impactoclastic density current emplacement of terrestrial meteorite impact ejecta and the formation of dust pellets and accretionary lapilli evidence from stac fada scotland abstractwe present an impactoclastic density current model for terrestrial meteorite impact ejecta emplacement that is based on a hitherto unreported threefold sequence of lithofacies in the recently identified stac fada impact ejecta outflow blanket in scotland massive suevite division a grades seamlessly up into massive suevite containing whole and broken accretionary lapilli division b and this is overlain by a thin layer of clast supported dust pellets division c the sequence closely resembles sequences in accretionary lapilli bearing ignimbrites emplaced by pyroclastic density currents at volcanoes and we deduce similar emplacement mechanisms we propose that 99 of the thickness of the stac fada ejecta blanket as seen at outcrop was deposited from a single depletive spatially decelerating granular fluid based density current that rapidly waxed and then waned a previously unreported thin layer of pellets division c records direct fallout from a residual atmospheric dust plume we pro

soft sediment deformation in upper proterozoic torridonian sandstones applecross formation at torridon northwest scotland abstract the applecross formation upper proterozoic torridon group northwest scotland was deposited as planar laminated and cross bedded coarse sands in braided streams soft sediment deformation is described from a 97 m thick study section most of which comprises contorted stratification the main structures are cusps and complex recumbent folded cross bedding formed by water escape during fluidization four assemblages of structures are recognized deformation as isolated structures in an otherwise undeformed unit may have been triggered by fluid turbulence discrete and chaotic units record widespread liquefaction and fluidization respectively of the most recently deposited sediment deformation may have been triggered by upwelling groundwater multilayer complexes formed when water escape from a buried layer generated fluidization in the overlying sediment this may have been triggered by seismically induced liquefaction some parts of the succession were disturbed on separate occasions by different processes sediment laden currents are unlikely to have been a major trigger for deformation it is concluded that specific soft sediment deformation structures result from the combination of driving forces deformation mechanism and initial geometry assemblages of structures relate to the timing of deformation and are of more value in identifying the trigger for deformation

origin of the northern atlantic s heinrich events as first noted by heinrich 1988 glacial age sediments in the eastern part of the northern atlantic contain layers with unusually high ratios of ice rafted lithic fragments to foraminifera shells he estimated that these layers are spaced at intervals of roughly 10 000 years in this paper we present detailed information documenting the existence of the upper five of these layers in odp core 609 from 50 n and 24 w their ages are respectively 15 000 radiocarbon years 20 000 radiocarbon years 27 000 radiocarbon years about 40 000 years and about 50 000 years we also note that the high lithic fragment to foram ratio is the result of a near absence of shells in these layers although we are not of one mind regarding the origin of these layers we lean toward an explanation that the heinrich layers are debris released during the melting of massive influxes of icebergs into the northern atlantic these sudden inputs may be the result of surges along the eastern margin of the laurentide ice sheet

the quaternary geology of the labrador shelf the regional distribution of glacial and postglacial sediments on the labrador shelf was mapped by medium and high resolution seismics and shows a succession of tills that extend to the continental shelf edge at approximately 600 m the uppermost of these tills is less laterally extensive and does not occur on the labrador shelf bank tops in water depths less than 160 m we infer that the last glacial episode was less extensive than the preceding ones glaciomarine stratified silts and postglacial muds are found overlying the upper till in the deep basins that separate the banks the upper till glaciomarine silts and postglacial muds are interpreted to constitute one complete deglacial cycle radiocarbon age determination suggests this deglacial sequence began at approximately 20 000 years bp on the northernmost labrador shelf and hudson strait seismic evidence indicates a late glacial readvance locally deposited a till that overlies the glaciomarine sediments associated with the last glacial advance

well preserved late precambrian paleosols from northwest scotland abstract two paleosol profiles were developed side by side on gneiss sheigra clay paleosol and amphibolite staca clay paleosol along the same unconformable contact of lewisian metamorphic rocks 2 7 1 7 ga overlain by torridonian sandstones 0 8 ga in northwestern scotland although altered during burial by compaction and illitization these profiles are well preserved compared with most other precambrian paleosols that are metamorphosed to at least greenschist facies in addition deformation of pegmatite veins in a manner similar to soil creep can be taken as an indication that upper horizons of these paleosols have been preserved soil development in both profiles was characterized by the oxidation of iron in biotite and magnetite and by hydrolysis of plagioclase orthoclase and hornblende clay skins soil like microfabrics and pedogenic smectite have been preserved in the profiles alteration of silicate grains other than quartz microcline and biotite has been intense but chemical analyses failed to show dramatic removal of alkalis and alkaline earths from the profiles microscopic concretions similar to pedogenic carbonate were found in subsurface horizons of the staca clay rare earth elements are more abundant within the paleosols than in their parent materials these paleosols developed on a well drained landscape that was buried by torridonian alluvial fans atmospheric oxidation was greater than revealed by studies of archean paleosols but probably still short of modern levels paleoclimate was subhumid and probably seasonal organic carbon and its light isotopic composition in the paleosol is evidence for photosynthetic microbial crusts paleosols of such great antiquity do not fit comfortably within modern soil taxonomies and are for the moment assigned to the informal category of green clays

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

cryosphere ocean interactions at the margin of the laurentide ice sheet during the younger dryas chron se baffin shelf northwest territories cores hu82 034 057 and hu84 035 008 resolution basin se baffin shelf contain 200 and 450 cm respectively of sediment that spans the younger dryas chron in both cores the interval is bracketed by 14c dates on foraminifera or molluscs these sites were close to the margin of the late wisconsin foxe ice sheet as it flowed toward the labrador sea prior to 11 ka both cores record moderate to high accumulations of foraminifera relatively high del 18o values in planktonic foraminifera and low values of detrital carbonate the diatom and percent opal records imply occasional seasonally open water conditions during part of the younger dryas chron both the diatom and opal analyses imply a shutoff of biogenic silica production suggesting surface water conditions affected by increased sea ice and or reduced nutrients in addition the younger dryas interval is marked by an increase in coarse sand and detrital carbonate a decrease in total organic carbon and foraminifera and high rates of sediment accumulation the inferred environment during the younger dryas is ice proximal in hu82 034 057 the foraminifera and other data suggest a change in conditions during the middle part of the younger dryas chron

searching for travertines calcretes and speleothems in deep time processes appearances predictions and the impact of plants common models for modern calcite precipitation in and around caves soils springs and streams involve co 2 supplied by thick high p co 2 biogenic soils which were probably thin or non existent before vascular plants indeed plant influenced chemical weathering might have caused accelerated terrestrial carbonate production from the devonian onwards however terrestrial carbonates have also been documented from the archaean proterozoic cambrian ordovician and silurian mechanisms which could have caused non marine carbonates to precipitate without organic rich soils are described and some geological events likely to have influenced non marine carbonate precipitation up to the origin of vascular plants are highlighted as organisms have evolved so have the petrographic characteristics of non marine carbonates some examples of this are also given here

remarkably preserved prokaryote and eukaryote microfossils within 1ga old lake phosphates of the torridon group nw scotland abstract the 1000 ma torridon group of northwest scotland are here shown to contain a rich diversity of organic walled microfossils preserved in exceptional detail within sedimentary phosphate the phosphatic nodules and bands in which they occur are autochthonous as are the organic fossils they contain the associated sedimentology of this phosphate is shown to be consistent with a lacustrine setting informal taxonomic treatment of the microfossils allows statistical assessment of the relative abundances of different morphotypes across a range of lacustrine environments exceptional preservation of pristine organic walled eukaryotes and prokaryotes together with a taphonomic spectrum in both physicochemical and biological degradational conditions allows a picture to be built of the earliest lacustrine ecological communities

late quaternary paleoceanography of the western baffin bay region evidence from fossil diatoms seven marine cores from the east coast of baffin island were examined for variations in diatom content the cores have good age control and were used for holocene paleoceanographic reconstruction

palaeozoic landscapes shaped by plant evolution fluvial landscapes diversified markedly over the 250 million years between the cambrian and pennsylvanian periods the diversification occurred in tandem with the evolution of vascular plants and expanding vegetation cover in the absence of widespread vegetation landscapes during the cambrian and ordovican periods were dominated by rivers with wide sand beds and aeolian tracts during the late silurian and devonian periods the appearance of vascular plants with root systems was associated with the development of channelled sand bed rivers meandering rivers and muddy floodplains the widespread expansion of trees by the early pennsylvanian marks the appearance of narrow fixed channels some representing anabranching systems and braided rivers with vegetated islands we conclude that the development of roots stabilized the banks of rivers and streams the subsequent appearance of woody debris led to log jams that promoted the rapid formation of new river channels our contention is supported by studies of modern fluvial systems and laboratory experiments in turn fluvial styles influenced plant evolution as new ecological settings developed along the fluvial systems we suggest that terrestrial plant and landscape evolution allowed colonization by an increasingly diverse array of organisms

microbially induced sedimentary structures a new category within the classification of primary sedimentary structures abstract cyanobacterial films and mats syndepositonally influence erosion deposition and deformation of sediments the biomass levels surface morphologies and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing growing microbial mats dredge grains from their substrate upwards whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle trap and bind suspended particles these and similar biotic physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures we distinguish structures on bedding planes leveled bedding surfaces wrinkle structures microbial mat chips erosional remnants and pockets multidirectional ripple marks and mat curls and internal bedding structures sponge pore fabrics gas domes fenestrae structures sinoidal laminae oriented grains benthic ooids biolaminites mat layer bound grain sizes we propose to place this group of microbially mediated structures as a fifth category bedding modified by microbial mats and biofilms in pettijohn and potter s 1964 existing classification of primary sedimentary structures

the criteria for the biogeneicity of microbially induced sedimentary structures miss in archean and younger sandy deposits abstract the identification of fossils or biogenic sedimentary structures in rocks of archean age is difficult because similar lithological features could rise from purely physical or chemical processes alone therefore it is important to define criteria that serve the secure definition of a fossil or structure in question as of biological origin such criteria have been established for stromatolites and microfossils this contribution discusses the 6 criteria of biogeneicity of microbially induced sedimentary structures miss those structures are found in sandy deposits of early archean age to the present and rise from the interaction of benthic microbiota with physical sediment dynamics the six criteria for their biogeneicity are i miss occur in rocks of not more than lower greenschist facies ii in stratigraphic sections miss correlate with turning points of regression trangressions iii miss correlate with a characteristic depositional facies that enhances the development and the preservation of microbial mats iv the distribution of miss correlates with the ancient average hydraulic pattern v the geometries and dimensions of fossil miss correspond to that of the modern ones vi the miss include at least one of 9 specific microtextures

fluvial channel belts floodbasins and aeolian ergs in the precambrian meall dearg formation torridonian of scotland inferring climate regimes from pre vegetation clastic rock records abstract the interpretation of climate regimes from facies analysis of precambrian clastic rocks has been challenging thus far hindering full reconstructions of landscape dynamics in pre vegetation environments yet comparisons between different and co active sedimentary realms including fluvial channelised floodplain and aeolian hold the potential to shed further light on this thematic this research discusses a fluvial aeolian record from the 1 2 ga meall dearg formation part of the classic torridonian succession of scotland tentatively considered to date as a braided fluvial deposit this unit is here reappraised as the record of fluvial channel belts floodbasins and aeolian ergs fluvial deposits with abundant transitional to upper flow regime structures mostly cross beds with tangential sets and plane antidunal beds and simple low relief sediment bars indicate a low sinuosity ephemeral style floodbasin deposits consist of plane and cross beds ubiquitously bounded by symmetrical ripples and rare sediment bars related to the progradation of splay complexes in temporary flooded depressions aeolian deposits occur nearby basement topography and are dominated by large scale pin stripe laminated cross beds indicative of intermountain ergs neither ephemeral fluvial nor intermountain aeolian systems can be considered as reliable indicators of local climate since their sedimentary style is respectively controlled by catchment size and shape and basin topography relative to groundwater tables contrarily the occurrence of purely clastic rather than carbonate or evaporitic floodplain strata can be more confidently related to humid regimes in brief this study provides new insight into an overlooked portion of the torridonian succession of scotland and discusses climate inferences for precambrian clastic terrestrial rocks

changing the picture of earth s earliest fossils 3 5 1 9 ga with new approaches and new discoveries new analytical approaches and discoveries are demanding fresh thinking about the early fossil record the 1 88 ga gunflint chert provides an important benchmark for the analysis of early fossil preservation high resolution analysis of gunflintia shows that microtaphonomy can help to resolve long standing paleobiological questions novel 3d nanoscale reconstructions of the most ancient complex fossil eosphaera reveal features hitherto unmatched in any crown group microbe while eosphaera may preserve a symbiotic consortium a stronger conclusion is that multicellular morphospace was differently occupied in the paleoproterozoic the 3 46 ga apex chert provides a test bed for claims of biogenicity of cell like structures mapping plus focused ion beam milling combined with transmission electron microscopy data demonstrate that microfossil like taxa including species of archaeoscillatoriopsis and primaevifilum are pseudofossils formed from vermiform phyllosilicate grains during hydrothermal alteration events the 3 43 ga strelley pool formation shows that plausible early fossil candidates are turning up in unexpected environmental settings our data reveal how cellular clusters of unexpectedly large coccoids and tubular sheath like envelopes were trapped between sand grains and entombed within coatings of dripstone beach rock silica cement these fossils come from earth s earliest known intertidal to supratidal shoreline deposit accumulated under aerated but oxygen poor conditions

wisconsinan late glacial environmental change on the southeast baffin shelf southeast baffin island and northern labrador the importance of the baffin labrador region is explained in terms of the channelling through this area of the mass flux of meltwater from the laurentide ice sheet and the probable effects of this on the thermohaline circulation of the north atlantic the chronology of glaciation and deglaciation in the region is summarised reference is made to recent marine records from the baffin shelf isotopic changes recorded in foraminiferal assemblages obtained from marine sediments in the baffin shelf labrador sea area indicate the timing and possible magnitude of meltwater fluxes during the period 14 0 11 0 ka bp

nanoscale analysis of pyritized microfossils reveals differential heterotrophic consumption in the 1 9 ga gunflint chert the 1 88 ga gunflint biota is one of the most famous precambrian microfossil lagerstatten and provides a key record of the biosphere at a time of changing oceanic redox structure and chemistry here we report on pyritized replicas of the iconic autotrophic gunflintia huroniospora microfossil assemblage from the schreiber locality canada that help capture a view through multiple trophic levels in a paleoproterozoic ecosystem nanoscale analysis of pyritic gunflintia sheaths and huroniospora cysts reveals differing relic carbon and nitrogen distributions caused by contrasting spectra of decay and pyritization between taxa reflecting in part their primary organic compositions in situ sulfur isotope measurements from individual microfossils 34sv cdt 6 7 to 21 5 show that pyritization was mediated by sulfate reducing microbes within sediment pore waters whose sulfate ion concentrations rapidly became depleted owing to occlusion of pore space by coeval silicification three dimensional nanotomography reveals additional pyritized biomaterial including hollow cellular epibionts and extracellular polymeric substances showing a preference for attachment to gunflintia over huroniospora and interpreted as components of a saprophytic heterotrophic decomposing community this work also extends the record of remarkable biological preservation in pyrite back to the paleoproterozoic and provides criteria to assess the authenticity of even older pyritized microstructures that may represent some of the earliest evidence for life on our planet

a billion years of environmental stability and the emergence of eukaryotes new data from northern australia carbon isotopes through 6km of fully cored drill holes in 1 7 to 1 5 ga carbonates of the mount isa and mcarthur basins australia which host the earliest known eukaryote biomarkers provide the most comprehensive and best dated delta 13c stratigraphy yet obtained from such ancient rocks both basins reveal remarkably stable temporal delta 13c trends mean of 0 6 2 pdb peedee belemnite and confirm the impression of delta 13c stasis between 2 0 and 1 0 ga which together with other evidence suggest a prolonged period of stability in crustal dynamics redox state of surface environments and planetary climate this delta 13c stasis is consistent with great stability in the carbon cycle controlled we suggest by p limitation of primary productivity recent evidence shows that p depletion is a major factor in obligate associations between photosymbionts and host cells we argue that a billion years of stability in the carbon and nutrient cycles may have been the driving force that propelled prokaryotes toward photosymbiosis and the emergence of the autotrophic eukaryote cell

deglacial meltwater discharge north atlantic deep circulation and abrupt climate change high resolution paleogeochemical data from the north atlantic ocean indicate that in the interval 15 000 to 10 000 14c years before present b p north atlantic deep water nadw production was decreased or eliminated four times at about 14 500 and probably older 13 500 12 000 and 10 500 years b p each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land in addition each of these times may be associated with brief episodes of cooler climate in the north atlantic region the best example of which is the younger dryas cooling of 10 500 years ago these results support models linking meltwater discharge decreased nadw production and decreased north atlantic heat flux

life on land in the proterozoic evidence from the torridonian rocks of northwest scotland the stoer group and diabaig formation of the torridonian succession in northwest scotland are late mesoproterozoic to early neoproterozoic ca 1200 1000 ma features preserved on the top surfaces of fine to medium grained sandstone beds in a number of stratigraphically and geographically separated localities are attributable to microbially induced sedimentary structures these include wrinkle structures remnants of apparent microbial crusts and indications of original cohesiveness and pliancy in sand sized sediment the surfaces on which the microbial structures formed were exposed subaerially abundant deep desiccation cracks and locally pedogenic structures in alluvial interfluve and lacustrine margin settings and many of the structures developed in areas well away from the perennially wetted regions adjacent to shorelines and fluvial channels thus these features indicate that earth9s biosphere had adapted to and colonized land surfaces many hundreds of millions of years before the dawn of the phanerozoic

stratigraphy sedimentology chronology and palaeohydrology of quaternary lacustrine deposits at madigan gulf lake eyre south australia abstract madigan gulf is a large bay at the southern end of lake eyre north a major ephemerally flooded playa in arid central australia at the southwestern margin of a vast 1 300 000 km2 internal drainage basin the stratigraphy and chronology of the quaternary sequence in the gulf is described from 8 cores and a cliff exposure at the gulf margin a number of depositional environments are recognised and their distinctive sedimentary components are described facies recognised include deep and shallow water lacustrine environments dominated by surface water processes and dry or ephemerally flooded playa environments dominated by groundwater controlled processes sedimentary components include terrigenous clastics from river inflow and shoreline erosion carbonates of detrital inorganic or biological origin and gypsum and halite evaporites carbonates and gypsum evaporites precipitated within the basin are frequently reworked as clastic components the establishment of a preliminary chronology for the sequence by the application of thermoluminescence uranium thorium disequilibrium amino acid racemization and radiocarbon dating techniques has allowed a reconstruction of the last 130 ka of lake eyre palaeohydrology the wettest phase occurred during the last interglacial early in oxygen isotope stage 5 when an enlarged lake eyre was up to 25 m deep subsequently there has been a number of dry periods separating successively less effective wet phases culminating in the deposition of a substantial halite salt crust around the time of the glacial maximum the dry interludes are characterised by deflation of salts and sediment from the basin a process controlled by lowering of the watertable the record from madigan gulf demonstrates the dramatic and repetitive impact of lake deflation on the quaternary record of lake eyre in the early holocene a minor but mostly perennial lacustrine event was terminated at about 3 4 ka when the modern ephemeral playa regime was established the major catchment of lake eyre is located in the monsoon watered areas of northern australia as demonstrated by large floodings of the modern ephemeral regime major lacustrine episodes must indicate enhanced monsoon precipitation in northern australia in the holocene the lake has not risen to levels achieved during the early stage 5 lacustral phase indicating a marked reduction in the effectiveness of the monsoon in the present interglacial by comparison with its predecessor

enhanced organic carbon burial in large proterozoic lakes implications for atmospheric oxygenation abstract the burial of organic carbon in sedimentary systems has been a fundamental part of the carbon cycle throughout the geological record and was instrumental in major oxygenations of the atmosphere in the early palaeoproterozoic and neoproterozoic while much focus has been placed on the burial of carbon in precambrian marine carbonate and organic carbon rich rocks deposited around the time of these major oxygenations such deposits yield little information on the evolution of the atmosphere in the significant time between there is however growing evidence from terrestrially deposited sediments to suggest the surface environment may have been at least intermittently well oxygenated from the late mesoproterozoic hence proterozoic sediments deposited in terrestrial near surface environments are useful targets for the study of atmospheric evolution during a time which is hitherto poorly understood thus far little attention has been paid to the contribution of large lakes and intercontinental basins to the global burial of organic carbon and thus the progressive oxygenation of the atmosphere especially given that the highest rates of organic carbon burial in modern aquatic environments occur in lacustrine settings in stark contrast to the low rates observed in the contemporary marine realm here we report high burial rates of organic carbon in large lacustrine systems of late mesoproterozoic to early neoproterozoic age which are comparable with modern lacustrine systems and significantly higher than modern and ancient marine deposits these data emphasise the significance of lakes as a global repository for organic carbon and imply proterozoic lakes were at least as efficient and perhaps as important as modern lakes in the global burial of organic carbon such findings suggest large proterozoic lakes and epicontinental basins played a crucial role in the progressive oxygenation of the atmosphere before the major neoproterozoic oxygenation

late quaternary stage 2 and 3 meltwater and heinrich events northwest labrador sea two major meltwater events are documented in cores from the nw labrador sea one occurred ca 20 000 14c yr b p in association with deposition of a major detrital carbonate unit both prior to and after this event 18o values of near surface planktonic foraminifera were 4 5 indicating fully enriched glacial values a younger event ca 14 000 14 c yr b p is characterized by a dramatic change in 18o from 4 5 to 2 0 and coincided with the retreat of ice from the outer se baffin shelf possibly into hudson strait these meltwater events coincide with heinrich h layers 1 and 2 from north atlantic sediments the 14 000 14c yr b p meltwater event indicates that the eastern margin of the laurentide ice sheet also underwent rapid retreat at approximately the same time as other ice sheet margins around the ne north atlantic a third major detrital carbonate event at the base of hu87 033 009 possibly correlative with heinrich layer 3 occurred ca 33 960 675 14 c yr b p however this is older than the accepted date for h 3 of 27 000 14c yr b p and may be h 4

taphonomy of very ancient microfossils from the 3400ma strelley pool formation and 1900ma gunflint formation new insights using a focused ion beam abstract focused ion beam fib milling permits the accurate extraction of ultrathin c 100 nm cross sectional lamellae from microfossils found in geological thin sections subsequent tem analysis of these lamellae can provide unique insights into the ultrastructure chemistry and taphonomy of precambrian microfossils at the micrometer to nanometer scale combining serial fib milling with sem imaging extends this capability to three dimensional 3d tomographic reconstruction and visualization of precambrian microfossils revealing information not available in light microscopy here we apply these techniques to two iconic silicified microfossil assemblages from the 3400 ma strelley pool formation of western australia and the 1900 ma gunflint formation of canada all the examined microfossils have carbonaceous walls surrounded by pure silica impregnation of microfossil walls by nano grains of silica is common together with variable degrees of wall displacement and replacement by silica all microfossils are rigidly preserved in 3d and show little or no folding or compression however there are also notable differences in taphonomic preservation our examples of the spheroidal gunflint microfossil huroniospora showed the highest fidelity of preservation with a continuous carbonaceous wall fossilized by spheroidal nano silica grains that resemble those found on bacterial surfaces in modern silicifying hot spring environments the nucleation of these silica nano spheres on the microfossil walls has induced an artificial saw tooth like ridged wall texture that may subsequently hinder species level identification the strelley pool microfossils in comparison show a lower fidelity of preservation with small parts of the microfossil walls completely replaced by silica plus extensive recrystallization of spheroidal silica nano grains to angular micro quartz our examples of the sheath like filamentous gunflint microfossil siphonophycus showed the lowest fidelity of preservation with many gaps in the carbonaceous walls and significant redistribution of carbon by recrystallizing silica grains a model is presented to explain these observations criteria for distinguishing highly probable microfossils from non cellular carbonaceous microstructures e g botryoids and grain coatings using fib based imaging are put forward for the first time here using examples drawn from the strelley pool formation and comparisons with younger gunflint material the combined in situ techniques of fib tem and fib sem nano tomography potentially provide a wealth of new nano scale information regarding the biogenicity antiquity and taphonomy of precambrian microfossils however the destructive nature of both techniques makes their application to unique palaeontological specimens problematical

transpressive inversion of a mesozoic extensional forced fold system with an intermediate decollement level in the basque cantabrian basin spain 1 in the basque cantabrian basin spain normal faulting and associated folding occurred during late jurassic to early cretaceous rifting cenozoic pyrenean thick skinned transpressive inversion in the western parts of the basin preserved the first order extensional architecture integration of geological maps and seismic profiles has permitted to fully constrain the style of extensional deformation and subsequent inversion in the western portion of the basque cantabrian basin extensional faults offset the paleozoic basement up to lower triassic rocks the presence of an efficient decollement level represented by triassic evaporites produced the decoupling between basement rocks and the upper triassic to middle jurassic prerift cover sequence extensional forced folding occurred in the cover driven by basement faulting and the migration of evaporites toward the hanging wall of the extensional faults with salt welds developing away from them upper jurassic to lower cretaceous syn rift sediments deposited synchronously with forced folding which led to the development of extensional growth geometries associated with both master faults and nearly transverse faults syn rift growth sequences are characterized by downlap and onlap relationships with the underlying prerift units interpreted as the result of along strike variations of master fault extensional displacement rate cenozoic pyrenean contraction generated the right lateral transpressive inversion of basement master faults and the almost dip slip reactivation of transverse extensional faults

thickness of the chemical weathering zone and implications for erosional and climatic drivers of weathering and for carbon cycle feedbacks quantitative understanding of variability in weathering fluxes on the modern earth is limited because little is known about where the most important weathering reactions take place this is partly because the locus of weathering is difficult to measure empirically inverse analysis of a parametric model presented here provides first order constraints on variability in the thickness of the zone of active weathering results suggest that the effective thickness of the weathering zone varies relatively little across several orders of magnitude of denudation rate at low to moderate denudation rates reactions in soils may dominate weathering fluxes at the catchment scale but the contribution from soil weathering decreases at higher denudation rates consequently increased erosion leads to higher weathering fluxes sustained by progressively greater contributions from weathering in bedrock the effect of climate temperature and runoff on weathering fluxes is apparently weaker at low denudation rates than at high denudation rates such that erosion and potentially associated bedrock weathering may be important for maintaining climate stabilizing feedbacks in earth s carbon cycle

a precambrian proximal ejecta blanket from scotland ejecta blankets around impact craters are rarely preserved on earth although impact craters are ubiquitous on solid bodies throughout the solar system on earth they are rapidly effaced and few records exist of the processes that occur during emplacement of ejecta the stac fada member of the precambrian stoer group in scotland has previously been described as volcanic in origin however shocked quartz and biotite provide evidence for high pressure shock metamorphism while chromium isotope values and elevated abundances of platinum group metals and siderophile elements indicate addition of meteoritic material thus the unit is reinterpreted here as having an impact origin the ejecta blanket reaches 20 m in thickness and contains abundant dark green vesicular devitrified glass fragments field observations suggest that the deposit was emplaced as a single fluidized flow that formed as a result of an impact into water saturated sedimentary strata the continental geological setting and presence of groundwater make this deposit an analogue for martian fluidized ejecta blankets

ice sheet dynamics and glacial history of southeasternmost baffin island and outermost hudson strait southeasternmost baffin island is mantled by hudson strait drift it contains abundant limestone erratics and 20 to 50 carbonate in the matrix to the northwest it is replaced by drift dominated by locally derived rock of the canadian shield the sense and orientation of ice erosional features demonstrate that hudson strait drift is associated with northeasterly ice flow that crossed the tip of meta incognita peninsula local drift associated with ice flow s10 w along the hudson strait coast was derived from a dispersal center on the peninsula erratic lithologies contained in the hudson strait drift indicate a labradorean provenance large scale bedrock molding and the distribution of cirques indicate ne flowing ice has been dominant throughout the middle and late quaternary radiocarbon dates of in situ shells confirm that deglaciation began more than 11 000 yr ago with the frobisher bay coast becoming ice free by 9300 yr ago five dates from hudson strait suggest that the strait was deglaciated before 9000 yr bp however ice from the labradorean sector recrossed hudson strait during the cockburn substage about 8500 yr ago damming drainage from the west final retreat of labradorean ice from baffin island was complete by 8000 yr ago at which time the sea was able to penetrate hudson bay

tufted microbial cyanobacterial mats from the proterozoic stoer group scotland upward thinning stellate and polygonal carbonate structures are described and compared to modern and ancient stromatolites although tufted microbial mats are common in modern settings they have rarely been described from ancient rocks owing to their poor preservation potential the preservation of these from the stoer group is due to early replacement by calcite before their original tufted relief could be obliterated by compaction associated limestones with some similar features are interpreted as flat to mamillated microbial mats

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

diverse microbially induced sedimentary structures from 1 ga lakes of the diabaig formation torridon group northwest scotland abstract the siliciclastic lacustrine rocks of the 1000 ma diabaig formation northwest scotland contain a remarkable diversity of macroscopic structures on bedding planes that can be compared with various kinds of microbially induced sedimentary structures miss field sedimentological investigations combined with laboratory analysis of bedding planes and petrographic study of thin sections have allowed us to characterise a range of depositional environments and document the spectrum of biological structures miss are reported from frequently subaerial environments through commonly submerged facies and down to permanently sub wavebase settings palaeoenvironmental conditions water depth exposure hydrodynamic energy control the distribution of miss within these facies this demonstrates that mat forming microbial communities were arguably well adapted to low light levels or periodic exposure some miss from the diabaig formation are typical of precambrian microbial mats including reticulate fabrics and old elephant skin textures in addition to these a number of new and unusual fabrics of putative microbial origin are described including linear arrays of ridges and grooves cf arumberia and discoidal structures that are comparable with younger ediacaran fossils such as beltanelliformis these observations indicate that benthic microbial ecosystems were thriving in freshwater lake systems 1000 ma and indicate how microbially induced sedimentary structures may be applied as facies indicators for proterozoic lacustrine environments the discovery of structures closely resembling ediacaran fossils cf beltanelliformis also serves to highlight the difficulty of interpreting simple discoidal bedding plane structures as metazoan fossils

late quaternary chronology and environments of square lake torngat mountains labrador sediment pollen and diatom records from square lake a small lake dammed by a segment of saglek moraine cover the period of deposition of and deglaciation from the saglek moraine the basal radiocarbon date 18 210 1900 years bp is on sediment contaminated by reworked pollen and is thus a maximum age however the date was measured on organic carbon recovered from glaciolacustrine couplets associated with deposition of the saglek moraine and thus establishes a late wisconsinan age for the saglek moraine vegetation on the ice free upland surrounding square lake at this time was a sparse tundra vegetation dominated by grasses and herbs the absence of diatoms indicates perennial lake ice cover a major transition is recorded by pollen and diatoms at 8 5 ka vegetation probably remained sparse tundra but birch and willow may have arrived in the area by 8 ka diatoms are first dominated by alkaliphil species reflecting continued influence of glaciolacustrine sedimentation an abrupt change in depos

palaeomagnetic dating of sub torridon group weathering profiles nw scotland verification of neoproterozoic palaeosols a palaeoplain that cuts across lewisian gneiss in the cape wrath area northwest scotland is weathered to depths of 1 6 m beneath fluvial red conglomerates and sandstones of the applecross formation of the early neoproterozoic torridon group weathered lewisian rocks are enriched in k 2 o and fe 2 o 3 and contain hematite as an alteration product of biotite and magnetite palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration thermal step demagnetization of 87 oriented core specimens of biotite gneiss and amphibolite spanning four profiles at poll a mhurain and sheigra reveal a stable high temperature component with a mean direction of remanence of d 149 1 i 57 2 95 5 0 corrected for dip of the overlying applecross strata that gives a pole at 9 8 s 199 2 e dp 5 3 dm 7 3 this direction differs significantly from that for little weathered lewisian at those two localities and also from directions for the late mesoproterozoic stoer group the remanence direction for the weathered rocks is however near the dip corrected high temperature remanence direction d 122 3 i 50 5 95 10 8 obtained by us from the thermal step demagnetization of 39 sandstone specimens of the applecross formation in the cape wrath area furthermore the direction for the weathered rocks agrees closely with other dip corrected results for the torridon group such as the detrital remanent magnetization drm mean direction d 148 3 i 59 9 95 13 8 for the applecross formation on rum and that for the main outcrop of the aultbea formation d 139 7 i 52 6 95 14 5 the concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the cape wrath area acquired their remanence penecontemporaneously with deposition of the applecross and aultbea formations the palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of applecross age c 980 ma that formed at a palaeolatitude of 38 and constitute the oldest known palaeosols in the british isles views that the weathering is substantially older or younger than applecross deposition cannot be sustained our findings carry implications for applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early neoproterozoic time

early oxygenation of the terrestrial environment during the mesoproterozoic geochemical data from ancient sedimentary successions provide evidence for the progressive evolution of earth s atmosphere and oceans1 2 3 4 5 6 7 key stages in increasing oxygenation are postulated for the palaeoproterozoic era 2 3 billion years ago gyr ago and the late proterozoic eon about 0 8 gyr ago with the latter implicated in the subsequent metazoan evolutionary expansion8 in support of this rise in oxygen concentrations a large database1 2 3 9 shows a marked change in the bacterially mediated fractionation of seawater sulphate to sulphide of 34s 25 before 1 gyr to 50 after 0 64 gyr this change in 34s has been interpreted to represent the evolution from single step bacterial sulphate reduction to a combination of bacterial sulphate reduction and sulphide oxidation largely bacterially mediated3 7 9 this evolution is seen as marking the rise in atmospheric oxygen concentrations and the evolution of non photosynthetic sulphide oxidizing bacteria3 7 10 here we report 34s values exceeding 50 from a terrestrial mesoproterozoic 1 18 gyr old succession in scotland a time period that is at present poorly characterized this level of fractionation implies disproportionation in the sulphur cycle probably involving sulphide oxidizing bacteria that is not evident from 34s data in the marine record1 2 3 9 disproportionation in both red beds and lacustrine black shales at our study site suggests that the mesoproterozoic terrestrial environment was sufficiently oxygenated to support a biota that was adapted to an oxygen rich atmosphere but had also penetrated into subsurface sediment

anoxia in the terrestrial environment during the late mesoproterozoic a significant body of evidence suggests that the marine environment remained largely anoxic throughout most of the precambrian in contrast the oxygenation history of terrestrial aquatic environments has received little attention despite the significance of such settings for early eukaryote evolution to address this we provide here a geochemical and isotopic assessment of sediments from the late mesoproterozoic nonesuch formation of central north america we utilize rhenium osmium re os geochronology to yield a depositional age of 1078 24 ma while os isotope compositions support existing evidence for a lacustrine setting fe s c systematics suggest that the nonesuch formation was deposited from an anoxic fe rich ferruginous water column thus similar to the marine realm anoxia persisted in terrestrial aquatic environments in the middle to late proterozoic but sulfidic water column conditions were not ubiquitous our data suggest that oxygenation of the terrestrial realm was not pervasive at this time and may not have preceded oxygenation of the marine environment signifying a major requirement for further investigation of links between the oxygenation state of terrestrial aquatic environments and eukaryote evolution