deep sea biostratigraphy of prograding platform margins neogene bahamas key evidence linked to depositional rhythm abstract new foraminiferal evidence from two boreholes on the paleoshelf and slope of western great bahama bank has wide ranging implications for understanding formation and evolution of carbonate platform margins the new data abundant well preserved planktic foraminifera were obtained by disaggregating samples from intercalated pelagic layers and selected parts of thick hemipelagic limestone earlier efforts to obtain biostratigraphic ages identified six biostratigraphic units in each borehole provided biozonal age alternatives for both holes and resulted in different pliocene biozones between them the new data define six units in one hole and seven in the other bracket the biozones present and their ages indicate different sedimentation rates and show that within the limits of biostratigraphic resolution the biozones are correlative between the holes most importantly the revised ages show that the paleoshelf borehole probably penetrated the late miocene rather than middle miocene the oldest unit is on the paleoshelf and the youngest uppermost pliocene is on the slope between the holes the stratigraphic interval spans the temporal interval from an inferred maximum of 10 2 ma to a minimum of 1 6 ma although the biozones range sequentially from the neogloboquadrina acostaensis n16 zone to the basal part of the globorotalia truncatulinoides truncatulinoides n22 zone globorotalia crassaformis viola subzone absence of key species indicates that deposition was discontinuous numerous periods of erosion and or nondeposition are inferred the largest of which is a condensed section hiatus 1 2 myr above the paleoslope miocene pliocene boundary in addition the late pliocene globorotalia tosaensis tosaensis n21 zone is not recognized on the slope its absence is consistent with a widespread regional unconformity sedimentation rates and depths of series boundaries vary widely in both holes the paleoslope miocene pliocene boundary lies at 540 m below top of the hole the lower upper pliocene boundary is placed at or near 444 m position of the pliocene pleistocene boundary is less certain but is within the top 382 m of the hole its placement anywhere within this interval is a reasonable assessment considering an exceptionally high rate of sedimentation 562 m myr 168 6 m interval based on topmost foraminiferal sample 1 9 1 6 ma as expected the lowest sedimentation rate occurs in the condensed section overlying the miocene pliocene boundary 5 m myr 9 5 m 5 3 4 1 ma the paleoshelf miocene pliocene boundary lies below a hiatal condensed section 295 278 m below top of the hole that has a greater sedimentation rate 89 m myr 17 7 m 5 5 5 3 ma than that at the slope the lower upper pliocene boundary is placed at or near a depth of 236 m and the pliocene pleistocene boundary lies within the top 113 m of the hole sedimentation rates on the shelf range from 15 m myr above the condensed section 22 9 m 5 3 3 8 ma to a late pliocene high of 183 m myr 54 9 m interval based on the point at which the age depth line crosses the 1 9 myr mark between the topmost two fossiliferous samples 2 2 1 9 ma

hiatal duration of correlative sequence boundaries from oligocene pliocene mixed carbonate siliciclastic sediments of the south florida platform abstract sequence boundaries defined by regional unconformities in mixed carbonate siliciclastic sediments have a variable duration from the margin to the interior of the florida platform factors controlling the timing and duration of these unconformities include a combination of global eustatic and regional mechanisms sediment supply current intensification integrated chronostratigraphies developed from a transect of continuous cores in the south central interior to the southern margin of the florida platform show an increase in duration of the hiatal gaps along correlative sequence boundaries toward the platform edge two sequence boundaries are correlated across the south florida platform and mark formation contacts the lower sequence boundary sb a marks the suwannee limestone arcadia formation contact and is of significantly less duration in the platform interior southwest peninsular florida 2 0 m y than at the south platform margin middle florida keys 8 1 m y the hiatal gap along the upper sequence boundary sb b marking the arcadia formation overlain by miocene pliocene sediments that vary in age and composition regionally records the same increase from the central part of the platform southwest florida arcadia fm peace river fm 1 6 m y to the south platform edge lower florida keys arcadia fm stock island fm 11 5 m y eustatic fluctuations affected the relative sedimentation rates of the mixed sediments in the platform interior and at the margin most notably the large eustatic fall at the end of the early oligocene terminated carbonate deposition of the suwannee limestone sb a sediment accumulation on the platform interior was more temporally continuous than at the margin because of greater siliciclastic input from the north the strengthening of marine currents beginning in the middle miocene limited sediment accumulation across the southern platform margin sb b and possibly caused erosion of the margin carbonates increasing the duration of the hiatal gap on top of the arcadia formation

growth rate variations of manganese nodules and crusts induced by paleoceanographic events time markers at about 3 ma 6 5 ma and 14 ma are common to 16 manganese nodules and crusts from different localities in the world oceans these time markers correspond to variations in the growth rates in the inner structure and in the chemistry of the samples the observed time markers could be related to 1 changes of the bottom water velocity and 2 variations in the manganese and iron contents in the deep ocean the time markers were determined by high resolution 10be dating applying the accelerator mass spectrometry facility at the eidgenossische technische hochschule zurich

a specimen based approach to reconstructing the late neogene seabird communities of california abstract california has experienced major climate change from the early miocene 23 ma to the present fossil seabirds are useful for studying marine faunal responses to these changes because they are numerous in museum collections and easily identified to the family level from fragmentary remains because extant seabirds respond quickly to environmental changes we hypothesize that the fossil record of crown group miocene seabirds will reflect past environments previous studies of fossil seabird diversity through the cenozoic of the north pacific relied mainly on literature records to describe the appearance and disappearance of seabird species and correlated these patterns to geologic and climatic events our study uses an empirical specimen based approach to describe the seabird response to climate and tectonic change during 12 million years of coastal california s geologic history middle miocene to early pliocene the foundation of our dataset is 242 fossil seabird specimens from orange county the strata from orange county form the basis for delineating chronostratigraphic bins used for studying 285 additional seabird fossils from other parts of california for a total of 527 seabird fossils examined our results show a relationship between pelagic seabirds and offshore facies as well as a clear increase of pan alcid abundance and decline of sulid abundance through time this pattern may be the result of taphonomic bias combined with the effect of shallower facies preserved through time the increase in pan alcid abundance is also coincident with global climatic and tectonic changes and with enhanced and stable nutrient upwelling upwelling through this time also helps to explain morphological changes in salmon and speciation rates in marine mammals the specimen based methodology used here can be applied to contemporaneous taxa such as marine mammals to quantitatively analyze diversity during the late neogene and further explore the relationship between physical drivers and faunal change

biogeography of neogene calcareous nannofossils in the caribbean and the eastern equatorial pacific floral response to the emergence of the isthmus of panama abstract principal components analysis of nannofossil assemblages in five odp sites in the caribbean and the eastern equatorial pacific was used to monitor neogene surface water circulation changes in 15 83 10 71 ma floral distributions clearly show the existence of the circum tropical current between the caribbean and the eastern equatorial pacific and no surface water communication between the northern and southern caribbean in 10 71 9 36 ma the circum tropical current weakened and the northward intra caribbean current had been initiated northern and southern caribbean surface waters again became separated in 8 35 3 65 ma and the circum tropical current was regenerated after 2 76 ma the northward intra caribbean current has completely been established and the circum tropical current disappeared this suggests that a barrier to the circum tropical current had formed indicating the final closure of the isthmus of panama

the neogene genus streptochilus bronnimann and resig 1971 from the gulf of california abstract four species of the planktonic foraminiferal genus streptochilus from key neogene marine localities are documented in relation to the evolution of the gulf of california s globigerus s latus s macdougallae sp nov and s inglei sp nov planktonic foraminiferal bioevents and strontium isotopes in the bouse tirabuzon carmen and ojo de buey lithostratigraphic units constrain the local distribution range between 6 and 5 3 ma for the last three species whereas s globigerus appears locally at 5 5 ma and disappears between 3 79 and 3 46 ma in the imperial and trinidad formations the last occurrence of streptochilus latus and the first and last occurrences of s globigerus in the ancient gulf of california are correlated with bioevents calibrated in the equatorial pacific therefore they can be used as reliable local biostratigraphic markers the presence of streptochilus in the ancient gulf of california seems to correlate with upwelling in a pattern similar to that observed in the modern oceans

role of temperature and nutrients in extinction of turritelline gastropods cenozoic of the northwestern atlantic and northeastern pacific abstract turritelline gastropods mesogastropoda turritelline are common to abundant components in many fossil cretaceous cenozoic and recent marine benthic communities but the environmental factors affecting their evolution and extinction are poorly understood this is in large part because today they occur most abundantly in cooler waters associated with upwelling or other high nutrient conditions but thermal tolerances and preferences are unclear for extinct turritelline species turritellines were diverse and abundant throughout the new world for much of the cenozoic beginning in the early neogene through both extinction and range contraction however they started to disappear from higher latitudes on both atlantic and pacific coasts of north and central america the highest rate of extinction took place in the western atlantic in the plio pleistocene when almost all turritelline species in this area became extinct preliminary comparison of the available stratigraphic record suggests that this paleobiogeographic pattern is not an artefact of the record this biogeographic shift coincides with a postulated regional mass extinction among all marine mollusks in the western atlantic at around the same time causes for turritelline extinction however appear to be more complex than the decline in temperature that is cited most often as the probable cause of the western atlantic molluskan extinction event neogene turritellines in the western atlantic appear to have been adversely affected by collapse and or major reorganization of oceanic productivity associated with the formation of the isthmus of panama nutrients thus may have been more important than temperature change in controlling extinction probability in this group

cainozoic oceanic and climatic events the indo pacific foraminiferal biostratigraphic record abstract datums in the biostratigraphic record are based on phylogeny meaning speciation and extinction and sometimes on coiling changes we can also employ various nonphylogenetic configurationssuch as disjunct distributions extratropical excursions and correlations with isotopic spikes progress in cainozoic geohistory depends on rigorous refined correlations between all environments on the earth in practice that means an opportunistic groping outward from the most secure and refined realm of our chronology tropical oceanic succession based on planktonic skeletonized protists towards the more difficult realms such as larger benthonic foraminifera in neritic sediments extra tropical oceanic and neritic successions continental faunas floras and palynofloral successions and finally and most elusively continental geomorphic successions formal zones are necessary but their proliferation is not a late eocene example of coincidence in time between planktonic foraminiferal marine dinoflagellate and terrestrial pollen zones suggests that heterogeneous oppelzones may be the soundest basis for local and regional stratigraphic systems the overall cainozoic climatic deterioration was reversed substantially in the eocene and in the miocene by shortlived far reaching bipolar events signalled by extratropical excursions by tropical type foraminifera in some cases correlated solidly with isotopic spikes widespread oceanic hiatuses appear to correlate with episodes of global warming and transgression better than with episodes of global cooling marginal hiatus and of intensified oceanic circulation australia s northward drift since the middle eocene notwithstanding wholesale diachronism no longer is a useful paradigm we are better off thinking in terms of virtually isochronous shifts in climate watermass and in planktonic and benthonic assemblages

stratigraphy and sedimentology of phosphate rich sediments in malta and south eastern sicily latest oligocene to early late miocene the maltese archipelago and south eastern sicily include an uppermost oligocene to upper miocene hemipelagic sedimentary succession representing the malta hyblean plateau which limits the eastern mediterranean to the west this succession hosts a unique and well exposed series of condensed and allochthonous phosphate rich beds which were formed in a sedimentary regime of erosion sediment reworking and frequent gravity flow deposition the combination of nannofossil biostratigraphy and 87sr 86sr isotope stratigraphy allows for the precise attribution of ages to the phosphate deposits and for the distinction of three periods of major phosphogenesis the first phase occurred between 24 5 and 21 ma and 25 and 18 9 ma clustering of ages between 25 and 22 5 ma on malta and sicily respectively the second and third phases of phosphogenesis are documented from the maltese islands and are dated as 17 2 to 13 1 ma and 10 9 to 9 8 ma respectively the phosphate rich beds are associated with hiatuses and phases of important condensation which for the oldest phosphogenic period envelop the time period of 23 2 to 22 ma for the fomm ir rhi bay section malta and from 19 1 to 16 3 ma for the sections of sampieri and modica sicily for the second phase of phosphogenesis on the maltese islands a consistent hiatus was found which embraces the time period of approximately 17 to 15 ma also the third phase of phosphogenesis appears to be associated with a major hiatus which probably envelops the time period between 12 5 and 10 9 ma but a better age control is needed here the correspondence in timing of the maltese sicilian phases of phosphogenesis with major phases of phosphogenesis outside the mediterranean realm to maxima in oceanic phosphorus burial rates and maxima in the 13c benthic foraminiferal record suggests that the palaeoceanographic evolution of the eastern mediterranean was well in phase with that of other ocean basins until at least the early late miocene despite its increasing isolation due to the gradual closure of the eurasian arabian strait and progressive sea level fall

carbon isotopic profile across the bilara group rocks of trans aravalli marwar supergroup in western india implications for neoproterozoic cambrian transition abstract the rocks of marwar supergroup in the trans aravalli sector in western india are presumed to span the time interval between neoproterozoic and early cambrian this predominantly unfossiliferous marine sedimentary sequence is characterized by a lower arenaceous facies jodhpur group middle carbonate facies bilara group and upper argillaceous arenaceous facies nagaur group rocks the sedimentation has been essentially in a shallow basin described either as the fore land slope of the rising aravalli mountains or a sag basin which developed and evolved due to subsidence of the updomed crust during neoproterozoic malani magmatism that failed to open rifts the carbon isotopic profile for the bilara group carbonate rocks in the lower part shows marked oscillations and broadly negative 13 c character with negative anomalies as low as pdb observed near the base of dhanapa formation lower unit and pdb in the overlying gotan formation middle unit the upper part of the profile shows a gradual positive shift the carbon isotopic signatures of the bilara group rocks can be correlated with the end neoproterozoic early cambrian vendian tommotian carbon isotopic evolution curve extremely low 13 c values indicate the glaciation related cold climatic postulates of the end neoproterozoic followed by the warmer climatic conditions as indicated by the positive shift the carbon isotopic data for gotan formation carbonates at variance with the globally observed 13 c trends for early tertiary do not support the recently proposed tertiary age for the bilara group

sedimentary hiatuses as indicators of fluctuating oceanic water masses a new model the upper and sometimes lower boundaries of deep sea disconformities recognized as sedimentary hiatuses at ocean drilling sites are shown to be diachronous the age depth relationships of these hiatuses are used in conjunction with the geographical location of sites relative to individual water masses in order to explain the formation of hiatuses as the product of interaction between fluctuating oceanic water masses and the sea floor the fluctuations in the water masses are related to the glacial history of antarctica the method used is a departure from a traditional approach in which deep sea hiatuses were considered to have no diachronous component and were viewed as the products of instantaneous regional or global events sedimentary hiatuses have commonly been identified in deep sea cores recovered by the deep sea drilling project dsdp and the ocean drilling program odp the disconformity associated with a hiatus has three components firstly an upper boundary which represents a point in time when sedimentation recommenced secondly a lower boundary that is the product of either erosion i e of no temporal significance or non deposition i e of potential temporal significance and thirdly the time interval between these two boundaries termed intra hiatus in this paper earlier studies of hiatuses and hiatus forming events have been used to infer the influence of climatic e g keller barron 1987 and or tectonic e g watkins kennett 1972 changes on the development of erosive oceanic watermasses hiatus distributions have been interpreted as having ocean wide e g davies et al 1977 keller barron 1987 or even

deep water circulation in the northeast atlantic and climatic changes during the late neogene dsdp site 141 abstract oxygen and carbon stable isotope data of pyrgo murrhina and flux rates of calcium carbonate in the bio and magnetostratigraphically dated sediment sequence at dsdp site 141 were used for a reconstruction of the deep water circulation in the northeast atlantic during late miocene and pliocene times a distinct change towards reduced advection of deep water recorded near 5 4 ma is contemporaneous with the cessation of the outflow of the saline mediterranean water into the atlantic during the pliocene between 4 5 and 2 75 ma and between 2 1 and 1 8 ma north atlantic deep water nadw circulation was sluggish and site 141 possibly influenced by antarctic bottom water aabw near 2 75 ma the advection of well oxidized nadw was strongly intensified this change is related to an onset of major arctic ice growth and or a major cooling of nadw

the paleocene eocene transition in the antarctic indian ocean inference from planktic foraminifera isotopic depth stratification and relative abundance studies of planktic foraminifera at odp site 738 reveal three major faunal turnovers during the latest paleocene and early eocene reflecting the climatic and structural changes in the antarctic surface ocean faunal event 1 occurred near the paleocene eocene boundary and is characterized by a faunal turnover in deep dwellers decreased relative abundance in intermediate dwellers and increased relative abundance in surface dwellers this event marks a temporary elimination of the vertical structure in the surface ocean over a period of more than 63 000 years that is apparently associated with the sudden shutdown of the antarctic intermediate water production the appearance of morozovellids before this event suggests that polar warming is the cause for the shutdown in the production of this water mass at this time warm saline deep water may have formed at low latitudes faunal event 2 occurred near the ap5a ap5b subzonal boundary and is characterized by a faunal turnover in deep dwellers with no apparent change in surface and intermediate dwellers increased individual size wall thickness and relative abundance in deep dwelling chiloguembelinids suggests the formation of a deep oxygen minima in the antarctic oceans during the maximum polar warming possibly as a result of upwelling of nutrient rich deep water faunal event 3 occurred in subzone ap6 and is characterized by a faunal turnover in surface dwellers and a delayed diversification in deep dwellers this event marks the onset of antarctic cooling a drastic decrease in the 13c 18o values of the deep assemblage in zone ap7 suggests an intensified thermocline and reduced upwelling following the polar cooling

biogenic sediment regimes in the neogene equatorial pacific iodp site u1338 burial production and diatom community abstract the equatorial pacific experienced significant changes in productivity and microfossil assemblage since 16 million years ago ma we compiled a record of those changes from iodp site u1338 using a reconnaissance of diatom assemblages and high resolution xrf scan chemical profiles 1 2 kyr spacing productivity and caco3 dissolution intervals are defined by sediment component ratios in particular opal clay opal baso4 and caco3 baso4 or caco3 clay there are large abrupt changes in export production in the miocene especially compared to the pleistocene and pliocene and average levels of export production are higher throughout the miocene compared to the pliocene and pleistocene using diatom assemblages and bulk sediment composition the u1338 sediment record is divided into several distinct deposition regimes 1 a middle miocene regime 13 2 ma marked by low diatom numbers and a few short lived productivity intervals 2 a carbonate crash regime distinguished by an older substage 13 2 10 2 ma with relatively high abundances of productivity related diatoms and common productivity related depositional intervals interspersed with caco3 dissolution intervals and a younger carbonate crash substage 10 2 8 0 ma with weaker brief productivity intervals high caco3 dissolution and moderately high numbers of upwelling diatom species 3 the biogenic bloom regime 8 0 4 5 ma marked by extended periods of high opal and caco3 deposition with a maximum between 7 0 and 6 4 ma and 4 a pliocene modern regime 4 5 0 ma with lower productivity and high cyclic caco3 dissolution changes in production and dissolution result from reorganizations of nutrient supply to the equatorial pacific not by higher wind driven upwelling we propose that nutrients were more accessible in the miocene because of a combination of factors including higher organic matter degradation in the upper water column and deep intermediate nutrient pathways that efficiently recycled nutrients to upwelling zones closure of the central american seaway was likely a prime cause of the biogenic bloom

global miocene tectonics and the modern world abstract an amazing congruence of seemingly unrelated diverse global events began in the middle and upper miocene and established our modern world two global orogenic belts were active mostly in the middle and upper miocene while backarc basins formed along the eastern margin of asia coincident with these events global temperatures cooled in both the ocean and atmosphere desertification occurred from central asia into and across most of northern africa and also in australia and in southern south america coincident with the expansion of the antarctic ice cap at 14 ma there was initial widespread deep sea erosion and changes in patterns of deep sea sedimentation muddy pelagic sedimentation increased six fold in the north and central atlantic and pacific oceans and global changes in circulation lead to more diatomites in the pacific and fewer in the atlantic by the end of the miocene most of the mediterranean sea had evaporated broadly coincident with these events many old large river systems were destroyed and new ones formed as much of the world s landscape changed collectively these global on shore tectonic and ocean atmospheric events provide the foundation for our modern world a mixture of new and rejuvenated orogenic belts and their far field effects distant epiorogenic uplift rain shadow deserts large alluvial aprons and distant deltas as inherited gondwanan landscapes persisted remote from plate boundaries thus at the end of the miocene much of the world s landscape except for that changed by pleistocene continental glaciation would be recognizable to us today we argue that all of these events had the same ultimate common cause an internal earth engine that drove plate motions in two broad ways first the opening and closing of seven key gateways to deep water oceanic currents radically altered global heat transfer and changed a lingering greenhouse to an icehouse world secondly these events were in part coincident with renewed heat flow in the african and pacific superplumes that energized global plate motions in the middle and upper miocene we hope this global synthesis will stimulate more research on the many global events of the miocene to understand better both our modern world and earlier global orogenies

sedimentological evidence for the formation of an east antarctic ice sheet in eocene oligocene time middle eocene to late oligocene sediments recovered at ocean drilling program sites 689 and 690 on maud rise in the southernmost atlantic ocean and at sites 738 and 744 on kerguellen plateau in the southernmost indian ocean were analysed in order to study the depositional environment and the palaeoclimate special emphasis was placed on the reconstruction of the formation of a cryosphere on the antarctic continent the investigations include quantifications of carbonate and opal contents grain size analyses and studies of clay mineral assemblages the sedimentary sequence at all sites under investigation is highly pelagic with nannofossil oozes and chalks dominant the first indication of probable glacierization at sea level is in the form of isolated gravel and terrigenous sand grains which indicate ice rafting from middle eocene time at c 45 5 ma this is supported by enhanced concentrations of detrital chlorite and reworked kaolinite probably some glaciers reached the sea while most of the continent remained under the influence of a humid and warm to temperate climate the growth of the inland ice resulted in enhanced physical weathering and in increased contents of detrital chlorite and kaolinite from about 40 ma a strengthening of the glacial conditions and the onset of continental east antarctic glaciation is recorded in early oligocene sediments with an age of 36 ma all major sediment parameters document this event the clay mineralogy changed between c 36 3 ma and c 35 5 ma from smectite dominated assemblages to illite and chlorite dominated assemblages the latter being indicative of physical weathering under cooler climates large quantities of ice rafted gravel and sand accumulated on the kerguelen plateau between 36 0 ma and 35 8 ma at the same time an increase in opal content occurred as well as a decrease in carbonate the sediment parameters imply that the east antarctic continent was more or less totally buried beneath the ice during oligocene time the ice sheet however probably did not have a polar but a temperate character as indicated by the occurrence of nothofagus and relatively warm surface water temperatures prohibiting the long distance transport of debris by icebergs

first record of middle miocene marine diatoms from the colombian pacific nw south america and their paleoceanographic significance abstract middle miocene marine diatoms have been reported in the eastern equatorial pacific by the deep sea drilling project dsdp the ocean drilling program odp and the integrated ocean drilling program iodp few studies however have been completed on onshore sections from south america vallejo et al 2006 published the biostratigraphy of the ladrilleros juanchaco section on the northwest pacific colombian coast utilizing calcareous nannofossils planktonic foraminifera and diatoms in this work we present in detail the study of marine diatoms of that section in order to understand their implications with the age model proposed and provide a paleoceanographic interpretation the diatom recovery is restricted to two intervals at the top of the miocene sequence two biozones from the low latitude zonation can be recognized from base to top they are 1 the coscinodiscus lewisianus zone which contains the same species assemblage reported in antofagasta coastal sediments chile and 2 the craspedodiscus coscinodiscus zone we found that the diatoms represent older ages than proposed by calcareous nannofossils these different ages suggest diachronism between the events of microfossil groups nevertheless in a new interpretation of the distribution patterns of calcareous nannofossils in the upper part of the section the diatom age model could be synchronous the high abundance of thalassionema nitzschioides and thalassiothrix longissima reveal cool upwelling rich oceanic waters at the top of the sequence as well as an increase in paleoproductivity suggested by an increase of rhizosolenia spp and t longissima mats cool water trends support a strengthening of the peru chile current actinoptychus sp leptocylindrus danicus and paralia sulcata point to a nearshore environment the common occurrence of fresh water continental diatoms and palm silicophytoliths indicate an increased input of continental material since ichnofossils and benthic foraminifera reveal ocean depths below 1000 m this change may have related to hyperpycnal flows

reconstruction of oceanic circulation patterns in the tropical pacific across the early middle miocene boundary as inferred from radiolarian assemblages abstract although there have been attempts to infer neogene oceanic circulation patterns in the tropical pacific on the basis of multiple marine proxies oceanic circulation patterns across the early middle miocene boundary are still poorly understood despite paleoclimate having significantly changed during this interval in this study we reconstruct the changes in tropical oceanic circulation patterns in the pacific across the early middle miocene boundary based on radiolarian assemblages obtained at integrated ocean drilling program site u1335 in the eastern tropical pacific the radiolarian upwelling taxa increased during four intervals 18 4 18 1 ma 16 9 15 7 ma 15 2 14 6 ma 14 2 13 4 ma radiolarian temperature index rti was calculated using the relative abundances of the warm water cluster b1 and cool water taxa cluster b2 in order to evaluate relative temperature fluctuations of the surface water the sea surface temperature index was relatively high from 16 8 to 16 0 ma and gradually decreased from 16 0 to 14 6 ma subsequently the temperature index decreased stepwise at 13 9 and 13 4 ma and became relatively low from 13 4 to 12 7 ma comparison of the rti record in the eastern tropical pacific to the climatic index based on the benthic foraminiferal 18o record shows that variability in the two records follows a roughly similar trend from the early to the middle miocene however the sea surface temperature record does not indicate a sharp increase in warmth in the eastern tropical pacific during the mid miocene climatic optimum 1 mmco 1 as suggested by the benthic isotopic record this is likely because of increased upwelling of cool nutrient rich water at this time beginning in the latest early miocene 17 ma radiolarian assemblages were dominated by different taxa in the eastern and western tropical pacific this pattern is interpreted as indicating a shallower thermocline in the east and a deeper thermocline in the west based on the relative abundance of the upwelling taxa there is an overall increasing trend in this difference since the latest early miocene we tie these events to the effective closure of the indo pacific seaway and the development of a substantial western pacific warm pool along with the development of a strong equatorial undercurrent

radiolarian faunal turnover across the oligocene miocene boundary in the equatorial pacific ocean abstract the global warming trend of the latest oligocene was interrupted by several cooling events associated with antarctic glaciations these cooling events affected surface water productivity and plankton assemblages well preserved radiolarians were obtained from upper oligocene to lower miocene sediments at ocean drilling program odp leg 199 sites 1218 and 1219 in the equatorial pacific and 110 radiolarian species were identified four episodes of significant radiolarian faunal changes were identified middle late oligocene 27 5 to 27 3 ma latest oligocene 24 4 ma earliest miocene 23 3 ma and middle early miocene 21 6 ma these four episodes approximately coincide with increases and decreases of biogenic silica accumulation rates and increases in 18o values coded as oi and mi events these data indicate that antarctic glaciations were associated with change of siliceous sedimentation patterns and faunal changes in the equatorial pacific radiolarian fauna was divided into three assemblages based on variations in radiolarian productivity species richness and the composition of dominant species a late oligocene assemblage 27 6 to 24 4 ma a transitional assemblage 24 4 to 23 3 ma and an early miocene assemblage 23 3 to 21 2 ma the late oligocene assemblage is characterized by relatively high productivity low species richness and four dominant species of tholospyris anthophora stichocorys subligata lophocyrtis nomas and lithelius spp the transitional assemblage represents relatively low values of productivity and species richness and consists of three dominant species of t anthophora s subligata and l nomas the characteristics of the early miocene assemblage are relatively low productivity but high species richness the two dominant species present in this assemblage are t anthophora and cyrtocapsella tetrapera the most significant faunal turnover of radiolarians is marked at the boundary between the transitional early miocene assemblages we also reviewed changes in other microfossil assemblages in the low latitudes during the late oligocene through early miocene the microfossil assemblages of major groups show sequential changes near the oligocene miocene o m boundary 23 8 ma many extinction events and some first occurrences of calcareous nannofossils and many occurrences of radiolarians are found from about 24 8 to 23 3 ma and first occurrences of planktic foraminifers and diatoms followed from 23 2 through 22 ma hence the o m boundary is identified as a significant level for microfossil evolutions

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

the early neogene continental rise off the eastern united states abstract single and multichannel seismic data were used to investigate depositional components comprising the late oligocene middle miocene constructional phase of the united states atlantic continental rise in the georges bank blake plateau region the seismic sequence studied was bounded by the eocene oligocene age horizon a u unconformity at the base and a newly defined late middle miocene age horizon g at the top late oligocene middle miocene accretion onlapping the paleocontinental slope and downlapping onto horizon a u basinward was derived primarily from erosion of the uplifted central appalachian mountains this accretion up to 3 2 km thick was characterized by seven depocenters the lower paleocontinental slope upper rise depocenters situated off southern new england and the delmarva peninsula were interpreted as deep sea fan complexes or a broadly overlapping series of sedimentary blankets a whereas the closely associated oceanographer and wilmington depocenters were inferred to be levee complexes immediately north of the developing blake bahama outer ridge two n ne oriented sediment ridges that were probably built by gulf stream flow during an early miocene sea level lowstand were observed on the lower continental rise the proto hatteras and mytilus outer ridges from 0 4 to 0 8 km thick were probably formed from the wilmington and oceanographer levee complexes in early middle miocene time through the interaction of the growing levee complex with the sw moving abyssal current late middle miocene 12 11 5 ma bottom current erosion created the horizon g unconformity and brought to an end this initial stage of continental rise construction nevertheless the basic morphological framework that controlled late neogene and quaternary continental rise evolution was now in place

planktonic and benthic foraminiferal extinction events during the last 100 m y abstract five major planktonic and benthic foraminiferal extinction events occurred during the past 100 m y at the cenomanian turonian c t boundary at the cretaceous tertiary k t boundary in the latest paleocene in the late eocene and in the early middle miocene to clarify the character of the five events i estimated 1 paleo waterdepths and paleoenvironments where foraminifera were affected 2 the duration of each extinction event and 3 evaluated possible causes for each extinction event on the basis of recent literature these extinction events can be classified into three fundamentally different types 1 the mass extinction event at the k t boundary was abrupt and concentrated among surface dwelling species 2 extinctions at the c t boundary and during the latest paleocene were abrupt and mainly affected intermediate and deep water taxa over short periods 3 the late eocene and early middle miocene events affected surface and deep water species and involved stepwise or gradual extinctions over a long period 4 m y i attributed the extinction events to 1 a bolide impact for the k t event 2 a decrease in dissolved oxygen in intermediate and deep waters due to climatic warming for the c t and latest paleocene events and 3 long term global cooling for the late eocene and early middle miocene events the following relationships exist between the causes and types of extinction events 1 large extraterrestrial impacts strongly affected surface dwelling organisms in short term extinction events 2 intermediate and deep water low oxygen conditions induced by climatic warming have severely affected intermediate and deep water organisms over medium and short terms

neogene paleoceanography of the eastern equatorial pacific based on the radiolarian record of iodp drill sites off costa rica fil sandoval maria i universidad de costa rica costa rica universite de lausanne suiza

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

preservational paleoecological and evolutionary patterns in the paleogene of wyoming montana and the neogene of pakistan abstract contributions to this special issue have compared the tectonic settings fluvial systems paleoclimates paleofloras and faunas vertebrate taphonomy and mammalian paleoecology and evolution from two long continental records of cenozoic ecosystems in this concluding paper we summarize highlights of earlier papers to provide an overview of similarities and differences between these paleogene and neogene records the relative influences of tectonic climatic and fluvial processes on lithofacies environments of fossil preservation and the productivity of the fossil record are compared for each sequence tectonism was the primary control on sediment accumulation rate and the distribution of major depositional environments in each basin fossil productivity species richness and the distribution of fossil localities among fluvial environments change across formation boundaries in each sequence the taxonomic temporal and spatial resolution of vertebrate assemblages varies among fluvial environments and differs between the two sequences as a result the paleogene record is better suited for detailed study of evolutionary change within local lineages and the neogene record for detailed paleocommunity reconstruction several aspects of biotic change are considered 1 the timing and magnitude of mammalian faunal turnover in relation to climatic change are evaluated in terms of three models of evolutionary change within ecosystems van valen s red queen model stenseth and maynard smith s stationary model and vrba s turnover pulse model in both records climatic change was accompanied by change in faunal composition and ecological structure but both pulsed and diachronous biotic change at a resolution of about 0 5 m y also occurred over periods when no significant climatic change was discernible this pattern best matches the predictions of the red queen model 2 in both records the rate of mammalian faunal turnover first and last appearances of taxa is not highly correlated with standing richness suggesting that paleocommunities were not a equilibrium richness on these time scales 3 in the paleogene record plant species richness declined while mammalian species richness increased 4 in each sequence some episodes of mammalian faunal turnover were in step with changes in size or trophic structure while other changes in faunal composition entailed no corresponding change in ecological structure while both records would benefit from improved paleoclimatic temporal species level and ecomorphological resolution this initial synthesis suggests that both physical and biotic factors were important influences on faunal composition the timing and rate of turnover and ecological structure in these paleocommunities

the pan amazonian ucayali peneplain late neogene sedimentation in amazonia and the birth of the modern amazon river system abstract we review the neogene geologic history of lowland amazonia in an attempt to focus attention on areas of agreement as well as areas in dispute in this research arena we reinterpret pre existing hypotheses present new data and discuss new insights intended to support a unified synthesis of the amazon basin as a single sedimentary basin albeit on a vast scale during the late miocene to middle late pliocene we document the ucayali peneplain as an isochronous pan amazonian geologic feature that formed following the early to mid miocene quechua i orogenic phase of andean tectonism peneplanation began possibly as early as 15 ma and terminated abruptly near the beginning of the late miocene quechua ii orogenic event at 9 5 9 0 ma subsequently a thin cover of sediments comprising the madre de dios formation began blanketing most of lowland amazonia excepting only the eastern subandean fold and thrust belt and isolated highlands within the basin the buried peneplain is readily observed in river cutbanks throughout amazonia as the marked often angular ucayali unconformity that separates eroded older often folded faulted and weathered moderately to well consolidated tertiary formations from unconsolidated near horizontal upper neogene deposits the dates of formation of major unconformities and subsequent depositional events at widely separated areas within the andes of bolivia ecuador and peru are coincident with that of the ucayali unconformity and deposition of the madre de dios formation and suggest that the events are linked to a common cause which is interpreted to be the still on going collision between the south american and nazca tectonic plates the madre de dios formation has three members the oldest of which documents a short lived high energy depositional environment followed by a moderate energy depositional environment both occurring at a time when drainage from the basin was unobstructed the upper two members record fluctuations between moderate and low energy continental depositional environments during a period when drainage from the basin was obstructed disorganized and took place over long distances with extremely low gradients the sedimentology of the madre de dios formation particularly the thick massive beds of clay and the widespread presence of paleodeltas and associated geomorphic features on the amazonian planato are consistent with the hypothesis that much of the upper two members formed as lacustrine and deltaic deposits within a gigantic lake lago amazonas or more probably within a complex series of interconnected mega lakes that occasionally united to cover most or all of lowland amazonia to a shallow depth from the latest miocene until 2 5 ma the presence of the ucayali unconformity and the relatively uniform lithostratigraphy basin wide of the fluvial fluviolacustrine and lacustrine sediments of the upper neogene madre de dios formation are consistent with the hypothesis that the amazon basin acted as a single undivided sedimentary basin in the late neogene the biostratigraphic correlation across important modern drainage divides of both micro and macro sized late miocene fossil vertebrates recovered from basal conglomerates of the madre de dios formation and the absence therefrom of fossil vertebrates of any other age is also consistent with this hypothesis two 40ar 39ar dates on ash deposits within the madre de dios formation corroborate the upper miocene age of the basal horizons of that formation indicated by fossil vertebrates and support an upper pliocene age for the youngest sediments of the formation the modern amazon river drainage system was established in the late pliocene at 2 5 ma by the breaching of the eastern rim of the sedimentary basin as a result of the basin being overfilled or by headward erosion of the lower amazon river or both data on cenozoic mass accumulation rates and the chemistry of terrigenous sediments in the atlantic ocean obtained by ocean drilling project leg 154 ceara rise support the postulated timing of the establishment of the modern amazon river drainage system at 2 5 ma rather than the long held view that this event occurred in the late miocene we discuss the important role of ocean currents and sea level fluctuations on terrigenous mass accumulation rates on the ceara rise the postulated time of formation of the modern amazon river is nearly coincident with the onset of the plio pleistocene glacial climatic regime and the lowest sea level stands since the latest middle miocene this analysis indicates that modern amazonia is a product of terrain development within an erosional regime since 2 5 ma

the middle miocene climatic transition in the southern ocean evidence of paleoclimatic and hydrographic changes at kerguelen plateau from planktonic foraminifers and stable isotopes abstract middle miocene 14 8 11 9 ma deep sea sediments from odp hole 747a kerguelen plateau southern indian ocean contain abundant well preserved and diverse planktonic foraminiferal assemblages a detailed study of the climatic and hydrographic changes that occurred in this region during the middle miocene climatic transition led to the identification of an intense cooling phase the middle miocene shift abundance fluctuations of planktonic foraminiferal species with different paleoclimatic affinities and oxygen and carbon stable isotopes have been integrated in a multi proxy approach reconstruction of changes in foraminiferal faunal composition and diversity through time were the basis for identification of three foraminiferal biofacies the most prominent faunal change took place at 13 8 ma when a fauna with warm water affinity marked by high abundance of globorotalia miozea group and globoturborotalita woodi plexus was replaced by an oligotypic opportunistic fauna with typical polar characters and dominated by neogloboquadrinids this faunal change is interpreted as the result of foraminiferal migration from adjacent bioprovinces caused by modifications in climate and hydrography a positive 2 0 shift in 18o interpreted as the mi3 event and a related positive 1 0 shift in 13c corresponding to the cm6 event accompanied this faunal turnover these are interpreted to reflect substantial reorganization of southern ocean waters the northward migration of the polar front and a strong increase in primary productivity the second faunal change took place at 12 9 ma and was characterized by the gradual decrease in abundance of the neogloboquadrinids and the recovery of globorotalia praescitula scitula group and globigerinita glutinata a positive 1 5 shift in 18o interpreted as the mi4 event and a concurrent gradual negative shift in 13c accompanied this faunal change witnessing further modifications of the climate ocean system variations in sea surface temperature considered as the main factor causing changes of surface hydrography at the kerguelen plateau seem to have been driven by obliquity and long term eccentricity thus suggesting a key role played by the astronomical forcing on the evolution of southern ocean dynamics during the middle miocene also an evident 1 2 myr modulation of the 13c record suggests a main control of the long term obliquity cycles on the carbon cycle dynamics particularly the mi3 cm6 events exactly fit with a node of the 1 2 myr modulation cycles this confirms the key role played by orbital parameters on high latitude temperatures and antarctic ice volume and indirectly on global carbon burial and or productivity this climatic transition was marked also by changes in surface hydrography from 14 8 to 13 8 ma an intermediate strength thermocline controlled by seasonality developed just below the photic zone weaker seasonality characterized the interval from 13 8 to 12 9 ma when the thermocline became shallower and sharper and favored intermediate water foraminifers from 12 9 ma seasonality increased again and an intermediate strength thermocline re developed

bioturbated oxygenation event beds in siliceous facies monterey formation miocene california abstract predominatly laminated marine diatomaceous sequences deposited in oxygen deficient neogene basins typically contain bioturbated intervals that reflect episodes of improved benthic oxygenation excellent exposures of the siliceous facies of the miocene monterey formation near lompoc central california provided an opportunity to assess the ichnology and sedimentology of 200 such event beds in their general paleogeographic paleoclimatic and sea level context and hence to evaluate the character and causes of associated oxygenation episodes four recurring event bed types type i iv are distinguished on the basis of ichnologic characteristics these types fall into two general groups delineated on the basis of sedimentologic attributes primarily texture group 1 beds type i only have 1 homogeneous bioturbate primary strata that are more clastic rich and much coarser than surrounding laminated diatomites and 2 vertically extensive piped zones characterized by firmground thalassinoides paradoxicus glossifungites ichnofacies these characteristics indicate that group 1 events involve coincident increases in bottom current intensity clastic input and benthic oxygenation some group 1 beds may be the product of turbidity current transport of sediments trauma resistant crustaceans and allochtonous waters however most group 1 beds more likely record climate and sea level mediated migratio of incipient bank top conditions down the flank of a submergent topographic high group 2 beds types ii iii and iv are characterized by primary strata that are only slightly coarser and more clastic rich than surrounding laminites ichnofabrics and trace fossil assemblages which are dominated by burrows of deposit feeders thalassinoides suevicus meniscate backfilled structures and chondrites reflect normal softground substrates and quiet water pelagic deposition zoophycos ichnofacies differences in ichnologic parameters bed thickness adn presence diversity size and vertical extent of discrete burrows in primary strata and or piped zones among the bed types within this group reflect differences in magnitude duration and to a lesser extent frequency of oxygenationepisodes coincident increases in oxygenation and clastic content recorded by group 2 beds are attributed to e1 nino like climatic events gross temporal patterns in the intensity and frequency of these climatic events appear to be linked to the phases of a third order sea level cycle whereas shorter term variations 100 120 ka may have been mediated by milankovitch cyclicity relationships established between bioturbation events and relative water depth are of potential use in reconstructing basin paleogeography and or recognizing the signal of sea level change in the monterey formation and other neogene siliceous sequences

convection in the magnetosphere ionosphere system a multimission survey of its response to imf by reversals past studies have demonstrated that the interplanetary magnetic field imf by component introduces asymmetries in the magnetosphere ionosphere m i system though the exact timings involved are still unclear with two distinct mechanisms proposed in this study we statistically analyze convective flows from three regions of the m i system the magnetospheric lobes the plasma sheet and the ionosphere we perform superposed epoch analyses on the convective flows in response to reversals in the imf by orientation to determine the flow response timescales of these regions we find that the lobes respond quickly and reconfigure to the new imf by state within 30 40 min the plasma sheet flows however do not show a clear response to the imf by reversal at least within 4 hr postreversal the ionospheric data measured by the super dual auroral radar network superdarn match their counterpart magnetospheric flows with clear and prompt responses at 75 magnetic latitude mlat but a less pronounced response at 60 70 mlat we discuss the potential implication of these results on the mechanisms for introducing the imf by component into the m i system

post rift submarine volcanic complexes and fault activities in the baiyun sag pearl river mouth basin new insights into the breakup sequence of the northern south china sea abstract post rift volcanisms and fault activities are significant for understanding the tectonic sedimentary evolution of rifted continental margins and associated hydrocarbon entrapments in this study a series of post rift fault associated volcanic activities have been found in the northern continental margin of the south china sea scs however their episodes and relationships with the progressive continental breakup are still doubtful here new 2d 3d seismic borehole dating and biostratigraphic data are used to investigate the characteristics of the post rift volcanism and fault activities in the baiyun sag bys pearl river mouth basin the newly found fault associated volcanic complexes include thirteen submarine volcanic mounds and associated lava flows together with igneous sills that are mainly composed of basalts and tuffs seismic stratigraphic analysis combined with k ar dating from cores confirms that post rift volcanism in the bys climaxed along with the breakup of the scs mainly in three episodes 1 the first episode near oligocene miocene boundary 23 3 ma 2 the second at the middle early miocene 17 6 ma and 3 the new found third episode at the late early miocene 17 1 ma also fifty three syn sedimentary faults are interpreted which are characterized by a wnw trending normal fault group separated by an ene trending transcurrent fault group and there is evidence that these fifty three faults were long lived from ca 32 17 1 ma and served as important conduits for the upward migration of the magma the three episodes of volcanism and tectonic fault activities are related to the progressive continental breakup in the northern scs margin and they represent the tectonic signature of the early stage of the post rift subsidence after the lithospheric breakup which extends to the top of a hypothetical breakup sequence 32 17 1 ma in the bys furthermore this study proves that the diachronous continental breakup in the scs margin can result in corresponding changes in depositional architectures along with a breakup sequence characterized by the deposition of a sandstone carbonate shale regressive transgressive cycle the sedimentological changes and associated tectonic features recorded in the baiyun sag suggest an exceptionally active tectonic setting in the time of the formation of the breakup sequence early oligocene early miocene along the northern scs margin

neogene oceanographic variations recorded in manganese nodules from the somali basin abstract a detailed study of a nodule from the somali basin dated by 230 th excess was correlated with the paleoceanographic events recorded in site 236 leg 24 deep sea drilling project dsdp cores tentative indications are that the phase of nodule accretion starting with the development of pillar structure at a depth of 20 mm in the nodule around 13 ma coincides with increased antarctic bottom water aabw flow and an elevated calciumcarbonate compensation depth ccd the late miocene lowering of the ccd is represented by the mottled zones between 8 and 18 mm in the nodule is characterised by an abundant silicate component 20 of aeolian origin the miocene pliocene boundary 5 ma occurs at a depth of about 8 mm and is represented by the development of pillar structure and a minimum of aeolian dust 10 3 the increased biological productivity of the somali surface water since the middle miocene is demonstrated by the increasing c org content of the nodule from 0 11 to 0 19 towards its surface

paleoceanographic and tectonic controls on deposition of the monterey formation and related siliceous rocks in california abstract the timing of paleoceanographic and tectonic events that shaped the deposition of the monterey formation of california and related siliceous rocks has been determined by application of a refined biochronology the base of the monterey at 17 5 ma coincides with rising global sea level and a switch in biogenous silica deposition from the caribbean and low latitude north atlantic to the north pacific major polar cooling which began at 15 ma postdates the base of the monterey by more than 2 ma and cannot be invoked to cause the deposition of diatomaceous sediments occurring in the lowermost monterey later polar cooling in the early late miocene however apparently caused increased upwelling and deposition of purer diatomites in the upper monterey the top of the monterey at about 6 ma coincides with a major sea level drop and is commonly marked by an unconformity equivalent unconformities are widespread around the rim of the north pacific and typically separate more pelagic sediments from overlying sediments with a greater terrigenous component above the monterey diatoms persist in california sediments to 4 5 4 0 m y where their decline coincides with increased deposition of diatoms in the antarctic carbon isotope records in the pacific and indian oceans record storage of 12 c in the monterey formation and equivalent organic rich sediments around the rim of the north pacific a 1 0 excursion in 13 c beginning at 17 5 ma coincides with rising sea level and probably reflects storage of organic material in monterey like marginal reservoirs a reverse 1 0 shift at 6 2 ma closely approximates the top of the monterey and may represent erosion of these marginal reservoirs and reintroduction of stored organic carbon into the ocean atmosphere system initiation of transform faulting and extension in the california margin in the latest oligocene and early miocene caused the subsidence of basins which later received monterey sediments a major tectonic event centered at 10 9 ma which included rotation of the santa barbara basin and a change in pacific plate motion is recorded by an unconformity and or interval of compressed sediments in many monterey sections another change in plate motion at about 6 ma may be expressed by the angular discordance observed in many sections across the unconformity at the top of the monterey

testing global synchronism in peri pacific arc volcanism abstract episodicity of arc volcanism along the pacific rim is quantitatively analysed based on the record preserved by ash layers in deep sea sediments drilled by the deep sea drilling project and the ocean drilling program two major pulses of arc volcanism have been found one during middle miocene times 18 13 ma the other during pliocene quaternary times 5 0 ma these pulses are shown to relate to the tectonic stress regime of the arc lithosphere rather than to subduction rate these episodes of arc volcanism activity coincide with major tectonic events documented on circum pacific subduction zones as a consequence a global control of plate kinematics could influence both volcanic and tectonic evolution of pacific convergent margins

nd isotope evidence for the evolution of the paleocurrents in the atlantic and tethys oceans during the past 180 ma the nd isotopic composition of atlantic and tethys seawater as deduced from marine phosphorites varied considerably during the past 180 ma the early tethys and central atlantic seawater from 180 to 160 ma ago early middle jurassic had a nd isotopic signature identical to that of the pacific end 6 suggesting that pacific seawater entered the newly forming tethys basin however with time continental runoff draining into the young basin became more important and led to a decrease in the nd isotopic composition and finally a decoupling from the pacific ocean during the late early cretaceous 120 90 ma ago atlantic and tethys seawater reached continental crust like nd isotopic composition values end 10 which could reflect high weathering rates induced by the warm and humid climate at that time in the time span between 80 and 50 ma late cretaceous early tertiary the nd isotopic composition of both tethys and atlantic seawater strongly increased towards pacific seawater values in the case of the atlantic ocean this change has been correlated with the opening of the south atlantic which enabled the more radiogenic pacific seawater to travel westwards around the southern edge of africa into the south and finally the north atlantic in the case of tethys seawater we have to assume that during this period large masses of pacific seawater entered the tethys again directly through the indian tethys seaway the renewed importance of this seaway might be related to the late cretaceous early tertiary first order global sea level highstand the most abrupt change towards lower more crust like values occurred in the early miocene 25 17 ma ago in the south and north atlantic as well as in the tethys during this period the143nd 144nd isotope ratios dropped from 0 5124 end 4 2 down to 0 5122 end 8 this change indicates a diminishing influx of pacif seawater into the atlantic and tethys oceans and can be correlated with the breakdown of the circum equatorial circulation patterns of the world oceans due to plate tectonics from 20 to 17 ma ago miocene the nd isotopic composition of the atlantic ocean increased again and reached values as high as 0 5123 end 6 6 whereas tethys seawater remained at low values this decoupling can be correlated with the incipient complete isolation of the tethys the nd isotopic composition was henceforth controlled by the continental runoff from surrounding land masses the major increase in the nd isotopic composition observed in the atlantic can be related to the opening of the drake passage and establishment of the circum antarctic current system this allowed pacific seawater to enter directly the south atlantic around antarctica a renewed drop in the nd isotopic composition of atlantic seawater during the late miocene can be related to the onset of north atlantic bottom water formation and its subsequent upwelling

late neogene lacustrine record and palaeogeography in the quillagua llamara basin central andean fore arc northern chile abstract the cenozoic quillagua llamara basin northern chile central andes is an asymmetrical intramassif fore arc basin with a relatively wide northern sector separated from a narrower southward extension by a basement threshold the northern sector was characterised by a noticeable oligocene late neogene alluvial fan and lacustrine dominated deposition which resulted in sequences up to 900 m thick whereas the southern sector was often a bypass zone with thinner fluvial and lacustrine sediment accumulation the basin infill includes two third order alluvial lacustrine unconformity bounded units which include other higher frequency 4th to 5th order sequences the evolution of the late miocene pliocene lacustrine episodes in the quillagua llamara basin was not only controlled by the regional variations from arid to hyperarid palaeoclimate conditions due to the combined influence of the pacific high pressure cell the rain shadow effect exerted by the rising andes and the northward flowing cold oceanic currents but also by a the extensional tectonics and evolution and uplift of the fore arc region which defined the location and size of the depocentres b the resulting basement palaeorelief which affected sediment thickness and facies distribution during the late basin infill episodes and c the tectonic modifications of watersheds water divides and drainage networks in the precordillera which caused considerable changes of water income in the lacustrine systems understanding of this regional tectonosedimentary evolution is a necessary first step before analysing of the low to high order lacustrine sequence changes in the region lacustrine water supply was very sensitive to tectonics even gentle tectonic tilting and uplifting in critical water divide zones could result in changes in water balance in the lacustrine basins and trigger variations in the depositional record the very conspicuous lacustrine regime changes recorded in the quillagua llamara basin infill cannot be considered in themselves conclusive proof of an exclusive climatic forcing since they took place close to either major regional drainage changes or to gentle but noticeable tectonic reactivation in the fore arc region