abrupt global ocean anoxia during the late ordovician early silurian detected using uranium isotopes of marine carbonates widespread marine anoxia is hypothesized as the trigger for the second pulse of the late ordovician hirnantian mass extinction based on lithologic and geochemical proxies that record local bottom waters or porewaters we test the anoxia hypothesis using 238u values of marine limestones as a global seawater redox proxy the 238u trends at anticosti island canada document an abrupt late hirnantian 0 3 negative shift continuing through the early silurian indicating more reducing seawater conditions the lack of observed anoxic facies and no covariance among 238u values and other local redox proxies suggests that the 238u trends represent a global ocean redox record the hirnantian ocean anoxic event hoae onset is coincident with the extinction pulse indicating its importance in triggering it anoxia initiated during high sea levels before peak hirnantian glaciation and continued into the subsequent lowstand and early silurian deglacial eustatic rise implying that major climatic and eustatic changes had little effect on global ocean redox conditions the hoae occurred during a global 13c positive excursion but lasted longer indicating that controls on the c budget were partially decoupled from global ocean redox trends u cycle modeling suggests that there was a 15 increase in anoxic seafloor area and 80 of seawater u was sequestered into anoxic sediments during the hoae unlike other ocean anoxic events oae the hoae occurred during peak and waning icehouse conditions rather than during greenhouse climates we interpret that anoxia was driven by global cooling which reorganized thermohaline circulation decreased deep ocean ventilation enhanced nutrient fluxes stimulated productivity which lead to expanded oxygen minimum zones

soils surficial geology and geomicrobiology of saline sodic wetlands north platte river valley nebraska usa abstract saline sodic wetlands along a 200 km stretch of the north platte river valley in western nebraska usa lie within an important agricultural region but their processes salt mineralogy and geomicrobiology have not previously been investigated putative anthropogenic salinization has long been a concern yet early historical accounts of widespread surface salts in the area have never been applied as comparative standards surface salts in the area originate from soil capillarity and surface evaporation thenardite na 2 so 4 and or mirabilite naso 4 10 h 2 o dominate depending on ambient conditions bloedite na 2 mg so 4 2 4 h 2 o halite nacl burkeite na 6 co 3 so 4 2 and calcite caco 3 are minor constituents historical accounts indicate that salts accumulated naturally long before euramerican settlement apparently as a result of rock water interaction in nearby volcaniclastic sediments of the brule formation ephemeral to permanent water holding basins in the wetlands contain na rich waters that vary widely in electrical conductivity as high as 159 ms cm and in ionic composition but local spring waters are extremely dilute basin floors exhibit a unique type of microrelief which appears to form by the filling of microlows with water and the dispersal of soil material therein by na followed by dewatering and collapse of the soil with drying illite dominates basin surface soils but smectite dominates at depth high soil ph available k and frequent wetting drying cycles in the wetlands suggest that in situ illitization may have occurred soil crusts and vesicular surface horizons are common as are upward increases in electrical conductivity the activity of sulfate reducing microbes forms prominent near surface horizons of sulfate reduction in saturated soils which retract or disappear entirely during dry episodes saline sodic wetland soils in the study area change on daily to seasonal scales cycles of surface salt development microbial activity and microrelief genesis are all controlled by regular wetting drying cycles and the interaction of ponded surface waters and shallow groundwaters relatively unique aspects of microbial ecology and surface processes make the soils important as geomicrobial reactors wherein important parts of hydrological and geochemical cycles occur

a high resolution micromorphological record of changing landscapes and climates on the western loess plateau of china during oxygen isotope stage 5 abstract the micromorphological characteristics of a continuous set of block samples from 24 38 m depth in a well section on jiuzhoutai mountain lanzhou china are described the shape of the magnetic susceptibility curve together with previously published radiocarbon luminescence and geomagnetic dates indicates that this sample set encompasses the whole of the last interglacial s1 palaeosol complex and extends into the overlying and underlying loess l1 and l2 units seven pedosedimentary units are recognised on the basis of trends in the bulk properties and the key micromorphological indicators excrements textural concentrations shell fragments and calcite and gypsum features high rates of loess deposition with significant cryogenic activity gypsum precipitation and only weak bioturbation characterised oxygen isotope stage ois 6 and ois 4 at lanzhou depositional rates were moderately high in ois 5d and 5b with slightly higher bioturbation and no significant cryogenesis or gypsum precipitation ois 5e 5c and 5a were marked by low rates of accretion more extensive bioturbation and limited leaching this reconstruction reflects the pedosedimentary responses to fluctuating dominances of monsoonal controls during this period of global climatic change comparison of earlier micromorphological results from sites to west and east of lanzhou along a dry to humid climatic gradient indicate a clear regional pattern with reduced loess accumulation rates and enhanced bioturbation and leaching in the humid east tianshui and xian clay translocation occurred only in ois 5e at xian the warmest and most humid site on the transect

euxinia caused the late ordovician extinction evidence from pyrite morphology and pyritic sulfur isotopic composition in the yangtze area south china abstract the late ordovician mass extinction lome was a severe biocrisis during the phanerozoic and in this extinction trilobites brachiopods mollusks graptolites conodonts and other types suffered severe mortality and 85 of marine invertebrate species were eliminated the potential cause of this mass extinction remains unclear although glaciation and marine anoxia have received the greatest attention herein we present findings on pyrite morphology pyritic sulfur isotopic composition and total organic carbon content toc to analyze the evolution of redox conditions across the ordovician silurian boundary in the yangtze area of south china our results show that long term euxinic conditions interrupted by episodic dysoxic oxic events occurred from the upper katian stage to the lower rhuddanian stage the two extinction pulses of lome were coincident with euxinic conditions suggesting that marine euxinia may have been the main cause of this mass extinction event

a far field record of the end ordovician glaciation the ellis bay formation anticosti island eastern canada abstract the end ordovician was a critical period in earth history it was a time of global climatic change with large scale continental glaciations a wealth of recent work has documented the stratigraphical records from the near field areas i e from various gondwanan continental relics where glaciation was more severely felt there is an urgent need to re examine the stratigraphic record from the far field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing duration and character at the glaciation centred around the ordovician silurian o s boundary one of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on anticosti island in eastern canada the east west trending 200 km long ellis bay hirnantian outcrop belt with superb coastal exposures at both ends of anticosti island is slightly oblique to the paleoshoreline with near shore siliciclastic dominated facies restricted to the eastern part of the island and more offshore carbonate dominated facies present along the central and western parts these mixed carbonate siliciclastic facies accumulated along a storm dominated beach to offshore profile they are organized into a stacked set of shallowing upward offshore to shoreline facies successions typically coarsening upward separated by deepening upward facies successions typically fining upward formed during intervening transgressions high frequency glacio eustatic fluctuations were the dominant control on the development of tr sequences observed in the ellis bay succession of anticosti island although tectonic subsidence had a substantial secondary effect on thickness five tr sequences are recognized allowing for precise correlation along the ellis bay outcrop belt in the eastern sections relatively thin tr sequences are locally bounded by unconformities a typical sequence begins with thin transgressive limestone overlain by a maximum flooding calcareous shale grading up into regressive storm influenced proximal to transitional zone fine sandstone grainstone and shale which are sharply overlain by shoreface to foreshore sandstone grainstone in the central and western sections the thicker tr sequences have more symmetrical transgressive regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply two discrete glacial periods are recognized during the deposition of the hirnantian ellis bay succession with the younger one recording large sea level changes 50 m in magnitude caused by the development of very large ice sheets over western gondwana abundant ice conditions on the globe during the end ordovician and the resulting high amplitude eustasy appear to have left distinctive stratigraphical imprints on mixed paleotropical siliciclastic carbonate ramps

interaction between aggrading geomorphic surfaces and the formation of a late pleistocene paleosol in the palouse loess of eastern washington state abstract variable rates of loess deposition contributed to dramatic regional variation in a soil stratigraphic unit the washtucna soil in the palouse loess deposits in the channeled scabland of eastern washington state throughout most of the channeled scabland the morphology of the washtucna soil is that of a single buried soil but it bifurcates into two well developed and pedologically distinct buried soils in areas immediately downwind of the major source of loessial sediment regional loess stratigraphy confirms that the two well developed soils formed during the same interval of time during which only one soil formed in areas that are distal to loess source areas the variable and perhaps rapid rates of soil formation suggested by the stratigraphy resulted from an interaction between variable rates of loess deposition and the formation of superimposed calcic soils petrocalcic horizons with weak stage iv morphology formed as the zone of carbonate accumulation moved up into former a and cambic horizons that had been profusely burrowed by cicadas the development of cicada burrows in one phase of soil development that were subsequently engulfed by pedogenic carbonate under a rising land surface seems to have greatly accelerated the development of the petrocalcic horizons accelerated rates of formation of the petrocalcic horizons occurred when extrinsic pulses of loess deposition and intrinsic engulfment of burrowed horizons thresholds were exceeded stratigraphic evidence suggests that the soil formation that accompanied the rise in the land surface due to additional loess deposition may have occurred during the late wisconsin glaciation when giant glacial outburst floods in the channeled scabland triggered a new cycle of loess deposition

climatic and oceanic changes during the middle late ordovician transition in the tarim basin nw china and implications for the great ordovician biodiversification event abstract the middle late ordovician transition darriwilian to sandbian age witnessed a major pulse of the great ordovician biodiversification event gobe and distinctive oceanic geochemical fluctuations such as coeval negative c and sr isotope excursions in this study investigations into geochemical variations notably the hg abundance or hg toc have been carried upon the organic rich black shale of the middle upper ordovician saergan formation to unravel the causes of this pulse based on these data three phases were identified phase 1 0 to 3 m is characterized by rising hg toc up to 138 ppb wt and ti al values as well as high ciacorr corrected chemical index of alteration values 68 9 72 3 with negligible enrichment of redox sensitive elements rse and nutrient elements e g u 5 2 ppm v 153 ppm mo 1 8 ppm p2o5 0 2 suggesting intensified volcanism which could have emitted significant amounts of greenhouse gases thereby leading to climate warming in contrast phase 2 3 to 11 m is characterized by decreasing hg toc and ti al ratios relatively low though slightly fluctuating ciacorr values generally depleted in rse except moderately enriched u up to 14 6 ppm and increased p al and ba al ratios implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling phase 3 sandbian age 11 13 m witnessed continuous decrease in hg toc ratio an increase in ti al and ciacorr values fairly low values of rse enrichment and p al and ba al ratios indicating recurrent climate warming and the potential for slowed oceanic circulation and attenuated upwelling of nutrient rich deep waters onto the shallow shelf these changes could have diminished bioproductivity and organic output onto the seafloor this study offers insights into volcanic climatic oceanic interactions during a major pulse of the gobe around the middle late ordovician transition while black shales were extensively deposited

extinction intensity during ordovician and cenozoic glaciations explained by cooling and palaeogeography a striking feature of the marine fossil record is the variable intensity of extinction during superficially similar climate transitions here we combine climate models and species trait simulations to explore the degree to which differing palaeogeographic boundary conditions and differing magnitudes of cooling and glaciation can explain the relative intensity of marine extinction during greenhouse icehouse transitions in the late ordovician and the cenozoic simulations modelled the response of virtual species to cooling climate using a spatially explicit cellular automaton algorithm we find that palaeogeography alone may be a contributing factor as identical changes in meridional sea surface temperature gradients caused greater extinction in late ordovician simulations than in cenozoic simulations differences in extinction from palaeogeography are significant but by themselves are insufficient to explain observed differences in extinction intensity however when simulations included inferred changes in continental flooding and interval specific models of sea surface temperature predicted differences in relative extinction intensity were more consistent with observations from the fossil record our results support the hypothesis that intense extinction in the late ordovician is partially attributable to exceptionally rapid and severe cooling compared to cenozoic events high extinction intensity during late ordovician but not cenozoic transitions to glacial conditions can be attributed to both temperature and palaeogeography according to analysis combining climate models and simulations of virtual species

ultra high resolution multivariate record and multiscale causal analysis of pridoli late silurian implications for global stratigraphy turnover events and climate biota interactions abstract the upper silurian and especially pridoli epoch is a critical interval for the understanding the evolution of earth s biota since it witnessed series of powerful extinction events global reorganizations of paleocommunities and expansion of new clades which assumed dominance in the subsequent devonian period the stratigraphic record of the eastern part of the silurian baltic basin represents an excellent candidate for the fine scale analysis of paleoecological and paleoceanographic dynamics therefore we presenting here stratigraphic record of the milaiciai 103 core south western lithuania of conodonts brachiopods 13ccarb 18ocarb and a series of various geochemical and geophysical proxies most of which were sampled at a very high resolution 11 ka spanning uppermost ludlow pridoli and the lowermost devonian the analysis revealed that the pridoli was characterized by four distinct long term dynamic states here named i normal pridoli i ii silal negative carbon isotopic excursion and extinction event iii normal pridoli ii and iv klonk positive stable carbon isotopic event and excursion the most significant change in most variables is detected approximately in the middle of the pridoli at the beginning of the delotaxis detorta zone at the recovery after silal event of conodonts and at the onset of collapse of brachiopod communities the analysis of dynamic complexity revealed the strong evidence for the presence of 405 ka metronome milankovitch cycles in the record of 13ccarb which constrains the duration of the pridoli epoch to 4 3 0 2 ma the recurrence plot analyses revealed deterministic coordination between 13ccarband conodont paleocommunity dynamics at all scales finally lead lag analysis using conditional joint recurrence revealed that conodont dynamics as a rule were driven by perturbations in the carbon cycle although silal event interval shown opposite pattern thus confirming the unusual nature of this previously unrecognized coordinated climatic and biotic event

identification of multiple loess units within modern soils of clay county nebraska abstract modem soils in south central nebraska may have formed in parent material made up of more than one loess unit the aerial extent of these soils could be as much as 1 6 million ha the presence of lithologic discontinuities within these soils complicates the explanation of their genesis the objectives of this study were to determine if multiple loess units exist in the modem soils and to identify properties that could be used to differentiate between the loess units four pedons representing an extensive soil and landscape map unit were sampled and particle size distribution cation exchange capacity extractable bases organic carbon content and mineralogy were determined results indicated that at least two loess deposits associated with the bignell loess of holocene age and the peoria loess of late wisconsinan age were present within the pedons studied these loess units can be differentiated on the basis of the following properties particle size distribution chemistry volcanic ash distribution and clay mineralogy differences in these properties within the pedons can be attributed to changes in sediments carried by the platte river the assumed source of the loess

transport direction of peoria loess in nebraska and implications for loess sources on the central great plains abstract in the midwestern united states large rivers draining the laurentide ice sheet lis were the most important sources of peoria loess deposited during the last glaciation loess deposition near those rivers may have responded primarily to ice sheet dynamics rather than direct effects of climatic change in contrast it has been proposed that thick peoria loess on the central great plains was derived mainly from unglaciated landscapes northwest of the main loess deposits in this study transport directions inferred from more than 600 measurements of peoria loess thickness in nebraska are used to test the hypothesis that much of the peoria loess on the great plains is nonglaciogenic a strong northwest to southeast thickness trend indicates that most peoria loess in nebraska was transported from one or more unglaciated northwestern source areas rather than from glacially influenced river floodplains the missouri river draining the lis the platte river draining alpine glaciers and the elkhorn river unglaciated basin were secondary sources their contribution is not detectable beyond a distance of 40 60 km peoria loess deposition on the central great plains was largely a direct response to climatic change in the unglaciated source region

lipid biomarkers record fundamental changes in the microbial community structure of tropical seas during the late ordovician hirnantian glaciation the late ordovician mass extinction was linked to climate cooling and glaciation of gondwana during the terminal ordovician hirnantian age 444 7 443 4 ma extinction patterns have been well described for many marine taxa but much less is known about marine microbial communities through this interval to elucidate the structure of microbial communities in tropical marine basins through the late ordovician we analyzed lipid biomarkers in thermally well preserved strata from the taconic foreland anticosti island canada the cincinnati arch midwestern united states and the western continental margin vinini formation nevada united states despite clear oceanographic differences lipid biomarker profiles show similarities between these three localities major shifts in biomarker distributions of anticosti island and the vinini formation mainly hopane sterane ratios record changes in the balance of bacterial versus algal primary production bacterial contributions to sedimentary organic matter were highest during warm intervals both before and after hirnantian cooling in particular 3 methylhopanes likely sourced from aerobic methanotrophic bacteria occur in high relative abundance many times the phanerozoic average across laurentia throughout most of the interval studied 3 methylhopane abundances also reveal an overall positive relationship with paleotemperature proxies implying increased methane cycling during warm intervals these results suggest that enhanced methane cycling could have provided an important positive feedback on climate during extended intervals of early paleozoic time

spatial variation in late ordovician glacioeustatic sea level change abstract mass extinction of late ordovician marine fauna closely coincided with southern hemisphere glaciation the sequence stratigraphic architecture of shallow marine deposits informs estimates of glacioeustatic sea level change at sites both proximal and distal to the reconstructed ordovician ice sheet s and contemporaneous changes in ice volume a recent correlation framework for the stratigraphic architectures of one near and one far field late ordovician margin concluded that the late ordovician glaciation encompassed multiple long term cycles of ice volume growth and retreat with superimposed higher frequency cycles here we posit that similar to cenozoic glacial cycles glacial isostatic adjustment can preclude synchronous and similar magnitude or directional changes in late ordovician sea level between ice proximal and ice distal locations and hence distort a globally correlative sequence stratigraphy we explored whether long duration i e million year late ordovician glacial cycles should produce a globally coherent eustatic record of sea level change between ice proximal and ice distal margins using a gravitationally self consistent theory that accounts for the deformational gravitational and rotational perturbations to sea level on a viscoelastic earth model we adopted a late ordovician paleogeography and a synthetic continental ice sheet distribution and volume informed by the areal extent of glaciogenic deposits and geochemical records respectively we demonstrate that modeled million year late ordovician glacial cycles produce sea level histories on near and far field margins that differ from eustasy and from one another due primarily to elastic flexure and associated gravitational effects while predicted far field sea level histories faithfully preserve the temporal structure of modeled glacioeustasy their amplitude may differ from eustasy by as much as 30 40 the impact of glacial isostatic adjustment is largest at the margins of glaciated continents and these effects can be of the same order of magnitude as the eustatic and even induce a local sea level rise during an episode of ice growth and eustatic sea level fall and vice versa in this regard stratal surfaces of maximum regression and flooding expressed at near field margins need not reflect global eustatic trends in ice sheet growth and decay respectively and thus may not provide chronostratigraphic horizons for correlation with far field sequence stratigraphic architectures

loess record of dry climate and aeolian activity in the early to mid holocene central great plains north america thick holocene bignell loess sections provide new evidence for the timing of early to middle holocene aridity in the central great plains the immediate source of loess in these sections was dune fields just upwind based on thickness trends and grain size data thus periods of rapid loess accumulation indicate episodes of extensive dune activity under drier than present climate in typical bignell loess sections a thick zone of coarse textured loess with minimal pedogenic alteration is interpreted as a record of the most rapid sustained holocene loess accumulation optical ages indicate that loess in this zone was deposited beginning about 9000 10000 years ago at five study sites accumulation ended shortly after 6500 years ago at three sites but possibly earlier at the other two in two well dated sections the average rate of loess accumulation between 9000 10000 and 6500 years ago was about 1 6 3 5 times greater than in the late holocene thus we infer that the most extensive sustained holocene

temperature and biotic crises in the marine realm climatic change has been a prominent cause of marine mass extinction but areal restriction of seafloor during global regression has not late eocene and pliocene pleistocene cooling for example caused major extinctions but profound global oligocene and pleistocene regressions had little or no direct effect on benthic diversity recurrent themes of pre cenozoic marine crises suggest that global temperature change also served as a major and perhaps dominant agent of extinction in these events 1 mass extinctions have frequently been concentrated in the tropics which seem to have become a refrigerated trap from which there has been no escape biotas previously occupying high latitudes have shifted equatorward to replace disappearing tropical biotas 2 some crises were not instantaneous but followed protracted and pulsatile temporal patterns as would be predicted for complex global climatic crises 3 several mass extinctions coincided with recognized intervals of climatic cooling

isotopic and rock magnetic expression of environmental change at the pleistocene holocene transition in the central great plains abstract in a region too dry for the effective use of the traditional palynological approach loess deposits of the central great plains offer a source of proxy information for the reconstruction of past climates the brady soil a ubiquitous feature developed within the uppermost late pleistocene peoria loess recorded the character of vegetation change associated with climate shifts during the pleistocene holocene transition stable carbon isotope ratio analysis and two room temperature magnetic parameters were used to examine the nature of the change during this period 13 c values derived from soil organic matter indicate that in the late pleistocene minor populations of drought tolerant c4 plants were interspersed among the dominant cool season grasses c3 in the west while c3 grasses were accompanied by trees in the east by the early holocene c4 grasses were the dominant vegetation in the west and contributed substantially to the 13 c signal in the east magnitude of the 13 c shift suggests an increase in july mean daily temperature of 8 10 c according to existing models magnetic susceptibility and frequency dependence signal the cessation of loess deposition and development of the brady soil and indicate a spatial pattern in the intensity of weathering across the region

severe selenium depletion in the phanerozoic oceans as a factor in three global mass extinction events abstract selenium se is one of the key trace elements required by all animal and most plant life and se deficiencies in the food chain cause pathologies or death here we show from new geochemical analyses of trace elements in phanerozoic marine pyrite that sustained periods of severe se depletion in the past oceans correlate closely with three major mass extinction events at the end of the ordovician devonian and triassic periods these represent periods of se depletion 1 5 2 orders of magnitude lower than current ocean abundances being within the range to cause severe pathological damage in extant se reliant organisms se depletion may have been one of several factors in these complex extinction scenarios recovery from the depletion extinction events is likely part of a natural marine cycle although rapid rises in global oxygen from sudden major increases in marine productivity and plant biomass after each extinction event may also have played a crucial role

high resolution studies of the oldest cultivated soils in the southern loess plateau of china abstract the holocene loess soil sequence on the chinese loess plateau constitutes an excellent record of evolution of soil formation monsoonal climate eolian dust accumulation and influences of arable farming of over 8000 years a high resolution soil profile on the zhouyuan loess tableland to the west of xi an was studied using particle size analysis measurement of magnetic susceptibility total fe toc and caco3 the results provide new insights into the roles of eolian dust deposition and human impact of cultivation on soil development in this environmentally sensitive zone several loess layers are found interbedded with cinnamon soils in the profile this has been caused by changes in rainfall and intensity of dust deposition that is forced by changes in balance between the northwest continental monsoon and southeast maritime monsoon of east asia during the holocene the processes of soil formation were interrupted several times by intensified dust fall and climatic aridity in the last 11 500 years the initial clearance of forest for arable farming occurred at ca 7500 years bp on the loess tableland it was contemporaneous with the arrival of the wetter and warmer climate of the holocene megathermal 8500 3100 years bp thereafter arable farming has been maintained during both the time of soil formation and that of intensified dust accumulation though there could be temporary cessation or reduced intensity of cultivation in the drier intervals because of lack of rainfall and soil moisture increased intensity of dust accumulation has been ongoing since ca 3100 years bp in the loess plateau a loess deposit of 40 80 cm has overlaid the multiple or single welded cinnamon soils on the flat land over the plateau

marine redox variability from baltica during extinction events in the latest ordovician early silurian abstract it is well documented that upper ordovician and silurian successions record multiple marine turnover events including the second largest mass extinction in the phanerozoic widespread glaciation and multiple global carbon cycle perturbations whereas causal mechanisms for the late ordovician major mass extinction event involving climate paleoceanographic variation and 13c records have been published similar records remain poorly constrained for subsequent extinction events in the early silurian here we present new organic matter carbon isotope 13corg chemostratigraphy and corresponding paleoredox proxies fe speciation mn v mo and pyrite sulfur isotopes from two organic rich drill core sections in sweden and latvia that span the upper katian through lower wenlock stages 446 431 ma pyritized fe and bulk sedimentary mn concentrations from upper ordovician strata in southern sweden suggest a local redox shift to more reducing conditions in the late hirnantian possibly including euxinic anoxic and sulfidic water column conditions that coincide with the second mass extinction pulse the new high resolution 13corg and 34spyr datasets from the late aeronian early silurian interval within both drill cores show positive excursions that are broadly coincident with the associated sandvika and sedgwickii extinction events independently fe speciation and bulk sedimentary trace metal data from this late aeronian interval record locally euxinic conditions in both the deep basinal sweden and distal shelf latvia settings before and during the late aeronian positive 13c excursion this multiproxy paleoredox dataset provides the first direct evidence for local to regional expansion of reducing marine conditions coincident with this early silurian late aeronian biotic event and positive 13c excursion additionally new 34spyr values spanning the llandovery wenlock boundary interval in the latvia core show a positive excursion coincident with fe speciation and trace metal enrichments that imply a local redox perturbation with intermittently euxinic bottom waters during the rising limb of the ireviken positive 13c excursion the combination of these geochemical data for local to regional scale more data required for global interpretations changes in marine redox conditions with paleobiological records and evidence for eustatic sea level rise indicate that environmental stresses related to an expansion of anoxic to euxinic conditions were a probable driver for several extinction events during the latest ordovician early silurian

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

mineralogical and geochemical trends across the loess plateau north china abstract the mineralogy and major element geochemistry of malan loess the uppermost loess layer were studied along two transects from the baiyu mountains in the north to the wei river lowlands 350 km to the south the mineralogy of the coarse silt fine silt and clay fractions is generally uniform across the loess plateau however in the coarse silt fraction there is a decrease in the weight percentage of heavy minerals and in the hornblende epidote ratio southwards also in the clay fraction mica contents increase slightly southwards and smectite increases northwards along one transect the major element geochemistry of malan loess shows several clear trends silicon sodium and magnesium generally decrease from north to south while aluminium iron potassium titanium and manganese generally increase the observed mineralogical and chemical patterns appear to be mainly related to the changing particle size from sandy loess in the north to clayey loess in the south slightly higher potassium and phosphorus contents in the clayey loess suggest that this loess has a greater natural fertility overall the results confirm particle size evidence that malan loess was derived from a desert source area to the north and northwest of the loess plateau

ocean euxinia and climate change double whammy drove the late ordovician mass extinction the late ordovician mass extinction lome ca 445 ma was the first of the big five phanerozoic extinction events and comprised two extinction pulses proposed kill mechanisms include glacially induced global cooling and the expansion of water column anoxia and or euxinia sulfidic conditions but no general consensus has been reached with regard to the precise role of these mechanisms a more definitive understanding is hampered by poorly constrained temporal links between the extinction pulses and climate change and by uncertainty over the spatial distribution and intensity of euxinia here we utilize fe speciation and mo concentrations in addition to the chemical index of alteration weathering proxy to reconstruct ocean redox conditions and climate change across a late ordovician to early silurian shelf to slope transect on the yangtze shelf sea a siliciclastic dominated shelf basin in south china these data show two cycles of expanded euxinia corresponding to the two pulses of the lome suggesting a strong causal relationship significantly we show that intermittent or weak euxinia developed during the first extinction pulse which likely accounts for the loss of benthic fauna and some planktonic organisms and nektonic groups by contrast the development of more intense euxinia throughout the water column during the second pulse likely drove survival fauna to extinction superimposed upon this significant global cooling occurred across the first extinction phase reflecting a secondary role in driving the extinction of certain low latitude taxa

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

optical and radiocarbon ages of stacked paleosols and dune sands in the nebraska sand hills usa abstract optical ages for eolian sands from the nebraska sand hills indicate periods of extensive eolian activity at ca 115 25 840 70 2300 240 and 3560 340 a activity was also noted at single sampling locations at ca 6180 370 8430 510 and 13110 800 a many of these ages are similar to those noted by earlier authors optical ages from samples collected within paleosols indicate shorter and possibly less extensive periods of eolian activity at approximately 1220 150 1590 110 and possibly 1950 150 a during which the paleosol sands accumulated what was originally interpreted as a single 1 2 m thick paleosol is shown by optical dating to consist of three or more welded soils developed within eolian sands with optical ages of ca 3800 240 2740 240 1560 110 and possibly 1930 140 a each of which match eolian pulses recognized elsewhere scatter in some optical ages is attributable to intersection of sand filled rodent burrows extending in outcrop 1 5 m below the contact between paleosol and overlying topset beds a 5310 360 a optical age for one probable intersected burrow provides evidence for upward or lateral transport of older sands

pedosedimentary reconstruction of a thick loess paleosol sequence near lanzhou in north central china abstract a 38 m well section near lanzhou at the semiarid western margin of the loess plateau china contained a continuous high resolution loess paleosol sequence spanning the last 130 000 yr depth functions of micromorphological features magnetic susceptibility calcium carbonate organic carbon and median grain size provide the basis for pedosedimentary and associated paleoenvironmental reconstructions of three paleosol complexes s1 sm and s0 each pedosedimentary stage reflects the interaction of changing intensities of controlling monsoonal forces three periods of reduced dust inputs and enhanced pedogenic activity notably bioturbation and weak leaching can be identified from within the s1 pedocomplex these soil forming intervals were separated by phases of varying land surface instability characterized by arid dust deposition semiarid accretionary pedogenesis water reworking and crust formation or even freeze thaw modification the sm pedocomplex represents an interval of relatively rapid dust accumulation with only minimal syndepositional modification and minor pedogenic alteration at ephemeral land surfaces the s0 pedocomplex is clearly more pedogenically developed although erosion reworking and mixing by water have partly homogenized the property depth functions

pedogenesis by upbuilding in an extreme leaching and weathering environment and slow loess accretion south westland new zealand abstract soil formation is usually envisaged as acting from the surface downwards in many environments however the accumulation of sediment and soil formation are contemporaneous it is only where the rate of one process far exceeds the rate of the other that assumptions of topdown pedogenesis are appropriate the aspect of upbuilding pedogenesis that distinguishes it from topdown pedogenesis is that each increment of soil below the a horizon has experienced processes characteristic of all horizons above it this study examines the influence of upbuilding soil formation in a high rainfall environment of rapid soil formation and slow loess accretion an environment where upbuilding could have a noticeable influence on soil properties soils formed by topdown soil formation topdown soils are compared with soils formed in loess that accumulated from early last glaciation to the holocene upbuilding soils soils are compared in two environments alaquods on poorly drained terraces and placorthods or dystrochrepts on well drained moraine crests we have assumed that properties of the upbuilding soils inherited during the current topdown phase of pedogenesis beginning in the holocene are analogous to those in the topdown soils in this manner soil properties evolved during upbuilding are distinguished in well drained upbuilding soils on moraines subsoil organic carbon has mineralised so that profile trends are very similar to corresponding topdown soils upbuilding has enhanced weathering in subsoils which is reflected in lower ph lower total p and lower primary inorganic p than in topdown soil counterparts total p concentrations in upbuilding soils are much lower than unweathered loess and constant throughout the subsoil and there is almost no p in primary forms in contrast total p in c horizons of topdown soils reaches concentrations typical of the parent material and more than 30 of this is in a primary inorganic form the differences are attributed to each subsoil increment of upbuilding soils having been part of acid a and e horizons oxalate extractable fe and al data indicate that podzolisation was active during upbuilding poorly drained upbuilding soils on terraces have higher subsoil organic carbon inherited from former a or o horizons than comparable topdown soils buried a and o horizons inherited during upbuilding and preserved in the reducing conditions have acted as sinks for mobile al and now resemble podzolic illuvial bh horizons upbuilding through addition of loess to soils promotes a pedogenic pathway distinct from topdown pedogenesis consequently soil chronosequence studies that include soils on old loess mantled surfaces do not strictly satisfy the assumptions of soil chronosequences the nature and rate of soil processes inferred from these studies need revisiting the findings of this study are transferable to other environments where the rate of soil formation and sediment accumulation are comparable

late quaternary climate change loess sedimentation and soil profile development in the central great plains a pedosedimentary model research on soil genesis often assumes a top down model in which the soil profi le develops downward from a stable land surface this model is inapplicable to upland landscapes affected by frequent dust deposition where soils grow upward as they develop on the central great plains late quaternary loess sections proximal to immediate source areas contain the brady soil a prominent marker separating late pleistocene peoria loess from holocene bignell loess farther from immediate dust source areas the brady soil and bignell loess are not recognizable in the fi eld on loess tablelands in these distal regions surface soils typically contain a prominent clay rich b horizon below a thick silty a horizon assuming top down pedogenesis this could be interpreted as a postglacial soil profi le formed in peoria loess with the b horizon produced by weathering and clay illuviation we propose a strikingly different interpretation in which the upper b horizon at distal sites is the brady soil a horizon that has been transformed by burial organic matter loss and modern subsoil structure formation processes the overlying modern a horizon represents bignell loess properties of the brady soil at proximal sites a distinctive burrowed zone high clay content and tio 2 zro 2 and low volcanic glass content can be traced to the b horizon in distal soils a decrease in smectite abundance above the brady soil at proximal sites is identifi able at the top of the clay rich b horizon in distal soils the spatial variation of clay content in loess and soil horizons is best explained by eolian sedimentation patterns the higher clay content in the brady soil and distal b horizons defi nes a fi ne grained zone that represents a late phase of peoria loess accumulation evidence of chemical weathering is minimal and illuvial clay is rare to absent in the clay rich b horizons illuvial clay does often occur deep in the solum and is related to the depth below the top of the brady soil the depth of occurrence of illuvial clay is not related to modern climate parameters although depth to secondary carbonate appears to be in equilibrium with modern climate upland soils in the central great plains are composite soils their properties are the result of a pedosedimentary history linked to regional climate change that has infl uenced sedimentation and pedogenesis since the late pleistocene

changes in marine productivity and redox conditions during the late ordovician hirnantian glaciation abstract changes in marine productivity and redox conditions during the end ordovician hirnantian glaciation and ordovician silurian transition were investigated through mo isotope and major and trace element analyses of the wangjiawan hubei province and nanbazi guizhou province sections from the yangtze platform of south china katian shales of the wufeng formation which yield the graptolites dicellograptus complanatus dicellograptus complexus and paraorthograptus pacificus were deposited under euxinic conditions at both localities as shown by high mo ef u ef and 98 mo values a major sea level regression during the hirnantian glaciation resulted in shallowing and a shift toward better oxygenated conditions within the yangtze sea as well as deposition of thin bedded siliceous sediments calcirudite debris flows and limestone turbidites of the kuanyinchiao formation the base of which correlates with the first phase of the end ordovician mass extinction the termination of the hirnantian glaciation at the top of the kuanyinchiao formation was associated with a major sea level transgression a rapid expansion of euxinia in the yangtze sea as documented by a return of high mo ef u ef and 98 mo values and the second phase of the mass extinction during which the cool adapted hirnantian fauna went extinct the long term cooling trend of the middle and late ordovician which culminated in the hirnantian glaciation was driven by enhanced burial of organic carbon as documented by 13 c carb 13 c org and 34 s py records increased organic carbon burial was linked to high rates of marine productivity as shown by high toc and biogenic ba concentrations especially at the deeper less restricted wangjiawan locale producing the hice hirnantian isotopic curve excursion 13 c carb excursion the locus of organic carbon burial during the hirnantian crisis shifted to deeper water environments that were located outside the study region the relatively rapid onset and termination of the hirnantian glaciation were probably due to crossing of tipping points in the late ordovician climatic oceanic system

impact of holocene dust aggradation on a horizon characteristics and carbon storage in loess derived mollisols of the great plains usa the amount and vertical distribution of soil organic carbon soc storage is primarily a function of vegetation type and climate with influence from soil and landscape properties and human activities the influence of dust aggradation on landscapes during the holocene has largely been ignored as a factor in long term soc storage dust aggradation is important to soil horizonation and c sequestration since the dust deposit buries the landscape and increases solum thickness mollisols on loess covered upland tables in central nebraska have aggraded throughout the holocene as bignell loess accumulated above the late pleistocene brady soil in the loess tablelands the thickness of mollic colors a horizon properties and soc storage all decrease with distance from the source of bignell loess which is along a gradient of increasing precipitation for example along a 140 km transect of decreasing bignell loess thickness and increasing rainfall total thickness of a horizons decreases by 70 cm and the thickness of mollic colors decreases by 93 cm total soc storage decreases from 17 5 to 7 7 kg m2 a portion of the soc in subsurface horizons of modern soils was likely inherited from the brady soil which probably formed in a climate different than the present suggesting that soc storage is probably not in equilibrium with modern climatic conditions dust aggradation on a biologically active surface in a steppe environment has affected horizonation pigmentation and soc storage in central great plains loess derived mollisols

sources and paleoclimatic significance of holocene bignell loess central great plains usa abstract this paper reexamines the stratigraphy sources and paleoclimatic significance of holocene bignell loess in the central great plains a broadly similar sequence of loess depositional units and paleosols was observed in thick bignell loess sections up to 300 km apart suggesting that these sections record major regional changes in the balance between dust deposition and pedogenesis new optical ages together with previously reported radiocarbon ages indicate bignell loess deposition began 9000 11 000 yr ago and continued into the late holocene some bignell loess is

high resolution stable isotope stratigraphy of upper ordovician sequences constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation the two phases of the late ordovician mass extinction are approximately coeval with the periods of rapid climate change associated with the onset and demise of the gondwanan glaciation in this paper we argue that the distinctive late ordovician carbon isotope profile provides a chronostratigraphic ruler against which a sequence of environmental and biotic events may be located the ruler also allows regional and global high resolution correlation of successions representing very different environments cores from the upper ordovician succession of estonia and latvia record a large 1 3 c c a r b o n a t e excursion up to 6 with a similar profile shape the consistent relationship between the chemostratigraphy and biostratigraphy in the baltic region suggests that the isotope profile has a regional chronostratigraphic value the presence of similar profiles in nevada united states suggests that the excursion is a global chronostratigraphic signal this interpretation enables a detailed correlation to be made between upper ordovician shallow marine and basinal sequences that have wholly different faunas successions in the baltic area and in canada that do not display the model profile are interpreted as incomplete reinter pretation of these important successions significantly modifies the global database used to assess the pattern of diversity change during the mass extinction key levels of environmental change have been located against the carbon isotope profile new oxygen isotope data from brachiopod and ostracode calcite set tight limits on the start of the glacial events cooling and sea level fall started at the same stratigraphic level as the start of the carbon isotope excursion the later rise in sea level and fall in oxygen isotope values record the end of the glaciation these restrict the duration of the main glaciation to only 1 5 graptolite zones we propose that models of the carbon cycle should be adapted to be consistent with the temporal relationships between carbon cycling sea level fall and temperature change documented here the chronostratigraphic ruler provided by the carbon isotope profiles is used as a scale to determine the sequence of biotic changes and to allow high resolution correlation of biotic events at different locations this approach identifies regional similarities and differences in the patterns of extinction the main phase of graptolite extinction in the monitor range nevada for example is synchronous with the chitinozoan extinction in the baltic region but chitinozoan taxa survive to higher levels in nevada the benthic faunas in the baltic region demonstrate that the main extinction event corresponded with the beginning of the isotope excursion at the start of the hirnantian the level at which the marine environment started to change rapidly but that there were further extinctions of species within the early hirnantian the cold adapted hirnantia fauna did not appear immediately after the extinction in this area the relationship between the second phase of extinction and the carbon isotope excursion is less clear but the available data suggest that the extinction coincides with a time of rapid environmental change but not at the inception of environmental change as happened in the first phase

holocene dust accumulation and the formation of polycyclic cinnamon soils luvisols in the chinese loess plateau chinese loess palaeosol sequences are well known for their records of monsoonal climatic variations however the modern processes of dust accumulation and soil formation remain poorly understood a high resolution investigation on modern soils including the measurement of magnetic susceptibility particle size distribution total fe total organic carbon caco3 content and optical stimulated luminescence osl dating was carried out on the zhouyuan loess tableland in the southern loess plateau the results indicate that modern cinnamon soils luvisols have developed on contemporarily accumulated aeolian dust during the holocene the aeolian loess accumulated during the younger dryas was identi ed in the top part of the malan loess that underlay the modern soil by osl dating and proxy climatic data it indicates that the malan loess accumulated during the last glaciation marine isotope stages 2 4 does not serve as the parent material for the modern soils pedogenesis of the soils started with the increased precipitation and soil moisture that have occurred on the loess tableland since the early holocene precipitation driven pedogenesis and organic activities are responsible for the leaching of caco3 decomposition of mineral dust and the production of clay and ferromagnetic minerals drier intervals have interrupted soil formation several times and therefore pro les with multiple soils have been developed at many sites on the loess tableland at places where soil erosion was relatively strong either a single soil or welded soils are preserved in the holocene pro les this does not necessarily mean however that modern soils over the plateau have been developed without interruption under a constantly warmer moister climate this is signi cant for understanding the surface processes and climatic variation during the formation of the numerous palaeosols over the loess plateau in the quaternary copyright 2003 john wiley sons ltd

zircon hf o li isotopes of granitoids from the central asian orogenic belt implications for supercontinent evolution abstract deep lithospheric processes associated with the formation of major orogens including the removal of lower crust and underlying mantle through delamination associated with orogen building are poorly constrained with a view to evaluate the potential link between deep geodynamic processes and magmatic events we performed in situ zircon hf o li isotopic analyses of granitoids from the eastern and western junggar altai and beishan orogens within the central asian orogenic belt caob the ehf t and 18o values of magmatic zircons crystallized during 443 ma and 252 ma indicate diverse and heterogeneous magma sources the corresponding 7li peaks at 440 ma and 250 ma suggesting two distinct high temperature magmatic events based on a comparison with global data we argue that large scale delamination formed through gondwana and pangaea supercontinent assembly may have occurred at ordovician to silurian boundary osb and permian to triassic boundary ptb which we term as super delamination the subsequent widespread magmatism and volcanism might have made significant impact on the earth surface ecosystems ultimately leading to the osb and ptb mass extinction events we propose super delamination as a potential mechanism to explain the link between earth s internal and external processes thus providing novel insights into the trigger for mass extinction events

dynamic sulfur and carbon cycling through the end ordovician extinction revealed by paired sulfate pyrite δ34s geochemical records of the end ordovician hirnantian stage show parallel positive excursions in the stable isotope compositions of sedimentary pyrite sulfur 34spyr organic carbon 13corg and carbonate carbon 13ccarb these isotope excursions coincide with the end ordovician glaciation and mass extinction a relative increase in pyrite burial fpyr attributed to marine anoxia has been invoked to explain the sulfur isotope excursion and link oceanic redox conditions to the extinction of marine fauna an increase in fpyr would necessarily generate a parallel excursion of equal magnitude in the isotopic composition of coeval marine sulfate 34sso4 34sso4 here we present new high resolution paired sulfur isotope data from carbonate associated sulfate 34scas and pyrite from the hirnantian stage of western anticosti island quebec canada these data document a positive 20 enrichment in 34spyr comparable in magnitude to previous reports but no parallel excursion in 34scas this pattern provides new constraints on the origin of the 34spyr excursion and the nature of carbon sulfur coupling through hirnantian time specifically these observations preclude enhanced pyrite burial as the cause of the hirnantian 34spyr excursion and suggest the possible role of anoxia in the mass extinction may need to be reevaluated rather the global 34spyr excursion is best explained by a transient reduction in the isotopic fractionations expressed during microbial sulfur cycling epyr the epyr record shows a strong inverse correlation with 13c suggesting a mechanistic link between carbon cycling and processes controlling epyr during the hirnantian changes in sea level or marine redox state associated with glaciation could further impact the expression of the biological fractionation e g through syndepositional sediment reworking and or chemocline migration and resultant restricted exchange of porewater sulfate the magnitude of isotopic fractionation during microbial sulfate reduction is partially controlled by metabolic rates which are sensitive to the abundance type and lability of metabolically relevant substrates environmental change associated with the end ordovician glaciation may have elevated the flux of organic material to marine sediments or caused an increase in physical reworking of sediments leading to increased microbial sulfate reduction rates and reduced epyr as such the hirnantian 34spyr excursion may be viewed as a dynamic biological response to global climate change highlighting the connections between the carbon and sulfur biogeochemical cycles

large perturbations of the carbon and sulfur cycle associated with the late ordovician mass extinction in south china high resolution 13 c data of organic carbon from a continuous section of the late ordovician early silurian reveal two positive 13 c excursions that are associated with the mass extinction in south china the fi rst stratigraphic 34 s measurements on pyrite tied to well established biostratigraphy indicate a large perturbation of the sulfur cycle consistent with major sea level changes related to the glaciation the elevated 34 s values of pyrites and a large short lived negative 34 s excursion of 20 associated with the decay of the glaciation suggest deep water anoxia during the hirnantian stage in contrast to the conventional view that the global oceans were oxygenated we suggest that deep water anoxia may have contributed to the late ordovician mass extinction in south china and possibly elsewhere

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

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

covariation in macrostratigraphic and macroevolutionary patterns in the marine record of north america students of earth history have long recog nized the correlation between the quantity of preserved sedimentary rock and the diversity of life recorded as fossils but paleontologists have yet to determine whether this pattern refl ects a causal relationship or a unidirec tional sampling bias in fossil data imposed by preserved rock quantity distinguishing between these two alternatives has been com plicated by the fact that many of the basic patterns of paleontologic and lithologic co variation have yet to be quantifi ed rigorously here we present the fi rst analyses of the co variation between the macrostratigraphic and macroevolutionary histories of north america based on geographically and tem porally explicit co occurrences of rocks and fossils the analyses use independent quan titative summaries of the stratigraphic and fossil records by integrating the paleobiology database paleodb and macrostrat a mac rostratigraphy database for north america which allows a more direct comparison of the stratigraphic and biological histories of the continent than has heretofore been possible within the macrostrat database the rock re cord is divided into discrete packages of sedi ment that are bound by hiatuses resolvable at the stage level using per interval per pack age rates of sediment package initiation and truncation and genus fi rst and last appear ances herein regional origination and extinc tion we fia substantially stronger positive correlation between sediments and biology for extinction like parameters than we do for origination like parameters four of the largest coincident pulses of regional extinc tion and sediment truncation occur during the widely recognized end ordovician late permian end triassic and end cretaceous mass extinction intervals a further compari son of the global ranges of north american genera to north american macrostratigra phy indicates that the regional and global extinction of genera are more likely to oc cur in the same stage than are global and regional originations together these results suggest that our general understanding of biodiversity dynamics from the fossil record may not be strongly biased by the preserva tion of sediments and leaves open the pos sibility that certain large perturbations to the earth system are responsible for major changes of state in both the sedimentary and biological systems

global palaeobiogeography of brachiopod faunas during the early katian late ordovician greenhouse episode the early katian late ordovician was marked by a major greenhouse episode and the early stage of a first order marine transgression that led to the flooding of much of north america during the middle late katian multivariate analyses of 33 brachiopod faunas of early katian trentonian chatfieldian late caradoc age including 252 rhynchonelliform genera demonstrate that the trentonian brachiopod faunas of laurentia were already clearly differentiated into a scoto appalachian fauna and an epicontinental fauna globally four distinct faunal clusters are identified kazakhstan avalonia epicontinental laurentia and scoto appalachia the scoto appalachian fauna which originated during the sandbian persisted into the early katian and became most closely related to the brachiopod faunas along the western margins of laurentia the early katian epicontinental faunas of laurentia were more closely related to the coeval brachiopods of the lithuanian north estonian confacies belts of baltica than to the scoto appalachian fauna palaeoecological control on the faunal endemism within laurentia was attributable to isolation of appalachian foreland basin by faulting and peripheral bulging during the early taconic orogeny palaeolatitudinal faunal gradient from the subtropical appalachia to mid tropical epicontinental laurentia as well as different water depth water temperature and substrate types between the two palaeogeographical settings of laurentia

environmental controls on marine ecosystem recovery following mass extinctions with an example from the early triassic abstract the recovery of marine ecosystems following a mass extinction event involves an extended interval of increasing biotic diversity and ecosystem complexity the pace of recovery may be controlled by intrinsic ecosystem or extrinsic environmental factors here we present an analysis of changes in marine conditions following the end permian mass extinction with the objective of evaluating the role of environmental factors in the protracted 5 myr long recovery of marine ecosystems during the early triassic specifically our study examines changes in weathering productivity and redox proxies in three sections in south china chaohu daxiakou and zuodeng and one in northern india mud our results reveal 1 recurrent environmental perturbations during the early triassic 2 a general pattern of high terrestrial weathering rates and more intensely reducing marine redox conditions during the early griesbachian late griesbachian mid smithian and more weakly the mid spathian 3 increases in marine productivity during the aforementioned intervals except for the early griesbachian and 4 stronger and more temporally discrete intervals of environmental change in deepwater sections chaohu and daxiakou relative to shallow and intermediate sections zuodeng and mud our analysis reveals a close relationship between episodes of marine environmental deterioration and a slowing or reversal of ecosystem recovery based on metrics of biodiversity within community alpha diversity infaunal burrowing and ecosystem tiering we infer that the pattern and pace of marine ecosystem recovery was strongly modulated by recurrent environmental perturbations during the early triassic these perturbations were associated with elevated weathering and productivity fluxes implying that nutrient and energy flows were key influences on recovery more regular secular variation in deepwater relative to shallow water environmental conditions implies that perturbations originated at depth i e within the oceanic thermocline and influenced the ocean surface layer irregularly finally we compared patterns of environmental disturbance and ecosystem recovery following the other four big five phanerozoic mass extinctions to evaluate whether commonalities exist in general the pace of ecosystem recovery depends on the degree of stability of the post crisis marine environment

changes in marine isotopic composition and the late ordovician glaciation well preserved brachiopod shells and marine cements from limestone coquinas which cap carbonate mudmounds in the siljan area of central sweden have the heaviest stable oxygen and carbon isotope values yet reported for lower palaeozoic marine sediments the coquinas formed during the eustatic regression at the onset of the late ordovician glaciation the isotopic compositions reflect simultaneous shifts in both carbon and oxygen more than 5 13 c and up to 2o 18 o away from more normal lower palaeozoic values in similar carbonates from within the mounds oxygen isotope data are consistent with a change in the isotopic composition of the sea water probably accompanied by a decrease in temperature the changes in carbon values suggest enhanced deposition of organic carbon a process which would have decreased p co 2 in the ocean and the atmosphere and thus contributed to rapid global cooling

the end ordovician mass extinction a single pulse event abstract the end ordovician mass extinction eome is widely interpreted as consisting of two pulses associated with the onset and demise of the gondwana glaciation respectively with the second pulse eradicating the distinctive glacially related hirnantian benthic biota hbb a global review of occurrence data of latest ordovician benthic marine organisms reveals that virtually all warm water benthic assemblages previously assigned to the hbb comprise two distinct and clearly postglacial faunas both younger middle and late hirnantian respectively than the cool water hirnantia fauna latest katian to early hirnantian the newly recognised three transitional benthic faunas i e tbfs 1 3 can be closely tied to graptolite conodont and chitinozoan biozonations the hirnantian isotope carbon excursion hice and the glaciation thereby providing an integrated much higher resolution timescale for understanding the tempo and nature of the eome at this finer resolution we postulate a more profound impact of the first pulse of the eome than hitherto envisaged as evidenced by opportunistic expansion of the hirnantia fauna globally and the complete absence of metazoan reefs in its immediate aftermath we also argue based on high quality data from well documented benthic groups in south china i e brachiopods tabulate and rugose corals trilobites and sponges that the magnitude of the second pulse of the eome caused by the deglaciation has been overestimated because the two postglacial faunas i e tbfs 2 3 were part of a subsequent recovery phase of marine ecosystems rather than contributing to biodiversity decline thus it is more plausible to reinterpret the eome as a single pulse rapid event that was followed by a prolonged initial recovery intermittently impeded by climatic shocks through the hirnantian prior to the onset of a progressive reestablishment of marine ecosystems during the early silurian rhuddanian and aeronian associated with an overall amelioration of climatic conditions

polar front shift and atmospheric co2 during the glacial maximum of the early paleozoic icehouse our new data address the paradox of late ordovician glaciation under supposedly high pco2 8 to 22 pal preindustrial atmospheric level the paleobiogeographical distribution of chitinozoan mixed layer marine zooplankton biotopes for the hirnantian glacial maximum 440 ma are reconstructed and compared to those from the sandbian 460 ma they demonstrate a steeper latitudinal temperature gradient and an equatorwards shift of the polar front through time from 55 70 s to 40 s these changes are comparable to those during pleistocene interglacial glacial cycles in comparison with the pleistocene we hypothesize a significant decline in mean global temperature from the sandbian to hirnantian proportional with a fall in pco2 from a modeled sandbian level of 8 pal to 5 pal during the hirnantian our data suggest that a compression of midlatitudinal biotopes and ecospace in response to the developing glaciation was a likely cause of the end ordovician mass extinction

ecological disruption precedes mass extinction mass extinctions are dramatic features of the fossil record in which extinction risk is substantially elevated above background levels although extinction risk varies markedly over geologic time as well as geographically it was particularly elevated and global in extent during the so called big five events the late ordovician late devonian end permian end triassic and end cretaceous 1 these events were originally recognized by variations in extinction rate in marine animal families and their importance remains in analyses at the genus level and that account for variable preservation over geologic time 2 3 increasing attention has concentrated on understanding the ecological effects of mass extinction and other lesser but still significant extinction episodes 4 10 in pnas sheets et al 11 document the ecological changes in marine planktonic communities not only during but preceding the late ordovician 447 444 ma mass extinction

thermal barriers and the fate of perched faunas thermal barriers provide an explanation for the geologically sudden extinction of benthic faunas of epeiric seas when these seas disappeared by contracting to the open ocean biotic interactions could not have caused the sudden extinctions and neither could reduction of the regional area of seafloor because substantial areas of shallow seafloor remained along neighboring continental shelves when epeiric seas drained instead temperature contrasts must have been responsible epeiric seas had strongly seasonal climates and when some receded to continental margins many of their species would have encountered waters that failed to provide the maximum or minimum temperature required for reproduction when epeiric seas receded poleward equatorward or into coriolis driven currents many species faced lethal temperatures the history of the jurassic sundance sea provides a striking example of the fate of a warm adapted fauna driven westward into an area dominated by a cool coriolis driven current

soil climate system across the prairies in midwestern u s a abstract among mollisols across the prairie plains from eastern iowa to western kansas coincident systems of loess stratigraphy and sedimentology climate and soils are separated at 96 to 97 w longitude to the east udolls are formed peopeoria loess that dates back 14 000 radiocarbon years and are currently in a subhumid climate to the west ustrols are cormed in bignell loess that dates back to

models for simulating the fossil record computer modeling of the distribution of fossils within depositional sequences provides a means for generating hypotheses and increasing understanding of the fossil record the simulation presented here joins the basin scale stratigraphic modeling program strata a random branching model of species origination and extinction and a model relating the probability of species occurrence to sedimentary facies model results indicate that facies sensitivity of species has the strongest control on range offset i e the time between last appearance in a stratigraphic column and the time of extinction within a basin or the time between first appearance in a column and time of origination in a basin model results also predict the distribution of zones of increased range offset within depositional sequences in the results presented here increased range offset occurs within the transgressive systems tract of the deep basin in far updip settings and near the shelf break of the late highstand to early lowstand the model also demonstrates how incompleteness of the fossil record is partitioned among sampling bias facies bias and unconformity bias in this simulation sampling bias and facies bias are roughly equal in magnitude and constant across the basin but unconformity bias increasingly dominates over these two in updip positions

climate change and the selective signature of the late ordovician mass extinction selectivity patterns provide insights into the causes of ancient extinction events the late ordovician mass extinction was related to gondwanan glaciation however it is still unclear whether elevated extinction rates were attributable to record failure habitat loss or climatic cooling we examined middle ordovician early silurian north american fossil occurrences within a spatiotemporally explicit stratigraphic framework that allowed us to quantify rock record effects on a per taxon basis and assay the interplay of macrostratigraphic and macroecological variables in determining extinction risk genera that had large proportions of their observed geographic ranges affected by stratigraphic truncation or environmental shifts at the end of the katian stage were particularly hard hit the duration of the subsequent sampling gaps had little effect on extinction risk suggesting that this extinction pulse cannot be entirely attributed to rock record failure rather it was caused in part by habitat loss extinction risk at this time was also strongly influenced by the maximum paleolatitude at which a genus had previously been sampled a macroecological trait linked to thermal tolerance a model trained on the relationship between 16 explanatory variables and extinction patterns during the early katian interval substantially underestimates the extinction of exclusively tropical taxa during the late katian interval these results indicate that glacioeustatic sea level fall and tropical ocean cooling played important roles in the first pulse of the late ordovician mass extinction in laurentia

episodic holocene loess deposition in central nebraska abstract the mirdan canal section in central nebraska usa provides evidence for multiple episodes of holocene loess deposition in the central great plains this irrigation canal cut exposes more than 4 m of holocene sediment that overlies the late wisconsinan peoria loess the oldest holocene sediment is local slopewash that fills gullies but overlying beds are interpreted as holocene loess bignell loess three buried soil profiles occur within the holocene sediment and have yielded uncalibrated 14 c ages of 3010 yr bp uppermost buried soil and 8790 yr bp third buried soil from top a fourth buried soil yielded an age of 9330 yr bp and is correlated with the brady soil the bignell loess has a darker color slightly more organic matter more clay more coarse silt and sand less smectite and less volcanic glass than the underlying peoria loess the bignell loess is most plausibly interpreted as dust from local flood plains containing a mixture of reworked peoria loess and older silts with contrasting mineralogy episodes of loess deposition occurred in the early middle and late holocene probably during periods of dry climate but possibly also as a consequence of large erosive floods that made wind erodible sediment available on nearby flood plains

terminal ordovician carbon isotope stratigraphy and glacioeustatic sea level change across anticosti island quebec canada globally documented carbon isotope excursions provide time varying signals that can be used for high resolution stratigraphic correlation we report detailed inorganic and organic carbon isotope curves from carbonate rocks of the ellis bay and becscie formations spanning the ordovician silurian boundary on anticosti island quebec canada strata of the anticosti basin record the development of a storm dominated tropical carbonate ramp these strata host the well known hirnantian positive carbon isotope excursion which attains maximum values of 4 5 in carbonate carbon of the laframboise member or the fox point member of the becscie formation the excursion also occurs in organic carbon and 13 c carb and 13 c org values covary such that no reproducible 13 c 13 c carb 13 c org excursion is observed the most complete stratigraphic section at laframboise point in the west shows the characteristic shape of the hirnantian stage excursion at the global stratotype section and point gssp for the hirnantian stage in china and the silurian system in scotland we therefore suggest that the entire hirnantian stage on anticosti island is confined to the laframboise and lower fox point members by documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin we are able to reconstruct glacioeustatic sea level fluctuations corresponding to maximum glacial conditions associated with the end ordovician ice age the combined litho and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and becscie limestones the former transgresses from west to east and the latter progrades from east to west the sea level curve consistent with our sequence stratigraphic model indicates that glacioeustatic sea level changes and the positive carbon isotope excursion were not perfectly coupled although the start of the isotope excursion and the initial sea level drawdown were coincident the peak of the isotope excursion did not occur until after sea level had begun to rise carbon isotope values did not return to baseline until well after the anticosti ramp was reflooded the sea level 13ccarb relationship proposed here is consistent with the weathering hypothesis for the origin of the hirnantian 13 c carb excursion

the stability of gas hydrate field in the northeastern continental slope of sakhalin island sea of okhotsk as inferred from analysis of heat flow data and its implications for slope failures the sudden release of methane from seas due to ocean warming and or sea level drop leading to extensive mass wasting at continental margins has been suggested as a possible cause of global climate change in the northeastern continental slope of the sakhalin island sea of okhotsk numerous gas hydrate related manifestations have been reported including hydroacoustic anomaly gas flare in the water column pockmarks and mounds on the seafloor seepage structures and bottom simulating reflectors bsrs the gas hydrate found at 385 mbsl represents the shallowest occurrence ever recorded in the okhotsk sea in this study we modeled the gas hydrate stability zone ghsz using methane gas composition water temperature and geothermal gradient to see if it is consistent with the observed depth of the bsr an important distinction can be made between the seafloor containing seepage features and normal seafloor in terms of their thermal structure the depth of the bsr matches well with the base of ghsz estimated from the background heat flow geothermal gradient a large slope failure feature is found in the northern sakhalin continental slope we explore the possibility that this failure was caused by gas hydrate dissociation based on the past climate change history and inference from the ghsz calculation prediction of the natural landslide is difficult however new stratigraphic evidence from subbottom profiles suggests that the landslide occurred at 20 ka which is roughly consistent with the late stage of the last glacial maximum

did the amalgamation of continents drive the end ordovician mass extinctions abstract global biodiversity has been punctuated throughout the phanerozoic by extinction events that vary in their degree of intensity and devastation the mass extinction event that occurred at the end of the ordovician period rapidly removed a wide range of species because taxonomic loss occurred during an ice age this is believed to have initiated the extinctions and thus these extinctions have often been viewed as a deep time analogue to the loss in species diversity during the present day glacial interval the current study however indicates that temperature though arguably being a trigger was not the sole reason for the crisis based on a large bibliographic database of rhynchonelliform brachiopods that specifically operates within very narrow time slices where every locality has been precisely georeferenced for the upper ordovician lower silurian interval we show that the extinctions were not uniformly distributed nor was the succeeding recovery here we argue that changing plate tectonic configurations during the ordovician silurian interval may have exerted a primary control on biotic extinction and recovery in particular the proximity and ultimate loss of microcontinents and associated smaller terranes around laurentia may have restricted shelf and slope habitats during the latest ordovician but nevertheless in a contracted iapetus ocean provided migration routes to help drive a diachronous early silurian recovery the conclusion that plate tectonics was a primary factor controlling the extent of the extinctions and the subsequent diversity rebound demonstrates that a reduction in diversity was perhaps the most important manifestation of the end ordovician crisis and further raises the question whether this could be applied to other large phanerozoic perturbations in biodiversity levels

clay illuviation and calcium carbonate accumulation along a precipitation gradient in kansas one of the significant features of loess derived soils in kansas is the occurrence of clay rich subsurface horizons above a layer enriched with pedogenic carbonates in order to examine the extent of clay increase and pedogenic carbonate enrichment in a precipitation gradient ten soil profiles from three different precipitation regions were studied using micromorphological and mineralogical techniques the precipitation gradient was divided into three groups 400 550 mm 550 750 mm and 750 1100 mm regions the objectives were to 1 understand the cause of clay orientation in clay rich horizons 2 investigate the reasons for the clay increase and 3 observe the interaction of clay and pedogenic carbonate accumulation features along a precipitation gradient in kansas although clay films were identified in the field for soils in the 400 550 mm regions illuvial clay films were not observed in thin section analysis the clay accumulations mostly occurred as grain coatings the rest of the clay accumulations observed were very thin striated and mostly associated with voids the argillic horizons had a granostriated b fabric which indicates stress orientation of micromass caused by high shrink swell activity thick and continuous illuvial coatings were observed in the buried horizons of paleosols in the other two regions where precipitation exceeds 550 mm illuvial clay coatings with strong orientation were observed along with thin and striated stress oriented clay both types of clay orientations exceeded 1 of the cross sectional area for the thin section although illuvial clay features and pedogenic carbonates were observed in all soils at approximately the same depth complete obliteration of clay coatings was not observed in these horizons in situ weathering of biotite was one of the reasons for the clay increase in all soil profiles in all soils studied the clay increase and cause of clay orientation cannot be attributed to a single genetic process or event both illuviation and shrink swell activity were involved in the orientation of clay although orientation of clay and pedogenic carbonates were observed in all soils at approximately the same depth the decomposition of clay coatings was not observed in these horizons

response of late ordovician paleoceanography to changes in sea level continental drift and atmospheric pco2 potential causes for long term cooling and glaciation abstract we performed sensitivity experiments using an ocean general circulation model at two stages of the late ordovician caradoc 454 ma ashgill 446 ma under a range of atmospheric pco2 values 8 18 pal pre industrial atmospheric level at high and low sea level the model results indicate that the long term cooling trend during the late ordovician can be explained by progressive cooling of the global ocean in response to falling levels of atmospheric pco2 sea level change and paleogeographic change these results also explain the occurrence of low latitude cool water carbonates in north america in all simulations a drop in sea level led to a reduction in poleward ocean heat transport this indicates a possible positive feedback that could have enhanced global cooling in response to sea level drop during the late ordovician alterations in poleward ocean heat transport linked to changes of atmospheric pco2 also indicate that there is a threshold of 10 pal above which ocean current change cannot be responsible for glaciation in the late ordovician continental drift could explain the observed global cooling trend in the late ordovician through a combined poleward ocean heat transport feedback and increased ice albedo effect if atmospheric pco2 was low during the entire late ordovician the model results further indicate that the response of meridional overturning to changes in paleogeography atmospheric pco2 and sea level is stronger than the response of surface circulation to these perturbations because the overturning circulation is so strong meridional overturning was the dominant mechanism for described changes in heat transport in the late ordovician

end ordovician extinctions a coincidence of causes the end ordovician hirnantian extinction was the first of the five big phanerozoic extinction events and the first that involved metazoan based communities it comprised two discrete pulses both linked in different ways to an intense but short lived glaciation at the south pole the first occurring at or just below the normalograptus extraordinarius graptolite biozone mainly affected nektonic and planktonic species together with those living on the shallow shelf and in deeper water whereas the second within the n persculptus graptolite biozone was less focused eradicating faunas across a range of water depths in all about 85 of marine species were removed proposed kill mechanisms for the first phase have included glacially induced cooling falling sea level and chemical recycling in the oceans but a general consensus is lacking the second phase is more clearly linked to near global anoxia associated with a marked transgression during the late hirnantian most recently however new drivers for the extinctions have been proposed including widespread euxinia together with habitat destruction caused by plate tectonic movements suggesting that the end ordovician mass extinctions were a product of the coincidence of a number of contributing factors moreover when the deteriorating climate intensified causing widespread glaciation a tipping point was reached resulting in catastrophe

large carbonate associated sulfate isotopic variability between brachiopods micrite and other sedimentary components in late ordovician strata carbonate associated sulfate cas is trace sulfate incorporated into carbonate minerals during their precipitation its sulfur isotopic composition is often assumed to track that of seawater sulfate and inform global carbon and oxygen budgets through earth s history however many cas sulfur isotope records based on bulk rock samples are noisy to determine the source of bulk rock cas variability we extracted cas from different internal sedimentary components micro drilled from well preserved late ordovician and early silurian age limestones from anticosti island quebec canada mixtures of these components whose sulfur isotopic compositions vary by nearly 25 can explain the bulk rock cas range large isotopic variability of sedimentary micrite cas 34 s depleted from seawater by up to 15 is consistent with pore fluid sulfide oxidation during early diagenesis specimens recrystallized during burial diagenesis have cas 34 s enriched by up to 9 from hirnantian seawater consistent with microbial sulfate reduction in a confined aquifer in contrast to the other variable components brachiopods with well preserved secondary layer fibrous calcite a phase independently known to be the best preserved sedimentary component in these strata have a more homogeneous isotopic composition these specimens indicate that seawater sulfate remained close to about 25 v cdt through hirnantian end ordovician events including glaciation mass extinction carbon isotope excursion and pyrite sulfur isotope excursion the textural relationships between our samples and their cas isotope ratios highlight the role of diagenetic biogeochemical processes in setting the isotopic composition of cas