integrated analyses constraining the provenance of sandstones a case study the section peak formation beacon supergroup antarctica sandstones of the section peak formation late triassic middle jurassic northern victoria land antarctica record a change of palaeotectonic regime from a stable continental basement setting to the beginning of rifting indicated by widespread basaltic volcanism during the middle jurassic the provenance study of sandstones was performed by integrating conventional microscope study of detrital mode a varietal study of quartz grains the detection of mineral inclusions in detrital quartz and electron microprobe analyses of detrital garnets this provides the basis for discussing the validity and limits of each of these approaches highlighting the potential source of pitfalls in using a single provenance approach especially when clastics related to rapidly changing geotectonic regimes are studied the resulting palaeogeographic picture is of a braided stream system draining a low relief crystalline basement area composed mainly of granitoid and high grade metamorphic rocks intermittently interacting with coeval basaltic lava flows that were partially reworked by fluvial currents 1999 elsevier science b v all rights reserved

the namibian and brazilian southern south atlantic petroleum systems are they comparable analogues tectonic reconstructions made across the southern south atlantic ocean indicate a diversity of rift and drift basin characteristics on the conjugate margins that define them as different stratigraphic and structural entities in terms of petroleum systems the basins are not as unlike as some characteristics suggest given the lack of significant hydrocarbon discoveries to date south of the walvis ridge doubts have been cast on the presence in this area of the prolific lower cretac eous lacustrine and marine source rock systems which are well known in the greater campos basin and offshore angola oils and condensates from the basins south and north of the walvis ridge exhibit geochemical similarities suggesting that comparable source rock systems are present in both areas the condensate geochemical analysis results from the kudu field in namibia are compared with oils from marine and lacustrine sources in brazil indicating that the kudu condensates are derived from at least two different source rocks these results suggest that the underexplored basins offshore namibia contain thermally mature lower cretaceous lacus trine and marine source rocks offering a new frontier for petroleum exploration in africa s southern south atlantic

forcing mechanisms for mid cretaceous black shale formation evidence from the upper aptian and lower albian of the vocontian basin se france calcareous nannoplankton palynomorph benthic foraminifera and oxygen isotope records from the supraregionally distributed niveau paquier early albian age oceanic anoxic event 1b and regionally distributed niveau kilian late aptian age black shales in the vocontian basin se france exhibit variations that reflect paleoclimatic and paleoceanographic changes in the mid cretaceous low latitudes to quantify surface water productivity and temperature changes nutrient and temperature indices based on calcareous nannofossils were developed the nutrient index strongly varies in the precessional band whereas variations of the temperature index reflect eccentricity since polar ice caps were not present during the mid cretaceous these variations probably result from feedback mechanisms within a monsoonal climate system of the mid cretaceous low latitudes involving warm humid and cool dry cycles a model is proposed that explains the formation of mid cretaceous black shales through monsoonally driven changes in temperature and evaporation precipitation patterns the lower albian niveau paquier which has a supraregional distribution formed under extremely warm and humid conditions when monsoonal intensity was strongest bottom water ventilation in the vocontian basin was diminished probably due to increased precipitation and reduced evaporation in regions of deep water formation at low latitudes surface water productivity in the vocontian basin was controlled by the strength of monsoonal winds the upper aptian niveau kilian which has a regional distribution only formed under a less warm and humid climate than the niveau paquier low latitude deep water formation was reduced to a lesser extent and or on regional scale only the threshold for the formation of a supraregional black shale was not reached the intensity of increases in temperature and humidity controlled whether black shales developed on a regional or supraregional scale at least in the vocontian basin the increased preservation of organic matter at the sea floor was more significant in black shale formation than the role of enhanced productivity

middle cretaceous calcareous nannofossil biogeography and preservation in the atlantic and indian oceans implications for paleoceanography abstract the quantitative study of calcareous nannofossil preservation and biogeographic distribution puts constraints on paleoceanographic models for the atlantic and indian oceans during the middle cretaceous we establish a dissolution ranking of middle cretaceous calcareous nannofossils based on susceptibility to dissolution a positive correlation of organic carbon content of the sediments and the relative abundance of watznaueria indicates that much of the carbonate dissolution was caused by the release of carbon dioxide during oxidation of organic matter the depth distribution of the most dissolution resistant species watznaueria barnesae shows relative abundance maxima at the deepest sites and a maximum where intermediate waters impinge on slopes this reflects stagnant oxygen poor bottom waters and possibly a mid water oxygen minimum zone with increased deposition and catabolic breakdown of organic matter factor analyses are performed on various data sets for three different time slices ranging in age from late aptian to early albian middle albian and late albian to cenomanian the resulting factors can be related to paleoceanographic conditions such as colder water at high latitudes high fertility upwelling conditions and neritic conditions for all three time intervals a wide tropical to subtropical zone was flanked by boreal and austral zones upwelling conditions probably existed along the north african margin and over the falkland plateau zonal upwelling is tentatively postulated for the northwestern and northeastern margin of the north atlantic in the middle albian and for the eastern margin of the north atlantic off western europe for the late albian to cenomanian we speculate on surface water current patterns location of upwelling regions and haline driven deep circulation for the middle cretaceous

isotopic evidence for the source of ca and s in soil gypsum anhydrite and calcite in the atacama desert chile abstract the origin of pedogenic salts in the atacama desert has long been debated possible salt sources include in situ weathering at the soil site local sources such as aerosols from the adjacent pacific ocean or salt encrusted playas salars and extra local atmospheric dust to identify the origin of ca and s in atacama soil salts we determined 34 s and 87 sr 86 sr values of soil gypsum anhydrite and 87 sr 86 sr values of soil calcite along three east west trending transects our results demonstrate the strong influence of marine aerosols on soil gypsum anhydrite development in areas where marine fog penetrates inland results from an east west transect located along a breach in the coastal cordillera show that most soils within 90 km of the coast and below 1300 m in elevation are influenced by marine aerosols and that soils within 50 km and below 800 m in elevation receive 50 of ca and s from marine aerosols 34 s values 14 and 87 sr 86 sr values 0 7083 in areas where the coastal cordillera is 1200 m in elevation however coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced most pedogenic salts from inland soils have 34 s values between 5 0 to 8 0 and 87 sr 86 sr ratios between 0 7070 and 0 7076 these values are similar to average 34 s and 87 sr 86 sr values of salts from local streams lakes and salars 5 4 2 34 s and 0 70749 0 00045 87 sr 86 sr in the andes and atacama suggesting extensive eolian reworking of salar salts onto the surrounding landscape ultimately salar salts are precipitated from evaporated ground water which has acquired its dissolved solutes from water rock interactions both high and low temperature along flowpaths from recharge areas in the andes therefore the main source for ca and s in gypsum anhydrite in non coastal soils is indirect and involves bedrock alteration not surficially on the hyperarid landscape but in the subsurface by ground water followed by eolian redistribution of ground water derived salar salts to soils the spatial distribution of high grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local marine and salar salt supporting arguments for tropospheric nitrogen as the main source for soil nitrate

deep structure of the western south african passive margin results of a combined approach of seismic gravity and isostatic investigations abstract the passive margin of the south atlantic shows typical features of a rifted volcanic continental margin encompassing seaward dipping reflectors continental flood basalts and high velocity density lower crust at the continent ocean transition probably emplaced during initial seafloor spreading in the early cretaceous the springbok profile offshore western south africa is a combined transect of reflection and refraction seismic data this paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives the velocity modelling revealed a segmentation of the margin into three distinct parts of continental transitional and oceanic crust as observed at many volcanic margins the lower crust is characterised by a zone of high velocities with up to 7 4 km s the conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities found to be 3100 kg m 3 both approaches identified the body to have a thickness of about 10 km yet the gravity modelling predicted the transition between the high density body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal finally isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced the results imply isostatic equilibrium over large parts of the profile smaller deviations are supposed to be compensated regionally the calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km

crustal detachment during south atlantic rifting and formation of tucano gabon basin system a series of major sedimentary basins of mesozoic to tertiary age are associated with the brazilian continental margin between 8 and 14 s fig 1 this passive continental margin formed when africa and south america split apart with the initiation of seafloor spreading in the aptian albian3 the offshore basins which border the continental margins on both sides of the atlantic show evidence of both rifting and crustal attenuation before break up and thermal subsidence after it in contrast a combined stratigraphical and gravity study of the brazilian onshore basins which are separated from the offshore basins by a northward widening strip of precambrian suggests that up to 7 km of non marine sediments were deposited during the rifting stage of the south atlantic with no apparent extension of the lower crust beneath these basins and no subsequent thermal subsidence a mechanism for the linked development of these onshore and offshore basins is proposed here along the lines suggested by wernicke4 5 in which upper crustal extension in the onshore region is connected to and balanced against deeper lithospheric extension beneath the incipient passive margin by intracrustal detachments

a threshold in soil formation at earth s arid hyperarid transition the soils of the atacama desert in northern chile have long been known to contain large quantities of unusual salts yet the processes that form these soils are not yet fully understood we examined the morphology and geochemistry of soils on post miocene fans and stream terraces along a south to north 27 to 24 s rainfall transect that spans the arid to hyperarid transition 21 to 2 mm rain y 1 landform ages are 2 my based on cosmogenic radionuclide concentrations in surface boulders and ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2 1 my a chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall in the arid 21 mm rain y 1 soil total mass loss to weathering of silicate alluvium and dust 1030 kg m 2 is offset by net addition of salts 170 kg m 2 the most hyperarid soil has accumulated 830 kg m 2 of atmospheric salts including 260 kg sulfate m 2 and 90 kg chloride m 2 resulting in unusually high volumetric expansion 120 for a soil of this age the composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in soil chemical transformation and loss long term rates of atmospheric solute addition were derived from the ion inventories in the driest soil divided by the landform age and compared to measured contemporary rates with decreasing rainfall the soil salt inventories increase and the retained salts are both more soluble and present at shallower depths all soils generally exhibit vertical variation in their chemistry suggesting slow and stochastic downward water movement and greater climate variability over the past 2 my than is reflected in recent 100 y rainfall averages the geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold in wetter soils the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion the specific geochemistry of hyperarid soils is a function of atmospheric sources and is expected to vary accordingly at other hyperarid sites this work identifies key processes in hyperarid soil formation that are likely to be independent of location and suggests that analogous processes may occur on mars

testing the cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the turonian tropics on demerara rise glassy turonian foraminifera preserved in clay rich sediments from the western tropical atlantic yield the warmest equivalent 18o sea surface temperatures ssts yet reported for the entire cretaceous cenozoic we estimate turonian ssts that were at least as warm as conservative mean 30 c to significantly warmer warm mean 33 c than those in the region today however if independent evidence for high middle cretaceous pco2 is reliable and resulted in greater isotopic fractionation between seawater and calcite because of lower sea surface ph our conservative and warm sst estimates would be even higher 32 and 36 c respectively our new tropical ssts help reconcile geologic data with the predictions of general circulation models that incorporate high cretaceous pco2 and lend support to the hypothesis of a cretaceous greenhouse our data also strengthen the case for a turonian age for the cretaceous thermal maximum and highlight a 20 40 m y mismatch between peak cretaceous cenozoic global warmth and peak inferred tectonic co2 production we infer that this mismatch is either an artifact of a hidden turonian pulse in global ocean crust cycling or real evidence of the influence of some other factor on atmospheric co2 and or ssts a hidden pulse in crust cycling would explain the timing of peak cretaceous cenozoic sea level also turonian but other factors are needed to explain high frequency 10 100 k y instability in middle cretaceous ssts reported elsewhere

neogene climate change and uplift in the atacama desert chile comment reply we welcome comments by hartley regarding our neogene climate reconstruction of the atacama desert rech et al 2006 1 and the opportunity to further discuss the resolution and sensitivity of soils and sedimentological deposits for inferring hyperaridity in the geologic record in regard to our

role of subaerial volcanic rocks and mantle plumes in creation of south atlantic margins implications for salt tectonics and source rocks abstract seaward dipping reflectors sdrs represent flood basalts rapidly extruded during either rifting or initially subaerial sea floor spreading evaporites can form on this basaltic proto oceanic crust as in the afar triangle today evidence for sdrs in south atlantic deep water regions comes from proximity to the uniquely large parana etendeka volcanic province onshore the tristan and gough hot spots drilled volcanic rocks and seismic profiles showing sdr provinces more than 100 km wide as much as 7 km thick and thousands of kilometers long sdrs are clearest adjoining the aptian salt basins however we speculate that sdrs are also present but seismically obscured below the salt basins we argue that the conjugate aptian salt basins are post breakup not pre breakup they were separated from the start by a mid oceanic ridge distal salt accumulated on proto oceanic crust not rift basins this hypothesis is supported by seismic stratigraphy and structure magnetic anomalies plate reconstructions and hydrothermal potash evaporites an important implication for exploration is that thick basalts rather than rift age source rocks may underlie distal parts of the salt basins

modeling organic carbon burial during sea level rise with reference to the cretaceous after three decades of research on the correlation between sea level and various proxies there has been very little quantification of the first order influence of sea level change on nutrient inventory marine productivity and burial of organic carbon we present a model aimed at quantifying the burial of organic carbon as a function of sea level rise the biogeochemical model explicitly considers the mean surface area distribution of the earth as a function of elevation also included is dissolved inorganic phosphate dip liberated from coastal sediments during transgression we quantify how sea level rise of magnitudes inferred for the mid cretaceous influences the phosphorus cycle and burial of organic carbon the burial efficiency is greater on the shelf therefore in the model the larger shelf area under high sea level results in more efficient burial of marine organic carbon for a given dip concentration and associated new production of organic matter np with a late cretaceous model shelf area the residence time of dip decreases by 60 relative to the present day reference thus in the final steady state dip and np in the open ocean are reduced compared to today when the biologically reactive phosphorus preac fluxes to the ocean from land are held constant the lower new production reduces the oxygen demand for respiration and results in a significant oxygenation of the global ocean finally the global organic carbon burial decreases by 30 relative to today in the model this decrease results from feedbacks between the flux of organic carbon to the seafloor and the ratio of organic carbon to preac in buried sediments in contrast during sea level rise coastal erosion may increase the preac flux to the ocean by up to 20 relative to today and cause a temporary increase in dip concentration in the ocean the resulting increase in organic carbon burial is equivalent to a carbon isotope event of 0 5 to 1 larger carbon isotope events can be triggered by sea level rise only when the ocean is sufficiently close to anoxia in the oxygen minimum zone just prior to the sea level rise

neogene climate change and uplift in the atacama desert chile the relationship between andean uplift and extreme desiccation of the west coast of south america is important for understanding the interplay between climate and tectonics in the central andes yet it is poorly understood here we use soil morphological char acteristics salt chemistry and mass independent fractionation anomalies 17 o values in dated paleosols to reconstruct a middle miocene climatic transition from semiaridity to extreme hyperaridity in the atacama desert paleosols along the southeastern margin of the calama basin change from calcic vertisols with root traces slickensides and gleyed horizons to an extremely mature salic gypsisol with pedogenic nitrate we interpret this transition which occurred between 19 and 13 ma to represent a change in precipitation from 200 mm yr to 20 mm yr this drastic reduction in precipitation likely resulted from uplift of the central andes to elevations 2 km the uplift blocked moisture from the south american summer monsoon from entering the atacama the mid miocene gyp sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the cal ama basin significantly higher than modern nitrate soils which occur below 2500 m modern and quaternary soils in this elevation zone contain soil carbonate and lack ped ogenic gypsum and nitrate we infer that 900 m of local surface uplift over the past 10 m y displaced these nitrate paleosols relative to modern nitrate soils and caused a return to wetter conditions in the calama basin by decreasing local air temperatures and creating an orographic barrier to pacific air masses

milankovitch climatic origin of mid cretaceous black shale rhythms in central italy the earth s orbital variations reflected in pleistocene ice volume were also recorded in non glacial times carbonate production in pelagic mid cretaceous sediments quantified by calcium carbonate and optical densitometry time series reflects the orbital eccentricity and precessional cycles minimal eccentricity brought deep sea anoxia most of the sedimentary variability of this 100 myr old sequence lies within the milankovitch orbital frequency band

palaeoceanographic differences of early late aptian black shale events in the vocontian basin se france abstract a high resolution study focussing on the distribution of calcareous nannofossils and carbon isotopes was carried out to improve the understanding of mid cretaceous black shale formation the studied interval of the early late aptian is characterized by two major black shale couplets the niveau noire 4 nn4 and niveau noire calcaire 2 nnc2 of the serre chaitieu section in the vocontian basin se france breheret 1997 this interval occurs within a long term negative carbon isotope excursion of 1 5 following the early aptian oceanic anoxic event 1a oae1a in contrast to the local nn4 black shales the black shales of nnc2 are of regional significance and occur at the end of the major negative carbon isotope excursion of the early late aptian time equivalent black shales are suggested to be coeval with black shales found in the western tethys and atlantic ocean herrle et al 2004 calcareous nannofossil analyses and carbon isotopes indicate higher surface water productivity mesotrophic warmer surface water and higher sea level during the formation of the nn4 black shales in contrast the formation of the nnc2 black shales took place during a cooler phase lower surface water productivity and lower sea level a sea level fall may cause a restriction of water mass exchange between the open marine western tethys and the vocontian basin this resulted in a longer residence time of the bottom water decreased ventilation and less mixing of surface waters and thus enhanced preservation of organic matter at the sea floor our results indicate that the black shale formation of nn4 and nnc2 was caused by different processes such as increased surface water productivity and enhanced preservation of organic matter at the sea floor thus we emphasize the role of different forcing factors which control the formation of local and regional black shales the most important factors are sea level fluctuations increasing productivity and changes in precipitation and evaporation rates

calcareous nannofossils and milankovitch cycles the example of the albian gault clay formation southern england a quantitative study of calcareous nannofossil assemblages was carried out on the albian gault clay formation england this unit consists of grey marly claystones and marlstones with phosphatic nodules and glauconitic layers as minor lithologies two cores were analyzed in detail to focus on changes in nannofloral assemblages and consider their significance preservation of calcareous nannofossils is excellent and therefore diagenesis cannot be invoked to explain not even partially the changes registered in assemblages fast fourier spectral analysis was performed on percentages of selected nannofossil species fluctuations in abundance of both biscutum constans and watznaueria barnesae regarded as fertility and non fertility indices respectively appear to be orbitally driven because spectral analysis showed periodicities of approximately 41 kyr obliquity cycle and 100 kyr short eccentricity cycle in all the spectra the axial obliquityc cycle is a strong signal as expected at these high latitude sites from the boreal realm the same pattern was observed in another fertility controlled taxon zygodiscus cf erectus but cross correlation shows that its abundance plot is displaced with respect to that of b constans orbital perturbations induce variations in climate and ocean circulation because they influence the earth s insolation and seasonality during times of high seasonality strong gradients cause vigorous oceanic circulation a better cycling of nutrients and higher fertility whereas during times of low seasonality low gradients induce more sluggish circulation and lower fertility the carbonate content of the gault clay cores is not correlatable to the higher fertility indices as the abundance curves of b constans w barnesae and z cf erectus are displaced with respect to caco3 content variations this also means that cyclicity in nannofossil assemblages is not a result of diagenesis but an original signal abundance fluctuations of repagulum parvidentatum and combined parhabdolithus asper p splendens colder and warmer water indicators respectively provide evidence for orbital periodicities suggesting that also cyclic variations of surface water temperature might have influenced the nannofossil assemblage patterns within the gault clay

global calibration of gephyrocapsa coccolith abundance in holocene sediments for paleotemperature assessment 1 a global sea surface temperature calibration based on the relative abundance of different morphotypes within the coccolithophore genus gephyrocapsa in holocene deep sea sediments is presented there is evidence suggesting that absolute sea surface temperature for a given location can be calculated from the relative abundance of gephyrocapsa morphotypes in sediment samples with a standard error comparable to temperature estimates derived from other temperature proxies such as planktic foraminifera transfer functions a total of 110 holocene sediment samples were selected from the pacific indian and atlantic oceans covering a mean sea surface temperature gradient from 13 6 to 29 3 c standard multiple linear regression analyses were applied to this data set linking the relative abundance of gephyrocapsa morphotypes to sea surface temperature the best model revealed an r2 of 0 83 with a standard residual error of 1 78 c for the estimation of mean sea surface temperature this new proxy provides a unique opportunity for the reconstruction of paleotemperatures with a very small amount of sample material due to the minute size of coccoliths permitting examination of thinly laminated sediments e g a pinhead of material from laminated sediments for the reconstruction of annual sea surface temperature variations such fine scale resolution is currently not possible with any other proxy application of this new paleotemperature proxy may allow new paleoenvironmental interpretations in the late quaternary period and discrepancies between the different currently used paleotemperature proxies might be resolved

150 million years of climatic stability evidence from the atacama desert northern chile the sedimentary succession in the atacama desert records deposition under an arid to semiarid climate from the late jurassic 150 ma to the present day palaeomagnetic data indicate no significant latitudinal movement of this area since the late jurassic the present day location of the atacama within the dry subtropical climate belt is the principal cause of aridity this situation is likely to have prevailed since the late jurassic supplemented by 1 the continentality effect enhanced by the gondwanan landmass and 2 the presence offshore of a cold upwelling current from at least the early cenozoic onwards and possibly earlier resulting in conditions promoting climatic stability and desert development rapid and extreme climatic fluctuations during the plio pleistocene were not sufficient to destabilize the climate within the atacama comparison with other long lived deserts e g sw usa namib sahara and australia suggests that the atacama is the oldest extant desert on earth

gravity modeling of the ocean continent transition along the south atlantic margins 1 gravity magnetic and seismic data have been used to examine changes in crustal structure of conjugate portions of the south atlantic volcanic margins south of the walvis ridge rio grande rise hot spot tracks we have constructed 18 seismically constrained crustal scale gravity models of the ocean continent transition 12 across the african margin and 6 across the south american margin we attribute changes in character of the gravity anomalies to variations in crustal thickness sedimentation and magmatic underplating to investigate variations in the lower crust and upper mantle we have removed the relatively well constrained contributions to the gravity field from shallow sources the resulting residual anomalies delineate distinct crustal domains associated with crustal thinning and regions of underplating that correspond well with those independently identified from limited seismic refraction data near the hot spot tracks we find underplated crust to be symmetrically distributed with a horizontal extent of 240 km for each margin further south however the underplated region is asymmetric with significantly more beneath the african than the corresponding south american margin residual gravity anomalies indicate that the width of underplated crust decreases from 240 km to 150 km at 33 s on the african side and to 100 km at 38 s on the south american side the wider underplated region on the african side suggests an asymmetric rifting mechanism in which greater melts accumulate beneath an upper plate margin our results are consistent with recently proposed asymmetric rifting models that involve strain softening of a coupled cold frictional upper lithosphere at low rifting velocities such models also predict greater symmetry for areas with more heat such as those associated with the hot spot

high temperatures in the late cretaceous arctic ocean to understand the climate dynamics of the warm equable greenhouse world of the late cretaceous period it is important to determine polar palaeotemperatures the early palaeoceanographic history of the arctic ocean has however remained largely unknown because the sea floor and underlying deposits are usually inaccessible beneath a cover of floating ice a shallow piston core taken from a drifting ice island in 1970 fortuitously retrieved unconsolidated upper cretaceous organic rich sediment from alpha ridge a submarine elevated feature of probable oceanic origin a lack of carbonate in the sediments from this core has prevented the use of traditional oxygen isotope palaeothermometry here we determine arctic palaeotemperatures from these upper cretaceous deposits using tex86 a new palaeothermometer that is based on the composition of membrane lipids derived from a ubiquitous component of marine plankton crenarchaeota from these analyses we infer an average sea surface temperature of approximately 15 degrees c for the arctic ocean about 70 million years ago this calibration point implies an equator to pole gradient in sea surface temperatures of approximately 15 degrees c during this interval and by extrapolation we suggest that polar waters were generally warmer than 20 degrees c during the middle cretaceous approximately 90 million years ago

seismic stratigraphy and subsidence analysis of the southern brazilian margin campos santos and pelotas basins abstract the campos santos and pelotas basins have been investigated in terms of 2d seismo stratigraphy and subsidence the processes controlling accommodation space e g eustacy subsidence sediment input and the evolution of the three basins are discussed depositional seismic sequences in the syn rift barremian to the drift holocene basin fill have been identified in addition the subsidence uplift history has been numerically modeled including i sediment flux ii sedimentary basin framework iii relation to plate tectonic reconfigurations and iv mechanism of crustal extension although the initial rift development of the three basins is very similar basin architecture sedimentary infill and distribution differ considerably during the syn rift sag to the drift basin stages after widespread late aptian early albian salt and carbonate deposition shelf retrogradation dominated in the campos basin whereas shelf progradation occurred in the santos basin in the tertiary these basin fill styles were reversed since the paleogene shelf progradation in the campos basin contrasts with overall retrogradation in the santos basin in contrast long term cretaceous paleogene shelf retrogradation and intense neogene progradation characterize the pelotas basin its specific basin fill and architecture mainly resulted from the absence of salt deposition and deformation these temporally and spatially varying successions were controlled by specific long term subsidence uplift trends onshore and offshore tectonism in the campos and santos basins affected the sediment flux history distribution of the main depocenters and occurrence of hydrocarbon stratigraphic structural traps this is highlighted by the exhumation and erosion of the serra do mar serra da mantiqueira and ponta grossa arch in the hinterland as well as salt tectonics in the offshore domain the pelotas basin was less affected by changes in structural regimes until the eocene when the andean orogeny caused uplift of the source areas flexural loading largely controlled its development and potential hydrocarbon traps are mainly stratigraphic

structural evolution of the orange basin gravity driven system offshore namibia the orange basin records the development of the late jurassic to present day volcanic rifted passive margin of namibia regional extension is recorded by a late jurassic to lower cretaceous syn rift megasequence which is separated from a cretaceous to present day post rift megasequence by the late hauterivian ca 130 ma break up unconformity the late cretaceous post rift evolution of the basin is characterized by episodic gravitational collapse of the margin gravitational collapse is recorded as a series of shale detached gravity slide systems consisting of an up dip extensional domain that is linked to a down dip zone of contraction domain along a thin basal detachment of turonian age the extensional domain is characterized by basinward dipping listric faults that sole into the basal detachment the contractional domain consists of landward dipping listric faults and strongly asymmetric basinward verging thrust related folds growth stratal patterns suggest that the gravitational collapse of the margin was short lived spanning from the coniacian ca 90 ma to the santonian ca 83 ma structural restorations of the main gravity driven system show a lack of balance between up dip extension 24 km and down dip shortening 16 km gravity sliding in the namibian margin is interpreted to have occurred as a series of episodic short lived gravity sliding between the cenomanian ca 100 ma and the campanian ca 80 ma gravity sliding and spreading are interpreted to be the result of episodic cratonic uplift combined with differential thermal subsidence sliding may have also been favoured by the presence of an efficient detachment layer in turonian source rocks

evidence of a widespread paleo pockmarked field in the orange basin an indication of an early eocene massive fluid escape event offshore south africa abstract the detailed interpretation and seismic attribute analysis of 2d and 3d seismic reflection data sets revealed the presence of a distinct seismic horizon displaying circular and elongated depressions in early eocene sediments along the northwestern slope of the orange basin on the southwest african margin their occurrence within a single seismic horizon indicates a contemporaneous formation process we interpret the depressions as paleo pockmarks which occur as single pockmarks predominantly nne ssw aligned pockmark trains and coalescing elongated pockmarks the paleo pockmarked surface covers an area of 2800 km 2 and has been mapped by integrating the 3d dataset with industrial 2d seismic grids the spatial and temporal occurrence of the paleo pockmarks is associated to two episodes of rapid sedimentation during the late maastrichtian to paleocene comprising fine grained shelf eroded material we propose that they were formed during the early eocene by the seepage of both 1 overpressured pore fluids from disequilibrium compaction of upper maastrichtian sediments and 2 thermogenic gas from underlying cretaceous mature source rocks we show that the location of the pockmarks is controlled by the underlying faults system of the paleo slope the paleocene eocene transition is known for its series of hyperthermal events characterized by negative excursions of the carbon isotope curve the early eocene widespread event of fluid and thermogenic gas escape identified in this work can be an example of the type of carbon release processes occurring in hydrocarbon prone sedimentary basins during this time

major element analysis of cyclic black shales paleoceanographic implications for the early cretaceous deep western tethys lower cretaceous sediments are frequently characterized by a well expressed cyclicity while the processes influencing environments above the carbonate compensation depth ccd are reasonably well understood almost nothing is known about the deep ocean cretaceous sub ccd sediments from the tethys and atlantic oceans typically show rhythmic black green shale successions to gain insight into the nature of these black green shale cycles we performed detailed geochemical analyses x ray fluorescence rock eval and reactive iron analysis on a 3 m long section of latest aptian age the major element distribution of the analyzed shale sequence indicates a periodic change from a high productivity and well oxygenated green shale mode to a low productivity oxygen deficient black shale mode it is proposed here that the preservation of organic matter was dependent on the strength of salinity driven deepwater generation furthermore the data show that the corg content covaries with changes in the detrital composition therefore we hypothesize that tethyan deepwater circulation was sensitive to changes in the monsoonal system time series analysis suggests that these changes are periodic in nature although we are currently unable to prove that the dominant periodicity is related to the precession component of the milankovitch frequencies

sediment supply to the orange sedimentary system over the last 150 my an evaluation from sedimentation denudation balance the south african plateau is bordered by passive margin basins preserving the terrigeneous sediment produced during onshore erosion as such these basins potentially provide a record of the variation in onshore elevation and relief over time here we bring new constraints on the uplift and erosion of the southern african plateau over the last 150 ma from the perspective of the stratigraphic architecture of these basins we review published data to quantify the terrigeneous supply eroded in the drainage area and preserved in the basins the novel aspect of our approach is the integration of the evolution of the whole domain in sedimentation i e not only the platform as well as the onshore eroding region along the south african and namibian atlantic margins we determined the long term signal of sedimentary supply at the 10 ma scale from 3d mass balance calculations comparing sedimentary volumes deduced from offshore isopach maps with a particular attention to associated uncertainties on one hand and denudation volumes deduced from thermochronology data on the other we show that in this case the volumes of denudation and of sediments preserved in the basin are consistent both in total and incremental amount the latter is therefore a reliable estimation of the denudation in the associated continental domain subject to certain assumptions such as the drainage area over time the long term signal of sedimentary supply of the south african and namibian atlantic margins shows that the major part of the denudation occurred during the post rift lower cretaceous and the upper cretaceous and subsequently decreased this is not the expected behavior for the simple relaxation of topography acquired during rifting that would show a progressive decrease of the sediment supply over time this suggests a significant rejuvenation of relief during the upper cretaceous 50 ma after the rifting event if erosion is directly linked to elevation or relief variation this implies 2 generations the first before break up i e before 130 ma presumably during rifting and the second during the upper cretaceous

orbitally induced limestone marlstone rhythms in the albian cenomanian cismon section venetian region northern italy sedimentology calcareous and siliceous plankton distribution elemental and isotope geochemistry abstract a multidisciplinary study of the upper albian cenomanian portion of the cismon section venetian region northern italy was undertaken in order to characterize the cyclic alternations of carbonate rich and carbonate poor layers and to investigate their possible origin and cyclic patterns limestone semicouplets are characterized by abundant radiolarians and micarbs micron sized calcitic fragments common planktonic foraminifera strong bioturbation good oxygenation as expressed by the mn and v v ni parameters high si al ratio low k al in the absence of pyrite and organic matter the marlstone semicouplets are on the contrary frequently laminated rich in pyrite and organic matter with common fish remains in the absence of radiolarians the mn and v v ni parameters indicate that the marlstone deposition occurred in dysoxic to anoxic conditions which allowed the preservation of organic matter the low si al ratio and high k al ratio the latter related to abundant feldspars indicate that marlstones received a substantial contribution from a terrigenous source the abundance of biogenic silica suggests that during limestone deposition surface waters were rather fertile due to an efficient recycling of nutrients from deeper waters the enhanced fertility was coupled with an efficient current system at the welloxygenated sea floor which prevented the organic matter from accumulating during marlstone deposition surface waters were less fertile than in the limestone semicouplets but still within the mesotrophic spectrum as indicated by the presence of fertility index species among calcareous nannofossils whereas the bottom waters were oxygen depleted to accommodate fertility at the surface with stagnation at the bottom the scenario implies an enhanced stratification caused by an increase in freshwater runoff monitored by 18o depleted values in the meantime the water discharge supplied the nutrients to the surface waters necessary for the calcareous nannofossils to proliferate the cyclic organization of limestone marlstone couplets as revealed by power spectral analysis seems to represent climatic cycles the alternating conditions are 1 efficient mixing highly fertile surface and near surface waters and a well oxygenated seafloor to 2 enhanced water runoff or decreased evaporation or both and oxygen depleted bottom waters we suggest that this deterministic sedimentary cyclicity could be explained by periodic orbital climatic cycles in tune with the milankovitch cycles within this scenario the latest cenomanian bonarelli level is an exceptional event marked by a large positive 13c spike it is characterized by high to very high fertility conditions in surface waters caused by increased upwelling that engendered high nutrient levels in near surface waters the enhanced production of organic matter was in excess for the oxygen available at the bottom floor thus allowing the organic matter to accumulate in the sediments

tectonic signatures in arid continental basins an example from the upper miocene pleistocene calama basin andean forearc northern chile abstract the calama basin a transtensional basin situated within the andean forearc and its extension to the pampa del tamarugal central depression is filled with 700 m of sediments detailed sedimentary logging and 40ar 39ar dates from interbedded volcanic deposits have established a lithostratigraphic and chronostratigraphic framework for the upper 95 m thick upper miocene pleistocene succession five facies associations fluvio lacustrine diatomite palustrine carbonate massive anhydrite fluvial and alluvial fan have been identified within this part of the basin fill the following events are recognised in the basin evolution 1 deposition of diatomaceous lacustrine sediments in two separate depocentres in the pampa del tamarugal and the central calama basin with alluvial fan accumulation along the basin margins at 8 ma 2 localised deformation within the central calama basin between 7 and 6 ma but continuous sedimentation elsewhere 3 palustrine carbonate deposition across the entire area from 6 to 3 ma except in the pampa del tamarugal where diatomaceous sedimentation continued until 5 ma before being replaced by palustrine carbonate deposition 4 widespread folding after 3 ma and entrenchment as the drainage system cut down to reach a new base level caused by the rio loa breaching the coastal cordillera and reaching the sea localised lacustrine deposition in topographic lows or tectonically created dams also occurred during this period comparison with similar age facies elsewhere in northern chile has shown changes such as these to be localised and diachronous indicating that tectonic controls on accommodation space and drainage patterns are the primary influence on sedimentation in this tectonic setting the lacustrine deposits do not appear to be a reliable recorder of low frequency climatic change this is exemplified by the change from lacustrine to palustrine deposition at 6 5 ma which would appear to record a drying of the climate however a synchronous change from ephemeral alluvial fan deposition to more permanent fluvial deposition at the basin margins indicates increased run off

permanent quaternary hyperaridity in the negev israel resulting from regional tectonics blocking mediterranean frontal systems although various episodes of wet quaternary climates have been suggested in studies of the negev desert here we demonstrate that reg soils developed on flat alluvial surfaces and sensitive to minor changes in precipitation indicate that the southern negev has been permanently hyperarid at least since the middle pleistocene the wetter episodes were restricted to the northern negev currently mildly arid gypsic saline reg soils that de veloped on stable surfaces during 200 k y in the southern negev are cumulative and polygenetic but none exhibits even the weakest development of calcic horizons under current climate conditions in the negev calcic soils are widespread in areas with rainfall 80 mm yr 21 the fact that the hyperarid 50 mm yr 21 southern negev lacks any calcic horizons indicates that it never experienced an average rainfall of 80 mm yr 21 during the middle late pleistocene whereas the northern negev was wetter this negev dichotomy is explained by the main physical features controlling rainfall over the negev i e the southward decreasing depth of the atmospheric boundary layer with distance from the mediterranean and the altitude of the central negev highland 1000 m the interaction between these two features often prevents the passage of rain clouds into the southern negev the holocene and pleistocene gypsic salic soil distribution across the negev closely matches the current circumstances we suggest that hyperaridity has prevailed over the southern negev since the last stages of the uplift of the central negev highland in the late pliocene early pleistocene

three dimensional subsidence analysis and gravity modelling of the continental margin offshore namibia summary seismic re ection pro les and gravity anomaly data have been used to determine the structure and evolution of the namibian continental margin in comparison to other margins the gravity anomaly at the namibian margin shows a number of distinctive features it lacks an oishore gravity low and despite the presence of up to 9 km of sediments the gravity high is displaced landwards of the maximum sediment thickness in an attempt to explain these features the early cretaceous recent stratigraphic record is analysed using a combined 3 d backstripping and gravity modelling technique that enables constraints to be placed on the long term mechanical properties of the lithosphere a neutral depth of necking and a relatively high exural rigidity or equivalent elastic thickness te o f 25 km can explain part of the anomaly at least in the south of the margin however large residual anomalies exist that can be reduced by the presence of pre rift low density sediments within the zone of maximum stretching south of 22 0 s high density volcanic material at the eastern pinch out of the lower rift sequence and a lateral sediment density variation across the shelf slope and rise in addition the presence of a magmatic body at the base of the crust provided that the exural strength of the margin is high signi cantly improves the t between observed and calculated gravity anomalies recovered stretching factors enable predictions of the geometry of the moho to be made palaeobathymetry is estimated along the entire margin through time by comparing the subsidence determined from backstripping to that predicted by simple models of rifting these estimates are within the errors of observed palaeowater depths where well control exists the requirement of high te rather than te that increases with time suggests that stretched continental crust extends oishore for some distance and the continent ocean boundary occurs in relatively deep water near magnetic anomaly m4 reconstruction of the gravity eld of the south atlantic at 100 ma illustrates that in terms of their gravity signature the south atlantic margins are asymmetric

basin forming processes and the deep structure of the campos basin offshore brazil abstract this paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins the events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors shear zones thin layers of intrusive rocks underplated material gneissic banding fluids and heterogeneities in the crust and mantle several types of sedimentary basins have been recognized with the aid of deep seismic reflection data basins formed along active master faults basins formed along inactive faults basins with no fault control basins formed by low angle detachment faults and basins formed by pervasive pure shear or an approximately pure shear where the lower crust has been locally extended by a different amount than the upper crust the mechanisms of basin forming processes recognized in deep seismic profiles are a simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle b thinning of the lower crust and mantle by ductile processes convective mechanisms or flow in the mantle and the upper crust by faulting resulting in an overall pure shear and c thinning of the lower crust associated with phase changes in the mantle or non conservation of mass two end member models of basin formation involving lithospheric stretching may be recognized in the south atlantic simple shear stretching where only the upper crust is involved in the process and mantle uplift is offset relative to the basin and pure shear stretching where uplift of the mantle vertically balances the basin fill the first mechanism has been suggested for the onshore tucano basin the data obtained for the offshore campos basin suggests regional lithospheric stretching and crustal thinning with moho uplift compensating for sediment accumulation in the basin depocentre complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the campos basin

warm saline bottom water in the ancient ocean considerable isotopic evidence indicates that oceanic bottom water was much warmer in the geological past a simple convection model driven by multiple turbulent buoyant plumes and observations of present deep water formation have led to the development of a theory for the formation of warm saline bottom water it is suggested that changes in the size and configuration of marginal seas in net evaporation zones due to lithospheric plate motions and eustatic sea level change caused these seas to become sources of warm saline bottom water evidence from the palaeotemperature record and palaeogeography during the late cretaceous is used to reinforce the hypothesis climatic and chemical consequences of the formation of warm saline bottom water are discussed

late pliocene age for the atacama desert implications for the desertification of western south america the atacama desert forms one of the major hyperarid deserts of the world previous studies suggest that desertification commenced at 14 ma during global climate desiccation sedimentologic data from middle miocene to upper pliocene successions in the modern atacama desert indicate that a semiarid climate persisted from 8 to 3 ma punctuated by a phase of increased aridity at ca 6 ma as such hyperaridity did not commence until the late pliocene implications are 1 that the rain shadow generated by the andean cordillera has had a minor influence on climate change and 2 that the upwelling north flowing cold humboldt current although important in establishing the generally arid climate of western south america did not control the shift to hyperaridity the formation of the hyperarid atacama desert in the late pliocene accompanied the development of the current phase of aridification in the sahara and namib deserts and is attributed to a phase of global climate cooling

late quaternary paleohydrology of the central atacama desert lat 22 24 s chile in northern chile precipitation in the high andes 3500 m recharges groundwater systems that flow down the pacific slope and feed large aquifers in the hyperarid atacama desert wetlands which are often found along the base of the andes mark locations where the water table intersects the land surface at these locations paleo wetland deposits which are present as terraces between 3 and 20 m above modern wetlands record past water table heights along the andean front and are used to reconstruct changes in groundwater discharge paleo wetland deposits in the central atacama desert lat 22 24 s record an episode 15 4 9 ka of high water tables followed by an episode 8 3 ka of moderately high water tables elevated water tables result from increased groundwater discharge and ultimately from enhanced recharge in the andes the concordance of results from three separate hydrologic systems suggests that changes in groundwater discharge and recharge are regional and reflect climatic fluctuations this interpretation is supported by close agreement with other paleoclimatic records in the region the periods of greater groundwater discharge were separated by episodes 9 8 and 3 0 ka of significant groundwater lowering and stream incision implying greatly diminished discharge the central atacama and andes lat 22 24 s receive precipitation mainly from moist air masses transported from the amazon basin by the south american summer monsoon sasm increases in groundwater recharge are therefore thought to reflect an increase in the frequency and or moisture content of sasm air masses crossing the andes fluctuations in sasm precipitation have previously been linked to summer insolation in the southern hemisphere the wettest period in the central atacama 15 4 9 ka however coincides with a minimum in austral summer insolation at 10 ka suggesting that regional summer insolation is not a dominant influence on the sasm instead intensification of the sasm may be linked to extraregional forcings such as the walker circulation

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

andean uplift and climate change sedimentological data indicate that a semi arid arid climate prevailed across the central andes from 15 ma to 4 ma between 4 and 3 ma a switch to hyperaridity occurred along the western margin of south america palaeoaltitude data suggest that a substantial proto central andean mountain range was in place between 15 and 9 ma these data support the idea that the andean rain shadow existed by 15 ma and that it reinforced the pre existing climatic regime rather than changing it the change to hyperaridity in western south america is attributed to a combination of global climate cooling and enhanced upwelling of the humboldt current generated by closure of the central american seaway between 3 5 and 3 ma and not to the andean rain shadow

elevated primary productivity of calcareous nannoplankton associated with ocean anoxic event 1b during the aptian albian transition early cretaceous 1 previous isotopic investigations of aptian albian oceanic anoxic event oae 1b from the western north atlantic blake nose posited that increased sea surface temperatures and decreased salinity led to stratification of the upper water column resulting in lowered dissolved oxygen and enhanced organic matter preservation we examined calcareous nannofossils from the same site in the western north atlantic blake nose to evaluate changes in surface water conditions prior to during and after the aptian albian oae1b the results of our analysis conflict somewhat with the previous interpretation that oae1b at blake nose was linked to water column stratification the signal from calcareous nannofossils indicates that in the late aptian prior to oae1b oligotrophic conditions in the surface waters were replaced by mesotrophic conditions that persisted throughout the oae event and into the early albian we speculate that increased surface water productivity facilitated to some degree the development of oae1b at blake nose

low productivity and slow deep water circulation in mid cretaceous oceans paleofertility estimates are derived from organic carbon accumulation rates determined in nonbioturbated mid cretaceous deep sea deposits using organic carbon preservation factors observed in laminated sediments from anoxic holocene environments the estimated mid cretaceous primary production rates are as much as an order of magnitude lower than those measured in today s oceans an evaluation of dissolved oxygen budgets using a simple oceanic circulation model indicates very low deep water renewal rates in mid cretaceous oceans 28 references 2 tables 4 figures

the great west african tertiary coastal uplift fact or fiction a perspective from the angolan divergent margin 1 we explore exhumation in the coastal kwanza basin by combining analyses of tertiary hiatuses and apatite fission tracks planktonic biozones show five major hiatuses in the oligo miocene and plio pleistocene between gaps oligo miocene strata accumulated under marine conditions a marine setting refutes the idea of a massively raised coastal plateau in the mid tertiary marine conditions continued until 5 ma fission track data suggest three thermal events 150 ma during rifting and volcanism 100 70 ma during shortening and volcanism and 20 10 ma during exhumation tertiary uplift was spatially highly variable for the kwanza basin we infer that tertiary uplift on the west african margin is indeed a fact but that estimates of uplift timing and size are unreliable when extrapolated to adjoining areas massive uplift 2000 4000 m of the precambrian craton had little structural effect in the outer basin instead minor uplifts on the shelf drove late tertiary deformation on the slope

paleoenvironmental controls on the morphology and abundance of the coccolith watznaueria britannica late jurassic southern germany the coccolithophore species watznaueria britannica is dominant in middle upper jurassic calcareous nannofossil assemblages and presents morphological variation including different coccolith size shape and length of the central area and of the bridge six morphotypes can be recognized in the polarizing light microscope the aim of this work is to better understand the morphological variability of w britannica and determine if this variability is controlled by paleoecological factors in order to investigate the potential paleoecological controls on w britannica morphology and abundance we carried out a biometric study on a restricted temporal interval the late oxfordian in the swabian alb southern germany characterized by increasing carbonate production linked to climatic changes the balingen tieringen section where previous works on sedimentology nannofossil assemblage composition and 18o and 13c analyses were performed was selected for this study the variations in morphology and abundances ofw britannicawere studied on 40 samples of the balingen tieringen section presenting variable lithologies and calcium carbonate contents for each level seven biometric parameters coccolith length width and ellipticity central area length width and ellipticity and central area proportion with respect to the coccolith were measured or calculated on digitally captured images of the first 100 w britannica coccoliths observed in the light microscope the relationships between the different biometric variables were described using bivariate and principal component analyses biometric parameters and principal component factors extracted from nannofossil assemblages as well as other paleoenvironmental proxies were investigated using regression and their stratigraphic trends were compared principal component analysis of the six biometric variables 3938 measurements on w britannica coccoliths shows a reduced morphological variability compared to a significant size gradient an allometric trend recognized on the total placolith and on the central area within thew britannica assemblages suggests that the different morphotypesmay represent intra specific variability rather than different species the general trend through late oxfordian shows an increase in size ofw britannica coccoliths mainly driven by an increase in the contribution of the largemorphotypes increasing placolith size is associated with drier and warmer climatic conditions during the latest oxfordian

development of volcanic passive margins two dimensional laboratory models 1 continental breakup above an anomalously hot mantle may lead to the development of volcanic margins volcanic margins are characterized by 1 thick seaward dipping lava flow sequences 2 central intrusive complexes associated with dyke swarms parallel to the coast and 3 high seismic velocity bodies in the lower crust attributable to magma underplating a conceptual model for volcanic margins development has recently been proposed based on onshore studies of the greenland margins and the british tertiary igneous province it is proposed that the long lived central intrusions are genetically linked to underlying persistent zones of mantle fusion these localized melting domains or soft spots equivalent to small mantle diapirs may locally soften the extending continental lithosphere the low viscosity diapirs would 1 localize tectonic strain and 2 feed the volcanic margin with magma thus such soft spots can control the along strike magmatic and tectonic segmentation of volcanic margins recent geophysical investigations appear to show that the along strike structure of volcanic passive margins is compatible with such a segmentation process here we present a set of scaled experiments designed to study how such localized rheological heterogeneities in the sub moho mantle may have a mechanical effect on continental breakup four layer models were constructed using sand and silicone putties to represent the brittle and ductile layers of both crust and mantle the soft spots are simulated by low viscosity silicone putty emplaced within the brittle material at the scale of the entire breakup zone the soft spots display an oceanic type strength profile defining low strength zones where continental breakup is initiated the rift orientation and segmentation are strongly controlled by the distribution of the low viscosity heterogeneities rather than by the direction of regional extension the experiments are compared with the geometry and segmentation of the onshore part of the greenland volcanic margins

kinematic evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we present an updated series of kinematic reconstructions of the major plates around the tethys from the atlantic ocean to the pamirs between the early jurassic and the present this set is used elsewhere as a basis for paleogeographic maps of the entire region the problems related to the positions of the continents in the lower triassic are also discussed no direct analyses of magnetic anomalies and fracture zones in the atlantic have been made rather all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors the reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies except for the jurassic cretaceous boundary and the cretaceous tertiary boundary which correspond to interpolated positions paleolatitudes have been drawn using the study of westphal et al 1986 an attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching the resulting relative vector of motions along the northern boundary of the tethys shows a significant change 80 m y ago left lateral motion with compression dominates before whereas right lateral motion with compression dominates after to the east rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the late jurassic late cretaceous eocene and post middle miocene to the west north of apulia on the contrary the motion rate has not changed significantly since the early cretaceous and is close to 1 cm yr as an average these rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the africa eurasia pole of rotation and seem to be mostly governed by the tectonics of the tethys plate boundary

orbitally modulated black shale deposition in the upper albian amadeus segment central italy a multi proxy reconstruction abstract the upper albian amadeus segment which falls within oceanic anoxic event 1c exhibits cyclical alternations of marls black shales and carbonate rich beds that record evidence of orbital climate and sedimentary dynamics a combined micropalaeontological planktic and benthic foraminifera and palynomorphs and geochemical stable isotopes clay mineralogy and major element distribution investigation allowed recognition of a remarkable influence of continental material within the black shales moreover the palaeoproductivity record is characterised by a non linear response to the orbital forcing the micropalaeontological and geochemical record suggests that upper albian bedded cycles in the marne a fucoidi formation resulted from interplay of changes in atmospheric and hydrological parameters of the western tethys and its drainage basin in particular deposition of the black shales occurred during precession minima in times of increased precipitation enhanced continental runoff and stratification of the upper water column in this respect the black shales of the amadeus segment share similarities with the neogene eastern mediterranean sapropels

oxygen 17 excesses of the central namib gypcretes spatial distribution abstract we present here sulfate oxygen isotopic data 72 samples with both 18o and 17o systematically collected from the central namib desert surface soils from two shore inland west east transects exhibit a gradual increase in the sulfate oxygen 17 excess 17o 17o 0 52 18o until at ca 70 km inland where no continuous gypcrete deposit is observed further east inland the oxygen isotopic compositions for water soluble sulfates extracted from soils and gypcretes range from 8 3 to 13 3 and 0 06 to 1 11 for 18o and 17o respectively the lateral pattern is similar to what has been seen in the cold deserts of the antarctic dry valleys however unlike the dry valleys no discernible correlation is found between 18o and 17o or between the depth of soil horizon and 17o in the namib possible explanations include a relatively smaller component of dimethylsulfide dms derived sulfate in the total gypsum deposits and or more active surface processes e g flooding and leaching in the central namib desert than in the antarctic cold deserts although current state of knowledge is insufficient to delineate quantitatively the sulfate contributions from different sources and reactions the measurement of sulfate 17o does identify an unmistakable atmospheric sulfate component and provides additional independent information regarding sources and reactions

origin of rhythmic albian black shales piobbico core central italy calcareous nannofossil quantitative and statistical analyses and paleoceanographic reconstructions 1 the mid cretaceous 120 90 ma was a time of excess atmospheric co2 greenhouse climate and widespread o2 deficiency in the ocean the albian is punctuated by brief intermittent episodes of anoxia dysoxia recorded as cyclic black shales in the western tethys and atlantic oceans the albian section of the piobbico core central italy contains 30 m 10 ma of rhythmic black shales that were sampled at a high resolution and examined for calcareous nannofossil assemblages and c and o stable isotopes unlike oceanic anoxic events productivity was not the primary factor controlling the deposition of albian rhythmic black shales it is suggested that during warm humid climatic cycles higher temperatures and or increased precipitation and runoff produced density stratification at a regional scale recurrent lowered salinity in the early albian and warming in the late albian are credited with causing development of a pycnocline resulting in slower rates of deep water renewal and consequent anoxia

consolidation and break up of the south american platform in southeastern brazil tectonothermal and denudation histories the different tectonic stages that occurred at the end of the proterozoic and during the phanerozoic have an important bearing on the tectonothermal history of the south american platform and its consolidation geochronological data u pb monazite 40ar 39ar whole rock and apatite fission track analysis from precambrian rocks of the southeastern brazilian coastline permit the modeling of a long term thermal history of the crust and constrain variable denudation rates using these data a temperature time diagram reflects a period of accelerated exhumation during the end of the brasiliano orogeny followed by long stability and reactivation of the platform during the rifting phase of the south atlantic ocean u pb zircon and monazite blocking temperature of ca 650 c data from a series of igneous bodies suggest that a tangential and transpressional tectonic regime occurred between 625 and 610 ma during the following escape tectonics between 610 and 590 ma the exhumation process indicates cooling rates of ca 12 c ma 40ar 39ar biotite ages between 540 and 510 ma ca 300 c and a corrected fission track age on apatites 100 c of 480 ma indicate an exhumation event related to block tectonics with huge vertical displacement along shear zones a long stabilization phase with low exhumation and cooling rate around 0 25 c ma was recorded from the cambro ordovician to the mesozoic at 65 ma an acceleration of the exhumation through denudation and reworking of the south american surface with cooling rate of 1 5 c ma is observed the uplift of the mantiqueira and serra do mar mountain ranges along the southeast brazilian coastline works as a climatic barrier provoking lateral erosional processes causing long term scarp retreat combined with intense but progressive denudation towards the continent a denudation of 2 5 to 4 km was calculated for such processes this lateral retreat of escarpments and flexural response can provide important insights regarding marginal isostatic uplift and the evolution of offshore sedimentary basins of southeast brazil

evidence for the development of the andean rain shadow from a neogene isotopic record in the atacama desert chile abstract varying ages from triassic to pliocene have been proposed for the onset of hyperaridity in the atacama desert the exact timing for the initiation of hyperaridity is critical for determining potential causes which range from regional effects of global cooling to andean uplift above elevations conducive to extreme rain shadows analysis of the stable isotopic composition of lower miocene quaternary 21 0 015 ma palustrine and lacustrine carbonates in the calama basin reveals extreme changes in their oxygen and carbon isotopic composition during the miocene limestone 18 o values increased by 5 from middle to late miocene ranging from 5 5 at 12 ma to 1 at 6 ma carbon isotopic values increase by 9 over the neogene from average values of 3 at 21 ma to 3 at 12 ma and reaching a maximum of 6 at 5 ma the increase in oxygen isotopic values occurred over a time span in which the catchment area of the basin experienced significant uplift causing the 18 o value of precipitation to become more negative we attribute the shift towards higher 18 o values to enhanced evaporative enrichment both of soil water or snow prior to infiltration and within shallow lakes or wetlands prior to carbonate precipitation the large increase in 13 c values was likely caused by a transition from a vegetated landscape influenced primarily by soil respired co 2 to a landscape largely devoid of vegetation and influenced by atmospheric and volcanic co 2 isotopic values of palustrine carbonates therefore indicate that hyperaridity commenced in the calama basin during the middle to late miocene in agreement with other paleoclimatic records from the basin the cause for the onset of this climate change is thought to be due to the development of a strong andean rain shadow associated with the uplift of the andes to mean elevations 2 km