did a katian large igneous province trigger the late ordovician glaciation a hypothesis tested with a carbon cycle model abstract the ordovician is known as a period with high temperatures and high sea levels and a cooling event at the end of the period leading to the hirnantian glaciation and the first of the big five mass extinctions of marine life the cause of this cooling event remains unclear several authors correlate it with a drop in atmospheric p co 2 to a threshold permitting the ice accumulation on the gondwana supercontinent however what are the reasons of the atmospheric p co 2 decrease here we follow the hypothesis initiated by barnes 2004 that an ordovician superplume event occurred during the middle to late ordovician such an event would not only have a large impact on the ordovician biodiversification barnes 2004 but it would also be responsible for the climatic upheaval during the late ordovician by the emplacement of a low latitudinal continental basaltic province that had an impact on the atmospheric p co 2 there is no direct evidence of a superplume event or of basaltic traps and the present study is therefore a hypothetical modelling approach where we demonstrate with a numerical box model including carbon alkalinity and phosphorus cycles coupled with a 1d climate model energy balance model ebm francois and walker 1992 that such a scenario allows to explain both the mid ashgill katian global warming event known as the boda event fortey and cocks 2005 and the subsequent late ordovician hirnantian glaciation because silicate weathering is enhanced upon warm and wet climate we try to constrain the size and the latitudinal emplacement of the basaltic province leading to a sufficient consumption of atmospheric p co 2 to the threshold proposed by herrmann et al 2004 a b to initiate a glaciation on gondwana

geochemistry and geochronology of the amphibolite blocks in ophiolitic melanges along bangong nujiang suture central tibet abstract amphibolites occur as blocks within serpentinite matrix melange at dong tso and lagkor tso in the gertse area of central tibet where they are associated with the bangong nujiang suture bns geochemical data shows amphibolites have similar morb or arc like morb geochemical characteristics lagkor tso amphibolites show greater arc affinity whereas those from dong tso amphibolites are more morb like these characteristics are similar to basalts from continental back arc basin settings hornblendes from the amphibolites dated by the 40 ar 39 ar single grain laser fusion technique show similar metamorphic ages 170 177 ma which is interpreted to represent the timing of the main metamorphic event experienced in this region we suggest that the western and middle sectors of the bns contain the remnants of a short lived back arc basin the bangong nujiang ocean which partly opened and closed in the middle jurassic

carbon isotopes and event stratigraphy near the ordovician silurian boundary yichang south china abstract the organic carbon isotope data through the unweathered ordovician silurian boundary strata at the wangjiawan riverside section which is c 180 m southeast of the wangjiawan north section the gssp for the base of the hirnantian stage upper ordovician show that a positive 13 c org excursion begins just below the base of the hirnantian stage and peaks in the lower part of the n extraordinarius biozone this is followed by an interval of slightly reduced 13 c values and a second peak of 2 above pre hirnantian values which occurs in the lower part of the n persculptus biozone upper hirnantian the peaks in 13 c org values can be correlated well with episodes of glacial expansion described from africa and peri gondwanan europe evidence from sedimentological faunal and geochemical data from south china as well as the evidence of glacial sediments in north africa all suggests a short lived glaciation of 1 ma in the ordovician south pole region consisting of at least two major pulses the first phase of gradual glacial expansion began just before the hirnantian epoch and the second phase ended with rapid melting in the late hirnantian early n persculptus biozone

proximal provenance of the western songpan ganzi turbidite complex late triassic eastern tibetan plateau implications for the tectonic amalgamation of china abstract the late triassic songpan ganzi turbidite complex on the eastern tibetan plateau which covers an area of 2 2 10 5 km 2 is one of the largest flysch turbiditic basins on earth it is juxtaposed with major chinese continental blocks across several outstanding tethyan sutures and is critical to elucidating the tectonic amalgamation of china however the provenance of the turbidites remains the subject of intense debate detrital modes and heavy mineral spectra of sandstone samples from three turbidite profiles in the western songpan ganzi complex were determined in an attempt to elucidate their provenance and the type of tectonic setting in which they were deposited upper triassic turbidites in the western songpan ganzi complex have an overwhelming derivation from an orogen source as shown by the average framework compositions of the sandstones q 57 f 21 l 22 qm 52 f 21 lt 27 qp 23 lvm 0 lsm 77 the compositional and textural immaturity of the sandstones suggests limited transport and nearby sources both light and heavy mineral and lithic fractions indicate dominant metamorphic source rocks and subordinate ophiolitic and sedimentary source rocks the occurrence of c type garnet omphacite rutile and si rutile and the high silica content of phengites reveal that the source rocks underwent ultrahigh pressure conditions therefore we advocate that the western songpan ganzi late triassic turbidites were derived from nearby central qiangtang triassic collisional orogen sources rather than distant dabie qinling sources eroded after north china south china collision as previously proposed the analogy of the songpan ganzi turbidites to the bengal indus fans is not favored the central qiangtang metamorphic belt is unlikely to have occurred as an early mesozoic melange underthrust from the jinsa suture and is more likely to have been an in situ triassic ultrahigh pressure orogen our provenance interpretation implies that the paleotethys represented by the present jinsa and kunlun sutures was not yet closed when central qiangtang was being exhumed to supply the sediments of the songpan ganzi basin this challenges the conventional model for sequential amalgamation of china during the phanerozoic

pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols the depth to carbonate nodular bk horizon in soils d in cm is correlated with mean annual precipitation p in mm so that bk horizons are deep in subhumid regions but shallow in semiarid regions previous quantifications of this relationship are unchanged by this new compilation of 807 soils p 137 24 6 45 d 0 013 d 2 where r 2 0 52 and standard error s e 147 mm in most north american postglacial soils the bk horizon is thin and well defined whereas in monsoonal tropical soils of pakistan and kenya the bk horizon is thick and diffuse data from 675 modern soils define the relationship between the thickness of soil with nodules t in cm and mean annual range of precipitation m in mm difference between monthly means of wettest and driest months m 0 79 t 13 71 where r 2 0 58 and s e 22 mm the relationship between carbonate nodule size s in cm and soil age a in ka is quantified by 9 radiocarbon dated soils from las cruces new mexico a 3 92 s 0 34 where r 2 0 57 and s e 1 8 k y these transfer functions are applied to reconstructing paleoclimatic change from paleocene eocene paleosols of the north horn formation and flagstaff limestone of axhandle canyon utah the terminal paleocene spike of warmth recorded by fossil plants in nearby wyoming was coincident with a brief peak in both mean annual precipitation and mean annual range of precipitation in utah

multiple early triassic greenhouse crises impeded recovery from late permian mass extinction the late permian mass extinction was not only the most catastrophic known loss of biodiversity but was followed by unusually prolonged recovery through the early triassic opinion has been divided on whether delayed recovery was a legacy of especially profound ecological disruption or due to additional environmental perturbations new records from the sydney basin in southeastern australia now reveal five successive late permian and early triassic spikes of unusually high atmospheric co2 and profound chemical weathering these successive atmospheric co2 greenhouse crises coincided with unusually warm and wet paleoclimates for a paleolatitude of 61 s successive transient greenhouse crises punctuated long term cool dry and low co2 conditions and may account for the persistence of low diversity and small size in early triassic plants and animals

response of the inter tropical convergence zone to southern hemisphere cooling during upper ordovician glaciation stable isotope ratios of whole rock carbonates and faunas from three low latitude upper ordovician sections demonstrate a coherent pattern of shifting subtropical and tropical water masses and associated climate belts we suggest that tropical water beneath the inter tropical convergence zone 18o 10 to 15 was isotopically light reflecting high sea surface temperatures sst and reduced sea surface salinity sss subtropical water in both the northern and southern hemispheres is characterised by 18o values from 4 to 6 our interpretation is consistent with the published results from physical chemical modeling of ordovician seawater composition and climate models deriving absolute temperature and salinity values from ordovician 18ocarb data is problematic as they could reflect sst sss ice volume and potentially a variable diagenetic overprint and currently no independent constraints exist for any of these variables the itcz was positioned at 10 n of the equator during the early katian consistent with the likely position of the region of peak sea surface temperature sst during the late katian boda event the itcz moved across the equator to 10 s by analogy with the pliocene this resulted in a symmetrical distribution of sea surface currents about the equator this led to a net increase in heat flow towards the poles and lower latitudinal temperature gradients potentially explaining the migration of warm water faunas to higher southern latitudes during the boda event during the hirnantian the itcz shifted north of the equator resulting in a reduced poleward heat transport itcz position has been proposed as a likely forcing mechanism of neogene glaciation and the evidence from the ordovician suggests a discrete palaeo itcz whose position was controlled by solar insolation and latitudinal temperature gradients linked to ice volume changes and hadley cell dynamics we hypothesize that the position of the itcz was sensitive to the growing hirnantian ice sheet during the late ordovician we propose that latitudinal movement of the itcz during the late ordovician resulted in a major atmosphere ocean circulation re organization higher pco2 estimates for the ordovician caused more extreme changes in this circulation that were the likely primary cause of biotic mass extinction if the monsoon is considered an off equator itcz then we provide direct evidence of an ordovician monsoonal climate

climatic forcing of carbon oxygen isotopic covariance in temperate region marl lakes carbon and oxygen stable isotopic compositions of lacustrine carbonate from a southeastern michigan marl lake display linear covariance over a range of 4 0 peedee belemnite pdb in oxygen and 3 9 pdb in carbon mechanisms of delta 13 c delta 18 o coupling conventionally attributed to lake closure in arid region basins are inapplicable to hydrologically open lake systems thus an alternative explanation of isotopic covariance in temperate region dimictic marl lakes is required we propose that isotopic covariance is a direct record of change in regional climate in short residence time temperate region lake basins summer meteoric precipitation is enriched in 18o relative to winter values and summer organic productivity enriches epilimnic dissolved inorganic carbon in 13c thus climate change toward longer summers and or shorter winters could result in greater proportions of warm month meteoric precipitation longer durations of warm month productivity and net long term enrichment in carbonate 18o and 13c isotopic covariance observed in the michigan marl lake cores is interpreted to reflect postglacial warming from 10 to 3 ka followed by cooler mean annual temperature a shift toward greater proportions of seasonal summer precipitation a shortening of the winter season or some combination of these three factors

organic and carbonate rich soil formation 2 6 billion years ago at schagen east transvaal district south africa abstract a 17 m paleosol sequence at schagen south africa which developed on a serpentinized dunite intrusion in a granite gneiss terrain 2 6 ga ago is characterized by an alternating succession of thick 1 3 m carbonate rich dolomite and calcite zones and silicate rich serpentines talc and quartz zones the upper 8 m section is especially rich in organic c up to 1 4 wt petrologic and geochemical data suggest the upper 8 m section is composed of at least three soil profiles that developed on i silicate rich rock fragments and minerals that were transported from local sources serpentinite and granite by fluvial and or eolian processes and ii dolomite and calcite zones that formed by locally discharged groundwater the mg and fe in the paleosol sequence were largely supplied from local sources mostly serpentinite but the ca sr and hco3 were supplied by groundwater that originated from a surrounding granite gneiss terrain in the uppermost soil profile the fe is retained the fe3 fe2 ratio increases and ferri stilpnomelane is abundant these data suggest the atmospheric po2 was much greater than 10 3 7 atm 0 1 present atmospheric level pal the carbonaceous matter in the soils is intimately associated with clays talc chlorite and ferri stilpnomelane and occurs mostly as seams 20 m to 1 mm thick that parallel the soil horizons these occurrences crystallographic structures h c ratios and 13corg values 17 4 to 14 4 pdb suggest that the carbonaceous matter is a remnant of in situ microbial mats originally 1 to 20 mm thick the microbial mats developed a mostly on soil surfaces during the formation of silicate rich soils and b at the bottom of an evaporating anoxic alkaline pond during the precipitation of the fe rich dolomite these 13corg values are difficult to be explained by a current popular idea of a methane and organic haze rich archean atmosphere pavlov et al 2001 these values however can be easily explained if the microbial mats were composed of cyanobacteria and heterotrophs that utilized the remnants of cyanobacteria in a strongly evaporating environment

chemistry of modern soils as a guide to interpreting precambrian paleosols precambrian paleosols are potentially one of our best sources of information about the composition of the atmosphere early in earth s history to be used this way however an alteration profile must be shown to be an in situ product of vadose processes data from modern soils show that the ratios of less mobile elements such as zr ti and al can be used as a preliminary screen to test for the in situ nature of a profile if the ti zr ratio departs from that of the host rock by more than 40 or the ti al ratio departs by more than 50 for moderate degrees of weathering then a significant allochthonous contribution to the profile is indicated this technique correctly separates in place from allochthonous horizons when applied to independent data from two modern profiles a review of published precambrian profiles identifies two often cited examples denison and pronto as suspect based on these criteria in the interpretation of paleoatmospheres it is also necessary to show that fe is controlled not b

indepth iii seismic data from surface observations to deep crustal processes in tibet 1 during the summer of 1998 active source seismic data were collected along a transect running 400 km nnw sse across the central tibetan plateau as the third phase of project indepth international deep profiling of tibet and the himalaya the transect extends northward from the central lhasa block across the jurassic bangong nujiang suture bns at 89 5 e to the central qiangtang block a seismic velocity model for the transect to 25 km depth produced by inversion of p wave first arrivals on 3000 traces shows 1 a 50 km wide region of low velocity at least 5 less than surrounding velocities extending to the base of the model at the bns 2 sedimentary cover for the southern qiangtang block that is 3 5 km thick 3 a distinct interface between sedimentary cover and qiangtang basement or underplated jurassic melange in the central qiangtang block and 4 evidence that the bangoin granite extends to a depth of at least 15 km the bns has little geophysical signature and appears unrelated to the 5 km northward shallowing of the moho which is associated with the bns in central tibet geophysical data along the main indepth iii transect show little evidence for widespread crustal fluids in contrast to the seismic bright spots found in southern tibet and to magnetotelluric evidence of fluid accumulations in eastern tibet a comparison between the global average and tibetan velocity depth functions offers constraints for models of plateau uplift and crustal thickening taken together with the weak geophysical signature of the bns these velocity depth functions suggest that convergence has been accommodated largely through pure shear thickening accompanied by removal of lower crustal material by lateral escape likely via ductile flow although we cannot resolve the details we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of tibet as a series of accreted terranes

tectonics of western tibet between the tarim and the indus a tarim to indus traverse provides insight into the tectonics of western tibet the kunlun was the site of a mid paleozoic collision at least three phases of post paleozoic accretion have thickened the blanket of sediments that covers western tibet sizeable parts of western qiangtang have remained stable however since the mid mesozoic since the neogene deformation and volcanism have been localized near the edges of the plateau strike slip motion along the karakorum and altyn tagh faults has been coeval with overthrusting in the himalayas and kunlun such slip partitioning and the volcanism appear to result simply from northward subduction of india and southward subduction of the tarim as tibet is extruded eastwards by india s penetration into asia

equation for compaction of paleosols due to burial existing empirically derived compaction curves for marine sediments fail to estimate the compaction of paleosols flood plain sediments and peats as indicated by deformation of clastic dikes dinosaur footprints and sandstone paleochannels yet compaction estimates are vital to reconstructing paleoclimate and geochemical profiles of paleosols to modeling sediment accumulation rates of nonmarine rocks and to tectonic flexural modeling of molasse facies this paper presents a compaction equation and physically derived constants that make geologically realistic estimates of compaction of paleosols and other nonmarine sediments a protocol for the application of the equation is suggested that would allow the following equation to solve for burial compaction c as a fraction of original thickness given depth of burial d in km as c s i f e d k 1 this equation can be applied to nonmarine sedimentary rocks or paleosols given appropriate empirical data on the physical constants in the equation such as initial solidity s i initial porosity f and the corresponding curve fitting constant k data on physical constants useful for this equation are compiled here for a range of paleosol and nonmarine sediment types

mixing zone diagenesis in the subsurface of florida and the bahamas abstract three cores from great bahama bank and southern florida contain examples of mixing zone diagenesis where typical shallow water aragonite rich sediments were altered to low mg calcite the 40 50 m thick mixing zones are identified on the basis of downcore covariant trends from depleted meteoric 18o 3 2 0 7 13c 1 6 1 8 to enriched marine isotopic values 18o 1 0 0 3 13c 1 9 0 7 the age of the mixing zones varies from late pliocene to pleistocene so these cores provide three independent examples of mixing zone diagenesis mixing zone diagenesis produced a limestone composed of low mg calcite with petrographic fabrics including moldic porosity blocky to dogtooth calcite cementation and recrystallization of lime mud to micrite and microspar the mixing zone diagenetic model predicts alteration in only part of the mixing zone because the alteration recorded in these cores shows continuous alteration through the entire mixing zone we consider the mixing of waters itself to be largely irrelevant rather diagenesis was probably driven by the difference in solubility between aragonite and calcite with some additional dissolution of aragonite driven by the microbial oxidation of buried organic matter the mixing zone has been proposed as a likely location for dolomitization with the exception of some clearly sea floor dolomite in one core unda the mixing zones in these cores lack dolomite despite the sediment clearly having been altered in the mixing zone the lack of dolomite in these mixing zones combined with the extensive literature questioning the chemical reasoning for mixing zone dolomite supports rejection of the mixing zone dolomite model

sequence stratigraphy and relative sea level history of the middle and upper ordovician of the nashville dome tennessee abstract eleven depositional sequences are present in the middle and upper ordovician of the nashville dome all but one lack lowstand systems tracts and many display evidence of subaerial exposure such as paleokarst or regional erosional truncation at their sequence boundaries which are combined with transgressive surfaces features formed during the transgression such as hardgrounds are more prominent at most sequence boundaries than are those features related to subaerial exposure all sequences have maximum flooding zones with aggradational stacking rather than discrete maximum flooding surfaces condensation features are much more common at transgressive surfaces than within the maximum flooding zone the expression of these parasequences and sequences changes with the onset of the taconic orogeny although parasequences from equivalent depositional environments can be recognized before and after the onset of the orogeny their facies composition changes completely sequences following the orogeny display fewer paleokarst features more pyritized and iron stained hardgrounds at flooding surfaces and transgressive surfaces and more phosphatic lags and mineralization at transgressive surfaces than those preceding the orogeny a relative sea level curve for the nashville dome has a long term 5 m y component presumably dominated by subsidence and a short term 1 3 m y component consisting of 10 50 m fluctuations that is primarily the result of eustasy

eclogites from central qiangtang northern tibet china and tectonic implications abstract eclogites in central qiangtang northern tibet china are characterized by the garnet omphacite rutile phengite quartz assemblage hornblende plagioclase and sphene occur as retrograde amphibolite facies phases in the eclogites pressure temperature estimates for the primary eclogite assemblage are consistent with minimum pressures of 20 25 kbar at 482 to 625 c the retrograde amphibolite facies assemblage suggests conditions of 10 12 kbar and 460 600 c consequently the eclogites from central qiangtang have undergone unambiguous ultrahigh pressure uhp metamorphism the presence of uhp metamorphic rocks implies a likely deep seated continental subduction along the shuanghu suture within the qiangtang terrain the denudation of these uhp metamorphic rocks correlates well with the main interval that the songpan ganzi turbidites accumulated and could have provided a significant source for these sedimentary rocks

trace fossil evidence for late ordovician animals on land fossil burrows within newly recognized buried soils in the late ordovician juniata formation near potters mills in central pennsylvania represent the oldest reported nonmarine trace fossils they are thought to have been an original part of the soil because their greater density toward the top of the paleosols corresponds with mineralogical microstructural and chemical changes attributed to ancient weathering and because about half the burrows are encrusted with nodular carbonate interpreted as caliche associated fossil caliche the size distribution of the burrows and their w shaped backfills are evidence that the burrows may have been excavated by bilaterally symmetrical organisms that grew in well defined growth increments and were able to withstand desiccation among well known soil organisms millipedes are burrowing animals that satisfy these requirements but have a fossil record not quite this old this trace fossil evidence for animals on land together with recent palynological evidence for land plants of a bryophytic grade of evolution during late ordovician time are indications of terrestrial ecosystems of slightly greater antiquity and complexity than hitherto suspected

late ordovician mass extinction in the selwyn basin northwestern canada geochemical sedimentological and paleontological evidence we present a detailed study of the trace element and stable isotope geochemistry sedimentology and fossil distributions in two avalanche lake av4b av1 ordovician silurian boundary sections in the selwyn basin trilobites and conodonts indicate a profound extinction at the end of the ordovician which is constrained stratigraphically within a 3 in magnitude is recorded in the extinction interval iridium abundances 0 051 ppb in the extinction interval are low and fail to provide evidence for an impact the highest ir abundance is found to be associated with reduced sedimentation in a condensed horizon cerium anomalies indicate a s

mallee model for mammal communities of the early cenozoic and mesozoic abstract mallee is an endemic australian woodland and shrubland of semi arid summer dry regions between dry woodland and desert shrubland in other parts of the world such as africa and the americas such climatic regions support grassland ecosystems using australian and african climofunctions and models gives very different reconstructions of paleoclimate subhumid versus perhumid and paleovegetation woodland versus rainforest for north american fossil mammal faunas before cenozoic evolution of grassland ecosystems modern mammal faunas of africa and australia have different ecological spectra of taxonomic units body size feeding and locomotion of species of mammals on precipitation gradients gradients in proportions of such categories yield transfer functions for mean annual precipitation from percent species of artiodactyla or macropodidae percent species of moderately large animals 45 180 kg percent species of arboreal mammals and percent species of grazers these transfer functions can be applied to fossil mammal faunas to estimate paleoprecipitation in africa australia and north america modern transfer functions match well paleoprecipitation estimates based on depth to calcic horizons in paleosols at the same localities in kenya and inland australia back through the miocene for fossil mammal faunas of the rocky mountain region of north america african transfer functions fail but australian transfer functions predict paleoprecipitation back to the cretaceous tertiary boundary 66 ma furthermore modern mallee soils investigated in this study closely match cretaceous to eocene paleosols of the rocky mountains extinct mallee like vegetation such as pori woodlands of kenya and cunhaka woodlands newly defined of the rocky mountains better explains the dominance of small nocturnal insectivorous arboreal mammals of paleogene and mesozoic mammal faunas than comparisons with african grassland or rainforest faunas

co2 forced climate thresholds during the phanerozoic abstract the correspondence between atmospheric co2 concentrations and globally averaged surface temperatures in the recent past suggests that this coupling may be of great antiquity here i compare 490 published proxy records of co2 spanning the ordovician to neogene with records of global cool events to evaluate the strength of co2 temperature coupling over the phanerozoic last 542 my for periods with sufficient co2 coverage all cool events are associated with co2 levels below 1000 ppm a co2 threshold of below 500 ppm is suggested for the initiation of widespread continental glaciations although this threshold was likely higher during the paleozoic due to a lower solar luminosity at that time also based on data from the jurassic and cretaceous a co2 threshold of below 1000 ppm is proposed for the initiation of cool non glacial conditions a pervasive tight correlation between co2 and temperature is found both at coarse 10 my timescales and fine resolutions up to the temporal limits of the data set million year timescales indicating that co2 operating in combination with many other factors such as solar luminosity and paleogeography has imparted strong control over global temperatures for much of the phanerozoic

refining the pedogenic carbonate atmospheric co2 proxy and application to miocene co2 abstract quantifying uncertainty is essential for any paleoenvironmental proxy recent work on propagating error through the equation used to determine atmospheric co 2 concentrations from paleosol carbonates yields conflicting results small magnitude uncertainty from gaussian error propagation contrasts with larger magnitude uncertainty from monte carlo error propagation the discrepancy is reconciled here by revising partial differential equations for the gaussian approach uncertainties calculated using the two approaches are compared and applied to miocene calcic paleosols monte carlo propagated errors are asymmetrical but otherwise agree with gaussian errors three methods for assigning soil respired co 2 concentrations s z to paleosols are also compared a revised calibration of depth to the bk horizon as a direct proxy for s z is presented and although it results in lower atmospheric co 2 concentrations than the original calibration it results in significantly higher calculated atmospheric co 2 than the use of mean annual precipitation as a proxy for s z or assigning s z values based on soil order averaging atmospheric co 2 concentrations calculated using soil order based s z values from 10 penecontemporaenous and independent paleosols will likely result in useful constraints on earth system sensitivity the improvement of proxies for s z is needed to increase the accuracy and precision and to resolve potential volatility in co 2 time series

environmental controls on the post permian recovery of benthic tropical marine ecosystems in western palaeotethys aggtelek karst hungary climate warming during the late permian is associated with the most severe mass extinction event of the phanerozoic and the expansion of hypoxic and anoxic conditions in shallow shelf settings it has been hypothesised that wave aeration provided a habitable zone in the shallowest environments that allowed the survival and rapid recovery of benthic invertebrates during the early triassic we test this hypothesis by studying the rock and fossil records of the aggtelek karst hungary nearshore settings recorded in the bodvaszilas sandstone formation and units a and d of the szin marl formation are characterised by taxonomically homogenous fossil assemblages of low diversity and low evenness ecological and taxonomic recovery in this environmental setting was hampered by persistent environmental stress this stress is attributed to increased runoff related to climate warming during the early triassic that resulted in large salinity fluctuations increased sedimentation rates and eutrophication that led to seasonal hypoxia and an environment only favourable for opportunistic taxa in contrast shoal and mid ramp settings further offshore are characterised by high diversity faunas with a greater functional complexity prior to the late spathian tirolites carniolicus zone the shelly fossils and trace fossils are limited to settings aerated by wave activity which supports the habitable zone hypothesis in the tirolites carniolicus zone however the oxygen minimum zone retreats offshore and the habitable deeper shelf settings are rapidly colonised by shallow water taxa evidenced by the highest levels of diversity and bioturbation recorded in the study locally full recovery of marine ecosystems is not recorded until the illyrian with the establishment of a sponge reef complex

microbial precipitation of pedogenic calcite pedogenic calcite in desert soils has become increasingly important as an indicator of paleoclimate landscape stability and landscape age this study indicates that calcic and petrocalcic horizons in desert soils are not simply the result of inorganic precipitation of calcite soil microorganisms were found to be involved in calcite precipitation in a typical desert soil near las cruces new mexico fossilized remains of calcified fungal hyphae and microcodium structures are abundant in the petrocalcic horizon soil bacteria and fungi precipitated calcite when cultured on a ca rich medium in an experiment where soil columns were irrigated with ca rich solutions calcite formed in soils containing soil microorganisms but no calcite formed in sterile soils thus biomineralization of calcite by soil microorganisms appears to be an important mechanism of unknown magnitude

geobiology of the late paleoproterozoic duck creek formation western australia the ca 1 8 ga duck creek formation western australia preserves 1000 m of carbonates and minor iron formation that accumulated along a late paleoproterozoic ocean margin two upward deepening stratigraphic packages are preserved each characterized by peritidal precipitates at the base and iron formation and carbonate turbidites in its upper part consistent with recent studies of neoarchean basins carbon isotope ratios of duck creek carbonates show no evidence for a strong isotopic depth gradient but carbonate minerals in iron formations can be markedly depleted in 13 c in contrast oxygen isotopes covary strongly with depth 18 o values as positive as 2 vpdb in peritidal facies systematically decline to values of 6 to 16 in basinal rocks reflecting we posit the timing of diagenetic closure the duck creek formation contains microfossils similar to those of the gunflint formation canada they are restricted to early diagenetic cherts developed in basinal facies strengthening the hypothesis that such fossils capture communities driven by iron metabolism indeed x ray diffraction data indicate that the duck creek basin was ferruginous throughout its history the persistence of ferruginous waters and iron formation deposition in western australia for at least several tens of millions of years after the transition to sulfidic conditions in laurentia suggests that the late paleoproterozoic expansion of sulfidic subsurface waters was globally asynchronous

tectonic evolution of the early mesozoic blueschist bearing qiangtang metamorphic belt central tibet this is the published version copyright 2003 american geophysical union all rights reserved

early triassic trace fossils from the three gorges area of south china implications for the recovery of benthic ecosystems following the permian triassic extinction abstract the lower triassic daye and jialingjiang formations of the three gorges area south china record the recovery interval of benthic tracemaking invertebrates following the p tr mass extinction a total of 17 ichnospecies in 14 ichnogenera are documented from smithian and spathian strata our trace fossil data in combination with previously published studies show that ichnodiversity in the middle yangtze region increased markedly in the early spathian trace fossils in the smithian are dominated by simple small horizontal burrows including didymaulichnus and planolites whereas spathian trace fossils are diverse and abundant with moderate high bioturbation levels and complex burrow networks such as thalassinoides both burrow sizes and penetration depths increased gradually from the early spathian to the middle late spathian implying a gradual recovery pattern for benthic ecosystems early triassic ichnofossils are characterised by aspects of opportunistic behaviour e g low to moderate ichnodiversity low to moderate bioturbation small burrow widths and shallow tiering suggesting stressed environmental conditions the recovery tempo and pattern of ichnocoenoses in south china is likely structured by temporal and spatial changes of the refuge zone in the early triassic

carbon isotopic evidence for photosynthesis in early cambrian oceans were the first metazoan reefs ecologically similar to modern tropical reefs enabling them to persist under oligotrophic conditions we tested the hypothesis of ecological similarity by employing a geochemical approach petrography cathodoluminescence trace elements and stable isotope analyses of primary precipitates of the lower cambrian ajax limestone south australia indicate preservation of original c isotopic composition all primary carbonate components exhibit c isotopic values similar to the composition of inorganically precipitated fibrous marine cements suggesting that archaeocyaths and the calcimicrobe epiphyton precipitated skeletal carbonate in equilibrium with ambient seawater in the absence of vital effects such data do not support the contention that archaeocyaths possessed photosymbionts however a 0 55 shift in 13 c occurs in reefs developed under shallower water conditions relative to deeper reefs this shift suggests the stratification of primary production in early cambrian oceans the pattern is similar to that seen in the modern ocean whereby significant photosynthesis modulates the c isotopic composition of the photic zone

evidence from middle ordovician paleosols for the predominance of alkaline groundwater at the dawn of land plant radiation atmospheric carbon is estimated to have been 500 times more abundant in hadean time than at present and its concentration has been gradually decreasing since then due to its storage in sedimentary rocks consequently rain ph has been gradually increasing through geologic time leading to the common assumption that groundwaters are less acidic today than they were in the distant past however this assumption overlooks the fact that root forming land plants increase the carbonic acid concentration in soils by one or two orders of magnitude in the absence of rooted land plants reactions between minerals and rainwater are known to promote alkalinity it is hypothesized that groundwater ph must have been on average highest shortly before the late ordovician to silurian proliferation of root forming land plants to verify this hypothesis we studied the mineralogy and geochemistry of the youngest known pre silurian paleosols to have developed on primary rocks and provide evidence that they have evolved in predominantly alkaline groundwaters despite warm and humid paleoenvironmental conditions today a lush vegetation cover would thrive in such a climate and the system would be necessarily acidic due to large inputs of organic acids paired with previous observations indicating that early paleozoic sedimentary rocks are especially rich in detrital illite and k feldspar there is now enough evidence to believe that there was a greater tendency for alkalinity during this time period than during previous and subsequent geologic periods

field and laboratory tests for recognition of ediacaran paleosols abstract not a single paleosol had been described from rocks of ediacaran age until 2011 but 354 ediacaran paleosols have been described by 20 different authors since then some of these newly recognized paleosols have proven controversial so this paper reviews 20 distinct tests to determine whether a particular ediacaran bed could be a paleosol or not one problem has been that ediacaran paleosols are not precisely like modern soils because they lack root traces a diagnostic feature of silurian and geologically younger paleosols the principal problem for recognition of some ediacaran paleosols is the occurrence in them of megafossils assumed to have been marine although most of these fossils remain problematic for both biological and ecological affinities not all the tests discussed here are diagnostic of paleosols some are ranked permissive or persuasive permissive conditions for paleosols include ripple marks hummocky bedding pyritic limestones acritarchs or thalloid fossils low strontium isotopic ratios high 26 mg ratios and red color persuasive tests include loessites tsunamites desert playa minerals low boron content high 10 b isotopic ratios high carbon sulfur ratios and very low total reactive iron ratios diagnostic tests include matrix supported lapilli or crystal tuff parent materials ice wedges and other cryoturbation sepic birefringence fabrics evaporitic sand crystals and negative geochemical strain and mass transfer and highly correlated 13 c and 18 o like other geological periods the ediacaran is known from a variety of marine and non marine paleoenvironments

nd isotopes of siliciclastic rocks from tibet western china constraints on provenance and pre cenozoic tectonic evolution nd isotope data from pre cenozoic siliciclastic rocks from tibet are used to determine variations of source regions with time and hence tectonic history on average the tdm and tchur ages of the tibetan samples are up to 1 83 0 36 ga and 1 25 0 45 ga respectively indicating that tibet is underlain by lower proterozoic basement and has close affinity with the himalayas an nd isotope crisis occurred at the early permian with older samples defining a trend of slight decrease in end values with time while the lower permian rocks sharply increasing 6 5 end units this is related to the coeval widespread basaltic magmatism in tibet which could be responsible for tibet rifting away from northern gondwanaland the central qiangtang meta sedimentary samples have distinct nd isotope characteristics end 0 19 87 1 51 tdm 2 07 0 13 ga tchur 1 78 0 21 ga from the songpan ganzi samples end 0 12 77 2 87 tdm 1 49 0 18 ga tchur 1 08 0 23 ga suggestive that these two groups were derived from differentsourcesandthatthe former werenot theequivalents ofthe latterthat wasunderthrust fromthe jinsasuture andmayrepresent subducted continental crust the mid jurassic flysch rocks in northern lhasa have typical upper crust nd isotopic signatures and could have formed under a passive margin environment their bangong counterparts averagely 6 35 units higher in end values unveil derivations from a likely magmatic arc along the northern bangong margin basaltic pebblewith low nd content and endvalue from the berriasian valanginian rocks in central tibet along with the samples with generally high end values shows that the bangong ophiolites could have been a main source of sediments during the earliest cretaceous time a significant transition of sediment sources occurred at the beginning of the hauterivian stage 120 ma sedimentary basaltic tuff end t 2 27 in the hauterivian lower cenomanian samples in central tibet and common nd signatures for input of mantle like materials in these rocks indicate that mantle derived magmatic rocks could have constituted a significant source the huge hauterivian lower cenomanian sediments in central tibet are thus inferred to have been accumulated under a back arc rifting setting 2007 elsevier b v all rights reserved

the effectiveness of the paleoproterozoic biological pump a δ13c gradient from platform carbonates of the pethei group great slave lake supergroup nwt samples of carbonate cements collected along a depth transect of the pethei platform a 1 9 ga stromatolitic reef reveal a small 0 5 carbon isotope gradient between shallow and basinal facies the magnitude of this gradient would conventionally be interpreted as indicating low export of organic matter from ocean surface waters but steady state simulations using a two box model of the paleoproterozoic ocean suggest that the small carbon isotope gradient could instead be due to high partial pressures of carbon dioxide in the paleoproterozoic atmosphere which would increase the ocean9s dissolved inorganic carbon content and damp the effects of biological pumping if the paleoproterozoic atmosphere were indeed enriched in atmospheric co 2 in compensation for a less luminous sun these results indicate that the planktonic biota was a significant component of the precambrian global carbon and nutrient cycles affecting if not regulating as today the composition of both surface and deep ocean waters

classification of paleosols despite increased interest in paleosols during the past decade no satisfactory classification is in current use presented here is a largely descriptive classification system that utilizes those pedogenic features that have the highest preservation potential in the rock record emphasized in the classification are morphological and mineralogical features that are easily recognizable in the field and through the petrographic microscope the classification is based on evaluation of the relative prominence in a paleosol of six pedogenic features or processes organic matter content horizonation redox conditions in situ mineral alteration illuviation of insoluble minerals compounds and accumulation of soluble minerals the most prominent of these six features processes provides the key to classifying a paleosol into one of nine orders four of the order names are borrowed from soil taxonomy histosol spodosol oxisol vertisol whereas the other five order names are presented here for the first time calcisol gypsisol gleysol argillisol protosol the orders may be preceded by one or more subordinate modifiers that describe other important features of the paleosol the classification is relatively easy to apply to the rock record and should enhance communication and aid in the standardization of terminology

early jurassic microbial mats a potential response to reduced biotic activity in the aftermath of the end triassic mass extinction event wrinkle structures are microbially induced sedimentary structures miss formed by cyanobacteria and are common in pre cambrian and cambrian siltstones and sandstones but are otherwise rare in the phanerozoic geological record this paper reports the first discovery of mesozoic wrinkle structures from sweden these are preserved in fine grained and organic rich heterolithic strata of the lower jurassic hettangian hoganas formation in skane southern sweden the strata formed in a low energy shallow subtidal setting in the marginal parts of the danish rift basin palynological analyses of fine grained sandstones hosting the wrinkle structures show that the local terrestrial environment probably consisted of a wetland hosting ferns cypress and the extinct conifer family cheirolepidaceae palynostratigraphy indicates a hettangian age still within the floral recovery phase following the end triassic mass extinction event the finding of wrinkle structures is significant as the presence of microbial mats in the shallow subtidal zone in a deeper setting compared to where modern epibenthic microbial mats grow suggests decreased benthic biodiversity and suppressed grazing in shallow marine environments in the early aftermath of the end triassic mass extinction event

assessing pedogenic calcite stable isotope values can positive linear covariant trends be used to quantify palaeo evaporation rates abstract palaeoclimate models suggest enhanced evaporation rates in subtropical regions 15 30 latitude during greenhouse world conditions however there are no empirical data to support these estimates laboratory evaporation experiments have shown that calcites precipitated from variably saturated solutions yield a positive linear covariant trend plct in 18 o calcite versus 13 c calcite values the goal of the present controlled laboratory experiments is to develop a method to quantify regional palaeo evaporation rates from palaeosol calcrete plcts samples of powdered caco 3 were dissolved in de ionized water in pressure sealed containers with a range of elevated atmospheric p co 2 concentrations for 24 h the solution was then allowed to evaporate completely from an open container within an incubator with the time of calcite crystallization noted and aliquots removed for analysis every 24 h the precipitated calcite produced an array of 18 o calcite versus 13 c calcite values that fall upon a plct projected from a theoretical meteoric calcite line mcl water analyses yielded 18 o water enrichments from an initial value of 4 75 vsmow ranging up to between 10 0 and 14 8 the experimental results show that solutions formed under higher p co 2 conditions precipitated calcite very early on during evaporation and thus have 18 o calcite and 13 c calcite values that are slightly enriched relative to the mcl the solutions that formed under low p co 2 conditions precipitated calcite after much of the fluid had evaporated and thus yield more enriched 18 o calcite and 13 c calcite values repeat trials with varying temperature and relative humidity show that the plct is steeper under both higher temperature and low relative humidity the wide range of p co 2 temperature and relative humidity conditions used simulate meteoric phreatic and meteoric vadose conditions that may affect a calcrete horizon over time during numerous dissolution precipitation reactions the results of these experiments show that a dominant factor in the precipitation of vadose calcite is the saturation state of the fluid with respect to caco 3 while the primary factors affecting the magnitude and steepness of the plct are vadose zone temperature relative humidity the starting 18 o water value and saturation state of the fluid with respect to caco 3 since the p co 2 of the rooting zone is cross controlled by the local soil and surface ecology the magnitude of the plct enrichment may not be a direct proxy for palaeo evaporation rates

oligocene miocene antarctic continental weathering record and paleoclimatic implications cape roberts drilling project ross sea antarctica abstract the bulk chemistry of mudrocks in ross sea drillcores on the antarctic inner shelf is re evaluated to extract records of continental paleotemperatures and ice extent the chemical index of alteration cia of the sedimentary sequence derived from the composite of the cape roberts cores changes in accordance with pco 2 pagani m zachos j c freeman k h tipple b bohaty s 2005 marked change in atmospheric carbon dioxide concentrations during the paleogene science 309 600 603 the k 2 o na 2 o al 2 o 3 molar ratio of the mudrocks is used as a mean summer temperature proxy for continental antarctica and it shows similarities with benthic mg ca temperatures for the southern ocean billups k schrag d p 2002 paleotemperatures and ice volume of the past 27 myr revisited with paired mg ca and stable isotope measurements on benthic foraminifera paleoceanography 17 doi 10 1029 2000pa000567 the ratio is calibrated using intervals with significant pollen assemblages and the temperature proxy enables extrapolation of the paleoclimatic data over a larger portion of the drilled record the cia and k na al ratios support a persistent cooling and drying of climate through the oligocene miocene transition the results reinforce the strong coupling between east antarctic continental temperatures and ice volume with deep sea temperatures and concentrations of atmospheric carbon over long timescales

magnetic susceptibility of early paleozoic and precambrian paleosols abstract paleosols formed by weathering and plant colonization during breaks in eolian deposition of quaternary loesses are marked by enhanced magnetic susceptibility these paleosols can be recognized in the field from their soil structure and soil horizons but the most obvious and diagnostic feature of paleosols are root traces for paleosols of early paleozoic and precambrian age pre dating the evolution of rooted land plants root traces cannot be used as a diagnostic field criterion for recognizing paleosols surficial enhancement of magnetic susceptibility can be useful confirmation that some precambrian and early paleozoic rocks were paleosols unfortunately magnetic susceptibility enhancement of some paleosol profiles is compromised by burial gleization which reduces overall susceptibility values but especially susceptibility at the surface of the profile and produces a marked covariance of magnetic susceptibility with total iron content groundwater and surface water gleization during soil formation also depletes susceptibility over a wide range of iron content to give relatively flat depth functions of susceptibility in contrast well drained soils and paleosols have pronounced surficial susceptibility enhancement over a narrow range of iron content magnetic susceptibility peaks are useful supporting evidence for paleosols entirely red and oxidized but not for paleosols with common green gray reduction spots abundant organic matter iron manganese nodules or other evidence of gleization all known paleozoic and precambrian paleosols have susceptibility an order of magnitude less than in comparable quaternary paleosols and soils so that quaternary magnetic susceptibility climofunctions and chronofunctions cannot yet be applied to paleozoic and precambrian paleosols although their susceptibility values are comparable with those of modern gleyed soils paleozoic and precambrian paleosols maintain a depth function of surficial enhancement of susceptibility as well as red color deeply penetrating cracks and other features of well drained soils burial recrystallization of fine grained magnetite and maghemite may have compromised the magnitude of their susceptibility peaks alternatively they may not have been as biologically active in susceptibility enhancing microbes as modern soils

triassic nb enriched basalts magnesian andesites and adakites of the qiangtang terrane central tibet evidence for metasomatism by slab derived melts in the mantle wedge new chronological geochemical and isotopic data are reported for triassic 219 236 ma adakite magnesian andesite nb enriched basaltic rock associations from the tuotuohe area central qiangtang terrane the adakites and magnesian andesites are characterized by high sr y 25 45 la yb 14 42 and na2o k2o 12 49 ratios high al2o3 15 34 18 28 wt and moderate to high sr concentrations 220 498 ppm and end t 0 86 to 1 21 values low enrichments of th rb relative to nb and subequal normalized nb and la contents and enrichments of light rare earth elements combine to distinguish a group of nb enriched basaltic rocks nebs they have positive end t 2 57 to 5 16 values positive correlations between th la and nb and an absence of negative nb anomalies on mantle normalized plots indicate the nebs are products of a mantle source metasomatized by a slab melt rather than by hydrous fluids a continuous compositional variation between adakites and magnesian andesites confirms slab melt interaction with mantle peridotite the spatial association of the nebs with adakites and magnesian andesites define an adakitic metasomatic volcanic series recognized in many demonstrably subduction related environments e g mindanao arc philippines kamchatka arc russia and southern baja california arc mexico the age of the touhuohe suite and its correlation with triassic neb to the north indicates that volcanism derived from subduction modified mantle was abundant prior to 220 ma in the central qiangtang terrane

ordovician magnetostratigraphy a correlation of global data palaeomagnetic studies where primary magnetizations are established from well dated rocks have been compiled to construct a magnetostratigraphic time scale for the ordovician period an excellent correlation of magnetozones is observed between independent studies of the baltic and south siberian platform sequences supplemental datasets from other continents concur with the baltic and siberian polarity data early ordovician times were mainly characterized by a reverse field rapid reversals then occurred during llanvirn and llandeilo times late ordovician times were dominated by a normal polarity field some gaps remain in the polarity record notably for late caradoc early ashgill and mid tremadoc times

alluvial facies evolution during the palaeozoic greening of the continents case studies conceptual models and modern analogues the palaeozoic greening of the continents the appearance and expansion of embryophytes land plants in terrestrial environments was arguably the most fundamental phanerozoic change to the earth system thirteen case studies of cambrian to devonian fluvial deposits from north america and europe are documented here to illustrate the evolution of fluvial style during this period during the cambro ordovician prior to the advent of terrestrial vegetation fluvial systems laid down relatively coarse sands with little mud resulting in self formed channels and an architecture dominated by broad sheets of trough cross beds sheet braided style similar deposits formed across a wide range of latitudes and passed basinward into sandy coastal deposits from the mid ordovician onwards an increase in floodplain mudstone corresponds broadly with the appearance of embryophytes which would have progressively enhanced upland weathering mud production and floodplain storage of fines during the late silurian small heterolithic channel bodies with lateral accretion sets provide the first evidence of meandering channels and floodplain mudstones contain more varied palaeosols especially calcretes which appear abundantly for the first time by the early devonian channel deposits comprise sandstone lenses channelled braided style probably due to the increased bank strength and cohesion and reduced potential for sediment sorting imparted by sand mud mixtures muddy coastal deposits are prominent by the upper devonian fluvial deposits commonly contain fossil trees and large mainstem meandering channels with lateral accretion sets indicating that rooted vegetation stabilized channels the advent of stable floodplains with levees crevasse splays and organic litter would have encouraged the diversification of terrestrial invertebrates which left alluvial ichnological signatures from the late silurian onwards

cambrian triassic geodynamic evolution of central qiangtang tibet comment abstract we disagree on many basic aspects of the cambrian triassic tectonic reconstruction of central qiangtang tibet proposed by xu et al 2020 earth science reviews v 201 doi https doi org 10 1016 j earscirev 2020 103083 because their reconstructions are inconsistent with geological evidence the allochthonous fragments in the accretionary complex of the shuanghu suture zone should not be used as indicators of south qiangtang geology furthermore no evidence exists for repeated drift collision between north and south qiangtang along the shuanghu suture zone from the paleozoic to triassic because the shuanghu paleo tethys ocean likely existed from the cambrian to triassic as indicated by continuous ophiolitic rocks no evidence supports the occurrence of north qiangtang qaidam kunlun collision during the middle ordovician to early silurian because the northward subduction of the anyemaqen kunlun muztagh paleo tethyan branch between these two tectonic entities occurred from the paleozoic to triassic as inferred from radiolarian bearing ophiolitic rocks and coeval arc magmatism in the kunlun block in addition there is no evidence for the north qiangtang south china collision during the late carboniferous to early permian because the jinsa paleo tethyan oceanic branch was active at this time as evidenced by the presence of radiolarian bearing ophiolitic fragments in the suture zone

emplacement of cretaceous tertiary boundary shocked quartz from chicxulub crater observations on shocked quartz in cretaceous tertiary k t boundary sediments compellingly tied to chicxulub crater raise three problems first in north america shocked quartz occurs above the main k t ejecta layer second shocked quartz is more abundant west than east of chicxulub third shocked quartz reached distances requiring initial velocities up to 8 kilometers per second corresponding to shock pressures that would produce melt not the moderate pressure shock lamellae observed shock devolatilization and the expansion of carbon dioxide and water from impacted wet carbonate producing a warm accelerating fireball after the initial hot fireball of silicate vapor may explain all three problems

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet a 500 km long belt of metamorphic exposures in the qiangtang block provides an opportunity to study the internal structure of northern tibetan crust metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist bearing melange and are bounded by late triassic early jurassic domal low angle normal faults we propose that this melange was underplated to the qiangtang block and was subsequently exhumed by detachment faulting both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the jinsha suture this model predicts that the deeper crust of much of northern tibet consists of accretionary melange in contrast to the continental crystalline crust of southern tibet and may account for north south variations of cenozoic tectonism in tibet

the characteristics of carbonate system recovery during a relatively dry event in a mixed carbonate siliciclastic environment in the pelsonian middle triassic proximal marginal marine basins a case study from the tropical tethyan northwest gondwana margins abstract eight early mid pelsonian middle triassic marine sedimentary successions an outcrop and subsurface sections of the ra af formation from the northwest gondwana margins facing the southeast corner of the tropical western tethyan seaway assigned to the southern intertropical convergence zone itcz south israel levant basin north arabian plate are the focus of this work the successions especially with the exclusive outcrop at har arief are a golden opportunity to observe the still connected transitional zone between the huge siliciclastic domains derived from gondwana hinterland parts north arabian plate and the open tropical tethyan marine carbonate domain and their mutual relationships this multidiscipline research tracks a carbonate recovery attempt in a dominant siliciclastic system the recovery effort was characterized by starved sedimentary conditions basin constrained oxygen stressed environments and low diverse autochthonous burrowed sediments barren of other faunal remains low diverse fossiliferous allochthonous beds transported from higher sites on the slopes revealed differential distributions of echinoids mollusks and especially foraminifera large pilammina pilamminella versus small glomospirids these variances were attributed to different environmental conditions on the slopes of each basin type and illuminate some constraints on their distribution pattern and climate affinity in the triassic southern itcz mixed carbonate siliciclastic sedimentary system the carbonate recovery phase is ascribed to a relatively dry period in the hinterland rather than to sea level rise this dry period which interrupted the relatively humid climate in the northern tropical gondwana during the pelsonian is represented by the constant although with some perturbations siliciclastic sedimentary system that prevailed in the southeastern corner of the tethys seaway during the anisian insights from the symmetric and asymmetric correlation of climate fluctuations in the hinterlands of the southeastern european basins northern hemisphere and this work from the southern hemisphere both embracing basins ascribed to the western tropical tethyan seaway illuminate the possible effects of the ictz behavior pattern and possible extra tropical forces on the climate variations

periglacial paleosols and cryogenian paleoclimate near adelaide south australia abstract late cryogenian deformation of the reynella siltstone near hallett cove south australia has been interpreted as shallow marine methane seeps which were furthermore proposed to have played a role in deglaciation from snowball earth re examination and new analyses of these same outcrops confirms an earlier interpretation of this intrastratal deformation as periglacial paleosols which are evidence that even at maximum extent late cryogenian glaciers did not cover all continental lowlands let alone the world ocean known miocene and pliocene methane seeps of california examined for this study are very different in their gray color rounded nodules and clayey host rock compared with red highly deformed and silty to pebbly reynella siltstone of hallett cove stable isotopic composition of nodular carbonate in the reynella siltstone fails to show extreme carbon isotopic depletion or the meteoric oxygen lines characteristic of known methane seeps and sphaerosideritic waterlogged paleosols but instead shows covariance of carbon and oxygen isotopic composition comparable with that in pedogenic and subaerial carbonate of well drained soils mass balance geochemical analyses show modest weathering compatible with sand wedge structures and evaporitic pseudomorphs indicative of arid and frigid paleoclimate outcrops of reynella siltstone near hallett cove represent frigid low latitude floodplains not submarine methane seeps nor global glaciation

chemistry and mineralogy of precambrian paleosols at the base of the dominion and pongola groups transvaal south africa abstract we have studied alteration zones found between sedimentary rocks of the lowest proterozoic dominion and archean pongola groups and underlying granitic rocks transvaal south africa the alteration zones have a transitional lower boundary primary igneous minerals are gradually destroyed upward toward the contact with overlying sedimentary rocks ca mg na p and mn are progressively lost upwards in the zone the chemical and mineralogical variations and available stratigraphic evidence are all consistent with the interpretation that these zones are paleosols formed by precambrian weathering 2 8 3 0 ga ago after formation the paleosols underwent metasomatism and metamorphism sericite now present in the paleosols was probably formed by low temperature potassium metasomatism of original clays andalusite in the pre pongola paleosol was probably formed by metamorphism of original clays in impermeable zones which prevented groundwater flow and potassium metasomatism the dominion paleosol was investigated at three localities the effects of weathering were similar at the three sites except for the behavior of iron one site lost about 20 of iron whereas the other sites lost 75 and 80 of initial iron content the differences in iron loss may be due to variation in soil ventilation and diffusion of atmospheric gases as a function of grain size and topography variations in iron loss may be used to place some constraints on the composition of the precambrian atmosphere given constraints imposed by dominion paleosols overlying uraniferous conglomerates requirements for chemical weathering and the greenhouse effect and possible presence of non diffusing photochemical oxidants the partial pressures of oxygen and carbon dioxide gases probably are in the ranges po1 0 02 to 0 5 pal present atmospheric level and pco2 5 to 30 pal

a comprehensive approach to the study of methane seep deposits from the lincoln creek formation western washington state usa a comprehensive approach using palaeontology petrography stable isotope geochemistry and biomarker analyses was applied to the study of seven small methane seep carbonate deposits these deposits are in the oligocene part of the lincoln creek formation exposed along the canyon and satsop rivers in western washington each deposit preserves invertebrate fossils many representing typical seep biota authigenic carbonates with 1 3 c values as low as 51 pdb reveal that the carbon is predominately methane derived carbonates contain the irregular isoprenoid hydrocarbons 2 6 11 15 tetramethylhexadecane crocetane and 2 6 10 15 19 pentamethylicosane pmi lipid biomarkers diagnostic for archaea these lipids are strongly depleted in 1 3 c 1 3 c values as low as 120 pdb indicating that archaea were involved in the anaerobic oxidation of methane small filaments preserved in the carbonate may represent methanotrophic archaea archaeal methanogenesis induced the formation of a late diagenetic phase brownish calcite consisting of dumbbell shaped crystal aggregates that exhibit 1 3 c values as high as 7 pdb clotted microfabrics of primary origin point to microbial mediation of carbonate precipitation downward directed carbonate aggregation in the seeps produced inverted stromatactoid cavities large filaments interpreted as green algae based on their size shape arrangement and biomarkers imply that deposition occurred in places in water no deeper than 210 m

black shale deposition in an upper ordovician silurian permanently stratified peri glacial basin southern jordan abstract the lower palaeozoic upper ordovician silurian succession of north africa contains one of the world s most prolific black shale source rocks yet the origin of these rocks remains contentious the black shale of the batra formation in jordan was deposited at high palaeolatitude during rapid hirnantian to early silurian deglaciation here we report geological and organic geochemical results that provide evidence for an increase in photic zone primary productivity during ice melting the decay of this organic matter through oxidative respiration resulted in euxinia which enhanced the potential for organic matter preservation the occurrence of isorenieratane in all samples indicates euxinia extended from the photic zone to the sediment water interface the stratified basins and fjords of east antarctica provide a likely modern analogue

methane release from igneous intrusion of coal during late permian extinction events abstract unusually large and locally variable carbon isotope excursions coincident with mass extinctions at the end of the permian period 253 ma and guadalupian epoch 260 ma can be attributed to methane outbursts to the atmosphere methane has isotopic values 13c low enough to reduce to feasible amounts the carbon required for isotopic mass balance the duration of the carbon isotopic excursions and inferred methane releases are here constrained to 10 000 yr by counting annual varves in lake deposits and by estimating peat accumulation rates on paleogeographic maps the most marked carbon isotope excursions form linear arrays back to plausible methane sources end permian siberian traps and longwood bluff intrusions of new zealand and end guadalupian emeishan traps of china intrusion of coal seams by feeder dikes to flood basalts could create successive thermogenic methane outbursts of the observed timing and magnitude but these are unreasonably short times for replenishment of marine or permaf

deposition of a saline giant in the mississippian windsor group nova scotia and the nascent late paleozoic ice age abstract saline giants are vast marine evaporite deposits that currently have no modern analogues and remain one of the most enigmatic of chemical sedimentary rocks the mississippian windsor group ca 345 ma maritimes basin atlantic canada is a saline giant that consists of two evaporite rich sedimentary sequences that are subdivided into five subzones sequence 1 is composed almost entirely of thick halite belonging to subzone a osagean sequence 2 is in unconformable contact and comprised of stacked carbonate evaporite peritidal cycles of subzones b through e meramecian subzone b the focus of research herein documents the transition from wholly evaporitic to open marine conditions and thus preserves an exceptional window into the processes forming saline giants lithofacies stacking patterns in subzone b reveal that higher order fluctuations in relative sea level produced nine stacked parasequences interpreted to reflect high frequency glacioeustatic oscillations during the onset of the late paleozoic ice age each parasequence reflects progradation of intertidal and sabkha sediments over subtidal carbonate and evaporite deposits dissimilarities in cycle composition between sub basins imply the development of contrasting brine chemistries from differing recharge rates with the open ocean what the windsor group shows is that evaporite type is ostensibly linked to the amplitude and frequency of sea level rise and fall during deposition true saline giants like the basinwide evaporites of sequence 1 apparently require low amplitude long frequency changes in sea level to promote the development of stable brine pools that are only periodically recharged with seawater by contrast the high amplitude short frequency glacioeustatic variability in sea level that controlled the accumulation of peritidal evaporites in subzone b produce smaller subeconomic deposits with more complex facies relationships

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet comment and reply comment kapp et al 2000 1 suggested that the qiangtang metamorphic belt consists of underplated blueschist bearing melange and was exhumed by low angle normal faults and that both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the

new constraints on using paleosols to reconstruct atmospheric pco2 ecosystem and climatic changes due to anthro po genic emissions of co 2 are of increasing importance to society one way to predict these changes in the future is to study warming events in the geologic past the main control of earth s temperature over geologic time is the concentration of atmospheric co 2 however the precise relationship between co 2 and temperature is not fully understood therefore it is essential to be able to quantify atmospheric co 2 concentrations in the geologic past especially at times of rapid climate change analogous to current atmospheric changes one widely applied proxy relates the carbon isotopic composition of pedogenic carbonates to atmospheric pco 2 but one of the key variables soil respired co 2 s z is not well constrained this study presents a new proxy where soil respired co 2 is related to mean annual precipitation map by the following equation s z 5 67 map 269 9 r 2 0 59 se 681 ppm this proxy validated for modern atmospheric pco 2 levels constrains the primary source of uncertainty in the soil carbonate paleobarometer we apply this proxy to make atmospheric pco 2 reconstructions for examples from the cenozoic mesozoic and paleozoic and calculate co 2 estimates that are in better agreement with estimates from other independent proxies and model results than previous pedogenic carbonate reconstructions the implications of the uncertainties attributed to pedogenic carbonate paleobarometry as well as guidelines for using this method are discussed by making individual measurements rather than assumptions for each isotopic value and combining those analyses with an s z estimate for each paleosol considered using our new proxy described herein we can obtain increased precision for atmospheric pco 2 reconstructions

first plants cooled the ordovician the late ordovician period ending 444 million years ago was marked by the onset of glaciations the expansion of non vascular land plants accelerated chemical weathering and may have drawn down enough atmospheric carbon dioxide to trigger the growth of ice sheets

geochemical evidence supporting t c chamberlin s theory of glaciation in 1899 t c chamberlin proposed that the co sub 2 content of the atomsphere decreased during times of enhanced continental erosion ultimately resulting in glacial epochs he ascribed the increase in the rate of chemical weathering relative to the rate of supply of co sub 2 from earth s interior to increased orogenic activity and globally higher average elevations which promoted rapid chemical erosion of silicates the oceanic record of strontium isotopes preserved in marine sediment supports his suggestion that glacial climates during the phanerozoic are in part linked to increases in the rate of global chemical erosion relative to outgassing from earth s interior further the close correspondence of the major tectonic episodes of the late cenozoic to times of increased continental erosion and glaciation suggests that chamberlin s hypothesis of the cause of glacial periods should be revived

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

the age of the earliest club mosses the silurian baragwanathia flora in victoria australia it is argued that there is overwhelming evidence from a good graptolite record that the earliest club mosses on earth were of gorstian ludlow silurian age and that baragwanathia longifolia lang cookson and its associated flora persisted through the p idoli and into the early devonian showing some changes during this time

revised wonoka isotopic anomaly in south australia and late ediacaran mass extinction the global late ediacaran shuram wonoka carbon isotope anomaly has been regarded as the largest and longest known isotopic anomaly in the ocean assuming that all ediacaran carbonate is marine disregarding carbonate in south australia shown here to be palaeosol or palaeokarst the synchronous marine organic carbonate excursion is only 8 for 13 c organic and 6 for 13 c carbonate and lasted less than a million years this revised magnitude and duration is comparable with perturbations across the permian triassic boundary and correlative with a global late ediacaran acritarch mass extinction like permian triassic isotopic excursions the revised organic carbonate wonoka excursion may also have been a greenhouse palaeoclimatic warm spike which terminated valley incision and glacioeustatic drawdown during the mid ediacaran fauquier glaciation and preceded chill of the late ediacaran billy springs glaciation supplementary material measured sections and tables of mineral and grain size proportions major element and stable isotope analyses are available at www geolsoc org uk sup18756

late ordovician glaciation initiated by early land plant evolution and punctuated by greenhouse mass extinctions abstractlate ordovician hirnantian glaciation indicated by periglacial paleosols tillites and glacial pavements in saharan africa has been attributed to advances in weathering and carbon sequestration due to evolution of early land plants in the same way that devonian permian glaciation has been attributed to evolution of forests and quaternary glaciations to evolution of grasslands two problems for carbon cycle explanations of this glaciation are an hirnantian co2 greenhouse estimated at 4035 ppmv from mass balance models and possibly related hirnantian mass extinctions this estimate of high co2 is a 10 m yr average here evaluated against high resolution sequences of paleosols that give estimates not only of co2 but also of paleoclimate and vegetation through the late ordovician katian paleosols of the eastern united states show increases in depth and in degree of carbon consumption and storage comparable with those preceding glaciation of the devonian permian and quaternary early thalloid l

local and global perspectives on carbon and nitrogen cycling during the hirnantian glaciation abstract the impact of climate and ocean circulation changes on marine carbon and nitrogen cycling was investigated for a period covering the late ordovician hirnantian glaciation three sections were studied two from nevada one proximal and one distal and one from the yukon the opportunity to compare 13 c carb 13 c org and 13 c grap profiles between ocean and epeiric platform settings enabled a more thorough assessment of environmental and diagenetic factors influencing the preservation of seawater 13 c values at each stratigraphic section using the best preserved sedimentary components from the two nevada sections a lowstand sedimentary gradient of 4 was found between platform and ocean settings during the hirnantian glaciation gradients of similar magnitude have been reported from hirnantian platforms in the canadian arctic and baltic basins taking the outboard sections as the most representative of the ocean carbon pool a mean positive 13 c shift of just 2 7 0 4 is calculated which is significantly smaller than the 5 7 positive shifts recorded in epeiric platform settings the discrepancy is attributed to the effects of local carbon cycling in tropical and subtropical epeiric seas it is hypothesized that increased carbonate weathering caused 13 c values of local carbon weathering fluxes to increase which in turn caused seawater 13 c values to increase in basin proximal settings in addition nutrient dust fluxes may have contributed to increased levels of photosynthetic activity in proximal settings driving sedimentary 13 c values even higher much higher than sediment deposited in outboard settings where carbon exchanges with the surface ocean were less restrictive a 15 n profile reconstructed from deposits of the distal vinini creek section records a 1 5 positive shift through the hirnantian glacial interval interpreted as a major late ordovician upwelling zone high organic carbon 1 wt and high hydrogen indices 400 600 make the vinini creek section a suitable candidate for the preservation of original sedimentary 15 n records to explain the results a conceptual model of ocean nitrogen cycling under greenhouse climate conditions is presented that highlights the role of cyanobacteria as both primary producers and nitrogen fixers it is hypothesized that in greenhouse climates the fixed nitrogen inventory of the abyssal ocean was low due to efficient denitrification in the ocean s expansive oxygen minimum zones this caused algal productivity to become strongly dependent on continuous supplies of fixed nitrogen from cyanobacteria in the photic zone yielding sedimentary 15 n values between 0 and 2 by contrast the cooler icehouse climate of the hirnantian led to increased ocean ventilation greater partitioning of atmospheric oxygen into downwelling surface waters and oxygen minimum zone shrinkage as a result fixed nitrogen levels climbed in the ocean s abyssal depths due to the lower denitrification rates this in turn allowed nitrogen with high 15 n values to recycle back into the photic zone driving sedimentary 15 n values higher during the hirnantian glaciation

trilobite extinctions facies changes and the roece carbon isotope excursion at the cambrian series 2 3 boundary great basin western usa the mass extinction of the olenellid trilobites occurred around the cambrian series 2 series 3 boundary like many other crises it coincided with a negative carbon isotope excursion but the associated palaeoenvironmental changes remain unclear to investigate the causal mechanism for this event we report facies changes pyrite framboid petrography and carbon isotope values from cambrian series 2 series 3 traditionally early middle cambrian boundary strata of the carrara formation death valley region california and pioche formation nevada these data reveal regionally changing water depths from high energy nearshore facies oolitic grainstone to more offshore silty marl and finer grained carbonate mudstone in the carrara formation the series boundary occurs within a deepening succession transitioning from high energy nearshore facies oolitic grainstone and oncolitic packstone to offshore marl the latter of which contains pyrite framboid populations indicative of low oxygen dysoxic depositional conditions intermittent dysoxia persisted below sub wave base settings throughout the early and middle cambrian but did not intensify at the time of extinction arguing against anoxia as a primary cause in the olenellid trilobite extinction within both field areas the extinction interval coincided with a minimum in 13ccarb values which we interpret as the regional manifestation of the redlichiid olenellid extinction carbon isotope excursion roece the series 2 series 3 boundary is reported to closely coincide with a large amplitude sea level fall that produced the sauk i ii sequence boundary but the placement of the series 2 series 3 boundary within a transgressive interval of the carrara formation shows that this is not the case the main sequence boundary in the succession occurs much lower in the succession at the top of the zabriskie quartzite and therefore precedes the extinction of the olenellids and roece

first evidence for cambrian glaciation provided by sections in avalonian new brunswick and ireland additional data for avalon gondwana separation by the earliest palaeozoic abstract the first evidence for cambrian glaciation is provided by two successions on the avalon microcontinent the middle lowest cambrian middle terreneuvian series and fortunian stage stage 2 boundary interval has an incised sequence boundary overlain by a fluvial lowstand facies and higher olive green marine mudstone on hanford brook southern new brunswick this succession in the lower mystery lake member of the chapel island formation may be related to melting of an ice sheet in avalon the evidence for this interpretation is a muddy diamictite with outsized up to 10 cm in diameter proterozoic marble and basalt clasts that penetrated overlying laminae in the marine mudstone that eustatic rise was associated with the mudstone deposition is suggested by an approximately coeval rise that deposited sediments with watsonella crosbyi zone fossils 650 km away in avalonian eastern newfoundland a sea level rise within the watsonella crosbyi chron at ca 535 ma may correspond to a unnamed negative 13 c excursion younger than the basal cambrian excursion bace and the zhuce excursion in stage 2 of the upper terreneuvian series cambrian dropstones are now also recognized on the northern gander margin of avalon in continental slope rise sedimentary rocks in southeast ireland although their age early middle cambrian is poorly constrained dropstones in the booley bay formation provide additional evidence for cambrian glaciation on the avalon microcontinent besides providing the first evidence of cambrian glaciation these dropstone deposits emphasize that avalon was not part of or even latitudinally close to the terminal ediacaran cambrian tropical carbonate platform successions of west gondwana

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

ecological controls on devonian stromatoporoid dominated and coral dominated reef growth in the mackenzie basin northwest territories canada the horn plateau formation composed of isolated reefs is part of the devonian strata that formed in the mackenzie basin in the northwest territories canada the reefs stretch over a 350 m north

shelf paralic and fluvial environments and eustatic sea level fluctuations in the origin of the tuscarora formation lower silurian of central pennsylvania abstract the tuscarora formation lower silurian of central pennsylvania owes its origin to a combination of tectonic eustatic and depositional events tuscarora deposition began at the beginning of the silurian period at the time of both glacioeustatic sea level rise and renewed tectonic elevation of the taconic source terrain braided alluvial systems transported coarse sediment northwestward to a wave dominated coast yet sea level rise was so rapid that the shoreline retrograded from western to eastern pennsylvania at the time of transgression shoreline characteristics varied from high energy beaches in central pennsylvania to estuaries along the south central border region of pennsylvania seaward of the shoreline sand was fashioned into shelf sand wave complexes analogous to moder shoreface connected sand ridges much of the tuscarora formation west of the susquehanna river accumulated in this shallow marine shelf environment later in the early silurian eustatic sea level fall resulted in progradation of lower energy coastal sand mud flats northwestward over the former shelf shortly before a subsequent sea level rise terminated tuscarora development with the return of the sea in which the rose hill formation was deposited the basis for these interpretations is the analysis of five major lithofacies in the exposed tuscarora formation of central pennsylvania the basal part of the tuscarora consists of two lithofacies of coastal origin the horizontally laminated lithofacies high energy beach and the pink transitional lithofacies estuary the main body of the tuscarora comprises the eastern cross laminated lithofacies braided fluvial and the western cross laminated lithofacies shelf sand wave complexes capping the tuscarora is the red skolithos burrowed lithofacies coastal sand mud flats

geocarbsulf a combined model for phanerozoic atmospheric o2 and co2 abstract a model for the combined long term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the geocarb iii model berner r a kothavala z 2001 geocarb iii a revised model of atmospheric co2 over phanerozoic time am j sci 301 182 204 for co2 with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for o2 berner r a 2001 modeling atmospheric o2 over phanerozoic time geochim cosmochim acta 65 685 694 new isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models sensitivity analysis is done by determining 1 the effect on weathering rates of using rapid recycling rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks 2 the effect on o2 of using different initial starting conditions 3 the effect on o2 of using different data for carbon isotope fractionation during photosynthesis and alternative values of oceanic 13c for the past 200 million years 4 the effect on sulfur isotope fractionation and on o2 of varying the size of o2 feedback during sedimentary pyrite formation 5 the effect on o2 of varying the dependence of organic matter and pyrite weathering on tectonic uplift plus erosion and the degree of exposure of coastal lands by sea level change 6 the effect on co2 of adding the variability of volcanic rock weathering over time berner r a 2006 inclusion of the weathering of volcanic rocks in the geocarbsulf model am j sci 306 in press results show a similar trend of atmospheric co2 over the phanerozoic to the results of geocarb iii but with some differences during the early paleozoic and for variable volcanic rock weathering lower co2 values during the mesozoic atmospheric oxygen shows a major broad late paleozoic peak with a maximum value of about 30 o2 in the permian a secondary less broad peak centered near the silurian devonian boundary variation between 15 and 20 o2 during the cambrian and ordovician a very sharp drop from 30 to 15 o2 at the permo triassic boundary and a more or less continuous rise in o2 from the late triassic to the present

carbon isotopes support the presence of extensive land floras pre dating the origin of vascular plants abstract multiple lines of evidence indicate that earth s land masses became green some 2 7 ga ago about 1 billion years after the advent of life about 2 2 billion years later land plants abruptly appear in the fossil record and diversify marking the onset of ecologically complex terrestrial communities that persist to the present day given this long history of land colonization surprisingly few studies report direct fossil evidence of emergent vegetation prior to the continuous record of life on land that starts in the mid silurian ca 420 425 ma ago here we compare stable carbon isotope signatures of fossils from seven ordovician silurian 450 420 ma old appalachian biotas with signatures of coeval marine organic matter and with stable carbon isotope values predicted for ordovician and silurian liverworts bryocarb model the comparisons support a terrestrial origin for fossils in six of the biotas analyzed and indicate that some of the fossils represent bryophyte grade plants our results demonstrate that extensive land floras pre dated the advent of vascular plants by at least 25 ma the appalachian fossils represent the oldest direct evidence of widespread colonization of continents these findings provide a new search image for macrofossil assemblages that contain the earliest stages of land plant evolution we anticipate they will fuel renewed efforts to search for direct fossil evidence to track the origin of land plants and eukaryotic life on continents further back in geologic time

pedogenic slickensides indicators of strain and deformation processes in redbed sequences of the appalachian foreland pedogenic slickensides are convex concave slip surfaces that form during expansion contraction in expansive clay soils such as vertisols in the central appalachians they occur near the tops of fining upward cycles in paleozoic redbeds such as the bloomsburg catskill and mauch chunk formations pedogenic slickensides are found in association with other pedogenic or paleosol features such as clay skinned peds in situ calcareous nodules and root impressions repeated movements along these shear planes during pedogenesis produce strongly aligned clay particles adjacent to pedogenic slickensides as a result they are preserved as discrete fractures throughout diagenesis compaction and superimposed tectonic deformation during whole rock deformation pedogenic slickensides segregate penetratively deformed rocks into independent foliated packets and serve as discontinuities that are followed by later structural features because the original morphology of pedogenic slickensides is known they can be used as crude strain markers

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

isotope geochemistry of caliche developed on basalt enormous variations in oxygen and carbon isotopes occur in caliche developed on 3 ma basalts in 3 volcanic fields in arizona significantly extending the range of 18o and 13c observed in terrestrial caliche within each volcanic field 18o is broadly co variant with 13c and increases as 13c increases the most 18o and 13c enriched samples are for subaerial calcite developed on pinnacles knobs and flow lobes that protrude above tephra and soil the most 18o and 13c depleted samples are for pedogenic carbonate developed in soil atmospheres the pedogenic caliche has 18o fixed by normal precipitation in local meteoric waters at ambient temperatures and has low 13c characteristic of microbial soil co2 subaerial caliche has formed from 18o rich evapoconcentrated meteoric waters that dried out on surfaces after local rains the associated 13c enrichment is due either to removal of 12c by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over layered during burial by calcite carrying the isotopic signature of the soil environment the large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if mars had a warm wet early period and photosynthesizing microbes were present in the early soils the approach can be similarly applied to terrestrial precambrian paleocaliche in the search for the earliest record of life on land large variations reported for 18o of carbonate in martian meteorite alh84001 do not necessarily require high temperatures playa lakes or flood runoff if the carbonate is an example of altered martian caliche

non marine arthropod traces from the subaerial ordovician borrowdale volcanic group english lake district arthropod trace fossils are described from the borrowdale volcanic group a sequence otherwise devoid of faunal evidence two forms both made by the same probably myriapod like organism are assigned to the ichnogenera diplichnites and diplopodichnus the lithologies preserving the trace fossils are non marine and may have been deposited in a freshwater lacustrine environment some of the traces were probably made in temporarily emergent conditions the change from one form of trace to the other reflects drying out of the substrate the trace fossils probably record some of the earliest freshwater arthropods before the widespread colonization of land by the group

ediacaran and cambrian paleosols from central australia abstract ediacaran to cambrian red sandstones of the northern territory including arumbera sandstone and grant bluff and central mount stuart formations have been reexamined and sampled in order to reconstruct paleoenvironments from sedimentary facies and paleosols sedimentary facies include green gray calcareous sandstone member iii of arumbera sandstone only red siltstone with calcareous nodules also only member iii and also non calcareous red green siltstone interbedded red siltstone and sandstone and trough bedded sandstone the green gray facies has diverse and distinctive cambrian marine trace fossils not found in the red beds which have instead an assemblage of problematic body fossils including arumberia noffkarkys hallidaya ernietta trepassia and dickinsonia the red facies are interpreted as fluvial paleochannels trough cross bedded fluvial levees interbedded siltstone and sandstone poorly drained floodplain red green siltstone and well drained floodplain or low terrace calcareous nodular the carbonate nodules have micritic replacive fabrics and a significant positive correlation of 13c and 18o characteristic of pedogenic carbonate of calcid soils other red beds have pseudomorphous sand crystals after gypsum like those of gypsid soils individual red beds show negative strain and mass transfer for cationic bases typical of soils as well as gradational alteration down from fossiliferous tops characteristic of soils their varied degree of pedogenic differentiation is identified as psamment aquent and ochrept paleosols phosphorus depletion in the paleosols is evidence that organisms lived in the paleosols and were not introduced by transient marine incursions the problematic quilted fossils arumberia noffkarkys and hallidaya formed a distinctive inland terrestrial polsterland during the ediacaran and early cambrian

a new paleothermometer for forest paleosols and its implications for cenozoic climate climate is a primary control on the chemical composition of paleosols making them a potentially extensive archive applicable to problems ranging from paleoclimate reconstruction to paleoaltimetry however the development of an effective widely applicable paleosol temperature proxy has remained elusive this is attributable to the fact that various soil orders behave differently due to their respective physical and chemical properties therefore by focusing on an individual order or a subset of the 12 soil orders whose members exhibit similar process behavior a better constrained paleothermometer can be constructed soil chemistry data were compiled for 158 modern soils in order to derive a new paleosol paleothermometry relationship between mean annual temperature and a paleosol weathering index pwi that is based on the relative loss of major cations na mg k ca from soil b horizons the new paleothermometer can be applied to clay rich paleosols that originally formed under forest vegetation including inceptisols alfisols and ultisols and halves the uncertainty relative to previous approaches a case study using cenozoic paleosols from oregon united states shows that paleotemperatures produced with this new proxy compare favorably with paleobotanical temperature estimates global climatic events are also evident in the oregon paleosol record including a 2 8 c drop across the eocene oligocene transition comparable to marine records and a neogene peak temperature during the mid miocene climatic optimum

search for evidence of impact at the permian triassic boundary in antarctica and australia life on earth was almost destroyed some 250 m y ago in the most profound of all known mass extinction events we investigated the possible role of impact by an extraterrestrial bolide through chemical and mineralogical characterization of boundary breccias search for shocked quartz and analysis for iridium in permian triassic boundary sections at graphite peak and mount crean antarctica and wybung head australia thin claystone breccias at the isotopically and paleobotanically defined boundary at all three locations are interpreted as redeposited soil rather than impact ejecta the breccias at all three locations also yielded shocked quartz but it is an order of magnitude less abundant 0 2 vol and smaller only as much as 176 micrometers m diameter than shocked quartz at some cretaceous tertiary boundary sites faint iridium anomalies were detected up to 134 pg g 1 these values are an order of magnitude less than iridium anomalies at some cretaceous tertiary boundary sites furthermore peak iridium values are as much as 1 m below the isotopically and paleobotanically defined boundary the idea that impact caused the extinctions thus remains to be demonstrated convincingly

controls on the formation of microbially induced sedimentary structures and biotic recovery in the lower triassic of arctic canada microbially induced sedimentary structures miss are reportedly widespread in the early triassic and their occurrence is attributed to either the extinction of marine grazers allowing mat preservation during the permo triassic mass extinction or the suppression of grazing due to harsh oxygen poor conditions in its aftermath here we report on the abundant occurrence of miss in the lower triassic blind fiord formation of the sverdrup basin arctic canada sedimentological analysis shows that mid shelf settings were dominated by deposition from cohesive sand mud flows that produced heterolithic rippled sandstone facies that pass down dip into laminated siltstones and ultimately basinal mudrocks the absence of storm beds and any other event beds points to an unusual climatic regime of humid quiet conditions characterized by near continuous run off geochemical proxies for oxygenation mo al th u and pyrite framboid analysis indicate that lower dysoxic conditions prevailed in the basin for much of the early triassic the resultant lack of bioturbation allowed the development and preservation of miss including wrinkle structures and bubble textures the microbial mats responsible for these structures are envisaged to have thrived on sandy substrates within the photic zone in oxygen poor conditions the dysoxic history was punctuated by better oxygenated phases which coincide with the loss of miss thus permo triassic boundary and griesbachian mudrocks from the deepest water settings have common benthos and a well developed tiered burrow profile dominated by phycosiphon the presence of the intense burrowing in the earliest triassic contradicts the notion that bioturbation was severely suppressed at this time due to extinction losses at the end of the permian the notion that early triassic miss preservation was caused by the extinction of mat grazers is not tenable

temporal variations of mesozoic sandstone compositions in the qiangtang block northern tibet china implications for provenance and tectonic setting abstract one hundred and forty four and fifty two mesozoic sandstone samples from the qiangtang block ca 32 20 35 n 85 93 e northern tibet china were conducted for analysis of detrital modes and heavy mineral spectra respectively in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited the lower middle triassic sandstones in central qiangtang are characterized by complex derivations from both recycled orogen and continental block provenances most of the upper triassic and jurassic sandstones are dominated by a recycled orogen origin however sandstones derived from a continental block source also occur in upper triassic strata in the southern margin of southern qiangtang and in lower jurassic strata in the southern margin of northern qiangtang although the heavy mineral spectra of the upper triassic and jurassic sandstones are occupied mostly by the stable heavy minerals zircon tourmaline and rutile ztr most of the samples are generally accompanied by unstable heavy minerals such as apatite chlorite chrome spinel amphibole epidote garnet and pyroxene except for those from upper triassic strata in the southern margin of southern qiangtang and from lower jurassic strata in the southern margin of northern qiangtang the detrital modes and heavy mineral spectra of the mesozoic sandstones along with the lithologic evidence indicate that the central qiangtang anticlinorium must have been elevated and eroding due to orogenesis and most likely has contained rolling topographic relief at least since early triassic time it could have contained abundant ophiolitic fragments as evidenced by rich chrome spinel amphibole and other readily decomposed ferromagnesian minerals across the entire anticlinorium these mafic minerals may point to the presence of the shuanghu suture along the entire central qiangtang anticlinorium the blueschist bearing metamorphic complexes in central qiangtang could be accommodated in a large scale enclave in an irregular central qiangtang continental margin the upper triassic jurassic recycled orogen derived sandstones in northeastern qiangtang suggest continued orogenesis due to the suturing between qiangtang and asia such a contractional background does not support models that relate a tensile mechanism for back arc spreading along the bangong nujiang belt