geomagnetic field in the near east at the beginning of the 6th millennium bc evidence for alternating weak and strong intensity variations abstract this study presents new archeointensity results from the multilayered settlement yarim tepe i located today in northern iraq archeological evidences and new radiocarbon dates indicate that this site was occupied for about four centuries between the end of the 7th and the beginning of the 6th millennium bc leading to a 6 5 m thick sequence of archeological deposits a series of 16 groups of potsherds with a total of 76 fragments were collected from superimposed stratigraphic layers with thicknesses of 30 cm on average archeointensity measurements were carried out using the triaxe procedure which takes into account both anisotropy and cooling rate effects on thermoremanent magnetization acquisition in this study 114 specimens from 40 fragments have fulfilled our selection criteria and mean archeointensity results were derived from nine different groups of fragments with a minimum of three fragments per group according to an age model constructed using a bootstrap approach the new archeomagnetic record spans a time interval of 220 years between 6070 bce and 5850 bce no significant intensity variations were observed during this time interval with an overall mean intensity value of 42 0 1 6 t a comparison of the yarim tepe i data with other archeointensity results spanning the 7th and 6th millennia bc previously obtained from the near east as well as from eastern and western europe was conducted the yarim tepe i results enabled to constrain the short duration one century at most of an intensity peak evidenced around 5750 bce from the bulgarian database intensity variation rates associated with the ascending branch of the peak reached values as high as 0 12 0 15 t year summarizing the geomagnetic field intensity variations in the near east and eastern europe during the entire 6th millennium bc it appears that they were likely characterized by the occurrence of two short lasting intensity peaks at 5750 bce and 5500 bce with intensity variation rates similar or slightly higher than the maximum rates prevailing in the modern field finally it is worth mentioning that the four radiocarbon dates reported in our study provide new constraints for deciphering the temporal correlation between the cultural historical phases halaf and hassuna that were independently defined from excavations carried out in iraq and syria

polynya dynamics drive primary production in the larsen a and b embayments following ice shelf collapse 1 the climate driven collapses of the larsen a and b ice shelves have opened up new regions of the coastal antarctic to the influence of sea ice resulting in increases in seasonal primary production in this study passive microwave remote sensing of sea ice concentration and satellite imagery of ocean color are employed to quantify the magnitude of and variability in open water area and net primary productivity npp in the larsen embayments between 1997 and 2011 numerical model output provides context to analyze atmospheric forcing on the coastal ocean following ice shelf disintegration the embayments function as coastal sensible heat polynyas the larsen a and b are as productive as other antarctic shelf regions with seasonally averaged daily npp rates reaching 1232 and 1127 mg c m 2 d 1 and annual rates reaching 200 and 184 g c m 2 yr 1 respectively a persistent cross shelf gradient in npp is present with higher productivity rates offshore contrasting with patterns observed along the west antarctic peninsula embayment productivity is intimately tied to sea ice dynamics with large interannual variability in npp rates driven by open water area and the timing of embayment opening opening of the embayment is linked to periods of positive southern annular mode and stronger westerlies which lead to the vertical deflection of warm maritime air over the peninsula and down the leeward side causing increases in surface air temperature and wind velocity high productivity in these new polynyas is likely to have ramifications for organic matter export and marine ecosystem evolution

sedimentary processes across the continental rise of the southern antarctic peninsula abstract a series of large sediment mounds have been identified along the pacific portion of the antarctic peninsula continental rise these mounds are composed of sediment delivered to the continental rise during the advance and retreat of grounded ice across the shelf the stratigraphic development of one of these sediment deposits the tula sediment mound is examined to investigate how the onset of glaciation influenced the deep sea depositional environment along this portion of the margin the stratal relationships associated facies distribution and the physiography observed along the southern antarctic margin reflect the waxing and waning of the antarctic ice sheets various processes erode transport and deposit sediment along the outer shelf slope and rise throughout a glacial cycle a deep sea erosional unconformity is apparent at the base of the tula deposit this surface may reflect the first onset of intensified bottom water circulation along the margin perhaps induced by the tectonic opening of the drake passage the tula sediment mound is comprised predominantly of canyon overbank systems evidence for the onset of canyon cutting and the development of the thick overbank deposits 2 km is found immediately above the deep sea erosional unconformity relating increased canyon cutting across the continental rise to the fluctuation of ice across the shelf implies that the onset of predominantly glacial conditions commenced soon after the onset of intensified bottom water circulation along this margin the volume of sediment associated with the tula deposit and the apparent point source distribution of the channel systems suggests that much sediment was transported to the slope and rise perhaps by meltwater processes through these canyons

the present and past bottom current flow regime around the sediment drifts on the continental rise west of the antarctic peninsula throughout the last decade large sediment drifts located on the upper continental rise west of the antarctic peninsula were the target of oceanographic measurements bathymetric mapping seismic investigations shallow sediment coring and deep sea drilling by ocean drilling program odp leg 178 these studies concluded that for most of the late neogene and quaternary a generally sw ward directed bottom current affected the deposition on the drifts in particular during glacial periods the deposition was additionally influenced by nw ward directed transport of terrigenous detritus supplied by turbidity currents from the antarctic peninsula continental slope in a recent study however the palaeomagnetic signal of the drift sediments recovered at two odp leg 178 sites 1095 and 1101 was interpreted to provide spatial and directional information on the physical record of the ne ward flowing antarctic circumpolar current acc here we investigate the link between the clockwise flowing acc and the generally sw wards flowing near bottom contour current we show that at the odp leg 178 sites on the western antarctic peninsula continental margin the acc only affects the ocean circulation above ca 1000 m water depth therefore the signal of acc flow might only be archived in the ice rafted debris ird content of the drift sediments however the ird from the leg 178 sites accounts for only a small proportion of the drift sediments and is dominantly of local origin past variability of bottom water flow around the sediment drifts was reconstructed on the basis of seismic studies and clay mineralogical and grain size analyses these reconstructions provide useful information about both downstream direction and velocity changes of the bottom current and point to its sw ward flow along the upper rise during at least the last 9 4 ma

searching for geomagnetic spikes in numerical dynamo simulations abstract we use numerical dynamo simulations to investigate rapid changes in geomagnetic field intensity the work is motivated by paleomagnetic observations of geomagnetic spikes events where the field intensity rose and then fell by a factor of 2 3 over decadal timescales and a confined spatial region no comparable events have been found in the historical record and so geomagnetic spikes may contain new and important information regarding the operation of the geodynamo however they are also controversial because uncertainties and resolution limitations in the available data hinder efforts to define their spatio temporal characteristics this has led to debate over whether such extreme events can originate in earth s liquid core geodynamo simulations produce high spatio temporal resolution intensity information but must be interpreted with care since they cannot yet run at the conditions of earth s liquid core we employ reversing and non reversing geodynamo simulations run at different physical conditions and consider various methods of scaling the results to allow comparison with earth in each simulation we search for extremal events defined as the maximum intensity difference between consecutive time points at each location on a 2 latitude longitude grid at earth s surface thereby making no assumptions regarding the spatio temporal character of the event extremal events display spike shaped time series in some simulations though they can often be asymmetric about the peak intensity maximum rates of change reach 0 75 t yr 1 in several simulations the lower end of estimates for spikes suggesting that such events can originate from the core the fastest changes generally occur at latitudes 50 which could be used to guide future data acquisitions extremal events in the simulations arise from rapid intensification of flux patches as they migrate across the core surface rather than emergence of flux from within the core the prospect of observing more spikes in the paleomagnetic record appears contingent on finding samples at the right location and time to sample this particular phase of flux patch evolution

geomagnetic intensity spike recorded in high resolution slag deposit in southern jordan article i nfo in paleomagnetism periods of high field intensity have been largely ignored in favor of the more spectacular directional changes associated with low field intensity periods of excursions and reversals hence questions such as how strong the field can get and how fast changes occur are still open in this paper we report on data obtained from an archaeometallurgical excavation in the middle east designed specifically for archaeomagnetic sampling we measured 342 specimens from 72 samples collected from a 6 1 m mound of well stratified copper production debris at the early iron age 12th 9th centuries bce site of khirbat en nahas in southern jordan seventeen samples spanning 200 yr yielded excellent archaeointensity results that demonstrate rapid changes in field intensity in a period of overall high field values the results display a remarkable spike in field strength with sample mean values of over 120 t compared to the current field strength of 44 t a suite of 13 radiocarbon dates intimately associated with our samples tight control of sample location and relative stratigraphy provide tight constraints on the rate and magnitude of changes in archaeomagnetic field intensities

a recent sea ice retreat west of the antarctic peninsula satellite passive microwave data show a record decrease in sea ice extent in the bellingshausen sea from mid 1988 through early 1991 the change coincides with more southerly surface winds increased cyclonic activity and rising surface air temperatures which reached historic highs along the west coast of the antarctic peninsula in 1989 preceded by high ice cover in 1986 87 the retreat was most evident during summer in the formerly perennial sea ice field over the continental shelf ocean heat storage probably contributed to the persistence and coastal propagation of this anomaly

atmospheric circulation impacts on winter maximum sea ice extent in the west antarctic peninsula region 1979 2001 increasing evidence exists that the strong warming of the west antarctic peninsula wap region since the 1950s is related to reduced sea ice that is likely to be due to changes in the atmospheric circulation over twenty years of sea ice extent ice motion and reanalysed near surface wind data are used to establish that winter ice extent in the west antarctic peninsula wap region is largely determined by the meridional north south atmospheric circulation a remarkably strong ice extent wind relationship is found in the wap no other antarctic or comparable sub arctic sea ice region shows this ice motion data confirm wind induced drift is crucial for extensive winter ice to occur reasons for winter ice extent and winds being more strongly correlated in the wap than in other parts of the antarctic are discussed along with implications for understanding the observed warming

230th normalization an essential tool for interpreting sedimentary fluxes during the late quaternary there is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates notwithstanding its common incidence this process generally referred to as sediment focusing is often difficult to recognize if redistribution is near synchronous to deposition the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked ignoring it however can lead to serious misinterpretations of sedimentary fluxes particularly when past changes in export flux from the overlying water are inferred in many instances this problem can be resolved at least for sediments deposited during the late quaternary by normalizing to the flux of 230th scavenged from seawater which is nearly constant and equivalent to the known rate of production of 230th from the decay of dissolved 234u we review the principle advantages and limitations of this method notwithstanding its limitations it is clear that 230th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates

earth s core internal dynamics 1840 2010 imaged by inverse geodynamo modelling inverse geodynamo modelling seeks to estimate the dynamic state of earth s core from geo magnetic data and the statistical information brought by a prior self consistent 3 d numerical model of the geodynamo the method rests on the use of least squares inversions under constraints and estimates hidden quantities by taking advantage of linear relations and long range statistical correlations with the magnetic observations the data together with their error statistics are provided by geomagnetic field models cov obs gufm sat q3 and cm4 covering epochs 1840 2010 the prior numerical model is the recently published coupled earth dynamo the output of which presents a high degree of morphological semblance to the geomagnetic field while reproducing the main features of its secular variation an analysis of the inversion misfits to the data shows that the prior model generally accounts well for the main field and secular variation data within their specified errors throughout the investigated time period inverted core flows are confirmed to be mainly organized in columns parallel to the earth s rotation axis a previously observed giant eccentric columnar gyre of westward drifting flow is found to be present from epoch 1870 onwards and its structure is shown to slowly rotate westwards at a rate up to 0 1 yr 1 confirming an earlier prediction based on direct numerical modelling temporal variations in the axisymmetric part of the gyre accurately account for observed variations of the length of the day in recent epochs inverted magnetic structures support a mechanism of azimuthal flux expulsion by convective columns to explain the origin of low latitude magnetic flux patches existing beneath the atlantic the 1840 2010 time average of the inverted density anomaly field has a longitudinal hemispheric structure with most of the buoyancy in the eastern hemisphere consistent with rapid surges of convective columns imaged in this hemisphere with earlier proposals of a faster inner core freezing there and with a possible east to west convective translation of the solid inner core the typical timescales of flow variation observed in the inversions are two to three times shorter than those naturally produced by a direct simulation of the prior model underlining its limits in fully rendering the earth s core short timescale dynamics

analysis of geomagnetic field intensity variations in mesopotamia during the third millennium bc with archeological implications abstract we present new archeointensity results obtained at two multi layer archeological sites tell atij and tell gudeda northeastern syria dated from the early bronze period in the third millennium bc the archeointensity data were obtained using the experimental protocol developed for the triaxe magnetometer in total 68 fragments 204 specimens of 151 fragments analyzed passed our selection criteria allowing average intensity values to be estimated for 14 archeological layers nine at tell atij and five at tell gudeda based on the available archeological constraints the different archeological layers of tell atij and tell gudeda were dated between 2900 bc and 2600 bc and between 2550 bc and 2325 bc respectively the tell atij data show a significant increase in intensity over the dated period while the results from tell gudeda exhibit a v shape evolution using high quality data available from syria the levant and turkey a regional geomagnetic field intensity variation curve spanning the entire third millennium bc was constructed using a trans dimensional bayesian method it clearly shows two intensity peaks around 2600 bc and at 2300 bc associated with variation rates of 0 1 0 2 t yr this indicates that the occurrence of century scale intensity peaks with rates of variation comparable to or even slightly higher than the maximum rates observed in the modern geomagnetic field is an ubiquitous feature of the geomagnetic secular variation from an archeological point of view the new archeointensity data strengthen the hypothesis that the successive occupation of tell atij and tell gudeda was synchronous with the two first urban phases of mari making possible a sustained trade network between these settlements during the third millennium bc we further suggest that the end of mari s first urban phase contemporaneous with the abandonment of tell atij might have been caused by a regional drought episode around 2600 bc more generally the bayesian approach used to estimate the new reference intensity variation curve offers promising chronological constraints for archeological purposes

mechanism for geomagnetic polarity reversals recent studies of old magnetic observations1 and the seismic velocity of the lower mantle2 suggest that at least part of the geomagnetic secular variation is driven by temperature anomalies in the solid mantle3 in particular a patch of flux of opposite sign to that expected for a dipole field occurs beneath southern africa it has intensified throughout the twentieth century by a process that is believed to be flux expulsion associated with a particularly hot part of the lowermost mantle and is drifting westwards a similar patch lies beneath south america here i show that the present fall in the dipole moment is directly related to the intensification and southward movement of these patches and suggest the fall occasionally leads to polarity reversal the almost linear increase in the frequency of polarity reversals that has occurred since the cretaceous quiet interval is attributed to the growth of the hot patch of mantle presently under southern africa

developing sediment geochronologies for high latitude continental shelf deposits a radiochemical approach during january and february of 1986 47 box cores and 55 piston cores were collected from the bransfield strait gerlache strait and marguerite bay antarctica apparent sediment accumulation rates established for 100 yr time scales with excess 210pb chronologies range from 0 08 to 0 34 cm yr in the bransfield strait 0 15 to 0 51 cm yr in gerlache strait and from 0 10 to 0 27 cm yr in marguerite bay for the antarctic peninsula continental shelf 137cs appears to have a limited usefulness in assessing the effects of deep sediment mixing by benthic fauna on seabed excess 210pb profiles because the observed activities for the regions examined are close to counter detection limits conventional 14c ages of surface sediments from the peninsula region range from 1370 to 2820 yr the spatial variations in the conventional 14c surface ages are thought to be influenced predominantly by inputs of older eroded sediment from lateral transport current winnowing mass wasting and or a variable carbon source on a 1000 yr time scale 14c measurements of five piston cores from the bransfield strait yielded sediment accumulation rates ranging from 0 06 to 0 49 cm yr these data support the assertion by nelson 1988 that the excess 210pb sediment accumulation rates range of 0 08 to 0 34 cm yr for this region represent true rates the two sediment accumulation rates of 0 01 and 0 08 cm yr for the gerlache strait are significantly lower than the observed apparent 210pb accumulation rates mean 0 27 0 12 cm yr for this region likewise two piston cores from marguerite bay yielded 14c sediment accumulation rates of 0 02 and 0 05 cm yr which also are considerably lower than the excess 210pb apparent accumulation rates mean 0 17 0 06 cm yr for this area the one dimensional advection diffusion equation of goldberg and koide geochim cosmochim acta vol 51 1962 was solved simulatneously with 210pb and 14c data for bc18 and bc113 to resolved the effects of sediment mixing and accumulation in bransfield strait and marguerite bay box core samples respectively this model indicates that 210pb and 14c sediment accumulation rates agree within a factor of 2 in the bransfield strait whereas in marguerite bay the apparent 210pb accumulation rates may be as much as an order of magnitude too high in the southern high latitude environments of the antarctic peninsula radiocarbon measurements in the seabed are a better approach than 210pb for establishing accurate rates of sediment accumulation the 14c chronologies when coupled with 210pb data provide more reliable assessments of biological deep mixing and sediment accumulation rates than do excess 210pb data coupled with measurements of bomb produced 137cs the sediment accumulation rates for both 100 yr and 1000 yr time scales tend to decrease southward from the bransfield strait to marguerite bay which is consistent with glacial marine sedimentation processes as modulated by climate for the western antarctic peninsula seabed measurements of 210pb 137cs and 14c also provides valuable insights on water circulation and upwelling atmospheric inputs and sediment depositional processes for the antarctic peninsula region

new archeointensity data from french early medieval pottery production 6th 10th century ad tracing 1500 years of geomagnetic field intensity variations in western europe nineteen new archeointensity results were obtained from the analysis of groups of french pottery fragments dated to the early middle ages 6th to 10th centuries ad they are from several medieval ceramic production sites excavated mainly in saran central france and their precise dating was established based on typo chronological characteristics intensity measurements were performed using the triaxe protocol which takes into account the effects on the intensity determinations of both thermoremanent magnetization anisotropy and cooling rate intensity analyses were also carried out on modern pottery produced at saran during an experimental firing the results show very good agreement with the geomagnetic field intensity directly measured inside and around the kiln thus reasserting the reliability of the triaxe protocol and the relevance of the quality criteria used they further demonstrate the potential of the saran pottery production for archeomagnetism the new archeointensity results allow a precise and coherent description of the geomagnetic field intensity variations in western europe during the early medieval period which was until now poorly documented they show a significant increase in intensity during the 6th century ad high intensity values from the 7th to the 9th century with a minimum of small amplitude at the transition between the 7th and the 8th centuries and finally an important decrease until the beginning of the 11th century together with published intensity results available within a radius of 700 km around paris the new data were used to compute a master curve of the western european geomagnetic intensity variations over the past 1500 years this curve clearly exhibits five intensity maxima at the transition between the 6th and 7th century ad at the middle of the 9th century during the 12th century in the second part of the 14th century and at the very beginning of the 17th century ad some of these peaks are smoothed or nearly absent when the selection of the data is extended to a 1250 km radius around paris the apparent regularity in the occurrence of intensity maxima with a recurrence of 250 years is particularly intriguing and might reflect a new characteristic of the secular variation at least in western europe it clearly requires further investigation and in particular the acquisition of new data from older periods

large geomagnetic field anomalies revealed in bronze to iron age archeomagnetic data from tel megiddo and tel hazor israel abstract geomagnetic field measurements from the past few centuries show heightened secular variation activity in the southern hemisphere associated with the south atlantic anomaly saa it is uncertain whether geomagnetic anomalies at a similar scale have existed in the past owing to limited coverage and uncertainties in the paleomagnetic database here we provide new evidence from archaeological sources in the levant suggesting a large positive northern hemisphere anomaly similar in magnitude to the saa during the 9th 8th centuries bce called levantine iron age anomaly we also report an additional geomagnetic spike in the 8th century the new dataset comprises 73 high precision paleointensity estimates from ca 3000 bce to 732 bce and five directional measurements between the 14th and the 9th centuries bce well dated pottery and cooking ovens were collected from twenty archaeological strata in two large contemporaneous stratigraphical mounds tells in israel tel megiddo and tel hazor the new data are combined with previously published data and interpreted automatically using the pmagpy thellier gui program the tel megiddo and tel hazor data sets demonstrate excellent internal consistency and remarkable agreement with published data from mesopotamia syria the data illustrate the evolution of an extreme geomagnetic high that culminated in at least two spikes between the 11th and the 8th centuries bce iron age in the levant the paleomagnetic directional data of the 9th century bce show positive inclination anomalies and deviations of up to 22 from the averaged geocentric axial dipole gad direction from comparison of the levantine archaeomagnetic data with igrf model for 2015 we infer the levantine iron age anomaly between the 10th and the 8th centuries bce is a local positive anomaly the eastward extent of the anomaly is currently unknown

an asymmetric upwind flow yellow sea warm current 1 new observations in the western yellow sea 1 the winter water mass along the yellow sea trough yst especially on the western side of the trough is considerably warmer and saltier than the ambient shelf water mass this observed tongue shape hydrographic feature implies the existence of a winter along trough and onshore current often referred to as the yellow sea warm current yswc however the yswc has not been confirmed by direct current measurements and therefore skepticism remains regarding its existence some studies suggest that the presence of the warm water could be due to frontal instability eddies or synoptic scale wind bursts it is noted that in situ observations used in most previous studies were from the central and eastern sides of the yst even though it is known that the warm water core is more pronounced along the western side data from the western side have been scarce here we present a set of newly available chinese observations including some from a coordinated effort involving three chinese vessels in the western yst during the 2006 2007 winter the data show unambiguously the existence of the warm current on the western side of yst both the current and hydrography observations indicate a dominant barotropic structure of yswc the westward deviation of yswc axis is particularly obvious to the south of 35 n and is clearly associated with an onshore movement of warm water to the north of 35 n the yswc flows along the bathymetry with slightly downslope movement we conclude that the barotropic current is mainly responsible for the warm water intrusion while the ekman and baroclinic currents play an important but secondary role these observations help fill an observational gap and establish a more complete view of the yswc

extended and revised archaeomagnetic database and secular variation curves from bulgaria for the last eight millennia the efforts of geophysicists to describe geomagnetic field behaviour in the past lead to creation of different geomagnetic field models on the other hand the established regional palaeosecular variations of geomagnetic elements are increasingly used for dating purposes in archaeology both of these goals can be achieved if sufficient amounts of long archaeomagnetic data sets exist for different geographical regions the accumulation of archaeomagnetic determinations began at the middle of the last century parallel with the progressive development of experimental methodology and acceptance criteria the presence of great number of old determinations requires their critical assessment the important question about the reliability of the associated dating intervals should be also re assessed all this requires the continuous refinement and extension of the accumulated databases this paper presents the last synthesis of bulgarian archaeomagnetic database and the local palaeosecular variation curves obtained using a statistical treatment based on bayesian approach rencurve software the rock magnetic characteristics of the newly included non published results are summarized

magnetic flux expulsion from the core as a possible cause of the unusually large acceleration of the north magnetic pole during the 1990s 1 the north magnetic pole nmp has been drifting in a north northwesterly direction since the 19th century both local surveys and geomagnetic models derived from observatory and satellite data show that the nmp suddenly accelerated during the 1990s its speed increased from about 15 km yr in 1989 to about 60 km yr in 2002 after which it started to decrease slightly using a green s function we show that this acceleration is mainly caused by a large negative secular variation change in the radial magnetic field at the core surface under the new siberian islands this change occurs in a region of the core surface where there is a pair of secular variation patches of opposite polarities which we suggest could be the signature of a so called polar magnetic upwelling of the type observed in some recent numerical dynamo simulations indeed a local analysis of the radial secular variation and magnetic field gradient suggests that the secular variation change under the new siberian islands is likely to be accompanied by a significant amount of magnetic diffusion in agreement with such a mechanism we thus hypothesize that the negative secular variation change under the new siberian islands that produced the nmp acceleration could result from a slowdown of the polar magnetic upwelling during the 1990s we finally note that the nmp drift speed is determined by such a combination of factors that it is at present not possible to forecast its future evolution

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

core flow constraints on extreme archeomagnetic intensity changes abstract recent studies ben yosef et al 2009 shaar et al 2011 propose extreme archeomagnetic intensity changes termed spikes in the range 4 5 t year c a 1000 bc in the near east around 40 to 50 times larger than values typical of the present day in order to investigate whether such extreme changes are consistent with a model of the source region of the magnetic field namely the fluid flow at the surface of earth s core we construct upper bounds for instantaneous magnetic intensity change at an arbitrary site on the earth s surface these bounds are constrained by the amount of kinetic energy available to sustain the change taken here to be a prescribed value for the root mean squared surface velocity of 13 km yr as inferred from the current state of the core further we focus attention on two end members of optimised core surface flow structure unrestricted and purely toroidal as the derivation of the bounds demands complete knowledge of the geomagnetic field at the core surface we model the unknown field by means of a monte carlo approach extending to high degree the chaos 4 epoch 2010 ad and cals10k1 b epoch 1000 bc geomagnetic field models using 2000 realisations for each family of stochastic field models we find that optimised core flows are always large scale and that they tend to generate a non dipole quadrupole dominated secular variation at the earth s surface the dependence of the upper bounds as a function of site location reflects the large scale structure of the intensity itself stronger field permits more rapid change for the site in the near east purely toroidal flows have upper bounds of approximately 0 62 0 02 t year whereas unrestricted flows increase this bound to 1 20 0 02 t year we favour the former as more geophysically sound on the account of a large body of previous results from core surface flow inversions and consistency with the existence of a stratified layer at the top of the core even if we allow for a generous threefold increase in the prescribed rms velocity and a concomitant threefold increase in the bound we conclude that the reported occurrences of extreme intensity changes as suggested in the near east are not compatible with the commonly accepted structure of core surface flow however it may be that an explanation for spikes lies beyond our current perception of core dynamics and future work would be further motivated by seeking corroborative evidence of rapid intensity change from sites elsewhere on earth s surface we therefore also discuss the form that the secular variation would take in the case of simultanenous archeomagnetic spikes

accounting for magnetic diffusion in core flow inversions from geomagnetic secular variation summary we use numerical dynamos to investigate the possible role of magnetic diffusion at the top of the core we find that the contribution of radial magnetic diffusion to the secular variation is correlated with that of tangential magnetic diffusion for a wide range of control parameters the correlation between the two diffusive terms is interpreted in terms of the variation in the strength of poloidal flow along a columnar flow tube the amplitude ratio of the two diffusive terms is used to estimate the probable contribution of radial magnetic diffusion to the secular variation at earth like conditions we then apply a model where radial magnetic diffusion is proportional to tangential diffusion to core flow inversions of geomagnetic secular variation data we find that including magnetic diffusion does not change dramatically the global flow but some significant local variations appear in the non frozen flux core flow models termed diffusive the hemispherical dichotomy between the active atlantic and quiet pacific is weaker a cyclonic vortex below north america emerges and the vortex below asia is stronger our results have several important geophysical implications first our diffusive flow models contain some flow activity at low latitudes in the pacific suggesting a local balance between magnetic field advection and diffusion in that region second the cyclone below north america in our diffusive flows reconciles the difference between mantle driven thermal wind predictions and frozen flux core flow models and is consistent with the prominent intense magnetic flux patch below north america in geomagnetic field models finally we hypothesize that magnetic diffusion near the core surface plays a larger role in the geomagnetic secular variation than usually assumed

rapid deglaciation of marguerite bay western antarctic peninsula in the early holocene glacial geological evidence of rapid ice stream retreat is important for the potential insight it can bring to understanding of contemporary rapid ice sheet change here we report new chronological constraints on the deglacial history of the marguerite trough ice stream which formerly drained part of the western margin of the antarctic peninsula this ice stream has been previously identified on geomorphological grounds as an example of an outlet that underwent rapid retreat following the last glacial maximum but the precise timing of this retreat has not been conclusively determined we obtained cosmogenic surface exposure dates on erratic boulders that record the thinning history of ice flowing into the ice stream these dates show that the ice stream underwent 270 m of thinning very rapidly at 9 6 ka bp the timing of ice stream thinning agrees closely with previously published data on ice shelf retreat the age of the first post glacial beaches and minimum constraining ages for the onset of lacustrine sedimentation and post glacial sedimentation in inner shelf fjords we compare the timing of retreat to the similarly sized anvers trough ice stream further north and find that retreat in marguerite bay was more rapid and earlier than in the anvers trough we argue that the intrusion of circumpolar deep water is the most likely forcing mechanism for retreat in marguerite bay a key finding of this study is that rapid retreat occurred in deep water with reverse continent dipping slope but also in relatively shallow water with a positive slope this may have implications for understanding of oceanographically forced ice sheet change in other parts of west antarctica

evaluating the effects of regional climate trends along the west antarctic peninsula shelf based on the seabed distribution of naturally occurring radioisotopic tracers abstract measurements of 230th 14c 210pb and 234th activities were made on sediment cores collected along a n s transect exhibiting a gradient in annual sea ice duration off the west antarctic peninsula the resultant data were used to evaluate the effects of regional warming on particle flux reaching the seabed on timescales from millennial to seasonal shelf samples were collected at five stations over three cruises between february 2008 and march 2009 as part of the foodbancs2 project sea ice conditions the number of days ice free prior to core collection were evaluated at the five stations to understand the relationship between ice abundance and particle radionuclide flux based on the millennial tracer 14c rates of sediment accumulation along the peninsula decrease southward consistent with the observed sea ice gradient 230th data provide additional evidence on millennial timescales that sediment focusing i e lateral transport occurs to a greater extent in the northern reaches of the study area compared to the southernmost stations the distribution of steady state 210pb flux to the seabed representing centurial trends displays a similar trend to 14c showing higher radionuclide particle flux in the northern study area where sea ice duration is diminished and lower flux southward as sea ice duration increases additionally 210pb data suggest that lateral transport plays an important role in the sediment distributions of this radiotracer on hundred year timescales which is explainable by the relatively short circulation times of peninsular waters relative to 210pb s half life on seasonal and annual time scales the distribution of steady state 234th flux to the seabed shows an increase in radionuclide flux at the southernmost stations this increase in radionuclide flux on seasonal and annual time scales is consistent with the warming trend along the peninsula and the reduction in sea ice duration over the past decade a significant statistical relationship however could not be established between annual sea ice free days and 234th derived particle flux to the seabed the fluxes distributions of long lived particle reactive tracers 14c 230th and 210pb on the west antarctic peninsula shelf appear to be controlled primarily by the long term pattern of increasing annual sea ice duration in the southward direction whereas the fluxes of the short lived tracer 234th are consistent with the more recent decreases in sea ice duration associated with climate change that have occurred over the past decade primarily in the southern west antarctic peninsula stations

secular variation in numerical geodynamo models with lateral variations of boundary heat flow we study magnetic field variations in numerical models of the geodynamo with convection driven by nonuniform heat flow imposed at the outer boundary we concentrate on cases with a boundary heat flow pattern derived from seismic anomalies in the lower mantle at a rayleigh number of about 100 times critical with respect to the onset of convection the magnetic field is dominated by the axial dipole component and has a similar spectral distribution as earth s historical magnetic field on the core mantle boundary cmb the time scales of variation of the low order gauss coefficients in the model agree within a factor of two with observed values we have determined the averaging time interval needed to delineate deviations from the axial dipole field caused by the boundary heterogeneity an average over 2000 years the archeomagnetic time scale is barely sufficient to reveal the long term nondipole field the model shows reduced scatter in virtual geomagnetic pole positions vgps in the central pacific consistent with the weak secular variation observed in the historical field longitudinal drift of magnetic field structures is episodic and differs between regions westward magnetic drift is most pronounced beneath the atlantic in our model although frozen flux advection by the large scale flow is generally insufficient to explain the magnetic drift rates there are some exceptions in particular equatorial flux spot pairs produced by expulsion of toroidal magnetic field are rapidly advected westward in localized equatorial jets which we interpret as thermal winds

pre bomb radiocarbon and the reservoir correction for calcareous marine species in the southern ocean four antarctic marine mollusc shells which were collected alive between 1917 and 1940 were analyzed by accelerator mass spectrometry to provide the first pre bomb radiocarbon measurements of biogenic carbonates from the southern ocean after correcting for the impact of fossil fuel combustion suess effect radiocarbon activities of the pre bomb shells averaged 149 8 10 4 in contrast the 14c values for post bomb molluscs echinoderms brachiopods and foraminifera averaged 96 1 25 2 these biogenic carbonate 14c values are nearly identical to pre bomb estimates 148 to 152 and post bomb measurements 98 4 22 0 of the surface waters in the southern ocean average radiocarbon ages of the biogenic carbonates before and after 1950 1303 84 years and 811 205 years respectively along with those from seals and penguins indicate that the antarctic marine radiocarbon reservoir has decreased in age by nearly 500 years during the second half of the 20th century marine species and seawater measurements firmly place the radiocarbon reservoir correction at 1300 100 years for calcareous marine fossils which are widespread abundant and well preserved organic materials for interpreting ice sheet climate and sea level impacts on the antarctic marine ecosystem during the holocene

magnetic boundary layers in numerical dynamos with heterogeneous outer boundary heat flux abstract it has been proposed that magnetic flux expulsion due to outer core fluid upwellings may affect the geomagnetic secular variation on the core mantle boundary bloxham 1986 in this process intense horizontal field lines are concentrated below the outer boundary introducing small radial length scales and consequently strong radial diffusion here we explore such magnetic boundary layers in numerical dynamo simulations with heterogeneous outer boundary heat flux inferred from a tomographic model of lower mantle seismic shear waves velocity anomalies our scheme associates magnetic boundary layers to peak horizontal magnetic fields at the top of the shell in our models mean magnetic boundary layer thickness ranges 200 400 km and decreases with increasing magnetic reynolds number extrapolation or interpolation to earth s core conditions based on total core flow amplitude or its poloidal part gives mean magnetic boundary layer thickness of 220 and 260 330 km respectively we find magnetic boundary layers associated with the azimuthal field at the equatorial region whereas magnetic boundary layers associated with the meridional field are found at mid latitudes negative outer boundary heat flux anomalies yield preferred locations of expulsion of azimuthal field below africa and the pacific while positive outer boundary heat flux anomalies yield preferred locations of expulsion of meridional field below the americas and east asia furthermore we find a tendency of the azimuthal field to low latitudes of the northern hemisphere our results suggest that the local diffusion time is on the order of several kyr and the local magnetic reynolds number is on the order of 10 both much smaller than classical estimates

rapid geomagnetic field intensity variations in the near east during the 6th millennium bc new archeointensity data from halafian site yarim tepe ii northern iraq abstract we present new archeointensity results from a series of groups of pottery fragments that were collected from the multi layered archeological site yarim tepe ii in northern iraq northern mesopotamia dated to the 6th millennium bc this site comprises a 7 m thick sequence of archeological deposits encompassing the middle halaf late halaf and the halaf ubaid transitional hut between 5750 and 5000 bc according to the chronology currently considered for the halafian archeological period three new radiocarbon dates obtained from bone fragments confirm that yarim tepe ii was likely not occupied before the middle halaf as was independently established from archeological constraints archeointensity determinations were carried out using the protocol developed for the triaxe magnetometer this procedure takes into account thermoremanent magnetization anisotropy and cooling rate effects 114 fragments fulfilled our set of archeointensity selection criteria with intensity data obtained from at least two but most often three specimens per fragment mean archeointensity values were estimated for 23 groups of fragments well distributed across the entire stratigraphic sequence from the averaging of the data obtained from a minimum of three fragments per group these values were dated using a bootstrap procedure relying on the stratigraphic position of the different groups of fragments and on the different age constraints available inside the yarim tepe ii sequence the new data show a significant decrease in geomagnetic field intensity by 12 t between the middle halaf and the late halaf hut time interval this decrease was accompanied by a short intensity peak which may have lasted only a few decades around the middle halaf late halaf boundary at 5500 bc this evolution is quite similar to that observed from syrian and bulgarian archeointensity data even though the precise duration of the intensity peak is presently questionable the bulgarian data set further suggests that the intensity secular variation in the near east and in eastern europe during the 6th millennium bc was in fact principally punctuated by two successive short lasting intensity peaks the first around 5800 bc and the second around 5500 bc the scarcity of the intensity data available worldwide however prevents us constraining the geomagnetic dipole or non dipole origin of these features the variation rates associated with the rapid intensity fluctuations observed in yarim tepe ii are of 0 15 0 25 t yr this range of values appears similar to that of rapid intensity variations that sporadically occurred in more recent times such as in western europe around 700 bc and 1000 ad in contrast it is lower than the variation rates that were proposed for geomagnetic spikes our results also have interesting implications on halafian archeology in particular they suggest that the late halaf hut boundary was older by one century than previously considered

fast geomagnetic field intensity variations between 1400 and 400 bce new archaeointensity data from germany abstract thirty five mean archaeointensity data were obtained on ceramic sherds dated between 1400 and 400 bce from sites located near munich germany the 453 sherds were collected from 52 graves pits and wells dated by archaeological correlation radiocarbon and or dendrochronology rock magnetic analyses indicate that the remanent magnetization was mainly carried by magnetite data from thellier thellier experiments were corrected for anisotropy and cooling rate effects triaxe and multispecimen msp dsc protocols were also measured on a subset of specimens around 60 of the samples provide reliable results when using stringent criteria selection the 35 average archaeointensity values based on 154 pots are consistent with previous data and triple the western europe database between 1400 and 400 bce a secular variation curve for central western europe built using a bayesian approach shows a double oscillation in geomagnetic field strength with intensity maxima of 70 t around 1000 900 bce and another up to 90 t around 600 500 bce the maximum rate of variation was 0 25 t yr circa 700 bce the secular variation trend in western europe is similar to that observed in the middle east and the caucasus except that we find no evidence for hyper rapid field variations i e geomagnetic spikes virtual axial dipole moments from western europe the middle east and central asia differ by more than 2 1022 a m2 prior to 600 bce which signifies a departure from an axial dipole field especially between 1000 and 600 bce our observations suggest that the regional levantine iron age anomaly has been accompanied by an increase of the axial dipole moment together with a tilt of the dipole

magnetic field expulsion into a conducting mantle summary we look at a 2 d numerical model of the expulsion of magnetic field from the earth s core into a conducting mantle driven by a prescribed upwelling fluid motion we consider different possible conductivity profiles for the mantle and compare with the fully insulating mantle solution as studied by bloxham 1986 we find that even with a thin conducting layer the expulsion process can be restricted compared with the insulating case motivated by recent work on the conductivity of the lower mantle we later look at a conductivity profile with large lateral heterogeneity in conductivity this is found to have a strong effect on the solution with the magnetic field being distorted in the conducting part of the mantle which results in the field seen at the top of the mantle being considerably altered from the insulating case

geomagnetic field intensity variations in western europe over the past 1100 years 1 ten archeointensity results have been obtained from brick and ceramic fragments collected in france and precisely dated to between the tenth and eighteenth centuries intensity experiments were performed using the triaxe protocol taking into account cooling rate and thermoremanent magnetization anisotropy effects together with our previous results from france and belgium we computed a geomagnetic field intensity variation curve for western europe covering the past 1100 years this curve is characterized by a general decreasing trend at the millennial timescale punctuated by three short intensity peaks during the twelfth century around 1350 1400 ad and 1600 ad a similar evolution but with smoother variations due to data scatter is also observed in western europe and to a lesser extent in eastern europe when all available archeointensity data fulfilling quality criteria are used comparison of our archeointensity variation curve with the climatic record derived from fluctuations in length of the swiss glaciers shows a good temporal concordance between all geomagnetic field intensity maxima detected in western europe over the past millennium and colder episodes a comparison is further discussed between these intensity maxima and episodes of low rates of 14c production a common pattern of variations between both records is recognized between the middle of the tenth and of the beginning of eighteenth centuries if significant such coincidences suggest a dual geomagnetic and solar origin for the century scale climate and radionuclide production variations during at least the past millennium

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

geomagnetic field intensity how high can it get how fast can it change constraints from iron age copper slag article the intensity of the geomagnetic field varies over different time scales yet constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record the purpose of this study is to place firm limits on these fundamental properties by constructing a high resolution archaeointensity record of the levant from the 11th century to the early 9th century bce a period over which the geomagnetic field reached its maximum intensity in eurasia over the past 50 000 years we investigate a 14 c dated sequence of ten layers of slag material which accumulated within an ancient industrial waste mound of an iron age copper smelting site in southern israel depositional stratigraphy constrains relative ages of samples analyzed for paleointensity and 14 c dates from different horizons of the mound constrain the age of the whole sequence the analysis yielded 35 paleointenisty data points with accuracy better than 94 and precision better than 6 covering a period of less than 350 years most probably 200 years we construct a new high resolution quasi continuous archaeointensity curve of the levant that displays two dramatic spikes in geomagnetic intensity each corresponding to virtual axial dipole moment vadm in excess of 200 zam 2