carbonate dissolution and sedimentation on the mid atlantic continental margin the calcium carbonate content was determined for core tops from two transects on the upper slope to lower rise on the mid atlantic continental margin carbonate content in the sediment increases from 5 percent by weight on the upper slope to more than 30 percent on the upper rise a zone of low carbonate content extends from 3000 to 4400 meters below 4400 meters the percent carbonate increases an examination of dissolution indices in these core tops indicates that the low carbonate zone is associated with intense dissolution below 4400 meters dissolution decreases and carbonate is well preserved the decrease in dissolution occurs where the high velocity core of the western boundary undercurrent is first encountered

subducted slabs and the geoid 1 numerical experiments with temperature dependent viscosity one of the most powerful constraints on mantle viscosity comes from the correlation of the long wavelength degree 4 9 geoid with that predicted by a density model for subducted slabs the effect of lateral variations of viscosity should be most pronounced at subduction zones due to the strong effect of temperature on viscosity an idealized slab model with temperature dependent viscosity is considered with various lateral and vertical viscosity structures using a two dimensional finite element formulation the viscosity parameterization affects the amplitude of the long wavelength geoid anomaly but not the sign of the correlation between the geoid and density anomalies depth dependent viscosity models with a high viscosity lithospheric layer do not completely match the temperature dependent laterally varying results suggesting that the rheology of the slab does have an effect on the long wavelength surface topography and geoid however this affect is minor suggesting that the radial models of mantle viscosity inferred from surface observables are correct to first order in contrast the short wavelength features are dramatically affected by the rheology of the region surrounding the downwelling further study of the shorter wavelength geoid over subduction zones may provide better insight into subduction zone processes

westward propagating twin gyres in the equatorial indian ocean 1 a reduced gravity 1 layer model forced by daily climatological winds simulates twin anticyclonic gyres which propagate westward on either side of the equator the gyres form at the beginning of both the southwest monsoon and the northeast monsoon in the equatorial eastern indian ocean and subsequently propagate across the basin their existence is supported by velocity observations taken during woce in 1995 and by topex poseidon sea level observations during 1993 they are also present in the ecco model data product they form at the front of a rossby wave packet generated by the reflection of the equatorial jet ej from the eastern boundary of the basin they are likely either rossby solitons or result from the nonlinear interaction between the ej and the rossby wave front

late quaternary sedimentation in the western north atlantic stratigraphy and paleoceanography abstract a detailed analysis of stratigraphic indices was undertaken to study paleoclimatic change in the western north atlantic three stratigraphic indicators carbonate content globorotalia menardii and uvigerina peregrina were tested in ten radiocarbon dated cores from the north american continental margin of these carbonate content appears to be most accurate carbonate content exhibits two major increases from late glacial time to the present estimated ages of the average of the mid point of these increase are 13 200 b p and 8600 b p an absolute chronology was established down fifteen cores using these stratigraphic indicators and the radiocarbon dates sea surface temperature sst estimates derived from planktonic foraminifera were mapped at 2000 yr intervals from 2000 b p to 14 000 b p sst warms rapidly from 12 000 to 8000 b p reaches a maximum between 8000 and 6000 b p and gradually cools after 6000 b p bottom waters were monitored with uvigerina peregrina and dissolution indices the disappearance of uvigerina suggests that the water type it lives in low o 2 started to thin from both the top and bottom by 13 000 b p and by 7300 b p had disappeared fragmentation of planktonic foraminifera was used as an index of dissolution in most cores dissolution was intense during late glacial time lessened during early postglacial time and resumed in the late postglacial

comparisons of 210pb and pollen methods for determining rates of estuarine sediment accumulation abstract comparisons of sedimentation rates obtained by 210 pb and pollen analyses of 1 m cores collected throughout the potomac estuary show good agreement in the majority of cores that can be analyzed by both methods most of the discrepancy between the methods can be explained by the analytical precision of the 210 pb method and by the exactness with which time horizons can be identified and dated for the pollen method x radiographs of the cores and the distinctness of the pollen horizons preclude significant displacement by reworking and or mixing of sediments differences between the methods are greatest where uncertainties exist in assigning a rate by one or both methods i e 210 pb trends and or possible horizon assignments both methods show the same relative rates with greater sediment accumulation more common in the upper and middle estuary and less toward the mouth the results indicate that geochronologic studies of estuarine sediments should be preceded by careful observation of sedimentary structures preferably by x radiography to evaluate the extent of mixing of the sediments time horizons whether paleontologic or isotopic are generally blurred where mixing has occurred precluding precise identification whenever possible two methods should be used for dating sediments because a rate albeit erroneous can be obtained isotopically in sediments that are mixed accurate sedimentation rates are also difficult to determine where the time boundary is a zone rather than a horizon where the historical record does not provide a precise date for the pollen horizon or where scouring has removed some of the sediment above a dated pollen horizon

pollen distribution in the bottom sediments of the western north atlantic ocean abstract an extensive survey of pollen distribution in surficial sediments from the slope and rise between 29 and 72 n shows patterns which reflect the distribution of vegetation in eastern north america and the effects of aeolian fluvial and marine sedimentation the results of the analysis of 94 core tops show that pollen concentration ranges from 23 000 to 10 grains per gram and decreases offshore with distance from vegetation sources and northward in relation to lower vegetation density maximum concentration occurs in organic rich fine grained lutum deposited off rivers draining the southeastern united states low pollen concentrations are associated with high energy environments such as the western boundary undercurrent and the labrador current percentages of dominant pollen types generally correspond with onshore distribution of pollen and vegetation oak and other deciduous pollen types are the most abundant in marine sediments off the deciduous forests of eastern united states hemlock pollen is essentially restricted to sediments off the hemlock oak forests of northeastern u s and southeastern canada and spruce pollen is most abundant adjacent boreal forests and forest tundra formations the relative increase in pine pollen seaward reflects the selective effects of marine transport synthesis of pollen data by q mode factor analysis shows that major vegetation formations of eastern north america produce distinctive regional marine pollen assemblages

pollen distribution in recent marine sediments eastern canada analysis of 90 samples from coastal and continental shelf areas off eastern canada between 42 and 69 n shows that quaternary pollen and spores are present in recent marine muds throughout this region pollen and spore concentrations range from 50 000 to 100 grains per cm3 and they decrease both offshore as a function of distance from vegetation sources and northwards as a function of lower vegetation density north of the summer arctic frontal zone isopercentage contours show that maximum relative abundances of most palynomorphs occur in marine sediments adjacent to their regions of highest vegetation representation onshore however pinus and picea pollen tend to increase in percentage abundance with distance offshore presence of these tree pollen in marine sediments off baffin island implies that wind transport is an important pathway by which bisaccate pollen types enter the ocean in addition to fluvial transport q mode factor analysis shows that four major factors account for 89 of the variance amo

reconstructing pathways of aeolian pollen transport to the marine sediments along the coastline of sw africa the distribution of pollen in marine sediments is used to reconstruct pathways of terrigenous input to the oceans and provides a record of vegetation change on adjacent continents the wind transport routes of aeolian pollen is comprehensively illustrated by clusters of trajectories isobaric 4 day backward trajectories are calculated using the modelled wind field of echam3 and are clustered on a seasonal basis to estimate the main pathways of aeolian particles to sites of marine cores in the south eastern atlantic trajectories and clusters based on the modelled wind field of the last glacial maximum hardly differ from those of the present day trajectory clusters show three regional and two seasonal patterns determining the pathways of aeolian pollen transport into the south eastern atlantic ocean mainly transport out of the continent occurs during austral fall and winter when easterly and south easterly winds prevail south of 25 s winds blow mostly from the west and southwest and aeolian terrestrial input is very low generally a good latitudinal correspondence exists between the distribution patterns of pollen in marine surface sediments and the occurrence of the source plants on the adjacent continent the northern angola basin receives pollen and spores from the congolian and zambezian forests mainly through river discharge the zambezian vegetation zone is the main source area for wind blown pollen in sediments of the angola basin while the semi desert and desert areas are the main sources for pollen in sediments of the walvis basin and on the walvis ridge a transect of six marine pollen records along the south western african coast indicates considerable changes in the vegetation of southern africa between glacial and interglacial periods important changes in the vegetation are the decline of forests in equatorial africa and the north of southern africa and a northward shift of winter rain vegetation along the western escarpment

oceanic pollen transport and pollen dinocyst ratios as markers of late cenozoic sea level change and sediment transport abstract palynological studies of late cenozoic cores from nine sites show large peaks in the ratio of pollen and spores to dinocysts p d which reflect major increases in terrigenous sediment influx to the north atlantic ocean under normal pelagic conditions in the north atlantic i e in the absence of ice rafting or mass wasting p d in oceanic sediments is low usually

millennial and orbital scale climate variability in southeastern united states and in the subtropical atlantic during marine isotope stage 5 evidence from pollen and isotopes in odp site 1059 abstract we present directly correlative high resolution pollen and isotopic data from marine isotope stage mis 5 from a marine core taken on the continental margin off the southeastern united states 31 40 n 75 24 w 2985 m these data provide the first chronostratigraphically controlled pollen data for the last interglacial from this region comparison of the pollen and benthic isotope stratigraphies demonstrate that vegetation and climate development in southeastern united states did not always coincide with global ice volume changes deglacial terrestrial climate amelioration which was nearly synchronous with ice decay peaked slightly before the ice volume minimum in mis 5e cooling in the latter part of the last interglacial began at c27 prior to ice growth vegetation and climate were not stable during mis 5e suborbital climate oscillations persisted throughout mis 5 both onshore and in the subtropical atlantic offshore the largest correlative suborbital oscillations in the pinus pine and quercus oak forests of coastal georgia and south carolina and sea surface variability correspond to stadial interstadials documented in greenland ice cores however coupled oscillations also occur more frequently

the role of trench retreat on the geometry and stress regime in the subduction complexes of the mediterranean 1 numerical modelling of the aegean and calabrian subductions using geological reconstructions starting 75 80 myr before the present reproduces the present day geometries of the slabs and mantle wedges imaged by seismic tomography the change from a shallow subduction dip of about 15 from the surface to a depth of 200 km into a dip of 60 below this depth in the aegean subduction is caused by the south west migration of the trench at the velocity of 3 cm yr obtained from gps and geological data the largest dip of 70 calabrian subduction is caused by the termination of the active phase of opening of the tyrrhenian back arc basin and by the essentially purely gravitational sinking of the slab in agreement with the null trench migration velocity obtained from gps data in calabria these two different tectonic settings are the ultimate causes of the different styles of seismicity within the two subduction complexes

pollen sedimentation in the northwest atlantic effects of the western boundary undercurrent abstract sedimentation on the continental rise off eastern north america is strongly affected by the southerly flowing western boundary undercurrent wbuc recent studies of physical characteristics position and velocity field of this contour current show that the high velocity core is characterized by decreased carbonate dissolution coarse particle size and improved magnetic alignment of sedimentary grains in this report we suggest that the high velocity core of the wbuc also effects palynomorph sedimentation pollen concentration is lower and the percentage of redeposited pollen grains is higher under this bottom current

correlating pleistocene sequences across the new jersey margin abstract the biostratigraphic ranges of palynomorphs as well as climatostratigraphic changes recorded by both marine and terrestrial palynomorphs provide one of the few means of dating and correlating neritic environments where calcareous microfossils are rare the transport of terrestrial palynomorphs to marine environments also allows for direct land sea correlation in addition palynological assemblages provide insights into sediment transport and sea levels over 500 m of upper pleistocene sediments were recovered in odp hole 1073a on the upper new jersey slope palynomorphs in this hole record rapid progradation during the late pleistocene a prominent unconformity pp3 s forms the upper boundary of this thick progradational sequence palynological analysis from odp sites 1072 and 1073 allows this surface to be correlated from the outer shelf to the upper slope and also suggests that it was generated during a sea level lowstand probably during oxygen isotope stage 12 450 425 ka both the atlantic coastal plain and odp site 1072 on the outer new jersey shelf appear to have become erosional during this progradational phase which began around the time that the magnitude of northern hemisphere ice accumulation increased this suggests that sediments prograded across the new jersey margin as accommodation in inner neritic and coastal environments decreased in response to amplified glacioeustatic fluctuation and substantially increased sediment availability supplied by glacial erosion erosion associated with the generation of unconformity pp3 s appears to have increased accommodation on the new jersey margin allowing sediments to aggrade on the continental shelf and even to accumulate on the atlantic coastal plain during interglacial highstands the upper pleistocene architecture of the new jersey margin appears to have been controlled by glacioeustatic fluctuations and the accompanying increase in sediment supply in contrast unconformable surface pp4 s is more complex and separates upper miocene sediments deposited between 7 4 and 5 9 ma from lower pleistocene sediments deposited prior to approximately 1 4 ma in hole 1072a whereas in hole 1073a pp4 s is in sediments of early pleistocene age between 1 6 and 1 3 ma these data suggest that unconformity pp4 s was generated during the early pleistocene but there is no clear palynological evidence that glacioeustasy was responsible for the generation of this unconformity

late quaternary pollen transport processes western north atlantic data from box models cross margin and n s transects abstract surface pollen assemblages in coastal and neritic sediments of the western north atlantic compare well with eastern north american vegetation zones and late quaternary pollen in marginal marine sediments clearly correlate with changes in terrestrial vegetation and paleoclimates in contrast offshore assemblages are strongly affected by differential adaptation of pollen to long distance transport by wind and water marine pollen transport processes were studied by measuring air and water inputs to a coast shelf box model and by study of surface samples from cross margin transects in three different climatic and oceanographic regions at approximately 38 45 and 55 n latitude the box model shows that aerial transport is the main process by which pollen moves across the continental margin off nova scotia two clear seabed distribution patterns were found betula quercus and herb pollen decrease rapidly offshore in abundance grains per cubic centimetre and in relative abundance percentage pinus and picea have abundance peaks on the continental margin but percentages increase further offshore distributions of the main pollen and spore taxa were compared for late wisconsinan glacial oxygen isotopic stage 2 12 28 ka terminal pleistocene 10 12 ka and holocene sediments at 5 continental margin and 3 deep sea sites the largest changes were found in percentages of pinus picea and herb pollen during the late wisconsinan glacial and terminal pleistocene intervals at subpolar latitudes these data can be related to shifts in paleo vegetation winds and hydrology that accompany global climate change

modern pollen distribution in marine sediments from the northern part of the south china sea we analyzed the pollen and spore distributions in sediments from the northern south china sea to support reconstruction of the region s paleoenvironment 1 based on pca analysis in the two areas with water depths of 200 m pollen and spores found at water depths of 200 m wind transport is dominant 2 tree pollen was abundant on the continental shelves near guangdong and southeastern hainan island the center of the study area and deep basins in the south pinus pollen was abundant with the highest levels on the continental shelf near guangdong and in the marine area south of 15 degrees n pollen from tropical and subtropical broad leaved trees e g castanopsis quercus had low abundances castanopsis pollen appeared to originate near hainan island guangdong and the indochina peninsula herbaceous pollen was less abundant with pollen from the poaceae and artemisia scattered among the northern and southern regions with the highest levels near guangdong and a distribution that followed the rivers or sea surface circulation poaceae pollen was most abundant along the guangdong and vietnam coasts due to their proximity to tropical and subtropical grasslands and crops monolete spores and cyperaceae pollen were most abundant near the pearl river estuary southern hainan island and southwestern taiwan probably originating from fluvial river discharge trilete spores mostly dicranopteris were most abundant on the continental shelf of southeastern hainan island and the continental slope far from guangdong 3 pollen and spore assemblages from airborne sea surface water and sediment samples were compared from august to september airborne pollen and spores mainly belong to trees ulmaceae pinus and herbaceous pollen and have seasonal characteristics ocean surface waters in august september mainly contain d dichotoma rhizoma polypodiodis nipponicae and other trilete spores but no pinus pollen with clear seasonality characteristic pollen and spore assemblages in surface sediments are dominated by trilete spores pinus and herbaceous pollen and appear to integrate the pollen characteristics of the four seasons c 2014 elsevier b v all rights reserved

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

gravity signature of crustal subduction inferred from numerical modelling summary gravitational signatures of subduction are a major feature of the earth s gravity pattern being visible as lineaments parallel to the arc trench systems the novelty of our study resides in the simulation of this subduction fingerprint via a newly developed dynamic subduction model based on the finite element approach which includes the compositional stratification of the lithosphere into a light upper crust and a dense lithospheric mantle crustal material sinks in the mantle in proximity to the top of the slab due to the viscous dragging forces and off scraping of the overriding plate we show that the sinking of light crustal material from both the subducting and overriding plates to depths of the order of 200 km may be responsible for a trough of about 100 mgal extending 100 200 km in a direction perpendicular to the trench this anomaly is superimposed on a global positive gravity anomaly due to thermal contraction of the mantle lithosphere system the negative gravity anomaly due to compositional stratification of the crust has comparable magnitude and horizontal extent of that due to the negative dynamic topography of the trench the gravity patterns from compositional stratification of the crust and from dynamic trench topography are evaluated for different tectonic subduction styles related to active convergence with and without roll back in general we found that case without roll back produces a broader and smoother dynamic topography gravity anomaly than with roll back while the gravity signature from compositional stratification is less sensitive to the style of subduction

quantitative reassessment of brick red lutites evidence from reflectance spectrophotometry abstract brick red lutites are distinctive bright red sediments deposited in the western north atlantic these unique sediments were derived from permo carboniferous outcrops in the canadian maritime provinces transported out the gulf of st lawrence through the cabot straits and redistributed by the western boundary undercurrent along the north american continental margin in this study we used fourteen cores taken off eastern north america to more precisely examine the input and depositional history of brick red lutites our study differs from previous studies in the manner in which the red sediment was identified unlike previous studies which used color charts to assess the presence of brick red lutites we used visible light reflectance spectrophotometry the primary mineral imparting a red color to brick red lutites is hematite experiments with known quantities of hematite in a matrix mineralogically similar to western north atlantic sediment indicate that the presence of hematite can be detected at weight percentages of less than 0 1 using spectral reflectance data we characterized the distribution of brick red lutites for two broadly defined time levels latest glacial and early holocene as expected the maximum input of brick red lutite was during late glacial time and input decreased reaching a minimum during the early holocene since the early holocene there has been little change in the distribution of this unique red sediment on the north american margin however our reflectance spectra indicate renewed accumulation of hematite rich sediment at the present time in the cabot straits

the high velocity core of the western boundary undercurrent at the base of the u s continental rise the western boundary undercurrent is a high velocity contour following bottom current that flows southwesterly on the u s atlantic continental margin a high velocity core of the western boundary undercurrent is delineated by an analysis of underlying sediments which are characterized by coarse particle sizes and efficiently aligned magnetic grains in a zone from 4440 meters at the base of the rise to 5200 meters on the adjacent abyssal plain

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

exotic pollen as an indicator of variable atmospheric circulation over the labrador sea region during the mid to late holocene variability in the abundance of exotic non native pollen in sediment cores has long been considered as a potential proxy for changing atmospheric circulation but the difficulty of gaining sufficient total exotic pollen and the incomplete under standing of atmospheric pollen transport patterns has hindered its application in light of recent advances in the study of pollen transport we present an exotic pollen record from two fjord sediment cores taken from the west placentia bay newfoundland and east narsaq sund greenland labrador sea as a basis for studying variations in regional atmospheric circulation the two cores cover the last ca 5500 years and indicate a shift in dominant spring summer air masses at ca 2000 southern greenland and 3000 cal a bp newfoundland transporting reduced concentrations of pollen from southerly and south westerly vegetation zones this may suggest a shift away from more dominantly zonal atmospheric circulation a feature of positive north atlantic oscillation years to more frequent meridional circulation these results support sea ice sea surface temperature proxy reconstructions from newfoundland investigated as part of the same project which also suggest increased winter atmospheric circulation during the early part of the time period studied in this region more positive north atlantic oscillation years and therefore more zonal atmospheric circulation are associated with increased atmospheric circulation in both the winter and the summer seasons copyright 2011 john wiley sons ltd