climate stability across the eocene oligocene transition southern argentina fossil mammal teeth from mid latitude southern argentina 46 s that closely bracket the eocene oligocene transition show no resolvable change in oxygen isotope compositions in combination with paleofloral observations and geographic considerations this finding implies not only that climate was essentially constant despite interpretations elsewhere for major mid and high latitude cooling but also that evolution of hypsodonty did not coincide with climate change during the eocene oligocene transition one possible explanation for eocene oligocene transition climatic stability is that southern high latitude cooling increased latitudinal temperature gradients and strengthened ocean circulation gyres including the southward flowing brazil current in the western south atlantic regionally increased heat transport in the western atlantic offset global cooling producing a nearly constant temperature in southern south america a more radical interpretation supported by some marine data is that the paradigm of major global cooling at the eocene oligocene transition is largely false in that mean sea surface temperatures changed very little

antarctic climate at the eocene oligocene boundary climate model sensitivity to high latitude vegetation type and comparisons with the palaeobotanical record abstract simulated climate for the antarctic continent using the genesis version 2 1 global climate model with 34 ma boundary conditions is shown to be highly sensitive to polar vegetation type six experiments were run using different levels of atmospheric co2 orbital configurations ice sheet geometries and vegetation types to assess model sensitivity to antarctica being covered with either a needle leaf evergreen forest or tundra simulations using 2 pre industrial levels of co2 combined mean annual temperature mat 19 c are about 7 c cooler than minimum estimates from the antarctic cenozoic plant record mat 12 to 15 c however simulations using 3 co2 mat 7 c are in good agreement with our empirical estimates of mean annual temperature with ice sheets and orbits set up to represent early oligocene interglacial conditions the tundra climate is significantly cooler than the evergreen forest climate with local austral summer averages up to 6 c cooler in non glaciated areas and continental averages 2 5 c cooler in the model this is mainly due to higher albedo and decreases in net radiation and sensible and latent heat flux especially during spring and summer feedbacks between coastal and continental cooling marginal sea surface temperatures and sea ice also appear to be significant a review of the late palaeocene to earliest miocene plant fossil record in the antarctic peninsula and ross sea regions shows that the vegetation was in transition 34 ma from a relatively diverse mainly evergreen forest to a tundra vegetation the modelled sensitivity of continental temperatures to a change from forest to tundra suggests vegetation climate feedbacks during the eocene oligocene transition played a significant role in the initial rapid glaciation of the continent

refinements for footwall red bed diagenesis in the sediment hosted stratiform copper deposits model the emplacement of copper in sediment hosted stratiform copper deposits requires the circulation of a low temperature chloride rich brine in rift filling footwall red beds probably driven by meteoric recharge in adjacent highlands the descending oxygen rich meteoric water may become saline probably by leaching of footwall evaporites or possibly by mixing with brines draining down from contemporaneous evaporite pans and evolve toward a moderately oxidized brine as oxygen is consumed during the diagenetic reddening of the initially nonred coarse grained clastic rift sediments under these conditions the pore solution attains its maximum ability to leach and transport trace amounts of copper from the red beds sediment hosted stratiform copper deposits may form where the brine crosses a redox boundary into reduced sulfide rich gray beds with further consumption of oxygen during reddening the brine may become highly reduced and able to take iron into solution possibly explaining synsediment hosted stratiform copper iron oxide alterations and deposits the timing of copper deposition is clearly linked to the diagenetic reddening of immature footwall sediments in rift basins a process considered to extend over millions of years

cenozoic ice rafting history of the central arctic ocean terrigenous sands on the lomonosov ridge the cenozoic ice rafted debris ird history of the central arctic is reconstructed utilizing the terrigenous coarse sand fraction in iodp 302 cores from 0 to 273 meters composite depth this holocene middle eocene quantitative record of terrigenous sand accumulation on the lomonosov ridge along with qualitative information on grain texture and composition confirms the interpretation that ice initiation sea ice and glacial ice occurred 46 ma in the arctic and provides a long term pattern of arctic ice expansion and decay since the middle eocene ird mass accumulation rates range from 0 to 0 13 g cm2 ka in the middle eocene and from 0 to 0 36 g cm2 ka in the neogene ird mass accumulation rate mar maxima in the miocene and pliocene cooccur with either glacial initiation or intensification in the sub arctic the 46 25 ma ird onset in the central arctic slightly precedes the earliest evidence of ice in the antarctic and compares in timing with a 1000 ppm decrease in atmospheric concentrations of co2 the decline of pco2 in the middle eocene may have driven both poles across the temperature threshold that enabled the nucleation of glaciers on land and partial freezing of the surface arctic ocean especially during times of low insolation

the sediment hosted stratiform copper ore system sediment hosted stratiform copper deposits comprise disseminated to veinlet cu and cu fe sulfides in siliciclastic or dolomitic sedimentary rocks sediment hosted stratiform copper deposits are extremely common though economically significant deposits are rare they account for approximately 23 percent of the world s cu production and known reserves in addition to being significant sources of co and ag three sedimentary basins the paleoproterozoic kodaro udokan in siberia the neoproterozoic katangan in central africa and the permian basin of central europe contain supergiant 24 million metric tons mt contained cu deposits sediment hosted stratiform copper deposits are the products of evolving basin scale fluid flow systems that include source s of metal and s source s of metal and s transporting fluids the transport paths of these fluids a thermal and or hydraulic pump to collect and drive the fluids and the chemical and physical processes which result in precipitation of the sulfides metal sources are undoubtedly red bed sedimentary rocks containing fe oxyhydroxides capable of weakly binding metals sulfur may be derived from marine or lacustrine evaporites reduced seawater or hydrogen sulfide bearing petroleum metals appear to have been transported at low to moderate temperatures in moderately to highly saline aqueous fluids with the temperature of the fluid largely dependent on the time of fluid migration in the basin s burial history these basinal fluids were focused to potential metal precipitation sites by thinning of the red bed sequence at basin margins by faults by differentially permeable sedimentary units by paleotopography within the basin or along the margins of salt diapirs fluid movement produced widespread basin scale alteration that has commonly been overlooked but can form an important exploration guide sulfide precipitation occurred due to reduction typically caused by reaction with carbonaceous rocks or petroleum the amount of sulfides present at any deposit may be either metals or sulfide limited or could have been controlled by the amount of available reductant e g petroleum while understanding of sediment hosted stratiform copper ore genesis at the deposit scale is relatively robust there are still significant questions in regards its position in terms of basin evolution a wide variety of basin architectures and processes can lead to the formation of sediment hosted stratiform copper deposits despite general agreement that sulfides postdate sedimentation the absolute age of mineralization in many deposits has been difficult to document and the available evidence suggests that deposits can form throughout a basin s evolution from early diagenesis of ore host sediments to basin inversion and metamorphism supergiant and giant deposits formed in basins which underwent prolonged periods of fluid flow and in which unique conditions allowed for the accumulation of large amounts of metal bearing fluid sufficient reduced s and large amounts of reductants

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

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

no extreme bipolar glaciation during the main eocene calcite compensation shift major ice sheets were permanently established on antarctica approximately 34 million years ago1 2 3 close to the eocene oligocene boundary at the same time as a permanent deepening of the calcite compensation depth in the world s oceans4 until recently it was thought that northern hemisphere glaciation began much later between 11 and 5 million years ago1 2 3 5 this view has been challenged however by records of ice rafting at high northern latitudes during the eocene epoch6 7 and by estimates of global ice volume that exceed the storage capacity of antarctica8 at the same time as a temporary deepening of the calcite compensation depth 41 6 million years ago9 here we test the hypothesis that large ice sheets were present in both hemispheres 41 6 million years ago using marine sediment records of oxygen and carbon isotope values and of calcium carbonate content from the equatorial atlantic ocean these records allow at most an ice budget that can easily be accommodated on antarctica indicating that large ice sheets were not present in the northern hemisphere the records also reveal a brief interval shortly before the temporary deepening of the calcite compensation depth during which the calcite compensation depth shoaled ocean temperatures increased and carbon isotope values decreased in the equatorial atlantic the nature of these changes around 41 6 million years ago implies common links in terms of carbon cycling with events at the eocene oligocene boundary4 and with the hyperthermals of the early eocene climate optimum3 10 11 our findings help to resolve the apparent discrepancy between the geological records of northern hemisphere glaciation6 7 8 and model results12 13 that indicate that the threshold for continental glaciation was crossed earlier in the southern hemisphere than in the northern hemisphere

the role of migrating oil field brines in the formation of sediment hosted cu rich deposits oil field brines commonly contain ppm quantities of zn pb and h 2 s but negligible quantities of cu theoretical analysis of the water rock interactions expected during migration through evaporitic and red bed sequences suggests that these reducing oil field brines can become transformed into far more oxidized fluids capable of transporting cu pb zn and sulfate and of forming sediment hosted cu rich deposits calculation of the chemical mass transfer accompanying reaction of a typical reducing brine with anhydrite hematite and quartz followed by the assemblage quartz k feldspar muscovite hematite chlorite and cuprite reveals that the aqueous phase becomes greatly undersaturated with respect to sphalerite galena chalcopyrite bornite pyrite and pyrrhotite as it achieves log f sub o 2 values well above the magnetite hematite buffer consistent with saturation with respect to anhydrite hematite and chalcocite and ore forming concentrations of copper and sulfate consequently evaporites are not only possible sources of salinity and oxidized sulfur for evolving basinal brines but may be crucial to the development of the f sub o 2 conditions required to transport cu in ore forming concentrations reaction of the oxidized brine with organic matter approximated by graphite and diagenetic pyrite results in precipitation of the zonation sequence chalcocite bornite chalcopyrite and pyrite with variable amounts of hematite covellite sphalerite and galena depending on the relative rates of reaction of the graphite and pyrite

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

thresholds for cenozoic bipolar glaciation the long standing view of earth s cenozoic glacial history calls for the first continental scale glaciation of antarctica in the earliest oligocene epoch 33 6 million years ago1 followed by the onset of northern hemispheric glacial cycles in the late pliocene epoch about 31 million years later2 the pivotal early oligocene event is characterized by a rapid shift of 1 5 parts per thousand in deep sea benthic oxygen isotope values3 oi 1 within a few hundred thousand years4 reflecting a combination of terrestrial ice growth and deep sea cooling the apparent absence of contemporaneous cooling in deep sea mg ca records4 5 6 however has been argued to reflect the growth of more ice than can be accommodated on antarctica this combined with new evidence of continental cooling7 and ice rafted debris8 9 in the northern hemisphere during this period raises the possibility that oi 1 represents a precursory bipolar glaciation here we test this hypothesis using an isotope capable global climate ice sheet model that accommodates both the long term decline of cenozoic atmospheric co2 levels10 11 and the effects of orbital forcing12 we show that the co2 threshold below which glaciation occurs in the northern hemisphere 280 p p m v is much lower than that for antarctica 750 p p m v therefore the growth of ice sheets in the northern hemisphere immediately following antarctic glaciation would have required rapid co2 drawdown within the oi 1 timeframe to levels lower than those estimated by geochemical proxies10 11 and carbon cycle models13 14 instead of bipolar glaciation we find that oi 1 is best explained by antarctic glaciation alone combined with deep sea cooling of up to 4 c and antarctic ice that is less isotopically depleted 30 to 35 than previously suggested15 16 proxy co2 estimates remain above our model s northern hemispheric glaciation threshold of 280 p p m v until 25 myr ago but have been near or below that level ever since10 11 this implies that episodic northern hemispheric ice sheets have been possible some 20 million years earlier than currently assumed although still much later than oi 1 and could explain some of the variability in miocene sea level records

spore pollen evidence for early oligocene high latitude cool climatic episode in northern canada climatic cooling in the eocene leading to markedly lower temperatures in the oligocene has been documented in oxygen isotope studies in the north pacific1 2 new zealand3 south pacific4 and north sea5 these lower temperatures in the oligocene correspond to palaeoclimatic interpretations of north american leaf floras6 7 which suggest a profound cooling occurring towards the end of the eocene indicated also in the southeastern united states from studies on mid tertiary spores and pollen8 recently collinson et al 9 have shown palaeobotanically that in southern england cooling occurred gradually starting in the latest early eocene leading to two major periods of floristic change before the end of the eocene thick tertiary sections in the subsurface mackenzie delta region northwest territories10 11 provide well preserved palaeogene palynofloras12 discussed here which indicate a cooler climate in the oligocene following warm temperate conditions in the eocene this widespread early middle oligocene cool episode is thus represented in both high latitude and lower latitude floras and seems to have been of global extent persisting until towards the end of the oligocene but its precise dating remains a problem in northern canada it is followed by an amelioration of the climate in the late oligocene which persisted probably until the middle miocene the rate of cooling during the eocene and oligocene may have varied depending on locality but once established the cooler oligocene climate exerted a profound influence on biotic development

vanadium accumulation in carbonaceous rocks a review of geochemical controls during deposition and diagenesis abstract published data relevant to the geochemistry of vanadium were used to evaluate processes and conditions that control vanadium accumulation in carbonaceous rocks reduction adsorption and complexation of dissolved vanadium favor addition of vanadium to sediments rich in organic carbon dissolved vanadate v v species predominate in oxic seawater and are reduced to vanadyl ion v iv by organic compounds or h 2 s vanadyl ion readily adsorbs to particle surfaces and is added to the sediment as the particles settle the large vanadium concentrations of rocks deposited in marine as compared to lacustrine environments are the result of the relatively large amount of vanadium provided by circulating ocean water compared to terrestrial runoff vanadium rich carbonaceous rocks typically have high contents of organically bound sulfur and are stratigraphically associated with phosphate rich units a correspondence between vanadium content and organically bound sulfur is consistent with high activities of h 2 s during sediment deposition excess h 2 s exited the sediment into bottom waters and favored reduction of dissolved v v to v iv or possibly v iii the stratigraphic association of vanadiferous and phosphatic rocks reflects temporal and spatial shifts in bottom water chemistry from suboxic phosphate concentrated to more reducing euxinic conditions that favor vanadium accumulation during diagenesis some vanadium organic complexes migrate with petroleum out of carbonaceous rocks but significant amounts of vanadium are retained in refractory organic matter or clay minerals as carbon in the rock evolves toward graphite during metamorphism vanadium is incorporated into silicate minerals

cooler winters as a possible cause of mass extinctions at the eocene oligocene boundary the eocene oligocene boundary at about 33 7 myr ago marks one of the largest extinctions of marine invertebrates in the cenozoic period1 for example turnover of mollusc species in the us gulf coastal plain was over 90 at this time2 3 a temperature change across this boundary from warm eocene climates to cooler conditions in the oligocene has been suggested as a cause of this extinction event4 but climate reconstructions have not provided support for this hypothesis here we report stable oxygen isotope measurements of aragonite in fish otoliths ear stones collected across the eocene oligocene boundary palaeotemperatures reconstructed from mean otolith oxygen isotope values show little change through this interval in agreement with previous studies5 6 from incremental microsampling of otoliths however we can resolve the seasonal variation in temperature recorded as the otoliths continue to accrete new material over the life of the fish these seasonal data suggest that winters became about 4 c colder across the eocene oligocene boundary we suggest that temperature variability rather than change in mean annual temperature helped to cause faunal turnover during this transition

fossil biotas from the okanagan highlands southern british columbia and northeastern washington state climates and ecosystems across an eocene landscape the late early to early middle eocene okanagan highlands fossil sites spanning 1000 km north south northeastern washington state southern british columbia provide an opportunity to reconstruct biotic communities across a broad upland landscape during the warmest part of the cenozoic plant taxa from these fossil sites are characteristic of the modern eastern north american deciduous forest zone principally the mixed mesophytic forest but also include extinct taxa taxa known only from eastern asian mesothermal forests and a small number of taxa restricted to the present day north american west coast coniferous biome in this preliminary report paleoclimates and forest types are reconstructed using collections from republic in washington state usa and princeton quilchena falkland mcabee hat creek horsefly and driftwood canyon in british columbia canada both leaf margin analysis lma and quantitative bioclimatic analysis of identified nearest living relatives of megaflora indicated uppe

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

magnetostratigraphic calibration of eocene oligocene dinoflagellate cyst biostratigraphy from the norwegian greenland sea the presence of abundant age diagnostic dinoflagellate cysts in ocean drilling program odp hole 913b leg 151 deep sea drilling project hole 338 leg 38 and odp hole 643a leg 104 has enabled the development of a new biostratigraphy for the eocene oligocene interval in the norwegian greenland sea this development is important because the calcareous microfossils usually used for biostratigraphy in this age interval are generally absent in high latitude sediments as a result of dissolution in parallel with this biostratigraphic analysis we developed a magnetic reversal stratigraphy for these norwegian greenland sea sequences this has allowed independent age determination and has enabled the dinocyst biostratigraphy to be firmly tied into the global geomagnetic polarity timescale gpts the relatively high resolution of this study has enabled identification of dinoflagellate cyst assemblages that have affinities with those from the north sea and the north atlantic which allows regional correlation correlation of each site with the gpts has also allowed comparison of the stratigraphic record preserved in each drill hole hole 913b is the most complete and is the best preserved record of the eocene and oligocene in the northern hemisphere high latitudes and can serve as a reference section for palaeoenvironmental reconstructions of this age interval

emplacement of bitumen asphaltite veins in the neuquen basin argentina veins of solid bitumen asphaltite have been commercially exploited in the neuquen basin argentina for over 100 yr veins are up to 5 m wide and several kilometers in length over a region of 15 000 km2 these veins were emplaced in fractures both parallel and at high angles to bedding in close proximity to their source rocks in the vaca muerta and agrio formations late jurassic early cretaceous two or more phases of bitumen emplacement can be recognized in several localities structures bearing viscous oil are younger than structures having solid bitumen bitumen emplacement was vigorous and caused brecciation and spalling of the host rocks the bitumen was also viscous and supports rock debris ranging in size from sand grains up to meter scale slabs brecciation bedding parallel injection and wall rock impregnation suggest high fluid pressures during emplacement high fluid pressure may have been engendered by substantial hydrocarbon generation from rich source rocks in a low permeability sequence and probably caused the fractures into which the bitumen migrated the bedding parallel veins facilitated decollement during thrusting that took place during and after bitumen emplacement the timing of emplacement relative to thrusting and oil migration constrains bitumen emplacement to the eocene oligocene

alteration halos in the tordillos sediment hosted copper deposit of the neuquen basin argentina abstract the tordillos sediment hosted cu deposit is located in the north central sector of the huincul high structure in the neuquen basin argentina it is hosted in sedimentary rocks of the huincul formation an upper cretaceous redbed sequence deposited in a fluvial environment field mapping petrography and chemical analysis of the authigenic clay sulfate carbonate and sulfide minerals at tordillos have identified systematic variation in alteration and paragenesis with time and with respect to the main feeder zones the alteration associated with cu mineralization provides evidence of a multistage infiltration of fluids spatially associated with paleo hydrocarbon migration driven by tertiary tectonism early diagenesis formed hematite kaolinite and calcite1 that have been preserved in the reddish brown mudstones later influx of hydrocarbons and warm reduced formation waters from deeper reservoir rocks is associated with dissolution of early cements and clasts and precipitation of new minerals in the thickest paleo channels of the huincul formation dissolution of early cements hematite kaolinite calcite1 and barite1 and clasts locally increased the porosity of sandstones up to 24 and destroyed hematite that changed sandstones and conglomerates from red to yellowish grey bitumen impregnations are abundant along with calcite2 3 and 4 pyrite minor montmorillonite quartz overgrowth and coarse barite2 crystals with primary organic fluid inclusions at the contact with hydrocarbon tubes the mno enrichment of calcite in stages 3 and 4 reflects their precipitation under reduced conditions grey and greenish grey sandstones with montmorillonite v hematite smectite chlorite group minerals and pyrite respectively are exposed between the reddish brown mudstones and the yellowish grey sandstones and conglomerates the grey and greenish grey horizons are representative of abrupt changes in eh ph conditions between regional early oxidizing solutions responsible for the formation of early diagenetic redbeds and later infiltration of reducing fluids responsible for alteration of sedimentary rocks copper mineralization is located inside the altered sandstones and conglomerates zones forming halos surrounding vertical tubes containing bitumen these tubes are interpreted as the conduits through which migrated hydrocarbons formation waters and subsequent chloride and metal rich fluids in these places pyrite calcite and barite cements of the altered rocks have been dissolved and replaced by copper rich sulfides and then by late supergene cu minerals interconnection among stacked permeable paleo channels in the mid section of the huincul formation facilitated the migration of several generations of fluids during early diagenesis hydrocarbon up flow and exhumation of the neuquen group resulting in the development of alteration halos and cu mineralization that can be used both in outcrop and drill core during exploration for sediment hosted cu mineralization

large scale bleaching of red beds related to upward migration of hydrocarbons los chihuidos high neuquen basin argentina abstract the striking arrangement of colorful facies in the sandstones outcrops of the los chihuidos high argentina has been attributed to the surface expression of hydrocarbon induced alteration the huincul formation late cenomanian early turonian is part of the thick continental red bed dominated neuquen group which consists of meandering fluvial deposits developed in a foreland basin visual distinctions reveal four diagenetic facies red white gray and minor brown sandstones differences in the diagenetic mineralogy among sandstones facies are interpreted to depend on the degree of interaction with hydrocarbon bearing solutions and the relative position of the redox roll front developed during this process the red facies represents the original oxidized sandstone with hematite kaolinite quartz and albite overgrowths and calcite cement precipitated under oxidizing conditions during the regional burial diagenesis the white facies formed during the interaction of red beds with reduced fluids in presence of hydrocarbons or organic acids where clasts and cement were partially dissolved and iron was reduced to fe2 due to redox reactions and removed resulting in the sandstone decoloration and in a marked development of the secondary porosity minor montmorillonite replaced kaolinite and detrital feldspars and a poikilotopic calcite precipitated filling the pores the gray and the brown sandstone bound the redox front which is characterized by the presence of abundant clay minerals and oxide minerals enriched in vanadium and copper montmorillonite and secondary hematite are dominant in the more oxidized gray sandstones whereas interstratified chlorite montmorillonite is more abundant in the more reduced brown sandstones the coexistence of these minerals indicates metastable reducing oxidizing conditions at the redox front based on diagenetic characteristics tectonic framework maturation timing of source rocks and petroleum emplacement history hydrocarbons could have migrated into the huincul formation during the tertiary uplift but later erosion could have released the hydrocarbons and exhumed the reservoir this multi scale investigation of diagenetic features related to burial history and to structure controlled upflow of hydrocarbons in a succession of siliciclastic rocks enhances modeling of a regional scale redox system worldwide our study offers new insight into the application of authigenic minerals to trace hydrocarbon pathways along red beds and to evaluate the reservoir quality in the understanding the importance of fluid compositions mixing and fluid rock interaction along a major redox system

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

how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones observations and experiments observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones samples of deeply buried porous sandstones with different types of clay coats were split in two coats removed from one of each pair of splits and grain surfaces inspected with scanning electron microscopy quartz grains in a fluvial deltaic sandstone buried to 115c had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats though well developed overgrowths occurred on nearby naturally uncoated grains however in similar sandstones buried to 164c quartz grain surfaces exposed by chlorite coat removal were covered with small 5 m mainly anhedral syntaxial quartz overgrowths similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215c we conclude that clay coats may retard quartz nucleation at moderate temperatures but at high temperatures many coats permit quartz nucleation and preserve porosity by limiting cement growth to investigate cement growth limitation mechanisms samples with coats removed were subjected to quartz cementing conditions in a hydrothermal reactor during experiments the naturally occurring small overgrowths on clay cleaned grains coalesced and grew suggesting that clay particles in coats inhibit cement growth by forming barriers to early overgrowth coalescence although the fraction of grain surface coverage is the primary control on cement inhibition by coats cement growth interference textures vary with coat type providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness in deeply buried sandstones quartz cement can fill significant microporosity within diagenetic chlorite coats potentially affecting mechanical and petrophysical rock properties

ancient subaerial exposure and freshwater leaching in sandstones the authors present an example of leaching by meteoric water in the jurassic magnus sandstone member of the northern north sea the interpretation is based on information from several independent techniques including downhole natural gamma ray spectrometry detailed petrographic analysis and cross sectional time slice reconstructions from seismic data the initial lines of evidence that prompted further investigation into freshwater involvement in the magnus sandstone member were brackish formation water compositions and a potassium depletion shown by the gamma ray spectrometry log in sandstone beneath the unconformity at the base of the cretaceous section subsequent detailed petrographic study confirmed that the potassium depletion had been brought about by potassium feldspar dissolution resulting in precipitation of kaolinite and creation of secondary porosity the latter also revealed by conventional core analysis porosity and permeability measurements finally cross sectional time slice reconstructions from a regional seismic line across the magnus oilfield strongly suggested subaerial exposure of the magnus sandstone member in early cretaceous time and consequent freshwater leaching of the magnus sandstone member and generation of the unconformity at the base of the cretaceous the combination of several lines of evidence from the magnus field supports the process of subaerial exposure and freshwater leaching to create secondary porosity in sandstones

cenozoic uplift of nuussuaq and disko west greenland elevated erosion surfaces as uplift markers of a passive margin remnants of a high plateau have been identified on nuussuaq and disko central west greenland we interpret the plateau as an erosion surface the summit erosion surface formed mainly by a fluvial

increased seasonality through the eocene to oligocene transition in northern high latitudes a profound global climate shift took place at the eocene oligocene transition approximately 33 5 million years ago when cretaceous early palaeogene greenhouse conditions gave way to icehouse conditions during this interval changes in the earth s orbit and a long term drop in atmospheric carbon dioxide concentrations resulted in both the growth of antarctic ice sheets to approximately their modern size and the appearance of northern hemisphere glacial ice however palaeoclimatic studies of this interval are contradictory although some analyses indicate no major climatic changes others imply cooler temperatures increased seasonality and or aridity climatic conditions in high northern latitudes over this interval are particularly poorly known here we present northern high latitude terrestrial climate estimates for the eocene to oligocene interval based on bioclimatic analysis of terrestrially derived spore and pollen assemblages preserved in marine sediments from the norwegian greenland sea our data indicate a cooling of approximately 5 degrees c in cold month winter mean temperatures to 0 2 degrees c and a concomitant increased seasonality before the oi 1 glaciation event these data indicate that a cooling component is indeed incorporated in the delta 18 o isotope shift across the eocene oligocene transition however the relatively warm summer temperatures at that time mean that continental ice on east greenland was probably restricted to alpine outlet glaciers

microcodium revisited root calcification products of terrestrial plants on carbonate rich substrates abstract microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols in the cretaceous and tertiary continental and marine successions of the peri tethyan realm the main controversy about the microcodium structures is their origin and possible relation with calcified plant roots microcodium and rhizogenic root formed microfabrics were studied in calcrete profiles within the paleocene shallow marine carbonate succession in southwestern slovenia the prominent laminar calcrete horizons contain abundant calcite aggregates 150 m to 1 cm in size with perfectly preserved structural details of plant root tissues morphology and structure of these aggregates indicate that they formed through biologically controlled precipitation of calcium carbonate within the root cortical cells the morphologies intermediate between the typical microcodium aggregates composed of a single layer of individual elongate pyramidal or prismatic crystals of calcite measuring 100 500 m in length and 20 70 m in width and calcified roots with multilayer arrangement of isodiametric cells were observed and this supports previous rhizogenic interpretations of microcodium structures intermediate forms show that the typical microcodium aggregates formed through morphological transformation of the root tissue by growth of the calcite within the cortical cells which distorted the cell shape calcification of roots and the creation of microcodium structures can be explained as an effective nutrient acquiring mechanism used by certain types of terrestrial plants inhabiting nutrient poor calcareous substrates the widespread occurrence of microcodium in almost unaltered shallow marine limestones indicates that its formation took place during early stages of paleosol development probably reflecting specific types of vascular plants of a pioneer community that were able to colonize carbonate substrates during the early phases of subaerial exposure

large temperature drop across the eocene oligocene transition in central north america the eocene oligocene transition about 33 5 million years ago was a major global climate event the end of the eocene was unusually warm with no significant ice on antarctica but the oligocene saw the arrival of a permanent antarctic ice sheet two papers this week relate to the continental effects of this global change dupont nivet et al examined sedimentary records from the tibetan plateau and find a drop in atmospheric water which caused cooling and aridification coincident with antarctic cooling previous studies attributed this phenomenon to the rapid uplift of the tibetan plateau but this new work suggests that regional tibetan climate was influenced by global events in an unrelated paper on the same climate transition zanazzi et al explore the cooling in north america at the time using stable isotope measurements from fossil teeth and bones to create a proxy temperature record they find a large drop in mean annual temperature of 8 2 c a greater fall than seen in the oceans this continental transition may explain why many cold blooded reptiles and amphibians became extinct whereas mammals able to regulate their body temperature escaped relatively unscathed an analysis of fossil tooth enamel and fossil bones is used to derive a continental temperature record for the eocene oligocene transition and reports a large drop in mean annual temperature over roughly 400 000 years this cooling explains the decline in cold blooded terrestrial animals whereas most mammals in the region were unaffected the eocene oligocene transition towards a cool climate 33 5 million years ago was one of the most pronounced climate events during the cenozoic era1 the marine record of this transition has been extensively studied however significantly less research has focused on continental climate change at the time yielding partly inconsistent results on the magnitude and timing of the changes2 3 4 5 6 7 8 here we use a combination of in vivo stable isotope compositions of fossil tooth enamel with diagenetic stable isotope compositions of fossil bone to derive a high resolution about 40 000 years continental temperature record for the eocene oligocene transition we find a large drop in mean annual temperature of 8 2 3 1 c over about 400 000 years the possibility of a small increase in temperature seasonality and no resolvable change in aridity across the transition the large change in mean annual temperature exceeding changes in sea surface temperatures at comparable latitudes9 10 and possibly delayed in time with respect to marine changes by up to 400 000 years explains the faunal turnover for gastropods amphibians and reptiles whereas most mammals in the region were unaffected our results are in agreement with modelling studies that attribute the climate cooling at the eocene oligocene transition to a significant drop in atmospheric carbon dioxide concentrations

detrital zircon provenance from the neuquen basin south central andes cretaceous geodynamic evolution and sedimentary response in a retroarc foreland basin the surface response in terms of drainage pattern changes to the cretaceous geodynamic reorganization of the andean subduction zone between 36 s and 41 s is reconstructed through the geochronology based provenance study of alluvial detrital zircons the age spectra obtained by 500 spot u pb ages record an eastward provenance of detritus coming from the foreland during the early cretaceous backarc extensional stage followed by westward sourced clastics coming from the cordillera during the cenomanian this drainage pattern reversal fits the regional unconformity in the sedimentary record that is linked to the geodynamic reorganization of the continental margin from an extensional to a compressional regime forcing the neuquen basin to evolve from a retroarc to a foreland stage after this inversion the clastic systems progressively returned to be mainly fed by the foreland due to the uplift of the peripheral bulge as a consequence of the late cretaceous thrust front migration this tectonic evolution of the neuquen basin and the related response of the drainage pattern are thought to be the surface expression of the dip decrease of the benioff subduction zone

alternative sources of metals for stratiform copper deposits abstract the controversy between sedimentary and epigenetic emplacement of copper in stratiform copper deposits appears to be subsiding in favour of an essentially synsedimentary enrichment of sulphur e g disseminated pyrite and or sulphates which undergo biogenic reduction to sulphide followed by an early diagenetic addition of copper to the host sediment the source of the copper has yet to be conclusively identified since the copper is deposited upon migrating into reducing zones e g sulphide rich organic bearing black shales and sulphate bearing microbial laminated carbonates the closest source of copper could be the commonly adjacent red beds where oxidising conditions would favour solution of copper in the presence of interstitial brines the thickness of these red bed sections is typically measured in 100 s to 1000 s of metres and they commonly contain abundant immature detrital grains including mafic minerals which could provide sufficient trace element copper to form major ore deposits copper in red beds may also include metal initially absorbed on amorphous iron oxides and released during diagenetic reddening of these oxides to form haematite on the other hand the ubiquitous red beds might be discounted as a principal source of copper and could be considered for the most part to have behaved as permeable beds through which cupriferous solutions from other sources migrated toward the grey beds for example mafic volcanics are found in important quantities in stratigraphic sequences hosting several stratiform copper deposits and could contribute large volumes of metalliferous solutions during metamorphic dehydration alternatively these same volcanic rocks may suggest volcanic associated pene exhalative activity with hydrothermal fluids emptying into subsurface red bed aquifers rather than onto surface sediments as commonly proposed for exhalative models although fluids of 250 c are generally considered necessary to transport copper in significant quantities in sulphide bearing exhalations lower temperatures are sufficient for chloride complexing in the absence of sulphide the large volumes of red beds could also have a major cooling effect on initially hot solutions the pene exhalative model may be closely related to fault bounded hot spring models of the type envisaged for so called shale hosted submarine exhalative base metal deposits

stable isotope and fluid inclusion study of sediment hosted stratiform copper deposits from the neuquen basin argentina the los chihuidos and el porvenir deposits of the neuquen basin are examples of sediment hosted stratiform cu deposits generated by interaction of hydrocarbons and formation water with host red beds and metal charged basinal brines during early diagenesis of red beds precipitated hematite kaolinite at 60 c from meteoric water followed by calcite 1 13c 8 to 3 3 and barite 34s 4 1 with increasing temperature 75 85 c from an evolved surface fluid 18ofluids 0 9 to 2 7 during the tertiary andean orogeny hydrocarbons and formation water migrated and reacted with the red beds resulting in bleaching of the sandstone smectite fluid 18o 2 2 and d 73 7 chlorite smectite mixed layer minerals fluid 18o 6 9 and d 84 pyrite 34s 10 2 and calcite 2 13c 12 9 to 6 8 formed as by product of the redox reactions calcite 2 formed from low salinity fluids 0 4 to 5 9 wt nacl equiv at slightly higher temperatures 125 145 c at el porvenir and at los chihuidos deposit 80 105 c during subsequent uplift metal charged basinal brines flowed into the bleached sandstone and precipitated chalcopyrite bornite 34s 12 3 followed by chalcocite spionkopite 34s 64 to 4 1 calcites 3a 13c 19 to 10 1 and 3b 13c 31 4 to 9 5 that accompanied fe cu and cu sulfides respectively formed from saline fluids up to 21 3 wt nacl equiv at temperatures of 159 70 c the 18o values 5 6 to 11 4 of fluids in equilibrium with calcite 3a are similar to oilfield and basinal brines fluids in equilibrium with calcite 3 in los chihuidos and calcite 3b in el porvenir show much lighter 18o values 7 3 to 4 6 and 0 6 to 2 3 respectively which suggests the involvement of methane as the main reductant and possibly meteoric water interaction

evidence for glaciation in the northern hemisphere back to 44 ma from ice rafted debris in the greenland sea the widely accepted age estimate for the onset of glaciation in the northern hemisphere ranges between 2 and 15 million years ago ma however recent studies indicate the date for glacial onset may be significantly older we report the presence of ice rafted debris ird in 44 to 30 ma sediments from the greenland sea evidence for glaciation in the north atlantic during the middle eocene to early oligocene detailed sedimentological evidence indicates that glaciers extended to sea level in the region allowing icebergs to be produced ird may have been sourced from tidewater glaciers small ice caps and or a continental ice sheet

the smectite to chlorite transition in the chipilapa geothermal system el salvador clay mineralogical x ray diffraction and electron microprobe studies have been carried out on separated 2 m fractions from cutting and core material from three wells in the chipilapa geothermal system in el salvador the data indicate that the smectite to chlorite transition is prevalent but a secondary smectite to illite transition is also present at depths approximately 750 m smectite with very minor chlorite mixed layers approximately 15 is dominant and has a composition midway between a di and tri smectite at 750 m there is a very clear distinction and sharp transition into discrete chlorite with very minor smectite mixed layers approximately 10 corrensite is recorded only as a rare and minor phase smectite occurs in abundance at temperatures up to 200 c and the transition from a smectitedominant to chlorite dominant assemblage takes place over a narrow temperature range 150 to 200 c the stability range of smectite is very similar to that recorded in other geothermal systems whereas the smectite to chlorite transition differs greatly from that recorded in other systems the transition does not involve continuous chlorite smectite mixed layering but a marked step it is the sharpest and most discontinuous stepped sequence of this mineralogical transition recorded

eocene oligocene extinction and paleoclimatic change near eugene oregon thick ash fl ow tuffs provide marker beds through fossiliferous eocene and oligocene marine and non marine sedimentary rocks near eugene oregon new mapping radiometric dating and paleomagnetic stratigraphy of these tuffs and rocks now allow dating of local fossil fl oras the comstock goshen rujada and willamette fl oras have been widely used as evidence for eocene oligocene climatic cooling and drying eocene leaves from comstock and hobart butte included such thermophilic taxa as liquidambar the early oligocene goshen fl ora lacked liquidambar but retained many thermophilic species with large leaves that have entire margins and acuminate apices drip tips in contrast fossil leaves from later oligocene rujada and willamette fl oras are small and serrate and most lack drip tips marine faunas also indicate climatic cooling and local disappearance of thermophilic molluscs such as anadara ficus and conus our dating and compilation of plant and molluscan fossil occurrences indicate a steady rise in species diversity from 46 ma to maximal diversity of thermophilic taxa at 35 34 ma then extinctions of 60 of plant species after 33 4 ma and 32 of marine invertebrates after 33 2 ma both signifi cantly postdating the eocene oligocene boundary at 33 7 ma plant diversity rebounds during the early oligocene but marine invertebrates continue to decline into the oligocene in part due to the retreat of fully marine environments from the eugene area neither these data nor evidence from coeval fossil plants and soils in central oregon support the notion of a terminal eocene event nor any other single abrupt paleoclimatic shift or extinction the eocene oligocene biotic and climatic transition was drawn out over some 6 m y abrupt forcings such as meteorite impacts or volcanic eruptions are less likely explanations for cooling and diversity decline than long term processes such as mountain building changing ocean currents or reorganization of the carbon cycle by coevolution of grasses and grazers

mineralogical signature of hydrocarbon circulation in cretaceous red beds of the barda gonzalez area neuquen basin argentina the portezuelo formation is part of a red bed sequence in the neuquen basin assigned to the neuquen group lower cenomanian middle campanian in outcrops of the portezuelo formation in the barda gonzalez region iron oxide and hydroxide cements are only preserved in the less permeable claystone layers paleomigration of hydrocarbons and formation waters along the most permeable layers bleached the rocks due to partial to total dissolution of cements and grains and formation of several authigenic minerals 1 montmorillonite and pyrite in the medium to fine grained sandstones and mudstones and 2 multistage calcite and pyrite in the coarse grained sandstones and conglomerates the canyon diablo troilite of 24 4 to 60 values in pyrite point to microbial reduction of a sulfate precursor the oxidation of hydrocarbons and fluctuations in ph and carbon dioxide pressure caused by organic acids resulted in multiple stages of dissolution and precipitation of calcite calcite crystals host hydrocarbon bearing fluid inclusions pyrite with variable fluorescence color indicating that hydrocarbon composition changed with time the intermediate peedee belemnite 8 11 to 8 52 and 9 76 to 9 83 isotope composition of calcites may have resulted from mixtures of rich after the dissolution of local calcrete with poor from oxidizing hydrocarbons this contribution provides for the first time key information to predict the distribution of authigenic minerals in reservoir rocks in the basin and improves understanding of the diagenetic history of the portezuelo formation

effects of handling storage and chemical treatments on δ13c values of terrestrial fossil organic matter with the need to interpret small isotopic variations 13c analyses of sedimentary organic matter are more and more widespread in the field of paleo climatology recent developments require an evaluation of the reliability and reproducibility of the whole data acquisition chain literature abounds in protocols for sediment pretreatment prior to physical measurements these procedures differ at every step from sampling handling and storage conditions to leaching procedure without cross evaluation in this study we focus on two sediment samples a modern temperate soil and a 70 ka typical loess we review different protocols that characterize each step of the sediment pretreatment handling and storage conditions are tested e g finger skin contact mild to high temperature oven dry and freeze drying likewise different decarbonation protocols are compared wet decarbonation under cold 0 6 n hcl 2 n hcl and boiling 1 n hcl and acid fuming with 36 hcl this study identifies up to 1 5 isotopic shifts linked to experimental bias this large bias might be at the origin of erroneous paleoclimatic interpretation on the basis of these results we propose specific treatments adapted to the sample type

heat transport deep waters and thermal gradients coupled simulation of an eocene greenhouse climate for the first time a coupled general circulation model with interactive and dynamical atmospheric oceanic and sea ice components is used to simulate an eocene 50 ma greenhouse climate we introduce efficient ocean spin up methods for coupled paleoclimate modeling sea surface temperatures ssts and salinities evolve unconstrained producing the first proxy data independent estimates for these eocene climate parameters tropical and extratropical model predicted ssts are warmer than modern values by 3 and 5 c respectively salinity driven deep water formation occurs in the north atlantic and tethys the zonal average overturning circulation is weaker than modern eocene ocean heat transport is 0 6 pw less than modern in the northern hemisphere and 0 4 pw greater in the southern hemisphere the model predicted near modern vertical and meridional eocene temperature gradients imply that the dominant theory for maintaining low gradients increased ocean heat transport is incorrect or incomplete and other mechanisms should be explored

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