profiles of steady state suction stress in unsaturated soils application of the effective stress principle in unsaturated geotechnical engineering problems often requires explicit knowledge of the stress acting on the soil skeleton due to suction pore water pressure this stress is defined herein as the suction stress a theoretical formulation of suction stress profiles based on the soil water characteristics curve the soil permeability characteristic curve and previous shear strength experimental verification is developed the theory provides a general quantitative way to calculate vertical suction stress profiles in various unsaturated soils under steady flow rate in the form of infiltration or evaporation

precessional climate cyclicity in late cretaceous early tertiary marine sediments a high resolution chronometer of cretaceous tertiary boundary events we report well dated late cretaceous and early tertiary precessional climatic cycles recorded by rhythmic carbonate maxima and minima in south atlantic deep sea sites spectral analyses of digitized sediment color a suitable carbonate proxy show prominent regularities in the spacing marl carbonate beds magnetostratigraphic dating over a number of magnetic chrons constrains the duration of the cycles which can be detected over at least 20 myr of sedimentation at 7 coring locations their mean absolute period of23 5 4 4kyr agrees closely with the predicted late cretaceous precessional period of 20 8 kyr because they can be matched to a physical forcing mechanism with a known repeat time the cycles offer a new high resolution tool to measure rates of climate change before and after the cretaceous tertiary k t boundary from counts of carbonate cycles we derive the position of the k t boundary within c29r at 350 kyr after the base of the reversal the constancy of cycle thickness linearly related to sedimentation rate and amplitude up to the boundary clay does not give evidence for climate instability preceding the boundary orbital chronometry records a step function decrease in sediment accumulation rate at the cretaceous tertiary boundary that is consistent with a geologically instantaneous event

a sequence of events across the cretaceous tertiary boundary the lithological and biological sequence of events across the cretaceous tertiary k t as developed in thick and complete landbased sections and termed the standard k t event sequence is also found in many i sdp cores from all over the globe microtektite like spherules have been found in almost every core or outcrop with an iridium anomaly forming a worldwide strewnfield these microtektites are an excellent indicator of the k t boundary level the basal paleocene globigerina eugubma zone is nov established in all major ocean basins the difference in faunas between the pacific and tethys atlantic oceans may point to different recovery of the oldest paleocene planktonic foraminiferal assemblages in different ocean basins planktonic foraminifera and nannoplankton apparently show a different extinction recovery pattern at the k t boundary which suggests that the nannoplankton underwent final extinction later and recover later than planktonic foraminifera these patterns may be important for the evaluation of the type and range of environmental stress resulting from the hypothesized impact s at the k t boundary blocking of sunlight by the dust cloud still can account for the clear mass mortality at the k t boundary but seems less likely as explanation for the final nannoplankton extinction as photosynthetic nannoplankton appear to survive 1000 10 000 years into the tertiary

analysis of rainfall induced slope instability using a field of local factor of safety 1 slope stability analyses are mostly conducted by identifying or assuming a potential failure surface and assessing the factor of safety fs of that surface this approach of assigning a single fs to a potentially unstable slope provides little insight on where the failure initiates or the ultimate geometry and location of a landslide rupture surface we describe a method to quantify a scalar field of fs based on the concept of the coulomb stress and the shift in the state of stress toward failure that results from rainfall infiltration the fs at each point within a hillslope is called the local factor of safety lfs and is defined as the ratio of the coulomb stress at the current state of stress to the coulomb stress of the potential failure state under the mohr coulomb criterion comparative assessment with limit equilibrium and hybrid finite element limit equilibrium methods show that the proposed lfs is consistent with these approaches and yields additional insight into the geometry and location of the potential failure surface and how instability may initiate and evolve with changes in pore water conditions quantitative assessments applying the new lfs field method to slopes under infiltration conditions demonstrate that the lfs has the potential to overcome several major limitations in the classical fs methodologies such as the shape of the failure surface and the inherent underestimation of slope instability comparison with infinite slope methods including a recent extension to variably saturated conditions shows further enhancement in assessing shallow landslide occurrence using the lfs methodology although we use only a linear elastic solution for the state of stress with no post failure analysis that require more sophisticated elastoplastic or other theories the lfs provides a new means to quantify the potential instability zones in hillslopes under variably saturated conditions using stress field based methods

extended cretaceous tertiary boundary extinctions and delayed population change in planktonic foraminifera from brazos river texas high resolution planktonic foraminiferal analysis of three brazos river sections indicates a nearly continuous cretaceous tertiary k t boundary sedimentary record second only to the world s most complete record at el kef tunisia species extinctions occur over an extended period of time and with two major extinction episodes the first extinction episode with 46 of the species extinct occurs at and just below 10 15 cm a short hiatus at the base of a sandy shell hash and clay sand unit which was interpreted by bourgeois et al 1988 to represent a tsunami bed generated by the k t boundary bolide impact the top of this tsunami bed is about 17 20 cm below the k t boundary as defined by the first appearance of tertiary planktonic foraminifera the second extinction phase with 45 of the species extinct occurs 25 cm above the k t boundary zone p0 p1a boundary of the remaining seven surviving cretaceous species six gradually disappear during planktonic foraminiferal subzones p1a and basal p1b no species extinctions or major faunal assemblage changes are directly associated with the k t boundary iridium distribution is ambiguous with one peak in the upper part of the tsunami bed and a second peak at the micropaleontologically defined k t boundary relative abundances of dominant species are stable through the late maastrichtian and only minor abundance changes coincide with the first extinction episode or the k t boundary the first major faunal change in the dominant species group coincides with the second extinction episode and leads to decline and eventual extinction of this group in subzone p1a species disappearing at the two extinction episodes 46 and 45 constitute only a small percentage 8 and 5 of the individuals of the total planktonic foraminiferal population this suggests that weakened species with low numbers of individuals and sensitive to relatively minor environmental changes were primarily affected by these extinction episodes magnetostratigraphy indicates that the first extinction phase began about 310 000 years before the k t boundary and the second extinction phase occurred 50 000 years after the k t boundary this stepped pattern of species extinctions suggests a progressively stressed ecosystem in continental shelf settings which may be related to an observed sea level regression and global cooling the hypothesis of a global catastrophic mass extinction at the k t boundary caused by a large extraterrestrial impact is not supported by the brazos river planktonic foraminiferal data

the occurrence and formation of ladderback ripples in subtidal shallow marine sands zululand south africa abstract until recently ladderback ripples have been considered diagnostic of late stage emergence run off in intertidal environments observations have demonstrated that ladderback ripples may also occur in the subtidal environment new underwater observations off the zululand coast support reddering s assertion that ladderback ripples are not confined to tide dominated environments two types of ladderback ripple are described they form in shallow submarine bioclastic and terrigenous sediments on the landward margin of offshore coral reefs the main ripple set in each instance is oriented sub parallel to the shoreline and is produced by the dominant southeasterly swell while superimposed ripple sets in the troughs of the larger sets are formed by two mechanisms 1 the approach of low amplitude high frequency northeasterly to easterly swells which are not sufficiently strong to change the orientation of the dominant larger set and 2 longshore currents produced by oblique wind driven swells in small coastal embayments when conditions change the superimposed ripples are rapidly reworked and incorporated into the main ripple set they therefore have a low preservation potential both the occurrences documented here and those reported previously are in microtidal wave dominated settings

influence of the changing lunar orbit on the astronomical frequencies of pre quaternary insolation patterns the sensitivity of the frequencies of the earth s orbital elements involved in the astronomical theory of paleoclimates obliquity and climatic precession to the earth moon distance and consequently to the length of the day and to the dynamical ellipticity of the earth is investigated for the last billions of years but the new frequencies have been computed for the last 440 million years only a time interval over which data and a model of the earth were available moreover we consider only the impact of the changing precessional parameter assuming that the fundamental frequencies of the planetary motion are steady at the geological time scale under such hypotheses the shortening of the earth moon distance and of the length of the day back in time induces a shortening of the fundamental astronomical periods for obliquity and precession a shortening that is larger for longer periods

the analysis and interpretation of stratification cycles beds in carbonate successions often form regular patterns either there is a cyclic repetition of beds with different thicknesses or bundles of beds occur separated from each other by some distinct layer any such group will be called a stratification cycle cycles of this type are from 0 5 m to several meters in thickness and they seem to be present throughout the geological column the analysis of sedimentary cycles involves three stages 1 regional mapping is necessary to understand the facies of the sediment establishing the lateral persistence of bedding eliminates the possibility that bedding is caused by diagenetic processes 2 petrographic examination and in particular analyses of primary structures are used to reconstruct the sedimentation conditions during cycle formation such conditions are extremely important for judging the time content of individual beds 3 the quantitative analysis of vertical sections using power spectral methods can find superimposed cycles and enables one to judge their degree of regularity the latter approach is illustrated using lower carboniferous limestone sections from nw ireland as an example spectral analysis finds a stratification cycle of 1 to 3 m in thickness each cycle consists of between five to seven limestone beds upper jurassic limestone marl successions from southern germany and late cretaceous and early tertiary successions from italy show a very similar cyclic pattern the scale of the cycles and their regularity suggest that they are ultimately caused by variations in the earth s orbit possibly by climatic cycles as was postulated by milankovitch however the exact mechanism which links sedimentation with orbital variations is not known

hunting for paleoclimatic periodicities in a geologic time series with an uncertain time scale an 8 m section of a mid cretaceous albian deep sea sediment core from piobbico italy contains 2 cycles m compositional oscillations as postdepositional chemical migration in the core is weak the relative amount of carbonate is a primary indicator of cretaceous paleoclimate variability we report results of spectral analysis of two series caco3 wt measurements taken at roughly 2 cm intervals and a more densely sampled photodensitometer record of light dark variation the uncertainty in the age depth relation i e unknown fluctuations in sedimentation rate hampers a detailed assessment of the suggested eccentricity correlation we fit for long term time scale greater than 75 kyr fluctuations in sedimentation rate by tracking the local frequency modulation of the dominant harmonic in the data series assuming the true input signal to be monochromatic we estimate uncertainty in the observed local frequency by means of a statistical jackknife we fit a smoothed cubic spline to the set of discrete frequency modulation estimates specifying a 2 n misfit criterion where n is the number of frequency estimates from this analysis we infer fluctuations of a factor of two in the sedimentation rate which occur predominately on a time scale of 400 kyr we generate tuned series using several candidate age depth relations from the carbonate and densitometer series the tuned carbonate series returns greater correlation with the modern milankovitch periodicities the 2 cycles m energy resolves principally into two harmonic components whose frequency ratio corresponds to an observational accuracy of 1 2 to that of the 97 0 and 127 6 kyr eccentricity oscillations at frequencies lower than 2 cycles m we find harmonic oscillations whose phase correlates with the beating envelope of the 2 cycles m energy in a manner consistent with the modern milankovitch insolation we also observe a sinusoidal oscillation of lower amplitude with period scaled by those of the presumed eccentricity oscillations of 39 2 1 1 kyr which agrees with the period of the modern obliquity cycle after correction for the effects of tidal friction oscillations corresponding to milankovitch precessional frequencies are not established owing possibly to low signal level and unknown short term deposition fluctuations the tuned densitometer series recovers few new periodic features although its inferred age depth relation correlates well with that of the carbonate series we calibrate the depth variable downcore to the orbital cycles to estimate r 5 0 0 15 m m y as the effective sedimentation rate

oligocene paleoceanography of the south atlantic paleoclimatic implications of sediment accumulation rates and magnetic susceptibility measurements paleoceanographic changes during the oligocene in the south atlantic have been examined using paleomagnetic rock magnetic carbonate content and sediment accumulation rate sar data we show that magnetic susceptibility ms is largely controlled by carbonate content at deep sea drilling project site 522 which in turn is the primary control on sar at the site over 5000 ms measurements were averaged over magnetostratigraphic polarity intervals for use as indicators of carbonate content and sar proxies the study produced three primary results 1 a spectral analysis of the ms data from one upper oligocene core reveals two dominant periodicities 44 7 10 4 kyr and 26 4 3 6 kyr which approximate milankovitch obliquity 41 kyr and precession 23 kyr periods 2 four pulses of increased sar lasting 1 to 2 m y punctuate the section implying longer term climatic changes these events are also seen in the contessa quarry section of the italian tethyan oligocene and 3 two of the sar maxima correspond to times of global cooling and possible ice volume increases at 31 and 36 ma

hydrological responses and stability analysis of shallow slopes with cohesionless soil subjected to continuous rainfall understanding the hydrological and physical responses of shallow slopes subject to rainfall events is vital for the efficiency of a warning system setup in this research a series of experiments were undertaken to evaluate the hydrological responses of shallow slopes of varying steepness and when subjected to varying intensities periods and inter storm periods of rainfall an analysis of infinite slopes was also undertaken to develop a fundamental understanding of rainfall induced shallow slope failure characteristics the hydrological and physical responses were characterized in the infiltration and saturation phases during the infiltration phase the maximum magnitude of water content was found behind the wetting front termed as the water content behind the wetting front wb for a certain soil type the magnitude of wb was found to be dependent on the magnitude of rainfall intensity regardless of the slope gradient and initial water content based on the relative depth of the failure plane th

laboratory experiments on local scour around cylinder for clay and clay sand mixed beds abstract new data on scouring around cylinders embedded in clay sand mixed beds are reported and the mechanism of the scouring process is described the effects of clay content water content bed shear strength and pier froude number on maximum equilibrium scour depth equilibrium scour hole geometry scouring process and time variation of scour were investigated existing literature on scouring around cylinders in cohesive sediment beds suggested that the maximum equilibrium scour depth in clay beds was similar to that of sand while increase in the clay fraction for clay sand mixed beds reduced maximum equilibrium scour depth the present study attempted to fill this gap between observations made by different investigators in cylindrical pier scour in clay sand mixed sediment beds further equations for estimation of non dimensional maximum scour depth for cylinders embedded in clay sand mixtures were proposed as functions of pier froude number clay content water content and bed shear strength while the clay sand mixtures had clay content in the range of 20 100 and water content in the range of 20 45 92

milankovitch climatic origin of mid cretaceous black shale rhythms in central italy the earth s orbital variations reflected in pleistocene ice volume were also recorded in non glacial times carbonate production in pelagic mid cretaceous sediments quantified by calcium carbonate and optical densitometry time series reflects the orbital eccentricity and precessional cycles minimal eccentricity brought deep sea anoxia most of the sedimentary variability of this 100 myr old sequence lies within the milankovitch orbital frequency band

model for the prediction of shear strength with respect to soil suction experimental studies on unsaturated soils are generally costly time consuming and difficult to conduct shear strength data from the research literature suggests that there is a nonlinear increas

a numerical framework for infinite slope stability analysis under transient unsaturated seepage conditions abstract this paper presents a simple numerical framework for infinite slope stability analysis under transient unsaturated seepage conditions the advantage of the proposed framework from the practical point of view is to predict the variability in stability of partially saturated slope along with rainfall data moreover the presented framework is adaptable to different types of soil water characteristic curve swcc models hydraulic boundary conditions and heterogeneity in soil properties a series of stability analyses for hypothetical hillslopes under various conditions was performed to scrutinize the potential failure mechanisms induced by rainfall the examined factors include the soil texture rainfall intensity heterogeneity in soil properties and hydraulic boundary conditions also four widely used swcc models were applied to assess the influence of this component the swcc model was demonstrated to strongly dominate the results of the infinite slope stability analysis under transient unsaturated seepage conditions for homogeneous hillslopes with a fixed water table when the rainfall intensity q equals the saturated hydraulic conductivity ks slope failure was expected to occur after a short time of rainfall with a relatively shallow slip depth in contrast for heterogeneous hillslopes or hillslopes with impermeable bedrock the failure could take place when q is less than ks and the potential failure surface was close to the discontinuity interface or at the bottom of the hillslope finally three case studies of landslides documented in literature were utilized to demonstrate the predictability of the proposed framework in practical applications

late cretaceous eocene nannofossil and magnetostratigraphic correlations near gubbio italy abstract using a modified sample preparation technique we have been able to establish a detailed lower campanian to upper eocene nannofossil stratigraphy in the bottaccione and contessa highway sections near gubbio appearance and extinction levels of virtually all the commonly used calcareous nannofossil zonal markers have been recognized and can now be closely correlated with the planktonic foraminifera zonation and the magnetic reversal stratigraphy previously established in these sections comparisons with the nannofossil calibrations of the oceanic magnetic anomaly sequence in deep sea drilling project dsdp sites suggest that magmetic subchrons c17n and c25n are missing in the bottaccione section the observed variability of the relative stratigraphic position of most plankton events is confirmed to less than one magnetic subchron absolute abundance paleobiogeographic restriction and differential preservation render some of the traditionally used biostratigraphic events less reliable than others

latitudinal variation in the planktic foraminifer contusotruncana contusa in the terminal cretaceous ocean abstract the morphological variability coiling properties size and shape of the planktic foraminifer contusotruncana contusa cushman in the terminal cretaceous ocean was examined at eight deep sea sites and two continental sections from low 16 to middle 42 paleolatitudes in both hemispheres the material used in this study includes samples from the south atlantic dsdp sites 356 527 and 525a north atlantic sites 384 and 548a indian and pacific oceans dsdp site 465a and odp sites 761c and 762c and tethyan ocean outcrop sections from el kef and caravaca on average 45 specimens from two samples per location were analysed from an interval corresponding approximately to the last 60 kyr of the cretaceous no differences in coiling direction dextral proportions were 90 in all samples percentage of kummerform specimens usually 50 and number of chambers in the last whorl 4 5 were observed between the sites both test size expressed as spiral outline area and test volume and total number of chambers increase significantly towards lower latitudes similarly test conicity examined by shape coordinate and eigenshape methods and angularity of the spiral outline show a rather continuous slight increase towards lower latitudes kummerform specimens of c contusa were slightly larger and more conical than normalforms and possessed substantially more chambers both totally and in the last whorl a principal components analysis of the sample means of five variables describing size and shape clearly distinguished high latitude sites 525a 527 548a 761c and 762c from low latitude sites 384 465a caravaca and el kef specimens from site 356 are transitional with respect to those two groups the results indicate 1 considerable morphological variation in c contusa during the terminal cretaceous comparable to that known in many recent planktic foraminiferal species and 2 a geographical distribution of this variation corresponding to previously suggested biogeographic schemes based on quantitative analysis of planktic foraminiferal assemblages despite the differences in sample means the overall morphology of c contusa overlaps among the sites studied supporting the classification of all c contusa morphotypes as a single species similarly no discrete morphologic groups could be distinguished within any of the samples

hydro mechanical properties of unsaturated residual soil from a flysch rock mass abstract rainfall is a major triggering factor of landslides in flysch deposits along europe physical and mechanical changes of flysch rock masses caused by the weathering process result in a complex soil profile with residual soil typically present at the slope surface the research encompasses residual soils from the rjecina river valley in croatia built in flysch deposits the valley is known for several deep seated and shallow historical landslides this manuscript presents the hydro mechanical properties of the soil existing in unsaturated zone of the slope that could play an important role in landslide activation on flysch slopes presented unsaturated soil property functions are essential for modelling of the transient rainfall infiltration process and how it affects the stability of flysch slopes in time for the first time shear strength and hydraulic conductivity determination were performed in saturated and unsaturated soil conditions using both the intact as well as remolded samples of the residual soil form flysch rock mass the results indicate that in order to correctly define hydraulic and mechanical properties of the in situ soil measurements have to be performed on intact samples also the presented results highlight the importance of hysteresis effects and hydraulic paths that fine grained residual soil has been subjected to in the past

geochemical evidence for suppression of pelagic marine productivity at the cretaceous tertiary boundary the normal biologically productive ocean is characterized by a gradient of the 13c 12c ratio from surface to deep waters here we present stable isotope data from planktonic and benthic micro fossils across the cretaceous tertiary boundary in the north pacific which reveal a rapid and complete breakdown in this biologically mediated gradient the fluxes of barium a proxy for organic carbon and caco3 also decrease significantly at the time of the major marine plankton extinctions the implied substantial reduction in oceanic primary productivity persisted for 0 5 myr before the carbon isotope gradient was gradually re established in addition the stable isotope and preservational data indicate that environmental change including cooling began at least 200 kyr before the cretaceous tertiary boundary and a peak warming of 3 c occurred 600 kyr after the boundary event

infinite slope stability under steady unsaturated seepage conditions 1 we present a generalized framework for the stability of infinite slopes under steady unsaturated seepage conditions the analytical framework allows the water table to be located at any depth below the ground surface and variation of soil suction and moisture content above the water table under steady infiltration conditions the framework also explicitly considers the effect of weathering and porosity increase near the ground surface on changes in the friction angle of the soil the factor of safety is conceptualized as a function of the depth within the vadose zone and can be reduced to the classical analytical solution for subaerial infinite slopes in the saturated zone slope stability analyses with hypothetical sandy and silty soils are conducted to illustrate the effectiveness of the framework these analyses indicate that for hillslopes of both sandy and silty soils failure can occur above the water table under steady infiltration conditions which is consistent with some field observations that cannot be predicted by the classical infinite slope theory a case study of shallow slope failures of sandy colluvium on steep coastal hillslopes near seattle washington is presented to examine the predictive utility of the proposed framework

carbonate time series of the quaternary and late miocene sediments in the pacific ocean a spectral comparison abstract the variance of the quaternary carbonate record of core rc11 209 from the equatorial pacific is almost twice as large as that for the upper miocene of dsdp site 158 from the same region spectral analysis of the carbonate time series shows that the miocene record is dominated by very low frequency components with a concentration of variance near the frequency corresponding to a period of about 400 kyr the quaternary record is also dominated by low frequency components but has in addition concentrations of variance spectral peaks at frequencies corresponding to periods near 100 and 40 kyr cross spectral analysis indicates that both records are coherent with the calculated record of eccentricity of the earth s orbit and that the quaternary record in addition shows coherence with the calculated record of axial tilt these variations in the earth s orbital parameters can then be used to tune the time scales of these two records based on the similarity in the amount of variance in the two records near the 400 kyr period and the observed relationship between carbonate fluctuations and variations in global ice volume as indicated by oxygen isotope measurements it is proposed that the dominant 400 kyr cycle has been associated directly or indirectly with long term fluctuations in the antarctic ice cap since its formation in mid miocene times the major difference in the quaternary carbonate spectrum lies in the marked increase in variance at frequencies associated with the 100 kyr period this increase is thought to be linked directly or indirectly with glacial to interglacial fluctuations of the northern hemisphere ice sheets

differential dissolution of upper cretaceous planktonic foraminifera from a temperate region of the south atlantic ocean differential dissolution susceptibility is well known in quaternary planktonic foraminifera but few quantitative dissolution studies have been carried out on pre quaternary species a dissolution ranking scheme is proposed for 19 of the most common uppermost cretaceous planktonic foraminifera from a temperate region of the south atlantic ocean the study is based on quantitative faunal variations in an uppermost maastrichtian segment about 190 kyr long from dsdp site 527 leg 74 from the walvis ridge a calcite dissolution index was formulated from degree of fragmentation of planktonic foraminiferal tests through this segment relative abundance of benthonic foraminifera which are generally known to be more dissolution resistant than planktonic foraminifera covary with degree of fragmentation supporting that degree of fragmentation is a measure of dissolution a prominent change from a state of moderate dissolution to a state of stronger dissolution takes place near the top of magnetic anomaly 30 66 74 ma some species decrease whereas others increase in relative abundance between these dissolution regimes and dissolution susceptibility was ranked from quantified differences between dissolution regimes the most susceptible species are in order from least resistant to most resistant heterohelix globulosa globigerinelloides multispina rugoglobigerina rugosa pseudoguembelina costulata globotruncanita stuartiformis and heterohelix carinata the following eight species are unaffected by dissolution in the sense that they show no significant difference between regimes heterohelix glabrans pseudotextularia browni p elegans racemiguembelina fructicosa planoglobulina brazoensis gublerina cuvillieri globotruncana arca and rosita contusa the most resistant species were ranked in the following order from least resistant to most resistant globigerinelloides subcarinata pseudoguembelina palpebra p polypleura globotruncanella havanensis and abathomphalus mayaroensis

effect of soil suction on slope stability at notch hill the side slopes of a railway embankment in central british columbia constructed with local lacustrine silt on relatively flat ground began to fail several years after construction shallow instability ultimately developed on both sides of the embankment over a distance of several kilometres initially the soil had a significant apparent cohesive strength with time the strength appeared to diminish owing to the dissipation of negative pore water pressures the remaining frictional strength was not sufficient to maintain stability since the slopes were constructed at angles close to the peak effective friction angle of the soil this case history together with the laboratory saturated and unsaturated strength test results and field suction measurements demonstrates the dramatic effect of negative pore water pressures on near surface slope stability key words soil suction slope stability nonsaturated soils shear resistance tensiometers

a new approach to an absolute timescale from measurements of orbital cycles and sedimentary microrhythms when the sedimentary microrhythms of two groups of jurassic zones forming part of the chronostratigraphical scale are analysed against recent radiometric age scales there is good agreement with present day cycles resulting from the obliquity of the ecliptic sedimentary microrhythms resulting from orbital forcing can be used to establish either the relative or the true length of chronostratigraphical zones and thus provide a new approach towards an absolute timescale such a scale will be powerful in the analysis of other cycles and perturbations in the geological record

late cretaceous and paroxysmal cretaceous tertiary extinctions the various geological signatures at cretaceous tertiary time including iridium and other associated elements microspherules and shock deformation features are compatible with the suggestion that the transition is marked by a period of intense volcanism the volatile emissions from this volcanism would lead to acid rain reduction in the alkalinity and ph of the surface ocean global atmospheric temperature changes and ozone layer depletion these environmental effects coupled with those related to the major sea level regression of the late cretaceous provide the framework for an explanation of the selective nature of the observed extinction record

vostok ice core climatic response to co2 and orbital forcing changes over the last climatic cycle vostok climate and co2 records suggest that co2 changes have had an important climatic role during the late pleistocene in amplifying the relatively weak orbital forcing the existence of the 100 kyr cycle and the synchronism between northern and southern hemisphere climates may have their origin in the large glacial interglacial co2 changes

end cretaceous mass extinction event argument for terrestrial causation the end cretaceous mass extinctions were not a geologically instantaneous event and were selective in character these features are incompatible with the original alvarez hypothesis of their being caused by a single asteroid impact that produced a world embracing dust cloud with devastating environmental consequences by analysis of physical and chemical evidence from the stratigraphic record it is shown that a modified extraterrestrial model in which stepwise extinctions resulted from encounter with a comet shower is less plausible than one intrinsic to the earth involving significant disturbance in the mantle

a high resolution benthic stable isotope record for the south atlantic implications for orbital scale changes in late paleocene early eocene climate and carbon cycling abstract the late paleocene and early eocene were characterized by warm greenhouse climates punctuated by a series of rapid warming and ocean acidification events known as hyperthermals thought to have been paced or triggered by orbital cycles while these hyperthermals such as the paleocene eocene thermal maximum petm have been studied in great detail the background low amplitude cycles seen in carbon and oxygen isotope records throughout the paleocene eocene have hitherto not been resolved here we present a 7 7 million year myr long high resolution orbitally tuned benthic foraminiferal stable isotope record spanning the late paleocene and early eocene interval 52 5 60 5 ma from ocean drilling program odp site 1262 south atlantic this high resolution 2 4 kyr record allows the changing character and phasing of orbitally modulated cycles to be studied in unprecedented detail as it reflects the long term trend in carbon cycle and climate over this interval the main pacemaker in the benthic oxygen isotope 18 o and carbon isotope 13 c records from odp site 1262 are the long 405 kyr and short 100 kyr eccentricity cycles and precession 21 kyr obliquity 41 kyr is almost absent throughout the section except for a few brief intervals where it has a relatively weak influence during the course of the early paleogene record and particularly in the latest paleocene eccentricity paced negative carbon isotope excursions 13 c cies and coeval negative oxygen isotope 18 o excursions correspond to low carbonate caco 3 and coarse fraction cf values due to increased carbonate dissolution suggesting shoaling of the lysocline and accompanied changes in the global exogenic carbon cycle these negative cies and 18 o events coincide with maxima in eccentricity with changes in 18 o leading changes in 13 c by 6 5 kyr in the 405 kyr band and by 3 1 kyr in the higher frequency 100 kyr band on average however these phase lags are not constant with the lag in the 405 kyr band extending from 4 5 kyr to 21 2 kyr from the late paleocene to the early eocene suggesting a progressively weaker coupling of climate and the carbon cycle with time the higher amplitude 405 kyr cycles in the latest paleocene are associated with changes in bottom water temperature of 2 4 c while the most prominent 100 kyr paced cycles can be accompanied by changes of up to 1 5 c comparison of the 1262 record with a lower resolution but orbitally tuned benthic record for site 1209 in the pacific allows for verification of key features of the benthic isotope records which are global in scale including a key warming step at 57 7 ma

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr