annual biogenic particle fluxes to the interior of the north atlantic ocean studied at 34 n 21 w and 48 n 21 w abstract in order to clarify the annual quality quantity and export processes of biogenic particles from the euphotic zone to the deep ocean interior an array of automated time series sediment traps were deployed for 1 year from 4 april 1989 to 17 april 1990 at 34 n 21 w and 48 n 21 w as part of the joint global ocean flux program jgofs north atlantic bloom experiment nabe three sediment traps with 13 time series sediment collectors were placed at both stations approximately 1 and 2 km below the surface and 0 7 km above the bottom they collected settling particles during 26 14 day intervals for 376 days with an 20 day hiatus in september october 1989 for changeover of the trap moorings the collection periods of the six traps were synchronized forming a spatio temporal matrix of 156 samples the annual mass flux at about 2 km deep during this experiment was 22 and 27 g m 2 y 1 at the 34 and 48 n stations consisting of biogenic particles with traceable quantities of lithogenic particle flux the spring particle bloom characterized by the sedimentation of particles relatively enriched by norg began in january at the 34 n station and in march at the 48 n station the bloom continued for 4 5 and 3 months and provided 62 and 50 of the annual biogenic particle mass flux at 2 km at the 34 and 48 n stations the surface bloom penetrated to the ocean interior within a few weeks with apparently accelerated settling speed at deeper layers the order of susceptibility of biogenic elements to mineralization while settling in the 0 7 1 km a b water column was from least to most resistant p norg corg si and ca the c n p ratio at 0 7 km a b was 154 18 1 at the 34 n station and 148 18 1 at the 48 n station

accumulation of mineral ballast on organic aggregates 1 to address whether the incorporation of suspended minerals drives the sedimentation of particulate organic carbon in the ocean or vice versa incubations of phytoplankton detritus aggregates were set up in rolling tanks containing seawater and suspended clay illite or calcium carbonate at concentrations ranging between 10 and 50 000 g l 1 the suspended minerals were efficiently scavenged by and incorporated into the organic aggregates the volume and porosity of aggregates decreased with increasing mineral concentrations and at suspended mineral concentrations higher than 500 g l 1 the initially medium sized aggregates were fragmented into thousands of tiny dense aggregates because radius and density have opposite effects on aggregate sinking rates the relationship between aggregate particulate organic carbon poc to mineral ratio and sinking rate is not a straightforward one that necessarily results in higher sinking rates with increased mineral content in these experiments the saturating capacity of organic aggregates for mineral particles appeared to be 97 to 98 weight percent mineral i e a poc to dry weight ratio of 0 02 to 0 03 g c g 1 this parallels the values of 0 05 g c g 1 observed for sinking particles in the deep sea and suggests that it is the flux of poc that determines the flux of minerals to the deep and not the other way around

effects of winter convection on the deep layer of the southern adriatic sea in 2012 1 we analyze aspects concerning the thermohaline changes observed in the near bottom layer of the southern adriatic pit sap in the eastern mediterranean after the arrival of abundant and exceptionally dense water 30 kg m 3 produced in the shallow northern adriatic sea during winter 2012 for this purpose we use temperature t salinity s and current time series collected at the e2m3a deep ocean observatory of the southern adriatic and conductivity temperature depth data obtained both in the southern and middle adriatic from freely drifting profiling floats the dense water produced in the northern adriatic arrived in the central and deepest part of the sap as a series of individual pulses starting on 10 march 2012 while a stronger and prolonged signal that significantly modified the local deep water stratification arrived after 10 april 2012 as a consequence t and s suddenly decreased 0 15 c and 0 015 thus interrupting positive t and s bottom trends observed during the previous 5 years and producing a density increase of 0 02 kg m 3 such variability has been rarely observed in the area we ascribe its occurrence to the concomitance of exceptionally harsh and long lasting bora wind scarce precipitation and low river discharge over the northern adriatic during winter 2011 2012 eventually this newly formed addw reached the strait of otranto during july 2012 its characteristics profoundly differed from those observed in the previous decade hence a noticeable variability in structure and circulation of the abyssal layers of the ionian basin is likely to occur in the near future

meiofaunal distribution in the levantine basin eastern mediterranean spatial variability at different scales depths and distance to coast abstract deep sea ecosystems have been characterised in many ways such as deserts stable monotonous poor remote building on knowledge over the years we now know that these extreme environments are highly variable and diverse hosting rich benthic communities that are largely affected and structured by food availability and quality in the land locked oligotrophic mediterranean sea food supply to the deep sea is expected to be dependent on pelagic production but also to be affected by inputs from the surrounding landmasses having this in mind we used samples collected during the levar expedition on board r v meteor m71 2 winter 2006 2007 from two distinct deep sea areas in the levantine sea for describing meiobenthic communities in relation to factors that may drive their spatial distribution variability and patchiness at several scales area 1 within the pliny trench is located close to the island of crete at ca 4000 m depth whereas area 2 within herodotus abyssal plain is more distant from the island but shallower ca 2700 m independent multiple corer deployments performed at three stations within each area the use of several sediment cores from each deployment and the vertical sectioning of one sediment sample per deployment allowed to explore meiobenthic patterns at multiple scales a set of permutational analyses indicated differences between the two areas in terms of sedimentary food proxies that were coupled with corresponding differences in meiofauna richness composition and in abundances of the two most dominant meiobenthic metazoans i e nematodes and copepods though to a lesser extent variability in meiobenthic characteristics was also detected within and among stations a similar trend was not clearly evident within deployments changes in meiofauna distribution occurred in relation to sediment depth layer as well with evidence that the meiofauna vertical profile may differ between the two areas overall our study shows that higher meiobenthic variability in the eastern mediterranean deep sea is accounted for at the scale of area habitat inferences of our results in light of previous findings in the same area imply that the relationship between food and meiobenthic composition could not be attributed to bathymetry or distance from the land but is more probably explained by the high complexity and dynamics of trench habitats and their role as depositional environments acting as physical traps of organic material

an evaporitic facies in neoproterozoic post glacial carbonates the gifberg group south africa abstract a geological study of the hitherto poorly described neoproterozoic gifberg group with emphasis on lithogeochemistry and o c and sr isotopic composition of the carbonate dominated widouw formation vredendal outlier westernmost south africa revealed that the entire group is an equivalent of the relatively well constrained port nolloth group in the external paraautochthonous part of the pan african gariep belt further north thus the vredendal outlier can be regarded as the southern extension of the port nolloth zone two diamictite units are recognised in the vredendal outlier which can be correlated respectively with the c 750 ma kaigas formation diamictite and the 583 ma syn gaskiers numees formation diamictite in the gariep belt proper the dominating carbonate unit in the studied area is post glacial with respect to the older of the two diamictite units the combined textural structural and geochemical evidence suggests that parts of the variably dolomitised limestone succession represent former evaporite beds sedimentation in a restricted very shallow and proximal basin led to a wide range in c isotope ratios 13cpdb from 4 2 to 4 8 very high sr concentrations derived from original anhydrite and initial 87sr 86sr ratios that are significantly higher 0 70785 than those of coeval seawater as c and sr isotopes are commonly used for chemostratigraphic correlation and high sr concentrations in neoproterozic carbonates are often interpreted as evidence of former aragonite the findings of this study should be used as warning against uncritical use of geochemical and isotopic parameters for describing ancient seawater composition thus c and sr isotope ratios alone in neoproterozoic carbonates may be less powerful proxies of ancient seawater composition and high sr contents are not necessarily indicative of an aragonite sea as previously inferred

postglacial vegetation and fire history eastern klamath mountains california usa pollen and high resolution charcoal data from bluff lake and crater lake california indicate simi lar changes in climate vegetation and fire history during the last 15 500 years pollen data at bluff lake suggest that the vegetation between c 15 500 and 13 100 cal bp consisted of subalpine parkland with scattered pinus and abies after 13 100 cal bp a relatively closed forest of p monticola p contorta and abies developed and fire event frequency was low the inferred climate then was cooler and wetter than present pinus and quercus vaccinifoliadominated at both sites during the early holocene when conditions were warm and dry as climate became wetter and cooler in the late holocene abies spp at both sites and tsuga mertensi ana at crater lake increased in importance displacing pinus and quercus the two lake records have similar trends in fire history with high event frequencies at c 8400 4000 and 1000 cal bp and low values at c 4800 cal bp the fire and vegetation history at both sites suggests a similar response to large scale changes in climate during the holocene

isotopic dating of an archean bolide impact horizon hamersley basin western australia detailed geologic field work in the hamersley basin of western australia has identified a single horizon that contains predominantly sand sized spherules similar to those found in impact ejecta such as at the cretaceous tertiary boundary evidence suggests that these spherules represent a reworked distal strewn field formed by a large bolide impact in late archean time this so called spherule marker bed occurs throughout the main body of the hamersley basin and in stratigraphically equivalent but shallower water lithologies in the northeastern corner carbonate constitutes both a matrix component of the spherule marker bed and minor interbeds in the local stratigraphic section we have utilized carbonate pb pb dating methods to provide for the first time an age estimate 2541 18 15 ma for this important marker bed and therefore of the bolide impact event samples from both facies define within error an identical isochron age that is also consistent with the few zircon based age estimates for the hamersley succession these findings highlight the great potential of carbonate pb pb geochronology in the dating of archean and proterozoic sedimentary rocks

neoproterozoic tectono thermal evolution of the gariep belt and its basement namibia and south africa abstract within the pan african gariep belt in southwestern africa a para autochthonous predominantly sedimentary sequence deposited in a rift graben and subsequently on a passive continental margin with a kibaran basement is distinguished from an allochthonous predominantly oceanic terrane that was thrust on top of the former during collision of the rio de la plata and the kalahari plates 40ar 39ar mineral cooling ages presented here set the following constraints after kibaran high grade metamorphism the consolidated basement cooled through 300 c at 1006 ma rifting between 781 and 741 ma was followed by the formation of oceanic crust the lower parts of which cooled through 500 c sometime between 630 and 600 ma after inversion from extension to compression some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge a process that is indirectly dated between 573 576 ma by hornblende plateau ages continent continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547 543 ma as indicated by further hornblende plateau ages muscovite cooling ages for the internal zone are between 526 and 529 ma those for the tectonically deeper external zone are 483 ma in the north and between 495 506 ma in the south of the belt in conjunction with published data these results suggest successive closure of first the northern adamastor ocean kaoko belt followed by the khomas sea intracontinental damara belt and finally the southern adamastor ocean gariep belt

direct radiometric age determination of carbonate diagenesis using u pb in secondary calcite abstract carbonate rocks and corals from the middle devonian lucas formation ontario have 238 u 204 pb values ranging from 132 to 559 and give concordant apparent 206 pb 238 u and 207 pb 235 u ages in approximate agreement with the primary age of sedimentation a carbonate rock from one part of the section gives an anomalously young age and contains patches of secondary calcite spar this spar is predominantly pale purple under cathodoluminescence cl and contains 250 600 ppb u and 15 22 ppb pb the values range from 2 10 3 10 4 making the pb very radiogenic apparent 206 pb 238 u ages range from 208 to 249 ma and 232 th 238 u values are from 0 005 to 0 040 the younger ages are correlated with an increase in orange cl and higher apparent values the age scatter is attributed to mixing of a subordinate calcite spar which infiltrated the older spar the young calcite is bright orange under cl has 7200 ppb u of 6 2 10 4 a of 0 23 and is dated at 45 ma the age of the older spar has been determined by a sequential dissolution technique the resulting u pb data can be plotted as u pb age spectra and yield lplateau dates ranging from 231 5 1 4 to 244 9 1 6 ma and an integrated 206 pb 238 u age of 238 2 1 1 ma also the sequential dissolution increases the values progressively to a5 10 4 making these ages independent of the initial pb correction the occurrence of two young ages of diagenetic calcite in these strata is consistent with the idea of recurrent fluid activity along fracture zones in subsurface sediments in the michigan basin this study shows that it is now possible to date secondary carbonate diagenesis by u pb and by inference to determine precisely the timing of activity on ancient fracture zones in carbonate rocks

an advanced stepwise leaching technique for derivation of initial lead isotope ratios in ancient mafic rocks a case study of mesoproterozoic intrusions from the udzha paleo rift siberian craton abstract lead isotope systematics is a powerful tool for the interpretation of magma sources however a significant problem in the study of pb and u is their high mobility during weathering and secondary alteration of igneous rocks therefore recalculations of the time integrated lead isotopic ratios back to their initial values in ancient rocks should be approached with caution a standard procedure for deriving initial lead isotopic ratios includes the analyses of minerals which have low u pb elemental ratios e g potassium feldspar and or sulfides however these minerals may not always be present in the sample of interest too small and or intergrown with other minerals and thereby mechanically inseparable and or altered to circumvent the effects of secondary alteration a conventional approach is the preliminary treatment of minerals by strong acids e g concentrated hno3 or aqua regia as well as whole rock samples in order to remove altered components the residual phases are then analyzed for initial lead isotopic ratios however this procedure does not always yield satisfactory results in this study we developed an advanced stepwise leaching technique in which the rock powder was first treated by 2 2 n hcl then by more concentrated hcl and finally by concentrated hno3 this procedure allows for the chemical separation of sulfides which are dissolved by concentrated hno3 but unaffected by hcl at least three leaching steps and two isotopic analyses per sample should be performed we applied this stepwise leaching procedure to mafic intrusions of a known age 1386 30 ma from the mesoproterozoic udzha paleo rift in the northern part of the siberian craton the pb isotope results for the whole rock and final leachate fractions are presented for three samples as well as their u th and pb whole rock data which revealed completely closed u th pb isotopic behavior despite moderate alteration of the samples thereby supporting the 1 38 ga age of the magmatic event the obtained initial pb isotope composition of the mafic intrusion clearly indicates an origin from a long lived enriched source

first direct radiometric dating of archaean stromatolitic limestone here we report the first direct dating of the depositional age of a sedimentary carbonate rock using long lived radioactive decay schemes the mushandike stromatolitic limestone from the masvingo formerly fort victoria greenstone belt of southern zimbabwe1 2 yields a pb pb isochron age of 2 839 33 myr published claims3 for an age of 3 5 gyr for the mushandike limestone were based on its supposed intrusion by the mushandike granite for which a rb sr whole rock age of 3 445 260 myr had been reported4 new rb sr and pb pb isotope data for the mushandike granite indicate a probable emplacement age of 2 900 myr and new field evidence j l o p t j f w in preparation suggests that the granite forms part of the basement on which the limestone was deposited thus the new dates do not support an age of 3 5 gyr for the algal stromatolites at mushandike the pb pb isochron method may have wide applicability to dating the time of deposition of precambrian fossiliferous and unfossiliferous limestones and thus to the problem of the antiquity of life

fluid rock interaction during progressive migration of carbonatitic fluids derived from small scale trace element and sr pb isotope distribution in hydrothermal fluorite associated with the cretaceous okorusu carbonatite complex namibia is a hydrothermal fluorite mineralization hosted in pan african country rock marbles which resulted from fluid rock reaction between the marbles and orthomagmatic carbonatitic fluids expelled from the carbonatite yellow fluorite i was deposited in veins up to 5 cm away from the wallrock contact followed by purple and colorless fluorite ii smoky quartz and barite a mn rich crust on early calcite and pure calcite this clear cut sequence of mineral growth allows an investigation into fluid rock interaction processes between the marble and the migrating carbonatitic fluid and element fractionation patterns between the fluid and subsequent hydrothermal precipitates fluorite i shows a progressive change in color from dark yellow to colorless with purple laminations over time of deposition subsequent fluorite i precipitates show an increase in ca and a continuous decrease in f sr ree y th u and pb contents the ratios eu eu cn th pb and u pb increase whereas y ho th u and la yb cn decrease the sr isotopic composition remains constant at 87sr 86sr 0 70456 0 70459 but with varying highly radiogenic pb 206pb 204pb 32 190 238u 204pb 7 63 fluorite ii has 87sr 86sr 0 70454 0 70459 206pb 204pb 18 349 and 207pb 204pb 15 600 and a chemical composition similar to youngest fluorite i the mn rich crust on early calcite accumulated ree ba pb zr cs th and u developing into pure calcite with a prominent negative ce anomaly and successively more radiogenic sr the calculated degrees of fluid rock interaction f weight fraction of fluid fluid marble decrease from fluorite i and most fluorite ii f 0 5 to calcite f 0 2 0 3 and hydrothermal quartz f 0 1 a crush leach experiment for fluid inclusions in the hydrothermal quartz yielded a rb sr isochron age of 103 12 ma crush leach analysis for the carbonatitic fluid trapped in the wallrock yielded a trend from the fluid leachate to the host quartz 206pb 204pb 18 224 and 18 602 207pb 204pb 15 616 and 15 636 respectively extending from carbonatite towards crustal rocks calculated trace element distribution coefficients fluorite fluid are below unity throughout and increase from la to yb elements largely excluded from fluorite ba pb lree relative to hree were incorporated later into the mn rich crust on calcite the trace element patterns of the hydrothermal minerals are related to changing aco2 and af in the fluid during continued fluid marble reaction a predominance of carbonate over fluoride complexing in the fluid as reactions proceeded controlled the y ho th u and ree patterns in the fluid and the crystallizing phases deviations from these trends indicate discontinuous processes of fluid rock reaction

enhanced carbon export to the abyssal depths driven by atmosphere dynamics long term biogeochemical observations are critical to understand the natural ability of the oceans to fix co2 into organic carbon and export it to the deep as sinking particles here we present results from a 3 year 2010 2013 sediment trap deployment that allowed detecting interannual variations of carbon fluxes beyond 4000 m depth in the eastern mediterranean sea anomalous atmospheric conditions triggering strong heat losses in winter spring 2012 resulted in convective mixing nutrient uplifting and a diatom dominated bloom southeast of crete phytoplankton growth reinforced by the arrival of nutrients from airborne etna volcano ash was the highest in the last decade satellite derived chl a concentrations up to 1 9 mg m 3 this situation caused carbon export to increase by 2 orders of magnitude 12 2 mg m 2 d 1 with respect to typical values which demonstrates how pulses of sinking fresh phytodetritus linked to rare atmospheric processes can episodically impact one of the most oligotrophic environments in the world ocean

distribution patterns of carbon oxidation in the eastern mediterranean sea evidence of changes in the remineralization processes 1 deep remineralization patterns as carbon dioxide production were investigated in microplankton samples from several areas of eastern mediterranean sea the respiration data collected during six oceanographic surveys from 1993 through 1999 evidence biogeochemical peculiarities of the deep waters of mediterranean as compared to the open ocean and allow drawing up an hypothesis on the possible impact of the eastern mediterranean transient emt on remineralization processes the temporal trend of the oxidative processes in the basin is tightly coupled with the main features of the circulation pattern in the mediterranean sea the convective processes and the consequent advection of dense waters assume a relevant role in sustaining the amount of remineralization in the deep layers and appear to be more important than the sinking of particulate matter from the upper layers respiration in the deepest mediterranean waters accounts for a greater percentage of the upper aphotic zone respiration over 45 than that found in the open oceans below 21 in addition the increase in respiratory rates in only a few years in the ionian basin could represent a signal of the changes induced by the emt which replaced the adriatic deepwater supply with waters of aegean origin the ecological change induced by the emt negligible in the photic layers of the eastern mediterranean sea seems to concern the impact of the continental shelf pump by advection of younger less refractory organic matter utilizable by deep biota

the use of mineral magnetism in the reconstruction of fire history a case study from lago di origlio swiss alps abstract the use of magnetic measurements in the detection of fire signals has been neglected since the work of rummery et al 1979 yet considerable developments have been made in the interpretation of magnetic measurements over the last 16 years this paper presents a study of the fire history of lago di origlio in the southern swiss alps the study utilises the technique of mineral magnetism alongside the stratigraphic pollen spore and charcoal records correlation between the various proxy records indicates that a magnetic fire record is present within the sediments for the last 4 ka the magnetic fire record has a distinct mineralogical and magnetic grain size signature that can be recognised against the background sedimentary signal the results suggest that magnetic measurements may be usefully employed in the reconstruction of fire history their application is rapid and non destructive and the results may provide additional information in relation to the links between catchment fire events and the sedimentary record

trace element behaviour in seawater during etna s pyroclastic activity in 2001 concurrent effects of nutrients and formation of alteration minerals abstract the eruption of etna in sicily on 13 july 2001 marked the most intense activity of the volcano in the last 300 years the eruption occurred while the oceanographic cruise ansic 01 was being conducted to the east of sicily presenting a unique opportunity for the investigation of the chemical effects on the marine system during a period of significant 1 g m 2 ash deposition comparison of trace element data with measured concentrations from the oceanographic cruise juvenile 99 carried out two years before indicates large enrichments of v cr mn fe co cu and pb we attribute this behaviour to leaching of freshly erupted volcanic ash according to trace element concentrations measured in suspended particulate matter spm the exception is pb which is likely to be influenced by another possibly anthropogenic source trace element behaviour in the water column and during leaching experiments suggests that trace element distributions are driven by the occurrence of alteration minerals on glass surfaces given that the main trace element source in seawater appears to come from the soluble ash fraction trace element solubility is likely to be increased by organic complexation which accelerates the alteration of ash particles organic substances suitable for complex trace elements are probably formed during the lysis of phytoplankton cells produced during an increase in marine primary productivity associated with the volcanic fertilisation of surface waters

enhanced silica ballasting from iron stress sustains carbon export in a frontal zone within the california current 2015 american geophysical union all rights reserved nutrient dynamics phytoplankton rate processes and export were examined in a frontal region between an anticyclone and a pair of cyclones 120 km off the coast in the southern california current system sccs low silicic acid nitrate ratios si n and high nitrate to iron ratios n fe characteristic of fe limiting conditions in the sccs were associated with the northern cyclone and with the transition zone between the cyclones and the anticyclone phytoplankton growth in low si n high n fe waters responded strongly to added fe confirming growth limitation by fe of the diatom dominated phytoplankton community low si n waters had low biogenic silica content intermediate productivity but high export compared to intermediate si n waters indicating increased export efficiency under fe stress biogenic silica and particulate organic carbon poc export were both high beneath low si n waters with biogenic silica export being especially enhanced this suggests that relatively high poc export from low si n waters was supported by silica ballasting from fe limited diatoms higher poc export efficiency in low si n waters may have been further enhanced by lower rates of organic carbon remineralization due to reduced grazing of more heavily armored diatoms growing under fe stress the results imply that fe stress can enhance carbon export despite lowering productivity by driving higher export efficiency

recent changes in deep water formation and spreading in the eastern mediterranean sea a review abstract observations of the last decade testify that the characteristics of the deep thermohaline circulation in the eastern mediterranean sea have changed thoroughly the source of the most dense waters of the basin has moved from the adriatic to the aegean sea this new source has proved to be very efficient since the estimated formation rate for the period 1989 95 was more than 1 sv about three times more than the dense water formation rate of the adriatic sea these new waters with hydrological characteristics that are warmer and more saline have replaced almost 20 of the older deep waters of the basin and have uplifted the deep isopycnals by about 500 m this major event can be attributed to important meteorological anomalies in the eastern mediterranean and to changes of circulation patterns the extended dry period of 1988 93 and the exceptionally cold winters of 1987 and 1992 93 created favorable conditions for increased formation of dense water in the aegean furthermore changes in the circulation patterns in the intermediate water range levantine intermediate water liw and cretan intermediate water themselves possibly linked to meteorological anomalies appear to have played an important role in the redistribution of salt as a result of an interruption to exchanges between the ionian and levantine basin the salinities in the latter started to rise high salinity waters were diverted into the aegean malanotte rizzoli p manca b b ribera dacala m theocharis a 1998 the eastern mediterranean in the 80s and in the 90s the big transition emerged from the poem bc observational evidence rapport du commission international de la mer medittanee 35 174 175 and the westward transport of liw was reduced an additional effect of the deep water discharge from the aegean and the resulting uplifting of mid depth waters was to lower salinities in the liw layer this effect is most strongly felt in the ionian sea a 3 d primitive equation numerical model for the eastern mediterranean with a 20 km grid size is used to simulate the observed changes and understand the basic mechanisms which caused them under appropriate atmospheric forcing the model successfully reproduces the main characteristics of the transient these results indicate that the observed changes can be at least partially explained as a response of the eastern mediterranean and more specifically of the aegean to atmospheric forcing variability

carbonate pbpb ages of the wittenoom formation and carawine dolomite hamersley basin western australia with implications for their correlation with the transvaal dolomite of south africa abstract pb isotopic analyses of carbonates from the hamersley group western australia yield the following results 1 a late archean minimum age of 2 5 ga is definitely established for the deposition of both the deeper water basinal wittenoom formation and the shallow water carawine dolomite stratigraphic correlation of the transvaal dolomite south africa with these two formations is supported by the present pb isotope age data 2 a pb pb isochron age of 2505 37 ma for the paraburdoo member lower part of the wittenoom formation is best interpreted as a late diagenetic event which took place 100 ma after the initial deposition 3 a pb pb isochron age of 2346 38 ma for the bee gorge member most likely represents a still later local event of calcite replacement in fractures and veins 4 a pb pb isochron age of 2541 32 ma for carawine dolomite could represent a regional diagenesis slightly earlier than the paraburdoo carbonates if the carawine carbonates were deposited contemporaneously with the wittenoom formation alternatively it may represent the time of early diagenesis which closely followed the initial sedimentation a study of trace element abundances and stable isotope compositions has revealed a major difference in fe and mn contents and 18 o values between the wittenoom and carawine dolomite samples this suggests that the two suites of dolomites have undergone separate diagenetic histories with different chemical and oxygen isotopic modifications

petrogenesis of peralkaline granite dykes of the straumsvola complex western dronning maud land antarctica peralkaline syenite and granite dykes cut the straumsvola nepheline syenite pluton in western dronning maud land antarctica the average peralkalinity index pi molecular al na k of the dykes is 1 20 n 29 and manifests itself in the presence of the zr silicates eudialyte dalyite and vlasovite and the na ti silicate narsarsukite the dykes appear to have intruded during slow cooling of the nepheline syenite pluton and the petrogenetic relationship of the dykes and the pluton cannot be related to closed system processes at low pressure given the thermal divide that exists between silica undersaturated and oversaturated magmas major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition and internal mineral segregation prior to final solidification the distribution of accessory minerals is consistent with late stage crystallization of isolated melt pockets the dykes give an rb sr isochron age of 171 4 4 ma with variable initial 87sr 86sr ratio 0 7075 0 0032 and have an average e nd of 12 0 quartz phenocrysts have 18o values of 8 4 9 2 which are generally in o isotope equilibrium with bulk rock differences in the 18o values of quartz and aegirine average quartz aegirine 3 5 suggest aegirine formation temperatures around 500 c lower than expected for a felsic magma but consistent with poikilitic aegirine that indicates subsolidus growth the negative e nd 10 and magma 18o values averaging 8 6 assuming quartz magma 0 6 are inconsistent with a magma produced by closed system fractional crystallization of a mantle derived magma by contrast the nepheline syenite magma had mantle like 18o values and much less negative e nd average 3 1 n 3 the country rock has similar 18o values to the granite dykes average 8 0 n 108 this means that models for the petrogenesis of the granites by assimilation are unfeasible unless an unexposed high 18o contaminant is invoked instead it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali metasomatised gneiss that surrounds the nepheline syenite followed by fractional crystallization

late vendian closure of the adamastor ocean timing of tectonic inversion and syn orogenic sedimentation in the gariep basin abstract new pb pb age data obtained on stromatolitic dolomite from the oceanic marmora terrane in the pan african gariep belt support previous ar ar age data on early amphibole that has been related to sea floor metamorphism and indicate formation of oceanic crust in the neoproterozoic adamastor ocean gariep basin around 600 ma a maximum age for oceanic crust in the gariep basin is given by 741 6 ma which is the best available minimum constraint on continental rifting in the gariep belt tectonic inversion to basin closure overlaps in time with the end of a younger glacial period that is recorded both in the oceanic and continental realms and is correlated with the global c 580 ma marinoan glaciation the rock record up to that time in the marmora terrane is completely different to that in the para autochthonous continental port nolloth zone in the east of the belt but the post glacial sediment successions bear many lithological and geochemical similarities these younger successions are therefore interpreted as reflecting syn orogenic deposits in a foredeep the age of which is constrained between c 580 and 550 ma comparison of three chemostratigraphic profiles through the post glacial carbonate succession shows that enrichment in carbonate 13c is in the first instance a function of proximity to the basin margin and thus of the extent of ocean water stratification

an eburnean base metal source for sediment hosted zinc lead deposits in neoproterozoic units of namibia lead isotopic and geochemical evidence the lead isotopic composition of galena from the neoproterozoic sediment hosted zn pb sulphide deposit at rosh pinah and the oxidised zn deposit at skorpion in the pan african gariep belt southwestern namibia as well as that of galena from minor occurrences in the wider rosh pinah ore province was investigated and is compared with that of other major sediment hosted base metal ore deposits hosted by neoproterozoic strata in southwestern africa and brazil the isotope data were supplemented by a geochemical provenance study of the argillitic host rock to the rosh pinah deposit and its stratigraphic equivalents the pre orogenic rosh pinah deposit and the inferred progenitor for the secondary skorpion deposits have very similar pb isotopic compositions in contrast syn orogenic deposits e g tsumeb type show a larger proportion of radiogenic pb from the country rocks that were infiltrated by orogenic ore fluids

isotopic composition of disseminated sulfur in precambrian sedimentary rocks abstract isotopic analyses of trace sulfur in precambrian carbonates and clastic sedimentary rocks support the results of previous studies which have shown that 34s measurements of archean sedimentary sulfur cluster mostly around mantle like values and that a significant overall dispersion of 34 s 32 s ratios commenced at about 2 7 ga ago the outstanding feature of the proterozoic record is the predominance of heavy 34ssulfide values mostly in excess of 0 cdm consideration of these data in combination with the sparse 534s record for proterozoic sulfates suggests that 34ssulfide sulfide may have been smaller than was the case during the phanerozoic a possible scenario that is compatible with the above data is based on a proposition that the terrestrial exogenic sulfur cycle evolved in a tripartite manner in the archean it was coupled to and controlled by the magmatic endogenic cycle the proterozoic for the most part represents a transitional stage into a phanerozoic like exogenic cycle the latter essentially decoupled from its endogenic counterpart and its redox balance maintained and mediated by the biosphere the indication that the mean combined 34s of proterozoic sulfate and sulfide may have significantly exceeded the phanerozoic mean value of 2 cdm is enigmatic and poses problems for balance considerations

application of the double spike technique to pb isotope geochronology abstract new pb isotope data are presented for three suites of carbonate bearing sedimentary rocks from the archaean and proterozoic of western australia the measured data are corrected for instrumental mass fractionation using both conventional and double spike techniques with the results contrasted on isochron diagrams compared with the data corrected using conventional methods the double spike corrected data show a significant improvement in all cases the isochrons produced are more tightly constrained resulting in lower uncertainties in the age estimates obtained and marked improvements in mswd values while highlighting the enormous potential for the direct dating of carbonate sequences throughout the geological record this study demonstrates the improvements possible using double spike correction for mass fractionation an issue pertinent to all aspects of pb isotope geochronology

fan delta sequence in the archean fig tree group barberton greenstone belt south africa abstract the fig tree group of the barberton greenstone belt in south africa records the onset of tectonism and significant terrigenous and volcaniclastic sedimentation following a period of predominantly effusive volcanism in the southern part of the greenstone belt the lower fig tree conformably overlies the onverwacht group and can be grouped into either 1 volcaniclastic deposits consisting of pyroclastic and reworked volcanic detritus or 2 non volcanogenic terrigenous deposits of sandstone and conglomerate these petrologic groups are further divided into six lithofacies the lithofacies within the lower fig tree formation can be grouped into two facies associations 1 a subaqueous delta front association made up of fine grained airfall pyroclastic and slightly reworked pyroclastic debris along with resedimented land derived sandstone and conglomerate and 2 a subaerial association that is interpreted as forming from deposition of sandstone and conglomerate by flashy ephemeral stream processes on an alluvial fan surface the stratigraphic sequence coarsens upward from predominantly siltstone and fine grained sandstone at the base to pebble and cobble conglomerate at the top this sequence is interpreted as forming from the progradation of a fan delta into a body of relatively shallow quiet water composition of clasts in the conglomerates indicates that immediately underlying greenstone belt rocks or coeval volcanic rocks were the source for these coarse sedimentary accumulations erosion of uplifted greenstone belt terrain was apparently only to shallow levels both tectonic and volcanic activity played a role in producing several cycles of fan delta progradation

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

the impact of diagenesis on high precision upb dating of ancient carbonates an example from the late permian of new mexico we have applied upb trace element and sr isotopic analyses to subsamples of a single specimen of the capitan formation from the permian reef complex of new mexico with the goal of calibrating the late permian time scale and understanding the factors that set and later alter the initial u pb ratio and pb isotopic composition of the limestone the data indicate a limestone upb isochron age of 249 8 4 7 ma 2 and a pbpb age of 249 18 ma in addition with values 238u 204pb ranging up to 4200 among the highest ever recorded for a limestone we also obtain precise single sample 206pb 238u 207pb 235u and 207pb 206pb ages indicating averages of 250 2 3 2 ma 250 0 2 8 ma and 256 3 3 5 ma respectively comparison of the average pb u age of 250 3 ma for the latest guadalupian with published radiometric constraints for the late permian suggests preservation of essentially the depositional age trace element and sr isotopic analyses highlight four samples that preserve a chemical signature expected for primary marine aragonite including high sr 3000 g g high u 2900 ng g low initial pb 26 g g and 87sr 86sr 0 7073 as well as the lower u and higher pb mg and fe expected for a diagenetically altered carbonate these data clearly delineate the diagenetic alteration of the limestone and allow us to demonstrate the critical importance of diagenesis in controlling the spread in values primarily through the expulsion of u but with some incorporation of pb the high are simply a function of preservation of original carbonate chemistry and not of u scavenging by fe oxyhydroxides or phosphates although it has long been suspected that limestone upb and pbpb isochron ages reflect the timing of early diagenesis as opposed to a strictly depositional age this is the first study in which this is conclusively demonstrated to be the case the upb data also suggest a late stage neogene alteration that disturbed the system mainly via the redistribution of u intriguingly the four samples that best retain their primary marine aragonitic geochemistry also best maintained a closed upb system most of the subsamples with comparably high values but clearly altered trace element compositions showed upb systematics indicating late disturbance unfortunately it is not yet possible to determine which processes are responsible for this late stage alteration and it is unclear exactly how to avoid such alteration in future work

loss of isotopic nd o and chemical ree memory during metamorphism of komatiites new evidence from eastern finland komatiites are often considered to depict the chemical and isotopic composition of their source rocks in the archean mantle however a weakness of these rocks in tracking the initial compositional heterogeneity of the earth s mantle is the ubiquitous presence of metamorphic recrystallization which casts some doubt about the preservation of primary chemical and isotopic characteristics two spinifex textured komatiite flows from the 2 75 ga old kuhmo greenstone belt siivikkovaara area of eastern finland document this weakness both flows have experienced low to medium grade metamorphism t 450 50 c and now consist entirely of secondary metamorphic assemblages of amphibole chlorite plagioclase with minor proportions of magnetite and ilmenite mgo contents range from 25 to 8 which suggests that low pressure differentiation was likely controlled by olivine and clinopyroxene fractional crystallization however neither major nor trace elements fall on olivine and or clinopyroxene control lines this is particularly well illustrated by the ree as there is an overall 60 variation of ce sm n ratios 0 38 to 0 91 which far exceeds that expected from olivine and clinopyroxene fractionation alone in fact careful evaluation of petrographic including mineral composition data and chemical characteristics shows that most elements of geological interest including the reputedly immobile ree were mobile on a whole rock scale during metamorphic recrystallization of these two flows this view is fully supported by sm nd isotopic data since both whole rock and mineral amphibole and plagioclase samples lie on a single isochron relationship at t 1800 ma an age which corresponds to the time of regional metamorphism thus the meta komatiite flows from siivikkovaara document a case of komatiite flow units in which metamorphism has induced whole rock scale resetting of primary ree patterns and sm nd isotope systematics as regards the nature of the mechanism responsible for this resetting it is suggested here that the secondary mineralogy played an important role as there are correlations between whole rock 147sm 144nd ratios major and trace element chemistry oxygen isotopic ratios and modal proportions of secondary minerals consideration of oxygen isotopic data as well as previous results from a komatiite flow from the nearby tipasjarvi belt further enable us to propose that the ree carrier was likely a co2 rich fluid the process of secondary ree redistribution prevents estimation of the true initial nd isotopic compositions of the two flows taken as a whole the results presented in this paper show that great care should be be exercised in the use of meta komatiites as probes of archean mantle composition

u pb dating of a remagnetized paleozoic limestone the upper ordovician trenton group of new york state is one of a number of appalachian limestones which yields paleomagnetic poles suggesting disturbance in the late paleozoic calcite fractions dissolved from samples of the trenton limestone define a 238u 206pb age of 454 8 ma mswd 12 8 in excellent agreement with 40ar 39 ar and u pb ages for the deposition of the underlying diecke bentonite this requires that the u pb system in the calcite has been preserved since deposition and early diagenesis if late paleozoic remagnetization of this limestone was induced by fluids these fluids did not interact with significant amounts of carbonate the apparent robustness of the u pb system and the large spread in values 18 17 118 8 offers potential precision of 1 ma in determining the paleozoic time scale

postglacial vegetation and fire history of the southern cascade range oregon abstract the cascade range of southwestern oregon contains some of north america s most diverse forests but the ecological history of this area is poorly understood a 7900 yr long pollen and charcoal record was examined to better understand past changes in vegetation and fire activity in relation to large scale climate variability from 7900 to 3500 cal yr bp the dominance of xerophytic species and the frequent fires are consistent with a climate that was warmer and drier than at present the period from 3500 cal yr bp to present experienced an abundance of mesophytic taxa and reduced fire frequency suggesting cooler and wetter conditions the regional history of abies indicates that it was most widespread during the late glacial period its range contracted during the early holocene thermal maximum and it steadily expanded during the middle and late holocene in contrast pseudotsuga was restricted in range during the glacial period became abundant at low elevation sites in the coast and northern cascade ranges during the early holocene and was more prevalent in southern mid elevation sites as the climate became cooler and wetter in the late holocene the sensitivity of these species to past climate change suggests that biogeographic responses to future conditions will be highly variable in this region

interaction of dense shelf water cascading and open sea convection in the northwestern mediterranean during winter 2012 the winter of 2012 experienced peculiar atmospheric conditions that triggered a massive formation of dense water on the continental shelf and in the deep basin of the gulf of lions multiplatforms observations enabled a synoptic view of dense water formation and spreading at basin scale five months after its formation the dense water of coastal origin created a distinct bottom layer up to a few hundreds of meters thick over the central part of the nw mediterranean basin which was overlaid by a layer of newly formed deep water produced by open sea convection these new observations highlight the role of intense episodes of both dense shelf water cascading and open sea convection to the progressive modification of the nw mediterranean deep waters

middle holocene climates and human population densities in the great basin western usa much of western north america saw higher temperatures and lower precipitation during the middle holocene the great basin became much drier and warmer than it is today causing major shifts in lake levels treelines plant community composition and vertebrate distributions and abundances to assess the impact of climate change on middle holocene human population densities in this region we examine the frequency structures of radiocarbon dated archaeological sites through time in three separate parts of the great basin the bonneville basin fort rock basin and western lahontan basin the results of the analysis support the hypothesis that human population densities in many parts of the great basin dropped substantially in response to middle holocene climate change but also document that there were intervals during the middle holocene in all three areas that appear to have been marked by temporary population increases we hypothesize that these increases were associated with equally temporary increases

a novel approach to double spike pb pb dating of carbonate rocks examples from neoproterozoic sequences in southern africa application of the 207 pb 204 pb double spiking technique to correct for mass fractionation led to marked improvement in the quality of pb isotope data and resulting ages obtained on low grade metamorphosed neoproterozoic carbonate rocks from the pan african gariep and saldania belts in southern namibia and south africa carbonate rock powders were dissolved using 1 n acetic acid residue fractions were separated from the supernatant carbonate fractions and analysed separately marbles from the lower pickelhaube formation gariep belt yielded distinctly different 207 pbr 206 pb isochron ages of 728 32 mswds 1 5 and 545 13 ma mswds 7 4 for the carbonate and residue fractions respectively the former is interpreted as dating early diagenesis whereas the latter is in excellent agreement with the timing of regional metamorphism in contrast both the carbonate and residue fractions from the bloeddrif member holgat formation gariep belt and from the kombuis member matjies river formation saldania belt yielded indistinguishable 207 pbr 206 pb isochron ages the isotope data from both fractions combined define an isochron age for the bloeddrif member marbles of

the first deep sea mooring station in the eastern levantine basin deeplev outline and insights into regional sedimentological processes abstract the eastern levantine basin elb is an ultra oligotrophic warm and saline marine system significantly affected by climate change during the past several decades the deployment of the first moored station at the elb deeplev at 1500 m water depth 50 km offshore haifa israel aimed at enhancing synchronized measurements of physical and biogeochemical dynamics the station carries an array of sediment traps and sensors that measure physical chemical and biological attributes along the water column here we describe the station and present sediment flux data from the first 1 5 years of operation since november 2016 we show that despite deeplev s offshore location and the open sea characteristics of the water column lateral transport from the continental margin contributes the greatest fraction of particulate flux to the seafloor at this site this mostly occurs via intermediate nepheloid layer plumes entering the study area between depths of 180 and 800 m bottom currents are weak average sd 3 5 2 cm s 1 and are unlikely to cause significant resuspension based on sedimentation rates obtained from nearby sediment cores and sediment particle fluxes sampled in the traps calculated accumulation rates of total particulate matter and of particulate organic carbon at the seafloor were 95 108 g m 2 y 1 and 0 5 0 6 g m 2 y 1 respectively offshore changes in sedimentation rates and in surface sediment elemental ratios indicate that sediment originating from the continental margins is transported at least 100 km offshore

direct dating of stromatolitic carbonates from the schmidtsdrif formation transvaal dolomite south africa with implications on the age of the ventersdorp supergroup pb isotopic analyses of stromatolitic limestones from the schmidtsdrif formation transvaal dolomite of south africa give a late archean isochron age of 2 557 plus minus 49 2 sigma ma the new date significantly changes the position of the archean proterozoic boundary in the kaapvaal province and suggests that a possible division of the transvaal supergroup is necessary it also resolves the long standing controversy about the age of the ventersdorp supergroup whose zircon u pb data of ca 2 7 ga must be more acceptable than the rb sr dates of about 2 3 ga the study provides a new example that demonstrates the great potential of employing the u pb systems of carbonate rocks for direct dating of sedimentary sequences

sm nd rb sr pb pb dating of carbonates from the early archaean barberton greenstone belt south africa evidence for post depositional isotopic resetting at low temperature abstract two silicic carbonate sequences occurring in the onverwacht group og and the overlying fig tree group ftg of the early archaean barberton greenstone belt south africa contain silicified dolomites with primary structures and textures they occur with volcanic rocks dated by single zircons the associated onverwacht felsic volcanics have an age of 3440 3416 ma whereas the fig tree volcanics are 3225 ma old therefore the depositional age of both sequences is between 3440 and 3225 ma pb isotopic data for different parts of a single og whole rock sample form a linear array corresponding to a pb pb age of 2632 99 ma sm nd and rb sr isotopes were analysed in four whole rock samples of the ftg silicified carbonates and form an sm nd array corresponding to an age of 2612 84 ma with initial end 8 1 0 5 and an rb sr array of 2743 36 ma with initial 87 sr 86 sr 0 7013 0 0003 these whole rock samples have rb sr ratios of 1 and unusually high rb concentrations for carbonates 100 ppm which derive from processes accompanying silicification all three isotopic systems of whole rocks indicate ages of 2 7 ga much younger than the depositional age of the successions the agreement provides strong evidence that an alteration event reset all of the isotopic systems sm nd as well as rb sr and u pb we suggest that the ages represent the timing of silicification and sericitization of the carbonates by large scale fluid movement at an apparent temperature of 200 c

organic walled microfossils and biostratigraphy of the upper port nolloth group namibia implications for latest neoproterozoic glaciations the occurrence of organic walled microfossils is reported for the first time from the neoproterozoic port nolloth group gariep belt southern namibia acritarchs assigned to bavlinella faveolata occur in the hilda subgroup below the younger of two glaciogenic diamictite units numees formation within the port nolloth group the microfossil assemblage in the overlying upper holgat formation above the numees formation is characterized by low diversity six species dominance of soldadophycus bossii and absence of acanthomorphic or large sphaeromorphic acritarchs the agglutinated foraminifer titanotheca also occurs in the holgat formation combined with available chemostratigraphic data and pb pb ages this microfossil assemblage indicates an upper ediacaran age of around 555 ma for the holgat formation virtually identical microfossil assemblages negative to positive 13 c trends 87 sr 86 sr values between 0 7080 and 0 7085 as well as pb pb carbonate ages make it possible to correlate the holgat formation with the buschmannsklippe formation witvlei group central namibia the kombuis member cango caves group southern south africa and the uppermost polanco to lowermost cerro espuelitas formation arroyo del soldado group uruguay based on these data the underlying numees formation the age of which has been only loosely constrained so far and subject to considerable debate can now be assigned to the c 580 ma gaskiers or the possibly younger 13 c excursion and a strong sea level drop if this correlation is confirmed lack of glacial deposits there might have implications for the palaeogeographic extent of upper ediacaran glaciations our preliminary studies show that acritarch biostratigraphy can make a significant contribution to unravelling the stratigraphy of neoproterozoic glacial deposits especially when combined with c and sr isotopic data

extreme cooling and dense water formation estimates in open and coastal regions of the adriatic sea during the winter of 2012 generation of dense waters in the adriatic sea during an extreme cooling event in the winter of 2012 including its preconditioning and spreading phases have been investigated using the one way coupled roms and the aladin hr modeling system both climatological and real river fluxes are used in the simulations aside from the convenient dense water formation areas located at the northernmost adriatic shelf we found that a similar amount of dense water with slightly lower density was formed in the eastern and deeper adriatic coastal area which was subjected to extreme heat losses up to 2000 w m2 during peak cooling periods this part of the adriatic has been known for extreme cooling during wintertime bora outbreaks nevertheless no ocean model study has previously reproduced dense water formation in this area the most likely reason for that was an overestimate of river discharges introduced to ocean models from newly available data we estimated that the contribution of eastern adriatic rivers between the neretva river and rijeka bay is more than six times lower than what has been previously documented transport of dense water toward the middle adriatic had a peak value of about 0 6 sv while the speed of initial bottom density current surpassed 40 50 cm s which is several times faster than past events different pathways of the dense water toward the middle and south adriatic depressions have also been documented the contribution of the eastern coastal adriatic area to the overall north adriatic dense water formation has been quantified and discussed for average and low freshwater load conditions indicating that this part of the adriatic is a common place for dense water generation

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

factors controlling the flux of organic carbon to the bathypelagic zone of the ocean 1 particle fluxes measured with time series sediment traps deployed below 2000 m at 68 sites in the world ocean are combined with satellite derived estimates of export production from the overlying water to assess the factors affecting the transfer of particulate organic matter from surface to deep water multiple linear regression is used to derive an algorithm suggesting that the transfer efficiency of organic carbon defined as the settling flux of organic carbon normalized to export production increases with the flux of carbonate and decreases with water depth and seasonality the algorithm predicts 80 of the organic carbon transfer efficiency variability in diverse oceanic regions the influence of the carbonate flux suggests that the ballasting effect of this biogenic mineral may be an important factor promoting export of organic carbon to the deep sea by increasing the density of settling particles however the lack of a similar effect for biogenic opal suggests that factors other than particle density also play a role the adverse effect of increasing seasonality on the transfer efficiency of carbon to the deep sea is tentatively attributed to greater biodegradability of organic matter exported during bloom events in high latitude opal dominated regions with high f ratios and seasonality while a higher fraction of net production is exported a higher fraction of the exported organic matter is remineralized before reaching bathypelagic depths on the other hand in warm low latitude carbonate dominated regions with low f ratios and seasonality a higher fraction of the exported organic matter sinks to the deep sea

shear zones of the limpopo belt and adjacent granitoid greenstone terranes implications for late archaean collision tectonics in southern africa abstract granulite metamorphism in the 2 65 ga limpopo belt of southern africa is related to crustal thickening due to late archaean collision tectonics the central zone of the belt is a major thrust sheet of distinctive lithological structural and isotopic character separated from adjacent terranes by crustal scale shear zones and the northern and southern marginal zones are up thrust deep level granulite facies terranes of the adjacent cratons geometrically the marginal zones are mirror images and the limpopo belt is essentially symmetrical in n s section conventionally regarded as a n s collisional orogeny the gross structural and lithological differences between the central zone and the marginal zones kinematic data from shear zones and geophysical data showing the three dimensional geometry of the central zone are here used to suggest that the direction of emplacement of the central zone was also the collision direction this was from northeast to southwest with the upthrust outwardly directed marginal zones developing in mechanical response to this collision

postglacial vegetation fire and climate history of the siskiyou mountains oregon usa the forests of the siskiyou mountains are among the most diverse in north america yet the long term relationship among climate diversity and natural disturbance is not well known pollen plant macrofossils and high resolution charcoal data from bolan lake oregon were analyzed to reconstruct a 17 000 yr long environmental history of high elevation forests in the region in the late glacial period the presence of a subalpine parkland of artemisia poaceae pinus and tsuga with infrequent fires suggests cool dry conditions after 14 500 cal yr b p a closed forest of abies pseudotsuga tsuga and alnus rubra with more frequent fires developed which indicates more mesic conditions than before an open woodland of pinus quercus and cupressaceae with higher fire activity than before characterized the early holocene and implies warmer and drier conditions than at present in the late holocene abies and picea were more prevalent in the forest suggesting a return to cool wet conditions although fire episode frequency remained relatively high the modern forest of abies and pseudotsuga and the present day fire regime developed ca 2100 cal yr b p and indicates that conditions had become slightly drier than before sub millennial scale fluctuations in vegetation and fire activity suggest climatic variations during the younger dryas interval and within the early holocene period the timing of vegetation changes in the bolan lake record is similar to that of other sites in the pacific northwest and klamath region and indicates that local vegetation communities were responding to regional scale climate changes the record implies that climate driven millennial to centennial scale vegetation and fire change should be considered when explaining the high floristic diversity observed at present in the siskiyou mountains d 2005 university of washington all rights reserved

petrogenesis of low δ 18 o quartz porphyry dykes koegel fontein complex south africa this paper investigates the origin of low 18o quartz porphyry dykes associated with the 144 133 ma koegel fontein igneous complex which was intruded during the initial phase of breakup of africa and south america the 25 km diameter rietpoort granite is the largest and youngest phase of activity and is roofed by a 10 km diameter pendant of gneiss quartz porphyry qp dykes up to 15 m in width strike nw se across the complex the qp dykes that intruded outside the granite have similar quartz phenocryst 18o values average 8 0 0 7 n 33 to the granite average 8 3 1 0 n 7 the qp dykes that intruded the roof pendant have quartz phenocrysts with more variable 18o values average 1 6 2 1 n 55 in some cases quartz phenocrysts have 18o values as low as 2 5 the variation in 18o value within the quartz crystal population of individual dykes is small relative to the overall range and core and rim material from individual quartz phenocrysts in three samples are identical within error there is no evidence that quartz phenocryst 18o values have been affected by fluid rock interaction based on a quartz magma value of 0 6 magma 18o values must have been as low as 3 1 samples collected along the length of the two main qp dykes that traverse the roof pendant have quartz phenocryst 18o values that range from 1 1 to 4 6 and 2 3 to 5 6 respectively these 18o values correlate negatively r 0 96 with initial 87sr 86sr which can be explained by the event that lowered 18o values of the source being older than the dykes we suggest that the qp dykes were fed by magma produced by partial melting of gneiss which had been variably altered at high temperature by 18o depleted meteoric water during global glaciation at 550 ma the early melts had variable 18o value but as melt pockets interconnected during melting the 18o values approached that of average gneiss variable quartz phenocryst 18o values in the same dyke can be explained by vertical emplacement at variable rates of ascent along the dyke the lateral variation in quartz and hence magma 18o value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke and the age of the particular magma batch

silicate microspherules intercepted in the plume of etna volcano a possible volcanic origin has been suggested for the micrometre sized spherules that have been discovered in polar snows or ice cores1 5 and in the stratosphere6 7 however although such particles especially when black and magnetic have been identified among the tephra deposits of some volcanoes3 8 12 their volcanic emission has never been directly observed here we describe micro spherules intercepted in the plume of mount etna sicily during moderate recurring volcanic activity their study using analytical transmission and scanning electron microscopy atem and asem demonstrates the simultaneous presence in the plume of glassy silicate microspherules of various chemical compositions 47 98 sio2

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

impact of the spatial distribution of the atmospheric forcing on water mass formation in the mediterranean sea the impact of the atmospheric forcing on the winter ocean convection in the mediterranean sea was studied with a high resolution ocean general circulation model the major areas of focus are the levantine basin the aegean cretan sea the adriatic sea and the gulf of lion two companion simulations differing by the horizontal resolution of the atmospheric forcing were compared the first simulation med16 era40 was forced by air sea fields from era40 which is the ecmwf reanalysis the second simulation med16 ecmwf was forced by the ecmwf analyzed surface fields that have a horizontal resolution twice as high as those of era40 the analysis of the standard deviations of the atmospheric fields shows that increasing the resolution of the atmospheric forcing leads in all regions to a better channeling of the winds by mountains and to the generation of atmospheric mesoscale patterns comparing the companion ocean simulation results with available observations in the adriatic sea and in the gulf of lion shows that med16 ecmwf is more realistic than med16 era40 in the eastern mediterranean although deep water formation occurs in the two experiments the depth reached by the convection is deeper in med16 ecmwf in the gulf of lion deep water formation occurs only in med16 ecmwf this larger sensitivity of the western mediterranean convection to the forcing resolution is investigated by running a set of sensitivity experiments to analyze the impact of different time space resolutions of the forcing on the intense winter convection event in winter 1998 1999 the sensitivity to the forcing appears to be mainly related to the effect of wind channeling by the land orography which can only be reproduced in atmospheric models of sufficient resolution thus well positioned patterns of enhanced wind stress and ocean surface heat loss are able to maintain a vigorous gyre circulation favoring efficient preconditioning of the area at the beginning of winter and to drive realistic buoyancy loss and mixing responsible for strong convection at the end of winter

pb pb dating of young marbles from taiwan direct determination of the depositional age of a sedimentary sequence usually relies on isotopic analyses of authigenic minerals such as glauconite1 and fine grained clay minerals2 4 moorbath et al 5 recently reported the first direct dating of sedimentary carbonates using the whole rock pb pb method the success of this method depends on the chemical fractionation of u from pb and the consequent variation in 206pb 204pb and 207pb 204pb ratios the high values and large variation of u pb ratios in sedimentary carbonates long known to marine geochemists have however not been exploited by geochronologists until recently5 6 here i report a pb pb isochron age for a relatively young carbonate sequence in taiwan and demonstrate that the long lived u pb decay scheme can be used to determine events as young as mesozoic age the new results also lead to the identification of an early jurassic 170 myr bp thermal event which has never before been recognized in the polymetamorphic schist belt the tananao schist of taiwan but is widespread in southeastern continental china and other parts of eastern asia7 10

chemical variations in clay minerals of the archaean barberton greenstone belt south africa abstract four rock profiles of shales carbonates and greywacke type sediments were sampled from the mafic compositional influenced sedimentary fig tree group and from overlying more felsic related sedimentary moodies group of the archaean barberton greenstone belt in south africa clay separates of forty sedimentary rocks were examined by xrd and sem to study the mineralogical chemical and morphological variations of the constitutive illite and chlorite the clay minerals showed a significant k enrichment in the illites and mg and fe depletions in the chlorites upwards the stratigraphic sequence three independent parameters influenced these variations 1 a change in the origin of the source rocks 2 the host rock lithologies and 3 a tectono thermal recrystallization process

late holocene fire and vegetation reconstruction from the western klamath mountains california usa a multi disciplinary approach for examining potential human land use impacts the influence of native american land use practices on vegetation composition and structure has long been a subject of significant debate this is particularly true in portions of the western unite

whole rock pbpb isotopic ages of late archaean limestones karnataka india application of the pbpb isotopic dating technique to metamorphosed limestones from late archaean low and high temperature greenschist amphibolite facies terrains in karnataka southern india has revealed distinct differences in the evolution of each terrain and in their model 1 values the western low temperature terrain is characterised by tracts of late archaean volcanic and sedimentary rocks dharwar supergroup their basement of orthogneisses and granodiorites 3400 2900 ma and enclaves of older supracrustal rocks intrusions of late archaean granite 2600 ma occur sporadically in the low temperature terrain in contrast the eastern high temperature terrain is dominated by late archaean polyphase syn and post tectonic granites 2500 ma which intruded tracts of supracrustal rocks that are similar to the dharwar supergroup in the low temperature terrain the low and high temperature terrains are separated by a n to nnw trending steep belt of mylonites which is over 400 km in length the pbpb data show that metamorphic recrystallisation of the limestones in the low temperature terrain took place 2639 32 ma ago whereas limestones in the high temperature terrain recrystallised 2475 65 ma ago this finding is consistent with previously published ages of late archaean granites in the low and high temperature terrains limestones from the low temperature terrain have a 1 value of 7 79 whereas those from the high temperature terrain have a 1 value of 8 50 these values suggest that the u and pb that was incorporated in the limestones of the low temperature terrain came from a less evolved source than the u and pb in the limestones in the high temperature terrain and indicate a further contrast between these two divisions of the archaean craton in karnataka the nature of the sources is enigmatic but they may reflect differences in composition of the lithospheric plates that were involved in the dharwar collision system data from the high temperature terrain indicate disequilibrium between the measured pb isotopic ratios of carbonate and residue components of the limestones pb isotopic compositions of hci insoluble residues are consistently more radiogenic than those of associated carbonate minerals and they define a younger rotated palaeoisochron that has no age significance this is the first documentation of carbonate residue disequilibrium on the scale of a single hand specimen it highlights the need for a thorough evaluation of pb isotope systematics and the potential effects of acid leaching for multi component limestones

atlantic forcing of the mediterranean oligotrophy 1 the mediterranean sea shows a peculiar anomaly in its nutrient pattern compared to the global ocean as there is decrease in nutrient concentration from west to east this feature has been attributed to the antiestuarine circulation at the strait of gibraltar where an eastward flow of atlantic nutrient poor surface waters is compensated by a westward countercurrent of mediterranean nutrient rich deep waters this water exchange has been suggested as the ultimate cause for the oligotrophy of the mediterranean basin even though only a few studies have accurately examined the magnitude of the nutrient flux through the strait of gibraltar in this work data from the gibraltar fixed time series gift between 2005 and 2008 were used to assess nutrient distributions applying a two layer model of water mass exchange and using the mediterranean outflow recorded in situ the net export of nutrients from the mediterranean to the atlantic was calculated as 139 and 4 8 gmol yr 1 of nitrate and phosphate respectively the results also demonstrated that the atlantic inflow is not nutrient depleted and in particular contains significant levels of phosphate which is the limiting factor for biological productivity in the eastern mediterranean the distribution of the quasi conservative parameter n in the western and eastern basins indicated that nitrate deficient surface waters are transformed into phosphate deficient bottom waters by internal cycling processes therefore phosphate depletion in the mediterranean does not have its origin in the entry of a phosphorus impoverished atlantic inflow through the strait of gibraltar

marine evaporites from an oceanic island in the neoproterozoic adamastor ocean abstract we report a hitherto unknown occurrence of ancient neoproterozoic evaporite deposits from an allochthonous terrane in the pan african gariep belt in namibia low contents of rb cs ba zr hf th and u flat chondrite normalised rare earth element ree patterns 87 sr 86 sr ratios as low as 0 7075 na cl br systematics of fluid inclusion leachates and high 11 b values for stratiform tourmalinites together with geologic evidence such as association with oceanic basalt gabbro and stromatolitic dolomite point to a marine evaporitic origin an atoll environment on an oceanic island is envisaged as a likely depositional setting in the associated mafic sequence we found a diamictite with metre sized ice rafted detritus suggesting the presence of sea ice cover at relatively low latitude around the time of evaporite deposition based on chemostratigraphic 87 sr 86 sr 13 c comparison with passive continental margin sediments in the para autochthonous external part of the gariep belt a correlation of the mafic diamicitite with the global varangian 590 560 ma glaciation is proposed

regional and local controls on postglacial vegetation and fire in the siskiyou mountains northern california usa article i nfo the siskiyou mountains of northwestern california and southwestern oregon are a floristic hotspot and the high diversity of conifers there likely results from a combination of geological ecological climatological and historical factors to evaluate how past climate variability has influenced the composition structure and fire regime of the siskiyou forests pollen charcoal and lithological evidence was examined from two lakes along a moisture gradient to reconstruct the vegetation fire and climate history the late glacial period was characterized by subalpine parkland and infrequent fire at both sites during the late glacial early holocene transition period subalpine parkland was replaced by a closed forest of pinus cupressaceae abies and pseudotsuga and more frequent fires a 1000 years earlier at the wetter site and it is likely that reduced pacific ocean upwelling created warmer drier conditions at the coast in the early holocene pinus cupressaceae were less abundant and fire less frequent at the coastal site during a period of increased coastal upwelling and fog production in the late holocene abies pseudotsuga pinus and quercus vaccinifolia increased in the forest at both sites suggesting a widespread response to cooling fewer fires at the wetter site may account for the abundance of picea breweriana within the last 1000 years the comparison of the two records implies that large scale controls in climate during the last 14 000 cal yr bp have resulted in major changes in vegetation and fire regime asynchrony in the ecosystem response of wetter and drier sites arises from small scale spatial variations in effective moisture and temperature resulting from topographically influenced microclimates and coastal to inland climate gradients

the nama foreland basin sedimentation major unconformity bounded sequences and multisided active margin advance abstract the nama foreland basin developed in neoproterozoic times as a peripheral foreland basin between the kalahari craton in the east and the partly contemporaneous damara and gariep orogenic belts in the north and west the foreland basin is subdivided into three basins from north to south the zaris witputs and vanrhynsdorp basins deposits of the nama group proper fill the first two basins whereas the third contains the vanrhynsdorp group which differs in many aspects from the nama group only during its latest phase of development did the foreland basin form one continuous depository the stratigraphic sequences in all three basins are suggested to be genetically related to orogenesis flexural bulging and sea level changes that affected the basin throughout its history palaeocurrent studies show that clastic sediments were initially derived from the craton and transported west and southwestwards in fluvial environments along the entire length of the basin as mountain belts emerged along the interactive northern and western orogenic margins transport directions switched to southerly and southeasterly foredeep flysch and ramp carbonates grade upwards into reddish conglomeratic molasse derived entirely from these orogens three major unconformity bounded sequences composite sequences probably of second order cyclicity can be recognised the uppermost composite sequence cs3 comprises the fish river subgroup whereas the lower composite sequence cs1 is composed of the flaminkberg formation and the kwanous kuibis and lower and middle schwarzrand subgroups cs2 comprises the knersvlakte brandkop and upper schwarzrand subgroups cs1 is composed of transgressive and highstand sequence sets onlapping the craton and truncated by a type 1 unconformity at the base of the nomtsas formation cs2 contains a conformable lowstand flysch delta partly derived from cs1 deposits eroded on flexural bulges as well as an orogenic source in the northwest cs3 is entirely composed of orogen derived molasse onlapping a basal type 2 unconformity and overstepping older sequences in a cratonward direction a number of intrabasinal unconformities is ascribed to flexural bulging in response to tectonic and sedimentary loading the age of the nama and vanrhynsdorp groups is estimated at between 600 ma and 530 ma as determined by the age of the basement the age of detrital micas and the age of subsequent thrust and fold tectonic events that vary in vergence from southeast to northeast which affected nama and vanrhynsdorp rocks along the active basin margins marginal tectonogenesis of the nama foreland basin resulted from northwesterly subduction of the kalahari plate underneath the congo and south american plates through a process of sinistral transpression along the western kalahari margin leading to sequential closure of the proto atlantic adamastor ocean from north to south

blue amphiboles and their significance for the metamorphic history of the pan african gariep belt namibia the metamorphic history of mafic exotic blocks from a tectonic melange zone within an allochthonous ophiolitic terrane marmora terrane of the pan african gariep orogenic belt in south western namibia was studied based on mineral parageneses and amphibole composition glaucophane described previously from these rocks could not be verified instead two types of blue amphiboles were distinguished i rims of ferro edenitic to pargasitic to barroisitic hornblende composition around brownish amphibole phenocrysts replacing magmatic clinopyroxene and ii deep blue porphyroblasts of magnesio riebeckite with little ferro glaucophane component in a highly metasomatized albite rich rock textural and mineralogical evidence particularly the existence of up to three different amphibole generations in metagabbro samples supports a multiphase metamorphic history experienced by these exotic blocks the first metamorphic event m1 is interpreted as very low p hydrothermal oceanic metamorphism that affected the igneous protoliths at up to amphibolite facies temperatures subsequent m2 metamorphism was syntectonic and is characterized by temperatures similar to those attained during m1 but higher pressures indicating burial to 15 20 km this event is related to a subduction process the third metamorphic event m3 was low grade and of regional nature it is the only one recorded in the sedimentary envelope of the exotic blocks the formation of magnesio riebeckite is considered a retrograde reaction at greenschist facies during m2 the results indicate that in the gariep belt subduction and subsequent obduction have occurred although blueschist facies metamorphism has not been reached

geochemistry and tectonic setting of magmatic units in the pan african gariep belt namibia abstract major trace rare earth element and rb sr and sm nd isotope data of the various low grade metamorphosed magmatic units in the pan african gariep belt southwestern namibia presented in this paper provide the basis for the reconstruction of the evolution of the late proterozoic adamastor ocean between the south american and kalahari cratons the gariep belt is subdivided into two major zones a para autochthonous predominantly sedimentary rift and passive continental margin succession port nolloth zone pnz and the allochthonous predominantly mafic marmora terrane the latter has been thrust in southeasterly direction over the former all of the magmatic activity in the pnz can be related to lithospheric stretching the earliest magmatic activity is expressed as bimodal volcanism with continental within plate affinity along an embryonic rift graben rosh pinah formation and is found within the older sediment package of the para autochthonous external zone of the orogen after 24 ma this was followed by the intrusion of mafic tholeftic dykes into the basement and the lower parts of the pnz heralding the opening of the gariepian basin at 717 ma there is evidence of a series of seamounts or an aseismic ridge with oceanic within plate characteristics in the marmora terrane such volcanic piles make up most of the schakalsberge complex and parts of the chameis complex both complexes are separated by turbidites oranjemund complex which are interpreted as continental slope deposits in the chameis complex metagabbros predominate most of them are part of the oceanic seamounts but some have a markedly different chemistry indicative of a mid ocean ridge setting during pan african collision the marmora terrane formed an accretionary wedge that was thrust over the passive continental margin succession peak metamorphism in the thickened crust was reached at 545 ma with erosion of the orogen having commenced as early as 540 ma

age constraints on the neoproterozoic baikal group from combined sr isotopes and pb pb dating of carbonates from the baikal type section southeastern siberia abstract depositional ages for the middle part of the neoproterozoic sedimentary succession exposed along lake baikal in the southern margin of the siberian platform have been constrained by the application of sr isotope chemostratigraphy and pb pb dating of carbonate rocks the thick siliciclastic carbonate baikal group is the type section of the regional siberian succession although many of the baikal group carbonate rocks have undergone pervasive dolomitization by mixed meteoric marine fluids oolitic limestones in the 800 m thick carbonate member of the uluntui formation preserve primary textures and high sr contents up to 3200 ppm suggesting that aragonite might have been the original precursor the 87sr 86sr in the least altered high sr limestones mg ca 8 pdb range within 0 70842 0 70876 values we interpret as representing the best proxy for coeval seawater stepwise dissolution using 0 5 n hbr was applied to enrich samples in primary carbonate from the uluntui formation and nine data points from the five least altered high sr limestones yielded a pb pb isochron age of 560 30 ma with mswd 0 6 and probability 0 8 this age matches that inferred from sr isotope stratigraphy 580 550 ma and further supports the inference that the baikal paleobasin was connected to the contemporary oceans our sr chemostratigraphy and pb pb isochron age of the uluntui limestones cast doubt on previously proposed correlations of the baikal group as late mesoproterozoic to cryogenian instead the data indicate that the entire siliciclastic carbonate succession of the baikal group was most likely deposited during the ediacaran or vendian of the russian stratigraphic scale

stratigraphy of the port nolloth group of namibia and south africa and implications for the age of neoproterozoic iron formations uncertainties in the number and age of glacial deposits within the port nolloth group have hindered both structural and stratigraphic studies in the neoproterozoic gariep belt of namibia and south africa these uncertainties are compounded by major lateral facies changes that complicate correlations locally herein we report the results of integrated geological mapping chemo and litho stratigraphic and sedimentological studies that shed light on the age and stratigraphic architecture of the port nolloth group particularly we have distinguished an additional glacial deposit herein referred to as the namaskluft diamictite which is succeeded by a ca 635 ma basal ediacaran cap carbonate this interpretation indicates that the stratigraphically lower iron bearing numees diamictite is not marinoan or gaskiers in age as previously suggested but is instead a ca 716 5 ma sturtian glacial deposit a sturtian age for the numees formation is further supported by the discovery of microbial roll up structures in the dark limestone of the bloeddrif member that caps the diamictite a re evaluation of the age constraints indicates that all neoproterozoic iron formations may be of sturtian age and thus indicative of secular evolution of the redox state of the ocean

the oxygen isotope composition of karoo and etendeka picrites high δ18o mantle or crustal contamination oxygen isotope compositions of karoo and etendeka large igneous province lip picrites and picrite basalts are presented to constrain the effects of crustal contamination versus mantle source variation olivine and orthopyroxene phenocrysts from lavas and dykes mg 64 80 from the tuli and mwenezi nuanetsi regions of the ca 180 ma karoo lip have 18o values that range from 6 0 to 6 7 they appear to have crystallized from magmas having 18o values about 1 1 5 higher than expected in an entirely mantle derived magma olivines from picrite and picrite basalt dykes from the ca 135 ma etendeka lip of namibia and karoo age picrite dykes from dronning maud land antarctica do not have such elevated 18o values a range of 18o values from 4 9 to 6 0 and good correlations between 18o value and sr nd and pb isotope ratios for the etendeka picrites are consistent with previously proposed models of crustal contamination explanations for the high 18o values in tuli mwenezi picrites are limited to 1 alteration 2 crustal contamination and 3 derivation from mantle with an abnormally high 18o previously a variety of models that range from crustal contamination to derivation from the enriched mantle lithosphere have been suggested to explain high concentrations of incompatible elements such as k and average end and esr values of 8 and 16 in mwenezi nuanetsi picrites however the primitive character of the magmas mg 73 combined with the lack of correlation between 18o values and radiogenic isotopic compositions mgo content or mg is inconsistent with crustal contamination thus an 18o enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition is indicated high 18o values are accompanied by negative nb and ta anomalies consistent with the involvement of the mantle lithosphere whereas the high 18o themselves are consistent with an eclogitic source magma 18o values about 1 higher than expected for mantle derived magma are also a feature of the bushveld mafic and ultramafic magmas and the possibility exists that a long lived 18o enriched mantle source has existed beneath southern africa a mixed eclogite peridotite source could have developed by emplacement of oceanic lithosphere into the cratonic keel during archaean subduction

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation