global events across the mesoproterozoic neoproterozoic boundary c and sr isotopic evidence from siberia thick unmetamorphosed successions of siliciclastic and carbonate rocks in eastern and western siberia preserve a record of middle riphean to early upper riphean sedimentary environments and geochemistry consistent with data from other continents our studies in the uchur maya region in southeastern siberia and the turukhansk uplift in northwestern siberia suggest a first order shift in 13 c from values near 0 in the early mesoproterozoic to values near 3 5 after about 1300 ma over this same interval primary 87 sr 86 sr values decrease from 0 7060 to 0 7053 combining lithologic biostratigraphic and geochemical data sets with available geochronologic constraints we present a refined correlation between these two key proterozoic successions in siberia and add this dataset to a growing body of c and sr isotopic data from this time interval carbon isotope chemostratigraphy from these regions supports the occurrence and timing of a first order 3 5 positive shift ca 1250 1300 ma approximately coeval with the onset of active margin activity that predates the main phase of rodinia assembly sr isotopic data may also be interpreted within the context of the evolving mesoproterozoic tectonic regime available data suggest that no dramatic rise in 87 sr 86 sr heralds the main phase of rodinia assembly in the terminal mesoproterozoic suggesting that significant juvenile crust was involved in mountain building that relative hydrothermal flux from mid ocean ridges remained high throughout the assembly of rodinia and or that increased continental runoff related to intense erosion of grenvillian mountain belts terminated shortly after orogeny 2001 elsevier science b v all rights reserved

diagenetic history of the proterozoic carbonates and its role in the oil field development in the baikit anteclise southwestern siberia abstract the meso to neoproterozoic kamo group and ediacaran carbonate rocks of the baikit anteclise southwestern siberian platform russia were studied to reconstruct the history of development of the reservoir rocks and oil field the kamo group is composed of 3 5 to 4 km thick dolostone and rarely limestone succession that was deformed before the ediacaran time post depositional processes played the key role in forming the reservoir properties of the kamo group during the stage of deposition and subsidence 1500 720 ma silicification cementation and recrystallization caused development of a dense and low permeability matrix pronounced recrystallization led to changes in isotopic and geochemical characteristics of the primary sediments including increase in mn up to 0 093 and fe up to 3 11 contents and 87sr 86sr ratios up to 0 7239 decrease in sr concentrations down to 11 ppm and the development of flat ree normalized patterns folding and uplift in the baikit basin began about 720 ma ago and caused fracturing and subsequent healing with vein dolomite and calcite precipitated from hot 70 130 c and highly saline up to 28 7 wt fluids geochemical e g paas normalized patterns of ree and isotopic 87sr 86sr 18o and 13c signatures of host dolostones and crosscutting dolomite veins are very similar suggesting that dolomites within the veins formed through dissolution of the host rocks uplift caused subaerial erosion intense dissolution and karstification which penetrated down to 200 300 m below the surface folding uplift and peneplenization were important for porosity and permeability development in the dolostones of the kamo group the cap rocks were subsequently formed during the ediacaran and paleozoic sedimentation cycle the oil field developed in the paleozoic oil produced from the source rocks of the madra vedreshev and iremeken formations of the kamo group was dissipated during the uplift and erosion before the ediacaran sedimentation after deposition of the ediacaran and cambrian strata source rocks of the iremeken formation might have released oil when they passed the oil window but most of oil in the kuyumba field was derived from younger ediacaran source rocks and migrated through fault network from the flanks the kureika and cis sayan yenisey syneclises into the reservoir of the baikit anteclise that was a topographic high at that time

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

the 1996 ad delta collapse and large turbidite in lake brienz abstract in spring 1996 ad the occurrence of a large mass transport was detected by a series of events which happened in lake brienz switzerland turbidity increase and oxygen depletion in deep waters release of an old corpse into surface waters and occurrence of a small tsunami like wave this mass transport generated a large turbidite deposit which is studied here by combining high resolution seismic and sedimentary cores this turbidite deposit correlates to a prominent onlapping unit in the seismic record attaining a maximum of 90 cm in thickness it is longitudinally graded and thins out towards the end of the lake basin thickness distribution map shows that the turbidite extends over 8 5 km2 and has a total volume of 2 72 106 m3 which amounts to 8 7 yr of the lake s annual sediment input it consists of normally graded sand to silt sized sediment containing clasts of hemipelagic sediments topped by a thin white clay sized layer the source area the exact dating and the possible trigger of this turbidite deposit as well as its flow mechanism and ecological impact are presented along with environmental data river inflow wind and lake level measurements the combined results indicate that no particular cause i e earthquake explosion flood wind storm lake level change seiche or sediment dredging triggered the 1996 ad large turbidite deposit instead it was due to a delta slope failure most likely caused by normal sediment accumulation

the evolution of a middle precambrian dolostone sequence a spectrum of dolomitization regimes abstract most precambrian dolostones are considered here to represent replacement of original limestone or calcareous sediment and hence such dolostone sequences can be interpreted in much the same way as phanerozoic dolostone sequences the sequential development of dolomite is recorded in a middle precambrian carbonate clastic platform succession in the belcher islands the timing of dolomite formation ranges from penecontemporaneous eogenetic to late burial mesogenetic the platform sequence is divided into three distinctive megacycles 1 a siliciclastic dominated phase wherein pisolitic tufas formed in the upper reaches of a supratidal flat incipient penecontemporaneous dolomite formed in a setting somewhat analagous to the modern coorong alkaline lakes ii a mixed clastic carbonate and stromatolitic carbonate phase where penecontemporaneous dolomite formed under conditions of high evaporation rates and high salinity and is comparable to recent dolomite in sabkha environments and iii a stromatolite bui dup slope basin phase where incipient dolomitization may have taken place within a mixed seawater freshwater aquifer system however pervasive dolomitization of the platform sequence did not occur until a later stage of burial diagenesis and postdated the main stage of silica diagenesis recrystallization was a late mesogenetic event that resulted in only minor crystal enlargement stylolitization also produced local crystal growth but was not responsible for pervasive dolomitization fracture filling dolomite accumulated as a result of deformation and uplift during the hudsonian orogeny telogenesis

dolomitization and isotope stratigraphy of the vazante formation sao francisco basin brazil the vazante formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the vazante carbonates dolomites can be classified based on petrography and geochemistry into four generations ranging in crystal size between 4 m almost micritic and 3 mm and occurring as both replacements and cements the sr ca molar ratios calculated for the dolomitizing fluid 0 0006 0 0138 suggest a contribution from a non marine possibly meteoric water component the 18 oa nd 13 c values vary from 0 1 to 14 3 pdb and 0 2 to 2 3 pdb respectively fluid inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 c a mixing zone model of dolomitization for dolomites i iii satisfies the constraints from elemental chemistry 18 o 87 sr 86 sr and fluid inclusions the petrographic and chemical criteria of the latest generation dolomite iv reflect conditions of deep burial environment at temperatures above 120 130 c the 13 c values show only small variations at the base of the formation followed by a major negative plunge 4 at the top this plunge correlated with the sturtian glacial phase in contrast to c isotopes only few samples may have retained their near primary 87 sr 86 sr values of 0 70614 0 70734 the lowest 87 sr 86 sr value 0 70614 from fibrous cement in the upper part of the formation correlates with the negative 13 c shift and is also consistent with the sr isotope signature proposed for the glacial sturtian seawater the 34 s values obtained from sulfates trapped in carbonates range between 10 8 and 16 9 with a jump to 21 3 in the overlying formation all isotope signals are within the range suggested for the early neoproterozoic seawater but the precambrian baseline is poorly known and additional work is required to confirm this tentative assignment 2001 elsevier science b v all rights reserved

palaeoproterozoic evaporites in fennoscandia implications for seawater sulphate the rise of atmospheric oxygen and local amplification of the δ13c excursion this paper addresses global oxygenation and establishment of a marine sulphate reservoir in the palaeoproterozoic we report syn depositional marine anhydrite containing pseudomorphs after ca sulphates as widespread throughout the tulomozero formation in the se fennoscandian shield implying that surface waters were oxidized and a large so42 marine reservoir was developed as early as 2100 ma the ca sulphates and associated magnesite and halite precipitated syn depositionally from oxidized evolved and modified seawater in coastal playa sabkha and intertidal flat settings sr 87 sr 86 and delta 13 c of associated c 13 rich stromatolitic dolostones were environmentally controlled with the highest ratios occurring in playa and sabkha carbonates the results imply that the palaeoproterozoic delta 13 c carb excursion was amplified by 8 by local environmental factors and calls into question many observations of putative delta 13 c global signals reported previously from similar palaeoproterozoic evaporitic dolostones the local environmental amplification can explain a large regional and intercontinental delta 13 c discrepancy observed in synchronous carbonates

evidence for paleoproterozoic cap carbonates in north america the early paleoproterozoic snowy pass supergroup of the medicine bow mountains and sierra madre wyoming usa and huronian supergroup ontario canada were deposited along the present day southern flank of the wyoming and superior cratons whereas three discrete levels of glacial diamictite are developed in both successions carbonate strata are known only directly above the middle diamictite vagner and bottle creek formations in medicine bow mountains and sierra madre respectively and espanola formation in southern ontario in these thick correlative siliciclastics dominated strata the carbonates from each succession record negative 13 c values 4 0 to 0 8 v pdb and attenuated carbon isotopic difference between organic and inorganic phases oxygen in carbonates is strongly depleted in 18 o suggesting exchange with hot fluids which is consistent with pervasive recrystallization of carbonates and remobilization of elements however the stratigraphic coherence of carbon isotopic compositions and the general lack of correlation between 13 c and either 18 o values or trace element concentrations supports a primary origin for 13 c depleted carbonates which are interpreted here to reflect anomalous oceanic compositions the intimate association of thick carbonate units containing abundant carbonate debris flows with immediately underlying glacial strata indicates that chemical precipitation resulted from a rapid flux of carbonate alkalinity onto ocean margins during post glacial transgression although these early paleoproterozoic carbonates are similar to neoproterozoic cap dolomites in stratigraphic position and carbon isotopic compositions the older post glacial accumulations begin with limestone and lack many of the sedimentary structures typical of neoproterozoic deposits furthermore it is not understood why carbonates only occur above the middle of the three glacial horizons whereas these deposits are ubiquitous above neoproterozoic diamictites the differences might reflect lower overall carbonate saturation in early paleoproterozoic oceans which contrasts sharply with archean and later paleoproterozoic intervals and higher siliciclastic inputs in rift environments which shut down carbonate deposition geological and geochemical indicators suggest a stepwise increase in atmospheric oxygen across the paleoproterozoic glacial epoch the tempo and mode of atmospheric oxygen rise has significant consequences for the abundance of the important greenhouse gases ch4 and co2 and hence for oceanic acidity if we accept that atmospheric oxidation of methane to carbon

the structure and dynamics of internal waves in baldeggersee1 analysis of long records of temperature depth distribution determined at lo or 20 min intervals at several stations and extending over three stratification seasons provides a detailed picture of the characteristics dynamics and seasonal variation in activity of internal waves in a small lake baldeggersee switzerland wind stress in general was low with occasional bursts of higher energy the ensuing slightly damped internal seiches govern the dynamics of the stratified lake the vertical structure displays a dominant response of the first vertical mode responses in the second vertical mode hitherto rarely recorded are also demonstrated with the help of spectral analysis and models the horizontal structure of the seiches motion displays dominance and persistence of the fundamental basin mode spectral analysis also discloses responses of the second and third basin modes multiplying of the spectra with the local buoyancy frequency leads to a collapse of all spectra into a narrow band pointing toward the possibility of a universal spectrum new light is shed on apparently common examples of waveform asymmetry and on their interpretation it is concluded that in this small basin low energy example internal waves provide motions at all levels but little vertical mixing

temporal constraints on the paleoproterozoic lomagundi jatuli carbon isotopic event the paleoproterozoic lomagundi jatuli positive 13c excursion in sedimentary carbonates represents an event whose magnitude and duration is unique in earth history although precise absolute chronology of this event remains poorly constrained in northeastern fennoscandia an 1300 m thick sedimentary volcanic succession of the pechenga greenstone belt records decline of this isotopic excursion zircons from sedimentary rocks that occur within the decline have yielded 207pb 206pb dates at 2058 2 ma 6 ma including u decay constant uncertainties and provide the first maximum age constraint on the termination of the lomagundi jatuli event combined with existing constraints these data indicate an 140 m y interval characterized by 13c rich carbonate accumulation

a widespread positive 13ccarbanomaly at around 2 33 2 06 ga on the fennoscandian shield a paradox carbon isotope measurements carried out on 201 carbonate samples from the early proterozoic of the kola peninsula n karelia and norway yield 13c pdb spanning 20 5 to 11 a general 13c secular trend shows that prior to 2 33 ga values are typically normal marine averaging around 3 0 between 2 33 and 2 06 ga in jatulian time there follows a rapid excursion to positive 13c of around 6 post jatulian time is characterized by 13c of sedimentary carbonates fluctuating between 5 and 3 also it is remarkable for the first pronounced development of diagenetic carbonates which have 13c between 14 and 6 the c 6 positive 13c shift with a duration of about 270 myr coincides with a maximum in the diversity and abundance of stromatolites and with widespread development of red beds but does not coincide with the maximum of buried corg mass the fennoscandian shield represents the largest isotoically anomalous carbonate province yet reported and the positive 13c excursion together with a series of major global palaeoenviromental changes seems to be more intense than the precambrian cambrian transition events however it is still not clear what kind of mechanism this phenomenon could be attributed to an increase of the ronov ratio and or broecker ratio and other possible models are discussed as the target for future investigations

inertial oscillations and related internal beat pulsations and surges in lakes michigan and ontario records of water temperature current and wind made during campaigns on lakes michigan 1963 and ontario 1972 are searched for inertial responses to wind action i circle tracking currents and ii isotherm depth undulations i e long internal wave manifestations of forcing by wind with stratification absent in winter only response i is seen and then only rarely with periods very close to the local inertial period tin after whole basin stratification is complete both responses i and ii occur frequently as internal kelvin waves shore trapped and not further treated here and cross basin poincare internal seiche modes mode periods range between 1 and 15 less than tin depending on which of the here modeled modes have responded and on differences between modes in their partitioning of kinetic and potential energy when in both lakes short duration wind impulses were followed by a week or more of relative calm poincare mode combinations produced beat pulsations in both responses i and ii diagnostic features of which sometimes permitted participating modes to be identified at a nearshore downwelled front in lake ontario another type of poststorm adjustment was seen periodically released from the front internal surges migrated across the basin through fields of inertially rotating response i currents

fluid rock interaction history during stabilization of early dolomites upper knox group lower ordovician u s appalachians abstract the lower ordovician upper knox group is characterized by shallowing upward peritidal cycles that are extensively dolomitized to form massive dolomite most 85 replacive dolomite is stratigraphically controlled composed of very fine to medium crystalline planar to nonplanar early dolomite composed of zones 1 2 and 3 dolomites remaining dolomite occurs as medium to coarsely crystalline nonplanar late replacive dolomite zones 2 and 3 late dolomites that selectively replaces limestone associated with stylolites and fractures or forms massive dolomite fronts complexly zoned zone 2 to 6 saddle dolomite cements make up less than 5 of all dolomite and primarily fill solution voids and fractures early dolomites are near stoichiometric and relative to early ordovician marin carbonates are similar to or significantly depleted in 18o and sr slightly to significantly enriched in mn and fe and have similar 87sr 86sr values relative to early dolomites late zones 2 and 3 dolomites are depleted in 18o and st enriched in mn and fe and have similar 87sr 86sr values early dolomites are interpreted to have formed from modified seawater during seaward progradation of tidal flats within each high frequency fifth and fourth order cycle duration late dolomites based on their texture and geochemistry formed as a result of recrystallization of early dolomites as direct replacement of limestone at elevated temperatures and as cement in fractures and secondary solution voids covariant trends between textures and geochemical compositions for early dolomite indicate that presentday compositions of knox early dolomite record a history of progressive diagenetic modification stabilization or recrystallization involving varying degrees of changes of textural and geochemical compositions during multiple episodes of dolomitization early dolomites wit the most extensively altered textures are more stoichiometric more depleted in 18o and sr and more enriched in mn and fe relative to texturally less altered early dolomite initial stabilization may have occurred syndepositionally in modified seawater continued stabilization of early dolomite likely occurred in fresh or mixed waters of a middle ordovician meteoric aquifer associated with knox unconformity development final stabilization occurred by deep burial brines at elevated temperatures as indicated by 1 extensive replacement and overgrowth of early dolomite by late dolomites 2 a progressive increase in crystal size and abundance of nonplanar crystal boundaries with increase in replacement by late dolomites and 3 the similarity of the geochemical compositions of texturally most extensively altered early dolomites and late dolomites

sedimentary and volcanic facies recording the neoarchaean continent breakup and decline of the positive δc excursion abstract the palaeoproterozoic kalix greenstone belt at the northern end of the bothnian bay in sweden comprises three groups the lower group a ca 3000 m thick unit of subaerially erupted tholeiitic intraplate basalts interbedded with fluviatile conglomerates represents the volcano sedimentary succession deposited in a rift setting these units are truncated by a break up unconformity above which is the middle group an 800 m thick succession of dolograinstones stromatolitic dolostones arenites volcanoclastic and mafic volcanic rocks stromatolite morphologies and the occurrence of distinctive sedimentary structures including planar and herringbone cross bedding hummocky cross stratification symmetrical and asymmetrical ripples desiccation cracks and tepees indicate markedly variable depositional environments associated with transition from a marine influenced rifted margin to a rimmed carbonate shelf sedimentation was accompanied by subaerial mafic volcanism and syndepositional tectonics the upper group a more than 2000 m thick unit of deep water shales was deposited on the drowned carbonate platform of the middle group in response to tectonically enhanced subsidence this study provides a detailed palaeoproterozoic depositional history of the fennoscandian shield from rifting to passive margin development associated with both dispersal of the neoarchaean supercontinent and decline of the proterozoic positive isotopic excursion of 13 c 12 c in sedimentary carbonates in contrast to many previously reported 13 c rich palaeoproterozoic carbonates which were deposited in non marine to restricted or partly restricted settings the carbonate facies reported herein were well connected to the oceanic realm and thereby represent a good opportunity to test restricted marine versus open marine models for how the carbon cycle functioned in deep time

transvaal supergroup carbonates implications for paleoproterozoic δ18o and δ13c records abstract the paleoproterozoic interval is characterized by the first evidence for incontrovertibly free oxygen in the ocean atmosphere system in this study carbonates from the transvaal group the silverton lucknow rooinekke and particularly the duitschland formations were analyzed for isotopes of carbon oxygen and strontium in order to constrain models dealing with the patterns of seawater evolution although the trace element data showed variable alteration of the carbonate samples selected samples with a lesser degree of alteration yielded interpretable results the best measured 87sr 86sr isotope value of 0 7031 at 2 35 ga ago fits well with the published curve for precambrian seawater confirming that paleoproterozoic budget of sr was dominated by input from continental sources during the paleoproterozoic times carbonates with 13c values of about 10 are known to have been present worldwide and they have been studied also during this research the observation that in the duitschland formation such high isotope values and normal marine values were both present within a single formation indeed within its single beds suggested that the large spread of 13c values reflects local or regional rather than global phenomena the oxygen isotope data show a large spread of values from 3 to 16 while their corresponding 13c signals are almost unchanged over this entire range with the lucknow and silverton formations clustering around 10 and the rooinekke formation around 2 the duitschland formation has a bimodal distribution with both 13c populations present the last formation also contains samples with 18o as low as 20 particularly in the vicinity of the bushveld intrusive body these highly 18o depleted samples show homogenization of 13c values towards 5 modelling suggests that the trends could have resulted from post depositional alteration by basinal fluids at near surface to about 50 c temperatures in a rock buffered system the duitschland formation samples particularly at duitschland farm the locality closest to the bushveld complex were then further altered at temperatures up to about 80 c in response to this intrusion

high δ13c paleoproterozoic carbonates from the transvaal supergroup south africa marbles from the transvaal supergroup kaapvaal craton south africa record two positive carbon isotope excursions to average 13 c carbonate values of 6 to 8 relative to v pdb separated by carbonates with normal marine 13 c values 0 2 these elevated values have been preserved through metamorphic devolatilization together with an already documented high 13 c excursion in overlying rocks three separate positive carbon isotope excursions occurred on the kaapvaal craton in the interval 2 43 to 1 93 ga it is not yet clear how many of these excursions reflect global rather than local processes

criteria for the retention of original seawater 87sr86sr in ancient shelf limestones abstract a suite of 40 pennsylvanian and permian shelf limestones from kansas and oklahoma has been analyzed for major and trace elements and for c o and sr isotopes the chemistry and c and o isotope ratios were compared to the difference between the measured 87 sr 86 sr and original seawater ratio concentrations of mn sr and fe varied most systematically with the change in the seawater sr ratio all samples containing 700 ppm mn yielded altered ratios fe and sr contents were also found to have a less well defined correlation with the retention of the original sr ratio all limestones with 900 ppm sr and 20 of 22 samples with a sr mn 2 0 yielded correct ratios the geochemical guidelines were applied to the chemical and isotopic results from 76 shelf limestones mostly of pennsylvanian age from texas and oklahoma more than 92 of the samples that contain sr mn 2 0 yielded correct seawater ratios every sample with sr mn 2 0 retained the original seawater ratio

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

surface manifestations of internal oscillations in a highly saline lake the dead sea 1 the manifestation of internal oscillations at the surface of the dead sea during summer 198 1 is described during that time the dead sea was highly stratified with a density difference of 1 7 x 1o 2 g liter l between its two layers measurements of lake level limnograms show oscillations with periods of internal seiches as confirmed by thermistor chain measurements as well as by a numerical model of internal oscillations wind measurements exhibit predominantly diurnal and semidiurnal periods during autumn 1984 the dead sea turned over and its layered structure thereby destroyed in contrast to the measurements of summer 198 1 the limnograms for this timespan show only oscillations associated with periods corresponding to surface seiches but no sign of internal oscillations

correlated geophysical geochemical and volcanological manifestations of plume ridge interaction along the galapagos spreading center 1 as the galapagos hot spot is approached from the west along the galapagos spreading center there are systematic increases in crustal thickness and in the k ti nb zr 3he 4he h2o and na2o content of lavas recovered from the spreading axis these increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth residual mantle bouguer gravity anomaly magma chamber depth average lava mg ca al ratio and the frequency of point fed versus fissure fed volcanism magma chamber depth and axial morphology display a threshold effect in which small changes in magma supply result in large changes in these variables these correlated variations in geophysical geochemical and volcanological manifestations of plume ridge interaction along the western galapagos spreading center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes and the consequences of these variations on magma supply axial thermal structure basalt chemistry and styles of volcanism

dolomitization of a carbonate platform during late burial lower to middle cambrian shady dolomite virginia appalachians abstract outer platform carbonates of the lower to middle cambrian shady dolomite virginia appalachians record a lengthy and complex diagenetic history ranging from marine cementation to burial diagenesis during late paleozoic overthrusting analysis of petrographically well preserved marine cements has provided an estimate of the isotopic composition of lower cambrian marine carbonate 18o 7 5 to 6 1 pdb 13c 0 2 to 0 8 pdb 87sr 86sr 0 70869 to 0 70975 establishing a base line with which to assess later diagenetic carbonate meteoric dissolution and early calcite cement postdate marine cementation and formed during subaerial exposure the platform margin carbonates were extensively dolomitized dolomitization postdates calcite cementation and the initial development of stylolites and tectonic fractures several episodes of dolomite replacement and cementation are evident zone 1 dolomite consists of rare relict dolomite cores which were replaced and overgrown by zone 2a dolomite the dominant replacement phase zone 2a dolomite is depleted in 18o 18o 10 2 to 7 0 and enriched in 13c 13c 1 0 to 1 6 relative to marine carbonate reflecting precipitation from warm formation fluids buffered by the host carbonate uniformly low sr contents 25 to 65 ppm and nonradiogenic 87sr 86sr 0 70900 to 0 70971 also attest to pore water equilibration with the host rock replacement dolomitization was followed by widespread carbonate dissolution which formed vuggy porosity and local solution breccias the host dolomite locally is encrusted by mississippi valley type mvt occurrences of ore mineralization consisting of sphalerite galena and pyrite this major generation of ore minerals is overgrown by three generations of dolomite cement the first dolomite cement zone 2b has similar 18o composition 13o 10 3 to 7 4 but slightly depleted 13c values 13c 0 4 to 1 1 relative to zone 2a dolomite and has similar sr contents 30 to 45 ppm and 87sr 86sr ratios 0 70933 to 0 70966 zone 2b dolomite cement exhibits petrographic isotopic and geochemical affinities with zone 2a replacement dolomite implying that the two dolomite types formed from fluids of similar composition and temperature primary fluid inclusions in zone 2b dolomite indicate precipitation from warm minimum 120 to 150 c brines 23 26 wt nacl equiv these temperatures imply that the dolomitizing fluid was enriched in 18o due to extensive water rock interaction with host carbonate which also buffered the fluid 13c sr and 87sr 86sr content zones 3 and 4 dolomite cement are depleted in 18o 13o 13 8 to 11 3 relative to the previous dolomites implying a temperature increase to 150 to 210 c the late dolomite cements have 13c compositions 13c 0 7 to 0 9 ranging from those similar to the earlier dolomites to more depleted values zones 3 and 4 dolomite cement are enriched in sr 60 to 135 ppm which is radiogenic 87sr 86sr 0 71025 to 0 71445 these later cements thus record fluids that were no longer buffered by the host carbonate dolomite cements locally were overgrown by sphalerite followed by authigenic quartz and calcite cement secondary three phase fluid inclusions in quartz and calcite indicate hot 175 to 225 c saline 30 to 33 wt nacl equiv fluids temperatures and pressures interpreted from the fluid inclusions are compatible with estimated burial depths in excess of 5 km the dolomites formed during deep burial coeval with late paleozoic alleghanian age thrusting the mg required for burial dolomitization was derived from pressure solution of structurally lower platform interior dolomites that were overridden by the shady dolomite thrust sheet tectonic uplift and deformation resulted in regional fluid flow which transported this mg to the shady dolomite outer platform during burial dolomitization

forced resonant oscillations as a response to periodic winds in a stratified reservoir the response of the sau reservoir to a wind field characterized by having marked periodicities of 12 h and 24 h has been studied measurements of temperature with a thermistor string and currents with an acoustic doppler current meter show that the reservoir also responds with the same water periodicities during certain times of the stratified period some of the natural oscillation modes of the reservoir are close to these forcing wind periods in particular in mid july the vertical mode v2 is close to 12 h and in mid to end of september the vertical mode v3 is close to 24 h in these situations these modes are selected out of the spectrum of possible internal waves and the reservoir behaves as a forced oscillator in resonance with the wind the structure and the period of these vertical modes have been elucidated by using the 3d model elcom both modes are affected by the earth s rotation at the widest part of the reservoir

high frequency internal waves in lake geneva trains of high frequency internal waves have been observed in the thermocline of lake geneva within a few hours after the passing of disturbances which cause a rapid increase or jump in the depth of the thermocline three possible mechanisms for the source of the internal waves are proposed and compared with available data i that the waves are a soliton packet following the jump ii that the waves are generated as the jump passes over or around a local region of rough topography and iii that the jump produces a moving disturbance near the shoreline with internal waves forming a wake behind it the investigation of the last possibility involves the solution of the problem of the form of an internal wave wake in the presence of mean currents which vary in depth both in their strength and direction the wave pattern of the wake in the observed current distribution and stratification of the lake is described and compared with that in the absence of shear an expression for the energy of internal waves propagating in a shear flow is also derived and it is found that the rate of energy loss from the jump to the waves is small the minimum richardson number resulting from the co existence of the mean shear and the internal waves is marginally greater than 1 4 suggesting that the waves may be limited by shear flow instability however none of the mechanisms for their generation and propagation is fully compatible with the available data

near inertial waves in lake erie near inertial poincare waves with a period t p similar to 17 h are the dominant wind induced internal wave motions in central lake erie and consequently have a substantial influence on lake circulation mixing and biogeochemistry however due to the complex three basin bathymetry in lake erie the vertical and horizontal modal structure of these waves remain poorly understood in this study we analyze field data to show wind events energize frequent vertical mode one poincare waves the horizontal modal structure was also investigated in a sensitivity analysis using a calibrated three dimensional hydrodynamic transport model forced with observed and idealized spatially uniform wind events strong horizontal mode one poincare wave cells form in both the central and eastern basins when wind events have a duration of 0 25 t p to 0 5 t p are impulsive and periodic at t p or have anticyclonic rotation with a duration of t p momentum transfer from longer wind events 0 5 tp will oppose the coriolis force rotated currents and damp poincare wave generation in agreement with theory the most efficient wind events are observed and computationally modeled to have a duration of 0 25 t p causing an excitation peak at similar to 0 4 t p and converting similar to 0 8 of the wind energy input to poincare waves the efficiency of wind work in generating poincare wave kinetic energy is given by 1 cos 2 pi f t t 1 where f is the inertial frequency and t is the wind duration therefore the efficiency peaks during each nt p period where n is a non negative integer and decreases significantly for longer wind events

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

depositional environments and an apparent age for the geci meta limestones constraints on the geological history of northern mozambique strongly contrasting rocks were juxtaposed during the long tectonometamorphic history of the mozambique orogenic belt in northern mozambique the latest depositional event was expressed by accumulation of marine sediments which were subsequently mildly metamorphosed and tectonically juxtaposed with granulite facies complexes the low grade metasedimentary rocks comprise the geci group which was mapped and studied petrographically geochemically and isotopically in order to provide constraints on the depositional environments age and latest history of the mozambique orogenic belt the group occurs as several large tectonically dissected intensively sheared folded and mylonitised sw ne trending lenses within unango complex granulite rocks in places primary depositional features are well preserved the dominant rocks are calcarenites dolarenites calcite matrix supported and dolostone clast supported carbonate breccias forming beds with erosional bases normal and reverse graded bedding and well developed bouma sequences dolomicritic microbial and oolitic dolostone clasts were apparently derived from the margin of a shallow water carbonate platform and redeposited by turbidity currents on a continental slope with calcareous sedimentation the geci meta carbonate rocks have low sio2 and al2o3 contents mgo cao ratios fluctuate between 0 05 and 0 70 averaging 0 15 0 18 1 n 111 acid soluble constituents have moderate concentrations of fe 777 310 ppm mn 131 85 ppm and sr 566 145 mn sr ratios are relatively low 0 26 0 19 13c and 18o values are invariably high with only moderate scatter 4 0 0 6 v pdb and 25 8 0 5 v smow respectively the least altered 87sr 86sr ratio of calcite is 0 70708 whereas dolomite is enriched in 87sr with the lowest ratio of 0 70730 the least altered 87sr 86sr and 13c ratios suggest an apparent depositional age of either 595 585 or 630 625 ma this provides a lower age limit for juxtaposition of the low grade geci group rocks and granulite facies rocks of the unango complex

holocene dolomitization of calcium carbonate sediments from the coastal sabkhas of abu dhabi u a e a stable isotope study secondary dolomitization of holocene calcium carbonate sediments occurs beneath the coastal sabkhas of abu dhabi u a e a stable isotope study of the sabhka sediments and brines indicates that the

two billion year old evaporites capture earth s great oxidation major changes in atmospheric and ocean chemistry occurred in the paleoproterozoic era 2 5 1 6 billion years ago increasing oxidation dramatically changed earth s surface but few quantitative constraints exist on this important transition this study describes the sedimentology mineralogy and geochemistry of a two billion year old and 800 m thick evaporite succession from the onega basin in russian karelia the deposit consists of a basal unit dominated by halite 100 m followed by anhydrite magnesite 500 m and dolomite magnesite 200 m dominated units the evaporite minerals provide a robust constraint that marine sulfate concentrations were at least 10 mmol kg representing an oxidant reservoir equivalent to over 20 of the modern ocean atmosphere oxidizing capacity these results show that substantial amounts of surface oxidant accumulated during this critical transition in earth s oxygenation

coastal sabkha dolomites and calcitised sulphates preserving the lomagundi jatuli carbon isotope signal palaeoproterozoic tulomozero formation carbonates of the onega basin were targeted by icdp far deep hole 10b the recovered rocks contain abundant evidence for palaeoproterozoic gypsum and anhydrite particularly in the form of pseudomorphs of inclusion containing nodules swallow tail twinned crystals and evaporite dissolution breccias similarly sand patch fabric indicates the former presence of surface covering salt crusts in the palaeoproterozoic atmospheric oxygen and seawater sulphate levels at that time must have been sufficient to allow such sulphates to form carbonate 13c values are considered sedimentary or early diagenetic predating greenschist facies metamorphism associated with the svecofennian orogeny these carbonate 13c values range from 7 7 to 15 7 with the highest and lowest values found in dolostones the dolostones as well as calcitised calcium sulphates record the lomagundi jatuli signal however evidence for activity of sulphate reducing and or methanogenic bacteria seems to be lacking dolomite precipitation or dolomitisation calcium sulphate calcitisation and production of the high 13c values processes which typically involve such bacteria in the phanerozoic are thus inferred to have proceeded without the direct influence of sulphate reducers and methanogens the most plausible explanation for the lomagundi jatuli excursion seems to remain significant sequestration of organic carbon in a location yet to be identified

internal wave weather in a stratified lake thermistor chain data collected in lake biwa japan during a period of strong stratification were used to determine the nature of the water column response to wind forcing the evolution of the basin scale waves in the lake were reproduced in three dimensional numerical simulations and the time variability of the internal wave spectrum is discussed based on spectral analysis of the thermistor chain records severe winds associated with the passage of three typhoons during the sampling period excited basin scale kelvin and poincare waves which by interaction with the topography of the lake and non linear effects generate trains of high frequency internal waves time frequency analysis of the temperature signals suggests that the 2 slope of the internal wave spectrum is probably the result of the composition of groups of waves with high intermittence travelling through the wave guide the observed rapid decay of internal wave energy after severe wind events seemed to be due mostly to high dissipation at the sloping boundaries of the lake

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

carbon isotope chemostratigraphy of the middle riphean type section avzyan formation southern urals russia signal recovery in a fold and thrust belt abstract the avzyan formation of the southern ural mountains russia forms part of the middle riphean 1300 1000 ma type section its age is constrained to be younger than 1348 ma and older than 1080 ma and thus may preserve all or part of the mid mesoproterozoic isotopic shift this shift represents a significant reorganization of the carbon cycle and separates pre 1300 ma carbonates with 13c near 0 from post 1200 ma mesoproterozoic successions having average 13c values greater than 3 5 because the avzyan formation is a significant reference section for the mesoproterozoic and because it has the potential to record this mid mesoproterozoic shift a chemostratigraphic profile was constructed for this strongly folded and thrust faulted succession best preserved 13c values from carbonates of the avzyan formation lie mainly between 0 and 2 but the unit contains a single excursion represented by several data points to values greater than 4 the chemostratigraphy of the avzyan formation does not provide a definitive match to a better dated succession however the character of the carbon isotopic curve is most similar to that of the dismal lakes group arctic canada suggesting that the avzyan formation was deposited during the mid mesoproterozoic isotopic shift and may have an age near 1270 ma best preserved sr isotopic values of 0 70587 are also consistent with a later mesoproterozoic age

geochemical preservation potential of high grade calcite marble versus dolomite marble implication for isotope chemostratigraphy the neoproterozoic hekkelstrand marble from the north central norwegian catedonides contains a calcite marble unit hosting a commercially exploited white dolomite marble deposit which underwent polyphase amphibolite facies scandian metamorphism major and trace element geochemistry delta 18 o delta 13 c and sr 87 sr 86 ratios have been used to define isotopic differences between interbedded calcite and dolomite marbles and to assess their post depositional isotopic resetting a high strontium concentration in the calcite marble up to 1200 ppm suggests an aragonitic lime mudstone precursor and its least altered delta 13 c 4 parts per thousand v pdb delta 18 o 27 parts per thousand v smow and sr 87 sr 86 0 70615 to 0 70624 ratios are consistent with 700 600 ma seawater the dolomite marbles contain minor relics of the calcite aragonite precursor despite pervasive dolomitisation the average mn 44 ppm and fe 1041 ppm contents of the dolomite marbles are comparable with those measured in the host calcite marbles and much lower than those reported from non metamorphosed dolostones of a similar neoproterozoic age the dolomite marbles have an average sr content of 115 ppm an average mg ca ratio of 0 56 is lower than that for stoichiometric dolomite 0 62 and is controlled mainly by remnant inclusions of precursor calcite sr 87 sr 86 ratios range between 0 70725 and 0 71034 the least altered delta 13 c of 6 parts per thousand and delta 18 o of 30 parts per thousand suggest a high degree of isotopic preservation of the dolomite the dolomite marbles are isotopically heavier both delta 18 o and delta 13 c by 2 parts per thousand on average compared with the calcite marble thus suggesting a pre metamorphic lower temperature isotopic equilibrium the dolomite was apparently originally precipitated from 600 to 700 ma seawater dominated fluids the sr 87 sr 86 ratio was modified by post depositional processes the high delta 13 c and delta 18 o values and the low mn content of the bulk dolomite marbles are not consistent with common meteoric diagenesis or metamorphic alteration the highest sr 87 sr 86 ratio of 0 70868 suggests that the earlier dolomite was apparently modified by ca 560 ma seawater or by meteoric water considerably buffered by ca 560 ma limestones the most significant geochemical and isotopic modification of the dolomite took place during metamorphism this has been observed across the dolomite marble silicate rock contacts over a limited distance of lt 0 5 in where c and 0 isotope systems were reset by up to 4 5 parts per thousand and 9 7 parts per thousand respectively the sr 87 sr 86 ratio increased by up to 0 0012 whereas the fe mn and sr concentrations have increased by factors of similar to 80 similar to 15 and similar to 1 5 respectively the dolomite marbles despite amphibolitc facies polyphase metamorphic alterations have preserved high delta 18 o and delta 13 c values which can be utilised for chronostratigraphic purposes whereas sr 87 sr 86 ratios provide insights into the composition of the post depositional fluids

dissolution methods for strontium isotope stratigraphy guidelines for the use of bulk carbonate and phosphorite rocks diverse carbonate dissolution methods have been applied to bulk carbonate rocks in order to target least altered components for strontium isotope stratigraphy sis this is especially important for precambrian and cambrian studies for which no suitable skeletal material is available one proven method for bulk limestones involves the removal of up to a third of the powdered sample using an acid pre leach before partial dissolution of the rest of the sample using a weak acid or acid buffer solution we applied a similar technique to dissolve a range of lithologies limestone dolostone and phosphorite and compared the strontium isotopic composition of various leaches to contemporaneous seawater our results vindicate this approach and allow us to conclude that some dolomitic and phosphatic rock components may retain a near primary seawater 87sr 86sr composition once contaminant strontium from secondary calcite and other phases has been removed commonly applied trace element e g mn sr mg ca and isotopic c o screening cut offs were also examined in the light of these results and proved unreliable particularly in the case of dolostones and partially dolomitized limestones we recommend that rigid sample selection and sequential leaching procedures be applied to all sis studies of bulk materials and propose a general protocol for strontium isotope studies on marine authigenic rocks which may also be applicable to rare earth element and other geochemical studies of marine authigenic minerals in the future

two billion year old isotopically heavy carbon evidence from the labrador trough canada forty eight samples of bulk carbonates were studied from the dunphy portage alder uve denault and abner formations and the undivided pistolet subgroup in the labrador trough quebec canada the carbonate units occur as a number of thick beds up to 300 m within 100 600 m thick formations they are interbedded with siliciclastic sediments and are composed of crystalline sparry micritic allochemical and stromatolitic dolostones the carbonate rocks are recrystallized and metamorphosed up to amphibolite grade they are chemically dolostones and include chemical or biochemical laminated and massive dolostones dolorudites dolarenites and dololitites which were deposited in shallow water dunphy denault and abner formations to relatively deep water uve formation marine environments reef facies are present in the denault and abner formations the dunphy portage alder and uve formations deposited between 2 17 and 2 14 ga yield isotopically heavy values of 13c pdb ranging from 5 3 to

chemostratigraphy of the paleoproterozoic duitschland formation south africa implications for coupled climate change and carbon cycling the paleoproterozoic duitschland formation lies stratigraphically beneath the timeball hill formation which contains the only unequivocal glacial unit of this era in the transvaal basin south africa lithologic evidence in paleoproterozoic successions of north america however indicates the existence of three discrete and potentially global ice ages within this 300 my interval carbonates of the duitschland formation are significantly enriched in 13c up to 10 1 permil in the upper part of the succession above a notable sequence boundary in contrast the lower part of this unit contains carbonates with consistently negative 13c values trace and major element compositions of these carbonates as well as carbon isotopic compositions of coexisting organic matter support a primary origin for the markedly positive carbon isotope anomaly the stratigraphic constraints indicate that 13c enriched carbonates were deposited prior to paleoproterozoic glaciation in southern africa similar to carbonates stratigraphically beneath neoproterozoic glacial diamictites worldwide also mirroring the neoproterozoic record are strongly negative 13c values in cap carbonates atop glacial diamictites in paleoproterozoic strata of wyoming and ontario the litho and chemostratigraphic constraints indicate that the interval of negative carbon isotope values in well preserved carbonates of the lower duitschland formation may reflect a second paleoproterozoic ice age in the transvaal succession this interpretation is further supported by recently discovered bullet shaped clasts with striations in diamictite from the basal part of the succession thus the emerging temporal pattern of carbon isotope variations and glaciation in the paleoproterozoic has a close analogue to neoproterozoic events suggesting a coupling of climatic and biogeochemical changes at both ends of the eon

aphebian mafic ultramafic magmatism in the labrador trough new quebec its age and the nature of its mantle source the magmatic events occurring within the two main cycles in the south central part of the labrador trough new quebec have been dated in cycle 1 a granophyre dike related to the cramolet lake gabbro sill which intrudes the seward subgroup has a u pb zircon age of 2169 2 ma in cycle 2 the tholeiitic basalts of the willbob hellancourt formation and the related mafic ultramafic sills are dated at ca 1900 ma by the pb pb method these data confirm the existence of at least two main magmatic cycles separated by about 270 ma the magma source was depleted upper mantle and the magma did not experience any significant crustal contamination as indicated by the 1 ratio 7 9 and 4 for the tholeiitic basalts the 1 value for the intrusive rocks 8 03 and the average value for the gabbroic rocks of cycle 2 2 8 and for the granophyre of cycle 1 1 05 could be the result of slight crustal contamination or of mantle heterogeneity whatever the cause of these values the data indicate the prolong

interbasin exchange and mixing in the hypolimnion of a large lake the role of long internal waves we conducted a combined field and numerical study of the effects of episodic internal kelvin type waves on bottom boundary turbulence and the exchange of a passive tracer between the main basin and the side basin of a large lake lake geneva high resolution measurements of the vertical current structure near the entrance of the 25 km long and 70 m deep side basin revealed that hypolimnetic current speeds frequently exceed 0 2 m s 21 leading to a turbulent bottom boundary layer several meters thick with logarithmic current profiles and to a region of strong shear across the thermocline the time series and vertical structure of the currents were reproduced by a threedimensional numerical model of the lake it was demonstrated that after episodes of strong winds from the northeast and southwest exchange flows due to internal kelvin waves were able to temporarily half or double the hypolimnetic volume of the side basin leading to an irreversible exchange of up to 40 of the hypolimnetic water in the side basin of lake geneva within only a few wave cycles with the help of the numerical model it was shown that the key mechanisms of exchange are horizontal dispersion by resolved scales and to a small extent shear dispersion in the bottom boundary layer it is suggested that bottom boundary layer turbulence and the interbasin exchange can explain the structural differences in the oxygen profiles observed in the side basin and the main basin respectively

geochemistry of precambrian carbonates vii belt supergroup montana and idaho usa carbonates from the 1100 1450 ma old proterozoic belt supergroup were collected from stratigraphic sections throughout montana and idaho usa the sampled sequences in ascending stratigraphic order include the newland altyn spokane greyson transition empire wallace helena siyeh snowslip shepard and libby formations an increase in the degree of postdepositional alteration of belt limestones is reflected in a diminution of sr and mg contents an increase in mn and depletion in 13c and 18o two diagenetic trends can be resolved for the limestones one affecting the presumed originally aragonite rich sediments includes carbonates from the lower belt newland formation in contrast the middle belt carbonate wallace helena siyeh formations may have been originally of high mg calcitic mineralogy projection of the alteration trends for the lower and middle belt limestones suggest 21 smow and 2 5 to 1 0 pdb as the best preserved values for the 18o and 13c of seawater respectively both comparable to results from other mesoproterozoic carbonate sequences the oxygen isotope data for limestones show a regional westward depletion of 8 in 18o possibly reflecting a higher temperature of postdepositional alteration in the western belt basin this depletion in 18o is accompanied by a comparable decrease in 13c values most likely because a higher proportion of carbon was incorporated from co2 generated by thermal cracking of hydrocarbons at depth dolostones in the belt basin are dominantly micritic with good preservation of depositional textures chemically and isotopically their alteration trends mimic those of limestones leading to comparable projected best values for 13c 87sr 86sr values of belt carbonates range between 0 70484 and 0 74991 progressive diagenesis as indexed by decreasing concentrations of sr and depletions in 18o and 13c results in an increase in 87sr 86sr values the least radiogenic measurement from a lower belt newland limestone sample appears to fit reasonably well into the general trend for proterozoic seawater as discussed in mirota and veizer 1994 this and the consistency of 18o and 13c in limestones with other coeval sequences suggests that the lower and middle belt carbonate sections are marine deposited in an environment that was not continuously separated from the open ocean

sm nd and pb isotopic study of mafic rocks associated with early proterozoic continental rifting the peropohja schist belt in northern finland the perapohja schist belt in northern finland rests unconformably on archaean granitoids and marks the early stages of proterozoic crustal evolution in the fennoscandian baltic shield 2440 ma old layered mafic intrusions predate the supracrustal and ca 2200 ma old sills of the gabbro wehrlite association intrude the lowest quartzites and volcanics runkaus of the sequence the sm nd mineral isochron of the penikat layered intrusion gives an age of 2410 64 ma the initial nd values of the penikat intrusion nd 2440 1 6 and the runkausvaara sill nd 2200 0 suggest that these mafic magmas were contaminated by older crustal material the sm nd and pb isotopic results on the 2 44 2 2 ga old runkaus volcanics indicate mobility of pb fractionation of sm nd during late greenschist facies metamorphism and crustal contamination the pb pb data provide an age of 1972 80 ma with a high initial 207pb 204pb ratio 1 8 49 while scattered sm nd data result in an imprecise age of 2330 180 ma with an initial nd value of about zero secondary titanite gives an u pb age of ca 2250 ma the jouttiaapa basalts in contrast ascended from the mantle without interaction with older crust these lree depleted tholeiites mark a break in continental sedimentation and yield a sm nd age of 2090 70 ma their initial nd 4 2 0 5 implies that the subcontinental early proterozoic mantle had been depleted in lree for a long period of time the first lava flows are strongly depleted in lree suggesting that their source was significantly more depleted than the source of mid ocean ridge basalts today

geochemistry of precambrian carbonates iv early paleoproterozoic 2 25 0 25 ga seawater in an effort to narrow down the first and second order variations in isotopic composition of the early paleoproterozoic 2 25 0 25 ga seawater we report a comprehensive study of mineralogy chemistry and isotopic composition of the malmani dolomite transvaal supergroup south africa duck creek dolomite wyloo group australia and bruce limestone member of the espanola formation huronian supergroup canada based on trace element data their diagenetic rank increases in the order duck creek bruce malmani the interpolation of alteration trends to best values yields an estimate of 0 70550 for 87sr86sr comparable to our previously published value of 0 70496 for 13c the measured range of 0 1 5 pdb is similar to that observed for phanerozoic marine carbonates in contrast the best 18o value for dolostones is 5 pdb depleted in 18o relative to phanerozoic counterparts but comparable to the estimates obtained by similar approach for archean facies the isotope geochemistry and mineralogy of bruce limestone member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment possibly related to the recession of the brace glaciers if such interpretation proves unacceptable from a sedimentological standpoint the origin of the unusual isotopic signatures must be sought despite the above trace element interpretation in as yet unspecified post depositional alteration phenomena in contrast to phanerozoic carbonates the early paleoproterozoic as well as the previously described archean counterparts are enriched in mn and fe

sr isotopic composition of paleoproterozoic 13c rich carbonate rocks the tulomozero formation se fennoscandian shield abstract rb sr systematics has been studied in 13c rich carbonate rocks of the paleoproterozoic 2 09 0 07 ga tulomozero formation in the northern onega lake area the se fennoscandian shield the formation is divided into eight members a f consisting of greenschist facies grade variegated sandstones siltstones mudstones stromatolitic dolostones and subordinate crystalline limestones samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the tulomozero formation prior to isotope analysis the rocks powders were treated with 1n ammonium acetate for partial removal of the late epigenetic carbonate phases major resetting of the rb sr systems in the tulomozero carbonate rocks appears to take place during the svecofennian regional metamorphic event and it was screened by using mn sr fe sr mg ca and 18o 16o ratios high sr content up to 2080 g g in limestones and 530 g g in dolostones coupled with low fe mn 0 40 in dolostones of the upper part of member b and of members g and h is indicative of seawater influxes sea transgression into the tulomozero basin the 87sr 86sr values in the least altered mn sr

geochemistry of precambrian carbonates vi aphebian albanel formations quebec canada abstract the 1 85 0 15 ga old albanel formations of northern quebec canada an example of paleoproterozoic carbonate buildups are lithologically composed of cherty dolostones subordinate limestones are encountered only near the base of the sequence coincident with maximal transgression the chemistry of the bulk rocks is almost entirely controlled by noncarbonate phases such as chert quartz plagioclase illite k spar pyrite and fe mn oxides all abundant particularly in the basal part of the section the chemistry of the carbonate portion of limestones is strongly reset by post depositional phenomena but the relict high sr contents suggest that aragonite may have been an original precursor phase geochemically the carbonate portion of dolostones can be subdivided into two populations reflecting the stratigraphie bottom and top of the sequence trace element and oxygen isotope criteria suggest that the entire sequence was subjected to meteoric and subsequent burial diagenesis with the less deeply buried top portion of the sequence having recrystallized and stabilized at lower temperatures but in a more open system the latter reflects the greater lithological variability and permeability of the younger strata the 13c varies up section from about 3 to 1 perhaps a reflection of progressively more open marine circulation the associated four generations of post depositional pyrites have overlapping 34s from 7 1 to 54 with a mean of 24 a reflection of dissimilatory bacterial sulfate reduction in a relatively closed diagenetic system with perhaps a limited sulfate availability and or replenishment the best measured 87 sr 86 sr of 0 70600 although possibly not a direct reflection of the coeval seawater may still be a near original value as indicated by its coherency with the roughly co stratigraphic paleo proterozoic sequences evaluation of the existing data indicates that strontium isotopic composition of the precambrian seawater was characterized by oscillations similar to those of the phanerozoic the interval of 0 8 1 5 ga ago being dominated by relatively nonradiogenic values followed by a radiogenic period at 1 6 2 2 ga ago near mantle values during the late archean and possibly another radiogenic peak at 3 5 ga ago

petrogenesis of 1 9 ga limestones and dolostones and their record of paleoproterozoic environments abstract fibrous calcite cements from limestones of the paleoproterozoic pethei group in northwest canada have relatively constant 13 c values near 1 that indicate the c exogenic cycle of ca 1 9 ga ago had similar balances between oxidized and reduced reservoirs of carbon as has existed throughout most of the phanerozoic 87 sr 86 sr ratios of these cements are near 0 7051 and support a progressively increasing influx of radiogenic sr from continental weathering into the paleoproterozoic oceans 18 o values of the cements vary from 14 to 8 5 although the original values of the cements were probably higher because the cements have been subseqently altered as evidenced by their variable trace element contents proposed higher temperatures or substantially lower 18 o values of ancient seawater based on such values may be in error fabric retentive dolomites generally regarded as syndepositional also have variable 18 o values the highest of which are ca 30 greater than the highest values of the fibrous calcite cements the dolomites have more radiogenic sr isotopic compositions indicating that they formed from fluids that interacted with rocks such as those of the clastic sediments which underlie the limestones these fluids may have partially altered the 18 o values and chemical compositions of the fibrous calcite cements late stage calcitic and dolomitic cements that formed from late diagenetic fluids have similar 18 o values and radiogenic sr isotopic ratios as the fabric retentive dolomites if these potentially syndepositional dolomites have had their primary isotopic compositions reset through interaction with diagenetic fluids this would have significant implications to other studies using early dolomite to constrain ancient environmental conditions

pb o and c isotopes in silicified mooidraai dolomite transvaal supergroup south africa implications for the composition of paleoproterozoic seawater and dating the increase of oxygen in the precambrian atmosphere abstract the mooidraai dolomite formation is a unit of marine sedimentary carbonates in the upper transvaal supergroup which recorded significant changes in the composition of the earth s atmosphere and ocean previously available data had suggested that this dolomite was about 2 2 ga old and showed 13ccarb values around 0 8 pdb and hence was an exception to the positive excursion of 13ccarb values observed worldwide in marine carbonates deposited between 2 25 and 2 05 ga ago we studied the pb o c isotope systematics of drill core samples from a highly silicified and aluminosilicate free sub unit of the mooidraai dolomite formation that yields well preserved micritic dolomite grains selective leaching of the silicified dolomite revealed a significant pb isotopic contrast between the carbonate and the quartz fraction the former shows only a narrow range of 208pb 204pb 35 35 35 58 which does not correlate with 206pb 204pb indicating the absence of th derived detrital pb the carbonate bound pb defines an isochron that corresponds to an age of 2394 26 ma 2 mswd 1 8 n 22 which we interpret as the diagenetic age of the mooidraai dolomite the 13ccarb values range from 0 51 to 0 64 pdb and confirm the previous data the 18ocarb values range from 2 08 to 0 18 pdb and are amongst the heaviest reported yet from early precambrian marine sedimentary carbonates the quartz fraction is considerably lighter than the carbonate fraction and shows 18oqtz values between 9 22 and 6 73 pdb 21 40 and 23 97 smow together with evidence from fluid inclusion microthermometry suggesting that the quartz formed at minimum temperatures between 130 and 155 c this indicates that the intense silicification was post depositional comparison of 18ocarb values of silicified and non silicified mooidraai samples suggests that the post depositional silicification had sealed the system and thereby protected the primary isotopic compositions of the micritic dolomite the heavy 18ocarb values suggest that 2 4 ga ago the oxygen isotopic composition of seawater differed only little from that of today the new pb pb carbonate age for the mooidraai dolomite indicates that the upper transvaal supergroup is about 200 my older than previously thought this explains the normal carbon isotopic composition of the mooidraai dolomite and suggests that the pronounced increase of the oxygen content in the precambrian earth s atmosphere that is seen in strata underneath the mooidraai dolomite occurred before 2 4 ga ago on this background it appears unlikely that the positive excursion of the carbon isotopic ratios of 2 25 2 05 ga old marine sedimentary carbonates is related to this increase in atmospheric oxygen

carbon isotope record for the onset of the lomagundi carbon isotope excursion in the great lakes area north america carbonate units of the chocolay group in the lake superior area usa and the gordon lake formation of the correlative upper huronian supergroup ontario canada were deposited after the last paleoproterozoic glacial event and an episode of intense chemical weathering the huronian supergroup contains at the base 2 45 ga volcanics and is intruded by the 2 22 ga nipissing diabase dykes and sills while the chocolay group is bracketed in age between 2 29 and 2 20 ga the lomagundi 2 22 2 1 ga carbon isotope excursion started after the paleoproterozoic glacial epoch and before a plume breakout event at 2 22 ga therefore the chocolay and upper huronian carbonates were deposited either before or during the onset of the lomagundi event notably thin carbonates of the basal gordon lake formation and thick carbonate succession of the kona dolomite in the northeastern exposures of the chocolay group record 13 c values as high as 9 5 versus v pdb similar to other successions deposited during the lomagundi event both units contain pseudomorphs and molds after sulfates this observation suggests that seawater sulfate contents rose dramatically in association with the onset of the lomagundi event and the rise of atmospheric oxygen carbonates in the western and southern exposures of the chocolay group randville and bad river dolomites and saunders formation previously assumed to be equivalent to the gordon lake formation and kona dolomite have carbon isotope values close to 0 based on basin analysis we infer that these carbonate units were deposited during a negative carbon isotope excursion after the lomagundi event started and are slightly younger than the kona dolomite and gordon lake formation this interpretation implies that the carbonate platform in the great lakes area transgressed to the west over shallow marine and fluvial deposits the negative carbon isotope excursion in the lake superior area might correspond to similar 13 c values of the mooidraai dolomite in the griqualand west basin south africa supporting correlation between paleoproterozoic successions of north america and south africa and the notion of three global glaciations in the paleoproterozoic era carbonates of the mille lacs group trout lake glen township and denham formations in minnesota have 13 c values ranging from 1 2 to 2 5 combined with geochronologic constraints these data suggest that these units were deposited after the lomagundi excursion and are related to the rifting event that led to development of the so far unrecognized 2 0 ga passive margin in the lake superior area 2006 elsevier b v all rights reserved

links between palaeoproterozoic palaeogeography and rise and decline of stromatolites fennoscandian shield through a review of literature and new data we have documented two major events in the palaeoproterozoic history of stromatolites as indicated by palaeontological and palaeoenvironmental studies with a time resolution of between 40 and 200 ma we confirm semikhatov and raaben s and awramik s albeit approximately maximum in diversity and abundance of stromatolites between 2330 and 2060 ma ago jatulian diversification we suggest that this taxonomic diversity was driven by a major phase of cratonisation formation of the karelian carbonate platform and numerous rift related shallow water carbonate basins supersaturated with ca 2 mg 2 and co2 the jatulian stromatolite explosion is synchronised with a positive 13ccarb shift of jatulian age carbonates we also document stromatolite decline which occurred on the fennoscandian shield somewhere between 2060 and 1900 ma ago this decline both in abundance and in taxonomic diversity is interpreted as having been caused by the first phase of oceanisation the oceanisation led to the considerable reduction in ecological niches that could be utilised by cyanobacteria the post jatulian decline of stromatolites coincides with an abrupt downward 13corg shift from 19 to 38 and is roughly coeval with the appearance of the first eukaryotic algae documented elsewhere the systematics of the fennoscandian diversity of palaeoproterozoic stromatolites is identical to that reported from india and china and reveals a dissimilarity with abundance and diversity patterns in australia and northern america

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

the structure of basin scale internal waves in a stratified lake in response to lake bathymetry and wind spatial and temporal distribution lake iseo italy field data theoretical analyses and numerical simulations were used to investigate the effects of lake basin bathymetry and spatial and temporal structures of the wind field on the basin scale internal wave spectrum in lake iseo this deep italian basin is characterized by the presence of a large island imparting attributes of an annulus to the lake during the summer of 2010 the internal wave activity was dominated by a basin scale internal wave of vertical and horizontal modes 1 v1h1 superimposed on which were occasional higher vertical modes v2h1 and higher horizontal modes v1h5 trapped by the main island the occurrence of these motions was interpreted as forcing by the wind components with similar horizontal structures and with energies at frequencies near the natural oscillations of the excited modes the modifications of the wind field by the topography in particular controlled the excitation of an anticyclonic wave trapped around the island whose features were investigated on the basis of an extension of the circular model to the case of an annular basin of constant depth

dissolution methods for strontium isotope stratigraphy whole rock analysis the preparation of samples for strontium isotope stratigraphy sis by whole rock dissolution results in contributions of sr from contaminant phases as well as from the target carbonate phase here we highlight and quantify the errors in 87sr 86sr values that might result when calcium carbonate is dissolved using an inappropriate method we report the variation in 87sr 86sr value of sr released from four samples of differing calcite content and degrees of cementation when subject to different methods of dissolution we show that errors of up to 0 000076 in 87sr 86sr which correspond to errors of 3 68 ma in age can be avoided by careful preleaching of the samples

spatial structure of the dominant basin scale internal waves in lake kinneret field data and numerical simulations were used to investigate the effects of basin shape continuous stratification and rotation on the three dimensional structure of the dominant natural basin scale internal wave modes in lake kinneret a stratified lake large enough so that the earth s rotation influences the wave motion the structure of the modes was inferred from power spectral density of measured and simulated isotherm vertical displacements and from rotary power spectral density of isopycnal velocities obtained from numerical simulations the shape of the lake at the level of the thermocline in conjunction with the dispersion relationship determines the horizontal configuration of the natural modes the dominant response to wind forcing was an azimuthal and vertical mode 1 kelvin wave with a natural period of 22 6 h that propagated around the entire basin the second most dominant response was a vertical mode 1 wave with a natural period close to 10 5 h and composed of two counter rotating

horizontal structure and excitation of primary motions in a strongly stratified lake a modal analysis in the horizontal plane was extended to a layer stratified basin with irregular bathymetry and the theory was applied to lake biwa to investigate the horizontal structure and excitation of the basin scale internal waves and gyres the horizontal structure of the basin scale internal waves consisted of cyclonic and anticyclonic elliptic cells each of which appeared to follow the dispersion relationship of kelvin and poincare waves in elliptic basins the internal waves were preferentially excited depending on the arrangement of the cells and the wind direction but the spatial distribution of wind stress curl over the lake primarily determined the horizontal structure of the ensuing gyres decoupled evolutionary equations for the individual modes provided a good approximation for excitation of the internal waves and early stages of excitation of the gyres before nonlinear effects and damping become significant the modal decomposition of hydrodynamic simulation results also showed that the primary action of the wind was to excite the internal waves however these internal waves were damped within a few days and the dynamics during calm periods were dominated by the gyres illustrating the importance of internal waves on mixing and gyres on long term horizontal transport

shallow burial dolomitization and dedolomitization of mid cenozoic cool water calcitic deep shelf limestones southern australia abstract oligocene to mid miocene deep shelf bryozoan rich limestones across southern australia are variably altered to gray to orange ca rich medium crystalline sucrosic dolomite degree of replacement ranges from scattered rhombs to complete dolostone units several tens of meters thick and many kilometers in areal extent the locale and timing of dolomitization are tightly constrained to shallow burial and mid to late miocene dolostone varies from friable to dense and well lithified dolomitization is fabric specific muddy sediments are preferentially replaced calcite bryozoans and brachiopods form biomolds geochemistry suggests that dolomitization was predominantly by seawater limestone interaction but admixing of continental possibly meteoric groundwater is required by data on stable and radiogenic isotopes and trace elements sr isotopes confirm a mid miocene age for the dolomite if precipitated from seawater with no inherited limestone values dolomite crystals have undergone variable degrees of dissolution and meteoric calcite cement has locally filled the resultant rhombic voids dissolution began in the crystal cores implying some sort of metastability and expanded outward until in some ins ances the whole dolomite crystal was dissolved leaving dolomolds in otherwise unaffected limestone this dolomite occurrence well constrained by regional geology shows that widespread porous and permeable fabric destructive medium crystalline sucrosic dolostone can form in the shallow subsurface soon after sediment deposition such dolomite however may be metastable in the presence of variably mixed continental marine waters and newly formed crystals can undergo dedolomitization soon after precipitation

internal seiche dynamics in lake geneva we analyzed season long water level records at 12 stations around the lake of geneva local name leman for evidence of internal seiches modified by coriolis force and compared the results with predictions from a two layer numerical model with real bottom topography for typical wind situations results are also compared with those obtained from current and temperature measurements in the lake agreement was satisfactory in all cases model predictions and measurements both indicated that only three internal seiche modes are excited the 1st mode and the 3rd mode which are kelvin seiche oscillations and the 12th mode which is a poincare seiche the model driven by winds from different directions demonstrates that the wind field constrained by the local topography determines which of the modes is generated

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less

geochemistry of precambrian carbonates v late paleoproterozoic seawater in an effort to obtain better constrained data for the first and second order variations in the isotopic composition of late paleoproterozoic 1 8 0 2 ga seawater we report a study of mineralogy chemistry and isotopic composition of the coronation supergroup 1 9 ga nwt canada and the mcarthur group 1 65 ga nt australia petrologically both carbonate sequences are mostly dolostones the mcarthur population however contains more abundant textural features that attest to the former presence of sulfates and halite and the facies investigated herein probably represent ancient equivalents of modern evaporitic sabkhas and lacustrine playa lakes this is reflected in 18 o enrichment up to 2 9 pdb and high mn fe and sr concentrations nonetheless the available 18 o data indicate that the best estimate for the archean to late paleoproterozoic limestones is about 7 pdb with dolostones some 3 2 heavier the observed 18 o doiomitc calcite is thus within the range of the proposed fractionation factor for a cogenetic mineral pair this together with the consistency of isotopic patterns for sedimentological facies over almost 2 billion years from the late paleoproterozoic to the present suggest that dolomitization was an early diagenetic event and that the 18 o depletion of the archean to late paleoproterozoic carbonates is not an artefact ofpost depositional alteration the 13 c although within the range of other precambrian and phanerozoic carbonates indicates a possible secular evolution from 0 2 pdb at 1 9 ga ago to 1 1 5 at 1 65 ga ago the secular evolution for reflects the 87 sr increase from 0 7031 0 0008 at 3 5 0 1 ga ago through 0 7025 0 0015 at 2 8 0 2 ga 0 7033 at 2 5 ga 0 7050 at 2 25 0 25 ga 0 7047 at 1 91 to 0 706 at 1 65 ga ago

flood stratigraphies in lake sediments a review abstract records of the frequency and magnitude of floods are needed on centennial or millennial timescales to place increases in their occurrence and intensity into a longer term context than is available from gauged river flow and historical records recent research has highlighted the potential for lake sediment sequences to act as a relatively untapped archive of high magnitude floods over these longer timescales abyssal lake sediments can record past floods in the form of coarser grained laminations that reflect the capacity for river flows with greater hydrodynamic energy to transport larger particles into the lake this paper presents a framework for investigating flood stratigraphies in lakes by reviewing the conditioning mechanisms in the lake and catchment outlining the key analytical techniques used to recover flood records and highlighting the importance of appropriate field site and methodology selection the processes of sediment movement from watershed to lake bed are complex meaning relationships between measureable sedimentary characteristics and associated river discharge are not always clear stratigraphical palaeoflood records are all affected to some degree by catchment conditioning fluvial connectivity sequencing of high flows delta dynamics as well as within lake processes including river plume dispersal sediment focussing re suspension and trapping efficiency with regard to analytical techniques the potential for direct e g laser granulometry and indirect e g geochemical elemental ratios measurements of particle size to reflect variations in river discharge is confirmed we recommend care when interpreting fine resolution geochemical data acquired via micro scale x ray fluorescence xrf core scanning due to variable down core water and organic matter content altering x ray attenuation we also recommend accounting for changes in sediment supply through time as new or differing sources of sediment release may affect the hydrodynamic relationship between particle size and or geochemistry with stream power where these processes are considered and suitable dating control is obtained discrete historical floods can be identified and characterised using palaeolimnological evidence we outline a protocol for selecting suitable lakes and coring sites that integrates environmental setting sediment transfer processes and depositional mechanisms to act as a rapid reference for future research into lacustrine palaeoflood records we also present an interpretational protocol illustrating the analytical techniques available to palaeoflood researchers to demonstrate their utility we review five case studies of palaeoflood reconstructions from lakes in geographically varied regions these show how lakes of different sizes and geomorphological contexts can produce comprehensive palaeoflood records these were achieved by consistently applying site validated direct and proxy grain size measurements well established chronologies validation of the proxy process interpretation and calibration of the palaeoflood record against instrumental or historical records