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

automated composite depth scale construction and estimates of sediment core extension 1 a composite section which reconstructs a continuous stratigraphic record from cores of multiple nearby holes and its associated composite depth scale are important tools for analyzing sediment recovered from a drilling site however the standard technique for creating composite depth scales on drilling cruises does not correct for depth distortion within each core additionally the splicing technique used to create composite sections often results in a 10 15 offset between composite depths and measured drill depths we present a new automated compositing technique that better aligns stratigraphy across holes corrects depth offsets and could be performed aboard ship by analyzing 618 cores from seven ocean drilling program odp sites we estimate that 80 of the depth offset in traditional composite depth scales results from core extension during drilling and extraction average rates of extension are 12 4 1 5 for calcareous and siliceous cores from odp leg 138 and 8 1 1 1 for calcareous and clay rich cores from odp leg 154 also average extension decreases as a function of depth in the sediment column suggesting that elastic rebound is not the dominant extension mechanism

linear and non linear response of late neogene glacial cycles to obliquity forcing and implications for the milankovitch theory constraints are given for the geometry and time lags of the prominent obliquity paced glacial stages 100 98 and 96 which mark a major phase in northern hemisphere nh glaciations during the late pliocene 2 56 2 4 ma ago for this purpose a high resolution benthic 18o record was constructed from the astronomically tuned mediterranean odp site 967 and decomposed into an ice volume and an annual nh 40 80 n temperature component using an inverse modelling approach our results indicate that the dominant 41 ky component in 18o lags obliquity by 6 5 0 6 ky which approximates late pleistocene estimates maximum minimum ice volume growth occurred in phase with obliquity minima maxima which invoked low high total summer energy conditions that reduced increased ice sheet ablation sea level dropped 60 70 m during full glacial conditions similar to late pleistocene ice core and marine 18o records our late pliocene 18o record reveals significant power at 28 ky which appear to be bound to the major glacial terminations we argue that this beat most likely reflects the sum frequency of the 41 ky prime and its multiples of 82 and 123 ky supporting the theory that the late neogene glacial cycles are primarily determined by the linear and non linear response mechanisms of the ice sheets to the obliquity forcing evidence for such a scenario may come from the alignment between the devils hole 18o chronology and the sum of the filtered linear and non linear obliquity related components of late pleistocene climate records suggesting that the 28 ky beat is intrinsic to the climate system or at least an important constituent of the annual high latitude nh temperature changes that have set the stage for the geometry of the glacial interglacial variability throughout the course of the pliocene and pleistocene

the late miocene to early pliocene climate transition in the southern ocean abstract we have constructed a relatively high resolution 5 10 kyr globigerina bulloides stable isotope record from ocean drilling program site 1088 south atlantic sector of the southern ocean together with the two published records from the high latitudes of the southern hemisphere that each span at least a portion of this interval of time we begin to assemble a spatial picture of sea surface hydrography during the late miocene early pliocene climate transition 8 3 ma we hypothesize that meridional movements in the frontal zones polar front and subtropical convergence with their impact on hydrographic gradients would be evident in temporal changes in the planktonic foraminiferal stable isotopic gradients among the sites our results indicate that there were no large changes in 18o gradients during the interval of study relatively small changes such as a gradual warming of about 3 4 c at site 1088 during the late miocene fall well within the realm of modern ocean seasonality in this region the early pliocene 18o records largely parallel the global decrease defined by benthic foraminifera in the deep pacific suggesting no amplified warming of surface waters in the southern ocean the planktic 13c records vary positively with the 18o records both in the long term trend as well as on the scale of individual data points we believe that this association reflects primarily reduced nutrient utilization during warmer intervals although a small component of the signal would also be consistent with reduced carbon isotope fractionation during air sea gas exchange during relatively warmer conditions if correct this argument would have implications for carbon cycling during the early pliocene and would support scenarios of elevated co2 levels

relative partitioning of acoustic and seismic energy during strombolian eruptions the relative elastic energy partitioning during strombolian eruptions can be estimated from acoustic pressure and seismic velocity records we outline methods for determining acoustic and seismic energies for sensors deployed within several kilometers of erupting vents we use these energies to introduce the concept of the volcano acoustic seismic ratio g or vasr which is the ratio of elastic energy propagated through the atmosphere and into the earth eruption vasr is a physical diagnostic of explosive degassing that is appropriate for comparing eruption mechanisms at individual and between various volcanoes here we assess acoustic and seismic energies and corresponding vasr for discrete strombolian explosive events at karymsky and erebus volcanoes we attribute the relatively high and stable vasr at erebus g 8 standard deviation 41 to repeatable source conditions occurring at the surface of a persistent lava lake with accompanying strong coupling to the atmosphere lower and more variable vasr at karymsky g 0 18 in 1998 to g 1 51 in 1991 with standard deviations of 93 and 313 respectively is attributed to changing conditions within a narrow partially choked conduit variable seismoacoustic energy partitioning for karymsky as manifested by the large vasr standard deviation suggests that conduit conditions affecting vasr which include magma properties conduit obstruction or fragmentation depths can evolve both during the course of an explosion and between successive events d 2005 elsevier b v all rights reserved

hydrodynamic aspects of caldera forming eruptions numerical models comparison of results from a two dimensional numerical eruption simulation kachina to calculations based upon a shock tube analog supports the conclusion that the hydrodynamics during the initial minutes of large caldera forming ash flow eruptions may be dominated by blast wave phenomena field evidence for this phenomenology is pyroclastic surge deposits commonly occurring both directly below caldera related ash flow sheets on top of a preceding plinian fall deposit ground surge and separating individual ash flow units we model the eruption of the tshirege member of the bandelier tuff 1 1 ma b p from the valles caldera new mexico in the model a magma chamber at 100 mpa 1 kbar and 800 c is volatile rich with an average h2o abundance above saturation greater than 8 7 wt increasing to nearly 100 wt near the very top of the chamber using a shock tube analogy decompression of the chamber through a wide open dikelike vent 0 1 km wide and 1 to 5 km long forms a shock wave of 3 mpa 3o atm with a velocity greater than 1 0 km s 1 steady flow of material erupted from the vent begins after 20 to 100 s based upon a 7 km depth from the ground surface to a reflective density boundary in the chamber and a rarefaction wave velocity of 100 to 600 m s 1 the velocity of the ash front behind the shock wave is 300 to 500 m s 1 the shock tube model serves as a basis to evaluate the consistency of the kachina code results which are similar to a one dimensional problem along the symmetry axis the results of the kachina simulation show in some detail the effect of multiple reservoir rarefaction reflections and possibly prandtl meyer expansion in generating compressive wave fronts following the initial shock the rarefaction resonance not only prolongs unsteady flow in the vent but tends to promote surging flow of ash behind the leading shock furthermore these results are consistent with a blast wave characterized as a shock front followed by one or more pulses of entrained ash the blast wave shocks ambient air to higher pressures and temperatures the magnitudes of which depend strongly on the initial chamber overpressure distance and direction from the vent in consideration of volcanic hazards our numerical model shows that a shock wave compressed the atmosphere to pressures of 0 2 to 0 7 mpa 2 7 atm and temperatures of 200 to 300 c for distances to 10 km from the bandelier vent s

magnetic analysis of some large seamounts in the north atlantic abstract an analysis of the magnetic structure of plato and atlantis seamounts in the north atlantic was made using the phase shifting technique of schouten 9 the possibility of distinguishing the age of the seamounts from the age of their adjacent seafloor using magnetic data was investigated the method described proved simple and effective but showed that age determinations cannot reliably be made from magnetics for the seamounts in question since the variation of the position of the palaeomagnetic poles during the cainozoic is not large enough to produce appreciable phase shift differences in this part of the atlantic the polarization is normal since smaller seamounts also show a predominance of normal magnetization this may mean that viscous remanent magnetization vrm dating from the present normal period the brunhes is involved vrm may lead to strong magnetizations in coarse grained rock as shown by dsdp results this may result into a general overprinting of thermoremanence trm by vrm thus glossing over reversed thermoremanent polarizations large seamounts thus may appear as normally polarized bodies the statistics of reversely versus normally polarized seamounts may be explained by the same effect this implies that palaeomagnetic pole and age determinations based on apparent magnetization directions modelled from the magnetic anomaly pattern over seamounts may be inaccurate

application of benthic foraminiferal mg ca ratios to questions of cenozoic climate change abstract we investigate the evolution of cenozoic climate and ice volume as evidenced by the oxygen isotopic composition of seawater 18osw derived from benthic foraminiferal mg ca ratios to constrain the temperature effect contained in foraminiferal 18o values we have constructed two benthic foraminiferal mg ca records from intermediate water depth sites ocean drilling program sites 757 and 689 from the subtropical indian ocean and the weddell sea respectively together with the previously published composite record of lear et al science 287 2002 269 272 and the neogene record from the southern ocean of billups and schrag paleoceanography 17 2002 10 1029 2000pa000567 we obtain three almost complete representations of the 18osw for the past 52 myr we discuss the sensitivity of early cenozoic mg ca derived paleotemperatures and hence the 18osw to assumptions about seawater mg ca ratios we find that during the middle eocene 49 40 ma modern seawater ratios yield mg ca derived temperatures that are in good agreement with the oxygen isotope paleothermometer assuming ice free conditions intermediate waters cooled during the middle eocene reaching minimum temperatures by 40 ma the corresponding 18osw reconstructions support ice growth on antarctica beginning by at least 40 ma at the eocene oligocene boundary mg ca ratios and hence temperatures from weddell sea site 689 display a well defined maximum we caution against a paleoclimatic significance of this result and put forth that the partitioning coefficient of mg in benthic foraminifera may be sensitive to factors other than temperature throughout the remainder of the cenozoic the temporal variability among 18osw records is similar and similar to longer term trends in the benthic foraminiferal 18o record an exception occurs during the pliocene when 18osw minima in two of the three records suggest reductions in global ice volume that are not apparent in foraminiferal 18o records which provides a new perspective to the ongoing debate about the stability of the antarctic ice sheet maximum 18osw values recorded during the pleistocene at southern ocean site 747 agree well with values derived from the geochemistry of pore waters schrag et al science 272 1996 1930 1932 further highlighting the value of the new mg ca calibrations of martin et al earth planet sci lett 198 2002 193 209 and lear et al geochim cosmochim acta 66 2002 3375 3387 applied in this study we conclude that the application of foraminiferal mg ca ratios allows a refined view of cenozoic ice volume history despite uncertainties related to the geochemical cycling of mg and ca on long time scales

late miocene stable isotope stratigraphy of se atlantic odp site 1085 relation to messinian events abstract high resolution benthic oxygen isotope and xrf fe and ca records from site 1085 drilled in the mid cape basin odp leg 175 are used to investigate global climate changes during the late miocene in relation to messinian geological events the cyclic fluctuations of the time series at site 1085 enable us to establish a reliable chronology for the time interval 7 3 4 7 ma spectral analysis of the 18o record indicates that the 41 kyr period of orbital obliquity dominates the late miocene record a global climate record was extracted from the oxygen isotopic composition of benthic foraminifera both long and short term variabilities in the climate record are discussed in terms of sea level and deep water temperature changes the time interval 7 3 6 25 ma characterized by low amplitude 18o variations is followed by a period marked by maximum in the 18o values 6 25 5 57 ma at about 5 56 ma a rapid decrease in 18o values is documented that may reflect a warming of deep water temperature associated with a global warming period comparison between the timing of the oceanic isotope events and the chronology of the mediterranean salinity crisis suggest that global eustatic processes were not essential in the mediterranean salinity crisis history from our data we infer that the global warmth documented in the early mid pliocene probably started during the late miocene at 5 55 ma at the same time the onset of evaporite deposition in the central basin of the mediterranean sea took place sharp changes in the sedimentation rates mainly driven by terrigenous input at this site are observed during the messinian stage

model of a strong volcanic blast and a method of estimating the mass ejected summary a hydrodynamic model of a strong volcanic blast associated with large calderaforming ash gas flow eruptions is presented we show that the mass erupted from a vent influences considerably the damping of the shock wave if the ejected mass flux is constant the shock amplitude as a function of distance will decrease slower in accordance with the inverse 4 3 power law than that of an atomic explosion the inverse cube power law from the modelling we extract a relation connecting the evective energy of a blast with the mass ejected during the blast phase the mass released during the largest explosion of the 1883 eruption of krakatau and the lateral blast energy at mount st helens on 1980 may 18 are estimated on the basis of the equation obtained

correlation of late miocene to early pliocene sequences between the mediterranean and north atlantic ocean drilling program odp site 982 in the north atlantic contains a complete latest miocene to early pliocene section that was tuned to the astronomical timescale by correlating the record of gamma ray attenuation gra bulk density to summer insolation at 65 n and the benthic 18o signal to orbital obliquity for the interval from 4 6 to 7 5 ma the astronomical tuning of the site 982 record permits a direct bed to bed correlation to the cyclostratigraphy of messinian sections in the mediterranean krijgsman et al 1999a 2001 the benthic 18o signal at site 982 records a latest miocene glacial period that lasted from 6 26 to 5 50 ma and consisted of 18 glacial to interglacial oscillations that were controlled by the 41 kyr cycle of obliquity although the intensification of glaciation at 6 26 ma may have contributed to the restriction of the mediterranean it preceded the depositional onset of the lower evaporite unit at 5 96 ma by some 300 kyr the transition from stage tg12 to tg11 at 5 5 ma marks the end of the latest miocene glacial period and precedes the miocene pliocene boundary by 170 kyr although benthic 18o values are relatively low and 18o of bulk carbonate reaches a minimum at the miocene pliocene boundary at 5 33 ma there is no single event that would indicate deglaciation and sea level rise as the cause of the reflooding of the mediterranean we conclude that glacioeustatic changes alone were not responsible for either the start or end of evaporite deposition during the messinian suggesting that tectonic or local climate changes in the mediterranean region were the dominant cause s

alkenone and boron based pliocene pco2 records abstract the pliocene period is the most recent time when the earth was globally significantly 3 c warmer than today however the existing pco2 data for the pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo pco2 this disagreement coupled with the general low temporal resolution of the published records does not allow a robust assessment of the role of declining pco2 in the intensification of the northern hemisphere glaciation inhg and a direct comparison to other proxy records are lacking for the first time we use a combination of foraminiferal 11b and organic biomarker alkenone derived carbon isotopes proxies to determine the concentration of atmospheric co2 over the past 5 ma both proxy records show that during the warm pliocene pco2 was between 330 and 400 ppm i e similar to today the decrease to values similar to pre industrial times 275 285 ppm occurred between 3 2 ma and 2 8 ma coincident with the inhg and affirming the link between global climate the cryosphere and pco2

δ13c values of miocene pacific benthic foraminifera correlations with sea level and biological productivity 13 c 12 c ratios of miocene benthic foraminifera from 22 pacific ocean sites vary with time but are similar at almost all sites in any restricted interval 13 c values are correlated with sea levels inferred from onlap offlap curves reflecting the deposition of greater amounts of organic matter on the continental shelves during transgressions differences in 13 c between sites are correlated with local differences in biological productivity in the overlying surface waters 13 c 12 c values of benthic foraminifera show promise as indicators of marine paleoproductivity

middle to late miocene oxygen isotope stratigraphy of odp site 1085 se atlantic new constrains on miocene climate variability and sea level fluctuations abstract the middle miocene 18o increase represents a fundamental change in earth s climate system due to a major expansion and permanent establishment of the east antarctic ice sheet accompanied by some effect of deepwater cooling the long term cooling trend in the middle to late miocene was superimposed by several punctuated periods of glaciations mi events characterized by oxygen isotopic shifts that have been related to the waxing and waning of the antarctic ice sheet and bottom water cooling here we present a high resolution benthic stable oxygen isotope record from odp site 1085 located at the southwestern african continental margin that provides a detailed chronology for the middle to late miocene 13 9 7 3 ma climate transition in the eastern south atlantic a composite fe intensity record obtained by xrf core scanning odp sites 1085 and 1087 was used to construct an astronomically calibrated chronology based on orbital tuning the oxygen isotope data exhibit four distinct 18o excursions which have astronomical ages of 13 8 13 2 11 7 and 10 4 ma and correspond to the mi3 mi4 mi5 and mi6 events a global climate record was extracted from the oxygen isotopic composition both long and short term variabilities in the climate record are discussed in terms of sea level and deep water temperature changes the oxygen isotope data support a causal link between sequence boundaries traced from the shelf and glacioeustatic changes due to ice sheet growth spectral analysis of the benthic 18o record shows strong power in the 400 kyr and 100 kyr bands documenting a paleoceanographic response to eccentricity modulated variations in precession a spectral peak around 180 kyr might be related to the asymmetry of the obliquity cycle indicating that the response of the dominantly unipolar antarctic ice sheet to obliquity induced variations probably controlled the middle to late miocene climate system maxima in the 18o record interpreted as glacial periods correspond to minima in 100 kyr eccentricity cycle and minima in the 174 kyr obliquity modulation strong middle to late miocene glacial events are associated with 400 kyr eccentricity minima and obliquity modulation minima thus fluctuations in the amplitude of obliquity and eccentricity seem to be the driving force for the middle to late miocene climate variability

the decompression of volcanic jets in a crater during explosive volcanic eruptions during explosive volcanic eruptions fragmented silicic magma and volatiles exit the vent with pressures typically in the range 10 100 atm and at the speed of sound of the mixture we show that for magmatic volatile contents no in the range 0 03 no 0 06 this has the approximate value 0 95 0 05 nort 12 where r is the gas constant and t is the eruption temperature this speed is nearly independent of the vent pressure and vent radius by assuming there is negligible mixing with the air we have modelled the decompression of such jets following eruption from the vent for free decompression into the atmosphere the velocity of the decompressed jet has the approximate value 1 85 0 05 nort 12 this is nearly independent of the eruption rate for decompression into a crater the process is more complex at low eruption rates with low vent pressure the material becomes underpressured as it rises in the crater a shock then forms in the crater this recompresses the magma volatile mixture which then issues from the crater as a relatively slow subsonic jet at atmospheric pressure as the eruption rate increases such shocks move towards the top of the crater and ultimately cannot form in the crater instead the material issues from the crater as a high speed supersonic jet the upward thrust provided by the crater walls on this high pressure jet then increases the upward velocity above that of a freely decompressing jet the presence of a crater may therfore cause collapse in relatively small eruptions whereas it may promote formation of buoyant eruption columns at higher eruption rates if a crater grows through erosion during an eruption column collapse will typically ensue even if the mass flux remains steady

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

in search of the late miocene early pliocene biogenic bloom in the atlantic ocean ocean drilling program sites 982 925 and 1088 1 we reconstruct paleoproductivity at three sites in the atlantic ocean ocean drilling program sites 982 925 and 1088 to investigate the presence and extent of the late miocene to early pliocene biogenic bloom from 9 to 3 ma our approach involves construction of multiple records including benthic foraminiferal and caco3 accumulation rates uvigerina counts dissolution proxies and geochemical tracers for biogenic and detrital fluxes this time interval also contains the so called late miocene carbon isotope shift a well known decrease in benthic foraminiferal 13c values we find that the timing of paleoproductivity maxima differs among the three sites at site 982 north atlantic benthic foraminifera and caco3 accumulation were both at a maximum at 5 ma with smaller peaks at 6 ma the paleoproductivity maximum was centered earlier 6 6 6 0 ma in the tropical atlantic site 925 in the south atlantic site 1088 paleoproductivity increased even earlier between 8 2 ma and 6 2 ma and remained relatively high until 5 4 ma we note that there is some overlap between the interval of maximum productivity between sites 925 and 1088 as well as the minor productivity increase at site 982 we conclude that the paleoproductivity results support hypotheses aiming to place the biogenic bloom into a global context of enhanced productivity in addition we find that at all three sites the 13c shift is accompanied by carbonate dissolution this observation is consistent with published studies that have sought a relationship between the late miocene carbon isotope shift and carbonate preservation

no major deglaciation across the miocene pliocene boundary integrated stratigraphy and astronomical tuning of the loulja sections bou regreg area nw morocco 1 an integrated high resolution stratigraphy and orbital tuning is presented for the loulja sections located in the bou regreg area on the atlantic side of morocco the sections constitute the upward continuation of the upper messinian ain el beida section and contain a well exposed continuous record of the interval straddling the miocene pliocene m p boundary the older loulja a section which covers the interval from 5 59 to 5 12 ma reveals a dominantly precession controlled color cyclicity that allows for a straightforward orbital tuning of the boundary interval and for detailed cyclostratigraphic correlations to the mediterranean the high resolution and high quality benthic isotope record allows us to trace the dominantly obliquity controlled glacial history our results reveal that the m p boundary coincides with a minor partly precession related shift to lighter interglacial values in d18o this shift and hence the m p boundary may not correlate with isotope stage tg5 as previously thought but with an extra weak obliquity controlled cycle between tg7 and tg5 consequently the m p boundary and basal pliocene flooding of the mediterranean following the messinian salinity crisis are not associated with a major deglaciation and glacio eustatic sea level rise indicating that other factors such as tectonics must have played a fundamental role on the other hand the onset of the upper evaporites in the mediterranean marked by hyposaline conditions coincides with the major deglaciation step between marine isotope stage tg12 and tg11 suggesting that the associated sea level rise is at least partly responsible for the apparent onset of intermittently restricted marine conditions following the main desiccation phase finally the loulja a section would represent an excellent auxiliary boundary stratotype for the m p boundary as formally defined at the base of the trubi marls in the eraclea minoa section on sicily

late quaternary vegetation dynamics in the southern amazon basin inferred from carbon isotopes in soil organic matter abstract carbon isotopes of soil organic matter som were used to evaluate and establish the chronology of the vegetation dynamics of an ecosystem presently composed of savannas surrounded by forests the study was carried out on a 200 km transect along highway br 319 on the border of amazonas and rondonia states in southern amazon brazil large ranges in 13 c values were observed in som collected from profiles in the savanna 27 to 14 and forest regions 26 to 19 reflecting changing distribution of 13 c depleted c 3 forest and 13 c enriched c 4 savanna vegetation in response to climate change these results indicate that from about 17 000 to 9000 14 c yr b p the study area was covered by forest vegetation between approximately 9000 and 3000 14 c yr b p savanna vegetation expanded at the expense of the forest although the expansion of savanna did not occur with the same intensity along the study transect this process was very clearly registered by 13 c enrichment in the som since 3000 14 c yr b p the carbon isotope data suggest that forested regions have expanded this study adds to the mounting evidence that extensive forested areas existed in the amazon during the last glaciation and that savanna vegetation expanded in response to warm and dry conditions during the early to middle holocene

dynamic behaviour of the east antarctic ice sheet during pliocene warmth warm intervals within the pliocene epoch 5 33 2 58 million years ago were characterized by global temperatures comparable to those predicted for the end of this century1 and atmospheric co2 concentrations similar to today2 3 4 estimates for global sea level highstands during these times5 imply possible retreat of the east antarctic ice sheet but ice proximal evidence from the antarctic margin is scarce here we present new data from pliocene marine sediments recovered offshore of adelie land east antarctica that reveal dynamic behaviour of the east antarctic ice sheet in the vicinity of the low lying wilkes subglacial basin during times of past climatic warmth sedimentary sequences deposited between 5 3 and 3 3 million years ago indicate increases in southern ocean surface water productivity associated with elevated circum antarctic temperatures the geochemical provenance of detrital material deposited during these warm intervals suggests active erosion of continental bedrock from within the wilkes subglacial basin an area today buried beneath the east antarctic ice sheet we interpret this erosion to be associated with retreat of the ice sheet margin several hundreds of kilometres inland and conclude that the east antarctic ice sheet was sensitive to climatic warmth during the pliocene

biogenic sediment regimes in the neogene equatorial pacific iodp site u1338 burial production and diatom community abstract the equatorial pacific experienced significant changes in productivity and microfossil assemblage since 16 million years ago ma we compiled a record of those changes from iodp site u1338 using a reconnaissance of diatom assemblages and high resolution xrf scan chemical profiles 1 2 kyr spacing productivity and caco3 dissolution intervals are defined by sediment component ratios in particular opal clay opal baso4 and caco3 baso4 or caco3 clay there are large abrupt changes in export production in the miocene especially compared to the pleistocene and pliocene and average levels of export production are higher throughout the miocene compared to the pliocene and pleistocene using diatom assemblages and bulk sediment composition the u1338 sediment record is divided into several distinct deposition regimes 1 a middle miocene regime 13 2 ma marked by low diatom numbers and a few short lived productivity intervals 2 a carbonate crash regime distinguished by an older substage 13 2 10 2 ma with relatively high abundances of productivity related diatoms and common productivity related depositional intervals interspersed with caco3 dissolution intervals and a younger carbonate crash substage 10 2 8 0 ma with weaker brief productivity intervals high caco3 dissolution and moderately high numbers of upwelling diatom species 3 the biogenic bloom regime 8 0 4 5 ma marked by extended periods of high opal and caco3 deposition with a maximum between 7 0 and 6 4 ma and 4 a pliocene modern regime 4 5 0 ma with lower productivity and high cyclic caco3 dissolution changes in production and dissolution result from reorganizations of nutrient supply to the equatorial pacific not by higher wind driven upwelling we propose that nutrients were more accessible in the miocene because of a combination of factors including higher organic matter degradation in the upper water column and deep intermediate nutrient pathways that efficiently recycled nutrients to upwelling zones closure of the central american seaway was likely a prime cause of the biogenic bloom

miocene to pliocene development of surface and subsurface temperatures in the benguela current system 1 the initiation of the benguela upwelling has been dated to the late miocene but estimates of its sea surface temperature evolution are not available this study presents data from ocean drilling program odp site 1085 recovered from the southern cape basin samples of the middle miocene to pliocene were analyzed for alkenone based u37k sstuk and glycerol dialkyl glycerol tetraether gdgt based tex86 temptex water temperature proxies in concordance with global cooling during the miocene sstuk and temptex exhibit a decline of about 8 c and 16 c respectively the temperature trends suggest an inflow of cold antarctic waters triggered by antarctic ice sheet expansion and intensification of southern hemisphere southeasterly winds a temperature offset between both proxies developed with the onset of upwelling which can be explained by differences in habitat alkenone producing phytoplankton live in the euphotic zone and record sea surface temperatures while gdgt producing thaumarchaeota are displaced to colder subsurface waters in upwelling influenced areas and record subsurface water temperatures we suggest that variations in subsurface water temperatures were driven by advection of cold antarctic waters and thermocline adjustments that were due to changes in north atlantic deep water formation a decline in surface temperatures an increased offset between temperature proxies and an increase in primary productivity suggest the establishment of the benguela upwelling at 10 ma during the messinian salinity crisis between 7 and 5 ma surface and subsurface temperature estimates became similar likely because of a strong reduction in atlantic overturning circulation while high total organic carbon contents suggest a biogenic bloom in the pliocene the offset between the temperature estimates and the cooling trend was reestablished

coupled productivity and carbon isotope records in the southwest pacific ocean during the late miocene early pliocene biogenic bloom biogenic components of sediment accumulated at high rates beneath frontal zones of the indian and pacific oceans during the late miocene and early pliocene the 13c of bulk and foraminiferal carbonate also decreased during this time interval although the two observations may be causally linked and signify a major perturbation in global biogeochemical cycling no site beneath a frontal zone has independent records of export production and 13c on multiple carbonate phases across the critical interval of interest deep sea drilling project dsdp site 590 lies beneath the tasman front tf an eddy generating jetstream in the southwest pacific ocean to complement previous 13c records of planktic and benthic foraminifera at this location late neogene records of caco3 mass accumulation rate mar ca ti ba ti al ti and of bulk carbonate and foraminiferal 13c were constructed at site 590 the 13c records include bulk sediment bulk sediment fractions 63 m and 5 25 m and the planktic foraminifera globigerina bulloides globigerinoides sacculifer with and without sac and orbulina universa using current time scales caco3 mars ca ti al ti and ba ti ratios are two to three times higher in upper miocene and lower pliocene sediment relative to overlying and underlying units a significant decrease also occurs in all 13c records all evidence indicates that enhanced export production the biogenic bloom extended to the southwest pacific ocean between ca 9 and 3 8 ma and this phenomenon is coupled with changes in 13c the chron c3ar carbon shift however caco3 mars peak ca 5 ma whereas elemental ratios are highest ca 6 5 ma foraminiferal 13c starts to decrease ca 8 ma whereas bulk carbonate 13c begins to drop ca 5 6 ma temporal discrepancies between the records can be explained by changes in the upwelling regime at the tf perhaps signifying a link between changes in ocean atmosphere circulation change and widespread primary productivity

the initial giant umbrella cloud of the may 18th 1980 explosive eruption of mount st helens abstract the initial eruption column of may 18th 1980 reached nearly 30 km altitude and released 10 17 joules of thermal energy into the atmosphere in only a few minutes ascent of the cloud resulted in forced intrusion of a giant umbrella shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m s the mushroom cloud expanded 15 km upwind forming a stagnation point where the radial expansion velocity and wind velocity were equal the cloud was initiated when the pyroclastic blast flow became buoyant the flow reduced its density as it moved away from the volcano by decompression by sedimentation and by mixing with and heating the surrounding air observations indicate that much of the flow covering an area of 600 km 2 became buoyant within 1 5 minutes and abruptly ascended to form the giant cloud calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant assuming an initial temperature of 450 c and a magmatic origin for the explosion these calculations indicate that the flow became buoyant when its temperature was approximately 150 c and the flow consisted of a mixture of 3 25 10 11 kg of pyroclasts and 5 0 10 11 kg of air if sedimentation is considered these figures reduce to 1 1 10 11 kg of pyroclasts and 1 0 10 11 kg of air

orbitally paced climate oscillations across the oligocene miocene boundary the late oligocene and early miocene periods some 21 to 27 million years ago have generally been viewed as times of moderate global warmth and ice free conditions yet several lines of evidence suggest that this interval was punctuated by at least one and possibly several episodes of high latitude cooling and continental glaciation1 2 3 here we present stable isotope and per cent coarse fraction data from an equatorial western atlantic deep sea sediment core that provide high resolution records of the climate variability across the oligocene miocene transition 22 5 25 7 million years ago a strong 40 kyr periodicity in the oxygen isotope record is consistent with a high latitude orbital obliquity control on ice volume and temperature orbital influences are also apparent at precession and eccentricity frequencies including a series of 400 kyr oscillations that culminate in distinct maxima at the oligocene miocene boundary about 23 7 million years ago covariance between the carbon and oxygen isotope records suggests that the oceanic carbon cycle may have contributed to global cooling during the 400 kyr cycles particularly at the oligocene miocene boundary

late miocene through early pliocene deep water circulation and climate change viewed from the sub antarctic south atlantic benthic foraminiferal stable isotope records for the past 11 myr from a recently drilled site in the sub antarctic south atlantic site 1088 ocean drilling program leg 177 41 s 15 e 2082 m water depth provide for the first time a continuous long term perspective on deep water distribution patterns and southern ocean climate change from the late miocene through the early pliocene i have compiled published late miocene through pliocene stable isotope records to place the new south atlantic record in a global framework carbon isotope gradients between the north atlantic south atlantic and pacific indicate that a nutrient depleted watermass probably of north atlantic origin reached the sub antarctic south atlantic after 6 6 ma by 6 0 ma the relative proportion of the northernprovenance watermass was similar to today and by the early pliocene it had increased to greater than the modern proportion suggesting that thermohaline overturn in the atlantic was relatively strong prior to the early pliocene interval of inferred climatic warmth site 1088 oxygen isotope values display a two step increase between v7 4 ma and 6 9 ma a trend that parallels a published n 18 o record of a site on the atlantic coast of morocco this is perhaps best explained by a gradual cooling of watermasses that were sinking in the southern ocean i speculate that relatively strong thermohaline overturn at rates comparable to the present day interglacial interval during the latest miocene may have provided the initial conditions for early pliocene climatic warmth the impact of an emerging central american seaway on atlantic pacific ocean upper water exchange may have been felt in the north atlantic beginning in the latest miocene between 6 6 and 6 0 ma which would be v1 5 myr earlier than previously thought 9 2002 elsevier science b v all rights reserved

middle to late miocene stepwise climate cooling evidence from a high resolution deep water isotope curve spanning 8 million years 1 we present high resolution 2 3 kyr benthic foraminiferal stable isotopes in a continuous well preserved sedimentary archive from the west pacific ocean ocean drilling program site 1146 which track climate evolution in unprecedented resolution over the period 12 9 to 8 4 ma we developed an astronomically tuned chronology over this interval and integrated our new records with published isotope data from the same location to reconstruct long term climate and ocean circulation development between 16 4 and 8 4 ma this extended perspective reveals that the long eccentricity 400 kyr cycle is prominently encoded in the 13c signal over most of the record reflecting long term fluctuations in the carbon cycle the 18o signal closely follows variations in short eccentricity 100 kyr and obliquity 41 kyr in particular the obliquity cycle is prominent from 14 6 to 14 1 ma and from 9 8 to 9 2 ma when high amplitude variability in obliquity is congruent with low amplitude variability in short eccentricity the 18o curve is additionally characterized by a series of incremental steps at 14 6 13 9 13 1 10 6 9 9 and 9 0 ma which we attribute to progressive deep water cooling and or glaciation episodes following the end of the miocene climatic optimum on the basis of 18o amplitudes we find that climate variability decreased substantially after 13 ma except for a remarkable warming episode at 10 8 10 7 ma at peak insolation during eccentricity maxima 100 and 400 kyr this transient warming associated with a massive negative carbon isotope shift is reminiscent of intense global warming events at eccentricity maxima during the miocene climatic optimum

punctuated evolution of global climate cooling during the late middle to early late miocene high resolution planktonic foraminiferal and oxygen isotope records from the mediterranean high resolution planktonic foraminiferal and oxygen isotope records are presented from a mediterranean deep marine succession dated astronomically between 12 12 and 9 78 ma planktonic and benthic oxygen isotope records are punctuated by two episodes of 18o increase which have astronomical ages of 11 4 and 10 4 ma and correspond to the mi5 and mi6 events of miller et al 1991a these ice growth events coincide with low amplitude variations in the 1 2 myr obliquity cycle and are accompanied by significant faunal changes in the mediterranean such as the arrival of neogloboquadrinids the increase in abundance of the g apertura g obliquus group and the areal differentiation between n atlantica and n acostaensis short term variations in the planktonic foraminiferal and oxygen isotope records correspond to dominantly precession controlled sedimentary cycles features of the sapropel grey marl layers indicate that the short term astronomically controlled circum mediterranean climate changes remained basically the same over the last 12 myr

the 1883 eruption of krakatau the 1883 eruption of krakatau was a modest ignimbrite forming event the deposits are primarily coarse grained dacitic non welded ignimbrite large explosions produced pyroclastic flows that entered the sea generating destructive tsunami grain size studies of the ignimbrite suggest that these explosions were not driven by magma seawater interaction the total bulk volume of pyroclastic deposits including co ignimbrite ash is estimated to be 18 21 km3

late miocene atmospheric co2 concentrations and the expansion of c4 grasses the global expansion of c4 grasslands in the late miocene has been attributed to a large scale decrease in atmospheric carbon dioxide co2 concentrations this triggering mechanism is controversial in part because of a lack of direct evidence for change in the partial pressure of co2 pco2 and because other factors are also important determinants in controlling plant type distributions alkenone based pco2 estimates for the late miocene indicate that pco2 increased from 14 to 9 million years ago and stabilized at preindustrial values by 9 million years ago the estimates presented here provide no evidence for major changes in pco2 during the late miocene thus c4 plant expansion was likely driven by additional factors possibly a tectonically related episode of enhanced low latitude aridity or changes in seasonal precipitation patterns on a global scale or both

late neogene benthic stable isotope record of ocean drilling program site 999 implications for caribbean paleoceanography organic carbon burial and the messinian salinity crisis we report on epibenthic foraminiferal 18o and 13c and percentage coarse fraction records from caribbean ocean drilling program odp site 999 12 44 n 78 44 w water depth 2828 m spanning the interval from 8 5 to 5 3 ma low epibenthic 13c values and low amounts of sand sized particles mostly foraminifer shells indicate a poorly ventilated deep caribbean throughout the late miocene at this time the deep caribbean was dominated by a nutrient rich and corrosive water mass a generally constant 13c gradient between the caribbean and deep atlantic records during the late miocene suggests that the fluctuations in 13c reflect global changes in 13c of the dissolved inorganic carbon due to varying erosion of organic carbon from terrigenous soils and shelf sediments the observed 100 kyr cyclicity of epibenthic 13c is in good accordance with the variability of terrigenous input to the equatorial atlantic as recorded by magnetic susceptibility records of the ceara rise however some short term gradient changes between 7 0 and 4 5 ma indicate a poorer ventilation of the deep atlantic related to a reduced production of deep water in the atlantic the messinian salinity crisis between 6 0 and 5 3 ma did not affect the intermediate to deep water gradient between the caribbean and the atlantic comparison to the bahama platform record of odp site 1006 however indicates a poorer ventilation of the shallower northern caribbean basins coincident with the isolation of the mediterranean sea

new evidence for changes in plio pleistocene deep water circulation from southern ocean odp leg 177 site 1090 abstract changes in atlantic deep water circulation were reconstructed by comparing the benthic foraminiferal 13c record at odp site 1090 in the south atlantic with similar records from the north atlantic sites 982 607 925 929 and deep pacific site 849 oceans important deep water circulation changes occurred in the early pleistocene at 1 55 myr and during the mid pleistocene transition at 0 9 myr at 1 55 myr glacial 13c values in the southern ocean became significantly lower than those in the deep pacific establishing a pattern that persisted throughout the late pleistocene we propose that the lowering of 13c values of southern component water scw at this time resulted from expansion of sea ice and reduced ventilation of deep water during glacial periods after marine isotope stage 52 accompanying this change in southern ocean deep water circulation was enhanced interhemispheric coupling between the north and south atlantic after 1 55 myr at 0 9 myr the magnitude of glacial to interglacial variability in 13c increased and shifted to a longer frequency 100 kyr along with oceanic 18o ice volume calculation of percent northern component water ncw using site 1090 as the scw end member yielded 20 30 less reduction of ncw during glacial periods of the late pleistocene also a trend toward reduced glacial suppression of ncw during the past 400 kyr is not evident the apparent decoupling of ice volume and deep water circulation reported previously may be an artifact of using a pacific rather than a southern ocean carbon isotopic record to calculate past mixing ratios of ncw and scw

evaluating climatic response to external radiative forcing during the late miocene to early pliocene new perspectives from eastern equatorial pacific iodp u1338 and north atlantic odp 982 locations orbital scale climate variability during the latest miocene early pliocene is poorly understood due to a lack of high resolution records spanning 8 0 3 5 ma which resolve all orbital cycles assessing this variability improves understanding of how earth s system sensitivity to insolation evolves and provides insight into the factors driving the messinian salinity crisis msc and the late miocene carbon isotope shift lmcis new high resolution benthic foraminiferal cibicidoides mundulus 18o and 13c records from equatorial pacific international ocean drilling program site u1338 are correlated to north atlantic ocean drilling program site 982 to obtain a global perspective four long term benthic 18o variations are identified the tortonian messinian miocene pliocene and early pliocene oxygen isotope lows 8 7 5 9 4 9 and 4 8 3 5 ma and the messinian oxygen isotope high moh 7 5 9 ma obliquity paced variability dominates throughout except during the moh eleven new orbital scale isotopic stages are identified between 7 4 and 7 1 ma cryosphere and carbon cycle sensitivities estimated from 18o and 13c variability suggest a weak cryosphere carbon cycle coupling the msc termination coincided with moderate cryosphere sensitivity and reduced global ice sheets the lmcis coincided with reduced carbon cycle sensitivity suggesting a driving force independent of insolation changes the response of the cryosphere and carbon cycle to obliquity forcing is established defined as earth system response esr observations reveal that two late miocene early pliocene climate states existed the first is a prevailing dynamic state with moderate esr and obliquity driven antarctic ice variations associated with reduced global ice volumes the second is a stable state which occurred during the moh with reduced esr and lower obliquity driven variability associated with expanded global ice volumes

late miocene early pliocene stratigraphy and paleoceanography of the south atlantic and southwest pacific oceans a synthesis the stratigraphy and paleoceanography of the late miocene and early pliocene have been examined at six sites in the south atlantic and southwest pacific oceans deep sea drilling project dsdp sites 284 516a 519 588 and 590 and two piston cores from chain cruise 115 a consistent stratigraphy was developed among sites using graphic correlation which resulted in age models for all sites that are tied to the revised paleomagnetic time scale of berggren et al 1985 applying these chronologies we assessed latitudinal and interocean contrasts in the stratigraphic ranges of late miocene early pliocene planktonic foraminiferal and nanno fossil datums salient stratigraphic results include 1 the last appearance datum lad of globoquadrina dehiscens is a late miocene 6 4 ma event in the subtropics and is not useful for the placement of the miocene pliocene m p boundary in this biogeographic province 2 the first appearance datum fad of globorotalia crassaformis occurred at 5 1 ma in the south atlantic near the m p boundary suggesting that gr crassaformis may have first evolved in the south atlantic and later migrated to other regions 3 in the southwest pacific the fads of gr margaritae 5 97 ma gr puncticulata 5 09 ma and gr crassaformis 4 87 ma are significantly time transgressive between temperate and warm subtropical regions time lags of 1 0 m y were required for these species to adapt to physical and or biotic conditions peripheral to their endemic biogeographic provinces 4 between the subtropics of the south atlantic and southwest pacific many planktonic foraminiferal datums fad of dentogloboquadrina altispira gr cibaoensis gr conomiozea gr margaritae and gq dehiscens and lad of gr cibaoensis markedly depart from the correlation suggested by magnetostratigraphy indicating that these datum levels are unreliable for correlation between these ocean basins 5 in contrast available calcareous nannofossil datum levels fall on or near the paleomagnetic correlation line indicating synchroneity of events within the subtropics 6 biostratigraphic magnetic and 87sr 86sr correlation between sites 588 and 519 and the m p neostratotype at capo rossello sicily suggests that the base of the zanclean stratotype occurs at 5 1 5 0 ma in the lower reversed subchron of the gilbert about 2 3 105 years above the gilbert chron 5 boundary oxygen isotopic results from dsdp sites 284 519 and ch115 piston cores confirm a prolonged benthic 18o increase in the latest miocene between 5 6 and 5 0 ma as originally proposed by shackleton and kennett 1975 at dsdp site 588 the benthic 18o record in the latest miocene is marked by high frequency fluctuations with amplitude variations of 0 5 and a long period wavelength component of 400 000 years maximum 18o values however occurred during the late miocene kapitean stage between 5 5 and 5 1 ma the late miocene 18o changes resulted from mid and high latitude cooling and pulses of ice sheet expansion and contraction glacial events were most intense during the latest miocene kapitean stage and occurred at 5 50 5 35 ma and at 5 10 ma glacial events are estimated to have lowered sea level by 40 to 60 m and contributed to the isolation and desiccation of the mediterranean basin during the late messinian interglacial conditions prevailed at 5 2 ma and between 5 0 and 4 1 ma ihe early pliocene the beginning of the pliocene was marked by changes in many proxy climatic indicators at all sites suggesting a prolonged interval of warm interglacial conditions between 5 0 and 4 1 ma during the earliest pliocene supplementary tables are available with entire article onmicrofiche order from american geophysical union 2000florida avenue n w washington dc 20009 documentp86 001 2 50 payment must accompany order

rise and fall of the paratethys sea during the messinian salinity crisis abstract extremely thick evaporite units were deposited during the so called messinian salinity crisis msc 5 96 5 33 ma in a deep mediterranean basin that was progressively disconnected from the atlantic ocean a crucial but still poorly understood component in messinian evaporite models is the connectivity between mediterranean and paratethys i e the former black sea domain inadequate stratigraphic correlations and insufficient age control for paratethys sediments have so far hampered a thorough understanding of hydrological fluxes and paleoenvironmental changes here we present a new chronology for the eastern paratethys by integrating biostratigraphic and paleomagnetic data from mio pliocene sedimentary successions of romania and russia we show that a major flooding event from the mediterranean surged the paratethys basins at the maeotian pontian boundary 6 04 million years ago this indicates that sea level in both mediterranean and paratethys was high at the beginning of the msc we argue in favor of changes in paratethys mediterranean connectivity to initiate the msc in combination with elusive tectonic processes in the gibraltar arc a subsequent fall of paratethyan water level closely coincides to the mediterranean isolation event corresponding in age to the glacial cycles tg12 14 5 60 5 50 myr in the latest messinian a major climate change towards more humid conditions produced a positive hydrological balance and likely a transgression in the paratethys although our stratigraphic resolution cannot exclude a possible relation to the pliocene flooding of the mediterranean here

paleoceanographic changes in the eastern equatorial pacific over the last 10 myr to examine the late neogene evolution of tropical pacific oceanography we determined multiple geochemical proxy records for temperature u 37 k and tex86h indices and primary productivity algal biomarkers and diol indices in sediments recovered at odp site 1241 in the east equatorial pacific eep spanning a record of the last 10 myr the tex86h temperatures are lower than those recorded by u 37 k indices exhibiting large fluctuations and suggesting strong warming during the mid pliocene warm period mpwp 4 5 3 2 ma and significantly colder temperature during the late miocene cooling period 7 5 ma and after the middle pliocene warm period mpwp such variations could reflect changes in the eep thermocline temperatures but we suggest that they instead reflect changes in the depth of export production of glycerol dialkyl glycerol tetraether lipids in response to changes in the upper ocean structure a combination of temperature records inferred to represent different and likely varying depths in the water column as well as algal biomarker records for export production and ecosystem structure suggest that both productivity and inference upwelling were reduced in the eep during warmer periods such as the mpwp and prior to 7 ma in contrast stronger upwelling conditions and associated increased productivity likely prevailed from 7 to 5 ma and for the past 3 myr both corresponding to globally cool intervals a further increase in eep productivity occurred at ca 1 8 ma coincident with the development of the e w pacific sst gradient these results confirm previous work that protracted el nino like conditions prevailed during warmer intervals of the pliocene before ultimately descending into the current climate state

miocene deepwater oceanography a global synthesis of miocene benthic foraminiferal carbon and oxygen isotopic and faunal abundance data indicates that miocene thermohaline circulation evolved through three regimes corresponding approximately to early middle and late miocene times there is evidence for major qualitative differences between the circulation of the modern ocean and the miocene ocean prior to 11 ma the 13c 12c ratios of the benthic foraminifera cibicidoides are interpreted in terms of water mass aging i e the progressive depletion of dissolved o2 and lowering of 13c values as the result of oxidation of organic matter as water flows further from its sources at the surface of the oceans both isotopic and faunal data indicate that the early miocene regime from 22 to 15 ma was the most different from today s during that interval intermediate and deep waters of both the atlantic and the pacific oceans aged in a northward direction and the intermediate waters of the indian the south atlantic and the south pacific oceans were consistently the youngest in the global ocean we speculate that early miocene global thermohaline circulation may have been strongly influenced by the influx of warm saline water tethyan indian saline water from the tethys into the northern indian ocean the isotopic and faunal data suggest that flow from the tethyan region into the indian ocean diminished or terminated at about 14 ma isotopic and faunal data give no evidence for north atlantic deep water nadw formation prior to about 14 5 ma with the exception of a brief episode in the early miocene from 14 5 to 11 ma nadw formation was weak and circumpolar and antarctic water flooded the deep south atlantic and south pacific as the antarctic ice cap grew from about 10 ma to the end of the miocene thermohaline circulation resembled the modern circulation in many ways in latest miocene time 6 to 5 ma circulation patterns were very similar to today s except that nadw formation was greatly diminished the distribution pattern of siliceous oozes in miocene sediments is consistent with our proposed reconstruction of thermohaline circulation major changes which occurred in circulation during the middle miocene were probably related to the closing of the tethys and may have contributed to rapid middle miocene growth of the antarctic ice cap appendices 1 4 6 and 7 are available withentire article on microfiche order fromamerican geophysical union 2000 floridaavenue n w washington dc 20009 document 88p 002 5 00 payment mustaccompany order

late pleistocene paleoclimatology of the central equatorial pacific sea surface response to the southeast trade winds proxy indicators of sea surface temperature and equatorial divergence based on radiolarian assemblage data and of trade wind intensity based on eolian grain size data show similar aspects of variability during the late pleistocene all indicators fluctuate at higher frequencies than the 100 000 year glacial interglacial cycle display reduced amplitude variations since 300 000 years ago exhibit a change in the record character at about 300 000 years ago the mid brunhes climatic event and have higher amplitude variations in sediments 300 000 850 000 years old time series analyses were conducted to determine the spectral character of each record 18o of planktonic foraminifer sea surface temperature values equatorial divergence indicators and wind intensity indicators and to quantify interrecord coherence and phase relationships the record was divided at the 300 000 year clear change in climatic variability nonstationarity the 18o based time scale is better lower in the core so our spectral analyses concentrated on the interval from 402 000 774 000 years the 18o spectra show 100 000 and 41 000 year power in the younger portion 0 300 000 years and 100 000 41 000 and 23 000 year power in the older interval all highly coherent and in phase with the specmap average stacked isotope record unlike the isotope record the dominant period in both the eolian grain size and equatorial divergence indicators is 31 000 years this period is also important in the sea surface temperature signal where the dominant spectral peak is 100 000 years the 31 000 year spectral component is coherent and in phase between the eolian and divergence records confirming the link between atmospheric and ocean surface circulation for the first time in the paleoclimate record since the 31 000 year power appears in independent data sets within this core and also appears in other equatorial records j imbrie personal communication 1987 we assume it to be real and representative of both a nonlinear response to orbital forcing possibly a combination of orbital tilt and eccentricity and some resonance phenomenon required to amplify the response at this period so that it appears as a dominant frequency component the mid brunhes climatic event is an important aspect of these records but its cause remains unknown

the closing of a seaway ocean water masses and global climate change the late neogene witnessed various major paleoceanographic changes that culminated in intense northern hemisphere glaciation nhg the cause and effects of these changes are still debated we use a multiproxy approach to determine the relative timing of the closure of the panama gateway changes in atlantic circulation global cooling and ice sheet growth benthic foraminiferal mg ca records from a pacific and an atlantic site have been produced and are interpreted in terms of bottom water temperatures these mg temperature records are combined with published benthic 13c 18o and erosion records to reconstruct the flow of proto north atlantic deep water proto nadw over the past 12 ma the results suggest that between 12 5 and 10 5 ma and again between about 8 5 and 6 ma a nutrient depleted water mass that was colder by 1 2 c and fresher than the intervening deep water mass filled the atlantic basin this proto nadw became warmer by 1 c and saltier between 6 and 5 ma coincident with the restriction of surface water flow through the central american seaway the mg temperature records define a subsequent global cooling trend of 3 5 c between 5 ma and today early nhg in the late miocene was perhaps related to the formation of the relatively cold fresh proto nadw the formation of the warmer and saltier proto nadw in the early pliocene may have initially limited northern hemisphere ice growth however the increased moisture released at high northern latitudes associated with formation of warm proto nadw coupled with the global temperature decrease of deep and hence polar surface waters likely helped initiate the intense nhg of the plio pleistocene

late neogene history of deepwater ventilation in the southern ocean we compiled carbon isotope records from the north atlantic south atlantic and pacific oceans to estimate changes in vertical and interbasinal 13c gradients for the last 9 myr benthic 13c values of deep water in the south atlantic decreased in a series of steps at 7 2 75 and 1 55 ma away from the north atlantic and toward the pacific the benthic 13c of intermediate water in the south atlantic evolved differently from deep waters resulting in an increase in the intermediate to deep 13c gradient 13cid the 13cid increased in steps at 2 75 and 1 55 ma marking the development and intensification of a chemical divide in the atlantic ocean between well ventilated intermediate water and poorly ventilated deep water we suggest these changes in interbasinal and vertical gradients resulted from decreasing northern component water and reduced ventilation of southern component water scw the latter was caused by several interrelated processes in antarctic sources areas including increased sea ice cover enhanced surface water stratification decreased ekman induced upwelling and reduced vertical mixing across the thermocline because antarctic surface water processes and deepwater ventilation rates ultimately influence atmospheric co2 concentration these processes may have acted as a positive feedback that contributed to the major cooling steps in late neogene climate

middle miocene ice sheet dynamics deep sea temperatures and carbon cycling a southern ocean perspective 1 relative contributions of ice volume and temperature change to the global 1 18o increase at 14 ma are required for understanding feedbacks involved in this major cenozoic climate transition a 3 ma benthic foraminifer mg ca record of southern ocean temperatures across the middle miocene climate transition reveals 2 2 c cooling 14 2 13 8 ma indicating that 70 of the increase relates to ice growth seawater 18o calculated from mg ca and 18o suggests that at 15 ma antarctica s cryosphere entered an interval of apparent eccentricity paced expansion glaciations increased in intensity revealing a central role for internal climate feedbacks comparison of ice volume and ocean temperature records with inferred pco2 levels indicates that middle miocene cryosphere expansion commenced during an interval of southern ocean warmth and low atmospheric pco2 the antarctic system appears sensitive to changes in heat moisture supply when atmospheric pco2 was low suggesting the importance of internal feedbacks in this climate transition

convergent cenozoic co2 history reconstructions of atmospheric carbon dioxide concentrations over the past 65 million years are heading towards consensus it is time for systematic testing of the proxies against measurements and against each other

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

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