oxygen isotope studies of achondrites abstract oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites among the differentiated achondrites three isotopic groups are recognized l snc mars 2 earth and moon and 3 hed howardites eucrites diogenites the hed group also contains the mesosiderites main group pallasites and silicates from iiiab irons the angrites may be marginally resolvable from the hed group within each of these groups internal geologic processes give rise to isotopic variations along a slope 1 2 fractionation line as is well known for terrestrial materials variations of 17o from one planet to another are inherited from the inhomogeneities in the solar nebula as illustrated by the isotopic compositions of chondrites and their constituents among the undifferentiated achondrites five isotopic groups are found 1 aubrites 2 winonaites and iab iiicd irons 3 brachinites 4 acapulcoites and lodranites and 5 ureilites the isotopic compositions of aubrites coincide with the earth and moon and also with the enstatite chondrites these bodies apparently were derived from a common reservoir the isotopic composition of which was established at the chondrule scale by nebular processes isotopic similarities between chondrites and achondrites are seen only for the following instances 1 enstatite chondrites and aubrites 2 h chondrites and he irons and 3 l or ll chondrites and iva irons the isotopic data also support the following genetic associations 1 winonaites and iab iiicd irons 2 acapulcoites and lodranites and 3 ureilites and dark inclusions of c3 chondrites an attempt to reconcile the whole planet isotopic compositions of earth mars and the eucrite parent body with mixing models of their chemical compositions failed it is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive solar system components

factors affecting natural rates of slope development summary two locations in southwestern australia 120 km apart are comparable in terms of soils natural vegetation lithology and to a lesser extent topography but not in terms of climate the low rainfall area 400 mm has erosion rates an order of magnitude greater than the higher rainfall location 1400 mm however data from unbounded hillslope erosion plots in other parts of australia do not yield such a clear relationship neither are slope erosion rates related to differences in vegetation lithology or slope angle differences in denudation rates obtained from stream sediment data in australia and worldwide support a climatic interpretation but the comparability between these data and those obtained from hillslope plots is unclear it is suggested that this comparison may provide a fruitful area for further research

experimental shock decomposition of siderite and the origin of magnetite in martian meteorite alh 84001 shock recovery experiments to determine whether magnetite could be produced by the decomposition of iron carbonate were initiated naturally occurring siderite was first characterized by a variety of techniques to be sure that the starting material did not contain detectable magnetite samples were shocked in tungsten alloy holders w 90 ni 6 cu 4 to further ensure that any iron phases in the shock products were contributed by the siderite rather than the sample holder each sample was shocked to a specific pressure between 30 to 49 gpa transformation of siderite to magnetite as characterized by tem was found in the 49 gpa shock experiment compositions of most magnetites are 50 fe 2 in the octahedral site of the inverse spinel structure magnetites produced in shock experiments display the same range of sizes 50 100 nm compositions 100 magnetite to 80 magnetite 20 magnesioferrite and morphologies equant elongated euhedral to subhedral as magnetites synthesized by golden et al 2001 and as the magnetites in martian meteorite allan hills alh 84001 fritz et al 2005 previously concluded that alh 84001 experienced 32 gpa pressure and a resultant thermal pulse of 100 110 c however alh 84001 contains evidence of local temperature excursions high enough to melt feldspar pyroxene and a silica rich phase this 49 gpa experiment demonstrates that magnetite can be produced by the shock decomposition of siderite as a result of local heating to 470 c therefore magnetite in the rims of carbonates in martian meteorite alh 84001 could be a product of shock devolatilization of siderite as well

shock temperatures of sio2 and their geophysical implications the temperature of sio 2 in high pressure shock states has been measured for samples of single crystal quartz and fused quartz pressures between 60 and 140 gpa have been studied using projectile impact and optical pyrometry techniques at lawrence livermore national laboratory both data sets indicate the occurrence of a shock induced phase transformation at 70 and 50 gpa along the and fused quartz hugoniots respectively the suggested identification of this transformation is the melting of shock synthesized stishovite with the onset of melting delayed by metastable superheating of the crystalline phase some evidence for this transition in conventional shock wave equation of state data is given and when these data are combined with the shock temperature data it is possible to construct the stishovite liquid phase boundaries the melting temperature of stishovite near 70 gpa pressure is found to be 4500 k and melting in this vicinity is accompanied by a relative volume change and latent heat of fusion of 2 7 and 2 4 mj kg respectively the solid stishovite hugoniot centered on quartz is well described by the linear shock velocity particle velocity relation u s 1 822 up 1 370 km s while at pressures above the melting transition the hugoniot centered on quartz has been fit with u s 1 619 u p 2 049 km s up to a pressure of 200 gpa the melting temperature of stishovite near 100 gpa suggests an approximate limit of 3500 k for the melting temperature of sio 2 bearing solid mantle mineral assemblages all of which are believed to contain si 4 in octahedral coordination with o 2 thus 3500 k is proposed as an approximate upper limit to the melting point and the actual temperature in the earth s mantle moreover the increase of the melting point of stishovite with pressure at 70 gpa is inferred to be 11 k gpa using various adiabatic temperature gradients in the earth s mantle and assuming creep is diffusion controlled in the lower mantle the current results could preclude an increase of viscosity by more than a factor of 10 3 with depth across the mantle

nannobacterial alteration of pyroxenes in martian meteorite allan hills 84001 in martian meteorite allan hills alh 84001 this scanning electron microscope study was focused on the ferromagnesian minerals which are extensively covered with nanometer size bodies mainly 30 100 nm in diameter these bodies range from spheres to ovoids to caterpillar shapes and resemble both in size and shape nannobacteria that attack weathered rocks on earth and that can be cultured dense colonies alternate with clean smooth cleavage surfaces possibly formed later statistical study shows that the distribution of presumed nannobacteria is very clustered in addition to the small bodies there are a few occurrences of ellipsoidal 200 400 nm objects that are within the lower size range of normal earthly bacteria we conclude that the nanobodies so abundant in alh 84001 are indeed nannobacteria confirming the initial assertion of mckay et al 1996 however whether these bodies originated on mars or are antarctic contamination remains a valid question

palynological approach in upper quaternary terrestrial carbonates of central italy anything but a mission impossible a palynological survey has been carried out on two late quaternary travertine s l deposits of central italy at serre di rapolano and bagnoli tuscany region the principal aim was to improve the understanding of factors that affect the accumulation and preservation of pollen grains in thermal i e travertine and ambient temperature i e calcareous tufa terrestrial carbonates for this purpose 52 526 pollen grains belonging to 118 pollen taxa from local to extra regional sources were analysed in 200 samples from different travertine and calcareous tufa lithofacies of these 97 samples generally from thermal travertines were barren in palynomorphs by contrast pollen grains in the remaining samples were well preserved and did not show differential preservation features these observations suggest that the ph of the solutions from which travertine or tufa precipitate is not a limiting factor in causing corrosion and or destruction of pollen grains rather the barrenness recorded in many samples appears to reflect a paucity of pollen rather than post depositional destruction results suggest that the depositional energy of the environment during the deposition of travertine plays an important role in controlling pollen concentration as verified in both depressions lower energy to higher pollen concentration and slopes higher energy to lower pollen concentration respectively analysis of pollen data suggests that relatively high depositional temperatures and rates of carbonate deposition are also major limiting factors for efficient accumulation of pollen grains results of this study reveal that understanding pollen taphonomy processes in travertine and calcareous tufa is critical to developing accurate palaeoenvironmental and palaeoclimatic reconstructions

synoptic conditions of fine particle transport to the last interglacial red sea dead sea from nd sr compositions of sediment cores abstract the sediments deposited at the depocenter of the dead sea comprise high resolution archive of hydrological changes in the lake s watershed and record the desert dust transport to the region this paper reconstructs the dust transport to the region during the termination of glacial marine isotope stage 6 mis 6 135 129 ka and the last interglacial peak period mis5e 129 116 ka we use chemical and nd and sr isotope compositions of fine detritus material recovered from sediment core drilled at the deepest floor of the dead sea the data is integrated with data achieved from cores drilled at the floor of the red sea thus forming a red sea dead sea transect extending from the desert belt to the mediterranean climate zone the dead sea accumulated flood sediments derived from three regional surface cover types settled desert dust mountain loess soils and loess soils filling valleys in the dead sea watershed termed here valley loess the valley loess shows a distinct 87sr 86sr ratio of 0 7081 1 inherited from dissolved detrital calcites that originate from dried waterbodies in the sahara and are transported with the dust to the entire transect our hydro climate and synoptic conditions reconstruction illustrates the following history during glacial period mis6 mediterranean cyclones governed the transport of saharan dust and rains to the dead sea watershed driving the development of both mountain soils and valley loess then at heinrich event 11 dry western winds blew saharan dust over the entire red sea dead sea transect marking latitudinal expansion of the desert belt later when global sea level rose the dead sea watershed went through extreme aridity the lake retreated depositing salt and accumulating fine detritus of the valley loess during peak interglacial mis 5e enhanced flooding activity flushed the mountain soils and fine detritus from all around the dead sea and red sea marking a significant contraction of the desert belt at the end of mis 5e the effect of the regional precipitation diminished and the dead sea and red sea areas re entered sever arid conditions with extensive salt deposition at the dead sea

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

northward intrusions of low and mid latitude storms across the saharo arabian belt during past interglacials the rain regime of the levant during the late quaternary was controlled primarily by mediterranean cyclonic systems associated with north atlantic climate shifts lake levels in the dead sea basin have been robust recorders of the regional hydrology and generally indicate highstand wet conditions throughout glacial intervals and lowstands dry during interglacials however sporadic deposition of travertines and speleothems occurred in the negev desert and arava valley during past interglacials suggesting intrusions of humidity from southern sources probably in association with enhanced activity of mid latitude red sea synoptic troughs and or low latitude tropical plumes the southerly incursions of wetness were superimposed on the long term interglacial levantine arid conditions as refl ected by the current prevailing hyperaridity and could have had an important impact on human migration through the red sea dead sea corridor

seasonal rainfall in the sinai desert during the late quaternary inferred from fluorescent bands in fossil corals skeletal bands of alternating high and low density in massive coral species have been used to record their growth history1 2 in the red sea living colonies of the genus porites deposit low density skeletal bands during summer and high density bands during winter3 additionally yellow green fluorescence can sometimes be seen in these massive corals imparted to them by the incorporation of humic material carried by coastal runoff4 annual banding of fluorescent sequences in living scleractinian corals has proved to be useful in the study of terrestrial runoff in the near shore environment5 6 here we report the finding of similar yellow green fluorescent bands in fossil porites from late quaternary reef terraces in southern sinai which are absent from living porites in the nearby fringing reefs the periodic sequences of the fluorescing humics were found to be superimposed on the low density sub bands of the fossil corals we interpret our observations as evidence that during the late quaternary reef forming peaks the climate was wetter than the extreme desert conditions now prevailing with a possible summer rainfall regime

evidence for exclusively inorganic formation of magnetite in martian meteorite alh84001 magnetite crystals produced by terrestrial magnetotactic bacterium mv 1 are elongated on a 111 crystallographic axis in a so called truncated hexa octahedral shape this morphology has been proposed to constitute a biomarker i e formed only in biogenic processes a subpopulation of magnetite crystals associated with carbonate globules in martian meteorite alh84001 is reported to have this morphology and the observation has been taken as evidence for biological activity on mars in this study we present evidence for the exclusively inorganic origin of 111 elongated magnetite crystals in alh84001 we report three dimensional 3 d morphologies for 1000 magnetite crystals extracted from 1 thermal decomposition products of fe rich carbonate produced by inorganic hydrothermal precipitation in laboratory experiments 2 carbonate globules in martian meteorite alh84001 and 3 cells of magnetotactic bacterial strain mv 1 the 3 d morphologies were derived by fitting 3 d shape models to two dimensional bright field transmission electron microscope tem images obtained at a series of viewing angles the view down the 110 axes closest to the 111 elongation axis of magnetite crystals 111 110 0 provides a 2 d projection that uniquely discriminates among the three 111 elongated magnetite morphologies found in these samples 111 elongated truncated hexa octahedron 111 tho 111 elongated cubo octahedron 111 eco and 111 elongated simple octahedron 111 eso all 111 elongated morphologies are present in the three types of sample but in different proportions in the alh84001 martian meteorite and in our inorganic laboratory products the most common 111 elongated magnetite crystal morphology is 111 eco in contrast the most common morphology for magnetotactic bacterial strain mv 1 is 111 tho these results show that 1 the morphology of 111 elongated magnetite crystals associated with the carbonate globules in martian meteorite alh84001 is replicated by an inorganic process and 2 the most common crystal morphology for biogenic mv 1 magnetite is distinctly different from that in both alh84001 and our inorganic laboratory products therefore 111 elongated magnetite crystals in alh84001 do not constitute as previously claimed a robust biosignature and in fact an exclusively inorganic origin for the magnetite is fully consistent with our results furthermore the inorganic synthesis method i e the thermal decomposition of hydrothermally precipitated fe rich carbonate is a process analogue for formation of the magnetite on mars namely precipitation of carbonate globules from carbonate rich hydrothermal solutions followed at some later time by a thermal pulse perhaps in association with meteoritic impact or volcanic processes on the martian surface

co2 release due to impact devolatilization of carbonate results of shock experiments a study of pure single crystal calcite shocked to pressures from 9 0 to 60 8 gpa was conducted to address contradictory data for carbonate shock behavior the recovered materials were analyzed optically and by transmission electron microscopy tem as well as by thermogravimetry tga x ray diffraction xrd and raman spectroscopy in thin section progressive comminution of calcite is observed although grains remain birefringent to at least 60 8 gpa tga analysis reveals a positive correlation between percent of mass loss due to shock and increasing shock pressure r 0 77 and suggests that shock loading leads to the modest removal of structural volatiles in this pressure range xrd patterns of shocked iceland spar samples produce peaks that are qualitatively and quantitatively less intense more diffuse and shift to lower o2 however the regularity observed in these shocked powder patterns suggests that structures with very uniform unit cell separations persist to shock pressures as high as 60 8 gpa raman spectral analyses indicate no band asymmetry and no systematic peak shifting or broadening tem micrographs display progressively diminishing crystallite domain sizes selected area electron diffraction saed patterns reveal no signatures of amorphous material these data show that essentially intact calcite is recovered at shock pressures up to 60 8 gpa with only slight mass loss 7 this work suggests that the amount of co2 gas derived from shock devolatilization of carbonate by large meteorite impacts into carbonate targets has been substantially overestimated

martian surface paleotemperatures from thermochronology of meteorites the temporal evolution of past martian surface temperatures is poorly known we used thermochronology and published noble gas and petrographic data to constrain the temperature histories of the nakhlites and martian meteorite alh84001 we found that the nakhlites have not been heated to more than 350 c since they formed our calculations also suggest that for most of the past 4 billion years ambient near surface temperatures on mars are unlikely to have been much higher than the present cold 0 c state

lateritic duricrust and relief inversion in australia abstract ollier and galloway have suggested that australian ferricretes were formed in low lying positions in the landscape and are now exposed in high situations as a result of relief inversion however denudation rates reported here from a number of locations across australia are far too slow to permit this interpretation on the other hand evidence from two sites is presented in support of their suggestion that the formation of lateritic duricrusts is not necessarily genetically linked with deep weathering

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

variability of sediment removal in a semiarid watershed field and documentary data from walnut gulch watershed an instrumented semiarid drainage basin of approximately 150 km 2 57 mi 2 in southeastern arizona show that 83 of the alluvium removed from the basin during a 15 year erosion episode beginning about 1930 was excavated from the highest order stream the amount of alluvium removed in the erosion episode would have been equal to a covering of about 4 cm 1 6 in over the entire basin the rate of sediment removal during the erosion episode was 18 times greater than the rate of present channel sediment transport production of sediment from slopes and channel throughput at present rates are approximately equal and refilling will not occur under present conditions the channel forms left by the massive evacuation of sediment impose controls on the spatial distribution of tractive force and total stream power that make renewed storage of sediment likely in only a few restricted locations modern instrumented records of a decade or more provide an inadequate perspective on long term sediment movement

freshwater on the route of hominids out of africa revealed by u th in red sea corals a fundamental issue in the evolution of human culture concerns the route and conditions of anatomically modern hominids during the migration out of africa a particular question is how could anatomically modern hominids cross the hyperarid arabian deserts here we outline the evidence for significant presence of freshwater along the red sea shores during the last interglacial period freshwater caused an extensive recrystallization of coral reefs from aragonite to calcite indicating uplift of the reefs through well developed phreatic freshwater coastal aquifers applying novel open system u th dating methodology to the calcitic corals we dated the freshwater recrystallization to ca 140 ka consistent with other geological lines of evidence placing the migration of anatomically modern hominids out of africa at the onset of the last interglacial

ejection of martian meteorites we investigated the transfer of meteorites from mars to earth with a combined mineralogical and numerical approach we used quantitative shock pressure barometry and thermodynamic calculations of post shock temperatures to constrain the pressure temperature conditions for the ejection of martian meteorites the results show that shock pressures allowing the ejection of martian meteorites range from 5 to 55 gpa with corresponding post shock temperature elevations of 10 to about 1000 c with respect to shock pressures and post shock temperatures an ejection of potentially viable organisms in martian surface rocks seems possible a calculation of the cooling time in space for the most highly shocked martian meteorite allan hills alh 77005 was performed and yielded a best fit for a post shock temperature of 1000 c and a meteoroid size of 0 4 to 0 6 m the final burial depths of the sub volcanic to volcanic martian rocks as indicated by textures and mineral compositions of meteorites are in good agreement with the postulated size of the potential source region for martian meteorites during the impact of a small projectile 200 m as defined by numerical modeling artemieva and ivanov 2004 a comparison of shock pressures and ejection and terrestrial ages indicates that on average highly shocked fragments reach earth crossing orbits faster than weakly shocked fragments if climatic changes on mars have a significant influence on the atmospheric pressure they could account for the increase of recorded ejection events of martian meteorites in the last 5 ma

rock magnetic properties related to thermal treatment of siderite behavior and interpretation detailed analyses of rock magnetic experiments were conducted on the oxidation products of high purity natural crystalline siderite that were thermally treated in air atmosphere susceptibilities increase sharply between 400 and 530 c indicative of some new ferrimagnetic mineral phase generation both a drop between 540 and 590 c on the heating cycle and a dramatic increase from 590 c to 520 c on the cooling cycle occurred and are well consistent with the characteristic of magnetite a distinct hopkinson type susceptibility peak indicates that hematite is the terminal product if siderite is heated to 700 c over and over it has been revealed in detail that the original inverse magnetic susceptibility fabric contributed by the crystalline anisotropy of siderite in siderite bearing specimens is changed to a normal magnetic fabric during incremental heating over 410 490 c this is a result of dominant contributions from the distribution anisotropy of newly transformed ferromagnetic minerals a strong chemical viscous remanent magnetization could be produced during siderite oxidation in an external field rock magnetic experimental results show that magnetite maghemite and hematite are the transformation products of high temperature oxidation of siderite in air maghemite was not completely inverted to hematite even at temperature as high as 690 c during incremental thermal treatments the mineral transformation processes were confirmed by conventional optical microscopic observation x ray diffractometry and mossbauer spectroscopic analyses these results indicate that the rock magnetic methods used here are reliable and highly sensitive in detecting very small magnetic phase changes in rocks we conclude that these temperaturedependent variations of magnetic properties can be used as criteria for identification of siderite in rocks and sediments furthermore it is clear that great care should be exercised in thermal demagnetization of siderite bearing rocks in paleomagnetic magnetic anisotropy and rock magnetic studies

siderite formation in anoxic deep sea sediments a synergetic bacteria controlled process with improtant implications in paleomagnetism recent work on magnetic properties of limestones has demonstrated that the mineral siderite can be very important in paleomagnetism for two reasons first oxidation of siderite produces secondary daughter magnetic minerals magnetite maghemite and hematite either before during or after sampling these daughter products can completely change the magnetic properties of limestone samples and if unrecognized may be one of the primary reasons why many paleomagnetic studies of limestones especially paleozoic limestones are unsuccessful second siderite in weakly magnetized rocks may indicate the potential for successful paleomagnetic results because the presence of siderite indicates that the primary magnetic carriers are still intact appropriate demagnetization methods should yield successful results we conclude that microenvironmental conditions in anoxic marine sediments may permit the formation of siderite from iron ii produced during bacterial dissimilatory iron reduction

magmatic water in the martian meteorite nakhla abstract mars does not recycle crustal materials via plate tectonics for this reason the magmatic water reservoir of the martian mantle has not been affected by surface processes and the deuterium hydrogen d h ratio of this water should represent the original primordial martian value following this logic hydrous primary igneous minerals on the martian surface should also carry this primordial d h ratio assuming no assimilation of martian atmospheric water during crystallization and no major hydrogen fractionation during melt degassing hydrous primary igneous minerals such as apatite and amphibole are present in martian meteorites here on earth providing these minerals have not been affected by terrestrial weathering martian atmospheric water or shock processes after crystallization they should contain a good approximation of the primordial martian d h ratio as nakhla was seen to fall in the egyptian desert in 1911 terrestrial contamination is minimized in this meteorite the nakhlites are also among the least shocked of the martian meteorites therefore apatite within nakhla could contain primordial martian hydrogen isotope ratios we produced in situ measurements of the d h ratios in nakhla apatite grains using a cameca ims 1280 ion microprobe our measurements produced d h values in nakhla apatite similar to terrestrial values despite strong evidence that our samples were not significantly contaminated by terrestrial hydrogen these results suggest that water trapped in the martian mantle has a similar d h to that of the earth therefore the water of these two planets may have originated from the same source material the d h ratios of the carbonaceous chondrite meteorites and the jupiter family comet 103p hartley 2 are similar to the d h of the two planets making both these primitive inner solar system materials strong candidates for the source of the terrestrial planets water these results support recent dynamical models of the formation of the solar system which suggest material in the inner solar system was homogenized by the migration of jupiter

petrological evidence for shock melting of carbonates in the martian meteorite alh84001 the meteorite alh84001 a shocked igneous rock of probable martian origin contains chemically and isotopically heterogeneous carbonate globules associated with which are organic and inorganic structures that have been interpreted as possible fossil remains of ancient martian biota a critical assumption underlying this suggestion is that the carbonates formed from low temperature fluids penetrating the cracks and voids of the host rock here we report petrological studies of alh84001 which investigate the effects of shock on the various mineralogical components of the rock we find that carbonate plagioclase and silica were melted and partly redistributed by the same shock event responsible for the intense local crushing of pyroxene in the meteorite texture and compositional data show that during the period of shock decompression monomineralic melts were injected into pyroxene fractures that were subsequently cooled and resealed within seconds our results therefore suggest that the carbonates in alh84001 could not have formed at low temperatures but instead crystallized from shock melted material this conclusion weakens significantly the arguments that these carbonates could host the fossilized remnants of biogenic activity

after the deluge mediterranean stagnation and sapropel formation in an east mediterranean marine core the upper sapropel begins soon after the start of a global event a very heavy precipitation which occurred in the equatorial latitudes during the late glacial early holocene this heavy precipitation in africa channelled by the nile river across 35 of latitude produced a low salinity surface layer in the east mediterranean in this confined basin with high bottom salinity the steep salinity gradient stratified the water column the stagnant bottom waters triggered the sapropel formation cretaceous sapropels in the tropical oceans may result from the same chain of events in warm humid climates with contrasting wet and dry seasonal rhythm

postshock temperatures in silica experimental postshock temperatures for crystalline quartz released from shock states between 86 and 127 gpa and for fuset silica released from 59 to 73 gpa have been determined using previously measured spectral radiance data the temperatures range from 3660 to 4150 k and are consistent with the interpretation based on shock temperature measurements that the hugoniot of sio sub 2 crosses the phase boundary between stishovite and liquid the new postshock temperatures were used to determine the grueneisen parameter of liquid silica at high pressures and temperatures and the specific volume of the release state gamma v 3 5 mg m sup 3 and f sub r 0 266 m sup 3 mg respectively although there is some ambiguity as to whether this volume corresponds to a partial or total release state to show the validity of these postshock temperatures a radiative transport model was extended to times after free surface arrival of the shock wave copyright american geophysical union 1988

epitaxial growth of nanophase magnetite in martian meteorite allan hills 84001 implications for biogenic mineralization crystallographic relationships between magnetite sulfides and carbonate rosettes in fracture zones of the allan hills alh 84001 martian meteorite have been studied using analytical electron microscopy we have focused on those magnetite grains whose growth mechanisms can be rigorously established from their crystallographic properties individual magnetite nanocrystals on the surfaces of carbonates are epitaxially intergrown with one another in stacks of single domain crystals other magnetite nanocrystals are epitaxially intergrown with the surfaces of the carbonate substrates the observed magnetite carbonate hkl miller indices orientation relationships are 1 1 3 m 1 1 0 c and 1 1 1 m 0 0 3 c with lattice mismatches of approximately 13 and approximately 11 respectively epitaxy is a common mode of vapor phase growth of refractory oxides like magnetite as is the spiral growth about axial screw dislocations previously observed in other magnetite nanocrystals in alh 84001 epitaxy rules out intracellular precipitation of these magnetites by martian organisms provides further evidence of the high temperature 120 degrees c inorganic origins of magnetite in alh 84001 and indicates that the carbonates also have been exposed to elevated temperatures

quaternary palaeogeomorphologic evolution of the wadi faynan area southern jordan abstract the faynan area comprises a complex assemblage of deposits and landforms of varying nature age and position in the landscape the dominant facies are fluvial slope and aeolian at least eight palaeochannels are present at different heights within the wadis dana and ghuwayr fewer terraces are preserved in the wadi faynan planated surfaces leaving low gradient straths rock cut channels and fluvial deposits are all evident the continuing drop in regional base level as a result of subsidence of the dead sea trough has driven incision and aided in the retention of many of the terrace deposits as they have been abandoned as the wadi systems have eroded downwards calcretisation has enhanced the preservation potential of many of the fluvial deposits a large proportion of these deposits are comparable to those found in the modern wadi systems and it is thought that they formed under arid conditions similar to those that exist today two of the terrace deposits appear to reflect slightly wetter conditions as they consist of finer grained better sorted materials and display evidence of biological activity there are at least four distinct alluvial fan and slope deposits that have formed at the edge of various mountain fronts the fans have been entrenched to varying degrees and are likely to have formed at different times in the quaternary possibly when conditions were more humid or during periods of increased tectonic activity aeolian deposits indicate a marginally drier phase in the late glacial maximum and similar sediments of mid holocene age could either reflect increasing aridity at this time as has been suggested by frumkin et al 1991 and or be a result of the increasing impact of humans on the landscape many of the deposits are beyond the age range of optically stimulated luminescence osl and radiocarbon dating techniques but a proposed stratigraphy is supported by some numerical dates on the younger deposits and associated archaeological artefacts

palynology of pleistocene travertines from the arava valley israel abstract pollen samples from the arava travertines dated to oxygen isotope stages 5 and 7 show relatively high arboreal pollen levels these spectra are probably a result of an expansion of the mediterranean maquis in transjordan which indicates periods of wetter climatic conditions these periods probably represent humid fluctuations within stages 5 and 7

dead sea pollen provides new insights into the paleoenvironment of the southern levant during mis 6 5 abstract the paleoclimate of the southern levant especially during the last interglacial lig is still under debate reliable paleovegetation information for this period as independent evidence to the paleoenvironment was still missing in this study we present a high resolution pollen record encompassing 147 89 ka from the dead sea deep drilling core 5017 1a the sediment profile is marked by alternations of laminated marl deposits and thick massive halite indicating lake level fluctuations the pollen record suggests that steppe and desert components predominated in the dead sea surroundings during the whole investigated interval the late penultimate glacial 147 3 130 9 ka and early last glacial 115 5 89 1 ka were cool and relatively dry with sub humid conditions confined to the mountains that sustained moderate amounts of deciduous oaks prior to the lig optimum a prevalence of desert components and a concomitant increase in frost sensitive pistachio trees demonstrate the occurrence of an arid initial warming phase 130 9 124 2 ka the lig optimum 124 2 ka 115 5 ka was initiated by an abrupt grass expansion that was followed by a rapid spread of woodlands in the mountains due to increased moisture availability the remarkable sclerophyllous expansion points to a strong seasonal moisture deficit these results contradict previous dead sea lake level investigations that suggested pluvial glacials and a warm dry lig in the southern levant prominent discrepancies between vegetation and dead sea lake stands are also registered at 128 115 ka and the potential causes are discussed in particular while the pollen spectra mirror increased effective moisture during the lig optimum the massive halite deposition is indicative of an extremely low lake level given that the climate amelioration triggered the migration of early modern humans to the southern levant we speculate that the diverse ecosystems in the region provided great potential for their residence across the eastern mediterranean region an analogous vegetation succession pattern is observed

the tempo of geomorphological change evidence from southeastern australia a revised denudational chronology based on k ar dating of basalts for much of southeastern australia shows that many major topographic features are far older than was previously realized some upland surfaces originated in the mesozoic and others in the early tertiary in some areas the last major uplift and onset of canyon cutting occurred prior to the mid oligocene apart from the slow headward extension of canyons and localized high level stripping much of the region had taken on approximately its present day form by the mid miocene paradoxically however none of the mechanisms that have been proposed to explain the survival of paleoforms hold here these instances may well be typical of large parts of the interior of tectonic plates and a revision of widely held estimates of modal rates of denudation seems required

impact generated carbonate melts evidence from the haughton structure canada evidence is presented for the melting of dolomite rich target rocks during formation of the 24 km diameter 23 ma haughton impact structure on devon island in the canadian high arctic field studies and analytical scanning electron microscopy reveal that the s 200 m thick crater fill deposit which currently covers an v60 km 2 area in the center of the structure comprises fragmented target rocks set within a carbonate silicate matrix the silicate component of the matrix consists of si al mg rich glass the carbonate component is microcrystalline calcite containing up to a few wt si and al the calcite also forms spherules and globules within the silicate glass with which it develops microtextures indicative of liquid immiscibility dolomite clasts exhibit evidence of assimilation and may show calcite and rare dolomite overgrowths some clasts are penetrated by calcite and silicate injections along with the carbonate silicate glass textures the presence of pigeonite and spinifex textured diopside suggests that the matrix to the crater fill deposit was originally molten and was rapidly cooled this indicates that the impact event that generated haughton caused fusion of the predominantly dolomitic target rocks it appears that the ca mg component of the dolomite may have dissociated with mg entering the silicate melt phase while the ca component formed a caco3 dominant melt the silicates were derived by the fusion of lower paleozoic sandstones siltstones shales and impure dolomites evidence for melting is corroborated by a review of theoretical and experimental work which shows that caco3 melts at s 10 gpa and s 2000 k instead of dissociating to release co2 this work indicates that carbonate rich sedimentary targets may also undergo impact melting and that the volume of co2 released into the atmosphere during such events may be considerably less than previously estimated fl 2001 elsevier science b v all rights reserved

the history of allan hills 84001 revised multiple shock events the geologic history of martian meteorite allan hills alh 84001 is more complex than previously recognized with evidence for four or five crater forming impacts onto mars this history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient martian life in alh 84001 allan hills 84001 formed originally from basaltic magma its first impact event i1 is inferred from the deformation d1 that produced the granular textured bands crush zones that transect the original igneous fabric deformation d1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater an intense thermal metamorphism followed d1 and may be related to it the next impact i2 produced fractures fr2 in which carbonate pancakes were deposited and produced feldspathic glass from some of the igneous feldspars and silica after i2 carbonate pancakes and globules were deposited in fr2 fractures and replaced feldspathic glass and possibly crystalline silicates next feldspars feldspathic glass and possibly some carbonates were mobilized and melted in the third impact i3 microfaulting intense fracturing and shear are also associated with i3 in the fourth impact i4 the rock was fractured and deformed without significant heating which permitted remnant magnetization directions to vary across fracture surfaces finally alh 84001 was ejected from mars in event i5 which could be identical to i4 this history of multiple impacts is consistent with the photogeology of the martian highlands and may help resolve some apparent contradictions among recent results on alh 84001 for example the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition whereas the elongate magnetite grains epitaxial on carbonates could be ascribed to vapor phase deposition during i3

transmission electron microscopy of minerals in the martian meteorite allan hills 84001 we have studied carbonate and associated oxides and glasses in a demountable section of allan hills 84001 alh 84001 using optical scanning and transmission electron microscopy tem to elucidate their origins and the shock history of the rock massive fracture zone and fracture filling carbonates in typical locations were characterized by tem x ray microanalysis and electron diffraction in a comprehensive study that preserved textural and spatial relationships orthopyroxene is highly deformed fractured partially comminuted and essentially unrecovered lamellae of diaplectic glass and other features indicate shock pressures 30 gpa bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene carbonate crystals are 5 m in size untwinned and very largely exhibit the r3c calcite structure evidence of plastic deformation is generally found mildly only in fracture zone and fracture filling carbonates even adjacent to highly deformed orthopyroxene and appears to have been caused by low stress effects including differential shrinkage high dislocation densities like those observed in moderately shocked calcite are absent carbonate contains impact derived glasses of plagioclase silica and orthopyroxene composition indicating brief localized impact heating stringers and lenses of orthopyroxene glass in fracture filling carbonate imply flow of carbonates and crystallization during an impact periclase mgo occurs in magnesite as 30 50 nm crystals adjacent to voids and negative crystals and as 1 m patches of 3 nm crystals showing weak preferred orientation consistent with 111 mgo 0001 carb as observed in the thermal decomposition of caco3 to cao magnetite crystals that are epitaxially oriented at voids negative crystals and microfractures clearly formed in situ fully embedded faceted magnetites are topotactically oriented in general with 111 mag 0001 carb so that their oxygen layers are aligned in optically opaque rims magnetites are more irregularly shaped and except for the smallest crystals poorly aligned all magnetite and periclase crystals probably formed by exsolution from slightly non stoichiometric co2 poor carbonate following impact induced thermal decomposition any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity

evidence of past environmental conditions during the evolution of a calcretised wadi system in southern jordan using stable isotopes abstract a stratigraphically and temporally ordered sequence of channel calcretes preserved along the wadi dana southern jordan records the quaternary evolution of the formation and infilling of rock cut channels and their subsequent incision in a tectonically subsiding basin it is currently unknown under what palaeoenvironmental conditions these non pedogenic calcretes formed stable isotope analyses have been used to investigate whether any past topographical hydrological vegetational diagenetic and or temporal signatures can be identified from the channel calcretes the results of this research indicate that channel calcrete development is influenced by altitudinal variation affecting vegetation and hydrology within the landscape as well as location within individual wadi channels which has an affect on diagenetic processes the lack of calcretisation of a terrace that is between 1 and 1 5 m above the modern wadi floor supports the idea that the environment is currently too arid for calcrete to develop thus the presence of older wadi dana channel duricrusts suggests wetter conditions when they formed the 18o data of the calcrete implies cooler conditions than today as there is little evidence of a strong evaporative effect any temporal control is limited and mainly a function of stratigraphic position with some numeric dating

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

towards a high resolution global deep sea chronology for the last 750 000 years abstract variations in the ratio of 18 o 16 o as measured in shells of marine calcareous microfossils are primarily dominated by changes in global ice volume hence these variations provide a set of global time lines in deep sea sediments it is likely that the timing of major changes in oxygen isotope values is strongly influenced if not controlled by variations in the geometry of the earth s orbit since the variation of orbital parameters can be accurately calculated the opportunity exists for transforming this orbital chronology into a geological chronology through careful correlation of oxygen isotope records in a set of deep sea cores from the sub antarctic south atlantic and equatorial pacific we have assembled a composite isotopic section spanning the last 750 000 years with an average sedimentation rate of 2 3 cm 1000 years a new chronology for this time period was developed by adjusting the ages of the oxygen isotope stage boundaries in this composite section so as to extend the consistent phase relationships that exist between variations in oxygen isotope ratios and changes in obliquity and precession during the last 300 000 years to the entire 750 000 year record previously identified difficulties in phase locking precession with the filtered isotopic signal between 365 000 and 465 000 years b p have been resolved with the recognition that precessional variations have an average period of 19 000 years and not 23 000 years during this interval since this new age model yields the best match between variations in obliquity and precession and their corresponding frequency components in the oxygen isotope record we believe that it presents the most accurate chronology yet developed for deep sea sediments with this new age model providing the time control power spectral analyses of south atlantic and sub antarctic chemical and biotic indices show that there is a strong tendency for variance to be concentrated at frequencies corresponding to periods of 100 000 41 000 and 23 000 years

uranium series dating of quaternary deposits abstract this is a review of u series dating methods as applicable to terrestrial sedimentary materials alpha counting techniques allow the dating of carbonates and phosphates with a precision of 5 to 10 over the time range from 1 ka to 350 ka atom counting methods mass spectrometry and ams substantially increase precision and may increase the range problems of contamination and open system behavior are discussed correction for detrital contaminant is demonstrated in the dating of speleothem from a cave in italy

two populations of carbonate in alh84001 geochemical evidence for discrimination and genesis we present major and trace element oxygen isotope textural and structural data for carbonates and related phases in the snc meteorite alh84001 these data document the existence of at least two distinct carbonate populations one composed of finely zoned chemically and isotopically heterogeneous concretions of magnesio siderite with distinct white magnesite rims and a second composed of relatively homogeneous isotopically and compositionally simple domains of ankeritic carbonate and intimately intergrown glass and fine grained pyroxene we suggest on the basis of textural evidence and geochemical systematics that the first population consists of low temperature aqueous precipitates and the second is produced by shock melting of the first values of 18 o and sr ca ratios are correlated with one another in magnesio siderite concretions the trend formed by these data is consistent with the predicted relationship for inorganic precipitation of carbonate from a solution of constant composition between temperatures of 190 c for concretion cores to 20 c for magnesite rich concretion rims given the assumptions inherent in this temperature estimate the aqueous fluid parental to carbonate concretions is constrained to have a 18 o of 5 vsmow significantly mass fractionated compared with expected juvenile martian volatiles and minor element abundances broadly similar to terrestrial seawater

desert speleothems reveal climatic window for african exodus of early modern humans one of the fi rst movements of early modern humans out of africa occurred 130 100 thousand years ago ka when they migrated northward to the levant region the climatic conditions that accompanied this migration are still under debate using high precision multicollector inductively coupled plasma mass spectrometry mc icp ms u th methods we dated carbonate cave deposits speleothems from the central and southern negev desert of israel located at the northeastern margin of the saharan arabian desert speleothems grow only when rainwater enters the unsaturated zone and this study reveals that a major cluster of wet episodes the last recorded in the area occurred between 140 and 110 ka this episodic wet period coincided with increased monsoonal precipitation in the southern parts of the saharanarabian desert the disappearance at this time of the desert barrier between central africa and the levant and particularly in the sinai negev land bridge between africa and asia would have created a climatic window for early modern human dispersion to the levant

a simple inorganic process for formation of carbonates magnetite and sulfides in martian meteorite alh84001 we show experimental evidence that the zoned mg fe ca carbonates magnetite and fe sulfides in martian meteorite alh84001 may have formed by simple inorganic processes chemically zoned carbonate globules and fe sulfides were rapidly precipitated under low temperature 150 c hydrothermal and non equilibrium conditions from multiple fluxes of ca mg fe co2 s h2o solutions that have different compositions chemically pure single domain defect free magnetite crystals were formed by subsequent decomposition of previously precipitated fe rich carbonates by brief heating to 470 c the sequence of hydrothermal precipitation of carbonates from flowing co2 rich waters followed by a transient thermal event provides an inorganic explanation for the formation of the carbonate globules magnetite and fe sulfides in alh84001 in separate experiments kinetically controlled 13c enrichment was observed in synthetic carbonates that is similar in magnitude to the 13c enrichment in alh84001 carbonates

paleoclimatic implications of u series dates for lake sediments and travertines in the arava rift valley israel abstract the sayif travertines and lacustrine limestones cropping out in the northern arava valley a region of extreme aridity have been dated by the 230 th 234 u disequilibrium method the consistency between the stratigraphic position and the apparent ages at each of the sampling sites strongly supports the reliability of the ages most of the ages fall within two clusters that are coincident with ages representative of oceanic oxygen isotope stages 5 and 7 the colder intervening stage 6 was not a period of substantial travertine formation during the warmer global episodes of isotopic stages 5 and 7 the arid south of israel apparently was considerably wetter than at present

petrography diagenesis and facies controls on porosity in shannon sandstone hartzog draw field wyoming on the basis of lithology and sedimentary structures six sedimentary facies a f were distinguished in cores of shannon sandstone in hartzog draw field campbell county wyoming core porosities and permeabilities are highest in facies a average 15 and 16 md moderately high in facies b average 13 and 11 md low in facies c average 11 3 and 3 5 md and very low in facies d and e average 9 10 and 1 2 md dispersed detrital clay is a primary control on porosity but in cross bedded sandstones of facies a and b the main productive interval dispersed detrital clay is sparse and variations in porosity are due mainly to diagenesis microporosity does not contribute to permeability it is low in facies a and b high in facies c e and is associated mainly with detrital clay as microporosity increases water saturation increases the paragenetic sequence inferred is 1 siderite cementation 2 formation of a brown grain coating clay possibly illite smectite 3 quartz cementation 4 calcite and dolomite cementation 5 dissolution of carbonate cements and some framework grains 6 a second generation of quartz cementation along with some chlorite and interlayered illite smectite formation and 7 oil emplacement

geomagnetic field intensity recorded after impact in the ries meteorite crater germany summary we carried out palaeointensity experiments on 45 samples collected throughout the ca 15 ma ries crater germany to test whether meteorite impacts can influence the geodynamo a thellier thellier procedure with alteration tail and additivity checks yields a weighted average palaeointensity of 19 1 1 1 t from 15 samples that pass stringent data selection criteria the corresponding virtual dipole moment of 3 2 0 2 1022 am2 is relatively low but indistinguishable from the global palaeointensity database between 20 and 10 ma our results suggest that either the energy released during the ries meteorite impact event was too low to affect the geodynamo the ries impact lithologies cooled too fast to record any effect or the time averaged by the impact lithologies was long enough to record secular variation but that the geodynamo was remarkably stable over that time

sediment and pollen evidence for an early to mid holocene humid period in the eastern sahara a major problem in the study of holocene palaeoenvironments of the arid and wind deflated sahara is the low preservation potential of sediments from which a record of past climatic change can be established several indirect and imprecisely dated pieces of evidence have suggested humid episodes in the holocene that may have supported more productive ecosystems and denser human populations than today searches for reliable proxy data on past climates however have failed to yield satisfactory results we report here the discovery of buried lake muds in north west sudan in the hyperarid core of the eastern sahara which yield sedimentological and palynological data clearly interpretable as recording an early to mid holocene humid episode that supported a relatively deep stratified lake surrounded by tropical savanna woodland vegetation

shock and thermal history of martian meteorite allan hills 84001 from transmission electron microscopy microstructures in the allan hills 84001 meteorite were studied using optical and electron microscopy putting emphasis on shock effects which are widespread some orthopyroxene exhibits only 100 slip but more typical grains suffered extensive slip microfracturing and frequently contain 100 clino inversion lamellae in fracture zones shock deformation of orthopyroxene has produced all three effects in profusion together with intergranular pockets of orthopyroxene glass and intragranular glass lamellae which were apparently created by shearing on low index planes usually 100 or 110 both types of plane are loci that pseudo planar fractures tend to follow thus the glass lamellae which have not been observed in other meteorites probably formed by frictional heating during the sliding of microscale corrugated surfaces one over another leading to local melting we infer that the orthopyroxene glass and the fracture zones both formed from shear stresses created by strong shock ubiquitous undeformed micrometer and submicrometer euhedral chromites in orthopyroxene and plagioclase glasses and carbonate probably crystallized after shock heating and fracture zone formation nanocrystals of eskolaite cr2o3 coating silica glass grains are probably also a result of shock induced thermal decomposition of chromite iron sulfides pyrite and pyrrhotite were identified tended to be associated with plagioclase glass a carbonate disk showing no evidence for shock deformation had a substructure of elongated slightly misoriented subcells in the exterior interior regions had more eqiaxed subcells both microstructures probably formed during growth but the conditions are undetermined chemical composition varied on a micron scale but the rim of the disk was more ferroan oxide precipitates and voids were widely distributed as in fracture filling carbonates if the fracture zones and opx glass are the result of strong shock as we deduce it is very unlikely that pores could have filled by carbonate long after the fracture zones formed we infer that the carbonate like the phosphate olivine pyrrhotite eskolaite and many euhedral submicrometer chromites crystallized during the final stages of the impact that created the fracture zones and glasses with compositions of plagioclase silica and orthopyroxene

the link between biomineralization and fossilization of bacteria insights from field and experimental studies abstract fossil biominerals and fossils of microorganisms in ancient rocks contain important biogeochemical signals decoding this record may reveal crucial information about the evolution of life on earth and terrestrial paleoenvironments however the identification of traces of life especially in very old rocks is extremely challenging because the morphological and chemical signatures of biominerals and microfossils are subtle of microscopic size and inevitably altered with aging in this review we stress on the fact that biomineralization is often the first step of fossilization and produces particular chemical structural and morphological features that can be preserved in fossil biominerals or microfossils with a special focus on fe biomineralization the taphonomic processes affecting biominerals and microfossils and altering their morphology and or chemical composition over time are then discussed in light of experimental fossilization simulations and field sample analyses we suggest that taxonomic biases observed in the fossil record may be related to differential abilities of species to trigger biomineralization this calls for studies of the effective biomineralizing activity and fossilization potential of each species present in highly diverse natural microbial communities finally recent analytical developments leave little doubt that very substantial progress in the study of biomineralization processes and ancient biostructures will be achieved in the near future

magnetite in alh 84001 an origin by shock induced thermal decomposition of iron carbonate in martian orthopyroxenite alh 84001 pockets of feldspathic glass frequently contain carbonate masses that have been disrupted and dispersed within feldspathic shock melt as a result of impact s transmission electron microscope studies of carbonate fragments embedded within feldspathic glass show that the fragments contain myriad nanometer sized magnetite particles with cuboid irregular and teardrop morphologies frequently associated with voids the fragments of carbonate must have been incorporated into the melt at temperatures of 900 c well above the upper thermal stability of siderite feco3 which decomposes to produce magnetite and co2 below 450 c these observations suggest that most if not all of the fine grained magnetite associated with fe bearing carbonate in alh 84001 could have been formed as result of the thermal decomposition of the siderite feco3 component of the carbonate and is not due to biological activity