present status of the astronomical polarity time scale for the mediterranean late neogene sedimentary cycles may reflect orbitally induced climate oscillations and can then be used to construct astronomical time scales following the initial tuning of the late pleistocene the anchored astronomical time scale was extended to the base of the pliocene using palaeoclimatic records from ocean drilling project odp sites in the eastern equatorial pacific and north atlantic and sedimentary cycle patterns in marine successions exposed onland in the mediterranean in this paper we present a review of the progress subsequently made in establishing a late neogene astronomical polarity time scale a p ts in the mediterranean region major steps forward are 1 the evaluation of the initial time scale using high resolution climatic proxy records different astronomical solutions and the additional influence of obliquity on sedimentary cycle patterns 2 the extension of the a p ts into the middle miocene i e back to about 12 3ma 3 the closure of the messinian gap in the a p ts 4 the incorporation of the continental record and 5 the intercalibration of astronomical and radioisotopic time

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

the relative contribution of orbital forcing and greenhouse gases to the north american deglaciation understanding what drove northern hemisphere ice sheet melt during the last deglaciation 21 7 ka can help constrain how sensitive contemporary ice sheets are to greenhouse gas ghgs changes the roles of orbital forcing and ghgs in the deglaciation have previously been modeled but not yet quantified here for the first time we calculate the relative effect of these forcings on the north american deglaciation by driving a dynamical ice sheet model glimmer cism with a set of unaccelerated transient deglacial simulations with a full primitive equation based ocean atmosphere general circulation model famous we find that by 9 ka orbital forcing has caused 50 of the deglaciation ghg 30 and the interaction between the two 20 orbital forcing starts affecting the ice volume at 19 ka 2000 years before co2 starts increasing in our experiments a delay which partly controls their relative effect

simulating the impact of freshwater inputs and deep draft icebergs formed during a mis 6 barents ice sheet collapse 1 an intermediate complexity climate model is used to simulate the collapse of the barents ice sheet during marine isotope stage 6 mis 6 140 ka b p with the purpose of investigating whether a mass input of freshwater from the collapse could have affected the convection and deep water formation in the north atlantic ocean further experiments used a coupled dynamic and thermodynamic iceberg model to determine the effects of deep draft icebergs rather than freshwater alone on the ocean circulation the results predict that the collapse of the barents ice sheet had a significant impact on the meridional overturning circulation in both the atlantic and pacific oceans freshwater fluxes have more of an impact on the atlantic overturning circulation during the actual release period compared to icebergs but the bergs induce effects over longer time scales even after the pulse is removed freshwater fluxes of 0 15 sverdrup sv and iceberg surges of 0 1 sv trigger significant changes in the global patterns particularly in the north pacific where there is strengthening of the overturning circulation at the expense of that in the north atlantic and associated increases in pacific sea surface temperatures these results highlight the importance of simulating not only the correct flux but also the form of the freshwater input from ice sheet collapses appropriately

seismotectonic model of the kangra chamba sector of northwest himalaya constraints from joint hypocenter determination and focal mechanism the existing seismological network in the kangra chamba sector has been upgraded with 12 three component digital seismometers to obtain new insight on the nature and sources of continued clustered seismicity in this part of northwest himalaya a combination of travel time distance plots and travel time inversion of p and s phases have been used to derive a 1d velocity model for the region the mini mum 1d velocity model divides the average 44 km thick crust into four layers the top 10 km thick layer represents the metamorphosed sediments of the chamba nappe that dominates the surface geology of the study area suggestion of a thin low velocity layer at 15 km depth possibly marks the detachment zone separating the downgoing indian plate from the overriding wedge the improved locations of epicenters show close clustering of seismic events immediately northeast of the epicenter of the 1905 kangra earthquake while away from this zone the seismicity in the chamba sector has more even distribution in the later sector space depth distribution of hypocenters suggests that strain resulting from the ongoing collision of the indian plate with asia is being consumed by reverse fault movement on the chamba thrust the clustered seismicity in the kangra sector has three distinct source regions and mechanisms 1 southward displacement of the thick chamba nappe sheet over the panjal imbricate zone along the panjal thrust accounts for the seismicity at shallow depths of less than 7 km 2 the nucleation of strains where the northeast dipping main boundary thrust mbt merges with the detachment plane produces focused seismicity near this junc tion and 3 the seismicity in a small pocket below the plane of detachment appears to be a consequence of stresses generated at the base of the northeast dipping detachment plane by the transverse structure

geochemical and paleomagnetic evidence for the occurrence of missing sapropels in eastern mediterranean sediments the cyclic occurrence of sapropels appears to be associated with maxima in the 65 n summer insolation target curve studies of the most recent sapropel have revealed extensive oxidation of organic matter or even the complete removal of this unit geochemical and magnetic signals allow for the detection of such a missing sapropel in older sediments these signals are altered by ongoing diagenetic processes an important parameter to reveal missing sapropels is ba elevated concentrations of ba are always found in visible sapropels and even after oxidation of part of the sapropel the initial ba profile remains visible we use this tool to trace down missing sapropels in kc01b a 37 m kullenberg core from the calabrian ridge eastern mediterranean at least 8 of 11 sampled intervals around insolation maxima contain evidence of a completely oxidized sapropel ba trace metal distributions and magnetic parameters are similar to those observed around visible sapropels

kinematic and thermodynamic study of a shallow hailstorm sampled by the mcgill bistatic multiple doppler radar network abstract in this paper the authors examine the kinematic and thermodynamic characteristics of a shallow hailstorm sampled by the mcgill bistatic multiple doppler radar network on 26 may 1997 this storm consists of two main shallow convective cells depth less than 5 km aligned along a sw ne convective line propagating to the southeast the authors also analyze the interactions between the two cells during the life cycle of the convective line in particular it is shown that dynamic interactions play a major role in the intensification of the second cell this storm is found to evolve in a manner that shares some characteristics with both multicell and supercell storms a rotating updraft associated with a mesocyclone develops in the mature stage of the storm which is characteristic of a supercell however the lack of a vault structure on the precipitation field the relatively fast evolution of the cells and other characteristics detailed henceforth seem to indicate that this storm only shares a f

north atlantic sea surface temperatures for the last 1 1 million years summary a 1 2 ma time series of north east atlantic sea surface temperature sst has been assembled from the spliced record of piston core k 708 7 0 68 0 ma and hydraulic piston cores taken in nearby dsdp hole 552a 1 2 0 68 ma a no analogue fauna precludes making credible sst estimates in the interval 1 2 1 1 ma but the record above 1 1 ma is suitable for sst estimates and related time series analysis the amplitude of sst variation is considerably higher in the brunhes than in the upper matuyama this is due both to colder glacial sst minima after 0 85 ma and to increasingly warm interglacial sst maxima from 0 7 to 0 4 ma the dominant periodicity in the sst signal is centred near 95 000 years it increases in amplitude by a factor of four from the bottom of the record to the top with the largest increase occurring between 0 7 and 0 4 ma this suggests a relatively gradual evolution of 100 000 year power over half a million years of the late quaternary rather than a single abrupt change at 0 9 ma significant but progressively smaller spectral peaks occur at periods of 54 000 41 000 31 000 23 000 and 19 000 years both the 54 000 year and 31 000 year signals may be a response to insolation forcing by minor obliquity terms long term 18 o records show small responses by the ice sheets at or near these two periods indicating the probable initial response of the climate system to these insolation rhythms in phase relationships between 18 o and sst at these periods suggest that the ice sheets then impose these rhythms on the ocean with no lag the mechanism by which such small insolation and ice volume signals become enhanced in the sst record is not clear

single doppler radar observations of a mini supercell tornadic thunderstorm abstract during the early evening hours of 19 may 1989 the chill 10 cm doppler weather radar observed most of the lifetime of an unusually small tornadic thunderstorm throughout the event the parent thunderstorm echo top remained below 6 7 km msl the low altitude echo diameter as defined by the 25 dbz contour was only 15 km despite its small size both visual and radar observations indicated that this storm contained many of the organizational features often noted in large classical southern great plains supercells the synoptic setting in which this storm occurred was atypical for supercell development in that both the thermodynamic instability and vertical wind shear magnitudes were limited this documentation of a tornadic storm that developed in a nonthreatening environment mid that presented a small seemingly inconsequential radar appearance demonstrates some of the challenges that will be faced by automated doppler radar based severe weather detection algorithms

a detection of milankovitch frequencies in global volcanic activity a rigorous detection of milankovitch periodicities in volcanic output across the pleistocene holocene ice age has remained elusive we report on a spectral analysis of a large number of well preserved ash plume deposits recorded in marine sediments along the pacific ring of fire our analysis yields a statistically significant detection of a spectral peak at the obliquity period we propose that this variability in volcanic activity results from crustal stress changes associated with ice age mass redistribution in particular increased volcanism lags behind the highest rate of increasing eustatic sea level decreasing global ice volume by 4 0 3 6 k y and correlates with numerical predictions of stress changes at volcanically active sites these results support the presence of a causal link between variations in ice age climate continental stress field and volcanism

extending the astronomical polarity time scale into the miocene an astronomical time scale is presented for the late miocene based on the correlation of characteristic sedimentary cycle patterns in marine sections in the mediterranean to the 65 n summer insolation curve of la90 1 2 with present day values for the dynamical ellipticity of the earth and tidd dissipation by the moon this correlation yields ages for all sedimentary cycles and hence also for the recorded polarity reversals and planktonic foraminiferal and dinoflagellate events the tortonian messinian t m boundary placed at the first regular occurrence of the globorofuliu conomiozea group in the mediterranean is dated at 7 24 ma the duration of the messinian is estimated at 1 91 myr because the miocene pliocene boundary has been dated previously at 5 33 ma 3 the new time scale is confirmed by oar 3gar ages of volcanic beds and by the number of sedimentary cycles in the younger part of the mediterranean messinian

integrated summer insolation forcing and 40 000 year glacial cycles the perspective from an ice sheet energy balance model although the origins of the 40 000 year glacial cycles during the early pleistocene are readily attributed to changes in earth s obliquity also having a 40 000 year period the lack of ice volume variability at precession periods 20 000 years is difficult to reconcile with most parameterizations of the insolation forcing it was recently proposed that precession s influence on glaciation is muted because variations in the intensity of summer insolation are counterbalanced by changes in the duration of the summertime but no climate model has yet been shown to generate obliquity period glacial cycles in response to the seasonal insolation forcing here we present a coupled ice sheet energy balance model that reproduces the seasonal cycle and when run over long time periods generates glacial variability in response to changes in earth s orbital configuration the model is forced by the full seasonal cycle in insolation and its response can be understood within the context of the integrated summer insolation forcing the simple fact that obliquity s period is roughly twice as long as that of precession results in a larger amplitude glacial response to obliquity however for the model to generate almost exclusively obliquity period glacial variability two other conditions must be met first the ice sheet s ablation zone must reside poleward of 60 n because insolation intensity is more sensitive to changes in earth s obliquity at high latitudes second the ablation season must be long enough for precession s opposing influences on summer and fall insolation intensity to counterbalance one another these conditions are consistent with a warm climate and a thin ice sheet where the latter is simulated as a response to subglacial sediment deformation if a colder climate is prescribed or in the absence of basal motion ice sheets tend to be larger and undergo greater precession period variability in keeping with proxy observations of late pleistocene glaciation

the deep ocean during the last interglacial period oxygen isotope analysis of benthic foraminifera in deep sea cores from the atlantic and southern oceans shows that during the last interglacial period north atlantic deep water nadw was 0 4 0 2 c warmer than today whereas antarctic bottom water temperatures were unchanged model simulations show that this distribution of deep water temperatures can be explained as a response of the ocean to forcing by high latitude insolation the warming of nadw was transferred to the circumpolar deep water providing additional heat around antarctica which may have been responsible for partial melting of the west antarctic ice sheet

astronomical calibration of the matuyama brunhes boundary consequences for magnetic remanence acquisition in marine carbonates and the asian loess sequences we have compiled 19 records from marine carbonate cores in which the matuyama brunhes boundary mbb has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes by correlation of the 18o data to a timescale based on astronomical forcing we estimate astronomical ages for each of the mbb horizons in all but one record the mbb occurs within stage 19 most magnetostratigraphic sections in asian loess place the mbb within a loess interval since loess deposition is presumed to be associated with glacial intervals loess horizons should correspond to even numbered oxygen isotope stages a glacial age for the mbb is at odds with the results presented here which firmly place the mbb within interglacial stage 19 inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed the mean of the stage 19 age estimates for the mbb is 777 9 1 8 n 18 inclusion of the single stage 20 age results in a mean of 778 8 2 5 n 19 the astronomical age estimate of the mbb compares favorably with an unweighted mean of 778 2 3 5 n 10 from a compilation of 40ar 39ar results of transitional lava flows combining the two independent data sets yields a grand mean of 778 0 1 7 n 28 the new compilation shows virtually no trend in placement of the mbb within isotope stage 19 as a function of sediment accumulation rate we interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment water interface separating the cores into two geographic regions an indo pacific caribbean ipc group and an atlantic group results in a significant difference in the position of the mid point of the reversal with respect to the astronomical time scale the data presented here suggest a difference of several thousand years between the two regions this observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest systematic differences in remanence acquisition or by genuine differences in the timing of the directional changes between the two regions

interglacials of the last 800 000 years interglacials including the present holocene period are warm low land ice extent high sea level end members of glacial cycles based on a sea level definition we identify eleven interglacials in the last 800 000 years a result that is robust to alternative definitions data compilations suggest that despite spatial heterogeneity marine isotope stages mis 5e last interglacial and 11c 400 ka ago were globally strong warm while mis 13a 500 ka ago was cool at many locations a step change in strength of interglacials at 450 ka is apparent only in atmospheric co2 and in antarctic and deep ocean temperature the onset of an interglacial glacial termination seems to require a reducing precession parameter increasing northern hemisphere summer insolation but this condition alone is insufficient terminations involve rapid nonlinear reactions of ice volume co2 and temperature to external astronomical forcing the precise timing of events may be modulated by millennial scale climate change that can lead to a contrasting timing of maximum interglacial intensity in each hemisphere a variety of temporal trends is observed such that maxima in the main records are observed either early or late in different interglacials the end of an interglacial glacial inception is a slower process involving a global sequence of changes interglacials have been typically 10 30 ka long the combination of minimal reduction in northern summer insolation over the next few orbital cycles owing to low eccentricity and high atmospheric greenhouse gas concentrations implies that the next glacial inception is many tens of millennia in the future

on the identification of a pliocene time slice for data model comparison the characteristics of the mid pliocene warm period mpwp 3 264 3 025 ma bp have been examined using geological proxies and climate models while there is agreement between models and data details of regional climate differ uncertainties in prescribed forcings and in proxy data limit the utility of the interval to understand the dynamics of a warmer than present climate or evaluate models this uncertainty comes in part from the reconstruction of a time slab rather than a time slice where forcings required by climate models can be more adequately constrained here we describe the rationale and approach for identifying a time slice s for pliocene environmental reconstruction a time slice centred on 3 205 ma bp 3 204 3 207 ma bp has been identified as a priority for investigation it is a warm interval characterized by a negative benthic oxygen isotope excursion 0 21 0 23 centred on marine isotope stage km5c km5 3 it occurred during a period of orbital forcing that was very similar to present day climate model simulations indicate that proxy temperature estimates are unlikely to be significantly affected by orbital forcing for at least a precession cycle centred on the time slice with the north atlantic potentially being an important exception

red sea circulation during marine isotope stage 5e we have employed a regional massachusetts institute of technology oceanic general circulation model of the red sea to investigate its circulation during marine isotope stage mis 5e the peak of the last interglacial approximately 125 ka before present compared to present day conditions mis 5e was characterized by higher northern hemisphere summer insolation accompanied by increases in air temperature of more than 2 c and global sea level approximately 8 m higher than today as a consequence of the increased seasonality intensified monsoonal conditions with increased winds rainfall and humidity in the red sea region are evident in speleothem records and supported by model simulations to assess the dominant factors responsible for the observed changes we conducted several sensitivity experiments in which the mis 5 boundary conditions or forcing parameters were used individually overall our model simulation for the last interglacial maximum reconstructs a red sea that is colder less ventilated and probably more oligotrophic than at present day the largest alteration in red sea circulation and properties was found for the simulation of the northward displacement and intensification of the african tropical rain belt during mis 5e leading to a notable increase in the fresh water flux into the red sea such an increase significantly reduced the red sea salinity and exchange volume of the red sea with the gulf of aden the red sea reacted to the mis 5e insolation forcing by the expected increase in seasonal sea surface temperature amplitude and overall cooling caused by lower temperatures during deep water formation in winter

deglacial rapid sea level rises caused by ice sheet saddle collapses a simulation of ice sheet retreat reveals two meltwater pulses coinciding with mwp 1a and the 8 200 year event caused by saddle collapses between ice domes during the last deglaciation the retreating ice sheets occasionally released massive amounts of fresh water these meltwater pulses are often implicated as important influences on abrupt climate change but the underlying triggering mechanisms are not clear here lauren gregoire et al show that saddle zones that connect adjacent high domes on ice sheets can be lowered by ongoing warming to an elevation at which they rapidly degrade leading to separate domes and a meltwater pulse that is consistent with geological observations although the mechanism itself is not novel the authors establish that two important sudden cooling events meltwater pulse 1a and the 8 200 year event were probably caused by saddle separation the last deglaciation 21 to 7 thousand years ago was punctuated by several abrupt meltwater pulses which sometimes caused noticeable climate change1 2 around 14 thousand years ago meltwater pulse 1a mwp 1a the largest of these events produced a sea level rise of 14 18 metres over 350 years3 although this enormous surge of water certainly originated from retreating ice sheets there is no consensus on the geographical source or underlying physical mechanisms governing the rapid sea level rise4 5 6 here we present an ice sheet modelling simulation in which the separation of the laurentide and cordilleran ice sheets in north america produces a meltwater pulse corresponding to mwp 1a another meltwater pulse is produced when the labrador and baffin ice domes around hudson bay separate which could be associated with the 8 200 year event the most pronounced abrupt climate event of the past nine thousand years7 for both modelled pulses the saddle between the two ice domes becomes subject to surface melting because of a general surface lowering caused by climate warming the melting then rapidly accelerates as the saddle between the two domes gets lower producing nine metres of sea level rise over 500 years this mechanism of an ice saddle collapse probably explains mwp 1a and the 8 200 year event and sheds light on the consequences of these events on climate

glacial variability over the last two million years an extended depth derived agemodel continuous obliquity pacing and the pleistocene progression abstract an agemodel not relying upon orbital assumptions is estimated over the last 2 ma using depth in marine sediment cores as a proxy for time agemodel uncertainty averages 10 ka in the early pleistocene 2 1 ma and 7 ka in the late pleistocene 1 ma to the present twelve benthic and five planktic 18 o records are pinned to the agemodel and averaged together to provide a record of glacial variability major deglaciation features are identified over the last 2 ma and a remarkable 33 out of 36 occur when earth s obliquity is anomalously large during the early pleistocene deglaciations occur nearly every obliquity cycle giving a 40 ka timescale while late pleistocene deglaciations more often skip one or two obliquity beats corresponding to 80 or 120 ka glacial cycles which on average give the 100 ka variability this continuous obliquity pacing indicates that the glacial theory can be simplified an explanation for the 100 ka glacial cycles only requires a change in the likelihood of skipping an obliquity cycle rather than new sources of long period variability furthermore changes in glacial variability are not marked by any single transition so much as they exhibit a steady progression over the entire pleistocene the mean variance skewness and timescale associated with the glacial cycles all exhibit an approximately linear trend over the last 2 ma a simple model having an obliquity modulated threshold and only three adjustable parameters is shown to reproduce the trends timing and spectral evolution associated with the pleistocene glacial variability

evaluation of the astronomically calibrated time scale for the late pliocene and earliest pleistocene an astronomically calibrated time scale was recently established hilgen 1991a b for the pliocene and earliest pleistocene based on the correlation of dominantly precession controlled sedimentary cycles sapropels and caco3 cycles in mediterranean deep sea sections to astronomical solution ber90 berger and loutre 1991 this time scale which differed substantially from the conventional time scales then widely used is essentially confirmed by a new generation of radiometric dates using the ar 40 ar 39 laser fusion technique to evaluate this time scale we here extract both precession and obliquity related components from late pliocene earliest pleistocene climatic proxy records in the mediterranean and determine coherencies and phase time lags between these components and the respective orbital variations this work is found to support the new time scale because significant coherencies are found between the astronomically related components in our proxy records and the orbital variations not only at the main period of precession but also at the main period of obliquity and because of the eccentricity modulation of precession in the low frequency eccentricity bands of the spectrum but the calculated time lag between obliquity and 41 kyr components in the climatic proxy records shows a significant increase of 5 6 1 kyr with respect to the late pleistocene if the phase relation with precession is kept constant this increase can be explained by a change in the climate response time to obliquity or to precession by a small uncertainty in the astronomical solution or by a combination of these factors

timing and nature of the penultimate deglaciation in a high alpine stalagmite from switzerland abstract the timing and nature of the penultimate deglaciation also known as termination ii t ii is subject of controversial discussions due to the scarcity of precisely dated palaeoclimate records here we present a new precisely dated and highly resolved multi proxy stalagmite record covering t ii from the high alpine schafsloch cave in switzerland an area where climate is governed by the north atlantic the inception of stalagmite growth at 137 4 1 4 kyr before present bp indicates the presence of drip water and cave air temperatures of above 0 c and is related to a climate induced change in the thermal state from cold to warm based of the glacier above the cave the cessation of stalagmite growth between 133 1 0 7 and 131 9 0 6 kyr bp is most likely related to distinct drop in temperature associated with heinrich stadial 11 the resumption of stalagmite growth at 131 9 0 6 kyr bp is accompanied by an abrupt increase in temperature and precipitation as indicated by distinct shifts in the oxygen and carbon isotopic composition as well as in trace element concentrations the mid point of t ii is around 131 8 0 6 kyr bp in the schafsloch cave record is significantly earlier compared to the age of 129 1 0 1 kyr bp in the sanbao cave record from china the different ages between both records can be best explained by the competing effects of insolation and glacial boundary forcing on seasonality and snow cover extent in eurasia

orbital insolation ice volume and greenhouse gases abstract the specmap models of orbital scale climate change imbrie et al paleoceanography 7 1992 701 paleoceanography 8 1993 699 are the most comprehensive to date all major climatic observations were analyzed within the framework of the three orbital signals subsequently tuning of signals in vostok ice to insolation forcing has fixed the timing of greenhouse gas changes closely enough to permit an assessment of their orbital scale climatic role in addition evidence from several sources has suggested changes in the specmap 18 o time scale this new information indicates that the timing of co 2 changes at the periods of precession and obliquity does not fit the 1992 specmap model of a train of responses initiated in the north propagated to the south and later returning north to force the ice sheets in addition analysis of the effects of rectification on 100 000 year climatic signals reveals that all have a phase on or near that of eccentricity this close clustering of phases rules out the long time constants for 100 000 year ice sheets required by the 1993 specmap model a new hypotheses presented here revives elements of an earlier climap view hays et al science 194 1976a 1121 but adds a new assessment of the role of greenhouse gases as proposed by milankovitch summer mid july insolation forces northern hemisphere ice sheets at the obliquity and precession periods with an ice time constant derived here of 10 000 years changes in ice volume at 41 000 years drive ice proximal signals sst nadw dust that produce a strong positive co 2 feedback and further amplify ice volume changes at the precession period july insolation forces ice sheets but it also drives fast and early responses in ch 4 through changes in tropical monsoons and boreal wetlands and variations in co 2 through southern hemisphere processes these ch 4 and co 2 responses enhance insolation forcing of ice volume climatic responses at 100 000 years result from eccentricity pacing of forced processes embedded in obliquity and precession cycles increased modulation of precession by eccentricity every 100 000 years produces 23 000 year co 2 and ch 4 maxima that enhance ablation caused by summer insolation and drive climate deeper into an interglacial state when eccentricity modulation decreases at the 100 000 year cycle ice sheets grow larger in response to obliquity forcing and activate a 41 000 year co 2 feedback that drives climate deeper into a glacial state alternation of these forced processes because of eccentricity pacing produces the 100 000 year cycle the 100 000 year cycle began 0 9 myr ago because gradual global cooling allowed ice sheets to survive during weak precession insolation maxima and grow large enough during 41 000 year ice volume maxima to generate strong positive co 2 feedback the natural orbital scale timing of these processes indicates that ice sheets should have appeared 6000 3500 years ago and that co 2 and ch 4 concentrations should have fallen steadily from 11 000 years ago until now but new ice did not appear and co 2 and ch 4 began anomalous increases at 8000 and 5000 years ago respectively human generation of co 2 and ch 4 is implicated in these anomalous trends and in the failure of ice sheets to appear in canada

modeling water isotopologues during the last glacial implications for quantitative paleosalinity reconstruction quantitative paleosalinity reconstructions with reasonable uncertainties remain a challenge in paleoceanography in this study we focus on stable isotope based methods 18o and 2h to derive paleosalinity we use the water isotopes enabled fully coupled atmosphere ocean vegetation land surface three dimensional model of intermediate complexity iloveclim to simulate the climate and water isotopes during the last glacial maximum lgm and heinrich event 1 we investigate how the isotopes in water can be used as reliable proxies to quantitatively reconstruct past changes in seawater salinity our results demonstrate that a quantitative salinity reconstruction during glacial conditions based on present day regional 18o salinity relationships can lead to considerable errors up to 25 g kg in certain regions however we show that these eventual uncertainties on paleosalinity reconstruction can be reduced by allowing for model derived regional 18o salinity relationships to vary through time our results indicate a rather stable dependence between 2h and 18o in surface seawater during the lgm this suggest that quantitative reconstruction of salinity based on the 2h measurement of alkenones 2ha might be possible if the slope and the intercept of the regression between the fractionation factor 2ha 2hsw and salinity can be sufficiently tightly constrained in open ocean conditions we confirm that pairing water isotopologues has a strong potential to reduce uncertainties on quantitative paleosalinity reconstructions also under glacial boundary conditions

astronomical solutions for paleoclimate studies over the last 3 million years three solutions numerical qtd90 semi analytical la90 and analytical ber90 for the planetary and precessional elements of the earth have recently become available they allow the computation of the fundamental astroclimatic parameters eccentricity climatic precession and obliquity the agreement between these different solutions is remarkably good reflecting their accuracy in such a way that they can be considered reliable enough for paleoclimate studies over at least 3 myr the difference between these solutions in eccentricity is less than 0 005 over this time interval while the differences in obliquity and climatic precession are larger but mainly due to phase lag between the solutions at 3 myr bp ber90 leads la90 by 6 x 10 3 yrs and qtd90 by 9 x 10 3 yrs for the obliquity and it leads la90 by 2 x 10 3 yrs and qtd90 by 4 x 10 3 yrs for the climatic precession

does interplanetary dust control 100 kyr glacial cycles the cause of the 100 kyr glacial interglacial cycles during the past 800 kyr is one of the fundamental puzzles in paleoclimatology the widely accepted milankovitch theory relating earth s climate cycles to variations in insolation caused by periodic changes in orbital parameters has difficulties to explain the predominant 100 kyr rhythm although earth s eccentricity varies with a period of 100 kyr the resulting change in insolation is too small to produce the corresponding climate cycle by direct forcing imbrie et al paleoceanography 8 1993 699 in order to solve the 100 kyr problem muller and macdonald nature 377 1995 107 science 277 1997a 215 proc nat acad sci usa 94 1997b 8329 proposed an alternative orbital but non milankovitch mechanism attributing the glacial cycles to regular variations in the accretion of interplanetary dust particles idp caused by 100 kyr cycles in the orbital inclination of the earth to test this controversial hypothesis we study the idp accumulation in deep sea sediments from a period in the early pleistocene we find apparent 41 kyr cycles but no 100 kyr periodicity in the idp accumulation rate as there is no known mechanism to produce 41 kyr cycles in idp supply from space we conclude that the 41 kyr cycles are caused by the dynamics of sediment accumulation and that changes in the idp flux do not drive the pleistocene glacial cycles r 2004 elsevier ltd all rights reserved

infilling of the younger kathmandu banepa intermontane lake basin during the late quaternary lesser himalaya nepal a sedimentological study the kathmandu and banepa basins central nepal are located in a large syncline of the lesser himalayas the older kathmandu lake evolved during the pliocene and early pleistocene the younger kathmandu lake which is the focus of this study is infilled with late quaternary sediments three formations arranged in stratigraphical order the kalimati gokarna and thoka formations formed during the infilling stage of this lacustrine basin structural and textural sedimentological analyses a chemical survey across the basin and mineralogical investigations of fine grained sediments form the basis of this palaeogeographical study the basin under investigation was covered by a perennial freshwater lake before 30 000 yr bp the lake was infilled with alluvial and fluvial sediments delivered mainly from the mountains north of the basin a fairly low gradient was favourable for the formation of diatomaceous earths carbonaceous mudstones and siltstones which were laid down in the centre of the lake and in small ponds towards the basin edge lacustrine sediments gave way to deltaic deposits spread across the delta plain crevasse splays and anastomosing rivers mainly delivered suspended load for the widespread siltstones and mudstones the proximal parts of the alluvial fluvial sedimentary wedge contain debris flows that interfinger with fine grained floodplain deposits three highstands of the water level 30 000 yr bp 28 000 19 000 yr bp 11 000 4000 yr bp have been recognised in the sedimentary record of the younger kathmandu lake in the late quaternary second order water level fluctuations are assumed to be triggered by local processes damming by tectonically induced landslides first order water level fluctuations are the result of climatic changes copyright 2003 john wiley sons ltd

the role of co2 decline for the onset of northern hemisphere glaciation the pliocene pleistocene transition ppt from around 3 2 to 2 5 million years ago ma represented a major shift in the climate system and was characterized by a gradual cooling trend and the appearance of large continental ice sheets over northern eurasia and north america paleo evidence indicates that the ppt was accompanied and possibly caused by a decrease in atmospheric co2 but the temporal resolution of co2 reconstructions is low for this period of time and uncertainties remain large therefore instead of applying existent co2 reconstructions we solved an inverse problem by finding a schematic co2 concentration scenario that allows us to simulate the temporal evolution of key climate characteristics in agreement with paleoclimate records to this end we performed an ensemble of transient simulations with an earth system model of intermediate complexity from which we derived a best guess transient co2 scenario for the interval from 3 2 to 2 4 ma that gives the best fit between the simulated and reconstructed benthic 18o and global sea surface temperature evolution our data constrained co2 scenarios are consistent with recent co2 reconstructions and suggest a gradual co2 decline from 375 425 to 275 300 ppm between 3 2 and 2 4 ma in addition to a gradual decline the best fit to paleoclimate data requires the existence of pronounced co2 variability coherent with the 41 kyr 1 kyr 1000 years obliquity cycle in our simulations the long term co2 decline is accompanied by a relatively abrupt intensification of northern hemisphere glaciation at around 2 7 ma this is the result of a threshold behaviour of the ice sheets response to gradual co2 decrease and orbital forcing the simulated northern hemisphere ice sheets during the early pleistocene glacial cycles reach a maximum volume equivalent to a sea level drop of about 40 m both ice volume and benthic 18o are dominated by 41 kyr cyclicity our simulations suggest that before 2 7 ma greenland was ice free during summer insolation maxima and only partly ice covered during periods of minimum summer insolation a fully glaciated greenland comparable to its present day ice volume is modelled only during glacial maxima after 2 7 ma and more continuously after 2 5 ma

earth history sediments to planetary motion celestial mechanics has long been used as an aid for interpreting geological records the compliment is being repaid through analysis of mediterranean sediments

long term equilibrium tides extreme equilibrium tides or hypertides are computed in a new equilibrium tidal model combining algorithms of a version of the chapront elp 2000 82 lunar theory with the ber78 milankovitch astronomical expansions for the rerent past a high correspondence exists between computed semidiurnal tide levels and a record of coastal flooding demonstrating that astronomical alignment is a potential influence on such flooding for the holocene and near future maximum tides demonstrate cyclic variations with peaks at near 5000 b p and 4000 a p on the late quaternary timescale variations in maximum equilibrium tide level display oscillations with periods of approximately 10 000 100 000 and 400 000 years because of precessional shifts in tidal maxima between vernal and autumnal equinoxes while flooding occurs under the combined effects of tides and storms via storm surges the most extensive flooding will occur with the coincidence of storms and the rarer hypertides and is thus primarily influenced by hypertides therefore we suggest that astronomical alignment s relationship to coastal flooding is probabilistic rather than deterministic data derived from this model are applicable to 1 archaeological and paleoclimatic coastal reconstructions 2 long term planning for example radioactive waste site selection 3 sealevel change and paleoestuarine studies or 4 ocean meteorological interactions

the sensitivity of simulated supercell structure and intensity to variations in the shapes of environmental buoyancy and shear profiles abstract convective storm simulations are conducted using varying thermal and wind profile shapes subject to the constraints of strict conservation of convective available potential energy cape and hodograph trace small and large cape regimes and straight and curved hodographs are studied each with a matrix of systematically varying thermal and wind profile shapes having identical levels of free convection and bulk richardson numbers favorable to supercell development differences in storm intensity and morphology resulting from changes in the profile shapes can be profound especially in the small cape regime where for the moderate shears studied here storms are generally weak except when the buoyancy is concentrated at low levels in stronger cape regimes less dramatic relative enhancements of storm updraft intensity are found when both the buoyancy and shear are concentrated at low levels peak midlevel vertical vorticity correlates roughly with peak updraft speed in the small cape regime but

magnitude of climate variability during middle pliocene warmth a palaeoclimate modelling study abstract the nature and magnitude of climatic variability during the period of middle pliocene warmth ca 3 29 2 97 ma is poorly understood we present a suite of palaeoclimate modelling experiments incorporating an advanced atmospheric general circulation model gcm coupled to a q flux ocean model for 3 29 3 12 and 2 97 ma bp astronomical solutions for the periods in question were derived from the berger and loutre bl2 astronomical solution boundary conditions excluding sea surface temperatures ssts which were predicted by the slab ocean model were provided from the usgs prism2 2 2 digital data set the model results indicate that little annual variation 0 5 c in ssts relative to a control experiment occurred during the middle pliocene in response to the altered orbital configurations annual surface air temperatures also displayed little variation seasonally surface air temperatures displayed a trend of cooler temperatures during december january and february and warmer temperatures during june july and august this pattern is consistent with altered seasonality resulting from the prescribed orbital configurations precipitation changes follow the seasonal trend observed for surface air temperature compared to present day surface wind strength and wind stress over the north atlantic north pacific and southern ocean remained greater in each of the pliocene experiments this suggests that wind driven gyral circulation may have been consistently greater during the middle pliocene the trend of climatic variability predicted by the gcm for the middle pliocene accords with geological data however it is unclear if the model correctly simulates the magnitude of the variation this uncertainty is derived from a the relative insensitivity of the gcm to perturbation in the imposed boundary conditions b a lack of detailed time series data concerning changes to terrestrial ice cover and greenhouse gas concentrations for the middle pliocene and c difficulties in representing the effects of climatic history in snap shot gcm experiments

pleistocene climatic variability at astronomical frequencies abstract spectral analysis of geological records show periodicities corresponding to those calculated for the eccentricity 400 and 100 ka the obliquity 41 ka and the climatic precession 23 and 19 ka it is precisely the geological observations of this bi partition of the precessional peak confirmed to be real in astronomical computations which was one of the most impressive of all tests for the milankovitch theory concerning the question of whether or not the observed cycles account for most of the climatic variability having periods in the range predicted by the astronomical theory substantial evidence from cross spectral analysis coherency analysis and modelling is provided that at least near frequencies of variation in obliquity and precession a considerable fraction of the climate variance is driven in some way by insolation changes accompanying changes in the earth s orbit the variance components centered near a 100 ka cycle dominate most ice volume records and seem in phase with the eccentricity cycle although the exceptional strength of this cycle needs a non linear amplification by the glacial ice sheets themselves and associated feedbacks as the insolation spectra change with latitudes and with the type of parameters considered the diversity of the spectra of different climatic proxy data recorded in different places of the world over different periods is used to better understand how the climate system responds to the insolation forcing this study of the physical mechanisms involved is also achieved through the analysis of the log log shape of the geological records and through the comparison in frequency domain between simulated climatic time series and proxy data the evidence both in the frequency and in the time domain that orbital influences are felt by the climate system implies that the astronomical theory may provide a clock with which to date quaternary sediments with a precision several times greater than is now possible

a depth derived pleistocene age model uncertainty estimates sedimentation variability and nonlinear climate change 1 a new chronology of glaciation spanning the last 780 000 years is estimated from 21 marine sediment cores using depth as a proxy for time to avoid biasing this depth derived age estimate the depth scale is first corrected for the effects of sediment compaction to provide age uncertainty estimates the spatial and temporal variability of marine sediment accumulation rates are estimated and modeled as an autocorrelated stochastic process depth derived ages are estimated to be accurate to within 9000 years and within this uncertainty are consistent with the orbitally tuned age estimates nonetheless the remaining differences between the depth and orbitally tuned chronologies produce important differences in the spectral domain from the 18o record using the depth derived ages we infer that there are weak nonlinearities involving the 100 kyr and obliquity frequency bands which generate interaction bands at sum and difference frequencies if an orbitally tuned age model is instead applied these interactions are suppressed with the system appearing more nearly linear

simulating heinrich event 1 with interactive icebergs during the last glacial major abrupt climate events known as heinrich events left distinct fingerprints of ice rafted detritus and are thus associated with iceberg armadas the release of many icebergs into the north atlantic ocean we simulated the impact of a large armada of icebergs on glacial climate in a coupled atmosphere ocean model in our model dynamic thermodynamic icebergs influence the climate through two direct effects first melting of the icebergs causes freshening of the upper ocean and second the latent heat used in the phase transition of ice to water results in cooling of the iceberg surroundings this cooling effect of icebergs is generally neglected in models we investigated the role of the latent heat by performing a sensitivity experiment in which the cooling effect is switched off at the peak of the simulated heinrich event icebergs lacking the latent heat flux are much less efficient in shutting down the meridional overturning circulation than icebergs that include both the freshening and the cooling effects the cause of this intriguing result must be sought in the involvement of a secondary mechanism facilitation of sea ice formation which can disturb deep water production at key convection sites with consequences for the thermohaline circulation we performed additional sensitivity experiments designed to explore the effect of the more plausible distribution of the dynamic icebergs melting fluxes compared to a classic hosing approach with homogeneous spreading of the melt fluxes over a section in the mid latitude north atlantic na ocean the early response of the climate system is much stronger in the iceberg experiments than in the hosing experiments which must be a distribution effect the dynamically distributed icebergs quickly affect western nadw formation which synergizes with direct sea ice facilitation causing an earlier sea ice expansion and climatic response furthermore compared to dynamic thermodynamic icebergs a homogeneous hosing overestimates the fresh water flux in the eastern ruddiman belt causing a fresh anomaly in the eastern north atlantic leading to a delayed recovery of the circulation after the event

early pleistocene glacial cycles and the integrated summer insolation forcing long term variations in northern hemisphere summer insolation are generally thought to control glaciation but the intensity of summer insolation is primarily controlled by 20 000 year cycles in the precession of the equinoxes whereas early pleistocene glacial cycles occur at 40 000 year intervals matching the period of changes in earth s obliquity the resolution of this 40 000 year problem is that glaciers are sensitive to insolation integrated over the duration of the summer the integrated summer insolation is primarily controlled by obliquity and not precession because by kepler s second law the duration of the summer is inversely proportional to earth s distance from the sun

late quaternary variability of the sedimentary record in the sao tome deep sea system south brazilian basin abstract very few sediment records allow addressing the paleoceanographic changes that took place on orbital and or sub milankovitch time scales in the south west atlantic here a set of sediment cores located across a deep channel levee system in the south brazilian basin 3700 3900 m depth is used to infer the sediment dynamics that prevailed on this system during the late quaternary and to relate this pattern to climate changes the spatial distribution of deposits changes in mineralogy texture structure and sedimentation rate according to the morphology indicates bulk of sediment supplies is distributed on the sao tome system by bottom advective processes the silty sandy layers originate undoubtedly from turbidity currents whereas the fine grained muds and oozes that form the major part of the deposits are controlled by deep geostrophic currents in addition to variations forced by the orbital cycles more turbidites less carbonates and higher sedimentation rate during glacials the sediment records chiefly the caco 3 record document outstanding higher frequency regional changes in sedimentation according to the location of the cores at the present day interface between nadw and aabw we infer that changes in sedimentation are related to pulses of aabw versus nadw production these changes commonly marked by asymmetric carbonate pattern of about 10 kyr suggest that the relative influence of nadw versus aabw followed throughout the last 190 kyr a repetitive sequence of progressive increase and sharp decrease the rather sharp shut down observed during the late glacial could be associated to heinrich events when nadw production stopped but similar events are also well expressed during mis 5 mis 6 and at the end of mis 7

sensitivity of pliocene ice sheets to orbital forcing the stability of the earth s major ice sheets is a critical uncertainty in predictions of future climate and sea level change one method of investigating the behaviour of the greenland and the antarctic ice sheets in a warmer than modern climate is to look back at past warm periods of earth history for example the pliocene this paper presents climate and ice sheet modelling results for the mid pliocene warm period mpwp 3 3 to 3 0 million years ago which has been identified as a key interval for understanding warmer than modern climates jansen et al 2007 using boundary conditions supplied by the united states geological survey prism group pliocene research interpretation and synoptic mapping the hadley centre coupled ocean atmosphere climate model hadcm3 and the british antarctic survey ice sheet model basism we show large reductions in the greenland and east antarctic ice sheets gris and eais compared to modern in standard mpwp experiments we also present the first results illustrating the variability of the ice sheets due to realistic orbital forcing during the mid pliocene while gris volumes are lower than modern under even the most extreme cold mid pliocene orbit losing at least 35 of its ice mass the eais can both grow and shrink losing up to 20 or gaining up to 10 of its present day volume the changes in ice sheet volume incurred by altering orbital forcing alone means that global sea level can vary by more than 25 m during the mid pliocene however we have also shown that the response of the ice sheets to mpwp orbital hemispheric forcing can be in anti phase whereby the greatest reductions in eais volume are concurrent with the smallest reductions of the gris if this anti phase relationship is in operation throughout the mpwp then the total eustatic sea level response would be dampened compared to the ice sheet fluctuations that are theoretically possible this suggests that maximum eustatic sea level rise does not correspond to orbital maxima but occurs at times where the anti phasing of northern and southern hemisphere ice sheet retreat is minimised

a methane based time scale for vostok ice abstract tuning the vostok methane signal to mid july 30 n insolation yields a new ice core gas time scale this exercise has two rationales 1 evidence supporting kutzbach s theory that low latitude summer insolation in the northern hemisphere controls the strength of tropical monsoons and 2 interhemispheric ch 4 gradients showing that the main control of orbital scale ch 4 variations is tropical monsoonal sources the immediate basis for tuning ch 4 to mid july insolation is the coincident timing of the most recent pre anthropogenic ch 4 maximum at 11 000 10 500 calendar years ago and the most recent july 30 n insolation maximum all ages in this paper are in calendar years unless specified as 14 c years the resulting ch 4 gas time scale diverges by as much as 15 000 years from the gt4 gas time scale petit et al nature 399 1999 429 prior to 250 000 years ago but it matches fairly closely a time scale derived by tuning ice core 18 o atm to a lagged insolation signal shackleton science 289 2000 1897 most offsets between the ch 4 and 18 o atm time scales can be explained by assuming that tropical monsoons and ice sheets alternate in controlling the phase of the 18 o atm signal the ch 4 time scale provides an estimate of the timing of the vostok co 2 signal against specmap marine 18 o often used as an index of global ice volume on the ch 4 time scale all co 2 responses are highly coherent with specmap 18 o at the orbital periods co 2 leads 18 o by 5000 years at 100 000 years eccentricity but the two signals are nearly in phase at 41 000 years obliquity and 23 000 years precession the actual phasing between co 2 and ice volume is difficult to infer because of likely sst overprints on the specmap 18 o signal co 2 could lead or be in phase with ice volume but is unlikely to lag behind the ice response