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

middle miocene southern ocean cooling and antarctic cryosphere expansion magnesium calcium data from southern ocean planktonic foraminifera demonstrate that high latitude similar to55degreess southwest pacific sea surface temperatures ssts cooled 6degrees to 7degreesc during the middle miocene climate transition 14 2 to 13 8 million years ago stepwise surface cooling is paced by eccentricity forcing and precedes antarctic cryosphere expansion by similar to60 thousand years suggesting the involvement of additional feedbacks during this interval of inferred low atmospheric partial pressure of co2 pco 2 comparing ssts and global carbon cycling proxies challenges the notion that episodic pco2 drawdown drove this major cenozoic climate transition sst salinity and ice volume trends suggest instead that orbitally paced ocean circulation changes altered meridional heat vapor transport triggering ice growth and global cooling

seasonal to interannual changes in planktonic foraminiferal assemblages in the northwestern north pacific sediment trap results encompassing a warm period related to el nino abstract planktonic foraminifera provide a record of the upper ocean environment through their species assemblage and individual tests to investigate the relationship between foraminifera and oceanographic conditions and the impact of el nino on foraminifera we analyzed foraminiferal fluxes and relative abundances by using sediment trap samples collected biweekly at three sites in the northwestern north pacific site 40n 39 60 n 165 00 e site knot 43 58 n 155 03 e and site 50n 50 01 n 165 02 e from 1998 2001 a period that included an el nino event based on foraminiferal production and assemblage composition we divided the sampling duration into several periods during which certain characteristic oceanographic properties were observed these sampling periods were classified into five types i v based upon four factors 1 the predominant foraminiferal group 2 total foraminiferal fluxes tffs 3 organic matter om fluxes and 4 hydrographic conditions which included sea surface temperature sst and thermal structure our results suggest that our observed seasonal changes in foraminifera were closely related to water mass properties in addition to sst if species compositions were the same then water mass properties were the most important factors affecting the seasonal variation of foraminiferal abundance in the northwestern north pacific although one of the major controlling factors for foraminiferal fluxes is food availability the controlling factors for each type types i v are different because of specific oceanographic situations such as phytoplankton blooms which result in an excess food supply for foraminifera at site knot high surface temperatures and weak winds related to an el nino period in 1998 would have caused a low nutrient supply and water column stratification and resulted in the relatively low fluxes of total foraminifera neogloboquadrina pachyderma and globigerina bulloides and high fluxes of neogloboquadrina dutertrei the impact of el nino on foraminifera was variable depending on the strength of the el nino event and site location however there were some instances of common annual foraminiferal patterns the patterns observed at sites 40n and knot in 1998 were similar to those observed off chile in 1997 1998 and in 1991 1992 respectively

middle miocene paleoceanography of the western equatorial pacific dsdp site 289 and the evolution ofgloborotalia fohsella abstract evolution of the planktic foraminiferal lineagegloborotalia fohsella occurred during the miocene between 23 7 and 11 8 ma and forms the basis for stratigraphic subdivision of the early middle miocene zones n10 through n12 important morphologic changes within theg fohsella lineage included a marked increase in test size a transition from a rounded to an acute periphery and the development of a keel in later forms we found that the most rapid changes in morphology ofg fohsella occurred between 13 and 12 7 ma and coincided with an abrupt increase in the 18o ratios of shell calcite comparison of isotopic results ofg fohsella with other planktic foraminifers indicate that 18o values of the lineage diverge from surface dwelling species and approach deep dwelling species after 13 0 ma indicating a change in depth habitat from the surface mixed layer to intermediate depth near the thermocline isotopic and faunal evidence suggests that this change in depth stratification was associated with an expansion of the thermocline in the western equatorial pacific after adapting to a deeper water habitat at 13 0 ma theg fohsella lineage became extinct abruptly at 11 8 ma during a period when isotopic and faunal evidence suggest a shoaling of the thermocline following the extinction ofg fohsella the ecologic niche of the lineage was filled by thegloborotalia menardella group which began as a deep water form and later evolved to an intermediate water habitat we suggest that the evolution ofg fohsella andg menardella were tightly linked to changes in the structure of the thermocline in the western equatorial pacific

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

numerical models of salt diapir formation by down building the role of sedimentation rate viscosity contrast initial amplitude and wavelength formation of salt diapirs has been described to be due to upbuilding i e rayleigh taylor like instability of salt diapirs piercing through a denser sedimentary overburden or syndepositional down building process i e the top of the salt diapir remains at the surface all the time here we systematically analyse this second end member mechanism by numerical modelling four parameters are varied sedimentation rate nu sed salt viscosity eta salt amplitude delta of the initial perturbation of the sedimentation layer and thewavenumber k of this perturbation the shape of the resulting salt diapirs strongly depends on these parameters small diapirs with subvertical side walls are found for small values of nu sed and eta salt or large values of delta whereas taller diapirs with pronounced narrow stems build for larges values of nu sed and eta salt or small values of delta two domains are identified in the four parameter space which separates successful down building models from non successful models by applying a simple channel flow law the domain boundary can be described by the non dimensional law nu sedcrit c 1 1 2 delta 0 rho sed k 2 k 2 c2 where rho sed is the sediment density scaled by the density contrast delta rho between sediment and salt the wavelength is scaled by the salt layer thickness h salt and velocity is scaled by eta salt h salt 2 delta rho g where eta salt is the salt viscosity and g is the gravitational acceleration from the numerical models the constants c 1 and c 2 are determined as 0 0283 and 0 1171 respectively

volcanic iron fertilization of primary productivity at kerguelen plateau southern ocean through the middle miocene climate transition abstract odp site 747 central kerguelen plateau contains a nearly complete record of the middle miocene climate transition mmct its unusually well preserved and rich foraminiferal assemblages have resulted in the mmct interval at site 747 becoming a focus of high resolution paleoclimatic studies that have established this sequence as a southern ocean reference we combine x ray fluorescence xrf elemental counts of fe ti and al and concentration dependent magnetic parameters to trace volcanic ash input to sediments at odp hole 747a we observe systematic variations in al ti and fe ti ratios that are consistent with increased primary productivity at times of increased terrigenous input and therefore increased nutrient supply a spike in terrigenous material which we interpret as volcanic ash coincides with major changes in species abundance and stable oxygen and carbon isotopic shifts within the mmct interval this correlation suggests that nutrient flux associated with volcanic iron fertilization played a significant role in controlling phytoplankton communities at site 747 we demonstrate that rapidly obtained xrf and magnetic parameters can provide useful proxies for terrigenous input and therefore nutrient fluxes in pelagic marine settings that can help to resolve local versus global drivers of paleoenvironmental and paleoceanographic changes

seasonal variability in the δ18o and δ13c of planktonic foraminifera from an upwelling environment sediment trap results from the san pedro basin southern california bight stable isotopic analyses 18o and 13crpar on four species of planktonic foraminifera collected from high resolution time series sediment trap sampling indicate that rapid and significant seasonal variability occurred in response to rapid changes in both the hydrographic environment and the isotopic composition of ambient seawater specimens were collected at a depth of 500 m for 28 consecutive 1 week periods in the san pedro basin within the southern california bight a region seasonally influenced by upwelling hydrographic monitoring of the study area revealed a 6 week period late april to mid june during which pulses of upwelling occurred causing significant increases in the oxygen isotopic composition of surrounding waters while decreasing the carbon isotopic composition analyses of globigerina bulloides orbulina universa neogloboquadrina dutertrei and neogloboquadrina pachyderma right coiling indicate that during the upwelling period these species live respectively at the surface in the surface mixed layer within the thermocline and below the thermocline globigerina bulloides ascends from well below the thermocline to the surface at the onset of upwelling and increases production to its maximum simultaneously as its morphology changes to less calcified open forms both n dutertrei and n pachyderma follow isothermal ranges migrating to shallower waters during upwelling and subsequently descending after upwelling ceases neogloboquadrina dutertrei appears to inhabit the region associated with both the thermocline base and chlorophyll maximum when it is most abundant following upwelling orbulina universa increases its depth habitat as solar irradiance increases but this species decreases in abundance during upwelling the 18o signal of o universa appears to be a useful indicator of seasonal temperature variations above the thermocline whereas the 18o of g bulloides documents near surface temperatures during upwelling the 13c signal of both n dutertrei and n pachyderma appears to record the 13c of equilibrium calcite for the regions of the thermocline and just below the thermocline respectively due to the relative preservation potentials of the four species analyzed only n dutertrei and n pachyderma are likely to be preserved in the sedimentary record in regions of oxygenated bottom waters thus an actual record of hydrographic conditions during upwelling as would be found in tests of g bulloides is not preserved instead conditions before n pachyderma and just after n dutertrei are more likely recorded

middle to late miocene oxygen isotope stratigraphy of odp site 1085 se atlantic new constrains on miocene climate variability and sea level fluctuations abstract the middle miocene 18o increase represents a fundamental change in earth s climate system due to a major expansion and permanent establishment of the east antarctic ice sheet accompanied by some effect of deepwater cooling the long term cooling trend in the middle to late miocene was superimposed by several punctuated periods of glaciations mi events characterized by oxygen isotopic shifts that have been related to the waxing and waning of the antarctic ice sheet and bottom water cooling here we present a high resolution benthic stable oxygen isotope record from odp site 1085 located at the southwestern african continental margin that provides a detailed chronology for the middle to late miocene 13 9 7 3 ma climate transition in the eastern south atlantic a composite fe intensity record obtained by xrf core scanning odp sites 1085 and 1087 was used to construct an astronomically calibrated chronology based on orbital tuning the oxygen isotope data exhibit four distinct 18o excursions which have astronomical ages of 13 8 13 2 11 7 and 10 4 ma and correspond to the mi3 mi4 mi5 and mi6 events a global climate record was extracted from the oxygen isotopic composition both long and short term variabilities in the climate record are discussed in terms of sea level and deep water temperature changes the oxygen isotope data support a causal link between sequence boundaries traced from the shelf and glacioeustatic changes due to ice sheet growth spectral analysis of the benthic 18o record shows strong power in the 400 kyr and 100 kyr bands documenting a paleoceanographic response to eccentricity modulated variations in precession a spectral peak around 180 kyr might be related to the asymmetry of the obliquity cycle indicating that the response of the dominantly unipolar antarctic ice sheet to obliquity induced variations probably controlled the middle to late miocene climate system maxima in the 18o record interpreted as glacial periods correspond to minima in 100 kyr eccentricity cycle and minima in the 174 kyr obliquity modulation strong middle to late miocene glacial events are associated with 400 kyr eccentricity minima and obliquity modulation minima thus fluctuations in the amplitude of obliquity and eccentricity seem to be the driving force for the middle to late miocene climate variability

surface water cooling and salinity decrease during the middle miocene climate transition at southern ocean odp site 747 kerguelen plateau paleoceanographic variability at southern high latitude ocean drilling program odp site 747 was investigated in this study through the interval which spans the middle miocene climate transition mmct between 15 0 and 12 2 million years ago ma foraminiferal 18 o records derived from both benthic cibicidoides spp and planktonic taxa globorotalia praescitula and globigerina bulloides reveal a history of changes in water column thermal and salinity structure and a strong imprint of seasonality prior to the mmct in the interval between 14 35 and 13 9 ma g bulloides displays relatively high 18 o values similar to those of g praescitula interpreted to indicate weakening of the thermocline and or increased seasonality with cooler early spring and or late fall temperatures following this interval g bulloides 18 o values diverge significantly from benthic and g praescitula values with g bulloides values remaining relatively low for at least 600 kyr following the benthic foraminiferal 18 o shift during the mmct at 13 9 ma this divergence in 18 o records occurs in direct association with the mi3 cooling and glaciation event and may suggest 1 a strengthening of the vertical temperature gradient with greater cooling of deep waters than surface waters 2 changes in the depth habitat of g bulloides 3 changes in the dominant season of g bulloides calcification 4 modification of surface water 18 o values in association with enhanced sea ice formation 5 increased surface water carbonate ion concentration and or 6 a significant decrease in surface water salinity across the mmct the first of these possible scenarios is not likely particularly in light of recent mg ca evidence for significant surface water cooling in the southern ocean associated with the mmct of the remaining possibilities we favor a change in surface salinity to explain the observed trends in 18 o values and hypothesize that surface salinity may have decreased by up to 2 salinity units at 13 9 ma in this scenario the development of a lower salinity antarctic surface layer coincided with regional cooling of both surface and deep waters of the southern ocean during the mi3 glaciation of east antarctica and contributed into the dominance of neogloboquadrina spp between 13 8 and 13 2 ma additionally the distinct patterns observed in planktonic foraminiferal 18 o records spanning the mmct correspond with changes in the vertical 13 c gradient between planktonic and benthic foraminiferal records and major changes in planktonic foraminiferal assemblages at site 747 providing further evidence of the environmental significance of this climatic transition

evolution of depth ecology in the planktic foraminifera lineage globorotalia fohsella after over 15 m y of evolution in near surface waters of the western pacific and caribbean planktic foraminifera in the lineage globorotalia fohsella abruptly invaded thermocline habitats at 13 ma stable isotope data demonstrate that this habitat shift took 50 ka to complete a duration similar to the fastest measured rates of speciation in marine protistans the ecological change occurred near the end of a long series of gradual morphological transitions and demonstrates that at least some of the chronospecies in this lineage had distinctive ecologies and were probably biological species other than this one shift in depth distribution skeletal evolution is largely disconnected from depth changes in globorotalia fohsella morphological trends of globorotaliid foraminifera may not record changes in depth ecology contrary to existing models for foraminiferal diversification

the oxygen isotope composition of planktonic foraminifera from the cariaco basin venezuela seasonal and interannual variations material collected during a three year sediment trapping experiment in the cariaco basin venezuela january 1997 to december 1999 is used to examine both temporal and inter species variability in the oxygen isotope composition of planktonic foraminifera specifically this study compares the oxygen isotope composition of six species of planktonic foraminifera globigerinoides ruber pink globigerina bulloides neogloboquadrina dutertrei orbulina universa globorotalia menardii and globorotalia crassaformis with the climatology and hydrography of the region and evaluates the application of each species for use in paleoceanographic reconstructions the isotope results are consistent with known depth habitats for all six species the lowest 18 o values 1t o 2 were measured on g ruber pink and g bulloides two species that live in the surface mixed layer values for deeper dwelling species such as n dutertrei g menardii and g crassaformis are higher predominantly ranging from 0 to 0 5 temperature estimates derived using species specific paleotemperature equations indicate that g ruber pink accurately estimates sea surface temperatures ssts throughout the year while g bulloides temperature estimates are similar to measured surface temperatures only during the upwelling season january april for the remainder of the year the 18 o derived temperatures for g bulloides typically are lower than the measured sst although the maximum flux of all species occurs during upwelling the flux weighted annual mean isotopic composition of the six species indicates that only g bulloides is biased towards this season therefore we conclude that the sediment 18 o record of g ruber pink is most suitable for estimating past values of mean annual sst while g bulloides provides information on conditions during spring upwelling the depth of calcification of n dutertrei varies seasonally in response to changes in the depths of the thermocline and chlorophyll maximum as a result the 18 o difference between g ruber pink and n dutertrei provides an estimate of the annual surface to thermocline temperature gradient in the basin 2006 elsevier b v all rights reserved

stable isotope clue to episodic sea ice formation in the glacial north atlantic sea ice is a determinant parameter of the climate system which acts as amplifier through positive ice albedo feedbacks and induces deep water formation through brine production it is also one of the most elusive parameters in the paleoclimate record most proxies of sea ice provide information on its presence and extent of e g diatoms or dinocysts but do not permit assessment on its rate of formation however seemingly off equilibrium low 18o values in mesopelagic planktic foraminifer species as observed today in the arctic ocean are thought to relate to the production of isotopically light brines during sea ice formation and might thus provide a clue on the rate of sea ice growth with reference to the isotopic properties of modern planktic foraminifers in the arctic ocean and using multi proxy data sets from the last glacial stage in the northwest north atlantic we re examined some of the current interpretations of planktic isotope records in the glacial nw north atlantic the large amplitude light isotopic excursions recorded in neogloboquadrina pachyderma left coiling during heinrich events 1 for example correspond to extensive sea ice cover as reconstructed from dinocysts and do not seem unequivocally linked to low salinity pulses such light isotopic excursion more likely responded to enhanced rates of sea ice formation resulting in the production and sinking of isotopically light brines on the contrary the isotopically heavy planktic foraminifers of the last glacial maximum lgm interval stricto sensu would rather suggest relatively low rates of brine production thus low sea ice growth rates in the area this would imply that the lgm distribution of sea ice in the north atlantic was primarily linked to spreading and drifting from marginal and nordic source areas and not from enhanced in situ production at the scale of glacial interglacial cycles isotopic distillation processes relating to sea ice production at high latitudes could be one of the factors controlling the 18o salinity relationship in deep ocean water masses that might deserve closer examination as it would be independent of continental ice volume changes

the influence of air sea exchange on the isotopic composition of oceanic carbon observations and modeling although the carbon isotropic composition of ocean waters after they leave the surface ocean is determined by biological cycling air sea exchange affects the carbon isotopic composition of surface waters in two ways the equilibrium fractionation between oceanic and atmospheric carbon increases with decreasing temperature in southern ocean surface waters this isotopic equilibrium enriches delta sup 13 c relative to the delta sup 13 c expected from uptake and release of carbon by biological processes alone similarly surface waters in the subtropical gyres are depleted in delta sup 13 c due to extensive air sea exchange at warm temperatures countering the tendency toward isotopic equilibration with the atmosphere a relatively slow process are the effects of the equilibration of co sub 2 itself a much faster process in regions where there is a net transfer of isotopically light co sub 2 from the ocean to the atmosphere e g the equator surface waters become enriched in sup 13 c whereas in regions where isotopically light co sub 2 is entering the ocean e g the north atlantic surface waters become depleted in sup 13 c a compilation of high quality oceanic delta sup 13 c measurements along with experiments performed using a zonally averaged three basin dynamic ocean model are used to explore these processes 40 refs 14more figs 1 tab less

miocene diatom biostratigraphy at odp sites 689 690 1088 1092 atlantic sector of the southern ocean abstract four odp sites located between 64 s and 41 s in the eastern atlantic sector of the southern ocean were investigated to refine the miocene diatom biostratigraphic zonation tied to the geomagnetic chronology the miocene diatom stratigraphy from two sites located on maud rise odp leg 113 is revised considering the progress in diatom biostratigraphic research diatom taxonomy and magnetostratigraphic age assignment during the past 10 years a new diatom zonation was erected for site 1092 odp leg 177 located on meteor rise integrating a magnetostratigraphic interpretation of the shipboard data this zonation was also applied to site 1088 odp leg 177 located on astrid ridge the study is focused to middle and upper miocene sequences it reveals latitudinal differentiations in stratigraphic species ranges and species occurrence pattern that are related to latitudinal differences in surface water masses reflecting the climatic development of the antarctic cryosphere considering the latitudinal differences two stratigraphic zonations are proposed that are applicable to the northern and southern zone of the southern ocean respectively the southern southern ocean miocene diatom biostratigraphic zonation consists of 16 zones in which 11 represent new or modified zones the northern biostratigraphic zonation contains 10 diatom zones allowing a stratigraphic resolution in the range of 0 2 2 myr this paper also includes the taxonomic transfer of seven miocene diatom taxa from genus nitzschia hassal to fragilariopsis hustedt

salt diapirs of the southwest nordkapp basin analogue modelling the geometry and evolution of the salt diapirs in the southwestern segment of the nordkapp basin sw nkb were interpreted on reflection seismic data reflection seismic profiles were used to build

numerical models of complex diapirs abstract numerically modelled diapirs that rise into overburdens with viscous rheology produce a large variety of shapes this work uses the finite element method to study the development of diapirs that rise towards a surface on which a diapir induced topography creeps flat or disperses erodes at different rates slow erosion leads to diapirs with mushroom shapes moderate erosion rate to wine glass diapirs and fast erosion to beer glass and column shaped diapirs the introduction of a low viscosity layer at the top of the overburden causes diapirs to develop into structures resembling a napoleon hat these spread lateral sheets

the middle miocene climatic transition east antarctic ice sheet development deep ocean circulation and global carbon cycling abstract the middle miocene represents a major change in state in cenozoic climatic evolution following the climax of neogene warmth in the late early miocene at 16 ma the early stage of this climatic transition from 16 to 14 8 ma was marked by major short term variations in global climates east antarctic ice sheet eais volume sea level and deep ocean circulation in the later stage from 14 8 to 12 9 ma climatic developments included major growth of the eais and associated antarctic cooling a distinct increase in the meridional temperature gradient large fluctuations in sea level followed by a global sea level fall and important changes in deep water circulation including increased production of southern component water east antarctic ice sheet growth and polar cooling also had large effects on global carbon cycling and on the terrestrial biosphere including aridification of mid latitude continental regions increased stability of the eais after 14 8 ma represents a crucial step in the establishment of late neogene global climate systems what controlled these changes in polar climates and the east antarctic ice sheet deep ocean circulation changes probably played a major role in the evolution and variation in polar climates as they have throughout the cenozoic oxygen and carbon isotopic evidence for warm saline deep water production in the eastern tethyan northern indian ocean indicates that meridional heat transport to the antarctic inhibited cenozoic polar cooling and eais growth during the early middle miocene from 16 to 14 8 ma inferred competition between warm low latitude sources derived from the eastern tethyan northern indian ocean and a cold high latitude source southern component water from 16 to 14 8 ma may have been associated with instability in the antarctic climate and cryosphere reduction of warm saline deep water flow to the southern ocean at 14 8 ma may have decreased meridional heat transport to the antarctic cooling the region and leading to increased production of southern component water these middle miocene climatic and cryospheric changes in the antarctic had profound effects on marine and terrestrial climates as the meridional surface temperature gradient increased boundaries between climatic zones strengthened leading to increased aridification of mid latitude continental regions in australia africa and north and south america enhancing the development of grasslands and stimulating the evolution of grazing mammals

vertical distribution and isotopic fractionation of living planktonic foraminifera from the panama basin a model1 2 which explains the vertical and seasonal distribution of planktonic foraminifera was developed using data from the western north atlantic and is comprised of three tenets 1 the conditions in the photic zone upper 80 100 m exclusively dictate planktonic foraminifera species composition and abundance throughout the water column 2 species are vertically stratified within the photic zone according to their temperature preference depth distribution according to temperature preference has been proposed by several authors3 based on isotope analyses of sediments due to seasonal temperature variations seasonal variations in the 18o composition of seawater and species specific isotope fractionation rigorous proof of this hypothesis cannot be provided from sediment analyses alone 3 the chlorophyll maximum is a major food source zone which is preferentially exploited by foraminifera our data from the panama basin which are presented here are confirmation of this model the panama basin in the eastern equatorial pacific which has a shallow and steep thermocline was sampled for living planktonic foraminifera a multiple opening closing net and environmental sensing system mocness 4 was used to collect 13 sets of eight vertically stratified oblique samples we report here the quantitative distribution and stable isotopic composition of planktonic foraminifera from four mocness tows taken over 5 days during august 1979 rv knorr cruise 73 at 5 20 n 8 50 w each tow consisted of eight samples which integrated 12 5 m intervals from 0 to 100 m for moc 126 25 m intervals from 0 to 200 m for moc 127 100 150 m intervals from 0 to 1 000 m on moc 128 and 250 m intervals from 0 to 2 000 m on moc 131

rheological and tectonic modeling of salt provinces abstract technological improvements and a more rigorous research approach have led the petroleum industry to renew and expand its interest in physical modeling of faulting associated with halokinesis salt tectonics and the formation of structural hydrocarbon traps can be reproduced in models using viscous polymers for the substratum and frictional plastic granular materials for the overburden the fault patterns simulated in these dynamically scaled models are remarkably similar to those in depth converted seismic sections scale models are therefore useful for generating new concepts and for reconstructing the structural evolution of hydrocarbon traps although even unsealed models may help generating ideas the full transfer of quantitative data requires that models be dynamically scaled to their natural prototypes previously published scaling theory for salt tectonics has erroneously assumed fluid behaviour of the overburden this paper provides a sound scaling approach for geological deformations involving brittle overburdens on ductile substrata rheological data from overburden rocks and salt are compiled and compared to model rheologies using dimensions of laboratory analogs and other intrinsic material properties scaling approaches are compared and rules are formulated for achieving dynamic similarity in models of salt tectonics including those with synkinematic sedimentation

rankine models for time dependent gravity spreading of terrestrial source flows over subplanar slopes geological mass flows extruding from a point source include mud lava and salt issued from subsurface reservoirs and ice from surface feeders the delivery of the material may occur via a salt stock a volcanic pipe for magma and mud flows or a valley glacier for ice all these source flows are commonly skewed by a superposed far field velocity vector imposed by the topographic slope and thus develop plumes having a wide range of shapes the morphological evolution of the perimeter of the plumes in plan view can be simulated by varying the key parameters in a simple analytical flow description on the basis of rankine equations our model systematically varies the strength of the point source relative to the downslope far field velocity of its expelled mass the flow lines are critically controlled by the relative speed of the two rates which can be concisely expressed by the dimensionless rankine number rk introduced in this study for steady flows plume widths can be expressed as a function of rk the viscosity of the rock mud or lava mass involved in the gravity flow affects rk and thus the appearance of the plumes for unsteady source strength rk becomes time dependent and the plume width varies over time the model flow shapes suggest that the plume shapes of natural gravity flows of terrestrial surface materials mud lava salt and ice commonly express fast initial flux of the source followed by an exponential decline of the source strength flows having initially higher rk but otherwise equal life cycles create broader plumes peaks in the source flux due to magmatic pulsing during the eruption cycle can explain the formation of pillow lavas rather than instantaneously reaching full strength before declining some natural source flows start by swelling slowly leading to the creation of unique plume shapes like a flying saucer

the origin of aqueous sulfate in frio pore fluids and its implication for the origin of oil field brines abstract the o and s isotopic composition of aqueous s0 4 in saline formation waters from the frio formation of s texas can be used to deduce the origin of the s0 4 the average o and s isotopic composition of aqueous s0 4 in frio brines is 13 4 smow and 8 2 cdt respectively the o isotopic compositions fall within the range of mesozoic and cenozoic seawater values and are interpreted to be derived from dissolved jurassic evaporites the s values appear to have been modified by the addition of light s derived from the oxidation of pyrite we conclude therefore that some of the chemical components in frio brines are derived from water rock interactions that occurred in much deeper units and that these components have migrated into the frio

paleotemperatures and ice volume of the past 27 myr revisited with paired mg ca and 18o 16o measurements on benthic foraminifera 1 we explore the applicability of paired mg ca and 18o 16o measurements on benthic foraminifera from southern ocean site 747 to paleoceanographic reconstructions on pre pleistocene timescales we focus on the late oligocene through pleistocene 27 0 ma history of paleotemperatures and the evolution of the 18o values of seawater 18osw at a temporal resolution of 100 200 kyr absolute paleotemperature estimates depend on assumptions of how mg ca ratios of seawater have changed over the past 27 myr but relative changes that occur on geologically brief timescales are robust results indicate that at the oligocene to miocene boundary 23 8 ma temperatures lag the increase in global ice volume deduced from benthic foraminiferal 18o values but the smaller scale miocene glaciations are accompanied by ocean cooling of 1 c during the mid miocene phase of antarctic ice sheet growth 15 13 ma water temperatures cool by 3 c unlike the benthic foraminiferal 18o values which remain relatively constant thereafter temperatures vary by 3 c and reach maxima at 12 and 8 5 ma the onset of significant northern hemisphere glaciation during the late pliocene is synchronous with an 4 c cooling at site 747 a comparison of our 18osw curve to the haq et al 1987 sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater 18o 16o ratios at 12 11 ma in particular when benthic foraminiferal 18o values do not support a further increase in ice volume the 18osw curve comes to a maximum that corresponds to a major mid miocene sea level regression the agreement between the character of our mg ca based 18osw curve and sequence stratigraphy demonstrates that benthic foramaniferal mg ca ratios can be used to trace the 18osw on pre pleistocene timescales despite a number of uncertainties related to poorly constrained temperature calibrations and paleoseawater mg ca ratios the mg ca record also highlights that deep ocean temperatures can vary independently and unexpectedly from ice volume changes which can lead to misinterpretations of the 18o record

refining a pedogenic carbonate co2 paleobarometer to quantify a middle miocene greenhouse spike abstract estimates of respired soil co2 concentration in paleosols are needed to calculate past atmospheric co2 levels from the difference between organic and carbonate carbon isotopic values in paleosols a new compilation of co2 concentrations late in the growing season in modern calcareous soils can be used to estimate respired soil co2 by subtracting atmospheric co2 concentrations furthermore respired co2 shows a significant relationship with depth to calcic horizon and this relationship can be used to estimate respired co2 concentrations in paleosols these values as well as estimates of 13c of atmospheric co2 from 13c of soil organic matter and of paleotemperature from bw horizon chemical composition refine the cerling co2 paleobarometer these refinements are applied to quantify the middle miocene greenhouse which has been controversial because it was not detected by marine alkenone and boron paleobarometers new estimates of middle miocene atmospheric co2 from paleosols in railroad canyon idaho reveal levels at 16 ma of 852 86 ppmv this is similar to predicted values by 2100 and 3 times pre industrial values pil 280 ppmv

early and middle miocene stable isotopes implications for deepwater circulation and climate the middle miocene 18o increase represents a fundamental change in the ocean atmosphere system which like late pleistocene climates may be related to deepwater circulation patterns there has been some debate concerning the early to early middle miocene deepwater circulation patterns specifically recent discussions have focused on the relative roles of northern component water ncw production and warm saline deep water originating in the eastern tethys our time series and time slice reconstructions indicate that ncw and tethyan outflow water two relatively warm deepwater masses were produced from 20 to 16 ma ncw was produced again from 12 5 to 10 5 ma another feature of the early and middle miocene oceans was the presence of a high 13c intermediate water mass in the southern hemisphere which apparently originated in the southern ocean miocene climates appear to be related directly to deepwater circulation changes deep waters warmed in the early miocene by 3 c 20 to 16 ma and cooled by a similar amount during the middle miocene 18o increase 14 8 to 12 6 ma corresponding to the increase 20 ma and subsequent decrease 16 ma in the production of ncw and tethyan outflow water large 0 6 relatively rapid 0 5 m y 18o increases in both benthic and planktonic foraminifera i e the mi zones of miller et al 1991a and wright and miller 1992a were superimposed in the long term deepwater temperature changes they are interpreted as reflecting continental ice growth events seven of these m y glacial interglacial cycles have been recognized in the early to middle miocene two of these glacial interglacial cycles mi3 and mi4 combined with a 2 to 3 c decrease in deepwater temperatures to produce the middle miocene 18o shift

mid miocene isotope stratigraphy in the deep sea high resolution correlations paleoclimatic cycles and sediment preservation mid miocene pelagic sedimentary sections can be correlated using intermediate and high resolution oxygen and carbon isotopic records of benthic foraminifera precision of a few tens of thousands of years is readily achievable at sites with high sedimentation rates for example deep sea drilling project sites 289 and 574 the mid miocene carbon isotope records are characterized by an interval of high 13c values between 17 and 13 5 ma the monterey excursion of vincent and berger 1985 upon which are superimposed a series of periodic or quasi periodic fluctuations in 13c values these fluctuations have a period of approximately 440 kyr suggestive of the 413 kyr cycle predicted by milankovitch theory vincent and berger proposed that the monterey excursion was the result of increased organic carbon burial in continental margins sediments the increased 13c values called 13c maxima superimposed on the generally high mid miocene signal coincide with increases in 18o values suggesting that periods of cooling and or ice buildup were associated with exceptionally rapid burial of organic carbon and lowered atmospheric co2 levels it is likely that during the monterey excursion the ocean atmosphere system became progressively more sensitive to small changes in insolation ultimately leading to major cooling of deep water and expansion of continental ice we have assigned an absolute chronology based on biostratigraphic and magneto biostratigraphic datum levels to the isotope stratigraphy and have used that chronology to correlate unconformities seismic reflectors carbonate minima and dissolution intervals intervals of sediment containing 13c maxima are usually better preserved than the overlying and underlying sediments indicating that the 13c values of tco2 in deep water and the corrosiveness of seawater are inversely correlated this again suggests that the 13c maxima were associated with rapid burial of organic carbon and reduced levels of atmospheric co2 the absolute chronology we have assigned to the isotopic record indicates that the major mid miocene deepwater cooling ice volume expansion took 2 m y and was not abrupt as had been reported previously the cooling appears abrupt at many sites because the interval is characterized by a number of dissolution intervals the cooling was not monotonic and the 2 m y interval included an episode of especially rapid cooling as well as a brief return to warmer conditions before the final phase of the cooling period the increase in 18o values of benthic foraminifera between 14 9 and 12 9 ma was greatest at deeper water sites and at sites closest to antarctica the data suggest that the 18o value of seawater increased by no more than about 1 1 per mil during this interval and that the remainder of the change in benthic 18o values resulted from cooling in antarctic regions of deepwater formation equatorial planktonic foraminifera from sites 237 and 289 exhibit a series of 0 4 per mil steplike increases in 13c values only one of these increases in planktonic 13c is correlated with any of the features in the mid miocene benthic carbon isotope record

changes in coiling direction habitat depth and abundance in two menardellid species abstract we present a detailed study of menardellid speciation abundance and coiling changes between 6 34 and 5 1 ma latest miocene and earliest pliocene menardellids are widely distributed in tropical to subtropical waters from miocene to modern time revealing a stable isotopic signature characteristic for the top of the thermocline and shallower rather than deeper habitat depth menardellid response to its environment reveals a tripartite pattern the first interval from 6 34 to 5 75 ma is marked by a dominance of menardella limbata frequent changes in coiling direction and variable abundance with strongest reductions during intervals of dextral dominance the onset of an interval of cooling and ice build up at 5 82 ma coincides with a weakening of the mixed layer leading to nearly 70 kyr of strongly reduced menardellid abundance at 5 75 ma menardella multicamerata replaces m limbata as the prevailing menardellid species at the same time as the overall abundance of menardellids stabilises initial stabilisation of sinistral coiling until the end of the cold interval at 5 49 ma suggests the development of a temperature preference in sinistral cold and dextral warm m multicamerata a strong increase in menardellid 13c values relative to the thermocline gradient difference between the 18o records of surface dwelling globigerinoides trilobus and menardella spp following 5 77 ma suggests a move to shallower habitat depths of both m multicamerata morphotypes initiated by mixed layer destabilisation both the development of a temperature preference and the move to shallower habitat depth in m multicamerata appear to identify its adjustment to a new niche driving the replacement of m limbata as the dominant menardellid species

b ca in planktonic foraminifera as a proxy for surface seawater ph boron isotope systematics indicate that boron incorporation into foraminiferal caco3 is determined by the partition coefficient k d frac left rm b rm ca right rm caco3 left rm b left rm oh right 4 rm hco 3 right rm seawater and b oh 4 hco3 seawater providing in principle a method to estimate seawater ph and pco2 we have measured b ca ratios in globigerina bulloides and globorotalia inflata for a series of core tops from the north atlantic and the southern ocean and in globigerinoides ruber white from ocean drilling program odp site 668b on the sierra leone rise in the eastern equatorial atlantic b ca ratios in these species of planktonic foraminifera seem unaffected by dissolution on the seafloor k d shows a strong species specific dependence on calcification temperature which can be corrected for using the mg ca temperature proxy a preliminary study of g inflata from southern ocean sediment core chat 16k suggests that temperature corrected b ca was 30 higher during the last glacial correspondingly ph was 0 15 units higher and aqueous pco2 was 95 atm lower at this site at the last glacial maximum the covariation between reconstructed pco2 and the atmospheric pco2 from the vostok ice core demonstrates the feasibility of using b ca in planktonic foraminifera for reconstructing past variations in ph and pco2

paleoproductivity during the middle miocene carbon isotope events a data model approach 1 to what extent are individual middle miocene eccentricity scale benthic foraminiferal carbon isotope maxima the so called cm events related to changes in marine export productivity here we use benthic foraminiferal accumulation rates from three sites in the pacific and southern oceans and a geochemical box model to assess relationships between benthic foraminiferal 13c records export productivity and the global carbon cycle results from deep sea drilling project hole 588 and ocean drilling program site 747 show a distinct productivity maximum during cm 6 at 13 8 ma the time of major expansion of ice on antarctica productivity maxima during other cm events are only recorded at high latitude site 747 a set of numerical experiments tests whether changes in foraminiferal 13c records cm events and export productivity can be simulated solely by sea level fluctuations and the associated changes in global weathering deposition cycles by sea level fluctuations plus global climatic cooling and by sea level fluctuations plus invigorated ocean circulation consistent with data the periodic forcing of sea level and albedo and associated weathering cycles produces 13c variations of the correct temporal spacing albeit with a reduced amplitude a productivity response of the correct magnitude is achieved by enhancing ocean circulation during cold periods we suggest that the pacing of middle miocene 13c fluctuations is associated with cyclical sea level variations the amplitude however is muted perhaps due to the competing effects of a time lagged response to sea level lowstands but an immediate response to invigorated ocean circulation during cold phases

an explanation of the effect of seawater carbonate concentration on foraminiferal oxygen isotopes abstract stable oxygen isotope ratios of foraminiferal calcite are widely used in paleoceanography to provide a chronology of temperature changes during ocean history it was recently demonstrated that the stable oxygen isotope ratios in planktonic foraminifera are affected by changes of the seawater chemistry carbonate system the 18 o of the foraminiferal calcite decreases with increasing co 3 2 concentration or p h this paper provides a simple explanation for seawater chemistry dependent stable oxygen isotope variations in the planktonic foraminifera orbulina universa which is derived from oxygen isotope partitioning during inorganic precipitation the oxygen isotope fractionation between water and the dissolved carbonate species s h 2 co 3 hco 3 co 3 2 decreases with increasing p h provided that calcium carbonate is formed from a mixture of the carbonate species in proportion to their relative contribution to s the oxygen isotopic composition of caco 3 also decreases with increasing p h the slope of shell 18 o vs co 3 2 of orbulina universa observed in culture experiments is 0 0022 mol kg 1 1 spero et al 1997 whereas the slope derived from inorganic precipitation is 0 0024 mol kg 1 the theory also provides an explanation of the nonequilibrium fractionation effects in synthetic carbonates described by kim and o neil 1997 which can be understood in terms of equilibrium fractionation at different p h the results presented here emphasize that the oxygen isotope fractionation between calcium carbonate and water does not only depend on the temperature but also on the p h of the solution from which it is formed

paleoceanographic implications of miocene deep sea hiatuses miocene paleoceanographic evolution exhibits major changes resulting from the opening and closing of passages the subsequent changes in oceanic circulation and development of major antarctic glaciation the consequences and timing of these events can be observed in variations in the distribution of deep sea hiatuses sedimentation patterns and biogeographic distribution of planktic organisms the opening of the drake passage in the latest oligocene to early miocene 25 20 ma resulted in the establishment of the deep circumpolar current which led to thermal isolation of antarctica and increased global cooling this development was associated with a major turnover in planktic organisms resulting in the evolution of neogene assemblages and the eventual extinction of paleogene assemblages the erosive patterns of two widespread hiatuses ph 23 0 22 5 ma and nh 1 20 18 ma indicate that a deep circumequatorial circulation existed at this time characterized by a broad band of carbonate ooze deposition siliceous sedimentation was restricted to the north atlantic and a narrow band around antarctica a major reorganization in deep sea sedimentation and hiatus distribution patterns occurred near the early middle miocene boundary apparently resulting from changes in oceanic circulation beginning at this time deep sea erosion occurred throughout the caribbean hiatus nh 2 16 15 ma suggesting disruption of the deep circumequatorial circulation and northward deflection of deep currents and or intensification of the gulf stream sediment distribution patterns changed dramatically with the sudden appearance of siliceous ooze deposition in the marginal and east equatorial north pacific by 16 0 to 15 5 ma coincident with the decline of siliceous sedimentation in the north atlantic this silica switch may have been caused by the introduction of norwegian overflow water into the north atlantic acting as a barrier to outcropping of silica rich antarctic bottom water the main aspects of the present oceanic circulation system and sediment distribution pattern were established by 13 5 to 12 5 ma hiatus nh 3 coincident with the establishment of a major east antarctic ice cap antarctic glaciation resulted in a broadening belt of siliceous ooze deposition around antarctica increased siliceous sedimentation in the marginal and east equatorial north pacific and indian oceans and further northward restriction of siliceous sediments in the north atlantic periodic cool climatic events were accompanied by lower eustatic sea levels and widespread deep sea erosion at 12 to 11 ma nh 4 10 to 9 ma nh 5 7 5 to 6 2 ma nh 6 and 5 2 to 4 7 ma nh 7

salt diapir downbuilding fast analytical models based on rates of salt supply and sedimentation abstract salt diapir shapes are influenced by the rate of salt supply into the base of the diapir the rate of sedimentation regional slope local deformation around the salt body and regional deformation prior graphical solutions to this complex problem consider only the salt supply and sediment aggradation rate but there are major problems with these existing plots precise computation of diapir shapes using a new fast analytical approach reveals that the shapes shown in previous publications were incorrect rise rate a proxy used for salt supply in previous models was inconsistently defined the fundamental controlling factors used in this study to predict salt diapir shapes are the volumetric rate of salt supply to the base of the diapir and the rate of sediment aggradation these are combined to create predictive charts of salt shapes for both radial and linear geometries we discuss the strengths and limitations intrinsic to our fast analytical models suggesting geological conditions where they may be appropriate simple downbuilding models may not be directly applicable to geological settings that involve subsequent deformation of the sediments or of the salt body however the modified salt downbuilding concept as elaborated in our study can explain many real world situations where the salt body flares or tapers upwards

middle miocene ice sheet dynamics deep sea temperatures and carbon cycling a southern ocean perspective 1 relative contributions of ice volume and temperature change to the global 1 18o increase at 14 ma are required for understanding feedbacks involved in this major cenozoic climate transition a 3 ma benthic foraminifer mg ca record of southern ocean temperatures across the middle miocene climate transition reveals 2 2 c cooling 14 2 13 8 ma indicating that 70 of the increase relates to ice growth seawater 18o calculated from mg ca and 18o suggests that at 15 ma antarctica s cryosphere entered an interval of apparent eccentricity paced expansion glaciations increased in intensity revealing a central role for internal climate feedbacks comparison of ice volume and ocean temperature records with inferred pco2 levels indicates that middle miocene cryosphere expansion commenced during an interval of southern ocean warmth and low atmospheric pco2 the antarctic system appears sensitive to changes in heat moisture supply when atmospheric pco2 was low suggesting the importance of internal feedbacks in this climate transition

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

calibrations for benthic foraminiferal mg ca paleothermometry and the carbonate ion hypothesis benthic foraminiferal magnesium calcium ratios were determined on one hundred and forty core top samples from the atlantic ocean the norwegian sea the indian ocean the arabian sea and the pacific ocean mostly at sites with bottom water temperatures below 5 c mg ca ratios are consistently lower by not vert similar 0 2 mmol mol in samples cleaned using oxidative and reductive steps than using oxidative cleaning differences between cibicidoides species have been identified mg ca of cibicidoides robertsonianus cibicidoides kullenbergi cibicidoides wuellerstorfi comparison with bottom water temperatures support observations of lowered mg ca of c wuellerstorfi at temperature below not vert similar 3 c compared with values predicted by published calibrations and from other cibicidoides species hydrographic data shows that carbonate ion saturation co32 decreases rapidly below this temperature an empirical sensitivity of co32 on mg ca has been established for c wuellerstorfi of 0 0086 0 0006 mmol mol mol kg a novel application using modern temperatures and last glacial maximum temperatures derived via pore fluid modelling supports a carbonate ion saturation state effect on mg incorporation this may significantly affect calculated 18oseawater obtained from foraminiferal 18o and mg ca temperature

the shaping of salt diapirs abstract six parameters shape the geometry of passive diapirs associated with stiff overburden rates of salt supply s dissolution d sediment accumulation a erosion er extension e and shortening sh these parameters change in space and time and hence influence the geometry of the structure as it forms a complex six parameter plot representing the evolution history of a salt diapir can be simplified into three separate graphs this study recommends adding to plots of s against a a second of rate of salt supply s this time with extension rate e and a third of rate of sediment accumulation a plotted against extension rate e integrating these three plots on a single diagram results in a complete description of the evolution history of a diapir when it is applied to three dimensional data however if applied to a profile the plot shows only the two dimensional evolution history of the diapir lateral forces extension or compression have a significant role in moulding the geometry of a salt diapir by influencing the space which it occupies by incorporating extension in the moulding plots of salt diapirs this study introduces the rate of sediment accumulation multiplied by extension e a as a significant factor in moulding salt diapirs by using this rate against the rate of salt supply this study redefines the conventionally accepted interpretation of upward narrowing upward widening and columnar diapirs upward narrowing diapirs form when the rate of salt supply is less than the rate of sediment accumulation multiplied by extension upward widening diapirs form when the salt supply is greater than the rate of sediment accumulation multiplied by extension columnar diapirs form when the rate of salt supply is equal to the rate of sediment accumulation multiplied by extension this new relationship explains the absence of columnar and upward widening diapirs in passive margins where thin skinned extension dominates and emphasizes the significance of lateral movement extension and shortening in moulding the geometry of salt diapirs

orbitally paced climate evolution during the middle miocene monterey carbon isotope excursion one of the most enigmatic features of cenozoic long term climate evolution is the long lasting positive carbon isotope excursion or monterey excursion which started during a period of global warmth after 16 9 ma and ended at similar to 13 5 ma approximately 400 kyr after major expansion of the antarctic ice sheet we present high resolution 1 9 kyr astronomically tuned climate proxy records in two complete sedimentary successions from the northwestern and southeastern pacific odp sites 1146 and 1237 which shed new light on the middle miocene carbon isotope excursion and associated climatic transition over the interval 17 1 12 7 ma we recog nize three distinct climate phases with different imprints of orbital variations into the climatic signals 1146 and 123 7 delta 18 o delta 13 c 1237 xrt fe fraction 63 pm 1 climate optimum prior to 14 7 ma characterized by minimum ice volume and prominent 100 and 400 kyr variability 2 long term cooling from 14 7 to 13 9 ma principally driven by obliquity and culminating with rapid cryosphere expansion and global cooling at the onset of the last and most pronounced delta 13 c increase 3 icehouse mode after 13 9 ma with distinct 100 kyr variability and improved ventilation of the deep pacific the monterey carbon isotope excursion 16 9 13 5 ma consists overall of nine 400 kyr cycles which show high coherence with the long eccentricity period superposed on these low frequency oscillations are high frequency variations 100 kyr which closely track the amplitude modulation of the short eccentricity period in contrast to delta 13 c the delta 18 o signal additionally shows significant power in the 41 kyr band and the 1 2 myr amplitude modulation of the obliquity cycle is clearly imprinted in the 1146 delta 18 o signal our results suggest that eccentricity was a prime pacemaker of middle miocene climate evolution through the modulation of long term carbon budgets and that obliquity paced changes in high latitude seasonality favored the transition into the icehouse climate c 2007 elsevier b v all rights reserved

miocene equatorial and southwest pacific paleoceanography from stable isotope evidence abstract stable isotope results from seven miocene deep sea drilling projects in the equatorial and southwest pacific ocean previously correlated using carbon isotope stratigraphy have been examined discussed and interpreted in terms of the development of the miocene pacific ocean the most obvious features of the benthonic foraminiferal stable isotopic records are a major increase in 18o 1 0 during the middle miocene a series of long term oscillations 2 3 my of amplitude 0 5 0 75 and a decrease in 13c values 0 5 1 0 during the latest miocene planktonic foraminiferal 18o records show different trends for high and low latitude regions in the equatorial pacific planktonic 18o values actually decrease during the miocene whereas in the higher southern latitudes planktonic 18o values become more positive in response to cooling surface waters planktonic 13c records show opposite trends to 18o with the high latitude values becoming more negative relative to the tropical regions the development of the miocene pacific ocean in terms of its vertical and horizontal thermal structure and isotopic composition is well illustrated by examining changes in the isotopic difference between planktonic and benthonic foraminifera 18ob p benthonic planktonic is a measure of the thermal structure of the water column 18oph pl high latitude low latitude planktonic values is a measure of the latitudinal temperature gradient 13cb p is an indirect measure of nutrient concentrations in the water column and 13cph pl measures differences in surface water nutrient concentrations between high and low latitude 18ob p increases during the miocene with the greatest increase occurring in the middel miocene at about 14 ma by the latest miocene the isotopic gradient at site 289 in the equatorial pacific approaches the present day isotopic gradient about 4 5 an increase in 18oph pl during the miocene suggests that the latitudinal temperature gradient increased by about 6 c to a value of 12 c in the latest miocene between sites 289 equator and 281 subantarctic 13cb p and 13cph pl values are relatively constant through the early miocene but begin to increase during the middle miocene bottom water 13c values respond similarly at all sites but surface water 13c values exhibit different trends because higher latitude values begin to decrease this decrease perhaps suggests that phosphate concentrations may have increased due to increased upwelling as the circum antarctic circulation system evolved its present day characteristics the isotopic data compiled in this paper suggest that the southwest pacific was responding uniformly to some global or at least pacific wide control during the early miocene in the middle miocene the response became more complex as the low and high latitudes began to show independent trends the changes in the thermal vertical and latitudinal structure probably occurred in respons to the build up of the east antarctic ice sheet intensification of bottom water circulation and an increase in zonal circulation in surface waters in the southern hemisphere the changes in 13c vertical and latitudinal gradients are due to some complex interaction of sea level continental hypsometry climate and biological processes coupled with oceanic circulation changes a strong correlation between estimated sea level changes and 13c values suggests that transgressions and regressions play a critical role in controlling the flux of oxidized organic carbon enriched in12c from the continental shelves and epicontinental seas to the open ocean

miocene evolution of atmospheric carbon dioxide changes in pco2 or ocean circulation are generally invoked to explain warm early miocene climates and a rapid east antarctic ice sheet eais expansion in the middle miocene this study reconstructs late oligocene to late miocene pco2 from ep values based on carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifera from deep sea drilling project sites 588 and 608 and ocean drilling program site 730 our results indicate that highest pco2 occurred during the latest oligocene 350 ppmv but decreased rapidly at 25 ma the early and middle miocene was characterized by low pco2 260 190 ppmv lower intervals of pco2 correspond to inferred organic carbon burial events and glacial episodes with the lowest concentrations occurring during the middle miocene there is no evidence for either high pco2 during the late early miocene climatic optimum or a sharp pco2 decrease associated with eais growth paradoxically pco2 increased following eais growth and obtained preindustrial levels by 10 ma although we emphasize an oceanographic control on miocene climate low pco2 could have primed the climate system to respond sensitively to changes in heat and vapor transport

numerical modelling of the displacement and deformation of embedded rock bodies during salt tectonics a case study from the south oman salt basin large rock inclusions are embedded in many salt bodies and these respond to the move ments of the salt in a variety of ways including displacement folding and fracturing one mode of salt tectonics is downbuilding whereby the top of a developing diapir remains in the same vertical position while the surrounding overburden sediments subside we investigate how the differential displacementofthe top saltsurface causedbydownbuilding inducesductilesaltflowand the associ ated deformation of brittle stringers by an iterative procedure to detect and simulate conditions for the onset of localization of deformation in a finite element model in combination with adaptive remeshing the model set up is constrained by observations from the south oman salt basin where large carbonate bodies encased in salt form substantial hydrocarbon plays the model shows that depending on the displacement of the top salt the stringers can break very soon after the onset of salt tectonics and can deform in different ways if extension along the inclusion dom inates stringersarebrokenbytensilefracturesandboudinageatrelativelyshallowdepth spacingof the boudin bounding faults can be as close as 3 4 times the thickness of the stringer in contrast salt shortening along the inclusion may lead to folding or thrusting of stringers

thinking outside the zone high resolution quantitative diatom biochronology for the antarctic neogene abstract developing a coherent neogene history of the antarctic ice sheet and southern ocean requires a high resolution biostratigraphic age model one that is applicable to the array of existing onshore and offshore paleoenvironmental records this study integrates comprehensive diatom biostratigraphy magnetostratigraphy and tephrostratigraphy from 32 neogene sections around the southern ocean and antarctic continental margin framing their correlation as a constrained optimization conop permits the use of computer implemented approaches that resemble multidimensional versions of graphic correlation the method establishes the most parsimonious sequence of events subject to an expert s selection of suitable input data determination of geological feasibility constraints and the most appropriate measure of fit to the data optimization quantitative output includes assessments of record quality and confidence intervals on all age assignments thorough review and testing of conop model settings and input datasets established the solution s sensitivity to these factors and guided the selective removal of unreliable data to produce a robust precise and reproducible biochronology two complementary models based on different assumptions about reworking yield independent estimates of average local ranges and total regional ranges of fossil taxa in the southern high latitudes the resulting composite sequences include range data of 116 diatom taxa as well as 52 paleomagnetic reversals and two radiometrically dated ashes which enable age calibration to within an average of 0 08 m y for first and last appearances as old as 18 ma deviations of new model ages from previously published estimates are generally small and reflect the different sample sizes available for traditional versus new quantitative biostratigraphic calibrations our results confirm the reliability and regional isochroneity of most commonly utilized diatom zonal marker events identify many new potentially useful events and provide up to an order of magnitude greater temporal resolution than traditional zonations the anticipated expansion of this new southern ocean chronostratigraphic framework to new localities and other microfossil groups will pave the way to further age refinement and broader application of this approach in the future

glacial interglacial changes in subantarctic sea surface temperature and δ18o water using foraminiferal mg laboratory culturing experiments with living globigerina bulloides indicate that mg ca is primarily a function of seawater temperature and suggest that mg ca of fossil specimens is an effective paleotemperature proxy using culturing results and a core top neogloboquadrina pachyderma calibration we have estimated glacial interglacial changes in sea surface temperature sst using planktonic mg ca records from core rc11 120 in the subantarctic indian ocean 43 s 80 e and core e11 2 in the subantarctic pacific ocean 56 s 115 w our results suggest that glacial sst was about 4 c cooler in the subantarctic indian ocean and 2 5 c cooler in the subantarctic pacific comparison of sst and planktonic 18o records indicates that changes in sst lead changes in 18o by on average 1 3 kyr the glacial interglacial temperature change indicated by the subantarctic mg ca records suggests that temperature accounts for 40 60 of the foraminiferal 18o change we have used the mg ca based sst estimates and 18o determinations to generate site specific seawater 18o records which suggest that seawater 18o was on average 1 more positive during glacial episodes compared with interglacial episodes

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

impacts of orbital forcing and atmospheric carbon dioxide on miocene ice sheet expansion the processes causing the middle miocene global cooling which marked the earth s final transition into an icehouse climate about 13 9 million years ago myr ago 1 4 remain enigmatic tectonically driven circulation changes 5 6 and variations in atmospheric carbon dioxide levels 7 8 have been suggested as driving mechanisms but the lack of adequately preserved sedimentary successions has made rigorous testing of these hypotheses difficult here we present high resolution climate proxy records covering the period from 14 7 to 12 7 million years ago from two complete sediment cores from the northwest and southeast subtropical pacific ocean using new chronologies through the correlation to the latest orbital model 9 we find relatively constant low summer insolation over antarctica coincident with declining atmospheric carbon dioxide levels at the time of antarctic ice sheet expansion and global cooling suggesting a causal link we surmise that the thermal isolation of antarctica played a role in providing sustained long term climatic boundary conditions propitious for ice sheet formation our data document that antarctic glaciation was rapid taking place within two obliquity cycles and coincided with a striking transition from obliquity to eccentricity as the drivers of climatic change

δ18o seasonality of planktonic foraminifera from southern ocean sediment traps latitudinal gradients and implications for paleoclimate reconstructions abstract the oxygen isotopic record obtained from globigerina bulloides globorotalia inflata and neogloboquadrina pachyderma s was analysed for 5 sediment traps moored in the southern ocean and southwest pacific the traps extend from subtropical to the polar frontal environments providing the first analysis of seasonal foraminiferal 18 o records from these latitudes comparison between the foraminiferal records and various equations for predicted 18 o of calcite reveals that the predicted 18 o is best captured by the equations of epstein et al 1953 epstein s buchsbaum r lowenstam h a urey h c 1953 revised carbonate water isotopic temperature scale geological society of america bulletin 64 1315 1326 and kim and o neil 1997 kim s t o neil j r 1997 equilibrium and non equilibrium oxygen isotope effects in synthetic carbonates geochimica et cosmochimica acta 61 3461 3475 the epstein equation shows a constant offset from the 18 o of g bulloides and n pachyderma s across the full range of latitudes the seasonal range in 18 o values for these two species implies a near surface habitat across all sites while g inflata most likely dwells at 50 m depth a significant finding in this study was that offsets from predicted 18 o for g bulloides do not correlate to changes in the carbonate ion concentration this suggests that co 3 2 in and of itself may not capture the full range of carbonate chemistry conditions in the marine system this sediment trap deployment also reveals distinct seasonal flux patterns for each species comparison between flux weighted isotopic values calculated from the sediment traps and the isotopic composition of nearby surface sediments indicates that the sedimentary records retain this seasonal imprint at the 51 s site g bulloides has a spring flux peak while n pachyderma s is dominated by summer production the imprint of these differing seasonal production patterns on the flux weighted 18 o values results in more depleted 18 o for n pachyderma s than for g bulloides interpretations of n pachyderma s isotopic composition as a record of subsurface conditions may therefore need to be refined with implications for interpretation of depleted glacial 13 c for this species sigman boyle 2000 sigman d m boyle e a 2000 glacial interglacial variations in atmospheric carbon dioxide nature 407 859 869

sediment accumulation rates in time and space paleogene genetic stratigraphic sequences of the northwestern gulf of mexico basin the paleogene fill of the northwestern gulf of mexico basin consists of eight genetic stratigraphic sequences bounded by regional marine flooding surfaces calculation of sediment accumulation rates along dip profiles through four subbasins shows that regional changes in accumulation rate and by inference rate of sediment supply of fivefold to tenfold repeatedly occurred over time spans of 1 to 3 m y major sequences record episodes of high supply and accommodation limited accumulation periods of declining and low supply were characterized by short term over accommodation and consequent transgressive flooding of the basin margin within sequences depositional rates varied with position relative to the contemporaneous shelf margin with type of depositional system and between subbasins normal patterns of load induced basin margin subsidence and creation of accommodation space were modified by perturbation of the crustal stress regime

the mid pleistocene climate transition onset of 100 ka cycle lags ice volume build up by 280 ka the mid pleistocene climate transition mpt is the complex climatic change which brought the late pleistocene ice ages we explore the mpt in the time and frequency domains by new methods of time series analysis high resolution oxygen isotope records reveal that the ice volume related increase in 18o mean amplitude 0 29 0 05 1 transition midpoint 922 12 ka duration 40 9 ka significantly preceded the abrupt increase in the amplitude of the 100 ka cycle at 641 9 ka this finding can be quantitatively simulated using a simple ice bedrock model in which due to the additional ice the calving threshold is exceeded the simulated calving events prior to 650 ka are separated by 77 ka whereas after 650 ka they occur pseudo periodically with a mean period of nearly 100 ka the cause for the delay of the 100 ka calving cycle was not a slow bedrock relaxation rather the coincidental combination of insolation existing ice mass and bedrock depression