the present day formation of apatite in mexican continental margin sediments abstract results of pore water and sediment analyses from the western mexican continental margin strongly suggest the present day formation of apatite the interstitial water phosphate and fluoride profiles indicate chemical removal at a depth which corresponds to a large maximum in the phosphorus content of the sediments apatite is identified within this maximum via x ray diffraction but is elsewhere undetectable in the core radioisotopic thorium uranium and radium data support the conclusion that this deposit is modern the present day depositional environment is consistent with those reported by other workers for phosphorite formation with the exception that pore water magnesium is not depleted below its seawater value

precipitation of dolomite using sulphate reducing bacteria from the coorong region south australia significance and implications dolomite was successfully precipitated in culture experiments that simulated microbiogeochemical conditions prevailing during late stages of evaporation in ephemeral hypersaline dolomitic lakes of the coorong region south australia analyses of lake and pore water samples document rapid geochemical changes with time and depth in both dolomitic and non dolomitic lakes extremely high sulphate and magnesium ion concentrations in lake waters decline rapidly with depth in pore waters throughout the sulphate reduction zone whereas carbonate concentrations in pore waters reach levels up to 100 times those of normal sea water ultimately sulphate is totally consumed and no solid sulphate is recorded in the dolomitic lake sediments most probable number calculations of lake sediment samples record the presence of large populations of sulphate reducing bacteria whereas sulphur isotope analyses of lake water samples indicate microbial fractionation in all the lakes studied viable populations of microbes from the lake sediments were cultured in anoxic conditions in the laboratory samples were then injected into vials containing sterilized clastic or carbonate grains or glass beads immersed in a solution that simulated the lake water falls in the levels of sulphate and rising ph in positive vials were interpreted as indicating active bacterial sulphate reduction accompanied by increased concentrations of carbonate within 2 months sub spherical sub micron size crystals of dolomite identical to those of lake sediments were precipitated it is concluded that bacterial sulphate reduction overcomes kinetic constraints on dolomite formation by removing sulphate and releasing magnesium and calcium ions from neutral ion pairs and by generating elevated carbonate concentrations in a hypersaline strongly electrolytic solution the results demonstrate that bacterial sulphate reduction controls dolomite precipitation in both the laboratory experiments and lake sediments it is proposed that dolomite formation through bacterial sulphate reduction provides a process analogue applicable to thick platformal dolostones of the past where benthic microbial communities were the sole or dominant colonizers of shallow marine environments

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

molybdenum isotope constraints on the extent of late paleoproterozoic ocean euxinia abstract the expansion of anoxic and sulfidic euxinic deep oceans may explain the termination of global large iron formation if deposition at 1 85 ga but the areal extent of late paleoproterozoic ocean euxinia is poorly constrained here we use the molybdenum mo isotope composition 98 95 mo of black shales from the hr fe t typically 0 38 and ratios of pyrite fe to highly reactive fe fe py fe hr typically 2 rich because the fe py fe hr ratios do not imply sustained water column euxinia the conversion of molybdate moo 4 2 to tetrathiomolybdate mos 4 2 in bottom waters and sediments was probably not quantitative and thus the 98 95 mo 0 1 to 0 8 from these shales provides only a minimum estimate for coeval seawater by contrast black shales from the middle rove formation have high fe py fe hr 0 8 and fe hr fe t 0 38 which implies persistent water column euxinia concentrations of free sulfide may have been sufficient to result in near quantitative formation of tetrathiomolybdate and its removal from bottom waters the observed narrow range in 98 95 mo of 1 19 0 28 2sd defined by most euxinic shales is thus inferred to represent the mo isotope composition of late paleoproterozoic global seawater mass balance modeling suggests that organic rich sediments deposited beneath euxinic bottom waters represented a substantial fraction of the oceanic mo sink at 1 8 ga the modeling results suggest that euxinic marine environments were common along late paleoproterozoic ocean margins these environments could have served as efficient sinks for fe 2 aq upwelling from anoxic deeper waters hence the mo isotope data are consistent with fe speciation and other geochemical evidence that suggest an expansion of euxinic environments led to the cessation of global large if deposition after 1 85 ga

tectonic systems and the deposition of iron formation abstract the iron formation group of chemically precipitated cherty iron rich sediments ranges in age from early precambrian to recent and is composed of a variety of lithological facies iron formations are classified as lake superior and algoma types to emphasize the contrasting conditions in the spectrum of sedimentary tectonic environments in which they formed thick stratigraphic units of lake superior type iron formation which form the most extensive iron ranges of the world are part of sedimentary tectonic systems that developed along the margins of cratons or continental platforms and were deposited over extended periods of time under relatively stable tectonic conditions the algoma type iron formations were deposited with volcanic rocks and greywacke in various tectonic volcanic systems formed under more dynamic tectonic conditions but are otherwise comparable to present day spreading ridges on the ocean floor the thick sequences of lake superior type iron formation deposited between 2 7 and 2 0 ga form part of major sedimentary tectonic systems that extended along the margins of continents direct biogenic factors and the composition of the atmosphere probably had only limited influence on the precipitation of these chemical sediments

secular change in the precambrian silica cycle insights from chert petrology chert deposits preserve a record of secular change in the oceanic silica cycle the evolutionary radiation of silica secreting organisms resulted in a transition from abiological silica deposition characteristic of the archean and proterozoic eons to the predominantly biologically controlled silica deposition of the phanerozoic comparative petrography of phanerozoic and precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the paleoproterozoic era ca 1 8 ga in neoproterozoic and mesoproterozoic strata early diagenetic chertification is largely restricted to peritidal environments these early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures the cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation some of the paleoproterozoic cherts associated with iron formations however are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed these primary cherts lack ghosts or inclusions of carbonate precursors have fine scale grain fracturing possibly from syneresis exhibit low grain packing densities and are not associated with unsilicified carbonate deposits of similar depositional composition cherts in some paleoproterozoic iron formations e g the gunflint formation northwestern lake superior region are composed of silica types similar to those in phanerozoic sinters e g the devonian rhynie and windyfield cherts scotland such cherts may provide evidence that basinal and perhaps global oceanic silica concentrations were higher during the paleoproterozoic era than at later times

geochronology of unconformity related uranium deposits in the athabasca basin saskatchewan canada and their integration in the evolution of the basin the importance of geochronology in the study of mineral deposits in general and of unconformity type uranium deposits in particular resides in the possibility to situate the critical ore related processes in the context of the evolution of the physical and chemical conditions in the studied area the present paper gives the results of laser step heating 40ar 39ar dating of metamorphic host rock minerals pre ore and syn ore alteration clay minerals and laser ablation inductively coupled plasma mass spectrometer la icp ms u pb dating of uraninite from a number of basement and sediment hosted unconformity related deposits in the athabasca basin canada post peak metamorphic cooling during the trans hudson orogen of rocks from the basement occurred at ca 1 750 ma and gives a maximum age for the formation of the overlying athabasca basin pre ore alteration occurred simultaneously in both basement and sandstone hosted mineralizations at ca 1 675 ma as indicated by the 40ar 39ar dating of pre ore alteration illite and chlorite the uranium mineralization age is ca 1 590 ma given by la icp ms u pb dating of uraninite and 40ar 39ar dating of syn ore illite and is the same throughout the basin and in both basement and sandstone hosted deposits the mineralization event older than previously proposed as well as several fluid circulation events that subsequently affected all minerals studied probably correspond to far field continent wide tectonic events such as the metamorphic events in wyoming and the mazatzal orogeny ca 1 6 to 1 5 ga the berthoud orogeny ca 1 4 ga the emplacement of the mckenzie mafic dyke swarms ca 1 27 ga the grenville orogeny ca 1 15 to 1 ga and the assemblage and break up of rodinia ca 1 to 0 85 ga the results of the present work underline the importance of basin evolution between ca 1 75 ga basin formation and ca 1 59 ga ore deposition for understanding the conditions necessary for the formation of unconformity type uranium deposits

sedimentary pyrite a window into the microbial past early diagenetic pyrite forms as a result of microbial processes in sediments but has received little attention as a potential source of microbial fossils sedimentary pyrite grains archean to jurassic in age were examined by scanning and transmission electron microscopy tem ion milling was essential for the preparation of electron transparent tem samples most pyrite grains revealed coccoid rod shaped and even filamentous features that are interpreted as microbial identification of microbial remains in sedimentary pyrite may greatly expand our knowledge of microbial life in the sedimentary record combined with geochemical characterization the study of pyritized microbes in the geologic record may in the future enable us to interpret changes in geochemical signatures more realistically in terms of different sulfate reducing communities sedimentary pyrite grains may also represent a good prospect to further our knowledge of past life on mars

redox state of the archean atmosphere evidence from detrital heavy minerals in ca 3250 2750 ma sandstones from the pilbara craton australia the presence of detrital uraninite and pyrite in fluvial placers of the witwatersrand basin south africa has been used to infer low levels of atmospheric oxygen during the archean 2500 ma however recent studies advocate a hydrothermal origin for these minerals thereby limiting their value as constraints on the composition of the early atmosphere in contrast ca 3250 2750 ma fluvial siliciclastic sediments from the pilbara craton in australia have never undergone significant hydrothermal alteration and their heavy minerals are of unequivocal detrital origin these heavy minerals include the redox sensitive phases pyrite uraninite and gersdorffite along with more inert zircon rutile chromite and monazite locally siderite is a major constituent to 90 of the heavy mineral population with grains displaying evidence for several episodes of erosion rounding and subsequent authigenic overgrowth detrital siderite is very rare in post archean sandstones largely due to its instability in oxidizing environments however its frequent survival of prolonged transport in well mixed and therefore well aerated archean river waters that contained little organic matter strongly implies that the contemporary atmosphere was indeed much less oxidizing than at present moreover concentrations of reduced sulfur species must have been very low in surface fluids for siderite to survive repeated transportation events without pyritization

molybdenum isotopes in late archean carbonate rocks implications for early earth oxygenation abstract we present carbonate mo isotope data of the 2 64 2 5 ga old ghaap group transvaal supergroup south africa to contribute to the debate on early earth oxygenation mo fractionation in carbonates has just recently been introduced as a potential new proxy for ocean redox chemistry and it was suggested that under favorable conditions non skeletal carbonates closely monitor the ambient seawater 98 95 mo composition the record presented here is characterized by three phases of moderately to strongly fractionated 98 95 mo with values of up to 1 6 interrupted by two shifts towards detrital levels this near continuous presence of heavy 98 95 mo suggests a quasi permanent availability of free atmospheric o 2 because fractionation relative to the continental background signature occurs exclusively when mo is present as dissolved molybdate ions moo 4 2 we compare our carbonate data to a previously published black shale mo record on samples from the same drill core the general interpretation of both records is identical a rise in o 2 preceded the commonly accepted transition to a consistently oxygenated atmosphere by 150 ma however in the ghaap group carbonates fractionated mo is found at a time when the coeval black shales still indicate detrital sedimentation a combination of detritus admixture and or redox controlled mo isotope fractionation during shale formation in suboxic to anoxic waters is suggested as cause for a period characterized by a persistent offset between lighter shale and heavier carbonate 98 95 mo the apparent offset can then be attributed to the different geochemical behavior of the two archives with respect to the local depositional conditions redox vs non redox dependent such changes in the sedimentary environment might also explain why the carbonates do not reproduce rapid fluctuations found throughout the shale record but show a continuous signature with scarce outliers thus by means of the combined archives the conspicuous 98 95 mo scatter found in the shale record could be attributed to variable depositional conditions during its formation rather than global oxic anoxic fluctuations simultaneously the carbonates provide a better approximation of the long term 98 95 mo seawater evolution this study introduces the mo isotopic composition of carbonates as a promising new tool for paleo redox studies in particular when combined with black shales

variations in pyrite texture sulfur isotope composition and iron systematics in the black sea evidence for late pleistocene to holocene excursions of the o2 h2s redox transition time dependent variations in the physicochemical properties of pyrite were investigated in four sediment cores from the black sea in the laminated deep basin sediments of unit i and unit ii 86 of pyrite particles are present as fine grained framboidal aggregates in these sediments the dominance of pyrite framboids with a narrow size distribution and maximum size 18 m is evidence of syngenetic water column pyrite formation subjacent to the o2 h2s boundary sediments from the basin margin collected below the impingement of the o2 h2s boundary contain an increased proportion of fine grained euhedral grains of pyrite relative to framboidal aggregates suggesting increased additions of diagenetic pyrite below the sediment water interface the more efficient reduction and sulfidation of iron in the water column of the central region of the basin implies a more reactive source of iron there compared to the basin margins and is tied to increasing pyrite as framboids and greater degrees of pyritization dop in the basin center relative to the basin margins an evaluation of fe s c relations indicates that pyrite formation was limited by sulfate availability during deposition of the oldest lacustrine sediments of unit iii c limited for upper unit iii due to the input of sulfate rich mediterranean seawater and fe limited in the laminated organic carbon rich sediments of units i and ii these changes in pyrite limiting factors occurred within 4 m of compacted burial representing 15 ka sediment fabric and framboid size distributions at a shallow basin margin site suggest that during two extended periods of unit ii deposition the o2 h2s transition retreated to depths below 200 m however during these same intervals the 34s of pyrite remains uniform compared to overlying and underlying laminated sediments implying that this chemical signature may not be uniquely tied to the position of the o2 h2s boundary relative to the sediment water interface but rather is likely related to biogeochemical processes

a reconstruction of archean biological diversity based on molecular fossils from the 2 78 to 2 45 billion year old mount bruce supergroup hamersley basin western australia abstract bitumens extracted from 2 7 to 2 5 billion year old ga shales of the fortescue and hamersley groups in the pilbara craton western australia contain traces of molecular fossils based on a combination of molecular characteristics typical of many precambrian bitumens their consistently and unusually high thermal maturities and their widespread distribution throughout the hamersley basin the bitumens can be characterized as probably of archean age accepting this interpretation the biomarkers open a new window on archean biodiversity the presence of hopanes in the archean rocks confirms the antiquity of the domain bacteria and high relative concentrations of 2 methylhopanes indicate that cyanobacteria were important primary producers oxygenic photosynthesis therefore evolved 2 7 ga ago and well before independent evidence suggests significant levels of oxygen accumulated in the atmosphere moreover the abundance of cyanobacterial biomarkers in shales interbedded with oxide facies banded iron formations bif indicates that although some archean bif might have been formed by abiotic photochemical processes or anoxygenic phototrophic bacteria those in the hamersley group formed as a direct consequence of biological oxygen production biomarkers of the 3 methylhopane series suggest that microaerophilic heterotrophic bacteria probably methanotrophs or methylotrophs were active in late archean environments the presence of steranes in a wide range of structures with relative abundances like those from late paleoproterozoic to phanerozoic sediments is convincing evidence for the existence of eukaryotes in the late archean 900 ma before visible fossil evidence indicates that the lineage arose sterol biosynthesis in extant eukaryotes requires molecular oxygen the presence of steranes together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least 1 of the present atmospheric level pal and may have been sufficient to support aerobic respiration

geochemical evolution of oligocene middle miocene sediments in the deep water area of the pearl river mouth basin northern south china sea abstract the deep water area of the pearl river mouth basin in the south china sea has received much scientific attention since the ocean drilling program odp leg 184 in 1999 due to its potential economic prospects and distinct tectono sedimentary evolutionary processes in this study we present the composition of major and trace elements from two newly sampled deep water boreholes by6 and lw3 in the baiyun sag of the southern pearl river mouth basin the geochemical evolution in the oligocene middle miocene as well as potential controlling factors are investigated based on a comparative study with previous data from odp site 1148 and borehole py33 the chemical index of alteration cia and a cn k plot reveal that the observed weathering trends are not compatible for the four discussed boreholes sedimentary sorting is primarily observed in borehole py33 where data trend away from the a apex to the feldspar join in the a cn k plot and show a spread of al 2 o 3 sio 2 and nb zr ratios compared to chemical weathering and hydrodynamic sorting provenance has a greater impact on sediment composition of the deep water area from the north the pearl river was the primary sediment supply however a positive eu anomaly and the provenance discrimination diagrams i e la th versus th yb and zr co versus th co reveal the mafic nature of borehole by6 sediments in the zhuhai lower zhujiang 32 0 18 5 ma and upper hanjiang 13 8 10 5 ma formations these compositions are unusual and differ from the well defined felsic sources in the majority of the baiyun sag these discrepancies are likely related to multistage magmatism the sediments at site 1148 are characterized by slightly enriched heavy rare earth elements and relatively high zr co ratios which could possibly be caused by zircon enrichment from local sources

coated phosphate grains proxy for physical chemical and ecological changes in seawater irrespective of geologic age phanerozoic coated phosphate grains deposited beneath productive surface waters in organic rich paleoenvironments are of only two types unconformity bounded grains contain internal discordances and erosional surfaces attributable to multiple episodes of phosphogenesis and sedimentary reworking during periods of stratigraphic condensation redox aggraded grains consist of concordant concentric phosphate laminae that are intimately interlayered with circumgranular layers containing pyrite chamosite or barite recording in situ diagenetic mineralization driven by changes in pore water redox potential such changes can be attributed to variations in biological oxygen demand within suboxic pore water environments resulting from fluctuations in sedimentation rate of organic carbon redox aggraded grains are thus sensitive indicators of variations in organic carbon export and record changes in primary productivity and or ecological dynamics of the surface ocean this concept of coated grain formation necessitates a long residence time just below the sediment water interface if sedimentation rate is too high grains are rapidly buried and so removed from the zone of active phosphate precipitation coated phosphate grains can therefore be considered the granular equivalents of condensed beds these concepts are equally applicable to the interpretation of other types of coated grains and concretions that contain eh sensitive minerals such as iron bearing ooids and polymineralic concretions

supercontinents and superplume events distinguishing signals in the geologic record abstract simultaneous supercontinent and superplume events may reinforce or cancel signals preserved in the geologic record alternatively one signal may overwhelm the other for instance relatively low sea level during a 2 7 ga superplume supercontinent event may reflect direct hits of superplumes beneath the supercontinent although both supercontinent formation and superplume events occurred at 0 28 1 9 and 2 7 ga global warming at these times indicates the dominance of superplume events in controlling climate enhanced deposition of black shales correlates with superplume events and with supercontinent breakup carbon isotopes in seawater however show positive excursions during supercontinent breakup at 2 2 2 1 and 0 8 0 6 ga but show mixed signals or no signal during other supercontinent or superplume events probably due to negative feedbacks peaks in marine organism originations at 100 280 and 480 ma correlate with possible superplume events whereas an overall decrease in origination rate in the early paleozoic correlates with the growth of pangea and destruction of shallow marine environments increased production rates of juvenile crust correlate with formation of supercontinents and with superplume events there may be two types of superplume events catastrophic events which are short lived

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

rise of atmospheric oxygen and the upside down archean mantle 1 the establishment of an oxygen rich atmosphere dramatically altered the evolution of life on earth most of the recent discussion of the topic has been focused on the timing of the event rather than on its mechanism here we draw upon recent developments in the understanding of earth s interior to propose that the rise of oxygen followed a geologically abrupt period of mantle overturn and or intense plume activity near the archean proterozoic transition 2470 2450 million years ago the magmatic event has already been linked to the widespread deposition of oxide facies banded iron formation and the rise of oxygen has been implicated as the trigger for iron deposition and earth s first major glaciation we argue that these events are all related to a change in redox state of volcanic gases brought about by deep seated late archean and earliest paleoproterozoic magmatism

significance of alteration assemblages for the origin and evolution of the proterozoic nabarlek unconformity related uranium deposit northern territory australia the proterozoic nabarlek unconformity related uranium deposit in the alligator rivers uranium field is hosted by paleoproterozoic amphibolite grade metamorphosed semipelitic sedimentary rocks and amphibolite schist high grade ore is confined to the nabarlek fault a reverse fault shear zone that crosscuts a series of interbedded muscovite quartz biotite schists and amphibolite petrographic studies on polished thin sections combined with electron microprobe analyses x ray diffraction fluid inclusion data o h and u pb isotope values as well as 40 ar 39 ar dating have identified up to eight significant fluid events beginning with the precipitation of early quartz veins during uplift of the myra falls metamorphics at 1830 ma and ending with limited uraninite recrystallization during reactivation of the nabarlek fault between 1380 and 750 ma quartz veins that likely formed toward the end of the top end orogeny represent the earliest recorded fluid event fluid inclusion data and 18 o and d values indicate that these veins formed from basement derived fluids that may have been heated by a cooling nabarlek granite during circulation through reverse faults shear zones the next fluid event represented by fine grained sericite and chlorite occurred when fluids passed into these faults and altered the metamorphic minerals following the exhumation of the basement and deposition of the kombolgie subgroup at ca 1760 ma the intrusion of the oenpelli dolerite at ca 1720 ma resulted in the local remobilization of silica and the precipitation of quartz associated with minor pyrite and dolomite around the reverse faults uranium mineralization is associated with an inner and outer alteration halo that extends as far as 1 km from the nabarlek fault alteration in the outer halo began as early as 1700 ma and is dominated by chlorite and sericite which formed when a 200 c fluid flowed into the nabarlek fault from the overlying kombolgie subgroup u pb and 207 pb 206 pb dating reveals that massive uraninite precipitated at ca 1640 ma and formed together with illite and hematite at ca 200 c chlorite was not coeval with uraninite precipitation stable isotope values indicate that the pre and synore alteration assemblage formed from basinal brines with 18 o fluid and d fluid values of 2 2 and 25 10 per mil respectively reactivation of the nabarlek fault at ca 1360 1100 and 900 ma is indicated by u pb and 207 pb 206 pb dating of uraninite these ages correlate with the intrusion of the maningkorrirr phonolitic dikes and the derim derim dolerite at ca 1316 40 and 1324 4 ma respectively the amalgamation of australia and laurentia during the grenville orogen at ca 1140 ma and the breakup of rodinia between 1000 and 750 ma fluid incursions associated with these events precipitated much of the chlorite that has previously been related to uraninite precipitation drusy quartz veins that host high salinity fluid inclusions and sulfides particularly galena also formed after the initial uraninite forming event finally erosion of the kombolgie subgroup and subsequent weathering of the deposit resulted in the recent formation of kaolinite and numerous secondary uranium minerals these data constrain individual events more precisely than previous studies and thus advance the current genetic model to a level that takes into account the multiple stages of fluid overprinting that occurred over a period of at least 800 m y

organic carbon cycling and modern phosphorite formation on the east australian continental margin an overview during 1987 the australian bureau of mineral resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern australia between 28 and 32 s the objectives of the work were to examine the processes controlling the cycling of organic carbon and bioactive elements nitrogen phosphorus sulphur and iron in the sediments and to investigate the roles which these processes played in the formation of the modern phosphorites bacterial productivities sulphate reduction rates sedimentary oxygen and pore water concentrations of nitrate ammonia phosphate iron sulphate and fluoride were measured at sea the highest rates of microbial productivity were found in the surficial 0 20 mm sediments of the modern phosphorite zone in 350 460 m water depth these rates were about double those in shallower shelf 300 m sediments and 3 4 fold those rates in mid slope 600 1000 m sediments aerobic and anaerobic oxidation rates of organic matter calculated from sediment oxygen profiles and sulphate reduction rates were highest in the surface sediments in the modern phosphorite zone the recycling of sedimentary iron via reductive dissolution of iron oxyhydroxides and reprecipitation at the oxic anoxic boundary results in a near surface sedimentary trap for iron in the phosphorite zone sediments phosphate released from organic matter in the interfacial sediments and fluoride from seawater are scavenged by iron oxyhydroxides in the top few centimetres of sediment phosphorus in this way is decoupled from organic carbon in the near surface sediments and linked to the redox cycling of iron phosphate and fluoride scavenged onto iron oxyhydroxides and concentrated in the surficial sediments are subsequently released to pore waters in the anoxic sediments when iron oxyhydroxides are buried and dissolve the recycling process releases phosphate and fluoride for incorporation into apatite fluoride is depleted from pore waters at depths 18 cm phosphorite nodules form within anoxic sediments at depths 18 cm and continue to accumulate iron and phosphorus while resident in the mixed layer combinations of rapid sediment mixing rates a slow sedimentation rate and a mixed layer to about 18 cm result in an average particle residence time in the phosphorite zone sediments which is about ten fold that of the mid slope sediments long residence times and rapid mixing promote the oxidation of organic carbon and release of phosphate while the continuous recycling of iron and phosphate concentrates the phosphorus for apatite precipitation and accumulation into phosphorite nodules phosphorite nodules are not found in mid slope sediments probably because of combinations of relatively rapid sedimentation rates ineffective iron phosphorus and fluoride recycling and trapping mechanisms plus dilution and dissemination of any incipient apatite

petrology and major element geochemistry of peru margin phosphorites and associated diagenetic minerals authigenesis in modern organic rich sediments abstract new petrographic and major element geochemical data from modern peru margin upper slope outer shelf phosphorites are presented which provide insight into their origin and paragenetic relationship with other authigenic minerals glauconite pyrite and dolomite occurring in organic rich sediments glauconites are precipitated relatively early following the partial reduction of ferric iron and following this process phosphate pyrite and then dolomite precipitation take place at progressively deeper levels in the sediment in association with microbial reduction of sulfate as in many ancient economic phosphorite deposits the phosphatic facies here consist of nodules crusts coatings and strata composed of phosphatic pelletal grains ooids structureless grains intraclasts clumps and biogenic grains in association with organic rich biosiliceous sediments all are considered to have formed within a few centimeters or within a few tens of centimeters below the sediment water interface important factors that influence which morphology will tend to develop include the amount of available pore space the presence of suitable nucleation sites the amount and size of siliciclastic detritus incorporated as inclusions and the relative solution chemistries of the precipitating solutions bacterial mediation may play an important but as yet unspecified role in the precipitation process textural data and factor analysis of chemical data suggest that structureless pellets are relatively inclusion free na f mg co3 substituted pore water precipitates whereas ooids are inclusion rich pore water precipitates poor in lattice substituted components variations in nodular cement birefringence and crystallinity are suggested to have been produced by similar lattice substitutions that directly reflect pore water carbonate ion concentrations and thus relative degrees of organic matter degradation phosphate and dolomite are intimately mixed yet mineralogically distinct phases in phosphatized dolomicrites depth stratified threshold carbonate ion concentrations may control the lower limit at which phosphatic minerals may precipitate below depths of a few centimeters excessive carbonate ion concentrations and diminished reactive iron and sulfate concentrations favor the development of dolomite while precluding further development of phosphatic minerals and pyrite periodic sediment reorganization bioturbation current winnowing and erosion mass wasting etc plays an important role in both concentrating pelletal grains and maintaining nodules and crusts at critical depth levels in the sediment as well as mixing ordered mineral parageneses into complicated sequences

pyrite paragenesis and multiple sulfur isotope distribution in late archean and early paleoproterozoic hamersley basin sediments abstract the sulfur isotope record in late archean and early paleoproterozoic rocks is of considerable importance because it provides evidence for changes in early earth atmospheric oxygen levels and potentially constrains the origin and relative impact of various microbial metabolisms during the transition from an anoxic to oxic atmosphere mass independently fractionated mif sulfur isotopes reveal late archean and early paleoproterozoic sulfur sources in different pyrite morphologies in western australia s hamersley basin multiple sulfur isotope values in late archean pyrite vary according to morphology fine grained pyrite has positive sulfur mif indicating a reduced elemental sulfur source whereas pyrite nodules have negative sulfur mif potentially derived from soluble sulfate via microbial sulfate reduction the hamersley basin 34 s 33 s record suggests that the extent of oxygenation of the surface ocean fluctuated through the late archean from at least 2 6 ga more than 150 million yr before the great oxidation event in the early paleoproterozoic there is less distinction between pyrite morphologies with respect to sulfur isotope fractionation and pyrite from the brockman iron formation trends toward modern sulfur isotope values an important exception to this is the strong negative mif recorded in layer parallel pyrite in paleoproterozoic carbonate facies iron formation this may suggest that deeper water hydrothermal environments remained anoxic while shallower water environments became more oxidised by the early paleoproterozoic the results of the current study indicate that sulfide paragenesis is highly significant when investigating archean and early paleoproterozoic multiple sulfur isotope compositions and sulfur sources

does the paleoproterozoic animikie basin record the sulfidic ocean transition the paleoproterozoic animikie basin in the lake superior region of north america is the benchmark for understanding one of the most significant changes in seawater chemistry in earth history the transition to a sulfidic ocean ca 1 84 ga this transition marks a dramatic increase in ocean sulfide produced by bacterial reduction of sulfate formed through the oxic chemical weathering of pyrite during the great oxidation event such a change is thought to have caused the cessation of widespread iron formation deposition and marks the beginning of a long lull in the evolution of eukaryotes the feasibility of using the animikie basin to understand this transition is re evaluated by comparing new sulfur isotope data 34 s pyrite from the michigamme formation in northern michigan united states to published results from the correlative rove formation in ontario canada these data suggest that microbial sulfate reduction was restricted to organic rich environments away from the influence of deltas which delivered low sulfate river water to the coast sulfur isotope data and the patchiness of sulfate reduction within the animikie basin suggest strong lateral and vertical gradients in seawater sulfate concentrations when viewed in the context of the most recent tectonic and sedimentologic models these observations are best interpreted as reflecting an evolving basin with restricted water circulation not open circulation with the global ocean as previously postulated thus the animikie basin does not contain an accurate record of open ocean structure and composition during the onset of sulfidic conditions what is clear is that marginal seas must be carefully evaluated before they are used to model the global ocean especially given the rarity of preserved precambrian deep sea sediments

primary haematite formation in an oxygenated sea 3 46 billion years ago the timing of the origin of photosynthesis remains controversial the discovery of ancient haematite crystals that formed in a jasper formation in australia which was created in a marine setting suggests that oxygen was being produced at least locally by photosynthesis as early as 3 46 billion years ago

diagenetic stability of the isotopic composition of phosphate oxygen palaeoenvironmental implications meteoric diagenesis of the lower pliocene varswater formation phosphate deposit cape province south africa has altered the chemistry of the francolite component the isotopic composition of the phosphate oxygen has been changed as have other chemical and isotopic parameters that are known to be susceptible to meteoric diagenesis from authors

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

sulphur isotope evidence for an oxic archaean atmosphere the presence of mass independently fractionated sulphur isotopes mif s in many sedimentary rocks older than approximately 2 4 billion years gyr and the absence of mif s in younger rocks has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2 4 gyr ago this is because the only mechanism known to produce mif s has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen poor atmosphere here we report the absence of mif s throughout approximately 100 m sections of 2 76 gyr old lake sediments and 2 92 gyr old marine shales in the pilbara craton western australia we propose three possible interpretations of the mif s geologic record 1 the level of atmospheric oxygen fluctuated greatly during the archaean era 2 the atmosphere has remained oxic since approximately 3 8 gyr ago and mif s in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere or 3 mif s in rocks was mostly created by non photochemical reactions during sediment diagenesis and thus is not linked to atmospheric chemistry

middle proterozoic ocean chemistry evidence from the mcarthur basin northern australia we report on the iron sulfur systematics and s isotopic compositions of upper paleoproterozoic carbonaceous shales from the wollogorang and lower reward formations mcarthur basin northern australia iron speciation in these rocks is similar to that of modern euxinic black sea sediments and distinct from sediments depositing from oxygenated water providing persuasive evidence of euxinic depositional environments in a mid proterozoic marine basin high values of degree of pyritization dop also indicate anoxic bottom water conditions at the time of shale deposition as does the relatively uniform isotopic composition of sedimentary sulfides the isotopic composition of the sulfides is however quite enriched in 3 4 s and for the lower reward formation similar to contemporaneous seawater sulfate these 3 4 s enriched sulfides are consistent with extensive or even complete sulfate depletion within the euxinic basins in which the sediments were deposited based on analogy with modern euxinic environments and further constrained by model results middle proterozoic seawater sulfate concentrations were probably within the range of 0 5 to 2 4 mm taken together our results indicate a relatively low sulfate late paleoproterozoic ocean and sulfidic deep waters in the mcarthur seaway a condition that could have encompassed much larger regions of the ocean

early quartz cements and evolution of paleohydraulic properties of basal sandstones in three paleoproterozoic continental basins evidence from in situ δ18o analysis of quartz cements quartz cement microstratigraphy and high precision in situ 18 o values obtained by secondary ion mass spectrometry sims from m size quartz cement zones have been used here to determine the timing of cementation and to evaluate precipitation mechanisms within the basal sandstones of three economically significant paleoproterozoic basins the athabasca and thelon basins canada and the mcarthur basin australia in these examples the earliest quartz cements have the highest 18 o values up to 33 mean 26 0 v smow indicative of low temperature precipitation at depths of 0 to 1 1 km some paragenetically early cement phases however have lighter isotopic values that suggest either precipitation from isotopically depleted water or precipitation at higher temperatures subsequent generations of quartz cements have progressively lower 18 o values mean 16 1 3 consistent with precipitation at higher temperatures these data combined with petrographic observations indicate quartz cementation began in near surface vadose and phreatic zones the next stage of cementation is characterized by syntaxial burial cement overgrowths regionally and stratigraphically well sorted sandstone facies that were initially aquifers preferentially experienced early quartz cementation which resulted in cement bridged pore throats at relatively shallow depths of 0 2 km and very early in their burial history transforming these units into diagenetic aquitards as a result these units did not experience mineralization compositionally and texturally immature braided fluvial facies did not experience early addition of quartz cement but are instead marked by minor syntaxial overgrowth cement that make upb5 of the intergranular volume formed at depths of 3 5 km and were variably replaced by illite and chlorite during burial these lithologies were still open to fluid movement at critical times in the basin when u and

reducing power of ferrous iron in the archean ocean 1 contribution of photosynthetic oxygen a quantitative model of the archean ocean is developed in order to estimate the oxidation rate of dissolved ferrous iron a key process for the deposition of banded iron formations the continuity equation including chemical reactions and vertical transport is solved for o2 and fe ii the oxidation rate depends on the oceanic ph profile which is calculated as a function of the atmospheric co2 pressure in this first part the oxidation due to the reaction with photosynthetic oxygen is considered the results are presented as a function of the oxygen concentration in the mixed layer which was varied between 10 10 and 10 6 mol kg 1 it is shown that this fe ii oxidation by o2 is very efficient and that relatively low o2 levels are required for the oxidation rate to be limited by upward transport of iron to the surface ocean possible evolutionary paths for the archean ocean atmosphere system are discussed by an analysis of the balance of the iron input flux

reconstructing earth s surface oxidation across the archean proterozoic transition the archean proterozoic transition is characterized by the widespread deposition of organic rich shale sedimentary iron formation glacial diamictite and marine carbonates recording profound carbon isotope anomalies but notably lacks bedded evaporites all deposits refl ect environmental changes in oceanic and atmospheric redox states in part associated with earth s earliest ice ages time series data for multiple sulfur isotopes from carbonateassociated sulfate as well as sulfi des in sediments of the transvaal supergroup south africa capture the concomitant buildup of sulfate in the ocean and the loss of atmospheric massindependent sulfur isotope fractionation in phase with sulfur is the earliest recorded positive carbon isotope anomaly convincingly linking these environmental perturbations to the great oxidation event ca 2 3 ga

a neogene seawater sulfur isotope age curve from calcareous pelagic microfossils abstract until now our knowledge of the sulfur isotopic composition of seawater through geologic time has depended on stable isotopic analysis of sulfate from evaporites owing to the sporadic occurrence of evaporites through time the secular sulfur isotope age curve contains many gaps with little or no data in order to fill in some of these gaps particularly the neogene we have analyzed the sulfur isotopic composition of carbonate associated sulfate in carbonate tests of planktonic foraminifera other investigators have shown that sulfate may occur in biogenic calcites either lattice bound as micro fluid inclusions in adsorbed phases or as protein polysaccharides whatever the origin the sulfur isotopic composition of this sulfate appears to be representative of that of the water in which the organism lived as shown by results on recent calcareous foraminifera and macrofossils using this approach for study of miocene to recent pelagic marine sediments supplemented by new data for miocene marine evaporites from the gulf of suez we have found that the 34 s of seawater has decreased about 2 5 over the past 25 m y and that most of the decrease has occurred over the past 5 m y parallelling a decrease in the 13 c of dissolved oceanic bicarbonate from the same interval sedimentary redox models based on isotope records suggest that organic carbon and sulfide burial have both decreased over the past 5 m y alternatively an increase in weathering rates over the past 5 m y would not require a decrease in organic carbon or sulfide burial as long as the isotopic effect of the increased river input exceeds the isotopic effect of the burial of the reduced species in either case the net result would be a decrease in atmospheric p o 2

comparative behavior of authigenic re u and mo during reoxidation and subsequent long term burial in marine sediments the trace element re is enriched in mildly reducing suboxic sediments close to or slightly later in the redox sequence than u and before reduction of mo in more reducing sulfidic sediments this work investigates the behavior of authigenic re during oxidation and long term burial in the sediments of several turbidites from the madeira abyssal plain exposure to bottom water o2 clearly remobilizes authigenic re as found in previous work but reimmobilization occurs in reducing sediments below the oxidation front over a depth range of 0 5 3 m the large spread of the re burndown peak must be kinetically controlled perhaps influenced by the corg content once reimmobilized however the authigenic re signal remains immobile for at least 3 4 myr authigenic u in contrast consistently forms peaks 0 5 m broad whereas authigenic mo is lost on oxidation and does not appear to be refixed unless authigenic pyrite is formed these results imply that any paleoenvironmental significance of authigenic re will be compromised in low sedimentation rate environments 10 cm kyr where there is significant exposure of authigenic re to bottom water o2 however authigenic re records from rapidly accumulating continental margin sediments with much less exposure to bottom water o2 still offer considerable potential as high fidelity recorders of past reducing conditions

evolution of the oceanic sulfur cycle at the end of the paleoproterozoic here we present new measurements of 32 s 33 s 34 s and 36 s in sedimentary sulfides and couple these measurements with modeling treatments to study the sulfur cycle of a late paleoproterozoic marine basin we target the transition in ocean chemistry from the deposition of paleoproterozoic iron formations gunflint formation biwabik formation trommald formation and mahnomen iron formations to the inferred sulfidic ocean conditions recorded by overlying shale rove formation the data suggest that certain features of the global sulfur cycle such as a control by sulfate reducing prokaryotes and low mm concentrations of oceanic sulfate were maintained across this transition this suggests that the transition was associated with changes in the structure of the basin scale sulfur cycle during deposition of these sediments sulfide data from the iron formations are interpreted to reflect sedimentary sulfides formed from microbial reduction of pore water sulfate that was supplied through steady state exchange with an overlying oceanic sulfate reservoir the sulfide data for the euxinic rove formation shales reflect the operation of a sulfur cycle that included the loss of sulfide by a rayleigh like process we suggest that the prevalence of large and variable heavy isotope enrichments observed in rove formation sulfide minerals reflect a sustained and significant net loss of sulfide from the euxinic water column either as a result of a shallow chemocline and degassing to the atmosphere or as a result of a water column pyrite sink the inclusion of 36 s measurements in addition to 32 s 33 s and 34 s illustrates the mass dependent character of these sedimentary environments ruling out contributions from the weathering of archean sulfides and pointing to at least modest levels of sustained atmospheric oxygen 10 5 present atmospheric levels of o2

paleoceanographic constraints on precambrian phosphorite accumulation baraga group michigan usa abstract the palaeoproterozoic baraga group ca 1850 1 ma of northern michigan is a 1200 m thick sedimentary succession of marine clastic iron formation chert and phosphatic sedimentary rocks that accumulated at the peak of the world s first major phosphogenic episode lithofacies stacking patterns are interpreted to record the flooding of the nuna continental margin during two sea level cycles the base of the first sequence is marked by a transgressive lag on archean basement that is transitional into a highstand accumulation of sandstones deposited in peritidal environments the bottom of the second sequence is characterized by a chert and carbonate unit with numerous subaerial exposure surfaces deposited in intertidal to supratidal environments this chert grades upward into subtidal deposits composed of interbedded organic rich mudstone and sandstone the highstand and falling stage systems tracts are composed of progradational deltaic deposits sequences were framed using the newly discovered sudbury impact ejecta horizon as a datum its emplacement approximately coincides with unrelated environmental changes that increased the delivery of river borne sediment to the nuna margin iron bearing minerals ankerite siderite and pyrite show that ferrous iron was present in peritidal to deep marine environments ankerite dominates nearshore settings where pyrite is absent pyrite occurs in deeper progradational deltaic deposits where sulfide was produced by bacterial sulfate reduction phosphorite is restricted to shallow water sediments of the first sequence and the lowstand of the second sequence precipitation is interpreted to be the result of fe redox pumping just below the sediment water interface where photosynthetically produced nearshore oxygen oases impinged on the seafloor such shallow water phosphorite accumulation is in stark contrast to many phanerozoic depositional systems in which phosphogenesis occurs across the shelf this difference likely reflects the dissimilarity in the oxygenation state of the seafloor in the precambrian fe redox pumping and thus phosphogenesis was restricted to shallow water settings with a suboxic seafloor in the phanerozoic phosphorite forms in the full spectrum of shelf environments because the entire seafloor is generally well oxygenated the concentration of bioavailable p in neritic environments during the proterozoic may have played a major role in the development of benthic microbial ecosystems and evolving eukaryotes

rise in seawater sulphate concentration associated with the paleoproterozoic positive carbon isotope excursion evidence from sulphate evaporites in the 2 2 2 1 gyr shallow marine lucknow formation south africa past oceanic sulphate concentration is important for under standing how the oceansredox state responded to atmospheric oxygen levels the absence of extensive marine sulphate evaporites before 1 2 gyr probably reflects low seawater sulphate and or higher carbonate concentrations sulphate evaporites formed locally during the 2 22 2 06 gyr lomagundi positive d 13 c excursion however the 2 2 2 1 gyr lucknow formation south africa provides the first direct evidence for seawater sulphate precipitation on a carbonate platform with open ocean access and limited terrestrial input these marginal marine deposits contain evidence for evaporite molds pseudomorphs after selenite gypsum and solid inclusions of ca sulphate in quartz carbon and sulphur isotope data match the global record and indicate a marine source of the evaporitic brines the apparent precipitation of gypsum before halite requires 2 5 mm l 1 sulphate concentration higher than cur rent estimates for the paleoproterozoic during the lomagundi event which postdates the 2 32 gyr initial rise in atmospheric oxygen seawater sulphate concentration rose from archean values of 200 lm l 1 but dropped subsequently because of higher pyrite burial rates and a lower oceanic redox state

oxygen isotope studies of diagenesis and pore water evolution in the western canada sedimentary basin evidence from the upper cretaceous basal belly river sandstone alberta abstract the oxygen isotope compositions and paragenetic sequence of diagenetic minerals from the upper cretaceous basal belly river sandstone southwestern alberta have been used to identify changes in pore water composition during diagenesis these changes can be related to major geologic events within the western canada sedimentary basin low 18o meteoric water has played an important role in the diagenesis of these upper cretaceous rocks on a basinwide scale during deposition and early diagenesis and following uplift of the basin in early eocene time diagenetic phases include early chlorite 18o 6 1 to 8 3 and calcite 18osmow 21 0 13cpdb 1 6 followed by quartz overgrowths 18o 13 1 to 18 0 calcite 18osmow 10 4 to 14 4 13cpdb 10 6 to 1 2 and then kaolinite 18o 7 8 to 11 9 illite smectite 18o 10 9 to 13 1 and smectite 18o 12 6 to 13 7 the interpretation of these results is that shallow diagenesis early in the burial history chlorite calcite occurred in the presence of brackish or fresh water with burial the 18o value of the pore water increased due to water rock interaction burial diagenesis was terminated in the early eocene by uplift laramide orogeny recharge of the basin by low 18o meteoric water occurred at this time the meteoric water then became involved in the formation of diagenetic quartz and calcite at or near maximum burial temperatures and in the crystallization at lower temperatures of kaolinite illite smectite and smectite as post eocene cooling and erosion progressed

rare earth elements in the phosphatic enriched sediment of the peru shelf abstract apatite enriched materials from the peru shelf have been analyzed for their major oxide and rare earth element ree concentrations the samples consist of 1 the fine fraction of sediment mostly clay material 2 phosphatic pellets and fish debris which are dispersed throughout the fine grained sediment 3 tabular shaped phosphatic crusts which occur within the uppermost few centimeters of sediment and 4 phosphatic nodules which occur on the seafloor the bulk ree concentrations of the concretions suggest that these elements are partitioned between the enclosed detrital material and the apatite fraction analysis of the fine grained sediment with which the samples are associated suggested that this detrital fraction in the concretions should have shale ree values the analysis of the fish debris suggested that the apatite fraction might have seawater values the seawater contribution of ree s is negligible in the nodules and crust in which the apatite occurs as a fine grained interstitial cement that is the concentration of ree s and the ree patterns are predominantly a function of the amount of enclosed fine grained sediment by contrast the ree pattern of the pelletal apatite suggests a seawater source and the absolute ree concentrations are relatively high the ree p 2 o 5 ratios of the apatite fraction of these samples thus vary from approximately zero in the case of the crust and nodules to as much as approximately 1 2 10 3 in the case of the pellets the range of this ratio suggests that rather subtle variations in the depositional environment might cause a significant variation in the ree content of this authigenic fraction of the sediment pelletal glauconite was also recovered from one sediment core its ree concentrations closely resemble those of the fish debris

an iron shuttle for deepwater silica in late archean and early paleoproterozoic iron formation iron formations are typically thinly bedded or laminated sedimentary rocks containing 15 or more of iron and a large proportion of silica commonly 40 in the ca 2590 2460 ma campbellrand kuruman complex transvaal supergroup south africa iron formation occurs as a sediment starved deepwater facies distal to carbonates and shales iron minerals primarily siderite define the lamination the silica primarily occurs as thin beds and nodules of diagenetic chert now microcrystalline quartz filling pore space and replacing iron formation minerals and co occurring deepwater lithologies mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production anoxygenic photosynthesis abiotic photochemistry and chemoautotrophy using fe ii as an electron donor the origin and mechanism of silica precipitation in these deposits have received less attention here we present a conceptual model of iron formation that offers insight into the deposition of silica the model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides which precipitate fe oh 3 binds silica and sinks from the surface ocean along with organic matter shuttling silica to basinal waters and sediments fe iii respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters driving precipitation of siderite from fe2 and respiration influenced co2 silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases this model explains many of the mineralogical textural and environmental features of late archean and earliest paleo proterozoic iron formation

chemostratigraphy of carbonates from the minas supergroup quadrilatero ferryifero iron quadrangle brazil a stratigraphic record of early proterozoic atmospheric biogeochemical and climactic change the ca 2 42 ga old gandarela formation overlies the finely laminated caue banded iron formation bif of the itabira group minas supergroup brazil and consists of red carbonate facies bif grading upsection into buff dolomites and limestones which are locally stromatolitic carbonates of the gandarela formation have 1 3 c values ranging narrowly from 1 6 to 0 4 permil versus pdb n 93 the cercadinho formation at the base of the overlying piracicaba group is a thick arenaceous succession with rare thin carbonates that have 1 3 c values ranging from 3 3 to 5 4 permil n 12 stromatolitic dolomites of the overlying fecho do funil formation have pb pb carbonate age 2 11 0 11 ga babinski and others 1995 that provide the minimum age for this unit and 1 3 c values ranging from 5 6 to 7 4 permil n 69 at present we find no evidence in brazil for the any of the three early paleoproterozoic glacial events recognized in 2 45 to 2 22 ga sedimentary successions in north america given the radiometric constraints however it is possible that geological and geochemical traces of these ice ages are missing along the unconformity between the gandarela and cercadinho formations if correct carbonates of the gandarela formation may provide a record of the carbon isotope composition of the seawater prior to any of the paleoproterozoic ice ages combined with other available 1 3 c data results from the gandarela formation suggest that seawater preceding the ice ages was not significantly enriched in 1 3 c on the other hand the carbon isotopic signature of the cercadinho carbonates and chemical composition of the cercadinho arenites are consistent with deposition after the end of the glacial epoch and likely during the early stage of ca 2 22 to 2 1 ga carbon isotope excursion the 1 3 c values of marine carbonates of the fecho do funil formation are consistent with carbon isotope record of contemporaneous carbonate successions worldwide providing additional evidence for a global biogeochemical anomaly at ca 2 22 to 2 1 ga these compositional and isotopic changes allow integration of the minas supergroup into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age

testing for eustatic sea level control in the precambrian sedimentary record abstract stratigraphic successions are deposited under the influence of three primary controls which tend to vary systematically through time these controls are tectonism basin subsidence and uplift sea level change and sediment supply the development of sequence stratigraphy has enabled the introduction of systematic quantitative methods for the analysis of these controls the use of the sequence approach requires the establishment of a regional sequence framework and detailed chronostratigraphic control working with the phanerozoic record this approach has revealed a variety of sequence types reflecting long term tectonostratigraphic processes caused by continental rifting continental collisions and cratonic dynamic topography high frequency thrust loading cycles in foreland basins and high frequency cycles caused by orbital forcing glacioeustasy 104 5 year periodicity and eustasy driven by changing rates of sea floor spreading 107 8 year periodicity have emerged as two significant global processes documenting the sequence record in the precambrian has proved difficult owing to fragmentary preservation and structural deformation methods of dating are at present not precise enough to permit reliable regional correlation or testing of high frequency sequence generating mechanisms some studies of precambrian sequence architecture have been achieved but these indicate tectonic processes acting at a much slower rate than during the phanerozoic further analysis of this problem is required

a marine biogeochemical perspective on black shale deposition abstract deposition of marine black shales has commonly been interpreted as having involved a high level of marine phytoplankton production that promoted high settling rates of organic matter through the water column and high burial fluxes on the seafloor or anoxic sulfidic water column conditions that led to high levels of preservation of deposited organic matter or a combination of the two processes here we review the hydrography and the budgets of trace metals and phytoplankton nutrients in two modern marine basins that have permanently anoxic bottom waters this information is then used to hindcast the hydrography and biogeochemical conditions of deposition of a black shale of late jurassic age the kimmeridge clay formation yorkshire england from its trace metal and organic carbon content comparison of the modern and jurassic sediment compositions reveals that the rate of photic zone primary productivity in the kimmeridge sea based on the accumulation rate of the marine fraction of ni was as high as 840 g organic carbon m 2 yr 1 this high level was possibly tied to the maximum rise of sea level during the late jurassic that flooded this and other continents sufficiently to allow major open ocean boundary currents to penetrate into epeiric seas sites of intense upwelling of nutrient enriched seawater would have been transferred from the continental margins their present location onto the continents this global flooding event was likely responsible for deposition of organic matter enriched sediments in other marine basins of this age several of which today host major petroleum source rocks bottom water redox conditions in the kimmeridge sea deduced from the v mo ratio in the marine fraction of the kimmeridge clay formation varied from oxic to anoxic but were predominantly suboxic or denitrifying a high settling flux of organic matter a result of the high primary productivity supported a high rate of bacterial respiration that led to the depletion of o2 in the bottom water a high rate of burial of labile organic matter albeit a low percentage of primary productivity in turn promoted anoxic conditions in the sediment pore waters that enhanced retention of trace metals deposited from the water column

alteration of cenozoic cool water carbonates to low mg calcite in marine waters gambier embayment south australia abstract the oligo miocene gambier limestone is a poorly lithified fine grained bryozoan rich grainstone wackestone and marl allochems in these rocks which are typical of many modern and ancient cool water carbonates were originally calcite lmc mg calcite imc and hmc and minor aragonite but most are now neomorphosed to lmc with the exception of calcretes most limestones have 5 cement 13c and 18o values from 0 to 1 5o oo and preserved 87sr 86sr ratios that indicate that neomorphism occurred in seawater rather than meteoric or evolved pore waters marine diagenesis of the limestones was facilitated by their slow accumulation rates which were ca 15 mm ky based on sr isotope stratigraphy relatively fine grain size and high porosities which locally reach 50 the sr and mg contents of grainstones are significantly lower than in the original sediments whereas a less porous marl has higher sr and mg contents the grainstones have about 300 ppm sr and 6000 ppm mg consistent with slow abiotic precipitation of calcite under near equilibrium conditions with seawater which is the thermodynamic drive for this diagenesis the higher sr and mg content of the marl results from variable preservation of original allochems of biotic calcite the results from this study and others on modern cool water carbonates indicate that the extent of diagenesis varies with lithology but much neomorphism occurred in seawater with minimal effects from meteoric waters as such the chemical characteristics of seawater may be effectively preserved in some ancient cool water carbonates

possible evidence for episodic epeiric marine and fluvial sedimentation and implications for palaeoclimatic conditions c 2 3 1 8 ga kaapvaal craton south africa abstract the debate on palaeoproterozoic atmospheric composition climate and the timing and nature of the postulated great oxidation event possibly at c 2 3 ga remain matters of debate within precambrian geology this paper contributes to the debate through examination of physical sedimentary structures preserved within two separate groups of sedimentary deposits from the central and northern portions of the kaapvaal craton south africa within the c 2 3 2 1 ga pretoria group study of epeiric marine sandstones and mudrocks magaliesberg and silverton formations c 2 1 ga suggests that fluvial input was predominant over low energy reworking by waves winds and tides this together with extensive evidence for microbial mat growth within the littoral sandstones of the magaliesberg formation points to strongly episodic sedimentation by both marine and fluvial processes within two younger basins belonging to the c 2 05 1 8 ga waterberg group anomalously high palaeoslopes calculated according to standard palaeohydrological methods combined with direct field evidence for sediment gravity flows and sheetflood deposition within an overall braided fluvial setting again suggest episodic sedimentation strongly episodic sedimentation can most easily be reconciled with a greenhouse palaeoclimate at least for that part of kaapvaal at that time taken together these data sets point to the postulate that the generally accepted change from a palaeoproterozoic greenhouse through a great oxidation event c 2 3 ga to less extreme climatic conditions may have been gradational and even diachronous across the earth s cratons at that time

microbial transformations of organic matter in black shales and implications for global biogeochemical cycles abstract the various roles that microorganisms play in transformation of organic matter in geologic environments are yet to be fully revealed many of these roles influence and perhaps control the composition of earth s atmosphere over geologic time by directly impacting global scale cycling of carbon dioxide oxygen and methane one example is the weathering of black shales exposure of organic matter rich sedimentary rocks to oxygenated earth surface conditions results in significant changes in outcrop scale rock geochemistry with implications for element cycling on a global scale this article reviews the progress of ongoing research of a black shale weathering profile exposed near clay city kentucky usa using tools of molecular biology microbial ecology isotope geochemistry and organic geochemistry this research explores the role of microorganisms in utilization and oxidation of sedimentary organic matter and sulfide mineral oxidation and examines the communities of microorganisms that may live in this environment from this and related studies we are developing greater awareness of the importance of microorganisms in transfer of organic materials among various reservoirs of the geosphere biosphere hydrosphere and atmosphere

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

redox stratification and anoxia of the early precambrian oceans implications for carbon isotope excursions and oxidation events abstract an updated compendium of 13c data offer compelling evidence that 13c positive excursions of unsurpassed magnitude in the recorded earth history 8 and up to 18 pdb occurred in the early proterozoic the lomagundi event questions whether or not these unprecedented positive 13c shifts were contemporaneous in various basins and represent local or global events remain unresolved the framework of major geologic events that occurred in the paleoproterozoic argues against a snowball earth scenario as a backdrop to these exceptional 13c enrichments substantial increases of carbon flux ratio fo fc eight times the phanerozoic average and organic carbon burial rate fo three times the phanerozoic average are required to account for the observed 13c enrichments under steady state and dynamic equilibrium modes respectively these enhanced ratios and rates are conditional on the availability of a flux of nutrients to the contemporaneous biota producers in excess of the riverine flux and a decoupling of the p and c cycles it is argued that these two conditions were met between 2 25 and 2 11 ga in a redox stratified ocean with weak upwelling and sluggish meridional surface circulation the alternative that the major disturbance in the terrestrial carbon cycle occurred in a rapidly ventilated ocean is assessed as being unlikely a large pulse of o2 equivalent to six to seven times the present terrestrial oxygen budget was rapidly scavenged and spent in the oxidation of reduced fe and s transported to the shelves by rivers this contention is consistent with data inferring low oxygen levels 10 5 to 10 2 pal in the paleoproterozoic atmosphere and the occurrence of ocean anoxia until the late precambrian

paleomagnetism and the evolution of fluids in the proterozoic athabasca basin northern saskatchewan canada in the athabasca basin diagenetic hematite of variable paragenesis occurs throughout the sandstones and underlying paleoregolith this hematite carries three distinct single component magnetizations a d 158 i 62 95 5 n 21 b d 11 i 36 95 7 n 6 and c d 18 i 79 95 3 n 27 in some areas of the sandstones such as near reactivated fault zones the diagenetic hematite has been altered to goethite which yields a very low intensity incoherent d magnetization ages for the a b and c magnetizations inferred from comparisons with paleomagnetic directions in precambrian rocks whose ages are known approximately are 1750 1600 1600 1450 and about 900 ma respectively the a magnetization is carried by the earliest formed hematite and its estimated age compares well with u pb ages of 1650 1700 ma for early diagenetic apatite u pb and rb sr ages of approximately 1500 and 900 ma for uraninite and illite coeval with hematite that carries the b and c magnetiza

depositional environment and diagenesis as controlling factors for petro physical properties and weathering resistance of siliciclastic dimension stones integrative case study on the wesersandstein northern germany middle buntsandstein

sedimentary phosphate formation in warm shallow waters new insights into the palaeoceanography of the permian phosphoria sea from analysis of phosphate oxygen isotopes abstract oxygen isotopic analyses of the structural phosphate 18 o p in sedimentary phosphorites of the upper permian phosphoria formation reveal that phosphogenesis occurred across a broad range of palaeoenvironments the 18 o p values from well preserved hand picked phosphate grain separates range from 20 2 to 13 7 0 5 smow with the lowest values in the most landward shallow water deposits and the highest values in the most basinward deeper water deposits the relationship between the maximum burial depth and 18 o p is opposite to that predicted if burial diagenesis was responsible for the isotopic signature of these phosphate grains this suggests that the regional trend in oxygen isotopic data is not due to burial diagenesis the range in oxygen isotopic values does correspond to the potential range of palaeotemperatures in the shallow and broad epicontinental phosphoria sea the range is 14 42 c 3 5 c assuming a seawater 18 o of 2 5 smow palaeotemperatures along an offshore to onshore transect indicate that upwelling involved temperate 14 26 c mean of 21 c but not cold waters and that warming across the palaeoshelf resulted in mean shallow water temperatures of 34 37 c the latter values approximate those predicted by climate and ocean circulation models these warm palaeotemperatures provide further evidence that phosphogenesis in the epicontinental phosphoria sea occurred under palaeoceanographic conditions that were different from modern phosphorite depositional systems this suggests that a purely actualistic approach to the interpretation of the phosphoria phosphorites is not valid

formation of syngenetic and early diagenetic iron minerals in the late archean mt mcrae shale hamersley basin australia new insights on the patterns controls and paleoenvironmental implications of authigenic mineral formation a section through the late archean mt mcrae shale comprising in ascending order a lower shale interval lsi a banded iron formation bif an upper shale usi and a carbonate c1 has been analyzed for total fe and al contents and authigenic fe present as carbonate oxide sulfide and silicate phases the authigenic mineralogy is controlled by the episodic addition of fe from hydrothermal activity and removal of fe by sulfide relative to rates of clastic sedimentation the lsi and bif have mean fet al values of 2 and 5 respectively that record iron enrichment from hydrothermal sources iron was precipitated primarily as siderite accompanied by fe rich chlorite from anoxic bottom waters rich in dissolved fe pyrite formation was probably limited by the availability of sulfate which was present at low concentrations and became rapidly depleted the usi has generally lower fet al values 0 6 1 3 similar to those found in paleozoic shales with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle this interval contains authigenic fe predominantly as pyrite where high values for dop 0 8 indicate the existence of a water column that became rich in dissolved sulfide euxinic when sulfate concentrations increased due to a transient or secular increase in ocean atmosphere oxygenation high concentrations of dissolved sulfide maintained low concentrations of dissolved fe which allowed only minor amounts of fe to be precipitated as carbonates and silicates the usi also has elevated concentrations of organic matter that most probably reflect increased productivity and likely limited euxinia to midportions of the water column on the basin margin the carbonate c1 represents a basinal chemistry where sulfide has been removed and fet al values are 1 indicating that hydrothermal activity again produced dissolved fe rich bottom waters detailed iron speciation of the mt mcrae shale can be used to recognize spatial and temporal variations in iron and sulfur inputs to the late archean hamersley basin just prior to the paleoproterozoic rise in atmospheric oxygenation and our refined methods have relevance to all fe rich deposits

molybdenum isotope evidence for widespread anoxia in mid proterozoic oceans how much dissolved oxygen was present in the mid proterozoic oceans between 1 8 and 1 0 billion years ago is debated vigorously one model argues for oxygenation of the oceans soon after the initial rise of atmospheric oxygen 2 3 billion years ago recent evidence for h 2 s in some mid proterozoic marine basins suggests however that the deep ocean remained anoxic until much later new molybdenum isotope data from modern and ancient sediments indicate expanded anoxia during the mid proterozoic compared to the present day ocean consequently oxygenation of the deep oceans may have lagged that of the atmosphere by over a billion years

oxygenation of the earth s atmosphere ocean system a review of physical and chemical sedimentologic responses abstract the great oxidation event goe is one of the most significant changes in seawater and atmospheric chemistry in earth history this rise in oxygen occurred between ca 2 4 and 2 3 ga and set the stage for oxidative chemical weathering wholesale changes in ocean chemistry and the evolution of multicelluar life most of what is known about this important event and the subsequent oxygenation history of the precambrian earth is based on either geochemistry or data mining published literature to understand the temporal abundance of bioelemental sediments bioelemental sediments include iron formation chert and phosphorite which are precipitates of the nutrient elements fe si and p respectively because biological processes leading to their accumulation often produce organic rich sediment black shale can also be included in the bioelemental spectrum thus chemistry of bioelemental sediments potentially holds clues to the oxygenation of the earth because they are not simply recorders of geologic processes but intimately involved in earth system evolution chemical proxies such as redox sensitive trace elements cu cr v cd mo u y zn and ree s and the ratio of stable isotopes 56fe 53cr 97 95mo 98 95mo 34s 33s in bioelemental sediments are now routinely used to infer the oxygenation history of paleo seawater the most robust of these is the mass independent fractionation of sulfur isotopes mif which is thought to have persisted under essentially anoxic conditions until the onset of the goe at ca 2 4 ga since most of these proxies are derived from authigenic minerals reflecting pore water composition extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously paleoenvironmental context is critical to understanding whether geochemical trends during earth s oxygenation represent truly global or merely local environmental conditions to make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diagenetically altered and often metamorphosed because relatively few studies consider alteration in detail our ability to measure geochemical anomalies through the goe now surpasses our capacity to adequately understand them in this review we highlight the need for careful consideration of the role sedimentology stratigraphy alteration and basin geology play in controlling the geochemistry of bioelemental sediments such an approach will fine tune what is known about the goe because it permits the systematic evaluation of basin type and oceanography on geochemistry this technique also provides information on how basin hydrology and post depositional fluid movement alters bioelemental sediments thus a primary aim of any investigation focused on prominent intervals of earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean atmosphere evolution

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

relationships between lithology and diagenesis in a limeston marl facies transition an electron microprobe study a 3 km long lateral facies transition from oligo miocene skeletal grainstones into packstones and mudstones displays systematic variation in carbonate skeletal fragment alteration and secondary carbonate mineral composition within the skeletal grainstone end member all aragonitic grains have dissolved all mg calcite skeletal fragments have altered to calcite and calcite has been precipitated as both void filling spar and as syntaxial overgrowths on echinoid fragments within the transitional grainstone packstone lithologies some skeletal fragments show preservation of their original mineralogy while calcite ferroan calcite spar and dolomite have precipitated in void spaces within the packstone mudstone end member of the transition aragonitic grains have been preserved and mg calcite skeletal fragments have retained most of their magnesium thin originally porous grainstone layers within the pack stone mudstones contain sideroplesite and ferroan calcite spar in void spaces and iron has been incorporated in skeletal fragments which were originally mg calcite in these grains iron has been incorporated at the expense of magnesium until the grains approach the composition of the surrounding ferroan calcite cement the lateral variation in diagenetic alteration across this facies transition is a function of differing groundwater movement and composition controlled ultimately by the composition porosity and permeability of the host lithologies

pge re os and mo isotope systematics in archean and early proterozoic sedimentary systems as proxies for redox conditions of the early earth abstract re os data and pge concentrations as well as mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the barberton greenstone belt south africa fig tree and moodies groups 3 25 3 15 ga the belingwe greenstone belt zimbabwe manjeri formation ca 2 7 ga and shales from the witwatersrand ventersdorp and transvaal supergroups south africa ranging from 2 95 to 2 2 ga moderately oxidizing conditions are required to mobilize re and mo in the environment mo fractionation only occurs in solution and these parameters thus have potential use as paleoredox proxies for the early earth pge re abundance patterns of barberton greenstone belt sediments are uniform and very similar in shape to those of komatiites this indicates 1 that the pge came from a source of predominantly ultramafic composition and 2 that pge were transported and deposited essentially in particulate form sediments from the younger belingwe greenstone belt show more fractionated pge re patterns and have re os ratios 10 to 100 higher than those of barberton sediments their pge abundance patterns and re os ratios are intermediate between those of the mid archean shales and neoproterozoic to recent black shales they reflect scavenging of re from solution in the sedimentary environment 98 95 mo values of black shales of all ages correlate with their concentrations the barberton greenstone belt samples have 1 3 ppm mo similar to a granitoid basaltic source this mo has 98 95 mo between 1 9 and 2 4 relative to present day mean ocean water molybdenum momo and is thus not isotopically fractionated relative to such a source similar to the pge this indicates transport in solid form sediments from the belingwe greenstone belt show in part enhanced mo concentrations up to 6 ppm and mo isotope fractionation 98 95 mo up to 1 4 relative to momo the combined pge re and mo data show mainly reducing conditions in the mid archean and suggest that by 2 7 ga the atmosphere and oceans had become more oxidizing substantially younger samples from the transvaal supergroup to ca 2 2 ga surprisingly have mainly low mo concentrations around 1 ppm and show no significant mo isotope fractionation relative to the continental source among possible explanations for this are a return to reducing atmospheric conditions after 2 7 ga reservoir effects or mo removal by sulfide precipitation following sulfate reduction in early proterozoic oceans

tectonic control of chemical and isotopic composition of ancient oceans the impact of continental growth a numerical model that couples carbon sulfur strontium and atmospheric oxygen cycles is used here to explore the impact of continental growth on the long term or 10 8 yrs evolution of the isotopic composition of seawater three growth scenarios are tested big bang generation of continents shortly after the accretion of the earth and two more gradual scenarios with a major growth episode around the archean proterozoic boundary the corresponding 87 sr 86 sr delta 34 s and delta 13 c of seawater and the sizes of the respective crustal sedimentary reservoirs are calculated for each scenario and compared to the available data the gradual continental growth scenarios yield a better fit to the existing 87 sr 86 sr and delta 34 s isotope data for ancient seawater than does the big bang model and can be in agreement also with the measured seawater delta 13 c providing c org participates in carbon subduction flux over the earth history these scenarios also generate a progressive oxygenation of the ocean atmosphere system with a large po 2 rise coincident with and due to the major continental growth event around the archean proterozoic transition in accord with the geologic record that indicates a major oxidation event in the early proterozoic the advancing oxygenation of the planetary exogenic system may therefore be a consequence of tectonic evolution rather than of biological innovations such as the photosystem 2 the latter may have predated considerably the impact of oxygenation visible in the geologic record in contrast to the above isotope systematics the model does not approximate well experimental observations of large delta 18 o variations at 10 8 yrs time scales at least during the phanerozoic the reason for this discrepancy may depend on the model structure that permits large variations in the oxygen isotopic composition of seawater only on time scales of 10 9 yrs

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

contourites associated with pelagic mudrocks and distal delta fed turbidites in the lower proterozoic timeball hill formation epeiric basin transvaal supergroup south africa abstract five genetic facies associations architectural elements are recognised for the epeiric sea deposits preserved in the early proterozoic timeball hill formation south africa basal carbonaceous mudrocks interpreted as anoxic suspension deposits grade up into sheet like laminated graded mudrocks and succeeding sheets of laminated and cross laminated siltstones and fine grained sandstones the latter two architectural elements are compatible with the te td and tc subdivisions of low density turbidity current systems thin interbeds of stromatolitic carbonate within these first three facies associations support photic water depths up to about 100 m laterally extensive sheets of mature cross bedded sandstone disconformably overlie the turbidite deposits and are ascribed to lower tidal flat processes interbedded lenticular immature sandstones and mudrocks comprise the fifth architectural element and are interpreted as medial to upper tidal flat sediments small lenses of coarse siltstone very fine grained sandstone analogous to modern continental rise contourite deposits occur within the suspension and distal turbidite sediments and also form local wedges of inferred contourites at the transition from suspension to lowermost turbidite deposits blanketing and progressive shallowing of the floor of the timeball hill basin by basal suspension deposits greatly reduced wave action thereby promoting preservation of low density turbidity current deposits across the basin under stillstand or highstand conditions a lowstand tidal flat facies tract laid down widespread sandy deposits of the medial klapperkop member within the formation salinity gradients and contemporaneous cold periglacial water masses were probably responsible for formation of the inferred contourites the combination of the depositional systems interpreted for the timeball hill formation may provide a provisional model for early proterozoic epeiric basin settings

rare earth elements in phosphorite and associated sediment from the namibian and south african continental shelves rare earth elements ree and major elements have been determined in twenty nine phosphorite and associated sediment samples from the southwestern coast of africa including thirteen samples of unconsolidated sediment collected during recent cruises to phosphate rich areas on the namibian shelf the concentrations of ree in various phosphorite types from southwestern africa differ by more than two orders of magnitude the samples mostly exhibit ree patterns which are similar to that of shale having no ce anomaly the ce content appears to have been controlled by the palaeoredox conditions of the localised environment of deposition or diagenesis and not to have been influenced by marine upwelling or by the general palaeoredox conditions of the water column the environments in which ree were incorporated into the francolite appear to have been anoxic or sub oxic in the case of both namibian unconsolidated authigenic phosphorites and the predominantly diagenetic phosphorites off the south african coast the ultimate ree source in both cases appears to have been mainly eolic continental detritus pelletal phosphorites are significantly enriched in ree relative to non pelletal varieties if sequestration of ree dissolved in the water column contributed to the high concentrations of ree in pelletal phosphorites then the main carrier phase appears to have been inorganic detritus or biogenic silica equilibrated with oxygen depleted bottom waters but not fish debris the contention that onland phosphorites are enriched in ree relative to their offshore counterparts is however refuted in the case of southwestern africa and the effects of groundwater ree enrichment are not obvious in the majority of onland phosphorites analysed in this study

iron formation in neoarchean deltaic successions and the microbially mediated deposition of transgressive systems tracts abstract neoarchean deltaic deposits present in superior province canada contain an excellent record of shallow water siliciclastic and iron formation if deposition the deltas were fed by braided fluvial systems which commonly resulted in the development of small active lobes that repeatedly cycled through progradation and abandonment although a thick progradational deltaic assemblage was also investigated magnetite rich chemical sediments interbedded with siltstones and slates are present on flooding surfaces overlying shore proximal fluvial channels and distributary mouth bars of the braid delta deposits magnetite laminae are also interbedded with some distributary mouth sediments draping reactivation surfaces in ripples to barforms thicker accumulations of if up to 73 m form portions of transgressive systems tracts where chemical sedimentation dominated offshore equivalents of these successions do not contain chemical sediments sedimentologic data imply that if deposition in this well mixed nearshore water mass was not compatible with slow rates of constant iron compound precipitation in the world ocean combined with siliciclastic sediment starvation the model presented for iron precipitation relies on an elevated flux of continentally derived nutrients in the nearshore stimulating photosynthetically induced oxidation of fe2 under these conditions elevated accumulation rates precluded the development of condensed intervals characteristic of transgressive systems tracts

precambrian superplumes and supercontinents a record in black shales carbon isotopes and paleoclimates prominent maxima in black shale abundance and in black shale total shale ratio occur at 2 0 1 7 ga with less prominent peaks in the late neoproterozoic 800 600 ma and in the late archean 2 7 2 5 ga peaks in chemical index of alteration cia of shales at the same times suggest corresponding warm paleoclimates the peaks in cia and black shale abundance are correlated in time at a 94 confidence level the black shale and cia peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2 7 and 1 9 ga mantle superplume events may have introduced large amounts of co2 into the atmosphere ocean system increasing depositional rates of carbon and increasing global warming increased black shale deposition may reflect some combination of 1 increased oceanic hydrothermal fluxes introducing nutrients 2 anoxia on continental shelves and 3 disrupted ocean currents the apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon peaks in black shale abundance at 2 1 ga and 800 600 ma correlate with peaks in 13 c in marine carbonates increases in atmospheric oxygen and with high cia values in shales these are all consistent with higher rates of organic carbon burial in black shales at these times these peaks may record the breakup of supercontinents at 2 2 2 0 ga and again at 800 600 ma which resulted in increased numbers of partially closed marine basins disruption of ocean currents and increased hydrothermal vents at ocean ridges all of which led to widespread anoxia 2001 elsevier science b v all rights reserved

early to middle devonian ironstone and phosphorite in the northwestern gondwana parnaiba basin brazil a record of an epeiric margin paleoceanographic changes abstract pragian to givetian successions in the caninde group parnaiba basin brazil present phosphorites ironstone and organic rich sediments that globally coincide with greenhouse conditions and an increase in continental weathering ocean oxygen levels and black shale deposition events these facies are associated with the evolution of the northern gondwana epeiric sea margin of the rheic ocean and can tell us how ocean chemistry changed and the paleoceanographic factors involved in this particular transition of the ocean record we conducted an integrated study based on sedimentology sequence stratigraphy petrography scanning electron microscopy and x ray diffraction to understand the paleoenvironmental stratigraphic and diagenetic controls lithofacies stacking patterns indicate that deposition occurred during marine transgressions resulting in four depositional sequences late eifelian and givetian maximum flooding surfaces are represented by phosphogenesis and francolite precipitation associated with condensed zones pragian emsian to givetian ironstones were represented by authigenic coated phosphatic iron silicate odinite chamosite clinochlore francolite and siderite precipitation these are related to normal regressions under lowstand and highstand systems tract oxic waters prevailed and episodic upwelling is interpreted as the source of nutrients that maintained fe si and p supply on a storm influenced to storm dominated shelf with an additional source of iron also associated with river discharges phosphorite was formed in a stratified ocean near storm wave base under anoxic sulfide reducing bacteria microbial communities and suboxic conditions productivity fueled by recycled p shallower anoxic conditions near the fair weather wave base existed possible related to a shift in the locus of upwelling and then resumption to oxic conditions indicate that water column stratification was not a stable feature due to variations in base level ventilation terrigenous or marine contribution and progressive microbial degradation at the beginning of late eifelian transgression an event of increased delivery of continental flux and eutrophication can also be proposed the link between organic rich intervals and third order black shale events daleje chot c and kacak suggests that extinction events can be tracked by analyzing sections in south america global correlation with ironstone precipitation and organic rich sediments shows that stepped periods of c sink conditions probably contributed to the ocean oxygenation

multiple sulfur isotopes of sulfides from sediments in the aftermath of paleoproterozoic glaciations geochemical evidence reported from paleoproterozoic sediments has long been used to evaluate the transition from the anoxic archean atmosphere to an oxygenated atmosphere sulfur isotopes 32s 33s 34s and 36s in sedimentary sulfides and sulfates are an especially sensitive means to monitor this transition such that the timing of the paleoproterozoic great oxidation event can be investigated using mass independently fractionated mif sulfur isotope systematics expressed as 33s here we report data from 83 individual analyses of pyrite pyrrhotite and chalcopyrite on a new suite of 30 different samples from finland south africa wyoming and ontario that span 600 my and follow one or several snowball earth events in the paleoproterozoic the samples were measured using a high resolution secondary ion mass spectrometry technique in multicollection mode that investigates multiple sulfur isotopes in microdomains 30 m within individual sulfide grains while preserving petrographic context we focused on sediments deposited in the aftermath of the paleoproterozoic glaciations between 1 9 and 2 2 ga to trace fluctuations in atmospheric o2 concentrations that were likely affected by an interplay of o2 sinks in the atmosphere and the upper ocean and continental crust and by the emergence and diversification of aerobic organisms our results demonstrate that mif sulfur isotopes are absent in sediments deposited after the period of protracted global cooling in the paleoproterozoic and independently confirm observations that mif ceased during this time we interpret our results by integrating 33s and 34s data in sulfides 13c data in carbonates and the estimated timing of glaciation events in the paleoproterozoic data strongly hint at the presence of microbial sulfate reduction and fluctuations in the concentration of dissolved seawater sulfate and or in 34ssulfate in the aftermath of glaciations and likely were affected by changing erosion rates and nutrient delivery to the oceans these changes modulated the population of primary producers especially oxygenic photosynthesizers and led to fluctuations in the abundance of atmospheric o2 co2 and ch4 our results support the interpretation that the world wide 13ccarb excursion observed between 2 25 and 2 05 ga karhu and holland 1996 was a period of significant accumulation of o2 in the atmosphere

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

tracking euxinia in the ancient ocean a multiproxy perspective and proterozoic case study the evolution and extinction of life are tied intimately to the oxygen state of the ocean and particularly to the presence of anoxic and h2s containing euxinic water on a global scale anoxia and euxinia were more common in the past relative to today s 0 5 euxinic seafloor we are able to constrain the distributions of these conditions through a combination of indirect numerical modeling methods and more direct geochemical proxies with particular emphasis on fe s mo analysis of fine grained siliciclastic rocks for the latter establishing the spatiotemporal pattern of oceanic redox is more difficult with very old rocks because of the limited availability of well dated well preserved materials that span shallow and deep environments across time lines despite these difficulties the multiple approaches synthesized in our case study point to global oxygen deficiency in the deep ocean and perhaps euxinia during most if not all of the proterozoic and likely extending into the early paleozoic

rare earth geochemistry of phosphorites abstract the abundance of rare earth elements ree s in francolite increases with increasing age as a result of post depositional enrichment abundances are higher in pelletal samples than in non pelletal ones because the higher surface mass ratio of pelletal francolite promotes enrichment during diagenesis onshore francolite has higher ree abundances than that found offshore distribution patterns in francolite may be similar to those reported for open ocean water or shale depending upon the dominant source of the ree s different patterns result from weathering and deep burial diagenetic modification of the original patterns and abundances negative ce anomalies occur in francolites containing structural sulphur that is isotopically indistinguishable from seawater of the same age zero or positive anomalies occur in francolites which contain sulphur that is isotopically lighter than seawater

isotope geochemistry of oxygen and carbon in phosphate and carbonate of phosphorite francolite oxygen isotope ratios in phosphate delta sup 18 o sub p were determined in 219 samples of phosphoritic rocks from all major phosphogenic provinces of the world ranging in age from lower proterozoic to recent in 58 samples oxygen was analyzed in the two oxygen lattice sites of francolite both in phosphate and in structural carbonate delta sup 18 o sub c delta sup 13 c was also analyzed in the latter and in coexisting calcite for 10 of them both delta sup 18 o sub p and delta sup 18 o sub c decrease sharply from the recent to the cretaceous and decrease with a gentler slope to the precambrian the nature of the time variation is elucidated by means of a delta sup 18 o sub p vs delta sup 18 o sub c plot which suggests that a the main cause of the time trends is the increase in temperature between recent and the cretaceous b the pre cretaceous trend is overprinted by both low temperature nonequilibrium exchange between sup 18 o depleted water and the rocks and higher temperature exchange between phosphate and carbonate c the observed trend cannot be explained by sup 18 o depleted cretaceous and paleozoic oceans the calcite coexisting with apatite in phosphatic rocks is similar in both delta sup 18 o and delta sup 13 c to the structural carbonatemore of the apatite even though phosphate may exchange its oxygen with environmental water to a greater extent than the authors originally claimed it is probably still a very good isotopic recorder of sedimentary environments less

multiple sulfur isotopes and the evolution of the atmosphere interest in multiple sulfur isotope analyses has been fueled by recent reports of mass independent sulfur isotope signatures in the geologic record a non zero multiple isotopic signature of sulfur 33s and 36s is produced primarily through photochemical reactions and it is an almost perfect tracer of the source of sulfur once the signature is passed on to a given sulfur reservoir it will be preserved unless there is addition of sulfur with a different 33s or 36s in the geological record this signature has been used to study the evolution of the earth s atmosphere and to trace movement of sulfur through geological systems early in earth s history recently small but significant mass independent signatures have been reported for some younger samples raising the possibility of additional applications of multiple isotope studies the purpose of this review is to introduce and discuss the implications of multiple isotope studies and to focus attention on these anomalous but not uncommon isotopic signatures

distribution of rare earth and some trace elements in tertiary phosphorites from the senegal basin and their weathering products abstract the distribution of rare earth and some trace elements is studied in the eocene phosphate deposits of farim saliquinhe lam lam and n diendouri ouali diala from the mauritania senegal guinea basin the depth of weathering varies between the three deposits the strong weathering of lam lam is marked by the leaching of apatites ca phosphate and the appearance of epigenetic crandallites al ca phosphate in the void spaces that have resulted from leaching of apatites and then of crandallites ferruginous and kaolinitic fillings are epigenised in millisites al ca phosphate all the studied phosphates have high ree ba and sr abundances and present shale normalized patterns with a negative ce anomaly of variable intensity clay minerals of sediments smectites and of weathered rocks kaolinites are characterized by low ree and high v zn cu cr sc and zr abundances they have no ce anomaly the comparison of the ree patterns and of the trace element abundances lead to three important conclusions at first the genesis of al ca phosphates is marked by specific leaching of heavy ree relative to the light ones then millisites have a ree abundance and a ce anomaly less pronounced than those of apatites and crandallites they show also v zn cu cr sc and zr abundances like clay minerals these facts can be related to their epigenetic formation from clay minerals last but not least the negative ce anomaly of epigenetic crandallites follows the same trend as in apatites therefore this anomaly cannot constitute a proof of the primary character for apatites if such variations should be related to the paleogeographic setting of studied deposits they must be considered with prudence

hematite formation by oxygenated groundwater more than 2 76 billion years ago geoscientific drilling in the marble bar area of the pilbara craton western australia resulted in the discovery of locally abundant hematite in archean basalts 200 m below the present land surface the hematized basalts occurring along a bedding parallel shear zone are cross cut by pyrite veinlets 3 mm in width and contain euhedral pyrite grains 10 500 m in diameter with sharp crystal edges indicating that the hematite formed before the pyrite we have dated the pyrite in the veinlets at 2 763 0 016 ga using the re os method therefore the hematite formed prior to 2 763 ga the basalts containing the hematite belong to the apex basalt of the warrawoona group and were erupted onto the archean seafloor at 3 46 ga due to 2 9 ga orogenic deformation and subsequent deep erosion the apex basalt was exposed at the surface of a continental landmass prior to 2 77 ga sometime in the period between 2 9 ga and 2 77 ga the basalt section we describe was less than 200 m below the late archean land surface and within range of groundwater percolation through the shear zone in the basalts geological mineralogical and geochemical lines of evidence strongly suggest that the infiltration of o2 rich groundwater through the bedding parallel shear in the basalts formed hematite prior to 2 76 ga and hence oxygenated surface environments at least localized and or short lived emerged more than 300 million years before the widely accepted great oxidation event during 2 45 and 2 32 ga

environmental and diagenetic variations in carbonate associated sulfate an investigation of cas in the lower triassic of the western usa an integrated stable isotope elemental and petrographic analysis of early triassic spathian carbonates and evaporites along a proximal to deep environmental transect reveals significant variations in 34s composition of carbonate associated sulfate cas the variations in the 34s of cas are strongly correlated with the ca mg composition of carbonates suggesting that the variations are driven by the degree of dolomitization the 34s of dolostones and evaporites are similar to one another and exhibit lower 34s values than limestones from all localities three hypotheses may explain the differences in 34s between proximal dolostones evaporites and inner middle shelf limestones 1 limestones experienced anaerobic sulfate reduction and subsequent incorporation of 34s enriched sulfate into cas during diagenesis while dolostones did not this is unlikely because of the lack of correlation between 34scas and toc as well as other indicators of diagenesis 2 dolomitization controlled the 34scas in proximal paleoenvironments where the source of the 34s depleted fluids was either continentally derived or the result of rayleigh distillation during evaporite formation and 3 a 34s depth gradient existed during the early triassic such that limestones formed in distal waters are more enriched in 34s versus evaporites and dolostones formed in proximal settings we do not favor this hypothesis because the strong correlation between ca mg and 34scas implies that dolomitization controls the 34scas in these samples results from subtidal well preserved non dolomitized limestones suggest that the 34s of spathian seawater sulfate may have been heavier than previously suggested from analyses of evaporite deposits alone

detrital pyrite in witwatersrand gold reefs x ray diffraction evidence and implications for atmospheric evolution pyrite is easily oxidized and therefore unambiguous evidence of detrital pyrite grains in metasediments is a significant constraint on when an oxygenated atmosphere developed compact rounded pyrite in the witwatersrand gold reefs of south africa has a detrital habit and is texturally equivalent to and spatially associated with detrital zircon and chromite x ray precession photography reveals that petrographically featureless as poor grains are untwinned single crystals of high diffraction quality this new evidence from crystallography is consistent with mechanically abraded pyrite from primary lode gold deposits and excludes an origin by replacement of a pre existing detrital phase further evidence of a detrital origin for the compact rounded pyrite is afforded by isolated grains of arsenian pyrite displaying truncated as rich growth bands the geographically extensive witwatersrand fluvial conglomerates evidently had a matrix of quartz and pyrite sand and pyritic mud in their unconsolidated state and thus the late archean atmosphere of earth was likely essentially anoxic

high primary productivity and nitrogen cycling after the paleoproterozoic phosphogenic event in the aravalli supergroup india the causes of atmospheric oxygenation in the paleoproterozoic are multifaceted yet poorly understood an increase in the level of primary productivity however may have been an important factor in producing large amounts of oxygen in the atmosphere a likely environmental consequence of atmospheric oxygenation is an increased abundance of seawater nitrate which would have resulted in a fundamental change from the archean nitrogen cycle in this study we investigate these questions using the carbon and nitrogen isotope compositions of carbonaceous shales from the paleoproterozoic aravalli supergroup india the shales occur stratigraphically above one of the oldest phosphogenic events that took place around 2 0 ga shales in the phosphate domain of the jhamarkotra formation from the lower aravalli group have distinct characteristics from coeval shales in the non phosphate domain carbonaceous shales of the phosphate domain contain relatively high v and u th typical of oxygen deficient environments these shales also have a high organic content of up to 14 wt generally homogeneous 13 corg values around 29 c nwr up to 360 and a large range of 15