Polycyclic aromatic hydrocarbons in the Jurassic–Cretaceous Tetori Group,central Japan

Primary data on the organic geochemistry of the Tetori Group provide basic information about depositional environments and thermal maturation of organic matter through two geological sections in the Hokuriku region, central Japan. The thermal maturity of organic matter was evaluated by the methylphenanthrene index‐1. The maturity progressively increases stratigraphically down through the Izumi section in Fukui Prefecture. The estimated vitrinite reflectance equivalent is <1.35% near the top of the section, gradually increasing to >2.0% near the base. The thermal maturation process can account for stratigraphic changes in abundance of PAHs through this section. However, the occasional occurrence of coronene in the middle of the Izumi section is attributed to possible paleo‐wildfires that supplied more coronene to the depositional site. The stratigraphic distribution of PAHs through the Tateyama section, Toyama Prefecture, also can be explained by the same scenario as envisaged for the Izumi section, but weathering and/or other secondary factors may have partly modified primary signals for this section. Polycyclic aromatic sulfur compounds were observed in nearly all samples from both sections, even in samples for which a freshwater paleoenvironment is surmised. Most plausibly, some reduced sulfur was re‐oxidized to elemental sulfur, which persisted in the sediments and may have contributed to the formation of aromatic sulfur compounds. Hence, polycyclic aromatic sulfur compounds do not provide an index to separate marine environments from the freshwater settings for the Tetori Group.     

Source,depositional environment and maturity levels of some crude oils in southwest Niger Delta,Nigeria

A suite of crude oil samples,that had not been previously characterized geochemically,was collected from two oil fields in the southwest Niger Delta Nigeria.The saturate biomarkers were used to evaluate geochemical characteristics such as depositional environments,sources of organic matter and extent of biodegradation using gas chromatography-flame ionization detector and gas chromatography-mass spectrometry.Distribution of n-alkanes(Pr/Ph,and isoprenoide/n-alkanes ratios),the abundance of hopanes,oleanane skeleton and C27-C29 steranes in the oils indicate that they were formed from mixed sources(marine and terrestrial kerogen) deposited in an oxic paleoenvironment with no particular maturity trend.These parameters also permit the source grouping of the oils into two families.     

Late Cretaceous paleoenvironment and lake level fluctuation in the Songliao Basin,northeastern China

Study of Late Cretaceous lacustrine sedimentary strata in the eastern Songliao Basin, China revealed that the paleoclimate was relatively arid and hot during sedimentation of the upper Santonian of the Yaojia Formation, but became relatively humid and warm during deposition of the lower Campanian Nenjiang Formation. The upper Yaojia Formation was deposited in a freshwater lake environment, while the lower Nenjiang Formation was deposited in a slightly brackish to brackish environment. The average total organic carbon content in the upper Yaojia Formation is 0.15%, while the hydrogen index is 36 mg_HC/g_TOC, implying poor source rock for oil generation and the organic matter comprised of a mixture of woody and herbaceous organic matter. In contrast, the hydrogen index of oil shale and black shale of the lower Nenjiang Formation is 619 mg_HC/g_TOC, and total organic carbon content on average is 3.37%, indicating a mixed algae and herbaceous source of kerogen and an increase in aquatic bioproductivity. The black shale and oil shale have low Pristane/Phytane and C₂₉ 5α,14α,17α(H) ? stigmastane 20R/(20R + 20S) ratios, with maximum concentration of n‐alkanes at n‐C₂₃, implying an anoxic depositional environment with algae, bacteria and higher plants providing most of the organic matter. Relatively abundant gammacerane and a higher Sr/Ba ratio in the oil shales suggest the presence of brackish water and development of salinity stratification in the lake. During sedimentation of the upper Yaojia through the lower Nenjiang Formations, the level of Songliao lake increased and a deep‐lake environment was formed with bottom waters being oxygen depleted. Concomitantly, as the lake deepened bottom conditions were changing from oxic to anoxic, and the input of organic matter changed from predominantly higher plants to a mixture of bacteria, algae and higher plants providing favorable conditions for oil source rock accumulation.     

Geochemical changes across the Ordovician-Silurian transition on the Yangtze Platform,South China

The trace element and rare earth element (REE) variations across the Ordovician-Silurian succession are presented from two outcrop sections on the Yangtze Platform: the Nanbazi section, Guizhou Province, deposited in a shallow platform interior setting, and the Wangjiawan section, Hubei Province, deposited in a deeper basinal environment. Geochemical analysis of closely spaced samples through three intervals, the Wufeng, Guanyinqiao and Longmaxi, revealed vast palaeoceanographic changes. Some geochemical proxies, including Th/U, Ni/Co, V/Cr, and V/(V+Ni) ratios, together with sedimentary facies and biotic data, indicate that an anoxic condition on the most of the Yangtze Platform during the Wufeng and Longmaxi intervals, but an oxic condition during the Guanyinqiao time. The shift of the anoxic to the oxic environment during the Guanyinqiao time coincided with a global sea-level lowstand, in parallel with the global glaciation. The Longmaxian anoxic environment was a result of a global sea-level rise, which may be synchronized with a mainly catastrophic event in the latest Ordovician. Although the two sections generally show similar variation patterns of trace and REE concentrations and some element ratios, a minor difference occurs between the Wangjiawan and Nanbazi sections, likely reflecting a difference in depositional setting during the accumulation. Such an oceanic oxygen-level variation may add a useful constraint to the current arguments on the cause and consequence of the latest Ordovician mass extinction. Supported by National Basic Research Program of China (Grant No. 2005CB422101)     

Effect of Resuspension on the Release of Heavy Metals and Water Chemistry in Anoxic and Oxic Sediments

Two types of river sediments with contrasting characteristics (anoxic or oxic) were resuspended and the release of heavy metals and changes in water chemistry were investigated. During resuspension of the anoxic sediment, the dissolved oxygen (DO) concentration and redox potential of the water layer decreased abruptly within the first 1 min, followed by increases toward the end of the resuspension period. Heavy metals were released rapidly in the first 6 h, probably due to the oxidation of acid volatile sulfide (AVS) of the anoxic sediment, and then the aqueous phase concentrations of the heavy metals decreased due to resorption onto the sediment until the 12‐h point. During resuspension of the oxic sediment, the DO concentration and redox potential remained relatively constant in the oxic ranges. The heavy metals were released from the oxic sediment gradually during a 24‐h resuspension period. The temporal maximum concentrations of Ni, Cu, Zn, and Cd in the aqueous phases in both experiments frequently exceeded the USEPA water quality criteria or the water quality guidelines of Australia and New Zealand. This suggests that a resuspension event could bring about temporal water quality deterioration in the two sediment environments.     

Early diagenetic growth of carbonate concretions in the upper Doushantuo Formation in South China and their significance for the assessment of hydrocarbon source rock

Mineralogical and textural characteristics and organic carbon composition of the carbonate concretions from the upper Doushantuo Formation (ca. 551 Ma) in the eastern Yangtze Gorge area reveal their early diagenetic (shallow) growth in organic-rich shale. High organic carbon content (up to 10%) and abundance of framboidal pyrites in the hosting shale suggest an anoxic or euxinic depositional environment. Well-preserved cardhouse clay fabrics in the concretions suggest their formation at 0-3 m burial depth, likely associated with microbial decomposition of organic matter and anaerobic oxidation of methane. Gases through decomposition of organic matter and/or from methanogenesis created bubbles and cavities, and anaerobic methane oxidation at the sulfate reduction zone resulted in carbonate precipitation, filling in bubbles and cavities to form spherical structures of the concretions. Rock pyrolysis analyses show that the carbonate concretions have lower total organic carbon (TOC) content but higher effective carbon than those in the host rocks. This may be caused by enclosed organic matter in pores of the concretions so that organic matter was protected from further modification during deep burial and maintained high hydrocarbon generating potential even in over-matured source rock. As a microbialite sensu latu, concretions have special growth conditions and may provide important information on the microbial activities in depositional and early burial environments.     

The relationship between methylated chromans and maturity of organic matter in the source rocks from Jianghan hypersaline basin

The distribution, the relative composition, and their variation of methylated chromans (MTTC) with maturation of organic matter in the source rocks from Mingjia 1 well, Jianghan hypersaline basin, have been analyzed and studied. There is a close relationship between the distribution of methylatd chromans and paleosalinity of depositional environment. In the Eq ₁–Eq ₃ source rocks deposited under hypersaline environment with lower Pr/Ph ratio, higher gammacerane index and abundant organic sulfur-containing compounds, methyl MTTC and dimethyl MTTC are main components, but in the Eq ₄ source rocks deposited non-hypersaline environment with relatively higher Pr/Ph ratio, lower gammacerane index and organic sulfur-containing compounds, trimethyl MTTC is a major compound in methylate chromans. Obviously, the distributions and the compositions of methylated chromans in the source rocks are in harmony with biomarker indicators indicating the paleaosalinity and redox of depositional environment. The relationship between the ratio of 5,8-dimethyl MTTC to 7,8-dimethyl MTTC (dimethyl MTTC ratio) and the maturity of organic matter in the source rocks from Mingjia 1 well has been discussed. The results show that dimethyl MTTC ratio increased with the burial depth and the maturation of organic matter in the source rocks, especially in immature level. It is noteworthy that this ratio is very susceptible to small variation in the maturity of organic matter in the source rocks when vitrinite reflectance R _o is less than 0.65%. It may imply that this ratio is a good and susceptible aromatic maturity indicator for the relative maturity of organic matter in immature source rocks.     

Euxinia as a dominant process during OAE1a (Early Aptian) on the Eastern Russian Platform and during OAE1b (Early Albian) in the Middle Caspian

A detailed study of the mineral composition and microstructure of the black shales associated with OAE1a (Eastern Russian Platform), OAE1b (Middle Caspian), and the host rocks has been carried out using X-ray diffraction, scanning electron microscopic, and microprobe analyses. The results provide important constraints for depositional environments in the sedimentary basins. Black shales with pyrite framboids imply euxinic (sulfidic) conditions with increased organic matter preservation. Disintegrating framboids suggest partial or complete dissolution of the organic matter inside the framboids due to increasing water oxygenation. OAE1a on the Eastern Russian Platform is heterogeneous as it includes thin interbeds of concretionary coccolith limestones within the interval of bituminous shales, and correlates with the Lower Aptian Rhagodiscus angustus nannofossil zone. The coccolith limestones indicate short intermittent episodes of interrupted stagnation, rapid oxygenation, and restoration of normal marine conditions. The presence of montmorillonite, albite, microcline, and diopside in the bituminous siltstones and in the host siltstones of OAE1a on the Eastern Russian Platform, as well as a high content of titanium in the black siltstones correlated with OAE1b in the Middle Caspian allow proposing significant input of pyroclastic material into the extant sea.     

Spatiotemporal variability of ocean chemistry in the early Cambrian,South China

Following the Ediacaran metazoan radiation, the “Cambrian Explosion” set up the major framework of todays’ animal phyla as well as modern marine ecosystem. Here, we present a preliminary investigation on the temporal and spatial (from shallow to deep waters) variations of the early Cambrian ocean chemistry in South China through analyzing a Fe-S-C systematic dataset integrated from literature. Our investigation indicates that the early Cambrian deep ocean in South China was still anoxic and Fe²⁺-enriched (i.e., ferruginous) although its surface was oxic, and in between a metastable euxinic (anoxic and sulfidic) water zone may have dynamically developed in anoxic shelf waters with an increasing weathering sulfate supply. Furthermore, accompanying marine transgression and regression cycles in the early Cambrian, such a “sandwich” structure in ocean redox chemistry demonstrates five evolutional stages, which can be well correlated to the spatiotemporal patterns of fossil records in South China. The good correlation between metazoan fossil occurrences and water chemistry in South China suggests that early animals possibly possessed ability to inhabit anoxic but generally not euxinic environments as free H₂S was fatal to most eukaryotes. This view can well explain why those small shell fauna and sponges disappeared from shelf to slope areas where sulfidic Ni-Mo-rich shales were widely deposited. Thus, we conclude that the spatiotemporal variations of ocean chemistry and its biological effects probably played a key role in the phased animal radiations and “extinctions” in the early Cambrian.     

Trace metal solubility in an anoxic fjord

Intermittent anoxia in the Saanich Inlet water column provides an easily accessible marine O₂/H₂S interface to study the response of metals to both a steep redox gradient and the availability of reactive reduced sulfur species. Our study indicates a strong anoxic zone sink for copper and cadmium and the characteristically enhanced solubility of manganese and iron. Thiosulfate and sulfite are below detection limits (1 μM and 0.1 μM, respectively) and thus not important in metal complexation. Elemental sulfur concentrations are high at the oxic/anoxic interface and throughout the anoxic zone, indicating the potential for metal complexation by polysulfides. A thermodynamic approach employing metal sulfide formation and class specific sulfidic ligand complexation to generate equilibrium profiles adequately describes the solubility of iron, copper, and cadmium. The extension of this scheme to other transition and class B metals in other marine environments with redox fronts is suggested.     

Palynostratigraphy and palaeoenvironments around the Albian-Cenomanian boundary interval(OAEld),North Bulgaria

The palynological assemblages from two Albian-Cenomanian boundary sections in North Bulgaria are described.The samples analyzed yielded a diverse palynological content including dinoflagellate cysts and miospores.Based on dinocyst nutrient and productivity indices a phase of enhanced nutrient availability and high primary productivity is inferred for the latest Albian interval.The pronounced predominance of peridinioid dinocysts in this interval,namely O.verrucosum,O.scabrosum and especially P.infusorioides is considered to reflect eutrophic conditions.It coincides with the increased phosphorus mass accumulation occurring at the top part of the Upper Albian Dekov Formation.OAE 1d is indicated in the Tolovitsa karst spring section,based on palynofacies dominated by high amounts of granular amorphous organic matter(AOM)related to anoxic environmental conditions.These sections serve as evidence suggesting a relationship between Cretaceous peridinioid cysts(including Palaeohystichphora infusorioides,Ovoidinium verrucosum,O.scabrosum)and anoxic/suboxic conditions and/or high primary productivity.The pollen spectrum inferred relatively stable vegetation patterns of surrounding continental areas during and after the Albian/Cenomanian boundary interval and the times of OAE 1d formation.The hinterland vegetation integrated mainly pteridophyte spores and gymnosperms.The area was part of the Southern Laurasian floral province which was characterized by warm temperate to subtropical humid climate.Angiosperms were still minor part of this vegetation.     

Spatial and temporal variations of nutrient concentration in the groundwater of a floodplain: effect of hydrology,vegetation and substrate

Spatio‐temporal variations in nitrogen and phosphorus concentrations in groundwater were analysed and related to the variations in hydrological conditions, vegetation type and substrate in an alluvial ecosystem. This study was conducted in the Illwald forest in the Rhine Plain (eastern France) to assess the removal of nutrients from groundwater in a regularly flooded area. We compared both forest and meadow ecosystems on clayey‐silty soils with an anoxic horizon (pseudogley) at 1·5–2 m depth (eutric gley soil) and a forest ecosystem on a clayey‐silty fluviosoil rich in organic matter with a gley at 0·5 m depth (calcaric gley soil). Piezometers were used to measure the nutrient concentrations in the groundwater at 2 m depth in the root layer and at 4·5 m depth, below the root layer. Lower concentrations of nitrate and phosphate in groundwater were observed under forest than under meadow, which could be explained by more efficient plant uptake by woody species than herbaceous plants. Thus NO₃‐N inputs by river floods were reduced by 73% in the shallow groundwater of the forested ecosystem, and only by 37% in the meadow. Compared with the superficial groundwater layer, the lowest level of nitrate nitrogen (NO₃‐N) and the highest level of ammonium nitrogen (NH₄‐N) were measured in the deep layer (under the gley horizon at 2·5 m depth), which suggests that the reducing potential of the anoxic horizon in the gley soils contributes to the reduction of nitrate. Nitrate concentrations were higher in the groundwater of the parcel rich in organic matter than in the one poorer in organic matter. Phosphate (PO₄‐P) concentrations in both shallow and deep groundwater are less than 62 to 76% of those found in surface water which can be related to the retention capacity of the clay colloids of these soils. Moreover, the temporal variations in nutrient concentrations in groundwater are directly related to variations in groundwater level during an annual hydrological cycle. Our results suggest that variations in groundwater level regulate spatio‐temporal variations in nutrient concentrations in groundwater as a result of the oxidation–reduction status of soil, which creates favourable or unfavourable conditions for nutrient bioavailability. The hydrological variations are much more important than those concerning substrate and type of vegetation. Copyright © 1999 John Wiley & Sons, Ltd.     

Palynostratigraphy and palaeoenvironments around the Albian-Cenomanian boundary interval (OAE1d), North Bulgaria

The palynological assemblages from two Albian-Cenomanian boundary sections in North Bulgaria are described. The samples analyzed yielded a diverse palynological content including dinoflagellate cysts and miospores. Based on dinocyst nutrient and productivity indices a phase of enhanced nutrient availability and high primary productivity is inferred for the latest Albian interval. The pronounced predominance of peridinioid dinocysts in this interval, namely O. verrucosum, O. scabrosum and especially P. infusorioides is considered to reflect eutrophic conditions. It coincides with the increased phosphorus mass accumulation occurring at the top part of the Upper Albian Dekov Formation. OAE 1d is indicated in the Tolovitsa karst spring section, based on palynofacies dominated by high amounts of granular amorphous organic matter (AOM) related to anoxic environmental conditions. These sections serve as evidence suggesting a relationship between Cretaceous peridinioid cysts (including Palaeohystichphora infusorioides, Ovoidinium verrucosum, O. scabrosum) and anoxic/suboxic conditions and/or high primary productivity. The pollen spectrum inferred relatively stable vegetation patterns of surrounding continental areas during and after the Albian/Cenomanian boundary interval and the times of OAE 1d formation. The hinterland vegetation integrated mainly pteridophyte spores and gymnosperms. The area was part of the Southern Laurasian floral province which was characterized by warm temperate to subtropical humid climate. Angiosperms were still minor part of this vegetation.     

Paleoenvironmental interpretations of Quaternary reef deposits based on comparisons of 10 selected modern and fossil larger foraminifera from the Ryukyu Islands, Japan

Abstract A new method for reconstructing depositional environments of larger foraminifera‐bearing limestones is proposed. First, depth and spatial distributions of empty tests of 10 foraminiferal taxa in a 1–2 mm size fraction were examined using 32 surface sediment samples collected from depths shallower than 200 m, located to the west of Miyako Island, Ryukyu Islands, northwest Pacific. Distributional ranges of empty tests in the 1–2 mm size fraction appear to be more limited than those including other size fractions in previous reports, partly because larger empty tests of each taxon are less easily transported than smaller ones. Multivariate analyses (Q‐mode cluster analysis and non‐metric multidimensional scaling ordination) based on binary (presence/absence) data of the 10 taxa delineate four sample groups, each of which corresponds to different depositional environments: nearshore zone/bay inlet; back‐reef to fore‐reef; outer shelf to shelf slope; and shelf slope distant from coral reefs. Next, these modern data were applied to reconstruct the depositional environment of a rock section distributed in the Shiratorizaki area (Irabu Island, Ryukyu Islands), which consists of larger foraminiferal limestone of the Pleistocene Ryukyu Group. Multivariate analyses were performed on the fossil plus the modern foraminiferal data to explore the possible relationship of the fossil associations with variations in modern associations, demonstrating that the fossil foraminiferal associations resemble the modern outer shelf associations. The modern analog technique was also applied to estimate paleobathymetry using fossil foraminiferal data. The results indicate that the section studied was deposited in outer shelf environments at depths between 53.5 and 98.6 m. These paleoenvironmental interpretations are consistent with previous studies based on sedimentary facies and the computer‐based expert system. The modern dataset and methods used in our work would be particularly useful for paleoenvironmental reconstructions of Quaternary reef and shelf carbonates along active margins of the northwest Pacific.     

Concurrent Salinization and Development of Anoxic Conditions in a Confined Aquifer,Southern Israel

An ancient, brackish, anoxic, and relatively hot water body exists within the Yarqon‐Tanninim Aquifer in southern Israel. A hydrogeological‐geochemical conceptual model is presented, whereby the low water quality is the outcome of three conditions that are met simultaneously: (1) Presence of an organic‐rich unit with low permeability that overlies and confines the aquifer; the confining unit contains perched horizons with relatively saline water. (2) Local phreatic/roofed conditions within the aquifer that enable seepage of the organic‐rich brackish water from above. The oxidation of the dissolved organic matter in the seeping water consumes the dissolved oxygen and continues through bacterial sulfate reduction, with H₂S as a product. These exothermic reactions result in some heating. (3) The seeping water comprises a relatively large portion of the water volume. In the presented case study, the latter condition first developed in the Late Pleistocene following climate change, which led to a dramatic decline in recharge. Consequently, water flow in the local basin has nearly ceased, as evident by old water ages, specific isotopic composition, and nearly equipotential water levels. The continuous seepage from above into the almost stagnant water body has resulted in degraded water quality. Seepages of organic‐rich brackish water exist at other sites throughout the aquifer but have limited impact on the salinity and redox conditions due to the dynamic water flow, which flushes the seeping water, that is, the third condition is not met. The coexistence of the above three conditions may explain the development of anoxic and saline groundwater in other aquifers worldwide.     

Towards a classification of organic enrichment in marine sediments based on biogeochemical indicators

A nomogram is developed to show that pH, redox potentials (Eh_NHE) and measures of dissolved sulfides (H₂S + HS⁻ + S²⁻)(total free S²⁻) can be used to classify organic enrichment impacts in marine sediments. The biogeochemical cycle of sulfur in marine sediments is described to show that changes in macrobenthic infauna community structure associated with high levels of organic matter supply result from stress due to oxygen deficiency (hypoxia and anoxia) and toxic effects of S²⁻. The changes reflect enhancement of microbial sulfate reduction under conditions of high organic matter sedimentation and the progressive formation of hypoxic–anoxic conditions measured by decreased Eh_NHE and increased concentrations of S²⁻. The nomogram provides a basis for classification of the oxic status of marine sediments based on changes in inter-related biological and biogeochemical variables along an organic enrichment gradient.     

The degree of trace metal pyritization in subtidal sediments of a mariculture area: application to the assessment of toxic risk

Four 2–3 m sediment cores were taken at the sites on the periphery of mussel raft concentrations in the subtidal zone of the inner Ría de Vigo (Galicia, NW Spain) with a view to evaluate the potential risk to mariculture from sediment-borne trace elements (As, Cd, Cr, Cu, Mn, Ni, Pb and Zn). The distribution of each of these elements in reactive, organic, pyrite and silicate-bound fractions was determined at 64 samples, and these data were used to calculate the degree of trace metal pyritization (DTMP) of each metal/metalloid. In the top 10–20 cm, relatively oxic conditions led to As, Cd, Cu, Pb and Zn having large reactive fractions due to their association with Fe and Mn oxyhydroxides. At lower levels, anoxic conditions favoured by intense diagenesis led to the precipitation of trace metals and metalloids as sulphides, with or without association with pyrite. Particularly large pyrite fractions in the 20–100 cm layer are attributed to the organic matter of this layer being more marine in origin than that of deeper sediments. DTMP was greatest for Cu and As, and least for Pb, Zn and Cr. The risk of trace element toxicity in the event of disturbances instituting oxic conditions in these sediments is discussed.     

A laboratory study of the biogeochemical cycling of Fe,Mn, Zn and Cu across the sediment-water interface of a productive lake

Laboratory incubation experiments were carried out on sediment cores collected from Esthwaite Water, U.K., during April 1987, when the sediments displayed a characteristic surface (1.5 to 2 cm) oxide floc. The experiments were undertaken at 10°C, in the dark, under variable redox and pH conditions for periods of ~ 720 h (30 d). In some cases, realistic amounts of decomposing lake algae were added to simulate the deposition of an algal bloom. Pore waters and overlying waters were obtained from the incubated sediment cores at various time intervals and the samples analysed for pH and dissolved Fe, Mn, Zn and Cu by AAS. The results demonstrated that trace metal concentrations at the sediment-water interface can show rapid, pulsed responses to episodic events associated with controlling factors such as algal deposition and mixing conditions. The variations in dissolved Fe and Mn concentrations could generally be explained by their well known redox behaviour. Appreciable loss of Mn from solution under conditions of well-developed anoxia was consistent with adsorption of Mn²⁺ by FeS. Cu and Zn were both rapidly (24 h) released into solution during incubation of sediment cores prior to the development of anoxia in the overlying waters. Their most likely sources were the reductive remobilization of Mn oxides and the decomposition of organic matter. The addition of decomposing algae to a series of cores resulted in even higher interfacial dissolved concentrations of Cu and Zn, probably through acting as a supplementary source of the metals and through increased oxide dissolution. Switching from anoxic to oxic conditions also rapidly increased dissolved Cu and Zn concentrations, possibly due to their release during the oxidation of metal sulphides. The enhanced releases of dissolved Cu and Zn were generally short-lived with removal being attributed to the formation of sulphides during anoxia and to adsorption by Fe and Mn oxides under oxic conditions.     

Stable isotopic characterisation of francolite formation

Stable isotopic data are presented for 112 samples of francolite from 18 separate phosphate deposits. Values ofδ¹³C andδ³⁴S in most offshore deposits suggest formation within oxic or suboxic environments either by carbonate replacement or direct precipitation of francolite from water of normal marine compositions. The exceptions are concretionary francolite from Namibia, which has an isotopic composition in keeping with its formation within organic-rich sediments, and that from offshore Morocco, which has an isotopic signature of the anoxic/suboxic interface. Onshore deposits from Jordan, Mexico, South Africa and, possibly, the Permian Phosphoria Formation in the western U.S.A., are substantially depleted in¹⁸O: they appear to be too altered for deductions to be made about their environments of formation. In other onshore deposits which are unaltered, or minimally altered, the isotopic composition suggests that some formed within sulphate-reducing sediments (Sedhura, Morocco) whilst francolite from the Georgina Basin of Australia formed at the oxic/anoxic boundary, where oxidation of biogenic H₂S decreases theδ³⁴S of pore water. In general, pelletal samples show non-oxic isotopic signatures, whilst non-pelletal samples show oxic isotopic signatures, but samples from Namibia, Peru (Ica Plateau) and the Californian and Moroccan margins are exceptions to this rule. Morphology may therefore be a misleading indicator of francolite genesis as no definitive relation exists between phosphorite type and isotopic signature.     

Occurrence conditions of hyperpycnal flows, and their significance for organic-matter sedimentation in a Holocene estuary, Niigata Plain, Central Japan

River floods influence sedimentary environments and ecosystems from the terrestrial to the deep-marine. This study documents the occurrence conditions of hyperpycnal flows generated by river floods and related organic-matter sedimentation for Holocene sediments of the Niigata Plain, Central Japan, based on detailed sedimentary facies, total sulfur and total organic carbon content, diatom assemblages and organic-matter composition. Holocene sediments of the Niigata Plain consist of sand, mud and gravel that were deposited in estuarine and fluvial systems during a sea-level rise (15 000–6800 years BP) and stillstand (after 6800 years BP) following the Last Glacial Maximum. Hyperpycnites are present in the upper part of the estuarine lagoon sediments. The depositional age is considered to be about 5000 years BP. The hyperpycnites comprise two successions of a top fining-up unit and a basal coarsening-up unit, and include abundant terrigenous organic matter and freshwater diatoms. A large volume of freshwater is inferred to have flowed into the lagoon during deposition of the upper part of the lagoon sediments. In consequence, hyperpycnal flows may have readily formed in the lagoon, because the halocline was weak. The hyperpycnal flows also produced a layer of concentrated terrigenous organic matter in the uppermost part of the hyperpycnites. The abundant organic matter on the estuarine floor is inferred to have produced anoxic bottom conditions owing to oxidative decomposition by benthic bacteria.