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

southern ocean frontal system changes precede antarctic ice sheet growth during the middle miocene abstract the middle miocene climate approximately 14 ma ago was characterized by the glaciation of antarctica deep ocean cooling and variations in the global carbon cycle although the southern ocean underwent significant oceanographic changes there is limited information on their spatial extent and timing however such knowledge is crucial for understanding the role of the southern ocean and the antarctic circumpolar current acc for antarctic glaciation and the coupling between the ocean and continental climate we have reconstructed surface temperatures and seawater oxygen isotopes at ocean drilling program odp site 1092 in the polar frontal zone of the atlantic sector of the southern ocean from foraminiferal oxygen isotopes 18 o and magnesium to calcium ratios mg ca sea surface cooling by 4 c and freshening indicated by the 1 reduction of seawater 18 o 18 o sw at 14 2 ma precede the major step in antarctic ice sheet growth at 13 8 13 9 ma this pattern qualitatively mirrors previous findings from the pacific sector and we interpret the surface hydrographic changes to reflect the circum antarctic northward shift of the southern ocean fronts and specifically at site 1092 the passage of the subantarctic front the magnitude of change in reconstructed 18 o sw requires a 18 o sw salinity gradient significantly higher than the modern value 0 52 and it possibly exceeded 1 1 this implies the polar frontal zone was influenced by freshwater derived from antarctica which in turn confirms higher than modern continental precipitation the latter has previously been suggested to have contributed to antarctic glaciation

oceanographic and climatic consequences of the tectonic evolution of the southern scotia sea basins antarctica abstract the scotia sea is a complex geological area located in the southern ocean which evolution is closely linked to the opening of the drake passage structural highs of continental nature derived from the former continental bridge between south america and the antarctic peninsula surround the abyssal plains of the scotia sea restricting small isolated sedimentary basins along its southern margin morpho structural and seismo stratigraphic analyses of multichannel seismic reflection profiles and additional geophysical data available in the region have been conducted decoding regional and global implications of the basins evolution the main aim of this work is to describe the stratigraphic evolution of the southern scotia sea basins from their opening in the back arc tectonic context of the scotia sea to the last oceanographic changes which have carried on global climatic implications the evolution of the south scotia sea occurred through two major tectonic stages registered in the sedimentary record of the region 1 the end of the subduction in the northwest part of the weddell sea during the early miocene which shortened the back arc subduction trench generating a major change in the regional tectonic field that determined the evolution of the southern basins towards two different types of passive margins magma poor and magma rich and 2 the full development of the southern scotia sea basins during the middle miocene that led to the opening of deep oceanic gateways along the south scotia ridge interplay among tectonics oceanography and climate is proposed to control the regional sedimentary stacking pattern with coeval changes globally identified

oxygen isotope calibration of the onset of ice rafting and history of glaciation in the north atlantic region we report here that dsdp site 552a cored with the hydraulic piston corer on the west flank of rockall bank recovered an undisturbed sequence of alternating white deep sea carbonate oozes and dark coloured layers that are rich in glacial debris oxygen isotope analysis of the sequence together with detailed nannofossil and palaeomagnetic stratigraphy shows that the first major horizon of ice rafting occurred at about 2 4 myr and was preceded by a minor pulse of ice rafting at about 2 5 myr the carbon isotope record shows that the site has been bathed by a water mass of similar characteristics to present day north atlantic deep water at least since 3 5 myr

transition from the cretaceous ocean to cenozoic circulation in the western south atlantic a twofold reconstruction abstract the cretaceous oceanic circulation has been quite different from the modern with a different distribution of the continents on the globe this has resulted in a much lower temperature gradient between poles and equator we have studied seismic reflection data and used numerical simulations of atmosphere and ocean dynamics to identify important steps in modifications of the oceanic circulation in the south atlantic from the cretaceous to the cenozoic and the major factors controlling them starting in the albian we could not identify any traces of an overturning circulation for the south atlantic although a weak proto antarctic circumpolar current acc was simulated no change in circulation was observed for the paleocene early eocene south atlantic which indicated that this period has witnessed a circulation similar to the albian and cenomanian turonian circulation the most drastic modifications were observed for the eocene oligocene boundary and the oligocene early miocene with the onset of an acc and atlantic meridional overturning circulation amoc and hence southern sourced deep and bottom water masses in the western south atlantic a modern amoc which intensified in strength after closure of the central american seaway cas and a strong acc have resulted in current controlled sedimentary features and wide spread hiatusses in the south atlantic since the middle miocene the opening of drake passage in early oligocene times and the closure of the cas at 6 ma i e tectonic processes have been identified as the key triggers for the observed most severe changes in oceanic circulation in the south atlantic

the progressive intensification of northern hemisphere glaciation as seen from the north pacific ocean drilling project odp site 882 50 22 n 167 36 e provides the first high resolution grape density magnetic susceptibility carbonate opal and foraminifera planktonic and benthic stable isotopes records between 3 2 and 2 4 ma in the northwest pacific we observed a dramatic increase in ice rafting debris at site 882 at 2 75 ma which is coeval with that found in the norwegian sea suggesting that the eurasian arctic and northeast asia were significantly glaciated from 2 75 ma onwards prior to 2 75 ma planktonic foraminifera 18o records indicate a warming or freshening trend of 4 c or 2 over 80 ka if this is interpreted as a warm pre glacial pliocene north pacific it may have provided the additional moisture required to initially build up the northern hemisphere continental ice sheet the dramatic drop in sea surface temperatures sst 7 5 c at 2 75 ma ended this suggested period of enhanced sst and thus the proposed moisture pump moreover at 2 79 and 2 73 ma opal mass accumulation rates mar decrease in two steps by five fold and is accompanied by a more gradual long term decrease in caco3 mars evidence from the southern ocean odp site 704 indicates that just prior to 2 6 ma there is a massive increase in opal mars the opposite to what is found in the north pacific this indicates that the intensification of northern hemisphere glaciation was accompanied by a major reorganisation of global oceanic chemical budget possibly caused by changes in deep ocean circulation the initiation of northern hemisphere glaciation occurred in the late miocene with a significant build up of ice on southern greenland however the progressive intensification did not occur until 3 5 3 ma when the greenland ice sheet expanded to include northern greenland following this stage we suggest that the eurasian arctic and northeast asia glaciated at 2 75 ma approximately 100 ka before the glaciation of alaska 2 65 ma and 200 ka before the glaciation of the north east american continent 2 54 ma

impact of restriction of the atlantic mediterranean gateway on the mediterranean outflow water and eastern atlantic circulation during the messinian 1 messinian foraminiferal stable oxygen and carbon isotopes of the montemayor 1 core guadalquivir basin sw spain have been investigated this record is exceptional to study the mediterranean outflow water mow impact on the atlantic meridional overturning circulation amoc and global climate during the messinian because the core is near the guadalhorce corridor the last betic gateway to be closed during the early messinian our results allow dating accurately its closure at 6 18 ma constant benthic 18o values high difference between benthic and planktonic 18o and low sedimentation rates before 6 18 ma indicate the presence of a two layer water column with bottom winnowing due to an enhanced mediterranean outflow current the enhanced contribution of dense mow to the north atlantic ocean likely fostered the formation of north atlantic deep water nadw after 6 18 ma benthic 18o values parallel that of the global glacioeustatic curve the difference between benthic and planktonic 18o is low and sedimentation rates considerably increased this indicates a good vertical mixing of the water column interruption of the mow and a dominant glacioeustatic control on the isotopic signatures according to the role of mow in the modern atlantic thermohaline circulation the reduction of the mow after the closure of the guadalhorce corridor might have resulted in a decreased nadw formation rate between 6 0 and 5 5 ma weakening the amoc and promoting northern hemisphere cooling after the gibraltar strait opening the restoration of the mow and related salt export from the mediterranean could have promoted an enhanced nadw formation

grain size variations in pelagic carbonate oozes from the walvis ridge se atlantic ocean odp site 1265 a low resolution miocene record of carbonate sedimentation and preservation abstract a record of variations in winnowing and dissolution was established for miocene 19 7 ma deep sea sediments at site 1265 from the walvis ridge south atlantic in this study we apply a new preservation index based on the abundance of coarse calcareous silt fraction 10 63 m the miocene record of this silt preservation index corresponds well with the conventional foraminiferal preservation index and variations in sand content preservation ranging from good to moderate during the miocene indicates that site 1265 is located above and close to the foraminiferal lysocline grain size mass accumulation rates of carbonate and organic carbon records indicate that changes in productivity and carbonate production play a minor role in variations of calcium carbonate concentration the miocene variation in carbonate content is caused mostly by a combination of winnowing and dissolution significant decreases of preservation at 17 3 16 3 13 8 13 0 11 6 and 10 4 ma coincide with miocene glacial events mi events suggesting that dissolution was caused by an increase of southern component water scw influence which occurred as a response to the intensification of antarctic ice sheet development the scw acts as the major deep water source during the global miocene cooling at 10 4 ma a change to overall better preservation points to a weakening of scw that occurred as a response to the initiation or strengthening of north atlantic deep water nadw

instability of the thermohaline circulation with respect to mixed boundary conditions is it really a problem for realistic models abstract this comment discusses two issues raised by a recent study of the stability of the thermohaline circulation tziperman et al a numerical problem is pointed out that may have affected the results of the reported calculations in addition it is argued that the state transition of the model circulation found in the above study is triggered by a convective instability mechanism not the advective mechanism discussed by its authors should this be the case then the conclusion of tziperman et al that the ocean is close to a stability transition point must be questioned

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

stratigraphy and sedimentology of phosphate rich sediments in malta and south eastern sicily latest oligocene to early late miocene the maltese archipelago and south eastern sicily include an uppermost oligocene to upper miocene hemipelagic sedimentary succession representing the malta hyblean plateau which limits the eastern mediterranean to the west this succession hosts a unique and well exposed series of condensed and allochthonous phosphate rich beds which were formed in a sedimentary regime of erosion sediment reworking and frequent gravity flow deposition the combination of nannofossil biostratigraphy and 87sr 86sr isotope stratigraphy allows for the precise attribution of ages to the phosphate deposits and for the distinction of three periods of major phosphogenesis the first phase occurred between 24 5 and 21 ma and 25 and 18 9 ma clustering of ages between 25 and 22 5 ma on malta and sicily respectively the second and third phases of phosphogenesis are documented from the maltese islands and are dated as 17 2 to 13 1 ma and 10 9 to 9 8 ma respectively the phosphate rich beds are associated with hiatuses and phases of important condensation which for the oldest phosphogenic period envelop the time period of 23 2 to 22 ma for the fomm ir rhi bay section malta and from 19 1 to 16 3 ma for the sections of sampieri and modica sicily for the second phase of phosphogenesis on the maltese islands a consistent hiatus was found which embraces the time period of approximately 17 to 15 ma also the third phase of phosphogenesis appears to be associated with a major hiatus which probably envelops the time period between 12 5 and 10 9 ma but a better age control is needed here the correspondence in timing of the maltese sicilian phases of phosphogenesis with major phases of phosphogenesis outside the mediterranean realm to maxima in oceanic phosphorus burial rates and maxima in the 13c benthic foraminiferal record suggests that the palaeoceanographic evolution of the eastern mediterranean was well in phase with that of other ocean basins until at least the early late miocene despite its increasing isolation due to the gradual closure of the eurasian arabian strait and progressive sea level fall

closure of the isthmus of panama the near shore marine record of costa rica and western panama the final closure of the isthmus of panama at 3 5 ma divided the american tropical ocean into two separate and different oceanographic regions consequences for the marine biota were profound but hitherto correlation of the pacific and caribbean coastal sections has not been precise enough to track biologic patterns we present here a correlation of 31 sections from the pacific and caribbean coasts of costa rica and western panama using calcareous nannofossils and planktonic foraminifera at both the tops and bottoms of each formation we estimate that the caribbean section ranges from 8 2 ma to 1 7 ma and the pacific sequence from 3 6 ma to 1 7 ma these intervals bracket postulated dates for final closure of the isthmus and provide the first well dated record of middle and late pliocene faunas from the region the caribbean and pacific sections include very different environments of deposition yet there is sufficient overlap and diversity of habitats to permit meaningful biological comparisons on the caribbean side formations tied together by the overlap of the upper pliocene markers sphenolithus abies and pseudoemiliana lacunosa 3 5 ma to 3 6 ma range from very shallow to shallow inner shelf 200 m and upper slope 200 800 m the pacific coast sections were mostly deposited in a trench slope environment which is absent on the caribbean side these sections fortuitously include abundant thick intra formational slumps containing shallow water fauna more appropriate for biological comparison with the caribbean biota similarly the 1 9 ma to 1 5 ma interval well constrained by various taxa includes middle to outer shelf and inner shelf to upper slope deposits on the caribbean side and marginal marine to inner shelf deposits on the pacific coast using our new biostratigraphic framework to correlate previously poorly constrained mollusc collections we show that evolutionary divergence of the pacific and caribbean near shore marine faunas had occurred by 3 5 ma this strongly suggests that the isthmus was effectively closed by this time

miocene to pliocene changes in south african hydrology and vegetation in relation to the expansion of c4 plants the miocene expansion of c4 plants mainly tropical grasses between 8 and 4 million years ma remains an enigma since regional differences in the timing of the expansion rules out decreased co2 pco2 as a dominant forcing e g tipple and pagani 2007 the early origins of terrestrial c4 photosynthesis annu rev earth planet sci 35 435 461 other environmental factors such as low latitude aridity and seasonality have been proposed to explain the low tree versus grass ratio found in savannahs and tropical grasslands of the world but conclusive evidence is missing here we use pollen and stable carbon 13c and hydrogen d isotope ratios of terrestrial plant wax from a south atlantic sediment core odp site 1085 to reconstruct miocene to pliocene changes of vegetation and rainfall regime of western southern africa our results reveal changes in the relative amount of precipitation and indicate a shift of the main moisture source from the atlantic to the indian ocean during the onset of a major aridification 8 ma ago we emphasize the importance of declining precipitation during the expansion of c4 and cam mainly succulent vegetation in south africa we suggest that the c4 plant expansion resulted from an increased equator pole temperature gradient caused by the initiation of strong atlantic meridional overturning circulation following the shoaling of the central american seaway during the late miocene

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

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

colder subarctic pacific with larger sea ice caused by closure of the central american seaway and its influence on the east asian monsoon a climate model study

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

closure of the panama seaway during the pliocene implications for climate and northern hemisphere glaciation the panama hypothesis states that the gradual closure of the panama seaway between 13 million years ago 13 ma and 2 6 ma led to decreased mixing of atlantic and pacific water masses the formation of north atlantic deep water and strengthening of the atlantic thermohaline circulation increased temperatures and evaporation in the north atlantic increased precipitation in northern hemisphere nh high latitudes culminating in the intensification of northern hemisphere glaciation nhg during the pliocene 3 2 2 7 ma here we test this hypothesis using a fully coupled fully dynamic ocean atmosphere general circulation model gcm with boundary conditions specific to the pliocene and a high resolution dynamic ice sheet model we carry out two gcm simulations with closed and open panama seaways and use the simulated climatologies to force the ice sheet model we find that the models support the panama hypothesis in as much as the closure of the seaway results in a more intense atlantic thermohaline circulation enhanced precipitation over greenland and north america and ultimately larger ice sheets however the volume difference between the ice sheets in the closed and open configurations is small equivalent to about 5 cm of sea level we conclude that although the closure of the panama seaway may have slightly enhanced or advanced the onset of nhg it was not a major forcing mechanism future work must fully couple the ice sheet model and gcm and investigate the role of orbital and co2 effects in controlling nhg

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

middle pliocene sea surface temperatures a global reconstruction identification and analyses of pliocene marine microfossils from 64 globally distributed stratigraphic sequences have been used to produce a middle pliocene sea surface temperature reconstruction of the earth this reconstruction shows little or no change from current conditions in low latitude regions and significant warming of the ocean surface at mid and higher latitudes of both hemispheres this pattern of warming is consistent with terrestrial records and suggests a combination of enhanced meridional ocean heat transport and enhanced greenhouse effect were responsible for the middle pliocene warmth

opal sedimentation shifts in the world ocean over the last 15 myr biogenic silica opal accumulation records were used to trace mechanisms consequence and geographic pattern of shifts in the main locus of opal deposition of the world ocean over the last 15 myr over this time interval the main opal sink seems to have moved from the north atlantic to the pacific equatorial pacific eastern equatorial pacific eastern boundary current upwelling systems california namibia peru and finally to the southern ocean the interplay between opal deposition and a series of climatic tectonic oceanographic and biologic events has been analyzed and discussed these events include the cenozoic global cooling trend intensified glaciation in antarctica late miocene early pliocene biogenic bloom development of northern hemisphere glaciation nhg closing of the panama seaway transition of the climate system from a monopolarto a bipolar glaciated world mid pleistocene revolution mpr nutrient availability evolution of diatoms and c4 plants and changes in continental weathering rates while the observed shifts are mostly traceable to oceanic reorganizations and global climatic evolution conditions favorable to opal deposition involve the above mentioned complex mix of processes for this reason the interpretation of opal deposition records might not always be straightforward we however believe that it can still provide clear indications of large scale oceanographic reorganizations in the geological past d 2004 elsevier b v all rights reserved

a 30 ma history of the amazon river inferred from terrigenous sediments and organic matter on the ceara rise abstract the history of the amazon river is a much discussed subject and the timing of the development of a transcontinental system in particular is a matter of some controversy with estimations varying between the early miocene and the pliocene or even the pleistocene to shed further light on this we studied the sediment provenance of an oligocene to late pleistocene marine sedimentary section from the ceara rise odp site 925 a topographic high in the central atlantic ocean using major element concentrations and nd isotopic composition in 85 samples in addition the carbon isotopic composition of bulk organic matter and changes in the distribution of glycerol dialkyl glycerol tetraethers gdgts were used to identify periods of increased river outflow on the basis of these results we suggest that the history of the development of the amazon river is characterized by specific steps during the late oligocene early miocene 30 18 3 ma the terrigenous mass accumulation rates tars were high and sediment and gdgt compositions suggest that a large river system existed which at times received weathering products from a younger and probably andean sediment source a shift to a younger andean sediment provenance after 8 7 ma indicates that the amazon river became permanently connected with the andes between 18 3 and 4 5 ma tars were generally low and gdgts were derived for the most part from in situ production in marine waters around 4 5 ma the river expanded probably due to ongoing tectonic activity and uplift in the andes increased andean rock erosion this led to a strong increase in terrigenous sediment deposition and enhanced organic matter preservation on the ceara rise and the delivery of terrestrial both soil and riverine branched gdgts to the ceara rise

sedimentary growth pattern on the northern argentine slope the impact of north atlantic deep water on southern hemisphere slope architecture large sedimentary deposits consisting of several major contourite drifts were studied by means of high resolution multichannel seismic data at the middle slope along the northern argentina continental margin to determine their evolutionary stages as well as to identify and assess the possible impact of northern source deep water nsdw on the slope architecture the imaged contouritic sediments allow decoding on the regional paleo oceanographic setting of the last 32 ma earliest contouritic sedimentation can be observed close to the eocene oligocene boundary based on an aggradational stacking pattern with a complex and wavy seismic facies pointing toward a hydrodynamically turbulent flow pattern this facies is most likely related to the opening of the drake passage associated with global cooling and a strengthening of surface intermediate and deep ocean currents in the southern ocean during the middle miocene plastered drift sequences with an aggradational reflection pattern were deposited their depositional style indicates weak non turbulent current conditions which are interpreted to be related to a vertical shift of water mass interfaces caused by the first formation of nsdw during the mid miocene climatic optimum on top the formation of plastered drift sequences led to the modern extent of the ewing terrace which was probably controlled by the continuous strengthening and thickening of nsdw until the final closure of the central american seaway cas during the pliocene and quaternary after the complete closure of the cas and under the influence of the full force of the nsdw mounded plastered drift sequences are built upon the ewing terrace generating the modern slope morphology therefore we suggest that deep water production in the northern hemisphere plays a significant role by controlling the shape of the continental slopes in the southwestern south atlantic since the middle miocene highlights slope of northern argentine continental margin is current controlled since 32 ma variability of northern source deep water nsdw controls sedimentary processes sedimentary processes are susceptible to changes of the brazil malvinas confluence impact of nsdw on slope processes is underestimated in the southern hemisphere

a short circuit in thermohaline circulation a cause for northern hemisphere glaciation the cause of northern hemisphere glaciation about 3 million years ago remains uncertain closing the panamanian isthmus increased thermohaline circulation and enhanced moisture supply to high latitudes but the accompanying heat would have inhibited ice growth one possible solution is that enhanced moisture transported to eurasia also enhanced freshwater delivery to the arctic via siberian rivers freshwater input to the arctic would facilitate sea ice formation increase the albedo and isolate the high heat capacity of the ocean from the atmosphere it would also act as a negative feedback on the efficiency of the conveyor belt heat pump

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

reorganization of miocene deep water circulation in response to the shoaling of the central american seaway 1 the response of ocean circulation to the shoaling of the central american seaway cas is investigated using the massachusetts institute of technology mit ocean general circulation model ogcm in contrast to earlier model studies it is found that significant amounts of deep water are formed in the north atlantic prior to the closure of the cas however the circulation pattern is fundamentally different from the modern ocean in the upper layers of the cas there is a relatively strong geostrophic flow from the pacific to the atlantic controlled by the pressure difference across the seaway however when the cas is deeper than the level of north atlantic deep water nadw outflow a significant amount of nadw passes through the cas to the pacific ocean in the pacific the deep water traverses the basin from east to west in a relatively narrow zonal jet and becomes a southward flowing western boundary current before it joins with the antarctic circumpolar current acc to the south this implies that deep sea sediment records from the miocene pacific ocean could have been influenced by relatively young nadw and provides a new framework for the interpretation of geochemical tracer data diversification of benthic foraminifer fauna suggests that the cas shoaled to a depth of about 1000 m toward the end of the middle miocene this would have prevented the passage of nadw to the pacific and established the modern deep water circulation pattern at that time

giant mounded drifts in the argentine continental margin origins and global implications for the history of thermohaline circulation abstract partially buried giant drifts are located in the southern most sector of the argentine continental margin generating a bathymetric jump at the base of the slope they are characterised as giant asymmetrical elongated mounded contourite drifts and are described in detail here for the first time this description is mainly based on the bathymetric and multichannel seismic reflection profiles data their origin and evolution in the argentine basin are decoded and their implications in an area crucial for geologic and palaeoceanographic reconstruction between the atlantic and antarctica are discussed the buried giant drifts are divided into two zones by a large seamount the major giant drift 50 km wide 300 km long and having a sedimentary thickness of nearly 1 km is located to the south of this seamount and oriented to the north where its summit outcrops at present seafloor here its asymmetric external shape is characterised by a steep western side and a gently dipping smooth eastern side with internal reflections prograding eastward based on its position morphology and internal characteristics it has been deduced that this giant drift was generated in the argentine basin by the southward branch of a confined large loop of the antarctic bottom deep water from the eocene oligocene boundary coevally with the drake passage opening until the middle miocene however after a major palaeoceanographic change in the mid to late miocene a new oceanographic scenario was established that fossilised these giant drifts these changes produced a new depositional style in the margin resulting in initiation of the present contourite depositional system and morphologic features over the slope extension of north atlantic deep water circulation into the southern hemisphere and deepening of antarctic bottom water circulation in the argentine basin are herein postulated as the factors that controlled said changes these giant drifts show strong seismic evidence of hydrates and free gas that are ripe for energy resources exploration thus demonstrating the economic potential of contourite deposits in deep marine environments

link between oceanic heat transport thermohaline circulation and the intertropical convergence zone in the early pliocene atlantic planktonic foraminiferal oxygen isotope records from the western and eastern tropical pacific and atlantic oceans suggest a southward shift in the intertropical convergence zone toward its modern location between 4 4 and 4 3 ma a concomitant shift in the carbon isotope compositions of atlantic benthic foraminifera provides strong evidence for an increased thermohaline overturn at this time we suggest that the southward shift of the intertropical convergence zone and associated change in trade wind circulation altered equatorial surface hydrography increased the advection of warmer and more saline surface waters into the subtropical and north atlantic and contributed to thermohaline overturn

global cenozoic paleobathymetry with a focus on the northern hemisphere oceanic gateways abstract the evolution of the northern hemisphere oceanic gateways has facilitated ocean circulation changes and may have influenced climatic variations in the cenozoic time 66 ma 0 ma however the timing of these oceanic gateway events is poorly constrained and is often neglected in global paleobathymetric reconstructions we have therefore re evaluated the evolution of the northern hemisphere oceanic gateways i e the fram strait greenland scotland ridge the central american seaway and the tethys seaway and embedded their tectonic histories in a new global paleobathymetry and topography model for the cenozoic time our new paleobathymetry model incorporates northeast atlantic paleobathymetric variations due to iceland mantle plume activity updated regional plate kinematics and models for the oceanic lithospheric age sediment thickness and reconstructed oceanic plateaus and microcontinents we also provide a global paleotopography model based on new and previously published regional models in particular the new model documents important bathymetric changes in the northeast atlantic and in the tethys seaway at eocene oligocene transition 34 ma the time of the first glaciations of antarctica believed to be triggered by the opening of the southern ocean gateways i e the drake passage and the tasman gateway and subsequent antarctic circumpolar current initiation our new model can be used to test whether the northern hemisphere gateways could have also played an important role modulating ocean circulation and climate at that time in addition we provide a set of realistic global bathymetric and topographic reconstructions for the cenozoic time at one million year interval for further use in paleo ocean circulation and climate models

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

the salinity contrast between the atlantic and pacific oceans during glacial time in today s ocean the salinity of low latitude surface waters in the pacific is on the average 1 per mil lower than in the atlantic oxygen isotope records for planktonic foraminifera suggest that an even larger salinity deficit may have existed for pacific surface water during glacial time if so the transport of water vapor from the atlantic to the pacific must have been as strong as or even stronger to balance the tendency toward salt buildup the atlantic would have had to export during glacial time waters saltier than it received from the other oceans

a coastal upwelling seesaw in the atlantic ocean as a result of the closure of the central american seaway 1 based on coupled climate model experiments we suggest an antiphase relationship between the two major coastal upwelling zones in the atlantic ocean off northwestern and southwestern africa we describe this as an upwelling seesaw the state of the coastal upwelling seesaw is controlled by variations in the oceanic meridional heat transfer between south and north atlantic in particular we suggest that the pliocene closure of the central american seaway induced a large scale redistribution of heat in the atlantic ocean leading to an intensification weakening of upwelling off southwest northwest africa

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

contourite depositional system on the argentine slope an exceptional record of the influence of antarctic water masses a significant contourite depositional system cds on the continental slope of the southern argentine margin is described here for the first time this system contains both erosive and depositional features that have resulted from several factors including topographic intensification of the antarctic sourced water masses the systematic northward decrease in speed of these water masses a northward increase of downslope sedimentary processes and local tectonic influences this system is an exceptional example of a cds that started to develop at the time of the eocene oligocene boundary potentially coeval with the opening of the drake passage however a new margin morphology characterized by a complex terraced slope lacking any continental rise developed after a major paleoceanographic change in the middle to late miocene we infer that this change resulted from the extension of north atlantic deep water circulation into the southern hemisphere and the deepening of antarctic bottom water circulation in the argentine basin

seismological constraints on the ultralow velocity zones in the lowermost mantle from core reflected waves abstract analyses of reflected seismic phases pcp and scp from the core mantle boundary cmb enable us to constrain the physical properties of the ultralow velocity zones ulvzs at the base of the mantle the cmb region beneath the western and southern pacific is investigated in detail by using a large amount of waveform data recorded by the short period array stations clear arrivals of postcursor to scp scsp are detected from several event receiver pairs which sample the cmb regions beneath philippine kalimantan and east of australia indicating the existence of ulvzs under the regions no scp precursors sdp and spcp are found in our data set except for a few events with a large epicentral distance over 45 no additional arrivals related to pcp are found in the region where scsp is observed in the scp wavelet elastic properties of the ulvz are precisely constrained by waveform modeling of both pcp and scp which sample the cmb region under philippine kalimantan different ulvz structures are found between the northeastern and southwestern parts of the philippine kalimantan the ratio of shear to compressional velocity reductions is 2 0 2 5 and the ratio of density to shear velocity changes is 0 5 to 0 33 in the northeastern part while minimal changes in compressional velocity and large shear velocity decrease in the southwestern part are found a certain degree of density increases and shear velocity decreases are also observed in the ulvz beneath east of australia increase in density and decrease in shear velocity throughout our observation imply that the iron incorporation into the lowermost mantle minerals may generate the ulvz anomaly however other mechanisms such as partial melting can explain the structural difference between the northeastern and southwestern parts of the ulvz beneath the philippine kalimantan region

plutonium activities and 240pu 239pu atom ratios in sediment cores from the east china sea and okinawa trough sources and inventories abstract plutonium concentrations and 240 pu 239 pu atom ratios in the east china sea and okinawa trough sediment cores were determined by isotope dilution inductively coupled plasma mass spectrometry after separation using ion exchange chromatography the results showed that 240 pu 239 pu atom ratios in the east china sea and okinawa trough sediments ranging from 0 21 to 0 33 were much higher than the reported value of global fallout 0 18 the highest 240 pu 239 pu ratios 0 32 0 33 were observed in the deepest okinawa trough sediment samples these ratios suggested the us nuclear weapons tests in the early 1950s at the pacific proving grounds in the marshall islands were a major source of plutonium in the east china sea and okinawa trough sediments in addition to the global fallout source it was proposed that close in fallout plutonium was delivered from the pacific proving grounds test sites via early direct tropospheric fallout and transportation by the north pacific equatorial circulation system and kuroshio current into the okinawa trough and east china sea the total 239 240 pu inventories in the cores were about 150 200 of that expected from direct global fallout about 46 67 of the total inventories were delivered from the pacific proving grounds much higher 239 240 pu inventories were observed in the east china sea sediments than in sediments of the okinawa trough because in the open oceans part of the 239 240 pu was still retained in the water column and continued pu scavenging was higher over the margin than the trough according to the vertical distributions of 239 240 pu activities and 240 pu 239 pu atom ratios in these cores it was concluded that sediment mixing was the dominant process in controlling profiles of plutonium in this area faster mixing in the coastal samples has homogenized the entire 240 pu 239 pu ratio record today slightly slower mixing and less scavenging in the okinawa trough have left the surface sediment ratios closer to the modern north pacific water end member and higher ratios 0 30 0 34 at the bottom of the cores

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

multiple equilibria of the global thermohaline circulation abstract a general circulation model with a highly idealized geometry is used to investigate which fundamentally different equilibria of the global thermohaline circulation may exist the model comprises two identical basins representing the atlantic and pacific oceans which are connected by a circumpolar channel in the south the model circulation is driven in addition to wind forcing by restoring the sea surface temperature to prescribed values and specified freshwater fluxes in the surface salinity budget mixed boundary conditions the boundary conditions are symmetric with respect to the equator and identical for both oceans four fundamentally different stable steady states are found under the same set of boundary conditions two of the equilibria show both oceans in the same state with high altitude deep water formation occuring either in both northern or in both southern oceans respectively two additional equilibria exist in which the thermohaline circulations of the basins differ fundament

arctic ocean evidence for late quaternary initiation of northern eurasian ice sheets a high resolution multiparameter stratigraphy allows the identification of late quaternary glacial and interglacial cycles in a central arctic ocean sediment core distinct sandy layers in the upper part of the otherwise fine grained sediment core from the lomonosov ridge lat 87 5 n correlate to four major glacials since ca 0 7 ma the composition of these ice rafted terrigenous sediments points to a glaciated northern siberia as the main source in contrast lithic carbonates derived from north america are also present in older sediments and indicate a northern north american glaciation since at least 2 8 ma we conclude that large scale northern siberian glaciation began much later than other northern hemisphere ice sheets

the initiation of northern hemisphere glaciation for well over a century scientists have speculated that variations in the atmospheric concentration of radiatively important trace gases such as carbon dioxide could control the earth s climate e g arrhenius 1 896 recent observations of significant glacial interglacial variation in atmo spheric co2 in ice cores put this idea on a firm observational as well as theoretical footing e g barnola et ai1 987 in this century as atmospheric co2 levels creep steadily upwards in response to the burning of fossil fuels and forests the specter of manmade global climate change looms before us ipcc 1990 a possibility that has heightened interest in the natural variability of climate in the past in particular the 3 c warming pre dicted for the next century has stimulated interest in past warm climates such as the middle pliocene around 3 0 million years ma ago an interval that has often been invoked as a greenhouse world budyko et al 1985 in this paper the climate transition from the warm mid pliocene around 3 2 ma to the onset of northern hemisphere ice ages around 2 4 ma is examined evidence for the initiation of significant northern hemisphere glaciation is examined as well as how this event affected climate around the globe while the cause of individual glacial interglacial oscillations is tied to milankovitch variations in the earth s orbit around the sun e g imbrie et ai 1 992 these insolation changes cannot account for the long term cooling trend which culminated in northern hemisphere glaciation in the final section of this paper mechanisms of long term climate change

reef building and carbonate production modes in the west central tethys during the cenozoic abstract changing components rock textures lithofacies platform types and architecture throughout time are unique characteristics of carbonate rocks characterizing these attributes has been approached by 1 building reference models for specific phanerozoic intervals 2 recognizing the climatic impact in modulating carbonate production and 3 analyzing the influence of changing bio geochemical conditions the reference model approach is mostly based on biological evolution the climatic impact approach emphasizes temperature and the bio geochemical approach considers the changes in mg ca ratios and ca concentrations in the oceans to date however an analysis integrating all of these factors is still missing the analysis presented here includes all these factors but also co 2 which is fundamental for both photosynthesis and caco 3 precipitation here we analyze the waxing and waning of cenozoic reef limestones from the central tethys region through several steps 1 on the basis of rock volume rock textures associated sediments and light dependent skeletal components as records of light penetration and wave energy depth 2 on global environmental conditions 13 c 18 o p co 2 temperature and 3 on the basis of functionality nutritional requirements and available resources through the cenozoic water motion whether induced by surface or internal waves or by currents increased as the thermal gradients strengthened both with depth and with latitude active water motion is essential for plankton catchers such as corals but less so for many larger benthic foraminifers lbf pycnoclines in the meso oligophotic zone would then favor the benthic plankton catchers such as corals but would be detrimental for many lbf warm temperatures favored lbf the eocene lbf families predominated during lowering of atmospheric p co 2 by using respiratory co 2 to enhance the symbiont production of photosynthates under oligotrophic conditions and limited turbulence whereas the miocene families had to adapt to a progressive increase in turbulence the eurythermal coralline red algae however became preponderant producers in the mesophotic zone during times when the 13 c was relatively high this explains two apparent paradoxes 1 corals thrive best when the earth s high latitudes cool and 2 the dominance of corals and lbf is inversely correlated despite they both require tropical conditions and have similar trophic strategies mixotrophy

consequences of shoaling of the central american seaway determined from modeling nd isotopes the central american seaway played a pivotal role in shaping global climate throughout the late cenozoic recent geological surveys have provided new constraints on timing of the seaway shoaling while neodymium isotopic e nd data measured on fossil teeth debris and ferromanganese crusts have helped define the history of water masses in the region here we provide the first 3 d simulations of e nd responses to the shoaling seaway our model suggests that a narrow and shallow seaway is sufficient to affect interoceanic circulation that inflow outflow balance between the caribbean and the antilles responds nonlinearly to sill depth and that a seaway narrower than 400 km is consistent with an active atlantic meridional overturning circulation during the late miocene simulated e nd values in the caribbean confirm that inputs from radiogenic pacific waters in the caribbean decrease as the seaway shoals despite model limitations a comparison between our results and e nd values recorded in the caribbean helps constrain the depth of the central american seaway through time and we infer that a depth between 50 and 200 m could have been reached 10 ma ago

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

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

primary productivity in the western tropical atlantic follows neogene amazon river evolution the amazon river nutrient rich plume currently triggers large scale phytoplankton blooms in the otherwise oligotrophic western tropical atlantic ocean little is known about the onset and development of this high productivity system although a direct link to the transcontinental amazon river evolution can be expected the amazon submarine fan located on the brazilian equatorial margin bem contains a unique sediment archive of the river s history and associated environmental changes in the marine realm this study represents the first marine microfossil multi proxy approach applied to any sedimentary record in the submarine fan area for the time interval encompassing the onset and development of the transcontinental amazon river system to reconstruct miocene to pleistocene changes in surface and bottom water conditions we analyzed organic walled dinoflagellate cyst and benthic foraminiferal assemblages respectively moreover terrestrial and freshwater palynomorph abundances were studied to provide a link between fluvial input and marine environmental changes in addition a planktonic foraminiferal biostratigraphy is constructed to verify the available calcareous nannofossil based age model our data show that in the early mid miocene up to 13 ma limited fluvial input reached the bem and primary productivity was elevated after which distinctly low productivity conditions prevailed after the birth of the transcontinental amazon at 9 ma surface water productivity initially increased slightly consistently high surface water productivity and decreased bottom water oxygenation followed increasing terrestrial input after the early pliocene the temporal consistency between records from the amazon fan and the more distant ceara rise reflects large scale marine environmental changes followed the development of the amazon river likely related to increased climatic variability in the amazon basin during the plio pleistocene

paleoceanographic implications of miocene deep sea hiatuses miocene paleoceanographic evolution exhibits major changes resulting from the opening and closing of passages the subsequent changes in oceanic circulation and development of major antarctic glaciation the consequences and timing of these events can be observed in variations in the distribution of deep sea hiatuses sedimentation patterns and biogeographic distribution of planktic organisms the opening of the drake passage in the latest oligocene to early miocene 25 20 ma resulted in the establishment of the deep circumpolar current which led to thermal isolation of antarctica and increased global cooling this development was associated with a major turnover in planktic organisms resulting in the evolution of neogene assemblages and the eventual extinction of paleogene assemblages the erosive patterns of two widespread hiatuses ph 23 0 22 5 ma and nh 1 20 18 ma indicate that a deep circumequatorial circulation existed at this time characterized by a broad band of carbonate ooze deposition siliceous sedimentation was restricted to the north atlantic and a narrow band around antarctica a major reorganization in deep sea sedimentation and hiatus distribution patterns occurred near the early middle miocene boundary apparently resulting from changes in oceanic circulation beginning at this time deep sea erosion occurred throughout the caribbean hiatus nh 2 16 15 ma suggesting disruption of the deep circumequatorial circulation and northward deflection of deep currents and or intensification of the gulf stream sediment distribution patterns changed dramatically with the sudden appearance of siliceous ooze deposition in the marginal and east equatorial north pacific by 16 0 to 15 5 ma coincident with the decline of siliceous sedimentation in the north atlantic this silica switch may have been caused by the introduction of norwegian overflow water into the north atlantic acting as a barrier to outcropping of silica rich antarctic bottom water the main aspects of the present oceanic circulation system and sediment distribution pattern were established by 13 5 to 12 5 ma hiatus nh 3 coincident with the establishment of a major east antarctic ice cap antarctic glaciation resulted in a broadening belt of siliceous ooze deposition around antarctica increased siliceous sedimentation in the marginal and east equatorial north pacific and indian oceans and further northward restriction of siliceous sediments in the north atlantic periodic cool climatic events were accompanied by lower eustatic sea levels and widespread deep sea erosion at 12 to 11 ma nh 4 10 to 9 ma nh 5 7 5 to 6 2 ma nh 6 and 5 2 to 4 7 ma nh 7

cenozoic ichthyolith biostratigraphy tofino basin british columbia five new late eocene pliocene ichthyolith zones are defined based on indigenous faunal occurrences in strata that outcrop along western vancouver island and samples from six offshore tofino basin wells five new interval zones are each defined based on distinct transported ichthyolith assemblages results are correlated within the framework of established cenozoic west coast and arctic foraminifer zones and ichthyolith occurrences in deep sea core samples and cretaceous queen charlotte group and nanaimo group strata of coastal british columbia the use of multiple data sets was important to interpret complex active tectonic margin sedimentation and structures the integrated ichthyolith and foraminifer biostratigraphy allowed an interpretation of well log data and lithology reports also we reassessed previous offshore seismic data evaluated fossil preservation and thermal alteration and distinguished transported from indigenous ichthyolith occurrences these data allowed a reinterpretation of tectonic segments and associated strata of the pacific rim and crescent terranes and the cascadia accretionary complex onshore and offshore vancouver island tectonically active phases during the eocene and oligocene early miocene resulted in transport of cretaceous to eocene ichthyoliths from structural highs to lows indicating proximity of the pluto i 87 and zeus d 14 wells to the tofino fault between the pacific rim and crescent terranes and suggesting derivation with coeval hesquiat peninsula strata an unconformity above the eocene crescent formation volcanics in prometheus h 68 and zeus d 14 wells is correlated with the pluto i 87 and apollo j 14 well stratigraphy an upper miocene unconformable surface coincides with accretion and uplift of the cascadia accretionary complex

isotopic paleoceanography of the caribbean and east pacific role of panama uplift in late neogene time comparisons of carbon isotopic data on benthic foraminifera from deep sea drilling project sites 502 western caribbean and 503 eastern pacific indicate that the difference between the atlantic and the pacific in the per mil enrichment in carbon 13 of total dissolved carbon dioxide increased about 6 million years ago and again 3 million years ago when the difference reached the modern level 1 per mil comparisons of planktonic foraminiferal oxygen isotopic data for the caribbean and the pacific suggest that the salinity of caribbean surface waters began increasing 4 million years ago possibly in response to shoaling of the panama isthmus these results suggest that modern circulation patterns in the caribbean and eastern pacific developed by 3 million years ago in concert with changing tectonic climatic and biogeographic patterns

on the mineral characteristics and geochemistry of the florida phosphate of four corners and hardee county mines abstract the florida phosphate deposits in four corners and hardee county mines are composed mainly of phosphate minerals and quartz in addition to subordinate proportions of feldspars dolomite calcite gypsum kaolinite attapulgite and montmorillonite these phosphorites contain three structurally different types of mudclasts massive mudclasts mudclasts with concentric structure and mudclasts consisting of agglomerates of apatite microparticles the latter are represented by particles resembling phosphatized fossil bacteria associated with microbial filaments and hollow apatite particles having surfacial coatings and connected to microbial filaments the florida phosphate particles are reworked and vary in mineral composition color and shape they are composed of a mixture of well crystalline species including carbonate fluorapatite francolite carbonate apatite and fluorapatite the color variation of the phosphate particles is related to difference in mineral composition extent of diagenetic effects and reworking the light colored mudclasts are characterized by the presence of carbonate apatite and aluminum hydroxide phosphate minerals whereas the dark mudclasts are rich in iron aluminum hydroxide phosphate minerals the florida phosphorites are suggested to be formed partially by authigenetic precipitation replacement of the sea floor carbonate and diatomite and microbial processes with respect to elemental geochemistry the analyzed particles contain small percentages of sulfur and iron which are related to the occurrence of pyrite traces of silica and alumina are recorded which may be attributed to the diagenetic some of the tested particles are relatively rich in phosphorous fluorine calcium and magnesium while poor in silicon potassium and sulfur whereas the bioclasts especially teeth are relatively rich in calcium phosphorous and fluorine while poor in silicon aluminum magnesium and potassium hence the microchemical analyses revealed that differential diagenesis affected mudclasts more than bioclasts there is a complete compositional gradation between clay and phosphate particles which reflects their interaction this involved kaolinitization of the phosphate particles phosphatization of the clay mineral particles and production of silica

controls on neogene carbonate facies and stratigraphic architecture of an isolated carbonate platform the caribbean island of bonaire abstract the neogene carbonate succession on the island of bonaire netherland antilles shows complex geometries associated with a sequence of depositional and erosional events which reflects the history of this isolated platform and the interaction between eustasy and tectonics three major episodes of carbonate platform deposition are defined which show contrasting depositional styles 1 aggradational platform lower middle miocene with sediments showing a fining upward trend from mixed coral rudstone to medium grained coralgal grain packstone partly dolomitized and tilted by tectonic deformation 2 progradational platform upper miocene pliocene which is formed of several shallowing upward prograding units mainly composed of reworked red algal grain packstone with significant dolomitization passing upward to shoreline and aeolian deposits formed of cor algal grain packstone and large benthic foraminifera grainstone and 3 flat topped platform pleistocene with a reefal framework composed of a rich variety of corals in a bioclastic pack wackestone matrix these platform episodes exhibit contrasting stacking patterns and are separated by erosional unconformities overprinting this depositional succession is a series of quaternary near horizontal shoreline erosional terraces and vertical cliffs which have been cut into the island stratigraphy and complicate the stratal field relationships however this terrace morphology clearly does not represent depositional episodes as has been suggested before the internal architecture of each of the three carbonate platform reflects of the dynamics of sedimentation with allogenic controls the latter relate to major oceanographic and tectonic events in the region including changing ocean circulation as a result of the closure of the panama isthmus and caribbean plate dynamics that affected sea floor and island topography the bonaire succession provides a model for understanding and predicting isolated carbonate platform development as well as architecture facies and potential diagenetic changes in an active tectonic setting

cenozoic climate changes a review based on time series analysis of marine benthic δ18o records the climate during the cenozoic era changed in several steps from ice free poles and warm conditions to ice covered poles and cold conditions since the 1950s a body of information on ice volume and temperature changes has been built up predominantly on the basis of measurements of the oxygen isotopic composition of shells of benthic foraminifera collected from marine sediment cores the statistical methodology of time series analysis has also evolved allowing more information to be extracted from these records here we provide a comprehensive view of cenozoic climate evolution by means of a coherent and systematic application of time series analytical tools to each record from a compilation spanning the interval from 4 to 61 myr ago we quantitatively describe several prominent features of the oxygen isotope record taking into account the various sources of uncertainty including measurement proxy noise and dating errors the estimated transition times and amplitudes allow us to assess causal climatological tectonic influences on the following known features of the cenozoic oxygen isotopic record paleocene eocene thermal maximum eocene oligocene transition oligocene miocene boundary and the middle miocene climate optimum we further describe and causally interpret the following features paleocene eocene warming trend the two step long term eocene cooling and the changes within the most recent interval miocene pliocene we review the scope and methods of constructing cenozoic stacks of benthic oxygen isotope records and present two new latitudinal stacks which capture besides global ice volume also bottom water temperatures at low less than 30 and high latitudes this review concludes with an identification of future directions for data collection statistical method development and climate modeling

coupled climate impacts of the drake passage and the panama seaway tectonically active gateways between ocean basins have modified ocean circulation over earth history today the atlantic and pacific are directly connected via the drake passage which forms a barrier to the time mean geostrophic transport between the subtropics and antarctica in contrast during the warm early cenozoic era when antarctica was ice free the drake passage was closed instead at that time the separation of north and south america provided a tropical seaway between the atlantic and pacific that remained open until the isthmus of panama formed in the relatively recent geological past ocean circulation models have previously been used to explore the individual impacts of the drake passage and the panama seaway but rarely have the two gateways been considered together and most explorations have used very simple atmospheric models here we use a coupled ocean ice atmosphere model gfdl s cm2mc to simulate the impacts of a closed drake passage both with and without a panama seaway we find that the climate response to a closed drake passage is relatively small when the panama seaway is absent similar to prior studies although the coupling to a dynamical atmosphere does increase the temperature change however with a panama seaway closing drake passage has a much larger effect due to the cessation of deep water formation in the northern hemisphere both gateways alter the transport of salt by ocean circulation with the panama seaway allowing fresh pacific water to be imported to the north atlantic and the drake passage preventing the flow of saline subtropical water to the circum antarctic a flow that is particularly strong when the panama seaway is open thus with a panama seaway and a closed drake passage the southern ocean tends to be relatively salty while the north atlantic tends to be relatively fresh such that the deep ocean is ventilated from the circum antarctic ensuing changes in the ocean heat transport drive a bi polar shift of surface ocean temperatures and the intertropical convergence zone migrates toward the warmer southern hemisphere the response of clouds to changes in surface ocean temperatures amplifies the climate response resulting in temperature changes of up to 9 c over antarctica even in the absence of land ice feedbacks these results emphasize the importance of tectonic gateways to the climate history of the cenozoic and support a role for ocean circulation changes in the glaciation of antarctica

miocene benthic foraminiferal cd ca records south atlantic and western equatorial pacific cadmium to calcium records in benthic foraminifera interpreted in conjunction with carbon isotope records reflect oceanic circulation patterns and oceanic inventories of cd carbon 13 carbon 12 and indirectly phosphorus cd ca ratios have been determined in benthic foraminifera cibicidoides spp for the late middle and early miocene from a site in the south atlantic deep sea drilling project dsdp site 525 leg 74 2500 m walvis ridge and from a site in the western equatorial pacific dsdp site 289 leg 30 2200 m ontong java plateau both sites have well established carbon isotope records for benthic foraminifera typical of the global record dominated by inventory changes cd ca ratios and carbon isotope values were averaged for the miocene in time intervals substantially longer than estimated oceanic residence times of cd c or p oceanic mean cd ca and carbon isotopes were estimated as weighted means of the values from both sites using weighting factors based on replicating modern oceanic means this may tend to underestimate oceanic inventories earlier than the late miocene because of changing water mass dominance at the south atlantic site from these means the oceanic cd inventory appears relatively stable during the miocene while there are large changes in carbon isotope inventories miocene cd inventory estimates are at least 20 lower than published estimates for quaternary stages 1 and 2 increasing oceanic cd inventories over the past 10 m y are consistent with hypotheses of increased river fluxes in the recent geologic past cd ca differences between sites may be indicative of the extent of basin basin fractionation from bottom water circulation although the validity of this comparison depends on how representative each of these two sites is through time for its respective ocean basin the pacific atlantic cd ca difference based on these two sites was larger in the late miocene compared to the middle and early miocene the relationship between cd ca and carbon isotope inventories and intersite gradients for these sites suggests that either oceanic phosphorus inventories or parameters related to marine organic matter composition must have been different in the miocene than at present large changes in the carbon isotope balance reflected as both positive and negative excursions in the carbon isotope inventories occurred with little effect on cd inventories changes in phosphorus fluxes associated with these carbon isotope flux changes may have been compensated for by changing deposition fluxes to other phosphorus sinks phosphorus inventories may have increased from the middle to late miocene with increasingly negative oceanic carbon isotope inventories however confirmation of these suggestions depend on collecting more cd ca and 13c data throughout the miocene oceans and on developing the application of other constraints for these interpretations

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

influence of the amazon river development and constriction of the central american seaway on middle late miocene oceanic conditions at the ceara rise abstract sediment samples from odp 154 site 926a ceara rise western equatorial atlantic ocean spanning the neogene from 12 8 to 9 2 ma were investigated on their calcareous dinoflagellate contents to better understand the oceanographic changes in relation to the closure of the central american seaway and the development of the amazon river intervals with increased cyst accumulation rates and dissolution sensitive species occur from 12 4 ma onward they correspond to periods of enhanced north atlantic deep water production indicating the presence of this water mass at the research site during these intervals this suggests that pulses of north atlantic deep waters sporadically flew into the south atlantic related to uplift phases of the panama sill at about 11 2 ma the first appearance of leonella granifera indicates river influence at the sample site this first indication of river influence in the western equatorial atlantic can be linked to the developing amazon river the cyst association changes at about 11 2 ma from almost monospecific to highly diverse and the permanent presence and increased abundance of l granifera suggest that river waters were able to reach the study site by now probably as a result of the southward flowing north brazil current after 10 5 ma the cyst association indicates a decrease in amazon influence at site 926a this change can be correlated to a reducing inflow of pacific waters through the central american seaway leading to a reverse of the north brazil current to its modern northwards flow pattern

changes in caribbean surface hydrography during the pliocene shoaling of the central american seaway pliocene d18o records of shallow and deep dwelling planktonic foraminifers from the caribbean ocean drilling program sites 999 and 1000 the tropical east pacific sites 1241 and 851 and the atlantic site 925 ceara rise and site 1006 western great bahama bank were used to examine atlantic caribbean pacific atmospheric and oceanic linkages associated with the progressive closure of the central american seaway 5 5 3 ma comparisons indicate the development of an inner caribbean salinity gradient in the mixed layer and salinity changes on precessional periodicities after 4 4 ma site 1000 when the pacific caribbean throughflow became significantly restricted the high amplitude variability in salinity is also observed at site 1006 monitoring the caribbean outflow into the atlantic comparisons of caribbean and atlantic planktonic d18o records suggest the north atlantic subtropical gyre as a major source for high salinity surface waters precession induced variations in the volume transport of pacific surface water masses through the panamanian seaway are considered a main factor to explain the caribbean salinity minima results from a coupled climate model point to changes in the el nin o southern oscillation state as a potential trigger for changes in the amount of pacific inflow into the caribbean