asymmetric impact of the north and south pacific on the equator in a coupled climate model 1 relative contributions of the north and south pacific to the equatorial ocean are estimated in a coupled climate model idealized experiments show that the south pacific contributes 30 50 more than the north pacific to the equatorial ocean temperature change because the warming in the north pacific can induce an additional cold rossby wave at around 20 n that tends to block the warm effect from the central north pacific while the south pacific influence can be easily convoyed to the equator through the interior pathway

oceanic plateau formation a cause of mass extinction and black shale deposition around the cenomanian turonian boundary the cenomanian turonian boundary 90 4 ma represents a major period of worldwide environmental disturbance the physical manifestations of this are elevated atmospheric and oceanic temperatures a significant sea level transgression and a period of widespread anoxia leading to the formation of oceanic black shales and the extinction of 26 of all genera elevated 13 c values and enrichment of trace elements in cenomanian turonian boundary sediments combined with a reduction in 87 sr 86 sr also imply a severe environmental perturbation at this time oceanic crustal production rates reached their highest level of the last 100 million years this was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere and the development of vast areas up to 1 10 6 km 2 of thickened oceanic crust in the pacific and indian oceans the anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c 90 ma these eruptions would also have resulted in the emission of large quantities of co 2 into the atmosphere leading to global warming additionally the emission of so 2 h 2 s co 2 and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate and further release of co 2 this run away greenhouse effect was probably put into reverse by the decline of the anomalous volcanic activity and by increased co 2 driven productivity in oceanic surface waters leading to increased organic carbon burial black shale deposition anoxia and mass extinction in the ocean basins

geochronology and calibration of global milankovitch cyclicity at the cenomanian turonian boundary new 40 ar 39 ar geochronology and global cyclostratigraphic calibration provide high resolution insights into the timing of geochemical fluctuations across oceanic anoxic event ii oae ii five new 40 ar 39 ar data from a single section in western canada suggest an age of 96 4 1 ma for the cenomanian turonian c t boundary this is 3 m y older than previously dated time series analysis e g wavelet transform reveals that oae ii was synchronously deposited worldwide over a period of 320 k y the preservation and stationarity of milankovitch cyclicity in sections across the atlantic indicates that no major fluctuations in sedimentation rate occur across the oae ii except for a minor short term decrease in the rate just before the c t boundary a 13 c org increase of 2 took place at the beginning of oae ii over a period of 110 k y and likely represents a period of worldwide excessive organic carbon burial during transgression leading to a 12 c depletion of seawater and sedimentation this interval was followed by a three step decrease to pre oae ii 13 c values

high resolution geochemical and biotic records of the tethyan bonarelli level oae2 latest cenomanian from the calabianca guidaloca composite section northwestern sicily italy abstract high resolution micropalaeontological and chemostratigraphic records for the upper cenomanian portion of the calabianca guidaloca composite section nw sicily provide new insight into the palaeoclimatic and palaeoceanographic evolution of the tethys ocean the bonarelli level equivalent was identified on the basis of lithology and well constrained by calcareous plankton biostratigraphy and radiolarian assemblages as well as by the 13 c curve showing a marked positive excursion up to 4 7 the bonarelli level equivalent deposition is characterized by highly eutrophic conditions as testified by radiolarian proliferation black shale samples from the calabianca guidaloca composite section contain very high toc up to 26 and moderate to high amounts of caco 3 on average 20 among the planktonic foraminifera hedbergellids and globigerinelloids tolerated the environmental stress induced by the oceanic anoxic event 2 oae2 the occurrence of some benthic foraminifera testifies to dysoxic rather than completely anoxic conditions at the sea floor during the deposition of some portions of the bonarelli level equivalent based on micropalaeontological results and geochemical proxies 13 c d rb ti v ni ba si cr the bonarelli level equivalent is interpreted as a high productivity event driven by increasingly warm and humid climatic conditions promoting an accelerated hydrological cycle we propose that periodically increased riverine influxes triggered by enhanced humidity resulted in a sluggish circulation mode and consequent anoxic euxinic conditions favouring the preservation of organic matter at the sea floor spectral analyses performed on selected geochemical signals reveal within the main high productivity event a strong orbital climatic forcing represented by a long term eccentricity steered humidity cyclicity that amplifies a short term precession controlled productivity fluctuation

source rock distribution model of the periadriatic region the periadriatic area is a mosaic of geological provinces comprised of spatially and temporally similar tectonic sedimentary cycles tectonic evolution progressed from a triassic early jurassic liassic continental rifting stage on the northern edge of the african craton through an early jurassic middle liassic late cretaceous eocene oceanic rifting stage and passive margin formation to a final continental collision and active margin deformation stage in the late cretaceous eocene to holocene this evolution occurred in a transtensional and transpressional framework resulting from the oblique separation and convergence of the african and european plates extensive shallow water carbonate platform deposits covered large parts of the periadriatic region in the late triassic platform breakup and development of a platform to basin carbonate shelf morphology began in the late triassic and extended through the cretaceous on the basis of this paleogeographic evolution the regional geology of the periadriatic region can be expressed in terms of three main upper triassic paleogene sedimentary sequences the platform sequence sequence a the platform to basin sequence sequence b and the basin sequence sequence c these sequences developed during the initial rifting and subsequent passive margin formation tectonic stages three distinct episodes of source bed deposition can be identified middle upper triassic to lower jurassic liassic organic rich oil prone and mature source beds generally are structurally controlled and are closely associated with transtensional intraplatform basins formed in the early continental rifting stage the principal triassic source basins and most of the surface hydrocarbon indications and economically important oil fields of the periadriatic region are associated with sequence b areas the second episode of organic rich rocks deposition includes both shallow and deeper water facies of jurassic cretaceous age thin immature deep water epioceanic organic rich marls and shales are generally bathymetrically controlled these units are related to basin wide anoxic events in the tethys ocean and its submerged continental margin and are associated with sequence b areas middle upper cretaceous shallow water oil prone facies occur in isolated anoxic basins within carbonate platforms these source sequences are generally thicker and are associated with sequence a areas no major hydrocarbon accumulation can be directly attributed to the jurassic cretaceous epioceanic and intraplatform source rock sequences the third episode of source bed deposition characterizes the final active margin deformation stage and is represented by upper tertiary organic rich terrigenous units mostly gas prone these are essentially associated with turbiditic and flysch sequences of foredeep basins and have generated the greater part of the commercial biogenic gases of the periadriatic region

ventilation of the subtropical north pacific the shallow salinity minimum abstract the shallow salinity minimum of the subtropical north pacific is shown to be a feature of the ventilated wind driven circulation subduction of low salinity surface water in the northeastern subtropical gyre beneath higher salinity water to the south causes the salinity minimum variation of salinity along surface isopycnals causes variations in density and salinity at the minimum a model of ventilated flow is used to demonstrate how the shallow salinity minimum can arise the model is modified to account for nonzonal realistic winds it is also extended to examine the three dimensional structure of the western shadow zone the boundary between the subtropical and subpolar gyres is given by the zero of the zonal integral of ekman pumping the western shadow zone fills the subtropical gyre at the base of the ventilated layers and decreases in extent with decreasing density for parameters appropriate to the north pacific the eastern shadow zone is of very limited extent observations of salini

pacific thermocline bridge revisited abstract the coupling on decadal time scales of the mid latitude and tropical pacific via an oceanic bridge in the thermocline is investigated using ocean general circulation model hindcasts and a coupled ocean atmosphere model results indicate that in the tropics decadal anomalies of isopycnal depth are forced by ekman pumping and are largely independent of the arrival of subducted anomalies in the thermocline that originate in the mid latitudes of either hemisphere in the coupled model temperature anomalies on isopycnals show little coupling from the tropics to the northern hemisphere but are lag correlated between southern hemisphere mid and low latitudes however anomaly magnitudes on the equator are small these results suggest that the oceanic bridge to the northern hemisphere explains only a small part of the observed decadal variance in the equatorial pacific coupling to the southern mid latitudes via temperature anomalies on isopycnals remains an intriguing possibility

jurassic and cretaceous paleomagnetic data from the southern alps italy jurassic and cretaceous paleomagnetic data from 46 sites in the venetian alps trento plateau and se lombardian basin have been compiled nine of these site mean directions have been previously published the sites can be divided into four age groups senonian hauterivian barremian kimmeridgian tithonian and callovian oxfordian for three of the age groups positive regional fold tests indicate at the 99 confidence level that the characteristic magnetization components predate the deformation structural mapping in the venetian alps allowed us to assign sites to individual thrust sheets and we observe no significant relative rotation of thrust sheets in the nappe pile indeed we observe remarkable consistency in mean direction throughout the entire sampling area for each of the four age groups the senonian and hauterivian barremian pole positions lie close to coeval poles on african apparent polar wander paths apwps this result is consistent with previous analyses of middle and late cretaceous data from the venetian alps the new jurassic poles deviate significantly from recently published african apwps reflecting either the poor definition of the african apwp in the late jurassic or about 20 anticlockwise rotation of the southern alps relative to africa during early cretaceous time the former of the two possibilities is favored by a new late jurassic early cretaceous pole from north america and by early permian paleomagnetic data from the southern alps which are consistent with coeval african data implying that the movement of the southern alps was similar to that of africa since early permian

on thermocline ventilation of active and passive tracers the ventilation propagation of temperature and passive tracer anomalies are compared in a numerical model although both anomalies tend to have pathways close to the mean ventilation flow the temperature anomaly could have a slower propagation speed and a more complex spatial structure the temperature propagation is better understood in terms of baroclinic planetary waves

cenomanian turonian and δ13c and δ18o sea level and salinity variations at pueblo colorado stable isotopes of the surface dwelling planktic foraminifera hedbergella planispira its abundance variations and mineralogical analysis of the cenomanian turonian at pueblo co reveal cyclic variations in surface salinity due to changes in precipitation freshwater influx marine incursions and long term sea level fluctuations hedbergella planispira is a proxy for salinity variations as indicated by 2 4 more negative 18o values in intervals of peak abundances as compared to intervals with reduced populations negative 18o values reflect periods of brackish surface waters caused by freshwater influx during wet humid periods accompanied by increased clastic transport more positive 18o values reflect more normal marine salinities as a result of arid periods and or marine incursions and correlate with intervals of increased biogenic carbonate deposition the magnitude of salinity variations during the low sea level of the hartland shale is twice that during the sea level transgression of the bridge creek limestone the rapid positive 13c shift that marks the onset of oceanic anoxic event 2 oae 2 at pueblo occurred over a period of about 100 ky 93 90 94 00 ma and coincided with the major sea level transgression that culminated in the deposition of the basal bridge creek limestone a positive 13c shift also occurred in the rotalipora cushmani zone prior to oae 2 and coincided with a sea level rise and enhanced preservation of terrestrial organic matter the likely cause for oae 2 is depletion of 12c in the water column as a result of high primary productivity whereas an earlier r cushmani zone event was primarily caused by increased input of terrigenous organic matter both 13c events are associated with enhanced organic matter preservation and anoxic or dysoxic bottom waters

new oxygen isotope evidence for long term cretaceous climatic change in the southern hemisphere a new composite 18 o record generated from calcareous fine fraction and bulk sediments from the exmouth plateau details long term cretaceous climatic change at mid latitudes in the southern hemisphere assessment of new and previously published 18 o data indicates that a mid cretaceous global climatic optimum was achieved sometime between the time of the cenomanian turonian boundary and the middle turonian when surface ocean paleotemperatures were the highest of the past 115 m y periods of cooling and warming that reversed the general patterns were superimposed on long term aptian turonian warming and turonian maastrichtian cooling trends respectively extrapolation of southern hemisphere paleotemperature trends to maastrichtian paleotemperature data from a low latitude pacific guyot implies that maximum mid cretaceous low latitude paleotemperatures could have been in excess of 33 c

the ventilation of the world s oceans maps of the potential vorticity field abstract maps of potential vorticity q within four different density layers are presented for the north and south atlantic north and south pacific and indian oceans potential vorticity is evaluated as f 0 z a prescription that is valid for large slow scales and that allows its mapping from hydrographic data alone here f is the coriolis frequency p0 the potential density and z the vertical coordinate it is shown that the character of a layer within the thermocline of a subtropical gyre will vary greatly depending on the extent to which the layer is isolated from surface boundary conditions and forcing fluid particles recirculating within ventilated layers have their potential vorticity and other properties reset with virtually every circuit round the gyre when they pass through the outcrop zone by contrast particles within unventilated or dynamically isolated layers have time to share their properties with neighboring particles and approach a common homogenized state the north

origin of upper ocean warming and el nino change on decadal scales in the tropical pacific ocean the cause of decadal scale variability in the tropical pacific ocean such as that marked by the 1976 77 shift in the el nino southern oscillation1 2 3 4 5 6 7 is poorly understood unravelling the mechanism of the recent decade long warming in the tropical upper ocean is a particularly important challenge given the link to el nino variability but establishing the hypothesized interannual decadal oceanic connections between middle latitudes and tropics has proved elusive8 here we present observational evidence that pacific upper ocean warming and decadal changes in the el nino southern oscillation after 1976 may originate from decadal mid latitude variability in the middle 1970s the north pacific ocean is observed to have undergone a clear phase transition a see saw subsurface temperature anomaly pattern that rotates clockwise around the subtropical gyre at middle latitudes a subsurface warm anomaly formed in the early 1970s from subducted surface waters and penetrated through the subtropics and into the tropics thus perturbing the tropical thermocline and driving the formation of a warm surface water anomaly that may have influenced el nino in the 1980s the identification of this teleconnection of extratropical thermal anomalies to the tropics through a subsurface ocean bridge may enable improved prediction of decadal scale climate variability

investigating the role of the tibetan plateau in the formation of atlantic meridional overturning circulation abstractthe tibetan plateau tp over the eurasian continent has significant effects on both regional and global climate it can even affect the remote atlantic meridional overturning circulation

planktonic foraminifers across the bonarelli event oae2 latest cenomanian the italian record abstract a high resolution quantitative study of the planktonic foraminiferal assemblages across the bonarelli level oae2 latest cenomanian of italy from different areas and geological settings has allowed the recognition of several events and biotic changes including acmes and crises of different genera it provides evidence of a progressive and rapid change of palaeoenvironmental conditions reaching a climax coincident with the bonarelli event and of the subsequent gradual although not complete resumption of previous conditions the observed pattern is illustrated by marked changes in the assemblages that indicate five discrete phases of environmental perturbation within the marine ecosystem though there are several similarities our analyses highlight that each section has its own peculiarities clearly indicating more or less extreme environmental conditions

influence of extratropical thermal and wind forcings on equatorial thermocline in an ocean gcm the equatorial thermocline variability in the pacific in response to the extratropical thermal and wind forcings is investigated with an ocean general circulation model modular ocean model version 3 mom3 sensitivity experiments show that the extratropical wind forcing and thermal forcing contribute equally to the equatorial variability the wind induced response is attributed to the off equatorial wind within 30 8 of the equator the thermal induced response can be traced to higher latitudes the thermal forcing affects the equator mainly through the equatorward transport of the perturbation temperature by mean subduction flow the wind forcing affects the equator by changing the strength of meridional overturning circulations it is also found that the southern hemisphere contributes more to the equatorial variability than the northern hemisphere under both external forcings

middle late cretaceous climate of the southern high latitudes stable isotopic evidence for minimal equator to pole thermal gradients a detailed 18 o and 13 c stratigraphy has been generated from analysis of well preserved albian early maastrichtian foraminifera from deep sea drilling project dsdp sites 511 and 327 falkland plateau 58 s 62 s paleolatitude in the southern south atlantic and cenomanian and coniacian santonian foraminifera from dsdp site 258 naturaliste plateau 58 s paleolatitude in the southern indian ocean these results when combined with previously published maastrichtian stable isotope data from ocean drilling program odp site 690 weddell sea 65 s paleolatitude provide new insight into the climatic and oceanographic history of the southern high latitudes during middle late cretaceous time the planktonic foraminifer 18 o curves reveal a gradual warming of surface waters from the albian through the cenomanian followed by extremely warm surface waters from the turonian through the early campanian long term cooling of surface waters began in the late early campanian and continued through the end of the maastrichtian the benthic foraminifer 18 o record generally parallels changes in the oxygen isotopic curves defined by shallow dwelling planktonic foraminifera the vertical oxygen and carbon isotopic gradients were relatively low during the albian cenomanian high from the turonian early campanian and then low during the late campanian and maastrichtian foraminiferal oxygen isotopic data from published sources and this study are averaged for each site corrected for latitudinal changes in salinity based on modern day surface water values and plotted versus paleolatitude for the late albian coniacian santonian and late maastrichtian differences between lowand high latitude surface water paleotemperatures are estimated at 14 c during the late albian and late maastrichtian but the coniacian santonian reconstruction reveals only a 0 4 c latitudinal temperature gradient uncertainty regarding cretaceous salinity gradients and possible diagenetic alteration of 18 o values introduce error into our estimates of paleolatitudinal thermal gradients however apparent low equator to pole temperature differences could indicate much higher poleward heat transport than at present

upper ocean thermal variations in the north pacific during 1970 1991 abstract a newly available extensive compilation of upper ocean temperature profiles was used to study the vertical structure of thermal anomalies between the surface and 400 m depth in the north pacific during 1970 1991 a prominent decade long perturbation in climate occurred during this time period surface waters cooled by 1 c in the central and western north pacific and warmed by about the same amount along the west coast of north america from late 1976 to 1988 comparison with data from coads suggests that the relatively sparse sampling of the subsurface data is adequate for describing the climate anomaly the vertical structure of seasonal thermal anomalies in the central north pacific shows a series of cold pulses beginning in the fall of 1976 and continuing until late 1988 that appear to originate at the surface and descend with time into the main thermocline to at least 400 m depth individual cold events descend rapidly 100 m yr 1 superimposed upon a slower cooling 15 m yr 1 the inter

subduction of decadal north pacific temperature anomalies observations and dynamics abstract observations of oceanic temperature in the upper 400 m reveal decadal signals that propagate in the thermocline along lines of constant potential vorticity from the ventilation region in the central north pacific to approximately 18 n in the western pacific the propagation path and speed are well described by the geostrophic mean circulation and by a model of the ventilated thermocline the approximate southward speed of the thermal signal of 7 mm s 1 yields a transit time of approximately eight years the thermal anomalies appear to be forced by perturbations of the mixed layer heat budget in the subduction region of the central north pacific east of the date line a warm pulse was generated in the central north pacific by a series of mild winters from 1973 to 1976 and reached 18 n around 1982 after 1978 a succession of colder winters initiated a cold anomaly in the central north pacific that propagated along a similar path and with a similar speed as the warm anomaly then arrived in the west

decadal thermocline variability in the north pacific ocean two pathways around the subtropical gyre yearly in situ temperature anomaly data in the north pacific ocean for 1961 90 have been analyzed along constant density surfaces isopycnals in order to better describe and understand decadal thermocline variability in the region various empirical orthogonal function analyses are performed on isopycnals to depict the dominant three dimensional patterns the major finding is of two preferential pathways associated with decadal temperature variability around the subtropical gyre a subduction pathway with a large signal in the upper thermocline originates from the north pacific central eastern outcrop regions about 408n 1508w and then basically follows the mean gyre circulation southwestward along isopycnals toward the western tropics a subtropical pathway extends from the eastern subtropical tropical and boundary regions and appears to continue predominantly westward across the southern part of the gyre between 158 and 308n and then along the kuroshio path toward the midlatitudes along these two pathways thermal anomalies show coherent phase relationships to one another in the surface layer and in the thermocline around the gyre with their source regions variability centers being out of phase on decadal timescales two examples of each type of anomaly pattern can be illustrated for the periods analyzed in the 1960s a negative temperature anomaly signal propagated predominantly westward across the subtropics followed by a subducted warm anomaly from the outcrop region in the early 1970s that subsequently moved southwestward along isopycnals toward the western tropics a similar pattern was observed in the late 1970s and in the 1980s but with the opposite sign a westward propagating positive temperature anomaly signal along the subtropics in the late 1970s through the 1980s and a subducted cold anomaly in the early 1980s that also made its way southwestward with the expected gyre circulation to the western tropics in the late 1980s it is suggested that the southwestward subduction pathway provides a mechanism that connects surface anomalies in the outcrop region to thermocline variations in the western subtropics and in the tropics and that the westward subtropical pathway presents a possible link of tropical subtropical variability to surface temperature anomalies around the kuroshio and its extension regions which may further force variations in the overlying atmospheric circulation in the midlatitudes the results provide an observational basis for verification of theoretical studies and model simulations

carbon and oxygen isotope stratigraphy of the english chalk and italian scaglia and its palaeoclimatic significance a detailed carbon and oxygen isotope stratigraphy has been generated from upper cretaceous coastal chalk sections in southern england east kent culver cliff isle of wight eastbourne and seaford head sussex norfolk coast and the british geological survey bgs trunch borehole norfolk data are also presented from a section through the scaglia facies exposed near gubbio italian apennines wherever possible the sampling interval has been one metre or less both the chalk and scaglia carbon isotopic curves show minor positive excursions in the mid cenomanian mid and high turonian basal coniacian and highest santonian lowest campanian there is a negative excursion high in the campanian in chalk sections that span that interval the well documented cenomanian turonian boundary spike is also well displayed as is a broad positive excursion centred on the upper coniacian a number of these positive excursions correlate with records of organic carbon rich deposition in the atlantic ocean and elsewhere the remarkable similarity in the carbon isotope curves from england and italy enables cross referencing of macrofossil and microfossil zones and pinpoints considerable discrepancy in the relative positions of the turonian coniacian and santonian stages the oxygen isotope values of the various chalk sections although showing different absolute values that are presumably diagenesis dependent show nonetheless a consistent trend the east kent section which is very poorly lithified indicates a warming up to the cenomanian turonian boundary interval then cooling thereafter regional organic carbon burial documented for this period is credited with causing drawdown of co 2 and initiating climatic deterioration inverse greenhouse effect data from other parts of the world are consistent with the hypothesis that the cenomanian turonian temperature optimum was a global phenomenon and that this interval represents a major turning point in the climatic history of the earth

thermocline forced by annual and decadal surface temperature variation abstract a two layer thermocline model is modified by adding an essentially passive mixed layer above it the surface temperature variation is simulated by a moving outcrop line it is found that in contrast to a surface wind stress a surface temperature variation causes strong variability in the ventilated zone through subducted water while it affects the shadow zone little two types of buoyancy forced solution are found when the outcrop line moves slowly the solutions are nonentrainment solutions for these solutions the surface beat flux is mainly balanced by the horizontal advection in the permanent thermocline the mixed layer never entrains the time mean thermocline is close to the steady thermocline with the time mean outcrop line when the outcrop line moves southward rapidly during the cooling season the solutions become entrainment solutions now deep vertical convection must occur because the horizontal advection in the permanent thermocline is no longer strong enough to balance the s

low productivity and slow deep water circulation in mid cretaceous oceans paleofertility estimates are derived from organic carbon accumulation rates determined in nonbioturbated mid cretaceous deep sea deposits using organic carbon preservation factors observed in laminated sediments from anoxic holocene environments the estimated mid cretaceous primary production rates are as much as an order of magnitude lower than those measured in today s oceans an evaluation of dissolved oxygen budgets using a simple oceanic circulation model indicates very low deep water renewal rates in mid cretaceous oceans 28 references 2 tables 4 figures

evolution of subduction planetary waves with application to north pacific decadal thermocline variability abstract the evolution of decadal subduction temperature anomalies in the subtropical north pacific is studied using a simple and a complex ocean model it is found that the amplitude of the temperature anomaly decays faster than a passive tracer by about 30 50 the faster decay is caused by the divergence of group velocity of the subduction planetary wave which is contributed to significantly by the divergent sverdrup flow in the subtropical gyre the temperature anomaly also seems to propagate southward slower than the passive tracer or mean ventilation flow this occurs because the mean potential vorticity gradient in the ventilated zone is directed eastward the associated general beta effect produces a northward propagation for the temperature anomaly partially canceling the southward advection by the ventilation flow

thermocline forced by varying ekman pumping part ii annual and decadal ekman pumping abstract thermocline variability forced by zonally uniform ekman pumping with annual to decadal periods is investigated both analytical and numerical solutions are obtained by the method of characteristics as found in part i there is little thermocline variability in the ventilated zone or pool zone in contrast strong variability may exist in the shadow zone for annual forcings nonlinearity is negligible however the linear solution is influenced substantially by the basic state thermocline structure as a result local responses dominate for a shallow interface while remote rossby waves dominate for a deep interface under a strong decadal forcing nonlinearity may become important the time mean thermocline in the shadow zone is shallower than the steady thermocline under the mean ekman pumping particularly in the western part of a shadow zone where the mean deviation may reach the order often meters this shallower mean thermocline is caused by the nonlinear rossby wave

high stress late maastrichtian paleoenvironment inference from planktonic foraminifera in tunisia high resolution v5 10 kyr planktonic foraminiferal analysis at elles tunisia reveals major changes in the structure of the tethyan marine ecosystem during the upper maastrichtian during the first 1 5 myr of the late maastrichtian 68 3 66 8 ma relatively stable environmental conditions and cool temperatures are indicated by diverse planktonic foraminiferal populations with abundant intermediate and surface dwellers a progressive cooling trend between v66 8 65 45 ma resulted in the decline of globotruncanid species intermediate dwellers this group experienced a further decline at the climax of a rapid warm event about 300 kyr before the k t boundary at the same time relative abundances of long ranging dominant species fluctuated considerably reflecting the high stress environmental conditions times of critical high stress environments during the late maastrichtian and particularly at the k t boundary are indicated by low species diversity and blooms of the opportunistic genus guembelitria at warm cool transition intervals during the last 100 kyr of the maastrichtian rapid cooling is associated with accelerated species extinctions followed by the extinction of all tropical and subtropical species at the k t boundary fl 2002 elsevier science b v all rights reserved

calcareous nannofossils and mesozoic oceanic anoxic events abstract twenty five years ago mesozoic oceanic anoxic events oaes were documented and formalised as intervals of widespread to global deposition of organic matter the toarcian early aptian oae1a and latest cenomanian oae2 oaes are truly global in nature commonly carbonate poor and typically represented by organic carbon rich black shales in some areas these oaes are also characterised by abundant radiolarian sands and silts they are associated with negative and positive excursions in the 87sr 86sr record in addition to large global carbon isotope anomalies in carbonate and or organic matter caused by a major perturbation of the global carbon budget increased rates of volcanism during the formation of the ontong java and manihiki and caribbean plates and the karoo ferrar traps are believed to have caused the geological responses associated with oae1a oae2 and the toarcian oae respectively excess volcanogenic co2 in the atmosphere most probably turned the climate into a greenhouse mode accelerating continental weathering and increasing nutrient content in oceanic surface waters via river run off higher fertility in the global ocean was also probably triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes because mesozoic oaes are often represented by carbonate poor sediments quantitative studies of calcareous nannofossils have been applied to explore a the causes and effects of igneous tectonic events and climate changes relative to nannofloral increases and crises as well as b dissolution events and c diagenetic modifications characterization of calcareous nannofloras in oae intervals can improve our understanding of the marine ecosystem and biological processes such as photosynthesis biological pump and biomineralisation carbonate pump that affect the organic and inorganic carbon cycle as well as adsorption of atmospheric co2 in the oceans types and rates of nannoplankton production and evolution are interpreted to trace the impact of major palaeoceanographic and palaeoclimatic events in selected sections it has been documented that calcareous nannofloras rapidly reacted to new conditions of fertility and higher pco2 by drastically reducing calcification as in the modern oceans during oaes the increase of nutrients and atmospheric co2 induced higher abundances of nannoplankton producing small placoliths and inhibited the deep photic zone nannoconids and schizosphaerellids similarly to the nannoconid crisis preceding deposition of the early aptian oae1a black shales a schizosphaerellid crisis is detected prior to the toarcian oae both oaes are further characterised by a rapid nannofloral speciation beginning approximately 1 5 myr before the oae but without extinctions global changes during the latest cenomanian oae 2 exerted different influences on calcareous nannoplankton that experienced a turnover due to most extreme environmental conditions this event in fact was a time of extinctions followed by originations within calcareous nannofossils precise timing of the events before during and after oae1a oae2 and the toarcian oae indicates that they were intervals of enhanced oceanic productivity and that anoxia dysoxia post dated biotic changes

utilisation d un modele stochastique de reseaux de fractures pour etudier les proprietes hydrauliques d un massif fissure this study of the hydraulic properties of a fractured rock mass is based on data from field injection tests and fracture measurements and on simulations of the fracture system in the bedrock upstream from the daniel johnson dam at manic 5 analysis of water injection tests indicates that the bedrock can be divided into two zones with respect to the permeability the more permeable zone which is the object of this study shows a log normal distribution of the hydraulic conductivities using several stochastic simulations of fracture networks the fracture aperture has been adjusted gradually to reproduce the rock mass permeability estimated from injection tests the results show that the fracture system geometry as well as the fracture porosity and the fracture lengths and densities influences widely the hydraulic properties of a fractured medium and particularly the fracture porosity also the estimation of the fracture porosity is sensitive to a number of other factors including the assumed hydrauli