calibration of the planktonic foraminiferal mg ca paleothermometer sediment trap results from the guaymas basin gulf of california 1 the mg ca ratio has been measured in two surface dwelling species of planktonic foraminifera globigerinoides ruber and globigerina bulloides from biweekly sediment trap samples collected in guaymas basin gulf of california between august 1992 and november 1997 the guaymas basin experiences significant seasonal changes in sea surface temperature sst 16 33 c and thus is an ideal location for testing the temperature dependence of mg incorporation into foraminiferal calcite the planktonic foraminiferal mg ca ratios are directly compared with concurrent temperature measurements from the study site the results from the sediment trap study reveal a strong positive correlation between sst and the mg ca ratio in both g ruber r2 0 86 and g bulloides r2 0 90 the mg ca ratio increases exponentially by 7 per 1 c change in temperature for g ruber and by 6 for g bulloides these results indicate that the mg ca ratio in g ruber and g bulloides accurately records the measured seasonal surface temperature cycle and interannual variability in the guaymas basin within 1 c the greatest deviation between mg ca estimated sst and observed sst occurs in the spring and fall for both species this variability is attributed to rapidly changing hydrographic conditions and possible offset between the time of calcification and the temperature measurements in addition g ruber and g bulloides exhibit very high mg ca ratios during the strong el nino southern oscillation years of 1992 and 1997 verifying their potential to record sst up to 33 c and 28 c respectively the trace metal data are compared with foraminiferal oxygen isotope and alkenone measurements made on the same suite of sediment trap samples to assess the internal consistency of these temperature proxies strong correlations exist between the 18o and mg ca for g ruber r2 0 86 and g bulloides r2 0 79 similarly sea surface temperature estimates from mg ca ratios are in good agreement with alkenone derived temperature estimates r2 0 89 for g ruber and r2 0 81 for g bulloides

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

inter hemispheric asymmetry in the early pleistocene pacific warm pool the position of the southern boundary of the pacific warm pool is shown to have been stable since the early pleistocene based upon a planktic foraminiferal mg ca derived reconstruction of subtropical sea surface temperature in the coral sea this contrasts with previous reconstructions showing warm pool contraction from the north and east and means that the early pleistocene warm pool was more hemispherically asymmetric than its present configuration the latter was not established until 1ma supporting a strengthening of the northern hadley cell which was not replicated in its southern counterpart prior to the mid pleistocene transition copyright 2010 by the american geophysical union

a study of cleaning procedures used for foraminiferal mg ca paleothermometry the various cleaning steps required for preparation of foraminiferal samples for mg ca and sr ca analysis are evaluated for their relative importance and effects on measured elemental ratios it is shown that the removal of silicate contamination is the most important step for the measurement of mg ca ratios in an example bulk sample mg ca decreases from 10 5 to 2 5 mmol mol 1 during clay removal oxidation of organic material causes a lowering of sample mg ca in the order of 10 or approximately 1 c when converted to temperature use of dilute acid leaching to remove adsorbed contaminants causes partial dissolution of the sample carbonate and a corresponding decrease in mg ca reductive treatment also causes dissolution of the sample and a decrease in the mg ca ratio of up to 10 15 sample preparation for sr ca analysis does not require the same degree of rigor as is necessary for mg ca work the within run reproducibility of the method described here for g ruber in a core top sample from the arabian sea was 1 8 mean sample ratio was 4 72 mmol mol 1 when converted to temperature this becomes 28 0 2 c the equivalent result for sr ca was 0 5 mean ratio 1 44 mmol mol 1

glacial interglacial ocean climate variability from planktonic foraminifera during the mid pleistocene transition in the temperate southwest pacific odp site 1123 abstract planktonic foraminiferal assemblages and artificial neural network estimates of sea surface temperature sst at odp site 1123 41 47 2 s 171 29 9 w 3290 m deep east of new zealand reveal a high resolution history of glacial interglacial g i variability at the subtropical front stf for the last 1 2 million years including the mid pleistocene climate transition mpt most g i cycles of 100 kyr duration have short periods of cold glacial and warm deglacial climate centred on glacial terminations followed by long temperate interglacial periods during glacial deglacial transitions maximum abundances of subantarctic and subtropical taxa coincide with sst minima and maxima and lead ice volume by up to 8 kyrs such relationships reflect the competing influence of subantarctic and subtropical surface inflows during glacial and deglacial periods respectively suggesting alternate polar and tropical forcing of southern mid latitude ocean climate the lead of ssts and subtropical inflow over ice volume points to tropical forcing of southern mid latitude ocean climate during deglacial warming this contrasts with the established hypothesis that southern hemisphere ocean climate is driven by the influence of continental glaciations based on wholesale changes in subantarctic and subtropical faunas the last 1 2 million years are subdivided into 4 distinct periods of ocean climate 1 the pre mpt 1185 870 ka has high amplitude 41 kyr fluctuations in sst superimposed on a general cooling trend and heightened productivity reflecting long term strengthening of subantarctic inflow under an invigorated antarctic circumpolar current 2 the early mpt 870 620 ka is marked by abrupt warming during mis 21 followed by a period of unstable periodicities within the 40 100 kyr orbital bands decreasing sst amplitudes and long intervals of temperate interglacial climate punctuated by short glacial and deglacial phases reflecting lower meridional temperature gradients 3 the late mpt 620 435 ka encompasses an abrupt decrease in the subantarctic inflow during mis 15 followed by a period of warm equable climate poorly defined low amplitude g i variations in ssts during this interval are consistent with a relatively stable stf and evenly balanced subantarctic and subtropical inflows possibly in response to smaller less dynamic polar icesheets 4 the post mpt 435 0 ka is marked by a major climatic deterioration during mis 12 and a return to higher amplitude 100 kyr frequency sst variations superimposed on a long term trend towards cooler ssts and increased mixed layer productivity as the subantarctic inflow strengthened and polar icesheets expanded

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

tropical cooling at the last glacial maximum and extratropical ocean ventilation1 1 the ncar csm is used to simulate the climate at last glacial maximum the csm simulates a tropical sst cooling of 2 1 c it is shown that about half of this tropical cooling is associated with the upper ocean circulation especially the ventilation of thermocline and intermediate waters from the mid high latitude south pacific

constraints on the magnitude and patterns of ocean cooling at the last glacial maximum observation based reconstructions of sea surface temperature from relatively stable periods in the past such as the last glacial maximum represent an important means of constraining climate sensitivity and evaluating model simulations1 the first quantitative global reconstruction of sea surface temperatures during the last glacial maximum was developed by the climate long range investigation mapping and prediction climap project in the 1970s and 1980s refs 2 3 since that time several shortcomings of that earlier effort have become apparent4 here we present an updated synthesis of sea surface temperatures during the last glacial maximum rigorously defined as the period between 23 and 19 thousand years before present from the multiproxy approach for the reconstruction of the glacial ocean surface margo project5 we integrate microfossil and geochemical reconstructions of surface temperatures and include assessments of the reliability of individual records our reconstruction reveals the presence of large longitudinal gradients in sea surface temperature in all of the ocean basins in contrast to the simulations of the last glacial maximum climate available at present6 7

seasonal and enso variations of sea surface salinity and temperature in the south pacific convergence zone during 1976 2000 1 sea surface salinity sss and temperature sst data collected from voluntary observing ships over 25 years 1976 2000 are analyzed in the southwestern tropical pacific 10 s 24 s 160 e 140 w this region lies under the south pacific convergence zone spcz at the southern edge of the western pacific warm pool between tahiti and darwin the two places whose atmospheric sea level pressure difference is used to define the southern oscillation index soi complementary data such as precipitation are used to assist in the analysis the mean and seasonal variations of these parameters are described an empirical orthogonal function eof analysis of low pass filtered time series is then performed to extract the interannual variability all parameters show an interannual signal which correlates well with the soi the southwestern tropical pacific ocean is saltier and colder during el nino than during la nina events in the southwestern part there is a shortage excess in precipitation during el nino la nina events the greatest anomalies appeared during the last la nina in 1996 and 1999 as regards sst and in 1999 and 2000 as regards sss sst and precipitation el nino southern oscillation enso related anomalies are an order of magnitude smaller than seasonal anomalies while the sss enso related signal is twice as strong as the seasonal signal these facts reflect the northeastward southwestward shift of the spcz during el nino la nina events while consistent with precipitation changes the enso related variability in sss can also be partly explained by the displacement of the salinity front that separates fresh warm pool waters from salty subtropical waters computation of surface geostrophic current anomalies from geosat 1987 1988 and topex poseidon 1993 2000 indicates that westward current anomalies developed during the 1987 88 and 1997 98 el nino and are linked to the displacement of the salinity front the southwestern tropical pacific salinity front moves westward eastward in contrast to the equatorial salinity front which moves eastward westward during an el nino la nina event

mg ca ratios of two globigerinoides ruber white morphotypes implications for reconstructing past tropical subtropical surface water conditions 1 tests of the planktonic foraminifer globigerinoides ruber white d orbigny have become a standard tool for reconstructing past oceanic environments paleoceanographers often utilize the mg ca ratios of the foraminiferal tests for reconstructing low latitude ocean glacial interglacial changes in sea surface temperatures sst we report herein a comparison of mg ca measurements on sample pairs n 20 of two g ruber white morphotypes g ruber sensu stricto s s and g ruber sensu lato s l from surface and downcore samples of the western pacific and indian oceans g ruber s s refers to specimens with spherical chambers sitting symmetrically over previous sutures with a wide high arched aperture whereas g ruber s l refers to a more compact test with a diminutive final chamber and small aperture the g ruber s s specimens generally show significantly higher mg ca ratios compared to g ruber s l our results from the mg ca ratio analysis suggest that g ruber s l specimens precipitated their shells in slightly colder surface waters than g ruber s s specimens this conclusion is supported by the differences in 18o and 13c values between the two morphotypes although it is still unclear if these two morphotypes represent phenotypic variants or sibling species our findings seem to support the hypothesis of depth and or seasonal allopatry within a single morphospecies

salinity bias on the foraminifera mg ca thermometry correction procedure and implications for past ocean hydrographic reconstructions 1 mg ca in foraminiferal calcite has recently been extensively used to estimate past oceanic temperatures here we show however that the mg ca temperature relationship of the planktonic species globigerinoides ruber is significantly affected by seawater salinity with a 1 psu change in salinity resulting in a 1 6 c bias in mg ca temperature calculations if not accounted for such a bias could lead for instance to systematic overestimations of mg ca temperatures during glacial periods when global ocean salinity had significantly increased compared to today we present here a correction procedure to derive unbiased sea surface temperatures sst and 18osw from g ruber tmg ca and 18of measurements this correction procedure was applied to a sedimentary record to reconstruct hydrographic changes since the last glacial maximum lgm in the western pacific warm pool while uncorrected tmg ca data indicate a 3 c warming of the western pacific warm pool since the lgm the salinity corrected sst result in a stronger warming of 4 c

evolution of the eastern tropical pacific through plio pleistocene glaciation a tropical pacific climate state resembling that of a permanent el nino is hypothesized to have ended as a result of a reorganization of the ocean heat budget 3 million years ago a time when large ice sheets appeared in the high latitudes of the northern hemisphere we report a high resolution alkenone reconstruction of conditions in the heart of the eastern equatorial pacific eep cold tongue that reflects the combined influences of changes in the equatorial thermocline the properties of the thermocline s source waters atmospheric greenhouse gas content and orbital variations on sea surface temperature sst and biological productivity over the past 5 million years our data indicate that the intensification of northern hemisphere glaciation 3 million years ago did not interrupt an almost monotonic cooling of the eep during the plio pleistocene sst and productivity in the eastern tropical pacific varied in phase with global ice volume changes at a dominant 41 000 year obliquity frequency throughout this time changes in the southern hemisphere most likely modulated most of the changes observed

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

deep sea temperature and ice volume changes across the pliocene pleistocene climate transitions earth has undergone profound changes since the late pliocene which led to the development 2 7 million years ago ma and intensification 0 9 ma of large scale northern hemisphere ice sheets recorded as transitions in the benthic foraminiferal oxygen isotope 18ob record here we present an orbitally resolved record of deep ocean temperature derived from benthic foraminiferal magnesium calcium ratios from the north atlantic which shows that temperature variations are a substantial portion of the global 18ob signal the record shows two distinct cooling events associated with the late pliocene lpt 2 5 to 3 ma and mid pleistocene mpt 1 2 to 0 85 ma climate transitions whereas the lpt increase in ice volume is attributed directly to global cooling the shift to 100 000 year cycles at the mpt is more likely to be a response to an additional change in ice sheet dynamics

the mid pleistocene transition in the tropical pacific a sea surface temperature sst record based on planktonic foraminiferal magnesium calcium ratios from a site in the western equatorial pacific warm pool reveals that glacial interglacial oscillations in sst shifted from a period of 41 000 to 100 000 years at the mid pleistocene transition 950 000 years before the present sst changes at both periodicities were synchronous with eastern pacific cold tongue ssts but preceded changes in continental ice volume the timing and nature of tropical pacific sst changes over the mid pleistocene transition implicate a shift in the periodicity of radiative forcing by atmospheric carbon dioxide as the cause of the switch in climate periodicities at this time

accuracy standardization and interlaboratory calibration standards for foraminiferal mg ca thermometry the use of liquid and solid standards for foraminiferal mg ca and sr ca determinations and interlaboratory calibration has been investigated preparation of single element standard solutions from primary solid standard material enables the preparation of mixed standard solutions with mg ca and sr ca ratios of known accuracy to better than 0 1 we also investigated commercial reference materials to determine whether existing carbonate standards could be used as reference material for mg ca determinations in foraminiferal calcite we propose that in the absence of a pure calcium carbonate standard certified for mg ca ecrm 752 1 a limestone crm containing mg ca within the range of typical foraminifera is a suitable solid standard for interlaboratory calibration replicate mg ca determinations showed that provided silicate phases are removed by centrifugation this material is homogenous within the precision of daily instrumental mg ca determinations over a range of sample weights from 10 to 1000 mg taken from two separate bottles of ecrm 752 1 results gave an average value of mg ca 3 75 mmol mol 0 015 s d 0 41 r s d on 118 determinations from the two bottles

an intensity ratio calibration method for the accurate determination of mg ca and sr ca of marine carbonates by icp aes an inductively coupled plasma atomic emission spectroscopy icp aes method for the accurate and precise simultaneous measurement of the mg ca and sr ca content of carbonates was established while a precision of 0 3 1 standard deviation sd is easily obtainable for both mg ca and sr ca analysis a ca matrix effect complicates achieving similar levels of accuracy with conventional calibration procedures an alternative ratio calibration procedure is proposed which overcomes the ca matrix effects and ensures the accuracy of the mg ca and sr ca analysis of marine carbonates to 0 3 almost an order of magnitude better than conventional calibration methods the longer term precision is 0 1 if the batch run average values are corrected for longer term drift the method is suitable for analysis of foraminiferal calcite and coral aragonite and can easily be adjusted for the analysis of other carbonates or microsamples

orbital and millennial antarctic climate variability over the past 800 000 years a high resolution deuterium profile is now available along the entire european project for ice coring in antarctica dome c ice core extending this climate record back to marine isotope stage 20 2 800 000 years ago experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation we assessed the general correspondence between dansgaard oeschger events and their smoothed antarctic counterparts for this dome c record which reveals the presence of such features with similar amplitudes during previous glacial periods we suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records

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

validity of the thin viscous sheet approximation in models of continental collision 1 the two dimensional thin viscous sheet approximation is widely used to describe large scale continental deformation it treats the lithosphere as a fluid layer in which deformation results from a balance between buoyancy forces and tectonic boundary conditions comparisons between two dimensional thin sheet and full three dimensional solutions of a simple indenter model show that appreciable differences exist especially when the indenter half width d is of the same order as the thickness of the deforming layer l i e d l 1 these differences are amplified by increasing the power law exponent of the viscous constitutive law n but decrease as the argand number ar is increased the greatest differences between two dimensional and three dimensional solutions are found at the indenter corner where the thin sheet consistently overestimates vertical strain rates differences between strain rates at the corner may be 50 or greater for small argand numbers other differences arise because a lithospheric root zone is formed in the three dimensional solutions and vertically averaged strain rate is decreased in regions close to the indenter this effect is absent from thin sheet calculations since the thickness of the load bearing layer is assumed constant in general the thin viscous sheet approximation provides a reasonably accurate estimate of long wavelength deformation for d l as low as 1 if n is less than 3 however even at large d l the solutions may be inaccurate close to strain rate concentrations at the indenter corners where horizontal gradients of deformation are large

enso like forcing on oceanic primary production during the late pleistocene late pleistocene changes in oceanic primary productivity along the equator in the indian and pacific oceans are revealed by quantitative changes in nanoplankton communities preserved in nine deep sea cores we show that variations in equatorial productivity are primarily caused by glacial interglacial variability and by precession controlled changes in the east west thermocline slope of the indo pacific the precession controlled variations in productivity are linked to processes similar to the southern oscillation phenomenon and they precede changes in the oxygen isotopic ratio which indicates that they are not the result of ice sheet fluctuations the 30 000 year spectral peak in the tropical indo pacific ocean productivity records is also present in the antarctica atmospheric co2 record suggesting an important role for equatorial biological productivity in modifying atmospheric co2

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

orbital controls on the el nino southern oscillation and the tropical climate the global synchroneity of glacial interglacial events is one of the major problems in understanding the link between milankovitch forcing and the climate of the late quaternary in this study we isolate a part of the climate system the tropical pacific and test its sensitivity to changes in solar forcing associated with changes in the earth s orbital parameters we use a simplified coupled ocean atmosphere model that is run for the past 150 000 years and forced with milankovitch changes in the solar insolation this system responds primarily to the precessional cycle in solar forcing and is capable of generating a mean response to the changes in the seasonal distribution of solar radiation even while the annual mean insolation is roughly constant the mean response to the precessional forcing is due to an interaction between an altered seasonal cycle and the el nifio southern oscillation enso changes in the enso behavior result in a mean tropical climate change the hypothesis is advanced that such a change in the tropical climate can generate a globally synchronous climate response to milankovitch forcing

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

uncertainties in seawater thermometry deriving from intratest and intertest mg ca variability in globigerinoides ruber intertest mg ca variability within these deep sea core top samples is a source of significant uncertainty in mg ca seawater temperature estimates and is notable for being site specific our results indicate that widely assumed uncertainties in mg ca thermometry may be underestimated we show that statistical power analysis can be used to evaluate the number of tests needed to achieve a target level of uncertainty on a sample by sample case a varying bias also arises from the presence and varying mix of two morphotypes g ruber ruber and g ruber pyramidalis which have different mean mg ca values estimated calcification temperature differences between these morphotypes range up to 5c and are notable for correlating with the seasonal range in seawater temperature at different sites

role of tropics in changing the response to milankovich forcing some three million years ago appeared in regions that today have intense oceanic upwelling the eastern equatorial pacific and the coastal zones of southwestern africa and california there was furthermore a significant change in the earth s response to milankovich forcing obliquity signals became large but those associated with precession and eccentricity remained the same the latter change in the earth s response can be explained by hypothesizing that the global cooling during the cenozoic affected the thermal structure of the ocean it caused a gradual shoaling of the thermocline around 3 ma the thermocline was sufficiently shallow for the winds to bring cold water from below the thermocline to the surface in certain upwelling regions this brought into play feedbacks involving

comment on deep sea temperature and ice volume changes across the pliocene pleistocene climate transitions sosdian and rosenthal reports 17 july 2009 p 306 used magnesium calcium ratios in benthic foraminifera from the north atlantic to reconstruct past bottom water temperatures they suggested that both ice volume change and ice sheet dynamics played important roles during the late pliocene and mid pleistocene climate transitions we present evidence that their record of deep ocean temperature is not reliable thus raising doubts about their conclusions

controls on shell mg ca and sr ca in cultured planktonic foraminiferan globigerinoides ruber white mg ca and sr ca ratios were determined on a single species of planktonic foraminiferan globigerinoides ruber white collected from the gulf of eilat and cultured in seawater at five different salinities 32 to 44 five temperatures 18 to 30 c and four ph values 7 9 to 8 4 the mg ca temperature calibration of cultured g ruber with an exponential slope of 8 3 c agrees well with previously published calibrations from core tops and sediment traps however the dependence of mg ca on salinity with an exponential slope of 5 3 psu is also significant and should be included in the calibration equation with this purpose we calculated a calibration equation for g ruber dependent on both temperature and salinity within the 95 confidence limits mg ca mmol mol exp 0 06 0 02 s psu 0 08 0 02 t c 2 8 1 0 r2 0 95 the influence of ph on mg ca ratios is negligible at ambient seawater ph 8 1 to 8 3 however we observe a dominating ph control on shell mg ca when the ph of seawater is lower than 8 0 sr ca in g ruber shows a significant positive correlation with average growth rate presumably part of the variability in shell sr ca in the geological record is linked to changes in growth rates of foraminifera as a response to changing environmental conditions

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

late pliocene equatorial pacific 1 late pliocene foraminiferal mg ca and 18o records from ocean drilling program hole 806b in the western equatorial pacific wep reveal warm pool climate evolution during the onset of northern hemisphere glaciation 3 1 2 3 myr b p mg ca data indicate an average late pliocene sea surface temperature sst of 27 8 c a small long term cooling of 0 3 c between 3 1 and 2 3 ma and a glacial interglacial g i sst range of 2 c throughout this time interval for comparison pleistocene ssts at this site over the last 0 9 myr average 27 7 c with a g i range of 3 c orbital scale variability in hole 806b ssts during the late pliocene occurs predominantly at 100 ka in contrast to foraminiferal 18o records which show a dominant 41 kyr period variability at a 41 kyr period out of phase with local annual insolation changes driven by obliquity is also observed in the new wep sst record the wep sst record suggests that an 3 c equatorial pacific sst zonal gradient prevailed during the late pliocene compatible with a weaker walker circulation adjustment of hole 806b ssts for past changes in seawater mg ca suggests that ssts higher than 30 c prevailed at 3 myr b p followed by a progressive cooling of the warm pool through the late pliocene the characteristics of late pliocene tropical climate evolution suggest that atmospheric greenhouse gas forcing played a major role in driving the observed g i sst changes

calibration of mg ca thermometry in planktonic foraminifera from a sediment trap time series 1 paired mg ca and 18o measurements on planktonic foraminiferal species g ruber white g ruber pink g sacculifer g conglobatus g aequilateralis o universa n dutertrei p obliquiloculata g inflata g truncatulinoides g hirsuta and g crassaformis from a 6 year sediment trap time series in the sargasso sea were used to define the sensitivity of foraminiferal mg ca to calcification temperature habitat depths and calcification temperatures were estimated from comparison of 18o of foraminifera with equilibrium calcite based on historical temperature and salinity data when considered together mg ca mmol mol of all species except two show a significant r 0 93 relationship with temperature t c of the form mg ca 0 38 0 02 exp 0 090 0 003 t equivalent to a 9 0 0 3 change in mg ca for a 1 c change in temperature small differences exist in calibrations between species and between different size fractions of the same species o universa and g aequilateralis have higher mg ca than other species and in general data can be best described with the same temperature sensitivity for all species and pre exponential constants in the sequence o universa g aequilateralis g bulloides g ruber g sacculifer other species this approach gives an accuracy of 1 2 c in the estimation of calcification temperature the 9 sensitivity to temperature is similar to published studies from culture and core top calibrations but differences exist from some literature values of pre exponential constants different cleaning methodologies and artefacts of core top dissolution are probably implicated and perhaps environmental factors yet understood planktonic foraminiferal mg ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures using g ruber pink g ruber white g sacculifer n dutertrei p obliquiloculata etc the existence of a single mg thermometry equation is valuable for extinct species although use of species specific equations will where statistically significant provide more accurate evaluation of mg ca paleotemperature

scenarios regarding the lead of equatorial sea surface temperature over global ice volume 1 recent proxy evidences indicate that the equatorial sea surface temperature sst may have led global ice volume by 3 kyr during the late pleistocene glacial cycles given the short timescales of equatorial dynamics equatorial climate variability is characterized by a timescale of no more than a few years it would seem somewhat surprising therefore that the equatorial ocean and atmosphere can determine and lead the long timescale 100 kyr glacial cycles two scenarios are presented according to which such a lead may be observed even when the equatorial ocean and atmosphere are not necessarily responsible for leading the glacial cycles they may still act as a strong amplifier first it is shown that if the plankton based proxy reflects the warm season temperature rather than an annual temperature it may lead the global temperature although the dynamics of the glacial cycles may still be dominated by the northern hemisphere ice sheets it is noted that a present day seasonal bias of the equatorial proxy record is still inconclusive and the possibility of a proxy bias only during glacial times is considered as well a second scenario is suggested in which global sea level rises before equatorial sst yet the later evolution of factors such as the atmospheric co2 and equatorial sst is faster and takes the lead over global ice volume if the initial rise of sea level is masked by a sufficiently large proxy noise because of instrumental and natural noise it may not be seen and the lead may be attributed to the equatorial sst and co2

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

the mid pleistocene transition in the subtropical southwest pacific 1 reconstructions of subtropical southwest pacific climate variability over the pleistocene were derived from coupled planktic foraminiferal 18o mg ca measurements taken from a southern coral sea sediment core a clear shift from 40 kyr to 100 kyr modes of reconstructed glacial interglacial sea surface temperature sst variability is seen over the mid pleistocene transition and these fluctuations are shown to have remained coherent with the orbital obliquity cycle across the transition the likely origin of this strong obliquity signal in subtropical southwest pacific sst is shown to be the southern high latitudes and comparison with existing sst reconstructions from the equatorial pacific is consistent with the communication of the signal occurring principally by greenhouse gas forcing in contrast to the sst reconstruction regional hydrological cycle variability based on the calculated local component of 18osw change does not show significant coherence with obliquity after 1000 ka the decoupling of the sst and hydrological cycle responses over the mid pleistocene transition allows constraints to be placed on the evolution and extent of orbitally paced fluctuations within the coupled low latitude ocean atmosphere system

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

exploring the structure of the mid pleistocene revolution with advanced methods of time series analysis the mid pleistocene climate transition is a complex global change leading to late pleistocene ice ages with increased mean ice volume and dominant 100 ka cycle a thorough understanding of this transition demands quantitative investigations in the time and frequency domains and in the stochastic domain three methods of time series analysis are presented which have been adapted for this purpose they are tested by means of predefined artificial time series and applied to benthic oxygen isotope d18o records which serve as ice volume indicator results are as follows a the time dependent mean shows an increase of 0 35 vs pdb from 942 to 892 ka b evolutionary spectral analysis reveals an abrupt in crease of 100 ka cycle amplitude at approximately 650 ka c probability density function exhibits a bifur cation behavior at approximately 725 ka these find ings point to a multiple transition from a more linear climate system to a strong nonlinear system the signif icant lead of the transition in mean in relation to the 100 ka amplitude change and bifurcation is left open for explanation

upwelling intensification as part of the pliocene pleistocene climate transition a deep sea sediment core underlying the benguela upwelling system off southwest africa provides a continuous time series of sea surface temperature sst for the past 4 5 million years our results indicate that temperatures in the region have declined by about 10 c since 3 2 million years ago records of paleoproductivity suggest that this cooling was associated with an increase in wind driven upwelling tied to a shift from relatively stable global warmth during the mid pliocene to the high amplitude glacial interglacial cycles of the late quaternary these observations imply that atlantic ocean surface water circulation was radically different during the mid pliocene

a test of different factors influencing the isotopic signal of planktonic foraminifera in surface sediments from the northern south china sea abstract the stable isotope composition of planktonic foraminifera is one of the most important proxies in paleoenvironmental research in this study three parameters affecting the stable isotope values of globigerinoides ruber from surface sediment from the northern south china sea were tested different cleaning methods different morphotypes and different size fractions our results show that in the small size fraction there is a small but significant effect on 13c by oxidizing the tests prior to measurement our data also confirm a small but significant difference between different morphotypes of g ruber however the variability caused by the seasonal effect stable isotope value is larger than the effect caused by different cleaning protocols or different morphotypes the large spread of the isotope values up to 2 have some implications to paleoceanographic reconstructions when measurements are performed on a small number of foraminiferal tests the isotope value does not necessarily reflect yearly average or a certain season but is a random value of the seasonal variability in that region

a 5 million year comparison of mg ca and alkenone paleothermometers geochemical sea surface temperature sst proxies such as the magnesium to calcium ratio mg ca in foraminifera and the alkenone unsaturation index uk 37 are becoming widely used in pre pleistocene climate records this study quantitatively compares previously published mg ca and uk 37 data from ocean drilling program odp site 847 in the eastern equatorial pacific to assess the utility of these proxies to reconstruct tropical sst over the last 5 ma foraminiferal mg ca sst calibrations that include a dissolution correction are most appropriate at this location because they provide sst estimates for the youngest sample that are close to modern mean annual sst the long term trends in the two records are remarkably similar and confirm a 3 5 c cooling trend from the early pliocene warm period to the late pleistocene noted in previous work absolute temperature estimates are similar for both proxies when errors in the dissolution correction used to estimate sst from mg ca are taken into account comparing the two sst records at odp site 847 to other records in the region shows that the eastern equatorial pacific was 2 4 c warmer during the early pliocene compared to today

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

atmospheric carbon dioxide concentration across the mid pleistocene transition the dominant period of pleistocene glacial cycles changed during the mid pleistocene from 40 000 years to 100 000 years for as yet unknown reasons here we present a 2 1 million year record of sea surface partial pressure of co2 pco2 based on boron isotopes in planktic foraminifer shells which suggests that the atmospheric partial pressure of co2 pco2 was relatively stable before the mid pleistocene climate transition glacial pco2 was approximately 31 microatmospheres higher before the transition more than 1 million years ago but interglacial pco2 was similar to that of late pleistocene interglacial cycles 450 000 years ago these estimates are consistent with a close linkage between atmospheric co2 concentration and global climate but the lack of a gradual decrease in interglacial pco2 does not support the suggestion that a long term drawdown of atmospheric co2 was the main cause of the climate transition

high latitude influence on the eastern equatorial pacific climate in the early pleistocene epoch many records of tropical sea surface temperature and marine productivity exhibit cycles of 23 kyr orbital precession and 100 kyr during the past 0 5 myr refs 1 5 whereas high latitude sea surface temperature records display much more pronounced obliquity cycles at a period of about 41 kyr ref 6 little is known however about tropical climate variability before the mid pleistocene transition about 900 kyr ago which marks the change from a climate dominated by 41 kyr cycles7 when ice age cycles and high latitude sea surface temperature variations were dictated by changes in the earth s obliquity8 9 to the more recent 100 kyr cycles of ice ages here we analyse alkenones from marine sediments in the eastern equatorial pacific ocean to reconstruct sea surface temperatures and marine productivity over the past 1 8 myr we find that both records are dominated by the 41 kyr obliquity cycles between 1 8 and 1 2 myr ago with a relatively small contribution from orbital precession and that early pleistocene sea surface temperatures varied in the opposite sense to local annual insolation in the eastern equatorial pacific ocean we conclude that during the early pleistocene epoch climate variability at our study site must have been determined by high latitude processes that were driven by orbital obliquity forcing