sediment carbonate mediated by metabolic CO2, regardless of the organic matter type. Rather, these results suggest that dissolution of metastable carbonate phases occurs in conjunction with reprecipitation of more stable carbonate phases. Simple closed system calculations support this suggestion, and are broadly consistent with results from more eutrophic Florida Bay sediments, where evidence of this type of carbonate dissolution/reprecipitation has also been observed. In conjunction with our previous work in the Bahamas, these observations provide further evidence for the important role that seagrasses play in mediating early diagenetic processes in tropical shallow water carbonate sediments. At the same time, when these results are compared with results from other terrigenous coastal sediments, as well as supralysoclinal carbonate-rich deep-sea sediments, they suggest that carbonate dissolution/reprecipitation may be more important than previously thought, in general, in the early diagenesis of marine sediments.  相似文献   

11.
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12.
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.  相似文献   

13.
《Chemical Geology》2004,203(1-2):153-168
The importance of the magnetic iron sulfide minerals, greigite (Fe3S4) and pyrrhotite (Fe7S8), is often underappreciated in geochemical studies because they are metastable with respect to pyrite (FeS2). Based on magnetic properties and X-ray diffraction analysis, previous studies have reported widespread occurrences of these magnetic minerals along with magnetite (Fe3O4) in two thick Plio-Pleistocene marine sedimentary sequences from southwestern Taiwan. Different stratigraphic zones were classified according to the dominant magnetic mineral assemblages (greigite-, pyrrhotite-, and magnetite-dominated zones). Greigite and pyrrhotite are intimately associated with fine-grained sediments, whereas magnetite is more abundant in coarse-grained sediments. We measured total organic carbon (TOC), total sulfur (TS), total iron (FeT), 1N HCl extractable iron (FeA), and bulk sediment grain size for different stratigraphic zones in order to understand the factors governing the formation and preservation of the two magnetic iron sulfide minerals. The studied sediments have low TS/FeA weight ratios (0.03–0.2), far below that of pyrite (1.15), which indicates that an excess of reactive iron was available for pyritization. Observed low TS (0.05–0.27%) is attributed to the low organic carbon contents (TOC=0.25–0.55%), which resulted from dilution by rapid terrigenous sedimentation. The fine-grained sediments also have the highest FeT and FeA values. We suggest that under conditions of low organic carbon provision, the high iron activity in the fine-grained sediments may have removed reduced sulfur so effectively that pyritization was arrested or retarded, which, in turn, favored preservation of the intermediate magnetic iron sulfides. The relative abundances of reactive iron and labile organic carbon appear to have controlled the transformation pathway of amorphous FeS into greigite or into pyrrhotite. Compared to pyrrhotite-dominated sediments, greigite-dominated sediments are finer-grained and have higher FeA but lower TS. We suggest that diagenetic environments with higher supply of reactive iron, lower supply of labile organic matter, and, consequently, lower sulfide concentration result in relatively high Eh conditions, which favor formation of greigite relative to pyrrhotite.  相似文献   

14.
The ion activity product of Fe and phosphate in interstitial waters from four sediment cores taken from Greifensee, Switzerland indicate the presence of vivianite [Fe3(PO4)2 · 8 H2O] in the solid phase. Analysis of the sediment using an electron microprobe and by electron microscopy revealed P-rich grains to be also enriched in Fe. The combined methods provide strong evidence that vivianite is forming authigenically in the sediments. Thermodynamic stability calculations demonstrate that the most stable Fe and phosphate minerals (pyrite, siderite and apatite) are not the ones controlling the pore water chemistry. The results emphasize the importance of rate processes of mineral formation in early diagenesis.Calculations based on the sediment phosphate concentration, and the degree of supersaturation of Fe and phosphate in the upper portion (0–15 cm) of the pore waters, indicate that the rate of vivianite mineral growth is controlled by a surface reaction rather than a diffusion mechanism. The response time of dissolved phosphate in the sediment pore waters with respect to mineral precipitation is on the order of 1–20 days. Less than 15% of the phosphate released by organic matter degradation at the sediment-water interface and below is retained in the sediments.  相似文献   

15.
Carbonate concretions from the Jet Rock (Upper Lias, Lower Jurassic) of NE England grew in uncompacted sediment, close to the sediment surface. Microbiological activity created isolated microenvironments in which dissolved carbonate and sulphide species were produced more rapidly than they could be dispersed by diffusion, so establishing the localised supersaturation of calcite and metastable iron sulphides. Precipitation of these minerals in the microenvironment formed a single concretion.Mass-balance calculations demonstrate that at least two different microbiological processes participated in concretionary growth. The early growth stages had an unidentifiable microbiological source of carbonate which declined in importance relative to sulphate reduction as growth proceeded. It is suggested that the diffusion of dissolved organic material was important in sustaining microbiological activity.Mineralogical zonations in the concretions result from changes in the chemistry of the microenvironment due to variations in the rates of addition/removal Ca2+, Fe2+, HCO?3 and HS? by microbiological activity, the crystallization of authigenic minerals and diffusion between the microenvironment and surrounding pore waters. Such changes are of only local significance and the resulting mineralogical zonations in a concretion cannot be used to deduce successive stages of diagenesis in the whole sediment.  相似文献   

16.
Berner (1971) has solved the differential equation governing the concentration in interstitial water of substances produced or consumed during steady-state diagenesis. We have shown that closed solutions exist for non-steady-state diagenesis as well, and that these solutions are best obtained by means of Green's functions. Non-uniform distribution of decomposable organic matter in sediments is a major cause of non-steady-state diagenesis. However, the effect of such non-uniform distributions on the composition of interstitial water in sediments is pronounced only when the rate of deposition exceeds ca. 200 cm/1000 yr. At slower deposition rates, diffusion is sufficiently rapid to damp out major fluctuations in the concentration of ions such as SO2?4, NH+4, PO3?4, and HCO?3. Concentration profiles of these ions therefore tend to be similar to steady-state profiles even if the concentration of decomposable organic matter is quite heterogeneous.The concentration of SO2?4 frequently approaches 0 in marine sediments rich in decomposable organic matter. In such sediments the total quantity of SO2?4 reduced during diagenesis is proportional to the concentration of SO2?4 in sea water even if the bacterial rate of decomposition of organic matter is nearly independent of the SO2?4 concentration in interstitial water. This implies that the rate of SO2?4 removal from sea water by reduction to sulfide is roughly proportional to the sulfate concentration in seawater.Solutions for the diagenetic equation exist for reasonable variations of the rate of ionic diffusion in interstitial waters and for changes in the rate of deposition due to compaction.  相似文献   

17.
Sediment cores were sampled from Xiamen Western Bay at five sites during the summer and winter of 2006 and Hg–Au microelectrodes were used to make on board measurements of the concentration gradients of dissolved oxygen, Mn2+, and Fe2+ within the sediments. The O2 concentrations decreased sharply from about 200 μmol L−1 in the bottom seawater to zero within a depth of a few millimeters into the sediment. Dissolved Mn2+ was detected below the oxic zones with peak concentrations up to 600 μmol L−1, whereas dissolved Fe2+ had peak concentrations up to 1,000 μmol L−1 in deeper layers. The elemental contents of organic carbon and nitrogen within the sediments were analyzed and their C/N ratios were in the range of 9.0 to 10.1, indicative of heavy terrestrial origin. Sediments from two sites near municipal wastewater discharge outlets had higher organic contents than those from the other sites. These high organic contents corresponded to shallow O2 penetration depths, high dissolved Mn2+ and Fe2+ concentrations, and negative redox potentials within the sediments. This indicated that the high organic matter content had promoted microbial respiration within the sediments. Overall, the organic content did not show any appreciable decrease with increasing sediment depths, so a quadratic polynomial function was used to fit the curve of O2 profiles within the sediments. Based on the O2 profiles, O2 fluxes across the seawater and sediment interface were estimated to be in the range 6.07 to 14.9 mmol m−2 day−1, and organic carbon consumption rates within the surface sediments were estimated to be in the range 3.3 to 20.8 mgC cm−3 a−1. The case demonstrated that biogeochemistry within the sediments of the bay was very sensitive to human activities such as sewage discharge.  相似文献   

18.
Depth distributions of bacterial respiration of O2, NO3− and SO42− were compared with geochemical data for Mn, Fe and S in coastal sediments from water depths between 26 and 520 m. As water depth increased, the zone of SO42− respiration was found deeper in the sediment and was eventually separated from the surface-located activity of O2 and NO2− respiration. At the deepest station additional SO42− reduction activity was observed in small, detrital aggregates on the sediment surface. Dissolved Mn2+ and Fe2+ appeared between the O2- plus NO3−-containing surface layer and the H2S-plus FeS-containing sediment below. This was a result of Mn and Fe reductions coupled to either the oxidation of sulfide or the mineralization of organic matter. Tracer experiments showed that both FeS, FeS2 and S0 were important radiolabelled products of sulfate respiration in this intermediate zone. In the same zone, the overall degradation of organic matter seemed to be underestimated by the assay of SO2-4 respiration and additional mineralization by Mn and Fe reductions was likely.  相似文献   

19.
Surface slices of 20 sediment cores, off southwestern Taiwan, and bed sediment of River Kaoping were measured for major and trace elements (Al, As, Ca, Cd, Cl, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S, Si, Ti, V, and Zn) to evaluate the geochemical processes responsible for their distribution, including elemental contamination. Major element/Al ratio and mean grain size indicate quartz-dominated, coarse grained sediments that likely derived from sedimentary rocks of Taiwan and upper crust of Yangtze Craton. Bi-plot of SiO2 versus Fe2O3T suggests the possible iron enrichment in sediments of slag dumping sites. Highest concentrations of Cr, Mn, P, S, and Zn found in sediments of dumping sites support this. Correlation analysis shows dual associations, detrital and organic carbon, for Cr, P, S, and V with the latter association typical for sediments in dumping sites. Normalization of trace elements to Al indicates high enrichment factors (>2) for As, Cd, Pb, and Zn, revealing contamination. Factor analysis extracted four geochemical associations with the principal factor accounted for 25.1% of the total variance and identifies the combined effects of dumped iron and steel slag-induced C–S–Fe relationship owing to authigenic precipitation of Fe–Mn oxyhydroxides and/or metal sulfides, and organic matter complexation of Fe, Mn, Ca, Cr, P, and V. Factors 2, 3, and 4 reveal detrital association (Ti, Al, Ni, Pb, Cu, and V), effect of sea salt (Cl, Mg, Na, and K) and anthropogenic component (As and Zn)-carbonate link, respectively, in the investigated sediments.  相似文献   

20.
Molar tooth (MT) structures are enigmatic, contorted millimetre‐ to decimetre‐long veins and spheroids of microcrystalline calcite that formed during very early diagenesis in Precambrian sediments. MT structures in the ca 2·6 Ga Monteville Formation are 600–800 Myr older than previously reported occurrences and establish that conditions necessary for MT genesis were met locally throughout much of the Precambrian. In the Monteville Formation, MT structures were formed shallow subtidally, extending to depths near storm wave base, in shale host sediments intercalated with storm‐generated carbonate sand lenses. They are filled with microcrystalline calcite and rare pyrite. Microcrystalline calcite identical to that in MT structures fills other pore space, including porosity between grains in carbonate sand lenses, moldic porosity in sand grains, sheet cracks in columnar stromatolites, and shallow cracks on sandy bedding planes. Relationships in the Monteville Formation demonstrate that microcrystalline CaCO3 precipitated in fluid‐filled cracks and pores; microcrystalline calcite characteristics, as well as the paucity of carbonate mud in host rocks, are inconsistent with injection of lime mud as the origin of MT structures. Locally, MT cracks were filled by detrital sediment before or during precipitation. Precipitation occurred in stages, and MT CaCO3 evolved from granular cores to a rigid mass of cores with overgrowths – allowing both plastic and brittle deformation of MT structures, as well as reworking of eroded MT structures as rigid clasts and lime mud. Crystal size distributions and morphology suggest that cores precipitated through nucleation, Ostwald ripening and size‐dependent crystal growth, whereas overgrowths formed during size‐independent crystal growth.  相似文献   

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