oxygen and carbon isotope trends and sedimentological evolution of a meromictic and saline lacustrine system the holocene medicine lake basin north american great plains usa abstract the succession of sedimentary subfacies in the medicine lake basin south dakota usa documents numerous changes in total dissolved solids tds lake level and the nature and stability of meromixis during the holocene the lake formed about 10 6 ka b p when coarse clastics and massive fine grained muds were deposited in a well mixed freshwater system subsequently the lake became saline and meromictic depositing a finely laminated unit composed of variegated and gray muds organic facies gypsum and aragonite laminae phototrophic bacterial plate communities bpc at the chemocline were significant producers of pelagic organic matter relatively high lake levels at least 10 m higher than today but saline and meromictic conditions predominated through the early to mid holocene at the end of this phase lake level dropped and stable meromixis was disrupted sands and massive or banded muds were deposited in sublittoral areas and benthic microbial communities bmc spread over the lake replacing bpcs as the predominant microbial producers when lake level rose again massive and laminated units were deposited under less saline conditions another pronounced episode of low lake level and high salinity followed the most recent sediments are sapropels massive or banded muds and littoral sands the 18o record from authigenic aragonite shows more positive values during the arid mid holocene interval with an abrupt increase at the end of this period followed by generally more negative values during the cooler and wetter late holocene the small range of 18o values 3 suggests that the lake was well buffered against high frequency climatic cycles probably as a result of groundwater modulation we postulate that meromixis and the type of photosynthetic microbial communities dominating the basinal region of the lake system either bpcs or bmcs exerted major controls on the carbon cycle and consequently on the development of isotopic trends linking 13c and 18o values more negative 13c values during the early and mid holocene are interpreted as a consequence of the predominance of 12c enriched microbial carbon from bpcs examples of both covariant and inverse covariant isotope trends between 13c and 18o values occur in the medicine lake record inverse covariant trends characterize long periods of stable meromixis when bpcs predominate over bmcs we hypothesize that well developed water stratification caused by greater influx of relatively 18o depleted freshwater induces higher productivity at the chemocline and consequently more positive 13c values in the precipitated carbonates unstable phases of meromixis or holomictic conditions are characterized by a predominance of bmcs or other biogenic producers over bpcs and a number of other factors combine to promote covariant trends the medicine lake record is consistent with the paleoclimatic interpretation for the northern great plains of a more arid early and middle holocene followed by an abrupt transition to a relatively cooler and wetter late holocene the sedimentary succesion in medicine lake documents a much more fluctuating and abrupt climate behavior than suggested by the vegetation paleorecord and suggest a late holocene climate punctuated by arid episodes

mixing of siliciclastic and carbonate sediments in shallow shelf environments the inhibiting effect that siliciclastic material has on carbonate secreting organisms has lead to the generalization that sediments composed of mixtures of carbonate and siliciclastic material should rarely form however many modern and ancient shelf deposits contain a spectrum of sediments that are of mixed composition the processes responsible for this mixing can be grouped into four categories 1 punctuated mixing where sporadic storms and other extreme periodic events transfer sediments from one depositional environment to another 2 facies mixing where sediments are mixed along the diffuse boundaries between contrasting facies 3 in situ mixing where the carbonate fraction consists of the autochthonous or parautochthonous death assemblages of calcareous organisms that accumulated on or within siliciclastic substrates and 4 source mixing where admixtures are formed by the uplift and erosion of nearby carbonate source terranes the allochemical constituents of mixed sediments are both coralgal and foram mollusc in composition the foram mollusc assemblage is the most common because of the effects of increased turbidity unstable substrates and the clogging of filter feeding mechanisms associated with a siliciclastic influx

palaeoceanographic redox environments for the lower cambrian hetang formation in south china evidence from pyrite framboids redox sensitive trace elements and sponge biota occurrence abstract the lower cambrian hetang formation a black shale sequence contains a stone coal a flammable organic rich mudstone unit abundant pyrite framboids occur in this unit with the average mean sizes falling in a narrow range from 4 6 to 5 4 m and the maximum diameters being around 10 m the size distribution pattern of the pyrite framboids indicates a euxinic depositional environment the redox sensitive trace metals of the stone coal samples reveal a large enrichment of mo 10 180 times v 4 40 times u 10 60 times and ni 2 20 times compared to the average upper continental crust value consistent with an anoxic environment for their deposition the redox sensitive element ratios th u v v ni v cr indicate that the depositional environment for the lower part of the stone coal unit was the most anoxic and euxinic in contrast a sponge biota including eleven species of demosponges and hexactinellids and two undetermined forms indicate an oxic or dysoxic environment to reconcile these two facts we propose that although an anoxic euxinic environment predominated during the deposition of the lower cambrian hetang formation black shales occasional currents may also have brought free oxygen to the bottom water column to allow the growth of the sponges on the sea floor

a stable isotope study of microbial dolomite formation in the coorong region south australia abstract the coorong region of south australia is a classic location for the study of modern dolomite formation a number of ephemeral lakes here contain dolomite whilst other lakes in close proximity contain aragonite with minor magnesite our previous field and experimental work has revealed that sulphate reducing bacteria play a key role in overcoming kinetic inhibitors to dolomite formation in this area we here use a combination of oxygen carbon and sulphur isotopes to help to further constrain the processes leading to dolomite precipitation in modern hypersaline environments we report isotopic data from four dolomite and three aragonite containing lakes 34 s data for dissolved lake and porewater sulphates reveal 34 s enrichment of residual sulphate in all seven lakes consistent with bulk closed system microbial fractionation by sulphate reducing bacteria calculations using temperature dependent oxygen isotope fractionation factors between dolomite and the solution from which it precipitated are consistent with a strong microbial component to carbonate precipitation in these lakes a comparison of 18 o data from dolomite and aragonite sediment and from their associated lakewaters indicates seasonal mineral precipitation occurs late in the evaporative cycle of the lakes within a yoghurt like saturated mud horizon coincident with the zone of sulphate reduction surprisingly 13 c data from the carbonates do not show the characteristic wide range of values often associated with organogenic carbonates but instead show that these bacterially mediated precipitates incorporate carbon primarily from the inorganic lakewater reservoir partially diluted by an organic component a coorong type microbial model of dolomite formation may be particularly applicable to for example precambrian sedimentary dolomites where benthic microbial communities which are today restricted to extreme environments dominated the shallow marine ecosystem prior to the emergence of metazoa

a modern vs permian black shale the hydrography primary productivity and water column chemistry of deposition the sediment currently accumulating in the cariaco basin on the continental shelf of venezuela has an elevated organic carbon content of approximately 5 is accumulating under o2 depleted bottom water conditions so42 reduction is composed dominantly of foraminiferal calcite diatomaceous silica clay and silt and is dark greenish gray in color upon lithification it will become a black shale recent studies have established the hydrography of the basin and the level of primary productivity and bottom water redox conditions these properties are used to model accumulation rates of cd cr cu mo ni v and zn on the seafloor the model rates agree closely with measured rates for the uppermost surface sediment the model is applied to the meade peak phosphatic shale member of the phosphoria formation a phosphate deposit of permian age in the northwest united states it too has all of the requisite properties of a black shale although the deposit is a world class phosphorite it is composed mostly of phosphatic mudstone and siltstone chert limestone and dolomite it has organic carbon concentrations of up to 15 is strongly enriched in several trace elements above a terrigenous contribution and is black the trace element accumulation defines a mean primary productivity in the photic zone of the phosphoria basin as moderate at 500 g m 2 year 1 organic carbon comparable to primary productivity in the cariaco basin the source of nutrient enriched water that was imported into the phosphoria basin upwelled into the photic zone and supported primary productivity was an o2 minimum zone of the open ocean the depth range over which the water was imported would have been between approximately 100 and 600 m the mean residence time of bottom water in the basin was approximately 4 years vs 100 years in the cariaco basin the bottom water was o2 depleted but it was denitrifying or no3 reducing rather than so42 reducing

hydrogen sulphide poisoning of shallow seas following the end triassic extinction the end of the triassic period was marked by a mass extinction biomarkers in black shales that formed at the time suggest that the repeated poisoning of shallow seas by hydrogen sulphide delayed the early jurassic recovery

holocene climate change in the eastern mediterranean region a comparison of stable isotope and pollen data from lake golhisar southwest turkey stable isotope and pollen data from golhisar golu a small intramontane lake located in southwest turkey provide complementary records of holocene climate change modern oxygen and hydrogen isotope water data are used as a means of comparing present day isotope composition of the lake water to the past oxygen isotope composition of the lake water as calculated from 18o 16o ratios in calcite precipitated in the summer months despite the lake system being chemically dilute the modern isotope data clearly establish that the lake water is evaporated in relation to its spring input suggesting that the palaeo data can be interpreted primarily in terms of changing precipitation evaporation ratios 18o and 13c values from authigenic calcite through the holocene show predominantly negative values indicating climatic conditions wetter than today particularly notable are low depleted isotope values during the earliest holocene ca 10 600 8800 cal yr bp a period for which pollen data imply drier conditions than at present this divergence between pollen inferred and stable isotope palaeoclimate data is found in other east mediterranean lake sediment records and suggests that vegetation may have taken several millennia to reach climatic equilibrium at the start of the holocene isotopic fluctuations during the early to mid holocene 8800 5100 cal yr bp suggest oscillations between aridity and humidity higher 18o and 13c values for the second half of the holocene indicate generally drier conditions than during the period before ca 5100 cal yr bp although there is some evidence for increased humidity coinciding with pollen evidence for increasing human impact and intensification of agriculture notably during the so called beyehir occupation phase classical and early byzantine periods the modern trend towards aridity started about 1300 yr ago

57fe mossbauer spectroscopic analysis of deep sea pelagic chert effect of secondary alteration with respect to paleo redox evaluation abstract chemical states of iron in the pre jurassic pelagic deep sea cherts are used as one of redox indicators of lost deep oceans e g the permian triassic boundary case primarily red hematite bearing cherts were often altered secondarily into greenish gray cherts we examined the pattern of secondary change in chemical state of iron associated with color change in the middle triassic chert beds at hisuikyo in central japan by 57fe mossbauer spectroscopy three sets of chert bed with the lateral color change from red to greenish gray were analyzed the analyses confirmed that the red cherts contain hematite paramagnetic fe3 and paramagnetic fe2 whereas the greenish gray cherts contain paramagnetic fe3 and paramagnetic fe2 without hematite the greenish gray parts contain larger amounts of paramagnetic fe2 component of amorphous siderite like mineral in contrast to the red parts have lesser amounts these results confirmed that the primary hematite has changed into paramagnetic fe2 bearing minerals in accordance with the color change by the secondary alteration further comparison with the pyrite bearing primary black to dark gray cherts was discussed with respect to the evaluation for primary redox change in the past oceans

evidence of possible induced seismicity in the wabash valley seismic zone from improved microearthquake locations to evaluate the spatial structure of seismicity in the wabash valley of southern indiana and illinois we analyzed data from a temporary seismic network that included 10 three component short period seismometers and a 10 component dense phased array we produced the first comprehensive catalog of local earth quakes recorded during the 211 day deployment from november 1995 through june 1996 the results are dominated by a cluster of 534 microearthquakes with magni tudes ranging from 0 6 to 1 8 near the town of new harmony indiana that were detected and recorded only by the phased array the remarkable similarity of the cluster events in terms of waveform magnitude and temporal distribution suggests a tightly spaced grouping from a single seismic source we relocated the earthquakes using improved relative arrival times of p and s waves computed by time domain cross correlation of the vertical beam traces and by complex cross correlation of the horizontal beam traces respectively additional constraints on absolute locations were applied using a graphical method of arrival time difference analysis for six earthquakes recorded by two network stations outside the phased array the resulting locations define a tight spatial grouping about a zone of post paleozoic faulting just west of the indiana illinois border in white county illinois average source depths from phased array location methods and waveform modeling with synthetic seis mograms indicate in contrast to larger events in the region that the earthquakes occurred at depths less than 4 km within the sedimentary section of the illinois basin we propose that these earthquakes are artificially induced events likely related to water injection for the purpose of secondary recovery of petroleum in the illinois basin the primary evidence for this includes 1 tight spatial clustering of earth quakes 2 unusually shallow earthquake depths 3 good spatial correlation of the relocated hypocenters to existing wells and oil fields and 4 highly repetitive events with only a small range in magnitude

rare earth element geochemistry of late devonian reefal carbonates canning basin western australia confirmation of a seawater ree proxy in ancient limestones rare earth element and yttrium ree y concentrations were determined in 49 late devonian reefal carbonates from the lennard shelf canning basin western australia shale normalized sn ree y patterns of the late devonian samples display features consistent with the geochemistry of well oxygenated shallow seawater a variety of different ancient limestone components including microbialites some skeletal carbonates stromatoporoids and cements record seawater like ree y signatures contamination associated with phosphate fe oxides and shale was tested quantitatively and can be discounted as the source of the ree y patterns co occurring carbonate components that presumably precipitated from the same seawater have different relative ree concentrations but consistent ree y patterns clean devonian early marine cements n 3 display ree y signatures most like that of modern open ocean seawater and the highest y ho ratios e g 59 and greatest light ree lree depletion average nd sn yb sn 0 413 sd 0 076 however synsedimentary cements have the lowest ree concentrations e g 405 ppb non contaminated devonian microbialite samples containing a mixture of the calcimicrobe renalcis and micritic thrombolite aggregates in early marine cement n 11 have the highest relative ree concentrations of tested carbonates average total ree 11 3 ppm stromatoporoid skeletons unlike modern corals algae and molluscs also contain well developed seawater like ree patterns samples from an estuarine fringing reef have very different ree y patterns with lree enrichment nd sn yb sn 1 possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged lree from a nearby riverine input source hence devonian limestones provide a proxy for marine ree geochemistry and allow the differentiation of co occurring water masses on the ancient lennard shelf although appropriate partition coefficients for quantification of devonian seawater ree concentrations from out data are unknown hypothetical devonian canning basin seawater ree patterns were obtained with coefficients derived from modern natural proxies and experimental values resulting devonian seawater patterns are slightly enriched in lree compared to most modem seawaters and suggest higher overall ree concentrations but are very similar to seawaters from regions with high terrigenous inputs our results suggest that most limestones should record important aspects of the ree geochemistry of the waters in which they precipitated provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high non seawater like ree contents hence we expect that many other ancient limestones will serve as seawater ree proxies and thereby provide information on paleoceanography paleogeography and geochemical evolution of the oceans copyright c 2004 elsevier ltd

the trace metal content of recent organic carbon rich sediments implications for cretaceous black shale formation organic carbon rich sediments are enriched in several redox sensitive and stable sulfide forming trace metals tm and form an important sink in global tm cycles in today s ocean such sediments are deposited in coastal upwelling areas like those in the gulf of california off peru or off the namibian coast and euxinic basins like the black sea in an attempt to distinguish both environments by their specific tm patterns some important conclusions may be drawn 1 the tm enrichment seen in both environments broadly is rather similar 2 upwelling sediments are often enriched in cd and p and depleted in co and mn 3 sapropels have a tendency towards higher enrichments in ba and mo s re as cu ni sb and fe reflecting the strong sulfidation in an anoxic water column 4 upwelling systems cover a broad range of environmental settings where the steepness of the slope the proximity of h2s to the sediment seawater interface and the intensity of bio accumulation and regeneration play an important role for tm accumulation 5 the tm content of anoxic basins is mainly controlled by tm availability in the water column and sedimentation rate 6 enhanced bio productivity is the main switch turning an oxygenated into an anoxic environment cretaceous black shales oae 2 exhibit tm signatures of both coastal upwelling areas and sapropels the strong enrichments in cd mo tl v cu ni bi and sb demonstrate that bio accumulation and presence of h2s in the water column and associated sulfidation processes persisted during this time interval in large parts of the proto atlantic the usefulness of ba as a paleoproductivity proxy is obscured by diagenesis the extreme accumulation in ag and zn requires either an additional tm source for instance hydrothermal input or a different tm seawater composition during the cretaceous the geochemical data suggest that the major driving force for the widespread occurrence of c t black shales seems to be the increase in volcanic activity and associated co2 input throughout the cretaceous the ocean atmosphere system operated in a mode different from today during this time interval after conversion of the nutrient co2 into organic matter the system switched back into the normal operational mode

paleobiology of complex trace fossils abstract complex trace fossils include structurally elaborate biogenic structures such as the large spiral forms of zoophycos and the intricately organized graphoglyptids compound structures consisting of connected parts that would receive different ichnotaxonomic labels if preserved in isolation and recurrent spatial associations of different trace fossils that probably represent strong ecologic interaction these complicated animal artefacts should not be viewed so much as preservational rarities or taxonomic puzzles but rather as neglected sources of information on the ethologic ecologic and evolutionary characteristics of trace producers trying to fit these structures into the traditional behavioral classification scheme of seilacher is bound to fail many complex trace fossils appear to have been occupied for long intervals and to record re engineering or active control of proximal habitats by the trace producing organisms attempting to understand their actual biologic significance will require new descriptive and interpretive approaches one possible descriptive approach involves making comprehensive inventories of structural elements that record particular kinds of adaptive behavior or interaction behavioral tokens portraying the spatial and temporal order of tokens could be done using a kind of systems diagram paleoethologic blueprint that summarizes construction operation and maintenance of complex biogenic structures new approaches to interpretation involve viewing the structures as evidence of physiologic projection of the trace producers or as the extended phenotypes of these organisms a reorganized approach to the study of complex biogenic structures emphasizing biologic properties would not only lead to some very old puzzles finally being solved but also would revitalize ichnology and forge new and productive connections to ecology and evolutionary biology

redox sensitive element uptake in north east atlantic ocean sediments benthic boundary layer experiment sites regularly increasing radiocarbon age depth profiles and near constant sediment composition with depth have demonstrated that sediment accumulation has been relatively constant during the late holocene at two north east atlantic sites on rockall bank and feni drift uk benthic boundary layer experiment sites in such a quasi steady state situation the geochemical responses to early diagenesis of the redox sensitive elements cd mn mo re se and u are readily discerned through changes in element concentrations with depth collectively a colour change in the sediments surficial mnox enriched layers and 210pbexcess profiles allow an estimate of the mean position of the oxic post oxic boundary in the sediments at both sites this boundary is situated deeper than the 210pbexcess surface mixed layer but shallower than the 14c surface mixed layer which represent mixing on 102 and 103 year time scales respectively this implies that the well defined surface mnox enrichment and its associated 226ra and mo fractions must have been efficiently recycled first by downwards mixing into anoxic conditions and then by subsequent reduction and diffusive migration back up into oxic conditions this occurs in 40 years at the feni drift site and in 103 years at the rockall bank site authigenic enrichments of se cd and u are evident immediately below the oxic post oxic boundary although re enrichment does not occur until a few centimetres deeper the long term fluxes of authigenic se cd u and re uptake are evaluated with steady state assumptions while diffusion from bottom waters can supply sufficient se u and re an additional source likely corg is required for cd

association of 34s depleted pyrite layers with negative carbonate δ13c excursions at the permian triassic boundary evidence for upwelling of sulfidic deep ocean water masses 1 a marine permian triassic boundary ptb section at nhi tao vietnam contains a series of at least 9 pyritic horizons characterized by concurrent decreases in pyrite s 34spy and carbonate c isotopic compositions 13ccarb the first and largest of the events that precipitated these pyritic horizons was coincident with the late permian mass extinction while subsequent events were generally smaller and occurred at quasiperiodic intervals of 20 to 30 ka a near complete lack of organic carbon to drive bacterial sulfate reduction in sediment pore waters among other considerations argues against a diagenetic control for these relationships rather the covariant patterns documented herein are most easily explained as the product of recurrent upwelling of anoxic deep ocean waters containing 34s depleted hydrogen sulfide and 13c depleted dissolved inorganic carbon the sulfide 34s record of the study section represents a mixture of a small amount of isotopically heavy authigenic pyrite formed via in situ bacterial sulfate reduction with a generally larger quantity of isotopically light syngenetic pyrite precipitated within the water column during upwelling episodes although upwelling of toxic deepwaters has been invoked in earlier studies as a mechanism for the late permian marine mass extinction this is the first study to 1 document patterns of pyrite 13ccarb covariation that strongly support upwelling as a major process at the ptb and 2 provide evidence of multiple quasiperiodic upwelling events that may reflect reinvigoration of global ocean overturn following a prolonged interval of late permian deep ocean stagnation

sulphur isotope geochemistry of pyrite from the upper cretaceous marshybank formation western interior basin abstract variations in the texture and sulphur isotopic composition of pyrite in the upper cretaceous marshybank formation are linked to depositional environment abundant framboidal and lesser euhedral pyrite precipitated in offshore marine rocks moderate quantities of pyrite also crystallized in brackish coastal plain rocks however in contrast to marine pyrite coastal plain pyrite is dominantly euhedral in texture reflecting direct precipitation from a porewater with a relatively low dissolved sulphide concentration in the marine rocks pyrite 34s values range from 35 7 to 27 4 avg 4 8 canyon diablo troilite cdt pyrite within carbonate concretions hosted in these marine rocks has a similar isotopic composition 49 8 to 10 6 cdt however isotopic values are often highly variable within individual concretions as a result of the heterogeneous nature of sulphate reduction and pyrite formation within marine sediments pyrite in coastal plain rocks is characterized by relatively high 34s values 4 2 to 35 5 avg 13 2 cdt while the overlying sideritized conglomerates have the lowest 34s values reported for cretaceous rocks from the western interior basin of north america 49 8 to 41 7 cdt very low 34s values which are only observed in the marine rocks are indicative of microbial sulphate reduction and pyrite formation in a sulphate replete i e open system higher 34s values up to 18 which were obtained for both the marine and coastal plain rocks are indicative of progressive pyrite crystallization in a sulphate limited i e closed system such conditions are expected in marine sediments as burial occurs and in brackish i e low sulphate sediments pyrite with very high 34s values 18 is common in the coastal plain rocks these high values are the result of influx of 34s enriched residual sulphide derived from overlying marine units a minor amount of 34s enriched pyrite is also present within and proximal to marine sedimentary unit h which was deposited at the same time as the coastal plain rocks we hypothesize that porewater containing 34s enriched residual sulphide flowed from the coastal plain and along unit h into the marine sediments thus producing anomalously high 34s pyrite

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

an evaluation of the inverted metamorphic gradient at langtang national park central nepal himalaya the crystalline core of the himalayan orogen in the langtang area of nepal located between the annapurna manaslu region and the everest region contains middle to upper amphibolite grade pelitic gneisses and schists these rocks are intimately associated with the main central thrust mct one of the major compressional structures in the northern indian plate which forms a 3 7 km wide zone containing rocks of both footwall and hangingwall affinity an inverted metamorphic gradient is noticeable from upper footwall through hangingwall rocks where metamorphic conditions increase from garnet grade near the mct zone to sillimanite k feldspar grade in the upper hangingwall petrographic data distinguish two metamorphic episodes that have affected the area a high pressure moderate temperature episode m1 and a moderate pressure high temperature episode m2 comparison with appropriate reaction boundaries suggests that conditions for m1 in the hangingwall were approximately 900 1200 mpa and 425 525 c thermobarometric results for 24 samples from the footwall mct zone and hangingwall reflect p t conditions during the m2 phase of 400 1200 mpa and 490 660 c the decrease in estimated palaeopressures from footwall to hangingwall approximate a lithostatic gradient of 27 mpa km 1 with slight fluctuations in the mct zone reflecting structural discontinuities in contrast to the palaeopressures palaeotemperatures are indistinguishable across the entire area sampled although field evidence suggests the presence of the inverted palaeothermal gradient well known in the himalaya quantitative thermobarometry indicates that temperatures of final equilibration were all within error of each other across 17 km of section at langtang change in pressure is responsible for the presence of the sequence of index minerals through the section i interpret these data to reflect diachronous attainment of equilibrium temperature conditions in a lithostatic palaeopressure profile after ductile faulting of the sequence

paleoenvironmental implications of uranium concentrations in lacustrine calcareous clastic evaporite deposits in the western qaidam basin abstract uranium u provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments its proxy potential under suboxic oxic conditions associated with salinization of surface water in closed hydrologic systems however has not been adequately explored this study examined the abundance and distribution of u in a sedimentary sequence of calcareous lacustrine clastics and salt layers from a high quality 938 5 m long core drilled in the western qaidam basin ne tibetan plateau this study examined u concentrations in calcareous clastic sediments including carbonates acid insoluble fractions as well as the amounts of iron and manganese oxides oxyhydroxides and salt layers in the calcareous clastic sediments variations in u concentrations within the acid insoluble fraction follow the fluctuations of reconstructed lake levels based on lithologic investigations providing a tool for tracing redox conditions in paleolake bottom water correlations between the variations in carbonate content with u and sr concentrations in the carbonate fractions demonstrate that the types and abundance of carbonate minerals control the u distribution in carbonates in the salt layers u and al concentrations correlate closely implying that both derived mainly from the detrital fraction in the salt layers however enrichments of u and mg are systematically higher in salt minerals formed in more concentrated brine and lower in those formed in less concentrated brine these results indicate that during the process of carbonate precipitation carbonate might effectively remove u from lake water during the subsequent stage of salt mineral precipitation u behavior is conservative and u resides for a long period in dissolved forms within the saline water giving rise to enhanced u enrichment in the more concentrated brines and the associated precipitated salt minerals our study suggests however that u related redox proxies should be interpreted with caution in suboxic oxic environments during the long term paleolake evolution

high frequency redox variations of the eocene cyclic lacustrine sediments in the yingxi area western qaidam basin china abstract the mineral assemblages of lacustrine mixed sediments have vital significance for reconstructing paleo lake environments and typical lacustrine mixed sediments deposited during the eocene are distributed throughout the yingxi area in the western qaidam basin accordingly the framboid size distributions and sulfur isotope 34spy characteristics of pyrite in addition to the carbon isotope 13ccarb characteristics and redox sensitive element ratios th u v v ni and v cr of authigenic carbonates were investigated to reconstruct the high frequency redox variations in cyclic lacustrine sediments corresponding to different redox conditions three types of mixed sediments developed during transgression and regression stages within the saline basin in the yingxi area during the eocene mixed terrigenous clasts formed in oxic dysoxic water column conditions during lake transgression mixed carbonates formed in anoxic water column conditions during early stages lake regression and mixed evaporites formed in anoxic sulfidic water column conditions during middle late stage lake regression the redox proxies correlate well with the water salinity exhibit little or no correlation with the water temperature and show an incongruity with the water depth suggesting that the redox conditions were predominantly controlled by the water salinity rather than the water depth or temperature therefore based on the water redox condition variations and the salinity three dynamic formation models of the high frequency sedimentary cycles in a saline basin are established a deep water low salinity oxic dysoxic model model i a medium water medium salinity anoxic model model ii and a shallow water high salinity anoxic sulfidic model model iii because the climate gradually turned dry and hot from lake transgression to regression the continued closure of the basin enhanced evaporation and salinization in addition mixed terrigenous clasts mixed carbonates and mixed evaporites deposited successively correspondingly salinity stratification gradually developed within the lake water increasing the extent of bacterial sulfate reduction in the bottom water

evidence for sulfidic deep water during the late permian in the east greenland basin a detailed study of the size distribution of framboidal pyrites in the black shales of the upper permian ravnefjeld formation was performed to evaluate the redox state of the late permian ocean in contrast to framboidal pyrites in bioturbated sediments the smaller and less variable size distribution of pyrite framboids in the laminated shales of the ravnefjeld formation provides persuasive evidence for sulfidic h 2 s rich bottom water conditions in the east greenland basin however the s isotope compositions of both pyrite populations show a similar distribution the widespread 3 4 s values of pyrites 41 2 to 28 2 in the black shales of the ravnefjeld formation indicate a large fractionation up to 52 7 relative to seawater sulfate and may record different pathways of sulfur cycling in sulfidic water columns as well as within sediments the new data from the east greenland basin indicate that environmental stress such as widespread sulfidic conditions could have caused the biotic crisis in the late permian

environmental controls on marine ecosystem recovery following mass extinctions with an example from the early triassic abstract the recovery of marine ecosystems following a mass extinction event involves an extended interval of increasing biotic diversity and ecosystem complexity the pace of recovery may be controlled by intrinsic ecosystem or extrinsic environmental factors here we present an analysis of changes in marine conditions following the end permian mass extinction with the objective of evaluating the role of environmental factors in the protracted 5 myr long recovery of marine ecosystems during the early triassic specifically our study examines changes in weathering productivity and redox proxies in three sections in south china chaohu daxiakou and zuodeng and one in northern india mud our results reveal 1 recurrent environmental perturbations during the early triassic 2 a general pattern of high terrestrial weathering rates and more intensely reducing marine redox conditions during the early griesbachian late griesbachian mid smithian and more weakly the mid spathian 3 increases in marine productivity during the aforementioned intervals except for the early griesbachian and 4 stronger and more temporally discrete intervals of environmental change in deepwater sections chaohu and daxiakou relative to shallow and intermediate sections zuodeng and mud our analysis reveals a close relationship between episodes of marine environmental deterioration and a slowing or reversal of ecosystem recovery based on metrics of biodiversity within community alpha diversity infaunal burrowing and ecosystem tiering we infer that the pattern and pace of marine ecosystem recovery was strongly modulated by recurrent environmental perturbations during the early triassic these perturbations were associated with elevated weathering and productivity fluxes implying that nutrient and energy flows were key influences on recovery more regular secular variation in deepwater relative to shallow water environmental conditions implies that perturbations originated at depth i e within the oceanic thermocline and influenced the ocean surface layer irregularly finally we compared patterns of environmental disturbance and ecosystem recovery following the other four big five phanerozoic mass extinctions to evaluate whether commonalities exist in general the pace of ecosystem recovery depends on the degree of stability of the post crisis marine environment

episodic euxinia in the changhsingian late permian of south china evidence from framboidal pyrite and geochemical data abstract a multiproxy study of a new upper permian lower triassic section xiaojiaba in sichuan province china documents large changes in marine productivity redox conditions and detrital input prior to the latest permian mass extinction marine productivity as proxied by total organic carbon content toc biogenic sio 2 and excess barium displays a long term decline through most of the changhsingian stage late late permian culminating in very low values around the permian triassic boundary concurrently redox proxies including pyrite framboid 34 s py mo auth and u auth and c org p document a shift from suboxic to dysoxic oxic conditions that was interrupted by several episodes of benthic euxinia and detrital siliciclastic proxies al hf nb and rees suggest an increased flux of weathered material from land areas the long term changes in productivity redox conditions and terrigenous detrital fluxes were probably caused by a regional sea level fall across the south china craton on the other hand the brief euxinic episodes occurring during the late permian had oceanographic causes probably related to the transient upward expansion of the chemocline at the top of the oceanic oxygen minimum zone these euxinic episodes may have been harbingers of the more widespread anoxia that developed concurrently with the latest permian mass extinction and that may have played a major role in triggering the largest biotic crisis of the phanerozoic

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

oxygenation history of bottom waters in the cariaco basin venezuela over the past 578 000 years results from redox sensitive metals mo v mn and fe we present results from analyses of the redox sensitive metals mo v mn and fe in sediment recovered from the cariaco basin ocean drilling program leg 165 site 1002 results are interpreted in the context of previous studies of 15n export production percent total organic carbon eolian input and hemipelagic deposition in the basin variations in redox metal ratios over the past 578 000 years were compared to variations in 18o at milankovitch frequencies and show a strong relationship between glacial interglacial cycles in sea level governed by the shallow sills encircling cariaco basin and bottom water oxygen content during 100 kyr and 41 kyr cyclicity enrichments of mo and v occur during highly productive interglacials indicating bottom water anoxia during glacials sediments are less depleted or enriched in mn and fe relative to the interglacials reflecting oxic conditions during 23 kyr and 19 kyr cyclicity however these redox metal patterns are not observed indicating that the cariaco basin responds differently to the higher frequency climate changes

chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in earth history the neoproterozoic shuram formation nafun group oman abstract oman records in its neoproterozoic nafun group huqf supergroup an essentially complete carbonate rich ediacaran succession the nafun group overlies the presumed marinoan rift related fiq member ca 635 ma and ends just below the precambrian cambrian boundary 542 ma tectonic subsidence caused by thermal contraction following fiq aged rifting allowed the preservation of about 1 km of postrift stratigraphy with no major stratigraphic breaks the nafun group above the marinoan cap carbonate hadash formation represents two siliciclastic to carbonate grand cycles both initiated by significant transgressions these cycles comprise the masirah bay khufai formations and the shuram buah formations the khufai shuram boundary is associated with the start of a major carbon isotope perturbation the uppermost ramp carbonates of the khufai formation record a smooth decrease in 13c from about 4 to values around 0 followed by two descending steps across which values plunge to a nadir of 12 in the overlying red siltstones and shales interbedded with thin limestones of the shuram formation this fall in isotopic values is temporally rapid and coincident in both shallow and deep water sections in the time span of a single parasequence the 13c nadir is then followed by 50 million years of monotonic recovery the shuram shift represents the largest 13c inorganic carbon negative excursion in earth history although the snowball earth theory links periods of depleted carbon isotopic ratios with periods of global glaciation the non glaciated context of the shuram formation suggests that the causal relationship between global glaciation and negative carbon isotopic excursions is non unique although the precise mechanism driving this major perturbation of the carbon cycle remains enigmatic the long term remineralization of an isotopically depleted organic carbon reservoir in ocean water is a promising candidate

relationship between inferred redox potential of the depositional environment and geochemistry of the upper pennsylvanian missourian stark shale member of the dennis limestone wabaunsee county kansas u s a analyses of 21 samples collected from a core of the 52 8 cm thick stark shale member of the dennis limestone in wabaunsee county kansas demonstrate four cycles with two orders of magnitude variations in contents of cd mo p v and zn and order of magnitude variations in contents of organic carbon cr ni se and u the observed variability in amounts and or ratios of many metals and amounts and compositions of the organic matter appear related to the cause and degree of water column stratification and the resulting absence presence of dissolved o2 or h2s high cd mo u v zn and s contents a high degree of pyritization dop 0 75 0 88 and high high v v ni 0 84 0 89 indicate the presence of h2s in a strongly stratified water column intermediate contents of metals and s intermediate dop 0 67 0 75 and intermediate v v ni 054 0 82 indicate a less strongly stratified anoxic water column whereas low metal contents and low v v ni 0 46 0 60 indicate a weakly stratified dysoxic water column high p contents at the top of the organic matter rich intervals within the stark shale member indicate that phosphate precipitation was enhanced near the boundary between anoxic and dysoxic water compositions relatively abundant terrestrial organic matter in intervals deposited from the more strongly stratified h2s bearing water column indicates a combined halocline thermocline with the fresher near surface water the transport mode for the terrestrial organic matter the predominance of algal organic matter in intervals deposited from a less strongly stratified water column indicates the absence of the halocline and the presence of the more generally established thermocline relatively low amounts of degraded hydrogen poor organic matter characterize intervals deposited in a weakly stratified dysoxic water column the inferred variability in chemistry of the depositional environments may be related to climate variations and or minor changes in sea level during the general phase of deeper water deposition responsible for this widespread shale member