cenozoic ice volume history development of a composite oxygen isotope record to view cenozoic ice volume and ocean temperature history a composite 18 o time series was constructed for tropical shallow dwelling planktonic foraminifers and for deep water benthic foraminifers we conclude that cenozoic benthic 18 o primarily reflect considerable deep ocean temperature variation and that tropical planktonic 18 o afford the best record of cenozoic ice volume the planktonic composite indicates the presence of a significant ice budget for the past 40 m y the difference between the benthic and planktonic 18 o composites suggests that low latitude production of warm saline bottom water dominated tertiary thermohaline circulation

stratigraphic response of a carbonate platform to relative sea level changes broken ridge southeast indian ocean in contrast to terrigenous margins the stratigraphic response of carbonate platforms to relative sea level changes is influenced by carbonate productivity dissolution and diagenesis using seismic reflection and ocean drilling program odp drilling data from broken ridge we assess the importance of these processes in controlling the stratigraphic response of a carbonate platform to relative sea level changes prior to middle eocene rifting broken ridge and kerguelen heard plateau constituted a large carbonate platform odp drilling data from the kerguelen heard plateau leg 120 and broken ridge leg 121 reveal that the carbonate platform had two major episodes of tectonic uplift the early maastrichtian uplift was confined to the southern section of the kerguelen eard plateau sites 747 748 and gently tilted the platform toward the north the middle eocene rift induced uplift affected both broken ridge and the northern section of the kerguelen heard plateau sites 752 755 watergun seismic reflection and drilling data indicate the following stratigraphic response of the platform to relative sea level changes 1 prograding clinoforms consisting of carbonate sediments were deposited during a long term relative sea level rise submergence of the uplifted parts of the platform southern kerguelen heard plateau increased the area available for productivity allowing the carbonate sediment to prograde northward into the basin during the rise 2 onlapping sequences consisting of carbonate detritus were deposited in the basin during the middle eocene relative sea level fall the carbonate strata on broken ridge were uplifted to at least wavebase and eroded the detritus was transported northward toward the basin and onlaps the preexisting strata the approx mate volume balance between the sediment eroded from the crest of broken ridge and the onlapping sediments deposited along the northern slope suggests that mechanical erosion and transportation of carbonate sediment into the deep sea were important processes controlling the stratigraphy at broken ridge during the relative sea level fall we propose that the stratagraphic response of broken ridge to the middle eocene relative sea level fall is consistent with stratigraphic predictions based on terrigenous margins however during a relative sea level rise biogenic carbonate production may allow carbonate sediment to prograde basinward despite the fact that water depths are increasing stratigraphic models based on terrigenous margins do not account for the increase in sediment supply along carbonate margins during a relative sea level rise and thus require minor modification in order to predict the development of the prograding clinoforms

the record of late pleistocene biogenic sedimentation in the eastern tropical pacific ocean we have generated 300 kyr records of biogenic opal calcite and organic carbon corg for three cores in the eastern and central equatorial pacific ocean and have compared the records to determine whether common periods of biogenic sedimentation have occurred throughout the region we find that corg has been deposited in common pulses throughout the area while opal has a much more local pattern of variation calcite varies regionally but the record is shaped by superimposed dissolution and productivity processes the most intense corg peak occurs at 18 ka and can have greater than 2 times the holocene corg content other major corg peaks occur 150 ka and perhaps at 280 ka we have compared the corg record in one of the cores v19 28 to a model deepwater oxygen record developed from 13c data in the nearby v19 30 to test whether the corg record has been mostly shaped by degradation or by the rain of organic matter from the euphotic zone we found no coherence between the two records implying that the corg record is primarily a measure of productivity by comparing the opal calcite and corg records in v19 28 a core which is at or above the lysocline we found that both increased calcite and opal deposition matches high corg accumulation we also found however that the calcite and opal records were uncorrelated so that episodes of high opal deposition do not necessarily accumulate calcite rapidly we hypothesize that at least two different plankton communities have been dominant in the waters above this site one rich in opal secreting plankton and one more dominated by calcite producers the opal rich plankton community was dominant during the intervals 10 15 ka and 35 60 ka supplement data table 2 is available with entire articleon microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document p88001 2 50 payment must accompany order

breakup between australia and antarctica a brief review in the light of new data abstract the arguments justifying the revised timing of breakup between australia and antarctica cande and mutter 1982 and the reconstruction of broken ridge and kerguelen plateau mutter and cande 1983 are reviewed and considered with respect to new subsidence data the age of breakup was revised from anomaly 22 time 55 my b p to anomaly 34 time 85 my b p the rough topography of the diamantina zone can be attributed to very slow spreading 5 mm yr beginning between the times of anomaly 34 and anomaly 19 the reconstruction of broken ridge and kerguelen plateau at anomaly 34 time shows overlap of these two features but the overlap problem is nearly resolved by anomaly 18 time 42 my b p normal seafloor spreading rates 22 mm yr commenced at anomaly 19 time 43 my b p subsidence patterns calculated from biostratigraphic data from wells drilled along australia s southern margin are interpreted as more consistent with the revised age of australia antarctic breakup subsidence curves systematically show rapid subsidence associated with the rift phase of margin development followed by much slower thermally controlled subsidence during the drift phase the timing of the rift to drift transition is believed to coincide with the age of breakup 60 to 110 my b p in addition the subsidence curves indicate a west to east propagation of breakup along the southern margin magnetic anomaly patterns and stratigraphie observations are consistent with this hypothesis

paleoceanography of the eastern indian ocean from odp leg 121 drilling on broken ridge broken ridge in the eastern indian ocean is overlain by about 1 600 m of middle cretaceous to pleistocene tuffaceous and carbonate sediments that record the oceanographic history of southern hemisphere mid to high latitude regions prior to about 42 ma broken ridge formed the northern part of the broad kerguelen broken ridge plateau during the middle eocene this feature was split by the newly forming southeast indian ocean ridge since then broken ridge has drifted north from about 55 to 31 s the lower part of the sedimentary section is characterized by turonian to santonian tuffs that contain abundant glauconite and some carbonate the tuffs record a large but apparently local volcanic input that characterized the central part of broken ridge into the early tertiary maestrichtian shallow water several hundred to 1 000 m depth limestones and cherts accumulated at some of the highest rates ever documented from the open ocean 4 to 5 g cm 2 10 3 yr 1 a complete with all biostratigraphic zones cretaceous tertiary boundary section was recovered from site 752 the first 1 5 m y of the tertiary is characterized by an order of magnitude reduction in the flux of biogenic sediments indicating a period of sharply reduced biological productivity at 55 s following which the carbonate and silica sedimentation rates almost reach the previous high values of the latest cretaceous we recovered a complete section through the paleocene that contains all major fossil groups and is more than 300 m thick perhaps the best pelagic paleocene section encountered in ocean drilling about 42 ma broken ridge was uplifted 2 500 m in response to the intra plateau rifting event subsequent erosion and deposition has resulted in a prominent eocene angular unconformity atop the ridge an oligocene disconformity characterized by a widespread pebble layer probably represents the 30 ma sea level fall the neogene pelagic ooze on broken ridge has been winnowed and thus its grain size provides a direct physical record of the energy of the southern hemisphere drift current in the indian ocean for the past 30 m y

paleoceanography of the cretaceous tertiary boundary event inferences from stable isotopic and other data we report the results of new stable oxygen and carbon isotope analyses on carbonate fine fraction whole rock and benthic foraminifers caco3 and coarse fraction percentage determinations and trace element sr analyses on carbonate constituents across the cretaceous tertiary k t boundary in deep sea drilling project dsdp sites 47 2 356 384 and 577 and compare them with published results from k t boundary sections in other dsdp sites we used the trace element data and scanning electron microscope examination to evaluate possible diagenetic alteration and relative preservation of the samples analyzed in this study the 18o data when interpreted as isotopic paleotemperatures indicate relative stable surface water and deepwater temperatures in the late maestrichtian followed by somewhat fluctuating temperatures in the early paleocene however there is no indication of either a significant warming or cooling at or following the k t boundary several sites do exhibit somewhat heavier 18o values by about 0 5 across the boundary which might suggest a 2 3 c cooling at most however we interpret these somewhat heavier 18o values as reflecting slightly better preservation of 18o of carbonate constitutents in relatively clay rich intervals e g lower diagentic potential characteristic of the k t boundary the 13c values of carbonate fine fraction and planktonic foraminifers indicate a major negative excursion in surface water total dissolved carbon across the boundary the surface water and deepwater 13c values from benthic foraminifers combined with calculated decreases in carbonate accumulation rates at all sites in the earliest paleocene are consistent with a major decrease in productivity across the k t boundary the decrease in caco3 accumulation rates is due not to increased rates of dissolution but to decreased production in surface waters the low productivity episode lasted at least 1 m y beyond the k t boundary crisis suggesting that hypothesized catastrophic events alone such as impact of an apollo object are not the only explanation for the observed long term decrease in productivity there is also an indication in the 13c curves from several sites that the surface water 13c decrease began in the latest maestrichtian suggesting that the decrease in productivity may have been independent of a hypothesized impact event

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

neogene history of the south pacific tradewinds evidence for hemispherical asymmetry of atmospheric circulation rea d k and bloomstme m k 1986 neogene history of the south pacific tradewmds evidence for hemlsphermal asymmetry of atmospherm circulation palaeogeogr palaeochmatol palaeoecol 55 55 64 eohan dust grains extracted from southeast pacific pelagic sediments provide a neogene record of southern hemisphere atmospherm processes the mass accumulation rate of dust has been low and generally constant at 1 4 mg cm 2 103 y since the late ohgocene eohan grains ze data show an increase from 8 20 3 40 t m older sediments to 7 20 6 80 t m younger materml this shift occurred about 10 5 m y ago and reflects a s gmfmant increase in the intensity of atmospheric circulation then there s no obwous response to the onset of northern hemisphere glacmtmn 2 5 m y ago m these eohan records from the south pacific compamson with existing lnformatmn from the northern hemisphere suggests that the southern hemisphere has had more intense atmospheric circulation throughout the neogene and that this asymmetry reached its greatest extent between 10 5 and 2 5 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

uvigerina proboscidea abundances and paleoceanography of the northern indian ocean dsdp site 214 during the late neogene abstract this study attempts to understand the significance of uvigerina proboscidea in paleoceanographic reconstructions at the northern tropical indian ocean dsdp site 214 from the late miocene through the pleistocene in this interval at this site u proboscidea is the most abundant species of the benthic assemblage and shows abrupt frequency changes about 1 74 based on relative percentages of u proboscidea calibrated with oxygen and carbon isotope record and the sediment accumulation rates the modern distribution of the species in the indian ocean and other evidence the peaks of abundance of u proboscidea are inferred to represent times of high surface productivity this productivity is related to intensified trade winds during strong southwest sw indian monsoons causing widespread upwelling along equatorial divergence in the indian ocean the sudden increase of u proboscidea abundance at approximately 8 5 7 5 ma reflects significant upwelling at the equatorial divergence this event corresponds to the permanent build up of west antarctic ice sheets and a major increase in sw indian monsoons related upwelling in the northwestern indian ocean the chron 6 carbon shift at approximately 6 2 ma is marked by another peak of abundance reflecting widespread ocean fertility the highest abundances of u proboscidea and highest sediment accumulation rates occur between 5 8 and 5 1 ma which coincides with the greatest development of antarctic ice sheets and strong southwest monsoons the higher percentages at 3 2 3 1 ma approximately 2 4 ma and 1 6 ma all represent phases of high productivity at the equatorial divergence

vertical heat transfer in the arctic ocean the role of double diffusive mixing 1 microstructure profiles collected in the central amundsen basin are analyzed in order to quantify the role of double diffusive mixing for vertical heat transfer from the atlantic layer in the profiles a persistent but laterally incoherent thermohaline staircase structure is identified in the 200 260 m depth range the staircase contains homogeneous layers with average height of 1 3 m and thin high gradient interfaces with average temperature and salinity jumps of about 0 065 c and 0 015 respectively when inferred from a commonly used diffusive convection parameterization the average vertical heat flux within the staircase is 0 6 w m 2 this is one order of magnitude larger than the molecular diffusion alone and of the same order as the overall heat loss from the atlantic layer in the deep basins of the arctic ocean the parameterization is evaluated using observed turbulent heat fluxes and is found to overestimate diffusive convective fluxes with up to an order of magnitude especially for weak thermal forcing small temperature jumps in staircase staircases coexist with thermohaline intrusions in the vertical temperature and salinity profiles lomonosov ridge is identified as a potential region for formation of intrusions it is found that salt fingering is the dominant process during intrusion growth whereas diffusive convection is the dominant process in maintaining the intrusions at steady state in the deep basins

two cretaceous volcanic episodes in the western pacific ocean stratigraphic analyses of cores recovered by the deep sea drilling project from the western pacific ocean confirm that cretaceous volcanic activity in that region was common and that two separate major episodes of activity can be distinguished the older episode aptian to cenomanian in age resulted in the formation of most of the oceanic plateaus in the western pacific in the younger santonian to maastrichtian episode several island and seamount chains formed especially those in the west central pacific that trend north northwest these volcanic events among the most extensive in the marine geologic record covered an area of at least 30 10 6 km 2 possibly the older episode was responsible for a cretaceous sea level high and concomitant epi continental transgression the younger episode of volcanism certainly covered an extensive area but its effect upon sea level may not have been as great as that of the older event

miocene stable isotope record a detailed deep pacific ocean study and its paleoclimatic implications deep sea drilling project site 289 in the western equatorial pacific has yielded an extremely detailed record of the carbon and oxygen isotopic changes in the miocene deep ocean the isotopic record reflects major changes in paleoclimate and paleoceanography probably dominated by a major phase of antarctic ice cap growth the transition from a relatively unglaciated world to one similar to today occurred between 16 5 x 10 6 and 13 x 10 6 years before the present with the greatest change occurring between approximately 14 8 x 10 6 and 14 0 x 10 6 years before the present

gas and particle emissions from soufriere hills volcano montserrat west indies characterization and health hazard assessment the soufriere hills volcano montserrat erupting since 18 july 1995 intensified its degassing in early 1996 with the continuing growth of the lava dome inside the summit crater during this period of increased activity between 11 and 18 march 1996 we measured gases and particles within the visible plume to determine whether at that time it posed a health risk to the population of plymouth the capital town which is 5 km southwest downwind and was then still occupied gravimetric measurements were made of total suspended particles tsp and particles having an aerodynamic diameter of less than 10 m pm10 measurements were made of sulphur dioxide so2 hydrochloric acid hcl hydrofluoric acid hf nitric acid hno3 acetic acid ch3cooh formic acid hcooh and particulate sulphate so4 2 chloride cl nitrate no3 fluoride f methanesulphonate ch3so3 acetate ch3coo formate hcoo ammonium nh4 sodium na and acidity h trace metals having human health implications chromium cr nickel ni cobalt co copper cu zinc zn arsenic as selenium se cadmium cd tin sn mercury hg and lead pb were also determined mean concentrations of hcl so2 and hf obtained in the town of plymouth were 14 0 5 9 and 0 8 ppbv respectively corresponding concentrations in the mixed plume on the crater edge were 533 168 and 22 ppbv there were no direct emissions of hno3 although nitrate was detected in coarse particles at the source higher concentrations of ch3cooh and hcooh were measured close to the crater mean tsp and pm10 were 64 and 15 g m 3 in plymouth and 455 and 47 g m 3 on the upper volcano slope aerosols were highly acidic at the source but rapidly neutralised during transport trace metals were enriched in the aerosol relative to crater surface material the concentrations of the acid gases sulphur dioxide in particular and particles were found to be too small to pose a health hazard at the time of these measurements when relatively modest volcanic activity was occurring

late miocene early pliocene stratigraphy and paleoceanography of the south atlantic and southwest pacific oceans a synthesis the stratigraphy and paleoceanography of the late miocene and early pliocene have been examined at six sites in the south atlantic and southwest pacific oceans deep sea drilling project dsdp sites 284 516a 519 588 and 590 and two piston cores from chain cruise 115 a consistent stratigraphy was developed among sites using graphic correlation which resulted in age models for all sites that are tied to the revised paleomagnetic time scale of berggren et al 1985 applying these chronologies we assessed latitudinal and interocean contrasts in the stratigraphic ranges of late miocene early pliocene planktonic foraminiferal and nanno fossil datums salient stratigraphic results include 1 the last appearance datum lad of globoquadrina dehiscens is a late miocene 6 4 ma event in the subtropics and is not useful for the placement of the miocene pliocene m p boundary in this biogeographic province 2 the first appearance datum fad of globorotalia crassaformis occurred at 5 1 ma in the south atlantic near the m p boundary suggesting that gr crassaformis may have first evolved in the south atlantic and later migrated to other regions 3 in the southwest pacific the fads of gr margaritae 5 97 ma gr puncticulata 5 09 ma and gr crassaformis 4 87 ma are significantly time transgressive between temperate and warm subtropical regions time lags of 1 0 m y were required for these species to adapt to physical and or biotic conditions peripheral to their endemic biogeographic provinces 4 between the subtropics of the south atlantic and southwest pacific many planktonic foraminiferal datums fad of dentogloboquadrina altispira gr cibaoensis gr conomiozea gr margaritae and gq dehiscens and lad of gr cibaoensis markedly depart from the correlation suggested by magnetostratigraphy indicating that these datum levels are unreliable for correlation between these ocean basins 5 in contrast available calcareous nannofossil datum levels fall on or near the paleomagnetic correlation line indicating synchroneity of events within the subtropics 6 biostratigraphic magnetic and 87sr 86sr correlation between sites 588 and 519 and the m p neostratotype at capo rossello sicily suggests that the base of the zanclean stratotype occurs at 5 1 5 0 ma in the lower reversed subchron of the gilbert about 2 3 105 years above the gilbert chron 5 boundary oxygen isotopic results from dsdp sites 284 519 and ch115 piston cores confirm a prolonged benthic 18o increase in the latest miocene between 5 6 and 5 0 ma as originally proposed by shackleton and kennett 1975 at dsdp site 588 the benthic 18o record in the latest miocene is marked by high frequency fluctuations with amplitude variations of 0 5 and a long period wavelength component of 400 000 years maximum 18o values however occurred during the late miocene kapitean stage between 5 5 and 5 1 ma the late miocene 18o changes resulted from mid and high latitude cooling and pulses of ice sheet expansion and contraction glacial events were most intense during the latest miocene kapitean stage and occurred at 5 50 5 35 ma and at 5 10 ma glacial events are estimated to have lowered sea level by 40 to 60 m and contributed to the isolation and desiccation of the mediterranean basin during the late messinian interglacial conditions prevailed at 5 2 ma and between 5 0 and 4 1 ma ihe early pliocene the beginning of the pliocene was marked by changes in many proxy climatic indicators at all sites suggesting a prolonged interval of warm interglacial conditions between 5 0 and 4 1 ma during the earliest pliocene supplementary tables are available with entire article onmicrofiche order from american geophysical union 2000florida avenue n w washington dc 20009 documentp86 001 2 50 payment must accompany order

geologic evolution of hess rise central north pacific ocean cores from four deep sea drilling project dsdp sites 310 464 465 and 466 and seismic reflection profiles provide data that are used to interpret the geological evolution and paleoenvironments of hess rise a prominent oceanic plateau in the central north pacific ocean hess rise apparently formed in the southern hemisphere along the western flank of the pacific farallon ridge 110 to 100 m y b p core stratigraphies and lithologies show the response of sedimentation to subsidence and northward movement of hess rise on the pacific plate oceanic islands which crowned hess rise during its early evolution were eroded and subsequently subsided below sea level major structural trends include three northwest trending 327 arms or ridges and an east northeast trending southern hess rise that parallels the mendocino fracture zone 060 normal faults offset basement as much as 3 000 m along the southern edge and 1 500 m on the western flank of hess rise many faults were active during sedimentation tholeiitic basalt from the base of hole 464 trachyte from the base of hole 465a and alkalic basalt clasts within sediment of hole 466 show the diversity of rock types that constitute the igneous basement a major rock unit is middle cretaceous limestone chalk and minor chert that form the basal sedimentary unit some limestone samples rich in organic carbon reflect accumulation above the carbonate compensation depth ccd within a mid water oxygen minimum zone the organic carbon rich sediments probably were deposited on the submarine slopes of islands and banks that were at upper bathyal depths as hess rise crossed the wide equatorial divergence where increased upwelling and biogenic productivity contributed to high accumulation rates the source of organic matter was mostly lipid rich autochthonous marine organic matter analyses of sediment samples from across the cretaceous tertiary boundary at site 465 show that there was a significant decrease in surface water temperature and biological productivity an abrupt increase in transition metals and iridium suggests that an outside source perhaps extraterrestrial was the cause for many of the sudden oceanographic geochemical and biological changes at the boundary

chert in the pacific biogenic silica and hydrothermal circulation abstract data on sections drilled by dsdp and odp in the pacific have been compiled for 187 sites 105 of which contain chert the spatial pattern of siliceous deposits in the pacific generally follows the pattern of biologic productivity there is some evidence that enhanced vertical diffusivity in the ocean during the eocene warm climate gave rise to siliceous deposits on and near bathymetric highs and basin margins the most common occurrence of chert is in eocene and older sections however cherts do occur in sections as young as 7 ma in the sites investigated over half of the youngest chert occurrences in sedimentary sections are within 150 m of basement nearly all the chert occurrences on crust younger than 60 ma are within 150 m of basement the deficit in heat flow over ocean crust younger than 60 ma is explained as the result of heat advection by hydrothermal waters circulating in the upper part of the ocean crust these waters appear to dissolve biogenic silica and deposit it as chert in the lower parts of sedimentary sections diffusion of these waters into the lower sediment section is enhanced by small scale faulting this faulting may be associated with intra plate stresses that develop during ridge jumps and reorientations of plate motion the relatively warm bottom waters of the early cenozoic may have also enhanced the dissolution of biogenic silica during hydrothermal circulation in the upper oceanic crust and basal sediment section

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

miocene deepwater oceanography a global synthesis of miocene benthic foraminiferal carbon and oxygen isotopic and faunal abundance data indicates that miocene thermohaline circulation evolved through three regimes corresponding approximately to early middle and late miocene times there is evidence for major qualitative differences between the circulation of the modern ocean and the miocene ocean prior to 11 ma the 13c 12c ratios of the benthic foraminifera cibicidoides are interpreted in terms of water mass aging i e the progressive depletion of dissolved o2 and lowering of 13c values as the result of oxidation of organic matter as water flows further from its sources at the surface of the oceans both isotopic and faunal data indicate that the early miocene regime from 22 to 15 ma was the most different from today s during that interval intermediate and deep waters of both the atlantic and the pacific oceans aged in a northward direction and the intermediate waters of the indian the south atlantic and the south pacific oceans were consistently the youngest in the global ocean we speculate that early miocene global thermohaline circulation may have been strongly influenced by the influx of warm saline water tethyan indian saline water from the tethys into the northern indian ocean the isotopic and faunal data suggest that flow from the tethyan region into the indian ocean diminished or terminated at about 14 ma isotopic and faunal data give no evidence for north atlantic deep water nadw formation prior to about 14 5 ma with the exception of a brief episode in the early miocene from 14 5 to 11 ma nadw formation was weak and circumpolar and antarctic water flooded the deep south atlantic and south pacific as the antarctic ice cap grew from about 10 ma to the end of the miocene thermohaline circulation resembled the modern circulation in many ways in latest miocene time 6 to 5 ma circulation patterns were very similar to today s except that nadw formation was greatly diminished the distribution pattern of siliceous oozes in miocene sediments is consistent with our proposed reconstruction of thermohaline circulation major changes which occurred in circulation during the middle miocene were probably related to the closing of the tethys and may have contributed to rapid middle miocene growth of the antarctic ice cap appendices 1 4 6 and 7 are available withentire article on microfiche order fromamerican geophysical union 2000 floridaavenue n w washington dc 20009 document 88p 002 5 00 payment mustaccompany order

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

paleodepth distribution of neocene deep sea hiatuses the depth of formation of miocene to middle pliocene deep sea hiatuses nh1 to nh8 has been determined using 152 deep sea drilling project sites in the atlantic pacific and indian oceans megahiatuses representing maxima in deep sea erosion occur at three water depths below 3800 m at intermediate depths between about 2000 and 3000 m and in shallower waters above 1500 m paleodepth both the paleodepths and distribution of these hiatus maxima suggest that flow of antarctic bottom water antarctic intermediate water and north atlantic deep water masses largely caused this erosion correlation of the hiatuses with oxygen isotope carbonate and sea level records indicates that prior to 11 ma brief hiatuses including those resulting from nondeposition in high productivity equatorial regions largely correlate with global cooling episodes high carbonate content and lowstands of sea level during the last 11 m y hiatuses also seem to correlate with cooling episodes but carbonate dissolution is characteristic and sea levels may have been rising or at a lowstand during the nh4 nh7 and nh8 intervals it is possible that a rise in sea level corresponds with polar cooling but there is uncertainty in correlation brief hiatuses during rising sea levels can be explained by basin shelf fractionation of carbonates hiatuses during sea level lowstands and cooling episodes may result from intensified bottom water circulation and increased corrosiveness of bottom water due to higher levels of co2 and increased productivity during increased upwelling supplement appendix 1 is availablewith entire article on microfiche orderfrom american geophysical union 2000florida avenue n w washington dc20009 document p87001 2 50 paymentmust accompany order

the early opening between broken ridge and kerguelen plateau abstract reconstructions to total closure of the australia antarctic continents causes an unacceptable overlap of broken ridge and kerguelen plateau this has been partially resolved in the past by supposing that the northern part of kerguelen that is principally involved in the overlap is younger than the remainder we have revised the early reconstructions using a newly proposed breakup chronology of australia and antarctica which suggests that opening began at least 90 m y b p at an initially slow rate this eliminates the overlap problem without invoking major age differences within the kerguelen plateau we also suggest that the northeastern flank of kerguelen plateau may be underlain by the missing westward continuation of the diamantina zone it may have been isolated on the antarctic plate by a ridge crest jump at about anomaly 24 time that also formed the ob trench

a 420 000 year record of cyclicity in oceanic and atmospheric processes from the eastern equatorial pacific samples from the upper portion of a cyclic pelagic carbonate sediment sequence in deep sea drilling project dsdp hole 503b 4 0 n 95 6 w are the first group to be analyzed for paleoceanographic and paleoclimatic proxy indicators of ice volume deep ocean and surface water circulation and atmospheric circulation in order to resolve the complex origin of the cyclicity temporal resolution is taken from the 18o time scale most other parameters are calculated in terms of their mass flux to the seafloor caco3 percent in the sediments fluctuates in the well known pacific pattern and is higher during glacial times the fluxes of opal and organic carbon have patterns similar to each other and show a variability of a factor of 2 5 to 4 the longer organic carbon record shows flux maxima during both glacial and interglacial times the accumulation patterns of both opal and organic carbon suggest that the variability in surface water productivity and or seafloor preservation of those materials is not simply correlated to glacial or interglacial periods eolian dust fluxes are greater during interglacial periods by factors of 2 to 5 indicating that eolian source regions in central and northern south america were more arid during interglacial periods the record of eolian grain size provides a semiquantitative estimation of the intensity of the transporting winds the eolian data suggest more intense atmospheric circulation during interglacial periods opposite to the anticipated results we interpret this observation as recording the southerly shift of the intertropical convergence zone to the latitude of hole 503b during glaciations