precise ages of the reunion event and huckleberry ridge excursion episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core the reunion event is one of the earliest recognized periods of normal polarity within the reversed matuyama chron named for the site at which it was first discovered on reunion island it has since purportedly been found globally in both volcanic rocks and sediments and thus has become a key chronostratigraphic marker however geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration but also the number of reunion events new 40ar 39ar ages from eight reunion island lavas in three distinct sections are indistinguishable from one another thereby placing the event at 2 200 0 007 0 010 ma2 200 0 007 0 010 ma 2 2 analytical total uncertainty note this format is used throughout the paper the paleomagnetic behavior recorded at two of the island sites shows that the opposite normal polarity was reached and sustained for a period during which several lava flows were erupted whether this can be classified as a very short subchron bounded by a rapid set of back to back reversals or as a special case of a geomagnetic excursion is unclear hence we choose to continue labeling the dynamo activity recorded by these reunion island lavas as an event this event preceded a 38 kyr 38 kyr period of normal polarity that we name the feni subchron after its locality of discovery at odp site 981 the feni subchron was succeeded by the huckleberry ridge excursion for which 40ar 39ar sanidine and u pb zircon ages of 2 077 0 001 0 003 ma2 077 0 001 0 003 ma and 2 084 0 012 0 013 ma2 084 0 012 0 013 ma respectively from member b of the huckleberry ridge tuff in idaho are in agreement these findings suggest that the full normal polarity recorded on reunion island is a singular brief period of unstable field behavior at the onset of a 125 kyr 125 kyr bundling of dynamo instabilities from 2 20 to 2 07 ma disturbances to the axial dipole component of earth s magnetic field during this period and by analogy similar periods of temporally clustered excursions during the early and late portions of the brunhes chron may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner core or core mantle boundaries a proposition that might be tested by future numerical dynamo simulations

marine derived 14c calibration and activity record for the past 50 000 years updated from the cariaco basin abstract an expanded cariaco basin 14c chronology is tied to 230th dated hulu cave speleothem records in order to provide detailed marine based 14c calibration for the past 50 000 years the revised high resolution cariaco 14c calibration record agrees well with data from 230th dated fossil corals back to 33 ka with continued agreement despite increased scatter back to 50 ka suggesting that the record provides accurate calibration back to the limits of radiocarbon dating the calibration data document highly elevated 14c during the glacial period carbon cycle box model simulations show that the majority of observed 14c change can be explained by increased 14c production however from 45 to 15 ka 14c remains anomalously high indicating that the distribution of radiocarbon between surface and deep ocean reservoirs was different than it is today additional observations of the magnitude spatial extent and timing of deep ocean 14c shifts are critical for a complete understanding of observed glacial 14c variability

chronostratigraphy of the 600 000 year old continental record of lake van turkey lake van sediment cores from the ahlat ridge and northern basin drill sites of the icdp project paleovan contain a wealth of information about past environmental processes the sedimentary sequence was dated using climatostratigraphic alignment varve chronology tephrostratigraphy argon argon single crystal dating radiocarbon dating magnetostratigraphy and cosmogenic nuclides based on the lithostratigraphic framework the different age constraints are compiled and a robust and precise chronology of the 600 000 year old lake van record is constructed proxy records of total organic carbon content and sediment color together with the calcium potassium ratios and arboreal pollen percentages of the 174 meter long ahlat ridge record mimic the greenland isotope stratotype ngrip therefore the proxy records are systematically aligned to the onsets of interstadials reflected in the ngrip or synthesized greenland ice core stratigraphy the chronology is constructed using 27 age control points derived from visual synchronization with the gicc05 timescale an absolutely dated speleothem record e g hulu sanbao linzhu cave and the epica dome c timescale in addition the uppermost part of the sequence is complemented with four ages from holocene varve chronology and two calibrated radiocarbon ages furthermore nine argon argon ages and a comparison of the relative paleointensity record of the magnetic field with reference curve piso 1500 confirm the accuracy of the age model also the identification of the laschamp event via measurements of 10be in the sediment confirms the presented age model the chronology of the ahlat ridge record is transferred to the 79 meter long event corrected composite record from the northern basin and supplemented by additional radiocarbon dating on organic marco remains the basal age of the northern basin record is estimated at 90 ka the variations of the time series of total organic carbon content the ca k ratio and the arboreal pollen percentages illustrate that the presented chronology and paleoclimate data are suited for reconstructions and modeling of the quaternary and pleistocene climate evolution in the near east at millennial timescales furthermore the chronology of the last 250 kyr can be used to test other dating techniques

discriminating assimilants and decoupling deep vs shallow level crystal records at mount adams using 238u 230th disequilibria and os isotopes article a suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the mount adams volcanic field has been analyzed for u th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis all of the lavas have 230 th 238 u n1 and span a large range in 230 th 232 th ratios

atmospheric radiocarbon calibration to 45 000 yr b p late glacial fluctuations and cosmogenic isotope production more than 250 carbon 14 accelerator mass spectrometry dates of terrestrial macrofossils from annually laminated sediments from lake suigetsu japan provide a first atmospheric calibration for almost the total range of the radiocarbon method 45 000 years before the present the results confirm the recently revised floating german pine chronology and are consistent with data from european and marine varved sediments and combined uranium thorium and carbon 14 dating of corals up to the last glacial maximum the data during the glacial show large fluctuations in the atmospheric carbon 14 content related to changes in global environment and in cosmogenic isotope production

loess stratigraphy using palaeomagnetism application to the poiana ciresului archaeological site romania abstract palaeomagnetic investigations were carried out at the archaeological site of poiana ciresului romania to check whether the loess records a reliable palaeomagnetic signal and to compare this signal to reference data the sediments are composed of loess and loess like sediments pedogenic alterations and a pronounced cultural layer occur in the section the archaeology bearing sedimentary geoarchive spans the time interval from ca 20 to ca 55 ka and was dated mainly by 14c and luminescence techniques the record of geomagnetic palaeosecular variation from this european loess site is discussed the presented palaeomagnetic record bears analogies to data from lac du bouchet france and is a reliable high resolution record mean sedimentation rates are ca 20 cm ky of palaeosecular variation the results imply that european loess geoarchives may record palaeosecular variation features even if loess is pedogenically altered the palaeosecular variation pattern of loess is here employed as a stratigraphic tool

evolution of ocean island rifts the northeast rift zone of tenerife canary islands the northeast rift zone of tenerife presents a superb opportunity to study the entire cycle of activity of an oceanic rift zone field geology isotopic dating and magnetic stratigraphy provide a reliable temporal and spatial framework for the evolution of the ne rift zone which includes a period of very fast growth toward instability between ca 1 1 and 0 83 ma followed by three successive large landslides the micheque and guimar collapses which occurred approximately contemporaneously at ca 830 ka and on either side of the rift and the la orotava landslide between 690 10 and 566 13 ka our observations suggest that canarian rift zones show similar patterns of development which often includes overgrowth instability and lateral collapses collapses of the rift flanks disrupt established fissural feeding systems favoring magma ascent and shallow emplacement which in turn leads to magma differentiation and intermediate to felsic nested eruptions rifts and their collapses may therefore act as an important factor in providing architectural and petrological variability to oceanic volcanoes conversely the presence of substantial felsic volcanism in rift settings may indicate the presence of earlier landslide scars even if concealed by postcollapse volcanism comparative analysis of the main rifts in the canary islands outlines this general evolutionary pattern 1 growth of an increasingly high and steep ridge by concentrated basaltic fissure eruptions 2 flank collapse and catastrophic disruption of the established feeder system of the rift 3 postcollapse centralized nested volcanism commonly evolving from initially ultramafic mafic to terminal felsic compositions trachytes phonolites and 4 progressive decline of nested eruptive activity

challenges in c 14 dating towards the limit of the method inferred from anchoring a floating tree ring radiocarbon chronology to ice core records around the laschamp geomagnetic field minimum the c 14 dating method is the cornerstone for inferring age estimates for natural archives covering the last 50 000 yrs however c 14 age calibration for the last ice age relies mostly on records that only indirectly reflect the atmospheric c 14 concentrations in consequence calendar age estimates are significantly more uncertain for the period of the last ice age compared to the past 14000 yrs where tree ring based calibration records exist here we connect a c 14 tree ring chronology from kauri trees in new zealand to ice core be 10 records via the common signal in the galactic cosmic ray flux around the period of the laschamp geomagnetic field minimum ca 41 000 yrs bp synchronous changes of modelled c 14 and c 14 inferred from u th dated speleothems support the ice core chronology independently and suggest that the published ice core time scale errors are rather conservative for this period our analysis puts c 14 age determinations directly into the context of ice core climate records and it shows that the c 14 records underlying the c 14 calibration curve overestimate the atmospheric c 14 concentration by more than 200 parts per thousand consequently c 14 age calibration presently yields too old calendar age estimates by about 1200 yrs for this period c 2014 elsevier b v all rights reserved less

late brunhes polarity excursions mono lake laschamp iceland basin and pringle falls recorded at odp site 919 irminger basin abstract component natural remanent magnetizations derived from u channel and 1 cm3 discrete samples from odp site 919 irminger basin indicate the existence of four intervals of negative inclinations in the upper brunhes chronozone according to the age model based on planktic oxygen isotope data these excursional intervals occur in sediments deposited during the following time intervals 32 34 ka 39 41 ka 180 188 ka and 205 225 ka these time intervals correspond to polarity excursions detected elsewhere known as mono lake laschamp iceland basin and pringle falls the isotope based age model is supported by the normalized remanence paleointensity record that can be correlated to other calibrated paleointensity records for the 0 500 ka interval such as that from odp site 983 for the intervals associated with the mono lake and laschamp excursions virtual geomagnetic poles vgps reach equatorial latitudes and mid southerly latitudes respectively for intervals associated with the iceland basin and pringle falls excursions repeated excursions of vgps to high southerly latitudes indicate rapid directional swings rather than a single short lived polarity reversal the directional instability associated with polarity excursions is not often recorded probably due to smoothing of the sedimentary record by the process of detrital remanence drm acquisition

millennial scale variations in hydrography and biogeochemistry in the eastern equatorial pacific over the last 100 kyr abstract in this study we use records of nitrogen isotope ratios 15n uk 37 temperature estimates organic carbon and opal percentages from high resolution sediment cores located in the eastern equatorial pacific eep to explore the mechanisms linking millennial scale changes in low latitude sea surface temperature water column denitrification and surface productivity to the timing of northern or southern polar climate during the last 100 000 yr our results support a hypothesis that the southern hemisphere and its connection to the low latitudes via shallow subsurface ocean circulation has a primary influence on the biogeochemistry of the eep in addition our results suggest that during the last glacial stage denitrification rates fluctuated on millennial timescales in response to water column ventilation rather than upstream oxidant demand in intermediate depth waters however due to the poor age constraints available for marine isotopic stage mis 3 the eep sedimentary data presented here could support two conflicting mechanisms one driven by enhanced intermediate overturning circulation in the southern ocean during heinrich events antarctic warm events implying that subsurface flow rates control thermocline ventilation and a second one consistent with more sluggish intermediate circulation during antarctic warm events and giving a central role to the temperature control on oxygen solubility in southern ocean surface waters

extending the radiocarbon calibration beyond 26 000 years before present using fossil corals fossil coral is an excellent archive to extend the radiocarbon 14c calibration beyond tree ring records however published coral data older than 26 000 years before present are too disparate for practical calibration we propose an explanation for these discrepancies trace amounts of secondary calcite and organic matter in fossil corals shift the 14c ages toward younger ages in a series of acid leaching experiments to simulate the standard cleaning procedure prior to 14c analysis we measure up to a 300 enrichment of diagenetic calcite in corals due to the solubility differences between calcite and aragonite in model calculations we show the 14c age offsets produced by acid leaching could be hundreds to thousands of years when typical amounts of calcite are present in samples we demonstrate the necessity and our ability to detect 0 2 calcite in aragonite by x ray diffraction and apply 0 2 calcite as our a priori criterion for our coral samples the estimated age offsets of samples with 0 2 calcite fall within the reported analytical uncertainty of 14c dating in addition we present 14c results from an extended hydrogen peroxide h2o2 pretreatment for coral samples to remove organic matter our data show that treated samples yield consistent or older 14c dates compared to non treated samples these rigorous but necessary screening and cleaning techniques provide precise reproducible and accurate radiocarbon calibration data

chlorine 36 evidence for the mono lake event in the summit grip ice core abstract a distinct peak has been discovered in the 36 cl data from the grip ice core between the dansgaard oeschger d o events 6 and 7 at approximately 32 kyr bp this peak can be attributed to a minimum of the geomagnetic dipole field associated with the mono lake event since the 36 cl peak reflects a higher production rate of all cosmogenic radionuclides it has an impact on the 14 c dating of the last ice age furthermore it provides an additional time marker similar to a peak found earlier corresponding to the laschamp event at approximately 39 kyr bp

a better radiocarbon clock reliable radiocarbon dating is essential in many fields from geology and archaeology to geochemistry and paleoclimatology but because the atmosphericc c ratio has changed over time obtaining accurate dating is difficult in their perspective bard et al discuss efforts to improve the accuracy of radiocarbon dating presented at a recent conference in new zealand stratigraphic results from ocean sediments off the coast of venezuela reported by hughen et al were presented at this meeting and are put into context by bard et al taken together these and other findings give hope that reliable radiocarbon calibration curves will be ready for the next such meeting in 2006

origin and age of the directions recorded during the laschamp event in the chaine des puys france abstract excursions of the geomagnetic field are likely even more frequent than reversals and thus must be seen as a major characteristic of the geodynamo the laschamp event discovered in lava flows of massif central france is the youngest and the most studied field excursion its geomagnetic origin has been controversial due to the existence of self reversal processes taking advantage of new dated sites the initial aim of this work was to find additional flows with intermediate or reverse directions but also to investigate the magnetic properties of normal polarity flows surrounding the event this study combines thermal and alternating field demagnetization of 272 samples from 21 units including 12 new localities and 12 k ar datings of main units no new site with intermediate or reverse polarity was found ten sites have a normal polarity and all sites studied at olby louchadiere and royat display intermediate but scattered directions magnetic mineralogy is characterized by primary titanomagnetite variable amounts of titanomaghemite and almost pure magnetite we confirmed that reverse polarity flows are affected by self reversals but we found that this is also the case for normal flows a direct consequence is that self reversals cannot be taken as responsible for the reverse directions but they likely contribute to generate dispersion however the overall coherency of the directions indicates that this process was limited and despite complex magnetization the geomagnetic origin of the laschamp in the chaine des puys is not questioned all volcanic pole positions vgps published so far show significant scatter but they remain consistent with each other interestingly they do not coincide with the longitudinal loops seen in the sedimentary records and are compatible with a dominant non dipolar field geometry new k ar datings provide a coherent chronology of the successive polarities the 37 ka old reverse directions of the olby flow are chronologically consistent with the 41 9 kyr old normal polarity flows preceding the event the royat flow is only marginally intermediate and indicates that the end of the event was not younger than 33 3 ka in the chaine des puys

beryllium 10 in the greenland ice core project ice core at summit greenland concentrations of the cosmogenic isotope 10be have been measured in more than 1350 samples from the greenland ice core project grip ice core drilled at summit greenland although a dust associated component of 10be retained by 0 45 m filters in some of the samples complicates the interpretations the results confirm that the first order origin of 10be concentration variations is changes in precipitation rate associated with different climate regimes this effect is seen not only between glacial and interglacial periods but also during the shorter dansgaard oeschger interstadials by contrast the 10be data do not support the interpretation of rapidly varying accumulation i e climate during the last interglacial they can however be used to help place limits on the origin of the ice in these events after taking into account variable snow accumulation effects variations in the 10be flux are observed probably caused by solar and geomagnetic modulation but possibly also by primary cosmic ray variations the most dramatic is a 10be peak 40 000 years ago similar to that found in the vostok ice core thus permitting a very precise correlation between climate records from arctic and antarctic ice cores the 36cl 10be ratio considering either total or only ice associated 10be shows significant variability over the whole core depth thus confirming the difficulty in using this parameter for dating ice cores

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

earth s magnetic field is probably not reversing the geomagnetic field has been decaying at a rate of 5 per century from at least 1840 with indirect observations suggesting a decay since 1600 or even earlier this has led to the assertion that the geomagnetic field may be undergoing a reversal or an excursion we have derived a model of the geomagnetic field spanning 30 50 ka constructed to study the behavior of the two most recent excursions the laschamp and mono lake centered at 41 and 34 ka respectively here we show that neither excursion demonstrates field evolution similar to current changes in the geomagnetic field at earlier times centered at 49 and 46 ka the field is comparable to today s field with an intensity structure similar to today s south atlantic anomaly saa however neither of these saa like fields develop into an excursion or reversal this suggests that the current weakened field will also recover without an extreme event such as an excursion or reversal the saa like field structure at 46 ka appears to be coeval with published increases in geomagnetically modulated beryllium and chlorine nuclide production despite the global dipole field not weakening significantly in our model during this time this agreement suggests a greater complexity in the relationship between cosmogenic nuclide production and the geomagnetic field than is commonly assumed

laschamp and mono lake geomagnetic excursions recorded in new zealand eight basaltic lavas from the auckland volcanic field new zealand record three distinct sets of excursional geomagnetic field directions and low paleointensities however the timing and therefore paleomagnetic significance of these records have been poorly understood radiocarbon k ar and thermoluminescence dating constrain these lavas to have erupted during the last 75 ka a period during which as many as three excursions have been recorded differentially at several northern and fewer southern hemisphere sites forty 40ar 39ar incremental heating experiments conducted on groundmass from seven of these excursional lavas indicate that they erupted during at least two periods at 39 1 4 1 and 31 6 1 8 ka coincident with 40ar 39ar astrochronologic and 14c ages determined for the laschamp and mono lake excursions respectively samples from a lava flow associated with a third cluster of virtual geomagnetic poles vgps are complicated by low concentrations of radiogenic argon in the presence of excess argon and thus yield discordant age spectra and an imprecise age of 26 6 8 1 ka our findings indicate that the laschamp and mono lake excursions until recently identified unequivocally and isotopically dated only in the northern hemisphere were globally synchronized at 40 1 and 32 1 ka however the vgps of lavas that record the laschamp excursion in new zealand and france are inconsistent with a simple clockwise looping geometry inferred from vgp paths obtained in eight marine sediment cores spanning the laschamp excursion we suggest that the differences in vgps recorded at the various sites are significant and may point to non axial dipole components and lower mantle control on transitional fields during short lived excursions

precise dating of dansgaard oeschger climate oscillations in western europe from stalagmite data the signature of dansgaard oeschger events millennial scale abrupt climate oscillations during the last glacial period is well established in ice cores and marine records but the effects of such events in continental settings are not as clear and their absolute chronology is uncertain beyond the limit of 14 c dating and annual layer counting for marine records and ice cores respectively here we present carbon and oxygen isotope records from a stalagmite collected in southwest france which have been precisely dated using 234u 230th ratios we find rapid climate oscillations coincident with the established dansgaard oeschger events between 83 000 and 32 000 years ago in both isotope records the oxygen isotope signature is similar to a record from soreq cave israel and deep sea records indicating the large spatial scale of the climate oscillations the signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western europe an important site in human cultural evolution we also find evidence for a long phase of extremely cold climate in southwest france between 61 2 0 6 and 67 4 0 9 kyr ago

geomagnetic field intensity north atlantic deep water circulation and atmospheric δ14c during the last 50 kyr we present simulated records of past changes in the atmospheric 14c for the last 50 kyr due to changes in geomagnetic field intensity and in the strength of the north atlantic deep water nadw a new geomagnetic record was used largely based on the napis 75 record laj et al phil trans r soc london a 358 2000 1009 1025 which has been extended for the 0 20 kyr interval using archeomagnetic and volcanic data past changes of the nadw were derived from a mineral magnetic study of the cores used in the construction of napis 75 two box models of different complexity 4 and 17 boxes were used to simulate the carbon cycle calculated records of 14c are consistent with experimental determinations for the last 24 kyr for older ages the records calculated with variable oceanic circulation conditions reach values as high as 600 with an average of 500 between 20 and 40 kyr with maxima around 21 30 and 38 kyr gisp2 age model while low values are observed prior to 42 kyr although large inconsistencies in experimental data preclude precise comparison the average record simulated with the 17 box model is overall consistent with the icelandic sea record voelker et al radiocarbon 40 1998 517 534 42 2000 437 452 except for the extremely high peak observed in this record at 40 5 kyr on the other hand the results recently reported from a stalagmite recovered from a submerged cave in the bahamas beck et al science 292 2001 2453 2458 are inconsistent with all our model simulations in the 20 45 kyr interval the improved geomagnetic record combined with the new nadw profile allows us to give a modeled evaluation of the relative contribution of these factors to changes in atmospheric 14c the average simulation provides a first order modeled correction for conventional radiocarbon ages older than 25 kyr for which no calibration curve is available as yet

paleointensity of the earth s magnetic field during the laschamp excursion and its geomagnetic implications the reversed paleomagnetic direction of the laschamp and olby flows represents a specific feature of the geomagnetic field this is supported by paleomagnetic evidence showing that the same anomalous direction was recorded at several distinct sites including scoria of the laschamp volcano to examine this anomalous geomagnetic fluctuation we studied the paleointensity of the laschamp and olby flows using the thellier method twenty five samples were selected for the paleointensity experiments and from seven we obtained reliable results because the paleointensity results of the olby and laschamp flows as well as laschamp scoria are very similar they can be represented by a single mean paleointensity f 7 7 t considering that this low paleointensity is less than 1 6 of the present geomagnetic field and is more characteristic of transitional behavior our results suggest that the paleomagnetic directions of the laschamp and olby flows were not acquired during a stable reversed polarity interval a more likely explanation is that the laschamp excursion represents an unsuccessful or aborted reversal

rapid magma ascent and generation of 230 th excesses in the lower crust at puyehue cordon caulle southern volcanic zone chile basaltic to rhyolitic lavas and tephras erupted over the last 70 kyr at the puyehue cordon caulle volcanic complex in the andean southern volcanic zone svz were analyzed for major and trace element sr isotope and u th isotope compositions to constrain the timescales of magmatic processes and identify the subducted and crustal components involved in magma genesis internal u th mineral isochrons from five lavas and three tephra fall deposits are indistinguishable from their eruption ages indicating a short period b1000 yr of crystal residence in the magma prior to eruption the 230 th 232 th ratios define a narrow range 0 80 0 83 compared to that of all svz lavas 0 72 0 97 suggesting that puyehue basalt was derived from a relatively uniform mantle source dacites and rhyolites have the largest u excesses and likely evolved via fractional crystallization of a plagioclase dominated mineral assemblage in contrast basalts have 1 to 6 230 th excesses a characteristic not previously observed in frontal arc stratovolcanoes of the andean svz the 230 th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust lower crustal processes therefore have dampened the 238 u excesses that were generated during fluid addition to the mantle wedge although prior 238 u 230 th 226 ra studies of lavas from other southern svz stratovolcanoes 36 to 41 s have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc our results suggest that mash processes envisioned by hildreth and moorbath w hildreth s moorbath crustal contributions to arc magmatism in the andes of central chile contrib mineral petrol 98 1988 455 489 in the northern svz also occur in the southern svz where the crust is relatively thin

on the age of the laschamp geomagnetic excursion abstract the laschamp geomagnetic excursion is a critical tie point found directly in deep sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice and to providing an independent calibration of the radiocarbon calendar its timing remains poorly known previous k ar 40 ar 39 ar and u th isochron determinations from lava flows at the type locality in the massif central france vary widely are imprecise and suggest a mean age of about 46 2 2 5 ka 2 results of 6 new unspiked k ar and 13 40 ar 39 ar incremental heating experiments on subsamples from three sites on the laschamp and olby flows are concordant and give a weighted mean age of 40 4 1 1 ka 2 uncertainty including analytical sources only that is 10 younger than the previous estimates considering that the 40 k 40 ar decay constant is not known to a precision better than 2 4 the most probable radioisotopic age for the laschamp excursion is 40 4 2 0 ka 2 analytical plus decay constant uncertainties this new age for the laschamp excursion agrees precisely with that deduced from the napis 75 deep sea sediment paleointensity stack when calibrated against the gisp2 ice core chronology using the o isotopes in ice and the magnetic properties of the marine cores

14c activity and global carbon cycle changes over the past 50 000 years a series of 14c measurements in ocean drilling program cores from the tropical cariaco basin which have been correlated to the annual layer counted chronology for the greenland ice sheet project 2 gisp2 ice core provides a high resolution calibration of the radiocarbon time scale back to 50 000 years before the present independent radiometric dating of events correlated to gisp2 suggests that the calibration is accurate reconstructed 14c activities varied substantially during the last glacial period including sharp peaks synchronous with the laschamp and mono lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in 14c production rate together with plausible changes in deep ocean ventilation and the global carbon cycle during glaciation

amplitude and timing of the laschamp geomagnetic dipole low from the global atmospheric 10 be overproduction contribution of authigenic 10 be 9 be ratios in west equatorial pacific sediments 1 authigenic 10be 9be ratios were measured along a sediment core collected in the west equatorial pacific in order to reconstruct cosmogenic 10be production variations near the equator where the geomagnetic modulation is maximum from 60 to 20 ka the single significant 10be production impulse recorded at 41 ka results from the geomagnetic dipole low that triggered the laschamp excursion no significant 10be overproduction signature is recorded at the age of the mono lake excursion 34 ka a compilation of authigenic 10be 9be records obtained from sediments was averaged over a 1 kyr window and compared with the 1 kyr averaged 10be flux record of greenland ice cores their remarkable similarity demonstrates that 10be production is globally modulated by geomagnetic dipole variations and redistributed by atmosphere dynamics after calibration using absolute values of the virtual dipole moment drawn from paleomagnetic database the authigenic 10be 9be stack allows reconstructing the geomagnetic dipole moment variations over the 20 50 ka time interval between 48 and 41 ka the dipole moment collapsed at a rate of 1 5 1022 a m2 kyr 1 which will be an interesting criterion for the assessment of the loss rate of the historical field and the comparison of dipole moment loss prior to excursions and reversals after a 2 kyr duration of the minimum dipole moment 1 1022 a m2 a slow increase started at 39 ka progressively reaching 5 1022 a m2 at 20 ka the absence of a significant dipole moment drop at 34 ka the age of the mono lake excursion suggests that the duration and amplitude of the dipole weakening cannot be compared with that of the laschamp this study provides a reliable basis to model the production of radiocarbon and in situ cosmogenic nuclides and to improve the calibration of these dating methods

visual stratigraphic dating of the gisp2 ice core basis reproducibility and application annual layers are visible in the greenland ice sheet project 2 ice core from central greenland allowing rapid dating of the core changes in bubble and grain structure caused by near surface primarily summertime formation of hoar complexes provide the main visible annual marker in the holocene and changes in cloudiness of the ice correlated with dustiness mark wisconsinan annual cycles both markers are evident and have been intercalibrated in early holocene ice layer counts are reproducible between different workers and for one worker at different times with 1 error over century length times in the holocene reproducibility is typically 5 in wisconsinan ice age ice and decreases with increasing age and depth cumulative ages from visible stratigraphy are not significantly different from independent ages of prominent events for ice older than the historical record and younger than approximately 50 000 years visible observations are not greatly degraded by brittle ice or many other core quality problems allowing construction of long consistently sampled time series high accuracy requires careful study of the core by dedicated observers

geomagnetic field excursion recorded 17 ka at tianchi volcano china new 40ar 39ar age and significance new 40ar 39ar dating of a comenditic lava atop tianchi volcano china indicates eruption at 17 1 0 9 ka the flow interior records a pair of transitional virtual geomagnetic poles and a low paleointensity of 25 t thus it records a geomagnetic field excursion that is younger than the 41 ka laschamp or 32 ka auckland excursions implications are 1 following a repose of several tens of kyr tianchi volcano became highly active immediately following termination of the last glaciation maximum the flare up of silicic eruptions may reflect rapid deglaciation of the edifice 2 a 17 ka age for the tianchi excursion provides the first direct radioisotopic evidence that excursional behavior which is imprecisely dated and less well documented magnetically at several other sites is a global feature of geodynamo behavior 3 during the brunhes chron 13 well dated excursions cluster into two periods including seven between 17 and 212 ka and six between about 530 and 730 ka

40ar 39ar and k ar chronology of pleistocene glaciations in patagonia during the pleistocene east of lago buenos aires argentina at 46 5 s at least 19 terminal moraines were deposited as piedmont glaciers from the patagonian ice cap advanced onto the semi arid high plains adjacent to the southern andes exceptional preservation of these deposits offers a rare opportunity to document ice cap fluctuations during the last 1 2 m y 4 0 ar 3 9 ar incremental heating and unspiked k ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence the 4 0 ar 3 9 ar isochron ages of three lavas that overlie till 90 km east of the cordillera at lago buenos aires and another 120 km from the andes along rio gallegos at 51 8 s that underlies till strongly suggest that the ice cap reached its greatest eastward extent ca 1100 ka at least six moraines were deposited within the 256 k y period bracketed by basaltic eruptions at 1016 10 ka and 760 14 ka similarly six younger more proximal moraines were deposited during an 651 k y period bracketed by an underlying 760 14 ka basalt and the 109 3 ka cerro volcan basalt flow that buried all six moraines coupled with in situ cosmogenic surface exposure ages of moraine boulders the 109 ka age of cerro volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6 further westward toward lago buenos aires six additional moraines younger than the cerro volcan basalt flow occur surface exposure dating of boulders on these moraines combined with the 1 4 c age of overlying varved lacustrine sediment indicates deposition during the last glacial maximum lgm 23 16 ka although antarctic dust records signal an important patagonian glaciation at 60 40 ka moraines corresponding to marine oxygen isotope stage 4 are not preserved at lago buenos aires apparently these were overrun and obliterated by the younger ice advance at 23 ka notwithstanding the overall pattern of glaciation in patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m y we hypothesize that tectonically driven uplift of the patagonian andes which began in the pliocene yet continued into the quaternary in part due to subduction of the chile rise spreading center during the past 2 m y maximized the ice accumulation area and ice extent by 1 1 ma subsequent deep glacial erosion has reduced the accumulation area resulting in less extensive glaciers over time

timing and lock in effect of the laschamp geomagnetic excursion in chinese loess 1 the laschamp geomagnetic excursion lge at 41 ka is a critical age control for hemispherical comparison of paleoclimate records from various kinds of geological archives e g ice core marine and lake sediments the timing of the lge in chinese loess however remains poorly constrained due to the lack of a reliable chronology and the complex acquisition processes of natural remanent magnetization here we systematically compare the paleomagnetic results of three optically stimulated luminescence dated loess sequences on the chinese loess plateau our results indicate that the timing of the lge in the northwestern loess plateau is slightly older than the absolute radiometric age determination and the timing inferred from greenland ice core 10be flux and marine i e relative paleointensity and authogenic 10be 9be stack records but younger than the counterparts in the central loess plateau we attribute the different timing of the lge in the three loess sections to a progressive southeastward increase in the lock in depth caused by the combined effects of postdepositional processes e g surface mixing chemical weathering and lock in delay on the remanence acquisition process we conclude that caution is needed to use the lge in chinese loess as a reliable tie event for high resolution chronological correlation to marine and ice core records only if the potential lock in effect can be precisely determined

atmospheric re organization during marine isotope stage 3 over the north american continent sedimentological and mineralogical evidence from the gulf of mexico abstract mineralogical and sedimentological records from the gulf of mexico gom reveal major changes in the terrigenous particles provenance during the marine isotopic stage 3 mis3 that likely resulted from modifications of the environmental conditions including glacial activity and precipitation distribution over the north american continent the southeastern margin of the laurentide ice sheet lis was active throughout the entire mis3 whereas the southwestern margin contributed to short term meltwater events only during late mis3 some of the major mineralogical changes cannot be attributed to glacial activity but rather result from the changes in precipitation distribution combining sedimentological records from the gom with previously published climate related archives from the north american continent allows the reconstruction of two main schematic patterns of moisture inflow and precipitation distribution that may have prevailed during mis3 meltwater discharges contribute to modifying the lis configuration and the gom hydrological properties ultimately affecting large scale oceanic circulation and may have influenced atmospheric re organizations although the insolation variation rate appears to be the main driver of the system

a critical evaluation of young near zero k ar ages an evaluation of the precision and resolution of the unspiked k ar dating method is presented with particular regard to the statistical significance of ages that are measured near or at the detection limit of the technique near zero historical ages can be measured by the unspiked k ar technique with a precision that is essentially controlled by the precision with which the 40ar 36ar of the sample can be resolved from the present day atmospheric value of 295 5 the best analytical precision on the isotopic ratio is 0 05 1 by this technique which currently limits the lower detection limit of unspiked k ar ages to samples featuring at least 0 14 of radiogenic 40ar the corresponding youngest resolvable k ar age depends on the k content and atmospheric contamination of the sample total fusion analysis of high k refractory minerals like sanidine is not practicable via k ar and the lowest resolvable age for medium k samples more amenable to complete fusion is around 1 5 ka on a single run basis it is argued that near zero age measured with a probability density straddling or narrowing the time origin cannot be handled without accounting for the non negativity constraint imposed by the physical requirement of a positive age the pertinent equations are derived both for the single run case and for the case of independent replicates made on a single sample we show that pooled k ar replicates can theoretically reduce the nominal uncertainty of individual unspiked ages typically 1 5 ka 2 to a value that is close to the smallest 40ar 39ar isochron age uncertainty achievable on sanidine in the 0 2 ka range 0 2 ka 2 however this performance is obtained at the cost of prohibitively large sample statistics n 15 for medium k feldspars datable via k ar coupled with the inability of the k ar approach to obviate the problems of excess fractionated 40ar and or xenocrystic contamination this makes the 40ar 39ar technique the method of choice for dating historical events by the k ar scheme

miocene pliocene shortening extension and mafic magmatism support small scale lithospheric foundering in the central andes nw argentina

palaeointensities of the auckland geomagnetic excursions by the ltd dht shaw method we report new palaeointensity results concerning the auckland geomagnetic excursions using the double heating technique of the shaw method with low temperature demagnetisation ltd dht shaw method the excursional palaeodirections recorded in six volcanoes of the auckland volcanic field new zealand have been classified into three groups north down nd west up wu and south up su directions in the present study five to six consistent palaeointensities have been obtained from each of five volcanoes recording the auckland geomagnetic excursions the wiri 27 ka crater hill and puketutu volcanoes nd group yielded mean palaeointensities of 10 6 1 2 1 11 8 2 8 and 11 1 0 4t respectively the hampton park volcano 55 ka wu group gave 9 5 1 2t while the mclennan hills volcano su group gave 2 5 0 5t it is notable that consistent palaeointensities have been obtained from the three different volcanoes which have almost the same palaeodirections nd group possibly supporting the reliability of the palaeointensity data these five palaeointensities for the auckland geomagnetic excursions correspond to virtual dipole moments vdms of 0 6 2 1 10 22 am 2 whereas three mean palaeointensities obtained from the auckland volcanoes having non excursional palaeodirections are 13 1 40 0 t giving stronger vdms of 2 1 6 9 10 22 am 2 these results suggest that the dipole component of the geomagnetic field reduced to about 2 10 22 am 2 or less during the auckland geomagnetic excursions

changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10be and 14c records the variations in atmospheric radiocarbon c 14 concentration during the last 50 000 years can be attributed to changes in the c 14 production rate due to changes in solar activity the geomagnetic field and or interstellar galactic cosmic ray flux and to changes in the global carbon cycle the relative contributions of these processes is the subject of current debate although the discrepancies between the various reconstructions of the past atmospheric radiocarbon concentration increase with age the relatively good agreement over the last 25 000 years allows a quantitative discussion of the causes of the observed c 14 variations for this period using be 10 measurements from greenland summit ice cores we show that in addition to solar and geomagnetically induced production rate changes significant changes in the carbon cycle have to be considered to explain the measured c 14 concentrations there is evidence that these changes are connected to 1 global deglaciation and 2 climate changes in the north atlantic region on centennial to millennial time scales related to changes in the ocean circulation differences between be 10 and geomagnetic field records however suggest that uncertainties of about 20 still exist in determinations of past changes in the c 14 production rate c 2004 elsevier b v all rights reserved less

feldspar 40 ar 39 ar dating of icdp paleovan cores abstract volcaniclastic fall deposits in icdp drilling cores from lake van turkey contain sodium rich sanidine and calcium rich anorthoclase which both comprise a variety of textural zoning and inclusions an age model records the lake s history and is based on climate stratigraphic correlations tephrostratigraphy paleomagnetics and earlier 40 ar 39 ar analyses stockhecke et al 2014b results from total fusion and stepwise heating 40 ar 39 ar analyses presented in this study allow for the comparison of radiometric constraints from texturally diversified feldspar and the multi proxy lacustrine age model and vice versa this study has investigated several grain size fractions of feldspar from 13 volcaniclastic units the feldspars show textural features that are visible in cathodoluminescence cl or back scattered electron bse images and can be subdivided into three dominant zoning types 1 compositional zoning 2 round pseudo oscillatory zoning and 3 resorbed and patchy zoning ginibre et al 2004 round pseudo oscillatory zoning records a sensitive alternation of fe and ca that also reflects resorption processes this is only visible in cl images compositional zoning reflects anticorrelated anorthite and orthoclase contents and is visible in bse eleven inverse isochron ages from total fusion and three from stepwise heating analyses fit the age model four experiments resulted in older inverse isochron ages that do not concur with the model within 2 uncertainties and that deviate from 1 ka to 17 ka minimum c and r type zoning are interpreted as representing growth in magma chamber cupolas as wall mushes or in narrow conduits persistent compositions of po type crystals and abundant surfaces recording dissolution features correspond to formation within a magma chamber c type zoning and r type zoning have revealed an irregular incorporation of melt and fluid inclusions these two types of zoning in feldspar are interpreted as preferentially contributing either heterogeneously distributed excess 40 ar or inherited 40 ar to the deviating 40 ar 39 ar ages that are discussed in this study

an analysis of west african dynamics using a linearized gcm abstract this study utilizes a linear primitive equation spherical model to study the development and propagation of easterly wave disturbances over west africa perturbations are started from an initial disturbance consisting of a barotropic vortex and the governing equations are integrated forward in time the perturbations are introduced into basic states corresponding to the observed dynamical and thermodynamical characteristics of two wet years in the sahel and two dry years the model simulations show consistent contrasts in wave activity between the wet and dry years the waves are markedly stronger in the wet years and show a barotropic structure throughout the troposphere the waves tend to extend throughout the troposphere to the level of the tropical easterly jet tej in the wet years but not in the dry years the upper tropospheric shear which is stronger in wet years appears to be a key factor in wave development this shear is dependent on the intensity of the tej suggesting that the t

dynamics of the earth magnetic field in the 10 75 kyr period comprising the laschamp and mono lake excursions new results from the french chaine des puys in a global perspective abstract we report here on a new paleomagnetic directions and intensities and coupled k ar and 40 ar 39 ar analysis of 35 different flows emplaced in the chaine des puys during the 75 to 10 kyr interval which contains the mono lake and laschamp excursions there is a remarkable agreement between the new set of absolute volcanic intensities and published sedimentary glopis 75 and cosmogenic 10 be and 36 cl records the laschamp and mono lake excursions are clearly revealed by a very significant intensity drop at 41 2 1 6 ka and 34 2 1 2 ka respectively the duration of the laschamp excursion is 1500 yr and about 640 yr when the drop of paleointensity or the directional change are considered respectively the intensity drop at the mono lake is twice as short in the 7 ka interval separating the two excursions the field intensity recovers to almost non transitional values the rate of decrease of the field intensity during these excursions attains 18 nt yr for the laschamp and even greater value 33 nt yr for the mono lake this figure is for the laschamp excursion similar to the present field intensity decrease in the last two centuries so that one may wonder whether such a high rate of change may be characteristic of an impending geomagnetic event reversal or excursion we suggest that the name auckland excursion should be used for the present day called mono lake

the greenland ice core chronology 2005 15 42 ka part 2 comparison to other records a new greenland ice core chronology gicc05 based on multi parameter counting of annual layers has been obtained for the last 42 ka here we compare the glacial part of the new time scale which is based entirely on records from the northgrip ice core to existing time scales and reference horizons covering the same period these include the grip and northgrip modelled time scales the meese sowers gisp2 counted time scale the shackleton fairbanks grip time scale sfcp04 based on 14c calibration of a marine core the hulu cave record three volcanic reference horizons and the laschamp geomagnetic excursion event occurring around greenland interstadial 10 gicc05 is generally in good long term agreement with the existing greenland ice core chronologies and with the hulu cave record but on shorter time scales there are significant discrepancies around the last glacial maximum there is a more than 1 ka age difference between gicc05 and sfcp04 and a more than 0 5 ka discrepancy in the same direction between gicc05 and the age of a recently identified tephra layer in the northgrip ice core both sfcp04 and the tephra age are based on 14c dated marine cores and fixed marine reservoir ages for the laschamp event gicc05 agrees with a recent independent dating within the uncertainties

the greenland ice sheet project 2 depth age scale methods and results the greenland ice sheet project 2 gisp2 depth age scale is presented based on a multiparameter continuous count approach to a depth of 2800 m using a systematic combination of parameters that have never been used to this extent before the ice at 2800 m is dated at 110 000 years b p with an estimated error ranging from 1 to 10 in the top 2500 m of the core and averaging 20 between 2500 and 2800 m parameters used to date the core include visual stratigraphy oxygen isotopic ratios of the ice electrical conductivity measurements laser light scattering from dust volcanic signals and major ion chemistry gisp2 ages for major climatic events agree with independent ages based on varve chronologies calibrated radiocarbon dates and other techniques within the combined uncertainties good agreement also is obtained with greenland ice core project ice core dates and with the specmap marine timescale after correlation through the 18 o of o 2 although the core is deformed below 2800 m and the continuity of the record is unclear we attempted to date this section of the core on the basis of the laser light scattering of dust in the ice

the 40ar 39ar dating of core recovered by the hawaii scientific drilling project phase 2 hilo hawaii 1 the hawaii scientific drilling project phase 2 hsdp 2 recovered core from a 3 1 km thick section through the eastern flanks of mauna loa and mauna kea volcanoes we report results of 40ar 39ar incremental heating by broad beam infrared laser of 16 basaltic groundmass samples and 1 plagioclase separate mostly from k poor tholeiites the tholeiites generally have mean radiogenic 40ar enrichments of 1 3 and some contain excess 40ar however isochron ages of glass poor samples preserve stratigraphic order in all cases a 246 m thick sequence of mauna loa tholeiitic lavas yields an isochron age of 122 86 kyr all errors 2 at its base beneath the mauna loa overlap sequence lie mauna kea s postshield and shield sequences a postshield alkalic lava yields an age of 236 16 kyr in agreement with an age of 240 14 kyr for a geochemically correlative flow in the nearby hsdp 1 core hole where more complete dating of the postshield sequence shows it to have accumulated at 0 9 0 4 m kyr from about 330 to 200 ka mauna kea s shield consists of subaerial tholeiitic flows to a depth of 1079 m below sea level then shallow submarine flows hyaloclastites pillow lavas and minor intrusions to core bottom at 3098 m most subaerial tholeiitic flows fail to form isochrons however a sample at 984 m yields an age of 370 180 kyr consistent with ages from similar levels in hsdp 1 submarine tholeiites including shallow marine vitrophyres clasts from hyaloclastites and pillow lavas were analyzed however only pillow lava cores from 2243 2614 and 2789 m yield reliable ages of 482 67 560 150 and 683 82 kyr respectively a linear fit to ages for shield samples defines a mean accumulation rate of 8 6 3 1 m kyr and extrapolates to 635 kyr at core bottom alternatively a model relating mauna kea s growth to transport across the hawaiian hot spot that predicts downward accelerating accumulation rates that reach 20 m kyr at core bottom depaolo and stolper 1996 is also consistent with all reliable ages except the deepest

contrasting timescales of crystallization and magma storage beneath the aleutian island arc abstract geologic chronologic and u th isotope data from pleistocene recent basaltic to rhyolitic lavas and their phenocrysts constrain the long term evolution of magmatic processes at seguam island aleutian island arc and suggest a model in which erupted magmas were derived from a single deep seated reservoir for 130 kyrs of its eruptive history the monotonic evolution in 230 th 232 th 0 ratios is consistent with radiogenic ingrowth of 230 th in a long lived magma reservoir between 142 and 9 ka internal u th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40 ar 39 ar dating which implies a short period i e 10 3 yrs or less of crystal residence in the magma prior to eruption these results can be reconciled if small batches of magma are repeatedly extracted from a deep thermally buffered basaltic reservoir followed by decompression driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering 12 km 2 the ignimbrite as well as subsequent rhyolitic and basaltic lava flows signal a decrease in 230 th 232 th 0 and 87 sr 86 sr ratios the abrupt change in magma composition at 9 ka most likely reflects disruption of the long lived reservoir by rapid ascent of basaltic magma with low 230 th 232 th and 87 sr 86 sr ratios and excess 226 ra while the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored the shift in isotopic composition and evidence for a protracted 100 kyr period of magma storage would have been missed had the focus been on only the most recent historical eruptions our results suggest that not all arc basalt produced in the mantle wedge separates and ascends rapidly magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano and u th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs provided a sufficiently long period of volcanic activity is examined

reconciling the greenland ice core and radiocarbon timescales through the laschamp geomagnetic excursion abstract cosmogenic radionuclides such as 10be and 14c share a common production signal with their formation in the earth s upper atmosphere modulated by changes to the geomagnetic field as well as variations in the intensity of the solar wind here we use this common production signal to compare between the radiocarbon intcal and greenland ice core gicc05 timescales utilising the most pronounced cosmogenic production peak of the last 100 000 years that associated with the laschamp geomagnetic excursion circa 41 000 years ago we present 54 new 14c measurements from a peat core tp 2005 from tenaghi philippon ne greece contiguously spanning between circa 47 300 and 39 600 cal bp demonstrating a distinctive tripartite structure in the build up to the principal laschamp production maximum that is not present in the consensus intcal13 calibration curve this is the first time that a continuous non reservoir corrected 14c dataset has been generated over such a long time span for this the oldest portion of the radiocarbon timescale this period is critical for both palaeoenvironmental and archaeological applications with the replacement of neanderthals by anatomically modern humans in europe around this time by placing our tenaghi philippon 14c dataset on to the hulu cave u series timescale of cheng et al 2018 via bayesian statistical modelling the comparison of tp 2005 14c with greenland 10be fluxes also implicitly relates the underlying u series and gicc05 timescales themselves this comparison suggests that whilst these two timescales are broadly coherent the intcal13 timescale contains erroneous structure circa 40 000 cal bp

mindanao dome variability over the last 160 kyr episodic glacial cooling of the west pacific warm pool we present sea surface upper thermocline and benthic delta 18 o data as well as temperature and paleoproductivity proxy data from the international marine global change study program images core md06 3067 6 degrees 31 n 126 degrees 30 e 1575 m water depth located in the western equatorial pacific ocean within the flow path of the mindanao current our records reveal considerable glacial interglacial and suborbital variability in the mindanao dome upwelling over the last 160 kyr dome activity generally intensified during glacial intervals resulting in cooler thermocline waters whereas it substantially declined during interglacials in particular in the early holocene and early marine oxygen isotope stage mis 5e when upwelling waters did not reach the thermocline during mis 3 and mis 2 enhanced surface productivity together with remarkably low sst and low upper ocean thermal contrast provide evidence for episodic glacial upwelling to the surface whereas transient surface warming marks periodic collapses of the mindanao dome upwelling during heinrich events we attribute the high variability during mis 3 and mis 2 to changes in the el nino southern oscillation state that affected boreal winter monsoonal winds and upper ocean circulation glacial upwelling intensified when a strong cyclonic gyre became established whereas el nino like conditions during heinrich events tended to suppress the cyclonic circulation reducing ekman transport thus our findings demonstrate that variations in the mindanao dome upwelling are closely linked to the position and intensity of the tropical convection and also reflect far field influences from the high latitudes

high resolution paleomagnetic secular variations and relative paleointensity since the late pleistocene in southern south america abstract paleomagnetic inclination declination and relative paleointensity were reconstructed from the sediments of laguna potrok aike in the framework of the international continental scientific drilling program icdp potrok aike maar lake sediment archive drilling project pasado here we present the u channel based full vector paleomagnetic field reconstruction since 51 2 ka cal bp the relative paleointensity proxy rpi was built by normalising the natural remanent magnetisation with the anhysteretic remanent magnetisation using the average ratio at 4 demagnetisation steps part of the chrm interval nrm arm 10 40 mt a grain size influence on the rpi was removed using a correction based on the linear relationship between the rpi and the median destructive field of the natural remanent magnetisation mdf nrm the new record is compared with other lacustrine and marine records and stacks from the mid to high latitudes of the southern hemisphere revealing consistent millennial scale variability the identification of the laschamp and possibly the mono lake geomagnetic excursions and a direction swing possibly associated to the hilina pali excursion at 20 ka cal bp nonetheless a global scale comparison with other high resolution records located on the opposite side of the earth and with various dipole field references hint at a different behaviour of the geomagnetic field around southern south america at 46 ka cal bp

paleointensity of the earth s magnetic field and k ar dating of the louchadiere volcanic flow central france new evidence for the laschamp excursion we report paleointensity results of the earth s magnetic field from a late pleistocene lava flow louchadiere central france which recorded an intermediate geomagnetic field direction 5 sites mean declination 114 1 inclination 58 2 k 130 95 6 7 new k ar age determinations confirm that this flow is contemporaneous with the laschamp and olby flows and that this excursion occurred around 45ka ago using the thellier double heating method reliable paleointensities have been obtained for ten samples from three different sites providing an average field strength of 12 9 t 3 3 this low value and previous results of the laschamp excursion from france and iceland confirm that the earth s magnetic field was in an intermediate state during the laschamp excursion

tracing time in the ocean a brief review of chronological constraints 60 8 kyr on north atlantic marine event based stratigraphies abstract well resolved event based stratigraphies in marine sediments spanning a significant portion of the last glacial period 60 8 kyr provide a unique opportunity for time stratigraphic correlation in the north atlantic region here we review the current methods available to chronologically constrain these event based stratigraphies highlighting in particular the value of tephrochronology as an independent tool to validate correlations between records while the intimate protocols lowe et al 2008 blockley et al 2011 are equally applicable to marine and terrestrial records spatially and temporally variable marine radiocarbon reservoir age effects mres provide a challenge to using marine radiocarbon in the former as an independent chronostratigraphic tool despite the inherent uncertainties associated with tuning we conclude that the mid points of the common abrupt warming transitions associated with the well defined millennial scale climate oscillations the dansgaard oeschger d o cycles observed in the oxygen isotopes of the greenland ice cores and north atlantic sea surface temperature sst records currently provide the most robust correlation tie points from which to derive age control in this invited intimate special issue article we propose a new protocol for establishing marine event based chronostratigraphies in the north atlantic region and focus on areas of chronological potential in palaeoceanographic research

the laschamp geomagnetic dipole low expressed as a cosmogenic 10 be atmospheric overproduction at 41 ka authigenic 10be 9be ratiomeasurementswere performed at high resolution along a portuguese margin deep sea core 37 48 n 10 09w spanning the 20 50 ka time interval in order to reconstruct variations in atmospheric cosmogenic 10be production rates and derive the related geomagnetic dipole moment modulation a complementary approach consisting in 230thxs normalized 10be deposition rate determination on selected samples confirmed the reliability of the authigenic 10be 9be record this study constitutes the first successful comparison of the two widely used normalization techniques of 10be concentrations for both methods the presented results herein evidence a factor of 2 cosmogenic nuclide overproduction linked to a minimum dipole moment associated with the laschamp excursion the latter is stratigraphically constrained beneath heinrich event 4 its age is estimated at 41 ka on the basis of direct correlation between the series of rapid paleoclimatic events recorded in the portuguese margin sediments and in the greenland ice sheet and is confirmed by calibrated radiocarbon dating carried out on the same sediments the remarkable agreement between the authigenic 10be 9be and the greenland ice cores 10be deposition rate records attests to their global significance this new authigenic 10be 9be record has been combined with that previously obtained at the same site to produce a stacked record that is calibrated using absolute values of virtual dipole moment determined on lava flows this provides a reconstruction of dipole geomagnetic moment variations over the 20 50 ka interval independent frompaleomagnetically constrainedmethods that documents the laschamp dipole low but fails to express any dipole low related to t e mono lake excursion this high resolution record responds to the necessity of supplementing the knowledge of the atmospheric 14c variations in the 30 45 ka interval during which the 14c calibration curve suffers from a lack of accurate data and during which a discrepancy of about 5500 yr between the 14c and u th ages is due to the laschamp geomagnetic dipole low such new high resolution datasets from records obtained from different latitudes will be required to make significant advances in understanding the causes of atmospheric 14c variations

a half million year record of paleoclimate from the lake manix core mojave desert california abstract pluvial lakes in the southwestern u s responded sensitively to past climate through effects on rainfall runoff and evaporation although most studies agree that pluvial lakes in the southwestern u s reached their highest levels coeval with glacial stages the specific timing of increased effective moisture and lake level rise is debated particularly for the southwesternmost lakes we obtained a 45 m core of lacustrine sediment from lake manix the former terminus of the mojave river prior to about 25 ka and supplemented data from the core with outcrop studies these sediments provide a robust record of mojave river discharge over the last half million years lake manix persisted from ois 12 through early ois 2 including during interstadial ois 3 and interglacials ois 5 7 and 9 the ostracode faunal record displays a shift from an unexpectedly warm summer dominated lake hydrology during ois 12 to predominantly colder winter dominated conditions afterwards the ostracode based stable isotope record displays a large degree of intra sample variability and does not mimic other well known isotopic records of climate change evaporation likely buffered the manix 18o record from most of the expected isotopic differences between interglacial and glacial interval discharge isotopically depleted and stable lakes occurred only four to six times most notably during ois 7 and ois 9 internal drainage basin changes also affected the isotopic record persistence of lakes in the manix basin during interglacials requires atmospheric or oceanic circulation controls on the mean position of the pacific storm track other than large ice sheets we propose that the relative strength and sign of the northern annular mode nam and its influence on atmospheric river derived precipitation is a potential explanation

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

the greenland ice core chronology 2005 15 42 ka part 1 constructing the time scale the greenland ice core chronology 2005 gicc05 is extended back to 42 ka b2k before 2000 ad i e to the end of greenland stadial 11 the chronology is based on independent multi parameter counting of annual layers using comprehensive high resolution measurements available from the north greenland ice core project ngrip these are measurements of visual stratigraphy conductivity of the solid ice electrolytical melt water conductivity and the concentration of na c a 2 s o 4 n o 3 n h 4 a n uncertainty estimate of the time scale is obtained from identification of uncertain annual layers which are counted as 0 570 5 years the sum of the uncertain annual layers the so called maximum counting error of the presented chronology ranges from 4 in the warm interstadial periods to 7 in the cold stadials the annual accumulation rates of the stadials and interstadials are on average one third and half of the present day values respectively and the onset of the greenland interstadials 2 3 and 8 based on 20 year averaged d 18 o

radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired 230 th 234 u 238 u and 14 c dates on pristine corals abstract radiocarbon dating is the most widely used dating technique in the world recent advances in accelerator mass spectrometry ams and sample preparation techniques have reduced the sample size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes today it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates the production of 14 c in the atmosphere varies through time due to changes in the earth s geomagnetic field intensity and in its concentration which is regulated by the carbon cycle as a result of these two variables a radiocarbon age is not equivalent to a calendar age four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12 000 calendar years before present this paper presents high precision paired 230 th 234 u 238 u and 14 c age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12 000 to 50 000 years before present we developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years taking full account of combined errors in input ages and calibration uncertainties our radiocarbon calibration program is publicly accessible at http www radiocarbon ldeo columbia edu along with full documentation of the samples data and our statistical calibration model

40ar 39ar k ar and 230th 238u dating of the laschamp excursion a radioisotopic tie point for ice core and climate chronologies abstract a brief period of enhanced 10 be flux that straddles the interstadial warm period known as dansgaard oeschger event 10 in greenland and its counterpart in antarctica the antarctic isotope maximum 10 is but one consequence of the weakening of earth s magnetic field associated with the laschamp excursion this 10 be peak measured in the grip ice core is dated at 41 250 y b2k before year 2000 ad in the most recent gicc05 age model obtained from the northgrip core via multi parameter counting of annual layers uncertainty in the age of the 10 be peak is however no better than 1630 y at the 95 confidence level reflecting accumulated error in identifying annual layers the age of the laschamp excursion guillou h singer b s laj c kissel c scaillet s jicha b 2004 on the age of the laschamp geomagnetic excursion earth planet sci lett 227 331 343 is revised on the basis of new 40 ar 39 ar unspiked k ar and 238 u 230 th data from three lava flows in the massif central france together with the 40 ar 39 ar age of a transitionally magnetized lava flow at auckland new zealand combined these data yield an age of 40 700 950 y b2k where the uncertainty includes both analytical and systematic 40 k and 230 th decay constant errors taking the radioisotopic age as a calibration tie point suggests that the layer counting chronologies for the northgrip and gisp2 ice cores are more accurate and precise than previously thought at depths corresponding to the laschamp excursion

the deccan trap cretaceous paleogene boundary connection new 40ar 39ar ages and critical assessment of existing argon data pertinent to this hypothesis abstract the cretaceous paleogene boundary kpgb was dated by the 40ar 39ar method herein from the western interior of north america at 65 48 0 12 ma 1 in good agreement with other recent published estimates for the deccan traps india new argon ages as well as others available in the literature are assessed for reliability based on a statistical reliability of plateau isochron sections and b freshness of material dated utilizing the alteration index method from tholeiitic lavas from the composite western ghats section cwgs only six ages are found to be reliable estimates of the time of crystallization these ages along with the magnetic polarity of the lavas agree with the geomagnetic polarity time scale gpts at 67 64 ma alkaline rocks from the anjar area of kutch provide three reliable ages that suggest a hiatus in lava extrusion around kpgb for the rajahmundry basalts the upper flow s age defines its formation during chron 29n a single age from the lower reversed polarity flow appears somewhat dichotomous when plotted against the gpts the reliable lava ages indicate the most voluminous reversed polarity sections of the cwgs were extruded at a time statistically indistinguishable from that of the kpgb the deccan trap kpgb faunal extinction hypothesis remains plausible but must compete with the latest report favoring a very close temporal connection 0 03 m y between the chixculub impact crater and the kpgb

new age for the skalamaelifell excursion and identification of a global geomagnetic event in the late brunhes chron article ar 39 ar geochronology paleomagnetism sixteen lava flows sampled from isolated hills including skalamaelifell hill on the reykjanes peninsula in southwest iceland record excursional geomagnetic field directions that correspond to virtual geomagnetic poles vgps grouped at 12 s 250 e in the eastern pacific ocean the vgps are similar to those obtained in earlier studies of these lavas for which the name skalamaelifell excursion was previously assigned based on unspiked k ar dating of nine samples from six hills it has been accepted for more than two decades that the transitionally magnetized lavas reflect the 41 ka laschamp excursion however nine new 40 ar 39 ar incremental heating experiments on four of the transitionally magnetized lavas give an age of 91 13 ka moreover 238 u 230 th isochrons determined for two of these lava flows yield a weighted mean age of 96 10 ka 2 the weighted meanof the

comment on radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired th 230 u 234 u 238 and c 14 dates on pristine corals by r g fairbanks et al quaternary science reviews 24 2005 1781 1796 and extending the radiocarbon calibration beyond 26 000 years before present using fossil corals by t c chin et al quaternary science reviews 24 2005 1797 1808

timing duration and transitions of the last interglacial asian monsoon thorium 230 ages and oxygen isotope ratios of stalagmites from dongge cave china characterize the asian monsoon and low latitude precipitation over the past 160 000 years numerous abrupt changes in 18o 16o values result from changes in tropical and subtropical precipitation driven by insolation and millennial scale circulation shifts the last interglacial monsoon lasted 9 7 1 1 thousand years beginning with an abrupt less than 200 years drop in 18o 16o values 129 3 0 9 thousand years ago and ending with an abrupt less than 300 years rise in 18o 16o values 119 6 0 6 thousand years ago the start coincides with insolation rise and measures of full interglacial conditions indicating that insolation triggered the final rise to full interglacial conditions

evidence for enhanced 10be deposition in mediterranean sediments 35 kyr bp in this letter we report the profiles 160 samples of 10be 9be zn in a mediterranean sediment core spanning the last 60 kyr we show the existence of a 10be peak whose absolute age is estimated to be 34 3 kyr bp based on its stratigraphic position between two tephra layers originating from two volcanic eruptions campanian ignimbrite and citara dated by k ar 40ar 39ar and 14c methods this peak is similar to the increase in the 10be concentration observed in vostok and dome c ice cores at the same age the increase with approximately similar amplitude in both ice and sediment cores at different latitudes and hemispheres cannot be explained by changes in the archives the present result supports the view of a well defined global enhancement of the 10be flux related to an increase of the 10be production rate and not to a redistribution of the 10be fallout between different latitudes

the detailed tephrostratigraphy of a core from the south east black sea spanning the last 60 ka one visible volcanic ash layer and 21 non visible cryptotephra horizons have been identified in the m72 5 25 gc1 core from the south east black sea that spans the last 60 ka glass chemistry suggests that the tephras derive from italian hellenic and turkish sources the visible tephra layer is trachytic in composition and has been correlated to the 39 ka campanian ignimbrite super eruption from campi flegrei caldera italy two cryptotephra layers are associated with eruptions from acigol volcano turkey around the last glacial maximum another is linked to activity at erciyes dagi in the early holocene perikartini or karagullu and a cryptotephra layer near the top of the core correlates to the 3 6k cal a bp minoan marine z2 tephra from santorini thera greece this extends the known dispersal area for all these volcanic ash units many other calc alkaline rhyolite layers preserved in the core are likely to be from central anatolian sources but the lack of detailed proximal data for many of the volcanoes in the region prohibits further correlations the tephrostratigraphy and glass compositional data presented here allow the tephra layers to be used to correlate sedimentary archives across the region and help to extend the mediterranean tephrochronology framework towards the east