environmental perturbations caused by the impacts of asteroids and comets we review the major impact associated mechanisms proposed to cause extinctions at the cretaceous tertiary geological boundary we then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions but we also stress that there are many uncertainties we conclude that impacts with energies less than about 10 mt are a negligible hazard for impacts with energies above 10 mt and below about 104 mt i e impact frequencies less than one in 6 104 years corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m respectively blast damage earthquakes and fires should be important on a scale of 104 or 105 km which corresponds to the area damaged in many natural disasters of recent history however tsunami excited by marine impacts could be more damaging flooding a kilometer of coastal plain over entire ocean basins in the energy range of 104 105 mt intervals up to 3 105 years corresponding to comets and asteroids with diameters up to 850 m and 1 4 km respectively water vapor injections and ozone loss become significant on the global scale in our nominal model such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects but if a higher fraction of pulverized rock than we think likely reaches the stratosphere stratospheric dust causing global cooling would also be important in this energy range thus 105 mt is a lower limit where damage might occur beyond the experience of human history the energy range from 105 to 106 mt intervals up to 2 106 years corresponding to comets and asteroids up to 1 8 and 3 km diameter is transitional between regional and global effects stratospheric dust sulfates released from within impacting asteroids and soot from extensive wild fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10 moreover the ejecta plumes of these impacts may produce enough no from shock heated air to destroy the ozone shield between 106 and 107 mt intervals up to 1 5 107 years corresponding to comets and asteroids up to 4 and 6 5 km diameter dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 107 km and the resulting smoke would reduce light levels even further at energies above 107 mt blast and earthquake damage reach the regional scale 106 km tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the world s ocean basins fires would be set globally light levels may drop so low from the smoke dust and sulfate as to make vision impossible at energies approaching 109 mt 108 years the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids the cretaceous tertiary impact in particular struck evaporate substrates that very likely generated a dense widespread sulfate aerosol layer with consequent climatic effects the combination of all of these physical effects would surely represent a devastating stress on the global biosphere

high noble metal concentrations in a late pliocene sediment a 2 3 myr old layer in a sediment from the antarctic ocean contains ir and au at levels comparable with those at the cretaceous tertiary boundary a sizable fraction of the noble metals is contained in vesicular millimetre sized poly mineralic grains that closely resemble ablation debris from chondritic meteorites and there is little doubt that the noble metals resulted from the accretion of a large extraterrestrial object no massive extinctions or other evidence of environmental stress seem to be associated with this accretionary event

composition of impact melt debris from the eltanin impact strewn field bellingshausen sea abstract the impact of the km sized eltanin asteroid into the bellingshausen sea produced mm to cm sized vesicular impact melt rock particles found in sediment cores across a large area of the ocean floor these particles are composed mainly of olivine and glass with minor chromite and traces of nife sulfides some particles have inclusions of unmelted mineral and rock fragments from the precursor asteroid although all samples of melt rock examined have experienced significant alteration since their deposition in the late pliocene a significant portion of these particles have interiors that remain pristine and can be used to estimate the bulk composition of the impact melt the bulk composition of the melt rock particles is similar to the composition of basaltic meteorites such as howardites or mesosiderite silicates with a contribution from seawater salts and a siderophile rich component there is no evidence that the eltanin impact melt contains a significant terrestrial silicate component that might have been incorporated by mixing of the projectile with oceanic crust if terrestrial silicates were incorporated into the melt then their contribution must be much

magnetofossil spike during the paleocene eocene thermal maximum ferromagnetic resonance rock magnetic and electron microscopy evidence from ancora new jersey united states previous workers identified a magnetically anomalous clay layer deposited on the northern united states atlantic coastal plain during the paleocene eocene thermal maximum petm the finding inspired the highly controversial hypothesis that a cometary impact triggered the petm here we present ferromagnetic resonance fmr isothermal and anhysteretic remanent magnetization first order reversal curve and transmission electron microscopy analyses of late paleocene and early eocene sediments in drillcore from ancora new jersey a novel paleogeographic analysis applying a recent paleomagnetic pole from the faeroe islands indicates that new jersey during the initial eocene had a 6 9 degrees lower paleolatitude 27 3 degrees for ancora and a more zonal shoreline trace than in conventional reconstructions our investigations of the petm clay from ancora reveal abundant magnetite nanoparticles bearing signature traits of crystals produced by magnetotactic bacteria this result the first identification of ancient biogenic magnetite using fmr argues that the anomalous magnetic properties of the petm sediments are not produced by an impact they instead reflect environmental changes along the eastern margin of north america during the petm that led to enhanced production and or preservation of magnetofossils

dating of sirius group tillites in the antarctic dry valleys with cosmogenic3he and21ne we measured in situ produced cosmogenic3he and21ne in boulder and bedrock surface samples in and near sirius group tillites from two localities in southern victoria land antarctica pyroxene quantitatively retains cosmogenic3he for millions of years the deposit at mt fleming has a minimum exposure age of 6 5 ma while 6 ma is a probable minimum age for the table mountain tillite these lower limits not taking into account erosion of the sampled surfaces are based on the samples with the highest concentrations of cosmogenic noble gases and currently accepted production rates since the plagioclase bearing mt fleming samples almost certainly lost part of their cosmogenic ne the minimum exposure age at this location is presumably even 50 higher than the stated value the concentrations of cosmogenic ne in our samples constrain uplift rates of the transantarctic mountains to 170 m ma these data contradict the hypothesis of a collapse of the east antarctic ice sheet in the pliocene coupled with high uplift rates and support the model of a stable ice sheet since the middle miocene

the detection of magnetotactic bacteria and magnetofossils by means of magnetic anisotropy an important characteristic of magnetotactic bacteria mtb is the anisotropy of one dimensionally aligned magnetite particles this paper introduces the use of ferromagnetic resonance spectroscopy fmr at two different frequencies to compare the anisotropic properties of magnetite chains of cultured intact mtb with those of lake sediments of holocene age in order to detect magnetofossils and to characterize their preservation in a geological system magnetite chains of intact mtb exhibit a predominantly uniaxial anisotropy in the lake sediments where diagenetic processes disintegrate the chains and diminish their uniaxiality magnetite chains or chain fragments and dissociated bulk magnetite particles differ in their anisotropy properties the two groups of assembly can be distinguished by empirical spectral separation of the fmr signal this straightforward use of the characteristics of magnetic anisotropy provides a way to detect magnetofossils experimentally thus allowing a better insight into microbial ecology during earth s history

sedimentary signatures and processes during marine bolide impacts a review abstract studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts this results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters in subaerial impacts the targets are commonly hard rocks frequently of igneous and or metamorphic origin whereas in submarine impacts the targets are mostly unconsolidated or poorly lithified sediments or sedimentary rocks with high volumes of pore water such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area impacts in shallow water marine neritic environments produced craters with low or absent rims and wide and shallow brims as characterize by both the montagnais on the scotian shelf the mjolnir in the barents sea 45 and 40 km in diameter respectively and the chesapeake bay 90 km in diameter lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity as the water in the crater is vaporized during this process resurge gullies can be cut across the rim while mass and debris flows turbidites and other gravity deposits are produced as results of tsunami and crater wall and central high collapse during and after the crater excavation stage such deposits are found both within and outside the crater structure the only difference between gravity deposits triggered by an impact or other rare events such as earthquakes is the admixture of various melt particles and possible enrichments in iridium in the former impacts near the shelf edge may cause partial collapse of the continental margin as shown by the montagnais and chicxulub impacts some of the gravity and debris flows generated by margin collapse may be channelized with final deposits up to several hundred meters thick extending for hundreds of kilometers from the impact site other impact features such as shatter cones tektites spherules and ni spinels shocked quartz isotropication and partial melts are common to both submarine and subaerial impacts theoretical calculations of the destructive forces of mega tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence a dearth of gigantic tsunami evidence for the chicxulub impact outside of the gulf of mexico where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep north atlantic basin should have been 1 m s could be the result of the mitigating effect of the bathymetry of surrounding area causing wave diffraction and interference calculated maximum horizontal orbital velocity near the seafloor at the shelf for the montagnais impact is 22 m s for a 200 m high wave 5 5 m s for a 50 m high wave at 500 km decreasing to 0 5 m s at 1000 km strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record however lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows

geomagnetic field intensity changes in sweden between 9000 and 450 cal bp extending the record of archaeomagnetic jerks by means of lake sediments and the pseudo thellier technique the pseudo thellier technique was applied to recover relative palaeointensity pt rpi estimates from four piston cores retrieved from a holocene lake sediment sequence in the province of smaland central southern sweden a time depth model between 11 000 and 450 cal bp was based on a combination of calibrated radiocarbon ages and the ages of established palaeomagnetic secular variation psv features that were transferred from a varve dated regional master curve in fennoscandia significant peaks in geomagnetic field intensity occurred at similar to8400 similar to6400 3900 and 2800 cal bp the maximum field intensity at 2800 cal bp was associated with the most rapid change in the direction of the geomagnetic vector while the high resolution sedimentary data show that significant century scale increases and decreases in relative field intensity between 4000 and 2000 cal bp were associated with abrupt changes jerks in the direction of the geomagnetic vector archaeomagnetic jerks younger than 2000 years were not reproduced in this multiple sediment core study on the other hand the pt rpi reconstructions display century to millennial scale trends between 5000 and 450 cal bp that are coherent with archaeomagnetic data sets from western europe and central asia this agreement indicates that palaeointensity may be used as a relative dating technique across this region fennoscandian psv and rpi records for the earlier holocene do not show the same rapid style of vector movement which suggests that northern europe has been affected by a more turbulent geodynamo since similar to4000 cal bp c 2004 elsevier b v all rights reserved less

geochronological evidence supporting antarctic deglaciation three million years ago the response of the antarctic ice sheets to increased global temperatures is an important unresolved issue in the assessment of future climate change in particular considerable controversy exists as to whether the east antarctic ice sheet suffered extensive deglaciation during the mid pliocene epoch 3 myr ago when temperatures were only slightly warmer than today although the ice sheet is widely assumed to have existed in something like its present form for the past 14 myr ref 1 marine diatoms eroded from the antarctic interior have been found in glacial till deposits high in the transantarctic mountains2 3 and have been bio stratigraphically dated at 3 myr before present this age has been disputed4 because it implies marine deposition in the antarctic interior and hence substantial deglaciation at a time when other evidence has been marshalled for the persistence of cold polar conditions4 here we report k ar and 40ar 39 ar ages for a volcanic ash bed in diatom bearing glaciomarine strata cored in ferrar fiord east antarctica by the ciros 2 drill hole5 which confirm the age of the diatoms at 3 myr and hence also confirm the mid pliocene deglaciation

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

laboratory investigations of marine impact events factors influencing crater formation and projectile survivability given that the earths surface is covered in around two thirds water the majority of impact events should have occurred in marine environments however with the presence of a water layer crater formation may be prohibited indeed formation is greatly controlled by the water depth to projectile diameter ratio as discussed in this paper previous work has shown that the underlying target material also influences crater formation e g gault and sonett 1982 baldwin et al 2007 in addition to the above parameters we also show the influence of impact angle impact velocity and projectile density for a variety of water depths on crater formation and projectile survivability the limiting ratio of water depth to projectile diameter on cratering represents the point at which the projectile is significantly slowed by transit through the water layer to reduce the impact energy to that which prohibits cratering we therefore study the velocity decay produced by a water layer using laboratory analytical and numerical modelling techniques and determine the peak pressures endured by the projectile for an impact into a water depth five times the projectile diameter the velocity of the projectile is found to be reduced to 26 32 its original value for deep water impacts we find that up to 60 of the original mass of the projectile survives in an oblique impact where survivability is defined as the solid or melted mass fraction of the projectile that could be collected after impact

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

on marine microfossil transport and pathways in antarctica during the late neogene evidence from the sirius group at mount fleming there are two extreme views of the evolution of the pliocene antarctic ice sheet dynamicists argue for ice sheet reduction and reexpansion on the basis of pliocene marine diatoms in a glacial deposit the sirius group that is widespread in the transantarctic mountains stabilists argue from other evidence that the antarctic cryosphere remained essentially constant in area and volume they propose marine diatom transport by eolian processes and emplacement into terrestrial glacial strata hence the inferred source area and transport mechanism of marine diatoms are of critical importance we tested the reduction hypothesis on an important outcrop of the sirius group at mount fleming south victoria land we observed very few unidentifiable marine diatom fragments in sirius group strata in contrast marine diatoms enclosed in a surface diamicton covering the sirius group were more abundant and identifiable our study further indicates that the sirius group at mount fleming was not deposited by the east antarctic ice sheet but rather by alpine ice originating on the transantarctic mountains on the basis of both data sets we infer that marine diatoms postdate sirius group deposition at mount fleming and that transport was by wind and we advance alternative scenarios for their source and transport pathways

early fracturing and impact residue emplacement can modelling help to predict their location in major craters understanding the nature and composition of larger extraterrestrial bodies that may collide with the earth is important one source of information lies in the record of ancient impact craters some of which have yielded chemical information as to the impacting body many deeply eroded craters have no remaining melt sheet or ejecta yet may contain impactor residue within basement fractures the emplacement mechanism for fractionated siderophile residues is likely to be gaseous although melt droplets and some solid materials may survive for breccia and melt filled fractures to contain extraterrestrial material they must form very early in the impact process most current numerical models do not dwell on the formation and location of early major fractures although fractures in and around small craters on brittle glass exposed to hypervelocity impact in low earth orbit have been successfully simulated modelling of fracture development associated with larger craters may help locate impact residues and test the models themselves

cenozoic marine sedimentation and ice volume variation on the east antarctic craton recycled cretaceous and cenozoic marine microfossils have been recovered from samples of the pliocene sinus formation samples were collected from outcrops in the reedy beardmore and ferrar glacier areas of the transantarctic mountains between lat 77 and 86 s the glaciogene sediments contained diatoms foraminifera calcareous nannoplankton silicoflagellates radiolarians sponge spicules palynomorphs and ostracodes of late cretaceous paleocene eocene late oligocene late miocene and pliocene age this suggests the presence of open marine basins on the east antarctic craton during late mesozoic and cenozoic time the apparent absence of early oligocene and early through middle and earliest late miocene assemblages suggests either that marine regression exposed the basin floors or that ice filled the basins during these times the high elevation setting of sirius formation outcrops suggests one of two hypotheses for their origin 1 they are in situ pliocene glaciomarine deposits that were uplifted 1 750 2 500 m with the transantarctic mountains to their present elevation 2 the sirius formation deposits are a mixture of derived sediments stripped from sub ice intracratonic basins and subsequently redeposited by ice flowing up the inland slope of the transantarctic mountains we favor the second hypothesis with transport to sites sometime within the past 3 m y

pliocene pleistocene diatoms in paleozoic and mesozoic sedimentary and igneous rocks from antarctica a sirius problem solved there are two competing scenarios on the behavior of the east antarctic ice sheet during the late tertiary in one scenario the ice sheet was very dynamic and underwent major drawdown and renewal as late as the pliocene in the other the ice sheet was relatively stable during the late neogene the presence of marine diatoms in sirius group sedimentary rocks in east antarctica is at the center of the disagreement one side regards the diatoms as the major piece of evidence to support the drawdown and renewal hypothesis and infers that they were introduced into the sirius during renewed glaciation of east antarctica others suggest that these diatoms were likely introduced into the sirius by atmospheric largely eolian processes we propose a simple test of the eolian hypothesis if diatoms were introduced into the sirius by eolian processes then they should also be present in older paleozoic and mesozoic sedimentary and igneous rocks samples from two units of the beacon supergroup devonian to jurassic from beacon valley east antarctica were analyzed the beacon heights orthoquartzite devonian and the feather conglomerate permian triassic also examined was sediment found in cracks of paleozoic and mesozoic devonian to cretaceous igneous rocks from marie byrd land west antarctica largely pliocene pleistocene planktonic marine diatoms were found in all sample sets because neither beacon supergroup sedimentary rocks nor igneous rocks from marie byrd land are pliocene pleistocene in age such findings strongly suggest that diatoms were introduced into them by eolian processes this same scenario can be applied to sirius group sedimentary rocks

detection of noninteracting single domain particles using first order reversal curve diagrams we present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles unisd in rocks and sediments the method is based on high resolution measurements of first order reversal curves forcs unisd particles have a unique forc signature that can be used to isolate their contribution among other magnetic components this signature has a narrow ridge along the hc axis of the forc diagram called the central ridge which is proportional to the switching field distribution of the particles therefore the central ridge is directly comparable with other magnetic measurements such as remanent magnetization curves with the advantage of being fully selective to sd particles rather than other magnetic components this selectivity is unmatched by other magnetic unmixing methods and offers useful applications ranging from characterization of sd particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments

sedimentary iron cycling and the origin and preservation of magnetization in platform carbonate muds andros island bahamas carbonate muds deposited on continental shelves are abundant and well preserved throughout the geologic record because shelf strata are difficult to subduct and peritidal carbonate units often form thick rheologically strong units that resist penetrative deformation much of what we know about pre mesozoic ocean chemistry carbon cycling and global change is derived from isotope and trace element geochemistry of platform carbonates paleomagnetic data from the same sediments would be invaluable placing records of paleolatitude paleogeography and perturbations to the geomagnetic field in the context and relative chronology of chemostratigraphy to investigate the depositional and early diagenetic processes that contribute to magneitzation in carbonates we surveyed over 500 core and surface samples of peritidal often microbially bound carbonate muds spanning the last not vert similar 1000 yr and deposited on top of pleistocene aeolianites in the triple goose creek region of northwest andros island bahamas sedimentological geochemical magnetic and ferromagnetic resonance properties divide the sediment columns into three biogeochemical zones in the upper sediments the dominant magnetic mineral is magnetite produced by magnetotactic bacteria and dissimiliatory microbial iron metabolism at lower depths above or near mean tide level microbial iron reduction dissolves most of the magnetic particles in the sediment in some cores magnetic iron sulfides precipitate in a bottom zone of sulfate reduction likely coupled to the oxidation of decaying mangrove roots the remanent magnetization preserved in all oriented samples appears indistinguishable from the modern local geomagnetic field which reflects the post depositional origin of magnetic particles in the lower zone of the parasequence while we cannot comment on the effects of late stage diagenesis or metamorphism on remanence in carbonates we postulate that early cemented thin laminated parasequence tops in ancient peritidal carbonates are mostly likely to preserve syn depositional paleomagnetic directions and magnetofossil stratigraphies

iridium concentrations and abundances of meteoritic ejecta from the eltanin impact in sediment cores from polarstern expedition ant xii 4 abstract the abundances of meteoritic ejecta from the eltanin asteroid impact have been examined in seven sediment cores recovered by the fs polarstern during expedition ant xii 4 using elemental concentrations of iridium and weights of coarse 500 m ejecta debris three cores with well preserved impact deposits ps2704 1 ps2708 1 and ps2709 1 each contain ir and ejecta fluences similar to those found previously in usns eltanin core e13 4 small ir anomalies and traces of ejecta were found in cores ps2706 1 and ps2710 1 but since these cores lack well defined deposits these are considered to be reworked and not representative of the fallout no evidence of ejecta was found in cores ps2702 1 and ps2705 1 these results confirm earlier speculation that the eltanin impact resulted in deposits of ejecta with up to 1 g cm2 of debris over a wide area of the ocean floor however there are still large uncertainties over the actual regional or global extent of this unique sediment deposit

ferromagnetic resonance and magnetic characteristics of intact magnetosome chains in magnetospirillum gryphiswaldense abstract the magnetic characteristics of intact magnetosome chains in magnetospirillum gryphiswaldense bacteria were investigated by means of static and dynamic magnetic analyses and ferromagnetic resonance spectroscopy the nano sized magnetosomes are generally in a stable single domain state but magnetosomes smaller than 30 nm characteristic of superparamagnetic magnetite particles were also found alternating current ac susceptibility indicates that all magnetosomes are blocked below 150 k at room temperature the anisotropy of m gryphiswaldense is dominated by the shape of the magnetosome chains low temperature ferromagnetic resonance fmr spectroscopy indicates that this dominant shape anisotropy can affect the detection of the verwey transition at 100 k the static and dynamic magnetic analyses show that the verwey transition is smeared and that our magnetotactic bacteria fail the moskowitz test this failure is explained by the biomineralization of non stoichiometric magnetosomes this interpretation is based on the increase in high field susceptibility and the distinct peak in the out of phase component of the ac susceptibility below 50 k these results are attributed to freezing of spins associated with defect structures in the core and at the surface of nano sized magnetosomes the results obtained from m gryphiswaldense demonstrate that intrinsic properties of nano sized magnetosomes are significantly influenced by non stoichiometry and by the anisotropy excited from their arrangement in the bacteria

late cenozoic glacial history of the ross embayment antarctica abstract knowledge of the cenozoic terrestrial and marine glacial history of antarctica has been developed through studies of on land surface outcrops and subsurface drillcores and through examination of drillcores and piston cores obtained from a variety of offshore marine geological settings these field and laboratory endeavors have been a major target of the antarctic earth science community for more than thirty years the ross sea sedimentary basins including the byrd basin west antarctica wilkes and pensacola basins east antarctica and the transantarctic mountains provide significant data on the cenozoic glacial record a record that has been traced back to at least 38 ma close to the eocene oligocene boundary this paper reviews the late cenozoic history of the transantarctic mountains and flanking sedimentary basins with emphasis on the debate between the stabilist view of antarctic ice sheet history see clapperton and sugden 1990 and our favored multi glacial dynamic ice sheet model

ferromagnetic resonance and low temperature magnetic tests for biogenic magnetite magnetite is both a common inorganic rock forming mineral and a biogenic product formed by a diversity of organisms magnetotactic bacteria produce intracellular magnetites of high purity and crystallinity magnetosomes arranged in linear chains of crystals magnetosomes and their fossils magnetofossils have been identified using transmission electron microscopy tem in sediments dating back to 510 570 ma and possibly in 4 ga carbonates in martian meteorite alh84001 we present the results from two rock magnetic analyses the low temperature moskowitz test and ferromagnetic resonance fmr applied to dozens of samples of magnetite and other materials the magnetites in these samples are of diverse composition size shape and origin biologically induced extracellular biologically controlled magnetosomes and chiton teeth magnetofossil synthetic and natural inorganic we confirm that the moskowitz test is a distinctive indicator for magnetotactic bacteria and provide the first direct experimental evidence that this is accomplished via sensitivity to the magnetosome chain structure we also demonstrate that the fmr spectra of four different strains of magnetotactic bacteria and a magnetofossil bearing carbonate have a form distinct from all other samples measured in this study we suggest that this signature also results from the magnetosomes unique arrangement in chains because fmr can rapidly identify samples with large fractions of intact isolated magnetosome chains it could be a powerful tool for identifying magnetofossils in sediments

magnetotactic bacterial response to antarctic dust supply during the palaeocene eocene thermal maximum distinct magnetic properties of marine sediments that record the palaeocene eocene thermal maximum petm have been suggested to be due to a bacterial magnetofossil signal that is linked to enhanced weathering conditions during the petm we document the dominance of bacterial magnetite in deep sea sediments from southern kerguelen plateau ocean drilling program hole 738c southern ocean not only during the petm but also before and after the thermal event this occurrence of magnetofossils throughout the petm indicates that the occurrence of bacterial magnetosomes is not due to a preservation effect instead we suggest that it is due to sustained mild iron reducing conditions that dissolved the most labile aeolian derived iron which favoured continued magnetotactic bacterial activity without being strong enough to dissolve the less reactive magnetite and haematite enhanced aeolian haematite abundances at the beginning of the petm indicate drier conditions on the neighbouring antarctic continent at those times our results provide evidence that iron fertilisation by aeolian dust was the main limiting factor that conditioned proliferation of magnetotactic bacteria in the deep sea at the southern kerguelen plateau with the exception of two short periods of rapidly changing palaeoenvironmental conditions at the onset and termination of the petm increased iron supply from aeolian dust that enhanced oceanic primary productivity and subsequent delivery of organic carbon to the seafloor along with mild iron reducing diagenetic conditions seem to have been necessary to provide the iron needed for magnetite biomineralization by magnetotactic bacteria to drive their marked increase in abundance in the studied petm record from southern kerguelen plateau our analyses of a deep sea petm record from hole 1051b at blake nose atlantic ocean failed to identify magnetofossils despite evidence for the occurrence of magnetite and haematite of probable aeolian origin contrasting magnetic properties at these petm sections indicate that further work is needed to understand the palaeoenvironmental and diagenetic factors whose interactions lead to production and preservation of magnetofossils in deep sea sediments

a strong angular dependence of magnetic properties of magnetosome chains implications for rock magnetism and paleomagnetism 1 single domain magnetite particles produced by magnetotactic bacteria magnetosomes and aligned in chains are of great interest in the biosciences and geosciences here we investigated angular variation of magnetic properties of aligned magnetospirillum magneticum amb 1 cells each of which contains one single fragmental chain of magnetosomes with measurements at increasing angles from the chain direction we observed that i the hysteresis loop gradually changes from nearly rectangular to a ramp like shape e g bc and remanence decrease ii the acquisition and demagnetization curves of irm shift toward higher fields e g bcr increases and iii the forc diagram shifts toward higher coercivity fields e g bc forc increases for low temperature results compared to unoriented samples the samples containing aligned chains have a much lower remanence loss of field cooled fc and zero field cooled zfc remanence upon warming through the verwey transition higher ratio fc zfc for the measurement parallel to the chain direction and lower ratio larger fc and zfc values for the perpendicular measurement micromagnetic simulations confirm the experimental observations and reveal that the magnetization reversal of magnetosome chain appears to be noncoherent at low angles and coherent at high angles the simulations also demonstrate that the angular dependence of magnetic properties is related to the dispersion degree of individual chains indicating that effects of anisotropy need to be accounted for when using rock magnetism to identify magnetosomes or magnetofossils once they have been preserved in aligned chains additionally this study experimentally demonstrates an empirical correspondence of the parameter bc forc to bcr rather than bc at least for magnetite chains with strong shape anisotropy this suggests forc analysis is a good discriminant of magnetofossils in sediments and rocks

ferromagnetic resonance of magnetite biominerals traces redox changes abstract redox variations govern a multitude of key geochemical and microbiological processes within lacustrine and marine systems yet the interpretation of these geological archives can be limited because redox sensitive microorganisms leave behind sparse fossil evidence here we assess a biologically controlled magnetic proxy through investigation of a well constrained sedimentary record covering a perturbation of redox conditions driven by a complete trophic cycle in lake constance ferromagnetic resonance spectroscopy of sediments reveals strong uniaxial anisotropy indicative of single domain magnetite particles in intact or fragmentary chain arrangements which are an unambiguous trait of magnetotactic bacteria mtb and their magnetofossil remains we show that biogenic magnetite formed intra cellularly in mtb faithfully records changing redox conditions at or close to the sediment water interface biogenic magnetite within sedimentary records points to the proliferation of mtb parallel to a decline in water column dissolved oxygen and the formation of sulfidic surface sediments in lake constance associated with an episode of eutrophication 1955 1991 we conclude that magnetofossils may serve as a sensitive geological proxy to reconstruct dynamic redox changes along the sediment water interface and bottom waters

magnetite as a prokaryotic biomarker a review 1 over the years nanometer sized magnetite fe3o4 crystals have been recovered from many modern and ancient environments including sediments and soils and even meteorites in some cases these crystals have been used as magnetofossils for evidence of the past presence of specific microbes magnetite nanocrystals can be formed by a number of different biological and inorganic mechanisms resulting in crystals with different physical and magnetic characteristics prokaryotes bacteria biomineralize magnetite through two methods that differ mechanistically including biologically induced mineralization bim and biologically controlled mineralization bcm magnetite nanocrystals produced by bim are known to be synthesized by the dissimilatory iron reducing bacteria are deposited external to the cell and generally are physically indistinguishable from magnetite particles formed inorganically bcm magnetites in contrast are synthesized by the magnetotactic bacteria and some higher organisms and are precipitated intracellularly as membrane bounded structures called magnetosomes these magnetites appear to have unique crystal morphologies and a narrow size range leading to their original use as magnetofossils because of the discovery of nanometer sized crystals of magnetite in the martian meteorite alh84001 the use of these criteria for the determination of whether magnetite crystals could constitute a prokaryotic biomarker was questioned thus there is currently great debate over what criteria to use in the determination of whether specific magnetite crystals are biogenic or not in the last decade additional criteria have been established e g the magnetite assay for biogenicity and new tools and technologies have been developed to determine the origin of specific types of magnetite crystals

bacterial magnetite and the magnetic properties of sediments in a swedish lake a comparison of the magnetic properties of lake sediments and catchment materials from pajep njakajaure north sweden demonstrates that high concentrations of single domain sd magnetite in the upper 70 cm of gyttja in the deepest part of the basin are not of detrital origin tem and light microscope examination revealed the existence of bacterial cells and magnetosomes in the upper sediments which are responsible for the presence of the sd magnetite most of the fossil magnetosomes appear to have been dissolved through reductive processes at lower depths although some are preserved in the lower sediments and carry a post depositional remanent magnetization pdrm the results highlight the complex effects of post depositional processes on mineral magnetic assemblages and natural remanent magnetization aquisition and demonstrate how mineral magnetic measurements may be used to identify magnetic grain size changes brought about by sediment diagenesis

laboratory impacts into dry and wet sandstone with and without an overlying water layer implications for scaling laws and projectile survivability scaling laws describing crater dimensions are defined in terms of projectile velocity and mass densities of the materials involved strength of the target and the local gravity here the additional importance of target porosity and saturation and an overlying water layer are considered through 15 laboratory impacts of 1 mm diameter stainless steel projectiles at 5 km s 1 into a an initially uncharacterized sandstone porosity 17 and b coconino sandstone porosity 23 the higher porosity dry sandstone allows a crater to form with a larger diameter but smaller depth than in the lower porosity dry sandstone furthermore for both porosities a greater volume of material is excavated from a wet target than a dry target by 27 30 comparison of our results with pi scaling dimensionless ratios of key parameters characterizing cratering data over a range of scales suggests that porosity is important for scaling laws given that the new data lie significantly beneath the current fit for ice and rock targets on a v versus 3 plot v gives cratering efficiency and 3 the influence of target strength an overlying water layer results in a reduction of crater dimensions with larger craters produced in the saturated targets compared to unsaturated targets a water depth of approximately 12 times the projectile diameter is required before craters are no longer observed in the targets previous experimental studies have shown that this ratio varies between 10 and 20 gault and sonett 1982 in our experiments 25 of the original projectile mass survives the impact

half a century of progress in research on terrestrial impact structures a review abstract the author who investigated the wolfe creek australia in 1962 and edited two benchmark sets of readings on meteorite craters and possible astroblemes in 1977 and 1979 reviews the state of knowledge at the present time the text is concerned with terrestrial impact structures geological features without any consideration of extraterrestrial analogues a handful of definitive publications are drawn on to present the story of terrestrial impact in a single article the text covers historical aspects briefly the effect of target variations the paucity of human observation of such large scale events distinction from volcanic endogenous structures modification by geological processes the transience of the crater initially formed on the target and its subsequent modifications the global geographic distribution of the 174 structures now listed of which a number are dubious attributions their distribution in geological time many ages being known only known to wide limits maximum or minimum values their size distribution calculations of impact frequencies shock effects processes on impact the stages of formation impact into shallow marine and deep sea targets impacts on ice about which little is known and finally the input of impact into biotic extinctions in this last lengthy section the summaries of the conclusions of scientists researching impact on earth and palaeontologists researching biotic impact are set side by side it is concluded that if the recent foraminiferal evidence obtained by gerta keller and associates is taken at its face value the case of impact as a sole agent in extinction is non existent biotic extinction is clearly a complex process involving a number of causes in some cases it was staggered in time and different sets of organisms responded quite differently and surprisingly even in the same extinction event extraterrestrial impact may have been one of the causes in some cases but it may have been regional rather than global in its effects we may never know how much input it had into the record of biotic extinction on earth an enormous amount of new knowledge has arisen from detailed studies of this new family of remarkable geological structures

regional variations in spinel compositions an important key to the cretaceous tertiary event spinel found in spherules from cretaceous tertiary boundary sediments exhibits a wide range in composition and is distinguished from typical igneous spinel by high mg al and ni relatively low ti and cr and high fe2o3 feo solid solution compositions range from nearly pure magnetite to magnesioferrite to relatively pure mgal2o4 spinel the ni spinel trevorite is also a common component a regional variation in composition is also observed spinel from two north pacific sites has higher mg and al and lower ni and fe than spinel from two european sites and one south atlantic site the most probable source of the spinel is crystallization from molten silicate droplets produced from a major impact event the regional variations may provide a key to locating the impact site s

magnetic detection and characterization of biogenic magnetic minerals a comparison of ferromagnetic resonance and first order reversal curve diagrams australian research council grant number dp120103952 australia new zealand iodp consortium anzic grant number le0882854

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

powder x ray diffraction analysis of homogeneous and heterogeneous sedimentary dolostones abstract the ca molar ca ca mg 100 in sedimentary dolomite which ranges from 48 ca to 62 5 ca can be determined by atomic absorption spectrometry electron microprobe analysis or powder x ray diffraction analysis the latter procedure which relies on the fact that the position of the d104 reflection depends linearly on the ca content offers an easy and relatively inexpensive method of determining ca in dolomite to date such analyses have assumed that the dolostone is homogeneous with respect to the ca content of its constituent dolomite crystals herein the xrd technique is extended to compositionally heterogeneous dolostones a considerable amount of important information can be derived from x ray diffractograms of dolostones if they are analyzed with peak fitting techniques pf xrd specifically this technique allows 1 determination of the ca with an accuracy of 0 5 ca 2 identification of heterogeneous dolostone samples formed of more than one population of dolomite crystals as defined by their ca content and 3 derivation of the proportion of each population of dolomite crystals with an accuracy of 10 the pf xrd technique is a powerful analytical tool that provides valuable information on the major element composition of low fe dolomite compared to other techniques the pf xrd technique is accurate rapid relatively inexpensive and easy to use

oceanic impacts a growing field of fundamental geoscience the importance of oceanic impacts of collisional events and resulting energy release are briefly described data collection methods from the eltanin a mesosiderite projectile deep water impact are presented

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

geochemistry and petrology of a middle tithonian limestone marl rhythmite in the neuquen basin argentina depositional and burial history

pliocene paleoenvironment and antarctic ice sheet behavior evidence from wright valley investigations in wright valley adjacent to the transantarctic mountains in east antarctica shed light on the question of whether high latitude pliocene climate was warm enough to cause widespread deglaciation of the east antarctic craton with a concurrent magellanic moorland like environment if pliocene age diatoms presently in glaciogenic deposits high in the transantarctic mountains had come from seaways on the east antarctic craton an expanding late pliocene ice sheet must have first eroded them from marine sediments and then deposited the diatoms at their present high altitude locations this hypothetical expanding glacier would have had to have come through wright valley glacial drift sediments from the central wright valley were mapped sampled analyzed and 40ar 39ar whole rock dated our evidence indicates that an east antarctic outlet glacier has not expanded through wright valley and hence cannot have overridden the dry valleys sector of the transantarctic mountains any time in the pa

basinal marine dolomicrite from the pennsylvanian wolfcampian horquilla formation new mexico abstract a 6 m thick sequence of black dolomicrite occurs within a shaley basinal portion of the pennsylvanian wolfcampian horquilla formation new mexico deep water lithologic associations absence of hypersaline and freshwater indicators elevated na and sr contents and marine derived 18o values all indicate that the dolomite precipitated from marine derived water during burial beneath the sediment water interface trace and major element contents and oxygen isotopic compositions of the dolomites exhibit significant covariance sr na and 18o all covary positively with excess ca in the dolomite structure and fe and mn covary negatively with excess ca in the dolomite structure such variations are suggestive of a diagenetic alteration trend ranging from primary marine compositions preserved in calcian dolomite to diagenetic compositions in more stoichiometric dolomite moreover dolomite which preserves primary 18o values exhibits only minor variation in 13c this is in contrast to the highly variable 13c values observed in tertiary deep water dolomites where sulfate reduction during organic diagenesis plays a key role in promoting dolomitization thus depletion of sulfate was not a prerequisite for dolomitization of basinal carbonates of the hor uilla formation rather dolomitization was promoted by the metastability of the precursor mineralogy while such a mechanism has been postulated for dolomitization of marginal slope facies of oceanic atolls this study extends its application to burial marine dolomitization of deep water micritic sequences

the diogenite eucrite links and the crystallization history of a crust of their parent body aspects of the crystallization history of the primitive crust of the hed howardites eucrites diogenites achondrite parent body have been deciphered from the microtextures and chemical compositions of pyroxene and plagioclase in some antarctic hed achondrites intermediate to diogenite and eucrite five new specimens related to yamato 75032 and three eucritic clasts in y791186 and y791208 have been investigated by electron microprobe analytical transmission electron microscope and x ray diffraction techniques their pyroxene compositions cover the entire range from fe rich diogenite to eucrite the pyroxene textures of the y75032 type achondrites indicate that many of them crystallized as low ca pigeonite and some as orthopyroxene later shock events modified the original inversion and exsolution textures the chemical compositions and the interstitial occurrence of plagioclase crystals in pyroxene indicate crystallization from intercumulus trapped liquid the pyroxene crystallization and the relation between the anorthite content an of plagioclase and the mg mg fe number of the coexisting pyroxene in these specimens reveal two crystallization trends one trend is intermediate to that of the mg rich suites of the lunar highland crust and that of ferroan anorthosite and the other is a trend with a steep decrease in the an content while the mg mg fe of the pyroxene remains effectively constant a model involving crystal fractionation in a shallow magma ocean or intrusion in the parent body has been proposed

classification of mafic clasts from mesosiderites implications for endogenous igneous processes we have analyzed thirteen igneous pebbles from the vaca muerta eet87500 and bondoc mesosiderites by electron microprobe and instrumental neutron activation and combined these data with literature data for forty three analyzed mesosiderite clasts we classify these well characterized clasts into the following five principal groups 1 1 polygenic and monogenic cumulates 39 are coarse grained gabbros that are highly depleted in incompatible elements relative to h chondrites they formed at moderate depth either as residues of low degree partial melting of pre existing cumulate eucrites or as cumulates from parent melts similar to cumulate eucrites 2 2 polygenic basalts 30 are finer grained rocks with positive europium anomalies la lu ratios 1 and lower rare earth element abundances than basaltic eucrites it seems likely that these rocks were formed near their parent body surface by remelting mixtures of major amounts of basaltic eucrites and lesser amounts of cumulate eucrites 3 3 quench textured rocks comprise two compositional groups a those which resemble basaltic eucrites 5 and b those which resemble cumulate eucrites 2 the quench textured rocks are probably monogenic they formed most likely when small scale impacts at their parent body surface totally melted small amounts of basaltic or cumulate eucrite material 4 4 monogenic basalts 11 resemble basaltic eucrites and formed by endogenous igneous processes on the mesosiderite parent body mpb the monogenic basalts and those quench textured rocks that resemble basaltic eucrites in composition plot subparallel to the nuevo laredo trend eucrites on diagrams of incompatible element concentration vs mg previous work has shown that the nuevo laredo trend eucrites were probably produced by fractional crystallization it seems likely that the monogenic basalts from the mpb had a similar origin 5 5 ultramafic rocks are cumulates consisting mainly of large crystals of orthopyroxene 9 or olivine 4 orthopyroxenite clasts closely resemble diogenites and were formed most likely by endogenous igneous processes olivine crystals vary significantly in major element composition some may be endogenous but others may have been derived from the disrupted mantle of a different parent body

unmelted meteoritic debris collected from eltanin ejecta in polarstern cores from expedition ant xii 4 abstract a total of 1 7 g of unmelted meteorite particles have been recovered from fs polarstern piston cores collected on expedition ant xii 4 that contain ejecta from the eltanin impact event most of the mass 1 2 g is a large single specimen that is a polymict breccia similar in mineralogy and chemistry to howardites or the silicate fraction of mesosiderites most of the remaining mass is in several large individual pieces 20 75 mg each that are polymict breccias fragments dominated by pyroxene and an igneous rock fragment the latter has highly fractionated ree similar to those reported in mafic clasts from mesosiderites other types of specimens identified include fragments dominated by maskelynite or olivine these pieces of the projectile probably survived impact by being blown off the back surface of the eltanin asteroid during its impact into the bellingshausen sea

ferromagnetic resonance characterization of greigite fe3s4 monoclinic pyrrhotite fe7s8 and non interacting titanomagnetite fe3 xtixo4 ferromagnetic resonance fmr spectroscopy has become an increasingly useful tool for studying the magnetic properties of natural samples magnetite fe3o4 is the only magnetic mineral that has been well characterized using fmr this limits the wider use of fmr in rock magnetism and paleomagnetism in this study we applied fmr analysis to a range of magnetic minerals including greigite fe3s4 monoclinic pyrrhotite fe7s8 magnetically non interacting titanomagnetite fe3 xtixo4 and synthetic magnetite chains to constrain interpretation of fmr analysis of natural samples and to explore applications of fmr spectroscopy we measured the fmr signatures of a wide range of well characterized samples at the x and q bands fmr spectra were also simulated numerically to compare with experimental results the effects of magnetic anisotropy mineralogy domain state and magnetostatic interactions on the fmr spectra are discussed for all studied minerals our experimental and theoretical analyses of magnetically non interacting tuff samples and magnetically interacting chains enable quantitative assessment of contributions of magnetostatic interactions and magnetic anisotropy to the fmr spectra our results also indicate that intact magnetosomes are a unique system with distinct fmr signatures while fmr analysis is useful for characterizing magnetic properties of natural samples care is needed when making interpretations because of overlaps in a range of fmr signatures of different magnetic minerals with different magnetic properties our analyses will help to constrain such interpretations in rock magnetic studies

combined use of magnetometry and spectroscopy for identifying magnetofossils in sediments identification of the mineral remains of magnetotactic bacteria mtb known as magnetofossils is of particular interest because their occurrence can be used for environmental and climatic reconstructions single domain magnetite particles which are biomineralized in the cell body of mtb have characteristic properties that can be used to detect their fossil remains acquisition of anhysteretic and isothermal remanent magnetization arm and irm first order reversal curve forc diagrams and ferromagnetic resonance fmr spectra were used to detect the magnetic mineral inventory in holocene lake sediments a comparative analysis in terms of the discriminatory power of these methods is presented the forc diagrams contain two distinct features a sharp horizontal ridge centered on the horizontal axis bc and a feature with symmetric spread along the vertical bb axis the coercivity spectra derived from the central ridge coincides with that derived from arm and irm acquisition curves and is compatible with the presence of noninteracting linear chains of single domain magnetite the second feature on forc diagrams is indicative of interacting particles in clusters in the fmr spectra from bulk sediment two populations are separated empirically based on the forc information an asymmetric signal is taken to describe the population which contains single domain particles in clusters empirical spectral separation of this contribution results in fmr spectra that are similar to those of intact mtb which strongly suggests that a fraction of linear magnetosome chains is present combination of fmr and forc results demonstrates the strong potential of these methods for identifying magnetofossils based on alignment and interaction patterns of magnetic particles

geological record and reconstruction of the late pliocene impact of the eltanin asteroid in the southern ocean in 1995 an expedition on board the research vessel fs polarstern explored the impact site of the eltanin asteroid in the southern ocean the only known asteroidimpact into a deep ocean basin analyses of the geological record of the impact region place the event in the late pliocene 2 15 myr and constrain thesize of the asteroid to be 1 km the explosive force inferred for this event places it at the threshold of impacts believed to have global consequences and its studyshould therefore provide a baseline for the reconstruction and modelling of similar events which are common on geological timescales

determining the impactor of the ordovician lockne crater oxygen and neon isotopes in chromite versus sedimentary pge signatures abundant chromite grains with l chondritic composition in the resurge deposits of the lockne impact crater 458 myr old dia similar to 10 km in sweden have been inferred to represent relict fragments of an impactor from the break up of the l chondrite parent body at 470 ma this view has been challenged based on it cr and platinum group element pge patterns of the same resurge deposits and a reinterpretation of the origin of the chromite grains an impactor of the non magmatic iron meteorite type was proposed instead here we show that single grain oxygen and noble gas isotope analyses of the chromite grains from the resurge deposits further support an origin from an l chondritic asteroid we also present pge analyses and ir cr ratios for fossil l chondritic meteorites found in mid ordovician marine limestone in sweden the l chondritic origin has been confirmed by several independent methods including major element and oxygen isotopic analyses of chromite although the meteorites show the same order of magnitude pge and cr concentrations as recent l chondrites the elements have been redistributed to the extent that it is problematic to establish the original meteorite type from these proxies different pge data processing approaches can lead to highly variable results as also shown here for the lockne resurge deposits we conclude that the lockne crater was formed by an l chondritic impactor and that considerable care must be taken when inferring projectile type from pges in sedimentary ejecta deposits c 2011 elsevier b v all rights reserved less

the preservation of fossil biomarkers during meteorite impact events experimental evidence from biomarker rich projectiles and target rocks a devonian siltstone from orkney scotland shows survival of biomarkers in high velocity impact experiments the biomarkers were detected in ejecta fragments from experiments involving normal incidence of steel projectiles at 5 6 km s 1 and in projectile fragments from impact experiments into sand and water at 2 5 km s 1 the associated peak shock pressures were calculated to be in the range of 110 147 gpa for impacts of the steel projectiles into the siltstone target and hydrocode simulations are used to show the variation of peak pressure with depth in the target and throughout the finite volume projectiles thermally sensitive biomarker ratios including ratios of hopanoids and steranes and the methylphenanthrene ratio showed an increase in thermal maturity in the ejecta and especially the projectile fragments measurement of absolute concentrations of selected biomarkers indicates that changes in biomarker ratios reflect synthesis of new material rather than selective destruction their presence in ejecta and projectile fragments suggests that fossil biomarkers may survive hypervelocity impacts and that experiments using biomarker rich rock have high potential for testing survival of organic matter in a range of impact scenarios

discovery of a 25 cm asteroid clast in the giant morokweng impact crater south africa meteorites provide a sample of solar system bodies and so constrain the types of objects that have collided with earth over time meteorites analysed to date however are unlikely to be representative of the entire population and it is also possible that changes in their nature have occurred with time large objects are widely believed to be completely melted or vaporized during high angle impact with the earth consequently identification of large impactors relies on indirect chemical tracers notably the platinum group elements here we report the discovery of a large 25 cm unaltered fossil meteorite and several smaller fragments within the impact melt of the giant 70 km diameter 145 myr old morokweng crater south africa the large fragment clast resembles an ll6 chondrite breccia but contains anomalously iron rich silicates fe ni sulphides and no troilite or metal it has chondritic chromium isotope ratios and identical platinum group element ratios to the bulk impact melt these features allow the unambiguous characterization of an impactor at a large crater furthermore the unusual composition of the meteorite suggests that the morokweng asteroid incorporated part of the ll chondrite parent body not represented by objects at present reaching the earth

the identification and biogeochemical interpretation of fossil magnetotactic bacteria magnetotactic bacteria which most commonly live within the oxic anoxic transition zone oatz of aquatic environments produce intracellular crystals of magnetic minerals specifically magnetite or greigite the crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria s search for optimal conditions within the sharp chemical gradients of the oatz the bacteria may also gain energy from the redox cycling of their crystals because magnetotactic bacteria benefit from their magnetic moments natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells these traits also allow crystals produced by magnetotactic bacteria called magnetofossils when preserved in sediments to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism magnetofossils are recognizable based on their narrow size and shape distributions distinctive morphologies with blunt crystal edges chain arrangement chemical purity and crystallographic perfection this article presents a scheme for rating magnetofossil robustness based on these traits the magnetofossil record extends robustly to the cretaceous and with lesser certainty to the late archean because magnetotactic bacteria predominantly live in the oatz the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column the magnetofossil record therefore provides an underutilized archive of paleoenvironmental information several studies have demonstrated a relationship between magnetofossil abundance and glacial interglacial cycles likely mediated by changes in pore water oxygen levels more speculatively a better developed magnetofossil record might provide constraints on the long term evolution of marine redox stratification more work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry

rock magnetic techniques complemented by ferromagnetic resonance spectroscopy to analyse a sediment record summary environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnostic tools to detect environmental changes concentration composition grain size and configuration of the carriers inferred from magnetic properties are key parameters because they are indicative of the formation conditions of magnetic phases and or of processes such as diagenesis and weathering we present a detailed ferromagnetic resonance fmr spectroscopy analysis in concert with routinely used rock magnetic measurements to determine these parameters in a sediment record that documents the development of lake soppensee central switzerland since latest pleistocene fmr spectroscopy monitors varying concentration of the predominant magnetite maghemite by the spectral signal intensity whereas the stable single domain and superparamagnetic states are determined by the signal shape at room and low temperature fitting and simulation of fmr spectra are successfully applied to samples with well defined magnetite components in the sediment matrix clear evidence for the colonization of magnetotactic bacteria mtb in lake soppensee was possible by applying empirical spectral separation to measured fmr signals that yield two magnetite populations differing in their configuration that is dispersed and aligned in chains low temperature measurements showed that these mtb can be preserved as pure or oxidized magnetite the fmr data set confirms and completes rock magnetic information obtained from the lacustrine sedimentary record the advanced application of fmr spectroscopy in the presented study critically highlights the benefit of this rapid and non destructive method for future analysis of magnetic properties in environmental studies

evidence for bacterial palaeoecological origin of mineral magnetic cycles in oxic and sub oxic tasman sea sediments abstract sediments from the tasman basin in the southwest pacific ocean show a cyclic pattern of variation in mineral magnetic properties including susceptibility and remanence over the brunhes chron the magnetic pattern is dominated by fluctuations in the concentration of a single domain ferrimagnetic component identified as bacterial magnetofossils by tem investigation of the magnetic fraction the intervals of low bacterial magnetite concentration occurring in glacial periods are accompanied by a change of the bacterial magnetosome morphology assemblage interpreted as recording a change in bacterial species assemblage there is no evidence of iron or sulphate reducing conditions during low bacterial magnetite intervals concentrations of bacterial magnetite are low when oxygen concentrations in surface sediment pore waters dropped either as the result of variation in bottom water oxygen concentration or sedimentation rate the variations in the magnetic signal are therefore interpreted as resulting primarily from bacterial palaeoecological control rather than dissolution or alteration the bacteria in this deep sea environment are apparently aerophilic and grow near the sediment surface within the oxic zone implying that magnetic remanence is also acquired near the surface but up to tens of centimetres below where the oxic zone is very thick

new evidence on the size and possible effects of a late pliocene oceanic asteroid impact debris from a late pliocene asteroid impact is spread across at least 600 kilometers of the ocean floor in the southeast pacific on the basis of iridium concentrations in sediments from six deep sea cores the asteroid diameter was at least 0 5 kilometer the impacting projectile may have been one of the largest in the last few million years the stratigraphic age of this impact is the same as that inferred for the onset of northern hemisphere glaciation

magnetic properties of pelagic marine carbonates abstract pelagic carbonates are deposited far from continents usually at water depths of 3000 6000 m at rates below 10 cm kyr and are a globally important sediment type recent advances with recognition of widespread preservation of biogenic magnetite the inorganic remains of magnetotactic bacteria have fundamentally changed our understanding of the magnetic properties of pelagic carbonates we review evidence for the magnetic minerals typically preserved in pelagic carbonates the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates and what magnetic properties can tell us about the open ocean environments in which pelagic carbonates are deposited we also discuss briefly late diagenetic remagnetisations recorded by some carbonates despite recent advances in our knowledge of these phenomena much remains undiscovered we are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation recently developed techniques have potential for testing how different magnetotactic bacterial species which produce different magnetite morphologies respond to changing nutrient and oxygenation conditions future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well calibrated modern magnetotactic bacterial occurrences a tantalizing link between giant magnetofossils and paleogene hyperthermal events needs to be tested much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies rather than being a well worn subject that has been studied for over 60 years the magnetic properties of pelagic carbonates hold many secrets that await discovery

numerical simulations of the mjolnir marine impact crater 1 the 40 km diameter mjolnir crater in the barents sea was created by a meteoroid impact into a 400 m shallow sea about 142 ma we have used a multimaterial hydrocode with depth depending target strength to model numerically the cratering and early modification stages of the mjolnir impact our models are constrained by the observed crater morphology and structure the best results were obtained for a composite target strength structure very low strength for the upper 3 km of sedimentary rocks and gradual increase from 3 to 6 5 km depth before using strength values typical for granitic rocks at greater depths the low strength of the target sediments at shallow depths led to exceptionally intensive slumping and gravitational collapse of the order of 2 0 2 5 considerably larger than the average expected values for typical terrestrial craters as a result we obtain a crater collapse in between simple and complex where slumping and gravitational collapse counteract the crater floor uplift and lead to suppression and burial of the central high the mjolnir impact in a shallow sea was responsible for a major disturbance of the water column following the impact induced outward water surge large amplitude tsunamis were formed and the rapid resurge of seawater into the excavated crater transported large amounts of ejecta and crater wall material back into the crater accounting for the redestribution of ejecta material and extensive infilling

evolutionary history of the mesosiderite asteroid a chronologic and petrologic synthesis abstract recent petrologic chemical and isotopic analyses of mesosiderite clasts and whole rocks allow reconstruction of the evolutionary history of the mesosiderite parent body mpb chronologic measurements indicate that initial melting of the mpb occurred shortly after accretion 4 56 ga ago at about the same time that the eucrites and angrites formed significant crustal remelting 4 47 ga ago probably associated with metal silicate mixing is responsible for forming abundant gabbroic clasts with extreme positive eu anomalies localized impact melting 4 5 3 9 ga ago produced quench textured clasts and matrices and may be responsible for causing mesosiderite metamorphism as well as the extensive melting of type 4 mesosiderites collisional disruption and gravitational reassembly 3 9 ga ago is indicated by thorough ar degassing at this time metallographic cooling rates of 1 c ma were likely initiated by this event the history of higher temperature events measured by thermoluminescence was annealed out impact excavation and ejection of deeply buried breccias occurred 3 9 ga ago the 10 160 ma cosmic ray exposure ages of mesosiderites represent the epoch when individual mesosiderites were excavated from the mpb or from a 10 m size fragment it is unlikely that mesosiderites and hed howardite eucrite diogenite samples come from the same parent body there is no evidence for metal silicate mixing with attendant redox effects or major crustal remelting on the hed body bulk howardites have flat chondrite normalized rare earth element ree patterns whereas many bulk mesosiderites have ree patterns with positive eu anomalies and relative enrichments in light ree the nature of the heating mechanism which caused the initial differentiation of mesosiderites and howardites is problematic

unmelted meteoritic debris in the late pliocene iridium anomaly evidence for the ocean impact of a nonchondritic asteroid ir bearing particles have been recovered from 2 piston cores in the antarctic basin in the southeastern pacific in core e13 3 the particles closely correspond to the late pliocene ir anomaly and have a fluence of 100 mg cm 2 in core e13 4 120 km to the southwest the particle fluence is 4 mg cm 2 particles with diameters from 0 5 to 4 mm contain at least 35 of the ir in this horizon three types of particles have been identified 1 vesicular 2 basaltic and 3 metal the vesicular particles appear to be shock melted debris derived from the oceanic impact of a howarditic asteroid containing a minor metal component these particles have recrystallized from a melt and impact into the ocean has resulted in the incorporation of na k cl and radiogenic sr from the ocean water target the basaltic clasts appear to be unmelted fragments of the original asteroid which may have separated from the main body prior to impact combined vesicular and basaltic particles are believed to have formed by collisions in the debris cloud estimates of the diameter of the projectile range from 100 to 500 m by many orders of magnitude this is the most massive achondrite sampled by a single meteorite fall

magnetic anisotropy magnetostatic interactions and identification of magnetofossils 1 single domain magnetite particles produced by magnetotactic bacteria mtb and aligned in chains called magnetosomes are potentially important recorders of paleomagnetic paleoenvironmental and paleolife signals rock magnetic properties related to the anisotropy of magnetosome chains have been widely used to identify fossilized magnetosomes magnetofossils preserved in geological materials however ambiguities exist when linking magnetic properties to the chain structure because of the complexity of chain integrity and magnetostatic interactions among magnetofossils that results from chain collapse during post depositional diagenesis in this paper magnetic properties of three sets of samples containing extracted magnetosomes of the culturedmagnetospirillum magneticumstrain amb 1 were analyzed to determine how chain integrity and particle concentration influence magnetic properties intact mtb and well dispersed magnetosome chains are characterized by strong magnetic anisotropy and weak magnetostatic interactions but progressive chain breakup and particle clumping significantly increase the degree of magnetostatic interaction this results in a change of the magnetic signature toward properties typical of interacting single domain particles i e a decrease of the ratio of anhysteretic remanent magnetization to the saturation isothermal remanent magnetization decreasing in the crossing point of the wohlfarth cisowski test and in the delta ratio between losses of field and zero field cooled remanent magnetization across the verwey transition as well as vertical broadening of the first order reversal curve distribution we propose a new diagram that summarizes the verwey transition properties with diagnostic limits for intact and collapsed chains of magnetosomes this diagram can be used in conjunction with other parameters to identify unoxidized magnetofossils in sediments and rocks

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