postseismic and interseismic displacements near a strike slip fault a two dimensional theory for general linear viscoelastic rheologies 1 we present an analytic solution for the deformation near an infinite strike slip fault in an elastic layer overlying a linear viscoelastic half space the theory is valid for any linear viscoelastic rheology and any earthquake sequence this is a generalization of the work of j c savage and colleagues which only holds for models with a uniform shear modulus maxwell viscoelastic lower half space and periodic rupture recurrence we demonstrate the theory for models of an elastic layer over a maxwell standard linear solid burgers and triviscous half space for each of these models we calculate example postseismic displacements interseismic displacements in a periodic earthquake sequence and interseismic displacements for a nonperiodic earthquake sequence our solution is for a simple geometry however the model presents an elegant tool to explore the evolution of displacements for relatively complex rheologies and rupture recurrence histories

the evolution of pleistocene climate a time series approach abstract in the later part of the pleistocene variations in global ice volume have been dominated by an approximate 100 000 year cycle analysis of 2 myr long oxygen isotope record from an equatorial pacific core indicates that this is true only for the last 900 000 years prior to this time the amplitude of the 100 000 year cycle is much reduced as is the variance of all oscillations with periods greater than 60 000 years based on results of time series analysis of this 2 myr long record the pleistocene glacial cycles can be divided into three sections 1 the late pleistocene 0 900 kyr b p where the variations in the isotope record are dominated by the 100 000 year cycle 2 the middle pleistocene 900 1450 kyr b p in which low frequency components are not as important as in the later period of the pleistocene and 3 the early pleistocene late pliocene 1450 2000 kyr b p where general reductions of importance at all frequencies is seen as compared to the later intervals recent modeling efforts which describe variations in global ice volume show that the dominant low frequency component observed in the late pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the earth s orbital parameters it is hypothesized that during the early pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice cap formation to below sea level when the ice caps became marine based more rapid decay of the ice became possible

rheological models and interpretation of postglacial uplift accepted 1989 february 1 received 1989 january 28 in original form 1988 september 26 summary paper investigates four generalized maxwebl bodies gmbs and the classical maxwell body cmb with respect to their potential to explain postglacial uplift the gmbs are the lomnitz body defined by a logarithmic creep law the q power law body defined by a power law dependence of the quality factor on frequency the caputo body defined by a special stress strain relation with memory and the burgers body the burgers body is a 4 parameter model the other gmbs have 3 and the cmb has only 2 parameters theoretical uplift curves are calculated for simple space time models of the canadian and fennoscandian deglaciations and hedgehog inversion yields the successful model parameters the gmbs explain the data better than the cmb the parameters of the burgers body are only rather weakly constrained because of trade offs among them from the success of the gmbs we conclude that long time scale transient creep is a possible rheological behaviour of mantle rocks viscosity extrapolation from the time constant of postglacial uplift about 10 ka to the time constant of mantle convection roughly 100 ma is made with the effective viscosity and yields approximately ld2 pas q power law body loz3 pas caputo body and loz5 pas lomnitz body hence postglacial uplift does not strongly constrain the long time scale mantle viscosity it provides only a lower bound about loz1 pas this value follows from data interpretation with the cmb model in which steady state creep is present alone rayleigh number estimates for whole mantle convection are clearly supercritical for the cmb the q power law body and the caputo body but only about critical or even subcritical for the lomnitz body both postglacial uplift and mantle convection is satisfactorily explained by the cmb the q power law body and the caputo body

viscosity of the lower mantle as inferred from rotational data until recently most estimates of the deep mantle viscosity were based on analyses of postglacial uplifts and there were no serious estimates of the sensitivity of the derived viscosity solutions to variations of the input parameters such as those associated with the surface loading in this paper the viscosity of the lower mantle is arrived at by making comparisons of the observed secular motions of the earth s rotation axis with theoretical results from a layered viscoelastic rotating earth which has been subjected to glacial forcing our model consisting of an elastic lithosphere a two layer adiabatically stratified viscoelastic mantle and an inviscid core is essentially analytical and this makes it economically feasible to use as an aid in conducting an extensive sensitivity analysis of the lower mantle viscosity from changes in the parameters connected with the deglaciation phenomenon both sets of rotational data polar wander and nontidal deceleration of the length of the day have been employed to constrain the viscosity structure on the basis of this type of investigation the viscosity of the lower mantle is found invariably to be larger than that of the upper mantle and lies between 1 and 4 1022 p our results which just rely on the second degree harmonic of the strain field also lead to a determination of an upper bound to the thickness of the globally averaged lithosphere this value ranging between 130 and 190 km suggests that strong lateral heterogeneities between oceanic and continental plates may extend a few hundred kilometers into the upper mantle

nonlinear response of pliocene climate to orbital forcing evidence from the eastern equatorial pacific a high resolution radiolarian record from the eastern equatorial pacific is used to examine paleoceanographic events during the pliocene from 1 8 to 4 9 ma these data provide a means to evaluate the response of the equatorial pacific to the onset of northern hemisphere glaciation near 2 47 ma and to the closure of the isthmus of panama near 3 2 ma responses are recorded both as long term trends and as variations within the milankovitch band 104 to 105 years radiolarian based sea surface temperature sst estimates indicate a mean cooling from lower to upper pliocene but no significant changes occur near the onset of northern hemisphere glaciation and no abrupt changes are observed before or after the closure of the isthmus spectral and cross spectral analyses of this sst time series indicate concentrations of variance within the precession 23 kyr and obliquity 41 kyr bands in several but not all time intervals variance is commonly concentrated at lower frequencies cross spectral analysis of sst time series and calculated orbital variations indicates that the sst record is not coherent with and therefore not linearly related to orbital variations detailed time series analysis of the sst and 18o time series indicates the absence of a direct linear relationship but rather the presence of a strongly nonlinear relationship between pliocene climatic variables and earth s orbit high coherence is seen between the amplitude of the 23 kyr component of sst and the amplitude modulation of precession the amplitude modulation of the 41 kyr component of the sst record is dissimilar to the amplitude modulation of obliquity but is extremely similar to and coherent with the amplitude of the 23 kyr component of sst the amplitude modulations of sst and 18o at a range of frequency bands have the same features features that roughly follow the modulation of precession this suggests a strongly nonlinear response of equatorial pacific sst and planktonic 18o to orbital forcing in the pliocene such a response extending over a number of frequency bands has not previously been seen in a paleoclimatic data set but may be consistent with proposed models describing paleoclimatic variability although the reasons for this response are still unclear it is evident that strong nonlinearities in climate are present during this time

geologic time series from deep sea sediments time scales and distortion by bioturbation abstract in order to evaluate the time and frequency domain characteristics of geologic time series obtained from deep sea sediments two important problems must be considered first an accurate time scale must be established for the sediment records to be studied an accurate time scale is a basic requirement for such techniques as spectral and cross spectral analysis secondly the distortion of a geologic time series by sedimentation processes such as bioturbation must be considered if inferences are to be made about the mechanisms controlling climatic and oceanographic processes reflected in the geologic record recently it has been suggested that the periodic components in paleoclimatic records which can be related to variations in the earth s orbital parameters can be used as a means for providing very detailed control for late pleistocene sediment sections a time scale is estimated by tuning an individual frequency component in a climate record to an orbital parameter the tuning procedure commonly uses the technique of bandpass filtering which removes all frequency components except the one equivalent to an orbital parameter and then adjusting the geologic time scale so that the filtered record and orbital record align since bandpass filtering will produce a periodic record from any data set a question is raised as to the circularity of this tuning approach a series of numerical experiments using synthetic time series are used to determine if the tuning of a geologic record artificially induces improved correlation and increased periodic character in a climatic record these experiments indicate that the tuning does not artificially induce these effects thus it is suggested that the careful use of orbital tuning provides a powerful means of defining a geologic time scale the degree to which bioturbation distorts a paleoclimatic record is studied by comparing the spectra of oxygen isotopic records obtained from cores with sedimentation rates ranging from 1 to 12 cm 1000 yr 1 it is shown that the systematic change in the variance spectra of these records that is predicted by models of bioturbation cannot be observed for sedimentation rates greater than 2 cm 1000 yr 1 this can be explained in part if bioturbation is a function of sedimentation rates if this is the case then bioturbation must proceed to depth of tens to hundreds of centimeters however it seems likely that the degree to which bioturbation alters the climate signal preserved in deep sea sediment cores has been overestimated

consequences of experimental transient rheology recent analyses of transient creep data for lower crustal and upper mantle substances show that the parameters of the burgers body rheological model used up to now by geodynamicists have much lower ratios of the short term to long term viscosities than the laboratory values we have used these data to constrain the viscosity contrast across the 670 km discontinuity our results show that the previously inferred viscosity variations are now reduced by about a factor of ten in the new transient models this finding is in agreement with the revised viscosity estimate based on long wavelength geoid anomalies and seismic tomography we find that time dependent perturbations to the gravity field from recent ice movements are not at all small

the effects of transient rheology on the interpretation of lower mantle viscosity we have reexamined the role played by transient rheology in the interpretation of mantle viscosity this investigation has been carried out by comparing the amplitude responses with the data of secular variation of the relative sea level histories at sites well within the ice margins and at the ice margin like the city of boston a linear burgers body rheology has been assumed in the lower mantle the data near the edge of the ice load proves most sensitive to the transient viscosity structure the non monotonic behavior of sea level data near boston can be explained both by a steady state lower mantle viscosity of 1022p with a thick lithosphere and also by a transient lower mantle rheology but with a thin lithosphere the long term viscosity of the lower mantle in this second model has a steady state value of around 5 1023p

pliocene variations in the position of the antarctic convergence in the southwest atlantic middle to late pliocene 4 1 1 9 ma variations in the positions of surface water masses and migrations of the polar front in the southwest atlantic are inferred from a factor analysis of radiolarian assemblages from dsdp site 514 faunal results suggest that surface water masses underwent progressive cooling as the polar front zone pfz advanced northward during the latest gilbert to late gauss chron the late gilbert chron 4 12 3 86 ma was the waning phase of a mid gilbert warm interval northern subantarctic surface water prevailed between 4 12 and 3 97 ma indicative of surface water temperatures warmer than those during the succeeding pliocene a long term sea surface cooling trend began between 3 97 and 3 86 ma which moved the pfz northward and closer to site 514 bringing cooler southern subantarctic surface waters to the site this movement northward of cooler surface waters is inferred to be related to the initial growth of sea ice and ice shelves throughout regions of west antarctica which were deglaciated or unglaciated during the prior warming interval of the gilbert chron a major late gilbert early gauss chron increase in the production of antarctic bottom water aabw resulted in an erosional or nondepositional hiatus at site 514 spanning 3 86 to 3 18 ma increased aabw activity at this time is linked to continued growth of expansive ice shelves and sea ice in the west antarctic region in the east antarctic margin and possibly in the marine embayments of east antarctica decreased aabw production led to a 3 2 ma resumption of sedimentation at site 514 and elsewhere in the southern ocean antarctic surface water assemblages above the hiatus 3 18 3 16 ma suggest that the pfz first passed over site 514 during the 3 86 3 18 ma interval of the hiatus following this initial invasion of antarctic surface waters between 3 16 and 2 94 ma the pfz retreated to a position near or south of the site this slight retreat of the pfz with a corresponding decrease in aabw activity is possibly related to a significant reduction in the area of shelf and sea ice as grounded ice expanded in the west antarctic region we interpret the widespread global enrichment of 18o at approximately 3 2 ma to be partially related to a cooling of surface waters the production of cooler bottom waters and an expansion of west antarctic ice volume associated with its grounding phase between 3 5 and 2 7 ma based on surface water mass migrations at site 514 the west antarctic ice volume increased significantly slightly prior to or at approximately 3 2 ma decreased between 3 14 and 2 7 ma and then increased by 2 67 ma to a more stable configuration with a volume at least as great as that at 3 2 ma during the late gauss 2 94 2 47 ma four northward advances of the pfz led to the migration of antarctic surface waters over the site by 2 67 ma and continued dominance of antarctic surface water through the remainder of the pliocene we suggest that this permanent change from subantarctic to antarctic surface water mass dominance was linked indirectly to the initial growth of northern hemisphere ice and a reduction of sea level we propose a model of simultaneous northern and southern hemisphere ice volume growth induced by interhemisphere positive feedback which is responsible for the major late gauss advance of antarctic surface waters and the global enrichment of 18o at 2 5 ma

new constraints on transient lower mantle rheology and internal mantle buoyancy from glacial rebound data a model of mantle rheology which has an explicit transient component of its relaxation spectrum is shown to reconcile two apparently conflicting inferences of lower mantle viscosity if recent analyses of isostatic geoid anomalies are correct in requiring a large increase of viscosity with depth then the weak viscosity stratification demanded by postglacial rebound data implies the importance of such rheological behaviour at least in the earth s lower mantle

glacial isostatic adjustment and the free air gravity anomaly as a constraint on deep mantle viscosity summary a gravitationally self consistent theory for relative sea level variations forced by pleistocene deglaciation events is employed to explore the extent to which rsl and free air gravity observations together constrain the viscosity of the mantle beneath the seismic discontinuity at 670 km depth a trade off is revealed between errors in the assumed deglaciation history and errors in the inferred value of the viscosity of the lower mantle taking full account of such uncertainty plausible values of the viscosity beneath the transition region are bounded above by 1023 poise cgs units the preferred value is at least a factor of 2 lower than this and is strongly constrained by the observed free air gravity anomaly over hudson bay the calculations described in detail here show for the first time that the relatively large gravity anomalies observed over sites of wurm wisconsin deglaciation do not require any pronounced increase of mantle viscosity with depth in order to explain them

slow physics of large continental ice sheets and underlying bedrock and its relation to the pleistocene ice ages very simple two component models of the earth s cryosphere lithosphere system are analyzed the models consist of a continental ice sheet coupled to a responding earth model because of relatively long response times of the bedrock at least over shorter wavelengths long period of the order of 50 000 years internal or free oscillations in the system occur over a wide range of parameters and parameterizations the oscillatory mechanism operates primarily in the ablation zone and in particular at the firn line these free oscillations could play a dynamic role in the response of the climate system to eccentricity forcing a simple analysis presented strongly suggests that for the simplest rheological models of the earth response to pleistocence ice sheets elastic response is confined to a layer considerably less than the thermal thickness of the lithosphere furthermore because of the striking lack of geological evidence for large depressions of bedrock at the leading edge of growing continental ice sheets predicted by viscoelastic models of the earth more sophisticated rheological models of the earth s uppermost layer appear necessary

change from atlantic type to pacific type carbonate stratigraphy in the middle pliocene equatorial pacific ocean a paleomagnetically dated piston core from the equatorial pacific ocean v28 179 contains levels of highest carbonate percentage in association with lower 18o values in early pliocene sediments this relationship is typical of the atlantic during the quaternary and is thus referred to as an atlantic type carbonate stratigraphy the change from an atlantic type to a pacific type stratigraphy high carbonate glacial episodes appears to have occurred during the earliest gauss magnetic chron 3 18 3 41 ma of middle pliocene age and may be related to the development of northern hemisphere glaciation samples deposited prior to the development of northern hemisphere glaciation in v28 179 have a significant proportion of low frequency 400 ka period variance in both the benthic o18 and carbonate spectra and an atlantic type correlation both the oxygen isotopic and the carbonate spectra of v28 179 have frequency specific correlation coherency with the calculated pliocene record of the eccentricity of earth s orbit thus orbital parameters may have affected equatorial pacific sedimentation in the pliocene the change from atlantic type to pacific type carbonate stratigraphy in the middle pliocene may have been related to a change in the dominant frequency of variance of carbonate oscillations related to orbital parameters and or to intensification of antarctic bottom water circulation during the middle pliocene near the base of the gauss chron

excitation of the earth s rotational axis by recent glacial discharges we study the effects of present day glacial dis charges and the growth of the antarctic ice sheet on exciting the earth s rotational axis glacial forcing could cause a maximum change in j of about one third of the observed amount for the maxwell rheology and for burgers body models with a long term lower mantle viscosity greater than about 1023p for transient rheologies the amount of excitation due to glacial melting decreases polar wander is not much excited by recent glacial melting for the various types of rheologies examined

transient creep in orthosilicates abstract an analysis of garnet olivine and ultramafic aggregate deformation data is performed using a burgers body rheology such that transient elasticities and viscosities can be obtained our results are indicative of transient shear moduli with magnitudes of about one tenth the isostatic shear moduli and transient viscosities of magnitudes 0 5 0 7 of the steady state viscosities these values differ significantly from those chosen by previous workers for burgers body parameters pertinent to transient creep in the lower mantle transient viscosity appears to have the same temperature dependence as the steady state viscosity which requires that the transient elastic modulus becomes rapidly weaker with increasing temperature hydrolytic weakening also seems to be accompanied by a weaker transient elastic modulus implying a longer relaxation period of the transient strains in water weakened experiments a discrepancy between previously reported transient behavior in dunite and the data analyzed here can be attributed to errors in the dunite experimental data

covariance patterns of foraminiferal dgr18o an evaluation of pliocene ice volume changes near 3 2 million years ago oxygen isotope data for a pliocene interval from 3 6 to 2 8 million years ago show a mean increase 0 5 per mil of benthic delta 18 o at about 3 2 million years ago whereas planktic delta 18 o does not increase this lack of covariance indicates that the event at 3 2 million years did not result in a permanent increase in the ice budget of either the northern or the southern hemisphere

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

isotopic paleoceanography of the caribbean and east pacific role of panama uplift in late neogene time comparisons of carbon isotopic data on benthic foraminifera from deep sea drilling project sites 502 western caribbean and 503 eastern pacific indicate that the difference between the atlantic and the pacific in the per mil enrichment in carbon 13 of total dissolved carbon dioxide increased about 6 million years ago and again 3 million years ago when the difference reached the modern level 1 per mil comparisons of planktonic foraminiferal oxygen isotopic data for the caribbean and the pacific suggest that the salinity of caribbean surface waters began increasing 4 million years ago possibly in response to shoaling of the panama isthmus these results suggest that modern circulation patterns in the caribbean and eastern pacific developed by 3 million years ago in concert with changing tectonic climatic and biogeographic patterns

mantle rheology and satellite signatures from present day glacial forcings we study the changes of the long wavelength portion of the earth s gravity field as a result of present day gladal discharges and the possible growth of the antarctic ice sheet we employ both maxwell and burgers body rheologies and find that there are significant differences in the responses between the two rheologies for time spans less than a century both present day gladal forcing and the ice buildup on antarctica can cause non negligible perturbations in j 2 of about one third the observed amount the contributions from the two mechanisms have opposite signs in j 2 only for i 3 and 6 do these cryospheric contributors act in concert with one another in disturbing the gravity field ali other degree harmonics up to 1 9 have opposite signs from these two excitations for a steady state lower mantle viscosity of 1022 pa s j 3 can attain a maximum value of around 1 5 x lo i1 yr 1 and j a value of 1 3 x io il yr i for current forcings these have signs opposite to those predicted for pleistocene deglaciation ali harmonics up to i 9 are sensitive to variations of the steady state vis cosity in the lower mantle even for time scales shorter than 100 years it is proposed that detailed satellite monitoring of present day ice movements in conjunction with geodetic satellite missions will eventually pro vide a viable alternative for estimating deep mantle viscosity useful in mantle convection models

on transient rheology and glacial isostasy theoretical predictions from analytical self gravitating layered earth models with maxwell burgers body and standard linear solid rheologies are employed to explore the extent to which transient creep may have influenced the inference of long term mantle viscosity from analysis of the various signatures derived from the process of glacial isostatic adjustment the compatibility of the various models is checked by their ability to fit simultaneously the relative sea level free air gravity and the rotational data we conclude that transient creep cannot operate throughout the entire mantle on account of the extreme sensitivity of the uplift and gravity data to any significant departures of the steady upper mantle viscosity from 1021 pa s transient rheology would most likely be unimportant in the upper mantle restricting transient rheology to the lower mantle we find that models with an adiabatic density jump at 670 km depth are not satisfactory because of their inability to produce sufficiently large gravity anomalies and polar wander speeds our calculations show that the preferred models with transient creep in the lower mantle are chemically stratified with a nonadiabatic density change of around 10 the long term viscosities in the lower mantle associated with compatible transient rheological models lie between 5 1022 and 1023 pa s these values are consistent with viscosity estimates derived from correlation of long wavelength geoid anomalies and the density perturbations in the deep mantle

viscoelastic relaxation of a burgers half space implications for the interpretation of the fennoscandian uplift summary we investigate the load induced response of a burgers earth model in particular using the incremental field equations and interface conditions of viscoelastodynamics we give closed form solutions for a two layer half space subjected to surface loading in the special case of homogeneity the solution shows that the burgers half space is characterized by two fundamental normal modes mo and m they correspond to the short and long time viscosities o and which describe the creep behaviour of the transient burgers rheology at short and long times after the onset of loading in the more general case of a burgers substratum overlain by an elastic layer the number of normal modes increases to four mo lo m l values of the parameters specifying the burgers substratum are estimated by predicting the observed post glacial uplift in fennoscandia with a simple model of the pleistocene glaciation history the calculations return the following results 1 an upper bound on cannot be determined 2 a comparison with uplift curves for a steady state maxwell substratum shows that the marginal and peripheral regions of the fennoscandian ice sheet are most sensitive to transient creep in the earth s mantle and 3 an estimate of cannot be obtained without a priori knowledge of the other parameters of burgers rheology

cross sectional stability of tidal inlets a comparison between numerical and empirical approaches article i nfo a process based morphodynamic model is used to simulate the evolution of tidal inlets towards equilibrium when subjected to a range of tide and wave conditions specific attention is given to the evolution of the cross sectional area of the inlet channel equilibrium values of cross sectional areas are found by approaching the equilibrium from two initial conditions i e by starting an experiment with an initial value of the cross sectional area that is smaller than the estimated equilibrium value and an initial value that is larger than the estimated equilibrium value similar to nature a good correlation between cross sectional area and tidal prism is found this suggests that the physical mathematical formulations in the model used to describe the hydrodynamic and sedimentary process are a good representation of the morphodynamics in the real world and that the conceptual idea of escoffier on the cross sectional stability of inlets is valid

sea surface temperature changes in the north pacific during the late miocene abstract twenty seven radiolarian species and species groups are identified and used in this study their stratigraphic ranges span from late miocene to modern times and they appear to have distributions which have changed little over this interval of time census data for these 27 taxa in surface sediment samples are used to define six assemblages q mode factors the distribution of these assemblages are similar to modern water mass distributions and can be statistically related to modern sea surface temperatures census data on these same 27 taxa have been collected in the upper miocene intervals of six pacific sites these faunal data are described in terms of the modern radiolarian assemblages and based on the multiple regression equations relating the modern assemblages to modern temperatures estimates of late miocene temperatures are made the estimated temperatures indicate that there is an overall cooling trend in the later part of the miocene magnetic chron 8 to chron 5 punctuated by several distinct cooling events in august the low latitude sites appear to be slightly cooler and the mid latitude sites warmer than in modern times in february both the low and mid latitude sites appear to be warmer than modern temperatures at the same locations the relatively warm temperatures near the base of the interval studied may be associated with the passage of warm tropical atlantic waters into the eastern pacific via the isthmus of panama the general cooling trend seen in these records is thought to be associated with the gradual closure of these straits and the increased influence of the high southern latitudes