slab weakening by the exothermic olivine spinel phase change numerical modelling of mantle convection with lateral variations of viscosity and both the 670 km depth endothermic and the 400 km exothermic phase changes reveals that downwellings are weakened upon crossing the exothermic phase change a high temperature low viscosity section is developed within the downgoing current owing to both the olivine to spinel phase transition and the slowing down of the flow due to the 670 km endothermic transition this mechanism may cause the creation of a weak zone at 400 km that results in a preferential location for horizontal or vertical slab tearing as revealed by seismic gaps in many slabs

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

fine details of the wadati benioff zone under indonesia and its geodynamic implications using about 5200 handpicked p s pp sp pcp and scp phases from digitally recorded seismograms together with international seismological centre reported phases we obtain improved hypocentral locations for 1790 earthquakes 983 of them having 90 confidence limits 50 km in the period 1962 to september 1996 along the indonesian subduction zone we use an improved joint hypocenter determination method in which the solution is more stable than in the original one we show that for deep earthquakes use of the core reflected phases is as good as use of depth phases and since such core phases often have larger amplitudes and sharper onsets than the depth phases their arrival times can be read very accurately the positions of the relocated hypocenters show that 1 a portion of the indonesian arc between 110 e and 123 e longitude and deeper than 500 km is dipping southward at an angle of 75 that is in a direction opposite to the upper part of the north dipping slab and 2 east of about 108 e the seismic zone is wider near 670 km than near the 500 km depth the first suggests southward lateral flow in the mantle relative to the plate motion vector here from the contortion of the seismic zone along the eastern portion of the indonesian arc we can estimate the average lateral shear strain rate in the 300 to 670 km depth range to be of the order of 10 16s 1 over the last 10 20 myr

sedimentation under deep sea storms fast currents high concentrations of suspended sediment and grooved mud beds are associated with erosion in frequent abyssal storms where a fast deep mean flow is augmented or reversed by intense intermittent currents this occurs about 5 km below the gulf stream or its rings the waning phase of a storm results in development of bedforms and rapid deposition of a mud blanket several other regions of the world ocean display evidence of abyssal storm activity

the 1996 june 17 flores sea and 1994 march 9 fiji tonga earthquakes source processes and deep earthquake mechanisms summary source parameters of the 1996 flores sea and 1994 fiji tonga deep earthquakes are derived from teleseismic body waves recorded by the global seismic network of broad band seismograph stations both events consisted of several subevents models to approximate the spatial and temporal extent of the source process include point sources propagating point sources and a combination of these for the flores sea event rupture lasted about 23 s and terminated some 70 km east of the nucleation point as inferred from the duration of p wave pulses in agreement with the findings reported by other investigators our preferred model suggests bilateral rupture propagation it consists of four point sources that have variable double couple radiation patterns and source time histories and explains well the large compensated linear vector dipole clvd component inferred from the harvard centroid moment tensor cmt solution the main moment release in the fiji tonga event lasted only about 15 s our best model consists of two point sources with a total moment release of 2 8 1020 n m rupture propagated subhorizontally from the nucleation point to the north the termination of rupture was located about 40 km to the north of rupture initiation the inferred velocity of moment release in the flores sea and fiji tonga events was 3 5 km s 1 a value which is higher than that inferred for the great bolivian earthquake of june 1994 other derived source parameters static stress drop radiated seismic energy and maximum seismic efficiency are also significantly different from those inferred for the 1994 bolivian event suggesting that deep earthquake processes do not follow an easily detectable common mechanism

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

paleoceanographic significance of sediment color on western north atlantic drifts ii late pliocene pleistocene sedimentation abstract color variations were interpreted in paleoceanographic terms for the late pliocene pleistocene sediments recovered by odp leg 172 on deep sea drifts at blake bahama outer ridge and northeastern bermuda rise the color derived parameters used in interpretation included predicted carbonate content terrigenous fluxes and hematite content abundance of upper carboniferous spores indicates that the hematite is probably derived from the permo carboniferous red beds of the canadian maritimes in the last 800 kyr sedimentation pattern changes on the blake bahama outer ridge were determined by the sediment delivery to the deep basin as well as circulation changes sediment delivery increased during glacials especially during the last 500 kyr and particularly since stage 6 a fundamental change in the thermohaline circulation occurred at about 500 ka corresponding to the end of the mid pleistocene transition period at the onset of the predominant 100 kyr climate cyclicity sedimentation related to wbuc had intensified at that time and had become more focused at depths below 3000 m changes in hematite content and sedimentation rate show a pulse of sediment via the st lawrence outlet at the pliocene pleistocene boundary suggesting that a likely change in the hydrography physiography of the laurentide ice sheet could have been involved in the climatic and ocean circulation changes at that time

calibration and comparison of the ldgo nephelometer and the osu transmissometer on the nova scotian rise gardner w d biscaye p e zaneveld j r v and richardson m j 1985 calibration and comparison of the ldgo nephelometer and the osu transmissometer on the nova scotian rise in a r m nowell and c d hollister editors deep ocean sediment transport preliminary results of the high energy benthic boundary layer experiment mar geol 66 323 344 during the survey of the hebble high energy benthic boundary layer experiments area two optical instruments were used to determine the spatial and temporal structure of the benthic nepheloid layer both instruments showed a linear response to suspended particle mass and volume over the typical range of concentrations found in the ocean 10 250 ug l at higher concentrations the transmissometer response remains linear but as previously noted and as shown here by theory the nephelometer has a predictable second order response due to multiply scattered light at the low concentrations found in mid water both instruments seem near their limit of sensitivity small differences between instruments at low concentrations may result from variations in response to particle types found in mid water or may be minor second order effects of temperature or pressure on the instruments the comparison made here shows that either instrument can be effectively used to study the nepheloid layer and that data from different study areas using these instruments can consistently be compared the hebble region was ideal for testing the range of these instruments because it yielded the highest concentrations of particles ever recorded in the deep sea

the little ice age and medieval warm period in the sargasso sea sea surface temperature sst salinity and flux of terrigenous material oscillated on millennial time scales in the pleistocene north atlantic but there are few records of holocene variability because of high rates of sediment accumulation holocene oscillations are well documented in the northern sargasso sea results from a radiocarbondated box core show that sst was approximately 1 degree c cooler than today approximately 400 years ago the little ice age and 1700 years ago and approximately 1 degree c warmer than today 1000 years ago the medieval warm period thus at least some of the warming since the little ice age appears to be part of a natural oscillation 39 refs 4 figs 1 tab

characterization of deep sea storms abstract to describe and quantify the physical forcing of sediment transport events at the hebble site velocity stress and sediment concentration data were obtained during a year long deployment through the year a variety of intense sediment transporting events were recorded careful analysis of these events reveals that only a small proportion of the peaks in suspended sediment concentration correspond to local erosion most sediment events were clouds of fine suspended material advected into the region analysis of progressive vector diagrams reveal that these advected clouds were bounded by regions with concentration gradients as high as 200 g 1 1 km 1 the high gradients suggest streams of sediment bearing water with sharp fronts of sediment concentration along their sidewalls were veering across the region the largest concentrations of suspended sediment were observed during local suspension events the growth and decay of the sediment clouds during the local events did not exactly track the boundary shear stress rapid erosion occurred as the stress exceeded a critical shear stress maximum suspended concentrations occurred shortly after erosion began indicating a source limited erosion process the depositional period lasted for several weeks whereas the erosion occurred in only a few days photographs of the bottom reveal depositional features crag and tail dunes which are aligned with the post storm flow not the erosional flow direction

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

deep seismicity and modes of deformation in tonga subduction zone the geometry of deep 400 km earthquakes in the subducting western edge of the pacific plate when related to the local distribution of seismic activity reveals a complex pattern of planes of shear failure enabling the modes of deformation by which the descending plate shortens and thickens to be identified the deepest part of the slab is displaced laterally suggesting that it fails to penetrate the discontinuity at 670 km

depression of the 660 km discontinuity beneath the tonga slab determined from near vertical scs reverberations the occurrence of the march 9 1994 mw 7 6 deep tonga earthquake during a temporary deployment of broadband seismographs provides an excellent opportunity to study the seismic discontinuity structure in the vicinity of a subducting slab near vertical scs reverberations from the 410 520 and 660 discontinuities are visible at 10 stations in the south pacific at frequencies between 0 003 0 033 hz without stacking we use a regional velocity model and ray theory to model the interval from scs to scs3 a grid search is then performed on the depth and impedance contrast at each of the seismic discontinuities to optimize the fit to both the underside and topside reflections stacked records indicate prominent discontinuities at 400 km and 665 km depth beneath and to the west of the slab and 410 km 500 km and 640 km to the east of the slab analysis of individual records suggests a local depression of 30 km in the 660 and a broadening of the transition zone directly beneath and to the west of the tonga slab in a region of possible near horizontal slab these results are consistent with a large cool region just to the west of the intersection of the tonga slab with the 660

a magnetic signature of bottom current erosion abstract to characterize the magnetic signal associated with bottom current erosion we have conducted downcore measurements of magnetic texture and fabric in three cores containing known erosional hiatuses using biostratigraphic and magnetostratigraphic analysis erosional hiatuses were identified for three cores from two current dominated environments the northern bermuda rise and the south indian basin the cores were contiguously subsampled and the magnetic susceptibility k the anisotropy of magnetic susceptibility ams and the anhysteretic susceptibility k arm were measured the ams parameters h and q were used to describe the relative magnitude and the shape respectively of the representative susceptibility ellipsoid the concentration independent and dimensionless ratio k arm k was used to indicate relative changes in magnetic grain size hiatuses in the three cores were marked by unusually high values of h and q indicating a greater development of internal fabric with a more linear fabric representation depressed values of k arm k indicated a coarser mean magnetic grain size the magnitude of this signal however did not have a simple relationship with hiatus duration t test results statistically confirmed the existence of an erosional signature to enhance the applicability of this technique a quantitative discriminant model was constructed and calibrated using the magnetic characteristics of erosion since this model was constructed from samples of known erosional and non erosional character the discriminant functions could then be used as reference equations to identify erosion in other cores biostratigraphic and oxygen isotopic data from a 14 m core from the northern bermuda rise extending back to isotopic stage 8 250 ka bp indicate two short intervals of low accumulation rate which for this region are most probably attributable to bottom current erosion magnetic data for this core were entered into the reference discriminant model and the two zones of suspected erosion were classified by the functions as being highly crosional in character zones of probable erosion were identified at several other levels in the core the timing of these short term crosional events was found to coincide closely with glacial intervals suggesting that bottom currents active on the northern bermuda rise were invigorated periodically during glacial maxima at c 18 58 64 85 130 140 165 180 225 and 250 ka bp

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

penetration of anthropogenic carbon into organic particles of the deep ocean 1 in the late 1980s bomb 14c was present in suspended particulate organic carbon poc from the north central pacific ncp and sargasso sea ss throughout most of the water column demonstrating that deep poc had exchanged with atmospheric co2 in the past 30 years upon reoccupation of these sites in 1999 and 2000 respectively we observed that the 13c values of suspended poc were lower than those measured a decade earlier this demonstrates that anthropogenic co2 from fossil fuel and biomass burning has penetrated a major organic matter pool in the deep ocean 14c measured in the suspended poc showed similar or higher values in the deep sargasso sea and decreased values in the deep ncp compared to those measured previously we use a box model to show that the differences in the radiocarbon results are likely due to the presence of resuspended sediment that is laterally advected from the continental margin to the deep sargasso sea

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 current influence on the southwest bermuda rise and surrounding region abstract echograms 3 5 khz and bottom photographs reveal that the northward flowing antarctic bottom water aabw has strongly influenced the modern depositional regime on the southwest bermuda rise the spatial distribution of echo character types the orientation and nature of current controlled structures and limited current meter data show that aabw flows with varying intensities along three primary pathways around and over the southwest bermuda rise the main core of aabw flows clockwise around the eastern and western flanks of the southern bermuda rise roughly parallel to the 5400 m isobath this current bifurcates at 28 30 n 69 w where a portion flows northeast over the southwest bermuda rise and the remainder continues north along the physiographic boundary between the southwest bermuda rise and the hatteras abyssal plain secondary ribbons of aabw branch off the main core of aabw during its southerly journey along the southeastern bermuda rise and flow west through fracture zones finally a diffuse northward flowing aabw sweeps the entire southwest bermuda rise a progression of current controlled bedforms occurs beneath the main path of the aabw reflecting the spatially varying current velocities and sediment supply the main core of aabw flows west through the narrow vema gap creating erosional furrows along the border between the southwest bermuda rise and the vema gap current velocities greater than 20 cm s 1 are inferred from the bedforms in this region farther north along the southwestern edge of the bermuda rise sediment waves become more prevalent this transition from erosional to more depositional bedforms results from diminished current velocities 5 15 cm s 1 and increased sediment supply although some of these bedforms on the southwest bermuda rise appear to be relict their orientation is consistent with current meter data and abyssal current direction inferred from bottom photographs

a study of the bottom boundary layer over the eastward scarp of the bermuda rise velocity and temperature measurements from the bottom boundary layer bbl over the eastward scarp of the bermuda rise in water depth of 4620 m show little variability over an 8 month period the free stream flow 62 m above the bottom was south southeasterly following the isobaths in the region with an average speed of 22 cm s the current vector in the bbl rotated an average of 5 in a counterclockwise sense between 62 and 0 8 m above the bottom the thickness of the bbl was 40 m and the average magnitude of the bottom stress was 0 7 dynes cm2 mean speed profiles height of the bbl and the magnitude of the bottom stress predicted by a model compare favorably with the observations but the model predicts a rotation of the current vector between 62 and 0 8 m more than twice that measured the time dependent nature of the flow field is also reproduced by the model the bermuda rise data and speed profile measurements at the base of the scotian rise show that the m2 clockwise polarized tide is damped more than the mean current as the bottom is approached in the bbl this phenomenon is reproduced by the model and can be explained by differing effective ekman layer thicknesses associated with tidal and steady components of the flow

high resolution data of the iceland basin geomagnetic excursion from odp sites 1063 and 983 existence of intense flux patches during the excursion based on discrete samples we report new high resolution records of the 185 kyr iceland basin ib geomagnetic excursion from ocean drilling project odp site 1063 on the bermuda rise sedimentation rate 32 cm kyr 1 and from odp site 983 in the far north atlantic sedimentation rate 18 cm kyr 1 two records from holes 1063a and 1063b are very consistent and provide the highest resolution of the detailed field behaviour during the ib excursion obtained so far inclination records from holes 983b and 983c in the far north atlantic are also very consistent whereas declination anomalies deviate more notably the pseudothellier pt technique was applied along with more conventional palaeointensity proxies nrm arm and nrm to recover relative palaeointensity rpi estimates from hole 1063a and hole 983b as expected these proxies indicate that the field intensity generally dropped at both sites during the ib excursion but also that the history of rpi from the two sites is different vgps from site 1063 indicate that the field at this location experienced some stop and go behaviour between patches of intense vertical flux over north america and the tip of south america areas which coincide fairly well with patches of preferred transitional vgp clustering from reversals and zones of high seismic velocity in the lower mantle changes in rpi at this location were generally gradual possibly due to the proximity of these flux patches and the first period of vgp clustering over north america was accompanied by a conspicuous increase in rpi vgps from site 983 track along a different path and the associated rpi changes are very abrupt and completely synchronous with the onset and termination of the excursion the differing vgp paths from sites 1063 and 983 indicate that the global field structure during the ib excursion was not dominated by a single dipole

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

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

storms and flow reversals at the hebble site abstract we examine a 20 month long abyssal current meter record obtained from the hebble site about 40 27 n 62 22 w as expected the westward flowing abyssal western boundary current described by richardson et al 1981 and weatherly and kelley 1982 generally passes over this site we call this current the cold filament current cfc and conclude that it is appreciably stronger than suggested by weatherly and kelley 1984 15 cm s 1 as compared to 10 cm s 1 after comparing the abyssal current record with charts showing the surface location of the gulf stream and rings we conclude that the majority of abyssal storms at the hebble site are due to the presence of the cfc passing by and the absence of a meander or ring overhead the presence of a ring or meander overhead apparently results in the cfc being displaced out of the hebble site as suggested by kelley 1984 at such times the near bottom flow appears to respond to the barotropic component of the overlying ring or meander

mechanism of slab thickening near 670 km under indonesia we examine a new data set of reliably relocated earthquakes deeper than 400 km under indonesia developed by schoffel and das 1999 the accuracy of this data allows us to demonstrate that the slab thickens shortens and weakens before penetrating below 670 km by shearing along conjugate fault planes on the upper and lower portions of the seismic zone we also show that the earthquakes in the lower portion of the wadati benioff zone width are generally fewer and smaller than those in the central portions with only 8 of the total moment released in the period 1976 1996 being due to earthquakes on the lower side

calcareous phytoplankton standing stocks fluxes and accumulation in holocene sediments off bermuda n atlantic abstract standing stocks and taxonomic composition of dominant coccolithophore taxa in the photic zone at bermuda hydrostation s are compared to their fluxes measured in a sediment trap at 3200 m water depth located 1200 m above the sea floor and to their accumulation in holocene sediments on the sea floor a pronounced monthly variability of coccolithophore cell densities in the photic zone over the three year plankton sampling interval 1991 1994 was observed in the sediment trap only an integrated seasonal signal is reflected in the fluxes of coccoliths and the fine fraction m carbonate which were determined for 12 biweekly sampling intervals during 1992 the average daily flux of all coccoliths during 1992 was 1 4 109 coccoliths m 2 d 1 but it varied by at least a factor of five with highest fluxes in late winter and late spring and low fluxes in late fall the fluxes of individual coccoliths of the two dominant species e huxleyi and f profunda accounted for 85 of the total coccolith fluxes the contribution of the other biogenic carbonate mostly calcispheres of thoracosphaerids to the fine fraction carbonate fluxes was minor compared to that of coccoliths fluxes of the fine fraction carbonate to the deep sargasso sea determined over 2 week intervals in 1992 were high in early spring up to 23 mg caco 3 m 2 d 1 and low in late fall minimum of 5 mg caco 3 m 2 d 1 with an annual mean of 12 mg caco 3 m 2 d 1 the surface sediments accumulating se of bermuda at 4300 m water depth consist of 11 by weight planktic foraminifera 56 fine fraction m carbonate and 33 inorganic clay the 14c age difference between the planktic foraminifera carbonate 1 5 ka and the fine fraction carbonate 6 7 ka at 13 cm core depth together with the fact that the accumulation rate of fine fraction carbonate in the sediment is about an order of magnitude higher than the corresponding fluxes to the trap confirms the previous reports of large scale resuspension of fine grained sediments in this area the taxonomic composition of the coccolith assemblages accumulating in holocene sediments near bermuda is very similar to the composition of coccoliths sampled in the trap and to that of the average living assemblage suggesting that transport and dissolution processes in this area affect all taxa in a similar way

determination of sediment provenance at drift sites using hydrogen isotopes and unsaturation ratios in alkenones the large 20 hydrogen isotopic gradient in surface waters of the northwest atlantic ocean is exploited to track changes in the source of alkenones to the bermuda rise sediment drift cultures of the predominant alkenone producing coccolithophorid e huxleyi were grown in deuterium enriched seawater and shown to possess alkenones with a d h ratio that closely tracked the water d h ratio r 2 0 999 n 5 isotopic enrichments with a fractionation factor between 0 732 and 0 775 a hydrogen isotopic depletion of 193 3 n 9 was measured in alkenones from suspended particles relative to seawater in the subpolar and subtropical northwest atlantic ocean this value was used to calculate the water d values in which alkenones from bermuda rise sediment were synthesized and by extension the water mass in which they were produced applying this technique we find that 60 to 100 of the alkenones in late holocene bermuda rise sediment were produced in deuterium depleted subpolar water to the northwest of the drift to reconcile values of the alkenone unsaturation ratio u k 37 a widely used proxy for sea surface temperature with the d values of alkenones in late holocene sediments from the bermuda rise at least three sources of sediment must be invoked a cold very isotopically depleted source almost certain to be the scotian margin a warm moderately isotopically depleted source likely to be the northwestern edge of the subtropical gyre and a cold isotopically enriched source which we hypothesize to be the subpolar waters overlying the main branch of north atlantic deep water flowing southwest from the nordic seas copyright 2005 elsevier ltd paleoclimate records with sufficient length and temporal resolution to study the occurrence and causal mechanisms of abrupt climate change are exceedingly rare rapidly deposited ocean sediments provide the best archive for studying these events through geologic time but such sites in the open ocean are limited to sediment drift deposits such as the bermuda rise in the northwest atlantic using multiple climate proxies in a single core is becoming more common in high resolution pa leoclimate investigations but a major potential concern for this approach arises from the possibility that the fine fraction of sediment e g 63 m and the climate proxies within it may represent conditions far from the deposition site here we propose and test the hypothesis that hydrogen isotope ratios of alkenones a class of lipids from phytoplankton may provide insight into the source of fine grained sediment because of their restricted sources broad geographic distri bution and excellent preservation properties alkenones are of particular interest in the emerging field of compound specific hydrogen isotopic analysis the sedimentary abundances ex tents of unsaturations and isotopic compositions of alkenones provide quantitative and near continuous records we isolated alkenones from cultured unicellular algae haptophyte emil iania huxleyi surface ocean particulate material and open ocean sediments to determine the extent and variability of hydrogen isotopic fractionation in the di tri and tetraunsat urated c37 compounds we then compared the d values of the alkenones in surface sediments between the bermuda rise and the scotian margin above which a large 20 d gradient exists we determined the fractionation between alkenones from suspended particulate samples and the water in which the phytoplanton lived and examined the variability of alkenone d values during the medieval warm period little ice age climate transition cf keigwin 1996 at the bermuda rise

mainshock and aftershock analysis of the june 17 1996 deep flores sea earthquake sequence implications for the mechanism of deep earthquakes and the tectonics of the banda sea one of the largest mw 7 8 deep earthquakes 595 km in recorded history occurred beneath the flores sea on june 17 1996 we analyze broadband body waveforms to determine the seismic source characteristics of this important earthquake the source time function has a small initial subevent 5 s duration followed by three major subevents with a total source duration of 29 s directivity analysis indicates an unilateral rupture to the east southeast with a subhorizontal rupture length of 75 km a time independent moment tensor timt inversion gives a total seismic moment of 5 4 1020 n m mw 7 8 a time dependent moment tensor tdmt inversion indicates rupture on a near vertical fault strike of 130 dip of 80 sw rake of 166 during the first 5 s releasing 8 of the seismic moment the major part of the moment was released along a shallower dipping plane strike of 100 dip of 55 rake of 45 this change of faulting geometry may imply that the rupture initiated at a structural thermal or phase boundary of high stress concentration and then triggered the major moment release along a regional weak zone the mainshock produced at least 23 mb 2 9 aftershocks within the first 3 days seventeen of the largest aftershocks were located using the joint hypocenter determination algorithm the aftershocks define a plane dipping antithetically to the dip of the subducting slab with lateral and depth dimensions of 145 km and 76 km respectively the aftershock locations correlate with the southeast striking nodal plane of the moment tensor mechanism

geological effects of the gulf stream system on the northern bermuda rise abstract the gulf stream system contains a clockwise rotating set of bottom currents which influence the sea bed in the northern north american basin it is possible to interpret the present day and historical record of current activity in this basin in terms of the deep flow of the gulf stream this interpretation provides a much more satisfactory and consistent explanation of the geologic record than previous interpretations based strictly on the influence of the classic abyssal circulation the present day circulation of the gulf stream actively resuspends large quantities of sediment on and near the northern bermuda rise and it is suggested that this resuspension may be due to the eddy field which is embedded in the westward flowing return flow during the late cenozoic the gulf stream system was responsible for depositing and shaping the major acoustic sedimentological units on the relatively smooth surface of the horizon a complex fine grained mont morillionite rich sediments derived from the chemically weathered saprolites of the hudson and st lawrence drainage basins were injected into the deep ocean basins by turbidity currents during the late paleogene and the neogene fine grained turbidites from the st lawrence spread southward down the southeastern sohm abyssal plain across the eastward flowing gulf stream and its westward flowing return flows portions of these fine grained sediments were entrained by the main and return flows and deposited downstream as acoustically non laminated accumulations on the corner and bermuda rises respectively the hudson river input was injected into the deep basin through the hudson canyon system the interaction of these fine grained turbidites with the northward flowing deep flow of the gulf stream system led to the formation of the gulf stream outer ridge here defined for the first time during the late neogene or early pleistocene the gulf stream outer ridge was partially eroded by the gulf stream system and portions subsequently covered by coarse grained pleistocene turbidites the coarse grained illite rich turbidites that flowed across the southeastern sohm abyssal plain also crossed the main and return flows of the gulf stream system the finer portions of these sediments were entrained and deposited as pleistocene acoustically laminated sediments on the plateaus of the bermuda and corner rises

surface and deep ocean variability in the northern sargasso sea during marine isotope stage 3 new geochemical proxy data from bermuda rise piston cores reveal ocean and climate conditions in the northern sargasso sea during marine isotope stage 3 using 18o on the planktonic foraminifer globigerinoides ruber we can correlate explicitly with every stadial interstadial change in greenland ice between 32 and 58 ka these data suggest repetitive changes of 4 c in the annual average sea surface temperature sst with maximum cooling comparable to or greater than sst during glacial maximum conditions the extent of sst depression is about the same for typical stadial events and for heinrich events 4 and 5 which we have identified on the bermuda rise by traces of ice rafting benthic foraminiferal 13c decreases during every stadial event consistent with reduced production of the deepest component of north atlantic deep water and shoaling of its interface with antarctic bottom water this interpretation is supported by benthic cd ca data from the climate cycle associated with interstadial 8

seismic stratigraphy of the northern bermuda rise abstract analysis of low frequency seismic profiles from the northern bermuda rise has led to the recognition of several seismic horizons and determination of a regional seismic stratigraphic framework horizon a is the deepest regionally significant reflector identified and horizon a c a t and a v of the horizon a complex are also mappable reflectors in the study area three additional regionally significant reflectors named yellow purple and silver have been identified in the area these seven reflectors allow speculation on the sedimentary evolution of the area sedimentation below the horizon a complex was primarily pelagic the distribution of reflectors within the horizon a complex indicates the area experienced turbidite deposition from both the u s east coast continental margin and nearby bermuda as well as pelagic deposition during the eocene sedimentation subsequently has been dominated by at least two distinct periods of abyssal current flow initially bottom currents brought in sediments from the north or west creating an anomalously thick accumulation of seismically poorly stratified sediments on the rise this period of accumulation was punctuated by an episode of erosion in latest miocene or earliest pliocene the most recent sediments are seismically well stratified sediments of primarily quaternary age carried from the east by the gulf stream system

on the interaction of the bottom boundary layer and deep rings abstract the interaction of rings extending down to the ocean bottom and the bottom boundary layer bbl is studied numerically with a primitive equation model in which the bbl is resolved using the mellor yamada level 2 5 closure scheme the lower portions below the thermocline of rings with radii of 50 100 and 200 km are considered for rings of comparable strength the larger ones are found to have the stronger velocity footprint on the bottom the bbl is found to spin down this portion of rings by 60 in times 0 6 months with most of the energy transfer lost irretrievably to bottom friction for the larger rings and to potential energy for the smaller rings the classical constant eddy viscosity bbl formulation is found to yield similar spindown results as the model provided the eddy viscosity is chosen to give realistic bottom stress rather than bbl thickness the numerical model indicates that ekman pumping out of the bbl affects the bottom mixed layer thickness more so than the ekman pumping into the bbl and that maximum bottom mixed layer thicknesses are found not in regions of maximum flows but in upwelling regions of relatively strong flows because of the presence of the cold filament the hebble site is found to be unsuitable for testing the model conclusions about bottom layers

variability of clay mineral composition on carolina slope nw atlantic during marine isotope stages 1 3 and its paleoceanographic significance abstract the clay mineral composition of marine isotope stages 1 3 sediments at the ocean drilling program odp site 1055 on the carolina slope consists mainly of illite kaolinite chlorite and smectite clay mineral variability is marked by a distinct increase in the relative amounts of kaolinite and smectite during the holocene and by high illite and to a lesser extent chlorite relative amounts during marine isotopic stages 2 and 3 this grouping of clay minerals as two different assemblages during different isotopic stages suggest their different source affinities the increase in the kaolinite smectite assemblage in the holocene is accompanied by a significant increase in the sedimentation rate the change in the sedimentation regime after the last glacial maximum is interpreted to be related to resuspension and advection of clays and silts by increased deep water activity over the bermuda rise followed by their transport to site 1055 on the carolina slope by shallow elements of the north atlantic deep water high amplitude variations with high illite amounts characterize marine isotope stage 3 and appear to be related to heinrich events

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

reply to the discussion of enigmatic super reduced phases in corundum from natural rocks possible contamination from artificial abrasive materials or metallurgical slags by litasov et al lithos v 340 341 p 181 190 by w l griffin v toledo and s y o reilly