structure of the upper ocean in the western equatorial pacific the western equatorial pacific has an important role in the el nino southern oscillation climate cycle1 2 and sea surface temperature anomalies in this area are known to affect the global atmospheric circulation3 little is known however about the factors that control the response of the upper ocean and hence sea surface temperature to changes in atmospheric conditions here we present observations of the density and current structure of the upper layers of the western equatorial pacific ocean we observed frontal features with temperature differences of 1 c extending from the surface down to a depth of 150 m a strong salinity front associated with a surface convergence and interleaving of high and low salinity waters in 10 m thick layers that extended for several hundred kilometres in the north south direction the large spatial and temporal variability in temperature and salinity observed must influence heat transport and hence the ocean response to atmospheric forcing in particular the response to westerly wind bursts which are thought to be precursors to an el nino we estimate the horizontal diffusion coefficient of the interleaving which indicates that the observed layering may con tribute to horizontal mixing within the thermocline

formation and evolution of the tancheng lujiang wrench fault system a major shear system to the northwest of the pacific ocean abstract in the northwestern peri pacific tectonic belt on the continental margin of east asia there is a series of major faults trending ne nne i e the tancheng lujiang wrench fault system this fault system extends more than 5000 km with a width of 800 1000 km in the continent and a general strike of n30 e on the basis of the results of special research on the major faults and a lot of new information about the fault system the authors discuss early mesozoic triassic jurassic period sinistral transcurrent faulting in the different regions it is suggested that in the eastern continent of east asia the e w trending tectonic pattern of the paleozoic and beginning of the mesozoic appear in the nuclei of the sino korean continent massif and the yangtze continent massif these ancient continent massifs accreted both southwards and northwards which finally led to their collision and suturing to the angura continent massif later this tectonic framework was changed greatly by the tancheng lujiang wrench fault system along which left lateral horizontal displacement of 1000 1500 km took place since the mesozoic the dislocation and deformation of horizontal displacement of the tancheng lujiang fault system produced a series of structures trending ne nne in the continental margin which have greatly transformed the ancient structures of the preceding mesozoic this is an important diktyogenese in the continent of east asia during the horizontal displacement motion of these faults the folds nappes and decollement structures pull apart basins and subductions within the continent occurred in the continent at the same time some large sinistral ductile shear zones of horizontal displacement appeared the depth of the deformed horizon of ductile shear which belongs to simple shear was controlled by the tectonic element and distance away from the continental margin whilst the ductile shear deformation occurred the fault dynamo thermal metamorphism and the fault remelting took place and magma activity appeared within the continent in the mesozoic the fault zones used to be high energy zones and recently became the seismic belts within the continent formation of the tan lu fault system was due to the collision between the southern and northern blocks its large sinistral horizontal displacement in the mesozoic was closely related to the kula plate moving northwards therefore the continent disintegrated by shearing which produced some new terranes the shear edge belt was formed between the continent and ocean the change of relative motion between the continent and pacific ocean plate transformed the tan lu fault system into first extension and then compression clarifying the characteristics of the tan lu fault system is an important key in understanding the evolution of the east asia continent since the mesozoic

oxygen isotopic composition of multiple species of planktonic foraminifera recorders of the modern photic zone temperature gradient abundances of 12 species of planktonic foraminifera collected in two plankton tows from the east tropical atlantic are compared to the chlorophyll content and the temperature of the sea water from which they were collected as expected from previous work in the tropics all dominant tropical species occur in greatest abundance within the photic zone many species occur in greatest abundances in the seasonal thermocline in association with the maximum chlorophyll concentration while a few algal symbiont bearing species occur in greatest abundance in the mixed layer the 18o measurements of planktonic foraminifera shells from core top sediment samples confirm the vertical stratification within the photic zone that is suggested by the relationship between hydrography and abundances found in the plankton tows and found in the statistical study by ravelo et al 1990 comparison between the measured 18o values of planktonic foraminifera with the predicted 18o profiles of the overlying water column at three core locations indicate that species abundances in the sediment record the seasonally integrated conditions of the photic zone and suggests that the abundance of a species in the sediment depends on whether the preferred ecological conditions of that species may be found within the photic zone of the overlying water column sometime during the year species which calcify below the photic zone have only trace relative abundances finally it appears that the total range of 18o values of the dominant species approximates the predicted annual 18o of calcite range in the upper 80 m of the water column

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

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

variations in the strontium isotopic ratio of seawater during the miocene stratigraphic and geochemical implications a composite strontium isotopic seawater curve was constructed for the miocene between 24 and 6 ma by combining 87sr 86sr measurements of planktonic foraminifera from deep sea drilling project sites 289 and 588 site 289 with its virtually continuous sedimentary record and high sedimentation rates 26 m m y was used for constructing the oligocene to mid miocene part of the record which included the calibration of 63 biostratigraphic datums to the sr seawater curve using the timescale of cande and kent 1992 across the oligocene miocene boundary a brief plateau occurred in the sr seawater curve 87sr 86sr values averaged 0 70824 which is coincident with a carbon isotopic maximum cm o m from 24 3 to 22 6 ma during the early miocene the strontium isotopic curve was marked by a steep rise in 87sr 86sr that included a break in slope near 19 ma the rate of growth was about 60 ppm m y between 22 5 and 19 0 ma and increased to over 80 ppm m y between 19 0 and 16 ma beginning at 16 ma between carbon isotopic maxima cm3 and cm4 of woodruff and savin 1991 the rate of 87sr 86sr growth slowed and 87sr 86sr values were near constant from 15 to 13 ma after 13 ma growth in 87sr 86sr resumed and continued until 9 ma when the rate of 87sr 86sr growth decreased to zero once again the entire miocene seawater curve can be described by a high order function and the first derivative d87sr 86sr dt of this function reveals two periods of increased slope the greatest rate of 87sr 86sr change occurred during the early miocene between 20 and 16 ma and a smaller but distinct period of increased slope also occurred during the late miocene between 12 and 9 ma these periods of steepened slope coincide with major phases of uplift and denudation of the himalayan tibetan plateau region supporting previous interpretations that the primary control on seawater 87sr 86sr during the miocene was related to the collision of india and asia the rapid increase in 87sr 86sr values during the early miocene from 20 to 16 ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans in the absence of another source of co2 these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric co2 and climatic cooling through a reversed greenhouse effect the paleoclimatic record however indicates a warming trend during the early miocene culminating in a climatic optimum between 17 and 14 5 ma we suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic co2 from the eruption of the columbia river flood basalts crfb between 17 and 15 ma the decrease in the rate of crfb eruptions at 15 ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in monterey deposits eventually led to cooling and increased glaciation between 14 5 and 13 ma the crfb hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the monterey at 17 5 ma as marked by increased 13c values and the climatic cooling and glaciation during the middle miocene as marked by the increase in 18o values which did not begin until 14 5 ma

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

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

geochemical assessment of the palaeoecology ontogeny morphotypic variability and palaeoceanographic utility of dentoglobigerina venezuelana to better understand the links between the carbon cycle and changes in past climate over tectonic timescales we need new geochemical proxy records of secular change in silicate weathering rates a number of proxies are under development but some of the most promising e g palaeoseawater records of li and nd isotope change can only be employed on such large samples of mono specific foraminifera that application to the deep sea sediment core archive becomes highly problematic dentoglobigerina venezuelana presents a potentially attractive target for circumventing this problem because it is a typically large 355 m diameter abundant and cosmopolitan planktic foraminifer that ranges from the early oligocene to early pliocene yet considerable taxonomic and ecological uncertainties associated with this taxon must first be addressed here we assess the taxonomy palaeoecology and ontogeny of d venezuelana using stable isotope oxygen and carbon and mg ca data measured in tests of late oligocene to early miocene age from ocean drilling program odp site 925 on ceara rise in the western equatorial atlantic to help constrain the depth habitat of d venezuelana relative to other species we report the stable isotope composition of selected planktic foraminifera species within globigerina globigerinoides paragloborotalia and catapsydrax we define three morphotypes of d venezuelana based on the morphology of the final chamber and aperture architecture we determine the trace element and stable isotope composition of each morphotype for different size fractions to test the validity of pooling these morphotypes for the purposes of generating geochemical proxy datasets and to assess any ontogenetic variations in depth habitat our data indicate that d venezuelana maintains a lower thermocline depth habitat at ceara rise between 24 and 21 ma comparing our results to published datasets we conclude that this lower thermocline depth ecology for the oligo miocene is part of an eocene to pliocene evolution of depth habitat from surface to sub thermocline for d venezuelana our size fraction data advocate the absence of photosymbionts in d venezuelana and suggest that juveniles calcify higher in the water column descending into slightly deeper water during the later stages of its life cycle our morphotype data show that 18o and 13c variation between morphotypes is no greater than within morphotype variability this finding will permit future pooling of morphotypes in the generation of the sample hungry palaeoceanographic records

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

mercury ores from the new idria mining district california geochemical and stable isotope studies abstract the mercury ores of the new idria mining district are composed of metacinnabar and cinnabar with minor amounts of stibnite and iron disulfides metacinnabar from the main ore zone at the new idria mine contains fe as a major impurity 0 96 to 2 80 wt whereas that from the co veins contains both se 0 30 to 6 15 wt and fe 0 13 to 1 18 wt in solid solution the impurity content varies within the same specimen and within individual crystals as well and demonstrates lack of chemical equilibrium impurities tend to concentrate at the interfaces between metacinnabar and cinnabar in partly transformed crystals thus increasing the barrier to nucleation of cinnabar and retarding or prohibiting any further transition of metacinnabar most of the pyrite and marcasite crystallized earlier than cinnabar and in rare cases contain nuclei of framboidal pyrite pyrite and marcasite crystallized as discrete phases and the latter does not show evidence of transition to pyrite dolomite shows normal marine carbonate 18 o 24 6 to 29 7 and 13 c 1 7 to 2 5 values quartz displays 18 o ranging from 18 5 to 21 5 distinctly lower than the associated dolomites suggesting that the two minerals may have formed at two different stages the range of 34 s values for metacinnabar 0 0 to 3 2 overlaps with that of cinnabar 1 3 to 4 4 the 34 s for pyrite ranges from 3 1 to 13 0 pyrite with framboidal nuclei shows the highest 34 s values the sulfur isotope data also suggest the lack of isotopic equilibrium the source of c in the ore forming fluids was marine carbonates leached from the franciscan sediments the source of s is uncertain because of lack of knowledge of the isotopic composition of total sulfur it is likely that mixed sources of sulfur leached from sulfides in the host rocks and magmatic sulfur are involved

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

chemostratigraphic documentation of a complete miocene intermediate depth section in the southern ocean ocean drilling program site 1120 campbell plateau off new zealand abstract an integrated chemostratigraphic 87sr 86sr 13c and 18o study of benthic foraminifera is presented for a 210 m thick intermediate depth upper middle bathyal transition miocene nannofossil ooze section of ocean drilling program site 1120 campbell plateau off new zealand our results indicate that new 87sr 86sr 13c and 18o profiles are wholly consistent with their respective miocene reference curves these observations facilitate identification of a total of five reliable chemostratigraphic datums which are based on the fundamental structural changes in the 87sr 86sr curve and paired simultaneous 13c and 18o events the resultant age depth relationship clearly shows that the miocene 20 5 ma biopelagic sedimentation on the campbell plateau was essentially continuous at a moderate to high linear sedimentation rate 17 5 m m y with an exception of the uppermost 13 m our findings do not support the shipboard biostratigraphic age model which assumes that the critical early middle miocene transition was interrupted by a major hiatus

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

pliocene pleistocene evolution of eastern tropical pacific surface water circulation and thermocline depth we evaluated the response of eastern equatorial pacific upper water column hydrography to the panama seaway closure and the initiation of large scale northern hemisphere glaciation the 18o gradient between globorotalia tumida and globigerinoides sacculifer indicates a general shoaling of the thermocline between 4 2 and 3 0 ma which may be related to a recorded decrease in southeast trade wind strength the 13c gradients suggest that surface water nutrient concentrations have increased during the last 5 myr and that upwelling intermediate waters began to change sources at 3 2 ma changes in the trans pacific east west temperature and nutrient gradients at 4 0 and 1 5 ma are coincident with the seaway closure and a major phase of northern hemisphere glaciation changes in the sensitivity of surface circulation to milankovitch forcing 18o records linearly related to orbital variations only after 3 2 ma are associated with the seaway closure pleistocene initiation of 100 and 41 kyr cycles in the thermocline depth proxy may suggest that the hydrographic response to milankovitch forcing is enhanced by air sea interactions which maintain the relatively steep pleistocene trans pacific gradients the 13c records are dominated by the 41 kyr period and are usually coherent with high latitude climate suggesting that associated changes in the global carbon reservoir dominate the tropical 13c signals

clinal variation in aperture shape in southwest pacific globorotalia partimlabiata and its ancestor about the time of the middle miocene climatic transition abstract to clarify the taxonomy and biostratigraphy of southwest pacific populations currently identified as fohsella peripheroronda a morphometric study is reported on the shape of its aperture in collections from dsdp sites 588a and 593 and from the stratotype of the new zealand lillburnian stage this material is compared with globorotalia partimlabiata from serravallian strata in sicily shape analyses use 50 equally spaced coordinates along the margin of the apertural re entrant of the last preserved chamber data are aligned by the procrustes algorithm and projected onto principal component axes canonical discriminant analysis of these scores shows that convexity of the margin increases clinally the cline is interpreted as primary evidence for a previously unrecognized lineage for which a provisional taxonomy is advanced globorotalia partimlabiata which has individuals with strongly convex margins represents the climax of the cline the highest occurrence of its ancestor recognized here informally as globorotalia cf peripheroronda is determined by the highest sample whose mean shape does not differ significantly from its earliest representative at site 593 there and at site 588a that datum is close to or within miocene isotope event mi 3b the architectural history of the lineage differs from the tropical fohsellids and its relation to them is unresolved

vertical thermal structure evolution of miocene surface waters western equatorial pacific dsdp site 289 abstract time series analyses from 18 7 to 5 8 ma of the oxygen isotopic composition of four shallow dwelling planktic foraminifera globorotalia mayeri dentoglobigerina altispira globoquadrina baroemoenensis andglobigerinoides immaturus one deep dwelling planktic foraminifer globoquadrina venezuelana and a benthic foraminifer cibicidoides spp from deep sea drilling project site 289 2206 m present day depth are used to determine the evolution of vertical thermal structure in the western equatorial pacific comparisons of composite oxygen isotopic curves representing the surface mixed layer the thermocline and deep waters reveal three distinct intervals of stability in the position and strength of the thermocline during the miocene 18 7 to 16 1 ma 11 1 to 9 9 ma and 7 5 to 5 8 ma separated by two intervals of instability of the thermocline 16 1 to 11 1 ma and 9 9 to 7 5 ma from 18 7 to 16 1 ma vertical thermal gradients were the weakest of the miocene interval studied and the water column was highly stable vertical thermal gradients strengthened between 16 1 and 11 1 ma as equatorial surface waters warmed and deep waters cooled during an inferred major growth phase of the east antarctic ice sheet as a consequence the strength and position of the thermocline became highly variable this was followed from 11 1 to 9 9 ma by relative climatic stability well defined vertical thermal gradients and a slight warming of the water column by 9 9 ma increasing restriction of surface water flow through the indonesian seaway led to the piling up of warm surface waters in the western equatorial pacific and a deepening of the thermocline deep waters continued to cool and vertical thermal gradients reached a miocene maximum by 7 5 ma from 7 5 to 5 8 ma vertical thermal gradients were stable and the water column became highly stratified and marked by a thick warm surface layer and a deep well defined thermocline based upon a previous assumption that ice volume growth during the middle miocene 16 1 to 11 1 ma contributed to an increase in 18o values of 0 5 to 0 7 we have estimated water column temperature changes at site 289 between 18 7 and 5 8 ma surface waters are inferred to have warmed 2 to 4 c between 18 7 and 5 8 ma with about half of the warming occurring from 16 1 to 11 1 ma between 11 1 and 9 9 ma the entire water column warmed and from 9 9 to 7 5 ma the mixed layer expanded and the thermocline deepened as warm waters piled up in the western equatorial pacific in response to the closure of the indonesian seaway the net decrease of thermoclinal 18o values between 11 1 and 7 5 ma reflects an upward change in position of the deep dwelling planktic foraminifera globoquadrina venezuelana relative to the position of the thermocline as the thermocline deepened after 7 5 ma relative 18o values indicate thatgloboquadrina venezuelana was a deep dwelling mixed layer species deep waters 2400 to 2200 m at site 289 cooled 2 to 4 c between 18 7 and 5 8 ma with about half of the cooling occurring from 16 1 to 11 1 ma and the other half from 9 9 to 7 5 ma

changes in the depth habitat of the oligocene planktic foraminifera dentoglobigerina venezuelana induced by thermocline deepening in the eastern equatorial pacific understanding planktic foraminiferal depth habitat along with consistent taxonomic concepts is key to accurate reconstruction of paleoceanographic records the oligocene pliocene long ranging and widely distributed species dentoglobigerina venezuelana lived in the mixed layer shallower during the early oligocene whereas the same species calcified at thermocline or subthermocline depths deeper during the late oligocene and miocene the exact timing of the species depth habitat change and its possible relationships with oligocene climate dynamics remain unknown here we reveal isotopic records of d venezuelana along with the paragloborotalia siakensis group a mixed layer dweller by using sediments at integrated ocean drilling program site u1334 in the eastern equatorial pacific throughout the oligocene a two step depth habitat change of d venezuelana is apparent 1 from upper to lower mixed layer 27 4 ma and 2 from lower mixed layer to thermocline depth 26 3 ma in addition the planktic foraminiferal faunal assemblage experienced a marked change from dominantly thermocline deeper species to abundant mixed layer shallower species suggesting that depth habitat shifts of d venezuelana were clearly related to thermocline deepening in the eastern equatorial pacific comparison of the first isotopic shift 27 4 ma at multiple sites u1334 u1333 and 1218 revealed a southward depth habitat change of d venezuelana within 200 kyr implying overall thermocline deepening with reduced steepness in the eastern equatorial pacific we consider that global warming conditions during the late oligocene likely caused thermocline deepening with upwelling decrease in the eastern equatorial pacific guiding d venezuelana to adapt to greater depths in the water column

morphology and relative frequency of planktic foraminiferal species in relation to oxygen isotopically inferred depth habitats abstract in modern low latitude open oceans planktic foraminiferal species with complex morphologies and or thickened shells usually dwell in the subsurface and deeper waters morphologically simple species with globular chambers and an umbilical aperture tend to be surface water dwellers this has led to the widely held belief that the relative depth habitats of extinct forms can be deduced from their test shape with planoconical or keeled species dwelling below globigerinids yet oxygen isotope ratios have shown some globorotaliids e g morozovella to have been surface dwellers by determining the relative depth position of many different fossil groups from their oxygen isotope ratios i found no relationship between test shape and preferred depth habitat many keeled and or planoconical forms globotruncana contusotruncana morozovella fohsella and turborotalia as well as unkeeled groups acarinina globigerinatheka some jenkinsella and globoconella were surface or subsurface dwellers deep dwellers include the keeled planomalina globotruncanita and planorotalites the unkeeled catapsydrax some jenkinsella and most paleogene globigerinids usually the most common group shows relatively low oxygen isotope ratios indicating that surface dwelling species are more abundant and or had a shorter life cycle with a larger shell output the tests of most inferred surface dwellers are relatively susceptible to dissolution with the globigerinathekids and acarinina senni as notable exceptions the low diversity oligocene faunas dominated by globigerinids followed the extinction of the morphologically diverse surface dwelling part of the late eocene fauna not the elimination of deep dwelling species

hantkeninid depth adaptation an evolving life strategy in a changing ocean the interplay between evolution paleoecology and environmental change is examined in a geochemical study of a group of eocene planktonic foraminifera the hantkeninids which are well known biostratigraphic inde fossils underwent spectacular long term evolution in the middle and upper eocene 49 0 33 7 ma a time when major global climate and oceanic changes were occurring we use oxygen and carbon isotope analysis of their shell calcite to investigate how their habitat changed as they evolved the hantkeninids originated in a deep water oxygen minimum environment but migrated into fully oxygenated near surface waters as global temperatures decreased and water column stratification declined this change in depth ecology coincided with pronounced morphological evolution involving changes in chamber shape and degree of inflation and modification of the primary aperture these developments are considered to be adaptations to a near surface habitat

vertical thermal gradient history in the eastern equatorial pacific during the early to middle miocene implications for the equatorial thermocline development knowledge of the equatorial thermocline is essential for understanding climate changes in the tropical pacific multispecies planktic foraminiferal analyses provide a way to examine temperature distributions and thus the structure of the thermocline although the secular thermocline development has been documented back to the late miocene the early to middle miocene interval has rarely been examined in addition relationships with the dynamic antarctic ice sheets remain unclear here we investigate the vertical thermal gradient in the upper water column at integrated ocean drilling program site u1337 in the eastern equatorial pacific eep throughout the early to middle miocene 23 1 to 11 7 ma the gradient increased over the miocene climatic optimum whereas it decreased during the east antarctic ice sheet expansion eaie comparison of the eep record with its western equatorial pacific wep counterpart suggests that sea surface temperature was more stable in the wep than in the eep we further estimated equatorial thermocline from two diagonal gradients between the eep and the wep thermocline shoaled from 16 7 to 15 7 ma and tilt weakened between 16 5 and 13 8 ma the onset of the monterey excursion and the reduced antarctic ice sheet volume would have affected thermocline depth and tilt respectively thermocline depth was likely much deeper compared to pliocene to modern conditions furthermore a 4 point based distribution of isotherms 4di index was used as a metric of the evenness or unevenness of the isotherm distributions the 4di index considerably reduced at around the eaie and other mi events reflecting the evenly distributed isotherms under a more glaciated antarctica