marine oxygenation lithistid sponges and the early history of paleozoic skeletal reefs abstract microbial carbonates were major components of early paleozoic reefs until coral stromatoporoid bryozoan reefs appeared in the mid ordovician microbial reefs were augmented by archaeocyath sponges for 15 myr in the early cambrian by lithistid sponges for the remaining 25 myr of the cambrian and then by lithistid calathiid and pulchrilaminid sponges for the first 25 myr of the ordovician the factors responsible for mid late cambrian microbial lithistid sponge reef dominance remain unclear although oxygen increase appears to have significantly contributed to the early cambrian explosion of marine animal life it was followed by a prolonged period dominated by greenhouse conditions as sea level rose and co2 increased the mid late cambrian was unusually warm and these elevated temperatures can be expected to have lowered oxygen solubility and to have promoted widespread thermal stratification resulting in marine dysoxia and hypoxia greenhouse conditions would also have stimulated carbonate platform development locally further limiting shallow water circulation low marine oxygenation has been linked to episodic extinctions of phytoplankton trilobites and other metazoans during the mid late cambrian we propose that this tendency to dysoxia hypoxia in shallow marine environments also limited many metazoan reef builders in contrast during the mid late cambrian the ability of lithistid sponges to withstand low oxygen levels allowed them to create a benthic association with microbial carbonates that dominated global reefs these conditions ameliorated during the ordovician as temperature decline promoted ocean ventilation the prolonged time gap occupied by low diversity reefs between the cambrian explosion and the great ordovician biodiversification event reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments

thin section and s e m textural criteria for the recognition of cement dissolution porosity in sandstones cement porosity relationships are described from the lower triassic sherwood sandstone group and the middle jurassic ravenscar group in the united kingdom calcite cemented sandstones display a variety of replacement textures with preferential replacement of grains and of overgrowth faces with high free surface energy dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains these range from small v shaped notches and pits through regular and irregular shaped embayments into large depressions these textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones and are interpreted to have been formed by the dissolution of pore filling and grain replacive authigenic carbonates this is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones these textures indicate that part of the intergranular porosity in these sediments is secondary in origin and has been generated by the dissolution of carbonate cements the identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface

a rudimentary mechanistic model for soil production and landscape development a rudimentary mechanistic model for soil production and landscape development is proposed the continuity equation of the model assumes that the change in soil thickness over time depends on the production of soil from the weathering of bedrock and the transport of soil through natural surface erosion the parameters for the model include the weathering rate and erosive diffusivity weathering rate is expressed as an exponential decay function of soil thickness which represents mechanical weathering erosive diffusivity can be estimated from soil erosion models the model is solved numerically using the finite difference approach and is applied to a numerical example simulation is performed for a hypothetical landscape with a series of hills and valleys results show the development rate of soil initially is very large and slows down until it reaches a steady state defined by a constant change in soil thickness the model also exhibits the characteristics of a nonlinear dynamic system nonlinearity of soil thickness and curvature and initial randomness appears to cause chaos instability in the system the results show promising progress in quantitative modelling of pedogenesis the limitations and suggested improvements of the model are also presented

trace fossils and substrates of the terminal proterozoic cambrian transition implications for the record of early bilaterians and sediment mixing the trace fossil record is important in determining the timing of the appearance of bilaterian animals a conservative estimate puts this time at 555 million years ago the preservational potential of traces made close to the sediment water interface is crucial to detecting early benthic activity our studies on earliest cambrian sediments suggest that shallow tiers were preserved to a greater extent than typical for most of the phanerozoic which can be attributed both directly and indirectly to the low levels of sediment mixing the low levels of sediment mixing meant that thin event beds were preserved the shallow depth of sediment mixing also meant that muddy sediments were firm close to the sediment water interface increasing the likelihood of recording shallow tier trace fossils in muddy sediments overall trace fossils can provide a sound record of the onset of bilaterian benthic activity

the ediacaran cambrian rise of siliceous sponges and development of modern oceanic ecosystems abstract the lack of unequivocal sponge fossils before the cambrian despite their probable deep origin during the cryogenian period has been a conundrum to geologists their impact on the dramatic evolution of ecosystems and the seawater silica cycle during the ediacaran cambrian e c transition is also speculative in this study abundant sponge spicules and spicule like structures that probably represent original sponge fossils were recovered from four sections of the e c boundary interval in the yangtze gorges south china the paleontological and geochemical data presented herein provides evidence for a continuous distribution of relatively abundant sponge spicules some even forming spiculites as well as the earliest biogenic deposition of silica by metazoans in the ediacaran cambrian boundary interval these results further confirm that a biological takeover of oceanic dissolved si co occurred with the evolution of silica biomineralization in the e c interval resulting in the widespread deposition of biogenic chert hydrothermal input in spicule bearing cherts is also observed such hydrothermal activity might have favored the blooming of sponges through significant nutrient supply these results provide the first conjunction of geochemical and paleontological proxies in support of previous models considering that sponges by ventilation filtration and oxygenation of seawater were important ecosystem engineers of the e c bioradiation event and of the related establishment of modern type ecosystems

the advent of hard part structural support among the ediacara biota ediacaran harbinger of a cambrian mode of body construction the apparent lack of taxonomic continuity between the precambrian and cambrian fossil records has led to controversial and conflicting interpretations about the ediacara biota and their place in the evolution of metazoan life on this planet this has been further complicated by the absence of similar modes of construction between these faunas and the rarity of precambrian skeletonized fossils we describe a new ediacaran organism that represents the oldest multielement organism with structural support through either biomineralization or chitin coronacollina acula gen et sp nov from the ediacara member rawnsley quartzite was constructed from a framework of rigid and brittle elements that disarticulated after death it reveals a constructional mode not recognized previously among members of this assemblage but one that was prevalent among cambrian organisms coronacollina consists of a truncated cone associated with spicules up to 37 cm in length diverging radially from the cone this constructional morphology is similar to the cambrian choia a low conical demosponge with a corona of long spicules providing a long predicted constructional link between the ediacara biota and the cambrian fossil record

influence of chemical denudation on hillslope morphology 1 models of hillslope evolution involving diffusion like sediment transport are conventionally presented as an equation in which the changes in land surface elevation or soil thickness are balanced by the divergence of soil transport and tectonic uplift soil production or both these models typically do not include the loss or gain of mass in hillslope soils due to processes of chemical weathering and deposition we formulate a more general depth integrated equation for the conservation of soil mass on a hillslope that includes a term representing chemical deposition or denudation this general depth integrated equation is then simplified to determine the one dimensional form of a steady state hillslope which experiences both mechanical and chemical denudation the differences in morphology between hillslopes only experiencing diffusion like mechanical sediment transport and hillslopes experiencing both diffusion like mechanical sediment transport and chemical denudation are explored under the conditions of a downslope increase in local soil lowering rate due to chemical weathering the hillslope profile will depart from the parabolic shape predicted by models that incorporate only linear diffusion like mechanical sediment transport in addition hillslopes that experience both chemical and mechanical denudation may have a convex concave profile at steady state a necessary condition for such steady state profiles is that the chemical denudation rate must exceed the mechanical denudation rate we further suggest that combinations of other physical parameter values such as total denudation rate average soil depth sediment diffusivity and the increase in soil depth away from the divide that lead to steady state convex concave hillslope profiles may exist in a wide variety of natural settings

spatial modelling and uncertainty assessment of pyroclastic cover thickness in the sorrento peninsula assessing spatial variability of soil thickness is a critical issue for understanding and predicting slope processes the present work was aimed at estimating the spatial scales at which the variation of pyroclastic cover thickness occurs in a sample area in the sorrento peninsula italy stochastic simulation was used to understand the spatial variability of pyroclastic cover thickness on mount pendolo and to assess its spatial uncertainty in the study area covering about 0 7 km2 thickness measurements were collected using electrical resistivity tomography profiles continuous core drillings and steel rod penetrometric tests variographic analysis revealed the occurrence of an anisotropic behaviour along the n50 and n140 directions in the latter anisotropic direction a nested variogram was fitted including 1 a long range component which could be related to large scale factors like the curvature of the slope and contributing area and 2 a shorter scale variation which is probably associated with the occurrence of denudation processes or to the articulate cover bedrock interface to assess the spatial variability and uncertainty of pyroclastic cover thickness a stochastic simulation algorithm was used and 500 equally probable images of cover thickness were yielded the results showed that a better thickness distribution map can be drawn by simulating the data collected on the slope and at the footslope separately the approach also allowed delineating the areas characterized by greater uncertainty suggesting supplementary measurements to further improve the cover thickness distribution model thus reducing the uncertainty

secular change in the precambrian silica cycle insights from chert petrology chert deposits preserve a record of secular change in the oceanic silica cycle the evolutionary radiation of silica secreting organisms resulted in a transition from abiological silica deposition characteristic of the archean and proterozoic eons to the predominantly biologically controlled silica deposition of the phanerozoic comparative petrography of phanerozoic and precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the paleoproterozoic era ca 1 8 ga in neoproterozoic and mesoproterozoic strata early diagenetic chertification is largely restricted to peritidal environments these early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures the cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation some of the paleoproterozoic cherts associated with iron formations however are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed these primary cherts lack ghosts or inclusions of carbonate precursors have fine scale grain fracturing possibly from syneresis exhibit low grain packing densities and are not associated with unsilicified carbonate deposits of similar depositional composition cherts in some paleoproterozoic iron formations e g the gunflint formation northwestern lake superior region are composed of silica types similar to those in phanerozoic sinters e g the devonian rhynie and windyfield cherts scotland such cherts may provide evidence that basinal and perhaps global oceanic silica concentrations were higher during the paleoproterozoic era than at later times

mineralogical constraints on the paleoenvironments of the ediacaran doushantuo formation assemblages of clay minerals are routinely used as proxies for paleoclimatic change and paleoenvironmental conditions in phanerozoic rocks however this tool is rarely applied in older sedimentary units in this paper the clay mineralogy of the doushantuo formation in south china is documented providing constraints on depositional conditions of the ediacaran yangtze platform that host the earliest animal fossils in the geological record in multiple sections from the yangtze gorges area trioctahedral smectite saponite and its diagenetic products mixed layer chlorite smectite corrensite and chlorite are the dominant clays through the lower 80 m of the formation and constitute up to 30 wt of the bulk rock saponite is interpreted as an in situ early diagenetic phase that formed in alkaline conditions ph 9 the absence of saponite in stratigraphically equivalent basin sections 200 400 km to the south indicates that alkaline conditions were localized in a nonmarine basin near the yangtze gorges region this interpretation is consistent with crustal abundances of redox sensitive trace elements in saponitic mudstones deposited under anoxic conditions as well as a 10 difference in the carbon isotope record between yangtze gorges and basin sections our findings suggest that nonmarine environments may have been hospitable for the fauna preserved in the yangtze gorges which includes the oldest examples of animal embryo fossils and acanthomorphic acritarchs

the age of the nantuo formation and nantuo glaciation in south china a u pb sensitive high resolution ion microprobe shrimp age of 654 5 3 8 ma from an ash bed immediately below the nantuo formation in south china provides the lowest age constraint for the nantuo glaciation which has been correlated with the global marinoan glaciation a u pb shrimp age of 636 3 4 9 ma from a fallout tuff within the basal layer of the nantuo formation along with the existing age of 635 2 0 6 ma from its overlying doushantuo cap carbonate suggests that the traditionally defined nantuo formation may have been deposited in a short time period at the end of the nantuo glaciation in combination with available ages globally the data support a relatively short duration and rapid termination of marinoan glaciations

biogenic silica dissolution in sediments of the southern ocean i solubility abstract a stirred flow through reactor technique was used to determine silica solubilities in sediments collected with a multicorer in the indian sector of the southern ocean antares i cruise the results show that the apparent silica solubility in the cores may decrease increase or remain constant with depth the silica solubility profiles are best explained by the early diagenetic interactions between biogenic silica and soluble aluminum derived from detrital material by combining the solubility data with measured dissolved silica profiles it is shown that the variable asymptotic pore water silica levels in the cores cannot be explained solely by differences in silica solubility in sediments that experience a significant detrital input the simultaneous reprecipitation of dissolved aluminum and dissolved silica prevents water silicic acid from reaching saturation with the dissolving biogenic silica the principal oceanographic control on pore water silica build up in the cores studied is the ratio of the deposition fluxes of biogenic silica and detrital material solubility differences inherited from the biomineralization process in the surface waters do not appear to have a significant effect on the observed pore water silica levels

ediacarian sponge spicule clusters from southwestern mongolia and the origins of the cambrian fauna carbon and strontium isotopic data are used to show that the earliest sponge spicule clusters and associated phosphatic sediments with anabarites from southwestern mongolia are of ediacarian age spicule morphologies include bundles of oxeas arranged in three dimensional quadrules linked together at junctions by tetracts pentacts hexacts or polyactines all are referred to the phylum porifera class hexactinellida these sponge spicules provide the oldest remains that can be assigned without question to an extant phylum and also the first firm evidence for filter feeding and metazoan silica biomineralization in the fossil record it is suggested that siliceous and phosphatic members of the cambrian fauna may have had their origins in eutrophic and outer shelf facies of the late proterozoic

spatial patterns and controls of soil chemical weathering rates along a transient hillslope article i nfo hillslopes have been intensively studied by both geomorphologists and soil scientists whereas geomorphologists have focused on the physical soil production and transport on hillslopes soil scientists have been concerned with the topographic variation of soil geochemical properties we combined these differing approaches and quantified soil chemical weathering rates along a grass covered hillslope in coastal california the hillslope is comprised of both erosional and depositional sections in the upper eroding section soil production is balanced by physical erosion and chemical weathering the hillslope then transitions to a depositional slope where soil accumulates due to a historical reduction of channel incision at the hillslope s base measurements of hillslope morphology and soil thickness were combined with the elemental composition of the soil and saprolite and interpreted through a process based model that accounts for both chemical weathering and sediment transport chemical weathering of the minerals as they moved downslope via sediment transport imparted spatial variation in the geochemical properties of the soil inverse modeling of the field and laboratory data revealed that the long term soil chemical weathering rates peak at 5gm 2 yr 1 at the downslope end of the eroding section and decrease to 1 5gm 2 yr 1 within the depositional section in the eroding section soil chemical weathering rates appear to be primarily controlled by the rate of mineral supply via colluvial input from upslope in the depositional slope geochemical equilibrium between soil water and minerals appeared to limit the chemical weathering rate soil chemical weathering was responsible for removing 6 of the soil production in the eroding section and 5 of colluvial influx in the depositional slope these were among the lowest weathering rates reported for actively eroding watersheds which was attributed to the parent material with low amount of weatherable minerals and intense coating of the primary minerals by secondary clay and iron oxides we showed that both the morphologic disequilibrium of the hillslope and the spatial heterogeneity of soil properties are due to spatial variations in the physical and chemical processes that removed mass from the soil

pedogenic hematitic concretions from the triassic new haven arkose connecticut implications for understanding martian diagenetic processes abstract the triassic new haven arkose locally contains mm scaled pedogenic hematitic concretions within red arkosic sandy mudstones that provide insights into concretion forming processes on earth and mars concretions represent 8 of the sediment by mass 7 by volume are irregularly distributed and have a near normal size distribution with a mean diameter of 1 53 mm and graphic standard deviation of 0 64 mm x ray diffraction xrd and attenuated total reflectance atr spectroscopy indicate that the concretions are composed of ambient sediments quartz montmorillonite and feldspar and a minor phosphatic phase that is also implied by geochemistry however the concretions preferentially exclude coarser grain sizes and are cemented by hematite and goethite optical observations and synchrotron x ray fluorescence xrf chemical mapping indicate that concretions are massive to weakly zoned with respect to hematitic cements compared to the surrounding sediments concretion rare earth elements ree are elevated in total abundances exhibit light rare earth element lree enrichment and possess negative ce anomalies th u ratios are lower due to elevated u mass balance calculations indicate that 20 of the concretions are composed of iron oxides and that 30 mm3 of ambient sediment is required to provide the hematitic cement in a single 1 5 mm diameter concretion element mobility during concretion formation was tested assuming different immobile elements al ti zr assuming zr immobility provides intermediate results and indicates the following gains in concretions pb 572 fe 322 mn 142 ree 438 116 v 138 u 124 ni 84 and nb 58 other elements show either gains or losses of less than 50 ce anomalies low th u and elevated v and u abundances point to a significant redox influence on element distributions in the concretions high iron content crude internal concentric banding and redox controls suggest that the concretions are formed in seasonally variable but generally moist soil conditions with annual precipitation 130 cm trace element enrichment patterns are broadly consistent with derivation from downward percolating weathering fluids although the new haven arkose pedogenic concretions have significant differences and were clearly formed by different mechanisms than hematitic spherules discovered on mars they nevertheless exhibit a number of textural e g sphericity size distributions and geochemical e g ni enrichments similarities that support models suggesting that the martian spherules are formed as sedimentary concretions

rinded iron oxide concretions hallmarks of altered siderite masses of both early and late diagenetic origin iron bearing concretions are valuable records of oxidation states of subsurface waters but the first concretions to form can be altered drastically during later diagenetic events distinctive concretions composed of heavy rinds of iron oxide that surround iron poor mud rich cores are common along bases of fluvial cross bed sets of the cretaceous dakota formation nebraska usa concretion rinds thicken inward and cores contain 46 to 89 void space millimetre scale spherosiderites are abundant in palaeosols that developed in floodplain facies evolution of rinded concretions began when intraformational clasts were eroded from sideritic soils transported abraded and deposited in river channels alteration of siderite and formation of rinds occurred much later perhaps in the quaternary when sandstone pore waters became oxic dakota concretions are analogous to rattlestones in pleistocene fluvial channels of the netherlands and their rinded structure is analogous to that of iron rich concretions in the aeolian navajo sandstone of utah in all three deposits rinded concretions formed when pre existing siderite cemented concretions were oxidized within a sand matrix unlike fluvial examples siderite in the navajo sandstone was autochthonous and of late diagenetic origin having precipitated from carbon dioxide and methane enriched waters moving through folded and jointed strata iron rich rinds formed in all these strata because concretion interiors remained anaerobic even as oxygen accumulated in the pore waters of their surrounding permeable matrix iron oxide first precipitated at redox boundaries at concretion perimeters and formed an inward thickening rind acid generated by the oxidation reaction drove siderite dissolution to completion creating the iron poor core iron oxide rinds are indicators of the former presence of siderite a mineral that forms only under reducing conditions during either early or late diagenesis siderite is vulnerable to complete oxidation upon exposure so the distinctive rinded concretions are valuable clues that aid in deciphering diagenetic histories and for recognizing methanic floodplain palaeoenvironments and wet palaeoclimate

the spatial variation of weathering and soil depth across a triassic sandstone outcrop in this study we used an archive of borehole logs from the british geological survey to collect information on the spatial structure of weathering that extends from the surface to competent bedrock across the triassic sherwood sandstone group outcrop 750 km2 in the east midlands uk the borehole logs were used to estimate the thickness of the soil n 280 and soil and saprolite ss the overburden of ice may have increased sub glacial pore water pressure with the fractures and faults acting as a drainage system for the removal of dissolved weathering products

enigmatic chambered structures in cryogenian reefs the oldest sponge grade organisms abstract previously undescribed chambered structures are common and widespread in the cryogenian post sturtian glacial carbonates of the oodnaminta reef complex adelaide geosyncline south australia the rasthof berg aukas formation namibia and the gauss formation namibia these carbonate structures have millimetre to centimetre scale chambers separated by well defined and generally thin micritic walls chamber walls now consist of dolomite but were probably originally aragonitic the chambers may have a lobate polygonal or dendritic morphology and are often further divided into smaller chambers chambered structures occur as reefal growth frameworks as cavity fillings in neptunean dykes and growth cavities and as intercolumnar material within stromatolite frameworks in the oodnaminta reef complex these structures are only present in the sub photic deep water framework these structures probably represent the calcified remains of an organism or community of organisms that was globally distributed and widespread for a significant time period following the sturtian glaciation no precisely analogous structures have been previously described from modern or ancient settings but the complexity and degree of organization suggests a significant evolutionary advance over older proterozoic fossils the closest morphological analogues for the structures are a some types of reef dwelling sponges and b some complex microbialites from archaean and paleoproterozoic carbonates the structures lack spicules and ostia found in sponges ruling out a true poriferan origin however it is plausible that they are proto sponges sponge grade organisms or complex microbial precursors to sponge grade organisms whatever their affinity we suggest these structures record a significant evolutionary event on the path towards organic complexity

organic walled microfossils from earliest cambrian or latest proterozoic tindir group rocks northwest canada abstract uppermost tindir group limestones exposed in the headwaters of tindir creek yukon territory canada contain a diverse and abundant microbiota of mineralized and organic walled microfossils preserved primarily in chert modules and chert beds the organic walled forms include mat building cyanobacterial filaments and coccoids accompanied by less common bacteria fungi and forms of uncertain affinity as well as planktonic coccoidal cyanobacteria and acritarchs several lines of evidence support interpretation of an early cambrian age whereas others suggest a late proterozoic age of the 54 organic walled taxa recognized 12 are newly described yukonosphaeridion interior awramik n gen n sp microagglomeratus borealis awramik n gen n sp phacelogeminus lineatus awramik n gen n sp palaeoanacystis magna awramik n sp cephalophytarion majesticum allison n sp heliconema bulbosa allison n sp trachyhystrichosphaera magna allison n sp fusilabellum brevistriatum awramik n gen n sp sphaeranasillos irregularis allison n gen n sp hyalocyrillium clardyi allison n gen n sp eophycomyces herkoides allison n gen n sp and archeomyces dimakeloides allison n gen n sp

overview of the microscopic imager investigation during spirit s first 450 sols in gusev crater the microscopic imager mi on the mars exploration rover spirit has returned images of mars with higher resolution than any previous camera system allowing detailed petrographic and sedimentological studies of the rocks and soils at the gusev landing site designed to simulate a geologist s hand lens the mi is mounted on spirit s instrument arm and can resolve objects 0 1 mm in size or larger this paper provides an overview of mi operations data calibration processing and analysis of mi data returned during the first 450 sols mars days of the spirit landed mission the primary goal of this paper is to facilitate further analyses of mi data by summarizing the methods used to acquire and process the data the radiometric and geometric accuracy of mi data products and the availability of archival products in addition scientific results of the mi investigation are summarized mi observations show that poorly sorted soils are common in gusev crater although aeolian bedforms have well sorted coarse sand grains on their surfaces abraded surfaces of plains rocks show igneous textures light toned veins or fracture filling minerals and discrete coatings the rocks in the columbia hills have a wide variety of granular textures consistent with volcaniclastic or impact origins case hardening and submillimeter veins observed in the rocks as well as soil crusts and cemented clods imply episodic subsurface aqueous fluid movement which has altered multiple geologic units in the columbia hills the mi also monitored spirit s solar panels and the magnets on the rover s deck

quantification of soil production and downslope creep rates from cosmogenic 10be accumulations on a hillslope profile average soil transport rates over a period of 3500 yr on a convex soil mantled hillslope have been quantified using a mass balance model that incorporates the soil concentration of the cosmogenic isotope 10 be the 10 be model results support the assumption used in most geomorphic models that the soil creep rate is proportional to surface gradient the predicted diffusion coefficient is 360 55 cm 3 yr 1 cm 1 contour length and the average rate of soil production is 0 026 0 007 cm yr within the uncertainty of this technique the data do not reject g k gilbert9s hypothesis that some hillslopes may exist in a condition of dynamic equilibrium with a uniform soil production rate however the model does not require an assumption of dynamic equilibrium and may be an approach that uniquely allows the quantification of a local soil production rate law

slope gradient and aspect effects on soils developed from sandstone in pennsylvania abstract this study was initiated to determine the basic characteristics and genesis of soils on gentle to very steep slopes on two contrasting slope aspects two transects a southwest sw and a northwest nw transect were laid out on a sandstone ridge across the summit and steepest convex shoulder positions the thickness of the o a and e horizons was not related to slope gradient across soils on all slope gradients and aspects this indicates that the present rate of soil formation is greater than erosion the influence of erosion on steep slopes could be off set by lateral movement of through flow water above the b horizon or the process of tree throw masks slope gradient effects solum depth b horizon thickness clay fe and al indexes decreased with increasing slope for both the nw and sw aspects organic carbon index decreased with increasing slope for the sw aspect decreases in effective precipitation over relatively larger surface areas at steep versus lower slope gradients a longer influence of periglacial erosion following the wisconsinan glacial advance and present slow but significant soil creep could cause these differences in soil development across the slope solum depth b horizon thickness clay fe and al indexes showed a smaller decrease across the nw compared to the sw slope organic carbon did not decrease across the nw slope the e horizon was also slightly thicker on the nw compared to the sw slope these aspect differences were probably caused by less evaporation on shaded nw compared to sunny sw slopes which produce generally higher moisture levels and more effective eluviation and illuviation on the nw slope

radiolarian palaeoecology and radiolarites is the present the key to the past abstract radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint evolutionary selection pressure among silica secreting marine plankton both radiolarians and diatoms has led toward more economic usage of rapidly shrinking nutrient resources including dissolved silica of the photic zone in the late cenozoic oceans and in particular a substantial modification of oceanic cycle by the diatom explosive radiation even if there is a proved link between biomineralization and dissolved silica loading among the phytoplankton only the relative independence of modern siliceous planktic biotas from the available silica pool reflects mainly their progressive physiological specialisation during evolutionary history oceanic chemistry and productivity as well as patterns of circulation upwelling have changed radically during the phanerozoic radiolarites apparently represent an anachronistic facies as exemplified by their long lived and ocean wide distribution in palaeo pacific and hitherto highlighted actualistic models of localized intra oceanic wind driven upwelling loci are of largely questionable applicability in addition to plate drift hypersiliceous domains and intervals are explainable mostly by a large scale volcano hydrothermal activity during major plate boundary reconfigurations which in many ways favoured siliceous biotas acme and their skeletal remains accumulation and preservation factors tied to rapid voluminous submarine eruptions such as thermal buoyant megaplumes and basin overturns offer a viable alternative for traditional climatic circulation scenarios in case of hypersiliceous high productivity events irrelevant to greenhouse to icehouse climatic change the evolving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering volcano hydrothermal and tectonic uplift events related mostly to extensive rifting and or accelerated oceanic spreading were the endogenous driving force that created this perturbation of the exogenous system the present biogeochemical cycle is representative only for the overall silica depleted post eocene oceanic ecosystems which broadly correlates with a major expansion of diatoms groups extremely efficient in silica removal and closely linking the silica budget with phosphorus and nitrogen cycles thus an orthodox uniformitarian approach to biosiliceous sedimentation based on a silica starved vigorous ocean is of limited significance when applied to the pre neogene settings especially in the peculiar planktic habitats of epeiric seas as well as during biotic crises marked by strong geotectonic overprint the major turnovers in marine siliceous biota composition in particular after the end permian radiolarite gap may have been coupled with discernible changes in an increasing biological control on the long term oceanic silica cycling punctuated equilibrium the evolutionary turnovers have induced a stepdown decrease of dissolved silica levels through the phanerozoic contemporaneously with the general secular trend of upward scaling of nutrient related ecological processes and increased effectiveness of resource utilization

sponge spicules from the lower cambrian in the yanjiahe formation south china the earliest biomineralizing sponge record abstract sponges are the earliest metazoans and according to molecular clocks have a deep history back into the cryogenian however the record of their biomineralized elements below the ediacaran cambrian boundary has been controversial here we present the first and earliest indisputable record of hexactinellid spicules in the lowest cambrian terreneuvian below the small shelly fossil anabarites trisulcatus protohertzina anabarica assemblage zone zone i spicules recovered from cherts of the lowest yanjiahe formation in the yichang hubei province south china include monaxons diaxons triaxons and that are all siliceous in composition this earliest record of silica biomineralization by filter feeding metazoan suggests sponges have contributed to the cycling of silica in the oceanic system as early as the beginning of the phanerozoic these results along with a review of previously reported sponge occurrences suggest a two step evolution on sponges of the early cambrian

neoproterozoic to early cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the yangtze platform china abstract small shelly fossils ssfs occur widely on the yangtze platform and have great potential for biostratigraphic subdivision of pre trilobitic early cambrian strata based on the ssf record of the shallow water realm five biozones can be recognized for the meishucunian stage in ascending order the biozones are anabarites trisulcatus protohertzina anabarica assemblage zone paragloborilus subglobosus purella squamulosa assemblage zone watsonella crosbyi assemblage zone formerly heraultipegma yunnanensis zone poorly fossiliferous interzone sinosachites flabelliformis tannuolina zhangwentangi assemblage zone in addition one ssf biozone is recognized for the overlying qiongzhusian stage pelagiella subangulata taxon range zone the formerly used siphogonuchites triangularis paragloborilus subglobosus zone and heraultipegma yunnanensis zone are discussed and redefined approximately 80 species of ssfs were screened for their spatial and temporal distribution on the yangtze platform variations in lithofacies and biofacies can be recognized throughout the yangtze platform extending over an area of 2000 900 km in a deeper water shelf setting the first zone is represented by the protohertzina anabarica kaiyangites novilis assemblage zone while younger zones are not recognized in the southern region at the northern platform margin the qiongzhusian is represented by the ninella tarimensis cambroclavus fangxianensis assemblage zone and the rhombocorniculum cancellatum taxon range zone the southeastern shaanxi western hubei region followed a slightly different lithological and faunal development than the rest of yangtze platform indicating a stronger similarity with parts of east gondwana some taxa such as w crosbyi p subangulata r cancellatum microdictyon effusum a trisulcatus protohertzina unguliformis and p anabarica occur nearly worldwide and support an international correlation of early cambrian sequences between the yangtze platform and smaller west gondwanan blocks siberia newfoundland and australia the six investigated zones of the yangtze platform comprise an interval spanning the early nemakit daldynian to the late atdabanian early botoman stage on the siberian platform palaeobiogeographic analysis revealed a strong taxic similarity between the yangtze platform and the tarim platform a smaller number of species are shared with other west gondwanan platform fragments such as india and iran palaeobiogeographic results do not support the previously reported position of the south china block between australia and siberia during the early cambrian

athena microscopic imager investigation 1 the athena science payload on the mars exploration rovers mer includes the microscopic imager mi the mi is a fixed focus camera mounted on the end of an extendable instrument arm the instrument deployment device idd the mi was designed to acquire images at a spatial resolution of 30 microns pixel over a broad spectral range 400 700 nm the mi uses the same electronics design as the other mer cameras but has optics that yield a field of view of 31 31 mm across a 1024 1024 pixel ccd image the mi acquires images using only solar or skylight illumination of the target surface a contact sensor is used to place the mi slightly closer to the target surface than its best focus distance about 66 mm allowing concave surfaces to be imaged in good focus coarse focusing 2 mm precision is achieved by moving the idd away from a rock target after the contact sensor has been activated the mi optics are protected from the martian environment by a retractable dust cover the dust cover includes a kapton window that is tinted orange to restrict the spectral bandpass to 500 700 nm allowing color information to be obtained by taking images with the dust cover open and closed mi data will be used to place other mer instrument data in context and to aid in petrologic and geologic interpretations of rocks and soils on mars

the humped soil production function eroding arnhem land australia we report erosion rates and processes determined from in situ produced beryllium 10 10be and aluminum 26 26al across a soil mantled landscape of arnhem land northern australia soil production rates peak under a soil thickness of about 35 cm and we observe no soil thicknesses between exposed bedrock and this thickness these results thus quantify a well defined humped soil production function in contrast to functions reported for other landscapes we compare this function to a previously reported exponential decline of soil production rates with increasing soil thickness across the passive margin exposed in the bega valley south eastern australia and found remarkable similarities in rates the critical difference in this work was that the arnhem land landscapes were either bedrock or mantled with soils greater than about 35 cm deep with peak soil production rates of about 20 m ma under 35 40 cm of soil thus supporting previous theory and modeling results for a humped soil production function we also show how coupling point specific with catchment averaged erosion rate measurements lead to a better understanding of landscape denudation specifically we report a nested sampling scheme where we quantify average erosion rates from the first order upland catchments to the main sixth order channel of tin camp creek the low 5 m ma rates from the main channel sediments reflect contributions from the slowly eroding stony highlands while the channels draining our study area reflect local soil production rates 10 m ma off the rocky ridge 20 m ma from the soil mantled regions quantifying such rates and processes help determine spatial variations of soil thickness as well as helping to predict the sustainability of the earth s soil resource under different erosional regimes copyright 2009 john wiley sons ltd

ries crater and suevite revisited observations and modeling part i observations we report results of an interdisciplinary project devoted to the 26 km diameter ries crater and to the genesis of suevite recent laboratory analyses of crater suevite occurring within the central crater basin and of outer suevite on top of the continuous ejecta blanket as well as data accumulated during the past 50 years are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations artemieva et al 2013 the properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting in a new approach we reconstruct the minimum maximum volumes of all allochthonous impact formations 108 116 km3 of suevite 14 22 km3 and the total volume of impact melt 4 9 8 0 km3 produced by the ries impact event prior to erosion these volumes are reasonably compatible with corresponding values obtained by numerical modeling taking all data on modal composition texture chemistry and shock metamorphism of suevite and the results of modeling into account we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement numerical modeling indicates that only a very small fraction of suevite can be derived from the primary ejecta plume which is possibly represented by the fine grained basal layer of outer suevite the main mass of suevite was deposited from a secondary plume induced by an explosive reaction fuel coolant interaction of impact melt with water and volatile rich sedimentary rocks within a clast laden temporary melt pool both melt pool and plume appear to be heterogeneous in space and time outer suevite appears to be derived from an early formed melt rich and clast poor plume region rich in strongly shocked components melt clasts and originating from an upper more marginal zone of the melt pool crater suevite is obviously deposited from later formed clast rich and melt poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper central zone of the melt pool genetically we distinguish between primary suevite which includes dike suevite the lower sublayer of crater suevite and possibly a basal layer of outer suevite and secondary suevite represented by the massive upper sublayer of crater suevite and the main mass of outer suevite

hillslope evolution by nonlinear creep and landsliding an experimental study landscape evolution models are widely used to explore links between tectonics climate and hillslope morphology yet mecha nisms of hillslope erosion remain poorly understood here we use a laboratory hillslope of granular material to experimentally test how creep and landsliding contribute to hillslope erosion in our experimental hillslope disturbance driven sediment transport rates increase nonlinearly with slope due to dilation driven gran ular creep and become increasingly episodic at steep slope angles as creep gives way to periodic landsliding we use spectral analysis to quantify the variability of sediment flux and estimate the slope dependent transition from creep to landsliding the power spec trum of sediment flux steepens with hillslope gradient exhibiting fractal 1 f scaling just below the creep landsliding transition by evolving the experimental hillslope under fixed base level boundary conditions we demonstrate how disturbance driven transport gen erates hillslope convexity the transient evolution is consistent with numerical predictions derived from a recently proposed nonlinear transport model as initially steep hillslopes are lowered rapidly by landsliding before slopes decay slowly by creep dominated transport

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

microfossils from the neoproterozoic gangolihat formation kumaun lesser himalaya their stratigraphic and evolutionary significance abstract a well preserved microbial assemblage from the gangolihat dolomite kumaun lesser himalaya is described in the present paper the present assemblage contains both filamentous septate and non septate and coccoidal microfossils besides acritarchs and sponge spicules the assemblage includes 23 species belonging to 12 genera siphonophycus typicum is the most common species in the entire assemblage the assemblage shows an abundance of filaments over coccoids and acritarchs the assemblage includes siphonophycus septatum siphonophyscus typicum siphonophycus robustum siphonophycus capitaneum siphonophycus kestron and siphonophycus punctatum gunflintia minuta gunflintia grandis gunflintia barghoornii oscillatoriopsis media oscillatoriopsis obtusa oscillatoriopsis cuboides oscillatoriopsis sp nostocomorpha chlorogloeaopsis contexta obruchevella parvissima sphaerophycus parvum myxococcoides minor leiosphaeridia crassa trachyhystrichosphaera vidalii trachyhystrichosphaera sp micrhystridium pallidum cymatiosphaera minuta and sponge spicules of monaxons and hexactinellid affinity the assemblage suggests a neoproterozoic age for gangolihat dolomite

tectonics fracturing of rock and erosion 1 we argue that by fracturing rock not by raising it relative to base level tectonics plays its most important role in causing rapid incision of valleys and rapid erosion of hillslopes tectonic deformation riddles the upper crust with fractures which not only provide avenues for water flow and thus promote weathering and further disintegration of rock but also fragment bedrock into debris that is readily extracted and transported by surface processes bends in active faults require straining of adjacent rock masses aftershocks that occur subsequent to slip on primary faults reflect penetrative brittle deformation of the upper crust at least some aftershocks must nucleate or lengthen cracks which contribute to the comminution of these rock masses scaling rules suggest that dimensions of ruptures for very small m 2 earthquakes can be meters or less the gutenberg richter recurrence relationship implies that such earthquakes are common as high magnification seismographs in low noise environments confirm moreover large differences among fault plane solutions for aftershocks show that the small faults on which they occur are not parallel to one another some faults must intersect thus the upper crust in tectonically active regions should be fragmented into blocks down to the scale of boulders or smaller dismembered rock arrives at the earth s surface already prepared to be transported away as a corollary both deeply exhumed lower crust and posttectonic igneous rock never deformed under brittle conditions and not deformed recently should be less susceptible to detachment and subsequent transport than fractured rock

ancient impact and aqueous processes at endeavour crater mars the rover opportunity has investigated the rim of endeavour crater a large ancient impact crater on mars basaltic breccias produced by the impact form the rim deposits with stratigraphy similar to that observed at similar sized craters on earth highly localized zinc enrichments in some breccia materials suggest hydrothermal alteration of rim deposits gypsum rich veins cut sedimentary rocks adjacent to the crater rim the gypsum was precipitated from low temperature aqueous fluids flowing upward from the ancient materials of the rim leading temporarily to potentially habitable conditions and providing some of the waters involved in formation of the ubiquitous sulfate rich sandstones of the meridiani region

coupling chemical weathering with soil production across soil mantled landscapes soil covered upland landscapes constitute a critical part of the habitable world our understanding of how they evolve as a function of different climatic tectonic and geological regimes is important across a wide range of disciplines and depends in part on understanding the links between chemical and physical weathering processes extensive previous work has shown that soil production rates decrease with increasing soil column thickness but chemical weathering rates were not measured here we examine a granitic soil mantled hillslope at point reyes california where soil production rates were determined using in situ produced cosmogenic nuclides 10be and 26al and we quantify the extent as well as the rates of chemical weathering of the saprolite from beneath soil from across the landscape we collected saprolite samples from the base of soil pits and analysed them for abrasion ph as well as for major and trace elements by x ray fluorescence spectroscopy and for clay mineralogy by x ray diffraction spectroscopy our results show for the first time that chemical weathering rates decrease with increasing soil thickness and account for 13 to 51 per cent of total denudation we also show that spatial variation in chemical weathering appears to be topographically controlled weathering rate decreases with slope across the divergent ridge and increases with upslope contributing area in the convergent swale furthermore to determine the best measure for the extent of saprolite weathering we compared four different chemical weathering indices the vogt ratio the chemical index of alteration cia parker s index and the silicon aluminium ratio with saprolite ph measurements of the cia were the most closely correlated with saprolite ph showing that weathering intensity decreases linearly with an increase in saprolite ph from 4 7 to almost 7 data presented here are among the first to couple directly rates of soil production and chemical weathering with how topography is likely to control weathering at a hillslope scale copyright 2006 john wiley sons ltd

the devensian weichselian lateglacial palaeoenvironmental record from gransmoor east yorkshire england a contribution to the north atlantic seaboard programme of igcp 253 termination of the pleistocene abstract palynological and coleopteran data are described from a section through late devensian weichselian deposits at gransmoor east yorkshire england the pollen evidence shows that following an initial open habitat episode first juniperus scrub and later betula woodland became established in the area during the lateglacial interstadial this woodland was replaced during the loch lomond younger dryas stadial by a predominatly herbaceous flora with arctic alpine steppe and halophytic elements the coleopteran evidence indicates a similar local environment and that the thermal maximum of the interstadial when mean july temperatures may well have exceeded 18 c occurred prior to the deposition of the earliest polleniferous sediments by the time of the juniperus expansion summer temperatures had fallen by 2 4 c to levels comparable with those of the present day a further temperature decline followed the expansion of betula during the later interstadial and another fall occurred at the lateglacial interstadial loch lomond stadial boundary during the loch lomond stadial mean july temperatures of 9 11 c are implied with winter temperatures as low as 15 to 20 c collectively the evidence reemphasises the point that for the first 1000 years or so of the lateglacial interstadial there was a disequilibrium between the palynological and coleopteran records in terms of climatic signal during the later interstadial however a closer relationship is apparent between both proxy data sources of particular significance in this respect is the fluctuation in reconstructed summer and winter temperatures and a rise and subsequent fall in betula pollen frequencies this may be the correlative of one of the pre younger dryas climatic oscillations recorded in other proxy data sources including palynological records from britain and eastern canada oxygen isotope traces from swiss lake sediments isotopic and electrical conductivity profiles from the greenland ice sheet and diatom and foraminiferal records in cores from the norwegian sea and the north atlantic ocean

assessing the veracity of precambrian sponge fossils using in situ nanoscale analytical techniques abstract paleontological inferences molecular clocks and biomarker fossils indicate sponges evolved in the cryogenian but precambrian sponge fossils are rare poorly substantiated and controversial spicule like microstructures slms hosted in phosphatized fossils from the ediacaran doushantuo formation 635 551 ma at weng an of south china have been interpreted as cylindrical siliceous monaxons and their hosting fossils as the oldest demosponges in the fossil record in order to assess their veracity as the oldest spiculate demosponges we utilize a suite of in situ nanoscale analytical techniques including scanning electron microscopy synchrotron x ray fluorescence mapping x ray absorption near edge structure xanes spectroscopy focused ion beam electron microscopy and transmission electron microscopy to evaluate the ultrastructures and elemental chemical and mineralogical compositions of the slms our data decisively shows that the slms are carbonaceous in composition and rectangular in transverse sections and therefore are not cylindrical siliceous spicules instead the slms may be microbial strands axial filaments of early hexactinellids or acicular crystals molded by organic matter regardless our new data invalidate the oldest and only precambrian demosponges with mineralized spicules these results indicate that interpretations of precambrian sponge fossils should be scrutinized with compositional mineralogical and ultrastructural data collected using in situ analytical techniques in addition our conclusions affirm that no unequivocal biomineralizing sponges occur below the ediacaran cambrian boundary if hexactinellids and demosponges did diverge in the cryogenian as suggested by molecular clocks and biomarkers they either evolved biomineralization long after their divergence or their biomineralized spicules were never preserved until after the ediacaran cambrian boundary in either case the dearth of biomineralizing sponge fossils in the precambrian and their abundance in the early cambrian must reflect a geobiologically significant aspect of the precambrian phanerozoic transition

trace fossil evidence for ediacaran bilaterian animals with complex behaviors abstract distinguishing ediacaran trace fossils from tubular body fossils can be a challenge and several ediacaran fossils previously interpreted as animal traces have been shown to be tubular body fossils nonetheless true ediacaran trace fossils are present but they are relatively few and morphologically simple dominated by horizontal trails and shallow burrows such simple morphologies have been taken as evidence for a modest behavioral complexity and limited geobiological impact of animal bioturbators before the cambrian explosion here we report three types of trace fossils horizontal tunnels surface tracks trails and vertical traces from the latest ediacaran dengying formation 551 541 ma in the yangtze gorges area of south china cross cutting tunnels and the presence of scratch marks indicate that these traces are unlikely tubular body fossils instead these three types of traces likely represent animal activities related to under mat feeding epibenthic locomotion and temporary dwelling respectively we show that these three types of traces were constructed by the same bilaterian animals that had moderately complex interactions with microbial mats to exploit nutrients and o 2 resources these animals heralded a new age in ecosystem engineering animal sediment interaction and biogeochemical cycling

hydrated minerals on endeavour crater s rim and interior and surrounding plains new insights from crism data 1 we have conducted a spectroscopic analysis of the rim and interior of endeavour crater using crism data in order to further constrain the mineralogical variability in the area and to identify targets of interest for in situanalysis by opportunity our analysis reveals that the spectral character of both the sulfates and the phyllosilicates in the area is more diverse than has been reported to date with phyllosilicates present on the rim and interior of endeavour crater as well as on the surrounding plains spectra of the sulfates adjacent to the rim and in the crater s interior mound exhibit features that are consistent with a component of ca sulfates the spectral character of the phyllosilicates is consistent with that of fe mg smectites but there are clear spectral differences between the rim and interior phyllosilicates specifically the phyllosilicates found inside the crater exhibit shallower more rounded fe mg oh bands a subtle 1 9 m hydration band and a strong 1 2 m spectral slope relative to the phyllosilicates on the rim suggesting that they have experienced modification relative to the rim phyllosilicates this modification may be attributed either to alteration via acidic leaching or to dehydration stratigraphically we find that these altered phyllosilicates unconformably overlie the sulfate bearing mound material suggesting that they were emplaced by the reworking of rim phyllosilicates after the interior mound had reached its present day form

fossil preservation through phosphatization and silicification in the ediacaran doushantuo formation south china a comparative synthesis phosphatized and silicified microfossils preserved through replication of organic templates by authigenic calcium phosphate and silica respectively form through mechanistically similar mineralization processes but under notably different conditions in order to characterize the environments and taphonomic processes associated with these preservational pathways we compare phosphatized and silicified fossil assemblages in the ediacaran doushantuo formation south china in terms of stratigraphic and paleogeographic distribution preservation and biodiversity several differences are recognized 1 phosphatization and silicification both occurred in sub tidal doushantuo sediments but phosphatized fossils generally occur in shallower sub tidal facies than silicified fossils 2 although both phosphatized and silicified fossils are preserved three dimensionally the latter are typically more compressed and 3 whereas phosphatized assemblages are numerically dominated by multicellular acritarchs putative metazoan eggs and embryos silicified assemblages are dominated by prokaryotes these differences can be explained by environmental and taphonomic distinctions phosphatized fossils occur in intraclastic phosphorites and phosphatic carbonates and were likely fossilized near the suboxic anoxic boundary in sediments under oxic bottom waters with redox and biogeochemical gradients conducive to p remineralization phosphate burial via iron pumping and calcium phosphate precipitation reworking and winnowing of phosphatized fossils preferentially concentrated certain taxa prior to final burial in contrast silicified fossils typically occur in chert nodules which formed deeper in the sediment column in ferruginous pore waters and perhaps under ferruginous bottom waters where bacterial sulfate reduction and pyrite formation created low ph conditions conducive to calcite dissolution and silica precipitation once formed chert nodules mostly remained in situ but some were reworked with turbidites and olistostromes developed in slope facies despite these distinctions phosphatized and silicified fossil assemblages in the doushantuo formation generally overlap in time and space and representatives of all major groups and many common genera of doushantuo organisms are present in both phosphorites and cherts consequently although both preservational pathways have paleoenvironmental and taphonomic biases their relative biases on taxonomic presence absence data are minimal given sufficient sampling efforts and paleobiologic and biostratigraphic information based on presence absence but not abundance data from the doushantuo formation appears to be robust

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

remarkable preservation of microbial mats in neoproterozoic siliciclastic settings implications for ediacaran taphonomic models abstract it is beyond doubt that the appearance of infaunal bioturbation and metazoan biomineralization across the ediacaran cambrian transition irreversibly affected the nature of marine sediment architecture and biogeochemistry here we review those changes in relation to their likely effect upon the processes of fossil preservation especially within siliciclastic sediments processes of soft tissue preservation in siliciclastic settings from the ediacaran period including microbes and microbial mats as well as ediacaran macrofossils are here reviewed within this context highlighted examples include the exceptional preservation of microbes found in association with wrinkle structures and ediacaran macrofossils in england and newfoundland replicated by silicate minerals and in the white sea region of russia replicated by iron sulphide these occurrences show that soft tissue preservation in siliciclastic settings went well beyond that typical for ediacaran macrofossils alone and also extended to similar modes of preservation in associated microbes using these new observations it can be argued that several existing explanations for ediacaran fossil preservation can be united within a biogeochemical model that involves evolution of the sediment mixed layer across this transition

terrestrial chondrules glass spherules and accretionary lapilli from the suevite ries crater germany abstract until recently no terrestrial analogues of meteoritic and lunar chondrules were known only rare glass spherules from the lonar crater india and black magnetic spherules from various localities have been recorded the impact breccia suevite of the no rdlinger ries crater germany contains both chondrules and glass spherules and in addition accretionary lapilli all of which are found imbedded within the fine grained matrix of the suevite the chondrules display many of the textural features characteristic of meteoritic and lunar chondrules lithic chondrules and fluid drop chondrules are present the latter having a composition quite similar to that of glass bombs and glass fragments in the suevite fluid drop chondrules developed from glass spherules by slow devitrification in the hot suevite ejecta masses after deposition on the whole fluid drop chondrules lithic chondrules and glass spherules are rare in the suevite with fluid drop chondrules prevailing detection of chondrules from a terrestrial impact crater supports theories of an impact origin for meteoritic and lunar chondrules accretionary lapilli also represent material formed as a result of impact

the neoproterozoic noonday formation death valley region california the neoproterozoic cambrian succession in the death valley region of the southwestern united states is among the best exposed and easily accessible in the world the largest single exposure of these strata occurs in the panamint range on the west side of death valley and it although variably metamorphosed contains the most complete sections of the noonday formation cap carbonate sequence new geological mapping measured sections and high resolution carbon isotope data for the noonday formation in this range enable establishment of a unified stratigraphy across the death valley region that consists of three units from the base upward they include the sentinel peak member of light gray massive to laminated fine dolostone locally containing vugs and tubes variably filled with micrite spar or quartz and rarely galena and varying from 2 to 200 m in thickness the radcliff member composed of feldspathic shale and sandstone and thin bedded limestone 0 200 m thick and the newly defined mahogany flats member of gray commonly stromatolitic thin to medium bedded fine dolostone that is 200 m thick at its type locality carbon isotopic trends in the panamint range match to within 1 2 reproducibility those known for the equivalent nonmetamorphosed strata in the eastern death valley sections a composite section of the noonday formation displays a chemostratigraphic profile with values near 3 through the cap dolostone of the sentinel peak member a decline to 6 in the lower part of the radcliff member followed by a recovery to near 0 and subsequent decline toward 2 in the remainder of the radcliff units and lower mahogany flats member and then a return to positive values 4 through the remainder of the mahogany flats member this pattern matches the ediacaran cap carbonate in namibia remarkably well and assuming our carbon isotopic correlations are correct it indicates that noonday deposition occurred at the beginning of the ediacaran period and that the immediately underlying wildrose diamictite of the kingston peak formation probably represents the younger cryogenian marinoan glacial episode

sorting and thickness of waste mantle material on a sandstone spur in central pennsylvania abstract utilizing a hammer seismograph and deep soil pits waste mantle material was studied on southwest and northwest aspects of a sandstone ridge in central pennsylvania soils on steep slope gradients 52 on both the sw and nw aspects showed preferred orientation of rock fragments in the direction of the slope aspect layers of 100 sorted and stratified rock fragments and an abrupt change from unconsolidated to underlying consolidated parent material soils on these sites were formed in waste mantle modified by periglacial denudation soils on lower slope gradients did not generally show preferred orientation of rock fragments but showed a gradual change from unconsolidated to consolidated bedrock parent material these sites were dominantly formed in residuum the upper 1 4 m across all sites were modified by periglacial denudation evidenced by lobate solifluction terraces and sub surface and surface rock sorting depth to bedrock ranged from 3 0 to 6 3 m across the srudy area and no strong trends with slope gradient or aspect were noted the differences found in the distribution and orientation of rock fragments and depth of unconsolidated parent materials may be best explained by deep weathering which occurred over many thousands of years followed by severe periglacial frost action and denudation during the last wisconsinan glacial advance steep slopes were eroded and material was deposited on them this material was derived from dominantly residual soil areas upslope with a lower slope gradient

possible animal body fossils in pre marinoan limestones from south australia the neoproterozoic era was punctuated by the sturtian about 710 million years ago and marinoan about 635 million years ago intervals of glaciation in south australia the rocks left behind by the glaciations are separated by a succession of limestones and shales which were deposited at tropical latitudes here we describe millimetre to centimetre scale fossils from the trezona formation which pre dates the marinoan glaciation these weakly calcified fossils occur as anvil wishbone ring and perforated slab shapes and are contained within stromatolitic limestones the trezona formation fossils pre date the oldest known calcified fossils of this size by 90 million years and cannot be separated from the surrounding calcite matrix or imaged by traditional x ray based tomographic scanning methods instead we have traced cross sections of individual fossils by serially grinding and scanning each sample at a resolution of 50 8 m from these images we constructed three dimensional digital models of the fossils our reconstructions show a population of ellipsoidal organisms without symmetry and with a network of interior canals that lead to circular apertures on the fossil surface we suggest that several characteristics of these reef dwelling fossils are best explained if the fossils are identified as sponge grade metazoans the trezona formation of south australia pre dates the 635 million year old marinoan glaciation fossils found at this location are up to several millimetres in size and share morphological characteristics with sponge grade animals

new lu hf and pb pb age constraints on the earliest animal fossils abstract the neoproterozoic doushantuo formation south china preserves a unique assemblage of early multicellular fossils and overlies rocks which are thought to have formed during an ice age of global extent the age of this formation is thus critical for understanding the important biological and climatic events that occurred towards the end of the proterozoic eon until now direct dating of sedimentary formations such as the doushantuo has been difficult and associated with large uncertainties here we show that dating of doushantuo phosphorites by a novel lu hf dating method and conventional pb pb geochronometry independently yield ages of 584 26 ma and 599 3 4 2 ma respectively these ages are in agreement with bio and chemostratigraphical observations and show that the doushantuo animal remains predate diverse ediacaran fossil assemblages making them the oldest unambiguous remains of metazoans currently known furthermore the pb pb age for the post glacial doushantuo rocks suggests that the neoproterozoic glaciation in china might predate glacial rocks in eastern north america commonly associated with the younger marinoan of two major neoproterozoic glaciations the combination of lu hf and pb pb dating shows considerable potential for dating other phosphorite successions and future application of these methods could therefore provide further constraints on proterozoic biological and environmental history

responses of soil clay mineralogy in the rothamsted classical experiments in relation to management practice and changing land use abstract changes in the clay mineralogy of soil samples collected from the rothamsted classical experiments over the past 150 years were examined comparisons were undertaken to assess the impact of i different fertiliser practices on the top soil and sub soil of the park grass experiment and ii the effects of changing land use from agriculture to woodland where soil ph either remained close to ph 7 or became acidic analyses were undertaken on the 0 4 a are reported in both top soil 0 23 cm and sub soil 46 69 cm and are likely related to i changes in soil k or nh4 status and or ii increasing soil acidity the greatest change was found in the park grass experiment nh4 2so4 plot where soil ph fell below 3 7 it is believed that solubilisation of hydroxyl al in smectite interlayers enabled renewed access to k or nh4 ions leading to increased collapse in d 001 spacing however the results show that over a diverse range of soil conditions i s minerals of the batcombe series soils showed considerable resilience to major change this is considered to be a result of competition for sorption sites by i the presence of competing ions added as fertiliser or liming materials ii an increase in h ions as soil acidity increases iii the possible role of organic carbon in protecting i s minerals and iv the role of the i s minerals as k reservoirs however the impact of decreasing soil ph should be considered when land is re forested as this may lead to decreases in the d 001 spacing of i s minerals thus compromising useful clay properties such as cec

interpreting late precambrian microfossils chia wei li et al describe microfossils found in doushantuo phosphate rocks from guizhou province south china that are about 580 million years old 1 1 well preserved macroscopic multicellular fossils associated with prokaryotic and eukaryotic microfossils from the same source rocks have been

phyllosilicates and sulfates at endeavour crater meridiani planum mars 1 phyllosilicates have been identified on the martian surface from orbit and are hypothesized to have formed under wet non acidic conditions early in the planet s history exposures of these minerals have not yet been examined by a landed mission using mars reconnaissance orbiter data we report the detection of phyllosilicate bearing outcrops that may be accessible by the mars exploration rover opportunity currently exploring meridiani planum the phyllosilicates are associated with layered polygonally fractured rocks exposed in the rim of the 20 km diameter crater endeavour these rocks may have formed via regional or global scale processes of aqueous alteration that predated the period of acid sulfate formation recorded in the rocks that opportunity has studied to date detailed characterization by opportunity could better constrain the conditions under which these phyllosilicates formed hydrated sulfates are also detected from orbit in the plains adjacent to endeavour s rim providing the first opportunity for ground truth of these detections

integrated ediacaran sinian chronostratigraphy of south china abstract the ediacaran period was a critical time in the history of life and earth understanding the complex interactions between geological and biological events during this interval requires high resolution chronostratigraphy thanks to its uniquely favorable palaeogeographic setting south china is an ideal region for constructing a framework for global chronostratigraphic correlation of ediacaran strata here continuous ediacaran successions contain both siliciclastic and carbonate rocks that range from shallow to deep marine and contain rich fossil assemblages showing a variety of preservation styles as part of the present study 62 reference sections distributed throughout the yangtze platform were investigated analyses of the sedimentary facies and sequence stratigraphy of more than 40 sections revealed four major sequences in the shallow marine facies three major sequence boundaries occur respectively in the middle and upper parts of the doushantuo formation and in the middle part of the dengying formation subdivision and correlation of the ediacaran successions of the yangtze platform based on sequence stratigraphy is congruent with the carbon isotope chemostratigraphy of 12 reference sections 1075 samples the sequence boundary sb2 in the middle part of the doushantuo formation is suggested here as the lower xiadongian upper yangtzean series boundary of the ediacaran which coincides with several globally recognizable events namely 1 the end of the neoproterozoic glaciations 2 the first appearance of animals 3 the sudden increase in the diversity of acanthomorphic acritarchs and 4 a negative 13 c excursion the ediacaran system of south china may be subdivided into five stages furthermore the criteria for defining these stages may be applicable throughout the world also the composite carbon isotope profile of the ediacaran of south china can be used as a global reference for chronostratigraphic subdivision and correlation in addition to the globally recognizable negative 13 c excursion in the cap carbonate at the base of the ediacaran the negative 13 c excursion in the uppermost doushantuo wonoka shuram interval represents another globally correlatable major excursion the positive 13 c excursion immediately following the uppermost doushantuo excursion and ending at 548 ma is also useful for global correlation current ediacaran chronostratigraphy and global correlations indicate that the upper doushantuo wonoka shuram negative 13 c excursion ranges from ca 560 ma to 551 ma and therefore coincides with the evolutionary renovation of life feedback between evolutionary innovation and geochemical cycling was the key mechanism for this unique and most strongly negative 13 c excursion in earth history our chronostratigraphic correlations also imply that the earliest known metazoan biotas namely the weng an and characteristic ediacaran biotas do not constitute two temporally separate evolutionary lineages as proposed previously but represent two parallel evolutionary pathways preceding the cambrian explosion of animal life

solving a conundrum of a steady state hilltop with variable soil depths and production rates bodmin moor uk the common wisdom is that steady state hilltops should display convex shape with both uniform soil thickness and uniform rates of lowering of the soil bedrock interface a 120 m hilltop transect on bodmin moor southwest uk displays a parabolic hilltop form which implies it is in geomorphic steady state yet the measured depths to bedrock range almost 6 fold from 14 to 80 cm and the soil production rates derived from 10be concentrations in rock at the soil interface range by 2 fold from 10 to 20 m my we address this conundrum by developing a theory to explain the scatter in soil depths that would result from block by block lowering of the soil bedrock boundary and support our theory with a simple model involving block release and diffusive transport we show that over long timescales a hilltop can indeed be in geomorphic steady state while at any point in time exhibiting a broad range of depths to bedrock variable depth to bedrock results in variable 10be concentrations in the rock sampled at the base of regolith which yields calculated soil production rates that vary about the long term average hence scatter in plots of soil production as a function of soil depth are to be expected in many landscapes and if the landscape is in geomorphic steady state this scatter reflects the stochastic nature of the block release mechanism by which the bedrock interface is lowering we find in our models that the commonly observed humped and exponentially declining soil production functions are only preserved during the transient approach toward geomorphic steady state

ancient aqueous environments at endeavour crater mars opportunity has investigated in detail rocks on the rim of the noachian age endeavour crater where orbital spectral reflectance signatures indicate the presence of fe 3 rich smectites the signatures are associated with fine grained layered rocks containing spherules of diagenetic or impact origin the layered rocks are overlain by breccias and both units are cut by calcium sulfate veins precipitated from fluids that circulated after the endeavour impact compositional data for fractures in the layered rocks suggest formation of al rich smectites by aqueous leaching evidence is thus preserved for water rock interactions before and after the impact with aqueous environments of slightly acidic to circum neutral ph that would have been more favorable for prebiotic chemistry and microorganisms than those recorded by younger sulfate rich rocks at meridiani planum

quantitative geochemical approach to pedogenesis importance of parent material reduction volumetric expansion and eolian influx in lateritization abstract using mass balance techniques we test the prevalent view that laterite genesis is dominated by in situ residual enrichment during chemical weathering of bedrock through calculation of net mass fluxes through the laterite soils in mali west africa we show that residual enrichment by removal of mobile elements with a corresponding increase in bulk porosity and decrease in bulk density contributes only a very minor fraction of the enrichment of al fe si and au instead we demonstrate that the abundance of these elements is due to the influx and accumulation by selective retention of chemically mature detritus of local and foreign origin clearly evident in micromorphological infilling features at the same sample depths that accumulation reaches extreme values we show that volumetric expansion in excess of 200 has occurred locally we infer that these spatially coincidental zones of mass influx of rock forming metals fe al si and also au with dilational hyperstrains result from a mutually reinforcing mechanical interaction between material influx and the effects of subsurface deformational processes such as shrink swell cycles and root growth and decay we propose that with progressive infilling of available connected voids by illuvial microsedimentary deposits of insoluble resistate and neoformed minerals the capacity for the combined skeleton and plasma to remain isovulumetric is exceeded we speculate that the resultant space problem is relieved by upwards expansion towards the overlying free surface continued translocation and void infilling occur and are limited to the depth where the size of translocational particles is smaller than that of the connected voids consequently progressive hyperstrains accumulate above this critical depth by the long term influence of a proposed translocational wedge of chemically resistant minerals against which numerous generations of plant roots have exerted stresses eclectic surficial contaminants involved are continuously derived from above leaving no indication of a relict source region within the present soil profile from which they might have been extracted instead the source region is largely the existing regolith column itself which releases local material and in addition is supplemented by deposition of colluvial detritus shed nearby by escarpment retreat

potential ediacaran sponge gemmules from the yangtze gorges area in south china abstract sponges are widely viewed as the most primitive metazoans sponge gemmule like structures have been recovered from the lower part of the doushantuo formation in the yangtze gorges area south china that were deposited between 635 and 580 ma gemmosclere like structures are embedded in the outer coat of the sponge structures electron probe microanalyses epma of the gemmoscleresindicate that they have siliceous composition the naked subspheroidal sponge structures are composed of three layers consistent with the preservation of a tri layered theca embedded with the gemmosclere like structures the potential fossils of the ediacaran sponge gemmules in the doushantuo formation may provide one of the earliest records of sponges

mineralogy and chemistry of altered icelandic basalts application to clay mineral detection and understanding aqueous environments on mars we used a suite of techniques including those emulating compositional data sets obtained from mars orbit and obtainable at the mars surface to examine aqueous alteration of basaltic rocks from iceland as a mineralogic and geochemical analog for noachian environments on mars a sample suite was collected for laboratory measurement of 1 whole rock visible near infrared vnir reflectance and thermal infrared tir emission spectra 2 vnir and tir reflectance spectra of particle size separates derived from the bulk rock and from materials extracted from fractures vesicles 3 x ray diffraction xrd patterns for determination of quantitative modal mineralogy 4 major element chemistry using flux fusion of whole rock powders and 5 electron microprobe analyses of minerals in thin sections conclusions about aqueous alteration can be influenced by technique for these basalts whole rock chemical data showed scant evidence for chemical fractionation but tir vnir and xrd measurements identified distinctive assemblages of hydrous silicate minerals differing by sample xrd provided the most complete and accurate quantitative determination of sample mineralogy however vnir spectroscopy was the technique most useful for determining composition of low abundance smectite clays and tir spectroscopy was the most useful for recognizing hydrated silicates in thin surface coatings high spatial resolution mineralogical and chemical data sets were useful for understanding the texture and distribution of alteration products and variations in fluid chemistry no single approach provides a complete assessment of the environment of alteration demonstrating the importance of employing multiple synergistic mineralogical and geochemical techniques and instruments in exploration of rock strata from aqueous paleoenvironments on mars

nonequilibrium regolith thickness in the ouachita mountains abstract interpretations of regolith and soil thickness in the context of landscape evolution are typically based on the notion that thickness is controlled by the interaction of weathering rates and erosion and tuned to topography on sideslopes of the ouachita mountains arkansas however there is a high degree of local spatial variability that is largely unrelated to topography this indicates nonequilibrium in the sense that there is no evidence of a balance between rates of weathering and removal as is postulated in some conceptual models in geomorphology and pedology johnson s soil thickness model is applied as an alternative to interpret local variations in regolith thickness at the study sites regolith thickness is not generally related to slope curvature elevation or pedogenic development in the solum this indicates that variability in thickness is related chiefly to processes and controls acting in the lower regolith below the solum the primary controls of variability are local lithol

stochastic processes of soil production and transport erosion rates topographic variation and cosmogenic nuclides in the oregon coast range landscapes in areas of active uplift and erosion can only remain soil mantled if the local production of soil equals or exceeds the local erosion rate the soil production rate varies with soil depth hence local variation in soil depth may provide clues about spatial variation in erosion rates if uplift and the consequent erosion rates are sufficiently uniform in space and time then there will be tendency toward equilibrium landforms shaped by the erosional processes soil mantle thickness would adjust such that soil production matched the erosion previous work in the oregon coast range suggested that there may be a tendency locally toward equilibrium between hillslope erosion and sediment yield here results from a new methodology based on cosmogenic radionuclide accumulation in bedrock minerals at the base of the soil column are reported we quantify how soil production varies with soil thickness in the southern oregon coast range and explore further the issue of landscape equilibrium apparent soil production is determined to be an inverse exponential function of soil depth with a maximum inferred production rate of 268 m ma 1 occurring under zero soil depth this rate depends however on the degree of weathering of the underlying bedrock the stochastic and large scale nature of soil production by biogenic processes leads to large temporal and spatial variations in soil depth the spatial variation of soil depth neither supports nor rejects equilibrium morphology our observed catchment averaged erosion rate of 117 m ma 1 is however similar to that estimated for the region by others and to soil production rates under thin and intermediate soils typical for the steep ridges we suggest that portions of the oregon coast range may be eroding at roughly the same rate but that local competition between drainage networks and episodic erosional events leads to landforms that are out of equilibrium locally and have a spatially varying soil mantle copyright 2001 john wiley sons ltd

thermodynamic and mass balance constraints on iron bearing phyllosilicate formation and alteration pathways on early mars 1 widespread occurrences of phyllosilicates predominantly nontronite and mg saponite in the noachian aged crust of mars place constraints on water availability and surface conditions however the noachian atmosphere likely lacked oxidants capable of forming nontronite from the basaltic crust and its origins are thus uncertain in this study scenarios for the formation of iron bearing phyllosilicates from the weathering and hydrothermal alteration of martian basalt were investigated using geochemical modeling to establish plausible pathways of nontronite generation and the effect of redox conditions on associated secondary phases hydrothermal alteration produced feii and mg saponite serpentine and zeolites at 100 c and chlorite prehnite and talc at 250 c consistent with phases observed in crater deposits on mars anoxic weathering produced primarily feii and mg saponite high water rock ratios produced al clays and amorphous silica and high pco2 conditions inhibited smectite formation because of carbonate precipitation oxic weathering produced primarily nontronite and mg saponite later oxidation of anoxic weathering products also formed nontronite mg saponite assemblages oxidation products of hydrothermal assemblages included nontronite mg saponite and either al smectites or residual zeolites or prehnite these calculations demonstrate that there are multiple plausible routes to form fe mg smectites on early mars and that an oxidizing atmosphere need not have existed in the noachian decoupling iron oxidation from initial clay formation raises the possibilities that mars transitioned to conditions favoring widespread iron oxidation after phyllosilicate formation ceased and that residual unoxidized ferrous iron phyllosilicates still exist today in the martian subsurface

the role of peri glacial active layer development in determining soil regolith thickness across a triassic sandstone outcrop in the uk this paper examines the weathering processes that have combined to produce the distribution of soil regolith sr thickness across the triassic sherwood sandstone group outcrop 750 km2 in nottinghamshire u k archive borehole logs n 282 taken across the outcrop showed that soil regolith thickness had mean and median depths of 1 8 and 1 5m respectively cores were taken from a forested site to depths 3m for geochemical analysis at this site the sr thickness was 1 7m analysis of the loss of elements compared to bedrock using mass balance calculations showed that all the calcite and gypsum cement had been removed to depths of 3m thus the major difference between the sr and the underlying saprolite was that the former exists as loose sand as opposed to a semi durable rock scanning electron microscopy sem analysis of core samples suggested that the non durable rock or saprolite had greater cementation of clay particles we propose that the mechanism through which the clay cement and other interlocking grain bonds were eased apart was through freeze thaw processes associated with the summer active layer development during the last glacial activity in the uk we tested this theory by developing a monte carlo simulation based on a simplified version of the stefan equation current arctic datasets of air and ground temperatures were obtained to provide reasonable starting conditions for input variables these were combined with known data for thermal conductivity bulk density and moisture content of the sherwood sandstone regolith model predictions n 1000 of the distribution of sr thickness accurately reflect the observed distribution thickness from the borehole logs this is strong evidence that freeze thaw and ald processes are major factors in determining the thickness of sr across this outcrop

geobiology of a palaeoecosystem with ediacara type fossils the shibantan member dengying formation south china abstract despite the importance of palaeoecosystems with ediacara type fossils for the early evolution of metazoans only little is known about the interplay of geological and biological processes in these environments the reason is that sedimentary structures biogenic structures and bio geochemical signatures e g hydrocarbon biomarkers are commonly not well preserved due to the predominance of volcanic and siliciclastic lithologies the shibantan member dengying formation south china is one of only few carbonate settings with ediacara type organisms worldwide and its lithology promises an excellent preservation of sedimentary facies and bio geochemical signatures here we provide the first comprehensive geobiological characterisation of the shibantan member in order to reconstruct the interplay between sedimentary and bio geochemical processes and to assess the microbial activities in the palaeoecosystem with ediacara type fossils facies analyses revealed that carbonate and organic matter were autochthonously formed by bio geochemical processes linked to microbial mats however the material was frequently reworked and re deposited within the same setting i e para autochthonous as evidenced by small scale hummocky cross stratification erosional contacts lenticular bedding and load casts negative ce anomalies ce ce and low v cr ratios demonstrate that molecular o 2 was present in the water column whereas characteristic ni co v v ni and v sc ratios suggest the contemporaneous presence of sub to anoxic water taken together these observations imply a temporarily stratified water body frequently mixed and ventilated by storms 13 c enrichments in the shibantan carbonates 13 c 3 29 to 3 98 vpdb together with 13 c depletions of syngenetic n alkanes cleaved from the extraction residue using catalytic hydropyrolysis hypy 13 c 31 7 to 36 3 vpdb could indicate a significant withdrawal of 12 c by primary producers that thrived within the mats at the same time sulphurised biomarkers in the bitumen and hypy pyrolysate hint at organic matter decomposition and concomitant sulphide production by sulphate reducing bacteria when oxygen was available at the sediment water interface due to mixing by storms sulphide oxidising bacteria were possibly temporarily favoured the results demonstrated that palaeoenvironmental conditions dynamically changed through a complex interplay of biogenic and abiogenic processes

the relationship between tectonic uplift and chemical weathering rates in the washington cascades field measurements and model predictions tectonic uplift is one of the key factors controlling the supply of material to weathering environments at present however there is debate about whether tectonics or climate plays a larger role in controlling chemical weathering rates on a global scale over geologic time we measured riverine weathering fluxes from twelve catchments along the skykomish river in the washington cascades where long term exhumation rates have been constrained by u th he thermochronom etry to examine the effect of tectonic uplift on chemical weathering rates we show that in the western washington cascades dissolved si fluxes increase from approxi mately 1900 to 3000 mol ha 1 yr 1 from west to east across the range and are correlated with rock exhumation rates we use dissolved si flux data from these catchments depth to bedrock measurements and long term exhumation rates to test a steady state weathering model that describes the relative importance of reaction kinetics and rock supply on weathering rates characterization of study basins with respect to two key variables in this model weathering zone depth and exhumation rate allows us to examine the relative effects of climate and tectonics on chemical weathering by analyzing the ratio of the timescale of reaction kinetics and the residence time in the weathering zone evaluation of this model shows that chemical weathering rates in the cascades are controlled by the rate of rock erosion and factors that influence the depth of the active weathering zone however the model predicts that in tectonically inactive areas the supply of material limits the overall rate of chemical weathering while in active tectonic or erosional environments the rate of supply exceeds the ability of a system to weather that material resulting in a landscape in which weathering rates are expected to respond strongly to changes in climatic conditions

the influence of freeze thaw cycles and soil moisture on aggregate stability of three soils in norway abstract winter conditions with seasonally frozen soils may have profound effects on soil structure and erodibility and consequently for runoff and erosion such effects on aggregate stability are poorly documented for nordic winter conditions the purpose of this study was to quantify the effect of variable freeze thaw cycles and soil moisture conditions on aggregate stability of three soils silt structured clay loam clay a and levelled silty clay loam clay b which are representative of two erosion prone areas in southeastern norway a second purpose was to compare aggregate stabilities measured by the norwegian standard procedure rainfall simulator and the more widely used wet sieving procedure surface soil was sampled in autumn field moist soil was sieved into the fraction 1 4 mm and packed into cylinders the water content of the soil was adjusted corresponding to matric potentials of 0 75 2 and 10 kpa the soil cores were insulated and covered and subjected to 0 1 3 or 6 freeze thaw cycles freezing at 15 c for 24 h and thawing at 9 c for 48 h aggregate stability was measured in a rainfall simulator all soils and a wet sieving apparatus silt and clay b the rainfall stability of silt was found to be significantly lower than of clay a and clay b clay a and clay b had similar rainfall stabilities even though it was expected that the artificially levelled clay b would have lower stability freezing and thawing decreased the rainfall stability of all soils but the effect was more severe on the silt soil there was no evident effect of water content on the stability probably due to experimental limitations the same effects were observed for wet sieved soil but the wet sieving resulted in less aggregate breakdown than the rainfall simulator rainfall impact seemed to be more detrimental than wet sieving on more unstable soil that is on silt soil and soil subjected to many freeze thaw cycles such conditions are expected to occur frequently during field conditions in unstable winters

erosion rates of alpine bedrock summit surfaces deduced from in situ 10be and 26al we have measured the concentration of in situ produced cosmogenic 10be and 26al from bare bedrock surfaces on summit flats in four western u s mountain ranges the maximum mean bare bedrock erosion rate from these alpine environments is 7 6 3 9 m my 1 individual measurements vary between 2 and 19 m my 1 these erosion rates are similar to previous cosmogenic radionuclide crn erosion rates measured in other environments except for those from extremely arid regions this indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments even though frost weathering should be intense in these areas our crn deduced point measurements of bedrock erosion are slower than typical basin averaged denudation rates 50 m my 1 if our measured crn erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion then relief in the mountain ranges examined here is probably increasing we develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady state erosion model to episodically eroding outcrops our simulations show that interpreting measurements with the steady state erosion model can yield erosion rates which are either greater or less than the actual long term mean erosion rate while errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples the mean value of many steady state erosion rate measurements provides a much better estimate of the long term erosion rate

silica rich deposits and hydrated minerals at gusev crater mars vis nir spectral characterization and regional mapping abstract the mars exploration rover mer spirit has discovered surprisingly high concentrations of amorphous silica in soil and nodular outcrops in the inner basin of the columbia hills in pancam multispectral observations we find that an absorption feature at the longest pancam wavelength 1009 nm appears to be characteristic of these silica rich materials however spectral analyses of amorphous silica suggest that the 1009 nm spectral feature is not a direct reflection of their silica rich nature based on comparisons with spectral databases we hypothesize that the presence of h 2 o or oh either free as water ice adsorbed or bound in a mineral structure is responsible for the spectral feature observed by pancam the gertrude weise soil which is nearly pure opaline silica may have adsorbed water cold trapped on mineral grains the origin of the 1009 nm pancam feature observed in the silica rich nodular outcrops may result from the presence of additional hydrated minerals specific sulfates halides chlorides sodium silicates carbonates or borates using the 1009 nm feature with other spectral parameters as a hydration signature we have mapped the occurrence of hydrated materials along the extent of spirit s traverse across the columbia hills from west spur to home plate sols 155 1696 we have also mapped this hydration signature across large panoramic images to understand the regional distribution of materials that are spectrally similar to the silica rich soil and nodular outcrops our results suggest that hydrated materials are common in the columbia hills

co evolution of eukaryotes and ocean oxygenation in the neoproterozoic era the oxygenation of the earth s deep oceans is often thought to have triggered the evolution of simple animals a review article proposes that instead the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea

pb pb ages of neoproterozoic doushantuo phosphorites in south china constraints on early metazoan evolution and glaciation events the pb pb isochron ages of 576 14 ma for the upper part of the upper phosphorite bed upb this study and 599 4 ma for the lower part of the upb earth planet sci lett 201 2002 203 in the doushantuo formation in the weng an area south china suggest that the deposition of the upb lasted at least 5 ma and indicate that the observed microfossil assemblage animal remains multicellular algae and giant process bearing acritarchs in the doushantuo formation predates all well dated diverse ediacaran assemblages and represents the earlier paleontological record of eukaryotes radiation on earth the 576 14 ma age in the upper part of the upb approximates the age of a negative delta 13 c excursion near the boundary between the doushantuo and the overlying dengying formation if the doushantuo dengying boundary at the weng an area is equivalent to that in the yangtze gorges area this negative excursion might correlate to a post marinoan glaciation in south china suggesting that the ice age following doushantuo sediment deposition is younger than 590 ma the 599 4 ma age in the lower part of the upb earth planet sci lett 201 2002 203 represents the minimum age of the nantuo diamictites which are generally correlated with the marinoan glaciation c 2004 elsevier b v all rights reserved

in situ imaging of organic sulfur in 700 800 my old neoproterozoic microfossils using x ray spectromicroscopy at the s k edge the study of very ancient microfossils has recently raised contentious issues regarding interpretation of the biogenicity of the structures in situ investigation of certain elements such as sulfur within potential microfossils is a powerful complement to other methods of investigation that can provide valuable information on biogenicity we present here a first such study on precambrian microfossils from the 700 800 my old neoproterozoic draken formation svalbard using scanning x ray microscopy sxm in the fluorescence mode and x ray absorption near edge spectroscopy xanes at the sulfur k edge sxm allowed mapping of up to 300 ppm of probably endogenous sulfur within the kerogenous walls of myxococcoides chlorelloidea microfossils xanes showed that the sulfur is most likely contained in heterocyclic organic compounds such as thiophene s

exceptionally preserved fossil assemblages through geologic time and space abstract geologic deposits containing fossils with remains of non biomineralized tissues i e konservat lagerstatten provide key insights into ancient organisms and ecosystems such deposits are not evenly distributed through geologic time or space suggesting that global phenomena play a key role in exceptional fossil preservation nonetheless establishing the influence of global phenomena requires documenting temporal and spatial trends in occurrences of exceptionally preserved fossil assemblages to this end we compiled and analyzed a dataset of 694 globally distributed exceptional fossil assemblages spanning the history of complex eukaryotic life 610 to 3 ma our analyses demonstrate that assemblages with similar ages and depositional settings commonly occur in clusters each signifying an ancient geographic region up to hundreds of kilometers in scale which repeatedly developed conditions conducive to soft tissue preservation using a novel hierarchical clustering approach we show that these clusters decrease in number and shift from open marine to transitional and non marine settings across the cambrian ordovician interval conditions conducive to exceptional preservation declined worldwide during the early paleozoic in response to transformations of near surface environments that promoted degradation of tissues and curbed authigenic mineralization potential we propose a holistic explanation relating these environmental transitions to ocean oxygenation and bioturbation which affected virtually all taphonomic pathways in addition to changes in seawater chemistry that disproportionately affected processes of soft tissue conservation after these transitions exceptional preservation rarely occurred in open marine settings excepting times of widespread oceanic anoxia when low oxygen levels set the stage with these patterns non marine cluster count is correlated with non marine rock quantity and generally decreases with age this result suggests that geologic processes which progressively destroy terrestrial rocks over time limit sampling of non marine deposits on a global scale future efforts should aim to assess the impacts of such phenomena on evolutionary and ecological patterns in the fossil record

the convenient fiction of steady state soil thickness abstract steady state regolith or soil thickness ssst whereby surface removals are approximately balanced by production of new soil by bedrock weathering is a common assumption in most models of hillslope and landscape evolution ssst is also a fundamental assumption in the use of cosmogenic radionuclides crn to estimate erosion and weathering rates the steady state concept is based on feedbacks between soil thickness and weathering at the base of the regolith such that sometimes after an optimal or threshold thickness is achieved thicker soils lead to lower weathering rates and vice versa ssst is thus only applicable to soils formed chiefly from weathering of the underlying bedrock where sufficient time has elapsed for regolith accumulation and where effects of processes other than weathering and surface removals on thickness are negligible even within this domain the widespread occurrence of deep weathering profiles regolith stripping and inherited regolith features makes ssst problematic as a conceptual model for pedogenesis or weathering profile development the ratio of soil thickness to total weathering profile thickness is proposed as a simple index of steady state steady state profiles formed on weathered bedrock should exhibit ratios close to unity data from the cumberland plateau region of eastern kentucky show that soil weathering profile ratios in shallow 20 m and generally significantly greater than soil thicknesses in the region suggesting ratios convenient fiction facilitating the use of some models and tools the potential pitfalls arise from the possibility that the utility of ssst as a convenient fiction in some contexts may be mistaken for a realistic representation of the dynamics of pedogenesis and weathering profile evolution