a cenozoic record of the equatorial pacific carbonate compensation depth atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks these feedbacks involve the erosion of silicate rocks and organic carbon bearing rocks the integrated effect of these processes is reflected in the calcium carbonate compensation depth which is the oceanic depth at which calcium carbonate is dissolved here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial pacific ocean the carbonate compensation depth tracks long term ocean cooling deepening from 3 0 3 5 kilometres during the early cenozoic approximately 55 million years ago to 4 6 kilometres at present consistent with an overall cenozoic increase in weathering we find large superimposed fluctuations in carbonate compensation depth during the middle and late eocene using earth system models we identify changes in weathering and the mode of organic carbon delivery as two key processes to explain these large scale eocene fluctuations of the carbonate compensation depth

depth ranges of alkenone production in the central pacific ocean we report analytical results of long chain c 37 c 39 alkenones in the surface sediments taken along a latitudinal transect at 175 e from 48 n to 15 s unsaturation degree of c 37 alkenones u k 37 indicated that alkenone temperature is the highest about 28 3 c in the tropical pacific and decreases toward 48 n about 10 1 c although the latitudinal trend of the alkenone temperature is generally similar to that of the observed sea surface temperature of the surface mixed layer the former exhibits markedly lower values than the latter does in the midlatitude 35 19 n on the basis of the comparisons between water column temperatures and alkenone temperatures in the surface sediments we estimated that in the midlatitudes alkenones are produced in the thermocline waters whereas in the high 48 40 n and low 10 n 2 s latitudes they are produced in the surface mixed layer this result is supported by the depth distribution of emiliania huxleyi and gephyrocapsa oceanica along a 155 w transect in the central pacific the latitudinal distribution of the alkenone production layers estimated from u k 37 is found to be consistent with that of the nitrate suggesting that the depth of alkenone production is mainly controlled by the nutrient supply from deeper waters alkenones in the sediments exhibited relatively large abundances in 43 27 n with a maximum at 30 n which may be explained by high productivity of haptophyte algae in kuroshio extension

oxygen isotopic composition of multiple species of planktonic foraminifera recorders of the modern photic zone temperature gradient abundances of 12 species of planktonic foraminifera collected in two plankton tows from the east tropical atlantic are compared to the chlorophyll content and the temperature of the sea water from which they were collected as expected from previous work in the tropics all dominant tropical species occur in greatest abundance within the photic zone many species occur in greatest abundances in the seasonal thermocline in association with the maximum chlorophyll concentration while a few algal symbiont bearing species occur in greatest abundance in the mixed layer the 18o measurements of planktonic foraminifera shells from core top sediment samples confirm the vertical stratification within the photic zone that is suggested by the relationship between hydrography and abundances found in the plankton tows and found in the statistical study by ravelo et al 1990 comparison between the measured 18o values of planktonic foraminifera with the predicted 18o profiles of the overlying water column at three core locations indicate that species abundances in the sediment record the seasonally integrated conditions of the photic zone and suggests that the abundance of a species in the sediment depends on whether the preferred ecological conditions of that species may be found within the photic zone of the overlying water column sometime during the year species which calcify below the photic zone have only trace relative abundances finally it appears that the total range of 18o values of the dominant species approximates the predicted annual 18o of calcite range in the upper 80 m of the water column

middle miocene paleoceanography of the western equatorial pacific dsdp site 289 and the evolution ofgloborotalia fohsella abstract evolution of the planktic foraminiferal lineagegloborotalia fohsella occurred during the miocene between 23 7 and 11 8 ma and forms the basis for stratigraphic subdivision of the early middle miocene zones n10 through n12 important morphologic changes within theg fohsella lineage included a marked increase in test size a transition from a rounded to an acute periphery and the development of a keel in later forms we found that the most rapid changes in morphology ofg fohsella occurred between 13 and 12 7 ma and coincided with an abrupt increase in the 18o ratios of shell calcite comparison of isotopic results ofg fohsella with other planktic foraminifers indicate that 18o values of the lineage diverge from surface dwelling species and approach deep dwelling species after 13 0 ma indicating a change in depth habitat from the surface mixed layer to intermediate depth near the thermocline isotopic and faunal evidence suggests that this change in depth stratification was associated with an expansion of the thermocline in the western equatorial pacific after adapting to a deeper water habitat at 13 0 ma theg fohsella lineage became extinct abruptly at 11 8 ma during a period when isotopic and faunal evidence suggest a shoaling of the thermocline following the extinction ofg fohsella the ecologic niche of the lineage was filled by thegloborotalia menardella group which began as a deep water form and later evolved to an intermediate water habitat we suggest that the evolution ofg fohsella andg menardella were tightly linked to changes in the structure of the thermocline in the western equatorial pacific

isotope data from cretaceous chalks and foraminifera environmental or diagenetic signals scanning electron microscopy of sediments microfossils and polished thin sections from the cenomanian turonian boundary section at dover england demonstrates that cement is concentrated within large pores such as foraminifera and calcispheres whereas the surrounding coccolith rich sediments are largely lacking in cement isotope analysis of single microfossils and cement samples indicates the presence of mixing lines between primary paleoceanographic compositions and cements consequently paleoceanographic and stratigraphic interpretations of these sediments and perhaps other similar sediments based on similar data may be suspect

microstructural and geochemical perspectives on planktic foraminiferal preservation glassy versus frosty in recent years it has become apparent that the cool tropic paradox of paleogene and cretaceous greenhouse climates arises because of the diagenetic alteration of tropical planktic foraminiferal calcite near the seafloor yielding artificially high 18o values because the mg ca compositions of foraminiferal and inorganic calcite are thought to be quite different mg ca measurements should be a sensitive way of monitoring diagenetic alteration here we examine the extent of diagenetic alteration of eocene planktic foraminiferal calcite using scanning electron microscope imaging of foraminiferal test microstructures and geochemical 18o and mg ca analyses we compare microstructural and geochemical characteristics between given species exhibiting two contrasting states of preservation those that appear frosty under reflected light and those that appear glassy microstructural evidence reveals extensive diagenetic alteration of frosty foraminiferal tests at the micron scale while 18o analyses document consistently higher 18o and therefore lower paleotemperatures in this material yet we find that 18o offsets between species in these frosty foraminiferal assemblages appear to be generally preserved suggesting that frosty foraminifera remain valuable for generating relatively short approximately 1 myr paleoceanographic time series that do not demand absolute estimates of paleotemperature we also find that the observed increase in mg ca for planktic foraminifera exhibiting diagenetic alteration compared to glassy taphonomies is far smaller than would be expected from the addition of inorganic calcite based on laboratory derived mg2 partition coefficients our findings imply that a much lower mg2 partition coefficient controls inorganic calcite formation in deep sea sedimentary sections in accordance with the findings of baker et al 1982

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

the evolution of the equatorial thermocline and the early pliocene el padre mean state the tropical pacific thermocline strength depth and tilt are critical to tropical mean state and variability during the early pliocene 3 5 to 4 5 ma the eastern equatorial pacific eep thermocline was deeper and the cold tongue was warmer than today which resulted in a mean state with a reduced zonal sea surface temperature gradient or el padre however it is unclear whether the deep thermocline was a local feature of the eep or a basin wide condition with global implications our measurements of mg ca of globorotalia tumida in a western equatorial pacific site indicate pliocene subsurface temperatures warmer than today thus el padre included a basin wide thermocline that was relatively warm deep and weakly tilted at 4 ma thermocline steepening was coupled to cooling of the cold tongue since 4 ma the basin wide thermocline cooled shoaled gradually with implications for thermocline feedbacks in tropical dynamics and the interpretation of tex86 derived temperatures

changes in pacific ocean circulation following the miocene onset of permanent antarctic ice cover we integrate micropaleontological and geochemical records benthic stable isotopes neodymium isotopes benthic foraminiferal abundances and xrf scanner derived elemental data from well dated pacific ocean successions 15 12 7 ma to monitor circulation changes during the middle miocene transition into a colder climate mode with permanent antarctic ice cover together with previously published records our results show improvement in deep water ventilation and strengthening of the meridional overturning circulation following major ice expansion at 13 9 ma neodymium isotope data reveal however that the provenance of intermediate and deep water masses did not change markedly between 15 and 12 7 ma we attribute the increased 13c gradient between pacific deep and intermediate water masses between 13 6 and 12 7 ma to more vigorous entrainment of pacific central water into the wind driven ocean circulation due to enhanced production of intermediate and deep waters in the southern ocean prominent 100 kyr ventilation cycles after 13 9 ma reveal that the deep pacific remained poorly ventilated during warmer intervals at high eccentricity whereas colder periods low eccentricity were characterized by a more vigorous meridional overturning circulation with enhanced carbonate preservation the long term 13c decline in pacific intermediate and deep water sites between 13 5 and 12 7 ma reflects a global trend probably related to a re adjustment response of the global carbon cycle following the last 400 kyr carbon maximum cm6 of the monterey excursion

the relationship between shell size and mg ca sr ca δ18o and δ13c of species of planktonic foraminifera the relationship between shell size and mg ca sr ca 18o and 13c of planktonic foraminiferal calcite has been investigated using seventeen species in six different size fractions mg ca increases and sr ca decreases with increasing size except for two globorotaliid species which show the opposite trend the changes in mg ca broadly follow 18o calcification temperatures except that surface and near surface dwelling species show larger changes in mg ca than can be accounted for by differences in calcification temperature derived from 18o the increases in mg ca and decreases in sr ca vary linearly with the well established increase in 13c with size this is consistent with smaller individuals calcifying faster than larger individuals and larger individuals forming calcite that more closely reflects seawater temperature and composition it appears that variations in calcification rate affect mg and sr shell chemistry the observations may account for part of the temporal variability in foraminiferal sr ca within a single size fraction that has been attributed to changes in seawater sr ca

late miocene decoupling of oceanic warmth and atmospheric carbon dioxide forcing measurements from several ocean cores reveal that ocean warmth persisted throughout the late miocene epoch despite co2 levels of only 200 350 p p m v probably driven by a deep thermocline that isolated climate responses from co2 variations geological records of past climate provide the only opportunity to investigate earth s response to large variations in atmospheric carbon dioxide concentrations during the late miocene between 12 million and 5 million years ago atmospheric co2 levels of only 200 to 350 parts per million by volume were accompanied by almost ice free conditions in the northern hemisphere and the continents were warmer than they are at present raising questions about the persistence of co2 climate couplings using measurements from several ocean cores lariviere et al show that ocean warmth persisted throughout the late miocene probably driven by a thick thermocline that isolated climate responses from co2 variations as the depth of the thermocline declined during the late miocene the more familiar relationship between co2 and climate was established deep time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration that is the atmospheric partial pressure of co2 1 although past studies have explored these responses during portions of the cenozoic era the most recent 65 5 million years myr of earth history comparatively little is known about the climate of the late miocene 12 5 myr ago an interval with values of only 200 350 parts per million by volume but nearly ice free conditions in the northern hemisphere2 3 and warmer than modern temperatures on the continents4 here we present quantitative geochemical sea surface temperature estimates from the miocene mid latitude north pacific ocean and show that oceanic warmth persisted throughout the interval of low 12 5 myr ago we also present new stable isotope measurements from the western equatorial pacific that in conjunction with previously published data5 6 7 8 9 10 reveal a long term trend of thermocline shoaling in the equatorial pacific since 13 myr ago we propose that a relatively deep global thermocline reductions in low latitude gradients in sea surface temperature and cloud and water vapour feedbacks may help to explain the warmth of the late miocene additional shoaling of the thermocline after 5 myr ago probably explains the stronger coupling between sea surface temperatures and climate that is characteristic of the more recent pliocene and pleistocene epochs11 12

geochemical assessment of the palaeoecology ontogeny morphotypic variability and palaeoceanographic utility of dentoglobigerina venezuelana to better understand the links between the carbon cycle and changes in past climate over tectonic timescales we need new geochemical proxy records of secular change in silicate weathering rates a number of proxies are under development but some of the most promising e g palaeoseawater records of li and nd isotope change can only be employed on such large samples of mono specific foraminifera that application to the deep sea sediment core archive becomes highly problematic dentoglobigerina venezuelana presents a potentially attractive target for circumventing this problem because it is a typically large 355 m diameter abundant and cosmopolitan planktic foraminifer that ranges from the early oligocene to early pliocene yet considerable taxonomic and ecological uncertainties associated with this taxon must first be addressed here we assess the taxonomy palaeoecology and ontogeny of d venezuelana using stable isotope oxygen and carbon and mg ca data measured in tests of late oligocene to early miocene age from ocean drilling program odp site 925 on ceara rise in the western equatorial atlantic to help constrain the depth habitat of d venezuelana relative to other species we report the stable isotope composition of selected planktic foraminifera species within globigerina globigerinoides paragloborotalia and catapsydrax we define three morphotypes of d venezuelana based on the morphology of the final chamber and aperture architecture we determine the trace element and stable isotope composition of each morphotype for different size fractions to test the validity of pooling these morphotypes for the purposes of generating geochemical proxy datasets and to assess any ontogenetic variations in depth habitat our data indicate that d venezuelana maintains a lower thermocline depth habitat at ceara rise between 24 and 21 ma comparing our results to published datasets we conclude that this lower thermocline depth ecology for the oligo miocene is part of an eocene to pliocene evolution of depth habitat from surface to sub thermocline for d venezuelana our size fraction data advocate the absence of photosymbionts in d venezuelana and suggest that juveniles calcify higher in the water column descending into slightly deeper water during the later stages of its life cycle our morphotype data show that 18o and 13c variation between morphotypes is no greater than within morphotype variability this finding will permit future pooling of morphotypes in the generation of the sample hungry palaeoceanographic records

changes in sea surface conditions in the equatorial pacific during the middle miocene pliocene as inferred from coccolith geochemistry abstract this study presents a new alkenone derived sea surface temperature sst record and 18onoelaerhabdaceae data of the 2 5 m carbonate fractions from the iodp site u1338 located in the eastern equatorial pacific eep over the miocene pliocene our data and those available from other sites of the same area show the establishment of a cold tongue during the early pliocene 4 4 3 6 ma sst and 18onoelaerhabdaceae time series indicate periods of significant salinity variations comparison with the 18obenthic curve from sediment cores of the equatorial pacific ocean allows us to distinguish between global changes and local surface salinity variations in the eep ice sheet growth and evaporation precipitation are then discussed as possible drivers of such changes along with the role of central american and the indonesian seaway restriction our data suggest a shallowing of the thermocline in the eep between 6 8 and 6 ma and its shoaling between 4 8 and 4 0 ma suggesting the appearance of the cold tongue steph et al 2010 the pliocene climate transition would therefore not be primarily driven by northern hemisphere glaciation

permanent el nino like conditions during the pliocene warm period during the warm early pliocene 4 5 to 3 0 million years ago the most recent interval with a climate warmer than today the eastern pacific thermocline was deep and the average west to east sea surface temperature difference across the equatorial pacific was only 1 5 0 9 c much like it is during a modern el nino event thus the modern strong sea surface temperature gradient across the equatorial pacific is not a stable and permanent feature sustained el nino like conditions including relatively weak zonal atmospheric walker circulation could be a consequence of and play an important role in determining global warmth

synchronous mid miocene upper and deep oceanic δ13c changes in the east equatorial pacific linked to ocean cooling and ice sheet expansion abstract we present mid miocene 12 0 16 0 ma high resolution 3 8 kyr deep thermocline planktonic foraminiferal 18 o and 13 c records at iodp site u1337 from the eastern equatorial pacific the benthic and planktonic 18 o records of site u1337 have a similar long term cooling trend but display obvious amplitude differences of glacial interglacial cycles after 13 9 ma planktonic 18 o cycles are usually larger than 1 0 nearly 2 times those of the benthic 18 o the post 13 9 ma change is probably caused by intensified upwelling along with ocean cooling but may include upper ocean circulation changes both the benthic and planktonic 13 c records at site u1337 reveal marked 400 kyr carbon isotope cycles during the mmco middle miocene climate optimum with a trend toward lower 13 c from 16 0 ma to 12 0 ma the similarity in response between surface and deep carbon isotopes indicates that the mid miocene carbon excursions in the east equatorial pacific involved the whole ocean basin water column and thus are a global signal box model simulations reveal that the long eccentricity 400 kyr paced carbon inputs from weathering changed the burial ratio of carbonates to organic carbon and helped to produce the significant 400 kyr cycles in oceanic 13 c the increased weathering of carbonate silicate and kerogen rocks are the major factors controlling the long term decrease in oceanic 13 c after 16 0 ma

global miocene tectonics and the modern world abstract an amazing congruence of seemingly unrelated diverse global events began in the middle and upper miocene and established our modern world two global orogenic belts were active mostly in the middle and upper miocene while backarc basins formed along the eastern margin of asia coincident with these events global temperatures cooled in both the ocean and atmosphere desertification occurred from central asia into and across most of northern africa and also in australia and in southern south america coincident with the expansion of the antarctic ice cap at 14 ma there was initial widespread deep sea erosion and changes in patterns of deep sea sedimentation muddy pelagic sedimentation increased six fold in the north and central atlantic and pacific oceans and global changes in circulation lead to more diatomites in the pacific and fewer in the atlantic by the end of the miocene most of the mediterranean sea had evaporated broadly coincident with these events many old large river systems were destroyed and new ones formed as much of the world s landscape changed collectively these global on shore tectonic and ocean atmospheric events provide the foundation for our modern world a mixture of new and rejuvenated orogenic belts and their far field effects distant epiorogenic uplift rain shadow deserts large alluvial aprons and distant deltas as inherited gondwanan landscapes persisted remote from plate boundaries thus at the end of the miocene much of the world s landscape except for that changed by pleistocene continental glaciation would be recognizable to us today we argue that all of these events had the same ultimate common cause an internal earth engine that drove plate motions in two broad ways first the opening and closing of seven key gateways to deep water oceanic currents radically altered global heat transfer and changed a lingering greenhouse to an icehouse world secondly these events were in part coincident with renewed heat flow in the african and pacific superplumes that energized global plate motions in the middle and upper miocene we hope this global synthesis will stimulate more research on the many global events of the miocene to understand better both our modern world and earlier global orogenies

paleogene tropical pacific clues to circulation productivity and plate motion 1 stratigraphic data from 63 deep sea drilling project and ocean drilling program odp sites that sample the lower neogene and paleogene sediments of the tropical pacific have been compiled and put on the biostratigraphic and paleomagnetic timescale refined by odp leg 199 scientists sediment accumulation rates have been calculated for ten intervals ranging in age from 10 to 56 ma and have been plotted for the midpoint of each interval at the associated paleoposition for each site used a fixed hot spot model was used for reconstruction of the pacific lithospheric plate all such reconstructed intervals show the development of a tongue of high accumulation rates associated with the oceanographic divergence at the geographic equator the development of this equatorial band is weakest between 46 and 56 ma the time of the peak warmth in paleogene climate possible motion of the hawaiian hot spot or true polar wander between 46 and 56 ma appears to have had little effect on the plate rotation estimate of the position of the equator in addition to temporal changes in the calcite compensation depth and in productivity the biggest change in the patterns of sediment accumulation rates in the eastern tropical pacific was the development of a relatively strong divergence between 6 and 10 n near the region of divergence between the modern north equatorial current and the north equatorial counter current changes in the equatorial circulation appear to be associated in time with the opening and closing of oceanic gateways particularly the complex closing of the caribbean pacific gateway

deformation and metamorphism at the south margin of the alps east of bellinzona switzerland deformation and metamorphism of rock series at the south margin of the alps e of bellinzona are studied magmatic relics and metamorphic minerals and rock types of the tonalite and of the augengneiss of melirolo are examined these bodies are regarded as of magmatic origin zoning of plagioclase is ascribed to rise of melts and cooling under loss of volatiles subvolcanic dykes branch off the augengneiss into the tonale series the magmatics are late alpine and intruded into the southerly cover plate on top of the subduction zone the cover plate was still cold and unaffected by alpine deformation during this intrusion later deformation and heating of cover plate and magmatics is late alpine the tonalite deformation was quantified by a study of xenolith deformation the tonalite was flattened to 1 20 of its original width we extrapolate this flattening to the southerly steep zone now 5 km broad we conclude that this zone was 100 km wide befor late alpine flattening this deformation occurred during the higher t part of lepontine heat rise a maximum t of ca 580 c and ph20 5 6 kbar is reached after the main deformation considerable stretching upward buried the niveau of intrusion formerly shallow under 15 20 km of rocks the tonale series suffered 3 fold rotational refolding under subsiding t after the main deformation an early alpine subduction eclogite was found imbricated into the tonale series it still contains parts of the subduction paragenesis preserved or pseudomorphosed and relics of subduction preferred orientation under late alpine deformation it is changed into amphibolite this proves continuation of subduction from val sesia to our area and further e the magmatics are interpreted as subduction magmatites they cannot be derived as partial melts from the subducted slice which underlies the tonale series from which the eclogite was imbricated into the tonale series during late alpine movements voll has found a more northerly subduction zone adula nappe misox zone this may have reached our area in sufficient depth to produce partial melts before late alpine flattening of the marginal zone last movements at the insubric line formed blastomylonites at 300 c indicating vertical movements in our area lepontine garnet staurolite kyanite schists abut directly against the insubric line and the seealpen lower t areas towards the s are cut off by the insubric line

in situ δ18o and mg ca analyses of diagenetic and planktic foraminiferal calcite preserved in a deep sea record of the paleocene eocene thermal maximum we report 18o and minor element mg ca sr ca data acquired by high resolution in situ secondary ion mass spectrometry sims from planktic foraminiferal shells and 100 500 m sized diagenetic crystallites recovered from a deep sea record odp site 865 of the paleocene eocene thermal maximum petm the 18o of crystallites 1 2 pee dee belemnite pdb is 4 8 higher than that of planktic foraminiferal calcite 3 6 pdb while crystallite mg ca and sr ca ratios are slightly higher and substantially lower than in planktic foraminiferal calcite respectively the focused stratigraphic distribution of the crystallites signals an association with petm conditions hence we attribute their formation to early diagenesis initially sourced by seafloor dissolution burndown ensued by reprecipitation at higher carbonate saturation the mg ca ratios of the crystallites are an order of magnitude lower than those predicted by inorganic precipitation experiments which may reflect a degree of inheritance from donor phases of biogenic calcite that underwent solution in the sediment column in addition sims 18o and electron microprobe mg ca analyses that were taken within a planktic foraminiferal shell yield parallel increases along traverses that coincide with muricae blades on the chamber wall the parallel 18o and mg ca increases indicate a diagenetic origin for the blades but their 18o value 0 5 pdb is lower than that of crystallites suggesting that these two phases of diagenetic carbonate formed at different times finally we posit that elevated levels of early diagenesis acted in concert with sediment mixing and carbonate dissolution to attenuate the 18o decrease signaling petm warming in whole shell records published for site 865

paleoenvironmental conditions for the development of calcareous nannofossil acme during the late miocene in the eastern equatorial pacific repeated monospecific coccolithophore dominance intervals acmes of specimens belonging to the noelaerhabdaceae family including the genus reticulofenestra and modern descendants emiliania and gephyrocapsa occurred during the neogene such acme was recognized during the late miocene 8 6 ma at a time of a major reorganization of nannofossil assemblages resulting in a worldwide temporary disappearance of larger forms of the genus reticulofenestra r pseudoumbilicus and the gradual recovery and dominance of its smaller forms 5 m in this study we present a multiproxy investigation of late miocene sediments from the east equatorial pacific integrated ocean drilling program site u1338 where small reticulofenestrid type placoliths with a closed central area known as small dictyococcites spp 3 m formed an acme we report on oxygen and carbon stable isotope records of multispecies planktic calcite and alkenone derived sea surface temperature our data indicate that during this 100 kyr long acme the east equatorial pacific thermocline remained deep and stable local surface stratification state fails to explain this acme and thus contradicts the model based hypothesis of a southern ocean high latitude nutrient control of the surface waters in the east equatorial pacific instead our findings suggest that external forcing such as an extended period of low eccentricity may have created favorable conditions for the small dictyococcites spp growth

neogene stratigraphy paleoceanography and paleobiogeography in northwest south america and the evolution of the panama seaway abstract the atrato basin in the pacific coastal region of nw south america colombia exhibits stratigraphic and biostratigraphic characteristics similar to other neogene pacific coastal basins of southern central america and northern south america an evaluation of the neogene stratigraphy and foraminiferal biostratigraphy with emphasis on paleoceanographic and paleobiogeographic phenomena indicate the following 1 previous to the middle miocene well aerated deep and open oceanic conditions and free and active water circulation prevailed along the steep continental margins of nw south america 2 during the early middle miocene right after the neogene hiatus nh 2 15 2 ma tectonic disturbances triggered the initial uplift of the panama sill bottom water circulation foraminiferal and sedimentation changes are observed in both the pacific and caribbean coastal areas of nw south america 3 during the middle miocene coinciding with the neogene hiatus nh 3 12 9 11 8 ma an abrupt paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the panama sill to about 1000 m reflecting the middle miocene tectonic disturbances in nw south america 4 during the late middle miocene immediately following this uplift a distinctive pacific benthic foraminiferal fauna abruptly appeared and extended from ecuador to california because these assemblages do not occur in the adjacent caribbean region a circulation barrier between the atlantic and pacific oceans as a result of intensification of the cool marginal california current is proposed anoxic middle to upper bathyal environments prevailed under well aerated cool surface waters in the pacific coastal areas of nw south america 5 during the latest miocene immediately following the neogene hiatus nh 6 7 0 6 3 ma surface water circulation between the caribbean and the pacific was re established and the previous influence of the california current disappeared a rapid filling and shallowing of the basin to mostly neritic depths is indicated along the pacific and caribbean coastal basins of colombia as well as a progressive increase in similarity between the coastal pacific and caribbean benthic biotas this shallowing in the early late miocene also appears to have been associated with the earliest terrestrial interchange of ground sloths to north america and raccoons and their allies to south america 6 immediately following early pliocene time the panamanian isthmus became completely emergent providing a terrestrial environment favourable for intermingling of terrestrial faunas and floras between north and south america i correlate this pliocene uplift with the neogene hiatus nh 8 3 7 3 1 ma four regional unconformities associated with tectonic disturbances in the nw corner of south america have also been recognized and correlated with neogene hiatuses recorded from the oceans early middle miocene middle miocene late miocene and early pliocene

early and middle miocene stable isotopes implications for deepwater circulation and climate the middle miocene 18o increase represents a fundamental change in the ocean atmosphere system which like late pleistocene climates may be related to deepwater circulation patterns there has been some debate concerning the early to early middle miocene deepwater circulation patterns specifically recent discussions have focused on the relative roles of northern component water ncw production and warm saline deep water originating in the eastern tethys our time series and time slice reconstructions indicate that ncw and tethyan outflow water two relatively warm deepwater masses were produced from 20 to 16 ma ncw was produced again from 12 5 to 10 5 ma another feature of the early and middle miocene oceans was the presence of a high 13c intermediate water mass in the southern hemisphere which apparently originated in the southern ocean miocene climates appear to be related directly to deepwater circulation changes deep waters warmed in the early miocene by 3 c 20 to 16 ma and cooled by a similar amount during the middle miocene 18o increase 14 8 to 12 6 ma corresponding to the increase 20 ma and subsequent decrease 16 ma in the production of ncw and tethyan outflow water large 0 6 relatively rapid 0 5 m y 18o increases in both benthic and planktonic foraminifera i e the mi zones of miller et al 1991a and wright and miller 1992a were superimposed in the long term deepwater temperature changes they are interpreted as reflecting continental ice growth events seven of these m y glacial interglacial cycles have been recognized in the early to middle miocene two of these glacial interglacial cycles mi3 and mi4 combined with a 2 to 3 c decrease in deepwater temperatures to produce the middle miocene 18o shift

tertiary ice sheet dynamics the snow gun hypothesis we observe strong negative correlation between tertiary low to mid latitude planktonic foraminiferal 18o and the difference between these data and coeval benthic foraminiferal 18o late quaternary data do not show this correlation coupling statistical model 18o comparisons and evidence for antarctic ice and ocean temperature variation we infer that tertiary ice volume recorded by tropical planktonic 18o increased as the deep ocean warmed because the isotopic signatures of deepwater temperature variation and ice volume change were of opposite sign the sum of these signals in tertiary benthic 18o became lost in the noise this renders low correlation between tertiary planktonic and benthic 18o time series compared to late quaternary data we contend that tertiary ice sheet growth was commonly driven by warming of deep water from low to mid latitude marginal seas snow gun hypothesis in contrast late quaternary ice sheets grew as deep water formed at high latitude cooled because tectonic forcing and orbital forcing at low latitude primarily controlled production and temperature variations of this warm saline deep water these influences largely dictated tertiary ice volume fluctuations through the tertiary we infer ice volume fluctuations to be an important component of sea level history on timescales between 103 and 107 years

timing and magnitude of miocene eustasy derived from the mixed siliciclastic carbonate stratigraphic record of the northeastern australian margin article eustasy is a key parameter to understand sedimentary sequences on continental margins and to reconstruct continental ice volume in the cenozoic but timing and magnitude of global sea level changes remain controversial especially for the miocene epoch we analyzed sediment cores recovered from the marion plateau offshore northeastern australia during ocean drilling program odp leg 194 to define the mechanisms and timing of sequence formation on mixed carbonate siliciclastic margins and to estimate the amplitude of miocene eustatic adjustments we identified sequence boundaries on seismic reflection lines significantly revised the existing biostratigraphic age models and investigated the sedimentary response to sea level changes across the marion plateau we subdivided the miocene sediments into three sequence sets comprising a set of prograding clinoforms a muddy prograding carbonate ramp evolving into an aggrading platform and a lowstand ramp evolving into a backstepping ramp we recognized eight individual sequences dated at 18 0 ma 17 2 ma 16 5 ma 15 4 ma 14 7 ma 13 9 ma 13 0 ma and 11 9 ma we demonstrate that sequences on the marion plateau are controlled by glacio eustasy since sequence boundaries are marked by increases in 18 o deep sea miocene isotope events mi1b mbi 3 mi2 mi2a mi3a mi3 mi4 and mi5 respectively which reflects increased ice volume primarily on antarctica our backstripping estimates suggest that sea level fell by 26 28 m at 16 5 ma 26 29 m at 15 4 ma 29 38 m at 14 7 ma and 53 81 m at 13 9 ma combining backstripping with 18 o estimates yields sea level fall amplitudes of 27 1 m at 16 5 ma 27 1 m at 15 4 ma 33 3 m at 14 7 ma and 59 6 m at 13 9 ma we use a similar approach to estimate eustatic rises of 19 1 m between 16 5 and 15 4 ma 23 3 m between 15 4 and 14 7 ma and 33 3 m between 14 7 and 13 9 ma these estimates can be combined into a eustatic curve that suggests that sea level fell by 53 69 m between 16 5 and 13 9 ma this implies that at least 90 of the east antarctic icesheet was formed during the middle miocene the new independent amplitude estimates are crucial as the miocene is the geologic epoch for which the new jersey margin sea level record is poorly constrained

variation in middle late miocene sedimentation rates in the northern south china sea and its regional geological implications abstract a complex suite of geological information from deep sea drilling cores can provide insights into the cenozoic evolution of the south china sea scs therefore the thick sediment sequence obtained during international ocean discovery program iodp expedition 368 at site u1501 in the northern scs provides excellent materials to investigate the paleoceanographic and geological evolution the generally well preserved calcareous nannofossil assemblages allowed us to establish a high resolution biostratigraphic framework and revealed dramatic fluctuations in sedimentation rates during the middle miocene through earliest pliocene ca 15 5 ma the sedimentation rate was high during the middle miocene decreased dramatically before the middle late miocene boundary and essentially ceased afterwards with resumption of deposition close to the late miocene pliocene boundary sediment source analysis with diffuse reflectance spectra shows that biogenic carbonate accounts for approximately half of the bulk sediments and that the dilution effect by terrigenous sediments partially controlled the carbonate content whereas the high sedimentation rates were fueled by both sources during the middle miocene the apparent 1 5 myr long hiatus 10 5 9 ma corresponds to the globally distributed carbonate crash which was truncated in this region by the regionally important dongsha movement the accompanying decrease in carbonate content and terrigenous materials may indicate decreased biogenic carbonate export to deep waters during the carbonate crash possibly resulting from the progressive constriction of the indonesian seaway the restricted distribution of discoaster hamatus at this site suggests that the dongsha movement commenced at 9 5 ma younger than previously proposed

obliquity and long eccentricity pacing of the middle miocene climate transition 1 the middle miocene east antarctic ice sheet expansion eaie which is indicated by an abrupt 1 increase in global benthic foraminiferal 18o at 13 8 ma marks the middle miocene climate transition mmct and has been related to astronomically modulated changes in the global carbon cycle here we present high resolution 3 4 kyr benthic foraminiferal 18o and 13c records from iodp site u1337 in the central equatorial pacific which spans the period 12 2 15 8 ma the isotopic records clearly demonstrate significant imprints from periodic variations in the earth s orbital parameters particularly the obliquity 40 kyr and the long eccentricity 400 kyr cycles while the benthic 18o and 13c exhibit nearly identical amplitudes for glacial interglacial cycles from 15 8 to 12 2 ma the long term trends in the benthic 18o and 13c had started to reverse after the beginning of the eaie within the 400 kyr band the benthic 18o and 13c displays a constant phase relationship between 15 8 and 12 2 ma at the 41 kyr band however a phase reversal reaching 180 between 18o and 13c occurs from 13 8 ma to 14 0 ma during the period of the eaie a similar phase relationship of benthic foraminiferal 18o and 13c at the 400 kyr band and the 41 kyr band is also observed at odp site 1146 from the northern south china sea this phase jump occurs when the long term trends in 18o and 13c split suggesting a decoupling of the global ice volume and ocean carbon reservoir changes during the middle miocene

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

vertical thermal structure evolution of miocene surface waters western equatorial pacific dsdp site 289 abstract time series analyses from 18 7 to 5 8 ma of the oxygen isotopic composition of four shallow dwelling planktic foraminifera globorotalia mayeri dentoglobigerina altispira globoquadrina baroemoenensis andglobigerinoides immaturus one deep dwelling planktic foraminifer globoquadrina venezuelana and a benthic foraminifer cibicidoides spp from deep sea drilling project site 289 2206 m present day depth are used to determine the evolution of vertical thermal structure in the western equatorial pacific comparisons of composite oxygen isotopic curves representing the surface mixed layer the thermocline and deep waters reveal three distinct intervals of stability in the position and strength of the thermocline during the miocene 18 7 to 16 1 ma 11 1 to 9 9 ma and 7 5 to 5 8 ma separated by two intervals of instability of the thermocline 16 1 to 11 1 ma and 9 9 to 7 5 ma from 18 7 to 16 1 ma vertical thermal gradients were the weakest of the miocene interval studied and the water column was highly stable vertical thermal gradients strengthened between 16 1 and 11 1 ma as equatorial surface waters warmed and deep waters cooled during an inferred major growth phase of the east antarctic ice sheet as a consequence the strength and position of the thermocline became highly variable this was followed from 11 1 to 9 9 ma by relative climatic stability well defined vertical thermal gradients and a slight warming of the water column by 9 9 ma increasing restriction of surface water flow through the indonesian seaway led to the piling up of warm surface waters in the western equatorial pacific and a deepening of the thermocline deep waters continued to cool and vertical thermal gradients reached a miocene maximum by 7 5 ma from 7 5 to 5 8 ma vertical thermal gradients were stable and the water column became highly stratified and marked by a thick warm surface layer and a deep well defined thermocline based upon a previous assumption that ice volume growth during the middle miocene 16 1 to 11 1 ma contributed to an increase in 18o values of 0 5 to 0 7 we have estimated water column temperature changes at site 289 between 18 7 and 5 8 ma surface waters are inferred to have warmed 2 to 4 c between 18 7 and 5 8 ma with about half of the warming occurring from 16 1 to 11 1 ma between 11 1 and 9 9 ma the entire water column warmed and from 9 9 to 7 5 ma the mixed layer expanded and the thermocline deepened as warm waters piled up in the western equatorial pacific in response to the closure of the indonesian seaway the net decrease of thermoclinal 18o values between 11 1 and 7 5 ma reflects an upward change in position of the deep dwelling planktic foraminifera globoquadrina venezuelana relative to the position of the thermocline as the thermocline deepened after 7 5 ma relative 18o values indicate thatgloboquadrina venezuelana was a deep dwelling mixed layer species deep waters 2400 to 2200 m at site 289 cooled 2 to 4 c between 18 7 and 5 8 ma with about half of the cooling occurring from 16 1 to 11 1 ma and the other half from 9 9 to 7 5 ma

cenozoic tectonic and depth age evolution of the indonesian gateway and associated back arc basins abstract we reconstruct the tectonic and paleo age and depth history of the indonesian seaway and associated southeast asian back arc basins to create a comprehensive paleogeographic framework for the region during the cenozoic we combine published tectonic models with revised models of the evolution of back arc basins based on available marine magnetic anomaly data and a recently published high resolution 1 min marine gravity anomaly grid which reveals many fine scale tectonic features previously not resolved reconstructions of the now subducted ceno tethys ocean floor are integrated with a revised absolute plate motion reference frame for the indian ocean based on moving hotspots all marginal seas north of australia formed in a back arc setting with the caroline and celebes seas opening north of a northward dipping subduction zone and the solomon sea south of a southward dipping subduction zone north of australia we suggest that the evolution of the two sub basins of the caroline sea from 36 to 25 ma followed different seafloor spreading patterns with the western sub basin being influenced by the manus hotspot our model suggests that collision between the melanesian and the caroline arcs led to cessation of oceanic crustal production in the caroline basin shortly before 25 ma combined geophysical and geological observations suggest that several major tectonic events occurred north of australia at around 45 ma roughly at the time when australia started moving northwards at fast rates similar to the present we suggest that this event is related to a relocation of the subduction zone nw of australia under the philippine sea plate due to a collision and subsequent accretion of old pacific plate material to the northward subducting australian plate our gridded oceanic paleo depth maps capture the history of marginal seas north of australia and the stepwise destruction of the ceno tethys ocean floor in the northern wharton basin and the west pacific providing boundary conditions for both paleoceanograpic and geodynamic models a regional analysis of basement depth anomalies reveals that median negative anomalous depth of several back arc basins is of the order of 650 800 m with a range of 300 1100 m accompanied by negative regional heatflow anomalies suggesting that mantle driven dynamic topography plays a larger role in modulating regional basement depth than previously suggested regional shear wave mantle tomography cross sections highlight that all marginal basins that exhibit large negative dynamic topography are underlain by massive buried slab material in the lower portion of the upper mantle and or below the transition zone supporting the notion that the observed paired negative dynamic topography and heatflow anomalies are due to basin formation above slab burial grounds

vertical thermal gradient history in the eastern equatorial pacific during the early to middle miocene implications for the equatorial thermocline development knowledge of the equatorial thermocline is essential for understanding climate changes in the tropical pacific multispecies planktic foraminiferal analyses provide a way to examine temperature distributions and thus the structure of the thermocline although the secular thermocline development has been documented back to the late miocene the early to middle miocene interval has rarely been examined in addition relationships with the dynamic antarctic ice sheets remain unclear here we investigate the vertical thermal gradient in the upper water column at integrated ocean drilling program site u1337 in the eastern equatorial pacific eep throughout the early to middle miocene 23 1 to 11 7 ma the gradient increased over the miocene climatic optimum whereas it decreased during the east antarctic ice sheet expansion eaie comparison of the eep record with its western equatorial pacific wep counterpart suggests that sea surface temperature was more stable in the wep than in the eep we further estimated equatorial thermocline from two diagonal gradients between the eep and the wep thermocline shoaled from 16 7 to 15 7 ma and tilt weakened between 16 5 and 13 8 ma the onset of the monterey excursion and the reduced antarctic ice sheet volume would have affected thermocline depth and tilt respectively thermocline depth was likely much deeper compared to pliocene to modern conditions furthermore a 4 point based distribution of isotherms 4di index was used as a metric of the evenness or unevenness of the isotherm distributions the 4di index considerably reduced at around the eaie and other mi events reflecting the evenly distributed isotherms under a more glaciated antarctica

orbitally paced climate evolution during the middle miocene monterey carbon isotope excursion one of the most enigmatic features of cenozoic long term climate evolution is the long lasting positive carbon isotope excursion or monterey excursion which started during a period of global warmth after 16 9 ma and ended at similar to 13 5 ma approximately 400 kyr after major expansion of the antarctic ice sheet we present high resolution 1 9 kyr astronomically tuned climate proxy records in two complete sedimentary successions from the northwestern and southeastern pacific odp sites 1146 and 1237 which shed new light on the middle miocene carbon isotope excursion and associated climatic transition over the interval 17 1 12 7 ma we recog nize three distinct climate phases with different imprints of orbital variations into the climatic signals 1146 and 123 7 delta 18 o delta 13 c 1237 xrt fe fraction 63 pm 1 climate optimum prior to 14 7 ma characterized by minimum ice volume and prominent 100 and 400 kyr variability 2 long term cooling from 14 7 to 13 9 ma principally driven by obliquity and culminating with rapid cryosphere expansion and global cooling at the onset of the last and most pronounced delta 13 c increase 3 icehouse mode after 13 9 ma with distinct 100 kyr variability and improved ventilation of the deep pacific the monterey carbon isotope excursion 16 9 13 5 ma consists overall of nine 400 kyr cycles which show high coherence with the long eccentricity period superposed on these low frequency oscillations are high frequency variations 100 kyr which closely track the amplitude modulation of the short eccentricity period in contrast to delta 13 c the delta 18 o signal additionally shows significant power in the 41 kyr band and the 1 2 myr amplitude modulation of the obliquity cycle is clearly imprinted in the 1146 delta 18 o signal our results suggest that eccentricity was a prime pacemaker of middle miocene climate evolution through the modulation of long term carbon budgets and that obliquity paced changes in high latitude seasonality favored the transition into the icehouse climate c 2007 elsevier b v all rights reserved

impacts of orbital forcing and atmospheric carbon dioxide on miocene ice sheet expansion the processes causing the middle miocene global cooling which marked the earth s final transition into an icehouse climate about 13 9 million years ago myr ago 1 4 remain enigmatic tectonically driven circulation changes 5 6 and variations in atmospheric carbon dioxide levels 7 8 have been suggested as driving mechanisms but the lack of adequately preserved sedimentary successions has made rigorous testing of these hypotheses difficult here we present high resolution climate proxy records covering the period from 14 7 to 12 7 million years ago from two complete sediment cores from the northwest and southeast subtropical pacific ocean using new chronologies through the correlation to the latest orbital model 9 we find relatively constant low summer insolation over antarctica coincident with declining atmospheric carbon dioxide levels at the time of antarctic ice sheet expansion and global cooling suggesting a causal link we surmise that the thermal isolation of antarctica played a role in providing sustained long term climatic boundary conditions propitious for ice sheet formation our data document that antarctic glaciation was rapid taking place within two obliquity cycles and coincided with a striking transition from obliquity to eccentricity as the drivers of climatic change

carbon and oxygen isotopes of bulk carbonate in sediment deposited beneath the eastern equatorial pacific over the last 8 million years to improve the understanding and utility of bulk carbonate stable carbon and oxygen isotope measurements we examine sediment from cores in the eastern equatorial pacific that span the last 8ma we measured c 13 and o 18 in 791 samples from integrated ocean drilling program site u1338 and deep sea drilling project site 573 both located close to the pacific equator in 100 samples we measured c 13 and o 18 on isolated 63 mu m and 38 mu m fractions which concentrates calcareous nannofossil carbonate and progressively excludes foraminiferal carbonate bulk carbonate c 13 and o 18 records are similar to published records from other sites drilled near the equator and seem to reflect mixed layer conditions albeit with some important caveats involving the precipitation of calcite by coccolithophores the comparatively lower c 13 and o 18 of the 63 mu m and 38 mu m fractions in sediments younger than 4 4ma is attributed to an increase in deep dwelling planktic foraminifera material in bulk carbonate shifting the bulk isotopic signals toward higher values bulk carbonate c 13 is similar over 2500km along the pacific equator suggesting covarying concentrations and c 13 of dissolved inorganic carbon within surface waters since 8ma greater bulk sediment c 13 and o 18 higher sedimentation rates and low content of coarse material suggest intensified wind driven upwelling and enhanced primary productivity along the pacific equator between 8 0 and 4 4ma although a full understanding of bulk carbonate records will require extensive future work