magnetite authigenesis and diagenetic paleotemperatures across the northern appalachian basin the concentration of magnetite in the lower and middle devonian helderberg and onondaga carbonate formations varies in a distinct pattern along an east west profile across new york state magnetite concentrations are uniformly low in the western half of the profile increase sharply eastward to a peak value near syracuse and decrease farther east the pattern strongly resembles previously reported variations in the extent of clay mineral diagenesis that reflect differences in paleotemperatures along the profile previous paleomagnetic studies have documented that the magnetite carries a well defined magnetization of pennsylvanian permian age and magnetite authigenesis is therefore no younger than late paleozoic we attribute the correlation between high magnetite concentration and high illite content to temperature dependent diagenesis triggered by orogenic fluids the large proportion of secondary magnetite indicates that over most of the area the mechanism of late paleozoic remagnetization was principally chemical thermal effects played a less direct role in the remagnetization of the strata by controlling the extent of the chemical processes that resulted in overprinting of the original remanence

influence of sedimentary setting on the use of magnetic susceptibility examples from the devonian of belgium bulk magnetic susceptibility measurements on sedimentological samples from all geological periods have been used widely in the last two decades for correlations and as a proxy for sea level variations this paper explores the link between magnetic susceptibility depositional setting and environmental parameters these environmental parameters include distal proximal transects microfacies successions and fourth order trends on different carbonate platform types platform ramp carbonate mound or atoll during different devonian stages eifelian givetian and frasnian average magnetic susceptibility values over a distal proximal trending facies succession vary markedly with depositional setting on carbonate platforms average magnetic susceptibility generally increases towards the top of shallowing upward sequences on a distal proximal transect average magnetic susceptibility is intermediate for the deepest facies decreases for the reef belts and increases to a maximum in the back reef zone in ramps and atolls magnetic susceptibility trends clearly differ average magnetic susceptibility generally decreases towards the top of shallowing upward sequences and is highest in the deepest facies the strong relationship between magnetic susceptibility facies and sequences implies a strong environmental influence however the different responses in the different platform types suggest that sea level changes leading to variation in detrital input is not the only parameter controlling average magnetic susceptibility values other primary or secondary processes also probably influenced magnetic mineral distribution primary processes such as carbonate production and water agitation during deposition are probably key factors when carbonate production is high the proportion of magnetic minerals is diluted and the magnetic susceptibility signal decreases high water agitation during deposition will also selectively remove magnetic minerals and will lead to low average magnetic susceptibility values these parameters explain the lowest values observed on the reef platform inner ramp and atoll crown which are all in areas characterized by higher carbonate production and greater water agitation during deposition the lowest values observed in the lagoon inside the atoll crown can be related to detrital isolation by the atoll crown however other parameters such as biogenic magnetite production or diagenesis can also influence the magnetic signal diagenesis can change magnetism by creating or destroying magnetic minerals however the influence of diagenesis probably is linked strongly to the primary facies permeability amount of clay or organic matter and probably enhanced the primary signal the complexity of the signal gives rise to correlation problems between different depositional settings thus while magnetic susceptibility has the potential to be an important correlation tool the results of this investigation indicate that it cannot be used without consideration of sedimentary processes and depositional environments and without strong biostratigraphical control

climatic versus tectonic controls of fan sequences lessons from the dead sea israel tectonic activity is commonly held responsible for abrupt coarsening of alluvial fan sequences and the role of climate is often relegated the pleistocene fans of the dead sea rift indicate clearly the problems of differentiating the sedimentary signatures of climatic and tectonic adjustments in a closed lake basin exposures are excellent the rivers are currently incising in response to the low lake levels of the holocene producing cliffs 50m high these reveal an interdigitation of lake deposits dominantly fine grained carbonates and the coarse river gravels which characterize the fan deposits the bounds of each facies are sharp river sediments are superimposed on lake sediments with little evidence of either reworking or loading in turn aragonite crusts are draped over alluvial gravels indicating the sudden return of still water conditions neither can be explained by gradual adjustment of the hinterland river system after tectonic shock instead it is clear that the coarse alluvium was deposited by discrete high magnitude floods that occurred only sporadically because of the infrequent nature of rainfall in this arid environment in interpreting basin development in ancient settings it is always tempting to assume that coarse deposits are indicative of tectonism however the dead sea fans demonstrate that climate is just as likely to be responsible for the conglomerates in a sedimentary sequence

deciphering the impact of diagenesis overprint on negative δ13c excursions using rock magnetism case study of ediacaran carbonates yangjiaping section south china rock magnetism is used here to investigate the genesis of one of the puzzling negative carbon isotopic excursions of the neoproterozoic in the yangtze platform south china a detailed characterization of the magnetic mineralogy which includes low temperature and high field magnetometry and classical magnetic measurement arm irm susceptibility was therefore performed along upper doushantuo and lower dengying formations outcropping in the yangjiaping section the derived magnetic parameters show variations that can be interpreted as variations in magnetic grains size and in oxide contents they show that the magnetic content is significantly reduced in samples presenting negative 13ccalcite values we interpret this as a result of magnetite dissolution and secondary carbonate precipitation during early diagenesis bacterial sulfate reduction combined with c and o isotopic data paleomagnetic techniques thus show that the upper doushantuo lower dengying negative excursion of the yangjiaping section is largely due to diagenesis although the preservation of a genuine 13c excursion of lower magnitude from 7 down to 0 instead of down to 9 as usually considered cannot be ruled out a corrected 13ccarbonate chemostratigraphic curve is therefore proposed the unambiguous identification of a strong diagenetic component for this excursion casts doubts on the primary nature of other potentially time equivalent negative excursions of the yangtze platform and thus to its correlation to negative excursions in other cratons i e shuram excursion more generally this study illustrates the potential of magnetic mineralogy characterization a low cost time efficient and non destructive technique as screening tool for diagenetic overprints of 13c and 18o

absolute dating of dedolomitization by means of paleomagnetic techniques paleomagnetic analysis in conjunction with petrographic studies is an approach whereby absolute dates can be placed on diagenetic events hematite is associated with dedolomite in the lower ordovician kindblade formation in south central oklahoma and is interpreted as a by product of the dedolomitization of ferroan dolomite paleomagnetic results date the time of dedolomitization as late pennsylvanian early permian dedolomitization was probably caused by oxidizing fluids with high calcium contents that migrated through the kindblade formation following uplift of the arbuckle mountains

replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism limestones are an important source of palaeomagnetic data with magnetite the dominant carrier of the magnetization because the magnetite is present in low concentrations however it has been directly observed only rarely and in trace amounts in acid treated separates such residues are composed primarily of spheres the ultimate origin of which has been enigmatic7 yet interpretation of the palaeomagnetic data depends on a knowledge of the origin of the magnetite which has been inferred to carry either a chemical or a viscous remanent magnetization here we present electron microscope data obtained from a sequence of samples of onondaga limestone from an east west traverse across northern new york which unambiguously show that magnetite spheres are derived by alteration and replacement of framboidal pyrite remagnetization is thus caused by chemical rather than viscous processes this chemical remanent magnetization is compatible with a fluid mediated event occurring on a regional scale induced by tectonic stress of alleghenian late palaeozoic age

magnetization of a dolomite bed in the monterey formation implications for diagenesis the effect of diagenesis upon natural remanent magnetization nrm has been studied at two sites within a single bed of dolomite from the monterey formation in santa barbara county california paleomagnetic direction demonstrates that the nrm is primary at one site and remagnetized at the other site magnetic separates from samples carrying a primary nrm yielded detrital magnetite separates from samples carrying secondary nrm yielded an unusual form of authigenic magnetite evidence suggests that the formation of the authigenic magnetite accompanied late stage dolomite diagenesis correlating the remagnetization direction to the paleomagnetic history of the region gives a probable age of 0 72 2 47 m y b p for both of these diagenetic events

paleomagnetic and petrographic investigation of the taum sauk limestone southeast missouri paleomagnetic results from 17 sites in one quarry of the cambrian dresbachian stage taum sauk limestone member bonneterre formation reveal two components of magnetization on the basis of thermal demagnetization coercivity spectrum analysis and petrographic evidence both components reside in hematite sites can be divided into three groups based on behavior during thermal demagnetization and petrographic criteria the first group consists of massive pink limestones and contains a component with declination 270 inclination 11 corresponding to a cambrian pole at 175 6 e 3 6 s the second group of sampling sites consists of specimens with numerous burrows filled with hematite intergrown with calcite the sites are dominated by the second component of magnetization which has declination 147 and inclination 10 and corresponds to a reversed pole position at 133 1 e 37 3 n a significant fraction of this component which is interpreted to be carboniferous in age resides in hematite intergrown with calcite in the burrows petrographic relationships indicate that the calcite and hematite were acquired before dolomite which places a maximum age on dolomitization the third group consists of massive pink limestones with variable amounts of burrowing and contains both directions of magnetization in roughly equal proportions with closely overlapping blocking temperature spectra the presence of two stable directions of magnetization from one location with neither being dominant or even present throughout the quarry indicates the significant effect that diagenetic and sedimentologic processes may have on the magnetization of sedimentary units

mineral magnetic study of lacustrine sediments from lake pumoyum co southern tibet over the last 19 ka and paleoenvironmental significance abstract high resolution proxy records from tibetan plateau are essential to understand the past climatic and environmental changes in this work we conducted systematic environmental magnetic studies on lacustrine sediments from lake pumoyum co southern tibet spanning from the last deglaciation 19 ka to present we correlated the magnetic proxies in the lake core with environmental changes the magnetic proxies from the 3 units were studied both unit 3 19 13 2 ka and unit 2 13 2 8 4 ka contain evidences for an anoxic or sub anoxic environment fine grained greigite dominates the bulk magnetic properties of unit 3 while both magnetite and greigite coexist in unit 2 overall this reveals a transition from an anoxic environment to an oxic environment in contrast unit 1 8 4 ka present spans the holocene when the lake environment was oxic there is an increasing trend in the concentration of magnetic minerals which reflects an increasing transport of magnetic minerals from the catchment into the lake due to the increased post glacial melt water flow although different mechanisms affect the magnetic assemblage and associated magnetic properties in the different units our results suggest that the magnetic proxies of the local lake environment on southern tibet are sensitive to environmental changes

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

use of strontium isotopes to constrain the timing and mode of dolomitization of upper cenozoic sediments in a core from san salvador bahamas the sup 87 sup 86 sr ratios and the activity ratios of sup 234 u sup 238 u and sup 230 th sup 238 u have been measured in dolomites from a 168 m deep core taken on the island of san salvador bahamas these data suggest two periods of dolomitization the first episode dolomitized miocene age sediments during the latest miocene and the second dolomitized the pliocene portion of the core and was still active as recently as 150 ka the late timing of the second episode argues against penecontemporaneous models of dolomitization for the pliocene sediments instead dolomitization is favored either as a result of mixing zone development during the large pleistocene sea level changes or by movement of seawater through the platform

textural and geochemical alternations in late cenozoic bahamian dolomites dolomitized intervals of a core from san salvador island bahamas exhibit variations of two texturally and geochemically distinct end members in the pliocene section of the dolomitized interval the two end members alternate in a pattern that may reflect originally and or diagenetically modified depositional facies formerly mud free intervals locally capped by exposure surfaces are massive crystalline mimetic dolomites cm muddier sediments are replaced by friable microsucrosic dolomites ms cm and ms dolomites also differ in porosity 30 permeability 100 md mol mgco3 44 9 vs 47 7 mol oxygen isotopic composition 1 7 vs 2 7 and strontium content 241 vs 106 ppm these data indicate that depositional and diagenetic fabric are the principal controls governing the distribution of dolomite types differences in texture and geochemistry are suggested as arising through differential rates of crystallization produced as a result of variations in permeability and reactivity of the precursor sediments and rocks

paleomagnetism of upper devonian reefal limestones canning basin western australia two conflicting models have been proposed for the middle paleozoic position of gondwana with respect to euramerica a pole from morocco msissi norite that may be of late devonian age suggests that a broad ocean 3 000 km separated the northern continents from gondwana whereas results from mauritania gneiguira supergroup indicate that this ocean had closed by devonian time detailed thermal and alternating field demagnetization experiments have been carried out on red and gray limestones from seven sites 89 samples in an essentially undeformed late devonian reef complex northern canning basin western australia after minor tilt correction the site mean characteristic directions average 42 7 28 9 declination inclination with k 61 6 and 95 7 8 these results position northwestern australia at 15 s latitude in the late devonian a paleolatitude consistent with the presence of a well developed reefal sequence an early age is inferred for these magnetizations on the basis of the occurrence of mixed polarities and the fact that no post devonian directions for australia resemble the observed declinations and inclinations if the smith and hallam reconstruction for gondwana is used the paleomagnetic pole calculated for the late devonian of western australia agrees with the msissi norite result from morocco and suggests that an ocean still separated gondwana from euramerica in the late devonian the pole measured from the gneiguira supergroup in mauritania may be based on remagnetizations of late paleozoic age the directions determined from canning basin limestones are similar in inclination to but differ by 40 in declination from paleomagnetic results from the upper devonian comerong volcanics lachlan fold belt southeastern australia canning basin samples are from the stable craton and comerong volcanics samples are from the tasman fold belt the discrepancy between the two australian results may be best explained by tectonic rotation of the comerong volcanics locality

nature and origin of magnetic minerals within the middle jurassic shallow water carbonate rocks of the paris basin france implications for magnetostratigraphic dating summary the middle and upper jurassic bathonian oxfordian shallow water carbonate rocks from the paris basin france consist mainly of oolitic and bioclastic limestones that are hydrocarbon reservoirs in the subsurface despite a preliminary positive study these deposits have been considered to be largely remagnetized rochette private communication and hence not amenable to palaeomagnetic dating to establish their magnetic mineralogy and test this remagnetization hypothesis we have used an integrated investigation combining petrographic geochemical rock magnetic and palaeomagnetic measurements on samples extracted from five cores from the paris basin and from outcrops in burgundy magnetic minerals in the bathonian oxfordian carbonates include 1 primary biogenic single domain magnetite and detrital multi domain ti magnetite and their oxidized form maghemite 2 authigenic spheres of magnetite probably related to hydrocarbons and 3 goethite either restricted to ferruginous ooid layers or resulting from surficial alteration notably replacement of pyrite framboids rock magnetic experiments carried out on 68 samples reveal hcr hc and mrs ms ratios ranging from 1 88 to 5 58 and 0 017 to 0 314 respectively these values are clearly distinct from diagnostic values for a chemical remagnetization pyrrhotite was not identified within these sediments moreover the average hcr hc ratio of 3 14 is significantly different from the value of 1 333 for natural pyrrhotite dekkers 1988 these results have a direct implication for the preservation of the primary magnetization consequently these deposits are selectively amenable for magnetostratigraphic dating and possible regional correlations

paleomagnetism and late diagenesis of jurassic carbonates from the jura mountains switzerland and france a crucial fold test for magnetizations measured in bajocian through kimmeridgian limestones from the jura mountain region shows that rare blue gray limestones found in this sequence preserve a prefolding magnetization in contrast tan limestones which predominate in this sequence contain secondary magnetizations acquired after folding in the late miocene the postfolding magnetizations in tan limestones appear to be carried in goethite and because reversed as well as normal polarity directions are present the magnetization in these limestones is probably a post miocene pre brunhes 700 000 yr chemical remanent magnetization crm field relations between tan and blue gray limestones which are present in some cases in the same rock unit for example the hauptrogenstein suggest that the tan limestones may have formed from alteration of blue limestones whereas goethite pseudomorphs after pyrite are found in tan limestones the occurrence of pyrite in the blue limestones supports this model indicating that goethite could have formed from oxidation of pyrite abundant meteoric water probably was available for such an oxidation process during uplift of these strata subsequent to deformation natural remanent magnetization in blue limestones appears to be carried in magnetite the positive fold test for the characteristic magnetization in these rocks is significant at the 99 confidence level k values for 24 site mean directions before and after tilt correction are 9 3 and 43 2 respectively the simplest interpretation for the origin of observed remanence in magnetite in the blue limestones is a primary magnetization of depositional or biogenic origin characteristic directions in these rocks yield a mean paleopole position of 77 7n 148 4e k 26 0 a95 5 9 this pole is in reasonable agreement with one other upper jurassic pole from western europe and with other middle to upper jurassic poles from the atlantic bordering continents after rotation of these continents to a reconstructed configuration for middle to late jurassic time

late paleozoic remagnetization of the trenton limestone samples of the middle ordovician trenton limestone were collected from 8 sites in quebec ontario and new york state for paleomagnetic study stepwise thermal demagnetization reveals a very well defined characteristic magnetization that has a corresponding paleomagnetic pole at 53 0 n 126 9 e k 97 7 a 95 2 7 this pole is not at all close to other ordovician poles from cratonic north america but is near late carboniferous and early permian ones indicating that the trenton was remagnetized in the late paleozoic an earlier paleomagnetic investigation of the trenton gave a pole that did not appear to be late paleozoic in age however we demonstrate that this earlier study failed to isolate the ancient characteristic magnetization rock magnetic experiments and analyses of magnetic extracts show that the carrier of magnetization is fe3o4 that does not contain ti or other elements in solid solution that would indicate a detrital source the magnetite and its associated remanence appear to have a diagenetic origin

magnetostratigraphic dating of the uplifted atoll of mare geodynamics of the loyalty ridge sw pacific the loyalty islands sw pacific are uplifted as they are progressively affected by the lithospheric flexure of the australian plate before its subduction under the new hebrides arc these geodynamic changes are constrained by magnetostratigraphically dating two sections from mare island where mineral extractions coupled with rock magnetic experiments suggest that the magnetic remanence is mostly carried by a mixture of single domain to multidomain magnetite maghemite with the help of faunal determinations and radiometric dating the sequences of polarity reversals correlated to the geomagnetic polarity timescale range from the top of chron c4n late miocene to the top of the gauss chron late pliocene this new chronostratigraphy refines the timing of two distinctive carbonate units rhodolith platform coral reefs separated by a hardground whose transition is known to coincide approximately with a regional event around the miocene pliocene boundary the magnetostratigraphic dating indicates that the hardground represents about a 1 9 m y hiatus and suggests variable sedimentation rates ranging from 4 7 to 65 4 m m y during the atoll construction the lithospheric bulge seems to have influenced the evolution of mare island some 3 1 m y ago leading to a diachronous emersion of the northeast and southwest rim of the atoll with a mean uplift rate of the order of 4 cm kyr

biogenic magnetite as a primary remanence carrier in limestone deposits abstract studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at sugarloaf key fl u s a and calcareous laminated sediments from laguna figueroa baja california mexico have revealed the existence of magnetotactic bacteria and ultrafine grained single domain magnetite in both environments magnetotactic bacteria were identified by light and electron microscopy the single domain magnetite was detected by coercivity spectra analysis with a squid magnetometer and examined under the transmission electron microscope the similarity in terms of size and shape between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine grained magnetite in these two marine environments was biologically formed these results combined with the common occurrences of ultrafine grained magnetite in limestone deposits detected rock magnetically suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks several cambrian limestone samples separately collected from siberia china and kazakhstan were examined for the presence of bacterial magnetite samples from the lower cambrian sinskian formation at siberia platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component post cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them assessing the potential role of biogenic magnetite as a primary remanence carrier in other phanerozoic limestone deposits ought to be further pursued

evidence for a relationship between hydrocarbons and authigenic magnetite establishing a relationship between hydrocarbon migration and the precipitation of authigenic magnetite in sedimentary rocks is of significant interest with respect to 1 elucidating mechanisms for remagnetization and establishing the origin of secondary magnetizations residing in magnetite 2 developing a method to date hydrocarbon migration events by determining the time of remanence acquisition by palaeomagnetic methods and 3 evaluating whether searching for anomalous concentrations of diagenetic magnetic minerals and or aeromagnetic anomalies is justified as a relatively inexpensive exploration tool the direct association of hydrocarbon migration and the precipitation of authigenic magnetite has however not been established in this paper we report palaeomagnetic rock magnetic petrographic and geochemical results of a study of samples from permian spelebthems and gilsonite found in the ordovician arbuckle group in southern oklahoma the results indicate that there is a genetic relationship between hydrocarbon migration and the precipitation of authigenic magnetite

diagenetic replacement controlled by force of crystallization many diagenetic replacements in sedimentary rocks occur along thin solution films between the authigenic and host phases rather than conventionally invoking bulk pore water undersaturation for host phase dissolution we propose that many diagenetic replacements occur by a force of crystallization controlled replacement mechanism whereby nonhydrostatic stresses resulting from authigenic crystal growth are principally responsible for host phase dissolution this model explains diagenetic features such as the restriction of host phase dissolution to authigenic host crystal contacts and euhedral authigenic crystal faces in planar contact with unreplaced host phases as well as the relative replacement tendencies of some common authigenic minerals

a middle late silurian paleomagnetic pole from limestone reefs of the wabash formation indiana u s a a middle late silurian paleomagnetic pole has been obtained from reefal limestones in north central indiana eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization one additional site gave the same characteristic direction after great circle analysis these 12 site level directions were used to calculate the mean paleomagnetic pole reef flank beds dip steeply about 38 away from reef centers field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12 a 12 tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement indicating that the magnetization was acquired early the presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence the mean of the site paleomagnetic poles with sites from steeply inclined flank beds corrected for 12 of tilt is 16 8 n 124 9 e k 74 a95 5 1 in good agreement with the middle silurian rose hill pole for north america

natural remanent magnetization acquisition in bioturbated sediment general theory and implications for relative paleointensity reconstructions we present a general theory for the acquisition of natural remanent magnetizations nrm in sediment under the influence of a magnetic torques b randomizing torques and c torques resulting from interaction forces dynamic equilibrium between a and b in the water column and at the sediment water interface generates a detrital remanent magnetization drm while much stronger randomizing torques may be provided by bioturbation inside the mixed layer these generate a so called mixed remanent magnetization mrm which is stabilized by mechanical interaction forces during the time required to cross the surface mixed layer drm is lost and mrm is acquired at a rate that depends on bioturbation intensity both processes are governed by a mrm lock in function the final nrm intensity is controlled mainly by a single parameter that is defined as the product of rotational diffusion and mixed layer thickness divided by sedimentation rate this parameter defines three regimes 1 slow mixing 10 with mrm acquisition and full drm randomization and 3 intermediate mixing because the acquisition efficiency of drm is larger than that of mrm nrm intensity is particularly sensitive to in case of mixed regimes generating variable nrm acquisition efficiencies this model explains 1 lock in delays that can be matched with empirical reconstructions from paleomagnetic records 2 the existence of small lock in depths that lead to drm preservation 3 specific nrm acquisition efficiencies of magnetofossil rich sediments and 4 some relative paleointensity artifacts

paleomagnetic evidence for fossil biogenic magnetite in western crete abstract previous paleomagnetic research has shown that the concentration of fine grained magnetite preserved in miocene marine clays of western crete drops during two successive geomagnetic transitions although the mechanism responsible for this effect was not known magnetite precipitation by magnetotactic bacteria offers a straightforward explanation for this decrease reduced field strengths during a reversal will strain the competitive advantage of having intracellular magnetite and will force bacteria to precipitate correspondingly more magnetite if they are to maintain the same alignment in the geomagnetic field their ability to do this is limited by several magnetophysical constraints which should act to reduce the numbers of magnetite precipitating cells particularly in fields of less than about 12 of the present level this would lead to a decrease in the supply of bacterial magnitude to the sediments recovery of intact magnetotactic bacterial fossils may lead to a new method for estimating paleointensities from sedimentary rocks

early dolomitization of platform carbonates and the preservation of magnetic polarity results from a combination of techniques are presented to evaluate the nature of magnetization in shallow water platform carbonates which have undergone recrystallization during early calcification and dolomitization magnetic grain separates coercivity spectra modified lowrie fuller tests magnetization efficiency and magnetostratigraphic constraints indicate that the ultrafine grained magnetite is preserved during early burial geochemical regimes inversion from aragonite high magnesium calcite to low magnesium calcite and even pervasive dolomitization these single domain crystals are thought to occur as interacting multigrain clusters some of which may exceed 1 m in diameter these large clusters may help prohibit magnetic reorientation during diagenesis furthermore during both fabric preserving and fabric destructive dolomitization the ultrafine scale replacement process restricts reorientation of the clusters thus preserving depositional or early postdeposition magnetic orientation this early dolomitization matrix stabilization may even help protect and extend the subsurface lifespan of the original polarity

occurrence of secondary magnetite within biodegraded oil samples of solid bitumen from the thornton quarry illinois and the cynthia quarry mississippi were found to be strongly magnetic and to have rock magnetic properties suggesting that the magnetizable grains present are magnetite studies of magnetic isolates revealed that magnetite is present primarily as spherical crystal aggregates that appear identical to magnetite spherules isolated from re magnetized paleozoic carbonate units from other localities organic geochemical analyses of the solid bitumen suggest an origin by microbial attack on what once was liquid crude oil the occurrence of secondary magnetite as inclusions within solid bitumen suggests a relationship between crude oil biodegradation and development of that mineral in our samples we infer that secondary magnetite in other geologic environments may be related to the presence of hydrocarbons the discovery of a natural association of secondary magnetite and hydrocarbons has important implications for paleomagnetism and for petroleum exploration

magnetostratigraphic dating of shallow water carbonates from mururoa atoll french polynesia implications for global eustasy magnetostratigraphic results are reported from a core penetrating the top 100 m of mururoa atoll french polynesia the sequence of polarity reversals based on relative up down direction in the unoriented core is correlated to the geomagnetic polarity time scale from the gauss early pliocene through the brunhes chron late pleistocene holocene the polarity sequence is matched with the help of an early brunhes radiometric date in a nearby core and the projected level of gephyrocapsa oceanica mid brunhes from an adjacent inclined core rock magnetic tests and extraction of magnetic components suggest a mixture of single and multi domain magnetite or maghemite carries the magnetic remanence newly refined atoll chronostratigraphy indicates a punctuated growth history and variable sedimentation rate of the reefal rim magnetostratigraphically dated periods of low accumulation correspond well with the increased frequency of subaerial exposure horizons these new data suggest that third order scale 1 m y sea level events are recorded in reefal deposits on moderate to slowly subsiding atolls mururoa and the bahamian island of san salvador have similar accumulation curves suggesting a global eustatic control

electron optical characterization of bacterial magnetite abstract magnetic grains isolated from magnetococcoid bacterial cells were studied by means of transmission electron microscopy electron diffraction and electron microprobe analysis observed in situ the magnetic grains are each surrounded by an organic membrane and are usually found in a random array although chains are also seen electron diffraction confirms the magnetite mineralogy and provides additional evidence in favor of vacancies in the structure electron microprobe analysis shows the magnetite to be slightly titaniferous electron microscopy indicates that the grains rather than being flake shaped are parallelepiped crystals with a mean length of 99 3 8 7nm a mean width of 62 3 6 1nm yielding a width to length ratio of 0 63 these data support the contention that the magnetic bacterial grains are single domain crystals capable of producing a natural remanent magnetization in sediments

magnetostratigraphic dating of an upper miocene shallow marine and continental sedimentary succession in northeastern morocco abstract a high resolution magnetostratigraphy has been developed for an upper miocene shallow marine carbonate complex equivalent basinal marls diatomites and lime mudstones and an overlying succession of marine to continental carbonate and siliciclastic sedimentary rocks in the melilla basin northeastern morocco units of the carbonate complex from bottom to top contain a bryozoan red algal ramp bioclastic platform fringing reef complex and a unit referred to as the terminal carbonate complex tcc 1 lithologies from these units contain at least three components of magnetic remanence 1 a low unblocking temperature component that is likely carried by a viscous remanent magnetization 2 a low coercivity intermediate unblocking temperature component that is probably carried by magnetite or magnetite and maghemite and 3 a high unblocking temperature component that is probably carried by hematite lithologies are typically dominated by the low coercivity component and single domain to pseudo single domain grain behavior rock magnetic studies suggest biogenic magnetite may contribute to the magnetic remanence of the low coercivity component the melilla basin magnetostratigraphy independently corroborated with new 40 ar 39 ar dating of volcanic ashes and foraminifer biostratigraphy has been correlated to the recently developed geomagnetic polarity time scale of shackleton et al 2 this correlation suggests deposition of the bioclastic platform began approximately at the tortonian messinian boundary which is defined as the base of chron 3ar 3 and continued into chron 3an 2n the fringing reefs span an interval that begins in chron 3an 2n until just above the base of chron 3r lower gilbert the overlying tcc and mixed carbonate and siliciclastic succession correlate to within chron 3r the new magnetostratigraphy is not without minor ambiguities possibly due to either remagnetization within the tcc or the presence of a short duration normal subchron in the lower portion of chron 3r results of this study have important implications for correlating between late miocene shallow marine carbonate sections and basinal sections within the mediterranean basin for improving the understanding of the history of the messinian salinity crisis and for deciphering the roles of eustacy tectonics and regional processes in the development of shallow marine carbonate sequences in the mediterranean region

late paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east tennessee evidence for a post ore chemical event thermal demagnetization studies on samples from mineralized and unmineralized rocks in the lower ordovician knox group of east tennessee reveal a mean direction of d i 157 18 95 7 7 k 52 9 after 60 unfolding combining stable endpoint and remagnetization circle data although not significant at the 95 confidence level the fold test is suggestive of a synfolding alleghenian age of the magnetization the magnetization cannot be postfolding i e permian or younger because the in situ directions do not resemble those for post carboniferous times the magnetization is unlikely to be much older than the folding because the directions with respect to bedding yield a paleopole characteristic of the early permian but unlike poles for pre late carboniferous times characteristic magnetizations from mineralized and unmineralized collapse breccias are in good agreement and fail the conglomerate test the best estimate for the paleopole resulting from this study falls at 40 n 126 e within the late carboniferous segment of the north american paleozoic apparent polar wander path thus the magnetization is interpreted as synfolding and secondary due to widespread kiaman remagnetization blocking temperatures of up to 520 c and the acquisition of isothermal remanent magnetization suggest magnetite as the carrier of the magnetization scanning electron microscopy and energy dispersive analysis of magnetic extracts from limestone samples reveal abundant magnetite spheres secondary dolomites have the same magnetic characteristics as the primary limestones and it is inferred that similar magnetite spheres are the carriers of their magnetization because this magnetization and therefore its carrier occurs in sphalerite and secondary dolomite it must be coeval with or younger than the ore which itself is thought to be of middle to late paleozoic age it is argued that the magnetization was acquired during growth of magnetite associated with a late paleozoic chemical event

biogenic magnetite from surface holocene carbonate sediments great bahama bank magnetic separates and rock magnetic tests from holocene carbonate surface sediments of the bahamas indicate that fine grained magnetite maghemite is common at or near the sediment water interface and forms a major portion of the magnetic fraction magnetite crystals range from 400 to 1000 a in diameter as observed under the transmission electron microscope tem and often occur as elongate chains or multi grain clusters crystals are very similar in dimension and grain habit to those of known biogenic origin mainly bacteria all measured grains are within the single domain stability field for magnetite based on crystal dimensions coercivity distributions are fairly broad 5 100 mt or greater likely the result of partial oxidation of the magnetic minerals no large multidomain grains were observed in the tem study individual grains occur in several shapes in the microscope image mainly hexagonal and cuboidal prismatic widespread occurrence of single domain magnetite in this isolated carbonate setting suggests that biogenic precipitation is a major contributor of early in situ depositional magnetization a contribution of magnetite from authigenic precipitation during burial has not been documented in these near surface environments

the great barrier reef the chronological record from a new borehole abstract a new borehole 210 mbsf meters below sea floor deep drilled in ribbon reef 5 on the great barrier reef off cooktown ne australia reveals a shallowing upwards succession the younger part of which is punctuated by a series of erosion surfaces nine depositional units have been defined by lithological changes and are numbered sequentially from the base of the hole upwards aminostratigraphy magnetostratigraphy radiocarbon dating uranium series dating and modeling together with strontium ratios have been applied in an attempt to establish a chronology of accumulation carbonate deposition began about 770 ka ago in a relatively deep water slope environment and is represented by a series of debris flows lithoclasts within these rocks indicate that older limestones already existed in the area subsequent accretion involved the downslope accumulation of grainstones and wackestones sometimes cross laminated characterized by intervals with abundant rhodoliths and scattered probably reworked corals four units at the base of the hole reflect deposition that probably began during isotope stage 16 and continued through stage 15 from about 770 to about 564 ka unit 5 probably extended to stage 11 about 400 ka and unit 6 to stage 9 330 ka typical reefal associations of corals and calcareous algae were established in this area only above depths of about 100 m in the borehole units 5 4 the succession is apparently unbroken to an erosion surface at 36 mbsf indicating subaerial emergence the lack of evidence of emergence below this surface reflects progressive accretion or progradation or both two younger erosion surfaces define further periods of lowered sea level unit 7 is attributed to deposition during isotope stage 7 but erosion during stage 8 resulted in the preservation of only 8 m of unit 7 limestones unit 8 is correlated with stage 5 125 ka and unit 9 is interpreted as holocene post 7 700 ka the limited thicknesses of units 7 8 and 9 are considered to reflect erosion the progressive shallowing brought the depositional surface within the zone exposed during lowstands and there is no sedimentological evidence that aggradation was restricted by a lack of accommodation

the origin of common dolomite fabrics clues from the pliocene abstract characteristic features common to dolomites of all ages are found in pliocene dolomites from the netherlands antilles these features include 1 pseudomorphic replacement 2 selective replacement 3 fossil moldic porosity 4 cloudy centered clear rimmed dolomite 5 sucrosic dolomite and 6 dolomite cement the development of a particular feature is controlled by 1 mineralogy of the material being replaced 2 the degree of saturation of the bulk solution with respect to the mineral being replaced and 3 the availability of dolomite nucleation sites low mg calcite lmc cement and fossils were often neither replaced by dolomite nor dissolved but aragonite and high mg calcite hmc were both replaced and dissolved the resistence of lmc to dolomitization was due to solutions saturated with respect to lmc and the lack of nucleation sites cryptocrystalline lmc was more readily replaced by dolomite because it provides more nucleation sites cloudy centered clear rimmed dolomite and sucrosic dolomite formed in sediments with abundant lmc and few nucleation sites fossil moldic porosity developed concurrent with pseudomorphic replacement suggesting that the dolomitizing solution was undersaturated with respect to the mineral being replaced dolomite cement commonly lines pores and fills intraparticle pore space dolomite cement occasionally has inclusions of a precursor lmc cement dolomite cement crystals are larger than replacement crystals because there are fewer nucleation sites for the cement crystals

ground penetrating radar stratigraphy and dynamics of megaflood gravel dunes ground penetrating radar was used to elucidate the stratigraphy of late pleistocene gravel dunes in the altai mountains of southern siberia that formed when a lake emptied as a result of ice dam failure survey lines across dunes had a resolution of decimetres with depth penetration of 20 m the reflections identify bounding surfaces and radar facies two classes of unconformities are identified 1 an erosional unconformity at the base of the dunes 2 steeply inclined unconformities that truncate underlying inclined reflections and are downlapped by overlying inclined reflections within the dunes unconformities define six radar facies rf rf 1 basal subhorizontal discordant reflections rf 2 poorly defined discordant reflections rf 3 planar inclined reflections rf 4 sigmoidal inclined reflections rf 5 trough fills rf 6 low angle inclined reflections the basal unconformity represents the flood cut surface across which the dunes migrated the inclined unconformities may be interpreted in two ways 1 erosional surfaces induced by unsteady flow within one flood or 2 erosional surfaces developed by a series of floods reactivating dunes left stranded by previous floods the evidence favours the latter model which is consistent with the occurrence of several dune forming events within the basin the broader implications of the study are considered with respect to investigations of megaflood bedforms worldwide

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

nrm irm s demagnetization plots an aid to the interpretation of natural remanent magnetization af demagnetization of thermoremanent magnetization trm and saturation isothermal remanent magnetization irm s are used to establish diagnostic criteria for primary trm and for secondary magnetization in igneous rocks in applying the approach to the interpretation of natural remanent magnetization nrm the demagnetization of irm s documents the nature of the magnetic phases in the rock while the demagnetization of the nrm gives information about the process of magnetization the ratio of nrm irm s measures the efficiency of the magnetization process

a pennsylvanian paleomagnetic pole from the mineralized late cambrian bonneterre formation southeast missouri one hundred eighty eight oriented samples of the late cambrian bonneterre carbonates of southeastern missouri were taken from viburnum trend mines and local surface outcrops initial natural remanent magnetization directions have south southeasterly declinations and downward inclinations progressive demagnetizations using thermal and af techniques give two directions one is a viscous present day field direction and the other is of reversed polarity with south southeasterly declinations and very shallow inclinations the resulting paleopole at 43 n 126 e is located near the pennsylvanian to early permian segment of the north american apparent polar wander path a negative conglomerate test supports this assignment for the magnetization age magnetic extracts from the mineralized as well as unmineralized zones of the bonneterre contain spherical crystal aggregates of pure magnetite of late diagenetic origin the secondary magnetization resides in the magnetite spheroids and we infer that the spheroids formed prior to sulfide mineralization

a lower ordovician paleomagnetic pole from the oneota dolomite upper mississippi river valley of 149 oriented specimens of the lower ordovician oneota dolomite subjected to stepwise thermal or alternating field demagnetization 50 contained a characteristic shallow east southeasterly or west northwesterly component of magnetization identifiable on orthogonal vector projections for 48 other specimens the demagnetization trajectories defined greats circle paths which intersect close to the characteristic direction the site mean directions for the characteristic component were determined by using both the characteristic directions and great circle data and these yield a formation mean direction obtained from 10 sites with a declination of 105 an inclination of 2 and a precision parameter k of 17 6 95 11 9 the mean of the site virtual poles falls at 10 4 n 166 4 e k 48 0 a95 7 0 which is close to previously published late cambrian paleopoles both polarity groups have predominantly positive inclinations suggesting the presence of an uncleaned downward component whose unblocking temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization we interpret this downward component as a drilling induced remanent magnetization and we hope to have removed its contribution by averaging the dual polarity results multicomponent magnetizations were also present in samples collected from blocks of oneota dolomite incorporated into the overlying new richmond sandstone lower ordovician but only six out of eight samples yielded well defined greats circle paths the best fit intersection of these defines a northerly and steep recent component while the directions of the higher stability components are different for each block due to the small number of samples and imperfect resolution of magnetic directions the conglomerate test is not conclusive nevertheless it suggests that the stable remanence in the oneota dolomite was acquired prior to the deposition of the new richmond sandstone

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

new constraints on the origin of the australian great barrier reef results from an international project of deep coring two new boreholes provide the first direct evidence of the age of the australian great barrier reef an inner shelf sequence total depth 86 m basal age 210 40 ka comprises a dominantly siliciclastic unit thickness 52 86 m overlain by four carbonate units total thickness 0 34 m a shelf edge and slope sequence total depth 210 m reveals three major sections 1 a lower section of resedimented flows deposited on a lower slope 2 a mid section including intervals of corals rhodoliths and calcarenites with low angle graded laminae and 3 an upper section of four shelf margin coral reef units separated by karst surfaces bearing paleosols sr isotope and magnetostratigraphic data indicate that the central great barrier reef is relatively young post bruhnes matuyama boundary time and our best estimate for the onset of reef growth on the outer barrier system is ca 600 280 ka this date suggests that reef initiation may have been related to the onset of full eccentricity dominated glacio eustatic sea level oscillation as inferred from large amplitude saw tooth 100 k y 18o cycles after marine isotope stage 17 rather than to some regional environmental parameter a major question raised by our study is whether reef margins globally display a similar growth history the possibility of a global reef initiation event has important implications for basin to shelf partitioning of caco3 atmospheric carbon dioxide levels and global temperature change during quaternary time

magnetofossil dissolution in a palaeomagnetically unstable deep sea sediment magnetofossils1 the fossil remains of bacterial magneto somes2 are found in various deep sea sediments and have been linked to the preservation of stable natural remanent magnetization in many of them1 3 5 they have also been extracted and identified from lithified carbonates of jurassic5 and precambrian6 age showing that magnetotactic bacteria have been a sedimentary source of fine grained magnetite for much of geological time some clay rich deep sea sediments however do not record a stable remanent magnetization for unknown reasons here we report results from a high resolution transmission electron microscope study on samples of this type collected by the deep sea drilling project dsdp the material contains a complex mixture of single domain magnetic minerals well preserved magnetofossils show the same crystal structures as those found in magnetosomes from recent bacteria7 10 whereas others in these same preparations display a wide range of dissolution corrosion and aggregation effects rock magnetic measurements are consistent with the presence of these alterations in many dsdp sediments including those that preserve reliable palaeomagnetic directions as we were unable to fina authigenic magnetic minerals in our sample and as the magnetic fraction is dominated by well preserved magnetofossils we suggest that the poor preservation of the magnetization is a result of diagenetic interactions between the magnetofossils and the clay minerals in the matrix

the rock magnetic fingerprint of chemical remagnetization in midcontinental paleozoic carbonates supported by nsf ear 90 05075 contribution number 9203 of the institute for rock magnetism the irm is supported by grants from the keck foundation the national science foundation and the university of minnesota

magnetostratigraphic dating of shallow water carbonates from san salvador bahamas magnetostratigraphic results are reported here from a sequence of late neogene quaternary shallow water carbonate sediments from a continuous core drilled on the island of san salvador bahamas on the basis of the remanent magnetism of 136 samples from a 91 m measured section of core the polarity sequence can be correlated with the magnetic polarity time scale from the gilbert chron early pliocene through the late brunhes chron late pleistocene holocene magnetic polarities were determined on the basis of relative up down direction in the unoriented core extraction studies of the magnetic particles reveal the presence of single domain crystals of magnetite resembling those produced by the magnetotactic bacteria and algae the sequence of reversals provides a minimum of six new major chronostratigraphic markers for the pliocene pleistocene of the bahamas it confirms and refines the local timing of both the lithologic change from skeletal to nonskeletal sediments and the disappearance of coral and molluscan species from the bahamas as upper late pliocene between 2 6 and 2 7 ma that the primary magnetic remanence is preserved in shallow water carbonates including replacement dolomites suggests that this technique could be used to date similar tertiary and possibly even older carbonate sequences the establishment of a reliable magnetostratigraphy provides refined dating of shallow water carbonates and regional faunal appearances or disappearances sediment accumulation rates subsidence and depositional events

two types of chemical remanent magnetization during the oxidation of magnetite abstract the chemical remanent magnetization crm m crm resulting from oxidation of equidimensional magnetite to a mixture of 90 hematite and 10 cation deficient magnetite is jointly controlled by the initial anhysteretic remanent magnetization m arm of the parent magnetite and the field h crm applied during oxidation m crm lies in the plane defined by h crm and m arm which were perpendicular in our experiments in an intermediate orientation determined by the relative strengths of h crm 0 05 0 1 and 0 2 mt and m arm 0 002 0 12 saturation remanence the crms are univectorial remanences that remain fixed in these stable intermediate directions throughout the course of stepwise alternating field and thermal demagnetization similar experiments in which acicular single domain cation deficient magnetite was oxidized to maghemite gave quite different results m crm was always parallel to m arm its direction was unaffected by field strengths as high as h crm 1 5 mt the essential factors controlling crm direction seem to be the number and crystallographic structure of oxidation products for single phase oxidation without change in crystal lattice crm remains parallel to the initial remanence of the parent mineral for multiphase oxidation to a mixture of rhombohedral and spinel phases both initial remanence and the field acting during oxidation influence the crm direction which therefore has an intermediate orientation that parallels neither m arm nor h crm

magnetotaxis and acquisition of detrital remanent magnetization by magnetotactic bacteria in natural sediment first experimental results and theory 1 the widespread occurrence of magnetotactic bacteria mtb in several types of marine and freshwater sediment and the role of fossil magnetosomes magnetofossils as main remanent magnetization carriers therein has important paleomagnetic and paleoenvironmental implications despite numerous studies on mtb biology and on magnetofossil preservation in geological records no detailed information is yet available on how magnetotaxis i e the ability to navigate along magnetic field lines is performed in sedimentary environments and on how magnetofossils possibly record the earth magnetic field we provide for the first time experimental evidence for these processes mtb living in sediment are poorly aligned with the geomagnetic field contrary to what is observed in water this can explain the seemingly excessive magnetic moment of most mtb the observed alignment is sufficient for supporting magnetotaxis across the typical thickness of chemical gradients experiments with magnetofossil rich sediment suggest that a natural remanent magnetization nrm is acquired by magnetofossils in the so called benthic mixed layer where natural mtb populations usually occur the acquired nrm is proportional to the applied field at least up to 160 t and its intensity is compatible with values observed in nature for same sediment types therefore if fossil magnetosome chains are not subjected to further alteration by early diagenetic processes they can provide useful relative paleointensities we propose a preliminary model to explain early stages of magnetofossil nrm acquisition as the result of a dynamic equilibrium between magnetic torques and randomizing forces due to sediment mixing

multiple remagnetizations of lower paleozoic limestones from the taconics of vermont the natural remanent magnetizations in some upper cambrian and middle ordovician limestones from the taconics of vermont 44 n 287 e are multicomponent and can be attributed to early paleozoic late paleozoic and cretaceous early tertiary remagnetizations a northwesterly and up magnetization found at 2 sampling localities characterized by maximum unblocking temperatures of about 350 c has directions prior to tilt correction corresponding to ordovician reference poles for north america and is likely to represent a taconic remagnetization a shallow southerly magnetization found at a third locality with unblocking temperatures that range to about 500 c has directions corresponding to a permian kiaman and alleghenian remagnetization common to all 3 sampling localities is a low up to 300 c unblocking temperature overprint similar in direction to cretaceous to paleocene reference poles

constraints and interpretation of 87sr 86sr ratios in cenozoic dolomites replacement dolomitization by seawater has been modeled in order to quantify the sr isotope signature in cenozoic dolomites as a function of precursor mineralogy and 87sr 86sr ratio reaction stoichiometry and 87sr 86sr ratio of the dolomitizing fluids high sr carbonates such as aragonite may introduce a significant precursor memory into an otherwise seawater dominated sr isotope signature if small quantities of seawater per unit volume of precursor carbonate are involved dolomitization of low sr carbonates i e low mg calcite are shown to create an isotopic signature indistinguishable from that of the seawater involved in the reaction therefore by comparison with the sr isotope evolution curve of seawater the 87sr 86sr ratios of the dolomites can be used to record the oldest possible age of dolomitization and the youngest age of deposition the implications for this approach have been applied to data obtained from a dolomitized core from little bahama bank bahamas two periods of dolomitization are recognized one in the early late miocene involving middle miocene or older rocks and a second one around 2 4 ma ago affecting early pliocene carbonates

paleomagnetism and rock magnetism of the mississippian leadville carbonate formation and implications for the age of sub regional dolomitization the mississippian leadville formation of central colorado is dolomitized near the sawatch uplift and hosts numerous pb zn ag mineral deposits paleomagnetic and rock magnetic data from several dolomite types away from centers of tertiary intrusive activity imply a late paleozoic late mississippian age for the dolomitization e g d 148 i 11 k 86 95 8 6 n 3 localities 31 samples in dolomitized units the magnetization is of diagenetic origin and was probably acquired during or soon after dolomitization tertiary thermochemical activity has overprinted dolomites close to intrusive centers data indicate that limestones removed from igneous activity were also remagnetized at the same time

chains clumps and strings magnetofossil taphonomy with ferromagnetic resonance spectroscopy magnetotactic bacteria produce intracellular crystals of magnetite or greigite the properties of which have been shaped by evolution to maximize the magnetic moment per atom of iron intracellular bacterial magnetite therefore possesses traits amenable to detection by physical techniques typically narrow size and shape distributions single domain size and arrangement in linear chains and often crystal elongation past strategies for searching for bacterial magnetofossils using physical techniques have focused on identifying samples containing significant amounts of single domain magnetite or with narrow coercivity distributions searching for additional of traits would however increase the likelihood that candidate magnetofossils are truly of biological origin ferromagnetic resonance spectroscopy fmr is in theory capable of detecting the distinctive magnetic anisotropy produced by chain arrangement and crystal elongation here we present analyses of intact and lysed magnetotactic bacteria dilutions of synthetic magnetite and sedimentary samples of modern carbonates from the great bahama bank oligocene miocene deep sea muds from the south atlantic and pleistocene lacustrine deposits from mono basin california we demonstrate that fmr can distinguish between intact bacterial magnetite chains collapsed chains and linear strings of magnetite formed by physical processes we also show that sediments in which the magnetization is likely carried by bacterial magnetite have fmr spectra resembling those of intact or altered bacterial magnetite chains