new archeointensity data from nw argentina 1300 1500 ce abstract a good characterization of the geomagnetic field strength at centennial to millennial time scales in the southern hemisphere is particularly crucial to disentangle the long term evolution of the south atlantic anomaly saa an intriguing geomagnetic feature currently observed at the earth s surface here we present 59 new archeointensities obtained from four well dated groups of potteries with ages ranging between 1300 and 1500 ce and collected in northwest argentina the new data were obtained in accordance with the thellier paleointensity method including partial thermoremanent magnetization ptrm checks and trm anisotropy and cooling rate corrections we have also performed a comparative study of the efficiency of magnetic susceptibility arm and trm anisotropy tensors to correct the trm anisotropy effect upon intensity estimates our results suggest that the magnetic susceptibility tensor systematically underestimates the trm anisotropy effect by 10 30 our new data together with selected archeointensities already published confirm that the decrease of the geomagnetic field intensity in south america started around 1600 1650 ce due to the arrival of the saa

relative geomagnetic intensity of the field during the last 140 ka relative magnetic field palaeointensities have been obtained from three marine cores in the somali basin western indian ocean the correlation between the records based on changes in the low field susceptibility and the inclination is extremely detailed a time versus depth correlation has been established from the 18o record the remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes coercivities and mineralogy the synthetic record of relative palaeointensity has been extended to cover the last 140 ka the quasi cyclic pattern observed during the last 80 ka confirms the results previously obtained in the mediterranean sea and thereby establishes its dominantly dipolar character between 140 and 80 ka the field intensity exhibits a much longer cycle spectral analysis of the intensity records shows the presence of two dominant periodicities but the length of the signal is too short to document the presence of a stationary process

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

orbital modulation of the earth s magnetic field intensity more than 20 years ago on the basis of data from a pacific sediment core it was suggested that geomagnetic field intensity may vary with the earth s orbital obliquity centred on a period of 41 kyr as a result of the effect of obliquity on precessional forces in the earth s core1 it had also been proposed that precession plays an important role in the energy budget of the earth s geodynamo2 but subsequent analyses indicated that the energy available from precession is at least an order of magnitude less than that required to drive the geodynamo3 here however we report a spectral analysis of sedimentary records of relative geomagnetic palaeointensity from two north atlantic sites which shows significant power both at orbital eccentricity 100 kyr and obliquity 41 kyr the eccentricity power is also present in bulk magnetic properties such as susceptibility and is therefore attributable to lithological variations controlled by eccentricity driven climate change the obliquity power however is not apparent in bulk magnetic properties and seems to be a property of the geomagnetic field itself thus providing evidence for the orbital forcing of geomagnetic field intensity

calcium carbonate as ikaite crystals in antarctic sea ice 1 we report on the discovery of the mineral ikaite caco3 6h2o in sea ice from the southern ocean the precipitation of caco3 during the freezing of seawater has previously been predicted from thermodynamic modelling indirect measurements and has been documented in artificial sea ice during laboratory experiments but has not been reported for natural sea ice it is assumed that caco3 formation in sea ice may be important for a sea ice driven carbon pump in ice covered oceanic waters without direct evidence of caco3 precipitation in sea ice its role in this and other processes has remained speculative the discovery of caco3 6h2o crystals in natural sea ice provides the necessary evidence for the evaluation of previous assumptions and lays the foundation for further studies to help elucidate the role of ikaite in the carbon cycle of the seasonally sea ice covered regions

the seasonal cycle of carbonate system processes in ryder bay west antarctic peninsula the carbon cycle in seasonally sea ice covered waters remains poorly understood due to both a lack of observational data and the complexity of the system here we present three consecutive seasonal cycles of upper ocean dissolved inorganic carbon dic and total alkalinity measurements from ryder bay on the west antarctic peninsula we attribute the observed changes in dic to four processes mixing of water masses air sea co2 flux calcium carbonate precipitation dissolution and photosynthesis respiration this approach enables us to resolve the main drivers of the seasonal dic cycle and also investigate the mechanisms behind interannual variability in the carbonate system we observe a strong asymmetric seasonal cycle in the carbonate system driven by physical processes and primary production in summer melting glacial ice and sea ice and a reduction in mixing with deeper water reduce the concentration of dic in surface waters the dominant process affecting the carbonate system is net photosynthesis which reduces dic and the fugacity of co2 making the ocean a net sink of atmospheric co2 in winter mixing with deeper carbon rich water and net heterotrophy increase surface dic concentrations resulting in ph as low as 7 95 and aragonite saturation states close to 1 we observe no clear seasonal cycle of calcium carbonate precipitation dissolution but some short lived features of the carbonate time series strongly suggest that significant precipitation of calcium carbonate does occur in the bay the variability observed in this study demonstrates that changes in mixing and sea ice cover significantly affect carbon cycling in this dynamic environment maintaining this unique time series will allow the carbonate system in seasonally sea ice covered waters to be better understood

a bayesian iterative geomagnetic model with universal data input self consistent spherical harmonic evolution for the geomagnetic field over the last 4000 years abstract changes of the earth s magnetic field have been observed with varying spatial and temporal resolution since the middle ages by means of compasses and later by refined mechanical inclination and intensity instruments until the modern use of quantum mechanical devices and satellite observations ancient records on the other hand rely on the natural magnetization of archaeological artifacts and other materials such as rocks and sediments being therefore indirect less accurate and sparser the combination of such heterogeneous records into a single self consistent global model of the geomagnetic field is very challenging the highly uneven data coverage both in space and time requires a careful handling of data uncertainties and error correlations previous models dealt with this problem using separated treatments of instrumental data and indirect records respectively as well as a different handling of the dipole field component with respect to non dipole terms here we present a global geomagnetic field model based for the first time on the simultaneous inversion of historical archaeomagnetic and volcanic records using a bayesian approach with minimal committing time regularization that minimizes the energy of secular variation a detailed assessment of data uncertainty and error correlation is used to minimize artifacts generated by the appearance of an overwhelming number of incomplete mostly declination only records associated with shipboard measurements in the early colonial period our model yields lower dipole energies which better match modern values as well as higher non dipole energies and a stronger secular variation of the dipole moment some model artifacts associated with extremely heterogeneous distributions of records in time and space are not completely eliminable and might be caused by incorrect a priori uncertainty assessments alternatively they might represent an intrinsic limit that can be overcome only by adding new records at critical locations and times

reconstructing holocene geomagnetic field variation new methods models and implications reconstructions of the holocene geomagnetic field and how it varies on millennial timescales are important for understanding processes in the core but may also be used to study long term solar terrestrial relationships and as relative dating tools for geological and archaeological archives here we present a new family of spherical harmonic geomagnetic field models spanning the past 9000 yr based on magnetic field directions and intensity stored in archaeological artefacts igneous rocks and sediment records a new modelling strategy introduces alternative data treatments with a focus on extracting more information from sedimentary data to reduce the influence of a few individual records all sedimentary data are resampled in 50 yr bins which also means that more weight is given to archaeomagnetic data during the inversion the sedimentary declination data are treated as relative values and adjusted iteratively based on prior information finally an alternative way of treating the sediment data chronologies has enabled us to both assess the likely range of age uncertainties often up to and possibly exceeding 500 yr and adjust the timescale of each record based on comparisons with predictions from a preliminary model as a result of the data adjustments power has been shifted from quadrupole and octupole to higher degrees compared with previous holocene geomagnetic field models we find evidence for dominantly westward drift of northern high latitude high intensity flux patches at the core mantle boundary for the last 4000 yr the new models also show intermittent occurrence of reversed flux at the edge of or inside the inner core tangent cylinder possibly originating from the equator less

carbon dynamics in sea ice a winter flux time series 1 a winter time series of the inorganic carbon system above within and beneath the landfast sea ice of the southern beaufort sea confirmed that sea ice is an active participant in the carbon cycle of polar waters eddy covariance measurements above the ice identified significant vertical co2 fluxes mostly upward away from the ice but with short periods of downward fluxes as well a novel in situ method revealed extremely high pco2 values within the ice that are not inconsistent with theory the total carbon content of the ice increased slightly through the winter season and increasing variability in the vertical profiles as spring began indicated that the inorganic carbon became mobile as the ice began to melt during early winter as the ice formed inorganic carbon concentrations in the surface waters increased dramatically along with salinity partly because of rejection from the ice and partly from advective mixing brine drainage was apparently not sufficient to initiate convection and the excess carbon remained in the surface waters into the summer

on the persistence of geomagnetic flux lobes in global holocene field models abstract to investigate the longevity and robustness of high latitude flux patches in the geomagnetic field at the core mantle boundary we present time dependent models of the geomagnetic field for the past 7000 years our models use the same data set as previously used for time dependent archaeomagnetic field modelling but constrained with additional priori models from time averages of field models covering the last 150 400 and 3000 years we find that the data are consistent with flux patches existing in both north and south hemispheres for the past 7000 years and that the northern hemisphere patches at least have highly dynamic behaviour simple averaging of the historical field may not be appropriate to obtain a characteristic time averaged model of the field for comparison with other geophysical observables our results should inform geodynamo studies of thermal core mantle coupling and of possible long term structure in the geomagnetic field

potassium argon ages from tristan da cunha south atlantic potassium argon ages measured on 11 volcanic rocks mainly lavas from tristan da cunha range from 0 21 0 01 to 0 01 0 02 ma and confirm the youthfulness of this volcano which lies on the walvis ridge just to the east of the crest of the mid atlantic ridge

relative changes of the geomagnetic field intensity during the last 280 kyear from piston cores in the acores area abstract relative changes in geomagnetic field intensity over the last 280 kyears have been recovered from the study of three marine cores from the acores area one core was precisely dated by oxygen isotope study and the other two records were linked to it using light reflectance analysis which allowed precise correlation rock magnetic analysis shows that the main magnetic mineral is magnetite with a very homogeneous grain size distribution in the pseudo single domain range for the three cores changes in the amount of magnetite do not exceed a factor of 10 therefore these cores appear to be suitable for relative palaeointensity determinations two mineral magnetic components with periodicities of 23 and 18 kyears are present in the records of bulk magnetic parameters and natural remanent magnetisation nrm but the power at these frequencies is not significant at the 95 level for the normalised remanence records for each core normalisation of the nrm using different normalising parameters yields virtually identical results using saturation isothermal remanent magnetisation sirm as the normalising parameter the results of the three cores were then combined into a stacked curve squared coherence analysis between this stacked curve and bulk mineral magnetic parameters reveals that the 18 kyear component is still present in the record but that the power spectrum is barely above noise level at this frequency some of the features of this north atlantic record are consistent with the main characteristics already documented in other sedimentary or volcanic records for instance distinct periods of low intensity are observed around 40 120 and 190 kyears and periods of high intensity at 50 and 80 kyears because of the uniformity of the mineral magnetic characteristics of the three cores we suggest that this record may be a suitable palaeointensity reference curve for the central north atlantic ocean region

long term secular variation of the geomagnetic field during the last 200 kyr recorded in sediment cores from the western equatorial pacific we have conducted a paleomagnetic study of five hemipelagic clay cores of 4 7 m in length obtained from the west caroline basin western equatorial pacific the purpose of this study is to clarify the long term secular variation of the geomagnetic field during the last 200 kyr using both intensity and directional changes one core was dated using oxygen isotope ratios 18o and characteristic magnetic susceptibility variations of about a factor of four allowed precise correlation between all cores the magnetic mineralogy is estimated to be dominated by magnetite of single domain sd and or pseudo single domain psd state the magnetic mineralogy and grain size are uniform throughout the cores which enabled us to obtain relative paleointensity variations as well as directional changes remanent intensity normalized by anhysteretic remanent magnetization arm indicates that sharp intensity drops to 20 of the present field occurred at about 40 and 190 kyr b p with a broader decrease around 110 kyr b p the former two are accompanied by anomalous remanent directions suggesting short polarity reversals or excursions their ages are close to those of the previously reported laschamp excursion and biwa i event a prominent intensity peak occurred immediately preceding the possible excursion at 40 kyr b p our results are in general similar to the recently reported datasets spanning the last 80 140 kyr from the somali basin the mediterranean and the sulu sea 1 3 the inclination record shows long term cyclic changes of several degrees in amplitude the intervals of the recurring variation are 40 50 kyr which is longer than the core s memory and is close to the milankovitch frequency associated with the change in obliquity of earth s rotational axis we infer that an external force such as orbital forcing could be a cause of the long term secular variation

palaeosecular variations of the geomagnetic field in africa during the holocene a review the importance of the full understanding of the holocene geomagnetic field spans from human history and archaeology to palaeoclimatic changes and engineering to geomagnetic field modelling and unravelling of the geomagnetic field characteristics and anomalies unfortunately the dearth of data from large under covered areas such as oceans the african and south american continents and the southern hemisphere only 4 6 of the global datasets dramatically limits our knowledge of the temporal and spatial evolution of the geomagnetic field and its application here a review of all data from the african continent is presented in order to encourage and motivate a new generation of palaeomagnetic and archaeomagnetic studies new data will sharpen the palaeomagnetism as a dating tool improve our knowledge of local global geomagnetic features and will help to finally answer some of the fundamental questions in palaeomagnetism like the temporal and spatial distribution of the palaeointensity peaks described and the origin and evolution of the south atlantic anomaly 2020 the author s published by the geological society of london all rights reserved

elevated paleomagnetic dispersion at saint helena suggests long lived anomalous behavior in the south atlantic earth s magnetic field is presently characterized by a large and growing anomaly in the south atlantic ocean the question of whether this region of earth s surface is preferentially subject to enhanced geomagnetic variability on geological timescales has major implications for core dynamics core mantle interaction and the possibility of an imminent magnetic polarity reversal here we present paleomagnetic data from saint helena a volcanic island ideally suited for testing the hypothesis that geomagnetic field behavior is anomalous in the south atlantic on timescales of millions of years our results supported by positive baked contact and reversal tests produce a mean direction approximating that expected from a geocentric axial dipole for the interval 8 to 11 million years ago but with very large associated directional dispersion these findings indicate that on geological timescales geomagnetic secular variation is persistently enhanced in the vicinity of saint helena this in turn supports the south atlantic as a locus of unusual geomagnetic behavior arising from core mantle interaction while also appearing to reduce the likelihood that the present day regional anomaly is a precursor to a global polarity reversal

alder creek sanidine acs 2 a quaternary 40ar 39ar dating standard tied to the cobb mountain geomagnetic event accurate 40ar 39ar dating of quaternary and holocene samples requires a standard neutron fluence monitor of appropriate age this standard should be isotopically homogenous at the scale of analysis and should be well intercalibrated with other standards several such standards have been proposed including sanidine from the 1 19 ma alder creek rhyolite acs turrin et al 1994 turrin b d donnely nolan j m hearn b c jr 1994 40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited geology 22 251 254 this sanidine is satisfactorily homogeneous well intercalibrated with fish canyon sanidine fcs renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 and is linked to the astronomically calibrated magnetic polarity time scale since the alder creek rhyolite lava is the type occurrence of the cobb mountain event cm we collected 408 kg of this rock from which we anticipate being able to extract more than 8 kg of 425 m sanidine crystals analyses of sanidine phenocryts from the acs 2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1 190 0 004 to 1 194 0 003 ma1 with a weighted mean age of 1 193 0 001 ma mswd 0 74 n 225 based on fcs 28 02 ma renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the value of the intercalibration factor renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 is rfcsacs 2 0 04229 0 00012 n 8 identical with that of the original acs measurements 0 04229 0 00012 renne et al 1998 renne p r swisher c c deino a l karner d b owens t l depaolo d j 1998 intercalibration of standards absolute ages and uncertainties in 40ar 39ar dating chem geol 145 117 152 the age of acs 2 is in agreement with the recent astronomical calibration of the cm event 1 215 to 1 190 ma channell j e t mazaud a sullivan p turner s raymo m e 2002 geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin j geophys res 107 6 10 1029 2001jb000491 the apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements aliquots from various grain sizes will be available upon request zircon is also potentially available which may be of interest to u th he researchers

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

paleointensity of the early geodynamo 2 45 ga as recorded in karelia a single crystal approach we report thellier paleointensity results from single plagioclase crystals derived from ca 2 45 ga border dikes of the burakovka layered intrusion karelia russia rock magnetic and transmission electron microscope analyses indicate that these plagioclase crystals contain pseudosingle to single domain magnetite inclusions fifteen determinations from four dikes meet reliability criteria and yield an average paleointensity of 43 2 10 8 t which corresponds to a virtual dipole moment of 8 43 2 11 x 10 2 2 am 2 although our result is unlikely to adequately represent the time averaged field the mean and range of values are similar to those of the present day field these values suggest that the inner core which may stabilize the geodynamo had started to grow by early proterozoic time

southern ocean co2 sink the contribution of the sea ice we report first direct measurements of the partial pressure of co2 pco2 within antarctic pack sea ice brines and related co2 fluxes across the air ice interface from late winter to summer brines encased in the ice change from a co2 large oversaturation relative to the atmosphere to a marked undersaturation while the underlying oceanic waters remains slightly oversaturated the decrease from winter to summer of pco2 in the brines is driven by dilution with melting ice dissolution of carbonate crystals and net primary production as the ice warms its permeability increases allowing co2 transfer at the air sea ice interface the sea ice changes from a transient source to a sink for atmospheric co2 we upscale these observations to the whole antarctic sea ice cover using the nemo lim3 large scale sea ice ocean and provide first estimates of spring and summer co2 uptake from the atmosphere by antarctic sea ice over the spring summer period the antarctic sea ice cover is a net sink of atmospheric co2 of 0 029 pg c about 58 of the estimated annual uptake from the southern ocean sea ice then contributes significantly to the sink of co2 of the southern ocean

high resolution record of the laschamp geomagnetic excursion at the blake bahama outer ridge summary geomagnetic excursions are brief deviations of the geomagnetic field from behaviour expected during normal secular variation the laschamp excursion at 41ka was one such deviation previously published records suggest rapid changes in field direction and a concurrent substantial decrease in field intensity associated with this excursion accurate dating of excursions and determination of their durations from multiple locations is vital to our understanding of global field behaviour during these deviations we present here high resolution palaeomagnetic records of the laschamp excursion obtained from two ocean drilling program odp sites 1061 and 1062 on the blake bahama outer ridge odp leg 172 high sedimentation rates 30 40cmkyr 1 at these locations allow determination of transitional field behaviour during the excursion palaeomagnetic measurements of discrete samples from four cores reveal a single excursional feature across an interval of 30cm associated with a broader palaeointensity low we determine the age and duration of the laschamp excursion using a stratigraphy linked to the 18 o record from the greenland ice cores this chronology dates the laschamp excursion at the blake ridge to 41 3ka the excursion is characterized by rapid transitions less than 200yr between stable normal polarity and a partially reversed polarity state the palaeointensity record is in good agreement between the two sites revealing two prominent minima the first minimum is associated with the laschamp excursion at 41ka and the second corresponds to the mono lake excursion at 35 5ka we determine that the directional excursion during the laschamp at this location was no longer than 400yr occurring within a palaeointensity minimum that lasted 2000yr the laschamp excursion at this location is much shorter in duration than the blake and iceland basin excursions

carbonate system properties in the gerlache strait northern antarctic peninsula february 2015 i sea air co2 fluxes abstract hydrographic and carbonate system properties were determined in the gerlache strait a coastal site in the northern antarctic peninsula nap during early february 2015 it was part of an effort to monitor changes in marine inorganic carbon chemistry in antarctic coastal zones here we describe surface observations performed during the nautilus i cruise and compare results with earlier studies in the region the focus of this first part of this study is to describe the spatial patterns of sea air carbon dioxide co2 fluxes and relating the results with physical chemical and biological parameters in order to determine the co2 fluxes we use several available parameterizations for estimation of the gas transfer velocity coefficient thus providing a range of the results uncertainties contrasting with results found twenty years ago the gerlache strait was estimated here to be a monthly net source for co2 of 0 50 0 45 mol m 2 month 1 during february 2015 based on shipboard wind measurements and wanninkhof s 2014 gas transfer velocity relationship that was probably driven by to warm water intrusions in combination with a phytoplankton community mainly dominated by small flagellates particularly cryptophytes yet in order to determine whether the region acts as an annual sink or source of co2 there is a need for additional studies focusing on short time scales throughout a year

experimental evidence for carbonate precipitation and co2 degassing during sea ice formation chemical and stable carbon isotopic modifications during the freezing of artificial seawater were measured in four 4 m3 tank incubations three of the four incubations were inoculated with a non axenic antarctic diatom culture the 18 days of freezing resulted in 25 to 27 cm thick ice sheets overlying the residual seawater the ice phase was characterized by a decrease in temperature from 1 9 to 2 2oc in the under ice seawater down to 6 7oc in the upper 4 cm of the ice sheet with a concurrent increase in the salinity of the under ice seawater and brine inclusions of the ice sheet as a result of physical concentration of major dissolved salts by expulsion from the solid ice matrix measurements of ph total dissolved inorganic carbon ct and its stable isotopic composition d13ct all exhibited changes which suggest minimal effect by biological activity during the experiment a systematic drop in ph and salinity normalized ct by up to 0 37 phsws units and 376 mol c kg 1 respectively at the lowest temperature and highest salinity part of the ice sheet were coupled with an equally systematic 13c enrichment of the ct calculations based on the direct ph and ct measurements indicated a steady increase in the in situ concentration of dissolved carbon dioxide co2 aq with time and increasing salinity within the ice sheet partly due to changes in the dissociation constants of carbonic acid in the low temperature high salinity range within sea ice the combined effects of temperature and salinity on the solubility of co2 over the range of conditions encountered during this study was a slight net decrease in the equilibrium co2 aq concentration as a result of the salting out overriding the increase in solubility with decreasing temperature hence the increase in the in situ co2 aq concentration lead to saturation or supersaturation of the brine inclusions in the ice sheet with respect to atmospheric pco2 3 5x10 4 atm when all physico chemical processes are considered we expect co2 degassing and carbonate mineral precipitation from the brine inclusions of the ice sheet which were saturated or highly supersaturated with respect to both the anhydrous calcite aragonite vaterite and hydrated ikaite carbonate minerals

subterranean clues to the future of our planetary magnetic shield the geomagnetic field generated in the liquid outer core provides a shield from cosmic radiation that can cause damage to man made satellites electrical power grids and the ozone layer 1 in the absence of this shield the solar wind might gradually erode the atmosphere eventually robbing the planet of its water 2 but the dipole magnetic field has decreased in strength over the last 160 y at an alarming rate 3 motivating continued spirited debate 4 6 over whether we are in the early stages of a geomagnetic pole reversal some even believe we are overdue but 160 y is just a blink of the eye in geological terms the record is too short to have great confidence about what these trends might mean about causal processes and the future in pnas trindade et al 7 provide insightful context for these modern changes in the form of a magnetic record from south america that extends 1 500 y into the past we know much about the recent waning geomagnetic field since the time of gauss in 1840 we have had a rich data source in the form of geomagnetic observatory records in the satellite era we have a deluge of data from magnetometers in orbit from analyses of these data we know that the field intensity decrease is concentrated in an area known as the south atlantic anomaly saa a vast swatch stretching from south africa to chile amazingly when one estimates the field structure beneath this region at the core mantle 1 1email john tarduno at rochester edu 1 xref corresp 1 1

a high resolution marine sedimentary record of geomagnetic intensity during the brunhes chron abstract this investigation of paleointensity employs data from four cores taken from two sites in the sulu sea during leg 124 of the ocean drilling program these sediments accumulated at a rate of about 10 cm ky yet they provide strong stable magnetizations a combination rarely found in the deep sea environment this study examines the interval of the last glacial cycle since 130 ka in most detail using paleomagnetic measurements from discrete samples to explore a variety of normalization parameters paleomagnetic remanence and susceptibility measurements taken aboard ship at odp site 769 extend the paleointensity record to the brunhes matuyama boundary after normalization by susceptibility the shipboard intensity data compare well with the more detailed measurements from discrete samples in the intervals for which such data are available and so complete a brunhes age 0 0 78 ma history of paleointensity for the sulu sea that record shows a number of short duration decreases in paleointensity that appear to be useful stratigraphic markers these results also indicate that the average geomagnetic dipole moment during the brunhes chron may have been considerably lower than the present value

relative paleointensity of the geomagnetic field during the brunhes chron abstract we conducted a paleomagnetic study of four sediment cores taken from different regions in the low latitudes of the pacific ocean in order to recover relative paleointensity variations during the brunhes chron three cores have good age control from oxygen isotope stratigraphy we extensively examined rock magnetic uniformity by the iso thermal remanent magnetization irm acquisition and the alternating field demagnetization of the anhysteretic remanent magnetization arm and the irm only rock magnetically homogeneous sediments were used biogenic magnetites were found in all but one core and their dissolution in early diagenesis was not significant the four cores showed coherent variations of the remanent intensity normalized by the arm implying that these variations represent the behavior of the geomagnetic field long term paleointensity variations during the brunhes chron are characterized by an asymmetric pattern rapid increases in the intensity occurred just after the brunhes matuyama transition in the mid brunhes from about 500 to 400 ka and during the last 100 ka and gradual decreases between them the first one supports the idea of rapid regeneration of the geomagnetic field after polarity reversals 1 the other two may have been triggered by very short polarity reversals

asymmetrical saw tooth pattern of the geomagnetic field intensity from equatorial sediments in the pacific and indian oceans abstract a record of relative paleointensity from marine sediments in the equatorial indian ocean spanning the last 4 ma completes a previous dataset from ocean drilling program odp leg 138 sites in the equatorial pacific 1 the timescale was defined with a precision better than 20 kyr with different and independent methods most of the features present in the record from the equatorial pacific are confirmed the overall saw tooth pattern is observed across every field reversal and the short term fluctuations superimposed on the slow intensity decrease preceding the reversals appear to be similar at both sites the global character of these features reinforces their interpretation in terms of changes in dipole field intensity and provides new important constraints on models of the geodynamo a synthetic curve of field intensity changes during the last 4 ma is proposed as a first paleointensity timescale for future stratigraphic studies

the argus multicollector noble gas mass spectrometer performance for 40ar 39ar geochronology we describe a new high sensitivity multicollector noble gas mass spectrometer argus that has been specifically designed for simultaneous collection of all ar isotopes and hence is ideally suited for 40ar 39ar geochronology the instrument uses a detector housing that holds five faraday collectors one collector is equipped with a 1011 ohm resistor for 40ar and the remaining four are fitted with 1012 ohm resistors for 36ar to 39ar argus has a mass resolution of 225 250 and a measured sensitivity of 7 10 14 mol v at 200 a trap current during the course of a 1 month run cycle 145 air calibrations yielded a weighted average 40ar 36ar of 300 67 0 07 2 eight data points rejected and mean square weighted deviation 1 18 the ages of three mineral standards taylor creek rhyolite sanidine heidelberg biotite and alder creek sanidine are within error of the accepted ages and uncertainties for two are improvements on previously published data argus decreases analytical time while allowing more measurements to be made at higher precision compared to standard peak jumping single collector mass spectrometers

co2 fluxes across the air sea interface in the southeastern beaufort sea ice free period 1 surface mixed layer co2 fugacities fco2 sw calculated from carbonate system parameters in the southeastern beaufort sea during the ice free period ranged from 240 to 350 atm in fall 2003 and from 175 to 515 atm in summer 2004 the surface mixed layer remains mostly undersaturated with respect to atmospheric co2 378 atm and therefore acts as a potential co2 sink throughout this period air sea co2 fluxes fco2 were first computed assuming ice free conditions and ranged from 32 4 to 8 6 mmol m 2 d 1 in fall 2003 and summer 2004 respectively then we included a reduction factor to account for ice cover ic and we computed the resulting fluxes fco2 ic in fall 2003 fco2 ic ranged from 4 7 mmol m 2 d 1 in the relatively open water of the cape bathurst polynya to 0 1 mmol m 2 d 1 in the southeastern beaufort sea limited by the presence of the multiyear sea ice in summer 2004 fco2 ic ranged from 13 1 mmol m 2 d 1 on the western mackenzie shelf to 8 6 mmol m 2 d 1 at cape bathurst the variability being ascribed to competing effects of vertical mixing temperature variations and possibly biological production on average a net sink of 2 3 3 5 mmol m 2 d 1 was estimated for the ice free period over the study area nevertheless the fco2 displays strong variability due to ice coverage freshwater input and upwelling events the potential responses direction and intensity of potential feedbacks of the carbon cycle in the study area to a changing arctic climate are discussed

re evaluation of greek archaeomagnitudes a new technique is used to determine the secular variation of the geomagnetic ancient field in athens the results indicate that published greek archaeomagnitudes were seriously affected by alteration it is shown that all intensities obtained using modifications of the thellier method may be affected by alteration and are therefore suspect

relative paleointensity across the last geomagnetic reversal from sediments of the atlantic indian and pacific oceans paleointensity records from marine sediments at three locations in the atlantic indian and pacific oceans show coherent and reproducible signals across the brunhes matuyama reversal the low field susceptibility signals are predominantly anticorrelated between the indo pacific and the atlantic sites while the normalization of the natural remanent magnetization by any rock magnetic parameter yield identical results we deduce that in these cases climatic components do not induce first order effects in the determination of relative paleointensity the results establish the worldwide character of the triangular pattern displayed by the decay and recovery phases of the field intensity variations across the brunhes matuyama reversal

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

emergence and evolution of the south atlantic anomaly revealed by the new paleomagnetic reconstruction shawq2k abstract the south atlantic anomaly saa is one of the most outstanding features of the present geomagnetic field thus a good knowledge of the saa is clue for a better understanding of the dynamical behavior of the geomagnetic field to achieve this goal paleomagnetic data are crucial since they provide the unique way to investigate past changes in the earth s magnetic field here we present a new global geomagnetic field reconstruction the shawq2k model which is based on a critical revision of the global archeomagnetic and volcanic dataset the new model provides an improved description of the geomagnetic field during the last 2 millennia and yields surprising outcomes about the emergence and development of the saa it shows that the reversed flux patch observed at the core mantle boundary and linked to the saa emerged in the southern hemisphere from at least 950 ad this patch moved westward from the equator to southern latitudes being clearly isolated after 1400 ad in addition since 1550 ad a second reversed flux patch moving northeastward is observed in the north atlantic the new data now available for the southern hemisphere coming from africa and south america together with the use of an appropriated weighting scheme in the modeling process have improved our understanding of past geomagnetic field behavior and showed new evolutionary aspects of the saa

padm2m a penalized maximum likelihood model of the 0 2 ma palaeomagnetic axial dipole moment summary we present a new time varying model for palaeomagnetic axial dipole moment padm for the past 2 myr and compare it with earlier virtual axial dipole moment vadm reconstructions which have been based on stacking and averaging scaled relative palaeointensity records the padm is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood pml approach to recover a cubic b spline representation of axial dipole field variations on million year timescales the pml method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average vadm reconstructions we apply the pml method to a set of 96 032 published data 1800 palaeointensities from igneous rocks 3300 archaeointensities and 86 relative palaeointensity time series of variable lengths and resolutions two models are discussed padm2mp is a trial model based on a subset of the nine longest available sedimentary records padm2m uses a comprehensive data set 76 records 81 446 data 10 records were eliminated and is our preferred model padm2m has a lower mean than existing vadm reconstructions but shows similarities in long period variability some differences in timing amplitude and resolution of certain features can be attributed to variations in age assignments others result from our more comprehensive data set and a reduction in bias attributable to pml modelling padm2m has an average axial dipole moment over 0 2 ma of 5 3 1022 am2 with a standard deviation of 1 5 1022 am2 the brunhes chron average 6 2 1022 am2 is higher than for earlier epochs of matuyama 4 8 1022 am2 as seen in some previous studies the power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 102 myr 1 we see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour

a 200 ka geomagnetic chronostratigraphy for the labrador sea indirect correlation of the sediment record to specmap abstract a chronostratigraphic framework for the last 200 ka based on relative variations of geomagnetic paleointensity has been derived for the labrador sea where 18o stratigraphies are compromised by lack of benthic foraminifera and environmental perturbations of the planktic record relative paleointensity records derived from three labrador sea piston cores are correlated to a recently developed 200 ka paleointensity composite sint 200 y guyodo j p valet earth planet sci lett 143 1996 23 36 the chronology for sint 200 was derived by correlating oxygen isotope stratigraphies from individual paleointensity records to the specmap chronostratigraphy the correlation of sint 200 to labrador sea paleointensity records allows the specmap chronology to be imported into the labrador sea although comparisons of the chronostratigraphy based on paleointensity correlations with those derived from ams 14c dates and 18o data indicate significant discrepancies support for the new chronostratigraphic framework is provided by 1 consistent correlation of labrador sea detrital layers to north atlantic heinrich layers 2 general consistency between planktic 18o values and the specmap reference curve and 3 feasible prediction for the age of ash layer 2 as the new chronostratigraphy is tied to specmap labrador sea sediment cores can now be directly compared with any specmap dated record

a link between geomagnetic reversals and events and glaciations abstract the apparent duration of geomagnetic polarity events in arctic ocean sediments is much longer than in sediments from lower latitudes in fact while the remanence of brunhes age sediment cores from the yermak plateau at 82 n is fully reversed for 30 of their lengths 1 the events often evade detection in many other continuously deposited sediments for example the laschamp event is absent in an otherwise high resolution record of secular variation from lac du bouchet 2 which is located near the laschamp volcanics where the event was first detected very short event durations of a few hundred years at the most have been suggested before 2 3 because sedimentation rates in the arctic ocean were increased during glaciations the exaggerated proportion of reverse polarities in sediments from high latitudes suggests a link between glaciation and field reversals this suggestion is supported by magnetostratigraphic results obtained from thick loess paleosol sequences in china 4 these demonstrate that all polarity boundaries separating chrons and subchrons since the gauss matuyama field reversal have been recorded in loess and thus during periods of cold climate although conflicting evidence exists for some boundaries furthermore the ages of 22 events and chron boundaries have been compared with the oxygen isotope record 5 thought to represent global ice volume all events and reversals younger than 2 6 ma may have occurred during periods of global cooling or during cold stages however some ages are still too poorly dated for a definite correlation climatic signals also exist in the two longest relative paleointensity records 6 7 but these are suspected to be caused by climatically driven variations in the rock magnetic parameters a mechanism for field reversals may be the acceleration of the earth s rotation caused by lowering of the sea level during glaciations the short duration of events also implies that the geomagnetic field can reverse an order of magnitude faster than commonly assumed

south atlantic and north atlantic geomagnetic paleointensity stacks 0 80 ka implications for inter hemispheric correlation recent coring of high deposition rate atlantic sediments has led to the development of north atlantic napis and south atlantic sapis geomagnetic paleointensity stacks the sapis stack comprises five records from the sub antarctic south atlantic 41 471s 6 101e four are from piston cores collected during the ocean drilling program odp leg 177 site survey cruise earth planet sci lett 175 2000 145 the other is odp site 1089 leg 177 j geophys res 2001 submitted for publication all but one 4 pc03 of the sapis cores have oxygen isotope records which are used in conjunction with lithologic and geomagnetic variability to derive an optimized correlation to site 1089 the site 1089 age model is derived by mapping the benthic and planktic isotope data to those from nearby core rc11 83 which has 14 calibrated radiocarbon ages in the 11 41 ka interval below this level the chronology is derived by matching the site 1089 benthic oxygen isotope data to the specmap stack the napis stack comprises six records from a wide area of the north atlantic philos trans r soc ser a 358 2000 1009 correlation between cores was based on marine isotopic stage boundaries augmented by millennial scale features heinrich events and fluctuations in concentration dependent magnetic parameters napis was placed on the gisp2 chronology using the marine to ice core oxygen isotope correlation proposed by voelker et al radiocarbon 40 1998 517 for one of the napis cores in the resulting age model the laschampevent recorded in five of six cores falls in a very narrow o1 kyr age range and coincides with the 10 be and 36 cl peak measured in the ice cores comparison of the two stacks placed on their own independent age models indicates that common millennial scale paleointensity features are preserved although more work is needed to define the true global content of the paleointensity record as well as the precise age of many features it is readily apparent that paleointensity can provide a global correlation tool at a resolution unattainable from isotope data alone r 2002 published by elsevier science ltd

reconstructing the holocene geomagnetic field abstract knowledge of the holocene evolution of earth s magnetic field is important for understanding geodynamo processes in the core is necessary for studying long term solar terrestrial relationships and can provide useful age constraints for archeologicaland stratigraphic applications continuous time varying global field models based on archeo and paleomagnetic data are useful tools in this regard we use a comprehensive data compilation and recently refined modelling strategies to produce cals10k 1b the first time varying spherical harmonic geomagnetic field model spanning 10 ky the model is an average obtained from bootstrap sampling to take account of uncertainties in magnetic components and ages in the data and hence has version number 1b instead of 1 this model shows less spatial and temporal resolution than earlier versions for 0 3 ka and particularly aims to provide a robust representation of the large scale field at the core mantle boundary cmb we discuss the geomagnetic dipole evolution and changes in holocene magnetic field morphology at the cmb as shown by the new reconstruction the results are compatible with earlier models cals3k 3 and cals3k 4 for 0 3 ka but reveal some clear deficiencies in the 0 7 ka cals7k 2 model prior to 3 ka cals10k 1b is able to resolve mobile and structurally evolving high latitude radial field flux lobes at the cmb in both hemispheres as well as persistent non zonal structure in the 10 ky average contributions to the average field from time varying structures in the equatorial indonesian australian region are particularly striking

estimates of heat flow in the deep mantle based on the power requirements for the geodynamo 1 regeneration of the magnetic field by convection in the core places demands on heat flow into the base of the mantle if the heat flow is too low thermal convection is shut off and the rate of generation of compositional buoyancy by the solidification of the core becomes too low to sustain the geodynamo conversely a large heat flow causes rapid growth of the inner core so that convection prior to the appearance of the inner core must be sustained by thermal buoyancy alone the attendant requirements on primordial heat become more severe as the age of the inner core decreases we show that estimates of the present day heat flow satisfy the power requirements for the geodynamo however the resulting thermal history is incompatible with estimates of mantle temperatures prior to 3 ga this discrepancy can be resolved by accumulating radioactive isotopes in d or adding heat sources to the core

dynamics of pco2 and related air ice co2 fluxes in the arctic coastal zone amundsen gulf beaufort sea we present an arctic seasonal survey of carbon dioxide partial pressure pco 2 dynamics within sea ice brine and related air ice co 2 fluxes the survey was carried out from early spring to the beginning of summer in the arctic coastal waters of the amundsen gulf high concentrations of pco 2 up to 1834 matm were observed in the sea ice in early april as a consequence of concentration of solutes in brines caco 3 precipitation and microbial respiration caco 3 precipitation was detected through anomalies in total alkalinity ta and dissolved inorganic carbon dic this precipitation seems to have occurred in highly saline brine in the upper part of the ice cover and in bulk ice as summer draws near the ice temperature increases and brine pco 3 shifts from a large supersaturation 1834 matm to a marked undersaturation down to almost 0 matm this decrease was ascribed to brine dilution by ice meltwater dissolution of caco 3 and photosynthesis during the sympagic algal bloom the magnitude of the co 2 fluxes was controlled by ice temperature through its control on brine volume and brine channels connectivity and the concentration gradient between brine and the atmosphere however the state of the ice interface clearly affects air ice co 2 fluxes

global contraction of antarctic bottom water between the 1980s and 2000s abstracta statistically significant reduction in antarctic bottom water aabw volume is quantified between the 1980s and 2000s within the southern ocean and along the bottom most southern branches of the meridional overturning circulation moc aabw has warmed globally during that time contributing roughly 10 of the recent total ocean heat uptake this warming implies a global scale contraction of aabw rates of change in aabw related circulation are estimated in most of the world s deep ocean basins by finding average rates of volume loss or gain below cold deep potential temperature surfaces using all available repeated hydrographic sections the southern ocean is losing water below 0 c at a rate of 8 2 2 6 106 m3 s 1 this bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near surface recovery apparently through a southward surge of circumpolar deep water from the north to the north smaller losses of bottom waters are see

relating the south atlantic anomaly and geomagnetic flux patches abstract the south atlantic anomaly saa is a region of weak geomagnetic field intensity at the earth s surface which is commonly attributed to reversed flux patches rfps on the core mantle boundary cmb while the saa is clearly affected by the reversed flux region below the south atlantic we show that the relation between the intensity minimum at earth s surface and rfps is not straightforward we map a field dependent intensity kernel constable 2007a to study the relation between the radial geomagnetic field at the cmb and the field intensity at earth s surface synthetic tests highlight the role of specific patches reversed and normal in determining the location of the surface intensity minimum and demonstrate that the saa can indeed be explained by a few intense patches we show that the level of axial dipolarity of the field determines the stability of the relation between the saa minimum and rfps the present position of the saa minimum is determined by the interplay among several robust geomagnetic flux patches at the cmb the longitude of the saa minimum appears near the longitude of the patagonia rfp due to the low latitude normal flux patches nfps near africa and mid atlantic which diminish the effect of the africa rfps the latitude of the saa minimum is lower than the patagonia rfp latitude due to the south pacific high latitude nfp and the axial dipole effect the motion of the saa minimum is explained by the motions and changes in intensity of these robust geomagnetic flux patches simple secular variation sv scenarios suggest that while the saa path can be explained by advection its intensity decrease requires magnetic diffusion in addition these sv scenarios provide some speculative predictions for the saa

late holocene multi proxy records of environmental change on the south atlantic island tristan da cunha sediment stratigraphies from three sites one lake one overgrown lake and an exposed section on the island tristan da cunha in the temperate south atlantic were analysed by means of pollen analysis total carbon nitrogen and sulphur content determination magnetic susceptibility measurements and detailed radiocarbon dating the aim of these studies was to reconstruct the late holocene vegetation and climatic variations the oldest sediment sequence extends back to 2300 cal years bp the vegetation was relatively stable up to the arrival of humans in the 17th century the appearance of the introduced taxon rumex acetosa acetosella at c 300 cal years bp and a subsequent decline in forest cover on the lowland plain provide evidence of substantial human influence on the vegetation well before the establishment of the first permanent settlement in the 19th century before the first anthropogenic influence centennial scale fluctuations in the proxy records are interpreted as reflections of local hydrological changes probably caused by variations in precipitation as inferred mainly from changing proportions of pollen derived from telmatic and terrestrial taxa and corresponding changes in the deposition of mineral matter by fluvial erosion lake levels were low between c 1450 and 1050 cal years bp and high between c 1050 and 300 cal years bp these variations coincide with known climatic changes in southern africa and in the north atlantic suggesting that the inferred hydrological changes on tristan da cunha were related to large scale variations in the general oceanic and atmospheric circulation in the atlantic region

air sea exchange of carbon dioxide in the southern ocean and antarctic marginal ice zone direct carbon dioxide flux measurements using eddy covariance from an icebreaker in the high latitude southern ocean and antarctic marginal ice zone are reported fluxes were combined with the measured water air carbon dioxide partial pressure difference pco2 to compute the air sea gas transfer velocity k normalized to schmidt number 660 the open water data showed a quadratic relationship between k cm h 1 and the neutral 10 m wind speed u10n m s 1 kopen 0 245 u10n2 1 3 in close agreement with decades old tracer based results and much lower than cubic relationships inferred from previous open ocean eddy covariance studies in the marginal ice zone the effective gas transfer velocity decreased in proportion to sea ice cover in contrast with predictions of enhanced gas exchange in the presence of sea ice the combined open water and marginal ice zone results affect the calculated magnitude and spatial distribution of southern ocean carbon flux

carbon cycling dynamics in the seasonal sea ice zone of east antarctica the carbon cycle of the seasonally ice covered region of the southwest indian ocean sector of east antarctica 30 80 e 60 69 s was investigated during austral summer january march 2006 large variability in the drivers and timing of carbon cycling dynamics were observed and indicated that the study site was a weak net source of carbon dioxide co2 to the atmosphere of 0 8 1 6 g c m 2 during the ice free period with narrow bands of co2 uptake observed near the continental margin and north of the southern antarctic circumpolar current front continuous surface measurements of dissolved oxygen and the fugacity of co2 were combined with net community production estimates from oxygen argon ratios to show that surface heat gain and photosynthesis were responsible for the majority of observed surface water variability on seasonal timescales winter sea ice cover reduced the flux of co2 to the atmosphere in the study area followed by biologically driven drawdown of co2 as the ice retreated in spring summer highlighting the important role that sea ice formation and retreat has on the biogeochemical cycling of the region

tristan da cunha constraining eruptive behavior using the 40ar 39ar dating technique the historically active volcanic ocean island of tristan da cunha exhibits a complex and dynamic history with numerous often compositionally distinct parasitic centers punctuating the large edifi ce to date the temporal relationship between differing styles of activity has been unclear we have applied high precision 40ar 39ar dating to 15 carefully selected samples from tristan da cunha to ascertain spatio temporal relationships of recent volcanism explore episodicity and establish if the most recent summit activity post dated eruptions from parasitic centers lower on the fl anks this has yielded a new suite of reliable holocene ages with the youngest dated deposit at 3 1 ka 1 a recent fl ow at the summit was constrained to 5 1 ka 1 confi rming that summit and parasitic activity on the volcano s fl anks overlap in time the oldest dated deposits were 118 4 ka 1 from a parasitic cone in the southern sector and 81 10 ka 1 from one of the lowest sub aerial shield forming lava fl ows in the northern sector large scale sector collapse is bracketed between 34 1 ka and 26 5 ka 1 via dating of the youngest headwall lava fl ow and oldest sub aerial scarp fi lling deposits no systematic relationship between the new temporal framework vent location and eruptive compositions was found although magmatic fl ux has been inferred to be relatively low tristan da cunha is capable of relatively frequent eruptions from a wide variety of vent locations across a broad range of compositions

carbonate system properties in the gerlache strait northern antarctic peninsula february 2015 ii anthropogenic co 2 and seawater acidification abstract during the nautilus i cruise in february 2015 the carbonate and associated physical properties were measured throughout the water column in the gerlache strait off the southern coast of the northern antarctic peninsula nap part i of this study kerr et al 2017 this issue focused on the net sea air carbon dioxide co2 flux whereas the same dataset was used here to estimate the extent of anthropogenic carbon cant storage in the gerlache strait deep basin in addition the impact of cant increases on ph and calcium carbonate saturation states for calcite and aragonite ca and ar respectively were evaluated the results indicate that up to the present 21 2 16 7 mol kg 1 of cant have been injected to the deep and bottom layers of the gerlache strait two mechanisms may have contributed to that fact i the pathway of the cant follows that of the high salinity shelf waters inflow coming from both the bransfield strait and the northwestern weddell sea shelf and ii the gerlache strait is absorbing a significant amount of cant from the atmosphere therefore ph ar and ca are decreasing in the deep waters of the gerlache strait since ar is already very close to 1 at depth any small additional input of cant will trigger the dissolution of aragonite

climatic and geomagnetic influences on the lac du bouchet palaeomagnetic sv record through the last 110 000 years abstract palaeomagnetic secular variation records stacked from five cores 20 m long extend back through the last glacial cycle to 100 000 years ago the time scale has been constructed from conventional and accelerator radiocarbon ages back to 45 ka bp and beyond this by comprehensive pollen studies which identify forest and steppe cycles that have been related to marine oxygen isotopic stages 4 and 5 spectral analyses of the intensity and directional records using both fourier and maximum entropy methods reveal strong low frequency components which fall in the range of the milankovitch obliquity and precession periodicities 40 20 ka it is concluded that although part of this signal may have fed indirectly into the natural remanence via the influence of climatic change on sediment facies part has been impressed directly by perturbation of the geomagnetic dynamo process at the core boundary by variations in the earth s obliquity and precession these long periods envelop a well defined pattern registered also by the independent mackereth core stack back to 50 ka bp of irregular secular oscillations of the geodynamo with periods of a few thousands of years

small scale structure of the geodynamo inferred from oersted and magsat satellite data the geodynamo in the earth s liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the earth s surface1 2 here we use data from the currently operating danish oersted3 satellite and from the us magsat2 satellite that operated in 1979 80 to identify and interpret variations in the magnetic field over the past 20 years down to length scales previously inaccessible projected down to the surface of the earth s core we found these variations to be small below the pacific ocean and large at polar latitudes and in a region centred below southern africa the flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs and retrograde vortices with lows in the historical 400 year average magnetic field4 5 this pattern is analogous to those seen in a large class of numerical dynamo simulations6 except for its longitudinal asymmetry if this asymmetric state was reached often in the past it might account for several persistent patterns observed in the palaeomagnetic field7 8 9 10 we postulate that it might also be a state in which the geodynamo operates before reversing

interpretation of recent southern hemisphere climate change climate variability in the high latitude southern hemisphere sh is dominated by the sh annular mode a large scale pattern of variability characterized by fluctuations in the strength of the circumpolar vortex we present evidence that recent trends in the sh tropospheric circulation can be interpreted as a bias toward the high index polarity of this pattern with stronger westerly flow encircling the polar cap it is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex which are due largely to photochemical ozone losses during the summer fall season the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the antarctic peninsula and patagonia and to the cooling over eastern antarctica and the antarctic plateau

normalized remanence in sediments of the western equatorial pacific relative paleointensity of the geomagnetic field piston cores from the ontong java plateau collected during the eurydice expedition have been used to estimate regional geomagnetic paleointensity rock magnetic measurements of the sediments show that 1 the remanence is carried by stable pseudo single domain magnetite 2 the grain sizes and concentrations are relatively uniform with respect to the normalization technique and 3 susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field we therefore have measured the remanent intensity both natural and demagnetized to 15 mt and susceptibility of archive quarters of three cores at centimeter intervals using an automated through bore magnetometer a further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores one of our cores displays high coherence over a broad range of frequencies while for two others coherence is insignificantly different from zero at the 95 level of confidence oxygen isotopic records paleontological datum levels and the brunhes matuyama boundary provide an independent temporal framework for inter core comparison when the two cores which pass our acceptance criteria are placed on a common time scale the coherence between the normalized intensity records is significantly different from zero at well above the 95 level of confidence suggesting some common input signal over the same band of frequencies furthermore there is a great deal of similarity between the records from the ontong java plateau and a record from core rc10 167 taken some 3000 km to the north the distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows 0 5 ma with a slightly smaller standard deviation the inter core agreement within a single region the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years over the period 0 350 ka this feature is however lacking for the period 350 700 ka

macronutrient supply uptake and recycling in the coastal ocean of the west antarctic peninsula nutrient supply uptake and cycling underpin high primary productivity over the continental shelf of the west antarctic peninsula wap here we use a suite of biogeochemical and isotopic data collected over five years in northern marguerite bay to examine these macronutrient dynamics and their controlling biological and physical processes in the wap coastal ocean we show pronounced nutrient drawdown over the summer months by primary production which drives a net seasonal nitrate uptake of 1 83 mol n m 2 yr 1 equivalent to net carbon uptake of 146 g c m 2 yr 1 high primary production fuelled primarily by deep sourced macronutrients is diatom dominated but non siliceous phytoplankton also play a role strong nutrient drawdown in the uppermost surface ocean has the potential to cause transient nitrogen limitation before nutrient resupply and or regeneration interannual variability in nutrient utilisation corresponds to winter sea ice duration and the degree of upper ocean mixing implying susceptibility to physical climate change the nitrogen isotope composition of nitrate 15nno3 shows a utilisation signal during the growing seasons with a community level net isotope effect of 4 19 0 29 we also observe significant deviation of our data from modelled and observed utilisation trends and argue that this is driven primarily by water column nitrification and meltwater dilution of surface nitrate this study is important because it provides a detailed description of the nutrient biogeochemistry underlying high primary productivity at the wap and shows that surface ocean nutrient inventories in the antarctic sea ice zone can be affected by intense recycling in the water column meltwater dilution and sea ice processes in addition to utilisation in the upper ocean

past and future evolution of the marine carbonate system in a coastal zone of the northern antarctic peninsula abstract it is arduous to gather a good spatial and temporal dataset of marine carbonate properties especially in the southern ocean in this study we have reconstructed the carbonate system in the gerlache strait a coastal zone of the northern antarctic peninsula we also analyzed the impact of ocean acidification by calculating the tipping points of the calcium carbonate saturation states and ph i e when saturation state and ph goes below one and 7 respectively hydrographic and carbonate data from three distinct data sets goal 2013 2016 fruela 1996 and world ocean database 1965 2004 have been joined and used to reconstruct the carbonate system from the past 50 years temporal annual mean trends were determined depending on the water column depth layer the northern gerlache strait showed a significant increasing trend of total inorganic carbon concentrations 1 0024 0 34 mol kg 1 and related ph decreasing trend 0 0026 0 0009 sws in the surface mixed layer 60 m the properties variability is relatively different magnitudes and signs between the northern and southern sectors of the gerlache strait which indicate that adjacent regions to the gerlache strait to the southwest and north respectively may major influence the regional carbonate dynamics results also show that episodic under saturation conditions in relation to aragonite within the surface mixed layer may already occur especially in regions close to large glaciers

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

paleointensity record from pleistocene sediments 1 4 0 ma off the california margin high resolution magnetic measurements of 45 m of sediment from the delgada fan area at ocean drilling program site 1021 39 5 n 127 46 w were performed in order to extract a reliable signal of relative paleointensity a detailed chronology was established by correlation of the magnetic susceptibility variations to the insolation curve for the past 1 4 myr covered by the record the remanence carrier is pseudo single domain sized magnetite that varies in concentration by a factor 3 stepwise alternating field demagnetization isolated a very stable natural remanent magnetization nrm which showed the succession of the cobb and jaramillo subchrons and directional changes associated with the punaruu event 1 105 ma two techniques have been used to extract the signal of relative paleointensity a significant climatic component remained present in the curve obtained from the slopes of the nrm versus the anhysteretic remanent magnetization arm in contrast the nrm and the susceptibility k lf both being driven by orbital forcing normalization by susceptibility is not contaminated by climatically driven changes and is thus taken as our best estimate of relative paleointensity comparison with other high resolution records and compilations reinforces the actual geomagnetic origin of the variations displayed by the nrm k signal overall this record confirms the succession of distinct highs and lows which are concomitant with full field reversals and short term geomagnetic events present during the matuyama chron the presence of large oscillations of the field with no periodicity confirms that the geomagnetic field at least his dipolar part is highly unstable and may have remained in this mode for a long period of time

the age of the inner core abstract the energy conservation law when applied to the earth s core and integrated between the onset of the crystallization of the inner core and the present time gives an equation for the age of the inner core in this equation all the terms can be expressed theoretically and given values and uncertainties of all relevant physical parameters the age of the inner core can be obtained as a function of the heat flux at the core mantle boundary and the concentrations in radioactive elements it is found that in absence of radioactive elements in the core the age of the inner core cannot exceed 2 5 ga and is most likely around 1 ga in addition to have an inner core as old as the earth concentrations in radioactive elements needed in the core are too high to be acceptable on geochemical grounds

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

earth science core values calculating the age of the earth s solid inner core has proved to be a tricky business but the suggestion that there is more potassium in the core than had been thought could help to reconcile differing estimates

feedbacks between ice cover ocean stratification and heat content in ryder bay western antarctic peninsula a multiyear all season time series of water column physical properties and sea ice conditions in ryder bay at the western antarctic peninsula wap is used to assess the effects on the ocean of varying ice cover reduced ice cover leads to increased mixing and heat loss in the winter the reduction in stratification persists into the following summer preconditioning the water column to a greater vertical extent of surface driven mixing this leads to an increased amount of heat from insolation being mixed down affecting approximately the top 100 m the increased heat uptake in summer exceeds the heat lost the preceding winter giving the initially counter intuitive effect that enhanced winter cooling generates warmer temperatures in the following summer and autumn this process is therefore a positive feedback on sea ice as reduced sea ice leads to increased heat content in the ocean the following autumn it also causes increased winter atmospheric temperatures due to the increased winter heat loss from the ocean in the deeper part of the water column heat and carbon stored in the circumpolar deep water cdw layer are released by deep mixing events at these depths conditions are restored by advection and vertical mixing on multiyear time scales in recent years stronger deep mixing events in winter have led to a persistent reduction in cdw temperatures at the study site ocean glider data demonstrate the representativeness of these results across the wider region of marguerite bay within which ryder bay is situated

ocean acidification and calcium carbonate saturation states in the coastal zone of the west antarctic peninsula the polar oceans are particularly vulnerable to ocean acidification the lowering of seawater ph and carbonate mineral saturation states due to uptake of atmospheric carbon dioxide co2 high spatial variability in surface water ph and saturation states for two biologically important calcium carbonate minerals calcite and aragonite was observed in ryder bay in the coastal sea ice zone of the west antarctic peninsula glacial meltwater and melting sea ice stratified the water column and facilitated the development of large phytoplankton blooms and subsequent strong uptake of atmospheric co2 of up to 55 mmol m 2 day 1 during austral summer concurrent high ph 8 48 and calcium carbonate mineral supersaturation aragonite 3 1 occurred in the meltwater influenced surface ocean biologically induced increases in calcium carbonate mineral saturation states counteracted any effects of carbonate ion dilution accumulation of co2 through remineralisation of additional organic matter from productive coastal waters lowered the ph 7 84 and caused deep water corrosivity aragonite 0 9 in regions impacted by circumpolar deep water episodic mixing events enabled co2 rich subsurface water to become entrained into the surface and eroded seasonal stratification to lower surface water ph 8 21 and saturation states aragonite 1 8 relative to all surface waters across ryder bay uptake of atmospheric co2 of 28 mmol m 2 day 1 in regions of vertical mixing may enhance the susceptibility of the surface layer to future ocean acidification in dynamic coastal environments spatially resolved studies are essential to elucidate the natural variability in carbonate chemistry in order to better understand and predict carbon cycling and the response of marine organisms to future ocean acidification in the antarctic coastal zone

link between excursions and paleointensity inferred from abnormal field directions recorded at la palma around 600 ka abstract an excursion of the geomagnetic field characterized by strongly abnormal directions associated with a field intensity as low as 7 t has been identified in a lava flow sequence from la palma canary islands the age of 602 12 ka obtained by k ar dating using the cassignol technique for a transitional flow represents our best estimate for the age of the occurrence of this episode of excursional directions this age is concordant with a minimum observed in the global sint800 composite record derived from worldwide deep sea records of relative paleointensity we show that a geomagnetic field model composed of a present day like non dipole field superimposed on an axial dipole reduced by a factor of 2 compared to its present value can account for excursional directions associated with low paleointensity in some areas while most other parts of the globe display apparently normal polarity directions we next generate through linear inversion a synthetic field model constrained by the actual la palma data and a set of directions at 30 sites distributed worldwide based on an axial dipole field with half present day intensity the energy spectrum of this synthetic field model is compatible with most of the non dipole part of the present day field and in that sense is earth like i e represents an acceptable configuration of the past geomagnetic field at 600 ka the model shows that very anomalous excursional directions covering 1 5 of the earth s surface may co exist with apparently normal directions elsewhere the new data and modeling strongly support the hypothesis that excursions are an integral aspect of secular variation there may be a continuum between normal sv and field reversals primarily controlled by the ratio of axial dipole field intensity relative to the remaining non axial dipole part

air sea co2 exchange in the ross sea antarctica although the ross sea is one of the most productive regions in antarctica it is not clear to what extent this region is an atmospheric co2 sink we calculate instantaneous co2 flux rates with in situ pco2 and wind speed data from 20 cruises in the ross sea in addition we estimate annual co2 fluxes into the ross sea with nutrient budgets from a late summer cruise we find that the ross sea is a lesser atmospheric co2 sink 7 5 0 5 tg c yr 1 1 3 0 1 mol c m 2 yr 1 than previously reported 13 tg c yr 1 1 7 to 4 2 mol c m 2 yr 1 one exception is terra nova bay tnb in the western ross sea with co2 flux rates 4 8 0 3 mol c m 2 january march that are 3 4 times greater than the ross sea mean the majority of the co2 flux into tnb occurs during the late summer with instantaneous co2 flux rates up to 246 mmol c m 2 d 1 these extraordinary co2 flux rates are caused by the unique coupling of strong katabatic winds and low surface pco2 values although strong katabatic winds deepen the mixed layer and entrain co2 rich water from below late season net community productivity maintains low surface water pco2 levels while tnb only covers 1 3600 km2 of the ross sea continental shelf extraordinary air to sea co2 fluxes during the late summer may be regular features in many of the major sea ice production polynyas 148 000 km2 combined including antarctic bottom water formation regions

the intensity of the earth s magnetic field over the past 160 million years in contrast to our detailed knowledge of the directional behaviour of the earth s magnetic field during geological and historical times1 2 data constraining the past intensity of the field remain relatively scarce this is mainly due to the difficulty in obtaining reliable palaeointensity measurements a problem that is intrinsic to the geological materials which record the earth s magnetic field although the palaeointensity database has grown modestly over recent years3 4 5 these data are restricted to a few geographical locations and more than one third of the data record the field over only the past 5 myr the most recent database5 covering the time interval from 5 to 160 myr contains only about 100 palaeointensity measurements here we present 21 new data points from the interval 5 160 myr obtained from submarine basalt glasses collected from locations throughout the world s oceans whereas previous estimates for the average dipole moment were comparable to that of the earth s present field6 the new data suggest an average dipole moment of 4 2 2 3 1022 a m2 or approximately half the present magnetic field intensity this lower average value should provide an important constraint for future efforts to model the convective processes in the earth s core which have been responsible for generating the magnetic field

paleomagnetic field properties at high southern latitude 1 statistical analyses of paleomagnetic data from lava flows are used to study geomagnetic field behavior on million year timescales previous paleomagnetic studies have lacked high latitude measurements necessary to investigate the persistence of geomagnetic anomalies observed in the recent and historical field and replicated in some numerical geodynamo simulations these simulations suggest that reduced convective flow inside the tangent cylinder may affect the magnetic field at high latitude whereas lower latitude observations are expressions of columnar helical flow outside the tangent cylinder this paper presents new paleointensity and paleodirectional data from 100 volcanic sites in the erebus volcanic province evp antarctica and 21 new age determinations by the 40ar 39ar incremental heating method the new evp data are combined with previously published paleomagnetic and geochronological results providing 133 sites 91 having radioisotopic dates modified thellier thellier paleointensity estimates are reported for 47 sites 37 have dates ages for the combined data set span 0 03 to 13 42 ma the 125 high quality evp directional data selected from the merged data set have a non fisherian distribution and a mean direction with an inclination anomaly of 3 but 95 confidence limits include the prediction from a geocentric axial dipole virtual geomagnetic pole vgp dispersions for brunhes matuyama and the combined 0 5 ma data set are consistently high compared with values from middle to low latitude regions regardless of the criterion used to determine transitional fields with vgp latitude cut off at 45 the dispersion 23 9 2 1 for the combined 0 5 ma evp data set is consistent with earlier high latitude data and paleosecular variation psv in model g but not with some more recent statistical psv models mean evp paleointensity of 31 5 2 4 t derived from 41 high quality sites is about half the current value at mcmurdo 63 t the result is essentially independent of data selection criteria high vgp dispersion and low intensity values support the global observation of anticorrelation between directional variability and field strength simulations of time varying dipole strength show that uneven temporal sampling may bias the mean evp intensity estimate but the possibility of persistently anomalous field behavior at high latitude cannot be excluded

the seasonal cycle of ocean atmosphere co2 flux in ryder bay west antarctic peninsula approximately 15 million km2 of the southern ocean is seasonally ice covered yet the processes affecting carbon cycling and gas exchange in this climatically important region remain inadequately understood here 3 years of dissolved inorganic carbon dic measurements and carbon dioxide co2 fluxes from ryder bay on the west antarctic peninsula wap are presented during spring and summer primary production in the surface ocean promotes atmospheric co2 uptake in winter higher dic caused by net heterotrophy and vertical mixing with circumpolar deep water results in outgassing of co2 from the ocean ryder bay is found to be a net sink of atmospheric co2 of 0 59 0 94 mol c m 2 yr 1 average of 3 years seasonal sea ice cover increases the net annual co2 uptake but its effect on gas exchange remains poorly constrained a reduction in sea ice on the wap shelf may reduce the strength of the oceanic co2 sink in this region

palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the south atlantic we present a dataset of a full vector palaeomagnetic study of late pleistocene lavas from the island tristan da cunha in the south atlantic ocean the current day geomagnetic field intensity in this region is approximately 25 mu t compared to an expected value of similar to 43 mu t this phenomenon is known as the south atlantic geomagnetic anomaly saa geomagnetic field models extending back to the last 10 ka find no evidence for this being a persistent feature of the geomagnetic field albeit all models are constructed from data which is particularly sparse in the southern hemisphere new ar 40 ar 39 incremental heating dating indicates the studied lavas from tristan da cunha extruded between 90 and 46 ka palaeointensity estimations of eight lava flows made using the thellier method yield an average palaeointensity of 18 6 mu t and virtual axial dipole moment vadm of 3 1 1 2 x 10 22 am 2 the lava flows demonstrate four time intervals comparable to the present day saa where the average vadm of the tristan da cunha lavas is weaker than the global vadm average this suggests a persistent or recurring low intensity anomaly to the main geomagnetic field similar to the saa existed in the south atlantic between 46 and 90 ka c 2016 elsevier b v all rights reserved

saw toothed variations of relative paleointensity and cumulative viscous remanence testing the records and the model a recent model kok and tauxe 1996a that attempts to explain asymmetrical sawtooth patterns of relative paleointensity from sediments valet and meynadier 1993 as the consequence of cumulative long term viscosity effects has been tested theoretically and experimentally a mathematical description of the model demonstrates that only a sharp distribution of relaxation times would be able to match the paleomagnetic record which implies that all the magnetic grains would be identical this has direct consequences for the distribution of the blocking temperatures tb which would not exceed 260 c and for the coercivities of the natural remanent magnetization nrm this tb spectrum derived from the model is inconsistent with the unblocking temperatures of the nrm and with the tb spectrum of the experimental thermoremanent magnetization trm of the sediment that recorded the sawtooth new results including thermal demagnetization and thellier thellier experiments across brunhes matuyama confirm the overall stability of the original records the stability of the older part of the record 2 to 4 ma has also been strengthed the sawtooth is unchanged between 20 mt and 65 mt and thermal demagnetization in a neutral atmosphere at 300 c and 450 c duplicates the results obtained by alternation field a f demagnetization thus viscous processes at least within the framework of a cumulative viscous model and for the existing data sets are not responsible for saw toothed paleointensity records

relative variations in geomagnetic intensity from sedimentary records the past 200 000 years abstract eighteen records of relative paleointensity from marine sediments have been compiled in order to retrace the global changes in geomagnetic intensity during the past 200 kyr before dealing with any other source of noise we focused on problems inherent to the depth versus time correlations the accuracy and resolution of dating mostly deduced from isotopic stratigraphy were identified as significant factors of distortion we found also that different time scales have been used by authors after readjustment with respect to the same time scale the records were stacked together and the statistical significance of the results tested by a bootstrap technique seventeen records shown to be coherent were combined into a synthetic curve defined as sint 200 other stacks combining various subsets of data were found to be identical to sint 200 except for changes in dispersion in fact the dispersion around sint 200 could be reduced further by improving the correlations without violating the stratigraphy a procedure which at least for some records emphasizes the need for more detailed depth time correlations another characteristic emerging from the results is that paleointensity features as short as 10 kyr are observed between 120 200 ka and can thus in principle be resolved however subsequent refinement is still necessary for this period which is documented by fewer records than the following 120 kyr finally we compared sint 200 with the records from the labrador sea and the lac du bouchet ldb both of which rely mostly on 14c chronology the two records show satisfactory agreement with sint 200 when transferred with respect to the age model established by correlating their 18o labrador and susceptibility features ldb with the 18o reference record the offset of 7 kyr at 40 ka measured in both cases between the 14c and 18o ages is higher than the value deduced from a recent model of a geomagnetic field control of 14c production 1 but is not inconsistent with recent u th dates

geomagnetic secular variation and the statistics of palaeomagnetic directions in this study we examine the role of palaeosecular variation psv in the use of statistics for palaeomagnetic studies and we provide new reliability criteria for palaeomagnetic poles or directions we first conclude that fisher statistics should not be applied to average palaeomagnetic directions but to virtual geomagnetic pole vgp distributions instead secondly we strongly advocate that typical properties of geomagnetic field behaviour are taken into account in the assessment of palaeomagnetic data sets the latitude dependent properties e s k provide useful guidelines for the reliability of a palaeomagnetic data set a reliable assessment of these properties depends on the sufficient number of palaeomagnetic samples being taken therefore as an additional instrument of assessing data sets we provide a n dependent a95 envelope bounded by an upper limit a95max and a lower limit a95min that helps to ascertain whether or not a distribution has sufficiently well sampled psv and therefore geomagnetic field behaviour applying these criteria is indispensable for studies of geomagnetic behaviour or for studies aiming at using tk03 gad for inclination error correction through the elongation inclination e i method for palaeomagnetic studies aimed at geological reconstructions they form helpful guidelines and increase the confidence in the rocks having faithfully recorded the field an analysis of published eastern mediterranean data shows that the vast majority of studies do not conform to the van der voo criteria in particular with respect to n and a95 we have provided criteria that are on the one hand more lenient lower n may still provide relevant information and on the other hand more strict for high n the criterion of a95 16 should be adapted to a requirement of lower a95 e g a95 80

relative palaeointensity records from the ontong java plateau summary rock magnetic palaeomagnetic and oxygen isotopic results are presented from core rndb 75p which was recovered from the ontong java plateau ojp a high degree of uniformity in magnetic properties characterized by relatively small changes in concentration and grain size in the upper 4 m of the core combined with a lack of coherence between the normalized remanence and rock magnetic data suggests that the natural remanence normalized by saturation remanence reflects variations in relative palaeointensity of the geomagnetic field the record from rndb 75p replicates other ontong java records spanning the last 400ka and extends the record back to some 700ka spectral analysis of the ontong java record suggests periodic behaviour in the relative palaeointensity record with a dominant period of between 30 and 40ka which appears not be be an artefact of lithologic variability this dominant period lies between functions describing climatic precession and obliquity changes in the earth s orbit comparison of the normalized remanence record with astronomical precession 26 ka period however is much more favorable none the less tuning the palaeointensity record to that of astronomical precession appears inconsistent with existing isotopic age constraints derived from the specmap time scale based on these data we must choose between assuming that the earth s orbit controlled ice volume inherent in the specmap time scale and assuming that the earth s magnetic field is driven by astronomical precession the former assumption has a substantial theoretical and observational base and we prefer to interpret the data presented here as suggesting that the earth s orbit has not played a detectable role in the modulation of the magnetic field plots of saturation remanence and magnetic susceptibility are very sensitive to quite subtle changes in magnetic grain size a slight shift within the pseudo singledomain grain size range toward the multidomain or superparamagnetic field was detected at about 4m in rndb 75p this change in grain size may reflect a diagenetic alteration of the magnetite such as dissolution and may be related to the phenomenon responsible for the loss of magnetic remanence at depth detected in other cores from the region

geomagnetic field stability reversal events and excursions abstract it has been suggested that there are several reverse subchrons in the brunhes and this suggestion is gaining some acceptance however such a suggestion has fundanmental implications for the reversal process because it implies that the reverse polarity state has been substantially less stable than the normal polarity state during the brunhes chron analysis of the latest polarity reversal timescale confirms that there is no reason to reject the hypothesis of a common stability for the two polarity states the cretaceous superchron appears to be a time when the reversal rate slowed to the point where the reversal process actually halted so does not indicate any difference in the stability of the two polarity states after examining the data we found no convincing evidence for even a single reversal event during the brunhes and we conclude that the only reversal excursions that are sufficiently well documented for the brunhes are the laschamp and the blake further we note that the question of whether of large excursions represent aborted reversals is unresolved so excursion data should not at this stage be grouped with transition polarity data finally we note that most of the time the nearly axially symmetric normal and reverse polarity states appear to be remarkably stable

fall in earth s magnetic field is erratic earth s magnetic field has decayed by about 5 per century since measurements began in 1840 directional measurements predate those of intensity by more than 250 years and we combined the global model of directions with paleomagnetic intensity measurements to estimate the fall in strength for this earlier period 1590 to 1840 a d we found that magnetic field strength was nearly constant throughout this time in contrast to the later period extrapolating to the core surface showed that the fall in strength originated in patches of reverse magnetic flux in the southern hemisphere these patches were detectable by directional data alone the pre 1840 model showed little or no evidence of them supporting the conclusion of a steady dipole up to 1840

dense waters of the weddell and scotia seas recent changes in properties and circulation the densest waters in the atlantic overturning circulation are sourced at the periphery of antarctica especially the weddell sea and flow northward via routes that involve crossing the complex bathymetry of the scotia arc recent observations of significant warming of these waters along much of the length of the atlantic have highlighted the need to identify and understand the time varying formation and export processes and the controls on their properties and flows here we review recent developments in understanding of the processes that control the changing flux of water through the main export route from the weddell sea into the scotia sea and the transformations of the waters within the scotia sea and environs we also present a synopsis of recent findings that relate to the climatic change of dense water properties within the weddell sea itself in the context of known atlantic scale changes among the most significant findings are the discovery that the warming of waters exported from the weddell sea has been accompanied by a significant freshening and that the episodic nature of the overflow into the scotia sea is markedly wind controlled and can lead to significantly enhanced abyssal stratification key areas for focusing future research effort are outlined

coastal southern ocean a strong anthropogenic co2 sink 1 large scale estimates of the southern ocean co2 sink do not adequately resolve the fluxes associated with antarctic continental shelves using a mechanistic three dimensional biogeochemical model of the ross sea we show that antarctic shelf waters are a strong sink for co2 due to high biological productivity intense winds high ventilation rates and extensive winter sea ice cover net primary production npp in these waters is 0 055 pg c yr 1 some of this carbon sinks to depth driving an influx of co2 of 20 50 g c m 2 yr 1 although currently unaccounted for the total atmospheric co2 sink on the ross sea continental shelf of 0 013 pg c yr 1 is equivalent to 27 of the most recent estimate of the co2 sink for the entire southern ocean given these results these and other highly productive waters around the antarctic continent need to be included in future budgets of anthropogenic co2