how are diagenesis and reservoir quality linked to depositional facies a deltaic succession edgeoya svalbard abstract middle to late triassic strata exposed at edgeoya eastern svalbard represents an uplifted part of the northwestern corner of the barents sea this interval is characterized by a predominantly mud dominated deltaic depositional system where rocks with petroleum reservoir potential are expected both in delta front and channelized sandstone deposits recent drilling campaigns into time equivalent rocks in the barents sea have however been disappointing with porosity and permeability below expectation this study improves the current understanding of reservoir potential within depositional systems of this kind by examining the link between different depositional facies diagenesis and their impact on reservoir quality five depositional facies were mapped and correlated channel floodplain shallow marine prodelta and offshore our study suggests that diagenetic signatures that control the quality of reservoir rocks vary systematically with these depositional facies mechanical compaction was the main cause of porosity destruction in the channel shallow marine and floodplain samples while early carbonate cementation occludes the intergranular volume in the prodelta samples moreover quartz cement reduced the porosity in the shallow marine depositional facies the estimated maximum burial temperature of 124 c indicates burial to several km depths depositional settings within a deltaic to shallow marine system do not exert a strong control on the abundance of chlorite but it does impact whether the chlorite is pore filling or grain coating and porosity preserving efficient chlorite coating have aided in preserving porosities up to 32 in the very well well sorted medium to fine grained sandstones deposited by fluvial dominated channels

neoproterozoic iron formation an evaluation of its temporal environmental and tectonic significance article neoproterozoic iron formation nif provides evidence for the widespread return of anoxic and ferruginous ba sins during a time period associated with major changes in climate tectonics and biogeochemistry of the oceans here we summarize the stratigraphic contextof neoproterozoic iron formation and its geographic and temporal dis tribution it is evident that most nif is associated with the earlier cryogenian sturtian glacial epoch although it is possible that some nif may be ediacaran there is no incontrovertible evidence to support this age assignment the paleogeographic distribution of nif is consistent with anoxic and ferruginous conditions occurring in basins within rodinia or in rift basins developed on its margins consequently nif does not require whole ocean anoxia simple calculations using modern day iron fluxes suggest that only models that invoke hydrothermal and or detrital sourcesofiron are capable ofsupplying sufficient ironto account forthe mass of the largernif occurrences this con clusion is reinforced by the available geochemical data that imply nif record is a mixture of hydrothermal and de trital components a common thread that appears to link most if not all nif is an association with mafi cv olcanics

application of k ϵ turbulence models to enclosed basins the role of internal seiches 1 a numerical model was developed for the prediction of the density stratification of lakes and reservoirs it combines a buoyancy extended k model with a seiche excitation and damping model to predict the diffusivity below the surface mixed layer the model was applied to predict the seasonal development of temperature stratification and turbulent diffusivity in two medium sized lakes over time periods ranging from 3 weeks to 2 years depending on the type of boundary condition for temperature two or three model parameters were optimized to calibrate the model the agreement between the simulated and the observed temperature distributions is excellent in particular if lake surface temperatures were prescribed as surface boundary condition instead of temperature gradients derived from heat fluxes comparison of different model variants revealed that inclusion of horizontal pressure gradients and or stability functions is not required to provide good agreement between model results and data with the aid of uncertainty analysis it is shown that the depth of the mixed surface layer during the stratified period could be predicted accurately within 1 m the sensitivity of the model to several parameters is discussed index terms 4211 oceanography general benthic boundary layers 4255 oceanography general numerical modeling 4568 oceanography physical turbulence diffusion and mixing processes keywords lake turbulence model seiche stratification simulation turbulence kinetic energy

iron isotopic variations of the cryogenian banded iron formations a new model abstract the fe isotope composition of banded iron formation bif is regarded as a powerful tracer in paleoceanography and the cryogenian banded iron formation associated with snowball earth events provides a precious record of environmental change in the neoproterozoic however fe isotope studies on cryogenian bifs are rare and previous interpretations of cryogenian fe isotope data are problematic here we present a systematic investigation of the fe isotope composition of the cryogenian xinyu bif sections from four localities in the yangtze region south china these bif sections comprise banded magnetite quartzite banded chlorite magnetite quartzite and magnetite bearing chlorite phyllite in stratigraphically ascending order the 56feirmm 014 values of the xinyu bif vary significantly and show an overall increase upsection from ca 0 to ca 1 5 this stratigraphic trend in fe isotope compositions is similar to those reported previously for cryogenian bifs in north america and australia and thus seems to be a common phenomenon we interpret fe isotope variation in cryogenian bif to be essentially controlled by varying degrees of fe precipitation in seawater rather than resulted from a water column fe isotope gradient as proposed previously the variation in the degree of fe precipitation can be controlled by changes of eh and or ph conditions in seawater resulting from transgression or ocean acidification

large iron isotope fractionation at the oxic anoxic boundary in lake nyos abstract the degassing of volcanic lake nyos cameroon provides the opportunity to study the strong isotopic variation of dissolved fe ii in a well constrained redox cycle and to identify the governing processes by reaction transport modeling two depth profiles sampled in the lake in march 2004 and 2005 reveal an increase in iron concentrations and 57 fe from around 1 mg l 1 and 1 88 at 55 m depth up to 344 mg l 1 and 0 83 at the bottom of the lake respectively a steep increase in 57 fe was observed across the oxic anoxic boundary as many biological and geochemical processes are known to fractionate fe isotopes we used a calibrated reaction transport model to disentangle the processes governing the fe cycle the model combines the isotopic signatures of dissolved fe ii and settling fe iii particles with the concentration profiles and settling fluxes of the fe particles in the lake we show that the strong shift in 57 fe is caused by isotopic fractionation via dissimilatory fe reduction across the oxic anoxic boundary of lake nyos the shift towards more positive values below the oxic anoxic interface could be attributed to vertical mixing of a heavier component from the bottom of the lake

iron isotope variations in holocene sediments of the gotland deep baltic sea holocene sediments from the gotland deep basin in the baltic sea were investigated for their fe isotopic composition in order to assess the impact of changes in redox conditions and a transition from freshwater to brackish water on the isotope signature of iron the sediments display variations in 56fe differences in the 56fe 54fe ratio relative to the irmm 14 standard from 0 27 0 09 to 0 21 0 08 samples deposited in a mainly limnic environment with oxygenated bottom water have a mean 56fe of 0 08 0 13 which is identical to the mean fe isotopic composition of igneous rocks and oxic marine sediments in contrast sediments that formed in brackish water under periodically euxinic conditions display significantly lighter fe isotope signatures with a mean 56fe of 0 14 0 19 negative correlations of the 56fe values with the fe al ratio and s content of the samples suggest that the isotopically light fe in the periodically euxinic samples is associated with reactive fe enrichments and sulfides this is supported by analyses of pyrite separates from this unit that have a mean fe isotopic composition of 1 06 0 20 for 56fe the supply of additional fe with a light fe isotopic signature can be explained with the shelf to basin fe shuttle model according to the fe shuttle model oxides and benthic ferrous fe that is derived from dissimilatory iron reduction from shelves is transported and accumulated in euxinic basins the data furthermore suggest that the euxinic water has a negative dissolved 56fe value of about 1 4 to 0 9 if negative fe isotopic signatures are characteristic for euxinic sediment formation widespread euxinia in the past might have shifted the fe isotopic composition of dissolved fe in the ocean towards more positive 56fe values

iron isotopes in the amazon river system weathering and transport signatures abstract trace metal clean plankton tows river samples aerosol leachates and porewater samples from the amazon river system were measured for their iron isotopic composition the overall isotopic range observed was 1 5 with fe isotopic variability observed between different types of tributaries dissolved fe and suspended load fe from two main channel sites were isotopically similar 0 1 to 0 3 in contrast the dissolved fe and suspended load fe from an organic rich tributary the negro river were isotopically distinct with the dissolved slightly isotopically heavy 0 3 and the suspended load isotopically light 0 9 the isotopically lightest sample from the amazon region was shelf porewater 1 3 in river water seawater mixing experiments the fe isotopic signal of dissolved fe of river water was modified by flocculation of isotopically heavy fe the variability of the fe isotopic composition of natural samples including biological and aqueous samples demonstrates that significant fractionation is associated with fe biogeochemistry in the environment

rayleigh fractionation of iron isotopes during pedogenesis along a climate sequence of hawaiian basalt abstract we measured iron isotopic composition of surface 10 20 cm and subsurface 50 70 cm basaltic soil horizons from the island of maui along a climate gradient mcg ranging from 2 2 to 4 2 m mean annual precipitation map all soil forming factors except climate were conserved the mcg has a documented decrease in fe with increasing rainfall that is highly correlated with decreasing mean annual eh values we found that increasing map from 2 8 to 4 2 m resulted in a surface plus subsurface average increase of 0 56 0 09 56fe with the subsurface consistently 0 33 0 06 56fe greater than the surface horizons based on loss of fe relative to nb rayleigh fractionation was observed with 103ln lost retained values of 0 37 0 03 and 0 34 0 04 for the surface and subsurface respectively equivalent 103ln lost retained values for the surface and subsurface soils suggests fe loss is driven by similar mechanisms throughout the soil profile our calculated fractionation factor is about 1 3 the magnitude of laboratory determined fractionation factors for fe reduction suggesting other processes organic complexation fe re precipitation modulate the net fe loss along the mcg these results offer field scale confirmation of laboratory experiments on model systems that show anoxic weathering reactions produce materials enriched in heavy fe isotopes

fe isotopic composition of the quaternary red clay in subtropical southeast china redoxic fe mobility and its paleoenvironmental implications abstract to probe fe isotopic signatures of the quaternary red clay qrc in subtropical southeastern china two qrc profiles located in southern anhui province southeast china were studied the whole profiles were generally in a narrow range in 56 fe of 0 040 0 061 0 117 0 048 the yellow brown earth ybe and uniform red clay urc of the upper profiles were similar in 56 fe to the xiashu loess along the yangtze river southeast china and the loess paleosols and tertiary red clay in the chinese loess plateaus northwest china as well as the baseline of terrestrial igneous rocks the slight variation of 56 fe from the ybe to the urc suggests that the chemical weathering and rubification on the aerobic surface did not fractionate fe isotopes significantly the reticulate red clay rrc underlying the ybe and urc fluctuated in 56 fe to some extent the white veins separated from the lower rrc were depleted of fe oxides and heavier in fe isotopes and the red veins on the contrary were isotopically lighter in fe 56 fe of the white veins 0 345 0 053 on average was significantly higher than the bulk samples 0 033 0 044 on average and the red veins 0 021 0 046 on average this suggests that the reticulate textures of the rrc were formed by the reductive removal re precipitation of fe due to the cyclical changes of redoxic conditions fe isotopic composition of the qrc was dominantly controlled by the redoxic mobility of fe rather than the degree of weathering 56 fe of the profiles was not correlated with the paleoclimatic events and thus cannot be used as a paleoclimatic proxy the fractionation of fe isotopes in the lower rrc however indicated the occurrence of over humid paleoclimate with alternations between drought and wetness in southern china during the pleistocene

the effect of early diagenesis on the fe isotope compositions of porewaters and authigenic minerals in continental margin sediments abstract iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the california continental margin where isotope compositions range from 56fe 3 0 to 0 4 at one site near monterey canyon off central california organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction dir and bacterial sulfate reduction whereas at our other site in the santa barbara basin dir appears to be comparatively small and production of sulfides fes and pyrite was extensive the largest range in fe isotope compositions is observed for fe ii aq in porewaters which generally have the lowest 56fe values minimum 3 0 near the sediment surface and increase with burial depth 56fe values for fes inferred from hcl extractions vary between 0 4 and 0 4 but pyrite is similar at both stations where an average 56fe value of 0 8 0 2 was measured we interpret variations in dissolved fe isotope compositions to be best explained by open system behavior that involves extensive recycling of feflux this study is the first to examine fe isotope variations in modern marine sediments and the results show that fe isotopes in the various reactive fe pools undergo isotopic fractionation during early diagenesis importantly processes dominated by sulfide formation produce high 56fe values for porewaters whereas the opposite occurs when fe iii oxides are present and dir is a major pathway of organic carbon respiration because shelf pore fluids may carry a negative 56fe signature it is possible that the fe isotope composition of ocean water reflects a significant contribution of shelf derived iron to the open ocean such a signature would be an important means for tracing iron sources to the ocean and water mass circulation

hydrothermal alteration of the ediacaran volyn brest volcanics on the western margin of the east european craton abstract the ediacaran continental flood basalts and associated tuffs were studied to identify and quantify alteration processes by means of xrd and chemistry supplemented by mossbauer and ftir spectroscopies petrography oxygen and iron isotopes k ar dating and organic geochemistry two superimposed alteration processes were identified the ediacaran hydrothermal alteration induced by meteoric waters heated and put in motion by the cooling basalt and the caledonian and or variscan potassic alteration the degree of basalt alteration was quantified using as an index the sum of primary minerals in the bulk rock the sequence of minerals dissolved and crystallized during the hydrothermal alteration was established the alteration resulted in the loss of ca dissolution of plagioclases compensated by the gain of water and mg crystallization of clays and proceeded from the edges of the basalt flows in an oxidizing environment evidenced by the increasing amount of hematite and fe3 fe2 ratio of the bulk rock cyanobacteria were active in the hydrothermal system most probably responsible for the measured negative 56fe values and more reducing conditions at the stage of intense alteration chlorophaeite palagonite following quartz as the earliest petrographically identifiable basalt alteration product was found to vary systematically from fully isotropic to birefringent the chlorophaeite was identified as a mixture of fe montmorillonite and fe saponite identical with griffithite and oxysmectites probably preceded by a finer grained ferrosaponite at the isotropic stage ree content of chlorophaeite indicates basaltic volcanic glass sideromelane as the major source of material ree in clays are contained mostly in the dioctahedral smectite while in the bulk rock mostly in phosphates the smectite characteristics and mg enrichment are indicative of the hydrothermal basalt alteration process which perhaps was dominant also on mars both mineral and chemical composition of tuffs vary continuously from basaltic to felsic the latter close to the measured rhyodacite composition dominated by quartz and feldspars the basaltic tuffs resemble the most altered basalts but contain also abundant albite and chlorite indicative of higher alteration temperatures up to 220 c tuff composition indicates stronger component of felsic volcanism in the trap formation than evidenced by the preserved bodies of effusive rocks

iron isotope fractionation in a sulfide bearing subterranean estuary and its potential influence on oceanic fe isotope flux we trace pathways of fe reactions in the indian river lagoon florida usa subterranean estuary using fe isotopes to provide new constraints on fe isotopic fractionation in a sulfide bearing subterranean estuary porewater 56fe values increase from 1 16 at 115cm depth to 0 2 at 7cm depth due to isotope fractionation in three distinct lithostratigraphic zones the deepest zone contains orange sands with elevated fe oxide contents 0 2wt that dissolve through diagenetic fe oxide reduction and elevate fe concentrations in porewaters 100 to 300 m l this reaction causes porewater 56fe values to be 1 lighter than the sediment 56fe values an intermediate zone contains white fe poor sands with fe oxide contents 0 1wt and dissolved fe concentrations 20 m l this zone is a sink for dissolved fe through adsorption of isotopically heavy dissolved fe ii onto mineral surfaces this adsorption results in porewater 56fe values that are as much as 1 8 lighter than sediment 56fe values the uppermost zone contains organic carbon and fe sulfide rich black sediments with low dissolved fe 1 m l and elevated porewater sulfide up to 600 m l concentrations precipitation of isotopically light fe sulfides increases the porewater 56fe values as much as 0 68 more than corresponding sediment 56fe values the near surface fe sulfide precipitation delivers to the lagoon dissolved fe with slightly positive 56fe values averaging about 0 24 via submarine groundwater discharge sgd iron sulfide precipitation in sulfide containing subterranean estuaries thus may result in a previously unidentified source of isotopically heavy fe to the coastal oceans

tracing water column euxinia in eastern mediterranean sapropels s5 and s7 abstract sapropels s5 and s7 formed in the semi enclosed eastern mediterranean sea ems during peak interglacial periods mis5e and mis7a respectively are considered among the most strongly developed quaternary sapropels this study investigates the redox dynamics of the water column during their formation via fe isotope and fe speciation studies of cores taken at 2550 m depth at site odp 967 both sapropels show an inverse correlation between 56fe and fet al with slopes mostly matching that found for the black sea pointing to a benthic shelf to basin shuttle of fe and subsequent precipitation of fe sulphides in euxinic bottom waters an exception to these black sea type trends occurs during the later peak stages of s7 where the negative 56fe fet al slope shallows fe speciation studies reveal that the dominant highly reactive fe phase fehr in the sapropels is pyrite with fe oxyhydr oxides forming the second major mineral component correspondingly fehr fet plots show increased strengthening of anoxic water conditions during the passage from pre sapropel sediment into the sapropel nevertheless despite the evidence for euxinic conditions from both fe isotopes and high mo concentrations in the sapropel fepy fehr ratios remain below values commonly used to identify water column euxinia this apparent contradiction is ascribed to the sedimentary preservation of a high flux of crystalline fe oxyhydr oxide minerals to the basin which resulted in a relatively low degree of sulphidation despite the presence of euxinic bottom waters thus the operationally defined ferruginous euxinic boundary for ems sapropels is better placed at fepy fehr 0 6 which is somewhat below the usually ascribed lower limit of 0 7 consistent with the significant presence of crystalline fe oxyhydr oxides the change in the 56fe fet al slope during peak s7 is ascribed to an enhanced monsoon driven flux of detrital fe iii oxides from the river nile into the ems basin and comcomitant diagenetic sulphidation euxinic water column conditions in sapropel s5 and s7 are interpreted here to reflect the positive balance between dissolved sulphide formation and rates of reductive dissolution of fe oxyhydr oxide minerals both of these parameters in turn depend on the extent to which water overturn times are reduced and export productivity increased during sapropel formation

iron isotopes in the early marine diagenetic iron cycle measurements of fe oxyhydroxides reactive fe iii in two shallow sediment profiles from reducing and oxic environments on the arabian sea margin demonstrate fe isotope fractionation during early marine diagenesis reactive fe iii has 56 fe values between 0 77 and 0 19 values are lowest at the top of the sediment profile and considerably lower than bulk sediment fe 56 fe 0 preferential reduction and dissolution of light fe isotopes during diagenesis leaves behind an increasingly smaller and heavier reactive fe iii residual initially the isotopic composition of reactive fe iii evolves down core according to the fractionation factor typical of microbial dissimilatory fe reduction deeper in the profile 56 fe values remain unchanged despite further fe reduction here another process with a different fractionation factor becomes dominant probably reduction by dissolved sulfide the 56 fe of the residual reactive fe iii suggests that 25 of the initially present reactive fe iii was reduced by microbial fe reduction when fe is diagenetically recycled between reducing sediments at depth and an oxic top layer the process depending on recycling efficiency may result in the accumulation of light fe in the top layer while the complementary heavier residual is buried fe diffusing from the seafloor back into the ocean should reflect the low 56 fe diagenetic source of dissolved fe

iron isotopes in an archean ocean analogue abstract iron isotopes have been extensively used to trace the history of microbial metabolisms and the redox evolution of the oceans archean sedimentary rocks display greater variability in iron isotope ratios and more markedly negative values than those deposited in the proterozoic and phanerozoic this increased variability has been linked to changes in either water column iron cycling or the extent of benthic microbial iron reduction through time we tested these contrasting scenarios through a detailed study of anoxic and ferruginous lac pavin france which can serve as a modern analogue of the archean ocean a depth profile in the water column of lac pavin shows a remarkable increase in dissolved fe concentration 0 1 1200 m and 56fe values 2 14 to 0 31 across the oxic anoxic boundary to the lake bottom the largest fe isotope variability is found at the redox boundary and is related to partial oxidation of dissolved ferrous iron leaving the residual fe enriched in light isotopes the analysis of four sediment cores collected along a lateral profile one in the oxic layer one at the redox boundary one in the anoxic zone and one at the bottom of the lake indicates that bulk sediments porewaters and reactive fe mostly have 56fe values near 0 0 0 2 similar to detrital iron in contrast pyrite 56fe values in sub chemocline cores 60 65 and 92 m are highly variable and show significant deviations from the detrital iron isotope composition 56fepyrite between 1 51 and 0 09 average 0 93 importantly the pyrite 56fe values mirror the 56fe of dissolved iron at the redox boundary where near quantitative sulfate and sulfide drawdown occurs suggesting limited iron isotope fractionation during iron sulfide formation this finding has important implications for the archean environment specifically this work suggests that in a ferruginous system most of the fe isotope variability observed in sedimentary pyrites can be tied to water column cycling foremost to the oxidation of dissolved ferrous iron this supports previous suggestions that enhanced iron isotope variability in the archean may record a unique stage in earth s history where partial ferrous iron oxidation in upwelling water masses was a common process probably linked to oxygenic or anoxygenic photosynthesis

geochemical insight into differences in the physical structures and dynamics of two adjacent maar lakes at mt vulture volcano southern italy 1 we report on the first geochemical investigation of the monticchio maar lakes mt vulture volcano southern italy covering an annual cycle that aimed at understanding the characteristic features of the physical structures and dynamics of the two lakes we provide the first detailed description of the lakes based on high resolution conductivity temperature depth ctd profiles chemical and isotopic h and o compositions of the water and the amounts of dissolved gases e g he ar ch4 and co2 the combined data set reveals that the two lakes which are separated by less than 200 m exhibit different dynamics one is a meromictic lake where the waters are rich in biogenic and mantle derived gases while the other is a monomictic lake which exhibits complete turnover of the water in winter and the release of dissolved gases our data strongly suggest that the differences in the dynamics of the two lakes are due to different density profiles affected by dissolved solutes mainly fe which is strongly enriched in the deep water of the meromictic lake a conceptual model of water balance was constructed based on the correlation between the chemical composition of the water and the hydrogen isotopic signature gas rich groundwaters that feed both of the lakes and evaporation processes subsequently modify the water chemistry of the lakes our data highlight that no further potential hazardous accumulation of lethal gases is expected at the monticchio lakes nevertheless geochemical monitoring is needed to prevent the possibility of vigorous gas releases that have previously occurred in historical time

the sedimentary and tectonic evolution of the yinggehai song hong basin and the southern hainan margin south china sea implications for tibetan uplift and monsoon intensification 1 the yinggehai song hong basin is one of the world s largest pull apart basins lying along the trace of the red river fault zone in the south china sea south of hainan island this basin crosscuts the rifted margin of the northern south china sea in this paper we present for the first time a regional compilation of multichannel seismic reflection data from both the strike slip and rifted margins the basins started to open after similar to 45 ma especially after similar to 34 ma the yinggehai basin was folded and inverted in the middle miocene after 21 ma in the north and 14 ma in the south before rapidly subsiding again after similar to 5 ma because of continued tectonism this subsidence has caused shale diapirism especially driven by associated sedimentation in the late pliocene 2 6 2 0 ma extension along the adjacent south hainan margin shows preferential lower crustal extension suggestive of lower crustal flow increasing toward the continent ocean transition during breakup sediment supply is reconstructed to peak in the middle miocene then falls between 14 and 10 3 ma to reach a low in the late miocene however rates rose again in the pliocene pleistocene the red river sediment budget is incompatible with climate models that propose stronger monsoon rains starting at 8 ma or with large scale river capture away from the red river after similar to 10 ma both lines of evidence point to major uplift in the red river drainage being middle miocene or older the recent preindustrial red river carried much more sediment than the average pleistocene accumulation rate indicating modest sediment buffering onshore at least in recent geologic time

iron isotopes constrain biologic and abiologic processes in banded iron formation genesis abstract the voluminous 2 5 ga banded iron formations bifs from the hamersley basin australia and transvaal craton south africa record an extensive period of fe redox cycling the major fe bearing minerals in the hamersley transvaal bifs magnetite and siderite did not form in fe isotope equilibrium but instead reflect distinct formation pathways the near zero average 56fe values for magnetite record a strong inheritance from fe3 oxide hydroxide precursors that formed in the upper water column through complete or near complete oxidation transformation of the fe3 oxide hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of 56fe values 1 addition of marine hydrothermal fe 2 aq 2 complete reduction by bacterial dissimilatory iron reduction dir and 3 interaction with excess fe 2 aq that had low 56fe values and was produced by dir most siderite has slightly negative 56fe values of 0 5 that indicate equilibrium with late archean seawater although some very negative 56fe values may record dir support for an important role of dir in siderite formation in bifs comes from previously published c isotope data on siderite which may be explained as a mixture of c from bacterial and seawater sources several factors likely contributed to the important role that dir played in bif formation including high rates of ferric oxide hydroxide formation in the upper water column delivery of organic carbon produced by photosynthesis and low clastic input we infer that dir driven fe redox cycling was much more important at this time than in modern marine systems the low pyrite contents of magnetite and siderite facies bifs suggests that bacterial sulfate reduction was minor at least in the environments of bif formation and the absence of sulfide was important in preserving magnetite and siderite in the bifs minerals that are poorly preserved in the modern marine record the paucity of negative 56fe values in older early archean and younger early proterozoic bifs suggests that the extensive 2 5 ga hamersley transvaal bifs may record a period of maximum expansion of dir in earth s history

water and gas chemistry of lake nyos and its bearing on the eruptive process abstract the isotopic and chemical composition of water samples collected from lake nyos some two and eight weeks after the eruption of august 21 1986 point to the existence of three distinct mixing regimes involving three water components an essentially homogeneous unmixed body of water at depths 100 m overlain by water increasingly affected by surface evaporation and a 5 10 m layer containing recent but pre eruption rain water the cationic constituents na k mg ca mn fe of the lake water correspond to the dissolution of around 0 1 g of local rock the waters are close to saturation with respect to siderite the composition of the gas dissolved in the deep lake waters 0 65 b w of co2 pco2 4 4b corresponds in mmol mol 1 to 996 co2 2 0 ch4 2 0 n2 0 05 ar 0 004 he 0 0002 h2 0 0001 ne co 2 13 c 3 4 and of he rair 5 5 suggest deep magmatic origins the 13c and 2h content of ch4 organic sedimentary origin the presence of aromatic hydrocarbons indicates very high temperatures of hydrocarbon formation exsolution of gas will first lead to the precipitation of siderite then iron hydroxide the chemistry of the lake waters points to a loss of some 240 000 t of co2 from the upper 100 m of the lake their isotopic composition is consistent with the assumption that the eruption was triggered by the accumulation of cold rain waters at the lake surface prior to the eruption inducing partial convective overturn there is no need to invoke addition of chemical or isotopic constituents from deeper levels during the eruption

iron isotope composition of some archean and proterozoic iron formations abstract fe isotopes can provide new insight into redox dependent biogeochemical processes precambrian iron formations if are deserving targets for fe isotope studies because they are composed predominantly of authigenic fe phases and record a period of unprecedented iron deposition in earth s history we present fe isotope data for bulk samples from 24 archean and proterozoic if and eight phanerozoic fe oxide rich deposits these data reveal that many archean and early paleoproterozoic iron formations were a sink for isotopically heavy fe in contrast to later proterozoic and phanerozoic fe oxide rich rocks the positive 56 fe values in if are best explained by delivery of particulate ferric oxides formed in the water column to the sediment water interface because if are a net sink for isotopically heavy fe there must be a corresponding pool of isotopically light fe in the sedimentary record earlier work suggested that archean pyritic black shales were an important part of this light sink before 2 35 billion years ago ga it is therefore likely that the persistently and anomalously low 56 fe values in shales are linked with the deposition of isotopically heavy fe in if in the deeper parts of basins if deposition produced a residual isotopically light dissolved fe pool that was captured by pyritic fe in shales local dissimilatory fe reduction in porewater and associated diagenetic reactions resulting in pyrite and carbonate precipitation may have further enhanced fe isotope heterogeneity in marine sediments and an iron shuttle may have transported isotopically light fe from shelf sediments to the basin nevertheless water column processing of hydrothermally delivered fe likely had the strongest influence on the bulk iron isotope composition of archean and paleoproterozoic iron formations and other marine sediments

iron and carbon isotope evidence for ecosystem and environmental diversity in the 2 7 to 2 5 ga hamersley province western australia abstract the largest excursion in kerogen 13c and bulk mineral 56fe values yet measured in the ancient rock record occurs in rocks of 2 7 to 2 5 ga age new fe isotope data integrated with previously collected c isotope data on the same samples document the metabolic diversity of microbial communities in the neoarchean hamersley province of the pilbara craton in western australia samples of shales carbonates and mixed carbonate shale lithologies were collected from three drill cores two cores from the depocenter of the province and one from the margin shallow water clastic carbonate rocks deposited in the center of the province tumbiana formation record kerogen 13c values that indicate c cycling by various anaerobic or aerobic methane pathways but the restricted range in 56fe values indicates little or no fe redox cycling deep water sediments deposited contemporaneously in both parts of the hamersley province jeerinah formation record slightly positive 56fe values in the relatively shallower and suboxic margin but strongly negative 56fe values in the deeper euxinic depocenter of the province a pattern consistent with fe cycling via a basin fe shuttle driven by bacterial dissimilatory iron reduction dir kerogen 13c values from these units indicate coupling of microbial fe cycling to aerobic methanotrophy or anaerobic oxidation of methane younger black shales intercalated with iron formation marra mamba iron formation in the depocenter record a shift to near zero 56fe values reflecting an fe budget dominated by hydrothermal and clastic sources however time equivalent fe rich carbonate shale lithologies deposited on the margin of the province carawine dolomite have 56fe values that steadily decrease from near zero to strongly negative values these relatively fe rich carbonates may reflect a carbonate trap of a dir driven fe shuttle similar to the sulfidic trap in the euxinic portion of the jeerinah formation in contrast younger shallow water carbonates deposited by turbidity currents in relatively deep water in the center of the province wittenoom formation have 56fe values that correlate with fe concentrations a pattern that indicates fe cycling by rayleigh fractionation through precipitation of iron oxides from aqueous ferrous iron

the biogeochemistry of ferruginous lakes and past ferruginous oceans abstract anoxic and iron rich ferruginous conditions prevailed in the ocean under the low oxygen atmosphere that occurred through most of the archean eon while euxinic conditions i e anoxic and hydrogen sulfide rich waters became more common in the proterozoic ferruginous conditions persisted in deep waters ferruginous ocean regions would have been a major biosphere and earth surface reservoir through which elements passed through as part of their global biogeochemical cycles understanding key biological events such as the rise of oxygen in the atmosphere or even the transitions from ferruginous to euxinic or oxic conditions requires understanding the biogeochemical processes occurring within ferruginous oceans and their indicators in the rock record important analogs for transitions between ferruginous and oxic or euxinic conditions are paleoferruginous lakes their sediments commonly host siderite and ca carbonates which are important precambrian records of the carbon cycling lakes that were ferruginous in the past or euxinic lakes with cryptic iron cycling may also help understand transitions between ferruginous and euxinic conditions in shallow and mid depth oceanic waters during the proterozoic modern ferruginous meromictic lakes which host diverse anaerobic microbial communities are increasingly utilized as biogeochemical analogues for ancient ferruginous oceans such lakes are believed to be rare but regional and geological factors indicate they may be more common than previously thought while physical mixing processes in lakes and oceans are notably different many chemical and biological processes are similar the diversity of sizes stratifications and water chemistries in ferruginous lakes thus can be leveraged to explore biogeochemical controls in a range of marine systems near shore off shore silled basins or those dominated by terrestrial or hydrothermal element sources ferruginous systems both extant and extinct lacustrine and marine host a continuum of biogeochemical processes that highlight the important role of iron in the evolution of earth s surface environment

impact of depositional facies on the distribution of diagenetic alterations in the devonian shoreface sandstone reservoirs southern ghadamis basin libya abstract the middle devonian shoreface quartz arenites present day burial depths 2833 2786 m are important oil and gas reservoirs in the ghadamis basin western libya this integrated petrographic and geochemical study aims to unravel the impact of depositional facies on distribution of diagenetic alterations and consequently related reservoir quality and heterogeneity of the sandstones eogenetic alterations include the formation of kaolinite pseudomatrix and pyrite the mesogenetic alterations include cementation by quartz overgrowths fe dolomite ankerite and illite transformation of kaolinite to dickite illitization of smectite intergranular quartz dissolution and stylolitization and albitization of feldspar the higher energy of deposition of the coarser grained upper shoreface sandstones combined with less extensive chemical compaction and smaller amounts of quartz overgrowths account for their better primary reservoir quality compared to the finer grained middle lower shoreface sandstones the formation of kaolin in the upper and middle shoreface sandstones is attributed to a greater flux of meteoric water more abundant quartz overgrowths in the middle and lower shoreface is attributed to a greater extent of stylolitization which was promoted by more abundant illitic clays this study demonstrated that linking the distribution of diagenetic alterations to depositional facies of shoreface sandstones leads to a better understanding of the impact of these alterations on the spatial and temporal variation in quality and heterogeneity of the reservoirs

iron isotope and ree y composition of the caue banded iron formation and related iron ores of the quadrilatero ferrifero brazil the minas supergroup banded iron formations bifs of the brazilian quadrilatero ferrifero qf mineral province experienced multiple deformational events synchronous with hypogene mineralization which resulted in the metamorphism of bifs to itabirites and their upgrade to high grade iron ore here we present rare earth element and yttrium ree y compositions together with iron isotope ratios of itabirites and their host iron orebodies from 10 iron deposits to constrain environmental conditions during bif deposition and the effects of hypogene iron enrichment the ree y characteristics of itabirites positive eu anomaly and lree depletion indicate hydrothermal iron contribution to the minas basin iron isotope data and ce anomalies suggest bifs were precipitated by a combination of anoxic biological mediated ferrous iron oxidation and abiotic oxidation in an environment with free oxygen such as an oxygen oasis perhaps related to increase in oxygen concentrations before the great oxidation event goe the similarity of the ree y composition of the itabirites from the different qf deformational domains as well as to other superior type bifs indicates that the metamorphism and synchronous hydrothermal mineralization did not significantly affect the geochemical signature of the original bifs however iron isotope compositions of iron ore vary systematically between deformational domains of the qf likely reflecting the specific mineralization features in each domain

experimental determination of fe isotope fractionation between aqueous fe ii siderite and green rust in abiotic systems abstract iron isotope fractionation between aqueous iron and siderite has been measured using abiotic synthesis experiments at 20 c applying a rayleigh distillation model to three experiments in which siderite feco3 was continuously precipitated from solutions over the course of tens of hours the derived fractionation factor is 103ln fe ii siderite 0 48 0 22 2 this experimentally determined fractionation factor is significantly smaller by 1 5 than that calculated from vibrational frequencies and mossbauer shifts geochim cosmochim acta 64 2000 849 865 geochim cosmochim acta 65 2001 2487 2497 and lies on the high end of that inferred from natural mineral assemblages where the fe ii aq siderite fractionation factor was inferred to lie between 1 7 and 0 3 contrib mineral petrol 144 2003 523 547 the measured fe ii aq siderite fractionation from this study suggests that siderite from archean and proterozoic rocks which have moderately negative 56fe values 1 5 to 0 5 may reflect precipitation from fe ii aq whose origin was hydrothermal in contrast natural siderites that have higher 56fe values apparently require non hydrothermal sources of fe and or different pathways of formation or may reflect the effects of cation substitution on the fe ii aq carbonate fractionation factor

iron stable isotopes beyond biosignatures abstract the stable isotope geochemistry of fe has attracted intense interest in the past five years this interest was originally motivated by the possible use of fe isotopes in biosignature applications particularly in sediments from the ancient earth or mars this application is still being developed with particular attention to fractionation mechanisms understanding such mechanisms should also provide new insights into the environmental biogeochemistry of fe at the same time the fe isotope system holds promise for other exciting frontiers including applications in oceanography solid earth geochemistry and biomedicine such applications will be increasingly attractive as fe isotope analysis becomes routine

iron isotope fractionation during pyrite formation in a sulfidic precambrian ocean analogue the chemical response of the precambrian oceans to rising atmospheric o2 levels remains controversial the iron isotope signature of sedimentary pyrite is widely used to trace the microbial and redox states of the ocean yet the iron isotope fractionation accompanying pyrite formation in nature is difficult to constrain due to the complexity of the pyrite formation process difficulties in translating the iron isotope systematics of experimental studies to natural settings and insufficient iron isotope datasets for natural euxinic i e anoxic and sulfidic marine basins where pyrite formation occurs herein we demonstrate that a large permil level shift in the isotope composition of dissolved iron occurs in the black sea euxinic water column during syngenetic pyrite formation specifically iron removal to syngenetic pyrite gives rise to an iron isotope fractionation factor between fe ii and fes2 of 2 75 permil the largest yet reported for reactions under natural conditions that do not involve iron redox chemistry these iron isotope systematics offer the potential to generate permil level shifts in the sedimentary pyrite iron isotope record due to partial drawdown of the oceanic iron inventory the implication is that the iron stable isotope signatures of sedimentary pyrites may record fundamental regime shifts between pyrite formation under sulfur limited conditions and pyrite formation under iron limited conditions to this end the iron isotope signatures of sedimentary pyrite may best represent the extent of euxinia in the past global ocean rather than its oxygenation state on this basis the reinterpreted sedimentary pyrite fe isotope record suggests a fundamental shift towards more sulfidic oceanic conditions coincident with the great oxidation event around 2 3 billion years ago importantly this does not require the chemical state of the ocean to shift from mainly de oxygenated to predominantly oxygenated in parallel with the permanent rise in atmospheric oxygen contrary to other interpretations based on iron isotope systematics

compaction of lithic sands experimental results and applications compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present these experiments produced a product that is texturally indistinguishable from natural lithic sandstones generally sedimentary lithic fragments are more ductile than metamorphic lithic fragments volcanic lithic fragments when altered to phyllosilicate minerals by weathering or diagenesis are extremely ductile mineralogy and intragranular microporosity influence ductility ratios based on weight percent of soft ductile minerals clay and micas to hard brittle minerals quartz and feldspar within the lithic fragments provides a good indication of ductility a ratio greater than 1 indicates the lithic material will be h ghly ductile microporous lithic fragments are more ductile than dense lithic fragments physical compaction models were developed based on experimental data from well sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress the type of lithic material and the quantity of lithic material these models are derived from laboratory sandstones with at least 25 lithic grains in the following categories moderately ductile metamorphic lithic fragments highly ductile shale lithic fragments and extremely ductile altered volcanic lithic fragments application of the compaction models of the experimental compaction techniques to modern subsurface sand samples or outcrop equivalents can provide valuable estimates of the preserved porosities i e reservoir potential for lithic sandstones that may be exploratory objecti es in frontier basins examples using the models and experimental compaction techniques for exploration are given experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation an early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack a later cement does not have this beneficial effect overpressure when developed early retards compaction by reducing the effective stress of course a late developing overpressure is not effective at preserving porosity because compaction is an irreversible process

iron isotope fractionation in sediments of an oligotrophic freshwater lake abstract in situ iron fe isotope compositions of pore water and solid bound fe phases were measured in sediments of an oligotrophic sulfate poor freshwater lake lake tantare quebec canada previous work has shown that dissimilatory fe iii reduction dir is the main fe reduction pathway in this setting hence the lake sediments provide a well characterized natural environment in which to assess the transferability of dir produced fe isotope fractionations obtained in laboratory model systems iron redox cycling within the sediments produces isotopically light pore water fe ii average 56 fe 2 1 0 6 and sorbed fe ii average 56 fe 1 2 0 2 compared to the fe iii oxyhydroxide average 56 fe 0 6 0 2 and tightly solid bound fe ii average 56 fe 0 3 0 2 sediment pools the apparent isotope fractionation factor between aqueous fe ii and fe iii oxyhydroxides of 2 6 0 5 derived for lake tantare sediments falls within the range reported for experimentally determined isotopic fractionations during dir our results yield the first comprehensive set of fe isotope signatures associated with microbially driven fe redox cycling obtained directly in sediments of an oligotrophic freshwater lake

origin and age of the lake nyos maar cameroon abstract lake nyos occupies a young maar crater in the precambrian granitic terrane of northwest cameroon the lake is partly surrounded by poorly consolidated ultramafic nodule bearing pyroclastic surge deposits that were explosively ejected from the nyos crater at the time of its formation radiocarbon dates show that the maar probably formed about 400 years ago field evidence suggests that carbon dioxide could have been the principal volatile involved in the formation of the nyos maar and that the role of water may have been minor the formation of the nyos maar was preceded by a brief period of effusive basaltic volcanism but the maar itself may have largely formed by cold dry explosive processes carbon dioxide may still be trapped interstitially in a diatreme inferred to underlie lake nyos its gradual release into the waters of lake nyos may have set the stage for the tragic gas release event of august 21 1986 only young maar lakes such as nyos may pose a danger of future lethal gas releases

extreme iron isotope fractionation between colloids and particles of boreal and temperate organic rich waters abstract significant and systematic variations of iron isotopic composition in surface water sample fractions obtained by frontal cascade filtration and ultrafiltration have been recorded in 1 subarctic organic rich boreal river and stream mire lake and soil solutions in northern taiga zone karelia nw russia and 2 temperate river and lake waters of the southern boreal zone central russia water samples were filtered in the field employing progressively decreasing pore size from 100 m to 1 kda followed by iron isotope analysis in all river samples there was a gradual increase of 57fe relative to irmm 14 with decreasing pore size from 0 4 0 1 at 100 m up to 4 2 0 1 at 10 kda fraction in the subarctic zone and from 0 024 0 2 at 100 m up to 1 2 0 2 at 10 kda in the temperate zone in the series of filtrates ultrafiltrates of subarctic and temperate streams and rivers the 57fe value decreases with increasing molar fe corg ratio therefore small size fe poor c rich colloids 1 10 kda and low molecular weight lmw fractions of oxygenated water exhibit strong enrichment in heavy isotope whereas high molecular weight fe rich colloids 100 kda 0 22 m and particles 1 100 m are isotopically lighter and closer to the continental crust fe isotope composition the relative enrichment of 1 10 kda ultrafiltrates in heavy isotopes suggests that low molecular weight ligands bind fe more strongly fe o c bonds than fe iii oxy hydr oxides fe o fe bonds in accord with quantum mechanics principles highly positive 57fe of the lmw fraction of labile and potentially bioavailable fe in small subarctic rivers may turn out to be a very important source of isotopically heavy fe in the arctic ocean the mechanisms involved in the production of this isotopically heavy fe may lead this tracer to become a new indicator of environmental changes occurring in the boreal zone

gas buildup in lake nyos cameroon the recharge process and its consequences the gases dissolved in lake nyos cameroon were quantified recently december 1989 and september 1990 by two independent techniques in situ measurements using a newly designed probe and laboratory analyses of samples collected in pre evacuated stainless steel cylinders the highest concentrations of co2 and ch4 were 0 30 mol kg and 1 7 mmol kg respectively measured in cylinders collected 1 m above lake bottom probe measurements of in situ gas pressure at three different stations showed that horizontal variations in total dissolved gas were negligible total dissolved gas pressure near the lake bottom is 1 06 mpa 10 5 atm 50 as high as the hydrostatic pressure of 2 1 mpa 21 atm comparing the co2 profile constructed from the 1990 data to one obtained in may 1987 shows that co2 concentrations have increased at depths to below 150 m based on these profiles the average rate of co2 input to bottom waters was 2 6 108 mol a increased deep water temperatures require an average heat flow of 0 32 mw into the hypolimnion over the same time period the transport rates of co2 heat and major ions into the hypolimnion suggest that a low temperature reservoir of free co2 exists a short distance below lake bottom and that convective cycling of lake water through the sediments is involved in transporting the co2 into the lake from the underlying diatreme increased ch4 concentrations at all depths below the oxycline and a high14c content 41 modern in the ch4 4 m above lake bottom show that much of the ch4 is biologically produced within the lake the ch4 production rate may vary with time but if the co2 recharge rate remains constant co2 saturation of the entire hypolimnion below 50 m depth would require 140a given present day concentrations

biogeochemical cycling of iron in the archean paleoproterozoic earth constraints from iron isotope variations in sedimentary rocks from the kaapvaal and pilbara cratons abstract iron isotope compositions of low metamorphic grade samples of archean paleoproterozoic sedimentary rocks obtained from fresh drill core from the kaapvaal craton in south africa and from the pilbara craton in australia vary by 3 in 56 fe 54 fe ratios reflecting a variety of weathering and diagenetic processes depositional ages for the 120 samples studied range from 3 3 to 2 2 ga and fe c and s contents define several compositional groups including samples rich in fe organic carbon carbonate and sulfide the 56 fe values for low c org low c carb and low s sedimentary rocks are close to 0 the average of igneous rocks this range is essentially the same as that of c org poor late cenozoic loess aerosol river loads and marine sediments and those of c org poor phanerozoic proterozoic shales that these 56 fe values are the same as those of igneous rocks suggests that fe has behaved conservatively in bulk sediments during sedimentary transport diagenesis and lithification since the archean these observations indicate that if atmospheric o 2 contents rose dramatically between 2 4 and 2 2 ga as proposed by many workers such a rise did not produce a significant change in the bulk fe budget of the terrestrial sedimentary system if the archean atmosphere was anoxic and fe was lost from bedrock during soil formation any isotopic fractionation between aqueous ferrous fe fe aq 2 and fe bearing minerals must have been negligible in contrast if the archean atmosphere was oxic fe would have been retained as fe 3 hydroxides during weathering as it is today which would produce minimal net isotopic fractionation in bulk detrital sediments siderite rich samples have 56 fe values of 0 5 0 5 and experimentally determined fe aq 2 siderite fractionation factors suggest that these rocks formed from fe aq 2 that had similar or slightly higher 56 fe values the 56 fe values calculated for fe aq 2 overlaps those of modern submarine hydrothermal fluids but it is also possible that fe aq 2 had 56 fe values higher than those of modern hydrothermal fluids depending upon the fe aq 2 fe carbonate fractionation factor that is used in contrast c org rich samples and magnetite rich samples have strongly negative 56 fe values generally between 2 3 and 1 0 and available fluid mineral fractionation factors suggest that the fe bearing minerals siderite and magnetite in these rocks formed in the presence of fe aq 2 that had very low 56 fe values between 3 and 1 reduction of fe 3 hydroxide by sulfide precipitation of sulfide minerals or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low 56 fe fe aq 2 we interpret microbial dissimilatory fe 3 reduction dir as the best explanation for producing such low 56 fe values for fe aq 2 and our results suggest that dir was a significant form of respiration since at least 2 9 ga

iron isotope fractionation between aqueous ferrous iron and goethite abstract the equilibrium fe isotope fractionation factor between aqueous fe ii and goethite has been experimentally measured to be 1 05 0 08 in 56 fe 54 fe 2 at 22 c using the three isotope method experiments were done using two sizes of goethite 81 11 nm and 590 42 nm and the experimental products were subjected to serial extraction using acid partial dissolution techniques to determine if surface fe iii atoms have different isotopic properties than the bulk goethite these experiments indicate that the interaction of fe ii aq and goethite is dynamic and results in complete or near complete fe isotope exchange over 30 days involving at least four components fe ii aq goethite sorbed fe ii and fe iii surface the equilibrium fractionation factor between fe ii aq and fe ii sorb is the same for both sizes of goethite at 56 fe fe ii aq fe ii sorb 1 24 0 14 this fractionation factor is significantly different than the results of previous studies on fe ii sorption to goethite the proportion of the fe iii surface component is greatest in the experiments that used the smallest goethite and the fe iii surface fe ii aq fractionation is estimated to be at least 2 1 the high fe iii surface fe ii aq fractionation may exert a significant influence on the fe isotope compositions of aqueous fe ii in natural systems that contain nanoparticulate goethite including those involving bacterial iron reduction these results demonstrate that the isotopic properties of nano scale minerals may be distinct from micron scale or larger minerals as is the case for other thermodynamic properties of nanoparticles

coupled fe and s isotope evidence for archean microbial fe iii and sulfate reduction direct fossil evidence for early microbial life on earth is rare microbiological data indicate that sulfate and iron reduction are both among the earliest forms of microbial respiration and direct evidence for the early origin of sulfate reduction comes from sulfur isotopic anomalies in ancient sediments fe isotope geochemistry potentially provides a new way of identifying microbial iron reduction early in earth9s history we present fe isotopic data for sedimentary pyrite from the 2 7 ga belingwe sedimentary basin in zimbabwe isotopically light fe and a remarkable covariation between fe and s isotopes provide strong evidence for coexisting fe and s reduction our results are consistent with an early origin for sulfate reduction and provide direct geochemical evidence for the antiquity of bacterial fe reduction the covariation of fe and s isotopes may provide a useful new tracer of microbial evolution early in earth history

simulation of co2 concentrations temperature and stratification in lake nyos for different degassing scenarios 1 a large gas cloud erupted unexpectedly in 1986 from lake nyos the larger of the two cameroonian killer lakes with devastating consequences regular monitoring subsequently revealed that the deep water of the lake was gradually recharged with co2 to preclude a similar event in the future a degassing pipe was installed in the lake in 2001 in the present study a one dimensional model is used to predict the effects of this pipe and other degassing options on the co2 concentrations and the stratification within the lake for the next 50 years the results of the simulations show that without degassing total co2 content would reach the preeruption value within a few decades the presently installed pipe is sufficient to reduce co2 pressures in the entire water column above the pipe inlet to 5 bar within 10 years and a steady state is reached within 50 years depending on the assessment of the risk due to the gas currently remaining in the lake and the costs involved the installation of additional pipes could be considered 1 to remove the gas more quickly and 2 as a backup for long term failures and maintenance once the steady state is reached degassing with one pipe is a practicable long term solution which can also be used for monitoring the approximate deep water co2 concentrations by nonprofessionals assuming a doubled deep water input in the future as an upper limit of the expected source strength the pipe is still able to prevent a co2 accumulation as a side effect the degassing operation strongly changes the stratification in the lake it transforms the lake from a meromictic to an oligomictic system and gradually removes the dissolved salts from the lake

the lake nyos gas disaster chemical and isotopic evidence in waters and dissolved gases from three cameroonian crater lakes nyos monoun and wum abstract to better understand the cause of the nyos gas disaster of august 21 1986 we conducted geochemical and limnological surveys in october 1986 of three lakes nyos monoun and wum which are located in the cameroon volcanic zone that is characterized by a prevalence of young alkaline basalts and basanitoids lake wum was studied as a non active control co2 is dissolved in significant concentrations about 1 5 of saturation in gas active lakes nyos and monoun but is virtually absent in lake wum stable isotopic ratios of total dissolved carbon 13c 3 for nyos and 5 5 for monoun and of helium 5 7 ratm for nyos and 3 6 ratm for monoun indicate a mantle origin of these gases however so42 and cl concentrations are found to be very low concentrations of dissolved chemical species like fe2 mg2 ca2 and hco3 are high in the two gas active lakes whereas they are very low in the gas inactive lake high salinities in the gas active lakes are probably due to dissolution of indigenous mafic rocks and transported lateritic soil in acidic co2 rich warm water the gas active lakes are characterized by increasing temperature and salinity with increasing depth indicating an active influx of heat and dissolved materials at the bottom density estimates show that the lake water is stably stratified in spite of the inverse temperature profile of the lakes on account of dissolved chemical species the concentrations of dissolved carbonate species co2 aq and hco3 are positively correlated with those of ionic dissolved species indicating their common occurrence in the bottom water the august 1986 gas bursts from lake nyos were most likely caused by rapid exsolution of dissolved co2 within the lake an explosive process such as a phreatic eruption or a co2 gas jetting from beneath the bottom is unlikely because of low concentrations of cl and so42 no oxygen isotopic shift low turbidity and no reported perturbation of the bottom sediments exsolution of co2 bubbles could occur if co2 saturated bottom water was displaced upwards by an increased influx of high salinity water from the bottom during the rainy season exsolution of co2 at the upper layers was possibly accelerated by upwelling of a two phase fluid co2 bubbles and solution a mechanism known as a pneumatic lift pump resulting in discharge of a large amount of co2 gas the h2s concentration in the gas cloud must have been kept far below the lethal level because of a high fe2 concentration of the lake water

the significance of siderite in the sediments from lake nyos cameroon abstract small amounts 1 3 wt of endogenic siderite feco3 and traces of pyrite fes2 are present in a 1 m long sediment core from lake nyos thermodynamic modeling shows that the deep waters are supersaturated with respect to siderite and pyrite no other carbonates were found to be stable this rare occurrence of siderite is symptomatic of the build up of large concentrations of co2 and fe2 in the lake nyos waters a search for siderite in the deeper sediments could reveal the existence of previous gas burst events that have occurred at lake nyos or in other cameroon lakes

anoxic development of sapropel s1 in the nile fan inferred from redox sensitive proxies fe speciation fe and mo isotopes abstract redox conditions and the mechanisms of redox development are a critical aspect of eastern mediterranean sapropels whose formation in oxygen depleted waters is closely related to water column stratification at times of global sea level rise and insolation maxima sapropels in the nile fan formed at relatively shallow water depths under the influence of the monsoon driven freshwater output from the river nile this work evaluates the redox evolution of holocene sapropel s1 in valpamed cruise core md9509 recovered at 880 mbsl in the ne nile fan using a combination of geochemical element proxies fe speciation fe and mo isotopes studies the productivity and redox proxies ba al mo al u al v al sb al show well defined enrichments in the sapropel but with a marked minimum at ca 8 2 ka indicative of reventilation corresponding to a well known global cooling event peak productivity and reducing signals occur close to the initiation of sapropel formation the proxy signals in sapropel 9509 are stronger and of longer duration than those of a second sapropel s1 recovered at the same depth but 380 km to the north md9501 supporting the notion suggested in previous studies of more reduced conditions in the nile fan the moef vs uef enrichment factor variations in core 9509 infer a transition from open marine suboxic conditions in the enclosing non sapropel sediments to anoxic non sulphidic water column conditions in the sapropel correspondingly the highly reactive fe pool fehr measured in fe speciation studies is dominated by fe oxyhydr oxide minerals in the background sediments whereas pyrite fepy becomes the dominant component of the fehr pool in the sapropel maximum fepy values in the sapropel coincide with peak productivity and reducing conditions implying a clear link between trace element uptake diagenetic bacterial sulphate reduction in anoxic porewater and fe mobilization in the sapropel iron isotope compositions 56fe in the sapropel do not show any departure from primary marine and detrital source sediment values and the absence of an fe al vs 56fe trend strongly argues against an fe shuttle molybdenum isotopes however show marked non conservative fractionation patterns background sediment 98 95mo values 0 2 to 0 7 are compatible with fractionation upon absorptive uptake by fe oxyhydr oxides and pyrite in contrast minimum 98 95mo values exhibited at peak sapropel reducing and pyrite producing conditions are most closely modeled by mo isotope fractionation during kinetically controlled conversion of aqueous molybdate to thiomolybdate species the conservative fe isotope behavior mo isotope fractionation minima in the sapropel may be a characteristic of organic rich sediment diagenesis below an anoxic non sulphidic water body without the operation of a benthic fe shuttle

evidence from molybdenum and iron isotopes and molybdenum uranium covariation for sulphidic bottom waters during eastern mediterranean sapropel s1 formation redox exerts a critical control on organic carbon rich sedimentation this is particularly true for eastern mediterranean sapropels where seawater stratification is regarded as a major driving force for oxygen depletion but in which sulphidic euxinic bottom waters occur only sporadically here we apply a powerful array of geochemical proxies fe and mo stable isotopes together with mo u ratios and redox sensitive trace elements rste to the determination of water redox evolution during the deposition of holocene s1 sapropel and its underlying and overlying sediments odp core 967d 2550 m depth rste are asymmetrically distributed within the sapropel with peak enrichments occurring in its lower early part negative correlations are found between fe57 and both fe al and s wt in the lower sapropel and are consistent with the benthic fe shuttle model fe enrichment in euxinic basins moef uef enrichment factor variations show well defined trends identical to those proposed for open marine settings in which sub oxic conditions in the background sediments give way to sulphidic waters at the rste peak in the lower sapropel the most notable features of the mo isotope profile are atypically light values mo98 95 0 7 in the lower sapropel such light mo isotope values relative to sea water mo98 95 2 3 have been related to oxic remobilisation however negative correlations between mo98 95 and fe al ba al mo al and s imply that the lowest mo isotopic compositions are associated with peak reducing conditions taken in conjunction with the evidence from the other proxies for a sulphidic water column the light mo isotope values in the lower sapropel are best explained by a large isotopic fractionation between sea water molybdate and thiomolybdate species in mildly euxinic bottom waters h2s aq 10 m the data from this study thus show that hitherto unrecognised euxinic conditions occur during the early stages of deposition of the holocene sapropel s1 molybdenum isotopes and ba al ratios identify a short lived sapropel re ventilation event timed to coincide with the 8 2 ka cold climatic event

iron isotope fractionation during biogeochemical cycle information from suspended particulate matter spm in aha lake and its tributaries guizhou china abstract iron isotope compositions of suspended particulate matters spm collected from the aha lake an artificial lake in the karst area of yun gui plateau and its tributaries in summer and winter were investigated for our understanding of the behavior of fe isotopes during iron biogeochemical cycling in lake 56fe values of spm display statistically negative shift relative to irmm 014 samples from the lake display a range from 1 36 to 0 10 in summer and from 0 30 to 0 07 in winter while river samples vary from 0 88 to 0 07 in summer and from 0 35 to 0 03 in winter the average iron isotope composition of aerosol samples is 0 10 which is very similar to that of igneous rocks 0 09 the spm in most rivers and water column showed seasonal variation in 56fe value the 56fe values of spm in summer were lower than in winter the seasonal variation in 56fe value of the riverine spm should be ascribed to the change in source of particulate fe and geochemical process in the watershed more particulate fe was leached from soil and produced by weathering of pyrite widely distributed in coal containing strata it is suggested that both allochthonous inputs and the redox iron cycling control the variations of 56fe values for spm in lake during summer stratification an fe cycle named ferrous wheel is established near the redox boundary where the upwardly diffusing fe ii is oxidized and the reactive fe oxides formed will continuously sink back into the reduction zone to complete the cycle the 56fe values for spm reach the minima 0 88 for db station and 1 36 for ljk station just near the redox boundary as a result of the fe cycling where a rough 45 to 76 of fe in these particles was produced by the repetitive cycle due to random transportation and diffusion 56fe values of the particles near the redox zone distributed into approximately a gaussian shape the good negative correlation existed between 56fe values and fe al ratios for db station suggesting that they together can be used as good indicators of the redox driven fe transformations

recent ph and co2 profiles at lakes nyos and monoun cameroon implications for the degassing strategy and its numerical simulation abstract in situ ph profiles are reported for the first time for lakes nyos and monoun the ph profiles were converted to co2 profiles using hco3 profiles calculated from conductivity data recent observations 1993 1996 at lake nyos indicates that co2 still accumulates below 180 m depth at a rate of 125 mmol year 1 at lake monoun the majority of co2 is present below a depth of 60 m only 25 m below the saturation depth consequently a potential danger of gas explosion is high at both lakes and artificial degassing of the lakes should be performed as soon as possible a system for industrial degassing of the lakes is proposed the system based on the self sustained gas lift principle consists of multiple pipes 14 cm in diameter with different intake depths 12 pipes for lake nyos four each at 185 195 and 205 m and three pipes for lake monoun at 70 80 and 90 m the stepped degassing at different depths is intended to keep the maximum stability of the lakes the proposed degassing operation was simulated using the dyresm code for both lakes in 5 years approximately 50 of currently dissolved co2 in lake nyos and 90 in lake monoun will be removed the expected changes in the thermal and chemical structures of the lakes as degassing proceeds will be most easily monitored with a carefully calibrated ctd equipped with a ph sensor the simulation indicates that the discharged degassed water will sink to a level of neutral buoyancy i e to a maximum of 70 m at lake nyos and 35 m at lake monoun there would be no possibility of triggering a gas explosion by this plunge of discharged water because the water present there would have already been replaced by water at lower co2 concentration during the degassing from shallower pipes

insights into iron sources and pathways in the amazon river provided by isotopic and spectroscopic studies abstract the present study investigated the weathering and transport mechanisms of fe in the amazon river a particular emphasis was placed on fe partitioning speciation and isotopic fractionation in the contrasting waters of the solimoes and negro rivers and their mixing zone at the beginning of the amazon river samples collected in the end member rivers and thirteen sites distributed throughout the mixing zone were processed through frontal vacuum filtration and tangential flow ultrafiltration to separate the different suspended solid fractions i e particulate p 0 45 m and p 0 22 m colloidal 0 22 m c 5 kda and truly dissolved elements td 57 fe values relative to irmm 14 while the particulate fractions yielded light isotopic compositions of 0 344 for p 0 22 m and 0 104 for p 0 45 m fractions the mineral particulate rich waters of the solimoes river had dissolved and colloidal fractions with light isotopic composition 0 532 and 0 176 respectively whereas the particulate fractions yielded 57 fe values close to those of the continental crust i e 0 029 for p 0 22 m and 0 028 for p 0 45 m ten kilometers downstream from the negro and solimoes junction the concentrations of colloidal and dissolved fe species deviate markedly from conservative mixing a maximum fe loss of 43 g l i e 50 of the dissolved and colloidal fe is observed 110 km downstream from the rivers junction the contrasting negro and solimoes rivers isotopic compositions along the pore sized water fractions is attributable to the biogeochemical processes involving different types of upland soils and parental materials for instance the isotopic composition of colloidal and dissolved fe from the negro river are consistent with fe oxidation and complexation mechanisms at the interface between waterlogged podzols and river networks as supported by strong organo fe complexes signals observed by epr conversely the particulate and colloidal fractions from the solimoes river have 57 fe consistent with strong mechanical erosion in the andean cordillera and upland soils as evidenced by high concentrations of fe 3 oxides sensu lato measured by epr the massive dissolved and colloidal fe removal is associated with the evolution of the physical and chemical composition of the waters i e ionic strength during mixing which influences organo fe 3 and fe 3 oxyhydroxides stability several models are discussed to explain fe non conservative behavior including dissociation of organo fe complexes and the subsequent formation of solid fe 3 oxyhydroxides and semiquinone free radicals as evidenced by epr spectra demonstrating that organo fe signals decrease as fe 3 oxyhydroxides and free radicals signals increase as in estuarine regions the mechanisms involving fe transfer and loss in the mixing zone has a negligible effect on the bulk water fe isotopic composition this result suggests that a tropical basin similar to the amazon river basin delivers to the ocean waters with an fe isotopic composition similar to that of the earth s continental crust

iron isotope fractionation in river colloidal matter temporal variations in the iron isotopic composition 56fe between 0 13 and 0 31 have been measured in the suspended fraction in a boreal river the major mechanism behind these variations is temporal mixing between two types of particles colloids fe oxyhydroxides and fe c colloids data in this study indicate that these two types of colloids have different fe isotope composition the fe c colloid has a negative 56fe value whereas the fe oxyhydroxide colloid is enriched in 56fe these two types of colloidal matter have different hydrogeochemical origin the fe c colloid reaches the river during storm events when the upper sections of the soil profile o and e horizons are flooded by a rising water table colloidal fe oxyhydroxides reach the river via inflow and subsequent oxidation of groundwater enriched in dissolved fe ii