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

late eocene to early miocene ice sheet dynamics and the global carbon cycle paired benthic foraminiferal trace metal and stable isotope records have been constructed from equatorial pacific ocean drilling program site 1218 the records include the two largest abrupt 1 myr increases in the cenozoic benthic oxygen isotope record oi 1 in the earliest oligocene 34 ma and mi 1 in the earliest miocene 23 ma the paired mg ca and oxygen isotope records are used to calculate seawater 18o w calculated w suggests that a large antarctic ice sheet formed during oi 1 and subsequently fluctuated throughout the oligocene on both short 0 5 myr and long 2 3 myr timescales between about 50 and 100 of its maximum earliest oligocene size the magnitudes of these fluctuations are consistent with estimates of sea level derived from sequence stratigraphy the transient expansion of the antarctic ice sheet at mi 1 is marked in the benthic 18o record by two positive excursions between 23 7 and 22 9 ma each with a duration of 200 300 kyr bottom water temperatures decreased by 2 c over the 150 kyr immediately prior to both rapid 18o excursions however the onset of each of these phases of ice growth is synchronous within the resolution of the records with the onset of a 2 c warming over 150 kyr we suggest that the warming during these glacial expansions reflect increased greenhouse forcing prompted by a sudden decrease in global chemical weathering rates as antarctic basement silicate rocks became blanketed by an ice sheet this represents a negative feedback process that might have operated during major abrupt growth phases of the antarctic ice sheet

the role of carbon dioxide during the onset of antarctic glaciation earth s modern climate characterized by polar ice sheets and large equator to pole temperature gradients is rooted in environmental changes that promoted antarctic glaciation 33 7 million years ago onset of antarctic glaciation reflects a critical tipping point for earth s climate and provides a framework for investigating the role of atmospheric carbon dioxide co2 during major climatic change previously published records of alkenone based co2 from high and low latitude ocean localities suggested that co2 increased during glaciation in contradiction to theory here we further investigate alkenone records and demonstrate that antarctic and subantarctic data overestimate atmospheric co2 levels biasing long term trends our results show that co2 declined before and during antarctic glaciation and support a substantial co2 decrease as the primary agent forcing antarctic glaciation consistent with model derived co2 thresholds

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

precise u pb zircon ages for the molson dyke swarm and the fox river sill constraints for early proterozoic crustal evolution in northeastern manitoba canada precise u pb zircon ages have been obtained for samples from the molson dyke swarm and the fox river sill in ne manitoba canada the ages determined for the cross lake and cuthbert lake dykes are 1 883 7 1 5 1 7and 1 883 2 ma respectively and are in excellent agreement with the 1 882 9 1 4 1 5ma age obtained for the fox river sill these results support the contention that the emplacement of the fox river sill and the molson dyke swarm was contemporaneous and also demonstrate the potential for correlating mafic igneous activity in widely spaced localities the timing of early proterozoic mafic magmatism in the western superior province appears to be synchronous with igneous activity in other parts of the circum superior belt and in the trans hudson orogen to the west the emplacement of the molson dyke swarm at 1 883 ma indicates a 700 ma interval of quiescence between the final igneous activity that is recorded in the archean basement and dyke intrusion the presence of deformed equivalents of molson dykes in the thompson nickel belt indicates that the intense deformation in this belt occurred sometime after 1 883 ma

resolving a late oligocene conundrum deep sea warming and antarctic glaciation abstract changes in ice volume and resulting changes in sea level were determined for the late oligocene 26 23 ma astronomical timescale ats by applying 18 o to sea level calibrations to deep sea 18 o records from odp sites 689 690 929 1090 and 1218 our results show that maximum global ice volume occurred during two late oligocene 18 o events oi2c 24 4 ma and mi1 23 0 ma inferred glacioeustatic lowering with volumes up to 25 greater than the present day east antarctic ice sheet eais ice volume during glacial minima was on the order of about 50 of the present day eais this is supported by late oligocene stratigraphic records from antarctica that contain evidence of cold climates and repeated episodes of glaciation at sea level and grounding lines of glacial ice on the antarctic continental shelf in the ross sea and prydz bay in contrast composite deep sea 18 o records show a significant decrease 1 between 26 7 and 23 5 ma which have long been interpreted as bottom water warming combined with deglaciation of antarctica however a close examination of individual 18 o records indicates a clear divergence after 26 8 ma between records from southern ocean locations i e ocean drilling program sites 689 690 744 and those of other ocean basins high 18 o values 2 9 3 3 from these southern ocean 18 o records are consistent with an ice sheet on the east antarctic continent equivalent to present day values and cold bottom water temperatures 2 0 c these differences suggest a reduction in deep water produced near the antarctic continent i e proto antarctic bottom water proto aabw which were quickly entrained and mixed with warmer and presumably more saline bottom water originating from lower latitudes expansion of a warmer deep water mass and the weakening of the proto aabw may explain the large intra basinal isotopic gradients that developed among late oligocene benthic 18 o records these conclusions are also supported by ocean modeling suggesting a reduction of deep water formed in the southern ocean strengthening of deep water from the northern hemisphere and decreasing temperatures in high southern latitudes occurred as the drake passage opened to deep water low 18 o values reported from deep sea locations other than the southern ocean are shown to bias estimates of antarctic ice volume calling for a re evaluation of the notion that antarctic ice volume was significantly reduced during the late oligocene

evidence for an earliest oligocene ice sheet on the antarctic peninsula there is growing consensus that development of a semipermanent ice sheet on antarctica began at or near the eocene oligocene e o boundary beyond ice rafted debris in oceanic settings however direct evidence for a substantial ice sheet at this time has been limited and thus far restricted to east antarctica it is unclear where glacier ice first accumulated and how extensive it was on the antarctic continent in the earliest oligocene sediments at the top of the eocene marine shelf section on seymour island antarctic peninsula include glacial marine deposits and a lodgment till with clasts derived from a variety of rock units on the peninsula dinoflagellate biostratigraphy and strontium isotope stratigraphy indicate an age at or very close to the e o boundary glacier ice extending to sea level in the northern peninsula at this time suggests the presence of a regionally extensive west antarctica ice sheet and thus an even more dramatic response to the forcing factors that facilitated high latitude ice expansion in the earliest oligocene

clay minerals in cenozoic sediments off cape roberts mcmurdo sound antarctica reveal palaeoclimatic history abstractthe clay mineral assemblages of the ca 1600 m thick cenozoic sedimentary succession recovered at the crp 1 crp 2 2aand crp 3 drill sites off cape roberts on the mcmurdo sound shelf antarctica were analysed in order to reconstruct thepalaeoclimate and the glacial history of this part of antarctica the sequence can be subdivided into seven clay mineral unitsthat reflect the transition from humid to subpolar and polar conditions unit i 35 33 6 ma is characterised by an almostmonomineralic assemblage consisting of well crystalline authigenic smectite and therefore does not allow a palaeoclimaticreconstruction unit ii 33 6 33 1 ma has also a monomineralic clay mineral composition however the assemblage consistsof variably crystallized smectite that at least in part is of detrital origin and indicates chemical weathering under a humidclimate the main source area for the clays was in the transantarctic mountains minor amounts of illite and chlorite appear forthe first time in unit iii 33 1 31 ma and suggest subordinate physical weathering the sediments of unit iv 31 30 5 ma have strongly variable smectite and illite concentrations indicating an alternation of chemical weathering periods and physicalweathering periods unit v 30 5 24 2 ma shows a further shift towards physical weathering unit vi 24 2 18 5 ma indicates strong physical weathering under a cold climate with persistent and intense illite formation unit vii 18 5 ma topresent documents an additional input of smectite derived from the mcmurdo volcanic group in the south d 2005 elsevier b v all rights reserved

foraminiferal mg ca evidence for southern ocean cooling across the eocene oligocene transition constraining the magnitude of high latitude temperature change across the eocene oligocene transition eot is essential for quantifying the magnitude of antarctic ice sheet expansion and understanding regional climate response to this event to this end we constructed high resolution stable oxygen isotope 18o and magnesium calcium mg ca records from planktic and benthic foraminifera at four ocean drilling program odp sites in the southern ocean planktic foraminiferal mg ca records from the kerguelen plateau odp sites 738 744 and 748 show a consistent pattern of temperature change indicating 2 3 c cooling in direct conjunction with the first step of a two step increase in benthic and planktic foraminiferal 18o values across the eot in contrast benthic mg ca records from maud rise odp site 689 and the kerguelen plateau odp site 748 do not exhibit significant temperature change the contrasting temperature histories derived from the planktic and benthic mg ca records are not reconcilable since vertical 18o gradients remained nearly constant at all sites between 35 0 and 32 5 ma based on the coherency of the planktic mg ca records from the kerguelen plateau sites and complications with benthic mg ca paleothermometry at low temperatures the planktic mg ca records are deemed the most reliable measure of southern ocean temperature change we therefore interpret a uniform cooling of 2 3 c in both deep surface thermocline waters and intermediate deep waters of the southern ocean across the eot cooling of southern ocean surface waters across the eot was likely propagated to the deep ocean since deep waters were primarily sourced on the antarctic margin throughout this time interval removal of the temperature component from the observed foraminiferal 18o shift indicates that seawater 18o values increased by 0 6 0 15 across the eot interval corresponding to an increase in global ice volume to a level equivalent with 60 130 modern east antarctic ice sheet volume

impact of antarctic circumpolar current development on late paleogene ocean structure global cooling and the development of continental scale antarctic glaciation occurred in the late middle eocene to early oligocene 38 to 28 million years ago accompanied by deep ocean reorganization attributed to gradual antarctic circumpolar current acc development our benthic foraminiferal stable isotope comparisons show that a large 13c offset developed between mid depth 600 meters and deep 1000 meters western north atlantic waters in the early oligocene indicating the development of intermediate depth 13c and o2 minima closely linked in the modern ocean to northward incursion of antarctic intermediate water at the same time the ocean s coldest waters became restricted to south of the acc probably forming a bottom ocean layer as in the modern ocean we show that the modern four layer ocean structure surface intermediate deep and bottom waters developed during the early oligocene as a consequence of the acc

the identification of mantle reflections below hungary from deep seismic profiling abstract in the 1970 s the eotvos lorand geophysical institute performed seismic reflection deep sounding experiments with multiple coverage in several parts of the pannonian basin posgay 1975 one of the most interesting results was that arrivals were received in the time range corresponding to the depth of the asthenosphere they were interpreted as reflections posgay et al 1981 conspicuous arrivals from the time range of the asthenosphere 15 20 s were also received along the pannonian geotraverse in 1987 in the majority of sections published in other countries the upper mantle seems to be transparent the present paper discusses investigations and preliminary results which lead to the conclusion that this apparent transparency is due to the frequency range below 10 hz being neglected in both signal generation and recording in most deep continental reflection surveys the seismogeological conditions of the pannonian basin are certainly favourable for reflection studies of the upper mantle the correctness of the assumption that arrivals from the time range of the asthenosphere are primary reflections is subjected to further investigation and conclusions are made concerning the structure of the asthenosphere

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

late oligocene climatic changes evidence from calcareous nannofossils at kerguelen plateau site 748 southern ocean abstract palaeoecological changes in nannofossil assemblages in the southern ocean during oligocene times are examined through high resolution quantitative analyses of samples from ocean drilling program odp site 748 kerguelen plateau we quantitatively characterized the palaeoecological preference of groups of species and compared their general trends with those determined at maud rise odp sites 690 and 689 persico d villa g 2004 eocene oligocene calcareous nannofossils from maud rise and kerguelen plateau antarctica palaeoecological and palaeoceanographic implications marine micropaleontology 52 153 179 we then attempt a correlation between the main assemblage variation and sea surface temperature sst changes in the southern ocean at this time relatively stable cool conditions are interpreted to have persisted from earliest to late oligocene times when an increase in abundance of temperate water taxa is recorded both at maud rise and kerguelen plateau before the mi 1 event this reveals a climatic event that probably involved both sites and which is comparable to that indicated by the global oxygen isotope curve miller k g wright j d fairbanks r g 1991 unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion journal of geophysical research 96 6829 6848 zachos j pagani m sloan l thomas e billups k 2001 trends rhythms and aberrations in global climate 65 ma to present science 292 686 693 in the uppermost oligocene at site 689 the temperate water taxa index temperate temperate cool 100 increases from 25 2 ma similarly at site 748 the temperate water taxa index indicates an increase in sst in the late oligocene from about 26 5 ma at the base of chron c8n 2n which is offset by about 1 m y between the two areas the reason for this time difference can probably be found in the location of site 748 with respect to site 689 and in the palaeoceanographic setting taking into account that site 748 lies north of a deep water passage that separates kerguelen plateau from antarctica and north of the present day front of the antarctic circumpolar current the presence of warm water taxa exclusively at site 748 corroborates this hypothesis

effects of the oligocene climatic events on the foraminiferal record from fuente caldera section spain western tethys abstract an expanded succession of upper priabonian upper eocene to chattian upper oligocene hemipelagic marls interbedded with turbiditic sandstone layers is present in the spanish fuente caldera section subbetic zone western tethys we analyzed foraminifera from this section quantitatively with emphasis on biostratigraphy and paleoecology benthic foraminifera indicate an upper to possibly upper middle bathyal depth of deposition for most of the studied section with paleobathymetric analysis made difficult because of the common presence of shallow water taxa some reworked by turbidites and others epiphytic taxa which may have been transported by turbidites or by floating plant material we identified three major biotic and paleoenvironmental events 1 the major planktonic foraminiferal turnover across the eocene oligocene boundary which includes several first and last occurrences as well as a decrease in the percentage of surface water dwellers possibly linked to global cooling 2 a dramatic sea level drop indicated by the presence of a 37 m thick sequence of calcarenites lower half of planktonic foraminiferal zone o2 31 5 ma with abundant trace fossils and allochthonous foraminifera this sea level fall which triggered erosion of material in shallow marine settings and transport by turbidity currents into the basin apparently post dated the major glacial expansion on the antarctic continent oi1 33 7 ma and predated the later major expansion oi2 through oi2b 27 30 5 ma thus may have been tectonically controlled 3 a warming event starting in the chattian lowermost part of zone o6 27 1 ma which could be correlated to the globally recognised late oligocene warming event but apparently started somewhat earlier 27 1 ma as compared to 26 5 ma

evidence for a heavily glaciated antarctica during the late oligocene warming 27 8 24 5 ma stable isotope records from odp site 690 high 18o values 3 0 from a 9 kyr resolution benthic foraminiferal stable isotope record from the ocean drilling program odp site 690b located on the maud rise antarctica indicate a heavily glaciated antarctic continent during late oligocene 27 8 24 5 ma values ranging 2 5 3 0 during interglacial periods and 3 0 3 6 during glacial intervals are consistent with an ice sheet near or larger than modern size in addition this record does not exhibit the long term late oligocene warming trend seen in records from low latitude drill sites oxygen isotope values from 26 0 24 5 ma are comparable ranging between 2 5 and 3 3 to values that preceded the 18o event oi2b at 26 7 ma indicating no significant glacial collapse occurred during the late oligocene a gradient between ocean basins during the oligocene has already been linked to the development of a modern multi layered ocean and worked to bathe the low to mid latitude deep sea records with a warmer water mass we suggest this masked the significant antarctic glaciation in low latitude paleoceanographic records additionally we propose a resolution for conflicting lines of evidence from some antarctic proximal records suggesting significant glaciation and others suggesting reduced glaciation during the late oligocene by allowing a modern sized ice sheet to grow on an antarctic continent as more land surface area existed above sea level during this time this could allow at least some portions of the antarctic coastline to remain ice free during glacial minima while still maintaining modern or near modern ice volume

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

oligocene eustasy from two dimensional sequence stratigraphic backstripping estimates of the magnitudes of changes in third order 0 522 m y eustasy were obtained for oligocene sequences defined by a suite of largely onshore boreholes of the new jersey coastal plain benthic foraminiferal biofacies and multiple age constraints in a sequence stratigraphic framework formed the database for this study the geometry of the margin through time was determined using two dimensional backstripping the depth ranges of benthic foraminiferal biofacies were determined from a combination of standard factor analysis techniques and the backstripped geometries benthic foraminiferal biofacies were then used to determine the depths of the oligocene margin profiles obtained from backstripping the water depths of 16 of these horizons were confirmed by independent benthic biofacies determinations from at least two wells this internal consistency indicates that the data and the twodimensional backstripping approach were robust where benthic biofacies require a vertical shift of the horizons generated by two dimensional backstripping eustatic changes were required and were readily calculated results indicate a major eustatic fall from the end of the eocene to the first record of oligocene deposition subsequent long term shoaling of sea level through the oligocene was 30 m in 10 m y superimposed on this long term trend were higher frequency third order variations in eustasy with amplitudes of 40 m or less

seismic reflections from the mantle represent relict subduction zones within the continental lithosphere in many areas of the world the continental lithospheric upper mantle contains bright continuous regionally extensive seismic reflectors despite the increasingly common observation of such mantle reflectors on deep seismic reflection profiles their significance and geologic origin remain obscure we report results from a series of seismic experiments acquired across two of the brightest of these reflectors and provide new constraints upon the composition and thickness of the reflecting region these new seismic data reveal regionally extensive dipping and subhorizonal slabs of high velocity 8 4 km s high density 3500 kg m 3 material several kilometres 2 km thick entrained within otherwise unremarkable upper mantle the geometry physical properties and geologic setting of these mantle reflectors suggest that they represent fragments of eclogitic oceanic crust a relict of pre caledonian oceanic subduction now preserved within the lower continental lithosphere such relict subduction zones appear to be widespread within the continental lithosphere and to exert an important influence upon the location and style of subsequent continental deformation

the application of u pb geochronology to mafic ultramafic and alkaline rocks an evaluation of three mineral standards abstract a key to proving the accuracy of u pb age determinations by the various techniques available is routine analysis of an internal well characterized matrix matched mineral standard of known age in general there are very few well studied natural mineral standards available to the geological community in this study the nature geochemistry and u pb systematics of baddeleyite titanite and perovskite from three potential mineral standards are evaluated a compilation of concordant to near concordant analyses of a single phalaborwa baddeleyite crystal yields a precise u pb age of 2059 60 0 35 ma a large variation in uranium 51 2124 ppm thorium 0 5 73 ppm and lead 19 782 ppm concentrations and the presence of some u bearing mineral inclusions zircon monazite and baddeleyite zircon intergrowths require some caution when using aliquots of this phalaborwa baddeleyite crystal as a mineral standard the khan titanite crystal investigated here overall has a relatively uniform chemical composition e g u 584 95 ppm th 473 73 ppm pb 57 8 ppm th u 0 81 0 05 but parts of the crystal contain abundant mineral inclusions the model 1 upper intercept age of 522 2 2 2 ma for nine khan titanite analyses is considered the current best estimate for its crystallization age some variability in the u pb systematics with 207pb 206pb ages that vary between 508 and 560 ma was observed and could be related to the presence of tiny u bearing mineral inclusions such as rutile or allanite so careful selection of material from this titanite crystal is also required a single perovskite crystal from the ice river alkaline intrusion has a relatively uniform chemical composition uranium 141 25 ppm thorium 1954 270 ppm lead 41 5 ppm and th u 14 5 1 3 the u pb results for eight perovskite fractions yield a weighted average 206pb 238u age of 356 5 1 0 ma although there are always some limitations to using natural mineral crystals as u pb standards such as the presence of mineral inclusions alteration and geochemical heterogeneity the three crystals investigated here are considered suitable to be used as internal u pb standards for any of the currently used u pb dating techniques

tectonics of the acadian orogeny in new england and adjacent canada paleogeographic analysis of post taconian rocks in new england and adjacent parts of canada has revealed the existence of two volcanic arcs which shut off at the time of the acadian orogeny one was built on arc basement previously accreted to north america during the taconic orogeny the other on precambrian continental basement of avalonia in the intervening merrimack fredericton trough metamorphosed and poly deformed turbidites and black shales record deep water conditions in silurian following mckerrow and ziegler 1971 this tract is interpreted as the site of an ocean which closed in siluro devonian by simultaneous subduction beneath both continental margins in the molucca sea in indonesia a comparable arc arc collision is in an early stage of development moore et al 1982 suggested that an accretionary prism built against one arc is overthrusting its counterpart which developed on the other side of the ocean an identical geometry with an avalonian accretionary prism overriding the conve

deep structure of the scottish caledonides revealed by the moist reflection profile the moine and outer isles seismic traverse moist is a deep crustal reflection profile shot at sea off the north coast of scotland in 1981 spectacular reflections are observed from the moho and from thrust zones within the caledonian fold belt and foreland deep reflection profiling of the continental crust of the united states under the direction of the cocorp group has amply demonstrated the value of this technique when applied to suitable geological problems see for example refs 1 2 a similar group the british institutes reflection profiling syndicate birps has now been set up in the uk to organize such projects funds are provided by the natural environment research council nerc through the core group based at cambridge university however a preliminary experiment of which moist is the result was organized through the institute of geological sciences igs

evidence for oligocene middle miocene abyssal circulation changes in the western north atlantic previous studies of benthic foraminiferal isotopic composition have demonstrated that a latest eocene earliest oligocene benthic foraminiferal 18o increase occurred in the pacific southern and atlantic oceans1 9 a middle miocene 18o increase has been noted in the pacific southern and south atlantic oceans1 3 7 10 11 and tentatively identified in the north atlantic12 13 due to the incomplete nature of the north atlantic stratigraphical record14 15 however the oligocene to middle miocene isotopic record of this ocean is poorly understood in the modern ocean the north atlantic and its marginal seas has a critical role in abyssal circulation influencing deep and bottom water hydrography as far away as the north pacific16 18 we now report oxygen isotope measurements on oligocene to middle miocene 12 36 myr bp benthic foraminifera in the western north atlantic which show two periods of enriched 18o values early oligocene and early middle miocene these enriched intervals are interpreted as resulting in part from the build up of continental ice sheets the oligocene to middle miocene 13c record shows three cycles of enrichment and depletion of large enough magnitude to be useful for time stratigraphical correlations within the biostratigraphical age resolution 18o and 13c records correlate with records from other oceans helping to establish a useful tertiary isotopic stratigraphy an atlantic pacific 13c contrast of 0 3 0 9 during the latest oligocene to middle miocene 12 26 myr bp indicates north atlantic deep and bottom water production analogous to modern north atlantic deep water nadw

paleocene to eocene benthic foraminiferal isotopes and assemblages implications for deepwater circulation early paleogene warm climates may have been linked to different modes and sources of deepwater formation warm polar temperatures of the paleocene and eocene may have resulted from either increased atmospheric trace gases or increased heat transport through deep and intermediate waters the possibility of increasing ocean heat transport through the production of warm saline deep waters wsdw in the tethyan region has generated considerable interest in addition general circulation model results indicate that deepwater source regions may be highly sensitive to changing basin configurations to decipher deepwater changes we examined detailed benthic foraminiferal faunal and isotopic records of the late paleocene through the early eocene 60 to 50 ma from two critical regions the north atlantic bay of biscay site 401 and the pacific shatsky rise site 577 these records are compared with published data from the southern ocean maud rise site 690 islas orcadas rise site 702 during the late paleocene similar benthic foraminiferal 18o values were recorded at all four sites this indicates uniform deepwater temperatures consistent with a single source of deep water the highest 13c values were recorded in the southern ocean and were 0 5 more positive than those of the pacific we infer that the southern ocean was proximal to a source of nutrient depleted deep water during the late paleocene upper paleocene reflector ab was cut on the western bermuda rise by cyclonically circulating bottom water also suggesting a vigorous source of bottom water in the southern ocean a dramatic negative excursion in both carbon and oxygen isotopes occurred in the latest paleocene in the southern ocean this is a short term 100 kyr globally synchronous event which also is apparent in both the atlantic and pacific records as a carbon isotopic excursion of approximately 1 faunal analyses from the north atlantic and pacific sites indicate that the largest benthic foraminiferal faunal turnover of the cenozoic was synchronous with the isotopic excursion lending support to the hypothesis that the extinctions were caused by a change in deepwater circulation we speculate that the southern ocean deepwater source was reduced or eliminated at the time of the excursion during the early eocene southern ocean 13c values remained enriched relative to the north atlantic and pacific however the southern ocean was also enriched in 18o relative to these basins we interpret that these patterns indicate that although the southern ocean was proximal to a source of cool nutrient depleted water the intermediate to upper deep water sites of the north atlantic and pacific were ventilated by a different source that probably originated in low latitudes i e wsdw

stepwise transition from the eocene greenhouse to the oligocene icehouse the eocene oligocene transition is the largest global cooling in the cenozoic period a comparison of three independent proxies from the continental shelf and deep ocean reveals a three step transition to cold glacial conditions with ice sheets 25 larger than their present size

the timan pechora basin northeastern european russia tectonic subsidence analysis and a model of formation mechanism abstract we study the tectonic subsidence of the timan pechora basin by using a number of exploration wells and lithologic stratigraphic sections along deep seismic sounding profiles the tectonic analysis reveals a rapid phase of subsidence in the late devonian followed by a slower one we suggest a two phase model of persistent subsidence in the basin 1 in the ordovician 505 438 ma b p and 2 in the silurian 438 408 ma b p the thinning of the lithosphere beneath the basin due to these rift phases led to the uplift of the asthenosphere and to partial melting we explain the vertical motions of the crust by a magmatism eclogitization model the formation mechanism includes accumulation of magmatic melt below the lithosphere asthenosphere boundary in the early middle devonian a phase transformation of magmatic material to eclogites during the cooling and a flow in the uppermost mantle induced by the evolved heavy bodies since the late devonian we construct a two dimensional numerical model of the flow in the asthenosphere compute this flow and describe the resulting changes in surface topography the model is in keeping with observed gravity seismic and subsidence data using the results of tectonic subsidence analysis and the model we interpret the geodynamic evolution of the timan pechora basin and discuss the effect of eclogitization in the uppermost mantle on the basin development

mechanism of formation of hydrocarbon basins the west siberia volga urals timan pechora basins and the permian basin of texas abstract many authors suggest that a large role in the formation of hydrocarbon basins is played by stretching or thrust loading our previous analysis artyushkov and baer 1983 1984a b 1986 has revealed a large number of deep basins on continental crust in fold belts that were formed by rapid subsidence without significant stretching or thrust loading this paper shows that the hydrocarbon basins of west siberia volga urals timan pechora and the permian basin in texas pertain to the same basin type water loaded basins 0 3 0 5 km deep were produced in these regions by rapid subsidence in 1 ma shallow water and deeper water strata cover the same area in the basins which precludes significant stretching during the subsidence there was no folding in the adjacent regions ai the epochs of the subsidence which precludes thrust loading a short duration of the subsidence precludes a significant role of thermal relaxation rapid subsidence was commonly preceded by slight crustal uplift in the volga urals and timan pechora it was also preceded by slight volcanism we suggest destruction of the lower crust under asthenospheric upwelling to the base of the crust as a cause of rapid subsidence the asthenospheric upwelling strongly increases the heat flow which ensures hydrocarbon maturation at shallow depth activization of local tectonic movements from crustal heating occurs after rapid subsidence which produces numerous traps for hydrocarbons deposition of highly organic rocks good source rocks and the formation of reefs and or clinoforms good reservoirs commonly occur after rapid subsidence deposition of shales and evaporites good seals takes place in deeper water basins formed by rapid subsidence diapirism of the latter rocks produces additional hydrocarbon traps the combination of all the above effects associated with rapid subsidence produces very favourable conditions for the generation and preservation of hydrocarbons almost all large hydrocarbon fields in the basins considered are located in regions of rapid subsidence there are no large fields in the adjacent sedimentary basins formed by slow subsidence although the sediment thickness is commonly high

gravity anomalies and possible formation mechanism of the dnieper donets basin abstract two dimensional gravity models are presented along several seismic lines intersecting the dnieper donets basin these models are based on velocity sections and the experimental data on the relationship between density and p wave velocity the detailed gravity modelling of gradient layered crustal and upper mantle features clearly reveals increased density in the upper mantle explainable by rock eclogitization using the gravity models we propose a possible mechanism of the dnieper donets basin evolution since the middle devonian the mechanism includes accumulation of magmatic melt in the asthenospheric bulge during post rifting phase transition to ecologite in the uppermost mantle and a flow induced by the evolved heavy bodies we construct a 2 d numerical model of the flow beneath the dnieper donets basin compute this flow and the resulting changes in the surface topography the subsidence curve calculated in the suggested model gives the best fit to the observed tectonic subsidence than curves of thermal subsidence predicted by the stretching model numerical results confirm the observed gravity high over the dnieper donets basin

numerical models of a subsidence mechanism in intracratonic basins application to north american basins summary mckenzie s model of sedimentary basin evolution and its modification widely used in geophysics sometimes fails to explain discrepancies between predicted and observed values of extension thinning and subsidence of the earth s crust as for the north sea we develop a numerical model of sedimentary basin evolution based on the mechanism suggested by lobkovsky in the course of rifting accompanied by thinning of lower parts of the lithosphere the roof of the underlying asthenosphere moves upward the material of the mantle lifts and partially melts owing to the reduction of pressure the density difference between the melt and the crystalline skeleton results in the filtration of the lighter melt and its accumulation in the form of a magmatic lens due to changed p t conditions the material of the lens undergoes the gabbro eclogite phase transformation the resultant anomalously heavy eclogite lens sinks in the surrounding material this induces a viscous flow changing the surface topography and forming a sedimentary basin we construct a 2 d numerical model describing a viscous flow induced by subsidence of a heavy body and compute changes of surface topography to compute the flow we employ the galerkin spline approach with modifications allowing for density discontinuities and time dependence of the phase transformation we apply the model to the cases of the illinois michigan and williston basins the computed and tectonic subsidence curves agree well for these cases the proposed model is compatible with the seismic structure of the crust and upper mantle below these basins the model is also consistent with gravity data the approach is applicable to other intracratonic basins

late oligocene to early miocene geochronology and paleoceanography from the subantarctic south atlantic mi2 as well as several other distinctive isotope events our data suggest that the d 18 o maximum commonly associated with the oligocene miocene o m boundary falls within c6cn 2r 23 86 ma the d 13 c maximum coincides within the temporal resolution of our record with c6cn 2n r boundary and hence to the o m boundary comparison of the stable isotope record from site 1090 to the orbitally tuned stable isotope record from odp site 929 across the o m boundary shows that variability in the two records is very similar and can be correlated at and below the o m boundary site 1090 stable isotope records also provide the first deep southern ocean end member for reconstructions of circulation patterns and late oligocene to early miocene climate change comparison to previously published records suggests that basin to basin carbon isotope gradients were small or nonexistent and are inconclusive with respect to the direction of deep water flow oxygen isotope gradients between sites suggest that the deep southern ocean was cold in comparison to the north atlantic indian and the pacific oceans dominance of cold southern component deep water at site 1090 at least until 17 ma suggests that relatively cold circumpolar climatic conditions prevailed during the late oligocene and early miocene we believe that a relatively cold southern ocean reflects unrestricted circumpolar flow through the drake passage in agreement with bathymetric reconstructions index terms 4267 oceanography general paleoceanography 1035 geochemistry geochronology 4870 oceanography biological and chemical stable isotopes keywords paleoceanography 1035 geochronology stable isotopes

geothermal evolution of the astrakhan arch region of the pricaspian basin the astrakhan arch region contains one of the largest sub salt structures of the pricaspian basin where perspectives for hydrocarbon generation and accumulation in the devonian to carboniferous deposits are considered to be high the paper addresses the problem of structural and geothermal evolution of the region deformed by salt movements initially we developed a model of the regional structural evolution along a geological profile using the volume balancing and back stripping methods and geo logical constraints on the sedimentation erosion and paleo water depths then we developed geothermal models along the study profile associated with the regional structural evolution the models were constrained by the temperatures measured in four deep boreholes along the profile we show that the present temperatures and heat flux are influenced by the presence of salt diapirs since the early carboniferous and till middle permian times the temperatures predicted by the models vary significantly due to the regional transgression and the presence of sea water the temperature of devonian carboniferous carbonates increases since the late permian time of post salt deposition and attains its maximum values in the sw part of the profile if the model assumptions concerning the constant vertical and zero lateral heat fluxes are valid we can conclude that hydrocarbons are most likely to be generated in the sw part of the region for the post early permian time

middle eocene through early oligocene climate history and paleoceanography in the southern ocean stable oxygen and carbon isotopes from odp sites on maud rise and kerguelen plateau mackensen a and ehrman w u 1992 middle eocene through early oligocene climate history and paleoceanography in the southern ocean stable oxygen and carbon isotopes from odp sites on maud rise and kerguelen plateau mar geol 108 l 27 paleogene stable oxygen and carbon isotopes were measured in foraminifera from odp sites 689 and 690 at maud rise in the atlantic ocean sector of the southern ocean and from sites 738 744 748 and 749 at the southern kerguelen plateau in the indian ocean sector these data were compared with sedimentological data from the same sample set both benthic and planktic 6180 values document a cooling trend beginning around 49 5 ma at all sites during the late middle eocene planktic lso values indicate a steepening latitudinal temperature gradient from 14 c at the northern sites towards 10 c at the southernmost sites terrigeneous sand grains of probably ice rafted origin and clay mineral assemblages point to the existence of a limited east antarctic ice cap with some glaciers reaching sea level as early as middle eocene time around 45 5 ma between 45 and 40 ma average paleotemperatures were between 5 and 7 c in deep and intermediate water masses while near surface water masses ranged between 6 and io c during the late eocene between 40 and 36 ma average temperatures further decreased to 4 5 c in the deep and intermediate water masses and to 5 8 c near the sea surface abruptly increasing 61so values at approximately 35 9 ma exactly correlate with a sharp pulse in the deposition of icerafted material on the kerguelen plateau a dramatic change in clay mineral composition and an altered southern ocean circulation indicated by a differentiation of benthic 61 c values between sites increasing opal concentrations and decreasing carbonate contents for planktic and benthic foraminifera this 80 increase ranges between 1 0 and 1 3 o and between 0 9 and 1 4 o respectively we favour a hypothesis that explains most of the 61so shift at 35 9 ma with a buildup of a continental east antarctic ice sheet consequently relatively warm oligocene antarctic surface water temperatures probably are explained by a temperate wet based nature of the ice sheet this would also aid in the fast build up of an ice sheet by enhancing the moisture transport on to the continent

glaciation across the oligocene miocene boundary in southern mcmurdo sound antarctica new chronology from the ciros 1 drill hole few palaeogene and neogene sediment cores from the antarctic continental margin have been dated with sufficient precision to enable establishment of direct linkages between glacial events on the antarctic continent and marked events in deep sea delta 18o records as a result much of our knowledge of the gradual but stepwise shift from greenhouse climates of the cretaceous to the ice house climates of the quaternary is inferred from well dated and more continuously deposited deep sea sediments in this study we present new magnetostratigraphic results from the ciros 1 drill core from mcmurdo sound antarctica along with a reinterpretation of a published diatom biostratigraphic zonation that is constrained by correlation to a high precision age model from the nearby crp 2 2a drill core our results suggest that most of the upper 350 m of the ciros 1 drill core represents rapid sediment accumulation during a short time interval spanning the oligocene miocene boundary chronostratigraphic control is precise enough to enable correlation of this interval of glacimarine sedimentation with the mi 1 deep sea delta 18o event which confirms that the mi 1 event was related to a major expansion of antarctic ice a major unconformity at 366 m in the ciros 1 drill core which is widely observed in regional seismic reflection studies represents 9 myr of missing time this unconformity can be traced offshore into the ross sea using seismic stratigraphy and is interpreted to indicate significant east antarctic ice sheet development during the mi 1 glaciation the stratigraphic expression of this 400 kyr glacial event is evidently multiphase and complex in the victoria land basin probably because it was punctuated by higher frequency orbitally induced glacial oscillations the presence of nothofagidites pollen throughout the ciros 1 drill core and the presence of a nothofagus southern beech leaf within the early miocene portion of the core indicate that antarctic mean summer temperatures did not decrease below 5 deg c throughout the mi 1 glaciation these temperatures are significantly warmer than present day mean summer temperatures at sea level in mcmurdo sound the persistence of a nothofagus forest in coastal southern victoria land throughout this time interval suggests that the present state of deep refrigeration was not reached until some time after the mi 1 glaciation

zircon isotopic age from the union ultramafic complex maine ultramafic and mafic rocks of the union complex maine u s a are interpreted as remnants of iapetus oceanic crust based upon geological geochemical and structural considerations these rocks were emplaced along a cryptic acadian suture during north american avalonian collision u pb analyses of zircons from the quartz diorite portion of the middle road unit yield a primary crystallization age of 410 7 ma marking a maximum age for collision and disappearance of iapetus ocean floor other studies have shown a minimum time of peak acadian deformation at 385 400 ma strongly suggesting that the acadian event occurred in a period of less than 20 ma in this portion of the northern appalachians

upper mantle velocity structure beneath the siberian platform summary we present a new velocity model for the continental upper mantle beneath central siberia based on observations of the 1982 rift deep seismic sounding dss profile three peaceful nuclear explosions pne were detonated to provide energy for the 2600 km long profile that extends from the yamal peninsula to the mongolian border se of lake baikal in this paper we model seismic recordings from the northernmost explosion since data from that shot shows unambiguous arrivals from the mantle transition zone discontinuities the analysis combines forward traveltime modelling and waveform matching using reflectivity synthetic seismograms our model for the lithosphere has velocities of 8 25 8 20 km s from the moho to 117km depth between 117 and 123km depth a strong velocity gradient 8 30 8 53 km s is required while a moderate gradient 8 53 8 55 km s l exists between 123 and 136km depth a low velocity zone from 136 to 210km depth terminates this phase arrival branch the gradient again rises between 210 and 233 km and depth culminating in a high gradient zone 8 63 8 80 km s between 233 and 235 km depth below the high gradient zone more moderate gradient 8 80 8 85 km s is required from 235 to 253 km depth terminating in a zone of lower velocity 8 62 8 64 km s from 253 to 400 km depth the upper mantle transition zone consists of two high gradient zones separated by a more moderate gradient the upper zone is best modelled as a 35 km thick velocity gradient 8 64 9 45 km s from 400 to 435 km depth the existence of the velocity gradient is based on the observation that arrivals from this feature can be identified starting at 1580 km range and rapidly become prominent with increasing distance a model with a first order discontinuity predicts significant arrivals at ranges closer than 1580 km our observations contain no compelling evidence for a 520 km discontinuity although a small discontinuity cannot be ruled out the lower transitionzone discontinuity is modelled as a 4 km thick gradient zone 10 25 10 62 km s i from 655 to 659 km depth

paleotemperatures and ice volume of the past 27 myr revisited with paired mg ca and 18o 16o measurements on benthic foraminifera 1 we explore the applicability of paired mg ca and 18o 16o measurements on benthic foraminifera from southern ocean site 747 to paleoceanographic reconstructions on pre pleistocene timescales we focus on the late oligocene through pleistocene 27 0 ma history of paleotemperatures and the evolution of the 18o values of seawater 18osw at a temporal resolution of 100 200 kyr absolute paleotemperature estimates depend on assumptions of how mg ca ratios of seawater have changed over the past 27 myr but relative changes that occur on geologically brief timescales are robust results indicate that at the oligocene to miocene boundary 23 8 ma temperatures lag the increase in global ice volume deduced from benthic foraminiferal 18o values but the smaller scale miocene glaciations are accompanied by ocean cooling of 1 c during the mid miocene phase of antarctic ice sheet growth 15 13 ma water temperatures cool by 3 c unlike the benthic foraminiferal 18o values which remain relatively constant thereafter temperatures vary by 3 c and reach maxima at 12 and 8 5 ma the onset of significant northern hemisphere glaciation during the late pliocene is synchronous with an 4 c cooling at site 747 a comparison of our 18osw curve to the haq et al 1987 sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater 18o 16o ratios at 12 11 ma in particular when benthic foraminiferal 18o values do not support a further increase in ice volume the 18osw curve comes to a maximum that corresponds to a major mid miocene sea level regression the agreement between the character of our mg ca based 18osw curve and sequence stratigraphy demonstrates that benthic foramaniferal mg ca ratios can be used to trace the 18osw on pre pleistocene timescales despite a number of uncertainties related to poorly constrained temperature calibrations and paleoseawater mg ca ratios the mg ca record also highlights that deep ocean temperatures can vary independently and unexpectedly from ice volume changes which can lead to misinterpretations of the 18o record

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

implications of late eocene to early miocene clay mineral assemblages in mcmurdo sound ross sea antarctica on paleoclimate and ice dynamics abstract the clay mineral assemblages of upper eocene to lower miocene sediments recovered at the ciros 1 and mssts 1 drill sites on the mcmurdo sound shelf antarctica were analyzed in order to reconstruct the cenozoic antarctic paleoclimate and ice dynamics the assemblages are dominated by smectite and illite with minor amounts of chlorite and kaolinite the highest smectite amounts and best smectite crystallinities occur in the upper eocene part of ciros 1 below 425 445 mbsf they indicate that during their deposition chemical weathering conditions prevailed on the nearby continent large parts of east antarctica were probably ice free at that time but some glaciers reached the sea and contributed to the glaciomarine sedimentation in contrast only minor total amounts of smectite are present in oligocene and younger sediments due to the shift to mainly physical weathering on an ice covered antarctic continent however relative smectite percentages rise to more than 60 during two late oligocene intervals ca 27 5 26 2 and 25 0 24 5 ma and during one early miocene interval starting at ca 23 3 ma these intervals are characterized by ice masses coming probably from the south where volcanic rocks acted as a source as also indicated by the composition of the sand and gravel fractions during the other intervals the ice came from the west where the physical erosion of basement rocks and sedimentary rocks of the beacon supergroup in the transantarctic mountains provided high illite concentrations because the two drill sites are only 4 km apart their clay mineral records can be correlated this led to a new interpretation of the oligocene paleomagnetic data of the mssts 1 site and to a more detailed lithostratigraphic correlation of the miocene parts of the cores

extensional basins of the former soviet union structure basin formation mechanisms and subsidence history abstract we review the structure and evolution of a number of riphean phanerozoic rifts and extensional basins within the territory of the former soviet union fsu horst and graben formation in strong crustal and subcrustal lithosphere layers can explain the multi trough character of rift systems observed in the russian platform the vilyuy rift the west siberian rift system the pechora kolva rift system and the laptev sea rift many features in the evolution of these rifted basins are incompatible with predictions of classical stretching models basin subsidence often occurs in the absence of any noticeable stretching and over time scales much longer than predicted by models of thermal subsidence other observations include a time gap between rifting and the onset of post rift basin subsidence of tens to hundreds ma and a correlation in timing of subsidence phases of rifted basins and platforms with opening and closure events of adjacent ocean basins these observations point to an important role for mechanisms such as ecologite formation within or beneath the lithosphere as well as intraplate compression and stress induced lithospheric deflection

upper eocene to oligocene isotope 87sr 86sr δ18o δ13c standard section deep sea drilling project site 522 we improved upper eocene to oligocene deep sea chronostratigraphic control by integrating isotope 87sr 86sr 18o 13c stratigraphy and magnetostratigraphy most previous attempts to establish the timing of isotope fluctuations have relied upon biostratigraphic age estimates which have uncertainties of 0 5 to over 4 0 m y deep sea drilling project dsdp site 522 contains the best available upper eocene to oligocene magnetostratigraphic record which allows first order correlations of isotope records 87sr 86sr 18o 13c to the geomagnetic polarity time scale gpts empirical calibrations between the 87sr 86sr of foraminifera and magnetochronology at site 522 allow more precise correlation of unknown samples with the gpts for example shallow water and high latitude sections may be tied into the deep sea record sr isotope stratigraphic resolution for the latest eocene to oligocene is approximately 2 m y

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

phase changes and thermal subsidence in intracontinental sedimentary basins summary a model of tectonic subsidence is developed to explain the late acceleration of subsidence observed in some intracratonic sedimentary basins the proposed mechanism combines the effect of thermal contraction of an initially hot lithosphere with the effect of a subcrustal phase transformation that moves under changing pressure and temperature conditions the subsidence following a sudden change in temperature at the base of the lithosphere is calculated the calculations show that 1 phase changes if present and activated contribute substantially to the subsidence of sedimentary basins 2 because the effect of phase change is delayed subsidence accelerates after a time on the order of 20 myr and 3 the duration of the subsidence is on the order of 100 to 150 myr during the late stages of subsidence the phase change is the dominating mechanism an application to the michigan basin is presented the calculated sediment accumulation history fits the record well when the effect of sea level changes is included in the model

temperature and carbonate ion effects on mg ca and sr ca ratios in benthic foraminifera aragonitic species hoeglundina elegans received 16 march 2005 revised 20 june 2005 accepted 3 november 2005 published 21 february 2006 1 core top samples from atlantic little bahama banks lbb and pacific hawaii and indonesia depth transects have been analyzed in order to assess the influence of bottom water temperature bwt and aragonite saturation levels on mg ca and sr ca ratios in the aragonitic benthic foraminifer hoeglundina elegans both the mg ca and sr ca ratios in h elegans tests show a general decrease with increasing water depth although at each site the decreasing trends are consistent with the in situ temperature profile mg ca and sr ca ratios in lbb are substantially higher than in indonesia and hawaii at comparable water depths with a greater difference observed with increasing water depth because we find no significant difference between results obtained on live and dead specimens we propose that these differences are due to primary effects on the metal uptake during test formation evaluation of the water column properties at each site suggests that in situ co3 ion concentrations play an important role in determining the h elegans mg ca and sr ca ratios the co3 ion effect is limited however only to aragonite saturation levels dco3 aragonite below 15 mmol kg 1 above this level temperature exerts a dominant effect accordingly we propose that mg ca and sr ca in h elegans tests can be used to reconstruct thermocline temperatures only in waters oversaturated with respect to the mineral aragonite using the following relationships mg ca 0 034 0 002 bwt 0 96 0 03 and sr ca 0 060 0 002 bwt 1 53 0 03 for dco3 aragonite 1 5mmol kg 1 the standard error associated with these equations is about 1 1 c reconstruction of deeper water temperatures is complicated because in undersaturated waters changes in mg ca and sr ca ratios reflect a combination of changes in co3 and bwt overall we find that sr ca rather than mg ca in h elegans may be a more accurate proxy for reconstructing paleotemperatures

late cretaceous neogene trends in deep ocean temperature and continental ice volume reconciling records of benthic foraminiferal geochemistry δ18o and mg ca with sea level history 1 we reconstruct trends in ice volume and deep ocean temperature for the past 108 myr resolving variations on timescales of 2 myr and longer we use a sea level record as a proxy for ice volume a benthic foraminiferal mg cabf record as a proxy for temperature and a benthic foraminiferal 18obf record as a proxy for both this allows us to construct dual estimates of temperature and ice volume variations for the interval 10 60 ma extracting temperature from 18obf by using sea level as a proxy for ice volume to constrain the 18osw component and extracting seawater 18osw which reflects ice volume from 18obf by using mg cabf to constrain the temperature component each of these approaches requires numerous assumptions but the range of plausible solutions are concordant on timescales 2 myr and within an uncertainty of 2 c temperature and 0 4 18osw the agreement between the two approaches for the last 50 myr provides empirical justification for the use of 18obf mg cabf and sea level records as robust climate proxies our reconstructions indicate differences between deep ocean cooling and continental ice growth in the late cenozoic cooling occurred gradually in the middle late eocene and late miocene pliocene while ice growth occurred rapidly in the earliest oligocene middle miocene and plio pleistocene these differences are consistent with climate models that imply that temperatures set by the long term co2 equilibrium should change only gradually on timescales 2 myr but growth of continental ice sheets may be rapid in response to climate thresholds due to feedbacks that are not yet fully understood

antarctic peninsular cryosphere early oligocene c 30 ma initiation and a revised glacial chronology strontium isotope stratigraphy provides a chronology for cenozoic cryogenic strata in the northern antarctic peninsula and allows an assessment of diachronism in onset of glacial conditions between east and west antarctica the earliest observed event on the antarctic peninsula was late early oligocene 29 8 0 6 ma at least c 4 ma later than in east antarctica with a second in the early early miocene 22 6 0 4 ma both glacials and intervening interglacial were continent wide phenomena two late neogene glaciations are distinguished early late miocene 9 9 0 97 ma hobbs glacier jones mountains and latest miocene alexander island a further late neogene glacial deposit weddell formation can be constrained only as c 10 ma

oligocene development of the west antarctic ice sheet recorded in eastern ross sea strata seismic reflection data from the easternmost ross sea image buried scour and fill troughs and flat topped ridges interpreted as having formed by glacial erosion and deposition during the oligo cene the nnw sse orientation of the troughs and lack of similar oligocene glacial features within the central ross sea suggests that the ice issued from the highlands of marie byrd land located 100 km away and that portions of the west antarctic ice sheet formed earlier than previously accepted existing global climate models gcms do not produce west antarctic ice caps for the oligocene in part due to low elevations modeled for that time evidence for oligocene ice beyond the paleocoast suggests a higher elevation for the early cenozoic marie byrd land and ross embayment than at present

resolving apparent conflicts between oceanographic and antarctic climate records and evidence for a decrease in pco2 during the oligocene through early miocene 34 16 ma abstract an apparent mismatch between published oxygen isotopic data and other paleoclimate proxies for the span from 26 16 ma is resolved by calibration against global sea level estimates obtained from backstripping continental margin stratigraphy ice volume estimates from calibrated oxygen isotope data compare favorably with stratigraphic and palynological data from antarctica and with estimates of atmospheric pco2 throughout the oligocene to early miocene 34 16 ma isotopic evidence for an east antarctic ice sheet eais as much as 30 larger than its present day volume at glacial maxima during that span is consistent with seismic reflection and stratigraphic evidence for an ice sheet covering much of the antarctic continental shelf at the same glacial maxima palynological data suggest long term cooling during the oligocene with cold near tundra environments developing along the coast at glacial minima no later than the late oligocene a possible mechanism for this long term cooling is a decrease in atmospheric pco2 from the middle eocene to oligocene reaching near pre industrial levels by the latest oligocene and remaining at those depressed levels throughout the miocene

bipolar atlantic deepwater circulation in the middle late eocene effects of southern ocean gateway openings we present evidence for antarctic circumpolar current acc like effects on atlantic deepwater circulation beginning in the late middle eocene modern ocean circulation is characterized by a thermal differentiation between southern ocean and north atlantic deepwater formation regions in order to better constrain the timing and nature of the initial thermal differentiation between northern component water ncw and southern component water scw we analyze benthic foraminiferal stable isotope 18obf and 13cbf records from ocean drilling program site 1053 upper deep water western north atlantic our data compared with published records and interpreted in the context of ocean circulation models indicate that progressive opening of southern ocean gateways and initiation of a circum antarctic current caused a transition to a modern like deep ocean circulation characterized by thermal differentiation between scw and ncw beginning 38 5 ma in the initial stages of drake passage opening in addition the relatively low 18obf values recorded at site 1053 show that the cooling trend of the middle late eocene was not global because it was not recorded in the north atlantic the timing of thermal differentiation shows that ncw contributed to ocean circulation by the late middle eocene 1 4 myr earlier than previously thought we propose that early ncw originated in the labrador sea based on tectonic reconstructions and changes in foraminiferal assemblages in this basin finally we link further development of meridional isotopic gradients in the atlantic and pacific in the late eocene with the tasman gateway deepening 34 ma and the consequent development of a circumpolar proto acc

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

mantle plumes and the metamorphism of the lower crust and their influence on basin evolution abstract since the stretching model appears to be not applicable to the subsidence of accretionary crust basins located on this crust type may have an alternative origin examples of such basins are the west siberia basin and the north german basin both basins showed intensive volcanism and magmatism during the initial phase of their development remarkably the west siberia basin is closely located to the hotspot related siberian flood basalts with a similar permo triassic age and the location of the north german basin in permian times is identical with the present day position of the tibesti hotspot in northern africa these two basins may have specific relations to hotspot heat sources of the earth s underlying mantle due to these heat sources thermal metamorphism within the lower layers of the accretionary crust may occur resulting in rock density increase and subsequent shrinkage of the affected rock volumes these shrinkage processes will lead to the development of topographic lows their filling with sediments and the subsequent start of an exponentially declining isostatic metamorphic basin subsidence in addition to the analyses of metamorphic processes potential field anomalies temperature fields and histories of subsidence have been integrated into one single model that can explain the development of the north german basin similarly the east african rift and eifel hotspots affected parts of the overriding continental plates the east african rift hotspot can be traced back to the afar flood basalts and the dniepr donets basin whereas the eifel hotspot can be linked to the north sea basin continental drift templates present day hotspot locations flood basalt areas metamorphic facies as function of temperature and crust categories are taken as published in recent literature

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

antarctic topography at the eocene oligocene boundary we present a reconstruction of the antarctic topography at the eocene oligocene ca 34 ma climate transition this provides a realistic key boundary condition for modeling the first big antarctic ice sheets at this time instead of using the present day bedrock topography which has changed significantly from millions of years of tectonism and erosion we reconstruct topography using a set of tools including ice sheet erosion models models of thermal subsidence and plate movement erosion estimates are constrained with offshore sediment volumes estimated from seismic stratigraphy maximum and minimum topographic reconstructions are presented as indicators of the range of uncertainty our results point to a significant upland area in the ross sea marie byrd land and weddell sea sectors in addition east antarctic coastal troughs are much shallower than today due to the restoration of material that has been selectively eroded by the evolving ice sheets parts of east antarctica have not changed since the e o boundary because they were protected under non erosive cold based ice the reconstructions provide a better defined boundary condition for modeling that seeks to understand interaction between the antarctic ice sheet and climate along with more robust estimates of past ice volumes under a range of orbital settings and greenhouse gas concentrations

early cenozoic benthic foraminiferal isotopes species reliability and interspecies correction factors 1 oxygen and carbon isotope records are important tools used to reconstruct past ocean and climate conditions with those of benthic foraminifera providing information on the deep oceans reconstructions are complicatedby interspecies isotopic offsets that result from microhabitat preferences carbonate precipitation in isotopically distinct environments and vital effects species specific metabolic variation in isotopic fractionation we provide correction factors for early cenozoic benthic foraminifera commonly used for isotopic measurements cibicidoides spp nuttallides truempyi oridorsalis spp stensioina beccariiformis hanzawaia ammophila and bulimina spp showing that most yield reliable isotopic proxies of environmental change the statistical methods and larger data sets used in this study provide more robust correction factors than do previous studies interspecies isotopic offsets appear to have changed through the cenozoic either 1 as a result of evolutionary changes or 2 as an artifact of different statistical methods and data set sizes used to determine the offsets in different studies regardless of the reason the assumption that isotopic offsets have remained constant through the cenozoic has introduced an 1 2 c uncertainty into deep sea paleotemperature calculations in addition we compare multiple species isotopic data from a western north atlantic section that includes the paleocene eocene thermal maximum to determine the most reliable isotopic indicator for this event we propose that oridorsalis spp was the most reliable deepwater isotopic recorder at this location because it was best able to withstand the harsh water conditions that existed at this time it may be the best recorder at other locations and for other extreme events also

ocean overturning since the late cretaceous inferences from a new benthic foraminiferal isotope compilation 1 benthic foraminiferal oxygen isotopic d 18 o and carbon isotopic d 13 c trends constructed from compilations of data series from multiple ocean sites provide one of the primary means of reconstructing changes in the ocean interior these records are also widely used as a general climate indicator for comparison with local and more specific marine and terrestrial climate proxy records we present new benthic foraminiferal d 18 o and d 13 c compilations for individual ocean basins that provide a robust estimate of benthic foraminiferal stable isotopic variations to 80ma andtentatively to 110ma first order variations ininterbasinal isotopicgradients delineate transitions from interior ocean heterogeneity during the late cretaceous 65 ma to early paleogene 35 65 ma homogeneity and a return to heterogeneity in the late paleogene early neogene 35 0 ma we propose that these transitions reflect alterations in a first order characteristic of ocean circulation the ability of winds to make water in the deep ocean circulate we document the initiation of large interbasinal d 18 o gradients in the early oligocene and link the variations in interbasinal d 18 o gradients from the middle eocene to oligocene with the increasing influence of wind driven mixing due to the gradual tectonic opening of southern ocean passages and initiation and strengthening of the antarctic circumpolar current the role of wind driven upwelling possibly associated with a tethyan circumequatorial current in controlling late cretaceous interior ocean heterogeneity should be the subject of further research

eocene oligocene magnetobiochronology of odp sites 689 and 690 maud rise weddell sea antarctica magnetostratigraphic studies of paleogene sediments piston cored on maud rise weddell sea odp sites 689 and 690 are a cornerstone of southern ocean paleogene and neogene chronostratigraphy however parts of previous magnetostratigraphic interpretations have been called into question and recent reinvestigation of the upper paleocene middle eocene portion of site 690 suggested that the records might be contaminated by spurious magnetizations which raises doubts about the reliability of these important records we undertook a high resolution magnetostratigraphic study of eocene oligocene u channel samples from odp holes 689b 689d 690b and 690c in order to address these concerns a pervasive overprint appears to be present below the middle eocene which compromises magnetobiostratigraphic interpretations for the upper cretaceous and lower paleogene nevertheless our new results provide a robust record of geomagnetic field behavior from 38 5 to 25 ma and confirm the reliability of these sediments for calibration of biostratigraphic datum events during a crucial phase of earth history when major antarctic ice sheets developed also comparison of magnetozone thicknesses in multiple holes at the same site indicates that 1 2 1 8 m of the stratigraphic record is missing at each core break which corresponds to time breaks of 120 360 k y lack of a continuous record within a single hole renders useless spectral analyses for investigating long geomagnetic and paleoclimatic time series this observation reinforces the need for coring of multiple offset holes to obtain continuous paleoceanographic records sedimentary hiatuses have been identified only at the deeper of the two investigated sites site 690 which could mark a local response to the onset of the antarctic circumpolar current

continental ice in greenland during the eocene and oligocene the eocene and oligocene epochs approximately 55 to 23 million years ago comprise a critical phase in earth history an array of geological records supported by climate modelling indicates a profound shift in global climate during this interval from a state that was largely free of polar ice caps to one in which ice sheets on antarctica approached their modern size however the early glaciation history of the northern hemisphere is a subject of controversy here we report stratigraphically extensive ice rafted debris including macroscopic dropstones in late eocene to early oligocene sediments from the norwegian greenland sea that were deposited between about 38 and 30 million years ago our data indicate sediment rafting by glacial ice rather than sea ice and point to east greenland as the likely source records of this type from one site alone cannot be used to determine the extent of ice involved however our data suggest the existence of at least isolated glaciers on greenland about 20 million years earlier than previously documented at a time when temperatures and atmospheric carbon dioxide concentrations were substantially higher