relation between depositional environment and the elemental composition of early diagenetic siderite early diagenetic siderites from marine and fresh water depositional environments are characterized by distinctive compositional trends siderite from fresh water environments is often relatively pure i e greater than 90 mol feco sub 3 and commonly attains end member composition siderite from marine environments however is always extremely impure and has extensive substitution of mg up to 41 mol and to a lesser extent ca up to 15 mol for fe in the siderite lattice in addition marine siderite generally contains less mn and has a higher mg ca ratio than fresh water siderite this compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters inasmuch as early marine pore waters generally have a higher mg sup 2 ca sup 2 ratio and contain less mn sup 2 and fe sup 2 and more ca sup 2 and mg sup 2 than meteoric waters

sedimentary rhythm of mn carbonate laminae induced by east asian summer monsoon variability and human activity in lake ohnuma southwest hokkaido northern japan abstract past and ongoing precipitations of mn carbonate rhodochrosite have occurred in the freshwater lake ohnuma which is situated about 8 km south of the hokkaido komagatake volcano southwest hokkaido northern japan lake sediments from a surface and a longer 4 m core contain the sedimentary rhythms of rhodochrosite laminae and volcanic deposits which were identified by nondestructive micro xrf fluorescence analyses the three tephra ash layers from the hokkaido komagatake volcano characterized by peaks of al si k ca and ti correspond to regionally recognized tephras 1640 1929 and 1996 ce based on ashfall isopachs and 137cs 210pb and 14c ages determined from the two cores the sedimentation rate showed a drastic change around 1929 ce 0 3 cm yr for 1640 1929 ce and 2 7 cm yr for 1929 2012 ce the mn content record of lake ohnuma sediments composed of rhodochrosite laminae showed a periodic variation with a decadal centennial scale over the interval 1640 2012 ce sediment petrography and hydrochemical measurements suggest that the metastable phase of rhodochrosite was deposited during summer after being precipitated followed by the crystal growth of rhodochrosite within the anoxic sediments on the basis of these results high mn content is interpreted as an indicator of strengthened summer stratification associated with reduced rainfall runoff the ti si ratios are representative of the relative abundance of detrital inputs from the watershed the combined signals of elevated mn content and ti si ratio for 1935 2012 ce indicate eutrophication and hypoxia in the lake and enhanced land erosion in the watershed associated with the post 1970s intensive land use such as livestock farming for 1640 1935 ce the lake ohnuma record shows an antiphase relationship between the mn content and ti si ratio which implies that the detrital inputs reflected the strength of the east asian summer monsoon easm and the reduced rainfall runoff resulted in weaker mixing followed by depleted o2 in the water column the antiphase variation in mn content and ti si ratio suggest weak easm during the little ice age 1850 ce followed by strong easm between 1860 and 1929 ce

geochemical study of a crater lake pavin lake france identification location and quantification of the chemical reactions in the lake abstract a study of the main geochemical characteristics of the pavin crater lake was performed in september 1992 associated with previous results on the hydrochemical budget of the lake the present study points out that 1 1 in the mixolimnion the major reactions are photosynthesis removal of silica by diatoms iron oxidation and alkali and alkaline earth incorporation in a solid phase 2 2 the major reduction reactions occur within the sediment and the products of the reaction are brought to the lake by diffusion 3 3 some reactions silica dissolution manganese oxide reduction sulfate sulfide transformations iron and phosphate precipitation occur within the monimolimnion 4 a complete budget of transfers and chemical reactions is presented for some components such as silica

meteoric sphaerosiderite lines and their use for paleohydrology and paleoclimatology sphaerosiderite a morphologically distinct millimeter scale spherulitic siderite feco 3 forms predominantly in wetland soils and sediments and is common in the geologic record ancient sphaerosiderites are found in paleosol horizons within coal bearing stratigraphic intervals and like their modern counterparts are interpreted as having formed in water saturated environments here we report on sphaerosiderites from four different stratigraphic units each of which has highly variable 13 c and relatively stable 18 o compositions the unique isotopic trends are analogous to well documented meteoric calcite lines which we define here as meteoric sphaerosiderite lines meteoric sphaerosiderite lines provide a new means of constraining ground water 18 o and thus allow evaluation of paleohydrology and paleoclimate in humid

origin of the lethal gas burst from lake monoun cameroun abstract on 15 august 1984 a lethal gas burst issued from a submerged 96 m deep crater in lake monoun in cameroun western africa killing 37 people the event was associated with a landslide from the eastern crater rim which slumped into deep water waters below 50 m are anoxic dominated by high fe 2 600 mg l and hco 3 1900 mg l anoxic and supersaturated with siderite which is a major component of the crater floor sediments the unusually high fe 2 levels are attributed to reduction of laterite derived ferric iron gradually brought into the lake as loess and in river input sulfur compounds are below detection limits in both water and gas gases effervescing from depressurized deep waters are dominantly co 2 with minor ch 4 having 13 c of 7 18 and 54 8 per mil respectively bacterial decomposition of organic matter may account for the methane but 14 c of lake water indicates that only 10 of the carbon is modern giving an apparent age of 18 000 years the dominant source of carbon is therefore attributed to long term emission of co 2 as volcanic exhalation from vents within the crater which led to gradual build up of hco 3 in the lake the density stratification of the lake may have been upset by an earthquake and underwater landslide on 15 august which triggered overturn of the lake and caused nucleation of co 2 in the deep water the resultant ebullition of co 2 from deep lake waters led to a gas burst at the surface and locally generated a water wave up to 5 m high people travelling through the gas cloud were asphyxiated presumably from co 2 and suffered skin discoloration from unidentified components

a young multilayered terrane of the northern mare imbrium revealed by chang e 3 mission china s chang e 3 ce 3 spacecraft touched down on the northern mare imbrium of the lunar nearside 340 49 e 44 12 n a region not directly sampled before we report preliminary results with data from the ce 3 lander descent camera and from the yutu rover s camera and penetrating radar after the landing at a young 450 meter crater rim the yutu rover drove 114 meters on the ejecta blanket and photographed the rough surface and the excavated boulders the boulder contains a substantial amount of crystals which are most likely plagioclase and or other mafic silicate mineral aggregates similar to terrestrial dolerite the lunar penetrating radar detection and integrated geological interpretation have identified more than nine subsurface layers suggesting that this region has experienced complex geological processes since the imbrian and is compositionally distinct from the apollo and luna landing sites

birth growth and morphologic evolution of the laghetto cinder cone during the 2001 etna eruption we have undertaken detailed observations of the formation of the laghetto cinder cone a new cone that formed during a 2 week period of intense activity in piano del lago on the upper slopes of mount etna in summer 2001 we describe the events leading to the formation of a small graben the formation of pit craters on the base of the graben the onset of phreatomagmatic activity a transition to intense strombolian activity and a return to phreatomagmatic activity as the eruption came to an end we discuss the reasons for these transitions and describe the morphological development of the cone during these events arcuate cracks on the southern part of the cone were related to withdrawal of magma at the end of the eruption other slope instabilities that developed during the eruption include the formation of small radial grain flows on the outer flanks of the cone and the collapse into the crater of part of the crater rim some of the failure planes we observed were first identified using a flir tm 695 thermal infrared camera this is the first time that infrared thermography has been used to detect instability of volcanic structures results obtained during this test case demonstrate that thermal cameras are a very useful tool for studies of volcanic instability

ground penetrating radar and its use in sedimentology principles problems and progress abstract ground penetrating radar gpr also referred to as ground probing radar surface penetrating radar subsurface radar georadar or impulse radar is a noninvasive geophysical technique that detects electrical discontinuities in the shallow subsurface

identification of subannual patterns in microbial community signatures from individual sedimentary laminae using a freeze coring approach discrete biological community signatures were identified in individual sub annually deposited sedimentary laminae of anoxic lake sediments from two lakes in the minneapolis st paul minnesota u s a urban area recognizing variation in microbial communities associated with discrete millimeter scale sedimentary horizons was made possible using a freeze coring method to recover bacterial dna for amplicon itag sequencing and terminal restriction fragment length polymorphism analyses variation in 16s rrna gene composition between laminae suggests that seasonal changes in cell transport from the water column impart a residual molecular signature on subsurface communities direct comparison of frozen in situ core samples to ambient temperature sediment indicates that freeze coring methodology imposes no significant bias on dna based community fingerprints this work further supports previous observations demonstrating the efficacy of freeze coring for high resolution analysis of microbial communities but here it is applied to resolving molecular signatures derived from sedimentary laminae

siderite clumped isotope thermometry a new paleoclimate proxy for humid continental environments abstract clumped isotope measurements can be used to exploit the paleoclimatic potential of pedogenic siderite feco3 however the applicability of this method is held back by the lack of clumped isotope calibrations of mineralogies other than calcite and aragonite here we present an inorganic calibration of siderites grown in the laboratory between 21 and 51 c linear regression of 47 values and temperature 106 t2 k yields the following relationship r2 0 997 47 rf 0 0356 0 0018 10 6 t 2 0 172 0 019 we demonstrate that this calibration is indistinguishable from calcite at current levels of analytical precision our observations suggest that there is likely no large systematic bias in the clumped isotope acid fractionation factors between the two different carbonate minerals we also present clumped isotope measurements of a natural siderite collected from holocene sediments of the mississippi river delta we find that siderites record warm season marsh water temperatures instead of mean annual temperatures as it has long been presumed this finding has important implications for the accuracy of siderite stable isotope and clumped isotope based climate reconstructions

assessing the utility of fe al and fe speciation to record water column redox conditions in carbonate rich sediments abstract geochemical proxies based on fe abundance fe al and fe speciation have been widely applied to marine sediments in order to unravel paleo depositional redox conditions though geological time to date however these fe proxies have only been calibrated in relation to modern and ancient fine grained siliciclastic marine sediments this clearly limits their use particularly in relation to carbonate rich sediments and rocks to address this we here explore the applicability of fe based redox proxies in carbonates through three approaches first we have compiled fe al data for modern marine sediments to investigate variability in fe enrichments as a function of carbonate content and depositional setting second we have expanded this approach with a compilation of new and existing fe speciation data for modern and ancient marine sediments deposited under oxic and euxinic anoxic sulfidic water column conditions finally we show new data from paired limestone and dolomite sample sets to demonstrate the potential significance of deep burial dolomitization on the fe speciation redox proxy modern marine sediments deposited under oxic conditions show no relationship between fe al and carbonate content these sediments have an average fe al ratio of 0 55 0 11 with some higher values potentially being attributable to steady state early diagenetic remobilization of fe towards the sediment water interface in contrast significant fe al enrichments occur as a consequence of water column fe mineral formation and deposition either under anoxic conditions or due to input of anoxic hydrothermal fluids into oxic seawater iron speciation data also show no direct correlation with carbonate content and instead three groups can be distinguished based on total fe fe t and organic c contents sediments deposited under oxic water column conditions with fe t 0 5 wt generally plot below the lower fe hr fe t siliciclastic reference threshold 0 38 for distinguishing anoxic environments regardless of carbonate content also consistent with siliciclastic calibrations carbonate rich sediments deposited under anoxic water column conditions tend to have fe hr fe t ratios 0 38 in contrast oxic carbonate rich sediments with low fe t hr fe t ratios suggesting that the use of fe speciation for such samples is not appropriate for evaluating water column redox conditions taken together our data suggest that an fe t content of 0 5 wt rather than carbonate content represents the most suitable basis for identifying whether carbonate rich sediments are appropriate for fe based redox proxy analyses analysis of burial dolostones suggests that the fe speciation proxy may also be compromised by deep burial dolomitization where there has been a source of mobile fe to enrich rocks during recrystallization however pre screening can identify such samples this new assessment expands the utility of fe based redox proxies to also incorporate appropriate carbonate rich rocks provided that care is taken to assess the possible impact of deep burial dolomitization

from sink to volcanic source unravelling missing terrestrial eruption records by characterization and high resolution chronology of lacustrine volcanic density flow deposits lake inawashiro ko fukushima japan

ground penetrating radar sounding in mafic lava flows assessing attenuation and scattering losses in mars analog volcanic terrains we conducted low frequency 16 to 100 mhz ground penetrating radar surveys on the eroded lava flows at craters of the moon idaho usa volcanic field to evaluate the potential of future radar sounding investigations on mars to map shallow subsurface features radar sounding profiles were obtained from three locations above a lava tube across a volcanic rift and over a scoria cone results were combined with laboratory permittivity and magnetic permeability measurements of field collected samples to deconvolve the electromagnetic attenuation and scattering losses from the total losses and therefore separately quantify both effects on the radar penetration depth our results demonstrate a constrained performance for low frequency sounding radars to characterize mafic arid volcanic terrains that contain a significant amount of ferro oxides 14 mainly in the form of olivine and magnetite penetration depths of 35 m were achieved at a frequency of 100 mhz and depths of 80 m were achieved at 16 mhz with an effective dynamic range of 60 db results indicate that for frequencies below 100 mhz the electromagnetic attenuation dominated the signal losses while above this frequency threshold the volume scattering dominated the losses over our frequency range the observed electromagnetic attenuation and penetration depths were strongly dependent on the magnetic losses ground porosities and degree of heterogeneity rather than the sounding frequency in light of these results we suggest average attenuation and scattering losses measured in terms of db m and discuss the expected penetration depth for the mars orbital radar sounding instruments sharad and marsis in mafic volcanic terrains

a review and analysis of varve thickness records from glacial lake ojibway ontario and quebec canada abstract varve thickness records from glacial lake ojibway provide a detailed picture of ice margin dynamics and paleohydrology for the laurentide ice sheet during the early holocene the demise of lake ojibway occurred when an ice dam of the shrinking laurentide ice sheet failed perhaps releasing enough freshwater to impede atlantic meridional overturning circulation and precipitate the 8200 cal bp climatic cooling event previously published records and new information from lacustrine sediment cores are combined to reconstruct a varve record over 2100 years long between varves numbered 1 to 1500 average rates of ice margin retreat increased from 140 m y to as high as 400 m y beginning prior to varve year 1800 and lasting to around year 2000 ice readvanced along a front at least 300 km wide during an episode called the cochrane the total distance of the advance was at least 75 km and averaged around 200 m y during the advance a series of 50 anomalously thick varves were deposited the cause of which is uncertain a widespread unconformity occurred during the end of the cochrane advance which was followed at around varve year 2060 by the return of varves which thicken abruptly unlike previous varves these varves include ice rafted pellets of lake ojibway sediment these varves may have resulted from rapid melting and calving rather than a second advance and may relate to basin re filling following a lowstand there are no direct dates on the lake ojibway varves but a sudden increase in varve thickness at year 1528 is a regional stratigraphic marker that may have resulted from glacial sediment and meltwater including lake agassiz overflow diverted from routes to the lake superior basin into the lake ojibway basin dates from the lake superior basin suggest that the diversion of this water occurred at around 9040 cal bp applying this date to varve 1528 constrains the age of the lake ojibway varves numbered 1 to 2100 to between 10 570 and 8470 200 cal bp

low pco2 in the pore water not in the archean air arising from m t rosing d k bird n h sleep c j bjerrum 744 747 2010 10 1038 nature08955 the solar luminosity during the archean 3 8 to 2 5 billion years ago was 20 25 lower than at present and was probably compensated at least in part by a stronger greenhouse effect1 2 3 rosing et al 4 estimate the archean partial pressure of carbon dioxide to have been about 10 3 bar on the basis of the simultaneous occurrence of magnetite fe3o4 and siderite feco3 in banded iron formations bifs a type of chemical sediment here we question a central assumption by rosing et al 4 that the mineralogy of bifs reflects near thermodynamic equilibrium with the atmosphere ocean system just as the presence of authigenic pyrite in modern sediments does not imply that the modern atmosphere is anoxic the mineralogy of bifs cannot be used to argue for a low archean atmosphere carbon dioxide is therefore still a viable greenhouse gas candidate with which to explain the warm archean climate

development of lava tubes in the light of observations at mauna ulu kilauea volcano hawaii during the 1969 1974 mauna ulu eruption on kilauea s upper east rift zone lava tubes were observed to develop by four principal processes 1 flat rooted crusts grew across streams within confined channels 2 overflows and spatter accreted to levees to build arched roofs across streams 3 plates of solidified crust floating downstream coalesced to form a roof and 4 pahoehoe lobes progressively extended fed by networks of distributaries beneath a solidified crust still another tube forming process operated when pahoehoe entered the ocean large waves would abruptly chill a crust across the entire surface of a molten stream crossing through the surf zone these littoral lava tubes formed abruptly in contrast to subaerial tubes which formed gradually all tube forming processes were favored by low to moderate volume rates of flow for sustained periods of time tubes thereby became ubiquitous within the pahoehoe flows and distributed a very large proportionof the lava that was produced during this prolonged eruption tubes transport lava efficiently once formed the roofs of tubes insulate the active streams within allowing the lava to retain its fluidity for a longer time than if exposed directly to ambient air temperature thus the flows can travel greater distances and spread over wider areas even though supply rates during most of 1970 1974 were moderate ranging from 1 to 5 m3 s large tube systems conducted lava as far as the coast 12 13 km distant where they fed extensive pahoehoe fields on the coastal flats some flows entered the sea to build lava deltas and add new land to the island the largest and most efficient tubes developed during periods of sustained extrusion when new lava was being supplied at nearly constant rates tubes can play a major role in building volcanic edifices with gentle slopes because they can deliver a substantial fraction of lava erupted at low to moderate rates to sites far down the flank of a volcano we conclude therefore that the tendency of active pahoehoe flows to form lava tubes is a significant factor in producing the common shield morphology of basaltic volcanoes

a lacustrine carbonate record of holocene seasonality and climate annually laminated varved holocene sediments from derby lake michigan display variations in endogenic calcite abundance reflecting a long term millennial scale decrease in burial punctuated with frequent short term decadal scale oscillations due to carbonate dissolution since 6000 cal yr b p sediment carbonate abundance has followed a decreasing trend while organic carbon abundance has increased the correlation between organic carbon abundance and the sum of march april october november insolation has an r 2 value of 0 58 we interpret these trends to represent a precession driven lengthening of the holocene growing season that has reduced calcite burial by enhancing net annual organic matter production and associated calcite dissolution correlations with regional paleoclimate records suggest that changes in temperature and moisture balance have impacted the distribution of short term oscillations in carbonate and organic matter abundance superimposed on the precession driven trends

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

iron formation carbonates paleoceanographic proxy or recorder of microbial diagenesis the chemical and isotopic compositions of carbonates are commonly used as proxies for ancient seawater or paleoenvironments iron formation if carbonates have been used as evidence for an anoxic fe ii rich archean and paleoproterozoic ocean and high atmospheric co 2 contents it has been proposed however that microbial fe cycling dominates the chemical and isotopic compositions of if carbonates suggesting less direct applicability as an oceanic proxy here were use an isotope tracer that is not affected by biological processes or isotopic fractionation the radiogenic 87 rb87 sr system to test the applicability of if carbonates as a paleoenvironmental proxy we focus on the 2 5 ga campbellrand platform transvaal basin south africa that records a shift from ca mg carbonates to if carbonates during a marine transgression when coupled with previously determined fe c and o isotope compositions it becomes clear that the if carbonates studied here do not refl ect seawater compositions but instead record extensive microbial fe cycling in the soft sediment prior to lithifi cation these results question the use of if carbonates to infer seawater compositions and paleoenvironmental conditions including estimates for atmospheric co 2 contents

stratiform siderites from the mesoproterozoic xiamaling formation in north china genesis and environmental implications abstract stratiform or layered siderites are much less common than siderite concretions in the stratigraphic record in the middle part of the mesoproterozoic xiamaling formation ca 1 40 1 35 ga layered siderites are widespread across the north china platform facies analysis indicates that the xiamaling siderites were formed in shallow water environments above the storm wave base high fe al and v al ratios and the lack of pyrites in siderite bearing strata indicate ferruginous bottom water conditions during the formation of siderites the low thermal maturity of organic matter and clay mineral structures suggest that the layered siderites were formed at burial temperatures 90 c the negative 13c values 24 to 12 of siderites and the presence of m scale hematite inclusions in siderite grains suggest that dissimilatory iron reduction dir during early diagenesis has played critical roles in siderite formation the source of fe oxyhydroxides required for dir could be from fe ii oxidation in shallow seawater above the redoxcline in response to enhanced hydrothermal activities or alternatively directly from continental fe oxide input the positive cerium ce and europium eu anomalies of clay minerals in siderite bearing strata and their positive correlation with total fe contents tfe2o3 seem to favor a hydrothermal origin of iron syndepositional hyalophane concretions intermittent tuff layers and remarkable enrichment of cu and zn in some of the siderite hosting rocks provide further support to hydrothermally sourced iron the enhanced hydrothermal activity for the development of stratiform siderites in the xiamaling formation probably related to the final breakup of the north china craton from the columbia supercontinent during which active hydrothermal systems provided sufficient fe ii for fe redox recycling and formation of layered siderites

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

δ13c δ18c covariance as a paleohydrological indicator for closed basin lakes the relationship between 18o and 13c in a closed basin lake is a function of hydrological change vapor exchange lake productivity and total co2 or carbonate alkalinity concentration covariance of 18o and 13c usually occurs in lake sediments under the condition of hydrological closure for time periods of the order of 5000 yr or longer on shorter time scales however certain subtleties in the use of the 13c 18o covariance as a hydrology history indicator must be recognized the covariant trend may not be found in hyper alkaline lakes because of the insensitivity of 13c to lake volume changes for less alkaline closed basin lakes covariance of 13c 18o will result from relatively rapid increase or decrease of lake volume with the rapid decrease condition giving rise to heavy and narrow ranged isotopic values for both 13c and 18o due to the coupled evaporation productivity effect when the lake volume remains stable as is the case for an over flowing open lake poor 13c 18o covariance will ensue due to the effect of vapor exchange with the atmosphere we have used the measured 13c 18o covariance in carbonate sediments of mono lake california to deduce paleo hydrological and lake water alkalinity and salinity variations and found the results in consonance with holocene lake level history derived from the 18o and other information

stable isotope geochemistry of cherts and carbonates from the 2 0 ga gunflint iron formation implications for the depositional setting and the effects of diagenesis and metamorphism abstract 18 o values for all but three of 97 samples of chert along a 220 km transect through the 2 0 ga gunflint iron formation of the animikie basin north america range from 21 3 to 24 7 o there is no correlation between 18 o and the type of chert i e deeper water lutitic chert shallower water arenitic chert and peritidal algal chert the fact that the gunflint iron formation has been exposed to thermal conditions only slightly above burial diagenesis suggests that the values of the chert have not been significantly affected by metamorphism field and petrographic observations and the small 18 o range of all the chert types suggests that silicification lithification and any conversion to quartz chert from a hydrous silica precursor occurred close to the sediment water interface the uniform 18 o values of all the different chert types suggest that all cherts precipitated over a relatively common temperature interval 20 c from a common parent water having a relatively uniform 18 o the isotopic data suggest silica saturation and precipitation was not the result of the mixing of seawater and meteoric water or the result of evaporative processes siderite in the deep water banded facies of the gunflint displays a 18 o range of 6 0 to 2 6 o whereas siderite in the more shoreward organic rich shale facies is significantly enriched in 13 c 13 c 2 5 to 0 5 o 18 c of siderite does not appear to have been altered as a result of neomorphic recrystallization and authigenic microsparitic siderite along stylolites displays the same 13 c range as primary and neomorphosed siderite however neomorphic and authigenic siderite are depleted in 18 o by up to 7 o relative to primary unaltered microspheroidal siderite the lower 13 c of banded facies siderite relative to the shale facies siderite does not seem to be the result of thermal decarboxylation or anaerobic oxidation abiotic or bacterial of organic matter but rather is better explained by primary precipitation in an ocean system that was layered with respect to carbon isotopes the more basinward banded facies siderite precipitated from a 13 c depleted fe 2 rich hydrothermal water mass that was moved by advective upwelling from submarine regions of magmatic volcanic hydrothermal activity onto the continental shelf the siderite of the shale facies precipitated from a more near shore shallow seawater mass that was comparatively enriched in 13 c 13 c 0 2 o ankerite in the gunflint iron formation occurs as a replacement of primary siderite in the banded facies and as a replacement of greenalite and chert in the arenite facies many ankerite samples have 13 c values that are similar to those of the shale facies siderite which suggests that shallow seawater was a major component of the diagenetic fluid later neomorphism of the replacement ankerite at higher burial temperatures and probably in the presence of 18 o depleted waters resulted in a wide range of 18 o values 13 8 22 5 o the unique mineralogy of the upper most gunflint iron formation calcite serpentine and magnetite compared to the majority of the formation the very low values and overlying diabase sills suggest that the non ferroan nature of the carbonate in the upper gunflint is the result of contact metamorphism

diagenetic siderite and other ferroan carbonates in a modern subsiding marsh sequence abstract mineralogical and carbon and oxygen stable isotope compositions in siderite rich concretions from a marsh within the mississippi river deltaic plain are highly variable the concretions have sideritic interiors and become more calcitic and dolomitic ankeritic near their margins 13c values of these concretions range from 10 0 to 7 4 pdb and 1 o values range from 2 4 to 8 9 pdb the stable isotope compositions exhibit two general trends one is a growth trend that shows 13c an 18o depletion from the centers to the margins of the nodules and bands the other is a progressive burial trend that shows 13c enrichments and 18o depletions at the centers of the concretions with depth a zonal model for carbonate precipitation with emphasis on an early fresh water non sulfidic methanic phase followed by a sulfidic stage associated with marine inundation of the marsh explains much of the observed carbon isotopic variations mixing of fluids plus precipitation in a partially closed chemical microsystem accounts for most of the oxygen isotope fractionations documented between growth layers and casts doubt on the utility of change in 18o shifts as an indicator of depth or relative temperature of concretion formation for concretions that form in coastal environments

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

oxygen and carbon isotopic composition of marine carbonate concretions an overview abstract compilation of stable isotope data 18o and 13c for marine calcite siderite and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends although many of the dolomites have 18o values that are compatible with precipitation from unaltered seawater at low temperatures most of the calcites and siderites are far too depleted in 12o for such an origin this is surprising as most of the concretions appear to have formed very early in the diagenetic history of the sediments often within several meters of the sediment water interface the most likely cause of the anomalously depleted oxygen values is a decrease in pore water 18o values during early diagenesis a result perhaps of a significant influx of meteoric water into the marine shelf environment or water mineral interaction e g precipitation of 18o enriched minerals the carbon isotopic field for dolomite is quite different from that for calcite and siderite highly positive 13c values are fairly common only for dolomites this indicates that while dolomite concretions often form in sediments where methanogenesis is rapid precipitation of calcite and siderite concretions is restricted to sediments where either n methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid 13c values this variation in carbon isotopic composition among the different minerals suggests that the rates of organic carbon oxidation and sedimentation may control the mineralogy of carbonate concretions high organic carbon oxidation rates favor dolomite and lower oxidation rates favor calcite or siderite

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

early holocene change in atmospheric circulation in the northern great plains an upstream view of the 8 2 ka cold event elk lake in northwestern minnesota contains numerous proxy records of climatic and environmental change contained in varved sediments with annual resolution for the last 10 000 years these proxies show that about 8200 calendar years ago 8 2 cal ka 7300 radiocarbon years elk lake went froma well stratified lake that was wind protected in a boreal forest to a wellmixed lake in open prairie savanna receiving northwesterly wind blown dust probably from the dry floor of lake agassiz this change in climate marks the initiation of the widely recognized mid holocene altithermal in central north america the coincidence of this change with the so called 8 2 cal ka cold event recognized in ice core and other records fromthe circum north atlantic and thought by some to be caused by catastrophic discharge of freshwater from proglacial lakes agassiz and ojibway suggests that the two events might be related our interpretation of the elk lake proxy records and of other records from less accurately dated sites suggests that change in climate over north america was the result of a fundamental change in atmospheric circulation in response to marked changes in the relative proportions of land water and especially glacial ice in north america during the early holocene this change in circulation probably post dates the final drainage of proglacial lakes along the southern margin of the laurentide ice sheet and may have produced a minor perturbation in climate over greenland that resulted in a brief cold pulse detected in ice cores r 2002 elsevier science ltd all rights reserved

palaeoenvironmental and diagenetic reconstruction of a closed lacustrine carbonate system the challenging marginal setting of the miocene ries crater lake germany chemostratigraphic studies on lacustrine sedimentary sequences provide essential insights on past cyclic climatic events on their repetition and prediction through time diagenetic overprint of primary features often hinders the use of such studies for palaeoenvironmental reconstruction here the potential of integrated geochemical and petrographic methods is evaluated to record freshwater to saline oscillations within the ancient marginal lacustrine carbonates of the miocene ries crater lake germany this area is critical as it represents the transition from shoreline to proximal domains of a hydrologically closed system affected by recurrent emergent events representing the boundaries of successive sedimentary cycles chemostratigraphy targets shifts related to subaerial exposure and or climatic fluctuations methods combine facies changes with 13c 18o chemostratigraphy from matrix carbonates across five closely spaced temporally equivalent stratigraphic sections isotope composition of ostracod shells gastropods and cements is provided for comparison cathodoluminescence and back scatter electron microscopy were performed to discriminate primary syn depositional from secondary diagenetic features meteoric diagenesis is expressed by substantial early dissolution and dark blue luminescent sparry cements carrying negative 13c and 18o sedimentary cycles are not correlated by isotope chemostratigraphy both matrix 13c and 18o range from ca 7 5 to 4 0 and show clear positive covariance r 0 97 whose nature differs from that of previous basin oriented studies on the lake negative values are here unconnected to original freshwater lacustrine conditions but reflect extensive meteoric diagenesis while positive values likely represent primary saline lake water chemistry noisy geochemical curves relate to heterogeneities in primary porosity resulting in selective carbonate diagenesis this study exemplifies that ancient lacustrine carbonates despite extensive meteoric weathering are capable to retain key information for both palaeoenvironmental reconstruction and the understanding of diagenetic processes in relation with those primary conditions also it emphasizes the limitation of chemostratigraphy in fossil carbonates and specifically in settings that are sensitive for the preservation of primary environmental signals such as lake margins prone to meteoric diagenesis this article is protected by copyright all rights reserved

a critical look at iron paleoredox proxies new insights from modern euxinic marine basins enrichments in reactive iron occur under euxinic marine conditions that is where dissolved sulfide is present in the water column these enrichments result primarily from the export of remobilized iron from the oxic shelf which is scavenged from the euxinic water column during syngenetic pyrite formation and deposited in the underlying sediments strongly elevated ratios of highly reactive iron to total iron fehr fet and total iron to aluminum fet al and high degrees of pyritization dop are each products of this enrichment process these paleoredox proxies are among the most faithful recorders of ancient euxinia contrary to previous arguments iron enrichment is decoupled from biogenic sediment inputs but it does appear to be a uniquely euxinic phenomenon in other words we can rule out a major contribution from preferential physical transport of fehr rich detrital sediment to the deep basin which could also operate under oxic conditions furthermore enrichment via the shuttling of iron remobilized from oxic shelves appears to be limited by inefficient transport and trapping processes in deep oxic basins elevated fet al ratios in the euxinic sediments also cannot be a product of internal enhancement of the reactivity of the detrital iron pool without net fehr addition these conclusions are supported by observations in the modern black sea orca basin and effingham inlet fet al ratios are unambiguous recorders of paleoredox even in sediments that have experienced high degrees of metamorphic alteration however this study suggests that high siliciclastic accumulation rates can swamp the enrichment mechanism resulting in only intermediate dop values for euxinic sediments and fet al ratios that mimic the oxic shelf such dilution effects are well expressed in black sea basinal turbidites and rapidly accumulating muds on euxinic basin margins under conditions of persistent euxinia varying extents of fehr enrichment can illuminate spatial and temporal gradients in siliciclastic sedimentation the magnitude of enrichment is a function of the source shelf to sink ocean basin areal ratio suggesting that iron proxies can also record ocean scale paleoenvironmental properties through muted enrichments at times of very widespread euxinia for the first time manganese data are interpreted in light of the redox shuttle model as for the iron data the black sea orca basin and effingham inlet show enrichments in total manganese in the deep euxinic basin suggesting export from the suboxic porewaters of the oxic shelf and scavenging and burial in the basin the black sea data reveal iron and manganese enrichment across the broad deep euxinic basin suggesting efficient lateral transport and deep water mixing tied to the physical properties of the water column

photosynthetic microbial mats in the 3 416 myr old ocean recent re evaluations of the geological record of the earliest life on earth have led to the suggestion that some of the oldest putative microfossils1 and carbonaceous matter were formed through abiotic hydrothermal processes2 3 similarly many early archaean more than 3 400 myr old cherts have been reinterpreted as hydrothermal deposits rather than products of normal marine sedimentary processes2 4 5 here we present the results of a field petrographic and geochemical study testing these hypotheses for the 3 416 myr old buck reef chert south africa from sedimentary structures and distributions of sand and mud we infer that deposition occurred in normal open shallow to deep marine environments the siderite enrichment that we observe in deep water sediments is consistent with a stratified early ocean6 7 we show that most carbonaceous matter was formed by photosynthetic mats within the euphotic zone and distributed as detrital matter by waves and currents to surrounding environments we find no evidence that hydrothermal processes had any direct role in the deposition of either the carbonaceous matter or the enclosing sediments instead we conclude that photosynthetic organisms had evolved and were living in a stratified ocean supersaturated in dissolved silica8 9 3 416 myr ago

isotopic composition of siderite as an indicator of depositional environment compilation of available oxygen and carbon isotopic data for siderites from different depositional environments indicates that marine and continental siderites are characterized by distinctive compositional fields the most notable difference between the fields is the generally higher 13 c values of continental vs marine siderites this difference appears to reflect the fact that marine sediments generally undergo a more extensive period of sulfate reduction than do the continental sediments in which siderite forms analysis of siderite geochemistry may assist in determination of depositional environment in cases where the available sedimentological evidence is ambiguous

biomineralization of iron phosphates in the water column of lake pavin massif central france the availabilities of iron and phosphorus have considerably impacted biological productivity in past and present natural aquatic environments and therefore have been key regulators of climate changes over geological time scales microbial organisms are known to play important roles in reactions that drive fe and p cycling at redox interfaces in earth s surface environments here we study the depth variations of fe and p speciation in lake pavin massif central france a deep bottom depth 92 m and permanently stratified lake with anoxic and ferruginous conditions in the water column below 60 m depth we particularly focus on the potential roles of microbes on fe and p transformations and traces left by these processes in the sedimenting particular matter bulk chemical analyses powder x ray diffraction and x ray absorption spectroscopy xas at the fe k edge were performed to characterize the mineralogy and fe oxidation state of solid particles at different depths in the water column and in sediments deposited at the bottom of the lake fe is mainly hosted by fe iii oxyhydr oxides and phyllosilicates in the shallower oxygenated water column of the lake 25 m the amount of fe in suspended matter increases with depth and an additional amorphous fe ii fe iii phosphate phase is detected close to the chemocline at 56 m depth while vivianite an fe ii phosphate with a formula of fe ii 3 po4 2 8 h2o becomes dominant in the deeper anoxic water 67 m and 86 m depths fe oxyhydr oxides are preserved down to these depths in the water column suggesting that fe reduction has little impact on the particulate fe budget over the monimolimnion these fe oxyhydr oxides undergo reductive dissolution at the surface of the sediments where vivianite is the main fe bearing phase these results are confirmed by imaging at the micrometer and nano scales using scanning electron microscopy sem transmission electron microscopy tem and synchrotron based scanning transmission x ray microscopy stxm as well as high spatial and energy resolution xanes spectra at the fe l2 3 edges and the c k edge moreover polyphosphate accumulating microorganisms were observed by microscopy and chemical imaging at all depths except in the deeper part of the lake 86 m and some fe was associated with these polyphosphate inclusions below 56 m finally the fe phosphates encrust microbial bodies suggesting that microbial activities might be involved in the formation of these phases based on comparisons with laboratory experiments we propose that fe oxidation in contrast to fe reduction may play a role in the precipitation of fe phosphates in the water column of lake pavin polyphosphate accumulating microorganisms could also be involved in fe phosphate formation in the lake for instance by increasing dissolved phosphate concentrations in the monimolimnion

the lacustrine carbon cycle as illuminated by the waters and sediments of two hydrologically distinct headwater lakes in north central minnesota u s a abstract the accumulation rates of caco3 and organic carbon oc in lake sediments are delicately balanced between production in the epilimnion and destruction in the hypolimnion the cycling of these two forms of carbon makes a carbon pump that greatly affects the biogeochemical cycles of other elements to further understand these biogeochemical dynamics the lakes streams and wetlands of the shingobee river headwater area of north central minnesota have been subjected to intensive hydrologic and biogeochemical studies williams lake situated close to the highest point in the regional flow system is hydrologically closed with no surface inlet or outlet and ground water and precipitation as the only sources of water shingobee lake situated at the lowest point in the regional flow system has the shingobee river as an inlet and outlet the surface waters of both lakes are oversaturated and the bottom waters undersaturated with respect to caco3 during the summer the small amount of caco3 that is precipitated in the epilimnion of williams lake during the summer is dissolved in the undersaturated hypolimnion and sediments with the result that no caco3 is incorporated into the profundal surface sediments because of the high phytoplankton productivity of shingobee lake sufficient caco3 is produced in the epilimnion that large amounts survive the corrosive hypolimnion and sediments and an average of 46 wt accumulates in surface sediments another consequence of higher phytoplankton productivity in shingobee lake is that the hypolimnion becomes oxygen deficient within a month after overturn in both the spring and fall because of reducing conditions that develop in the hypolimnion of shingobee lake high concentrations of dissolved fe and mn accumulate there during summer stratification precipitation of fe and mn oxyhydroxides during periods of fall and spring overturn results in high concentrations of fe and mn in surface sediments in williams lake high concentrations of fe and mn do not build up in the hypolimnion the concentration of caco3 is about 80 wt in lower holocene sediments of both lakes the lower holocene sediments in both lakes also contain high concentrations of fe and mn and the lower holocene sediments of shingobee are laminated the waters of both lakes had identical values of 13c and 18o during the early holocene but the waters of williams lake evolved during the early holocene increasing about 10 in both 13c and 18o deposits of lacustrine marl occur as much as seven meters above the present elevation of williams lake the highest of the two lakes taken together these observations suggest that the lakes were once connected to form a larger lake called lake willobee with a hypolimnion that was anoxic at least seasonally

paleohydrology of the late miocene ridge basin lake california uppermost miocene lacustrine deposits of the ridge basin california contain benthic microbial carbonates as fragmentary crusts on clasts in resedimented conglomerates of the violin breccia deposited adjacent to the fault controlled southwestern margin of the basin and as fragments and more extensive in situ stromatolitic units within the ridge route formation around the northeastern part of the basin ridge route microbialites formed in the open waters of the ridge basin paleolake and have 18 o values that are significantly higher than those from the violin breccia the latter may have formed in warm littoral pools or in streams maintained by local runoff their 18 o compositions are consistent with carbonate precipitation from meteoric waters supplied by rainfall dominated by 67 75 winter precipitation at temperatures of 16 17 c ridge route formation microbialites have a wide range of 13 c and 18 o values and indicate that the lake fluctuated between hydrologically open and closed conditions light carbon isotopic compositions for carbonate that precipitated during periods of open basin conditions suggest that inflow waters came from a fluvial system with a well vegetated flood or delta plain relatively high 18 o values imply that the lake was not maintained by local rainfall but depended on inflow that came either from an upstream lake or from a region to the east or northeast where the rainfall regime was dominated by isotopically heavy summer precipitation

a 700 year record of climate change based on geochemical and palynological data from varved sediments lac pavin france abstract the fe flux budget of the meromictic crater lac pavin french massif central is re evaluated in the light of new results from detailed sediment investigations geochemistry sedimentology pollen varve counting sedimentation of lac pavin during the last 700 years is characterised by high net accumulation of biogenic silica 100 500 g m 2 yr 1 sio 2 and autochthonous fe precipitates f fe exc 2 200 g m 2 yr 1 fe at overall high sediment accumulation e g ad 1530 1890 3 7 mm yr 1 high net accumulation of biogenic silica and fe exc is sustained by nutrient and fe influx via sub lacustrine springs the feeding of the groundwater inflow is closely correlated with the seepage of meteoric waters which declines in winter due to freezing of the ground in the context of the derived local vegetation dynamics balanced fe exc flux is interpreted as a proxy for rainfall during the growing season meteorological monitoring data document a significant inverse correlation between monthly temperature and rainfall between may and october which reflects enhanced cloudiness for precipitation above average beech growing under sub optimal conditions at the high altitude of the study site represents a major tree component in the catchment of lac pavin minor temperature decrease in the growing season can explain reduced flowering of beech in the following year distinct short term variability of beech characterised by an inverse correlation with the balanced f fe exc supports the data interpretation two periods with prevailing of overall wetter and cooler conditions in the growing season were derived ca ad 1525 1600 and ad 1630 1660 climate change to drier and warmer conditions is implied around ad 1450 and ad 1620 the late maunder minimum ad 1675 1715 is characterised by distinct short term variability of the palaeo hydrological conditions at the study site

comparison of kirchhoff and reverse time migration methods with applications to prestack depth imaging of complex structures the performance of two popular migration methods the kirchhoff integral and reverse time migrations is evaluated through applications to imaging complex structures using prestack shot records the migration results from the marmousi model data demonstrate that reverse time migration is more accurate than kirchhoff migration in imaging the steeply dipping faults however the improved accuracy of reverse time migration requires higher computational costs in the application to the alberta foothills data where a good estimate of the velocity model is available however both the kirchhoff and the reverse time migration methods produce almost identical results this implies that in the real world of exploration seismology it will be relatively difficult to identify which method performs better because we never know the exact answer of the subsurface

first discovery and formation process of authigenic siderite from gas hydrate bearing mud volcanoes in fresh water lake baikal eastern siberia 1 we report on the first authigenic siderite feco3 concretions recovered from near bottom sediments at gas hydrate bearing mud volcanoes in fresh water lake baikal eastern siberia the carbonates appear as firm plate type formations at the malenky mud volcano southern baikal basin and as soft nodules at the k 2 mud volcano central baikal basin calcium is the main divalent component which substitutes iron in the carbonate lattice 7 to 20 mol the 13c values of the carbonates 3 3 to 6 8 at malenky and 16 5 to 21 9 pdb at k 2 indicate that their formation is due to methanogenesis the latter was most likely caused by the microbial methyl type acetate fermentation that is suggested from the isotopic composition of the accompanying methane hydrates and dissolved methane general depletion of the siderites in 18o 11 6 to 9 9 at malenky and 13 9 to 12 3 pdb at k 2 is mainly inherited from the isotope composition of pore water 15 2 to 15 4 smow at ambient temperature 3 5 c

frequency and temperature dependence in electromagnetic properties of martian analog minerals 1 ground penetrating radar gpr has the potential to image the martian subsurface to give geological context to drilling targets investigate stratigraphy and locate subsurface water gpr depth of penetration depends strongly on the electromagnetic em properties complex dielectric permittivity complex magnetic permeability and dc resistivity of the subsurface these em properties in turn depend on the mineralogical composition of the subsurface and are sensitive to temperature in this study the em properties of martian analog samples were measured versus frequency 1 mhz 1 ghz and at martian temperatures 180 300 k results from the study found the following gray hematite has a large temperature dependent dielectric relaxation magnetite has a temperature independent magnetic relaxation and jsc mars 1 has a broad temperature dependent dielectric relaxation most likely caused by absorbed water two orbital radars marsis and sharad are currently investigating the subsurface of mars on the basis of the results of our measurements the attenuation rate of gray hematite is 0 03 and 0 9 db m magnetite is 0 04 and 1 1 db m and jsc mars 1 is 0 015 and 0 09 db m at marsis and sharad frequencies respectively and at the average martian temperature of 213 k with respect to using gpr for subsurface investigation on mars absorbed water will be a larger attenuator of radar energy as high concentrations of magnetite and gray hematite are not found globally on mars citation stillman d and g olhoeft 2008 frequency and temperature dependence in electromagnetic properties of martian analog minerals j geophys res 113 e09005 doi 10 1029 2007je002977

emplacement and inflation of pahoehoe sheet flows observations and measurements of active lava flows on kilauea volcano hawaii inflated pahoehoe sheet flows have a distinctive horizontal upper surface which can be several hundred meters across and are bounded by steep monoclinal uplifts the inflated sheet flows we studied ranged from 1 to 5 m in thickness but initially propagated as thin sheets of fluid pahoehoe lava generally 20 30 cm thick individual lobes originated at outbreaks from the inflated front of a prior sheet flow lobe and initially moved rapidly away from their source velocities slowed greatly within hours due to radial spreading and to depletion of lava stored within the source flow as the outward flow velocity decreases cooling promotes rapid crustal growth at first the crust behaves plastically as pahoehoe toes form after the crust attains a thickness of 2 5 cm it behaves more rigidly and develops enough strength to retain incoming lava thus increasing the hydrostatic head at the flow front the increased hydrostatic pressure is distributed evenly through the liquid lava core of the flow resulting in uniform uplift of the entire sheet flow lobe initial uplift rates are rapid flows thicken to 1 m in 1 2 hours but rates decline sharply as crustal thickness increases and as outbreaks occur from the margins of the inflating lobe one flow reached a final thickness of nearly 4 m after 350 hr inflation data define power law curves whereas crustal cooling follows square root of time relationships the combination of data can be used to construct simple models of inflated sheet flows as the flow advances preferred pathways develop in the older portions of the liquid cored flow these pathways can evolve into lavatube systems within a few weeks formation of lava tubes results in highly efficient delivery of lava at velocities of several kilometers per hour to a flow front that may be moving 1 2 orders of magnitude slower if advance of the sheet flow is terminated the tube remains filled with lava that crystallizes in situ rather than draining to form the cave like lava tubes commonly associated with pahoehoe flows inflated sheet flows from kilauea and mauna loa are morphologically similar to some thick icelandic and submarine sheet flows suggesting a similar mechanism of emplacement the planar sheet like geometry of flood basalt flows may also result from inflation of sequentially emplaced flow lobes rather than nearly instantaneous emplacement as literal floods of lava

mixing and its effects on biogeochemistry in the persistently stratified deep tropical lake matano indonesia in the 590 m deep tropical lake matano indonesia stratification is characterized by weak thermal gradients 2uc per 500 m and weak salinity gradients 0 14 per 500 m these gradients persist over seasons decades and possibly centuries under these nearly steady state conditions vertical eddy diffusion coefficients k z cannot be estimated by conventional methods that rely on time derivatives of temperature distributions we use and compare several alternative methods one dimensionalk emodeling three dimensional hydrodynamic modeling correlation with the size of thorpe instabilities and correlation with the stability frequency in the thermocline region at 100 m depth thek z is 5310 26 m 2 s 21 but below 300 m the small density gradient results in large 20 m vertical eddies and high mixing rates k z 10 22 m 2 s 21 the estimated timescale of water renewal in the monimolimnion is several hundred years intense evaporation depletes the surface mixed layer of 16 o and 1 h isotopes making it isotopically heavier the lake waters become progressively isotopically lighter with depth and the isotopic composition in the deep waters is close to those of the ground and tributary waters the vertical distribution ofk z is used in a biogeochemical reaction transport model we show that outside of a narrow thermocline region the vertical distributions of dissolved oxygen iron methane and phosphorus are shaped by vertical variations in transport rates rather than by sources or sinks

isotopic constraints on the late archean carbon cycle from the transvaal supergroup along the western margin of the kaapvaal craton south africa few existing studies illuminate the operation of the carbon cycle before the rise of atmospheric oxygen circa 2400 million years ago stable carbon isotopic measurements of shallow stromatolitic carbonates 0 vpdb and basinal carbonate minerals 6 in iron formation have been used to infer a strong isotopic depth gradient in archean ocean basins from new diamond drill cores obtained by the agouron drilling project from the griqualand west structural basin in the northern cape province south africa we present 13 c data from carbonates and organic matter that offer fresh insights into the late archean carbon cycle three drill cores cover the development progradation and ultimate demise by drowning of the campbellrand carbonate platform ca 2590 2500 ma one captures the platform top shallow marine and intertidal paleoenvironments the other two run through slope and basinal sections deposited adjacent to the platform margin increasing inwater depth likely to 1 km both shallowand deep water carbonates precipitated on the seafloor consistently show 13 c values around 0 5 incompatible with a strong late archean isotopic depth gradient a mathematical model suggests that these isotopic data are consistent with a reduced biological pump increased dissolved inorganic carbon in seawater due to higher atmospheric p co2 or both certain horizons do show distinct isotopic variability such areas are commonly shaly and they tend to be organic and or iron rich strong c isotopic variations occur on a cm scale and most likely stem from diagenetic remineralization of organic matter in sediment starved areas where iron formation developed siderite tends to be 13 c depleted sometimes by as much as 14 these observations suggest a carbon cycle in which iron respiration played a conspicuous role carbon isotope ratios from organic matter in shales are commonly 1 lighter than stratigraphically contiguous carbonates but there is no clear water depth trend in the organic carbon isotopic data taken as a whole the 13 c of organic matter can be explained by several non unique sets of processes including different autotrophic mechanisms of carbon fixation heterotrophic recycling including fermentation and methanotrophy and post depositional diagenesis the most striking feature is the occurrence of organic 13 c values 40 a feature that appears to be commonplace in late archean successions framed in the context of carbon cycle isotopic mass balance both organic and carbonate carbon isotopic data suggest that the proportion of carbon buried as organic matter was not radically different before the appearance of free environmental oxygen

carbon and oxygen stable isotopic signatures of methanogenesis temperature and water column stratification in holocene siderite varves abstract manganoan siderite fe mn co 3 occurs in abundance of up to 19 dry weight as the sole endogenic carbonate within a succession of holocene organic rich varved sediments from freshwater otter lake ol michigan radiocarbon dating and varve counts from a 7 m piston core constrain periods of major siderite accumulation to sediments older than 1200 cal yr bp sediment petrography suggests that siderite was a seasonal precipitate confined to the summer layer of the varve couplet bulk sediment chemistry reveals cycles in abundance of manganese and aluminum coincident with centennial scale cycles of siderite accumulation siderite 13 c and 18 o are enriched when the mineral is abundant and depleted when it is least abundant samples with high abundance of siderite precipitated in oxygen isotopic equilibrium with modern lake water but are 13 c enriched relative to modern waters samples from intervals of low siderite abundance are 13 c and 18 o depleted relative to equilibrium with modern lake water these data suggest that abundant siderite precipitation occurred when the ol water column was ferruginous iron meromictic allowing for enhanced ferrous iron concentrations and dissolved inorganic carbon dic enriched in 13 c below the chemocline where methanogenesis in waters and sediments influenced dic composition seasonal siderite precipitation was triggered by water column alkalinity fluctuations driven by summer calcite dissolution manganese substitution in siderite lowered kinetic barriers to low temperature mineral precipitation climate changes and basin filling influenced lake stratification and the rate of siderite precipitation the siderite occurrence reported here displays remarkable similarity to its occurrence in paleogene maar lake deposits suggesting that further studies of holocene lacustrine siderites may provide insight into ancient sedimentary systems and environments

ground penetrating radar for high resolution mapping of soil and rock stratigraphy ground penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions the need to better understanding overburden conditions for activities such as geochemical sampling geotechnical investigations and placer exploration as well as the factors controlling groundwater flow has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible the areas of application for ground penetrating radar are diverse the method has been used successfully to map ice thickness water depth in lakes bedrock depth soil stratigraphy and water table depth it is also used to delineate rock fabric detect voids and identify karst features the effective application of the radar for the high resolution definition of soil stratigraphy and fractures in bedrock is highlighted the basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories one example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination the corroboration of the radar results by borehole investigations demonstrates the power and utility of the high resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes with the advent of new instrumentation and field procedures the routine application of the radar method is becoming economically viable and the method will see expanded use in the future

stable isotope composition of siderite as an indicator of the paleoenvironmental history of oil shale lakes the sediments of two eocene oil shale lakes messel graben and eckfeld maar west germany were analyzed with respect to the distribution chemical and stable isotope composition of siderite the data allow a reconstruction of different stages in the sedimentary and early diagenetic history of the lakes in the messel lake siderite formation started when the sedimentation of laminated sapropelic corg rich sediments was already established the early stage of oil shale sedimentation was characterized by pyrite precipitation in the siderite stage the stable isotope composition of the carbonate suggests methanogenesis to have been an important process at the sediment water boundary layer in the eckfeld lake siderite formation started even before sedimentation of the first sapropelic laminae the stable isotopic composition of the carbonate again indicates an anoxic sediment water boundary layer and strong methanogenesis with the expansion of this environment into the deep water mass sapropel sedimentation became established for both sequences the chemistry and stable isotope composition of the siderites correlates with petrographic and paleontological information about climate dependent fluctuations in the ecological chemical and early diagenetic environment the strong relationship of siderite formation to methanogenesis may be explained by the rate of microbial organic matter degradation which governed both methanogenesis and pco2

topographic migration of georadar data implications for acquisition and processing application of conventional elevation static corrections and migration to wavefield data recorded on irregular surfaces may result in poor reconstructions of complex subsurface features particulary poor images may be obtained at locations where the depths to target structures are comparable to undulations in the surface topography for example topographic relief of only 1 2 m may be important for the processing of georadar data we describe an algorithm that allows georadar data to be migrated directly from gently to highly irregular acquisition surfaces when applied to a variety of complicated synthetic data sets topographically migrated images are observed to be markedly superior to those produced by two standard processing schemes extensive tests demonstrate that topographic migration should be considered in regions characterized by surface gradients 10 i e dips 6 for effective topographic migration lateral and vertical coordinates of the georadar antennas should be determined to better

observations on basaltic lava streams in tubes from kilauea volcano island of hawai i from 1986 to 1997 the pu u o o kupaianaha eruption of kilauea produced a vast pahoehoe flow field fed by lava tubes that extended 10 12 km from vents on the volcano s east rift zone to the ocean within a kilometer of the vent tubes were as much as 20 m high and 10 25 m wide on steep slopes 4 10 a little farther away from the vent some tubes formed by roofing over of lava channels lava streams were typically 1 2 m deep flowing within a tube that here was typically 5 m high and 3 m wide on the coastal plain 1 tubes within inflated sheet flows were completely filled typically 1 2 m high and several tens of meters wide tubes develop as a flow s crust grows on the top bottom and sides of the tubes restricting the size of the fluid core the tubes start out with nearly elliptical cross sectional shapes many times wider than high broad flat sheet flows evolve into elongate tumuli with an axial crack as the flanks of the original flow were progressively buried by breakouts temperature measurements and the presence of stalactites in active tubes confirmed that the tube walls were above the solidus and subject to melting sometimes the tubes began downcutting progressive downcutting was frequently observed through skylights a rate of 10 cm d was measured at one skylight for nearly 2 months

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

a thermodynamic model for water column precipitation of siderite in the plio pleistocene black sea it has been hypothesized that finely disseminated soft wheat like grains of siderite in the sideritic muds of the plio pleistocene section of the black sea are sub aqueous precipitates hsu and kelts 1978 in an attempt to quantify this hypothesis we 1 investigated the chemistry mineralogy and isotopic composition of the siderites and associated calcite and 2 developed a thermodynamic model of endogenic siderite precipitation that is directly precipitated from the water column driven by sequential evaporative concentration of acidic dissolved or anic carbon and iron rich waters considering the average com position of the present day satilla drainage of the southeastern umted states as approximating the composition of the waters that fed the neogene black sea and allowing this water to evaporate isothermally at 25 c in equilibrium with atinospheric c02 pc02 l 0 3 5 attn we calculated the aqueous concentrations at which various carbonate phase5 would precipitate out of the system it is shown that evaporation of this water to roughly one twelfth of its original volume can result in precipitation of the pure endmember siderite alone relating our geochemical and modeling results to the details of black sea climatostratigraphic and evolutionary history dur ing the times of deposition of the siderite units we corroborate the qualitative arguments of hsu and kelts 1978 we also conclude that climate was the chief forcing function in transforming episodically the calcite precipitating eutrophic water masses of the plio pleistocene basin to one of siderite ptecipitation based on our model calculations of the thickness of siderite precipitated per cm2 of basin area we further predict that the observed laminae interbeds of siderite could be a composite of several microlayers of siderite and clays

primary and diagenetic carbonates in the anoxic sediments of lake bosumtwi ghana cores of organic rich muds from the tropical meteorite crater lake bosumtwi ghana contain laminae of authigenic calcite polyhedra and aragonite needles as well as scattered diagenetic calcite mg calcite spherulites and aggregates of dolomite crystals their respective origins are traced by analyses of oxygen and carbon stable isotopes anoxic dolomite and mg calcite formed in pore waters reflecting highest degrees of evaporative concentration and high rates of methanogenesis carbon dioxide reduction must have been the dominant methanogenic pathway producing diagenetic carbonates with delta sup 13 c values up to 27 pdb this example shows the inherent problems of interpreting bulk samples with progressive mineralogical sequences of calcite to mg calcite to aragonite to dolomite merely in terms of mg sup 2 ca sup 2 in lake waters it provides a model for the interpretation of ancient lacustrine carbonates from organic rich environments

toward establishing precise 40ar 39ar chronologies for late pleistocene palaeoclimate archives an example from the lake suigetsu japan sedimentary record the varved suigetsu sg06 sediment core is potentially one of the most important and well constrained mid latitude palaeoclimate archives recording continuous deposition during the last 150 kyrs numerous visible and non visible volcanic ash layers form unique age markers within sg06 these ash layers are too fine and crystal poor to be directly dated using the 40ar 39ar technique so the tephra were correlated to proximal volcanic deposits using their glass shard compositions a high precision 40ar 39ar sanidine age of 10 0 0 3 ka 1 n 34 mswd 0 71 p 0 89 was obtained for the sg06 1288 u oki proximal tephra ulleungdo u4 40ar 39ar yields a precision of 3 near the younger limit of the method with improved precision possible for older sg06 samples such 40ar 39ar ages for tephra layers can provide invaluable tie points within the lake suigetsu sg06 sequence giving independent verification of the core s varve chronology allowing for the calibration of cosmogenic nuclide production and providing a precise chronology beyond the varve limit

distribution formation mechanisms and significance of lunar pits abstract lunar reconnaissance orbiter camera images reveal the presence of steep walled pits in mare basalt n 8 impact melt deposits n 221 and highland terrain n 2 pits represent evidence of subsurface voids of unknown extents by analogy with terrestrial counterparts the voids associated with mare pits may extend for hundreds of meters to kilometers in length thereby providing extensive potential habitats and access to subsurface geology because of their small sizes relative to the local equilibrium crater diameters the mare pits are likely to be post flow features rather than volcanic skylights the impact melt pits are indirect evidence both of extensive subsurface movement of impact melt and of exploitable sublunarean voids due to the small sizes of pits mare highland and impact melt and the absolute ages of their host materials it is likely that most pits formed as secondary features

evaluating a primary carbonate pathway for manganese enrichments in reducing environments abstract most manganese mn enrichments in the sedimentary rock record are hosted in carbonate minerals which are assumed to have formed by diagenetic reduction of precursor mn oxides and are considered diagnostic of strongly oxidizing conditions here we explore an alternative model where mn carbonates form in redox stratified water columns linked to calcium carbonate dissolution in ferruginous brownie lake in minnesota usa we document mn carbonates as an hcl extractable phase present in sediment traps and in reducing portions of the water column mn carbonate becomes supersaturated in the brownie lake chemocline where dissolved oxygen concentrations fall below 5 m and mn oxide reduction increases the dissolved mn concentration supersaturation is enhanced when calcite originating from surface waters dissolves in more acidic waters at the chemocline in the same zone sulfate reduction and microaerobic methane oxidation add dissolved inorganic carbon dic with negative 13 c these observations demonstrate that sedimentary mn enrichments may 1 develop from primary carbonate phases and 2 can occur in environments with dissolved oxygen concentrations 200 m and where mn and fe are partitioned by s cycling photoferrotrophy or microaerophilic fe oxidation a shallow lysocline enhances mn carbonate production by providing additional dic and nucleation sites for crystal growth this carbonate model for mn enrichments is expected to be viable in both euxinic and ferruginous environments and provides a more nuanced view of the relationships between mn and carbon cycling with applications throughout the rock record

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

possible lunar lava tube skylight observed by selene cameras 1 we discovered a vertical hole on the moon which is a possible lava tube skylight using data from selene s two high resolution cameras the terrain camera and the multi band imager the hole is nearly circular 65 m in diameter and located in a sinuous rille at the marius hills region a volcanic province on the lunar nearside we observed the hole at various solar illumination conditions and estimated its depth to be 80 to 88 m the depth diameter ratio is much larger than for typical impact craters there are neither conspicuous deposits indicating volcanic eruptions from the hole nor are there pit craters adjacent to the hole that could be related to an underlying fault or dike the area around the hole is covered by a thin 20 to 25 m lava sheet which may help protect the lava tube from collapse due to meteorite bombardment

themis observes possible cave skylights on mars 1 seven possible skylight entrances into martian caves were observed on and around the flanks of arsia mons by the mars odyssey thermal emission imaging system themis distinct from impact craters collapse pits or any other surface feature on mars these candidates appear to be deep dark holes at visible wavelengths while infrared observations show their thermal behaviors to be consistent with subsurface materials diameters range from 100 m to 225 m and derived minimum depths range between 68 m and 130 m most candidates seem directly related to pit craters and may have formed in a similar manner with overhanging ceilings that remain intact

mapping the structure and depth of lava tubes using ground penetrating radar 1 the formation of lava tubes is one of the most significant factors controlling the emplacement of lava flows however extents and structures of lava tubes are typically not precisely known due to the difficulty in finding lava tubes in the field we developed a new stepped frequency ground penetrating radar gpr system with shielded antennas which allows measurements that have both high spatial resolution and large penetration depth we performed two types of measurements over an inactive lava flow and show that this method can easily detect the existence of a lava tube importantly phase reversals of the reflection signals can help identify reflections from a lava tube using these reflection patterns we estimate the vertical dimension and the depth of a lava tube at fuji volcano which are validated by survey measurements the presented method may be the most practical way to map terrestrial and perhaps extraterrestrial lava tubes

authigenic minerals from the paola ridge southern tyrrhenian sea evidences of episodic methane seepage abstract paola ridge along the nw calabrian margin southern tyrrhenian sea is one of the few reported deep sea sites of precipitation of authigenic carbonates in the tyrrhenian sea here the changing composition of the seeping fluids and the dynamic nature of the seepage induced the precipitation of pyrite siderite and other carbonate phases the occurrence of this array of authigenic precipitates is thought to be related to fluctuation of the sulfate methane transition zone smtz concretions of authigenic minerals formed in the near sub bottom sediments of the paola ridge were investigated for their geochemical and isotopic composition these concretions were collected in an area characterized by the presence of two alleged mud volcanoes and three mud diapirs the mud diapirs are dotted by pockmarks and dissected by normal faults and are known for having been a site of fluid seepage for at least the past 40 kyrs present day venting activity occurs alongside the two alleged mud volcanoes and is dominated by co2 rich discharging fluids this discover led us to question the hypothesis of the mud volcanoes and investigate the origin of the fluids in each different domed structure of the study area in this study we used stable isotopes carbon and oxygen of carbonates coupled with rare earth element ree composition of different carbonate and non carbonate phases for tracing fluid composition and early diagenesis of authigenic precipitates the analyses on authigenic precipitates were coupled with chemical investigation of venting gas and sea water authigenic calcite aragonite concretions from surficial sediments on diapiric structures have depleted 13c isotopic composition and slightly positive 18o values by contrast siderite concretions generally found within the first 6 m of sediments on the alleged mud volcanoes yielded positive 13c and 18o values the siderite ree pattern shows consistent lree light ree fractionation mree medium ree enrichment and positive gd and la anomalies as shown by the ree distribution the 13c depleted composition and their association with chemosymbiotic fauna calcite aragonite precipitated at time of moderate to high methane flux close to the seafloor under the influence of bottom seawater authigenic siderite on the other hand formed in the subseafloor during periods of lower gas discharges under prolonged anoxic conditions within sediments in equilibrium with 13c rich dissolved inorganic carbon dic and 18o rich water likely related to methanogenesis and intermittent venting of deep sourced co2

microbial influence on the oxygen isotopic composition of diagenetic siderite abstract numerous early diagenetic siderite concretions previously described have been interpreted as the results of microbially mediated reactions interpretation of oxygen isotope data for such material requires an understanding of the effect of temperature on fractionation processes however whilst equilibrium fractionation of oxygen isotopes between siderite and water has been measured down to 33 c extrapolation to lower temperatures may be invalid furthermore inorganic measurements may not be applicable to microbial systems the specific iron reducing microorganism geobacter metallireducens has been cultured anaerobically in the laboratory using acetate as an organic substrate and amorphous feooh as an electron acceptor the acetate is oxidised to co2 with concurrent iron reduction and extracellular siderite precipitation rhombohedral siderite crystals up to 25 m in size have been precipitated over a range of temperatures 18 40 c stable isotopic analysis of these crystals and the solutions from which they precipitate shows that this microbial precipitation of siderite has an associated isotopic fractionation different from the published equilibrium and which is not simply a function of temperature in all cases 18o values of siderite are lower than predicted by inorganic equilibrium fractionation data this may explain the numerous anomalously low 18o values reported for early diagenetic marine siderites and previously attributed to mechanisms such as mixing with meteoric water sediment water interaction recrystallisation or variable isotopic fractionation despite a lack of supporting evidence

oxygen isotope fractionation between calcite and fluid as a function of growth rate and temperature an in situ study the 18o content of carbonates expressed as 18o is widely used in paleothermometry yet it is clear from comparison of experimental results and natural carbonate samples that chemical isotopic equilibrium is not always achieved in nature this observation underscores the importance of exploring possible effects of growth rate on isotopic fractionation which is the focus of this study in situ secondary ion mass spectrometry sims analyses of 18o were performed on single crystals of experimentally grown calcite the change in growth rate v over time within each crystal was monitored by addition of multiple rare earth element ree spikes into the fluid from which the calcite grew the values of 18o in bulk calcites and experimental fluids were measured by stable isotope ratio mass spectrometers the average sims 18o overlaps with those determined by conventional mass spectrometry within analytical uncertainty the fractionation factor expressed as 18o approaches its equilibrium value in slowly grown calcites v 0 02 nm s and decreases by 1 5 with increasing growth rate from 10 3 to 0 34 nm s here 18o 103 ln 18o and 18o 18o 16o calcite 18o 16o fluid our results provide the first in situ evidence that under equilibrium conditions 18o may be depleted in the near surface region of calcite relative to the bulk crystal lattice and that this 18o depleted zone can be captured during rapid crystal growth crystal growth rate is therefore a potentially important consideration when using 18o in natural carbonates as a proxy for ocean and terrestrial temperature