Seasonal variations of arsenic at the sediment–water interface of Poyang Lake,China

Arsenic species including arsenite, arsenate, and organic arsenic were measured in the porewaters collected from Poyang Lake, the largest freshwater lake of China. The vertical distributions of dissolved arsenic species and some diagenetic constituents [Fe(II), Mn(II), S(−II)] were also obtained in the same porewater samples in summer and winter. In sediments the concentration profiles of total As and As species bound to Fe–Mn oxyhydroxides and to organic matter were also determined along with the concentrations of Fe, Mn and S in different extractable fractions. Results indicate that, in the summer season, the concentrations of total dissolved As varying from 3.9 to 55.8 μg/L in sediments were higher than those (5.3–15.7 μg/L) measured in the winter season, while the concentrations of total As species in the solid phase varied between 10.97 and 25.32 mg/kg and between 7.84 and 30.52 mg/kg on a dry weight basis in summer and winter, respectively. Seasonal profiles of dissolved As suggest downward and upward diffusion, and the flux of dissolved As across the sediment–water interface (SWI) in summer and winter were estimated at 3.88 mg/m² a and 0.79 mg/m² a, respectively. Based on porewater profiles and sediment phase data, the main geochemical behavior of As was controlled by adsorption/desorption, precipitation and molecular diffusion. The solubility and migration of inorganic As are controlled by Fe–Mn oxyhydroxides in summer whereas they appear to be more likely controlled by both amorphous Fe–Mn oxyhydroxides and sulfides in winter. A better knowledge of the cycle of As in Poyang Lake is essential to a better management of its hydrology and for the environmental protection of biota in the lake.     

Distribution of arsenic in sediment of Hangjinhou Banner- Linhe transect in Hetao Basin,North China

In this research, we laid a Hangjinhou Banner-Linhe District drilling transect along western piedmont of Hetao Basin to the modern Yellow River, which passes through the Langshan Mountain piedmont alluvial-proluvial fan, the Yellow River paleochannel, alluvial-fluvial plain of the Yellow River and the modern Yellow River flood plain successively and based on which we collected sedimentary samples of different geomorphic units. Based on hydrogeology and sedimentology and by means of stratum tracing and transects comparison, the analysis result comes out that the arsenic (As) sediments in Hetao Basin is distributed in obvious zoning form and is closely related to the sedimentary environment. The paleochannel zone in northern Basin has the highest content of As sediments, average content reaching 12.24 μg/g. In the horizontal direction of the profile, the distribution regularity of As is consistent to that of elements like Mn, Fe, Al and TOC, since they have experienced the same geochemical process in sediments. In the vertical direction, the high-arsenic sediments are mainly formed of grey black silty-fine sand layers or interbedding layers of silty-fine sand and muddy clay, rich in organic matter. The sedimentary environments are mainly shallow lakes and depressions, where the content of As is related to that of elements like Sb, Mn, B, V, etc., and particularly has significant relation with Fe.     

大同盆地浅层含水层沉积物中砷的生物可利用性评价

以采自大同盆地野外试验场的含水层沉积物为研究对象,调查高砷含水层中生物可利用性砷的含量及其影响因素。经实验室测定,沉积物总体呈碱性,Al、Fe等金属元素含量较高。利用连续提取法发现沉积物中砷结合形态主要为无定形和低结晶铁铝氧化物结合态,其次是残余态砷。其中铁铝氧化物形态的砷所占比例达53.54%,非专性吸附态和专性吸附态占14.33%,生物可利用形态的砷较少,这与体外模拟方法测定的生物可利用性砷含量较低吻合。体外模拟实验还发现：肠阶段生物可利用性砷含量比胃阶段低,生理学提取法(PBET)和体外消化法(IVG)测定的生物可利用性砷差异较小。     

Geochemistry and mineralogy of shallow alluvial aquifers in Daudkandi upazila in the Meghna flood plain, Bangladesh

The shallow alluvial aquifers of the delta plains and flood plains of Bangladesh, comprises about 70% of total land area are mostly affected by elevated concentrations of arsenic (As) in groundwater exposing a population of more than 35 million to As toxicity. Geochemical studies of shallow alluvial aquifer in the Meghna flood plain show that the uppermost yellowish grey sediment is low in As (1.03 mg/kg) compared to the lower dark grey to black sediment (5.24 mg/kg) rich in mica and organic matter. Sequential extraction data show that solid phase As bound to poorly crystalline and amorphous metal (Fe, Mn, Al)-oxyhydroxides is dominant in the grey to dark grey sediment and reaches its maximum level (3.05 mg/kg) in the mica rich layers. Amount of As bound to sulphides and organic matter also peaks in the dark grey to black sediment. Vertical distributions of major elements determined by X-ray fluorescence (XRF) show that iron (Fe₂O₃), aluminum (Al₂O₃) and manganese (MnO) follow the general trend of distribution of As in the sediments. Concentrations of As, Mn, Fe, HCO₃ ⁻, SO₄ ²⁻ and NO₃ ⁻ in groundwater reflect the redox status of the aquifer and are consistent with solid phase geochemistry. Mineralogical analysis by X-ray diffraction (XRD) and scanning electron microscopy (SEM) fitted with energy dispersive X-ray spectrometer (EDS) revealed dominance of crystalline iron oxides and hydroxides like magnetite, hematite and goethite in the oxidised yellowish grey sediment. Amorphous Fe-oxyhydroxides identified as grain coating in the mica and organic matter rich sediment suggests weathering of biotite is playing a critical role as the source of Fe(III)-oxyhydroxides which in turn act as sink for As. Presence of authigenic pyrite in the dark grey sediment indicates active reduction in the aquifer.     

Partitioning and Speciation of Trace Metal Diagenesis in Differing Depositional Environments in the Sediments of the Oman Margin

Organic-rich sediment samples collected from a transect within, and below, the Oman Margin oxygen minimum zone (OMZ) were analysed using a sequential leaching technique to characterise the diagenetic behaviour and speciation of Mn and Fe in operationally defined sediment host fractions. Trace metals showed distinct diagenetic behaviour in the two contrasting environments that were sampled. The absence of non-detrital Mn in the cores below the OMZ site is attributed to the lack of easily reducible oxides in surficial sediments and to the reduction and export of any moderately reducible aged oxides. The reactive form of solid phase Mn showed a classic feature of enrichment in the upper layer of the sediments at the abyssal site, reflecting the presence of an oxidising sedimentary layer which acts as a Mn trap during its recycling. The diagenetic Mn enrichment was inferred from typical downcore colour changes and an upward-increasing Mn content in the upper core sections. An easily reducible Fe oxide layer was observed in the abyssal sediments at an identical depth to the Mn enrichment suggesting that Fe associated with Mn oxides also has undergone sub-oxic diagenesis. However, the association of Fe with organic matter did not indicate diagenetic modification; i.e., the binding strength of the metal with organic materials appears to be sufficiently strong to preserve the trace metal. The speciation signature of non-detrital Fe differed from that of Mn. The association of Fe with organic matter suggests that this metal does not undergo diagenetic modification and is preserved in abyssal sediments. The contrasting behaviour of Mn and Fe observed between cores within the OMZ were particularly interesting. Another interesting observation was that, for cores below the OMZ, the iron oxides were associated with the Mn-oxide peak, rather than deeper in the sediments as observed by earlier studies in the Atlantic [Froelich et al. (1979). Geochim. et Cosmochim Acta 43, 1075–1090].This revised version was published online in May 2005 with corrections to the article title.     

山西临汾-运城盆地土壤中As含量的变化规律

通过对临汾-运城盆地表层和深层土壤中As含量变化规律的研究,发现表层土壤的As含量稍高于全国土壤的背景值,不同相态的含量差异较大。土壤pH、磁化率、有机质、粘粒含量等理化性质对不同形态的As有着不同程度的影响：研究区水溶态的As受土壤pH值的控制,腐殖质结合态的As与有机质呈明显的正相关关系,磁化率、粘粒含量与土壤As的含量都有明显的正相关关系。研究结果表明,As与Fe、Mn、Cu、Zn、Mg、K、F等元素的相关性非常显著,说明研究区表层土壤中的As主要是自然成因的。进一步的回归分析表明,土壤的As含量随着铁氧化物、氢氧化物和硫化物含量的增加而增加。研究区土壤As的纵向分布特点是中层多、上下层少,这可能与研究区As的污染状况、土壤的碱性环境、植物的吸收作用、磷肥的施用等因素有关。     

白云岩成岩相与地球化学特征--以鄂尔多斯盆地中东部马家沟组马五5-马五1亚段为例

白云岩成岩相研究对恢复其成岩环境具有重要意义。以鄂尔多斯盆地中东部马家沟组马五₅-马五₁亚段为例,利用岩芯及岩石薄片研究了白云岩的成岩相类型及岩相特征,并根据氧、碳稳定同位素、锶同位素以及主、微量元素等分析了各类成岩相的地球化学特征。结果显示,鄂尔多斯盆地中东部马五₅-马五₁亚段白云岩成岩相主要包括近地表环境下的膏盐溶蚀角砾相、活跃回流渗透云化相和隐伏回流渗透云化相,以及表生淡水环境下的风化岩溶角砾相等四类,其中受淡水或混合水影响的近地表环境下的膏盐溶蚀角砾相、活跃回流渗透云化相及隐伏回流渗透云化相白云岩样品具有相对奥陶纪海水"负偏"的δ18O和δ13C值,相对高Mn或低Fe含量,中等-强阴极发光,高于同期海水的87Sr/86Sr值以及相对较低的Al和Ti含量;而表生淡水环境下的风化岩溶角砾相白云岩样品因受表生期淡水以及大量陆源碎屑物的影响,呈现出低于同期海水的δ18O和δ13C值,相对高Mn和过高Fe含量,弱或无阴极发光,明显高于同期海水的87Sr/86Sr值以及明显较高Al和Ti含量。研究结果表明,氧、碳稳定同位素、锶同位素及主、微量元素等地化数据是识别白云岩成岩相的有效指标。     

Evolution of heavy-metal profiles in river Weser Estuary sediments,Germany

Studies of heavy metal concentrations in porewaters and in sediments from the Weser Estuary, Germany, indicate that the depth distribution of Mn, Cd, and Cu in the solid phase is predominantly influenced by the subsurface redox regime, while Ni shows only moderate alteration. Based on solid-phase heavy metal data from eight locations in the study area, linear regressions of Fe, Mn, Cu, Ni, and Cd on Al allowed differences between the initial metal concentration and the concentration preserved in the sediments to be predicted. We calculate that, due to early diagenetic processes, Mn is enriched to 145 percent, while Cu and Cd are depleted to 71 percent and 46 percent, respectively, of the initial metal concentration in the near-surface sediment. Maximum depletion of Cd (84 percent), Cu (68 percent), Mn (54 percent), and Ni (24 percent) concentrations in the sediment, observed at a restricted area downcore, indicate the importance of post-depositional processes to metal preservation in the sediment. Without knowledge of the quantitative effects of diagenetic reactions on the preservation of metal concentrations in sediments, the depth distribution of metals in sediment cores may be misinterpreted as the effect of human activities.     

Manganese-rich brown layers in Arctic Ocean sediments: Composition, formation mechanisms, and diagenetic overprint

We present inorganic geochemical analyses of pore waters and sediments of two Late Quaternary sediment cores from the western Arctic Ocean (southern Mendeleev Ridge, RV Polarstern Expedition ARK-XXIII/3), focussing on the composition and origin of distinct, brown-colored, Mn-rich sediment layers. Carbonate enrichments occur in association with these layers as peaks in Ca/Al, Mg/Al, Sr/Al and Sr/Mg, suggesting enhanced input of both ice-rafted and biogenic carbonate. For the first time, we show that the Mn-rich layers layers are also consistently enriched in the scavenged trace metals Co, Cu, Mo and Ni. Distinct bioturbation patterns, specifically well-defined brown burrows into the underlying sediments, suggest these metal enrichments formed close to the sediment-water interface. The geochemical signature of these metal- and carbonate-rich layers most probably documents formation under warmer climate conditions with an intensified continental hydrological cycle and only seasonal sea ice cover. Both rivers and sea ice delivered trace metals to the Arctic Ocean, while enhanced seasonal productivity exported reactive organic matter to the sea floor. The coeval deposition of organic matter, Mn (oxyhydr)oxides and trace metals triggered intense diagenetic Mn cycling at the sediment-water interface. These processes resulted in the formation of Mn and trace metal enrichments, and the degradation of labile organic matter. With the onset of cooler conditions, reduced riverine runoff and/or a solid sea ice cover terminated the input of riverine trace metal and fresh organic matter, resulting in deposition of grayish-yellowish, metal-poor sediments. Oxygen depletion of Arctic bottom waters under these cooler conditions is not supported by our data, and did not cause the sedimentary Mn distribution. While the original composition and texture of the brown layers resulted from specific climatic conditions and corresponding diagenetic processes, pore water data show that diagenetic Mn redistribution is still affecting the organic-poor deeper sediments. Given persistent steady state conditions, purely authigenic Mn-rich brown layers may form, while others may be partly or completely dissolved. The degree of diagenetic Mn redistribution largely depends on the depositional environment, the Mn and organic matter availability, and apparently affected the Co/Mo ratios of Mn-rich layers. Thus, brown Arctic layers are not necessarily synchronous features, and should not be correlated across the Arctic Ocean without additional age control.     

Mobilization of arsenic and iron from Red River floodplain sediments, Vietnam

Sediments from the Red River and from an adjacent floodplain aquifer were investigated with respect to the speciation of Fe and As in the solid phase, to trace the diagenetic changes in the river sediment upon burial into young aquifers, and the related mechanisms of arsenic release to the groundwater. Goethite with subordinate amounts of hematite were, using Mössbauer spectroscopy, identified as the iron oxide minerals present in both types of sediment. The release kinetics of Fe, As, Mn and PO₄ from the sediment were investigated in leaching experiments with HCl and 10 mM ascorbic acid, both at pH 3. From the river sediments, most of the Fe and As was mobilized by reductive dissolution with ascorbic acid while HCl released very little Fe and As. This suggests As to be associated with an Fe-oxide phase. For oxidized aquifer sediment most Fe was mobilized by ascorbic acid but here not much As was released. However, the reduced aquifer sediments contained a large pool of Fe(II) and As that is readily leached by HCl, probably derived from an unidentified authigenic Fe(II)-containing mineral which incorporates As as well. Extraction with ascorbic acid indicates that the river sediments contain both As(V) and As(III), while the reduced aquifer sediment almost exclusively releases As(III). The difference in the amount of Fe(II) leached from river and oxidized aquifer sediments by ascorbic acid and HCl, was attributed to reductive dissolution of Fe(III). The reactivity of this pool of Fe(III) was quantified by a rate law and compared to that of synthetic iron oxides. In the river mud, Fe(III) had a reactivity close to that of ferrihydrite, while the river sand and oxidized aquifer sediment exhibited a reactivity ranging from lepidocrocite or poorly crystalline goethite to hematite. Mineralogy by itself appears to be a poor predictor of the iron oxide reactivity in natural samples using the reactivity of synthetic Fe-oxides as a reference. Sediments were incubated, both unamended and with acetate added, and monitored for up to 2 months. The river mud showed the fastest release of both Fe and As, while the effect of acetate addition was minor. This suggests that the presence of reactive organic carbon is not rate limiting. In the case of the river and aquifer sediments, the release of Fe and As was always stimulated by acetate addition and here reactive organic carbon was clearly the rate limiting factor. The reduced aquifer sediment apparently can sustain slower but prolonged microbially-driven release of As. The highly reactive pools of Fe(III) and As in the river mud could be due to reoxidation of As and Fe contained in the reducing groundwater from the floodplain aquifers that are discharging into the river. Deposition of the suspended mud on the floodplain during high river stages is proposed to be a major flux of As onto the floodplain and into the underlying aquifers.     

Decoupling of arsenic and iron release from ferrihydrite suspension under reducing conditions: a biogeochemical model

High levels of arsenic in groundwater and drinking water are a major health problem. Although the processes controlling the release of As are still not well known, the reductive dissolution of As-rich Fe oxyhydroxides has so far been a favorite hypothesis. Decoupling between arsenic and iron redox transformations has been experimentally demonstrated, but not quantitatively interpreted. Here, we report on incubation batch experiments run with As(V) sorbed on, or co-precipitated with, 2-line ferrihydrite. The biotic and abiotic processes of As release were investigated by using wet chemistry, X-ray diffraction, X-ray absorption and genomic techniques. The incubation experiments were carried out with a phosphate-rich growth medium and a community of Fe(III)-reducing bacteria under strict anoxic conditions for two months. During the first month, the release of Fe(II) in the aqueous phase amounted to only 3% to 10% of the total initial solid Fe concentration, whilst the total aqueous As remained almost constant after an initial exchange with phosphate ions. During the second month, the aqueous Fe(II) concentration remained constant, or even decreased, whereas the total quantity of As released to the solution accounted for 14% to 45% of the total initial solid As concentration. At the end of the incubation, the aqueous-phase arsenic was present predominately as As(III) whilst X-ray absorption spectroscopy indicated that more than 70% of the solid-phase arsenic was present as As(V). X-ray diffraction revealed vivianite Fe(II)₃(PO₄)₂.8H₂O in some of the experiments. A biogeochemical model was then developed to simulate these aqueous- and solid-phase results. The two main conclusions drawn from the model are that (1) As(V) is not reduced during the first incubation month with high Eh values, but rather re-adsorbed onto the ferrihydrite surface, and this state remains until arsenic reduction is energetically more favorable than iron reduction, and (2) the release of As during the second month is due to its reduction to the more weakly adsorbed As(III) which cannot compete against carbonate ions for sorption onto ferrihydrite. The model was also successfully applied to recent experimental results on the release of arsenic from Bengal delta sediments.     

河水入渗过程中近岸带地下水中砷的形成与影响因素研究

沈阳黄家水源地是我国北方地区典型的傍河地下水水源地,近岸带地下水中铁(Fe)、锰(Mn)、砷(As)含量严重超标。为查明地下水中As的来源与影响因素,对研究区河水、地下水以及土壤样品进行采集与测试,分析了水样常规指标与碳硫稳定同位素、土样中典型矿物、砷的含量及赋存形态。结果表明,研究区河水中As含量很低,而地下水中As含量普遍超标。河水入渗初期,氧化性河水使部分含As矿物发生氧化而释放As;随着河水入渗,地下水向还原环境转变,含As的Fe/Mn矿物发生还原性溶解,地下水中As含量逐渐升高。研究区典型矿物有黄铁矿、菱铁矿、软锰矿、赤铁矿、针铁矿、菱锰矿等,通过可交换态砷解吸、有机质结合态砷氧化、铁锰氧化物结合态砷还原性溶解等,介质中的As释放至地下水中。地下水中As含量与酸碱度(pH)、氧化还原电位(Eh)呈一定负相关,与溶解有机碳(DOC)、 {\mathrm{HCO}}_3^{-}、Fe、Mn含量呈正相关。     

Chemical forms of some heavy metals in Huanghe River sediments (China) and comparison with data from Rhine River sediments (West Germany)

The chemical forms of Fe, Mn, Zn, Cu, Cr, Pb and Cd in the Huanghe River sediments have been studied by sequential extraction techniques and the comparison with data from the Rhine River sediments has been made. In the Huanghe River sediments the average contents of metals, without exception, are below their respective contents in average shales and very close to their levels in Ca-poor granites. The major portion of metals is combined with the detrital and moderately reducible phases. Both in the Huanghe River and in the Rhine River sediments the distribution ratios of metals between the moderately reducible and the easily reducible phases are generally more than unity. However, the distribution ratios of Mn, Zn and Cd are obviously lower than those of Fe, Cr, Cu and Pb. As a result of contamination, the ratios of Fe, Cr, Cu and Pb show an apparent increase, but no remarkable ratio variation is observed for Mn, Zn and Cd. Metals in the Huanghe River sediments, especially Cu and Zn, show a tendency to be associated with the organic phase. The effect of carbonate on metal association preference seems to be less important than that in the Rhine River although there is higher content of carbonate in the Huanghe River sediments. Cd has a greater percentage of the exchangeable phase, which is similar to the result from the Rhine River sediments.     

Early diagenetic processes in coastal marine sediments disturbed by a catastrophic sedimentation event

Following a catastrophic flash flood in July 1996, as much as 50 cm of post-glacial clays were deposited in less than 2 days in the upper reaches of the Saguenay Fjord (Quebec, Canada), disrupting the normal sedimentation and diagenetic regimes. We report detailed geochemical analyses of sediments (porosity, Eh, organic and inorganic carbon, Fe and Mn reactive solid phases, and acid volatile sulfide) and porewaters (salinity, dissolved organic carbon (DOC), Fe(II), Mn(II), nitrate, ammonium, and sulfate) for seven stations located in the Saguenay Fjord. Three of these (SAG-05, SAG-09, and SAG-30) were visited in 1996 and once per year thereafter to document the chemical evolution of the sediment toward a new steady state. The flood deposits contain less organic carbon and more inorganic carbon than the indigenous fjord sediments. The flood deposit modified the distribution patterns of reactive Mn and Fe as a result of the reduction of Mn and Fe oxides delivered with the deposit and those concentrated at the now buried former sediment-water interface. Most of the Mn(II) migrated to the new sediment-water interface, where a Mn-rich layer was formed. In contrast, much of the Fe(II) was precipitated as sulfides and remained trapped at or close to the old interface. A nitrate peak developed in the porewater at the old sediment-water interface, possibly because of the oxidation of ammonia by Mn oxides. The distributions of porewater DOC within the flood deposit correlate with the distributions of dissolved Mn(II) and Fe(II), suggesting that adsorbed DOC was released when metal oxides were reduced.     

Geochemistry of two sediment cores from the west coast of India

Copper, Pb, Zn, Ni, Co, Mn, AI, Ca, magnetic susceptibility and loss on ignition of sediments from two cores near Mangalore along the western continental shelf of India have been studied. The sediments have high Al and organic matter contents due to the high sedimentation rate and their proximity to river mouths. Down-core variations of elements indicate a decrease of lithogenous component during probably the past few centuries. While abundance of calcareous shells in some zones has lead to the dilution of most of the metals, it appears that Pb and Mn are associated with this phase. Copper, Zn and Fe are associated with organic matter and detrital particles, whereas Ni and Co are predominantly associated with the insoluble fraction. Oxides/hydroxides of Fe and Mn are absent because of the reducing conditions and the high terrigenous influx. Geochemically, Mn and Fe are present in different phases of sediments (in the insoluble fraction and organic matter respectively). The Fe content of one of the cores is positively correlated with magnetic susceptibility.     

Trace metal remobilization following the resuspension of estuarine sediments: Saguenay Fjord,Canada

The Saguenay Fjord sediments are characterised by high trace metal, organic matter and Fe monosulfide (AVS) concentrations. The presence of important maritime activities, dredging operations around harbour installations, and natural slumpling events contribute to the resuspension of the Fjord sediments, their exposure to an oxidising environment, and the remobilization of a variety of contaminants to the water column.In order to simulate these processes, sediments collected from various depths in a box core recovered on the landward slope of the fjord's interior basin were resuspended in aerated seawater in the laboratory at room temperature. The slurries were sampled at various time intervals over a period of nearly 2000 h and the dissolved Fe, Mn and As concentrations were determined. Results of this study indicate that the amount of Fe and As released to the solution is strongly correlated to the acid volatile sulfide (AVS) content of the resuspended sediments. The release of Mn to the water phase, however, appeared to be mainly controlled by the dissolution of a solid phase, other than the Fe monosulfides but soluble in 1 N HCl, to which Mn (II) is associated. This phase could be a Mn carbonate or a mixed Mn–Ca carbonate.The dissolved metal concentrations measured at any time during the resuspension experiments reflect the reactivity of the carrier phases and the competing kinetics of the release and removal mechanisms. Upon their release to solution, Fe, Mn and As were scavenged by the newly precipitated Fe and Mn oxyhydroxides. The differential rate of formation of these phases and removal of associated metals in solution reflects their respective oxidation kinetics. The residence time of As in solution was very similar to that of Mn, implying that it was more efficiently removed with the slowly precipitated Mn oxyhydroxides. This observation is consistent with the faster oxidation kinetics of As (III) to As (V) in the presence of Mn oxyhydroxides. Nevertheless, results of As speciation analyses also suggest that a portion of the solubilized As was adsorbed and possibly oxidised by the rapidly precipitated Fe oxyhydroxides.     

湖北省巴东栖霞组沉积成岩作用地球化学特征研究

在详细的沉积作用、成岩作用研究基础上,利用多元统计分析方法重点对湖北省巴东水布垭剖面栖霞组碳酸盐岩地球化学特征进行了分析。研究表明,该组碳酸盐岩中 CaO 、Al₂O₃ 、K₂O 、Fe₂O₃ 、P、Ba 和 Mn的含量主要与沉积环境或原岩岩性有关,地层中SiO₂、MgO 、Sr 和 Na 的含量主要与成岩作用有关。建立了栖霞组沉积成岩作用地球化学模式。该研究合理地解释了本组岩石特殊的地球化学特征,深化了对该组岩石形成环境,特别是成岩环境的认识,区分了沉积作用和成岩作用对该组岩石地球化学特征的影响,对合理地选取有关地球化学参数研究该组岩石及类似地层的沉积作用和成岩作用具参考价值。     

鄂西地区上二叠乐平统大隆组硅质岩成因及有机质富集机理

晚二叠世是古生代环境、生物演化的关键时期,发育广泛的富有机质沉积,并成为南方重要的烃源岩层位。为了解该时期的富有机质堆积机理,我们选择了鄂西台内盆地晚二叠世大隆组富有机质硅质岩(TOC平均值为5.8%,峰值为18%)作为研究对象。硅质岩中常量元素Al/Fe均值为0.79,Mn/Ti均值为0.21,显示为大陆边缘环境下的生物成因,即非热水成因。∑REE值与Al2O3含量有着良好的正相关关系,表明当时陆源物质的输入对硅质岩的形成有很大影响。微量元素U/Th、V/(V+Ni)均说明硅质岩沉积环境为缺氧水体环境。通过对草莓状黄铁矿粒径的统计表明,大多数(64%～89%)的草莓状黄铁矿粒径小于5μm,反映缺氧甚至硫化的环境。而且,U、V、Mo的富集以及Ni、Cu元素的亏损说明水体的缺氧主要是由于海水分层,水体循环受阻造成的。TOC与氧化还原参数的变化步调基本一致,两者有着较好的正相关关系,而与生产力指标Ba/Al相关性很差,说明有机质富集主要是由水体的缺氧而不是初级生产力控制的。     

Riddle of arsenic in groundwater of Bengal Delta Plain—role of non-inland source and redox traps

In the Bengal Delta Plain (BDP) the primary arsenic sourcing appears to be different from the global scenario. Here, the Terminal Pleistocene–Holocene depositional platform, the interactive early Holocene depositional morphology with fluvio-estuarine and marine incursions played a crucial role for arsenic sourcing and enrichment. The lenticular silt-fine sand layer between anoxic clay beds favoured entrapment of dissolved organic carbon with decayed phyto-planktons debris. The Terminal Pleistocene–Holocene transgression and regression processes may have acted as major events in the BDP. Interestingly, at the end of the last glacial maxima, the Pleistocene delta had undergone block movements, wherein some parts of the platform were raised above the level of Holocene deposition. Those blocks were found to be free from arsenic in the groundwater. The sea, during re-emerging inundation (10–7 ka BP), has witnessed a monsoon-induced environment in the BDP with the resultant oscillation of sea level leading to higher upsurge towards the north. This might have resulted in the marine incursion and inundation in pre-existing land depressions. Meanwhile arsenic entrapments through marine incursion as well as enrichment in the presence of organic carbon/DOC and/or Fe/Mn/Al catalytic agents could have developed into localised redox traps. It may be of relevance that due to the repetitive transgressive–regressive phases in Holocene, resulting in periodic exposure and weathering of iron-bearing minerals and consequent iron enrichment in the aquifer system. The iron, thus present, had free charge to host arsenic as a sink. It appears that arsenic, wherever found, would likely be of atypical localised exhaustible phenomenon, both in horizontal and vertical context. It also rationalises the cause of the absence of arsenic in the other nearby Pleistocene platform, which has not come across Holocene interaction and marine incursion, as to the likely limiting condition for the search for arsenic in the BDP or beyond.     

Distribution and immobilization of heavy metals in Pliocene aquifer sediments in Wadi El Natrun depression, Western Desert

The Pliocene aquifer receives inflow of Miocene and Pleistocene aquifer waters in Wadi El Natrun depression. The aquifer also receives inflow from the agricultural activity and septic tanks. Nine sediment samples were collected from the Pliocene aquifer in Wadi E1 Natrun. Heavy metal (Cu, Sr, Zn, Mn, Fe, Al, Ba, Cr, Ni, V, Cd, Co, Mo, and Pb) concentrations of Pliocene aquifer sediments were investigated in bulk, sand, and mud fractions. The determination of extractable trace metals (Cu, Zn, Fe, Mn, and Pb) in Pliocene aquifer sediments using sequential extraction procedure (four steps) has been performed in order to study environmental pathways (e.g., mobility of metals, bounding states). These employ a series of successively stronger chemical leaching reagents which nominally target the different compositional fractions. By analyzing the liquid leachates and the residual solid components, it is possible to determine not only the type and concentration of metals retained in each phase but also their potential ecological significance. Cu, Sr, Zn, Mn, Fe, and Al concentrations are higher in finer sediments than in coarser sediments, while Ba, Cr, Ni, V, Cd, Co, Mo, and Pb are enriched in the coarser fraction. The differences in relative concentrations are attributed to intense anthropogenic inputs from different sources. Heavy metal concentrations are higher than global average concentrations in sandstone, USEPA guidelines, and other local and international aquifer sediments. The order of trace elements in the bulk Pliocene aquifer sediments, from high to low concentrations, is Fe?>?Al?>?Mn?>?Cr?>?Zn?>?Cu?>?Ni?>?V?>?Sr?>?Ba?>?Pb?>?Mo?>?Cd?>?Co. The Pliocene aquifer sediments are highly contaminated for most toxic metals, except Pb and Co which have moderate contamination. The active soluble (F0) and exchangeable (F1) phases are represented by high concentrations of Cu, Zn, Fe, and Mn and relatively higher concentrations of Pb and Cd. This may be due to the increase of silt and clay fractions (mud) in sediments, which act as an adsorbent, retaining metals through ion exchange and other processes. The order of mobility of heavy metals in this phase is found to be Pb?>?Cd?>?Zn?>?Cu?>?Fe?>?Mn. The values of the active phase of most heavy metals are relatively high, indicating that Pliocene sediments are potentially a major sink for heavy metals characterized by high mobility and bioavailability. Fe–Mn oxyhydroxide phase is the most important fraction among labile fractions and represents 22% for Cd, 20% for Fe, 11% for Zn, 8% for Cu, 5% for Pb, and 3% for Mn. The organic matter-bound fraction contains 80% of Mn, 72% of Cu, 68% of Zn, 60% of Fe, 35% of Pb, and 30% of Cd (as mean). Summarizing the sequential extraction, a very good immobilization of the heavy metals by the organic matter-bound fraction is followed by the carbonate-exchangeable-bound fraction. The mobility of the Cd metal in the active and Fe–Mn oxyhydroxide phases is the highest, while the Mn metal had the lowest mobility.