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

sediment flux distribution in the southern brazil basin during the late quaternary the role of deep sea currents detailed sedimentological and stratigraphic analyses were carried out on seven kullenberg cores collected across the brazilian continental margin during the french cruises byblos and apsara iii in order to highlight the factors controlling the sediment flux distribution in the southern brazil basin during the late quaternary on the continental slope and upper continental rise above 3000 m depth sediment fluxes are important and highly variable 4 2 14 2 g cm 2 10 3 yr the sediments show a pelagic or turbiditic character depending on the width of the shelf and proximity of canyons the material is characterized by high kaolinite contents and originates from the coastal rivers draining the south american continent north of rio de janeiro on the middle continental rise between 3000 and 4000 m depth sediment fluxes are the lowest observed in the area 0 9 g cm 2 10 3 yr because terrigenous input is trapped at shallower depths on the sao paulo plateau pelagic settling is the dominant process in the deep domains below 4000 m depth contouritic accumulations are developed on the path of the northwards moving antarctic bottom water aabw currents the deposits consist of fine grained silty clayey muds with very low carbonate contents the sediment fluxes 1 45 g cm 2 10 3 yr are higher than on the middle continental rise as a consequence of fine grained terrigenous supply derived from higher latitudes argentine basin and southern ocean and transported in the basin through the vema channel by the aabw currents this material is characterized by high smectite and chlorite contents these data reveal large sediment flux variations which are linked to distinct depth related domains such a distribution is the consequence of the presence of two available sources of terrigenous sediments 1 the brazilian continental areas with a downslope material transport and a sediment distribution controlled by the morphology of the margin and 2 the argentine basin with an alongslope material transport by deep sea currents which dominate the sedimentation in the abyssal domains

long period changes in the bottom water flowing through vema channel further details of the warming of bottom water flowing through the vema channel first reported by zenk and hogg 1996 are given because cross channel gradients of temperature are large and the channel is narrow careful analysis is required to determine whether or not the bottom water temperature minimum was adequately sampled by a particular station or cruise during a period from 1972 to 1996 at least 14 visits with quality hydrographic measurements have been made to the region of these 11 were judged successful and their data indicate an abrupt rise in potential temperature by 0 03 c from 0 18 c to 0 15 c in the early 1990s that has remained until the latest observation in the spring of 1996 although this observation is consistent with the report of warming of antarctic bottom water in the argentine basin by coles et al 1996 their associated conclusion that this water mass has also freshened by 0 008 psu on potential density surfaces is in contradiction with our finding of no measurable change

contour current accumulation off the vema channel mouth southern brazil basin pattern of a contourite fan abstract in the southern part of the brazil basin 3 5 khz echosounding reveals the existence of a cone shaped sedimentary accumulation 4480 m depth this sediment body is interpreted as deposited by the antarctic bottom water currents aabw at the downstream exit of deep valleys cutting through the rio grande rise it is located between two major channels that prolonged northeastward the valleys into the brazil basin the vema channel in the southeast and the sao paulo abyssal gap in the northwest as a consequence most of the aabw flow is channelized in two branches deposition of the current transported particles occurs below the shear zone which takes place between the two branches the quaternary sediments consist of silty clayey muddy contourites associated with manganiferous deposits the accumulation is built by lateral migration of secondary channels and by lateral construction of lobes between the two major channels which are both well stabilized on account of a morphotectonic control on their location the entire sediment body progrades upward and downstream toward the northeast on the basis of morphological seismic and lithological arguments we would like to propose the name of contourite fan for such a contour current controlled accumulation

near surface scattering effects observed with a high frequency phased array at pinyon flats california analysis of data collected by a high frequency array experiment con ducted at pinyon flat in southern california provides strong evidence that the high frequency wave field from local earthquakes at this hard rock site are strongly dis totted by near surface scattering the seismic array we deployed consisted of 60 2 hz natural frequency three component sensors deployed in a three dimensional array two of the sensors were located in boreholes at 150 and 275 m depth the other 58 sensors were deployed in an areal array above these boreholes thirty six of these were deployed in a 6 by 6 element grid array with a nominal spacing of 7 m centered over the borehole sensors the remaining 22 seismometers were laid out in two 11 element linear arrays radiating outward from the grid coherence calculations reveal a rapid loss of coherence at frequencies over 15 hz at all but the shortest length scales of this array three dimensional visualization techniques were used to closely examine the spatial stability of particle motions of p and s waves this reveals systematic variations of particle motion across the array in which the particle motion tracks tilt drastically away from the backazimuth expected for an isotropic medium these variations however are frequency dependent below around 8 hz the particle motions become virtually identical for all stations at progressively higher frequen cies the wave field particle motion becomes increasingly chaotic frequency wave number analysis of these data provide quantitative measures of the same phenomena we find that direct wave f k spectra are bathed in a background of signal generated noise that varies from 10 to 30 db down from the direct arrival signal this signal generated noise appears to be nearly white in wavenumber indicating the wavelength of this noise on the scale of tens of meters and less refraction measurements we made on two lines crisscrossing the array reveal that the weathered layer velocities are highly variable and define a very strong wave guide measured surface p wave velocities varied from 400 to 1300 rn sec and velocities at depth of approximately 15 m varied from 1600 to 2700 m sec previous measurements in the boreholes showed that the intact granite below about 65 m depth has a velocity of approximately 5400 m sec these results demonstrate the extreme velocity contrast and degree of velocity heterogeneity of the near surface at this site we conclude that all the ob servations we made can be explained by strong scattering of incident body wave signals into a complex mishmash of body wave and surface wave modes in this heterogeneous near surface wave guide

hiatuses in the late pliocene pleistocene stratigraphy of the ioffe calcareous contourite drift western south atlantic abstract the ioffe drift located in the antarctic bottom water pathway from the vema channel to the brazil basin provides a suitable site to study past variations in bottom contour currents and their contribution to erosion and accumulation of deep sea sediments our previous study of the reference core ai 2436 from the drift summit ivanova et al 2016a documented the stratigraphic sequence of the uppermost part of the sedimentary cover ranging from the recent through upper pliocene the reduced thickness and or absence of biostratigraphic zones and the occurrence of several hiatuses here we provide the multi proxy biostratigraphic magnetic susceptibility ms color reflectance and x ray fluorescence xrf data in the sediment cores ai 3318 from the summit and ai 3316 from the ne slope of the ioffe drift the new data report numerous long and short term stratigraphic gaps over the last 3 ma an interval of specific high amplitude peaks representing abrupt changes in volume ms and xrf variability is identified from 2 51 2 59 to 1 9 ma which can likely serve as a regional stratigraphic benchmark in the future studies of deep sea contourites the correlation of three sediment records from the drift suggests that the most pronounced series of hiatuses associated with enhanced aabw flow intensity occurred from 1 6 to 0 81 ma i e roughly during the mid pleistocene transition and from 2 51 2 59 to 1 9 ma i e covers the onset of the modern type deep water circulation in the south atlantic turnau and ledbetter 1989 comparison of the studied sediment records with dsdp site 516 reveals reduced thickness of all recovered biostratigraphic zones and more often occurrence of hiatuses in the ioffe drift than on the rio grande rise suggesting more vigorous contour currents in the former area

the interplay between turbidity and contour current processes on the columbia channel fan drift southern brazil basin abstract the distribution of sedimentary processes and associated deposits on the columbia channel fan drift system southern brazil basin were investigated on the basis of 3 5 khz echosounding profiles and core lithology this system is composed of a sediment levee elongated downslope with a w e then sw ne trend bounded to the south by the columbia channel and to the north by the vitoria trindade seamounts turbidites are widespread on the shallowest part of the system 1 thick turbidite sequences composed of quartz rich sandy material originating from the upper continental margin are found in the axis of the channel the turbidity currents do not significantly overflow the northern levee where only sparse and thin silty muddy turbidites occur close to the axis of the channel 2 the major part of the levee shows quartz poor mica and foram rich turbidite material originating from the vitoria trindade seamounts the abundance of this material decreases southwards and eastwards the whole area is swept by the northward flowing antarctic bottom water contour current deposited muds are dominant on the southern flank of the columbia channel free of any turbidite supply as a consequence of the coriolis effect and on the deepest part of the northern levee remote from turbidite source areas however the contour currents interplay with the turbidity currents on the shallowest part of the levee contour current deposited muds are interbedded with silty sandy turbidite sequences and a part of the muddy material has probably been pirated from the finer tails of the turbidity flows the other part being transported from southernmost areas by the bottom currents the interplay of both processes is also responsible for the deposition of silty turbidite beds top truncated by contour currents and overlain by bioturbated muddy contourites

the energy partitioning and the diffusive character of the seismic coda following recent theoretical developments of the radiative transfer the ory of elastic waves we propose to use the ratio r of energies of curl and divergence part of the ground displacement to distinguish between the different possible domi nant scattering regimes in the lithosphere a consequence of the diffusion regime is the partitioning of the energy between different vibrational modes which is inde pendent of time it results in the stabilization of r this behavior is not expected if low order diffraction such as single scattering is dominant we apply our technique to seismograms recorded by a small aperture seismic array operated during june august 1997 in guerrero mexico we estimate the energy ratio r in the coda of 13 earthquakes recorded by the array we find it to be very stable in the coda window while the energy level itself changes by several orders of magnitude the value of r is 7 1 independent of the earthquake location and the magnitude the observed stabilization of r is a strong indication of the diffusive regime of the seismic coda

closure of the panama seaway during the pliocene implications for climate and northern hemisphere glaciation the panama hypothesis states that the gradual closure of the panama seaway between 13 million years ago 13 ma and 2 6 ma led to decreased mixing of atlantic and pacific water masses the formation of north atlantic deep water and strengthening of the atlantic thermohaline circulation increased temperatures and evaporation in the north atlantic increased precipitation in northern hemisphere nh high latitudes culminating in the intensification of northern hemisphere glaciation nhg during the pliocene 3 2 2 7 ma here we test this hypothesis using a fully coupled fully dynamic ocean atmosphere general circulation model gcm with boundary conditions specific to the pliocene and a high resolution dynamic ice sheet model we carry out two gcm simulations with closed and open panama seaways and use the simulated climatologies to force the ice sheet model we find that the models support the panama hypothesis in as much as the closure of the seaway results in a more intense atlantic thermohaline circulation enhanced precipitation over greenland and north america and ultimately larger ice sheets however the volume difference between the ice sheets in the closed and open configurations is small equivalent to about 5 cm of sea level we conclude that although the closure of the panama seaway may have slightly enhanced or advanced the onset of nhg it was not a major forcing mechanism future work must fully couple the ice sheet model and gcm and investigate the role of orbital and co2 effects in controlling nhg

effects of a topographic gradient on coastal current dynamics 1 when a boundary current in a rotating environment encounters a significant change in depth two typical behaviors are observed the flow may bifurcate with one branch moving away from the boundary following the topographic contours and the other branch following the boundary alternatively the boundary current may rebound or retroflect from the topography with the formation of a dipole that moves in the upstream direction which behavior is observed is determined for the most part by the geometry of the system and not by the initial direction of the boundary current bifurcation is typical in a right handed geometry i e in the case with the boundary on the right side looking from deep to shallow water while retroflection is typical in a left handed geometry laboratory experiments in a rotating tank using dye visualization and particle tracking verify this basic dichotomy but also reveal new aspects of the problem not examined in previous studies in particular it is demonstrated that in the enclosed basin there is a return current flowing counter to the branch following the topographic contours the experimental design allows comparison of the large time flow with the circulation in the adriatic sea

geochemical evidence for the trindade hotspot trace columbia seamount ankaramite abstract the columbia seamount 825 km offshore from brazil at 20 s lies on the east west trace of the trindade hotspot continental and oceanic magmatism believed to have originated with this hotspot is alkalic and sio2 undersaturated and dates from 85 ma in southern brazil to

the oligocene neogene deep sea columbia channel system in the south brazilian basin seismic stratigraphy and environmental changes abstract the columbia channel ccs system is a depositional system located in the south brazilian basin south of the vitoria trindade volcanic chain it lies in a wnw ese direction on the continental rise and abyssal plain at a depth of between 4200 and 5200 m it is formed by two depocenters elongated respectively south and north of the channel that show different sediment patterns the area is swept by a deep western boundary current formed by aabw the system has been previously interpreted has a mixed turbidite contourite system more detailed study of seismic data permits a more precise definition of the modern channel morphology the system stratigraphy as well as the sedimentary processes and control the modern ccs presents active erosion and or transport along the channel the ancient oligo neogene system overlies a upper cretaceous paleogene sedimentary substratum unit u1 bounded at the top by a major erosive late eocene early oligocene discordance d2 this ancient system is subdivided into 2 seismic units u2 and u3 the thick basal u2 unit constitutes the larger part of the system it consists of three subunits bounded by unconformities d3 oligocene miocene boundary d4 late miocene and d5 late pliocene the subunits have a fairly tabular geometry in the shallow nw depocenter associated with predominant turbidite deposits they present a mounded shape in the deep ne depocenter and are interpreted as forming a contourite drift south of the channel the deposits are interpreted as a contourite sheet drift the surficial u3 unit forms a thin carpet of deposits the beginning of the channel occurs at the end of u1 and during the formation of d2 its location seems to have been determined by active faults the channel has been active throughout the late oligocene and neogene and its depth increased continuously as a consequence of erosion of the channel floor and deposit aggradation along its margins such a mixed turbidite contourite system or fan drift is characterized by frequent rapid lateral facies variations and by unconformities that cross the whole system and are associated with increased aabw circulation

upper quaternary deposits on the sao tome deep sea channel levee system south brazilian basin major turbidite versus contourite processes abstract this paper is an attempt to show how one may discriminate turbiditic and contouritic processes in the deposition of deep sea accumulations the case study is the sao tome deep sea channel levee system in the south brazilian basin this system is elongated parallel to the margin contour and was first interpreted as being controlled by contour current activity detailed analyses of 3 5 khz profiles and piston cores allowed to demonstrate that the upper quaternary sediments are predominantly deposited by turbidite and hemipelagic pelagic sedimentation processes on the levee and the transitional area towards the deeper rise frequent fine grained turbidites accounting for 25 45 of the entire depositional series are interbedded with the hemipelagic pelagic muds in the channel thicker and coarser turbidites 15 are associated with debris flows 20 deposit deformation in the form of slides slumps and diapir like structures largely affect the distal transitional area slight evidence of contour current activity only consists of some sediment wave fields and manganiferous rich layers or some top truncated sequences and foraminiferal sandy layers

paleogene neogene depositional history of the middle u s atlantic continental rise mixed turbidite and contourite depositional systems abstract the construction of the middle u s atlantic continental rise from late paleogene to late pliocene time included downslope sediment gravity flows and alongslope contourite deposition some previous studies have suggested that contour following bottom currents were the dominant control on depositional patterns depositional relief from two large contourite drifts largely built in late miocene to late pliocene time continue to dominate present day lower rise morphology detailed seismic sequence mapping presented here suggests that significant turbidite fan systems also were active and are preserved in the stratigraphic section a single thick depositional sequence is mapped for oligocene to middle miocene sediments deposited when inferred fan systems off the norfolk washington canyons were reworked by bottom currents along the lower rise to form the proto hatteras outer ridge detailed mapping of three late miocene to late pliocene depositional sequences indicates that fan channel systems were active at the same time large contourite drifts were being constructed sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the hatteras outer ridge and through entrainment and resuspension was an important source for regional contourite deposition controlled by the deep western boundary undercurrent we suggest that submarine fans and current controlled sediment drifts may develop simultaneously as companion systems types of fan drift interaction include current only modification of fans transitional fan to drift and adjacent or overlapping submarine fan and sediment drift deposition

sea surface temperature variability and deep water reorganisation in the subtropical north atlantic during isotope stage 2 4 abstract two sediment cores from a high accumulation sediment drift at 3000 m water depth in the western north atlantic are investigated in order to examine possible linkages between subtropical surface and bottom water properties and global climate oscillations during the last glacial interglacial cycle planktonic foraminiferal assemblages and the 18 o of globigerinoides ruber both document large sea surface temperature oscillations 3 4 c that appear to co vary with greenland ice core records short episodes of sea surface cooling as far south as the blake outer ridge at 33 n are inferred from increased abundance of the polar species neogloboquadrina pachyderma sinistral during the ice core stadials events nutrient proxy records suggest that the deep waters flooding the site alternated between a northern north atlantic deep water and a southern southern ocean water source in most cases warm surface water conditions interstadials were accompanied by a strong north atlantic deep water component in the bottom water circulation however the transitions into interstadial 14 and 12 seem to be marked by a 1500 yr lag between surface water warming and the deep water circulation shift as recorded by caco 3 and 13 c of benthonic foraminifera

anatomy of the high frequency ambient seismic wave field at the tcdp borehole 1 the taiwan chelungpu fault drilling project tcdp installed a vertical seismic array between 950 and 1270 m depth in an active thrust fault environment in this paper we analyze continuous noise records of the tcdp array between 1 and 16 hz we apply multiple array processing and noise correlation techniques to study the noise source process properties of the propagation medium and the ambient seismic wave field diurnal amplitude and slowness patterns suggest that noise is generated by cultural activity the vicinity of the recording site to the excitation region indicated by a narrow azimuthal distribution of propagation directions leads to a predominant ballistic propagation regime this is evident from the compatibility of the data with an incident plane wave model polarized direct arrivals of noise correlation functions and the asymmetric arrival shape evidence for contributions from scattering comes from equilibrated earthquake coda energy ratios the frequency dependent randomization of propagation directions and the existence of correlation coda waves we conclude that the ballistic and scattered propagation regime coexist where the first regime dominates the records but the second is weaker yet not negligible consequently the wave field is not equipartitioned correlation signal to noise ratios indicate a frequency dependent noise intensity iterations of the correlation procedure enhance the signature of the scattered regime discrepancies between phase velocities estimated from correlation functions and in situ measurements are associated with the array geometry and its relative orientation to the predominant energy flux the stability of correlation functions suggests their applicability in future monitoring efforts

a 600 000 year record of antarctic bottom water activity inferred from sediment textures and structures in a sediment core from the southern brazil basin at the northern exit of the vema channel in the southern brazil basin deep currents linked with antarctic bottom water flow aabw below 4200 m have formed contouritic accumulations along the continental rise lithologic and textural investigations were carried out on a kullenberg core in order to establish a chronology of late pleistocene holocene fluctuations in aabw flow the core spanning the last 600 000 years was recovered from a field of sediment waves the deposits consist of fine grained muds carbonate contents are very low because deposition takes place near the present day carbonate compensation depth the core stratigraphic framework is based on calcareous nannofossil and excess 230th analyses two main types of facies can be identified 1 yellowish brown muds with frequent manganese enrichments forming dark laminae and 2 homogeneous gray green muds two indicators of paleocurrent activity have been considered 1 erosional sediment features that give evidence for high amplitude and short term current events and 2 grain size fluctuations percentage of panicles greater than 10 m indicating low amplitude and long term variations two periods can be defined the first one circa 600 to circa 350 kyr b p is characterized by an instability in current activity with strong flow events recorded as erosional surfaces long term fluctuations reveal the existence of several episodes of increased velocity occurring approximately every 50 kyr the second period 350 kyr b p to present is marked by globally weaker current activity and long term fluctuations of lower amplitude and longer duration maximum velocities occur preferentially during periods of climatic cooling these fluctuations might be correlated with the 100 kyr eccentricity cycle of the earth s orbit

control of north atlantic deep water circulation by the greenland scotland ridge coherent bathymetric features along the reykjanes ridge indicate that there were significant changes in the flux of buoyant material within the icelandic hot spot during the neogene the radial extent of the topographic swell associated with this hot spot is of the order of 1000 to 2000 km and therefore these changes affected the greenland scotland ridge gsr at present sill depths along the gsr are generally less than 500 m with the deepest passage being less than 1000 m making the overflow water sensitive to even small changes in the ridge depths reconstructions of neogene mantle plume activity correlate with the deepwater circulation patterns in the north atlantic times of high mantle plume activity caused northern component water ncw production to cease ncw fluxes resumed once this phase of high mantle plume activity slowed the long term climate change during the neogene must be controlled by factors other than ncw production however climatic optima during the late early miocene and early pliocene appear to have been augmented by high ncw fluxes while subsequent middle miocene and middle pliocene coolings correlate with uplifts on the gsr and reduced ncw flux we suggest that reductions in ncw may have contributed to both of these cooling events

sediment drifts and contourites on the continental margin off northwest britain abstract seismic reflection profiles and short cores from the continental margin off northwest britain have revealed a variety of sediment drift styles and contourite deposits preserved in the northeast rockall trough and faeroe shetland channel the sediment drifts include 1 distinctly mounded elongate drifts both single and multi crested 2 broad sheeted drift forms varying from gently domed to flat lying and 3 isolated patch drifts including moat related drifts fields of sediment waves are locally developed in association with the elongate and gently domed broad sheeted drifts the contrasting styles of the sediment drifts most probably reflect the interaction between a variable bottom current regime and the complex bathymetry of the continental margin the bulk of the mounded gently domed drifts occur in the northeast rockall trough whereas the flat lying sheet form deposits occur in the faeroe shetland channel a much narrower basin which appears to have been an area more of sediment export than drift accumulation patch drifts are present in both basins in the northeast rockall trough the along strike variation from single to multi crested elongate drifts may be a response to bottom current changes influenced by developing drift topography muddy silty muddy and sandy contourites have been recovered in sediment cores from the uppermost parts of the drift sequences on the basis of their glaciomarine origin these mid to high latitude contourites can be referred to collectively as glacigenic contourites both partial and complete contourite sequences are preserved the former consist largely of sandy mid only and top only contourites sandy contourites by their coarse grained nature and their formation under strongest bottom current flows are the most likely to be preserved in the rock record however the very large scale of sediment drifts should be borne in mind with regard to the recognition of fossil contourites in ancient successions

empirical synthesis of time asymmetrical green functions from the correlation of coda waves we demonstrate the existence of long range field correlations in the seismic coda of regional records in alaska the cross correlations between the different components of coda records at two points are measured for a set of distant earthquakes remarkably while individual correlations have a random character the correlations averaged over source and time exhibit deterministic arrivals that obey the same symmetry rules as the green tensor between the two points in addition the arrival times of these waves coincide with propagating surface waves between the two stations thus we propose to identify the averaged correlation signals with the surface wave part of the green tensor we observe the causal and anticausal parts of the green function however we find experimentally that amplitudes at positive and negative times are not equal we explain this observation by the long lasting anisotropy of the diffuse field we show that the flux of energy coming from the source can still dominate the late coda and result in nonsymmetric cross correlations when the distribution of earthquakes is not isotropic around the stations the extraction of green functions from coda waves allows new types of measurements with seismic waves along paths between stations that could not be obtained with the waves produced by earthquakes

site effect evaluation using spectral ratios with only one station the spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity the purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station the technique originally proposed by nakamura to analyze rayleigh waves in the microtremor records is presented briefly and it is discussed why it may be applicable to study the intense s wave part in earthquake records results are presented for three different cities in mexico oaxaca oax acapulco gro and mexico city these cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city each time we compare the results of nakamura s technique with standard spectral ratios in all three cases the results are very encouraging we conclude that if site effects are caused by simple geology a first estimate of dominant period and local amplification level can be obtained using records of only one station

bottom current pathways in the argentine basin revealed by mean silt particle size regional circulation patterns in the deep sea are very difficult to determine because of time constraints on the acquisition of large physical oceanographic data sets from abyssal depths in addition the large number of long term 1 yr current meter arrays needed to delineate bottom current pathways are extremely costly an alternative method for inferring benthic flow patterns described here is to map the mean size of the silt fraction of sediment on the sea floor because of the availability of a large number of core samples in the argentine basin the particle size approach provides a useful method for determining bottom current pathways in that deep basin

basin forming processes and the deep structure of the campos basin offshore brazil abstract this paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins the events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors shear zones thin layers of intrusive rocks underplated material gneissic banding fluids and heterogeneities in the crust and mantle several types of sedimentary basins have been recognized with the aid of deep seismic reflection data basins formed along active master faults basins formed along inactive faults basins with no fault control basins formed by low angle detachment faults and basins formed by pervasive pure shear or an approximately pure shear where the lower crust has been locally extended by a different amount than the upper crust the mechanisms of basin forming processes recognized in deep seismic profiles are a simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle b thinning of the lower crust and mantle by ductile processes convective mechanisms or flow in the mantle and the upper crust by faulting resulting in an overall pure shear and c thinning of the lower crust associated with phase changes in the mantle or non conservation of mass two end member models of basin formation involving lithospheric stretching may be recognized in the south atlantic simple shear stretching where only the upper crust is involved in the process and mantle uplift is offset relative to the basin and pure shear stretching where uplift of the mantle vertically balances the basin fill the first mechanism has been suggested for the onshore tucano basin the data obtained for the offshore campos basin suggests regional lithospheric stretching and crustal thinning with moho uplift compensating for sediment accumulation in the basin depocentre complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the campos basin

shelf fed turbidite system model and its application to the oligocene deposits of the campos basin brazil despite the large number of models involving the genesis and sedimentary facies of deep water sandstones none of these models adequately explain the origin and evolution of the extremely clean widespread over 6000 km2 predominantly massive thick over 150 m blanket like sandstones deposited in the deep water environment of the campos basin during the oligocene consequently to explain this sandstone i propose a shelf fed turbidite system model which is strongly based on the campos basin data set the basic framework necessary for the development of a shelf fed turbidite system includes 1 deposition of a large volume of clastics during the buildup of a shelf sand rich unit which later constitutes the main source of sediment for the system 2 localized tectonic pulses that modify the outer shelf declivity and trigger mass flows and 3 a relative fall of sea level which causes the subaqueous exposure of the shelf sediments to reworking in a shallow high energy marine environment these three basic elements are equally important for shelf fed turbidite system development but relative sea level position controls the development of the progradational aggradational and retrogradational depositional phases within the system submarine canyons commonly are scoured during al three phases on the outer shelf and lower slope environments the shelf fed turbidite system model may apply to other sedimentary basins principally to those of the atlantic continental margins that have a thick evaporite sublayer halokinesis can provide the necessary room for the shelf sedimentary unit buildup the tectonic pulses that trigger the flows and even localized relative sea level oscillations that can accelerate or abort any one of the depositional phases of the system

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

timing and climatic consequences of the opening of drake passage age estimates for the opening of drake passage range from 49 to 17 million years ago ma complicating interpretations of the relationship between ocean circulation and global cooling secular variations of neodymium isotope ratios at agulhas ridge southern ocean atlantic sector suggest an influx of shallow pacific seawater approximately 41 ma the timing of this connection and the subsequent deepening of the passage coincide with increased biological productivity and abrupt climate reversals circulation productivity linkages are proposed as a mechanism for declining atmospheric carbon dioxide these results also indicate that drake passage opened before the tasmanian gateway implying the late eocene establishment of a complete circum antarctic pathway

late quaternary variability of the sedimentary record in the sao tome deep sea system south brazilian basin abstract very few sediment records allow addressing the paleoceanographic changes that took place on orbital and or sub milankovitch time scales in the south west atlantic here a set of sediment cores located across a deep channel levee system in the south brazilian basin 3700 3900 m depth is used to infer the sediment dynamics that prevailed on this system during the late quaternary and to relate this pattern to climate changes the spatial distribution of deposits changes in mineralogy texture structure and sedimentation rate according to the morphology indicates bulk of sediment supplies is distributed on the sao tome system by bottom advective processes the silty sandy layers originate undoubtedly from turbidity currents whereas the fine grained muds and oozes that form the major part of the deposits are controlled by deep geostrophic currents in addition to variations forced by the orbital cycles more turbidites less carbonates and higher sedimentation rate during glacials the sediment records chiefly the caco 3 record document outstanding higher frequency regional changes in sedimentation according to the location of the cores at the present day interface between nadw and aabw we infer that changes in sedimentation are related to pulses of aabw versus nadw production these changes commonly marked by asymmetric carbonate pattern of about 10 kyr suggest that the relative influence of nadw versus aabw followed throughout the last 190 kyr a repetitive sequence of progressive increase and sharp decrease the rather sharp shut down observed during the late glacial could be associated to heinrich events when nadw production stopped but similar events are also well expressed during mis 5 mis 6 and at the end of mis 7

testing equipartition for s wave coda using borehole records of local earthquakes abstract equipartition is a state in which energy of all possible wave modes becomes equal due to multiple scattering among the modes this is important to identify the scattering regime and constitutes the basic principle underlying seismic interferometry in this study we calculate partitioning of total kinetic energy into the horizontal and vertical components similar to the horizontal to vertical ratio for s wave coda of 60 local earthquake records at three borehole stations in japan s wave coda in lapse times of 40 80 s and in the 2 16 hz frequency band is used at the surface receivers of two rock sites the horizontal components are dominant and their contribution gradually increases with frequency at a soft site the energy partitioning shows stronger variations with frequency reflecting the presence of low velocity layers subsurface receivers at about 100 m depths show larger contribution of the horizontal components irrespective of site conditions we quantitatively explain the observations with random wave fields at equipartition in horizontally layered structures derived from well logging through the modeling we test three formulations of equipartition a mixture of 1 only body waves 2 only surface waves and 3 both body and surface waves subsurface receivers are critically important to distinguish among the three formulations for s wave coda at frequencies lower than about 5 hz equipartition holds among both body and surface waves for higher frequencies equipartition holds predominantly among body waves equipartition of coda waves enables a simple modeling of the horizontal to vertical ratio which contrasts with ambient noise