a new cretaceous tertiary boundary site at flaxbourne river new zealand biostratigraphy and geochemistry abstract an exceptionally complete rock sequence across the cretaceous tertiary k t boundary has been discovered near the flaxboume river marlborough province south island new zealand the boundary is marked by a large ir anomaly 21 ng g on a decalcified basis with an integrated abundance of 134 ng cm2 after correction for background above the boundary there is a 30 cm transition zone in which a few cretaceous foraminiferal taxa such as hedbergella monmouthensis and guembelitria cretacea survived though with reduced abundance and size only ca 1 2 to 1 4 of normal apparently reflecting environmental stresses inaa and icp analyses show that in addition to ir the boundary clay is also enriched in cr and ni mainly from meteoritic material and as co cu sb and zn from terrestrial sources volcanic sources even when scaled to the 10 7 km 3 volume of the deccan basalts fail by three orders of magnitude to account for the ir and as at the k t boundary and by even larger factors for sb zn cu etc comparison of our data with those from six other k t boundary sites shows that the zn sb as sb and zn as ratios generally fall between crustal and oceanic values suggesting contributions from both sources mass balance calculations show that as and sb could be derived from only 300 500 m of ocean water or also from modest amounts 20 36 g cm 2 of average crustal rock copper and zn on the other hand can only be derived from crustal or mantle rock 5 15 g cm 2 presumably impact ejecta such an amount of ejecta is fairly close to the global fallout of boundary clay 2 5 g cm 2

terminal ordovician extinction geochemical analysis of the ordovician silurian boundary anticosti island quebec elemental abundances including ir carbon and oxygen ratios in carbonates mineral content and thin sections have been measured in samples collected across the conodont defined ordovician silurian o s boundary exposed on anticosti island quebec the ir concentrations ranged from 5 to a maximum at the boundary of 58 parts per trillion ppt however there is no evidence on the basis of these ir results for the association of a large body earth impact with the o s extinction because the ir concentrations like those of most other trace elements are simply proportional to the clay al content in the carbonate sequence the sup 13 c sup 12 c and sup 18 o sup 16 o ratios decrease abruptly at the boundary then just as abruptly increase to a long period of higher than preboundary ratios these patterns are probably related to the salinity in the seaway which was shallowing up to boundary time and then deepened and developed patch reefs fresh water input from rivers would have been most influential during the shallow water conditions 32 references 4 figures

a study of the onset of melt over the arctic ocean in radarsat synthetic aperture radar data 1 the regional melt onset signal of lagrangian elements of arctic sea ice in radarsat synthetic aperture radar sar data during the spring of 1998 is examined the melt signal is clearly detectable not only in backscatter changes over multiyear ice but also first year ice and mixtures of these ice types this allows a more complete mapping of the progression of melt over the sea ice cover for the spring of 1998 the onset dates range between 10 may and 29 june the spatial pattern of melt onset is characterized by sharp boundaries delineating distinct regions with relatively uniform onset dates this pattern appears to be associated with moisture and warm air brought in by a lowpressure trough and the timing of a rain event reported at the surface heat budget of the arctic ocean sheba camp measurements from the sheba camp show good correspondence between the timing of melt in sar imagery and the onset of albedo drop off associated with the beginning of summer onset dates are compared with the timing of the zero crossing of the temperature records from drifting buoys and the onset dates derived from satellite passive microwave brightness temperature fields the timing of our estimates is within 1 2 days of the zero crossing of the buoy temperature records onset dates derived from passive microwave observations appear to be biased toward a later stage of melt comparison with results derived from a sar data set from the spring of 1992 show that the onset of melt in 1998 derived from radarsat in the beaufort sea occurred 2 weeks earlier index terms 4540 oceanography physical ice mechanics and air sea ice exchange processes 4207 oceanography general arctic and antarctic oceanography 4227 oceanography general diurnal seasonal and annual cycles 4275 oceanography general remote sensing and electromagnetic processes 0689 keywords melt onset arctic ice cover radar remote sensing lagrangian ice motion

late ordovician sand wave complexes on anticosti island quebec a marine tidal embayment laterally discontinuous mixed carbonate siliciclastic sandstones in the upper vaureal and lower ellis bay formations of anticosti island were deposited on an equatorial carbonate ramp with a slope of less than 1 the 10 18 m thick sandstones are interpreted as subaqueous sand wave complexes analogous to detached parts of modern shoreface connected sand ridges these record storm enhanced tidal modification of a northerly derived shoal retreat massif that may have formed in response to recovery from global sea level lowstands in the late ordovician ashgill late rawtheyan hirnantian the sand wave complexes formed within a tidal embayment that was confined by the precambrian shield to the north and northwest by rising tectonic highlands of the humber zone in newfoundland to the east and by active tectonic highlands in the quebec appalachians gaspesie to the south paleocurrent distributions parallel to the western margins of the strait of belle isle suggest that the north end of the embayment w

a high resolution continuous δ13c record spanning the ordovician silurian boundary on anticosti island eastern canada1 one of the best exposed and most complete stratigraphic records from paleotropical areas spanning the ordovician silurian o s boundary is located on anticosti island eastern canada our study is

the great latest ordovician extinction on the south china plate chemostratigraphic studies of the ordovician silurian boundary interval on the yangtze platform continuous samples from four ordovician silurian boundary sections of graptolitic shales and mudstones in the yangtze basin yangtze platform on the south china plate spanning the latest ordovician mass extinction of many unrelated groups of organisms were chemically analyzed for the abundances of about 40 common and trace elements and iridium it is observed that the abundances of ir and other siderophile and chalcophile elements e g co cr as mo sb and v show elevated values in the extinction horizon at the base of the graptolite persculptus zone which corresponds to the stratigraphic level for the local systematic boundary in china between the ordovician and silurian iridium abundances show maxima at this extinction level in three of the four sections analyzed with varying strength 230 ppt at yichang 92 ppt at jinxian and 88 ppt at youyang but 50 ppt at xiushan these geochemical patterns appear to be of global significance since similar geochemical anomalies including weak ir anomaly were also recorded at about the same stratigraphic level in many other sections previously studied in quebec yukon arctic and northwest territories canada and scotland although impact by a comet with low platinum group element abundances could have been responsible for the geochemical anomalies we find it is most plausible to attribute the geochemical signal to the effects of a lower sedimentation rate and reducing water conditions at this horizon the beginning of a rapid transgression induced by rapid melting of the ice caps on gondwana carbon isotope excursions in organic material at yichang together with other carbon isotope data from central sweden northwestern canada latvia quebec and wisconsin suggest that after a period of decreasing atmospheric and marine pco2 in the latest ordovician the hirnantian a sudden large pco2 increase in the ocean and atmosphere may have occurred at the extinction horizon at the base of the persculptus zone a greenhouse effect might have existed for a short time what triggered such a sudden pco2 increase is uncertain but a rapid co2 degassing caused by volcanism tectonics and or extraterrestrial impact could have been responsible

first record of the hirnantian upper ordovician δ13c excursion in the north american midcontinent and its regional implications the most prominent of the two major global delta c 13 excursions in the ordovician the hirnantian delta c 13 excursion hice which is previously recorded from the uppermost ordovician in a few sections in nevada quebec arctic canada baltoscandia scotland and china is documented for the first time from the north american midcontinent samples through the girardeau limestone and leemon formation in missouri and illinois show elevated delta c 13 values of 4 parts per thousand to 5 parts per thousand although not determined precisely the beginning of the hice is likely to be in the upper part of the orchard creek shale and it ends in the upper leemon formation being extraordinarily useful chronostratigraphically the presence of the hice makes it possible to provide a firm dating of the study interval whose age has long been controversial comparison between the study sections and coeval hice sequences in north america and europe show striking similarities especially in sea level history indicating that major local lowstands reflect eustatic sea level changes a comparison with hirnantian diamictite successions in north and south africa and argentina suggests that these lowstands correspond to two major gondwanan glacial episodes less

the distribution and geochemistry of platinum group elements as event markers in the phanerozoic abstract the discovery of the platinum group element pge enrichment in the cretaceous tertiary k t boundary impact layer popularized the use of pges as geochemical event markers and prompted an explosion of interest in these elements whose geochemical behavior is poorly understood instead of concentrating on the more easily analyzed pges e g iridium many studies now include the entire group analytical methods have been improved to the point where small samples 5 g can be analyzed confidently at the sub ppb level allowing the determination of the distribution of pges in the oceans atmosphere and biosphere the presence or absence of a pge anomaly is often used as a criterion for determining whether an event horizon is impact related however to date only the k t boundary yields a distinct pge anomaly with positive interelement correlations and solar interelement ratios terrestrial sources such as volcanic emissions are also being investigated as potential contributors to pge anomalies at other phanerozoic event horizons primary signatures e g interelement ratios characteristic of the pge source may be altered during various high and or low temperature processes for example fractionation during impact melting vaporization condensation remobilization redistribution and diagenesis can obscure original signatures and make determination of the pge source difficult future research might be directed towards establishing the behavior of the pges during such processes

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

revision of the position of the ordovician silurian boundary in southern ontario regional chronostratigraphic implications of δ13c chemostratigraphy of the manitoulin formation and associated strata delta c 13 values of 142 samples from the manitoulin formation and subjacent strata collected from 14 exposures and two drill cores on manitoulin island bruce peninsula and the region south of georgian bay suggest that the manitoulin formation is latest ordovician hirnantian rather than earliest silurian in age a delta c 13 excursion identified as the hirnantian isotope carbon excursion hice which has a magnitude of nearly 2 5 parts per thousand above baseline values is present in an interval from the upper queenston formation to the lower to middle part of the manitoulin formation in most of bruce peninsula and in the area south of georgian bay whereas on manitoulin island the rice appears to be absent this indicates that a significant part of the manitoulin formation is older on the bruce peninsula and in its adjacent region than on manitoulin island the chemostratigraphically based conclusions are consistent with biostratigraphic data from conodonts and brachiopods the hirnantian delta c 13 curve from anticosti island quebec is closely similar to those of southern ontario traditionally the ordovician silurian boundary has been placed at the base of the manitoulin formation but the new results suggest that it is more likely to be at or near the base of the overlying cabot head formation these new results have major implications for the interpretation of the geologic history and marine depositional patterns of the latest ordovician of a large part of the north american midcontinent

high resolution stable isotope stratigraphy of the upper cambrian and ordovician in the argentine precordillera carbon isotope excursions and correlations abstract we report the occurrence of important carbon isotope excursions in early paleozoic formations of the eastern and central argentine precordillera the steptoean positive isotope carbon excursion spice is known from north america kazakhstan south china australia and south america and the negative isotope carbon excursion snice has been described for the first time in south america we report here the record of the spice and snice in a single section in the eastern precordillera in the central precordillera a minor middle darriwilian positive carbon isotope excursion mdice and a late sandbian positive isotope carbon excursion the gice 3 c bicornis biozone are reported from two sections one pre gice positive carbon isotope excursion sandbian sa1 n gracilis biozone in the central precordillera with a 13c peak of 2 is perhaps equivalent to the positive spechts ferry 13c excursion of north america a positive 13c excursion 6 n persculptus biozone recorded at the base of the late hirnantian la chilca formation probably corresponds to hice these carbon isotope excursions resulted from more than one factor where paleoceanographic events probably played a major role a sea level fall and vigorous fluctuations in the steptoean spice b sea level rise in the sunwaptan snice c important transgression in the sandbian pre gice and gice and d sea level fall in the late hirnantian hice in the darriwilian and sandbian stages organic burial has led to a large 12c sequestration in the deep anoxic ocean with saline bottom water recorded by the graptoliferous black shales of the gualcamayo and los azules formations in central precordillera helped the building of the mdice and gice anomalies 18o values for the upper cambrian carbonates are likely near primary isotope signals that point to progressive cooling from the spice to the snice whereas for sandbian carbonates they suggest strong temperature fluctuations the 13c peak of the gice coincides with a cooler period with temperatures warming up towards the late hirnantian to rhuddanian the upper cambrian to the middle ordovician of the precordillera recorded a decrease of seawater 87sr 86sr ratios in accordance to the global picture this decrease probably reflects the influence of widespread volcanic activity from arc terranes in low latitude settings along eastern margins of laurentia and in the argentine precordillera in the early to middle ordovician five carbonate samples from san juan gualcamayo and los azules formations yielded end values that along seven already published values seem to plot along the nd isotopic evolution trend of the iapetus ocean together the record of global upper cambrian and ordovician carbon isotope excursions in the argentine precordillera is a valuable proxy in refining early paleozoic stratigraphy establishing of regional global high resolution correlations and sea level change history in south america

an organic carbon isotope record of late ordovician to early silurian marine sedimentary rocks yangtze sea south china implications for co2 changes during the hirnantian glaciation studies of 13c of marine sedimentary organic carbon in six clastic sections with sediments deposited from the latest ordovician to earliest silurian in the yangtze sea on the south china plate show a positive 13corg excursion of up to 4 in the latest ordovician hirnantian stage corresponding in time to a major continental glaciation on gondwana various factors that might be responsible for the 13corg increase are considered including input and contribution from exogenic carbon sources secondary processes such as heterotrophy and thermal maturation and changes in yangtze sea surface water temperature productivity primary producers dissolved co2 concentrations and the isotopic composition of dissolved inorganic carbon current data are consistent with the interpretation that the observed 13corg increase may have resulted from a decrease in the surface water dissolved co2 in the yangtze sea during the hirnantian glaciation probably reflecting a significant decrease in glacial ocean atmosphere pco2 levels at that time

constituent energy of regional seismic coda seismic coda is composed of scattered wavelets originated from various heterogeneities the phase composition of regional seismic coda still remains unknown despite its use for several decades this is caused partly because the ray paths of scattered wavelets in coda are not on the great circle path between a source and receiver we examine the constituent original phases of regional coda with the help of a source array analysis a set of uniform sources that are essential for a source array analysis is organized with underground nuclear explosions strong rg origin energy is observed in the coda at frequencies of 0 2 0 8 hz and it lasts more than 700 sec until the end of records the coherent energy in the coda reduces with frequency it constitutes about 20 of the total coda energy at frequencies of 0 2 0 4 hz and 12 at frequencies of 0 4 0 8 hz the other 80 of coda energy in 0 2 0 8 hz is mixed with complex phases from various untraceable origins the rg energy is the most influential component in the construction of low frequency regional coda on the other hand the coda at higher frequencies 0 8 3 2 hz is observed to be mixed with complex phases that cause the wave field to be diffused the observation of rg origin energy at the regional coda suggests that scattered energy from phase coupling of rg is not significant compared to rg to rg scattered energy

stratigraphic variation in marine carbonate carbon isotope ratios stratigraphic variation in the carbon isotopic 13c value of marine carbonate and organic matter preserved within it has become a popular tool for supporting paleoclimatic hypotheses and refining stratigraphic correlation virtually every important biostratigraphic and sequence stratigraphic event of the last 800 million years has had a 13c dataset collected to aid in interpreting the causal environmental factors that contributed to the event the stratigraphic resolution obtainable using 13c has yielded critical temporal constraints on the interplay between carbon cycle variation and environmental events such as mass extinction and eustasy although the non uniqueness of the carbon isotopic response to different environmental stimuli has frustrated the identification of a unified mechanism linking marine 13c variation with distinct environmental forcing functions the interpretation of 13c variation can nonetheless provide important insights into processes that influence major global climatic and surficial systems early studies of marine carbonate minerals suggested that the 13c value of ancient oceans was essentially 0 versus the peedee belemnite standard pdb clayton and degens 1959 schidlowski et al 1975 see also keith and weber 1964 it was later recognized however that departures from the pdb standard represented secular variation in the 13c values of the marine environment rather than analytical noise that the record was potentially rich with stratigraphic and paleoenvironmental information scholle and arthur 1980 veizer et al 1980 wadleigh and veizer 1982 arthur et al 1985 holser et al 1986 zachos and arthur 1986 and that variation was intimately linked to the global carbon oxygen and sulfur cycles kump and garrels 1986 holser et al 1988 it is now well established that carbonate 13c values vary within 3 from the pdb standard for

mass extinctions the view of a sceptic the concept of periodic mass extinctions by extraterrestrial impacts has contributed very strongly to the current revival of interest in the problem of mass extinctions in this context several radical claims have been made asserting that the newly emerging picture of mass extinctions as a separate class of biological phenomena caused by a separate class of processes demands a far reaching revision of the the neodarwinian paradigm of evolution and implies a vindication of the cuvierian perspective on earth9s history the validity of these claims however is questionable at all levels from the interpretation of particular events to the concept of mass extinctions as a special class of phenomena to their proposed causation by impacts and to their alleged historical pedigree and implications for current evolutionary theory mass extinctions may actually be clusters of more or less accidentally aggregated in time but causally distinct extinction episodes

a unique geochemical record at the permian triassic boundary a 330 metre core drilled through the marine permian triassic boundary in the carnic alps of austria allows closely correlated studies of geochemistry petrography and palaeontology across the boundary the isotope shifts and metal concentrations are extended multiple and complex and do not resemble those seen at the cretaceous tertiary boundary both the carbon isotope shifts and the chemical events including an indium anomaly may have causes related to a major regression of the sea

geochemical analysis through the transitional zone of conodont faunal turnover in the ordovician silurian boundary interval anticosti island quebec geochemical analysis of the ordovocian silurian boundary interval on anticosti island quebec has revealed low ir abundances for the lower becscie formation 0 005 0 039 ppb comparable to those reported for the upper ellis bay formation 0 005 0 058 ppb when normalized as caco3 free values and ratioed to al a small amount of excess ir over background ir levels was observed in and on the upper surface of an oncolite platform bed in member 7 laframboise of the ellis bay formation except for this bed the abundances of ir and other elements are relatively uniform throughout the section but small enrichments are apparent in a few clay layers as in other areas previously studied south china northwestern canada scotland the ir maxima on anticosti island occur near the base of the graptolite persculptus zone and are attributable to condensation and slower sedimentation rates associated with the hardgrounds found within and on the upper surface of the oncolite platform bed although we cannot p

the extraterrestrial component in marine sediments description and interpretation the influx of extraterrestrial matter to the earth is dominated by two size fractions sub millimeter interplanetary dust and impacting asteroids and comets over geologic time the major contribution of extraterrestrial matter is from the largest impactors interplanetary dust is largely vaporized by atmospheric entry but some surviving material is found in sediments as cosmic spherules impacting asteroids and comets can produce fallout layers either regionally or globally which may contain a significant fraction of meteoritic material as well as shock metamorphosed and shock melted terrestrial material iridium is a sensitive tracer of extraterrestrial matter although high concentrations of ir in marine sediments probably have an extraterrestrial origin they do not necessarily indicate the presence of a major impact event of the thousands of impact horizons which must be resolvable in the sedimentary record to date only seven probable impact horizons have been identified in the entire phanerozoic the marine geochemistry and occurrence of ir is still poorly understood but the thousands of ir analyses performed in the last several years have demonstrated that the global occurrence high ir 10 ng g concentrations in cretaceous tertiary boundary sediments is a truly anomalous phenomenon

refining 2 km of ordovician chronostratigraphy beneath anticosti island utilizing integrated chemostratigraphy new high resolution chemostratigraphy in combination with updated biostratigraphy refines the chronostratigraphic resolution of the nearly 2 km thick ordovician section below anticosti island a total of 1414 horizons spaced at 1 0 1 5 m intervals were sampled from the new associated consolidated paper nacp drill core and analyzed for major and trace elemental composition by portable x ray fluorescence analyzer pxrf select micrite and calcareous shale powders were then analyzed for 13ccarb number of samples n 364 and 87sr 86sr n 25 our results indicate a floian to early darriwilian f3 dw1 age for the romaine formation a middle darriwilian to sandbian dw2 sa1 age for the mingan formation a lower to mid katian ka1 ka2 age for the macasty formation an upper katian ka3 age for the lower vaureal formation an uppermost katian ka4 age for the upper vaureal and a hirnantian h1 age for the ellis bay formation this integrated chemostratigraphic and biostratigraphic synthesis es

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events

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 stratigraphic and geochemical imprints of late ordovician glaciation on far field neritic carbonates anticosti island eastern canada abstract the storm dominated open marine neritic carbonate facies continuously exposed in coastal cliffs and wide rocky tidal flat exposures at the western end of anticosti island form a 315 m thick expanded section spanning the ordovician silurian boundary these carbonate facies display deepening shallowing trends of different orders resulting in a hierarchical stacking of sedimentary cycles the carbonate facies within these multi order cycles represent sediments that were deposited from shoreline to deeper outer carbonate ramp 80 m in water depth and exhibit sea level changes of several tens of metres in magnitude field petrographic and geochemical evidence support a well preserved primary 18o signal in close equilibrium with the original paleoenvironmental conditions the 18o intracycle variations up to 1 0 in magnitude reveal repeated decreasing and increasing values during the transgressive and regressive portions of decametre tr cycles respectively although temperature salinity and ice volume influenced the recorded primary 18o signal orbital scale glacioeustasy ultimately controlled facies and cycle development and 18o signal during the katian immediately before the latest ordovician hirnantian glaciation the 3 5 myr long 18o signal recorded by our entire succession supports a protracted period of cooling leading into the main phase of latest ordovician hirnantian glaciation spectral analysis of the 18o record also reveals decametre variations which are associated with decametre scale transgressive regressive tr cycles possibly reflecting eccentricity orbital signals our study shows the importance of a thick expanded stratigraphic succession sampled at the highest resolution possible to decipher potential linkages between facies changes multi order stratigraphic cycles 18o signal and the earth s orbital parameters