dolomitized cells within chert of the permian assistencia formation parana basin brazil abstract dolomitic microscopic structures in the form of microspheres horseshoe shaped objects and thin botryoidal crusts found within microfossiliferous chert within stromatolites of the evaporite bed eb of the permian assistencia formation irati subgroup parana basin brazil have been investigated by means of optical microscopy x ray fluorescence scanning electron microscopy raman spectrometry and energy dispersive x ray spectrometry the microspheres were identified as dolomitized coccoidal cyanobacteria based on similarity in size spheroidal and paired hemispheroidal morphologies and colonial habit to co occurring silicified organic walled cyanobacteria embedded within the same microfabric and rock samples the co occurrence of dolomite pyrite framboids and abundant dispersed carbonaceous material and silicified cells is consistent with a hypersaline depositional environment with abundant cyanobacterial mats and elevated mg2 ca2 ratios and reducing conditions with active anoxic microbial processes near the water bio sediment interface the abundance of extracellular polymeric substances facilitated anoxic microbial processes sulfate reduction providing essential conditions for possible primary microbially induced dolomitization in most of the dolomitized cells dolomite occurs only as an external layer in fully dolomitized cells magnesium is richest in the outermost layer presumably the dolomitization process was favored by the presence of anoxic microbial degraders and negatively charged functional groups at the surface of the cyanobacterial cells botryoidal dolomite rims of silica filled fenestrae formed by a similar process and inherited the botryoidal morphology of the cell as originally lining the fenestrae silicification interrupted the dolomitization of the largely organic biosediment mostly by permineralization but locally by substitution thereby preserving not only dolomitic microspheres but also huge numbers of structurally well preserved organic walled cyanobacteria and portions of microbial mat clearly dolomitization began very early in the microbial mats prior to compaction of the sediment or full obliteration of cellular remains followed very closely by silicification thereby impeding continued degradation and providing a window onto very well preserved permian microbial mats

palaeointensity determinations from historical and holocene basalt lavas in iceland summary palaeomagnetic measurements were made for 11 post glacial basalt lavas in iceland three pre 2 ka lavas seven historical lavas and one present day lavas palaeodirections were easy to determine at most sites but three of the sites seem to have suffered from some block movement at the lava front making determination of palaeodirections difficult reliable site mean palaeodirections were successfully obtained at two of these sites by analysing the remanence components with low blocking temperature but one site was rejected due to low convergence of remanence directions indicating a possible post cooling block movement a total of 45 samples 52 specimens from nine lava flows were selected for palaeointensity experiments using the thellier method based on the thermal stability of remanence and other rock magnetic features together with hysteresis parameters to show single domain to pseudo single domain natures 26 of the 45 samples 29 of the 52 specimens produced successful results subsidiary experiments using shaw s method were also carried out on one specimen from each of eight lava flows and all of these produced successful results combining these thellier and shaw results eight site mean palaeointensities were obtained including one 1984 ad lava giving a specimen level success rate of 56 per cent for the experiments using the thellier method and 100 per cent for those using shaw s method our results together with those of previous studies indicate that variations in palaeointensity and palaeodirection in iceland are basically similar to those in nw europe for the last 2000 yr variations in palaeointensity converted to virtual dipole moment or virtual axial dipole moment are in good agreement with both nw europe and global models some differences in inclination and declination are recognized between iceland and nw europe but overall variation curves are similar and good agreement with the global models is also observed

calcification of cyanobacterial filaments girvanella and the origin of lower paleozoic lime mud comment and reply sedimentology of microbial carbonate is a challenging subject with controversial views and complex links to petrography geochemistry and microbiology pratt 2001 1 presented an intriguing hypothesis that a considerable part of paleozoic carbonate mud may have been derived from disintegrated

evaluation of the domain state corrected multiple specimen absolute palaeointensity protocol a test of historical lavas from iceland summary palaeointensity estimates were made for a suite of five historical lava flows from mt askja iceland with the purpose of testing the newly proposed domain state corrections to the multispecimen parallel differential ptrm palaeointensity method msp where ptrm stands for partial thermoremanent magnetization before beginning the experimental procedure the chemical stability on heating to the determined ptrm induction temperature was assessed some flows were found to be unaltered by heating some displayed inconsistent repeatability while one flow was severely affected by heating to implement the domain state corrections and interpret the data successfully we had to implement two new approaches to the msp method 1 we used vector addition subtraction in our calculations and 2 we introduced a new selection rejection criterion that improves the robustness of the estimates by preferentially removing outliers based on the error analyses of the linear regression this rejection criterion typically rejected one or two points and was found to significantly improve the estimates with the exception of the one flow that displayed significant chemical alteration during the experiment the palaeointensity estimates determined using the original msp protocol yielded estimates within 3 t of the known field of 49 5 t the domain state corrected msp estimates were all within 4 t with no clear relationship between the uncorrected and domain state corrected palaeointensity estimates for this data set

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

sedimentological and stable isotope evolution of lakes in the vestfold hills antarctica the sedimentological chemical and isotopic characteristics of sediment cores from three slightly saline to hypersaline lakes highway ace and organic lakes and two marine inlets ellis fjord and taynaya bay in the vestfold hills antarctica have been examined sections of the cores deposited in marine environments are characterized by uniform regularly laminated fine grained organic rich sediments with uniform organic 13c values 18 0 to 19 4 vs pdb and sulfur contents in contrast sediments deposited in lacustrine environments are extremely heterogeneous varying from finely laminated mat like sequences to poorly sorted clastic rich sediments authigenic monohydrocalcite and aragonite occur in some lake sediments the 13c values of organic matter in the lacustrine sediments exhibit an extremely wide range 10 5 to 25 3 that can be related to variations in physico chemical conditions in the lake waters strongly negative organic 13c values coupled with high sulfur contents are indicative of an anoxic zone in the overlying lake waters whereas less negative organic 13c values coupled with low sulfur contents are indicative of well mixed oxic conditions particularly high organic 13c values result during high levels of microbial activity in the lakes due to high rates of photosynthetic co2 fixation the large shifts in organic 13c are not necessarily accompanied by any change in macroscopic sedimentological characteristics illustrating the utility of isotopic investigations in these environments the 13c composition of authigenic carbonate in hypersaline organic lake sediments provides a record of changes in palaeoproductivity while the 18o of the carbonate provides information on rates of meltwater input and evaporation in the lake 14c dating suggests that highway lake was isolated from the sea by isostatic uplift at least 4600 yr before present b p whereas organic lake was isolated at approximately 2700 yr b p apparent emergence rates calculated from the 14c ages range from 1 0 to 2 1 mm yr 1 the reservoir effect in the lacustrine and marine environments is variable but probably does not exceed 1000 yr in any of the lakes examined

post glacial microbialite formation in coral reefs of the pacific atlantic and indian oceans abstract the occurrence of microbialites in post glacial coral reefs has been interpreted to reflect an ecosystem response to environmental change the greater thickness of microbialites in reefs with a volcanic hinterland compared to thinner microbial crusts in reefs with a non volcanic hinterland led to the suggestion that fertilization of the reefal environment by chemical weathering of volcanic rocks stimulated primary productivity and microbialite formation using a molecular and isotopic approach on reef microbialites from tahiti pacific ocean it was recently shown that sulfate reducing bacteria favored the formation of microbial carbonates to test if similar mechanisms induced microbialite formation in other reefs as well the tahitian microbialites are compared with similar microbialites from coral reefs off vanuatu pacific ocean belize caribbean sea atlantic ocean and the maldives indian ocean in this study the selected study sites cover a wide range of geological settings reflecting variable input and composition of detritus the new lipid biomarker data and stable sulfur isotope results confirm that sulfate reducing bacteria played an intrinsic role in the precipitation of microbial carbonate at all study sites irrespective of the geological setting abundant biomarkers indicative of sulfate reducers include a variety of terminally branched and mid chain branched fatty acids as well as mono o alkyl glycerol ethers isotope evidence for bacterial sulfate reduction is represented by low 34s values of pyrite 43 to 42 enclosed in the microbialites and compared to seawater sulfate slightly elevated 34s and 18o values of carbonate associated sulfate 21 9 to 22 2 and 11 3 to 12 4 respectively microbialite formation took place in anoxic micro environments which presumably developed through the fertilization of the reef environment and the resultant accumulation of organic matter including bacterial extracellular polymeric substances eps coral mucus and marine snow in cavities within the coral framework tof sims analysis reveals that the dark layers of laminated microbialites are enriched in carbohydrates which are common constituents of eps and coral mucus these results support the hypothesis that bacterial degradation of eps and coral mucus within microbial mats favored carbonate precipitation because reefal microbialites formed by similar processes in very different geological settings this comparative study suggests that a volcanic hinterland is not required for microbialite growth yet detrital input derived from the weathering of volcanic rocks appears to be a natural fertilizer being conductive for the growth of microbial mats which fosters the development of particularly abundant and thick microbial crusts

two opposing effects of sulfate reduction on carbonate precipitation in normal marine hypersaline and alkaline environments comment sulfate reduction sr has been linked to carbonate mineral precipitation and has been invoked as a possible mechanism for microbialite formation lyons et al 1984 1 kempe 1990 2 although there is evidence to support the contribution of sr in carbonate mineral precipitation from both

the role of sulphate reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the coorong region south australia abstract the dolomitic distal ephemeral lakes of the coorong region of south australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input during late evaporation active bacterial sulphate reduction occurs in uppermost anoxic lake sediments as rising salinity and then h2s toxicity kills off the macrobiota these hypersaline lakewaters are characterised by high ph and elevated concentrations of carbonate and sulphate ions magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly electrolytic alkaline lake brines from lake margin stromatolites and mats during desiccation and may explain the remarkable increase in mg levels recorded at this time continued evaporation of the lakewaters produces a halite crust overlying the dolomite sulphate free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate reducing bacteria dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters and both the morphology and size distribution of dolomite grains is identical to bacterial cells it is not proven exactly when dolomite precipitation occurs but the submicron sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate which is known to inhibit dolomite formation bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation in adjacent non dolomitic lakes sulphate is present throughout evaporation so that other minerals principally aragonite and hydromagnesite are precipitated

the notch peak contact metamorphic aureole utah petrology of the big horse limestone member of the orr formation the upper cambrian big horse limestone member of the orr formation is contact metamorphosed by the jurassic notch peak quartz monzonite in the central portion of the house range utah two lithologic types were sampled from specific stratigraphic horizons within the member over a lateral distance of 6 km at locations reflecting a range in metamorphic grades the rocks were metamorphosed at 2 kbar on the basis of estimates of stratigraphic overburden at the time of intrusion the lithologies are relatively pure dolomitic limestones and impure argillaceous limestones argillites the progressive metamorphism has resulted in the successive appearance of talc tremolite scapolite diopside and forsterite in the meta limestones and the appearance of biotite tremolite diopside plagioclase scapolite vesuvianite grossular and wollastonite in the argillites mineral assemblages of the limestones compared with isobaric 2 kbar phase equilibria in the cao mgo sio 2 h 2 o co 2 system suggest that 1 fluid buffering by metamorphic reactions has occurred 2 domains of equilibrium are small and 3 the limestones behaved as relatively closed systems during metamorphism maximum temperatures and x co 2 composition consistent with assemblages in the limestone are see pdf for table temperatures estimated by the calcite dolomite geothermometer are consistent with these temperature estimates temperatures estimated from limestone mineral assemblages are used to establish the sequence of prograde reactions in the multi component argillite system compositions of fluids in equilibrium with low grade argillites are poorly constrained but may have reached a maximum x co 2 of 0 75 fluid compositions in equilibrium with medium to high grade argillites were more h 2 o rich than x co 2 0 20 indicated primarily by the presence of wollastonite at temperatures between 475 and 600 c therefore the argillites were either initially more water rich than limestones at the same grade or they were more open to h 2 o rich fluids during metamorphism

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

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

occurrence and formation of dolomite in organic rich continental margin sediments dolomite is presently forming throughout a large part of the oceans in continental margin marine sediments accumulating at rates less than 500 m m y and having organic carbon contents greater than 0 5 wt at higher sedimentation rates the dolomite content of the sediments is greatly diluted at lower organic carbon content the porewater chemistry precludes dolomite formation the reactants for dolomite formation have the following sources magnesium supplied from overlying seawater calcium supplied by the dissolution of calcium carbonate and carbon supplied from carbonate dissolution and organic carbon oxidation the rate of dolomite formation appears to be limited by the rate of calcium carbonate dissolution this rate can be quantitatively calculated from the magnesium diffusional and advective fluxes conversely in ancient sediments the depth and timing of dolomite formation can be calculated from the amount of dolomite and the sedimentation rate in dolomite rich sediments these calculations demonstrate that most of the dolomite formed within a few tens of meters of the sea floor within the zone of microbial sulfate reduction such dolomites have ne ative values of dgr13c dolomite in dolomite poor sediments may form at greater burial depth below the zone of sulfate reduction resulting in positive values of dgr13c the distribution coefficient of strontium in dolomite has been determined to have an average value of 0 06 using this value predicted strontium contents for submarine dolomites range from 150 ppm in dolomite rich sections to 1 290 ppm in dolomite poor sections

microbially mediated carbonate precipitation in a hypersaline lake big pond eleuthera bahamas microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability this can result in the precipitation of carbonate minerals and thus the formation of microbialites here the microbial metabolic activity the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake big pond bahamas were determined to identify the driving forces in microbialite formation a conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented geochemical modelling indicated an oversaturation with respect to carbonates including calcite aragonite and dolomite but these minerals were never observed to precipitate at the mat water interface this failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium a layer of high mg calcite was present 4 to 6 mm below the surface of the mat just beneath the horizons of maximum photosynthesis and aerobic respiration this carbonate layer was associated with the zone of maximum sulphate reduction it is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface i e the cyanobacterial layer of the mat bind calcium both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances each process accounting for approximately half of the total consumption and low molecular weight organic carbon liberating calcium and producing inorganic carbon the combination of these geochemical changes can increase the carbonate saturation index which may result in carbonate precipitation in conclusion the formation and degradation of extracellular polymeric substances as well as sulphate reduction may play a pivotal role in the formation of microbialites both in marine and hypersaline environments

reliable absolute palaeointensities independent of magnetic domain state knowledge of palaeointensity variation is required for determining the full vector variation of the geomagnetic field as a function of geological time this provides essential constraints for numerical geodynamo models to date most palaeointensity determination methods are laborious characterised by rather low success rates and demand substantial processing time the rocks under investigation must obey stringent criteria to yield faithful palaeointensities the magnetic particles must be single domain the natural remanent magnetisation must be a thermoremanent magnetisation and during the successive heating steps in the laboratory no chemical alteration should occur here we describe a new method that allows all magnetic domain states to be processed i e it does not require single domain particles the method makes use of the linearity of partial thermoremanent magnetisation ptrm with the applied laboratory field multiple specimens are used so that every sample is exposed only once to a laboratory field warranting that all samples experienced the same magnetic history through the limited number of thermal steps alteration effects are reduced as well the laboratory ptrm and natural remanent magnetisation nrm of the specimens are oriented parallel to minimise the effects of high temperature tails that affect multidomain minerals the ptrm acquisition temperature is selected below the temperature at which chemical alteration sets in and above the temperature trajectory where secondary viscous nrm components occur the procedure requires a lower number of steps than any other palaeointensity method reducing significantly the total time needed per rock unit we propose to name the new protocol multispecimen parallel differential ptrm method i t provides the correct answer to 5 for artificial samples and natural rocks containing multidomain magnetic particles that were given a laboratory trm of known intensity application to the paricutin september december 1943 lava flow three sites 7 specimens per site yields a weighted mean of 45 9 1 25 t within uncertainty margin of the expected value of 45 0 t

paleomagnetism of the late cambriancrepicephalus aphelaspis trilobite zone boundary in north america divergent poles from isochronous strata paleomagnetic samples from the nolichucky formation late cambrian sampled at two sites in the valley and ridge province of east tennessee yield a possibly penecontemporaneous characteristic magnetization that appears to reside in detrital magnetite the paleomagnetic pole positions are paleozoic but differ site i lat 41 n long 109 e dp 1 dm 2 site ii lat 39 n long 131 e dp 4 dm 7 the difference in poles reflects a significant difference in declination between the site mean directions and this declination difference probably reflects relative tectonic rotation as the sites are in different thrust sheets the paleontologic age of both sections is exceptionally well constrained as they are sampled across an abrupt biomere boundary between contrasting trilobite faunas comparison of these results with paleomagnetic data from coeval strata elsewhere in north america reveals gross discrepancies so that at least some of the published data must reflect remagnetization and or tectonic rotation

remagnetized cratonic cambrian strata from southern nevada stratigraphic sections of cambrian strata in southeastern nevada are akin to thin cratonic facies exposed in the grand canyon their structural setting is much more complicated however from mesozoic and cenozoic tectonism paleomagnetic samples from two sections through these strata appear to have been completely remagnetized coarse grained lightcolored sandstone from the tapeats sandstone yields scattered magnetizations residing in hematite that appear to reflect protracted diagenetic acquisition of remanence a hematitic sandstone in the bright angel shale yields relatively consistent paleozoic directions of magnetization but petrographic study shows that the hematite results from diagenetic oxidation and stratigraphic arguments suggest that the oxidation was not penecontemporaneous gray limestones of the jangle and muav limestones yield a magnetization residing in magnetite that may reflect late tertiary remagnetization being imposed during uplift related to the onset of basin and range deformation in any case this magnetization differs greatly in direction from a hematite magnetization reported from slightly younger muav limestone in the grand canyon both the sampling sites also appear to have been tectonically rotated but whether this rotation is true or a geometric artifact of the tilt correction cannot be determined from the present data these results suggest that paleomagnetic data from rocks as old as cambrian must be scrutinized very carefully before their magnetizations can be accepted as penecontemporaneous and such scrutiny must include geologic data

cryptalgalaminate dolomite of the dunphy formation labrador trough diagenetic and tectono metamorphic evolution related to copper mineralization the strata bound cu fe s mineralization of the lower proterozoic dunphy formation forms the largest known sediment hosted copper deposit of the labrador trough occurring in three thin stratiform dolomitic intervals the sulfides are mainly 1 disseminated in a primary porosity birdseyes cemented by dolospar and 2 vein bound in early tectonic spar filled tension gashes the first of these formed during diagenesis the second during synkinematic metamorphism their parageneses and chemical and sulfur isotopic compositions are similar the history of cementation of the host dolomite of one of the intervals has been traced by cathodoluminoscopy of dolospar sulfide filled birdseyes four concentric rim cements locally disrupted are followed by two to three central cements electron probe microanalysis shows increasing fe and mn in solid solution in these cements from rim to center pointing to a decrease in eh toward the center during cementation corroborated by the presence of semi centrally located

paleomagnetism and geochronology of sills of the doherty mountain area southwestern montana implications for the timing of fold and thrust belt deformation and vertical axis rotations along the southern margin of the helena salient paleomagnetic and 40ar 39ar data from intermediate composition sills exposed along the southern margin of the helena salient in southwestern montana help refine the age of late cretaceous contraction in this part of the north american cordillera the structures in the doherty mountain fold complex deform cambrian through mississippian strata in the hanging wall of the jefferson canyon lombard fault system sills exposed within these strata which are complexly deformed into northeast to northwest plunging anticlines and a northwest plunging syncline give well defined remanent magnetizations and after correction for fold plunge followed by bedding tilt paleomagnetic data from 20 of 21 sites from four folds yield positive fold tests at 100 percent unfolding the magnetizations from these sites are interpreted to be primary remanent magnetizations acquired during sill emplacement prior to fold and thrust deformation although pre folding remagnetization due to widespread latest cretaceous magmatism in the area cannot be ruled out one site yields a magnetization that appears to postdate folding this site has either been remagnetized or is in a distinct intrusive phase emplaced subsequent to folding biotite mineral separates from sills from two folds yield identical 40ar 39ar plateau dates of 77 ma although the group mean directions from sills in each of the folds may represent incomplete samplings of the geomagnetic field and the number of independent readings is low their grand mean direction declination dec 347 inclination inc 66 k 71 9 95 9 n 5 independent observations based on results from 21 sites is similar to the expected late cretaceous reference direction for the study area based on results from the essentially coeval adel mountain volcanics therefore these data suggest that deformation began after 77 ma campanian maestrichtian and that upper plate strata in the lombard thrust sheet of the helena salient in this area have not experienced significant local vertical axis rotation due to thrust movement and impingement with foreland structures along the southwest montana transverse zone the absence of appreciable vertical axis rotations is consistent with data obtained from other paleomagnetic studies in the region

relations between hinterland and foreland shortening sevier orogeny central north american cordillera the tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens the classic central cordilleran foreland fold and thrust belt in the united states late jurassic to early tertiary sevier belt and the more internal zones to the west central nevada thrust belt provide data critical to understanding the developmere of internal and external parts of orogens the garden valley thrust system part of the central nevada thnkst belt crops out in south central nevada within a region generally considered to be the hinterland of the jurassic to eocene sevier thrust belt the thrust system consists of at least four principal thrust plates composed of strata as young as pennsylvanian in age that are unconformably overlain by rocks as old as oligocene suggesting that contraction occurred between those times new u pb dates on intrusions that postdate contraction combined with new paleomagnetic data showing significant tilting of one area prior to intrusion suggest that regionally these thrusts were active before 85 100 ma the thrust faults are characterized by long relatively steeply dipping ramps and associated folds that are broad and open to close upright and overturned although now fragmented by cenozoic crustal extension individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike we correlate the structurally lowest thrust of the garden valley thrust system the golden gate mount irish thrust southward with the gass peak thrust of southern nevada this correlation carries the following regional implications at least some of the slip across jurassic to mid cretaceous foreland thrusts in southern nevada continues northward along the central nevada thrust belt rather than noaheastward into utah this continuation is consistent with age relations which indicate that thrusts in the type sevier belt in central utah are synchronous with or younger than the youngest thrusts in southern nevada this in turn implies that geometrically similar sevier belt thrusts in utah must die out southward before they reach nevada that slip along the southem nevada thrusts is partitioned

remagnetization and tectonic rotation of upper precambrian and lower paleozoic strata from the desert range southern nevada in the desert range of southern nevada miogeoclinal sedimentary rocks mostly shallow water limestones of latest precambrian through early ordovician age yield three components of magnetization having different blocking temperature spectra 1 a low blocking temperature component near the direction of the present axial dipole field 2 an intermediate blocking temperature component with northerly declination and inclination 60 and 3 a characteristic component with southeasterly declination and inclination 20 combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite the intermediate magnetization probably reflects a viscous partial thermoremanent magnetization vptrm imposed between the late cretaceous and mid cenozoic it was probably acquired when the strata were dipping slightly eastward the characteristic magnetization is probably a vptrm imposed during regional uplift in the late permian the degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the ordovician rocks additionally the characteristic magnetization in those rocks is younger than bedding disruption caused by major late stylolitization red purple mudstones from the middle member of the wood canyon formation in which a directionally similar characteristic magnetization resides in fine grained hematite also appear to have been remagnetized in these rocks the remagnetization probably reflects partial recrystallization as the blocking temperatures are too high to have been reset by burial heating the sampled sections have undergone relative tectonic rotation about a vertical axis consistent with late tertiary oroflexural bending that had been proposed on independent geologic evidence the characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations as the regional geology suggests the desert range strata were essentially horizontal throughout the paleozoic the total observed relative rotation is 44 5 representing absolute counterclockwise rotation 27 4 of the northern part of the desert range and absolute clockwise rotation 17 5 of the southern part the only unit in the desert range sequence that may retain a primary magnetization is the late precambrian rainstorm member of the johnnie formation the characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite after correction for 18 of counterclockwise rotation the resulting pole 5 n 151 e is near other late hadrynian poles from north america

the fissure swarm of the askja volcanic system along the divergent plate boundary of n iceland divergent plate boundaries such as the one crossing iceland are characterized by a high density of subparallel volcanic fissures and tectonic fractures collectively termed rift zones or fissure swarms when extending from a specific volcano volcanic fissures and tectonic fractures in the fissure swarms are formed during rifting events when magma intrudes fractures to form dikes and even feeds fissure eruptions we mapped volcanic fissures and tectonic fractures in a part of the divergent plate boundary in northern iceland the study area is 1 800 km2 located within and north of the askja central volcano the style of fractures changes with distance from askja close to askja the swarm is dominated by eruptive fissures the proportion of tectonic fractures gets larger with distance from askja this may indicate that magma pressure is generally higher in dikes close to askja than farther away from it volcanic fissures and tectonic fractures are either oriented away from or concentric with the 3 4 identified calderas in askja the average azimuth of fissures and fractures in the area deviates significantly from the azimuth perpendicular to the direction of plate velocity as this deviation decreases gradually northward we suggest that the effect of the triple junction of the north american eurasian and the hreppar microplate is a likely cause for this deviation shallow tectonic earthquakes in the vicinity of askja are often located in a relatively unfractured area between the fissure swarms of askja and kverkfjoll these earthquakes are associated with strike slip faulting according to fault plane solutions we suggest that the latest magma intrusions into either the askja or the kverkfjoll fissure swarms rotated the maximum stress axis from being vertical to horizontal causing the formation of strike slip faults instead of the dilatational fractures related to the fissure swarms the activity in different parts of the askja fissure swarm is uneven in time and switches between subswarms as shown by a fissure swarm that is exposed in an early holocene lava nw of her ubrei but disappears under a younger 3500 4500 bp lava flow we suggest that the location of inflation centres in askja central volcano controls into which part of the askja fissure swarm a dike propagates the size and amount of fractures in the kollottadyngja lava shield decrease with increasing elevation we suggest that this occurred as the depth to the propagating dike s was greater under central kollottadyngja than under its flanks due to topography

mt pelee martinique may 8 and 20 1902 pyroclastic flows and surges abstract the distribution stratigraphic relationships and fragmental components of the may 8 and 20 1902 pyroclastic flows from mt pelee martinique together with eyewitness accounts suggest the following explanation for those eruptions the eruptions were vertically directed magmatic perhaps initiated phreatically and contained abundant juvenile lithics from congealed magma of the dome and neck this resulted in a two part eruption column having 1 a dense lithic charged part which collapsed into the crater and flowed out of a pre existing notch in its side giving rise to pyrochlastic flows and 2 a magmatically derived column containing gases juvenile vitric material and crystals which largely by passed the neck and dome and escaped into the atmosphere all of the energy of the flows was apparently focused through the notch they emerged fully turbulent and flowed down riviere blanche gravity segregation of large and abundant fragments soon resulted in a dense high concentration poorly fluidized block and ash flow confined to the valley leaving above a fully turbulent high energy ash cloud surge as the ash cloud surge moved down the mountain it continued to expand outward the process of gravity segregation continued as the ash cloud surge expanded resulting in secondary block and ash underflows toward st pierre the secondary block and ash flows developed on a gently sloping upland surface 100 m or more above the valley of riviere blanche the turbulent fragment depleted surges above the secondary block and ash flows maintained sufficient energy to devastate the landscape outward to about 3000 m including st pierre the surges refracted around obstacles and in one place moved up a small valley in a direction opposite to the main flows

the notch peak contact metamorphic aureole utah paleomagnetism of the metasedimentary rocks and the quartz monzonite stock the notch peak pluton a late jurassic granitic stock intrudes early paleozoic miogeoclinal strata in the house range western utah as part of a detailed study to determine textural and chemical changes related to the intrusion a paleomagnetic reconnaissance has been made of the pluton and its contact aureole steep scattered magnetizations with both polarities present are found in the pluton upon alternating field af demagnetization and they may reflect an original thermoremanence in magnetite that was acquired over at least one polarity reversal limestones and argillites in the big horse canyon member of the orr formation late cambrian were sampled at four stratigraphically controlled sites ranging from unaltered country rock to marble the site farthest from the pluton is well behaved upon progressive thermal demagnetization and yields a low inclination reversed paleozoic direction by 400 c the sites nearer the stock generally have a diffusely defined two polarity high inclination magnetization that reflects remagnetization by the pluton this magnetization is removed above 300 c and af demagnetization suggests that it resides in fine grained hematite of these sites one appeared nearly unaltered in thin section whereas the others range from hornfels to skarn the remagnetization resulted from a late oxidation event related to the intrusion perhaps from mobilization of groundwater thus the country rock has been remagnetized chemically rather than thermally these results suggest that paleomagnetism may be a useful tool for detecting altered zones around plutons in addition no consistent high blocking temperature magnetization was found in the aureole rocks the magnetization of the hornfels was easily measurable at higher steps but directions changed erratically and this behavior in part results from the presence of pyrrhotite

microbial dolomite precipitation using sulfate reducing and halophilic bacteria results from qinghai lake tibetan plateau nw china past studies have revealed the important roles of certain types of microbes in precipitating dolomite in coastal hypersaline lakes and lagoons and marine sediments however it is still poorly known if other types of microbes can precipitate dolomite in low salinity lakes without any marine influence the objective of this study was therefore to expand the microbial dolomite precipitation model by investigating the origin of dolomite in the sediments of qinghai lake an inland slightly saline lake salinity 12 5 g l on the tibetan plateau nw china this objective was accomplished via an integrated approach combining geochemical investigations of natural dolomite present in the lake sediments with laboratory precipitation experiments a 5 m sediment core was collected and pore water chemistry was analyzed calculation of saturation index indicated that dolomite was oversaturated in sediment pore water sub spherical grains of dolomite with abundant pores and microstructures similar to microbially precipitated dolomite were found in the sediments suggesting a biogenic origin dolomite precipitation experiments were set up in the laboratory to simulate the natural conditions of the qinghai lake dolomite was precipitated using sulfate reducing bacterium srb desulfotomaculum ruminis and aerobic halophilic bacterium halomonas marina present in the qinghai lake sediments aqueous geochemistry and mineral characterization data collectively suggest that dolomite precipitation was accomplished via increase of ph and alkalinity to overcome the kinetic inhibitors and exopolymeric substances eps and cell walls as possible nucleation sites these results expand the current microbial dolomite precipitation model and have implications for dolomite formation in slightly saline inland lake environments

paleomagnetic and cooling constraints on the duration of the pahute mesa trail ridge eruptive event and associated magmatic evolution black mountain volcanic center southwestern nevada two successive ash flow cooling units of the black mountain volcanic center the pahute mesa and trail ridge members of the late miocene thirsty canyon tuff possess nearly indistinguishable reverse polarity remanent magnetization directions of about 178 declination and 10 inclination these are interpreted to reflect thermoremanent magnetization either during a single geomagnetic field excursion or polarity transition or possibly from a field oriented near the limit of secular variation regardless of the type of geomagnetic field behavior the pahute mesa member was apparently overlain by the trail ridge member before appreciable shift of the anomalous geomagnetic field this strongly suggests that the time between eruption of the two ash flow sheets was very short perhaps less than 100 years a conclusion that is independently supported by the local absence of a complete cooling break between the two units the interval between eruption of the pahute mesa and trail ridge members was too brief for reestablishment of chemical stratification in the black mountain magma system nevertheless a small volume of highly evolved comendite magma developed and was erupted as lava within the black mountain caldera and as a component of the trail ridge member the short repose interval and rapid development of the comendite together with evidence for mixing of magmas of silicic to near mafic composition vogel et al this issue and the presence of air fall tuff at the base of the trail ridge lead us to conclude that eruption of the trail ridge member was probably triggered by injection of hot relatively unevolved magma into the black mountain magma body

thellier paleointensity theory and experiments for multidomain grains 1 we extend theories of thermoremanent magnetization trm and partial trm ptrm in multidomain md grains to model thermal demagnetization and ptrm acquisition steps in thellier paleointensity determination because of the interleaving of zero field and in field heating cooling steps to increasing temperatures the initial state for any step is complex and theoretical modeling is more intricate than for ptrm production or thermal demagnetization separately at low to moderate temperature t trm lost exceeds ptrm regained causing convex down arai plots sagging below the ideal single domain sd line at moderate to high t ptrm acquisition outweighs trm loss as t approaches tcurie ptrm gain exactly equals trm loss and the arai plot becomes ideal when ptrms are produced perpendicular to the original trm and measured directly rather than by differencing field on and field off results there is less deviation from ideality at low to moderate t our theory agrees semiquantitatively with results for parallel and perpendicular ptrms for large md magnetites 135 m smaller md magnetites 6 and 20 m have less curved arai plots and the smallest magnetites 0 6 and 1 m have almost linear plots positive ptrm checks demonstrate that curved arai plots of md grains are reproducible unlike curved plots for rocks that alter physicochemically in the thellier experiment while negative ptrm tail checks indicate undemagnetized ptrm residuals low temperature demagnetization improves linearity only slightly practical applications of this work include using the predicted threshold t below which no net ptrm is produced in a parallel thellier experiment to screen data used for paleointensity fits straight line fits through low and medium t points in arai plots of md 135 m grains overestimated the paleofield by as much as 100 and for small pseudo single domain psd 0 6 and 1 m grains overestimated by about 25 however by using linear segments of medium to high t data with f values 0 5 it may be possible to obtain reasonable paleointensity estimates even for larger psd 6 and 20 m and md grains middle to high t fits for 0 6 m grains gave paleointensities within 4 of the correct value utilizing essentially the entire data set f 0 9 perpendicular data always gave superior linear fits orienting samples with their natural remanent magnetizations perpendicular to the laboratory field is therefore recommended for rocks containing psd and md grains however double heatings are preferable to single heatings because they allow ptrm tail checks to be carried out

microstructure variability in freshwater microbialites pavilion lake canada abstract calcite microbialites in pavilion lake british columbia exhibit a diverse range in macro morphology biomass abundance porosity and mineral content to evaluate the role of microorganisms in their formation samples collected from a range of depths were examined by scanning electron microscopy sem and synchrotron radiation based micro x ray fluorescence xrf spectroscopy to characterize both their outer surfaces as well as internal structures observed trends in both surface colonization as well as microbialite framework with increasing lake depth include decreasing microbial abundance on outer surfaces as well as increasing ratios of carbonate biomass in the microbialites microscopic investigations of the interiors show bacteria and algae entrapped within calcite with this calcite exhibiting micropores and casts similar in size and shape to microorganisms based on these observations it is hypothesized that microbialite development in pavilion lake initiates calcite precipitation in phototrophic microbial mats i e combined phototrophy and heterotrophy followed by heterotrophic oxidation of organic matter leading to eventual carbonate infilling of the microbial mineral matrix in addition an observed shift from cyanobacteria to algae with increasing lake depth suggests variability in contemporary conditions controlling microbialite growth and diagenesis high photosynthetic growth rates at shallower depths result in significant porosity and friability due to biomass accumulation outpacing carbonate precipitation at intermediate depths lower light levels and slower growth rates of phototrophs lead to a greater proportion of the microbialite matrix being in filled by carbonate carbonates precipitate initially within the bacteria eps matrix with abundant uncalcified algae maintaining microbialite porosity in the deepest waters the presence of only sparse algal colonization as well as fine grained laminated metal rich sediments covering microbialites suggests that present day insolation levels are too low to support the development of photosynthetic microbial mats as a consequence heterotrophic carbonate precipitation has progressively in filled these microbialite interiors to create lithified calcite fabrics that exhibit minimal porosity but preserve the casts of microorganisms as biosignatures while the origin of microbialites in pavilion lake remains unknown current observations provide valuable information in evaluating how environmental conditions influence microbialite growth in a freshwater lacustrine environment

depth dependent δ 13 c trends in platform and slope settings of the campbellrand malmani carbonate platform and possible implications for early earth oxygenation highlights carbon cycle of neoarchean carbonate platform and potential oxygen oasis carbon isotopes reveal a shift to aerobic biosphere and increasing oxidation state rare earth element patterns reveal decrease in open ocean water influx rimmed margin architecture was crucial for evolution of aerobic ecosystems abstract the evolution of oxygenic photosynthesis is widely seen as the major biological factor for the profound shift from reducing to slightly oxidizing conditions in earth s atmosphere during the archean proterozoic transition period the delay from the first biogenic production of oxygen and the permanent oxidation of earth s atmosphere during the early paleoproteorozoic great oxidation event goe indicates that significant environmental modifications were necessary for an effective accumulation of metabolically produced oxygen here we report a distinct temporal shift to heavier carbon isotope signatures in lagoonal and intertidal carbonates 13ccarb from 1 6 to 0 2 relative to vpdb and organic matter 13corg from about 40 to 25 relative to vpdb from the 2 58 2 50 gy old shallow marine campbellrand malmani carbonate platform south africa this indicates an increase in the burial rate of organic matter caused by enhanced primary production as well as a change from an anaerobic to an aerobic ecosystem trace element data indicate limited influx of reducing species from deep open ocean water into the platform and an increased supply of nutrients from the continent both supporting primary production and an increasing oxidation state of the platform interior these restricted conditions allowed that the dissolved inorganic carbon dic pool in the platform interior developed differently than the open ocean this is supported by coeval carbonates from the marginal slope setting which had a higher interaction with open ocean water and do not record a comparable shift in 13ccarb throughout the sequence we propose that the emergence of stable shallow water carbonate platforms in the neoarchean provided ideal conditions for the evolution of early aerobic ecosystems which finally led to the full oxidation of earth s atmosphere during the goe

beachrock occurrence characteristics formation mechanisms and impacts beachrocks are hard coastal sedimentary formations consisting of various beach sediments lithified through the precipitation of carbonate cements the objectives of this contribution are to a collate and review information on the reported occurrences characteristics and formation mechanisms of beachrocks and b consider their impacts on the coastal zone the analysis of the available information has shown that a beachrock formation is a global and diachronic phenomenon and b the great majority of beachrocks are found in tropical subtropical and low temperate latitude microtidal coasts the cementing agents of beachrocks are composed predominantly of the metastable carbonate phases high magnesian calcite hmc and aragonite ar appearing in a diverse crystalline morphology it has been suggested that cement precipitation in the coastal environment is controlled by i the physicochemical conditions ii the presence of organic compounds and microbes iii the magnitude and distribution of the wave energy along the coast and iv the textural characteristics of the constituent sediments various theories have been proposed to explain beachrock formation itself linking the phenomenon to either physicochemical or biological processes these theories however do not seem to be of universal validity and acceptance as each is able to explain only some of the reported occurrences the presence of beachrocks appears to affect beach morphodynamics by i locking the beach profile ii modifying the nearshore hydrodynamics iii changing the porous character of the beach and thus its response to wave forcing and iv differential bed erosion at the margins of the beachrock outcrops that can alter significantly the long and particularly the cross shore sediment transport therefore although relict submerged beachrock outcrops may provide some coastal protection by reducing the wave energy impinging onto the coastline modern beachrocks may promote offshore loss of unconsolidated beach sediments and buried beachrock outcropping finally the presence of beachrocks may have also significant ecological impacts as the indigenous mobile substrate fauna and flora of the beach is replaced by hard substrate benthic assemblages which are commonly arranged in hydrodynamically controlled zones

pervasive late paleozoic triassic remagnetization of miogeoclinal carbonate rocks in the basin and range and vicinity sw usa regional results and possible tectonic implications abstract relatively unmetamorphosed paleozoic miogeoclinal carbonate rocks in the basin and range of e nevada sw nevada and adjacent california and w utah yield low inclination magnetizations that reflect pervasive regional remagnetization around the close of the paleozoic the rocks range in age from mid cambrian through pennsylvanian and lie generally in a broad belt between the mid paleozoic roberts mountain thrust and the late cretaceous sevier thrusts most of the magnetizations reside in magnetite but at one site the magnetization is evidently carried by pyrrhotite preliminary rock magnetic data suggest samples with magnetite borne remanences have wasp waisted hysteresis curves typical of remagnetized carbonates the origin of the remagnetization is problematic and probably polygenetic both the permo triassic sonoma orogeny and deformation associated with the ancestral rockies seem too spatially limited but magnetite from smectite destruction seems difficult to reconcile with the great stratigraphic extent of late paleozoic remagnetization unless combined with thermal resetting of the lowermost units a number of sites also appear to have undergone some vertical axis rotation and the sense and magnitude of these rotations are grossly consistent with independent geologic evidence however the probably large age range of the low inclination components complicates their use for resolving tectonic rotations younger intermediate stability components of magnetization probably of cretaceous or cenozoic age are also found in many sites and also probably have multiple origins at sites farther w the late paleozoic component is not found which probably reflects its destruction by later mesozoic or cenozoic heating at sites farther e on and near the colorado plateau gray carbonates yield only cenozoic magnetizations some reddish oxidized carbonates there locally contain a hematite borne magnetization of late paleozoic age however it is probably related to the development of thick continental redbed sequences in overlying strata on the plateau rather than to the remagnetization process es in the miogeocline

calcification of cyanobacterial mats in solar lake sinai pore water samples were obtained from the shallow water part of solar lake sinai where luxurious cyanobacterial mats grow these samples were analyzed for ca sup 2 mg sup 2 sr sup 2 cl sup so sub 4 sup 2 and titration alkalinity ta to determine the role of cyanobacterial growth and degradation on the calcification of the mats the data are modeled thermodynamically to predict mineral pore water equilibria the data support earlier bacterial and sedimentological studies suggesting that the degradation of the cyanobacterial mat via sulfate reduction is of major importance in the calcification process 34 references 1 figure 2 tables

stonewall mountain volcanic center southern nevada stratigraphic structural and facies relations of outflow sheets near vent tuffs and intracaldera units directly south and southeast of stonewall mountain nevada a depression and north facing caldera scarp were formed during and or after eruption of the spearhead member of the late miocene stonewall flat tuff abundant large lithic and juvenile blocks are present in the spearhead member within 0 5 km of this topographic margin but absent elsewhere in the ash flow sheet consistent with eruption from vents in the stonewall mountain area within about 100 000 years comendite tuff of the overlying civet cat canyon member of the stonewall flat tuff buried the depression and associated scarp the civet cat canyon member is traceable continuously to the north from an outflow sheet capping northwestern pahute mesa into near vent tuff on the southeastern flank of stonewall mountain proximal outflow sheet tuff locally exhibits strong rheomorphic disruption and is overlain without a cooling break by surge flow and fall deposits of trachytic composition much of stonewall mountain is composed of welded tuff and megabreccia interpreted as intracaldera tuff of the civet cat canyon member strongly suggesting that the vent area of the member was largely within stonewall mountain welded tuff of trachytic composition comprises an important part of the intracaldera civet cat canyon member which was intruded by dikes and plugs of trachyte and rhyolite juvenile inclusions of basalt dispersed in near vent facies trachyte tuff provide direct evidence for the high level involvement of basaltic magma in the evolution of the highly potassic stonewall mountain center complex discordant compaction foliations and the widespread presence of megabreccia within the intracaldera tuff suggest following foley 1978 cauldron subsidence by piecemeal collapse during eruption of the civet cat canyon member the elevation of intracaldera tuff and intrusions in stonewall mountain above the surrounding ashflow sheet suggests a significant amount of magmatic uplift perhaps involving the emplacement of plugs dikes and small stocks within the intracaldera tuff prism

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

microbialite formation in seawater of increased alkalinity satonda crater lake indonesia abstract the crater lake of the small volcanic island satonda indonesia is unique for its red algal microbial reefs thriving in marine derived water of increased alkalinity the lake is a potential analogue for ancient oceans sustaining microbialites under open marine conditions current reef surfaces are dominated by living red algae covered by non calcified biofilms with scattered cyanobacteria and diatoms minor caco3 precipitates are restricted to the seasonally flooded reef tops which develop biofilms up to 500 m thick dominated by the cyanobacteria pleurocapsa calothrix phormidium and hyella microcrystalline aragonite patches form within the biofilm mucilage and fibrous aragonite cements grow in exopolymer poor spaces such as the inside of dead lysed green algal cells and reef framework voids cementation of lysed hadromerid sponge resting bodies results in the formation of wetheredella like structures hydrochemistry data and model calculations indicate that co2 degassing after seasonal mixis can shift the carbonate equilibrium to cause caco3 precipitation increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium therefore photosynthesis induced cyanobacterial calcification does not occur instead passive diffusion controlled eps mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place in contrast to extreme soda lakes the release of ca2 from aerobic degradation of extracellular polymeric substances does not support caco3 precipitation in satonda because the simultaneously released co2 is insufficiently buffered subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous unidentified mg si phase the microstromatolites probably formed when lake satonda evolved from seawater to ca2 depleted raised alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong co2 degassing the latter effect caused rapid growth of fibrous aragonite while mg si layers replaced the initially mg calcite impregnated biofilms this could be explained by dissolution of siliceous diatoms and sponge spicules at high ph followed by mg calcite dissolution and mg silica precipitation at low ph due to heterotrophic activity within the entombed biofilms

cryptic intertidal microbialites in beachrock heron island great barrier reef implications for the origin of microcrystalline beachrock cement microbialites with microcolumnar growth morphology occur on the surfaces of solution enlarged crypts in modern beachrock an heron island great barrier reef australia the microbialites consist predominantly of felted aragonite but blocky mg calcite and fringes of acicular aragonite also occur a variety of microbial bodies and filaments occur within and on the surface of the microbialites although most heron beachrock is cemented by isopachous fringes of acicular aragonite irregular micritic cement is common adjacent to the microbialites the micritic cement consists predominantly of felted aragonite and contains microbial filaments and organic matter similar to those in the microbialites both the microbialite and the micritic cement are interpreted to be biologically induced although physico chemical models including co2 degassing and evaporation are commonly held to be sufficient to promote the formation of beachrock the similarity of heron cryptic beachrock microbialite with micritic beachrock cements supports the contention that some beachrock cements are biologically induced additionally the primary difference between fringes of acicular aragonite and the felts of aragonite in microbialites appears to be that the former nucleated on a well defined surface of organic matter whereas the latter nucleated within a framework of organic matter that contained irregularly disposed nucleation sites hence aragonite fringe cements may also be localized and or induced by films of organic matter c 1999 elsevier science b v all rights reserved

first evidence of pleistocene ochre production from bacteriogenic iron oxides a case study of the upper palaeolithic site at the san teodoro cave sicily italy abstract the use of iron pigments is well documented in the archaeological horizons of the different parts of the world since the middle pleistocene the mineralogical and chemical composition of the pigments allowed defining in most cases their inorganic origin which were then used after a limited transformation and manipulation the use of a biogenic ochraceous pigment and its manipulation has recently been described in a late holocene archaeological horizon of the american continent here we describe the earliest case of archaeological use of ferrous pigment produced by iron oxidising bacteria feob the first identified in a european epigravettian late upper palaeolithic layer at the san teodoro site in sicily italy samples of the ochraceous archaeological deposit overlying a large burial site were analysed according to current methods of physical analysis and sem highlighting a matrix of bacterial structures and a chemical composition coherent with biogenic productions the physical chemical analysis of the archaeological material from the palaeolithic site and of the modern bacteriogenic iron sediments from two close springs gave consistent results even after heat treatment in absence of terra rossa or other easily available inorganic ferrous materials in the hinterland of their site the hunter gatherers identified several possible water sources rich of pigment used for covering a multiple burial the implications of these results influence interpretations of the ecology of the late glacial and epigravettian sites in europe especially in relation to problems such as the increasing complexity of behaviours the need to establish rituals and the search for materials

multiple specimen absolute paleointensity determination an optimal protocol including ptrm normalization domain state correction and alteration test abstract a recent proposal of a multiple specimen technique promises to be a viable alternative to the classical thellier thellier method of absolute paleointensity determination however to exploit the full potential of the multiple specimen approach a thorough understanding of its theoretical foundation and a detailed experimental verification of its implicit assumptions is required here the validity of the multiple specimen technique is studied on a collection of synthetic samples covering grain sizes ranging from single domain sd over intermediate pseudo single domain psd to multidomain md the experimental data indicate that the multiple specimen method in its present form systematically overestimates paleointensity for intermediate psd to md particle sizes this finding is investigated theoretically by a statistical theory of weak field thermoremanence and quantified by a phenomenological thermoremanence model based on this theoretical framework and on the new experimental evidence an extended version of the multiple specimen technique is designed which is more reliable in the critical domain state range the new measurement scheme improves normalization and quantifies the psd and md overestimate which then can even be corrected for furthermore the proposed measurement scheme includes a thermal repeat measurement to assess the effect of alteration upon the accuracy of the final paleointensity result the new technique is verified experimentally for the synthetic samples investigated

dolomitization of holocene shallow marine deposits mediated by sulfate reduction and methanogenesis in normal salinity seawater northern belize abstract dolomite constitutes an average of 12 of the holocene organic rich sediments over a 15 km2 area of the cangrejo shoals mudbank in northern belize although it defines a laterally persistent stratiform body that averages 3 m thick it is present throughout the 7 6 m thick sediment section these transgressive sediments are less than 6400 years old and were deposited in shallow marine environments of normal salinity the dolomite is dominantly cement and average crystal size is 7 m there are no significant correlations among amount of dolomite vs sediment texture mineralogy porosity or mole mgco3 in associated particulate high mg calcite depth or location on the shoals the dolomites are poorly ordered and calcic 39 5 44 5 mole mgco3 with low mean mn 210 ppm and relatively high mean sr 1034 ppm concentrations there is no evidence of recrystallization or geochemical alteration of the dolomite 18o values of the dolomites range from 0 5 to 2 8o oopdb and the mean value 2 1o oo suggests that the dolomite precipitated from normal salinity pore water dolomite 13c values range from 5 2o oo to 11 6o oopdb mean seawater 13c 0 5o oo which suggests dolomitization promoted by both bacterial sulfate reduction and methanogenesis in environments with anoxic pore water dolomitization attending these organodiagenetic reactions apparently was reversible over time and episodic rather than continual precipitation is indicated requisite mg and ca were provided by seawater and by some dissolution of host sediments the most rapid period of dolomitization may have been during early transgression when relatively high sedimentation rates sustained high levels of organodiagenesis and pore water alkalinities

application of the multispecimen palaeointensity method to pleistocene lava flows from the trans mexican volcanic belt abstract despite the many advances recently made in palaeointensity pi studies more high quality pi data are required for a better understanding of the full vector evolution of the ancient geomagnetic field in this study we applied the recently proposed multispecimen parallel differential ptrm ms method for absolute pi determination to rocks older than those studied hitherto with this method fifty one lava flows from within the trans mexican volcanic belt which are generally younger than 1 ma returned 32 new pi estimates rock magnetic investigations revealed that in most cases the remanence was carried by ti poor titanomagnetites of pseudo single domain magnetic structure although titanomagnetites with higher ti contents and additional contributions of titano haematite were also occasionally present comparisons with reconstructions of the global dipole moment reveal that the pi estimates from this study were on average around 30 higher than expected other mainly thellier type pi data from mexico were also observed to be high relative to global records which could indicate that persistent non dipole features might be responsible for the higher than expected results however the paucity of available data obscures the significance of this observation and the balance of evidence rather suggests an artificial biasing of most measurements towards high values our results seem to corroborate results from previous studies on historical lava flows and synthetic samples in which domain state effects were found to cause overestimates of the pi by up to 30 in the ms method we expect that the degree of the overestimate in the majority of these new ms results is no larger than what might be expected from thellier experiments performed on samples with a similar given degree of multidomain behaviour however seven of the lava flows which were studied here using the ms method were subjected to a preceding demagnetisation step of 200 c in order to erase viscous remanences and these may have produced results which were further biased towards higher intensities this additional bias is expected from theoretical considerations and may represent a challenge to future application of the ms method to older rock units it could however potentially be avoided in future studies by performing measurements of the sample moment at temperatures higher than the blocking temperature of any overprint

carbonate and silicate biomineralization in a hypersaline microbial mat mesaieed sabkha qatar roles of bacteria extracellular polymeric substances and viruses in a modern peritidal microbial mat from qatar both biomediated carbonates and mg rich clay minerals palygorskite were identified the mat ca 5 cm thick shows a clear lamination reflecting different microbial communities the initial precipitates within the top millimetres of the mat are composed of ca mg si al s amorphous nanoparticles few tens of nanometres that replace the ultrastructure of extracellular polymeric substances the extracellular polymeric substances are enriched in the same cations and act as a substrate for mineral nucleation successively crystallites of palygorskite fibres associated with carbonate nanocrystals develop commonly surrounding bacterial bodies micron sized crystals of low mg calcite are the most common precipitates together with subordinate aragonite very high mg calcite dolomite and ankerite pyrite nanocrystals and framboids are present in the deeper layers of the mat calcite crystallites form conical structures circular to triangular hexagonal in cross section evolving to crystals with rhombohedral terminations some crystallite bundles develop into dumb bell and stellate forms spheroidal organo mineral structures are also common within the mat nanospheres a few tens of nanometres in diameter occur attached to coccoid bacteria and within their cells these are interpreted as permineralized viruses and could be significant as nuclei for crystallite crystal precipitation microspheres 1 to 10 m in diameter result from intracellular permineralization within bacteria or the mineralization of the bacteria themselves carbonates and clay minerals are commonly aggregated to form peloids tens of microns in size surrounded by residual organic matter magnesium silicate and carbonate precipitation are likely to have been driven by ph saturation index redox changes within the mat related to microenvironmental chemical changes induced by the microbes extracellular polymeric substances viruses and their degradation

microscale observations of sulfate reduction correlation of microbial activity with lithified micritic laminae in modern marine stromatolites we report for the first time micrometer scale correlation of geologic and microbial processes in modern marine stromatolites precipitation of micritic laminae in these stromatolites was studied by comparing microstructure as observed in petrographic thin sections with microbial sulfatereduction activity two dimensional mapping of sulfate reduction rates was implemented by incubating a vertical section of a stromatolite face on silver foil coated with 35 so 4 2 our results show that sulfate reduction activity is high in zones of caco 3 precipitation and indicate that microbial activity produces lithified micritic laminae near the surface of the stromatolites similarities with micritic laminae in ancient stromatolites suggest that sulfate reduction may also have been an important mechanism of carbonate precipitation in these fossilized structures

testing the independence of partial thermoremanent magnetizations of single domain and multidomain grains implications for paleointensity determination 1 the thellier method of paleointensity determination requires that partial thermoremanent magnetizations ptrms be additive mutually independent and reciprocal in their thermal blocking and unblocking we tested the independence law by thermally demagnetizing a sum of orthogonal ptrms produced by replacing part of an original trm tc t0 h by ptrm t t0 h with h rotated 90 between the trm and the ptrm tc curie temperature t0 room temperature the composite remanence simulates thermal remagnetization in nature for single domain grains thermal demagnetization resolved the orthogonal ptrms cleanly two remagnetization temperatures were tried t 400 c and 550 c the latter gave a better test because the two ptrms were almost equal directions were recovered to within 5 for three of the four ptrms trajectories on vector diagrams intersected at exactly 400 c or 550 c and the arai plots were linear and yielded the correct field intensity for both ptrms however multidomain magnetite violated the independence law a remagnetization temperature t 400 c produced almost equal ptrms but in thermal cleaning because of overlap of their unblocking spectra they no longer appeared orthogonal each was displaced 13 15 toward the direction of the other as judged by quasi linear segments on a vector diagram the angle between the fields that produced the ptrms thus appeared to be 60 not 90 arai plots for both ptrms were strongly curved and yielded no usable paleointensity estimate phenomenological modeling correctly predicted the main results of the independence experiments

the trace metal geochemistry of pore water brines from two hypersaline lakes abstract pore water brines from modern hypersaline lake sediments have been analyzed to determine the nature of brine trace metal interaction in these environments brine samples were collected from the surface waters and sedimentary pore waters of two hypersaline lakes solar lake eastern sinai and goto meer bonaire netherlands antilles metal concentrations in the oxic brines appear to be related to the strength of their respective chloride complexes with pb and zn more enriched than cu pore water cu zn and fe concentrations increased across the sediment water interface in both lakes all three metal profiles follow a similar pattern with depth in the pore water higher concentrations were observed in the goto meer brines strontium isotopes were utilized to qualify better the mixing of the brines in solar lake although these metals were observed at higher concentrations than sea water in these sulfidic brines their concentrations are not close to what is needed for the formation of ore deposits

origin and evolution of major salts in the darling pans western cape south africa abstract sediment and water samples from 12 saline pans on the semi arid west coast of south africa were analysed to determine the origin of salts and geochemical evolution of water in the pans pans in the area can be subdivided into large gypsiferous coastal pans with 79 150 g kg total dissolved salt tds small inland brackish to saline 2 64 g kg tds pans and small inland brine 168 531 g kg tds pans that have a layer of black sulphidic mud below a halite crust the salinity of coastal pan waters varies with the seasonal influx of dilute runoff and dissolution of relict pleistocene marine evaporite deposits in contrast inland pans are local topographic depressions bordered on the north by downslope lunette dunes where solutes are concentrated by evaporation of runoff throughflow and groundwater seepage the composition of runoff and seepage inflow waters is determined by modification of coastal rainfall by weathering calcite precipitation and ion exchange reactions in the predominantly granitic catchment soils evaporation of pan waters leads to precipitation of calcite mg calcite dolomite gypsum and halite in a distinct stratigraphic succession in pan sediments bicarbonate limits carbonate precipitation ca limits gypsum precipitation and na limits halite precipitation dolomitisation of calcite is enhanced by the high mg ca ratio of brine pan waters brine pan waters evolve seasonally from na cl dominated brines in the wet winter months to mg cl dominated brines in the dry summer months when 5 20 cm thick halite crusts cover pan surfaces pan formation was probably initiated during a drier climate period in the early holocene more recent replacement of natural vegetation by cultivated land may have accelerated salt accumulation in the pans

paleomagnetism of the notch peak contact metamorphic aureole revisited pyrrhotite from magnetite pyrite under submetamorphic conditions 1 on the basis of new demagnetization and rock magnetic experiments a scattered two polarity remagnetization extending throughout the contact metamorphic aureole surrounding the jurassic notch peak granitic stock in western utah is now thought to reside in metamorphic pyrrhotite rather than in fine grained hematite in part the pyrrhotite formed by in situ reduction of magnetite by pyrite and organic compounds it could not have been formed by the large scale introduction of reducing fluids as isotopic evidence indicates that these metalimestones remained impermeable at higher grades some pyrrhotite probably also formed by the reaction of pyrite and fe bearing silicates as magnetite destruction and pyrrhotite formation extends into rocks that are not obviously metamorphosed the reactions responsible could proceed at very low less than 200 c temperatures and the feasibility of such low temperature reactions in a closed system is shown by detailed thermodynamic calculations this study demonstrates that under reducing conditions in the presence of sufficient pyrite pyrrhotite can form at the expense of magnetite under very mild conditions indeed magnetite may be destroyed merely by deep burial such a mechanism may help explain the paucity of primary magnetite in ancient sedimentary rocks particularly since the single domain and pseudo single domain grains capable of retaining a stable paleomagnetic signature over geologic time are extremely fine grained and hence particularly vulnerable to destruction

paleomagnetic directional dispersion produced by plastic deformation in a thick miocene welded tuff southern nevada implications for welding temperatures two voluminous rhyolitic to quartz latitic ash flow sheets the tiva canyon and topopah spring members of the paintbrush tuff were erupted from the miocene claim canyon cauldron in southern nevada although lithologically similar these units differ greatly in their recording of the ancient geomagnetic field the reversely magnetized tiva canyon member yields remanent directions that are well grouped both within and between sites the normally magnetized topopah spring member however yields remanent directions that vary both laterally and vertically remanent directions from a 100 m thick section of nonwelded to partially welded topopah spring are well grouped precision parameter k 200 and give a mean direction d 293 i 69 that is probably a good approximation of the geomagnetic field at the time of emplacement and cooling in three 300 m thick sections of more densely welded tuff inclinations from the upper parts of the topopah spring are steep 70 but flatten with depth to about 20 and then steepen abruptly near the base of the unit in general the directional dispersion increases with the thickness of the section k 30 for sections that are about 300 m thick much of the directional variation in the topopah spring can be attributed to an inclination error produced by subblocking temperature rotation of the magnetic carriers during welding the possibility of subblocking temperature deformation in an ash flow sheet is enhanced by 1 high blocking temperature spectra of the primary magnetic phases 2 relatively low emplacement temperatures and 3 great stratigraphic thickness thermal demagnetization results from the topopah spring member indicate that welding may have continued to temperatures as low as 475 c in the basal vitrophyre of the ash flow sheet remanent directions from the tiva canyon member were unaffected by the welding process because this unit possesses significantly lower blocking temperatures and does not attain the great thicknesses of the topopah spring

eruptive history of the rhyolitic kane springs wash volcanic center nevada the 19 13 km kane springs wash caldera formed 14 ma ago following eruption of the three youngest members of the kane wash tuff two older cooling units of the kane wash tuff are dated 15 7 0 2 ma and 14 7 0 2 ma and apparently erupted from source areas west of the exposed caldera volcanic eruptions from the kane springs wash center began about 14 ma ago with extrusion of trachyte lavas now preserved north of the caldera member v1 of the kane wash tuff overlies the trachytes and is zoned from rhyolite to trachyte in contrast overlying members v2 and v3 are slightly peralkaline rhyolites comendite eruption of these tuffs 14 1 0 2 ma ago initiated caldera collapse following collapse trachyte lavas indistinguishable in age from the ash flow tuffs erupted within the caldera forming a 5 m diameter cumulodome whose interior portions crystallized to syenite this magmatic resurgence formed an extrusive complex within the caldera rather than doming the cauldron block as in classic resurgent calderas metaluminous ferroedenite rhyolite lavas and associated ash flow tuffs then erupted in the moat surrounding the central trachyte syenite complex pyroclastic deposits from the domes were covered by trachyandesite and basalt lava flows about 13 4 ma ago biotiterhyolite domes dated 13 4 0 2 ma continued to erupt in the northeastern moat and appear to be the last silicic eruptions from the kane springs wash magma system a 13 4 ma metaluminous biotite rhyolite dome containing vapor phase topaz overlies the trachyandesite lavas and appears to be the youngest biotite rhyolite dome these intracaldera units filled the moat resulting in a caldera filled with its own eruptive products local basalt flows covered both intracaldera units and outflow sheets between 12 7 and 11 5 ma ago prior to basin and range faulting the dominant volume of the system was trachytic as trachyte lavas erupted both prior to and following caldera collapse and trachyte inclusions occur as scoriaceous bombs at the top of member v2 fractionation of this trachyte magma produced the slightly peralkaline rhyolitic magma that erupted as the kane wash tuff the shift from an anhydrous mineral assemblage in the tuffs to hydrous assemblages in the later intracaldera rhyolites shows that the system was cooling and was possibly more hydrous in its late stages petrographic characteristics of trachyandesite lavas erupted late in the history of the center suggest that they represent basaltic magmas contaminated with silicic material

paleomagnetism of tertiary tuffs from the lava dam southern house range utah at lava dam utah the tunnel spring tuff yields a reversed polarity characteristic magnetization two units within the tuff yield similar but distinct directions of magnetization lower d 156 i 73 95 2 upper d 137 i 73 95 2 no bedding corrections applied if this difference reflects paleosecular variation it suggests that the two units reflect different eruptions perhaps a few tens of years apart the cottonwood wash tuff member d 348 i 50 95 2 and the wah wah springs tuff member d 344 i 47 95 3 of the needles range formation yield normal polarity magnetizations the polarity of the cottonwood wash is consistent with the results of other workers but the wah wah springs has been reported to have reversed polarity whether the normal polarity at lava dam is due to pervasive remagnetization of that section or to miscorrelation of the wah wah springs cannot be determined from the present data but miscorrelation seems more likely

late proterozoic glacial carbonates in northeast spitsbergen new insights into the carbonate tillite association carbonate rich glacial deposits from two discrete vendian glacial periods are described the older is represented by the 24 40 m thick petrovbreen member e2 of the elbobreen formation which contains abundant detrital dolomite clasts in e2 and their possible source rocks have positive 13 c and negative 18 o pdb values in contrast the carbonate mud fraction of e2 sediments has different cathodoluminescence characteristics from clasts slightly negative 13 c values and higher fe and mn concentrations than clasts oxygen isotopes vary from 2 5 to 4 5 pdb thought to be related to various seawater meltwater mixtures in the depositional environment preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy ostwald s ripening and coinciding with sulphate reduction the younger glaciation is represented by the wilsonbreen formation 170 m which has a distinctive glaciolacustrine middle carbonate member w2 w2 contains precipitated periglacial carbonates with high mn fe ratio both limestone rhythmitic and stromatolitic and dolostone rhythmites stromatolites and dolomite rich sandstones which are compared with carbonates in modern antarctic lakes evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values up to 10 5 pdb high na concentration and evidence for dissolved evaporites high mn concentration in detrital dolostones in w2 is suggestive of syn sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones the carbonate tillite association ultimately arises from the erosion of underlying carbonates which originated in this case under radically different climatic conditions glacial depositional waters then became carbonate saturated as a result of dissolution of detrital carbonate massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis in glacial lakes carbonate precipitated in response to evaporation or photosynthesis carbonate precipitation as the result of seawater freezing or in warm interglacial conditions is not yet established oxygen isotope value are inconsistent too heavy with the presence of high latitude meltwaters implying that glaciation extended to low latitudes as proposed by harland

paleomagnetic evidence for the timing of collapse and resurgence of the lake city caldera san juan mountains colorado rocks of the 23 1 m y old lake city caldera consist of the compositionally zoned sunshine peak tuff postcollapse intracaldera lava flows and resurgent quartz syenite intrusions declinations of reversely magnetized i 45 to 75 sunshine peak tuff change from easterly d 93 130 throughout most of the tuff to southerly d 195 207 within the late eruptive phases the postcollapse lava flows are also reversely magnetized and their emplacement appears to have spanned resurgence of the caldera in contrast the uppermost levels of the resurgent intrusion record two distinct components of magnetization a reversed component isolated by thermal demagnetization typically at temperatures greater than 400 c and a normal component defined by thermal demagnetization at lower temperatures a baked contact test implies that emplacement and initial cooling of the intrusion occurred during a time of reversed polarity and that continued cooling at lower blocking temperatures took place during normal polarity assuming that the reversed directions of the extrusive and intrusive rocks record a single period of reversed polarity as suggested by the paleomagnetic and radiometric age data the time for caldera development from eruption of the ash flow tuffs to emplacement of the resurgent intrusion was less than about 300 000 years on the basis of best estimates of durations of reversed polarities in the early miocene the estimated 300 000 year time span for the development of the lake city caldera is very close in duration to the radiometrically determined time spans of much younger pleistocene calderas having similar dimensions and similar volumes of erupted material

evaluation of the multispecimen parallel differential ptrm method a test on historical lavas from iceland and mexico summary obtaining reliable geomagnetic palaeointensity pi estimates is still a difficult objective most common techniques in pi studies are based on various modifications of the thellier method but high failure rates and large uncertainties are common furthermore only rocks which obey stringent criteria are assumed to yield faithful pi estimates in general magnetic particles must be of single domain size the magnetization must be a thermoremanent magnetization and chemical alteration of magnetic carriers must be avoided during the laboratory experiment recently a new method for obtaining pis was proposed which on the grounds of results from two historic lavas and three simulated experiments is assumed to be independent of magnetic domain state this method uses multiple samples subjected to only one heating step at moderate temperatures here we present a test of the multispecimen parallel differential ptrm method on 11 historical lavas from iceland and mexico from which the actual field intensity is known either from magnetic observatory data or from magnetic field models our results show that the majority of pi estimates after the multispecimen parallel differential ptrm method yielded results that are very close or indistinguishable within the range of error from the expected intensity and thus largely confirm the findings from the initial study however there is in general a small overestimate which we show is associated with multidomain material when compared to a thellier type pi experiment carried out on sister samples from five icelandic lava flows the average multispecimen pi is closer to the actual field intensity samples from one lava characterized by low curie temperatures failed the thellier approach due to laboratory induced alteration but here the multispecimen method returned a reasonable pi even though there is evidence that the method is not entirely independent of magnetic domain state as previously proposed we regard this method as a valuable alternative in future pi studies

origins of carbonate in neoproterozoic stromatolites and the identification of modern analogues abstract the viewpoint in this paper is that the distinction of detrital from in situ precipitated carbonate in stromatolites is vital so that the appropriate modern analogues can be sought reviewing literature on modern stromatolites it is clear that marine stromatolites are dominated by detrital sediment although diagenetic processes can obscure the detrital nature of carbonate in mats in low salinity conditions carbonate is dominantly of precipitate origin being triggered by photosynthesis in mats in saline conditions petrographically similar precipitates occur apparently triggered variously by photosynthesis evaporation and organic matter degradation in different cases light carbon isotopes in the carbonate are a good pointer to organic degradation as a carbon source a review of criteria proposed for identifying the origin of carbonate in precambrian stromatolites shows that in different cases reliable evidence both for sediment incorporation and carbonate precipitation has been obtained although many arguments are equivocal the utility of luminescence petrography for positive identification of detritus and carbon isotope data for identification of carbon sources is outlined new petrographic and carbon isotope data are presented from four units canyon formation of east greenland draken and wilsonbreen formations of spitsbergen and atar group mauritania although environmentally diverse each displays evidence for both sediment incorporation and localized carbonate precipitation although in widely varying proportions it is recommended that luminescence studies be carried out for stromatolite microstructural work carbon isotope data indicate no distinction between early inorganic cements stromatolitic and non stromatolitic dolo micrites and indicate a common origin related to water column and microbial mat photosynthesis variably accompanied by evaporation

modern cyanobacterial analogs of paleozoic stromatoporoids recent and subfossil calcareous structures resembling cystose and subclathrate paleozoic stromatoporoids have been discovered in a sea linked stratified alkaline crater lake on satonda island indonesia the structures are produced by mats of coccoid cyanobacteria growing along the lakeshore from the water surface down to the o2 h2s interface located at a depth of 22 8 meters calcification of the mats is controlled by seasonal changes in calcium carbonate supersaturation in the epilimnion the internally complex structures are a product of two different calcification processes i periodic in vivo calcification of the surficial cyanobacterial layers by low mg calcite and ii early postmortem calcification of the cyanobacterial aggregates below the mat surface by microbially precipitated aragonite the finding supports the idea that paleozoic stromatoporoids represent fossilized cyanobacteria stromatolites it also implies that the stromatoporoid generating epicontinental seas during the early paleozoic may have been more alkaline and had a higher carbonate mineral supersaturation than modern seawater

thermoremanent magnetization of nonuniformly magnetized grains a simple and elegant interpretation of thermoremanent magnetization trm in uniformly magnetized single domain sd grains was given by neel 50 years ago but the trm acquisition processes in larger nonuniformly magnetized grains are more varied and difficult to describe theoretically sd trm is a frozen high temperature partition between two microstates spins parallel or antiparallel to an applied magnetic field nonuniformly magnetized grains have a much greater choice of microstates local energy minimum or lem states and partitioning among various lem states continues to change during cooling these changes may involve barkhausen jumps of domain walls between positions of minimum local energy or nucleation of new domains and walls because of the lower remanence capacity of nonuniform microstates compared to the uniform sd state trm intensity decreases as grain size increases although certain microstates e g single vortex states seem to contribute little to trm thermal demagnetization of trm begins just above room temperature and continues to the curie point quite unlike the sharp unblocking of sd trm this continuous demagnetization resulting from changes in microstates driven by the changing internal demagnetizing field during heating profoundly affects the separation of different components of natural remanent magnetization and the determination of paleomagnetic field intensity

carbonate stromatolites from a messinian hypersaline setting in the caltanissetta basin sicily petrographic evidence of microbial activity and related stable isotope and rare earth element signatures lower messinian stromatolites of the calcare di base formation at sutera in sicily record periods of low sea level strong evaporation and elevated salinity thought to be associated with the onset of the messinian salinity crisis overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions occasionally influenced by an influx of meteoric water evidence of bacterial involvement in carbonate formation is recorded in three dolomite rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre scale lamination and small fenestrae the dominant microfabrics are i peloidal and clotted dolomicrite with calcite filled fenestrae ii dolomicrite with bacterium like filaments and pores partially filled by calcite or black amorphous matter and iii micrite in which fenestrae alternate with dark thin wispy micrite the filaments resemble beggiatoa like sulphur bacteria under scanning electron microscopy the filaments consist of spherical aggregates of dolomite interpreted to result from calcification of bacterial microcolonies the dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces biofilm like remains occur in voids between the filaments the dolomite consistently shows negative d 13 c values down to 11ae3 and very positive d 18 o mean value 7ae9 that suggest formation as primary precipitate with a substantial contribution of organic co2 very negative d 13 c values down to 31ae6 of early diagenetic calcite associated with the dolomite suggest contribution of co2 originating by anaerobic methane oxidation the shale normalized rare earth element patterns of sutera stromatolites show features similar to those in present day microbial mats with enrichment in light rare earth elements and m type tetrad effects enrichment around pr coupled to a decline around nd and a peak around sm and eu taken together the petrography and geochemistry of the sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation