formation and export of dense shelf water from the adelie depression east antarctica 1 coastal polynyas in the adelie depression are an important source of antarctic bottom water to the australian antarctic basin we present time series april 1998 to may 1999 and august 1999 to february 2000 of data from temperature salinity sensors in both the adelie depression and the known outflow region of the adelie sill to describe the annual cycle of shelf water densities from april through september salinification beneath the polynya produces dense shelf waters during september october shelf water densities in the depression peak at 27 94 kg m 3 and the cooling and freshening signature of ice shelf water is observed north of buchanan bay in november december shelf water densities decrease as intrusions of warm and relatively fresh modified circumpolar deep water enter east of the adelie sill from january march the surface layer is conditioned by the cooling of the atmosphere which overturns the upper water column at the adelie sill observed daily mean currents were approximately 10 cm s 1 with intense instantaneous currents greater than 50 cm s 1 at the sill depth using an idealized outflow region with a rectangular cross sectional area 6 106 m2 we present the first estimates of shelf water export by potential density class assuming shelf water with a minimum density of 27 88 kg m 3 has sufficient negative buoyancy for downslope mixing and a fourfold volume increase 1 3 mixing ratio from entrainment the dense shelf water export of 0 1 0 5 sv results in an annual average production of bottom water in this region of between 0 4 and 2 0 sv the wide range in bottom water estimate results from data limitations and a narrowing of this range requires further mooring observations

the sulfur isotope signatures of marinoan deglaciation captured in neoproterozoic shallow to deep cap carbonate from south china a b s t r a c t we present high resolution sulfur and carbon isotope records from the cap carbonate of the doushantuo formation south china a unique suite of depositional facies spanning across a pronounced paleo oceanic depth gradient the i34s of carbonate associated sulfate cas decreases across a shelf to slope depth transect we propose that the deeper water slope profile represents syn glacial sulfur isotope compositions of the seawater sulfates with impact of 34 s depleted hydrothermal sulfur inputs while the i 34 s of the shallow inner shelf were dominantly controlled by the 34 s enriched terrigenous input we suggest that the terrigenous sulfur inputs were mainly from weathering and erosion of the pre marinoan datangpo formation glacially ground and then freshly exposed during rapid deglaciation the widespread datangpo formation deposited between the sturtian and marinoan glacial intervals is noted for the unusually high i34s values of its pyrite the low sulfate conditions in the ocean allowed for strong local controls and consequent heterogeneities in seawater chemistry the post marinoan ocean was stratified in the immediate wake of glaciation with anoxic deep waters overlain by an oxic melt water layer during the transgression the shallow water was disturbed by upwelling the oxidation of dissolved organic carbon doc from deep water would lead to transiently enhanced 13c depletions but 34s enrichments in the shallow waters

change in dense shelf water and adelie land bottom water precipitated by iceberg calving antarctic bottom water supplies the deep limb of the global overturning circulation and ventilates the abyssal ocean antarctic bottom water has warmed freshened and contracted in recent decades but the causes remain poorly understood we use unique multiyear observations from the continental shelf and deep ocean near the mertz polynya to examine the sensitivity of this bottom water formation region to changes on the continental shelf including the calving of a large iceberg postcalving the seasonal cycle of dense shelf water dsw density almost halved in amplitude and the volume of dsw available for export reduced in the deep ocean the density and volume of adelie land bottom water decreased sharply after calving while oxygen concentrations remained high indicating continued ventilation by dsw this natural experiment illustrates how local changes in forcing over the antarctic continental shelf can drive large and rapid changes in the abyssal ocean

a study on the regulation of the concentrations of some trace metals rb sr zn pb cu v cr ni mn and mo in saanich inlet sediments british columbia canada abstract bulk metal concentrations were measured in the sediments of saanich inlet in an attempt to establish the occurrence of trace metal enrichment in the anoxic central basin ba ni v cr zn pb cu and mo were found to be enriched in the anoxic ooze over the possible contributions from lithogenic sources spatial and seasonal variations in the chemical composition of the settling particulates collected with interceptor traps gave further indications of the mode of incorporation of these metals biogenic ba and cr appeared to be associated with opaline silica although alternative explanations are also possible particularly for ba zinc seemed to be added to the sediment essentially in association with planktonic materials while cu required an additional source directly linked to the anoxic environment similarly ni v and mo were added to the anoxic sediments by reactions occurring at the sediment water interface in the nearshore environment studied here these metals particularly ni and mo were not associated to any significant extent with planktonic materials of all the elements analyzed mo showed the largest enrichment in the anoxic sediments of saanich inlet

sea ice growth and water mass modification in the mertz glacier polynya east antarctica during winter in july september 1999 an extensive oceanographic survey 87 conductivity temperature and depth measuring stations was conducted in the mertz glacier polynya over the adelie depression off the antarctic coast between 145 and 150 e we identify and describe four key water masses in this polynya highly modified circumpolar deep water hmcdw winter water ww ice shelf water isw and high salinity shelf water hssw combining surface velocity data from an acoustic doppler current profiler with three hydrographic sections we found the hmcdw to be flowing westward along the shelf break 0 7 sv the ww and hssw flowing eastwards underneath mertz glacier 2 0 sv and that there was a westward return flow of isw against the continent 1 2 sv using a simple box model for the exchanges of heat and fresh water between the principal water masses we find that the polynya was primarily a latent heat polynya with 95 of the total heat flux caused by sea ice formation this heat flux results from a fresh water equivalent sea ice growth rate of 4 9 7 7 cm d 1 and a mass exchange between hmcdw and ww of 1 45 sv the inferred ocean heat flux is 8 14 w m 2 and compares well with other indirect estimates

zn sr isotope records of the ediacaran doushantuo formation in south china diagenesis assessment and implications abstract recent studies show that zinc isotopes could provide valuable clues to environmental change and biogeochemical cycle of the past oceans this study reports a modified procedure for leaching the carbonate fractions in sedimentary rocks a thorough evaluation of diagenetic effects and systematic variations of zn and sr isotope ratios in lower part of the ediacaran stratigraphic unit deposited in the aftermath of the marinoan glaciation in south china the influence of post depositional diagenesis on zn isotope compositions of the studied samples is assessed by comparing 66zn to other geochemical indexes 87sr 86sr 13c 18o and mn sr ratios in the five studied cap carbonate sections member i of the doushantuo formation dolostones from four sections have 66zn values positively correlated with 18o values and negatively correlated with 87sr 86sr ratios 0 7081 0 7204 these correlations suggest that these cap dolostone samples have been modified by post depositional diagenesis the light 66zn value 0 02 suggests that initial zn isotope ratios of cap dolostones could have been reset by hydrothermal fluids with relatively high zn concentration and low 66zn values by contrast carbonates from member ii of the doushantuo formation above cap dolostones are relatively pristine based on their low 87sr 86sr ratios 0 7079 0 7086 being indistinct from the proposed early ediacaran seawater 87sr 86sr values chemical and isotopic variations in these samples are interpreted to reflect primary signals that record paleo environmental changes of the early ediacaran ocean a rapid increase of 66zn from 0 3 to 1 1 occurs in the middle part of member ii accompanying by relatively invariant 87sr 86sr ratios that imply insignificant changes in input from continental weathering considering the limited change in atmospheric oxygen during this period the rapid 66zn raise indicates an increase in buried organic matters which is consistent with the coupled positive shift of 13ccarb as well as the fossil records found in the same strata these results provide insights into zn cycling in the post marinoan oceans and facilitate the application of zn isotopes in carbonates as a proxy for the fate of marine organic matter

landslide susceptibility mapping prediction using support vector machine for mandakini river basin garhwal himalaya india abstract in recent years various machine learning techniques have been applied for landslide susceptibility mapping in this study three different variants of support vector machine viz svm proximal support vector machine psvm and l 2 support vector machine modified finite newton l 2 svm mfn have been applied on the mandakini river basin in uttarakhand india to carry out the landslide susceptibility mapping eight thematic layers such as elevation slope aspect drainages geology lithology buffer of thrusts faults buffer of streams and soil along with the past landslide data were mapped in gis environment and used for landslide susceptibility mapping in matlab the study area covering 1625 km 2 has merely 0 11 of area under landslides there are 2009 pixels for past landslides out of which 50 1000 landslides were considered as training set while remaining 50 as testing set the performance of these techniques has been evaluated and the computational results show that l 2 svm mfn obtains higher prediction values 0 829 of receiver operating characteristic curve auc area under the curve as compared to 0 807 for psvm model and 0 79 for svm the results obtained from l 2 svm mfn model are found to be superior than other svm prediction models and suggest the usefulness of this technique to problem of landslide susceptibility mapping where training data is very less however these techniques can be used for satisfactory determination of susceptible zones with these inputs

oxygenation of ediacaran ocean recorded by iron isotopes abstract the increase in atmospheric oxygen during the late neoproterozoic era ca 800 542 ma may have stimulated the oxygenation of the deep oceans and the evolution of macroscopic multicellular organisms however the mechanism and magnitude of neoproterozoic oxygenation remain uncertain we present fe isotopes fe species and other geochemical data for two sections of the doushantuo formation ca 635 551 ma deposited after the nantuo glacial episode in the yangtze gorge area south china it is highlighted that highly positive 56 fe values reflect a lower oxidation rate of fe ii aq under ferruginous conditions and in turn near zero 56 fe values indicate oxidizing conditions our study suggests that during the deposition of the bottom of member ii of the doushantuo formation the shallow seawater was oxic but the deep water was characterized by ferruginous conditions which is consistent with a redox chemical stratification model subsequent anoxic conditions under shallow seawater represented by positive 56 fe and negative 13 c carb excursions should be ascribed to the upwelling of fe ii aq and dissolved organic carbon doc rich anoxic deep seawater the oxidation of fe ii aq and doc rich anoxic deep seawater upon mixing with oxic shallow water provides an innovative explanation for the well known negative 13 c carb excursions enc2 and positive 56 fe excursions in the middle of doushantuo formation meanwhile the upwelling fe ii aq and doc rich anoxic deep seawater could increase photosynthetic activity the balance between oxygen consumption and production was most important criteria for the oxygenation of early ediacaran ocean and diversity of eukaryotic organisms

regulation of algal blooms in antarctic shelf waters by the release of iron from melting sea ice during summer 1995 96 we measured iron in the water column and conducted iron enrichment bottle incubation experiments at a station in the central ross sea 76 30 s 170 40 w first in the presence of melting sea ice and 17 days later in ice free conditions we observed a striking temporal change in mixed layer dissolved iron concentrations at this station from 0 72 2 3 nm with sea ice present to 0 16 0 17 nm in ice free conditions these changes were accompanied by a significant drawdown in macronutrients and an approximate doubling of algal diatom biomass our incubation experiments suggest that conditions were iron replete in the presence of sea ice and iron deficient in the absence of sea ice we surmise that bioavailable iron was released into seawater from the melting sea ice stimulating phytoplankton production and the biological removal of dissolved iron from the mixed layer until iron limited conditions developed these observations suggest that the episodic release of bioavailable iron from melting sea ice is an important factor regulating phytoplankton production particularly ice edge blooms in seasonally ice covered antarctic waters

determination of 88sr 86sr mass dependent isotopic fractionation and radiogenic isotope variation of 87sr 86sr in the neoproterozoic doushantuo formation abstract we measured both mass dependent isotope fractionation of 88sr 88sr 86sr and radiogenic isotopic variation of sr 87sr 86sr for the neoproterozoic doushantuo formation that deposited as a cap carbonate immediately above the marinoan related nantuo tillite the 88sr and 87sr 86sr compositions showed three remarkable characteristics 1 high radiogenic 87sr 86sr values and gradual decrease in the 87sr 86sr ratios 2 anomalously low 88sr values at the lower part cap carbonate and 3 a clear correlation between 87sr 86sr and 88sr values these isotopic signatures can be explained by assuming an extreme greenhouse condition after the marinoan glaciation surface seawater mixed with a large amount of freshwater from continental crusts with high 87sr 86sr and lighter 88sr ratios was formed during the extreme global warming after the glacial event high atmospheric co2 content caused sudden precipitation of cap carbonate from the surface seawater with high 87sr 86sr and lighter 88sr ratios subsequently the mixing of the underlying seawater with unradiogenic sr isotope compositions and normal 88sr ratios probably caused gradual decrease of the 87sr 86sr ratios of the seawater and deposition of carbonate with normal 88sr ratios the combination of 87sr 86sr and 88sr isotope systematics gives us new insights on the surface evolution after the snowball earth

community structure and benthic habitats across the george v shelf east antarctica trends through space and time physical and biological characteristics of benthic communities are analysed from underwater video footage collected across the george v shelf during the 2007 2008 ceamarc voyage benthic habitats are strongly structured by physical processes operating over a range of temporal and spatial scales iceberg scouring recurs over timescales of years to centuries along shallower parts of the shelf creating communities in various stages of maturity and recolonisation upwelling of modified circumpolar deep water mcdw onto the outer shelf and cross shelf flow of high salinity shelf water hssw create spatial contrasts in nutrient and sediment supply which are largely reflected in the distribution of deposit and filter feeding communities long term cycles in the advance and retreat of icesheets over millennial scales and subsequent focussing of sediments in troughs such as the mertz drift create patches of consolidated and soft sediments which also provide distinct habitats for colonisation by different biota these physical processes of iceberg scouring current regimes and depositional environments in addition to water depth are important factors in the structure of benthic communities across the george v shelf the modern shelf communities mapped in this study largely represent colonisation over the past 8000 12 000 years following retreat of the icesheet and glaciers at the end of the last glaciation recolonisation on this shelf may have occurred from two sources deep sea environments and possible shelf refugia on the mertz and adelie banks however any open shelf area would have been subject to intense iceberg scouring understanding the timescales over which shelf communities have evolved and the physical factors which shape them will allow better prediction of the distribution of antarctic shelf communities and their vulnerability to change this knowledge can aid better management regimes for the antarctic margin

a role for scavenging in the marine biogeochemical cycling of zinc and zinc isotopes zinc zn and cadmium cd are important biologically active trace metals in the ocean to date the marine distributions of these elements have been understood primarily in terms of biological assimilation by growing phytoplankton and regeneration of sinking biological material initial studies of zn and cd concentrations and stable isotope ratios 66zn and 114cd have therefore focused on their use as simple tracers of assimilation and regeneration in the oceans however these two processes are insufficient to explain new data on the marine distribution of zn and 66zn here using the first high resolution paired marine depth profiles of zn cd 66zn and 114cd we suggest that scavenging of zn onto organic matter plays a major yet largely unconsidered role in the marine cycling of zn this hypothesis is supported by culture experiments which show that zn released from degrading phytoplankton is rapidly scavenged back onto organic matter and that adsorbed zn is isotopically heavier than the dissolved phase by 0 58 in contrast very little cd or phosphate was scavenged and cd isotopes were not significantly fractionated during degradation our hypothesis is further supported by one dimensional modeling which reproduces observed marine 66zn profiles with 1 of zn adsorbed to particles understanding how zn cycling in the oceans is a balance between assimilation scavenging and regeneration is necessary in order to investigate 66zn as a tracer of marine productivity we anticipate that paired analyses of 66zn and 114cd will prove to be valuable new tools in constraining patterns of global primary productivity providing key information for the marine carbon cycle during periods of past and present global climate change

zn isotopic evolution in early ediacaran ocean a global signature abstract the zn isotope composition of sedimentary archives reveals key biosphere lithosphere interactions throughout geologic history a recent zn isotope study of ediacaran cap carbonates from south australia revealed that the local isotopic profile reflects global changes in oceanic chemistry to examine this critical assumption this study investigates a high resolution zinc isotope profile of marinoan post glacial cap carbonates from the yangtze platform south china stratigraphic variations in zn isotope data from this study are similar to previously reported sections in south australia and south china the remarkable isotopic similarity between the two sections that were deposited on distant geographically separated continental blocks demonstrates that the zn isotopic variations reflect changing global signatures of the early ediacaran ocean this study highlights the value of zn isotopes in sediments as a global proxy for paleoceanographic studies we interpret the systematic stratigraphic changes in the zn isotope composition of the studied post glacial marinoan carbonates as a global oceanic trend controlled mainly by varying inputs and outputs of zn to the ocean decreasing 66zn values in the lowermost part of the section are attributed to the global precipitation of carbonate enriched in the heavier zn isotope a positive shift in the middle part of the section is explained by the non quantitative removal of isotopically lighter zn by sulfide minerals lower 66zn values in the uppermost part likely reflect the changing contributions of continental weathering and sulfide precipitation our data suggest that non quantitative removal of zn by sulfides is a previously unrecognized sink for zinc in the oceans

the cycling of iron zinc and cadmium in the north east pacific ocean insights from stable isotopes abstract dissolved stable isotope ratios of the transition metals provide useful information both for understanding the cycling of these bioactive trace elements through the oceans and tracing their marine sources and sinks here we present seawater dissolved fe zn and cd concentration and stable isotope ratio 56fe 66zn and 114cd profiles from two stations in the pacific ocean the safe station 30 n 140 w in the subtropical north east pacific from the geotraces ic2 cruise and the marginal san pedro basin 33 8 n 118 4 w within the south california bight these data represent to our knowledge the first full water column profiles for 66zn and 56fe from the open ocean north pacific and the first observations of dissolved 66zn and 114cd in a low oxygen marginal basin at the safe station 56fe is isotopically lighter throughout the water column 0 6 to 0 1 relative to irrm 014 compared to the north atlantic suggesting significant differences in fe sources or fe cycling between these two ocean basins a broad minimum in 56fe associated with the north pacific oxygen minimum zone omz

deglacial ocean and climate seasonality in laminated diatom sediments mac robertson shelf antarctica the palaeoceanography and climate history of the east antarctic margin eam are less well understood than those of west antarctica yet the eam plays an important role in deep ocean circulation and the global ocean system and has likely done so in the past deglacial age marine sediments from the eam provide clues about its past role during this critical period of rapid climate change several deep basins across the eam such as iceberg alley 67 s 63 e on the mac robertson shelf mrs accommodate thick marine sequences that archive the deglaciation in the form of diatom rich continuously laminated varved sediments these laminated sediments are pristinely preserved and contain seasonal and long term information on the cryospheric and palaeoceanographic changes associated with the rapid retreat of the glacial ice sheet across the mrs we present results of microfabric analysis of the lower 2 m of deglacial varves from jumbo piston core jpc43b iceberg alley backscattered electron imagery bsei of polished thin sections and scanning electron microscope secondary electron imagery sei of lamina parallel fracture surfaces are used to analyze the varves one hundred and ninety two laminations are investigated and their nature and temporal significance are discussed in terms of seasonal deposition and cyclicity of diatom species our high resolution palaeodata record exceptionally high diatom production and silica flux associated with the retreat of the east antarctic ice sheet and seasonal sea ice changes along the eam this information is invaluable for assessing cryospheric oceanographic variation and therefore the local and regional response to this period of rapid climate change varves are made up of lamina couplets comprising i thickly laminated to thinly bedded orange orange brown very pure diatom ooze dominated by hyalochaete chaetoceros spp vegetative cells and resting spores and ii brown blue grey terrigenous angular quartz sand silt and clay with an abundant mixed diatom flora the colour variation between these two types of lamination is striking using floristic and textural information we interpret the diatom oozes as spring flux and the terrigenous laminae as summer flux each couplet pair represents one annual cycle and reflects seasonal changes in nutrient availability and stratification associated with the cyclical advance and retreat of seasonal sea ice the diatom oozes can reach up to 7 5 cm in thickness indicating enormous silica flux to the sea floor associated with ice sheet retreat

glacier tongue calving reduced dense water formation and enhanced carbon uptake 1 dense shelf water formed in the mertz polynya supplies the lower limb of the global overturning circulation ventilating the abyssal indian and pacific oceans calving of the mertz glacier tongue mgt in february 2010 altered the regional distribution of ice and reduced the size and activity of the polynya the salinity and density of dense shelf water declined abruptly after calving consistent with a reduction of sea ice formation in the polynya breakout and melt of thick multiyear sea ice released by the movement of iceberg b9b and the mgt freshened near surface waters the input of meltwater likely enhanced the availability of light and iron supporting a diatom bloom that doubled carbon uptake relative to precalving conditions the enhanced biological carbon drawdown increased the carbonate saturation state outweighing dilution by meltwater input these observations highlight the sensitivity of dense water formation biological productivity and carbon export to changes in the antarctic icescape

modern distribution of diatoms in sediments from the george v coast antarctica abstract currently little information exists concerning high frequency climate change in the antarctic data that are essential to gaining a more complete understanding of global climate change sediments from the george v coast continental margin potentially contain a high resolution record of the late holocene the relatively high rate of sedimentation 3 mm year and the presence of laminated intervals which contain undisturbed sequences make these sediments valuable for future downcore work in order to utilize diatom assemblages as proxies of paleoclimatic and paleoceanographic parameters a preliminary study of the surface sediment distribution of diatoms from the george v coast continental margin was completed nitzschia curta van heurck hasle and nitzschia kerguelensis o meara hasle present great potential for tracking the holocene retreat of ice across the continental shelf of this region concentrations of n curta an ice related form decreases in an offshore direction whereas n kerguelensis an oceanic species associated with circumpolar flow displays an increase in sediments offshore the genus thalassiosira cleve displays a clear association with areas of open water primary production as has been noted in others parts of the antarctic downcore variability in relative abundance of this genus will be useful in understanding the history of sea ice versus open water production on the george v coast shelf in contrast the distribution of chaetoceros ehrenberg resting spores appears to be more complex but may be related to the position of the stationary ice edge associated with the annual minimal extent of sea ice finally an understanding of the downcore variability of thalassiothrix antarctica schimper ex karsten a species which is very abundant in the laminated sections of the sediment cores will have to rely on studies of its distribution in other areas of the antarctic as it is sparsely distributed along the george v coast today

the paragenetic history of a marinoan cap carbonate abstract carbonates that cap late neoproterozoic glacial diamictites contain large atypical tepee like structures that have been interpreted as evidence for seafloor dolomite cementation the nuccaleena formation in the adelaide fold thrust belt is the stratotype marinoan age cap dolostone at parachilna gorge the nuccaleena formation contains paired cogenetic tepees and growth faults that demonstrate these structures grew well below the sediment water interface these tepees and their associated sheet veins were the result of pore fluid overpressure driven by expansive early diagenetic dolomite cementation fluid overpressure resulted in two forces 1 macroscale lateral compression that overcame the lithostatic weight to buckle sediments upward forming the tepee structure and 2 macro to microscale extension that overcame the lithologic tensile strength to form sheet veins the physical processes of fluid overpressure within the parachilna gorge cap carbonate are in many ways analogous to the fluid overpressure dynamics in mud volcanoes and were sufficiently high to generate veins extending 17 m below the cap carbonate overpressure driven dewatering removed pore fluid clay and organic matter from the dolomicrospar matrix into dewatering structures and fabrics the latter are hierarchically arranged from small linear clay fabrics to clay lined dendritic arrays to clay choked curvilinear veins to large sheet veins packets of red clay laminae were deposited when pore fluids vented to the sea floor the early diagenetic chemical evolution of the parachilna gorge cap carbonate was equally dynamic in which an initial reducing trend was supplanted by an oxidation trend brought about by tepee sheet vein deformation once established the sheet vein network also reinvigorated diffusional processes that likely enhanced early diagenetic cementation of the cap dolostone the detailed paragenetic history also suggests that calcite not dolomite was the initial carbonate precipitated from the post glacial adelaidean sea the nuccaleena formation exhibits the dolomicrospar isotopic signatures and tepee like structures that distinguish marinoan age cap carbonates at parachilna gorge these typical features are a product of early diagenesis suggesting that other marinoan age cap carbonates may also be early diagenetic in origin

neoproterozoic cap dolostone deposition in stratified glacial meltwater plume abstract neoproterozoic cap carbonates host distinctive geochemical and sedimentological features that reflect prevailing conditions in the aftermath of snowball earth interpretation of these features has remained contentious with hypotheses hinging upon timescale and synchronicity of deposition and whether or not geochemical signatures of cap carbonates represent those of a well mixed ocean here we present new high resolution sr and mg isotope results from basal ediacaran cap dolostones in south australia and mongolia least altered sr and mg isotope compositions of carbonates are identified through a novel incremental leaching technique that monitors the purity of a carbonate sample and the effects of diagenesis these data can be explained by the formation of these cap dolostones involving two chemically distinct solutions a glacial meltwater plume enriched in radiogenic sr and a saline ocean residue with relatively lower 87sr 86sr ratios model simulations suggest that these water bodies remained dynamically stratified during part of cap dolostone deposition most likely lasting for 8 thousand years our results can potentially reconcile previous conflicts between timescales estimated from physical mixing models and paleomagnetic constraints geochemical data from cap carbonates used to interpret the nature of snowball earth and its aftermath should be recast in terms of a chemically distinct meltwater plume

constraints on the nature of the marinoan glaciation cyclic sedimentary records from the nantuo formation south china abstract detailed sedimentological investigations of the cryogenian glacial deposits are critical for constraining the true nature of the sturtian and marinoan glaciations the entire succession of the nantuo formation which corresponds to the marinoan glaciation is well developed in south china here a detailed lithological and sedimentological analysis of two cores of the nantuo formation in guizhou south china is presented in which two continuous ice advance retreat cycles are identified the base consists of a massive diamictite lithofacies that is representative of glacier dominated subaqueous debris flow deposits these proximal glaciomarine deposits are overlain by a gravel free sandy mudstone lithofacies that is lacking in diamictites which implies distal glaciomarine deposits this ice advance retreat sedimentary cycle is overlain by a weakly laminated diamictite lithofacies which represents subaqueous debris flow deposits that formed during the second episode of ice advance these proximal glaciomarine deposits are overlain by an interbedded sandstone mudstone lithofacies which is indicative of distal glaciomarine deposits that formed during another episode of ice retreat these sedimentary cycles imply that the climate switched twice from relatively warm to extremely cold and back during the marinoan glaciation the global sea level and sedimentary environments fluctuated in response to these climate changes this co evolution of the climate sea level and sedimentary environments which is represented by the lithofacies and sedimentary sequence of the nantuo formation as well as the corresponding strata of other sections worldwide suggests that the marinoan glaciation is characterized by a progressive and dynamic course

the surface windfield over the antarctic ice sheets the intense radiative cooling of air over the ice slopes of antarctica generates a surface wind regime that is strongly controlled by topography and plays a key role in determining the behaviour of the atmosphere and ocean in high southern latitudes1 7 resultant surface winds are intimately linked to the orientation of the ice terrain fig 1 and display the highest degree of persistence found on earth the close coupling between wind and topography allows estimation of the former if the latter is known with some precision here we report on time averaged near surface airflow over the antarctic continent during winter diagnosed from a recent accurate synthesis of terrain slopes and from estimates of the lower atmospheric temperature structure the simulated drainage pat tern exhibits strong spatial variability with the airflow concentrated into several zones near the coastal margin these confluence regions are responsible for strong persistent katabatic winds over downstream coastal stretches and are indicative of zones of greatest katabatic potential

the classification of arctic sea ice types and the determination of surface temperature using advanced very high resolution radiometer data the accurate quantification of new ice and open water areas and surface temperatures within the sea ice packs is a key to the realistic parameterization of heat moisture and turbulence fluxes between ocean and atmosphere in the polar regions multispectral noaa advanced very high resolution radiometer 2 avhrr 2 satellite images are analyzed to evaluate how effectively the data can be used to characterize sea ice in the bering and greenland seas both in terms of surface type and physical temperature the basis of the classification algorithm which is developed using a late wintertime bering sea ice cover data is that frequency distributions of 10 8 m radiances provide four distinct peaks representing open water new ice young ice and thick ice with a snow cover the results are found to be spatially and temporally consistent possible sources of ambiguity especially associated with wider temporal and spatial application of the technique are discussed an ice surface temperature algorithm is developed for the same study area by regressing thermal infrared data from 10 8 and 12 0 m channels against station air temperatures which are assumed to approximate the skin temperatures of adjacent snow and ice the standard deviations of the results when compared with in situ data are about 0 5 k over leads and polynyas to about 0 5 1 5 k over thick ice this study is based upon a set of in situ data limited in scope and coverage cloud masks are applied using a thresholding technique that utilizes 3 74 and 10 8 m channel data the temperature maps produced show coherence with surface features like new ice and leads and consistency with corresponding surface type maps further studies are needed to better understand the effects of both the spatial and temporal variability in emissivity aerosol and precipitable atmospheric ice particle distribution and atmospheric temperature inversions

iron deficiency limits phytoplankton growth in antarctic waters enrichment experiments were performed in the ross sea to test the hypothesis that iron deficiency is responsible for the phytoplankton s failure to use up the luxuriant major nutrient supplies found in these and all other offshore antarctic ocean waters nitrate uptake rates in the controls without added trace elements ranged from 0 58 to 1 22 mol kg 1 d 1 the addition of 1 to 5 nmol of unchelated fe per liter resulted in rates that were 2 to 10 times higher 2 54 to 6 00 mol no3 kg 1 d 1 rates in bottles with 2 nmol mn added were identical to those in the controls 0 57 to 1 04 mol no3 kg 1 d 1 total decreases in no3 were balanced by increases in particulate organic n these results suggest that fe deficiency is the primary reason that the present day southern ocean biological pump is shut off in contrast iron was 50 times more abundant during the last glacial maximum greater fe availability may have stimulated the biological pump and contributed to the ice age drawdown of atmospheric c02 these results also imply that large scale southern ocean fe fertilization is feasible at least in terms of the total amounts of fe required i e 100 000 to 500 000 tons yr 1

spatial and year to year patterns in new and primary productivity in sea ice melt regions of the eastern bering sea abstract four years of data on for new nitrate regenerated ammonium and primary carbon productivity 2007 through 2010 confirm the elevated productivity associated with the marginal ice zone miz of the eastern bering sea ebs in spring the greatest levels of productivity were associated with ice melt regions along the western edge of the ice pack population seeding and the shallowing of the mixed layer depth near the ice edge were less important in the development of elevated miz productivity than was the amount of ice melt entering surface water suggesting that ice contributed substances stimulating phytoplankton growth based on ice melt as an index of bloom development primary productivity continued to increase after new productivity peaked at the miz environments with optimal growth conditions new productivity averaged 18 6 mmol n m 2 d 1 with a ratio of 0 61 and primary productivity averaged 492 mmol c m 2 d 1 although these features were consistently recorded in late april early may they varied with the dynamics of ice cover whose retreat was later in 2008 relative to 2007 and 2009 the intense but localized miz blooms appear to be under sampled in ocean color imagery under ice productivity was generally low ave no3 0 45 mmol n m 2 d 1 ave primary prod 4 16 mmol c m 2 d 1 although larger values were measured under the western edge of the ice pack and in the st lawrence polynya region the advection of ice and water may contribute to the large values under the western ice edge and the highly transparent new ice may play a role in the polynya new and primary productivity rates of under ice samples were significantly inhibited when exposed to ice free irradiance levels in situ productivity could be estimated to 35 from rates measured on deck by adjusting for the impact of ice on submarine light the carbon to nitrate uptake ratio was closely related to specific carbon production in the in situ incubations

paleobathymetric influence on the development of the late ediacaran yangtze platform hubei hunan and guizhou provinces china abstract a compiled facies map based on 48 sections positioned in three chinese provinces hunan guizhou and hubei and field data emphasize the changes in depositional environments on the shelf of the ediacaran yangtze platform the facies interpretation suggests three depositional environments 1 a deep water basin with a slope facies on its margins maoping and wuhe sections hubei province 2 intertidal or shoals environments subjected to wave energy yangjiaping and zhongling sections hunan province wengan section guizhou province 3 a shallow water subtidal shelf environment subjected to currents xikou section guizhou province during the doushantuo formation time period 635 ma 551 ma condon d zhu m y bowring s yang a jin y g 2005 u pb ages from the neoproterozoic doushantuo formation china science science 308 95 98 the shelf showed an irregular bathymetry interpreted as inherited from the rifting history of the yangtze platform following the break up of rodinia approx 700 ma the tilted blocks influenced the platform development and the facies distribution locally more than the doushantuo formation time period 84 ma has been necessary to compensate the variations of bathymetry inherited of the tilted blocks geometry

major factors controlling holocene δ13corg changes in a seasonal sea ice environment adelie land east antarctica 1 in an effort to investigate the controlling factors behind holocene 13corg changes in east antarctica we report high resolution down core records of bulk organic matter carbon isotopic ratios 13corg diatom census counts total organic carbon and biogenic silica content taken from one core recovered in the adelie trough a good correspondence between the 13corg record and records of small large and pennate centric ratios in many of the core segments indicates that diatom species composition affected holocene 13corg variations possibly via diatom shape and size effects variations in the surface water co2 concentration and in the isotopic composition of the source during the holocene cannot be ruled out although they cannot explain rapid and large amplitude 13corg changes within the limit of our investigation our results argue against active carbon acquisition through a carbon concentration mechanism as evidenced in low latitude upwelling systems the bulk organic matter thus represents a mixing of diatom taxa having different fractionation affinity to aqueous co2 our results confirm previous evidence which demonstrates that 13corg down core records should be used with great caution to reconstruct past co2 content in surface waters

zinc and strontium isotope evidence for climate cooling and constraints on the frasnian famennian 372 ma mass extinction abstract the frasnian famennian f f mass extinction is one of the big five biotic crises during the phanerozoic whereas the cause of this catastrophic event still remains debated here we present for the first time high resolution zn and sr isotope data obtained by a sequential leaching procedure for carbonate rocks across the f f boundary from the fuhe section in south china both 66zn and 87sr 86sr values rapidly increase in the lower and upper kellwasser horizons and palmatolepis delicatula platys to p minuta minuta conodont zones and the positive isotope shifts are exactly located in the same horizons characterized by low 13c values equivalent to low atmospheric pco2 level reflecting climate cooling and sea level fall stages the strong temporal coupling of 66zn values raise with elevated 87sr 86sr ratios is most likely to have been caused by input of isotopically heavy zn and radiogenic sr from carbonate dominated weathering and or uptake of isotopically light zn into primary producers during climate cooling climate cooling could cause relative sea level falls resulting in widespread exposure of carbonate platform enhance carbonate influx proportion through differential weathering of carbonate and silicate rocks and cause greater productivity by promoting oceanic thermohaline circulation and enhancing upwelling of bio limiting nutrients all of these processes would have contributed to the positive shifts of 66zn as observed this proposed scenario is in accordance with the fact that the f f mass extinction was almost limited to the shallow water tropical stenothermal faunas whereas the high latitude and deep water faunas were less affected our data support and represent the composite effect of a series of events linked to climate cooling which may have induced this catastrophic event

primary productivity off the coast of east antarctica 80 150 e january to march 1996 during february and march 1996 an interdisciplinary research project was undertaken off the coast of east antarctica south and southwest of australia from approximately 63 to 66 25 s and 80 to 150 e coastal continental shelf and open ocean waters were sampled encompassing the antarctic slope front asf and the antarctic divergence ad sea ice coverage during the previous winter exhibited a gradual decline in northwards extent from 80 e where its edge was located at 57 s to 150 e where it reached 62 63 s productivity versus irradiance p i experiments were conducted to calculate primary productivity and measurements of the photochemical quantum efficiency of photosynthesis fv fm were made using a fast repetition rate frr fluorometer phytoplankton pigment hplc and nutrient nitrate phosphate and silicate concentrations also were obtained mean chlorophyll a concentrations in the mixed layer were highest 1 g l 1 on the continental shelf along 93 5 e and in the shelf and shelf break waters of a meridional transect along 120 e integrated production over the mixed layer ranged from greater than 700 mg c m 2 d 1 slightly north of the ad along 80 e to less than 50 mg c m 2 d 1 in open ocean water at the northeastern corner of the survey area the elevated biomass and productivity west of approximately 120 e was attributed to macro and micro nutrient release and upper water column stratification following the spring summer ice melt that preceded and coincided with the sampling period macronutrient concentrations were not found to be limiting and in the upper 75 m of the water column ranged between 20 8 and 32 7 m for nitrate from 0 75 to 2 3 m for phosphate and from 22 6 to 74 9 m for silicate nutrient uptake rates inferred from depletion in the mixed layer indicated that silicate uptake was uncoupled from that of nitrate and phosphate in the northeastern region of the survey area the mixed layer deepened slightly and critical depths shoaled by a factor of three as the survey moved eastwards from 80 to 150 e a pattern most likely due to the onset of winter daily irradiance also decreased between january and march and manipulations of the light field used in the production modelling of the eastern transects resulted in productivity estimates similar to the western transects these observations together with the analysis of the frr data suggest that the phytoplankton community east of approximately 120 e may have been co limited by iron concentrations and incident irradiance the lower biomass and productivity in the east also coincided with greater southwards extension of warmer oceanic waters and a shift from a krill diatom to salp picoplankton biological community

holocene productivity changes off adelie land east antarctica 1 this study presents the first high resolution multiproxy investigation of primary productivity pp during the holocene from the antarctic continental margins micropaleontological and geochemical data from the sediment core md03 2601 associated to sea ice model outputs give unprecedented insights into the biological pump of the antarctic coastal area off adelie land in response to climatic changes plurimillennial and millennial changes of pp are observed in the study area in response to changes in nutrient availability stratification and growing season duration which are linked to sea ice upwelling wind and glacier dynamics the precessional cycle seems to be responsible in the pp long term variations while forcing factors involved at the millennial timescale remain more enigmatic our results emphasize enhanced biological pump during warmer and windier holocene phases because of a longer growing season and greater nutrient input antarctic coastal and continental shelf zones may therefore represent a more intense carbon sink in the future

ocean oxidation during the deposition of basal ediacaran doushantuo cap carbonates in the yangtze platform south china abstract precipitation of cap carbonate lithologies is a key feature of cryogenian global glaciations negative carbonate carbon isotopic compositions 13ccarb of these cap carbonates have been variably interpreted as massive drawdown of atmospheric co2 via extensive continental chemical weathering methane oxidation or postglacial upwelling each of these interpretations argues a non steady state of carbon cycle in the aftermath of marinoan global glaciation to further explore the postglacial marine carbon cycle we measured 13ccarb of cap carbonates from six localities in the yangtze platform south china the studied cap carbonates were deposited in a variety of sedimentary environments ranging from the open shelf slope to basin facies cap carbonates deposited in different environments show distinct stratigraphic trends of 13ccarb in the open shelf 13ccarb profile of the songlin section remains almost constant 3 to 4 while the 13ccarb of the jiulongwan section records a negative excursion decreasing from 3 5 to 7 13ccarb of cap carbonates deposited in the slope environment does not show stable stratigraphic trend in the basin environment 13ccarb demonstrates a sharp decline in the middle part of cap carbonates to the nadir value of 11 the negative 13ccarb excursion is best interpreted in terms of oxidation of dissolved organic carbon doc thus recording a pulse of ocean oxidation during cap carbonate precipitation clearly absence of negative 13ccarb excursion in all slope and most open shelf sections may imply that such oxidation event was not ubiquitous in the yangtze platform we speculate that the renewed thermohaline circulation during deglaciation brought oxic surface water into ocean interior which oxidized the basin environment of the yangtze platform however the deglacial thermohaline circulation was not strong enough to cause complete oxidation of the ocean the sporadic oxidation in the open shelf on the other hand might result from the terrestrial influx of oxidant from postglacial continental weathering our study suggests that ocean oxidation though sporadic might have occurred during cap carbonate precipitation and predated the first appearance of putative animal embryos

extreme ocean anoxia during the late cryogenian recorded in reefal carbonates of southern australia abstract the neoproterozoic was a time of great change in the earth s surface and marine environments including extensive climate variability the widespread oxygenation of the oceans and the accompanying rise of animal life however the timing of ocean oxygenation remains uncertain particularly in regard to cryogenian seas which were disrupted by large periods of global glaciation interglacial cryogenian reef complexes in the northern adelaide fold belt of australia contain abundant primary marine dolomite cements these cements have well preserved textural and growth zonation indicating they preserve their original marine chemistry and can be used as geochemical proxies for late cryogenian paleooceanography analysis of marine cements from peritidal nearshore facies shallow platformal facies and deep framework facies of the reef complexes reveals significant geochemical gradients with paleo depth while nearshore cements have low fe contents and commonly contain iron oxide inclusions the shallow and deep cements have very high fe concentrations chalcophile elements cu cd pb zn etc are most abundant in the nearshore cements whereas rare earth elements are found in highest concentrations in the deeper facies cements rare earth element profiles are unusual with shallow and deep facies having convex profiles with negligible ce ce anomalies and positive eu eu anomalies being constrained by sedimentology this carbonate geochemistry provides a window into interglacial cryogenian ocean chemistry and structure the marine cements reveal pronounced chemical stratification in this late cryogenian ocean a thin veneer of oxic surface waters existed at the ocean surface in peritidal facies with increasingly anoxic and fe rich seawater at depth the distribution of strongly chalcophile elements like cd and cu across the chemocline suggests that although ferruginous deeper anoxic waters probably contained some dissolved sulphide these conditions describe a ferro sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late precambrian a return to archean like ocean conditions at this time suggests large scale disruption of the ocean system during the neoproterozoic

dolomitization of a carbonate platform during late burial lower to middle cambrian shady dolomite virginia appalachians abstract outer platform carbonates of the lower to middle cambrian shady dolomite virginia appalachians record a lengthy and complex diagenetic history ranging from marine cementation to burial diagenesis during late paleozoic overthrusting analysis of petrographically well preserved marine cements has provided an estimate of the isotopic composition of lower cambrian marine carbonate 18o 7 5 to 6 1 pdb 13c 0 2 to 0 8 pdb 87sr 86sr 0 70869 to 0 70975 establishing a base line with which to assess later diagenetic carbonate meteoric dissolution and early calcite cement postdate marine cementation and formed during subaerial exposure the platform margin carbonates were extensively dolomitized dolomitization postdates calcite cementation and the initial development of stylolites and tectonic fractures several episodes of dolomite replacement and cementation are evident zone 1 dolomite consists of rare relict dolomite cores which were replaced and overgrown by zone 2a dolomite the dominant replacement phase zone 2a dolomite is depleted in 18o 18o 10 2 to 7 0 and enriched in 13c 13c 1 0 to 1 6 relative to marine carbonate reflecting precipitation from warm formation fluids buffered by the host carbonate uniformly low sr contents 25 to 65 ppm and nonradiogenic 87sr 86sr 0 70900 to 0 70971 also attest to pore water equilibration with the host rock replacement dolomitization was followed by widespread carbonate dissolution which formed vuggy porosity and local solution breccias the host dolomite locally is encrusted by mississippi valley type mvt occurrences of ore mineralization consisting of sphalerite galena and pyrite this major generation of ore minerals is overgrown by three generations of dolomite cement the first dolomite cement zone 2b has similar 18o composition 13o 10 3 to 7 4 but slightly depleted 13c values 13c 0 4 to 1 1 relative to zone 2a dolomite and has similar sr contents 30 to 45 ppm and 87sr 86sr ratios 0 70933 to 0 70966 zone 2b dolomite cement exhibits petrographic isotopic and geochemical affinities with zone 2a replacement dolomite implying that the two dolomite types formed from fluids of similar composition and temperature primary fluid inclusions in zone 2b dolomite indicate precipitation from warm minimum 120 to 150 c brines 23 26 wt nacl equiv these temperatures imply that the dolomitizing fluid was enriched in 18o due to extensive water rock interaction with host carbonate which also buffered the fluid 13c sr and 87sr 86sr content zones 3 and 4 dolomite cement are depleted in 18o 13o 13 8 to 11 3 relative to the previous dolomites implying a temperature increase to 150 to 210 c the late dolomite cements have 13c compositions 13c 0 7 to 0 9 ranging from those similar to the earlier dolomites to more depleted values zones 3 and 4 dolomite cement are enriched in sr 60 to 135 ppm which is radiogenic 87sr 86sr 0 71025 to 0 71445 these later cements thus record fluids that were no longer buffered by the host carbonate dolomite cements locally were overgrown by sphalerite followed by authigenic quartz and calcite cement secondary three phase fluid inclusions in quartz and calcite indicate hot 175 to 225 c saline 30 to 33 wt nacl equiv fluids temperatures and pressures interpreted from the fluid inclusions are compatible with estimated burial depths in excess of 5 km the dolomites formed during deep burial coeval with late paleozoic alleghanian age thrusting the mg required for burial dolomitization was derived from pressure solution of structurally lower platform interior dolomites that were overridden by the shady dolomite thrust sheet tectonic uplift and deformation resulted in regional fluid flow which transported this mg to the shady dolomite outer platform during burial dolomitization

the distribution and formative processes of latent heat polynyas in east antarctica a study of polynya characteristics in east antarctica was undertaken using ice concentrations derived from special sensor microwave imager data for the period july 1987 august 1994 the areas of polynyas were defined as having an ice concentration 75 the analysis revealed a total of 28 coastal polynyas within the study region the spatial and temporal variability in areal extent was quantified the timing of mean maximum areal extent varied from june to october the bathymetry and wind regime at each polynya site was examined to gauge the relative importance of these parameters in polynya formation and maintenance in 20 locations shallow banks and shoals form grounding zones for icebergs and anchoring sites for fast ice which form barriers to the predominantly westward drift of the pack ice elsewhere north south coastal protrusions or alignments form similar barriers the subsequent removal of newly formed sea ice from the lee of such barriers by katabatic and synoptic winds maintains areas of reduced ice concentration and open water very few coastal polynyas are attributed solely to katabatic outflow the combined influence of bathymetry topography and winds is responsible for the characteristics of the majority of polynyas many were considered to be marginal characterised by occasional periods when the ice concentration falls below 75 an analysis of annual winter totals of areas with ice concentrations 75 shows no trends in total polynya areal extent over the period 1987 94 known locations of emperor penguin aptenodytes forsteri rookeries were also found to be associated with the locations of coastal polynyas

zinc isotope variations in deep sea carbonates from the eastern equatorial pacific over the last 175 ka the carbonate fraction of sediment core odp 849 leg 138 located in the eastern equatorial pacific mostly consisting of coccoliths was separated and analyzed for its zn isotopic composition the overall variation in zn isotopic composition as determined by multiple collector magnetic sector inductively coupled plasma mass spectrometry was found to be on the order of 1 expressed in 66zn where xzn xzn 64zn sample xzn 64zn standard 1 103 and x 66 67 or 68 over the last 175 ka the analytical precision was 0 04 and the overall reproducibility was usually better than 0 07 the zn isotopic composition signal exhibits several marked peaks and a high frequency variability a periodogram of the 66zn signal showed two periodicities of 35 2 and 21 2 ka we suggest that the latter is caused by the precession of the earth s axis of rotation the periodogram exhibits a minimum at 41 1 ka thus showing that the zn isotopic composition is independent of the obliquity in the eastern equatorial pacific the range of 66zn values observed for the carbonate fraction of odp 849 overlaps with the range observed for fe mn nodules in the world s oceans which suggests that seawater carbonate zn isotope fractionation is weak we therefore assume that most of the zn isotope variability is a result of the selective entrainment of the light isotopes by organic matter in the surface ocean the odp 849 66zn record seems to follow the changes in the insolation cycles changes in the late summer fall equatorial insolation modulate the intensity of the equatorial upwelling hence the mixing between deep and surface waters we propose that during decreased summer fall equatorial insolation when a steep thermocline can develop el nino like conditions the surface waters cannot be replenished by deep waters and become depleted in the lighter zn isotopes by biological activity thus resulting in the progressive increase of the 66zn values of the carbonate shells presumably in equilibrium with surface seawater

zinc and cadmium stable isotopes in the geological record a case study from the post snowball earth nuccaleena cap dolostone abstract zinc and cadmium are important biologically cycled nutrients in the modern ocean recent analytical advances and sampling efforts such as geotraces have led to measurements of the global distribution of zn and cd and their stable isotope ratios 66zn and 114cd providing an understanding of how biological processes cycle these elements and their isotopes in the modern ocean this opens the door to better application of 66zn and 114cd as tracers of biogeochemical processes archived in the geological record here we present new measurements of cd and 114cd on samples from the nuccaleena cap dolostone in south australia which was deposited during deglaciation of the ca 635 ma old marinoan snowball earth our new data are combined with previously measured 66zn data from the same samples the combined record of 114cd and 66zn is used to explore various hypotheses about the sources and sinks of cd and zn in the post marinoan ocean key features of this record are that zn and cd remain relatively similar throughout that 66zn and 114cd change at the same stratigraphic level and in the same direction and that 66zn changes are about four times larger than 114cd changes even though both quantities reflect a similar fractional mass difference we suggest that both zn and cd concentration and isotopic data are consistent with a change in the sink by which zn and cd are removed from the oceans we propose that no isotope fractionation is expressed during removal of zn and cd as sulfides while zn and cd removed with biological material are about 0 6 and 0 2 lighter than deep ocean 66zn and 114cd respectively many previous studies attribute the burial of isotopically light biological zn to preferential assimilation of isotopically lighter zn by phytoplankton in contrast we propose that lighter biological 66zn occurs mostly because isotopically heavy zn was scavenged by organic matter which causes dissolved 66zn in the surface ocean to be lighter than deep ocean 66zn the organic matter originated from primary producers that thrived in the surface ocean during deglaciation for cd whose isotope cycling in the modern ocean is dominated by preferential assimilation of isotopically lighter cd by phytoplankton much less biological fractionation is expressed the combined analyses of 66zn and 114cd provide insight into the biogeochemical processes which occurred after the marinoan snowball earth glaciation and may be a useful new tool to explore the history of life on earth in other geological records as well

physical forcing of phytoplankton dynamics in the southwestern ross sea coastal zone color scanner czcs imagery of phytoplankton pigments and passive microwave imagery of sea ice distributions in the southwestern ross sea are presented for three different seasons 1978 1989 1979 1980 and 1981 1982 and were analyzed in conjunction with meteorological data obtained from a series of automatic weather stations aws dynamics of the phytoplankton bloom in terra nova bay differed from those in the ross sea owing to spatial differences in katabatic wind fields which determine when the surface waters stratify interannual variation in the timing of formation of the ross sea polynya appears to be controlled by winter temperatures which determine sea ice thickness and integrity rather than variability or intensity in wind stress together czcs aws and passive microwave data suggest that when the ross sea polynya forms early stronger and more frequent katabatic winds result in increased advective losses of phytoplankton in surface waters and a delay in the phytoplankton bloom if polynya formation is delayed until after the winds diminish in frequency the phytoplankton bloom will develop earlier the observation that diatoms dominate both the marginal ice zone and terra nova bay which are hydrographically similar while phaeocystis antarctica is found in unstable waters north of the ross ice shelf suggests that stratification plays an important role in determining species composition in the ross sea

climate mediated size variability of diatom fragilariopsis kerguelensis in the southern ocean abstract fragilariopsis kerguelensis o meara hustedt is the most abundant open ocean diatom species in southern ocean sediments and its average valve area has recently been used to infer glacial interglacial paleoceanographic conditions studies from the atlantic sector of the southern ocean demonstrated how larger average valve area of f kerguelensis during the last glacial compared to the interglacial possibly relate to greater availability of iron through wider sea ice coverage and higher eolian dust input we present here data on average valve area of f kerguelensis from three sediment cores covering the last 42 cal ka bp from different zones of the southern ocean our records confirm previous results from the atlantic sector but highlight a different pattern from the indian sector where the largest valves of f kerguelensis are encountered during the holocene fragilariopsis kerguelensis average valve area variations in the antarctic polar front apf of the atlantic sector and subantarctic front of the indian sector are in phase with records of opal burial while this correlation does not hold for the apf of the indian sector variations in circum polar upwelling were suggested as the main controlling factor of opal production during the last 20 000 years we here hypothesize that high nutrient input from the antarctic peninsula during the last deglaciation may have exerted a stronger control on f kerguelensis average valve area and opal export in the atlantic sector of the southern ocean than inferred changes in circum polar upwelling

sea ice diatom contributions to holocene nutrient utilization in east antarctica combined high resolution holocene 30sidiat and 13cdiat paleorecords are presented from theseasonal ice zone east antarctica both data sets re ect periods of increased nutrient utilization by diatomsduring the hypsithermal period circa 7800 to 3500 calendar years cal years b p coincident with a higherabundance of open water diatom species fragilariopsis kerguelensis increased biogenic silica productivity bsi and higher regional summer temperatures the neoglacial period after circa 3500 cal years b p isre ected by an increase in sea ice indicative species fragilariopsis curta and fragilariopsis cylindrus upto50 along with a decrease in bsi and 13cdiat 18 to 23 however over this period 30sidiatdata show an increasing trend to some of the highest values in the holocene record average of 0 43 competing hypotheses are discussed to account for the decoupling trend in utilization proxies including ironfertilization species dependent fractionation effects and diatom habitats based on mass balance calculations we highlight that diatom species derived from the semi enclosed sea ice environment may have a confoundingeffect upon 30sidowncorecompositions of the seasonal sea ice zone a diatom composition with approximately28 of biogenic silica derived from the sea ice environment diat si can account for the increased averagecompo sition of 30sidiatduring the neoglacial these data highlight the signi cant role sea ice diatoms can playwith relation to their export in sediment records which has implications on productivity reconstructions fromthe seasonal ice zone

the southern ocean at the last glacial maximum a strong sink for atmospheric carbon dioxide analysis of satellite ocean color sea surface temperature and sea ice cover data reveals consistent patterns between biological production iron availability and physical forcings in the southern ocean the consistency of these patterns in conjunction with information on physical conditions during the last glacial maximum lgm enables estimates of export production at the lgm the lgm southern ocean experienced increased wind speeds colder sea surface and atmospheric temperatures increased deposition of atmospheric dust and a greatly expanded winter sea ice cover these variations had strong effects on southern ocean ecology and on air sea fluxes of co2 the seasonal ice zone siz was much larger at the lgm 30 million km2 than at present 19 million km2 the antarctic polar front pf likely marked the northern boundary of this expanded siz throughout the southern ocean as it does today in the drake passage region a large northward shift in the position of the pf during glacial times is unlikely due to topographic constraints north of the pf the increased flux of aeolian dust during glacial times altered phytoplankton species composition and increased export production and as a result this region was a stronger sink for atmospheric co2 than in the modern ocean south of the pf interactions between the biota and sea ice strongly influence air sea gas exchange over seasonal timescales the combined influence of melting sea ice and increased aeolian dust flux with its associated iron increased both primary and export production by phytoplankton over daily monthly timescales during austral spring summer resulting in a strong flux of co2 into the ocean heavy ice cover would have minimized air sea gas exchange over much of the rest of the year thus an increased net flux of co2 into the ocean is likely during glacial times even in areas where annual primary production declined we estimate that export production in the southern ocean as a whole was increased by 2 9 3 6 gt c yr 1 at the lgm relative to the modern era altered seasonal sea ice dynamics would further increase the net flux of co2 into the ocean thus the southern ocean was a strong sink for atmospheric co2 and contributed substantially to the lowering of atmospheric co2 levels during the last ice age

abundance of zinc isotopes as a marine biogeochemical tracer 1 the zn isotopic compositions of ferromanganese nodules sediment trap samples sediments and organic reference samples have been analyzed by multiple collector inductively coupled plasma mass spectrometry icp ms the range of isotopic variations 1 for 66zn 64zn is significant with respect to the analytical precision 0 04 at the 95 confidence level marine argillaceous sediments show rather constant values consistent with those of continental shales and loess and with the value of a basalt from reunion island we infer that the depletion of the light zn isotopes in marine particles and in ferromanganese nodules presumably equilibrated with seawater results from biological activity in the upper water column the 66zn values of sediment trap samples collected near the upwelling off the coast of mauritania central atlantic show a seasonal isotopic fluctuation consistent with biological pumping during the high productivity period higher values of 66zn in nodules appear to be associated with the amplitude of seasonal variations rather than with the mean values of the biological productivity

historical variations in the isotopic composition of atmospheric zinc deposition from a zinc smelter abstract in order to test the usefulness of stable zinc isotopes as an atmospheric source tracer we analyzed the zinc isotopic composition of two sediment cores taken at 1 km distance of the former zinc smelter in lommel belgium the peat bog lake sediments accumulate mainly atmospheric particles have high organic matter contents 12 60 wt are anoxic and highly contaminated with heavy metals up to 4 7 wt zn and 1 1 wt pb with a sulfide mineralogical control on mobility down core variations in 66 zn relative to the jmc 3 0749l standard were small ranging from 0 07 to 0 39 but are nevertheless eight times the external reproducibility of 0 04 good agreement was found between the two cores and despite 30 zn mobilization to deeper layers no evidence of associated diagenetic zn isotope fractionation was found sediments deposited in the early 20th century have 66 zn of 0 30 0 05 2sd n 5 and a shift takes place between 1945 and 1950 to 66 zn values of 0 14 0 09 2sd n 7 in the 2nd half of the century in order to understand this shift in 66 zn we analyzed 32 ore grade sphalerite zns samples from african australian and european origin together with 29 published 66 zn values for ore grade zns we find remarkably homogeneous isotopic compositions which when averaged for mining location gives 66 zn of 0 16 0 20 2sd n 10 mines n 61 analyses early 20th century zn deposition with 66 zn of 0 30 is significantly different p 66 zn of 0 16 we suggest that this reflects the presence of an atmospheric smelting residue slag component enriched in the heavy zn isotopes due to rayleigh type fractionation during zn refining

inputs of glacially derived dissolved and colloidal iron to the coastal ocean and implications for primary productivity 1 glacial meltwaters draining shield bedrock under the greenland ice sheet gis contain 0 4 m total dissolved fe tdfe with an average flow weighted concentration of 53 nm the concentrations of 0 03 and 0 03 0 4 m fe vary over the ablation period with weighted means for each of these fractions being respectively 22 4 nm and 30 8 nm these concentrations are lower estimates as an adjacent larger glacier a more representative source of glacial meltwater had higher dissolved fe concentrations and reactions of meltwaters with proglacial sediments could also enhance dissolved fe concentrations this source of tdfe is additional to the reactive oxyhydr oxide phases identified by raiswell et al 2006 that are also introduced to adjacent polar seas from glaciers the fe concentrations in the shield bedrock underlying the gis are lower than those of other crustal rocks 4 0 cf 6 2 but we argue that these fe concentrations are not limiting on the total dissolved fe concentrations we measure the biogeochemical weathering processes operating on the subglacial debris and suspended sediment in our catchment are likely to be similar to those in other glaciated catchments therefore the meltwater fe concentrations reported here can be used to give a first estimate of global fluxes of meltwater dissolved fe to coastal polar waters a lower estimate of the global flux of tdfe from glacial meltwaters is 75 106 moles fe a this glacial meltwater input of fe to adjacent polar waters will be greatest around greenland where there are highest annual meltwater discharges however the greatest impact of this source of glacial meltwater fe is anticipated to be in antarctic high nutrient low chlorophyll hnlc waters where phytoplankton productivity is typically limited by availability of fe for antarctic waters the estimated meltwater fe tdfe input is about 10 of that suggested to come from sea ice melting but glacial inputs continue throughout the austral summer ablation period after sea ice melt is complete

authigenic cadmium enrichments in suboxic sediments precipitation and postdepositional mobility the postdepositional mobility of cd and u has been investigated in north atlantic turbidites in order to validate their utility as proxies of paleo organic carbon fluxes in this region pelagic sediments are interspersed with relatively organic rich turbidites following instantaneous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite thereby leading to redistribution of authigenic phases according to their affinity to different redox conditions authigenic cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portion than in the deeper unoxidized part of the turbidite this distribution indicates that cd has been continuously remobilized by the oxidation front thereby suggesting high sensitivity of the cd species to the redox state and an efficient immobilization mechanism for cd under reducing conditions we propose the precipitation of cds as a plausible mechanism for removal of cd in reducing sediments rather than adsorption onto mineral surfaces this proposal is consistent with porewater data indicating diffusion of cd toward and uptake by suboxic sediments solubility calculations suggest that precipitation of cds may occur in suboxic sediments in the presence of trace levels of h2s below routine detection limits there is no evidence for release of cd or u back into seawater upon oxidation of reducing sediments the strong coupling between precipitation of authigenic cd and u and the early diagenetic degradation of organic carbon offers the potential of reconstructing paleo productivity using records of cd and u however postdepositional redistribution of authigenic cd and u may significantly alter records of authigenic cd and u thereby rendering paleo productivity reconstructions uncertain we propose that the cd u authigenic ratio may provide a qualitative indicator to assess the relative role of steady state incorporation relative to transient burn down enrichments

iron in east antarctic snow implications for atmospheric iron deposition and algal production in antarctic waters to evaluate the deposition and solubility of aerosol iron in the antarctic seasonal sea ice zone ssiz iron was measured in snow samples collected from three areas in the ssiz prydz bay dumont d urville sea and ross sea and one continental area princess elizabeth land of east antarctica concentrations of total dissolvable iron that soluble at ph 2 ranged from 20 2950 pg g 1 with the lowest concentrations measured in snow from the dumont d urville sea using estimates of snow accumulation rates we calculate atmospheric iron deposition fluxes of 0 017 0 11 mg m 2 yr 1 0 30 2 0 mol m 2 yr 1 which are generally lower than previously published estimates measurements of iron in filtered meltwaters of snow samples from prydz bay and princess elizabeth land suggest that 10 90 of the total atmospheric iron is readily soluble assuming our results to be broadly representative of atmospheric deposition over seasonally ice covered high nutrient antarctic waters we use our mean estimates of atmospheric iron deposition 1 1 mol m 2 yr 1 and solubility 32 to calculate that atmospheric iron potentially supports annual phytoplankton production of 1 1 1012 mole c in the antarctic ssiz which is less than 5 of the estimated total annual primary production in this ocean region

the biogeochemical cycling of zinc and zinc isotopes in the north atlantic ocean zinc zn is a marine micronutrient with an overall oceanic distribution mirroring the major macronutrients especially silicate seawater zn isotope ratios 66zn are a relatively new oceanographic parameter which may offer insights into the biogeochemical cycling of zn to date the handful of published studies of seawater 66zn show the global deep ocean to be both remarkably homogeneous approximately 0 5 and isotopically heavier than the marine sources of zn 0 1 to 0 3 here we present the first high resolution oceanic section of 66zn from the u s geotraces ga03 north atlantic transect from lisbon to woods hole throughout the surface ocean biological uptake and release of isotopically light zn together with scavenging of heavier zn leads to large variability in 66zn in the ocean below 1000 m 66zn is generally homogeneous 0 50 0 14 2 sd though deviations from 0 5 allow us to identify specific sources of zn the mediterranean outflow is characterized by 66zn of 0 1 to 0 3 while margin sediments are a source of isotopically light zn 0 5 to 0 8 which we attribute to release of nonregenerated biogenic zn mid atlantic ridge hydrothermal vents are also a source of light zn close to 0 5 though zn is not transported far from the vents understanding the biogeochemical cycling of zn in the modern ocean begins to address the imbalance between the light 66zn signature of marine sources and the globally homogeneous deep oceans 66zn of 0 5 on long timescales with overall patterns pointing to sediments as an important sink for isotopically light zn throughout the oceans

geochemistry of ediacaran cap dolostones across the yangtze platform south china implications for diagenetic modification and seawater chemistry in the aftermath of the marinoan glaciation cap dolostones of the ediacaran doushantuo formation yangtze platform south china from various palaeo water depths were studied to evaluate the extent of their diagenetic alteration and to assess temporal and spatial variations of seawater chemistry in the aftermath of the marinoan glaciation diagenetic fluid overprint is common in cap dolostone lithologies however the mobilization of trace elements and the modification of sr and o isotopic compositions are variable and were controlled by multiple stages of fluid overprint the highest 87 sr 86 sr ratios up to 0 7246 occur in cap dolostones from the palaeo slope environment at the huanglianba section whereas cap dolostones deposited in platform settings and in one of the distal basinal settings reveal 87 sr 86 sr ratios close to the proposed late neoproterozoic seawater composition 0 7077 shale normalized ree y patterns of carbonate leachates display enrichments of heavy over light ree and superchondritic y ho ratios typical of seawater however y ho ratios in the cap dolostones are always lower than modern seawater values which is interpreted to reflect dilution of the seawater signal by continent derived meltwater influx during deglaciation negative ce anomalies in carbonate leachates from platform and slope sections suggest that oxidized conditions existed in shallow marine environments shortly after the marinoan glaciation whereas positive or no resolvable ce anomalies in basin settings indicate that the latter remained anoxic redox stratification of the yangtze margin at the beginning of the ediacaran is further supported by the relative enrichment of redox sensitive trace metals in basinal sections these data may indicate moderately anoxic less than 10 m dissolved o 2 and presumably manganous conditions during the deposition of cap dolostones in the deeper realms of the yangtze basin supplementary material supplementary data for this paper are available at https doi org 10 6084 m9 figshare c 3770957

bioavailability of zinc in marine systems through time zinc is a marine nutrient that may have been limited in the early oceans estimates of marine zinc availability through time suggest that values were instead near modern during the proterozoic eon

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

the oceanic mass balance of copper and zinc isotopes investigated by analysis of their inputs and outputs to ferromanganese oxide sediments abstract the oceanic biogeochemical cycles of the transition metals have been eliciting considerable attention for some time many of them have isotope systems that are fractionated by key biological and chemical processes so that significant information about such processes may be gleaned from them however for many of these nascent isotopic systems we currently know too little of their modern oceanic mass balance making the application of such systems to the past speculative at best here we investigate the biogeochemical cycling of copper cu and zinc zn isotopes in the ocean we present estimates for the isotopic composition of cu and zn inputs to the oceans based on new data presented here and published data the bulk isotopic composition of dissolved cu and zn in the oceans 65cu 0 9 66zn 0 5 is in both cases heavier than their respective inputs at around 65cu 0 6 and 66zn 0 3 respectively implying a marine process that fractionates them and a resulting isotopically light sedimentary output for the better known molybdenum isotope system this is achieved by sorption to fe mn oxides and this light isotopic composition is recorded in fe mn crusts hence we present isotopic data for cu and zn in three fe mn crusts from the major ocean basins which yield 65cu 0 44 0 23 mean and 2sd and 66zn 1 04 0 21 thus for cu isotopes output to particulate fe mn oxides can explain the heavy isotopic composition of the oceans while for zn it cannot the heavy zn in fe mn crusts and in all other authigenic marine sediments measured so far implies that a missing light sink is still to be located these observations are some of the first to place constraints on the modern oceanic mass balance of cu and zn isotopes

dissolved fe in the upper waters of the pacific sector of the southern ocean the concentration of dissolved fe was determined in upper ocean waters along 170 degreesw between 53 and 72 degreess during four cruises between october 1997 and march 1998 as part of the us jgofs southern ocean antarctic polar frontal zone cruises fe concentrations were extremely variable in both space and time as a result of advection of meso scale eddys through the region observed values ranged from a maximum of 0 34 nm at 64 degreess in november next to the retreating ice edge to 0 075 nm in march at 71 degreess in the northern ross sea gyre in general the highest average mixed layer fe concentrations and the largest seasonal changes were observed in the two frontal zones at similar to 60 degreess and similar to 64 degreess formation of deep mixed layers during winter and the entrainment of sub surface waters enriched in fe is the primary source of this element to surface waters in this region of the southern ocean additionally upwelling of circumpolar deep water in the circumpolar current along its northern boundary at the polar front and its southern boundary at the southern antarctic circumpolar current front is an important mechanism supplying fe to surface waters in these areas throughout the summer comparison of integrated fe concentrations and th derived poc export and n drawdown all suggest that the maximum changes in these properties are comparable to those that would be predicted by fe availability fe concentrations determined by shipboard flow injection analysis measures et al mar chem 50 1995 3 were significantly higher than those found in replicate samples determined by shored based flameless atomic absorption spectrophotometry after preconcentration by chelation solvent extraction johnson et al mar chem 57 1997 137 the concentration differences which appear to be restricted to the upper 200 300 m do not appear to arise from different sampling or handling processes but instead are believed to result from the different sensitivity of the methods to organic fractions of fe in seawater c 2001 elsevier science ltd all rights reserved

zn isotope evidence for immediate resumption of primary productivity after snowball earth the ediacaran period began with the deglaciation of the ca 635 ma marinoan snowball earth and the deposition of cap dolostones on continental shelves worldwide during post glacial sea level rise these carbonates sharply overlie glacial sediments deposited at low paleolatitudes and preserve negative carbon isotope excursions the snowball earth hypothesis invokes an almost complete cessation of primary productivity in the surface ocean because assimilatory uptake of zn appears to fractionate its isotopes zn isotope ratios measured in carbonate precipitated in the surface ocean should track fluctuations in primary productivity here we report the first zn isotopic data together with carbon and oxygen isotopic profiles from a neoproterozoic cap dolostone the nuccaleena formation in the flinders ranges south australia we interpret the zn isotopic data in terms of a two stage evolution of the deglacial ocean slightly 66 zn enriched values at the base of the cap dolostone indicate immediate resumption of the biological pump upon melting of the surface ocean but this signal was diluted by intense surface runoff that drove 66 zn 66 zn 64 zn versus the jmc lyon reference values down to the composition of continentally derived zn a subsequent rise in 66 zn records a vigorous increase in primary production and export from a nutrient laden surface ocean

summer plankton production and nutrient consumption patterns in the mertz glacier region of east antarctica abstract we report on a biological and nutrient survey conducted off the eastern adelie and george v coasts of east antarctica in late december 2000 and january 2001 the region was dominated by the mertz glacier tongue mgt and its associated polynya that controlled the amount of open water through the season and influenced stratification near forming and melting ice as in other regions of the southern ocean large concentrations of nitrate phosphate and silicic acid were found chlorophyll levels varied from 0 3 mg m 3 in pack ice and slope regions to greater than 10 mg m 3 in a phaeocystis bloom encountered among grounded ice northeast of the mgt net utilization rates of nutrients were derived from the deficit between measured concentrations and estimated winter values nitrate n n and silicic acid utilization rates ranged from 2 to 12 and 14 to 30 mmol m 2 d 1 respectively and were greatest in commonwealth bay the ratio of nitrate to phosphate utilization was low 10 lower silicic acid to n n ratios were found in regions with substantial populations of phaeocystis sp in the eastern grounded ice region however there was significant variation in the silicic acid to n n utilization that may be induced by mixed layer iron levels that varied from

iron and silicic acid concentrations regulate si uptake north and south of the polar frontal zone in the pacific sector of the southern ocean abstract we investigated the relative roles of fe and silicic acid availabilities in regulating si uptake rates across the polar frontal zone in the pacific sector of the southern ocean 59 68 s 170 w during the us jgofs antarctic environment southern ocean process study aesops meridional gradients in silicic acid concentration si oh 4 of about 0 25 0 56 m km 1 were observed in this area during austral spring and summer 1997 1998 with si oh 4 ranging from 32 si oh 4 and 15 no 3 uptake rates biogenic silica concentrations and si oh 4 no3 uptake ratios fe addition had little or no effect on si uptake rates in enrichments conducted in the low si waters north of the si gradient however fe addition increased si uptake rates 3 5 times over controls in enrichments conducted in the high si waters south of the gradient in both spring and summer fe addition decreased si oh 4 no3 uptake ratios by 2 5 times largely due to stimulation of no3 uptake rates zn addition had no effect on si oh 4 and no3 uptake rates short term 24 h si additions had varying effects on si uptake rates depending on season and location in spring additions of 40 m si to water from bottle enrichments conducted north of the si gradient in situ si oh 4 15 m did not increase si uptake rates initially but did increase uptake rates after 8 days in the summer enrichment north of the si gradient in situ si oh 4 5 m 50 m si additions doubled in situ si uptake rates in the initial water collected for the enrichment and increased si uptake rates as much as 16 fold during the experiment south of the si gradient where in situ si oh 4 was 40 m in both spring and summer si addition had no effect on in situ si uptake rates in the initial enrichment water nor on any si uptake rates measured during the experiment our results indicate that both fe and si availabilities regulate si uptake rates and silica production in the southern ocean along 170 w fe limitation appears to restrict si uptake rates south of the si gradient and plays a role in preventing si depletion south of the acc where ambient si oh 4 never fell below 40 m during 1997 1998 our experiments in the seasonal ice zone at 62 suggest that si uptake in this area switched from being fe limited in the spring when in situ si oh 4 was 40 m to si limited in the summer when si oh 4 was

carbonate chemistry in the mertz polynya east antarctica biological and physical modification of dense water outflows and the export of anthropogenic co2 1 dense shelf water dsw is formed in east antarctica from enhanced sea ice production driven by air sea interaction in coastal polynyas cross shelf export and downslope mixing of this dsw produces antarctic bottom water contributing to the lower limb of the global overturning circulation we present biogeochemical observations from the mertz polynya region in summer 2007 2008 with additional observations from spring 2001 and winter 1996 the seasonal changes in mixed layer carbonate chemistry are driven by a combination of air sea co2 exchange biological activity and the formation and melt of sea ice the air sea fluxes in 2008 were 15 mmol c m 2 d 1 and net community production estimated from the summertime surface dissolved inorganic carbon dic deficit ranged from 0 9 to 1 2 mol c m 2 yr 1 we show that biological modification of carbonate chemistry over the shelf in summer preconditions the dsw outflows from the adelie depression this process appears to supply both organic material and water depleted in co2 and with enhanced carbonate saturation state relative to inflowing water to coral communities on the slope we combined model based transports of exported dsw with the natural and anthropogenic cant carbon concentrations and estimate that the annual outflows of dic and cant from the mertz polynya range from 320 to 560 tg c yr 1 and from 3 to 6 tg cant yr 1 respectively the formation and export of dense water from this region and by extension all similar polynyas around antarctica is an effective mechanism for the transfer of anthropogenic carbon into the deep ocean

oxygenation of a cryogenian ocean nanhua basin south china revealed by pyrite fe isotope compositions abstract the nature of ocean redox chemistry between the cryogenian sturtian and marinoan glaciations ca 663 654 ma is important for understanding the relationship between environmental conditions and the subsequent emergence and expansion of early animals the cryogenian to ediacaran stratigraphic succession of the nanhua basin in south china provides a nearly complete sedimentary record of the cryogenian including a continuous record of interglacial sedimentation here we present a high resolution pyrite fe isotope record for a 120 m long drill core zk105 through sturtian glacial diamictites and the overlying interglacial sediments in the nanhua basin to explore changes in marine chemistry during the late cryogenian our pyrite fe isotope profile exhibits significant stratigraphic variation interval i comprising middle to upper tiesi ao diamictites correlative with the sturtian glaciation is characterized by light modern seawater like fe isotope compositions interval ii comprising uppermost tiesi ao diamictites and the basal organic rich datangpo formation is characterized by an abrupt shift to heavier fe isotope compositions and interval iii comprising organic poor grey shales in the middle datangpo formation is characterized by the return of lighter seawater like fe isotope compositions we infer that interval i pyrite was deposited in a predominantly anoxic glacial nanhua basin through reaction of dissolved fe2 and h2s mediated by microbial sulfate reduction msr the shift to heavier pyrite fe isotope values in interval ii is interpreted as partial oxidation of ferrous iron to ferric iron and subsequent near quantitative reduction and transformation of fe oxyhydroxides to pyrite through coupling with oxidation of organic matter in the local diagenetic environment in interval iii near quantitative oxidation of ferrous iron to fe oxyhydroxides followed by near quantitative reduction and conversion to pyrite in the local diagenetic environment was likely responsible for the return of lighter seawater like fe isotope compositions in pyrite our pyrite fe isotope profile thus records increased oxygenation in the nanhua basin between the sturtian and marinoan glaciations the increased oxygenation of nanhua basin seawater deduced from pyrite fe isotopes could have resulted from either local or global controls further work will be needed to determine whether this increasing oxygenation extended to the global scale

natural radioactivity and formation of oxygen in earth s atmosphere decay of radioactive 40k and radiolysis of ocean water abstract the important role of radioactivity in the conversion of primary hydro and atmosphere is substantiated considering the decay of 40 k isotope in seawater by now the total energy resulting from the decay of 40 k in the ocean amounts to 6 3 1026 j the calculations show that the concentration of oxygen formed over the last 3 8 ga is approximately 8 1017 kg i e it is of the same order of magnitude as the content of oxygen in the modern atmosphere the ocean was an intermediate reservoir in which oxygen was formed and which was purified from toxic components the radiation provides the threshold concentrations of oxygen urey pasteur and berkner marshall points important for the development of oxygen breathing life the radical and ionic products of water radiolysis induced chemical transformations of the compounds dissolved in the ocean the calculations show that their total concentration over a period of 3 8 ga was approximately 0 3 mol l 1 seawater radiolysis resulted in water purification saturation with oxygen and formation of simple organic molecules

phytoplankton biomass and photosynthetic competency in the summertime mertz glacier region of east antarctica abstract vertical profiles of water temperature salinity beam transmission density pressure wind speed wind direction phytoplankton biomass chlorophyll a chl a plus phaeophytin a and photosynthetic competency by fast repetition rate fluorometry are presented for the mertz glacier region east antarctica for a 3 week period during the austral summer 2000 2001 injection of low salinity water from the melting of the ice pack formed a shallow ca 25 m mixed layer offshore two distinct deep mixing features were observed associated to varying degrees with high chlorophyll levels and high photosynthetic competency along the adelie land coast a deep mixing layer 1400 m with chl a concentrations of 386 mg m 2 and elevated fv fm 0 5 was observed at the eastern end of the study area along a seaward extension of fast ice a bloom of phaeocystis antarctica bloom formed within a shallow 24 m mixed layer with surface chl a concentrations of ca 8 mg m 3 and elevated fv fm 0 5 this feature also had high surface salinity 34 and was contiguous with a deep mixing feature 236 m frrf parameters fv fm and psii were strongly suppressed in surface waters

zinc isotope and transition element dynamics accompanying hydrozincite biomineralization in the rio naracauli sardinia italy abstract the rio naracauli in sw sardinia drains part of the ingurtosu zn pb mining district and contains extreme concentrations of dissolved zn at near neutral ph in the upper reaches of the stream ph alkalinity and zn concentrations are such that hydrozincite zn 5 co 3 2 oh 6 precipitates in a biologically mediated process facilitated by a microalga chlorella sp and a cyanobacterium scytonema sp values of 66 zn in water and solid samples ranged from 0 35 to 0 5 relative to the jmc 3 0749 lyon standard and closely follow a mass dependent fractionation line two composite samples of sphalerite the primary ore mineral in the ingurtosu deposits had an average 66 zn of 0 15 similar to sphalerite measured elsewhere in hydrothermal mineral deposits zinc isotope measurements of the stream water and the hydrozincite forming in the stream show a consistent preference for the heavy isotope 66 zn in the hydrozincite relative to 64 zn synthetic hydrozincites produced without added bacteria have 66 zn identical to the dissolved zn thus suggesting a biologically mediated mineralization process in rio naracauli the average fractionation hdz water is 0 35 the magnitude of which is consistent with other studies and suggests an extracellular mechanism of the biomineralization process zinc concentration and dissolved 66 zn steadily decrease in the reach of the stream where the biomineralization occurs the biomineralization process also leads to the sequestration of pb cu and ni in the hydrozincite lattice and the coeval precipitation of an amorphous cdco 3 solid prompting the suggestion that if optimized the biomineralization process might represent a feasible passive remediation strategy for streams with high zn and other metals and with near neutral ph

the behaviour of cu and zn isotopes during soil development controls on the dissolved load of rivers abstract the stable isotopes of copper cu and zinc zn are finding increasing applications in surface earth geochemistry an important early observation is that aqueous pools of cu rivers and oceans are isotopically heavier than rocks of the continental crust for zn the global average 66 zn for rivers is identical to estimates of the upper continental crust but the oceans are slightly heavier here we study cu and zn abundances and isotopes in soils to assess how processes in the weathering environment control isotopic values of rivers and ultimately the oceans we investigated a range of soils developed on granitoid scotland and basaltic hawaiian islands rock substrates representing soil development times from 0 1 to 400 kyr mean annual temperatures of 6 16 c and mean annual precipitation map of 1060 5050 mm soils developed on the island of maui over 400 kyr and under high rainfall show the clearest chemical weathering signatures here in oxic soils cu and zn are both depleted by 50 90 with preferential loss of heavy cu 65 cu up to 1 2 lower than parent material and small shifts in 66 zn we attribute this to equilibrium fractionation between aqueous organic species that are removed from the soil versus sorption to residual fe oxyhydroxides in soils that are 0 1 150 kyr old in scotland and hawaii this same pattern is overprinted by biological cycling and the addition of mineral aerosol cu and zn with heavier isotope compositions in reducing soils on maui developed under the highest rainfall cu and zn are almost completely stripped from the soil leaving a very small residual pool with 65 cu and 66 zn values that range from those for the oxic soils back to and beyond parent material this removal of the light sorbed cu seen in oxic soils is attributed to the dissolution of fe oxides under anaerobic conditions perhaps microbially mediated leaving a very small residual isotopically heavier pool overall these data suggest that processes in soils create significant variability in cu isotopes with the heavy isotope exported from oxic soils in contrast small isotope fractionations of zn are generated in the soil environment these observations are consistent with those made previously for cu and zn isotopes in rivers but the maximum amount of cu and zn stored by world soils equates to only 30 130 years of the dissolved riverine flux soils cannot be the ultimate repository of the residual cu and zn required balancing the riverine flux and this must ultimately reside in the erosive products of soil formation in continental and ocean margin sediments

biogeochemical cycling of zinc and its isotopes in the southern ocean abstract we report zn concentration and isotope data for seawater samples from the atlantic sector of the southern ocean collected during the ipy geotraces ant xxiv iii cruise along the greenwich zero meridian data are reported for the full depth range of the water column at three stations as well as a transect of surface samples using a new analytical approach that is presented in detail here zn concentrations increase with depth though due to proximity to upwelling sites surface concentrations are not as low as in some parts of the ocean such as further northward into the sub antarctic zone for two depth profiles south of the polar front zone the physical stratification of the upper water column is reflected in sudden near surface changes in zn concentration with depth in contrast beneath 100 300 m zn concentrations barely change with depth zn isotopic data beneath 1000 m for the southern ocean data presented here as well as published data from the north atlantic and north pacific are strikingly homogeneous with an average 66zn 0 53 0 14 2sd 2se 0 03 n 21 the surface southern ocean is more variable with 66zn ranging from 0 07 to 0 80 between the two is a thin horizon at 40 80 m which in the southern ocean as well as the north atlantic and north pacific is characterised by distinctly light isotopic signatures with 66zn about 0 3 lower than surface waters strong correlations between si and zn concentrations seen here and elsewhere coupled to the lack of any systematic relationship between si and zn isotopes in the southern ocean suggest that the removal of zn associated with diatom opal involves little or no isotopic fractionation regeneration of this zn also explains the homogeneous zn isotopic composition of the global deep ocean so far sampled however the low zn content of opal requires that deep ocean zn does not directly come from the opal phase itself but rather from associated organic material external to the diatom frustule during growth experimental data are consistent with little or no fractionation during incorporation of zn into this material on the other hand the light zinc at 40 80 m is most consistent with the regeneration of an intra cellular pool that both culturing experiments and field data suggest will be isotopically light the data thus imply two processes by which zn is taken up in the surface ocean that these pools have very different regeneration lengthscales and that physical mixing of the oceans cannot eradicate their isotopic signatures finally the deep 66zn ocean value is significantly higher than the current best estimate of the input to the oceans the most obvious candidate for the required light sink is the survival of some of the cellular zn to be buried in sediment

late summer biogeochemistry in the mertz polynya east antarctica a marked reconfiguration of the mertz polynya following the 2010 calving of the mertz glacier tongue has been associated with a decrease in the size and activity of the polynya we report observations of the oceanic carbonate co 2 system in late summer 2013 the third post calving summer season estimates of seasonal net community production ncp based on inorganic carbon deficits and the oxygen argon ratio indicate that the waters on the shelf to the east of commonwealth bay adjacent to the mertz glacier remain productive compared to pre calving conditions the input of residual or excess alkalinity from melting sea ice is found to contribute to the seasonal enhancement of carbonate saturation state and ph in shelf waters mean rates of ncp in 2012 2013 are more than twice as large as those observed in the pre calving summers of 2001 and 2008 and suggest that the new post calving configuration of the polynya favors enhanced net community production and a stronger surface ocean sink for atmospheric co 2 due at least in part to the redistribution of sea ice and associated changes in summer surface stratification

the influence of an alongshelf current on the formation and offshore transport of dense water from a coastal polynya the influence of an ambient alongshelf current on the formation and offshore transport of dense water from an idealized coastal polynya is examined by using a primitive equation numerical model brine rejection during ice formation within the polynya is modeled by a prescribed surface buoyancy flux over a limited region along a straight coastline a uniform alongshelf current is imposed at one end of the model domain the basic ocean response is unchanged by the ambient current i e an unstable density front forms around the polynya perimeter and breaks up into small scale 15 30 km eddies that carry the dense water away from the polynya the ambient current continuously carries water out of the polynya region thereby both increasing the alongshelf density flux and reducing the typical density anomaly generated the total volume of dense water formed is not appreciably altered the influence of a submarine canyon located downstream of the polynya in the direction of the imposed alongshelf current is examined almost no dense water enters a canyon oriented normally or diagonally to the coast because the ambient flow tends to follow isobaths and carries virtually all of the dense water around and past the canyon an appreciable fraction of densified water may flow down a canyon oriented parallel to the coast the details depending on the canyon geometry and the strength of the ambient current thus both the density anomalies produced by coastal polynyas and the pathways of dense water transport are sensitive to ambient shelf currents potentially providing a source of interannual variability in dense water formation and export from the shelf

zinc isotope evidence for intensive magmatism immediately before the end permian mass extinction the end permian extinction is typically ascribed to massive volcanic eruptions but direct geochemical evidence linking the two independent events is generally lacking zinc is an important micronutrient of marine phytoplanktons and zn isotope 66zn ratios of seawater are markedly higher than those of volcanic rocks and riverine waters we conducted high resolution zn concentration and zn isotope analyses on carbonate rocks across the permian triassic boundary ptb in the meishan section of south china an abrupt increase of zn concentration and a concomitant 0 5 decrease in 66zn occur 35 k y before the mass extinction and carbon isotope 13c minima mass balance calculation demonstrates that a 0 5 negative shift in 66zn within thousands of years requires rapid and massive input of isotopically light zn from volcanic ashes hydrothermal inputs and or extremely fast weathering of large igneous provinces a positive 66zn shift of as much as 1 0 following the mass extinction demonstrates that primary productivity recovered and reached a maximum in fewer than 360 k y our finding provides insights into the marine zn cycling across the ptb and clarifies the temporal relationship and duration of events including intensive volcanism carbon isotope excursion mass extinction and widespread ocean anoxia

the tonian beck spring dolomite marine dolomitization in a shallow anoxic sea abstract the reason for the abundance of dolomite lithologies in earth s early geological record compared to modern environments remains contentious this study provides new insight into this precambrian dolomite problem by revisiting one of the most controversial dolomite localities the beck spring dolomite of death valley usa consistent with some previous studies petrographic evidence indicates that although the beck spring dolomite now consists almost entirely of dolomite it was originally precipitated largely as aragonite and high mg calcite depositional constituents microbialites and ooids were likely originally aragonitic and early marine length fast cements now dolomite are suggested to have precipitated as high mg calcite then replaced syntaxially by dolomite based on petrographic and geochemical evidence we suggest that marine dolomitization was the dominant synsedimentary diagenetic process in the unit and for the most part involved syntaxial and mimetic replacement further a length slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement this is indicated by the length slow crystallographic structure of the cement and from its preserved geochemical and cathodoluminescence growth zonation this new evidence for tonian marine dolomite precipitation reinforces the idea of precambrian marine environmental conditions including the chemical composition of seawater promoting dolomite formation at this time the trace metal geochemical composition of well preserved marine components especially dolomite marine cements reveals information about redox conditions in this tonian shallow seawater in terms of rare earth element geochemistry the beck spring dolomite has no significant ce anomaly and a ubiquitous positive eu anomaly consistent with widespread oceanic anoxia during deposition furthermore the relatively low levels of iron and chalcophile elements co cu pb and zn in marine components compared to other neoproterozoic carbonates suggest euxinic conditions both anoxic and sulphidic prevailed during deposition of the beck spring dolomite

trace elements at the intersection of marine biological and geochemical evolution life requires a wide variety of bioessential trace elements to act as structural components and reactive centers in metalloenzymes these requirements differ between organisms and have evolved over geological time likely guided in some part by environmental conditions until recently most of what was understood regarding trace element concentrations in the precambrian oceans was inferred by extrapolation geochemical modeling and or genomic studies however in the past decade the increasing availability of trace element and isotopic data for sedimentary rocks of all ages has yielded new and potentially more direct insights into secular changes in seawater composition and ultimately the evolution of the marine biosphere compiled records of many bioessential trace elements including ni mo p zn co cr se and i provide new insight into how trace element abundance in earth s ancient oceans may have been linked to biological evolution several of these trace elements display redox sensitive behavior while others are redox sensitive but not bioessential e g cr u their temporal trends in sedimentary archives provide useful constraints on changes in atmosphere ocean redox conditions that are linked to biological evolution for example the activity of oxygen producing photosynthetic cyanobacteria in this review we summarize available precambrian trace element proxy data and discuss how temporal trends in the seawater concentrations of specific trace elements may be linked to the evolution of both simple and complex life we also examine several biologically relevant and or redox sensitive trace elements that have yet to be fully examined in the sedimentary rock record e g cu cd w and suggest several directions for future studies

an organodiagenetic model for marinoan age cap carbonates abstract the nuccaleena formation cap carbonate is the global stratotype for the marinoan glaciation and ediacaran gssp a designation that emphasises the importance of cap carbonates to our current understanding of global neoproterozoic biogeochemistry sedimentology and stratigraphy however to date there is no agreed depositional model for cap carbonates and there remains minimal detailed paragenetic data for developing process based depositional models here an early diagenetic organodiagenetic dolomite cementation model for marinoan cap carbonate formation is hypothesised and explored it is demonstrated how this process based model can explain the three main atypical sedimentary structures that are the basis for correlation of marinoan age cap carbonates giant tepee like structures sheet veins and tubestones these features are interpreted to be products of fluid overpressure deformation induced by organodiagenetic expansive cementation giant tepee like structures the hallmark of marinoan cap carbonates conform to a simple structural analysis that is consistent with the predicted stress field induced by expansive cementation driven fluid overpressure the undeformed to highly deformed gradient of deformation features in cap carbonates is consistent with the range of rheologies in a shallowly buried cementing carbonate modification of matrix textures and bedding surfaces also conforms to the range of expected textures within this model overall it is hypothesised that the aggradational condensed section architecture of cap carbonates was the primary control over the generation of these atypical sedimentary features the available paragenetic analysis suggests that the geochemical data particularly the c isotopic data can be reinterpreted as supportive of the organodiagenetic model and that the neoproterozoic ocean was perhaps similar to that of today the proposed model indicates that the current criteria for the placement of the ediacaran gssp are non unique and potentially non isochronous perhaps the most important aspect of this model is that it is testable and is a call for focused research on the much overlooked paragenesis of cap carbonates

globally synchronous marinoan deglaciation indicated by u pb geochronology of the cottons breccia tasmania australia u pb zircon data from the uppermost cottons breccia representing the marinoan glacial postglacial transition on king island tasmania provide the first direct age constraint on the cryogenian ediacaran boundary in australia zircons in four samples from the topmost meter of the cottons breccia dated by sensitive high resolution ion microprobe exhibit two modes ca 660 ma and ca 635 ma the younger component predominates in the uppermost sample a possibly volcanolithic dolomitic sandstone apparently lacking glacially transported debris in the transition to cap carbonate chemical abrasion thermal ionization mass spectrometry ca tims u pb dating of euhedral zircons from that sample yields a weighted mean age of 636 41 0 45 ma equivalence to published tims ash bed dates from cryogenian ediacaran transitional strata in namibia 635 51 0 82 ma within glacial deposit and china 635 23 0 84 ma 2 m above glacial deposit supports correlation of those strata to the australian type sections and globally synchronous deglaciation at the end of the cryogenian period

early season depletion of dissolved iron in the ross sea polynya implications for iron dynamics on the antarctic continental shelf 1 the ross sea polynya is among the most productive regions in the southern ocean and may constitute a significant oceanic co2 sink based on results from several field studies this region has been considered seasonally iron limited whereby a winter reserve of dissolved iron dfe is progressively depleted during the growing season to low concentrations 0 1 nm that limit phytoplankton growth in the austral summer december february here we report new iron data for the ross sea polynya during austral summer 2005 2006 27 december 22 january and the following austral spring 2006 16 november 3 december the summer 2005 2006 data show generally low dfe concentrations in polynya surface waters 0 10 0 05 nm in upper 40 m n 175 consistent with previous observations surprisingly our spring 2006 data reveal similar low surface dfe concentrations in the polynya 0 06 0 04 nm in upper 40 m n 69 in association with relatively high rates of primary production 170 260 mmol c m 2 d 1 these results indicate that the winter reserve dfe may be consumed relatively early in the growing season such that polynya surface waters can become iron limited as early as november i e the seasonal depletion of dfe is not necessarily gradual satellite observations reveal significant biomass accumulation in the polynya during summer 2006 2007 implying significant sources of new dfe to surface waters during this period possible sources of this new dfe include episodic vertical exchange lateral advection aerosol input and reductive dissolution of particulate iron

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

post glacial seasonal diatom record of the mertz glacier polynya east antarctica an ultra high resolution post glacial laminated sediment record from mertz ninnis trough east antarctic margin eam has been analysed using sem backscattered electron imagery secondary electron imagery and quantitative diatom abundance laminations are classified using visually dominant diatom species and terrigenous content four biogenic diatom ooze laminae types one diatom bearing terrigenous lamina type and one diatom bearing terrigenous sub lamina type have been identified diatom ooze lamina types comprise near monogeneric hyalochaete chaetoceros spp resting spore laminae laminae characterised by corethron pennatum laminae characterised by rhizosolenia spp and mixed diatom assemblage laminae diatom bearing terrigenous lamina and sub lamina types comprise mixed diatom assemblage terrigenous laminae and sub laminae characterised by porosira glacialis resting spores formation of each of these lamina types is controlled by seasonal changes in nutrients oceanographic regimes and the mertz glacier polynya dynamics

the effects of diagenesis on geochemical paleoredox proxies in sedimentary carbonates abstract metal and metal isotope records in carbonates have the potential to provide novel insights into ancient ocean atmosphere redox conditions paleoenvironmental conditions and biogeochemical cycling however trace element geochemical signatures in carbonates can record either diagenetic or depositional signatures here we explore the variability in uranium isotopes trace metal and rare earth element yttrium rey concentrations in carbonate successions that have undergone several common types of diagenetic alteration case studies include the cryogenian balcanoona reef australia marine dolomitization neomorphism the devonian canning basin reefs australia burial dolomitization karstification the paleozoic of the great basin usa high temperature and regional burial dolomitization and the carboniferous waulsortian limestone ireland pb zn mineralization in all of the examined cases there are significant heavier and or lighter shifts in u isotope values between the most petrographically pristine marine depositional components and altered or late stage diagenetic phases although we also found that rey patterns can be overprinted during diagenesis normalized rey profiles appear to be commonly retained through diagenetic recrystallization consistent with previous studies the direction and magnitude of the change in metal isotope systems during diagenesis cannot be generalized between case studies consistent with carbonate alteration being controlled by a wide range of factors including the composition and source of the alteration fluids in this light we build on framework developed from traditional isotope systems and support the view that future work on metal and metal isotope compositions of carbonates should include integrated sedimentological and petrographic analysis petrographic work not only represents a screening procedure for sedimentary geochemical work but can also significantly enhance the paleo environmental interpretation of metal isotope data

sediment delivery and depositional patterns off adelie land east antarctica in relation to late quaternary climatic cycles article i nfo this study focuses on the understanding of depositional patterns over late quaternary glacial interglacial cycles and of the influence of diagenesis on geochemical sedimentary records from the continental margin off adelie land east antarctica we provide a complete analysis of the 30 m long sediment core md03 2603 collected on a deep sea sediment mound deposit the lr04 benthic oxygen isotope stack vs down core records of ba al and ba ti atomic ratios reflectance data and micropaleontological evidence allow to determine a detailed chronology back to 490 ka bp long lived uranium and thorium 238 u 234 u 232 th and 230 th provide an assessment of lateral vs vertical sediment advection by constraining the sediment focusing factor is high in most glacial intervals when mostly detrital material was transported down slope and focused to the core area interglacials display highs in ba al and ba ti coupled to low values related to stronger contour current flow biogenic fractions are more abundant during interglacials and more diluted by detrital fractions during glacials the diatom assemblage is dominated by the open ocean species fragilariopsis kerguelensis but glacial intervals display an increase of sea ice related and coastal species the integrated approach high resolution record of major and minor elements uauth 15 n norg 13 c and corg provides key insights into how redox conditions affect glacial interglacial sedimentation processes our evidence shows that diagenetic effects due to development of reducing conditions and consequent higher preservation of organic matter may overprint the paleoclimatic significance of some proxies involved into a sequence of diagenetic remineralization pathways comparison of the marine record to ice core data epica dome c edc allows to extend the information to the whole east antarctic margin sediment proxies of increased burial of biogenic material or increased preservation of organic matter are linked to climatic amelioration on the continent highs in edc dust flux correlate with high mn al and mo al ratios within the sediment record confirming that establishment of colder conditions on the continent triggered changes in sedimentation rates at the glacial interglacial transition which in turn affected the development of reducing redox fronts within the sediment

marine controls on modern sedimentation on the antarctic continental shelf abstract the extreme depth and rugged topography of the antarctic continental shelf its seasonal to perennial sea ice cover and its large slowly calving ice shelves exert major influences on the adjacent ocean and sediments regionally variable exchanges of heat and momentum between the atmosphere and ocean result in net off shelf transport of sea ice and net downward salt flux this buoyancy partitioning drives a vertical convection that is followed by westward and northward flows of ventilated dense water across the continental shelves shelf waters meet mix with and modify circumpolar deep water at a dynamic slope front near the continental shelf break locally or periodically forming antarctic bottom water nutrient rich deep water upwells along this front regionally intruding onto the continental shelf in long thermally attenuated filaments these intrusions are frequently linked to mass balance processes at the base of ice tongues and ice shelves melting freezing and tidal currents beneath glacial ice are related to the larger scale circulation and to the fate of basal debris in the ice sheet the ocean circulation near ice fronts and decadal variability in ice front positions may account for several features of the sediment record time series 1 yr current measurements in the ross and weddell seas have revealed moderate to strong currents that are seasonally variable vertically coherent and dominated by the diurnal tide and longer term events few long term current measurements exist in the shelf and slope bottom boundary layers that are most important to sediment deposition and erosion icebergs may frequent certain routes e g in association with currents and fronts near the edge of the continental shelf and are capable of scouring and resuspending sediments down to shelf break depths sea ice plays a role in shelf sedimentation by rafting eolian and bottom debris away from the nearshore regions in addition generally offshore winds produce numerous leads and polynyas along the coastline and within the sea ice fields higher biological productivity in these polynyas may be linked to higher local sedimentation rates biogenic marine sediments apparently comprise a major percentage of the regionally variable modern shelf deposits despite the impressive glacial setting and polar environment

iodine records from the ediacaran doushantuo cap carbonates of the yangtze block south china abstract late neoproterozoic cap carbonates formed following the cryogenian glaciations may record information about the significant climate change and ocean chemistry evolution immediately after the snowball earth events large negative carbon isotope anomalies and enigmatic sedimentary structures have been identified from the post marinoan cap carbonates across the globe but their formation mechanism remains debatable due to difficulties of distinguishing overlapped chemical signatures and diagenetic alterations in this study we present i ca mg profiles of four sections from a shallow to deep water transect of the basal doushantuo cap carbonates one of the most extensively studied ediacaran units in south china most samples have i ca mg values lower than 1 mol mol with a few exceptions in the shelf sections up to 2 5 mol mol all calcite samples with extremely low 13c have iodine signal below detection limit suggesting carbonate precipitation from anoxic seawater or diagenetic fluids among the four sections the slope section has the lowest i ca mg values suggesting that it may have experienced more extensive fluid buffered diagenesis and more severe loss of io3 in combination with available major trace rare earth element and isotope data the temporal i ca mg variations support freshwater mixing and deep water upwelling in proximal sites and transient oxidation during deposition of the doushantuo cap carbonates