a high resolution paleointensity stack of the past 14 to 68 ka from black sea sediments abstract detailed paleo and mineral magnetic analyses of a sediment composite record from the southeastern black sea yielded a high resolution well dated paleointensity record though hampered by some larger hiatuses in some cores and contaminated by diagenetically formed greigite the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period relative variations of paleointensity inferred from the sediments magnetisations were converted into a record of the virtual axial dipole moment vadm lowest vadm values are linked with the laschamp 0 50 10 22 a m 2 at 41 0 ka the norwegian greenland sea 1 5 10 22 a m 2 at 64 5 ka and the mono lake 3 0 10 22 a m 2 at 34 5 ka geomagnetic excursions the fully reversed field during the laschamp excursion exhibits a vadm of 2 0 10 22 a m 2 which is more than 25 of the present day axial dipole moment 7 628 10 22 a m 2 rates of change calculated from the black sea vadm record also give some information on how to assess the global decay of the present day geomagnetic field which is significantly enhanced in the area of the south atlantic anomaly comparison with provided c 14 and 10 be records confirm partly in the very detail the non linear anti correlation of geomagnetic field intensity and the production of cosmogenic radionuclides in the earth s upper atmosphere however discrepancies in the timing of lows and highs in the compiled records points out that the combination of different data sets from different archives remains a challenge

matuyama brunhes polarity transition near bishop california summary exposed pleistocene lake sediments at three localities separated by as much as six kilometres near bishop california contain the change from reverse to normal palaeomagnetic polarity that occurred about 0 73 million years ago the matuyama brunhes m b transition a decrease in the relative field strength is used to define the length of the transition the width of that decrease exceeds by about one fifth the width where the palaeomagnetic directions show the switch in polarity because some of the usually six subsamples in a measured horizon in the central part of the transition have normal polarity while others have intermediate or reverse polarity the behaviour of the palaeomagnetic field in that region cannot be determined accurately there is however a short reversal within the upper quarter of the transition at one locality that can be documented at least 30 cm of normal polarity underlie reverse polarity at three sites spread over a distance of 150m taken as a whole the palaeomagnetic data for the full transition are a warning that some rapid short reversals within the transition that are reported in the literature might be caused by inaccurate recording of the palaeomagnetic field the palaeomagnetic polarity 17m beneath the m b transition at one locality places a minimum age of about 1 0 myr on the glass mountain d ash bed that is found in east central and southern california

why are geomagnetic excursions not always recorded in sediments constraints from post depositional remanent magnetization lock in modelling abstract many geomagnetic excursions have been documented during the brunhes chron however high resolution paleomagnetic studies of sediments often provide evidence of few if any excursions we have investigated the reasons for this observation by modelling the post depositional remanent magnetization pdrm lock in process a cubic lock in function produces more rapid lock in compared to linear and exponential functions proposed in the literature we therefore used a cubic function to model best case scenarios for the quality of the paleomagnetic record when a high frequency geomagnetic input signal is convolved with the sediment lock in function for a wide range of sedimentation rates even for a lock in depth of 10 cm where 95 of the pdrm is locked in 5 cm below the surface mixed layer an input signal containing abundant excursions with 1 kyr duration is smoothed so drastically at low sedimentation rates 1 3 cm kyr that no excursions are recorded excursions are further attenuated by increasing the lock in depth as well as by sampling at discrete stratigraphic intervals the pdrm process acts as a low pass filter for high frequency secular variation signals even at moderate sedimentation rates 10 cm kyr we have also modelled the effects of modulation of sedimentation rate by a climatic forcing function e g insolation even though the average sedimentation rate might be substantial pdrm recording becomes much less reliable in intervals where sedimentation rates are lowest in these cases some excursions are reliably recorded in detail while others may not be recorded at all our modelling suggests that in order to consistently detect the presence of geomagnetic excursions it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm kyr if the normal spectrum of geomagnetic field behaviour contains abundant excursions failure to document excursions in many apparently high resolution analyses probably results from pdrm lock in within relatively slowly deposited sediments or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates

paleomagnetic evidence for long term 104 year memory and periodic behavior in the earth s core dynamo process a new paleomagnetic secular variation psv record from the late quaternary wilson creek beds of mono lake california contains a distinctive periodic vector waveform that follows and is almost certainly related to the mono lake excursion a comparison of all published psv results from the wilson creek beds suggests that the magnetic field at mono lake went through an interval 36 000 28 000 ybp of very low amplitude psv followed by the mono lake excursion 28 000 27 000 ybp and four subsequent recurrences 27 000 12 500 ybp of the excursion waveform with relatively diminished amplitudes lower than the excursion amplitude but higher than psv amplitudes at mono lake since 12 500 ybp this pattern clearly suggests that the mono lake excursion is related to typical psv it also suggests that the core dynamo process responsible for psv is capable of near impulse onset persistence over a time scale of 15 000 years and quasi periodic behavior that is non wave length dispersive in time

long term secular variation in geomagnetic field inclination during brunhes chron recorded in sediment cores from ontong java plateau abstract a paleomagnetic study has been conducted on three gravity cores obtained from the ontong java plateau ojp in the western equatorial pacific in order to investigate long term secular variations during the brunhes chron the cores consist of nanofossil foraminiferal ooze and covers the last 500 000 800 000 years with average sedimentation rates of 6 9 m m y downcore changes of magnetic concentration represented by magnetic susceptibility are a factor of six or less and variations in magnetic grain size and mineralogy are estimated to be small from the hysteresis parameters ratio of anhysteretic remanent magnetization arm to saturation isothermal remanent magnetization sirm and s ratio these results indicate that the sediments are rock magnetically homogeneous in general and are hence suitable for relative paleointensity estimation i chose sirm as a normalizer for relative paleointensity the three cores could be precisely correlated with each other using relative paleointensity variations depth to age conversion was carried out by tying to the sint 800 paleointensity stack guyodo and valet 1999 the ages are consistent with the oxygen isotope stratigraphy available for one of the three cores inclination variations were obtained after alternating field demagnetization large inclination anomaly about 7 in average was observed as has been suggested to occur in the western equatorial pacific by some studies the inclination records of the three cores were integrated into a single record based on the correlation by the relative paleointensity long term inclination variations of the order of 10 000 100 000 years were recognized the inclinations tend to deflect toward negative when relative paleointensity is low the variations are similar in general to the record from the west caroline basin yamazaki and oda 2002 which supports their model that long term inclination variation is controlled by relative strength of the geocentric axial dipole gad and persistent non dipole components

regionally consistent western north america paleomagnetic directions from 15 to 35 ka assessing chronology and uncertainty with paleosecular variation psv stratigraphy abstract new paleomagnetic secular variation psv data from fish lake utah usa along with previously published regional records allow us to build an independently dated western north america psv stack wnam17 from about 35 to 15 ka that quantifies dating and paleomagnetic uncertainties in february 2014 we recovered a composite 11 m long sediment record from fish lake utah that retrieved glacially influenced and post glacial sediments computed tomography ct scans and magnetic susceptibility were used to build a precise depth scale while radiocarbon dating of terrestrial macrofossils and tephrostratigraphy provide tight age control glacially influenced sediments have well defined characteristic remanent magnetizations with all maximum angular deviation values

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

the holocene paleosecular variation record from elk lake minnesota the paleosecular variation record of the geomagnetic field from elk lake minnesota is presented and its implications are discussed the elk lake record is compiled from measurements of four parallel cores with a 2 cm sampling interval 15 years time control is provided by radiocarbon dates and by annual laminations varves in the sediment stratigraphic control is provided by the annual laminations and by fine scale 2 5 cm variations in magnetic susceptibility the varve chronology provides improved time control for the central north american paleosecular variation record back to about 4000 years the results of this study largely confirm the results of lund and banerjee from lake st croix and kylen lake and demonstrate the general utility of holocene sediment paleomagnetic records differences between the records appear to be due to experimental procedure so minimal procedures are recommended for future work comparison of north american and european records suggests that two drifting non dipole sources one having its advent during the last 5000 years can account for much of the secular variation

precambrian phosphorites in the bijawar rocks of hirapur bassia areas sagar district madhya pradesh india the precambrian phosphorites of bijawar group of rocks show characteristics of a epicontinental sea with restricted and very shallow marine environment of formation along some shoals which existed during the iron rich precambrian times these phosphorite deposits located in the hirapur bassia areas show extensive leaching of carbonate and phosphate minerals during episodes of weathering x ray diffraction studies indicated that carbonate flourapatite is the major apatitic phase in these phosphorites while crandallite developed on the surface outcrops there is a general tendency for the depletion of co2 in these apatites leading to formation of flourapatite this co2 is an indicator of hidden weathering in the rocks major and trace element determinations of phosphorite have been used to indicate various correlation factors responsible for the concentration of elements in these precambrian leached phosphorites

the implications of non suppressed geomagnetic secular variation during the permo carboniferous reversed superchron abstract we have investigated variations in the natural remanent magnetisation nrm in red beds which were deposited during the permo carboniferous reversed superchron pcrs near the paleo equator these red beds are reliable recorders of the geomagnetic field because the nrm is carried by detrital hematite we analysed the angular standard deviation asd and spectral content of the directional records to quantify the amount of paleosecular variation psv during the time of deposition the asd of a high resolution directional record of the dome de barrot which spans approximately 27 kyr is 10 this is comparable to recent 0 5 ma low latitude lavas from the psvrl database mcelhinny and mcfadden 1997 and to recent lake sediments from the secvr database mcelhinny and lock 1996 the frequency spectra of declination and inclination are also similar to those from the recent lake sediments spectral power significant at the 95 level is concentrated in periods of 1 5 kyr higher frequencies are not significant from the asd and spectral analyses it appears that the amount of psv during at least part of the pcrs was comparable to that of recent times several heuristic and numerical geodynamo models are discussed in the context of superchrons and psv there is a discrepancy between the amount of sv shown by our data and that suggested by numerical models which use lateral heat flux variations at the core mantle boundary cmb however computational limitations still force some model parameters to have values that are unrealistic for the earth for the phenomenological geodynamos low energy state models which suggest that psv is suppressed during a superchron are not supported by the dome de barrot data rather a high energy state of the geodynamo in which outer core convection is vigorous is favoured as a prerequisite for a superchron to occur

multiple constraints on the age of a pleistocene lava dam across the little colorado river at grand falls arizona the grand falls basalt lava flow in northern arizona was emplaced in late pleistocene time it flowed 10 km from its vent area to the little colorado river where it cascaded into and filled a 65 m deep canyon to form the grand falls lava dam lava continued 25 km downstream and 1 km onto the far rim beyond where the canyon was filled subsequent fluvial sedimentation filled the reservoir behind the dam and eventually the river established a channel along the margin of the lava flow to the site where water falls back into the preeruption canyon the ca 150 ka age of the grand falls flow provided by whole rock k ar analysis in the 1970s is inconsistent with the preservation of centimeter scale flow top features on the surface of the flow and the near absence of physical and chemical weathering on the flow downstream of the falls the buried little colorado river channel and the present day channel are at nearly the same elevation indicating that very little if any regional downcutting has occurred since emplacement of the flow newly applied dating techniques better define the age of the lava dam infrared stimulated luminescence dating of silty mudstone baked by the lava yielded an age of 19 6 1 2 ka samples from three noneroded or slightly eroded outcrops at the top of the lava flow yielded 3he cosmogenic ages of 16 1 ka 17 1 ka and 20 1 ka a mean age of 8 19 ka was obtained from averaging four samples using the 40ar 39ar step heating method finally paleomagnetic directions in lava samples from two sites at grand falls and one at the vent area are nearly identical and match the curve of magnetic secular variation at ca 15 ka 19 ka 23 ka and 28 ka we conclude that the grand falls flow was emplaced at ca 20 ka

late quaternary paleomagnetic field secular variation from two minnesota lakes detailed paleomagnetic records from the wet sediments of two minnesota lakes are the basis for estimating the local secular variation of the earth s magnetic field during late quaternary time results from lake st croix cover the interval 0 9500 years b p and results from kylen lake cover the interval 4000 14 000 years b p paleomagnetic data were recovered from two replicate cores in each lake by sampling the sediments at 3 to 10 cm intervals analysis of the potential errors in the results indicates that all cores were correctly oriented in the vertical plane but that some azimuthal offsets of individual core segments did occur various methods for correcting the occasional azimuthal offsets are described the final paleomagnetic data sets from each lake show a high degree of internal consistency these results also show close correspondence 1 between the two lakes 2 with the known historic magnetic field variation and 3 with paleomagnetic results from dated archeological material and lava flows from the western united states rock magnetic analysis of the lake sediments indicates that the natural remanent magnetization is the result of a physical remanence acquisition process detrital remanent magnetization active at or near the sediment water interface there is no evidence for a systematic inclination error in the results statistical analysis of vector time series determined from the paleomagnetic data indicates that both the field vector and virtual geomagnetic pole vgp distributions are non fisherian and essentially axial dipolar when averaged over 10 000 years spectral analysis of the scalar inclination and declination suggests that the field behavior is separable into a dominant low frequency dipolelike waveform period of 9500 years and a higher frequency band irregular waveform fundamental period of 2400 years with multiples at 1200 800 and 600 years that may represent the nondipole field this irregular waveform is interpreted as resulting from zonal drift of the nondipole field past minnesota on the basis of waveform morphology and vgp circularity zonal drift of the nondipole field plus a low frequency dipole variation can account for almost all of the observed paleomagnetic field variation

geomagnetic excursions recorded in chinese loess in the last 70 000 years detailed paleomagnetic investigations conducted on the malan loess l1 at a well dated section weinan in the chinese loess plateau revealed two distinct anomalous directional intervals adi accompanied by lower relative paleointensity rock magnetic properties and anisotropy of magnetic susceptibility ams of samples taken from sedimentary horizons carrying the anomalous directions are similar to those taken from outside the anomalous directional intervals thus the anomalous directions are interpreted as geomagnetic excursions based on 14c and tl data the excursions occurred at 27 1 26 0 and 46 8 37 4 kyr b p the ages are consistent with the ages assigned to the mono lake excursion mle and laschamp excursion le respectively the result indicates that the mle and le were independent geomagnetic excursions the morphology of le at this section suggests that the le might be an aborted geomagnetic reversal

low to high amplitude oscillations and secular variation in a 1 2 km late miocene inclination record abstract a miocene section of 2 km thickness was continuously cored near szombathely nw hungary a detailed magnetostratigraphic study has been integrated with results of lithologic sedimentologic and paleontologic studies progressive alternating field a f and thermal demagnetization and rock magnetic and mineralogic studies indicate that the natural remanent magnetization nrm resides in magnetite and the strata contain only minor secondary magnetizations major polarity zones have been defined by inclinations and correlated with the geomagnetic polarity time scale for the interval 10 9 ma inclinations for samples collected at 1 2 m intervals display fine scale oscillations representing secular variation with period times of 400 700 years fluctuating amplitudes of oscillation range from low less than 10 peak to peak to high more than 40 reflecting apparently varying stabilities of the geomagnetic field additionally amplitudes of oscillation progress from low to high and return to low forming oscillation cycles with a periodicity of 6 2 1 8 kyr the oscillations and boundaries of oscillation cycles are generally unrelated to lithology and stratigraphy many oscillation cycles appear to be incomplete and interrupted by high level oscillations some incomplete cycles appear to arise from brief interruptions to the depositional record whereas other interruptions may irregularly arise from a separate component of the geomagnetic field

paleomagnetic investigation of the bonneville alloformation lake bonneville utah abstract paleomagnetic secular variation in a portion of the bonneville alloformation is compared with secular variation in lacustrine sediments in the mono basin california and with secular variation in lake lahontan sediments in the northwestern great basin the comparison places an age of about 18 000 yr b p and a span of 1000 to 3000 yr on part of a transgressive stage of lake bonneville near delta utah that is coeval with a wet period in the lahontan basin

correlation of late pleistocene glaciolacustrine and marine deposits by means of geomagnetic secular variation with examples from northern new york and southern ontario abstract the geomagnetic secular variation record retained by glaciolacustrine and marine sediments at nine sites in northern new york and southern ontario provides a means for stratigraphic correlation of glacial deposits for the time period between about 12 600 to 9900 14c yr b p measurement of the depositional remanent magnetism of sediments deposited in glacial lake iroquois and the champlain sea has produced a geomagnetic secular variation curve that represents the time period immediately following deglaciation about 12 600 14 c yr b p the curve varies from about 358 to 344 declination and 51 to 61 inclination over approximately 180 valve years marine sediments of the champlain sea have preserved a record approximately 1500 yr long that varies from about 2 to 29 declination and 47 to 60 inclination these combined glacial paleomagnetic records may also correlate with those from glacial sequences beyond our study area the shape and amplitude of the secular variation record in glaciolacustrine and marine sediments from the western adirondack borderland show agreement with other glacial varve secular variation records and suggest possible correlations with secular variation curves from lake cores

the rise and fall of lake bonneville between 45 and 10 5 ka a sediment core taken from the western edge of the bonneville basin has provided high resolution proxy records of relative lake size change for the period 45 1e10 5 calendar ka hereafter ka age control was provided by a paleomagnetic secular variation psv based age model for blue lake core bl04 4 continuous records of d 18 o and total inorganic carbon tic generally match an earlier lake level envelope based on outcrops and geomorphic features but with differences in the timing of some hydrologic events states the stansbury oscillation was found to consist of two oscillations centered on 25 and 24 ka lake bonneville appears to have reached its geomorphic highstand and began spilling at 18 5 ka the fall from the highstand to the provo level occurred at 17 0 ka and the lake intermittently overflowed at the provo level until 15 2 ka at which time the lake fell again bottoming out at w14 7 ka the lake also fell briefly below the provo level at w15 9 ka carbonate and d

paleomagnetic investigation of lake lahontan sediments and its application for dating pluvial events in the northwestern great basin abstract a comparison of paleomagnetic secular variation in sediment of pleistocene lake lahontan in the northwestern great basin with secular variation in lake sediment in the mono basin california indicates that lake lahontan was in the valley of the truckee river between pyramid lake and wadsworth nevada from about 19 000 to 13 000 yr b p the secular variation in older lake lahontan sediment in the truckee river valley has the general features of secular variation in middle pleistocene lacustrine sediments near rye patch dam nevada 125 km to the east on the basis of field mapping and tephrochronology the sections of older lacustrine sediments are not coeval the apparent but erroneous correlation of those sediments emphasizes the need for multiple dating methods when paleomagnetic secular variation is used to date stratigraphy

dating late pleistocene pluvial events and tephras by correlating paleomagnetic secular variation records from the western great basin abstract paleomagnetic records are used to correlate sedimentary sequences from pluvial lakes chewaucan and russell in the western great basin this correlation is the basis for age control in the relatively poorly dated sequence from lake chewaucan the resulting chronology supports a lack of sedimentation in lake chewaucan during the interval 27 400 to 23 200 yr b p an assertion supported by the presence of a lag deposit at the corresponding stratigraphic horizon because the lake chewaucan outcrop near summer lake oregon is near the bottom of the lake basin we conclude that lake chewaucan was at a lowstand during this time interval the chewaucan lowstand is coeval with the lowstand accompanying the wizard s beach recession isotope stage 3 previously seen in the geologic record from nearby pluvial lake lahontan the ages of six tephra layers including the trego hot springs tephra were also estimated using the paleomagnetic correlation together the new age of the trego hot springs tephra 21 800 yr b p and the lake surface level prehistory of lake chewaucan imply a revised model for the lake surface level prehistory of lake lahontan the revised model includes a longer duration for the wizard s beach recession and the occurrence of a younger lowstand of short duration soon after the lowstand corresponding to the wizard s beach recession

correcting distorted paleosecular variation in late glacial lacustrine clay abstract an undisturbed horizontal chronostratigraphic marker horizon of laminated red glacio lacustrine clay crops out over 25 000 km 2 in northern ontario canada the primary clastic hematite laminae possess two stable vector components of magnetization we sampled a 1 6 m vertical section at overlapping 2 cm intervals in cubic specimens 8 cm 3 n 106 precisely oriented in geographic coordinates which permitted measurement of inclination and declination alternating field demagnetization 12 17 steps per specimen isolated a characteristic chrm primary component coercivity 40 mt approximately 30 shallower than the mean geomagnetic field inclination at this latitude with distorted paleosecular secular variation psv a lower coercivity overprint 20 40 mt is 6 shallower than the present geomagnetic field and similarly inclined to the depositional remanence deflection when the clay was re sedimented in the laboratory we believe this angle to be representative of the inclination of the deflected primary remanence inclination during deposition caused by the magnetic anisotropy of the clay using this as a proxy for the initial inclination the chrm hard component inclinations were restored to their original values assuming vertical compaction which averages to 51 the restored inclinations are compatible with site latitude and the restored secular variation loops centre reasonably on the geographic north pole the structural correction based on homogeneous vertical shortening over simplifies the reality of heterogeneous particulate flow in which grain rotations depend on shape size and packing and effectively combines influences of compaction and depositional settlement nevertheless this correction makes the psv data more useful and interpretable in terms of paleopole migration the more logical correction technique using anisotropy of anhysteretic remanence aarm jackson m j banerjee s k marvin j a lu r gruber w 1991 detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results geophys j int 104 95 103 is foiled here due to the high coercivity of the remanence bearing grains and their unknown orientation distribution and shape distribution

repeating waveform initiated by a 180 190 ka geomagnetic excursion in western north america implications for field behavior during polarity transitions and subsequent secular variation new paleomagnetic lithologic and stratigraphic data are presented from the sediments of lake chewaucan in the summer lake basin oregon the new data place better age constraints on the sediments and improve the accuracy of the previously published paleomagnetic record from this locality a complex yet distinct waveform is observed in all three components of the paleomagnetic vector the waveform begins as the 180 190 ka pringle falls long valley summer lake ii geomagnetic excursion and continues for two cycles after the excursion until the record is interrupted by an unconformity that we correlate to the oxygen isotope stage 6 5e boundary the waveform s directional morphology in virtual geomagnetic pole vgp space is defined by two clockwise loops followed by a distinctive counterclockwise clockwise counterclockwise looping sequence the vgp paths of the two cycles after the excursion are rotated 180 about earth s spin axis with respect to the vgp paths of the excursion cycle the waveform also consists of a relative paleointensity variation which repeats during the two cycles after the excursion the average paleointensity of the postexcursion waveform repetitions is high relative to the extremely low values that occur during the excursion this observation indicates that excursion initiated secular variations can occur after the field fully recovers from the low intensities which commonly typify excursions because of the similarities noted previously between this excursion and full polarity transitions trie et al 1991 our new observations constrain models for a wide range of field behavior including polarity transitions excursions and secular variation

application of stratigraphic sedimentological forward modeling of sedimentary processes to predict high quality reservoirs within tight sandstone abstract the prediction of high quality reservoirs within tight sandstone is presently a popular and challenging research topic many geophysical characteristics of tight sandstone reservoirs are similar to those of the surrounding rock making it difficult to predict the distribution of these reservoirs in this paper a stratigraphic sedimentological forward modeling is proposed to predict the distribution of high quality tight sandstone reservoirs in the xihu depression east china sea stratigraphic sedimentological forward modeling can evaluate the influences of different geological parameters on sedimentary processes and the results to reproduce the spatial distribution of sedimentary strata in a progradational delta and the overall stratigraphic stacking pattern different parameters were tested by numerical simulation according to the influence of the parameters on the sedimentary process the parameters can be divided into two types high sensitivity parameters transport capacity of the river and sand ratio and low sensitivity parameters maximum initial water depth lake level change and tectonic subsidence the analysis results are suitable for continental sedimentary basins with relatively high deposition rates in the process of optimizing the simulation parameters 236 simulations were carried out by adjusting the high sensitivity parameters and then the low sensitivity parameters an optimal model that can reflect the development characteristics of high quality tight sandstone reservoirs was obtained according to the characteristics of high quality reservoirs the product of the sand ratio and reservoir thickness from the sedimentary simulation results is used as the parameter for evaluating high quality reservoirs the ratio can effectively overcome the difficulties of insufficient amounts of data and seismic prediction and effectively predict the locations of high quality reservoirs in low exploration areas of tight sandstone reservoirs this method also provides a new idea for the prediction of high quality reservoirs in this kind of area in the future

a 20 15 ka high resolution paleomagnetic secular variation record from black sea sediments no evidence for the hilina pali excursion abstract a comprehensive magnetostratigraphic investigation on sixteen sediment cores from the southeastern black sea yielded a very detailed high quality paleosecular variation psv record spanning from 20 to 15 ka the age models are based on radiocarbon dating stratigraphic correlation and tephrochronology further age constraints were obtained by correlating four meltwater events described from the western black sea ranging in age from about 17 to 15 ka with maxima in k ti ratios obtained from x ray fluorescence xrf scanning and minima in s ratios reflecting increased hematite content in the studied cores since the sedimentation rates in the investigated time window are up to 50 cm ka 1 the obtained psvs records enabled a stacking using 50 yr bins a directional anomaly at 18 5 ka associated with pronounced swings in inclination and declination as well as a low in relative paleointensity rpi is probably contemporaneous with the hilina pali excursion originally reported from hawaiian lava flows however virtual geomagnetic poles vgps calculated from black sea sediments are not located at latitudes lower than 60 n which denotes normal though pronounced secular variations during the postulated hilina pali excursion the vgps calculated from black sea data migrated clockwise only along the coasts of the arctic ocean from ne canada 20 0 ka via alaska 18 6 ka and ne siberia 18 0 ka to svalbard 17 0 ka then looping clockwise through the eastern arctic ocean

research note mono lake excursion in the lahontan basin nevada summary in the lahontan basin of north western nevada the mono lake excursion is recorded in a wave cut bank at pelican point on the west side of pyramid lake the record is almost identical to one across the lake near pyramid island 10 km away and in the carson sink 60 km further to the east at each locality the older half of the excursion is present only as a reduction in the relative field intensity while the younger half shows the expected easterly declination and steep positive inclination that result in a small counterclockwise loop of the virtual geomagnetic poles the younger half occurs during high relative field intensity and the entire intensity pattern is similar to the one for the excursion in the mono basin california 250 km to the south the presence of only part of the mono lake excursion in the lahontan basin might have resulted from the slow sedimentation rate in pyramid lake estimated to be about 12 cm kyr 1 with the field behaviour being more extensively integrated in the subsamples used in an alternate explanation the absence of the older part of the excursion can be attributed to overprinting when magnetic grains that were free to rotate in unconsolidated sediment were realigned during the high relative field intensity in the younger part of the excursion

40ar 39ar and paleomagnetic constraints on the evolution of volcan de santa maria guatemala 40 ar 39 ar dating of 15 lava flows indicates that volcan de santa maria grew episodically to 8 km 3 in size at an average rate of 0 12 km 3 ka between 103 and 35 ka the composite cone grew in four phases including two periods of intense activity at ca 72 ka and ca 35 ka during which 1 5 km 3 and 3 km 3 of basaltic to andesitic magmas were erupted there is no evidence of further volcanism after ca 35 ka until the great dacitic eruption in 1902 the average eruptive rate is 0 16 km 3 ka if products of the 1902 eruption and subsequent santiaguito dome are included whereas the mono lake excursion is not clearly recorded at volcan de santa maria as had been inferred from earlier studies virtual geomagnetic poles vgps of the 35 ka cone forming lavas exhibit high amplitude paleosecular variation that may correspond in time to the mono lake excursion two older packages of lava flows are each associated with a distinctive cluster of vgps which supports the 40 ar 39 ar age model and the conclusion that cone building was episodic during the final 60 of cone growth lavas evolved from basaltic to andesitic 51 57 sio 2 with time but with a regression to slightly less evolved compositions during the onset of the final cone building phase despite the relatively small volume of santa maria cone growth processes and geochemical evolution through time mirror observations at other currently active volcanoes along the central american volcanic arc and may prove useful as an analogy in assessing long term hazards posed by other predominantly basaltic andesitic composite volcanoes

record of the younger part of the pringle falls excursion at long valley california summary the younger of two closely spaced palaeomagnetic excursions at pringle falls oregon is recorded in lacustrine silts that crop out in long valley california assigned an age of about 220 000 years the virtual geomagnetic poles of the younger excursion form a clockwise loop that reached 35 s latitude east of south america before returning to the northern hemisphere in the pacific ocean west of central america the poles then form a narrow band across north america while moving to high northern latitudes this record matches extremely well feature b of the original excursion record from pringle falls reported by herrero bervera et al 1994 and is similar to this excursion at summer lake oregon negrini et al 1994 in that the pole path is confined primarily to the east central pacific ocean on the basis of an assumed sedimentation rate of 30 cm per thousand years the younger excursion feature b at pringle falls spans an estimated 1200 years and followed by about 1000 years a larger excursion feature a at pringle falls previously discovered at the same long valley site at a second long valley site 30 m away the younger excursion feature b is only partially recorded because of a presumed small hiatus in the sedimentary section

regional character of geomagnetic field directional circularity holocene east asia abstract this study summarizes the pattern of directional paleomagnetic secular variation psv in five previously published holocene records from east asia each record is high in resolution and well dated we have developed equi spaced 100 year increment time series for the declinations and inclinations for the five sites and compared their overall directional variability we see evidence of nine correlatable scalar features in both inclination and declination these comparisons suggest that to first order these five psv records all estimate the same regional pattern of psv for east asia we specifically focus on the clockwise or counter clockwise motion of paleomagnetic directions termed circularity in these psv records we have used two techniques to estimate the directional circularity of each psv record we have first calculated the incremental rate and direction of motion clockwise or counter clockwise in 200 year increments for each record over the last 3000 8000 years we have separately used a more traditional technique of looking for discernable looping full loops down to 1 2 loops in individual records we estimate the loop sizes durations and circularity direction we largely see the same pattern of circularity in both measurement techniques there are eight intervals of oscillating circularity clockwise versus counter clockwise in all five sites they generally agree in timing and direction of circularity we see evidence for 5 discernable loops alternating between clockwise and counter clockwise in all five records both techniques suggest an oscillating teeter totter like behavior to psv circularity that must be due to the pattern of fluid flow magnetic flux regeneration in the outer core this teeter totter behavior is unbalanced with almost twice the time spent in clockwise motion as in counter clockwise motion less than 10 of the overall time series duration is spent in linear not circulating behavior the loops tend to have a distribution of sizes and durations with smaller loops being shorter in duration 300 600 years and bigger loops having longer durations 800 1500 years one particular feature of these circularity patterns bears special note all five psv records show short term 200 400 years significant acceleration in circularity rate combined with change in circularity direction we see evidence for three of these short intervals near 200 bce 400 ce and 1300 ce these features also are intervals of the fastest regular secular variation rates in all the records these features are broadly analogous to geomagnetic jerks courtillot and lemouel 1976 1984 in that they are short intervals 1 2 years of anomalous acceleration within high resolution historical

paleomagnetism of pleistocene volcanic rocks from pantelleria island sicily channel italy abstract a paleomagnetic investigation has been carried out on pleistocene volcanic rocks from pantelleria in the sicily channel this island is characterised by a bimodal volcanism of peralkaline rhyolite to trachyte and basalt composition the radiometric data indicate that volcanic activity started around 324 ka bp samples have been collected from 30 sites in 16 volcanic units spanning activity during the last 150 ka magnetic properties vary systematically with lithotype and indicate high ti titanomagnetite to magnetite as the main carriers of magnetisation stable characteristic remanent magnetisation chrm directions isolated by alternating fields af demagnetisation show normal polarity consistent with emplacement during the brunhes chron and vary within the paleosecular variation psv range however the inclinations are low as compared with the geocentric axial dipole gad inclination at pantelleria especially for volcanic units younger than 50 ka the mean chrm direction computed from 27 sites is d 358 8 i 46 9 k 30 95 5 2 and the inclination anomaly is i 8 1 both tectonic movements and the presence of large magnetic anomalies around the island of pantelleria fail to explain this low inclination which may therefore be related to a significant long lived non dipolar field contribution in the area over the past 150 kyr

the blake geomagnetic event transition geometry dynamical characteristics and geomagnetic significance abstract two records of the blake geomagnetic event have been obtained from marine cores in the mediterranean the upper bound for duration of the event calculated from accurate oxygen isotope stratigraphy and or tephrochronology is 4000 years one record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved the directional changes do not occur at constant speed rather a stop and go behaviour is observed as in detailed records of reversals the virtual geomagnetic pole vgp path confined to two bands of longitude over the americas and antipodal to them is virtually identical to the only other path of this event obtained from lacustrine sediments in the western usa suggesting that a dipolar component may be present in the transitional field during the blake event the two bands of longitude coincide with those of many recent reversals suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time

continuous lake sediment records of glaciation in the sierra nevada between 52 600 and 12 50014c yr b p the chemistry of the carbonate free clay size fraction of owens lake sediments supports the use of total organic carbon and magnetic susceptibility as indicators of stadial interstadial oscillations owens lake records of total organic carbon magnetic susceptibility and chemical composition of the carbonate free clay size fraction indicate that tioga glaciation began 24 500 and ended by 13 60014c yr b p many of the components of glacial rock flour e g tio2 mno bao found in owens lake sediments achieved maximum values during the tioga glaciation when valley glaciers reached their greatest extent total organic carbon and sio2 amorphous concentrations reached minimum values during tioga glaciation resulting from decreases in productivity that accompanied the introduction of rock flour into the surface waters of owens lake at least 20 stadial interstadial oscillations occurred in the sierra nevada between 52 600 and 14 00014c yr b p total organic carbon data from a pyramid lake sediment core also indicate oscillations in glacier activity between 39 500 and 13 60014c yr b p alpine glacier oscillations occurred on a frequency of 1900 yr in both basins suggesting that millennial scale oscillations occurred in california and nevada during most of the past 52 600 yr

palaeomagnetic secular variation and rock magnetic studies of holocene sediments from a maar lake hoya de san nicolas in central mexico summary three up to 520 cm long sediment cores from hoya san nicolas in guanajuato mexico were analysed for various magnetic properties in order to better define a palaeomagnetic secular curve for central mexico the results magnetic susceptibility hysteresis cycles anhysteretic and isothermal remanent magnetization suggest that the remanent magnetization of the sediments is controlled by ferrimagnetic minerals pseudo single domain magnetite which are suitable recorders of the geomagnetic field the age depth model indicates average deposition rates of 0 32 interval 146 198 cm and 0 45 mm yr 1 interval 200 520 cm and a basal age of about 11 600 calibrated years bp cal yr bp we used magnetic susceptibility and natural remanent magnetization to correlate the three cores a composite palaeomagnetic secular variation sv record was obtained from the cores with a stretching and stacking process and a chronology established with accelerator mass spectrometer dates on microscopic charcoal and stratigraphic correlations with other well dated holocene records from mexico and guatemala the declination and inclination results show oscillatory behaviour varying in a narrow range although a distinctive directional swing is evident between 9060 and 9810 cal yr bp the san nicolas palaeomagnetic sv curve is similar to palaeomagnetic master curves from europe and north america in shape occurrence and synchronicity of directional features especially with respect to inclination

middle late pleistocene relative palaeointensity of the geomagnetic field from lacustrine sediments lake chewaucan western united states summary detailed palaeomagnetic and rock magnetic studies of a 15 m succession of middlellate pleistocene lacustrine sediments from ancient lake chewaucan southern oregon western united states indicate that the remanence bearing grains are sufficiently uniform to be applicable to relative palaeointensity studies we have used arm sirm and x for normalization of the nrm all three parameters give essentially identical results in their relative stratigraphic variations which indicates that the normalizations efficiently remove the effects of variation in magnetic mineral concentration patterns in grain size variation as indicated by small scale quasi cyclic fluctuations in hysteresis parameters may be due to environmental changes such as lake level variation however these fluctuations are within the acceptable range of grain sizes for palaeointensity studies and cannot be correlated with any of the features of the normalized remanence record we therefore conclude that the large scale variations in the normalized remanence record are due to geomagnetic palaeointensity fluctuations parts of the normalized remanence record where firm chronological constraints exist may correlate with features of relative palaeointensity records from deep sea sediments our results also confirm the observation that low geomagnetic field intensities dominate during geomagnetic excursions further studies of relative palaeointensity of the geomagnetic field may enable the development of an independent time scale which would make possible the direct correlation of palaeoclimate records from deep sea and continental environments

dynamo waves and palaeomagnetic secular variation summary a dynamic model is proposed for large amplitude geomagnetic secular variation with periodicity in the range of 1000 3000yr thermal convection in the lower mantle imposes a spatially variable heat flux at the core mantle boundary if the total heat loss from the core exceeds approximately 3 x 10 2w then both i a convective layer and ii large scale quasigeostrophic shear flows are likely to exist beneath the core mantle boundary the combined effects of convection and shear at the top of the outer core can generate three dimensionally propagating magnetic field disturbances called cuw dynamo waves an analysis is made of the surface magnetic field disturbances due to dynamo wave propagation in the outer core in the presence of turbulent convection and an east west shear flow nonaxisymmetric disturbances exhibit the following characteristics i dynamo waves propagate in each hemisphere with a poleward or equatorward component depending on the sign of the a effect ii both eastward and westward propagation is allowed althcugh westward propagation is preferred the apparent velocity at the earth s surface is the sum of propagation and drift velocities in the core iii dynamo wave disturbances are transient with exponential growth and decay phases iv traces of inclination versus declination of individual fourier components obey runcorn s circularity rule a comparison is made with lake sediment secular variation records from central north america measured by lund banerjee the band passed time series and the major inclination declination loops can be modelled from 2600 to 10 000 yr bp by a single dynamo wave periodic in 2400 yr propagating under the site along an azimuth of 345 between 0 and 2600 yr bp the record can be modelled by addition of a wave with 1200yr period propagating along an azimuth of 30 this indicates that the palaeomagnetic secular variation record in north america to 10 000 yr may be dominated by poleward propagating wave like disturbances

latest olduvai short lived reversal episodes recorded in chinese loess 1 a duplicate high resolution magnetic record was obtained from two parallel loess sections separated by about 300 m near baoji city southern chinese loess plateau this reveals at least eight short lived reversed polarity episodes in the uppermost part of the olduvai normal polarity subchronozone rock magnetic experiments and anisotropy of low field magnetic susceptibility confirm that the multiple occurrence of short lived reversals is not due to rock magnetic and or sedimentary causes assuming a constant accumulation rate within the olduvai subchron the short episode zone was estimated to span about 24 30 ka and 24 31 ka for the two sections respectively the duration of each short episode ranges from about 0 3 ka to 2 1 ka the present results together with previous worldwide observations show that the occurrence of numerous short reversals is peculiar to the latest olduvai subchron the geodynamo may have been in an anomalous state at that time

paleoclimatic context of geomagnetic dipole lows and excursions in the brunhes clue for an orbital influence on the geodynamo the hypothesis of an influence of the astronomical precession on the geodynamo energy budget was recently reappraised by theoreticians paleomagnetic indications of such an influence remain controversial because reconstructions of paleointensity variations from sediments are suspected to be contaminated by lithological paleoclimatically induced influences three sets of complementary indicators are however available 1 records of the direction of magnetization in sediments 2 records of magnetic anomalies of the deep sea floor basalts and 3 records of production variations of cosmogenic isotopes from sediment and ice cores these records confirm the genuine geomagnetic origin of paleointensity lows and their narrow link with excursions or short reversals recorded in various materials and often dated by radiometric methods the analysis of these time series and their comparison with 18o records of the paleoclimate suggest that such globally recorded geomagnetic dipole lows have preferentially occurred in the context of interglacial or transitional paleoclimates at the time of low or decreasing obliquity the dominant periods extracted with the complex continuous wavelet transform technique range from 40 to 125 ka further suggesting a link with orbital parameters these results encourage future efforts of research to improve the precision the resolution and the dating of the time series of geomagnetic dipole low in order to better decipher orbital signatures and understand their origin an important implication of this topic is that the next geomagnetic dipole low should be related with the present interglacial

a sixty thousand year paleomagnetic record from gulf of california sediments secular variation late quaternary excursions and geomagnetic implications a unique inclination record of geomagnetic secular variation for the past 60 000 years 60 ka has been obtained from continuously deposited sediments in the gulf of california recovered during deep sea drilling project dsdp leg 64 by hydraulic piston coring at site 480 the chronology of hole 480 was determined by 18o stratigraphy and varve counts indicating sedimentation rates approaching 1 m ka the paleomagnetic results of the upper 49 m show 1 the average inclination over this interval is identical to the geocentric axial dipole value at the sampling site 2 excursion directions occur between about 53 and 22 ka before present bp during this time the geomagnetic field was generally noisier than in the overlying and underlying sections with greater dispersion of the inclination 3 the laschamp excursion was apparently recorded at hole 480 between about 51 and 49 ka bp and the mono lake excursion between about 29 and 26 ka bp in addition a narrow 0 3 0 4 m zone near 23 ka bp has a very similar paleomagnetic signature as the excursion observed at summer lake oregon 1 and we suggest that the summer lake is distinct from and younger than the mono lake excursion by 3 5 ka and of considerably shorter duration lasting no more than a few hundred years 4 recurring inclination fluctuations were identified at site 480 characterized by end points with steep inclinations and shallow intermediate value s as compared with the geocentric axial dipole the inclination cycles are particularly apparent from 54 to 24 ka bp with a characteristic period of about 4 4 ka 5 the noisier inclination record between 54 and 24 ka bp might be related to the generally reduced dipole moment between about 20 and 50 ka and particularly low paleointensities for the laschamp and mono lake excursions

overprinting of natural magnetic remanence in lake sediments by a subsequent high intensity field there has been considerable debate recently on how well sedimentary rocks record transitional field directions during a polarity reversal1 5 this has not been an easy question to answer because the input geomagnetic signal for a given sedimentary record is not known independently and because the process by which remanent magnetization is acquired by sediments is difficult to duplicate in the laboratory the geomagnetic excursion recorded by sediments outcropping around the shores of mono lake california6 8 offers an unusually good opportunity to examine this question we report here a study of the mono lake excursion mle at a new locality on the southeast shore where although the records satisfy usually applied standards of palaeomagnetic quality it is clear from comparison with records from other localities that the field directions during a time of low field intensity have not been faithfully preserved the distinctive directions of natural remanence in the discrepant part of the record show that the southeast shore sediments were overprinted by the ensuing higher intensity field acting on the still unconsolidated sediment apparently by realigning some magnetic grains that had previously been locked in

paleomagnetic and tephra evidence for tens of missoula floods in southern washington paleomagnetic secular variation and a hiatus defined by two tephra layers confirm that tens of floods from glacial lake missoula montana entered washington s yakima and walla walla valleys during the last glaciation in these valleys the field evidence for hiatuses between floods is commonly subtle however paleomagnetic remanence directions from waterlaid silt beds in three sections of rhythmically bedded flood deposits at zillah touchet and burlingame canyon display consistent secular variation that correlates serially both within and between sections the secular variation may further correlate with paleomagnetic data from fish lake oregon and mono lake california for the interval 12 000 17 000 14 c yr b p deposits of two successive floods are separated by two tephras derived from mount st helens washington the tephras differ in age by decades indicating that a period at least this long separated two successive floods the beds produced by these two floods are similar to all of the 40 beds in the slack water sediment sequence suggesting that the sequence is a product of tens of floods spanning a period of perhaps a few thousand years

mono lake excursion recorded in sediment of the santa clara valley california 1 two intervals recording anomalous paleomagnetic inclinations were encountered in the top 40 meters of research drill hole ccoc in the santa clara valley california the younger of these two intervals has an age of 28 090 330 radiocarbon years b p calibrated age 32 8 ka this age is in excellent agreement with the latest estimate for the mono lake excursion at the type locality and confirms that the excursion has been recorded by sediment in the san francisco bay region the age of an anomalous inclination change below the mono lake excursion was not directly determined but estimates of sedimentation rates indicate that the geomagnetic behavior it represents most likely occurred during the mono lake laschamp time interval 45 28 ka if true it may represent one of several recurring fluctuations of magnetic inclination during an interval of a weak geomagnetic dipole behavior noted in other studies in the region

a full vector paleomagnetic secular variation record psv from pyramid lake nevada from 47 17 ka evidence for the successive mono lake and laschamp excursions abstract we have carried out a paleomagnetic study of late pleistocene pyramid lake core plc08 1 1680 cm our goals were to develop a full vector paleomagnetic secular variation psv record for the core establish a paleomagnetic chronostratigraphy for the lake based on correlation of the psv record to other dated psv records in the region compare that chronostratigraphy with previously developed radiocarbon and ash chronologies and search for evidence of the mono lake excursion and laschamp excursion we have recovered a full vector psv record inclination declination relative paleointensity for the interval 47 ka to 17 ka twenty radiocarbon dates and four dated ashes provided a chronostratigraphic framework for this record we have also used the link between our psv and other dated psv records to develop an independent psv chronostratigraphy for the core the psv chronostratigraphy is not significantly different from that estimated by the radiocarbon and ash chronologies we note the existence of two intervals of anomalous paleomagnetic directions the younger interval centered at 34 1 0 4 ka has the characteristic vector component features of the mono lake excursion the older interval centered at 40 9 0 5 ka has the characteristic paleomagnetic signature of the laschamp excursion this is the first time both intervals of excursional behavior have been found in the same sediment record from the western usa our new psv record also corroborates previous estimates of the mono lake excursion directional field behavior liddicoat and coe 1979 and age benson et al 2003