late eocene southern ocean cooling and invigoration of circulation preconditioned antarctica for full scale glaciation during the eocene oligocene transition eot 34 33 5 ma antarctic ice sheets relatively rapidly expanded leading to the first continent scale glaciation of the cenozoic declining atmospheric co2 concentrations and associated feedbacks have been invoked as underlying mechanisms but the role of the quasi coeval opening of southern ocean gateways tasman gateway and drake passage and resulting changes in ocean circulation is as yet poorly understood definitive field evidence from eot sedimentary successions from the antarctic margin and the southern ocean is lacking also because the few available sequences are often incomplete and poorly dated hampering detailed paleoceanographic and paleoclimatic analysis here we use organic dinoflagellate cysts dinocysts to date and correlate critical southern ocean eot successions we demonstrate that widespread winnowed glauconite rich lithological units were deposited ubiquitously and simultaneously in relatively shallow marine environments at various southern ocean localities starting in the late eocene 35 7 ma based on organic biomarker paleothermometry and quantitative dinocyst distribution patterns we analyze southern ocean paleoceanographic change across the eot we obtain strong indications for invigorated surface and bottom water circulation at sites affected by polar westward flowing wind driven currents including a westward flowing antarctic countercurrent starting at about 35 7 ma the mechanism for this oceanographic invigoration remains poorly understood the circum antarctic expression of the phenomenon suggests that rather than triggered by tectonic deepening of the tasman gateway progressive pre eot atmospheric cooling played an important role at localities affected by the antarctic countercurrent sea surface productivity increased and simultaneously circum antarctic surface waters cooled we surmise that combined these processes contributed to preconditioning the antarctic continent for glaciation

eocene bipolar glaciation associated with global carbon cycle changes the transition from the extreme global warmth of the early eocene greenhouse climate 55 million years ago to the present glaciated state is one of the most prominent changes in earth s climatic evolution it is widely accepted that large ice sheets first appeared on antarctica 34 million years ago coincident with decreasing atmospheric carbon dioxide concentrations and a deepening of the calcite compensation depth in the world s oceans and that glaciation in the northern hemisphere began much later between 10 and 6 million years ago here we present records of sediment and foraminiferal geochemistry covering the greenhouse icehouse climate transition we report evidence for synchronous deepening and subsequent oscillations in the calcite compensation depth in the tropical pacific and south atlantic oceans from 42 million years ago with a permanent deepening 34 million years ago the most prominent variations in the calcite compensation depth coincide with changes in seawater oxygen isotope ratios of up to 1 5 per mil suggesting a lowering of global sea level through significant storage of ice in both hemispheres by at least 100 to 125 metres variations in benthic carbon isotope ratios of up to 1 4 per mil occurred at the same time indicating large changes in carbon cycling we suggest that the greenhouse icehouse transition was closely coupled to the evolution of atmospheric carbon dioxide and that negative carbon cycle feedbacks may have prevented the permanent establishment of large ice sheets earlier than 34 million years ago between 55 and 34 million years ago earth switched from a warm greenhouse ocean climate system to the icehouse that we now enjoy frustratingly the few oceanographic datasets that exist for this key period are of low resolution tripati et al now present measurements of sediment and foraminiferal geochemistry that provide the first continuous high resolution records for this period the data suggest that northern hemisphere glaciation started 34 million years ago rather than between 10 and 8 million years ago as previously thought

eocene stratigraphy of southeastern tierra del fuego island argentina recognizing folded and thrusted eocene foreland strata of the oil producing austral basin provides new insights on the paleogene evolution of tierra del fuego island and the adjacent atlantic shelf field mapping and petrographic and micropaleontologic studies at the punta torcida anticline depict an eocene composite section about 1600 m thick assigned to the la despedida group this group is divided into three formations punta torcida 215 m leticia 520 m and cerro colorado 855 m these formations are subdivided into informal members in the punta torcida formation the lower member is composed of dark gray mudstones with thin sandstone beds the middle member of the punta torcida contains mudstones with small sandstone lenses both the lower and middle members have a high radiolarian content the upper member contains mudstones lithologic succession presence of pyrite and a well preserved benthic assemblage having little diversification suggest a regressive sequence formed in shelf settings with a dysaerobic to anoxic bottom planktonic foraminifera indicate a ypresian age equivalent to upper zone p6 to basal zone p9 tropical planktonic zones in the leticia formation the lower and upper members are glauconite rich highly bioturbated sandstones in the middle member rich in pyrite and carbonized plant material the glauconite is rare or absent large channels filled with cross stratified sandstone lenses and having a general absence of planktonic foraminifera suggest estuarine restricted marine settings planktonic foraminifera indicate a late middle eocene age basal zone p12 to middle of zone p14 indicating a hiatus with the underlying punta torcida formation the cerro colorado formation a coarsening and thickening upward succession of mudstones and sandstones is a major shelfal transgressive regressive sequence punctuated by minor deepening and shallowing its four members record different benthic assemblages that reflect cooling waters the uppermost member bears abundant radiolarians and planktonic foraminifera typical of an oxygen minimum habitat and indicating a late eocene age upper zone p15 to upper zone p16 the microfauna and stratigraphic position of the lower three members are consistent with a latest middle eocene age middle of zone p14 to lower zone p15 in preliminary observations on the outcrop area of late cretaceous paleogene strata between cabo san pablo and bahia thetis two important angular unconformities were recorded one at the contact between the estratos de policarpo and the rio bueno la despedida group and the other at the contact between the la despedida group and the cabo pena rio leona formations

middle miocene southern ocean cooling and antarctic cryosphere expansion magnesium calcium data from southern ocean planktonic foraminifera demonstrate that high latitude similar to55degreess southwest pacific sea surface temperatures ssts cooled 6degrees to 7degreesc during the middle miocene climate transition 14 2 to 13 8 million years ago stepwise surface cooling is paced by eccentricity forcing and precedes antarctic cryosphere expansion by similar to60 thousand years suggesting the involvement of additional feedbacks during this interval of inferred low atmospheric partial pressure of co2 pco 2 comparing ssts and global carbon cycling proxies challenges the notion that episodic pco2 drawdown drove this major cenozoic climate transition sst salinity and ice volume trends suggest instead that orbitally paced ocean circulation changes altered meridional heat vapor transport triggering ice growth and global cooling

tectonic evolution of the west scotia sea joint inversion of isochron and flowline data from the flanks of the extinct west scotia ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron c8 26 5 ma and extinction around chron c3a 6 65 9 ma when they are placed in a regional plate circuit the rotations predict plate motions consistent with known tectonic events at the margins of the scotia sea oligocene extension in powell basin miocene convergence in tierra del fuego and at the north scotia ridge and miocene transpression at the shackleton fracture zone the inversion results are consistent with a spreading history involving only two plates at rates similar to those between the enclosing south america and antarctica plates after chron c5c 16 7 ma but that were faster beforehand the spreading rate drop accompanies inception of the east scotia ridge back arc spreading center which may therefore have assumed the role of the west scotia ridge in accommodating eastwards motion of the trench at the eastern boundary of the scotia sea this interpretation is most easily incorporated into a model in which the basins in the central parts of the scotia sea had already formed by chron c8 contrary to some widely accepted interpretations and which has significant implications for paleoceanography and paleobiogeography

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

structure of the southern falkland islands continental shelf initial results from new seismic data abstract acquisition of a modern marine dataset has revealed the internal structure of sedimentary basins to the south of the falkland islands for the first time the new data reveal a variety of structural styles and indicate the presence of a sedimentary section ranging from 2 to 10 km in thickness developed in three linked sub basins 1 the eastern part of the malvinas basin lies south west of the islands a prominent regional unconformity developed throughout the area marks the top of gently folded rocks of probable palaeozoic age and their cover of late jurassic volcanics to the north west of the island the underlying strata approach the surface in a structural high which forms a prominent gravity low and may record the continuation of the palaeozoic basin exposed on the islands themselves strata of probable mesozoic and tertiary age rest on the unconformity throughout the area and show a regional southward dip these deposits increase in thickness south westwards towards the depocentre of the malvinas basin midway between the islands and argentina 2 the south falkland basin and burdwood bank lie south of the islands thicknesses increase towards an area of southward dipping extensional fault blocks within the south falkland basin downdip these are buried beneath a prograding and north vergent stack of blind and emergent thrusts at the northern margin of the burdwood bank which is a structural highly lying a shallow water south of the islands thrust loading has resulted in the development of a foreland basin north of the thrust front complex antiformal structures are associated with the thrusting whereas a shallow sedimentary basin developed on the crest of the burdwood bank is interpreted as a piggy bank basin riding on and sourced dominantly from within the thrust pile 3 the falkland plateau basin lies to the east and south east of the islands and is characterized by a varied and extensive mesozoic to tertiary succession of submarine fan deposits thickening eastwards into deeper water igneous intrusives deep in the section may correlate with jurassic volcanic rocks present within the malvinas basin to the west or may record younger rifting associated with the opening of the south atlantic these initial results indicate considerable structural heterogeneity in this part of the falkland islands continental shelf in the light of the hydrocarbon discoveries already made further west significant economic potential is likely stratigraphic traps may occur at regional unconformities and within submarine fan deposits whereas potential structural traps include mesozoic extensional fault blocks and thrust and sub thrust structures generated by tertiary and quaternary compression

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

review of erosion dynamics along the major n s climatic gradient in chile and perspectives abstract chile is an elongated country running in a north south direction for more than 30 along a subduction zone its climate is progressively wetter and colder from north to south this particular geography has been used positively by a growing number of studies to better understand the relationships between erosion processes and climate land use slope tectonics volcanism etc here we review the erosion rates factors and dynamics over millennial to daily periods reported in the literature in addition 21 new catchment mean erosion rates suspended sediment and 10be are provided and previous suspended sediment derived erosion rates are updated a total of 485 local and catchment mean erosion rates are reported erosion rates vary between some of the smallest values on earth 10 5 mm a to moderate values 0 5 mm a compared to other active ranges this review highlights strong limitations concerning the quantification of local erosion factors because of uncertainties in sampling point location slope and rainfall data for the mean erosion rates e for the millennial and decennial catchments a model of the form e s 1 s 0 6 2 r with 0 3 0 8 accounts for 40 to 70 of the erosion variance confirming a primary role of slope s compared to precipitation rate r over this time scale over the long term this review points to the long 5 to 10 ma response time of rivers to surface uplift in north central arid chile over millennia data provide evidence for the progressive contribution of extreme erosion events to millennial averages for drier climates as well as the link between glacier erosion and glacier sliding velocity in this period of time a discrepancy exists between the long term offshore sedimentological record and continental decennial or millennial erosion data for which no single explanation appears still little information is available concerning the magnitude of variation of millennial erosion rates over centuries data show the variable role of groundwater in the dynamics of suspended load and document a decrease in erosion over hundreds of years probably associated with historical harvesting

continental stretching preceding the opening of the drake passage evidence from tierra del fuego age estimates for the onset of a seaway through the drake passage range from middle eocene to early miocene complicating interpretations of the relation between ocean circulation and cenozoic global cooling here we present evidence for the presence of a latest paleocene early eocene extensional basin i e lateral rift in tierra del fuego an accurately dated postrift unconformity indicates that extensional faulting ended in the studied area ca 49 ma in concurrence with a previously reported eightfold increase in south america antarctica separation rate and the proposed onset of oceanic basins in the incipient drake passage the coincidence of these facts indicates progressive strain concentration on the zone of future crustal separation i e drake passage after abandonment of lateral rifts ca 49 ma although the described extensional depocenters did not allow the exchange of water between the pacifi c and atlantic oceans they represent a direct indication of continental lithospheric stretching preceding the recently proposed eocene opening of embryonic basins in the west scotia sea

the tectonic history of drake passage and its possible impacts on global climate this study provides an integrated review of plate tectonic models of the evolution of the antarctica patagonia connection compared to geological records collected on land in patagonia and tierra del fuego and offshore along the northern edge of the scotia sea a temporal framework for the sedimentary and tectonic events of the north scotia ridge and tierra del fuego is constructed with additional data compiled from entire patagonia and the austral basin this review provides robust correlations of seaways and tectonic events along the scotia and south america plates and indicates that the opening of the drake passage was not steady state since ca 30 ma rather the regions forming the present day northern limit of this gateway experienced important paleogeographic changes from deep marine basins to shallow ridges and emerged regions during the late oligocene and early middle miocene time our compilation of geological data shows that emergence along the north scotia ridge and tierra del fuego was achieved at 23 22 ma and has been followed by elimination of the patagoniano sea in patagonia starting at 22 23 ma and achieved at 20 ma this transition towards more continental sedimentation in southern south america is correlated with more shallow marine conditions in the austral basin this succession of events had a strong influence on the general geometry of the drake passage corresponding to a constriction of its northern limit starting in the window 29 22 ma and achieved at 21 ma this period of active deformation in southern south america also corresponds to a period of the global climate having two anomalies well known from the isotopic records the late oligocene warming around 26 ma and the mid miocene climatic optimum which ended between 15 and 14 ma the possible effects of the post oligocene tectonic evolution of the drake passage region on general oceanic circulation are discussed causes for the synchronicity between tectonic events and these global warming events are examined

lead isotope provinciality of central north pacific deep water over the cenozoic understanding the pre anthropogenic pb cycle of central north pacific deep water has attracted a lot of attention in recent years partly because of its unique geographical location in that it is a remote gyre system characterized by high dust fluxes and sluggish overturning circulation however the factors controlling pb isotope evolution in this area over the cenozoic are still controversial and various mechanisms have been proposed in previous studies here we report new pb and nd isotope time series of four ferromanganese crusts two from the western pacific near the mariana arc and the other two from the central pacific together with previously published records we discuss for the first time the significance of a persistent and systematic pb isotopic provinciality recorded by central north pacific crusts over the cenozoic we propose that globally well mixed stratosphere volcanic aerosols could contribute pb but have not been the major factors controlling the pb isotope distribution in the central north pacific over time island arc input and probably enhanced hydrothermal input between about 45 and 20 ma likely controlled the pb isotope provinciality and evolution prior to 20 ma when coeval pb isotope records in different crusts showed large differences and atmospheric silicate dust flux was extremely low after the eocene in particular after 20 ma asian dust input has become an isotopically resolvable source while island arc derived pb has remained important to balance the dust input and to produce the observed pb isotope distribution in the central north pacific during this period

geomorphology and shallow structure of a segment of the atlantic patagonian margin abstract we study an area little known of the atlantic patagonian margin from 44 30 s to 47 40 s and from 59 w to nearly 61 w the multi beam bathymetry coupled with high resolution seismic reflection profiles has provided details on the morphology and shallow acoustic structure on this area the main morphological characteristics of the seafloor features on the shelf and middle slope are described the atlantic patagonian continental shelf north of 45 40 s is located at a depth of 170 200 m south of this latitude the shelf edge is at 128 to 200 m the shelf surface is marred by circular depression and ridges oriented oblique to the shelf edge the upper slope and upper middle slope are plowing by icebergs from antarctica in pleistocene and local reefs of cold water coral further enhance the topography of the area in the middle slope there are two terraces the 20 to 60 km wide nagera and the 15 to 60 km wide perito moreno terraces showing moats hollows pot holes sediment drifts and sediment waves the terraces may have been formed in late miocene whereas the other forms are of pleistocene age other features are a sediment swell south of 47 s and seven submarine canyons on the middle slope these incipient canyons have been developed in the middle slope by retrogressive erosion some terminating on the upper middle slope and others on the upper slope and the canyon 6 breaching the shelf edge individual seafloor features existing on the atlantic patagonian margin have been classified into two main groups according to their origin along and across slope processes these primary agents were supplemented by endogenic processes such as expulsion of gas water diapirism of high pressure mud and folding faulting the results suggest that today down slope processes on the slope are practically non existent and that the morphology of the upper and middle slope is slowly being remolded by along slope bottom currents

autopsy on a dead spreading center the phoenix ridge drake passage antarctica new bathymetric and magnetic anomaly data from the phoenix ridge antarctica show that extinction of all three remaining segments occurred at the time of magnetic chron c2a 3 3 0 2 ma synchronous with a ridge trench collision south of the hero fracture zone this implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision spreading rates slowed abruptly at the time of chron c4 7 8 0 3 ma probably as a result of extinction of the west scotia ridge which would have led to an increase in slip rate and transpressional stress across the shackleton fracture zone spectacular high relief ridges flanking the extinct spreading center mapped for the first time using multibeam swath bathymetry are interpreted as a consequence of a reduction in spreading rate involving a temporary magma oversupply immediately prior to extinction

oligocene paleoceanographic changes based on an interbasinal comparison of cibicidoides spp δ18o records and a new compilation of data abstract oligocene paleoceanographic changes are useful for assessing future changes in the warming world because the levels of atmospheric pco2 in the oligocene were similar to those expected within the next century however the overall oligocene history is still unclear even though our understanding has been improved by previous studies in this paper we review the preexisting hypotheses for the eocene oligocene and oligocene miocene changes as well as the long term global changes based on newly compiled cibicidoides spp 18o 18ocib records we divide the oligocene paleoceanographic changes into five phases an abrupt increase in 18ocib values during the eocene oligocene transition eot phase 1 a gradual decrease in the early oligocene phase 2 a long term slow increase in the early to middle oligocene phase 3 a gradual decrease in the late oligocene phase 4 and an abrupt increase near the oligocene miocene transition omt phase 5 these recurring sequential changes in the global scale 18ocib profile following the eot are likely not directly connected to the strengthening and stratification of ocean currents during the oligocene as a result of the unidirectional strengthening of the antarctic circumpolar current acc and the northern component water ncw instead we suggest that the strengthened ocean currents contributed significantly to interbasinal differences in 18ocib values for a more robust explanation of oligocene long term 18ocib changes following the eot future studies will require high resolution and more reproducible records of changes in the volume of antarctic ice and atmospheric pco2 as well as information about the tectonic history

middle eocene to late oligocene antarctic glaciation deglaciation and southern ocean productivity during the eocene oligocene transition earth cooled significantly from a greenhouse to an icehouse climate nannofossil assemblages from southern ocean sites enable evaluation of paleoceanographic changes and hence of the oceanic response to antarctic ice sheet evolution during the eocene and oligocene a combination of environmental factors such as sea surface temperature and nutrient availability is recorded by the nannofossil assemblages of and can be interpreted as responses to the following changes a cooling trend started in the middle eocene was interrupted by warming during the middle eocene climatic optimum and by short cooling episodes the cooling episode at 39 6 ma preceded a shift toward an interval that was dominated by oligotrophic nannofossil assemblages from 39 1 to 36 2 ma we suggest that oligotrophic conditions were associated with increased water mass stratification low nutrient contents and high efficiency of the oceanic biological pump that in turn promoted sequestration of carbon from surface waters which favored cooling after 36 2 ma we document a large synchronous surface water productivity turnover with a dominant eutrophic nannofossil assemblage that was accompanied by a pronounced increase in magnetotactic bacterial abundance this turnover reflects a response of coccolithophorids to changed nutrient inputs that was likely related to partial deglaciation of a transient antarctic ice sheet and or to iron delivery to the sea surface eutrophic conditions were maintained throughout the oligocene which was characterized by a nannofossil assemblage shift toward cool conditions at the eocene oligocene transition finally a warm nannofossil assemblage in the late oligocene indicates a warming phase

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

polynostratigraphy and palaeoenvironments across the oligocene miocene boundary within the centinela formation southwestern argentina abstract palynological analysis of the centinela formation exposed in the foothills of the patagonian andes has revealed the presence of pollen dinoflagellate cysts and chlorococcalean and prasinophycean algae these groups are here reported from the centinela formation for the first time sporomorph and dinoflagellate cyst assemblages suggest a late oligocene and early miocene age these results coincide with a 87 sr 86 sr age close to the age of the oligocene miocene boundary obtained from the lower part of the section palynological information from the centinela formation permits correlation with upper oligocene and lower miocene units cropping out along the atlantic patagonian coast assemblages from the lower part of the section suggest that the beds were deposited under marine near shore palaeoenvironmental conditions with a strong continental influence in the middle part of the section high dinoflagellate cyst ratios coincide with a maximum flooding surface recorded in the centinela formation towards the top of the centinela formation the sporomorph assemblages reflect the development of vegetation adapted to coastal environments which agrees with the sparse occurrence of marine palynomorphs a new dinoflagellate species hystrichostrogylon sulcatum is proposed this species appears to range across the oligocene miocene boundary and is particularly abundant in the lowest miocene

cooling of the oceanic lithosphere evidence from geoid anomalies across the udintsev and eltanin fracture zones abstract the often step like variation in the marine geoid across fracture zones is believed to reflect the change in age of the ocean floor and hence in temperature structure of the plate across the fracture zone the rate of change of the geoid with age has been estimated as a function of age from geoid offsets across fracture zones and used to constrain thermal models of lithospheric cooling results from three studies of the geoid offsets across the mendocino fracture zone are consistent with a thermal plate cooling model having a plate thickness between 100 and 125 km this is in contrast to results obtained from the combined measurements from fracture zones in the south pacific which are not consistent with either the plate model or boundary layer model of cooling to examine this inconsistency we have measured geoid offsets on seasat profiles across the udintsev and eltanin fracture zones in the south pacific and tested the robustness of the measurement technique we used a second order polynomial that included a step function centered on the fracture zone to estimate the regional geoid and the geoid offset the polynomials were fit to the geoid profiles using least squares minimization criteria data within a certain distance of the fracture zone were excluded from the fitting procedure in order to minimize the effects of horizontal cooling and lithospheric deformation near the fracture zone we find that the estimate of the geoid offset is very sensitive to the length of data excluded and that generally an exclusion width of 100 200 km is needed to avoid anomalies unrelated to the regional geoid offset near the fracture zone exclusion widths of less than 100 km generally lead to unpredictable errors in the geoid offset estimates using exclusion widths of between 150 and 200 km on most profiles we constructed geoid slope geoid offset divided by the age offset versus average age plots for each of the limbs of the two fracture zones similar trends were found on both the east and west limbs of the eltanin and the east limb of the udintsev the trends were inconsistent with a single thermal plate thickness and instead showed an initial decrease in slope between 0 and 35 m y followed by an abrupt increase followed again by a decrease in slope in contrast the west branch of the udintsev shows an increase in geoid slope values between 20 and 35 m y followed by a decrease the difference in observed trends across the western limb of the udintsev may be due to the effects of the louisville ridge north of the fracture zone on the regional geoid the similarity in trends on the remaining three limbs argues against the effects of isolated thermal or bathymetric anomalies and appears instead to reflect a general feature of the geoid slope versus average age relationship across fracture zones although the thermal plate cooling model is successful in predicting both seafloor depths and heat flow values out to ages of at least 80 m y b p it cannot explain the observed geoid slope values for these two fracture zones it is not clear at this point whether this is due to inadequacies in the cooling model or to peculiarities in fracture zone evolution possibly caused by small scale convection driven by the horizontal temperature gradients beneath fracture zones

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

drake passage and cenozoic climate an open and shut case drake passage opening has often been viewed as a single discrete event possibly associated with abrupt changes in global circulation and climate at or near the eocene oligocene boundary a new plate tectonic model based on recent reinterpretations of the opening history of basins in the scotia sea suggests that an effective ocean gateway may have developed even earlier during the middle eocene this is consistent with a growing body of evidence from sediment core proxy data for eocene changes in southern ocean circulation and biological productivity the period between earliest opening after 50 ma and the latest eocene was characterized by the evolution of various current pathways across the subsiding continental shelves and intervening deep basins this shallow opening may have caused important changes in southern ocean circulation contributing to eocene cooling and the growth of antarctic ice sheets

the curved magallanes fold and thrust belt tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study the magallanes fold and thrust belt ftb presents a large scale curvature from n s oriented structures north of 52 s to nearly e w in tierra del fuego island we present a paleomagnetic and anisotropy of magnetic susceptibility ams study from 85 sites sampled in cretaceous to miocene marine sediments magnetic susceptibility is lower than 0 0005 si for 76 sites and mainly controlled by paramagnetic minerals ams results indicate that the sedimentary fabric is preserved in the undeformed areas of tierra del fuego and the more external thrust sheets units where an incipient lineation due to layer parallel shortening is recorded prolate ams ellipsoids indicating a significant tectonic imprint in the ams fabric are observed in the internal units of the belt ams results show a good correlation between the orientation of the magnetic lineation and the fold axes however in peninsula brunswick the ams lineations are at 20 counterclockwise to the strike of the fold axes pretectonic stable characteristic remanent magnetizations chrm were determined in seven sites a counterclockwise rotation 21 2 9 2 is documented by chrm data from four sites near the hinge of the belt in peninsula brunswick and near canal whiteside while there is no evidence of rotation near the nearly e w oriented vicuna thrust within tierra del fuego the curved shape of the cenozoic magallanes ftb is not related to vertical axis rotation and thus the magallanes ftb can be considered as a primary arc

climatic and ecologic changes during miocene surface uplift in the southern patagonian andes abstract the up to 4 km high southern patagonian andes form a pronounced topographic barrier to atmospheric circulation in the southern hemisphere westerlies and cause one of the most drastic orographic rain shadows on earth geologic data imply that this climatic pattern has been established or significantly enhanced during miocene surface uplift of this andean segment we report evidence for important climatic and ecologic changes in the eastern foreland of the patagonian andes that appear to be the result of this uplift to provide constraints on miocene plant ecosystems and precipitation in the eastern leeward foreland of the patagonian andes we determined carbon and oxygen isotope values of pedogenic carbonate nodules from a 500 m thick section of the continental santa cruz formation the age of these deposits was constrained by ar ar dating of intercalated tuffs which range from 22 to 14 ma at 16 5 ma the 13 c values increase by 3 the 18 o values decrease by 2 and the scatter in the oxygen isotope data increases significantly we interpret these changes as the consequence of 1 km surface uplift in this andean segment from the 18 o values and increased aridity to its east from the 13 c values and the increased scatter in the 18 o values sediments overlying the santa cruz formation are very limited in extent and volume and dominated by coarse conglomerates related to pleistocene and older glaciations it thus seems that by 14 ma deposition in the eastern foreland of the southern patagonian andes had essentially ceased as the result of rain shadow formation

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

pliocene extensional tectonics in the eastern central patagonian cordillera geochronological constraints and new field evidence recent field work and review of radiometric data obtained from neogene lavas and plutonic rocks exposed in the eastern central patagonian cordillera 46 48os which overlie subducted segments of the south chile ridge suggest important late miocene to pleistocene morphological changes in relation to base level variations and or tectonic events we present new field observations from a region south of lago general carrerabuenos aires between the main cordillera and the meseta del lago buenos aires demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5 4 ma and after 3 ma we also show that the 12 ma basaltic flows of the meseta del lago buenos aires 2000 m a s l have been subjected to deep incision with younger lavas dated at 1 2 ma partially filling the valleys these incisions are thought to reflect progressive eastward tilting of the entire meseta our new observations together with additional features from central patagonia strongly suggest that tectonic events led to a regional widespread morphological change after 5 ma the coincidence in time and space between the subduction of segments of the south chile ridge at 6 and 3 ma causing opening of a slab window and strong base level variations in the studied area suggests a cause and effect relationship in central patagonia compressional tectonics ended well before extensional events reported here causes of uplift and further extension are probably completely disconnected the uplift is purely tectonic in origin and occurred prior to the subduction of the south chile ridge extension should be a consequence of this subduction

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

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

origin signature and palaeoclimatic influence of the antarctic circumpolar current the antarctic circumpolar current acc is today the strongest current in the world s ocean with a significant influence on global climate its assumed history and influence on palaeoclimate while almost certainly equally profound are here called into question in this paper we review 30 years of accumulated data interpretation and speculation about the acc deriving mainly from dsdp and odp drilling in the southern ocean for most of this time a conventional view of acc development signature and influence has held sway among palaeoceanographers and marine geologists in this view the acc began at about 34 ma close to the eocene oligocene boundary the time of onset of significant antarctic glaciation and the time of creation of a deep water gap tasmanian seaway between australia and antarctica as the south tasman rise separated from north victoria land this is the smoking gun of synchroneity the southern ocean sediment record shows a latest eocene development and subsequent geographic expansion of a siliceous biofacies its northern limit taken to indicate the palaeoposition of the acc axis in addition the acc was considered to have caused antarctic glaciation by isolating the continent within a cold water annulus reducing north south heat transport a different and later date for antarctic south american opening drake passage was proposed but the timing of acc onset there was disputed and the simple story survived recent developments however call it into question modern physical oceanography shows that all or most of present day acc transport is confined to narrow jets within deep reaching circumpolar fronts and numerical modelling has suggested that a steady reduction in greenhouse gas concentration through the cenozoic could cause antarctic glaciation with or without a contribution from ocean circulation change the rapidity of antarctic glacial onset at the eocene oligocene boundary and coeval creation of a deep water gap south of tasmania both survive but in light of the new information the presence of a siliceous biofacies cannot be claimed as evidence of the existence of a continuous deep reaching oceanic front and therefore of an acc and the possibility arises that cool and cold sea surface temperatures were effects of antarctic glaciation rather than evidence of a major contributor to its cause in considering future work we emphasise the importance of additional information from ancillary fields better definition of the necessary and sufficient properties of oceanic fronts additional determinations of cenozoic atmospheric pco2 and further developments in models of antarctic glaciation but also suggest the way forward in marine geology our knowledge of the development and palaeoclimatic significance of the acc will be best served by grainsize studies of bottom current strength at selected locations and geochemical or mineralogical studies of clays and ird as a way of examining provenance and therefore surface and bottom current directions and the existence of interocean connections

early oligocene initiation of north atlantic deep water formation dating the onset of deep water flow between the arctic and north atlantic oceans is critical for modelling climate change in the northern hemisphere1 2 and for explaining changes in global ocean circulation throughout the cenozoic era3 from about 65 million years ago to the present in the early cenozoic era exchange between these two ocean basins was inhibited by the greenland scotland ridge3 4 but a gateway through the faeroe shetland basin has been hypothesized3 5 previous estimates of the date marking the onset of deep water circulation through this basin on the basis of circumstantial evidence from neighbouring basins have been contradictory5 6 7 8 9 ranging from about 35 to 15 million years ago here we describe the newly discovered southeast faeroes drift which extends for 120 km parallel to the basin axis the onset of deposition in this drift has been dated to the early oligocene epoch 35 million years ago from a petroleum exploration borehole we show that the drift was deposited under a southerly flow regime and conclude that the initiation of deep water circulation from the norwegian sea into the north atlantic ocean took place much earlier than is currently assumed in most numerical models of ancient ocean circulation

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

anatomy and formation of oblique continental collision south falkland basin 1 the south falkland basin is a partially filled cenozoic foreland basin located south of the falkland plateau it was formed by flexure of this southern edge of the south american plate when the load represented by burdwood bank collided this continental fragment belongs to the predominantly oceanic scotia plate flexure probably started in early cenozoic times and has continued to the present day the whole region is submarine and so the detailed stratigraphy and structure of the basin has been well imaged by seismic reflection profiling the clarity of this imagery has made analysis of structures within the collision zone possible the plate boundary itself is an active oblique thrust fault which has controlled the growth of a frontal fold there is evidence for older phases of thrusting and folding further south within and beneath the sediments which blanket the flexed south american plate normal faulting occurs on a variety of scales episodes of stratigraphic growth associated with the largest of these faults demonstrates that they were active during flexural bending we have modeled the development of the south falkland basin using two different approaches both of which are based upon the simplest elastic model inverse modeling of free air gravity and bathymetric profiles suggest that the elastic thickness of the loaded crust is 5 20 km a complementary approach based upon the spectral analysis of free air gravity and bathymetry shows that the elastic thickness is 15 5 km both techniques indicate that the flexed continental lithosphere is weak a conclusion supported by the presence of normal faults within the flexed plate a small increase in elastic thickness from west to east appears consistent with a change in the density and penetration of normal faulting

magellan strait part of a neogene rift system the magellan strait joins the atlantic and pacific oceans separating tierra del fuego from southernmost continental south america the strait cuts both the andean cordillera and the magellan foreland basin other arms of the sea intrude the axial zone of the magellan basin these depressions have long been interpreted as glacial valleys on the basis of landsat images digital topography and field data we interpret the depressions as rifts or half rifts in general active rifts developed within foreland basins are unusual but in patagonia they are consistent with regional deformation and its plate tectonic setting during the neogene

indications for control of the iceland plume on the eocene oligocene greenhouse icehouse climate transition the eocene oligocene boundary at about 33 5 myr ago marks the transition from greenhouse to icehouse world accompanied by a sudden cooling of ocean bottom water we show that this global event is simultaneous with a deep rooted mantle process an abrupt suppression of the iceland plume triggered rapid deepening of the greenland scotland ridge gsr the sill moderating deep circulation between the nordic seas and north atlantic striking coincidence of several sets of events reflects the abrupt suppression of the iceland plume and a rapid removal of its influence on the nearby reykjanes ridge rr 1 a sudden segmentation of the paleo rr seen on seafloor magnetic anomalies 2 a drop in spreading rate of the north atlantic 3 a transition from thick to normal oceanic crust and 4 a rapid deepening and accelerated subsidence of the gsr inferred from the sedimentary record of dsdp site 336 the plume suppression and the concomitant gsr deepening coincide with the initiation of north atlantic deep water nadw at the eocene oligocene e o transition attested by onset of drift sedimentation in the faroe shetland channel fsc the deepest spill point on the gsr and in the north atlantic the feni drift these processes have influenced global deepwater composition and temperature as indicated by the striking correlation with the jump in global 18o 1 measured on benthic foraminifers that reflects the e o global cooling and with enrichment of unradiogenic nd isotopes in the southeastern atlantic and southern ocean the initiation of atlantic thermohaline circulation at that time is inferred from the abrupt split between planktonic and benthic 18o indicating the building of ocean water stratification this scenario is further corroborated by a reversal in benthic 18o at the late oligocene coincident with the renewal of vigorous iceland plume some 25 myr ago causing a considerable retardation in nadw fluxes the plume renewal is inferred from the emergence of the iceland plateau the transition to oblique unsegmented rr axis the cessation in deepening of the gsr and rapid increase in spreading rate of the north atlantic these events coincide with decreasing difference in planktonic benthic in global 18o by the late oligocene all these inferences suggest the role of the nadw sourced at the nordic seas to form background cooler conditions in the long time scale since the early oligocene or to form permanent conditions of invigorated thermohaline circulation that forces co2 trap in the oceans

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

timing and origin of deformation along the patagonian fold and thrust belt the andean orogeny in the patagonian cordillera of southern south america reflects the consequences of the mesozoic and cenozoic subduction of an oceanic plate beneath the south american continental margin the geological evolution of the region has been influenced by the eocene collision and subduction of the farallon aluk ridge and the miocene recent subduction of the chile ridge another aspect of plate interaction during this period was two intervals of rapid plate convergence one at 50 42 ma and the other at 25 10 ma between the south american and the oceanic plates it has been proposed that the collision of the chile ridge with the trench was responsible for the development at least in part of the patagonian fold and thrust belt this belt extends for more than 1000 km along the eastern foothills of the southern andes between 46 and 54 s along the southwestern rim of the austral basin the interpretation of a link between subduction of the ridge and formation of the fold and thrust belt is based on assumed time coincidences between contractional tectonism and the collision of ridge segments during middle and late miocene times the main tertiary contractional events in the patagonian fold and thrust belt took place during latest cretaceous palaeocene eocene and during miocene times although the timing of deformation is still poorly constrained the evidence currently available suggests that there is little or no relationship between the timing of the fold and thrust belt and the collision of ridge segments most if not all of the contractional tectonism pre dated the latest episodes of ridge collision collision of a ridge crest with the continental margin has been active for the past 14 to 15 million years contrary to the suggestion of a relationship between ridge subduction and compression the main result of this collision has been fast uplift and extensional tectonism the initiation of the patagonian fold and thrust belt in latest cretaceous or early tertiary times coincided with a fundamental change in the tectonic evolution of the austral basin throughout the cretaceous most of this basin subsided as a broad backarc continental shelf only in latest cretaceous times and coinciding with the initiation of the fold and thrust belt the basin underwent a transition to a retro arc foreland basin this change to an asymmetrically subsiding foreland basin with an associated foreland fold and thrust belt was related to uplift of the andean orogenic belt in the west

geochronology and geochemistry of the northern scotia sea a revised interpretation of the north and west scotia ridge junction abstract understanding the tectonic evolution of the scotia sea is critical to interpreting how ocean gateways developed during the cenozoic and their influence on ocean circulation patterns and water exchange between the atlantic and southern oceans we examine the geochronology and detrital age history of lithologies from the prominent submerged barker plateau of the north scotia ridge metasedimentary rocks of the north scotia ridge share a strong geological affinity with the fuegian andes and south georgia indicating a common geological history and no direct affinity to the antarctic peninsula the detrital zircon geochronology indicates that deposition was likely to have taken place during the mid late cretaceous a tonalite intrusion from the barker plateau has been dated at 49 6 0 3 ma and indicates that magmatism of the patagonian fuegian batholith continued into the eocene this was coincident with the very early stages of drake passage opening the expansion of the proto scotia sea and reorganization of the fuegian andes the west scotia ridge is an extinct spreading center that shaped the scotia sea and consists of seven spreading segments separated by prominent transform faults spreading was active from 30 6 ma and ceased with activity on the w7 segment at the junction with the north scotia ridge reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the w7 spreading segment is distinct to the other segments of the west scotia ridge basaltic lava samples from the eastern flank of the w7 segment have been dated as early mid cretaceous in age 137 93 ma and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the w7 segment instead the w7 segment is likely to represent a downfaulted block of the north scotia ridge of the fuegian andes continental margin arc or is potentially related to the putative cretaceous central scotia sea

evolution of atlantic thermohaline circulation early oligocene onset of deep water production in the north atlantic the flow of deep water masses is a key component of heat transport in the modern climate system yet the role of deep ocean heat transport during periods of extreme warmth is poorly understood the present mode of meridional overturning circulation is characterized by deep water formation in both the north atlantic and the southern ocean however a different mode of meridional overturning circulation operated during the extreme greenhouse warmth of the early cenozoic during which time the southern ocean was the dominant region of deep water formation the combination of general global cooling and tectonic evolution of the atlantic basins over the past 55 m y ultimately led to the development of a mode of overturning circulation characterized by both southern ocean and north atlantic deep water sources the change in deep water circulation mode may in turn have affected global climate however unraveling the causes and consequences of this transition requires a better understanding of the timing of the transition new nd isotope data from the southeastern atlantic ocean indicate that the initial transition to a bipolar mode of deep water circulation occurred in the early oligocene ca 33 ma the likely cause of significant deep water production in the north atlantic was tectonic deepening of the sill separating the greenland norwegian sea from the north atlantic

sand boils without earthquakes sedimentary deformation caused by liquefaction has become a popular means for inferring prehistoric strong earthquakes in this report we describe a new mechanism for generating such features in the absence of earthquakes sand boils and a 180 m long sand dike formed in fremont valley california when sediment laden surface runoff was intercepted along the upslope part of a 500 m long preexisting ground crack flowed subhorizonally in the crack and then flowed upward in the downslope part of the crack where it discharged as sand boils on the land surface if the sand boils and their feeder dike were stratigraphically preserved they could be misinterpreted as evidence for earthquake induced liquefaction

contrasting palaeoenvironments of the mid late miocene dunedin volcano southern new zealand climate or topography abstract at the mid late miocene boundary a northward shift occurred in the subtropical front neighbouring the new zealand continental landmass the terrestrial palaeoenvironment of new zealand concurrently underwent a period of cooling the dunedin volcanic group in southern new zealand spans the mid late miocene boundary and the sedimentary deposits from kaikorai valley and double hill within this group contain diverse and well preserved floral assemblages we used terrestrial palaeoclimate proxies clamp climate leaf analysis multivariate program and ba bioclimatic analysis to determine the paleoenvironment at southern mid latitudes during the mid late miocene transition our results indicate that the mid late miocene climate of the dunedin volcano was warm temperate to subtropical mean annual temperature mat 17 19 c at double hill similar to modern coastal queensland and cool to warm temperate mat 12 14 c at kaikorai valley similar to modern northern new zealand winter temperatures at kaikorai valley were distinctly cooler 6 8 c differing aspect i e north vs south facing may have played a part in determining solar radiation particularly in an area of high relief such as the dunedin volcano because kaikorai valley was situated facing south and double hill facing north the two sites would have received differing radiative fluxes particularly in winter thereby possibly influencing the local environment biota and potentially also the palaeoclimate signal produced by floral proxies this study provides evidence for the temperature decline in terrestrial palaeoenvironments at mid latitudes caused by the northward movement of the subtropical front and strengthening of the antarctic circumpolar current during the mid late miocene

role of the drake passage in controlling the stability of the ocean s thermohaline circulation the role of a southern ocean gateway in permitting multiple equilibria of the global ocean thermohaline circulation is examined in particular necessary conditions for the existence of multiple equilibria are studied with a coupled climate model wherein stable solutions are obtained for a range of bathymetries with varying drake passage dp depths no transitions to a northern hemisphere nh overturning state are found when the drake passage sill is shallower than a critical depth 1100 m in the model described herein this preference for southern hemisphere sinking is a result of the particularly cold conditions of the antarctic bottom water aabw formation regions compared to the nh deep water formation zones in a shallow or closed dp configuration this forces an exclusive production of deep bottom water in the southern hemisphere increasing the depth of the drake passage sill causes a gradual vertical decoupling in atlantic circulation removing the influence of aabw from the upper 2000 m of the atlantic ocean when the dp is sufficiently deep this shifts the interaction between a north atlantic deep water nadw cell and an aabw cell to an interaction between an shallower antarctic intermediate water cell and an nadw cell this latter situation allows transitions to a northern hemisphere overturning state

a geologic perspective on the miocene evolution of the antarctic circumpolar current system abstract the dominant geologic feature of pelagic sediment accumulation patterns at intermediate water depths in the southern ocean is the sequential northward migration and replacement of a calcareous ooze sedimentary facies by a siliceous ooze facies following the late oligocene opening of the drake passage and the subsequent initiation of the antarctic circumpolar current closely spaced whole core measurements of bulk density are evaluated for four sites drilled in the southern ocean during ocean drilling program odp legs 113 sites 689 and 690 and 120 sites 747 and 751 these sites are all located on submarine rises in open ocean pelagic depositional environments south of the polar front a major oceanographie and sedimentologic boundary high resolution profiles of bulk density are used to correlate between adjacent drill holes on the maud rise where multiple overlapping sedimentary sequences were recovered using the advanced hydraulic piston core barrel apc stratigraphie age models have been applied to the bulk density data to provide a temporal framework for a discussion of the sedimentologic basis for paleoceanographic interpretations and also to constrain future acoustic research studies the early to middle miocene intervals at sites 689 and 690 are marked by distinct sequences of meter scale alternating carbonate and siliceous units separated by several proposed stratigraphie hiatuses at least six dissolution productivity events are identified in the middle to late miocene on the maud rise and others are noted at the odp sites on the kerguelen plateau the downhole profiles are used to identify the density signature and position of sediment facies changes and proposed stratigraphie hiatuses in sediment cores of late oligocene to late miocene age based on the reversals inflections and discontinuities in the downhole bulk density trends an analysis of the observed bulk density trends at these sites is combined with a review of other tectonic sedimentologic oceanographie and modeling studies to develop a hypothesis for explaining the observed sediment accumulation patterns at intermediate water depths of the southern ocean

continental response to active ridge subduction 1 apatite fission track ages from a 2000 m elevation transect from the patagonian fold and thrust belt 47 5 s allow us to quantify the denudational and orographic response of the upper plate to active ridge subduction accelerated cooling started at 17 ma predating the onset of ridge collision 14 10 ma and was followed by reheating between 10 and 6 ma thermal modeling favors reheating on the order of 60 c at 28 c ma due to east migration of a slab window after the ridge trench collision final rapid cooling since 4 ma of 18 c ma geothermal gradient of 14 c km correlates with the presence of an orographic barrier and 1 km rock uplift in this region between 17 1 and 6 3 ma increased precipitation and erosion since 4 ma caused asymmetric exhumation with 3 4 km on the leeside repeated crustal unroofing in response to active ridge subduction can explain the positive gravity anomaly south of the chile triple junction

opening of drake passage gateway and late miocene to pleistocene cooling reflected in southern ocean molluscan dispersal evidence from new zealand and argentina thirty two benthic marine molluscan genus group taxa are newly recognised as common to new zealand and argentina previously suggested examples including panis athlopecten austrovenus large cirsotrema lucinoma aulacomya and zygochlamys are confirmed from new collections the total of 46 cenozoic dispersed taxa now recognised falls into four main groups a oligocene early miocene dispersals from south america to new zealand 8 taxa b oligocene early miocene dispersals from new zealand to south america 22 c latest miocene pliocene dispersals from south america to new zealand 9 and d pleistocene holocene dispersals from south america to new zealand at least 7 taxa dispersal seems to have occurred in bursts both to and from south america when the antarctic circumpolar current acc initiated i e when drake passage opened late in the oligocene while dispersal from south america to new zealand only seems to have occurred again at two later times 1 during the latest miocene pliocene enhancement of acc circulation caused by the initiation of west antarctic glaciation and 2 during pleistocene glaciations however the incompleteness of the argentinian record exaggerates the size of the initial burst and allows no assessment of post middle miocene dispersal to south america formation of the acc through the opening of a passage south of australia at ca 43 40 ma and of drake passage at ca 23 ma not only led to the glaciation of antarctica but also greatly altered southern ocean biogeography

review secular tectonic evolution of archean continental crust interplay between horizontal and vertical processes in the formation of the pilbara craton australia the archean pilbara craton contains five geologically distinct terranes the east pilbara karratha sholl regal and kurrana terranes all of which are unconformably overlain by the 3 02 to 2 93 ga de grey superbasin the 3 53 3 17 ga east pilbara terrane ep represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3 53 3 43 3 35 3 29 and 3 27 3 24 ga each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3 72 ga and melting of crust to generate first tonalite trondhjemite granodiorite ttg and then progressively more evolved granitic magmas in each case plume magmatism was accompanied by uplift and crustal extension the combination of conductive heating from below thermal blanketing from above and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust these mantle melting events caused severe depletion of the subcontinental lithospheric mantle making the ep a stable buoyant unsubductable continent by c 3 2 ga extension accompanying the latest event led to rifting of the protocontinent margins at between 3 2 and 3 17 ga after 3 2 ga horizontal tectonic forces dominated over vertical forces as revealed by the geology of the three terranes karratha sholl and regal of the west pilbara superterrane the c 3 12 ga whundo group of the sholl terrane is a fault bounded 10 km thick volcanic succession with geochemical characteristics of modern oceanic arcs including boninites and evidence for flux melting that indicate steep archean subduction at 3 07 ga the 3 12 ga sholl terrane 3 27 ga karratha terrane and c 3 2 ga regal terrane accreted together and onto the ep during the prinsep orogeny this was followed by development of the de grey superbasin an intracontinental sag basin and widespread plutonism 2 99 2 93 ga as a result of orogenic relaxation and slab break off craton wide compressional deformation at 2 95 2 93 ga culminated with 2 91 ga accretion of the 3 18 ga kurrana terrane with the ep this compression caused amplification of the dome and keel structure in the ep final cratonization was effected by emplacement of 2 89 2 83 ga post tectonic granites

evidence for late oligocene establishment of the antarctic circumpolar current removal of the last impediment to circum antarctic flow opening of the drake passage between antarctica and south america is essential to establishment of the acc and has been placed in both the earliest oligocene and earliest miocene on the basis of paleomagnetic reconstructions the tasman gateway between antarctica and tasmania notably deepened following 33 5 ma completion of the acc circuit has been seen as the key to major growth of the east antarctic ice sheet by thermal isolation of the continent an alternative argument attributes ice sheet growth to changes in atmospheric pco2 we present evidence on current strength and water mass properties suggesting a clear increase in flow speed and homogeneity of southern ocean water masses in the latest oligocene after about 23 95 ma marking establishment of the acc attributable to deep opening of drake passage the mi 1 glaciation may be a consequence establishment of a permanent low altitude ice sheet does not appear to take place prior to mid miocene cooling

stratigraphic and structural evolution of the tertiary cosmelli basin and its relationship to the chile triple junction at the taitao peninsula in southern chile 46 5 s an active mid ocean ridge is being subducted under the south american continent continental crust south of the current triple junction has experienced subduction of three ridge segments since the mid miocene a series of mid late tertiary sedimentary basins lies inboard of the triple junction the cosmelli basin lies in the region which has experienced the earlier ridge subduction events within the cosmelli basin fill abrupt facies dislocations give clear evidence of marked base level changes four sequence boundaries are delineated with an upward increase in degree of facies dislocation which is believed to reflect increasing magnitudes of base level fall the lower part of the basin fill is folded and then thrust eastward as a series of imbricate slices while the overlying greater thickness of fluvial sediments is only gently tilted westwards this geometry indicates that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited these processes controlled accommodation space and thus stratigraphic architecture during much of the basin life this complex basin history plus the presence of basaltic sheet intrusions may reflect the underlying slab history as successive ridge segments were subducted

environmental evolution and geological significance of the miocene carbonates of the eratosthenes seamount odp leg 160 abstract miocene carbonates of the eratosthenes seamount located offshore from cyprus in the eastern mediterranean odp leg 160 site 966 have been re analysed the use of sedex sequential extraction to evaluate the nutrient concentrations and of ct scan for the identification of large benthic foraminifera has resulted in a more detailed palaeoenvironmental reconstruction and an improved stratigraphic framework three intervals are recognised in the succession and are attributed to the lower middle and upper miocene respectively the lower miocene interval is dominated by large benthic foraminifera and echinoids this association colonized the top of the seamount following its uplift from deep water to the photic zone the middle miocene interval is characterised by rhodoliths and corals at its base and near the top whereas seagrass seaweed assemblages with epiphytic foraminifera and hooked coralline algal crusts dominate its central part the upper miocene is represented by coral reef facies near its base and lagoonal facies toward its top the succession was deposited in an oligotrophic environment and exhibits a general shallowing upward trend that was mainly controlled by tectonic activity related to the northward subduction of the african plate the succession of the eratosthenes seamount is characterised by a diverse large benthic foraminifera assemblage in its lower part and by corals in its uppermost part whereas corallines characterise the central interval this trend is common in miocene mediterranean carbonates and its occurrence on an isolated seamount confirms its connection with the general evolution of the basin the decline of miogypsinids and lepidocyclinids after the early miocene is notable because the eratosthenes seamount was geographically close to the indo pacific ocean where both groups also thrived during the middle miocene this decline was probably caused by the early miocene closure of the deep water connection between the indo pacific and the mediterranean

structural and stratigraphic evolution of the western malvinas basin argentina the malvinas basin is one of the few basins on the argentine continental shelf that contains a proven petroleum system however uneconomical oil discoveries keep the basin at a frontier exploration status the malvinas basin evolved through three main tectonic phases rift sag and foredeep the sedimentary fill of the basin is closely related to its tectonic history middle jurassic rifting resulted in north northwest oriented grabens that filled with volcanic and pyroclastic continental rocks diminished faulting and generalized subsidence during the late jurassic neocomian early sag phase were accompanied by deposition of a basal transgressive marine wedge the aptian maastrichtian interval was characterized by tectonic quiescence and deposition of offshore mud prone sediments southerly localized early paleogene transtensional tectonism accompanied the early development of a foredeep trough outer shelf glauconite rich sandstones basinal claystones and localized carbonate buildups partially filled the basin by the middle eocene oligocene a strong deepening event marked the initiation of the foredeep sensu stricto phase this phase resulted in the full development of the malvinas foredeep and the formation of compressional structures in the foreland the foredeep basin was replenished by a westerly derived offlapping siliciclastic wedge of oligocene miocene age noncommercial hydrocarbon discoveries in 5 of the 17 wells drilled in the basin suggest the presence of an undercharged lower inoceramus springhill petroleum system

balancing the cenozoic carbon and alkalinity cycles constraints from isotopic records a simple model of the carbon alkalinity and strontium cycles is built up and used to interpret the carbon and strontium isotopic evolution of seawater over the cenozoic as recorded in marine limestones the idea that weathering fluxes have increased globally over the cenozoic in response to the himalayan uplift is critically examined it is shown that such an increase of global weathering postulated to explain the sharp increase in the 87sr 86sr ratio is not compatible with the carbon isotopic record if the budgets of carbon alkalinity and strontium are considered simultaneously another scenario of the cenozoic evolution of global weathering consistent with the carbon isotopic record is proposed in which the global weathering flux varies only slightly over the cenozoic with a small increase between 15 ma and the present this second scenario appears more consistent with the reconstructed carbonate sedimentation rate and the evolution of the carbon isotopic fractionation in continental and oceanic environments

evolution of cenozoic seaways in the circum antarctic region abstract a complete circum antarctic seaway did not open until both the south tasman rise cleared the oates land coast of east antarctica and drake passage opened between the southern tip of south america and the northern end of the antarctic peninsula major plate motions based on dated seafloor spreading anomalies and distinct fracture zone lineations constrain the age of the opening of a seaway between the south tasman rise and antarctica as very close to the eocene oligocene boundary with an unrestricted opening deeper than 2000 m dating from 32 ma timing of the opening of drake passage is more circumstantial because the exact motions of certain micro continental fragments are not known the motion of africa with respect to south america as well as the motion of east antarctica with respect to africa are well constrained for the cenozoic these major plate motions are used with the reasonable assumption of no cenozoic motion of the antarctic peninsula with respect to east antarctica to constrain the location of the antarctic peninsula with respect to the southern tip of south america for the critical period of late eocene to late oligocene uncertainty of motion of the south georgia and south orkney microcontinents and other possible continental fragments make an exact time for opening of drake passage difficult to ascertain even so the early oligocene position of the antarctic peninsula with respect to south america requires a through going deep water seaway to have been open at drake passage prior to 28 ma even given the unconstrained motion of various high standing crustal fragments in the scotia sea with reasonable assumptions concerning motion of the crustal fragments in the western and central scotia sea it is likely that drake passage or passage through powell basin was open to deep water circulation by 31 2 ma

contourite deposits in the central scotia sea the importance of the antarctic circumpolar current and the weddell gyre flows abstract new swath bathymetry with multichannel and high resolution seismic profiles shows a variety of contourite drift sediment wave morphologies and seismic facies in the central scotia sea the deposits are to be found at the confluence between the two most important bottom current flows in the southern ocean the eastward flowing antarctic circumpolar current acc and the northward outflow of the weddell sea deep water wsdw the contourite drifts are wedge like deposits up to 1 km thick that exhibit aggradational reflectors along axis thinning towards the margins the contourite drifts occur in areas of weaker flows along the margins of contourite channels and in areas protected by obstacles the elongate mounded drifts are best developed along the left hand margins of channelized bottom current flows due to the coriolis force a contourite fan has a main channel and two distributary channels that expand over a gentle seafloor the proximal fan exhibits sediment waves with the distal fan incised by furrows sediment wave fields are well developed in areas of intensified bottom flows without channels particularly at the confluence of the acc and the wsdw small sediment waves occur where unidirectional bottom current flows predominate sediment waves may develop under the influence of internal waves produced by the interaction of the flows and sea bottom relief the stratigraphic sequence above the oceanic crust of early to middle miocene age contains six seismic units separated by major reflectors all the units were shaped under the influence of strong bottom current flows although they exhibit distinct seismic facies changes that record the variations of the bottom current pathways over time the age of the units was calculated based on the age of the oceanic crust and sedimentation rates of deep sea deposits in the region the oldest units vi iv are of early to middle miocene age and developed under the influence of the acc they are characterized by a southward progradational pattern of the seismic units and sedimentation rates of 5 8 cm ky unit iii with an estimated middle miocene age evidences the first incursion of wsdw into the central scotia sea when plate movement caused openings in the south scotia ridge and allowed the connection with the northern weddell sea through jane basin and gaps in the ridge unit ii estimated to be of late miocene to early pliocene age extends over the area and is characterized by internal unconformities a major unconformity at the base of unit ii records an important reorganization of bottom current flows that may predate the onset of grounded ice sheets on the antarctic peninsula shelf unit i of late pliocene to quaternary age shows intensified bottom currents the unconformity at the base of unit i probably predates the onset of major northern hemisphere glaciations and the greater expansion of antarctic ice sheets during the late pliocene the extensive distribution of contourite deposits above the oceanic crust testifies to the long term production of antarctic bottom water cold deep water was swept northward from the weddell gyre interacting with the acc and possibly exerting profound influences on the global circulation system and the onset of major glaciations

circum antarctic palaeobathymetry illustrated examples from cenozoic to recent times abstract we use a revised set of seafloor spreading isochrons and updated plate tectonic reconstructions to produce palaeo age and basement depth grids for the southern ocean the palaeo depth maps illustrate the evolution of the ocean basins widening and deepening and oceanic gateway development surrounding antarctica we provide examples of palaeo basement depth profiles at selected times between 45 ma and the present across the tasman gateway and drake passage the digital grids provide improved boundary conditions for palaeoceanographic circulation models as well as for backtracking sediments from odp drilling

the onset of modern like atlantic meridional overturning circulation at the eocene oligocene transition evidence causes and possible implications for global cooling a compilation of benthic 18o from the whole atlantic and the southern ocean atlantic sector shows two major jumps in the interbasinal gradient of 18o 18o during the eocene and the oligocene one at 40 ma and the second concomitant with the isotopic event of the eocene oligocene transition eot 33 7 ma ago from previously published circulation models and proxies we show that the first 18o jump reflects the thermal isolation of antarctica associated with the proto antarctic circumpolar current acc the second marks the onset of interhemispheric northern sourced circulation cell similar to the modern atlantic meridional overturning circulation amoc the onset of amoc like circulation slightly preceded 100 300 ky the eot as we show by the high resolution profiles of 18o and 13c previously published from dsdp odp sites in the southern ocean and south atlantic these events coincide with the onset of anti estuarine circulation between the nordic seas and the north atlantic which started around the eot and may be connected to the deepening of the greenland scotland ridge we suggest that while the shallow proto acc supplied the energy for deep ocean convection in the southern hemisphere the onset of the interhemispheric northern circulation cell was due to the significant eot intensification of deepwater formation in the north atlantic driven by the nordic anti estuarine circulation this onset of the interhemispheric northern sourced circulation cell could have prompted the eot global cooling

tectonic events reflected by palaeocurrents zircon geochronology and palaeobotany in the sierra baguales of chilean patagonia abstract the sierra baguales situated north of the torres del paine national park in the magallanes region of southern chile shows a well exposed stratigraphic sequence ranging from the late cretaceous to late pliocene which presents a unique opportunity to study the evolution of sedimentological styles and trends palaeoclimate changes and tectonic events during this period the depositional environment changed from a continental slope and shelf during the cenomanian campanian tres pasos formation to deltaic between the campanian maastrichtian dorotea formation and estuarine in the lutetian bartonian man aike formation during the rupelian a continental environment with meandering rivers and overbank marshes was established rio leona formation this area was flooded in the early burdigalian estancia 25 de mayo formation during the patagonian transgression but emerged again during the late burdigalian santa cruz formation measured palaeocurrent directions in this mesozoic cenozoic succession indicate source areas situated between the northeast and east southeast during the late cretaceous east southeast during the middle eocene and southwest during the early oligocene to early miocene this is confirmed by detrital zircon age populations in the different units which can be linked to probable sources of similar ages in these areas the east southeastern provenance is here identified as the antarctic peninsula or its northeastern extension which is postulated to have been attached to fuegian patagonia during the eocene the southwestern and western sources were exhumed during gradual uplift of the southern patagonian andes coinciding with a change from marine to continental conditions in the magallanes austral basin as well as a decrease in mean annual temperature and precipitation indicated by fossil leaves in the rio leona formation the rain shadow to the east of the andes thus started to develop here during the late eocene early oligocene 34 ma long before the quechua phase of andean tectonics 19 18 ma that is generally invoked for its evolution at lower latitudes

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

productivity and sea surface temperature changes recorded during the late eocene early oligocene at dsdp site 511 south atlantic this study investigates paleoenvironmental changes during the eocene oligocene transition eot at deep sea drilling project dsdp site 511 south atlantic as inferred from lipid biomarker long chain diols alkenones and calcareous nannofossil accumulation rates as well as changes in sedimentation regime i e relative contributions of total organic carbon toc calcium carbonate and biogenic silica sea surface temperatures ssts reconstructed from the alkenone unsaturation index uk 37 indicate a progressive but significant cooling 8 c from 34 5 ma to 33 6 ma consistent with estimates derived from other temperature proxies tex86 18o at the same site and for the same time interval this cooling is associated with a marked increase in primary productivity as indicated by high accumulation rates of biogenic silica toc alkenones long chain diols and calcareous nannofossils together these results are consistent with an enhancement of upwelling conditions favorable to the development of siliceous organisms at dsdp site 511 possibly induced by the oi 1 glaciation in antarctica that occurred during this period

effect of the drake passage throughflow on global climate abstract the role of the southern ocean in global climate is examined using three simulations with a coupled model employing geometries different only at the location of drake passage dp the results of three main experiments are examined 1 a simulation with dp closed 2 an experiment with dp at a shallow 690 m depth and 3 a realistic dp experiment the climate with dp closed is characterized by warmer southern hemisphere surface air temperature sat little antarctic ice and no north atlantic deep water nadw overturn on opening the dp to a shallow depth of 690 m there is an increase in antarctic sea ice and a cooling of the southern hemisphere but still no north atlantic overturn on fully opening the dp the climate is mostly similar in the southern hemisphere to dp at 690 m but the model now simulates nadw formation and a warming in the northern hemisphere this suggests the north atlantic thermohaline circulation depends not only on the existence of a dp throughflow but also on the dep

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

equatorial heat accumulation as a long term trigger of permanent antarctic ice sheets during the cenozoic abstract growth of the first permanent antarctic ice sheets at the eocene oligocene transition eot 33 7 million years ago indicates a major climate shift within long term cenozoic cooling the driving mechanisms that set the stage for this glaciation event are not well constrained however owing to large uncertainties in temperature reconstructions during the eocene especially at lower latitudes to address this deficiency we used recent developments in coccolith biogeochemistry to reconstruct equatorial atlantic sea surface temperature sst and atmospheric pco2 values from pelagic sequences preceding and spanning the eot we found significantly more variability in equatorial ssts than previously reported with pronounced cooling from the early to middle eocene and subsequent warming during the late eocene thus we show that the antarctic glaciation at the eocene oligocene boundary was preceded by a period of heat accumulation in the low latitudes likely focused in a progressively contracting south atlantic gyre which contributed to cooling high latitude austral regions this prominent redistribution of heat corresponds to the emplacement of a strong meridional temperature gradient that typifies icehouse climate conditions our equatorial coccolith derived geochemical record thus highlights an important period of global climatic and oceanic upheaval which began 4 million years before the eot and superimposed on a long term pco2 decline drove the earth system toward a glacial tipping point in the cenozoic

a review of tertiary climate changes in southern south america and the antarctic peninsula part 2 continental conditions abstract climate changes in southern south america and the antarctic peninsula during the tertiary show a strong correlation with ocean warming and cooling events which are in turn related to tectonic processes during periods of accelerated sea floor spreading and mid ocean ridge activity sea levels rose so that parts of the continents were flooded and forests were destroyed however this was balanced by the large scale release of co2 during volcanic outgassing and carbonate precipitation on the continental shelves which caused rising air temperatures and the poleward expansion of sub tropical and temperate forests cooling episodes generally caused an increase in the north south thermal gradient because of an equatorward shift in climate belts so that the westerly winds intensified and brought higher rainfall to the lower latitudes an increase in wind blown dust caused temperatures to drop further by reflecting sunlight back into space the rising andes range had a marked influence on climate patterns up to the middle miocene it was still low enough to allow summer rainfall to reach central and north central chile but after about 14 ma it rose rapidly and effectively blocked the spill over of moisture from the atlantic ocean and amazon basin at this time the cold humboldt current was also established which together with the andes helped to create the arid diagonal of southern south america stretching from the atacama desert to the dry steppes of patagonia this caused the withdrawal of subtropical forests to south central chile and the expansion of sclerophytic vegetation to central chile however at the same time it intercepted more rain from the northeast causing the effect of the south american monsoon to intensify in northwestern argentina and southern bolivia where forest communities presently occur in patagonia glaciation started as early as 10 5 ma but by 7 ma had become a prominent feature of the landscape and continued apparently uninterruptedly into the pleistocene the antarctic peninsula saw its first mountain glaciation between 45 and 41 ma with major ice sheet expansion commencing at about 34 ma isolated stands of nothofagus forests were still present in low lying areas suggesting that the glaciers were initially wet based but dry based glaciers were established at around 8 ma although temperatures rose briefly during the messinian pliocene transition causing sub antarctic flora to retreat to higher elevations of the transantarctic mountains the present cold polar conditions were finally established by about 3 ma

coupled tectonic evolution of andean orogeny and global climate abstract the largest tectonic relief breaking the earth s surface 13 km vertically is at the subduction margin of the andes which generates routinely megathrust earthquakes mw 8 5 and drives the paradigmatic andean orogen here we present key geologic evidence to reassess first order features of geomorphology and tectonics across the central andes where the orogen includes the altiplano plateau and attains its maximum integrated height and width the andean subduction margin has a stepped morphology dominated by the low relief atacama bench which is similar to a giant uplifted terrace slopes gently over a width of 60 100 km from the andes to the pacific and runs over more than 1000 km of coastal length we find that the genesis of stepped morphology at the andean seaboard is due to concomitant development of large west vergent thrusts parallel to the subduction interface and increasing aridity in the atacama desert which keeps an unprecedented large scale record of interplaying tectonics and cenozoic climate change incorporating our results with published geological knowledge demonstrates that andean orogeny is characterized by trench perpendicular bivergent and trench parallel bilateral growth over the past 50 myr associated with positive trench velocity toward the continent trench advance and subduction of a wide slab under south america we hypothesize that a global plate tectonic reorganization involving long lasting viscous mantle flow has probably forced both andean orogeny and global climate cooling since 50 ma in contrast two important stepwise pulses of increasing aridity and trench perpendicular andean growth appear to be results of changes in erosion rates due to global late eocene and middle miocene cooling events

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

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

miocene evolution of atmospheric carbon dioxide changes in pco2 or ocean circulation are generally invoked to explain warm early miocene climates and a rapid east antarctic ice sheet eais expansion in the middle miocene this study reconstructs late oligocene to late miocene pco2 from ep values based on carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifera from deep sea drilling project sites 588 and 608 and ocean drilling program site 730 our results indicate that highest pco2 occurred during the latest oligocene 350 ppmv but decreased rapidly at 25 ma the early and middle miocene was characterized by low pco2 260 190 ppmv lower intervals of pco2 correspond to inferred organic carbon burial events and glacial episodes with the lowest concentrations occurring during the middle miocene there is no evidence for either high pco2 during the late early miocene climatic optimum or a sharp pco2 decrease associated with eais growth paradoxically pco2 increased following eais growth and obtained preindustrial levels by 10 ma although we emphasize an oceanographic control on miocene climate low pco2 could have primed the climate system to respond sensitively to changes in heat and vapor transport

eocene to miocene terrigenous inputs and export production geochemical evidence from odp leg 177 site 1090 abstract changes in ocean circulation and climate during the cenozoic led to the development of the antarctic circum polar current acc and permanent antarctic ice sheets however the timing of the opening of the drake passage and the establishment of the acc is poorly constrained we present geochemical proxies of terrigenous inputs and export production at a single site in the southeastern atlantic ocean that suggest the drake passage had opened to intermediate and deep water by 32 8 ma two separate styles of sedimentation and geochemical composition of terrigenous material are observed with the change in sediment geochemical characteristics occurring at 32 8 ma middle to late eocene records are highly variable while latest eocene to early miocene records exhibit cyclic variations in elemental records and terrigenous material based on al ti ratios metal sources change from continental crust to oceanic crust sources at the eocene oligocene transition which we suggest reflects a change in deep water circulation phosphorus metal ratios indicate that there are two distinct intervals of enhanced export production one in the middle eocene and one throughout the eocene oligocene transition elevated al ti ratios greater than any lithic source in the middle eocene provide evidence of particulate scavenging and thus increased export production barium ratios further support changes in productivity in the middle eocene and at the eocene oligocene transition permanent changes in the ba concentration record in the early oligocene further support a change in deep water circulation due to the opening and deepening of the drake passage at 32 8 ma

a 9 myr cycle in cenozoic δ13c record and long term orbital eccentricity modulation is there a link abstract the 65 myr long cenozoic carbon isotope record 13 c of zachos et al 2001 2008 documents a strong long term cycle with a mean pseudoperiodicity close to 9 myr this cyclicity modulates the 2 4 myr eccentricity cycle amplitude hinting at a possible link between long term astronomical and geological variations some phase shifts between 9 myr 13 c and astronomical cycles suggest that additional processes e g tectonics contribute to these long term carbon cycle variations the strong response of 13 c to long term eccentricity periods 9 myr 2 4 myr 400 kyr supports the hypothesis that the long time residence of carbon in the oceans amplifies lower frequency or dampens higher frequency orbital variations additionally the strong expression of low amplitude 9 myr eccentricity cycle in the 13 c record could be explained by energy transfer process from higher to lower frequency cycles and all eccentricity components modulate the carrier climatic precession cycles finally the paleocene eocene thermal maximum petm 55 9 ma event which corresponds to a pronounced 13 c negative excursion is situated within a strong decrease in the most prominent 9 myr 13 c cycle hinting at a link between accelerated rates in 13 c variations and the petm this specific 9 myr 13 c cycle seems to be amplified by non orbital mechanisms in atmosphere continent ocean system such as previously suggested methane release from gas hydrate and volcanism

effect of orogeny plate motion and land sea distribution on eurasian climate change over the past 30 million years the eurasian climates of today 10 million and 3o million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions the resulting climates compare well with various palaeoclimate records the retreat of the paratethys an epicontinental sea shifts the central asian climate from temperate to continental conditions and plays as important a role as uplift of the himalayan tibetan plateau in driving the asian monsoon changes

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

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

neogene to quaternary tectonic evolution of the patagonian andes at the latitude of the chile triple junction abstract we conducted a preliminary structural and tectonic analysis of the region north and south of the lake general carrera buenos aires lgcba southern patagonian andes chile and argentina in order to study the effects of the subduction of the active south chile spreading ridge scr on the development of the tectonic front of the patagonian andes at the latitude of the chile triple junction ctj here one segment of the scr entered the trench at 6 ma and the corresponding slab window is now located below the study area we emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments a to d trending mainly n170 and right laterally offset this geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction the four segments of the tectonic front are separated and locally deformed by n170 oriented more recent sinistral strike slip fault zones kinematic analysis of minor fault and striaes confirms the abundance of strike slip faults and suggests rotation of the stress tensor after at least 9 ma a consequence of the subduction of the scr beneath the south american plate at the latitude of the lgcba is a two step history 1 pre ridge subduction oblique to the western margin of the south america plate showing contractional regime with its main axis trending ne sw responsible for dextral transpressive deformation and 2 post ridge subduction more orthogonal convergent regime recent uplift occurred later starting at 3 5 ma and could still be active neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial lacustrine terraces anomalies in stream development can be found although evidences for recent to active faulting can be shown effects of syn glacial and post glacial related processes cannot be ruled out the evolution of this region during the late cenozoic can be divided into three distinct periods 1 20 15 ma deposition of foreland continental detrital sediments rio zeballos group followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10 9 ma 2 9 4 ma erosion peneplanation and emplacement of voluminous plateau basalts coeval with fluvial glacial sedimentation and 3 3 0 ma renewed orogenic activity uplift and disruption of former erosional surfaces in relation to activation of strike slip fault zones and north trending thrusts onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the chile ridge at 14 15 ma in southernmost patagonia inducing a different stress regime in the frontal arc region