bipolar seesaw control on last interglacial sea level our current understanding of ocean atmosphere cryosphere interactions at ice age terminations relies largely on assessments of the most recent last glacial interglacial transition1 2 3 termination i t i but the extent to which t i is representative of previous terminations remains unclear testing the consistency of termination processes requires comparison of time series of critical climate parameters with detailed absolute and relative age control however such age control has been lacking for even the penultimate glacial termination t ii which culminated in a sea level highstand during the last interglacial period that was several metres above present4 here we show that heinrich stadial 11 hs11 a prominent north atlantic cold episode5 6 occurred between 135 1 and 130 2 thousand years ago and was linked with rapid sea level rise during t ii our conclusions are based on new and existing6 7 8 9 data for t ii and the last interglacial that we collate onto a single radiometrically constrained chronology the hs11 cold episode5 6 punctuated t ii and coincided directly with a major deglacial meltwater pulse which predominantly entered the north atlantic ocean and accounted for about 70 per cent of the glacial interglacial sea level rise8 9 we conclude that possibly in response to stronger insolation and co2 forcing earlier in t ii the relationship between climate and ice volume changes differed fundamentally from that of t i in t i the major sea level rise clearly post dates3 10 11 heinrich stadial 1 we also find that hs11 coincided with sustained antarctic warming probably through a bipolar seesaw temperature response12 and propose that this heat gain at high southern latitudes promoted antarctic ice sheet melting that fuelled the last interglacial sea level peak

the southeast indian ridge between 88 e and 118 e variations in crustal accretion at constant spreading rate the temperature of the mantle and the rate of melt production are parameters which play important roles in controlling the style of crustal accretion along mid ocean ridges to investigate the variability in crustal accretion that develops in response to variations in mantle temperature we have conducted a geophysical investigation of the southeast indian ridge seir between the amsterdam hotspot and the australian antarctic discordance 88 e 118 e the spreading center deepens by 2100 m from west to east within the study area despite a uniform intermediate spreading rate 69 75 mm yr 1 the seir exhibits the range in axial morphology displayed by the east pacific rise and the mid atlantic ridge mar and usually associated with variations in spreading rate the spreading center is characterized by an axial high west of 102 45 e whereas an axial valley is prevalent east of this longitude both the deepening of the ridge axis and the general evolution of axial morphology from an axial high to a rift valley are not uniform a region of intermediate morphology separates axial highs and mar like rift valleys local transitions in axial morphology occur in three areas along the ridge axis the increase in axial depth toward the australian antarctic discordance may be explained by the thinning of the oceanic crust by 4 km and the change in axial topography the long wavelength changes observed along the seir can be attributed to a gradient in mantle temperature between regions influenced by the amsterdam and kerguelen hot spots and the australian antarctic discordance however local processes perhaps associated with an heterogeneous mantle or along axis asthenospheric flow may give rise to local transitions in axial topography and depth anomalies

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 last spreading episode of the west philippine basin revisited in 1996 bathymetric data and backscattering imagery were gathered along a 20 km wide nw se transect across the west philippine basin wpb between taiwan and the palau kyushu ridge these data reveal fine scaled structures of the fossil spreading axis from which we infer episodes of oblique deformation and diminished magmatic supply resulting from cessation of spreading an original ne sw seafloor fabric is observed in the basin northeast of the benham volcanic plateau since this trend is oblique to the more common e w and nw se seafloor fabrics known in the wpb it could be the result of overlap of two segments of the spreading axis several rhomboidal structures exist in the vicinity of the rift valley suggesting that dextral shear occurred along the central basin rift axis during the last spreading phase the e w spreading segments associated with n s nontransform discontinuities that occur between 127 e and 132 e were probably cross cut during a final largely amagmatic extensional phase to produce a n130 trending deep rift valley

derivation of isotope ratios errors and error correlations for u pb geochronology using 205pb 235u 233u spiked isotope dilution thermal ionization mass spectrometric data a comprehensive treatment of the derivation of u pb isotope ratios and their corresponding uncertainties from isotope dilution thermal ionization mass spectrometric measurements is presented standard parametric statistical methods of error propagation are utilized to convolve uncertainties associated with instrumental mass fractionation tracer subtraction blank pb and u subtraction and initial common pb correction derivations include errors and error correlations for total sample u pb and pb isotope ratios including radiogenic and initial common pb for two and three dimensional isochron calculations radiogenic u pb and pb isotope ratios for concordia and radiogenic model age calculations and the propagation of model age errors from radiogenic isotope ratios

u pb dating of a speleothem of quaternary age we demonstrate that u pb dating is a promising method for secondary carbonate materials of quaternary age and older by obtaining a 206pb 238u age for a speleothem with high u 10 g g 1 and very low pb 10 ng g 1 that is supported by an independent 230th age thermal ionisation mass spectrometry was used to determine the u and pb isotopic ratios and concentrations for subsamples of a stalactite from winnats head cave peak district uk we obtained 206pb 204pb ratios up to 50 and determined a 206pb 238u age of 248 10 ka which is within error of the 207pb 235u age of 333 79 ka and a spectrometric u th age of 255 ka for samples of tertiary and quaternary age the initial state of u series disequilibrium is an important consideration and as with most radiometric dating techniques the mineral must have remained closed to u th pb and all intermediate daughters we show that dense calcite speleothems are ideal in this respect and that no loss of rn has occurred unlike u series disequilibrium methods u pb dating has no upper limit and hence materials of quaternary age older than 0 6 ma can be analysed to investigate landscape development paleoclimate hominid evolution or hydrogeochemistry in carbonate terrains

evidence for obliquity forcing of glacial termination ii variations in the intensity of high latitude northern hemisphere summer insolation driven largely by precession of the equinoxes are widely thought to control the timing of late pleistocene glacial terminations however recently it has been suggested that changes in earth s obliquity may be a more important mechanism we present a new speleothem based north atlantic marine chronology that shows that the penultimate glacial termination termination ii commenced 141 000 2500 years before the present too early to be explained by northern hemisphere summer insolation but consistent with changes in earth s obliquity our record reveals that terminations i and ii are separated by three obliquity cycles and that they started at near identical obliquity phases

persistent influence of obliquity on ice age terminations since the middle pleistocene transition radiometric dating of glacial terminations over the past 640 000 years suggests pacing by earth s climatic precession with each glacial interglacial period spanning four or five cycles of 20 000 years however the lack of firm age estimates for older pleistocene terminations confounds attempts to test the persistence of precession forcing we combine an italian speleothem record anchored by a uranium lead chronology with north atlantic ocean data to show that the first two deglaciations of the so called 100 000 year world are separated by two obliquity cycles with each termination starting at the same high phase of obliquity but at opposing phases of precession an assessment of 11 radiometrically dated terminations spanning the past million years suggests that obliquity exerted a persistent influence on not only their initiation but also their duration

stalagmite evidence for the onset of the last interglacial in southern europe at 129 1 ka multi proxy data from an italian stalagmite constrain the commencement of full last interglacial conditions in southern europe at 129 1 ka consistent with absolutely dated records currently available from both hemispheres the post glacial transition towards warmer and wetter conditions commenced at 134 2 ka oxygen isotope evidence suggests this was interrupted briefly at 130 2 ka an event probably related to the termination ii pause associated with heinrich event 11 for most of the stalagmite the pattern of delta 18 o variation mimics the air temperature record from the vostok ice core especially through marine isotope stage 5 there is no obvious evidence for substantial early interglacial warming

a systematic analysis of the mid atlantic ridge morphology and gravity between 15 n and 40 n constraints of the thermal structure multibeam bathymetry data obtained along a 2400 km long section of the mid atlantic ridge mar from 15 n to the azores platform 40 n and satellite derived gravity data were used to calculate the mantle bouguer anomaly mba along this portion of the mar both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference a long wavelength influence of the azores hot spot is characterized by a gentle continuous slope of the average ridge axial depth and a general gradient in the along axis mba profile this thermal influence of the azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 30 n south of 26 30 n the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments except on the azores platform the systematic along axis 11 90 km long wavelength segmentation is independent of the long wavelength influence of the azores at the segment center the axial morphology is linked to the thermal state of the segments between 1 hotter segments characterized by a smooth axial morphology a well defined shallow inner valley high mba and a long length 2 colder segments which present a rough axial morphology with a deep wide and well defined rift valley a low mba and a small length for hotter segments the formation of the abyssal hills is mainly due to a magmato tectonic cycle over periods of 0 3 to 1 myr whereas on colder segments the axial morphology is mainly controlled by a tectonic rift valley formation we propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years

four climate cycles of recurring deep and surface water destabilizations on the iberian margin centennial climate variability over the last ice age exhibits clear bipolar behavior high resolution analyses of marine sediment cores from the iberian margin trace a number of associated changes simultaneously proxies of sea surface temperature and water mass distribution as well as relative biomarker content demonstrate that this typical north south coupling was pervasive for the cold phases of climate during the past 420 000 years cold episodes after relatively warm and largely ice free periods occurred when the predominance of deep water formation changed from northern to southern sources these results reinforce the connection between rapid climate changes at mediterranean latitudes and century to millennial variability in northern and southern polar regions

complexities in the tectonic evolution of the west philippine basin abstract the oceanic west philippine basin can be subdivided into a main basin a northwest sub basin and a southern sub basin these basins have undergone different tectonic histories magnetic anomalies 21 17 are present in the main basin and indicate that it formed by two limbed seafloor spreading the now extinct spreading center is located within a zone of rough topography associated with the northwest trending central basin fault cbf the morphology of the cbf is primarily the result of post spreading tectonism the magnetic anomaly pattern of the main basin does not extend into the southern sub basin or the northwest sub basin geometrical constraints indicate that the southern sub basin did not form as part of the southern limb of the main basin the southern sub basin may have formed immediately before a ridge jump to the main basin spreading axis alternatively possible shallower crustal depth in the southern sub basin may indicate that the sub basin is younger than the main basin and formed separately from it in the northwest sub basin which originated as part of the main basin the dominant topographic trends are perpendicular to those in the main basin this difference as well as higher heat flow and shallower depth in the northwest sub basin indicate that it has undergone a deformational event which did not affect the main basin this deformation may be related to subduction along the east luzon margin in mid tertiary time the gagua ridge an uplifted sliver of oceanic crust along the western edge of the northwest sub basin may have originated during this event

800 000 years of abrupt climate variability we constructed an 800 000 year synthetic record of greenland climate variability based on the thermal bipolar seesaw model our greenland analog reproduces much of the variability seen in the greenland ice cores over the past 100 000 years the synthetic record shows strong similarity with the absolutely dated speleothem record from china allowing us to place ice core records within an absolute timeframe for the past 400 000 years hence it provides both a stratigraphic reference and a conceptual basis for assessing the long term evolution of millennial scale variability and its potential role in climate change at longer time scales indeed we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the middle to late pleistocene

the middle pleistocene transition characteristics mechanisms and implications for long term changes in atmospheric pco2 abstract the emergence of low frequency high amplitude quasi periodic 100 kyr glacial variability during the middle pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system this middle pleistocene transition mpt began 1250 ka and was complete by 700 ka its onset was accompanied by decreases in sea surface temperatures ssts in the north atlantic and tropical ocean upwelling regions and by an increase in african and asian aridity and monsoonal intensity during the mpt long term average ice volume gradually increased by 50 m sea level equivalent whereas low frequency ice volume variability experienced a 100 kyr lull centered on 1000 ka followed by its reappearance 900 ka although as a broad band of power rather than a narrow persistent 100 kyr cycle additional changes at 900 ka indicate this to be an important time during the mpt beginning with an 80 kyr event of extreme sst cooling followed by the partial recovery and subsequent stabilization of long term north atlantic and tropical ocean ssts increasing southern ocean sst variability primarily associated with warmer interglacials the loss of permanent subpolar sea ice cover and the emergence of low frequency variability in pacific ssts and global deep ocean circulation since 900 ka ice sheets have been the only component of the climate system to exhibit consistent low frequency variability with the exception of a near universal organization of low frequency power associated with marine isotope stages 11 and 12 all other components show an inconsistent distribution of power in frequency time space suggesting a highly nonlinear system response to orbital and ice sheet forcing most hypotheses for the origin of the mpt invoke a response to a long term cooling possibly induced by decreasing atmospheric pco2 none of these hypotheses however accounts for the geological constraint that the earliest northern hemisphere ice sheets covered a similar or larger area than those that followed the mpt given that the mpt was associated with an increase in ice volume this constraint requires that post mpt ice sheets were substantially thicker than pre mpt ice sheets indicating a change in subglacial conditions that influence ice dynamics we review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline precambrian shield bedrock became exposed by glacial erosion of a thick mantle of regolith this exposure of a high friction substrate caused thicker ice sheets with an attendant change in their response to the orbital forcing marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the mpt if this carbon came from terrigenous sources an increase in atmospheric pco2 would be likely which is inconsistent with evidence for widespread cooling apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith a more likely source of organic carbon and nutrients which would mitigate pco2 rise is from shelf and upper slope marine sediments which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the mpt modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long term silicate weathering rates in good agreement with marine sr and os isotopic records we use a carbon cycle model to show that a post mpt increase in silicate weathering rates would lower atmospheric pco2 by 7 12 ppm suggesting that the attendant cooling may have been an important feedback in causing the mpt

fractal nature of the planetary boundary layer depth in the trade wind cumulus regime the fractal nature of the planetary boundary layer depth in the trade wind cumulus regime was explored the boundary layer depths were derived from discontinuities in airborne lidar backscatter profiles obtained during the frontal air sea interaction experiment fasinex in february 1986 scale invariant behaviour was observed over horizontal distances ranging from 50 m to 50 km the degree of intermittency and non stationarity of the signals were calculated using standard multifractal statistical methods

oxygen and hydrogen isotope signatures of northeast atlantic water masses abstract only a few studies have examined the variation of oxygen and hydrogen isotopes of seawater in ne atlantic water masses and data are especially sparse for intermediate and deep water masses the current study greatly expands this record with 527 18o values from 47 stations located throughout the mid to low latitude ne atlantic in addition d was analyzed in the 192 samples collected along the geotraces north atlantic transect ga03 ga03 e kn199 4 and the 115 iberia forams cruise samples from the western and southern iberian margin an intercomparison study between the two stable isotope measurement techniques cavity ring down laser spectroscopy and magnetic sector isotope ratio mass spectrometry used to analyze ga03 e samples reveals relatively good agreement for both hydrogen and oxygen isotope ratios the surface 0 100 m and central 100 500 m water isotope data show the typical evaporation related trend of increasing values equatorward with the exception for the zonal transect off cape blanc nw africa off cape blanc surface water isotope signatures are modified by the upwelling of fresher antarctic intermediate water aaiw that generally has isotopic values of 0 0 to 0 5 for 18o and 0 to 2 for d along the iberian margin the mediterranean outflow water mow is clearly distinguished by its high 18o 0 5 1 1 and d 3 6 values that can be traced into the open atlantic isotopic values in the ne atlantic deep water neadw are relatively low 18o 0 1 to 0 5 d 1 to 4 and show a broader range than observed previously in the northern and southern convection areas the neadw is best observed at ga03 e stations 5 and 7 in the central ne atlantic basin antarctic bottom water isotope values are relatively high indicating modification of the original antarctic source water along the flow path the reconstructed 18o salinity relationship for the complete data set has a slope of 0 51 i e slightly steeper than the 0 46 described previously by pierre et al 1994 j mar syst 5 2 159 170 for the tropical to subtropical northeast atlantic this slope decreases to 0 46 for the subtropical north atlantic central water nacw and the mow and to 0 32 for the surface waters of the upper 50 m the d salinity mixing lines have estimated slopes of 3 01 for the complete data 1 26 for the mow 3 47 for the nacw and 2 63 for the surface waters the slopes of the 18o d relationship are significantly lower than the one for the global meteoric water line with 5 6 for the complete data set 2 30 for the mow 4 79 for the nacw and 3 99 for the surface waters the lower slopes in all the relationships clearly reflect the impact of the evaporation surplus in the subtropics

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

crustal structure of an extinct rift axis in the labrador sea preliminary results from a seismic refraction survey abstract a preliminary crustal structure for the extinct rift axis in the labrador sea is presented based on a one dimensional analysis of ocean bottom seismometer obs refraction profiles within the extinct rift low p wave velocities are observed for crustal 3 6 6 3 km s and upper mantle 7 3 8 1 km s refractors the velocity depth structures for obss sampling crust within the extinct rift suggest a marked thinning and variability in crustal thickness from 4 7 km to 2 6 km in comparison to typical thicknesses of 6 4 km observed in the adjacent flanks the crustal structures at the extinct rift resemble those observed at the active and inactive transforms of oceanic fracture zones following arguments for crustal thinning at fracture zones we speculate that the first order thinning of the lower crust is due to a reduction in the supply of partial melt to the accretionary axis at the extinct rift the reduced supply is attributed to changes in the thermal field and spreading rate as rifting ceased this is consistent with thermal models which predict that crustal thickness decreases appreciably for spreading rates less than 20 mm yr when plate spreading predominates over buoyancy in driving mantle upwelling

assessing amino acid racemization variability in coral intra crystalline protein for geochronological applications over 500 free amino acid faa and corresponding total hydrolysed amino acid thaa analyses were completed from eight independently dated multi century coral cores of massive porites sp colonies this dataset allows us to re evaluate the application of amino acid racemization aar for dating late holocene coral material 20 years after goodfriend et al gca56 1992 3847 first showed aar had promise for developing chronologies in coral cores this re assessment incorporates recent method improvements including measurement by rp hplc new quality control approaches e g sampling and sub sampling protocols statistically based data screening criteria and cleaning steps to isolate the intra crystalline skeletal protein we show that the removal of the extra crystalline contaminants and matrix protein is the most critical step for reproducible results and recommend a protocol of bleaching samples in naocl for 48 h to maximise removal of open system proteins while minimising the induced racemization we demonstrate that aar follows closed system behaviour in the intra crystalline fraction of the coral skeletal proteins our study is the first to assess the natural variability in intra crystalline aar between colonies and we use coral cores taken from the great barrier reef australia and jarvis island in the equatorial pacific to explore variability associated with different environmental conditions and thermal histories chronologies were developed from thaa asx d l ala d l glx d l and faa asx d l for each core and least squares monte carlo modelling applied in order to quantify uncertainty of aar age determinations and assess the level of dating resolution possible over the last 5 centuries aar within colonies follow consistent stratigraphic aging however there are systematic differences in rates between the colonies which would preclude direct comparison from one colony to another for accurate age estimation when aar age models are developed from a combined dataset to include this natural inter colony variability thaa asx d l glx d l and ala d l give a 2 age uncertainty of 19 38 and 29 year for the 20th c respectively in comparison 2 age uncertainties from a single colony are 12 12 and 14 year this is the first demonstration of faa d l for dating coral and following strict protocols 2 precisions of 24 years can be achieved across different colonies in samples from the last 150 years and can be 10 years within a core from a single colony despite these relatively large error estimates aar would be a valuable tool in situations where a large number of samples need to be screened rapidly and cheaply e g identifying material from mixed populations in beach or uplift deposits prior to and complementing the more time consuming geochronological tools of u th or seasonal isotopic timeseries

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

ice age terminations 230th dated oxygen isotope records of stalagmites from sanbao cave china characterize asian monsoon am precipitation through the ends of the third and fourthmost recent ice ages as a result am records for the past four glacial terminations can now be precisely correlated with those from ice cores and marine sediments establishing the timing and sequence of major events in all four cases observations are consistent with a classic northern hemisphere summer insolation intensity trigger for an initial retreat of northern ice sheets meltwater and icebergs entering the north atlantic alter oceanic and atmospheric circulation and associated fluxes of heat and carbon causing increases in atmospheric co2 and antarctic temperatures that drive the termination in the southern hemisphere increasing co2 and summer insolation drive recession of northern ice sheets with probable positive feedbacks between sea level and co2

ocean circulation ice sheet growth and interhemispheric coupling of millennial climate variability during the mid pleistocene ca 800 400ka abstract stable carbon and oxygen isotopes from benthic and planktic foraminifers planktic foraminifer assemblages and ice rafted debris from the north atlantic site u1314 integrated ocean drilling program expedition 306 were examined to investigate orbital and millennial scale climate variability in the north atlantic and its impact on global circulation focusing on the development of glacial periods during the mid pleistocene ca 800 400 ka glacial initiations were characterized by a rapid cooling 6 10 c in less than 7 kyr in the mean annual sea surface temperature sst increasing benthic 18o values and high benthic 13c values the continuous increase in benthic 18o suggests a continuous ice sheet growth whereas the positive benthic 13c values indicate that the flow of the iceland scotland overflow water isow was vigorous strong deep water formation in the norwegian greenland sea promoted a high transfer of freshwater from the ocean to the continents however low ssts at site u1314 suggest a subpolar gyre cooling and freshening that may have reduced deep water formation in the labrador sea during glacial initiations once the 3 5 threshold in the benthic 18o record was exceeded ice rafting started and ice sheet growth was punctuated by millennial scale waning events which returned to the ocean part of the freshwater accumulated on the continents ice rafting events were associated with a rapid reduction in the isow benthic 13c values dropped 0 5 1 and followed by millennial scale warmings the first two millennial scale warm intervals of each glacial period reached interglacial temperatures and were particularly abrupt 6 10 c in 3 kyr subsequent millennial scale warm events were cooler probably because the amoc was rather reduced as suggested by the low benthic 13c values these two abrupt warming events that occurred at early glacial periods were also observed in the antarctic temperature and co2 records suggesting a close correlation between both hemispheres the comparison of the sea surface proxies with the benthic 18o record as the southern sign indicates the presence of a millennial scale seesaw pattern similar to that seen during the last glacial period

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

radiocarbon evidence for alternating northern and southern sources of ventilation of the deep atlantic carbon pool during the last deglaciation recent theories for glacial interglacial climate transitions call on millennial climate perturbations that purged the deep sea of sequestered carbon dioxide via a bipolar ventilation seesaw however the viability of this hypothesis has been contested and robust evidence in its support is lacking here we present a record of north atlantic deep water radiocarbon ventilation which we compare with similar data from the southern ocean a striking coherence in ventilation changes is found with extremely high ventilation ages prevailing across the deep atlantic during the last glacial period the data also reveal two reversals in the ventilation gradient between the deep north atlantic and southern ocean during heinrich stadial 1 and the younger dryas these coincided with periods of sustained atmospheric co2 rise and appear to have been driven by enhanced ocean atmosphere exchange primarily in the southern ocean these results confirm the operation of a bipolar ventilation seesaw during deglaciation and underline the contribution of abrupt regional climate anomalies to longer term global climate transitions

the asian monsoon over the past 640 000 years and ice age terminations oxygen isotope records from chinese caves characterize changes in both the asian monsoon and global climate here using our new speleothem data we extend the chinese record to cover the full uranium thorium dating range that is the past 640 000 years the record s length and temporal precision allow us to test the idea that insolation changes caused by the earth s precession drove the terminations of each of the last seven ice ages as well as the millennia long intervals of reduced monsoon rainfall associated with each of the terminations on the basis of our record s timing the terminations are separated by four or five precession cycles supporting the idea that the 100 000 year ice age cycle is an average of discrete numbers of precession cycles furthermore the suborbital component of monsoon rainfall variability exhibits power in both the precession and obliquity bands and is nearly in anti phase with summer boreal insolation these observations indicate that insolation in part sets the pace of the occurrence of millennial scale events including those associated with terminations and unfinished terminations records of the asian monsoon have been extended to 640 000 years ago and confirm both that the 100 000 year ice age cycle results from integral numbers of precessional cycles and that insolation influences the pacing of major millennial scale climate events prior records of the asian monsoon have revealed cyclic variations over hundreds of thousands of years probably driven by variations in insolation caused by the precession of earth s orbit hai cheng and colleagues now provide a speleothem record from chinese cave samples that extends earlier records to 640 000 years ago close to the maximum age possible with uranium thorium dating this spectacular record confirms that the characteristic 100 000 year ice age cycle corresponds to an integral number four or five of precession cycles and that insolation influences millennial scale variations in monsoon strength

early middle pleistocene transitions linking terrestrial and marine realms abstract marked by a progressive increase in the amplitude of climate oscillations an evolving waveform and a shift towards a quasi 100 ky frequency the early middle pleistocene transition empt previously known as the mid pleistocene transition or mid pleistocene revolution 1 4 0 4 ma represents a fundamental transformation in the earth s climate state the empt began with a substantial change in climate dynamics and ended with the mid brunhes event signaling the establishment of a new steady state the reasons for the empt while uncertain appear to involve a non linear response of the earth climate system the physical and biotic responses to this transition amplified by the growth of northern hemisphere ice sheets have been profound two important chronostratigraphic markers characterize the empt the jaramillo subchron 1 070 0 988 ma and the matuyama brunhes chron boundary 773 ka the latter has been chosen as the primary guide for the lower middle pleistocene subseries boundary as it lies at the approximate midpoint of the empt and aids in global recognition both in marine and terrestrial deposits the jaramillo subchron has received less attention but the late early pleistocene is important in europe because it saw the progressive transition from the villafranchian to galerian mammal faunas and expansion of hominins into western and northern europe the jaramillo subchron is represented by marine isotope stages mis 31 to 28 with mis 30 already showing the asymmetrical sawtooth pattern characteristic of the middle pleistocene indeed while variation in the 40 ky band obliquity remains strong throughout the empt low frequency variability begins at around 1250 1200 ky which coincides with a progressive increase in global ice volume against a backdrop of increasingly severe glacial cycles notably during mis 36 34 24 22 the so called 0 9 ma event 16 and 12 pronounced phases of warming are also documented globally including the super interglacial mis 31 the early phase of the empt is characterized by important glaciations beginning with mis 36 and continuing to mis 24 22 a major intensification of the east asian monsoon system intensification of loess deposition in northern europe development of open landscapes in western siberia increased fluvial incision higher amplitude sea level change and spread of large mammals across northern eurasia and a strong reduction in the north atlantic thermohaline circulation in europe the loss of thermophilous plant taxa during the empt and indeed throughout the quaternary is a reminder of the progressive cooling that took place here and elsewhere

the central basin spreading center in the philippine sea structure of an extinct spreading center and implications for marginal basin formation 1 newly acquired marine geophysical data along the central basin spreading center cbsc the extinct spreading axis of the west philippine basin wpb display along axis variations of spreading style we have described and analyzed the tectonic spreading fabric and segmentation patterns along a 1000 km long section of the fossil ridge between 126 00 and 133 30 e slow spreading features deep rift valleys and nodal basins rough abyssal hills on the ridge flanks and mantle bouguer anomaly lows beneath segment centers are observed in the eastern cbsc in contrast fast spreading features overlapping spreading centers volcanic axial ridges and smooth abyssal hill fabric are identified along the western cbsc we attribute the large morphological and geophysical variations along the cbsc to higher melt supply in the western region caused by high mantle temperature and or mantle heterogeneity which may be related to a relatively small scale mantle plume forming the oceanic plateaus located in the wpb another prominent feature of the cbsc is the development of a deep valley oblique to the spreading fabric reconstructions of the plate boundary geometry through time using abyssal hill geometry as well as other measurements provide evidence for a later stage of amagmatic extension i e reactivation along the cbsc after the formation of the main basin this stage is dominated by tectonic deformation with minor amounts of volcanism mainly located in eastern segments resulting in the observed surface brittle deformation distributed within a broad zone of ductile deformation

thermohaline circulation crisis and impacts during the mid pleistocene transition the mid pleistocene transition mpt marked a fundamental change in glacial interglacial periodicity when it increased from 41 thousand year to 100 thousand year cycles and developed higher amplitude climate variability without substantial changes in the milankovitch forcing here we document by using nd isotopes a major disruption of the ocean thermohaline circulation thc system during the mpt between marine isotope stages miss 25 and 21 at 950 to 860 thousand years ago which effectively marks the first 100 thousand year cycle including an exceptional weakening through a critical interglacial mis 23 at 900 thousand years ago its recovery into the post mpt 100 thousand year world is characterized by continued weak glacial thc the mpt ocean circulation crisis facilitated the coeval drawdown of atmospheric co2 and high latitude ice sheet growth generating the conditions that stabilized 100 thousand year cycles

the last glacial termination a major puzzle of paleoclimatology is why after a long interval of cooling climate each late quaternary ice age ended with a relatively short warming leg called a termination we here offer a comprehensive hypothesis of how earth emerged from the last global ice age a prerequisite was the growth of very large northern hemisphere ice sheets whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation the southern hemisphere westerlies shifted poleward during each northern stadial producing pulses of ocean upwelling and warming that together accounted for much of the termination in the southern ocean and antarctica rising atmospheric co2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres

impact of suborbital climate changes in the north atlantic on ice sheet dynamics at the mid pleistocene transition pervasive feature across our study negative excursions in the benthic d 18 o time series observed at the times of ird events may be related to glacio eustatic changes due to ice sheets retreats and or to changes in deep hydrography time series analysis on surface water proxies ird sst and planktonic d 18 o of the interval between mis 31 to mis 26 shows that the timing of these millennial scale climate changes are related to half precessional 10 kyr components of the insolation forcing which are interpreted as cross equatorial heat transport toward high latitudes during both equinox insolation maxima at the equator

evolution of ocean temperature and ice volume through the mid pleistocene climate transition earth s climate underwent a fundamental change between 1250 and 700 thousand years ago the mid pleistocene transition mpt when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing over this time an increase occurred in the amplitude of change of deep ocean foraminiferal oxygen isotopic ratios traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep ocean temperature we have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios our results suggest that the mpt was initiated by an abrupt increase in antarctic ice volume 900 thousand years ago we see no evidence of a pattern of gradual cooling but near freezing temperatures occur at every glacial maximum

palaeoclimatic implications of the growth history and stable isotope delta o 18 and delta c 13 geochemistry of a middle to late pleistocene stalagmite from central western italy the age structure and stable isotope composition of a stalagmite cc i from an upland cave in central western italy were studied to investigate regional response to global climatic changes four growth phases are constrained by 28 thermal ionization and multi collector inductively coupled plasma mass spectrometry th u ages and reveal intermittent deposition through the period between marine isotope stage mis 11 and 3 similar to380 and similar to43 kyr most of the growth took place between similar to380 and similar to280 kyr a period punctuated briefly by a hiatus in deposition through the glacial maximum of mis 10 growth was terminated abruptly at 280 kyr just prior to the mis 8 glacial maximum with a present day chamber temperature of 7 5 degreesc the timing of hiatuses close to these glacial maxima point to freezing conditions at the time no deposition was recorded through the entirety of mis 7 and most of mis 6 whilst two minor growth phases occurred at similar to141 125 and similar to43 kyr growth at 141 kyr indicates temperatures 0 degreesc at a time when mis 6 ice volumes were close to their maximum high stable carbon isotope delta 13 c values similar to2 8parts per thousand to 3 1parts per thousand throughout the stalagmite s growth reflect a persistently low input of biogenic co2 indicating that the steep barren and alpine like recharge area of today ha s been in existence for at least the last similar to380 kyr during mis 9 the lowest delta 13 c values occur well after maximum interglacial conditions suggesting a lag in the development of post glacial soils in this high altitude karst the stable oxygen isotope delta 18 o trends match the main structural features of the major climate proxy records specmap vostok and devils hole suggesting that the delta 18 o of cc1 has responded to global scale climate changes whilst remarkable similarity exists between cc1 delta 18 o and regional sea surface temperature reconstructions from north atlantic core odp980 and southwest pacific marine core md97 2120 through the most detailed part of the cc1 record mis 9 8 the results suggest that cc1 and other stalagmites from the cave have the potential to capture a long record of regional temperature trends particularly in regards to the relative severity of pleistocene glacial stages c 2004 elsevier b v all rights reserved

the east ridge system 28 5 32 s east pacific rise implications for overlapping spreading center development abstract we report here on geophysical data from the east ridge and surrounding areas of the large offset overlapping spreading centers oscs that accommodate pacific nazca opening between 28 5 and 32 s the east ridge overlaps and is offset from the west ridge system by 120 km forming the largest known pair of oscs in this area spreading rates reach the fastest currently active on earth of 149 mm yr although the east ridge is composed of 4 morphologically defined segments separated by 3 small oscs other geophysical characteristics imply 1 upwelling segment all the active ridge segments in this area including the propagating tips of the east and west ridges form relative topographic highs with respect to the flanking sea floor however identified abandoned ridge tips form deeps we interpret these data in terms of a model in which the propagating segment represents an overshoot of a surficial rupture of the brittle lithospheric layer only partially coupled to the diverging flow of a more broadly distributed ductile deformation zone ddz surrounding the steady state ridges and crossing the offset between the oscs the topographic high of the propagating segment may be maintained primarily by along axis melt migration from the stable spreading segments rather than by direct upwelling from beneath the ridge the large overlapping ridges are inherently unstable and continued extension causes the overlapping axes to become offset from the stably spreading segments cut off from the supply of melt and replaced by a new set the failed rift tips for a period of time overlie the broad ddz and preferentially undergo continued extension and subsidence the ddz surrounding the ridge axes may be very broad in this area because of the very fast spreading rate creating a very thin lithosphere susceptible to perturbation by relatively small mantle heterogeneities advected near the ridge axis leading to the formation of the smaller oscs observed

gravity anomalies over extinct spreading centers a test of gravity models of active centers gravity fields over active mid ocean ridges display long wavelength free air gravity highs with superimposed short wavelength anomalies over the spreading axis these axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30 70 mgal and wavelengths of 40 60 km but decrease in amplitude and wavelength with increasing spreading rate until at fast spreading centers the anomalies are positive 10 20 mgal with wavelengths of about 20 30 km similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo spreading rate dependency suggesting that a large component of the anomaly is not dynamically maintained calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows the amplitudes of the residual lows at slow paleo spreading rates are 25 45 mgal with half widths of about 20 45 km and decrease with increasing spreading rate to about 5 mgal amplitude and 10 15 km half width over fast paleo spreading ridges the amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers most previously proposed models of mid ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid ocean ridges this research provides a further constraint on geometric models of accretion as these models predict vastly different gravity fields results indicate that a frozen in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis this conclusion is consistent with the presence of a low density gabbroic root situated at shallow mantle depths beneath the ridge axis this root tends to narrow with increasing spreading rate producing the required residual gravity field variation

abrupt temperature changes in the western mediterranean over the past 250 000 years a continuous high resolution western mediterranean sea surface temperature sst alkenone record spanning the past 250 000 years shows that abrupt changes were more common at warming than at cooling during marine isotope stage mis 6 sst oscillated following a stadial interstadial pattern but at lower intensities and rates of change than in the dansgaard oeschger events of mis 3 some of the most prominent events occurred over miss 5 and 7 after prolonged warm periods of high stability climate during the whole period was predominantly maintained in interglacial interstadial conditions whereas the duration of stadials was much shorter

gravity anomalies isostasy and mantle flow at the east pacific rise crest bathymetry and gravity data obtained during a detailed hydrosweep survey of the southern east pacific rise from 7 s to 9 s are used to investigate isostasy at the axis of a fast spreading mid ocean ridge in particular we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160 km long 7 12 s 8 38 s ridge segment the crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km the summit is broad and flat and an axial summit caldera is present for the entire length of the ridge segment however the width and cross sectional shape of the crestal high vary systematically along the axis it is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment the ridge crest is marked by a free air gravity anomaly high about 15 20 km wide with an amplitude of 10 15 mgal relative to the ridge flanks mantle bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment the axial mantle bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross sectional area of the crestal bathymetric high the effects of cooling and subsidence away from the axis were removed from the bathymetry and free air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest the residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis the lithosphere was treated as a broken plate either with a constant flexural rigidity or an effective elastic thickness te which grows at a rate proportional to the square root of distance from the axis the best fitting values of te for the constant rigidity case are in the range of 0 3 0 6 km for the growing plate model te increases at a rate of 0 2 0 3 km the gravity constrains the mass deficiency to extend to a depth of 20 30 km for both lithospheric models we interpret this low density material as a region of partial melt feeding magma to the ridge axis the best fitting density anomalies imply that a 4 9 melt fraction is present beneath the crestal high upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis the mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment it thus appears that differences between the along axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three dimensional focused upwelling at slow spreading ridges to two dimensional sheetlike upwelling at fast spreading ridges rather the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along axis magma distribution

age depth correlation of the philippine sea back arc basins and other marginal basins in the world abstract statistical analysis of underway bathymetric profiles of the philippine sea back arc basins the mariana trough the shikoku basin the parece vela basin and the west philippine basin reveals an age depth correlation in the basins the basement depths of the philippine sea range from 3200 to 6000 m with their ages from 0 to 60 ma the age depth curve of the whole philippine sea can be represented by the following equation depth m 3222 366 age ma the equation shows that the basement depth of the philippine sea is about 800 m deeper than that of the major ocean floors of the same age the coefficient of 366 in this equation is close to that for major oceans 350 this suggests that the cooling processes of the lithospheres in the philippine sea and major oceans are similar to each other whereas the structure and composition of the deeper layers of the philippine sea seem to be different from those of major oceans our analysis of etop digital bathymetric data however indicates that this relationship does not hold for some other marginal basins marginal basins other than back arc basins generally follow the age depth curve of major oceans young back arc basins

east asian hydroclimate modulated by the position of the westerlies during termination i speleothem oxygen isotope records have revolutionized our understanding of the paleo east asian monsoon yet there is fundamental disagreement on what they represent in terms of the hydroclimate changes we report a multiproxy speleothem record of monsoon evolution during the last deglaciation from the middle yangtze region which indicates a wetter central eastern china during north atlantic cooling episodes despite the oxygen isotopic record suggesting a weaker monsoon we show that this apparent contradiction can be resolved if the changes are interpreted as a lengthening of the meiyu rains and shortened post meiyu stage in accordance with a recent hypothesis model simulations support this interpretation and further reveal the role of the westerlies in communicating the north atlantic influence to the east asian climate

partitioning of mg sr ba and u into a subaqueous calcite speleothem abstract the trace element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate with a particular emphasis on elements whose concentrations vary according to hydrological conditions at the cave site e g mg sr ba and u an important step in interpreting trace element abundances is understanding the underlying processes of their incorporation this includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients where the partitioning d of element x dx is the molar ratio x ca in the calcite divided by the molar ratio x ca in the parent water and evaluating the degree of spatial variability across time constant speleothem layers previous studies of how these elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in natural cave settings or have used synthetic solutions under cave analogue laboratory conditions to produce similar dripstones however dripstones are not the only speleothem types capable of yielding useful palaeoclimate information in this study we investigate the incorporation of mg sr ba and u into a subaqueous calcite speleothem cd3 growing in a natural cave pool in italy pool water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to the deep cave setting enabling the calculation of precise solution x ca we determine the trace element variability of modern subaqueous calcite from a drill core taken through cd3 to derive dmg dsr dba and du then compare these with published cave cave analogue and seawater analogue studies the dmg for cd3 is anomalously high 0 042 0 002 compared to previous estimates at similar temperatures 8 c the dsr 0 100 0 007 is similar to previously reported values but data from this study as well as those from tremaine and froelich 2013 and day and henderson 2013 suggest that na sr might play an important role in sr incorporation through the potential for na to outcompete sr for calcite non lattice sites dba in cd3 0 086 0 008 is similar to values derived by day and henderson 2013 under cave analogue conditions whilst du 0 013 0 002 is almost an order of magnitude lower possibly due to the unusually slow speleothem growth rates

stalagmite evidence for the precise timing of north atlantic cold events during the early last glacial evidence of millennial scale cold events following the last interglacial are well preserved in north atlantic marine cores greenland ice and pollen records from europe however their timing was previously undetermined by radiometric dating we report the first precise radiometric ages for two such events c23 105 1 0 9 ka to 102 6 0 8 ka and c24 112 0 0 8 ka and 108 8 1 0 ka based on stable carbon and oxygen isotope measurements on a stalagmite from italy cc28 in addition to providing new information on the duration of these events in southern europe the age data provide invaluable tuning points for the melisey i c24 and montaigu c23 pollen zones identified in western europe the former event is of particular significance because it represents the end of the eemian interglacial forest phase in western europe the new age data will also allow fine tuning of the timing and duration of greenland stadial 24 equivalent to c23 in the north greenland ice core project ice core and via a common gasage chronology tuning of the vostok and epica european project for ice coring in antarctica ice cores

enigmatic extinct spreading center in the west philippine backarc basin unveiled the central basin fault in the center of the west philippine basin was first discovered 50 yr ago it is a 1000 km long ridge oriented northwest to southeast and is cut by north south trending fracture zones hypotheses about the origin and development of the central basin fault have remained unresolved until recently submersible observations and seabeam surveys show that the central basin fault is a segmented spreading ridge having a morphology similar to that of a slow spreading ridge with a nontransform offset a nodal deep and an inside corner high the distance from the ridge versus the depth of the sea floor the obliqueness of sets of small trough and ridge structures and heat flow values both of the crestal and off axis areas of the central basin fault suggest that the fault is not a simple spreading center but rather underwent multiple spreading episodes the texture and chemistry of basalts obtained from the ridge suggest that the lavas were formed in a backarc basin setting these data confirm that the central basin fault is a slow backarc spreading center that has a more complicated evolutionary history than previously realized

the structure and segmentation of the southeast indian ridge abstract the southeast indian ridge seir spreads at a relatively narrow range of intermediate rates 59 75 km ma but exhibits the full range of slow to fast spreading morphology and segmentation satellite gravity data reveal transitions in the structure of the spreading center where it is influenced by the amsterdam and kerguelen hotspots and at the australian antarctic discordance aad although the spreading rate between the hotspots and the aad is nearly constant the ridge exhibits a variety of distinct styles of morphology and segmentation not observed at fast or slow spreading centers recently collected multibeam bathymetry data reveal a transition from east pacific rise style overlapping axial highs near 92 e to mid atlantic ridge style axial valleys with non transform offsets near 116 e the intervening segmentation is characterized by propagating offsets coexisting with stationary transforms which exhibit different degrees of temporal stability currently there are 10 transform offsets between the hotspots and the aad but only five of these have persisted since seafloor spreading stabilized at 35 ma the other five appear to have formed since 35 ma and several more have disappeared by transform shortening or coalesced by along axis propagation there is a transition from monotonic offset propagation near the hotspots to oscillatory propagation approaching the aad this change in offset stability corresponds to transitions in depth axial morphology and offset structure through much of the transitional region higher order segmentation is characterized by en echelon offsets of a diffuse spreading axis that generally lacks a well defined neovolcanic zone since the spreading rate is nearly constant the regional variation in axial morphology and segmentation appears to be controlled by an upper mantle thermal gradient possibly a result of flux of asthenosphere from the hotspots to the aad this is consistent with the gradual increase in average ridge flank depths along this part of the plate boundary but segment scale changes in axial depth reveal spatio temporal variability in the dynamic topography that are not preserved on older lithosphere intrasegment transitions in axial morphology and en echelon offsets within first order segments suggest that local variations in mantle thermal structure introduce short lived instabilities in higher order segmentation and dominate the short term evolution of the plate boundary

a biomarker approach to the organic matter deposited in the north atlantic during the last climatic cycle abstract the study of the composition of total organic carbon toc c37 alkenones and c23 c33 n alkanes in the north atlantic ocean cores su90 08 and su90 39 has allowed the development of a model for the differentiation of marine and terrigenous toc this model gives rise to results in good agreement with inorganic markers such as magnetic susceptibility ms and non carbonate content according to this model the terrigenous toc accounts for most of the organic matter in the glacial sediments thus the higher toc of the glacial periods 0 1 0 45 vs 0 05 0 15 in the interglacials is due to the increase in terrigenous toc the changes in marine toc those associated to sea surface productivity are independent of the glacial interglacial evolution the terrigenous toc is more important at higher latitudes probably due to higher terrigenous detrital inputs associated with iceberg transport in this respect the correlation between n alkanes and ms strongly suggests that the main source of these hydrocarbons are ice rafted materials and that aeolian inputs only represent minor contributions the four peaks of reworked n alkanes in the su90 08 core that are coincident with the heinrich layers h1 h2 h4 and 115 are in agreement with this hypothesis on the other hand the abrupt marine toc peaks of su90 08 show a 23 ka periodicity indicating that the marine productivity at 43 n in the north atlantic ocean was precessionally driven the productivity maxima in this core correlate with low latitude oceanographic processes such as periods of enhanced trade wind intensity and equatorial upwelling which are also tuned to precession and independent of the global glacial interglacial evolution

iberian margin sea surface temperature during mis 15 to 9 580 300 ka glacial suborbital variability versus interglacial stability 1 past sea surface water conditions of the western iberian margin were reconstructed based on biomarker analyses of a marine deep sea core md03 2699 from the estremadura spur north off lisbon providing new insights into orbital and suborbital scale climate variability between marine isotope stage mis 15 to mis 9 580 to 300 ka we use biomarker based proxy records such as the alkenone unsaturated index to estimate sea surface temperature sst the total alkenone concentration to reconstruct phytoplankton productivity and terrestrial biomarkers to evaluate the continental input the results extend the existing biomarker record namely the sst for the iberian margin back to the sixth climatic cycle 580 ka a general trend of stable interglacials contrasts with glacial periods and glacial inceptions which are marked by high frequency variability thus several short lived climatic coolings were identified by large sst decreases the occurrence of ice rafted detritus and high percentages of the tetraunsaturated alkenone c37 4 some of these events were extremely cold and similar in their general trends to the well known heinrich events of the last glaciation we identified eight heinrich type events between 580 and 300 ka the general deglaciation pattern detected between mis 15 and mis 9 is similar in their general trends to that characterizing the more recent climatic cycles i e marked by two coolings separated by a short warming episode which may reflect the southward northward and southward migration of the polar front

a simple rule to determine which insolation cycles lead to interglacials the pacing of glacial interglacial cycles during the quaternary period the past 2 6 million years is attributed to astronomically driven changes in high latitude insolation however it has not been clear how astronomical forcing translates into the observed sequence of interglacials here we show that before one million years ago interglacials occurred when the energy related to summer insolation exceeded a simple threshold about every 41 000 years over the past one million years fewer of these insolation peaks resulted in deglaciation that is more insolation peaks were skipped implying that the energy threshold for deglaciation had risen which led to longer glacials however as a glacial lengthens the energy needed for deglaciation decreases a statistical model that combines these observations correctly predicts every complete deglaciation of the past million years and shows that the sequence of interglacials that has occurred is one of a small set of possibilities the model accounts for the dominance of obliquity paced glacial interglacial cycles early in the quaternary and for the change in their frequency about one million years ago we propose that the appearance of larger ice sheets over the past million years was a consequence of an increase in the deglaciation threshold and in the number of skipped insolation peaks

uranium series chronology and environmental applications of speleothems an increasing number of scientists recognize the value of speleothems1 as often extremely well preserved archives of information about past climate vegetation hydrology sea level nuclide migration water rock interaction landscape evolution tectonics and human action well constrained data are required to document past changes reconstruct past patterns and predict future responses of the earth system at a wide range of spatial and temporal scales speleothems are particularly useful in this regard because they can be found in many locations of the globe sampled at high resolution and reliably dated using high precision uranium series techniques speleothems are bodies of mineral material formed in caves as the result of chemical precipitation from groundwater flowing or dripping in a cave most speleothems are composed of calcite formed by slow degassing of co2 from supersaturated groundwater but aragonite and gypsum forms are also common particularly near cave entrances where evaporative effects are important a host of different speleothem types decorate the walls ceilings and floors of caves and their mineralogy and morphology is a function of fluid flow and chemistry of waters feeding a particular location as well as the ambient conditions temperature chemistry light in the air or water filled void subaerial forms include the familiar stalagmites stalactites draperies flowstones subaqueous forms include rimstone pools rafts mammillary calcite wall coatings and dog tooth spar for an extensive review of the types of speleothem that have been observed see hill and forti 1997 speleothems are used in a multitude of ways to explore past environmental conditions perhaps the most fundamental of which is their very presence or absence deposition of speleothems relies on sufficient water supply and soil co2 to enable dissolution and transport of reactants in the vadose zone to underlying caves in arid or glacial times conditions may not have been favorable for speleothem formation

a 1 ma record of sea surface temperature and extreme cooling events in the north atlantic a perspective from the iberian margin abstract the iberian margin is a sensitive area to track high and low latitude processes and is a key location to understand major past climatic and oceanographic changes here we present new biomarker data from iodp site u1385 shackleton site 1017 336 ka that when combined with existing data from cores md01 2443 4 last 335 ka allows us to assess the evolution of sea surface temperature sst and meltwater influx over the last 1 ma at the iberian margin interglacial periods throughout the last 1 ma show sst close to 20 c even during the so called luke warm interglacials that are marked by relatively low atmospheric co2 concentrations during glacial periods extremely cold stadial events are recognized at the iberian margin and are very likely related to meltwater discharges from the european and british irish ice sheets into the ne atlantic which were transported southwards by the portugal current we subdivided the record into four intervals on the basis of the timing and the magnitude of these extremely cold stadials 1 from 1017 to 900 ka only minor sporadic freshwater input occurred during deglaciations 2 from 900 to 675 ka extreme cold events occur as terminal stadial events at the beginning of the deglaciations which results in abrupt deglacial sst shifts 3 from 675 to 450 ka only a few very short lived events are recorded and seldom is there freshwater input at the iberian margin 4 during the last 450 ka the extreme cold events occurred under full glacial conditions with particularly severe events during mis 6 and 8 we propose these mid glacial events are associated with a strong discharges of european ice sheet eis the fact that these extreme cold events do not coincide with deglaciations questions the role of european ice sheet discharges in triggering deglaciations

similarities and dissimilarities between the last two deglaciations and interglaciations in the north atlantic region abstract sea surface temperatures ssts recorded by alkenones and oxygen isotopes in the alboran basin are used here to describe at an unprecedented fine temporal resolution the present interglaciation pig initiated at 11 7 ka bp the last interglaciation lig onset approximately at 129 ka and respective deglaciations similarities and dissimilarities in the progression of these periods are reviewed in comparison with ice cores and stalagmites cold spells coeval with the heinrich events h described in the north atlantic include multi decadal scale oscillations not previously obvious up to 4 c in less than eight centuries within the stadials associated with h1 and h11 ca 133 ka and 17 ka respectively these abrupt oscillations precede the accumulation of organic rich layers deposited when perihelion moves from alignment with nh spring equinox to the summer solstice a reference for deglaciations events observed during the last deglaciation at 17 ka 14 8 ka and 11 7 ka are reminiscent of events occurred during the penultimate deglaciation at ca 136 ka 132 ka and 129 ka respectively the sst trend during the pig is no more than 2 c from 20 c to 18 c up to 0 2 c ka the trend is steeper during the lig i e up to a 5 c change from the early interglaciation to immediately before the glacial inception from 23 c to 18 c up to 0 4 c ka events are superimposed upon a long term trend towards colder ssts beginning with sst maxima followed by temperate periods until perihelion aligned with the nh autumn equinox before ca 5 3 ka for the pig and 121 ka for the lig a cold spell of around eight centuries at 2 8 ka during the pig was possibly mimicked during the lig at ca 118 ka by a sst fall of around 1 c in a millennium these events led interglacial sst to stabilise at around 18 c the glacial inception barely evident at the beginning ca 115 ka north atlantic event c25 after perihelion passage in the nh winter solstice culminated with a sst drop of at least 2 c in two millennia event c24 ca 111 ka the little ice age 0 7 ka also occurred after the latest perihelion passage in the nh winter solstice and could be an example of how a glacial pre inception event following an interglaciation might be

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

spreading rate dependence of three dimensional structure in oceanic spreading centres segmentation by transform faults and other types of along axis discontinuity1 is a well described but poorly explained characteristic of oceanic spreading centres here we use numerical experiments to explore the dynamics of mantle flow and melting beneath a mid ocean ridge buoyant upwelling driven by compositional density variations resulting from the extraction of the melt that forms the ocean crust exhibits a spreading rate dependent transition between two dimensional and three dimensional upwelling structures for low spreading rates and mantle viscosities an initial two dimensional structure transforms into a three dimensional one at high spreading rates an initially two dimensional structure remains two dimensional these results suggest that the origin of spreading centre segmentation may be different at fast and slow spreading rates

combined obliquity and precession pacing of late pleistocene deglaciations a new statistical test shows that precession as well as obliquity contributes to the pacing of deglaciations in the late pleistocene the glacial cycles that characterize earth s climate history are paced by variations in earth s orbital orientation relative to the sun into the middle pleistocene about a million years ago glacial cycles were paced by 40 000 year cycles in earth s tilt or obliquity since then 100 000 year variations in orbital eccentricity seem to have dominated although it has been speculated that a combination of obliquity and precession orbital axis orientation might conspire to pace deglaciations firm quantitative evidence has not emerged peter huybers now uses advances in timing and statistics to show that indeed obliquity and precession simultaneously pace deglaciations milankovitch1 proposed that earth resides in an interglacial state when its spin axis both tilts to a high obliquity and precesses to align the northern hemisphere summer with earth s nearest approach to the sun this general concept has been elaborated into hypotheses that precession2 obliquity3 4 or combinations of both5 6 7 8 could pace deglaciations during the late pleistocene9 10 earlier tests have shown that obliquity paces the late pleistocene glacial cycles4 11 but have been inconclusive with regard to precession whose shorter period of about 20 000 years makes phasing more sensitive to timing errors4 11 12 no quantitative test has provided firm evidence for a dual effect here i show that both obliquity and precession pace late pleistocene glacial cycles deficiencies in time control that have long stymied efforts to establish orbital effects on deglaciation are overcome using a new statistical test that focuses on maxima in orbital forcing the results are fully consistent with milankovitch s proposal but also admit the possibility that long southern hemisphere summers contribute to deglaciation

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