variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

dynamical excitation of the tropical pacific ocean and enso variability by little ice age cooling tropical pacific ocean dynamics during the medieval climate anomaly mca and the little ice age lia are poorly characterized due to a lack of evidence from the eastern equatorial pacific we reconstructed sea surface temperature el nino southern oscillation enso activity and the tropical pacific zonal gradient for the past millennium from galapagos ocean sediments we document a mid millennium shift mms in ocean atmosphere circulation around 1500 1650 ce from a state with dampened enso and strong zonal gradient to one with amplified enso and weak gradient the mms coincided with the deepest lia cooling and was probably caused by a southward shift of the intertropical convergence zone the peak of the mca 900 1150 ce was a warm period in the eastern pacific contradicting the paradigm of a persistent la nina pattern

the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols the depth to carbonate nodular bk horizon in soils d in cm is correlated with mean annual precipitation p in mm so that bk horizons are deep in subhumid regions but shallow in semiarid regions previous quantifications of this relationship are unchanged by this new compilation of 807 soils p 137 24 6 45 d 0 013 d 2 where r 2 0 52 and standard error s e 147 mm in most north american postglacial soils the bk horizon is thin and well defined whereas in monsoonal tropical soils of pakistan and kenya the bk horizon is thick and diffuse data from 675 modern soils define the relationship between the thickness of soil with nodules t in cm and mean annual range of precipitation m in mm difference between monthly means of wettest and driest months m 0 79 t 13 71 where r 2 0 58 and s e 22 mm the relationship between carbonate nodule size s in cm and soil age a in ka is quantified by 9 radiocarbon dated soils from las cruces new mexico a 3 92 s 0 34 where r 2 0 57 and s e 1 8 k y these transfer functions are applied to reconstructing paleoclimatic change from paleocene eocene paleosols of the north horn formation and flagstaff limestone of axhandle canyon utah the terminal paleocene spike of warmth recorded by fossil plants in nearby wyoming was coincident with a brief peak in both mean annual precipitation and mean annual range of precipitation in utah

high resolution variability of the south american summer monsoon over the last seven millennia insights from a speleothem record from the central peruvian andes stable oxygen isotope d 18 o measurements of two speleothems collected from huagapo cave in the central peruvian andes and with overlapping age from 1 1 to 1 4 ka characterize tropical south american climate variability over the last 7150 years in the study region precipitation d 18 o d 18 op is inversely correlated to rainfall amount upstream in the amazon basin and the intensity of convection associated with the south american summer monsoon sasm speleothem long axis profiles yield an average age resolution of five years and permit investigation of climate over orbital to decadal timescales variations in the isotopic composition of huagapo cave calcite d 18 oc are in good agreement with several pre

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

late holocene environmental changes as recorded in the sediments of high andean laguna chepical central chile 32 s 3050 m a s l abstract we present a reconstruction of environmental changes from sediments of high altitude laguna chepical in the subtropical andes of central chile 32 16 s 70 30 w 3050 m a s l for the past 3100 years based on subfossil pollen microscopic charcoal and diatoms we inferred changes in moisture related to precipitation and ice cover ice free season related to summer temperature at decadal to millennial scales sustained wetter and colder summer temperatures than today prevailed between 1100 bc and ca ad 1 afterward decreasing pollen accumulation rates and increased fire activity suggest drier conditions and possibly enhanced seasonality and or inter annual climate variability frequent changes between cold and warm summers were observed particularly for the last 1000 years about ad 1250 during the medieval climate anomaly wet years and early break up of ice cover occurred in central chile which is today typical for el nino like mean conditions conversely and with the exception of a few wet pulses a generally dry period with extended ice cover cool summers was observed between ad 1400 and ad 1850 little ice age this can be interpreted as a trend toward more la nina like mean conditions recent climate change and human disturbances during the last 100 years have prompted changes in diatom and plant communities that are unprecedented in the late holocene first planktonic diatoms increased as a result of hydraulic interventions in the lake during the late 19th century and secondly the andean vegetation shifted upward as result of recent warming and the frequency of arboreal taxa was significantly reduced at the same time peaks of fire activity were observed

phanerozoic atmospheric co2 change evaluating geochemical and paleobiological approaches abstract the theory and use of geochemical modeling of the long term carbon cycle and four paleo pco 2 proxies are reviewed and discussed in order to discern the best applications for each method geochemical models provide pco 2 predictions for the entire phanerozoic but most existing models present 5 10 m y means and so often do not resolve short term excursions error estimates based on sensitivity analyses range from 75 200 ppmv for the tertiary to as much as 3000 ppmv during the early paleozoic the 13 c of pedogenic carbonates provide the best proxy based pco 2 estimates for the pre tertiary with error estimates ranging from 500 1000 ppmv pre devonian estimates should be treated cautiously error estimates for tertiary reconstructions via this proxy are higher than other proxies and models 400 500 ppmv and should not be solely relied upon we also show the importance of measuring the 13 c of coexisting organic matter instead of inferring its value from marine carbonates the 13 c of the organic remains of phytoplankton from sediment cores provide high temporal resolution up to 10 3 10 4 year high precision 25 100 ppmv for the tertiary to 150 200 ppmv for the cretaceous pco 2 estimates that can be near continuous for most of the tertiary its high temporal resolution and availability of continuous sequences is advantageous for studies aiming to discern short term excursions this method however must correct for changes in growth rate and oxygen level at elevated pco 2 750 1250 ppmv this proxy loses its sensitivity and is not useful the stomatal density and stomatal index of land plants also provide high temporal resolution 2 year high precision 10 40 ppmv for the tertiary and possibly cretaceous pco 2 estimates and so also is ideal for discerning short term excursions unfortunately this proxy also loses sensitivity at some level of pco 2 above 350 ppmv which currently is largely undetermined our analysis of the recently developed 11 b technique shows that it currently is not yet well constrained most importantly it requires the assumption that the boron isotopic composition of the ocean remains nearly constant through time in addition it assumes that there are no biological or temperature effects and that diagenetic alteration of the boron isotopic composition does not occur a fifth co 2 proxy based on the redox chemistry of marine cerium has several fundamental flaws and is not discussed in detail here

constraint of the co2 rise by new atmospheric carbon isotopic measurements during the last deglaciation 1 the causes of the 80 ppmv increase of atmospheric carbon dioxide co2 during the last glacial interglacial climatic transition remain debated we analyzed the parallel evolution of co2 and its stable carbon isotopic ratio 13co2 in the european project for ice coring in antarctica epica dome c ice core to bring additional constraints agreeing well but largely improving the taylor dome ice core record of lower resolution our 13co2 record is characterized by a w shape with two negative 13co2 excursions of 0 5 during heinrich 1 and younger dryas events bracketing a positive 13co2 peak during the bolling allerod warm period the comparison with marine records and the outputs of two c cycle box models suggest that changes in southern ocean ventilation drove most of the co2 increase with additional contributions from marine productivity changes on the initial co2 rise and 13co2 decline and from rapid vegetation buildup during the co2 plateau of the bolling allerod

rapid crustal accretion and magma assimilation in the oman u a e ophiolite high precision u pb zircon geochronology of the gabbroic crust 1 new high precision u pb zircon geochronology from the oman united arab emirates u a e ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust the new data come from a well preserved and exposed crustal section in the center of the wadi tayin massif single grain and grain fragment 206pb 238u dates from upper level gabbros tonalites trondhjemites and gabbroic pegmatites corrected for initial th exclusion range from 112 55 0 21 to 95 50 0 17 ma with most data clustered between 96 40 0 17 to 95 50 0 17 ma zircon dates from upper level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half rates of 50 100 km ma dates from tonalites trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper level gabbros suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates three of the dated upper level gabbros and a single gabbroic pegmatite from the base of the crust have 1 ma intrasample variability in single grain dates suggesting assimilation of older crust during the formation or crystallization of the magmas whole rock end t of seven samples including the upper level gabbros with variable zircon dates have tightly clustered initial values ranging from end 96 ma 7 59 0 23 to 8 28 0 31 the end values are similar to those from other gabbros within the ophiolite suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas the new dates suggest that the studied section formed at a fast spreading mid ocean ridge between 96 4 95 5 ma the large intrasample variability in zircon dates in some samples is unexpected in this setting and may be related to propagation of a younger ridge into older oceanic lithosphere

crystallization depth beneath an oceanic detachment fault odp hole 923a mid atlantic ridge oceanic detachment faults are increasingly recognised as playing an integral role in the seafloor spreading process at slow and ultraslow spreading mid ocean ridges with significant consequences for the architecture of the oceanic lithosphere although melt supply is considered to play a critical control on the formation and evolution of oceanic detachments much less well understood is how melts and faults interact and influence each other few direct constraints on the locus and depth of melt emplacement in the vicinity of detachments are available gabbros drilled in odp hole 923a near the intersection of the mid atlantic ridge and the kane transform fault 23 n the mark area represent magmas emplaced into the footwall of such a detachment fault and unroofed by it we here present u pb zircon dates for these gabbros and associated diorite veins which when combined with a tectonic reconstruction of the area allow us to calculate the depths at which the melts crystallized th corrected single zircon u pb dates from three samples range from 1 138 0 062 ma to 1 213 0 021 ma we find a crystallization depth of 6 4 1 7 1 3 km and estimate that the melts parental to the gabbros were initially emplaced up to 1 5 km deeper at 8 km below the seafloor the tectonic reconstruction implies that the detachment fault responsible for the exposure of the sampled sequence likely crossed the ridge axis at depth suggesting that melt emplacement into the footwall of oceanic detachment faults is an important process the deep emplacement depth we find associated with detachment mode spreading at 1 2 ma appears to be significantly greater than the depth of magma reservoirs during the current magmatic mode of spreading in the area suggesting that the northern mark segment preserves a recent switch between two temporally distinct modes of spreading with fundamentally different lithospheric architecture this article is protected by copyright all rights reserved

high precision zircon u pb geochronology of astronomically dated volcanic ash beds from the mediterranean miocene several orbitally tuned miocene sedimentary sequences around the mediterranean contain abundant intercalated volcanic ash beds these sequences provide the rare opportunity to directly compare radioisotopic dating methods with independent and accurate deposition ages derived from astrochronology we present a large data set n 16n 16 n 166n 166 of zircon u pb dates obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims techniques for ash beds from an almost continuous orbitally tuned messinian to langhian 6 2 15 4 ma sedimentary sequence exposed along the adriatic coast south of ancona italy we use this unique data set to evaluate 1 the accuracy of zircon u pb dates 2 the significance of initial intermediate daughter product disequilibria for zircon u pb geochronology of young rocks 3 the effect of prolonged pre eruption zircon crystallization and zircon recycling on u pb derived ash bed deposition ages and 4 discuss the implications for the intercalibration of radioisotope geochronometers and the calibration of the geologic time scale

accretion and oxidation of a superfast spread axial melt lens tims and sims zircon analyses of the iodp hole 1256d gabbros abstract iodp hole 1256d penetrates a complete section of upper oceanic crust spread from the east pacific rise at a full rate of up to 220 mm yr here we present u pb zircon data from hole 1256d gabbroic rocks using isotope dilution thermal ionization mass spectrometry id tims and secondary ion mass spectrometry sims three gabbro samples yielded th corrected weighted mean 206pb 238u sims dates of 15 03 0 14 ma 15 13 0 35 ma and 15 23 0 12 ma an id tims th corrected weighted mean 206pb 238u date of 15 191 0 040 ma from the first of the three samples provides verification and refinement of the timing of magmatism the u pb dates agree with the predicted age of the crust based on marine magnetic anomalies suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis zircons from two of the studied samples have anomalously high th u ratios likely reflecting oxidation of u prior to or during zircon crystallization the elevated o2 of late stage melts in the axial melt lens may be related to progressive crystallization assimilation of hydrothermally altered sheeted dikes and gabbros and or contamination of the melt lens by seawater derived saline brines

influence of the intraseasonal variability on heat waves in subtropical south america the influence of the intraseasonal variability on heat wave development over subtropical south america during austral summer is analyzed the role of the south atlantic convergence zone sacz on this development is documented results show that intraseasonal variability can explain on average at least 32 of summer temperature variance moreover 73 of the heat waves in subtropical south america develop in association with an active sacz the analysis of pentad maps shows that warm conditions in the region under study develop in association with the strengthening of an anticyclonic anomaly which is discernible over the subtropical regions at least 15 days before temperature peak occurrence that circulation anomaly is embedded in a large scale rossby wave train extending along the south pacific ocean that is linked to convection anomalies at the equatorial western and central pacific ocean in addition the development of the anticyclonic circulation over subtropical south america appears to be strengthened by the subsidence conditions promoted by the active sacz which result in temperature rise in the subtropical region under relatively dry conditions on the other hand during the last 2 days of evolution sacz activity weakens and the progressive temperature rise in the region is dominated by warmer and moister air being anomalously advected from the north results confirm the important role that sacz activity on intraseasonal time scales has in inducing persistent circulation anomalies at the subtropical regions that can result in the development of persistent heat waves and very extreme daily temperature

use of carbon isotopes in paleosols as an indicator of the p co2 of the paleoatmosphere measurements of carbon isotopic composition of coexisting paleosol organic matter and carbonate results in improved estimates of the paleo p co2 content of the paleoatmosphere measurements from tertiary paleosols indicate that the levels of co2 in the early eocene middle and late miocene and pliocene were less than 1000 ppmv early cretaceous levels of p co2 were significantly higher of the order of 1500 to 3000 ppmv

the climate of the altiplano observed current conditions and mechanisms of past changes abstract the large scale controls on the climate of the south american altiplano are investigated using local observations reanalysis data and general circulation model experiments the objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial interglacial our results suggest that on all timescales the climatic conditions on the altiplano are closely related to the upper air circulation with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions different factors influence the upper air circulation on the different timescales intraseasonal variability is a reflection of the position and intensity of the bolivian high which is modulated by rossby waves emanating from the midlatitude south pacific the annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere rather than direct insolation forcing over the altiplano or moisture changes in the source area interannual variability is primarily related to changes in the mean zonal flow over the altiplano reflecting changes in meridional baroclinicity between tropical and subtropical latitudes which in turn is a response to sea surface temperature changes in the tropical pacific orbitally forced changes in the land sea contrast drive continental scale circulation changes which significantly alter the zonal flow over the altiplano on glacial interglacial timescales the contrast in heating between northern and southern hemispheres during the glacial leads to upper air easterly anomalies throughout the tropics on modern timescales the marked submonthly seasonal and interannual changes of moisture over the altiplano cannot be accounted for by moisture changes in the humid tropical lowlands however the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands which counteracts the effect of the upper level circulation resulting in little overall change in precipitation this observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the altiplano on a wide range of timescales

basin floor fans in the north sea sequence stratigraphic models vs sedimentary facies discussion original article shanmugan g r b bloch s m mitchell g w j beamish r j hodgkinson j e damuth t straume s e syvertsen and k e shields 1995 aapg bulletin v 79 p 477 512

climate in the western cordillera of the central andes over the last 4300 years abstract the distichia peat core obtained in the carhuasanta valley near nevado mismi cordillera chila provides information on climatic and environmental conditions over the last 4300 years the relative changes in the stable carbon isotope composition of plant remains preserved in the core reflect major temperature fluctuations in the western cordillera of the southern peruvian andes these temperature variations can be additionally linked with the changes in precipitation patterns by analysing c and c n ratio in the core relatively warm and moist conditions prevailed from 4280 to 3040 cal yrs bp bc 2330 1090 with a short colder dry episode around 3850 cal yrs bp bc 1900 the most prominent climate changes recorded in the peat occurred between 3040 and 2750 cal yrs bp bc 1090 800 when the initial warming turned to a rapid cooling to temperatures at least 2 c lower than the mean for the late holocene initially drier conditions within this event turned to a short wet phase after 2780 cal yrs bp bc 830 when the temperature increased again this event coincides with significant changes in peat and ice core records in the central andes matching the timing of the global climate event around 2 8 cal ka bp climatic conditions in the study area became relatively dry and stable after the event for about 800 years highly variable temperatures and humidity prevailed during the last 2000 years when an extended warm and relatively humid period occurred between 640 and 155 cal yrs bp ad 1310 1795 followed by predominantly colder and drier conditions the established 13 c peat record represents the first continuous proxy for the temperature in the southern peruvian andes dated by the ams 14 c distichia peat is wide spread in the andes and the proposed approach can be applied elsewhere in high altitudes where no other traditional climate proxies are available

quantitative uncertainty analyses of ancient atmospheric co2 estimates from fossil leaves the relationship between atmospheric co 2 and ancient climate is of fundamental importance for gauging the climate sensitivity of the earth system to a changing co 2 regime one of the most widely adopted paleobiological co 2 proxies for reconstructing earth s atmospheric co 2 history exploits the inverse relationship between leaf stomatal index the fraction of leaf epidermal cells that are stomatal structures and atmospheric co 2 however fossil leaf based co 2 reconstructions make a prioii assumptions about the form of the empirical relationship between si and co 2 required for transfer functions and have failed to correctly propagate error terms these effects can translate into erroneous interpretations that undermine the value of the proxy here we report the development and application of a rigorous generalized statistical framework overcoming these limitations that generates probability density functions for each atmospheric co 2 estimate the utility of our statistical tools is demonstrated by showing how they revise earlier atmospheric co 2 estimates from fossil cuticles of ginkgo and metasequoia trees during the early eocene and middle miocene warm periods upwards by 150 to 250 ppm to 450 to 700 ppm the revised co 2 reconstructions therefore help to resolve the paradox of warm paleogene and neogene greenhouse climates co existing with near present day levels of co 2 and support the emerging view from independent paleoclimate studies for a high climate sensitivity of the earth system the statistical tools presented are sufficiently versatile to permit their use in other investigations of paleoco 2 estimates from fossil leaves

structural pattern of the galapagos microplate and evolution of the galapagos triple junctions a 13 000 km2 microplate intervenes to prevent the junction of the nazca cocos and pacific plates in the eastern equatorial pacific ocean west of the galapagos islands the microplate is rotating clockwise at 6 m y about an absolute pole 400 km southeast of its center so that although its northern margin has a similar motion to adjacent parts of the cocos plate its southern margin is 50 slower the perimeter of the galapagos microplate is best defined by bathymetry and has been surveyed with sea beam and examined by deep tow two of the three triple junctions on its margins are ridge ridge ridge rrr junctions 160 km apart on the crest of the east pacific rise epr the most like a classic rrr junction is the pacific nazca galapagos intersection at 1 10 n where the epr axis is joined by a narrow volcanic ridge 70 km long and 1 km high the cocos nazca galapagos junction at the west end of the cocos nazca spreading center is inferred to be of the rff type but the transform fault branches are represented by large nontransform offsets lack of stable transform faults is related to rapid westward migration of the junction caused by lengthening of the cocos nazca axis at 60 70 mm yr this rate which was once required to maintain a stable nazca cocos pacific rrr junction has persisted even after birth of the microplate about 1 ma continuing propagation of the cocos nazca axis has opened up the hess deep rift basin in the eastern epr flank in advance of the eruptive propagator tip this rift valley has a relief of more than 3 km from its uplifted rims to the floor where severe attenuation exposes basal layers of the epr crust the similar dietz deep rift basin at the southeastern margin of the microplate 70 km from hess deep was formed when a lengthening galapagos nazca spreading axis rifted through another part of the epr flank

constraints on the magmatic evolution of the oceanic crust from plagiogranite intrusions in the oman ophiolite we present major and trace element as well as sr nd and hf isotope data on a suite of 87 plutonic rock samples from 27 felsic crustal intrusions in seven blocks of the oman ophiolite the rock compositions of the sample suite including associated more mafic rocks range from 48 to 79 wt sio2 i e from gabbros to tonalites the samples are grouped into a ti rich and relatively light rare earth element lree enriched p1 group ce yb n 0 7 resembling the early v1 lavas and a ti poor and lree depleted p2 group ce yb n 0 7 resembling the late stage v2 lavas based on the geochemical differences and in agreement with previous structural and petrographic models we define phase 1 p1 and phase 2 p2 plutonic rocks felsic magmas in both groups formed by extensive fractional crystallization of olivine clinopyroxene plagioclase apatite and ti magnetite from mafic melts the incompatible element compositions of p1 rocks overlap with those from mid ocean ridges but have higher ba nb and th nb trending towards the p2 rock compositions and indicating an influence of a subducting slab the p2 rocks formed from a more depleted mantle source but show a more pronounced slab signature these rocks also occur in the southern blocks with the exception of the tayin block of the oman ophiolite implying that the entire ophiolite formed above a subducting slab initial nd and hf isotope compositions suggest an indian morb type mantle source for the oman ophiolite magmas isotope compositions and high th nb in some p2 rocks indicate mixing of a melt from subducted sediment into this mantle

climatic and volcanic forcing of tropical belt northern boundary over the past 800 years the position of the northern boundary of the tropical belt affects the hydroclimate of many arid and semi arid regions in the northern hemisphere widening of the tropical belt since the 1970s has largely been attributed to anthropogenic forcing however the relative influence of natural drivers of tropical belt expansion and contraction before this time is poorly understood here we use data on tree ring widths from five mid latitude regions in the northern hemisphere to reconstruct the movement of the northern boundary of the early spring tropical belt over the past 800 years ad 1203 2003 our reconstruction explains 45 of the interannual variance in the latitudinal extent of the hadley circulation a metric of the position of the tropical belt boundary we find that the tropical belt contracted expanded during positive negative phases of the el nino southern oscillation and pacific north american teleconnection patterns the tropical belt also contracted significantly following major volcanic events that injected sulfur into the stratosphere the longest period of persistent tropical belt expansion occurred in the late sixteenth century during one of the coldest periods of the little ice age our results warn of potential socio economic consequences of future variations in tropical belt width driven by natural climate variability or stratospheric aerosol injections whether volcanic or artificial climate variability and volcanic forcing both influenced the latitudinal migration of the tropical belt over the past 800 years according to an analysis of tree ring widths in the northern hemisphere

formation of fast spreading lower oceanic crust as revealed by a new mg ree coupled geospeedometer abstract a new geospeedometer is developed based on the differential closures of mg and rare earth element ree bulk diffusion between coexisting plagioclase and clinopyroxene by coupling the two elements with distinct bulk closure temperatures this speedometer can numerically solve the initial temperatures and cooling rates for individual rock samples as the existing mg exchange thermometer was calibrated for a narrow temperature range and strongly relies on model dependent silica activities a new thermometer is developed using literature experimental data when the bulk closure temperatures of mg and ree are determined respectively using this new mg exchange thermometer and the existing ree exchange thermometer this speedometer can be implemented for a wide range of compositions mineral modes and grain sizes applications of this new geospeedometer to oceanic gabbros from the fast spreading east pacific rise at hess deep reveal that the lower oceanic crust crystallized at temperatures of 998 1353 c with cooling rates of 0 003 10 2 c yr stratigraphic variations of the cooling rates and crystallization temperatures support deep hydrothermal circulations and in situ solidification of various replenished magma bodies together with existing petrological geochemical and geophysical evidence results from this new speedometry suggest that the lower crust formation at fast spreading mid ocean ridges involves emplacement of primary mantle melts in the deep section of the crystal mush zone coupled with efficient heat removal by crustal scale hydrothermal circulations the replenished melts become chemically and thermally evolved accumulate as small magma bodies at various depths feed the shallow axial magma chamber and may also escape from the mush zone to generate off axial magma lenses

1 ma east pacific rise oceanic crust and uppermost mantle exposed by rifting in hess deep equatorial pacific ocean abstract a series of dives with the french submersible nautile has been conducted in the region of the galapagos triple junction at the tip of the cocos nazca propagator most of the dives were devoted to the study of hess deep a rift valley produced by rifting of oceanic crust created at the axis of the very fast spreading east pacific rise the dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra rift ridge located to the north of the axis of hess deep this shows that this situation is not specific not slow spreading ridges a nearly complete and unique log of young 1 ma crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered we propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust one model stresses symmetry and vertical tectonics through isostasy diapirism of serpentinized mantle another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low angle detachment faulting in both models which differ mainly in the structure at depth amagmatic extension of east pacific rise crust by the cocos nazca propagator tip results in uplift and denudation of lower crust and upper mantle

ten turbidite myths abstract during the past 50 years the turbidite paradigm has promoted many myths related to deep water turbidite deposition john e sanders 1926 1999 a pioneering process sedimentologist first uncovered many of these turbidite myths this paper provides a reality check by undoing 10 of these turbidite myths myth no 1 turbidity currents are non turbulent flows with multiple sediment support mechanisms reality turbidity currents are turbulent flows in which turbulence is the principal sediment support mechanism myth no 2 turbidites are deposits of debris flows grain flows fluidized flows and turbidity currents reality turbidites are the exclusive deposits of turbidity currents myth no 3 turbidity currents are high velocity flows and therefore they elude documentation reality turbidity currents operate under a wide range of velocity conditions myth no 4 high density turbidity currents are true turbidity currents reality ph h kuenen 1950 introduced the concept of turbidity currents of high density based on experimental debris flows not turbidity currents high density turbidity currents are sandy debris flows myth no 5 slurry flows are high density turbidity currents reality slurry flows are debris flows myth no 6 flute structures are indicative of turbidite deposition reality flute structures are indicative only of flow erosion not deposition myth no 7 normal grading is a product of multiple depositional events reality normal grading is the product of a single depositional event myth no 8 cross bedding is a product of turbidity currents reality cross bedding is a product of traction deposition from bottom currents myth no 9 turbidite facies models are useful tools for interpreting deposits of turbidity currents reality a reexamination of the annot sandstone in se france which served as the basis for developing the first turbidite facies model suggests a complex depositional origin by plastic flows and bottom currents myth no 10 turbidite facies can be interpreted using seismic facies and geometries reality individual turbidity current depositional events commonly centimeters to decimeters in thickness cannot be resolved in seismic data all turbidite myths promote falsehood and should be abandoned

a late eocene date for late triassic bird tracks arising from r n melchor s de valais j f genise nature 417 936 938 2002 bird like tracks from northwest argentina have been reported as being of late triassic age1 they were attributed to an unknown group of theropods showing some avian characters however we believe that these tracks are of late eocene age on the basis of a new weighted mean 206pb 238u date isotope dilution thermal ionization mass spectrometry method on zircons from a tuff bed in the sedimentary succession containing the fossil tracks in consequence the mentioned tracks are assigned to birds and its occurrence matches the known fossil record of aves

reconstructing el nino southern oscillation enso from high resolution palaeoarchives el nino southern oscillation enso is one of the most important coupled ocean atmospheric phenomena to cause global climate variability on interannual timescales efforts to understand recent apparently anomalous enso behaviour are hampered by the phenomenon s unstable non stationary nature and the limitations inherent in palaeoclimate records in this paper the complexities associated with isolating enso signals in observational and palaeoclimate records are reviewed the utility and limitations of high resolution tree ring coral speleothems ice and documentary proxy data for enso reconstruction are discussed to overcome the regional biases contained within each palaeoclimatic source it is necessary to compare complementary signals derived from multiple proxy climate records to date there have been limited attempts to reconstruct large scale enso using these multiproxy methodologies a critique of the complexities associated with previous approaches of reconstructing discrete enso events and atmospheric oceanic indices is provided abundant potential remains to better characterise teleconnection patterns propagation signatures and non stationary features of large scale enso behaviour if key uncertainties in enso dynamics such as the response of extreme events to natural human forcing are to be adequately assessed then complementary attempts must be made to model the historic synoptic conditions with apparent changes in reconstructed indices copyright 2006 john wiley sons ltd

the stillwater complex integrating zircon geochronological and geochemical constraints on the age emplacement history and crystallization of a large open system layered intrusion a reply to the comment by rais latypov on wall et al j petrology 59 153 190 2018

distributed deformation ahead of the cocos nazca rift at the galapagos triple junction the galapagos triple junction is not a simple ridge ridge ridge rrr triple junction the cocos nazca rift c n rift tip does not meet the east pacific rise epr instead two secondary rifts form the link incipient rift at 2 40 n and dietz deep volcanic ridge the southern boundary of the galapagos microplate gmp at 1 10 n recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the gmp 1 5 ma south of c n rift a band of northeast trending cracks cuts epr generated abyssal hills it is a mirror image of a band of cracks previously identified north of c n rift on the same age crust in both areas the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the epr spreading center and cut eastward into pre existing topography each crack formed a short lived triple junction until it was abandoned and a new crack and triple junction initiated nearby between 2 5 and 1 5 ma the pattern of cracking is remarkably symmetric about c n rift providing support for a crack interaction model in which crack initiation at the epr axis is controlled by stresses associated with the tip of the westward propagating c n rift the model also shows that offsets of the epr axis may explain times when cracking is not symmetric south of c n rift cracks are observed on seafloor as old as 10 5 ma suggesting that this triple junction has not been a simple rrr triple junction during that time

rayleigh distillation and the depth profile of 13c 12c ratios of soil organic carbon from soils of disparate texture in iron range national park far north queensland australia a depth and particle size specific analysis of soil organic carbon soc and its isotopic composition was undertaken to investigate the effects of soil texture or particle size on the depth profile of stable carbon isotopic composition of soc 13csoc in two tropical soils depth specific samples from two soil profiles of markedly different texture coarse grained and fine grained were separated into particle size classes and analyzed for the mass mass concentration of soc c and 13csoc within 1 m of the soil surface 13csoc in the coarse textured soil increases by 1 3 to 1 6 while 13csoc from the fine textured soil increase by as much as 3 8 to 5 5 this increasing depth trend in the coarse textured soil is approximately linear with respect to normalized c while the increase in the fine textured soil follows a logarithmic function with respect to normalized c a model of rayleigh distillation describing isotope fractionation during decomposition of soil organic matter som accounts for the depth profile of 13csoc in the fine textured soil but does not account for the depth profile observed in the coarse textured soil despite their similar climate vegetation and topographic position these results suggest that kinetic fractionation during humification of som leads to preferential accumulation of 13c in association with fine mineral particles or aggregates of fine mineral particles in fine textured soils in contrast the coarse textured soil shows very little applicability of the rayleigh distillation model rather the depth profile of 13csoc in the coarse textured soil can be accounted for by mixing of soil carbon with different isotopic ratios

zircon dating of oceanic crustal accretion most of earth s present day crust formed at mid ocean ridges high precision uranium lead dating of zircons in gabbros from the vema fracture zone on the mid atlantic ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery combined with results from previous dating studies this finding suggests that two distinct modes of crustal accretion occur along slow spreading ridges individual samples record a zircon date range of 90 000 to 235 000 years which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks

the temporal and spatial distribution of magmatism during lower crustal accretion at an ultraslow spreading ridge high precision u pb zircon dating of odp holes 735b and 1105a atlantis bank southwest indian ridge abstract ocean drilling program hole 735b at atlantis bank on the southwest indian ridge sampled 1508 m of plutonic oceanic crust hosted in the footwall of an oceanic detachment fault we present new high precision isotope dilution thermal ionization mass spectrometry id tims u pb zircon dates from samples spanning the length of hole 735b and from the shallower adjacent hole 1105a 158 m the new dates provide the most complete and precise record of both the spatial and temporal distribution of magmatism during accretion of the lower oceanic crust to date whole rock and mineral geochemistry from hole 735b define three main igneous series weighted mean 206pb 238u dates suggest each igneous series intruded beneath the preceding series weighted mean 206pb 238u dates range from 12 175 to 11 986 ma in series 1 11 974 to 11 926 ma in series 2 and 11 936 to 11 902 ma in series 3 0 015 to 0 069 ma weighted mean 206pb 238u dates from hole 1105a range from 11 9745 to 11 9573 ma 0 0082 to 0 0086 ma the hole 1105a dates are coeval with series 2 in hole 735b consistent with previous correlations of fe ti oxide rich layers between the two holes suggesting individual magmatic series formed sheet like bodies that were 250 m thick and extended 1 1 km parallel to the ridge axis e w and 0 48 km in the spreading direction n s the data suggest a total duration of magmatism in hole 735b of 0 214 0 032 ma corresponding to accretion over a horizontal distance of 2 6 0 4 km the crust at atlantis bank was formed during active detachment faulting and the successive underplating of each magmatic unit may have been favored in this environment the combined u pb dates and reported ti in zircon temperatures are consistent with magmatic cooling rates of 10 3 10 4 c ma over the temperature interval of 900 700 c

u th ra disequilibria and the extent of off axis volcanism across the east pacific rise at 9 30 n 10 30 n and 11 20 n there is widespread interest in the distance that mid ocean ridge magmatism extends beyond the neovolcanic zone off axis magmas also provide a means to map out variations across the melting zone we present 238u 230th 226ra data for 35 well characterized samples that extend up to 50 km away from the ridge axis across the east pacific rise at 9 30 n 10 30 n and 11 20 n the 230th 238u ratios range from 1 00 to 1 19 and the 226ra 230th ratios range from 1 to 2 78 the samples have a bimodal 230th 238u distribution with approximately half overlying published axial data on the u th diagram and the remainder lying close to the equiline the u series disequilibria in the majority of the samples can be explained by aging subsequent to eruption in a zone 8 km wide about the neovolcanic zone consistent with visual evidence for sample age nevertheless seven of the samples lie above calculated 230th 238u axial decay curves and or have 226ra excesses implying eruption tens of kilometers off axis these are consistent with evidence from seamounts and seismic interpretations that magmatism can extend up to 20 km off axis the implication is that magma is not as efficiently focused beneath the ridge axis as has generally been believed there is a decrease in initial 230th 238u in both these and published samples inferred to have formed off axis but there is no compelling evidence that this reflects source heterogeneity simple modeling suggests that this could be explained by a decrease in fertility and melt column length as the overlying lithosphere thickens with age and the solidus shallows

the geological and structural evolution of the cerro tuzgle quaternary stratovolcano in the back arc region of the central andes argentina abstract the aim of our paper is to contribute to a better knowledge of the volcanism in the back arc region of the central andes and its relationships with the basement geology the stress field and the tectonic evolution by studying in detail the stratigraphy and the structure of the quaternary cerro tuzgle stratovolcano in the puna plateau field mapping and remote sensing analysis reveal the stratigraphic architecture the geological evolution and the volcanotectonic interactions in the cerro tuzgle area for the first time in a volcano of the puna plateau synthemic units bounded by unconformity surfaces have been defined unrevealing the temporal and spatial relationships between constructive and destructive phases of the volcano history our study indicates that after the emplacement of a small ignimbrite deposit and of few scattered lava domes the central cerro tuzgle volcano built up throughout three distinct phases of edifice construction the first of these constructive phases ended with a previously unreported destructive event consisting of 0 5 km 3 catastrophic sector collapse of the volcanic edifice whose stratigraphic position and main characteristics have been identified the study suggests that the regional stress regime and the topography of the substrata are the main non magmatic factors controlling the constructive and destructive phases of the volcano including the directions of magmatic intrusions faulting and gravitational sector failure of the volcano the integration of synthemic stratigraphy and volcanotectonic analysis in the study of volcanic edifices showed to be an effective methodological approach for the understanding of the magmatic and tectonic evolution of the puna plateau

rainfall variations in central indo pacific over the past 2 700 y tropical rainfall variability is closely linked to meridional shifts of the intertropical convergence zone itcz and zonal movements of the walker circulation the characteristics and mechanisms of tropical rainfall variations on centennial to decadal scales are however still unclear here we reconstruct a replicated stalagmite based 2 700 y long continuous record of rainfall for the deeply convective northern central indo pacific ncip region our record reveals decreasing rainfall in the ncip over the past 2 700 y similar to other records from the northern tropics notable centennial to decadal scale dry climate episodes occurred in both the ncip and the southern central indo pacific scip during the 20th century current warm period cwp and the medieval warm period mwp resembling enhanced el nino like conditions further we developed a 2 000 y long itcz shift index record that supports an overall southward itcz shift in the central indo pacific and indicates southward mean itcz positions during the early mwp and the cwp as a result the drying trend since the 20th century in the northern tropics is similar to that observed during the past warm period suggesting that a possible anthropogenic forcing of rainfall remains indistinguishable from natural variability

carbon isotope ratio of cenozoic co2 a comparative evaluation of available geochemical proxies 1 the carbon isotope ratio 13c of plant material is commonly used to reconstruct the relative distribution of c3 and c4 plants in ancient ecosystems however such estimates depend on the 13c of atmospheric co2 13cco2 at the time which likely varied throughout earth history for this study we use benthic and planktonic 13c and 18o records to reconstruct a long term record of cenozoic 13cco2 confidence intervals for 13cco2 values are assigned after careful consideration of equilibrium and non equilibrium isotope effects and processes as well as resolution of the data we find that benthic foraminifera better constrain 13cco2 compared to planktonic foraminiferal records which are influenced by photosymbiotes depth of production seasonal variability and preservation furthermore sensitivity analyses designed to quantify the effects of temperature uncertainty and diagenesis on benthic foraminifera 13c and 18o values indicate that these factors act to offset one another our reconstruction suggests that cenozoic 13cco2 averaged 6 1 0 6 1 while only 11 2 million of the last 65 5 million years correspond to the pre industrial value of 6 5 with 90 confidence here 13cco2 also displays significant variations throughout the record at times departing from the pre industrial value by more than 2 thus the observed variability in 13cco2 should be considered in isotopic reconstructions of ancient terrestrial plant ecosystems especially during the late and middle miocene times of presumed c4 grassland expansion

protracted timescales of lower crustal growth at the fast spreading east pacific rise the formation of oceanic crust at mid ocean ridges is a fundamental component of plate tectonics a large fraction of the new crust is created when magmas in the lower crust cool to form gabbroic rocks the duration of magmatism during formation of the new gabbroic crust is expected to vary with plate spreading rate and has been constrained by dating gabbroic rocks at the slow spreading southwest indian ridge and mid atlantic ridge1 2 3 4 here we present high precision u pb dating of zircon minerals from gabbroic rocks formed at the fast spreading east pacific rise and exposed at hess deep we find that the zircons formed between 1 420 and 1 271 0 006 0 081 million years ago within individual samples the zircon minerals exhibit a range in formation dates of up to 0 124 myr consistent with either protracted crystallization from a magma or assimilation into the magma of older zircons from adjacent rocks the variability of zircon dates is comparable to that measured at the slow spreading mid atlantic ridge3 we conclude that the timescales of magmatic processes in the lower crust may be similar at slow and fast spreading ridges implying that the duration of crust formation at mid ocean ridges is not only controlled by spreading rate

tectonic development of the samail ophiolite high precision u pb zircon geochronology and sm nd isotopic constraints on crustal growth and emplacement 1 new high precision single grain u pb zircon geochronology and whole rock nd isotopic data provide insight into the magmatic and tectonic development of the samail ophiolite the analyzed rocks can be broadly divided into two groups based on their structural position dates and isotopic composition an older group related to on axis magmatism and a younger group of post ridge dikes sills and stocks on axis gabbros tonalites and trondhjemites yielded th corrected 206pb 238u dates from 96 441 0 062 to 95 478 0 056 ma these dates combined with dates from rioux et al 2012 suggest that most of the ophiolite crust formed at an oceanic spreading center in 1 ma the post ridge intrusions come from all depths in the crust the upper mantle and the metamorphic sole post ridge gabbros tonalites and trondhjemites from the crust and mantle yielded th corrected 206pb 238u dates of 95 405 0 062 to 95 077 0 062 ma a small trondhjemitic pod from the metamorphic sole yielded younger th corrected 206pb 238u dates of 94 90 0 38 to 94 69 0 12 ma isotopic data suggest two distinct sources for the post ridge magmas five of the gabbros and tonalites from the crust have end 96 ma 6 90 0 12 to 7 88 0 16 and two trondhjemites from the upper mantle and metamorphic sole have end 96 ma 7 77 0 08 and 7 01 0 16 the negative end t and u pb dates from the mantle dike require that subduction or thrusting was established below the ophiolite 0 25 0 5 ma after formation of the crust the bimodal isotopic composition of post ridge magmas may reflect coeval decompression and or fluid fluxed melting of the mantle and melting dehydration or assimilation of sediment in the down going plate at this time the new data place temporal constraints on mid ocean ridge and supra subduction zone models for ophiolite formation

cracking of lithosphere north of the galapagos triple junction the galapagos triple junction is a ridge ridge ridge triple junction where the cocos nazca and pacifi c plates meet around the galapagos microplate directly north of the large scarps of the cocos nazca rift a 250 km long and 50 km wide band of northwest southeast trending cracks with volcanics at their western ends crosscuts and blankets the north south trending abyssal hills of the east pacifi c rise it appears that the roughly northeast southwest extension of east pacifi c rise generated seafl oor has been accommodated by a succession of minor rifts that during at least the past 4 m y had their triple junctions with the east pacifi c rise at distances of 50 100 km north of the tip of the propagating cocos nazca rift we propose that the rift locations are controlled by stresses associated with the dominant cocos nazca rift and scaled by the distance of its tip to the east pacifi c rise we speculate that similar ephemeral rifts occurred south of the cocos nazca rift and were instrumental in the origin of the rotating galapagos microplate ca 1 5 ma

magma emplacement differentiation and cooling in the middle crust integrated zircon geochronological geochemical constraints from the bergell intrusion central alps abstract u th pb zircon geochronology is an essential tool for quantifying the emplacement differentiation and thermal evolution of crustal magmatic systems however the power of u pb zircon dates can be enhanced through complementary characterization of mineral texture and geochemistry as this permits more detailed interpretations of geochronological datasets than conventionally achieved our approach to better relating zircon dates and geological processes consists of a multi method analytical workflow including cathodoluminescence imaging cl in situ la icpms epma zircon geochemistry u pb zircon id tims geochronology and solution icpms zircon trace element analysis u pb tims tea these methods are here applied to zircon from the bergell intrusion a composite alpine pluton preserving a 10 km mid crustal transect hand samples of tonalite granodiorite and hybridized granitoid each record 250 700 kyr of autocrystic zircon growth bergell zircons are ubiquitously zoned with ca 10 4 10 6 yr growth histories as evidenced by id tims analysis of microsampled fragments from single crystals u pb tims tea data exhibit compositional trends on multiple spatiotemporal scales including the handsample scale representing in situ differentiation at the emplacement level e g th u lithology scale defining trajectories corresponding to the production of tonalitic versus granodioritic magmas lu hf and pluton scale indicating increasingly evolved melts over 1 6 myr of pluton assembly zr hf these absolute tims tea temporal trends are corroborated by relative la icpms epma core to rim geochemistry we compare records of trace element evolution from tims tea bergell whole rock geochemistry and a global compilation of whole rock geochemical data these findings support zircon compositional evolution as a robust indicator of differentiation at local and crustal scales and provide key empirical constraints on melt differentiation and cooling timescales in the middle crust

a 2 300 year long annually resolved record of the south american summer monsoon from the peruvian andes decadal and centennial mean state changes in south american summer monsoon sasm precipitation during the last 2 300 years are detailed using an annually resolved authigenic calcite record of precipitation 18o from a varved lake in the central peruvian andes this unique sediment record shows that 18o peaked during the medieval climate anomaly mca from a d 900 to 1100 providing evidence that the sasm weakened considerably during this period minimum 18o values occurred during the little ice age lia between a d 1400 and 1820 reflecting a prolonged intensification of the sasm that was regionally synchronous after the lia 18o increased rapidly particularly during the current warm period cwp a d 1900 to present indicating a return to reduced sasm precipitation that was more abrupt and sustained than the onset of the mca diminished sasm precipitation during the mca and cwp tracks reconstructed northern hemisphere and north atlantic warming and a northward displacement of the intertropical convergence zone itcz over the atlantic and likely the pacific intensified sasm precipitation during the lia follows reconstructed northern hemisphere and north atlantic cooling el nino like warming in the pacific and a southward displacement of the itcz over both oceans these results suggest that sasm mean state changes are sensitive to itcz variability as mediated by western hemisphere tropical sea surface temperatures particularly in the atlantic continued northern hemisphere and north atlantic warming may therefore help perpetuate the recent reductions in sasm precipitation that characterize the last 100 years which would negatively impact andean water resources

quantifying and understanding the uncertainty of atmospheric co2 concentrations determined from calcic paleosols 1 a computer program pbuq that uses monte carlo simulations to propagate uncertainty through regression equations and the equation for the paleosol carbonate co2 paleobarometer is presented pbuq includes options for all of the common approaches to determining values for input variables and incorporates several recent advancements relevant to determining values for soil respired co2 concentrations 13c values of respired co2 13c values of atmospheric co2 and temperatures of soil carbonate formation pbuq is intended to improve confidence in paleoatmospheric co2 research by helping researchers draw statistically significant conclusions pbuq can also be used to attribute and partition error among various sources and thereby advance this technique sensitivity analysis indicates that s z is the largest source of uncertainty for most paleosols and that uncertainty is minimized for soils in which co2 is an evenly balanced mixture between soil derived and atmospheric components evenly balanced mixtures are most likely for paleosols formed in deserts and for weakly developed paleosols development of proxies for soil respired co2 concentrations and 13c values of soil respired co2 specifically for such soils is perhaps the most crucial next step for improving this technique currently calcic paleosols are best used to test the significance of trends and or differences among time slices in paleoatmospheric co2 concentration application to quantifying earth system sensitivity will require large scale averaging of determinations from individual paleosols and or reduced uncertainty associated with input variables

a record of the southern oscillation index for the past 2 000 years from precipitation proxies the el nino southern oscillation enso is a coupled ocean atmosphere climate phenomenon in the tropical pacific ocean the interannual climate variations have been shown to modify both the hadley and walker meridional and zonal atmospheric circulations with strong impacts on global climate 1 3 proxy based reconstructions of the southern oscillation index on a multi decadal scale have shown that the strength and frequency of el nino occurrences have varied over the past millennium 4 7 here we compile reconstructions of precipitation 8 15 from regions that experience substantial enso variability to extend the multidecadal scale southern oscillation index to include the past 2 000 years we find that the medieval warm period similar to ad 800 1300 was characterized by a negative index which indicates more el nino dominated conditions whereas during the little ice age similar to ad 1400 1850 more la nina dominated conditions prevailed the southern oscillation index we derive is significantly correlated with reconstructions of solar irradiance and mean northern hemisphere temperature fluctuations

plate tectonics piecing together rifts earth s crust is formed where tectonic plates rift apart and upwelling magma solidifies disparate observations from rifts beneath the oceans and on land provide insights into the dynamics of rifting and opportunities for synthesis

ammonium concentration in ice cores a new proxy for regional temperature reconstruction 1 we present a reconstruction of tropical south american temperature anomalies over the last 1600 years the reconstruction is based on a highly resolved and carefully dated ammonium record from an ice core that was drilled in 1999 on nevado illimani in the eastern bolivian andes concerning the relevant processes governing the observed correlation between ammonium concentrations and temperature anomalies we discuss anthropogenic emissions biomass burning and precipitation changes but clearly favor a temperature dependent source strength of the vegetation in the amazon basin that given the reconstruction reveals that medieval warm period and little ice age type episodes are distinguishable in tropical south america a region for which until now only very limited temperature proxy data have been available for the time period from about 1050 to 1300 ad our reconstruction shows relatively warm conditions that are followed by cooler conditions from the 15th to the 18th century when temperatures dropped by up to 0 6 c below the 1961 1990 average the last decades of the past millennium are characterized again by warm temperatures that seem to be unprecedented in the context of the last 1600 years

petrochronology of zircon and baddeleyite in igneous rocks reconstructing magmatic processes at high temporal resolution zircon zrsio4 and baddeleyite zro2 are common accessory minerals in igneous rocks of felsic to mafic composition both minerals host trace elements substituting for zr among them hf th u y rees and many more the excellent chemical and physical resistivity of zircon makes this mineral a perfect archive of chemical and temporal information to trace geological processes in the past utilizing the outstanding power and temporal resolution of the u pb decay schemes baddeleyite is a chemically and physically much more fragile mineral it preserves similar information only where it is shielded from dissolution and physical fragmentation as an inclusion in other minerals or in a fine grained or non reactive rock matrix it offers the potential for dating the solidification of mafic rocks with high precision through its crystallization in small pockets of zr enriched melt after extensive olivine and pyroxene fractionation zircon and baddelelyite u pb dates are for an overwhelming majority of cases and where we can assume a closed system considered to reflect the time of crystallization the development of the u pb dating tool ca id tims chemical abrasion isotope dilution thermal ionization mass spectrometry since 2005 has led to unprecedented precision of better than 0 1 in 206pb 238u dates bowring et al 2005 increased sensitivity of mass spectrometers and low laboratory blanks due to reduction of acid volumes allow routine u pb age determinations of micrograms of material at sufficiently high radiogenic common lead ratios see schoene and baxter 2017 this volume in situ u pb age analysis using laser ablation or primary ion beam sputtering allows analysis of sub microgram quantities of zircon material from polished internal sections or zircon surfaces with spot diameters ranging from 30 m for laser ablation inductively coupled plasma mass spectrometry la icp ms to 10 m for secondary ion mass spectrometry sims lateral resolutions of 2 5 m for nanosims

characterization of the in situ magnetic architecture of oceanic crust hess deep using near source vector magnetic data marine magnetic anomalies are a powerful tool for detecting geomagnetic polarity reversals lithological boundaries topographic contrasts and alteration fronts in the oceanic lithosphere our aim here is to detect lithological contacts in fast spreading lower crust and shallow mantle by characterizing magnetic anomalies and investigating their origins we conducted a high resolution near bottom vector magnetic survey of crust exposed in the hess deep tectonic window using the remotely operated vehicle rov isis during rrs james cook cruise jc21 in 2008 hess deep is located at the western tip of the propagating rift of the cocos nazca plate boundary near the east pacific rise epr 2 15 n 101 30 w rov isis collected high resolution bathymetry and near bottom magnetic data as well as seafloor samples to determine the in situ lithostratigraphy and internal structure of a section of epr lower crust and mantle exposed on the steep 20 dipping south facing slope just north of the hess deep nadir ten magnetic profiles were collected up the slope using a three axis fluxgate magnetometer mounted on rov isis we develop and extend the vertical magnetic profile vmp approach of tivey 1996 by incorporating for the first time a three dimensional vector analysis leading to what we here termed as vector vertical magnetic profiling approach we calculate the source magnetization distribution the deviation from two dimensionality and the strike of magnetic boundaries using both the total field fourier transform inversion approach and a modified differential vector magnetic analysis overall coherent long wavelength total field anomalies are present with a strong magnetization contrast between the upper and lower parts of the slope the total field anomalies indicate a coherently magnetized source at depth the upper part of the slope is weakly magnetized and magnetic structure follows the underlying slope morphology including a bench and lobe shaped steps imaged by microbathymetry the lower part of the slope is strongly magnetized with a gradual reduction in amplitude from east to west across the slope surface morphology and recent drilling results indicate that the slope has been affected by mass wasting but the observation of internally coherent magnetization distributions within the upper and lower slopes suggest that the disturbance is surficial we attribute the spatial differences in magnetization distribution to the combination of changes in in situ lithology and depth to the source these survey lines document the first magnetic profiles that capture the gabbro ultramafic and possibly dike gabbro boundaries in fast spreading lower crust

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

atmospheric circulation anomalies during episodes of enhanced and reduced convective cloudiness over uruguay abstract regional and large scale circulation anomalies associated with periods of enhanced and reduced convective cloudiness over uruguay are studied for austral spring and summer when rainfall associated with deep convection is more frequent in this region the analysis was performed at a submonthly timescale considering that the essential nature of the mechanisms producing rainfall is not well captured by anomalies calculated on a monthly or seasonal basis in regions where precipitation is highly episodic periods of enhanced and reduced convective cloudiness over uruguay are characterized by a marked dipolar structure in the outgoing longwave radiation anomaly field along eastern south america from 10 to 40 s with the centers of the dipole located over the south atlantic convergence zone sacz and over a broad region including uruguay southern brazil and northeastern argentina this dipole which corresponds to one of the key factors of climate dynamics in south america during spring and summer

dating the growth of oceanic crust at a slow spreading ridge nineteen uranium lead zircon ages of lower crustal gabbros from atlantis bank southwest indian ridge constrain the growth and construction of oceanic crust at this slow spreading midocean ridge approximately 75 of the gabbros accreted within error of the predicted seafloor magnetic age whereas 25 are significantly older these anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or more likely crystallization of older gabbros at depths of 5 to 18 kilometers below the base of crust in the cold axial lithosphere which were uplifted and intruded by shallow level magmas during the creation of atlantis bank

small and large scale soft sediment deformations in a triassic lacustrine delta caused by overloading and seismicity in the ordos basin central china abstract the instability of an upper triassic lacustrine deltaic system in the southwestern ordos basin central china is examined based mainly on detailed measurements and descriptions of core data nine lithofacies associations are identified including undeformed and deformed channel deposits sandy and muddy slumps sandy debrites sandy dykes undeformed and deformed fringe silty deposits and muddy deposits quantification of the vertical and horizontal spatial distribution of these sediments reveals the deformed sediments can comprise more than one third of the entire cored interval in more than half of the wells indicating the significance of soft sediment deformation structures as essential components of this deltaic system a depositional model is constructed which highlights the location of deformed and remobilized sediments and their relationships with undeformed sediments in the proximal part of the delta the undeformed and deformed channel deposits and sandy slumps dominate the sandy sediments which are interbedded to form thick amalgamated sandstones in areas distant from these massive sandstones undeformed and deformed fringe fine grained deposits are well developed which could generate muddy slumps in the distal part of the delta besides undeformed and deformed fringe fine grained deposits sandy debrites are present as isolated sandbodies large scale sandy dykes formed between thick sandstones and adjacent mudstones two possible trigger mechanisms for initial deformation development are identified sediment overloading induced by sudden deposition of remobilized sediments and seismicity which caused large scale remobilization in the study area the high percentage of soft sediment deformation structures in this delta influences the reservoir heterogeneity of the three types of sandstones undeformed channel deposits sandy slumps and sandy debrites porosity permeability analysis reveals that reservoir quality of these three types varies significantly and thus distinguishing deformed sediments from undeformed deltaic sediments is essential for hydrocarbon development in this area

environmental change in the great plains an isotopic record from fossil horses abstract carbon and oxygen isotope ratios of fossil horse tooth enamel from nebraska and texas show evidence for late neogene environmental changes in the great plains the earliest unambiguous c4 dietary signal among texas equids coincides with the age of the classic late hemphillian age coffee ranch fauna which we suggest is 6 6 ma based on volcanic ash correlations c4 vegetation was present in the diets of a small fraction of late hemphillian equids in nebraska and was thereafter ordinary in the diets of both nebraska and texas equids there is no unequivocal evidence for abundant c4 vegetation in the diets of pre late hemphillian equids and we suggest that the ensuing dietary change reflects c4 biomass expansion in the latest miocene carbon isotope ratios of post hemphillian horses in nebraska can be divided into two statistically distinct populations on the basis of whether tortoise remains co occur with horse remains indicating that the two proxies carbon isotopes and presence absence of tort

evidence for initial excess 231pa in mid ocean ridge zircons abstract a limiting factor in the accuracy and precision of u pb zircon dates is accurate correction for initial disequilibrium in the 238u and 235u decay chains the longest lived and therefore most abundant intermediate daughter product in the 235u isotopic decay chain is 231pa t1 2 32 71 ka and the partitioning behavior of pa in zircon is not well constrained here we report high precision thermal ionization mass spectrometry tims u pb zircon data from two samples from ocean drilling program odp hole 735b which show evidence for incorporation of excess 231pa during zircon crystallization the most precise analyses from the two samples have consistent th corrected 206pb 238u dates with weighted means of 11 9325 0 0039 ma n 9 and 11 920 0 011 ma n 4 but distinctly older 207pb 235u dates that vary from 12 330 0 048 ma to 12 140 0 044 ma and 12 03 0 24 to 12 40 0 27 ma respectively if the excess 207pb is due to variable initial excess 231pa calculated initial 231pa 235u activity ratios for the two samples range from 5 6 1 0 to 9 6 1 1 and 3 5 5 2 to 11 4 5 8 the data from the more precisely dated sample yields estimated dpazircon duzircon from 2 2 3 8 and 5 6 9 6 assuming 231pa 235u of the melt equal to the global average of recently erupted mid ocean ridge basaltic glasses or secular equilibrium respectively high precision id tims analyses from nine additional samples from hole 735b and nearby hole 1105a suggest similar partitioning the lower range of dpazircon duzircon is consistent with ion microprobe measurements of 231pa in zircons from holocene and pleistocene rhyolitic eruptions schmitt 2007 schmitt 2011 the data suggest that 231pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions and excess initial 231pa may be more common in zircons than acknowledged the degree of initial disequilibrium in the 235u decay chain suggested by the data from this study and other recent high precision datasets leads to resolvable discordance in high precision dates of cenozoic to mesozoic zircons minor discordance in zircons of this age may therefore reflect initial excess 231pa and does not require either inheritance or pb loss

seismic analysis of the axial magma chamber reflector along the southern east pacific rise from conventional reflection profiling the thickness and internal properties of the magma sill located at the top of the axial magma chamber amc along the southern east pacific rise epr have been investigated through a combination of waveform modeling the near vertical incidence reflections from this body and analysis of reflection amplitude variation as a function of source receiver offset or slowness our results show that the amc reflector observed along the southern epr is best modeled by a thin 100 m thick sill of partial melt vs 0 km s sandwiched between higher velocity material and that the thickest sills are generally associated with the lowest p and s wave velocities the comparatively high p wave velocities and nonzero shear wave velocities inferred for this sill indicate that it is filled with partially molten magma which in some locations has a high crystal content this may have important implications for eruption mechanisms and along axis mixing of magma at the epr there is no simple relationship between morphologic indicators of magma supply e g axial depth or volume and sill thickness depth or velocity magma sill properties may be closely tied to the eruption and replenishment cycle of the amc and thus may vary on a much shorter spatial and temporal scale than axial morphology which reflects longer term variations in magma supply to the ridge

the petrology geochronology and tectono magmatic setting of igneous rocks in the suckling dayman metamorphic core complex papua new guinea abstract southeastern papua new guinea has hosted magmatism throughout the cenozoic with the latest phase being concurrent with active extension in the woodlark rift there the suckling dayman metamorphic core complex sdmcc has exhumed middle crustal rocks in the past few million years along a still active low angle normal fault the mai iu fault uplift of the sdmcc has exposed metasedimentary and metaigneous rocks of late cretaceous pleistocene age that record an evolution from oceanic spreading to subduction to collision and finally to subduction inversion and extensional exhumation we present new petrographic whole rock geochemical and geochronologic zircon u pb la icp ms data from the sdmcc to reconstruct its long term evolution the dominant footwall lithology of the sdmcc the goropu metabasalt has a morb composition detrital zircons from metasedimentary beds intercalated with these basalts yield u pb based maximum estimates for deposition of 103 and 72 ma suggesting a late cretaceous spreading age of the oceanic protolith bulk compositions of low grade tholeiitic meta gabbroic and tonalitic rocks yau igneous complex that intrude the goropu metabasalt reveal both enriched and depleted light rare earth element patterns zircon u pb ages from the yau igneous complex range between 60 and 57 ma providing a minimum age for the formation of the goropu metabasalt much younger syn extensional granitoids in the mafic footwall of the sdmcc have calc alkaline to high k compositions and u pb ages on zircon between 3 7 and 2 0 ma our data indicate that the mai iu fault had re activated a paleogene thrust as an extensional detachment fault by 3 7 0 2 ma u pb ages of detrital zircons in modern streams draining the footwall of the sdmcc essentially reflect the ages revealed by the paleocene and plio pleistocene intrusions xenocrystic zircons in the plio pleistocene granitoids imply that the crust underlying the metabasaltic carapace of the sdmcc consists chiefly of australian continent derived sedimentary rocks

climate pco2 and terrestrial carbon cycle linkages during late palaeozoic glacial interglacial cycles the late palaeozoic was marked by a series of glacial interglacial cycles geochemical and fossil data suggest a role for terrestrial vegetation carbon cycle feedbacks in the climate response to orbital forcing

a fossil melt lens in the oman ophiolite implications for magma chamber processes at fast spreading ridges from seismic evidence we know that a small body of magma is present at mid crustal levels beneath most fast spreading ridges and that this small sill or melt lens overlies a much broader area of hot but largely solid material a crystal mush that constitutes the lower ocean crust beyond this however we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens nor the role the melt lens plays in the storage and aggregation of mid ocean ridge basalts morb we here offer constraints on the processes of melt transport modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the oman ophiolite concentrating on the uppermost plutonic rocks and transition into sheeted dykes we find that almost the entire oman plutonic suite the layered gabbro and most of the high level gabbro is dominated by cumulate rocks containing very low proportions of trapped interstitial melt the exception to this is the very uppermost part of the section within 150 metres of the base of the sheeted dyke complex at which level the gabbros instead display a marked heterogeneity of structure texture and composition and plutonic rocks of generally basaltic composition occur for the first time associated pegmatitic ferrogabbros formed by in situ fractionation of these liquids this horizon passes up into a more homogeneous microgabbroic facies also of basaltic composition within which isolated asymmetric doleritic chilled margins are found and thence into doleritic sheeted dykes we argue that the thin heterogeneous gabbro horizon is a fossilised melt lens containing pooled liquids of net basaltic composition mg of 65 expelled from the crystal mush beneath existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens but this is not supported by the field relationships instead we propose that the fabrics record the buoyant ascent of magma through the mush pile probably in part by porous flow and reflect crystallisation essentially in situ without significant vertical transport of the mush low residual melt porosities in the sub melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust

seismic structure and indicators of magma budget along the southern east pacific rise in this paper we re examine the relationship between seismically constrained variations in crustal structure along the southern east pacific rise sepr and the segment scale variations in axial depth morphology basalt geochemistry and hydrothermal activity that have often been attributed to along axis differences in the supply of magma to the mid ocean ridge along 800 km of the fast spreading sepr good correlations exist between axial depth ridge cross sectional area mantle bouguer anomaly and the mgo weight percent of basalts recovered from the rise axis these correlations indicate along axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of 100 000 years in contrast we show that the depth and width of the midcrustal magma sill the thickness of seismic layer 2a and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross sectional area we suggest that the emplacement geometry width of the intrusion zone and flow lengths not magma supply controls extrusive layer seismic layer 2a thickness we hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events dike intrusion eruptions faulting which occur on much shorter timescales 10 100 years than the longer term variations in magma supply reflected in along axis changes in the shape and depth of the ridge axis

inclusion of the weathering of volcanic rocks in the geocarbsulf model the carbon cycle model geocarbsulf is extended by dividing the weathering of silicates into volcanic and non volcanic rocks the proportion of volcanic weathering is calculated as a function of time from the oceanic record of 87sr 86sr the volcanic proportion is then used to modify the equations for calculating atmospheric co2 by the addition of a new non dimensional volcanic weathering factor the effect of uplift and physical erosion on weathering is also modified by using only the distribution over time of the abundance of sandstones and shales and not sr isotopic data that had been used previously results indicate moderate change from geocarbsulf in the distribution of atmospheric co2 over phanerozoic time this includes an increased minimum in co2 during the late ordovician in agreement with the presence of a continental glaciation at that time and a shift of maximum mesozoic values from the jurassic to the early cretaceous

paleoclimate and paleovegetation of lower narmada basin gujarat western india inferred from stable carbon and oxygen isotopes abstract surface sediment samples depth 0 160 cm and a paleosol depth 10 15 m both part of a sediment profile exposed in a cliff section of narmada river western india were analyzed for stable isotopes 13 c and 18 o three organic carbon samples from top 160 cm and two paleosol samples from top and bottom of the paleosol were dated by the radiocarbon method 13 c of both soil carbonates and soil organic carbon fall in the range of mixed c3 and c4 vegetation except for the paleosol which shows differences attributable to the contribution of bedrock carbonate during calcite formation the vegetation was dominantly c3 in the late pleistocene from 15 10 ka bp which shifted to mixed c3 c4 during the holocene enhanced monsoon was observed at 12 5 and 1 6 ka bp the presence of c4 grasses in the late holocene is detectable the present day vegetation is dominantly c3 type 13 c 25 4 probably due to anthropogenic activity where grasslands c4 have been replaced by c3 vegetation recently last 50 years the enhanced atmospheric co 2 concentration in recent times could be a probable agent for the current dominance of c3 vegetation

protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole u1309d 30 n mar sensitive high resolution ion microprobe shrimp u pb zircon ages of 24 samples from oceanic crust recovered in integrated ocean drilling program iodp hole u1309d and from the surface of atlantis massif mid atlantic ridge mar 30 n document a protracted history of accretion in the footwall to an oceanic detachment fault ages for 18 samples of evolved fe ti oxide gabbro and felsic dikes collected 40 1415 m below seafloor in u1309d yield a weighted mean of 1 20 0 03 ma mean square of weighted deviates 7 1 however the ages range from 1 08 0 07 ma and 1 28 0 05 ma indicating crustal construction occurred over a minimum of 100 200 ka the zircon ages along with petrologic observations indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka the oldest ages are observed below 600 mbsf an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault the data are most consistent with a multiple sill model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1 4 km zircon ages from broadly spaced samples collected along the southern ridge of atlantis massif yield a detachment fault slip rate of 28 7 6 7 mm a and imply significant asymmetric plate spreading up to 100 on the north american plate for at least 200 ka during core complex formation

atmospheric carbon dioxide linked with mesozoic and early cenozoic climate change carbon isotopes of fossil plants and model simulations suggest that atmospheric carbon dioxide levels were variable during the period 200 to 60 million years ago the large decreases in the partial pressure of co2 coincide with glaciations providing evidence against climate co2 decoupling during the mesozoic

mesoscale convective systems over southeastern south america and their relationship with the south american low level jet abstract prior studies have shown that the low level jet is a recurrent characteristic of the environment during the initiation and mature stages of mesoscale convective systems mcss over the great plains of the united states the south american low level jet sallj over southeastern south america sesa has an analogous role advecting heat and moisture from the amazon basin southward into the central plains of southeastern south america generating ideal environmental conditions for convection initiation and growth into mcss this research has two purposes one is to describe the characteristics of a 3 yr mcs sample in south america south of the equator and its related geographical distribution of convection frequency the other is to advance the knowledge of the evolution of favorable environmental conditions for the development of large mcss specifically those that mature under sallj conditions high horizontal and temporal resolution satellite images are used to detect mcss in the area for the

holocene changes in eastern tropical pacific climate inferred from a galapagos lake sediment record abstract paleoclimate records from the tropical pacific suggest the early to mid holocene was a period of reduced el nino southern oscillation enso variability with a transition to modern increased enso frequency occurring some time in the last few thousand years however the nature and timing of this shift remains uncertain due to the discontinuous nature and or coarse resolution of many enso proxies as well as a lack of agreement between previously published records a new continuous climate record from el junco crater lake in the galapagos islands reveals several abrupt changes in lake level and precipitation through the holocene hydroclimatic model simulations suggest that el junco lake level responds sensitively to increases in precipitation associated with el nino events rising during wet el nino events and falling during the intervening dry periods grain size data from el junco sediment cores indicate past lake level variability likely associated with changing seasonal precipitation and enso frequency the grain size data suggest increased precipitation intensity prior to 9000 120 cal years bp and after 4200 130 cal years bp as well as a two step increase in precipitation at 3200 160 and 2000 100 cal years bp maximum holocene precipitation and inferred enso variability occurred between 2000 100 and 1500 70 cal years bp during the same period that six other independent proxy records suggest higher enso frequency and longer stronger el nino events decreasing sediment carbon nitrogen c n ratios in el junco sediments indicate rising lake levels from the early holocene to present corroborating the grain size data the inferred increase in precipitation at 4200 130 cal years bp and at 2000 100 cal years bp coincides with decreasing southwest asian and east asian monsoon intensity suggesting tropical pacific climate and the asian monsoon were interconnected systems at centennial to millennial timescales during the holocene a weakening trend in the asian monsoon and the trend toward wetter conditions at el junco also coincide with a trend toward cooler and drier conditions inferred from cariaco basin sediment proxies from the mid holocene to present suggesting the migration of the intertropical convergence zone itcz likely influenced hydrological changes in both the eastern tropical pacific and the asian monsoon region during the holocene

evaluating the temporal link between the karoo lip and climatic biologic events of the toarcian stage with high precision u pb geochronology the karoo large igneous province k lip is considered to have substantially influenced global warming marine anoxia and concomitant marine extinction in the toarcian lower jurassic the sequence and duration of these events has been uncertain thus precluding direct comparison between sedimentary successions and the k lip two relatively narrow time intervals of the early toarcian one approximately within the lower portion of the h falciferum ammonite zone and the other near the pliensbachian toarcian boundary pl to b have received much attention because they are associated with carbon isotope excursions cies and marine extinctions potential causal mechanisms have been proposed such as the release of massive amounts of carbon to the atmosphere via a catastrophic magmatic intrusive episode related to the k lip here we test the timing with four new high precision ca id tims u pb zircon ages from the toarcian stage collected from tephra interstratified with ammonite bearing carbonates in southernmost peru we also present new u pb zircon and baddeleyite data from three sills and one rhyolite of the k lip in south africa our new data permit a substantial reinterpretation of the toarcian timescale the pliensbachian toarcian boundary pl to b a prominent global extinction level must be older than 183 5 ma on the basis of our oldest tephra the h falciferum cie equivalent to the so called toarcian oceanic anoxic event is bracketed by two ash fall bed dates between 183 22 0 26183 22 0 26 and 181 99 0 13 ma181 99 0 13 ma all 2 analytical and tracer errors the cie duration is estimated at approximately 300 ka on the basis of minimum error interpolation of ammonite zone boundaries the base of the h falciferum cie appears to correlate in time with the onset k lip sill emplacement dated by the oldest sill at 183 014 0 072 ma183 014 0 072 ma however sill emplacement appears to have lasted for a minimum of 2 million years at least until 181 31 0 19 ma181 31 0 19 ma which is in disagreement with a short duration of the h falciferum negative cie furthermore our data suggest that the timing of the main volume of the k lip may have been approximately 4 5 ma this prolonged and possibly pulsed magmatic activity of the k lip may be correlated with extinction pulses and climate changes in the middle and late toarcian stage each event occurring within a unique set of global environmental conditions

evidence and implications of crustal magmatism on the flanks of the east pacific rise article i nfo article history formation of oceanic crust along the east pacific rise is thought to be complete within a few kilometers of the rise axis here however we present evidence for magmatism 20 km from the spreading center in 300 ka old crust seismic data reveal an intrusive complex 2 km beneath the seafloor which is limited in lateral extent b5 km and comprises a melt lens underlain by low velocity high attenuation crust the crustal magma body overlies a region of off axis delivery of mantle melt that results from a long lived skew between the axes of mantle upwelling and plate spreading magmatism on the flanks of the epr may be commonplace thus affecting the architecture of oceanic crust and providing the necessary conditions to drive off axis hydrothermal activity

in situ enrichment of ocean crust microbes on igneous minerals and glasses using an osmotic flow through device the integrated ocean drilling program iodp hole 1301a on the eastern flank of juan de fuca ridge was used in the first long term deployment of microbial enrichment flow cells using osmotically driven pumps in a subseafloor borehole three novel osmotically driven colonization systems with unidirectional flow were deployed in the borehole and incubated for 4 years to determine the microbial colonization preferences for 12 minerals and glasses present in igneous rocks following recovery of the colonization systems we measured cell density on the minerals and glasses by fluorescent staining and direct counting and found some significant differences between mineral samples we also determined the abundance of mesophilic and thermophilic culturable organotrophs grown on marine r2a medium and identified isolates by partial 16s or 18s rdna sequencing we found that nine distinct phylotypes of culturable mesophilic oligotrophs were present on the minerals and glasses and that eight of the nine can reduce nitrate and oxidize iron fe ii rich olivine minerals had the highest density of total countable cells and culturable organotrophic mesophiles as well as the only culturable organotrophic thermophiles these results suggest that olivine a common igneous mineral in seawater recharged ocean crust is capable of supporting microbial communities that iron oxidation and nitrate reduction may be important physiological characteristics of ocean crust microbes and that heterogeneously distributed minerals in marine igneous rocks likely influence the distribution of microbial communities in the ocean crust

soil co2 efflux simulations using monte carlo method and implications for recording paleo atmospheric pco2 in pedogenic gibbsite abstract monte carlo simulation testing of the sensitivity of the fickian diffusion model used for paleo pco 2 proxies indicates that assuming annual average soil respiration rate results in over estimation of pco 2 in all cases the natural variability of the factors of the model including soil porosity and inverse tortuosity and the 13 c value of the soil produced co 2 were not able to account for discrepancies between measured 13 c values from a modern soil and modeled results using established assumptions assuming that there is negligible carbon isotope fractionation during the occlusion of co 2 in pedogenic gibbsite the flaw in the model appears to be the assumed value of soil respiration rate if correct the implication is that previous paleo pco 2 estimates from soil hydroxides may be too high by as much as a factor of ten

annually resolved ice core records of tropical climate variability over the past 1800 years ice cores from low latitudes can provide a wealth of unique information about past climate in the tropics but they are difficult to recover and few exist here we report annually resolved ice core records from the quelccaya ice cap 5670 meters above sea level in peru that extend back 1800 years and provide a high resolution record of climate variability there oxygen isotopic ratios 18 o are linked to sea surface temperatures in the tropical eastern pacific whereas concentrations of ammonium and nitrate document the dominant role played by the migration of the intertropical convergence zone in the region of the tropical andes quelccaya continues to retreat and thin radiocarbon dates on wetland plants exposed along its retreating margins indicate that it has not been smaller for at least six millennia

imaging bed geometry and architecture of massive sandstones in the fontanelice channels italian apennines using new digiscoping techniques abstract in this study we present digital images and sedimentological data from a channel fill succession in the italian apennines that is dominated by massive sandstones although the studied outcrop is largely inaccessible valuable data have now been obtained using the new technique of digiscoping which allows features of about 75 80 of the channel fill is composed of massive sandstone beds 1 m thick with overall sandstone shale ratios of 9 1 massive sandstones are poorly sorted and overall show little or no normal grading they are commonly amalgamated and always have sharp bed tops massive sandstone beds show abrupt pinch outs at the channel margin whereas overlying thin bedded siltstone mudstone layers taper gradually and drape up the margin more extensively this suggests that the depositing flows were stratified into a lower thin hyper concentrated density flow and an upper more dilute turbidity current in summary the digiscoping technique is shown to be a cheap and efficient method for imaging distant and or inaccessible outcrops and providing information on bed geometry and architecture

an algorithm for u pb isotope dilution data reduction and uncertainty propagation this is the publisher s version also available electronically from http onlinelibrary wiley com