climate and vegetation in southeastern australia respond to southern hemisphere insolation forcing in the late pliocene early pleistocene terrestrial climate responses to orbital forcing during the late pliocene early pleistocene are poorly understood particularly in the southern hemisphere but are important for determination of the timing of regional climate evolution early in the history of the glaciated quaternary world we present a pollen record from southeastern australia that shows marked cyclic change over some 280 000 yr straddling the pliocene pleistocene boundary rainforest communities responded to climate forcing primarily within the precession and eccentricity bands suggesting that major vegetation changes were driven directly by summer insolation rather than by obliquity dominated glacial cycles

an outsider s review of the astronomical theory of the climate is the eccentricity driven insolation the main driver of the ice ages abstract although the astronomical theory of the climate also known as the milankovitch theory explains a number of key features of the paleoclimate time series especially frequency and phase distribution in terms of orbital influence on the waxing and waning of ice sheets it still faces several important unresolved challenges in this review we discuss five main challenges we believe the theory must resolve in order to survive 1 the large amplitude of the 100 ky ice age cycles 2 the switch from 41 ky to 100 ky long glaciations around 0 9 ma bp 3 the absence of significant spectral power at 413 ky over the past 1 2 ma 4 the variation in glacial cycle duration in the last 800 ky and 5 the presence of non orbital spectral peaks in the climate record but in spite of these problems we conclude that the milankovitch theory may still be a viable one at least in the original sense of hays et al science 194 1976 1121 that orbital changes somehow influenced the ice ages many investigators ourselves included appear to see the impact of orbital forcing in the data clear evidence of astronomical forcing as well as evidence which suggests that the climate system responds nonlinearly to all milankovitch orbital frequencies

in retrospect forty years of linking orbits to ice ages in 1976 it was demonstrated that tiny wobbles in earth s orbit led to the great ice age cycles of the past few million years this finding had wide implications for climate science and the details remain hotly debated today

incidence of obliquity and precession forced milankovitch cycles in the western mediterranean early messinian sedimentation in the sorbas basin almeria southern spain spectral analysis of the messinian abad marls in the cariatiz section sorbas basin south eastern spain reveals three relevant orders of cyclicity the most significant cycle is in the lowest frequency average thickness of 365 cm 4 5 cycles in the section it is recorded in the composition of planktic foraminiferal assemblages indicative of surface water temperature planktic and benthic stable isotope signals and carbonate proportions the planktic assemblages isotope values and carbonate proportions also record a middle frequency cycle with an average of 177 cm 9 10 cycles in the section the highest frequency cycle average of 132 cm 12 13 cycles in the section is mainly reflected in siliciclastic and calcite proportions age constraints and cycle patterns suggest that the lowest frequency cycle was forced by orbital obliquity whereas the two higher frequency ones are related to precession obliquity seems to have controlled major changes in surface water temperature in the sorbas basin during the early messinian surface water temperature was also affected by precession with changes in weathering and run off spectral analysis has also been applied to vertical shifts of reef facies throughout the progradation of the cariatiz reef this reef is coeval with the cariatiz section vertical shifts of reef talus breccias point to the existence of 4 5 major cycles of sea level change whereas 7 9 higher frequency cycles are reflected in the repeated occurrence of lowstand non reefal deposits correlation with the cycles observed at the cariatiz section suggests that obliquity forced glacio eustatic sea level oscillations in the western mediterranean during the late miocene

iron supply and demand in the upper ocean is extraterrestrial dust a significant source of bioavailable iron interplanetary dust particles accrete on the earth at a rate of 40 ktons yr 1 some 90 of this material evaporates in the atmosphere producing a bioavailable iron flux of 3 10 7 mol fe m 2 yr 1 this extraterrestrial fe flux is 30 300 of the eolian flux of bioavailable iron transported from terrestrial sources in remote marine regions and 20 of the upwelled fe flux in the southern ocean extraterrestrial fe may play an important role in regulating the marine carbon cycle in these regions

did glacials start with global warming abstract correlation of paleoclimatic evidence with orbital changes shows that the build up of polar ice accelerated when low obliquity coincided with perihelion in northern hemisphere winter under low obliquity the insolation was channeled to the tropics at the expense of both polar caps as perihelion moved from winter solstice toward spring equinox the solar beam in astronomic winter and spring became stronger than in summer and autumn this orbital configuration under climate conditions like today would lead to warming of tropical oceans but cooling of the polar regions the areally weighted global mean surface temperature which is dominated by the low latitudes would increase consequently during the first millennia the early glacial ice build up was most likely accompanied by global warming it was the associated increase of meridional insolation and temperature gradients which were instrumental in the transition to a glacial a significant part of the current global warming is due to the gradual temperature increase of the tropical oceans as the changing orbital configuration today resembles that of the last interglacial glacial transition the warming is likely to have a natural component

quantitative estimate of the milankovitch forced contribution to observed quaternary climate change a number of records commonly described as showing control of climate change by milankovitch insolation forcing are reexamined the fraction of the record variance attributable to orbital changes never exceeds 20 in no case including a tuned core do these forcing bands explain the overall behavior of the records at zero order all records are consistent with stochastic models of varying complexity with a small superimposed milankovitch response mainly in the obliquity band evidence cited to support the hypothesis that the 100 ka glacial interglacial cycles are controlled by the quasi periodic insolation forcing is likely indistinguishable from chance given the small sample size andnear integer ratios of 100 ka to the precessional period s at the least the stochastic background noise is likely to be of importance r 2004 elsevier ltd all rights reserved

ages of regional metamorphism and ductile deformation in the central and southern appalachians abstract a summary of available geochronology of deformational fabrics formed at greenschist and higher grades in the central and southern appalachians is presented as a map showing the ages of regional metamorphism and ductile deformation the ca 480 435 ma taconic event affected most of the piedmont and blue ridge provinces the ca 380 340 ma acadian event was milder and was centered in the west central piedmont and eastern blue ridge the 330 230 ma alleghanian event exhibits an abrupt ductile deformation front in the eastern piedmont and extends an unknown distance eastward below the atlantic coastal plain

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 late pleistocene holocene sapropel record of climatic and oceanographic variability in the eastern mediterranean sapropels and intercalated marls in a piston core from the eastern mediterranean are chemically and mineralogical distinct kaolinite smectite and total s contents fe a1 ba a1 co a1 mo a1 and v a1 are higher whereas quartz mg calcite illite contents si a1 ti a1 k a1 rb a1 and zr a1 are lower in the sapropels missing and ghost sapropels are identified by mineralogical and chemical properties that are not prone to diagenesis primary production was higher ba a1 and bottom water and or interstitial oxygenation was lower co a1 mo a1 ni a1 and v a1 during sapropel formation wind speeds quartz clay si a1 and zr a1 and bottom water salinities mg calcite calcite were higher during periods of marl formation sapropels represent a fundamentally different sedimentary facies whose formation is linked to changes in the hydrological balance in the basin driven by precessionally modulated changes in monsoon strength and subtropical precipitation changes which altered circulation production and sediment source areas

consequences of pacing the pleistocene 100 kyr ice ages by nonlinear phase locking to milankovitch forcing 1 the consequences of the hypothesis that milankovitch forcing affects the phase e g termination times of the 100 kyr glacial cycles via a mechanism known as nonlinear phase locking are examined phase locking provides a mechanism by which milankovitch forcing can act as the pacemaker of the glacial cycles nonlinear phase locking can determine the timing of the major deglaciations nearly independently of the specific mechanism or model that is responsible for these cycles as long as this mechanism is suitably nonlinear a consequence of this is that the fit of a certain model output to the observed ice volume record cannot be used as an indication that the glacial mechanism in this model is necessarily correct phase locking to obliquity and possibly precession variations is distinct from mechanisms relying on a linear or nonlinear amplification of the eccentricity forcing nonlinear phase locking may determine the phase of the glacial cycles even in the presence of noise in the climate system and can be effective at setting glacial termination times even when the precession and obliquity bands account only for a small portion of the total power of an ice volume record nonlinear phase locking can also result in the observed quantization of the glacial period into multiples of the obliquity or precession periods

amplitude and frequency modulations of the earth s obliquity for the last million years the variability of the earth s obliquity is a result of the deviation of the obliquity from its main component at 41 kyr and is generated by amplitude and frequency modulation for the last million years the amplitude modulation of the obliquity explains 8 of the obliquity s total variance and the frequency modulation explains 10 3 the spectra of both the amplitude and frequency modulations display significant power at 171 and 97 kyr the contribution of these two modulations to the variance of the insolation is evaluated at 65 degreesn in june the amplitude modulation of the obliquity explains 1 3 of the variance of the insolation and the frequency modulation explains 1 6 it is shown that the obliquity s frequency modulation is physically questionable and could only be meaningful for its components at 41 54 and 29 kyr taken separately

new perspectives on orbitally forced stratigraphy this survey of the current status of research into earth s orbitally forced paleoclimatic record summarizes recent developments in the theory of earth s orbital parameters and reviews how various techniques of data collection and analysis have fared in the search and recovery of orbital signals in ancient stratigraphy the emerging significance of the quasi periodicity of earth s orbital variations as a prin cipal tool in the analysis of orbitally forced stratigraphy is discussed in detail five case studies are presented that illustrate new directions in research a time series analysis of discontinuous strata b measurement of ultra high resolution stratigraphic signals c new perspectives on the 100 kyr pleistocene glaciation problem d strati graphic evidence for solar system resonance modes and e evaluating phanerozoic length of day from orbitally forced stratigraphy the study of how earth s orbital parameters came to be recorded in ancient stra tigraphy has recently moved into a new phase of scientific inquiry and discovery notable advances in celestial mechanics and in the analysis of stratigraphic data have given rise to new ways of solving old problems and have uncovered new problems for the first time new tools have facilitated the recovery of extended high resolution orbital signals from stratigraphy advanced analytical techniques have now been fully incorporated into the field an updated theory of earth s orbital parameters provides an accurate ephemeris for the past 16 million years for precision correlation between earth s orbital parameters and orbitally forced stratigraphy some orbital eccentricity modes can be used for orbital stratal cal ibrations to several hundred million years ago these breakthroughs have transformed the study of orbitally forced stratigra phy into an applied field that addresses long standing problems in disciplines as far ranging as geochronology geodynamics and astrodynamics orbital signals provide the framework for a high resolution orbital chronostratigraphy as far back

simultaneous presence of orbital inclination and eccentricity in proxy climate records from ocean drilling program site 806 ocean drilling program site 806 in the western pacific shows evidence of a remarkably constant average sedimentation rate this feature allows us to analyze ancient climate proxies without the need for orbital tuning a standard procedure in prior work but one that can lead to biased results spectral analysis of stable oxygen isotope ratios at this site a proxy for global ice volume shows a single narrow peak with a period 100 k y a result that supports our model which links glacial cycles to variations in the inclination of the earth s orbit in contrast spectral analysis of the coarse component fraction of the sediment primarily foraminifera shows a structure characteristic of standard milankovitch theory with a triplet of peaks with periods near those expected from the earth s eccentricity 95 125 and 400 k y bispectral analysis confirms these linkages but suggests that orbital inclination also plays some role in the coarse fraction from the clear presence of both signals in different proxies at the same site we conclude that although eccentricity affected the local climate it is orbital inclination that drove the variations in the global ice volume for the past million years

the missing glaciations of the middle pleistocene global glaciations have varied in size and magnitude since the early middle pleistocene transition 773 ka despite the apparent regular and high amplitude 100 ka pacing of glacial interglacial cycles recorded in marine isotope records the evidence on land indicates that patterns of glaciation varied dramatically between different glacial interglacial cycles for example marine isotope stages mis 8 10 and 14 are all noticeably absent from many terrestrial glacial records in north america and europe however globally the patterns are more complicated with major glaciations recorded in mis 8 in asia and in parts of the southern hemisphere such as patagonia for example this spatial variability in glaciation between glacial interglacial cycles is likely to be driven by ice volume changes in the west antarctic ice sheet and associated interhemispheric connections through ocean atmosphere circulatory changes the weak global glacial imprint in some glacial interglacial cycles is related to the pattern of global ice buildup this is caused by feedback mechanisms within glacial systems themselves that partly result from long term orbital changes driven by eccentricity

early onset and origin of 100 kyr cycles in pleistocene tropical sst records abstract the large 100 kyr cycles evident in most late pleistocene 0 0 6 ma paleoclimatic records still lack a satisfactory explanation previous studies of the nature of the transition from the early pleistocene 1 2 1 8 ma 41 kyr dominated climate regime to the 100 kyr world have been based almost exclusively on benthic foraminiferal oxygen isotopic 18o data it is generally accepted that the late pleistocene 100 kyr cycles represent a newly evolved sensitivity to eccentricity precession superimposed on an earlier and largely constant response to obliquity and precession forcing however orbitally resolved pleistocene sea surface temperature sst records from a variety of oceanic regions paint a rather different picture of the global climate transition across the mid pleistocene transition mpt 0 6 1 2 ma reanalysis of these sst records shows that 1 an early onset of strong 100 kyr like cycles in two low frequency bands 120 145 kyr and 60 80 kyr derived from the bundling of two three obliquity cycles into grand cycles obliquity subharmonics occurred in tropical sst records during the early pleistocene 2 these two early pleistocene periods converge into the late pleistocene 100 kyr period in tropical sst records 3 the dominance of 100 kyr sst power in the late pleistocene coincides with a dramatic decline in the 41 kyr sst power and 4 the correlation of timing of glacial terminations with eccentricity precession variation could well extend back into the early pleistocene we demonstrate that most of these features also occur in 18o records but in a much more subtle manner these features could be explained in two plausible ways a shift in climate sensitivity from obliquity to eccentricity precession a modified version of the conventional view or an increasingly nonlinear response to orbital obliquity across the mpt however our examination of the development of 100 kyr cycles favors an obliquity bundling mechanism to form late pleistocene 100 kyr cycles we therefore suggest that the late pleistocene 100 kyr climatic cycles are likely a nonlinear response to orbital obliquity although the timing of late pleistocene 100 kyr climatic cycles and their early forms appears to be paced by eccentricity precession

polar synchronization and the synchronized climatic history of greenland and antarctica abstract stable isotope proxies from ice cores show subtle differences in the climatic fluctuations of the arctic and antarctic and recent analyses have revealed evidence of polar synchronization at the millennial time scale at this scale we analogize the polar climates of the last ice ages to two coupled nonlinear oscillators which adjust their natural rhythms until they synchronize at a common frequency and constant phase shift heat and mass transfers across the intervening ocean and atmosphere make the coupling possible here we statistically demonstrate the existence of this phenomenon in polar proxy records with methane matched age models and quantify their phase relationship we show that the time series of representative proxy records of the last glaciation recorded in greenland grip ngrip and antarctica byrd dome c satisfy phase synchronization conditions independently of age for periods ranging 1 6 ky and can be transformed into one another by a 2 phase shift with antarctica temperature variations leading greenland s based on these results we use the polar synchronization paradigm to replicate the 800 ky long antarctic epica time series from a theoretical model that extends greenland s 100 ky long grip record to 800 ky statistical analysis of the simulated and actual antarctic records shows that the procedure is stable to change in adjustable parameters and requires the coupling between the polar climates to be proportional mainly to the difference in heat storage between the two regions

pleistocene glacial terminations triggered by synchronous changes in southern and northern hemisphere insolation the insolation canon hypothesis throughout the last 900 kyr the late pleistocene earth has experienced periods of cold glacial climate punctuated by seven abrupt transitions to warm interglacials the so called terminations although most of glacial ice is located in the northern hemisphere nh the southern hemisphere sh seems to play a crucial role in deglaciation variation in the seasonal distribution of solar insolation is one candidate for the cause of these climatic shifts but so far no simple mechanism has been identified here we present a mathematical analysis of variations in midsummer insolation in both hemispheres at 65 latitude applying this analysis to the entire pleistocene the last 2 myr we find that prior to each termination the insolation in both hemispheres increases in concert with a sh lead introducing time and energy thresholds to these overlaps calculated times for the onsets of the seven terminations by this insolation canon exceptional overlaps meeting the two threshold prerequisites are 23 139 253 345 419 546 and 632 kyr bp perfectly matching the geologic record the timing originates from the interplay between the two orbital parameters obliquity and precession explaining why terminations occur at integer multiple of the precessional cycle there is no such constellation between 1 and 2 myr bp the early pleistocene in agreement with earth s climate at that time this change in orbital forcing coincides with the mid pleistocene revolution separating the late from the early pleistocene therefore we hypothesize that the insolation canon is the trigger for glacial terminations

deepwater dynamics and mixing processes during a major inflow event in the central baltic sea intrusions of large amounts of dense and oxygen rich waters during so called major baltic inflows mbis form an essential component of the baltic sea overturning circulation and deep water ventilation despite their importance however detailed observations of the processes occuring in the central basins during an mbi are virtually lacking here data from a long term deployment of an autonomous profiling platform located in the center of one of the main basins are presented providing the first direct and detailed view of the deep water modifications and dynamics induced by one of the largest mbis ever recorded mbi 2014 2015 approximately 21 gmol of oxygen were imported during three distinct inflow phases with an unexpectedly large contribution of oxic intrusions at intermediate depth oxygen consumption rates during the stagnation period immediately following the inflow phase was found to be 87 g m 2 yr 1 with a dominant contribution of sedimentary oxygen demand the most energetic deep water processes topographic and near inertial waves were only marginally affected by the inflow however sub inertial energy levels associated with intrusions and eddies were strongly enhanced turbulence microstructure data revealed that the deep interior regions remain essentially non turbulent even during the energetic conditions of an mbi emphasizing the importance of boundary mixing warm intrusions frequently showed a temperature fine structure with vertical scales of the order of 0 1 m without any signs of active turbulence at the upper flanks of these intrusions double diffusive staircases were often found to develop suggesting an important alternative mixing process during inflow conditions

calcareous microfossil based orbital cyclostratigraphy in the arctic ocean abstract microfaunal and geochemical proxies from marine sediment records from central arctic ocean cao submarine ridges suggest a close relationship over the last 550 thousand years kyr between orbital scale climatic oscillations sea ice cover marine biological productivity and other parameters multiple paleoclimate proxies record glacial to interglacial cycles to understand the climate cryosphere productivity relationship we examined the cyclostratigraphy of calcareous microfossils and constructed a composite arctic paleoclimate index api stack from benthic foraminiferal and ostracode density from 14 sediment cores following the hypothesis that api is driven mainly by changes in sea ice related productivity the api stack shows the arctic experienced a series of highly productive interglacials and interstadials every 20 kyr these periods signify minimal ice shelf and sea ice cover and maximum marine productivity rapid transitions in productivity are seen during shifts from interglacial to glacial climate states discrepancies between the arctic api curves and various global climatic sea level and ice volume curves suggest abrupt growth and decay of arctic ice shelves related to climatic and sea level oscillations

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

co2 and northern hemisphere ice volume variations over the middle and late quaternary the atmospheric co sub 2 concentrations have been reconstructed over the past 600 ka based on regression between the vostok co sub 2 data and the specmap oxygen isotope values a lag of 4 5 ka co sub 2 preceding delta sup 18 o gives the best results a polynomial of order 5 explains 66 of the vostok co sub 2 variance over the last 220 ka the northern hemisphere ice sheet volume was simulated over the past 575 ka using the lln 2 d model forced by insolation and these statistically reconstructed atmospheric co sub 2 concentrations the simulated ice volume fluctuations resemble the deep sea oxygen isotope variations co sub 2 of interglacial level is necessary for explaining both the interglacial at oxygen isotopic stage 11 and our present day interglacial

the early middle pleistocene transition of asian summer monsoon abstract the global climate cyclicity transferred from an early pleistocene mode dominated by the obliquity periodicity to a new middle pleistocene state dominated by the eccentricity periodicity in the absence of any significant changes in orbital forcing known as the early middle pleistocene transition empt is an unresolved issue of milankovitch orbital theory here we contribute new insights into the orbital characteristic and dynamics of the asian summer monsoon precipitation changes across the empt spectral analyses of precipitation over the past 1 6 myr reconstructed from a new loess magnetic susceptibility stack suggest that the empt of the summer monsoon precipitation characterized by a shift from obliquity to eccentricity rhythm occurred over a 640 kyr long term period from 1 24 to 0 6 ma rather than quasi instantaneously on a much shorter duration the eccentricity cyclicity initiated as early as 1 24 ma subsequently intensified and became the dominated pacing from 1 1 ma while obliquity weakened at 1 1 ma and significantly weakened at 0 7 ma these empt characteristics of the summer monsoon precipitation reflect complex impacts from global climate systems and regional climate changes in the northern and southern hemispheres and in both low and high latitude regions

a simple ice sheet model yields realistic 100 kyr glacial cycles records of global ice volume for the past 700 kyr based on oxygen isotopic data from deep sea cores and reflecting mainly the changing northern hemispheric ice sheets show a dominant cycle of roughly 100 kyr period the records also show smaller amplitude oscillations with spectral peaks at roughly 40 and 20 kyr periods which are well correlated with the milankovich insolation variations due to perturbations in the earth s orbital parameters however no model has accurately simulated the 100 kyr glacial cycle recently birchfield et al 1 and oerlemans2 have obtained encouraging agreement with some features of the glacial cycle by using a simple ice sheet model with a realistic time lag in the response of the bedrock to the ice load this study extends their basic model first by including topography to represent high ground in the north improved results can then be obtained but only with unrealistic parameter values and for some aspects of the record further improvements areobtained by crudely parameterizing possible calving at the equatorward ice sheet tip during deglaciation by proglacial lakes and or marine incursions from the atlantic as emphasized by andrews3 the resulting ice volume curves agree fairly well with the observed records and their power spectra over the past 700 kyr

the role of orbital forcing in the early middle pleistocene transition abstract the early middle pleistocene transition empt is the term used to describe the prolongation and intensification of glacial interglacial climate cycles that initiated after 900 000 years ago during the transition glacial interglacial cycles shift from lasting 41 000 years to an average of 100 000 years the structure of these glacial interglacial cycles shifts from smooth to more abrupt saw toothed like transitions despite eccentricity having by far the weakest influence on insolation received at the earth s surface of any of the orbital parameters it is often assumed to be the primary driver of the post empt 100 000 years climate cycles because of the similarity in duration the traditional solution to this is to call for a highly nonlinear response by the global climate system to eccentricity this eccentricity myth is due to an artefact of spectral analysis which means that the last 8 glacial interglacial average out at about 100 000 years in length despite ranging from 80 000 to 120 000 years with the realisation that eccentricity is not the major driving force a debate has emerged as to whether precession or obliquity controlled the timing of the most recent glacial interglacial cycles some argue that post empt deglaciations occurred every four or five precessional cycle while others argue it is every second or third obliquity cycle we review these current theories and suggest that though phase locking between orbital forcing and global ice volume may occur the chaotic nature of the climate system response means the relationship is not consistent through the last 900 000 years

the plio pleistocene development of atlantic deep water circulation and its influence on climate trends abstract using benthic stable isotope records from 10 sites in the atlantic ocean including two new records from walvis ridge in the southeast atlantic sites 1264 and 1267 we review changes in atlantic deep water circulation in the context of plio pleistocene climate overall we find non linear responses of atlantic deep water circulation to a cooling climate with differently evolving glacial and interglacial states our main conclusion is that peak north atlantic deep water nadw production was reached between 2 0 and 1 5 ma most prominently seen by a maximum in ventilated high 13 c conditions in the mid depth southeast atlantic site 1264 we infer that a major source of nadw at this time was the export of dense overflow water from the nordic seas into the abyssal east atlantic sea surface temperature records from the north and south atlantic support this notion and indicate that the peak nadw production between 2 0 and 1 5 ma was compensated by a stronger warm surface water return flow i e atlantic meridional overturning circulation amoc was enhanced causing long term 10 5 year heat piracy from the south to the north atlantic in the wider picture of plio pleistocene climate evolution we find that a long term enhancement in the average state of amoc 2 4 1 3 ma coincides with the 41 kyr world hence we speculate that the transitory negative feedback response of enhanced amoc to a cooling climate supplied heat to key areas of ice sheet growth acting to limit their size and maintain the 41 kyr world once a threshold in global cooling was reached the strength of amoc lessened providing a positive feedback for the early middle pleistocene transition and the associated build up of northern hemisphere ice sheets

global glacier dynamics during 100 ka pleistocene glacial cycles ice volume during the last ten 100 ka glacial cycles was driven by solar radiation flux in the northern hemisphere early minima in solar radiation combined with critical levels of atmospheric co 2 drove initial glacier expansion glacial cycles between marine isotope stage mis 24 and mis 13 whilst at 100 ka periodicity were irregular in amplitude and the shift to the largest amplitude 100 ka glacial cycles occurred after mis 16 mountain glaciers in the mid latitudes and asia reached their maximum extents early in glacial cycles then retreated as global climate became increasingly arid in contrast larger ice masses close to maritime moisture sources continued to build up and dominated global glacial maxima reflected in marine isotope and sea level records the effect of this pattern of glaciation on the state of the global atmosphere is evident in dust records from antarctic ice cores where pronounced double peaks in dust flux occur in all of the last eight glacial cycles glacier growth is strongly modulated by variations in solar radiation especially in glacial inceptions this external control accounts for 50 60 of ice volume change through glacial cycles internal global glacier climate dynamics account for the rest of the change which is controlled by the geographic distributions of glaciers

are the 41 kyr glacial oscillations a linear response to milankovitch forcing abstract the characteristics of glacial oscillations changed drastically 0 8 ma ago at the mid pleistocene transition during the past 0 8 ma the 100 kyr glacial interglacial oscillations were strongly asymmetric i e long glacial intervals of growth followed by rapid intervals of deglaciation the 40 kyr oscillations prior to the mid pleistocene transition were of a smaller amplitude and less asymmetrical looking the smaller amplitude apparently symmetric form and period that matches that of obliquity suggests that these oscillations were a linear response to milankovitch forcing while the 100 kyr oscillations are attributed either to some nonlinear self sustained variability due to a mechanism internal to the climate system itself or to nonlinear amplification of the insolation forcing the significant strengthening of the 100 kyr eccentricity power in the past 800 kyr is one of the intriguing questions of climate history here we show that glacial interglacial oscillations pre mid pleistocene transition are in fact significantly asymmetric this asymmetry may contradict a straight forward linear milankovitch explanation and we therefore suggest that the glacial oscillations before and after the transition may both be explained as self sustained variability although the possibility of nonlinear response to insolation forcing still exists the role of milankovitch forcing is in setting the phase of the oscillations e g time of terminations and their period rather in being the main driving force of the oscillations this is demonstrated using a simple model based on the sea ice switch mechanism of gildor and tziperman paleoceanography 15 2000 605

is the spectral signature of the 100 kyr glacial cycle consistent with a milankovitch origin spectral analysis techniques traditionally applied to proxy climate records have involved the use of specialized windowing functions because of the finite length of the record available these have the effect of artificially broadening the spectral peaks such that closely spaced peaks may not be fully resolved alternatively use of a simple boxcar window following muller and macdonald 1997b will maximize the spectral frequency resolution this is achieved at the expense of potential truncation artifacts manifested as spurious peaks in the spectra although in the spectral analyses presented here such additional peaks are not significant in addition simple methods like this are not able to place any confidence intervals on the results global ice volume proxy records obtained

mid brunhes century scale diatom sea surface temperature and sea ice records from the atlantic sector of the southern ocean odp leg 177 sites 1093 1094 and core ps2089 2 abstract diatom assemblages from odp leg 177 sites 1093 1094 and core ps2089 2 from the present antarctic sea ice free zone and close to the polar front were analyzed in order to reconstruct the climate development around the mid brunhes event 400 000 yr ago as reflected by summer sea surface temperature ssst and sea ice distribution dense sample spacing allows a mean temporal resolution during marine isotope stage mis 11 423 362 ka of 300 400 yr ssst values were estimated from diatom assemblages using a transfer function technique the distribution pattern of sea ice diatoms indicates that the present day ice free antarctic zone was seasonally covered by sea ice during the cold mis 12 and mis 10 these glacial periods are characterized by sea ice fluctuations with a periodicity of 3 and 1 85 kyr suggesting the occurrence of dansgaard oeschger style millennial scale oscillations in the atlantic sector of the southern ocean during the glacial stages mis 12 and mis 10 termination v mis 12 11 is characterized by a distinct temperature increase of 4 6 c intersected especially at the southern site 1094 and core ps2089 2 by two distinct cooling events reminiscent of the younger dryas which are associated with a northward shift of the winter sea ice edge in the antarctic zone the ssst record is characterized by distinct temperature intervals bounded by stepwise rapid changes maximum temperatures were reached during termination v and the early mis 11 exceeding modern values by 2 c over a period of 8 kyr this pattern indicates a very early response of the southern ocean to global climate on milankovitch driven climate variability the ssst optimum is marked by millennial scale temperature oscillations with an amplitude of ca 1 c and periodicities of ca 1 85 and 1 47 kyr probably reflecting changes in the ocean circulation system the sssts during the mis 11 temperature optimum do not exceed values obtained from other interglacial optima such as the early periods of mis 5 or mis 1 from the antarctic zone however the total duration of the warmest period was distinctly longer than observed from other interglacials the comparison of the south atlantic climate record with a high resolution record from odp leg 162 site 980 from the north atlantic shows a strong conformity in the climate development during the studied time interval

glacial cycles and astronomical forcing narrow spectral features in ocean sediment records offer strong evidence that the cycles of glaciation were driven by astronomical forces two million years ago the cycles match the 41 000 year period of earth s obliquity this supports the croll milankovitch theory which attributes the cycles to variations in insolation but for the past million years the spectrum is dominated by a single 100 000 year feature and is a poor match to the predictions of insolation models the spectrum can be accounted for by a theory that derives the cycles of glaciation from variations in the inclination of earth s orbital plane

hydrographic variations in deep ocean temperature over the mid pleistocene transition abstract during the mid pleistocene transition the dominant 41 ka periodicity of glacial cycles transitioned to a quasi 100 ka periodicity for reasons not yet known this study investigates the potential role of deep ocean hydrography by examining oxygen isotope ratios in benthic foraminifera oxygen isotope records from the atlantic pacific and indian ocean basins are separated into their ice volume and local temperature hydrography components using a piece wise linear transfer function and a temperature calibration although our method has certain limitations the deep ocean hydrography reconstructions show that glacial deep ocean temperatures approached freezing point as the mid pleistocene transition progressed further analysis suggests that water mass reorganisation could have been responsible for these temperature changes leading to such stable conditions in the deep ocean that some obliquity cycles were skipped until precessional forcing triggered deglaciation creating the apparent quasi 100 ka pattern this study supports previous work that suggests multiples of obliquity cycles dominate the quasi 100 ka glacial cycles with precession components driving deglaciations

feedback between aeolian dust climate and atmospheric co2 in glacial time 1 enhanced aeolian supply of iron to the biota of the southern ocean during glacial periods is suspected to be an important contributory mechanism to the concurrently low observed mixing ratios of atmospheric co2 declining rates of dust deposition prior to the glacial terminations may be critical in driving the initial deglacial rise in co2 but the reasons behind the dust decline itself are as yet unknown here we show that the dust record from the vostok ice core can be qualitatively derived from a few general assumptions regarding the formation and aging of patagonian sources of aeolian material and the efficiency with which it is transported through the atmosphere we suggest that during glacial periods this dust supply becomes particularly sensitive to changes in global climate and that in turn climate is responsive to the dust due to iron fertilization this positive feedback may mean that during glacial periods the carbon cycle exhibits two quasi steady states characterized by distinct co2 concentrations recognition of this glacial subcycle may help to account for the timing and sequence of events at the terminations

modelling northern hemisphere ice volume over the last 3 ma the northern hemisphere ice sheet volumes over the last 3 ma were simulated using the lln 2 d model the forcings were both insolation and co2 concentration different atmospheric co2 scenarios were used because of a lack of co2 reconstruction over this remote past with constant co2 concentrations the simulated ice volume does not show any gradual increase as recorded in the marine sediments moreover its spectrum changes according to the chosen value of co2 when co2 is fixed at 220 ppmv the simulated ice volume is dominated by the similar to 100 ka period if a pre industrial co2 concentration of 280 ppmv is used the simulation is dominated by the similar to 41 ka period by using a linearly decreasing co2 concentration going from 320 ppmv at 3 ma bp to 200 ppmv at present the simulated changes in the power spectrum are in agreement with those obtained from the sedimentary records roughly before 1 ma bp the dominating period is similar to 41 ka afterwards the similar to 100 ka period becomes dominant this transition is accompanied by a gradual increase of the ice volume potential mechanisms for this transition are discussed using cyclic co2 fluctuations over the last 0 6 ma does not change significantly the spectral characteristics of the simulated ice volume but amplifies the amplitude of its variations c 1998 elsevier science ltd all rights reserved

the initiation of ice sheet growth milankovitch solar radiation variations and the 100 ky ice age cycle much work has gone into deciphering the causes of the large scale glacial interglacial variations in the climate system over the last 900 000 years while variations on the 41 thousand year ky and 23 ky time scales seem to be linearly linked to the variations in the distribution of solar radiation at the top of the atmosphere milankovitch solar radiation variations the causes of the dominant 100 ky cycle in the geologic record are still unknown one of the aspects of this cycle that is not well understood is how large scale ice sheet growth is initiated here we describe the mechanisms by which large scale ice sheet growth may have been initiated by the changes in the seasonal and latitudinal distribution of solar radiation over the past 160 ky this is done through the use of a coupled energy balance climate thermodynamic sea ice model that includes a hydrologic cycle which computes precipitation and a land surface energy balance which determines the net accumulation of snow and ice results indicate that the initiation of ice sheet growth is possible during times of extremely low summer solstice solar radiation as a result of a large decrease in ablation during the critical melt season

resolving milankovitch consideration of signal and noise milankovitch climate theory provides a fundamental framework for the study of ancient climates although the identification and quantification of orbital rhythms are commonplace in paleoclimate research criticisms have been advanced that dispute the importance of an astronomical climate driver if these criticisms are valid major revisions in our understanding of the climate system and past climates are required resolution of this issue is hindered by numerous factors that challenge accurate quantification of orbital cyclicity in paleoclimate archives in this study we delineate sources of noise that distort the primary orbital signal in proxy climate records and utilize this template in tandem with advanced spectral methods to quantify milankovitch forced paced climate variability in a temperature proxy record from the vostok ice core vimeux and others 2002 our analysis indicates that vostok temperature variance is almost equally apportioned between three components the precession and obliquity periods 28 a periodic 100 000 year cycle 41 and the background continuum 31 a range of analyses accounting for various frequency bands of interest and potential bias introduced by the saw tooth shape of the glacial interglacial cycle establish that precession and obliquity periods account for between 25 percent to 41 percent of the variance in the 1 10 kyr 1 100 kyr band and between 39 percent to 66 percent of the variance in the 1 10 kyr 1 64 kyr band these results are approximately two to four times greater than those published by wunsch 2004 for the same vostok time series in all cases most of the remaining variance is accounted for by the 100 000 year cycle which is distinct from a background continuum that resembles autoregressive red noise our analysis high lights the importance of a comprehensive assessment of the climate signals in geologic records and underscores the significance of orbital forcing pacing as a primary agent of pleistocene climate change

links between eccentricity forcing and the 100 000 year glacial cycle the 100 000 year glacial cycles are generally thought to be driven by the eccentricity of the earth s orbit statistical analyses of climate variability and orbital forcing over the past five million years indicate that the glacial cycles are the result of an internal climate oscillation phase locked to the 100 000 year eccentricity cycle variations in the eccentricity 100 000 yr obliquity 41 000 yr and precession 23 000 yr of earth s orbit have been linked to glacial interglacial climate cycles it is generally thought that the 100 000 yr glacial cycles of the past 800 000 yr are a result of orbital eccentricity1 2 3 4 however the eccentricity cycle produces negligible 100 kyr power in seasonal or mean annual insolation although it does modulate the amplitude of the precession cycle alternatively it has been suggested that the recent glacial cycles are driven purely by the obliquity cycle5 6 7 here i use statistical analyses of insolation and the climate of the past five million years to characterize the link between eccentricity and the 100 000 yr glacial cycles using cross wavelet phase analysis i show that the relative phase of eccentricity and glacial cycles has been stable since 1 2 myr ago supporting the hypothesis that 100 000 yr glacial cycles are paced8 9 10 by eccentricity4 11 however i find that the time dependent 100 000 yr power of eccentricity has been anticorrelated with that of climate since 5 myr ago with strong eccentricity forcing associated with weaker power in the 100 000 yr glacial cycle i propose that the anticorrelation arises from the strong precession forcing associated with strong eccentricity forcing which disrupts the internal climate feedbacks that drive the 100 000 yr glacial cycle this supports the hypothesis that internally driven climate feedbacks are the source of the 100 000 yr climate variations12

on the origin of the 100 kyr cycles in the astronomical forcing 1 investigations during the last 25 years have demonstrated that the astronomically related 19 23 and 41 kyr quasiperiodicities actually occur in long records of the quaternary climate however the same investigations also identified the largest climatic cycle as being about 100 kyr long as the 100 kyr variations in standing insolation due to eccentricity change are too small they cannot be the direct cause of the ice ages this is the reason why most of the modeling studies attempting to explain the relation between the astronomical forcing and climatic change have focused on this 100 kyr cycle in this paper we will show the astronomical origin of the periods at about 100 kyr that characterize the long term variations of eccentricity of its first derivative of the frequency modulation of obliquity and of the inclination of the earth s orbit on the invariable plane of reference five independent values are found between 95 and 107 kyr and a wavelet signature is suggested to test the possible relationships between the astronomical and climatic variables proxy records from deep sea cores and european programme for ice coring in antarctica ice core and modeling results from the louvain la neuve two dimensional model are used for illustration