co2 over the past 5 million years continuous simulation and new δ11b based proxy data during the past five million yrs benthic 18o records indicate a large range of climates from warmer than today during the pliocene warm period to considerably colder during glacials antarctic ice cores have revealed pleistocene glacial interglacial co2 variability of 60 100 ppm while sea level fluctuations of typically 125 m are documented by proxy data however in the pre ice core period co2 and sea level proxy data are scarce and there is disagreement between different proxies and different records of the same proxy this hampers comprehensive understanding of the long term relations between co2 sea level and climate here we drive a coupled climate ice sheet model over the past five million years inversely forced by a stacked benthic 18o record we obtain continuous simulations of benthic 18o sea level and co2 that are mutually consistent our model shows co2 concentrations of 300 to 470 ppm during the early pliocene furthermore we simulate strong co2 variability during the pliocene and early pleistocene these features are broadly supported by existing and new 11b based proxy co2 data but less by alkenone based records the simulated concentrations and variations therein are larger than expected from global mean temperature changes our findings thus suggest a smaller earth system sensitivity than previously thought this is explained by a more restricted role of land ice variability in the pliocene the largest uncertainty in our simulation arises from the mass balance formulation of east antarctica which governs the variability in sea level but only modestly affects the modeled co2 concentrations

orbital forcing of the east antarctic ice sheet during the pliocene and early pleistocene the pliocene and early pleistocene between 5 3 and 0 8 million years ago span a transition from a global climate state that was 2 3 c warmer than present with limited ice sheets in the northern hemisphere to one that was characterized by continental scale glaciations at both poles growth and decay of these ice sheets was paced by variations in the earth s orbit around the sun however the nature of the influence of orbital forcing on the ice sheets is unclear particularly in light of the absence of a strong 20 000 year precession signal in geologic records of global ice volume and sea level here we present a record of the rate of accumulation of iceberg rafted debris oshore from the east antarctic ice sheet adjacent to the wilkes subglacial basin between 4 3 and 2 2 million years ago we infer that maximum iceberg debris accumulation is associated with the enhanced calving of icebergs during ice sheet margin retreat in the warmer part of the record between 4 3 and 3 5 million years ago spectral analyses show a dominant periodicity of about 40 000 years subsequently the powers of the 100 000 year and 20 000 year signals strengthen we suggest that as the southern ocean cooled between 3 5 and 2 5 million years ago the development of a perennial sea ice field limited the oceanic forcing of the ice sheet after this threshold was crossed substantial retreat of the east antarctic ice sheet occurred only during austral summer insolation maxima as controlled by the precession cycle

concurrence of folding and remagnetization events in the monterrey salient ne mexico abstract carbonate formations from many locations reveal magnetizations that are secondary attaching a numerical age to these remagnetizations would provide information on orogenic processes to contribute to the effort of assigning an age to remagnetization events lower cretaceous formations from the monterrey salient in northeast mexico were sampled individual fold tests reveal eight site pair synfolding remagnetizations set a of eocene age the remanence is a chemical remanent magnetization crm carried by magnetite the magnetization of set b is possibly due to an early carbonate alteration that produced a remanence significantly older than folding a complexity of set b is that the site pairs reveal k versus unfolding percentage diagrams with maximum unfolding peaks at 100 to 125 an explanation of multiple deformation phases with an earlier horizontal folding phase around a non plunging horizontal axis and a later vertical axis rotation is suggested for the past 100 peaks of the b sites the latter deformation rotation about a vertical axes is revealed by a declination strike test in set a sites and site means of the full collection that deviate counterclockwise from the reference declination by combining synfolding results with published 40ar 39ar illite ages of folding fitz diaz et al 2016 remagnetization ages in set a sites of 48 52 ma are obtained the magnetization in set b sites is likely older though not well constrained interestingly prior results from the central sierra madre oriental to the south show two remagnetization events late cretaceous and early eocene in age in a succession from w to e nemkin et al 2015 the age of folding and remagnetization acquisition in set a sites along the monterrey salient are concurrent with the frontal remagnetized folds in the central sierra madre oriental thus the timing of major remagnetization in the monterrey salient occurred in the eocene

a new ice thickness and bed data set for the greenland ice sheet 1 measurement data reduction and errors ice thickness data collected between 1993 and 1999 using a coherent ice penetrating radar system developed at the university of kansas have been combined with data collected by the technical university of denmark in the 1970s to produce a new ice thickness grid for greenland crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5 km spacing grid using a kriging interpolation procedure a high resolution land ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where in the past the relative errors have been largest the ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of greenland the ice thickness grid was compared with the currently available data set differences in the center of the ice sheet where the ice is thickest were of the order of a few percent near the margins however large differences of as much as a factor of 10 were found the total volume of ice contained in the ice sheet was reestimated and found to have a value of 2 93 x 10 6 km 3 the ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow

stratigraphy and provenance of strata along the san marcos fault central coahuila mexico stratigraphic and petrologic studies supported by geologic mapping north of the san marcos fault central coahuila indicate that as much as 3 000 m of coarse upper jurassic and lower cretaceous detritus was derived from south of the fault two coarse jurassic submarine conglomerate units las palomas and sierra el granizo beds of the la casita formation reflect separate episodes of displacement younger jurassic eolianite and marine sandstone colorado and tanque cuatro palmas beds of the la casita formation record subsequent tithonian fault inactivity neocomian rejuvenation of the fault produced alluvial deposits san marcos formation finer grained paralic or marine strata that are distal and suprajacent and manifest the same faulting are the la mula formation interruption of neocomian displacement is indicated by a tongue of carbonate padilla and barril viejo formations that divides the san marcos into upper and lower members the principal source of this detritus was the wildflysch of the late paleozoic arcproximal las delicias basin and the plutonic rocks that cut it rb sr ages on clasts and from outcrops of granitic rocks are triassic and probably reflect late stage magmatism within the las delicias arc although this arc supplied most of the detritus to the las delicias basin devonian ages on some clasts reveal that an unknown source area lay somewhere nearby during the late part of the paleozoic era the san marcos fault may have served as part of the system of transform faults that connected sea floor spreading ridges of the atlantic with those of the pacific if so then the jurassic units record periods of jurassic activity and inactivity of that system neocomian displacment may have been caused by subsequent isostatic adjustment

assessing orbitally forced interglacial climate variability during the mid pliocene warm period abstract the traditional view of the pliocene is one of an epoch with higher than present global mean annual temperatures 2 to 3 c and stable climate conditions published data model comparisons for the mid pliocene warm period mpwp 3 3 to 3 million years ago have identified specific regions of concordance and discord between climate model outputs and marine terrestrial proxy data due to the time averaged nature of global palaeoenvironmental syntheses it has been hypothesised that climate variability during interglacial events within the mpwp could contribute to site specific data model disagreement the hadley centre coupled climate model version 3 hadcm3 is used to assess the nature of climate variability around two interglacial events within the mpwp that have different characteristics of orbital forcing marine isotope stages km5c and k1 model results indicate that 20 kyr on either side of the mis km5c orbital forcing produced a less than 1 c change in global mean annual temperatures regionally mean annual surface air temperature sat variability can reach 2 to 3 c seasonal variations exceed those predicted for the annual mean and can locally exceed 5 c simulations 20 kyr on either side of mis k1 show considerably increased variability in relation to km5c we demonstrate that orbitally forced changes in surface air temperature during interglacial events within the mpwp can be substantial and could therefore contribute to data model discord this is especially likely if proxies preserve growing season rather than mean annual temperatures model results indicate that peak mis km5c and k1 interglacial temperatures were not globally synchronous highlighting leads and lags in temperature in different regions this highlights the potential pitfalls in aligning peaks in proxy derived temperatures across geographically diverse data sites and indicates that a single climate model simulation for an interglacial event is inadequate to capture peak temperature change in all regions we conclude that the premise of sustained global warmth and stable pliocene climate conditions is incomplete we also contend that the likely nature of pliocene interglacial climate variability is more akin to interglacial events within the quaternary where the character of interglacials is known to be diverse in the future the utility of pliocene data model comparisons is dependent upon 1 establishing precise chronology of the proxy data 2 providing climate models with fully proxy consistent boundary conditions and 3 in utilising ensembles of climate simulations that can adequately capture orbital variability around any studied interval

magnetostratigraphy of mid cretaceous limestones from the sierra madre of northeastern mexico we present the results of a palaeomagnetic study of four mid cretaceous limestone sections exposed in northeastern mexico the limestones are weakly magnetized and exhibit two to three component magnetizations these magnetization components appear to be carried by both a sulphide mineral and a magnetite titanomagnetite mineral the sulphide mineral carries a reverse polarity overprint that often makes it difficult to isolate definitively the higher unblocking temperature component the high unblocking temperature component is well defined in the upper portion of the santa rosa canyon section and in the cienega del toro section and passes the fold test the characteristic remanent magnetization chrm inclinations agree well with predicted mid cretaceous inclinations for these sites although the declinations differ by more than 100 the relative rotation between these two sites probably occurred as the thrust sheets were emplaced during laramide deformation at two of the sections namely cienega del toro and the overturned los chorros sections only normal polarity directions are observed the la boca canyon and santa rosa canyon sections exhibit zones of both normal and reverse polarity magnetization correlation of these polarity zones with the geomagnetic polarity timescale provides a time framework for lithostratigraphic and palaeoceanographic studies of these sections

paleomagnetism of the cretaceous morelos and mezcala formations southern mexico a paleomagnetic study of platform facies carbonate rocks of the mid cretaceous morelos formation and deep water carbonate rocks of the overlying upper cretaceous mezcala formation sampled at zopilote canyon in guerrero state southern mexico indicates that their characteristic magnetization was acquired contemporaneously with folding of these rocks during the late cretaceous laramide orogeny the remanence carrier is interpreted to be magnetite although other mineral phases of high coercivity carry recent secondary overprints the overall mean is of dec 323 1j and inc 36 5j k 162 7 a95 2 7j n 18 sites 64 unfolding comparison with the north america reference direction indicates that this area has experienced a small yet statistically significant counterclockwise direction of 19 2f4 0j similar rotations are documented in other localities from southern mexico rotations are linked to mid tertiary deformation associated with the left lateral strike slip fault system that accommodated motion of the chortis and xolapa blocks d 2002 elsevier science b v all rights reserved

a paleomagnetic constraint on the late cretaceous paleoposition of northwestern baja california mexico we present palaeomagnetic data for cretaceous sedimentary rocks from the el rosario formation of northwestern baja california which imply a northward displacement of 15 0 3 8 and a 30 8 4 2 clockwise rotation for the baja peninsula relative to the north american craton since late cretaceous time paired irm acquisition x ray diffraction and mossbauer spectra indicate that fine grained magnetite is the dominant magnetic mineral present with goethite and other more oxidized phases rare to absent biochemical evidence constrains the amount of diagenesis undergone by the sediments we subjected oriented samples to both progressive alternating field and thermal demagnetization and in general found univectorial magnetizations yielding a mean inclination of 44 6 and declination of 5 3 with an 95 of 2 0 the corresponding paleolatitude for these values is 26 3 1 6 most of the samples were of normal polarity which is consistent with their deposition within the upper campanian chron 33n we collected samples from younger maastrichtian sediments which yield consistent reversed directions

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

mid cretaceous paleomagnetic reference field for north america restudy of 100 ma intrusive rocks from arkansas we have obtained new paleomagnetic data from three middle cretaceous alkalic intrusive complexes in central arkansas magnet cove and potash sulfur springs intrusions about 100 ma 20 sites and granite mountain stock about 88 ma 6 sites out of 254 specimens measured 147 from 20 sites were accepted the remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region the geologic evidence indicates that the intrusions have not been tilted since emplacement the directions of magnetization calculated with respect to present day horizontal yield a mean paleopole at 192 5 e 74 1 n n 20 k 34 a95 5 7 this does not differ at 95 confidence from that determined previously by scharon and hsu but the error is diminished by half the angular dispersion sd of site virtual geomagnetic poles vgps is 13 6 similar to the value of sd about 15 predicted by current paleosecular variation models the revised arkansas paleopole is combined with four other cratonic north american paleopoles of mid cretaceous age to yield a revised middle cretaceous reference pole at 196 e 71 n a95 4 9 the mean of nine paleopoles derived from overprints in rock units from the craton and craton margin which range in age from proterozoic to jura cretaceous is 202 e 68 n a95 8 2 this is not different from the reference paleopole indicating that overprinting was very widespread across north america in mid cretaceous time

palaeolatitudes from dsdp leg 73 sediment cores implications for the apparent polar wander path for africa during the late mesozoic and cenozoic summary palaeolatitudes estimated from dsdp sediments provide important constraints on the apparent polar wander path apwp of africa during the cenozoic a revised apwp is suggested based on new information about palaeomagnetic poles from the african continent and predicted palaeolatitudes are compared with those determined from leg 73 sediments other published paths are discussed

tectonic transpression in the sierra madre oriental northeastern mexico an alternative model visual analysis of the shuttle imaging radar sir a of northeastern mexico reveals the existence of several geologic features including 1 a well developed pattern of en echelon folds 2 juxtaposition of tectostratigraphic domains 3 fold structures varying from fan shaped asymmetric to recumbent doubly plunging anticlines 4 anticlinal synclinal trends associated with regional plunging and tilting of the structures and 5 strike slip faults oblique to the fold trends these structures are interpreted as the result of late tertiary transpressional stress related to a complex anastomosed wrench faulting system in the basement reactivated several times since the late jurassic this transpressional orogenic belt in northeastern mexico is linked to the spreading phase in the gulf of mexico

enhancement factors for grounded ice and ice shelves inferred from an anisotropic ice flow model polar ice is known to be one of the most anisotropic natural materials for a given fabric the polycrystal viscous response is strongly dependent on the actual state of stress and strain rate within an ice sheet grounded ice parts and ice shelves have completely different stress regimes so one should expect completely different impacts of ice anisotropy on the flow the aim of this work is to quantify through the concept of enhancement factors the influence of ice anisotropy on the flow of grounded ice and ice shelves for this purpose a full stokes anisotropic marine ice sheet flowline model is used to compare isotropic and anisotropic diagnostic velocity fields on a fixed geometry from these full stokes results we propose a definition of enhancement factors for grounded ice and ice shelves coherent with the asymptotic models used for these regions we then estimate realistic values for the enhancement factors induced by ice anisotropy for grounded ice and ice shelves

shallowing upward sequences in purbeckian peritidal carbonates lowermost cretaceous swiss and french jura mountains purbeckian carbonates in the swiss and french jura goldberg formation lower berriasian comprise shallow subtidal intertidal supratidal low energy high energy marine brackish freshwater and hypersaline facies these facies are arranged in small 0 2 1 5 m thick sequences which display a dominant shallowing upward component and which form the fundamental units of the highly structured purbeckian sedimentary record six types of small scale sequences can be recognized a intertidal to supratidal overprinting of shallow lagoonal facies b algal marsh sequence with frequent dolomitization c sabkha sequence often associated with collapse breccia d tidal flat sequence with desiccation features e lacustrine sequence f terrestrial overprinting of subtidal or intertidal facies episodic event deposits such as tempestites are superimposed thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences green marls and black pebbles are common at the top and indicate long subaerial exposure the sequences are often incomplete as subtidal facies may be absent or their upper part can be eroded lateral facies changes are common which is due to the very shallow and partly emergent purbeckian platform where various depositional environments were juxtaposed however many sequence boundaries are well developed and can be correlated over large parts of the study area the purbeckian shallowing upward sequences were generated by climatically controlled sea level changes autocyclic processes occurred locally but were overprinted by drops of sea level affecting the entire platform the small scale sequences are most probably related to the 20 000 year cycle of the precession of the equinoxes larger sequences with usually well developed emersion surfaces are attributed to the 100 000 and 400 000 year eccentricity cycles of the earth s orbit identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces which often cut across facies boundaries and thus to interpret in detail the palaeogeographic sedimentological and diagenetic evolution of the purbeckian peritidal carbonate environments

palaeomagnetic palaeolatitudes of the ontong java plateau from 120 to 55 ma implications for the apparent polar wander path of the pacific plate we present palaeomagnetic inclinations from late cretaceous early tertiary deep water carbonates obtained during odp leg 192 drilling at the ontong java plateau ojp for the following periods magnetochrons c33n 73 6 79 1 ma c32 71 1 73 6 ma c27r c31n 61 3 68 7 ma and c25 c26 55 9 60 9 ma compaction induced inclination shallowing is considered to be negligible for the ojp sediments examined here because i their palaeomagnetic inclinations are in excellent accord with those recently obtained from underlying ojp basement rocks ii the studied sediments have anisotropy of anhysteretic remanent magnetization fabrics that appear poorly correlated with individual characteristic remanent magnetization inclinations and iii in the few cases where we observe a significant difference between our new ojp sedimentary palaeomagnetic data and data from other parts of the pacific plate obtained mainly from seamount magnetization studies and skewness analyses the sedimentary inclinations are not systematically lower and therefore cannot be explained in terms of inclination shallowing combining our new data with existing ojp palaeomagnetic data we obtain an internally consistent data set that we interpret to indicate northward motion of the plateau from its formation at similar to 120 ma until 55 ma at 120 ma the central plateau was located at 24 degrees 4 degrees s approximately 45 myr later similar to 76 ma the plateau is located near 21 degrees s our data therefore indicate that the ojp was either stationary or exhibited a slow northward latitudinal drift during this interval in contrast from similar to 76 ma until 68 ma the plateau moved rapidly through approximately 10 degrees of latitude to similar to 10 degrees s these intervals of slow and rapid motion track similar motions previously proposed for the entire pacific plate during the cretaceous but extends the interval of slow motion into the late cretaceous from 68 ma until 56 ma the plateau was again almost latitudinally stationary before moving slowly northward more direct palaeomagnetic data are necessary to better define pacific plate motions during this time interval

a paleomagnetic study of the lower cretaceous cupido limestone northeast mexico evidence for local rotation within the sierra madre oriental we collected 401 oriented samples from the lower cretaceous cupido limestone in northeast mexico at an average stratigraphic spacing of 2 m analysis of the remanent magnetization reveals an unstable component that is removed upon heating to 300 c and a component that remains stable to 550 c thermal demagnetization and isothermal remanent magnetization irm studies indicate that the carrier of this stable remanence is magnetite the magnetite is believed to be detrital and therefore responsible for a remanence associated with the time of deposition with the exception of a small number of spurious samples all samples are normally magnetized this is in contrast to studies elsewhere in which as many as three reversed intervals have been identified during the time of cupido deposition the pole position obtained 31 0 n 169 8 w 95 3 1 k 6 1 n 401 lies some 35 to 40 south of other post jurassic mexico pole positions suggesting that the sampling site has rotated counterclockwise by a similar amount relative to stable mexico the sampling site is located in a northeast trending portion of the sierra madre oriental that offsets the main southeast trending ranges by 50 km the northeast trend is interpreted as the result of a wrapping of the folds around the southeast corner of the old coahuila peninsula which acted as a barrier to laramide decollement deformation the counterclockwise rotation suggested by the paleomagnetic results therefore is thought to be due to local rotation of the folds and not related to major plate motions between north america and mexico

paleomagnetic evidence for tectonic rotation of northern mexico and the continuity of the cordilleran orogenic belt between nevada and chihuahua results of paleomagnetic studies of volcanic and sedimentary rocks from chihuahua northern mexico suggest that a counterclock wise tectonic rotation relative to stable north america occurred during early tertiary time the results permit development of a model for the tectonic evolution of southern north america in which northern mexico south of the texas lineament is rotated in order to restore it to its eocene oligocene position this explains the anomalous zone of the cordilleran orogenic belt between southern nevada and northern chihuahua which suggests that the belt was once continuous and that the complications are the result of later tectonic events the restoration may also explain the dislocation of the mesozoic batholith belt of western mexico and the large nappe structures of the sierra madre oriental within the continuation of the belt into mexico

a continuous simulation of global ice volume over the past 1 million years with 3 d ice sheet models sea level records show large glacial interglacial changes over the past million years which on these time scales are related to changes of ice volume on land during the pleistocene sea level changes induced by ice volume are largely caused by the waxing and waning of the large ice sheets in the northern hemisphere however the individual contributions of ice in the northern and southern hemisphere are poorly constrained in this study for the first time a fully coupled system of four 3 d ice sheet models is used simulating glaciations on eurasia north america greenland and antarctica the ice sheet models use a combination of the shallow ice and shelf approximations to determine sheet shelf and sliding velocities the framework consists of an inverse forward modelling approach to derive a self consistent record of temperature and ice volume from deep sea benthic 18o data over the past 1 million years a proxy for ice volume and temperature it is shown that for both eustatic sea level and sea water 18o changes the eurasian and north american ice sheets are responsible for the largest part of the variability the combined contribution of the antarctic and greenland ice sheets is about 10 for sea level and about 20 for sea water 18o during glacial maxima however changes in interglacials are mainly caused by melt of the greenland and antarctic ice sheets with an average time lag of 4 kyr between melt and temperature furthermore we have tested the separate response to changes in temperature and sea level for each ice sheet indicating that ice volume can be significantly influenced by changes in eustatic sea level alone hence showing the importance of a simultaneous simulation of all four ice sheets this paper describes the first complete simulation of global ice volume variations over the late pleistocene with the possibility to model changes above and below present day ice volume constrained by observations of benthic 18o proxy data

synfolding magnetization in the jurassic preuss sandstone wyoming idaho utah thrust belt the jurassic preuss sandstone exposed in five thrust plates of the wyoming idaho utah thrust belt carries directions of remanent magnetization that group most tightly after only partial unfolding field petrographic and rock magnetic evidence indicates that the carrier of this magnetization is detrital low ti titanomagnetite the detrital titanomagnetite was remagnetized at low temperatures 75 150 c probably completely during folding anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain we suggest that low temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization the acquisition of which was enhanced by stress or a piezoremanent magnetization that involved stress induced movement of domain walls during intracrystalline strain or was a combination of the two mechanisms stress may promote remagnetization at temperatures much lower than predicted by current theoretical models other mechanisms such as acquisition of chemical remanent magnetization during folding deflection of a prefolding magnetization by internal strain or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the preuss the locus of acquisition of synfolding magnetization in the preuss migrated in conjunction with deformation in the thrust belt a model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps a lack of reverse polarity directions remains a puzzling feature of the remanence the remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in late cretaceous and early tertiary periods of predominantly normal polarity

plio pleistocene ice volume antarctic climate and the global δ18o record we propose that from 3 to 1 million years ago ice volume changes occurred in both the northern and southern hemispheres each controlled by local summer insolation because earth9s orbital precession is out of phase between hemispheres 23 000 year changes in ice volume in each hemisphere cancel out in globally integrated proxies such as ocean 18 o or sea level leaving the in phase obliquity 41 000 years component of insolation to dominate those records only a modest ice mass change in antarctica is required to effectively cancel out a much larger northern ice volume signal at the mid pleistocene transition we propose that marine based ice sheet margins replaced terrestrial ice margins around the perimeter of east antarctica resulting in a shift to in phase behavior of northern and southern ice sheets as well as the strengthening of 23 000 year cyclicity in the marine 18 o record

paleomagnetic results from northeastern mexico evidence for large mesozoic rotations preliminary paleomagnetic evidence indicates that a significant part of mexico rotated 130 counterclockwise during early mesozoic time coeval with the opening of the gulf of mexico and that a major structural discontinuity must exist between mexico and cratonic north america measurements made on samples from fourteen isolated outcrops of the huizachal group a red bed sequence exposed in northeastern mexico indicate that these sediments were deposited in rifted basins and could range in age from post middle permian to pre early cretaceous of samples analyzed from the overlying zuloaga limestone oxfordian and the cuesta del cura formation aptian albian the oldest yield a paleomagnetic pole position that implies that the sampling area was originally oriented about 130 clockwise relative to stable north america progressive counterclockwise rotation continued to middle jurassic time because of the limited geographical distribution of sampling sites it is not possible to assess whether most of mexico shared this rotation or whether the proposed left lateral motion along major west northwest trending faults caused the rotation of a smaller crustal block in either case our results strongly suggest a major structural discontinuity in northern mexico

high resolution alkenone palaeobarometry indicates relatively stable pco2 during the pliocene 3 3 to 2 8 ma temperature reconstructions indicate that the pliocene was approximately 3 c warmer globally than today and several recent reconstructions of pliocene atmospheric co2 indicate that it was above pre industrial levels and similar to those likely to be seen this century however many of these reconstructions have been of relatively low temporal resolution meaning that these records may have failed to capture variations associated with the 41 kyr glacial interglacial cycles thought to have operated in the pliocene here we present a new high temporal resolution alkenone carbon isotope based record of pco2 spanning 3 3 2 8 ma from ocean drilling program site 999 our record is of high enough resolution approx 19 kyr to resolve glacial interglacial changes beyond the intrinsic uncertainty of the proxy method the record suggests that pliocene co2 levels were relatively stable exhibiting variation less than 55 ppm we perform sensitivity studies to investigate the possible effect of changing sea surface temperature sst which highlights the importance of accurate and precise sst reconstructions for alkenone palaeobarometry but demonstrate that these uncertainties do not affect our conclusions of relatively stable pco2 levels during this interval

the transient response of ice volume to orbital forcing during the warm late pliocene examining the nature of ice sheet and sea level response to past episodes of enhanced greenhouse gas forcing may help constrain future sea level change here for the first time we present the transient nature of ice sheets and sea level during the late pliocene the transient ice sheet predictions are forced by multiple climate snapshots derived from a climate model set up with late pliocene boundary conditions forced with different orbital forcing scenarios appropriate to two marine isotope stages miss mis km5c and k1 our results indicate that during mis km5c both the antarctic and greenland ice sheets contributed to sea level rise relative to present and were relatively stable insolation forcing between the hemispheres was out of phase during mis k1 and led to an asynchronous response of ice volume globally therefore when variations of precession were high inferring the behavior of ice sheets from benthic isotope or sea level records is complex

high tide of the warm pliocene implications of global sea level for antarctic deglaciation we obtained global sea level eustatic estimates with a peak of 22 m higher than present for the pliocene interval 2 7 3 2 ma from backstripping in virginia united states new zealand and enewetak atoll north pacifi c ocean benthic foraminiferal 18 o values and mg ca 18 o estimates statistical analysis indicates that it is likely 68 confi dence interval that peak sea level was 22 5 m higher than modern and extremely likely 95 that it was 22 10 m higher than modern benthic foraminiferal 18 o values appear to require that the peak was 20 21 m our estimates imply loss of the equivalent of the greenland and west antarctic ice sheets and some volume loss from the east antarctic ice sheet and address the longstanding controversy concerning the pliocene stability of the east antarctic ice sheet

modelling west antarctic ice sheet growth and collapse through the past five million years changes in earth s orbit are known to influence climate shifts from cold glacials to warm interglacials how the vast west antarctic ice sheet responds to these fluctuations is uncertain but because its collapse could raise sea levels by about 5 metres of great interest naish et al have analysed the and 1b ocean sediment core extracted from beneath the ross ice shelf as part of the andrill drilling project and find evidence that the ice sheet collapsed periodically during the early pliocene 3 5 million years ago when atmospheric co2 levels were similar to or slightly higher than today s the pattern of collapse suggests an influence of approximately 40 000 year cycles in the tilt of earth s rotational axis obliquity also in this issue of nature in a numerical modelling study focused on the past 5 million years in antarctica david pollard and robert deconto combine ice sheet land supported and ice shelf water supported modelling approaches to simulate the movement of the grounding line the border between land and sea ice their results show that over the past 5 million years the west antarctic ice sheet transitioned between full intermediate and collapsed states in just a few thousand years this means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 c if the west antarctic ice sheet wais melted sea levels would rise by about 5 m such changes are thought to have occurred in the past but could not be simulated by models pollard and deconto combine ice sheet with ice shelf modelling and show that over the past 5 million years the wais transitioned among full intermediate and collapsed states in only a few thousand years suggesting possible disintegration of the wais if ocean temperatures in the area rise by 5 c the west antarctic ice sheet wais with ice volume equivalent to 5 m of sea level1 has long been considered capable of past and future catastrophic collapse2 3 4 today the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams grounding lines are several hundred metres below sea level and the bed deepens upstream raising the prospect of runaway retreat3 5 projections of future wais behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms6 7 its variation since the last glacial maximum is best known with grounding lines advancing to the continental shelf edges around 15 kyr ago before retreating to near modern locations by 3 kyr ago8 prior collapses during the warmth of the early pliocene epoch9 and some pleistocene interglacials have been suggested indirectly from records of sea level and deep sea core isotopes and by the discovery of open ocean diatoms in subglacial sediments10 until now11 however little direct evidence of such behaviour has been available here we use a combined ice sheet ice shelf model12 capable of high resolution nesting with a new treatment of grounding line dynamics and ice shelf buttressing5 to simulate antarctic ice sheet variations over the past five million years modelled wais variations range from full glacial extents with grounding lines near the continental shelf break intermediate states similar to modern and brief but dramatic retreats leaving only small isolated ice caps on west antarctic islands transitions between glacial intermediate and collapsed states are relatively rapid taking one to several thousand years our simulation is in good agreement with a new sediment record andrill and 1b recovered from the western ross sea11 indicating a long term trend from more frequently collapsed to more glaciated states dominant 40 kyr cyclicity in the pliocene and major retreats at marine isotope stage 31 1 07 myr ago and other super interglacials

the evolution of middle america and the gulf of mexico caribbean sea region during mesozoic time a plate tectonic model for the evolution of middle america and the gulf of mexico caribbean sea region is presented the model which is based upon the existence of the mojave sonora megashear incorporates into the triassic pangea reconstruction three microplates between north and south america thus avoiding the overlap of the bullard fit these plates are the yaqui bounded on the north by the mojave sonora megashear the east and west maya plates bounded on the north by the mexican volcanic zone and on the south by a predecessor of the motagua fault zone and the chortis plate parts of guatemala and honduras during late jurassic time as north america split away from europe africa and south america shear with left lateral sense of displacement occurred along the transform faults that bounded the micro plates if 800 km of left lateral displacement along the mojave sonora megashear 300 km along the mexican volcanic belt and 1 300 km along a proto motagua megashear are restored and if yucatan and cuba are rotated to fit against northern south america then 1 a curvilinear belt of late paleozoic rocks that show lithologic as well as paleontologic similarities extends across the reconstruction and links outcrops in texas eastern mexico nuclear central america and colombia 2 a mediterranean like sea is delineated that was a precursor of most of the present gulf of mexico 3 correlation is implied between the distinctive quartzose san cayetano formation of cuba and the caracas and juan griego groups of venezuela geometric constraints suggest that probably shear initially occurred along the mexican volcanic zone near the end of the middle jurassic subsequently probably about 160 m y ago displacements that total 800 km began along the mojave sonora megashear contemporaneously yucatan and fragments of pre cretaceous rocks that compose parts of central and western cuba migrated northward toward their present positions rotation of yucatan was facilitated by considerable displacement along the proto motagua zone and along a zone that is probably coincident with the modern salina cruz fault accumulation of widespread major salt units of late jurassic callovian to early oxfordian age in the gulf basin probably occurred contemporaneously with the arrival of these blocks at their present positions clastic units that interfinger with some of the youngest salt units and rim the gulf of mexico have not recorded major recognized translations since their accumulation clockwise rotation of south america and the chortis plate occurred during early cretaceous time this movement which was manifested by subduction of jurassic ocean floor against the previously rifted precursor of the island of cuba and under parts of hispaniola and puerto rico is recorded by circum caribbean orogeny abrupt changes in the relative motions between north and south america during late cretaceous time may have resulted in extension and outpourings of basalt upon the jurassic rocks of the ocean floor of the venezuelan basin west of beata ridge sea floor spreading formed the colombian basin related subduction occurred as the chortis plate including part of central america the nicaraguan rise and southeastern cuba was sutured against the maya east plate along the present motagua fault and cayman trench our model is constrained by published geologic data the relative positions of north and south america from atlantic sea floor magnetic anomalies and the requirement that the major transform faults be compatible with the poles of rotation for the appropriate relative motions between north and south america paleomagnetic data from middle america are sparse but do not conflict with the predicted motions of some of the microplates especially chortis

the last glacial cycle transient simulations with an aogcm a number of transient climate runs simulating the last 120 kyr have been carried out using famous a fast atmosphere ocean general circulation model aogcm this is the first time such experiments have been done with a full aogcm providing a three dimensional simulation of both atmosphere and ocean over this period our simulation thus includes internally generated temporal variability over periods from days to millennia and physical detailed representations of important processes such as clouds and precipitation although the model is fast computational restrictions mean that the rate of change of the forcings has been increased by a factor of 10 making each experiment 12 kyr long atmospheric greenhouse gases ghgs northern hemisphere ice sheets and variations in solar radiation arising from changes in the earth s orbit are treated as forcing factors and are applied either separately or combined in different experiments the long term temperature changes on antarctica match well with reconstructions derived from ice core data as does variability on timescales longer than 10 kyr last glacial maximum lgm cooling on greenland is reasonably well simulated although our simulations which lack ice sheet meltwater forcing do not reproduce the abrupt millennial scale climate shifts seen in northern hemisphere climate proxies or their slower southern hemisphere counterparts the spatial pattern of sea surface cooling at the lgm matches proxy reconstructions reasonably well there is significant anti correlated variability in the strengths of the atlantic meridional overturning circulation amoc and the antarctic circumpolar current acc on timescales greater than 10 kyr in our experiments we find that ghg forcing weakens the amoc and strengthens the acc whilst the presence of northern hemisphere ice sheets strengthens the amoc and weakens the acc the structure of the amoc at the lgm is found to be sensitive to the details of the ice sheet reconstruction used the precessional component of the orbital forcing induces 20 kyr oscillations in the amoc and acc whose amplitude is mediated by changes in the eccentricity of the earth s orbit these forcing influences combine to first order in a linear fashion to produce the mean climate and ocean variability seen in the run with all forcings

the amplitude and origin of sea level variability during the pliocene epoch earth is heading towards a climate that last existed more than three million years ago ma during the mid pliocene warm period 1 when atmospheric carbon dioxide concentrations were about 400 parts per million global sea level oscillated in response to orbital forcing2 3 and peak global mean sea level gmsl may have reached about 20 metres above the present day value4 5 for sea level rise of this magnitude extensive retreat or collapse of the greenland west antarctic and marine based sectors of the east antarctic ice sheets is required yet the relative amplitude of sea level variations within glacial interglacial cycles remains poorly constrained to address this we calibrate a theoretical relationship between modern sediment transport by waves and water depth and then apply the technique to grain size in a continuous 800 metre thick pliocene sequence of shallow marine sediments from whanganui basin new zealand water depth variations obtained in this way after corrections for tectonic subsidence yield cyclic relative sea level rsl variations here we show that sea level varied on average by 13 5 metres over glacial interglacial cycles during the middle to late pliocene about 3 3 2 5 ma the resulting record is independent of the global ice volume proxy3 as derived from the deep ocean oxygen isotope record and sea level cycles are in phase with 20 thousand year kyr periodic changes in insolation over antarctica paced by eccentricity modulated orbital precession6 between 3 3 and 2 7 ma thereafter sea level fluctuations are paced by the 41 kyr period of cycles in earth s axial tilt as ice sheets stabilize on antarctica and intensify in the northern hemisphere3 6 strictly we provide the amplitude of rsl change rather than absolute gmsl change however simulations of rsl change based on glacio isostatic adjustment show that our record approximates eustatic sea level defined here as gmsl unregistered to the centre of the earth nonetheless under conservative assumptions our estimates limit maximum pliocene sea level rise to less than 25 metres and provide new constraints on polar ice volume variability under the climate conditions predicted for this century sea level varied by 13 5 metres on average but up to 25 metres over glacial interglacial cycles during the pliocene epoch due to partial collapses of antarctic ice sheets

facies sequences on a carbonate ramp the carboniferous limestone of south wales during early carboniferous times a major sea level rise led to the development of an extensive carbonate ramp over what is now south wales differential subsidence and sea level changes resulted in distinctive facies sequences in the ramp succession and a model is offered which recognizes three distinct geomorpho tectonic settings inner mid and outer ramp the inner ramp zone occurs in the more landward part of the province and was an area undergoing little or no subsidence the sequence is dominated by oolitic grainstones and peritidal limestones representing shoal and back shoal environments the peritidal units are transgressive deposits consisting of stacked asymmetrical shallowing up cycles the sequence contains many subaerial breaks and tectonic uplift resulted in base level changes and fluvial incision the mid ramp zone sequence is intermediate in thickness between the inner and outer ramp successions and consists mainly of bioclastic limestones deposited below fairweather wave base sedimentation periodically exceeded sea level rise and subsidence and regressive progradational oolitic sand bodies developed the thickest of which are stacked units with up to four individual sand bodies storm processes were of major importance in this setting the outer ramp zone is represented by a thick sequence of muddy bioclastic limestones deposited below storm wave base and major waulsortian reef mounds also developed none of the shallowing phases seen in the other ramp zones can be detected in this sequence subsidence and eustatic sea level rise seem to have been the major controls on deposition but the recognition of eustatic sea level falls is difficult the detailed facies model for ramp carbonates presented here may be applicable elsewhere in the geological record

palaeomagnetic evidence for local and regional post eocene rotations in northern mexico summary 122 oriented core samples were collected from folded lower cretaceous units at four localities in the sierra madre oriental mountains near torreon northeast mexico palaeomagnetic analyses of these samples identify a stable synfolding remanence at three localities that was acquired during the laramide orogeny eocene the fourth locality exhibited a post folding or late synfolding remanence each locality has undergone a regional rotation and at least three localities have undergone local rotations since remanence acquisition by examining the relationships between the apparent polar wander paths for north america and mexico pole positions generated by this study and observed structural trends it has been possible to evaluate the different contributions of local versus regional rotations and to calculate a least locally rotated eocene pole position for the torreon region this pole lat 69 9 n long 172 8 e n 27 k 30 ag5 5 2 agrees well with a published mexican eocene pole position for a locality located 450 km northnorthwest of the present study area on the basis of this information we conclude that a large fragment of northern mexico has undergone a 10 15 post eocene counterclockwise rotation with respect to the rest of north america in agreement with the model proposed by urrutia fucugauchi 1981

counterclockwise rotations in the late eocene oligocene volcanic fields of san luis potosi and sierra de guanajuato eastern mesa central mexico abstract we used paleomagnetic and structural data to investigate the late eocene oligocene tectonic evolution of the mesa central area in mexico the mesa central was affected by nw trending faults tepehuanes san luis fault system coeval with a late eocene oligocene ignimbrite flare up and by post 27 ma nne trending grabens related to the basin and range we obtained reliable paleomagnetic directions from 61 sites within the late eocene oligocene volcanic series 30 to 27 ma of the san luis potosi volcanic field and sierra de guanajuato for each site we also measured the anisotropy of magnetic susceptibility ams tilt corrections were made using ams data for 33 sites where in situ bedding measurements were not available paleomagnetic directions indicate counterclockwise rotations of about 10 with respect to stable north america after 30 25 ma structural data suggest that the volcanic succession was mainly affected by normal faults however we also found evidences for oblique or horizontal striae showing a left lateral component along nw trending faults and a right lateral component along ne trending faults both motions are consistent with a n s extension oblique to the tepehuanes san luis fault system previous paleomagnetic studies in northern and southern mexico show the prevalence of minor left lateral shear components along regional scale transpressional and transtensional lineaments our paleomagnetic data may reflect thus small vertical axis rotations related to a minor shear component coeval with the oligocene intra arc extension in central mexico

plio pleistocene climate sensitivity evaluated using high resolution co2 records theory and climate modelling suggest that the sensitivity of earth s climate to changes in radiative forcing could depend on the background climate however palaeoclimate data have thus far been insufficient to provide a conclusive test of this prediction here we present atmospheric carbon dioxide co2 reconstructions based on multi site boron isotope records from the late pliocene epoch 3 3 to 2 3 million years ago we find that earth s climate sensitivity to co2 based radiative forcing earth system sensitivity was half as strong during the warm pliocene as during the cold late pleistocene epoch 0 8 to 0 01 million years ago we attribute this difference to the radiative impacts of continental ice volume changes the ice albedo feedback during the late pleistocene because equilibrium climate sensitivity is identical for the two intervals when we account for such impacts using sea level reconstructions we conclude that on a global scale no unexpected climate feedbacks operated during the warm pliocene and that predictions of equilibrium climate sensitivity excluding long term ice albedo feedbacks for our pliocene like future with co2 levels up to maximum pliocene levels of 450 parts per million are well described by the currently accepted range of an increase of 1 5 k to 4 5 k per doubling of co2

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