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

comparison of magnetic hysteresis parameters of unremagnetized and remagnetized limestones for white magnetite bearing mesozoic pelagic limestones from italy which carry a primary magnetization the values of mrs ms and hcr hc generally lie in the pseudo single domain psd field of the day et al 1977 plot the logarithmic plot of mrs ms against hcr hc gives a straight line r 0 814 with slope and intercept close to the empirical mixing line of parry 1982 for single domain sd and multidomain md magnetite for one of the white pelagic limestone formations maiolica formation samples with hysteresis ratios closer to the md field display increased paramagnetic susceptibility and are from the upper part of the formation characterized by increased detrital clay we therefore associate the increased md magnetite with increased detrital influx for pinkish and reddish varieties of the italian pelagic limestones the presence of hematite is manifest by high saturation fields a wide range of hcr hc and wasp waisted hysteresis loops attributed to the mixing of magnetite and high coercivity authigenic hematite the hysteresis ratios for a collection of paleozoic and mesozoic remagnetized magnetite bearing limestones from britain nevada alaska and the appalachians lie mainly outside the psd field and appear to follow a power law trend following jackson et al 1993 the high values of hcr hc and the characteristically wasp waisted hysteresis loops can be interpreted in terms of a fine grained subspherical high coercivity sd magnetite mixed with a high proportion of superparamagnetic magnetite the slope and intercept of the power law relationship for mrs ms and hcr hc in the remagnetized limestones are distinct from those observed for the italian limestones and may provide a means of fingerprinting magnetite of primary as opposed to diagenetic origin

rapid stepwise onset of antarctic glaciation and deeper calcite compensation in the pacific ocean the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution is termed the calcite compensation depth at present this depth is 4 500 m with some variation between and within ocean basins the calcite compensation depth is linked to ocean acidity which is in turn linked to atmospheric carbon dioxide concentrations and hence global climate1 geological records of changes in the calcite compensation depth show a prominent deepening of more than 1 km near the eocene oligocene boundary 34 million years ago 2 when significant permanent ice sheets first appeared on antarctica3 4 5 6 but the relationship between these two events is poorly understood here we present ocean sediment records of calcium carbonate content as well as carbon and oxygen isotopic compositions from the tropical pacific ocean that cover the eocene oligocene boundary we find that the deepening of the calcite compensation depth was more rapid than previously documented and occurred in two jumps of about 40 000 years each synchronous with the stepwise onset of antarctic ice sheet growth the glaciation was initiated after climatic preconditioning7 by an interval when the earth s orbit of the sun favoured cool summers the changes in oxygen isotope composition across the eocene oligocene boundary are too large to be explained by antarctic ice sheet growth alone and must therefore also indicate contemporaneous global cooling and or northern hemisphere glaciation

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

reducing disparity in radio isotopic and astrochronology based time scales of the late eocene and oligocene a significant discrepancy of up to 0 6 myr exists between radio isotopically calibrated and astronomically tuned time scales of the late eocene oligocene we explore the possible causes of this discrepancy through the acquisition of high precision 206pb 238u dating of zircons from 11 volcanic ash beds from the umbria marche sedimentary succession which hosts the global stratotype section and point for the base of the oligocene our results indicate that the four 40ar 39ar dates from the umbria marche succession which underpin the late eocene oligocene portion of the paleogene geomagnetic polarity time scale in the 2012 edition of the geological time scale are anomalously old by up to 0 5 myr conversely when integrated with the established magnetic polarity record of the umbria marche succession 206pb 238u zircon data from this study result in oligocene magnetic reversal ages that are generally equivalent to those obtained through the tuning of ocean drilling program odp site 1218 equatorial pacific furthermore our results indicate that the late eocene tuning of odp site 1218 and international ocean discovery program iodp sites u1333 1334 equatorial pacific to the 405 kyr eccentricity signal is accurate at least back to 36 ma propagating the full uncertainty of our radio isotopic data set and where appropriate taking into account locally derived astronomical time scales we arrive at an age of 34 09 0 08 ma for the eocene oligocene boundary and 28 11 0 17 ma for the base of the chattian

a new view of italian seismicity using 20 years of instrumental recordings in this paper we show the seismicity of the past 20 years that occurred in italy and surrounding regions hypocentral locations have been obtained by using p and s wave arrival times from the ingv national and several regional permanent seismic networks more than 48 000 events selected from an original data set of about 99 780 are used to reconstruct the most complete seismic picture of the italian region so far the seismicity distribution allows inference on seismotectonics of this complex region of subduction versus continental collision our results clearly reveal the geometry of the adria and the ionian subduction and a continuous normal fault belt in the upper crust following the apennines mountain range the depth of the seismogenic layer is computed from the cut off of seismicity at depth and shows large variations along and across the seismic active regions earthquakes are generated by the different velocity of slab retreat and the subsequent asthenospheric upwelling d 2004 elsevier b v all rights reserved

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

the middle eocene climatic optimum event in the contessa highway section umbrian apennines italy we report a high resolution paleomagnetic investigation constrained by new qualitative and semiquantitative analyses of planktic and benthic foraminifera nannofossil assemblages integrated with oxygen and carbon isotope measurements for the middle eocene scaglia limestones of the contessa highway section central italy calcareous plankton assemblages enable recognition of several biostratigraphic events from planktic foraminiferal zone p11 to the lower part of zone p15 and from calcareous nannofossil zone np15 to the upper part of zone np17 which results in refi nement of the magnetobiostratigraphy of the contessa highway section correlation of the paleomagnetic polarity pattern with the geomagnetic polarity time scale provides a direct age interpretation for strata around the middle eocene scaglia limestones of the contessa highway section from chrons c21n 47 ma through to subchron c18n 1n 38 5 ma bulk carbon isotope values indicate a distinct carbon isotopic shift at 40 ma that is interpreted to represent the fi rst evidence in the northern hemisphere of the middle eocene climatic optimum which has recently been observed as a stable isotope anomaly in multiple records from the indian atlantic sector of the southern ocean this demonstrates a global response of the carbon cycle to the proposed transient increased pco 2 levels during the late middle eocene and consequent global co 2 driven climate change

ecological and evolutionary response of tethyan planktonic foraminifera to the middle eocene climatic optimum meco from the alano section ne italy abstract the enigmatic middle eocene climatic optimum meco is a transient 500 kyr warming event that significantly interrupted at 40 ma the long term cooling through the middle and late eocene eventually resulting in establishment of permanent antarctic ice sheet this event is still poorly known and data on the biotic response are so far scarce here we present a detailed planktonic foraminiferal analysis of the meco interval from a marginal basin of the central western tethys alano section northeastern italy the expanded and continuous alano section provides an excellent record of this event and offers an appealing opportunity to better understand the role of climate upon calcareous plankton evolution a sapropel like interval characterized by excursions in both the carbon and oxygen bulk carbonate isotope records represents the lithological expression of the post meco event in the study area and follows the 18 o negative shift interpreted as representing the meco warming high resolution quantitative analysis performed on both 38 m and 63 m fractions reveals pronounced and complex changes in planktonic foraminiferal assemblages indicating a strong environmental perturbation that parallels the variations of the stable isotope curves corresponding to the meco and post meco intervals these changes consist primarily in a marked increase in abundance of the relatively eutrophic subbotinids and of the small low oxygen tolerant streptochilus chiloguembelina and pseudohastigerina at the same time the arrival of the abundant opportunist eutrophic jenkinsina and pseudoglobigerinella bolivariana typical species of very high productivity areas also occurs the pronounced shift from oligotrophic to more eutrophic opportunist low oxygen tolerant planktonic foraminiferal assemblages suggests increased nutrient input and surface ocean productivity in response to the environmental perturbation associated with the meco particularly critical environmental conditions have been reached during the deposition of the sapropel like beds as testified by the presence of common giant and or odd morphotypes this is interpreted as evidence of transient alteration in the ocean chemistry the enhanced surface water productivity inferred by planktonic foraminiferal assemblages at the onset of the event should have resulted in heavier 13 c values the recorded lightening of the carbon stable isotope preceding the maximum warmth therefore represents a robust indication that it derives principally by a conspicuous increase of p co 2 the increased productivity of surface waters also supported by geochemical data may have acted as mechanism for p co 2 reduction and returned the climate system to the general eocene cooling trend the oxygen depleted deep waters and the organic carbon burial following the peak of the meco event represent the local response to the meco warming and suggest that high sequestration of organic matter if representing a widespread response to this event might have contributed to the decrease of p co 2 as well though the true mechanisms are still obscure several lines of evidence indicate a potential pressure on planktonic foraminiferal evolution during the meco event including permanent changes besides transient and ecologically controlled variations

significant southern ocean warming event in the late middle eocene a prominent middle eocene warming event is identified in southern ocean deep sea cores indicating that long term cooling through the middle and late eocene was not monotonic at sites on maud rise and the kerguelen plateau a distinct negative shift in 1 8 o values 1 0 is observed ca 41 5 ma this excursion is interpreted as primarily a temperature signal with a transient warming of 4 c over 600 k y affecting both surface and middle bathyal deep waters in the indian atlantic region of the southern ocean this isotopic event is designated as the middle eocene climatic optimum and is interpreted to represent a significant climatic reversal in the midst of middle to late eocene deep sea cooling the lack of a significant negative carbon isotope excursion as observed during the paleocene eocene thermal maximum and the gradual rate of high latitude warming suggest that this event was not triggered by methane hydrate dissociation rather a transient rise in pco 2 levels is suspected possibly as a result of metamorphic decarbonation in the himalayan orogen or increased ridge arc volcanism during the late middle eocene

constraints on glaciation in the middle eocene 46 37 ma from ocean drilling program odp site 1209 in the tropical pacific ocean the presence of glacial ice in the late middle eocene has been vigorously debated recently published sedimentary data from the high latitudes is suggestive of episodic cooling events and near freezing sea surface temperatures during parts of the late middle eocene constraints on ice volumes and the significance of excursions in open ocean foraminifera and seawater 18o reconstructions are less clear and there are few high resolution 18o records we present a new detailed record of benthic foraminiferal 18o from site 1209 that exhibits variations 18o benthic of 0 6 1 3 different approaches have previously been used to interpret 18o benthic including 1 an a priori assumption of a 50 contribution of temperature similar to what is reconstructed for the last glacial maximum recent change 2 applying oligocene calibrations between apparent sea level asl and 18o benthic or 3 assuming temperature and seawater 18o contributions can be partitioned through comparison with benthic mg ca using assumption 1 the record from site 1209 indicates changes in seawater 18o of 0 3 0 7 equivalent to 33 72 m m of asl assuming mean ice 18o of 45 using assumption 2 and two different end member calibrations the 18o benthic record implies changes in asl of 23 50 m or 50 108 m the third approach yields changes in seawater 18o of up to 0 6 to 1 4 we explore the compatibility of the results of each of these approaches with other studies that discuss evidence for ephemeral glaciations during the middle eocene with variable ice storage at one or both poles

organic carbon burial following the middle eocene climatic optimum in the central western tethys we present trace metal geochemistry and stable isotope records for the middle eocene alano di piave section ne italy deposited during magnetochron c18n in the marginal tethys ocean we identify a sim 500 kyr long carbon isotope perturbation event we infer to be the middle eocene climatic optimum meco confirming the northern hemisphere expression and global occurrence of meco interpreted peak climatic conditions are followed by the rapid deposition of two organic rich intervals le 3 toc and contemporaneous positive delta 13 c excursions these two intervals are associated with increases in the concentration of sulphur and redox sensitive trace metals and low concentrations of mn as well as coupled with the occurrence of pyrite together these changes imply low possibly dysoxic bottom water o 2 conditions promoting increased organic carbon burial we hypothesize that this rapid burial of organic carbon lowered global it p co 2 following the peak warming and returned the climate system to the general eocene cooling trend

redox state of the archean atmosphere evidence from detrital heavy minerals in ca 3250 2750 ma sandstones from the pilbara craton australia the presence of detrital uraninite and pyrite in fluvial placers of the witwatersrand basin south africa has been used to infer low levels of atmospheric oxygen during the archean 2500 ma however recent studies advocate a hydrothermal origin for these minerals thereby limiting their value as constraints on the composition of the early atmosphere in contrast ca 3250 2750 ma fluvial siliciclastic sediments from the pilbara craton in australia have never undergone significant hydrothermal alteration and their heavy minerals are of unequivocal detrital origin these heavy minerals include the redox sensitive phases pyrite uraninite and gersdorffite along with more inert zircon rutile chromite and monazite locally siderite is a major constituent to 90 of the heavy mineral population with grains displaying evidence for several episodes of erosion rounding and subsequent authigenic overgrowth detrital siderite is very rare in post archean sandstones largely due to its instability in oxidizing environments however its frequent survival of prolonged transport in well mixed and therefore well aerated archean river waters that contained little organic matter strongly implies that the contemporary atmosphere was indeed much less oxidizing than at present moreover concentrations of reduced sulfur species must have been very low in surface fluids for siderite to survive repeated transportation events without pyritization

conditions of subaerial weathering of basalts in the neoarchean and paleoproterozoic abstract precambrian weathering profiles or regoliths were formed on the earth s surface in contact with meteoritic water this signifies that their mineral and bulk composition should reflect the environmental conditions at the time of weathering the aim of this study is to reconstruct the main mineralogical and chemical trends involved in the formation of precambrian weathering regoliths by means of geochemical modeling verified through laboratory experimentation we have demonstrated that in a wide range of modeling conditions weathering profiles of basalts consist mostly of clay minerals smectites and illite carbonate minerals may also be deposited within the profile at the initial stage of weathering which are later dissolved during a further process the time necessary for complete dissolution of primary minerals and crystallization of authigenic minerals is between 100 and 140 kyr modeling results indicate that neoarchean and paleoproterozoic 2 8 2 0 ga weathering profiles were formed under an atmosphere with pco2 not much higher than 25 pal and climatic conditions close to those found in modern times weathering under co2 rich pco2 1 and 10 bar and ch4 rich atmospheres as well as high temperature 50 c and 75 c conditions do not produce compounds corresponding to the analyzed samples

atmospheric carbon dioxide through the eocene oligocene climate transition geological and geochemical evidence1 2 3 indicates that the antarctic ice sheet formed during the eocene oligocene transition4 33 5 34 0 million years ago modelling studies5 6 suggest that such ice sheet formation might have been triggered when atmospheric carbon dioxide levels fell below a critical threshold of 750 p p m v but the timing and magnitude of relative to the evolution of the ice sheet has remained unclear here we use the boron isotope ph proxy7 8 on exceptionally well preserved carbonate microfossils from a recently discovered geological section in tanzania9 10 to estimate before during and after the climate transition our data suggest that a reduction in occurred before the main phase of ice growth followed by a sharp recovery to pre transition values and then a more gradual decline during maximum ice sheet growth was between 450 and 1 500 p p m v with a central estimate of 760 p p m v the ice cap survived the period of recovery although possibly with some reduction in its volume implying as models predict11 a nonlinear response to climate forcing during melting overall our results confirm the central role of declining in the development of the antarctic ice sheet in broad agreement with carbon cycle modelling12 and help to constrain mechanisms and feedbacks associated with the earth s biggest climate switch of the past 65 myr

eocene oligocene paleoceanographic changes in the stratotype section massignano italy clues from rock magnetism and stable isotopes 1 we have conducted high resolution paleomagnetic and rock magnetic studies in addition to stable isotope analyses of the massignano sedimentary section which is the global stratotype section and point gssp for the eocene oligocene boundary our research builds upon the many past studies of the massignano section in seeking to understand the timing and nature of the paleoenvironmental variations that occurred during the transition for the earth s climate system from greenhouse to icehouse the new paleomagnetic results provide a refined magnetostratigraphy of the section and new age for the eocene oligocene boundary at 33 7 ma abrupt and large alternations in magnetic concentration composition and grain sizes that occur in the high resolution rock magnetic record are interpreted to be the result of rapid bimodal shifts in deep sea circulation that affect sediment sources or transport we speculate that currents flowing through the gateway between the atlantic and indo pacific oceans may have turned on and off as the gateway was progressively closing resulting in the deposition of two different assemblages of magnetic minerals at massignano finally stable isotope 18o and 13c data collected on ostracod valves also suggest significant changes in sea bottom circulation in the neo tethys ocean at and about 2 m y before the eocene oligocene boundary

palaeoclimatology evidence for hot early oceans arising from f robert m chaussidon 969 972 2006 10 1038 nature05239 robert chaussidon reply the oxygen isotopes in sedimentary cherts siliceous sediments have been used to argue that the precambrian oceans were hot with temperatures of up to 70 c at 3 3 gyr before present1 robert and chaussidon2 measure silicon isotopes in cherts and arrive at a similar conclusion we suggest here that both isotope trends may be caused by variations in seawater isotope composition rather than in ocean temperatures if so then the climate of the early earth may have been temperate as it is today and therefore more consistent with evidence for precambrian glaciations and with constraints inferred from biological evolution

integrated magnetobiostratigraphy of the middle eocene lower oligocene interval from the monte cagnero section central italy the monte cagnero sedimentary section which crops out in the northeastern apennines near urbania in the umbria marche basin italy contains well exposed strata spanning the middle eocene to lower oligocene interval we use an integrated magnetobiostratigraphic approach to generate a high resolution age model for the monte cagnero section with the goal of obtaining a reliable chronostratigraphic framework for studying eocene oligocene palaeoceanographic changes during the switch from greenhouse to icehouse conditions the studied sediments consist of alternating reddish and greenish limestones and marlstones a new integrated age model for the section is based on high resolution palaeomagnetic analyses combined with detailed planktonic foraminiferal and calcareous nannofossil biostratigraphic results rock magnetic measurements show that the magnetic mineralogy is dominated by a mixture of highand low coercivity minerals probably representing a combination of hematite and magnetite a robust magnetostratigraphic signal together with the identification of key planktonic foraminiferal and nannofossil biostratigraphic events allows construction of a detailed age model for the section based on these results we infer that the section spans a continuous interval within magnetochron resolution from the middle eocene to lower oligocene c 41 27 ma chrons c18r c12r the monte cagnero section therefore represents a sequence that is suitable for studying the impact of the neo tethyan gateway closure on subtropical eocene circulation and determining the nature and timing of palaeoceanographic changes in the tethys through the late middle eocene to early oligocene interval the cenozoic era which represents the last 65 million years myr from the end of the cretaceous to the present is a transformational time in the evolution of earth s palaeoclimate benthic foraminiferal oxygen isotope do records from all major ocean basins indicate a cooling trend over the past 50 myr since the early eocene climatic optimum c 53 51 ma e g zachos et al 2001 2008 the middle eocene to early oligocene was a key interval for earth s transition from warmer greenhouse to cooler icehouse conditions this interval is characterized by the expansion of antarctic large ice sheets to 50 the size of the present day volume e g zachos et al 1996 lear et al 2000 coxall et al 2005 katz et al 2008 coxall wilson 2011 culminating in the eocene oligocene from jovane l herrero bervera e hinnov l a housen b a eds 2013 magnetic methods and the timing of geological processes geological society london special publications 373 79 95 first published online march 25 2013 http dx doi org 10 1144 sp373 13 the geological society of london 2013 publishing disclaimer www geolsoc org uk pub ethics 2013 at universita degli studi di urbino carlo bo on july 29 http sp lyellcollection org downloaded from

weathering and sediment generation in the archean an integrated study of the evolution of siliciclastic sedimentary rocks of the 3 2ga moodies group barberton greenstone belt south africa abstract alluvial and braided fluvial deposits of the 3 2 ga moodies group in the barberton greenstone belt south africa are analyzed petrographically and geochemically to identify their source rocks provenance and assess the degree and type s of weathering required to produce the observed moodies sediment compositions conglomerate sandstone and shale data give somewhat differing pictures of provenance and weathering due to derivation from different components within the source terrane and size and compositional fractionation during transport the results suggest that shale geochemistry provides the most accurate estimate provenance although both rare earth element and trace element th sc zr cr and ti data must be used in combination to give the best results the source area for moodies group sediments was dominated by tonalite felsic volcanic rock komatiite basalt and granite based on mineralogical and major element divergence from estimated source area composition the moodies group sediments are remnants of an aggressive weathering environment labile materials such as komatiite basalt and coarse plagioclase grains decomposed almost entirely to clays and solutes and the chemical index of alteration for moodies shale is well above the global average an aggressive weathering environment in the archean may have been achieved by increased rainfall higher temperatures and or higher atmospheric p c o 2 more likely a combination of these conditions worked to offset the inhibitory weathering effects of a plant free environment

high resolution 104 years deep sea foraminiferal stable isotope records of the eocene oligocene climate transition we have constructed high resolution 104 105 years benthic foraminiferal 13c and 18o records for the upper eocene through lower oligocene of two pelagic sequences deep sea drilling project dsdp site 522 in the angola basin south atlantic ocean and ocean drilling program odp site 744 in the southern indian ocean these records provide improved constraints on both the timing and magnitude of marine oxygen and carbon isotope events from 30 to 35 ma the oxygen isotope records indicate that the ubiquitous 18o increase oi 1 which marks the rapid expansion of continental ice sheets and a minimum of 3 to 4 c of cooling of bottom waters in the earliest oligocene 33 6 ma occurred in 350 kyr more than half the transition occurred over the final 40 50 kyr this period of lower temperatures and widespread continental glaciation persisted for roughly 400 kyr i e the duration of magnetochron c13n these records also indicate that this interval was characterized by at least two 100 kyr waxing and waning cycles oi 1a and oi 1b and possibly several higher frequency events the benthic foraminiferal 13c records show a positive 0 8 excursion that is nearly isochronous with the oi 1 oxygen isotope increase similar magnitude 13c increases at other sites indicate this was a global phenomenon suggestive of an unusually large perturbation to the carbon cycle this excursion was followed by smaller amplitude 13c oscillations with periods of roughly 400 kyr we suspect that the ubiquitous oi 1 13c excursion resulted from a brief but substantial increase in export production and carbon burial

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

modeling of continental weathering under high co2 atmospheres during precambrian times batch experiments were conducted to simulate abiotic weathering of the continental crust under high co2 atmospheric conditions during precambrian times i e corresponding to the general archaean conditions as well as to the immediate aftermath of the neoproterozoic snowball earth ice ages three types of rock basalt granodiorite and tonalite representative of the archaean to proterozoic continental crust were reacted in the form of powders for 1 year at 40 c with pure water under various water rock ratios in oxic and anoxic atmospheres containing 10 co2 that is under conditions assumed to characterize the greenhouse effect which prevailed at that time chemical and mineralogical data collected during the course of the experiments reflect alteration phenomena occurring in two steps 1 rapid leaching of the fresh surfaces probably related to the proton donor capacity of the co2 and 2 a steady state reaction under near neutral conditions these observed dissolution rates can be satisfactorily modeled by kinetic data available in the literature using a 1 d weathering model reproducing the two steps we evaluate the atmospheric co2 consumption rate at the snowball earth aftermath focusing on the relative contribution of surface leaching versus steady state reaction the results show that the process is dominated by surface exchange

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

new biostratigraphic magnetostratigraphic and isotopic insights into the middle eocene climatic optimum in low latitudes the middle eocene climatic optimum meco was a warming event that interrupted the long term eocene cooling trend while this event is well documented at high southern and mid latitudes it is poorly known from low latitudes and its timing and duration are not well constrained because of problems of hiati microfossil preservation and weak magnetic polarity in key sedimentary sections here we report the results of a study designed to improve the bio magneto and chemostratigraphy of the meco interval using high resolution records from two low latitude sections in the atlantic ocean ocean drilling program odp sites 1051 and 1260 we present the first detailed benthic foraminiferal stable isotope records of the meco from the low latitudes as well as the biostratigraphic counts of orbulinoides beckmanni and new magnetostratigraphic results our data demonstrate a 750 kyr long duration for the meco characterized by increasing 13c and decreasing 18o with minimum 18o values lasting 40 kyr at 40 1 ma coincident with a short lived negative 13c excursion thereafter 18o and 13c values recover rapidly the shift to minimum 18o values at 40 1 ma is coincident with a marked increase in the abundance of the planktonic foraminifera o beckmanni consistent with its inferred warm water preference o beckmanni is an important eocene biostratigraphic marker defining planktonic foraminiferal zone e12 with its lowest and highest occurrences lo and hos our new records reveal that the lo of o beckmanni is distinctly diachronous appearing 500 kyr earlier in the equatorial atlantic than in the subtropics 40 5 versus 41 0 ma we also show that at both sites the ho of o beckmanni at 39 5 ma is younger than the published calibrations increasing the duration of zone e12 by at least 400 kyr in accordance with the tropical origins of o beckmanni this range expansion to higher latitudes may have occurred in response to sea surface warming during the meco and subsequently disappeared with cooling of surface waters

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

integrated stratigraphy of the oligocene pelagic sequence in the umbria marche basin northeastern apennines italy a potential global stratotype section and point gssp for the rupelian chattian boundary the oligocene represents an important time period from a wide range of perspectives and includes significant climatic and eustatic variations the pelagic succession of the umbria marche apennines central italy includes a complete and continuous sequence of marly limestones and marls with volcaniclastic layers that enable us to construct an integrated stratigraphic framework for this time period we present here a synthesis of detailed biostratigraphic magnetostratigraphic and chemostratigraphic studies along with geochronologic results from several biotite rich volcaniclastic layers which provide the means for an accurate and precise radiometric calibration of the oligocene time scale from this study the interpolated ages for the rupelian chattian stage boundary located in the upper half of chron 10n at meter level 188 in the monte cagnero section and corresponding to the o4 o5 planktonic foraminiferal zonal boundary are 28 36 ma paleomagnetic interpolation 28 27 0 1 ma direct radioisotopic dating and 27 99 ma astrochronological interpolation these ages appear to be slightly younger than those reported in recent chronostratigraphic time scale compilations the monte cagnero section is a potential candidate for defining the chattian global stratotype section and point gssp and some reliable criteria are here proposed for marking the rupelian chattian boundary according to international union of geological sciences iugs recommendations

lithologic and climatologic controls of river chemistry abstract the chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff in all 101 different rivers each draining a primary lithology were examined across the united states puerto rico and iceland to minimize seasonal effects only rivers with at least two years of data were used basaltic catchments were examined in the greatest detail in a survey of hawaiian island watersheds the average river chemistries could be related to the distribution of soil associations within each catchment an analysis of cation activity relationships among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium ion exchange control on cation ratios among different lithologies unique weathering rate relationships were developed with yields at typical present day runoff rates 1 100 cm y increasing in the order sandstones granites basalts shales and carbonates the bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering thus a warm wet climate or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal

radiolarian diversity changes during the late cambrian early ordovician transition as recorded in the cow head group of newfoundland canada abstract upper cambrian to lower ordovician pelagic carbonate rocks of the cow head group in western newfoundland canada were sampled for the study of their radiolarian fauna well preserved and diverse faunal assemblages were obtained from a number of levels of the shallow bay and green point formations three different assemblages including a total of five families eight genera and 20 species are recognized in three different sections situated on a palaeobathymetric profile the various recognized assemblages are compared with previous studies conducted on the cow head group they improve considerably our understanding of the pattern of changes in radiolarian diversity during the cambrian ordovician transition the radiolarian biotic record of western newfoundland unveils a two step faunal change across the cambrian ordovician transitional interval one situated at the base of the uppermost cambrian stage 10 and the second at a poorly constrained interval situated towards the top of the same stage 10 these biotic changes consist of genus level selective extinctions as well as within clade species faunal turnover i e echidnina paleospiculum decrease of species abundance species extinction and within clade episodes of species diversification i e protoentactinia parechidnina

astronomical calibration of the middle eocene contessa highway section gubbio italy abstract the eocene climatic system experienced an important transition from warm paleocene greenhouse to icehouse oligocene conditions this transition could first appear as a long term cooling trend but at an up close look this period is a complex combination of climatic events and for most of them causes and consequences are still not fully characterized in this context a study has been carried out on the middle eocene sedimentary succession of the contessa highway section central italy which is proposed as the global stratotype section and point gssp for the lutetian bartonian boundary at the top of the chron 19n with an astronomically calibrated age of 41 23 ma through a cyclostratigraphic analysis of the rhythmic sedimentary alternations and combination with the results of time series analysis of the proxy record we provide an orbital tuning of the middle eocene and astronomical calibration of the bio magnetostratigraphic events particularly for the c19n c18r chron boundary recognized at the contessa highway section

review and revision of cenozoic tropical planktonic foraminiferal biostratigraphy and calibration to the geomagnetic polarity and astronomical time scale planktonic foraminifera are widely utilized for the biostratigraphy of cretaceous and cenozoic marine sediments and are a fundamental component of cenozoic chronostratigraphy the recent enhancements in deep sea drilling recovery multiple coring and high resolution sampling both offshore and onshore has improved the planktonic foraminiferal calibrations to magnetostratigraphy and or modified species ranges this accumulated new information has allowed many of the planktonic foraminiferal bioevents of the cenozoic to be revised and the planktonic foraminiferal calibrations to be reassessed we incorporate these developments and amendments into the existing biostratigraphic zonal scheme in this paper we present an amended low latitude tropical and subtropical cenozoic planktonic foraminiferal zonation we compile 187 revised calibrations of planktonic foraminiferal bioevents from multiple sources for the cenozoic and have incorporated these recalibrations into a revised cenozoic planktonic foraminiferal biochronology we review and synthesize these calibrations to both the geomagnetic polarity time scale gpts of the cenozoic and astronomical time scale ats of the neogene and late paleogene on the whole these recalibrations are consistent with the previous work however in some cases they have led to major adjustments to the duration of biochrons recalibrations of the early middle eocene first appearance datums of globigerinatheka kugleri hantkenina singanoae guembelitrioides nuttalli and turborotalia frontosa have resulted in large changes in the durations of biochrons e7 e8 and e9 we have introduced upper oligocene zone o7 utilizing the biostratigraphic utility of paragloborotalia pseudokugleri for the neogene period major revisions are applied to the fohsellid lineage of the middle miocene and we have modified the criteria for recognition of zones m7 m8 and m9 with additional adjustments regarding the globigerinatella lineage to zones m2 and m3 the revised and recalibrated datums provide a major advance in biochronologic resolution and a template for future progress of the cenozoic time scale

enhanced primary productivity and magnetotactic bacterial production in response to middle eocene warming in the neo tethys ocean abstract earth s climate experienced a warming event known as the middle eocene climatic optimum meco at 40 ma which was an abrupt reversal of a long term eocene cooling trend this event is characterized in the deep southern atlantic pacific and indian oceans by a distinct negative 18 o excursion over 500 kyr we report results of high resolution paleontological geochemical and rock magnetic investigations of the neo tethyan monte cagnero mca section northeastern apennines italy which can be correlated on the basis of magneto and biostratigraphic results to the meco event recorded in deep sea sections in the mca section an interval with a relative increase in eutrophic nannofossil taxa and decreased abundances of oligotrophic taxa spans the culmination of the meco warming and its aftermath and coincides with a positive carbon isotope excursion and a peak in magnetite and hematite goethite concentration the magnetite peak reflects the appearance of putative magnetofossils while the hematite goethite apex is attributed to an enhanced detrital mineral contribution likely as aeolian dust transported from the continent adjacent to the neo tethys ocean during a drier more seasonal climate during the peak meco warming based on our new geochemical paleontological and magnetic records the meco warming peak and its immediate aftermath are interpreted as a period of high primary productivity sea surface iron fertilization is inferred to have stimulated high phytoplankton productivity increasing organic carbon export to the seafloor and promoting enhanced biomineralization of magnetotactic bacteria which are preserved as putative magnetofossils during the warmest periods of the meco event in the mca section together with previous studies our work reinforces the connection between hyperthermal climatic events and the occurrence or increased abundance of putative magnetofossils in the sedimentary record

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

dinoflagellate cyst stratigraphy of the eocene oligocene transition in central italy abstract eight dinoflagellate cyst zones based on the massignano and monte cagnero sections in central italy provide high resolution control for the eocene oligocene e o transition most of these zones have been recognized in the time equivalent contessa highway section and are calibrated against previously established magneto and biostratigraphies the invasion of the mediterranean area by several higher latitude dinoflagellate taxa marks the e o boundary as characterized by the last occurrence of the hantkeninids planktic foraminifers calibrated against a horizon just above the youngest normal polarity event within chron c1 3r quantitative and qualitative dinoflagellate events across the e o boundary allow the recognition of warmer and cooler successive intervals of sea surface temperature in addition to apparent sea level fluctuations our data indicate that latest eocene warm conditions and a sea level highstand were rapidly replaced by short term cooler conditions and a lowstand across the e o boundary horizon following a shortterm warmer interval and a highstand during the earliest oligocene a second cool and lowstand episode occurred this cool interval was of greater magnitude and duration than the one at the e o boundary horizon and may correspond to the globally recognized cooling event reported from the basal part of chron c1 3n

eocene oligocene ocean de acidification linked to antarctic glaciation by sea level fall one of the most dramatic perturbations to the earth system during the past 100 million years was the rapid onset of antarctic glaciation near the eocene oligocene boundary some 34 million years ago this climate transition was accompanied by a deepening of the calcite compensation depth the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution but the mechanism linking glaciation to the deepening of calcite compensation depth remains unclear merico et al use a global biogeochemical box model to test competing hypotheses put forward to explain the eocene oligocene transition they find that only shelf to deep sea carbonate partitioning can explain the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor this work sheds new light on the mechanisms linking glaciation and ocean acidity change during this important climate transition a global biogeochemical box model is used to test competing hypotheses put forward to explain the eocene oligocene transition it is found that only shelf to deep sea carbonate partitioning is capable of explaining the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor this work sheds new light on the mechanisms linking glaciation and ocean acidity change across the climate transition of the cenozoic one of the most dramatic perturbations to the earth system during the past 100 million years was the rapid onset of antarctic glaciation near the eocene oligocene epoch boundary1 2 3 34 million years ago this climate transition was accompanied3 by a deepening of the calcite compensation depth the ocean depth at which the rate of calcium carbonate input from surface waters equals the rate of dissolution changes in the global carbon cycle4 rather than changes in continental configuration5 have recently been proposed as the most likely root cause of antarctic glaciation but the mechanism linking glaciation to the deepening of calcite compensation depth remains unclear here we use a global biogeochemical box model to test competing hypotheses put forward to explain the eocene oligocene transition we find that of the candidate hypotheses only shelf to deep sea carbonate partitioning is capable of explaining the observed changes in both carbon isotope composition and calcium carbonate accumulation at the sea floor in our simulations glacioeustatic sea level fall associated with the growth of antarctic ice sheets permanently reduces global calcium carbonate accumulation on the continental shelves leading to an increase in pelagic burial via permanent deepening of the calcite compensation depth at the same time fresh limestones are exposed to erosion thus temporarily increasing global river inputs of dissolved carbonate and increasing seawater 13c our work sheds new light on the mechanisms linking glaciation and ocean acidity change across arguably the most important climate transition of the cenozoic era

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

uranium in iron formations and the rise of atmospheric oxygen abstract the concept of the great oxidation event goe during which atmospheric oxygen rose precipitously and perhaps to near modern levels around 2 4 2 1 billion years ago ga has become entrenched in our views on secular atmospheric evolution multiple proxies confirm a permanent shift towards more oxygenated conditions at some time near the archean proterozoic boundary however it remains unclear precisely when this transition occurred due in part to the likely temporal variability in those early levels and different sensitivities of the proxies utilized to track atmospheric oxygen partial pressures here we provide a new look at the timing and magnitude of early atmospheric oxygenation through the record of uranium u concentrations in iron formations if just as if are important archives of the redox state of seawater concentrations of redox sensitive u in if are faithful proxies for oxidative continental weathering and associated delivery of dissolved u to seawater our dataset suggests that there was an increase in u redox cycling and transport at ca 2 47 ga just before the permanent loss of mass independent sedimentary sulfur isotope anomalies traditionally used to define the onset of the goe further there is significant temporal variability in the if u record that we propose reflects dynamic precambrian redox conditions we provide additional support for earlier suggestions that the goe was a protracted event marked by vacillating oxygen levels

a model for late archean chemical weathering and world average river water abstract interpretations of the geologic record of late archean near surface environments depend very strongly on an understanding of weathering and resultant riverine transport to the oceans the late archean atmosphere is widely recognized to be anoxic p o 2 g 10 5 to 10 13 bars p h 2 g 10 3 to 10 5 bars detrital siderite feco3 pyrite fes2 and uraninite uo2 in late archean sedimentary rocks also suggest anoxic conditions however whether the observed detrital minerals could have been thermodynamically stable during weathering and riverine transport under such an atmosphere remains untested similarly interpretations of fluctuations recorded by trace metals and isotopes are hampered by a lack of knowledge of the chemical linkages between the atmosphere weathering riverine transport and the mineralogical record in this study we used theoretical reaction path models to simulate the chemistry involved in rainwater and weathering processes under present day and hypothetical archean atmospheric boundary conditions we included new estimates of the thermodynamic properties of fe ii smectites as well as smectite and calcite solid solutions simulation of present day weathering of basalt calcite by world average rainwater produced hematite kaolinite na mg saponite and chalcedony after 10 4 moles of reactant minerals kg 1 h2o were destroyed combination of the resultant water chemistry with results for granitic weathering produced a water composition comparable to present day world average river water warw in contrast under late archean atmospheric conditions p co 2 g 10 1 5 and p h 2 g 10 5 0 bars weathering of olivine basalt calcite to the same degree of reaction produced kaolinite chalcedony and na fe ii rich saponite late archean weathering of tonalite trondhjemite granodiorite ttg formed fe ii rich beidellite and chalcedony combining the waters from olivine basalt and ttg weathering resulted in a model for late archean warw with the composition na ca2 fe2 mg2 cl hco 3 sio2 aq the ph of the water was 6 3 and it is much richer in hco 3 and in mg fe relative to ca na compared to present day warw at higher ph2 g e g 10 4 0 bars organic acid anions could be metastable our results are consistent with the thermodynamic stability of fe ii clays pyrite uraninite and under some conditions siderite during weathering and riverine transport overall our results provide a basis for assessing the formation of organic hazes and the mobility of trace elements and nutrients due to fluctuations of the late archean atmosphere

astronomic calibration of the late eocene early oligocene massignano section central italy we present an astronomically tuned polarity timescale for the late eocene early oligocene based on a cyclic limestone marls sedimentary succession exposed at massignano italy gssp for the eocene oligocene boundary here we applied spectral analysis techniques to the high resolution magnetic susceptibility record the results provide clear evidence of a regular and cyclic signal that allows us to discard a stochastic mechanism as controlling these fluctuations conversely these outcomes reveal the presence of stable and notable periodicities that are in tune with those associated with orbital forcing through a cyclostratigraphic analysis of the rhythmic sedimentary alternations and combination with the results of time series analysis of the proxy record we refine the late eocene portion of the gpts through astronomical tuning of the rhythmic sedimentary alternations to recent astronomically calculated variations of earth s orbit we recalibrated the polarity boundaries ages and the main stratigraphic events recognized throughout the sedimentary record among these an age of 33 714 ma is proposed for the e o boundary

environmental magnetic record of paleoclimate change from the eocene oligocene stratotype section massignano italy 1 a high resolution environmental magnetic study of the massignano section italy global stratotype section and point for the eocene oligocene boundary has been performed to test whether a clear magnetic signature associated with climatic change is recognizable in this record our results reveal the existence of alternating intervals with high and low magnetic mineral concentrations similar to the pattern of rock magnetic property variations observed from an environmental magnetic study of the ciros 1 sediment core from antarctica these results suggest that an external forcing mechanism drove the sedimentary response to global climate change prior to the major oi 1 cooling event at the eocene oligocene boundary

timing of diagenetic haematite growth in red pelagic limestones from gubbio italy abstract magnetite haematite and to a minor extent maghaemite are recognised in the cretaceous and paleocene red pelagic limestones at gubbio the magnetite is detrital or biological whereas the haematite grew during diagenesis from a goethitic precursor thermal and af demagnetization of samples collected from close to reversal boundaries indicate that the various magnetization components do not record the polarity reversal at exactly the same stratigraphic level in the few tens of centimetersbelow a recorded geomagnetic reversal defined by the magnetite magnetization some of the haematite grains are magnetized in the post reversal field the blocking temperature spectra of this haematite fraction with post reversal magnetization are found to shift toward higher temperatures as the reversal boundary is approached the blocking temperature spectra reflect the grain size spectra of the haematite which we interpret as arising by the continual nucleation of grains down to a certain burial depth where the conditions are no longer conducive to further haematite growth the depth below reversal boundaries to which haematite with post reversal magnetization can occur is estimated to be about 60 cm after compaction and is equivalent to a time of about 105 years for these particular sediments a detailed study of the magnetization components at reversal boundaries indicates that the first diagenetic growth of haematite through the single domain critical volume occurs prior to the mechanical fixation of the detrital or biological magnetite subsequently the diagenetic haematite grains do not rotate in response to the ambient geomagnetic field polarity as easily as the magnetite because of their occurrence as pigmentary coatings on larger non magnetic grains

thermodynamic and mass balance constraints on iron bearing phyllosilicate formation and alteration pathways on early mars 1 widespread occurrences of phyllosilicates predominantly nontronite and mg saponite in the noachian aged crust of mars place constraints on water availability and surface conditions however the noachian atmosphere likely lacked oxidants capable of forming nontronite from the basaltic crust and its origins are thus uncertain in this study scenarios for the formation of iron bearing phyllosilicates from the weathering and hydrothermal alteration of martian basalt were investigated using geochemical modeling to establish plausible pathways of nontronite generation and the effect of redox conditions on associated secondary phases hydrothermal alteration produced feii and mg saponite serpentine and zeolites at 100 c and chlorite prehnite and talc at 250 c consistent with phases observed in crater deposits on mars anoxic weathering produced primarily feii and mg saponite high water rock ratios produced al clays and amorphous silica and high pco2 conditions inhibited smectite formation because of carbonate precipitation oxic weathering produced primarily nontronite and mg saponite later oxidation of anoxic weathering products also formed nontronite mg saponite assemblages oxidation products of hydrothermal assemblages included nontronite mg saponite and either al smectites or residual zeolites or prehnite these calculations demonstrate that there are multiple plausible routes to form fe mg smectites on early mars and that an oxidizing atmosphere need not have existed in the noachian decoupling iron oxidation from initial clay formation raises the possibilities that mars transitioned to conditions favoring widespread iron oxidation after phyllosilicate formation ceased and that residual unoxidized ferrous iron phyllosilicates still exist today in the martian subsurface

coupled greenhouse warming and deep sea acidification in the middle eocene 1 the middle eocene climatic optimum meco is an enigmatic warming event that represents an abrupt reversal in long term cooling through the eocene in order to further assess the timing and nature of this event we have assembled stable isotope and calcium carbonate concentration records from multiple deep sea drilling project and ocean drilling program sites for the time interval between 43 and 38 ma revised stratigraphy at several sites and compilation of 18o records place peak warming during the meco event at 40 0 ma chron c18n 2n the identification of the 18o excursion at sites in different geographic regions indicates that the climatic effects of this event were globally extensive the total duration of the meco event is estimated at 500 ka with peak warming lasting 100 ka assuming minimal glaciation in the late middle eocene 4 6 c total warming of both surface and deep waters is estimated during the meco at the study sites the interval of peak warming at 40 0 ma also coincided with a worldwide decline in carbonate accumulation at sites below 3000 m depth reflecting a temporary shoaling of the calcite compensation depth the synchroneity of deep water acidification and globally extensive warming makes a persuasive argument that the meco event was linked to a transient increase in atmospheric pco2 the results of this study confirm previous reports of significant climatic instability during the middle eocene furthermore the direct link between warming and changes in the carbonate chemistry of the deep ocean provides strong evidence that changes in greenhouse gas concentrations exerted a primary control on short term climate variability during this critical period of eocene climate evolution

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

magnetic properties of pelagic marine carbonates abstract pelagic carbonates are deposited far from continents usually at water depths of 3000 6000 m at rates below 10 cm kyr and are a globally important sediment type recent advances with recognition of widespread preservation of biogenic magnetite the inorganic remains of magnetotactic bacteria have fundamentally changed our understanding of the magnetic properties of pelagic carbonates we review evidence for the magnetic minerals typically preserved in pelagic carbonates the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates and what magnetic properties can tell us about the open ocean environments in which pelagic carbonates are deposited we also discuss briefly late diagenetic remagnetisations recorded by some carbonates despite recent advances in our knowledge of these phenomena much remains undiscovered we are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation recently developed techniques have potential for testing how different magnetotactic bacterial species which produce different magnetite morphologies respond to changing nutrient and oxygenation conditions future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well calibrated modern magnetotactic bacterial occurrences a tantalizing link between giant magnetofossils and paleogene hyperthermal events needs to be tested much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies rather than being a well worn subject that has been studied for over 60 years the magnetic properties of pelagic carbonates hold many secrets that await discovery

integrated stratigraphy magneto bio and chronostratigraphy and geochronology of the palaeogene pelagic succession of the umbria marche basin central italy

early oligocene glaciation and productivity in the eastern equatorial pacific insights into global carbon cycling the onset of sustained antarctic glaciation across the eocene oligocene transition eot marks a pivotal change in earth s climate but our understanding of this event particularly the role of the

arabia eurasia collision and the forcing of mid cenozoic global cooling the end of the eocene greenhouse world was the most dramatic phase in the long term cooling trend of the cenozoic era here we show that the arabia eurasia collision and the closure of the tethys ocean gateway began in the late eocene at 35 ma up to 25 million years earlier than in many reconstructions we suggest that global cooling was forced by processes associated with the initial collision that reduced atmospheric co2 these are 1 waning volcanism across southwest asia 2 increased organic carbon storage in paratethyan basins e g black sea and south caspian 3 increased silicate weathering in the collision zone and 4 a shift towards modern patterns of ocean currents associated with increased vigour in circulation and organic productivity

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

the middle eocene climatic optimum meco a multiproxy record of paleoceanographic changes in the southeast atlantic odp site 1263 walvis ridge the middle eocene climatic optimum meco 40 ma was a transient period of global warming that interrupted the secular cenozoic cooling trend we investigated the paleoceanographic paleoenvironmental and paleoecological repercussions of the meco in the southeast atlantic subtropical gyre ocean drilling program site 1263 tex86 and 18o records support an 4 c increase in surface and deepwater temperatures during the meco there is no long term negative carbon isotope excursion cie associated with the early warming consistent with other sites and there is no short term negative cie 50 kyr during the peak of the meco in contrast to what has been observed at some sites this lack of a cie during the peak of the meco at site 1263 could be due to poor sediment recovery or geographic heterogeneity of the 13c signal benthic and planktic foraminiferal mass accumulation rates markedly declined during meco indicating a reduction of planktic foraminiferal production and export productivity vertical 13c gradients do not indicate major changes in water column stratification and there is no biomarker or micropaleontological evidence that hypoxia developed we suggest that temperature dependency of metabolic rates could explain the observed decrease in foraminiferal productivity during warming the kinetics of biochemical reactions increase with temperature more so for heterotrophs than for autotrophs steady warming during meco may have enhanced heterotroph i e foraminiferal metabolic rates so that they required more nutrients these additional nutrients were not available because of the oligotrophic conditions in the region and the lesser response of primary producers to warming the combination of warming and heterotroph starvation altered pelagic food webs increased water column recycling of organic carbon and decreased the amount of organic carbon available to the benthos

cenozoic deep sea temperatures and global ice volumes from mg ca in benthic foraminiferal calcite a deep sea temperature record for the past 50 million years has been produced from the magnesium calcium ratio mg ca in benthic foraminiferal calcite the record is strikingly similar in form to the corresponding benthic oxygen isotope 18o record and defines an overall cooling of about 12 c in the deep oceans with four main cooling periods used in conjunction with the benthic 18o record the magnesium temperature record indicates that the first major accumulation of antarctic ice occurred rapidly in the earliest oligocene 34 million years ago and was not accompanied by a decrease in deep sea temperatures