terrigenous sediment supply in the scotia sea southern ocean response to late quaternary ice dynamics in patagonia and on the antarctic peninsula geochemical and mineralogical compositions of modern and late quaternary marine sediments from the scotia sea trace sources and transport paths of terrigenous sediment we discuss downcore variations of compositional data of two sediment cores from the northern and southern scotia sea that correlate with fluctuations in magnetic susceptibility sediments were derived from very different sources at both localities as revealed by contrasting clay mineral assemblages however a common feature is the input of more basic and undifferentiated crustal material with the potential of high magnetic susceptibility during glacial periods indicated by variable quartz feldspar ratios and major trace and rare earth elements terrigenous sediments mainly originate from nearby terrestrial sources or are introduced through interbasinal sediment transfer from adjacent seas the observed temporal compositional variations have to be attributed to changes in the relative detrital contributions from the diverse source areas ice mass extensions in southern patagonia on the antarctic peninsula and adjacent islands likely control the supply of glaciogenic detritus to the open ocean during times of glacial expansion diluting the sediment input of interbasinal origin current transport is mainly responsible for sediment dispersal to the pelagic scotia sea and may amplify the glaciological source signals during glacial climate periods because of a stronger wind forcing of the antarctic circumpolar current

facies distribution and textural variation in faro drift contourites velocity fluctuation and drift growth abstract detailed study of seismic profiles piston cores and bottom photographs from the faro drift on the southern margin of portugal has led to a better understanding of drift development and its relationship to bottom current circulation data on the contourite facies characteristics and the surface microphysiography have been published elsewhere here we concentrate on sediment distribution and geometry longitudinal trends in facies types mean grain size sedimentary structures and composition can be interpreted in terms of relative intensity of currents over different parts of the drift these are generally more intense in the marginal channels and at the upstream or eastern end of the drift three different scales of vertical variation of facies can be identified at the large scale 300 500 m of sediment has accumulated over 4 6 ma in a regular vertical succession due to the northward progradation of the drift at the medium scale the upper 20 30 m of sediment shows alternating phases of active lateral progradation and uniform vertical accumulation that may correlate with episodes of more and less current activity related to high and low sea level stands respectively over the past 0 3 ma at the small scale the topmost 2 3 m of sediment deposited in approximately 0 03 ma shows three zones of coarser grained sediments separated by finer grained contourites this sequence can also be interpreted in terms of long term fluctuation in bottom current activity although the signal is clearly complex this kind of analysis of sedimentary drifts can lead to more accurate reconstruction of paleocirculation patterns

quaternary history of the antarctic circumpolar current evidence from the scotia sea within the scotia sea the axis of the antarctic circumpolar current acc is geographically confined and sediments therefore contain a record of palaeo flow speed uncomplicated by acc axis migration we outline holocene and last glacial maximum lgm current controlled sedimentation using data from 3 5 khz profiles cores and current meter moorings geophysical surveys show areas of erosion and deposition controlled by neogene basement topography deposition occurs in mounded sediment drifts or flatter areas where 500 1000 m of sediment overlies acoustic basement 3 5 khz profiles show parallel continuous sub bottom reflectors with highest sedimentation rates in the centre of the drifts and reflectors converging towards marginal zones of non deposition locally on the flanks of continental blocks e g south georgia downslope processes are dominant the absence of mudwaves on the sediment drifts may result from the unsteadiness of acc flow a core transect from the acc axis south to the boundary with the weddell gyre shows a southward decrease in biogenic content controlled by the polar front and the spring sea ice edge both these features lay farther north at lgm the cores have been dated by relative abundance of the radiolarian cycladophora davisiana and by changes in the biogenic ba content a palaeoproductivity indicator sedimentation rates range from 3 to 17 cm ka the grain size of holocene sediments shows a coarsening trend from south to north consistent with strongest bottom current flow near the acc axis though interpretation is complicated by the presence of biogenic grains year long current meter records indicate mean speeds from 7 cm s in the south to 12 cm s in the north with benthic storm frequency increasing northwards lgm sediments are predominantly terrigenous and show a clearer northward coarsening trend with well sorted silts in the northern scotia sea assuming a constant terrigenous source this implies stronger acc flow at the lgm contrasting with weaker weddell gyre flow deduced from earlier work

stable isotope stratigraphy from the antarctic continental margin during the last one million years abstract a stable isotope record from the eastern weddell sea from 69 s is presented for the first time a 250 000 yr record from the southern ocean can be correlated in detail to the global isotope stratigraphy together with magnetostratigraphic sedimentological and micropalaeontological data the stratigraphic control of this record can be extended back to 910 000 yrs b p a time scale is constructed by linear interpolation between confirmed stratigraphic data points the benthic 18o record epistominella exigua reflects global continental ice volume changes during the brunhes and late matuyama chrons whereas the planktonic isotopic record neogloboquadrina pachyderma may be influenced by a meltwater lid caused by the nearby antarctic ice shelf and icebergs the worldwide climatic improvement during deglaciations is documented in the eastern weddell sea by an increase in production of siliceous plankton followed with a time lag of approximately 10 000 yrs by planktonic foraminifera production peak values in the difference between planktonic and benthic 13c records which are 0 5 higher during warm climatic periods than during times with expanded continental ice sheets also suggest increased surface productivity during interglacials in the southern ocean

petrology of glacial sand implications for the origin and mechanical durability of lithic fragments the petrology of first cycle unmodified and second cycle reworked sand at the termini of eleven valley glaciers eroding coarse to fine grained bedrock types is determined in order to evaluate the origin and mechanical durability of lithic sands first cycle sands are coarse to medium grained poorly sorted fine skewed non modal lithic sands with an average composition of q21f6l73 grain size distributions do not vary with composition or source rock types although sands derived from finer grained source rocks contain more lithic fragments than sands from coarser grained sources by contrast second cycle sands are medium grained poorly sorted fine to coarse skewed arkosic to lithic sands with an average composition of q19f40l41 and contain fewer lithic fragments than do first cycle sands we propose that the origin mechanical durability and survival potential of lithic fragments are related to the types and abundances of their internal planes of weakness and the particular stress field of the transporting medium as a result of abrasion glacial clasts and lithic fragments are subjected to shear stress so that fractures propagate along intracrystal and intercrystal planes of weakness resulting in a continuous grain size spectrum of lithic fragments and monomineralic grains irrespective of the type of source rock upon subsequent aqueous transport in the proglacial environment lithic fragments are subjected to point loading during saltation in addition to shear stress point loading produces extensional fractures which preferentially propagate along intercrystal planes of weakness so the lithic fragments break into smaller monomineralic grains and or lithic fragments lithic fragments of very coarse sand size are abundant in first cycle sands thus refuting at least for glacial sands the widely held view that grains of this size are deficient in nature the presence of fewer grains of this size in second cycle sands indicates that very coarse sand size lithic fragments can survive recycling but in greatly reduced proportions due to breakage

dinoflagellate cysts from sediment traps deployed in the bellingshausen weddell and scotia seas antarctica dinoflagellate cysts have been recovered from six long term 1 2 yr sediment trap moorings deployed in the bellingshausen weddell and scotia seas antarctica these traps mostly moored near the sea bed to sample the nepheloid layer were located both within and to the north of the maximum sea ice limit the numbers of cysts together with the composition of the assemblages reinforce the importance of the maximum sea ice limit as a modern biogeographic boundary for the distribution of dinoflagellate cysts cysts derived from heterotrophic dinoflagellates make up the highest proportions within the assemblages recovered from the traps one trap sampled the export production revealing little difference in cyst flux over those sampling the nepheloid layer cyst flux appears to be highest in areas closest to the antarctic convergence north of the maximum sea ice limit and to high nutrient availability there are however differences between the sediment trap assemblages and those recovered from core top samples at the same or nearby sites these differences in the greater number of cysts and in the higher numbers of round brown protoperidinium cysts in the traps may reflect annual differences in the primary productivity and or cyst production in the area in some areas the sediment record may preserve little information about local surface water productivity because of the activity of bottom water currents for example those arising from the antarctic circumpolar current

late quaternary changes in antarctic bottom water velocity inferred from sediment grain size in the northern weddell sea abstract newly formed bottom water 0 7 c in the northern weddell sea flows e or ne at speeds up to 10 15 cm s with velocity decreasing towards the centre of the weddell gyre preliminary results from long term current meter moorings upper quaternary sediments from this area contain a fine grained terrigenous component from the nepheloid layer plus biogenic silica mostly diatoms with a small amount of ice rafted debris in cores from between 61 and 66 s and from 3300 m to 4700 m water depth the proportion of biogenic silica increases northwards corresponding to increasing seasonal extent of open water vs sea ice cover and the proportion of silt and well sorted fine sand in surface sediments increases with average current velocity downcore diatom rich and diatom poor sediments alternate on a scale of 1 3 m and intervals with more diatoms contain a higher proportion of silt to clay preliminary stratigraphy suggests the cyclicity in composition and texture is related to glacial interglacial cyclicity during warm periods indirectly correlated with isotope stages 1 3 5 etc biogenic silica production takes place during several months of each year and silt and fine sand are transported by bottom currents during glacial periods with greater sea ice cover than at present biogenic productivity was suppressed and no silica was preserved in the sediments in addition a lower proportion of terrigenous silt implies that bottom currents were weaker at sites with a present day average velocity of 10 cm s a last glacial maximum average velocity of 1 cm s or less is inferred from grain size measurements

surface lithofacies biofacies and diatom diversity patterns as models for delineation of climatic change in the southeast atlantic ocean abstract distribution of diatom species in surface sediments of the southeast atlantic ocean is regulated by present day oceanographic and hydrodynamic processes five assemblages vectors defined by factor vector analysis reflect different environments and conditions assemblage a is a high diversity flora associated with the nutrient rich relatively cold waters south of the polar front in the diatom ooze belt occurrence of this flora on maud rise and vicinity may reflect periodic occurrence of a polynya in that area assemblage b is a reworked assemblage that dominates the southern portion of the study area productivity there is low reflecting sea ice cover during most of the year the area north of the polar front is dominated by assemblage c whose characteristic species reflect the relatively warm subantarctic surface water winnowing and frustule breakage have altered assemblage d found in three isolated samples by removing relatively delicate forms leaving lag deposits of more robust species assemblage e is a low diversity stress flora reflecting unstable unpredictable environments along the polar front antarctic slope and the northern boundary of winter sea ice these sites are characterized by the sinking of cold water downcore analysis of cores lying adjacent to the polar front and the diatom ooze pelagic clay boundary show evidence for past climatic variation the low trophic level occupied by diatoms and the subsequent sensitivity of these organisms to abiotic environmental parameters such as light quality make relative diatom abundance a useful tool for monitoring fluctuations of winter sea ice and the temperature changes responsible for these fluctuations the position of the polar front has migrated at least three times within the last 0 015 m y b p within the last 0 3 m y b p warm maxima have occurred at approximately 0 0 0 015 0 125 and 0 3 m y b p

an analysis of the optical features of the near bottom and bottom nepheloid layers in the area of the scotian rise profiles of light transmission versus depth have been studied in the region of the scotian rise of the north atlantic at bottom depths between 4900 and 5000 m a component model has been developed and consists of three components of transmission which can be combined to duplicate accurately any given transmission profile two of the components are shown to be representative of several basic phenomena the particle concentration within a uniform flow and the separation of a benthic nepheloid layer in the case of a uniform flow it is the relative magnitudes of settling and eddy diffusion which determine the shape of the transmission profile separation of the benthic nepheloid layer was inferred to be caused by an occasional cross slope velocity component the physical interpretation of the third component was ambiguous eulerian and lagrangian transformations of the data show that benthic storms that were detected at one location and time appeared nearly identical at a later time downstream and that benthic storms can be detected over a large distance distance and time scales obtained from these transformations show the area of the scotian rise to be one characterized by bottom storms which keep their general form over periods of at least 2 weeks and for distances traveled of at least 400 km

weddell sea abyssal sediments a record of antarctic bottom water flow abstract deep circulation in the weddell sea is a clockwise gyre with bottom water formed by mixing along the southern and western margins most antarctic bottom water originates here leaving the weddell sea to the east depths 4500 m or to the north depths 16 m ma sediments from the deep weddell basin are hemipelagic muds with ash laminae and mainly in the centre of the gyre distal turbidites derived from the southwest thin debris flows occur near seamounts the hemipelagic muds become coarser from the centre to the edge of the gyre winnowing by strong bottom currents is localised along the northern margin of the basin the absence of diatoms from the weddell basin sediments may result from dissolution as well as low productivity sediments from jane basin consist of alternating diatomaceous and barren hemipelagic muds considered to represent interglacial and glacial conditions respectively the diatomaceous sediments contain more silt suggesting that bottom water flow increases during interglacial periods

faulting process of the august 8 1993 guam earthquake a thrust event in an otherwise weakly coupled subduction zone we study a large mw 7 7 earthquake that occurred on june 8 1993 slightly offshore and under the island of guam in the southern mariana island arc from a complete study of p and sh body waves a relocation of the aftershocks and the subevents of the main shock we propose a relatively simple model of the rupture process of this event we propose that this earthquake ruptured a shallow dipping thrust fault that corresponds to the subduction interface under guam like many other earthquakes this event started with a small foreshock and was followed by two large energy release events located to the northeast of the epicenter along the subduction zone the rupture process had a relatively short duration of about 32 s with a weak starting phase that lasted about 8 s seismic moments estimated from body waves surface waves and global positioning system gps are very similar of the order of 4 5 1020 n m the displacement field produced by our best model was compared to the gps measurements of coseismic slip obtained by beavan et al 1994 we find an excellent agreement both in displacement direction and magnitude between the predicted and observed gps displacements this appears to be then the largest earthquake to have occurred on a shallow dipping thrust fault in the mariana subduction zone during this century its occurrence requires a reassessment of the concept of seismic coupling in this subduction zone

the large normal faulting mariana earthquake of april 5 1990 in uncoupled subduction zone a large ms 7 5 shallow earthquake occurred beneath the mariana trench on april 5 1990 from the relocated aftershock distribution the fault area is estimated to be 70 40 km2 a tsunami observed on the japanese islands verifies that the depth of the main shock is shallow for waveform analysis we use long period surface waves and body waves recorded at global networks of gdsn iris geoscope and erios the centroid moment tensor cmt solution from surface waves indicates normal faulting on a fault whose strike is parallel to the local axis of the mariana trench with the tension axis perpendicular to it the seismic moment is 1 4 1020 nm 1027 dyn cm which gives mw 7 3 far field p and sh waves from 13 stations are used to determine the source time function since the sea around the epicentral region is about 5 km deep body waveforms are contaminated with water reverberations the inversion results in a source time function with a predominantly single event with a duration of 10 sec a seismic moment of 2 1 1020 nm and a focal mechanism given by strike 198 dip 48 slip 90 the short duration indicates a small area of the rupture the location of the main shock with respect to the aftershock area suggests that the nodal plane dipping to the west is preferred for the fault plane the local stress drop of the single subevent is estimated to be 150 mpa 1 5 kbars the mariana earthquake is considered to have occurred in an uncoupled region in response to the gravitational pull caused by the downgoing pacific plate

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

circulation in the glacial north atlantic inferred from grain size measurements records of nutrient proxies in marine sediments indicate that the nutrient distribution and hence circulation of the glacial north atlantic ocean was markedly different from that of today1 2 but these tracers are influenced by several biogeochemical factors unrelated to ocean circulation3 6 and thus do not provide a direct measure of the vigour of circulation distributions of grain size in marine sediments can provide such information owing to the sorting effects of currents7 8 if the characteristics of the input sediment flux are known here we present a record inferred from grain size measurements of variations in the relative strength of deep and intermediate currents in the eastern north atlantic over the past 25 000 years we find that glacial intermediate water flowed rapidly at depths of between 1 100 and 2 000 m in contrast deep water circulation was sluggish during the last glaciation but increased in strength shortly after the glacial maximum this would have resulted in increased heat flux to high latitudes and may have triggered sudden deglaciation deep current strengths declined again at the start of termination stage ia and during the younger dryas perhaps as a result of iceberg discharges the so called heinrich events9 10 a remarkably similar record of grain size variations has been found in the western north atlantic11 indicating that these changes in circulation are ocean wide

quaternary history of calcium carbonate fluctuations in the western equatorial indian ocean somali basin abstract a study of the quaternary history of calcium carbonate fluctuations in the equatorial indian ocean based on the swedish deep sea expedition core 154 demonstrates that the calcite compensation depth ccd was shallower during interglacial periods than during glacials higher percentages of calcium carbonate lower percentages of dissolution resistant planktonic foraminifera and lower values of the berger dissolution index suggesting better calcium carbonate preservation are recorded during glacial periods and or at glacial interglacial transitions in contrast interglacial periods are marked by stronger calcium carbonate dissolution the data indicate that dissolution is the main controlling factor of calcium carbonate in the western equatorial indian ocean higher dissolution during interglacial periods and enriched 13 c are due to higher noncarbonate productivity and enhanced antarctic bottom water aabw ventilation in the equatorial indian ocean

chronology and correlation of quaternary magnetostratigraphy and nannofossil biostratigraphy in norwegian greenland sea sediments paleomagnetic analyses of a series of sediment cores from the norwegian greenland sea document the existence of seven short lived geomagnetic events during the last about 500 000 years calcareous nannofossil biostratigraphy correlated to oxygen isotope chronostratigraphy provides a time framework to infer age and duration of these periods of extreme divergence of the earth s magnetic paleofield from the regular normal polarity configuration of the brunhes chron the geomagnetic events typically extend over 5 to 10 kyr two events are observed within the oxygen isotope stage 2 4 interval a short event is recorded at the stage 4 5 boundary and another just below the stage 5 6 boundary additional three geomagnetic events are identified in oxygen isotope stage 6 at the stage 9 10 boundary and near the top of oxygen isotope stage 13 respectively

late tertiary quaternary magnetostratigraphy and biostratigraphy of vema channel sediments abstract twenty one piston cores in a transect of the vema channel near 31 s were analyzed magnetostratigraphically and biostratigraphically in order to ascertain the depositional continuity in the late tertiary and quaternary the cores span a depth interval of 2941 4235 m and represent three distinct physiographic provinces each of which shows a relatively consistent accumulation pattern the deepest cores are from the broad terraces which flank the vema channel axis on both the west 3650 mm and east 4150 4250 m these cores show the highest accumulation rates in the region 20 m m y 2 and no evidence of unconformities twelve cores from the flanks of the rio grande rise 2941 3950 m show depositional continuity in the late quaternary and normal pelagic accumulation rates 5 10 m m y 2 with evidence in two of the cores for minor erosional hiatuses on a local scale perhaps due to slumping a relatively narrow transition zone between the rio grande rise flanks and the eastern terrace 3950 4150 m shows buried manganese pavements and near outcrops of miocene pliocene strata indicating intermittent erosion during the late tertiary and quaternary perhaps due to episodes of intensified aabw flow modern flow within this transition zone and over the eastern terrace is relatively weak gu 2 cm s 2 and southward the presence of well developed sediment waves on the terrace and buried erosional unconformities upslope to the east suggests substantially higher current velocities during the late tertiary than those present today

sedimentology and stratigraphy of box cores from the hebble site on the nova scotian continental rise abstract fifty box cores were obtained from the 2 4 km site of the high energy benthic boundary layer experiment hebble on the lower nova scotian continental rise at 4820 m the oldest unit occurring in two gravity cores is a pre holocene dark grey gravelly mud probably of debris flow origin above this is a soft grey mud overlain by brown sandy mud whose sand is largely foraminiferal with some terrigenous material from interbedded turbidites in some places this unit has a stiff brown gravelly mud at its base which is always overlain by sandy mud containing dispersed gravel it is suggested that the dispersed gravel originated from the underlying gravelly mud and has been dispersed upwards by the action of burrowing organisms foraminifera indicate a holocene age down to about 65 cm and thus a net accumulation rate of about 6 cm ka 1 the surface unit of brown mud named the hebble mud is spatially variable in both thickness and sand content which average about 4 cm and 6 respectively sand percentage is not related to silt size or percentage but varies inversely with deposition rate and thickness suggesting a relation to local winnowing the size and peak height of the main silt mode 10 17 m are closely correlated and the peak height is inversely related to the percentage of clay this is attributed to fractionation during deposition of the mud wherein fine silt and clay deposition is suppressed at flow speeds from 10 to 18 cm s 1 leaving coarser silt there is more clay on the upstream and downstream sides of a low spur suggesting spatial variation of shear stress that may be due to a standing internal wave added to the mean flow silt mode size is variable downcore although clay percentages are similar to those at the surface the overall silt size is probably controlled by turbidite input thus silt size is likely to be an unreliable indicator of current speeds down core though peak height of the silt mode may be satisfactory

origin of detrital fluxes in the southeast indian ocean during the last climatic cycles because of a close relationship between detrital flux variations and magnetic susceptibility ms flux ms cm 3 of bulk sediment multiplied by the linear sedimentation rate variations in the southeast indian basin of the southern ocean ms flux profiles have been used to examine the spatial and temporal detrital flux changes in this basin during the last climatic cycle results indicate a general increase in detrital material input during the coldest periods suggesting a widespread phenomenon at least on the basin scale mineralogical data geochemical data and 87sr 86sr isotopic ratios have been used to determine the origin and transport mechanisms responsible for increased detrital flux during glacial periods mineralogical and geochemical data show that these glacial highs are due to increases in both kerguelen crozet volcanic and antarctic detrital inputs the 87sr 86sr isotopic composition of the 45 m fraction indicates that the kerguelen crozet province contributes to at least 50 of the coarse particule input to the west this contribution decreases eastward to reach less than 10 these tracers clearly indicate that the crozet kerguelen province was a major source region of detrital in the western part of the basin during glacial times in contrast material of antarctic origin is well represented in the whole basin fine and coarse fractions because of the minor amount of coarse particles in the sediments volcanic particles from kerguelen and crustal particles from antarctica have most probably been transported by the antarctic bottom water current and or the circumpolar deepwater current during glacial periods as is the case today nevertheless the presence of coarse particles even in low amount suggests also a transport by ice rafting sea ice and icebergs originated from both kerguelen and antarctic sources however the relative importance of both hydrographic and ice rafting modes of transport cannot be identified accurately with our data during low sea level stands glacial maximum periods increasing instability and erosion of the continental platform and shallow plateaus could have resulted in a more efficient transfer of crustal and volcano detrital material to the southeast indian basin at the same time extension of the grounded ice shelves over the continental margins and increase in the erosion rate of the antarctic ice sheet could have induced a greater input of ice rafted detritus ird to southern ocean basins enhancement of the circumpolar deepwater current strength might have also carried a more important flux of detrital material from kerguelen however an increase in the bottom water flow is not necessarily required

origin signature and palaeoclimatic influence of the antarctic circumpolar current the antarctic circumpolar current acc is today the strongest current in the world s ocean with a significant influence on global climate its assumed history and influence on palaeoclimate while almost certainly equally profound are here called into question in this paper we review 30 years of accumulated data interpretation and speculation about the acc deriving mainly from dsdp and odp drilling in the southern ocean for most of this time a conventional view of acc development signature and influence has held sway among palaeoceanographers and marine geologists in this view the acc began at about 34 ma close to the eocene oligocene boundary the time of onset of significant antarctic glaciation and the time of creation of a deep water gap tasmanian seaway between australia and antarctica as the south tasman rise separated from north victoria land this is the smoking gun of synchroneity the southern ocean sediment record shows a latest eocene development and subsequent geographic expansion of a siliceous biofacies its northern limit taken to indicate the palaeoposition of the acc axis in addition the acc was considered to have caused antarctic glaciation by isolating the continent within a cold water annulus reducing north south heat transport a different and later date for antarctic south american opening drake passage was proposed but the timing of acc onset there was disputed and the simple story survived recent developments however call it into question modern physical oceanography shows that all or most of present day acc transport is confined to narrow jets within deep reaching circumpolar fronts and numerical modelling has suggested that a steady reduction in greenhouse gas concentration through the cenozoic could cause antarctic glaciation with or without a contribution from ocean circulation change the rapidity of antarctic glacial onset at the eocene oligocene boundary and coeval creation of a deep water gap south of tasmania both survive but in light of the new information the presence of a siliceous biofacies cannot be claimed as evidence of the existence of a continuous deep reaching oceanic front and therefore of an acc and the possibility arises that cool and cold sea surface temperatures were effects of antarctic glaciation rather than evidence of a major contributor to its cause in considering future work we emphasise the importance of additional information from ancillary fields better definition of the necessary and sufficient properties of oceanic fronts additional determinations of cenozoic atmospheric pco2 and further developments in models of antarctic glaciation but also suggest the way forward in marine geology our knowledge of the development and palaeoclimatic significance of the acc will be best served by grainsize studies of bottom current strength at selected locations and geochemical or mineralogical studies of clays and ird as a way of examining provenance and therefore surface and bottom current directions and the existence of interocean connections

faulting within the pacific plate at the mariana trench implications for plate interface coupling and subduction of hydrous minerals we investigate faulting within the incoming pacific plate at the mariana subduction trench to understand stresses within the bending plate regional stresses acting upon the plate interface and the extent of possible faulting induced mantle serpentinization we determine accurate depths by inverting teleseismic p and sh waveforms for earthquakes occurring during 1990 2011 with global centroid moment tensor gcmt solutions for earthquakes with mw 5 0 we determine centroid depths and source time functions and refine the fault parameters results from central mariana indicate that all earthquakes are extensional and occur at centroid depths down to 11 km below the moho at the southern mariana trench extensional earthquakes continue to 5 km below the moho one compressional earthquake at 34 km below the seafloor suggests stronger plate interface coupling here in addition we model the stress distribution within the pacific plate along two bathymetric profiles extending seaward from the mariana subduction trench axis to better understand whether our earthquake depth solutions match modeled scenarios for plate bending under applied external forces results from our flexure models match the locations of extensional and compressional earthquakes and suggest that the pacific plate at southern mariana is experiencing larger compressional stresses possibly due to greater interplate coupling additionally we conclude that if extensional faulting promotes the infiltration of water into the subducting plate mantle then the top 5 15 km of the pacific plate mantle are partially serpentinized and a higher percentage of serpentinization is located near the central mariana trench where extensional events extend deeper

a sediment record of cyclic bottom current variability from the northwest weddell sea cores and 3 5 khz sub bottom profiles collected from the continental slope and rise near 64 s 52 w off the tip of the antarctic peninsula reveal transport and depositional processes in a high energy environment a large channel runs downslope from the shelf break the remaining slope area is smooth and shows no acoustic penetration except locally in the lee of topography sediments on the slope and rise were mainly deposited by suspension from a 150 to 300 m thick nepheloid layer periodic resuspension and winnowing by bottom currents has produced coarse lags of ice rafted debris throughout these deposits and has also produced an elongated field of mudwaves running parallel to the base of the slope these low amplitude asymmetric features were formed by currents flowing from west to east parallel to the local slope short gravity cores show a compositional and textural cyclicity developed between two distinct lithologies intervals of poorly sorted sediment barren of fauna and with mud dominated size distributions are interpreted to result from a cold climate and low bottom current strength production of weddell sea bottom water probably lessened during these times and there was a greater duration of sea ice cover these sediments are interbedded with winnowed and better sorted deposits comprising fine sands and silts that contain abundant planktonic foraminifera interpreted as implying warmer climatic conditions and stronger current activity a varied calcareous benthic foraminiferal assemblage also indicates that these intervals lay above the ccd during deposition renewed and invigorated production of bottom water is implied contemporaneous with a shorter duration of sea ice cover petrographic evidence indicates an antarctic peninsula source throughout the cycles are considered to be the result of climatic fluctuations occurring over the timespan of glacial interstadials although confirmation of this timescale awaits acquisition of an absolute chronology

bottom current speed in the vema channel recorded by particle size of sediment fine fraction abstract the particle size distribution of the carbonate free silt fraction was determined in forty two core top samples from the east flank of the vema channel in order to allow comparison of the mean particle size with near bottom current speed based on nearby ctd and current meter observations the silt mean particle size fluctuates insignificantly between 1450 3950 m but coarsens markedly below 4000 m under antarctic source bottom currents the top of the zone of coarse sizes approximates the top of lower circumpolar water lcpw at 4000 m the steep gradient to coarsest sizes in the axis of the channel marks the transition to antarctic bottom water aabw the vertical profile of mean particle size on the east flank correlates in general respects with the vertical velocity gradient in the channel axis although the core sites on the east flank are as far as 200 miles from the current meter arrays nevertheless this relationship is used to infer that the non carbonate particle sizes deposited on the east flank of the channel may be an indication of bottom current speed and therefore the mean size is used to reconstruct profiles of mean current speed for three time slices 18 120 and 140 ka b p during glacial isotopic stages 2 and 6 the particle sizes deposited on the east flank of the channel are characterized by finer sizes at nearly all depths and the zone of coarse sizes corresponding to lcpw and aabw deepens during interglacial isotopic stage 5e the particlesize profile is very similar to the modern profile however lcpw and aabw appear to be deeper than at present the finer particle sizes deposited at nearly all depths during glacial periods indicates that the velocity of deep circulation in the atlantic was reduced in response to the ice ages deep circulation during isotopic stage 5e closely resembled modern circulation however aabw production rates may have been less that at present

late quaternary deep stratification climate coupling in the southern ocean implications for changes in abyssal carbon storage the southern ocean plays an important role in modulating pleistocene atmospheric co2 concentrations but the underlying mechanisms are not yet fully understood here we report the laser grain size distribution and mn geochemical data of a 523 kyr long sediment record core ant30 p1 02 off prydz bay east antarctica to trace past physical changes in the deep southern ocean the core sediments are predominantly composed of clay and silt sized material three grain size end members em as well as three sensitive grain size classes sc were discerned interpreted as ice rafted debris em1 and sc1 and coarse em2 and sc2 and fine em3 sc3 materials deposited from bottom nepheloid layers respectively ratios of em2 em2 em3 and sc2 sc3 reveal changes in the local bottom current strength which is related to the deep ocean diapycnal mixing rate showing higher values during interglacial periods and lower values during glacial periods mno was enriched at each glacial termination probably caused by abrupt elevations in antarctic bottom water aabw formation rate lower aabw formation rate and reduced deep diapycnal mixing during glacial periods enhanced deep southern ocean stratification contributing to glacial atmospheric co2 drawdown the elevated aabw formation and enhanced deep diapycnal mixing during glacial terminations alleviated such deep stratification promoting deeply sequestered co2 to outgas

calibration of a point counting technique for estimation of biogenic silica in marine sediments the simple technique of point counting is very quick and is suitable for shipboard use or in any study where high accuracy is not necessary point counting smear slides yields results different from measurements of weight percent silica if there are systematic differences in shape and density of biogenic and terrigenous grains in this note i present calibration curves for synthetic biogenic terrigenous mixtures to show how the accuracy of point counting may be improved

seismogenic characteristics of the northern mariana shallow thrust zone from local array data 1 the northern mariana seismogenic zone has no shallow thrust earthquakes larger than ms 7 4 in the historical seismological record and is traditionally considered decoupled or aseismic during the 2003 2004 mariana subduction factory imaging experiment we recorded local shallow earthquakes throughout the central and northern regions of the mariana forearc using an array of terrestrial broadband and ocean bottom seismographs accurate locations for both the 2003 2004 local seismicity as well as earthquakes with global centroid moment tensor gcmt solutions from 1976 to 2008 were obtained using the hypocentroidal decomposition relocation method and a local velocity model additionally focal mechanisms for the largest 2003 2004 earthquakes were determined using regional waveform inversion thrust faulting earthquakes occur along the mariana megathrust between depths of 20 60 km showing that the lack of great shallow thrust earthquakes does not result from a narrow seismogenic zone and that most seismicity occurs where the downgoing plate contacts the overriding mantle wedge clusters of small plate interface earthquakes with ml 1 6 4 7 occur within patches 100 120 km west of the trench at depths of 30 45 km furthermore the larger gcmt earthquakes mw 4 9 5 8 occur mostly updip and downdip of the patches of smaller earthquakes recorded by our local array and is suggestive of changes in the fault properties with depth clusters of small forearc earthquakes occur discontinuously along the length of the mariana subduction zone showing that northern mariana is variable both along the strike of the margin and with depth along the seismogenic zone we propose that the lack of great mw 8 thrust faulting earthquakes is due in part to the variable frictional heterogeneity along the megathrust

models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences abstract this paper argues that glaciomarine environments can be regarded as special glacially influenced types of continental margin environments e g continental shelf slope rise and basin plain knowledge of the stratigraphic architecture and typical sedimentary sequences of non glacial margins is becoming well known but remains limited for those that have been glacially modified the principal influences on sedimentation in these environments relate to the glacial sediment input controlled by relief of basin margin glacier thermal regime and ice flow dynamics and depositional environments influence of traction currents substrate relief and proximity to nearby ice margins typical ranges of sedimentation rates can be established for glacially influenced continental margin environments and these may provide a framework for ancient sequences starvation of sediment supply to glacially influenced continental margins is common the nature of sub ice shelf sedimentation a model that has been applied to many ancient glacial sequences is critically reviewed the significance of such sedimentation in the rock record has probably been exaggerated because of oversimplistic interpretations of diamictite sequences existing process models of glaciomarine sedimentation derived from study of modern environments are sometimes difficult to employ in investigation of ancient sedimentary sequences because simple lithofacies criteria and typical vertical profiles are not available to aid in interpretation in addition compositional data emphasized by many workers for distinguishing glaciomarine from continental glacial diamict ite s frequently fingerprint sediment source and not mode of deposition the importance of facies analysis methods for isolating depositional environments is illustrated by three examples of ancient glaciomarine sequences these are the early proterozoic gowganda formation 2 3 ga of northern ontario canada the late proterozoic port askaig formation 670 ma of scotland and ireland and the late cenozoic yakataga formation 20 ma to recent of the gulf of alaska these examples illustrate the significance of detailed genetic studies of ancient glacial rocks in the interpretation of palaeogeographic and tectonic settings diamictite units in ancient glaciomarine sequences cannot be easily interpreted in terms of climatic or ice advance retreat cycles because of the varied controls on diamict accumulation and diamictite preservation in marine basins

sea ice winter convection and the temperature minimum layer in the southern ocean the structure of the near surface waters in the southern ocean poleward of the antarctic polar front but away from continental margins is investigated with a three dimensional time dependent numerical model which resolves the annual sea ice cycle the growth and decay of the ice field is predicted using one of semtner s 1976 thermodynamic ice models in terms of specified atmospheric data and computed thermohaline characteristics of the ocean layers the ice field is found to be sensitive to the lateral advection of heat by the oceanic circulation as well as vertical heat transports due to deep winter convection the model treats the temperature minimum layer in the southern ocean as the remnant of a deep winter mixed layer which becomes capped by surface heating and precipitation in summer the predicted thermohaline characteristics of the temperature minimum layer and the surface mixed layer are in good agreement with observations finally the annual air sea heat exchange predicted by the model is discussed the model s predicted area averaged heat loss experienced by the ocean south of the atlantic polar front is much smaller than was previously estimated this is attributed to errors in the southern region of the model domain it is suggestedmore that models which include the interactions between sea ice and surface deep and bottom waters along the continental margins are needed to investigate this region less

high resolution diatom stratigraphy of quaternary sediments from the scotia sea upper quaternary pelagic and hemipelagic sediments from the scotia sea and south scotia ridge range from diatom ooze to diatom bearing mud diatom content increases northwards and at most sites diatom rich and diatom poor sediments alternate downcore on a scale of metres a local stratigraphy is based on relative abundance of six prominent diatom taxa eucampia antarctica rhizosolenia spp thalassiosira spp chaetoceros spores nitzschia kerguelensis and othernitzschia species and one silicoflagellate species distephanus speculum these stratigraphic units defined using diatoms are correlated with radiolarian abundance stratigraphy cycladophora davisiana and with palaeomagnetic stratigraphy information from modern environments phytoplankton and sediment trap studies indicates that changes in diatom species composition are related to n s movement of the winter ice edge and of the antarctic convergence

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

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

the april 5 1990 mariana islands earthquake and subduction zone stresses abstract the april 5 1990 mariana islands earthquake m w 7 5 involved normal faulting at shallow depth in the pacific plate under the mariana trench about 200 km east of the central mariana islands the earthquake is significant for its tectonic setting focal mechanism and large size the event occurred in the vicinity of a proposed slab segment boundary and may in part owe its unusually large size to lateral variation in intraplate strain as appears to be the case for other large normal faulting events near trenches detailed analysis of the seismic source parameters of the earthquake and of the relationship between seismicity and tectonics in the mariana islands is conducted to improve our understanding of the mechanical behavior and seismotectonics in island arcs with aseismic convergence the 1990 mariana earthquake rupture initiated with very low levels of energy release for an interval of about 7 2 s followed by a large single impulsive event with a duration of about 6 s body wave inversion yields a strike 215 dip 49 rake 95 and a moment 1 2 10 20 nm with a centroid time of 11 12 s the source mechanism is further constrained by the long period rayleigh and love wave data with inversions yielding a strike 184 5 dip 67 rake 90 5 and moment 2 2 0 5 10 20 nm the long period centroid depth is 23 km with a centroid time of 20 3 s

a model for sedimentation by tidewater glaciers sampling of sediment from the fjord floor in front of tidewater glaciers in glacier bay alaska has provided information about processes in this restricted glacimarine dreimanis 1979 setting sediment sampling in conjunction with oceanographic and glacial dynamics data has also enabled the discrimination of sediment types and their facies associations deposits are strongly controlled by sea water characteristics position and sediment discharge of melt water streams iceberg calving and rate of glacierfront retreat five distinct facies associations have been found to reflect glacier fjord regimes the facies associations and ice fjord conditions responsible for them form the basis for constructing a preliminary model for glacimarine sedimentation by tidewater glaciers the model can be used to predict i rapid retreat of an actively calving ice front ii slow retreat or stabilization of a calving ice front at a channel constriction iii stabilization of a melting very rarely calving ice front when the glacier base is near tidewater elevation and iv large outwash delta progradation into a fjord when the ice front retreats onto land this model can be used to interpret facies associations found in a stratigraphic record

the margins of the south orkney microcontinent marine geophysical studies of the south orkney microcontinent show two intersecting tectonic fabrics of different age the older fabric is oriented e w and represents the magmatic arc and fore arc created by subduction related processes at the pacific margin during the mesozoic the younger oriented nnw to sse comprises half graben related to e w rifting at both the eastern and western margins of the microcontinent both fabrics provide constraints on a 40 ma reconstruction which shows a highly systematic distribution of arc and fore arc provinces and points to the importance of e w strike slip in regional tectonic evolution the rifting apparently of oligocene age is associated with back arc extension which created the oceanic basins to either side of the microcontinent this strongly supports the existence of subduction with the same orientation as the present south sandwich arc as far back as 35 or 40 ma

subducting oceanic crusts of the philippine sea and pacific plates and weak zone normal compression in the kanto district japan summary oceanic crusts subducting into the mantle were detected by analyses of mantle earthquakes in the kanto district central japan earthquakes in a depth range of 30 80 km in the southwestern ibaraki prefecture were relocated by a modified method of joint hypocentre determination we obtained a good distribution map of hypocentres and focal mechanisms a boundary layer between the eurasian and philippine sea plates was found where thrust earthquakes with p axes in the nw se direction and of which low angle nodal planes dip northward occurred in a layer with a thickness of about 4 km the p wave velocity in this layer is estimated to be less than about 7 0kms l another boundary layer between the philippine sea and pacific plates exists about 15 km beneath the above boundary layer thrust earthquakes with p axes in the ew direction and of which low angle nodal planes dip westward occurred in this layer that also has a thickness of about 4km furthermore normal faulting earthquakes of which the p axes are almost perpendicular to the boundary layers were found immediately outside the boundary layers combining these results we can conclude that the boundary layers are oceanic crusts that exist at the top of the subducting philippine sea and pacific plates weak zone normal compression was proposed to result from the existence of low velocity oceanic crusts at plate boundaries this hypothesis can be explained because only stress caused by relative motion of the two plates is consumed inside the weak oceanic crust the remnant stress after subtracting the above stress from the tectonic one applied at the plate boundary is the stress of which the p axis is normal to the plate boundary

quick aftershock relocation of the 1994 shikotan earthquake and its fault planes the 1994 shikotan earthquake of mw 8 2 occurred near the western kurile trench where the pacific plate is subducting beneath the north american plate although its mechanism is thrust type it is not a typical low angle thrust event with a nodal plane parallel to the plate boundary that occurs at the plate boundary in order to determine which of the two nodal planes of the main shock was the fault plane we relocated the main shock and all aftershocks with m 5 3 using a modified joint hypocenter determination method earthquakes on the northwestern side of the aftershock area including the main shock are located along a plane dipping east southeastward with a dip of 70 this plane is about 80 to 150 km long and 70 km wide the main shock occurred at the north northeastern bottom of this plane which is almost parallel to one of the two nodal planes of the main shock determined by harvard university which has a strike of n51 e and a dip of 76 thus we conclude that this plane is the fault plane that the fault broke at the northeastern bottom of the fault plane and that the rupture propagated to the surface and also to the southeast in addition to the aftershocks aligned with this fault plane there is a secondary alignment which is subparallel to the main fault plane and about 50 km east where the largest aftershock of mw 7 3 occurred on oct 9 since several aftershocks occurred on this secondary fault plane on the same day as the main shock it seems reasonable to suppose that this secondary fault plane was generated together with the main fault plane at the time of the main shock

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

interpretation of the 1993 2001 and 2002 guam earthquakes as intraslab events by a simultaneous relocation of the mainshocks aftershocks and background earthquakes it has been unclear whether the 1993 2001 and 2002 guam earthquakes m w 7 8 7 0 and 7 1 respectively in the southern mariana subduction zone were interplate events or intraplate ones within the pacific slab we relocated their mainshocks aftershocks and numerous long term background earthquakes simultaneously with high relative accuracy using the modified joint hypocenter determination mjhd method as a result their mainshocks and three day aftershocks are clearly distributed within the background slab seismicity therefore we conclude that the 1993 2001 and 2002 guam earthquakes were not interplate events which scarcely occur in the mariana subduction zone but intraslab earthquakes within the subducted pacific plate the 1993 fault plane is inferred to be the near horizontal nodal plane of the mechanism solution though the 2001 and 2002 fault planes remain unknown the relocation of target events simultaneously with background earthquakes has turned out very effective for discriminating between large interplate and intraslab earthquakes in subduction zones

deep sea bottom currents differentiated from texture of underlying sediment abstract forty five core top samples taken in the vema channel southwest atlantic within the deduced boundaries of two important bottom currents have been analyzed for the particle size distribution of the noncarbonate silt size fraction this was done in order to characterize the sediment beneath each water mass using statistical and graphical methods it was found that sediment beneath the high velocity antarctic bottom water in the bottom of the channel exhibits the coarsest mean size poor sorting and a large percent of coarse silt left as a lag deposit sediment beneath the overlying low velocity north atlantic deep water is marked by fine mean size good sorting no coarse silt and a large percent of fine silt sediment beneath the intervening transition zone is characterized by fine t mean size moderate sorting largest percent fine silt and a small lag component of coarse silt the methods developed for delineating bottom currents in the vema channel may be applied to a wide range of benthic oceanographic settings as well as the geologic record in order to examine relative paleovelocity fluctuations of major bottom currents

notes on southern ocean hydrography sea ice and bottom water formation abstract the southern ocean circulation and sea ice distribution is briefly described the formation of extremely cold bottom water in the weddell sea and its relation to the floating ronne filchner ice shelves is discussed it is shown that a concentrated swift eroding bottom current with anomalous low 18 o ratio transports the cold and dense ice shelf water from the shelf towards large depths comments are made on possible implications of this process for the large scale deep water circulation and for the interpretation of sediment cores

analysis of seismological and tsunami data from the 1993 guam earthquake the fault parameters of the guam earthquake of august 8 1993 are estimated from seismological analyses and the possibility of identifying the actual fault plane from tsunami waveforms is tested the centroid moment tensor solution of long period surface waves shows one nodal plane shallowly dipping to the north and the other nodal plane steeply dipping to the south the seismic moment is 3 5 1020 nm and the corresponding moment magnitude is 7 7 the moment tensor rate function inversion of p waves also yields a similar focal mechanism and seismic moment the point source depth is estimated as 40 50 km

where and why do large shallow intraslab earthquakes occur we try to find how often and in what regions large earthquakes m 7 0 occur within the shallow portion 20 60 km depth of a subducting slab searching for events in published individual studies and the harvard university centroid moment tensor catalogue we find twenty such events in e hokkaido kyushu sw japan s mariana manila sumatra vanuatu n chile c peru el salvador mexico n cascadia and alaska slab stresses revealed from the mechanism solutions of these large intraslab events and nearby smaller events are almost always down dip tensional except for e hokkaido manila and sumatra the upper plate shows horizontal stress gradient in the arc perpendicular direction we infer that shear tractions are operating at the base of the upper plate in this direction to produce the observed gradient and compression in the outer fore arc balancing the down dip tensional stress of the slab this tectonic situation in the subduction zone might be realized as part of the convection system with some conditions as shown by previous numerical simulations 2003 elsevier b v all rights reserved

post miocene disconformities and paleoceanography in the atlantic sector of the southern ocean abstract the magnetostratigraphy and biostratigraphy of 166 islas orcadas piston cores in the atlantic sector of the southern ocean reveal disconformities in the sedimentary record which are attributed to bottom current erosion the hiatus frequency as a function of water depth reveals both shallow erosion by circumpolar deep water cdw and deep erosion by antarctic bottom water aabw the post miocene history of erosion by both bottom currents is delineated by analysis of areal and temporal fluctuations of hiatus frequency erosion by cdw caused disconformities on the maud rise astrid ridge indian antarctic ridge scotia sea and islas orcadas rise the shallow disconformities increased in frequency and size from 4 0 to 1 0 ma with two periods of distinctly widespread disconformities from 5 5 to 4 5 ma and 2 5 to 1 5 ma after 1 0 ma the frequency and extent of the hiatuses decreased erosion by aabw caused disconformities in the weddell abyssal plain argentine basin and through a narrow conduit in the south sandwich basin downstream from the bullard fracture zone the deep disconformities increased in frequency and size from 4 5 to 5 0 ma 3 5 to 4 0 ma and from 1 0 to 2 5 ma after 1 5 ma the frequency and extent of hiatuses decreased fluctuations in hiatus frequency are consistent with results of a similar study in the southeast indian ocean and may be correlated with paleoceanographic events southern ocean cooling and possible re establishment of the west antarctic ice sheet and fringing ice shelves at 4 3 3 5 ma was accompanied by increased aabw and cdw activity the formation expansion of the northern hemisphere ice sheets at 2 5 3 0 ma resulted in a decrease in aabw velocity and was accompanied by increased cdw activity both deep and shallow bottom currents increased in velocity during the late matuyama chron 2 0 1 5 ma as a result of antarctic cooling after 1 0 ma however the frequency and extent of shallow and deep hiatuses was reduced and the modern extent of hiatuses is the lowest within the post miocene