controls of eustasy and diagenesis on the 238u 235u of carbonates and evolution of the seawater 234u 238u during the last 1 4 myr using a leaching protocol designed for the study of u isotopes in recent carbonates we measured the u isotope composition both 238 u 235 u and 234 u 238 u of modern and ancient corals n 6 a limestone and a dolostone as well as 43 shallow water carbonate sediments from the odp leg 166 site 1009 drill core on the slope of the bahamas platform although bulk corals record the seawater 238 u value within 0 02 differences of up to 0 30 in the 238 u of individual leachates suggest a control of the coral structure and a more positive 238 u 235 u ratio in the centers of calcification the drill core 238 u data shows that the 238 u 235 u ratio of shallow water carbonates is controlled mainly by 1 variations in sea level through the mixing of different amounts of platform derived sediments with 238 u 0 50 0 60 heavier than seawater and pelagic sediments with seawater like 238 u values 2 authigenic u enrichment via pore watercirculation and u reduction both on the platform and down to 5 m below the surface mbsf after deposition of the sediment and to a lesser extent by 3 early diagenetic processes i e carbonate dissolution and or recrystallization during sediment burial the global effect of these processes leaves the 238 u values of shallow water carbonates offset relative to that of seawater by carbonates sw 0 24 0 06 95 ci including all samples this shift can be used in seawater paleoredox reconstructions based on carbonates deposited on shallow water platform shelf and slope environments i e most of the carbonate sedimentary record prior to the mesozoic to account for the average effect of carbonate diagenesis assuming that the 238 u 235 u ratio of carbonate platform sediments directly records the seawater 238 u 235 u ratio would underestimate the extent of ocean seafloor anoxia by at least a factor 10 the rapid fluctuations in 238 u values due to sea level changes i is a factor that should be considered before interpreting 238 u variations as reflecting changes in oceanic paleoredox conditions and ii reinforces the need for statistically meaningful data sets the 234 u data suggest that the 234 u 238 u ratio of the seawater has remained within 20 of the modern seawater value during the last 1 1 4 myr furthermore we find that small scale 1 15 variations in seawater 234 u mirror sea level changes during the penultimate glacial interglacial period 140 to 200 ka thus confirming the record of lower 234 u sw during periods of low sea level stand and expanding it to at least the last two glacial interglacial events i e 0 23 ma such fluctuations in 234 u initial values should be taken into account when screening carbonate sediments u th ages on the basis of the initial 234 u 238 u ratios of the samples

uranium stable isotope fractionation in the black sea modern calibration of the 238 u 235 u paleo redox proxy abstract the isotopic compositions of redox sensitive metals including uranium u in marine sediments have recently emerged as powerful diagnostic tracers of the redox state of the ancient ocean atmosphere system reliable interpretation of sedimentary isotopic information requires a thorough understanding of the environmental controls on isotopic fractionation in modern anoxic environments before being applied to the paleo record in this study the relationship between ocean anoxia and the isotopic fractionation of u was investigated in the water column and sediments of the black sea the world s largest anoxic basin paired measurements of 238 u 235 u and u concentration supported by other redox parameters were obtained for water column and sediment samples collected during the 2013 ga04n geotraces expedition to the black sea removal of u from the water column occurs during the redox transition of soluble u vi to relatively insoluble u iv resulting in up to 43 of u being removed from solution in euxinic bottom waters uranium reduction and removal is accompanied by a progressive shift in 238 u 235 u towards isotopically light values in the water column as heavier 238 u is preferentially exported to sediments over lighter 235 u this gives rise to apparent isotope enrichment factors of e 0 63 0 09 and e 0 84 0 11 when u removal is modelled by rayleigh and closed system equilibrium isotope fractionation respectively these e values fall within the range determined for bacterial u reduction experiments and together with a striking correlation between the distributions of u and h 2 s implicate microbially mediated u vi u iv reduction as the primary mechanism controlling u isotopic shifts in the black sea the 238 u 235 u of underlying sediments is related to the the 238 u 235 u of black sea bottom waters through the isotope enrichment factor of the u reduction reaction but the relationship between sedimentary and water column 238 u 235 u is complicated by mass transport processes and the modelled e values may underestimate the true isotope enrichment factors these new results for the black sea provide important constraints on the use of 238 u 235 u as a proxy of the redox state of ancient oceans

geochronological evidence supporting antarctic deglaciation three million years ago the response of the antarctic ice sheets to increased global temperatures is an important unresolved issue in the assessment of future climate change in particular considerable controversy exists as to whether the east antarctic ice sheet suffered extensive deglaciation during the mid pliocene epoch 3 myr ago when temperatures were only slightly warmer than today although the ice sheet is widely assumed to have existed in something like its present form for the past 14 myr ref 1 marine diatoms eroded from the antarctic interior have been found in glacial till deposits high in the transantarctic mountains2 3 and have been bio stratigraphically dated at 3 myr before present this age has been disputed4 because it implies marine deposition in the antarctic interior and hence substantial deglaciation at a time when other evidence has been marshalled for the persistence of cold polar conditions4 here we report k ar and 40ar 39 ar ages for a volcanic ash bed in diatom bearing glaciomarine strata cored in ferrar fiord east antarctica by the ciros 2 drill hole5 which confirm the age of the diatoms at 3 myr and hence also confirm the mid pliocene deglaciation

enigmatic origin of the largest known carbon isotope excursion in earth s history carbonate rocks from the middle ediacaran period in locations all over the globe record the largest excursion in carbon isotopic compositions in earth history this finding suggests a dramatic reorganization of earth s carbon cycle named the shuram excursion for its original discovery in the shuram formation oman the anomaly closely precedes impressive events in evolution including the rise of large metazoans and the origin of biomineralization in animals instead of a true record of the carbon cycle at the time of sedimentation the carbon isotope signature recorded in the shuram excursion could be caused by alteration following deposition of the carbonate sediments a scenario supported by several geochemical indicators however such secondary processes are intrinsically local which makes it difficult to explain the coincident occurrence of carbon isotope anomalies in numerous records around the globe both possibilities are intriguing if the shuram excursion preserves a genuine record of ancient seawater chemistry it reflects a perturbation to the carbon cycle that is stronger than any known perturbations of the modern earth if however it represents secondary alteration during burial of sediments then marine sediments must have been globally preconditioned in a unique way to allow ordinary and local processes to produce an extraordinary and widespread response

dating of sirius group tillites in the antarctic dry valleys with cosmogenic3he and21ne we measured in situ produced cosmogenic3he and21ne in boulder and bedrock surface samples in and near sirius group tillites from two localities in southern victoria land antarctica pyroxene quantitatively retains cosmogenic3he for millions of years the deposit at mt fleming has a minimum exposure age of 6 5 ma while 6 ma is a probable minimum age for the table mountain tillite these lower limits not taking into account erosion of the sampled surfaces are based on the samples with the highest concentrations of cosmogenic noble gases and currently accepted production rates since the plagioclase bearing mt fleming samples almost certainly lost part of their cosmogenic ne the minimum exposure age at this location is presumably even 50 higher than the stated value the concentrations of cosmogenic ne in our samples constrain uplift rates of the transantarctic mountains to 170 m ma these data contradict the hypothesis of a collapse of the east antarctic ice sheet in the pliocene coupled with high uplift rates and support the model of a stable ice sheet since the middle miocene

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

errata neogene fjordal sedimentation on the western margin of the lambert graben east antarctica the lambert graben is occupied by the world s largest fjord system through which flows the lambert glacier the amery ice shelf and their tributaries along the western margin of the graben in the northern prince charles mountains remnants of uplifted miocene and pliocene strata of the glacigenic fjordal pagodroma group total more than 800 m in thickness these sediments provide evidence for a dynamic east antarctic ice sheet during the neogene period each of the four pagodroma group formations defined from this region rests unconformably on either proterozoic or permo triassic rocks the unconformity surfaces represent parts of the walls and floors of neogene fjords for these surfaces to have been eroded the ice must have been grounded out as far as the continental shelf in prydz bay the pagodroma group was deposited by wet based glaciers discharging into a fjordal setting and includes lithofacies that are quite different from those produced by modern antarctic ice masses the principal lithofacies are massive diamicts and soulder gravels deposited both close to a calving grounded glacier terminus and from icebergs the few stratified diamicts are the product of more distal iceberg sedimentation an ice transported gravel lithofacies includes rockfall debris derived from palaeofjord walls and mixed with subglacially derived diamicts some lithofacies contain evidence of subaquatic slumping and gravity flowage volumetrically minor lithofacies include laminites with some exposures exhibiting large ice rafted clasts the laminites represent less proximal mainly ice free fjordal sediments resulting either from tidal current sorting of suspended sediment originating from subaquatic glaciofluvial discharge or from turbidity currents derived from unstable subaquatically deposited glacigenic sediment the pagodroma group provides a record of multiple glaciation by dynamic sliding glaciers carrying large amounts of both basal and supraglacial debris the closest modern analogues in terms of the thermal and dynamic characteristics of the neogene lambert glacier appear to be the fast flowing tidewater glaciers of east greenland these glaciers originate from the interior ice sheet and discharge large volumes of icebergs the resulting lithofacies are predominantly diamicts

global ocean redox variations across the smithian spathian boundary linked to concurrent climatic and biotic changes abstract the smithian spathian boundary ssb was an interval characterized by a major global carbon cycle perturbation climatic cooling from a middle late smithian boundary hyperthermal condition and a major setback in the recovery of marine necto pelagic faunas from the end permian mass extinction although the ssb has been linked to changes in oceanic redox conditions key aspects of this redox variation e g duration extent and triggering mechanisms and its relationship to coeval climatic and biotic changes remain unresolved here we report a high resolution middle smithian to middle spathian u isotope 238u record based on marine carbonates of the zuodeng south china and jesmond british columbia sections to investigate the timing and global extent of ocean redox variation across the ssb our 238u record reveals values similar to or slightly heavier than modern seawater 0 39 during the middle smithian a rapid negative shift to highly negative values during the early late smithian a positive shift at the ssb and a rapid shift back to more negative values in the early middle spathian a simple u isotope mass balance modeling suggests that the global area of anoxic seafloor expanded strongly during the late smithian and the early middle spathian covering 11 of total seafloor area but that it contracted sharply during the ssb 2 the redox pattern documented by our 238u record shows a good first order correspondence to tropical sea surface temperature sst data for the smithian spathian in particular peak anoxia coincided with the middle late smithian boundary hyperthermal event and diminished anoxia with a pronounced decline in ssts at the ssb the temporal correlation between anoxia and low biodiversity levels of many marine clades e g conodonts and ammonoids during the late smithian indicates that oceanic anoxia may have played a role in the ssb biocrisis

anaerobic oxidation of methane and sulfate reduction along the chilean continental margin anaerobic oxidation of methane aom and sulfate reduction sr were investigated in sediments of the chilean upwelling region at three stations between 800 and 3000 m water depth major goals of this study were to quantify and evaluate rates of aom and sr in a coastal marine upwelling system with high organic input to analyze the impact of aom on the methane budget and to determine the contribution of aom to sr within the sulfate methane transition zone smt furthermore we investigated the formation of authigenic carbonates correlated with aom we determined the vertical distribution of aom and sr activity methane sulfate sulfide ph total chlorins and a variety of other geochemical parameters depth integrated rates of aom within the smt were between 7 and 1124 mmol m 2 a 1 effectively removing methane below the sediment water interface single measurements revealed aom peaks of 2 to 51 nmol cm 3 d 1 with highest rates at the shallowest station 800 m the methane turnover was higher than in other diffusive systems of similar ocean depth this higher turnover was most likely due to elevated organic matter input in this upwelling region offering significant amounts of substrates for methanogenesis sr within the smt was mostly fuelled by methane aom led to the formation of isotopically light dic 13c 24 6 vpdb and of distinct layers of authigenic carbonates 13c 14 6 vpdb

pliocene pleistocene diatoms in paleozoic and mesozoic sedimentary and igneous rocks from antarctica a sirius problem solved there are two competing scenarios on the behavior of the east antarctic ice sheet during the late tertiary in one scenario the ice sheet was very dynamic and underwent major drawdown and renewal as late as the pliocene in the other the ice sheet was relatively stable during the late neogene the presence of marine diatoms in sirius group sedimentary rocks in east antarctica is at the center of the disagreement one side regards the diatoms as the major piece of evidence to support the drawdown and renewal hypothesis and infers that they were introduced into the sirius during renewed glaciation of east antarctica others suggest that these diatoms were likely introduced into the sirius by atmospheric largely eolian processes we propose a simple test of the eolian hypothesis if diatoms were introduced into the sirius by eolian processes then they should also be present in older paleozoic and mesozoic sedimentary and igneous rocks samples from two units of the beacon supergroup devonian to jurassic from beacon valley east antarctica were analyzed the beacon heights orthoquartzite devonian and the feather conglomerate permian triassic also examined was sediment found in cracks of paleozoic and mesozoic devonian to cretaceous igneous rocks from marie byrd land west antarctica largely pliocene pleistocene planktonic marine diatoms were found in all sample sets because neither beacon supergroup sedimentary rocks nor igneous rocks from marie byrd land are pliocene pleistocene in age such findings strongly suggest that diatoms were introduced into them by eolian processes this same scenario can be applied to sirius group sedimentary rocks

uranium isotope fractionation in saanich inlet a modern analog study of a paleoredox tracer abstract uranium isotope fractionation was studied in the sediment and water of saanich inlet a seasonally anoxic fjord on the east coast of vancouver island the concentration of dissolved u is the same above and below the redoxcline at 120 m depth with an average 238u value of 0 45 0 06 2 s e which is indistinguishable from seawater values reported in other studies this finding is consistent with water renewal times for the inlet that are roughly seasonal in frequency thus hiding the impact of the u losses to the anoxic sediment of the inlet complete digests of the anoxic sediment yielded higher 238u values than seawater 0 21 0 11 2 s e by contrast complete digests of the sediment trap samples yielded lower 238u values than seawater 0 52 0 10 2 s e and marine plankton from the inlet yielded the lowest 238u value of 1 24 0 17 2 cross plots of 238u and 234u vs th u reveal strong correlations indicative of two component mixing when the data from traps and anoxic inlet sediment are combined one mixing end member is fractionated uranium of seawater origin with a 238u value of 0 17 0 14 2 the other is detrital uranium with a 238u value of 0 83 0 12 2 the detrital end member is lower than the range of 238u values reported in the literature for granitoid igneous rocks 0 44 to 0 17 2 telus et al 2012 suggesting that continental weathering fractionates uranium isotopes with preferential release of 238u development and application of u isotopes as a paleoredox proxy has its basis in the nuclear volume fractionation the data from saanich inlet meets this expectation with a positive fractionation factor aq vi sed iv of 0 62 0 17 2 calculated as the difference in 238u between authigenic u in anoxic sediments and sediment traps 0 17 and u dissolved in seawater 0 45 however it is widely believed that u vi reduction in the marine environment occurs on the surfaces of particles and that the negative isotope effect associated with u vi sorption to plankton seawater plankton 0 79 0 17 2 opens up the possibility that the particulate pool of seawater derived u vi is fractionated from the dissolved pool accordingly the reduction of u vi in the marine environment might involve two steps 1 u vi sorption to particles with a negative fractionation and 2 reduction to u iv on particles with a positive fractionation environmental factors that are not yet well understood may influence the relative reaction rates for the two steps thus affecting the magnitude and sign of the overall fractionation this has the potential to explain conflicting results reported for u vi reduction experiments in the literature laboratory and field where both positive and negative fractionations have been observed and in some instances no fractionation at all variation in the fractionation factor has the potential to complicate application of u isotopes as a paleoredox proxy but this study and a study of the black sea romaniello 2012 give the same results within their respective uncertainties suggesting that 0 62 0 17 2 is a robust assessment of the apparent u isotope fractionation factor associated with reductive deposition of seawater u vi in both anoxic and euxinic marine sediments of the present day

modeling of middle pliocene climate with the ncar genesis general circulation model abstract a climate modeling study was carried out which used boundary conditions created by the usgs prism group the model used for this study is the national center for atmospheric research ncar genesis climate model genesis is an atmospheric general circulation model gcm that is coupled to a land surface model and contains submodels for snow and soil boundary conditions incorporated into the model which were specific to the middle pliocene 3 ma are 1 present day continent configuration modified by a 35 m rise in sea level 2 modified present day elevations 3 reduced continental ice cover on both greenland and antarctica 4 pliocene vegetation and 5 pliocene sea surface temperatures ssts and sea ice distribution atmospheric pco 2 and orbital parameters were set at current values the greatest influence of the specified warm ssts upon the resulting climate was occurrence of warmer and more humid high latitudes than observed in the current climate especially in the winter season the global mean surface temperature was 3 6 c warmer than in a present day control case and global mean precipitation increased by 5 relative to the control case with most excess precipitation occurring over the african continent and the oceans zonal winds weakened slightly in the pliocene case but hadley cell extent and jet stream locations were unchanged relative to the control case model results predict specific areas in which middle pliocene climate should have differed substantially from the present climate which can be tested with paleoclimate data

enhanced framboidal pyrite formation related to anaerobic oxidation of methane in the sulfate methane transition zone of the northern south china sea abstract many studies have confirmed that size distributions of framboidal pyrite can be an effective indicator of bottom water redox conditions in modern as well as ancient sedimentary environments however one environment in which production of framboidal pyrite has not been sufficiently studied to date is the sulfate methane transition zone smtz in which anaerobic oxidation of methane aom is coupled with microbial sulfate reduction msr to enhance iron sulfide mineral precipitation e g fes2 fes and fe3s4 here we document for the first time size distributions for pyrite framboids from the smtz based on data from two sites in the methane hydrate bearing region of the northern south china sea on the basis of framboid size pyrite concentration and sulfur isotope data we propose new insights into the formation process of authigenic pyrite framboids within the smtz we conclude that 1 enhanced anaerobic oxidation of methane aom not only plays a dominant role in the accumulation of 34s enriched pyrite but also is responsible for formation of highly variable and sometimes exceptionally large pyrite framboids in the smtz 2 most framboids occur in framboid clusters which commonly exhibit rod like shapes secondary overgrowths heavier 34s values and unusual size distributions e g mean size 20 m and standard deviation 3 0 m in the smtz 3 pyrite framboids formed in the smtz which have characteristics different from those formed in the sulfate reduction zone srz do not comment on redox conditions of the overlying water column and 4 framboid occurrences with similar characteristics in ancient marine deposits may be considered indicators of enhanced aom and mark the former position of the smtz in the paleo marine system in addition significant quantities of elemental sulfur were observed in the smtz possibly related to anaerobic oxidation of hydrogen sulfide and fluctuations of the smtz

carbon isotope excursions and the oxidant budget of the ediacaran atmosphere and ocean a possible global drop in marine carbon isotope values to as low as 12 peedee belemnite pdb recorded in the ediacaran shuram formation of oman has been attributed to the non steady state oxidation of oceanic dissolved organic carbon doc resulting from the rise in atmospheric oxygen to near modern values at the end of the precambrian geologic constraints indicate that the excursion lasted between 25 and 50 m y requiring a doc pool thousands of times to 10 000 times the modern inventory to conform with carbon isotope mass balance calculations for a 12 excursion at the consequent rates of doc oxidation oceanic sulfate and oxygen in the atmosphere and oceans are exhausted on a time scale of 800 k y oxidant depletion is incompatible with independent geochemical and biological indicators that show oceanic sulfate and oxygen levels were maintained or increased during the shuram excursion furthermore a doc driven excursion does not explain strong covariation between the carbon and oxygen isotope record these indicators show that negative isotope excursions recorded in the shuram and other ediacaran sections are unlikely to represent a global ocean signal

minimum 10be exposure ages of early pliocene for the table mountain plateau and the sirius group at mount fleming dry valleys antarctica using accelerator mass spectrometry ams we have measured 10be and 26al in quartz from granites and sandstones from table mountain and mount fleming antarctica our data show that the plateau surface at table mountain had formed by early pliocene time at the latest granites fringing but within the sirius group at table mountain give a minimum exposure age of 2 6 ma for this deposit a sandstone clast on the ferrar dolerite surface just outside and below and thus postdating the sirius group has a minimum age of 2 9 ma two samples from the sirius group at mount fleming have 10be concentrations that have reached secular equilibrium this deposit is at least 4 8 m y old the sirius group at mount fleming cannot have been deposited after 3 0 2 5 ma as implied by biostratigraphic data our dates contradict the hypothesis that in the pliocene east antarctica was deglaciated and the climate was significantly warmer and wetter the preservation of these surfaces indicates a continuous cold desert in the dry valleys since the beginning of the pliocene the high 10be concentrations we have measured cannot be reconciled with uplift of the transantarctic mountains at a rate of 1 km m y during the past 3 m y

paired carbonate and organic carbon isotope variations of the ediacaran doushantuo formation from an upper slope section at siduping south china abstract intensive carbon isotope analyses from the doushantuo formation ca 635 551 ma in south china reveal decoupled carbonate and organic carbon isotopes 13ccarb and 13corg that have been interpreted as recording a large dissolved organic carbon doc reservoir in the ediacaran ocean or chemoautotrophic methanotrophic recycling modification of organic matter in a redox stratified ocean existing paired 13ccarb 13corg data however are mostly from sections of an intrashelf lagoon near the yangtze gorges area where basin restriction may have resulted in local isotope departure from the open ocean to better understand the spatial and temporal variations of the ediacaran 13c excursions we have conducted high resolution paired 13ccarb 13corg analyses of the doushantuo formation from an upper slope section at siduping hunan province south china the data reveal three negative 13ccarb excursions at the basal middle and upper doushantuo formation two prominent negative 13corg anomalies are also observed in black shales and shaly dolostones at the base and top of the doushantuo formation for the majority of the doushantuo formation however 13ccarb and 13corg values are decoupled similar to those observed in the yangtze gorges area both 13ccarb and 13corg show large up to 10 variations among shallow and deep water sections that are consistent with a large carbon isotope gradient in a stratified ediacaran ocean the large 13ccarb gradient between shallow and deep water sections implies the availability of sulfate in deep water environments where the addition of authigenic carbonate films cements into surface ocean derived carbonate particles in anoxic water column or pore water led to more negative 13ccarb values significant spatial 13corg variations suggest that the decoupled 13ccarb 13corg may not have been resulted from direct buffering of a large doc reservoir but from recycling of organic matter by chemoautotrophs and methanotrophs compilation of available paired 13ccarb 13corg data from the yangtze platform reveals alternating intervals of coupled and decoupled 13ccarb 13corg values up to the early cambrian 520 ma intervals with coupled 13ccarb and 13corg coincide with evidence for ocean oxygenation events this phenomenon may be explained by episodic growth and oxidation of a moderate doc reservoir 6 12 times of the modern ocean the upper doushantuo 13ccarb anomaly temporally crosses a major stratigraphic unconformity indicating that part of the upper doushantuo shuram 13ccarb excursion was formed during sea level regression large spatial and temporal variations of the upper doushantuo 13ccarb excursion may record a mixed signature of open ocean seawater overprinted by facies dependent isotope variation stratigraphic truncation and authigenic diagenetic modification along the unconformity the real primary 13ccarb excursion may be shorter lived than previously thought

fossil weevils coleoptera curculionidae from latitude 85 s antarctica two species of fossil listroderine weevils coleoptera curculionidae rhytirhinini listroderina are reported from the meyer desert formation at a locality on the beardmore glacier in the transantarctic mountains about 500 km from the south pole associated fossils include wood leaves and pollen of nothofagus stems and leaves of several species of mosses achenes of ranunculus shells of freshwater molluscs and a fish tooth the age of the fossiliferous strata is contentious but probably within the range of pliocene to mid miocene the fossils represent organisms that colonised the margins of a glacier at the head of a fjord during an interglaciation the autecology of the listroderine species indicates that temperatures during summer months averaged 5 c the mean annual temperature with winter temperatures constrained by 6 months of darkness is estimated to have been about 38 c compared to the 326 c estimated for sea level at latitude 85 s today the closest evolutionary link of the fossil listroderines is with south american rather than australian or new zealand taxa divergence of the taxa at least at the level of tribe had most probably occurred on gondwana before the continent broke apart the fossil species are considered to be the descendants of antarctic lineages which evolved on the continent in the late cretaceous or palaeogene and survived until the neogene extinction of the listroderines and most other antarctic terrestrial biota occurred with the growth of the polar ice sheets and the change to the polar desert climate 9 2003 elsevier science b v all rights reserved

a burial diagenesis origin for the ediacaran shuram wonoka carbon isotope anomaly marked negative 13c excursions in ediacaran age carbonate sediments have been identified in several sections globally but are not recognized in all sections of similar age the presence of 13ccarb values as low as 12 has been interpreted as recording fundamentally different processes in the global carbon cycle than those recognized today the 13ccarb anomalies are strongly correlated with 18ocarb values but are not represented in 13corg records while no primary depositional processes have been identified that can produce the correlated 18o 13c arrays simulations show that fluid rock interaction with high pco2 fluids is capable of producing such arrays at geologically reasonable pco2 and water rock ratios variations in the mg ca ratio and sulfate concentration of the altering fluid determine the extent of dolomite vs calcite and anhydrite in the resulting mineral assemblage incorporation of an initially aragonitic mineralogy demonstrates that high sr low mn sr and modest alteration of 87sr 86sr in ancient carbonates are all compatible with a burial diagenesis mechanism for generation of the 13c anomalies and do not necessarily imply preservation of primary values the profound ediacaran negative 13c anomalies can be adequately explained by well understood diagenetic processes conflated with the difficulty of correlating precambrian sections independently of chemostratigraphy they are not a record of primary seawater variations and need not have independent stratigraphic significance

provenance of the sirius group and related upper cenozoic glacial deposits from the transantarctic mountains antarctica relation to landscape evolution and ice sheet drainage abstract this study uses bulk x ray diffraction xrd heavy mineral analysis and analysis of detrital modes of the sirius group and related upper cenozoic deposits to evaluate their provenance the data suggest that deposition of the sirius group occurred as a result of long term glacial denudation of the transantarctic mountains such a conclusion is in contrast with the traditional view of deposition on a pre existing high relief glacial topography during one relatively short term phase of glacial expansion by the east antarctic ice sheet two petrofacies are distinguished which have different geomorphological settings and paleo ice flow directions one petrofacies includes much material from the higher elevated sedimentary sequences of the transantarctic mountains and represents overriding of the mountains and deposition of part of the sirius group during or after the late oligocene to early miocene the second petrofacies is characterized by a large input from crystalline basement sources and represents active erosion and deposition in glacial troughs during the neogene the second petrofacies of the sirius group shows similarities to the record from dry valley drilling project core 11 dvdp 11 which indicates that east antarctic ice was last flowing through the dry valleys in the late miocene to early pliocene

the oldest ice on earth in beacon valley antarctica new evidence from surface exposure dating abstract beacon valley antarctica contains unique remnants of glacier ice underneath a till layer covering the valley floor to constrain the age and evolution of this important indicator of antarctic paleoclimate we analyzed two dolerite erratics from the till surface and one from within the ice for cosmogenic helium and neon a conservative minimum exposure age of the older surface sample is 2 3 ma but taking into account erosion the true exposure age of this boulder is likely to be considerably higher the buried sample contains more than 20 times less cosmogenic noble gases than the old surface sample although its current shielding would imply only a three times lower production rate this indicates that the ice level has slowly been lowered by sublimation at the rate of a few m ma the high exposure age of the surface sample as well as the very low sublimation rate of the relict ice both support the conclusion that the remnant ice in beacon valley was deposited many million years ago sugden et al nature 376 1995 412 414 and has never been thinner than at present in addition we found that cosmogenic helium and neon are released quantitatively from pyroxene at temperatures of 1000 c respectively

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

a marine and terrestrial sirius group succession middle beardmore glacier queen alexandra range transantarctic mountains antarctica a succession of sirius group glacigene sediments which crop out along the western margins of the beardmore valley between cherry icefall and hewson glacier below the cloudmaker is designated as the stratotype of the cloudmaker formation this new formation overlies a multiply glaciated pavement dominion erosion surface cut into the precambrian goldie formation and is disconformably overlain by the meyer desert formation sirius group the cloudmaker formation comprises bedded and massive diamictons bedded sands and silts and laminated clays assemblages of foraminifera occur throughout the cloudmaker formation and indicate that these basal sirius group sediments were deposited in brackish glacial marine environments the general absence of diatoms suggest these marine waters were ice covered similar marine assemblages are also present in basal sirius group sediments at oliver bluffs dominion range recycled marine diatom assemblages in the sirius group at the latter locality indicate that the host sediments have an age of 3 8 ma pliocene the cloudmaker formation is placed in the pliocene although a latest miocene age for the basal sediments cannot be ruled out stratigraphie sedimentologic and paleontologic evidence suggests that beardmore valley was occupied by a fjord and tidewater glacier system that extended at least 165 km through the transantarctic mountains from the southwestern ross sea the stratigraphy of the cloudmaker formation consists of a succession of members separated by disconformities it is hypothesised that these strata were deposited by a dynamic valley glacier system that underwent a history of glacier advance and grounding alternating with glacier retreat and flotation over a marine water column a combination of fjord basin sediment filling and sea level oscillations may also have influenced the pattern of glacier ice advance and retreat within beardmore paleofjord the marine cloudmaker formation is overlain by the terrestrial diamicton dominated meyer desert formation at oliver bluffs the meyer desert formation diamictons are interbedded with fluvial and lacustrine sediments successions that contain in situ vascular plant fossils principally the southern beech nothofagus mosses and beetle remains a magellanic type flora and fauna occupied the coastal margins of the beardmore paleofjord the vertical transition from the basal marine cloudmaker formation to terrestrial meyer desert formation provides a sea level datum that can be used to assess the extent of post sirius group tectonic uplift uplift rates at the cloudmaker section 90 km inland from the transantarctic mountain front or rift shoulder margin in the queen alexandra mountain block are determined to be 429 or 350 m myr this assumes a total uplift of 1331 m for the uppermost marine sediments of the cloudmaker formation and maximum diatom based ages for the sirius group of 3 1 ma or 3 8 ma gross similarities in stratigraphy and interpreted paleoenvironments are apparent between the cloudmaker succession beardmore paleofjord and the upper miocene pliocene successions at the mouth of taylor paleofjord 800 km to the north contrasting present day elevational settings for these two widely separated marine successions indicates the post sirius rate of tectonic uplift for the transantarctic mountains has been significantly greater in the queen alexandra block

facies phosphate and fossil preservation potential across a lower cambrian carbonate shelf arrowie basin south australia abstract the effects of sedimentological depositional and taphonomic processes on preservation potential of cambrian small shelly fossils ssf have important implications for their utility in biostratigraphy and high resolution correlation to investigate the effects of these processes on fossil occurrence detailed microfacies analysis biostratigraphic data and multivariate analyses are integrated from an exemplar stratigraphic section intersecting a suite of lower cambrian carbonate palaeoenvironments in the northern flinders ranges south australia the succession deepens upsection across a low gradient shallow marine shelf six depositional facies sequences are identified ranging from protected fs1 and open fs2 shelf lagoonal systems high energy inner ramp shoal complex fs3 mid shelf fs4 mid to outer shelf fs5 and outer shelf fs6 environments non metric multi dimensional scaling ordination and two way cluster analysis reveal an underlying bathymetric gradient as the main control on the distribution of ssfs unlike groups that produced primary organophosphatic biominerals taxa that built calcareous skeletons are more taphonomically controlled which is further exacerbated by sampling and processing biases a strong facies association with condensed and reworked horizons suggests the stratigraphic occurrence of calcareous groups reflects conditions conducive to preservation phosphogenesis and phosphatization rather than true stratigraphic ranges consequently organophosphatic taxa should take precedence in the erection of biostratigraphic zones for subdivision of lower cambrian successions

periodic mid cretaceous oceanic anoxic events linked by oscillations of the phosphorus and oxygen biogeochemical cycles 1 a series of oceanic anoxic events oaes occurred in the mid cretaceous warm period 120 80 ma that have been linked with high rates of organic carbon burial warm high and low latitude temperatures and sea level changes oaes have been studied individually but a causal mechanism that connects them has been lacking we show that peaks in phosphorus accumulation in marine sediments broadly coincide with oaes 1a 1b 1d 2 and 3 and exhibit a 5 6 myr periodicity which for reactive p is prominent over 100 80 ma oxic anoxic oscillations of this frequency are also found in a model of the coupled n p c and o2 biogeochemical cycles these oscillations are maintained by positive feedbacks between phosphate concentration biological productivity and anoxia in the global ocean and counteracting but slower negative feedbacks involving changes in atmospheric oxygen an increase in phosphorus weathering rate above a critical threshold can shift the system into self sustaining oscillation this could have been caused by tectonic and volcanic forcing increasing atmospheric co2 and global warmth 120 80 ma augmented by the rise of flowering plants around 100 ma with a plausible forcing scenario we are able to reproduce the approximate timing of oaes 1a 1b 1d 2 and 3 in the model

uranium isotope variations in a dolomitized jurassic carbonate platform tithonian franconian alb southern germany abstract the uranium u isotopic composition 238u of limestones is increasingly used to quantitatively track changes in paleoredox conditions of the global ocean however many limestones have undergone significant dolomitization during diagenesis to assess the potential impact of diagenetic changes on the u isotope composition of dolomitized rocks we examined the uranium isotopic composition of a jurassic carbonate platform of the franconian alb in southern germany this platform underwent dolomitization during shallow burial due to the supply of magnesium rich fluids in the form of slightly modified seawater this type of dolomitization is common in the geologic record the carbonate platform of the franconian alb can therefore serve as an example for many dolomitized carbonate platforms a positive correlation between the concentrations of redox sensitive elements e g re and v and 238u confirms that the authigenic uptake of u under reducing conditions preferentially incorporates u enriched in 238u this positive correlation between redox sensitive elements and 238u does not change within the dolomitized interval suggesting that the 238u is not altered during shallow burial dolomitization of limestones therefore our results indicate that dolostones can retain information about secular variations in seawater 238u the diagenetic uptake of u shifts 238u of bulk carbonate sediments to values heavier than contemporaneous seawater the magnitude of this offset correlates with the level of authigenic enrichment of redox sensitive elements in our sample set samples with high re and v concentrations and high re mo ratios are the most enriched in 238u this positive correlation could be used to constrain the amount of diagenetic offset of bulk 238u values from seawater this is particularly important in carbonate systems that had a primary calcitic mineralogy primary calcite precipitates have low u concentrations and so small amounts of 238u uptake during diagenesis can have a significant impact on the bulk composition the samples with the most depleted 238u values have high mo concentrations and low re mo ratios as carbonate sediments are generally mo poor the high mo concentrations suggests that the depleted 238u values might be linked to a manganese cycle that operated across the sediment water interface during the deposition of this jurassic carbonate platform

carbon isotope variability across the ediacaran yangtze platform in south china implications for a large surface to deep ocean δ13c gradient new isotope data obtained from relatively conformable carbonate rich strata of the ediacaran yangtze platform in south china reveal substantial 13 c variability in platform sections four negative 13 c anomalies with a nadir down to 8 pdb are present in the interval between the cap carbonate level 635 ma and the precambrian cambrian boundary 542 ma while in slope and basinal sections 13 c values are negative through the entire doushantuo formation 635 551 ma if these 13 c values are close to their primary seawater signature they imply a strong 10 surface to deep ocean 13 c gradient that is consistent with long term deep ocean anoxia and the presence of a large dissolved organic carbon doc reservoir the two prominent negative 13 c excursions within the doushantuo formation above the cap carbonate level are associated with shoaling and local exposure of the platform the anomalies may thus record remineralization of a large oceanic doc pool via sulfate reduction that transferred 13 c depleted carbon from the oceanic doc reservoir to the surface ocean during regression inconsistencies in ediacaran 13 c profiles globally and variations in south china in particular highlight the need for further evaluation of local departures in 13 c from an inferred average seawater signature

diatoms in south pole ice implications for eolian contamination of sirius group deposits marine and nonmarine diatoms occur throughout a south pole ice core spanning the past 2000 yr similar diatoms occur in east and west antarctica at siple and taylor domes because there are no local diatom sources at these locations diatoms must have been carried by winds from coastal or extra antarctic sites our data demonstrate widespread historical eolian transport of diatoms to the antarctic ice sheets supporting the view that sirius group sediments were contaminated by late neogene diatoms long after they were deposited if so there is no reason to postulate a pliocene deglaciation event in east antarctica

cosmogenic noble gas studies in the oldest landscape on earth surface exposure ages of the dry valleys antarctica extraordinarily high surface exposure ages have been determined for sirius group tillites of mt fleming and mt feather as well as at localities in the inner dry valleys using cosmogenic helium and neon ages of 10 ma at mt fleming 5 3 ma at mt feather and 6 5 ma at insel mountain are among the highest nominal exposure ages published so far these values are minimal ages as they are based on the assumption of zero erosion and uplift the mt feather sample independently confirms the pre pliocene age of the sirius group sediments in the dry valleys as previously determined at mt fleming the insel mountain samples provide evidence for a landscape formation of the inner dry valleys not later than late miocene time assuming conservatively low values of 2 5 cm ma 1 for erosion rate and 50 m ma 1 for uplift rate we infer that the sirius group tillites at mt fleming were deposited earlier than 20 ma ago this indicates that the overriding of the dry valleys block of the transantarctic mountains by the east antarctic ice sheet occurred not later than the early miocene maximum long term erosion rates in the inner dry valleys must be 15 cm ma 1 down to altitudes 1000 m since such low erosion rates require permanently cold and hyperarid conditions the response of antarctica to the pliocene warm climatic episode must have been small cosmogenic nuclide data from both the inner dry valleys and the sirius group sediment localities support the hypothesis of a stable east antarctic ice sheet since at least late miocene time implying that the climate of antarctica was decoupled from that of lower southern latitudes we present also new elemental 21ne production rates of p21 mg 196 atoms g 1 yr 1 and p21 al 55 atoms g 1 yr 1 at sea level and high geomagnetic latitude these figures are consistent with a 3he production rate of p3 110 atoms g 1 yr 1 similar to previously published values this consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 ma cosmogenic ne in quartz and pyroxene has a 22ne 21ne cos ratio of 1 266 0 040 and 1 159 0 040 respectively

fossil preservation through phosphatization and silicification in the ediacaran doushantuo formation south china a comparative synthesis phosphatized and silicified microfossils preserved through replication of organic templates by authigenic calcium phosphate and silica respectively form through mechanistically similar mineralization processes but under notably different conditions in order to characterize the environments and taphonomic processes associated with these preservational pathways we compare phosphatized and silicified fossil assemblages in the ediacaran doushantuo formation south china in terms of stratigraphic and paleogeographic distribution preservation and biodiversity several differences are recognized 1 phosphatization and silicification both occurred in sub tidal doushantuo sediments but phosphatized fossils generally occur in shallower sub tidal facies than silicified fossils 2 although both phosphatized and silicified fossils are preserved three dimensionally the latter are typically more compressed and 3 whereas phosphatized assemblages are numerically dominated by multicellular acritarchs putative metazoan eggs and embryos silicified assemblages are dominated by prokaryotes these differences can be explained by environmental and taphonomic distinctions phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates and were likely fossilized near the suboxic anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to p remineralization phosphate burial via iron pumping and calcium phosphate precipitation reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial in contrast silicified fossils typically occur in chert nodules which formed deeper in the sediment column in ferruginous pore waters and perhaps under ferruginous bottom waters where bacterial sulfate reduction and pyrite formation created low ph conditions conducive to calcite dissolution and silica precipitation once formed chert nodules mostly remained in situ but some were reworked with turbidites and olistostromes developed in slope facies despite these distinctions phosphatized and silicified fossil assemblages in the doushantuo formation generally overlap in time and space and representatives of all major groups and many common genera of doushantuo organisms are present in both phosphorites and cherts consequently although both preservational pathways have paleoenvironmental and taphonomic biases their relative biases on taxonomic presence absence data are minimal given sufficient sampling efforts and paleobiologic and biostratigraphic information based on presence absence but not abundance data from the doushantuo formation appears to be robust

uranium isotope fractionation during coprecipitation with aragonite and calcite abstract natural variations in 238u 235u of marine calcium carbonates might provide a useful way of constraining redox conditions of ancient environments in order to evaluate the reliability of this proxy we conducted aragonite and calcite coprecipitation experiments at ph 7 5 and 8 5 to study possible u isotope fractionation during incorporation into these minerals small but significant u isotope fractionation was observed in aragonite experiments at ph 8 5 with heavier u isotopes preferentially enriched in the solid phase 238u 235u of dissolved u in these experiments can be fit by rayleigh fractionation curves with fractionation factors of 1 00007 0 00002 0 00003 1 00005 0 00001 and 1 00003 0 00001 in contrast no resolvable u isotope fractionation was observed in an aragonite experiment at ph 7 5 or in calcite experiments at either ph equilibrium isotope fractionation among different aqueous u species is the most likely explanation for these findings certain charged u species are preferentially incorporated into calcium carbonate relative to the uncharged u species ca2uo2 co3 3 aq which we hypothesize has a lighter equilibrium u isotope composition than most of the charged species according to this hypothesis the magnitude of u isotope fractionation should scale with the fraction of dissolved u that is present as ca2uo2 co3 3 aq this expectation is confirmed by equilibrium speciation modeling of our experiments theoretical calculation of the u isotope fractionation factors between different u species could further test this hypothesis and our proposed fractionation mechanism these findings suggest that u isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater u particularly changes in seawater ph p co 2 ca2 or mg2 concentrations in general these effects are likely to be small

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

congruent permian triassic δ238u records at panthalassic and tethyan sites confirmation of global oceanic anoxia and validation of the u isotope paleoredox proxy

redox architecture of an ediacaran ocean margin integrated chemostratigraphic δ13c δ34s 87sr 86sr ce ce correlation of the doushantuo formation south china abstract early diagenetic silicification and phosphatization of the ediacaran doushantuo formation ca 635 to 551 ma in south china offer extraordinary taphonomic windows into the early evolution of multicellular eukaryotes including various algal groups and potentially animals in order to understand how the ecological and taphonomic distribution of these ediacaran eukaryotes was controlled by oceanic redox conditions it is critical to reconstruct the redox architecture of the sedimentary basin recently two alternative redox models have been proposed to account for the geochemical and sedimentary features of the doushantuo formation one argues that the unit was deposited on a continental margin where a metastable sulfidic wedge was dynamically maintained by a sulfate concentration gradient between shelf and basinal environments the other contends that the sulfidic water mass was largely restricted to the intra shelf basin behind a rimmed margin these two models make different predictions about the stratigraphic completeness and correlation of the doushantuo formation to test these predictions we generated high resolution time series trends of multiple isotopic and elemental tracers including 34s 87sr 86sr and ce ce to facilitate an integrated chemostratigraphic correlation between inner shelf xiaofenghe intra shelf jiulongwan and outer shelf yangjiaping and zhongling sections our correlations suggest that both the inner and outer shelf sections are stratigraphically incomplete relative to the intra shelf section the euxinic wedge model should be reconsidered insofar as it is based on a miscorrelation between sections viewed from our revised chemostratigraphic framework euxinic conditions on the platform appear to have been largely restricted to the intra shelf basin carbonates in the upper doushantuo formation at jiulongwan and their stratigraphic equivalents are characterized by a profound negative carbon isotope anomaly i e the shuram excursion coincident with a drop in pyrite sulfur isotope values and a significant rise in 87sr 86sr from 0 7080 to 0 7090 the integrated stratigraphic data from south china suggest that the onset of the shuram excursion is associated with enhanced oxidative continental weathering that delivered radiogenic strontium as well as sulfate to the ediacaran basin

multiple negative molybdenum isotope excursions in the doushantuo formation south china fingerprint complex redox related processes in the ediacaran nanhua basin abstract the ediacaran doushantuo formation offers one of the most complete and extensively studied records of end neoproterozoic biotic and environmental change here we report multiple coeval negative molybdenum mo isotope excursions to as low as 98monist 0 25 2 24 0 10 2sd in shales from four separate sites in south china rongxi taoying wuhe and yuanjia that preserve the doushantuo formation the negative 98mo excursions appear coincident with previously discovered and seemingly peculiar redox sensitive element rse patterns in the same sedimentary rocks we propose that these geochemical trends can be explained by some combination of a enhanced local marine oxygenation in the sedimentary basin where the doushantuo formation was originally deposited the nanhua basin and b changes in the degree of connectivity between this paleo basin and the open ocean enhanced local marine oxygenation by exposing more sediments in the nanhua basin to h2s poor conditions could have hindered quantitative tetrathiomolybdate formation within these sediments local marine oxygenation could have also stimulated the operation of a mn oxide shuttle today both of these processes are shown to promote the retention of lighter mass mo isotopes in sediments and also govern rse enrichment patterns alternatively or in addition the nanhua basin may not have maintained an uninterrupted connection with the open ocean during the entirety of the ediacaran period the negative 98mo excursions occur coincident with sea level highstands that could have also exposed more sediments in the basin to h2s poor conditions and or catalyzed the operation of a local mn oxide shuttle when trying to infer temporal changes in ancient global ocean redox it is important to consider the influence of sea level changes and associated variations in local depositional conditions on stratigraphic trends in rse enrichments and isotope compositions

rapid expansion of oceanic anoxia immediately before the end permian mass extinction tive of an expansion of oceanic anoxia and higher th u ratios from 0 06 to 0 42 indicative of drawdown of u concentrations in seawater using a mass balance model we estimate that this isotopic shift represents a sixfold increase in the flux of u to anoxic facies implying a corresponding increase in the extent of oceanic anoxia the intensification of oceanic anoxia coincided with or slightly preceded the eh and persisted for an interval of at least 40 000 to 50 000 y following the eh these findings challenge previous hypotheses of an extended period of whole ocean anoxia prior to the end permian extinction

was the ediacaran shuram excursion a globally synchronized early diagenetic event insights from methane derived authigenic carbonates in the uppermost doushantuo formation south china abstract the ediacaran period is characterized by the most profound negative carbon isotope 13c excursion in earth history the shuram excursion various hypotheses including the massive oxidation of dissolved organic carbon doc in the oceans the weathering of terrestrial organic carbon or the release and oxidation of methane hydrates and or expelled petroleum from the subsurface have been proposed as sources of the 13c depleted carbon more recently it has been suggested that global scale precipitation of early authigenic carbonates driven by anaerobic microbial metabolism in unconsolidated sediments may have caused the shuram excursion but empirical evidence is lacking here we present a comprehensive analysis of a shuram associated interval from the uppermost doushantuo formation in south china our study reveals petrographic evidence of methane derived authigenic calcite formed as early diagenetic cements and nodules that are remarkably depleted in 13c suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane aom and systematically depleted in 18o relative to co occurring dolomite early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels in light of the finding of methane derived authigenic carbonates at zhongling and based on our basin scale stratigraphic correlation we hypothesize that the marked 13c and 18o depletion including their co variation noted worldwide in the shuram excursion may reflect an episode of authigenesis occurring within a sulfate methane transition zone smtz if true the shuram excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the ediacaran ocean driven by the neoproterozoic oxidation of surface environments this paleo oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals our study highlights the significance of an integrated approach that combines petrography mineralogy and texture specific micro drilling geochemistry in chemostratigraphic studies

re evaluation of the holocene sea level maxima for the state of parana brazil abstract the best defined relative sea level curves for the brazilian coast such as the salvador state of bahia curve exhibit three holocene maxima with progressively decreasing heights dated at about 5100 3600 and 2500 years b p although the maximum reached a height of 3 to 5 m in different sectors of the brazilian coast previous workers martin et al 1988 postulated that for the coast of paranastate southern brazil it would have been less than 2 5 m and that the other maxima would likewise also have been lower than in other sectors re evaluation of available evidence including new data on vermetidae indicated an ancient sea level of 2 9 0 5 m above the present at 3500 60 years b p approximately coincidental with the second holocene maximum in parana if the relative differences in height between the maxima that are observed elsewhere along the brazilian coast hold for the paranacoast as well then it is possible that the first maximum may have been 4 m above the present sea level in this state consequently great changes must be made in the distinction between the pleistocene and holocene coastal palaeogeography of parana moreover if the difference in height of the first maximum along the brazilian coast must now be reduced from 2 5 m to less than 1 m the prevailing hypothesis attributing the larger value to a migrating geoidal depression during the past 5000 years must be modified or even called into question

magnitude of climate variability during middle pliocene warmth a palaeoclimate modelling study abstract the nature and magnitude of climatic variability during the period of middle pliocene warmth ca 3 29 2 97 ma is poorly understood we present a suite of palaeoclimate modelling experiments incorporating an advanced atmospheric general circulation model gcm coupled to a q flux ocean model for 3 29 3 12 and 2 97 ma bp astronomical solutions for the periods in question were derived from the berger and loutre bl2 astronomical solution boundary conditions excluding sea surface temperatures ssts which were predicted by the slab ocean model were provided from the usgs prism2 2 2 digital data set the model results indicate that little annual variation 0 5 c in ssts relative to a control experiment occurred during the middle pliocene in response to the altered orbital configurations annual surface air temperatures also displayed little variation seasonally surface air temperatures displayed a trend of cooler temperatures during december january and february and warmer temperatures during june july and august this pattern is consistent with altered seasonality resulting from the prescribed orbital configurations precipitation changes follow the seasonal trend observed for surface air temperature compared to present day surface wind strength and wind stress over the north atlantic north pacific and southern ocean remained greater in each of the pliocene experiments this suggests that wind driven gyral circulation may have been consistently greater during the middle pliocene the trend of climatic variability predicted by the gcm for the middle pliocene accords with geological data however it is unclear if the model correctly simulates the magnitude of the variation this uncertainty is derived from a the relative insensitivity of the gcm to perturbation in the imposed boundary conditions b a lack of detailed time series data concerning changes to terrestrial ice cover and greenhouse gas concentrations for the middle pliocene and c difficulties in representing the effects of climatic history in snap shot gcm experiments

uranium isotopes fingerprint biotic reduction knowledge of paleo redox conditions in the earth s history provides a window into events that shaped the evolution of life on our planet the role of microbial activity in paleo redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record the ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo history and to correlate specific biogeochemical events with the corresponding microbial metabolism here we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium u i e the accumulation of the heavy isotope in the u iv phase is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments thus isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought after tool to probe for biotic processes because uranium is a common element in the earth s crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of u vi this tool is applicable to a multiplicity of geological epochs and terrestrial environments the findings of this study indicate that biological activity contributed to the formation of many authigenic u deposits including sandstone u deposits of various ages as well as modern cretaceous and archean black shales additionally engineered bioremediation activities also exhibit a biotic signature suggesting that although multiple pathways may be involved in the reduction direct enzymatic reduction contributes substantially to the immobilization of uranium

comparison of ediacaran platform and slope δ238u records in south china implications for global ocean oxygenation and the origin of the shuram excursion abstract the ediacaran shuram negative carbon isotope excursion se records major paleoceanographic changes during the late neoproterozoic possibly linked to a global oceanic oxygenation event yet its cause s remain uncertain earlier studies of the upper ediacaran doushantuo formation in south china based on local redox proxies have documented strong spatial redox heterogeneity along shelf to basin transects but variations of 238u a global redox proxy have not yet been examined in deep water se carbonates in this study we examined 238u variations through the se in the upper slope siduping section similar to platform se sections siduping exhibits a shift toward higher 238u values correlative with the peak of the se i e maximum negative 13ccarb confirming inferences of global ocean oxygenation during the se this raises an apparent paradox because a global negative carbon isotope excursion implies net oxidant consumption requiring an ocean based oxygenation mechanism we hypothesize that an increase in the efficiency of phosphorus burial due to a plankton driven shift from dominantly dissolved organic matter dom cycling to greater particulate organic matter pom export depleted the ocean of nutrient phosphorus by producing a steep redox gradient close to the sediment water interface we suggest that ocean oxygenation also triggered a globally simultaneous diagenetic event in which isotopically light 13ccarb was precipitated in authigenic carbonate minerals this scenario can account for 238u differences between shallow water and deep water carbonates which reflect precipitation of relatively larger amounts of authigenic carbonate minerals in shallow water settings generating both a larger negative 13ccarb shift and a larger early diagenetic 238u offset

diagenetic effects on uranium isotope fractionation in carbonate sediments from the bahamas abstract uranium isotope variations 238u recorded in sedimentary carbonate rocks are a promising new proxy for the extent of oceanic anoxia through geological time however the effects of diagenetic alteration on the u isotopic composition in carbonate sediments which are crucial to understand the accurate reconstruction of marine 238u are currently poorly constrained here we examine the effects of the aragonite to calcite transition in the pleistocene key largo limestone of south florida and assess the effects of vadose meteoric phreatic meteoric and phreatic marine diagenesis on u isotope fractionation in carbonate sediments from the bahamas transect including the well studied clino unda and odp site 1006 drill cores our results suggest that early diagenetic processes in bahamas carbonate sediments fractionate u isotopes by an average of 0 27 0 14 1 sd heavier than contemporaneous seawater downcore variations of 238u in slope and basin sediments display little if any correlation with u concentration and common geochemical indicators of diagenesis 13c 18o mn sr mg ca sr ca enrichments of redox sensitive elements or rare earth elements anomalies we propose two possible mechanisms to interpret the positive change in the 238u during carbonate diagenesis authigenic enrichment of isotopically positive u iv in carbonates and preferential incorporation of isotopically positive aqueous u vi species into carbonates these processes likely operate during early syndepositional diagenesis on the banktop further diagenesis during deeper burial is limited by the low solubility of u iv under reducing pore water conditions the early diagenetic behavior of u isotopes in bahamas carbonate sediments is likely broadly representative of carbonate diagenesis in the geological past we suggest that the mean diagenetic offset determined in this study be applied when reconstructing seawater 238u from ancient carbonates furthermore early diagenesis induces significant statistical variability in sediment 238u values pointing to the need for large high resolution data sets in order to average out stochastic variations in individual bulk sediment samples

geological record and reconstruction of the late pliocene impact of the eltanin asteroid in the southern ocean in 1995 an expedition on board the research vessel fs polarstern explored the impact site of the eltanin asteroid in the southern ocean the only known asteroidimpact into a deep ocean basin analyses of the geological record of the impact region place the event in the late pliocene 2 15 myr and constrain thesize of the asteroid to be 1 km the explosive force inferred for this event places it at the threshold of impacts believed to have global consequences and its studyshould therefore provide a baseline for the reconstruction and modelling of similar events which are common on geological timescales

does the global stratigraphic reproducibility of δ13c in neoproterozoic carbonates require a marine origin a pliocene pleistocene comparison the large and systematic negative shifts in the 13 c values 12 of carbonate dominated rocks that preceded neoproterozoic glacial successions have been interpreted to record a dramatic series of global environmental and evolutionary events these values are widely considered to be marine rather than diagenetic in origin because stratigraphic patterns of change are systematic and reproducible from basin to basin distinct in magnitude and associated with recognizable stratigraphic markers such as glacial deposits in contrast diagenetic systems are commonly considered to have a more local and stochastic influence on 13 c values cores taken in quaternary carbonate platform sediments however reveal a curious similarity in magnitude thickness and core to core reproducibility where diagenetic alteration has occurred in response to sea level fall sea level changes produced similar 13 c and 18 o stratigraphic records at globally disparate locations which are unrelated to the global marine 13 c signal and bear no relation to the global carbon cycle by analogy with the pliocene pleistocene we propose that spatial reproducibility of 13 c in some neoproterozoic successions might be attributed to causes other than secular variation of the global carbon cycle including diagenesis this observation does not negate the stratigraphic utility of the carbon isotopic values only the origin of the signal

integrated geochemical petrographic insights from component selective δ238u of cryogenian marine carbonates emerging geochemical proxies have improved our understanding of the broad scale history of earth s oxygenation however paleoredox work does not always include extensive consideration of sample preservation and paleoenvironmental setting this is particularly an issue with marine carbonates which although being potentially ideal ocean redox archives are commonly altered during diagenesis here we provide new insight into the robustness of uranium isotopes 238u 235u ratios 238u values as paleoredox tracers by determining texture specific 238u values from a well described cryogenian balcanoona reef complex in south australia we found high variability in 238u values between different carbonate components even within a single sample petrographically the best preserved components from the balcanoona reef are marine cements which have a mean 238u value of 0 23 essentially unfractionated from riverine inputs these values are interpreted as reflecting a marine system with widespread anoxic and iron rich settings less well preserved phases have 238u values spanning almost the entire extent of the documented isotopic range this integrated petrographic geochemical work demonstrates the need for petrographic analysis and careful sample selection on a case by case basis in future carbonate metal isotope geochemistry

co evolution of eukaryotes and ocean oxygenation in the neoproterozoic era the oxygenation of the earth s deep oceans is often thought to have triggered the evolution of simple animals a review article proposes that instead the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea

evidence of pliocene nothofagus in antarctica from pliocene marine sedimentary deposits dsdp site 274 microfossil assemblages in pliocene sediments from dsdp site 274 68 59 81 s 173 25 64 e provide data on the age of the sediments and suggest the presence of nothofagus southern beech in antarctica during the pliocene a suite of 17 samples was collected in an interval from samples 28 274 6r 1 83 87 cm to 28 274 11r 4 73 77 cm 48 33 100 29 mbsf bio stratigraphic study of the abundant diatom assemblages combined with published radiolarian data indicates that the sample interval ranges in age from 5 0 to 2 2 ma with an apparent unconformity between about 3 8 and 3 2 ma nothofagidites the genus for fossil pollen referable to nothofagus occurs throughout the interval as well as pollen and spores with known stratigraphic ranges that unequivocally indicate reworking from older rocks species of nothofagidites recovered include n asperus n brachyspinulosus n flemingii n senectus and n sp cf n lachlaniae the latter form is previously known from the sirius group in the transantarctic mountains abundant palynomorphs were recovered in only three of the samples from site 274 samples 28 274 9r 2 15 19 cm 28 274 9r 2 48 52 cm and 28 274 9r 2 65 69 cm based on the diatom and radiolarian biostratigraphic data the ages of these samples range from 3 00 to 3 01 ma the relative abundance of n sp cf n lachlaniae in the three samples is an order of magnitude higher than relative abundances for the other species of nothofagidites in the same samples the significantly higher relative abundance of n sp cf n lachlaniae suggests that this pollen was derived from trees of nothofagus that were living in antarctica during the mid pliocene diatom assemblages from these three samples indicate that sediments in this interval were rapidly deposited as biogenic oozes in an open ocean setting relatively free of sea ice thus decreasing the possibility of reworking from a single source bed rich in n sp cf n lachlaniae clearly more detailed work in additional well dated cores from around antarctica is needed before a clear picture of the neogene history of antarctic terrestrial vegetation emerges

benthic phosphorus regeneration net primary production and ocean anoxia a model of the coupled marine biogeochemical cycles of carbon and phosphorus we examine the relationships between ocean ventilation primary production water column anoxia and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon c and phosphorus p the elemental cycles are coupled via the redfield c p ratio of marine phytoplankton and the c p ratio of organic matter preserved in marine sediments the model assumes that on geologic timescales net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone the model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water sediment interface evidence from marine and lacustrine settings modern and ancient demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia dysoxia expands steady state simulations show that a reduction in the rate of thermohaline circulation or a decrease of the oxygen content of downwelling water masses intensifies water column anoxia dysoxia and at the same time increases surface water productivity the first effect reflects the declining supply of oxygen to the deeper parts of the ocean the second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides sedimentary burial of organic carbon and authigenic calcium phosphate mineral francolite on the other hand is promoted by reduced ocean ventilation according to the model global scale anoxia dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system consequently higher rates of primary production and organic carbon burial can be achieved even when the continental supply of reactive phosphorus to the oceans remains unchanged

phosphogenesis associated with the shuram excursion petrographic and geochemical observations from the ediacaran doushantuo formation of south china abstract the ediacaran period witnessed one of the largest phosphogenic events in earth s history coincidently some phosphorite deposits in south china are associated with the largest known carbon isotope negative excursion i e shuram excursion suggesting an intimate coupling of the biogeochemical carbon and phosphorous cycles however the geomicrobiological linkage between these anomalies remain poorly understood in this study we investigated the phosphorite samples from the uppermost doushantuo formation in south china carbon isotope compositions of authigenic calcite cements and nodules in the phosphorites are as low as 34 vpdb petrographic and geochemical investigations indicate that the 13 c depleted carbonates likely formed as the result of microbial sulfate and iron reduction that released phosphorous from iron oxyhydroxide concentrating phosphorous in pore waters and thereby promoting phosphate mineralization the timing of this event appears to coincide with enhanced sulfate delivery to seawater through continental weathering the basin scale distribution of doushantuo phosphorites suggests a redox control on the availability of iron oxyhydroxide and the recycling of pore water phosphorous both inner and outer shelf regions were likely characterized by an oxic water column and were the main loci for phosphogenesis on the contrary intra shelf and slope regions which are lean in phosphorite were subjected to euxinic or ferruginous water column conditions the intimate coupling between ediacaran phosphogenesis and the shuram excursion suggests strong links among seawater redox conditions c s p fe cycling and fossil phosphatization increased microbial sulfate reduction driven by enhanced sulfate reservoir in the ediacaran ocean may have played an essential role on these biogeochemical events

uranium isotope evidence for an expansion of anoxia in terminal ediacaran oceans abstract anoxic and iron rich oceanic conditions prevailed throughout most of the archean and proterozoic 4000 to c 540 million years ago ma but the oceans are hypothesised to have become progressively oxygen rich during the ediacaran cambrian transition interval coincident with the rise of animal life we utilise the uranium isotope ratio of seawater 238u 235u reformulated as 238 u an effective tracer of oceanic redox conditions as a proxy for changes in the global proportion of anoxic seafloor we present a new 238 u dataset for carbonate rocks from the lower nama group namibia deposited in a shelf ramp succession during the terminal neoproterozoic 550 to 547 ma these data capture a transition from 238 u similar to the modern ocean towards persistently low 238 u average 0 81 0 06 such low 238 u are consistent with enhanced u drawdown from the water column under anoxic conditions and the preferential export of heavy 238u to sediments following u vi u iv reduction placing our results into a steady state ocean box model suggests at least a third of the global seafloor was covered by anoxic bottom waters compared with only 0 3 in today s oxygenated oceans comparison with 238 u from older sediments deposited in other basins further supports an expansion of anoxic bottom waters towards the end of the ediacaran our data are consistent with an emerging picture of a dominantly anoxic ediacaran ocean punctuated by brief ocean oxygenation events in the nama group the transition towards globally widespread anoxic conditions post dates the first appearance of both skeletal metazoans and soft bodied fauna of the nama assemblage this suggests that the global expansion of anoxia did not coincide with the decline of the ediacaran biota or drive the biotic turnover between the white sea and nama assemblages the impact of this global redox change on metazoan ecosystems is unclear since the expansion of anoxia if contained mainly within deeper waters may not have impinged significantly upon continental shelves that host the majority of biodiversity

constraints on age erosion and uplift of neogene glacial deposits in the transantarctic mountains determined from in situ cosmogenic 10be and 26al 10 be and 26 al data from sandstone boulders in three neogene glacial deposits in the mcmurdo sound dry valleys region of southern victoria land antarctica indicate minimum exposure ages of approx 3 ma and maximum long term erosion rates of approx 5 12 cm m y supporting the suggestion that polar desert conditions have persisted in the dry valleys since at least late pliocene time variation of cosmogenic nuclide production rate with altitude also allows constraints on past uplift rates model calculations employing 10 be data indicate little or no uplift in the dry valleys region in the past 3 m y precluding rapid 1 km m y late pliocene uplift previously suggested for some parts of the transantarctic mountains

conditions required for oceanic anoxia euxinia constraints from a one dimensional ocean biogeochemical cycle model article i nfo widespread black shale depositional intervals termed oceanic anoxic events oaes occurred repeatedly during the phanerozoic eon here we developed a new vertical one dimensional ocean biogeochemical cycle model that involves several chemical reactions in an oxic anoxic sulfidic water column to explore the theoretical constraints for global oceanic anoxia euxinia quantitatively and systematically we conducted sensitivity analyses of the proposed causal mechanisms including elevated rates of riverine phosphorus p input ocean stagnation and lowered oxygen solubility due to climate warming we gave special attention to the vertical chemical structure of the ocean and also to the characteristic behaviors of the marine p cycle under anoxic conditions because the relationship between the depth of anoxia and the benthic phosphorus flux could be important for the occurrence of oceanic anoxia euxinia steady state simulations indicated that 1 a decrease in ocean stagnation or oxygen solubility is not enough by itself to achieve widespread anoxia with the present reactive p river input rate and 2 shallow water anoxia followed by massive p liberation from surface sediments can lead to widespread eutrophication and anoxia euxinia we conclude that elevated riverine flux of reactive p is the most important factor for triggering global anoxic events via a positive feedback loop among ocean anoxia phosphorus regeneration and surface biological productivity

causes and consequences of mid proterozoic anoxia evidence for low po2 and a ferruginous ocean characterize the mid proterozoic 1 8 0 8 ga considerations of redox sources and sinks imply that generation of o2 via organic carbon corg burial must be low to maintain a low po2 atmosphere for geologically long intervals yet low oxygen should result in increased corg preservation marine export production must therefore be low to limit corg burial and o2 generation formation of ferrous phosphate can buffer deepwater phosphate pi to levels an order of magnitude or more below those in the modern ocean limiting export production low deepwater pi is consistent with the hiatus in sedimentary phosphorite deposits during the mid proterozoic and low po2 limits formation of sedimentary iron deposits bif we propose that low po2 was maintained by p limitation resulting from ferrous phosphate buffering the near absence of bif and phosphorite deposition is direct and indirect consequences of the low po2 respectively