a sulfate control on marine mid depth euxinia on the early cambrian ca 529 521 ma yangtze platform south china abstract given the high oxygen demands of complex metazoans a sound understanding of marine redox conditions is essential to reconstruct the factors that lead to the dramatic increases in ecosystem complexity and biological diversity in the early cambrian in this study we report new fe mo c s geochemical data from black shale rich strata of the early cambrian continental margin yangjiaping section in the yangtze platform south china integrated fe speciation and mo abundance data from the yangjiaping section in combination with previously reported data from shatan inner shelf songtao slope and longbizui basinal sections suggest mid depth euxinic waters coexisting with ferruginous deep waters and oxic surface waters this redox structure is similar to conditions previously suggested for the ediacaran yangtze platform and other proterozoic marine basins indicating that proterozoic layered ocean redox structure in some basins persisted into the early cambrian furthermore the fe mo data demonstrate that the mid depth euxinia was widespread during a marine transgression but shrank gradually along with widespread shelf oxygenation when the basin evolved into subsequent regression excess highly reactive iron fe hr in deep water with a hydrothermal origin may have had a control on the mid depth euxinia given geochemical and geological evidence for basinal hydrothermal activities on the other hand a sulfate limitation on bacterial sulfate reduction bsr and sulfide generation was suggested by a weak correlation r 2 0 22 between pyrite iron versus fe hr and total organic carbon toc form anoxic deep water sections and high toc averagely 6 in the deepest ferruginous water longbizui section furthermore a spatial gradient was observed in sulfur isotopic composition of pyrite averagely from 0 2 to 21 0 consistent with a lateral gradient of sulfate in concentration or and in isotope from shore to distal waters on the transgressive yangtze platform we accordingly proposed that two distinct sources riverine input and ocean reservoir provided sulfate for bsr dominantly in near shore and distal areas respectively and the euxinic retreat observed for the regression can be explained as a response to the progressive sulfate consumption of the ocean reservoir by intensive bsr and pyrite formation during preceding transgression therefore our results support a sulfate control on the spatiotemporal distribution of mid depth marine euxinia on the yangtze platform in early cambrian when complex metazoans rapidly evolved

marine oxygenation lithistid sponges and the early history of paleozoic skeletal reefs abstract microbial carbonates were major components of early paleozoic reefs until coral stromatoporoid bryozoan reefs appeared in the mid ordovician microbial reefs were augmented by archaeocyath sponges for 15 myr in the early cambrian by lithistid sponges for the remaining 25 myr of the cambrian and then by lithistid calathiid and pulchrilaminid sponges for the first 25 myr of the ordovician the factors responsible for mid late cambrian microbial lithistid sponge reef dominance remain unclear although oxygen increase appears to have significantly contributed to the early cambrian explosion of marine animal life it was followed by a prolonged period dominated by greenhouse conditions as sea level rose and co2 increased the mid late cambrian was unusually warm and these elevated temperatures can be expected to have lowered oxygen solubility and to have promoted widespread thermal stratification resulting in marine dysoxia and hypoxia greenhouse conditions would also have stimulated carbonate platform development locally further limiting shallow water circulation low marine oxygenation has been linked to episodic extinctions of phytoplankton trilobites and other metazoans during the mid late cambrian we propose that this tendency to dysoxia hypoxia in shallow marine environments also limited many metazoan reef builders in contrast during the mid late cambrian the ability of lithistid sponges to withstand low oxygen levels allowed them to create a benthic association with microbial carbonates that dominated global reefs these conditions ameliorated during the ordovician as temperature decline promoted ocean ventilation the prolonged time gap occupied by low diversity reefs between the cambrian explosion and the great ordovician biodiversification event reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments

the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

extreme change in sulfide concentrations in the black sea during the little ice age reconstructed using molybdenum isotopes the black sea is the largest and most studied anoxic basin in the modern world much of this research has focused on the redox structure of the water column specifically on the driving forces behind variations in the position stability and structure of the oxic anoxic interface chemocline however none of these studies has been able to quantify the historical sulfide concentrations associated with the changes in chemocline depth using the isotopic composition of molybdenum in sediments as a proxy we show for the first time that varying concentrations of dissolved sulfide can be fingerprinted in historical systems our molybdenum isotope data indicate that in the region of the bosporus inlet the chemocline rose more than 65 m reaching concentrations over 100 m sulfide in the bottom water ca 300 yr b p this historical shoaling of the chemocline and extreme change in bottom water sulfide concentration exceeds the modern changes that have been observed directly and attributed to anthropogenic influences on the black sea chemistry hydrology the first cold interval of the little ice age when temperature and circulation changes occurred in the black sea basins may have provided the natural trigger for this extreme rise in bottom water sulfide concentrations

variations of redox conditions in the atmosphere and yangtze platform during the ediacaran cambrian transition constraints from cr isotopes and ce anomalies abstract the ediacaran cambrian transition was characterized by the evolution from ediacaran soft body to cambrian skeletal organisms and fluctuations of the marine carbon cycle marine redox changes should have played an essential role in both ecological and environmental evolutions however temporary changes and spatial distribution of redox conditions in the ediacaran cambrian oceans remain elusive and highly debated in this study we analyzed chromium cr isotope and cerium ce anomaly profiles in carbonates from the upper ediacaran lower cambrian dengying and zhujiaqing formations at the xiaotan section south china in an attempt to provide new insights helping unravel earth s surface redox changes during the ediacaran cambrian transition our results show that these carbonates record generally positive 53crauth values up to 1 15 with fluctuations and prominent negative ce anomalies ce ce values down to 0 35 implying a broad increase in oxygenation in the atmosphere and the inner shelf of the yangtze platform during the ediacaran cambrian transition the first appearance and subsequent diversification of the cambrian animals may have benefited from this oxygenation event the pronounced fluctuations of 53crauth and 13ccarb values can be explained by episodic oxidation of a deep water doc pool in a stratified water column across the chemocline through upwelling releasing light carbon isotopes and organic matter bound isotopically unfractionated cr iii into shallow waters

molybdenum drawdown during cretaceous oceanic anoxic event 2 during the cretaceous greenhouse episodes of widespread ocean deoxygenation were associated with globally occurring events of black shale deposition possibly the most pronounced of these oceanic anoxic events oae s was the cenomanian turonian oae2 94 ma however although certain redox sensitive trace metals tend to be preferentially sequestered in sediments deposited under anoxic conditions with mo drawdown being specifically prone to euxinic settings these elements are generally somewhat depleted in sediments deposited during oae2 to understand the driving factors responsible for this depleted trace metal drawdown we have studied a low latitude section from the proto north atlantic ocean tarfaya s57 where existing biomarker and iron sulphur data point to a dominantly euxinic water column with periodic transitions to ferruginous fe rich water column conditions we utilise a variety of redox proxies fe speciation redox sensitive trace metals and mo isotopes which in combination allows us to evaluate the detailed nature of ocean redox conditions and hence controls on trace metal drawdown the results suggest that seawater 98mo values may have ranged between 0 6 and 1 1 during oae2 likely connected to changes in the local mo reservoir as a consequence of low and probably heterogeneous concentrations of mo in the ocean the very low mo toc ratios at tarfaya and elsewhere in the proto north atlantic may support a model in which deep water circulation was partially restricted within and between the north atlantic and other ocean basins we propose that the combination of a low and possibly heterogeneous 98mo of seawater mo together with low mo toc ratios points to a large decrease in the global oceanic mo reservoir during oae2 reflecting a major global scale increase in mo drawdown under persistent euxinic conditions 2016 elsevier b v

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

dense bottom gravity currents and their impact on pelagic methanotrophy at oxic anoxic transition zones we show that inflows of oxygenated waters into sulfidic layers have a strong impact on biogeochemical transformation at oxic anoxic transition zones taking the pelagic methane dynamics in the gotland basin as an example we performed our studies when one of the largest inflows ever recorded entered the baltic sea in march 2015 an inflowing gravity current transported oxic waters into the sulfidic deep layers and freshly generated a near bottom secondary redox interface at the upper slope where the inflowing water masses were vigorously turbulent and the main and secondary redox interfaces in close contact to each other methane oxidation rates inside the transition zone were found to be higher compared to the weakly turbulent basin interior at the main redox interface in the basin center lateral intrusions of oxygenated waters into intermediate water depth may have stimulated the growth of the methanotrophic community and their activity

characterization of airborne carbonate over a site near asian dust source regions during spring 2002 and its climatic and environmental significance 1 during spring 2002 three dust storm events were monitored by filter sampling in xi an near an asian dust source region of northwest china the carbonate co32 fraction was determined by sample acidification and thermal evolution the co32 accounted for 8 0 0 8 of particles with aerodynamic diameter 2 5 m pm2 5 during dust storms and 4 7 3 0 of pm2 5 between storms the ratios of calcium to carbonate carbon were consistent with those of calcite caco3 the 13c and 18o abundances in dust storm samples were 2 7 0 7 and 5 8 1 5 which differed from 8 3 1 9 for 13c and 10 8 2 0 for 18o during normal conditions the 13c is positively correlated with 18o values r 0 78 this first measurement of isotopic abundance in asian dust indicates the potential to quantify its contribution at distant locations using receptor models by increasing the alkalinity of ocean water in the pacific ocean and buffering the atmospheric acidity of east asia the large amounts of airborne co32 as high as 44 8 tg yr 1 entrained by asian dust may provide an important atmospheric alkaline carbon reservoir for large scale climatic and environmental changes

new paleomagnetic results from the ediacaran doushantuo formation in south china and their paleogeographic implications abstract a new paleomagnetic pole is obtained from the top of member 3 of the doushantuo formation in the jiulongwan section of the yangtze gorges area in south china a total of 191 samples from 23 sites were collected and subjected to stepwise alternating field af and thermal demagnetization after the removal of a soft component sc of viscous magnetic remanence acquired in present geomagnetic field a high temperature component hc likely carried by magnetite was isolated the hc includes vectors from 147 samples using principal component analysis and arc constraints from 44 samples using remagnetization great circle analysis both the vectors only and the combined vectors and arc constraints of the hc passed a reversal test this is the first ediacaran paleomagnetic remanence from the south china block scb that passes a reversal test and it is interpreted as a primary remanence the hc vectors were thus used to calculate the paleomagnetic pole and in combination with arc constraints were used for polarity interpretation the vectors give a mean direction of d 75 2 i 41 0 95 2 5 n 147 after bedding correction and a corresponding paleomagnetic pole at 23 9 n 187 0 e dm dp 3 0 1 8 this pole differs significantly from previously published results of the doushantuo formation which may have been subjected to paleozoic remagnetization our results provide a 23 5 1 8 n paleolatitude for the yangtze gorges area of the scb and suggest several alternative paleogeographic options for the ediacaran cambrian amalgamation of east gondwana depending on chronostratigraphic correlations

intermittent euxinia reconciliation of a jurassic black shale with its biofacies isorenieratane is ubiquitous in the organic carbon rich peterborough member callovian uk of the oxford clay formation indicating a temporal overlap of the euphotic and sulphidic zone in the water column

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

oligocene early miocene river incision near the first bend of the yangze river insights from apatite u th sm he thermochronology abstract the southeastern tibetan plateau is deeply incised by three parallel rivers the salween the mekong and the yangtze the river incision and surface uplift histories of this landscape are hotly debated this study presents bedrock apatite u th sm he data from a 1800 m vertical profile located near the first bend of the yangtze river ages range from 20 to 30 ma indicating an oligocene early miocene phase of moderate river incision at a rate of 0 10 0 18 mm yr this is considerably older than elsewhere in the region but consistent with a previously proposed phase of eocene surface uplift inferred from stable isotope geochemistry we consider the implications of the new data under two different tectonic models if the surface uplift and river incision resulted from lower crustal flow the new results require such flow to have commenced at oligocene early miocene time rather than during the previously proposed late miocene alternatively oligocene to early miocene plateau growth might have resulted from transpressional deformation in the southeastern tibetan plateau

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

the impact of the three gorges dam on the downstream distribution and texture of sediments along the middle and lower yangtze river changjiang and its estuary and subsequent sediment dispersal in the east china sea abstract the grain size of river sediments changes systematically downstream from source to sink and is influenced by catchment lithology geomorphology hydrology oceanography and in modern settings anthropogenic impacts compared with small gravel bedded river systems less is known about large sandy bed rivers particularly from the river source to marine sink in the present study we examine longitudinal changes in sediment grain size along the middle and lower yangtze river downstream of the three gorges dam tgd and along the major sediment dispersal pathway into the east china sea over a total length of 2100 km we also examine the spatial patterns of seabed sediment grain size in the east china sea adjacent to the yangtze estuary 70 000 km 2 in area in particular we consider the impact of the tgd on the grain size of the riverbed and seabed sediments before the construction of the tgd the relationship between median grain size and distance along the sandy bed of the middle and lower yangtze showed a downstream fining trend that was exponential in form after the tgd was built erosion caused an abrupt gravel sand transition to develop in the section immediately downstream of the tgd in the yangtze estuary flocculation and subsequent deposition of suspended riverine mud during the slack water period between flood and ebb tides led to the formation of an abrupt sand mud transition muddy deposits along the major longshore sediment dispersal route also show an exponential fining trend however towards the open east china sea the modern riverine muds are replaced seawards by older sands the mud margin there was found to have retreated landward significantly over the post tgd period due to erosion driven by the significant decrease in sediment supply from the yangtze river we expect that the impact of the tgd on the grain size of bed sediments in the yangtze river and the east china sea will continue for some time and the change in the grain size of bed sediments there will become more pronounced

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

nitrogen cycle perturbations linked to metazoan diversification during the early cambrian abstract the rapid diversification of metazoans during the late ediacaran early cambrian is one of the most remarkable evolutionary events in earth history however the triggering mechanism remains highly contentious particularly concerning the relationship of this biological event to contemporaneous changes in ocean redox and nutrient cycling in this study we report new bulk nitrogen isotope 15n and organic carbon isotope 13corg data from the lower cambrian niutitang formation in two deep water sections in south china to provide a better understanding of the relationship of the nitrogen cycle ocean redox and biological evolution during this key period the 15n data from two study sections are characterized by consistently low values 1 to 2 throughout the niutitang formation despite large variations in 13corg we interpret these low 15n values to have resulted from assimilation of recycled ammonium nh4 and n2 fixation compilation of available 15n data reveals large temporal and spatial 15n variations from 1 to 7 in time equivalent strata in south china such 15n variations imply the co existence of aerobic and anaerobic n cycles and highly heterogeneous ocean redox conditions during the early cambrian in south china which is consistent with inferences from other geochemical proxies temporal fluctuations and spatial heterogeneity in ocean redox conditions may have controlled the distribution and evolution of metazoans during the early cambrian

towering sponges in an early cambrian lagerstatte disparity between nonbilaterian and bilaterian epifaunal tierers at the neoproterozoic cambrian transition epifaunal suspension feeding bilaterian animals in the cambrian lived close to the sediment water interface and hence their ecological tiering levels were low 10 cm here we report an early cambrian diandongian or probably tommotian atdabanian lagerstatte from the hetang formation in anhui province south china the hetang biota is characterized by high tiering to 50 cm sponges and small 0 5 cm bilaterians including orthothecid hyoliths and bivalved arthropods nonbilaterian suspension feeders sponges cnidarians and archaeocyathids as high tiering animals and bilaterian suspension feeders as low tiering animals also characterize other neoproterozoic cambrian assemblages such as the ediacaran chengjiang burgess shale and sinsk biotas these data are consistent with medium to high tiering levels in neoproterozoic cambrian epifaunal communities but suggest that nonbilaterians achieved such tiering levels long before bilaterian suspension feeders did so in the early ordovician the disparity between bilaterian and nonbilaterian tierers during the neoproterozoic cambrian transition and the delayed appearance of high tiering bilaterians demand phylogenetic and ecological explanations the cambrian substrate revolution may have triggered a cascade of ecological evolution including the rise of bilaterian animals in high tiering levels during the ordovician radiation of the paleozoic fauna

rapid high amplitude variability in baltic sea hypoxia during the holocene hypoxia oxygen concentrations of 2 ml l and so called dead zones are a growing concern in coastal marine environments the baltic sea is a shelf sea that is highly sensitive to hypoxia and may serve as a laboratory for studying the interplay between natural and anthropogenic forcing of redox conditions in the global coastal zone past occurrences of hypoxia in the baltic sea have been shown by previous studies but high resolution quantitative reconstructions of past hypoxia intensity are lacking here we present bulk sediment geochemical records from the deep basins of the baltic sea that show multicentennial oscillations during intervals of past hypoxia suggesting rapid alternations between hypoxic and relatively oxic conditions while the onset of past hypoxic events was likely forced by climatic variability these events intensified and terminated rapidly due to feedbacks in the phosphorus p cycle the modern intensity of hypoxia is similar to several past events suggesting that hypoxia in the baltic sea has a maximum potential intensity however using ultrahigh resolution laser ablation inductively coupled plasma mass spectrometry scanning of sediment blocks we show that modern hypoxia intensified more rapidly than any past event this confirms the role of anthropogenic nutrient loading in driving this system into its current hypoxic state

diagenetic mobilization of ti and formation of brookite anatase in early cambrian black shales south china abstract titanium ti is typically hosted in detrital minerals in marine sediments and has long been considered to be immobile during diagenesis in this study the authigenic titania minerals brookite and anatase are observed in early cambrian carbonaceous shales from the meishucun and zhajin sections of south china respectively black shales in the meishucun section have total organic carbon toc contents from 1 6 to 3 9 wt and hi hydrogen index values from 3 8 to 20 mg hc g toc whereas black shales in the zhajin section have much higher toc 7 1 15 6 wt but lower hi

global variations of surface ocean productivity in low and mid latitudes influence on co2 reservoirs of the deep ocean and atmosphere during the last 21 000 years based on detailed reconstructions of global distribution patterns both paleoproductivity and the benthic 13c record of co2 which is dissolved in the deep ocean strongly differed between the last glacial maximum and the holocene with the onset of termination i about 15 000 years ago the new export production of low and mid latitude upwelling cells started to decline by more than 2 4 gt carbon year this reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric co2 as recorded in ice cores and with a slight delay of more than 1000 years to a large scale gradual co2 depletion of the deep ocean by about 650 gt c this estimate is based on an average increase in benthic 13c by 0 4 0 5 the decrease in new production also matches a clear 13c depletion of organic matter possibly recording an end of extreme nutrient utilization in upwelling cells as shown by sarnthein et al 1987 the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades which in turn was linked via a shrinking extent of sea ice to a massive increase in high latitude insolation i e to orbital forcing as primary cause

sedimentary corg p ratios paleocean ventilation and phanerozoic atmospheric po2 the corg p ratios of organic rich facies tocn1 exhibit considerable variation through the phanerozoic from b10 1 in the permian phosphoria formation to n1000 1 in some devonian black shales relative to the composition of phytoplankton c p 106 1 which provide the bulk of organic c and p to organic rich marine sediments the range of phanerozoic sedimentary corg p ratios largely reflects the influence of benthic redox conditions on diagenetic pathways of c and p remobilization enhanced preservationoforganiccatlowerredoxpotentials combinedwithenhancedsedimentaryretentionofremineralizedorganicpathigher redox potentials results in a strong relationship between benthic redox conditions and sedimentary corg p ratios the strong secular coherence of the phanerozoic corg p record i e limited variation within narrow time slices suggests that these ratios are controlled primarily by factors influencing benthic redox conditions on a global scale such as atmospheric po2 inversion and scaling of the phanerozoic corg p record yields a new atmospheric po2 model that has strong similarities to existing models based on elemental and isotopic mass balances yet differs in some potentially significant respects in particular the new model suggests that atmospheric po2 wasmarkedlylowerduringtheearlytomiddlepaleozoicthanpreviouslyinferred duringmostofthedevonian atmospherico2levels mayhavebeensufficientlylow b13 astopreventthesustainedcombustionofplantmaterial thisfeatureofthemodelmayaccount for ananomalous gapinthefossilcharcoalrecordduringthedevonian theredox dependentburialofpintheoceanresults inpositive and negative feedback mechanisms that tend to stabilize atmospheric oxygen concentrations on geologic timescales temporal variations in the burial of p relative to c suggest that the strength of these feedback mechanisms varied over the phanerozoic 2007 elsevier b v all rights reserved

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

molybdenum isotope behaviour accompanying weathering and riverine transport in a basaltic terrain the application of the molybdenum mo isotope system as a proxy for determining changes in the redox state of the oceans is predicated on the assumption that the composition of continental input can be characterised from crustal rock types and remains constant however it has recently been shown that the 98 95mo composition of global rivers varies between 0 15 and 2 4 and is therefore systematically heavier than the average composition of the continental crust 0 in order to understand the processes that control mo isotope fractionation during weathering this study presents 98 95mo and mo abundance data for rivers and estuarine samples from iceland that drain predominantly basaltic terrains resolvable differences are observed in the isotopic composition of the riverine mo sources ice 98 95moice 1 8 basaltic bedrock 98 95mobedrock 0 0 and hydrothermal waters 98 95mohydrothermal 3 4 systematic changes in the dissolved mo isotope composition are also observed within river catchments with 98 95mo values increasing from 0 in glacial rivers close to the source to 1 downstream consistent with mo isotopes being fractionated during weathering analysis of other riverine phases bedload colloids and iron precipitates demonstrates that these phases preferentially incorporate light mo isotopes and remain coupled to the dissolved load during riverine transportation a 98 95mo profile through the borgarfjor eth ur estuary exhibits a predominantly conservative mixing behaviour but suggests that the release of isotopically light mo from the particulate and or colloidal phases may occur in the low salinity part of the estuarine mixing zone 2010 elsevier b v

dust sources in arid and semiarid china and southern mongolia impacts of geomorphological setting and surface materials abstract analysis of the geomorphological settings and surface materials in arid and semiarid china and southern mongolia revealed that the dominant dust sources are gobi deserts and alluvial fans adjacent to gobi deserts in the piedmonts of the kunlun qilian helan and gobi altai mountains these areas have persistent supplies of dust sized particles deposited by intermittent floods from the mountains and fine materials are also generated by weathering and aeolian processes acting on the coarse fractions of gobi surfaces clear spatial differences in mineral types and contents and in the salt contents of surface materials also strongly affected the intensity of dust emissions inconsistencies in monitoring simulation and calculation of dust emissions in previous studies of northern china and mongolia may have been caused by insufficiently detailed considerations of variations in the geomorphological setting where the geomorphological settings in these areas are not considered in sufficient detail deviations arise in the estimation and prediction of dust using current models although the present study did not quantify the persistence and replenishment of dust sources in the study area our conclusions about the importance of geomorphology can improve future studies of dust sources and controlling factors improve assessments of the effects of climate change on dust emissions and facilitate the interpretation of paleoclimate records

the link between metazoan diversity and paleo oxygenation in the early cambrian an integrated palaeontological and geochemical record from the eastern three gorges region of south china abstract the link between environmental perturbations especially oceanic redox conditions and evolution during the cambrian biodiversification event is still highly controversial although this issue has attracted considerable attention for decades we here report on the fossil record of the early cambrian biota discovered in the uppermost yanjiahe and shuijingtuo formations that crop out in the luojiacun and qiaojiaping sections eastern three gorges area south china the uppermost member of the yanjiahe formation contains abundant benthic algae based on the fossil record four distinct intervals were recognized in the shuijingtuo formation i few sessile suspension feeders such as sponges were recovered from interval i ii interval ii is dominated by bivalved arthropods living in the water column in this interval are also found a few sponges and eodiscoid trilobites iii no fossils other than few sponge spicules were recovered from interval iii iv diverse benthic communities composed of sponges cnidarians brachiopods as well as bradoriids dominate interval iv comprehensive research on framboidal pyrite morphology whole rock geochemistry as well as in situ la icp ms analyses were carried out on samples from the luojiacun section in order to assess paleo redox conditions the results show that the overall evolution of the paleo redox conditions was characterized by a double episode of sharp anoxic events ae observed at the base of intervals i and iii followed by a gradual oxygenation event oe evidence of oxygenation fluctuations were detected especially within interval iv a case study is presented herein which brings together early cambrian changes in metazoan diversity and changes in environmental conditions it is suggested that periodical oxygenation of the water column increased the biological diversity in the restricted basins of the eastern three gorges thus four intervals of habitat succession can be tentatively reconstructed in the studied area for the cambrian age 2 age 3 interval indicating a transition from uninhabited sediments to well populated habitats finally both the temporal and spatial characteristics of the cambrian aes oes and the fossil record in south china are compared and links between them are discussed in the context of their specific paleo geographical setting

structure and tectonic geomorphology of the qujiang fault at the intersection of the ailao shan red river fault and the xianshuihe xiaojiang fault system china abstract the northwest striking qujiang fault is located at the southeastern tip of the sichuan yunnan block which is bounded by the ailao shan red river fault and the xianshuihe xiaojiang fault system the qujiang fault is one of the most seismically active faults in western yunnan china and is considered to be the seismogenic fault of the 1970 ms 7 7 tonghai earthquake in this study we examine the geometry kinematics and geomorphology of this fault through field observations satellite images and dems it is a holocene active fault characterized by dextral strike slip movements with dip slip components it can be divided into northwest and southeast segments depending on different kinematics the northwest segment from baiyizhai to wujie shows right lateral strike slip with normal components whereas it is characterized by dextral movements with the northeast wall thrusting over the opposite in the southeast segment from wujie to miaobeishan the geophysical data combined with our investigations indicate that the fault dips steeply to the ne at depth while secondary faults with different dip directions developed at shallow surface owing to fault bifurcation during the tonghai earthquake a blind fault may have developed on the southwest wall resulting in aftershocks concentrated to the southwest of the surface raptures multiple tectonic landforms are developed along the fault the right lateral holocene slip rate was determined to be 2 84 3 27 mm year based on radiometrically dated offset gullies our analyses show that the present deformation at the intersection of the ailao shan red river fault and the xianshuihe xiaojiang fault system is partitioned into a shortening distributed over a region 150 km wide and a right lateral strike slip localized on the qujiang and jianshui faults since the late pleistocene the qujiang fault has been playing an important role in accommodating the ongoing se directed movements of the sichuan yunnan block creating strong earthquakes to release strain energy

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

paleoclimatic significance of eolian carbonates supplied to the japan sea during the last glacial maximum laminated sediments deposited under anoxic bottom waters in the japan sea during the last glacial maximum lgm contain extremely well preserved calcareous microfossils and eolian carbonates the radiocarbon age difference between bulk sediment and monospecific planktonic foraminifera in discrete laminae from a core in the southern japan sea implies that 40 of the total carbonates in the sediments at the lgm are of eolian origin extrapolation of this result yields a rate of supply of eolian carbonates of 2800 tons d 1 to the entire japan sea during the lgm the climatic significance of this flux potentially lies in its broader geographic extension particularly in the interaction of the carbonate bearing dust with shallow corrosive north pacific waters and with rain in the atmosphere by increasing the alkalinity of such waters and by enhancing the biological pump the dust flux could have increased co2 absorption by both the ocean and rain during the lgm

a direct link between the china loess and marine δ 18 o records aeolian flux to the north pacific most studies of quaternary climates conducted during the past 150 years have focused on glacial sequences and for the past 25 years marine sediments have provided significant further understanding of glacial and interglacial palaeoclimates an important goal of palaeoclimatology however to link the well known detailed continental records to marine records of similar resolution has yet to be realized the classic loess sequences of china provide an apparently continuous record of continental climate over the past 2 4 million years1 2 the oxygen isotope 18o timescale developed from the marine record3 gives us a global chrono stratigraphy of late pleistocene climates previously it has been suggested in studies of the aeolian component in pelagic sediments that it may be possible to link these two records directly by finding a suitable core from the north west pacific downwind from china4 5 here we report the results of such an effort for core v21 146 fig 1 and tie the last 500 000 years of the chinese loess soil sequence directly to the 18o timescale and in so doing refine the dating of regional loess and soil horizons

polystage deformation of the gaoligong metamorphic zone structures 40ar 39ar mica ages and tectonic implications abstract the gaoligong metamorphic zone is located southeast of the eastern himalayan syntaxis in western yunnan china the zone is characterized by four stages of deformation d1 d4 d1 structures record early compressive deformation during the indosinian orogeny which formed tight to isoclinal f1 folds of bedding with a penetrative s1 foliation developed parallel to fold axial planes mid crustal horizontal shearing during d2 resulted in overprinting of d1 structures d1 and d2 structures are associated with granulite facies metamorphism d3 doming resulted in late crustal thickening and the development of a regional nw se trending f3 antiform synchronous with or slightly subsequent to d3 deformation the zone experienced d4 ductile strike slip shearing resulting in its exhumation to shallow crustal levels and retrograde metamorphism granitic d4 mylonites predominantly yield 40 ar 39 ar mica ages of 15 16 ma indicating that d4 dextral strike slip shearing occurred in the miocene weakly deformed leucogranite and protomylonite yield 40 ar 39 ar ages of 10 11 ma suggesting that ductile strike slip shearing continued to the late miocene the new 40 ar 39 ar data indicate that escape related deformation along the gaoligong strike slip shear zone occurred in the miocene in association with recent geophysical studies and on the basis of the structural crystal preferred orientation cpo and geochronological data presented in this paper we suggest that the gaoligong metamorphic zone formed in response to intracontinental transpression in the southeast of tibet characterized as intense deformation and metamorphism at middle upper crustal levels

recent human impacts on the morphological evolution of the yangtze river delta foreland a review and new perspectives abstract this paper reviews the morphological change in the yangtze river delta due to increasing human impacts from three major aspects the first is the reduction of sediment supply to the ocean due to dam construction soil conservation and sand mining within the yangtze river basin the reduced sediment supply has decreased the progradation rate of the delta and triggered erosion in the front of the delta the second impact relates to the reclamation of intertidal wetlands by human activities since the 1950s approximately 1100 km 2 of intertidal land has been embanked resulting in the disappearance of salt marshes and even the entire intertidal zone along some sections of the coastline the third change in the delta due to human interference is the construction of deep waterway structures at the mouth bar which has greatly modified the local hydrodynamics and morphology sediment accretion has increased significantly in these areas as a result of sheltering by these deep waterway structures this review shows that human activities have severely altered the natural balance among the hydrodynamics and sediment supply affecting the morphological features of the yangtze river watershed and delta human impacts on the morphological evolution of deltaic coasts in general are becoming an increasingly concern and more attention should be paid to the management and mitigation of these effects

fluctuation of shelf basin redox conditions in the early ediacaran evidence from lantian formation black shales in south china abstract finely laminated pyrite and organic rich black shales that bear a euxinic interpretation dominate the ediacaran lantian formation in southern anhui south china however these black shales preserve benthic sessile algae and possible metazoans in situ suggesting that the lantian shelf basin may have had at least episodically oxidized bottom water during the early ediacaran period redox conditions of the fossiliferous lantian black shales were examined by petrological analysis pyrite framboid and 34s measurements and redox sensitive trace element rste analysis lamina by lamina pyrite framboid measurements show that lantian black shales were deposited under fluctuating redox conditions the 34s values of pyrites range from 20 5 to 2 3 for different laminae and display less negative values than the earliest ediacaran black shales supporting a return to a relatively low seawater sulfate concentration after the early ediacaran oxidation event similarly the lantian black shale rste concentrations e g mo u and v are low compared to previously published results from the early ediacaran and constant from lamina to lamina suggesting a drop in ocean wide rste concentrations occurring independently of frequent local redox condition changes these frequent redox condition changes in the lantian shelf basin may have facilitated the productivity and preservation of macroscopic lantian biota

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

the cause of the glacial to interglacial atmospheric co2 change a polar alkalinity hypothesis in an attempt to create a scenario for the cause of the glacial to interglacial co2 change recorded in air trapped in polar ice we call on an increase in the alkalinity of polar surface waters in this way we circumvent a major deficiency of the polar nutrient scenarios of sarmiento and toggweiler 1984 siegenthaler and wenk 1984 and knox and mcelroy 1984 namely our scenario does not require a drop in the nutrient content of polar surface waters in conformity with the demonstration by boyle 1988a b that the cadmium content of planktonic foraminifera from polar regions did not decrease from late glacial to holocene time the rise in alkalinity required by our model is a natural consequence of the demise during glacial time of north atlantic deep water as a major force in ocean circulation and of the nutrient maximum deepening of boyle 1988b rather than being original our hypothesis builds on the concept basic to the polar nutrient hypotheses namely that the co2 partial pressure in polar waters controls that for both the atmosphere and warm surface ocean it also requires the alkalinity increase in surface waters produced by boyle s nutrient deepening

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

remotely induced warming of antarctic bottom water in the eastern weddell gyre 1 four repeat hydrographic sections across the eastern weddell gyre at 30 e reveal a warming by 0 1 c and lightening by 0 02 0 03 kg m 3 of the antarctic bottom water aabw entering the gyre from the indian sector of the southern ocean between the mid 1990s and late 2000s historical hydrographic and altimetric measurements in the region suggest that the most likely explanation for the change is increased entrainment of warmer mid depth circumpolar deep water by cascading shelf water plumes close to cape darnley where the indian sourced aabw entering the weddell gyre from the east is ventilated this change in entrainment is associated with a concurrent southward shift of the antarctic circumpolar current s acc southern boundary in the region this mechanism of aabw warming may affect wherever the acc flows close to antarctica

a horodyskia nenoxites dominated fossil assemblage from the ediacaran cambrian transition liuchapo formation hunan province its paleontological implications and stratigraphic potential abstract because of the scarcity of fossil data life evolution in deep water settings through the ediacaran cambrian transitional interval is still poorly understood this study describes a horodyskia nenoxites dominated fossil assemblage from the chert of the liuchapo formation in liujiata village western hunan province which was deposited in a slope to basinal setting at the terminal ediacaran and earliest cambrian in this assemblage horodyskia covers the longest range possibly from the upper part of the lower ediacaran to the base of the cambrian other members including three types of nenoxites helanoichnus shaanxilithes and two affinity undetermined forms appear in the upper part of the liuchapo formation the whole assemblage exhibits an upward increasing complexity and fossil abundance which reaches its acme at a few meters below the cambrian xiaoyanxi formation regardless of the lithology of the host rock these fossils are exclusively preserved with clay minerals and devoid of any detectable concentration of carbon such a preservation mode is dominant among the known records of these fossils worldwide and possibly related to an unknown biological commonality of these organisms microscope observations support a body fossil origin of nenoxites and contradict the trace fossil interpretation meanwhile some large horodyskia show features implying a close relation to testate amoebae however despite some resemblances the differences between these fossils and any suggested relatives are considerable taking all the information together we tentatively interpret this fossil assemblage as a diverse and complex deep water community of protist grade organisms the worldwide occurrence of some members in this assemblage endows them with a biostratigraphic potential nevertheless to establish such a tool more quality data of fossil occurrences and referential radiometric ages and a better understanding of the interplay between these organisms and sedimentary environments are required

transient shallow ocean oxidation associated with the late ediacaran nama skeletal fauna evidence from iodine contents of the lower nama group southern namibia abstract the terminal ediacaran interval from 550 ma to 541 ma witnessed the first appearance of skeletal metazoans and complex ecosystems in earth history this biotic innovation event is thought to be related to increase of oxygen in earth s surface environments but many studies suggested that pervasive oceanic anoxia continued through the late ediacaran and early cambrian to further evaluate the redox conditions of the terminal ediacaran interval and their potential relationship with biotic changes we analyzed the iodine contents in the lower nama group 550 547 ma from the driedoornvlagte and zebra river sections southern namibia the i ca mg values fluctuate from 0 08 to 6 20 mol mol with an average of 0 94 mol mol in the driedoornvlagte section and 0 70 mol mol in the zebra river section high i ca mg values 2 6 mol mol occur immediately above the boundary between the lower and upper omkyk members with a peak up to 6 20 mol mol indicative of well oxygenated surface waters comparable with those of the modern marine environments abundant skeletal fossils including cloudina namacalathus namapoikia and trace fossils are observed only in the intervals after the high i ca mg peak the results are consistent with the interpretation that the nama skeletal communities grew in oxic o2 20 70 m and dysoxic o2 10 m waters above the chemocline the i ca mg ratios show significant tempo spatial variations which is also consistent with previous studies that suggested redox stratified terminal ediacaran sedimentary basins and highly heterogeneous oceanic redox conditions

early cambrian ocean mixing recorded by phosphorite successions in the nanhua basin south china abstract phosphorite successions in the nearshore meishucun section and the offshore gezhongwu section of the nanhua basin south china were investigated to decipher the circulation pattern of the early cambrian ocean the 539 535 ma phosphorites from the meishucun section contain authigenic phosphorus rich mineral aggregates collophanite grains with much lower bio limiting elements zn cd and ni than those from the gezhongwu section in contrast collophanite grains in the 535 526 ma phosphorites from both sections have similar concentrations of these elements this change indicates that the ocean likely transformed from a chemically stratified state to a homogeneous state at 535 ma phosphorites from both sections have constant 87sr 86sr ratios and end t values supporting the interpretation of a relatively constant terrestrial input into the nanhua basin between 539 ma and 526 ma instead negative 13c excursions and similar 208pb 204pb ratios of the 535 526 ma phosphorites are indicative of an extensive upwelling of the deep oceanic water in combination with previous studies of deep ocean oxygenation our work provides evidence for the vertical mixing of the ocean between 535 ma and 526 ma which introduced o2 rich water at the ocean bottom to promote early cambrian biodiversity

nitrate dependent iron oxidation limits iron transport in anoxic ocean regions abstract iron is an essential element for life on earth and limits primary production in large parts of the ocean oxygen free continental margin sediments represent an important source of bioavailable iron to the ocean yet little of the iron released from the seabed reaches the productive sea surface even in the anoxic water of oxygen minimum zones where iron solubility should be enhanced most of the iron is rapidly re precipitated to constrain the mechanism s of iron removal in anoxic ocean regions we explored the sediment and water in the oxygen minimum zone off peru during our sampling campaign the water column featured two distinct redox boundaries separating oxic from nitrate reducing i e nitrogenous water and nitrogenous from weakly sulfidic water the sulfidic water mass in contact with the shelf sediment contained elevated iron concentrations 300 nm at the boundary between sulfidic and nitrogenous conditions iron concentrations dropped sharply to

hydrogen sulphide release to surface waters at the precambrian cambrian boundary changes in environmental conditions at the precambrian cambrian transition around 542 million years ago have been suggested as a possible explanation for the apparent rapid increase in abundance of multicellular organisms known as the cambrian explosion the nature of the environmental changes is still a matter of debate however wille et al now report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of about 542 million years with the help of a box model of the oceanic molybdenum cycle they find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal this suggests that the early cambrian animal radiation may have been triggered by a major change in ocean circulation following a long period during which the ocean was stratified with sulphidic deep water this paper reports molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 million years the findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water animal like multicellular fossils appeared towards the end of the precambrian followed by a rapid increase in the abundance and diversity of fossils during the early cambrian period an event also known as the cambrian explosion 1 2 3 changes in the environmental conditions at the precambrian cambrian transition about 542 myr ago have been suggested as a possible explanation for this event but are still a matter of debate1 2 3 here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 myr we find a transient molybdenum isotope signal immediately after the precambrian cambrian transition using a box model of the oceanic molybdenum cycle we find that intense upwelling of hydrogen sulphide rich deep ocean water best explains the observed early cambrian molybdenum isotope signal our findings suggest that the early cambrian animal radiation may have been triggered by a major change in ocean circulation terminating a long period during which the proterozoic ocean was stratified with sulphidic deep water

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

marine redox stratification on the earliest cambrian ca 542 529 ma yangtze platform abstract in order to explore the marine redox structure on the earliest cambrian ca 542 529 ma yangtze platform south china new fe mo c s geochemical data from the slope xugongping section for the liuchapo and the base of niutitang formation are reported here integrated fe speciation and mo abundance data from the xugongping section in combination with previously published data from slope basinal longbizui section and others suggest oxic surface waters and ferruginous deep waters coexisting with occasional mid depth euxinic waters during the fortunian stage and with widespread euxinic waters during the early stage 2 marine transgression a compilation of new and published sulfur isotope data of pyrite for the liuchapo and the base of niutitang formations presents high isotopic values mostly 10 and distinctly different spatiotemporal gradient characteristics during the fortunian stage and the beginning of the stage 2 we propose that redox stratification developed in the earliest cambrian protected basin and eustatic change and the connectivity of the protected basin with the open ocean controlled the materials supply from riverine input and open ocean reservoir and thus spatiotemporal variability of ocean chemistry

uranium and molybdenum isotope evidence for an episode of widespread ocean oxygenation during the late ediacaran period abstract to improve estimates of the extent of ocean oxygenation during the late ediacaran period we measured the u and mo isotope compositions of euxinic anoxic and sulfidic organic rich mudrocks orm of member iv upper doushantuo formation south china the average 238 u of most samples is 0 24 0 16 2sd relative to standard crm145 which is slightly higher than the average 238 u of 0 02 0 12 for restricted black sea deep water unit i euxinic sediments and is similar to a modeled 238 u value of 0 2 for open ocean euxinic sediments in the modern well oxygenated oceans because 238 u is preferentially removed to euxinic sediments compared to 235 u expanded ocean anoxia will deplete seawater of 238 u relative to 235 u ultimately leading to deposition of orm with low 238 u hence the high 238 u of member iv orm points to a common occurrence of extensive ocean oxygenation ca 560 to 551 myr ago the mo isotope composition of sediments deposited from strongly euxinic bottom waters h 2 s aq 11 m either directly records the global seawater mo isotope composition if mo removal from deep waters is quantitative or represents a minimum value for seawater if mo removal is not quantitative near the top of member iv 98 mo approaches the modern seawater value of 2 34 0 10 high 98 mo points to widespread ocean oxygenation because the preferential removal of isotopically light mo to sediments occurs to a greater extent in o 2 rich compared to o 2 deficient marine environments however the 98 mo value for most member iv orm is near 0 relative to standard nist srm 3134 0 25 suggesting extensive anoxia the low 98 mo is at odds with the high mo concentrations of member iv orm which suggest a large seawater mo inventory in well oxygenated oceans and the high 238 u hence we propose that the low 98 mo of most member iv orm was fractionated from contemporaneous seawater possible mechanisms driving this isotope fractionation include 1 inadequate dissolved sulfide for quantitative thiomolybdate formation and capture of a seawater like 98 mo signature in sediments or 2 delivery of isotopically light mo to sediments via a particulate fe mn oxyhydroxide shuttle a compilation of mo isotope data from euxinic orm suggests that there were transient episodes of extensive ocean oxygenation that break up intervals of less oxygenated oceans during late neoproterozoic and early paleozoic time hence member iv does not capture irreversible deep ocean oxygenation instead complex ocean redox variations likely marked the transition from o 2 deficient proterozoic oceans to widely oxygenated later phanerozoic oceans

marine carbon and sulfur cycling across the ediacaran cambrian boundary in tarim block and its implications for paleoenvironmental changes abstract the ediacaran cambrian transition was a critical period of earth history because of the coincidence among climate change bioevolution and tectonic activity the nature of carbon and sulfur cycling processes spanning the ediacaran cambrian boundary remains debatable this study considers high resolution stable isotope chemostratigraphy of the sugetbrak and yutixi sections of the keping area of the tarim basin northwestern china the sections display a pronounced negative 13ccarb shift the basal cambrian carbon isotope excursion that spans the ediacaran cambrian boundary deposits overlying the negative excursion are characterized by heavier 13ccarb values ranging from 7 8 to 4 1 the biostratigraphically calibrated 13ccarb profiles likely reflect widespread perturbation of the global oceanic carbon cycle and associated mass extinction and subsequent biological evolution the 34spy profiles of the studied sections display a wide distribution of values ranging from 16 4 to 23 8 the variable 34spy signatures of these sections are partially controlled by microbial sulfate reduction rates which are sensitive to local sulfate concentration availability of organic matter and the marine redox state the strong coupling between enhanced organic carbon and pyrite burial the appearance of cambrian type animals and a shift to a more oxidizing marine environment might suggest a temporal causality of higher oxygen levels of the atmosphere ocean realm with bio events during the ediacaran cambrian transition the development of ecosystems across the ediacaran cambrian transition may have significantly impacted the oceanic carbon and sulfur biogeochemical processes in earth s history and in turn affecting themselves

early mesozoic deep crust reworking beneath the central lhasa terrane south tibet evidence from intermediate gneiss xenoliths in granites abstract understanding the rheological behavior of the tibetan plateau and its response to geodynamic processes requires a clear knowledge of the composition evolution and lithological properties of the deep crust here we present u pb hf isotopes of zircons bulk rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host early mesozoic granites 205 6 ma in the central lhasa terrane to probe the deep crust beneath southern tibet the xenoliths contain plagioclase amphibole titanite allanite quartz biotite and muscovite with accessory fe ti oxides apatite and zircon bulk rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite facies metamorphism equilibration at pressures of 0 46 0 83 gpa and temperatures of 650 c before they were captured by the host granite at 205 ma zircons in these xenoliths show complex microstructures including inherited cores magmatic or metamorphic bands and high u th hydrothermal rims inherited zircon cores record u pb ages from 2277 ma to 517 ma igneous zircons show a range of concordant u pb ages suggesting a protracted magmatism from 236 ma to 203 ma metamorphic zircon zones record the timing of amphibolite facies metamorphism from 224 to 192 ma while the high u th hydrothermal rims show a subsequent fluid activity until 150 ma unradiogenic hf isotopic compositions of both xenoliths and host granites xenolith e hf t 11 2 to 0 host granite e hf t 17 3 to 3 3 indicate that the early mesozoic deep crust in the central lhasa terrane originated mainly from ancient i e proterozoic crust with little or no interaction with juvenile magmas this study suggests a possible continental differentiation mechanism during crustal reworking progressive melting may initiate from the lower mafic crust at ca 236 ma and gradually migrate into the sediment rich upper crust until ca 203 ma the reworking results in the transition from small fluxes of intermediate magmas to voluminous peraluminous s type granite in a convergent depth

is there a component of pleistocene co2 change associated with carbonate dissolution cycles the only processes which could have decreased atmospheric co2 during glacial climates without appreciably changing the carbon isotope distribution in the sea appear to be 1 dissolved calcium carbonate input to the ocean e g coral reef buildup and erosion 2 the ocean solubility pump due to changes in surface temperature or air sea exchange or 3 decreased biological production of calcium carbonate it is assumed here that one of these mechanisms caused part of the atmospheric co2 changes recorded in the 200 kyr long vostok ice core two residual co2 records are generated by scaling the 13c difference between planktonic and benthic foraminifera in marine sediment cores to 82 ppm co2 per 1 increase in 13c and subtracting from this the measured co2 concentration in the ice core both residual co2 records exhibit two broad maximums between about 20 50 ka and 140 200 ka indicating that during these times about 40 ppm of the co2 decrease from interglacial levels cannot be explained by the interaction of the ocean s biological and vertical mixing cycles the shape of the residual co2 curve is similar qualitatively to the variation of calcium carbonate in central equatorial pacific sediments during this time period which would imply that changes in dissolved carbonate input to the ocean contributed the added component of co2 change however recent models of atmospheric co2 change in response to changing alkaline input to the ocean exhibit about a 25 to 35 ppm decrease per 1013 mol yr 1 increase in dissolved caco3 input if compensation for the changing input is occurring mostly within an area of about 40 106 km below the lysocline in the indo pacific the change in carbonate accumulation rate corresponding to a 40 ppm co2 change would be a minimum of 3 mg cm 2 yr 1 this can be compared to glacial increases of 0 5 to 1 0 mg cm 2 yr 1 during the last 200 kyr in central equatorial pacific sediments thus the added glacial accumulation of carbonate does not seem to match quantitatively with the 40 ppm amplitude of the residual co2 signal leading one to suspect that solubility plays a greater role than expected on the basis of a 2 c cooler surface ocean

authigenic molybdenum isotope signatures in marine sediments we present new mo isotope data from the mexican continen tal margin that in conjunction with previous data allow us to propose a mechanistic description of the mo isotope system in ma rine sediments we hypothesize that there are unique environmen tally dependent mo isotope signatures recorded in marine sedi ments that reflect the mechanisms responsible for authigenic mo accumulation open ocean anoxic sites defined as having dissolved oxygen and sulfide concentrations near zero in the overlying water exhibit a 98 95 mo isotope signature of 1 6 we believe this val ue reflects mo sulfide formation via diagenetic processes within sediments quantitative formation of mo sulfide within the sulfidic water column of euxinic environments results in sediment isotope values similar to the modern seawater value 2 3 as typified by samples from the highly sulfidic black sea in contrast to these reducing settings manganese oxide rich sediments have measured mo isotope values that are more negative relative to seawater than any other sediment samples analyzed to date similar to fe mn crusts 0 7 most measured mo isotope compositions of marine sediments from open ocean settings appear to reflect a mix ture of lithogenic mo 0 0 and the mo signature of a specific authigenic mo accumulation mechanism we therefore suggest that mo isotopes may record unique signatures that reflect the domi nant chemical mechanism for mo sequestration into sediments

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

sulfidity controls molybdenum isotope fractionation into euxinic sediments evidence from the modern black sea molybdenum mo isotope fractionation has recently been introduced as a new proxy in oceanography and biogeochemistry it is therefore fundamental to understand the processes controlling mo partitioning into modern marine environments this study identifies the availability of dissolved sulfide as the dominant control on overall mo removal from the water column in euxinic systems mo isotopic composition of surface sediments from different localities of the black sea demonstrates complete fixation of mo only below 400 m water depth above a critical concentration of 11 mol l 1 aqueous hydrogen sulfide in the bottom water the mo isotopic composition of these sediments reflects the homogeneous seawater isotopic composition of 2 3 in contrast significant mo isotope fractionation into less euxinic sediments is evident at shallower depths in the black sea as well as in temporarily euxinic deeps of the baltic sea consistent with the observed lower maximum sulfide concentrations in the respective water columns therefore mo isotope signatures in the modern black sea constrain the processes responsible for global mo removal from the ocean by euxinic sediments furthermore models of past ocean anoxia reconstruction have to consider that the seawater mo isotopic composition is not per se archived in euxinic sediments

iron limitation in the western interior seaway during the late cretaceous oae 3 and its role in phosphorus recycling and enhanced organic matter preservation the sedimentary record of the coniacian santonian oceanic anoxic event 3 oae 3 in the north american western interior seaway is characterized by a prolonged period of enhanced organic carbon oc burial this study investigates the role of fe in enhancing organic matter preservation and maintaining elevated primary productivity to sustain black shale deposition within the coniacian santonian aged niobrara formation in the usgs 1 portland core iron speciation results indicate the development of a reactive fe limitation coeval with reduced bioturbation and increased organic matter preservation suggesting that decreased sulfide buffering by reactive fe may have promoted enhanced organic matter preservation at the onset of oae 3 an fe limitation would also provide a feedback mechanism to sustain elevated primary productivity through enhanced phosphorus recycling additionally our results demonstrate inconsistencies between fe based and trace metal redox reconstructions iron indices from the portland core indicate a single stepwise change whereas the trace metal redox proxies indicate fluctuating redox conditions during and after oae 3 using fe speciation to reconstruct past redox conditions may be complicated by a number of factors including fe sequestration in diagenetic carbonate phases and efficient sedimentary pyrite formation in a system with limited fe supply and high levels of export production

tracking the evolution of seawater mo isotopes through the ediacaran cambrian transition abstract the rapid increase in animal diversity and abundance over the ediacaran cambrian boundary has been attributed to among other parameters pronounced earth surface oxygenation at that time however whether atmospheric oxygen elevated to modern levels or whether the extent was smaller and oxygen remained relatively muted until the late paleozoic is currently debated here we present new mo isotope datasets of drill core material for two key black shale units the ediacaran doushantuo formation and the lower cambrian jiumenchong formation located on the yangtze block these data capture a 0 7 isotope fractionation between sediments experiencing quantitative and non quantitative mo removal by identifying this offset we provide integrated estimates for global seawater mo isotope compositions during the deposition of these two formations 1 2 1 4 and 1 8 2 respectively when paired with a mass balance model our results indicate that a large proportion of the early cambrian ocean was bathed under oxic waters we also discuss how recent discoveries have provided insights into the geochemical processes underpinning the oxygenation of the oceans

spatial and temporal evolution of ediacaran carbon and sulfur cycles in the lower yangtze block south china abstract major radiations of microscopic and macroscopic eukaryotes occurred respectively in the early and middle ediacaran period various hypotheses have been proposed to attribute these evolutionary events to changes in ocean redox conditions to date published models of the ediacaran ocean in south china have largely been focused on the upper and middle yangtze blocks as opposed to the slope and basin sections of the lower yangtze block where the oldest complex macrofossils of the ediacaran i e the 600 ma lantian biota are preserved to achieve a holistic understanding of the record of ocean chemistry in the entire yangtze block we carried out carbon 13ccarb and 13corg and sulfur 34scas and 34spyr isotopic investigations of four new sections i e wujialing meishuxia wangfu and zhushuwu in the lower yangtze block in addition to one previously reported drill core at lantian the doushantuo and lantian formations at these sections represent shelf margin upper slope lower slope and deep basin environments in conjunction with previous work on sections of the upper and middle yangtze blocks our study reconstructs the redox structure of the entire ediacaran ocean in south china the new results show profound 13ccarb negative excursions in the slope and deep basinal sections which can be correlated to the en3 and shuram excursions at a global scale notably the 13ccarb and 34spyr profiles from shelf to basinal sections show distinct spatial and temporal patterns which can be explained by model of a redox stratified ocean during the early and middle ediacaran period the finding of isotopically light 34s depleted pyrite from this time suggests that the oceanic chemocline extended to intermediate depth integrating this study with existing ecological and taphonomic data it is likely that marine redox stratification in the early ediacaran limited the spatial distribution of early metazoans in the lower yangtze block

sr isotopic and elemental characteristics of calcites in the chinese deserts implications for eolian sr transport and seawater sr evolution abstract calcite content sr concentrations and isotopes of calcites in the chinese deserts are systematically studied in this paper calcite contents which are calculated according to acid soluble ca contents in the deserts are generally higher in the sandy deserts than in the sandy lands and decrease roughly from northwest to northeast of china acid soluble sr is well correlated with calcite in the chinese deserts implying acid soluble sr comes mainly from the calcite dissolution sr concentrations in calcites calculated on the basis of calcite contents and acid soluble sr concentrations in the deserts have an inverse relation to calcite contents essentially mirroring the degree of sr substitution for ca in the calcite development desert calcites have regional variations in sr isotopic ratios calcite sr isotopic ratios depend on geological settings and chemical weathering the badain jaran and tengger deserts are probably affected by additional factors such as the remote groundwater cycle or overturning of underlying sand deposits only four deserts taklimakan qaidam badain jaran and tengger appear to be potential sources of eolian deposits in the chinese loess plateau clp isotopic signatures of calcite sr and silicate nd further indicate that the tengger desert was not an important source for eolian deposits in the clp eolian calcite was probably enriched due to wind sorting from the potential sources to the clp and suffered weathering leaching after it accumulated in the clp sr isotopic compositions and ca sr molar ratios of calcites are different between the deserts and the lingtai profile due to the integrated effect of wind sorting and weathering leaching it is essential to calculate accurately the 87sr 86sr ratio and sr concentration of eolian calcite entering the oceans according to geochemical data of the chinese deserts because of the importance of the chinese deserts in the global dust cycle the calculated sr concentration and 87sr 86sr ratio of eolian calcite entering the north pacific ocean are 11 75 mol g and 0 71032 respectively the calculated values in this study are close to the recommended values by jacobson jacobson a d 2004 has the atmospheric supply of dissolved calcite dust to seawater influenced the evolution of marine 87sr 86sr ratios over the past 2 5 million years geochem geophys geosyst 5 12 1 9 q12002 doi 10 1029 2004gc000750 using the same model as that of jacobson 2004 the effect of asia dust on the evolution of seawater sr isotopes is evaluated 87sr 86sr seawater increases by 0 3 10 5 if the lower dust flux of 2 34 108 mol sr yr is used in the model suggesting the little effect of asian dust on the seawater sr record in the quaternary the increase in 87sr 86sr seawater is 1 5 10 5 if the higher value of 1 17 109 mol sr yr is used as observed in the quaternary sr record these results further support the suggestions of jacobson 2004

paleo environmental variation and its control on organic matter enrichment of black shales from shallow shelf to slope regions on the upper yangtze platform during cambrian stage 3 abstract to understand the evolution of oceanic redox conditions and organic matter om accumulation in the shelf and slope regions of the upper yangtze platform during cambrian stage 3 high resolution geochemical data were obtained from two wells respectively representing shelf and slope environments based on redox proxies widespread anoxic euxinic waters with intermittent sulfidic water columns were formed from shallow shelf to deep water slope and basinal regions during stage 2 and early stage 3 then widespread anoxic waters were reduced and oxic waters expanded during the middle stage 3 after which fluctuating suboxic and oxic occasionally anoxic environments became predominant during late stage 3 the presence of hyalophane and positive eu anomalies suggest strong ba rich hydrothermal activities in the shelf and deep basin regions during early and middle stage 3 the ba rich hydrothermal fluids and upwelling events enhanced the primary productivity in surface waters and subsequently promoted organic matter enrichment during stage 2 and early stage 3 the high primary productivity driven by coastal upwelling and hydrothermal activity were the main factors controlling om enrichment during middle stage 3 the local anoxic water condition became responsible for om enrichment when oxic waters began to gradually expand during late stage 3 both primary productivity and redox conditions became the main control factors for om enrichment according to the analysis of primary productivity redox conditions hydrothermal activity and upwelling events an evolution model is proposed to account for the paleo environmental variation and the om enrichment from shallow shelf to slope regions during cambrian stage 3

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

highly heterogeneous poikiloredox conditions in the early ediacaran yangtze sea abstract although there has been a sustained effort to reconstruct ocean redox conditions throughout the ediacaran a lack of high resolution data across time equivalent strata shelf to basin has hindered our ability to understand redox dynamics at the basinal scale we measured iron species trace and major element concentrations toc and corg p ratios of black shale from six high resolution sections from the early ediacaran yangtze sea i e the inner shelf jiulongwan outer shelf zhongling slope siduping and mingle and basinal yuanjia and long e sections which were integrated with published data from these sections and seven other correlative sections from the same region for a basinwide perspective on redox tracers these results document strong spatiotemporal redox variability during the early ediacaran including transitions between ferruginous euxinic suboxic and oxic conditions at high frequencies and short distances a pattern for which we coin the term poikiloredox conditions our results also suggest that multiple geochemical proxies including iron species redox sensitive trace element concentrations and enrichments and corg p ratios show different responses to poikiloredox conditions accounting for conflicting redox interpretations in early ediacaran studies this study thus demonstrates the importance of multiple proxies high stratigraphic resolution and a basin scale perspective in paleomarine redox studies underlining the potential for overgeneralization of redox patterns based on analysis of a single or a limited number of proxies or sections

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day

the molybdenum isotopic compositions of i s and a type granitic suites this study reports mo isotopic compositions for fifty two palaeozoic granitic rocks with contrasting source affinities a i and s type from the lachlan fold belt lfb and the new england batholith neb both in se australia and three compositionally zoned plutons loch doon criffell and fleet located in the southern uplands of scotland the results show relatively large variations in 98mo for igneous rocks ranging from 1 73 to 0 59 with significant overlaps between different types no relationships between 98mo and 18o or asi alumina saturation index are observed indicating that mo isotopes do not clearly distinguish igneous vs sedimentary source types instead effects of igneous processes source mixing regional geology as well as hydrothermal activity control the mo isotope compositions in these granites it is found that mo is mainly accommodated in biotite and to a lesser extent in hornblende hornblende and fe3 rich minerals may preferentially incorporate light isotopes as reflected by negative correlations between 98mo and k rb and fe2o3 there is a positive correlation between initial 87sr 86sr and 98mo in i type granitic rocks reflecting the admixing of material from isotopically distinct sources granitic rocks from scotland and australia display strikingly similar curvilinear trends in 98mo vs initial 87sr 86sr despite the differing regional geology localized hydrothermal effects can result in low 98mo in granite as seen in three samples from loch doon and criffell which have anomalously light 98mo of 1 based on this study an estimate of 98mo 0 14 0 07 95 s e for the phanerozoic upper crust is proposed this is slightly heavier than basalts indicating an isotopically light lower crust and or a systematic change to the crust resulting from subduction of isotopically light dehydrated slab and or pelagic sediment over time

transient deep water oxygenation in the early cambrian nanhua basin south china abstract many late neoproterozoic to early cambrian fossils of multicellular eukaryotes including those of benthic animals are found preserved under anoxic and even euxinic bottom water conditions which is contradictory to the consensus that oxygen is essential to eukaryotes to investigate this conundrum we conducted an integrated study of iron speciation redox sensitive trace elements and mo isotopes 98 mo on the black shale interval of the lower cambrian hetang formation 535 521 ma at lantian south china in which benthic sponge fossils are abundant in the lower member lm but absent in the upper member um iron speciation data point to uniformly anoxic ferruginous conditions in the lm and euxinic conditions in the um whereas the trace element and 98 mo data show greater secular variation in redox conditions the lm shows higher mean trace element concentrations mo 108 ppm u 36 ppm v 791 ppm and lower and more variable 98 mo 0 13 to 1 76 relative to the um mo 45 ppm u 18 ppm v 265 ppm 98 mo 1 59 to 1 67 and ratios of redox sensitive trace element concentrations to total organic carbon are significantly more variable and higher on average in the lm relative to the um the appearance of sponge fossils and lower 98 mo values correlate strongly with gray i e lighter colored layers in the lm these patterns can best be interpreted as recording mainly euxinic conditions throughout deposition of the study units with more intense background euxinia in the lm relative to the um but also with frequent transient oxygenation events in the lm that do not appear in the um the transient oxygenation events of the lm led to the initial colonization of the deep nanhua basin by sponges and the termination of these events in the um caused sponge faunas to disappear until a general rise in o 2 levels later in the cambrian permitted their return to deeper water habitats our study also illustrates that multiple geochemical and paleobiological proxies exhibit different responses in poikiloredox environments i e characterized by small scale spatial and high frequency temporal variations which can lead to apparent contradictions between metazoan fossil occurrences and their inferred watermass redox conditions

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

surface water oxygenation and bioproductivity a link provided by combined chromium and cadmium isotopes in early cambrian metalliferous black shales nanhua basin south china abstract the intrinsic link between atmospheric oxygenation surface seawater redox and bioproductivity in the upper water column of oceans offers to combine cadmium with chromium isotopes in marine archives to deduce information on past controls of biological metal and nutrient cycling we here use a novel approach in combining chromium and cadmium isotope signatures in metalliferous and organic matter om rich shales from the early cambrian niutitang formation south china to reconstruct the surface water redox and cycling of bioessential metals in a period directly following the cambrian animal evolution the inventory of authigenic cd in the black shales is controlled by their sulfide abundance whereas authigenic cr is dominantly controlled by iron oxy hydroxides and om cadmium isotopes measured on bulk sediments and 3 n hno3 leachates exhibit e114cd values from 1 5 to 4 5 with mo ni rich sulfidic shales revealing elevated values 2 5 chromium isotopes define 53cr values from 0 4 to 1 7 with v rich non sulfidic shales yielding more positively fractionated signatures than mo ni rich sulfidic shales the negative 53cr e114cd correlation trend recorded in the black shales suggests a high e114cd sulfide endmember of authigenic cd preserved in anoxic bottom conditions the other endmember is constituted by om with the isotopically heavy cr and light cd signals reflecting efficient phytoplankton uptake in the photic zone the endmembers constrain the cd and cr isotope signatures of surface waters in the early cambrian nanhua basin to e114cd of 7 to 17 and to 53cr of 0 7 to 1 9 these values are compatible with those of modern ocean surface waters and attest to a cambrian surface ocean water sustaining elevated primary bioproductivity in the aftermath of late ediacaran snowball earth glaciations our study emphasizes the potential of the cr cd double tracer in studies aimed at the reconstruction of paleoproductivity and bio geochemical metal cycling in marine paleo basins

asian provenance of glacial dust stage 2 in the greenland ice sheet project 2 ice core summit greenland samples of dust from the greenland ice sheet project 2 gisp2 ice core summit greenland dated within marine isotope stage 2 between 23 340 and 26 180 calendar years b p around the time of the coldest local last glacial temperatures have been analyzed to determine their provenance to accomplish this we have compared them with approximately coeval aeolian sediments mostly loesses sampled in possible source areas psas from around the northern hemisphere the 5 m grain size fraction of these samples was analyzed on the basis that it corresponds to the atmospheric dust component of that time and locale which was sufficiently fine grained to be transported over long distances on the basis of comparison of the clay mineralogy and sr nd and pb isotope composition with ice dust and psas and assuming that we have sampled the most important psas we have determined that the probable source area of these gisp2 dusts was in eastern asia the dust was not derived from either the midcontinental united states or the sahara two more proximal areas that have been suggested as potential sources based on atmospheric circulation modeling except for a brief period during an interstadial when dust transport was exceptionally low for glacial times and had a mineralogical composition indicative of a slightly more southern provenance the source area of the dust did not change significantly during times of variably higher fluxes of dust with larger mean grain size or lower fluxes of dust with smaller mean grain size this includes the high dust period that correlates with the heinrich 2 period of major iceberg discharge into the north atlantic variable wind strengths must therefore be invoked to account for these abrupt and significant changes in dust flux and grain size

an overview of the crustal structure of the tibetan plateau after 35 years of deep seismic soundings abstract since the pioneer wide angle seismic profile along the yadong gulu rift acquired in 1974 by the ex institute of geophysics chinese academy of sciences cas several research programs aimed to deep geophysics performed thanks to the participation of chinese national and international institutions have been developed during last 35 years including 23 wide angle seismic profiles with total length of about 6000 km these profiles are unevenly distributed most of them in eastern tibet and few profiles in western tibet in this paper we make a summarized presentation of all these wide angle seismic profiles and provide an overall view of the seismic velocity structure of the crust beneath the broad tibetan plateau which is the product of the continuous convergence and collision of the indian and eurasian plates since about 50 ma ago different patterns of crustal thickness variation related to the tectonic blocks and along suture zones of the region are displayed the crust thickness is confirmed to be about 70 75 km under southern tibet and 60 65 km under northern northeastern and southeastern tibet the leading edge of the subducted lithosphere reaches the northern margin of the plateau and directly contacts with tarim basin westward of the 90 e boundary the indian crust is moving towards the northern edge of the plateau and collides with tarim basin at 80 e while reach the bangong nujiang suture belt at 88 e eastward of the 90 e boundary the northern edge of the crust should be at 50 100 km south of bangong nujiang suture the results supply helpful constrains to understand the mechanism of the continent continent collision and its consequences in the plateau and neighbouring areas

when do black shales tell molybdenum isotope tales molybdenum mo isotopes in ancient sediments are promising recorders of global ocean paleoredox conditions organic rich black shales can be used to reconstruct ancient ocean mo isotope compositions if these sediments record the isotopic composition of contemporaneous seawater comparison of 98 95 mo in two devonian shale sequences of similar age the new york oatka creek and geneseo formations reveals that this assumption cannot be applied to all organic rich shales although both sequences contain laminated intervals elevated organic carbon and enrichments of redox sensitive metals the mean 98 95 mo differs systematically between the formations by 0 59 independent paleoredox indicators reveal that portions of the oatka creek formation were deposited under pervasively euxinic anoxic and sulfidic conditions whereas conditions during deposition of the geneseo formation were intermittently euxinic to suboxic oxygen deficient but not sulfidic in the water column we infer that reconstruction of ancient ocean 98 95 mo from organic rich shales requires independent verification of persistent local euxinia with these considerations in mind our data point to 98 95 mo in the devonian oceans 0 6 lighter than in today9s oceans consistent with expanded anoxia

controls on organic matter accumulation on the early cambrian western yangtze platform south china abstract the origin of widespread organic rich shales on the early cambrian yangtze platform of nanhua basin south china has been attributed to high primary productivity to test this hypothesis we generated multiple paleoproductivity ocar and par paleoredox corg p ratios upwelling restriction coef mnef co ppm mn mo toc and cd mo ratios and terrigenous flux proxies al and unreactive fe feu for the lower cambrian niutitang formation in the outer shelf jinsha section which records a typical transition from the organic rich black shales to overlying organic lean mudstones or siltstones these new data were integrated with published paleoredox proxies iron speciation and redox sensitive trace metals for the same section our results suggest that the organic rich toc 4 6 1 8 lower member lm 0 37 m of the niutitang formation records ferruginous conditions high productivity levels and low siliciclastic fluxes and that the organic poor toc 0 1 0 1 upper member um 37 329 m was characterized by dominantly oxic conditions declining productivity levels and high siliciclastic fluxes our results suggest a redox shift from ferruginous to oxic conditions and enhanced terrigenous fluxes in response to falling sea level from the lm to the um secular variations in primary productivity and redox conditions are strongly correlated which is consistent with a positive feedback high primary productivity led to increasing water column anoxia which enhanced organic p recycling stimulating further productivity variations in primary productivity may reflect variable upwelling intensity in a semi restricted shelf setting linked to sea level changes taken together these findings suggest that a combination of primary productivity redox conditions and terrigenous fluxes driven by sea level changes played a key role in organic matter accumulation on the early cambrian western yangtze platform

deep water dissolved iron cycling and reservoir size across the ediacaran cambrian transition abstract the majority of the deep ocean was likely under ferruginous conditions during the first four billion years of earth s history as the atmosphere was gradually oxygenated the sources sinks redox cycling and reservoir size of dissolved iron in the deep ocean are likely to have changed dramatically whether deep water was thoroughly oxygenated by the time of the ediacaran cambrian transition and the relationship of this oxygenation to the cambrian explosion remains debated to explore the degree of oceanic oxygenation and its effect on cambrian explosion we measured the iron isotopic composition 56fe of bulk rock i e cherts and mudstones shales through the piyuancun and hetang formations using samples collected from the chunye 1 core on the lower yangtze block in western zhejiang the limited variation in 56fe values 0 55 during the end ediacaran and cambrian stages 1 3 to

photic zone euxinia during the permian triassic superanoxic event carbon and sulfur isotopic data together with biomarker and iron speciation analyses of the hovea 3 core that was drilled in the perth basin western australia indicate that euxinic conditions prevailed in the paleowater column during the permian triassic superanoxic event biomarkers diagnostic for anoxygenic photosynthesis by chlorobiaceae are particularly abundant at the boundary and into the early triassic similar conditions prevailed in the contemporaneous seas off south china our evidence for widespread photiczone euxinic conditions suggests that sulfide toxicity was a driver of the extinction and a factor in the protracted recovery

spatiotemporal evolution and causes of marine euxinia in the early cambrian nanhua basin south china abstract marine redox conditions and their dynamic variations in the early cambrian are thought to have strongly influenced the evolution of early animals however the patterns and mechanisms of contemporaneous marine redox variation remain poorly understood the nanhua basin south china was a failed rift basin located between the yangtze block nw margin and cathaysia block se margin in the early cambrian in this study we conducted a comprehensive study of marine redox conditions sulfur cycling and chemical weathering intensity of source areas for the lower cambrian shales of the qingxi formation in the basinal shimensuidao section which was geographically close to the cathaysia block we integrated these results with previously published data from the nearby silikou section as well as the coeval inner shelf xiaotan upper slope daotuo lower slope longbizui and basinal zhalagou sections where were closer to the yangtze block at shimensuidao iron species and redox sensitive trace element data suggest dominantly euxinic conditions for the organic rich black shale interval lower member lm and variable redox conditions oxic ferruginous euxinic for the overlying organic lean shales upper member um of the qingxi formation integrated redox results show a similar lessening of euxinic conditions from lm to um across the entire basin and that euxinia developed along both the yangtze and cathaysia basin margins but it was mainly in upper slope areas e g daotuo on the yangtze margin and reached basinal areas e g silikou and shimensuidao on the cathaysia margin a decline of 34spy from lm to um was observed at shimensuidao and other sections which can be explained by decreasing microbial sulfate reduction rates due to decreasing organic availability and increasing riverine sulfate flux at study sections lower 34spy values in shallow marine relative to deep marine areas suggest increased terrestrial sulfate availability to the shallower sites chemical index of alteration cia values at shimensuidao are uniformly high for both the lm ciacorr1 82 5 0 4 ciacorr2 88 6 0 5 and um ciacorr1 80 2 2 3 ciacorr2 86 2 2 5 compared with the low to moderate values at daotuo ciacorr2 61 9 3 9 for lm 71 3 4 8 for um this pattern is consistent with intense chemical weathering of source areas on the cathaysia block and weaker chemical weathering of source areas on the yangtze block based on these observations we propose that strong chemical weathering of the cathaysia block led to regionally high terrigenous sulfate fluxes resulting in expansion of mid depth euxinia into deep basinal areas on the cathaysia margin of the nanhua basin in contrast weak chemical weathering of the yangtze block yielded lower terrigenous sulfate fluxes limiting euxinia to upper slope areas on the yangtze margin of the basin weaker euxinia during um deposition may have been due to its decreasing organic availability and or small seawater sulfate reservoir caused by persistent euxinic consumption during lm deposition our results highlight the complex controls and the key role of continental weathering and resulting terrigenous sulfate fluxes in marine euxinic variations in early cambrian marginal seas providing new insights into the co evolution of continents oceans and early animals

implications from the kinematic pattern of the longmenshan region abstract the boundaries of a plate are usually correlated with the locations of major already known faults which are well defined by local geology regional seismicity it facilitates for the case of the tibetan plateau with boundaries along the large scale faults such as the altyn tagh fault nevertheless for smaller scale regions it might become much more difficult to determine the block boundaries especially when lacking of neither active faults nor historical earthquake records the geodetic including gps observations may become useful tools to determine the potential faults for the purpose of obtaining the kinematic pattern across the longmenshan fault we performed k means clustering analysis for 145 gps sites in the longmenshan region longitude 99 110 e and latitude 28 35 n and obtained a modified ten block system almost best fits the gps velocity fields rigid rotations of these blocks are well determined and the velocity filed can be explained well by rigid block motion and fault slip across block boundaries the mean residual velocity is 1 26 mm year less than the 2 3 mm year uncertainty of the gps data with 90 confidence all the block boundaries except for these of ah lms and gz kd are along major faults the ah lms boundary is regarded to be along longriba fault and translates 5 5 mm year right lateral rates and reduces the southeastward velocity of the plateau as shortening rate of

repeated enrichment of trace metals and organic carbon on an eocene high energy shelf caused by anoxia and reworking petroleum source rocks are strongly enriched in organic carbon oc and their trace metal tm contents often reach low grade ore levels the mechanisms leading to these coenrichments are important for understanding how extreme environmental conditions support the formation of natural resources we therefore studied organic rich eocene marls and limestones oil shale from the central jordan amzaq hazra subbasin part of a cretaceous paleogene shelf system along the southern neo tethys margin geochemical analyses on two cores show highly dynamic depositional conditions consistent with sedimentological and micropaleontological observations maximum and average contents respectively in oc 26 and 10 wt sulfur 7 and 2 4 wt phosphorus 10 and 2 wt molybdenum 400 and 130 ppm chromium 500 and 350 ppm vanadium 1600 and 550 ppm and zinc 3800 and 900 ppm are exceptional in particular without any indication of hydrothermal or epigenetic processes we propose a combination of two processes physical reworking of oc and metal rich 30 material from locally exposed cretaceous paleogene sediments as supported by reworked nannofossils and high marine productivity fueled by chemical remobilization of nutrients and metals on land that sustained anoxic sulfidic conditions burial of high quality organic matter hydrogen index 600 700 mghc goc was related to strongly reducing conditions punctuated by only short lived oxygenation events and to excess h2s promoting organic matter sulfurization these processes likely caused the oc and tm coenrichments in a high energy shallow marine setting that contradicts common models for black shale formation but may explain similar geochemical patterns in other black shales