neoproterozoic snowball earth simulations with a coupled climate ice sheet model ice sheets may have reached the equator in the late proterozoic era 600 800 myr ago according to geological and palaeomagnetic studies possibly resulting in a snowball earth but this period was a critical time in the evolution of multicellular animals posing the question of how early life survived under such environmental stress here we present computer simulations of this unusual climate stage with a coupled climate ice sheet model to simulate a snowball earth we use only a reduction in the solar constant compared to present day conditions and we keep atmospheric co2 concentrations near present levels we find rapid transitions into and out of full glaciation that are consistent with the geological evidence when we combine these results with a general circulation model some of the simulations result in an equatorial belt of open water that may have provided a refugium for multicellular animals

circulations and rainfall on the lee side of the island of hawaii during harp using aircraft data and portable automated mesonet pam data during the hawaiian rainband project harp 11 july 24 august 1990 the circulation on the lee side of the island of hawaii is analyzed the largest temperature deviations 2 3 k and negative mixing ratio deviations 2 2t o2 4gk g 21 are found in the northwestern and southwestern leeside areas of the island because of the descending trade winds aloft in the lee of low 2 km mountain ridges and tops rainfall in these areas is much less compared to the central leeside coast e g kona coast in these areas the sea breeze duration is short compared to the kona coast because of the presence of opposing trade winds the wind speed is greater because of larger land sea thermal contrast during the stronger trade wind days adiabatic descent in the lee is more significant and the lcl is higher as a result in the northern and southern leeside areas localized trade wind rain showers driven by local circulations occur mainly under weak trades in the kona area the mean surface winds are calm behind the massive mountains with a daytime upslope onshore and nighttime downslope offshore flow along the axis of the return flow in the large wake off the kona coast the water vapor content is higher than the ocean upstream possibly caused by the low level convergence between the counterrotating eddies along the wake axis along the kona coast the rainfall in the afternoon hours continues in the early evening and reaches the daily maximum after the onset of the land breezes possibly because of the lifting of stronger moist return flow by the land breeze front under stronger trade winds the return flow would be stronger because of stronger island blocking more rainfall occurs there when trades are stronger

sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

a case study of trade wind rainbands and their interaction with the island induced airflow abstract a case study of trade wind rainbands observed on 22 august 1990 during the hawaiian rainband project is presented it shows that the interaction between the morning rainbands and the island induced airflow is important for the evolution of the rainbands in the early morning of 22 august there are two convective periods 0400 0600 and 0700 0900 hst hawaii standard time on the windward side of the island of hawaii for both periods preexisting rain cells are observed in the trade wind flow at least 40 km upstream of the island and move westward toward the island at night and in the early morning the offshore flow opposes the trade winds resulting in a convergent region over the area immediately upstream of the island as the first group of rain cells 0400 0600 hst moves toward the island the low level convergent airflow provides a favorable kinematic background for the enhancement of the coming rain cells these rain cells merge in the convergent zone and become a well defined rainband

tectonostratigraphic evolution of the northern porcupine basin irish atlantic margin during the late jurassic early cretaceous implication for a regional compressional event abstract the conventional interpretation of the jurassic lower cretaceous succession in the porcupine basin suggests an extensional setting with progressive deepening of the basin however well data show a prominent gap of several million years between the upper jurassic and lower cretaceous a data base of 15 key wells and approximately 5 000 km of seismic reflection data were examined in the northern porcupine basin in order to understand the nature controls and mechanisms of this unconformity seven seismic markers constrained by well data are mapped it is shown that during the late jurassic possibly the oxfordian kimmeridgian the basin experienced extension and synrift deposition during the latest jurassic earliest cretaceous possibly the tithonian early berriasian a series of north trending structural highs and lows developed and extensive areas in the northern porcupine basin experienced folding uplift and erosion evidence from the study suggests that compression uplift and erosion played an important role in the shaping of the depositional and structural architecture of the basin and caused formation of the regional base cretaceous unconformity in the northern basin it is suggested that the deformation in the northern porcupine basin during the latest jurassic earliest cretaceous may be related to the initial closure of the alpine tethys during the late tithonian this tectonic event may also have resulted in compressional deformation and formation of the base cretaceous unconformity elsewhere in western europe

the effects of the island of hawaii on offshore rainband evolution abstract the diurnal cycle of airflow on the island of hawaii and the structure of the low level flow separation line between the island induced offshore flow and incoming trade winds are reasonably well understood this study examines the formation and organization of nocturnal and early morning offshore hawaiian rainbands the flow patterns associated with these rainbands and their relationship to the flow separation line this investigation uses the complete radar and aircraft dataset from the 1990 hawaiian rainband project the formation and evolution of nocturnal and early morning rainfall on the windward side of the island were found to be related to four major factors a the existence or absence of trade wind cloud patches arriving from the northeast pacific b the effect of island blocking on the trade wind flow as it approaches the island c the low level convergence and thermal and moisture stratification near the flow separation line and d the position of the flow separation line con

co2 levels required for deglaciation of a near snowball earth geologic evidence suggests that in the late neoproterozoic 600 ma almost all land masses were glaciated with sea level glaciation existing even at the equator a recent modeling study has shown that it is possible to simulate an ice covered earth glaciation with a coupled climate ice sheet model however separate general circulation model experiments suggest that a second solution may exist with a substantial area of ice free ocean in the tropics although 0 1 to 0 3 of an atmosphere of co 2 300 to 1000 x is required for deglaciation of a snowball earth the exit co 2 levels for an open water solution could be significantly less in this paper we utilize a coupled climate ice sheet model to demonstrate four points 1 the open water solution can be simulated in the coupled model if the sea ice parameter is adjusted slightly 2 a major reduction in ice volume from the open water equatorial ice solution occurs at a co level of about 4x present values about two orders of magnitude less than required for exit from the hard snowball initial state 3 additional co 2 increases are required to get fuller meltback of the ice and 4 the open water solution exhibits hysteresis properties such that climates with the same level of co 2 may evolve into either the snowball open water or a warmer world solution with the trajectory depending on initial conditions these results set useful targets for geochemical calculations of co 2 changes associated with the open water solution

a multiwinter analysis of channeled flow through a prominent gap along the northern california coast during caljet and pacjet abstract experimental observations from coastal and island wind profilers aircraft and other sensors deployed during the california land falling jets experiment of 1997 98 and the pacific land falling jets experiment of 2000 01 2003 04 were combined with observations from operational networks to document the regular occurrence and characteristic structure of shallow 400 500 m deep cold airstreams flowing westward through california s petaluma gap from the central valley to the coast during the winter months the petaluma gap which is the only major air shed outlet from the central valley is 35 50 km wide and has walls extending at most a modest 600 900 m above the valley floor based on this geometry together with winter meteorological conditions typical of the region e g cold air pooled in the central valley and approaching extratropical cyclones this gap is predisposed to generating westward directed ageostrophic flows driven by along gap pressure differences two case studies and a fiv

ediacaran δ13c chemostratigraphy of south china abstract we report new 13 c chemostratigraphic data of the ediacaran system in south china using litho and biostratigraphic markers as independent calibrators we combine new and previously published 13 c data to construct a composite 13 c profile for the ediacaran system in south china the composite curve shows in chronostratigraphic order 1 a negative 13 c excursion en1 in the doushantuo cap carbonate that overlies the ghaub age nantuo diamictite 2 a pronounced positive 13 c excursion ep1 in the lower doushantuo formation where doushantuo type acanthomorphic acritarchs red algae and animal embryos first appear 3 a mild negative 13 c excursion en2 in the middle doushantuo formation 4 a positive 13 c excursion ep2 in the upper doushantuo formation where doushantuo type acanthomorphic acritarchs micrometazoans and stem group florideophyte red algae become abundant and diverse 5 a pronounced negative 13 c excursion en3 near the uppermost doushantuo formation 6 a strongly positive 13 c excursion up to 6 3 pdb ep3 in the lower dengying formation 7 a stable 13 c plateau ca 2 5 pdb ei in the middle and upper dengying formation where abundant vendotaenid algae macroscopic ediacaran fossils biomineralized animal tubes and mostly horizontal animal traces occur and 8 a 10 negative 13 c excursion en4 immediately below the ediacaran cambrian boundary the general pattern of this composite 13 c curve is robust despite a considerable amount of variation among the 13 c profiles of different sections this composite 13 c curve provides a first order chemostratigraphic framework for the subdivision and correlation of the ediacaran system in south china

kinematic models of basement cover interaction insights from the malargue fold and thrust belt mendoza argentina abstract we investigate through natural examples from the malargue fold and thrust belt the manner in which the basement structures interact with the cover in a thick skinned fold and thrust belt our work suggests that there is a broad spectrum of basement cover interaction mechanisms with two end members coupled and uncoupled interactions the kind of bonding between basement and cover is related to the propagation to slip ratio p s because high values of this ratio indicate rapid forward propagation of shortening into the cover while low values favour welding between basement and cover structures we analyse the factors controlling the kinematics of basement cover deformation and find that not only the strength of the cover its thickness and its mechanical anisotropy control the p s and basement cover bonding but also the strain rate the thickness of synorogenic strata and the presence of pre existing faults

structural analysis and kinematic evolution of the inverted central south celtic sea basin abstract the central south celtic sea basin shows excellent examples of tectonics and sedimentation relationships as evidenced by detailed geological interpretation of a local grid of seismic profiles these relationships together with balanced restored geological cross sections and block diagrams have been used to unravel the post variscan evolution of this part of the celtic sea basin three sedimentary mega sequences separated by two erosional surfaces have been recognized overlying a deformed palaeozoic basement in this area 1 permo triassic jurassic syn rift mega sequence sedimentation of the lowest part of this succession is strongly controlled by extensional faults whereas the rest of the sequence indicates much less fault control lateral extent of these rocks is also controlled by the disposition of the palaeozoic basement which appears as an elevation in the pembrokeshire ridge area where no sedimentation occurs 2 cretaceous post rift mega sequence 3 tertiary post rift mega sequence the cretaceous and tertiary successions are more extensive than permo triassic jurassic sediments covering the central south celtic sea basin and the pembrokeshire ridge the structural framework displayed by the syn rift and post rift sequences is very different thus the general structure beneath the jurassic cretaceous late cimmerian unconformity consists of a wsw ene trending fault system and related folds this resulted from an extensional phase which led to a sub basin compartmentalization of the area and a subsequent inversional event which caused significant reverse movement along previous extensional structures early cretaceous erosion was responsible for an important and unequal denudation the timing of the switch between extension and inversion cannot be determined due to early cretaceous erosion overlying the jurassic cretaceous unconformity cretaceous sediments are slightly deformed by broad anticlines formed during early tertiary mainly as a result of tightening of previous folds the tertiary sequence shows a subhorizontal disposition responsible for the present day smooth bathymetric relief of the south celtic sea basin

extension tectonics in the wessex basin southern england the permian to cretaceous tectonic evolution of the wessex basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere the gross morphologies of its constituent structures were governed by the location of variscan thrust and wrench faults in the upper and middle crust which suffered extensional reactivation in tensional stress fields oriented approximately nw se several episodes of crustal extension can be resolved in early permian early triassic early jurassic and late jurassic early cretaceous times these were characterized by the rapid subsidence of fault bounded basins and commonly by erosion of adjacent upfaulted blocks superimposed upon the fault controlled subsidence dominant during periods of fault quiescence and becoming increasingly important with time a component of regional subsidence is considered to have a thermal origin this suggests that crustal extension was accompanied by some form of not necessarily uniform lithospheric thinning subsidence analyses assuming local airey isostasy give cumulative crustal extension factors of 20 28 beneath the grabens a more reasonable assumption of regional airey compensation indicates basinwide crustal extension of 13 17 which is consistent with birps offshore deep seismic reflection data

aspects of basin inversion in southern britain the wessex basin of southern england a permian to cretaceous extensional basin can be structurally divided into a set of constituent asymmetrical graben bounded by major e w trending zones of en echelon syn depositional normal faults the graben were inverted in late cretaceous and tertiary times by compressive stresses oriented roughly north south inversion structures fall into two related categories regional upwarps overlie earlier graben depocentres and were formed by bulk shortening of the graben fill superimposed upon and geographically delimiting the regional upwarps are roughly east west trending linear zones of en echelon inversion structures these coincide with the earlier graben bounding faults and typically have the form of monoclinal or periclinal flexures each underlain by a partially reversed normal fault the linear reverse fault monocline inversion structures were a relatively inefficient method of basin shortening because upper crustal faults in the region steepen upward markedly reversal of these faults under compression resulted in a shortening discrepancy at shallow depths locally particularly in south dorset this was overcome by bucklefolding and low angle reverse faulting which increased the amount of shortening attainable across the linear inversion structures elsewhere however the shortening discrepancy was accommodated by bulk shortening and regional upwarp of the graben sedimentary fill this occurred preferentially in those graben containing young poorly lithified and therefore weak sediments thus the early cretaceous depocentres of the weald and channel basins were strongly upwarped with axial uplifts of over 1000 m conversely graben having older more lithified sequences suffered little regional upwarp shortening primarily by fault reversal along the linear inversion structures the amount of crustal shortening which accompanied inversion was considerably less than the earlier crustal extension

structure and quantification of processes controlling the evolution of the inverted ne german basin abstract a ne sw reflection seismic transect crossing the entire ne german basin negb was investigated in detail to study the interaction between tectonics and sedimentation during the multiphase evolution of the region by restoring a geological section across the post rotliegend the kinematic evolution was reconstructed and rates of important geological processes such as erosion or basin shortening were quantified the present day structural style of the negb is the result of several major deformation events 1 regional subsidence accompanied by local thin skinned e w extension during the permo triassic jurassic was followed by basin inversions at the 2 jurassic cretaceous and at the 3 cretaceous tertiary boundary zechstein salt played an important role since mobile zechstein evaporites decoupled the supra salt from the sub salt succession and allowed the transmission of deformation over a large distance of major significance to the late cretaceous structural development was the crustal scale upthrusting of the calfoerde block along the gardelegen fault at the southern basin margin the accompanying horizontal shortening has a minimum value of 9 km and led to the development of thrusts and folds within the post zechstein succession

sulfate oxygen 17 anomaly in an oligocene ash bed in mid north america was it the dry fogs 1 the oxygen and sulfur isotope compositions of sulfate particularly the oxygen 17 anomaly provide clues to the origin of sulfate we report a variable oxygen 17 anomaly for water and acid leached sulfate from an oligocene volcanic ash bed in the northern high plains u s a we find no sulfate o 17 anomaly however in freshly collected ashes from recent eruptions around the world the oligocene paleoclimatic and sedimentary evidence argues against a hyperarid condition in the high plains thus a long term background atmospheric sulfate accumulation is less likely the origin combining sulfur isotope data we suggest that the anomalous sulfate was associated with volcanic eruptions in the west but was not directly carried by ash falls extreme dry fog sulfate haze events resulting from tropospheric oxidation of volcanic sulfur gases might provide a viable explanation for the sulfates in the ash bed that carry the oxygen 17 anomaly

persistence of a freshwater surface ocean after a snowball earth geochemical data from cap carbonates deposited above cryogenian glacial deposits have been widely used to infer the conditions that prevailed in the aftermath of snowball earth however the time scale over which these carbonates were deposited and the degree to which they record the chemistry of a globally well mixed ocean have remained poorly constrained during deglaciation a large volume of meltwater entered the ocean creating two distinct layers the fresh hot and light upper layer and the salty cold and dense lower layer here we estimate the ocean mixing time scale based on energetic constraints we find that the mixing time scale is 10 4 10 5 yr with a best estimate of 5 10 4 yr or up to 100 times longer than that of the modern ocean mixing of the surface temperature anomaly implies a delayed sea level rise of 40 50 m associated with pure thermal expansion this result reconciles geological geochemical and paleomagnetic data from basal ediacaran cap carbonates with physical oceanographic theory in particular our model suggests that 1 the cap dolostones formed predominantly in a freshwater environment 2 the waters in which the dolostones formed were not well mixed with saline deep water allowing for large geochemical differences between the cap dolostones and the deep ocean and 3 the cap carbonate sequences formed in a two phase transgression that lasted 10 4 yr which is consistent with both local sea level records and the preservation of magnetic excursions and reversals

mesozoic reactivation of variscan thrusting in the bristol channel area uk new seismic reflection data from the bristol channel area provide direct evidence of a major variscan thrust in the palaeozoic basement beneath the mesozoic bristol channel syncline the thrust has an average southerly dip of about 25 and it rises into the major fault the central bristol channel fault zone cbcfz at the northern margin of the bristol channel syncline the cbcfz is attributed to reactivation in extension of the variscan thrust probably during the early cretaceous there is no evidence for major synsedimentary movement on the cbcfz and the bristol channel syncline does not represent a rollover structure section restoration suggests that the cbcfz has cut up through a previously tilted sequence of triassic and jurassic strata pervasive nnw sse cross faults are interpreted as original variscan transfer faults that have been reactivated in the mesozoic and tertiary

structural and tectonic setting of the permo triassic basins of northwest europe abstract the permo triassic basins of nw europe formed by i late orogenic spreading of the variscan mountain belt eventually leading to ocean floor formation in the mid atlantic ii continental rifting propagating south from the arctic and iii thermal subsidence following carboniferous volcanic activity in the north sea spreading values in the northernmost north sea west of shetland and west and southwest of britain are relatively high but the sediments are dominantly continental the permo triassic rifting affected lithosphere thickened during caledonian and variscan orogenesis however subsidence associated with the arctic rifting together with global sea level changes allowed marine incursions into the north sea producing the large zechstein salt basins interference of the different basin formation mechanisms led to a complex three dimensional fault pattern and multidirectional extension across britain and the north sea this fault pattern formed the basic framework for subsequent jurassic and cretaceous basin formation

triassic jurassic development of the st george s channel basin offshore wales uk abstract isopach maps and backstripped tectonic subsidence plots from the triassic jurassic of the st george s channel basin reveal significant departures from current models of extensional basin development i rifting was highly protracted with discrete increments of crustal extension spanning ca 120 my ii substantial syn kinematic sediment on the footwalls of major faults records pure shear sub seismic scale deformation to have been important during regional triassic extension forward modelling of depth converted seismic interpretations is used to test the notion of combined pure and simple shear and to quantify the distribution of crustal extension between large and sub seismic scale faults the models indicate that triassic subsidence was generated by ca 2 5 km extension on major faults and 10 km extension by sub seismic scale faulting in fault bounded compartments jurassic extension was dominated by large scale faults whereas in the areas of overlap between major fault segments jurassic extension was taken up chiefly by sub seismic scale faulting comparison of measured vitrinite reflectance vr and vr modelled from heatflow history yielded by the forward models supports this scheme of episodic pure shear dominated extension this research highlights the inherent difficulty in using seismic interpretation in isolation from quantitative subsidence and or thermal history data to identify syn and post kinematic sedimentary sequences furthermore it provides an opportunity to more fully reconstruct mesozoic basin development in the west of uk area which in the cheshire central and east irish sea basins is largely precluded by erosion of post triassic strata

tectonic framework of the celtic sea and adjacent areas with special reference to the location of the variscan front abstract structural trends in the celtic sea area indicate that variscan deformation patterns were inherited from caledonian basement structures and that the regional fold alignment is arcuate with a regional wsw ene direction rather than wnw ese armorican there is no lateral structural continuity between southern ireland and south wales southwest england three major structural provinces arranged en echelon across the variscan foldbelt are recognised a southwest england where there was complex deformation of a major basin b the south wales mendips foreland area with strong basement cover interaction and c the southern ireland graben and flanking platform province late palaeozoic depositional patterns indicate that southern ireland and southwest england were separated by a wsw ene trending platform bounded on the north by the inherited wexford boundary lineament and to the south by a previously unidentified major palaeozoic fault zone here termed the bristol channel lineament the south wales mendips variscan successions accumulated on this intervening wales celtic sea platform and were partly influenced by rejuvenated caledonian fault lines it is suggested that the northern margin of the rheno hercynian foldbelt the variscan front be taken along the bristol channel lineament which can be traced for some 400 km southwestwards towards the goban spur on the continental margin this permits a rationalisation of both tectonic and major facies boundaries in locating the front it is also suggested that the structurally localised nature of the southern ireland basin be recognised by designating it as the southern ireland zone of the variscan foldbelt the sites of mesozoic rifting in the celtic sea and adjacent areas although complex in detail appear to have been located along the wexford boundary and bristol channel lineaments

benefits and limitations of different 2d algorithms used in cross section restoration of inverted extensional faults application to physical experiments abstract in recent years 2d restoration techniques have been systematically used to restore cross sections through inverted basins the accuracy of these techniques and in particular which method better restores the inverted extensional faults to previous stages is uncertain and difficult to assess in natural examples to address this drawback the applicability of flexural slip and vertical oblique slip restoration techniques executed with section restoration software is tested through restoration of physical experiments of inverted extensional faults to their pre inversion stage the experiments chosen consist of simple listric and planar faults in which 1 the original state and the kinematic path followed by the rocks to reach the final state is known 2 the boundary conditions are known 3 erosion is absent and 4 the orientation of extension and compression vectors is equal comparing the restored sections with their corresponding actual pre inversion stage reveals that flexural slip is the best restoration method whereas the combination of different slip angles method gives the worst results the accuracy of these techniques depends to a great extent on the master fault geometry the coefficient of friction along it and the amount of inversion the best results are obtained for a physical model that consists of a listric fault with 60 dip at the top of the rigid footwall a shallow detachment low coefficient of friction along the fault and mild amount of inversion since the deformation mechanisms and the geometry of the inverted structures are non identical in physical experiments and in natural examples the results obtained in our study should be cautiously applied to cross sections across natural inverted basins

neogene basin inversion and recent slip rate distribution of the northern termination of the alhama de murcia fault eastern betic shear zone se spain the alhama de murcia fault is one of the main active structures of the eastern betic shear zone se spain characterized by the presence along its trace of neogene basins developed under early to late miocene extensional tectonics a dominant nnw sse shortening direction is active from late miocene driven by the present day plate convergence we present the structural analysis of the northeastern section of this fault where reliable estimations of slip rates were unknown due to the lack of geomorphological evidence of recent activity the recent tectonic evolution and reactivation of the northeastern section is closely related to the fortuna basin development and tectonic inversion we approach the structural analysis through the interpretation of seismic reflection profiles well data fieldwork and 3d geological modeling we estimated a maximum long term slip rate of 0 32 0 18 0 13 mm yr in the librilla sector last 4 8 7 6 ma based on cross section restorations and assuming current motion trends from gps data according to the results from the cross sections restored along the section and a vertical displacement analysis based on a 3d model the slip rate distribution shows a decrease of activity toward the northeastern tip of the studied fault section this supports a transference of deformation between the alhama de murcia fault and the carrascoy fault which seems to absorb part of the shortening during the plio quaternary explaining the lower relief created by the activity of the northeastern section the slip rates obtained have important implications in seismic hazard assessments and in the distribution of deformation along the region

evolution of downslope flow under strong opposing trade winds and frequent trade wind rainshowers over the island of hawaii abstract the evolution of downslope flow on the windward side of the island of hawaii during 7 8 august 1990 is investigated this period is characterized by atypical strong 11 m s 1 upstream trade winds and frequent nocturnal rainshowers in the late afternoon the onset of downslope flow first occurs on the upper slope which is frequently dominated by orographic clouds with mean weak surface winds on the windward slopes because of island blocking the downslope flow onset occurs after the slope surface becomes negatively buoyant along the hilo coastal areas that are well exposed to decelerating trade wind flow the offshore flow onset there occurs much later 2 h than the hawaiian rainband project harp mean as the drainage front moves slowly from the windward lowlands toward the coast against the atypical strong incoming trade wind flow the downslope flow onset along the coast occurs after a band of trade wind rainshowers has moved over the coast these rainshowers produce evaporatively coole

backstacking apatite fission track stratigraphy a method for resolving the erosional and isostatic rebound components of tectonic uplift histories measuring tectonic uplift of the mean surface of earth using thermochronological data to document uplift histories is often plagued by confusion arising from the independent nature of the thermal and sea level frames of reference the technique of backstacking apatite fission track stratigraphy differentiates between vertical movement of rocks toward earth9s surface due to erosional or tectonic denudation and tectonic uplift of earth9s surface apatite fission track analysis can also be used to identify episodes of erosion where no other evidence for the erosional event exists in addition to providing information about the timing magnitude and geographic distribution of erosion

crustal structure beneath southern england from deep seismic reflection profiles seismic reflection events from recently acquired profiles in southern england are interpreted in terms of stratigraphic and structural interfaces between rock units in particular evidence is brought forward that a series of reflection events dipping between 22 and 27 with azimuth averaging 160 represents 2 major thrust zones occurring in the variscan floor beneath mesozoic cover a geological model based upon borehole information and regional geological considerations is presented the tentative recognition of crystalline basement overlain and overstepped by late proterozoic rocks of variable thickness suggests that basement in this area was unstable from late precambrian times the mesozoic structural evolution can be related to tectonic re activation of major concealed thrust zones in the palaeozoic 9basement9

apatite fission track data from southeast ireland implications for post variscan burial history the results of a reconnaissance apatite fission track study from southeast ireland are compared with modelled ages and length distributions predicted using both traditional thermal histories and the thermal history indicated by the fission track age and length data all samples have experienced total annealing during the variscan orogeny there is strong evidence for significant post variscan pre mid jurassic heating and cooling the modelling also recognizes a third thermal event subsequent to 100 ma the implications of these results are examined in terms of the timing and extent of subsidence and inversion along the northern margin of the north celtic sea basin

cenozoic deformation history of the qaidam basin nw china results from cross section restoration and implications for qinghai tibet plateau tectonics abstract the deformation and uplifting history of the qinghai tibet plateau is a controversial issue and so far there has been insufficient evidence for interpretation investigations of the cenozoic deformation in the northeastern edge of this plateau may provide some relevant information in this paper the cenozoic deformation history of the qaidam basin has been studied based on results from the balanced cross section restoration of six geological sections they indicate that 1 the qaidam basin has been undergoing continuous shortening since the beginning of cenozoic with two relatively fast shortening phases the first during 42 8 40 5 ma and the second during 2 8 ma present 2 the shortening rate of the qaidam basin reached its maximum value since 2 8 ma and approximately 32 of the average total cenozoic shortening took place in this period and 3 the shortening of all sections reached approximately 50 of their respective total shortening by miocene the cenozoic deformation history of the qaidam basin shows good synchroneity with uplifting history of the qinghai tibet plateau it is therefore proposed that the deformation and uplifting in the south and north edges of the qinghai tibet plateau was almost synchronous with the altyn tagh fault formed at the beginning of india asia collision accordingly it is postulated that since the india asia collision at the beginning of cenozoic the indian plate moved northward continuously with two relatively fast phases the first during 42 8 40 5 ma and the second during 2 8 ma present approximately 50 of the total cenozoic northward displacement of the indian plate of the total amount of crustal shortening and uplifting of the qinghai tibet plateau and of the total cenozoic strike slip displacement of the altyn tagh fault were probably reached by the early miocene moreover another 30 40 might have taken place during 2 8 ma present the periods during 42 8 40 5 ma and during 2 8 ma present may also be the two fast phases for crustal shortening and uplifting of the qinghai tibet plateau and strike slip displacement of the altyn tagh fault

structural style and early stage growth of inversion structures 3d seismic insights from the egersund basin offshore norway abstract high quality three dimensional 3d seismic reflection and borehole data from the egersund basin offshore norway are used to characterise the structural style and determine the timing of growth of inversion related anticlines adjacent to a segmented normal fault system two thick skinned normal faults which offset permian clastics and evaporites delineate the north eastern margin of the basin these faults strike nnw sse have up to 1900 m of displacement and are separated by an ese dipping c 10 km wide relay ramp both of these faults display exclusively normal separation at all structural levels and tip out upwards into the upper part of the lower cretaceous succession at relatively shallow structural levels in the hangingwalls of these faults a series of open low amplitude fault parallel anticlines are developed these anticlines which are asymmetric and verge towards the footwalls of the adjacent faults are interpreted to have formed in response to mild inversion of the egersund basin the amplitude of and apparent shortening associated with the anticlines vary along strike and these variations mimic the along strike variations in throw observed on the adjacent fault segments we suggest that this relationship can be explained by along strike changes in the propensity of the normal faults to reactivate during shortening wider damage zones and lower angles of internal friction coupled with higher pore fluids pressures at the fault centre mean that reactivation is easier at this location than at the fault tips or in the undeformed country rock seismic stratigraphic analysis of growth strata indicate that the folds initiated in the latest turonian to earliest coniacian c 88 6 ma and santonian c 82 6 ma the control on this c 6 myr diachroneity in the initiation of fold growth is not clear but it may be related to strain partitioning during the early stages of shortening anticline growth ceased in the maastrichtian and the inversion event is therefore interpreted to have lasted at least c 20 myr this study indicates that 3d seismic reflection data is a key tool to investigate the role that normal fault segmentation can play in controlling the structural style and timing of inversion in sedimentary basins furthermore our results highlight the impact that this structural style variability may have on the development of structural and stratigraphic hydrocarbon traps in weakly inverted rifts

occurrence of upslope flows at the pico mountaintop observatory a case study of orographic flows on a small volcanic island 1 upslope flows caused by mechanical forcing in strong synoptic winds or by buoyant forcing driven by solar heating under weak synoptic winds can influence the air composition at mountaintop observatories using meteorological and trace gas measurements at the pico nare observatory on pico mountain azores islands north atlantic ocean the frequency and impact of such orographic flows on a small volcanic subtropical island was examined to determine the origin of mechanically lifted air upstream kinetic energy was balanced against potential energy gained during uplift sheppard s model mechanically forced upslope flow is most important during october through april when the calculated probability of observing marine boundary layer mbl air at the observatory near the summit ranges from 35 to 60 per month in contrast lower synoptic wind speeds and a more stable lower free troposphere during may september result in a reduced frequency of mbl impacts 20 buoyant upslope flows buf were quantified through meteorological measurements on the mountain slope in summer 2004 diurnal cycles of wind direction on the mountain slope consistent with daytime upslope and nighttime downslope flow were found on 24 of the days during late june july and august 2004 buoyant forcing can also occur in the presence of moderate synoptic winds resulting in enhancement of the mechanically forced upslope flow on the windward side of the mountain such conditions were found on 15 of the summer days in 2004 however on buf days the specific humidity at the mountaintop was significantly smaller than on the slope indicating turbulent mixing during ascent or vertical decoupling of air masses impacts of buf or a mixture of buoyant and mechanical upslope flow on o3 or nitrogen oxides mixing ratios at the mountaintop station were rare or extremely small and no significant diurnal cycle of o3 expected if daytime buf of mbl air occurred regularly was present midday increases in isoprene concentrations a nonmethane hydrocarbon nmhc expected to be emitted from vegetation more than 700 m below the pico nare station were found on 54 of the days during may august 2005 no corresponding increase in n butane used for heating and cooking at sea level residences was detected suggesting that the air did not originate from as low as sea level these results indicate that the latitude size and topography of pico island combine to prevent frequent transport of mbl air to the pico nare station in the summer

the north celtic sea reflector a possible caledonian basement structure offshore southern ireland abstract a series of seismic reflectors were traced across the north celtic sea area using a combination of three deep seismic profiles and over 3000 km of multichannel seismic reflection data the dominant dipping reflector package the north celtic sea reflector can be traced across much of the region and is interpreted as a basement structure the orientation of this structure trending from 072 in the east to 067 in the west is consistent with a possible caledonian origin although subsequent reactivation particularly during variscan times is also suggested the stacked and imbricate geometry of the structures indicates that they were formed by repeated thrust episodes leading to the development of stacked thrust sheets and localised crustal thickening the location of these basement structures was one of the dominant controls on subsequent mesozoic basin location and development

an early diagenetic deglacial origin for basal ediacaran cap dolostones abstract the beginning of the ediacaran period 635 ma is marked by conspicuous dolostone units that cap marinoan glacial deposits worldwide the extent and sedimentary characteristics of the cap dolostones indicate that anomalous carbonate over saturation coincided with deglacial sea level rise and ocean warming however the geochemical variability within cap dolostones both between continents across single continental margins and within individual stratigraphic sections has been difficult to reconcile with depositional models using a compilation of new calcium and magnesium isotope measurements in marinoan cap dolostone successions worldwide we show that the geochemical variability can be explained by early diagenetic dolomitization of aragonite along a spectrum of fluid and sediment buffered conditions dolostones from the outer platform formed under fluid buffered conditions whereas dolostones on the inner platform and foreslope environment formed under sediment buffered conditions this spatial pattern of dolomitizing conditions is consistent with buoyant recirculation of glacial seawater within carbonate platforms driven by the deglacial sea level rise and development of a meltwater surface ocean using a numerical diagenetic model to evaluate the geochemical differences between sediment and fluid buffered cap dolostone units we constrain the chemical and isotopic composition of both the dolomitizing fluid glacial seawater 13 c 0 2 the meltwater lens 13 c 11 and the primary aragonite sediment 13 c 6 to 3 these model end members do not imply that primary geochemical variability did not exist but demonstrates that it is not necessary to change the chemistry of seawater to explain the global stratigraphic variability in the geochemistry of basal ediacaran cap dolostones our results provide a novel framework for understanding the geochemical variability of cap dolostone units including large excursions in carbon isotopes and how this variability is the product of local diagenetic processes expressed globally in continental margin environments following the last snowball earth

the vendian succession of northeastern spitsbergen petrogenesis of a dolomite tillite association abstract the sedimentary record of late precambrian time is magnificently displayed in the highland snowfields of northeastern spitsbergen svalbard vendian strata are represented essentially by the polarisbreen group which consists mostly of dolostone and includes two dolomitic glacial units the oldest sediments in the polarisbreen group compose the elbobreen formation c 400 m which is divided into four laterally persistent members the lower carbonate member e1 125 m contains a distinctive basal dark grey limestone with microspar filled synaeresis cracks suggested to be of lagoonal origin and associated with minor dolostone shale and chert higher parts of the member are dominantly dolostone partly stromatolitic with some shale and sandstone shallow subtidal to intertidal deposition is indicated by the dominance of intraclastic lithologies and relics of anhydrite penecontemporaneous dolomite is partially overprinted by microsparry dolomite thought to be of groundwater origin the redefined petrovbreen member e2 consists of diamictite and other detrital dolostone pronounced thickness variations 2 40 m are thought to be original depositional features the member represents the deposits of a short glacial period in which the following depositional processes are inferred lodgement massive diamictite subaqueous meltout massive and bedded diamictite ice rafting lithologies bearing dropstones and possibly also diamictite redeposition by sediment gravity flows some diamictite and conglomerate rhythmite and shale current winnowing thin tabular conglomerate subaerial or subaqueous meltwater action channelled conglomerate and sandstone periglacial shrinkage diamictite wedge fillings the macdonaldryggen member e3 230 m is a monotonous succession of shaly dolostone of lagoonal origin it grades up into the slangen member e4 25 m which consists of subtidal to intertidal dolarenite with anhydrite relics succeeded by fenestral dolostone that was fractured and cemented by saline groundwaters in an emergent environment the wilsonbreen formation 160 m represents a return to glacial deposition but this time longer lasting and with substantial extra basinal material represented the gropbreen member w1 28 107 m and the ormen member w3 44 139 m consist dominantly of dolomitic diamictite with subordinate conglomerate and sandstone and are separated by a middle carbonate member w2 3 30 m which contains distinctive rhythmitic and stromatolitic limestone as well as sandstone the same depositional processes can be recognised as in the petrovbreen member but the wilsonbreen formation is overall of somewhat more continental aspect lower proportion of rhythmite and dropstone structures in addition there are basal breccia and crack fillings formed by frost shattering of the underlying cemented dolostone tabular sandstone thought to be formed by wave reworking of outwash a striated terrestrial cobble pavement supraglacially derived breccia horizons periglacial wedges filled by sand and the w2 assemblage of possible lacustrine origin the dracoisen formation 525 m represents an abrupt return to non glacial conditions an upward deepening wave dominated succession of pure dolostone d1 20 m and impure dolostone d2 105 m is succeeded by offshore black shale d3 150 m and then by a very shallow water succession of evaporite lacustrine aspect with a dolostone containing evaporite relics d5 10 m separating dolomitic sandstone and shale d4 80 m and d6 150 m the contact with the transgressive cambrian sandstones above is poorly exposed deposition of the succession dominantly under marine conditions is inferred but it is difficult to rule out a lacustrine environment at any particular horizon this dolomite tillite association can be explained by penecontemporaneous and minor secondary dolomite formation in marginal environments with warm climatic indicators at some levels being sharply interrupted because of rapid climatic changes by glacial sediments containing abundant detrital dolomite since the latter sediments make up only 17 of the 1080m thick succession glacial conditions only occupied a small proportion of vendian time

the tectonic evolution of the north celtic sea and cardigan bay basins with special reference to basin inversion abstract the morphology and structural evolution of the north celtic sea basin ncsb and cardigan bay basin cbb are in large part controlled by reactivation of basement features of caledonian and possibly variscan origin from earliest triassic until maastrichtian times sedimentation was almost continuous initially in a rapidly subsiding rift system and finally in a progressive passive sag phase at the onset of the alpine orogeny both basins experienced spectacular inversion along readily identifiable axes the inversion takes the form of a compressive reactivation and reversal of the pre existing growth faults and a broad regional upwarping of the cretaceous depocentre the amount of inversion varies depending on the location within the basin but values in excess of 1000 m are possible by utilising regional seismic data the timing of inversion can be estimated as occurring between maastrichtian and eocene times which correlates with the so called laramide pulse of the alpine orogeny

the sea level fingerprint of a snowball earth deglaciation cap dolostones are thought to represent deposition from seas transgressing over formerly glaciated continental margins during marinoan snowball deglaciation nevertheless facies associations within some cap dolostones indicate that an episode of regional regression punctuated these transgressive sequence tracts to date inferences of sea level change during and after the marinoan snowball deglaciation have been interpreted using simple qualitative arguments in the present study we explore the full spatio temporal variability of sea level change during snowball deglaciation and its aftermath using a gravitationally self consistent theory that accounts for the deformational gravitational and rotational perturbations to sea level on a viscoelastic earth model the theory is applied to model marinoan snowball deglaciation on a generalized ediacaran paleogeography with a synthetic continental ice sheet distribution we find that sea level change following a synchronous rapid 2 kyr collapse of snowball ice cover will exhibit significant geographic variability including site specific histories that are characterized by syn deglacial sea level fall or stillstand moreover some sites that experience syn deglacial transgression will continue to experience transgression in the post deglacial phase taken together these results suggest that sea level change recorded by strata capping snowball glaciogenic units may reflect a more complicated trajectory than previously thought including deposition that was not limited to the deglaciation phase these simulations as well as others that explore the response to asynchronous melting and deglaciation phases of longer duration demonstrate that an episode of regional regression interrupting a cap dolostone transgressive sequence tract may reflect one of several processes or their combination 1 near field adjustment associated with rapid local melting during an otherwise global hiatus in deglaciation 2 post glacial uplift of sites during a period of slowing deglaciation and 3 a transition at some sites from a sea level fall dominated by post glacial uplift to a phase of sea level rise due to eustasy and peripheral bulge subsidence throughout an extended order 50 kyr or greater snowball deglaciation

tertiary erosion magnitudes in the east midlands shelf onshore uk sonic velocities from the upper and middle chalk the kimmeridge clay the lias and the bunter sandstone were used to quantify apparent erosion i e amount of missing section in the welton 1 and cleethorpes 1 wells of the east midlands shelf onshore uk apparent erosion was determined with reference to sonic velocities in 150 wells the uk southern north sea area apparent erosion at cleethorpes 1 is estimated as 1 4 km from the kimmeridge clay lias and bunter sandstone and 1 6 km from the upper and middle chalk and at welton 1 as 1 6 km from the bunter sandstone and 1 7 km from the lias the consistency of results from the different units analysed increases confidence in the results with respect to compaction studies based on a single stratigraphic unit estimates of missing section in this study are generally greater than those previously published for the east midlands shelf but they are consistent with recent estimates from apatite fission track analysis

evolution of the western approaches trough abstract the western approaches trough corresponds to a major mesozoic rift which became partly inverted during the late oligocene and miocene sediments contained in this basin range in age from permian to neogene and are some 7000 m thick the rifting stage of the western approaches trough spans permian to aptian times there is only minor evidence for volcanic activity important wrench deformations preceded the accumulation of wealden clastics in the rapidly subsiding southwest channel basin and in the brittany trough early aptian wrench deformations coincided with crustal separation in the bay of biscay during the albian to early oligocene the western approaches trough was tectonically inactive and subsided regionally late oligocene to miocene compressional deformations induced the upwarping of a major anticlinorium having a structural relief of some 3000 m

phosphogenesis aragonite fan formation and seafloor environments following the marinoan glaciation abstract carbonates in the sete lagoas formation sao francisco craton brazil preserve a record of chemical biological and oceanographic changes that occurred during the ediacaran period the base of this formation constitutes a post glacial cap carbonate which contains seafloor precipitates carbonate and barite crystal fans as well as various authigenic and diagenetic minerals apatite pyrite and barite here we present petrographic and geochemical data on this unit and discuss the significance of its mineral association for marine environments following the marinoan snowball earth glaciation for the first time we report well developed apatitic cements in a neoproterozoic cap carbonate isopachous and intergranular void filling cements encrust and surround seafloor precipitated fan crystals that precipitated as aragonite we propose a model for the origin of this mineral association which relates phosphogenesis and aragonite fan formation to a single set of environmental conditions according to this model the boundary between oxic and anoxic conditions was located at or just below the sediment water interface burial of iron oxyhydr oxides below this boundary liberated phosphate to pore water and provided fuel for iron reduction iron reduction released fe2 which inhibited nucleation of carbonate and allowed for aragonite growth on the seafloor concurrently iron pumping shuttled phosphate from the water column to the sediment and perhaps in conjunction with organic phosphorus remineralization via anaerobic microbial pathways created conditions conducive to phosphate mineralization this model corroborates the hypotheses that aragonite crystal fan formation requires the presence of an inhibitor to carbonate nucleation in addition to high alkalinity and that fe2 serves as this inhibitor overall our work documents a close association between aragonite crystal fan formation and phosphogenesis at the beginning of the ediacaran illuminates the paleoenvironments of cap carbonates with seafloor precipitates and contributes to understanding of phosphogenesis following low latitude glaciations

marinoan glaciation in east central brazil abstract remnants of a neoproterozoic glaciation in east central brazil are represented by thin diamictite layers jequitai formation and correlative units locally overlying striated pavements on the sao francisco craton the diamictites are covered by the sete lagoas formation of the basal bambui group which is generally accepted to be a typical cap carbonate sequence although most authors have preferred a mid cryogenian post sturtian age for it based mainly on pb pb whole rock data the sete lagoas formation bears lithostratigraphic and isotopic characteristics that are identical to early ediacaran cap carbonates worldwide including a basal thin 0 10 m pale and flinty cap dolostone preserving a drop in 13c values from around 3 2 to 4 5 with associated 18o around 5 and crystal fan facies interpreted as aragonite pseudomorphs ediacaran zircons have been recovered from the middle of the sete lagoas formation constraining the deposition of its upper half to be younger than 610 ma rodrigues 2008 although there is an unconformity below the point where the zircons were collected it is short lived as suggested by the identical typically ediacaran 87sr 86sr values above and below 0 7074 0 7076 carbonate clasts from the jequitai formation and correlative diamictite bearing units in the fold belts that surround the sao francisco craton canabravinha and serra do catuni formations display similar ranges in 13c 6 7 to 2 6 suggesting the erosion of a pre glacial carbonate platform with negative 13c values i e the islay and or trezona anomalies the cratonic carrancas formation on the other hand yielded pale dolostone clasts with 13c in a small range between 4 2 and 3 4 and 18o values around 6 5 these clasts could be derived from the cap dolostone unit itself in which case the carrancas formation would represent resedimented basal sete lagoas formation and imply that sections of the sete lagoas formation sitting atop the carrancas formation are incomplete the base truncated sections have confused previous attempts to correlate the sete lagoas formation with other cap carbonate successions in light of the available lithostratigraphic isotopic and u pb zircon data we propose that the sete lagoas formation represents a basal ediacaran cap carbonate sequence 635 610 ma deposited after the marinoan glaciation in east central brazil

forcing of flow reversal along the windward slopes of hawaii abstract hawaii is an island approximately 4 km high the lower portion of which is immersed in an easterly trade wind layer that is typically 2 km deep blockage of the trade wind combined with diurnal thermally driven circulations leads to a general flow stagnation along the windward slopes and to a reversal of flow at night this westerly downslope flow is confluent with the incoming trade wind and usually extends offshore from observations in the hawaiian rainband project this flow is examined to determine if it is primarily driven by blocking dynamics or thermal forcing it is determined that the westerly downslope flow is principally a thermally driven circulation and that it closely resembles a density drainage current this flow is normally initiated by evaporative cooling principally from orographic rainfall but perhaps also from evapotranspiration under less cloudy and precipitation free conditions downslope flow can be initiated and maintained by radiative cooling of the land surface b

sedimentary constraints on the duration of the marinoan oxygen 17 depletion mosd event the 635 ma marinoan glaciation is marked by dramatic earth system perturbations deposition of nonmass dependently 17o depleted sulfate so42 in worldwide postglacial sediments is thus far unique to this glaciation it is proposed that an extremely high pco2 atmosphere can result in highly 17o depleted atmospheric o2 or the marinoan oxygen 17 depletion mosd event this anomalous 17o signal was imparted to sulfate of oxidative weathering origin however 17o depleted sulfate occurs in limited sedimentary intervals suggesting that earth surface conditions conducive to the mosd had a finite duration an mosd duration can therefore provide much needed constraint on modeling earth system responses at that time unfortunately the sulfate 17o record is often sparse or lacks radiometric dates here we report 11 barite layers from a post marinoan dolostone sequence at wushanhu in the south china block the 17o depletion fluctuates in magnitude in lower layers but is persistently absent up section providing the most confident first and last sedimentary appearance of the anomaly 13c chemostratigraphy is used to correlate the wushanhu section to two proximal sections on the same shallow platform that lack barite layers but have published u pb dates that occur in dolostone and shale assuming a similar pattern and rate for carbonate and shale deposition among the different sections we estimate the mosd duration at 0 0 99 my this number can be further constrained by new radiometric dates from equivalent sequences worldwide thus underpinning models on the nonsteady state earth system response in the immediate aftermath of the marinoan meltdown

airflow within major alpine river valleys under heavy rainfall summary this study documents the aire ow within major alpine river valleys in the presence of precipitation the analysis seeks a conceptual understanding of the effect of rain on the valley e ow ground based and airborne doppler radar and surface data from the mesoscale alpine programme show that during persistent rainy periods the subsidence caused by melting and evaporation of precipitation particles contributes to the formation of a down valley e ow this down valley eow opposes moist southerly e ow aloft which is lifted over the topographic barrier and in which precipitation particles are formed

role of sulfide oxidation in dolomitization sediment and pore water geochemistry of a modern hypersaline lagoon system dolomite precipitation has been reported in sediments of marginal marine hypersaline lagoons near rio de janeiro brazil and has been related to microbial sulfate reduction our field study of pore water and sediment geochemistry in two lagoons reveals only modest degrees of so4 reduction and even h2s oxidation significant differences in dolomite contents salinity and carbon sulfur cycling occur in the two lagoons pore waters from the most dolomite rich sediments exhibit significantly elevated so4 cl ratios relative to seawater moreover the o isotope composition of so4 in all lagoonal waters has evolved significantly from seawater values this suggests active sulfur recycling via h2s oxidation which provides acid that decreases pore water saturation state for high mg calcite and aragonite thermodynamically favoring the precipitation of dolomite the open hydrogeochemical nature of these lagoons maintains so4 concentrations below gypsum saturation allowing dolomite to form from fluids with normal marine mg ca

kinematic evolution and fracture prediction of the valle morado structure inferred from 3 d seismic data salta province northwest argentina the valle morado structure located in northwest argentina is an approximately 7 km long and 4 km wide north northeast south southwest trending anticline cut by faults that involve paleozoic basement and mesozoic cenozoic sedimentary cover geological interpretation of a three dimensional seismic survey that covers the structure plus a well and two dimensional regional scale seismic lines indicates that the valle morado is an andean pop up structure formed by foreland directed faults resulting from reverse reactivation of cretaceous extensional faults dipping westward and newly formed hinterland directed reverse faults dipping eastward the orientation of the preexisting extensional faults is nearly perpendicular to the andean tectonic transport direction in the northern part of the structure and oblique toward the south resulting in almost pure contraction in the north and transpression southward curvature analysis performed on two interpreted reservoir seismic horizons suggests that the maximum density of open fractures is likely to occur along the anticline axis in the northern portion of the fold and along the northeast southwest striking foreland directed reactivated faults to the south the direction of the open fractures interpreted from ultrasonic borehole image data along the well coincide with that of the open fractures predicted from the curvature analysis

duration and nature of the end cryogenian marinoan glaciation the end cryogenian glaciation marinoan is portrayed commonly as the archetype of snowball earth yet its duration and character remain uncertain here we report u pb zircon ages for two ash beds from widely separated localities of the marinoan equivalent ghaub formation in namibia 639 29 0 26 ma and 635 21 0 59 ma these findings verify for the first time the key prediction of the snowball earth hypothesis for the marinoan glaciation i e longevity with a duration of 4 m y they also show that the nonglacial interlude of cryogenian time spanned 20 m y or less and that glacigenic erosion and sedimentation and at least intermittent open water conditions occurred 4 m y prior to termination of the marinoan glaciation

evolution of katabatic flow on the island of hawaii on 10 august 1990 abstract the data collected on 10 august 1990 from the hawaiian rainband project were analyzed to study the evolution of katabatic flow results from this study show that for a relatively dry case thermal forcing is important to account for the onset evolution depth and offshore extension of the katabatic flow on the windward side of the island of hawaii where the mean winds are weak because of island blocking during the evening transition the initiation of katabatic flow on the windward slopes was mainly driven by the diurnal heating cycle at the windward coast the wind shift was caused by the arrival of the drainage front from the windward lowlands after the arrival of the drainage front at hilo slopewise instability was generated because of rain showers along the coast and diminishing orographic clouds and showers on the slopes along the coast because of the reduced radiative cooling due to cloud cover and vertical mixing associated with rains a warm period was observed at hilo on the slop

sulfate in modern playa settings and in ash beds in hyperarid deserts implication for the origin of 17o anomalous sulfate in an oligocene ash bed abstract a large amount of calcite pseudomorphs after gypsum were discovered in a volcanic ash bed and its underlying sand siltstone in the gering formation late oligocene northwestern ne usa the highly positive 17o anomalies in the residual sulfate indicate that the sulfate could only have come from the oxidation of sulfur gases by ozone in the atmosphere we previously proposed that it was most likely the result of an extreme tropospheric sulfate haze dry fog event that was associated with volcanic eruptions in the west to test the dry fog hypothesis we examine here two alternative scenarios that also involve atmospheric sulfate deposition but do not imply a dramatic volcanic and dry fog source 1 is it possible that some playas in arid or semiarid regions accumulate the type of atmospheric sulfate deposits as we see in the mid gering bed by the inflow of surface water or groundwater 2 could a period of hyperaridity and stability during the late oligocene have caused a significant accumulation of atmospheric background sulfate onto the mid gering ash bed surface sulfates from a variety of playa settings were collected and measured for their 17o anomalies the sulfate isotope both o and s and concentration profiles of two volcanic ash beds that are currently exposed in the central atacama desert chile and in taylor valley of the antarctica dry valleys were studied along with their sedimentary features these two ash beds were chosen for comparison because of their significant levels of atmospheric sulfate accumulated during prolonged and hyperarid climatic conditions results showed no or minimal 17o anomalies in any of the playa sulfate samples from around the world and none of the parameters in the mid gering ash profile matches those in either of the two volcanic ash profiles in modern hyperarid settings conclusion the observed 17o anomalous gypsum deposit in the mid gering ash bed is most likely the result of a series of extreme dry fogs that resulted in an intense pulse of atmospheric sulfate deposition to a semiarid playa setting

chalk porosity and tertiary uplift western approaches trough sw uk and nw french continental shelves porosities have been derived from sonic logs for the cenomanian maastrichtian chalk of 22 hydrocarbon exploration wells in the western approaches trough although the burial depth of the chalk studied ranges from 40 1500 m below sea bed porosities are all within the range 0 45 the most deeply buried chalk sequences exhibit close to normal porosity depth relations for offshore chalks abnormally low porosities in the remainder of the wells result from burial at greater depth than that presently observed low porosities were preserved during tertiary alpine related uplift of the area for a given well the magnitude of uplift not reversed by subsequent subsidence apparent uplift equals the displacement of its porosity depth relation from the normal relation along the depth axis apparent uplift in the western approaches trough ranges from 0 1 3 km in the western approaches trough chalk porosity data provide evidence of more widespread tertiary uplift than do seismic reflection records alpine related compression and uplift fault bounded basin inversion is apparent only on seismic records in the southern part of the trough brittany and south west channel basins whereas chalk porosity data suggest regional uplift occurred throughout the trough and adjacent highs chalk porosity data were also used to quantify tertiary uplift in the cleethorpes l well of the east midlands shelf the resultant value of 1 1km is considerably greater than that previously derived from shale sonic velocities approximately 0 3 km and slightly less than that based on apatite fission track analysis 1 3 1 7 km

zn isotope evidence for immediate resumption of primary productivity after snowball earth the ediacaran period began with the deglaciation of the ca 635 ma marinoan snowball earth and the deposition of cap dolostones on continental shelves worldwide during post glacial sea level rise these carbonates sharply overlie glacial sediments deposited at low paleolatitudes and preserve negative carbon isotope excursions the snowball earth hypothesis invokes an almost complete cessation of primary productivity in the surface ocean because assimilatory uptake of zn appears to fractionate its isotopes zn isotope ratios measured in carbonate precipitated in the surface ocean should track fluctuations in primary productivity here we report the first zn isotopic data together with carbon and oxygen isotopic profiles from a neoproterozoic cap dolostone the nuccaleena formation in the flinders ranges south australia we interpret the zn isotopic data in terms of a two stage evolution of the deglacial ocean slightly 66 zn enriched values at the base of the cap dolostone indicate immediate resumption of the biological pump upon melting of the surface ocean but this signal was diluted by intense surface runoff that drove 66 zn 66 zn 64 zn versus the jmc lyon reference values down to the composition of continentally derived zn a subsequent rise in 66 zn records a vigorous increase in primary production and export from a nutrient laden surface ocean

signature of cryptic sedimentary basin inversion revealed by shale compaction data in the irish sea western british isles 1 inversion is one of the most common modes of intraplate deformation and a primary mechanism of sedimentary basin exhumation the key criterion for identification of basin inversion is the contractional reactivation of extensional faults reverse displacement associated with inversion is focused along the upper segments of faults so the evidence for inversion may be removed in deeply exhumed basins we believe this is the case in the irish sea western british isles previous authors have attributed the severe exhumation of this basin to plume related uplift and have ruled out inversion as a cause of uplift we present a method for identifying tectonic inversion in a basin where exhumation has removed evidence of reverse displacement on the upper segments of faults we utilize compaction based mapping of erosion patterns recorded by anomalously high sonic velocities in upper triassic shales we show that the amount of section removed during exhumation in the irish sea varies between 1 3 and 3 3 km and that exhumation patterns are markedly heterogeneous neither the heterogeneity of exhumation nor its focusing over hanging wall blocks of major faults is consistent with solely plume related uplift and we believe that our results record cryptic basin inversion that is superimposed on epeirogenic exhumation some of which may be plume related the direct evidence for this inversion has been removed because erosion has proceeded beneath the null lines of reactivated faults and we propose erosion of reactivated fault segments as one of the major reasons why shortening is frequently underestimated in inverted basins

the snowball earth aftermath exploring the limits of continental weathering processes article i nfo carbonates capping neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball earth aftermath it is commonly assumed that post snowball climate dominated by co2 partial pressures several hundred times greater than modern levels would be characterized by extreme temperatures a vigorous hydrological cycle and associated high continental weathering rates however the climate in the aftermath of a global glaciation has never been rigorously modelled here we use a hierarchy of numerical models from an atmospheric general circulation model to a mechanistic model describing continental weathering processes to explore characteristics of the earth system during the supergreenhouse climate following a snowball glaciation these models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse indeed constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation even at 400 times the present day pressure of atmospheric co2 continental runoff is only 1 2 times the modern runoff under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux consequently it takes millions of years for the silicate weathering cycle to reduce post snowball co2 levels to background neoproterozoic levels regarding the origin of the cap dolostones we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume

the south celtic sea bristol channel basin origin deformation and inversion history abstract the south celtic sea bristol channel basin is a relatively narrow and elongated sedimentary trough located within the northern part of the hercynian fold belt and filled with permo triassic redbeds to jurassic marine sediments its present configuration is the result of a three fold sequence of tectonic phases initiated during the late palaeozoic the origin of the basin appears to be related to localised extensional reactivation of hercynian thrust zones during the permian and early triassic followed by regional subsidence during the late triassic and jurassic uplift of the basin margins together with dextral strike slip movements in an east west sense and large scale erosion during the late jurassic to early cretaceous resulted in the present elongated configuration of the lower mesozoic basin regional subsidence from aptian time onwards resulted in widespread deposition of a relatively thin section of wealden elastics and upper cretaceous chalk unconformably overlying folded pre cretaceous strata sedimentation was interrupted during the early tertiary by a series of tectonic events that included emplacement of the lundy igneous mass during the early eocene and mild basin inversion during the oligo miocene inversion was accompanied by dextral reactivation of the nw se oriented sticklepath fault zone the total amount of uplift probably not exceeding 350 m reflects the rather stable configuration of the underlying lithospheric crust as indicated by seismic reflection and refraction data

numerical simulations of airflow and cloud distributions over the windward side of the island of hawaii part ii nocturnal flow regime abstract numerical experiments are performed with the fifth generation mesoscale model to study the evolution of island airflow thermodynamic fields and clouds over the island of hawaii at night the model has successfully simulated the major observed features associated with the nocturnal flow regime these include the formation of nocturnal inversion development gradual offshore extension and deepening of the katabatic flow shifting of the overall cloudy areas from the windward slopes to the ocean and generation of clouds within the simulated offshore convergence zone furthermore it is also shown that rain evaporative cooling affects the depth strength and offshore extension of the katabatic flow in the early evening the nocturnal cooling provides the land sea thermal contrasts for the development of the simulated downslope flow on the windward slopes with continued nocturnal cooling and rain evaporation the simulated katabatic flow extends toward the coast the simulated convergence zone

the thermal and mechanical development of the wessex basin southern england summary the wessex basin of southern england is an e w trending late palaeozoic to tertiary extensional basin formed on early palaeozoic variscan crust extension occurred over a protracted period from late carboniferous to cretaceous times resulting in the evolution of four main depocentres or sub basins the channel basin winterborne kingston trough vale of pewsey and weald basins each sub basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults the basement faults represent primary structures associated with the fold and thrust terrain of the devonian carboniferous variscan orogeny subsequent but synchronous reactivation of both thrust and transfer faults tended to compartmentalize the basement leading to discrete depocentres and the production of general rhomboidal shaped sub basins characteristic of pull apart basins in strike slip terrains to investigate the thermo mechanical properties of the lithosphere during basin formation 50 boreholes within the wessex basin were back stripped to isolate the driving tectonic subsidence responsible for basin formation the resultant driving subsidence is characterized by a gentle sometimes negative exponential form with superimposed rapid discrete subsidence events as the geological development of the wessex basin was characterized by polyphase extension and the utilization of crustal detachments the lithospheric model used to determine crustal extension above the detachment and stretching below the detachment included the effects of depth dependent stretching and finite re rifting from the back stripping analysis the basin wide averaged upper crustal extension and lower crustal sub crustal lithospheric stretching were 11 and 5 respectively the distribution of is strongly influenced by the location of growth faults and hence basement thrust fault reactivation suggesting that is reflecting the brittle failure and collapse of hanging wall blocks repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence in contrast is uniformly distributed across the basin suggestive of a ductile failure of the lower crust and responsible for the low magnitude thermal subsidence strain compatibility between the upper crust and lower crust sub crustal lithosphere is maintained partly within the basin 5 and partly external to the basin as the variscan basal thrust was the fundamental relay in the sense of wernicke responsible for the wessex basin formation we postulate that strain balancing induced a cumulative essentially thermal isostatic uplift of 60 m under the northern margin of the armorican massif

slow net sediment accumulation sets snowball earth apart from all younger glacial episodes during the sturtian and marinoan snowball earth episodes ice cover is thought to have extended from polar to tropical latitudes we test the supposition that such an extreme glacial climate not repeated in the subsequent 635 m y of earth history would have reduced the vigor of the hydrologic cycle and thus diminished sediment flux to the oceans with 500 sediment accumulation rates to characterize sturtian and marinoan deposits we find median accumulation rates at least four to 15 times slower than expected for phanerozoic glaciomarine deposits as characterized by 10 000 rates our comparison is conservative with respect to time span latitude and distance from the ice margin phanerozoic accumulation rates decrease systematically when averaged over longer time spans comparisons were drawn therefore at 5 and 57 m y time spans to match minimum marinoan and sturtian durations respectively cenozoic glaciomarine accumulation also slows with increasing latitude from temperate to polar climates and with increasing distance from the ice margin after accounting for time span snowball earth deposits at low latitude are found to be thinner than would be expected either for high latitude cenozoic glacial deposits or for very distal glaciomarine abyssal muds with ice rafted debris the rate discrepancy is not readily attributed to overestimates of the total marinoan or sturtian durations if sediment fluxes during warm melt intervals did approach phanerozoic rates these intervals must have occupied a much smaller proportion of snowball earth episodes than in younger glacial climates

relation between continental rifting and uplifting evidence from the suez rift and northern red sea abstract the suez rift and the northern red sea are flanked by high standing shoulders with elevations often exceeding 2 km and with a structural relief reaching 4 5 km in this area pre rifting erosion was small not more than a few hundred meters there is no evidence that a precursory uplift formed along the site of future rifting uplifting and erosion began essentially at the same time as rift faulting and magmatism in late oligocene and early miocene limes in the gebel serbal area sinai where the uplifting history is resolved best the tectonic relief reached about 800 m by the end of the early miocene and a similar amount of tectonic uplifting developed in later times other parts of the rift shoulders probably had a similar uplifting history but the total uplifting was often less considerable erosion in places more than 3 km occurred mainly since middle miocene times and caused considerable isostatic rebound this magnified the uplifting and caused the structural and topographic relief to be considerably greater than the tectonic uplifting initial uplifting and contemporaneous magmatic activity were probably caused by rising of deep hot material which thinned the lithosphere and heated it such deep seated processes probably continued to operate also during the later stages of tectonic uplifting though igneous activity no longer occurred on the surface stretching of the lower lithosphere under the rift shoulders could have also contributed to their uplifting the features of uplifting history of the studied region may well be typical of other active rifts

sedimentary basin inversion and intra plate shortening abstract sedimentary basin inversion the shortening of formerly extensional basins is accommodated mainly by compressional reactivation of extant faults and fractures across a wide range of scales as such inversion is a large scale manifestation of byerlee friction the dynamic criterion for fault reactivation governing the effective shear strength of the shallow crust basin inversion generates distinctive deformational architecture and it is implicated strongly in sedimentary basin exhumation as a principal source of horizontal stress inversion drives significant sedimentary porosity reduction and resultant fluid flow upper crustal deformation is critically dependent on fluid overpressure i e pore fluid pressures greater than would be calculated from a hydrostatic gradient and perhaps more than in any other tectonic setting overpressures are potentially large during sedimentary basin inversion this review therefore includes discussion of the role of fluid overpressure in inversion and the evidence for it collectively inversion has profound implications good and bad for the prospectivity of many petroliferous sedimentary basins thus recognizing the evidence for basin inversion quantifying its magnitude and understanding the mechanisms that accommodate inversion and other phenomena affected by it have become essential components of the basin analyst s remit

tropical island convection in the absence of significant topography part i life cycle of diurnally forced convection abstract diurnally forced convection was observed over the tiwi islands north of the australian continent as part of the maritime continent thunderstorm experiment immature peninsula scale 5 15 km sea breezes were observed to initiate moist convection early each day principally through convergence that results from the confluence or collision of peninsula breeze fronts convection initiated by peninsula scale breezes usually fails to organize beyond a small cluster of cells and dissipates as a local event mature island scale 100 km breezes develop by late morning and subsequently play a pivotal role in the forcing and evolution of organized convection the initiation of mesoscale convective systems mcss is observed to be a direct consequence of breeze front collisions for only 20 of the days on which organized convection develops this is referred to as type a forcing and it occurs when normal convective development is delayed or otherwise suppressed type a forcing is nature s backup mech

numerical simulations of island scale airflow over maui and the maui vortex under summer trade wind conditions abstract the fifth generation pennsylvania state university ncar mesoscale model mm5 coupled with the noah land surface model lsm is employed to simulate island scale airflow and circulations over maui county hawaii under summer trade wind conditions during july august 2005 the model forecasts are validated by surface observations with good agreement in this study it is shown that a previously known closed circulation over the central valley of maui or the maui vortex represents the northern cyclonic vortex of the dual counter rotating vortices in the lee of haleakala which extend up to the base of the trade wind inversion with a westerly reversed flow 2 m s 1 at low levels the northern cyclonic vortex is more pronounced than the southern anticyclonic vortex the asymmetric structure of the dual vortices is related to the shape of haleakala and the flow deflection by the west maui mountains the maui vortex has a relatively narrow east west extent in the lowest levels especially at nigh

toward the snowball earth deglaciation the current state of knowledge suggests that the neoproterozoic snowball earth is far from deglaciation even at 0 2 bars of co2 since understanding the termination of the fully ice covered state is essential to sustain or not the snowball earth theory we used an atmospheric general climate model agcm to explore some key factors which could induce deglaciation after testing the models sensitivity to their parameterizations of clouds co2 and snow we investigated the warming effect caused by a dusty surface associated with ash release during a mega volcanic eruption we found that the snow aging process its dirtiness and the ash deposition on the snow free ice are key factors for deglaciation our modelling study suggests that under a co2 enriched atmosphere a dusty snowball earth could reach the deglaciation threshold

seawater buffered diagenesis destruction of carbon isotope excursions and the composition of dic in neoproterozoic oceans carbonate sediments of nonglacial cryogenian 659 to 649 ma and early ediacaran 635 to 590 ma age exhibit large positive and negative 13ccarb excursions in a shallow water marine platform in northern namibia the same excursions are recorded in fringing deep sea fans and in carbonate platforms on other paleocontinents however coeval carbonates in the upper foreslope of the namibian platform and to a lesser extent in the outermost platform have relatively uniform 13ccarb compositions compatible with dissolved inorganic carbon dic in the modern ocean we attribute the uniform values to fluid buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection this attribution which is testable with paired ca and mg isotopes implies that large 13ccarb excursions observed in neoproterozoic platforms while sedimentary in origin do not reflect the composition of ancient open ocean dic

snowball earth ice thickness on the tropical ocean 1 on the tropical oceans of a neo proterozoic snowball earth snow free ice would have existed in regions of net sublimation photosynthesis could have continued beneath this bare ice if it was sufficiently thin and sufficiently clear the steady state ice thickness is determined by the necessity to balance the upward conduction of heat with three subsurface heating rates the heat flux from the ocean to the ice base the latent heat of freezing to the ice base and the solar energy absorbed within the ice a preliminary study using a broadband model for solar radiation and assuming a large freezing rate had indicated that tropical ice might be only a few meters thick here we show that the vertical throughput of ice by surface sublimation and basal freezing would be too slow to keep the ice thin and that the broadband model had exaggerated the absorption depth of sunlight we use a spectral model for solar absorption computing radiative transfer at 60 wavelengths considering absorption by the ice and scattering by bubbles with the spectral model the computed ice thickness is much greater for a solar flux of 320 w m 2 at the equatorial surface and expected albedo of 0 5 for bare sea ice we find that surface temperatures below 12 c generate ice layers too thick for photosynthesis 100 m if the albedo were as low as 0 4 thick ice would occur only for surface temperatures below 25 c but such low temperatures would be difficult to maintain with such low albedo for surface temperatures warmer than these limits the ice becomes thin 1 m and is unlikely to represent a coherent ice layer however glacial deformation of thick floating ice from nearby oceanic regions may preclude the existence of thin or ice free patches

dynamic model constraints on oxygen 17 depletion in atmospheric o2 after a snowball earth a large perturbation in atmospheric co2 and o2 or bioproductivity will result in a drastic pulse of 17o change in atmospheric o2 as seen in the marinoan oxygen 17 depletion mosd event in the immediate aftermath of a global deglaciation 635 mya the exact nature of the perturbation however is debated here we constructed a coupled four box and quick response biosphere atmosphere model to examine both the steady state and dynamics of the mosd event our model shows that the ultra high co2 concentrations proposed by the snowball earth hypothesis produce a typical mosd duration of less than 106 y and a magnitude of 17o depletion reaching approximately 35 both numbers are in remarkable agreement with geological constraints from south china and svalbard moderate co2 and low o2 concentration e g 3 200 parts per million by volume and 0 01 bar respectively could produce distinct sulfate 17o depletion only if postglacial marine bioproductivity was impossibly low our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean atmosphere exchange only if partial pressure of atmospheric o2 is larger than 0 02 bar during the snowball period and records of weathering derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown in any case a snowball earth is a precondition for the observed mosd event

eocene deformation on the continental margin sw of the british isles two narrow e w trending zones of late eocene compressional deformation one on the northern margin of the bay of biscay the other in the southern porcupine seabight are described both appear to follow pre existing geological lineaments the deformation is related to the nw movement of iberia between latest cretaceous and late eocene time

late proterozoic glacial carbonates in northeast spitsbergen new insights into the carbonate tillite association carbonate rich glacial deposits from two discrete vendian glacial periods are described the older is represented by the 24 40 m thick petrovbreen member e2 of the elbobreen formation which contains abundant detrital dolomite clasts in e2 and their possible source rocks have positive 13 c and negative 18 o pdb values in contrast the carbonate mud fraction of e2 sediments has different cathodoluminescence characteristics from clasts slightly negative 13 c values and higher fe and mn concentrations than clasts oxygen isotopes vary from 2 5 to 4 5 pdb thought to be related to various seawater meltwater mixtures in the depositional environment preservation of information about glacial sedimentary environments is attributed to early diagenetic recrystallization forced by excess surface free energy ostwald s ripening and coinciding with sulphate reduction the younger glaciation is represented by the wilsonbreen formation 170 m which has a distinctive glaciolacustrine middle carbonate member w2 w2 contains precipitated periglacial carbonates with high mn fe ratio both limestone rhythmitic and stromatolitic and dolostone rhythmites stromatolites and dolomite rich sandstones which are compared with carbonates in modern antarctic lakes evaporitic environments for dolomitic sandstones and stromatolitic dolostones are indicated by heavy oxygen isotope values up to 10 5 pdb high na concentration and evidence for dissolved evaporites high mn concentration in detrital dolostones in w2 is suggestive of syn sedimentary dolomite recrystallization in freshwater diamictites and haematitic siltstones the carbonate tillite association ultimately arises from the erosion of underlying carbonates which originated in this case under radically different climatic conditions glacial depositional waters then became carbonate saturated as a result of dissolution of detrital carbonate massive recrystallization of glacially transported carbonate is proposed as a geologically significant process with considerable potential for palaeoenvironmental analysis in glacial lakes carbonate precipitated in response to evaporation or photosynthesis carbonate precipitation as the result of seawater freezing or in warm interglacial conditions is not yet established oxygen isotope value are inconsistent too heavy with the presence of high latitude meltwaters implying that glaciation extended to low latitudes as proposed by harland

stretching the envelope of past surface environments neoproterozoic glacial lakes from svalbard the oxygen isotope composition of terrestrial sulfate is affected measurably by many earth surface processes during the neoproterozoic severe snowball glaciations would have had an extreme impact on the biosphere and the atmosphere here we report that sulfate extracted from carbonate lenses within a neoproterozoic glacial diamictite suite from svalbard with an age of similar to 635 million years ago falls well outside the currently known natural range of triple oxygen isotope compositions and indicates that the atmosphere had either an exceptionally high atmospheric carbon dioxide concentration or an utterly unfamiliar oxygen cycle during deposition of the diamictites

pre quaternary sea level changes while the paramount importance of global sea level changes has been widely accepted by quaternary geologists throughout this century it is only within the last decade that such changes in the rest of the phanerozoic have received much attention there are two principal reasons for this arousal of interest first increased exploration of the stratigraphic record across the earth both on land and under the sea and significant improvement in biostratigraphic correlation have drawn attention to what appear to have been globally synchronous events second oceanographic research has demonstrated the importance of tectonically controlled changes in the cubic capacity of the ocean basins notably variations in ridge volume which provide a ready mechanism for sea level changes even at nonglacial times the adjective eustatic was first proposed for global changes of sea level by suess 1906 in his great treatise the face of the earth the corresponding noun is eustasy not eustacy as it is commonly misspelt the unconvinced reader need only think of ecstasy eustatic sea level changes can be inferred as suess recognized by a plotting the temporal spread of marine sediments over the continents and b estimating the depositional depth variations in marine stratal sequences which can be correlated across and between continents to these methods can be added seismic stratigraphy in which biostratigraphically age determined seismic sequences on the con tinental margins are analyzed in terms of cycles of coastal onlap and offiap when the results of these methods are broadly in accord confidence in a eustatic interpretation is increased but the range of confidence can vary considerably it is widely accepted that there was an exceptionally high sea

globally synchronous marinoan deglaciation indicated by u pb geochronology of the cottons breccia tasmania australia u pb zircon data from the uppermost cottons breccia representing the marinoan glacial postglacial transition on king island tasmania provide the first direct age constraint on the cryogenian ediacaran boundary in australia zircons in four samples from the topmost meter of the cottons breccia dated by sensitive high resolution ion microprobe exhibit two modes ca 660 ma and ca 635 ma the younger component predominates in the uppermost sample a possibly volcanolithic dolomitic sandstone apparently lacking glacially transported debris in the transition to cap carbonate chemical abrasion thermal ionization mass spectrometry ca tims u pb dating of euhedral zircons from that sample yields a weighted mean age of 636 41 0 45 ma equivalence to published tims ash bed dates from cryogenian ediacaran transitional strata in namibia 635 51 0 82 ma within glacial deposit and china 635 23 0 84 ma 2 m above glacial deposit supports correlation of those strata to the australian type sections and globally synchronous deglaciation at the end of the cryogenian period

a cryogenian chronology two long lasting synchronous neoproterozoic glaciations the snowball earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons here we present four new re os geochronology age constraints on sturtian 717 660 ma and marinoan 635 ma termination glacial deposits from three different paleocontinents a 752 7 5 5 ma age from the base of the callison lake formation in yukon canada confirms nonglacial sedimentation on the western margin of laurentia between ca 753 and 717 ma coupled with a new 727 3 4 9 ma age directly below the glacigenic deposits of the grand conglomerate on the congo craton africa these data refute the notion of a global ca 740 ma kaigas glaciation a 659 0 4 5 ma age directly above the maikhan uul diamictite in mongolia confirms previous constraints on a long duration for the 717 660 ma sturtian glacial epoch and a relatively short nonglacial interlude in addition we provide the first direct radiometric age constraint for the termination of the marinoan glaciation in laurentia with an age of 632 3 5 9 ma from the basal sheepbed formation of northwest canada which is identical within uncertainty to u pb zircon ages from china australia and namibia together these data unite re os and u pb geochronological constraints and provide a refined temporal framework for cryogenian earth history