authigenic uranium relationship to oxygen penetration depth and organic carbon rain we have measured u in benthic incubation chambers sediment pore waters and in sediments along the california continental margin sedimentary u uptake rates based on a combination of sediment pore water profiles and benthic incubation chambers generally agree with those predicted from sediment accumulation rate data this agreement supports the view that most of the continental margin sedimentary u is delivered by diffusion across the seawater sediment boundary the average rate of authigenic u accumulation for all the sites examined here is 0 2 nmol cm 2 y 1 which is consistent with published global estimates of sedimentary u uptake in addition the accumulation rate of u in sediments exhibits a nonlinear relationship with the oxygen penetration depth and a linear relationship with the organic carbon rain rate these relationships highlight the potential utility for the u accumulation rate as a proxy for these processes

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

molybdenum isotope constraints on the extent of late paleoproterozoic ocean euxinia abstract the expansion of anoxic and sulfidic euxinic deep oceans may explain the termination of global large iron formation if deposition at 1 85 ga but the areal extent of late paleoproterozoic ocean euxinia is poorly constrained here we use the molybdenum mo isotope composition 98 95 mo of black shales from the hr fe t typically 0 38 and ratios of pyrite fe to highly reactive fe fe py fe hr typically 2 rich because the fe py fe hr ratios do not imply sustained water column euxinia the conversion of molybdate moo 4 2 to tetrathiomolybdate mos 4 2 in bottom waters and sediments was probably not quantitative and thus the 98 95 mo 0 1 to 0 8 from these shales provides only a minimum estimate for coeval seawater by contrast black shales from the middle rove formation have high fe py fe hr 0 8 and fe hr fe t 0 38 which implies persistent water column euxinia concentrations of free sulfide may have been sufficient to result in near quantitative formation of tetrathiomolybdate and its removal from bottom waters the observed narrow range in 98 95 mo of 1 19 0 28 2sd defined by most euxinic shales is thus inferred to represent the mo isotope composition of late paleoproterozoic global seawater mass balance modeling suggests that organic rich sediments deposited beneath euxinic bottom waters represented a substantial fraction of the oceanic mo sink at 1 8 ga the modeling results suggest that euxinic marine environments were common along late paleoproterozoic ocean margins these environments could have served as efficient sinks for fe 2 aq upwelling from anoxic deeper waters hence the mo isotope data are consistent with fe speciation and other geochemical evidence that suggest an expansion of euxinic environments led to the cessation of global large if deposition after 1 85 ga

emplacement of the 18 may 1980 lateral blast deposit ene of mount st helens washington facies variations east northeast of mount st helens preserve a record of depositional processes in the 18 may 1980 lateral blast cloud this paper reports new field grain size and component data from the ene sector of the timber blowdown zone and presents a model for blast flow and sedimentation the first erupted ejecta was rich in juvenile components and extends to the distal blowdown limit the last erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge the blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high density turbidity current the possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns depositional conditions were strongly influenced by the rate of suspended load fallout from the blast within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered settling zone able to detach gravitationally and drain into local depressions the resulting proximal facies resembles a low aspect ratio ignimbrite fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered settling zone component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers with increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions building up antidune like bedforms wavelengths of these bedforms range from 20 to 1 m and decrease away from vent there is a systematic relationship between antidune migration direction and depositional slope the transition from proximal ignimbrite like to distal surge like facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high velocity pyroclastic flows

shallow stratification prevailed for 1700 to 1300 ma ocean evidence from organic carbon isotopes in the north china craton abstract the late paleoproterozoic to early mesoproterozoic from 1700 ma to 1300 ma was highlighted by the assembly of the nuna supercontinent expansion of euxinic marine environments and apparent stasis in the diversity of eukaryotes the isotopic composition of carbonate carbon 13ccarb was surprisingly constant during this interval but little is known about the secular variation in the organic carbon isotopic composition 13corg here we report 13corg data from the latest paleoproterozoic 1650 ma to early mesoproterozoic 1300 ma succession in north china the 13corg values range from 25 to 34 and are dependent on sedimentary facies in subtidal and deeper environments 13corg values are low and constant ca 32 but relatively enriched and more variable in shallower intertidal and supratidal environments we attribute the facies dependent variation in 13corg to the presence of a shallow chemocline a probable result of a shallow chemocline is that it supported significant contributions of organic matter produced by chemoautotrophic and or anaerobic photoautotrophic microbes in relatively deep environments from the latest paleoproterozoic to early mesoproterozoic continental shelf of north china

oxygen facies and secular controls on the appearance of cryogenian and ediacaran body and trace fossils in the mackenzie mountains of northwestern canada the causes behind the appearance of abundant macroscopic body and trace fossils at the end of the neoproterozoic era remain debated iron geochemical data from fossiliferous ediacaran successions in newfoundland suggested that the first appearances correlated with an oxygenation event a similar relationship was claimed to exist in the mackenzie mountains canada although later stratigraphic studies indicated that the sections analyzed for geochemistry were incorrectly correlated with those hosting the fossils to directly connect fossil occurrences with geochemistry in the mackenzie mountains we conducted a multiproxy iron carbon sulfur and trace element geochemical analysis of stratigraphic sections hosting both the cryogenian twitya discs at bluefish creek as well as ediacaran fossils and simple bilaterian traces at sekwi brook there is no clear oxygenation event correlated with the appearance of macroscopic body fossils or simple bilaterian burrows however some change in environment a potential partial oxygenation is correlated with increasing burrow width higher in the blueflower formation data from sekwi brook suggest that these organisms were periodically colonizing a predominantly anoxic and ferruginous basin this seemingly incongruent observation is accommodated through accounting for differing time scales between the characteristic response time of sedimentary redox proxies versus that for ecological change thus hypotheses directly connecting ocean oxygenation with the appearance of macrofossils need not apply to all areas of a heterogeneous ediacaran ocean and stably oxygenated conditions on geological time scales were not required for the appearance of these avalon assemblage ediacaran organisms at least in the mackenzie mountains the appropriate facies for fossil preservation appears to be the strongest control on the stratigraphic distribution of macrofossils

shallow water anoxia in the mesoproterozoic ocean evidence from the bashkir meganticlinorium southern urals abstract the apparent lag between the first permanent rise of atmospheric oxygen to appreciable levels and oxygenation of the deep ocean has focused efforts in deciphering the evolution of seawater chemistry across the proterozoic eon 2 5 0 542 ga it is generally accepted that from 1 85 ga oxic shallow marine waters were widespread while the deep ocean remained dominantly ferruginous fe ii rich with episodic euxinia confined to productive continental margins and intracontinental basins the geochemical record that informs this picture however is currently sparse and further studies are required to adequately evaluate temporal and spatial variability in ocean redox conditions across this vast expanse of time here we report fe s c systematics alongside major al mn and trace metal mo u data for 1 6 0 8 ga marine sediments from the bashkir meganticlinorium bma southern urals russia our fe speciation data reveal a water column dominated by ferruginous conditions no evidence for euxinia and oxygenated waters confined to only the shallowest near coastal settings trace metal data support these findings with mo and u enrichment factors in anoxic sediments implying the operation of a particulate fe oxyhydr oxide shuttle under ferruginous conditions as the main mechanism for generating moderate authigenic mo enrichments sulfur isotope systematics imply that predominantly low dissolved sulfate concentrations prevailed throughout the deposition of the bma succession which in combination with a low organic carbon loading promoted the development of ferruginous rather than euxinic water column conditions the restriction of oxic conditions to the shallowest near coastal depositional settings within the bashkir basin contrasts to other redox studies from this period such discrepancies highlight major uncertainty in our understanding of the temporal and spatial evolution of water column redox chemistry across the mesoproterozoic era and signify the need for further detailed work to constrain the chemical evolution of the oceans during this crucial time period

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

oceanic molybdenum drawdown by epeiric sea expansion in the mesoproterozoic abstract molybdenum is a bioessential micronutrient whose abundance in the global oceans may have played a primary role in evolution of the earth s nitrogen cycle and ultimately in the timing of the ecologic expansion of early eukaryotes because molybdenum mo is delivered to the ocean under conditions of oxic weathering and is removed in suboxic and euxinic environments the concentration of mo in the oceans reflects a complex function of global redox and the hydrologic conditions that influence the areal extent of euxinic sedimentation recent compilations of mo within euxinic shales scott et al 2008 och and shields zhou 2012 sahoo et al 2012 indicate that the oceanic reservoir of mo did not rise substantially above crustal values until the great oxidation event 2 3 ga and a modern mo cycle did not develop until the contraction of ocean euxinia in the latest proterozoic at present a paucity of data from euxinic shales of the late mesoproterozoic 1 3 to 1 0 ga limits our ability to relate marine redox evolution to biological innovation during a critical interval of eukaryotic development here we present data from marine shales of the 1 1 ga atar and el mreiti groups mauritania which were deposited during sea level highstand when shallow epeiric seas covered much of the west african craton epicratonic strata of the el mreiti group were deposited under fluctuating redox conditions near a shallow chemocline as evidenced by iron speciation and record generally low mo concentrations

depositional mechanics and sequences of base surges songaksan tuff ring cheju island korea the songaksan mount in the southwestern part of cheju island korea is a taalian tuff ring produced by phreatomagmatic explosions at an aquifer a detailed analysis of proximal to distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies one lateral facies sequence lfs and three vertical facies sequences vfs the vfs 1 and 2 are representative of facies relationships in horizontal near vent deposits the vfs 1 comprises scour fill bedded tuff inversely graded tuff massive tuff and laminated tuff from base to top the vfs 2 is a variant of the vfs 1 replaced by an inversely graded lapilli tuff unit at the base the sequences suggest traction carpet suspension and minor traction sedimentation from a high concentration near vent base surge the lfs 1 and the vfs 3 are distilled from outward dipping flank deposits both sequences begin with disorganized lapilli tuff followed successively by stratified lapilli tuff dune bedded lapilli tuff very thinbedded tuff and accretionary lapilli they are suggestive of waning base surge which decreases in particle concentration suspended load fall out rate and flow regimes with an increase in traction and sorting these facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space a near vent base surge is turbulent uniformly mixed and highly concentrated and produces scour fill bedded tuff as capacity decreases the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part gravity transformation depositing inversely graded massive and normally graded lapilli tuff ensuing loss of sediment load and mixing of ambient air result in flow dilution surface transformation stratified and dune bedded units are produced by tractional processes of turbulent and low concentration surge further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension

microbenthic distribution of proterozoic tidal flats environmental and taphonomic considerations silicified carbonates of the late mesoproterozoic to early neoproterozoic society cliffs formation baffin island contain distinctive microfabrics and microbenthic assemblages whose paleo environmental distribution within the formation parallels the distribution of these elements through proterozoic time in the society cliffs formation restricted carbonates including microdigitate stromatolites laminated tufa and tufted microbial mats consist predominantly of synsedimentary cements these facies and the cyanobacterial fossils they contain are common in paleoproterozoic successions but rare in neoproterozoic and younger rocks less restricted tidal flat facies in the formation are composed of laminated microbialites dominated by micritic carbonate lithified early yet demonstrably after compaction these strata contain cyanobacteria that are characteristic in neoproterozoic rocks within the formation the facies dependent distribution of microbial populations reflects both the style and timing of carbonate deposition because of the strong substrate specificity of benthic cyanobacteria a reasonable conclusion is that secular changes in microbenthic assemblages through proterozoic time reflect a decrease in the overall representation of rapidly lithified carbonate substrates in younger peritidal environments as well as concomitant changes in the taphonomic window of silicification through which early life is observed

development of a sequential extraction procedure for iron implications for iron partitioning in continentally derived particulates abstract the development of a sequential extraction procedure for iron in modern and ancient sediments is presented the scheme recognizes seven operationally derived iron pools 1 carbonate associated fe fe carb including siderite and ankerite 2 easily reducible oxides fe ox1 including ferrihydrite and lepidocrocite 3 reducible oxides fe ox2 including goethite hematite and akaganeite 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u as such this is the first extraction scheme specifically developed to allow the separate identification of magnetite and the first to allow a complete evaluation of fe carbonate phases such as siderite and ankerite the scheme was developed following tests on pure mineral phases to evaluate the minerals solubilized by each technique and to determine optimum extraction times further tests on mixtures of pure minerals and on grain size separated sediments from two major us rivers and two glacial meltwaters validate the specificity of the scheme for different pools of iron minerals and demonstrate a high degree of reproducibility for each analytical stage the data obtained for the riverine and glacial sediments are additionally discussed in relation to the dominant modes of transport of different iron minerals in fine grained continental sediments

rhyolitic fallout and pyroclastic density current deposits from a phreatoplinian eruption in the eastern aegean sea greece abstract the kos plateau tuff consists of pyroclastic deposits from a major quaternary explosive rhyolitic eruption centred about 10 km south of the island of kos in the eastern aegean greece five main units are present the first two units a and b were the product of a phreatoplinian eruption the eruption style then changed to dry explosive style as the eruption intensity increased forming a sequence of ignimbrites and initiating caldera collapse the final waning phase returned to phreatomagmatic eruptive conditions unit f the phreatomagmatic units are fine grained poorly sorted and dominated by blocky vitric ash thickly ash coated lapilli and accretionary lapilli they are non welded and were probably deposited at temperatures below 100 c all existing exposures occur at distances between 10 km and 40 km from the inferred source unit a is a widespread 42 km from source thin upwind on kos to very thick downwind internally laminated dominantly ash bed with mantling sheet like form upwind unit a and the lower and middle part of downwind unit a are ash rich ash rich facies whereas the upper part of downwind unit a includes thin beds of well sorted fine pumice lapilli pumice rich facies unit a is interpreted to be a phreatoplinian fall deposit although locally the bedforms were influenced by wind surface water and topography the nature and position of the pumice rich facies suggests that the eruption style alternated between wet phreatoplinian and dry plinian during the final stages of unit a deposition unit b is exposed 10 19 km north of the inferred source on kos overlying unit a it is a thick to very thick internally stratified bed dominated by ash coated medium and fine pumice lapilli in an ash matrix unit b shows a decrease in thickness and grain size and variations in bedforms downcurrent that allow definition of several different facies and laterally equivalent facies associations unit b ranges from being very thick coarse and massive or wavy bedded in the closest outcrops to source to being partly massive and partly diffusely stratified or cross bedded in medial locations pinch and swell clast supported pumice layers are also present in medial locations in the most distal sections unit b is stratified or massive and thinner and finer grained than elsewhere and dominated by thickly armoured lapilli unit b is interpreted to have been deposited from an unsteady density stratified pyroclastic density current which decelerated and progressively decreased its particle load with distance from source condensation of steam during outflow of the current promoted the early deposition of ash and resulted in the coarser pyroclasts being thickly ash coated the distribution texture and stratigraphic position of unit b suggest that the pyroclastic density current was generated from collapse of the phreatoplinian column following a period of fluctuating discharge when the eruptive activity alternated between wet and dry the pyroclastic density current was transitional in particle concentration between a dilute pyroclastic surge and a high particle concentration pyroclastic flow unidirectional bedforms in unit b suggest that the depositional boundary was commonly turbulent and in this respect did not resemble conventional pyroclastic flows however unit b is relatively thick and poorly sorted and was deposited more than 19 km from source implying that the current comprised a relatively high particle concentration and in this respect did not resemble a typical pyroclastic surge

middle proterozoic ocean chemistry evidence from the mcarthur basin northern australia we report on the iron sulfur systematics and s isotopic compositions of upper paleoproterozoic carbonaceous shales from the wollogorang and lower reward formations mcarthur basin northern australia iron speciation in these rocks is similar to that of modern euxinic black sea sediments and distinct from sediments depositing from oxygenated water providing persuasive evidence of euxinic depositional environments in a mid proterozoic marine basin high values of degree of pyritization dop also indicate anoxic bottom water conditions at the time of shale deposition as does the relatively uniform isotopic composition of sedimentary sulfides the isotopic composition of the sulfides is however quite enriched in 3 4 s and for the lower reward formation similar to contemporaneous seawater sulfate these 3 4 s enriched sulfides are consistent with extensive or even complete sulfate depletion within the euxinic basins in which the sediments were deposited based on analogy with modern euxinic environments and further constrained by model results middle proterozoic seawater sulfate concentrations were probably within the range of 0 5 to 2 4 mm taken together our results indicate a relatively low sulfate late paleoproterozoic ocean and sulfidic deep waters in the mcarthur seaway a condition that could have encompassed much larger regions of the ocean

proterozoic ocean redox and biogeochemical stasis the partial pressure of oxygen in earth s atmosphere has increased dramatically through time and this increase is thought to have occurred in two rapid steps at both ends of the proterozoic eon 2 5 0 543 ga however the trajectory and mechanisms of earth s oxygenation are still poorly constrained and little is known regarding attendant changes in ocean ventilation and seafloor redox we have a particularly poor understanding of ocean chemistry during the mid proterozoic 1 8 0 8 ga given the coupling between redox sensitive trace element cycles and planktonic productivity various models for mid proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients with potential ecological constraints on emerging eukaryotic life here we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates we find that the metal enrichment record implies a proterozoic deep ocean characterized by pervasive anoxia relative to the phanerozoic at least 30 40 of modern seafloor area but a relatively small extent of euxinic anoxic and sulfidic seafloor less than 1 10 of modern seafloor area our model suggests that the oceanic mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of mo and chromium enrichments through time is consistent with the possibility of a mo n colimited marine biosphere during many periods of earth s history

anisotropy of magnetic susceptibility studies of depositional processes in the campanian ignimbrite italy the late pleistocene trachytic campanian ignimbrite underlies much of the campanian plain near naples italy and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source the campi flegrei caldera at sites within 15 km of the campi flegrei anisotropy of magnetic susceptibility ams principal directions indicate that in the absence of significant topography deposition came from a flow moving in a roughly radial direction ams studies of the more distal ignimbrite reveal downhill and or downvalley flow directions prior to deposition even where these directions are at high angles to a generally radial transport direction from the vent on the flanks of roccamonfina volcano flow was directly downhill as if the source of the ignimbrite was the summit of the volcano in most localities the ignimbrite consists of a single massive deposit in a few localities in the apennine mountains however the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys a detailed study of the ignimbrite in the flat titerno river valley near massa shows that the ams fabrics are not due to late stage creeping during deposition or compaction well defined but non parallel ams fabrics from vertical and lateral sections in the massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit clast compositions and ams axes at mondragone indicate that the pyroclastic flow encountered the monte massico massif and was partially blocked so that flow during deposition was toward the campi flegrei similar ams data from sites along the edge of the campanian plain indicate back flow off the first ridge of the apennine mountains reached at least 5 km from their base the campanian ignimbrite was deposited from a density stratified pyroclastic flow the depositional system consisted of the lower denser portion of the current and was controlled by topography the grouping of the ams axes is interpreted to indicate that deposition occurred under laminar flow conditions

manganese rich deposits in the mesoproterozoic gaoyuzhuang formation ca 1 58 ga north china platform genesis and paleoenvironmental implications abstract the redox state of the surface ocean atmosphere system has long been considered to have been remarkably stable during the mid proterozoic ca 1800 800 ma however there is a growing body of evidence that supports dynamic redox fluctuations during this time sedimentary mn enrichment in the ca 1 58 ga gaoyuzhuang formation north china may represent an important archive of mid proterozoic paleoenvironmental conditions we present a sedimentological and geochemical study of the mn rich carbonate succession in the gaoyuzhuang formation in order to further elucidate the genesis and paleoredox implications of this mn rich deposit we suggest that the mn rich interval was deposited in deep subtidal environments close to storm wave base manganese occurs either as millimeter sized mn oxide pyrolusite nodules or as mn rich dolomite which features moderately positive i ca mg values 0 5 1 7 mol mol and ce anomalies 1 1 1 2 in contrast the underlying and overlying carbonates that lack mn oxides were deposited in a shallow subtidal environment above fair weather wave base they have relatively low i ca mg values

uranium isotope evidence for limited euxinia in mid proterozoic oceans abstract reconstructing earth s oxygenation history is key to deciphering environmental controls on early biospheric evolution during the mid proterozoic eon low but potentially variable atmospheric po2 led to highly heterogeneous marine redox conditions with most studies indicating a relatively shallow depth of oxygen penetration the relative proportion of oxic anoxic and iron rich ferruginous and anoxic and sulfide rich euxinic conditions on the global seafloor is difficult to quantify however due to a general reliance on local redox proxies applied to the temporally discontinuous black shale record it is particularly important to constrain the global prevalence of euxinic bottom waters because sulfide toxicity has been implicated as a primary constraint on evolution in mid proterozoic oceans and limits the solubility of bioessential trace metals such as copper zinc and molybdenum here we present a suite of new uranium u isotope data from marine carbonate rocks that span the entire mid proterozoic interval 1 8 to 0 8 ga u isotopes in well preserved carbonate rocks represent a powerful new proxy that can be used to quantitatively constrain the global extent of marine euxinia the median 238 u value for mid proterozoic carbonate samples is 0 43 which is lower than the median value for modern post depositional carbonate sediments from the bahamas yet significantly higher than carbonate 238 u values recorded during other times of expanded anoxia in earth history such as during the end permian mass extinction when paired with a three sink isotope mass balance model our data indicate that no more than 7 of the global seafloor was euxinic during the mid proterozoic eon although transient pulses of expanded euxinia are recorded although these results challenge early expectations of a sulfide rich mid proterozoic ocean they are consistent with more recent inferences from the black shale trace metal record evidence therefore indicates that euxinic bottom waters were relatively limited in their spatial extent in the mid proterozoic oceans

dilute gravity current and rain flushed ash deposits in the 1 8 ka hatepe plinian deposit taupo new zealand two groups of poorly sorted ash rich beds previously interpreted as rain flushed ashes occur in the ca ad 180 hatepe plinian pumice fall deposit at taupo volcano new zealand two ash beds with similar dispersal patterns and an aggregate thickness of up to 13 cm make up the lowermost group a group a beds extend 45 km north east of the vent and cover 290 km2 in the southern part of the group a distribution area a coarse ash to lapilli size plinian pumice bed deposit b separates the two group a beds the scarcity of lapilli material seen elsewhere from the still depositing pumice fall in group a beds indicates that they were rapidly transported and deposited however this rapid transportation and deposition did not produce cross bedding nor did it erode the underlying deposits it is proposed that thick 600 m but dilute gravity currents generated from the collapsing outer margin of the otherwise buoyant hatepe plinian eruption column deposited the group a beds the upper ash beds group c consist of one to seven layers attain an aggregate thickness of 35 cm and vary considerably in thickness and number of beds with respect to distance from vent group c beds contain variable amounts of ash mixed with angular plinian pumices and are genuine rain flushed ashes several recent eruptions at other volcanoes ukinrek maars vulcan rabaul la soufrere de guadeloupe and soufriere st vincent have produced gravity currents similar in style but much smaller than those envisaged for group a deposits the overloaded margins of otherwise buoyant eruption plumes generated these gravity currents laboratory studies have produced experimental gravity current analogues hazards from dilute gravity currents are considerable but often overlooked thus the recognition of gravity current deposits will contribute to more thorough volcanic hazard assessment of prehistoric eruption sequences

sources of iron for pyrite formation in marine sediments more than two hundred aerobic continental margin aerobic deep sea dysaerobic and anaerobic euxinic sediments have been examined for their variations in different operationally defined iron fractions each of which represents a different reactivity towards dissolved sulfide aerobic continental margin deep sea and dysaerobic sediments contain similar contents of highly reactive iron dithionite soluble iron plus pyrite iron poorly reactive iron iron soluble in hcl less that soluble in dithionite and unreactive iron total iron less that soluble in hcl by contrast non turbidite euxinic samples from the black sea as well as euxinic samples from the cariaco basin and framvaren are enriched in highly reactive iron these sediments contain a small lithogenous fraction and a large biogenous organic c rich fraction which decays by sulphate reduction in an iron rich water column to form pyrite rich sediment other anaerobic euxinic samples from the black sea orca basin and kau bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron degrees of pyritization dop for all the aerobic dysaerobic and anaerobic euxinix samples except those low in biogenous material are consistent with analogous ancient sediments and indicate that most pyrite formation occurs form the highly reactive iron fraction

marine carbon reservoir corg ccarb coupling and the evolution of the proterozoic carbon cycle the post 2 0 ga proterozoic c isotope record reveals two distinct yet interrelated trends a stepwise increase in average 13c from 0 calculated with respect to the peedee belemnite isotope standard prior to ca 1 3 ga to 5 in the neoproterozoic and a concomitant increase in the magnitude of isotopic excursions steady state and non steady state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon dic reservoir the dic reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation major rearrangements of carbon cycling during the proterozoic in part related to the evolving marine carbon reservoir permit elevated 13c values to be sustained for geologically long time spans recognition of this dependence on dic reservoir size provides for the first time a direct link between changing carbonate precipitation styles and the marine c isotope record and may help constrain estimates of proterozoic p co2

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

stable isotopic compositions of carbonates from the mesoproterozoic bangemall group northwestern australia marine carbonate rocks from the mesoproterozoic bangemall group of northwestern australia show little deviation 1 3 in whole rock delta 13c carb values about a mean of 0 5 this narrow range persists despite close sampling every 10 20 m through long sections up to 2500 m that are geographically widespread up to 250 km apart over many depositional environments supralittoral to outer shelf sediment sources stromatolitic bioherms to detrital calcilutites and rock types pure limestones to dolomitic shales the only major excursions from the norm seem related to unusual environmental or post depositional processes as they are correlated with large enrichments to 3 or depletions to 16 in 18o relatively heavy delta 13c values up to 2 5 occur in a single bed of brecciated ferruginous dolostone at a single locality these abnormal values may result from local evaporitic conditions limey and shaley nodular dolostones have delta 13c values as low as 4 3 probably caused by remineralization of organic matter during late and patchy dolomitization most notably sharp negative excursions in delta 13c up to 8 4 occur in bleached kerogen free rocks with mineral assemblages of dolomite quartz calcite tremolite talc reflecting isotopic re equilibration in thick metamorphic aureoles around dolerite intrusions general environmental variations are minor with delta 13c values of peritidal facies tending to be slightly positive whereas those of subtidal facies are slightly negative there are no strong secular trends but subtle fluctuations within the range 2 to l can be correlated along the northwestern margin of the basin this resembles the pattern seen in other mesoproterozoic successions but is markedly unlike the heavy background 5 and extreme variations up to l0 in delta 13c evident in neoproterozoic successions of similar thickness and environmental setting hence in contrast to the neoproterozoic the global rate of organic carbon burial was probably fairly constant during deposition of the bangemall group and perhaps generally during the mesoproterozoic as was the redox state of the atmosphere and hydrosphere

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

concentration oxidation state and particulate flux of uranium in the black sea abstract in contrast to its unreactive behavior in the open ocean uranium is removed from seawater to sediments of anoxic marine basins four biogeochemical processes have been proposed to account for this removal active uptake by organisms whose remains are preserved by the anoxic conditions of the sediments complexation of u vi by particulate organic matter chemical reduction of soluble u vi to insoluble u iv which is scavenged to the sediments by settling particles and precipitation of uranium within the sediments themselves the black sea is the largest anoxic marine basin in the world today waters below 150 200 m are anoxic and h2s levels build up to about 400 mol 1 1 in the bottom water maximum depth 2200 m concentrations of total u and the oxidation state of dissolved u were measured throughout the water column while particulate u fluxes were determined from time series sediment trap samples collected in the deep anoxic zone bottom waters were depleted in u by 40 relative to the initial u content of the water two independent methods gave residence times of dissolved u in the deep water column of about 103 years uranium occurred throughout the water column as soluble chemically labile reducible u vi rather than the thermodynamically favored u iv uranium is neither reduced to u iv by the high levels of h2s nor is it rapidly scavenged from the water column as would be expected for strongly hydrolyzed tetravalent actinides like u iv fluxes of particulate authigenic u measured with sediment traps are at least two orders of magnitude less than the rate of authigenic u burial in black sea sediments therefore the first three processes listed above contribute negligibly to the deposition of u in the sediments of anoxic marine basins such as the black sea the dominant removal process must involve precipitation of u in the sediments rather than scavenging from the water column

mesoproterozoic paleo redox changes during 1500 1400 ma in the yanshan basin north china abstract the mesoproterozoic 1600 1000 million years ago ma is thought to have experienced important changes in ocean chemistry and eukaryote evolution our understanding of global redox conditions remains in its infancy and redox data derived from the mesoproterozoic thus far indicate heterogeneity and complexity in the realm of ocean redox which hampers our understanding of the potential significance of variations in ocean redox on eukaryote evolution in this study we report analyses of fe species and redox sensitive trace metals of u and mo from the jixian group including the wumishan hongshuizhuang and tieling formations as well as the xiamaling formation in the yanshan basin in north china the fe speciation data from the wumishan hongshuizhuang and tieling formations first reported in this study help bridge the global data gap of fe species data during 1500 1400 ma our data suggest likely oxic bottom water conditions for the wumishan formation and ferruginous bottom water conditions for the tieling formation the fe species data can distinguish ferruginous conditions for the upper hongshuizhuang from sulfidic depositional conditions for the lower middle hongshuizhuang formation based on fe species and trace metal data redox conditions likely varied from anoxic conditions with transient euxinia in the lower middle xiamaling formation to oxic conditions in the upper xiamaling formation the new fe speciation data from the yanshan basin when combined with previously published data show a surprisingly high proportion of middle proterozoic samples with very low highly reactive to total iron ratios this suggests either a relatively large extent of oxic seafloor or non uniformitarian iron cycling resolving this discrepancy will be key to understanding the extent of suitable habitats for complex eukaryotes

molybdenum isotope evidence for widespread anoxia in mid proterozoic oceans how much dissolved oxygen was present in the mid proterozoic oceans between 1 8 and 1 0 billion years ago is debated vigorously one model argues for oxygenation of the oceans soon after the initial rise of atmospheric oxygen 2 3 billion years ago recent evidence for h 2 s in some mid proterozoic marine basins suggests however that the deep ocean remained anoxic until much later new molybdenum isotope data from modern and ancient sediments indicate expanded anoxia during the mid proterozoic compared to the present day ocean consequently oxygenation of the deep oceans may have lagged that of the atmosphere by over a billion years

isotopic composition of organic and inorganic carbon from the mesoproterozoic jixian group north china implications for biological and oceanic evolution a b s t r a c t analyses of marine carbon isotope profiles have provided much of our current understanding of the evolution of earth surface environments particularly in the latter portion of the proterozoic eon earlier mesoproterozoic successions however have received comparatively little attention due to the relatively subdued nature of carbon isotope variation in this study we present high resolution isotopic profiles from three sections in the yanshan basin north china craton that combined comprise the entirety of the early mesoproterozoic 1600 1400 ma calymmian period jixian group high resolution profiles of both carbonate and organic carbon provide critical data for global comparison and permit us to better con strain both the pattern and origin of isotopic variation in the mesoproterozoic marine carbonate rocks of the jixian group show generally muted isotopic variation with average values near 0 consistent with previous observations from the early mesoproterozoic data furthermore record an increase in isotopic variation through the succession that is interpreted to reflect a long term decrease in pco2 and conse quently in the isotopic buffering capacity of marine dissolved inorganic carbon dic by contrast the isotopic composition of marine organic matter suggests facies dependent differences in carbon cycling organic carbon compositions suggest a dominance of autotrophic carbon fixation and aerobic decom position in shallow water environments and increased remineralization by anaerobic heterotrophs in deeper water environments correlation between organic carbon composition and depositional environ ment are interpreted to reflect differences in carbon cycling within benthic microbial mats under low oxygen conditions and dynamically maintained stratification of marine waters

hybrid fall deposits in the bishop tuff california a novel pyroclastic depositional mechanism hybrid fall deposits in the bishop tuff show features common to both archetypal fall and surge deposits like normal fall deposits they have an overall plane parallel bedding and flat lying pumice clasts but also like surge deposits they show variable development of cross bedding some crystal and pumice sorting and some rounding of pumice clasts all variations exist from normal fall deposits through streaky material with incipient development of cross bedding to the hybrid fall deposits with well developed cross bedding the streaky and hybrid deposits are interpreted as fall material contemporaneously redeposited by strong up to 40 m s swirling winds comparable to firestorm whirlwinds generated by air currents associated with coeval emplacement of pyroclastic flows recognition of hybrid fall deposits is important in interpreting the dynamics of explosive eruptions and correctly assessing volcanic hazards however although such deposits may be commonly produced by explosive eruptions especially where pyroclastic flows accompany fall activity they are likely to be overlooked or wrongly interpreted as surge deposits or secondary reworked material

alluvial and colluvial sediment storage in the geul river catchment the netherlands combining field and modelling data to construct a late holocene sediment budget abstract we used a combined approach of a two dimensional erosion and hillslope sediment delivery model watem sedem and detailed geomorphological reconstructions to quantify the different components in a sediment budget for the geul river catchment southern netherlands since the high middle ages hillslope erosion and colluvium deposition were calculated using the model while floodplain storage was estimated using field data our results show that more than 80 of the total sediment production in the catchment has been stored as colluvium mostly generated by hillslope erosion while almost 13 is stored in the floodplain since the high middle ages this situation resembles a capacity limited system model results for the period prior to the high middle ages with a nearly completely forested catchment show that far less sediment was generated and that most of the sediments were directly transported to the main river valleys or out of the catchment a supply limited system geomorphological analysis of a large alluvial fan shows the sensitivity of the study area to changes in the percentage of arable land our combined field data modeling study presents an elegant method to calculate a catchment sediment budget for a longer period and is able to identify and quantify the most important sediment storage elements furthermore it provides a valuable tool to calculate a sediment budget while only limited dated fluvial sediment sequences are available

sulfur isotope composition of carbonate associated sulfate from the mesoproterozoic jixian group north china implications for the marine sulfur cycle abstract the mesoproterozoic has been traditionally viewed as a period of prolonged stability in terms of environmental and biological evolution yet growing body of evidence suggests subtle yet dynamic changes in marine biogeochemical cycles in this study we present a high resolution analysis of carbonate associated sulfate cas from the mesoproterozoic jixian group 1 6 1 4 ga north china combined with previously reported carbon isotopes from the same sections sulfur isotope data provide insight into both oceanic chemistry and biospheric evolution during this period the sulfur isotopic composition of cas in the jixian group displays high amplitude variations with a total range from 3 7 to 51 9 stratigraphic variation in 34scas is most noted in the wumishan formation and records shifts of 10 over stratigraphic thickness of 100 200 m indicating a short residence time resulting from a limited oceanic sulfate reservoir differences in the pattern of isotopic variability between the gaoyuzhuang and wumishan formations suggest complex behavior of the marine sulfur cycle based on proximity of depositional environments to a local or regional chemocline and the degree of mixing between adjacent environments deposition under predominantly oxic conditions favored a stable sulfate reservoir whose isotopic composition was controlled primarily by pyrite burial by contrast depositional environments proximal to euxinic waters were subject to rapid changes in both sulfate reservoir size and isotopic composition resulting from growth and subsequent oxidation of a reactive hydrogen sulfide reservoir together these units demonstrate the difficulty of interpreting marine sulfur cycling in a low oxygen low sulfate world subject to a range of environmental controls data also reveal a long term positive trend in average 34scas and 13ccarb from approximately 10 to 20 and 0 5 to 0 5 respectively which is interpreted as reflecting prolonged organic carbon and pyrite burial burial of reduced carbon and sulfur may have resulted in a slow but steady release of oxygen to the biosphere ultimately driving a gradual increase in oceanic sulfate levels our study combined with previous contributions from the late mesoproterozoic supports hypotheses of protracted biospheric oxygenation marked by a modest increase in oceanic sulfate concentrations through the mesoproterozoic

morphology of molar tooth structures in precambrian carbonates influence of substrate rheology and implications for genesis abstract molar tooth mt is an enigmatic carbonate fabric composed of variously shaped cracks and voids filled with a characteristically uniform equant microspar mt is both abundant and widespread in mesoproterozoic and early neoproterozoic strata where void filling microspar comprises up to 90 of individual beds and 5 25 of preserved carbonate the temporal restriction of this fabric suggests a potential link between mt formation and the biogeochemical evolution of marine environments detailed petrographic relationships among mt crack morphology distribution of mt microspar and composition of the surrounding substrate suggest that crack formation and microspar precipitation are intimately linked to the decomposition of sedimentary organic matter in the presence of supersaturated proterozoic seawater laboratory experiments have shown that gas generated within unconsolidated mud can reproduce a variety of mt crack morphologies yet current gas expansion and migration models do not explicitly consider the role of substrate variability in determining morphologies of mt cracks a detailed petrographic examination of mt structures from the mesoproterozoic belt supergroup montana permits interpretation of the microscale relationship between crack morphology and lithologic and potentially rheologic variability of the surrounding substrate by tracing the distribution of petrographically distinctive mt microspar observations of lateral offset of mt cracks at bedding planes or within coarser grained siltstone or sandstone layers termination of cracks beneath clay or organic rich horizons grain collapse into underlying mt cracks and the presence of mt microspar as a pore filling precipitate suggest that grain size substrate lithology and substrate cohesion all play critical roles in the development of mt cracks by contrast the presence of a wide range of mt crack morphologies within petrographically homogeneous substrates and an apparent relationship between crack diameter and sinuosity suggest that the void forming process itself also played a role in determining the final morphology of mt cracks together these petrographic observations are used to define a model of microscale gas sediment interactions that can be used to interpret crack morphology in terms of gas pressure and the strength of sedimentary substrates the presence of characteristic void filling microspar is integral to preservation of mt structures cathodoluminescence cl identification of this characteristic microspar within mt voids in pore space of coarse grained facies and interstitially within fine grained facies adjacent to mt voids suggests that mt voids and cement share a common genesis because microspar cores are similar in size and morphology to vaterite precipitated experimentally in the presence of a variety of dissolved organic molecules we suggest that precipitation of mt microspar was intimately linked with gas production during organic decomposition within the host substrate in this scenario gas production would result in pore fluids with elevated concentrations of dissolved organic molecules which would initiate precipitation of mt microspar when the pore fluids come in contact with supersaturated proterozoic seawater restriction of mt largely to mesoproterozoic and early neoproterozoic strata likely reflects a critical level of carbonate saturation that limited early substrate lithification thereby allowing void production but remained high enough that organic catalysts were able to initiate precipitation of mt microspar

evidence for episodic oxygenation in a weakly redox buffered deep mid proterozoic ocean abstract over the last two decades popular opinion about prevailing conditions in the mid proterozoic deep ocean has evolved from fully oxygenated to globally euxinic sulfidic to a more heterogeneous stratified water column with localized pockets of euxinia existing in predominantly iron rich ferruginous deep waters the animikie basin in the lake superior region has been essential in shaping our view of marine redox evolution over this time period in this study we present a multi proxy paleoredox investigation of previously unanalyzed strata of the late paleoproterozoic animikie basin using drill cores through the 1 85 ga stambaugh formation paint river group in the iron river crystal falls district of the upper peninsula of michigan usa based on previous tectonic reconstructions and analysis of sedimentary regimes the iron river crystal falls section captures strata from among the deepest water facies of the animikie basin in contrast to previous work on sedimentary rocks in this basin we find evidence from iron speciation trace metal and mo isotope data for episodes of at least local deep water oxygenation within a basin otherwise dominated by ferruginous and euxinic conditions while trace metal enrichments and iron speciation data suggest predominantly anoxic conditions the occurrence of mn rich intervals up to 12 3 wt mno containing abundant mn fe carbonate and a wide range of mo isotope data with extremely negative values 98 95mo 1 0 to 1 1 record the shuttling of mn oxides from surface waters through oxic or suboxic waters to the sediment water interface we propose that such conditions are analogous to those of locally restricted modern and holocene basins in the baltic sea which receive episodic inflow of oxygenated water producing similar geochemical signatures to those observed for the animikie basin we argue that the mid proterozoic was characterized by a lack of a strong redox buffer low sulfide ferrous iron and oxygen contents and thus was vulnerable to dramatic and at least local redox shifts including briefly oxygenated bottom waters a refined view of the mid proterozoic ocean is emerging one that was still predominantly anoxic but marked by regional heterogeneities and short term redox variability that may in part reflect a transitional state between prevailingly anoxic archean and predominantly oxic phanerozoic oceans

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

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

oxygenation of the mesoproterozoic ocean and the evolution of complex eukaryotes the mesoproterozoic era 1 600 1 000 million years ago ma has long been considered a period of relative environmental stasis with persistently low levels of atmospheric oxygen there remains much uncertainty however over the evolution of ocean chemistry during this period which may have been of profound significance for the early evolution of eukaryotic life here we present rare earth element iron speciation and inorganic carbon isotope data to investigate the redox evolution of the 1 600 1 550 ma yanliao basin north china craton these data confirm that the ocean at the start of the mesoproterozoic was dominantly anoxic and ferruginous significantly however we find evidence for a progressive oxygenation event starting at 1 570 ma immediately prior to the occurrence of complex multicellular eukaryotes in shelf areas of the yanliao basin our study thus demonstrates that oxygenation of the mesoproterozoic environment was far more dynamic and intense than previously envisaged and establishes an important link between rising oxygen and the emerging record of diverse multicellular eukaryotic life in the early mesoproterozoic

oxygenated mesoproterozoic lake revealed through magnetic mineralogy terrestrial environments have been suggested as an oxic haven for eukaryotic life and diversification during portions of the proterozoic eon when the ocean was dominantly anoxic however iron speciation and fe al data from the ca 1 1 billion year old nonesuch formation deposited in a large lake and bearing a diverse assemblage of early eukaryotes are interpreted to indicate persistently anoxic conditions to shed light on these distinct hypotheses we analyzed two drill cores spanning the transgression into the lake and its subsequent shallowing while the proportion of highly reactive to total iron fehr fet is consistent through the sediments and typically in the range taken to be equivocal between anoxic and oxic conditions magnetic experiments and petrographic data reveal that iron exists in three distinct mineral assemblages resulting from an oxycline in the deepest waters reductive dissolution of iron oxides records an anoxic environment however the remainder of the sedimentary succession has iron oxide assemblages indicative of an oxygenated environment at intermediate water depths a mixed phase facies with hematite and magnetite indicates low oxygen conditions in the shallowest waters of the lake nearly every iron oxide has been oxidized to its most oxidized form hematite combining magnetics and textural analyses results in a more nuanced understanding of ambiguous geochemical signals and indicates that for much of its temporal duration and throughout much of its water column there was oxygen in the waters of paleolake nonesuch

decline in oceanic sulfate levels during the early mesoproterozoic abstract multiple sulfur isotope compositions 32 s 33 s 34 s and 36 s were analyzed for paired carbonate associated sulfate cas and disseminated pyrite py from the 1 6 ga gaoyuzhuang formation of the north china craton to reconstruct the history of sulfate levels in proterozoic oceans the 200 m thick study interval yielded relatively constant values for 34 s cas 13 0 1 8 34 s py 8 0 2 3 and 34 s cas py 5 as well as relatively constant 33 s 0 0 05 and 36 s 0 35 0 15 for both cas and pyrite limited variation in 34 s py and slightly lower 33 s of pyrite relative to cas suggest water column precipitation of pyrite limited fractionation of sulfur during microbial sulfate reduction as documented by 34 s cas py implies low seawater sulfate concentrations in the early mesoproterozoic ocean we quantitatively constrained paleo seawater so 4 2 using a novel modeling approach based on measured values of 34 s cas py and 34 s cas t max for the study unit 34 s cas py is 5 4 1 4 n 17 and 34 s cas t max is 6 8 34 myr 1 based on sedimentation rates of 30 150 m myr 1 these data indicate early mesoproterozoic seawater so 4 2 of p o 2 after great oxidation event i

origin of layer 1 deposits in ignimbrites types of layer 1 deposits in ignimbrite sequences include ground surge ground layer and jetted deposits ground surge deposits are generally thought to represent deposition from turbulent low concentration pyroclastic density currents resulting from ash hurricane initial stages of plinian eruption column collapse or blast wave phenomena ground layers form by sedimentation from the fluidized head of a pyroclastic flow and jetted deposits form by explosive jetting from the flow head as a result of heating of ingested air theoretical evidence suggests some layer 1 deposits may result from turbulent boundary layers at the base of pyroclastic flows deposits formed from this process will have characteristics that depend on local flow conditions and thus may be largely independent of distance from vent the turbulenit boundary layer model may explain the occurrence of surgelike deposits at distances from sources that appear too far for transportation by a low concentration turbulent pyroclastic surge

long runout pyroclastic surge on a cretaceous alluvial plain republic of korea pyroclastic surge is a dilute and turbulent flow of volcanic gas and tephra that is commonly generated during explosive volcanic eruptions and can threaten lives along its flow paths assessing its travel distance and delineating future volcanic hazards have therefore been major concerns of volcanologists historical eruptions show that most pyroclastic surges travel a few tens of kilometres or less from their sources aeolian or aquagene processes have therefore been evoked for the emplacement of supposed surge deposits much beyond this distance here we show that a cretaceous tuff bed in korea was emplaced by an exceptionally powerful pyroclastic surge that flowed as far as the most powerful pyroclastic flows that formed the low aspect ratio ignimbrites lari this has significant implications for interpreting ancient volcanic eruptions and delineating volcanic hazards by pyroclastic surges and casts intriguing questions on the eruption dynamics and physics of long runout pyroclastic surges and their distinction from lari forming pyroclastic flows

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

distinctive characteristics of a streamflow dominated alluvial fan deposit sanghori area kyongsang basin early cretaceous southeastern korea abstract in the northwestern part of the non marine kyongsang basin early cretaceous conglomeratic deposits occur at intervals along the northern basin margin and grade basinward into interbedded sandstone and mudstone one of these conglomeratic deposits referred to as the sanghori deposit is 150 m thick and characterized by amalgamated sheets of conglomerate and sandstone facies assemblages 1 and 2 alternating with interbedded sandstone and purple siltstone facies assemblage 3 facies assemblage 1 comprises clast supported disorganized and organized sheet conglomerates interbedded with massive to stratified sandstones this assemblage is interpreted as the product of proximal gravel bed stream deposition facies assemblage 2 consists of trough and planar cross stratified and horizontally stratified sandstones it represents sandy braided stream deposits in which sand bars and dunes were common sedimentary facies and architecture of the coarse grained facies assemblages suggest that the gravel bed and sandy braided streams were wide and shallow and rapidly aggraded during flash floods facies assemblage 3 consists of interbedded purple siltstone and tabular sandstone which show evidence of weak pedogenic modification and bioturbation these characteristics are interpreted as the result of floodplain processes sediments were supplied from the north and dispersed radially forming a streamflow dominated alluvial fan system the sanghori alluvial system was most probably fed from a large drainage basin resulting in the predominance of streamflow deposits and rounded clasts the alternation of the gravel bed stream and floodplain deposits was due to autocyclic switching of active fan sectors under the conditions of relatively slow and steady basin subsidence

formation of shallow water glaucony in weakly oxygenated precambrian ocean an example from the mesoproterozoic tieling formation in north china abstract authigenic glaucony precipitation in phanerozoic oceans takes place mostly in middle shelf to upper slope environments with low depositional rate or sediment starvation precambrian glaucony however is more common in stratigraphic successions deposited from shallower water environments with high and variable depositional rates this phenomenon has long been noticed in literature but the controlling factors of shallow water glaucony precipitation in precambrian oceans have not been adequately investigated to better understand the glauconitization processes in precambrian oceans we have conducted an integrated study of the glaucony in stromatolitic carbonates of the mesoproterozoic tieling formation ca 1437 ma in north china using sedimentological mineralogical and geochemical data obtained from field observations petrography xrd sem quantitative eds and icp ms analyses macro and microscopic observations show that the tieling glaucony fills voids of varying sizes and shapes and records different maturation stages of glauconitization geochemical analyses show that the tieling glaucony has high k 2 o avg 8 but low and variable total fe 2 o 3 tfe 2 o 3 contents 1 92 13 65 wt the tfe 2 o 3 contents increase with maturation of glaucony titration results show that the tieling glaucony contains both fe 3 and fe 2 ions but has fe 2 fe 3 ratios much higher than that of the phanerozoic glaucony ree results of glaucony hosting carbonates show weak negative and positive ce anomalies with average ce ce ratio close to 1 0 suggesting carbonate precipitation near the redoxcline of fe mn oxides all these features suggest that the tieling glaucony was precipitated in seawater around the fe redoxcline where both fe 2 and fe 3 were available throughout the glaucony maturation stages high fe 2 fe 3 ratios in the depositional environments led to fe 2 occupation at octahedral sites of glaucony and negative charges on octahedrons which resulted in high k content to balance the negative charges on octahedrons and low tfe 2 o 3 limited by dioctahedral structure the formation of the tieling glaucony and other similar precambrian glauconies is likely controlled by low oxygen concentration in seawater and a shallow redoxcline that controls the availability of fe and k cations during initial precipitation and maturation of glaucony the shift of authigenic glaucony precipitation from shallow water in the precambrian to deep water in the phanerozoic may record the deepening of ocean chemocline in response to increased ocean oxygenation

oxygen climate and the chemical evolution of a 1400 million year old tropical marine setting the xiamaling formation is an exceptionally well preserved sedimentary succession deposited on a marine passive margin about 1400 million years ago we used a multi proxy approach including iron speciation trace metal dynamics and organic geochemistry to explore the evolution of ocean chemistry through most of the xiamaling formation this evolution is put in the context of the paleogeography and the sedimentological evolution of the xiamaling depositional system overall the xiamaling formation is informally divided into six units based on both sedimentological and geochemical criteria of the six units we fully explored four of them unit 4 the lowest unit we studied is comprised of deep water red muds periodically interrupted by green colored silt and sandy turbidites iron extraction results show that the red muds are enriched in highly reactive iron indicating a water column source for the iron however the low organic carbon contents low hydrogen index hi values and the oxidized nature of the reactive iron pool indicate deposition in oxygenated bottom waters we interpret unit 4 to represent a low productivity ferruginous oxygen minimum zone omz environment underlain by oxygenated bottom waters the transition to unit 3 reflects an increase in primary productivity and the development of a more biologically active omz that supported anoxygenic phototrophic bacteria still in this unit the bottom waters remained oxygenated the overlying unit 2 represents the transition to deep water deoxygen ation and anoxic waters at the sediment surface these waters were ferruginous in the bottom part of the unit and sulfidic euxinic towards the top in the uppermost unit 1 euxinic conditions continued punctuated by more frequent water column oxygenation towards the upper part of the unit we place the evolution of these chemical dynamics in the context of climate and climate change and in particular the placement of the xiamaling formation in relation to the intertropical convergence zone itcz and the resulting hadley cell dynamics also while our results demonstrate the persistence of anoxic water column conditions high in the water column during the deposition of the xiamaling formation they also demonstrate bottom water oxygenation near the seafloor during the deposition of three of the four units and over a time interval extending to 10 s of millions of years less

a model for uranium rhenium and molybdenum diagenesis in marine sediments based on results from coastal locations abstract the purpose of this research is to characterize the mobilization and immobilization processes that control the authigenic accumulation of uranium u rhenium re and molybdenum mo in marine sediments we analyzed these redox sensitive metals rsm in benthic chamber pore water and solid phase samples at a site in buzzards bay massachusetts usa that has high bottom water oxygen concentrations 230 300 mol l and high organic matter oxidation rates 390 mol c cm2 y the oxygen penetration depth varies from 2 to 9 mm below the sediment water interface but pore water sulfide is below detection release of rsm to pore waters during the remineralization of solid phases occurs near the sediment water interface at depths above the zone of authigenic rsm formation this release occurs consistently for mo at both sites but only in the winter for re in buzzards bay and intermittently for u at the buzzards bay site re removal to the solid phase extends to the bottom of the profile while the zone of removal is restricted to 2 9 cm for u and mo authigenic re formation is independent of the anoxic remineralization rate which is consistent with an abiotic removal mechanism the rate of authigenic u formation and its modeled removal rate constant increase with increasing anoxic remineralization rates and is consistent with u reduction being microbially mediated authigenic mo formation is related to the formation of sulfidic microenvironments the depth and extent of mo removal from pore water is closely associated with the balance between iron and sulfate reduction and the consumption of pore water sulfide via iron sulfide formation pore water rsm reach constant asymptotic concentrations in sulfidic sediments but only pore water re is constant at depth in buzzards bay the increases in pore water u at the buzzards bay site are consistent with addition via irrigation and subsequent upward diffusion to the removal zone deep pore water mo concentrations exceed its bottom water concentration due to irrigation induced oxidation and remobilization from the solid phase in sulfidic sediments there is no evidence for higher pore water u or mo concentrations at depth due to the absence of irrigation and or the presence of more stable authigenic rsm phases there are good correlations between benthic fluxes and authigenic accumulation rates for u and mo in sulfidic sediments however results from buzzards bay suggest irrigation ultimately results in the partial loss of u and mo from the solid phase with accumulation rates that are 20 30 of the modeled flux irrigation can augment re possibly u or compromise u mo authigenic accumulation in sediments and is important when determining burial rates in continental margin sediments

re os age constraints and new observations of proterozoic glacial deposits in the vazante group brazil abstract a new re os radiometric age date for an organic rich shale horizon from the vazante group in brazil coupled with geological observations provide evidence for late mesoproterozoic glacial episodes conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time field observations of a reverse fault juxtaposing older mesoproterozoic sedimentary rocks above younger neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of u pb detrital zircon ages through the succession combined the geochronological geochemical and stratigraphic evidence suggest that the vazante group sediments accumulated along a passive margin of the sao francisco craton and are correlative with the neighboring paranoa group biomarker sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post glacial transgression and deposition of one of the bituminous shale horizons the relationship of the glaciogenic vazante group to other late mesoproterozoic successions such as the non glacial atar group in west africa and bylot supergroup in arctic canada however remains enigmatic

early oxygenation of the terrestrial environment during the mesoproterozoic geochemical data from ancient sedimentary successions provide evidence for the progressive evolution of earth s atmosphere and oceans1 2 3 4 5 6 7 key stages in increasing oxygenation are postulated for the palaeoproterozoic era 2 3 billion years ago gyr ago and the late proterozoic eon about 0 8 gyr ago with the latter implicated in the subsequent metazoan evolutionary expansion8 in support of this rise in oxygen concentrations a large database1 2 3 9 shows a marked change in the bacterially mediated fractionation of seawater sulphate to sulphide of 34s 25 before 1 gyr to 50 after 0 64 gyr this change in 34s has been interpreted to represent the evolution from single step bacterial sulphate reduction to a combination of bacterial sulphate reduction and sulphide oxidation largely bacterially mediated3 7 9 this evolution is seen as marking the rise in atmospheric oxygen concentrations and the evolution of non photosynthetic sulphide oxidizing bacteria3 7 10 here we report 34s values exceeding 50 from a terrestrial mesoproterozoic 1 18 gyr old succession in scotland a time period that is at present poorly characterized this level of fractionation implies disproportionation in the sulphur cycle probably involving sulphide oxidizing bacteria that is not evident from 34s data in the marine record1 2 3 9 disproportionation in both red beds and lacustrine black shales at our study site suggests that the mesoproterozoic terrestrial environment was sufficiently oxygenated to support a biota that was adapted to an oxygen rich atmosphere but had also penetrated into subsurface sediment

heterogeneous redox conditions and a shallow chemocline in the mesoproterozoic ocean evidence from carbon sulfur iron relationships abstract oxygenation of earth s oceans in the proterozoic was a protracted process that began prior to the great oxidation event goe and culminated in deep ocean ventilation following neoproterozoic glacial episodes with both of these major thresholds accompanied by profound changes in the evolution of earth s biosphere our understanding of the redox structure of mesoproterozoic oceans however is considerably less certain limited proxy investigations of oceanic redox in the mesoproterozoic have focused largely on early mesoproterozoic shelf to basinal settings with comparatively little attention paid to epeiric sea environments that dominate the late mesoproterozoic sedimentary record here we report multi proxy shale redox data from epicratonic and pericratonic environments of the 1 1 ga atar and el mreiti groups taoudeni basin mauritania these strata were deposited across the west african craton during global sea level highstand and provide insight into the redox structure of mesoproterozoic epeiric seas iron speciation pyrite sulfur isotope and trace metal concentration data suggest a shallow chemocline and highly heterogeneous redox conditions with fundamental heterogeneities in sulfate concentration and the extent and persistence of euxinia between onshore and offshore environments we suggest that a protracted increase in biospheric oxygenation in the late mesoproterozoic was accompanied by unstable and heterogeneous redox conditions in shallow nearshore areas contributing to the fragmentary pattern of early eukaryotic evolution and diversification

emplacement of the taupo ignimbrite by a dilute turbulent flow an ignimbrite is a pumice rich deposit that records the passage of a ground hugging ash flow a pyroclastic flow generated by the collapse of a volcanic eruption column1 geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions the 1 800 yr old taupo ignimbrite of new zealand has been interpreted to represent en masse emplacement by an avalanche like flow with a volumetric solids concentration in excess of 30 ref 2 the evidence for this is equivocal however and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3 we present an isothermal hydraulic model the results of which taken together with existing observations suggest that the total flow rate of solids and gas was about 40km3s 1 for around 15 minutes this intense flux resulted in a flow which had a near vent solids concentration of 0 3 by volume was about 1 km thick and travelled outward from the vent with a typical speed of 200 ms 1 in view of the good agreement between predicted and observed radial trends in the taupo deposit we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered

the iron paleoredox proxies a guide to the pitfalls problems and proper practice oceanic anoxia including euxinic settings defined by the presence of water column hydrogen sulfide h2s is minor in the ocean today such conditions however were common or even dominant in the past particularly during the precambrian and phanerozoic oceanic anoxic events the latter are associated with massive petroleum and mineral reserves and many of the major extinction events in the paleontological record our ability to recognize ancient oxygen deficiencies relies strongly on paleontological data viewed in combination with geochemical tracers and geochemistry is typically our only window onto ancient marine redox during the precambrian when diagnostic skeletal and behaviorial traces of oxygen dependent animals are mostly missing so far no approach has gained wider acceptance than the iron proxies which rely generally on quantification of the extent to which reactive iron as oxides principally is converted to pyrite the promise of these approaches lies in part with the relative ease of measurement but it is this ease and the corresponding widespread use that has also led to misuses much of the recent confidence in the iron paleoredox proxies lies with sophisticated deconstruction of the reactive fe pool via mineral calibrated wet chemical speciation these validations and calibrations mostly in the modern ocean expose the challenges while at the same time opening other doors of opportunity as the catalog of controlling factors extends beyond water column redox to include sedimentation rate sedimentary fe remobilization signals of oscillatory redox and hydrothermal versus other primary fe inputs to the ocean among other factors also key is a deep understanding of the limitations imposed or at least the due diligence required as linked to mineral transformations during burial and metamorphism this review seeks to highlight many of the key issues including appropriate sample choices as a roadmap for those keen to apply fe proxies in their studies of ancient oceans and their relationships to co evolving life among the critical messages to take away is the value of robust fe based measures of local redox that when combined with elemental mass balances and isotopic proxies dependent on those local conditions can shed light on the global redox state of the oceans through time and related implications for the history of life on earth