hydrocarbon biomarkers of neoproterozoic to lower cambrian oils from eastern siberia the neoproterozoic era is of widespread geobiological interest because it marks the critical transition from a world of microbes to one where animals become an established feature of the landscape much research into this time period has focused on the ventilation of the oceans as this is widely considered a primary factor driving the diversification of complex multicellular life in this study proterozoic to cambrian aged oils from eastern siberia were analyzed for their hydrocarbon biomarker contents and compound specific carbon isotopes in order to further our understanding of the prevailing environment and its microbial and metazoan communities geochemically these oils are broadly comparable to those of the ediacaran cambrian sedimentary rocks and oils of the south oman salt basin organic matter in the source sedimentary rocks included significant contributions from green algae demosponges and bacteria including cyanobacteria and methanotrophic proteobacteria although the ages of the siberian oils and putative parent source rock intervals are poorly constrained the geochemical similarities between the ediacaran oman huqf and nepa botuoba katanga family of siberian oil samples are impressive leading to the inference that their source rocks are coeval on the other hand oils from the baykit high are distinctive likely older and possibly of cryogenian age

discussion of the applicability of the generalized clausius clapeyron equation and the frozen fringe process abstract based on the literature and our laboratory experiments we discuss the applicability of the generalized clausius clapeyron equation gcce for describing the phase change process between ice and water and specific applicability conditions for the gcce during soil freezing are proposed we then put forth two models a static model and a dynamic model both models include the variation tendencies of the pore water pressure ice stress effective stress and pore pressure the static model is an ideal model whereas the dynamic model is more similar to the real freezing process of soil in addition the initial conditions of ice lens are considered and developed in the models by introducing a concept of constraining force based on the dynamic model such phenomena as consolidation zoning and thickness of frozen fringe could be well explained

late quaternary hydrological dynamics in the middle kalahari forcing and feedbacks the kalahari region has become a major source of quaternary palaeoenvironmental data derived primarily from the analysis of geomorphological proxies of environmental change one suite of data from palaeolacustrine landforms has recently provided a new record of major hydrological changes in the last 150 ka burrough s l thomas d s g bailey r m 2009 mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system quaternary science reviews in press here we present an improved analysis of the drivers and feedbacks of lake level change utilising information from three main sources data from the lake system itself from analyses of other late quaternary records within the region and from climate modelling simulations using the hadley centre coupled climate model hadcm3 suggest that once triggered the lake body was large enough to potentially affect both local and regional climates surface waters and their interactions with the climate are therefore an important component of environmental dynamics during the late quaternary through its capacity to couple middle kalahari environments to distant forcing mechanisms and to itself force environmental change we demonstrate that the existence or absence of megalake makgadikgadi adds a new level of complexity to the interpretations of environmental proxy records in southern africa s summer rainfall zone

a eukaryote assemblage intercalated with marinoan glacial deposits in south australia composite hematite silica structures recovered from a siltstone bed in the elatina formation of south australia include 1 sub circular to whorl shaped forms 2 elongate to half moon shaped forms and 3 and lozenge shaped forms locally linked into chains they range from 200 to 500 m in diameter and are interpreted as eukaryote tests evidence for internal etching of a calcite core of some tests indicates that at least some of the hematite silica fabrics were acquired through replacement carbon isotope values of 20 13 c are suggestive of precipitation by microbial activity and imply a change in ambient fluid chemistry associated with a ph reduction the tests occur within sandstone beds that were deposited on a tidally modulated braidplain during the marinoan glaciation at the end of the cryogenian the quartz grains in the sandstone sample lack the typical textures surface striae internal fractures or irregular grain boundaries expected for glacially transported material thus on textural grounds we argue that the eukaryote tests represent a proglacial ecosystem during a late cryogenian snowball earth event supplementary material video files of digital x ray tomographs ct in the longitudinal and transverse planes are available at https doi org 10 6084 m9 figshare c 2209723

tonian cryogenian boundary sections of argyll scotland abstract the tonian cryogenian system boundary is to be defined at a gssp global boundary stratigraphic section and point beneath the first evidence of widespread glaciation a candidate lies within the dalradian supergroup of scotland and ireland which is least deformed and metamorphosed in argyll western scotland we present new stratigraphic profiles and interpretations from the isle of islay and the garvellach islands update the chemostratigraphy of the appin group tonian carbonates underlying the thick ca 1 km glacigenic port askaig formation paf and demonstrate an environmental transition at the contact the appin group forms a regionally extensive 4 km thick succession of limestones shales and sandstones deposited on a marine shelf on islay the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure oolite stromatolitic dolomite and intraclastic microbial mounds significantly deeper erosion at the unconformity at the base of the overlying paf is demonstrated in the southern section carbonate facies show a gradual decline in 13 c vpdb from 5 to 2 upwards in ne garbh eileach garvellach islands a continuously exposed section of appin group carbonates 70 m thick here designated the garbh eileach formation gef lies conformably beneath the paf the gef and the gef paf boundary relationships are re described with new sedimentological logs petrological and stable isotope data interstratified limestone and dolomicrosparite with 13 c of 4 to 7 a feature named the garvellach anomaly replacing the term islay anomaly are overlain by dolomite in which the isotope signature becomes weakly positive up to 1 upwards shallow subtidal conditions become peritidal upwards with evidence of wave and storm activity gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the gef the basal diamictite d1 of the paf is rich in carbonate clasts similar to slightly deeper water parts of the underlying succession d1 is typically several metres thick with interstratified sandstone and conglomerate but dies out laterally scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft sediment deformation is interbedded with carbonate below and above d1 dolomite beds with derived intraclasts and gypsum pseudomorphs are found above d1 or equivalent position where d1 is absent published and new sr isotope studies including successive leach data demonstrate primary tonian 87 sr 86 sr values of 0 7066 0 7069 on islay decreasing to 0 7064 0 7066 in the younger gef limestones on the garvellachs with 1700 2700 ppm sr other typically tonian characteristics of the carbonates are the sr rich nature of limestones molar tooth structure and dolomitized peritidal facies with evidence of aridity seabed surveys suggesting uniformly dipping strata and shallow borehole core material illustrate the potential for extending the tonian record offshore of the garvellachs a candidate tonian cryogenian gssp is proposed on garbh eileach within the smooth 13 c profile at the cross over to positive 13 c signatures 4 m below the first occurrence of ice rafted sediment and 9 m below the first diamictite although lacking radiometric constraints or stratigraphically significant biotas or biomarkers the scottish succession has a thick and relatively complete sedimentary record of glaciation coherent carbon and strontium chemostratigraphy lateral continuity of outcrops and 100 exposure at the proposed boundary interval

the anomalous ca cycle in the ediacaran ocean evidence from ca isotopes preserved in carbonates in the three gorges area south china abstract the ediacaran cambrian was one of the most important periods for the evolution of life recent studies have provided detailed chemostratigraphies that decipher the linkages between ambient surface environmental changes and biological evolution the occurrence of skeletal fossils in ediacaran rocks suggests that the possible onset of ca biomineralization started in the latest neoproterozoic molecular clocks also predict the emergence of animals containing ca carbonate spicules in the neoproterozoic therefore it is important to estimate the transition of the ca cycle in seawater ancient ca cycles in the oceans are estimated from the calcium isotopic compositions of carbonate rocks however the lack of continuous ca isotopic data in the ediacaran leaves the ca cycle unresolved the almost continuously exposed ediacaran and cambrian strata in south china mainly comprise carbonate rocks with subordinate black shales and sandstones which contain many fossils suitable for the study of environmental and biological changes we conducted drilling from the liantuo via the nantuo doushantuo dengying and yanjiahe to the shuijingtuo formation at four sites in the three gorges area in order to obtain continuous fresh samples without surface alteration and oxidation we analyzed 44 ca 42 ca ratios in carbonate rocks with a multiple collector inductively coupled plasma mass spectrometer mc icp ms screened for diagenetic alteration the resultant 44 42 ca ratios exhibit a smooth curve the values of 44 42 ca are anomalously high relative to those in the phanerozoic and they gradually decrease towards the end of the ediacaran our new ca isotope chemostratigraphy suggests that a different ca cycle had existed during the ediacaran the long term high 44 42 ca is not simply explicable by changes in temperature differences in carbonate mineralogy or changes in the input output fluxes of ca to the ocean we propose three possible explanations for the observed high 44 42 ca ratios in the ediacaran high 44 42 ca ratios of ca inputs potential undiscovered sinks of ca with low 44 42 ca values and negligible isotope fractionation between carbonate and seawater we consider that one or all of these contributed to the high 44 42 ca ratios of these we prefer the small isotope fractionation which implies that the ca concentration in seawater was initially low in the early ediacaran and increased throughout that period

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

organic walled microfossil assemblages from glacial and interglacial neoproterozoic units of australia and svalbard before the onset of the neoproterozoic snowball earth glaciations eukaryotes had begun diversifying and in their aftermath macroscopic life including both animals and macroalgae became abundant and widespread although glacially driven mass extinctions have been hypothesized little is known about the biosphere during and between these glaciations here we present new data from organic walled microfossil assemblages from five successions in australia and svalbard that collectively span the first sturtian glaciation and interglacial interval and integrate them with data derived from a critical evaluation of the literature to produce a new estimate of eukaryotic diversity from 850 to 650 ma these new glacial and interglacial assemblages consist of only smooth walled spheroids leiosphaerids aggregates of cells and filaments in contrast to the much more diverse organic walled microfossil assemblages found in early neoproterozoic rocks this contrast is not attributed to biases in deposition or preservation but is instead interpreted as reflecting an interval of lowered eukaryotic diversity that spanned the glaciations and that may have begun millions of years prior to their onset

interpreting geoproxies of late quaternary climate change in african drylands implications for understanding environmental change and early human behaviour abstract climate and environmental change in the late quaternary are widely regarded as key drivers of early human development and dispersal in africa robust records of change are required to assess the nature of potential impacts today s late quaternary palaeoenvironmental palaeoclimate reconstructions from the interior southern african basin kalahari are more spatially detailed have a stronger chronometric underpinning and span a longer time range than those of 10 20 years ago but are no less controversial reconstructions for the last 150 ka are usually interpreted in terms of changes in hydrological balance from conditions more humid or more arid than today and expressed in climatological terms as the changing latitudinal interplay between monsoon easterlies and tropical and mid latitude westerlies with more data than ever before why then is the record not yet delivering a more coherent picture of regional climate change this paper deliberately addresses current challenges and controversies including issues often ignored or neglected with the view that it is essential to do so if more robust interpretations of the past are to be generated in the future sources of palaeo data from the interior are predominantly geoproxies reconstructions not based on highly resolved organic or isotope records but on interpretation of spatially extensive geomorphological proxies of past environments despite being increasingly datable using osl their inclusion in syntheses has proved complex not least when interpretations appear to conflict with records from other sources consequently they are sometimes removed from regional syntheses of climate change these omissions are often replaced by spatially exaggerated interpretations of higher quality local records allowing more spatially coherent timeslice interpretations to be presented yet this is counter intuitive to the spatial complexity of conditions present in the palaeoenvironmental proxies or in climatic and environmental systems today in addition the quest to link regional records to master records from ice and ocean cores can force interpretation within a temporal scale that may be inappropriate for their analysis underplaying the reality of complexity within the environmental system there are not necessarily good or bad records of terrestrial palaeoenvironments in interior southern africa rather there are records that contain different suites of information about past land surface hydrological and climate conditions preserved at temporal scales which can sometimes be incomparable consideration of these factors may resolve apparent conflicts between datasets resulting in a better understanding of past environmental dynamics and revealing the nature of environmental variability which may be a key facet to which human development has responded

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

stratigraphy palaeontology and geochemistry of the late neoproterozoic aar member southwest namibia reflecting environmental controls on ediacara fossil preservation during the terminal proterozoic in african gondwana abstract common ediacaran fossils are well preserved in a late neoproterozoic ca 545 ma shallow marine sequence described here as the aar member of the dabis formation kuibis subgroup nama group near aus in southwest namibia this 31 38 m thick shale dominant unit records the transition from fluvial shallow marine kliphoek sandstone to open marine limestone of the mooifontein member of the zaris formation deposited on a subsiding continental margin during a major regional transgression thin sandstone beds contain fossils at a number of levels throughout the aar member concentrations of pteridinium were mostly transported in flood derived sheets while some ernietta assemblages are preserved close to in situ rangea has also been transported and is mostly confined to thin sandstone lenses incised into mudstone limestone beds common throughout include at least two marker horizons that can be followed regionally and show local evidence of storm reworking systematic sampling and analyses of limestone reveals enrichment in both 13 c and 18 o higher in the section with negative 13 c near the base rising to moderate positive values near the top the negative to positive transition in 13 c values is more pronounced in the east with all of the lower aar member samples consistently depleted in 13 c while this may reflect greater degrees of alteration by meteoric or dewatering fluids the same carbonates are notably enriched in 18 o relative to those at the same stratigraphic position to the west the overall rise in 13 c is attributed to greater proportional burial of organic matter and release of oxygen to surface environments while the spatial variability is likely the result of a strong surface to deep carbon isotopic gradient in seawater a number of the fossils especially rangea are encrusted with jarosite an iron bearing sulphate mineral and common weathering product of pyrite this observation suggests that preservation of the fossils may have resulted from the rapid encrustation of pyrite on the surface of the organisms as they decomposed and were consumed by sulphate reducing bacteria within the sandy near shore sediments insofar as pyrite formation requires iron which is soluble and reactive in anoxic solutions it is likely that the deeper subtidal environments lacked oxygen in situ pyritized forms like ernietta may have developed the capacity to survive under episodically anoxic or sub oxic environmental conditions while pteridinium and rangea lived within an oxygenated estuarine or fluvial setting and were transported during storms to anoxic ferruginous environments where they were exquisitely preserved

early neoproterozoic origin of the metazoan clade recorded in carbonate rock texture early neoproterozoic reefs older than 779 younger than 1083 ma contain a carbonate rock texture already familiar from sponge rich phanerozoic limestones characterized by authigenic ca carbonate and irregular secondary voids containing internal sediment poly mud fabric in holocene sediment this texture develops by calcification of degrading extra cellular collagenous matrix ecm of siliceous sponges ecm calcification of siliceous sponges is evident throughout the phanerozoic and the related polymud fabric is a diagnostic petrographic feature of phanerozoic sponge rich carbonate mudmounds the authigenic ca carbonate of polymuds is interpreted to result from connective tissue calcification just beneath the seafloor that in a kind of taphonomic race takes place at the same time as tissue oxidation and associated void formation it is intriguing that ecm is a fundamental character of the metazoan clade and so the presence in early neoproterozoic rocks of a texture directly associated with it implies the existence of metazoan grade organisms at that time this observation pushes back the earliest geological evidence for animals by about 200 m y the timing corroborates results of an integrated phylochronology and supports the concept of a biosphere that persisted through the snowball earth interval

internal stresses in frozen ground small transducers that were inserted 1 2 cm apart in a frozen soil subjected to a temperature gradient have shown differences in pressure of more than 0 1 mpa the pressures are generated by growth of ice lenses taking place against the resistance of the surrounding frozen soil the observations which are compatible with reports by others from various field studies illustrate the importance of both the thermodynamic and mechanical constraints on the process of frost heave and the development of frost heave pressures key words frozen ground frost heave frost heave pressures phase relations internal stresses

sedimentological and geochemical evidence for palaeo environmental change in the makgadikgadi subbasin in relation to the moz rift depression botswana abstract this work considers new evidence for palaeo environmental change taking place during the pleistocene in northern botswana duricrusted strandlines along the northeastern margin of sua pan provide palaeo environmental data pertaining to the makgadikgadi subbasin msb with inferences regarding the larger makgadikgadi okavango zambezi moz rift depression field xrd and geochemical data show that msb strandlines comprise calcretes lu1 type mgo rich calcretes with silica lu2 type sil calcrete lu3 type and silcrete lu4 type early freshwater episodes appear to have been followed by calcrete dominated drying phases interspersed with repeated silcretisation calcretisation through pan littoral sediments may have been both biogenically and environmentally induced calcite precipitation was in part controlled by the mg ca ratio of pore water in the pan littoral zone suggesting closed basin type evaporative conditions which were followed by a major desiccation interval phases of silcrete precipitation appear to be related to periods when the geochemistry of the lake littoral more closely resembled present day na co 3 so 4 cl type brines silica saturated acidic moderately saline groundwater preceded si precipitation which took place as the ph reduced si mobilisation occurred inter alia as a result of quartz grain dissolution enhanced by diatoms bacteria and algal growth in the moist pan littoral sio 2 rich pore waters migrated through cracked and desiccated calcrete into areas of lower salinity and lower ph resulting in preferential calcite removal and silcrete precipitation approximate tl dates imply that exposed littoral sand underwent calcretisation during the drying phases of extensive palaeo lakes which occurred prior to 110 ka 80 90 ka and 41 43 ka these wet periods compare fairly well with vostok core chronologies for southern africa

multiphase quaternary highstands at lake ngami kalahari northern botswana large lake basins now almost entirely devoid of water in the kalahari s semi arid environment can be clearly identified from landsat imagery of northern botswana these basins are delineated by a series of beach ridges marking the former shorelines of an inundated area that at its largest extent encompassed 60 000 km 2 in the last 30 yr over 150 radiocarbon ages have been produced from three component basins that make up the palaeo makgadikgadi system consisting of the ngami mababe and makgadikgadi basins thepaucityofwellpreservedorganicmaterialintheregionhasledtothese 14 cagesbeingderivedfrominorganiccarbonates whose open system behaviour and polygenetic origin frequently result in unreliable dates from uncertain contexts and then only within the temporal limits of the radiocarbon method the application of optically stimulated luminescence osl dating to beach ridge sediments from lake ngami shaw p a bateman m d thomas d s g davies f 2003 holocene fluctuations of lake ngami middle kalahari chronology and responses to climatic change quaternary international 111 1 23 35 improved chronological resolution but was limited by the use of hand dug pits the new application of osl to samples derived from deep coringofbeachridgeshasallowedaccesstoamuchlonger threedimensional geomorphicrecordthanpreviouslyavailable seventyfour ages new osl ages from shorelines are presented allowing for the first time the opportunity to unravel the complex history of lake stages in the ngami basin multiple lake highstands are identified in the period 0 3 ka to 140 ka with several causes of these events includingtropicalandlocalclimatesandupstream channelavulsion identified tounravelthefullpaleaoclimatic significance of these events it will be necessary to identify the timing of high stands in the other terminal basins within the okavango system 2007 elsevier b v all rights reserved

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

the morphology of barchan shaped sand banks from western torres strait northern australia abstract large barchan shaped sand deposits have been observed in the north west of torres strait these deposits share characteristics of both subaerial barchan dunes and subaqueous sand banks a study of satellite imagery indicate that the deposits migrate in the direction indicated by their horns 10 15 m west per year and that sediment is shed from their horns features that are characteristic of barchan dunes however the orientations of sand dunes superimposed upon the sand banks indicate the presence of mutually evasive channels and circulation of sediment around the sand bank a characteristic of subaqueous sand banks the presence of mutually evasive channels is the criteria used to categorise the deposits as sand banks barchan forms are known to exist in regions with limited sediment supply and unidirectional current or wind regimes in the torres strait both these criteria are met previous work has demonstrated the presence of a net westward current through the torres strait that is driven by the southeast trade winds the relatively high displacement of the wind driven currents during the trade wind season relative to the monsoon appears to provide the necessary unidirectional regime to form barchans the low and typically eastwards displacement of the residual monsoon season current appears to have a negligible affect on the barchan form while seasonal wind driven currents appear to maintain the barchan shape of the sand banks tidal currents actively maintain mutually evasive channels observed by variations in dune orientation on the sand banks a sediment starved environment combined with bedload transport attributed to both wind driven and tidal currents is concluded to create a unique hydrodynamic environment where sand banks can attain a barchan form

mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system abstract a distinct series of beach ridges marking the former shorelines of large inter connected lacustrine basins in the kalahari can be clearly identified from landsat imagery and shuttle radar topography mission srtm data these basins which form the terminal sump of the okavango system in northern botswana are now almost completely dry during the quaternary they were intermittently occupied by large stable lake bodies and are thought to have periodically filled to a point of coalescence inundating an area that at its largest extent encompassed 66 000 km 2 poor chronological control has previously limited the utility of this important palaeo archive as part of a region wide lake palaeo shoreline research programme a systematic optically stimulated luminescence osl dating programme has utilised a lightweight hydraulic auger to take samples at depth from relict shoreline features twenty drill sites have generated 140 samples for dating establishing a firm chronology for multiple lake full phases in all three component basins ngami mababe and makgadikgadi of this mega lake this paper presents the final set of ages in the programme derived from four cores from the western and north eastern shorelines of makgadikgadi and uses these ages to establish a chronology of mega lake high stands during the last 300 ka providing a rare directly dated long terrestrial record of positive hydrological excursions within the southern hemisphere

nitrogen cycle perturbations linked to metazoan diversification during the early cambrian abstract the rapid diversification of metazoans during the late ediacaran early cambrian is one of the most remarkable evolutionary events in earth history however the triggering mechanism remains highly contentious particularly concerning the relationship of this biological event to contemporaneous changes in ocean redox and nutrient cycling in this study we report new bulk nitrogen isotope 15n and organic carbon isotope 13corg data from the lower cambrian niutitang formation in two deep water sections in south china to provide a better understanding of the relationship of the nitrogen cycle ocean redox and biological evolution during this key period the 15n data from two study sections are characterized by consistently low values 1 to 2 throughout the niutitang formation despite large variations in 13corg we interpret these low 15n values to have resulted from assimilation of recycled ammonium nh4 and n2 fixation compilation of available 15n data reveals large temporal and spatial 15n variations from 1 to 7 in time equivalent strata in south china such 15n variations imply the co existence of aerobic and anaerobic n cycles and highly heterogeneous ocean redox conditions during the early cambrian in south china which is consistent with inferences from other geochemical proxies temporal fluctuations and spatial heterogeneity in ocean redox conditions may have controlled the distribution and evolution of metazoans during the early cambrian

discrete ice lens theory for frost heave in soils the existing segregation potential sp method for frost heave prediction in soils is semiempirical in nature and does not explicitly predict the relationship between heave rate temperature gradient and other more fundamental soil properties the sp method assumes that the heave rate is directly related to the temperature gradient at the frost front but acknowledges that the sp parameter is dependent on pressure suction at the frost front cooling rate soil type and so forth this paper extends and modifies an approximate analytical technique of gilpin and accounts for the effects of distributed phase change within the freezing fringe in both the head and mass transfer components of the formulation the approach requires as input a relationship between frozen hydraulic conductivity and temperature and predicts the discrete location of each ice lens within the freezing soil the solution can be carried out quickly on a microcomputer to obtain the heave suction at the frost front ice lens temperatur

chronology of early cambrian biomineralization data on the first appearances of major animal groups with mineralized skeletons on the siberian platform and worldwide are revised and summarized herein with references to an improved carbon isotope stratigraphy and radiometric dating in order to reconstruct the cambrian radiation popularly known as the cambrian explosion with a higher precision and provide a basis for the definition of cambrian stages 2 to 4 the lophotrochozoa and probably chaetognatha were first among protostomians to achieve biomineralization during the terreneuvian epoch mainly the fortunian age fast evolutionary radiation within the lophotrochozoa was followed by radiation of the sclerotized and biomineralized ecdysozoa during stage 3 the first mineralized skeletons of the deuterostomia represented by echinoderms appeared in the middle of cambrian stage 3 the fossil record of sponges and cnidarians suggests that they acquired biomineralized skeletons in the late neoproterozoic but diversification of both definite sponges and cnidarians was in parallel to that of bilaterians the distribution of calcium carbonate skeletal mineralogies from the upper ediacaran to lower cambrian reflects fluctuations in the global ocean chemistry and shows that the cambrian radiation occurred mainly during a time of aragonite and high magnesium calcite seas

ca and mg isotope constraints on the origin of earth s deepest δ13c excursion abstract understanding the extreme carbon isotope excursions found in carbonate rocks of the ediacaran period 635 541 ma where 13 c of marine carbonates 13 c carb reach their minimum 12 for earth history is one of the most vexing problems in precambrian geology known colloquially as the shuram excursion the event has been interpreted by many as a product of a profoundly different ediacaran carbon cycle more recently diagenetic processes have been invoked with the very negative 13 c values of ediacaran carbonates explained via meteoric alteration late stage burial diagenesis or growth of authigenic carbonates in the sediment column thus challenging models which rely upon a dramatically changing redox state of the ediacaran oceans here we present 257 44 40 ca and 131 26 mg measurements along with mg mn and sr data from carbonates of the ediacaran aged wonoka formation fm of south australia to bring new isotope systems to bear on understanding the shuram excursion data from four measured sections spanning the basin reveal stratigraphically coherent trends with variability of 1 5 in 26 mg and 1 2 in 44 40 ca this ca isotope variability dwarfs the 0 2 0 3 change seen coeval with the permian triassic mass extinction the largest recorded in the rock record and is on par with putative changes in the 44 40 ca value of seawater seen over the phanerozoic eon changes in both isotopic systems are too large to explain with changes in the isotopic composition of ca and mg in global seawater given modern budgets and residence times and thus must be products of alternative processes relationships between 44 40 ca and sr and 26 mg and mg are consistent with mineralogical control e g aragonite vs calcite limestone vs dolostone on calcium and magnesium isotope variability the most pristine samples in the wonoka dataset preserving sr concentrations in the 1000s of ppm range and 44 40 ca values inherited from an originally aragonitic polymorph have 13 c carb of 8 to 7 thereby providing strong geochemical evidence that extremely negative 13 c carb values are primary products of the ediacaran surface environment

timing of the termination of sturtian glaciation sims u pb zircon dating from south china abstract radiometric calibration of the cryogenian glaciations is critical in understanding the nature of the climatic geochemical and biological events in the cryogenian period however the timing of termination of the sturtian glaciation remains unsettled here we carried out new sims u pb zircon dating from two tuff layers at the non glacial datangpo formation at the jiangjunshan section in guizhou south china the sims u pb zircon analyses yield u pb concordia ages of 659 3 2 4 ma and 657 3 3 2 ma for the two tuff layers respectively the age of 659 3 2 4 ma obtained from the tuff layer occurring at the boundary between sturtian diamictites and the non glacial datangpo formation is regarded as the best age estimate for the termination of sturtian glaciation in south china this new u pb age is well consistent with those from the previous studies suggesting a globally synchronous termination of the sturtian glaciation at ca 659 ma the onset of the marinoan glaciation has been estimated to occur at ca 649 ma based on the radiometric age obtained in this study and previous cyclostratigraphic results thus it is argued that the marinoan glaciation was a long lived glaciation and spanned approximately 14 million years

a syn depositional age for earth s deepest δ13c excursion required by isotope conglomerate tests terra nova 24 318 325 2012 abstract the most negative carbon isotope excursion in earth history is found in carbonate rocks of the ediacaran period 635 542 ma workers have interpreted the event as the oxidation of the ediacaran oceans rothman et al proc natl acad sci usa100 2003 8124 fike et al nature444 2006 744 mcfadden et al proc natl acad sci usa105 2008 3197 or as diagenetic alteration of the 13c of carbonates 13ccarb knauth and kennedy nature460 2009 728 derry earth planet sci lett 294 2010 152 here we present chemo stratigraphic data from the ediacaran aged wonoka formation fm of south australia that require a syn depositional age for the extraordinary range of 13ccarb values 12 to 4 observed in the formation in some locations the wonoka fm is 700 metres m of mixed shelf limestones and siliclastics that record the full 16 13ccarb excursion in other places the wonoka fm is host to deep 1 km palaeocanyons which are partly filled by tabular clast carbonate breccias that are sourced from eroded wonoka canyon shoulders by measuring the isotopic values of 485 carbonate clasts an isotope conglomerate test we show that canyon shoulder carbonates acquired their 13ccarb 18ocarb values before brecciation and redeposition in the palaeocanyons

cryogenian glaciation and the onset of carbon isotope decoupling global carbon cycle perturbations throughout earth history are frequently linked to changing paleogeography glaciation ocean oxygenation and biological innovation a pronounced carbonate carbon isotope excursion during the ediacaran period 635 to 542 million years ago accompanied by invariant or decoupled organic carbon isotope values has been explained with a model that relies on a large oceanic reservoir of organic carbon we present carbonate and organic matter carbon isotope data that demonstrate no decoupling from approximately 820 to 760 million years ago and complete decoupling between the sturtian and marinoan glacial events of the cryogenian period approximately 720 to 635 million years ago growth of the organic carbon pool may be related to iron rich and sulfate poor deep ocean conditions facilitated by an increase in the fe s ratio of the riverine flux after sturtian glacial removal of a long lived continental regolith

late quaternary lake level fluctuations in the mababe depression middle kalahari palaeolakes and the role of zambezi inflows abstract a systematic drilling and optical dating programme on middle kalahari beach ridge relict shoreline sediments has enabled the identification of multiple episodes of lake high stands of an extensive palaeolake system at the terminus of the okavango delta northern botswana this paper presents 23 ages from the mababe depression and establishes four shoreline construction phases in the late quaternary coeval with other sub basin lake high stands lake ngami these synchronous lake phases result from a coalescence of the sub basins into a unified palaeolake lake thamalakane covering an area of 32 000 km2 six additional ages are also presented from the chobe enclave to the north of the basin where shoreline ridges were emplaced at the same time as lake thamalakane phases this suggests that increased flow in the chobe and zambezi system significantly contributed to the middle kalahari lake phases in both the post glacial and holocene periods the integration of these new data and their compatibility with other regional and tropical palaeo archives is discussed in the light of understanding quaternary climate drivers within the kalahari

vegetation induced subsurface precipitation of carbonate as an aggradational process in the permanent swamps of the okavango delta fan botswana abstract islands constitute an important geomorphological component of the permanent swamps of the okavango alluvial fan studies of the topography soil chemistry groundwater chemistry and vegetation cover across several islands indicate that many islands form as a result of the subsurface precipitation of calcite and probably amorphous silica which produces vertical expansion creating topographic relief the main agent responsible for this precipitation is transpiration especially by large deep rooted trees which increases the salinity of the groundwater leading to saturation in calcite and silica the process ultimately leads to the development of highly saline groundwater and results in marked zonation of vegetation across the islands numerical modelling of the process suggests that salinization of the groundwater beneath an island takes between 100 and 200 yr high calcite contents in island soils appear to be the product of several cycles of salinization which are interspersed with periods of swamp abandonment during which the more soluble alkali salts are leached from the islands island formation represents an important process in the overall aggradation of the alluvial fan

possible early foraminiferans in post sturtian 716 635 ma cap carbonates foraminifera are an ecologically important group of modern heterotrophic amoeboid eukaryotes whose naked and testate ancestors are thought to have evolved 1 ga ago however the single chambered agglutinated tests of these protists appear in the fossil record only after ca 580 ma coinciding with the appearance of macroscopic and mineralized animals here we report the discovery of small slender tubular microfossils in the sturtian ca 716 635 ma cap carbonate of the rasthof formation in namibia the tubes are 200 1300 m long and 20 70 m wide and preserve apertures and variably wide lumens folds constrictions and ridges their sometimes flexible walls are composed of carbonaceous material and detrital minerals this combination of morphologic and compositional characters is also present in some species of modern single chambered agglutinated tubular foraminiferans and is not found in other agglutinated eukaryotes the preservation of possible early foraminifera in the carbonate rocks deposited in the immediate aftermath of sturtian low latitude glaciation indicates that various morphologically modern protists thrived in microbially dominated ecosystems and contributed to the cycling of carbon in neoproterozoic oceans much before the rise of complex animals

pan floor barchan mounds ntwetwe pan makgadikgadi botswana their origin and palaeoclimatic implications abstract the variety of forms assumed to be dunes and palaeolakes of northern botswana have long been perceived as geoproxies for the elucidation of quaternary palaeoclimates mounds on the floor of ntwetwe pan have been regarded as modified barchans pertaining to a time in the past when conditions were drier than at present the morphology hydrogeology and biology of these placed in the wider context of the palaeolacustrine hinterland indicate that they are not dunes but subaqueous features spring mounds the morphology is inconsistent with barchans conspicuous features are the elevated eyes within the enclosing arms of the crescentic forms that occur furthest from the pan edge seepages discharge from their flanks commonly about 7 m higher than the dry saline pan floor on the top of forms nearer the pan margins permanent pools of potable water occur similarly elevated above the floor hydraulic pressure to sustain the upward discharge of groundwater is provided by the excellent recharge through silicate karst depressions in the hinterland particularly those of the 936 m palaeolake floor groundwater discharge still operating today was formerly much stronger and associated with lacustrine conditions a chronological framework is provided by archaeological remains on the palaeolake floors of the hinterland these indicate that the oldest palaeolake floor pertaining to the quaternary is the 920 m level periods of spring mound formation are associated with both this and the 912 m palaeolake the implications for future luminescence dating targeting palaeoenvironmental reconstruction are discussed in the light of this new model of mound formation

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

the ion chemistry of lakes and late holocene desiccation in the badain jaran desert inner mongolia china abstract in the markedly arid badain jaran desert one of the least studied areas in the world over a hundred permanent lakes exist among the dunes the geochemistry of water samples from nine lakes and from the groundwater in the desert and on its periphery was investigated ion chemistry of the water from these desert lakes changes considerably on a regional scale we argue that there is a positive correlation between the ages of these lakes and their totally dissolved solids tds varying between 1 2 and 398 2 g l tritium analysis of groundwater in wells gives an age generally older than four decades and younger than 100 years the lacustrine deposits freshwater fauna and shorelines indicate the existence of large water bodies with low salinities in the inter dune hollows of this desert during the early and middle holocene the precipitation in this desert during the early and middle holocene was estimated to be ca 200 mm year on average at least twice as high as at present on the basis of paleoecology tl and radiocarbon ages it was concluded that the huge enrichment of ions in the lakes and decrease of water availability originated from climatic desiccation during the last 4000 years the paleoclimate of the badain jaran desert reported here was remarkably consistent with the late quaternary climatic histories of the regions to the immediate north west east and south of the study area

chambered structures from the ediacaran dengying formation yunnan china comparison with the cryogenian analogues and their microbial interpretation enigmatic chambered structures have been reported forming reef frames in cryogenian interglacial carbonates prior to the commonly acknowledged microbial metazoan reefs at the terminal ediacaran and interpreted as fossils of possible sponge grade organisms a better constraint on the affinity of these structures is partly hindered by few analogues in other time periods this study describes similar structures from peritidal dolostones of the ediacaran denying formation from yunnan china samples were investigated using optical microscopy and three dimensional 3 d reconstruction based on grinding tomography the dengying chambered structures are comparable with cryogenian structures in basic construction but are not frame building and show variations in overall shape and inhabiting facies two dimensional 2 d cross sections show that thin homogeneous micritic laminae are the basic building blocks of the chamber walls thick walls represent parallel accretion of these laminae and thin walls developed from the angular growth of a single lamina or merging of multiple laminae in 3 d space the laminae primarily correspond to continuous surfaces which sometimes contain sub circular holes while a few represent filamentous elements connected to the surfaces the morphological features and growth pattern of the dengying chambered structures indicate that they are likely to be calcified microbial constructions rather than skeletal remains of basic metazoans however aside from the cryogenian and dengying examples comparable chambered constructions with laminae based architecture are yet unknown from other fossil or extant microbialites further work investigating related structures is needed to determine the microbial consortia and controlling environmental factors that produced these chambered structures

spatial variability and hydrologic evolution of australian lake basins analogue for pleistocene hydrologic change and evaporite formation abstract processes evident in basins across hydrologic gradients from humid to arid regions today provide analogues for systems that may have operated in some individual basins during periods of past hydrologic change thus lakes in humid regions today with low salinity sedimentary facies and with morphology controlled largely by wave action may closely resemble past lacustral pluvial phases experienced in zones which today are relatively arid and saline conversely playas controlled today by groundwater and in some cases subject to defletion may resemble those arid and dune building stages experienced by basins in sub humid regions today as they underwent glacial age desiccation in any such sequence of spatial or temporal gradients from humid to more arid conditions the frequency of basin filling and drying modulates the rate of evaporitive loss of interstitial waters from the exposed basin surface this in turn controls the salinity evolution of the interstitial and local groundwaters the notion of closed systems for terminal lakes while satisfactory for analysis of surface water budgets is quite inapplicable to the evolution of brines where surfacewater groundwater exchange during the high salinity drying stages becomes a controlling factor affecting all salt budgets in these systems evaporites may only be preserved when the sub surface waters have evolved to salinites near saturation for the salts concerned if sub surface waters are frequently diluted by surfacewater groundwater exchange evaporites will not be preserved in the record even though the surface system appears hydrologically closed the process of interstitial water evolution which controls efflorescence deflation and salt weathering exerts a dominating influence on basin geomorphic expression as well as on hydrochemical fractionation

effect of phycosiphoniform burrows on shale hydrocarbon reservoir quality unconventional gas tight gas coalbed methane and shale gas has become an increasingly significant source of energy economic production from such low permeability reservoirs relies upon identifying regions of the reservoir that will yield the highest gas production rates currently available gas recovery technologies are highly dependent on the fracturability of the reservoir zones of enhanced brittleness and permeability within shale gas reservoir horizons are a prerequisite for successful shale gas recovery such brittle zones are directly linked with increased quartz and or carbonate content within the mudstone in mudstones with high clay mineral content quartz may be concentrated and redistributed as a result of burrowing activities of infaunal organisms high quality porosity and permeability zones in shale petroleum reservoirs may be present in the form of silty and sandy tortuous strips of selectively concentrated grains of quartz that constitute burrow halos grain selective burrows therefore can improve reservoir capacity permeability and fracturability and thus control the storativity of the shale petroleum reservoir this study presents three dimensional reconstructions of three different types of phycosiphon like burrows and investigates the possible fluid flow paths caused by the ichnofabric the volumetric approach to the bioturbation generated by phycosiphoniform burrow makers used herein shows that the volume of sediment that becomes more porous and more permeable media within such bioturbated interval can range from 13 to 26 of the total volume the quartzose strips of sediment caused by bioturbation are highly tortuous and interconnected vertically and horizontally thereby increasing both horizontal and vertical permeability additionally the quartz frameworks created by the burrows may locally increase fracturability within otherwise nonbrittle mudstones

microbial nucleation of calcium carbonate in the precambrian microbial sulfate reduction is thought to stimulate carbonate precipitation in modern stromatolites yet whether this metabolism was important in shaping precambrian stromatolites is unknown here we use geochemical modeling to suggest that the influence of sulfate reduction on the saturation index of calcite si is negligible when seawater is in equilibrium with high pco 2 as is thought for the precambrian our laboratory experiments with heterotrophic bacteria in a medium mimicking precambrian seawater chemistry show that even if sulfate reduction does not significantly change the si the presence of bacteria stimulates calcite precipitation over sterile controls by effectively increasing the si over a ph range from 7 3 to 7 8 under our experimental conditions dead cells stimulate in situ carbonate precipitation equally if not more than active sulfate reducing bacteria heterogeneous nucleation of calcite by microbial cell material appears to be the driving mechanism that explains this phenomenon

shell structure and distribution of cloudina a potential index fossil for the terminal proterozoic cloudina bearing biosparites and biomicrites in the lower part of the nama group namibia contain a wide morphological diversity of shell fragments that can all be attributed to the two named species c hartmannae and c riemkeae the curved to sinuous tubular shells of cloudina were multi layered each shell layer was 8 to 50 micrometers thick and in the form of a slightly flaring tube with one end open and the other closed growth appears to have been periodic with successive shell layers forming within older layers each added layer was slightly elevated from the previous layer at the proximal end and was asymmetrically placed within the older layer so that only a portion of the new shell layer was fused to the previous layer this type of growth left a relatively large unminerialized area between the shell layers which was often partially or fully occluded by early marine cements the thin shell layers exhibit both plastic and brittle deformation and were likely formed of a rigid caco3 impregnated organic rich material often the shell layers are preferentially dolomitized suggesting an original mineralogy of high magnesian calcite both species in the nama group formed thickets or perhaps bioherms and this sedentary and gregarious habit suggests that cloudina was probably a filter feeding metazoan of at least a cnidarian grade of organization the unusual shell structure of cloudina gives rise to a characteristic suite of taphonomic and diagenetic features that can be used to identify cloudina bearing deposits within the nama group and in other terminal proterozoic deposits around the world species of cloudina occur in limestones from brazil spain china and oman in sequences consistent with a latest proterozoic age assignment in addition supposed lower cambrian pre trilobitic shelly fossils from northwest mexico and the white inyo mountains in california and nevada including sinotubulites nevadatubulus and wyattia are all either closely related to or con generic with cloudina hence it is probable that these outcrops are latest proterozoic in age and that cloudina or cloudina like organisms were widely distributed at that time it is possible moreover to suggest that metazoan biomineralization occurred on a global scale by the latest proterozoic at the same time that evidence for complex multicellularity and locomotion in animals appears in siliciclastic ediacaran rocks in the form of body and trace fossils

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

molecular fossils extracted from the early cambrian section in the three gorges area south china abstract environmental investigation of the early cambrian is assessed through molecular fossils based from three core samples drilled in the three gorges area south china the core samples record environmental information dating from the earliest part of the early cambrian to nearly the end of the early cambrian making this investigation unique from previous investigations because a more far reaching temporal record is assessed this record includes unusually high abundance of longer chain n alkanes lca around n c 27 having been recognized in the earliest cambrian strata based on no odd even preference of the lca and comparisons to hydrocarbons in cultured cells of microorganisms the lca may be derived from hydrocarbons of sulfate reducing bacteria on the other hand a phototrophic origin of the sca is supported by a clear positive correlation with pristane the relative abundance of lca to sca increased in the earliest cambrian suggesting predominance of sulfate reducers against phototrophs this lca abundance with simultaneous decrease of pristane phytane may indicate a euxinic condition where sulfate reducers remineralize organic carbon in the water column this is consistent with the concomitant decrease of the 13 c carb value probably due to a massive reoxidation of organic carbon the inferred reducing condition is further supported by the enhanced appearance of squalane that could be derived from an archeal lipid our results suggest the euxinia of the yangtze platform in the earliest cambrian that may relate to the emergence of the large nektonic animals which require sufficient level of free oxygen

uranium isotope evidence for temporary ocean oxygenation in the aftermath of the sturtian snowball earth abstract the appearance and radiation of animals are commonly attributed to neoproterozoic oceanic oxygenation yet independent geochemical evidence for such an event remains equivocal strata deposited between the sturtian and marinoan snowball earth glaciations 660 to 640 ma contain the earliest animal biomarkers and possible body fossils to quantify the extent of seafloor oxygenation during this critical interval we present uranium isotope ratios 238u 235u denoted as 238 u from limestone of the taishir formation in mongolia through two stratigraphic sections that are separated by 75 km within the same depositional basin above the sturtian glacial deposits through 150 m of stratigraphy 238 u compositions have a mean value of 0 47 this interval is followed by a 0 3 decrease in 238 u coincident with the taishir negative carbon isotope excursion thereafter 238 u values remain relatively low until the erosional unconformity at the base of the marinoan glacial deposits using a box model we show that the best explanation for the higher 238 u values of the post sturtian limestones is extensive but temporary oxygenation of the seafloor and is inconsistent with a scenario involving only increased delivery of uranium to the oceans due to post snowball weathering the decline in 238 u in overlying strata coincident with the taishir negative 13 c excursion indicates a subsequent decrease in seafloor oxygenation the u isotopic data combined with modeling results challenge the notion of a simple unidirectional oxygenation of neoproterozoic oceans

effectiveness of natural zeroing of the thermoluminescence in sediments the last exposure to sunlight of detrital feldspars and quartz grains in quaternary sediments can be dated directly by thermoluminescence tl methods but the effectiveness of zeroing of the light sensitive tl signal by daylight varies widely depending upon details of the sedimentation processes to document this variability and to help develop objective criteria useful for future sample selection and laboratory procedures i have collected and analyzed several eolian fluvial marine deltaic and lacustrine samples from less than 100 year old deposits as well as suspensions from sediment plumes also underwater spectral irradiance was measured during recovery of suspensions tl properties of polyrnineralic feldspar dominated 2 4 m and 4 11 m size fractions were measured in the laboratory unexpectedly some light sensitive tl was found to remain in all samples however the light sensitive tl was more effectively zeroed in some samples a proximal loess a sandy braided channel deposit tidal flat and upper delta front muds and thin clayey lacustrine laminae than in others a silty braided channel deposit submarine fan muds and silt bearing lacustrine laminae for the first group the minimum relict paleodose obtained with the partial bleach technique is about 0 4 0 3 gy corresponding to a relict tl age estimate of about 200 years for these sediments thus similar sediments excluding fluvial deposits as young as about 2 ka can be accurately and routinely dated without significant correction for a zero point error if one uses the partial bleach technique coupled with integrated optical bleaching energy fluences of less than 1 j cm 2 and wavelengths only above about 550 nm also some simple optical bleaching tests are reported that have a potential use in sample selection and in provenance studies of some surficial detrital sediments finally a simple mathematical model is presented that helps account for the gross features of the observed bleaching response curves

periglacial paleosols and cryogenian paleoclimate near adelaide south australia abstract late cryogenian deformation of the reynella siltstone near hallett cove south australia has been interpreted as shallow marine methane seeps which were furthermore proposed to have played a role in deglaciation from snowball earth re examination and new analyses of these same outcrops confirms an earlier interpretation of this intrastratal deformation as periglacial paleosols which are evidence that even at maximum extent late cryogenian glaciers did not cover all continental lowlands let alone the world ocean known miocene and pliocene methane seeps of california examined for this study are very different in their gray color rounded nodules and clayey host rock compared with red highly deformed and silty to pebbly reynella siltstone of hallett cove stable isotopic composition of nodular carbonate in the reynella siltstone fails to show extreme carbon isotopic depletion or the meteoric oxygen lines characteristic of known methane seeps and sphaerosideritic waterlogged paleosols but instead shows covariance of carbon and oxygen isotopic composition comparable with that in pedogenic and subaerial carbonate of well drained soils mass balance geochemical analyses show modest weathering compatible with sand wedge structures and evaporitic pseudomorphs indicative of arid and frigid paleoclimate outcrops of reynella siltstone near hallett cove represent frigid low latitude floodplains not submarine methane seeps nor global glaciation

late quaternary paleoenvironments in the southwestern kalahari abstract landforms of the southwestern kalahari in southern africa include linear dune systems pans and associated lunette dunes and the valleys of ephemeral rivers present day climates in the region are transitional from arid to semi arid making the area sensitive to climatic change the linear dunes are part of an extensive system of fixed dunes developed in the surface kalahari sands the dunes were in existence prior to 32 000 yr b p but have been reactivated on a number of occasions since then in conditions of increased wind velocities more nw winds and reduced rainfall especially in northern and eastern parts of the region periods of dune activity probably occurred at 19 000 17 000 10 000 6000 and 4000 3000 yr b p the development of pans in the region has been strongly influenced by changes in groundwater conditions deflation from pans and formation of lunettes occurred in periods when groundwater levels were higher and fluctuated seasonally more than they do today such periods probably occurred 25 000 20 000 yr b p around 10 000 yr b p and just prior to 1000 yr b p still higher groundwater discharges promoted by increased regional rainfall resulted in the formation of spring tufas and seasonal or permanent lakes in the pans which were also fed by surface runoff during the intervals between 33 000 28 000 23 000 21 000 17 000 15 000 and 12 000 11 000 yr b p the headwaters of the major ephemeral streams lie outside the region but within the same climatic zone increased flow in the molopo river occurred prior to 19 000 and between 15 000 and 13 000 yr b p

from landform to process morphology and formation of lake bed barchan dunes makgadikgadi botswana abstract a suite of crescentic landforms is visible from remotely sensed imagery within the ntwetwe panpan in the makgadikgadi basin botswana we investigate the most distinct of these landforms using morphometric measurements sedimentary data and optically stimulated luminescence osl signal analysis comparative analysis with previously published barchan morphological data sets suggest the ntwetwe features fall within the spectrum of morphometric parameters found in a range of barchan dunefields from around the world there is currently insufficient comparative morphometric data from sub aqueous dunefields to be able to distinguish the particular formative environment of the dune osl signal analyses however support the hypothesis of grove 1969 grove a t 1969 landforms and climatic change in the kalahari and ngamiland geographical journal 135 191 212 that the last deposition of the sediments within the ntwetwe forms was most likely aeolian in origin luminescence signal analysis is employed to investigate potential transport and bleaching environments of the sediments forming the features but results in this case do not shed further light on the formative conditions of these enigmatic landforms

bedform transport rates for the lowermost mississippi river 1 new methods of data collection and processing are developed to provide quantitative reach scale measurements of bedform transport mass within the tidally influenced mississippi river a multibeam swath profiler was used to collect daily bathymetry over a range of water discharges and bed elevation changes induced by dune migration are measured these values are coupled with bulk physical properties of the bed sediment to constrain mass flux and annual bedform transport is estimated at 2 2 x 10 6 metric tons mt the total annual sand flux from the mississippi river calculated by combining measured bedform transport rates and suspended sediment flux is estimated to be 20 x 10 6 mt survey data also provide information about the spatial distribution of dunes across the channel bottom straight reach segments are commonly mantled by dunes for the entire cross section while bends are typically areas of focused scour devoid of bedforms presumably any sediments associated with migrating dunes are propelled into suspension within bends before redepositing in the subsequent straight reach movement via suspension is therefore an important component for the downriver transport of bed materials in the lower mississippi river

influence of microbial framework on cryogenian microbial facies rasthof formation namibia abstract the rasthof formation is a mid cryogenian cap carbonate succession deposited in namibia following the sturtian glaciation it includes a microbial member typically 100 m thick this member exhibits contorted intervals and is divisible into two informally defined units the lower unit microbial member 1 mm1 comprises thickly laminated microbialites 1 6 mm the upper unit mm2 is characterized by thinly laminated microbialites sub millimetre layering contortion of the microbialite deposits a recurrent feature of this succession is interpreted to result from soft sediment deformation deformed intervals and styles range from metre to decimetre scale chaotic folds in mm1 to a few centimetre scale localized roll up structures in mm2 study of the microfacies of mm1 and mm2 reveals two essentially different architectures in mm1 the microfacies is dominated by an alternation of thin micritic laminae with thicker cemented intervals this probably gave less rigidity to the sediments than in mm2 where the laminated fabric is also present but connected vertically as well forming a continuous framework we suggest that the continuity of this framework limited the frequency and scale of soft sediment deformation in the rasthof formation the microarchitecture is thus suggested to translate into different degrees of rigidity of the macrofacies

relative and absolute dating of land surfaces abstract the dating of land surfaces has long posed problems for geomorphologists relative methods stratigraphic geomorphic topographic are sound and convincing exhumed forms may complicate identification and relationships for both epigene and etch forms have been buried and exhumed but in tectonically undisturbed areas the higher surfaces are older than those preserved at lower levels also surfaces have an age range the relationship of surfaces with volcanic deposits old shorelines and genetically related sedimentary sequences provides sound ages and correlation with dated duricrusts and faults is also useful there are no temporal limits to relative dating for the methods are equally applicable to the dating say of proterozoic surfaces as of those of pleistocene age the disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed the so called absolute physical numerical methods and especially those based on exposure age dating with in situ cosmogenic radionuclides are appealing because they produce direct numerical ages and appear to be widely applicable but there are severe temporal limitations and sampling problems complicate and may invalidate interpretation absolute age determinations must be consistent with the stratigraphic and geomorphologic settings the best results are obtained when physicists and earth scientists pool their knowledge and experience a background in local and regional geology is especially important

stratigraphic expression of earth s deepest δ13c excursion in the wonoka formation of south australia the most negative carbon isotope excursion in earth history is found in carbonate rocks of the ediacaran period 635 541 ma known colloquially as the shuram excursion workers have long noted its broad concordance with the rise of abundant macro scale fossils in the rock record collectively known as the ediacaran biota thus the shuram excursion has been interpreted by many in the context of a dramatically changing redox state of the ediacaran oceans for example a result of methane cycling in a low o2 atmosphere the final destruction of a large pool of recalcitrant dissolved organic carbon doc and the step wise oxidation of the ediacaran oceans more recently diagenetic interpretations of the shuram excursion have challenged the various redox models with the very negative 13c values of ediacaran carbonates explained via sedimentary in growth of very 13c depleted authigenic carbonates meteoric alteration or late stage burial diagenesis a stratigraphic and sedimentological context is required to discriminate between these explanatory models and to determine whether the shuram excursion can be used to evaluate oxygenation in terminal neoproterozoic oceans here we present chemo stratigraphic data 13c 18o and trace element abundances from 15 measured sections of the ediacaran aged wonoka formation fm of south australia in some locations the wonoka fm is 700 meters m of mixed shelf limestones and siliciclastics that record a 17 permil 13c excursion 12 to 5 further north in the basin the wonoka fm is host to deep 1 km paleocanyons which are partly filled by tabular clast carbonate breccias canyon filling ceased during ongoing sedimentation on the shelf interior the canyon shoulder as evidenced by upper canyon shoulder units that overlie and cap certain canyon fill sequences the unprecedented size of the chemostratigraphic dataset presented here 2671 13c 18o measurements from the canyon shoulder 1393 13c 18o measurements from canyon clasts and 11 different trace elements measured on 247 wonoka fm carbonate samples when coupled with the unique canyon shoulder to canyon fill depositional system of the wonoka fm allows for new insights into ediacaran carbon isotope systematics the excursion is preserved in a remarkably consistent fashion across 12 000 km2 of basin area fabric altering diagenesis where present occurs at the sub meter vertical scale results in

the palaeobiology and geochemistry of precambrian hydrocarbon source rocks abstract organic carbon productivity and formation of hydrocarbon source rocks during the early precambrian was almost exclusively the product of the growth of microbial mats indirect evidence of microbial mats can be traced back to at least 2 6 2 7 ga neoarchaean with the earliest evidence of mat development in siliciclastic sediments coming from the 2 9 ga mesoarchaean predominantly marine sedimentary rocks of the mozaan group in south africa the earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2 75 ga palaeoproterozoic fluviolacustrine sediments of the hardey formation of the pilbara craton in western australia well preserved proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere eukaryotic steranes biomarker for eukaryotic cells and therefore evolved forms of life are present in the geological record from about 2 7 ga but they are not abundant or diverse within archaean communities which tend to be dominated by archaea isoprenoids some hydrocarbons have been generated and migrated from archaean organic rich shales but they were probably not of sufficient volume to be of commercial interest the world s oldest significant hydrocarbon source rocks are palaeoproterozoic in age and include the shungite deposits 2 0 ga in the lake onega region of arctic russia there is strong evidence of a global biospheric oxygenation event at c 1300 1250 ma mesoproterozoic in conjunction with a first order positive shift in the marine carbon isotope record this is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest unambiguously multicellular eukaryotes at this time this oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons hydrocarbon source rocks of late mesoproterozoic and early neoproterozoic age are widespread and include highly organic rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks by c 850 ma the neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes including those of multicellular eukaryotes red and green algae as well as molecular evidence for heterotrophic protists the most widespread hydrocarbon source rocks of mid to late neoproterozoic age are commonly transgressive organic rich black shales associated with inter glacial and post glacial phases of the neoproterozoic global scale glaciations the relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late neoproterozoic and earliest cambrian perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and or nutrient supply precambrian and infracambrian petroleum systems are relatively abundant and widespread the oldest live oil recovered to date is sourced from mesoproterozoic rocks within the velkerri formation roper group of the mcarthur basin of northern australia dated at 1361 21 ma and 1417 29 ma re os dates with at least the initial phase of oil generation and migration having taken place before 1280 ma however the geologically oldest commercial production is currently from the somewhat younger mid to late neoproterozoic cryogenian ediacaran petroleum systems of the lena tunguska province in east siberia and in southern china from the latest neoproterozoic to early cambrian huqf supergroup in oman and potentially in the near future from the age equivalent mawar supergroup in western india

holocene fluctuations of lake ngami middle kalahari chronology and responses to climatic change abstract lake ngami a 3000 km 2 basin at the distal end of the okavango delta botswana has a complex suite of shorelines ranging from the lake sump at 919 m asl up to 945 m asl linked to topographic thresholds and inflows current and fossil in the okavango system the lake has been ephemeral throughout the 20th century and completely dry since 1982 yet held a substantial body of water up to a level of 930 m asl when visited by european travellers in the mid nineteenth century the historical decline in the lake has been well documented and used as evidence for regional desiccation but the holocene record based on archaeological studies and radiocarbon dating of inorganic carbonates is more speculative a study of a 30 km 2 diatomite at the eastern end of the basin indicates that a substantial freshwater lake at a level of at least 932 m asl was present throughout the holocene linked to a now abandoned inflow of the kunyere river a programme of 18 luminescence age determinations of the peripheral dautsa and magotlwanen shoreline sequences suggests that they are old features that have been constructed on a lacustrine substrate dating back to 90 100 ka or earlier and have been modified by holocene fluctuations in lake level and subsequent aeolian reworking the water level peaked at 936 m asl between 4 and 3 ka returned to 934 m at 2 8 2 ka ponding water into the phatane gap and fell rapidly thereafter to 932 m and thence to historical levels the 936 m level can be correlated with increased throughflow in the okavango delta hence increased rainfall in the catchment the subsequent decline can be attributed to episodic closure and rerouting of okavango distributaries in particular the thaoghe perhaps aided by anthropogenic activities as such the desiccation of the lake is not indicative of regional climatic change

luminescence dating applications in geomorphological research abstract chronological control remains critical to the interpretation of environmental change and to establishing long term rates of geomorphological processes radiocarbon dating will continue to be the central technique for age assessment of carbon bearing sediments deposited in the last ca 40 ka but alternative absolute dating strategies are required for older or non carbon bearing sediments they include uranium series dating of precipitated sediments k ar and ar ar dating of primary volcanic materials and cosmogenic isotopic dating of surface exposure ages an additional family of methods collectively termed luminescence techniques is increasingly providing a basis for the age assessment of detrital sedimentary deposits these methods directly date the deposition of detrital mineral grains typically quartz and feldspar and have been demonstrated to provide accurate deposition chronologies for up to 800 ka although 150 ka is more typical in the past decade a subset of luminescence techniques which employ optical sample stimulation methods called optical dating has made it possible to date samples from a much wider range of depositional systems given that the methods directly date sedimentation they allow direct assessment of depositional dynamism in numerous terrestrial and subaqueous including marine sedimentary systems and have demonstrated particular potential in the elucidation of chronologies from aeolian dune and loess fluvial colluvial coastal and volcaniclastic environments this paper provides a summary of the principles of the luminescence especially optical dating methods and shows how recent developments specifically single aliquot dating procedures have dramatically enhanced the scope and potential of the method the luminescence techniques are increasingly being incorporated into a wide range of geomorphological studies which seek to establish the nature of landscape changes over the past interglacial glacial cycle and beyond

deflation in the dustiest place on earth the bodele depression chad the bodele depression in chad is the largest single source of dust on earth because of a combination of strong near surface winds and a large reservoir of readily eroded and easily entrained low density sediment during the holocene and late pleistocene the bodele depression was host to a great lake palaeolake megachad that is now completely desiccated leaving the lake bed sediments exposed to deflation deflation in the bodele depression is revealed in erosional remnants of lake bed deposits exposed as yardangs on the lake floor erosional remnants indicate that locally up to 4 m of sediment has been removed by deflation optically stimulated luminescence dating shows that this occurred in less than 2400 years giving a rate of deflation of at least 1 6 mm yr 1 a range of other geomorphological features indicate that in some areas rates of deflation may be greater than or equal to 10 mm yr 1 deflation appears to be greatest in an arc around the northeastern margins of the lake basin where the diatomite deposits have been removed and revealed an underlying network of distributary channels preserved as gravel capped ridges the ridges which are part of an inverted topography are interpreted as deltaic distributary channels from an inland delta similar to the okavango delta examples of barchan dunes exist that were flooded by the palaeolake and which have now been exhumed from beneath layers of diatomite the distribution of extensive deflation around the northeasterly upwind end of the bodele depression probably results from a combination of large wind velocities an upwind source of abrasive sand and thinner diatomite deposits around the lake margin satellite image analysis shows that outcropping diatomite covers 24 049 km2 of the bodele depression furthermore the area from which diatomite has been removed by deflation is around 14 000 km2 combining this information with the ages and rates of deflation mentioned above indicates that some 61 000 km3 of diatomite has been eroded from the bodele depression during the past 1000 years

the jormungand global climate state and implications for neoproterozoic glaciations 1 geological and geochemical evidence can be interpreted as indicating strong hysteresis in global climate during the neoproterozoic glacial events 630 ma and 715 ma standard climate theory only allows such strong hysteresis if global climate enters a fully glaciated snowball state however the survival of photosynthetic eukaryotic marine species through these glaciations may indicate that there were large areas of open ocean a previously proposed slushball model for neoproterozoic glaciations could easily explain the survival of these organisms because it has open ocean throughout the tropics but there is only a small amount of hysteresis associated with the slushball state in this paper a new state of global climate the jormungand state is proposed in this state the ocean is very nearly globally ice covered but a very small strip of the tropical ocean remains ice free the low ice latitude of the jormungand state is a consequence of the low albedo of snow free bare sea ice if the ice latitude propagates into the subtropical desert zone it can stabilize without collapsing to the equator because subtropical ice covered regions have a relatively low top of atmosphere albedo as a result of the exposure of bare sea ice and relatively lower cloud cover moreover there is strong hysteresis associated with the jormungand state as greenhouse gas levels are varied because of the high albedo contrast between regions of bare and snow covered sea ice the jormungand state is illustrated here in two different atmospheric global climate models and in the budyko sellers model by offering a scenario that could explain both strong hysteresis in global climate and the survival of life the jormungand state represents a potential model for neoproterozoic glaciations although further study of this issue is needed

fossil spring deposits in the southern great basin and their implications for changes in water table levels near yucca mountain nevada during quaternary time fossil spring deposits are common in the southern great basin and their distribution provides important constraints on the hydrologic response of the regional water table to climate change this information is crucial because the proposed high level nuclear waste repository at yucca mountain will be located 200 400 m above the modern water table water tables will rise in response to a future return to glacial climates but the magnitude of the change and the consequences for radionuclide travel times and overall repository integrity are key uncertainties increased recharge during past pluvial periods in the spring mountains and sheep range caused water tables to rise and ground water to discharge over broad expanses of the las vegas valley system and in nearby pahrump sandy and coyote springs valleys in contrast other valleys in the region contain only small areas of pleistocene discharge resulting from local damming of ground water by faults that cut valley alluvium in these instances which include the valley wells area and piute and northern coyote springs valleys smaller ranges such as the clark and new york mountains supplied the moisture the change in water table levels since the last full glacial period varies between and within valleys from as little as 10 m in several areas to 95 m in the coyote springs valley at yucca mountain the water table has probably changed by 115 m in response to climate change the spring deposits and the mollusk faunas found with them often misinterpreted as lacustrine in origin share many essential features with active spring systems in northeast nevada deposits associated with discharge mainly consist of pale brown silt and sand that is entrapped by dense stands of phreatophytes covering valley bottoms when water tables are high pale green mud containing a mix of aquatic semiaquatic and moist terrestrial mollusks accumulates in wet meadows and marshes associated directly with spring discharge the record in subbasins of the las vegas valley system is dominated by late wisconsin age sediments although pond sediments and alluvium belonging to at least one older pre wisconsin pluvial period are also locally exposed deposits from two even earlier episodes of spring discharge both of which also occurred during rancholabrean time 10 to 450 ka are exposed in the pahrump chicago piute and coyote springs valleys and in the valley wells the records from the pahrump and coyote springs valleys are especially well exposed and likely extend back to at least early pleistocene time

paleolandscape and paleoenvironmental interpretation of spring deposited sediments in dakhleh oasis western desert of egypt abstract iron rich sediments in dakhleh oasis western desert of egypt have been recognized as spring mounds and as archaeological sites where paleolithic materials have been recovered detailed sedimentologic investigation and mapping of these features reveal that spring mound sediments were deposited in a shallow vegetated wetland formed by the discharge of iron rich nubian aquifer waters along the southern margin of the oasis controlled largely by localized faulting and the variable paleotopography of the basin floor the spring sediments represent peri lacustrine or lake independent features and can be differentiated from fully lacustrine deposits on the basis of their sedimentary characteristics as well as the presence of goethite and jarosite in a region where authigenic deposition during pleistocene pluvial activity principally resulted in tufas and lacustrine marls spring mound formation incorporated sediment through the ponding of surface water aeolian entrapment by local vegetation and the formation of iron precipitates in a low energy oxidative and acidic environment that was not dependent upon surface water inputs into the dakhleh basin the potentially interpluvial nature of this water resource makes it an important sedimentary archive for archaeological investigations in the basin the dakhleh spring mounds record a unique groundwater controlled paleoenvironment providing the first evidence of a bog iron in egypt and one of the few occurrences of iron rich wetland remnants in the modern sahara

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

agglutinated tests in post sturtian cap carbonates of namibia and mongolia abstract paleomagnetic data suggest that the early cryogenian sturtian glaciation extended to sea level at low latitude the impact of this dramatic environmental change on biota and the composition of ecosystems in the immediate aftermath of the sturtian glaciation remain virtually unknown here we report the discovery of abundant agglutinated tests in organic rich carbonates directly overlying sturtian glacial deposits from two different paleocontinents the rasthof formation of the congo craton in northern namibia and the tsagaan oloom formation of the dzabkhan terrane in mongolia the most abundant tests preserve morphological and compositional characters consistent with those found in at least two different families of modern lobose testate amoebae amoebozoa a group of heterotrophic microbial eukaryotes the presence of spatially and compositionally variable clay minerals quartz and microcline on the test walls is a signature of widespread biological agglutination the post glacial fossil assemblages differ from the most common pre sturtian vase shaped fossil testate amoebae perhaps as a result of different preservational mechanisms or of the appearance of new forms after the glaciation the apparent local abundance of eukaryotic body fossils in the post sturtian carbonates suggests that the cryogenian limestones and dolostones may host a currently unexplored fossil record of modern eukaryotes

the role of decay and mineralization in the preservation of soft bodied fossils abstract fossil deposits that preserve soft bodied organisms provide critical evidence of the history of life usually only more decay resistant materials e g cuticles survive as organic remains as a result of selective preservation and subsequent diagenesis to more resistant biopolymers permineralization the permeation of tissues by mineralizing fluids may preserve remarkable detail particularly of plants however evidence of more labile tissues e g muscle normally requires the replication of their morphology by rapid in situ growth of minerals i e authigenic mineralization this process relies on the steep geochemical gradients generated by decay microbes the minerals involved and the level of detail preserved which may be subcellular depend on a number of factors including the nature of microbial activity and amount of decay availability of ions and the type of organism that is fossilized understanding these controls is essential to determining the conditions that favor exce

eolian dunes computer simulations and attractor interpretation a simple computer simulation algorithm for the transport of sand by wind produces forms resembling barchan crescentic ridge linear and star natural dune classes sand is moved as slabs composed of many grains that are picked up at random transported in a specified direction and deposited 1 with a probability that depends on the local presence or absence of sand or 2 in shadow zones in the lee of dunes the simulated dune fields are interpreted as complex systems with sand dune classes being dynamical attractors of these systems the evolution of dunes once formed becomes decoupled from the details of eolian sand transport

degradation of linear dunes in northwest ngamiland botswana and the implications for luminescence dating of periods of aridity the long morphodynamic history of the linear dune system of northwest ngamiland comprises both construction and degradation processes here we describe features resulting from the latter crest lowering during wet periods is associated with lateral transfer of sand into the straats weathering and leaching have mobilised iron to the base of the dune profiles and have corroded the quartz grains satellite image interpretation indicates that dune sand has been diluted by bioturbation which transferred damaran saprolite from below the dunes to the surface flooding and waterlogging have modified and in places completely destroyed the original crisply linear morphology small linear collapse features across the linear dunes are associated with open fractures or faults where water infiltration is facilitated the implications for luminescence dating of dune sands to identify periods of aridity are discussed the dunes cannot simply be regarded as providing a stacked record of such periods they are very old and much of the record is missing as a result of crest lowering the chronostratigraphy includes degradational components which must be distinguished from those of construction bioturbation cannot be disregarded as a complication which overprints the luminescence signals

proliferation of miss forming microbial mats after the late neoproterozoic glaciations evidence from the kimberley region nw australia abstract a total of 20 morphological types of microbially induced sedimentary structures miss are recognized from the late neoproterozoic tillite bearing successions of the kimberley northwestern australia which include alpha petees triradiate cracks multidirectional linear ridges millimeter ripples erosional remnants and pockets and so on of these triradiate cracks multidirectional linear ridges and millimeter ripples are reported for the first time from the neoproterozoic worldwide most kimberley miss are morphologically comparable with their ancient and present day counterparts whose biogenicity have been well demonstrated in the kimberley during the interglacial period miss have colonization preference to relatively coarse sandy substrates ranford formation and their abundance decreased towards the younger strata throssell shale in contrast miss abundance increased from the older strata yurabi formation towards the younger strata flat rock formation during the postglacial period however overall both miss abundance and bedding plane coverage percentage increased from the marinoan younger marinoan gaskiers interglacial successions to younger marinoan gaskiers postglacial successions indicating a proliferation of microbial mats after the younger marinoan gaskiers glaciation the increase of microbial mats could have ameliorated marine biogeochemical conditions and ecosystems and thus laid the foundation for the rise of ediacaran biota in australian basins