the progressive intensification of northern hemisphere glaciation as seen from the north pacific ocean drilling project odp site 882 50 22 n 167 36 e provides the first high resolution grape density magnetic susceptibility carbonate opal and foraminifera planktonic and benthic stable isotopes records between 3 2 and 2 4 ma in the northwest pacific we observed a dramatic increase in ice rafting debris at site 882 at 2 75 ma which is coeval with that found in the norwegian sea suggesting that the eurasian arctic and northeast asia were significantly glaciated from 2 75 ma onwards prior to 2 75 ma planktonic foraminifera 18o records indicate a warming or freshening trend of 4 c or 2 over 80 ka if this is interpreted as a warm pre glacial pliocene north pacific it may have provided the additional moisture required to initially build up the northern hemisphere continental ice sheet the dramatic drop in sea surface temperatures sst 7 5 c at 2 75 ma ended this suggested period of enhanced sst and thus the proposed moisture pump moreover at 2 79 and 2 73 ma opal mass accumulation rates mar decrease in two steps by five fold and is accompanied by a more gradual long term decrease in caco3 mars evidence from the southern ocean odp site 704 indicates that just prior to 2 6 ma there is a massive increase in opal mars the opposite to what is found in the north pacific this indicates that the intensification of northern hemisphere glaciation was accompanied by a major reorganisation of global oceanic chemical budget possibly caused by changes in deep ocean circulation the initiation of northern hemisphere glaciation occurred in the late miocene with a significant build up of ice on southern greenland however the progressive intensification did not occur until 3 5 3 ma when the greenland ice sheet expanded to include northern greenland following this stage we suggest that the eurasian arctic and northeast asia glaciated at 2 75 ma approximately 100 ka before the glaciation of alaska 2 65 ma and 200 ka before the glaciation of the north east american continent 2 54 ma

observations on attenuation and shear wave velocity in fine grained marine sediments compressional p wave attenuation shear s wave attenuation and shear s wave velocity measurements compiled for fine grained unconsolidated sediments show trends that do not support empirical relationships hamilton j acoust soc am 68 1313 1340 1980 in acoustics and ocean bottom f a s e specialize conf madrid 1987 commonly used for geoacoustic modeling the exception is compressional wave attenuation data from 10 hz to 500 khz that essentially follow a frequency dependence of f1 2 standard deviations p wave attenuation with depth is difficult if not impossible to predict given the variability in the data examination of several attenuation depth profiles indicates that those of mitchell and focke j acoust soc am 67 1582 1589 1980 may be the best choice for predicting p wave attenuation with depth at the present time the assumption that s wave and p wave attenuations are proportional hamilton j acoust soc am 60 334 338 1976c in acoustics and ocean bottom

estimating seabed scattering mechanisms via bayesian model selection a quantitative inversion procedure is developed and applied to determine the dominant scattering mechanism surface roughness and or volume scattering from seabed scattering strength data the classification system is based on trans dimensional bayesian inversion with the deviance information criterion used to select the dominant scattering mechanism scattering is modeled using first order perturbation theory as due to one of three mechanisms interface scattering from a rough seafloor volume scattering from a heterogeneous sediment layer or mixed scattering combining both interface and volume scattering the classification system is applied to six simulated test cases where it correctly identifies the true dominant scattering mechanism as having greater support from the data in five cases the remaining case is indecisive the approach is also applied to measured backscatter strength data where volume scattering is determined as the dominant scattering mechanism comparison of inversion results with core data indicates the method yields both a reasonable volume heterogeneity size distribution and a good estimate of the sub bottom depths at which scatterers occur

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

late quaternary paleoproductivity and deep water circulation in the eastern south atlantic ocean evidence from benthic foraminifera abstract two late quaternary sediment cores from the northern cape basin in the eastern south atlantic ocean were analyzed for their benthic foraminiferal content and benthic stable carbon isotope composition the locations of the cores were selected such that both of them presently are bathed by north atlantic deep water nadw and past changes in deep water circulation should be recorded simultaneously at both locations however the areas are different in terms of primary production one core was recovered from the nutrient depleted walvis ridge area whereas the other one is from the continental slope just below the coastal upwelling mixing area where present day organic matter fluxes are shown to be moderately high recent data served as the basis for the interpretation of the late quaternary faunal fluctuations and the paleoceanographic reconstruction during the last 450 000 years nadw flux into the eastern south atlantic ocean has been restricted to interglacial periods with the strongest dominance of a nadw driven deep water circulation during interglacial stages 1 9 and 11 at the continental margin high productivity faunas and very low epibenthic 13c values indicate enhanced fluxes of organic matter during glacial periods this can be attributed to a glacial increase and lateral extension of coastal upwelling the long term glacial interglacial paleoproductivity cycles are superimposed by high frequency variations with a period of about 23 000 yr enhanced productivity in surface waters above the walvis ridge far from the coast is indicated during glacial stages 8 10 and 12 during these periods cold nutrient rich filaments from the mixing area were probably driven as far as to the southeastern flank of the walvis ridge

the importance of the oxygen minimum zone and sediment geochemistry in the distribution of recent benthic foraminifera in the northwest indian ocean abstract the relationship between the distribution of benthic foraminifera and sediment type and depositional environment in the arabian sea is discussed the benthic foraminiferal fauna were sampled in nineteen recent surface sediment samples and geochemical variables of the sediment of the same samples were measured the water depths for the box core samples varies from 440 to 4040 m a total of 103 species and six species complexes were identified the geochemical properties were found to correspond well to the sediment type and depositional environment and six different sediment depositional environment types could be distinguished analysis of the benthic foraminiferal fauna reveals specific faunal assemblages that are closely related to these sediment depositional environment types

recent benthic foraminifera from the continental margin of northwest africa community structure and distribution abstract sediment grab samples were collected at 107 locations along the continental margin of northwest africa these samples form a series of depth transects between the straits of gibraltar and dakar senegal the greater than 250 m size fraction was retained for a census of both the live and dead foraminifera after deleting trace occurrences jaccard and correlation coefficient based cluster analyses were performed to decipher the community structure for this margin the geographic distribution of the four major faunal provinces recognized is conventional in that for both the live and dead assemblages three are related to depth upper slope and shelf middle slope and lower slope and continental rise the biotopes and thanatotopes within these provinces are strongly restricted along vertical or latitudinal boundaries when the data are analysed using jaccard coefficients dendrograms constructed from correlation coefficients emphasize depth related faunal communities dominant species such as cibicides lobatulus trifarina fornasinii planulina ariminensis uvigerina finisterrensis and c wuellerstorfi are generally distributed within a definite depth range along this margin and strongly influence the correlation coefficient based dendrograms other distributions are clearly not depth related but correspond to various environmental variables for example cancris auriculus coarse substrate bolivina subaenariensis oxygen minimum cibicidoides kullenbergi and hoeglundina elegans low organic carbon uvigerina peregrina is also a notable exception to depth dependence in that this dominant species and the province it represents are generally confined to the continental slope south of cape blanc its preference for that region may arise from the high organic carbon and fine grain size of the sediment there from low salinity of the bottom water or from other unknown variables the most important vertical faunal boundary occurs between cape barbas and cape blanc between 22 and 21 n latitude in contrast the canary island ridge intersects the continental margin at a saddle depth of about 1500 m but that physiographic barrier has little effect on provincialism of the deep water benthic foraminifera

southward shifting of the tasman front at 4 4 ma early pliocene paleobiogeographic and oxygen isotopic evidence abstract the tasman front represents a thermal boundary across the north tasman sea located between 31 s and 35 s over the tasman sea ridge system in the south west pacific census data of planktic foraminiferal assemblages and oxygen isotopic variations of planktic and benthic foraminifera obtained from three dsdp sites 284 590a and 588 were used to investigate the pliocene history of the tasman front the results indicate that at about 4 4 ma the tasman front shifted to the south of 31 s as reflected by a major change in the biogeographic distribution of planktic foraminifera this change caused warming of the sea surface particularly in the area of dsdp site 590a temperature gradients between surface water and intermediate water indicated by 18 o planktic benthic at dsdp590a also increased suggesting that the site has been subsequently located beneath a more vertically stratified water column than prior to 4 4 ma the southward shifting of the tasman front might have been a response to the intensification of the east australian current and the enlargement of the west pacific warm pool in the western equatorial pacific both of these oceanographic developments may have been caused by the curtailing of the westerly flowing south equatorial current due to tectonic closure of the new guinea seaway involving northward motion of the australian plate and an associated pliocene orogeny in central new guinea

intensification of the asian monsoon and a chemical weathering event in the late miocene early pliocene implications for late neogene climate change several sedimentary geochemical and isotopic records indicate that the hypothesized intensification of the asian monsoon at about 8 ma triggered a physical and chemical weathering event in the himalayan tibetan plateau records of sediment input and clay composition from the northern indian ocean reveal clear weathering changes in the plateau at this time and ge si ratios of opaline silica as well as biogenic sedimentation rates indicate that increased dissolved element fluxes from this weathering event had an oceanwide effect it is likely that weathering intensity also increased in the andes and amazon basin in this interval as well perhaps the most important impact of this weathering event was to increase the net flux of the biolimiting nutrient phosphorus to the ocean as evidenced by a peak in phosphorus accumulation rates at this time the temporary increase in nutrient inputs to the ocean triggered increased oceanic productivity and organic carbon burial recorded in carbon isotopic records and paleooxygen concentrations the net result of this weathering event may have been an increase in the rate of drawdown of atmospheric co2 through the late miocene early pliocene via direct silicate weathering reactions and increased burial of organic carbon in the ocean this increased rate of atmospheric co2 drawdown may have destabilized the climate system by the late pliocene thus initiating a period of intense cooling and ice buildup leading to the present

neogene evolution of the himalayan tibetan region constraints from odp site 758 northern ninetyeast ridge bearing on climatic change abstract magnetic susceptibility remanence and lithostratigraphic profiles for the neogene quaternary sequence at site 758 odp leg 121 on the northern ninetyeast ridge show distinct changes dated from biostratigraphy and detailed magnetostratigraphy at 17 5 10 4 10 0 8 8 6 5 5 4 5 1 2 7 2 5 1 9 and 1 2 1 1 ma these magnetic and lithologic changes appear to reflect changes in the supply and character of terrigenous material from the himalayan tibetan region resulting from changes in gradient of the ganges brahmaputra and probably the ancient indus drainage systems the sedimentary changes can be correlated with changes in uplift sensitive markers such as the oceanic 87 sr 86 sr ratio and monsoonal induced upwelling but not clearly so with sealevel variations we interpret these sedimentary changes therefore to primarily reflect changes in the tectonic evolution of the himalayan tibetan region the changes in the distal marine sedimentary record of the northern ninetyeast ridge are compared with isotopic control on the timing of himalayan tibetan tectonic phases and magnetostratigraphic control on their reflection in the proximal siwalik molasse record this comparison indicates that the distal ninetyeast ridge record can be used to detail and to place minimal age constraints on tectonic phases in the wider himalayan region and on evolution of the proximal molasse sequence with a time lag determined for the four earliest changes at less than 1 m y the changes at 17 5 ma and 5 4 5 1 ma can be interpreted in terms of the causative chain enhanced plate motion uplift and sedimentation change climatic change supporting arguments that the late cainozoic global climatic deterioration is driven by uplift of large plateaus such as the himalayan tibetan region and the western cordillera

the ecology of living stained deep sea benthic foraminifera from the sulu sea the distribution of living rose bengal stained deep sea benthic foraminifera was determined in the upper 20 cm of sediments of eight soutar box cores taken from two depth transects 510 4515 m in the thermospheric 10 c sulu sea despite the uniformity of bottom water temperatures salinities and dissolved oxygen levels below 1000 m significant faunal differences exist at different depths in the low oxygen 1 25 ml l below 1000 m basin the shallowest site 510 m is dominated 10 of the calcareous fauna by cibicidoides uvigerina 150 m and bolivina 63 m while siphonina is codominant with cibicidoides and uvigerina in the 1005 m core the 2000 m cores are dominated by cibicidoides gyroidinoides and oridorsalis while cibicidoides bradyi and oridorsalis umbonatus dominate the 3000 and 4000 m cores infaunal assemblages of valvulineria mexicana are found in the sediments of the 4515 m core relatively low bottom water oxygen values do not necessarily yield typical low oxygen taxa such as bolivina uvigerina chilostomella bulimina and globobulimina changes in the abundances of these taxa in fossil assemblages have been used as indicators of changes in ancient bottom water oxygen levels but may instead reflect organic carbon contents of the sediments an examination of the vertical distributions of foraminiferal assemblages from the 63 m and 150 m fractions reveals that taxa have microhabitat preferences similar to those observed in other regions taxa found in the upper 0 to 1 cm interval epifaunal include cibicidoides wuellerstorfi and hoeglundina elegans while taxa such as chilostomella and globobulimina reach maximum abundances in subsurface sediments and have infaunal microhabitat preferences cibicidoides bradyi and o umbonatus live in sediment depths from 0 to 4 cm and have transitional preferences with both epifaunal and infaunal occurrences intrageneric differences in test morphologies including pore distribution rounded peripheries and variable spire height are observed in cibicidoides and gyroidinoides and are suggested to be related to microhabitat preferences vertical distributions of a number of taxa found in both the 63 to 150 m and 150 m fractions are similar suggesting that juveniles and adults live under similar microhabitat conditions ontogenetic changes in microhabitat preferences of most species are not observed in this study and therefore would not be expected to account for isotopic vital effects reported for some taxa in previous studies

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

the latest miocene early pliocene biogenic bloom a revised indian ocean perspective abstract numerous investigations of neogene sediment sequences from the indian and pacific oceans have led to a paleoceanographic hypothesis that recently has been coined the biogenic bloom this hypothesis suggests that primary productivity increased substantially at indo pacific upwelling zones during the latest miocene early pliocene although it is clear that recognition of a widespread biogenic bloom provides a fundamental framework for understanding late neogene global change it also is apparent that investigations of this phenomena require syntheses of a variety of information from time correlated sediment sequences that span large oceanic regions late neogene polarity chron boundaries and nannofossil events are placed on a common and current time scale for 12 ocean drilling program odp sediment sequences in order to construct a coherent description of the biogenic bloom in the indian ocean carbonate mass accumulation rates mars were significantly higher than present day in the north and west indian ocean between 9 0 and 3 5 ma sediment deposited during this time interval also is dominated by planktic faunal assemblages that are indicative of high productivity and redox conditions that are explained by an increase in the supply of organic carbon further sediment of this age contains benthic faunal assemblages and chemical characteristics that are consistent with low dissolved o 2 in intermediate waters these observations collectively suggest that between 9 0 and 3 5 ma productivity in the north and west indian ocean was significantly higher than present day and the intermediate water oxygen minimum zone omz was more extensive than present day this paleoceanographic condition was most intense between 6 0 and 5 0 ma in the indian ocean comparison of the indian ocean sites with two locations in the pacific ocean suggests that the biogenic bloom was time coincident in the indian and pacific oceans the biogenic bloom therefore implies an important change in global nutrient cycling occurred during the latest miocene early pliocene although a satisfactory explanation for the change remains elusive

sensitivity of eurasian climate to surface uplift of the tibetan plateau experiments with climate models are used to illustrate how earth s climate could be expected to change if surface uplift occurred the experiments contrast two climate simulations one with all continents at sea level and one with present day mountains and plateaus the simulations in the eurasia sector show that surface uplift increased elevation produces marked increases in the intensity of summer and winter monsoons of asia with development of wetter climates to the south and east of the tibetan plateau and drier climates to the north and west of the plateau there is a general cooling in northern polar latitudes comparison of simulations from two different models shows that the magnitude of the simulated changes depends upon how the models calculate the land surface hydrology and the ocean temperature the simulated changes produced by surface uplift agree in a general way with many geologic indicators of late cenozoic climate change the results suggest that surface uplift of mountains and platea

discrimination between discrete and continuum scattering from the sub seafloor there is growing evidence that seabed scattering is often dominated by heterogeneities within the sediment volume as opposed to seafloor roughness from a theoretical viewpoint sediment volume heterogeneities can be described either by a fluctuation continuum or by discrete particles in at sea experiments heterogeneity characteristics generally are not known a priori thus an uninformed model selection is generally made i e the researcher must arbitrarily select either a discrete or continuum model it is shown here that it is possible to acoustically discriminate between continuum and discrete heterogeneities in some instances for example when the spectral exponent 3 4 the volume scattering cannot be described by discrete particles conversely when 3 2 the heterogeneities likely arise from discrete particles furthermore in the range 2 3 4 it is sometimes possible to discriminate via physical bounds on the parameter values the ability to so discriminate is important because there are few tools for measuring small scale o 10 2 to 10 1 m sediment heterogeneities over large areas therefore discriminating discrete vs continuum heterogeneities via acoustic remote sensing may lead to improved observations and concomitant increased understanding of the marine benthic environment

closure of the central american isthmus and its effect on deep water formation in the north atlantic modern ocean thermohaline driven circulation influences global climate by transporting heat to high latitudes1 2 and by affecting the exchange of co2 between ocean and atmosphere3 north atlantic deep water nadw plays a key role in this circulation and quaternary climate cycles have been linked to changes in nadw flow4 general circulation model simulations indicate that before closure some 3 4 million years ago of the central american isthmus the narrow strip of land linking north and south america the direct flow of low salinity water from the pacific to the atlantic ocean would have led to a smaller nadw flow5 6 sedimentation patterns7 and nutrient proxies8 11 support these model results by indicating an increase in nadw flow around the time of isthmus closure but these records do not allow changes in different nadw sources to be distinguished and the overall effect of closure on global ocean circulation is poorly known here we present nd pb and sr isotope records preserved by a hydrogenous ferromanganese crust from the nadw flow path in the western north atlantic ocean these records indicate that the isotopic signal associated with nadw strengthened around 3 4 million years ago showing that deep water that formed in the labrador sea made a gradually increasing contribution to nadw flow these data taken together with those from the central pacific ocean12 indicate an increased nadw flow since isthmus closure and suggest that the closure established today s general pattern of ocean circulation

analyses of high frequency bottom and subbottom backscattering for two distinct shallow water environments high frequency 40 khz bottom backscattering data obtained by the benthic acoustic measurement system from two different shallow water locations were analyzed one of the sites has a hard sandy bottom and the other a soft silty bottom since the data were recorded on two slightly separated arrays the differential phase technique was used to locate the scatterers it is found that scattering at the sandy site originated from the seafloor interface while the scattering at the silty bottom was caused by a scattering layer about 1 m beneath the seafloor believed to be methane gas voids the backscattering strengths at the silty bottom were about 3 db higher than those at the sandy bottom a simple sub bottom scattering model for the silty bottom is assumed and compared with the data when the sound refraction and attenuation are taken into account the model data fit yields reasonable results

northeastern atlantic benthic foraminifera during the last 45 000 years changes in productivity seen from the bottom up we studied benthic foraminifera from the last 45 kyr in the 63 mu m size fraction in biogeochemical ocean flux studies bofs cores 5k 50 degrees 41 3 n 21 degrees 51 9 w depth 3547 m and 14k 58 degrees 37 2 n 19 degrees 26 2 w depth 1756 m at a time resolution of several hundreds to a thousand years the deepest site showed the largest fluctuations in faunal composition species richness and benthic foraminiferal accumulation rates the fluctuations resulted from changes in abundance of epistominella exigua and alabaminella weddellensis in the present oceans these species bloom opportunistically when a spring plankton bloom results in seasonal deposition of phytodetritus on the seafloor the phytodetritus species had very low relative abundances and accumulation rates during the last glacial maximum a strong increase in absolute and relative abundance of e exigua and a weddellensis during deglaciation paralleled the decrease in abundance of the polar planktonic foraminifer neogloboquadrina pachyderma s and the increase in abundance of warmer water planktonic species such as globigerina bulloides this strong increase in relative abundance of the phytodetritus species and the coeval increase in benthic foraminiferal accumulation rate were thus probably caused by an increase in the deposition of phytodetritus to the seafloor and thus probably of surface productivity when the polar front retreated to higher latitudes the abundance of phytodetritus species decreased during the younger dryas but not to the low levels of fully glacial conditions during heinrich events periods of excessive melt water formation and ice rafting benthic accumulation rates were very low as were the absolute and relative abundances of the phytodetritus species supporting suggestions that surface productivity was very low during these events in both cores pullenia and cassidulina species were common during isotope stages 2 3 and 4 as were bolivinid buliminid and uvigerinid species high relative abundances of these species have been interpreted as indicative either of sluggish deep water circulation or of high organic carbon fluxes to the seafloor in our cores relative abundances of these species are negatively correlated with benthic foraminiferal accumulation rates and we can thus not interpret them as indicative of increased productivity during glacials the percentage of these low oxygen species calculated on a phytodetritus species free basis decreased slightly at deglaciation at 5k but not at 14k this suggests that decreased production of north atlantic deep water during the last glacial might have slightly affected benthic foraminiferal faunas in the eastern north atlantic at 3547 m depth but not at 1756 m in conclusion major changes in deep sea benthic foraminiferal faunas over the last 45 000 years in our cores from the northeastern atlantic were the result of changes in surface water productivity not of changes in deep water circulation productivity was lower during the glacial probably because of extensive ice cover

the emplacement time of the hegenshan ophiolite constraints from the unconformably overlying paleozoic strata abstract controversy has long surrounded the emplacement time of the hegenshan ophiolite that is considered to mark a suture zone called the hegenshan heihe suture resulting from the closure of a back arc basin in the paleo asian ocean the hegenshan ophiolite in the xiaobaliang area is unconformably overlain by a sequence of paleozoic strata called the zhesi formation that consists of conglomerate sandstone siltstone and limestone some of which contain permian marine fossils of brachiopods therefore the ages of these paleozoic strata can be used to constrain the emplacement time of the hegenshan ophiolite four samples of the zhesi formation collected in the xiaobaliang area yield the detrital zircon u pb ages of 285 272 ma with the peak at 279 ma 315 288 ma with a peak at 300 ma 320 358 ma with a peak at 336 ma and 406 3 ma of which the 280 and 300 ma age groups are remarkably similar to the ages of latest carboniferous early permian gegenaobao dashizai formation or a type granites which formed under a post collisional setting however the age groups of 320 to 358 ma with a peak at 336 ma show the features of mafic ultramafic zircons in cl image most likely derived from local mafic ultramafic rocks of the hegenshan ophiolite in the xiaobaliang area which is supported by the fact of the ophiolite unconformably overlain by the middle permian zhesi formation therefore we propose that the emplacement time of the hegenshan ophiolite must have happened at some time before the middle permian 280 ma most likely between 300 and 335 ma not in the silurian devonian or mesozoic as previously considered

insolation cycles as a major control of equatorial indian ocean primary production analysis of a continuous sedimentary record taken in the maldives indicates that strong primary production fluctuations 70 to 390 grams of carbon per square meter per year have occurred in the equatorial indian ocean during the past 910 000 years the record of primary production is coherent and in phase with the february equatorial insolation whereas it shows diverse phase behavior with 18o depending on the orbital frequency eccentricity obliquity or precession examined these observations imply a direct control of productivity in the equatorial oceanic system by insolation in the equatorial indian ocean productivity is driven by the wind intensity of westerlies which is related to the southern oscillation therefore it is suggested that a precession forcing on the southern oscillation is responsible for the observed paleoproductivity dynamics

variations of the monsoon regime during the upper quaternary evidence from carbon isotopic record of organic matter in north indian ocean sediment cores abstract the 13 c 12 c ratios and contents of organic carbon in deep sea sediment cores of the north indian ocean have been compared for the different climatic stages as determined by oxygen isotope stratigraphy using the differences in the 13 c values between marine and terrestrial organic matter the terrestrial contribution to the eastern gulf of bengal and the andaman sea has been determined for modern and glacial times the geographical distribution of the terrestrial organic carbon linked to sea surface circulation shows that during the last glacial period the ne monsoon circulation was the dominant feature the organic carbon abundance within the sediment which is related to local productivity was greater during glacial times in the areas where production depends on the ne monsoon strength coast of north india in the arabian sea and andaman sea suggesting its reinforcement

latest miocene pleistocene productivity and deep sea ventilation in the northwestern indian ocean deep sea drilling project site 219 knowledge of the biology of deep sea benthic foraminifera was used to interpret the results of multivariate analyses factor and cluster on relative abundance data of benthic foraminifera at deep sea drilling project site 219 southeastern arabian sea 1764 m depth in combination with carbon and oxygen isotope data faunal data document major changes in deep sea ventilation and productivity over the past 5 5 ma including the end of the miocene pliocene indo pacific biogenic bloom period at 4 0 ma interestingly there is no simple correlation between high productivity and low oxygenation productivity fluctuated but became overall more pulsed whereas overall oxygenation increased we interpret the records as a combination of local to regional fluctuations in productivity probably caused by changes in monsoonal development particularly its seasonality and changes in oxygenation of intermediate depth waters in the indian ocean caused by global changes in climate and ocean circulation

changes in the distribution of uvigerinidae species over the past 775 kyr implications for the paleoceanographic evolution of the japan sea abstract the distribution of benthic foraminifera of the uvigerinidae family which are abundant in middle to upper pleistocene samples from the yamoto basin in the japan sea have been analyzed quantitatively and used for paleoceanographic interpretations twenty three uvigerinidae species are reported from iodp site u1426 6 of which are present in at least three or more samples with 10 or more abundance and considered as dominant species the abundance and ecological preferences of dominant uvigerinidae species comprised of uvigerina mediterranea uvigerina yabei uvigerina sp 1 uvigerina peregrina suggest dysoxic conditions with high influx of organic matter between 775 and 475 ka in response to the enhanced flow of the tsushima warm current the end of mis 13 marked the onset of more extreme glacial interglacial climate cycles in the japan sea glacial interglacial alternations of uvigerina peregrina and trifarina angulosa from 475 ka to the holocene indicate major fluctuations in circulation and productivity within this semi enclosed basin both u peregrina and t angulosa respond to 100 kyr variability reflecting the paleoclimatic shift and global ice volume changes associated with the middle pleistocene transition

nonstationary phase of the plio pleistocene asian monsoon paleoclimate records indicate that the strength of the asian summer monsoon is sensitive to orbital forcing at the obliquity and precession periods 41 000 and 23 000 years respectively and the extent of northern hemisphere glaciation over the past 2 6 million years the timing phase of strong monsoons has changed by 83 degrees in the precession and 124 degrees in the obliquity bands relative to the phase of maximum global ice volume inferred from the marine oxygen isotope record these results suggest that one or both of these systems is nonstationary relative to orbital forcing

seabed roughness parameters from joint backscatter and reflection inversion at the malta plateau this paper presents estimates of seabed roughness and geoacoustic parameters and uncertainties on the malta plateau mediterranean sea by joint bayesian inversion of mono static backscatter and spherical wave reflection coefficient data the data are modeled using homogeneous fluid sediment layers overlying an elastic basement the scattering model assumes a randomly rough water sediment interface with a von karman roughness power spectrum scattering and reflection data are inverted simultaneously using a population of interacting markov chains to sample roughness and geoacoustic parameters as well as residual error parameters trans dimensional sampling is applied to treat the number of sediment layers and the order zeroth or first of an autoregressive error model to represent potential residual correlation as unknowns results are considered in terms of marginal posterior probability profiles and distributions which quantify the effective data information content to resolve scattering geoacoustic structure results indicate well defined scattering roughness parameters in good agreement with existing measurements and a multi layer sediment profile over a high speed elastic basement consistent with independent knowledge of sand layers over limestone

impact of productivity events on the benthic foraminiferal fauna in the arabian sea over the last 150 000 years we have studied the sediment geochemistry and benthic foraminiferal fauna in a piston core from the northwest arabian sea covering the last 150 000 years and compared the results with the stable isotopic record from both benthic uvigerina peregrina and planktonic globigerinoides sacculifer foraminifera the 18o record of the planktonic foraminiferal species shows excursions towards heavier 18o values when compared to the benthic record and the specmap stack particularly in stage 3 between approximately 30 and 45 50 kyr we call this the stage 3 event species diversity and relative abundance data on benthic foraminifera reveal that some species e g cibicidoides wuellerstorfi oridorsalis umbonatus pullenia bulloides pyrgo spp and the uvigerinids increased in relative abundance during glacial stages 2 and 4 and during the stage 3 event whereas other e g astrononion novozealandicum cibicidoides bradyi eggerella bradyi gyroidina altiformis hoeglundina elegans pullenia subcarinata and sigmoilina edwardsii decreased using correspondence analysis we determined that approximately 50 of the total sample variation may be accounted for by three first factors which all are interpreted as primarily related to surface water productivity and the quality of the organic matter that reaches the seafloor the relative abundance of some species appears to vary on a precessional cycle perhaps responding to monsoon driven upwelling and associated increased food supply whilst the abundances of other species are related to the 100 kyr ice volume cycle and may be responding to changes in intermediate and deep water chemistry geochemical paleoproductivity tracers such as the ba content of the sediment have a very similar distribution to specific foraminiferal factors in both time and frequency domains certain planktonic foraminifera e g globigerina bulloides also display obvious maxima in the stage 3 event the stage 3 event occurs at a time of reduced rate of amplitude change in both solar radiation and monsoon pressure index we conclude that there was a period 15 to 20 kyr duration during stage 3 with cooler than normal surface waters enhanced productivity and rapid settling of biogenic matter

bistatic bottom scattering model experiments and model data comparison a model is presented for bistatic scattering from ocean sediments it treats scattering due to both roughness of the seabed and volume inhomogeneities within the sediment accordingly the scattered intensity is assumed to be a sum of two terms one proportional to the roughness scattering cross section and the other proportional to the volume scattering cross section the model is tested against data acquired as part of the coastal benthic boundary layer cbbl research program as part of that program an autonomous circularly scanning sonar system was deployed in well characterized regions this sonar operated at 40 khz had a 5 horizontal beam and acquired backscattering data over a 50 m radius during part of the deployment it operated in conjunction with a mobile receiving array so as to acquire bistatic data the experimental apparatus and procedures are presented and results are compared with model predictions

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

miocene benthic foraminiferal isotope records a synthesis abstract 18 o 16 o and 13 c 12 c ratios of miocene benthic foraminifera from a number of atlantic pacific and indian ocean dsdp sites 71 77b 206 208 238 279 289 296 329 357 and 366a have been compiled these provide a rather detailed history of miocene deep water especially in the pacific ocean bottom water temperatures rose during the early miocene and then declined rapidly during the middle miocene this decline was accompanied by an increase in antarctic glaciation late miocene bottom temperatures and antarctic ice volumes are inferred to be similar to today s but exhibited some fluctuation the early miocene ocean was less thermally stratified at intermediate and abyssal depths while the late miocene deep ocean had a thermal structure generally similar to the modern ocean foraminiferal carbon isotope ratios at most of the sites varied quasi sympathetically throughout the miocene these variations must reflect comparable variations in the mean 13 c 12 c of marine hco 3 however the causes of such variations are not yet clear

high frequency scattering from saturated sand sediments measurements of bottom scattering strengths were made with a multi frequency echo sounder mounted on a tower on a sandy bottom off west destin fl data were measured at five frequencies in the range 265 1850 khz at subcritical grazing angles ranging from 9 to 20 and at azimuths up to 90 around the tower scattering strength increased at about f1 4 up to 400 khz peaked between 700 and 1100 khz and decreased at higher frequencies

recent abyssal benthic foraminiferal biofacies of the eastern equatorial indian ocean abstract study of recent abyssal benthic foraminifera from core top samples in the eastern equatorial indian ocean has identified distinctive faunas whose distribution patterns reflect the major hydrographic features of the region above 3800 m indian deep water idw is characterized by a diverse and evenly distributed biofacies to which globocassidulina subglobosa pyrgo spp uvigerina peregrina and eggerella bradyi are the major contributors nuttalides umbonifera and epistominella exigua are associated with indian bottom water ibw below 3800 m within the ibw fauna n umbonifera and e exigua are characteristic of two biofacies with independent distribution patterns nuttalides umbonifera systematically increases in abundance with increasing water depth the e exigua biofacies reaches its greatest abundance in sediments on the eastern flank of the ninetyeast ridge and in the wharton cocos basin the hydrographic transition between idw and ibw coincides with the level of transition from waters supersaturated to waters undersaturated with respect to calcite and with the depth of the lysocline carbonate saturation levels possibly combined with the effects of selective dissolution on the benthic foraminiferal populations best explain the change in faunas across the idw ibw boundary and the bathymetric distribution pattern of n umbonifera the distribution of the e exigua fauna cannot be explained with this model epistominella exigua is associated with the colder more oxygenated ibw of the wharton cocos basin the distribution of this biofacies on the eastern flank of the ninetyeast ridge agrees well with the calculated bathymetric position of the northward flowing deep boundary current which aerates the eastern basins of the indian ocean

neogene carbonate stratigraphy of ontong java plateau western equatorial pacific three unexpected findings the neogene carbonate stratigraphy of five sites drilled on ontong java plateau during leg 130 reveals a number of patterns which are unexpected and which we refer to as loss paradox equatorial insensitivity and climate paradox they denote the following unresolved questions 1 the loss of carbonate at depth as derived from differences in accumulation rates is much greater than suggested by the change in carbonate percentages calculated under the assumption that carbonate dissolution is the cause of loss this indicates an important role for redeposition processes such as winnowing bottom currents sifting resuspension and catabatic flow and episodic sloughing or solifluction presumably stimulated by earthquakes 2 accumulation rates are not markedly increased at the time a site crosses the equator there are several possible reasons equatorial upwelling may be unimportant in controlling sedimentation rates this far in the western pacific or its output may be spread over a considerable distance from the equator alternatively increased supply below the equator is compensated for by increased removal e g from resuspension by bioturbation combined with catabatic flow it is conceivable that errors in the timescale could also produce the effect seen 3 there is an overall tendency for agreement between the stratigraphic patterns of carbonate content and of accumulation rates but neither pattern is readily explained by reference to changes in climate represented by benthic 18o or in sea level as derived from sequence stratigraphy

a benthic foraminiferal proxy of pulsed organic matter paleofluxes in the temperate open ocean of the modern northeast atlantic the spring bloom of phytoplankton leads to a seasonal pulse of detrital organic material phytodetritus to the ocean floor opportunistic benthic foraminifera rapidly colonise this food resource producing large numbers of individuals whose tests are ultimately added to the sediment one of these taxa epistominella exigua shows periodic peaks in abundance in the fossil record at many open ocean sites previously such peaks have been commonly interpreted to result from changes in physicochemical properties of bottom water mass and thus deep sea circulation the main purpose of this paper is to propose thate exigua may be used as a proxy of pulsed organic matter inputs to the deep ocean and therefore as an indicator of relative changes in productivity

late miocene early pliocene manganese redirection in the central indian ocean expansion of the intermediate water oxygen minimum zone decomposition of organic matter combined with density stratification generate a pronounced intermediate water oxygen minimum zone omz in the northwest indian ocean this zone currently lies between water depths of 200 and 2000 m and extends approximately 5000 km southeast from the arabian coast based upon benthic foraminiferal assemblage changes it has been suggested that this omz was even more extensive during the late miocene early pliocene 6 5 3 0 ma with a maximum volume and or intensity at approximately 5 0 ma while this inference may contribute to an understanding of the history of northwest indian ocean upwelling corroborating geochemical evidence for this interpretation has heretofore been lacking ocean drilling program odp sites 752 754 and 757 on broken and ninetyeast ridges are located within central indian ocean intermediate water depths 1086 1650 m but outside the present lateral dimensions of the indian ocean omz high resolution chemical analyses of sediment from these sites indicate significant reductions in the flux of mn and normalized mn concentrations between 6 5 and 3 0 ma that are most pronounced at approximately 5 0 ma because late miocene pliocene paleodepths for these sites were essentially the same as at present and because extremely low sedimentation rates 0 3 1 3 cm ky most likely precluded sedimentary metal oxide diagenesis we suggest that the observed mn depletions reflect diminished deposition of reducible mn oxyhydroxide phases within o2 deficient intermediate waters and that this effect was most intense at approximately 5 0 ma this interpretation implies that waters with less than 2 0 ml l o2 extended at least 1500 km beyond their present limits and is consistent with changes in benthic foraminifera assemblages we further suggest this expanded indian ocean omz is related to regionally and or globally increased biological productivity

response of deep sea benthic foraminifera to late quaternary climate changes southeast indian ocean offshore western australia the late quaternary benthic foraminifera of four deep sea cores off western australia odp 122 760a odp122 762b bmr96gc21 and rc9 150 have been examined for evidence of increased surface productivity to explain the anomalously low sea surface paleotemperatures inferred by planktic foraminifera for the last and penultimate glaciations the 13c trends ofcibicidoides wuellerstorfi and differences between the 13c trends of planktics globigerinoides sacculifer and benthicsc wuellerstorfi in the four cores indicate that during stage 6 bottom waters were significantly depleted in 13c and strong 13c gradients were established in the water column while during stage 2 and the last glacial maximum 13c trends did not differ greatly from that of the holocene two main assemblages of benthic foraminifera were identified by principal component analyses one dominated byuvigerina peregrina another dominated byu proboscidea abundance of these uvigerinids and of taxa preferring an infaunal microhabitat and ofepistominella exigua andbulimina aculeata indicate that episodes of high influx of particulate organic matter were established in most sites during glacial episodes and particularly so during stage 6 while evidence for upwelling during the last glacial maximum is less strong the penultimate glaciation upwellings were established within the areas of low sea surface paleotemperature indicated by planktic foraminifera during the last interglacial climax upwelling appears to have been established in an isolated region offshore from a strengthened leeuwin current off north west cape last glacial maximum 13c values ofc wuellerstorfi at waterdepths of less than 2000 m show smaller than global mean glacial interglacial changes suggesting the development of a deep hydrological front a similar vertical stratification bathyal front was also established during the penultimate glaciation

late quaternary deep ocean circulation a review of paleoceanographic studies dealing with late quaternary deep water circulation in the oceans is presented these studies which are based on the analysis of deep sea benthic foraminiferal faunal and geochemical data and sedimentological data are discussed in light of physical oceanographic conditions in the modern ocean we include in this review a re evaluation of benthic foraminiferal data from north atlantic and southern ocean piston cores and suggest that the dominance of uvigerina during glacial intervals reflects increased amounts of organic carbon at the sea floor compared to modern values a number of geochemical studies have suggested that the production or characteristics of north atlantic deep water nadw changed during glacial times although these changes have been thought to result from a cessation of overflow water from the norwegian greenland seas it is suggested here that seasonal sea ice cover was possible over southern portions of the norwegian sea during glacial intervals the presence of seasonally open water would have allowed norwegian sea overflow water to have been produced although perhaps at lower volumes and with different hydrographic properties than at present the record of antarctic bottom water aabw circulation does not show a simple relationship with paleoclimatic oscillations indicating that changes in oceanographic conditions in the southern ocean had little effect on aabw formation the aabw record contrasts with the glacial interglacial cycles of nadw suggesting no direct link between aabw and nadw circulation a variety of data suggests that changes in pacific deep water circulation occurred as a result of glacial production of north pacific deep water or from an increased flux of southern ocean water

productivity and continental denudation history from the south china sea since the late miocene late miocene recent micropaleontological and geochemical records from ocean drilling program odp site 1143 in the southern south china sea scs indicate that increase and decrease in abundance of siliceous plankton may be controlled mainly by the input of nutrients derived from land and provided by upwelling a high export production event a biogenic bloom event occurred in the southern scs between 12 and 6 ma during this period high ratios of smectite illite chlorite smectite quartz and al k indicate a high weathering intensity of the asian continent possibly due to the intensification of the east asian summer monsoon easm which may have increased the net flux of nutrients to the ocean both directly through terrestrial input and indirectly through upwelling activity a drop in ba ti al ti and ca ti values around 6 ma may indicate a lowering of productivity possibly due to the large consumption of sea surface nutrients by the biogenic bloom alternatively it may indicate a shift in terrigenous input source area at about 5 4 ma a decrease in weathering intensity as indicated by a sudden decrease in the values of smectite illite chlorite smectite quartz and al k might have led to a sudden decrease of terrestrial nutrient input to the scs we suggest that the biogenic bloom ended when nutrients in surface waters were exhausted because of a decrease in supply as well as a decrease in upwelling intensity due to weakening of the easm as a result radiolarians were absent in the studied area between similar to 6 and 3 2 ma at similar to 3 2 ma radiolarians began to recover possibly because the start of northern hemispheric glaciation and the rapid uplift of the tibet plateau led to intensification of the east asian monsoon after the mid pleistocene climate transition at 0 9 ma the abundance and mass accumulation rates of radiolarians increased probably as a result of increased upwelling activity driven by the increasing intensity of the summer monsoon crown copyright c 2009 published by elsevier b v all rights reserved

early pliocene closing of the indonesian seaway evidence from north east indian ocean and tropical pacific deep sea cores abstract deep sea cores from sites 214 and 758 a ninetyeast ridge north east indian ocean 761 b wombat plateau north east indian ocean and 586 b ontong java plateau tropical pacific are ideally located for comparison of late neogene planktic foraminiferal biogeography and paleoceanographic records of tropical indian and pacific oceans to infer the timing of closing of the indonesian seaway a consistent stratigraphy was developed between sites 214 and 586 b using graphic correlation and was integrated with the paleomagnetic time scale of berggren et al 1985 to provide an accurate chronology to compare interocean stratigraphic ranges of late neogene planktic foraminifera tropical planktic foraminifera occur throughout each sequence at all sites at each site the miocene pliocene boundary is defined by the first appearance of globorotalia tumida 5 2 ma the early late pliocene boundary by the first appearance of globigerinoides fistulosus 3 2 ma and the pliocene pleistocene boundary by the last appearance of g fistulosus 1 6 ma neogene planktic foraminiferal assemblages at sites 214 758a 761 b and 586b are generally similar until the beginning of the pliocene 5 2 ma when the faunal record indicates divergence a notable difference is complete absence of early pliocene taxon pulleniatina spectabilis from all the indian ocean sites this difference suggests that the indonesian seaway became an effective biogeographic barrier to planktic foraminifera at the beginning of the pliocene however there is still exchange of surface waters through this seaway earlier studies suggested a middle to late miocene occurrence for this biogeographic barrier p spectabilis evolved from p primalis in the equatorial pacific at about 5 2 ma it is a short ranging early pliocene species spanning about 1 3 my and occurred only in pacific unlike earlier suggestions of a broader distribution into the indian ocean

environmental distribution of recent benthic foraminifera on the northeast united states continental slope several bathymetrically consistent recent benthic foraminiferal biofacies have been recognized from the continental slope south east of cape cod faunal assemblages are dominated by globobulimina bulimina upper slope less than 1 000 m and uvigerina peregrina lower slope uppermost rise water depths between 1 000 and 2 500 m the globobulimina bulimina assemblage coincides with the oxygen minimum zone this agrees with the occurrence of globobulimina in mediterranean sapropels organic rich sediments deposited under anoxic conditions previous studies have noted the association of u peregrina with bathyal water depths deep water temperatures of 3 to 4 c and water of low oxygen content however correlations between abundance of u peregrina and water depth or temperature are not the same everywhere and no correlation with dissolved oxygen in the water column is seen on the united states continental slope and rise the highest abundances of u peregrina coincide with maxima of organic carbon and silt within the slope sediments this suggests that the distribution of u peregrina in this region may be influenced more by low oxygen in the sediments rather than by low oxygen content of the overlying water

benthic foraminiferal response to the emergence of the isthmus of panama and coincident paleoceanographic changes abstract late cenozoic benthic foraminiferal faunas from the caribbean deep sea drilling project dsdp site 502 3052 m and east pacific dsdp site 503 3572 m were analyzed to interpret bottom water masses and paleoceanographic changes occurring as the isthmus of panama emerged major changes during the past 7 myr occur at 6 7 6 2 3 4 2 0 and 1 1 ma in the caribbean and 6 7 6 4 4 0 3 2 2 1 1 4 and 0 7 ma in the pacific prior to 6 7 ma benthic foraminiferal faunas at both sites indicate the presence of antarctic bottom water aabw after 6 7 ma benthic foraminiferal faunas indicate a shift to warmer water masses north atlantic deep water nadw in the caribbean and pacific deep water pdw in the pacific flow of nadw may have continued across the rising sill between the caribbean and pacific until 5 6 ma when the pacific benthic foraminiferal faunas suggest a decrease in bottom water temperatures after 5 6 ma deep water to intermediate water flow across the sill appears to have stopped as the bottom water masses on either side of the sill diverge the second change recorded by benthic foraminiferal faunas occurs at 3 4 ma in the caribbean and 4 0 3 2 ma in the pacific at this time the caribbean is flooded with cold aabw which is either gradually warmed or is replaced by glacial bottom water gbw at 2 0 ma and by nadw at 1 1 ma these changes are related to global climatic events and to the depth of the sill between the caribbean and atlantic rather than the rising isthmus of panama benthic foraminiferal faunas at east pacific site 503 indicate a gradual change from cold pdw to warmer pdw between 4 0 and 3 2 ma the pdw is replaced by the warmer poorly oxygenated piw at 2 1 ma although the pdw affects the faunas during colder intervals between 1 4 and 0 7 ma the piw remains the principal bottom water mass in the guatemala basin of the east pacific

modeling of bottom backscattering from three dimensional volume inhomogeneities and comparisons with experimental data in this paper backscattering from 3d volume inhomogeneities in the seabed is modeled and the results compared with experimental data at 250 650 hz the experiment was part of the acoustic reverberation special research program arsrp and the data were obtained in a sediment pond on the western flank of the mid atlantic ridge a volume scattering model based on first order perturbation theory is developed incorporating contributions from both sound speed and density fluctuations with the propagators i e the green s functions handled accurately through numerical wave number integration and random fluctuations generated effectively by a new scheme modified from the spectral method the model is capable of simulating monostatic backscattered fields in the frequency domain as well as in the time domain owing to 3d volumetric sediment inhomogeneities the model compares favorably and consistently with the arsrp backscattering data over the entire frequency band with the fluctuations of sound speed and

recent deep sea benthic foraminiferal distributions in the south china and sulu seas thirty nine surface sediment samples collected from the south china and sulu seas were examined to determine the bathymetric distribution of benthic foraminifera in these two basins and the factors responsible for these distributions the benthic foraminiferal faunas of both basins are highly diverse but have low absolute abundances benthic number and species diversity systematically decrease with increasing water depth four faunal assemblages were identified for each basin byq mode principal component analysis theglobocassidulina subglobosa uvigerina assemblage 1500 m and theuvigerina assemblage 1400 m represent the shallowest assemblages in the south china sea and sulu sea respectively and are found within the oxygen minimum zone of each basin where the sedimentary organic carbon content is highest and the pore water oxygen penetration depth zo is shallowest thebulimina aculeata assemblage occurs at water depths between 1700 and 2000 m in the southeastern south china sea and is also associated with relatively high sedimentary organic carbon content theastrononion pusillum assemblage is found in south china sea at the depths between 1500 and 3200 m below the lysocline 3200 m in the south china sea this assemblage is replaced by the agglutinatedrhabdammina abyssorum assemblage this assemblage reflects the presence of a water mass that is highly undersaturated with respect to calcite in the sulu sea thepyrgo murrhina assemblage occupies water depths from 1400 to 2200 m below 2200 m two deep water assemblages are present that are both dominated byoridorsalis umbonatus a correlation between various environmental and benthic foraminiferal variables indicates that salinity temperature sediment grain size water depth and bottom water oxygen concentration are not key factors controlling the benthic foraminiferal distribution in either the south china sea or the sulu sea rather organic carbon content and oxygen penetration depth in sediments appear to play important roles in determining the distribution of benthic foraminifera in both basins