pacific ocean and cenozoic evolution of climate 1 the pacific ocean has played a major role in climate evolution throughout the cenozoic 65 0 ma it is a fundamental component of global heat transport and circulation the dominant locus of primary productivity and consequently the largest reservoir for carbon exchange between the oceans and the atmosphere a satisfactory understanding of the cenozoic evolutionary history of the pacific and its impact on global climate is currently data limited nevertheless the large dynamic range of cenozoic conditions sets the stage to greatly expand our understanding of global climate and biogeochemical cycles past earth experiments are particularly useful to understand interactions between climate and geosystems under different greenhouse gas loads we highlight in this review four important problems in which the pacific played a major role the effect of changing geographic boundary conditions on ocean circulation interactions between the carbon cycle and climate the pacific ocean s influence on north american climate and its water cycle and the gradual evolution of climate systems

chemical weathering indices applied to weathering profiles developed on heterogeneous felsic metamorphic parent rocks abstract chemical weathering indices are commonly used for characterizing weathering profiles by incorporating bulk major element oxide chemistry into a single metric for each sample generally on homogeneous parent rocks weathering indices change systematically with depth however the weathering of heterogeneous metamorphic rocks confounds the relationship between weathering index and depth in this paper we evaluate previously defined chemical weathering indices for their suitability in characterizing weathering profiles developed on felsic heterogeneous metasedimentary bedrock from coweeta hydrologic laboratory north carolina usa among the weathering indices evaluated here the weathering index of parker wip is the most appropriate for application to weathering profiles on heterogeneous and homogeneous parent rock because the wip includes only the highly mobile alkali and alkaline earth elements in its formulation it yields values that differ greatly from those of the parent rock in addition the wip allows for aluminum mobility unlike other weathering indices these characteristics combine to make the weathering index of parker the most applicable index for studying the weathering of heterogeneous metasedimentary rocks however the wip should be applied judiciously as alkali and alkaline earth metals may be readily depleted during weathering in addition to reflecting weathering the chemical index of alteration cia chemical index of weathering ciw plagioclase index of alteration pia and vogt s residual index v may be sensitive to subtle geochemical changes such as hydrothermal alteration along a fault and or alteration at the water table weathering indices that include iron in their calculations were not included in this study because they do not distinguish between ferric and ferrous iron the oxidation state and distribution of iron in a weathering profile is influenced by the proportion of oxidation states in the pre weathered metamorphic rock and interaction with groundwater during weathering

paleogene calcite compensation depth in the eastern subtropical pacific answers and questions 1 ocean drilling program leg 199 drilled a north south transect across the eocene paleoequator in the eastern pacific permitting reconstruction the calcite compensation depth ccd since earliest eocene time the ccd was relatively shallow near the early eocene pacific equator 3200 m and unlike modern latitudinal ccd gradients deepened to the north to 3600 m paleolatitude 10 n at 41 ma the ccd underwent a brief sharp transient deepening of 700 m then remained shallow until the eocene oligocene boundary at the e o boundary the ccd deepened by 1200 m in less than 300 kyr this rapid deepening served to more than double the area of seafloor subject to caco3 deposition sea level fall associated with ice volume buildup and ensuing shelf basin fractionation is unlikely to be the sole cause of the increased deep ocean caco3 burial rather a sudden rapid increase in the amount of ca entering the ocean appears necessary to explain the observations

effect of ocean gateways on the global ocean circulation in the late oligocene and early miocene 1 we investigate the effect of changes in the tectonic boundary conditions on global ocean circulation patterns using a fully coupled climate model in an idealized setup we compare situations corresponding to the late oligocene the early miocene and present day the model results show the existence of a flow reversal through the panama seaway between the oligocene and miocene this flow reversal is induced by global tectonic changes related to the widening of the southern ocean passages and the closing of the tethys seaway it mainly involves the wind driven ocean circulation in agreement with previous model studies the global thermohaline circulation in the oligocene and miocene simulations is significantly different from the present day conveyor circulation as there is deepwater formation in both the north atlantic and the north pacific oceans in particular in the oligocene simulation the salinity contrast between the atlantic and pacific oceans is reduced because of water mass exchange through the low latitude connections between the two oceans

chronological study of oxygen isotope composition for the solar protoplanetary disk recorded in a fluffy type a cai from vigarano abstract fluffy type a ca al rich inclusions cais containing reversely zoned melilite crystals are suggested to be aggregates of direct condensates from solar nebular gas we conducted an investigation of 26 al 26 mg systematics of a fluffy type a cai from vigarano named v2 01 with known oxygen isotopic distributions of reversely zoned melilite crystals we also conducted oxygen isotope measurements of coexisting minerals two of six reversely zoned melilite crystals show continuous variations in magnesium isotopic composition with 25 mg decreasing along the inferred direction of crystal growth which supports the idea that they originated through condensation petrography suggests that the constituent minerals of v2 01 formed in the following order first spinel and fassaite enclosed by melilite then reversely zoned melilite crystals and spinel and diopside in the wark lovering rim the spinel enclosed by melilite has 16 o rich compositions 17 o 24 and on an al mg evolutionary diagram plots along model isochron with an initial value of 26 al 27 al 0 5 6 0 2 10 5 the fassaite enclosed by melilite crystals shows variable oxygen isotopic compositions 17 o 12 and 17 and plots on an isochron with 26 al 27 al 0 5 6 0 2 10 5 the oxygen isotopic compositions of reversely zoned melilite showed continuous variations in 17 o along the inferred direction of crystal growth suggesting that surrounding nebular gas during the formation of the reversely zoned melilite changed from 16 o poor 17 o values larger than 10 to 16 o rich 17 o 25 the six reversely zoned melilite crystals show indistinguishable initial 26 al 27 al values with an average 26 al 27 al 0 of 4 7 0 3 10 5 which is clearly distinguishable from the value of enclosed spinel and fassaite indicating a younger formation age than the enclosed spinel and fassaite the spinel and diopside from the wark lovering rim show 16 o rich compositions 17 o 23 with 26 al 27 al 0 4 5 0 4 10 5 the values of 26 al 27 al 0 are consistent with the formation sequence inferred from petrography the formation period for the v2 01 cai is estimated to be 0 18 0 07 myr from the difference in initial 26 al 27 al values these data suggest that the oxygen isotopic composition of solar nebular gas surrounding the cai changed from 16 o rich to 16 o poor and back to 16 o rich during the first 0 2 myr of solar system formation

basement unconformity control on alteration st francois mountains se missouri the basement lamotte formation unconformity in southeastern missouri shows evidence of several distinctive alteration events some of which may be related to migration of ore forming fluids or k rich basinal brines the alterations are spatially related to the unconformity suggesting that the unconformity was a preferred pathway for fluid migration unconformity related rocks in five cores penetrating both the lamotte formation and the basement have been characterized by bulk chemistry mineralogy textures and paleotopographic position the alteration history along the unconformity includes 1 pre lamotte hydrothermal alteration affecting basement 2 pre lamotte weathering and 3 post lamotte hydrothermal and diagenetic events the third group is particularly complex including early diagenetic hematite and calcite cementation later quartz cementation secondary biotite growth k metasomatism that generated adulaira and k leaching that left a narrow dickite rich zone along the unconformity sev

comparison of 226ra 238u and 228th 228ra disequilibrium with weathering indices in beach sand sediments from greece weathering indices can be used to quantify weathering of rocks at various stages of weathering processes and provide a better understanding of element mobility during weathering various weathering indices have been proposed in the literature worldwide in the present work radioactive secular disequilibrium has been used to determine weathering extent thirty seven samples from kavala and thirty from sithonia beach sands all of them being the weathering products of the local granitic plutons are studied for their secular disequilibrium in both 238u and 232th series 226ra 238u and 228ra 228th respectively the results are compared to weathering indices already in use worldwide and suggest a consistent correlation with wip alkaline and alkaline earth element leaching and pwi silica leaching for the well and very well sorted sands and for the heavy minerals of kavala this study suggests that with an appropriate choice of the weathering index and samples both u series and weathering index can recover the extent of chemical weathering

archaean proterozoic transition geochemistry provenance and tectonic setting of metasedimentary rocks in central fennoscandian shield finland the central part of the fennoscandian shield in finland is composed of the palaeoproterozoic svecofennian domain and the archaean karelian craton with a palaeoproterozoic allochthonous and autochthonous cover a cryptic suture separating these areas and another tentative suture dividing the svecofennian into central and southern parts have been proposed the chemical composition of sedimentary rocks n 300 within the study area including the effects of palaeoweathering hydraulic sorting depositional environment and post depositional processes have been studied in order to delineate sediment source components the main proposed source components for the archaean sedimentary rocks are weathered 3 0 3 2 ga greenstone granite ttg and local 2 7 ga sources autochthonous 2 2 1 9 ga cover rocks were mainly derived from a mixture of chemically weathered palaeosol 2 2 2 35 ga sedimentary rocks derived from the palaeosol and mafic dykes and plateau volcanics mainly 2 2 2 1 ga although in places locally derived non weathered archaean sources dominated archaean crust and 2 0 1 92 low k bimodal rocks from a primitive island arc are the proposed source for the allochthonous western kaleva cover rocks these formed in a subsiding foredeep during initial collision from orogenic detritus in the same oblique collision zone the central svecofennian sedimentary rocks can be divided into local arc derived rocks 1 89 ga and older 1 91 ga rocks from a mixture of western kaleva sources and a 1 91 2 0 ga mature island arc active continental margin source rifting followed by increased subsidence during initial collision in the ne and subsequent arc reversal caused rapid erosion from the mountain belt exposing diverse source compositions as seen in the large variation of th sc 2 0 5 and deposition into an oblique hinterland basin further developing into a subduction related foredeep mature greywackes from the southern svecofennian in the study area resemble passive margin sediments with a source dominated by inferred alkaline affinity complexes and archaean rocks less mature rocks also occur and had sources dominated either by island arc active continental margin rocks or local picritic rocks in the sedimentary record the archaean proterozoic transition up to 2 1 ga was dominated by input of mainly mafic plateau type volcanic contribution to the archaean detritus palaeoproterozoic sediments having a crustal component 2 1 ga show higher th sc th cr and lower sm nd and eu eu relative to the archaean rocks but locally low th cr ratios complicate the situation ba depletion relative to k rb and th is a characteristic feature of the sedimentary rocks of the central fennoscandian shield indicating high amounts of ba lost from the clastic record during 2 3 1 9 ga and further recycled back to the mantle forming a subduction component and an enriched mantle component ba depletion seems to have been especially characteristic of chemical weathering during 2 35 2 2 ga under co2 rich and low o2 atmosphere whether this strong ba depletion is characteristic of the archaean proterozoic transition and quiet supercontinent stages in general remains to be determined

geochemistry of calcretes calcic palaeosols and hardpan coimbatore southern india formation and paleoenvironment abstract there is no major chemical variation in the late quaternary calcic nodules formed within vertisols in the foot hill regions and hardpan calcretes greater than 200 ka formed over the precambrian substrate that occur as duricrust horizons in the plains around coimbatore region tamil nadu india the bulk chemistry of calcic nodules and the hardpan calcretes show very little variability in cao sio2 al2o3 and fe2o3 content the trend of trace metal content in calcic nodules and in the hardpan calcrete is mn zn ni cr pb co cu and zn mn ni co pb cu cr respectively paas normalised ree data of the calcretes calcic nodules and the hardpan demonstrate a positive euanomaly this could be attributed to feldspar apatite mineral alteration and soil digenetic processes differential weathering leaving behind plagioclase phenocrysts and apatite grains which are enriched in eu the stable isotope values of the calcic nodules do not exhibit a wide range 18o 3 39 to 5 84 and 13c 3 01 to 6 64 compared to the hardpan calcretes 18o 2 91 to 12 98 and 13c 0 05 to 7 4 the palaeoenvironment during the formation of the calcretes nodules was dominated by the c4 plants with sparse vegetation cover and that the parent material bedrock supported a thin veneer of soil column with sparse vegetation cover as present day calcic nodule accumulations in the soils has resulted from differential weathering and chemical histories within the soil profiles while the hardpan calcretes formed by complex pedogenic and chemical groundwater processes in arid semi arid conditions over along geological period during the late mis3 to the lgm period the south west monsoon was weak with mean annual rainfall mar between 300 and 500 mm yr the sea level was low and the rainfall at the time of carbonate formation would have remained damp enough to allow silicate weathering of the soil sediments

record of climatic fluctuations and high ph weathering conditions in a thick ordovician palaeosol developed in rhyolite of the dunn point formation arisaig nova scotia canada a thick ordovician intra rhyolitic palaeosol was studied to investigate the peculiarities of early palaeozoic continental environments shortly before the development of vascular plants and to compare its pedogenic patterns with those of previously studied intra basaltic profiles from the same succession well defined k eluviation from the top of the palaeosol is proportionally met by k illuviation at the base of the profile a pedogenic behaviour that was masked in the underlying intra basaltic profiles due to subsequent eodiagenetic k enrichment associated with rhyolite emplacement greater stability of k and eventually mg than si in the intra rhyolitic profile suggests high ph soil water at the time of pedogenesis despite the low base contents of the acidic host rock and despite evidence for humid weathering conditions we also observe that at the base and at the top of the profile al2o3 zr ratios are substantially lower than those of the host rock suggesting al leaching and therefore extreme weathering conditions whereas the middle portion of the profile shows al2o3 zr ratios that are similar to those of the host rock suggesting al stability and therefore less extreme conditions we interpret these variations in al2o3 zr ratios as signatures of a cyclic change from sub humid periods during which al was stable throughout the profile to more humid periods with a well defined seasonality during which only the juvenile zone of weathering at the base of the profile couldhavedevelopedsufficientlyhighphduringdryseasonsforaltobeleached leavingtheal2o3 zr signatures from previous stages untouched higher in the profile

eocene circulation of the southern ocean was antarctica kept warm by subtropical waters 1 near the eocene s close 34 million years ago the climate system underwent one of the largest shifts in earth s history antarctic terrestrial ice sheets suddenly grew and ocean productivity patterns changed previous studies conjectured that poleward penetration of warm subtropical currents the east australian current eac in particular caused eocene antarctic warmth late eocene opening of an ocean gateway between australia and antarctica was conjectured to have disrupted the eac cooled antarctica and allowed ice sheets to develop here we reconstruct eocene paleoceanographic circulation in the tasmanian region using 1 biogeographical distributions of phytoplankton including data from recently drilled ocean drilling program leg 189 sites and 2 fully coupled climate model simulations we find that the eac did not penetrate to high latitudes and ocean heat transport in the region was not greater than modern our results do not support changes in thermal isolation as the primary driver of the eocene oligocene climatic transition

paleogene tropical pacific clues to circulation productivity and plate motion 1 stratigraphic data from 63 deep sea drilling project and ocean drilling program odp sites that sample the lower neogene and paleogene sediments of the tropical pacific have been compiled and put on the biostratigraphic and paleomagnetic timescale refined by odp leg 199 scientists sediment accumulation rates have been calculated for ten intervals ranging in age from 10 to 56 ma and have been plotted for the midpoint of each interval at the associated paleoposition for each site used a fixed hot spot model was used for reconstruction of the pacific lithospheric plate all such reconstructed intervals show the development of a tongue of high accumulation rates associated with the oceanographic divergence at the geographic equator the development of this equatorial band is weakest between 46 and 56 ma the time of the peak warmth in paleogene climate possible motion of the hawaiian hot spot or true polar wander between 46 and 56 ma appears to have had little effect on the plate rotation estimate of the position of the equator in addition to temporal changes in the calcite compensation depth and in productivity the biggest change in the patterns of sediment accumulation rates in the eastern tropical pacific was the development of a relatively strong divergence between 6 and 10 n near the region of divergence between the modern north equatorial current and the north equatorial counter current changes in the equatorial circulation appear to be associated in time with the opening and closing of oceanic gateways particularly the complex closing of the caribbean pacific gateway

tectonic processes in papua new guinea and past productivity in the eastern equatorial pacific ocean phytoplankton growth in the eastern equatorial pacific ocean today accounts for about half of the new production the fraction of primary production fuelled by externally supplied nutrients in the global ocean the recent demonstration that an inadequate supply of iron limits primary production in this region1 supports earlier speculation that in the past fluctuations in the atmospheric deposition of iron bearing dust may have driven large changes in productivity2 but we argue here that only small 2 nm increases in the iron concentration in source waters of the upwelling equatorial undercurrent are needed to fuel intense diatom production across the entire eastern equatorial pacific ocean episodic increases in iron concentrations of this magnitude or larger were probably frequent in the past because a large component of the undercurrent originates in the convergent island arc region of papua new guinea which has experienced intensive volcanic erosional and seismic activity over the past 16 million years cycles of plankton productivity recorded in eastern equatorial pacific sediments may therefore reflect the influence of tectonic processes in the papua new guinea region superimposed on the effects of global climate forcing

elemental distributions and neodymium isotopic compositions of silurian metasediments western maine usa redistribution of the rare earth elements abstract pelitic schists from the lower garnet to lower staurolite zones from the rangeley perry mountain and smalls falls formations of western maine were analyzed for major elements trace elements and neodymium isotopes these formations were derived from highlands created during the taconian orogeny deposited into a trough and metamorphosed during subsequent orogenic events most major and trace element abundances relative to al 2 o 3 were statistically identical between zones of the same formation as well as between formations although the average major element composition of these formations are the same there are systematic variations in some elements notably plots of sio 2 vs al 2 o 3 and k 2 o vs al 2 o 3 suggest that most of the variation could be produced by mixing of a fairly constant ratio of clay minerals and feldspar with varied amounts of quartz due to sorting in the sedimentary system different amounts of these minerals should not influence the shape of the ree patterns of the metapelites but higher amounts of quartz and feldspar may dilute the rees and most elemental abundances of the clay minerals and lead to lower elemental abundances the major difference between the samples within the perry mountain formation are different lree and mree abundances relative to al 2 o 3 which are not correlated to differences in major element or other trace element abundances relative to al 2 o 3 the samples in the perry mountain with higher lree and mree abundances have for example 42 3 8 3 ppm and those with low abundances have 5 6 3 6 ppm the samples with the high ree abundances of the perry mountain formation are similar in abundances and ree patterns to those of the rangeley and smalls falls formations typical of mudstones derived from granitoids another difference between the low and high ree abundance samples are the calculated t dm model ages the high ree abundance samples of the perry mountain formation show t dm similar to the samples of the rangeley formation with ages of about 1 7 1 8 ga the perry mountain samples with low ree abundances however give unrealistically old t dm s between 2 5 and 5 3 ga these unrealistically old t dm s are due to the relatively high sm nd ratios compared to crustal values which are characteristic of samples of the perry mountain formation with lower ree abundances we therefore suggest that these samples may be indicators for open system behavior of the neodymium isotopic system the timing of this disturbance of the neodymium isotope system is difficult to determine and cannot be tied to weathering or a definite postdepositional event the complexities of the data suggest more than one resetting event the most likely event that could have produced much of the movement of the lrees and mrees could have been due to small scale migration between anoxic hemipelagites and turbidite mudstones during diagenesis but some migration may have continued during metamorphism in order to reconcile the neodymium isotopic data

mass independently fractionated sulfur in archean paleosols a large reservoir of negative δ33s anomaly on the early earth abstract results of multiple sulfur isotope analyses from five archean paleosols show the widespread presence of mass independently fractionated sulfur in the regolith developed on the pre 2 5 ga earth analysis of sulfur from a small set of diamictite samples gave similar results all values of 33 s are negative indicating that the archean surface environments preferentially retained atmospheric s from the so 4 2 pathway which carried a negative 33 s signal whereas a portion of the s from the s 8 pathway with a positive 33 s was transferred to the oceans the soil so 4 2 was then converted to sulfide by bacterial sulfate reduction with terrestrial organic matter in the weathering horizon acting as a reductant some s from the s 8 pathway also included which resulted in a net soil 33 s anomaly from atmospheric s in the range 0 3 to 0 6 similar to values for pyrites from floodplain sediments excess s 8 derived s was carried to the oceans resulting in the negative positive dichotomy between terrestrial and marine systems pyrite that formed in paleosols and pyrite grains that formed in flood plain deposits both carrying the terrestrial 33 s signal were then recycled into detrital pyrite grains now found in sandstones and conglomerates deposited before the rise of atmospheric oxygen therefore the earth s early regolith constituted a reservoir of s with negative 33 s values that could be important for balancing the predominantly positive 33 s signature found in marine sediments

a paleosol developed on hydrothermally altered granite from the hinterland of the witwatersrand basin characteristics of a source of basin fill near delmas in the transvaal a paleoweathering surface is developed beneath the black reef on hydrothermally altered granite hag au in the weathering profile averages 10 ppb compared with 5 ppb in the hag the hydrothermal alteration converted all plagioclase to muscovite and k feldspar removing na and ca from the rock and converting ferromagnesian minerals to chlorite discontinuous zones in the core were enriched in as sb co au and la superimposed on this hydrothermal alteration was a weathering event that penetrated to about 15 m k rb cs sr and ba were leached mg and fe were depleted in about equal proportion whereas ti and al were conserved consistent with weathering under a low o 2 atmosphere magnetite in the soil was destroyed and fe ti oxides converted to anatase sc and ta were also leached as a result of destruction of sphene and fe ti oxide without formation of secondary iron oxyhydroxides that re adsorb these elements in younger soils a later diagenetic event added

226ra 238u and 228th 228ra disequilibrium as weathering indices in beach sand sediments associated with granitoids from cyclades greece abstract radioactive secular disequilibrium measured as the absolute difference from 1 of the 226ra 238u and 228ra 228th ratios have been used to determine weathering extent twenty eight samples from beach sands of cyclades islands all of them being the weathering products of the local granitic plutons are studied for their secular disequilibrium in both 238u and 232th series 226ra 238u and 228th 228ra respectively the results are compared to weathering indices already in use worldwide and suggest a significant linear correlation of u series disequilibrium with wip alkaline and alkaline earth element leaching and pwi silica leaching for the beach sands studied and significant non linear correlations with cia chemical index of alteration and pia plagioclase index of alteration considering only the heavy mineral enriched samples significant correlations exist between u series disequilibrium and cia ciw chemical index of weathering and pia therefore with an appropriate choice of the weathering index and samples both u series and weathering index can recover the extent of chemical weathering on the other hand the use of th series disequilibrium as a weathering index is limited due to 228th and 228ra geochemistry and especially due to their short radionuclide half lives u series disequilibrium can therefore be used as a relatively sensitive weathering index in sediments consistent with cia ciw and pia indices however the mineralogical composition must be considered more specifically the mineral content of the sediments should involve considerable amounts of sensitive to weathering minerals e g fe mg minerals as well as feldspars

climatic and tectonic influences on fan deltas and wave to tide controlled shoreface deposits evidence from the archaean keskarrah formation slave province canada the 2 6 ga keskarrah formation located in the central slave province northwest territories canada is a late orogenic tectonically controlled sedimentary sequence that developed under unusual climatic and depositional conditions the formation is adjacent to the crustal scale north trending beniah lake fault and overlies the 3 15 ga augustus granite the 2 69 2 7 ga mafic volcanic peltier formation and the turbiditic contwoyto formation unconformably principal lithofacies in the keskarrah formation include conglomerate sandstone and siltstone sandstone the conglomerate lithofacies represents coalescing gravelly streamflow dominated fan deltas adjacent to topographic highs up section quartz rich arenites and quartz arenites of the sandstone lithofacies are interpreted to be shallow water shoreface deposits influenced by wave action and tides the overlying feldspathic litharenites of the siltstone sandstone lithofacies are consistent with a lower shoreface to proximal offshore environment dominated by wave and tide interaction tidal influence in both sandstone dominated lithofacies is inferred from the presence of mudstone laminae between bedforms and on foresets of cross beds as well as from abundant reactivation surfaces with local mudstone drapes intense chemical weathering during the archaean resulting from elevated atmospheric co2 levels higher temperatures and moist climatic conditions played an important role in the development of quartz rich arenites that appear to be first cycle deposits few lithic fragments and feldspar grains are preserved due to in situ host rock weathering chemical weathering during transport and wave and tide action hydraulic sorting and abrasion in the shoreface environment contributed to the continued breakdown and transport of labile minerals increased proportions of lithic fragments in sandstone beds of the conglomerate lithofacies are the result of shorter transport distances from source areas to the depositional environment abundant conglomerate with up to 4 m large granitic boulders derived from the adjacent augustus granite and mafic clasts from the peltier formation indicate high relief and fault related uplift and subsidence the intimate association of fan deltas and wave and tide influenced shallow marine deposits in association with quartz rich sandstones forming in a high relief area make the keskarrah formation remarkable in the rock record

provenance and paleoweathering reconstruction of the mesoproterozoic hongshuizhuang formation 1 4 ga northern north china this is the first study presenting major and trace elemental data from the mesoproterozoic hongshuizhuang formation shales in yanshan basin north china in order to reconstruct its provenance and chemical weathering history the shales are strongly depleted in na2o and sr and enriched in y and transition metal elements relative to upper continental crust low zr concentrations and various discriminant plots e g th sc zr sc and al2o3 tio2 zr indicate insignificant mineral sorting or recycling of these shales the rocks show light rare earth element ree enrichment la ybcn 3 99 6 92 flat heavy ree and significantly negative eu anomalies euan 0 57 0 68 in chondrite normalized ree patterns similar to post archean australian average shales the fairly uniform ree patterns and trace element ratios indicate that the hongshuizhuang formation shales were derived from a felsic source area with granodiorite as the dominant contributor mixing calculations suggest a mixture of 30 granite porphyry 5 basalt and 65 granodiorite as the possible source of the shales also supporting that granodiorite was the predominant source intense chemical weathering of the source terrain is indicated by high values of the premetasomatized chemical index of alteration plagioclase index of alteration rb sr a strong positive correlation between tio2 and al2o3 depletion of cao na2o and sr and mineral compositions such strong chemical weathering suggests a warm and wet paleoclimate perhaps due to high atmospheric co2 and ch4 concentrations and a near equatorial location of the north china craton in the columbia supercontinent at 1 4 ga

timing and nature of the deepening of the tasmanian gateway 1 tectonic changes that produced a deep tasmanian gateway between australia and antarctica are widely invoked as the major mechanism for antarctic cryosphere growth and antarctic circumpolar current acc development during the eocene oligocene e o transition 34 33 ma ocean drilling program odp leg 189 recovered near continuous marine sedimentary records across the e o transition interval at four sites around tasmania these records are largely barren of calcareous microfossils but contain a rich record of siliceous and organic walled marine microfossils in this study we integrate micropaleontological sedimentological geochemical and paleomagnetic data from site 1172 east tasman plateau to identify four distinct phases a d in the e o tasmanian gateway deepening that are correlative among odp leg 189 sites phase a prior to 35 5 ma minor initial deepening characterized by a shallow marine prodeltaic setting with initial condensation episodes phase b 35 5 33 5 ma increased deepening marked by the onset of major glauconitic deposition and inception of energetic bottom water currents phase c 33 5 30 2 ma further deepening to bathyal depths with episodic erosion by increasingly energetic bottom water currents phase d 30 2 ma establishment of stable open ocean warm temperate oligotrophic settings characterized by siliceous carbonate ooze deposition our combined evidence indicates that this early oligocene tasmanian gateway deepening initially produced an eastward flow of relatively warm surface waters from the australo antarctic gulf into the southwestern pacific ocean this proto leeuwin current fundamentally differs from previous regional reconstructions of eastward flowing cool water e g a proto acc during the early oligocene and thereby represents an important new constraint for reconstructing regional to global scale dynamics for this major global change event

significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data how and where the ocean tides dissipate their energy are long standing questions1 that have consequences ranging from the history of the moon2 to the mixing of the oceans3 historically the principal sink of tidal energy has been thought to be bottom friction in shallow seas4 5 there has long been suggestive evidence6 7 however that tidal dissipation also occurs in the open ocean through the scattering by ocean bottom topography of surface tides into internal waves but estimates of the magnitude of this possible sink have varied widely3 8 9 10 11 here we use satellite altimeter data from topex poseidon to map empirically the tidal energy dissipation we show that approximately 1012 watts that is 1 tw representing 25 30 of the total dissipation occurs in the deep ocean generally near areas of rough topography of the estimated 2 tw of mixing energy required to maintain the large scale thermohaline circulation of the ocean12 one half could therefore be provided by the tides with the other half coming from action13 on the surface of the ocean

evidence for an early opening of the bering strait the first opening of the bering strait was an important palaeogeographical and biogeographical event for marine and terrestrial biotas in asia and north america1 2 3 and an oceanographic event of global importance4 5 6 this event however has never been precisely dated so it has not been accurately incorporated into models of global biogeography and oceanography a recent find of the bivalve mollusc astarte in southern alaska is a clear signal that the strait had opened by at least the late miocene or earliest pliocene epochs because this genus otherwise dwelled only in the arctic and north atlantic oceans at that time1 2 here we show that marine diatoms with astarte are correlative with subzone b of the neodenticula kamtschatica zone of the north pacific diatom biochronology yielding an age range of 4 8 5 5 myr refs 7 8 stratigraphic information suggests that this may be a minimum age range for the strait s first opening which evidently occurred between 4 8 and 7 3 7 4 myr ago our results contrast with past studies9 10 11 12 13 that suggested an age of 3 1 4 1 myr for the initial opening of the bering strait

neoarchean paleoweathering of tonalite and metabasalt implications for reconstructions of 2 69ga early terrestrial ecosystems and paleoatmospheric chemistry abstract field and laboratory investigations of a 2690 83 ma 207pb 206pb age of saganaga tonalite unconformity exposed in outcrop in northeastern minnesota usa reveal evidence for development of a deep paleoweathering profile with geochemical biosignatures consistent with the presence of microbial communities and weakly oxygenated conditions weathering profiles are characterized by a 5 50 m thick regolith that consists of saprolitized saganaga tonalite and paulson lake succession basaltic metavolcanic rocks retaining rock structure which is cross cut by a major unconformity surface marking development of a successor basin infilled with alluvial deposits the regolith and unconformity are overlain by thick conglomerate deposits that contain both intrabasinal saprock as well as extrabasinal detritus thin section microscopy and electron microprobe analyses reveal extensive hydrolysis and sericitization of feldspars exfoliation and chloritization of biotite and weathering of fe mg silicates and cu fe sulfides weathering of fe ti oxides was relatively less intense than for other minerals and evidence was found for precipitation of fe oxides geochemical analyses of the tonalite assuming immobile tio2 during weathering ti j show depletion of sio2 al2o3 na2o cao mgo and mno and to a lesser degree of k2o relative to least weathered parent materials significant fe was lost from the tonalite a paleoatmospheric pco2 of 10 50 times pal is estimated based on geochemical mass balance of the tonalite profile and assuming a formation time of 50 500 kyr interpretations of metabasalt paleoweathering are complicated by additions of sediment to the profile and extensive diagenetic carbonate dolomite overprinting patterns of release of p and fe and retention of y and cu in tonalite are consistent with recent laboratory experiments of granite weathering and with the presence of acidic conditions in the presence of organic ligands produced for example by a primitive microbial community during weathering cu metal in the profile may document lower po2 than present day at the surface comparison with previous studies of weathered tonalite and basalt denison 2 45 2 22 ga in ontario canada reveal general similarities in paleoweathering with our study as well as important differences related to lower paleoatmospheric po2 and terrestrial biosignature for the older minnesota profile a falling water table in the alpine lake locality is presumed to have promoted formation of this gossan like deep weathering system that extends to 50 m depth

intra basaltic soil formation sedimentary reworking and eodiagenetic k enrichment in the middle to upper ordovician dunn point formation of eastern canada a rare window into early palaeozoic surface and near surface conditions mafic flows of the middle to upper ordovician dunn point formation of eastern canada were deeply weathered under warm and relatively humid conditions before being buried by subsequent flows in the absence of superior plants and in the context of relatively low atmospheric carbon levels the soils developed alkaline groundwater conditions through mineral water interactions which resulted in an enhanced mobility of al relative to si in most palaeosols of that formation although the vegetation cover was volumetrically insignificant compared with that of subsequent geological times it was apparently producing very efficient chelates which for most palaeosols of the succession generated a well defined cheluviation pattern for not only al and fe but also and mainly ti which is typically immobile in modern soils the resulting soils developed an al fe ti depleted upper horizon that was enriched in si probably through periodic ground saturation long term climatic variations related to orbital cycles are inferred to have accounted for a second type of soil in the succession which contrasts with the former by showing a si depleted and less al fe ti depleted upper horizon some soil material was substantially reworked by surface runoff but such occurrences can be easily differentiated from in situ soil material in terms of texture structure and composition a thick overlying rhyolite flow is thought to be responsible for providing abundant k in solution which was incorporated in the underlying basalt palaeosols as exchangeable cations within a probably montmorillonitic clay precursor to the fe mg rich phengite that later developed during deep burial and orogenic compression

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

neogene evolution of atlantic thermohaline circulation perspective from walvis ridge southeastern atlantic ocean 1 we present new nd isotope records from walvis ridge ocean drilling program odp sites 1262 1264 southeastern atlantic spanning the past 24 ma to investigate the neogene evolution of atlantic thermohaline circulation the new data indicate that deepwater nd t values from odp site 1262 decrease from 11 0 at 10 6 ma to 12 5 by 7 3 ma this decrease parallels the nd isotope trends contained in fe mn crust records from the northwestern atlantic however the shift at odp site 1262 4755 m water depth occurred up to 6 ma earlier than the corresponding changes in crust records from the atlantic and earlier than any atlantic site shallower than 2700 m paleowater depth recent interpretations of the rapid decrease in fe mn crust nd nd t ssvalues invoke changes in weathering inputs to the labrador sea region rather than a fundamental change in deepwater convection in the labrador sea however the new evidence for significant depth stratification of the nd isotope signal in the southeastern atlantic between 10 6 and 7 3 ma suggests that the onset of deepwater convection in the labrador sea may have played a role in the deepwater decrease in nd isotopic composition climatic conditions during the middle to late miocene likely favored an increase in the importance of glacially induced mechanical weathering while at the same time promoting deep convection in the labrador sea