north pacific halocline and cold climate induced by panamanian gateway closure in a coupled ocean atmosphere gcm 1 the influence of the closure of the panamanian gateway during the late cenozoic on climate in and around the north pacific is investigated by using a coupled ocean atmosphere general circulation model with an open and closed gateway in the case of an open closed gateway deep convection is present absent in the north pacific the deep convection is associated with surface saline water transported from the subtropical atlantic through the open gateway to the north pacific on the other hand with the closed gateway the lack of saline water transport from the atlantic induces halocline formation over the subarctic pacific with cold climate the deep convection in the north pacific leads to a vigorous thermohaline circulation with larger meridional heat transport and causes warmer climate in and around the north pacific these results are generally consistent with paleoceanographic and paleoclimatic estimates related to the closure of the panamanian gateway

small basins in the scotia sea the eocene drake passage gateway after isostatic correction for their sedimentary fill the depths of two small oceanic basins in the southern scotia sea suggest that both started to open in mid to late eocene times models of the short magnetic anomaly profiles across the basins provide support for these determinations the location of the basins adjacent to the present day drake passage marine gateway and the timing of continental stretching leading up to their opening during the ypresian early eocene onset of global cooling mean that their importance is potentially far greater than their small size implies extension in the region of the two basins would have opened drake passage to shallow or intermediate depth oceanic circulation between the pacific and atlantic oceans for the first time this coincided with a reorganisation of vertical mixing patterns in the global ocean a shift in the site of carbon sequestration from coal swamps and peatlands to ocean sediments and the onset of a long decline in atmospheric carbon dioxide concentrations cenozoic global cooling may therefore have begun as a result of the shallow opening of drake passage

a modeling assessment of the interplay between aeolian iron fluxes and iron binding ligands in controlling carbon dioxide fluctuations during antarctic warm events 1 we add a prognostic biogeochemical model to the bern3d ocean circulation model to test the impact of increased aeolian iron fluxes in various regions of the ocean on long time scales atmospheric co2 is most sensitive when modern dust flux is increased 100 fold in the southern ocean for 1000 years resulting in a reduction of 10 ppmv seeding the indian ocean and south pacific results in increased export production and co2 drawdown in the southern ocean due to interbasinal transport of iron the non sea salt calcium record from dome c antarctica is used to scale aeolian iron deposition in the southern ocean in transient simulations over four antarctic warm events of the last glacial period our results suggest changes in dust flux to the southern ocean played a limited role in modulating co2 variations the impact of iron fluxes on co2 is dependent on parameter values chosen for the iron binding ligand

effect of ocean gateways on the global ocean circulation in the late oligocene and early miocene 1 we investigate the effect of changes in the tectonic boundary conditions on global ocean circulation patterns using a fully coupled climate model in an idealized setup we compare situations corresponding to the late oligocene the early miocene and present day the model results show the existence of a flow reversal through the panama seaway between the oligocene and miocene this flow reversal is induced by global tectonic changes related to the widening of the southern ocean passages and the closing of the tethys seaway it mainly involves the wind driven ocean circulation in agreement with previous model studies the global thermohaline circulation in the oligocene and miocene simulations is significantly different from the present day conveyor circulation as there is deepwater formation in both the north atlantic and the north pacific oceans in particular in the oligocene simulation the salinity contrast between the atlantic and pacific oceans is reduced because of water mass exchange through the low latitude connections between the two oceans

weathering versus circulation controlled changes in radiogenic isotope tracer composition of the labrador sea and north atlantic deep water geological reconstructions and general circulation models suggest that the onset of both northern hemisphere glaciation 2 7 myr ago and convection of labrador sea water lsw were caused by the closure of the panama gateway 4 5 myr ago time series data that have been obtained from studies of ferromanganese crusts from the northwestern atlantic suggest that radiogenic isotopes of intermediate ocean residence time pb and nd can serve as suitable tracers to reconstruct these events however it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering in this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in lsw but can be explained in terms of increases of erosion levels due to the glaciation of greenland and canada furthermore we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic pb and nonradiogenic nd from continental detritus eroding into the labrador sea this can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic pb should provide such a rich supply of radiogenic pb to the oceans an important general conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources perhaps more importantly the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater

influence of the central american seaway and drake passage on ocean circulation and neodymium isotopes a model study the sensitivity of the neodymium isotopic composition nd to tectonic rearrangements of seaways is investigated using an earth system model of intermediate complexity the shoaling and closure of the central american seaway cas is simulated as well as the opening and deepening of drake passage dp multiple series of equilibrium simulations with various intermediate depths are performed for both seaways providing insight into nd and circulation responses to progressive throughflow evolutions furthermore the sensitivity of these responses to the atlantic meridional overturning circulation amoc and the neodymium boundary source is examined modeled nd changes are compared to sediment core and ferromanganese fe mn crust data the model results indicate that the north atlantic nd response to the cas shoaling is highly dependent on the amoc state i e on the amoc strength before the shoaling to shallow depths preclosure three scenarios based on different amoc forcings are discussed of which the model data agreement favors a shallow preclosure miocene amoc 6 sv the dp opening causes a rather complex circulation response resulting in an initial south atlantic nd decrease preceding a larger increase this feature may be specific to our model setup which induces a vigorous cas throughflow that is strongly anticorrelated to the dp throughflow in freshwater experiments following the dp deepening odp site 1090 is mainly influenced by amoc and dp throughflow changes while odp site 689 is more strongly influenced by southern ocean meridional overturning circulation and cas throughflow changes the boundary source uncertainty is largest for shallow seaways and at shallow sites

circulation through the central american seaway during the miocene carbonate crash changes in circulation associated with the shoaling of the isthmus of panama and the caribbean carbonate crash in the miocene were investigated using nd isotopes from fossil fish teeth and debris from two sites in the caribbean basin ocean drilling program sites 998 and 999 and two sites in the eastern equatorial pacific sites 846 and 1241 the total range for nd values measured from 18 to 4 5 ma in the caribbean is 7 3 to 0 these values are higher than atlantic water masses 11 and range up to values equivalent to contemporaneous pacific water masses confirming that flow into the caribbean basin was composed of a mixture of pacific and atlantic waters with an upper limit of almost pure pacific sourced waters throughout the caribbean record particularly during the carbonate crash 10 12 ma low carbonate mass accumulation rates mars correlate with more radiogenic nd values indicating increased flow of corrosive pacific intermediate water into the caribbean basin during intervals of dissolution this flow pattern agrees with results from general ocean circulation models designed to study the effect of the shoaling of the central american seaway low carbonate mars and high nd values also correlate with intervals of increased northern component water production and therefore enhanced conveyor circulation suggesting that the conveyor may respond to changes in circulation associated with shoaling of the central american seaway reduced pacific throughflow related to shoaling of the seaway led to a gradual increase in carbonate preservation and more atlantic like nd values following the carbonate crash

evidence for middle eocene and younger land emergence in central panama implications for isthmus closure the rise of the isthmus of panama linked to a number of climatic paleoceanographic and biological events has been studied mostly from indirect often distal geochemical and biotic evidence we have upgraded existing geologic mapping in central panama with more than 2000 field stations over 40 petrographic analyses and more than 30 new geochronological and thermochronological analyses this data set suggests that the isthmus was an uninterrupted chain above sea level from late eocene until at least late miocene times the basement complex of central panama is a folded faulted 3 km thick arc sequence intruded by granitoid bodies and onlapped by mildly deformed upper eocene and oligocene strata six u pb zircon ages in the granitoids along with published geochronological data reveal intense late paleocene to middle eocene magmatism 58 39 ma a temporary cessation of magmatic activity between 38 and 27 ma and renewed magmatism between 25 and 15 ma in a position 75 km south of the former magmatic axis thermochronological analyses in zircon eight u th he ages and in apatite crystals four u th he ages and nine fission track ages obtained from a subset of 58 54 ma granitoid bodies record a concordant lutetian age 47 42 ma cooling from 200 c to 70 c in 5 m y and cooling below 40 c between 12 and 9 ma cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed upper eocene to oligocene terrestrial to shallow marine strata above exhumation and erosion of the basement complex are independently confirmed by lower miocene strata that have a detrital zircon signature that closely follows the central panama basement complex age distribution these results greatly restrict the width and depth of the strait separating southern central america from south america and challenge the widely accepted notion that the central american seaway closed in late pliocene time when the ice age began

closing of the central american seaway and the ice age a critical review 1 what role did the closing of the central american seaway play in enabling continental ice sheets to wax and wane over north america and fennoscandia a summary of relevant evidence presented here permits a causal relationship between them but can be interpreted to show none the common denominator of such evidence is the approximate simultaneity of that closing with global cooling and the first major ice advance at the resolution of paleoclimate however geologic evidence from central america places only weak constraints on when interchange between pacific and caribbean water ceased the strongest evidence for when north and south america became connected by a continuous land bridge the great american exchange of vertebrates assigns climate change the causal role for an arid central american climate typical of glacial times seems necessary for animals that inhabited savannas to pass through panama paleoceanographic and geochemical studies of environments and water masses in the eastern pacific western caribbean and atlantic in general call for continual change since 6 ma or earlier if evidence concurrent with global change near 3 5 3 ma seems more widespread than for other times hypothesized connections between a closed seaway and glaciation commonly call for profoundly different north atlantic ocean circulation but simulations using general circulation models provide a spectrum of differences in circulation for open and closed seaways might much not all of the evidence implicating the closing of the seaway in global climate change in fact be a consequence of that change and blind to central america s tectonic development

the ocean circulation in the southern hemisphere and its climatic impacts in the eocene abstract opening of southern ocean gateways may have had a controlling influence on the evolution of global climate the largest step in the history of cenozoic climate from an eocene greenhouse toward the modern glaciated state is correlated with opening of a deep water connection between the southern indian and southern pacific oceans this study synthesizes results from previous work assesses the possibility that ocean heat transport was much greater than modern during the globally warm eocene and explores the sensitivity of polar climate to large changes in ocean heat transport the magnitudes of these sensitivities are compared with those due to greenhouse gas concentration changes we find that ocean heat transport was unlikely to have been sufficiently greater than modern during the eocene to explain the change in the climate specifically in order to have significantly affected antarctic climate ocean heat transport changes would have had to change by an order of magnitude much greater than the largest change produced in our simulations in coupled climate model simulations changes in co2 concentrations from 1120 to 560 ppm supported by proxy records lead to significant antarctic responses in the absence of ocean heat transport changes the results show that antarctic snow accumulation is primarily controlled by changes in summer temperature not precipitation and that snow did not remain on antarctica during summer months in eocene even at low greenhouse gas concentrations greenhouse gas concentration changes and subsequent processes such as ice albedo and weathering feedbacks were more likely to have been the primary cause of antarctic glaciation than ocean heat transport changes

deep long offset transient electromagnetic surveys for crustal studies in the u s a abstract controlled source electromagnetic methods are often thought of as being used for detailed studies of earth conductivity at shallow depths such as is appropriate for minerals exploration however for the past quarter of a century the colorado school of mines has placed considerable effort into the development of similar techniques for exploration deeper into the earth s crust the main difference between these techniques and shallow methods is primarily that the source moment is generally one to two orders of magnitude greater for the former these deep electromagnetic sounding studies allow for a considerable flexibility in survey design so that optimum results may be obtained in diverse geological settings four time domain electromagnetic sounding surveys in different geological settings across continental u s a are reviewed the surveys on the u s west coast in the pacific northwest new york state and southern colorado provide insight into not only the diversity of the technique but also its ability to gather knowledge of deep electrical structures

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

antarctic glaciation caused ocean circulation changes at the eocene oligocene transition a climate model is used to show that the growth of the antarctic ice sheet at about 34 myr ago drove changes in ocean circulation but the opening of ocean gateways had relatively little impact antarctica developed an ice sheet about 34 million years ago following a period of ice free conditions known as the super greenhouse climate previous research has suggested that ice sheet growth was driven by either the thermal isolation of antarctica arising from the opening of ocean gateways or by progressively lower atmospheric co2 content matt huber and colleagues now use a climate model to show that the growth of the ice sheet forced by changes in co2 drove the changes in ocean circulation in contrast the opening of the gateways had relatively little impact on ocean circulation two main hypotheses compete to explain global cooling and the abrupt growth of the antarctic ice sheet across the eocene oligocene transition about 34 million years ago thermal isolation of antarctica due to southern ocean gateway opening1 2 3 4 and declining atmospheric co2 refs 5 6 increases in ocean thermal stratification and circulation in proxies across the eocene oligocene transition have been interpreted as a unique signature of gateway opening2 4 but at present both mechanisms remain possible here using a coupled ocean atmosphere model we show that the rise of antarctic glaciation rather than altered palaeogeography is best able to explain the observed oceanographic changes we find that growth of the antarctic ice sheet caused enhanced northward transport of antarctic intermediate water and invigorated the formation of antarctic bottom water fundamentally reorganizing ocean circulation conversely gateway openings had much less impact on ocean thermal stratification and circulation our results support available evidence that co2 drawdown not gateway opening caused antarctic ice sheet growth and further show that these feedbacks in turn altered ocean circulation the precise timing and rate of glaciation and thus its impacts on ocean circulation reflect the balance between potentially positive feedbacks increases in sea ice extent and enhanced primary productivity and negative feedbacks stronger southward heat transport and localized high latitude warming the antarctic ice sheet had a complex dynamic role in ocean circulation and heat fluxes during its initiation and these processes are likely to operate in the future

paleogene opening of drake passage the timing of events leading to the earliest connection between the pacific and atlantic oceans at drake passage is controversial but important because gateway opening probably had a profound effect on global circulation and climate a rigorous new analysis of marine geophysical data demonstrates a major change in the motion of the south american and antarctic plates at about 50 ma from n s to wnw ese accompanied by an eightfold increase in separation rate this would have led to crustal extension and thinning and perhaps the opening of small oceanic basins with the probable formation of a shallow b1000 m gateway during the middle eocene no change in south american antarctic motion is observed near the eocene oligocene boundary but a deep water connection developed between 34 and 30 ma when continued extension led to the initiation of seafloor spreading at the west scotia ridge these timings correlate with events seen in the oxygen isotope record from benthic foraminera and support the view that drake passage opening was the trigger for abrupt eocene oligocene climate deterioration and the growth of extensive antarctic ice sheets d 2005 elsevier b v all rights reserved

closure of the central american isthmus and its effect on deep water formation in the north atlantic modern ocean thermohaline driven circulation influences global climate by transporting heat to high latitudes1 2 and by affecting the exchange of co2 between ocean and atmosphere3 north atlantic deep water nadw plays a key role in this circulation and quaternary climate cycles have been linked to changes in nadw flow4 general circulation model simulations indicate that before closure some 3 4 million years ago of the central american isthmus the narrow strip of land linking north and south america the direct flow of low salinity water from the pacific to the atlantic ocean would have led to a smaller nadw flow5 6 sedimentation patterns7 and nutrient proxies8 11 support these model results by indicating an increase in nadw flow around the time of isthmus closure but these records do not allow changes in different nadw sources to be distinguished and the overall effect of closure on global ocean circulation is poorly known here we present nd pb and sr isotope records preserved by a hydrogenous ferromanganese crust from the nadw flow path in the western north atlantic ocean these records indicate that the isotopic signal associated with nadw strengthened around 3 4 million years ago showing that deep water that formed in the labrador sea made a gradually increasing contribution to nadw flow these data taken together with those from the central pacific ocean12 indicate an increased nadw flow since isthmus closure and suggest that the closure established today s general pattern of ocean circulation

actual timing of neodymium isotopic variations recorded by femn crusts in the western north atlantic hydrogenetic ferromanganese femn crusts from the western north atlantic record variations in the nd and pb isotopic composition of cenozoic deep water preserved during their growth the timing and cause of the most striking change have been the subject of debate some have proposed that the shift took place after 4 ma in response to the closure of the panama gateway others have argued that the major change in isotope composition occurred as early as 8 ma this study presents high resolution nd isotope records for crusts previously dated using 10be 9be chronology these data confirm that the shifts in nd occurred after 4 ma consistent with a likely relationship with the closure of the central american isthmus and intensification of northern hemisphere glaciation and in accordance with changes seen in other physical and chemical records these results illustrate the need for both a robust chronological framework and high resolution records before a reliable paleoceanographic interpretation can be made of the variations recorded by femn crusts

how important are southern hemisphere wind changes for low glacial carbon dioxide a model study 1 the response of atmospheric co2 to modifications in the strength and position of southern hemisphere westerlies is examined with the bern3d ocean model the model responds more sensitively to changes in the wind amplitude than to variations in latitudinal position depending on the model setup a 50 reduction in wind strength leads to a co2 drawdown of 3 34 ppm while a 50 increase results in a rise of 10 24 ppm a poleward shift of 5 lowers co2 by 2 16 ppm whereas an equatorward shift of 5 induces a co2 increase of 2 14 ppm physical and biological mechanisms equally contribute to the modeled changes in atmospheric co2 our results are in conflict with the hypothesis that southern hemisphere wind changes are responsible for the low atmospheric co2 concentrations during glacial periods

testing the influence of the central american seaway on orbitally forced northern hemisphere glaciation 1 utilizing a climate model of intermediate complexity we test the hypothesis that the pliocene closure of the central american seaway was a necessary precondition for orbitally triggered northern hemisphere glaciation we conduct a series of sensitivity experiments in order to analyze the isolated and combined effects of panama closure and orbital forcing on northern hemisphere perennial snow cover while orbital forcing efficiently controls the extension of perennial snow cover the model results suggest that the closure of the central american seaway did not intensify orbitally forced glaciation in high northern latitudes

neogene overflow of northern component water at the greenland scotland ridge in the north atlantic ocean flow of north atlantic deep water nadw and of its ancient counterpart northern component water ncw across the greenland scotland ridge gsr is thought to have played an important role in ocean circulation over the last 60 ma the iceland plume has dynamically supported an area which encompasses the gsr consequently bathymetry of the gsr has varied with time due to a combination of lithospheric plate cooling and fluctuations in the temperature and buoyancy within the underlying convecting mantle here we reassess the importance of plate cooling and convective control on this northern gateway for ncw flow during the neogene period following wright and miller 1996 to tackle the problem benthic foraminiferal isotope data sets have been assembled to examine 13c gradients between the three major deep water masses i e northern component water southern ocean water and pacific ocean water composite records are reported on an astronomical timescale and a nonparametric curve fitting technique is used to produce regional estimates of 13c for each water mass confidence bands were calculated and error propagation techniques used to estimate ncw and its uncertainty despite obvious reservations about using long term variations of 13c from disparate analyses and settings and despite considerable uncertainties in our understanding of ancient oceanic transport pathways the variation of ncw through time is consistent with independent estimates of the temporal variation of dynamical support associated with the iceland plume prior to 12 ma 13c patterns overlap and ncw cannot be isolated significant long period variations are evident which are consistent with previously published work from 12 ma when lithospheric cooling probably caused the gsr to submerge completely long period 13c patterns diverge significantly and allow reasonable ncw estimates to be made our most robust result is a dramatic increase in ncw overflow between 6 and 2 ma when dynamical support generated by the iceland plume was weakest between 6 and 12 ma a series of variations in ncw overflow have been resolved

paleoclimatic response of the closing of the isthmus of panama in a coupled ocean atmosphere model the paleoclimatic effects of the closure of the isthmus of panama 3 ma are investigated using a coupled atmosphere ocean model consistent with earlier ocean only modelling studies it is shown that prior to closure there is an absence of deep water formation in the north atlantic hence there is a reduction in oceanic heat transport this is largely compensated for by the atmosphere such that only small changes in total planetary heat transport occur the model climate of the north atlantic is significantly warmer after isthmus closure in addition the regions surrounding the pacific ocean and south atlantic are generally cooler while the indian ocean is generally warmer in the model present day climate finally possible relationships to glaciation and initiation of northern hemisphere glacial cycles are discussed

a coupled dynamical ocean energy balance atmosphere model for paleoclimate studies abstract the bern3d coupled three dimensional dynamical ocean energy balance atmosphere model is introduced and the atmospheric component is discussed in detail the model is of reduced complexity developed to perform extensive sensitivity studies and ensemble simulations extending over several glacial interglacial cycles on large space scales the modern steady state of the model compares well with observations in a first application several 800 000 yr simulations with prescribed orbital greenhouse gas and ice sheet forcings are performed the model shows an increase of atlantic meridional overturning circulation strength at glacial inceptions followed by a decrease throughout the glaciation and ending in a circulation at glacial maxima that is weaker than at present the sensitivity of ocean temperature to atmospheric temperature atlantic meridional overturning circulation amoc and antarctic bottom water aabw strength is analyzed at 23 locations in a second application the climate sensitivi

effect of the deepening of the tasman gateway on the global ocean 1 we examine the effect of the deepening of the tasman seaway at the end of the eocene in a climate model with realistic late eocene bathymetry and winds for this we have constructed an eocene numerical model based on the university of victoria climate model with wind forcing derived from a fully coupled eocene simulation the model climate state is characterized by an oceanic meridional overturning circulation moc involving southern hemisphere sinking and a northward atmospheric moisture transport across the equator the deepening of the tasman seaway in the presence of an open drake passage and the associated establishment of the antarctic circumpolar current acc have a limited climatic impact on antarctica nonetheless the antarctic deep sinking regions cool sufficiently to lead to a global deep ocean cooling of 3 c no initiation of northern component water is found indicating that this may require the development of a more mature acc previous studies suggest that the ross sea gyre cools the east coast of australia and expected the deepening of the tasman seaway to lead to a warming east of australia due to the introduction of warmer water from the australo antarctic gulf we here find that this warming is limited to close to the australian coast and that widespread cooling prevails further off shore

on the relationship between nd isotopic composition and ocean overturning circulation in idealized freshwater discharge events using a cost efficient climate model the effect of changes in overturning circulation on neodymium isotopic composition nd is systematically examined for the first time idealized sequences of abrupt climate changes are induced by the application of periodic freshwater fluxes to the north atlantic na and the southern ocean so thus mainly affecting either the formation of north atlantic deep water nadw or antarctic bottom water aabw variations in nd reflect weakening and strengthening of the formation of nadw and aabw changes in ndof end members are relatively small relationships between nd and the strength of nadw or aabw are more pronounced for aabw than for nadw atlantic patterns of variations in nd systematically differ between na and so experiments additionally the signature of changes in nd in the atlantic and the pacific is alike in na but opposite in so experiments discrimination between na and so experiments is therefore possible based on the atlantic pattern of variations in nd and the contrariwise behavior of nd in the atlantic and the pacific in further experiments we examined the effect of variations in magnitudes of particle export fluxes within the examined range and although settling particles represent the only sink of nd their effects on nd are relatively small our results confirm the large potential of nd as a paleocirculation tracer but also indicate its limitations of quantitative reconstructions of changes in the atlantic meridional ocean circulation

deciphering the role of southern gateways and carbon dioxide on the onset of the antarctic circumpolar current 1 growth of antarctic ice sheet during the cenozoic 34 million years ago appears as a potential tipping point in the long term cooling trend that began 50 ma ago for decades the onset of the antarctic circumpolar current acc following the opening of the drake passage and of the tasman seaway has been suggested as the main driver of the continental scale antarctic glaciation however recent modeling works emphasized that the eocene oligocene atmospheric carbon dioxide co2 lowering could be the primary forcing of the antarctic glaciation questioning the acc theory here we investigate the response of the acc to changes in co2concentrations occurring from the late eocene to the late oligocene we used a fully coupled atmosphere ocean model foam with a mid oligocene geography we find that the opening of southern oceanic gateways does not trigger the onset of the acc for co2typical of the late eocene 840 ppm a cooler background climatic state such as the one prevalent at the end of the oligocene is required to simulate a well developed acc in this cold configuration the intensified sea ice development around antarctica and the resulting brine formation lead to a strong latitudinal density gradient in the southern ocean favoring the compensation of the ekman transport and consequently the acc our results imply that the acc has acted as a feedback rather than as a driver of the global cooling

north atlantic deep water export to the southern ocean over the past 14 myr evidence from nd and pb isotopes in ferromanganese crusts the intensity of north atlantic deep water nadw production has been one of the most important parameters controlling the global thermohaline ocean circulation system and climate here we present a new approach to reconstruct the overall strength of nadw export from the north atlantic to the southern ocean over the past 14 myr applying the deep water nd and pb isotope composition as recorded by ferromanganese crusts and nodules we present the first long term nd and pb isotope time series for deep southern ocean water masses which are compared with previously published time series for nadw from the nw atlantic ocean these data suggest a continuous and strong export of nadw or a precursor of it into the southern ocean between 14 and 3 ma an increasing difference in nd and pb isotope compositions between the nw atlantic and the southern ocean over the past 3 myr gives evidence for a progressive overall reduction of nadw export since the onset of northern hemisphere glaciation nhg the nd isotope data allow us to assess at least semiquantitatively that the amount of this reduction has been in the range between 14 and 37 depending on location

a coupled climate ice sheet modeling approach to the early cenozoic history of the antarctic ice sheet abstract the sudden widespread glaciation of antarctica and the associated shift toward colder temperatures near the eocene oligocene boundary 34 ma represents one of the most fundamental reorganizations of the global climate system recognized in the geologic record this glacial inception and the subsequent evolution of the early east antarctic ice sheet eais are simulated using a new coupled global climate dynamical ice sheet model accounting for the paleogeography greenhouse gas concentrations changing orbital parameters and varying ocean heat transport suites of long 105 yr climate ice sheet simulations are used to investigate the effects of declining atmospheric co2 compared to those of the tectonic opening of southern ocean gateways and the timing of mountain uplift in the antarctic interior in contrast to the established paradigm for the glaciation of antarctica which centers on the opening of the southern ocean gateways and the thermal isolation of the continent our results show that declining cenozoic pco2 may have played the dominant role first small isolated ice caps formed on the highest antarctic plateaus then as a co2 threshold between 3 and 2 pre industrial level pal was crossed height mass balance feedbacks were initiated during orbital periods with cold austral summers triggering much larger highly dynamic terrestrial ice sheets as co2 continued to decline these isolated ice caps eventually merged into a permanent continental scale eais in our model neither the opening of the southern ocean gateways nor mountain uplift significantly affected the timing of the major ice sheet transition given a scenario of gradually declining co2 from 4 to 2 pal over 10 million years around the eocene oligocene boundary

new constraints on the pb and nd isotopic evolution of ne atlantic water masses 1 time series of lead pb and neodymium nd isotope compositions were measured on three ferromanganese crusts recording the evolution of ne atlantic water masses over the past 15 ma the crusts are distributed along a depth profile 700 4600 m comprising the present day depths of mediterranean outflow water and north east atlantic deep water a pronounced increase of the 206pb 204pb in the two deeper crusts starting at 4 ma and a decrease in 143nd 144nd in all three crusts took place between 6 4 ma and the present these patterns are similar to isotope time series in the western north atlantic basin and are consistent with efficient mixing between the two basins however the changes occurred 1 3 ma earlier in the eastern basin indicating that the northeastern atlantic led the major change in pb and nd isotope composition probably due to a direct supply of labrador seawater via a northern route the pb isotope evolution during the pliocene pleistocene can generally be explained by mixing between two end members corresponding to mediterranean outflow water and north east atlantic deep water but external sources such as saharan dust are likely to have played a role as well the pb isotope composition of the shallowest crust that grew within the present day mediterranean outflow water does not show significant pb isotope changes indicating that it was controlled by the same pb sources throughout the past 15 ma

early pliocene increase in thermohaline overturning a precondition for the development of the modern equatorial pacific cold tongue unraveling the processes responsible for earth s climate transition from an el nino like state during the warm early pliocene into a modern like la nina dominated state currently challenges the scientific community recently the pliocene climate switch has been linked to oceanic thermocline shoaling at 3 million years ago along with earth s final transition into a bipolar icehouse world here we present pliocene proxy data and climate model results which suggest an earlier timing of the pliocene climate switch and a different chain of forcing mechanisms we show that the increase in north atlantic meridional overturning circulation between 4 8 and 4 0 million years ago initiated by the progressive closure of the central american seaway triggered overall shoaling of the tropical thermocline this preconditioned the turnaround from a warm eastern equatorial pacific to the modern equatorial cold tongue state about 1 million years earlier than previously assumed since 3 6 3 5 million years ago the intensification of northern hemisphere glaciation resulted in a strengthening of the trade winds thereby amplifying upwelling and biogenic productivity at low latitudes

timing and climatic consequences of the opening of drake passage age estimates for the opening of drake passage range from 49 to 17 million years ago ma complicating interpretations of the relationship between ocean circulation and global cooling secular variations of neodymium isotope ratios at agulhas ridge southern ocean atlantic sector suggest an influx of shallow pacific seawater approximately 41 ma the timing of this connection and the subsequent deepening of the passage coincide with increased biological productivity and abrupt climate reversals circulation productivity linkages are proposed as a mechanism for declining atmospheric carbon dioxide these results also indicate that drake passage opened before the tasmanian gateway implying the late eocene establishment of a complete circum antarctic pathway

arc continent collision and orocline formation closing of the central american seaway closure of the central american seaway was a local tectonic event with potentially global biotic and environmental repercussions we report geochronological six u pb la icp ms zircon ages and geochemical 19 xrf and icp ms analyses data from the isthmus of panama that allow definition of a distinctive succession of plateau sequences to subduction related protoarc to arc volcaniclastic rocks intruded by late cretaceous to middle eocene intermediate plutonic rocks 67 6 1 4 ma to 41 1 0 7 ma paleomagnetic analyses 24 sites 192 cores in this same belt reveal large counterclockwise vertical axis rotations 70 9 6 7 and moderate clockwise rotations between 40 4 1 and 56 2 11 1 on either side of an east west trending fault at the apex of the isthmus rio gatun fault consistent with isthmus curvature an oligocene miocene arc crosscuts the older deformed and segmented arc sequences and shows no significant vertical axis rotation or deformation there are three main stages of deformation 1 left lateral strike slip offset of the arc 100 km and counterclockwise vertical axis rotation of western arc segments between 38 and 28 ma 2 clockwise rotation of central arc segments between 28 and 25 ma and 3 orocline tightening after 25 ma when this reconstruction is placed in a global plate tectonic framework and published exhumation data is added the central american seaway disappears at 15 ma suggesting that by the time of northern hemisphere glaciation deep water circulation had long been severed in central america

modelling nd isotopes with a coarse resolution ocean circulation model sensitivities to model parameters and source sink distributions the neodymium nd isotopic composition nd of seawater is a quasi conservative tracer of water mass mixing and is assumed to hold great potential for paleoceanographic studies here we present a comprehensive approach for the simulation of the two neodymium isotopes 143nd and 144nd using the bern3d model a low resolution ocean model the high computational efficiency of the bern3d model in conjunction with our comprehensive approach allows us to systematically and extensively explore the sensitivity of nd concentrations and nd to the parametrisation of sources and sinks previous studies have been restricted in doing so either by the chosen approach or by computational costs our study thus presents the most comprehensive survey of the marine nd cycle to date our model simulates both nd concentrations as well as nd in good agreement with observations nd covaries with salinity thus underlining its potential as a water mass proxy results confirm that the continental margins are required as a nd source to simulate nd concentrations and nd consistent with observations we estimate this source to be slightly smaller than reported in previous studies and find that above a certain magnitude its magnitude affects nd only to a small extent on the other hand the parametrisation of the reversible scavenging considerably affects the ability of the model to simulate both nd concentrations and nd furthermore despite their small contribution we find dust and rivers to be important components of the nd cycle in additional experiments we systematically varied the diapycnal diffusivity as well as the atlantic to pacific freshwater flux to explore the sensitivity of nd concentrations and its isotopic signature to the strength and geometry of the overturning circulation these experiments reveal that nd concentrations and nd are comparatively little affected by variations in diapycnal diffusivity and the atlantic to pacific freshwater flux in contrast an adequate representation of nd sources and sinks is crucial to simulate nd concentrations and nd consistent with observations the good agreement of our results with observations paves the way for the evaluation of the paleoceanographic potential of nd in further model studies

simulating the impact of the panamanian seaway closure on ocean circulation marine productivity and nutrient cycling abstract the closure of the panama isthmus 14 3 ma caused large reorganizations of ocean circulation here we use a global climate ocean ecosystem model to investigate the effects of the closing gateway on ocean circulation marine productivity and nutrient distributions several sensitivity experiments with different sill depths and vertical diffusivities are performed consistent with previous model studies we find constricted throughflow due to shallowing of the sill leads to intensification of the north atlantic thermohaline overturning we demonstrate a strong coupling between the flows through the tropical gateways of panama and indonesia in a way that reduced outflow of upper ocean pacific waters via the panama gateway into the atlantic is compensated by increased flow through the indonesian archipelago the simulated rates of north atlantic deep water formation strongly depend on the vertical diffusion in the model particularly for a deep sill for the first time we document by model results shifts in nutrient distributions associated with reorganizations of ocean circulation with repercussions on marine productivity patterns reduced flow of nutrient rich sub surface waters from the pacific into the atlantic reduces biological productivity in the north atlantic in the eastern tropical pacific restriction of the nutrient rich outflow leads to nutrient accumulation which in turn maintains a strong increase in productivity these results seem to be largely consistent with the paleoproductivity proxy record a massive drop of opal accumulation rates however as found in north pacific sediments for the time of the final closure 2 75 ma cannot be simulated generally global marine net primary productivity npp is found to increase with proceeding gateway closure

heat transport deep waters and thermal gradients coupled simulation of an eocene greenhouse climate for the first time a coupled general circulation model with interactive and dynamical atmospheric oceanic and sea ice components is used to simulate an eocene 50 ma greenhouse climate we introduce efficient ocean spin up methods for coupled paleoclimate modeling sea surface temperatures ssts and salinities evolve unconstrained producing the first proxy data independent estimates for these eocene climate parameters tropical and extratropical model predicted ssts are warmer than modern values by 3 and 5 c respectively salinity driven deep water formation occurs in the north atlantic and tethys the zonal average overturning circulation is weaker than modern eocene ocean heat transport is 0 6 pw less than modern in the northern hemisphere and 0 4 pw greater in the southern hemisphere the model predicted near modern vertical and meridional eocene temperature gradients imply that the dominant theory for maintaining low gradients increased ocean heat transport is incorrect or incomplete and other mechanisms should be explored

are repetitive slumpings during sapropel s1 related to paleo earthquakes abstract the recurrence of mass flow units within sapropel s1 an organic carbon rich lower holocene marker bed in the eastern mediterranean sea was used to study the interplay between earthquakes and sedimentation along the seismically active calabrian arc ionian sea nine turbidite beds interrupt anoxic conditions during the deposition of sapropel s1 each of these turbidites is associated with sharp grain size and geochemical elemental anomalies high al and si low ca and coarse grained basal part marked by zr peaks and with displaced foraminiferal species from different bathymetric ranges we used these proxies to identify turbidite beds also above and below the sapropel where turbidite signature is less clear due to the absence of major color changes turbidite structure and composition as well as comparison with historical seismoturbidites suggest a seismic triggering for such mass flow events the peculiar color well known composition geochemistry and age of sapropel s1 make this unit a key bed within which turbidites may be considered a sort of sedimentary bar code recording high energy events within the background pelagic sedimentation deciphering this code will reconstruct paleo seismicity in this well defined stratigraphic interval the pelagic units bracketing turbidite beds were radiometrically dated and the age of the sapropel s1 deduced through age modeling is between 6 0 and 10 2 kyr cal bp the emplacement age of each turbidite was estimated considering the average time interval between successive turbidite beds from pelagic sediment thickness and sedimentation rate subsequently these ages were further refined through age modeling in this way we compiled a catalogue of mass flow events during sapropel s1 deposition a time span long enough to include several earthquake cycles and allow reliable seismic and tsunami hazard assessment in this area