forcing of atlantic equatorial and subpolar millennial cycles by precession abundance cycles of the marine alga florisphaera profunda centered on a period of 7600 carbon 14 years 8400 calendar years are present in high resolution records from the equatorial atlantic spanning 0 to 45 000 years ago these cycles correlate with heinrich events 1 through 5 which document rapid changes in continental ice melting around the subpolar north atlantic these variations in f profunda are a direct response to modulation in zonal wind driven divergence produced by a precessional component of orbital variation during a time of reduced eccentricity modulation

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

timing and structure of the mid pleistocene transition records from the loess deposits of northern china abstract the mid pleistocene transition mpt from obliquity to eccentricity forced climate variations has been extensively documented within the oceanic realm using a collection of orbitally tuned records of palaeoclimatic change from the loess sequences of northern china we have analysed terrestrial palaeomonsoon behaviour since 1 5 ma and characterised the structure of the mpt wavelet analysis of a stacked magnetic susceptibility record from the loess sequences indicates that the onset of 100 kyr cyclicity was synchronous in both global ice volume and the east asian monsoon system however analysis in the time domain reveals that during the mpt interim period the east asian winter monsoon appears to have weakened whilst global ice volume was increasing indicating that the palaeomonsoon was not the primary forcing mechanism responsible for driving the global climatic change

millennial scale fluctuations in north atlantic heat flux during the last 150 000 years abstract surface water and deep ocean palaeoclimate records obtained from north atlantic core su90 03 40 n 32 w exhibit pronounced fluctuations at sub orbital time scales surface temperature variability in the mid latitude atlantic over the last 150 kyr is dominated by forcing at precession 21 kyr and semi precession harmonics 11 kyr abrupt changes in sea surface temperatures show that the northward transport of heat by the north atlantic current was curtailed during ice rafting events sea surface temperatures were about 12 c colder than modern values during the most intense cooling episodes associated with major ice rafting events heinrich events 1 4 6 and 11 and 10 c below modern values during the later part of isotope stage 3 40 30 ka during these cold intervals the north atlantic current was displaced to south of 40 n as the cold meltwaters penetrated southwards into what is now the subtropical ocean these changes in surface ocean heat exchange were most probably allied to a latitudinal migration and increase in zonality of atmospheric circulation patterns concurrent shifts in ird concentration sea surface temperatures and benthic 13 c values provide evidence of coupling between sea surface processes and the deep water circulation and suggest that decreases in surface temperature and salinity during ice rafting events culminated in a significant reorganisation of north atlantic deep water production

links between the onset of modern walker circulation and the mid pleistocene climate transition sea surface temperatures ssts reconstructed from ocean drilling program sites 806 and 849 in the western and eastern equatorial pacific provide a history of the strength of the walker circulation between 1 5 and 0 5 ma a progressive cooling in the eastern equatorial pacific is paralleled by intensification of the equatorial zonal sst gradient that began 1 17 ma prior to the expansion of global ice volume associated with the mid pleistocene climate transition the ssts in the eastern equatorial pacific and the zonal gradient reached modern values by 0 9 ma we propose that this onset and intensification of the modern walker circulation reduced heat flux but increased moisture transport to high latitudes leading to the development of more extensive ice sheets and the shift toward the 100 ka world

phase relationships between millennial scale events 64 000 24 000 years ago a core recovered on the iberian margin off southern portugal can be correlated with greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over greenland our age model identifies the base of every interstadial between 64 000 and 24 000 years ago and uses the greenland ice core project grip timescale the oxygen isotope signal in benthic foraminifera on this grip based timescale is quite different from the planktonic record and resembles the temperature record over antarctica when this is synchronized with greenland using the record of methane in the atmospheric air in the polar ice cores we interpret the benthic record as indicating significant fluctuations in ice volume during millennial events and we suggest that antarctic temperature changed as a function of ice volume

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

timing and significance of maximum and minimum equatorial insolation variations in summer insolation at high northern latitudes on a timescale of 100 ka are very small thus a common belief is that the pronounced 100 ka glacial cycles are not directly linked to the very weak 100 ka insolation periodicity here we show analytically and numerically that the annual maximum and minimum of daily equatorial insolation has pronounced eccentricity periodicities with timescales of 400 ka and 100 ka as well as a pronounced half precession periodicity with timescale of 11 ka the timing of the maximum and minimum annual equatorial insolation may change around the equinoxes solstices alternating between the vernal and autumnal equinoxes summer and winter solstices where the time of the maximum minimum equatorial insolation may occur up to more than 1 month from the equinoxes solstices we also show that when considering the mean insolation of periods larger than 1 d the 11 ka periodicity becomes less dominant and it vanishes when the averaging period is half a year for the later case the maximum minimum may occur for any day in the annual cycle the maximum equatorial insolation may alter the timing and amplitude of the maximum surface temperature of the summer hemisphere and in this way may drastically affect the hadley circulation changes in hadley circulation affect the heat and moisture transport from low to high latitudes affecting the buildup of the high latitude northern hemisphere ice sheets

early onset and tropical forcing of 100 000 year pleistocene glacial cycles between 1 5 and 0 6 myr ago the period of the earth s glacial cycles changed from 41 kyr the period of the earth s obliquity cycles to 100 kyr the period of the earth s orbital eccentricity1 2 which has a much smaller effect on global insolation the timing of this transition and its causes pose one of the most perplexing problems in palaeoclimate research3 here we use complex demodulation to examine the phase evolution of precession and semiprecession cycles the latter of which are phase coupled to both precession and eccentricity in the tropical and extra tropical atlantic ocean we find that about 1 5 myr ago tropical semiprecession cycles with periods of about 11 5 kyr started to propagate to higher latitudes coincident with a growing amplitude envelope of the 100 kyr cycles evidence from numerical models suggests that cycles of about 10 kyr in length may be required to explain the high amplitude of the 100 kyr cycles4 combining our results with consideration of a modern analogue we conclude that increased heat flow across the equator or from the tropics to higher latitudes around 1 5 myr ago strengthened the semiprecession cycle in the northern hemisphere and triggered the transition to sustained 100 kyr glacial cycles

persistent suborbital climate variability in marine isotope stage 5 and termination ii new surface water records from two high sedimentation rate sites located in the western subtropical north atlantic near the axis of the gulf stream provide clear evidence of suborbital climate variations through marine isotope stage mis 5 persisting even into the warm peak of the interglaciation substage 5e we found that the amplitude of suborbital climate oscillations did not vary significantly for the whole of mis 5 implying that ice volume has little or no influence on the amplitude of suborbital climate variability in this region although some records suggest that longer suborbital variations 4 10 kyr during mis 5 are linked to deepwater changes none of the existing records is of sufficient resolution to assess if a linkage occurred for oscillations shorter than 4 kyr however when examined in conjunction with published data from the norwegian sea new evidence from the subpolar north atlantic suggests that coupled surface deepwater oscillations occurred during the penultimate deglaciation this supports the hypothesis that during glacial and deglacial times ocean ice interactions and deepwater variability amplify suborbital climate change at higher latitudes we suggest that during the penultimate deglaciation the north atlantic deepwater source varied between nordic sea and open north atlantic locations in parallel with surface temperature oscillations

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

analysis of seismic signals recorded on a prone to fall rock column vercors massif french alps summary a small aperture 40 m short period seismic array was installed during four months on the vercors massif western french alps at the top of a limestone column which collapsed one month and a half later during this monitoring period 193 seismic events were recorded by the seven seismometers of the array signal analysis yielded three main types of nearby seismic events to be identified from temporal and spectral characteristics microearthquakes single or multiple events individual block falls and rock falls it turned out that 60 per cent of the 193 events were classified as microearthquakes exhibiting distinct p and s waves while 17 per cent of these events remained unclassified out of the microearthquakes 40 events with a good signal to noise ratio were selected and processed p and s wave traveltimes were picked on the records and the inferred hypocentral distances agree with the two zones of the scarp exhibiting fresh ruptures after the fall polarization analysis of the 3 c records along with numerical simulations allowed discriminating between the two possible rupture mechanisms toppling and sliding shear rupture sliding was the predominant mode in the lower part of the column whereas traction rupture toppling affected the upper part finally the comparison between the ground motions recorded on the column and on the rock mass showed a systematic amplification on the column signal processing and numerical modelling both suggest that this amplification resulted from the excitation of the natural frequencies of the column and is particularly high 3 for microearthquakes occurring at the column to mass interface

millennial scale variability of deep water temperature and δ18odw indicating deep water source variations in the northeast atlantic 0 34 cal ka bp 1 paired measurements of mg ca and 18occ calcite 18o in benthic foraminifera from a deep sea core recovered on the iberian margin md99 2334k 37 48 n 10 10 w 3 146 m have been performed in parallel with planktonic 18occ analyses and counts of ice rafted debris ird the synchrony of temperature changes recorded in the greenland ice cores and in north atlantic planktonic 18occ allows the proxy records from md99 2334k to be placed confidently on the gisp2 time scale this correlation is further corroborated by ams 14c dates benthic mg ca measurements in md99 2334k permit the reconstruction of past deep water temperature tdw changes since 34 cal ka bp calendar kiloyears before present using these tdw estimates and parallel benthic 18occ measurements a record of deep water 18o 18odw has been calculated results indicate greatly reduced tdw in the deep northeast atlantic during the last glaciation until 15 cal ka bp when tdw warmed abruptly to near modern values in parallel with the onset of the bolling allerod interstadial subsequently tdw reverted to cold glacial values between 13 4 and 11 4 cal ka bp in parallel with the younger dryas cold reversal and the h0 ice rafting event similar millennial scale tdw changes also occurred during the last glaciation indeed throughout the last 34 cal ka millennial 18odw and tdw changes have remained well coupled and are linked with ird pulses coincident with heinrich events 3 2 1 and the younger dryas when transitions to lower tdw and 18odw conditions occurred in general millennial tdw and 18odw variations recorded in md99 2334k describe an alternation between colder low 18odw and warmer high 18odw conditions which suggests the changing local dominance of northern sourced north atlantic deep water nadw versus southern sourced antarctic bottom water aabw the observed similarity of the tdw and gisp2 18oice records would therefore suggest a common component of variability resulting from the coupling of nadw formation and greenland climate a link between greenland stadials and the incursion of cold low 18odw aabw in the deep northeast atlantic is thus implied which contributes to the relationship between greenland climate and the millennial benthic 18occ signal since 34 cal ka bp

changes in calcareous nannofossil assemblages during the mid pleistocene revolution the distribution of calcareous nannofossils are quantitatively analysed in high resolution deep sea core samples from north atlantic dsdp site 607 and eastern mediterranean odp site 967 between marine oxygen isotope stage mis 35 and 15 in order to observe the response of calcareous nannofossils to the mid pleistocene revolution mpr nannofossil abundance patterns show major variations through mis 25 22 and distinct higher amplitude changes from mis 21 upward sometime clearly correlatable to glacial interglacial cycles high amplitude short term fluctuations of small placoliths and f profunda characterize the interval through mis 24 to 22 21 suggesting strong instability in nutricline dynamics and surface water productivity principal component analysis and shannon weaver indices diversity and dominance highlight significant shifts mainly related to variations in the characteristics of surface waters in both sites a minimum in diversity is recorded at mis 22 21 corresponding to a maximum in the dominance of reticulofenestra spp at site 607 and of pseudoemiliania lacunosa at site 967 above mis 21 the increase in abundance of various taxa c leptoporus umbilicosphaera spp syracosphaera spp rhabdosphaera spp oolithotus spp and a general increase in diversity are related to a distinct trend toward more stable and oligotrophic surface waters in contrast to the higher productivity and less stratified surface waters of the early pleistocene in addition power spectral analysis was performed on calcareous nannofossil abundances shannon index diversity values and factor 1 of principal component analysis these show significant peaks in the obliquity 41 kyr and eccentricity 100 kyr frequency bands indicating that the quasi periodic oscillations in the climate proxy data are controlled by these orbital parameters a relationship between changes in abundance of calcareous nannofossils and production of glacial north atlantic deep water is inferred

rapid changes in the mechanism of ocean convection during the last glacial period high amplitude rapid climate fluctuations are common features of glacial times the prominent changes in air temperature recorded in the greenland ice cores1 2 are coherent with shifts in the magnitude of the northward heat flux carried by the north atlantic surface ocean3 4 changes in the ocean s thermohaline circulation are a key component in many explanations of this climate flickering5 here we use stable isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate change events deep water was generated more or less continuously in the nordic seas during the latter part of the last glacial period 60 to 10 thousand years ago but by two different mechanisms the deep water formation occurred by convection in the open ocean during warmer periods interstadials but during colder phases stadials a freshening of the surface ocean reduced or stopped open ocean convection and deep water formation was instead driven by brine release during sea ice freezing these shifting magnitudes and modes nested within the overall continuity of deep water formation were probably important for the structuring and rapidity of the prevailing climate changes

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

abrupt climate events 500 000 to 340 000 years ago evidence from subpolar north atlantic sediments subpolar north atlantic proxy records document millennial scale climate variations 500 000 to 340 000 years ago the cycles have an approximately constant pacing that is similar to that documented for the last glacial cycle these findings suggest that such climate variations are inherent to the late pleistocene regardless of glacial state sea surface temperature during the warm peak of marine isotope stage 11 mis 11 varied by 0 5 to 1 c less than the 4 to 4 5 c estimated during times of ice growth and the 3 c estimated for glacial maxima coherent deep ocean circulation changes were associated with glacial oscillations in sea surface temperature

eustatic sea level change at the mid pleistocene climate transition new evidence from the shallow marine sediment record of japan abstract based on the excellent sedimentary record we have identified five depositional sequences in the upper part of the omma formation exposed along the japan sea coast of central japan the base of this stratigraphic section is placed just below the top of the jaramillo subchron and the top is located below the brunhes matuyama boundary biostratigraphy supports the interpretation that the lower three sequences correlate with marine oxygen isotope stages mis 28 26 26 24 and 24 22 respectively we have reconstructed a eustatic sea level curve from analyses of sedimentary facies fossil molluscs basin subsidence and sediment accumulation the resulting sea level curve shows that mean eustatic sea level dropped between 1 0 and 0 9 ma and that sea level was lowest at mis 22 which has the heaviest 18o value between mis 28 and 22 the total fall in mean sea level is inferred to be 20 30 m this independent sea level record demonstrates that ice sheet volume rapidly increased at the mid pleistocene climate transition mpt and culminated in mis 22 as inferred from the 18o record the upper two depositional sequences imply the contribution of precession to changes in ice sheet volume would have arisen in mis 21 just after the mpt

gulf stream and western boundary undercurrent variations during mis 10 12 at site 1056 blake bahama outer ridge abstract the stable isotopic composition of two planktonic foraminifer species globigerinoides sacculifer and neogloboquadrina dutertrei and two benthic foraminifer species cibicidoides wuellerstorfi and uvigerina peregrina was measured at sub orbital resolution through the marine isotope stages miss 10 11 and 12 345 460 ka at site 1056 on the blake bahama outer ridge planktonic foraminifers were counted for the interval 405 450 ka at 2 4 kyr resolution site 1056 32 29 n 76 20 w is located on the continental slope water depth 2167 m beneath the gulf stream the average rate of sediment accumulation through the interval is 11 4 cm kyr but sediment accumulation is much more rapid during glacial intervals 15 17 cm kyr the decline in percent carbonate during glacial intervals and its rise during interglacials indicates that the increased sediment supply is of terrigenous origin low carbonate values and low benthic 13c which are both associated with a weak western boundary undercurrent and low north atlantic deep water production persist for 6 kyr after the benthic 18o record indicates that ice volume has begun to decrease recovery of carbonate and benthic 13c values is abrupt and rapid millennial scale variation 3 4 kyr is apparent in the glacial intervals of the planktonic 18o records and is more pronounced in a 18o record which represents the temperature range in the photic zone semi precessional 10 12 kyr cycles are apparent in the planktonic 13c record the millennial scale cycles are largely caused by an increase in g sacculifer 18o and represent surface warming they are interpreted as representing periodic increases in westward intensification of the gyre the semi precessional cycles are driven by changes in the n dutertrei 13c and represent fluctuations in the gulf stream itself and therefore likely have a tropical origin planktonic foraminifer census counts did not show an expected response to one of the largest glacial interglacial transitions of the late pleistocene the most obvious change was an increase in faunal diversity during mis 12 2 the interval of maximum 18o values this suggests that cool slope water and warm subtropical gyre water penetrated a more sluggish gulf stream with greater frequency at this time the millennial scale maxima in the 18o record are accompanied by decreases in diversity which is consistent with the interpretation of surface warming during these events

north atlantic millennial scale climate variability 910 to 790 ka and the role of the equatorial insolation forcing abstract the mid pleistocene transition mpt was the time when quasi periodic 100 kyr high amplitude glacial variability developed in the absence of any significant change in the character of orbital forcing leading to the establishment of the characteristic pattern of late pleistocene climate variability it has long been known that the interval around 900 ka stands out as a critical point of the mpt when major glaciations started occurring most notably in the northern hemisphere here we examine the record of climatic conditions during this significant interval using high resolution stable isotope records from benthic and planktonic foraminifera from a sediment core in the north atlantic integrated ocean drilling program expedition 306 site u1313 we have considered the time interval from late in marine isotope stage mis 23 to mis 20 910 to 790 ka our data indicate that interglacial mis 21 was a climatically unstable period and was broken into four interstadial periods which have been identified and correlated across the north atlantic region these extra peaks tend to contradict previous studies that interpreted the mis 21 variability as consisting essentially of a linear response to cyclical changes in orbital parameters cooling events in the surface record during mis 21 were associated with low benthic carbon isotope excursions suggesting a coupling between surface temperature changes and the strength of the atlantic meridional overturning circulation time series analysis performed on the whole interval indicates that benthic and planktonic oxygen isotopes have significant concentrations of spectral power centered on periods of 10 7 kyr and 6 kyr which is in agreement with the second and forth harmonic of precession the excellent correspondence between the foraminifera 18 o records and insolation variations at the equator in march and september suggests that a mechanism related to low latitude precession variations advected to the high latitudes by tropical convective processes might have generated such a response this scenario accounts for the presence of oscillations at frequencies equal to precession harmonics at site u1313 as well as the occurrence of higher amplitude oscillations between the mis22 21 transition and most of mis 21 times of enhanced insolation variability

roles of sea ice surface wind feedback in maintaining the glacial atlantic meridional overturning circulation and climate abstractduring glacial periods climate varies between two contrasting modes the interstadials and stadials these climate changes are often associated with drastic reorganizations of the atlantic

evolution of millennial scale climate variability during the mid pleistocene 1 we use the oxygen isotopic composition of planktonic foraminifera globigerinoides ruber white from ocean drilling program site 1058 in the subtropical northwestern atlantic to construct a high resolution 800 year climate record spanning the mid pleistocene climate transition 410 ka to 1350 ka we investigate whether or not millennial scale instabilities in the proxy record are associated with the extent of continental glaciation g ruber 18o values display high frequency fluctuations throughout the record but the amplitude about mean glacial and interglacial 18o values increases at marine isotope stage mis 22 880 ka and is highest during mis 12 these observations support that millennial scale climate instabilities are associated with ice sheet size time series analysis illustrates that these variations have significant concentration of spectral power centered on periods of 10 12 ka and 5 ka the timing of these fluctuations agrees well or coincides with the periodicities of the second and fourth harmonics respectively of precessional forcing at the equator an insolation based origin of the millennial scale instabilities would be independent of ice volume and explains the presence of these fluctuations before the mid pleistocene climate transition as well as during interglacial intervals e g mis 37 and 17 because the amplitude of the millennial scale variations increases during the mid pleistocene transition feedback mechanisms associated with the growth of large 100 ka paced polar ice sheets may be important amplifiers of regional surface water hydrographic changes

the mid pleistocene climate transition onset of 100 ka cycle lags ice volume build up by 280 ka the mid pleistocene climate transition mpt is the complex climatic change which brought the late pleistocene ice ages we explore the mpt in the time and frequency domains by new methods of time series analysis high resolution oxygen isotope records reveal that the ice volume related increase in 18o mean amplitude 0 29 0 05 1 transition midpoint 922 12 ka duration 40 9 ka significantly preceded the abrupt increase in the amplitude of the 100 ka cycle at 641 9 ka this finding can be quantitatively simulated using a simple ice bedrock model in which due to the additional ice the calving threshold is exceeded the simulated calving events prior to 650 ka are separated by 77 ka whereas after 650 ka they occur pseudo periodically with a mean period of nearly 100 ka the cause for the delay of the 100 ka calving cycle was not a slow bedrock relaxation rather the coincidental combination of insolation existing ice mass and bedrock depression