on the reliability of mesozoic dipole low new absolute paleointensity results from parana flood basalts brazil 1 thellier paleointensity experiments were carried out on early cretaceous parana flood basalts forty two samples from 11 lava flows yielded apparently reliable absolute intensity determinations the mean paleointensity values per flow obtained in this study are ranging from 19 4 4 0 to 46 7 7 0 t and the corresponding virtual dipole moments are ranging from 4 0 0 6 to 10 5 1 1 10 22 am 2 these yield a mean value of 7 2 2 3 x 10 22 am 2 which is about 92 of the present geomagnetic axial dipole our results in conjunction with tarduno et al s 2001 data suggest that the paleostrength during early cretaceous period might be comparable or even much higher than recent field intensity and not anomalousely low as previously suggested thus the mesozoic dipole low mdl may probably be considered as unreliable

palaeomagnetism and 40ar 39ar age from a cretaceous volcanic sequence inner mongolia china implications for the field variation during the cretaceous normal superchron abstract an integrated palaeomagnetic and 40ar 39ar dating study was carried out on an early cretaceous volcanic lava sequence from the suhongtu section inner mongolia to determine the field behavior within the cretaceous normal superchron cns 40ar 39ar ages were obtained from 12 lava flows indicating that the studied lava was formed around 114 1 0 3 ma for the lower interval and 110 6 0 1 ma for the upper interval rock magnetic experiments and electron microprobe analyses indicate that the primary fe ti oxides are the main magnetic carriers all lava flows carry normal palaeomagnetic directions which can be grouped into 31 units by an f test with a fisher mean characteristic remanent magnetization chrm of d i 12 8 58 6 95 2 3 the corresponding palaeomagnetic pole is located at 80 3 n and 200 3 e a95 3 2 k 64 4 which is indistinguishable at the 95 confidence level from the eurasia pole derived from the apparent polar wander path for the early cretaceous using a modified thellier palaeointensity method with stringent acceptance criteria we obtained two time series of palaeointensity records from 15 independent palaeomagnetic units total 136 samples the virtual dipole moment vdm values varied from 2 53 1022 am2 to 9 92 1022 am2 the mean vdms for the upper and lower intervals are 5 38 2 06 1022 am2 and 4 61 2 67 1022 am2 respectively the observed time series of palaeointensity together with the previously available data for the cns suggest that magnetic field strength during the cns fluctuated significantly with time

a theoretical treatment of paleointensity determination experiments on rocks containing pseudo single or multi domain magnetic particles a phenomenological theory of thermoremanent magnetization trm is applied to model the possible results of paleointensity determination experiments using the thellier method it is shown that intrinsic curvature of the arai plot for samples containing pseudo single or multi domain particles can be attributed exclusively to the presence of remanences which are unblocked below their blocking temperature thus the thermal demagnetization tail of partial trm ptrm is not a direct measure for a sample s tendency to yield a curved arai plot here it is proposed to use the advancing decay of a ptrm t1 t2 which during heating occurs already below the lower temperature t1 t2 as a more reliable pre selection criterion in addition the effect of an average magnetostatic interaction on the thellier experiment turns out to be negligible an extended thellier method which compares the thermal demagnetization of natural remanent magnetization with that of an artificial trm according to the applied trm theory should result in a perfectly linear modified arai plot even for pseudo single or multi domain samples if no alteration or viscous effects occur

geochronology of zhunsujihua molybdenum deposit in sonid left banner inner mongolia and its geological significance a precise early permian re os isochron age of 298 1 3 6 ma for the molybdenite and a zircon shrimp u pb age of 298 2 3 1 ma for the ore bearing granite from the zhunsujihua porphyry molybdenum deposit in sonid left banner of inner mongolia provide new information for the ore forming epoch of the zhunsujihua porphyry molybdenum deposit the rhenium content of molybdenite suggests a crust mantle mixed source for the ore forming substance combined with the mineralization ages of the porphyry deposits in northern central inner mongolia and the adjacent mongolia from the literature the authors recognized four metallogenic episodes i e late devonian early permian triassic and cretaceous on the basis of the metallogenic evolution the porphyry mineralization type is divided into the late devonian copper ore forming stage in an arc tectonic setting and the early permian and subsequent molybdenum ore forming stage in an extensional tectonic setting the difference and evolution of mineralization types in geohistory indicate variation of tectonic setting which gradually changed from the subduction accretion related tectonic setting through the subduction related back arc spreading tectonic setting to the continental extensional setting these data also provide great potential for exploration of molybdenum deposits hosted in the extensional setting

characteristic magnetic behavior of subaerial and submarine lava units from the hawaiian scientific drilling project hsdp 2 1 this study presents rock magnetic properties and the magnetic mineralogy of subaerial and submarine lava flows of mauna loa and mauna kea volcanoes collected from the 3109 m deep hsdp 2 drill hole in hawaii three different groups of magnetic behavior are recognized in the subaerial lava flows related to the degree of high temperature oxidation during extrusion group 1 shows homogenous titanomagnetite with low xmt low curie temperatures tc 100 200 c and weak median demagnetizing fields 20 mt further subdivision into 1a and 1b subgroups is based on the low temperature behavior of magnetic susceptibility ms and hysteresis loops which indicate a contribution from ferrimagnetic cr al spinel below ca 160 c in the 1b type samples group 2 samples with exsolution lamellae of ilmenite in the titanomagnetites have higher tc 480 580 c and higher coercive forces 20 40 mt group 3 the highest oxidation stage is characterized by titanohematite bearing assemblages with enhanced median demagnetizing fields 35 85 mt and a significantly different low temperature ms behavior ms core logging shows a systematic variation occurs in the subaerial lava flows directly related to the degree of high temperature oxidation and their flow morphology aa lava flows have higher mean ms than other lava flow types besides these factors ms appears to be also affected by the magma composition of the various shield building stages mauna loa subaerial lava flows generally show lower mean susceptibilities 4 6 3 10 3 si than subaerial mauna kea lava flows 9 8 5 10 3 si as submarine lava flows show no group 3 assemblages no high temperature oxidation influenced these rocks some hyaloclastites and pillow breccias show low ms 1 10 3 si small amounts of nearly pure magnetite tc 580 c and high coercive forces up to 110 mt suggesting single domain and or superparamagnetic behavior the controlling mechanism of the magnetic properties in the submarine lava units is the cooling and quenching rate of lava flows which creates large grain size variations in titanomagnetites of varying compositions hydrothermal alteration as described from ocean floor or icelandic basalts is not an important process that influences the magnetic properties in the ocean island basalts from the hsdp 2 drill hole

the kamikatsura event and the matuyama brunhes reversal recorded in lavas from tjornes peninsula northern iceland abstract we report paleomagnetic results from four overlapping stratigraphic sections 55 flows through the lava pile in tjornes peninsula north iceland the initial aim of our study was to check the existence of intermediate directions previously interpreted as belonging to the matuyama brunhes geomagnetic reversal mbr and complete this record with thellier thellier paleointensity determinations and 40ar 39ar radioisotopic dating the directional results corroborate the findings by kristjansson et al 1988 a sequence of reversed transitional normal and transitional polarity r t n t is found in each section the polarity change is characterized by a jump from reversed virtual geomagnetic poles vgps to a quasi cluster of transitional vgps located over china followed by a second jump to a cluster of normal polarity before moving again towards intermediate polarities reversely magnetized flows exhibit a magnetic mineralogy well suited for paleointensity determination of these 25 flows 20 yielded paleointensity estimates of good quality unfortunately the systematic presence of coarse multidomain md titanomagnetite in the transitional and normal polarity flows hampered paleointensity estimates for these flows the virtual dipole moments vdm calculated for the reversed polarity flows vary from 4 9 to 7 0 1022 am2 with an arithmetic mean value of 5 5 0 8 1022 am2 this value is identical to the mean vdm obtained for the 0 3 5 ma time window and thus strengthens the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0 3 ma 40ar 39ar isochron ages were obtained with the incremental heating technique on groundmass separates or phenocryst poor whole rock samples six of the transitionally magnetized lavas yielded isochron ages that are indistinguishable from one another at the 95 confidence level the weighted average age is 862 51 ka in the upper part of this volcanic sequence the normally magnetized flows yielded a weighted average age of 787 40 ka this finding supports the conclusion that a geomagnetic excursion within the matuyama chron is recorded in the lower part of the tjornes volcanic sequence whereas only the latest portion of the mbr is recorded by flows in the upper part of the lava pile this conclusion is supported via comparison of the vgps obtained from the lavas with those predicted by a geomagnetic field model

jurassic cretaceous boundary age and mid ocean ridge type mantle source for shatsky rise basalt sills cored in lowermost berriasian sediments on shatsky rise in the northwest pacific yield a mean 4 0 ar 3 9 ar incremental heating age of 144 6 0 8 ma providing a minimum estimate on the age of the jurassic cretaceous boundary the largest part of the rise was emplaced at rates similar to those of several continental flood basalts the rise formed at an oceanic triple junction and the sills as well as lavas from two dredge sites exhibit an ocean ridge type nd pb sr isotope signature this signature is provisionally consistent with either a perisphere or meteorite impact model but is not predicted by the plume head model of plateau formation however no model can yet be rejected given the very sparse sampling of shatsky rise

paleointensity of the earth s magnetic field in early cretaceous time the parana basalt brazil paleointensity experiments were carried out with the coe and thellier methods on early cretaceous basalt 133 my old from the parana basin in southern brazil paleointensity estimates could be obtained from only six lava flows three of normal and three of reversed polarity of the 71 sampled moreover the quality of the determinations is fairly low with q ratio usually less than 10 mainly as a result of chemical and or physical changes of the magnetic oxides during heating this study strongly underscores the difficulty of finding natural rocks suitable for paleointensity experiments even so the directional results previously obtained were good the most reliable result give an intensity of 28 1 t while the others range from 20 8 t to 37 7 t the corresponding virtual dipole moments vdms range from 4 7 x 10 22 to 7 9 x 10 22 am 2 this dispersion is attributed mainly to inaccurate estimates of the ancient field rather than to secular variation although the number of cooling units for which paleointensity estimates could be obtained is limited the results nevertheless indicate that the early cretaceous vdm was significantly lower than that for the recent field but greater than has been reported for this portion of the mesozoic dipole low period

transfer of lipid molecules and polycyclic aromatic hydrocarbons to open marine waters by dense water cascading events abstract settling particles were collected by a set of moored sediment traps deployed during one year in the western gulf of lion along cap de creus and lacaze duthiers submarine canyons and on the adjacent southern open slope these traps collected particles during periods of pelagic settling and also during events of deep water flushing by dense shelf water cascading dswc analyses of lipid biomarkers n alkanes n alkan 1 ols sterols and c37 c38 alkenones and polycyclic aromatic hydrocarbons pahs showed much higher transfer of terrestrial lipids and pahs to open deep waters during dswc than in the absence of cascading the area of highest lateral fluxes was mostly located at 1000 m depth but also at 1500 m depth and extended along the canyons and to the adjacent slope higher fluxes were observed near the bottom 30 m above bottom mab than at intermediate waters 500 mab which is consistent with the formation and sinking of dense water over the continental shelf and its transport through the canyons towards the continental slope and deep basin dswc involved the highest settling fluxes of terrestrial lipids and pahs ever described in marine continental slopes and the pelagic domain as illustrated by peak values of c23 c33 odd carbon numbered alkanes 405 ng m 2 d 1 c22 c32 even carbon numbered alkan 1 ols 850 ng m 2 d 1 sitosterol sitostanol 4800 ng m 2 d 1 and pahs 55 g m 2 d 1 the algal lipids also showed higher transfer to deep waters during dswc but to a lower extent than the terrigenous compounds however the c37 c38 alkenones constituted an exception and their settling fluxes were not influenced by dswc the lack of influence of the dswc on the c37 c38 alkenone settling is consistent with absence of haptophyte algal inputs from the continental shelf and reinforces the reliability of these molecules for palaeothermometry and palaeoproductivity measurements in pelagic systems

linear and nonlinear thellier paleointensity behavior of natural minerals 1 we tested single domain sd and multidomain md theories of the acquisition and thermal demagnetization of thermoremanent magnetization trm and partial trm ptrm in thellier paleointensity determination using as samples plagioclase and mafic minerals extracted from matachewan diabase dikes susceptibility hysteresis and coercivity data indicate that the plagioclase contains mainly sd grains of magnetite while the mafic minerals clinopyroxene biotite and actinolite contain mainly md magnetite the extracted minerals were used in simulated thellier experiments with laboratory trm as a proxy for natural remanent magnetization nrm the plagioclase behaved ideally as predicted by neel sd theory giving a linear nrm ptrm arai plot with positive ptrm checks the mafic minerals behaved nonideally with strongly curved arai plots sagging below the ideal sd line in accord with observations for md magnetites and with predictions that in heating to moderate temperatures t more trm is lost than can be regained as ptrm causing convex down arai plots in experiments with ptrm parallel to nrm all ptrm tail checks were zero but ptrm checks increasingly deviated from the arai curve toward the sd line as t increased however in repeat thellier experiments ptrms at all t agreed with the original ptrms the negative ptrm checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original ptrms the inability of ptrm checks to reliably distinguish between md behavior and irreversible alteration could lead to overestimates of paleointensity from straight line fits to the low t unaltered part of the arai curve or underestimates from fits to high t data alternating field cleaning to 15 mt before each thellier step made the arai plot only marginally less curved low temperature demagnetization ltd zero field cycling through the 120 k verwey transition hardly changed the curvature of the arai plot for ptrm parallel to nrm for ptrm perpendicular to nrm ltd led to an inflected arai curve straddling the ideal sd line data of either form would not give reliable paleointensity values no presently available pretreatment renders the paleointensity data of md grains usable sd and nearly sd grains which behave ideally in the thellier experiment remain the only useful recorders of absolute paleointensity

observations of pigment and particle distributions in the western north atlantic from an autonomous float and ocean color satellite profiling floats with optical sensors can provide important complementary data to satellite ocean color determinations by providing information about the vertical structure of ocean waters as well as surface waters obscured by clouds here we demonstrate this ability by pairing satellite ocean color data with records from a profiling float that obtained continuous high quality optical data for 3 yr in the north atlantic ocean good agreement was found between satellite and float data and the relationship between satellite chlorophyll and floatderived particulate backscattering was consistent with previously published data upper ocean biogeochemical dynamics were evidenced in float measurements which displayed strong seasonal patterns associated with phytoplankton blooms and depth and seasonal patterns associated with an increase in pigmentation per particle at low light surface optical variables had shorter decorrelation timescales than did physical variables unlike at low latitudes suggesting that biogeochemical rather than physical processes controlled much of the observed variability after 2 25 yr in the subpolar north atlantic between newfoundland and greenland the float crossed the north atlantic current to warmer waters where it sampled an unusual eddy for 3 months this anticyclonic feature contained elevated particulate material from surface to 1000 m depth and was the only such event in the float s record this eddy was associated with weakly elevated surface pigment and backscattering but depthintegrated backscattering was similar to that previously observed during spring blooms such seldom observed eddies if frequent are likely to make an important contribution to the delivery of particles to depth

is there a precursor to the cretaceous normal superchron new paleointensity and age determination from liaoning province northeastern china abstract although the geomagnetic reversal record for the past 160 myr is relatively well documented whether a precursor exists before the onset of the cretaceous normal superchron cns remains a contentious topic in the study of the evolution of geomagnetic reversals in the present study we have tried to find out if new paleointensity data can provide more information about this topic a paleointensity geochronologic and paleomagnetic investigation has been conducted on four andesitic basalt lava flows from hulahada in liaoning province northeastern china radiometric 40 ar 39 ar dating experiments indicate the volcanism occurred at about 122 7 0 6 myr corresponding to marine anomaly mln thermomagnetic experiments and hysteresis loops show that the main magnetic carrier of remanence are titanomagnetites with curie temperatures of 580 c thirty eight samples from three lavas underwent thellier thellier paleointensity experiments with systematic partial thermoremanent magnetization ptrm checks ten samples from the studied lavas yield reliable paleointensity estimates with a mean virtual dipole moment vdm of 4 8 0 2 10 22 am 2 about half the strength of the modern day field our paleointensity results in conjunction with previous data suggest that there is no precursor to the cns

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

rapid formation of the shatsky rise oceanic plateau inferred from its magnetic anomaly shatsky rise in the northwest pacific ocean is probably the oldest extant oceanic plateau and as with most such features its origin is uncertain both oceanic plateaus and continental flood basalts are thought to be formed by rapid voluminous eruptions that occur when the head of a newly born mantle plume ascends to the base of the lithosphere1 3 high eruption rates have been estimated for flood basalts for example 1 5km3 yr 1 for the deccan traps2 from dating of lava flows but the inaccessibility of oceanic plateaus makes it necessary to extrapolate dating information from a small number of samples and sites4 here we estimate the eruption rate of shatsky rise by a method that is indirect but has the virtue of sampling the entire volume of the plateau above the surrounding sea floor the main southern part of the plateau has a positive magnetic anomaly corresponding to a reversed geomagnetic polarity at the time of eruption using age constraints to identify the longest period of reversed polarity during which the plateau could have formed we estimate that 2 106 km3 of material erupted at a minimum rate of 1 7km3 yr 1 this issomewhat less than the rate of 8 22 km3 yr 1 estimated for the ontong java plateau4 but still represents a massive eruption consistent with the plume head hypothesis

long term trends in paleointensity the contribution of dsdp odp submarine basaltic glass collections abstract the deep sea drilling project and the ocean drilling program have been collecting fresh appearing submarine basaltic glass from the world s oceans for over three decades this glass has proved nearly ideal for estimating paleointensity variations of the earth s magnetic field we compile here data for 726 paleointensity experiments from six publications on paleointensity using dsdp odp glass we also include new data for an additional 225 specimens these were obtained through the so called izzi paleointensity experiment of tauxe l staudigel h 2004 strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite geochem geophys geosyst 5 2 q02h06 doi 10 1029 2003gc000635 whereby infield zerofield steps are alternated with the zerofield infield steps to enhance quality assessment of the resulting data the entire collection of data from 951 experiments was prepared for uploading to the magic data base http earthref org including original measurements interpretations and useful metadata excellent results were obtained throughout the depth 1400 mbsf and age 0 160 ma range sampled dsdp odp glass data are compared with published paleointensity data meeting minimal acceptance criteria from the time interval 1 160 ma paleolatitudes were estimated for all cooling units in a self consistent manner for use in calculating virtual axial dipole moments we conclude 1 there is about a 20 difference in mean values between the sbg and the lava flow data 48 36 and 57 29 zam2 respectively the difference is caused by the fact that there are more higher values in the lava flow data than in the sbg data set rather than a difference in the minimum values lava flows cooling over a periods of days to months can account for the discrepancy 2 the positive relationship between polarity interval length and average paleofield intensity first hypothesized by cox a v 1968 lengths of geomagnetic polarity intervals j geophys res 73 3247 3260 is supported by data compiled here the brunhes data for which we have only a minimum estimate for polarity interval length are consistent with a long polarity interval suggesting that instead of racing toward reversal hulot g eymin c langlais b mandea m olsen n 2002 small scale structure of the geodynamo inferred from oersted and magsat satellite data nature 416 620 623 we could instead be in the midst of a long stable polarity interval 3 because the average value appears to be a function of polarity interval length it is probably not useful to calculate a mean value nonetheless it appears that most of the time apart from the brunhes and the longest polarity intervals the average dipole moment is substantially less than the present day value as suggested by juarez t tauxe l gee j s pick t 1998 the intensity of the earth s magnetic field over the past 160 million years nature 394 878 881

high geomagnetic intensity during the mid cretaceous from thellier analyses of single plagioclase crystals recent numerical simulations have yielded the most efficient geodynamo having the largest dipole intensity when reversal frequency is low reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled we report data from 56 thellier thellier experiments on plagioclase crystals separated from basalts of the rajmahal traps 113 to 116 million years old of india that formed during the cretaceous normal polarity superchron these data suggest a time averaged paleomagnetic dipole moment of 12 5 1 4 1022 amperes per square meter three times greater than mean cenozoic and early cretaceous late jurassic dipole moments when geomagnetic reversals were frequent this result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

palaeointensities determined from the middle cretaceous basalt in liaoning province northeastern china abstract a palaeomagnetic and geochronologic investigation has been conducted on a basalt lava sequence at jianguo in liaoning province northeastern china radiometric 40 ar 39 ar dating experiments indicate the volcanism occurred in relatively short time intervals between 105 5 and 102 2 ma thermomagnetic experiments and electron microprobe show that main primary carrier of remanence to be titanomagnetite with curie temperature of 300 c a modified version of the thellier thellier technique with systematic partial thermoremanent magnetization ptrm checks was used for palaeointensity determinations fourteen out of 61 samples from the studied lavas yield reliable palaeointensity estimates with a mean virtual dipole moment vdm of 4 07 0 2 1022 am2 about half the strength of the modern day field

evidence for weak geomagnetic field intensity prior to the cretaceous normal superchron abstract a palaeomagnetic and geochronologic investigation has been carried out on a sequence of 19 lava flows at sihetun in northeastern china age determinations by k ar dating indicate that the sequence spans 124 133 ma that is the time just prior to cretaceous normal superchron cns rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo single domain magnetite stepwise thermal demagnetization successfully isolated a well defined direction of characteristic magnetization chrm in all the lavas the mean palaeodirection determined is d i 5 4 58 7 95 2 4 and d i 175 9 60 0 95 6 4 for 15 normally magnetized flows and 3 reversely magnetized flows respectively one lava was found to possess an intermediate direction of magnetization the inferred palaeolatitude of the sampling site is very similar to the present day position supporting the contention that suturing between the mongolia and north china plates was probably completed by the late jurassic and that subsequently little rotation or translation occurred in the region west of the tanlu fault zone thellier thellier palaeointensity experiments with systematic partial thermoremanent magnetization ptrm checks were conducted on 66 samples from 13 lava flows results from the 29 samples 44 which met strict reliability criteria indicate a range of virtual dipole moments vdms from 2 83 to 4 17 1022 a m2 with an average of 3 53 0 02 1022 a m2 this finding helps confirm that a weak geomagnetic field probably occurred just prior to the cns

origin and dynamics of mesoscale eddies in the catalan sea nw mediterranean insight from a numerical model study past observations and satellite sea surface temperature imagery indicate the presence of mesoscale anticyclonic eddies drifting along the catalan coast in september 2001 one of these anticyclonic eddies was surveyed over the shelf break during an oceanographic cruise which permitted the 3 d description of its structure in this work we investigate the origin and dynamics of such catalan eddies using a numerical circulation model of the northwest mediterranean at 3 km resolution driven by high resolution atmospheric analyses and compare model eddies with the observations in the catalan sea we identify two zones of eddy formation in the gulf of lions in front of the city of marseille and at the southeast of coast of roussillon from which anticyclonic eddies are observed to drift toward the catalan sea the hydrology and dynamics of the structures observed in the simulations are characterized sensitivity experiments and energy analysis are performed which allow us to identify the mechanisms associated with their generation properties of the eddy observed during the 2001 cruise at the catalan shelf break are found to compare well with model eddies generated at the southeast of the roussillon coast the model relates the origin of these eddies to the separation of the coastal current downstream from creus cape flow separation is linked to intense downwelling taking place in front of the roussillon coast when strong northwesterly winds events occur

palaeointensity determinations and rock magnetic properties on basalts from shatsky rise new evidence for a mesozoic dipole low iodp expedition 324 cored igneous rocks from shatsky rise an oceanic plateau in the northwest pacific ocean that formed mainly during late jurassic and early cretaceous times we selected 60 samples from 3 different holes for thellier thellier palaeointensity determinations induced and remanent magnetization curves measured at low and high temperature suggest a diverse and complex magnetic mineralogy with large variations in ti content and oxidation state hysteresis and forc measurements show that most samples contain singledomain magnetic grains after carrying out the palaeointensity determinations only 9 samples satisfied all reliability criteria these gave palaeointensity values between 16 5 and 21 5 t which correspond to average vdm values of 4 9 0 2 1022 am2 for an estimated age of 140 142 ma this value is lower than that for the recent field which agrees with the hypothesis of a mesozoic dipole low

geomagnetic palaeointensity just prior to the cretaceous normal superchron abstract a combined geochronologic k ar and palaeomagnetic study has been conducted on a basalt lava sequence at zhuanchengzi in liaoning province northeastern china the new k ar age obtained from these lava flows is 120 93 0 88 ma marine anomaly m0 detailed investigations were conducted on two samples from each lava flow to determine their rock magnetic properties the temperature dependences of the saturation magnetization j t and the initial susceptibility t show evidence of low titanium magnetite and a curie temperature of 560 585 c with generally good reversibility of heating and cooling curves hysteretic properties ratios of mrs ms and hcr hc indicate that pseudo single domain psd grains of magnetite are dominant in these lavas stepwise thermal demagnetization isolated well defined characteristic magnetization chrm in all the 20 lava flows with a mean direction of d i 174 9 60 1 with 95 2 3 the modified version of the thellier and thellier palaeointensity method coe 1967a with systematic partial thermoremanent magnetization ptrm checks was used forty two of 71 samples 59 yielded reliable palaeointensity results virtual dipole moment vdm values range from 1 71 1022 to 5 27 1022 a m2 with an average of 3 96 0 07 1022 a m2 this low dipole field intensity value is approximately half the present day vdm consistent with older results obtained from the caribbean submarine basalt glasses for the period of 125 1 ma pick and tauxe 1993 this suggested that a weak geomagnetic field probably occurred just prior to the cretaceous normal superchron cns

physical characteristics and dynamics of the coastal latex09 eddy derived from in situ data and numerical modeling 1 we investigate the dynamics of a coastal anticyclonic eddy in the western part of the gulf of lion gol in the northwestern mediterranean sea during the latex campaign in the summer 2009 latex09 the sampling strategy combines sea surface temperature satellite imagery hull mounted acoustic doppler current profiler data conductivity temperature depth casts and drifter trajectories our measurements reveal an anticyclonic eddy latex09 eddy with a diameter of 23 km and maximum depth of 31 m centered at 3 34 e 42 33 n we use a high resolution three dimensional primitive equation numerical model to investigate its generation process and evolution the model is able to reproduce the observed eddy in particular its size and position the model results suggest that the latex09 eddy is induced by a large anticyclonic circulation in the northwestern part of the gol pushed and squeezed toward the coast by a meander of the northern current this represents a new generation mechanism that has not been reported before the post generation dynamics of the eddy is also captured by the model the collision of the latex09 eddy with cape creus results in a transient structure which is depicted by the trajectories of two lagrangian drifters during latex09 the transient structure and its advection lead to a transfer of mass and vorticity from the gol to the catalan shelf indicating the importance of mesoscale structures in modulating such exchanges in the region

the combined effect of transient wind driven upwelling and eddies on vertical nutrient fluxes and phytoplankton dynamics along ningaloo reef western australia we investigate the influence of wind stresses stratification and coastal mesoscale eddies on upwelling intensity vertical nutrient fluxes and phytoplankton biomass on the continental shelf off ningaloo reef in northwestern australia during an austral spring summer period a three dimensional 3 d hydrodynamic model roms regional ocean modeling system was coupled with a four component nitrogen based biogeochemical npzd model nitrogen phytoplankton zooplankton detritus to resolve the shelf circulation as well as the coupled nutrient and phytoplankton dynamics within a broad shelf region surrounding ningaloo reef the simulated currents temperatures and chlorophyll a concentrations generally agreed well with both the remotely sensed satellite images and observational data collected during a field experiment from september to november 2010 scenario tests for an individual wind driven upwelling event under a variety of hypothetical physical forcing conditions showed that shelf currents and biogeochemical variables were largely a function of wind stress and stratification however the functional relationships derived from this single wind event could not be extrapolated to other periods of the upwelling season due to the additional influence of 3 d mesoscale processes on the shelf the presence intensification and propagation of a coastal anticyclonic eddy during the study period strongly influenced the spatial and temporal variations in nutrient profiles which in turn caused fluctuations in vertical nutrient fluxes that were largely independent of wind stress these results emphasize that it is necessary to fully capture the 3 d details of the mesoscale and submesoscale coastal dynamics to properly predict upwelling induced coastal phytoplankton dynamics in eddy intensive shelf regions such as ningaloo reef

new absolute paleointensity results from the parana magmatic province uruguay and the early cretaceous geomagnetic paleofield 1 thellier paleointensity experiments are carried out on the early cretaceous arapey formation which belongs to the parana magmatic province pmp in northern uruguay twenty seven samples out of 225 analyzed yield reliable absolute intensity determinations for these samples the nrm fraction f used for determination ranges between 0 33 to 0 83 and the quality factor q ranges from 4 5 to 14 1 being generally greater than 5 the flow mean paleointensity values obtained in this study range from 16 3 2 3 to 41 5 3 8 t with corresponding virtual dipole moments vdms from 3 4 0 6 to 8 2 0 8 1022 am2 this corresponds to mean vdm value of 6 4 2 3 1022 am2 40ar 39ar dating on the pmp sequence by feraud et al 1999 gave a mean age of 132 2 0 5 ma our results in conjunction with previous studies kosterov et al 1998 goguitchaichvili et al 2002 suggest that the paleostrength during the early cretaceous might be as variable and strong as brunhes data and not anomalously low as previously suggested the scatter observed in our study is similar to recent findings by granot et al 2007 for the troodos ophiolite

palaeointensity determinations and magnetic properties on eocene rocks from izu bonin mariana forearc iodp exp 352 iodp expedition 352 cored igneous rocks from the izu bonin mariana forearc crust cores from sites u1440 and u1441 recovered eocene basalts and related rocks and cores from sites u1439 and u1442 recovered eocene boninites and related rocks we selected samples from holes u1439c u1440b and u1442a for palaeointensity measurements hysteresis measurements and high and low temperature magnetization curves show that samples from hole u1440b undergo magneto chemical changes when heated and are mostly composed of single domain sd or pseudo single domain psd titanomaghemite in contrast the same measurements show that most selected samples from holes u1439c and u1442a are thermally stable and are composed of either sd or psd titanomagnetite with very little titanium content or sd ferromagnetic grains with a large paramagnetic contribution thellier thellier palaeointensity experiments carried out on u1439c and u1442a samples give a good success rate of 25 60 and virtual dipole moment vdm values between 1 3 and 3 5 1022 am2 multispecimen palaeointensity experiments with the domain state corrected method carried out on 55 samples from hole u1440b divided into four groups and 20 from hole u1439c gave poor quality results but indicated a vdm around 4 6 1022 am2 in hole u1440b forearc basalts these results are in agreement with the few low vdm values previously measured on eocene rocks however they do not support an inverse relationship between field intensity and reversal rate for this period of time since the eocene reversal rate was low

exploiting the potential of an improved multimission altimetric data set over the coastal ocean until now most satellite altimetry studies of the coastal ocean have been based on along track data from a single mission whereas up to four missions were operative in 2002 2005 here to monitor the coastal ocean we have applied specialized corrections and dedicated processing strategies to compute a multimission data set at a mean distance of 32 km of the coast the resulting altimetric data set is compared with sea level data from three in situ stations over a coastal zone of the northwestern mediterranean the mean rms difference between this data set and the sea level stations is 2 9 cm against 3 7 cm when using the aviso altimetric product comparison of altimeter derived geostrophic velocities with a mooring also shows that the spatial and temporal variability of the surface current field is well reproduced the agreement with in situ measurements extends to intraseasonal time scales showing a significant improvement compared to previous studies in the 50 km coastal band

low temperature magnetic behavior of titanomagnetites titanium substitution has a significant effect on the temperature dependence 5 300 k of low temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite fe3 xtixo4 0 x 0 4 and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range these features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples moreover the apparent similarities between the thermal decay of low temperature remanence of titanomagnetite with pyrrhotite pure magnetite and superparamagnetic phases can lead to complications in interpreting low temperature remanence data when titanomagnetite is present

paleointensity of the earth s magnetic field in the jurassic new results from a thellier study of the lesotho basalt southern africa thellier paleointensity experiments carried out on the jurassic 180 m y old karoo basalt lesotho southern africa provided paleointensity estimates for 40 samples from 10 distinct lava flows with reversed magnetization mean paleointensity was found to be equal to 24 2 11 3 t standard deviation with a minimum of 12 t and a maximum of 41 3 t this corresponds to a mean virtual dipole moment vdm of 4 6 2 0 1022 am2 the between flow dispersion is ascribed to paleosecular variation which thus would approach up to 40 of the mean paleointensity as is observed from the published paleointensity data set covering last 20 m y this result yields further evidence for the period of low geomagnetic field in jurassic to early cretaceous time mesozoic dipole low a symmetrical intensity versus time pattern is suggested for this time interval with a minimum vdm of 3 1022 am2 in late jurassic and substantially higher 4 5 1022 am2 vdms in early to middle jurassic and early cretaceous respectively

nutrients and plankton spatial distributions induced by a coastal eddy in the gulf of lion insights from a numerical model a plankton functional types model forced by a hydrodynamic model is used in the present work to study the impacts of a coastal eddy on the distribution of nutrients and plankton in the western part of gulf of lion nw mediterranean sea this study based on a realistic simulation of the year 2001 focuses on a long life anticyclonic eddy detected during summer the studied anticyclonic eddy has some biogeochemical characteristics of those observed in the open ocean as for example the low productivity at their core and rising up of nutricline on their edges however the functioning and consequences of such coastal eddy on nutrients and plankton distributions are complicated by potential interactions with topography wind induced upwelling along the languedoc coast and nearby northern current nc especially the proximity of the southern edge of the eddy with the nc makes possible the exchanges of organic matter plankton at times during the eddy s life the coastal eddy thus transports organic matter from the coastal zone to the offshore domain the coupled model also suggests the importance of offshore to coastal transport by nc through a seeding process of the eddy in plankton at the beginning of its life a detailed biogeochemical functioning of the studied eddy all along its lifetime is proposed from the model results at the beginning the eddy mainly acts as a transporter of the plankton wind induced bloom occurring along the languedoc coast several days before the eddy set up at the same time the eddy shows upwards vertical velocities on its edges creating upwelling of nutrient enriched waters the process of eddy induced upwelling is notably enhanced on the eddy western side along the roussillon coast due to the shelf topography the around eddy advection and relative lower temperatures prevent the large development of phytoplankton along the roussillon coast and the eddy induced bloom is predicted to occur near the northwestern side of the eddy the eddy induced and wind induced blooms of phytoplankton then merge on the eastern side by advection and this process of aggregation may explain the persistent occurrence of a filament of high chlorophyll observed on the offshore edge of the eddy throughout the study period the changes in plankton composition due to a combination of top down and bottom up processes during the coast to offshore transport may explain the disappearance of the filament south of 43 n at the surface

low intensity of the geomagnetic field in early jurassic time from a large collection of jurassic continental tholeiites cropping out in europe and africa we selected 90 samples for paleointensity determinations the samples were carefully selected to avoid any secondary magnetizations especially viscous magnetization use of the thellier method reveals that magnetic property changes due to heating begin often at quite low temperatures but fortunately without modifying noticeably their natural remanent magnetization thermoremanent magnetization ratio twenty eight well clustered paleointensity estimates were obtained from two european dikes that were emplaced during early jurassic time the kerforne dike at brenterc h in brittany northwestern france and the messejana dike on the iberian peninsula spain and portugal virtual dipole moments calculated from both magmatic units are similar and only about one third of present day values these new data lend support to the recently postulated low dipole moment of the mesozoic geomagnetic field

absolute palaeointensity of oligocene 28 30 ma lava flows from the kerguelen archipelago southern indian ocean we report palaeointensity estimates obtained from three oligocene volcanic sections from the kerguelen archipelago mont des ruches mont des tempetes and mont rabouillere of 402 available samples 102 were suitable for a palaeofield strength determination after a preliminary selection among which 49 provide a reliable estimate application of strict a posteriori criteria make us confident about the quality of the 12 new mean flow determinations which are the first reliable data available for the kerguelen archipelago the virtual dipole moments vdm calculated for these flows vary from 2 78 to 9 47 10 2 2 am 2 with an arithmetic mean value of 6 15 2 1 10 2 2 am 2 compilation of these results with a selection of the 2002 updated iaga palaeointensity database lead to a higher 5 4 2 3 10 2 2 am 2 oligocene mean vdm than previously reported goguitchaichvili et al 2001 riisager 1999 identical to the 5 5 2 4 10 2 2 am 2 mean vdm obtained for the 0 3 5 ma time window however these kerguelen palaeointensity estimates represent half of the reliable oligocene determinations and thus a bias toward higher values nonetheless the new estimates reported here strengthen the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0 3 ma

in situ estimates of submesoscale horizontal eddy diffusivity across an ocean front in the last decade the rapid advancements in computational power have favored the development of high resolution numerical models capable of directly resolving small scale structures such as fronts and filaments such models have greatly improved our understanding of submesoscale dynamics at the same time the small dimensions and short duration of these structures still pose major challenges for small scale dedicated field experiments for this reason submesoscale studies from in situ observations are still relatively scarce and quantitative estimates of key physical parameters for high resolution numerical models such as horizontal eddy diffusivity are still lacking this study presents a novel approach for computing in situ horizontal eddy diffusivity associated with frontal structures by combining cross front widths derived from surface thermosalinograph sections with stirring rates estimated from lagrangian drifter trajectories the method is applied to the measurements collected across a frontal structure observed in the western part of the gulf of lion during the latex10 campaign lagrangian transport experiment 1 24 september 2010 a total of 76 estimates of eddy diffusivity were obtained for strain rates of 0 70 and 1 21 day 1 and front widths horizontal scales ranging between 1 and 4 km the estimates are log normally distributed with 70 of the values ranging between 0 4 and 5 m 2 s 1 further analysis based on high resolution simulations and remote sensed observations as well as dedicated field experiments will help to assess the robustness of some of the assumptions at the base of the proposed approach and to extend the results to different ocean regions

cretaceous geomagnetic paleointensities thellier experiments on pillow lavas and submarine basaltic glass from the ontong java plateau 1 we present new thellier paleointensity results on pillow lavas and pillow rim submarine basaltic glass sbg from 120 ma basalts erupted on the ontong java plateau all pillow lavas have suffered low temperature alteration and are unsuitable for thellier paleointensity experiments in contrast pillow rim sbg is virtually impermeable to seawater and therefore has suffered little or no low temperature alteration out of 21 selected sbg samples with magnetic moment larger than 10 10 am2 18 samples from 12 chilled sbg margins yielded paleointensity estimates of good technical quality rock magnetic measurements show the primary carrier of remanence is low ti titanomagnetite occurring in a continuous range of compositions starting from nearly pure magnetite as suggested by a recent study magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400 440 c which is in accordance with the partial thermoremanent magnetization checks failing significantly above the same temperature we suggest that paleointensity estimates obtained below this critical temperature represent true determination of the paleomagnetic field strength for several pillow margins two to three samples were measured and in such cases the data agree well altogether we obtain reliable estimates from 12 sbg pillow rims yielding a mean paleointensity of 27 1 13 4 t and a corresponding virtual axial dipole moment of 5 8 2 8 1022 am2 this value is slightly higher than whole rock data from similar aged subaerial lava flows the paleosecular variation of the intensity estimates is slightly larger than previously reported for paleointensity determinations for the cretaceous normal superchron

magnetic lineations within shatsky rise northwest pacific ocean implications for hot spot triple junction interaction and oceanic plateau formation oceanic plateaus are major ocean features yet their origins and development are poorly understood many are huge piles of basalt and it is widely accepted that mantle plumes are their source perhaps from eruptions of the voluminous head of an emerging plume shatsky rise is a basaltic plateau that formed during a period of geomagnetic reversals unlike many mid cretaceous plateaus so magnetic data can help us understand its tectonic history in this study we analyzed magnetic anomaly data from 131 cruises over and around shatsky rise and constructed a magnetic lineation chart for tectonic interpretation a significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs indicating nearly normal lithosphere and between large volcanic edifices many lineations form bights near the rise axis and show former locations of the pacific izanagi farallon triple junction they indicate that the junction was in a ridge ridge ridge rrr configuration and closely followed the rise axis from chron m20 146 ma to chron m4 127 ma lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times often apparently in response to volcanic activity from the mantle plume some jumps however appear related to ridge reorientations numerous features within the rise are parallel to lineations or lineation offsets implying that the plateau formed near the triple junction and was modified by ridge faulting most of our observations support the hypothesis that shatsky rise was formed by a mantle plume that captured a triple junction

the cold core eddy and strong upwelling off the coast of new south wales in early 2007 abstract during the austral summer of 2006 07 a series of extreme oceanic events occurred in the tasman sea following a series of strong wind driven upwelling events an intense cold core eddy developed off sydney australia a data assimilating eddy resolving ocean model is used to create a three dimensional time varying reanalysis of these events the reanalysis indicates that the cold anomalies associated with the upwellings were in excess of 5 c near the coast where sea level decreased by as much as 0 2 m the reanalysed three dimensional structure of the cold core eddy shows the eddy leaning to the west north west in towards the continental shelf the diameter of the eddy is about 100 km and the sea level anomaly at the eddy centre peaks at around 1 m with an associated sub surface temperature anomaly in excess of 8 c at 200 m depth corresponding to an upward isotherm excursion of 600 m the circulation around the cyclonic eddy is ageostrophic with upwelling in the southern sector of the eddy where flow is onshore and climbing the continental slope and downwelling in the northern sector where flow is descending off the slope three dimensional trajectories of water parcels around the eddy involve 50 100 m vertical excursions based on the reanalysed circulation and composite satellite images of chlorophyll a we hypothesise that the circulation around the eddy led to significant nutrient enrichment in the euphotic zone around the perimeter of the eddy

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

a persistent upwelling off the central cantabrian coast bay of biscay abstract north atlantic central waters were detected near the surface in some locations off the central cantabrian coast bay of biscay this feature suggests an upwelling process taking place in the season of the thermal stratification which is persistent throughout the summer the exact mechanism of formation is unknown but the wind driven currents are probably the main force the prevailing winds off the cantabrian coast are northeast during summer generating westward superficial currents that produce an eckman transport offshore factors such as shape of the coast and slope topography maybe modulators although the upwelling was subsuperficial our results suggest that cold and nutrient rich waters reached the surface previously the biomass distribution of phytoplankton varied according to the upwelling and different populations could be recognized in stations not affected by upwelling phytoplankton occured mainly on top of the thermocline whereas in those affected by upwelled waters it was concentrated near the surface the mean size and composition of the populations were also different and two distinct types could be recognized and related to nutrient concentrations

geomagnetic palaeointensities during the cretaceous normal superchron measured using submarine basaltic glass high quality palaeointensity data have been obtained from thellier thellier experiments on recent and cretaceous submarine basaltic glasses whereas the recent samples faithfully yield today s geomagnetic intensity at the site the palaeointensities for the beginning and end of the cretaceous normal superchron are only 45 and 25 respectively of today s value the data thus extend the mesozoic dipole low into the cretaceous superchron and confirm that submarine basaltic glass is an excellent material for palaeointensity studies

possible mechanisms causing failure of thellier palaeointensity experiments in some basalts summary the normally magnetized zone of the jurassic lesotho basalts although providing apparently quite reliable palaeofield directions kosterov perrin 1996 shows anomalous behaviour when studied in vacuum using the thellier palaeointensity method typically the slope of the natural remanent magnetization thermoremanent magnetization nrm trm curves is very steep at intermediate temperatures 200 to 400 460 c in order to elucidate the reasons for such an anomalous behaviour six representative samples from a total of 74 studied using this method were subjected to a variety of analyses these experiments indicate that the magnetic properties are dominated by pseudo single domain psd magnetite grains some 1 mm in size resulting from high temperature oxidation of titanomagnetite laboratory heatings in vacuum up to the curie point do not change significantly the roomtemperature hysteresis characteristics or the initial susceptibility k similarly the k t curves in vacuum are with a single exception rather reproducible since the laboratory trms yield almost ideal nrm trm plots the anomalous nrm trm plot is presumably due to some peculiarity of the natural trm the partial trm ptrm acquisition capacity in the moderate temperature range cooling from 200 to 20 c is generally very strongly reduced after heating to 270 c which indicates that some magnetic alteration has already occurred at these temperatures hysteresis measurements between room temperature and the curie temperature t c show that some small less than 10 per cent but significant irreversible changes in hysteresis characteristics also occur during heating in particular the coercive force h c0 at room temperature is typically reduced after heating at a moderate temperature 175 c but increases after treatments at 475 c and more pronouncedly at 580 c the saturation magnetization j s0 remains unchanged except for a very small decrease less than 5 per cent occurring in some samples after the two latter treatments these changes are most clearly seen on h c t j s t bilogarithmic plots which show that the moderate temperature change in coercivity can extend up to 200 250 c thus hysteresis measurements as a function of temperature over a promising tool for sample pre selection for thellier experiments alternating field demagnetization and cycling of ptrms at liquid nitrogen temperature suggest that the blocking mechanism is largely multidomain like near room temperature but becomes less so as the curie point is approached the main reason for the failure of the thellier experiments is the loss of a fraction of the nrm natural trm at temperatures apparently lower than the blocking temperatures in nature it is suggested that this anomalous behaviour results from the reorganization of the domain structure of the psd grains during heating this transformation which seems to be triggered by the coercivity decrease observed at very moderate temperatures can reduce the nrm intensity without requiring any correlated ptrm acquisition

mechanisms for vertical nutrient transport within a north atlantic mesoscale eddy abstract prompted by observational evidence for an enhanced source of surface nutrients within an anticyclonic eddy in the ne atlantic we investigate vertical transport processes that may produce such a phenomenon for the eddy investigated the dominant mechanism is found to be ageostrophic circulation resulting from a perturbation of the circular flow of the eddy this can produce upwelling velocities of order 10 m d 1 ekman pumping due to wind stress on the eddy also produces upwelling within but on a smaller scale of 0 5 m d 1 there is no evidence that self propagation of the eddy leads to an enhanced nutrient flux by displacement upwards of nutrient laden deeper isopycnals over the core of the eddy deeper winter mixing within the eddy relative to the surroundings however may contribute to the elevated nutrient levels

observation and modeling of the winter coastal oceanic circulation in the gulf of lion under wind conditions influenced by the continental orography fetch experiment 1 hydrological and currentmeter observations were collected on the continental shelf and slope of the gulf of lion during the fetch experiment 13 march to 15 april 1998 results from the first part of the cruise characterized by strong northern winds are presented the hydrological structures evidence well mixed water masses on the eastern and western ends of the shelf in the central part the situation is more complex with the influence of the rhone river s freshwater plume in the first 40 m of the water column and closer from the bottom with the confrontation of downwelled coastal cold water and upwelled warmer and saltier slope water current measurements show the path of the cyclonic circulation along the slope which is part of the general circulation of the western mediterranean and suggest the presence of large and temporary eddies on the shelf this oceanic circulation is simulated with a free surface three dimensional model using realistic forcing the model outputs are in agreement with the main hydrological and circulation patterns the results further indicate that coastal eddies are generated by the mesoscale structure of the wind field

palaeointensity study of the hawaiian 1960 lava implications for possible causes of erroneously high intensities summary it is known that the hawaiian 1960 lava sometimes yields higher palaeointensities than the igrf 36 2 t in order to clarify the causes we have performed a comprehensive investigation of palaeointensity measurements on 19 cores from the lava according to various rock magnetic analyses they are classified into three groups with degrees of the deuteric oxidation in coe s version of the thellier method a significantly larger mean palaeointensity of 49 0 9 6 t n 17 was observed similar to previous studies these palaeointensities showed a dependence on the oxidation indices which can be explained by thermochemical remanent magnetization tcrm acquisition during the natural cooling stage of the lava although it is generally difficult to distinguish the tcrm from thermoremanent magnetization trm a lowtemperature demagnetization ltd treatment on nrm and trm in the thellier experiment seems to allow us to detect the influence of the tcrm on nrm another possible tool for tcrm detection is the double heating technique dht of the shaw method combined with ltd ltd dht shaw method it gave a better average of 39 4 7 9 t n 9 though the success rate was much lower most of the inappropriate results were screened by a combination of the double heating test the arm correction and the ltd treatment if two outliers are further excluded the average is improved to be 35 7 3 3 t n 7 which agrees well with the expected intensity the results of both methods emphasize the importance of taking and measuring samples from the parts of various oxidation degrees in a lava flow