rare earth elements in the permian phosphoria formation paleo proxies of ocean geochemistry abstract the geochemistry of deposition of the meade peak member of the phosphoria formation mpm in southeast idaho usa a world class sedimentary phosphate deposit of permian age that extends over 300 000 km2 is ascertained from its rare earth element ree composition ratios of ree al2o3 suggest two sources seawater and terrigenous debris the seawater derived marine fraction identifies bottom water in the phosphoria sea as o2 depleted denitrifying suboxic most of the time and seldom sulfate reducing anoxic this interpretation is supported by earlier research that showed progressively greater ratios in the marine sediment fraction of cr ni v ni mo ni relative to their ratios in seawater for which marine cr v and mo can have a dominantly o2 depleted bottom water source and ni a photic zone largely algal source the water chemistry was maintained by a balance between bacterial oxidation of organic matter settling through the water column determined largely by primary productivity in the photic zone and the flux of oxidants into the bottom water via advection of seawater from the open ocean samples strongly enriched in carbonate fluorapatite the dominant ree host mineral have variable er sm tm sm and yb sm ratios their distribution may represent greater advection of seawater between the phosphoria sea and open ocean during deposition of two ore zones than a center waste and greater upwelling of nutrient enriched water into the photic zone however the mean rate of deposition of marine ni a trace nutrient of algae and po43 a limiting nutrient indicate that primary productivity was probably high throughout the depositional history an alternative interpretation of the variable enrichments of er tm and yb relative to sm is that they may reflect temporally variable carbonate alkalinity of open ocean seawater in permian time a more strongly negative ce anomaly for all phosphatic units than the ce anomaly of modern pelletal phosphate is further indicative of an elevated o2 concentration in the permo carboniferous open ocean as proposed by others in contrast to the depletion of o2 in the bottom water of the phosphoria sea itself the oceanographic conditions under which the deposit accumulated were likely similar to conditions under which many sedimentary phosphate deposits have accumulated and to conditions under which many black shales that are commonly phosphate poor have accumulated a shortcoming of several earlier studies of these deposits has resulted from a failure to examine the marine fraction of elements separate from the terrigenous fraction

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

k ar dating of miocene magmatism and related epithermal mineralization of the northeastern altiplano of bolivia this paper describes the age and geology of 13 centers of miocene magmatism with related epithermal precious and base metal mineralization on the eastern side of the northern altiplano of bolivia part of the central andean polymetallic belt twenty eight new k ar ages are reported together with a compilation of 12 from the literature the data show four magmatic episodes the two middle miocene episodes 17 15 ma 13 5 12 ma are characterized by clusters of small volcanic and hypabyssal porphyries which host the most important epithermal mineralization in contrast large volcanic centers formed during the two late miocene episodes 10 8 ma 6 5 ma no significance is attached to the secondary age groups the middle and late miocene magmatic mineralization episodes correlate with those of the southern part of the tin belt of the eastern cordillera and formed during the main arc broadening of the central andes

isotope stratigraphy of neoproterozoic cap carbonates in the araras group brazil abstract the neoproterozoic carbonate sequence on the southeastern border of the amazon craton is divided into three lithostratigraphic units a basal cap dolomite an intermediate limestone limestone mudstone unit and an upper dolarenite dolorudite unit sections of the cap carbonate were measured from the inner shelf to the outer shelf carbon isotope ratios relative to pdb vary between 10 5 and 1 7 in cap dolomite and between 5 4 and 0 1 in laminated limestone and mud limestone limestones and mud limestones exhibit 87sr 86sr ratios ranging from 0 70740 to 0 70780 a comparative isotope stratigraphy between the inner shelf and the middle shelf basin shows differences in carbon isotope ratios the cap dolomite and limestones have lower 13c ratios on the border of the basin inner shelf than in the middle shelf of the basin these lower values can be related to shallower environmental conditions and to a stronger influence of the continental border the 87sr 86sr ratios are the same in both areas and are consistent with seawater composition at around 600 ma

paleodepositional conditions in the orca basin as inferred from organic matter and trace metal contents abstract the orca basin located in the gulf of mexico is a highly stratified basin that collects sedimentary particles of clastic origin supplied by the mississippi river and of biogenic origin marine surface productivity the strong pycnocline induces anoxic bottom conditions that are expected to be favorable to organic matter om accumulation and prone to enrichment of sediments in redox sensitive trace metals here we report on om and trace metal contents in the upper 750 cm below sea floor cmbsf of core md02 2552 mostly holocene and deposited under permanently stratified bottom conditions the organic content is dominated by marine derived amorphous om contrary to expectations sedimentary om in the basin is highly degraded probably due to a long residence time of organic particles at the basin s basin s halocline at 2240 m also unexpected is the limited sedimentary enrichment of redox sensitive trace metals we suggest that the non enrichment in u and v could be explained by the so called basin reservoir effect that is limited metal availability because of low rates of water renewal in a strongly restricted subpycnoclinal water mass the non enrichment in cu could have the same cause plus the fact that the relatively degraded om may not readily form organometallic complexes however a marked enrichment is observed for mo and a moderate one for ni and cr which could be accounted for by sporadic presence of h 2 s in bottom waters such occurrences of h 2 s would not be sufficient to promote a detectable fes to fes 2 conversion but they would induce the capture of mo in the form of thiomolybdate and accessorily ni cr as ni cro 4 complexes

the inorganic geochemistry of cretaceous black shales dsdp leg 41 in comparison to modern upwelling sediments from the gulf of california

the hercynian gabbro tonalite granite leucogranite suite of iberia geochemistry and fractionation detailed geochemical and geochronological study of hercynian granites and related rocks in portugal and w spain the ibergranite project has inspired new ideas about their origin formerly regarded as the sole products of crustal anatexis trace element petrology shows descent by crystal fractionation from mantle derived parent magmas of basic composition

extreme winds and waves in the aftermath of a neoproterozoic glaciation the most severe excursions in the earth s climatic history are thought to be associated with proterozoic glaciations according to the snowball earth hypothesis the marinoan glaciation which ended about 635 million years ago involved global or nearly global ice cover at the termination of this glacial period rapid melting of continental ice sheets must have caused a large rise in sea level here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant wave ripples these structures occur at homologous stratigraphic levels in australia brazil canada namibia and svalbard our hydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds significantly longer than those typical for today s oceans the reconstructed wave conditions could only have been generated under sustained high wind velocities exceeding 20 metres per second in fetch unlimited ocean basins we propose that these extraordinary wind and wave conditions were characteristic of the climatic transit and provide observational targets for atmospheric circulation models

low latitude and multiple geomagnetic reversals in the neoproterozoic puga cap carbonate amazon craton palaeomagnetic study of the carbonates that ubiquitously cap glacial deposits may constrain the latitudinal extent of neoproterozoic glaciations and the duration of the greenhouse recovery we present the first palaeomagnetic data on the neoproterozoic cap carbonates covering the amazon craton which are folded along the paraguay belt samples collected at deformed beds along the paraguay belt present a single polarity secondary magnetization acquired by the end of the brasiliano orogeny 540 520 ma in the cratonic area a dual polarity component was isolated in dolostones at the base of the sequence the presence of a stratabound reversal stratigraphy along with high unblocking temperatures strongly suggest that this magnetization is primary this result implies a low palaeolatitude 22 6 5 for the amazon block just after deposition of puga diamictites in addition the presence of multiple reversals across the first 20 m of the cap carbonate sequence suggests that their sedimentation must have spanned hundreds of thousands of years at least

paleoenvironmental reconstruction of the ediacaran araras platform western brazil from the sedimentary and trace metals record ocean and atmosphere oxidation in the ediacaran period paved the way for the dawn of animals in an attempt to better document the record of seawater redox state at the onset of the ediacaran we performed a paleoenvironmental study of post marinoan 635 ma carbonate strata from the southeastern margin of the amazon craton western brazil five sections were sampled along the araras carbonate platform outcrop based facies analysis complemented by petrographic description of representative samples was performed on these sections seven facies associations fa were recognized four fa are encountered in the inner shelf from the basal glacio marine deposits of the puga formation to shallow and moderately deep platform facies which are systematically covered by deeper caco3 over saturated facies five fa occur on the outer shelf including storm wave influenced facies below storm wave base facies and slope intraformational breccia these facies associations indicate a transgressive systems tract over the carbonate platform trace metals u mo zn pb cd cu ni v and al concentrations pyrite abundance and total organic carbon toc contents in these sections are generally low they nonetheless present from base to top of the sections significant stratigraphic variations which can be traced along the platform with successive enrichments of i pb and zn ii u and iii both u and mo in the thin marl levels containing the highest amount of organic carbon and pyrite 0 4 and 1 9 respectively we interpret this succession to record the progressive evolution of sediment pore waters from oxic to more reducing conditions driven by an increase in sedimentary organic matter accumulation the lack of evidence for persistent sulphide mineralization and associated enrichments in mo suggests that sulfidic conditions were only achieved in sediment pore waters in the aftermath of the marinoan glaciation the water column must then have been essentially oxic on the araras platform with anoxia only sporadically reaching the sediment water column interface in the deepest parts of the platform

stratigraphy sedimentology and isotopic geochemistry of australian neoproterozoic postglacial cap dolostones deglaciation delta 13 c excursions and carbonate precipitation abstract thin d13c excursions in the geologic record they are interpreted to record a brief postglacial anomaly in ocean chemistry in australia the marinoan varanger equivalent 600 ma cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe including fine grained dolomitic mineralogy lateral persistenc at basinal scales thin laminae graded beds intervals with abundant marine cements crystal fans and a distinctive negative d13c isotopic signature of up to 5 pdb on the basis of these features the australian examples are interpreted here to be deeper water deposits below storm wave base resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression detailed isotopic analysis of australian cap dolostones indicates d13c values ranging from 1 to 5 and gen rally becoming more depleted in 13c upsection trace element data indicate some diagenetic stabilization but textural evidence and the presence of similar profiles in different basins argue against a pervasive recrystallization event the range in d13c values between sections as well as more negative 13c values appear to correlate with greater paleobathymetry within basins this implies either 1 only partial preservation of the complete oceanic variation of d13c or 2 precipitation of this peculiar facies owes its genesis to basinal specific oceanographic processes such as proximi y of a given section to a postglacial upwelling zone in the later case d13c values would not represent whole ocean values stratigraphic constraints and paleoenvironmental interpretations suggest that much of this excursion lies within the geologically brief period of postglacial transgression this implies that the cause s of d13c variation likely operated on time scales significantly less than the residence time of carbon in the oceans 105 yr the continent wide and perhaps global nature of cap dolostones indicates that large volumes of carbonate were precipitated during the postglacial transgressive period such large scale carbonate precipitation over the hypothesized short time interval of postglacial transgression must have caused or have been the product of profound changes in the carbon cycle and global climate at tha time similar evidence for transgression and increased carbonate deposition in the holocene are attributed to the changing basin shape and ph of the oceans the coral reef hypothesis of berger 1982 cap dolostones may record a proterozoic equivalent of this process with the substitution of abiotic carbonate precipitation for skeletal precipitation by reef organisms

glacially influenced sedimentation in the later proterozoic of the paraguay belt mato grosso brazil abstract late proterozoic probably early vendian 610 590 ma glaciogenic and turbiditic sediments and metasediments are present along the transitional zone between the amazonian craton to the northwest and the paraguay belt to the southeast in the cuiaba region mato grosso brazil a few exposures of diamictites are associated with conglomerates and graded pebbly sandstones on the cratonic fringe and pass southeastwards into fine grained sediments deposited in the deeper part of the probably marine basin in the paraguay belt these rocks have been affected by the brasiliano orogeny possibly around 570 ma they show increasing deformation and metamorphism from the craton towards the fold belt the sedimentation scheme proposed for the western border of the paraguay belt and the eastern edge of the amazonian craton involves three probably marine main depositional systems to the west on the craton the platform domain can be subdivided into a western inner shelf and an eastern outer shelf the deposits on the inner shelf subdomain show an alternation of dominant massive diamictites sandstones and fine grained sediments with few dropstones this diamictite association is tentatively interpreted as glaciomarine in origin in the outer shelf subdomain the massive diamictites are progressively replaced southeastwards by an association of massive diamictites stratified diamictites and fine grained sediments this diamictite association is interpreted as a whole as resedimented glacial debris deposited by subaqueous debris flows the slope domain located at the transition between craton and fold belt is characterized by strong reworking of the glaciomarine sediments by gravity flows responsible for a type of submarine fan deposition the coarse clastic facies which result from a reworking by rapid downslope transport of cohesive debris flows and gravity flows with high viscosity fill feeder channels in the upper fan eastward inside the belt a progressive sorting in the deeper portions of the fan is shown by the transition from diamictites massive gravels sandstone intercalations with occasional inverse and or normal grading debris flow turbidity current to normal graded finer sediments turbidity current sandstone and siltstone intercalations represent interchannel deposits formed by turbidity currents east of the slope well bedded fine grained sediments were generated by low density turbidity currents direct glacial influence is reduced to the presence of a few isolated clasts or dropstones two distinct upper proterozoic glaciogenic units are known in brazil the older and more widespread one dated at about 950 ma is located on the sao francisco craton and in the adjacent brasiliano fold belts the younger one 600 ma is restricted to the paraguay belt and the adjacent border of the amazonian craton in southern western gondwana late proterozoic glaciation is of mountain type and developed at latitudes of 60 on highs probably generated by pan african brasiliano rifting at the same time in its northern part a widespread ice sheet covered the west african craton

paleoceanographic applications of trace metal concentration data abstract recent studies have identified a range of new applications of trace metal concentration data in the analysis of paleoceanographic systems in restricted anoxic marine systems trace metal toc ratios can provide insight into the degree of watermass restriction and estimates of deepwater renewal times in such systems secular changes in sediment trace metal ratios may provide evidence of the chemical evolution of basinal deepwaters in response to differential rates of trace metal removal to the sediment the degree of deepwater restriction in silled basins is generally controlled by eustatic elevations with higher lower sea levels resulting in lesser greater watermass restriction short term ca million year drawdown of the trace metal inventory of seawater has occurred repeatedly in conjunction with oceanic anoxic events which result in elevated rates of trace metal removal to widespread anoxic facies long term eon scale changes of the trace metal composition of seawater have occurred in response to secular changes in atmospheric oceanic redox conditions and their effects on trace metal cycling caution must be exercised in evaluating trace metal patterns in paleomarine systems however 1 hydrographic analyses based on trace metal toc relationships can be undertaken only on systems that had anoxic deepwaters and 2 the influence of redox variation on trace metal accumulation patterns outweighs that of hydrographic factors in some paleomarine systems

chemostratigraphy of the neoproterozoic mirassol d oeste cap dolostones mato grosso brazil an alternative model for marinoan cap dolostone formation abstract we have conducted a detailed study of the neoproterozoic mirassol d oeste cap dolostones that overlay the glacial diamictites of the puga formation 635 ma amazon craton brazil in order to understand the formation of these post glacial dolostones petrographic features indicate that the dolostones are primary to early diagenetic in origin and precipitated in a moderately shallow water platform corresponding to a carbonate ramp during transgressive conditions major and trace element contents as well as c and o isotopic signatures are consistent with an anoxic sediment influenced by sulphate reducing bacteria such an environment is known to provide favourable conditions for the precipitation of dolomite as observed nowadays in modern hypersaline lagoons isotopic compositions of tube like structures suggest local upward fluid seepage from the underlying cap dolostone our data concur with geochemical data from other neoproterozoic cap dolostones to support a microbially mediated model in specific environmental conditions for the formation of these unusual deposits worldwide

neptunian dykes and associated features in southern spain mechanics of formation and tectonic implications neptunian dykes and sills in jurassic carbonate platform strata of the betic region of spain occupy spaces created by wholly mechanical fracturing and displacement of the host strata and later filled by pelagic sediments from above or by precipitation of calcite from circulating solutions in some places joint bounded blocks of platform carbonates have been wholly removed possibly by sliding down submarine slopes leaving a staircase topography commonly fe encrusted that was subsequently filled by pelagic sediments other cavities that formed during cretaceous times were developed by dissolution and current erosion in moderately deep submarine environments and then filled by pelagic sediments from above none of the cavities hosted by either platform or pelagic strata contain evidence for their formation by dissolution in a subaerial environment the jurassic and cretaceous history of subsidence of the betic margins is thus simpler than in versions requiring repeated emergence to form subaerial karstic cavities

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

cycling of trace metals mn fe mo u v cr in deep pore waters of intertidal flat sediments trace metals mn fe mo u cr v were studied in pore waters of an intertidal flat located in the german wadden sea the study system is an example of a permeable tidal flat system where pore water exchange is affected by tidal driven pressure gradients besides diffusion permanently installed in situ samplers were used to extract pore waters down to 5 m depth throughout one year the samplers were either located close to the tidal flat margin or in central parts of the tidal flat despite dynamic sedimentological and hydrological conditions the general trends with depth in deep tidal flat pore waters are remarkably similar to those observed in deep sea environments rates of trace metal cycling must be comparably large in order to maintain the observed pore water profiles trace metals further show similar general trends with depth close to the margin and in central parts of the tidal flat seasonal sampling revealed that v and cr vary concurrent with seasonal changes in dissolved organic carbon doc concentration this effect is most notable close to the tidal flat margin where sulphate doc and nutrients vary with season down to some metres depth seasonal variations of mn fe mo and u are by contrast limited to the upper decimetres of the sediment their seasonal patterns depend on organic matter supply redox stratification and particulate matter deposited on sediment surfaces pore water sampling within one tidal cycle provides evidence for pore water advection in margin sediments during low tide pore water flow towards the creekbank is generated by a hydraulic gradient suggesting that deep pore waters may be seeping out of creekbank sediments owing to the enrichment of specific elements like mn in pore water compared to sea water seeping pore waters may have an impact on the chemistry of the open water column mass balance calculations reveal that the impact of deep pore waters on the mn budget in the open water column is below 4 mn deep pore water discharge of the whole wadden sea is estimated to be about 9 of the total dissolved riverine mn input into the southern north sea

contrasting molybdenum cycling and isotopic properties in euxinic versus non euxinic sediments and sedimentary rocks refining the paleoproxies abstract the redox sensitive transition element molybdenum is present in the earth s crust at trace concentrations but is abundant in seawater marine sediments and ancient sedimentary rocks these occurrences have led to the development of a suite of paleoredox proxies based on the bulk concentration of mo its isotopic composition and the covariation of mo with total organic carbon toc in ancient black shales however these proxies have almost exclusively targeted the identification and interpretation of euxinic environments where bottom waters are both anoxic and sulfidic here we present a discussion of the mo geochemistry of non euxinic sediments where hydrogen sulfide is sometimes present yet always restricted to pore waters we propose a new paleoredox application that uses mo concentrations to help distinguish between environments where sulfide was present in the bottom waters and environments where sulfide was restricted to pore waters under ideal conditions it is also possible to infer the presence of mn oxides recycling in non euxinic paleoenvironments based on the mo isotope composition of ancient black shales providing a critical constraint on bottom water oxygenation

the vendian succession of northeastern spitsbergen petrogenesis of a dolomite tillite association abstract the sedimentary record of late precambrian time is magnificently displayed in the highland snowfields of northeastern spitsbergen svalbard vendian strata are represented essentially by the polarisbreen group which consists mostly of dolostone and includes two dolomitic glacial units the oldest sediments in the polarisbreen group compose the elbobreen formation c 400 m which is divided into four laterally persistent members the lower carbonate member e1 125 m contains a distinctive basal dark grey limestone with microspar filled synaeresis cracks suggested to be of lagoonal origin and associated with minor dolostone shale and chert higher parts of the member are dominantly dolostone partly stromatolitic with some shale and sandstone shallow subtidal to intertidal deposition is indicated by the dominance of intraclastic lithologies and relics of anhydrite penecontemporaneous dolomite is partially overprinted by microsparry dolomite thought to be of groundwater origin the redefined petrovbreen member e2 consists of diamictite and other detrital dolostone pronounced thickness variations 2 40 m are thought to be original depositional features the member represents the deposits of a short glacial period in which the following depositional processes are inferred lodgement massive diamictite subaqueous meltout massive and bedded diamictite ice rafting lithologies bearing dropstones and possibly also diamictite redeposition by sediment gravity flows some diamictite and conglomerate rhythmite and shale current winnowing thin tabular conglomerate subaerial or subaqueous meltwater action channelled conglomerate and sandstone periglacial shrinkage diamictite wedge fillings the macdonaldryggen member e3 230 m is a monotonous succession of shaly dolostone of lagoonal origin it grades up into the slangen member e4 25 m which consists of subtidal to intertidal dolarenite with anhydrite relics succeeded by fenestral dolostone that was fractured and cemented by saline groundwaters in an emergent environment the wilsonbreen formation 160 m represents a return to glacial deposition but this time longer lasting and with substantial extra basinal material represented the gropbreen member w1 28 107 m and the ormen member w3 44 139 m consist dominantly of dolomitic diamictite with subordinate conglomerate and sandstone and are separated by a middle carbonate member w2 3 30 m which contains distinctive rhythmitic and stromatolitic limestone as well as sandstone the same depositional processes can be recognised as in the petrovbreen member but the wilsonbreen formation is overall of somewhat more continental aspect lower proportion of rhythmite and dropstone structures in addition there are basal breccia and crack fillings formed by frost shattering of the underlying cemented dolostone tabular sandstone thought to be formed by wave reworking of outwash a striated terrestrial cobble pavement supraglacially derived breccia horizons periglacial wedges filled by sand and the w2 assemblage of possible lacustrine origin the dracoisen formation 525 m represents an abrupt return to non glacial conditions an upward deepening wave dominated succession of pure dolostone d1 20 m and impure dolostone d2 105 m is succeeded by offshore black shale d3 150 m and then by a very shallow water succession of evaporite lacustrine aspect with a dolostone containing evaporite relics d5 10 m separating dolomitic sandstone and shale d4 80 m and d6 150 m the contact with the transgressive cambrian sandstones above is poorly exposed deposition of the succession dominantly under marine conditions is inferred but it is difficult to rule out a lacustrine environment at any particular horizon this dolomite tillite association can be explained by penecontemporaneous and minor secondary dolomite formation in marginal environments with warm climatic indicators at some levels being sharply interrupted because of rapid climatic changes by glacial sediments containing abundant detrital dolomite since the latter sediments make up only 17 of the 1080m thick succession glacial conditions only occupied a small proportion of vendian time

environmental analysis of paleoceanographic systems based on molybdenum uranium covariation abstract modern low oxygen marine systems exhibit patterns of molybdenum uranium covariation that can be linked to specific attributes and processes of the depositional system including 1 variation in benthic redox conditions 2 the operation of particulate shuttles within the water column and 3 the evolution of watermass chemistry the importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum mo auth and uranium u auth and from the mo u auth ratio of the sediment relative to the seawater mo u molar ratio of 7 5 7 9 in open ocean systems with suboxic bottomwaters u auth enrichment tends to exceed that of mo auth owing to onset of u auth accumulation at the fe ii fe iii redox boundary resulting in sediment mo u auth ratios less than that of seawater as bottomwaters become increasingly reducing and at least occasionally sulfidic the rate of accumulation of mo auth increases relative to that of u auth and sediment mo u auth ratios equal or exceed that of seawater in restricted marine systems with permanently sulfidic deepwaters the relative enrichment of mo auth and u auth depends on additional factors in the cariaco basin which has an aqueous mo u ratio similar to that of seawater the operation of a particulate mn fe oxyhydroxide shuttle serves to accelerate the transfer of mo to the seafloor leading to strong enrichments in mo auth relative to u auth in the black sea the chemistry of the deep watermass has evolved to the point where its aqueous mo u ratio is only 0 04 that of seawater as a consequence of which sediments deposited under deepwater influence are depleted in mo auth relative to u auth these mo u covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low oxygen marine systems analysis of anoxic facies from two north american paleomarine systems the late pennsylvanian midcontinent sea lpms and the late devonian seaway lds reveals authigenic mo u relationships similar to those of the modern marine environments above implying similar redox and hydrographic controls the observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the lpms and with markedly restricted deepwater circulation in silled basins of the lds patterns of authigenic mo u covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

progressive oxidation of anoxic and ferruginous deep water during deposition of the terminal ediacaran laobao formation in south china abstract the laobao formation a black chert sequence was deposited in deep water in the terminal ediacaran nanhua basin the chert samples have finely laminated fabric and have no distinct recrystallization or metasomatic relict texture there is an extremely limited detrital component and no obvious hydrothermal influence therefore the chert appears to be an authigenic deep water precipitate the trace elements in the chert were mainly derived from seawater and thus could be a relatively unique proxy for late precambrian seawater conditions redox sensitive trace elements and relevant ratios were used to reconstruct the depositional environment of the chert sequence combined with iron species results of the laobao formation enrichments in redox sensitive trace elements u v mo depleted mn very low th u ratios 7 9 and v v ni 0 6 ratios suggest an overall anoxic and ferruginous conditions over the deposition of the chert the pyrite sulfur isotope compositions are between 15 4 and 11 0 and show a decreasing trend toward the top of the section possibly implying sulfate concentration gradually increased in the terminal ediacaran nanhua basin which is consistent with implication of their pyrite framboid diameter characteristics our data confirm that deep water in the nanhua basin was still anoxic and ferruginous and it was progressively oxidized during the terminal ediacaran period which could create necessary conditions for the emergence and rapid evolution of multicellular metazoans

searching for an oxygenation event in the fossiliferous ediacaran of northwestern canada abstract late neoproterozoic ediacaran strata from northwestern canada provide a thick and rich sedimentological record preserving intercalated carbonates and shale extending from the 635 million year old marinoan glacial deposits up through the 541 million year old precambrian cambrian boundary this region also holds one of the classic localities for the study of early animal life with the ensuing suggestion that this temporal interval captures a gross change in the o 2 content of the earth s atmosphere to test this hypothesis and bring records of northwestern canada into line with other ediacaran fossil bearing basins we provide a detailed geochemical reconstruction from the wernecke mountains of the yukon where possible we also extend these records to the ogilvie mountains to the west and previously published data from the mackenzie mountains to the east our work in the wernecke mountains is set against a composite 13 c record for carbonate that preserves three distinct ediacaran isotope excursions the lowermost of which preserved in the gametrail formation is a putative shuram excursion equivalent what emerges from a multi proxy fe speciation sulfur isotopes major and trace element analyses reconstruction is a picture of a persistently anoxic and ferruginous ediacaran ocean notably absent is geochemical evidence for a prominent oxygenation event an expectation given the appearance of animals and large swings in 13 c the new insight gained through these data challenges the idea of an ediacaran jump in atmospheric oxygen which in turn muddles the link between animal evolution and local geochemical environments

pitfalls of normalization of marine geochemical data using a common divisor abstract normalization of trace element contents of sediment through division by the content of an immobile element usually aluminum is common practice in marine geochemical studies this might appear to be a simple way of correcting for dilution by sedimentary phases barren of a particular trace element for instance carbonates or silica and for comparison with its content in a standard clay or shale the purpose of this paper is to revive the awareness of the pitfalls of this practice by giving numerical examples of the often unexpected results of normalization taking a statistical perspective it is first shown that uncorrelated variables acquire spurious correlations when normalized but normalization can also increase decrease change sign of or even blur the correlations between unmodified variables next a number of realistic scenarios are worked out to show that the correlations between normalized element contents still suffer from the closure effect only in a few simple cases it is possible to extract from the normalized data realistic estimates of trace element contents in distinct sedimentary phases such as organic matter this is ascertained from data on copper contents in sediment from the black sea and arabian sea when the coefficient of variation i e standard deviation divided by the mean of the aluminum data is relatively low and much lower than for the original values of the trace elements comparisons of correlations can equally well or even better be made on the basis of the unmodified values then the comparison of trace element data with their values for standard shales is discussed the inherent problem is that the composition of the commonly used standard shale and consequently the reference values of normalized elements are not necessarily representative of the local or regional sediments in the study area also this method falls short of identifying the processes responsible for enrichment or diminution in trace element contents finally consideration is given to some aspects of a proper use of al normalization

organic carbon isotope constraints on the dissolved organic carbon doc reservoir at the cryogenian ediacaran transition abstract prominent negative carbonate carbon isotope 13 c carb anomalies from some ediacaran successions are accompanied by invariant or decoupled organic carbon isotope 13 c org values and have been interpreted as resulting from the remineralization of a large dissolved organic carbon doc reservoir capable of buffering carbon isotopes of organic matter this inferred oceanic doc reservoir was thought to have initiated with the onset of cryogenian glaciations ca 720 ma and lasted for millions of years until the late ediacaran period 13 c org around 26 vpdb and relatively low 13 c 22 2 and 2 the overlying organic rich black shale and shaly dolostone have more negative 13 c org 28 to 35 and higher 13 c 28 30 both 13 c carb and 13 c org show a 6 shift within a 4 m thick interval overlying the doushantuo cap carbonate the 13 c org values of the cap carbonate are associated with low toc mostly 13 c carb and 13 c org following cap carbonate deposition is best interpreted as resulting from a rapid increase in organic carbon burial which may have resulted in the rise of oxygen and heralded the first appearance of animals a few meters above the doushantuo cap carbonate the data suggest that a large oceanic doc reservoir did not exist in the early ediacaran ocean excess oceanic doc required to explain the ediacaran shuram and upper doushantuo 13 c excursions if it existed had to be developed during the ediacaran period after cap carbonate deposition

a new particulate mn fe p shuttle at the redoxcline of anoxic basins abstract pelagic redoxclines of anoxic basins and deeps form the suboxic transition between oxygenated surface and anoxic or even sulfidic bottom waters intense element cycling favoured by elevated microbial activity causes steep gradients of physico chemical parameters nutrients and redox sensitive trace metals this study presents a conceptual model for authigenic particle formation at pelagic redoxclines which is based on the tight coupling of mn fe and p cycles besides the well known occurrence of mn oxides textural sem edx and geochemical icp oes icp ms analyses of particles from the redoxclines of the black sea and the baltic sea gotland basin landsort deep evidence the existence of earlier postulated fe oxyhydroxo phosphates and emphasize mixed phases consisting of mn oxides and fe oxyhydroxo phosphates as a new solid species most of the analyzed particles are star shaped of about 5 m in size and occur as single particles or aggregates without any morphological differences between mn oxides fe oxyhydroxo phosphates and mixed phases throughout the redoxcline these minerals show a general succession with maximum abundance of mn oxides above the redoxcline followed by mixed phases and almost pure fe phosphates within and below the redoxcline respectively molar fe p ratios of single particles argue against the formation of known pure fe phosphates like vivianite or strengite at the lower end of the redox transition zone but are consistent with recent experimental findings for colloidal p bearing hydrous ferric oxides moreover morphological similarities suggest the formation of irregular fe oxyhydroxo coatings due to oxidation of upward diffusing fe 2 by oxygen and stepwise replacement of mn iv by fe iii on sinking mno x particles followed by immediate adsorption or even co precipitation of phosphate batch type experiments using biogenic mno x particles demonstrate the efficient potential of fe 2 oxidation by sinking mno x particles when entering sulfidic waters mno x particles are progressively reduced leading to an increasing relative abundance of fe and p rich particles in deeper parts of the water column these particles are also reductively dissolved thereby releasing fe 2 and phosphate to the water column this mn fe p shuttle likely affects phosphate transport throughout the water column and thus impacts primary production at least over longer time scales furthermore the particulate mn fe p shuttle must have played an important role for the cycling of p and certain trace metals in ancient ocean basins e g during certain periods of cretaceous black shale formation and should be considered in future mass balances and modeling approaches dealing with oxic anoxic interfaces of aquatic ecosystems

analysis of marine environmental conditions based onmolybdenum uranium covariation applications to mesozoic paleoceanography article i nfo article history accepted 13 september 2011 available online 17 september 2011 patterns of uranium molybdenum covariation in marine sediments have the potential to provide insights re garding depositional conditions and processes in paleoceanographic systems specifically such patterns can be used to assess bottom water redox conditions the operation of metal oxyhydroxide particulate shuttles in the water column and the degree of water mass restriction the utility of this paleoenvironmental proxy is due to the differential geochemical behavior of u and mo 1 uptake of authigenic u by marine sediments begins at the fe ii fe iii redox boundary i e suboxic conditions whereas authigenic mo enrichment re quires the presence of h2s i e euxinic conditions and 2 transfer of aqueous mo to the sediment may be enhanced through particulate shuttles whereas aqueous u is unaffected by this process in the present study we examine u mo covariation in organic rich sediments deposited mostly in the western tethyan region during oceanic anoxic events oaes of early jurassic to late cretaceous age our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regard ing water mass restriction and the operation of particulate shuttles in depositional systems these insights will help to address contentious issues pertaining to the character and origin of mesozoic oaes such as the degree to which regional paleoceanographic factors controlled the development of the oaes

fast or slow melting of the marinoan snowball earth the cap dolostone record article i nfo the end of the neoproterozoic era is punctuated by two global glacial events marked by the presence of glacial deposits overlaid by cap carbonates duration of glacial intervals is now consistently constrained to 3 12 million years but the duration of the post glacial transition is more controversial due to the uncertainty in cap dolostone sedimentation rates indeed the presence of several stratabound magnetic reversals in brazilian cap dolostones recently questioned the short sedimentation duration a few thousand years at most that was initially suggested for these rocks here we present new detailed magnetostratigraphic data of the mirassol d oeste cap dolostones mato grosso brazil and bomb spike calibrated ams 14 c data of microbial mats from the lagoa vermelha rio de janeiro brazil we also compile sedimentary isotopic and microbiological data from post marinoan outcrops and or recent depositional analogues in order to discuss the deposition rate of marinoan cap dolostones and to infer an estimation of the deglaciation duration in the snowball earth aftermath taken together the various data point to a sedimentation duration in the range of a few 10 5 years

formerly aragonitic limestones associated with tillites in the late proterozoic of death valley california abstract the upper proterozoic virgin spring and sour dough limestones occur within the glaciogenic kingston peak formation of the death valley region eastern california the virgin spring limestone is black thinly laminated and consists of microscopic spheroids and scattered ooids contained within coarse neomorphic spar an original aragonite mineralogy is inferred and this is supported by high contents of strontium up to 3 650 ppm thin sandstone beds occur within the limestone and their features suggest a storm origin the sour dough limestone has been metamorphosed and deformed but shows similar sedimentary structures and a high sr content both limestones are interpreted as deeper water outer shelf deposits and their sedimentation is related to a sea level rise through deglaciati n the virgin spring limestone has a typical marine 13c signature average 2 0 pdb but the 18o is very negative 15 8 pdb the latter is attributed more to the burial diagenetic calcitization of an originally aragonitic sediment than to precipitation from 18o depleted seawater

late proterozoic patsy springs canyon adelaide geosyncline submarine or subaerial origin a significant aspect of late proterozoic sedimentation in the adelaide geosyncline south australia is the presence of kilometre deep erosional incisions which have been termed canyons these structures were formerly described to be of submarine origin cut and filled in an inferred basin slope setting by subaqueous processes subsequent detailed research particularly on a specific incision known as patsy springs canyon indicates that sedimentary structures within some of the canyon filling sediments are indicative of deposition above fair weather wave base in addition an unusual carbonate unit which is observed to veneer upper portions of canyon shoulders and to contribute to carbonate breccias interbedded with canyon fill has a stable isotope signature which may imply a non marine origin the presence of the carbonate veneer where it is in situ suggests that at least upper portions of the canyons could have been emergent during the canyon filling phase considering these observations and combining them with regional stratigraphical relationships an alternative model for canyon genesis is proposed involving subaerial erosion and subsequent filling by coastal onlap such a model requires base level changes of the order of 1 km in order to account for observed canyon cutting and filling vertical movements associated with halokinesis or thermally induced uplift of the order of 1 km could have resulted in the observed erosional events alternatively a messinian style evaporitic lowering of base level is currently receiving serious attention with present knowledge this mechanism most satisfactorily explains all observations

a carbon isotope challenge to the snowball earth the snowball earth hypothesis contends that most of the planet was covered with thick ice for millions of years about three quarters of a billion years ago a dramatic warming event thought to have been fuelled by high atmospheric carbon dioxide levels eventually defrosted earth a new isotopic analysis of rocks that were laid down after the initial glaciation challenges the idea of a hard snowball earth the data are consistent with a rather low partial pressure of atmospheric co2 not the orders of magnitude above current levels that would provide the extreme warming necessary for escape from snowball earth this supports the idea that the initial glaciation was much less severe than has been proposed the snowball earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the neoproterozoic era in a self enhanced glaciation caused by the high albedo of the ice covered planet in a hard snowball picture the subsequent rapid unfreezing resulted from an ultra greenhouse event attributed to the buildup of volcanic carbon dioxide co2 during glaciation1 high partial pressures of atmospheric co2 from 20 000 to 90 000 p p m v in the aftermath of the marinoan glaciation 635 myr ago have been inferred from both boron and triple oxygen isotopes2 3 these values are 50 to 225 times higher than present day levels here we re evaluate these estimates using paired carbon isotopic data for carbonate layers that cap neoproterozoic glacial deposits and are considered to record post glacial sea level rise1 the new data reported here for brazilian cap carbonates together with previous ones for time equivalent units4 5 6 7 8 provide estimates lower than 3 200 p p m v and possibly as low as the current value of 400 p p m v our new constraint and our re interpretation of the boron and triple oxygen isotope data provide a completely different picture of the late neoproterozoic environment with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard snowball earth

late neoproterozoic cap carbonates mackenzie mountains northwestern canada precipitation and global glacial meltdown the 3 27 m thick cap carbonate overlying marinoan ice brook formation glacigene sediments and keele formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units a lower splintery buff weathering microcrystalline dolostone of extensive lateral uniformity comprises mm laminated peloidal sediment with local low angle hummocky like cross stratification micro ridges and synsedimentary tepees all elongated per pendicular to depositional strike this dolostone is unconformably overlain by an upper limestone that exhibits pro nounced facies variation from inboard peloidal lime grainstone and mudstone to shelf edge cementstone to outboard lime wackestone and mudstone calcite cementstones range from isolated crystal fans in laminated limestone to huge decimetre scale crystal arrays to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre scale reeflike structures crystal stromatolites are precipitates and replaced microbiolites that con structed biostromes and bioherms like those on many flat topped reef rimmed platforms the calcite crystals have all the physical and chemical attributes of neomorphosed aragonite this aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean all sediment is interpreted to have been caco3 originally low and constant 18 o values reflect diagenetic modification of these carbonates although chemical attributes such as sr and c isotopes in some lithologies are near pristine lower dolostones virtually identical to most other coeval marinoan caps worldwide were part of a global precipitation event of remarkable similarity upper limestones are a more local phenomenon deposited during sea level rise in an aragonitic sea returning to equilibrium after global glaciation low 87 sr 86 sr ratios and varying 13 c values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly prior to significant flu vioglacial runoff or that the influence of this runoff on the chemistry of seawater along continental shelves was mini mal the cap carbonate is thus interpreted to have formed in two steps 1 during initial marine ice melting accompanied by oceanic overturn and upwelling preceding continental margin rebound and 2 during initial stages of sea level rise accompanying continental deglaciation while confirming brief but extensive carbonate precipitation from an ocean highly perturbed by global glaciation the rocks also suggest that this event did not permanently affect either late neoproterozoic ocean chemistry or the contained marine biosphere 1262

a review of the origin and setting of tepees and their associated fabrics carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal lacustrine and subaerial carbonate shelf sediments these are commonly disrupted and brecciated when the surface area of these crusts increases in the subtidal environment megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand similar megapolygons form in the supratidal lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and or cement also causes the area of the hardgrounds to expand the arched up antiform margins of expansion megapolygons are known as tepees the types of tepees found in the geological record include 1 submarine tepees which form in shallow carbonate saturated waters where fractured and bedded marine grainstones are bound by isopachous marine phreatic acicular and micritic cements the surfaces of these brecciated crusts have undergone diagenesis and are bored unlike tepees listed below they contain no vadose pisolites or gravity cements 2 peritidal and lacustrine tepees are formed of crusts characterized by fenestral pisolitic and laminar algal fabrics this similarity in fabric makes these tepees of different origins difficult to separate peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical they are associated with fractured and bedded tidal flat carbonates their fracture fills contain geopetal asymmetric travertines of marine vadose origin and or marine phreatic travertines and or terra rossa sediments the senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common they include groundwater tepees which form over evaporitic boxwork carbonates and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas but the dominant sediment type is carbonate mud these latter tepee crusts are coated and crosscut by laminated micrite the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions in time intervals of days to years cement morphologies reflect this and the crusts often contain gravitational meniscus vadose cements as well as phreatic isopachous cement rinds 3 caliche tepees which are developed within soil profiles in a continental setting they are formed by laminar crusts which contain pisolites and fractures filled by micritic laminae microspar spar and terra rossa most of the cements are gravitational and or meniscoid in ancient carbonates when their cementation and diagenetic fabric can be interpreted tepee structures can be used as environmental indicators they can also be used to trace the evolution of the depositional and hydrological setting

rhenium in the black sea comparison with molybdenum and uranium rhenium concentrations were measured in the black sea to determine the behavior of this redox sensitive element in an anoxic basin under reducing conditions re is removed from solution leading to its depletion from black sea deep waters compared with uranium water column re and mo profiles show evidence of removal from mid depths suggesting that these elements are scavenged either in the water column or slope sediments to a greater degree than u black sea surface waters are enriched in re reflecting re in rivers draining into the northern black sea that are enhanced up to 80 fold over other large world rivers this enrichment is most likely anthropogenic and may be due to coal burning in the region a simple two box model is used to demonstrate that re concentrations in the black sea are not in steady state with respect to current riverine inputs and authigenic fluxes riverine inputs of u and mo are not as significant perhaps reflecting the fractionation of re from the other elements during a high temperature combustion process re2o7 is volatile above 270 c

trace element behavior and redox facies in core shales of upper pennsylvanian kansas type cyclothems the black shale submember of core shales of pennsylvanian kansas type cyclothems is highly enriched in redox sensitive trace elements tes relative to the overlying gray shale submember as well as to many other modern and ancient organic rich deposits controls on trace element behavior in the black shale can be inferred from trace element enrichment factors efs relative to gray shale and the relationship of tes to total organic carbon toc most tes conformed to one of two patterns 1 mo u v zn and pb exhibited moderate efs and strong covariation with toc in samples having 10 wt toc and 2 cu ni cr and co exhibited low efs and moderate to strong covariation with toc in all samples these patterns are inferred to represent different responses to benthic redox conditions the first pattern is characteristic of tes of strong euxinic affinity i e those taken up in solid solution by fe sulfide or involved in other reactions catalyzed by free h2s and resident mainly in authigenic phases whereas the second pattern is characteristic of tes of weak euxinic affinity i e those not strongly influenced by the presence of free h2s and resident mainly in the organic carbon or detrital fractions of the sediment these inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples euxinic conditions were considered to have existed if at least two of four redox indicator trace elements mo u v zn showed euxinic levels of enrichment and nonsulfidic anoxic conditions were inferred otherwise the validity of the procedure is indicated by 1 agreement among all four redox indicators for a large majority of samples 69 of 185 2 among samples yielding a mixed redox signal a systematic sequence of te enrichment v zn mo u and 3 for tes of weak euxinic affinity reduced variance among samples representing each redox facies sequential enrichment may be a response to differential redox thresholds for accumulation e g v or postdepositional remobilization of trace elements e g u and may provide a basis for finer assessment of redox conditions in low oxygen paleoenvironments than permitted by simple classification schemes the multiproxy procedure for redox facies analysis developed in this study is likely to be more reliable than widely used single proxy indicators based on trace elements e g mo authigenic u or v v ni or c s fe systematics e g s toc or dop d 2004 elsevier b v all rights reserved

carbon and strontium isotope fluctuations and paleoceanographic changes in the late neoproterozoic araras carbonate platform southern amazon craton brazil the late neoproterozoic araras group is exposed along the southern border of the amazon craton and at the northern paraguay belt central brazil it comprises carbonate rocks over 600 m thick overlying marinoan glaciogenic diamictites and includes the first welldocumented cap carbonate sequence in south america 13 c values for araras carbonates interpreted as representative of the original seawater 18 on 10 exhibit a trend from strongly negative 13 c values around 9 at the base to enriched 13 c compositions towards the top of the succession reaching values above 0 the cap carbonate succession is composed of deep platform deposits of dolomite mirassol d oesteformation coveredbylimestonerichincrustsandcements lower guiaformation theserocks show 13 c values around 5 and anomalous sedimentary structures such as vertical tubes megaripple bedding tepee like structure and crystal fans of aragonite pseudomorphs the anoxic deep platform limestone succession guia formation that overlies the cap succession presentsahomogeneousc isotopecurve with 13 cvaluesaround 2 5to 1 upsection valuesswitchtopositive 0 1to 0 3 in platform dolomites and shoreface to shallow platform deposits serra do quilombo formation but exhibit oscillations to slightly negative values in the overlying peritidal facies nobres formation 87 sr 86 sr values follow the general c isotope trend with values increasing upsection from 0 7074 to 0 7088 striking shifts in 13 ca nd 87 sr 86 sr values are observed close to stratigraphic surfaces the correlationbetweentheisotoperecordandtherelativesea levelcurvessupportsthecloseassociationbetweenpaleoceanographicchanges

authigenic molybdenum formation in marine sediments a link to pore water sulfide in the santa barbara basin pore water and sediment mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the santa barbara basin to examine the connection between authigenic mo formation and pore water sulfide concentration only at the deepest site 580 m where pore water sulfide concentrations rise to 0 1 mm right below the sediment water interface was there active authigenic mo formation at shallower sites 550 430 and 340 m where pore water sulfide concentrations were consistently 0 05 mm mo precipitation was not occurring at the time of sampling a sulfide concentration of 0 1 mm appears to be a threshold for the onset of mo fe s co precipitation a second threshold sulfide concentration of 100 mm is required for mo precipitation without fe possibly as mo s or as particle bound mo mass budgets for mo were constructed by combining pore water and sediment results for mo with analyses of sediment trap material from santa barbara basin as well as sediment accumulation rates derived from 210 pb the calculations show that most of the authigenic mo in the sediment at the deepest site is supplied by diffusion from overlying bottom waters there is however a non lithogenic particulate mo associated with sinking particles that contributes 15 to the total authigenic mo accumulation analysis of sediment trap samples and supernant brine solutions indicates the presence of non lithogenic particulate mo a large fraction of which is easily remobilized and perhaps associated with mn oxides our observations show that even with the very high flux of organic carbon reaching the sediment of santa barbara basin active formation of sedimentary authigenic mo requires a bottom water oxygen concentration below 3 mm however small but measurable rates of authigenic mo accumulation were observed at sites where bottom water oxygen ranged between 5 and 23 mm indicating that the formation of authigenic mo occurred in the recent past but not at the time of sampling copyright 2000 elsevier science ltd

birdseyes fenestrae shrinkage pores and loferites a reevaluation abstract birdseyes birdseye limestone fenestrae fenestral fabric shrinkage pores and loferites are considered similar if not synonymous when occurring in lime mudstone or syndepositional dolomite especially in association with mudcracks and stromatolites compaction experiments indicate however that in the absence of early cementation these rugs can be completely obliterated whereas mudcracks and stromatolites remain relatively unchanged lumping of burrows and root tubes under the general terms birdseyes or fenestrae such as for example tubular fenestrae burrow fenestrae or root tube fenestrae is discouraged they should be called burrows or root tubes because the birdseyes fenestrae and shrinkage pores are so intimately associated with tidal flats in the minds of most geologists submarine cementation of pelletal and oolitic botryoidal grainstone under 5 to 6 m of water on the bahama banks has resulted in rugs provocatively similar to many birdseyes and fenestrae generally attributed to peritidal conditions in the literature birdseyes and fenestrae in ancient grainstones cannot therefore be reliably identified without consideration of the sedimentary sequence and assoc ated sedimentary structures

contrasting sulfur geochemistry and fe al and mo al ratios across the last oxic to anoxic transition in the cariaco basin venezuela an abrupt transition from oxic to anoxic sulfidic euxinic marine bottom waters occurred in the cariaco basin in response to increasing productivity resulting from the late pleistocene post glacial rise in sea level and corresponding increase in surface water nutrient availability the microlaminated sediments of the euxinic interval which span the last f14 5 ky suggest a predominance of water column syngenetic pyrite formation based on 1 high pyrite sulfur spy concentrations in the surficial sediment layers 2 values for degree of pyritization dop that generally do not increase appreciably with increasing burial 3 ratios of total iron fet to al that are elevated above the continental baseline recorded in the underlying oxic sediments and 4 spy isotope trends that largely mimic the d 34 shsof the modern water column intermediate dop values in the microlaminated deposits and fet al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of fe bearing siliciclastic accumulation as predicted from the relative rates of siliciclastic delivery fet al and dop data lie between end member values observed in the modern black sea as viewed broadly fet al and dop relationships in euxinic sediments reflect the balance between syngenetic fe scavenging and temporal and spatial gradients in siliciclastic input pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon corg however the upper 80 cm of the fe rich corg poor bioturbated sediment show evidence for a strong diffusional hsoverprint from the overlying fe limited euxinic marine environment this post glacial transition manifests in pyrite overprints that are strongly 34 s depleted relative to those in restricted presently euxinic marine settings elsewhere in the world such as the black sea where the sedimentary sequence spanning the last glacial interglacial transition begins with a shift from freshwater to corg poor oxic marine deposition and thus dominantly sulfate diffusion trends for mo al ratios in the microlaminated sediments suggest that mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits organic matter plays a role by enhancing hsproduction and or by providing a substrate for mo scavenging significant mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite hs rich pore waters as recorded in the heavy iron sulfide overprint we have

enhanced trapping of molybdenum by sulfurized marine organic matter of marine origin in mesozoic limestones and shales there have been many studies devoted to trace metals and their value in assessing the paleoredox conditions of ancient marine deposition among them molybdenum mo is frequently cited as an effective proxy for sediments and sedimentary rocks recently helz et al helz g r miller c v charnock j m mosselmans j l w pattrick r a d garner c d vaughan d j 1996 mechanisms of molybdenum removal from the sea and its concentration in black shales exafs evidences geochim cosmochim acta 60 3631 3642 and adelson et al adelson j m helz g r miller c v 2001 reconstructing the rise of recent coastal anoxia molybdenum in chesapeake bay sediments geochim cosmochim acta 65 237 252 suggested that mo does not behave conservatively in the water column when h2s reaches a threshold concentration above this concentration a switch operates and mo is scavenged by forming bonds with metal rich notably iron particles sulfur rich organic molecules and pyrite in this paper mo trapping by sulfur rich organic matter om in ancient marine deposits is emphasized the following mesozoic geological formations were selected for study because of their relatively high concentration of sulfurized om the akkuyu formation turkey the calcaires d orbagnoux france and kimmeridge clay uk and its timeequivalent in boulonnais france the kashpir oil shales russia and the la luna formation venezuela the sulfur rich om is identified by either measured organic s abundance or kerogen microscope observation our results show that mo is systematically more enriched relative to the other redox sensitive sulfide forming elements studied u v ni cu zn cr and mo enrichment is positively correlated with the amount of sulfurized om but not with pyrite abundance these results illuminate the role played by sulfurized om in geologic scale mo capture and retention but they also underline the role played by reactive iron significant om sulfurization is only possible when reactive iron is limited nevertheless pyrite formation though limited could act as an initial mo trap prior to mo uptake by om that is sulfurized after the pyritization step in future paleoenvironmental reconstructions attention must be paid to this enhanced mo enrichment in the presence of sulfurized organic matter in such cases the use of mo could lead to overestimation of the reducing conditions of the depositional environment

soft sediment deformation at the base of the neoproterozoic puga cap carbonate southwestern amazon craton brazil confirmation of rapid icehouse to greenhouse transition in snowball earth stratigraphic and isotopic data identify the lower 45 m of the araras group on the southwest margin of the amazon craton as a neoproterozoic platform cap carbonate deposited below wave base upon varanger glacial diamictites of the puga formation the basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube like structures and fenestral nonmicrobial planar laminites with tepee like features above the basal carbonates are deep water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans pseudomorphs after aragonite the basal contact of the puga cap exhibits soft sediment deformational structures principally load casts that are here attributed to rebound induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite these features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball earth model of neoproterozoic glaciation

a multilayered water column in the ediacaran yangtze platform insights from carbonate and organic matter paired δ13c abstract large carbon isotope fluctuations have been systematically reported for ediacaran carbonate rocks the meaning of which remains controversial in order to better understand the mechanisms behind such variations we present new paired 13c data on carbonates 13ccarb and their associated organic matter 13corg from a shelf margin section yangjiaping of the doushantuo formation in this section the doushantuo formation starts with cap dolostones presenting 13ccarb values around 5 and 13corg values between 30 3 and 27 6 up section phased variations in 13ccarb and 13corg describe positive and negative excursions while their difference 13cdol org 13ccarb 13corg remains around 29 2 these new data allow the first reconstruction of lateral variations of 13ccarb 13corg and 13ccarb org for a shelf to basin cross section of the yangtze platform south china after integration with results reported previously for two other sections across the yangtze platform the isotope signals reveal strong lateral heterogeneities with complex variations of 13ccarb and 13cdol org in the inner shelf section phased variations in the shelf margin section with positive 13ccarb and 13cdol org close to 29 and dominantly negative 13ccarb with 13corg as low as 35 in the basin these variations are incompatible with the idea that the 13ccarb can systematically be used as a proxy of ocean surface waters assuming that 13ccarb are acquired in bottom waters and or upper sediments we show that the heterogeneous 13ccarb and 13cdol org are compatible with a stratified water column composed of up to three layers i an oxic surface layer where dissolved inorganic carbon dic is probably in isotope equilibrium with the atmosphere ii an intermediate euxinic layer with a lower 13cdic due to organic matter oxidation by ongoing sulphate reduction iii a bottom euxinic layer that seems to be restricted to the inner shelf lagoonal facies lacking sulphate containing methane and with a higher 13cdic due to dic production by methanogenesis if our model holds true it suggests that not only negative but also positive ediacaran carbon isotope excursions may reflect ocean stratification the positive excursion possibly recording a sulphate free methanogenic layer at the bottom of restricted basins

seismites in the mesoproterozoic altyn formation belt supergroup montana a test for tectonic control of peritidal carbonate cyclicity the mesoproterozoic altyn formation of western montana contains abundant seismites that record the physical state of the sediment at the time of earthquake induced ground motion earthquakes were likely generated by normal faulting during subsidence because part of the altyn is composed of cyclic upward shallowing peritidal carbonates it may afford an opportunity to test whether spasmodic subsidence provides a viable mechanism for the episodic generation of the requisite accommodation space at cycle tops no stratigraphic relation between the seismites and peritidal cycles is perceived however so it appears that this process may not be a control of cyclicity

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

rates of reaction between silicate iron and dissolved sulfide in peru margin sediments porewaters in site 680 peru margin sediments contain dissolved sulfide over a depth of approximately 70 m which at a sedimentation rate of 0 005 cm yr 1 gives a sediment exposure time to dissolved sulfide of about 1 4 106 yr reactions with dissolved sulfide cause the site 680 sediments to show a progressive decrease in a poorly reactive silicate iron fraction defined as the difference between iron extracted by dithionite fed at room temperature and that extracted by boiling concentrated hcl feh normalised to the total iron content fet straight line plots are obtained for 1n feh fed fet against time of burial from which a first order rate constant of 0 29 10 6 yr 1 equivalent to a half life of 2 4 106 yr can be derived for the sulfidation of this silicate iron comparable half lives are also found for the same poorly reactive iron fraction in the nearby site 681 and 684 sediments this silicate fe fraction comprises 0 8 1 0 fe only 30 60 of which reacts even with 1 5 3 million years exposure to dissolved sulfide diagenetic models based on porewater concentrations of sulfate and sulfide and solid phase iron contents at site 680 are consistent in indicating that this poorly reactive iron fraction is only sulfidized on a million year time scale silicate iron not extracted by hcl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments

testing models for post glacial cap dolostone deposition nuccaleena formation south australia sedimentologically and geochemically distinctive carbonate sequences consistently drape the glacial deposits associated with the younger cryogenian ice age the presence of ice rafted debris in the basal dolostone implies that at least the lower portion of the cap sequence records deglaciation an isochronous model proposes that cap dolostones were deposited synchronously around the world regardless of water depth whilst a diachronous model proposes that deposition tracked glacioeustatic flooding during deglaciation the nuccaleena formation of the adelaide rift complex arc south australia exhibits many of the unique sedimentary features observed in other younger cryogenian cap dolostones around the world some bedforms are the product of wind driven oscillatory flow thus constraining post glacial dolostone deposition to 400 m these sedimentary features are absent in the deepest basinal facies in the northern arc suggesting that this region was below storm wave base even at the glacial sea level lowstand in the north flinders ranges there is a distinct relationship between lateral facies variability in the pre syn and post glacial sediments and the axes of 50 km scale structural folds the northern fold limbs are characterised by basinal facies whilst the southern limbs are associated with upper slope facies we interpret the abrupt facies transitions to reflect lower slope to upper slope shelf breaks across a series of linked south facing half grabens the majority of cap dolostone carbon isotope records show monotonic declines in 13c of 2 with starting points between 0 5 and 3 5 an isochronous model implies a 3 0 lateral gradient from platform to lower slope that varies dramatically on a short spatial scale and non systematically with palaeobathymetry if pre and syn glacial facies are used to infer palaeobathymetry and cap dolostones are deposited diachronously as sea level rises during deglaciation then 13c values becomes progressively lighter with time implying that cap dolostone deposition tracked the glacioeustatic sea level rise over a series of half grabens that deepened to the north the carbon isotope dataset cannot rule out uniquely isochronous or diachronous models given the high frequency spatial variability of 13c values temperature cannot be the dominant control on the isotopic variability of the cap dolostone

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

al to oxide and ti to organic linkages in biogenic sediment relationships to paleo export production and bulk al ti abstract to increase our understanding of the mechanisms that control the distribution of al and ti within marine sediment we performed sequential extractions targeting the chemical signatures of the loosely bound exchangeable carbonate oxide organic opal and residual fraction of sediment from a carbonate dominated regime equatorial pacific and from a mixed opal terrigenous regime west antarctic peninsula we observe a systematic partitioning of al and ti between sediment phases that is related to bulk al ti we show that where we can quantify an alexcess component the dissolved al is preferentially affiliated with the oxide fraction resulting in al ti molar ratios of 500 3000 this is interpreted as the result of surface complexation in the water column of dissolved al onto oxyhydroxides we also observe a previously undetected tiexcess with as much as 80 of the total ti in the organic fraction which is most likely a function of metal organic colloidal removal from the water column in samples where the excess metals are obscured by the detrital load the al and ti are almost exclusively found in the residual phase this argues for the paired removal of al preferentially by the oxide component and ti preferentially by the organic component from the water column by settling particulate matter this research builds upon earlier work that shows changes in the bulk ratio of al to ti in carbonate sediment from the central equatorial pacific that coincide with changes in the sedimentary bulk accumulation rate bar the ratios that are observed are as much as three times higher than typical shale values and were interpreted as the result of scavenging of dissolved al onto particles settling in the water column because this non terrigenous alexcess accounts for up to 50 of the total sedimentary al inventory and correlates best with bar the bulk al ti may be a sensitive tracer of particle flux and therefore export production because we show that the excess metals are the result of scavenging processes the bulk al ti may be considered a sensitive proxy for this region