could early cenozoic thermohaline circulation have warmed the poles the paleocene and eocene epochs provide examples of oceanographic conditions when the earth had a stable climate much warmer than that of the modern world lower cenozoic sediments thus provide a way to investigate how oceanographic processes might respond to extreme warming because high latitude sea surface temperatures were significantly warmer during the early cenozoic while tropical sea surface temperatures were similar to or somewhat less than modern there may have been a much higher poleward oceanic heat transport at that time shackleton and boersma 1981 barron 1987 sloan et al 1995 here i develop a simple ocean thermohaline model to demonstrate that poleward oceanic heat transport could strongly warm the polar regions only if the strength of vertical mixing vertical eddy diffusion in the oceans is sensitive to changes in oceanic density structure if this is true the early cenozoic oceans because they had significantly less density stratification could have had a thermohaline circulation as much as 3 times as strong as that of the modern ocean despite reduced temperature gradients the model also predicts that an optimum bottom water temperature exists for maximum poleward heat transport continued warming of the earth does not continue to increase oceanic heat transport to the poles the hypothesized sensitivity of oceanic vertical mixing to the density gradient should provide a positive feedback to both the transition to ice ages and to a future transition out of them a test of the sensitivity of vertical eddy diffusion to density gradients can be studied through map and depth transects in the horizontal direction high vertical diffusion would be marked by relatively strong bottom water temperature gradients away from bottom water source regions in the vertical direction high vertical diffusion should be indicated by very small temperature gradients

abyssal circulation and benthic foraminiferal changes near the paleocene eocene boundary a major change in cenozoic deep sea benthic foraminifera occurred in the atlantic indian and pacific oceans near the paleocene eocene boundary benthic foraminiferal abundance changes began at about 61 5 ma at pacific deep sea drilling project dsdp site 577 a major extinction event followed at 58 57 ma between zones p6a and p6b and a series of first appearances continued until circa 55 5 ma zone p6c these faunal changes occurred during a 6 c warming of pacific bottom water and may indicate that the primary cause was changing temperature other potential causes of the faunal turnover include global changes in surface ocean productivity and changing bottom water source regions comparison of benthic and planktonic 13c records requires no change in the ratio of oceanic phosphorous to carbon during the late paleocene to early eocene which weakens the case for but does not disprove a change in surface ocean productivity at this time interbasinal comparisons of benthic foraminiferal 13c records document that water with high 13c values filled the cape basin during the late paleocene and possibly the early eocene circa 61 57 ma but apparently did not extend into the western basins of the atlantic this pattern suggests a supply of antarctic source water for the cape basin and possible tectonic isolation of the western atlantic basins during at least part of the late paleocene carbon isotope comparisons show that bottom water supply to the cape basin was reduced in the early eocene eolian grain size data suggest that a decrease in zonal wind intensity occurred at the end of the paleocene these late paleocene climatic changes bottom water warming and decreased wind intensity correspond with evidence for an important global tectonic reorganization and extensive subaerial volcanism which may have contributed to climatic warming through increased supply of co2

diffusion of ar in k feldspar present and absent the mechanisms of ar release from k feldspar samples in laboratory experiments and during their geological history are assessed here modern petrology clearly established that the chemical and isotopic record of minerals is normally dominated by aqueous recrystallization the laboratory critique is trickier which explains why so many conflicting approaches have been able to survive long past their expiration date current models are evaluated for self consistency especially arrhenian non linearity leads to paradoxes the models testable geological predictions suggest that temperature based downslope extrapolations often overestimate observed geological ar mobility substantially an updated interpretation is based on the unrelatedness of geological behaviour to laboratory experiments the isotopic record of k feldspar in geological samples is not a unique function of temperature as recrystallisation promoted by aqueous fluids is the predominant mechanism controlling isotope transport k feldspar should therefore be viewed as a hygrochronometer laboratory degassing proceeds from structural rearrangements and phase transitions such as are observed in situ at high temperature in na and pb feldspars these effects violate the mathematics of an inert fick s law matrix and preclude downslope extrapolation the similar upward concave non linear shapes of arrhenius trajectories of many silicates hydrous and anhydrous are likely common manifestations of structural rearrangements in silicate structures

towards the general circulation of the north atlantic ocean abstract after reviewing the inverse method we apply it to deducing the general circulation of the north atlantic ocean we argue that the method is purely classical in nature being nothing more than a mathematical statement of the principles upon which nearly all previous circulation schemes have been based the smoothed solution is shown to represent the components of the flow field that are determinable independently of the initial reference level we then produce two circulation schemes based upon two different initial reference levels 2000 decibars and the bottom called north atlantic 1a and north atlantic 1b respectively the models share many features in common and are strikingly similar to several previous schemes most notably those of jacobsen and defant in the region west of bermuda no simple level of no motion emerges in the flow fields rather the velocity sections exhibit a complex cellular structure zonally integrated meridional cells of models and of the uniquely determined components are very similar showing a poleward movement of warm saline water compensated at depth by a return flow of cold fresher water the magnitudes of the implied polar sea overflows and the heat fluxes are in good agreement with previous estimates finally it is argued that neither these model circulations nor any other circulation pattern based upon the existing data can be regarded as actually representing the true time average ocean circulation because the data are aliased in time the frequency wavenumber spectrum of the ocean is inadequately known to determine the resulting errors

mantle derived noble gases in ore forming fluids of the granite related yaogangxian tungsten deposit southeastern china more than 90 of the tungsten resources of china are in the nanling region of south china and the yaogangxian vein deposit is the largest tungsten deposit in this region the tungsten deposits have ages of 150 160 ma and are spatially temporally and genetically related to granites which were previously believed to be produced by crustal anatexis this paper provides he and ar isotope data of fluid inclusions in pyrite and arsenopyrite from the yaogangxian w veins 3he 4he ratios range from 0 41 to 3 03 ra where ra is the 3he 4he ratio of air 1 39 10 6 and 40ar 36ar ratios from 328 to 1 191 moreover there are excellent correlations between he and ar isotopic compositions the results suggest that the ore forming fluids are a mixture between a crustal fluid containing atmospheric ar and crustal 4he and a fluid containing mantle components it is likely that the former is a low temperature meteoric fluid and the later is a fluid exsolved from the w associated granitic magma which formed by crustal melting induced by intrusion of a mantle derived magma

paleogene deepwater mass composition of the tropical pacific and implications for thermohaline circulation in a greenhouse world 1 the pacific ocean likely exerted the greatest marine influence on climate during the greenhouse interval of the paleogene the first step toward assessing the role of pacific deepwater circulation in global heat transport is a comprehensive reconstruction of deepwater mass composition and circulation patterns we present new deepwater mass nd isotope data from odp leg 199 sites 1215 1217 1219 and 1221 to augment the existing coverage provided by ferromanganese crust and fossil fish debris data the new data corroborate the notion that the pacific ocean operated in a bimodal pattern with deepwater sources in both the southern ocean and north pacific for much of the early to middle paleogene in addition the data indicate that there was little to no influence of atlantic waters on the deepwater composition of the tropical pacific the available deepwater nd isotope data provide refined constraints on the geographic and depth range of influence of both a southern ocean and north pacific deepwater mass from 65 to 45 ma the deep tropical pacific was the mixing location of deep waters sourced from the southern ocean and the north pacific the southern ocean deepwater mass extended from 1800 m to abyssal depths until 53 6 ma it circulated northward in the western pacific at least as far as the paleolocation of fe mn crust cld01 5 n paleolatitude and in the eastern portion of the pacific as far as site 1215 10 n paleolatitude a north pacific deepwater mass extended from at least 2400 m to 2900 m water depth from 65 to 54 ma south to shatsky rise paleolatitude 10 15 n and possibly as far as site 1215 700 km to the south of shatsky rise sites 1209 and 1211

silica burp in the eocene ocean the eocene was a time of greatly increased silica accumulation in the ocean and the peak was in the early middle eocene at about 50 ma the responsible geohistorical configuration included the following elements extensive volcanism about 4 m y earlier as part of the chron 24 plate reorganization early eocene warming with deep weathering to high latitudes and accumulation of the released silica in a sluggish ocean and sharp cooling in the earliest middle eocene stimulating oceanic upwelling and biosilicification it is possible on the evidence of carbon and oxygen isotopic patterns that the trigger for the exhalation of silica was a reverse greenhouse effect

k ar and ar ar dating the aim of this chapter is to present the k ar and ar ar dating techniques in the context of noble gas studies since there are already several recent texts on k ar and ar ar dating dickin 1995 mcdougall and harrison 1999 the focus of this section will be aspects of argon transport and storage in the crust which affect k ar and ar ar dating including ar loss from minerals by diffusion and ar gain by minerals or excess argon the k ar dating technique was one of the earliest isotope dating techniques developed soon after the discovery of radioactive potassium and provided an important adjunct to u pb and u he dating techniques the ease of measurement and ideal half life 1250 million years see table 2 below for dating geological events has made this the most popular of isotopic dating techniques aldrich and nier 1948 first demonstrated that 40ar was the product of the decay of 40k and soon after k ar ages were being measured in several laboratories most often using an absolute method such as a mcleod gauge to measure argon concentrations the first published k ar results by such a technique were those of smits and gentner 1950 who analyzed sylvite from the buggingen oligocene evaporite deposits obtaining an age of 20 million years mass spectrometers which simultaneously measured very small amounts of gas and the isotope ratios necessary to make corrections for atmospheric contamination quickly replaced manometric techniques crucially the use of static vacuum techniques pioneered by john reynolds at the university of california berkeley meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals although the earliest mass spectrometers were built in house the introduction of the commercially available ms10 farrar et al 1964 a small 180 metal mass spectrometer built for

neodymium isotopic reconstruction of late paleocene early eocene thermohaline circulation abstract high resolution fish tooth nd isotopic records for eight deep sea drilling project and ocean drilling program sites were used to reconstruct the nature of late paleocene early eocene deep water circulation the goal of this reconstruction was to test the hypothesis that a change in thermohaline circulation patterns caused the abrupt 4 5 c warming of deep and bottom waters at the paleocene eocene boundary the paleocene eocene thermal maximum petm event the combined set of records indicates a deep water mass common to the north and south atlantic southern and indian oceans characterized by mean nd values of 8 7 and different water masses found in the central pacific ocean nd 4 3 and caribbean sea nd 1 2 the geographic pattern of nd isotopic values before and during the petm suggests a southern ocean deep water formation site for deep and bottom waters in the atlantic and indian ocean basins the nd data do not contain evidence for a change in the composition of deep waters prior to the onset of the petm this finding is consistent with the pattern of warming established by recently published stable isotope records suggesting that deep and bottom water warming during the petm was gradual and the consequence of surface water warming in regions of downwelling

paleogene geochronology an integrated approach geochronology is the conceptual division of continuous time as measured geochronometry by the progression in an ordinal series of events this is best achieved by an approach which integrates four independent data sets magnetostratigraphy seafloor spreading magnetic lineation patterns biostratigraphy and isotopic ages this integrated approach results in an ordinal framework which can measure time with greater resolution though perhaps less accuracy than a radiometric approach alone a comparative analysis of two recently proposed paleogene geochronologic time scales is presented

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

ice sheet load cycling and fluid underpressures in the eastern michigan basin ontario canada strong fluid underpressures have been detected in paleozoic strata in the eastern michigan basin with hydraulic heads reaching 400 m below land surface 4 mpa underpressure and 200 m below sea level in strata where unusually low permeabilities 10 20 10 23 m2 were measured in situ multiple glaciations including three with as much as 3 km of ice cover at the site in the last 120 ka suggest a causal link with the underpressures we examined this possibility using a one dimensional groundwater flow model incorporating mechanical loading from both ice weight and lithospheric flexure because hydrologic and mechanical changes during glaciation are not well characterized and subsurface properties are imperfectly known the model was used inversely to estimate flexural loads and loosely constrained permeabilities by matching observed pressures acceptable matches were obtained for a surprisingly wide range of scenarios with permeabilities close to measured values and plausible flexural loads matches were not obtained when too many parameters were preselected or when permeabilities were constrained to be significantly larger than measured values in successful model runs groundwater expulsion under glacial mechanical loads caused the underpressuring and flexural loads were important if aquifer and sub glacial pressures were significantly elevated during glaciation simulated fluid pressures in the low permeability strata fluctuated by 30 40 mpa during glacial cycles but resulted in advective transport of only tens of meters or less although other mechanisms cannot be ruled out we conclude that glacial mechanical forcing of a water saturated system can explain the observed underpressures

a chlorine disinfectant for excess argon released from k felsspar during step heating abstract the release of excess 40 ar 40 ar e from virtually all alkali feldspars at low temperatures 40 ar 39 ar step heating experiments obscures potentially valuable age and thermal history information we report a method that takes advantage of the differential release of 40 ar e between contiguous isothermal heating steps due to the decrepitation of fluid inclusions and which permits derivation of a correction for the contaminating argon differences in age and cl k between successive temperature pairs commonly yield a single well correlated array on a 40 ar k versus cl k plot that identifies the cl correlated composition of 40 ar e 40 ar e cl affecting that sample eighteen of the twenty k feldspar samples we have measured yield a single 40 ar e cl component that varies from 1 1 10 3 to 4 2 10 6 making possible recovery of otherwise unobtainable thermochronological information the upper bound on the 40 ar e cl distribution of 10 3 may reflect ar saturation of pore fluids or the maximum duration between deuteric alteration and cooling below ar closure in k feldspar

seismic and geologic evidence for early paleogene deepwater circulation in the western north atlantic previous studies of paleogeography global climate and foraminiferal stable isotopes have suggested that paleogene deepwater circulation was significantly different from that of today extracting details of deepwater history from the western north atlantic has been hampered by oligocene erosion and the lack of ideally located drill holes we report the discovery of a seismic disconformity on the sw bermuda rise that we have tied to a late paleocene erosional event in deep sea drilling project dsdp boreholes from this region distribution of this new seismic marker reflector ab indicates that it was formed by cyclonically circulating bottom water entering the western north atlantic from the south we attribute this vigorous deep circulation to a southern ocean source analogous to the effects of modern antarctic bottom water this interpretation is supported by isotopic comparisons that show the paleocene southern ocean was enriched in 13c relative to the pacific we distinguish reflector ab from the overlying reflector au on the basis of superposition and ages obtained at dsdp sites and piston cores using one of the cores we have estimated that the youngest age for reflector au is earliest oligocene 36 6 34 0 ma while its oldest age has been previously established as late eocene laminated sediments beneath reflector ab are enriched in organic carbon indicating that the early paleocene deep western north atlantic was occasionally anoxic these low oxygen levels may have been the result of sluggish deep circulation and or low latitude production of warm saline deep water wsdw sediments immediately above reflector ab show that deposition resumed on the western bermuda rise site 387 by the latest paleocene to earliest eocene biochron np10 57 2 0 6 ma while deposition resumed on the central bermuda rise site 386 during the late paleocene erosion nondeposition continued across much of the bermuda rise widespread resumption of pelagic deposition in the early eocene is consistent with 13c evidence for reduced southern ocean influence at this time however lower eocene sediments record well oxygenated conditions and 13c and 18o comparisons indicate that there were two early eocene deepwater sources the southern ocean and wsdw pak and miller this issue

the thermal history of the new york basement determined from 40ar 39ar k feldspar studies constraints on the thermochronologic evolution of the new york crystalline basement are mainly restricted to the high temperature early cooling history following 1 1 ga grenville metamorphism u pb methods and the late cooling history fission track methods the thermal history for the one billion years between these intervals is herein assessed using 40ar 39ar hornblende muscovite biotite and k feldspar thermochronology hornblende preferred ages suggest cooling below 450 500 c between ca 900 and 950 ma total gas ages of biotites range from 746 1060 ma and their age spectra complexity is probably related to younger thermal events and or excess argon a single muscovite analysis yields a preferred age of 854 ma argon isotope analyses of k feldspar provide a quantitative evaluation of the thermal history between 175 and 350 c multi diffusion domain thermochronology on highly variable k feldspar results yield internally consistent thermal histories and along with geologic constraints suggest thermal maxima of 275 350 c at ca 700 470 450 and 300 ma combining all of the k feldspar analyses provides an internally consistent thermal history for the region and allows for the following inferences reheating at 700 ma associated with late precambrian rifting during the formation of the iapetus ocean apparently affected the entire adirondack region with the highest temperatures occurring in the eastern part of new york local reheating in the ordovician is inferred to have affected the eastern adirondack mountains and probably resulted from the combined effects of burial during the early paleozoic emplacement of taconic thrust sheets and migration of hot fluids along normal faults during the taconic orogeny high paleotemperatures for the devonian section of eastern new york are related to maximum burial of the basement during the carboniferous at ca 300 ma

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

glacially induced hydromechanical coupling in shale may have caused underpressured water in the eastern michigan basin despite the possible presence of gas phase methane

paleoceanographic conditions on the sao paulo ridge sw atlantic ocean for the past 30 million years inferred from os and pb isotopes of a hydrogenous ferromanganese crust abstract hydrogenous ferromanganese fe mn crusts can provide records of long term environmental changes during the cenozoic to understand the paleoceanographic conditions in the southwestern atlantic ocean we investigated depth profiles of major and trace element concentrations as well as os and pb isotopic compositions in a fe mn crust collected from the southern flank of the sao paulo ridge in the southwestern atlantic major and trace element data plotted on ternary mn fe 10 ni co cu and rare earth element plus yttrium rey discrimination diagrams indicate that the analyzed sample is a typical hydrogenous fe mn crust the obtained 187 os 188 os data were matched to the cenozoic seawater os isotope evolution curve reconstructed from pelagic sediments the result suggests that the fe mn crust has accreted over 30 myr with growth rates of 0 5 3 mm myr although the sample likely grew in two directions during the early stage of its growth we found no evidence of growth hiatus in the sample which may contrast with the growth histories of many pacific fe mn crusts hence the conditions favorable for the accretion of hydrogenous fe mn crusts were likely to have developed on the sao paulo ridge over the past 30 myr the pb isotopic compositions show very limited ranges e g 206 pb 204 pb 18 80 18 85 and are similar to those of pre anthropogenic seawater in the southern ocean as the sao paulo ridge is located near the vema channel which is presently a major path of antarctic bottom water we suggest that a strong northward bottom current has continuously swept detrital and biogenic sediments from the ridge and played a vital role in the fe mn crust formation since 30 ma

abyssal current character determined from sediment bedforms on the nova scotian continental rise abstract bottom photographs across the nova scotian continental rise show effects of an intense bottom current at 4800 5000 m that decreases in strength upslope to tranquil near bottom conditions at 3200 4000 m apparently weak currents are present above 3200 m long term geologically significant flow parallels the bathymetric contours toward the southwest but significant short term variability in direction is indicated by orientations of small scale bedforms the western boundary undercurrent documented elsewhere along the continental margin at depths less than 4000 m has relatively little effect in modifying the sea floor on the nova scotian rise but the deep currents 4000 m have a significant effect repeated photography of small seafloor areas suggests that many bedforms especially those of smaller scale are constructed and destroyed by strong variably directed current events benthic storms on time scales of 3 6 months or less thus a significant part of the geologic response of the seabed to currents may be event controlled on time scales not very different from those in coastal environments

heat flow observations on the bermuda rise and thermal models of midplate swells the bermuda rise is a broad topographic swell which is apparent in both residual depth and geoid anomaly maps of the western north atlantic the magnitudes of the depth and geoid anomalies associated with the bermuda rise are similar to the anomalies associated with other swells surrounding recent volcanic islands e g hawaii suggesting that despite the lack of recent volcanism on bermuda the rise has a similar origin to other midplate swells results are reported from 171 new heat flow measurements at seven carefully selected sites on the bermuda rise and the surrounding seafloor off the bermuda rise the basement depths are generally shallower and the heat flow higher than either the plate or boundary layer models predict with the measured heat flow apparently reaching a uniform value of about 50 mw m 2 on 120 m y old crust on the bermuda rise the heat flow is significantly higher 57 4 2 6 mw m 2 than off the swell 49 5 1 7 mw m 2 the magnitude of the anomalous heat flux 8 10 mw m 2 is comparable to that previously found along the older portion of the hawaiian swell near midway the existence of higher heat flow on both the hawaiian swell and bermuda rise indicates that these features fundamentally have a thermal origin the differences in the shape uplift and subsidence histories of the hawaiian swell and bermuda rise can be quantitatively explained by the different absolute velocities of the pacific and north american plates moving across a distributed heat source in the underlying mantle two dimensional numerical convection models indicate that the observed depth geoid and heat flow anomalies are consistent with simple convection models in which the lower part of the thermally defined plate acts as the upper thermal boundary layer of the convection

new constraints on the release of noble gases during in vacuo crushing and application to scapolite br cl i and 40ar 39ar age determinations abstract the release of irradiation produced noble gas isotopes 38arcl 80krbr 128xei and 39ark during in vacuo crushing scapolite has been investigated and is compared to quartz three thousand crushing strokes released 98 of fluid inclusion hosted noble gas from quartz in comparison 3000 crushing strokes released only 4 of the lattice hosted 38arcl from a scapolite gem in vacuo crushing released lattice ar preferentially relative to lattice kr or xe and prolonged crushing released 88 of the lattice hosted noble gas in 96 000 crushing strokes we suggest fast diffusion pathways generated by crushing are an important noble gas release mechanism and we demonstrate two applications of prolonged in vacuo crushing on irradiated scapolite firstly scapolite molar br cl and i cl values are shown to vary over a similar range as crustal fluids the cl rich scapolite gem from hunza pakistan has br cl of 0 5 0 6 10 3 and i cl values of 0 3 2 10 6 that are similar to fluids that have dissolved evaporites in contrast three out of four skarn related scapolites from the canadian grenville province have molar br cl values of 1 5 2 4 10 3 and i cl values of 11 24 10 6 that are broadly consistent with skarn formation by magmatic fluids the fourth grenvillian scapolite with only 0 02 wt cl has an exceptionally elevated molar br cl value of up to 54 10 3 and i cl of 284 10 6 it is unclear if these values reflect the composition of fluids formed during metamorphism or preferential incorporation of br and i in cl poor meionitic scapolite secondly the grenvillian scapolites give plateau ages of between 830 ma and 400 ma the oldest ages post date regional skarn formation by 200 myr but are similar to feldspar cooling ages in the province the age variation in these samples is attributed to a combination of factors including variable thermal history and the presence of mineral sub grains in some of the samples these sub grains control the release of 39ark 38arcl and 40ar during in vacuo crushing as well as the samples 40ar retentivity in nature scapolite is suggested as a possible analogue for k feldspar in thermochronologic studies

refined insight into 40ar 39ar progressive crushing technique from k cl ar correlations in fluid inclusions abstract hydrothermal fluid activity associated with geological processes can now be dated by 40ar 39ar progressive crushing of minerals thanks to recent advances in high precision mass spectrometry however methods of data analysis and interpretation for these high resolution measurements still need to be refined in this study we report further insights into using k cl ar diagrams as tools for interpreting ages of secondary and primary fluids from 40ar 39ar stepwise crushing experiments three paragenetic mineral pairs of muscovite and wolframites from a southern china tungsten deposit were investigated using the 40ar 39ar technique through laser incremental heating and progressive crushing of single minerals incremental heating analyses of muscovites yielded flat age spectra defining plateau ages of 156 153 ma progressive crushing experiments of wolframites yielded anomalously old apparent ages due to excess 40ar however the apparent ages decline quickly after the initial steps and stabilize to form age plateaus of 156 153 ma without excess 40ar the plateau age steps interpreted as ages of gases derived from primary fluid inclusions pfis yield well defined isochrons with ages ranging from 157 to 153 ma and initial 40ar 36ar values that are indistinguishable from the modern atmospheric value digitally created 2d and 3d diagrams of k cl ar correlations can easily reveal distribution trends and correlation planes from the crushing steps interpretations of these diagrams further reveal the ages of secondary fluid inclusions sfis 87 to 83 ma and primary fluid inclusions pfis 159 to 150 ma raw data from sfis degassed during the initial crushing steps form scatters on inverse isochron plots however 2d and 3d k cl ar diagrams yield sfi ages that agree with 40ar 39ar ages of k feldspar veins cutting the tungsten ore bodies

helium and argon isotope geochemistry of alkaline intrusion associated gold and copper deposits along the red river jinshajiang fault belt sw china abstract the red river jinshajiang strike slip fault zone on the eastern margin of the tibetan plateau was originally produced by the india eurasia collision 60 70 myr ago numerous post collisional mantle derived alkaline igneous rocks with ages of 40 30 ma have been intruded along this fault zone in recent years several copper and gold deposits associated with the alkaline intrusions of this region were discovered such as the yao an and beiya gold deposits and the yulong and machangqing copper deposits studied in this paper the mineralised intrusions are felsic with sio 2 ranging from 61 4 to 67 7 wt k 2 o na 2 o from 8 1 to 11 5 wt and k 2 o na 2 o 1 the deposits are located at both the exo and endo contact zones of the intrusions the mineral deposits are of hydrothermal origin with the ore forming temperatures mainly in the range 150 450 c this paper presents he and ar isotope analyses of these four deposits the concentrations of 4 he trapped in fluid inclusions of pyrites from the ores are 0 7 54 1 10 6 cm 3 stp g 1 and those of 40 ar are 0 6 7 3 10 6 cm 3 stp g 1 3 he 4 he ratios are 0 3 2 5 ra ra represents the 3 he 4 he ratio of air 1 39 10 6 40 ar 36 ar ratios are 316 1736 and 3 he 36 ar ratios are 0 2 11 2 10 3 generally the 3 he 4 he 40 ar 36 ar and 3 he 36 ar ratios for the gold deposits are higher than those for the copper deposits we suggest that the ore forming fluids of both gold and copper deposits were differentiated from the mantle derived alkaline magmas but were diluted by modified air saturated water masw that experienced intensive interaction with crustal rocks however the magmatic fluids responsible for the gold deposits were less extensively diluted by masw resulting in higher 3 he 4 he 40 ar 36 ar and 3 he 36 ar ratios than the copper deposits

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

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

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

timing of detachment faulting in the bullfrog hills and bare mountain area southwest nevada inferences from 40ar 39ar k ar u pb and fission track thermochronology crustal extension in the bullfrog hills and bare mountain area of southwest nevada is associated with movement along a regional detachment fault normal faulting in the upper plate and rapid cooling denudation of the lower plate were coeval with miocene silicic volcanism and with west northwest transport along the detachment fault a west northwest progression of tilting along upper plate normal faults is indicated by ages of the volcanic rocks in relation to angular unconformities near the breakaway tilting in the upper plate occurred between 12 7 and 11 6 ma continued less strongly past 10 7 ma and was over by 8 2 ma ten to 20 km west of the breakaway tilting occurred between 10 7 and 10 33 ma continued less strongly after 10 33 ma and was over by 8 1 ma the cooling histories of the lower plate metamorphic rocks were determined by thermochronologic dating methods k ar and ar 40 39 a on muscovite biotite and hornblende ar 40 39 a on k feldspar u pb on apatite zircon and sphene and fission track on apatite zircon and sphene lower plate rocks 10 km west of the breakaway cooled slowly from early cretaceous lower amphibolite facies conditions through 350 50 degrees to 300 50 degrees c between 57 and 38 ma then cooled rapidly from 205 50 degrees to 120 5o degrees c between 12 6 1 6 and 11 1 1 9 ma lower plate rocks 20 km west of the breakaway cooled slowly from early cretaceous upper amphibolite facies conditions through 500 50 degrees c at 78 67 ma passed through 350 50 degrees to 300 50 degrees c between 16 3 0 4 and 10 5 0 3 ma then cooled rapidly from 285 50 degrees to 120 50 degrees c between 10 2 and 8 6 ma upper plate tilting and rapid cooling denudation of the lower plate occurred simultaneously in the respective areas the early slow cooling part of the lower plate thermal histories was probably related to erosion at the earth s surface which stripped off about 9 km of material in 50 to 100 m y the results indicate an initial fault dip greater than or equal to 30 degrees and a 12 mm yr 1 west northwest migration of the locus of rapid tilting in the upper plate

cenozoic north atlantic deep circulation history recorded in contourite drifts offshore newfoundland canada abstract in the north atlantic ocean contour following deep currents have created regional erosional unconformities and deposited contourite drifts that exceed 2 km in thickness and extend for 100 s of km the stratigraphic records in the drifts have been used to reconstruct variations in north atlantic deep water circulation throughout the cenozoic however uncertainties remain about certain aspects of the timing intensity depth distribution and regional impact of these currents here we use an integrated dataset of seismic reflection profiles and iodp core data lithology biostratigraphy and magnetostratigraphy to document sedimentation history and the development of current effects in the cretaceous to present sedimentary record on the j anomaly ridge and southeast newfoundland ridge offshore newfoundland canada the newfoundland ridges are in a key location lying between well studied areas in the northern and western north atlantic and under the path of both the modern deep western boundary current and the gulf stream late cretaceous through early eocene sedimentation on the ridges was dominated by biogenic pelagic sedimentation but at 47 ma near the early middle eocene boundary well developed contourite drifts began to accrete in paleo water depths of 4000 4500 m accompanied by an order of magnitude increase in terrigenous sediment mass accumulation rates from this time forward drift deposition interrupted by brief episodes of erosion continued unabated this timing for the onset of persistent deep currents is coincident with reorganization of atlantic circulation inferred from a change from biosiliceous to non biosiliceous sedimentation in the western north atlantic horizon a c and with the current eroded intra eocene unconformity ieu in the northern north atlantic a change in sedimentation style occurred within the middle eocene to upper oligocene drift sequence and it likely was related to a shift to deeper more intense currents that eroded the widespread horizon a u along the margin of eastern north america about early oligocene time beginning in the late oligocene 25 ma a thick drift exhibiting seismically laminated mudwaves was deposited in a distinct belt at 3500 4500 m paleodepth on the southeast newfoundland ridge this development correlates with widespread late oligocene through miocene pliocene drift accumulation throughout the north atlantic the most recent phase of drift deposition since late pliocene time 3 ma occurred after a shift to the modern circulation system of deeper swifter currents and it includes mixed pelagic hemipelagic sediments and ice rafted debris that reflect glacial interglacial influences on sedimentation

argon loss by alkali feldspars abstract ar loss is generally thought to preclude the use of alkali feldspars for dating old rocks by the k ar method and has been thought to occur via structural features such as the boundaries of perthite lamellae or twins we have done a 40 ar 39 ar step heating study of a suite of alkali feldspars from the klokken syenite intrusion u pb age 1166 4 ma which have been fully characterized by transmission electron microscopy all are low albite low microcline intergrowths but some are non turbid strain controlled cryptoperthites while others are turbid coarse microperthites geological and textural evidence suggests that the turbidity developed by fluid rock interactions during the initial cooling of the pluton in the stability field of low microcline 5 a after crystallization the finest cryptoperthites have numerous fully coherent perthite lamellar boundaries lamellar periodicity 40 nm and twin composition planes the have retained radiogenic 40 ar and yield total degassing ages as high as 1125 16 ma and maximum ages on the age spectra of 1162 16 ma the coarse turbid microperthites with patchy intergrowths and twins on the mm scale gave total degassing ages as low as 650 ma maximum age 750 ma argon leakage occurs least in feldspars with the most lamellar interfaces but in proportion to the degree of turbidity which is thought to be caused by large numbers of micropores strain controlled intergrowths in perthites allow the recognition of volumes of alkali feldspar crystals which have not interacted with deuteric fluids these retain argon and can give reliable k ar ages in old rocks saddle shaped age spectra appear to result from the preferential degassing at 1000 c of part of the feldspar structure which has lost more of its argon in nature the laboratory degassing does not occur in a manner analogous to that which has occurred naturally

40ar39ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the klokken syenite south greenland laser probe andin vacuo crushing have been used to apply the40ar39ar technique to microtexturally well characterised pristine and turbid alkali feldspar from the 1166 ma klokken syenite individual crystals are several millimetres in size but sem and tem studies reveal complex microstructures which define a range of sub grain sizes over three orders of magnitude from 100 m in pristine areas with fine scale cryptoperthite down to 200 nm in turbid areas with coarsened perthites and numerous micropores laser probe40ar39ar measurements of turbid feldspar gave low apparent ages indicating significant40ar loss 40 average which can be accounted for by sustained heating of the small sub grains 1 m for 1166 ma at low temperature 150 c pristine feldspar gives high apparent ages indicating the presence of excess40ar this component was readily lost from the feldspar by laboratory heating at low temperature 600 c and was also released by crushing which indicates that it was released from fluid inclusions during crushing release of excess40ar is correlated with cl release and the40ar cl ratio of the fluid is similar to that of mantle fluids turbid feldspar also released a fluid during crushing but the40ar cl ratio was lower probably as a result of evolution of the original fluid during boiling anomalously high apparent ages are often a feature of low temperature ar release in stepped heating age spectra of feldspars the presence of excess40ar in fluid inclusions may substantially affect the lower temperature release in feldspar stepped heating spectra the character of the microtextures in the host feldspar provides information on the temperature of fluid trapping because inclusions formed above the coherent alkali feldspar solvus have little or no effect on the exsolution textures whereas fluids trapped at low temperatures are associated with major structural rearrangements leading to development of patch perthite

mafic dike swarms in the south shetland islands volcanic arc unravelling multiepisodic magmatism related to subduction and continental rifting eight groups of mafic dikes and related high level stocks cut triassic accretionary complex and mesozoic magmatic arc formations on livingston island some are affected by silicic sericitic alteration related to cretaceous hydrothermal activity and propylitic epidosite alteration analogous to that in ocean floor sheeted dikes alteration was accompanied by major and trace element metasomatism ar ar analysis of the freshest rocks indicates five intrusive events some of which are unexpectedly young groups 1 3 were intruded in the mid to late cretaceous 108 74 ma and were coeval with the calc alkaline arc between 70 and 50 ma relatively rapid and oblique plate convergence led to strike slip tectonism and a pause in magmatism at 52 ma orthogonal slow convergence resulted in extensional faulting and emplacement of calc alkaline group 2 and primitive tholeiitic dikes groups 4 6 between 51 and 45 ma extension of antarctic peninsula southern south american crust culminated in emplacement of mafic to intermediate medium grained plutons and group c porphyries between 44 and 36 ma localized hydrothermal flow along fault zones resulted in partial to complete argon loss from nearby cretaceous lavas and ar ar reset ages of 40 ma in mid cretaceous hydrothermal k feldspar primitive olivine basalts group d and epithermal carbonate veins 31 29 ma were emplaced during along arc extension accompanying the opening of drake passage and powell basin excess argon occurs in two forms strongly held in melt inclusions in the primitive tholeiites and weakly held in some secondary alteration there is no radiometric evidence in the area studied for magmatism related to late cenozoic subduction nor to the pleistocene recent opening of the back arc bransfield rift

direct dating of tin tungsten mineralization of the piaotang tungsten deposit south china by 40ar 39ar progressive crushing abstract dating ore minerals is the most direct method to determine the mineralization age of a deposit but only a few ore minerals can be directly dated by traditional isotopic geochronometers the aim of this study is to investigate the possibility of direct dating of the ore minerals cassiterite and wolframite from the piaotang tungsten deposit using the 40ar 39ar stepwise crushing technique comparing the age to coexisting k rich muscovite dated by 40ar 39ar laser stepwise heating the cassiterite wolframite and muscovite samples were separated from four pieces of ore hand specimens the 40ar 39ar isochron ages of cassiterite and wolframite are concordant with ages of their coexisting muscovite indicating that cassiterite and wolframite are suitable ore minerals for directly dating the ore forming event by 40ar 39ar stepwise crushing

excess argon crustal fluids and apparent isochrons from crushing k feldspar using a combination ofin vacuo crushing and stepped heating the40ar39ar technique has been applied to two k feldspar bearing sedimentary rocks from the proterozoic of north china with a view to studying ancient fluids trapped in these rocks and their chronology correlations between natural ar isotopes and those produced by neutron irradiation permitted a clear distinction to be made between different components of natural argon crushing released cl correlated excess40ar and palaeoatmospheric36ar trapped in moderately saline fluid inclusions in quartz and possibly the k feldspar the very high40ar cl ratios of these fluids 2 7 10 4 and1 7 10 4 is interpreted as indicating a metamorphic source for the40ar more surprisingly crushing also released from the k feldspar a k correlated component with a very well defined40ar k ratio which for both samples corresponded formally to an apparent age comparable to or slightly less than the stratigraphic age in contrast stepped heating yielded40ar k ratios corresponding to significantly younger ages which are interpreted as low grade metamorphic ages the explanation of the k correlated component released on crushing and the significance if any of the distinct40ar k ratio is obscure although several effects involving microporosity and a combination of39ar recoil and diffusion are suggested there is a contribution of this component to the stepped heating release pattern which may have implications for attempts to use k feldspar as a40ar39ar thermochronometer

new constraints on deformation slip rate and timing of the most recent earthquake on the west tahoe dollar point fault lake tahoe basin california high resolution seismic compressed high intensity radar pulse chirp data and piston cores acquired in fallen leaf lake fll and lake tahoe provide new paleoseismic constraints on the west tahoe dollar point fault wtdpf the western most normal fault in the lake tahoe basin california paleoearthquake records along three sections of the wtdpf are investigated to determine the magnitude and recency of coseismic slip chirp profiles image vertically offset and folded strata along the southern and central sections that record deformation associated with the most recent event mre on the wtdpf three faults are imaged beneath fll and the maximum vertical offset observed across the primary trace of the wtdpf is 3 7 m coregis tered piston cores in fll recovered sediment and organic material above and below the mre horizon radiocarbon dating of organic material constrained the age of the mre to be between 3 6 and 4 9 k y b p with a preferred age of 4 1 4 5 k y b p in lake tahoe near rubicon point approximately 2 0 m of vertical offset is observed across the wtdpf based on nearby core data the timing of this offset occurred be tween 3 10 k y b p which is consistent with the mre age in fll offset of tioga aged glacial deposits provides a long term record of vertical deformation on the wtdpf since 13 14 k y b p yielding a slip rate of 0 4 0 8 mm yr in summary the slip rate and earthquake potential along the wtdpf is comparable to the nearby genoa fault making it the most active and potentially hazardous fault in the lake tahoe basin

longitudinal triangular ripples in the blake bahama basin abstract longitudinal triangular ripples developed in cohesive sediments were studied in the blake bahama basin depth 4988 m western north atlantic with a deeply towed instrument package and the bathyscaphe trieste ii the 15 cm high by 75 cm wide by 2 10 m long ripples with triangular cross section vary in orientation over several kilometers these longitudinal bed forms are constructed of coarse sandy muds 20 sand and are created by bottom current flow near the extremities of the field they appear to be buried suggesting that the ripple field is not active at the present time these ripples may be formed by short lived high velocity bottom currents and they may be commonly developed in the sediments of submarine canyons deep sea channels and deep sea areas episodically swept by strong currents

geothermometry from 40ar39ar dating experiments abstract we have selected a variety of minerals from three basic intrusions within the high grade metamorphic terrane near haliburton ontario for k ar dating by application of 40 ar 39 ar incremental heating experiments we conclude that the discordant k ar mineral dates can best be explained by a slow cooling history from the stepwise heating experiments we calculated isotopic closure temperatures using dodson s theory for slow cooling and from these constructed a cooling curve this curve was then compared to independent constraints to test the overall validity of our procedures and assumptions the results are geologically reasonable insofar as 1 a the maximum calculated temperatures from hornblendes of 700 c are consistent with independent approximate metamorphic grid arguments 2 b the ar closure temperatures for biotites 350 400 c are consistent with independent evidence and 3 c minerals with older k ar dates have higher calculated closure temperatures the resultant geological cooling curve implies that a maximum temperature of 700 c was reached before 1000 ma ago and thereafter the temperature decreased via changes in uplift and cooling rates to below 200 c by 700 ma ago we have discovered that plutonic plagioclases are very sensitive indicators comparable to k felds pars of ar loss and therefore can usefully extend the range of application of the 40 ar 39 ar stepwise heating method we also describe experiments in which we have tested the effects on the ar systematics of acid leaching of plagioclases

he ar h o isotopic signatures in auag bearing ore fluids of the sunshin epithermal gold silver ore deposits south korea abstract helium 3he 4he argon 40ar 36ar and oxygen 18o 16o isotopic ratios in fluid inclusions trapped in quartz and pyrite from the sunshin epithermal gold silver ore deposits in south korea were analyzed to investigate the magmatic contribution to ore forming fluids mantle helium with high 3he 4he ratios up to 3he 4he 9ra where ra 1 4 10 6 for air were identified from fluid inclusions in auriferous quartz veins which are characterized as low sulfidation adularia sericite type the ratios provide direct evidence of links between low sulfidation ore fluids in epithermal gold deposits and magma from a mantle source interestingly the oxygen and hydrogen isotopic compositions of the ore fluids 18o 1 9 to 6 6 d 82 to 100 calculated from ore quartz indicate that meteoric waters were predominant during precipitation of the au ag ore microthermometric measurements for fluid inclusions indicate relatively low temperatures 300 c for the formation of the low sulfidation epithermal ore deposits

classification of sedimentary furrows and a model for furrow initiation and evolution sedimentary furrows are longitudinal bed forms which form in fine grained cohesive sediments of the deep sea floor as well as in similar sediments in estuaries and in large lakes deep sea furrows have been observed where bottom currents often flow in one direction at 5 to 20 cm sec currents are much stronger greater than 50 cm sec in furrowed estuarine environments but sediment accumulation rates are higher a review of furrows studied in a variety of locations and environments suggests that these bed forms fall into two broad morphological classes those developed in muddy cohesive sediments tend to have steep walls and flat floors while those developed in carbonate rich cohesive sediments are broad and have gently sloping walls at abyssal depths furrows can often be detected by their distinctive echo patterns on 3 5 khz echo sounders a series of similar evenly spaced hyperbolic echoes which often have their apices tangent to the sea floor however different furrow cross sections may generate distinctive hyperbolic echo patterns a model for furrow initiation and development in muddy cohesive sediments suggests that furrows develop in depositional environments swept by recurring directionally stable and episodically strong currents furrows are initiated when secondary circulations in the boundary layer align coarse generally light debris in sand ribbon like bed forms as the coarse light sediments move along the sea floor they abrade small longitudinal troughs into the muds the grooves may fill in during intervals between strong current events if sedimentation rates are too high once a furrow is established on the sea floor further development may be controlled by the balance between sediment deposition and erosion where deposition occurs continuously and erosion occurs only episodically during brief periods of increased current strength narrow furrows width 1 5 to 1 15 of spacing appear to develop where deposition exceeds erosion whereas wide furrows width 1 2 of spacing develop where erosion exceeds or equals deposition dissolution may play an important role in the formation and evolution of furrows in carbonate rich cohesive sediments secondary circulation patterns over established furrows may be strongly controlled by bed form topography

40ar39ar dating of the quartz samples from two mineral deposits in western yunnan sw china by crushing in vacuum abstract the primary aim of this study is to investigate the possibility of dating fluid inclusions of quartz samples using the 40 ar 39 ar method by crushing in vacuo the samples are from the lushui tin tungsten deposit and the shangmanggang gold deposit in western yunnan sw china coexisting muscovite s 90ms and quartz s 90qw in a piece of tungsten ore from the former deposit were selected and the isochron age of s 90qw by crushing 38 6 1 9 ma shows excellent coincidence with that of s 90ms by stepped heating 38 6 0 8 ma the isochron age and plateau age by crushing of the quartz smg 8qz from the latter deposit indicate that the gold mineralization occurred at 144 ma these ages are respectively agreeable with the evaluated ages from the geologic facts in addition since the potassium concentration calculated from 39 ar released by crushing in the quartz sample smg 8qz was as high as 0 24 electron probe analysis was applied to investigate the occurrence of potassium and the results reveal that the majority of potassium scattered in the microcrystal quartz grains

abyssal currents and advection of resuspended sediment along the northeastern bermuda rise abstract an integrated study using current meters time series nephelometers and water samplers for measurements of particulate matter concentration revealed an intense nepheloid layer centered on the slopes of the eastward scarp the northeastern boundary of the bermuda rise these slopes also contained extensive furrows truncated seismic reflectors and wisconsin age and older surface sediments the abyssal circulation in this region appears to be accelerated when it encounters the steep topography of the eastward scarp eroding sediments creating the erosional furrows and moving eroded sediment upslope instruments on top of the bermuda rise recorded episodic events benthic storms where particle concentrations at 25 m above the bottom increased from a background level of 60 g 1 1 to maximum of 251 g 1 1 and current velocities were nearly 20 cm s 1 net transport of both water and entrained sediment was to the west northwest during the largest benthic storm water temperature decreased by 0 1 c consistent with a 300 600 m upslope transport of water and resuspended sediments from the upper slopes of the eastward scarp when currents carry resuspended sediment over the rim of the eastward scarp to the plateau region the current slows depositing its sediment load thus the plateau is growing upwards slowly at the expense of the surrounding scarps

brief duration of hydrothermal activity at round mountain nevada determined from ar 40 ar 39 geochronology the round mountain gold silver deposit is one of the world s largest volcanic hosted precious metal deposits reserves plus production total at least 16 million oz 500 000 kg au the deposit occurs in ash flow tuff that is ponded within its source caldera along and overlying the caldera ring fracture host rocks have undergone propylitic potassic and high level silicic and intermediate argillic alteration these are interpreted either as a temporal sequence from early propylitic to late silicic and argillic or as a spatial progression from a potassic core outward to a propylitic halo 40 ar 39 ar ages determined by single crystal fusion of sanidine from host tuff and incremental heating of adularia indicate a most likely duration of hydrothermal activity of approximately 0 1 m y possibly no more than 0 05 m y and a maximum of 0 5 m y eruption of host ash flow tuff and caldera collapse occurred at 26 5 ma a late or postalteration tuff that rests upon mineralized rock is 26 0 ma hydrothermal circulation is unlikely to have been established immediately after caldera collapse so duration was probably considerably less than 0 5 m y eight adularia samples give ages between 25 94 or 0 09 and 26 09 or 0 05 ma and one is 26 20 or 0 05 ma the mean of all nine is 26 03 or 0 08 ma the maximum and minimum values and their uncertainties allows a duration of as much as 0 4 m y but for the following reasons the longevity is probably much less the absolute ages of seven samples that can be placed in the possible temporal alteration sequence do not correlate with that sequence this does not disprove the temporal sequence but does indicate that alteration occurred in a time span which is not resolvable even with precise 40 ar 39 ar dating furthermore all samples show flat age spectra which themselves argue against a long continuously active system kinetic studies of two adularia reveal diffusion domains with ar closure temperatures between approximately 230 degrees and approximately 390 degrees c adularia that crystallized and remained at temperatures of 275 degrees c the maximum temperature inferred from fluid inclusion data would yield age spectra with age gradients spanning the lifetime of the system no spectra are consistent with a thermal history that incorporates sustained 0 1 m y temperatures at or above 275 degrees c episodic activity to account for the one older adularia date cannot be precluded but is inconsistent with most data therefore the hydrothermal system probably was active for less than the uncertainty in the ages of individual samples possibly as little as 50 000 years we conclude that hydrothermal activity at round mountain was a brief event at approximately 26 0 ma and was closely linked to caldera structure and magmatism this activity followed initial ash flow eruption by 0 5 m y and may have been driven by a ring fracture intrusion

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

the relative importance of gravity induced versus current controlled sedimentation during the quaternary along the mideast u s outer continental margin revealed by 3 5 khz echo character abstract a detailed echo character map based on more than 50 000 km of 3 5 khz echograms reveals the relative importance of contour currents turbidity currents and mass wasting processes for the upper 50 100 m of sea floor sediments along the u s continental margin between southern georges bank and cape hatteras nineteen individual echo types are mapped and interpreted as deposits produced either by current controlled or gravity induced processes based on echo type distribution analysis of selected sediment core samples and the published results of other workers turbidity currents and mass wasting have apparently been the primary depositional agents throughout 60 of the region in contrast contour currents alone appear to have influenced only 10 of the region however combined processes i e non separable interaction between downslope and parallel to contour processes have influenced an estimated 25 of the margin the remaining 5 of the study area constitutes the outermost continental shelf echo types here reflect the influence of fluvial deltaic and shallow marine sedimentary processes changes in the relative distributions of various echo types with sea floor depth indicate that the dominant sedimentary processes change downslope and are significantly different within each physiographic province deposits formed by mass wasting dominate the continental slope where 50 of the sea floor is eroded and gullied by slides slumps and the heads of debris flows deposits developed under the influence of combined processes are most wide spread on the upper continental rise and cover 40 of the sea floor primarily in the form of deep sea channel levees modified by contour currents on the lower continental rise turbidites form more than 50 of the sea floor deposits the downslope deposits turbidity currents slumps debris flows etc were emplaced primarily during the pleistocene glacioeustatic sea level lowering moved beaches and rivers across the exposed continental shelf to the shelf edge where massive fluxes of terrigenous sediment could be delivered to the continental slope and rise by turbidity currents significant amounts of this sediment were transported through submarine canyons and leveed deep sea channels to the lower continental rise and formed the lower continental rise terrace a flat turbidite basin ponded behind the hatteras outer ridge southwest of the wilmington channel this glacially derived influx was diminished however depositional patterns indicate that downslope processes continued to be dominant at least as far south as cape hatteras where rapid deposition on the continental slope and uppermost continental rise created instabilities slope failure produced slumps slides and debris flows and provided an additional volumetrically important redistribution of sediment downslope the influence of contour currents is clearly observed only in those areas topographically shielded from the masking effects of downslope processes in particular along the seaward side of the buried chesapeake drift and the hatteras outer ridge

high precision 40ar 39ar age of the gas emplacement into the songliao basin the problem of determining an exact isotopic age of hydrocarbon emplacement is complex because minerals suitable for dating with common isotopic methods are often lacking in the sedimentary domain however the igneous quartz from the cretaceous volcanic rocks that host the gas reservoir in the songliao basin northeastern china contains abundant secondary fluid inclusions with high concentrations of k and high partial pressures of methane trapped during gas emplacement quartz with abundant k rich fluid inclusions provides an excellent closed system well suited for 40 ar 39 ar dating three igneous quartz samples were measured by stepwise crushing to release the inclusion based argon gas all three samples yielded well defined isochrons with ages in close agreement precisely constraining the gas emplacement at 42 4 0 5 ma 2 below the daqing oil field in the songliao basin extending possible gas reservoirs from the upper cretaceous to the middle eocene

abyssal current influence on the southwest bermuda rise and surrounding region abstract echograms 3 5 khz and bottom photographs reveal that the northward flowing antarctic bottom water aabw has strongly influenced the modern depositional regime on the southwest bermuda rise the spatial distribution of echo character types the orientation and nature of current controlled structures and limited current meter data show that aabw flows with varying intensities along three primary pathways around and over the southwest bermuda rise the main core of aabw flows clockwise around the eastern and western flanks of the southern bermuda rise roughly parallel to the 5400 m isobath this current bifurcates at 28 30 n 69 w where a portion flows northeast over the southwest bermuda rise and the remainder continues north along the physiographic boundary between the southwest bermuda rise and the hatteras abyssal plain secondary ribbons of aabw branch off the main core of aabw during its southerly journey along the southeastern bermuda rise and flow west through fracture zones finally a diffuse northward flowing aabw sweeps the entire southwest bermuda rise a progression of current controlled bedforms occurs beneath the main path of the aabw reflecting the spatially varying current velocities and sediment supply the main core of aabw flows west through the narrow vema gap creating erosional furrows along the border between the southwest bermuda rise and the vema gap current velocities greater than 20 cm s 1 are inferred from the bedforms in this region farther north along the southwestern edge of the bermuda rise sediment waves become more prevalent this transition from erosional to more depositional bedforms results from diminished current velocities 5 15 cm s 1 and increased sediment supply although some of these bedforms on the southwest bermuda rise appear to be relict their orientation is consistent with current meter data and abyssal current direction inferred from bottom photographs

rough smooth basement boundary in the western north atlantic basin evidence for a seafloor spreading origin a distinctive change in the character of acoustic basement has been mapped within the mesozoic keathley magnetic anomaly sequence in the western north atlantic basin this boundary known as the rough smooth basement boundary separates relatively smooth acoustic basement on the older western side of the boundary from much rougher basement to the east south of bermuda the boundary is well defined and nearly isochronous corresponding to anomaly m 12 or m 13 128 m y b p north of bermuda the boundary is diachronous and more transitional with the change in basement relief occurring in a zone between anomaly m 21 and m 16 age crust 132 to 142 m y b p both the well defined and transitional changes in basement relief found in the western basin are also identifiable in seismic reflection records from the eastern canary basin and an analysis of magnetic anomaly patterns in the central north atlantic basin indicates that these changes occurred simultaneously on both sides of the ridge these results strongly suggest a primary seafloor spreading origin for the rough smooth basement boundary with the greater basement relief associated with slower spreading rates changing directions of relative plate motion and ridge jumps of isolated accretionary segments beginning in late jurassic early cretaceous time

visions of ice sheets in a greenhouse world abstract backstripped eustatic estimates from new jersey and the russian platform show large 25 m and rapid 18 o record highlights the enigma this was the warmest interval of the past 200 my yet it was bracketed by two inferred eustatic falls of 25 m that were associated with two large 0 75 deep sea 18 o increases 92 93 ma mid turonian and 96 ma mid cenomanian we reconcile records of warm high latitudes with glacioeustasy by proposing that late cretaceous early eocene ice sheets generally reached maximum volumes of 8 12 10 6 km 3 20 30 m glacioeustatic equivalent but did not reach the antarctic coast hence coastal antarctica hence deep water remained relatively warm even though there were significant changes in sea level as the result of glaciation unlike the oligocene and younger icehouse world these ice sheets only existed during short intervals of peak milankovitch insolation leaving antarctica ice free during much of the greenhouse late cretaceous to middle eocene these results highlight the need to re evaluate the paradigm that polar ice sheets did not exist during times of warm high latitude climates

ocean nutrient distribution and oxygenation limits on the formation of warm saline bottom water over the past 91 m y it has been proposed that deep water formation in the oceans would be quite different during geologic intervals with reduced equator to pole temperature gradients salinity rather than temperature differences might drive the deep ocean circulation saline water would tend to form at subtropical latitudes where evaporation exceeds precipitation we point out a likely consequence of warm saline bottom water formation on ocean chemistry the tendency to drive the ocean toward anoxia this effect is not due to the previously noted lower oxygen solubility with increasing ocean temperature but rather to the increased efficiency with which plankton will extract nutrients from convecting waters at low latitudes the present ocean nutrient content is sufficient to induce deep water anoxia if circulation were to change yet anoxia is rare in the paleoceanographic record a simple ocean chemical model makes explicit the tradeoffs between mean ocean nutrient content and circulation parameters that will satisfy the geologic observations of an oxygenated ocean since the mid cretaceous barring decreases of ocean phosphate on the order of 30 50 deep water formation at high latitudes was a major source of ocean ventilation in the warmer past

recent sedimentary processes in the prestige site area galicia bank nw iberian margin evidenced by high resolution marine geophysical methods an echo character analysis of the oil tanker prestige wreck area was undertaken using high resolution marine geophysical methods topas and airgun seismic reflection profiles multibeam echosounder and tobi sidescan sonar integration and comparison of the results using all these methods is presented given some practical applications for indirect near surface and seafloor interpretations ten different echo types were identified and grouped into four main classes i distinct ii indistinct iii irregular and iv hyperbolic echo character distribution enabled us to determine recent sedimentary processes in the area two major depositional systems can be found through the identification of these sedimentary processes a a slope depositional system sds located in the eastern and central area where mass movement processes are dominant and b a turbiditic main channel depositional system tds located in the western area where channel related processes are dominant both of these interact over the half graben structure of the southwestern sector of the galicia bank where the prestige wreck is located within the sds erosive and depositional mass movement processes characterised a complex depositional system erosive processes occur on the fault scarp channels head inter lobe channels and distal part of the sedimentary lobes moreover depositional processes take place on the top of the fault scarp sedimentary wedges sedimentary lobes and on the west flank of the main channel both depositional systems interact but the sds should be more active during fault scarp reactivation periods through relief rejuvenation and new exposed deposits microearthquake activity would favour the available materials being weathered eroded and transported by mass movements in such a situation the tds acts as the main collector of eroded sediment derived from the fault scarp throughout the sds being responsible for its evacuation into the iberian abyssal plain however outside of fault reactivation periods the sds is less active such as during the present situation

revealing mineralization and subsequent hydrothermal events insights from 40ar 39ar isochron and novel gas mixing lines of hydrothermal quartzs by progressive crushing abstract increasingly more data have indicated that the 40ar 39ar crushing technique provides a very useful approach to determine the mineralization ages of hydrothermal ore deposits in this study novel gas mixing lines are proposed based on the data of two quartz samples by 40ar 39ar crushing electron microprobe analyses indicate that the quartz grains contain k rich silicate microlites both quartzs by crushing yield two mixing lines with ages corresponding respectively to those of k feldspar or sericite post mineralization fluids and muscovite samples ore forming fluids this is the first report that both ages of secondary and primary fluid inclusions sfis and pfis are confirmed correspondingly by those of the k rich minerals this study strongly indicates that the 40ar 39ar crushing technique is very useful to determine the ages of geofluid activities the newly proposed gas mixing patterns are shown on the inverse and normal isochron diagrams indicating 1 the mixtures are in different proportions of the pfis and sfis in different crushing steps 2 the gas mixing lines yield pfi ages with low initial 40ar 36ar ratios 3 an intermediate age between pfi and sfi ages cannot be obtained because the progressive crushing technique never release gas mixtures with a constant pfi sfi proportion for each step the gas mixing lines are thus interpreted to yield significant geological ages the mixing patterns will help us better understand the 40ar 39ar dating results and be used for references to other isotopic mixing isochron systems

geophysical evidence for widespread cenozoic bottom current activity from the continental margin of nova scotia canada abstract two dimensional and three dimensional multichannel seismic reflection data are utilized to examine previously unrecognized contourite depositional systems along the continental margin of nova scotia canada prior to this study these features were thought to be of limited extent in the study area the new data show that contourite drifts are widespread with greatest drift development during the late miocene to pliocene giant sediment waves form stacked aggrading sequences of bedforms and their development is linked to pre existing seafloor morphology small sediment drifts developed locally throughout the late paleogene and neogene either southwest and down current of seafloor obstacles or form channel fills major erosional pulses form regional seismic markers first along the continental rise in the early oligocene then along the continental slope during the late miocene and pliocene three dimensional seismic data show that erosion surfaces preserve along slope amplitude anomalies two dimensional low relief sediment waves and barchanoid bedforms the geophysical evidence implies that a northeast to southwest along slope bottom current influenced cenozoic depositional patterns throughout the study area

40 ar 39 ar thermochronology using alkali feldspars real thermal history or mathematical mirage of microtexture the multiple diffusion domain mdd theory of ar loss based on the 40ar 39ar method has been used to obtain thermal histories over tens of millions of years in the range 500 c to near surface temperatures of alkali feldspars from slowly cooled geological environments and recovered in situ from sedimentary basins and geothermal systems it assumes that ar diffuses in nature over geological time from the same domains and by the same processes that control its diffusion during laboratory step heating experiments mathematically the model has many stringent requirements the domains must form at t above those being modelled and must remain unchanged during geological time and during step heating must not interact must occur in discrete size ranges sharing one simple geometrical shape contain no compositional gradients in 40k and release ar into an infinite reservoir we describe the nature of possible diffusion domains and the fast pathways for diffusion between them known to exist in alkali feldspars from direct electron microscopic observations and explore their time temperature evolution based on established phase behaviour both over geological time and during step heating strain controlled microtextures whose morphology is controlled by the minimization of coherency strain energy do not provide fast pathways for ar loss and behave as crystallographically complex but single domains for ar subregular dislocations may form over a range of t in some strain controlled intergrowths but in nature the majority form closed loops and do not provide pathways to the crystal surface deuteric microtextures cut across strain controlled microtextures and are composed of microporous subgrain and grain mosaics with many pathways and a wide spectrum of possible domain sizes and shapes the deuterically altered zones provide pathways for ar diffusion out of the crystal although some ar is retained in fluid inclusions all microtextures likely to define diffusion domains form below 500 c over a range of t and in slowly cooled rocks of times the most important involve fluid feldspar reactions which can reset ar and continue to surface t feldspars in geothermal systems and young sedimentary basins are subject to ongoing chemical and microtextural changes because the microtextures are metastable during step heating experiments in the t range over which most ar is released domains and pathways for rapid diffusion of ar between domains will be significantly modified during step heating particularly above 750 c when departures from a simple arrhenius diffusion law are most marked pre melting is likely to be an important cause of complexity in ar loss behaviour few if any of the requirements of the mdd model are met and we conclude that thermal histories cannot be obtained from alkali feldspars using this method

seafloor zonation in sediment texture on the nova scotian lower continental rise abstract grain size analyses of surface sediment samples from the continental rise below 4000 m off nova scotia clearly show a slope parallel zonation in textural parameters zones of sediment with high sand content 20 are located between 4800 and 4900 m and below 5000 m a zone rich in clay 40 occurs between 4000 and 4800 m the silt clay ratio increases steadily with depth between 4000 and 5100 m these textural trends suggest predominantly slope parallel sedimentary processes on the modern sea floor and conversely little evidence of downslope processes the textural zonations are thought to result from a zonation of bottom currents which are weak at 3500 m but increase regularly to peak speeds greater than 50 cm s 1 on the lower rise at 5000 m the mean position of the northern edge of the gulf stream lies across the southern part of the continental rise over the 4600 to 4900 m depth contours the clay rich band lies upslope from the gulf stream edge beneath a weak to moderate mean abyssal flow toward the southwest in contrast the sand rich zone lies beneath the region of stronger mean southwest flow and beneath the cold filament mapped by weatherly and kelly 1982 this region is also intermittently occupied by the gulf stream and episodic reinforcement of flow in the mean southwest current by bottom sensing eddies from the gulf stream probably creates intermittent high velocity currents that account for the coarse surface texture

early eocene palaeoceanography and palaeoclimatology of the eastern north atlantic stable isotope results for dsdp hole 550

ocean and climate response to north atlantic seaway changes at the onset of long term eocene cooling abstract around the early middle eocene boundary the first occurrence of contourite drift sediments and widespread deep ocean erosion indicate changes in the north atlantic ocean circulation interestingly these changes coincide with the first steps of cenozoic cooling from the paleogene greenhouse climate towards the modern icehouse the cause for this ocean circulation reorganization is poorly understood since modern water mass tracers may have worked fundamentally different in the past and the paleoceanographic proxy record is limited in both time and space as a result it is challenging to reliably reconstruct the climatic and tectonic boundary conditions e g atmospheric greenhouse gas concentration and the depth and geometry of developing and closing passages between ocean basins in this study we attempt to identify thresholds in tectonic gateway passages and atmospheric co2 concentration using the fully coupled earth system model cosmos indeed the simulation of earth s past climates can unravel the physical processes driving deep water formation in a greenhouse world specifically we use cosmos to evaluate the impact of changes in the north atlantic gateways at the early middle eocene boundary on the north atlantic deep western boundary currents under low obliquity configuration we find that northern component waters start to form when the greenland scotland ridge reaches a threshold depth of deeper than 200 m while the arctic ocean is still shut off from the north atlantic in this scenario the relatively deep greenland scotland ridge allows for sufficient inflow of warm salty atlantic surface waters into the nordic seas to initiate convection during winter cooling opening the seaway towards the arctic leads to a cessation of northern component water formation as it allows for inflow of brackish surface waters into the northern nordic seas hindering northern component water formation

deep water temperature carbonate ion and ice volume changes across the eocene oligocene climate transition 1 paired benthic foraminiferal stable isotope and mg ca data are used to estimate bottom water temperature bwt and ice volume changes associated with the eocene oligocene transition eot the largest global climate event of the past 50 myr we utilized odp sites 1090 and 1265 in the south atlantic to assess seawater 18o w antarctic ice volume and sea level changes across the eot 33 8 33 54 ma we also use benthic 13c data to reconstruct the sources of the deep water masses in this region during the eot our data together with previously published records indicate that a pulse of northern component water influenced the south atlantic immediately prior to and following the eot benthic 18o records show a 0 5 increase at 33 8 ma eot 1 that represents a 2 c cooling and a small 10 m eustatic fall that is followed by a 1 0 increase associated with oi 1 the expected cooling of deep waters at oi 1 33 54 ma is not apparent in our mg ca records we suggest the cooling is masked by coeval changes in the carbonate saturation state co32 which affect the mg ca data to account for this the bwt ice volume and w estimates are corrected for a change in the co32 of deep waters on the basis of recently published work corrected bwt at sites 1090 and 1265 show a 1 5 c cooling coincident with oi 1 and an average w increase of 0 75 the increase in ice volume during oi 1 resulted in a 70 m drop in global sea level and the development of an antarctic ice sheet that was near modern size or slightly larger

warm saline bottom water in the ancient ocean considerable isotopic evidence indicates that oceanic bottom water was much warmer in the geological past a simple convection model driven by multiple turbulent buoyant plumes and observations of present deep water formation have led to the development of a theory for the formation of warm saline bottom water it is suggested that changes in the size and configuration of marginal seas in net evaporation zones due to lithospheric plate motions and eustatic sea level change caused these seas to become sources of warm saline bottom water evidence from the palaeotemperature record and palaeogeography during the late cretaceous is used to reinforce the hypothesis climatic and chemical consequences of the formation of warm saline bottom water are discussed

hydrothermal fluids argon isotopes and mineralization ages of the fankou pb zn deposit in south china insights from sphalerite 40ar 39ar progressive crushing abstract hydrothermal fluid geochemistry and mineralization timing are two important factors in the genesis of a hydrothermal deposit 40 ar 39 ar analyses of fluid inclusions not only provide time constraints for the mineralization but also help to clarify the k ca cl ar characteristics for the ore forming fluids in this study six sphalerite samples collected from the fankou lead zinc sulfide deposit are investigated by 40 ar 39 ar in vacuo crushing gases liberated from the early and late crushing steps exhibit distinct ar isotopic compositions and 40 ar 39 ar apparent ages argon released in the early steps has much higher 40 ar and 38 ar cl and lower 37 ar ca contents than those in the late steps the significant excess ar 40 ar e extracted in the early crushing steps shows a strong correlation with 38 ar cl with very high 40 ar e 38 ar cl ratios in contrast those of the late steps mainly consist of atmospheric ar and k correlated radiogenic ar with a constant 40 ar r 39 ar k ratio and the atmospheric initial 40 ar 36 ar ratio as a result all samples yield similar declining age spectra the fore segments with anomalously old apparent ages decline quickly in the early crushing steps and the rear ones are flat with concordant apparent ages in the late crushing steps the data points of the early steps define linear correlations in plots of 40 ar na 39 ar k vs 38 ar cl 39 ar k and 38 ar cl 40 ar na vs 39 ar k 40 ar na na for non atmospheric and most yield ages of 240 230 ma on the other hand the data of the late steps always construct well defined isochrons in the plots of 36 ar a 40 ar na vs 39 ar k 40 ar na with consistent ages of 300 ma we interpret that gases released in the early steps were from the secondary fluid inclusions sfis due to their distribution characteristics along cracks leading to be easily extracted and those released in the later steps represented the contribution of the primary fluid inclusions pfis the initial 40 ar 36 ar ratios of sfis much higher than the modern atmospheric ratio suggest that their fluids were most likely from deep sources however these ratios of pfis close to the modern atmospheric ratio might imply that the ore forming fluids were mainly meteoric accordingly our results show that a major lead zinc mineralization occurred at about 300 ma and a subsequent hydrothermal interruption at 240 230 ma the sphalerite 40 ar 39 ar crushing results suggest that the ore forming fluid might strongly interact with the sedimentary rocks resulting in the enrichment of ore deposit

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

the high velocity core of the western boundary undercurrent at the base of the u s continental rise the western boundary undercurrent is a high velocity contour following bottom current that flows southwesterly on the u s atlantic continental margin a high velocity core of the western boundary undercurrent is delineated by an analysis of underlying sediments which are characterized by coarse particle sizes and efficiently aligned magnetic grains in a zone from 4440 meters at the base of the rise to 5200 meters on the adjacent abyssal plain

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

paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing abstract the 40 ar 39 ar stepwise crushing technique is applied for the first time to date garnet from ultra high pressure metamorphic uhpm eclogites three garnet samples from the bixiling eclogites analyzed by 40 ar 39 ar stepwise crushing yield regular predictable age spectra and a clear separation between excess 40 ar and concordant plateau and isochron ages all three age spectra begin with high apparent ages followed by step by step decreasing ages and finally age plateaux with apparent ages in the range from 427 20 to 444 10 ma the data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 34 to 459 58 ma corresponding to initial 40 ar 36 ar ratios from 292 1 4 5 to 294 5 6 7 statistically indistinguishable from the modern air the high initial ages are interpreted to derive from secondary fluid inclusions containing excess 40 ar whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess 40 ar the plateau ages are interpreted to approximate the time of garnet growth during initial uhpm metamorphism phengite analyzed by laser stepwise heating yielded a complicated two saddle age spectrum with a scattered isochron corresponding to age of 463 116 ma and initial 40 ar 36 ar ratio of 1843 1740 indicative of the presence of extraneous 40 ar within phengite these concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that dabie uhpm eclogites were first formed in the early paleozoic during the same event that caused the qinling northern qaidam basin altyn tagh eclogites

modelling the effect of arbitrary p t t histories on argon diffusion in minerals using the macargon program for the apple macintosh argon diffusion in mineral grains has been numerically modelled using p t t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the earth s crust for long durations the macargon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion it can be run on an apple macintosh computer with arbitrary p t t histories used as input finite difference equations are used in the calculation of 40ar concentration profiles across individual diffusion domains the associated macspectrometer generates model spectra after a p t t history has been specified the form of model 40ar39ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40ar39ar apparent age spectra particularly in cases involving metamorphic rocks where complex p t t histories might apply although modelled spectra cannot be directly compared to experimentally determined 40ar39ar age spectra especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion they do provide insight into theoretically expected age spectra for samples that have experienced complex p t t histories

heterogeneity enhanced gas phase formation in shallow aquifers during leakage of co2 saturated water from geologic sequestration sites a primary concern for geologic carbon storage is the potential for leakage of stored carbon dioxide co2 into the shallow subsurface where it could degrade the quality of groundwater and surface water in order to predict and mitigate the potentially negative impacts of co2 leakage it is important to understand the physical processes that co2 will undergo as it moves through naturally heterogeneous porous media formations previous studies have shown that heterogeneity can enhance the evolution of gas phase co2 in some cases but the conditions under which this occurs have not yet been quantitatively defined nor tested through laboratory experiments this study quantitatively investigates the effects of geologic heterogeneity on the process of gas phase co2 evolution in shallow aquifers through an extensive set of experiments conducted in a column that was packed with layers of various test sands soil moisture sensors were utilized to observe the formation of gas phase near the porous media interfaces results indicate that the conditions under which heterogeneity controls gas phase evolution can be successfully predicted through analysis of simple parameters including the dissolved co2 concentration in the flowing water the distance between the heterogeneity and the leakage location and some fundamental properties of the porous media results also show that interfaces where a less permeable material overlies a more permeable material affect gas phase evolution more significantly than interfaces with the opposite layering

seismic stratigraphy of the northern bermuda rise abstract analysis of low frequency seismic profiles from the northern bermuda rise has led to the recognition of several seismic horizons and determination of a regional seismic stratigraphic framework horizon a is the deepest regionally significant reflector identified and horizon a c a t and a v of the horizon a complex are also mappable reflectors in the study area three additional regionally significant reflectors named yellow purple and silver have been identified in the area these seven reflectors allow speculation on the sedimentary evolution of the area sedimentation below the horizon a complex was primarily pelagic the distribution of reflectors within the horizon a complex indicates the area experienced turbidite deposition from both the u s east coast continental margin and nearby bermuda as well as pelagic deposition during the eocene sedimentation subsequently has been dominated by at least two distinct periods of abyssal current flow initially bottom currents brought in sediments from the north or west creating an anomalously thick accumulation of seismically poorly stratified sediments on the rise this period of accumulation was punctuated by an episode of erosion in latest miocene or earliest pliocene the most recent sediments are seismically well stratified sediments of primarily quaternary age carried from the east by the gulf stream system

the source and fate of massive carbon input during the latest paleocene thermal maximum lithologic faunal seismic and isotopic evidence from the blake nose subtropical western north atlantic links a massive release of biogenic methane 55 5 million years ago to a warming of deep ocean and high latitude surface waters a large perturbation in the combined ocean atmosphere carbon cycle the largest of the past 90 million years a mass extinction event in benthic faunas and a radiation of mammalian orders the deposition of a mud clast interval and seismic evidence for slope disturbance are associated with intermediate water warming massive carbon input to the global exogenic carbon cycle pelagic carbonate dissolution a decrease in dissolved oxygen and a benthic foraminiferal extinction event these events provide evidence to confirm the gas hydrate dissociation hypothesis and identify the blake nose as a site of methane release

steps toward dating early diagenetic k feldspar by the 40ar 39ar method abstract authigenic sedimentary low temperature k feldspar separated from albian turonian carbonates in israel was dated by the step heating 40ar 39ar method in contrast to high temperature k feldspars ca 90 of the radiogenic ar was released at temperatures lower than 600 c and fusion mostly occurred below 750 c though formed under low temperature ar loss in all but one sample is estimated to be less than 2 nevertheless the effect of 39ar recoil is evident in some separates probably due to their very fine size 4 10 m the plateau age of one sample with the highest content of authigenic k feldspar 93 96 ma is slightly younger than the assumed stratigraphic age and thus defines the timing of an early diagenetic event within a few million years after deposition other samples where a plateau age could be determined yield ages slightly older strong acid etching 7 n hno3 85 c of the k feldspar enriched fraction may have improved the 39ar 40ar spectra but did not eliminate the presence of minute quantities of detrital k bearing minerals though the present set of data is insufficient for a clear cut conclusion the activation energy of the authigenic k feldspar is in the range of 14 26 kcal mol 1 s 1 much lower than that of magmatic and hydrothermal k feldspars and can probably be attributed to the heating schedule and the small crystal size it is not clear whether the formation at low temperatures

a silicon depleted north atlantic since the palaeogene evidence from sponge and radiolarian silicon isotopes despite being one of earth s major geochemical cycles the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica dsi concentration through geologic time remain poorly constrained silicon isotope ratios expressed as 30si in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling in particular the 30si of siliceous sponge spicules has been demonstrated to be a useful proxy for past dsi concentrations we analysed 30si in radiolarian tests and sponge spicules from the blake nose palaeoceanographic transect odp leg 171b spanning the palaeocene eocene ca 60 30 ma our 30si results range from 0 32 to 1 67 and 0 48 to 0 63 for the radiolarian and sponge records respectively using an established relationship between ambient dissolved si dsi concentrations and the magnitude of silicon isotope fractionation in siliceous sponges we demonstrate that the western north atlantic was dsi deplete during the palaeocene eocene throughout the water column a conclusion that is robust to a range of assumptions and uncertainties these data can constitute constraints on reconstructions of past ocean circulation previous work has suggested ocean dsi concentrations were higher than modern ocean concentrations prior to the cenozoic and has posited a drawdown during the early palaeogene due to the evolutionary expansion of diatoms our results challenge such an interpretation we suggest here that if such a global decrease in oceanic dsi concentrations occurred it must predate 60 ma

a review of methane and gas hydrates in the dynamic stratified system of the blake ridge region offshore southeastern north america abstract the blake ridge region lies on the passive margin of southeastern north america and contains a large amount of methane gas hydrate the methane and methane gas hydrate are predominantly biogenic apparently produced by co2 reduction reflection seismics indicate that bottom simulating reflectors bsrs enclose 55 000 km2 with high amplitude bsrs covering 26 000 km2 ocean drilling program odp leg 164 drilled three deep holes on a 10 km long transect sites 994 995 and 997 water depth 2770 2798 m based on sampling and geochemical thermal seismic and borehole geophysical measurements gas hydrates are most likely present between 190 and 450 m in sediment column gas hydrate is most often disseminated throughout the sediment column although concentrations occur within specific sedimentary horizons within supposed fault zones and at the base of gas hydrate stability bghs where methane recycling produces more pervasive concentrations of gas hydrate estimates of gas hydrate inventory are based on a variety of methods including geochemical proxies vertical seismic profiling electric logging and measurements of in situ methane over the entire sediment column at least 2 4 of pore space volume 1 2 sediment volume is occupied by methane gas hydrates but average and maximum estimates are 5 4 and 12 respectively extrapolation of vertical gas hydrate and methane inventory over the area containing high amplitude bsrs yields estimates of 67 406 gt gigatons 1015 g of methane gas hydrate or 9 52 gt of methane and 2 6 27 gt of methane occurring as gas bubbles below the bghs average values are 185 gt of gas hydrate and 24 gt of methane as gas hydrate any gas hydrate occurring outside the area underlain by bsrs as suggested by geochemical evidence or that associated with low amplitude bsrs may increase these estimates by an unknown factor various data give conflicting pictures of mass transport with blake ridge sediments the data can be reconciled by viewing the upper sedimentary section accumulation of gas hydrate in the blake ridge sediments depends on the amount of methane leaving the system versus the amount of methane entering the gas hydrate stability zone ghsz over geologic time although there are some point sources of methane loss from the sediments e g seafloor seeps odp site 996 of unknown magnitude most of the documented methane loss occurs through diffusion and consumption at the sulfate methane interface smi by anaerobic methane oxidation amo 2 108 mol year 1 methane entering the ghsz at a rate of 1 3 109 mol year 1 indicates a methane trapping efficiency of 85 129i measurements suggest that the blake ridge system has accumulated gas hydrate over as much as 55 million years

a subdivision of the precambrian of china abstract precambrian rocks are widely distributed in china the precambrian is divided into two time units i e the archaean and proterozoic eon each of these is separated into three chronological intervals also with the status of eras with the prefixes early middle or late the time boundary between the archaean and proterozoic eon is placed at 2500 ma according to the present isotopic data the proposed subdivision for the archaean of china is two fold the age of the fuping group is younger than 2800 2900 ma and that of the qianxi group and the corresponding stratigraphic units of eastern liaoning are older than 2800 ma so that 2800 ma is selected as the boundary between the early middle and late archaean based on the representative stratigraphic units the wutai and huto groups and an intervening major unconformity formed by the wutaiian orogeny at 2200 2300 ma the early proterozoic is further divided into two periods with a time demarcation at 2200 ma a major episode of orogeny known as the luliangian movement occurred at the end of the early proterozoic at 1900 ma this disturbance was very extensive and is in a way responsible for the difference in geological conditions between the lower and middle upper proterozoic in china the boundary 1900 ma that relates to the luliangian movement is more important than the boundary corresponding to the age of 1600 ma which is recommended as the time boundary between proterozoic i and ii so we propose to use 1900 ma as the boundary between the early and middle proterozoic in china the time boundary between the middle proterozoic including the changcheng system and the jixian system and the late proterozoic which is composed of the qingbaikou and sinian systems is 1000 ma the age for the boundary between cambrian and precambrian based upon the recent isochron data is inferred to be 610 ma

hydraulic testing of low permeability silurian and ordovician strata michigan basin southwestern ontario summary straddle packer hydraulic testing was performed in 31 silurian intervals and 66 ordovician intervals in six deep boreholes at the bruce nuclear site located near tiverton ontario as part of site characterization activities for a proposed deep geologic repository dgr for low and intermediate level radioactive waste the straddle packer assembly incorporated a hydraulic piston to initiate in situ pulse tests within low hydraulic conductivity horizontal hydraulic conductivities of the silurian test intervals range from approximately 4e 14 to 4e 8 m s the average horizontal hydraulic conductivities of the ordovician intervals range from 2e 16 to 2e 10 m s the lower member of the cobourg formation the proposed host formation of the dgr between 660 and 688 meters below ground surface was found to have a horizontal hydraulic conductivity of 4e 15 to 3e 14 m s the formation pressures inferred from the hydraulic testing confirmed by long term monitoring show that the upper ordovician and middle ordovician trenton group are significantly underpressured relative to a density compensated hydrostatic condition and relative to the overlying silurian strata and underlying black river group and cambrian strata these underpressures could not persist if hydraulic conductivities were not as low as those measured