ring index a new strategy to evaluate the integrity of tex86 paleothermometry the tex86 tetraether index of 86 carbon atoms sea surface temperature proxy has been increasingly applied in the reconstruction of mesozoic and cenozoic ocean temperatures however the archaeal lipids that compose tex86 indices glycerol dialkyl glycerol tetraethers gdgts can derive from water column production terrestrial sources and or within sediments tex86 sea surface temperature estimates can also be influenced by non temperature factors such as growth phase and nutrient levels here we show that the weighted average of cyclopentane moieties known as the ring index ri can be used to determine if tex86 temperature estimates are influenced by non thermal factors and or deviate from modern analogues we demonstrate that ri and tex86 indices from the published global core top data set and mesocosm cultures are significantly correlated as predicted through the influence of temperature on lipid biochemistry we further show that when ri and tex86 indices in modern or ancient records deviate from the modern global tex86 ri relationship gdgt distributions are not solely controlled by environmental temperature and or tex86 based temperature reconstructions are questionable

opening of a trans pangaean marine corridor during the early jurassic insights from osmium isotopes across the sinemurian pliensbachian gssp robin hood s bay uk the hispanic corridor represents a significant phase of continental reorganisation of the early jurassic that eventually provided connectivity between the western tethyan and eastern pacific oceans along the central atlantic rift zone although the initiation of this marine corridor profoundly impacted oceanic circulation and marine faunal exchange patterns the timing of its formation hitherto remains poorly constrained with estimates spanning both the hettangian and sinemurian the sinemurian pliensbachian global stratotype section and point gssp at robin hood s bay uk comprises a succession of well exposed immature organic rich sediments only previously characterised by strontium oxygen and carbon isotope geochemistry new re and os isotope profiling indicates substantial variation in seawater chemistry at this time initial osmium isotope data become increasing unradiogenic 0 40 to 0 20 across the boundary providing evidence for a continual flux of unradiogenic os into the oceans during the latest sinemurian the initial unradiogenic 187os 188os values indicate the occurrence of low temperature hydrothermal activity associated with the formation of the hispanic corridor during the breakup of pangaea therefore combined with biogeography and faunal exchange patterns the os isotope data demonstrates that connectivity between the eastern pacific and tethyan oceans initiated during the latest sinemurian as a result this study better constrains the timing of establishment of the hispanic corridor which was previously limited to poorly defined biogeography

new constraints on atmospheric co2 concentration for the phanerozoic earth s atmospheric co2 concentration ca for the phanerozoic eon is estimated from proxies and geochemical carbon cycle models most estimates come with large sometimes unbounded uncertainty here we calculate tightly constrained estimates of ca using a universal equation for leaf gas exchange with key variables obtained directly from the carbon isotope composition and stomatal anatomy of fossil leaves our new estimates validated against ice cores and direct measurements of ca are less than 1000 ppm for most of the phanerozoic from the devonian to the present coincident with the appearance and global proliferation of forests uncertainties obtained from monte carlo simulations are typically less than for ca estimates from other approaches these results provide critical new empirical support for the emerging view that large 2000 3000 ppm long term swings in ca do not characterize the post devonian and that earth s long term climate sensitivity to ca is greater than originally thought key points a novel co2 proxy calculates past atmospheric co2 with improved certainty co2 is unlikely to have exceeded 1000 ppm for extended periods post devonian earth s long term climate sensitivity to co2 is greater than originally thought

a potential global stratotype for the sinemurian pliensbachian boundary lower jurassic robin hood s bay uk ammonite faunas and isotope stratigraphy a coastal exposure at wine haven robin hood s bay north yorkshire uk fulfils the criteria for definition as the global stratotype section and point gssp for the base of the pliensbachian stage lower jurassic this marine sequence was deposited during a long term trans gression and is relatively expanded stratigraphically a rich fauna of ammonites above and below the boundary interval allows recognition of the leptechioceras gr meigeini paltechioceras aureolum and paltechioceras tardecrescens horizons of latest sinemurian age and the bifericeras donovani and apoderoceras gr aculeatum horizons of earliest pliensbachian age a suitable level for the boundary is characterized by the faunal association of bifericeras donovani dommergues meister and apoderoceras sp strontium isotope stratigraphy based on analysis of belemnites yields a calcite 87 sr 86 sr ratio for the suggested boundary level of 0 707425 0 000021 combined uncertainties based on line fit to stratigraphic dataset 0 000004 and measurement of the standard 0 000017 alternative uncertainties of 0 000008 are associated with the most extreme interpretation of sedimentation rate history allowed by the strontium isotope data that is abrupt changes in sedimen tation rate at precisely the boundary level however sedimentological considerations and measured strontium isotope values at the boundary support condensation rather than hiatus belemnite oxygen isotope data suggest a significant temperature drop 5 c across the boundary at this locality

distribution of crenarchaeota tetraether membrane lipids in surface sediments from the red sea abstract the red sea represents an extreme marine environment with high salinity high temperature and low level of nutrients complicating the application of standard geochemical palaeotemperature proxies in order to investigate the applicability of the tex 86 tetraether index of gdgts with 86 carbons proxy for sea surface temperature sst in the red sea the distribution of glycerol dialkyl glycerol tetraether membrane lipids gdgts in sediments from the red sea and the gulf of aden were examined against expectations tex 86 values for the red sea do not show a simple linear relationship with sst and deviate from the global core top calibration in the northern red sea at temperatures between 25 and 28 c the values increase linearly with sst whereas in the southern red sea at temperatures above 28 c tex 86 decreases with increasing temperature factors like seasonality and depth of production salinity and nutrient availability as well as diagenetic overprint or influence of allochtonous terrestrial lipids cannot explain this pattern however the observed tex 86 relationship with sst could be explained by the presence of a hypothetical endemic crenarchaeota population in the red sea with a specific tex 86 vs sst relationship in the southern red sea a two component mixing model implies an exponential decrease in the endemic population towards the gulf of aden thus the application of the tex 86 as a palaeotemperature proxy in the red sea is likely only possible for the northern red sea area with the specific sst vs tex 86 relationship determined in this study and potentially for the whole red sea basin during glacials when water exchange with the indian ocean was more restricted than today and the endemic archaeal lipid distribution was not affected by transport from the indian ocean our results suggest that distinct populations of crenarchaeota in extreme environments such as evaporitic basins may have different membrane composition necessitating application of another calibration than the global core top calibration for tex 86 palaeothermometry

climatic ups and downs in a disturbed jurassic world the tropical warm and equable climate of the jurassic world is regularly challenged by geoscientists especially since oxygen isotopes 18 o of fossil hardparts have been used to reconstruct the paleotemperature history of seawater by applying the innovative sizer sig nifi cant zero crossings of the derivatives statistical approach to a newly compiled 18 o data base for the jurassic we demonstrate the occurrence of major and multiscale 18 o changes mainly related to climate disturbances for the fi rst time two long term anomalies in 18 o are identifi ed during the toarcian and the late jurassic in conjunction with intensive volcanism in large igneous provinces these results support a strong infl uence of repeated volcanic pulses on the modulation of pco 2 temperatures and polar ice cap volumes over protracted periods at shorter time scales 13 relatively rapid 0 5 1 m y and signifi cant warming and cooling events are identifi ed the causes of which include transient fl uctuations in greenhouse gas con centrations related to still debated mechanisms

high sea surface temperatures during the early cretaceous epoch the early cretaceous epoch about 145 100 million years ago is generally thought of as a greenhouse period with high atmospheric co2 concentrations1 and high global mean temperatures2 but evidence for episodes of cooler conditions and even transient glaciations has been proposed3 4 5 6 7 8 9 here we present sea surface temperature records spanning the period from 142 to 128 million years ago berriasian barremian ages from low and mid latitudes reconstructed using the tex86 palaeotemperature proxy during this period we find sea surface temperatures exceeding 32 c at 15 20 n and averaging 26 c at 53 s these temperatures substantially exceed modern temperatures at equivalent latitudes and are incompatible with the notion of consistently cooler conditions in the earliest cretaceous moreover we find little variability in the sea surface temperature records even during the valanginian carbon isotope excursion 138 135 million years ago which was thought to be associated with marked temperature fluctuations5 we conclude that the earliest cretaceous was characterized by a warm stable climate with a lower meridional temperature gradient than today

a novel proxy for terrestrial organic matter in sediments based on branched and isoprenoid tetraether lipids we propose a novel tracer for terrestrial organic carbon in sediments based on the analysis of tetraether lipids using high performance liquid chromatography mass spectrometry hplc ms analysis of terrestrial soil and peats shows that branched tetraether lipids are predominant in terrestrial environments in contrast to crenarchaeol the characteristic membrane lipid of non thermophilic crenarchaeota which is especially abundant in the marine and lacustrine environment based on these findings an index was developed the so called branched and isoprenoid tetraether bit index based on the relative abundance of terrestrially derived tetraether lipids versus crenarchaeol this bit index was applied to surface sediments from the angola basin where it was shown to trace the outflow of the congo river furthermore analyses of particulate organic matter from the north sea showed relatively higher bit indices in water column particulate organic matter near large river inputs a survey of globally distributed marine and lacustrine surface sediments shows that the bit index in these environments correlates with the relative fluvial input of terrestrial organic material making this index generally applicable the new proxy allows the rapid assessment of the fluvial input of terrestrial organic material in immature sediments up to 100 ma old

chemostratigraphy of the jurassic system applications limitations and implications for palaeoceanography current chemostratigraphical studies of the jurassic system primarily involve the use of one sedimentary component marine organic carbon one divalent transition metal substituted in carbonate manganese and two isotopic tracers strontium isotope ratios 87 sr 86 sr and carbon isotope ratios 13 c carb and 13 c org in carbonate and in organic matter other parameters such as mg ca and sr ca ratios in calcite oxygen isotope ratios 18 o in carbonate sulphur isotope ratios 34 s in carbonate hosted sulphate nitrogen isotope ratios 15 n org in organic matter osmium isotope ratios 187 os 188 os in black shales and neodymium isotope ratios 143 nd 144 nd in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors stratigraphical patterns in total organic carbon toc can be of either local or regional significance depending on the lateral extent of the former nutrient rich and productive water mass divalent manganese follows a similar pattern being concentrated most probably as a very early diagenetic phase only in oxygen depleted waters that typically underlie zones of elevated organic productivity shifts in mg ca and sr ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes strontium isotope ratios from pristine skeletal calcite provide a global signal 13 c values from carbonates with minimal diagenetic overprint potentially do the same although small spatial differences in palaeo water mass composition may have been locally significant oxygen isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful because they reflect palaeotemperature the evaporation precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase nitrogen isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis neodymium isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems preliminary data on sulphur isotope ratios in carbonates and osmium isotope ratios in organic rich shales both potentially offering global correlation indicate that these tracers may be valuable although the records at present are not sufficiently well established to allow high resolution regional correlation in all cases biostratigraphically well dated reference sections against which the relevant geochemical data have been calibrated are required in the first instance to date studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern boreal and southern tethyan europe carbon isotope stratigraphy has been undertaken primarily but not exclusively on bulk pelagic sediments from the alpine mediterranean or tethyan domain and strontium isotope stratigraphy has been undertaken largely on calcitic skeletal material belemnites and oysters from northern and southern europe in many sections including those containing ammonites multi parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means strontium isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record

a paleoceanographic model for the early evolutionary history of planktonic foraminifera the first major radiations of planktonic foraminifera occurred during the mid cretaceous however this period of great diversification was interrupted by two intervals of pronounced paleoceanographic paleoecologic change one during the latest aptian early albian and a second associated with the cenomanian turonian boudary both of these times corresponded with widespread organic carbon burial during intervals commonly referred to as oceanic anoxic events oae 1 and 2 respectively this synthesis focuses on the evolutionary history of aptian early cenomanian planktonic foraminifera and the relationship with oae 1 dynamic paleoceanographic changes controlled the course of planktonic foraminiferal evolution during the mid cretaceous heightened plate tectonic activity a major long term rise of global sea level and an overall increase of global temperatures characterized aptian albian time as proposed here the evolutionary history of aptian early cenomanian planktonic foraminifera was influenced most strongly by changes in the density structure and stability of the upper water column via changes in the sites and rates of water mass production through time and perhaps by subtle changes in oceanic productivity including trophic structure and shifts in productivity centers with rising sea level the creation and growth of epicontinental seas played a major role in regional climate change water mass production and water column structure epicontinental seas were also an important factor in creating vast new niche space and increased biotic diversity for many marine organisms the amplified seasonality and instability of epicontinental seas resulted in habitation of the upper water column by morphologically simple eurytopic planktonic foraminifera several new species of epicontinental sea dwelling planktonic foraminifera appeared during the mid cretaceous however much of the diversification of morphologic form took place in oceanic areas during times of oceanic stability and upper water column stratification during mid to late aptian and late albian time a decline in diversity and return to simple morphotypes occurred when oceanic stability was disrupted and the upper water column was poorly stratified during latest aptian early albian time

palaeo digital elevation models for use as boundary conditions in coupled ocean atmosphere gcm experiments a maastrichtian late cretaceous example abstract palaeogeography specifically palaeotopography and palaeobathymetry provides an essential boundary condition for computer based atmosphere and ocean modelling it also provides the geographic context for understanding surface processes palaeodrainage palaeoweathering and biotic interactions palaeoecology palaeobiogeography with increased model resolution coupled ocean atmosphere general circulation models gcms and the addition of vegetation soil weathering ice and chemical modules there is now a need for more robust detailed palaeotopographies and palaeobathymetries that are fully integrated with the processes being modelled especially the hydrological system here we present a new geographic information system gis based hydrologically correct palaeo digital elevation model dem for the maastrichtian late cretaceous we describe the methods and concepts used to construct the map and draw attention to the limits imposed by scale and uncertainty and how these factors must be considered as part of the error analysis of derived model results the underlying palaeogeography is one of a series of 27 global maps that represent the stages of the cretaceous and sub epochs of the cenozoic each map is generated at a scale of 1 30 million in arcview gis and arcinfo using data from the lead author s own databases of lithologic tectonic and fossil information the lithologic databases of the paleogeographic atlas project the university of chicago a survey of published literature and dsdp odp data interpretations of elevation are derived following the methods outlined in ziegler et al ziegler a m rowley d b lottes a l sahagian d l hulver m l gierlowski t c 1985 paleogeographic interpretation with an example from the mid cretaceous annual review of earth and planetary sciences 13 385 425 an understanding of the tectonic regime and evolution of each feature and the age depth relationship for the ocean the global palaeo dem was derived using the elevation contours from the paleogeography and the suite of hydrological tools available in arcinfo grid the palaeo dem has been constrained by defining areas of internal palaeodrainage palaeoriver mouths and known palaeoriver courses the maastrichtian has been completed first to provide the boundary conditions for a series of coupled atmosphere ocean experiments when integrated with the results of the coupled ocean atmosphere model the result is a powerful tool for understanding surface processes and an important step towards the development of a continuous series representing a fully evolving palaeolandscape

an offset in tex86 values between interbedded lithologies implications for sea surface temperature reconstructions abstract the tex 86 tetraether index of tetraethers consisting of 86 carbon atoms sea surface temperature sst proxy is based on the distribution of isoprenoid glycerol dialkyl glycerol tetraether gdgt membrane lipids of pelagic thaumarchaeota that are preserved in marine sediments it is a valuable tool for reconstructing past ssts from sedimentary archives however there are still major uncertainties as to the effects of variables other than temperature on the proxy here we present the first study of gdgt variability across early cretaceous interbedded pelagic and shelf sourced turbiditic sediments from two deep sea drilling project dsdp sites in the western north atlantic the results indicate that a small but consistent offset in tex 86 ratios equivalent to 1 2 c of temperature difference occurs between interbedded lithologies of a similar age the offset can be attributed to spatial differences in sea surface temperatures or thaumarchaeotal populations between the shelf and the open ocean or to secondary diagenetic effects related to oxic degradation of the gdgts of these a difference in either thaumarchaeotal taxa or ecology between those living in the shelf and ocean areas seems most plausible regardless of the root cause of the offset these findings highlight the necessity of careful sample selection prior to tex 86 analysis to ensure robust interpretation of palaeotemperature trends

secular environmental precursors to early toarcian jurassic extreme climate changes the early toarcian oceanic anoxic event t oae about 183 myr ago was a global event of environmental and carbon cycle perturbations which deeply affected both marine biota and carbonate production nevertheless the long term environmental conditions prevailing prior to the main phase of marine extinction and carbonate production crisis remain poorly understood here we present a similar to 8 myr long record of early pliensbachian middle toarcian environmental changes from the lusitanian basin portugal in order to address the long term paleoclimatic evolution that ultimately led to carbonate production and biotic crises during the t oae paleotemperature estimates derived from the oxygen isotope compositions of well reserved brachiopod shells from two different sections reveal a pronounced similar to 5 degrees c cooling in the late pliensbachian margaritatus spinatum ammonite zones boundary this cooling event is followed by a marked similar to 7 10 degrees c seawater warming in the early toarcian that after a second cooling event in the mid polymorphum zone culminates during the t oae calcium carbonate caco3 contents the amount of nannofossil calcite and the mean size of the major pelagic carbonate producer schizosphaerella all largely covary with paleotemperatures indicating a coupling between climatic conditions and both pelagic and neritic caco3 production furthermore the cooling and warming episodes coincided with major marine regressions and transgressions respectively suggesting that the growth and decay of ice caps may have exerted a strong control on sea level fluctuations throughout the studied time interval this revised chronology of environmental changes shows important similarities with neogene and paleozoic episodes of deglacial black shale formation and thus prompts the reevaluation of ice sheet dynamics as a possible agent of mesozoic events of extinction and organic rich sedimentation c 2010 elsevier b v all rights reserved

compilation and time series analysis of a marine carbonate δ18o δ13c 87sr 86sr and δ34s database through earth history abstract the sr s o and c isotope database of marine carbonates contains over 55 000 published isotope values of low mg calcite from diagenetically little altered phanerozoic fossil shells as well as samples of whole rocks and calcite cements of ordovician to archean age carbon and oxygen isotope data for the shell material are divided into habitat subsets high mid low latitude and deep sea and whole rock data are separated by mineralogy into calcite dolomite subsets trend correlation wavelet and spectral analyses on gaussian filtered isotope records were applied to detect and quantify similarities and patterns in temporal records with the following results 1 oxygen isotope trends from the high latitude and deep sea habitats are almost indistinguishable through the last 115 ma consistent with the existence of the oceanic conveyor belt throughout this interval 2 all oxygen isotope habitat records show a strong coherent 30 45 ma 38 ma cyclicity throughout the cretaceous and the cenozoic 3 up to 70 of the multi million year variability in the 18o record of the last 115 ma can be simulated by the following equation 18 o 0 64 sin 2 t 120 ma 0 9 x sin 2 t 38 3 ma 1 1 with x ranging from 0 4 0 6 for the low high latitude and deep sea habitats to 0 8 for the mid latitude realm 1 a 120 20 ma cycle occurs in the paleozoic and neoproterozoic 18o record consistent with paleoclimate variability as interpreted from sedimentological and faunal records 2 the offset of 13c values between deep water and high latitude vs surficial habitats at lower latitudes is consistent with the operation of a biological pump in the oceans since at least the cretaceous 3 sr and s isotope records exhibit a 60 70 ma cyclicity throughout the phanerozoic

water mass exchange and variations in seawater temperature in the nw tethys during the early jurassic evidence from neodymium and oxygen isotopes of fish teeth and belemnites article i nfo oxygen and neodymium isotope analyses performed on biostratigraphically well dated fish remains recovered from the hettangian to toarcian of the paris basin were used to reconstruct variations of early jurassic seawater temperature and to track oceanographic changes in the nw tethys our results indicate a strong correlation between 18 o trends recorded by fish remains and belemnites confirming the paleoenvironmental origin of oxygen isotope variations interestingly temperatures recorded by pelagic fishes and nektobenthic belemnites and bottom dwelling fishes are comparable during the late pliensbachian sea level lowstand but gradually differ during the early toarcian transgressive episode recording a difference in water temperatures of 6 c during the bifrons zone this could suggest that the surface bottom water temperature difference was not large enough during regressive phases to be recorded by organisms living near the lower and upper part of the water column the globally unradiogenic nd budget of euro boreal waters through the early jurassic suggests that these waters were strongly affected by continental neodymium input from surrounding emerged areas and that exchange with more radiogenic waters from the tethys and panthalassa oceans remained limited this supports the existence of a southward directed current in the euro boreal area for most of the early jurassic the only exception is observed at the early late pliensbachian transition where a positive end excursion is recorded suggesting northward influx of low latitude tethyan or panthalassan waters which may have contributed to the warming of nw tethyan seawater recorded at this time the absence of a marked negative excursion in end concomitant with a negative 18 o shift recorded during the falciferum zone exaratum subzone argues against the influence of less radiogenic arctic water influxes with low 18 o values during this interval instead we suggest that

mid cretaceous albian santonian sea surface temperature record of the tropical atlantic ocean paleoclimate records of geologic time periods characterized by extreme global warmth such as the mid cretaceous are important for a better understanding of the earth9s climate system operating in an exceptionally warm mode here we applied an organic geochemical proxy tex 86 on organic matter rich albian santonian sediments recovered from ocean drilling program leg 207 sites 1258 and 1259 on demerara rise to reconstruct sea surface temperatures ssts in the western equatorial atlantic preceded by a stepwise cenomanian warming trend 31 35 c the onset of the cretaceous thermal maximum coincided here with the cenomanian turonian boundary event once established this extreme warm climate regime characterized by averaged tropical ssts close to 35 c lasted up to the turonian coniacian transition two pronounced cooler intervals 2 3 c interrupt this otherwise remarkably stable record providing the first 18 o independent evidence for middle turonian cooling that previously has been attributed to glacioeustatic sea level lowering coniacian ssts decline stepwise reaching a minimum in the santonian 32 33 c where cooling is most pronounced presumably concomitant with the first progressive opening of a deep water passage through the equatorial atlantic gateway

mesozoic climates and oceans a tribute to hugh jenkyns and helmut weissert the study of past greenhouse climate intervals in earth history such as the mesozoic is an important relevant and dynamic area of research for many sedimentary geologists geochemists palaeontologists and climate modellers the mesozoic sedimentary record provides key insights into the mechanics of how the earth system works under warmer conditions providing examples of natural climate change and perturbations to ocean chemistry including anoxia that are of societal relevance for understanding and contextualizing ongoing and future environmental problems furthermore the deposition of widespread organic carbon rich sediments black shales during the mesozoic means that this is an era of considerable economic interest in july 2015 an international group of geoscientists attended a workshop in ascona switzerland to discuss all aspects of the mesozoic world and to celebrate the four decade long contributions to our understanding of this fascinating era in earth history made by hugh jenkyns university of oxford and helmut weissert eth zurich this volume of sedimentology arose from that meeting and contains papers inspired by and co authored by hugh and helmi here a brief introduction to the volume is provided that reviews aspects of hugh and helmi s major achievements contextualizes the papers of the thematic issue and discusses some of the outstanding questions and areas for future research this article is protected by copyright all rights reserved

isotopic evidence for glaciation during the cretaceous supergreenhouse the turonian 93 5 to 89 3 million years ago was one of the warmest periods of the phanerozoic eon with tropical sea surface temperatures over 35 c high amplitude sea level changes and positive 18 o excursions in marine limestones suggest that glaciation events may have punctuated this episode of extreme warmth new 18 o data from the tropical atlantic show synchronous shifts 91 2 million years ago for both the surface and deep ocean that are consistent with an approximately 200 000 year period of glaciation with ice sheets of about half the size of the modern antarctic ice cap even the prevailing supergreenhouse climate was not a barrier to the formation of large ice sheets calling into question the common assumption that the poles were always ice free during past periods of intense global warming

a kaolinitic weathering crust in tomsk west siberia interpretation in the context of weathering crusts in russia and elsewhere abstract two exposures of a thick kaolinitic weathering crust located in the banks of the river tom in tomsk west siberia were investigated by x ray diffraction scanning electron microscopy and x ray fluorescence analyses the weathering crusts are developed upon a near vertical series of laminated fine grained sandstones and siltstones of carboniferous age that have been deformed by the variscan orogeny and are overlain by horizontal seams of coals and bedded sediments of palaeocene age the geological evidence indicates that the likely age of the weathering crust is mesozoic however there is only slight evidence of vertical mineralogical and chemical zonation of the weathering crust and in the blue cliff exposure chlorite in the parent rock persists apparently unchanged into the upper parts of the profile these features may be accounted for if the profiles examined are of the linear type which typically develop upon tectonically affected features such as faults fractures veins and steeply dipping sediments extend to great depths lack well developed mineralogical and chemical zonation and are significantly affected by lateral drainage and an unstable landscape in contrast areal weathering crusts are of limited thickness and are often truncated extend over a wider area on stable sites and show well developed mineralogical and chemical zonation the distinction between these two types of weathering crust was described by petrov 1958 who further attributed the weathering to a period of virtually unprecedented tectonic quiescence during the late triassic to early jurassic during this period the pangea super continent enjoyed a warm humid and equable climate with no polar ice caps and well vegetated soils were developed on a peneplained surface the evidence supporting this concept is briefly reviewed and indicates that an initial period of early jurassic weathering may well have affected other weathering profiles that have been attributed exclusively to much later periods

distribution of tetraether lipids in the 25 ka sedimentary record of lake challa extracting reliable tex86 and mbt cbt palaeotemperatures from an equatorial african lake the distribution of isoprenoid and branched glycerol dialkyl glycerol tetraether gdgt lipids was studied in the sedimentary record of lake challa a permanently stratified partly anoxic crater lake on the southeastern slope of mt kilimanjaro kenya tanzania to examine if the gdgts could be used to reconstruct past variation in regional temperature the study material comprised 230 samples from a continuous sediment sequence spanning the last 25 ka with excellent age control based on high resolution ams c 14 dating the distribution of gdgts showed large variation through time in some time intervals i e from 20 4 to 15 9 ka bp and during the younger dryas 12 9 11 7 ka bp crenarchaeol was the most abundant gdgt whereas at other times i e during the early holocene branched gdgts and gdgt 0 were the major gdgt constituents in some intervals of the sequence the relative abundance of gdgt 0 and gdgt 2 was too high to be derived exclusively from lacustrine thaumarchaeota suggesting a sizable contribution from methanogens and other archaea this severely complicated application of tex86 palaeothermometry in this lake and limited reliable reconstruction of lake water temperature to the time interval 25 13 ka bp i e the last glacial maximum and the period of post glacial warming the tex86 inferred timing of this warming is similar to that recorded previously in two of the large african rift lakes while its magnitude is slightly or much higher than that recorded at these other sites depending on which lake based tex86 calibration is used application of calibration models based on distributions of branched gdgts developed for lakes inferred temperatures of 15 18 degrees c for the last glacial maximum and 19 22 degrees c for the holocene however the mbt cbt palaeothermometer reconstructs temperatures as low as 12 degrees c for a lateglacial period centred on 15 ka bp variation in down core values of the bit index are mainly determined by the varying production rate of crenarchaeol relative to in situ produced branched gdgts the apparent relationship of the bit index with climatic moisture balance can be explained either by the direct influence of lake level and wind strength on nutrient recycling or by influx of soil nutrients promoting aquatic productivity and nitrification this study shows that gdgts can aid in obtaining climatic information from lake records but that the obtained data should be interpreted with care

offset of latest pleistocene shoreface reveals slip rate on the hosgri strike slip fault offshore central california abstract the hosgri fault is the southern part of the regional hosgri san gregorio dextral strike slip fault system which extends primarily in the offshore for about 400 km in central california between morro bay and san simeon high resolution multibeam bathymetry reveals that the eastern strand of the hosgri fault is crossed by an 265 m wide slope interpreted as the shoreface of a latest pleistocene sand spit this sand spit crossed an embayment and connected a western fault bounded bedrock peninsula and an eastern bedrock highland a paleogeography resembling modern coastal geomorphology along the san andreas fault detailed analysis of the relict shoreface with slope profiles and slope maps indicates a lateral slip rate of 2 6 0 9 mm yr considered a minimum rate for the hosgri given the presence of an active western strand this slip rate indicates that the hosgri system takes up the largest share of the strike slip fault budget and is the most active strike slip fault west of the san andreas fault in central california this result further demonstrates the value and potential of high resolution bathymetry in characterization of active offshore faults online material 1 high resolution bathymetric data for the cross hosgri slope and 2 metadata that describes data collection processing and formatting

early jurassic north atlantic sea surface temperatures from tex86 palaeothermometry early jurassic marine palaeotemperatures have been typically quantified by oxygen isotope palaeothermometry of benthic and nektonic carbonate and phosphatic macrofossils however records of early jurassic sea surface temperatures that can be directly compared with general circulation model simulations of past climates are currently unavailable the tex86 sea surface temperature proxy is based upon the relative abundance of glycerol dialkyl glycerol tetraethers preserved in organic carbon bearing sediments this proxy has been used extensively on cretaceous and cenozoic materials and in one study on middle and upper jurassic sediments here tex86 is applied for the first time to lower jurassic sinemurian pliensbachian sediments cored at deep sea drilling project site 547 in the north atlantic the abundance of glycerol dialkyl glycerol tetraethers in these sediments is very low despite biomarker and rock eval data suggesting that thermal maturity is generally low sea floor oxygenation and a high input of reworked terrestrially sourced organic matter may explain the low concentrations for samples from which it was possible to quantify the relative abundance of glycerol dialkyl glycerol tetraethers tex86 values range from 0 78 to 0 88 equating to sea surface temperatures in excess of 28 c these temperatures are broadly comparable with new general circulation model simulations of the sinemurian and pliensbachian stages and support the general view of a predominantly warm climate the new proxy data suggest that under favourable geological conditions it is possible to extend the record of tex86 based sea surface temperatures back into the early jurassic

evolution of middle to late cretaceous oceans a 55 m y record of earth s temperature and carbon cycle a new 55 m y global compilation of benthic foraminifera 13 c and 18 o for the middle to late cretaceous shows that there was widespread formation of bottom waters with temperatures 20 c during the cretaceous greenhouse world these bottom waters filled the silled north atlantic and probably originated as thermocline or intermediate waters in the tropical oceans carbon burial during the cretaceous oceanic anoxic events produced a positive 13 c shift in global carbon reservoirs but this is not particularly large especially by comparison with the remarkable late paleocene carbon maximum the interbasin 13 c gradient was unusually large during the cretaceous hot greenhouse probably because the north atlantic sills prevented the free exchange of waters in the deep basin the hot greenhouse ended when the equatorial atlantic gateway opened sufficiently to flood the deep north atlantic with relatively cool polar waters formed in the southern ocean

a guide to phanerozoic cold polar climates from high latitude ice rafting in the cretaceous the high latitude extent of warm climate indicators at certain times in earth history has been considered as evidence that the globe was ice free for long intervals despite theoretical considerations and results from numerical modelling experiments1 2 indicating that this was unlikely one of the warmest periods the cretaceous displays faunal and floral evidence for cool temperate to sub tropical conditions very near to the poles however our studies of lower cretaceous mudstones of central australia that contain outsized exotic blocks have led to the conclusion that the blocks were emplaced by ice rafting implying that high latitude ice was present at sea level strata of a similar origin of mid jurassic to mid cretaceous age occur on other continents that were positioned between 65 and 78 palaeolatitude indeed there is a record of high latitude ice rafting throughout the phanerozoic suggesting that ice was present on earth for much of its history and that ice free conditions could have been at most only episodic over the past 600 myr

climatic influences on the paleogene evolution of alkenones application of the alkenone paleotemperature proxy uk 37 for marine sediments is typically constrained by three factors i an absence of alkatrienones in sediments deposited where ocean temperatures exceed 28 c ii loss of alkenones in thermally altered sediments and iii poor preservation of alkenone signals due to oxidative degradation in addition there appears to be a temporal limit on the occurrence of alkatrienones which are conspicuously absent in all alkenone containing sediments from the early aptian to the immediate aftermath of the early eocene climatic optimum eeco when they first appeared in arctic ocean sediments compilation of reported and previously unpublished alkenone distributions for the paleogene coupled with assessment of co occurring calcareous nannoplankton genera within the noelaerhabdaceae provide evidence that evolutionary developments in alkenone occurrences include biosynthetic responses likely triggered by climate change the timing of emergence of alkatrienones post eeco and their subsequent appearance at all latitudes during the middle eocene accompanies expansion of the calcareous nannoplankton genus reticulofenestra coincident with significant climate driven changes in oceanic conditions including i modification of trophic structure associated with weakened thermal stratification ii higher productivity facilitated by enhanced nutrient influx and iii changes in seasonality initially at high latitudes related to greater latitudinal temperature gradients collectively these changes would serve to favor eurythermal eurytrophic algae like reticulofenestra with a biomechanism to store energy through production of lipid bodies rich in alkenones during episodes of higher nutrient availability this ability likely enhanced the viability of this marine haptophyte when nutrients were limiting ultimately ensuring its evolutionary success

evidence for rapid and widespread emplacement of volcanic plains on venus stratigraphic studies in the baltis vallis region geological mapping of the area around baltis vallis the longest 6 800 km canali type channel on venus has led to the determination of stratigraphic relations which appear to be consistent over this entire region comprising 20 x 106 km 2 or 5 of the total surface of venus the most abundant unit in the study area plains with wrinkle ridges pwr consists of two subunits separated by the formation of baitis vailis a feature which is interpreted by most workers to have formed essentially geologically instantaneously the wrinkle ridges deform both subunits of pwr while ridge belts are embayed by the pwr plains consistent age relationships between the pwr subunits wrinkle ridges baltis vallis and ridge belts along the entire 6 800 km channel length is evidence that unit pwr is a real stratigraphic marker unit and not a mosaic of plains emplaced at significantly different times at least in the 5 of the surface of venus comprising the study area the baltis stratigraphic sequence was compared with stratigraphic relations found in earlier studies of thirty six 1 000 x 1 000 km sites randomly distributed over the surface of venus basilevsky and head 1995 and was found to be comparable this supports the interpretation that these stratigraphic units represent generally globally quasi synchronous geologic events rather than the products of geologic activity in different areas of the planet at different times stratigraphic complexes and a number of structures typically formed at several different periods during the observed geologic history this model may correspond to either of two different options for the geologic history of venus 1 the proposed stratigraphic units and equivalent geologic events were generally synchronous over the whole planet or 2 the proposed stratigraphic sequence represents typical sequences of local events which occurred in different places at different times the first option is consistent with the global resurfacing model and the second is consistent with the equilibrium resurfacing model in our previous observations we never saw a case of superposition of one local stratigraphic sequence on another one with repetition of all or part of the sequence of units which would seem to be inevitable at the boundaries of neighboring local activity zones if the second scenario is correct on the basis of this we concluded that the first option consistent with the global resurfacing model agrees better with the observations than the second one however the validity of this conclusion remains tentative for other areas and requires additional testing stratigraphic studies in the area of baltis vallis is a further step in this direction because of the potential for the use of canali type channels as stratigraphic time markers

the early evolution and palaeobiogeography of mesozoic planktonic foraminifera this is a digitised version of a thesis that was deposited in the university library if you are the author and you have a query about this item please contact pearl admin pearladmin plymouth ac uk

descent toward the icehouse eocene sea surface cooling inferred from gdgt distributions the tex86 proxy based on the distribution of marine isoprenoidal glycerol dialkyl glycerol tetraether lipids gdgts is increasingly used to reconstruct sea surface temperature sst during the eocene epoch 56 0 33 9 ma here we compile published tex86 records critically reevaluate them in light of new understandings in tex86 palaeothermometry and supplement them with new data in order to evaluate long term temperature trends in the eocene we investigate the effect of archaea other than marine thaumarchaeota upon tex86 values using the branched to isoprenoid tetraether index bit the abundance of gdgt 0 relative to crenarchaeol gdgt 0 and the methane index mi we also introduce a new ratio gdgtrs which may help identify red sea type gdgt distributions in the geological record using the offset between tex86h and tex86l h l and the ratio between gdgt 2 and gdgt 3 2 3 we evaluate different tex86 calibrations and present the first integrated sst compilation for the eocene 55 to 34 ma although the available data are still sparse some geographic trends can now be resolved in the high latitudes 55 there was substantial cooling during the eocene 6 c our compiled record also indicates tropical cooling of 2 5 c during the same interval using an ensemble of climate model simulations that span the eocene our results indicate that only a small percentage 10 of the reconstructed temperature change can be ascribed to ocean gateway reorganization or paleogeographic change collectively this indicates that atmospheric carbon dioxide pco2 was the likely driver of surface water cooling during the descent toward the icehouse

the effect of maturity and depositional redox conditions on archaeal tetraether lipid palaeothermometry recently we proposed a new organic sea surface temperature proxy tex86 based on the distribution of archaeal tetraether lipids here we have examined the effect of oxic degradation and maturity on this new temperature proxy our results show that oxic degradation does not appear to affect the tex86 values within the analytical error hydrous pyrolysis experiments suggest that thermal maturity does have an effect on tex86 sediments with relatively high amounts of 22s hopanes 22s 22s 22r ratios 0 1 or 17 21 h hopanes ratios 0 5 may have tex86 values which are biased towards lower values and thus lower sea surface temperatures

oxygen carbon isotope composition of middle jurassic cretaceous molluscs from the saratov samara volga region and main climate trends in the russian platform caucasus

distributional variations in marine crenarchaeotal membrane lipids a new tool for reconstructing ancient sea water temperatures it has recently been shown that membrane lipids of marine crenarchaeota a ubiquitous and abundant component of plankton occur in relatively high concentrations in recent and ancient sediments in this study we investigated the environmental controls on the relative distribution of these lipids in surface sediments we especially focussed on temperature as it is known from the thermophilic genetic relatives of marine crenarchaetoa that the composition of their membrane strongly depends on growth temperature indeed a significant linear correlation r2 0 92 is found between the number of cyclopentane rings in sedimentary membrane lipids derived from marine crenarchaeota and the annual mean sea surface temperatures this suggests that the mechanism of physical adaptation of their membrane compositions to temperature is identical to that of their thermophilic relatives in turn archaeal lipid distributions in sediments may thus allow the reconstruction of sea water temperatures of ancient marine environments

constraining the applicability of organic paleotemperature proxies for the last 90 myrs abstract we evaluated changes in the distributions of long chain alkenones long chain diols and gdgts lipids commonly used for paleothermometry over the last 90 myrs for sediments deposited on the new jersey shelf the bass river site and assessed potential effects of different ancestral producers and diagenesis on their distributions and their impact on the associated temperature proxies as reported before the paleogene distributions of alkenones are generally similar to those in modern haptophytes but unusual alkenone distributions characterized by a dominant di unsaturated c40 alkenone are observed for late cretaceous sediments suggesting different ancestral source organisms for alkenones in this interval the isoprenoid gdgt distributions remained comparable to modern day distributions suggesting that tex86 can be applied up to ca 90 ma the miocene long chain diol distributions are similar to modern day distributions but the older sediments reveal unusual distributions dominated by the c28 1 12 and c26 1 13 diols suggesting different source organisms before 30 ma accordingly the ldi does not match other paleotemperature proxies suggesting its applicability might be compromised for sediments older than the miocene our results indicate that of the three proxies the tex86 seems to be the most applicable for deep time temperature reconstructions

tex86 and stable δ18o paleothermometry of early cretaceous sediments implications for belemnite ecology and paleotemperature proxy application abstract recent studies have cast doubt on the unadjusted usage of jurassic and cretaceous 18 o paleotemperature data derived from belemnites since the latter data often reflect cooler paleotemperature estimates than would be expected in this study we address this problem by analysing rocks of barremian to early aptian age from two outcrops in northern germany using tex 86 paleothermometry along with 142 belemnite guards studied for their stable isotope 13 c 18 o and trace element composition magnesium strontium iron and manganese both tex 86 and 18 o bel indicate very warm water temperatures for a distinctive black shale sequence of late early barremian age hauptblatterton with temperatures of up to 29 c and 23 c respectively we observe a constant offset of tex 86 temperatures versus the 4 to 5 c cooler 18 o belemnite signal for this interval the late barremian sequence shows an increase of the 18 o bel values from 1 to 0 reflecting temperatures around 16 to 12 c while the contemporaneous tex 86 temperatures vary between 26 and 32 c the common occurrence of belemnites in the anoxic sediments of the early barremian implies however clearly a nektonic way of life similar to that of recent teuthids rather than a nektobenthic one like sepia this in turn suggests that the belemnites investigated here genera praeoxyteuthis aulacoteuthis oxyteuthis and neohibolites were active swimmers which inhabited a deeper habitat below the thermocline in an epicontinental sea of perhaps 100 to 250 m water depth the offset of the tex 86 and 18 o bel data is therefore interpreted to reflect temperature signals from two different depth habitats i e the tex 86 is selectively derived from warm sea surface waters and the belemnites likely occupied deeper and cooler waters with relative increasing salinities in the late barremian this study stresses the importance that the taxonomy paleobiology and ecology of the belemnite taxa genera species used for paleotemperature reconstructions must be considered before the 18 o bel signal can be further interpreted the variation of the 18 o signature gained from one belemnite population of 22 specimens by 1 1 suggests that the 18 o data of individual belemnites should be used with caution for reconstructing long termed paleotemperature trends

extremely high sea surface temperatures at low latitudes during the middle cretaceous as revealed by archaeal membrane lipids the middle cretaceous 125 88 ma greenhouse world was characterized by high atmospheric co 2 levels the general absence of polar ice caps and much higher global temperatures than at present both 1 8 o based and model based temperature reconstructions indicate extremely high sea surface temperatures ssts at high latitudes however there are a number of uncertainties with sst reconstructions based on 1 8 o isotope data of foraminifera due to diagenetic overprinting effects and tenuous assumptions with respect to the 1 8 o value of cretaceous seawater the paleoecology of middle cretaceous marine organisms and seawater ph here we applied a novel sst proxy i e text tetraether index of 86 carbon atoms based on the membrane lipids of marine crenarchaeota derived from middle cretaceous sedimentary rocks deposited at low latitudes the text proxy indicates that tropical ssts in the proto north atlantic were at 32 36 c during the early albian and late cenomanian early turonian this finding agrees with sst estimates based on 1 8 o paleothermometry of well preserved foraminifera as well as global circulation model calculations the text proxy indicates cooler ssts 27 32 c for the equatorial pacific during the early aptian which is in agreement with sst estimates based on 1 8 o paleothermometry

thermal evolution of cretaceous tethyan marine waters inferred from oxygen isotope composition of fish tooth enamels 1 the evolution of subtropical 30 35 n upper ocean temperatures through the cretaceous is inferred from the oxygen isotope compositions of 64 fish teeth enamel coming from the western tethyan platform mean 18o values of 22 at the berriasian valanginian boundary decrease with oscillations to 18 5 around the cenomanian turonian boundary and progressively increase to 21 5 by the end of the cretaceous the similarity of this oxygen isotope curve for bioapatites from platform environments with those for foraminifera and bulk carbonates that were deposited in deeper waters and at other paleolatitudes indicates that they record global climatic signals major cooling events at the million year scale can be distinguished 1 at the berriasian valanginian boundary and 2 during the earliest late valanginian a third cooling event is detected during the earliest aptian these events already proposed as icehouse interludes during the lower cretaceous are also recorded at subtropical latitudes a progressive warming is identified from the aptian to the cenomanian turonian interval that corresponds to a thermal optimum and then upper ocean temperatures decreased to the maastrichtian minimum isotopic temperatures range from 15 c to 28 c assuming a 18oseawater of 1 for an ice free world taking more realistic 18oseawater values of 0 for tropical waters during glacial periods within the berriasian valanginian interval and earliest aptian or with above average salinities possibly the maastrichtian temperatures are increased by 4 5 c temperature differences between climatic extremes of the valanginian and cenomanian turonian are estimated to have been 10 c latitudinal thermal gradients for the albian cenomanian turonian and maastrichtian were 0 2 0 3 c latitude and thus weaker than modern oceanic values at about 0 4 c latitude

methane index a tetraether archaeal lipid biomarker indicator for detecting the instability of marine gas hydrates abstract gas hydrates represent one of the largest pools of readily exchangeable carbon on earth s surface releases of the greenhouse gas methane from hydrates are proposed to be responsible for climate change at numerous events in geological history many of these inferred events however were based on carbonate carbon isotopes which are susceptible to diagenetic alterations here we propose a molecular fossil proxy i e the methane index mi to detect and document the destabilization and dissociation of marine gas hydrates mi consists of the relative distribution of glycerol dibiphytanyl glycerol tetraethers gdgts the core membrane lipids of archaea the rational behind mi is that in hydrate impacted environments the pool of archaeal tetraether lipids is dominated by gdgt 1 2 and 3 due to the large contribution of signals from the methanotrophic archaeal community our study in the gulf of mexico cold seep sediments demonstrates a correlation between mi and the compound specific carbon isotope of gdgts which is strong evidence supporting the mi methane consumption relationship preliminary applications of mi in a number of hydrate impacted and or methane rich environments show diagnostic mi values corroborating the idea that mi may serve as a robust indicator for hydrate dissociation that is useful for studies of global carbon cycling and paleoclimate change

bivalve palaeobiogeography and the hispanic corridor time of opening and effectiveness of a proto atlantic seaway abstract the hispanic corridor is an embryonic seaway between the eastern pacific and western tethyan oceans that has been postulated to have preceded middle jurassic drifting and the birth of the atlantic ocean by many millions of years in this study the distribution of pectinoid bivalve morphotypes is analysed to examine the origin of the hispanic corridor and its effectiveness during early jurassic times a comparison of pectinoid faunal similarities based on similarity coefficients between various regions at opposite ends of the corridor suggests that it opened progressively during early jurassic times however with this approach it is difficult to exclude alternative dispersal routes and to determine when faunal interchange began analysis of the percentage of pectinoid morphotypes that were 1 present at opposite sides of the corridor 2 simultaneously absent in the western pacific eastern tethys and 3 confined to relatively low palaeolatitudes provides evidence that the hispanic corridor developed from an effective barrier in earlier early jurassic hettangian to sinemurian times into a filter allowing the passage of a few morphotypes during later early jurassic pliensbachian and toarcian times the apparently two way faunal exchange through the hispanic corridor is consistent with the establishment of a megamonsoonal circulation for pangaea which may have caused seasonally changing directions of oceanic surface currents through the corridor

pangaean climate during the early jurassic gcm simulations and the sedimentary record of paleoclimate results from new simulations of the early jurassic climate show that increased ocean heat transport may have been the primary force generating warmer climates during the past 180 m y the simulations conducted using the general circulation model gcm at the goddard institute for space studies include realistic representations of paleocontinental distribution topography epeiric seas and vegetation in order to facilitate comparisons between model results and paleoclimate data three major features of the simulated early jurassic climate include the following 1 a global warming compared to the present of 5 c to 10 c with temperature increases at high latitudes five times this global average average summer temperatures exceed 35 c in low latitude regions of western pangaea where eolian sandstones testify to the presence of vast deserts 2 simulated precipitation and evaporation patterns agree closely with the moisture distribution interpreted from evaporites and coal deposits high rainfall rates are associated primarily with monsoons that originate over the warm tethys ocean unlike the megamonsoons proposed in previous studies these systems are found to be associated with localized pressure cells whose positions are controlled by topography and coastal geography 3 decreases in planetary albedo occurring because of reductions in sea ice snow cover and low clouds and increases in atmospheric water vapor are the positive climate feed backs that amplify the global warming similar to other mesozoic climate simulations our model finds that large seasonal temperature fluctuations occurred over mid and high latitude continental interiors refuting paleoclimate evidence that suggests more equable conditions sensitivity experiments suggest that some combination of ocean heat transport increase high levels of co2 and improved modeling of ground hydrological schemes may lead to a better match with the geologic record we speculate also that the record itself is biased toward equable climatic conditions a suggestion that may be tested by comparing gcm results with more detailed phytogeographic analyses

recognition of alkenones in a lower aptian porcellanite from the west central pacific odp leg 198 drilling on shatsky rise recovered a lower aptian porcellanite 120 5 ma deposited during oceanic anoxic event oae 1a that contains c36 c39 alkadienones c37 2 and c39 2 alkadien 2 ones and c36 2 and c38 2 alkadien 3 ones this alkenone distribution differs from that typical of contemporary sediments and haptophyte algae but resembles that of cretaceous sediments from the blake bahama basin the discovery of alkenones in the early aptian extends their sedimentary record by 15 m y to 120 5 m y and demonstrates the potential for long term survival of these diagnostic functional lipids under favorable depositional conditions and subsequent shallow burial it also contributes to the understanding and reconstruction of evolutionary developments in alkenone distributions and biosynthesis over geologic time 2003 elsevier ltd all rights reserved

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels