evidence for a rapid release of carbon at the paleocene eocene thermal maximum the paleocene eocene thermal maximum petm and associated carbon isotope excursion cie are often touted as the best geologic analog for the current anthropogenic rise in pco2 however a causal mechanism for the petm cie remains unidentified because of large uncertainties in the duration of the cie s onset here we report on a sequence of rhythmic sedimentary couplets comprising the paleocene eocene marlboro clay salisbury embayment these couplets have corresponding 18o cycles that imply a climatic origin seasonal insolation is the only regular climate cycle that can plausibly account for 18o amplitudes and layer counts high resolution stable isotope records show 3 5 13c decrease over 13 couplets defining the cie onset which requires a large instantaneous release of 13c depleted carbon during the cie a clear 13c gradient developed on the shelf with the largest excursions in shallowest waters indicating atmospheric 13c decreased by 20 our observations and revised release rate are consistent with an atmospheric perturbation of 3 000 gigatons of carbon gtc

paleomagnetic study of the eocene quxu pluton of the gangdese belt crustal deformation along the indus zangbo suture zone in southern tibet abstract forty five samples have been collected at nine sites on the 42 5 ma quxu pluton 90 50 e 29 20 n in the gangdese batholith westerly declination d 48 and 83 is observed in primary magnetizations from two sites about 25 km from the indus zangbo suture zone after thermal demagnetization this direction is consistent with the westerly paleomagnetic directions of the crustal blocks in other areas along the indus zangbo suture zone the quxu pluton of the gangdese belt was rotated in a domino style deformation process as a part of a long 840 km and narrow less than 100 km deformed zone between the india eurasia continents associated with the collision of india since 42 5 ma the pluton between 11 km and 14 km from the suture acquired the secondary magnetization d 28 and 39 during a cataclastic metamorphic process at sometime during the domino style deformation the primary magnetization was completely destroyed in the pluton within 11 km of the suture during slow cooling at the uplift stage and was replaced by thermoviscous remanent magnetization parallel to the present axial dipole field

multicomponent magnetizations from the mississippian mauch chunk formation of the central appalachians and their tectonic implications previous paleomagnetic study of mississippian red beds of the mauch chunk formation from the central appalachians reported shallow inclination directions a positive fold test was obtained after thermal demagnetization typically to only 550 c the near equatorial position for north america indicated by these results has been used to support the idea of a circa 15 latitudinal tectonic offset of the acadia displaced terrain in the northern appalachians the present study of 153 samples from 29 sites occupied in the mauch chunk formation of eastern and southern pennsylvania reveals multicomponent magnetizations a characteristic component declination d 161 0 inclination i 27 8 alpha95 a95 7 9 with unblocking temperatures usually concentrated above 650 c and with dual polarity 10 sites normal 13 sites reversed is isolated in 23 sites and passes a fold test at the 99 confidence level it is interpreted as an early acquired hematitic detrital or chemical magnetization of pre alleghanian orogeny and pre kiaman reverse polarity interval age a southern hemisphere paleolatitude of 15 for the sampling area is indicated another component has generally lower unblocking temperatures which however can extend up to 650 c or more and therefore is also likely to be carried by hematite this secondary magnetization d 170 6 i 5 3 a95 5 0 isolated in 25 sites is of uniform reverse polarity and application of fold tests suggests that it is of synfolding origin a near equatorial paleolatitude 3 n at the time of acquisition is obtained consistent with permo carboniferous kiaman directions from north america incomplete removal of the secondary component can explain the previous results from the mauch chunk comparison of the present paleomagnetic results with those from similar age rocks in the canadian maritimes does not support a latitudinal offset of acadia with respect to interior north america in the early carboniferous in fact according to these and other paleomagnetic data thought to be representative of this age the atlantic bordering continents already were near to a pangea configuration the alleghanian and particularly the hercynian orogenies therefore may not be associated with the closure of a large latitudinal ocean later in the carboniferous and permian alleghanian deformation however may have resulted in the partial bending of the pennsylvanian salient on the basis of apparently systematic deviations in paleomagnetic declinations

what was the paleogene latitude of the lhasa terrane a reassessment of the geochronology and paleomagnetism of linzizong volcanic rocks linzhou basin tibet the paleogene latitude of the lhasa terrane southern tibet can constrain the age of the onset of the india asia collision estimates for this latitude however vary from 5 n to 30 n and thus here we reassess the geochronology and paleomagnetism of paleogene volcanic rocks from the linzizong group in the linzhou basin the lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes we report consistent 40ar 39ar and u pb zircon dates of 52 ma for the upper linzizong and 40ar 39ar dates 51 ma from the lower linzizong are significantly younger than u pb zircon dates 64 63 ma suggesting that the lower linzizong was thermally and or chemically reset paleomagnetic results from 24 sites in lower linzizong confirm a low apparent paleolatitude of 5 n compared to the upper part 20 n and to underlying cretaceous strata 20 n detailed rock magnetic analyses end member modeling of magnetic components and petrography from the lower and upper linzizong indicate widespread secondary hematite in the lower linzizong whereas hematite is rare in upper linzizong volcanic rocks of the lower linzizong have been hydrothermally chemically remagnetized whereas the upper linzizong retains a primary remanence we suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower linzizong we estimate that the paleogene latitude of lhasa terrane was 20 4 n consistent with previous results suggesting that india asia collision likely took place by 52 ma at 20 n

paleomagnetism of the upper ordovician juniata formation of the central appalachians revisited again two components of magnetization were isolated in the upper ordovician juniata formation sampled in the area of the pennsylvania salient the thermally distributed reversed polarity b component was most likely acquired during alleghenian deformation and although it is poorly grouped it is similar to other appalachian synfolding magnetizations the pre alleghenian age c magnetization is entirely of normal polarity and shows a difference in declinations between the mean magnetizations isolated on the northern and southern limbs of the salient of 24 23 this anomaly is consistent with the sense and magnitude of declination anomalies observed in pre alleghenian magnetizations isolated in other throughgoing appalachian red beds of silurian devonian and early carboniferous age the mean inclination of 44 7 suggests a paleolatitude of about 26 s for the central appalachians in the late ordovician this paleolatitude fits a trend of southward motion of north america from the ordovician to the early devonian followed by northward drift through the remainder of the paleozoic

paleomagnetism of the cambrian moore hollow group texas evidence for a primary magnetization carried by detrital magnetite petrographic and paleomagnetic studies of the cambrian moore hollow group texas establish the presence of three magnetic minerals and several components of magnetization hematite is the dominant carrier of the magnetic remanence in most samples and is primarily a diagenetic replacement of preexisting fe bearing phases hematite dominant samples have characteristic directions which range from shallow easterly to shallow southerly this distribution suggests multiple components of magnetization acquired at various times in the paleozoic the paleopole obtained for an unmineralized breccia 135 7 e 15 4 n represents either a chemical remanent magnetization of middle ordovician to early silurian age or a vector addition of two or more cambrian through silurian components mineralized breccias at the silver creek mississippi valley type deposit yield a permian direction 117 0 e 51 6 n because hematite in this deposit predates mineralization the permian age represents a maximum age for the mineralization at silver creek many samples contain goethite as a replacement of fe dolomite glauconite and pyrite the direction of magnetization carried by goethite is near the present geomagnetic field or in a minority of samples antipodal to it rounded ti magnetite was identified by sem in magnetic extracts from siliciclastic poor facies irm acquisition and thermal demagnetization confirms the presence of magnetite in these samples some samples contain an apparent modern viscous remanent magnetization the dominant component 72 samples 10 sites has a site mean declination of 93 8 and inclination of 14 5 with an 95 8 5 corresponding to a pole position at 163 0 e 0 6 s the presence of detrital magnetite polarity changes lack of evidence for prolonged burial and the proximity of this pole position to other late precambrian cambrian poles suggest that this component of magnetization indeed reflects the late cambrian paleopole for cratonic north america

coupled and decoupled responses of continental and marine organic sedimentary systems through the paleocene eocene thermal maximum new jersey margin usa 1 the flux of sediment and organic carbon from continents to the coastal ocean is an important factor governing organic burial in coastal sediments and these systems preserve important records of environmental and biogeochemical conditions during past global change events burial of organic materials in coastal systems can be promoted by chemical resilience or through protection by association with mineral surfaces but the role and influence of these processes on organic records from ancient sediments is poorly known we studied sediment and organic matter burial as particulate organic matter pom and mineral bound organic matter mom in near shore marine sediments from the wilson lake core new jersey usa that span the paleocene eocene thermal maximum petm a climatic perturbation 55 9 myr ago our results show that distinct pom and mom fractions can be isolated from sediments both fractions appear to be dominated by terrestrial material but pom consisted primarily of recently synthesized material whereas mom included a significant fraction of pre aged organic matter from soils or ancient sediments variation in organic burial through the petm is associated with changes in inorganic nitrogen burial clay mineralogy and clastic grain size that we associate with enhanced continental weathering erosion and redeposition of ancient kaolinites and eustatic sea level variation respectively these results provide a new perspective on factors governing carbon burial and carbon isotope records in ancient marine margin settings and offer information on rate and phasing of late paleocene early eocene earth system changes that may constrain interpretations of the cause of the petm climate change event

karakoram glacier surge dynamics quincey d j braun m glasser n f bishop m p hewitt k luckman a 2011 geophysical research letters 38 article number l18504

regional patterns of magnetite authigenesis in the appalachian basin implications for the mechanism of late paleozoic remagnetization we report paleomagnetic and rock magnetic data from sedimentary carbonates of the appalachian basin that have implications for the mechanism of widespread late paleozoic remagnetization in this rock type sampling was accomplished mostly along two across basin transects one in devonian carbonates of new york state and the other in ordovician and mississippian carbonates of tennessee and alabama paleomagnetic investigations of the new york devonian carbonates along a transect from albany to buffalo indicate that these rocks were completely remagnetized in alleghenian time rock magnetic studies show that magnetite concentration varies in a regular fashion along the transect with a local maximum near syracuse and decreasing gradually to the east and west this pattern shows a striking correlation with the degree of diagenetic alteration of clay minerals in a devonian bentonite horizon which took place during alleghenian time we conclude that most of the magnetite present in these carbonates is of authigenic origin and that clay mineral alteration and magnetite authigenesis were coeval late paleozoic events that were controlled by the same diagenetic factors thermoviscous remagnetization processes cannot be ruled out in this setting in light of the observed degree of thermal maturity however our results suggest that chemical factors played a critical role in remagnetization since they allow us to infer that magnetite authigenesis and remagnetization are about the same age we propose a geochemical model for magnetite authigenesis wherein the iron is derived from detrital smectites during diagenetic illitization which is triggered by the introduction of potassium rich brines results from the tennessee transect from nashville to chattanooga show a very different pattern paleomagnetic studies indicate that late paleozoic remagnetization has affected ordovician carbonates of the nashville dome paleozoic carbonates in the overthrust belt near chattanooga also carry the late paleozoic remagnetization however mississippian carbonates between the dome and the overthrust belt are very weakly magnetized and show no evidence for late paleozoic remagnetization samples show only a present field magnetization or at one locality in northwestern alabama a dual polarity magnetization of probable mississippian age rock magnetic studies indicate that higher concentrations of magnetite are present in the remagnetized rocks of the nashville dome and the overthrust belt than in the nonremagnetized mississippian carbonates we therefore conclude that magnetite authigenesis and remagnetization are related events in this setting and that the remagnetization must be due to chemical processes the presence of a remagnetization shadow in the younger rocks now exposed between the fold thrust belt and the flexural arch is attributed to westward flow of magnetite forming aqueous fluids through deep sub carboniferous aquifers during alleghenian time upward migration of the fluids through the mississippian carbonates may have been prevented by the devono mississippian chattanooga shale an impermeable stratum that is present throughout the region

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

regional trends in the timing of alleghanian remagnetization in the appalachians pole positions related to remagnetized components isolated in appalachian limestone and redbed rock units range over about 60 m y of the permian carboniferous apparent polar wander path for north america apparent ages of remagnetization are older in the southern appalachians and younger to the north if the remagnetizations are associated with fluids expelled during the alleghany orogeny then the apparent remagnetization age trend could describe the timing of thrust sheet emplacement

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

thermoviscous remagnetization in some appalachian limestones experimental evidence shows that blocking temperature relaxation time theory for single domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in devonian limestones of new york state thermoviscous remagnetization at moderately elevated burial temperatures during the late paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks more generally these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite bearing rocks the prob leto

diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates a rock magnetic study hysteresis measurements lowrie fuller tests cisowski tests and low temperature demagnetization experiments on samples of the knox dolomite trenton limestone and onondaga limestone indicate that the remanence in these remagnetized carbonate units resides principally in single domain magnetite in contrast to previous interpretations volumetrically however a significant fraction of the magnetite is coarser grained as previous scanning electron microscopic work on magnetic extracts has shown these conclusions are based mainly on the characteristic wasp waisted shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity both of which clearly indicate a bimodal distribution of coercivities grain sizes the magnetization is controlled mainly by cubic magnetocrystalline anisotropy consistent with the spheroidal particles commonly observed in extracts hysteresis trends show that especially the finest particles lack shape anisotropy in view of these conclusions the two components of natural remanence in these rocks are interpreted as 1 a late paleozoic kiaman chemical remanence carried by single domain diagenetic magnetite and 2 a cenozoic thermoviscous overprint residing principally in coarser pseudosingle domain and multidomain magnetite also of late paleozoic diagenetic origin

siluro devonian palaeomagnetism terrane emplacement and rotation in the caledonides of western ireland summary the palaeomagnetism of a silurian succession lough mask formation lavas and sediments and intrusive rocks kilbride dolerites and andesites and microgranodiorites killary harbour area in the connemara mayo segment of the irish caledonides is described magnetizations are related to the regional tectonic history with the aid of fold conglomerate and contact tests they describe sequences of shallow to intermediate inclinations in both the eastern and western sectors of this caledonian inlier the shallow components appear to pre date siluro devonian folding while the intermediate ones post date this episode the change in palaeofield inclination is identical to the transition described by contemporaneous deep level silurian to shallow and surface level lower devonian bodies in the british and scandinavian caledonides but declinations are rotated consistently to the west the lower middle silurian vector is rotated clockwise by c 100 this difference is progressively reduced to c 50 by the time of the siluro devonian d2 folding showing that this fold generation now oriented e w developed in parallelism with contemporaneous folds on the ne strike continuation of the caledonides post folding remanences continue to deviate in a clockwise sense from the remainder of the caledonides showing that rotation continued into middle upper devonian times and finally ceased during carboniferous times the integrated effect of this rotation is recognized in ordovician magnetizations from the dalradian metamorphic terrane of south connemara it can be explained by block rotations within a zone of distributed deformation according to the mckenzie jackson model and illustrated by many neovolcanic zones palaeomagnetism defines a tectonic regime commencing with rotations of c 3 myr and lasting from mid silurian to carboniferous times western ireland is sited within a closure gap along the iapetus suture extending from ireland to newfoundland and these terranes were apparently emplaced by progressive elimination of this gap during sinistral transpression across the caledonian orogen

on the duration of the paleocene eocene thermal maximum petm 1 the paleocene eocene thermal maximum petm is one of the best known examples of a transient climate perturbation associated with a brief but intense interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean atmosphere system one key to quantifying the mass of carbon released identifying the source s and understanding the ultimate fate of this carbon is to develop high resolution age models two independent strategies have been employed cycle stratigraphy and analysis of extraterrestrial helium heet both of which were first tested on ocean drilling program odp site 690 these two methods are in agreement for the onset of the petm and initial recovery or the clay layer main body but seem to differ in the final recovery phase of the event above the clay layer where the carbonate contents rise and carbon isotope values return toward background values here we present a state of the art age model for the petm derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during odp leg 208 walvis ridge southeastern atlantic integrated with published records from site 690 weddell sea southern ocean odp leg 113 during leg 208 five paleocene eocene p e boundary sections sites 1262 to 1267 were recovered in multiple holes over a depth transect of more than 2200 m at the walvis ridge yielding the first stratigraphically complete p e deep sea sequence with moderate to relatively high sedimentation rates 1 to 3 cm ka where a is years a detailed chronology was developed with nondestructive x ray fluorescence xrf core scanning records on the scale of precession cycles with a total duration of the petm now estimated to be 170 ka the revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and heet

integrated stratigraphy of the paleocene eocene thermal maximum in the new jersey coastal plain toward understanding the effects of global warming in a shelf environment 1 in the new jersey coastal plain a silty to clayey sedimentary unit the marlboro formation represents deposition during the paleocene eocene thermal maximum petm this interval is remarkably different from the glauconitic sands and silts of the underlying paleocene vincentown and overlying eocene manasquan formation we integrate new and published stable isotope biostratigraphic lithostratigraphic and ecostratigraphic records constructing a detailed time frame for the petm along a depth gradient at core sites clayton wilson lake ancora and bass river updip to downdip the onset of the petm marked by the base of the carbon isotope excursion cie is within the gradual transition from glauconitic silty sands to silty clay and represented fully at the updip sites wilson lake and clayton the cie core interval is expanded at the updip sites but truncated the cie core is complete at the bass river and ancora sites where the early part of the recovery is present most complete at ancora the extent to which the petm is expressed in the sediments is highly variable between sites with a significant unconformity at the base of the overlying lower eocene sediments our regional correlation framework provides an improved age model allowing better understanding of the progression of environmental changes during the petm high resolution benthic foraminiferal data document the change from a sediment starved shelf setting to a tropical river dominated mud belt system during the petm probably due to intensification of the hydrologic cycle the excellent preservation of foraminifera during the petm and the lack of severe benthic extinction suggest there was no extreme ocean acidification in shelf settings

further studies of carbonate remagnetization in the northern appalachian basin paleomagnetic and rock magnetic studies of sedimentary carbonates in the northern appalachian basin demonstrate that the late paleozoic remagnetization in this area is probably related to authigenesis of single domain magnetite fifteen sites within middle devonian carbonates were sampled along a transect extending from albany new york to the algonquin arch in southeastern ontario paleomagnetic studies show that all sites except the two westernmost sites on the algonquin arch were remagnetized during alleghenian time the very weakly magnetic unremagnetized sites retain a dual polarity magnetization of probable devonian age in the remagnetized sites both the size of the kiaman components and magnetite concentration estimated from low field susceptibility x are highest in the central part of the transect and lower on either side x ray powder diffraction analyses demonstrate that k feldspar is also most abundant in the middle of the transect with lower concentrations to the west and east we suggest that formation of authigenic magnetite and k feldspar may be related in this setting sites with the largest kiaman components have the highest unblocking temperatures and the largest ratios of anhysteretic remanent magnetization arm to bulk susceptibility arm x indicating that these sites have the greatest amounts of single domain magnetite several remagnetized sites exhibit northerly shallow magnetic components which may be kiaman normal directions we conclude that remagnetization along the new york ontario transect is a chemical remanent magnetization resulting primarily from authigenesis of single domain magnetite we present a model in which pulses of heated fluids triggered magnetite formation scanning electron microscope observation of magnetic extracts from 14 sites show that most of the resolvable magnetite grains are multidomain grains of irregular shape many sites also contain a small percentage of magnetic spheres and or euhedral magnetite particles a majority of the magnetite grains contain fe as the only cation but many also contain minor amounts of other cations including various combinations of cr ni ti zn mn si al ca and mg single domain magnetite is not resolvable with our observational techniques but its presence is inferred from rock magnetic tests much of this magnetite may be of authigenic origin

remagnetization and northward coastwise transport of franciscan complex rocks northern california a reinterpretation of the paleomagnetic data previous paleomagnetic studies of accreted oceanic rocks within the franciscan complex of northern california have concluded that these rocks originated far to the south of their present positions with respect to the north american continent based on positive fold tests the characteristic remanent magnetizations were inferred to predate accretion related deformation and metamorphism and to have been acquired during or soon after deposition of these rocks thus the paleomagnetic data were thought to provide direct information on ancient oceanic plate motions however the plate motions implied by some of these paleomagnetic data are problematic e g exceptional plate velocities and uniform polarity magnetizations in almost all of these rocks indicate the possibility of remagnetization recent work on oceanic rocks in similar subduction complexes of japan and mexico have shown that they were most likely chemically remagnetized during accretion prior to disruption of the original stratigraphic sequences modern analogs indicate that the oceanic rocks in mexico were probably remagnetized while still part of a shallow dipping subducting slab 10 at the base of an accretionary prism assuming these rocks were near horizontal at the time of remagnetization paleolatitudes at which these rocks were subducted and subsequent arc parallel displacements along the western margin of north america can be inferred in this paper franciscan rocks in northern california are reinterpreted as also having been remagnetized prior to accretion related deformation this scenario satisfies both geologic and paleomagnetic constraints for these rocks and resolves conflicts between data indicating both remagnetization and tectonic displacement transport of the laytonville limestone from the southern hemisphere is not required paleolatitudes of subduction and remagnetization in the northern hemisphere 12 to 33 appear to be inversely proportional to age of accretion middle cretaceous to oligocene for the franciscan rocks subsequent northward diplacements 800 to 3700 km and clockwise rotations 56 and 154 of these rocks inferred from the paleomagnetic data are consistent with potential displacements along the western margin of north america during late mesozoic and cenozoic time calculated using examples of modern subduction zones and current plate reconstruction models

paleomagnetism of the upper devonian catskill formation from the southern limb of the pennsylvania salient possible evidence of oroclinal rotation multiple components of magnetization were isolated in the natural remanent magnetiza tion of samples of the upper devonian catskill formation red beds taken from the southern limb of the pennsylvania salient the dominant ther mally distributed component sf previously thought to predate folding is demonstrably syn folding in origins the mean direction for sf based on data from the current study and a pre vious study is declination inclination 161 6o 7 9 o a95 3 9 o pole position 127 3oe 43 1on a95 3 1 o n 14 sites although the remagnetization is clearly synfolding in most areas the relative ages of folding and remagne tization vary locally a subordinate high unblocking temperature component ht has a mean tilt corrected direction of 160o 36 o a95 16 o pole position 123 5oe 26 ion a95 15 4 o s n m 7 samples comparison of ht with the prefolding magnetization isolated in the northern limb of the salient suggests that the paleolatitude of this part of north america was about 16os and that part of the curvature of the salient was acquired during orogenesis

comment on early silurian palaeolatitude of the springdale group redbeds of central newfoundland a palaeomagnetic determination with a remanence anisotropy test for inclination error by j p hodych and k l buchan summary recent palaeomagnetic results from coeval silurian sedimentary and volcanic rocks of newfoundland yield contradictory results with shallow characteristic directions recorded by the redbeds and steeper characteristic directions recorded by the volcanics in their recent paper on the springdale group redbed hodych buchan 1994a argue that the redbed magnetization is more reliable based on a fold test dual polarity magnetization conglomerate test and an irm acquisition experiment specifically designed to test for compaction or inclination error accordingly these authors dismiss the volcanic results as well as the silurian reference poles for laurentia citing possible remagnetization or other complications however we argue that the irm experiments of hodych buchan 1994a are flawed and do not necessarily resolve the question of inclination error moreover we point out that the conglomerate test is incomplete and that the proposed tectonic reconstruction based solely on the redbed magnetization implies a unique tectonic scenario that is incompatible with out present understanding of silurian palaeogeography

acquisition of viscous remanent magnetization abstract viscous remanent magnetization vrm has been frequently suggested as a possible source of magnetic anomalies in ocean crust and as an important contributor of natural remanent magnetization in some volcanic and sedimentary rocks most references to vrm assume a linear dependence with log t yet long term experimental data on vrm while scarce often disagree with this simple notion here we investigate the angular dependence grain size dependence initial state dependence and time t dependence of vrm acquisition we observe a non linear log t dependence of vrm acquisition over all time intervals 104 s for all the samples regardless of various experimental conditions echoing earlier studies we also find that vrm acquisition is strongly dependent on the initial magnetic state of the samples samples in a thermally demagnetized initial state were most susceptible to vrm acquisition when vrm is produced parallel to the initial thermal remanence vrm acquisition is negligible vrm acquisition is more prevalent for magnetites in single domain and multidomain md grains than in the pseudo single domain range in md magnetites frequent removal and restoration of samples for the purpose of zero field remanence measurement apparently increases the efficiency of vrm acquisition

origin of hematite carrying a late paleozoic remagnetization in a quartz sandstone bed from the silurian rose hill formation virginia usa abstract paleomagnetic and petrographic studies of a quartz sandstone bed from the silurian rose hill formation of the central appalachians show that hematite carrying a late paleozoic remagnetization is authigenic comparison of paleopole positions from the rose hill formation with late paleozoic poles for north america suggests a synfolding remagnetization the sandstone cement is characterized by pore lining hematite pore filling berthierine and intergrowths of hematite berthierine and quartz these cements postdate pressure solution of detrital quartz grains late paleozoic cementation is a consequence of diagenesis associated with alleghanian deformation detrital fe mg silicate grains are believed to have been a local source of fe for some authigenic hematite and berthierine although mass balance calculations suggest that an external source is also required some berthierine aggregates are pseudomorphic after detrital fe bearing silicate grains trace amounts of detrital tourmaline are the only remnants of the detrital fe mg silicates

the remagnetization of devono carboniferous sediments from the ardenno rhenish massif abstract a paleomagnetic study was conducted on the paleozoic sedimentary rocks of the ardenno rhenish massif and the brabant massif lower carboniferous and middle devonian carbonates in the dinant synclinorium and inde syncline carry a thermoviscous remagnetization residing in magnetite lower devonian red beds in the dinant synclinorium carry a chemoremanent remagnetization residing in hematite the sediments were remagnetized by burial metamorphism during the asturian phase of the variscan orogeny with a few exceptions in all formations the magnetizations are of inverse polarity corresponding to the kiaman paleofield in the permo carboniferous the virtual geomagnetic poles vgps of the inde syncline agree with the permo carboniferous pole position of europe vgps of the dinant synclinorium which deviate from the permo carboniferous reference direction are due to syntectonic remagnetizations and subsequent 10 20 maximum 40 dextral shear deformations these deformations can be correlated with the northwards overthrusting of the dinant nappe for the inde syncline post asturian rotational deformations can be excluded should a n s transport of this part of the dinant nappe not have occurred the declination pattern indicates that the western part of the dinant nappe was transported further north than the eastern part

palaeomagnetism of red beds of the late devonian worange point formation se australia summary gently folded strata of the late devonian merrimbula group along the south coast of new south wales are similar to numerous deposits of late devonian to early carboniferous subaerial to shallow marine quartzose sandstone that are known as the lambie facies of se australia because the lambie sands overlap the early palaeozoic tectonic elements of the lachlan fold belt large displacement since the late devonian of any lachlan terranes with respect to interior australia is precluded we collected oriented core samples from 37 sites primarily in reddish quartzose litharenites of the worange point formation the remanent magnetization is carried by haematite incremental thermal demagnetization reveals a dominant well defined steep upward north component of magnetization that post dates the mid carboniferous folding the south pole position 146 4 e 68 6 s ag5 3 1 derived from the overprinted specimens is close to both the late carboniferous and mid cretaceous reference poles as well as the spin axis of today the overprint is attributed to both viscous partial thermoremanent and chemical remanent magnetization vptrm and crm its exclusively normal polarity is consistent with a mid cretaceous acquisition perhaps related to the rifting of the se australian margin a characteristic component of magnetization is isolated between 660 and 680 c in 30 per cent of the samples although a regional fold test of the overall result is inconclusive because the directions of characteristic magnetization are generally close to the axes of folding two sets of samples from opposite limbs of a local syncline pass a fold test at 90 per cent confidence indicating a pre folding origin of the remanence the presence of both polarities in thin horizons some of which are palaeosols and diagenetic haematite associated with incipient cleavage formed in an early stage of deformation before folding indicates that the acquisition of remanent magnetization occurred over a protracted period an analysis that combines great circle demagnetization paths and set points facilitates the incorporation of more data but gives a biassed result although normal and reverse polarity subsets are roughly antipodal residual post folding components cause the normal subset to fail a fold test a south pole position 19 7 e7 70 8 s ag5 7 1 is derived from the specimens with characteristic component of reverse polarity in which the isolation of a characteristic component is clear the pole position is close to that of a previous palaeomagnetic study from a widely separated occurrence of lambie facies sediments both results are applicable not only to the lachlan fold belt but also to the rest of australia and gondwana as a whole the position of the two poles derived from the lambie facies overlap assemblage in the lachlan fold belt supports the interpretation that the progression

an evaluation of 14c age relationships between co occurring foraminifera alkenones and total organic carbon in continental margin sediments 1 radiocarbon age relationships between co occurring planktic foraminifera alkenones and total organic carbon in sediments from the continental margins of southern chile northwest africa and the south china sea were compared with published results from the namibian margin age relationships between the sediment components are site specific and relatively constant over time similar to the namibian slope where alkenones have been reported to be 1000 4500 years older than co occurring foraminifera alkenones were significantly similar to 1000 years older than co occurring foraminifera in the chilean margin sediments in contrast alkenones and foraminifera were of similar age within 2 sigma error or better in the nw african and south china sea sediments total organic matter and alkenone ages were similar off namibia age difference toc alkenones 200 700 years chile 100 450 years and nw africa 360 770 years suggesting minor contributions of preaged terrigenous material in the south china sea total organic carbon is significantly 2000 3000 years older owing to greater inputs of preaged terrigenous material age offsets between alkenones and planktic foraminifera are attributed to lateral advection of organic matter physical characteristics of the depositional setting such as seafloor morphology shelf width and sediment composition may control the age of co occurring sediment components in particular offsets between alkenones and foraminifera appear to be greatest in deposition centers in morphologic depressions aging of organic matter is promoted by transport age offsets are correlated with organic richness suggesting that formation of organic aggregates is a key process

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

the demagnetization temperature necessary to remove viscous remanent magnetization recent experimental studies have shown that the temperature necessary to demagnetize viscous remanent magnetization vrm in magnetite bearing rocks is significantly higher than predicted by standard neel single domain theory the walton time temperature equation which has been invoked by some to explain the enhanced thermal stability actually relates intensities of vrms acquired at different temperatures rather than describing the unblocking of a previously acquired vrm we show that the low field approximation lfa used in all previous formulations which predicts equal vrm acquisition and decay rates is not valid for a magnetite assemblage in the earth s field a medium field approximation mfa predicts significantly faster vrm acquisition and therefore greater thermal stability in decay for the larger particles in the assemblage conventional theory does apply to red bed acquisition decay experiments since the lfa is valid for hematite

paleomagnetism and paleothermometry of the sydney basin 2 origin of anomalously high unblocking temperatures the milton monzonite of southeastern australia was thermoviscously remagnetized as a result of cretaceous burial and uplift thermal demagnetization separates the low unblocking temperature lt overprint from the high unblocking temperature ht primary remanence with a relatively sharp junction between lt and ht components in vector projections for single domain grains the junction temperature tl between two such vectors corresponds to the maximum blocking temperature tr reactivated in nature apart from a correction for the difference between natural and laboratory timescales however measured tl values are distributed over an implausibly wide range 250 c for burial remagnetization of an untilted intrusion like the milton monzonite furthermore many tl values are anomalously high compared to the predictions of single domain theory multidomain grains are the cause of these anomalies samples pretreated before thermal demagnetization by zero field cycling to liquid nitrogen temperature so as to erase multidomain remanence and isolate single domain remanence do have the theoretically expected tl values in these samples realistic remagnetization time and temperature tr tr conditions in nature are predicted using the t t contours of pullaiah et al 1975 the anomalously high tl values before low temperature treatment are due to multidomain grains which carry 50 of the lt overprint the lt thermal demagnetization curve in samples dominated by multidomain grains is quasi exponential in shape with a high temperature tail extending almost to the curie point as predicted by multidomain theory these high lt unblocking temperatures which are much greater than plausible remagnetization temperatures reached in nature overlap and mask the lower part of the ht unblocking temperature spectrum driving up tl values and leading to inflated estimates of tr although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the milton monzonite chemical overprinting may be a factor in other lithologies and tectonic settings

palaeomagnetism and rock magnetism of the permian redbeds from the velebit mt karst dinarides croatia dating of the early alpine tectonics in the western dinarides by a secondary magnetization abstract the studied area of the velebit mt a part of the adria microplate belonged to a ne margin of gondwana during the carboniferous and permian while the carboniferous to the early permian was characterised by deposition of clastic rocks younger sedimentation was dominated by a thick sequence of carbonate rocks the lower permian deposits of the core part of the velebit mt at kosna and crne grede localities were investigated using palaeomagnetic and rock magnetic measurements the main remanence carriers were recognized as haematite with an increasing contribution of sp sd magnetite in younger subsections the ams fabric with low anisotropy ratio 1 3 is strongly oblate at kosna and weakly prolate at crne grede reflecting differences in the contribution of magnetic phases a significant remagnetization of the permian rocks as proved by results of a conglomerate test probably caused by a combination of elevated temperatures and fluid migration may be assigned to burial related processes that affected the rocks before the final uplift of the dinarides characteristic remanent magnetizations recorded in haematite are apparently similar to the permian direction for africa shallow inclination with nnw declination as expected for velebit mt coordinates paradoxically this orientation is observed in situ within the almost vertically dipping beds we explain this relationship assuming a syn folding cretaceous remagnetization of the rocks at their subhorizontal position ca 30 s in which a mean vector of the secondary remanence overlaps with the cretaceous direction expected for africa at the velebit mt geographical coordinates consequently our results indirectly point to the cretaceous time of incipient stages of the dinaric tectonism and suggest african geotectonic affinity of the velebit rocks no important vertical axis rotation is implied by our results in contrast to previously published data the puzzling complete remagnetization carried by haematite at relatively low to moderate temperatures is also discussed

anomalous unblocking temperatures viscosity and frequency dependent susceptibility in the chemically remagnetized trenton limestone abstract chemical remagnetization of many carbonate rock units occurred by diagenetic alteration of iron sulfide precursors and by authigenesis of fine grained magnetite which can commonly be recognized by a distinctive set of rock magnetic attributes these carbonates also typically exhibit anomalously high unblocking temperatures for recent thermoviscous components that have overprinted the characteristic chemical remanence recent work has shown that departures from neel theory as applied in the model of pullaiah et al can generally be attributed to vrm carried by multidomain particles to which the theory does not apply in the remagnetized carbonates however rock magnetic data all point to an overwhelming dominance of fine superparamagnetic and stable single domain particles therefore we explore possible alternative causes for the anomalous unblocking temperatures including maghemitization and magnetocrystalline rather than shape anisotropy new blocking unblocking models for these cases however fail to match the observed behavior leading to the conclusion that the anomalous unblocking temperatures are indeed probably attributable to multidomain carriers a possible alternative explanation is an increase in coercivity due to maghemitization of single domain particles after acquisition of the viscous overprint causing a significant increase in their unblocking temperatures new frequency dependent susceptibility measurements at low temperatures allow us to focus on the time temperature behavior of the particles at the superparamagnetic stable single domain threshold and we find that these do closely follow the predictions of neel theory as formulated in the models of pullaiah et al and walton middleton schmidt

redbeds and thermoviscous magnetization theory thermal demagnetization character istics of a brunhes age viscous overprint in appalachian redbeds and a thermoviscous component acquired in the laboratory at moderate tempera ture agree well with relaxation time blocking temperature relations for hematite proposed by pullaiah et al 1975 supporting evidence was obtained from redbeds associated with an igneous dike intrusion although the interpretation of these data are complicated by magnetochemical alterations paradoxically experimental data for magnetite in some limestones were shown to agree more closely with an alternative theory of walton 1980 the hematite in redbeds is predominantly singledomain sd whereas magnetite in the lime stones studied extends well into the multidomain md range thus experiment and theory could be reconciled if it is assumed that pullaiah eta applies strictly to sd material whereas waltoh s theory somehow describes thermoviscous effects dominated by larger md grains

paleothermometry of the sydney basin evidence from overprinting of magnetizations of late permian and mesozoic rocks and from the rank of permian coals and mesozoic phytoclasts coal particles suggests that surface rocks in the sydney basin eastern australia have been raised to temperatures of the order of 200 sup 0 c or higher as vitrinite reflectance an index of coal rank or coalification is postulated to vary predictably with temperature and time estimates of the paleotemperatures in the sydney basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin these estimates give maximum paleotemperatures of present day surface rocks in the range 60 249 sup 0 c depending on factors such as location in the basin the thickness of the sediment eroded and the maximum paleogeothermal gradient higher coal rank and consequently larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the tasman sea on the basin s eastern margin a theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperaturesmore in the sydney basin this estimate places the maximum paleotemperature in the range 250 300 sup 0 c along the coastal region both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures which in places exceed 200 sup 0 c and the loss of sediment thicknesses in excess of 1 km due to erosion less

palaeomagnetic studies within the ballantrae ophiolite southwest scotland magnetotectonic and regional tectonic implications abstract the early arenig slockenray formation within the ballantrae ophiolite southwest scotland displays a multivectorial magnetisation structure two components s and m are identified delineated by differing blocking temperature coercivity spectra component s is removed around 200 c 10 mt and is regarded to be of recent viscous origin component m forms the characteristic formation magnetisation and resides in both magnetite and haematite extensive sampling of all exposed lithologies reveals an in situ non fisherian distribution of the characteristic magnetisation defining an envelope from se moderate positive to sw shallow negative directions a negative infra formation conglomerate test identifies this component as a pervasive overprint a second conglomerate test performed in the overlying benan conglomerate of llandeilo age reveals dispersely directed magnetisation with a stability range equivalent to that of component m this field test therefore defines a maximum remagnetisation window of 30 million years for the characteristic remanence hard viscous magnetisations are identified both in the benan conglomerate and at some sites within the slockenray formation structurally corrected site mean results from the slockenray formation define a non fisherian distribution and form a small circle partial arc centred on a vertical axis nw moderate positive to sw moderate positive directions a combined palaeomagnetic fold and fault test suggests that acquisition of component m pre dates both folding and faulting the resulting palaeolatitude of remanence acquisition 28 8 s implies a tectonic position close to the southern laurentian margin for the ballantrae ophiolite in arenig times

slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the mid atlantic ridge south of the kane fracture zone mark area 1 the remanent magnetization of gabbroic material of the mid atlantic ridge south of the kane fracture zone mark area provides constraints on both the thermal structure and tectonic history of the lower crust in this slow spreading environment the remanence of these gabbroic samples is often complex with the juxtaposition of intervals of apparently normal and reversed polarity rocks over small spatial scales tens of centimeters to a few meters moreover several samples when thermally demagnetized have a reversed polarity magnetization component between higher and lower stability normal polarity components given the nominal age 1 ma of the crust we suggest that this pattern of normal reversed normal polarity most plausibly reflects emplacement and or cooling through three successive polarity intervals jaramillo normal polarity interval 1 07 0 99 ma a portion of the matuyama reversed polarity interval 0 99 0 78 ma and the brunhes normal polarity interval 0 78 ma to present a small number of samples with three well defined magnetization components have magnetic characteristics compatible with a remanence carried by fine grained possibly single domain magnetite laboratory unblocking temperatures in these samples therefore allow estimation of lower crustal temperatures at the time of the jaramillo matuyama 0 99 ma and matuyama brunhes 0 78 ma polarity transitions together with depth estimates derived from fluid inclusion studies these results suggest that middle and lower crustal temperatures remained as high as 350 475 c for a minimum of 0 21 m y after emplacement we suggest that continued injection of liquid in the form of sills or small magma bodies over a broad region half width of 3 km is responsible for this slow cooling in addition inclinations of the highest stability component from these drill sites are remarkably similar to that expected from an axial geocentric dipole suggesting that little if any resolvable tilt occurred during uplift of these rocks to the seafloor

paleomagnetism of the silurian devonian andreas redbeds evidence for an early devonian supercontinent two components of magnetization were isolated in the silurian devonian andreas redbeds of the central appalachians of pennsylvania lat 40 75 n long 75 78 w a thermally distributed synfolding b component and a thermally discrete pre alleghenian age folding c component the c component mean direction and associated pole position correspond to a silurian devonian paleolatitude for the andreas location of about 35 s which in conjunction with early devonian results from gondwana is consistent with an early devonian supercontinent configuration

paleomagnetism and rock magnetism of quaternary volcanic rocks and late paleozoic strata vc 1 core hole valles caldera new mexico with emphasis on remagnetization of late paleozoic strata paleomagnetic and rock magnetic data obtained from azimuthally unoriented core samples collected at approximately 1 to 3 m intervals of continental scientific drilling program core hole vc 1 have prompted reinterpretations of the quaternary volcanic stratigraphy intersected by the bore and have aided in evaluating the thermal regime within late paleozoic strata attending fluid circulation and mineral deposition during and after development of the toledo and valles calderas the results from quaternary units banco bonito obsidian i 35 4 a95 2 8 inclination only determinations n 33 battleship rock tuff d 359 6 i 42 4 a95 2 8 n 5 site means surface sites vc 1 rhyolite i 39 2 a95 12 8 n 7 upper vc 1 tuff i 37 2 a95 10 7 n 13 middle vc 1 tuff i 42 1 a95 2 1 n 39 south mountain rhyolite d 350 9 i 49 9 a95 3 4 n 10 one surface site are consistent with isotopic age data indicating that the entire moat volcanic sequence intersected is less than 650 kyr monitoring of natural remanent magnetization nrm intensity nrm directions directions of magnetizations isolated during progressive demagnetization median destructive forces and rock magnetization parameters has identified systematic variations within the thick banco bonito obsidian and vc 1 tuff units the permian abo formation pennsylvanian to earliest permian madera limestone and pennsylvanian sandia formation typically contain a moderate positive inclination magnetization component abo formation i 52 2 a95 7 4 n 16 madera limestone i 58 4 a95 2 8 n 105 sandia formation i 53 9 a95 4 8 n 21 when residing in magnetite it is usually unblocked in the laboratory by 350 c when carried by hematite it is unblocked by 550 c a moderate negative inclination e g madera and abo strata d 173 1 i 46 6 a95 5 5 n 47 samples assuming a north seeking positive inclination rm providing a useful means of core orientation oppositely directed magnetization is occasionally isolated and removed at higher laboratory unblocking temperatures up to 550 c in magnetite dominated and 600 c in hematite dominated lithologies where present magnetizations of shallower inclination e g madera and abo strata d 160 1 i 10 9 a95 4 4 n 28 are removed over higher ranges of unblocking temperatures and these are probably of late paleozoic age the moderate negative and positive inclination magnetizations are in all likelihood viscous partial thermoremanent magnetizations trms vptrms these were activated at moderate 300 c temperatures between 1 45 and 0 97 ma attending and following the main stages of toledo valles caldera development during the matuyama and at near present downhole temperatures 100 180 c during the brunhes and possibly within the last 10 kyr respectively importantly the preservation of negative inclination rm components in magnetite dominated lithologies of the madera limestone dictates that temperatures during the brunhes chron past 730 kyr did not exceed approximately 300 350 c the inferred temperatures for thermal activation of the vptrms based upon the middleton and schmidt thermal activation curves for both magnetite and hematite derived following walton agree with independent estimates of thermal activity using fluid inclusion and k ar isotopic age determination data during the quaternary the results further illustrate the potential usefulness of thermal activation theories for inferring temperatures of past geologic events especially when they can be applied to magnetizations of similar age residing in both magnetite and hematite and of paleomagnetic and rock magnetic studies in general in continuous coreholes in complex geologic environments

time temperature relations for the remagnetization of pyrrhotite fe7s8 and their use in estimating paleotemperatures abstract paleotemperature controls the maturation of coal and hydrocarbons in sedimentary basins and is also important in determining paleogeothermal gradient and hence tectonic style in exhumed metamorphic terrains one method of estimating paleotemperature analyses the partial remagnetization of a rock due to heating in thick volcanic or sedimentary sequences over subcrustal heat sources such as plumes or at convergent plate margins the overprinted natural remanent magnetization nrm of a rock records both the age and the paleotemperature of remagnetization but a temperature correction from laboratory to geological time scales is required using theoretical time temperature relations time temperature relations are well known for magnetite fe 3 o 4 but are reported here for the first time for pyrrhotite fe 7 s 8 another common nrm carrier data for each mineral separately yield independent estimates of paleotemperature if geologically reasonable estimates of heating time can be made paleotemperature can be estimated without geological input if data for both minerals are combined together with the age of remagnetization determined from the paleomagnetic pole of the nrm overprint these paleotemperature estimates can be used to infer the history of heating and uplift following burial as a test case we examine thermally acquired nrm overprints carried by pyrrhotite fe 7 s 8 and magnetite fe 3 o 4 in the milton monzonite of southeastern australia these overprints record a heating event about 100 ma ago probably thermal doming prior to rifting of the tasman sea that upgraded coal rank in the sydney basin extrapolating from laboratory to geological times using the new time temperature contours for pyrrhotite we estimate that the presently exposed sydney basin in the vicinity of the milton monzonite was remagnetized by heating to 165 30 c for 100 ka assuming a paleogeothermal gradient of 70 c km appropriate for young or incipient rifts the depth of burial at the time of remagnetization is estimated to have been 2 3 0 4 km this figure is in excellent agreement with independent estimates based on reflectance data for the coal accessory mineral vitrinite

flow duration of a dike constrained by palaeomagnetic data summary we have conducted a palaeomagnetic study of a permian red sandstone layer intruded by a 70cm wide plio pleistocene basaltic dike with the ultimate goal of estimating the flow duration of the magma before its solidification 31 samples were collected at various distances from the dike sandstone interface and were thermally demagnetized in order to separate the overprinted pliopleistocene magnetization component from the primary permian one and to determine for each sample the unblocking temperature tub necessary to erase totally the overprint this temperature can be used to determine the maximum palaeotemperature reached at any distance from the contact provided the secondary magnetization is a p trm and the thermal fluctuation effects and actual cooling rates are taken into account examination of various magnetic parameters nrm intensity 15 days vrm intensity initial magnetic susceptibility and hysteresis curves allowed us to establish the nature of the overprinted magnetization and consequently to exclude samples carrying a crm overprint an apparent palaeotemperature profile and two corrected ones were thus deduced from the samples in which the dike emplacement produced no crystallization of new magnetic minerals these profiles were compared with the calculations of a new conductive thermal model which includes the basalt s latent heat of crystallization the flow duration of the dike the width of the circulating magma and the temperature dependence of the thermal conductivity k in the basalt and in the sandstone or alternatively convection simulated by an increase of the k values good fits with the experimental data are obtained for a flowing width consistent with field observations and when the latent heat is taken into account in the two following cases first when k is temperature dependent in the basalt and in the sandstone which implies a magma flow duration of 5 days second when a limited convection process is assumed in the sandstone only here a flowing time of only 1 day is required the corrected palaeotemperature profiles argue in favour of the second hypothesis suggesting that when a dike is emplaced heat transfer by fluids may play an important role in the surrounding rock

a model for interpreting continental shelf hydrographic processes from the stable isotope and cadmium calcium profiles of scallop shells abstract stable oxygen and carbon isotope and cadmium calcium profiles from the shells of scallops collected from the outer continental shelf of the south atlantic bight and the virginia bight appear to trace the hydrographic processes of the two regions a model is developed to distinguish two principal modes of nutrient and trace metal input to the outer shelf from the biogeochemical profiles of the shell the oxygen isotope profiles record bottom water temperature changes caused by seasonal temperature cycles and water mass movements nutrient input and subsequent phytoplankton blooms are recorded in the carbon isotope and cd ca profiles the shell profiles from the south atlantic bight record the dominant effect of upwelling caused by gulf stream meandering the shell profiles from the virginia bight show the influence of spring river runoff and summer water column stratification biogeochemical profiles in mollusk shells are potentially important tools for studying the dynamics of both modern and ancient continental shelf upwelling systems

release of methane from a volcanic basin as a mechanism for initial eocene global warming a 200 000 yr interval of extreme global warming marked the start of the eocene epoch about 55 million years ago negative carbon and oxygen isotope excursions in marine and terrestrial sediments show that this event was linked to a massive and rapid approximately 10 000 yr input of isotopically depleted carbon it has been suggested previously that extensive melting of gas hydrates buried in marine sediments may represent the carbon source and has caused the global climate change large scale hydrate melting however requires a hitherto unknown triggering mechanism here we present evidence for the presence of thousands of hydrothermal vent complexes identified on seismic reflection profiles from the voring and more basins in the norwegian sea we propose that intrusion of voluminous mantle derived melts in carbon rich sedimentary strata in the northeast atlantic may have caused an explosive release of methane transported to the ocean or atmosphere through the vent complexes close to the palaeocene eocene boundary similar volcanic and metamorphic processes may explain climate events associated with other large igneous provinces such as the siberian traps approximately 250 million years ago and the karoo igneous province approximately 183 million years ago

thermoviscous remanent magnetism of columbia river basalt blocks in the cascade landslide we studied sixteen basalt samples from a landslide in the columbia river gorge to determine if they had acquired a thermoviscous remanent magnetism tvrm since the slide was emplaced about 800 years ago all samples were thermally demagnetized at 20 heating steps until 360 c where a large change in susceptibilit y was noted analysis of the directional changes during demagnetization indicates that the samples contain up to four tvrm components in addition to an nrm component the tvrm components with the lowest blocking temperatures are tightly clustered around the present field direction while the nrm directions are consistent with a random distribution as expected for a landslide deposit measurements of hysteresis parameters and thermomagnetic analyses of the samples demonstrate that the dominant magnetic mineral in the basalt is single domain magnetite the temperature at which the first tvrm component was removed ranges from 70 c to 100 c this result can be compared to nomographs that relate the time and temperature used to demagnetize a tvrm with the time and temperature at which the tvrm was acquired our results are more consistent with the nomograph of pullaiah et al 1975 than with the nomograph of middleton schmidt 1982