influence of organics and silica on fe ii oxidation rates and cell mineral aggregate formation by the green sulfur fe ii oxidizing bacterium chlorobium ferrooxidans kofox implications for fe ii oxidation in ancient oceans most studies on microbial phototrophic fe ii oxidation photoferrotrophy have focused on purple bacteria but recent evidence points to the importance of green sulfur bacteria gsb their recovery from modern ferruginous environments suggests that these photoferrotrophs can offer insights into how their ancient counterparts grew in archean oceans at the time of banded iron formation bif deposition it is unknown however how fe ii oxidation rates cell mineral aggregate formation and fe mineralogy vary under environmental conditions reminiscent of the geological past to address this we studied the fe ii oxidizer chlorobium ferrooxidans kofox a gsb living in co culture with the heterotrophic geospirillum strain kofum we investigated the mineralogy of fe iii metabolic products at low high light intensity and in the presence of dissolved silica and or fumarate silica and fumarate influenced the crystallinity and particle size of the produced fe iii minerals the presence of silica also enhanced fe ii oxidation rates especially at high light intensities potentially by lowering fe ii toxicity to the cells electron microscopic imaging showed no encrustation of either kofox or kofum cells with fe iii minerals though weak associations were observed suggesting co sedimentation of fe iii with at least some biomass via these aggregates which could support diagenetic fe iii reduction given that gsb are presumably one of the most ancient photosynthetic organisms and pre date cyanobacteria our findings on the one hand strengthen arguments for photoferrotrophic activity as a likely mechanism for bif deposition on a predominantly anoxic early earth but on the other hand also suggest that preservation of remnants of fe ii oxidizing gsb as microfossils in the rock record is unlikely

nutrient irrigation of the north atlantic abstract the north atlantic as all major oceans has a remarkable duality in primary production manifested by the existence of well defined high and low mean primary production regions the largest region is the north atlantic subtropical gyre nastg an anticyclone characterized by bowl shaped isopycnals and low production the nastg is surrounded at its margins by smaller cyclonic high production regions where these isopycnals approach the sea surface the most extensive cyclonic regions are those at the latitudinal extremes i e the subpolar and tropical oceans though smaller ones do occur at the zonal boundaries in this article we review historical data and present new analyses of climatological data and a selected number of hydrographic cruises in the western northwestern and eastern southeastern boundaries of the nastg with the objective of investigating the importance of upward epipycnal advection of nutrient rich subsurface layers irrigation in maintaining high primary production in the euphotic layers in the north atlantic subpolar gyre naspg irrigation implies intergyre exchange caused by the outcropping extension of the gulf stream gs following the formation of the deep winter mixed layer in the eastern boundary of the nastg irrigation is attained through a permanent upwelling cell which feeds the canary upwelling current cuc in the southeastern corner irrigation occurs in fall when the guinea dome gd is reinforced and in winter when the cuc reaches its southernmost extension other characteristics of the north south extension of the gs cuc are the seasonal nutrient replenishing of subsurface layers spring restratification of naspg and winter relaxation of the gd and the maintenance of high levels of diapycnal mixing during the last phase of nutrient transfer to the euphotic layers off the mid atlantic bight the gs transports a total of about 700 kmol s 1 of nitrate with almost 100 kmol s 1 carried in the surface

annual textural banding in holocene estuarine silts severn estuary levels sw britain patterns cause and implications a high resolution textural study has been made of laminated and banded estuarine silts exposed intertidally at representative localities and horizons in the holocene deposits of the severn estuary levels the laminae on a submillimetre to millimetre scale are sharp based graded couplets formed of a lower silty part overlain by a finer textured clayey element the centimetre to decimetre scale banding is formed of laminae in alternating gradually intergrading sets of relatively coarse and relative fine grained examples at outcrop in the field the banding is recognizable because the coarse sets prove to be recessive to varying degrees under the influence of weathering and current action independent evidence at two localities points toward an annual origin for the banding at a third it arose during part of what appears to have been a relatively short period quantified physical arguments suggest that the textural banding is a response of suspended fine sediment to marked seasonal changes in sea tempe

silicon isotope fractionation during microbial reduction of fe iii si gels under archean seawater conditions and implications for iron formation genesis abstract microbial dissimilatory iron reduction dir is a deeply rooted metabolism in the bacteria and archaea in the archean and proterozoic the most likely electron acceptor for dir in marine environments was fe iii si gels it has been recently suggested that the fe and si cycles were coupled through sorption of aqueous si to iron oxides hydroxides and through release of si during dir evidence for the close association of the fe and si cycles comes from banded iron formations bifs which consist of alternating bands of fe bearing minerals and quartz chert although there has been extensive study of the stable fe isotope fractionations produced by dir of fe iii si gels as well as studies of stable fe isotope fractionations in analogous abiologic systems no studies to date have investigated stable si isotope fractionations produced by dir in this study the stable si isotope fractionations produced by microbial reduction of fe iii si gels were investigated in simulated artificial archean seawater aas using the marine iron reducing bacterium desulfuromonas acetoxidans microbial reduction produced very large 30 si 28 si isotope fractionations between the solid and aqueous phase at 23 c where 30 si solid aqueous isotope fractionations of 3 35 0 16 and 3 46 0 09 were produced in two replicate experiments at 32 fe iii reduction solid phase fe ii fe total 0 32 this isotopic fractionation was substantially greater than that observed in two abiologic controls that had solid phase fe ii fe total 0 02 0 03 which produced 30 si solid aqueous isotope fractionations of 2 83 0 24 and 2 65 0 28 in a companion study the equilibrium 30 si solid aqueous isotope fractionation was determined to be 2 3 for solid phase fe ii fe total 0 collectively these results highlight the importance of fe ii in fe si gels in producing large changes in si isotope fractionations these results suggest that dir should produce highly negative 30 si values in quartz that is the product of diagenetic reactions associated with fe si gels such si isotope compositions would be expected to be associated with fe bearing minerals that contain fe ii indicative of reduction such as magnetite support for this model comes from recent in situ si isotope studies of oxide facies bifs where quartz in magnetite rich samples have significantly more negative 30 si values than quartz in hematite rich samples

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

geomorphic change of saltmarsh tidal creek networks in the dyfi estuary wales abstract planimetric changes of saltmarsh tidal creek networks in the dyfi estuary wales have been mapped from aerial photographs dating from 1966 to 1989 rates of these changes have been evaluated with reference to creek length and creek density creek densities have been increasing during their development ranging from 8 up to 11 km km 2 these changes are mainly attributable to the instability of the main dyfi tidal channel flood and storm surges saltmarsh vegetation development elevational differences of the estuarine intertidal zone and sea level fluctuations dissipation of tide and wave energies associated with storm surges is chiefly responsible for the morphological development of the dyfi tidal creeks the discontinuous stage prism asymmetry hydrodynamic model is applicable to explaining the dyfi estuarine tidal creek development a form process feedback between the vertical growth of the dyfi saltmarsh surface and the hydrodynamics of its network is also important in governing the dyfi tidal creek network development the rapid development of the dyfi tidal creek networks could result in the loss of the dyfi estuarine wetland

sedimentary facies of the tide dominated paleo changjiang yangtze estuary during the last transgression abstract a large estuary was formed by marine inundation of the paleo changjiang yangtze incised valley during the transgression after the last glacial maximum this paper presents the sedimentary facies and architecture of the estuary fill based on the analysis of three sediment cores cm97 js98 and hq98 obtained from the present changjiang delta plain estuary fill deposits showing an upward fining succession were grouped into five depositional facies tidal river distributary channel muddy intertidal to subtidal flats transgressive lag and estuary front based on sedimentary textures lithology and physical sedimentary structures sand mud couplets are common in these deposits indicating that tides played a significant role in producing these sedimentary structures and that the estuary type was tide dominated some of the successive sand mud couplets probably recorded neap spring cycles as well as semidiurnal tidal cycles the nature of the estuary was very different from other representative tide dominated estuaries in sediment facies its distribution and sediment source for estuarine fill unlike the other estuaries that receive sediments mainly from the sea the paleo changjiang estuarine fill deposits were supplied largely from the river this difference would also have a great influence on the sedimentological and morphological component in the estuary the sediment distribution of the estuary showed fining seaward and estuary mouth sand bodies fed by marine source sand were absent the architecture model of tide dominated estuaries should be divided into two types by the degree of fluvial sediment supply the paleo changjiang estuary shows a good example for an estuary of large rivers

influences of storm erosion and deposition on rhythmites of the upper wenchang formation upper ordovician around tonglu zhejiang province china abstract tonglu rhythmites upper ordovician in zhejiang province east central china display three orders of cyclicity in sandstone and mudstone layer thickness millimeter thick alternations of sandstone and mudstone laminae are ascribed to single tidal cycles centimeter thick alternations of sand dominated layers sdls and mud dominated layers mdls are interpreted to be related to alternation of storm and calm weather conditions with a periodicity longer than that forced by neap spring tidal cyclicity the sdls are interpreted as storm deposits on the basis of presence of scour structures abundant intraformational mud pebbles oscillation ripples and thinning upward trends in the sandstone laminae a third meter thick cycle of variations in sandstone lamina thickness is interpreted as a reflection of cross shore changes in coastal dynamics and water depth in the subtidal intertidal environment storm waves usually considered to be random destructive factors to normal cyclic deposits are here highlighted as effective agents of sediment transport and deposition of the sand dominated layers this study aims at improving our facies level understanding of the genesis and preservation of storm related tidal flat rhythmites on open coasts and highlights the fact that storm related facies can mimic the cyclicity that is commonly ascribed to neap spring tidal variation

fe isotope fractionation during fe ii oxidation by the marine photoferrotroph rhodovulum iodosum in the presence of si implications for precambrian iron formation deposition abstract the iron fe isotopic composition of precambrian iron formations ifs besides providing geological context through its mineralogical properties was suggested to function as a biosignature that can be used to infer a potential microbial role in the formation of the deposited fe minerals anoxygenic phototrophic fe ii oxidizing bacteria photoferrotrophs capable of oxidizing fe ii anoxically using light energy were potentially involved in fe ii oxidation in anoxic or suboxic precambrian oceans the effect of si on fe isotopic fractionation between aqueous fe ii and fe si co precipitates has been investigated before however it is currently unknown how stable fe isotopes are fractionated during enzymatic fe ii oxidation under marine hydrogeochemical conditions and particularly how the presence of si affects the fe isotope composition and the isotopic exchange among different fe phases we therefore studied fe isotope fractionation during fe ii oxidation by the marine photoferrotroph rhodovulum iodosum in simulated precambrian seawater amended with 1 mm dissolved si our results show that the change in the fe isotope compositions over time for both the initial aqueous fe ii fe aq and the fe iii precipitates fe ppt follow a rayleigh distillation model moreover the fractionation e 56 fe ppt aq determined independently from either 56 fe aq or 56 fe ppt data resulted in a value of 2 3 0 3 2sd n 6 this value differs from the fractionation factor determined previously for fe ii oxidation by r iodosum in the absence of si where the fractionation calculated from 56 fe aq i e 0 96 1 18 was different from that calculated from 56 fe ppt 1 96 1 98 this difference was attributed to isotopic exchange processes with soluble and sorbed fe species the present study suggests that si present in precambrian oceans retards fe isotopic exchange likely through combined effects of complexation of dissolved fe species by si and sorption of si to fe iii minerals thus lowering sorption of fe ii to the fe iii minerals which is necessary for isotopic exchange in summary our data suggests that si in ancient oceans played a key role for the fe isotope composition of fe iii minerals that were deposited by photoferrotrophic iron oxidation in precambrian oceans by minimizing subsequent isotope exchange and recrystallization processes with aqueous fe ii

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

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

iron isotopes in hot springs along the juan de fuca ridge we present fe isotope data for high temperature hydrothermal fluids sulfides and basalts from the juan de fuca ridge usgs basalt standard bcr 2 and the canyon diablo iron meteorite the fe isotopic compositions of all samples fall within a range of 1 however there is a small but statistically significant isotopic variation between fe dissolved in hydrothermal fluids and fe in canyon diablo specifically the hydrothermal fluids are lighter compared to canyon diablo yielding 56fe 56fe 54fe sample 56fe 54fe canyon diablo 1 1000 values from 0 30 to 0 77 in contrast the bcr 2 basalt and canyon diablo are isotopically indistinguishable measurements of 56fe in juan de fuca ridge basalts and sulfides suggest that fe isotopes are fractionated in hydrothermal systems however the data do not exclude the possibility of small 56fe differences between the rocks from which fluid fe is derived and other igneous fe in which case fractionation in igneous systems is indicated regardless these results are evidence of non biological mass fractionation of fe isotopes in nature further venting of hydrothermal fluids into the overlying water column could provide a source of isotopically light fe to the deep oceans contributing to fe isotopic variations observed in deep sea sediments

physical and biological mechanisms for planetary waves observed in satellite derived chlorophyll we examine the evidence for global propagation of planetary wavelike features in sea surface chlorophyll over much of the midlatitude ocean westward propagating signals are seen that travel at the same speed as that predicted for long planetary waves we then test three mechanisms for production of this signal these are horizontal passive north south advection by the wave against a mean background gradient vertical upwelling of nitrate which is converted into chlorophyll and vertical upwelling of chlorophyll itself the tests involve comparisons of the amplitude and phase of the predicted signal with observations the horizontal advective process predicts an amplitude for chlorophyll fluctuations that is in fair agreement with the data though both overestimating and underestimating in places the predictions for the phase difference between the chlorophyll and sea surface height signatures are in good agreement with the data the upwelling biological mechanism could potentially give a large signal in the chlorophyll field but the predicted amplitude patterns and the predicted phase difference which is everywhere negative are not in accord with the observations except in a few regions the amplitude predicted by upwelling of chlorophyll is small compared with the horizontal advection mechanism we conclude that over most of the ocean the chlorophyll signal is well explained by horizontal advective processes although we cannot rule out that there exist locations where additional biological mechanisms may be responsible for at least part of the signal

ediacarian sponge spicule clusters from southwestern mongolia and the origins of the cambrian fauna carbon and strontium isotopic data are used to show that the earliest sponge spicule clusters and associated phosphatic sediments with anabarites from southwestern mongolia are of ediacarian age spicule morphologies include bundles of oxeas arranged in three dimensional quadrules linked together at junctions by tetracts pentacts hexacts or polyactines all are referred to the phylum porifera class hexactinellida these sponge spicules provide the oldest remains that can be assigned without question to an extant phylum and also the first firm evidence for filter feeding and metazoan silica biomineralization in the fossil record it is suggested that siliceous and phosphatic members of the cambrian fauna may have had their origins in eutrophic and outer shelf facies of the late proterozoic

tidal bedding and tidal cyclicities within the intertidal sediments of a microtidal estuary dyfi river estuary west wales u k abstract in the microtidal dyfi river estuary west wales the characteristic sedimentary structure within the intertidal sediments is tidal bedding this consists of sand mud couplets by means of thickness measurements and detailed observations the following conclusions can be drawn 1 the thicknesses of and structures in both mud layer and sand layer of each couplet vary systematically downward in the studied cores and trenching profiles variations are mainly due to differences in the current speed and tidal range between tides to difference of individual neap spring tides and to the possible effect of filtering of currents due to being in the intertidal zone 2 the patterns of sand mud couplet thickness variations reflect two main types of tidal cyclicities diurnal inequality and the neap spring neap tidal cycle which is in turn related to the lunar cycle 3 the mud layer of each couplet represents deposition from suspension at times of tidal slack no ebb slack water period during low tide in the intertidal zone thus no ebb mud deposition during low water whilst the sand layer is deposited by asymmetrical bidirectional tidal currents in either the ebb or flood half of a tidal cycle 4 the tidal regime derived from these variation patterns is characterized by a mixed predominantly semi diurnal form which is comparable with the present tidal regime 5 finally it can be concluded that tidal bedding patterns are the structural expressions of tidal cyclicity

post glacial sedimentary evolution of a microtidal estuary dyfi estuary west wales u k abstract the dyfi estuary is a microtidal estuary located on the shores of cardigan bay on the west coast of wales the inundation of the post glacial sea level rise has produced transgressive muddy or silly late glacial and post glacial unconsolidated sediments in the pre existing valley these sediments are complicated by sea level fluctuations and changes in tidal range modern facies distribution patterns and sedimentary characteristics extensive core data and chronostratigraphic cross sections provide a detailed history of the post glacial sedimentary evolution of the dyfi estuary the post glacial estuarine evolution of the dyfi estuary has been subdivided into four phases phase 1 15 000 10 000 yr bp shallow water high energy fluvially dominated facies phase 2 10 000 6 000 yr bp deep water low energy estuarine dominated facies phase 3 6 000 3 500 yr bp shallow water high energy tidally dominated facies phase 4 3 500 yr bp present shallow water low energy estuarine salt marshes dominated facies

seasonal climatologies of nutrients and hydrographic properties on quasi neutral surfaces for four coastal upwelling systems seasonal climatologies of upper ocean hydrographic properties and nutrients are presented for four different major coastal upwelling centers the california peru humboldt canary and benguela regions the climatologies are built from historical hydrographic data primarily to serve as a tool for comparative studies of productivity variations and nutrient fluxes in the near coastal zone care was taken to ensure that quantities were interpolated vertically to and averaged horizontally on approximately neutral surfaces to constrain averaging among only those water masses that would naturally mix easily rather than rely on potential density surfaces that are not coincident with neutral surfaces in regions with large vertical excursions of isopycnals the neutral surfaces were properly approximated through the use of specific volume anomaly surfaces and were spaced closely in the vertical approximately 0 1 unit apart horizontal averaging to a 1 6 spatial grid was accomplished via the application of bicubic splines under tension and provided sufficient spatial resolution for resolving upwelling patterns near the coastline seasonal mean nutrient to nutrient relationships also were constructed as another diagnostic tool for inter regional comparisons and are consistent with expected redfield ratios a consistent high resolution climatology is now publicly available as a tool for local and comparative studies of four different upwelling zones

evidence of deposition from individual tides and of tidal cycles from the francis creek shale host rock to the mazon creek biota westphalian d pennsylvanian northeastern illinois abstract a silt clayrock sequence in the francis creek shale pennsylvanian westphalian d of northeastern illinois is characterized by 1 the presence of numerous clayrock paired bands and 2 cyclic variations in thickness of silt between and within these clayrock pairs the sequence is interpreted to result from tidal influence on inner estuarine sediments the clayrocks representing deposition from suspension at times of tidal slack the interbedded silts and fine sands representing deposition from bottom traction during ebb and flood tidal flow the cyclic variations in thickness is interpreted to represent lunar neap spring forcing of tidal range in a dominantly diurnal tidal system the association of tidal characteristics provides a means whereby the rate of sedimentation can be quantified at about 1 0 m yr of compacted sediment during times of active deposition such a high rate of sedimentation would account for the rapid burial and preservation of soft bodied organisms as indicated by the associated mazon creek biota similar rates in similar settings i e tidal estuaries have recently been reported although the depositional setting of many westphalian sequences was favorable to the preservation of tidal features few examples have been recorded it is suggested that evidence of westphalian tides may have been misinterpreted or overlooked in field studies elsewhere

integrated fe and s isotope study of seafloor hydrothermal vents at east pacific rise 9 10 n abstract in this study we report on coupled fe and s isotope systematics of hydrothermal fluids and sulfide deposits from the east pacific rise at 9 10 n to better constrain processes affecting fe isotope fractionation in hydrothermal environments we aim to address three fundamental questions 1 is there significant fe isotope fractionation during sulfide precipitation 2 is there significant variability of fe isotope composition of the hydrothermal fluids reflecting sulfide precipitation in subsurface environments 3 are there any systematics between fe and s isotopes in sulfide minerals the results show that chalcopyrite precipitating in the interior wall of a hydrothermal chimney displays a limited range of 56fe values and 34s values between 0 11 to 0 33 and 2 2 to 2 6 respectively the 56fe values are on average slightly higher by 0 14 relative to coeval vent fluid composition while 34s values suggest significant s isotope fractionation 0 6 0 2 during chalcopyrite precipitation in contrast systematically lower 56fe and 34s values relative to hydrothermal fluids by up to 0 91 and 2 0 respectively are observed in pyrite and marcasite precipitating in the interior of active chimneys these results suggest isotope disequilibrium in both fe and s isotopes due to s isotopic exchange between hydrothermal h2s and seawater so42 followed by rapid formation of pyrite from fes precursors thus preserving the effects of a strong kinetic fe isotope fractionation during fes precipitation in contrast 56fe and 34s values of pyrite from inactive massive sulfides which show evidence of extensive late stage reworking are essentially similar to the hydrothermal fluids multiple stages of remineralization of ancient chimney deposits at the seafloor appear to produce minimal fe isotope fractionation similar affects are indicated during subsurface sulfide precipitation as demonstrated by the lack of systematic differences between 56fe values in both high temperature fe rich black smokers and lower temperature fe depleted vents

decoupling photochemical fe ii oxidation from shallow water bif deposition abstract oxidized fe minerals in archean paleoproterozoic banded iron formations bifs are commonly taken to indicate the presence of biogenic o 2 or photosynthetic fe ii oxidizing bacteria in the oceans photic zone however at least one viable abiogenic oxidation mechanism has been proposed prior to the rise of atmospheric oxygen and the development of a protective ozone layer the earth s surface was subjected to high levels of ultraviolet radiation bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved fe ii under such conditions dissolved ferrous iron species such as fe 2 and fe oh would have absorbed radiation in the 200 400 nm range leading to the formation of dissolved ferric iron fe iii which in turn would have hydrolyzed to form ferric hydroxide fe oh 3 at circumneutral ph cairns smith a g 1978 precambrian solution photochemistry inverse segregation and banded iron formations nature 76 807 808 braterman p s cairns smith a g and sloper r w 1983 photo oxidation of hydrated fe 2 significance for banded iron formations nature 303 163 164 this process has been invoked to account for bif deposition without need for biology francois l m 1986 extensive deposition of banded iron formations was possible without photosynthesis nature 320 352 354 here we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models the experiments simulate the chemistry of ambient precambrian seawater mixing with fe ii rich hydrothermal fluids with and without uv irradiation we find that if fe ii was effused from relatively shallow seamount type vent systems directly into an anoxic photic zone the photochemical contribution to solid phase precipitation would have been negligible instead most of the fe ii would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite fe 3 si 2 o 5 oh 4 and or the ferrous carbonate siderite feco 3 depending on different simulated atmospheric pco 2 levels conversely in experiments where fe ii was exposed either to phototrophic fe ii oxidizing bacteria or to o 2 ferric hydroxide formed rapidly and the precipitation of ferrous iron phases was not observed if as suggested on mass balance grounds bif deposition requires that fe be sourced from shallow seamount type systems then we are driven to conclude that oxide facies bif are the product of a rapid non photochemical oxidative process the most likely candidates being direct or indirect biological oxidation and that a significant fraction of bif could have initially been deposited as ferrous minerals

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

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

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

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

evidence for oxygenic photosynthesis half a billion years before the great oxidation event the early earth was characterized by the absence of oxygen in the ocean atmosphere system in contrast to the well oxygenated conditions that prevail today atmospheric concentrations first rose to appreciable levels during the great oxidation event roughly 2 5 2 3 gyr ago the evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise but it has proved difficult to constrain the timing of this evolutionary innovation1 2 the oxidation of manganese in the water column requires substantial free oxygen concentrations and thus any indication that mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing mn oxides are not commonly preserved in ancient rocks but there is a large fractionation of molybdenum isotopes associated with the sorption of mo onto the mn oxides that would be retained here we report mo isotopes from rocks of the sinqeni formation pongola supergroup south africa these rocks formed no less than 2 95 gyr ago3 in a nearshore setting the mo isotopic signature is consistent with interaction with mn oxides we therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen

tidal deposits associated with low sulfur coals brazil fm lower pennsylvanian indiana abstract the interval between the lower and upper block coal members of the brazil formation lower pennsylvanian indiana records a transgression of brackish water tidal deposits over a coastal domed peat field followed by a regression and then re establishment of the peat forming environment within the study area the interval averages 8 m in thickness and consists of a basal sequence of laminated mudstone that coarsens upwards to thinly interlayered sandstone and mudstone lenticular wavy and flaser bedding this is capped by an upward fining sequence into a rooted mudstone sedimentation rates can be estimated by derailed thickness measurements of vertically stacked mudstone laminae and sandstone layers that thicken and thin rhythmically this rhythmic thickness variation can be elated to neap spring tidal current cyclicity and indicates that sedimentation varied from 1 cm yr to 1 m yr locally as much as 8 m of accommodation space for the sediments appears to have been generated and mostly filled in a matter of decades to a few centuries such rapid rates of filling indicate the possibility that the accommodation space was generated by compaction of the transgressed peat and mud rich sediments

initiation of the changjiang yangtze delta and its response to the mid holocene sea level change abstract the changjiang yangtze delta in eastern china is one of the largest tide dominated deltas in the world to obtain a more detailed information on its initiation we drilled three sediment cores pk01 hg01 and xj03 from the uppermost region of the delta plain on the basis of sedimentary facies and ams 14c dating we identified three depositional systems 1 a fluvial system 2 an estuarine system and 3 a deltaic system including an initial delta estuary delta transition and a typical delta the maximum flooding surface separates the estuarine and deltaic systems and occurred at ca 8 0 cal kyr bp when brackish water intruded landward and reached the nanjing area the changjiang delta was initiated at the area between nanjing and yizheng when the sea level rise decelerated after the rapid rise in sea level during 9 0 8 2 cal kyr bp the subsequent evolution of the changjiang delta has been shown as a seaward development model of river mouth bars with shoreline migration the oldest river mouth bar called hongqiao sand body which is located most landward was dated at 6 0 5 5 cal kyr bp in this study nearly one thousand years younger than the previous estimates a tide dominated funnel shaped estuary was formed at 8 0 cal kyr bp and lasted until the hongqiao sand bar became exposed this stage is regarded as an aggradation dominated initial stage of the changjiang delta followed by a progradation dominated stage that is characterized by the formation of a series of seaward migrating river mouth bars a relative sea level highstand in the mid holocene may be associated with glacio hydro isostatic tilting

iron silicate microgranules as precursor sediments to 2 5 billion year old banded iron formations banded iron formations bifs are chemical sedimentary rocks comprising alternating layers of iron rich and silica rich minerals that have been used to infer the composition of the early precambrian ocean and ancient microbial processes however the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues petrographic examination of the ca 2 5 ga dales gorge member of the brockman iron formation hamersley group western australia reveals the presence of abundant silt sized microgranules composed of stilpnomelane the microgranules are most common in the least altered bif where they define sedimentary laminations implying a depositional origin we suggest that the precursor mineral was an iron rich silicate that formed either in the water column or on the seafloor the microgranular texture may have developed due to clumping of amorphous mud forming silt sized floccules the microgranules were resedimented by dilute density currents and deposited in lamina sets comprising a basal microgranular rich lamina overlain by amorphous mud with dispersed microgranules the lamina sets collectively define plane lamination structure probably of the lower flow regime the microgranular textures are preserved only where early diagenetic silica prevented the compaction of lamina sets episodic resedimentation of iron silicates alternating with periods of nondeposition and seafloor silicification provides an explanation for some of the characteristic banding in bif we propose that for most of the early precambrian the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation

abiologic silicon isotope fractionation between aqueous si and fe iii si gel in simulated archean seawater implications for si isotope records in precambrian sedimentary rocks abstract precambrian si rich sedimentary rocks including cherts and banded iron formations bifs record a 7 spread in 30si 28si ratios 30si values yet interpretation of this large variability has been hindered by the paucity of data on si isotope exchange kinetics and equilibrium fractionation factors in systems that are pertinent to precambrian marine conditions using the three isotope method and an enriched 29si tracer a series of experiments were conducted to constrain si isotope exchange kinetics and fractionation factors between amorphous fe iii si gel a likely precursor to precambrian jaspers and bifs and aqueous si in artificial archean seawater under anoxic conditions experiments were conducted at room temperature and in the presence and absence of aqueous fe ii fe ii aq results of this study demonstrate that si solubility is significantly lower for fe si gel than that of amorphous si indicating that seawater si concentrations in the precambrian may have been lower than previous estimates the experiments reached 70 90 si isotope exchange after a period of 53 126 days and the highest extents of exchange were obtained where fe ii aq was present suggesting that fe ii fe iii electron transfer and atom exchange reactions catalyze si isotope exchange through breakage of fe si bonds all experiments except one showed little change in the instantaneous solid aqueous si isotope fractionation factor with time allowing extraction of equilibrium si isotope fractionation factors through extrapolation to 100 isotope exchange the equilibrium 30si 28si fractionation between fe iii si gel and aqueous si 30sigel aqueous is 2 30 0 25 2 in the absence of fe ii aq in the case where fe ii aq was present which resulted in addition of 10 fe ii in the final solid creating a mixed fe ii fe iii si gel the equilibrium fractionation between fe ii fe iii si gel and aqueous si 30sigel aqueous is 3 23 0 37 2 equilibrium si isotope fractionation for fe si gel systems is significantly larger in magnitude than estimates of a near zero solid aqueous fractionation factor between pure si gel and aqueous si indicating a major influence of fe atoms on si o bonds and hence the isotopic properties of fe si gel larger si isotope fractionation in the fe ii bearing systems may be caused by incorporation of fe ii into the solid structure which may further weaken fe si bonds and thus change the si isotope fractionation factor the relatively large si isotope fractionation for fe si gel relative to pure si gel provides a new explanation for the observed contrast in 30si values in the precambrian bifs and cherts as well as an explanation for the relatively negative 30si values in bifs in contrast to previous proposals that the more negative 30si values in bifs reflect hydrothermal sources of si or sorption to fe oxides hydroxides

precipitation of iron silicate nanoparticles in early precambrian oceans marks earth s first iron age the early ocean was characterized by anoxic iron rich ferruginous conditions before the rise of atmospheric oxygen 2 45 b y ago a proxy for ferruginous conditions in the ancient ocean is the deposition of banded iron formations bifs which are iron and silica rich chemical sediments whose constituents were largely derived from seawater although experiments simulating ancient ocean chemistry support the rapid growth of iron silicate phases the main iron precipitates are hypothesized to have been ferric oxyhydroxides the paradox between the prevailing reducing conditions and the deposition of oxidized iron phases is explained by biologically mediated oxidation in the water column new high resolution microscopy of bifs and shales throughout the 2 63 2 45 b y old hamersley group australia reveals the presence of vast quantities of nanometer sized iron silicate particles in laminated chert the nanoparticles are finely disseminated in early diagenetic chert and locally define sedimentary lamination indicating that they represent relicts of the original sediments by inference from experimental studies simulating the composition of the early precambrian ocean we suggest that the nanoparticles precipitated from anoxic seawater enriched in silica and dissolved iron and were silicified upon deposition the prevalence of iron silicate nanoparticles implies that they were pervasive background precipitates in ferruginous silica enriched oceans forming the primary sediments of bifs during periods of enhanced submarine mafic volcanism our results imply that silicate precipitation was a major sink of seawater iron and silica before the great oxidation event and because of the reactivity of nanoparticle surfaces may also have influenced the transport and geochemical cycling of trace metals and nutrients our hypothesis that the basic building blocks of bifs were predominantly iron silicate muds rather than iron oxides and or hydroxides may lead to new insights into seawater chemistry on the early earth and the role of biology in the deposition of bifs

lower cretaceous tides revealed by cross bedding with mud drapes the sand dominated folkestone beds widely exposed around the weald of south east england are of upper aptian to lower albian lower cretaceous age and range in thickness up to 84 m refs 1 3 their content of ammonites bivalves and sponges indicates a marine origin while their relative coarseness and the incorporation of drifted timber point to a shallow water deposition near land one striking sedimentary facies is formed by medium to coarse grained cross bedded sands in which several foreset and or bottomset surfaces support mud drapes although it was suggested4 that this facies may have been formed by the storm induced movement of sand waves i now consider that the facies is best explained by the migration of sand waves under the influence of strongly asymmetrical tides and that the mud drapes are formed at slack water a model of this process has been described elsewhere5 and i now describe measurements of a section which supports my earlier conjecture6that the separation of the mud drapes is systematically related to the phase of the spring neap tidal cycle at the time of deposition although criteria for recognizing tidal deposits have long been available it has not previously been possible to analyse a series of ancient sediments with a view to characterizing the tidal regime

a multistage origin for neoarchean layered hematite magnetite iron formation from the weld range yilgarn craton western australia abstract the origins of iron formations remain somewhat enigmatic despite much progress over the past decades this is due in part to continuing research demonstrating that these deposits do not have a single origin but rather can be formed under various and variable conditions this study describes a formation pathway for an algoma type banded iron formation bif from the 2 75 billion year old weld range of western australia based on petrographic and fe isotope analyses of drill core samples the bif is composed of alternating layers of jaspilitic and fe poor chert magnetite layers of varying thickness and abundance and rare pyrite petrographic analysis indicates the minerals formed in the order hematite magnetite and then pyrite with the latter two clearly replacive of primary bedded hematite hematite 56fe values from all portions of the drill core are positive and essentially identical 56fe 0 61 0 05 magnetite 56fe values are positive as well 56fe 0 51 0 07 but vary systematically with total magnetite content of the core samples pyrite 56fe values are positive and higher than both hematite and magnetite 56fe 1 03 0 05 combined these analyses reveal a multistage formational model for the weld range bif initial deposition occurred by the partial oxidation of hydrothermally sourced aqueous fe ii to form a fe iii si co precipitate probably by anoxygenic photosynthetic iron oxidizers this material would have been converted to hematite under equilibrium conditions with excess aqueous fe ii magnetite formed by a later intrusion of reducing fluids mostly along bedding planes but also as cross cutting veins this produced large quantities of magnetite layers parallel and sub parallel to the original bedding but that do not represent a primary depositional feature pyrite was produced in some portions of the unit likely under equilibrium conditions by later infiltration of reducing fluids containing sulfide the entirely positive 56fe values for all fe phases in the weld range bif stands in stark contrast to the more massive superior type bifs of the paleoproterozoic hamersley and transvaal basins of western australia and south africa respectively which have 56fe values that average close to 0 but vary over nearly the entire range of 56fe values measured in nature 56fe 2 5 to 2 6 the weld range bif 56fe values are similar to those of older algoma type bifs such as the paleoarchean isua bif of greenland but are less positive on average this overall trend of decreasing average 56fe values through time likely records an overall increasing trend of oxygenation of the archean surface ocean from 3 8 to 2 45 billion years ago

nickel partitioning in biogenic and abiogenic ferrihydrite the influence of silica and implications for ancient environments fe iii oxyhydr oxides are ubiquitous in modern soils and sediments and their large surface area leads to scavenging of trace elements experimental trace element partitioning between fe iii oxyhydr oxides and aqueous solutions have been used to elucidate the geochemical composition of the precambrian oceans based on the trace element concentrations in precambrian banded iron formations bifs however previous partitioning experiments did not consider the potential influence of microbially derived organic material even though it is widely believed that bacterial phytoplankton was involved in fe ii oxidation and the deposition of bif primary minerals therefore the present study focuses on sorption of ni to and co precipitation of ni with both biogenic ferrihydrite precipitated by the freshwater photoferrotroph rhodobacter ferrooxidans sw2 and the marine photoferrotroph rhodovulum iodosum as well as chemically synthesized ferrihydrite we considered the influence of cellular organic material medium composition and the availability of dissolved silica our results show a preferential association of ni with ferrihydrite and not with the microbial cells or extracellular organic substances we found that the addition of silica 2 mm did not influence ni partitioning but led to the encrustation of some cells with ferrihydrite and amorphous silica the two to threefold lower ni fe ratio in biogenic as compared to abiogenic ferrihydrite is probably due to a competition between ni and organic matter for sorption sites on the mineral surface additionally the competition of ions present at high concentrations in marine medium for sorption sites led to decreased ni sorption or co precipitation based on our data we conclude that if the fe iii minerals deposited in bifs were at least to some extent biological then the ni concentrations in the early ocean would have been higher than previously suggested this study shows the importance of considering the presence of microbial biomass and seawater ions in paleomarine reconstructions

iron isotope fractionation during microbial reduction of iron the importance of adsorption in experiments investigating the causes of fe isotope fractionation the d 56 54 fe value of fe ii remaining in solution fe ii aq after reduction of fe iii goethite by shewanella putrefaciens is 21 2 relative to the goethite in agreement with previous research the addition of an electron shuttle did not affect fractionation suggesting that fe isotope fractionation may not be related to the kinetics of the electron transfer furthermore in abiotic anaerobic fecl2 aq experiments in which approximately one third of fe ii aq is lost from solution due to adsorption of fe ii onto goethite the d 56 54 fe value of fe ii aq remaining in solution is shifted by 20 8 relative to fecl 2 this finding demonstrates that anaerobic nonbiological interaction between fe ii and goethite can generate signif icant fe isotope fractionation acid extraction of sorbed fe ii from goethite in experi ments reveals that heavy fe preferentially sorbs to goethite simple mass balance modeling indicates that the isotopic composition of the sorbed fe ii pool is 11 5 to 12 5 heavier than fe in the goethite 2 7 3 7 heavier than aqueous fe ii mass balance is also consistent with a pool of heavy fe that is not released to solution during acid extraction

fossil evidence of iron oxidizing chemolithotrophy linked to phosphogenesis in the wake of the great oxidation event the oxygenation of earth s atmosphere allowed for the diversification of metabolisms to include those that rely on oxygen and its derivatives for example chemolithotrophic oxidation of sulfide and iron both require oxygen or nitrate as terminal electron acceptors a growing number of oxygen utilizing chemolithotrophs are known to accumulate intracellular polyphosphate as an energy reserve that allows them to adapt to the fluctuating redox conditions in their distinctive gradient habitats polyphosphate is also thought to play an important role in the formation of phosphatic mineral deposits here we present fossil evidence of iron oxidizing bacteria preserved as filamentous iron oxides within phosphatic paleoproterozoic stromatolites the filaments include twisted stalks similar to those produced by modern iron oxidizing bacteria that are known to metabolize polyphosphate and inhabit steep redox gradients fossil iron oxidizing bacteria preserved within some of the oldest known phosphorites serve as indicators of o 2 fe ii gradients that may have supported microbially mediated phosphogenesis via polyphosphate metabolism and or an active iron redox pump

high resolution paleoproductivity fluctuations during the past 24 kyr as indicated by benthic foraminifera in the marginal arctic ocean abstract analyses of benthic foraminifera in sediment cores taken at about 1000 m water depth at the yermak plateau and the barents sea slope adjacent to the position of the ice sheet edge during the last glacial maximum show that paleoproductivity was reduced to about a third of its present level during peak glacial stadials these reduced values are still higher than values for modern permanently ice covered regions suggesting that the core locations were at least partially ice free even during stadials paleoproductivity at the core locations was higher than or equal to that of today during initial deglaciations and warm substages peak paleoproductivity occurred in samples with low salinity surface waters as indicated by oxygen isotope values of planktonic foraminifera and slightly after increased abundance of atlantic species suggesting that enhanced advection of warmer waters from the atlantic supported the initial ice sheet retreat during the holocene climatic optimum productivity was much less than at present on the yermak plateau high at the barents sea site perhaps because of increased advection of atlantic water to the latter site and heavier ice coverage at the former after this optimum paleoproductivity at both sites was similar with slightly lower values during cold periods such as the little ice age

late quaternary stratigraphy of the yeongsan estuary southwestern korea abstract the 18 820 m long sediment core mw 1 from the yeongsan estuary consists of five primary unconsolidated to semi consolidated units ranging in age from late pleistocene to holocene overlying a cretaceous aged basement from oldest to youngest these units include units 1 2 3 fluvial units unit 4 paleosol unit and unit 5 holocene estuarine deposits units 1 2 and 3 are interpreted as in channel or overbank deposits plant materials included within the units and the toc tn total organic carbon total nitrogen ratios 11 14 are also indicative of terrestrial origin unit 2 yields radiocarbon date of 37 700 14 c yrbp unit 4 the paleosol unit is estimated to be originally deposited in fluvial setting such as the area of floodplain or backswamp away from the main channel based on the sporadic distribution of the stagnant freshwater acritarch pseudoschizaea and algae spirogyra the timing of deposition of the original sediments of the paleosol unit is constrained by radiocarbon age of underlying fluvial sediments unit 2 the timing of pedogenesis is around the last glacial maximum when sea level of the yellow sea was lowered temperature was lowered rainfall was reduced and the inland was sparsely vegetated estuarine deposits unit 5 are interpreted to be deposited during the holocene marine transgression

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

ancient geochemical cycling in the earth as inferred from fe isotope studies of banded iron formations from the transvaal craton variations in the isotopic composition of fe in late archean to early proterozoic banded iron formations bifs from the transvaal supergroup south africa span nearly the entire range yet measured on earth from 2 5 to 1 0 in 56fe 54fe ratios relative to the bulk earth with a current state of the art precision of 0 05 for the 56fe 54fe ratio this range is 70 times analytical error demonstrating that significant fe isotope variations can be preserved in ancient rocks significant variation in fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments and the range measured for bifs stands in marked contrast to the isotopic homogeneity of igneous rocks which have 56fe 0 00 0 05 as well as the majority of modern loess aerosols riverine loads marine sediments and proterozoic shales the fe isotope compositions of hematite magnetite fe carbonate and pyrite measured in bifs appears to reflect a combination of 1 mineral specific equilibrium isotope fractionation 2 variations in the isotope compositions of the fluids from which they were precipitated and 3 the effects of metabolic processing of fe by bacteria for minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis the relative order of 56fe values appears to decrease in the order magnetite siderite ankerite similar to that estimated from spectroscopic data although the measured isotopic differences are much smaller than those predicted at low temperature in combination with on going experimental determinations of equilibrium fe isotope fractionation factors the data for bif minerals place additional constraints on the equilibrium fe isotope fractionation factors for the system fe iii fe ii hematite magnetite fe carbonate 56fe values for pyrite are the lowest yet measured for natural minerals and stand in marked contrast to the high 56fe values that are predicted from spectroscopic data some samples contain hematite and magnetite and have positive 56fe values these seem best explained through production of high 56fe 54fe reservoirs by photosynthetic fe oxidation it is not yet clear if the low 56fe values measured for some oxides as well as fe carbonates reflect biologic processes or inorganic precipitation from low 56fe ferrous fe rich fluids however the present results demonstrate the great potential for fe isotopes in tracing the geochemical cycling of fe and highlight the need for an extensive experimental program for determining equilibrium fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments

late pleistocene silty tidal rhythmites in the macrotidal flat between youngjong and yongyou islands west coast of korea abstract late pleistocene silty tidal rhythmites developed in the macrotidal flat between youngjong and yongyou islands demonstrate hierarchical tidal cycles ranging from daily to monthly variations associated with synodic tropical and anomalistic cycles on the basis of rhythmicities and textural composition the rhythmites were likely deposited in an upper intertidal flat under a mixed dominantly semi diurnal tidal regime the rhythmites showing gradational contact with overlying cryoturbated silty facies indicating subaerial exposure and subsequent weathering during the last glacial maximum represent the tidal environment during the last highstand or ensuing regression phase of sea level in the late pleistocene

deposition of banded iron formations by anoxygenic phototrophic fe ii oxidizing bacteria the mechanism of banded iron formation bif deposition is controversial but classically has been interpreted to reflect ferrous iron fe ii oxidation by molecular oxygen after cyanobacteria evolved on earth anoxygenic photoautotrophic bacteria can also catalyze fe ii oxidation under anoxic conditions calculations based on experimentally determined fe ii oxidation rates by these organisms under light regimes representative of ocean water at depths of a few hundred meters suggest that even in the presence of cyanobacteria anoxygenic phototrophs living beneath a wind mixed surface layer provide the most likely explanation for bif deposition in a stratified ancient ocean and the absence of fe in precambrian surface waters

influence of ph and dissolved si on fe isotope fractionation during dissimilatory microbial reduction of hematite microbial dissimilatory iron reduction dir has been identified as a mechanism for production of aqueous fe ii that has low 56 fe 54 fe ratios in modern and ancient suboxic environments that contain ferric oxides or hydroxides these studies suggest that dir could have played an important role in producing distinct fe isotope compositions in precambrian banded iron formations or other marine sedimentary rocks however the applicability of experimental studies of fe isotope fractionation produced by dir in geochemically simple systems to ancient marine environments remains unclear here we report fe isotope fractionations produced during dissimilatory microbial reduction of hematite by geobacter sulfurreducens in the presence and absence of dissolved si at neutral and alkaline ph hematite reduction was significantly decreased by si at alkaline but not neutral ph presumably due to si polymerization at the hematite surface the presence of si altered fe isotope fractionation factors between aqueous fe ii or sorbed fe ii and reactive fe iii reflecting changes in bonding environment of the reactive fe iii component at the oxide surface despite these changes in isotopic fractionations our results demonstrate that microbial fe iii oxide reduction produces fe ii with negative d 56 fe values under conditions of variable ph and dissolved si similar to the large inventory of negative d 56 fe in neoarchean and paleoproterozoic age marine sedimentary rocks

alternating si and fe deposition caused by temperature fluctuations in precambrian oceans precambrian banded iron formations contain thin alternating layers of iron and silica rich minerals microbial culture experiments show that these alternations could be caused by changes in the relative dominance of microbial iron oxidation and abiotic silica precipitation driven by fluctuating ocean temperature

iron isotope fractionation in a buoyant hydrothermal plume 5 s mid atlantic ridge fe isotopes are a potential tool for tracing the biogeochemical redox cycle of fe in the ocean specifically it is hypothesized that fe isotopes could enable estimation of the contributions from multiple fe sources to the dissolved fe budget an issue that has received much attention in recent years the first priority however is to understand any fe isotope fractionation processes that may occur as fe enters the ocean resulting in modification of original source compositions in this study we have investigated the fe inputs from a basalt hosted deep sea hydrothermal system and the fractionation processes that occur as the hot chemically reduced and acidic vent fluids mix with cold oxygen rich seawater the samples collected were both end member vent fluids taken from hydrothermal chimneys and rising buoyant plume samples collected directly above the same vents at 5 s mid atlantic ridge our analyzes of these samples reveal that for the particulate fe species within the buoyant plume 25 of the fe is precipitated as fe sulfides the isotope fractionation caused by the formation of these fe sulfides is fe ii fes 0 60 0 12 the source isotope composition for the buoyant plume samples collected above the red lion vents is calculated to be 0 29 0 05 this is identical to the value measured in end member vent fluids collected from the underlying tannenbaum chimney the resulting isotope compositions of the fe sulfide and fe oxyhydroxide species in this buoyant plume are 0 89 0 11 and 0 19 0 09 respectively from mass balance calculations we have been able to calculate the isotope composition of the dissolved fe fraction and hypothesize that the isotope composition of any stabilised dissolved fe species exported to the surrounding ocean may be heavier than the original vent fluid such species would be expected to travel some distance from areas of hydrothermal venting and hence contribute to not only the dissolved fe budget of the deep ocean but also it s dissolved fe isotope signature

annual cycles of nutrients and oxygen in the upper layers of the north atlantic ocean abstract monthly climatologies of nutrients and oxygen in the upper north atlantic ocean are analyzed and used to estimate the rates of spring summer new production and remineralization the annual cycle of surface nutrients is characterized by a spring summer biological drawdown and a fall winter increase due to vertical mixing the drawdown is accompanied by maximum oxygen supersaturations in the subsurface layer 100 200 m the decrease of oxygen concentrations during the spring summer period reflects the remineralization of organic matter the spring summer drawdown of surface nutrients implies new production rates that are high in the high latitudes around 40 g c m2 on average and low in the low and temperate latitudes about 15 g c m2 on average in the subsurface layer respiration rates are computed from the oxygen concentration decrease during spring summer period results show that most of the new production in the low latitudes and half or less of it in the high latitudes is remineralized above 200 m the spring summer new production in the north atlantic ocean is 1 34 pg c considering that the spring summer production that is not mineralized in the top 200 m of the ocean is mainly composed of particulate organic matter would lead to an export lying between 0 4 and 0 6 gt c period for the whole north atlantic ocean

biogenic fe iii minerals from formation to diagenesis and preservation in the rock record abstract fe metabolizing bacteria are intimately linked to the cycling of fe in modern environments and have likely been key players in the evolution of the earth s biogeosphere fe minerals have also been suggested as a key preservative of cell organic matter in sediments keeping otherwise labile phases conserved at least on time scales of 100 000 years the interpretation of a biological influence on the fe rock record is difficult without a deeper understanding of the mechanisms of biogenic fe iii and fe ii mineral formation the character of these minerals and their diagenesis over short and long time scales here we present the recent advances in the study of abiogenic and biogenic fe iii minerals in particular we focus on the role of fe ii oxidizing bacteria in the deposition of ancient banded iron formations bif we discuss this work within the framework of the main challenge separating biogenic from abiogenic processes over deep time we describe how efforts in isotope geochemistry biomarker research mineral analysis and biogeochemistry are helping to establish a window to the past finally we present some new approaches that help investigate the main processes leading to the formation and potential fate of fe organic matter aggregates

the record of tidal cycles in mixed silici bioclastic deposits examples from small plio pleistocene peripheral basins of the microtidal central mediterranean sea the pliocene pleistocene peripheral marine basins of the mediterranean sea in southern italy from basilicata and western calabria to northern and eastern sicily represent tectonically formed coastal embayments and narrow straits here units of cross stratified mixed silici bioclastic sand 25 to 80 m thick record strong tidal currents the central mediterranean sea has had a microtidal range of ca 35 cm and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out of phase reversal of the tidal prism in narrow straits linking the tyrrhenian and ionian basins the siliciclastic sediment was generated by local bedrock erosion whereas the bioclastic sediment was derived from the contemporaneous foramol type cool water carbonate factories the cross strata sets represent small to medium sized 10 to 60 cm thick two dimensional dunes with mainly unidirectional foreset dip directions these tidalites differ from the classical tidal rhythmites deposited in mud bearing siliciclastic environments firstly the foreset strata lack mud drapes and instead show segregation of siliciclastic and bioclastic sand into alternating strata secondly the thickness variation of the successive silici bioclastic strata couplets measured over accretion intervals of 2 to 3 m and analysed statistically reveal only the shortest term diurnal and semi diurnal tidal cycles thirdly the record of diurnal and semi diurnal tidal cycles is included within the pattern of neap spring cycles differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea with the tidal currents locally enhanced in peripheral basins it is suggested that this particular facies of mud free silici bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub tidal environment of straits and embayments and the shortest term tidal cycles

tidal cross strata and the sediment transport rate problem a geologist s approach abstract cross strata of large scale tidal ripples allow a detailed reconstruction of the kinematics of ripple migration they reflect tidal periodicities such as neap spring tide cycles or daily inequalities which can be used together with random thickness variations to study the sediment transport dynamics quantitatively as a function of the stream power since only a few current data are necessary the method is called the geologist s approach in this study the thickness of ancient some hundred years old tidal cross strata bundles is compared with the tidal range at neap and at spring the effect of randomly distributed current deviations due to wind driven current components is used as a control parameter sternberg s 1972 version of bagnold s sediment transport formula is shown to explain satisfactorily the cross strata structure even as high as several g cm sec this is more than one order of magnitude greater than the highest rates observed in the laboratory or in the field the only difference with sternberg concerns the limit of the exponential growth of bagnold s transport coefficient k

comparison of iron isotope variations in modern and ordovician siliceous fe oxyhydroxide deposits abstract formation pathways of ancient siliceous iron formations and related fe isotopic fractionation are still not completely understood investigating these processes however is difficult as good modern analogues to ancient iron formations are scarce modern siliceous fe oxyhydroxide deposits are found at marine hydrothermal vent sites where they precipitate from diffuse low temperature fluids along faults and fissures on the seafloor these deposits exhibit textural and chemical features that are similar to some phanerozoic iron formations raising the question as to whether the latter could have precipitated from diffuse hydrothermal fluids rather than from hydrothermal plumes in this study we present the first data on modern fe oxyhydroxide deposits from the jan mayen hydrothermal vent fields norwegian greenland sea the samples we investigated exhibited very low 56fe values between 2 09 and 0 66 due to various degrees of partial oxidation the fe oxyhydroxides are with one exception either indistinguishable from low temperature hydrothermal fluids from which they precipitated 1 84 and 1 53 in 56fe or are enriched in the heavy fe isotopes in addition we investigated fe isotope variations in ordovician jasper beds from the lokken ophiolite complex norway which have been interpreted to represent diagenetic products of siliceous ferrihydrite precursors that precipitated in a hydrothermal plume in order to compare different formation pathways of fe oxyhydroxide deposits iron isotopes in the jasper samples have higher 56fe values 0 38 to 0 89 relative to modern high temperature hydrothermal vent fluids ca 0 40 on average supporting the fallout model however formation of the ordovician jaspers by diffuse venting cannot be excluded due to lithological differences of the subsurface of the two investigated vent systems our study shows that reliable interpretation of fe isotope variations in modern and ancient marine fe oxyhydroxide deposits depends on comprehensive knowledge of the geological context furthermore we demonstrate that very negative 56fe values in such samples might not be the result of microbial dissimilatory iron reduction but could be caused instead by inorganic reactions

fe c and o isotope compositions of banded iron formation carbonates demonstrate a major role for dissimilatory iron reduction in 2 5 ga marine environments abstract combined fe c and o isotope measurements of 2 5 ga banded iron formation bif carbonates from the kuruman iron formation and underlying bif and platform ca mg carbonates of the gamohaan formation south africa constrain the biologic and abiologic formation pathways in these extensive bif deposits vertical intervals of up to 100 m were sampled in three cores that cover a lateral extent of 250 km bif fe carbonates have significant fe isotope variability 56fe 1 to 1 and relatively low 13c down to 12 and 18o values 18o 21 in contrast gamohaan and stratigraphically equivalent campbellrand ca mg carbonates have near zero 13c values and higher 18o values these findings argue against siderite precipitation from seawater as the origin of bif fe rich carbonates instead the c o and fe isotope compositions of bif fe carbonates reflect authigenic pathways of formation in the sedimentary pile prior to lithification where microbial dissimilatory iron reduction dir was the major process that controlled the c o and fe isotope compositions of siderite isotope mass balance reactions indicate that the low 13c and low 18o values of bif siderite relative to those expected for precipitation from seawater reflect inheritance of c and o isotope compositions of precursor organic carbon and ferric hydroxide that were generated in the photic zone and deposited on the seafloor carbon fe isotope relations suggest that bif fe carbonates formed through two end member pathways low 13c low 56fe fe carbonates formed from remobilized low 56fe aqueous fe2 produced by partial dir of iron oxide whereas low 13c high 56fe fe carbonates formed by near complete dir of high 56fe iron oxides that were residual from prior partial dir an important observation is the common occurrence of iron oxide inclusions in the high 56fe siderite supporting a model where such compositions reflect dir in place in the soft sediment in contrast the isotopic composition of low fe carbonates in limestone dolomite may constitute a record of seawater environments although our petrographic studies indicate that the presence of pyrite in most low fe carbonates may influence the fe isotope compositions the combined fe c and o isotope data from kuruman bif carbonates indicate that bif siderites that have negative near zero or positive 56fe values may all record biological fe cycling where the range in 56fe values records differential fe mobilization via dir in the sediment prior to lithification our results demonstrate that the inventory of low 56fe marine sedimentary rocks of neoarchean to paleoproterozoic age although impressive in volume may represent only a minimum of the total inventory of fe that was cycled by bacteria

modern weathering in outcrop samples versus ancient paleoredox information in drill core samples from a mesoarchaean marine oxygen oasis in pongola supergroup south africa there is growing evidence from mo and fe stable isotope tracers that shallow water sediments of the 2 95 ga mozaan group pongola supergroup in south africa were deposited under oxidizing conditions while mineralogical elemental and s isotopic evidence favors the contemporaneous atmosphere to have remained reducing in contrast the first reported cr stable isotope data for white mfolozi riverbed outcrop samples of the ijzermijn iron formation if mozaan group and drill core samples of the underlying paleosol of the nsuze group were interpreted to require oxidative weathering conditions during the time of deposition more recent cr isotope data of at least the ijzermijn if samples of the same white mfolozi riverbed outcrop however suggested that modern weathering effects can superimpose oxidative signatures on paleo redox proxies this study compares 234u 238u activity ratios along with stable mo and fe isotope data of corresponding drill core and white mfolozi riverbed outcrop samples in order i to test 234u 238u disequilibria as a potential tool to identify samples affected by modern weathering and ii to further investigate weathering effects on outcrop samples particularly with regard to the stable mo and fe isotope systematics all drill core samples from the ijzermijn if have 234u 238u activity ratios that are within uncertainties of secular equilibrium i e a value of 1 and thus were not significantly affected by modern weathering related u gain or loss or other sub recent rock water interaction by contrast the outcrop if and shale samples from the white mfolozi riverbed show very strong deviations from the secular equilibrium 234u 238u activity ratio furthermore these deviations are accompanied by the observations that i outcrop samples have on average heavier 56 54fe values 0 96 than drill cores 1 59 and ii positive correlations in drill core samples between log fe mn and 98 95mo or 56 54fe are disrupted by inclusion of the outcrop samples as are negative correlations between log mno and 98 95mo however 56 54fe and 98 95mo of our outcrop samples do not correlate with the extent of 234u 238u disequilibria as such 234u 238u activity ratios fail to act as a direct identification tool for modern weathering effects on stable fe and mo isotopes in outcrop samples overall this study confirms the adsorption of isotopically light mo onto mno2 particles in oxic surface water conditions in the epicontinental pongola sea reduction of the mn oxides together with microbial fe reduction in the reduced sediment pile of the pongola basin led to the formation of mn and fe carbonates during early diagenesis storing the observed stable fe and mo isotopic signatures we thus extend the so called mno2 shuttle first proposed for the neoarchean 2 46 ga old koegas if to late mesoarchean shallow marine depositional settings

biological fe oxidation controlled deposition of banded iron formation in the ca 3770 ma isua supracrustal belt west greenland the redox balance of the archean atmosphere ocean system is among the most significant uncertainties in our understanding of the earliest history of earth s surface zone most workers agree that oxygen did not constitute a significant proportion of the atmosphere until after ca 2 45 ga after the great oxidation event but there is less agreement on when o2 production began and how this may have been consumed by reduced species such as fe ii in the oceans the fe redox cycle through time has been traced using banded iron formations bifs and fe isotopes are increasingly used to constrain the conditions of earth s paleoenvironments including the pathways of formation of bifs iron isotope analyses of bifs from the 3 7 to 3 8 ga isua supracrustal belt isb obtained by micro sampling of magnetite rich layers and conventional analysis as well as by in situ femtosecond laser ablation fs la icp ms indicate a consistently narrow range of non zero 56fe values analysis of magnetite by fs la icp ms allows for precise and accurate micron scale analyses without the problems of orientation effects that are associated with secondary ion mass spectrometry sims analyses magnetite 56fe values range from 0 4 to 1 1 among different bands but within individual layers magnetite grains are mostly homogeneous although these bifs have been metamorphosed to amphibolite facies the metamorphism can neither explain the range in fe isotope compositions across bands nor that between hand samples the isotopic compositions therefore reflect primary low temperature sedimentary values the positive 56fe values measured from the isb magnetites are best explained by deposition of fe iii oxides produced by partial oxidation of fe ii rich ocean water a dispersion reaction model which accounts for rates of hydrothermal fe ii aq input rates of oxidation and rates of fe oh 3 settling suggests exceptionally low o2 contents 0 001 of modern o2 contents in the photic zone such low levels suggest an anoxygenic pathway is more likely and the data can be well modeled by anoxygenic photosynthetic fe ii oxidation comparison of the fe isotope data from the isua bifs with those from the 2 5 ga bifs from the hamersley and transvaal basins australia and south africa respectively suggests a striking difference in fe sources and pathways the 2 5 ga magnetite facies bifs of australia and south africa have 56fe values that range from 1 2 to 1 2 over small scales and are on average close to 0 which is significantly lower than those reported here from the isua bifs the wide range in fe isotope compositions for the hamersley and transvaal bifs in concert with c and o isotope data have been interpreted to reflect bacterial dissimilatory fe iii reduction dir the absence of low 56fe values in the isua bifs as well as the lack of fine scale isotopic heterogeneity may indicate formation prior to widespread dir

aerobic iron and manganese cycling in a redox stratified mesoarchean epicontinental sea abstract redox conditions in the marine realm prior to the great oxidation event goe 2 46 2 32 ga ago during which the atmospheric oxygen level rose dramatically for the first time are still debated here we present c o fe and mo stable isotope systematics of fe mn and carbonate rich shales deposited at different water depths in association with iron formations ifs of the mesoarchean mozaan group pongola supergroup south africa 13 c values between 22 3 and 13 5 vpdb and 18 o values between 21 1 and 8 6 vpdb for fe mn rich carbonate minerals indicate their precipitation out of equilibrium with seawater instead early diagenetic reduction of fe mn oxyhydroxide precursor minerals along with microbially induced oxidation of organic matter om formed these carbonates 56 fe irmm 014 values between 1 27 and 0 14 and 98 mo nist 3134 0 25 values between 0 46 and 0 56 co vary with mn concentrations and inferred water depth of deposition this suggests that despite the diagenetic origin of the fe mn carbonates the primary light fe and mo isotopic signature of fe mn oxyhydroxides that originally precipitated from seawater is still preserved while isotopically light mo implies that mn ii was oxidized to mn iv due to the availability of free photosynthetically produced o2 mn enrichment suggests that the water column was redox stratified with a mn redoxcline situated at a depth below the storm wave base a trend to highly negative 56 fe values with increasing mn fe ratios and decreasing depositional depth suggests progressive oxidation of fe ii as deep waters upwelled across a redoxcline towards shallow locally oxygenated waters where mn iv oxyhydroxides precipitated combined 56 fe and 98 mo data indicate pervasive oxygenation of seawater with the o2 content in the photic zone likely reaching levels higher than the maximum value of 10 m proposed for archean oxygen oases since abiotic mn ii oxidation is kinetically very slow in marine environments it is likely that mn oxidizing microorganisms catalyzed mn oxidation in the oxygenated pongola surface waters during deposition of ifs this implies that aerobic metabolism had evolved before the goe in shallow aquatic habitats where it exerted a first order control on the deposition of shallow marine mn rich iron formations

ge si in hamersley bif as tracer of hydrothermal si and ge inputs to the paleoproterozoic ocean abstract ge si and ge fe ratios have been measured by laser ablation icp ms in selected sections of the hamersley banded iron formation bif including chert siderite bands i e carbonate and silicate facies bif the major ge carriers are magnetite and hematite in the oxide facies and the fe rich silicates greenalite minnesotaite and stilpnomelane in chert siderite bands while quartz has the lowest ge si ratios 0 15 0 7 mol mol in accord with discrimination against ge in quartz the homogeneous si fe ratio of 1 25 for dales gorge oxide facies bif is consistent with si ferrihydrite as precursor phase this si concentration is sufficiently high for formation of iron free silica clusters at the surface of ferrihydrite so that only a minor fractionation of the ge si ratio should result from preferential ge adsorption relative to si hematite chert bands that have been minimally altered by metamorphic reactions and or fluid metasomatism are the preferred material to determine the sources of silica in hamersley bif variations of fe si and ge si ratios in chert mesobands are interpreted as the result of diagenetic mineral segregation a consistent ge si 3 mol mol is then derived by extrapolation of measured ge si ratios to the average fe si composition of dales gorge bif simple mass balance calculations indicate hydrothermal inputs to the ocean of the order of 25 for si and 90 for ge assuming ge si 10 mol mol for high temperature hydrothermal fluids the high si concentration in siderite chert bands si fe 4 demands only a small amount of si ferrihydrite precursor and some excess silica likely precipitated in pore water oversaturated with regard to amorphous silica the average ge si 0 80 0 1 mol mol for bulk chert siderite samples including some stilpnomelane rich cherts is within the compositional range of modern riverine and aeolian inputs pointing to a continental si source during their deposition

the effect of plume processes on the fe isotope composition of hydrothermally derived fe in the deep ocean as inferred from the rainbow vent site mid atlantic ridge 36 14 n abstract the rainbow hydrothermal vent site which is the largest known point source for dissolved fe delivered to the deep north atlantic ocean has remained invariant in its fe isotope composition over at least the past 16 000 years based on analysis of metalliferous sediments beneath the plume because of the conservative behavior of fe in the rainbow plume 56 fe values of particles in the neutrally buoyant plume 0 18 0 05 and underlying sediments 0 19 0 05 are indistinguishable from the 56 fe values of the high temperature fluid sources 0 23 0 04 particles from the near vent buoyant stage of the plume however have higher 56 fe values 0 15 to 1 20 relative to the original vent fluid consistent with fractionation during oxidation of fe ii aq to fe iii aq isotope compositions become invariant in the plume once all fe ii aq is fully oxidized preserving the original composition of the vent fluid the constant fe isotope compositions of the vent fluids over time implies that changes in seawater fe isotope composition of the north atlantic ocean as they are recorded in fe mn crusts requires changes in the relative fluxes of fe to the ocean