oxygen isotope analyses and pleistocene ice volumes abstract the oxygen isotope record from fossil foraminifera in deep sea sediments is commonly used as a proxy for global ice volume the linkage between 18 o and ice volume however is probably nonlinear we have developed a simple numerical model of the isotopic response of the oceans to ice volume change the major features it simulates are 1 the changing mean isotopic composition of snow as a function of ice volume colder snow temperatures forced by climate change and higher elevation accumulation areas imply more negative mean 18 o 2 the nonequilibrium isotopic composition of ice sheets the past history of an ice sheet is integrated into its mean isotopic composition which introduces a lag of isotopic ice volume i e the measured 18 o record scaled to ice volume units behind true ice volume 3 selective preservation of isotopically more negative colder higher latitude ice this geographic effect can selectively amplify or dampen the isotopic response to the ice volume signal we illustrate the response of our model to simple hypothetical ice volume transitions of ice growth and ice decay sensitivity tests are illustrated for all model parameters the results suggest that oxygen isotope records reproduce the general patterns of ice volume change fairly accurately the foraminiferal isotope record however may misrepresent the true amplitude of the ice volume signal and lag true ice volume by 1000 to 3000 yr

evidence of a collision between the yucatan block and mexico in the miocene we present the evidence for an anomalous southwest dipping slab in southern mexico the main evidence comes from a clear receiver function image along a seismic line across the isthmus of tehuantepec and is also supported by a previous global tomographic model the slab dips at 35 is approximately 250 km in length and appears to truncate the cocos slab at about 120 km depth we hypothesize that the slab was created by subduction of oceanic lithosphere prior to the collision of the yucatan block with mexico at approximately 12 ma this scenario would explain the chiapas fold and thrust belt as the product of this collision and its age constrains the date of the event to be in the miocene

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

alkenone molecular stratigraphy in an oceanic environment affected by glacial freshwater events the sedimentary record of a ratio uk37 of long chain c37 unsaturated alkenones is a useful indicator of glacial interglacial climatic change in the late quaternary northern gulf of mexico where a planktonic foraminiferal 18o caco3 record is complicated by meltwater and or fluvial events williams and kohl 1986 application of a laboratory temperature calibration of the uk37 ratio prahl and wakeham 1987 to a pigmy basin hydraulic piston core record deep sea drilling project core 619 suggested that the minimum glacial surface mixed layer sml temperature was 8 1 c colder than the holocene high sml temperature of 25 6 0 5 c this predicted glacial interglacial temperature difference was significantly larger than the differences predicted by either the foraminiferal 18o or foraminiferal assemblage temperature methods 0 8 2 0 c this large difference may be caused by local prymnesiophyte assemblage changes in response to climatically induced hydrographic changes interglacial periods may be dominated by pelagic prymnesiophyte assemblages while glacial periods may be dominated by neritic assemblages a combined mechanism of both climatically varying prymnesiophyte species assemblage and depth of alkenone biosynthesis may account for the large difference between the temperature methods

deglacial warming of the northeastern atlantic ocean correlation with the paleoclimatic evolution of the european continent abstract isotopic micropaleontologic and pollen analyses of deep sea cores from the bay of biscay and the northeastern atlantic ocean show that the deglacial warming of this oceanic area was closely correlated with the paleoclimatic evolution of the adjacent european continent temperatures were at least as warm as those of today in the bay of biscay between 13 300 and 11 000 b p coinciding with the combined bolling allerod warm continental events a major spread of polar water occurred between 11 000 and 10 000 b p during this event which coincides with the younger dryas continental cold event marine temperatures were almost as low as those of the last glacial maximum the final deglacial warming of the norteastern atlatntic ocean occurred during the following 3000 yr

spatial variations in stress within the 1987 whittier narrows california aftershock sequence new techniques and results the ml 5 9 whittier narrows california earthquake of october 1 1987 triggered a complex aftershock sequence the aftershocks had many different focal mechanisms including thrusting on e w striking north dipping planes similar to the main shock right lateral motion on nw se striking planes similar to the ml 5 3 aftershock on october 4 1987 thrusting on n s striking planes and left lateral motion on n s striking planes i attempt to interpret these mechanisms in terms of a stress field that may have caused them previously stress inversion methods based on focal mechanisms assumed that the stress field was spatially uniform in order to test for the spatially varying effect of the main shock s dislocation on the aftershocks focal mechanisms a new inversion technique was developed that solves for a uniform stress field that is superimposed on a given spatially varying stress field a series of simulations were performed to test both the new method and an older method that uses only the uniform component of the stress field these simulations show that both techniques work but will be limited by the amount of noise in the data and the accuracy to which the given spatially varying stress field resembles the actual spatially varying stresses most importantly the simulations show that the older methods do work when the stress field has a uniform component that is at least as large as the spatially varying component the simulations also show that when applying the older stress inversion techniques one can determine if the results are valid and the amount of spatially varying stresses present by examining the misfit between the data and the model application of these techniques to the whittier narrows aftershocks suggests that the uniform component of the stress field and the complex faulting corresponds to the response of an elastic halfspace to a simple regional n s compression the misfit between the data and the uniform model demonstrate the presence of spatially varying stresses at whittier narrows however their form as shown by the simulations can not be determined with this data set and these techniques

surface water temperature salinity and density changes in the northeast atlantic during the last 45 000 years heinrich events deep water formation and climatic rebounds we developed a new method to calculate sea surface salinities sss and densities ssd from planktonic foraminiferal 18o and sea surface temperatures sst as determined from planktonic foraminiferal species abundances sst sss and ssd records were calculated for the last 45 000 years for biogeochemical oceanic flux study bofs cores 5k and 8k recovered from the northeast atlantic the strongest feature is the dramatic drop in all three parameters during the heinrich ice rafting events we modelled the possibility of deepwater formation in die northeast atlantic from the ssd records by assuming that the surface waters at our sites cooled as they flowed further north comparison with modelled north atlantic deepwater densities indicates that there could have been periods of deepwater formation between 45 000 and 30 000 14c years b p interrupted by iceberg meltwater input of heinrich event 3 and 4 at 27 000 and 38 000 14c years b p and during the holocene no amount of cooling in the northeast atlantic between 30 000 and 13 000 years could cause deep water to form because of the low salinities resulting from the high meltwater inputs from icebergs our records indicate that after each heinrich event there were periods of climatic rebound with milder conditions persisting for up to 2000 years as indicated by the presence of warmer and more saline water masses after these warm periods conditions returned to average glacial levels these short term cold and warm episodes in the northeast atlantic are superimposed on the general trend towards colder conditions of the last glacial maximum lgm heinrich event 1 appears to be unique as it occurs as insolation rose and was coeval with the initial melting of the fennoscandian ice sheet we propose that meltwater input of heinrich event 1 significantly reduced north atlantic deep water formation reducing the heat exchange between the low and high latitudes thus delaying deglaciation by about 1500 radiocarbon years 2000 calendar years

slab break off abrupt cut or gradual detachment new insights from the vrancea region se carpathians romania slab break off is a plate tectonic process which does not only return lithospheric material into the deeper mantle but also has severe effects on surface movements and seismic hazard slab pull induced seismicity is reduced when the subducted slab decouples from the overlying crust in the vrancea region se carpathians strong earthquakes frequently occur at intermediate depths 70 180 km in a laterally small volume while the crust shows low but distributed seismicity the stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust both features indicate a decoupling between slab and overlying crust either by slab break off or delamination however the strong vertical elongation of the slab requires that the upper end of the slab is fixed vertically thus a soft coupling is assumed that is strong enough to enable elongation of the slab but weak enough to inhibit quasi static stress transfer to the overlying crust

determination of stress from slip data faults and folds a new technique is derived to invert slickenside data for the stress field that caused the faulting episode this inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes at the time of rupture is similar study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes comparison of the inversion results with the geometry of the folds shows this application to be successful greatly increasing the number of data sets that can be used to find the paleostress field

a reexamination of the bucaramanga colombia earthquake nest the bucaramanga earthquake nest is situated at a depth of about 160 km beneath colombia at 6 8 n 73 w and it produces about eight earthquakes of mb 4 7 or more each year from a source region having dimensions of about 10 km or less in this study we evaluate nest properties using reported locations focal mechanisms and moment tensors as well as relocations determined from teleseismic p pp and pkp arrivals we also introduce a novel method for determining the volume of a group of hypocenters and use this volume as a statistic for evaluating the accuracy of nest hypocenters we have relocated 26 nest earthquakes occurring between 1964 and 1986 using the jhd method and carefully reread teleseismic p pp and pkpdf arrivals the relocated earthquakes cluster within a region having a volume of 776 km3 and east west north south and vertical dimensions of 13 18 12 km a smaller region than any found previously from analysis of teleseismic data in practical terms this represents an approximate upper bound on the accuracy of relative earthquake locations possible using teleseismic data and standard relocation methods this compares with a volume of 11 km3 and dimensions of 5 1 3 4 3 8 km as determined by schneider et al 1987 from observations of microearthquakes collected over 16 days with a temporary local network these dimensions are comparable to those of the rupture dimensions of the largest observed nest earthquakes as determined from an analysis of their seismic moments 5 1024 dyne cm furthermore an evaluation of cumulative moment rate since 1964 indicates that strain rates within the nest volume must be between 10 13 sec and 10 11 sec the b values 2 0 and values 0 9 determined from available magnitudes and moments are remarkably high suggesting that earthquakes of all observed sizes 3 1020 to 5 1024 dyne cm contribute significantly to the moment release focal mechanisms determined from teleseismic observations exhibit a consistent pattern most having westward dipping p axes and eastward dipping t axes this differs from the more varied pattern reported by schneider et al 1987 for microseismic earthquakes

seasonal stable isotopic variations in living planktonic foraminifera from bermuda plankton tows abstract seasonal variations in surface water temperature and the 18o 16o ratios of calcite tests of living planktonic foraminifera collected in the upper 10 mare are coincident in the sargasso sea off bermuda the 18o of seasonally restricted globorotalia truncatulinoides and globigerinoides ruber pink and white may be used to determine the seasonal temperature contrast of the oceans in the past a preliminary model based on yearly absolute abundance and isotopic composition of these two species is able to approximate the composite 18o of fossil specimens of these species carbon and oxygen isotopic compositions of most species are size dependent with the largest differences occuring in 13c as a function of size the calcite tests of all living species were depleted in 13c relative to the 13c of total dissolved co2 at 0 and 50 m with no seasonal pattern in 13c being discernible in any species in almost all cases fossil specimens from box core sediments are significantly enriched in 13c and slightly enriched in 18o relative to their counterparts in the living fauna because the sediment populations are generally composed of larger sized specimens compares to the plankton of larger size specimens compared to the plankton

late quaternary paleochemistry of high latitude surface waters recent studies have stressed the role of high latitude nutrient levels and productivity in controlling the carbon isotopic composition of the deep sea and the co2 content of the atmosphere we undertook a study of the chemical composition of the polar planktonic foraminifer neogloboquadrina pachyderma s sinistral coiling from 30 late holocene samples and 49 down core records from the high latitude north and south atlantic oceans to evaluate the history of sea surface chemical change from glacial to interglacial time stable isotopic analysis of coretop samples from the atlantic pacific and southern oceans shows no significant correlation between the 13c of n pachyderma and either 13c or po4 in seawater conversely cd ca ratios in planktonic foraminifera are consistent with the po4 content of surface waters the level of maximum glaciation 18 000 yr b p identified by climap and 18o was chosen for mapping isopleths of 18o on n pachyderma s in the north atlantic reveal a pattern largely influenced by sea surface temperature s s t and generally support the s s t reconstruction of climap differences between the two suggest significantly lower salinity in north atlantic surface waters at high latitudes than in lower latitudes down core 13c records of n pachyderma confirm that low 13c values occurred in the northeast atlantic during the latest glacial maximum labeyrie and duplessy 1985 however a map of 13c for the 18 000 yr b p level for a much larger region in the north atlantic shows that minimum n pachyderma 13c occurred in temperate waters n pachyderma 13c decreased toward the southwest reaching a minimum of 1 at 37 n despite the variability seen in 13c records of n pachyderma none of our cores show significant temporal variability in cd ca from the combined cd ca and 13c data we can see no evidence for an upwelling gyre in the eastern north atlantic during the latest glacial maximum nor evidence that the southern and northern oceans had significantly different levels of preformed nutrients than today

petrochemistry of igneous rocks of the california vetas mining district santander colombia implications for northern andean tectonics and porphyry cu mo au metallogeny abstract porphyry mo and cu mineralization in the california vetas mining district is contemporaneous with 10 9 to 8 4 ma granodiorite porphyry stocks and overprinted by au ag mineralization of epithermal affinity mineralization is hosted by grenvillian aged paragneisses bucaramanga gneiss of the santander massif and late triassic to early jurassic granitic rocks all intrusive rocks are high k calc alkaline late triassic to early jurassic rocks include peraluminous granites with more than 70 wt sio2 as well as metaluminous diorites tonalites and granodiorites with sio2 between 54 9 and 60 4 wt late miocene rocks are weakly peraluminous granodiorite porphyries with sio2 between 61 and 67 wt sio2 late miocene rocks share some characteristics with adakite like rocks which are widely associated with porphyry and epithermal style mineralization elsewhere in the andes they have high ba 930 to 1500 ppm and high ba la 28 to 50 high sr 850 to 1100 ppm and sr y 48 78 and depleted middle rare earth elements mree compared to the mesozoic granites which have 400 to 700 ppm ba ba la 14 to 25 and 80 to 150 ppm sr sr y 2 5 to 14 and mesozoic diorites and tonalites which have 900 to 1200 ppm ba ba la 20 to 32 and 610 to 750 ppm sr sr y 22 to 25 miocene granodiorite porphyries in contrast to mesozoic intrusive rocks have only weak negative eu anomalies the miocene rocks have 87sr 86sr ratios of 0 7052 to 0 7067 and end of 1 9 to 5 4 and are significantly more isotopically primitive than all other rocks in the study area including the mesozoic diorites to tonalites 87sr 86sr 0 7082 and 0 7092 end 6 7 and 7 2 granites 87sr 86sr 0 730 n 2 end 8 2 and 8 3 and bucaramanga gneiss 0 718 to 0 743 end 10 8 to 14 1 lead isotope data are broadly consistent with the sr and nd isotope data and the miocene porphyries have the lowest 207pb 204pb ratios but overlap with the mesozoic diorites to tonalites in their 206pb 204pb and 208pb 204pb ratios the geochemical signature suggests that late miocene granodiorite porphyries represent oxidized and water rich melts however in contrast to many other porphyry districts in the andes they do not have low y

variations in slab dip along the subducting nazca plate as related to stress patterns and moment release of intermediate depth seismicity and to surface volcanism 1 the subducting nazca plate shows a high degree of along strike heterogeneity in terms of intermediate depth seismicity 70 300 km orientations of slab stress and volcanism we compile the intermediate depth earthquakes of south america from the harvard centroid moment tensor cmt catalogue to determine along strike dip variations and we explore the variable level of correlation between these observations primary results are three fold 1 consistency among orientation of tension axes highest level seismicity and occurrence of volcanism is associated with steeply dipping regions 20 30 2 volcanism and intermediate depth seismicity exhibit no direct correlation suggesting that earthquake faulting is not a necessary condition for transport of dehydrated fluids out to the mantle wedge for initiation of melting 3 comparison of slab thermal structures between regions with and without volcanism suggests that for certain wedge widths the maximum temperature in the mantle wedge is higher for the former than for the latter

volume and routing of late glacial runoff from the southern laurentide ice sheet melting of the laurentide ice sheet during the last deglaciation added large volumes of water to many rivers and lakes of north america and to the world s oceans the volume and routing of this meltwater not only helped shape the land s surface but also played a role in the evolution of late glacial climate a computerized model was prepared to quantify meltwater generation from seven drainage areas along the southern side of the laurentide ice sheet at 500 yr time slices between 14 000 and 8000 yr b p nearly all waters reaching the oceans flowed through the st lawrence hudson or mississippi river valleys discharge through the mississippi river to the gulf of mexico during late glacial time varied by more than a factor of 5 ranging between 17 400 m3 sec 1 550 km3 yr 1 and 98 200 m3 sec 1 3200 km3 yr 1 discharge entering the north atlantic ocean through the st lawrence and hudson valleys ranged between 20 300 m3 sec 1 640 km3 yr 1 and 65 300 m3 sec 1 2060 km3 yr 1 with very abrupt twofold changes at about 11 000 10 000 and 9500 yr b p as a result of the rerouting of water from the lake agassiz basin as the areal extent and mass of the laurentide ice sheet diminished the total volume of meltwater plus runoff due to precipitation from its southern side declined from 3800 km3 yr 1 at about 14 000 yr b p to 2100 to 2600 km3 yr 1 between 11 500 and 8000 yr b p no meltwater entered the gulf of mexico after 9500 yr b p after the demise of the ice sheet over hudson bay about 8000 yr b p the modern continental drainage network was established and flows through the st lawrence declined to modern values of about 320 km3 yr 1

upper and middle crustal velocity structure of the colombian andes from ambient noise tomography investigating subduction related magmatism in the overriding plate e p acknowledges support from the coordenacao de aperfeicoamento de pessoal de nivel superior capes through a 4 year scholarship to complete his ph d j j thanks cnpq for his research fellowship cnpq grant 308171 2012 8 m s acknowledges support by brazilian science without border program grant 40 2174 2012 7

evidence for a tropical freshwater spike during the last glacial interglacial transition in the venezuela basin δ18o and δ13c of calcareous plankton abstract oxygen and carbon isotope analysis of planktonic foraminifera and calcareous nannofossils from five box cores from the venezuela basin collected at depths between 3500 and 4800 m has revealed a detailed paleoglacial and paleoceanographic history for the last 18 000 yrs several climatic events have been recorded in these cores such as shift towards 18 o enriched values at about 13 000 yrs b p in planktic foraminifera which may be related to a sudden re advance of the northern hemisphere ice sheet and a tropical fluvial spike at about 8000 yrs b p during the completion of deglaciation comparison of carbon isotopes of nannofossils and globigerinoides sacculifer have revealed cyclical variations which has been correlated with changes in surface water circulation and productivity in the present day oceans cycles in surface water productivity have been found in the more northerly shallower cores analyzed these cycles are believed to be related to variations in intensity of upwelling during the last 18 000 yrs in the northern part of the basin the southern part of the basin had increased upwelling and productivity which resulted in increased carbonate deposition during glacial times during the present interglacial foraminifera show increased dissolution with depth but during the glacial period foraminifera are well preserved throughout the basin indicating increased watercolumn turnover rates nannofossils are well preserved during the glacial and interglacial periods analysis of the relationship of water depth to the preserved isotopic record of these two major calcareous plankton groups indicates that in this region nannofossils preserve a better record of surface water isotopic variations than planktonic foraminifera

potential productivity and surface circulation in the northern indian ocean during the last glacial to interglacial transition application of a new paleoceanographic model abstract a new paleoceanographic model is presented which approximates concentrations of total po 4 for the upper water column for the recent geologic past using the isotopes of carbon and oxygen from the tests of monospecific assemblages of planktic foraminifera these calculated concentrations of total po 4 are in turn used to estimate relative changes in potential productivity as well as changes in the position and intensity of surface currents for the recent geologic past this new model is based uponthe statistical constancy of the redfield o 2 po 4 ratio in the modern oceans and has been shown to be an accurate and reliable predictor of total po 4 the down core application of this model thus far limited to five core sites in the northern indian ocean has yielded results which are consistent with a number of other studies in characterizing the last glacial maximum in the northern indian ocean as a time of reduced potential productivity and weak surface currents in contrast by 9 ky b p the potential productivity and the surface circulation had increased dramatically in the northern indian ocean conditions in the holocene appear to be very similar to conditions which were in existence during the last glacial maximum the observed general trends in surface circulation and potential productivity during the last 18 000 years in the northern indian ocean suggest a milankovitch type astronomically induced changes in solar insolation orbital forcing

an insight into the bucaramanga nest abstract we used the local seismicity for the period of 1993 to 2001 in the northeast of colombia to show the existence of two slabs in the north and south of the bucaramanga nest the northern slab has a dip angle of about 25 and the southern slab has a 50 dip angle while the dip in the bucaramanga nest is about 29 in order to explain the nature of the bucaramanga nest we proposed the scenario of collision between these two slabs using a 3d finite element model fem we show that collision can concentrate modify and perturb the stress field the active process of dehydration embrittlement at intermediate depths and the concentrated stress field in the collision zone may explain the high rate of seismic activity inside the bucaramanga nest the perturbed and modified stress field resulting from the simultaneous effect of collision between two subducted slabs and subduction of the lithosphere under its own weight can explain the variation in the focal mechanism of micro earthquakes and the complexity in the source of the moderate size earthquakes in the bucaramanga nest

source complexity of the 1988 armenian earthquake evidence for a slow after slip event we analyzed teleseismic p and s waves using a multiple event deconvolution method to investigate the source process of the spitak armenia earthquake of december 7 1988 teleseismic long period body waves exhibit complex waveforms significantly more complex than those normally seen for an event of this size we identified two groups of subevents one is a group of strike slip events during the first 20 s the other is a dip slip event initiating at about 30 s after the initial rupture the tensor sum of all the subevents is essentially strike slip strike 302 dip 59 rake 143 with a scalar moment of 1 47 10 19 n m the seismic moment of the dip slip event strike 89 dip 60 rake 88 is 5 6 10 18 n m nearly 40 of the total and the moment release rate is slower than the other subevents with a strike slip mechanism we call this event a slow after slip event the mechanism of the after sup event is consistent with the present day deformation pattern in the caucasus where north south compression is predominant due to the continental collision between the arabian plate and the russian platform this result suggests that in a region where both strike slip motion and crustal shortening are taking place like southern california a large strike slip event may accompany large thrust events and vice versa in order to determine the total coseismic deformation pattern it is essential to resolve these multiple events in time space and mechanism

the last deglaciation event in the eastern central arctic ocean oxygen isotope records of cores from the central arctic ocean yield evidence for a major influx of meltwater at the beginning of the last deglaciation 15 7 thousand years ago 16 650 calendar years b c the almost parallel trends of the isotope records from the arctic ocean the fram strait and the east greenland continental margin suggest contemporaneous variations of the eurasian arctic and greenland laurentide ice sheets or increased export of low saline waters from the arctic within the east greenland current during the last deglaciation on the basis of isotope and carbon data the modern surface and deep water characteristics and seasonally open ice conditions with increased surface water productivity were established in the central arctic at the end of termination lb about 7 2 thousand years ago or 6 000 calendar years b c

clay mineral distribution patterns as influenced by depositional processes in the southeastern levantine sea analysis of the clay minerals in stratigraphically defined cores 23 000 years bp to the present in the southeastern levantine nile cone sector of the eastern mediterranean indicates that depositional processes are very significant in determining the distribution of clay assemblages the interplay of long distance transport by water mass circulation downslope mass gravity transport and wind dispersal is recorded by the clay assemblages in each of the stratigraphic layers the temporal variations and spatial distribution of smectite the dominant clay mineral of the river nile can be related closely to downslope gravity related processes and to deposition from water mass flow increased kaolinite in part of wind blown origin from north africa correlates with areas receiving low terrigenous input but influenced by enhanced suspended sediment transport illite and chlorite distributions are most closely associated with a northern levantine provenance and dispersal by the circulation of eastern mediterranean water masses climatically induced changes may have altered the clay minerals in the region and minor diagenetic changes may have occurred but these factors do not fully explain observed vertical clay mineral changes in the late quaternary we conclude that palaeoclimatic interpretations based on vertical clay mineralogical changes at single core localities should be approached cautiously in small ocean basins such as the mediterranean

tectonic implications of tomographic images of subducted lithosphere beneath northwestern south america we used seismic tomography to investigate the complex structure of the upper mantle below northwestern south america images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted caribbean pacific lithosphere beneath the study area beneath western venezuela and colombia we distinguish two slabs a maracaibo and a redefined bucaramanga slab the maracaibo slab coinciding with most of the bucaramanga slab previously defined by w d pennington dips in a direction of 150 at an angle of 17 to a depth of 275 km and correlates to the subducted late cretaceous oceanic plateau of the caribbean plate farther south a second slab dips at an angle of 50 in a direction of 125 to a depth of at least 500 km and correlates to the subducted oceanic crust of the nazca plate and the downdip extension of the panama island arc we refer to this slab as the redefined bucaramanga slab because it is different from the bucaramanga slab segment defined by pennington the area of the south american plate overriding both slabs is characterized by the absence of an active volcanic arc an anomalously wide topographically uplifted and tectonically active area and the northward escape of the maracaibo block along active strike slip faults in support of earlier studies we attribute this to the underthrusting of the caribbean oceanic plateau our shallowly dipping maracaibo slab along the base of the south american lithosphere and to the recent collision of the panama island arc rafted in on more steeply dipping crust of the nazca plate our redefined bucaramanga slab

transient slab flattening beneath colombia subduction of the nazca and caribbean plates beneath northwestern colombia is seen in two distinct wadati benioff zones one associated with a flat slab to the north and one associated with normal subduction south of 5 5 n the normal subduction region is characterized by an active arc whereas the flat slab region has no known holocene volcanism we analyze volcanic patterns over the past 14 ma to show that in the mid miocene a continuous arc extended up to 7 n indicating normal subduction of the nazca plate all along colombia s pacific margin however by 6 ma we find a complete cessation of this arc north of 3 n indicating the presence of a far more laterally extensive flat slab than at present volcanism did not resume between 3 n and 6 n until after 4 ma consistent with lateral tearing and re steepening of the southern portion of the colombian flat slab at that time

role of glacial arctic ocean ice sheets in pleistocene oxygen isotope and sea level records abstract a comparison of the oxygen isotope signal in deep sea benthic foraminifera with the record of glacio eustatic sea level for the last 160 000 years reveals that the amplitude of the benthic 18 o records predicts more continental ice volume than appears to be reflected in lowered sea level stands these differences between the benthic 18 o ice volume estimates and radiometrically dated records of eustatic sea level are consistent with the presence of a large floating arctic ocean ice mass during glacial intervals the presence of an arctic ocean ice sheet during glacial intervals may account for the two climatic modes observed in oxygen isotope records which span the entire pleistocene the early pleistocene 1 8 to 0 9 myr b p interval is characterized by low amplitude high frequency 18 o fluctuations between glacial and interglacial periods while the late pleistocene 0 9 myr b p to present is characterized by large amplitude low frequency 18 o changes these two climatic modes can be explained by the initiation of earth orbital conditions favoring the co occurrence of glacial period arctic ocean ice sheets and large continental ice sheets approximately 900 000 years before present

tectonic implications of tomographic images of subducted lithosphere beneath northwestern south america tomographic images and deep seismicity of the mantle beneath northwestern south america support the existence of extensive and continuous subducted slabs which may be important controls on the neotectonic deformation of the overriding south america plate using tomographic data we have estimated lateral continuity regional dip downdip length and approximate convergence direction of two largely aseismic slabs tomographic data suggests that the cauca slab previously defined using earthquakes beneath southern colombia extends 300 km into the mantle beneath northern colombia has an average dip of 50 degrees and is the downdip extension of eocene to miocene panama island arc and oligocene miocene oceanic crust of the nazca plate the maracaibo slab of northern colombia and western venezuela extends up to 450 km into the mantle at an average angle of 16 degrees and is the downdip extension of late cretaceous oceanic plateau crust of the caribbean sea the boundary between the two slabs is less than 50 km in width the maracaibo and northern cauca slabs are generally aseismic and not associated with an overlying volcanic arc instead both slabs are overlain by broad zones of diffuse active faulting and topography in the northern andes the shallow dip angle of oblique subduction andmore thickness of the maracaibo slab may produce crustal deformation including northward tectonic escape of the maracaibo block a triangular wedge of continental crust in the overriding south america plate less

low radiation efficiency of the intermediate depth earthquakes in the japan subduction zone robust determination of earthquake source parameters over a continuous depth range is central to inferring rupture mechanisms dominant at different depths we employed a cluster event method to constrain the source parameters as well as along path attenuation for earthquakes over 0 150 km depths and 4 orders of seismic moments in the japan subduction zone we found that corner frequency and stress drop increase with depth whereas the radiated energy scaled by seismic moment declines with depth slightly as a result the radiation efficiency exhibits a notable deficit for events deeper than 60 km together these suggest an increased energy dissipation during faulting in ductile deformation regime consistent with shear heating instability as an important faulting mechanism for intermediate depth earthquakes

oceanic mechanisms for amplification of the 23 000 year ice volume cycle situated adjacent to the largest northern hemispher ice sheets of the ice ages the mid latitude north atlantic ocean has an important role in the earth s climate history it provides a significant local source of moisture for the atmosphere and adjacent continents forms a corridor that guides moisture bearing storms northward from low latitudes and at times makes direct contact along its shorelines with continental ice masses evidence of major ice ocean air interactions involving the north atlantic during the last 250 000 years is summarized outflow of icebergs and meltwater initially driven by summer insolation over the ice sheets affects midlatitude ocean temperatures summer heat storage winter sea ice extent and global sea level these oceanic responses in turn influence the winter moisture flux back to the ice sheets as well as ablation of land ice by calving spectral data indicate that the oceanic moisture and sea level feedbacks in part controlled by glacial melt products amplify milankovitch insolation forcing of the volumetrically dominant mid latitude ice sheets at the 23 000 year precessional cycle

modeling earthquake cycles in the shumagin subduction segment alaska with seismic and geodetic constraints deformation associated with the earthquake cycle in the shumagin islands segment of the alaska aleutian subduction zone is analyzed with the use of a two dimensional finite element model the model consists of an oceanic plate dipping under an upper plate both of which respond elastically to stress fluctuations in the earthquake cycle and these are underlain by asthenospheric mantle and mantle wedge regions which respond viscoelastically it is tailored to the geometry of the shumagin islands region by using seismicity to define the position of the interplate interface and partially coupled region along it the model is preconditioned by forcing this interface to undergo periodically repeated slips up to and including the time of the may 31 1917 event ms 7 4 in that region with each chosen to be consistent with the moment and estimated rupture area of that event we investigated the dependence of model results for geodetic signals on the strength of seismic coupling between the plates and viscoelastic relaxation of deviatoric stresses in the mantle including in the mantle wedge close to the plate junction and along the aseismic downdip continuation of the thrust interface in models with significant relaxation in the wedge or downdip thrust zone results show that as the intraseismic stage matures there is a region of diminished compressional strain rates and even of locally extensional rates on the earth s surface above the downdip end of the seismically coupled zone based on the seismic estimates of the location of the coupled zone this region is in the area of the shumagin islands we find that if approximately 20 of the convergence takes place seismically compatible with the previous seismic history and if an extensive region of relaxed deviatoric stress is assumed to be present in the wedge and or along the downdip interface then deformations predicted by the model can be made consistent with the measured strain data from the shumagin islands geodetic network lisowski et al 1988 larson and lisowski 1994 as well as uplift and tilt data savage and plafker 1991 beavan 1992 our model simulations here suggest that the shumagin segment is capable of large earthquakes the hypothesis of totally aseismic subduction is not similarly consistent with all geodetic constraints

new faunal and isotopic evidence on the late weichselian holocene oceanographic changes in the norwegian sea downcore studies of planktonic and benthonic foraminifera and 18o and 13c in the planktonic foraminifer neogloboquadrina pachyderma sin in two piston cores from the southern part of the norwegian sea suggest large changes in the oceanic circulation pattern at the end of oxygenisotope stage 2 and in the early part of stage 1 prior to oxygen isotope termination ia 16 000 13 000 yr b p an isolated watermass with lower oxygen content and temperature warmer than today existed below a low salinity ice covered surface layer in the norwegian sea close to termination ia well oxygenated deep water probably with positive temperatures was introduced this deep water which must have had physical and or chemical parameters different from those of present deep water in the norwegian sea could have been introduced from the north atlantic or been formed within the basin by another mechanism than that which forms the present deep water of the norwegian sea a seasonal ice cover in the southern part of the norwegian sea is proposed for the period between termination ia and the beginning of ib close to 10 000 yr b p the present situation with strong influx of warm atlantic surface water and deep water formation by surface cooling was established at termination ib

comparison of foraminiferal coccolithophorid and radiolarian paleotemperature equations assemblage coherency and estimate concordancy the imbrie kipp method of paleotemperature estimation is rigorously tested by comparing atlantic temperature equations independently derived from the microfossils of three biotic groups the foraminifera coccolithophorida and radiolaria this method consists of two steps factor analysis of the modern sea bed data of the individual groups which resolves discrete biogeographic assemblages and regression analysis of the modern assemblage data with observed sea surface temperature data to obtain paleotemperature equations assemblage biogeography shows a simple subdivision into warm low latitude and cold high latitude for all biotic groups between biotic groups there is greater similarity among high latitude assemblages than low latitude ones correlating the assemblage data with observed sea surface temperatures to produce temperature distribution patterns shows differences of less than 2 c in their optimum and critical temperatures regression analysis produced accurate temperature equations for each biotic group all with standard errors of estimate of less than or equal to 2 c multiple correlation coefficients were all greater than 0 970 applying these equations to two multiple biotic data sets the modern and ice age sea bed data and comparing their temperature estimates using the standard error pooled shows over 87 concordancy for both data sets unlike the modern data the discordancy among temperature estimates of the ice age data shows a distinct geographic distribution its cause is believed to be oceanographic a difference in the water mass structure between the modern and ice age ocean

modelling the younger dryas abstract an energy balance climate model ebcm having land sea surface air and latitudinal resolution and which simulates the seasonal growth and decay of sea ice in terms of a number of explicit physical processes is used to test the following hypotheses for the cause of the younger dryas cold episode 1 a low salinity meltwater lens from ablation of continental ice sheets which would have increased the freezing point and stability of the oceanic mixed layer thereby allowing more extensive sea ice 2 a flood of icebergs into the north atlantic from collapse of continental ice sheets or of a hypothetical arctic ocean ice shelf 3 a reduction or cessation in the rate of formation of north atlantic deep water nadw which would have led to a reduction in the northward oceanic heat flux in the north atlantic ocean and 4 a reduction in atmospheric co 2 concentration a low salinity meltwater lens leads to zonally averaged atmospheric cooling over land of up to 2 5 c in winter but warming of up to 1 5 c in summer temperature effects outside the latitudes of hypothesized low salinity meltwater are negligible an iceberg flood covering 30 of the ocean between latitudes 45 and 70 n causes a winter atmospheric cooling over land of up to 8 c and a summer cooling of up to 4 c in the northern hemisphere but maximum winter cooling at high southern hemisphere latitudes of only 1 3 c if the icebergs are initially 100 m thick and allowed to ablate without replenishment then the icebergs completely disappear in only 20 50 years and negligible temperature response occurs in the southern hemisphere a reduction in the northward meridional oceanic heat flux by 0 6 10 15 w at 45 n leads to maximum winter and summer atmospheric coolings over land of 5 6 c and 1 2 c respectively at high latitudes in the northern hemisphere but a warming at low latitudes and in the southern hemisphere however a reduction in the rate of nadw formation would also lead to a reduction in the upward heat flux at the base of the mixed layer at high latitudes in the southern hemisphere this is a parameter of the model used here and reducing its value by 20 causes a mean annual temperature cooling of up to 3 3 c in the southern hemisphere a co 2 decrease from 300 ppmv to 250 ppmv causes a global mean cooling of about 0 7 c and maximum mean annual atmospheric temperature responses of about 1 c in both hemispheres with greater response in winter this study identifies a number of specific types of paleoclimatic data which can help in discriminating among different hypothesized causes of the younger dryas in particular a seasonal patterns of temperature change b differences between high and low latitude and between northern hemisphere and southern hemisphere temperature changes as well as regional patterns of climatic change c phase lags of temperature change between different regions and d a more precise determination of the duration of the younger dryas in various localities

multiple plates subducting beneath colombia as illuminated by seismicity and velocity from the joint inversion of seismic and gravity data abstract subduction beneath the northernmost andes in colombia is complex based on seismicity distributions multiple segments of slab appear to be subducting and arc volcanism ceases north of 5 n here we illuminate the subduction system through hypocentral relocations and vp and vs models resulting from the joint inversion of local body wave arrivals surface wave dispersion measurements and gravity data the simultaneous use of multiple data types takes advantage of the differing sensitivities of each data type resulting in velocity models that have improved resolution at both shallower and deeper depths than would result from traditional travel time tomography alone the relocated earthquake dataset and velocity model clearly indicate a tear in the nazca slab at 5 n corresponding to a 250 km shift in slab seismicity and the termination of arc volcanism north of this tear the slab is flat and it comprises slabs of two sources the nazca and caribbean plates the bucaramanga nest a small region of among the most intense intermediate depth seismicity globally is associated with the boundary between these two plates and possibly with a zone of melting or elevated water content based on reduced vp and increased vp vs we also use relocated seismicity to identify two new faults in the south american plate one related to plate convergence and one highlighted by induced seismicity

linking late cretaceous to eocene tectonostratigraphy of the san jacinto fold belt of nw colombia with caribbean plateau collision and flat subduction collision with and subduction of an oceanic plateau is a rare and transient process that usually leaves an indirect imprint only through a tectono stratigraphic analysis of pre oligocene sequences in the san jacinto fold belt of northern colombia we show the late cretaceous to eocene tectonic evolution of northwestern south america upon collision and ongoing subduction with the caribbean plate we linked the deposition of four forearc basin sequences to specific collision subduction stages and related their bounding unconformities to major tectonic episodes the upper cretaceous cansona sequence was deposited in a marine forearc setting in which the caribbean plate was being subducted beneath northwestern south america producing contemporaneous magmatism in the present day lower magdalena valley basin coeval strike slip faulting by the romeral wrench fault system accommodated right lateral displacement due to oblique convergence in latest cretaceous times the caribbean plateau collided with south america marking a change to more terrestrially influenced marine environments characteristic of the upper paleocene to lower eocene san cayetano sequence also deposited in a forearc setting with an active volcanic arc a lower to middle eocene angular unconformity at the top of the san cayetano sequence the termination of the activity of the romeral fault system and the cessation of arc magmatism are interpreted to indicate the onset of low angle subduction of the thick and buoyant caribbean plateau beneath south america which occurred between 56 and 43 ma flat subduction of the plateau has continued to the present and would be the main cause of amagmatic post eocene deposition

out of our depth on the impossibility of fathoming eustasy from the stratigraphic record the evolution of a sedimentary basin s gross morphology depends upon 1 basement movement 2 sediment accumulation and compaction and 3 variations of eustasy with time thus if an accurate history of eustasy could be derived it would aid in our understanding of basin stratigraphy and help in the prediction of sediment geometry from described areas techniques which attempt to determine the magnitude of eustatic sea level excursions include the measurement of 1 the amount of sedimentary onlap onto the continental margins with and without the use of hypsometric curves 2 the thickness of marine sedimentary cycles and the elevation and distance between indicators of old strandlines 3 the perturbations on individual thermo tectonic subsidence curves and stacked crustal subsidence curves 4 the variations in deep ocean oxygen isotopes found in sediments and 5 the size of variables used in graphical and numerical simulations of basin fill in terms of tectonic behavior rates of sediment accumulation and eustasy which invert the problem to date a combination of the use of relative sea level charts derived from sediment onlap of the continental margin with dimensioning by oxygen isotopes responding to glacial events offers the best potential for relative tectono eustatic sea level curves but even this method can not produce a unique solution for absolute eustatic variations mathematical modelling shows that at best it is possible to obtain only the sum of tectonic basement subsidence and sea level variations from the above methods and at worst not even that simple a combination thus every proposed scheme to measure eustatic sea level excursions assumes some behavior for two of the three underlying processes tectonic movement of the basement sedimentary accumulation and eustatic sea level each scheme then determines the third process relative to the assumed model behavior of the other two it would seem that like king canute we cannot command the sea though we can still use undifferentiated relative tectono eustatic sea level curves to generate a family of solutions which are the product of a variety of absolute sea level curves and tectonic models each solution has then to be assessed in terms of geologic setting and the hypotheses it generates

stress rotation during the coalinga aftershock sequence this study considers spatial and temporal changes of the stress regime during the 1983 coalinga aftershock sequence in both cases the observed changes manifest themselves as rotations of the most compressive stress axis inversion of 165 m 3 aftershocks shows that in the southern aftershock zone the azimuth of the most compressional stress axis is oriented 24 e of north while in the northern and central zones it is oriented 51 e of north statistical analysis by use of non parametric bootstrap resampling shows that this rotation is significant at above the 95 confidence level inverting the focal mechanisms of 122 m 3 aftershocks in the central and northern aftershock zones reveals a rotation of the principal stress axes during the first 500 days after the main shock the compressional axis rotates from an azimuth of 62 e of north to 47 e of north statistical analysis by use of nonparametric bootstrap resampling shows that this rotation is just significantly different from zero at the 95 confidence level subject to the assumption that 80 of the fault planes can be correctly picked from the two nodal planes as the number of fault planes picked correctly varies from 50 random choices to 100 the significance of the rotation varies from 87 to 98 further investigation fails to find the signature of the stress changes predicted by dislocation theory in the focal mechanisms of the aftershocks active folds in the area suggest that the pre main shock stress regime had a compressional axis with a ne sw orientation thus the rotation during the aftershock sequence is thought to be a rebound from a stress anomaly induced by the main shock this stress anomaly may be causally related to the occurrence of the aftershocks

after the deluge mediterranean stagnation and sapropel formation in an east mediterranean marine core the upper sapropel begins soon after the start of a global event a very heavy precipitation which occurred in the equatorial latitudes during the late glacial early holocene this heavy precipitation in africa channelled by the nile river across 35 of latitude produced a low salinity surface layer in the east mediterranean in this confined basin with high bottom salinity the steep salinity gradient stratified the water column the stagnant bottom waters triggered the sapropel formation cretaceous sapropels in the tropical oceans may result from the same chain of events in warm humid climates with contrasting wet and dry seasonal rhythm

active tectonics of the pamirs and karakorum the source parameters of moderate to large sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three dimensional configuration of the subducted lithosphere in the pamir karakorum intracontinental convergence zone the continental collision has resulted in intense deformation in the upper crust the deformation is distributed over a broad area and is absorbed by thrust and strike slip faulting along the northern margin of the pamirs where the deformation due to continental collision is most intense the northern pamir thrust and several major fault zones mark the present boundary most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region along the pamir front thrust faulting dominates while on the western and eastern edges of the pamirs the style of deformation is characterized by oblique thrusting with a component of strike slip motion thrust type events beneath the tadjik depression indicate that both the sedimentary rocks and the basement are involved in shortening right lateral strike slip motion is observed on the eastern edge of the pamirs where they border the tarim basin and in the talas fergana kun lun fault zone a southward dipping seismic zone beneath the pamirs and karakorum indicates that the asian lithosphere has been subducted along the pamir front to a depth of at least 200 km this interpretation is consistent with published geological and geophysical data focal mechanism solutions of some intermediate depth events beneath the pamirs show strike slip faulting with approximately n s horizontally oriented p axes indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation a 90 km deep event beneath the karakorum is interpreted as occurring in the subducted asian lithosphere the fact that the p axis of this event is oriented parallel to the descending plate suggests downdip compression within the asian lithosphere the overall tectonics of the region is interpreted as a consequence of the underthrusting asian lithosphere being impinged upon by the shallow northward underthrusting indian lithosphere

late quaternary stage 2 and 3 meltwater and heinrich events northwest labrador sea two major meltwater events are documented in cores from the nw labrador sea one occurred ca 20 000 14c yr b p in association with deposition of a major detrital carbonate unit both prior to and after this event 18o values of near surface planktonic foraminifera were 4 5 indicating fully enriched glacial values a younger event ca 14 000 14 c yr b p is characterized by a dramatic change in 18o from 4 5 to 2 0 and coincided with the retreat of ice from the outer se baffin shelf possibly into hudson strait these meltwater events coincide with heinrich h layers 1 and 2 from north atlantic sediments the 14 000 14c yr b p meltwater event indicates that the eastern margin of the laurentide ice sheet also underwent rapid retreat at approximately the same time as other ice sheet margins around the ne north atlantic a third major detrital carbonate event at the base of hu87 033 009 possibly correlative with heinrich layer 3 occurred ca 33 960 675 14 c yr b p however this is older than the accepted date for h 3 of 27 000 14c yr b p and may be h 4

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

coseismic stress changes induced by the 1989 loma prieta california earthquake earthquake focal mechanisms from before and after the 1989 loma prieta california earthquake are used to infer the coseismic stress change before the main shock most earthquakes correspond to right lateral slip on planes sub parallel to the san andreas fault and imply a generally n s most compressional stress axis and a vertical intermediate stress axis aftershocks within the main shock rupture zone however display almost every style and orientation of faulting implying an extremely heterogeneous stress field this suggests that the main shock relieved most if not all of the shear stress acting on its fault plane aftershocks that lie on the perimeter of the rupture agree with spatially uniform stress states but only when considered in three groups north south and above the main shock rupture area in each of these areas the stress state may reflect stress transfer by the main shock

subduction system and flat slab beneath the eastern cordillera of colombia seismicity at the northern terminus of the nazca subduction is diffused over a wide area containing the puzzling seismic feature known as the bucaramanga nest we relocate about 5000 earthquakes recorded by the colombian national seismic network and produce the first 3 d velocity model of the area to define the geometry of the lithosphere subducting below the colombian andes we found lateral velocity heterogeneities and an abrupt offset of the wadati benioff zone at 5 n indicating that the nazca plate is segmented by an e w slab tear that separates a steeper nazca segment to the south from a flat subduction to the north the flat nazca slab extends eastward for about 400 km before dip increases to 50 beneath the eastern cordillera where it yields the bucaramanga nest we explain this puzzling locus of intermediate depth seismicity located beneath the eastern cordillera of colombia as due to a massive dehydration and eclogitization of a thickened oceanic crust we relate the flat subducting geometry to the entrance at the trench at ca 10 ma of a thick buoyant oceanic crust likely a volcanic ridge producing a high coupling with the overriding plate sub horizontal plate subduction is consistent with the abrupt disappearance of volcanism in the andes of south america at latitudes 5 n

current states of stress in the northern andes as indicated by focal mechanisms of earthquakes abstract systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern andes reveals relatively homogeneous patterns of crustal stress in three main regions the first region presently under the influence of the caribbean plate includes the northern segment of the eastern cordillera of colombia and the western flank of the central cordillera north of 4 n it is characterized by wnw ese compression of dominantly reverse type that deflects to nw se in the merida andes of venezuela where it becomes mainly strike slip in type a major bend of the eastern thrust front of the eastern cordillera near its junction with the merida andes coincides with a local deflection of the stress regime sw ne compression suggesting local accommodation of the thrust belt to a rigid indenter in this area the second region includes the sw pacific coast of colombia and ecuador currently under the influence of the nazca plate in this area approximately e w compression is mainly reverse in type it deflects to wsw ene in the northern andes south of 4 n where it is accommodated by right lateral displacement of the romeral fault complex and the eastern front of the northern andes the third and most complex region is the area of the triple junction between the south american nazca and caribbean plates it reveals two major stress regimes both mainly strike slip in type the first regime involves sw ne compression related to the interaction between the nazca and caribbean plates and the panama micro plate typically accommodated in an e w left lateral shear zone the second regime involves nw se compression mainly related to the interaction between the caribbean plate and the north andes block which induces left lateral displacement on the uramita and romeral faults north of 4 n deep seismicity about 150 170 km concentrates in the bucaramanga nest and cauca valley areas the inversion reveals a rather homogeneous attitude of the minimum stress axis which dips towards the e this extension is consistent with the present plunge of the nazca and caribbean slabs suggesting that a broken slab may be torn under gravitational stresses in the bucaramanga nest this model is compatible with current blocking of the subduction in the western northern andes inhibiting the eastward displacement of slabs which are forced to break and sink in to the asthenosphere under their own weight

deep root of andesitic volcanoes new evidence of magma generation at depth in the benioff zone abstract the association of andesitic volcanoes with island arcs and continental margins is well known and the close spatial coincidence of these volcanoes with inclined deep seismic zones suggests a direct relation of cause and effect between earthquakes at depth 100 300 km beneath the recent volcanoes and their activities since 1962 the author has drawn attention to the temporal correlation between earthquakes at depth beneath volcanoes and their eruptions as observed in the new hebrides and other island arc regions to test these time correlations attempts have been made to predict volcanic eruptions in the new hebrides and in new zealand from 1970 until 1977 the author has supplied predictions to be tested against the records of volcanic activity collected in the new hebrides by the geological survey a significant number of successful eruption predictions 65 notably for ambrym lopevi and karua volcanoes has emerged activity at these volcanoes is actually correlated to specific intermediate depth earthquakes within nests which are located around aseismic gaps seated straight beneath these volcanoes in 1977 and 1978 investigation carried out in new zealand particularly on white island volcano shows a similar correlation between occurrence of subcrustal earthquakes and volcanic activities crater floor level changes on white island are also correlated with seismic activity at depth beneath the volcano successful tests of prediction of recent eruptions at white island using a simple relation with the depth of earthquakes get confirmation of a deep root of andesitic volcanoes which is seated in the benioff zone and where seismic activity is related to magma generation

microseismicity and focal mechanisms of the intermediate depth bucaramanga nest colombia the bucaramanga nest of intermediate depth seismicity in northern colombia was investigated from microearthquakes m sub b less than or equal to4 3 recorded from june 20 to july 6 1979 by a local array of 12 digital three component seismographs of 161 events studied 142 are from the nest and 19 define a thin wadati benioff w b zone mainly from 110 to 190 km depth the w b zone part of the bucaramanga segment strikes n10 sup 0 e with a dip increasing wtih depth from 30 sup 0 e to near vertical beneath the nest the 89 best located events from within the nest define a volume of about 8 km nw by 4 km ne by 4 km depth centered at 161 km depth with relative location accuracies of 0 5 to 1 0 km double couple fault plane solutions of 59 events were determined from an objective procedure using p wave polarities and s p amplitude ratios there is no dominant trend in focal mechanisms nor any reasonable correlation with regional stress patterns or previously determined mechanisms from teleseismic data the subduction and assimilation of a linear feature such as an island arc undergoing a phase transformation may explain the existence of the bucaramanga nest however the extraordinarilymore high rate of seismicity suggests that the nest may be moving rate several orders of magnitude faster than the rate of relative plate motion alone the intense seismicity and variation in focal mechanisms suggest that the nest represents some aspect of stress release associated with the ascent of fluids perhaps brine or magma that will ultimately result in the initiation of volcanic activity over this recently emplaced w b zone copyright american geophysical union 1987 less

stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle an elementary mechanical model of a coupled subduction zone is presented it is used to study the transfer of extensional stresses along the underthrusting plate and shear stress interactions between the plate itself and its thrust contact zone with the overriding plate during the earthquake cycle the model assumes an elastic plate which deforms one dimensionally has shear drag interactions treated approximately by the elsasser procedure with a maxwellian viscoelastic mantle and is subject to steady gravitational sinking forces great thrust earthquakes are simulated as sudden stress relieving displacements at the contact zone between the slab and overriding plate the model allows analysis of the space and time variable stresses in the main thrust zone the adjacent oceanic plate and the subducting slab over each earthquake cycle stress variations leading to outer rise events are discussed as well as stress changes responsible for intraplate seismicity downdip from the main thrust zone based on simulations of periodic great earthquakes the results of simulated slow progression of deformation slip from below into the thrust contact zone are also presented such preslip occurring over a time scale that is small in comparison with the whole seismic cycle e g of the order of a tenth or so is shown to transiently slow the rate of buildup of or even relieve extensional stresses in the slab in the absence of preslip those stresses build up steadily in time during the latter part of the cycle the results have implications for induction of sesimic quiescence as well as for explaining patterns of seismicity in relation to spatial position along the underthrusting plate and to time throughout the cycle correlations are reported between the model simulations and seismicity observations from coupled subduction zones around the world the observations include tensional outer rise events after large subduction earthquakes and compressional ones in the latter parts of the cycle normal tensional intraplate earthquakes preceding the main thrust events and positioned downdip from the thrust zones and interlaced periods of seismic activity in the lower part of the main thrust zone and in the descending slab

early deglaciation of the greenland sea during the last glacial to interglacial transition abstract isotopic and sedimentological analysis of a core from the southern greenland sea northwest of the present polar front exhibits a high input of ice rafted acidic continental material between 9000 and 18 000 yrs b p replacing basic icelandic products which are commonly found before and after that period this indicates that summer melting of sea ice in the norwegian greenland basin began as early as 18 000 b p permitting the southward drift of icebergs produced by rapid ablation of northern ice sheets

cyclic stressing and seismicity at strongly coupled subduction zones we use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle deformation is assumed to be uniform along strike plane strain on a cross section normal to the trench axis and periodic earthquake slip is imposed consistent with the long term rate of plate convergence and degree of coupling simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field the oceanic plate descending slab and continental lithosphere are assumed here to respond elastically to these fluctuations and the remaining mantle under and between plates is assumed to respond as maxwell viscoelastic in the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event in particular trench normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone compressional earthquakes when they occur at all have the opposite pattern our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface the second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone with the strain rate constant between mainshocks however with significant relaxation present a localized region of unusually low compressional or even slightly extensional strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone the low strain rate starts in the middle or late part of the cycle depending on position this result suggests that the negligible or small contraction measured on the shumagin islands alaska during 1980 to 1991 may not invalidate an interpretation of that region as being a moderately coupled subduction zone in contrast mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate even when the maxwell time is an order of magnitude less than the recurrence interval

flexure of subducted slabs the subducted lithosphere is regarded as a thin elastic plate that bends as a consequence of slab pull the pressure of the asthenospheric flow induced by the subduction motion and the pressure exerted by the asthenospheric motion relative to the lithosphere in westward subductions the latter factor enhances the slab pull but in eastward subductions it opposes it as a result the subduction angle changes continuously with depth following an elastic profile it is smaller in eastward subductions and larger in those having a westward direction the application of the model to 13 subducted slabs shows a good fit between the observed and the calculated shapes of the slabs

earthquakes at depth beneath volcanoes forerunners of their activities application to white island new zealand abstract the concept of a time depth correlation between tectonic earthquakes at depth beneath some volcanoes and their eruptions developed by the author since 1962 has been confirmed by new observations and successful prediction of renewed volcanic activity in new zealand regular earthquake migrations are observed along the benioff zone and volcanic eruptions are found to be related to these seismic migrations beneath the volcanoes as follows therefore in island arcs and continental margins volcanic activity is the result of two processes occurring beneath the volcanoes 1 a tectonic process a migration of strain release along the downgoing lithosphere of which the earthquakes are the manifestation 2 a magmatic process a relatively fast vertical ascent of magmatic material from the deep root of the volcano where the observed shocks may be the starting signal from this level the rate of migration of tectonic earthquakes increases with depth in the upper mantle an empirical time relationship between the earthquakes occurring at depth beneath a volcano and its eruptions has been successfully tested for renewed activity at white island in new zealand over the period 1977 1978

intermediate depth seismicity in northern colombia and western venezuela and its relationship to caribbean plate subduction the recent intermediate depth seismicity in northern colombia and western venezuela was analyzed to understand its origin and its presumed relationship to a subducted lithospheric slab in northwestern south america the area included in this study is located to the north and east of the bucaramanga nest which is a particular region beneath bucaramanga northern colombia that presents a high concentration of intermediate depth earthquakes to the north of the nest the seismicity of the area is sparse and most of the events are of low magnitude mb 5 l thus only 23 earthquakes were large enough to be investigated using teleseismic data the focal parameters of the two largest events mb 5 4 were obtained from the formal inversion of long period body waves recorded at teleseismic distances the focal mechanisms of 10 more events were determined from first motion data in total the focal mechanisms of 12 events were determined from both the inversion of p and s h waveforms and the polarities of first arrivals for the smaller earthquakes the focal depths were calculated by measuring the observed pp p interval time and comparing it to the theoretical travel time tables the isodepth curves reflect a slab striking in a nne ssw direction and dipping approximately at 25 32 to the southeast this observation is corroborated by the direction and plunge of the t axes of the focal mechanisms which are generally parallel to the gradient of the slab defined by the spatial distribution of hypocenters these results indicate that the intermediate depth earthquakes in western venezuela and northern colombia are apparently related to the presence of a continuous lithospheric slab subducted near the northern coast of colombia the two largest earthquakes located at a significant distance from the bucaramanga nest present similar fault plane solutions moreover they also agree with those of the two largest earthquakes reported inside the nest this similarity suggests that the bucaramanga nest lies on the same subducted slab where the other earthquakes occur there is not enough shallow seismicity to define the location where the caribbean lithosphere subducted beneath the south american plate however the extension of the slab toward the surface inferred from the intermediate depth seismicity suggests that the subducted lithosphere may still be attached to the caribbean plate

do trench sediments affect great earthquake occurrence in subduction zones seismic energy release is dominated by the underthrusting earthquakes in subduction zones and this energy release is further concentrated in a few subduction zones while some subduction zones are characterized by the occurrence of great earthquakes others are relatively aseismic this variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties age of the subducting lithosphere and convergence rate these two properties do not explain all the variance in maximum earthquake size i propose that a third subduction zone property trench sediments explains part of the remaining variance in maximum earthquake size subduction zones are divided into two groups 1 those with excess trench sediments and 2 those with horst and graben structure at the trench thirteen of the 19 largest subduction zone events including the three largest occur in zones with excess trench sediments about half the zones with excess trench sediments are characterized by great earthquake occurrence most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age rate correlation two notable exceptions are the oregon washington and middle america zones overall the presence of excess trench sediments appears to enhance great earthquake occurrence one speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer which at elevated temperature and pressure forms a homogeneous and strong contact zone between the plates

amazon cone isotopic stratigraphy evidence for the source of the tropical freshwater spike abstract the isotopic stratigraphy of piston cores from the upper fan of the amazon cone reveals a negative planktic foraminiferal 18o deviation from the global oxygen isotope signal that extends back into the last deglacial period the benthic foraminiferal 18o signal agrees with the global oxygen isotope signal and supports the lithological interpretation that the glacial interglacial boundary occurs at the iron rich crust on the amazon cone the negative planktic foraminifera 18o deviation from the global 18o signal is interpreted as evidence of increased deglacial amazon freshwater discharge to the equatorial atlantic ocean 14c dates suggest that the amazon tropical freshwater discharge initiated at 13 5 ky b p peaked at 9 6 ky b p and terminated at approximately 6 ky b p the timing of the tropical meltwater spike showers and margolis 1985 which peaked in the southern venezuelan basin at 9 6 ky b p can be linked to the flooding of the amazon shelf and the reinitiation of the brazilian coastal current this change in equatorial circulation and advection of amazon river freshwater to the north atlantic via the caribbean surface current the gulf of mexico loop current the western atlantic gulf stream and the north atlantic drift may be the cause for the rapid termination of the younger dryas cool period

tidal coulomb failure stresses in the northern andean intermediate depth seismic clusters implications for a possible correlation between tides and seismicity abstract a recent statistical analysis of the relationship between tides and seismic activity in colombia has suggested the existence of correlation anomalies for the case of intermediate depth events in the bucaramanga nest and the cauca cluster moncayo et al 2019 in this work we explore in detail the hypothesis that tides may be triggering seismic activity in these regions and extend the analysis to two other seismic clusters in northern central south america specifically in the areas of el puyo ecuador and pucallpa peru for this purpose we use the available focal mechanism information for seismic events at these locations and calculate for each event the tidal coulomb failure stress tcfs as obtained from estimations of the tidal strain tensor tidal strains are computed considering the earth body tides and the effect of the ocean tidal loading since our purpose is to elucidate the role of tides in earthquake nucleation calculations of the tcfs are conducted not only for the time of earthquake but also for the time of the closest maximum strain within a window of a few hours before the events our results tend to support the hypothesis that tidal stresses are contributing to earthquake generation in all the studied areas this trend is especially stronger when tcfs are calculated at pre earthquake times the physical explanation for the positive contribution of tides to earthquake triggering in intermediate depth clusters may lie in the fact that a wide diversity of fault plane orientations is possible within a relatively small volume of subducted lithosphere so making the tides more likely to help loosening up the blocks at the fault plane to promote slip

geodynamics and intermediate depth seismicity in vrancea the south eastern carpathians current state of the art abstract the vrancea region of the south eastern carpathians is a remarkable site of intra continental intermediate depth seismicity a large set of geological geophysical and geodetic observations has been accumulated for the last few decades and utilised to improve our knowledge of the shallow and deep structures beneath vrancea the crustal and mantle dynamics and the linkage between deep and surface processes in the region in this article we review geology and tectonics of the vrancea region including post collisional to recent deformations syn to post collisional magmatism and orogenic exhumation along the east and south carpathians the regional seismicity is analysed and the recent seismic studies including reflection refraction body and surface wave tomography are reviewed we discuss new geodetic measurements of horizontal and vertical movements in the region geoelectric studies density gravity and thermal modelling qualitative and quantitative including retrospective geodynamic models developed for vrancea are analysed the knowledge of regional tectonics geodynamics seismicity lithospheric deformation and stress regime in the vrancea earthquake prone region assists in an assessment of strong ground motion seismic hazard and risk the earthquake simulation seismic hazard and earthquake forecasting models have also been reviewed providing a link between deep geodynamic processes and their manifestation on the surface finally we discuss unresolved problems in vrancea in order to improve our understanding of the regional evolution present tectonics mantle dynamics intermediate depth seismicity and surface manifestations of the lithosphere dynamics and to enhance our ability to forecast strong earthquakes in the vrancea region the problems to be solved include i the origin of the high velocity body revealed by seismic tomography studies oceanic versus continental ii the lithospheric scale mechanism driving the miocene subsidence of the transylvania basin iii sub crustal structure between 40 and 70 km iv contemporary regional horizontal and vertical movements and v a comprehensive seismic hazard assessment in the region

paleomagnetism and magnetic fabric of the eastern cordillera of colombia evidence for oblique convergence and nonrotational reactivation of a mesozoic intracontinental rift we report the paleomagnetic and magnetic fabric results of 58 sites from cretaceous miocene marine and continental strata from the eastern cordillera ec and the cucuta zone at the junction between the santander massif and the merida andes of colombia the ec is an intracontinental doubly vergent range inverting a triassic to early cretaceous rift zone twenty three sites reveal nonsystematic tectonic rotations including unrotated areas of the ec range with respect to stable south america our data show that the ec inverted a nne oriented rift zone and that the orientation of the mesozoic rift and the mountain chain roughly correspond interestingly magnetic lineations from anisotropy of magnetic susceptibility analysis do not trend parallel to the chain but rather are oblique to the main orogenic trend by also considering gps evidence of a 1 cm yr ene displacement of central western colombia accommodated by the ec we suggest that the miocene recent deformation event of this belt arises from ene oblique convergence reactivating a nne oriented rift zone oblique shortening was likely partitioned into pure dip slip shear characterizing thick skinned frontal thrust sheets well known along both chain fronts and by range parallel right lateral strike slip faults which have not been identified yet but likely exist in the axial part of the ec finally the 35 9 clockwise rotation observed in four post miocene magnetically overprinted sites from the cucuta zone reflects late cenozoic and ongoing right lateral strike slip displacement occurring along faults parallel to the bocono fault system possibly connected with the right lateral faults inferred to exist along the axial part of the ec

subduction in the southern caribbean images from finite frequency p wave tomography 1 the eastern boundary of the caribbean plate is marked by subduction of the atlantic under the lesser antilles the southeastern plate boundary is characterized by a strike slip margin while different configurations of subduction of the southwest caribbean under south america have been proposed we investigate the slab geometry in the upper mantle using multiple frequency teleseismic p wave tomography waveforms from p and pkpdf phases from 285 mb 5 0 events occurring at epicentral distances from 30 to 90 and greater than 150 were bandpass filtered and cross correlated to obtain up to three sets of delay times for each event the delay times were inverted using approximate first fresnel zone sensitivity kernels our results show the subducting atlantic slab as well as a second slab in the west of the study area that we interpret as a subducting fragment of the caribbean plate both slabs have steep dips where imaged and can be traced to depths greater than 600 km these results are consistent with transition zone boundary topography as determined by receiver function analysis the atlantic slab extends continent ward south of the plate bounding strike slip margin we interpret this extension as continental margin lithospheric mantle that is detaching from beneath south america and subducting along with the oceanic atlantic slab the steep subduction of the caribbean occurs 500 km landward from the trench implying an initial stage of shallow subduction as far to the east as the lake maracaibo merida andes region as has been inferred from intermediate depth seismicity

crustal density structure in northwestern south america derived from analysis and 3 d modeling of gravity and seismicity data abstract this paper presents a three dimensional 3 d interpretation of new gravity and seismicity datasets for northern south america a 3 d forward density model was constructed on the basis of deep wide angle seismic refraction sections moho depth from receiver functions and surface geology density values were estimated from published borehole data for sediments by using empirical velocity density functions and considering mineralogical chemical composition variations under typical pressure temperature conditions for upper and lower crustal rocks the modeled 3 d density structure was kept as simple as possible the continental and oceanic plates were formed by two sedimentary bodies one crustal body and one mantle lithosphere body overlying a sub lithospheric mantle the caribbean plate was modeled with an atypical crustal thickness of 18 km including sediments the geometry of the caribbean plate was modeled using a combination of gravity modeling and analyses of the seismicity and focal mechanism solutions intermediate seismicity and the orientation of the t axes appeared aligned along the predicted position of the slab as a result the estimated slab dip angle under maracaibo and the merida andes was 15 and increases up to 20 after 100 km depth the model shows two orientations in the slab strike n150 e 5 in western colombia and southward underneath the maracaibo block the modeling results suggest that the northern south american upper and lower crusts are relatively light and the density of the caribbean crust is typical for an oceanic crust

meltwater and deglaciation se baffin shelf ne margin laurentide ice sheet between 13 5 and 7 ka from o and c stable isotopic data stable isotopic values on planktonic foraminifera in a suite of cores from basins across the se baffin shelf are used to extract a record of meltwater events during termination i deglaciation resolution and hatton basins lie on the se baffin shelf at water depths 500 m seaward of major conduits for ice drainage from the eastern sector of the laurentide ice sheet lis accelerator mass spectrometry 14c dates are used to constrain our chronology of events in ten cores in resolution basin three cores have 14c ams dates on foraminifera of 20 ka at their bases whereas hatton basin cores terminate in sediments 13 kyr sedimentation rates varied between 0 1 to 4 5 m ka stable oxygen and carbon isotopic ratios were obtained on 146 samples of the planktonic foraminifera neogloboquadrina pachyderma ehrenberg sinistral from seven of the ten cores no evidence was found to indicate that test morphology or size affected 18o between 7 and 13 5 ka the surface water on the shelf was on average 1 lower than the open ocean signal significant temporal variations were found in both 18o and 13c evidence for significant low 18o events occurred between 13 and 8 ka the 13c record from the planktonic foraminifera suggests a threefold division of events between 13 and 7 ka with positive values between 10 8 and 13 0 ka negative values between 9 and 10 8 ka and positive values from 7 to 9 ka the 18o data suggest the presence of meltwater on the shelf some 3 000 years prior to the first late glacial dates on terrestrial deglaciation at circa 10 4 ka hudson strait must be the real key to the importance of the calving process during deglaciation because it is potentially the largest marine outlet for the laurentide ice sheet and because it leads into the very center of the ice sheet the rates of calving through hudson strait during the period of initial 18o rise unfortunately are unknown w f ruddiman 1987 p 151

dynamics of meltwater discharge from northern hemisphere ice sheets during the last deglaciation during the last ice age ice sheets covered large portions of north america europe asia and the arctic ocean the ice sheets were composed of meteoric water enriched in the light isotope of oxygen 16o leaving oceanic seawater correspondingly enriched1 2 in 18o the volume of past continental ice sheets during the last glacial maximum 18 000 yr ago has been estimated from terrestrial evidence of ice limits3 glaciological modelling4 the 18o of foraminifera from deep sea cores5 6 and sea level evidence from raised coral terraces7 8 oxygen isotope studies of planktonic foraminifera reveal that 18o shifts during deglaciations in areas near sources of ice meltwater like the gulf of mexico and labrador sea baffin bay are often larger than the corresponding average global oceanic shift9 11 and that these variations reflect the timing of ice decay without the ocean mixing lag contained in both distal and benthic 10o variations12 we show here that geographically distinct planktonic 18o gradients can be related to the volume of meltwater discharged through specific meltwater outlets and thus to the initial volume of the ice sheets at the commencement of deglaciation and to the subsequent dynamics of ice sheet decay

seismic anisotropy above a subducting plate shear wave splitting observed in northeastern colombia has provided evidence of seismic anisotropy in a shear zone immediately above a subducting plate in an upper mantle composed mainly of olivine 57 and orthopyroxene 17 the splitting can be interpreted by wave propagation in an anisotropic medium of orthorhombic symmetry that results from alignment of these intrinsically anisotropic minerals the mechanism of alignment is most likely the shearing associated with the subduction aided by fluids migrating from the subducting plate when the plate exceeds 100 km in depth

deep water formation and quaternary glaciations a large fraction of climatic varability on the quaternary time scale can be explained by nonlinear interactions between the radiation balance of the global atmosphere ocean system and the mass balance of northern hemisphere ice sheets recent analyses of paleoclimatic proxy data indicate a further important contribution to this variability from changes in deep water formation occurring in the north atlantic subpolar sea we study the effects of these changes on variations in global temperature and ice volume characteristic of the late quaternary the novel framework of boolean delay equations bdes is used to formulate a conceptual model of the climatic system under study and to analyze this formal model selfsustained oscillations in the intensity of the atlantic ocean s thermohaline circulation result from the interaction of sea ice formation with the waxing and waning of continental ice sheets the comparison of model results with paleoclimatic records suggests a considerable slowing down of the abyssal circulation during glacial episodes