abrupt changes in deep atlantic circulation during the transition to full glacial conditions 1 six ocean drilling program odp sites in the northwest atlantic have been used to investigate kinematic and chemical changes in the western boundary undercurrent wbuc during the development of full glacial conditions across the marine isotope stage 5a 4 boundary 70 000 years ago sortable silt mean grain size ss measurements are employed to examine changes in near bottom flow speeds together with carbon isotopes measured in benthic foraminifera and planktic foraminiferal fragmentation as proxies for changes in water mass chemistry a depth transect of cores spanning 1 8 4 6 km depth allows changes in both the strength and depth of the wbuc to be constrained across millennial scale events ss measurements reveal that the flow speed structure of the wbuc during warm intervals interstadials was comparable to modern holocene conditions however significant differences are observed during cold intervals with higher relative flow speeds inferred for the shallow component of the wbuc 2 km depth during all cold stadial intervals including heinrich stadial 6 and a substantial weakening of the deep component 3 4 km during full glacial conditions our results therefore reveal that the onset of full glacial conditions was associated with a regime shift to a shallower mode of circulation involving glacial north atlantic intermediate water that was quantitatively distinct from preceding cold stadial events furthermore our chemical proxy data show that the physical response of the wbuc during the last glacial inception was probably coupled to basin wide changes in the water mass composition of the deep northwest atlantic

antarctic link to deep flow speed variation during marine isotope stage 3 in the western north atlantic the western boundary undercurrent wbuc off eastern america is an important component of the atlantic meridional overturning circulation and is the principal route for southward transport of north atlantic waters and southward return of southern source water ssw here a direct flow speed proxy mean grain size of the sortable silt is used to infer the vigour of flow of the palaeo wbuc at blake outer ridge odp site 1060 depth 3481 m during marine isotope stage mis 3 the overall shape of the flow speed proxy record shows a complex pattern of variability with generally more vigorous flow and larger scale flow variations between 35 and 60 ka than in the younger part of mis 3 and mis 2 b35 ka six events of reduced bottom flow vigour slow events ses occur these appear uncorrelated with heinrich events but are instead synchronous with the warming phases of antarctic warm events a 1 to a 4 with one new one a 1a and one poorly defined a 0 this indicates that antarctic climate exerts a stronger control on deep flow vigour in the north atlantic during mis 3 than northern hemisphere climate the correspondence of ses with antarctic warming suggests a weaker wbuc flow due to reduced volume flux at ssw source or reduced ssw density because the variability of the lower limb of the wbuc was not connected to sharp north atlantic changes in temperature it is unlikely that the dansgaard oeschger cycles were associated with a mode of moc variation involving wholeocean overturn but more likely with perturbations of only the shallow glacial gulf stream glacial northern source intermediate water cell nutrient proxies benthic carbon isotopes and cd ca of uvigerina peregrina at this site show similar trends to the grip 18 o record this correlation has previously been attributed mainly to hydrographic and flow changes but is here shown to be better explained by variations in surface ocean productivity and subsequent decomposition of 12 c rich organic material on the

active tectonics and human survival strategies tectonic movements continuously remould the surface of earth in response to plate motion yet such deformation is rarely taken into account when assessing landscape change and its impact on human land use except perhaps as an occasional hazard to human life or a temporary disruption in the longer term parems of human history however active tectonics also create and sustain landscapes that can be beneficial to human survival forming a complex topography of potentially fertile sedimentary basins enclosed by mountain bard that can facilitate the control and exploitation of food resources especially animal prey we discuss the tectonic history of northwest greece and show how the paleolithic sites of the region are located to take advantage of tectonically created features at both a local and a regional scale we suggest that the association of significant concentrations of early paleolithic sites with tectonically active regions is not coincidental and that on the longer time spans of human biological evolution active tectonics has been an importam selective agent contributing to the development of the human species as an intelligent predator

transient changes in well water level in bore wells in western india due to the 2004 mw 9 3 sumatra earthquake large water level changes were observed in six bore wells in the koyna region in western india following the great sumatra earthquake of m w 9 3 on 2004 december 26 this earthquake occurred at 00 58 50 coordinated universal time utc off the coast of sumatra located about 3000 km from the koyna region the arrival time of the p wave recorded at the koyna seismic station was 01 04 24 45 utc the anomalous water level changes were observed at 01 15 utc at all of these wells where the sampling rate was 15 min no abnormal changes were noticed in the preceding sampling time at 01 00 utc clearly indicating that these changes occurred due to the passage of seismic waves generated by the sumatra earthquake however due to the large sampling interval it is difficult to correlate the water level changes either to shear or surface waves no local earthquake was recorded by the network during this period the anomalous water level changes were of the order of 2 65 cm and show either a spikelike or a steplike change these types of changes are attributed to the dynamic strain induced by the passage of seismic waves most probably long period surface waves

pronounced changes in atmospheric circulation and dust source area during the mid pleistocene as indicated by the caotan loess soil sequence in north china the quaternary loess soil sequence in north china is of significant value for tracing variations in atmospheric circulation in this study we measured the anisotropy of magnetic susceptibility ams of the caotan ct loess soil sequence from the northeastern margin of the tibetan plateau tp and analyzed the surface textural features of quartz grains of representative samples the ams results indicate a primary sedimentary magnetic fabric without significant disturbance by pedogenesis the dec kmax values indicate a significant shift of the dominant surface wind direction from nearly n s to w e since 300 ka while the surface textural features of the quartz grains indicate an increase in the contribution from fluvioglacial sediments derived from the tp at the same time as there is no evidence of significant climatic changes around 300 ka we attribute the coeval changes of atmospheric circulation and dust source area to the uplift of the tp during the mid pleistocene and the resulting climatic cooling cooling of the tp may have led not only to the enhancement of the plateau monsoon but also to the significant accumulation of plateau glaciers which would have resulted in accelerated physical erosion of the underlying rocks thus increasing the quantity of fine grained fluvioglacial sediments to be transported to the study site

surface sediment response to deepwater circulation on the blake outer ridge western north atlantic paleoceanographic implications abstract two transects of box core hydrographic and photographic stations were made along the blake outer ridge on the southeastern u s continental rise to study how circulation patterns affect surface sediment properties circulation is strongest at 4200 m on the flanks of the ridge and is reflected in the suspended particulate distribution a second turbidity maximum at 3600 m suggests that there may be a second axis of circulation at this depth the intense circulation on the flanks of the ridge has resulted in winnowing of the sediment revealed by coarsening and flattening of the grain size distribution in the detrital silt fraction and concentration of carbonate by the formation of a foraminiferal lag deposit the mean grain size of the detrital silt fraction on the crest of the ridge decreases southeastwards with increasing distance from upstream sediment sources shifts in the position and strength of the western boundary undercurrent affect the distance that relatively coarse silt is transported downcore changes in grain size can therefore be used as paleoceanographic proxy for changes in deepwater circulation

a centennial scale record of vegetation and climate variability from 312 to 240 ka marine isotope stages 9c a 8 and 7e from tenaghi philippon ne greece abstract based on a new centennial scale resolution pollen record from the terrestrial site of tenaghi philippon ne greece we report a series of high amplitude millennial scale forest expansion events during the interval from 312 to 240 ka equivalent to isotopic substages c a of marine isotope stage mis 9 mis 8 and the onset of mis 7e pollen based indices support the climatic interpretation of the forest expansion events as indicators of abrupt increases in temperature and moisture availability in the mediterranean region long term changes in vegetation cover and composition during the studied interval permit the definition of glacial substages early middle and late analogous to the glacial substages of the last climatic cycle within this framework forest expansion events occurred during the early glacial equivalent to mis 9c a and the later part of the middle glacial mid to late mis 8 but are absent from the early part of the middle glacial early mis 8 forest expansion events are also detected during the lateglacial and onset of mis 7 associated with a millennial scale climate oscillation across the glacial interglacial transition i e termination iii t iii a peak for peak match between forest expansion events and antarctic temperature maxima suggests interhemispheric coupling of millennial scale climate variability via global oceanic circulation change and synergistic variability in atmospheric circulation consistent with the bipolar see saw the resolution of our record mean 290 yr also permits the characterisation of climate evolution over the course of interstadial episodes following forest expansion events and highlights differences in the characteristics of early and middle glacial interstadials saw tooth vs plateau profile these differences point to an influence of long term boundary conditions especially global ice volume on the characteristics of millennial scale events

size sorting in marine muds processes pitfalls and prospects for paleoflow speed proxies the basis for and use of fine grain size parameters for inference of paleoflow speeds is reviewed here the basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field experimental data on suspended sediment transport and deposition and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed in the deep sea sorting events occur under resuspension deposition events in benthic storms at flow speeds below 10 15 cm s 1 size in the noncohesive sortable silt 10 63 m range is controlled by selective deposition whereas above that range removal of finer material by winnowing also plays a role the best particle size instruments to measure a flow speed related grain size employ the settling velocity method while laser diffraction sizers can yield misleading results because of particle shape effects potential problems including source effects downslope supply on continental margins spatial variability of flow over bedforms and influence of ice rafted detritus are examined a number of studies using the sortable silt flow speed proxy are reviewed and inverse modeling of grain size distributions is examined outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed

deepwater circulation on blake outer ridge western north atlantic during the holocene younger dryas and last glacial maximum three depth transects containing a total of 33 sediment cores were investigated along the blake outer ridge in the western subtropical north atlantic sortable silt mean grain size and stable isotope records were used to assess the position and relative intensity of the western boundary undercurrent wbuc during the holocene the last glacial maximum lgm and the younger dryas yd intervals the holocene reconstruction is consistent with modern physical and chemical hydrographic measurements in the area suggesting a deep position for the fast flowing core of the wbuc 3000 4000 m deepening to 4500 m water depth on the ridge flanks and a water column dominated by north atlantic deep water nadw the lgm and yd reconstructions show that a comparable hydrographic regime was present during both these intervals suggesting a similar mode of circulation that was appreciably different from the holocene reconstruction the wbuc s zone of maximum flow speed during these intervals is suggested to have shifted above 2500 m water depth consistent with nutrient depleted glacial north atlantic intermediate water formation with an increasing influence of southern source water ssw beneath below 4000 m water depth results hint at increased ssw flow vigor during both the lgm and yd with higher flow speeds than during the holocene this study provides a framework for aiding the interpretation of time series records of paleocurrent flow speed changes in the region of the wbuc

fluctuations in deep western north atlantic circulation on the blake outer ridge during the last deglaciation variations in the mean grain size of the detrital silt fraction from four piston cores along a depth transect on the blake outer ridge were used to examine paleocirculation patterns and intensity between 2650 and 3818 m for the late pleistocene and holocene prior to 12 kyr circulation was focused on the upper part of the ridge above 2700 m and may have corresponded to enhanced levels of intermediate water production in the north atlantic deepwater circulation during deglaciation underwent a two step decrease to minimum speeds from 15 5 to 13 5 kyr followed by a three step increase to maximum speeds at 10 kyr minima in circulation at 13 5 and 12 kyr can be directly associated with oxygen isotopic evidence for advection of reduced salinity surface water to the north atlantic additional circulation minima at 15 and 10 5 kyr may also be the result of reduced salinity from high latitude meltwater discharge evidence for a drop to moderate circulation intensity during the holocene is supported by isolated geochemical records but could also be explained by decoupling of ocean ventilation rates from bottom circulation or a change in sediment provenance at the end of deglaciation

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

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

deep water bottom current evolution in the northern south china sea during the last 150 kyr evidence from sortable silt grain size and sedimentary magnetic fabric abstract deep water bottom current dwbc plays a central role in global climate relative to the atlantic due to poor preservation of marine carbonate in the pacific sediments the evolution of pacific deepwater circulation is still unclear the south china sea scs connection to the pacific through luzon strait at a depth of about 2400 m offers a unique opportunity to monitor the changes of western pacific deep water circulation we present a magnetic and grain size analysis of two sediment cores pc111 and pc83 located in xisha trough northern scs in order to reconstruct past changes of dwbc during the last 150 kyrs variations in the mean size of sortable silt are interpreted to indicate past changes of dwbc which suggest abrupt millennial scale increasing dwbc strength during heinrich stadials hs1 hs2 hs3 hs4 hs5 hs6 hs7 hs8 hs9 hs10 hs11 for past 150 kyr the flow orientation of dwbc revealed by the anisotropy of magnetic susceptibility ams are mostly nw se direction in core pc111 and core pc83 which are both parallel to the local topography combined with the published studies we suggest the increase bottom current in northern scs during the heinrich stadials hs1 to hs11 times of weak north atlantic deep water formation demonstrating the tight coupling between the atlantic and pacific oceans

measurement of the sortable silt current speed proxy using the sedigraph 5100 and coulter multisizer iie precision and accuracy the weight percentage and mean size of the 10 63 m terrigenous silt fraction termed sortable silt of fine grained marine sediments have recently been argued to be proxies for near bottom palaeocurrent intensity this paper details the accuracy and precision of the sedigraph 5100 and the coulter multisizer iie in determining the weight percentage and mean size of the sortable silt at abundances characteristic of deep ocean sediments three sortable silt standards and a fine silt clay diluent were generated from natural deep sea sediment of the iceland basin these were used to produce silty clays with standard additions of sortable silt ranging from 1 to 20 of the total by weight accuracy was measured relative to the size of the pure spike measured by each instrument at sortable silt abundance 5 the sedigraph and the coulter multisizer estimate both weight percentage and mean grain size precisely and accurately while below 5 instrumental noise degrades their performance on the basis of the sedigraph results a series of percentage errors is defined which may be applied to sortable silt mean size and weight percentage estimates used in palaeoceanographic near bottom current reconstruction it is suggested that ideally palaeocurrent reconstructions should be based on sedigraph determinations of the 0 63 m grain size distribution yielding both mean and abundance or on coulter multisizer determinations of the sortable silt grain size mean the coulter multisizer may prove especially useful in regions of very depleted 5 sortable silt component

deep western boundary current variability in the subtropical northwest atlantic ocean during marine isotope stages 12 10 high resolution 125 500 year temporal resolution sortable silt mean grain size data of subtropical northwest atlantic ocean drilling program odp leg 172 site 1061 monitor fluctuations of the deep western boundary current dwbc throughout marine oxygen isotope stages mis 12 10 330 460 kyr in great detail comparison with foraminiferal stable isotope data from the same site and odp site 1063 bermuda rise data of poli et al 2000 shows that the dwbc shoals during glacial times in relation to a reduced north atlantic deep water production and increased advection of southern ocean waters superimposed fluctuations in the flow speed position of the dwbc at a semiprecession scale are persistent throughout mis 10 12 and independent of the glacial state additional millennial scale variations in dwbc flow speed parallel surface water cooling events and ice rafted debris episodes especially at the transition of mis 11 10 indicating the sensitivity of deep current flow variations to changes in the surface water conditions these millennial scale variations are amplified when benthic foraminiferal 18o values exceed 3 8 and may be related to the transition of an ice volume threshold where climate variability is possibly amplified by feedback mechanisms associated with ice sheet growth

use of grain size and magnetic fabric analyses to distinguish among depositional environments the combined use of grain size and magnetic fabric analyses provides the ability to discriminate among depositional environments in deep sea terrigenous sediments we analyzed samples from three different depositional settings turbidites pelagic or hemipelagic interlayers and sediment drifts results indicate that sediment samples from these different environments can be distinguished from each other on the basis of their median grain size sorting as well as the intensity and shape of magnetic fabric as determined from an examination of the anisotropy of magnetic susceptibility we use these discriminators to interpret downcore samples from the bermuda rise sediment drift we find that the finer grains of the bermuda rise relative to the blake outer ridge do not result from lower depositional energy current speed and so may reflect a difference in the nature of sediment being delivered to the site i e distance from source between the two locations

paleoceanography of the deep western boundary undercurrent on the north american continental margin for the past 25 000 yr temporal fluctuations of silt mean particle size and foraminiferal indices were used to examine fluctuations in velocity and position of the axis of the deep western boundary undercurrent dwbuc on the north american continental margin from 25 to 17 ka the dwbuc axis was 500 m shallower and farther inshore than its present position on the lower rise current speed was high from 25 to 20 ka and then waned until 17 ka between 17 and 7 ka the major axis of the dwbuc shallowed to 4000 m current speed increased from 17 to 11 ka but slowed from h to 7 ka since 7 ka the dwbuc axis has deepened to its modern position at 4900 m the zone of maximum carbonate dissolution remained below the dwbuc the most intense dissolution was from 22 to 9 ka and 5 to 2 ka highest abundance of uvigerina and highest percent benthic foraminifera were immediately above the dwbuc from 22 to 7 ka after which values of both parameters fell to near zero

abyssal currents and advection of resuspended sediment along the northeastern bermuda rise abstract an integrated study using current meters time series nephelometers and water samplers for measurements of particulate matter concentration revealed an intense nepheloid layer centered on the slopes of the eastward scarp the northeastern boundary of the bermuda rise these slopes also contained extensive furrows truncated seismic reflectors and wisconsin age and older surface sediments the abyssal circulation in this region appears to be accelerated when it encounters the steep topography of the eastward scarp eroding sediments creating the erosional furrows and moving eroded sediment upslope instruments on top of the bermuda rise recorded episodic events benthic storms where particle concentrations at 25 m above the bottom increased from a background level of 60 g 1 1 to maximum of 251 g 1 1 and current velocities were nearly 20 cm s 1 net transport of both water and entrained sediment was to the west northwest during the largest benthic storm water temperature decreased by 0 1 c consistent with a 300 600 m upslope transport of water and resuspended sediments from the upper slopes of the eastward scarp when currents carry resuspended sediment over the rim of the eastward scarp to the plateau region the current slows depositing its sediment load thus the plateau is growing upwards slowly at the expense of the surrounding scarps

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

intermediate water links to deep western boundary current variability in the subtropical nw atlantic during marine isotope stages 5 and 4 records from ocean drilling program sites 1057 and 1059 2584 m and 2985 m water depth respectively have been used to reconstruct the behavior of the deep western boundary current dwbc on the blake outer ridge bor from 130 to 60 kyr b p marine isotope stage mis 5 and the 5 4 transition site 1057 lies within labrador sea water lsw but close to the present day boundary with lower north atlantic deep water lnadw while site 1059 lies within lnadw high resolution sortable silt mean inline equation grain size and benthic 13c records were obtained and changes in the dwbc intensity and spatial variability were inferred comparisons are made with similar proxy records generated for the holocene from equivalent depth cores on the bor during mis 5e inline equation evidence at site 1057 suggests slower relative flow speeds consistent with a weakening and a possible shoaling of the lsw sourced shallower limb of the dwbc that occupies these depths today in contrast the paleocurrent record from the deeper site suggests that the fast flowing deep core of the dwbc was located close to its modern depth below 3500 m during this interval the benthic 13c suggests little chemical stratification of the water column and the presence of a near uniform lnadw dominated water mass after 111 kyr b p the inline equation record at site 1057 increases to reach values similar to site 1059 for the rest of mis 5 the strengthening of flow speeds at the shallow site may correspond to the initiation of glacial north atlantic intermediate water formation also suggested by a divergence in the benthic 13c records with site 1057 values increasing to 1 2 coupled suborbital oscillations in dwbc flow variability and paleohydrography persisted throughout mis 5 comparison of these data with planktonic 18o records from the sites and alkenone derived sea surface temperature sst estimates from the nearby bermuda rise suggest a hitherto unrecognized degree of linkage between oscillations in subtropical north atlantic sst and dwbc flow

abyssal bedforms as indicators of changing bottom current flow examples from the u s east coast continental rise direct indications of circulation patterns in the deep sea especially along continental rises and sediment drifts can be obtained through the analysis of abyssal bedforms such as furrows and mud waves such direct evidence of current flow will supplement indirect indicators of circulation provide by paleochemical indicators preserved in ocean sediments water that sinks in the northern north atlantic forms part of the western boundary undercurrent along the continental rise of eastern north america and the understanding of temporal changes in the speed and depth of flow along this continental rise can provide important information on how the speed at which the global conveyor system is operating changes with time studies of furrows and mud waves along the blake bahama outer ridges show that they are active sedimentary features today and that bottom flow speeds near the time of the younger dryas were increased over later intervals a more detailed analysis of bedform structure and sediments is needed to more clearly show the timing and reason for this change in sedimentation pattern

changes in grain size and magnetic fabric at blake bahama outer ridge during the late pleistocene marine isotope stages 8 10 abstract odp sites 1055 1062 on the blake bahama outer ridge bbor are characterized by high sedimentation rates which allow high resolution analyses to reconstruct the behavior of the deep western boundary current dwbc as a function of depth along the bbor the odp sites provide an intermediate and deep water transect from 1800 to 4760 m water depths we focused on the time interval from 350 to 250 ka marine isotope stages mis 10 2 8 3 grain size and magnetic properties were analyzed with centennial to millennial scale time resolution integrating the results of grain size analyses lightness and magnetic properties the changes in the vertical position and the intensity of the dwbc core were inferred during glacial periods mis 10 2 and 8 4 8 3 the dwbc core was located at around 2200 m water depth whereas it deepened to 3000 m or more during warm periods interglacial stage 9 3 and interstadial 8 5 between these periods the dwbc core moved to shallower depth 2500 3000 m and its intensity increased which generally provided coarser sediments to a broad depth range from 2100 to 4800 m furthermore the magnetic properties imply that the dwbc supplied carbonate material other than terrigenous sediments to the deeper sites during mis 9 the depositional processes at site 1062 are characterized by sudden influxes of fine carbonate rich sediments which gradually decreased with time paleocurrent directions inferred from the direction of the great axis of the magnetic fabric are consistent with the modern topography at sites 1055 1056 1058 and 1060 during the relatively warm period from stages 9 3 to 8 5 at site 1062 the deepest site which exhibits a mud wave field the inferred flow direction is parallel to the bathymetric contours during warm periods while the nearly cross wave direction seemingly dominated during the other periods it appears that topographic position may have affected the scattering of azimuth plots moreover there is a possibility that the changes in flow direction correlate with climate changes

pleistocene fluctuations in the western boundary undercurrent on the blake outer ridge the mean grain size of the detrital silt fraction was analyzed in two piston cores from the crest of the blake outer ridge using electronic particle analysis the resultant profiles in the two cores are highly correlatable and are interpreted to reflect the paleospeed of the western boundary undercurrent wbuc for the past 90 kyr the record can be divided into three periods the first encompasses the past 12 000 years and begins with an abrupt acceleration marking the transition from slow glacial to more vigorous holocene conditions the later holocene is a period of strong but variable flow the second period extends from 30 to 12 10 years and is characterized by a steady reduction of wbuc strength through the last ice age the current continued to decelerate until 12 10 years when the polar front finally retreated northward past the passages connecting the norwegian sea to the north atlantic the third period from 30 to about 90 10 years was a time of strong and variable flow that was independent of the glacial interglacial signal that is seen in the oxygen isotope record during this time the core of maximum flow appears to have shifted up and down the flank of the blake outer ridge and may have been influenced as much by salinity as by temperature variations in the source areas of the wbuc

magnetic anisotropy and environmental changes in two sedimentary cores from the norwegian sea and the north atlantic abstract we have studied the magnetic properties and the magnetic anisotropy ams and aarm of cores md95 2010 from the voering plateau and su90 33 from the south icelandic basin in the first core the study has been focussed on climatic stage 3 during which the record of magnetic susceptibility is correlated with the 18o signal from the greenland ice cores minima maxima in susceptibility anhysteretic remanent magnetization and isothermal remanent magnetization coincide with cold warm periods and reflect a modulation in the amount of deposited magnetite in the second core six climatic stages have been studied the ams and the aarm changes are tightly correlated showing that both are related to changes in the organization of magnetite grains in the sediment both magnetic fabrics are oblate and the variations of the degree of anisotropy are climatically controlled with higher values during short warm events or interglacial periods compared to cold periods different possible mechanisms leading to the observed fabrics are discussed it appears that although a differential compaction might have been active the fabrics mainly result from depositional effects in connection with climatic changes although the exact mechanisms of the link between magnetic fabrics and bottom current dynamics are as yet open to discussion the study of magnetic anisotropy which reflects the sedimentary structure may ultimately lead to a better understanding of the coupling between atmospheric temperature changes and paleocirculation pattern in the north atlantic and nordic seas

depositional changes along the blake bahama outer ridge deep water transect during marine isotope stages 8 to 10 links to the deep western boundary current odp sites 1055 1062 recover a bathymetric transect from 1800 to 4800 water depth in the subtropical nw atlantic carolina slope blake bahama outer ridge this sediment drift region is known for high deposition rates 40 cm kyr and offers the excellent opportunity to investigate the history of water mass circulation and chemistry as well as depositional changes during the quaternary a late pleistocene time interval from 250 to 350 kyr marine isotope stages 8 10 was investigated with centennial to millennial scale time resolution stable isotope records of benthic foraminifera provide new detailed insights on variations in climate and deep water ventilation the 13c records indicate well ventilated north atlantic water masses between 2200 and 3000 m water depth during the time interval from stage 9 to interstadial 8 5 during the glacial stages 8 4 and 10 2 however the decrease in 13c reflects an extension of nutrient rich antarctic bottom water aabw up to 2200 m water depth this is paralleled by a shoaling of the lysocline as indicated by the carbonate records a comparison between carbonate dissolution proxies points out that the carbonate dissolution at the hemipelagic sites was not only influenced by the different carbonate ion concentration of the water masses aabw contra north atlantic deep water but also by the organic carbon flux to the sea floor especially at the shallower sites whereby the decay of organic matter enhanced carbonate dissolution in the sediments additionally the varying depth position and strength of the deep western boundary current dwbc plays an important role on the depositional system along the blake bahama outer ridge comparisons between current intensities as inferred from grain size analyses sand and carbonate contents suggest that high intensities of the dwbc during cold stages caused an erosion of the fine carbonate and an enrichment of the sand fraction at shallower depth during warm stages the main core of dwbc moved to greater depth and allowed the settling of finer material at shallower depth synchronously this led to an increased supply of carbonate and extremely high carbonate concentrations below 3500 m water depth

transport and bottom boundary layerobservations of the north atlantic deep western boundary current at the blake outer ridge abstract the north atlantic deep western boundary current dwbc was surveyed at the blake outer ridge over 14 days in july and august 1992 to determine its volume transport and to investigate its bottom boundary layer bbl this site was chosen because previous investigations showed the dwbc to be strong and bottom intensified on the ridge s flanks and to have a thick bbl the primary instrument used was the absolute velocity profiler a free falling velocity and conductivity temperature depth device in two sections across the width of the dwbc volume transports of 17 1 sv and 18 1 sv were measured for all water flowing equatorward below a potential temperature of 6 c 1 sv 1 10 6 m 3 s 1 transport values were derived using both absolute velocities and avp referenced geostrophic velocities and were the same within experimental uncertainty good agreement was found between our results and historical ones when both were similarly bounded and referenced although this was a short term survey the mean of a 9 day time series of absolute velocity profiles was the same as the means of year long current meter records at three depths in the same location a turbulent planetary bbl was found everywhere under the current the thickness of the bottom mixed layer bml where concentrations of density nutrients and suspended sediments were vertically uniform was asymmetrical across the current and up to 5 times thicker than the bbl there was no velocity shear above the bbl within the thicker bmls and the across slope density gradient was very small the extra thick bml is perhaps maintained by a combination of processes including turbulence downwelling ekman transport a weak up slope return flow above the bbl and buoyant convection from the bbl into the bml the frictional bottom stress was mostly balanced by a down stream change in the current s external potential energy evidenced by a drop in the velocity core of the current

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

paleomagnetic overprints in ocean sediment cores and their relationship to shear deformation caused by piston coring 1 we use paleomagnetic data from cores collected at ocean drilling program site 1062 on the bahama outer ridge as well as observations from prior cruises to quantify the nature and to assess the causes of drilling overprints the largest overprint arises from an isothermal remanent magnetization that is imparted to the core during coring or core retrieval a second type of overprint is apparent only in soft sediments collected with a piston corer and has coercivities and unblocking temperatures that completely overlap those of the primary remanent magnetization which means it cannot be removed by standard demagnetization methods this demagnetization resistant overprint can produce significant biases in paleomagnetic observations particularly those from split core sections we hypothesize that shearing which occurs as a result of friction on the inside wall of the piston corer as it rips through sediments and cuts a core deflects the ancient magnetization we develop and test a model that predicts how shear deformation can rotate remanence carrying grains and deflect the paleomagnetic remanence of a split core section using the shear model predictions and directions measured on split core sections and u channel samples from the undeformed core center we estimate the average deformation sustained and then correct the split core data for biases caused by shear deformation the reduction in the deviations between corrected split core data and u channel data is statistically significant indicating that the shear model is capable of accounting for a significant portion of the systematic biases observed in paleomagnetic observations obtained from piston cores