global iridium anomaly mass extinction and redox change at the devonian carboniferous boundary iridium abundance anomalies have been found on a global scale in the devonian carboniferous d c boundary interval which records one of the largest phanerozoic mass extinction events an event that devastated many groups of living organisms such as plants ammonoids trilobites conodonts fish foraminiferans brachiopods and ostracodes at or very close to the d c boundary there exists a geographically widespread black shale interval and ir abundances reach anomalous maxima of 0 148 ppb montagne noire france 0 138 ppb alberta canada 0 140 ppb carnic alps austria 0 156 ppb guangxi china 0 258 ppb guizhou china and 0 250 ppb oklahoma the discovery of global d c ir anomalies argues for an impact extinction model however nonchondritic ratios of ir to other important elements and a lack of physical evidence shocked quartz microtektites do not support such a scenario the fact that all ir abundance maxima are at sharp redox boundaries in these sections leads us to conclude that the ir anomalies likely resulted from a sudden change in paleo redox conditions during deposition and or early diagenesis 36 refs 2 figs 1 tab

the stratigraphic distribution of ni rich spinels in cretaceous tertiary boundary rocks at el kef tunisia caravaca spain and hole 761c leg 122 abstract ni rich spinels have been found throughhout the world at the cretaceous tertiary boundary these minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris suggesting that the cretaceous tertiary boundary spinels result from an extraterrestrial event their stratigraphic distribution which has been observed at the cretaceous tertiary boundary sites of el kef tunisia caravaca spain and hole 761c odp leg 122 indian ocean are markedly different from site to site and are all much less extended than the ir anomalies regional differences in ni rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions the wider stratigraphic dispersion of ir is attributed to postdepositional chemical processes at el kef the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate the 1 2 mm thick ni rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years this upper limit is consistent only with the collision of an asteroid or a comet at the close of the cretaceous

siderophile interelement variations in the cretaceous tertiary boundary sediments from caravaca spain a detailed profile of the siderophiles fe co ni pd re os ir pt and au from the cretaceous tertiary boundary at caravaca spain indicates that the major fractionation mechanisms are local redistribution reworking and diagenetic mobilization large fractions of the fe co and re and perhaps a small amount of pd and au appear to be terrestrially derived but nearly all of the ni os ir and pt must be derived from an extraterrestrial source only the basal layers at caravaca and stevns klint denmark are representative of the initial fallout and integrated ir fluences across entire sections can be misleading these suggest a concentration oft 120mg g projectile in the fallout and a global fluence of chondritic matter oft 140mg cm 2

multiple impacts across the cretaceous tertiary boundary abstract the stratigraphy and age of altered impact glass microtektites microkrystites ejecta layers from the chicxulub crater are documented in late maastrichtian and early danian sediments in mexico guatemala belize and haiti in northeastern mexico two to four ejecta layers are present in zone cf1 which spans the last 300 ky of the maastrichtian the oldest ejecta layer is dated at 65 27 0 03 ma based on sediment accumulation rates and extrapolated magnetostratigraphy all younger ejecta layers from the maastrichtian and early danian parvularugoglobigerina eugubina zone pla l may represent repeated episodes of reworking of the oldest layer at times of sea level changes and tectonic activity the k t boundary impact event 65 0 ma is not well represented in this area due to widespread erosion an early danian pla l ir anomaly is present in five localities bochil actela coxquihui trinitaria and haiti and is tentatively identified as a third impact event at about 64 9 ma a multiimpact scenario is most consistent with the impact ejecta evidence the first impact is associated with major deccan volcanism and likely contributed to the rapid global warming of 3 4 c in intermediate waters between 65 4 and 65 2 ma decrease in primary productivity and onset of terminal decline in planktic foraminiferal populations the k t boundary impact marks a major drop in primary productivity and the extinction of all tropical and subtropical species the early danian impact may have contributed to the delayed recovery in productivity and evolutionary diversity

experimental tests for particle size dependent bioturbation in the deep ocean the potential for particle size dependent bioturbation rates was experimentally tested at 1 240 m in the santa catalina basin eastern pacific spherical glass bead tracers in five size classes 816 17 31 32 62 63 125 and 126 420 m were spread over the sediment surface and tube cored 997 d later downcore concentrations of glass beads were enumerated in each of the five size categories and page s l test was used to test the null hypothesis of equal vertical penetration of all size classes of tracer in all cores the null hypothesis was rejected finer tracers penetrated deeper into the sediment in two of the three cores vertical biodiffusivities were computed from concentration profiles of downcore tracers these also showed size dependence with biodiffusivities ranging from 1 cm2 yr for the 8 1 6 pm fraction to 0 1 cm2 yr for the 1 25 420 pm size class these data demonstrate that vertical bioturbation rates are particle size dependent in santa catalina basin the likely cause is preferential ingestion and downward transport of fine particles by deposit feeders

ru ir ratios at the cretaceous tertiary boundary implications for pge source and fractionation within the ejecta cloud abstract ruthenium and ir are the least mobile platinum group elements pges within the cretaceous tertiary k t boundary clay the ru ir ratio is therefore the most useful pge interelement parameter for distinguishing terrestrial and extraterrestrial contributions to the boundary clay the ru ir ratio of european marine k t sections 1 67 0 38 is statistically different from that of the north american continental sections 0 76 0 26 it is unlikely that this difference is due to secondary pge remobilization pge input to the boundary clay during multiple impacts or volcanic emissions the global difference in ru ir ratios in the boundary clay may therefore be primary the positive correlation between distance from the chicxulub impact structure yucatan mexico and ru ir ratio and the more than 1000 c difference in the condensation temperatures of these elements lead us to propose that fractionation of ru from ir during condensation from the ejecta cloud may have occurred resulting in the global difference in ru ir ratios

the global stratigraphy of the cretaceous tertiary boundary impact ejecta the chicxulub crater ejecta stratigraphy is reviewed in the context of the stratigraphy of underlying and overlying rock sequences the ejecta sequence is regionally grouped in a thick polymict and monomict breccia sequences inside the crater and within 300 km from the rim of the crater known from drill holes in and close to the breater and exposures near the border of yucatan and belize b gulf of mexico region 2500 m from the crater with up to 9 m thick complex tsunami wave influenced tektite bearing sequences in shallow marine 500 m deep environments and tektite bearing decimeter thick gravity flow deposits in deep water sites c an intermediate region between 2500 and 4000 km from the crater where centimeter thick tektite bearing layers occur and d a global distal region with a millimeter thin ejecta layer the distal ejecta layer is characterized by sub millimeter sized microkrystites often rich in ni rich spinels and altered clinopyroxene wherever present the ejecta layers mark exactly the sudden mass mortality horizon of the k t boundary what exactly caused the mass mortality is still uncertain but it appears the main event leading to the k t mass extinctions

acraman impact ejecta and host shales evidence for low temperature mobilization of iridium and other platinoids geochemical investigations of a widely dispersed late proterozoic impact ejecta horizon and its host shales bunyeroo formation australia have provided strong evidence for low temperature mobilization of the platinum group elements pge including ir pge anomalies are associated with both the ejecta horizon ir up to 2 0 ppb pt up to 270 ppb and the green shales that envelop it several pge anomalies 0 073 0 45 ppb ir 3 11 314 ppb pt also occur in thin green shale beds at other stratigraphic levels within a predominantly red shale sequence average red shale background of 0 019 ppb ir and 0 89 ppb pt all green shale horizons analyzed regardless of their stratigraphic position have relatively high levels of ir and other pge the nonextraterrestrial postdepositional origin of the pge at other stratigraphic levels away from the impact ejecta horizon is indicated by their occurrence in thin permeable green beds in a predominantly red shale sequence their association with enrichments in cu v zn ni and their nonchondritic pge interelement ratios the impact ejecta horizon has a more chondritic pge geochemistry consistent with a meteoritic origin a redox precipitation model similar to that invoked for redbed cu u v deposits is proposed to explain the pge anomalies in the green shales

positive eu anomaly development during diagenesis of the k t boundary ejecta layer in the agost section se spain implications for trace element remobilization k t boundary geochemical anomalies have been used previously to support the impact event at the end of the cretaceous however impact models and assessment of the extraterrestrial contribution to the boundary sediments should also consider the diagenetic alteration of the impact signatures mineralogical and geochemical studies centring on redox proxies reveal differences in trace element concentrations at agost and caravaca se spain two of the most complete k t boundary sections these differences probably derive from variations in the diagenetic evolution of the ejecta layer several redox proxies such as extensive pyrite formation high authigenic uranium concentration and positive eu anomalies indicate very strong reducing conditions at agost positive eu anomalies are extremely unusual in sediments and in this case are interpreted to indicate a highly reducing environment in such conditions certain trace elements such as ir may have been remobilized thereby masking the original signature of the impact

the cretaceous tertiary boundary at gubbio revisited vertical extent of the ir anomaly abstract the classical gubbio italy cretaceous tertiary boundary section has been resampled for both magnetostratigraphy and iridium paleomagnetic samples were taken over 7 m in the maastrichtian and 6 m in the paleocene previous results obtained a decade ago are confirmed the reversal sequence is well defined and the individual reversals are somewhat more precisely located a noticeable difference is the location of the position of the 29n 29r limit which may be lowered by 20 30 cm in the stratigraphic column this would imply that the ktb occurs near the middle of chron 29r iridium measurements were made on samples from both shales and surrounding limestone beds from 2 m below to 3 m above the boundary these measurements indicate that ir is associated with clay minerals concentrations in the two types of samples are indeed compatible when reduced to a carbonate free basis iridium concentrations stand above background over almost 3 m of section corresponding to half a million years based on magnetostratigraphy this is likely to indicate a protracted duration of the external or internal source of iridium on top of which the main short lived ktb anomaly proper stands

the chicxulub asteroid impact and mass extinction at the cretaceous paleogene boundary the cretaceous paleogene boundary similar to 65 5 million years ago marks one of the three largest mass extinctions in the past 500 million years the extinction event coincided with a large asteroid impact at chicxulub mexico and occurred within the time of deccan flood basalt volcanism in india here we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction notably a single ejecta rich deposit compositionally linked to the chicxulub impact is globally distributed at the cretaceous paleogene boundary the temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations for example darkness and cooling lead us to conclude that the chicxulub impact triggered the mass extinction

chemostratigraphic evidence of deccan volcanism from the marine osmium isotope record continental flood basalt cfb volcanism is hypothesized to have played a causative role in global climate change and mass extinctions uncertainties associated with radiometric dating preclude a clear chronological assessment of the environmental consequences of cfb volcanism our results document a 25 decline in the marine 187 os 188 os record that predates the cretaceous tertiary boundary ktb and coincides with late maastrichtian warming we argue that this decline provides a chemostratigraphic marker of deccan volcanism and thus constitutes compelling evidence that the main environmental consequence of deccan volcanism was a transient global warming event of 3 to 5 c that is fully resolved from the ktb mass extinction

special paper 356 two anomalies of platinum group elements above the cretaceous tertiary boundary at beloc haiti geochemical context and consequences for the impact scenario

two large meteorite impacts at the cretaceous paleogene boundary the end cretaceous mass extinction has been attributed by most to a single asteroid impact at chicxulub on the yucatan peninsula mexico the discovery of a second smaller crater with a similar age at boltysh in the ukraine has raised the possibility that a shower of asteroids or comets impacted earth close to the cretaceous paleogene k pg boundary here we present palynological and 13 c evidence from crater fi ll sediments in the boltysh impact crater our analyses demonstrate that a post impact fl ora formed on the ejecta layer was in turn devastated by the k pg event the sequence of flrecovery from the k pg event is directly comparable with that in middle north america we conclude that the boltysh crater predated chicxulub by 2 5 k y a time scale that constrains the likely origin of the bodies that formed the two known k pg craters

comparative analysis of teleseismic p coda and lg waves from underground nuclear explosions in eurasia this study has investigated the characteristics of p coda and lg waves recorded at the norsar and graefenberg arrays from presumed nuclear explosions in western russia lg waves were found to be more energetic relative to the preceding coda in the mid band frequency range 0 2 to 1 0 hz recorded on the broadband graefenberg instruments than in the high frequency band 0 6 to 3 0 hz at either array comparison of the log rms coda envelopes at the two arrays revealed that the early high frequency p coda at norsar is stronger relative to lg than at graefenberg also the amplitudes of p waves and early p coda waves within 55 sec after p recorded at the centers of the norsar subarrays are positively correlated indicating the effect of focusing defocusing on the p and coda waves however for subarrays which cluster spatially there is an inverse correlation which seems to be due to scattering attenuation suggesting that much of the early p coda is generated by local p wave scattering beneath the norsar array also the norsar coda envelopes flatten between about 219 and 352 sec after p whereas the graefenberg coda decays monitonically from the p wave out to the sn and lg arrival times the standard deviation across the norsar array of the log rms amplitudes of lg and s coda phases are higher by about 0 05 to 0 06 magnitude units than those of p coda phases these differences are probably related to differential site and propagation path effects on p s and lg waves in the p coda the flattening of the norsar coda envelopes appears to be a burst of energy not associated with any known regional phases the burst may be caused by forward scattering of lg to p waves along the path between the source and norsar based on a regional travel time curve for western russia the start and end time of the coda flattening is consistent with lg to p scattering in the distance range of 23 to 29 from norsar which brackets the ural mountains the lg propagation path from the source region to graefenberg does not appear to traverse the ural mountains which may explain why the coda flattening is not observed at graefenberg

post depositional mobility of platinum iridium and rhenium in marine sediments the discovery of high concentrations of iridium in cretaceous tertiary boundary sediments engendered the hypothesis1 that a meteorite collided with the earth 65 million years ago coincident with the mass extinction that occurred at that time iridium spikes of various magnitudes have subsequently been reported at more than 10 other extinction horizons2 11 it has been suggested on the other hand that geochemical processes might create or modify many of these spikes5 11 4 but a critical evaluation of these suggestions has been hindered by incomplete understanding of low temperature iridium geochemistry other platinum group elements ru rh pd re os pt au are often found to be associated with ir spikes and inter element ratios have been used to assess the cosmic or terrestrial nature of the enrichments5 7 9 28 29 but the geochemical influences on these relative abundances are also poorly constrained here we describe analyses of recent abyssal marine sediments which allow us to characterize the behaviour of pt re and ir during early diagenesis these elements are redistributed by changes in sedimentary redox conditions such changes can probably account for many of the small platinum group element spikes found in the geological record and may render ambiguous attempts to interpret inter element ratios

astronomical calibration of the paleocene time the first complete cyclic sedimentary successions for the early paleogene from drilling multiple holes have been retrieved during two odp expeditions leg 198 shatsky rise nw pacific ocean and leg 208 walvis ridge se atlantic ocean these new records allow us to construct a comprehensive astronomically calibrated stratigraphic framework with an unprecedented accuracy for both the atlantic and the pacific oceans covering the entire paleocene epoch based on the identification of the stable long eccentricity cycle 405 kyr high resolution x ray fluorescence xrf core scanner and non destructive core logging data from sites 1209 through 1211 leg 198 and sites 1262 1267 leg 208 are the basis for such a robust chronostratigraphy former investigated marine odp sites 1001 and 1051 and land based e g zumaia sections have been integrated as well the high fidelity chronology is the prerequisite for deciphering mechanisms in relation to prominent transient climatic events as well as completely new insights into greenhouse climate variability in the early paleogene we demonstrate that the paleocene epoch covers 24 long eccentricity cycles we also show that no definite absolute age datums for the k pg boundary or the paleocene eocene thermal maximum petm can be provided by now because of still existing uncertainties in orbital solutions and radiometric dating however we provide two options for tuning of the paleocene which are only offset by 405 kyr our orbitally calibrated integrated leg 208 magnetostratigraphy is used to revise the geomagnetic polarity time scale gpts for chron c29 to c25 we established a high resolution calcareous nannofossil biostratigraphy for the south atlantic which allows a much more detailed relative scaling of stages with biozones the re evaluation of the south atlantic spreading rate model features higher frequent oscillations in spreading rates for magnetochron c28r c27n and c26n

plutonium and210pb distributions in northeast atlantic sediments subsurface anomalies caused by non local mixing abstract the depth distributions of 210 pb and 239 240 pu measured in a suite of box cores collected from water depths of 4000 5000 m in the northeast atlantic ocean exhibit pronounced subsurface maxima caused by sediment reworking by benthic infauna small scale spatial heterogeneity in bioturbation rates is indicated by large differences in tracer profiles from duplicate cores separated only by a few centimeters 210 pb and 239 240 pu activity distributions from each subcore exhibit a high degree of correlation and most tracer profiles exhibit one or more subsurface maxima one dimensional biodiffusion analogue models do not adequately simulate the principal features of this data set however an inverse conveyer belt mixing model which simulates subsurface egestion or a functionally equivalent process of surficial material which is enriched both in organic debris and radioactive tracers can reproduce the subsurface tracer maxima single event and continuous subsurface egestion models have been formulated and solved for different feeding rates and background biodiffusive fields the single event model provides a better fit to the data and in particular ensures the observed high degree of correlation between the 210 pb and 239 240 pu activity profiles regardless of the different tracer input functions the most likely candidate responsible for subsurface tracer egestion is a large infaunal worm of the phylum sipunculida which dominates the biomass below a depth of 3 cm

40k 40ar dating of the main deccan large igneous province further evidence of ktb age and short duration most mass extinctions coincide in time with outpourings of continental flood basalts cfb some 20 years ago it was shown courtillot v besse j vandamme d montigny r jaeger j j cappetta h 1986 deccan flood basalts at the cretaceous tertiary boundary earth planet sci lett 80 361 374 courtillot v feraud g maluski h vandamme d moreau m g besse j 1988 deccan flood basalts and the cretaceous tertiary boundary nature 333 843 846 duncan r a pyle d g 1988 rapid eruption of the deccan flood basalts at the cretaceous tertiary boundary nature 333 841 843 that the age of the deccan traps was close to the cretaceous tertiary kt boundary and its duration under 1 myr we have undertaken a new geochronological study using the unconventional 40k 40ar cassignol gillot technique which is particularly well suited to the potassium poor deccan lavas the mean of 4 determinations from the topmost ambenali and mahabaleshwar formations is 64 5 0 6 ma they straddle the c29r c29n reversal boundary for which they provide a new constraint the mean age of 3 determinations from the oldest jawhar formation is 64 8 0 6 ma the difference in age between top and bottom of a 3500 m composite section probably comprising 80 of the total deccan volume is statistically insignificant with the overall mean age being 64 7 0 6 ma n 7 our results are consistent with the most recent 40ar 39ar determinations knight k b renne p r halkett a white n 2003 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps earth planet sci lett 208 85 99 knight k b renne p r baker j waight t white n 2005 reply to 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps discussion by a k baksi earth planet sci lett 239 374 382 confirming that there should be no systematic difference between the two methods when they are used in an optimal way an earlier smaller but significant pulse of volcanism between 68 and 67 ma extending over at least 500 km in latitude in the northern part of the deccan cfb has also been identified after 2 to 3 ma of quiescence the second major phase of volcanism occurred near 65 ma expanding over most of the area covered by the first pulse and another 500 km to the south consistent with drift of india by 300 to 450 km at not vert similar 150 mm yr during the quiescence period new paleontological data from the remote rajahmundry section keller g adatte t gardin s bartolini a bajpai s humler e in prep the cretaceous tertiary boundary in deccan traps of the krishna godavari basin of southeastern india epsl to be submitted suggest that this second pulse can itself be divided into two major pulses one starting in c29r and ending at the kt boundary the second starting in the upper part of c29r and ending within c29n

terminal cretaceous environmental events the geologic record of terminal cretaceous environmental events indicates that iridium and other associated elements were not deposited instantaneously but during a time interval spanning some 10 000 to 100 000 years the available geologic evidence favors a mantle rather than meteoritic origin for these elements these results are in accord with the scenario of a series of intense eruptive volcanic events occurring during a relatively short geologic time interval and not with the scenario of a single large asteroid impact event

fe oxide spherules infilling thalassinoides burrows at the cretaceous paleogene k p boundary evidence of a near contemporaneous macrobenthic colonization during the k p event a detailed ichnological analysis focused on thalassinoides structures recorded just below the cretaceous paleogene k p boundary layer of the agost section southeast spain aimed to evaluate macrobenthic colonization during the k p boundary event stereomicroscopy and field emission scanning electron microscopy analyses conducted on material filling in thalassinoides trace fossils reveal the presence of dark infill material that includes spherule like elements similar to those previously differentiated in the k p boundary layer energy dispersive x ray spectrometry indicates a ferruginous composition goethite spherules this represents the first record of spherules related to the k p boundary event within trace fossil infilling material the passive filling of this material into the thalassinoides bioturbation allows interpretation of the gravitational deposition just after thalassinoides trace maker colonization the absence of erosive sedimentary structures or other material deposited between the thalassinoides trace maker colonization and the spherule deposition is interpreted as evidence of rapid colonization during the k p boundary event nearly contemporary with the spherule layer deposition

biological pumping upwards of the coarse fraction of deep sea sediments abstract in a restricted part of an area intensively sampled by box coring approximately 50 samples from a 2 4 km rectangle a stiff muddy glacial gravel till is encountered at a depth of 30 40 cm x radiographs of the upper 15 to 20 cm of the cores reveal gravel in overlying foraminiferal mud only from the area where there is the deeper till layer the localization strongly suggests that the material in the top of the cores has its source in the deeper layer i postulate that burrowing organisms move gravel upward by pushing aside the particles that are too large to be eaten because the strength of the enclosing mud decreases upwards the gravel particles move preferentially upward the upward decrease in shear strength of he sediment is rapid at an interface occurring 4 cm below the surface on average thus hydrostatic pressures of 20 to 200 g cm2 exerted by vermiform burrowing animals act within an environment of asymmetrical sediment property permitting an asymmetrical response to the uniformly directed body pressure this asymmetrical response a pumping mechanism affecting material that does not pass through the gut is contrasted with the mixing of finer sediments that are eaten and excreted documented by many other workers dimensional arguments suggest the flux of material is related to the ratio of biological mixing rate to sedimentation rate the mechanism is also applicable to maintenance of mn nodules and their nuclei at the surface if macrofossils have behaved like gravel some s ell beds may have been generated and some stratigraphic anomalies caused by this means

bioturbational mixing depth and carbon flux at the seafloor radiocarbon dating series bulk sediment and organic carbon flux from various atlantic deep sea regions reveal that the thickness of the bioturbated zone increases by 2 cm if food supply increases by 1 gcm 2yr 1 r 0 8 bulk sediment accumulation rates do not influence the depth of bioturbational mixing under normal pelagic sedimentary conditions we believe that this relationship between nutrient supply and benthic mixing can be used for a quantitative and time variable unmixing procedure to improve high resolution stratigraphic correlations and paleoclimatic interpretations of deep sea records

platinum group elements pge and rhenium in marine sediments across the cretaceous tertiary boundary constraints on re pge transport in the marine environment the nature of re platinum group element pge pt pd ir os ru transport in the marine environment was investigated by means of marine sediments at and across the cretaceous tertiary boundary ktb at two hemipelagic sites in europe and two pelagic sites in the north and south pacific a traverse across the ktb in the south pacific pelagic clay core found elevated levels of re pt ir os and ru each of which is approximately symmetrically distributed over a distance of 1 8 m across the ktb the re pge abundance patterns are fractionated from chondritic relative abundances ru pt pd and re contents are slightly subchondritic relative to ir and os is depleted by 95 relative to chondritic ir proportions a similar depletion in os 90 was found in a sample of the pelagic ktb in the north pacific but it is enriched in ru pt pd and re relative to ir the two hemipelagic ktb clays have near chondritic abundance patterns the 1 8 m wide re pge peak in the pelagic south pacific section cannot be reconciled with the fallout of a single impactor indicating that postdepositional redistribution has occurred the elemental profiles appear to fit diffusion profiles although bioturbation could have also played a role if diffusion had occurred over 65 ma the effective diffusivities are 10 13 cm 2 s much smaller than that of soluble cations in pore waters 10 6 cm 2 s the coupling of re and the pges during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances if redistribution was caused by diffusion then the effective diffusivities are the same fractionation of os from ir during the ktb interval must therefore have occurred during aqueous transport in the marine environment distinctly subchondritic os ir ratios throughout the cenozoic in the south pacific core further suggest that fractionation of os from ir in the marine environment is a general process throughout geologic time because most of the inputs of os and ir into the ocean have os ir ratios 1 mass balance calculations show that os and re burial fluxes in pelagic sediments account for only a small fraction of the riverine os 10 and re 0 1 inputs into the oceans in contrast burial of ir in pelagic sediments is similar to the riverine ir input indicating that pelagic sediments are a much larger repository for ir than for os and re if all of the missing os and re is assumed to reside in anoxic sediments in oceanic margins the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes however putting all of the missing os and re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of os at pelagic ktb sites where at most 25 of the k t impactor s os could have passed through estuaries if os is preferentially sequestered in anoxic marine environments it follows that the os ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition there is thus a clear fractionation of os and re from ir in precipitation out of sea water in pelagic sections accordingly it is inferred here that re and os are removed from sea water in anoxic marine depositional regimes

the cretaceous tertiary boundary in the gosau basin austria the cretaceous tertiary boundary has been identified in the gosau beds elendgraben near salzburg austria the undisturbed 2 mm thick boundary clay in the palaeomagnetic g zone differs from the surrounding sediments in having significant colour no biogenic calcite and different contents of rare earth and siderophile elements carbon and magnetic minerals the clay also contains shocked quartz and plagioclase particles and indicates a dramatic change in sedimentation caused by a short lived event

the cretaceous tertiary transition the fossil sequences from cores across the cretaceous tertiary boundary show a range of transition times and transition time intervals depending on the fossil indicators and the location of the site these variations together with the pattern of iridium distribution with depth at some sites differences in total amounts of iridium variations in noble metal abundances normalized to extraterrestrial concentrations the depositional effects that might be expected in a reducing environment and the clay mineralogy of the boundary layer clays put into question the interpretation that an extraterrestrial event was the cause of the faunal changes and the iridium anomaly in the vicinity of the cretaceous tertiary transition it seems more likely that an explanation for the changes during the transition will come from continued examination of the great variety of terrestrial events that took place at that time including extensive volcanism major regression of the sea from the land geochemical changes and paleoclimatic and paleoceanographic changes

distribution of noble metals across the cretaceous tertiary boundary at gubbio italy iridium variation as a constraint on the duration and nature of cretaceous tertiary boundary events iridium pd pt and au were determined in sections from the bottaccione gorge and contessa valley gubbio italy by radiochemical neutron activation shales and limestones were sampled from 2 85 m above to 219 m below the cretaceous tertiary k t boundary metal enrichment was evaluated by comparing the boundary shale region with the lower part of the section background iridium is concentrated by 63 times in the boundary shales in comparison with the background whereas other metals are enriched by no more than 2 2 times the enrichment of ir is not confined to the boundary shales but extends approximately 2 m above and below this horizon within this ir rich region there are four distinct ir maxima in addition to the major ir enrichment in the k t boundary shales iridium maxima are stratigraphically coincident with maxima in abundances of shocked minerals characteristic of explosive volcanism limestones are much lower in noble metals than shales and their ir contents in the k t boundary region are largely accounted for by their minor clay mineral contents the time represented by the 4 m of ir rich section is at least 3 x 10 5 yr if published sedimentation rates are used to sustain an increased ir flux over this period and to account for the ir distribution near the k t boundary intense volcanic activity is a preferred alternative to impact of extraterrestrial material

what controls the mixed layer depth in deep sea sediments the importance of poc flux the depth of biogenic particle mixing i e the mixed layer depth l is fundamental in models of organic matter recycling and paleoreconstructions for deep sea sediments factors postulated to control l in the oxygenated deep sea include particulate organic carbon poc flux oxygen penetration into the sediment and a balance between the downward mixing and decay of labile poc we explore the dependence of l on biogeochemical characteristics by compiling from 36 sites in three oceans an internally consistent set of deep sea estimates of l poc flux biogenic mixing intensity db and poc reactivity we use excess 210 pb as a tracer for l and db to avoid the confounding effects of tracer dependent mixing we find that l estimated from the penetration depth of excess 210 pb varies systematically with poc flux with an asymptotic function explaining 88 of the variance in l stepwise multiple regression suggests that the penetration depth of excess 210 pb and estimated l is much more likely to be controlled by poc flux than by 1 the sediment inventory of excess 210 pb or 2 biogenic mixing intensity db in addition l is negatively related to oxygen penetration into the sediment r 52 0 629 and not significantly related to predictions of l from a recent mixing poc decay model we conclude that in the food poor deep sea poc flux substantially controls the size and activities of the sediment mixing benthos and in turn the thickness of the biogenic mixed layer thus in contrast to previous suggestions average mixed layer depth is not environmentally invariant but rather responds predictably to ecologically important parameters such as poc flux

metal precipitation in the cretaceous tertiary boundary clay at stevns klint denmark abstract alvarez et al 1980 explained anomalous ir concentrations in the cretaceous tertiary k t boundary informal type sediment the fish clay in denmark in terms of a global fall out of extraterrestrial matter however geochemical and mineralogical analyses of the various sublayers in the clay reveal that not only ir but also elements such as ni co zn mo as se and sb are substantially enriched in the clay the enrichments may be due to precipitation of elements as sulfides at a redoxcline the facts supporting this view are 1 1 the over all chalcophile element abundance pattern in the clay 2 2 the occurrence of a redoxcline deduced from the distribution of organic matter and oxidized iron in the clay 3 3 the large amounts of small pyrite spheroids present at this redoxcline 4 4 the high concentration of trace elements in these spheroids also ni co and ir elements that have previously been presumed to have a meteoritic origin are substantially enriched in the spheroids a model is suggested in which metal bearing pore solutions lose their metals by precipitation upon passing the boundary between the oxic cretaceous chalk and the overlying anoxic and hydrogen sulnde rich fish clay the great similarity in geochemistry and lithological appearance between several strongly ir enriched clays suggest that the model may also be applied to other k t boundary clays

determining chondritic impactor size from the marine osmium isotope record decreases in the seawater 187os 188os ratio caused by the impact of a chondritic meteorite are indicative of projectile size if the soluble fraction of osmium carried by the impacting body is known resulting diameter estimates of the late eocene and cretaceous paleogene projectiles are within 50 of independent estimates derived from iridium data assuming total vaporization and dissolution of osmium in seawater the variations of 187os 188os and os ir across the late eocene impact event horizon support the main assumptions required to estimate the projectile diameter chondritic impacts as small as 2 kilometers in diameter should produce observable excursions in the marine osmium isotope record suggesting that previously unrecognized impact events can be identified by this method

calcareous nannofossils at the k t boundary el kef no evidence for stepwise gradual or sequential extinctions a detailed quantitative study of well preserved nannofossil assemblages in closely spaced samples across the cretaceous tertiary k t boundary at el kef tunisia reveals no extinctions of nannoplankton in the uppermost 4 m of maastrichtian marls as has been reported for planktonic foraminifers from the same samples nannofossil assemblages below the boundary show no trends to indicate that ecologic stresses such as temperature change foreshadowed the k t event in the tethyan region on the other hand the disappearance of several species of the more depth sensitive planktonic foraminifers below the boundary at el kef is probably the result of sea level regression or sampling effects rather than preboundary climate change or other environmental stresses cretaceous nannofossil specimens present above the k t boundary at el kef are interpreted to have been reworked rather than having survived the k t event this interpretation is suggested by abundance patterns of common cretaceous species and a correlation with short term sea level lowstands

k t transition in deccan traps of central india marks major marine seaway across india deccan intertrappean sediments in central india are generally considered as terrestrial deposits of maastrichtian age but the cretaceous tertiary k t position is still unknown here we report the discovery of the k t transition a marine incursion and environmental changes preserved within the intertrappean sediments at jhilmili chhindwara district madhya pradesh integrative biostratigraphic sedimentologic mineralogic and chemostratigraphic analyses reveal the basal danian in the intertrappean sediments between lower and upper trap basalts that regionally correspond to c29r and the c29r c29n transition respectively intertrappean deposition occurred in predominantly terrestrial semi humid to arid environments but a short aquatic interval of fresh water ponds and lakes followed by shallow coastal marine conditions with brackish marine ostracods and early danian zone p1a planktic foraminifera mark this interval very close to the k t boundary this marine incursion marks the existence of a nearby seaway probably extending inland from the west through the narmada and tapti rift valleys the jhilmili results thus identify the k t boundary near the end of the main phase of deccan eruptions and indicate that a major seaway extended at least 800 km across india

mechanisms and models of iridium anomaly shape across the cretaceous paleogene boundary article i nfo the interpretation of the cretaceous paleogene k pg iridium anomaly and other impact ejecta as the result of a single large asteroid impact has been the subject of much debate in part due to the distribution of impact markers beyond the narrow confines of the k pg boundary sedimentary layer here we revisit the hypothesized processes leading to the shape of k pg iridium profiles including geochemical remobilization and or diffusion prolonged deposition volcanism multiple impacts and sediment mixing using evidence from the literature and modeling of one north pacific site we find that sediment mixing of a single impact event provides the most parsimonious mechanism for iridium profile shape in open ocean oxic sediments while the increase in background iridium bracketing the boundary likely has a volcanic origin in some past studies a sediment mixing mechanism for iridium profile shape was ruled out based on an overly simplified set of expectations for the effect of sediment mixing on markers of geologically instantaneous events thus we introduce and use a lagrangian sediment mixing model to illustrate the theoretical effects of mixing on records of rapid events the sediment mixing origin for iridium anomaly shape the correspondence in mixing extent between iridium and microfossils and the fit of sediment mixing models to an empirical iridium profile indicate that iridium may provide a better tracer of mixing than previously proposed k pg mixing tracers such as ni spinels

a high resolution marine 187os 188os record for the late maastrichtian distinguishing the chemical fingerprints of deccan volcanism and the kp impact event abstract a composite late maastrichtian 65 5 to 68 5 ma marine osmium os isotope record based on samples from the southern ocean odp site 690 the tropical pacific ocean dsdp site 577 the south atlantic dsdp site 525 and the paleo tethys ocean demonstrates that subaerially exposed pelagic carbonates can record seawater os isotope variations with a fidelity comparable to sediments recovered from the seafloor new results provide robust evidence of a 20 decline in seawater 187os 188os over a period of about 200 kyr early in magnetochron c29r well below the cretaceous paleogene boundary kpb confirming previously reported low resolution data from the south atlantic ocean new results also confirm a second more rapid decline in 187os 188os associated with the kpb that is accompanied by a significant increase in os concentrations complementary platinum pt and iridium ir concentration data indicate that the length scale of diagenetic remobilization of platinum group elements from the kpb is less than 1 m and does not obscure the pre kpb decline in 187os 188os increases in bulk sediment ir concentrations and decreases in bulk carbonate content that coincide with the os isotope shift suggest that carbonate burial flux may have been lower during the initial decline in 187os 188os we speculate that diminished carbonate burial rate may have been the result of ocean acidification caused by deccan volcanism

siderophile rich inclusions from the morokweng impact melt sheet south africa possible fragments of a chondritic meteorite an 870 m thick diameter 30 km melt sheet associated with the morokweng impact structure south africa contains cr rich silicate inclusions up to 30 mm in diameter as well as disseminated ni rich sulphides and oxides that commonly occur in mutual intergrowths although the silicate inclusions are highly altered the mineral chemistry and petrographic evidence along with broadly chondritic re os systematics and platinum group element pge signatures all support the view that the silicate inclusions represent relict fragments of an impactor with compositions similar to ordinary chondrites the ni rich sulphides and oxides are significantly enriched up to 35 chondrites in pge ni and cr although the bulk pge signature of the melt sheet is chondritic the sulphides and oxides display mm and sub mm scale fractionation of pge into different inclusion components a model is presented whereby pieces of the projectile react with the relatively more oxidising impact melt into which they fell resulting in oxidation and fractionation of the metal silicate fragments to form the inclusion assemblages this may be the first reported occurrence of a large melt sheet 300 km3 contaminated by swarms of discrete pebble sized fragments of asteroid material the high proportion of meteorite fragments in the morokweng melt sheet suggests either that the structure was formed by a relatively low velocity 18 km s impact event or that the angle of impact was optimum for retaining maximum amounts of meteoritic material within the crater

iridium and other platinum group elements as geochemical markers in sedimentary environments iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions however platinum group element pge anomalies may result from several different processes both syn and postdepositional acting separately or jointly the sources and processes of enrichment involved are extraterrestrial asteroid comet cosmic dust impact ejecta volcanogenic pge rich condensates precipitation from seawater low sedimentation rate anoxic conditions microbial concentration dissolution reprecipitation exhalative hydrothermal leaching transport and precipitation at a redox boundary geochemical characteristics of pge and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes 1 extraterrestrial chondritic ru ir and os ir ratios low 187os 186os ratio high ir compared to pt pd and au 2 volcanogenic high iridium and se as sb and in contents 3 exhalative hydrothermal high au pd and pt relatively low ir contents 4 precipitation from seawater high pt and low pd contents also high cd and ag contents under anoxic conditions and 5 reduction from intermediate and low temperature solutions high pge and cu or mo u ni contents