deep sea carbonates reading the carbon isotope signal the carbon isotope signal in deep sea sediments reflects a mix of 1 global changes in the rates of exchange of the ocean s carbon reservoir with biosphere soil and sediments 2 global and regional changes in surface water productivity 3 internal shifts in water mass structure and circulation basin basin fractionation oxygen minimum development and 4 organism specific fractionation effects due to changes in micro habitat and or ontogenic fractionation vital effects additional complications arise from differential preservation it is impossible to entirely isolate these various factors as a rule of thumb long period signals that are parallel for planktonic and benthic data reflect external global fractionation patterns whilst short period signals are more likely tied to internal patterns water mass fractionation the various approaches to interpretation are illustrated with three case studies the glacial holocene transition the messinian carbon shift and the miocene monterey excursion

snowball earth ocean chemistry driven by extensive ridge volcanism during rodinia breakup the cryogenian snowball earth glaciations were followed by the deposition of massive cap carbonates geochemical modelling suggests that shallow ridge volcanism supplied much of the alkalinity and cations that fuelled this deposition

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

stratigraphy and paleomagnetism of a 3 km thick miocene lava pile in the mjoifjordur area eastern iceland we have carried out stratigraphic mapping in the upper miocene basalt lava pile around the fjords mjoifjordur and seydisfjordur eastern iceland the mapping is based on conventional methods including the use of interbasaltic clastic horizons and petrographically distinct lava groups these units are also used to provide correlations with the nordfjordur area south of mjoifjordur we present a 3 km composite stratigraphic column for the area between mjoifjordur and seydisfjordur the geology of this area shows some differences from the classical model of walker for the structure of eastern iceland partly due to the fact that most of mjoifjordur is not in the vicinity of central volcanoes detailed laboratory measurements of remanent magnetization were carried out on oriented core samples from 363 lavas in 10 selected profiles the local paleomagnetic polarity stratigraphy supports correlations made on the basis of other criteria over 20 geomagnetic reversals are recorded in the eastern iceland lava pile in a period approximately 13 10 ma ago the geomagnetic field during this period averages to a central axial dipole field and its overall statistical properties resemble those obtained in earlier surveys in iceland

the carbon isotope shift at the permian triassic boundary in the southern alps is gradual carbon isotope ratios in marine carbonate rocks have been shown to shift at some of the time boundaries associated with extinction events for example cretaceous tertiary1 and ordovician silurian2 the permian triassic boundary the greatest extinction event of the phanerozoic3 is also marked by a large 13c depletion4 5 new carbon isotope results from sections in the southern alps show that this depletion did not actually represent a single event but was a complex change that spanned perhaps a million years during the late permian and early triassic these results suggest that the permian triassic p tr extinction may have been in part gradual and in part stepwise 6 7 but was not in any case a single catastrophic event

new palaeomagnetic results from icelandic neogene lavas summary in this paper some statistical properties of new collections of palaeomagnetic direction and intensity data are presented and compared with results from older collections the new data come mostly from three composite surveys in the basalt lava pile of iceland totalling about 1000 lavas of ages 7 14 myr the palaeomagnetism of all these lavas converges on a reproducible long term pattern in several important features of the neogene field behaviour previous conclusions which are confirmed in this paper include 1 the distribution of intermediate virtual geomagnetic poles vgps is not confined to preferred longitude intervals 2 normal and reverse polarity states of the geomagnetic field are essentially equivalent as regards the intensity of the field mean field directions and secular variation 3 the virtual geomagnetic dipole moment fdls off with decreasing vgp latitude by a factor of 3 5 4 between polar and equatorial latitudes 4 a significant reduction in the between flow angular dispersion of vgps may have occurrcd during the last 14 myr and 5 palaeomagnetic directions from icelandic basalt lavas are stable and reliable

crustal rifting and related thermo mechanical processes in the lithosphere beneath iceland recent observations of crustal rifting in the ne iceland axial zone are summarized and discussed the thermal state of the crust in the axial zone is discussed on the basis of thermal gradient measurements magnetotelluric soundings and modelling of the accretion mechanism the three independent methods are in good general agreement thermal modelling indicates that a zone of partial melting generally exists below approximately 6 km depth in the axial zone it is suggested that thermal stresses induced by the cooling of the lithosphere as it moves away from the axis are largely responsible for the off axis volcanism and that the thermal stresses also contribute to enhancing the vertical permeability for geothermal waters at intermediate crustal depths on the flanks of the axial zone

shifts of the plate boundary in iceland some aspects of tertiary volcanism this paper points out five anomalies of the icelandic upper crust that are not compatible with currently accepted construction models these anomalies are 1 assumption of a single time averaged plate boundary 2 irregular depth distribution to seismic layer 3 3 the assumption of a uniform vertical compositional zonation 4 the excess width of iceland and 5 abnormal variations in the duration and stratigraphic thickness of chron 5 time lavas in western northern and eastern iceland a model involving frequent shifts of the volcanic zones in iceland is outlined that suggests a way to account for the above structural anomalies the model also suggests that detailed studies of basalt lava facies types may be used effectively to trace small scale shifts of the tertiary plate boundary

a chemostratigraphic overview of the late cryogenian interglacial sequence in the adelaide fold thrust belt south australia within the neoproterozoic sequence of the adelaide fold thrust belt south australia two of the most severe ice ages in earth history sturtian and marinoan are recorded in the glacigenic rocks which mark the base and top of the 4 5 km thick umberatana group the focus of the present chemostratigraphic study rock units from three drillholes and ten measured outcrop sections located in the southern and central flinders ranges and on the stuart shelf were sampled for isotopic analysis of their co existing calcite 13ccal dolomite 13cdol and kerogen 13corg strontium and sulphur isotopic analyses were also run on selected samples the sample sites are in tectonic domains where regional metamorphism has been insufficient to significantly alter the isotopic composition of the kerogen the resulting data when integrated with the lithostratigraphy of the umberatana group yield one of the most complete records of secular variation in the c isotope signature of the late cryogenian ocean between 750 and 680 ma its composite 13ccarb curve begins with a sharp positive excursion 3 to 1 5 in the dolomitic tindelpina shale which caps sturtian glacial diamictites wilyerpa formation then follows a steady climb through the shoaling upward tapley hill formation reaching 4 in the brighton limestone and culminating in a plateau of 13c enrichment within the platformal carbonates of the etina formation 8 to 10 the upper part of the temporal trend shows a decline to 6 to 7 in the stromatolite reef facies of the enorama shale and a dramatic drop to 8 to 9 5 followed by a recovery to 3 in the youngest carbonate unit trezona formation beneath the marinoan glacigenic elatina formation the latter is capped by a thin dolostone the nuccaleena formation which records a consistent up section decrease in 13c 1 to 3 superimposed on this overall pattern of c isotopic variation are second and third order fluctuations attributable to eustatic sea level change pyritic sulphur in the lower tapley hill formation 34s 9 to 40 is distinctly heavier than that in the underlying wilyerpa formation 4 to 20 minimum 87sr 86sr values obtained from the brighton limestone 0 7071 and etina formation 0 7076 are consistent with their late cryogenian age this sturtian to marinoan interglacial succession is more complete than equivalent sequences elsewhere and thus provides a useful reference for global chemostratigraphic correlation notable features of its c isotope curve are the strong post wilyerpa positive excursion also seen in key sections from svalbard and western mongolia the etina plateau of 13c enrichment which is remarkably similar to maxima in the terminal proterozoic c isotope records of western mongolia north western canada and brazil and the spectacular depletion of 13c in the lower trezona relative to similar carbonate facies in the underlying etina formation and enorama shale a preglacial offset in 13ccarb values of such proportions 14 is almost unprecedented except perhaps for that recently documented in the upper ombaatjie formation beneath tillites of the ghaub formation otavi group namibia it represents a key piece of evidence in the case presented by the proponents of a neoproterozoic snowball earth

the age of the nantuo formation and nantuo glaciation in south china a u pb sensitive high resolution ion microprobe shrimp age of 654 5 3 8 ma from an ash bed immediately below the nantuo formation in south china provides the lowest age constraint for the nantuo glaciation which has been correlated with the global marinoan glaciation a u pb shrimp age of 636 3 4 9 ma from a fallout tuff within the basal layer of the nantuo formation along with the existing age of 635 2 0 6 ma from its overlying doushantuo cap carbonate suggests that the traditionally defined nantuo formation may have been deposited in a short time period at the end of the nantuo glaciation in combination with available ages globally the data support a relatively short duration and rapid termination of marinoan glaciations

mid neoproterozoic 830 750 ma isotope stratigraphy of australia and global correlation abstract carbon and strontium isotope studies of numerous drillcore and outcrop sections within individual australian neoproterozoic basins in a tectonic and sedimentologic framework have allowed the construction of isotopic records for the interval 830 750 ma this combined with the discovery of a restricted stratigraphic range for the acritarch cerebrosphaera buickii in mid neoproterozoic strata of australia makes possible correlation with successions in canada namibia and spitsbergen that part of the mid neoproterozoic succession in spitsbergen which contains c buickii was previously considered younger than the sturtian glacial epoch but this study has indicated that the akademikerbreen group is older than that a correlation scheme is proposed along five tie lines and is based on a correlation between australia and canada at 830 ma and australia spitsbergen and namibia at 760 ma the lowest ever recorded seawater 87 sr 86 sr ratios in the upper shaler supergroup of canada 0 70561 0 70670 compare with ratios in evaporites of the 830 ma bitter springs formation amadeus basin australia 0 70568 0 70720 comparable 13 c carb records support a correlation at about 760 ma a correlation is made between the heaviest mid neoproterozoic 13 c carb values of 7 2 upper burra group adelaide rift complex and 8 1 kanpa formation western officer basin in australia 8 5 in spitsbergen backlundtoppen formation akademikerbreen group and 8 5 in namibia ombombo subgroup congo craton the stratigraphic range of c buickii in australia and spitsbergen supports this correlation a compilation of 13 c carb and 87 sr 86 sr records between about 830 and 750 ma from australia canada namibia and spitsbergen reveals numerous negative excursions which are not associated with preserved glacial deposits it is necessary therefore to consider additional models to the snowball earth to explain the large and rapid carbon isotopic excursions

glacial origin of the cryogenian nantuo formation in eastern shennongjia area south china implications for macroalgal survival abstract the cryogenian period was characterized by global glaciations imposing extreme environmental stresses on early multicellular organisms globally distributed diamictite deposits are the major evidence for these glaciation events but the glacial origin of at least some of these diamictite deposits has been disputed complicating interpretations of the environmental conditions influencing the evolution of life in the cryogenian 720 635 ma in this study we conducted a systematic sedimentological investigation of the marinoan age 650 635 ma nantuo formation in six sections representing a 20 km long east west transect in the eastern shennongjia area of south china the nantuo formation which ranges in thickness from 2 m in the east to 300 m in the west comprises diamictite conglomerate lonestone bearing sandstone green siltstone and black shale facies the eastern sections e g longxi and wushanhu consist mainly of diamictite facies exhibiting micro scaled rotation structures microshear grain stacking grain to grain contacts and water escape structures supporting their deposition in a subglacial setting in contrast the western sections shijiahe liziping and songluo consist of massive and graded diamictite and conglomerate facies consistent with an origin as glacially induced submarine debris flows furthermore the lonestone bearing sandstone facies in these sections contain abundant dropstones and mud pellets within finely laminated turbidites indicating an ice rafted origin with locally reworking by turbidity currents the green siltstone and black shale facies lack glacigenic features and may represent late deglacial deposits based on thickness and facies variations among the study sections we infer that the nantuo formation records glacial sedimentation on a complex paleotopographic surface that was incised into the underlying bedrock with subglacial deposits to the east transitioning into glaciomarine deposits to the west of study area the distal songluo section likely records three cycles of glacial advance and retreat in the form of a facies alternation between diamictite and black shale consistent with a dynamically fluctuating ice margin in summary our sedimentological investigation supports a glacial origin for the nantuo formation in the eastern shennongjia area the occurrence of the benthic macroalgal dominated songluo biota in the black shale facies at songluo suggests that strong glacial retreats provided niches for macroalgal survival in the cryogenian glacial ocean

the iceland research drilling project this issue of the journal of geophysical research contains the collected results of the initial studies of a 3 km vertical section of icelandic crust the material described in these papers was largely presented at a meeting of the investigators working in this project held in reykjavik iceland from may 13 to may 15 1980 iceland presents a very well exposed example of crust formed at an accretional plate margin the crest of the mid atlantic ridge the desirability of a detailed study of a long vertical section of icelandic crust by deep drilling has been expressed in proposals and recommendations of scientists and international scientific committees since the early 1960 s a formal proposal with descriptions of several alternative drill sites was presented in 1975 working group on deep crustal drilling in iceland 1975 but difficulties in financing a drill hole that would penetrate into crustal layer 3 vp 6 5 km s delayed implementation of the recommendations

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

residence time of osmium in the oceans 1 estimates of osmium residence time in the oceans that are based on oceanic mass balance calculations 35 50 kyr appear irreconcilable with those inferred from the recent evolution of the osmium isotope composition of seawater 3 4 kyr it is argued that the osmium budget of the oceans is currently close to steady state and thus that the estimates made by the two methods should agree as the inventory of osmium in the oceans is relatively well constrained these disparate residence time estimates imply wildly different osmium input fluxes to the oceans an osmium residence time of 8 10 kyr is proposed by evaluating the uncertainties and limitations of both methods and it is argued that osmium inputs to the ocean are currently underestimated by a factor of 3 this reflects in part the underestimation of the river input of osmium to the oceans owing to a bias within the existing data set and in part the probable existence of sources of osmium to the oceans that have not yet been identified the very short residence time of 3 4 kyr inferred from the postglacial change in seawater composition assuming a single step change in input flux is rejected as it implies unreasonably high osmium input fluxes to the oceans it is concluded that a postglacial spike in osmium flux associated with a meltwater event must have driven part of the change in seawater composition however it is also shown that such a spike cannot be the dominant cause of the most recent shift in seawater 187os 188os

a major perturbation of the carbon cycle before the ghaub glaciation neoproterozoic in namibia prelude to snowball earth 1 a large 11 15 negative shift in 13c is observed in shallow water carbonates directly beneath neoproterozoic glacial deposits or correlative disconformity in northwest namibia ascribed to a snowball earth reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin and field relations show it predates the fall in sea level associated with the ghaub glaciation we crudely estimate the duration of the isotopic shift as 0 6 106 years from a simple thermal subsidence model similar or larger 13c anomalies are found directly beneath neoproterozoic glacial units in australia canada china scotland and svalbard after considering conventional interpretations for negative 13c anomalies we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale magnitude and geological context of the anomaly in namibia counterintuitively an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation

a spreading drop model for plumes on venus many of the large scale plume related features on venus can be modeled by a buoyant viscous drop or plume head as it rises and spreads laterally below a free fluid surface the drop has arbitrary density and viscosity contrast and begins as a sphere below the surface of a fluid half space the boundary integral method is used to solve for the motion of the plume head and for the topography geoid and stress at the fluid surface as the plume approaches the surface stresses in the fluid above it cause it to spread and become thin below the surface during the spreading the surface swell above evolves through various stages whose morphologies resemble several different plume related features observed on venus when the plume head first approaches the surface a high broad topographic dome develops with a large geoid and radial extensional deformation patterns at later stages the topography subsides and becomes plateau like the geoid to topography ratio gtr decreases and the dominant stress pattern consists of a band of concentric extension surrounded by a band of concentric compression we find that a low viscosity model plume head viscosity that is 0 1 times the mantle viscosity produces maximum topography that is 20 lower and swell features which evolve faster than for an isoviscous plume we compare model results with both the large scale highland swells and smaller scale features such as coronae and novae the dome shaped highlands with large gtrs such as beta atla and western eistla regiones may be the result of early stage plume motion while the flatter highlands such as ovda and thetis regiones which have lower gtrs may be later stage features comparison of model results with gtr data indicates that the highlands result from plume heads with initial diameters of about 1000 km on a smaller scale an evolutionary sequence may begin with novae domes having radial extensional deformation followed by features with radial and concentric deformation such as arachnoids and end with coronae with mostly concentric deformation the model predicts that the highlands evolve on a timescale of order 10 ma and the smaller scale features evolve in a 100 ma timescale

hot spot evolution and the global tectonics of venus the global tectonics of venus may be dominated by plumes rising from the mantle and impinging on the lithosphere giving rise to hot spots global sea floor spreading does not take place but direct convective coupling of mantle flow fields to the lithosphere leads to regional scale deformation and may allow lithospheric transport on a limited scale a hot spot evolutionary sequence comprises i a broad domal uplift resulting from a rising mantle plume ii massive partial melting in the plume head and generation of a thickened crust or crustal plateau iii collapse of dynamic topography and iv creep spreading of the crustal plateau crust on venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea floor spreading

cyclostratigraphic constraints on the duration of the datangpo formation and the onset age of the nantuo marinoan glaciation in south china abstract constructing an accurate timeline is critical for reconstructing the earth systems through critical transitions in climate geochemistry and life existing dates constrain synchronous initiation ca 717 ma and termination ca 660 ma of the sturtian glaciation from multiple continents the termination of the younger marinoan glaciation is also well dated at ca 635 ma but the onset of this glaciation is only roughly constrained as ca 654 ma south china and ca 639 ma namibia to test if the marinoan glaciation started close to ca 654 ma or ca 639 ma we have conducted a cyclostratigraphic study on the cryogenian non glacial datangpo formation that conformably overlies and underlies sturtian and marinoan glacial diamictites respectively in a deep water basin section in south china a total of 28 765 magnetic susceptibility ms measurements from a drillcore of the 292 m thick muddy siltstone and shale dominated datangpo formation are used for cyclostratigraphic analysis the results reveal significant decameter to meter scale sedimentary cycles of 16 12 m 3 6 3 0 m 1 0 0 8 m and 0 6 0 4 m the ratios of these cycle wavelengths match well with those of the milankovitch cycles calibrated for the cryogenian period the established astrochronologic time scale suggests that the duration of the datangpo formation is about 9 8 million years together with the radiometric age of ca 660 ma for the termination of the sturtian glaciation the cyclostratigraphic data suggest that the nantuo marinoan glaciation in south china initiated at ca 650 ma which is slightly younger than but consistent with the ca 654 ma u pb age from the top of the datangpo formation in shelf sections this age however is significantly older than the ages obtained from marinoan age glacial diamictites in south china ca 636 ma and namibia ca 639 ma given that most of the shelf sections may have suffered from glacial erosion obtaining the onset age of the marinoan glaciation should focus on relatively complete deep water successions

global distribution and characteristics of coronae and related features on venus implications for origin and relation to mantle processes coronae on venus range from 60 to over 2000 km across and are characterized by a complex range of morphologies the annuli around coronae range from about 10 to 150 km across and have tectonic features ranging from extensional to compressional to a combination of both topographically coronae are domes plateaus plateaus with interior lows and rimmed depressions a subset of features classified here as coronae corresponds to depressions and is interpreted to consist of large scale calderas a number of features have been identified with many of the basic characteristics of coronae similar interior deformation associations with volcanism high topography but lacking a distinct tectonic annulus these features tend to be somewhat smaller than coronae and may represent failed coronae or coronae in an early stage of evolution the size distribution of coronae and coronalike features with maximum widths greater than about 250 km is well represented by a power law of the form n d kd where n is the number of coronae with maximum widths greater than d km and 3 05 the spatial distribution of coronae is not random the features are concentrated in a few groups and along several chains coronae are similar in many morphologic characteristics to major volcanic shield structures and volcanic rises such as western eistla regio the largest corona artemis is actually larger than several volcanic rises on venus coronae and volcanic rises appear to be surface manifestations of mantle plumes there is no evidence of any systematic variation in age along chains of coronae as occurs in hotspot chains on earth instead a number of multiple and overlapping coronae may indicate limited movement of the surface above a hotspot or mantle plume the morphology and size distribution of coronae highlands and major shields suggest that mantle upwelling on venus operates either on several spatial scales with coronae representing smaller scale upflows and major volcanic rises representing larger convective upwellings or on several temporal scales with coronae representing shorter duration upflows and major volcanic rises representing long term upwellings

a detailed palaeomagnetic study of the oldest 15 myr lava sequences in northwest iceland summary we have determined the directions of primary remanence in over 400 lava flows in the northwestern peninsula of iceland near 66 1 n 23 3 w these lavas were sampled in 16 profiles spread across 75 km at the stratigraphic level of the oldest approximately 15 myr in age of several distinctive lignite bearing sediments in the peninsula we find that the pattern of polarity reversals in the lava pile is broadly similar in these profiles but it is generally not possible to trace short polarity zones and excursions over more than a few kilometres laterally our results which also include measurements on the lignite sediments at three sites are consistent with previous suggestions that these sediment beds represent a time gap of the order of 0 2 myr between the lavas above and below the mean virtual geomagnetic pole of this collection is far sided by approximately 7 and the scatter of individual poles is greater than that in palaeomagnetic surveys on younger lava series in iceland

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

chemical diagenesis of a multicomponent carbonate system 2 stable isotopes abstract this study examines the oxygen and carbon isotopic distribution in the brachiopods crinoids rugose corals and matrix cement from the mississippian burlington limestone iowa and missouri and the silurian read bay formation arctic canada the isotopic data can be reconciled with textural and trace element observations only if the 18o content of ancient oceans differed from that of the present oceans this postulated secular variation in the 18o content of seawater requires a separate evaluation of the mississippian and silurian carbonate components examination of the 18o content of the least altered low mg calcite fossil components of the two formations suggests that the 18o of mississippian seawater was on average about 1 5 lighter and that of silurian seawater was about 5 5 lighter than the 18o of present day ocean water the average diagenetic shift in 18o for the read bay formation which stabilized in a partly closed diagenetic phreatic meteoric system is about 1 for the original low mg calcite and about 2 for the original high mg calcite and aragonite components conversely for the burlington limestone which stabilized in an open diagenetic phreatic meteoric system the average diagenetic shift in 18o is about 3 for the original low mg calcite and about 4 f r the original high mg calcite components the 13c values of the originally low mg calcite or high mg calcite with less than 7 mole mgco3 brachiopods and rugose corals the originally high mg calcite crinoids and the originally aragonite lime mud of the burlington limestone and read bay formation are bimodally distributed this bimodal 13c distribution is independent of geologic age inferred original mineralogy of the component degree of diagenetic alteration and the type of diagenetic meteoric system with components from both the read bay formation and burlington limestone contributing to both 13c groups the mode of the light 13c group is 0 25 and for the heavy 13c group it is 3 25 pdb both 13c groups also show a decrease in 13c of about 1 with increasing diagenetic alteration stabilization

correlation between isotope records in marine and continental carbon reservoirs near the palaeocene eocene boundary changes in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land the carbon isotope compositions of marine carbonate sediments from the late palaeocene vary considerably exhibiting a sudden decrease close to the palaeocene eocene boundary which coincides with deep sea benthic extinctions1 and with changes in ocean circulation here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel in using palaeosol carbonates to reconstruct the co2 content of the ancient atmosphere2 isotope shifts of this sort will have to be taken into account the sharp decrease in 13c 12c ratios in the late palaeocene provides a datum for precise correlation of marine and continental records and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals

detailed paleomagnetic study of two volcanic polarity transitions recorded in eastern iceland abstract two sequences of 45 and 49 individual lava flows respectively have been sampled in eastern iceland the two sections range in age from 12 09 to 10 21 ma as reported by watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 and are labelled as profiles c and d stepwise alternating field af and thermal demagnetizations accompanied by investigations of rock magnetic properties indicate that the magnetization is primarily carried by titanomagnetite demagnetization experiments have identified eight and six transitional lavas respectively for these profiles the transitional virtual geomagnetic poles vgps of the r n profile c reversal are located mainly between patagonia and antarctica with a tendency of the virtual poles to move towards west coast of south america subsequently traveling to the northern hemisphere through several discrete steps located in the middle part of south america then to west africa and on to central asia before the poles settle into normal polarity the second and younger vgp path corresponding to profile d is a path characterized by a reverse to normal to reverse r n r motion of the virtual poles this path is characterized by poles located in west antarctica patagonia and the western and eastern part of south america the passage from the southern to the northern hemisphere is also through a discrete sets of steps along the southwestern pacific followed by a rapid motion to the northern siberian region continuing to the western equatorial part of south america before moving on to the central region of asia followed by a motion to the western pacific prior to the final move to the eastern part of antarctica the data from eastern iceland based on highly reliable transitional results as indicated by the demagnetizations and rock magnetic experiments derived from relatively spaced sites and different ages seem to indicate that the eastern icelandic high latitude 66 n profiles studied have several persistent transitional paleofield features that are uniquely observed at those sites particularly when compared to other volcanic records located at lower latitudes

paleomagnetic volcanic data and geometric regularity of reversals and excursions mostly on the basis of paleomagnetic sedimentary data it has been suggested that maps of virtual geomagnetic poles vgps corresponding to directions of the magnetic field at each site tend to fall along american and asian longitudes during reversals and excursions such geometric regularity in transitional fields may indicate that the core and mantle are dynamically coupled however studies of paleomagnetic lava data have thus far failed to show any pattern in transitional fields in this paper we examine a paleomagnetic lava database covering reversals and excursions which have occurred over the last 20 myr volcanic eruptions occur sporadically thus we normalize the data to account for the fact that reversal and excursional events at the various sites are recorded by different numbers of intermediate directions but we prefer not to use averaging methods of previous investigators who discarded or combined directions when they appeared to be similar we find that volcanic data give intermediate vgps which tend to fall along american and asian longitudes roughly consistent with the sedimentary data this result is not an apparent artifact arising from the nonuniform geographic distribution of volcanic sites provided the appropriate polarity is assigned to intermediate vgps we find that icelandic vgps tend to fall along asian longitudes other patterns in the data for example latitudinal clustering of vgps or distinguishing longitudinal preferences of excursions from reversals are not resolved however it appears that in general transitional fields are nondipolar

the marine osmium isotope record over the past decade the marine osmium isotope record has been developed into a new tracer in palaeoceanographic research several analytical developments particularly in the past few years have significantly increased our ability to study the behaviour of osmium in the surficial environment the 187os 188os and osmium concentration of seawater river water rain and hydrothermal vent fluids have been measured directly recently the behaviour of osmium in estuaries critical for estimating the marine residence time of osmium has been studied our knowledge of the surficial osmium cycle has thus significantly improved in addition reconstructions of past variations in the marine 187os 188os recently have been extended back into the mesozoic this review attempts to summarize our current understanding of the marine osmium system present and past the 187os 188os of seawater during the cenozoic to first order mimics the marine 87sr 86sr record it is therefore tempting to interpret both records as reflecting increased input of radiogenic osmium and strontium resulting from enhanced continental weathering regulated by climatic tectonic processes however the marine osmium isotope system differs fundamentally from the marine strontium isotope system this review emphasizes three important differences first large impacts are capable of resetting the 187os 188os to unradiogenic values without significantly affecting the marine strontium system second organic rich sediments are characterized by high 187re 188os resulting 187os 188os ingrowth trajectories are similar to the average slope of the cenozoic 187os 188os seawater record trends towards more radiogenic 187os 188os seawater therefore can be caused by weathering of organic rich sediments at a constant rate third the marine residence time of osmium is sufficiently short to capture short periodic glacial interglacial fluctuations that are inaccessible to the buffered marine strontium isotope system this offers the opportunity to discriminate between high frequency climatic and low frequency tectonic forcing

condensation origin for neoproterozoic cap carbonates during deglaciation neoproterozoic deglacial stratigraphy is commonly characterized by a sharp contact separating glacial sediments from laminated capping carbonates this stratigraphic relation is generally assumed to have time significance and to reflect an abrupt shift from icehouse to greenhouse conditions in contrast to this sequence stratigraphic field studies of an ediacaran ca 635 ma glacial to postglacial transition in the amadeus basin of central australia reveal a complex deglacial stratigraphy in which more than 175 m of conglomerate sandstone marl and carbonate at the basin margin and portions of four unconformity bounded sequences pass basinward into no more than 3 m of laminated dolomicrite of typical cap carbonate facies the unconformities which are characterized by as much as several tens of meters of erosional relief oblique sections of incised valleys separate intervals of contrasting sediment provenance and are confidently mapped on the basis of both criteria comparable unconformities are absent in the overlying neoproterozoic succession which is 2 km thick and encompasses many tens of millions of years the amadeus basin cap carbonate was thus deposited during a protracted interval of multiphase cyclical transgression more similar to phanerozoic cyclical sea level rise than to the single catastrophic deglaciation and instantaneous precipitation invoked by popular current models to explain the classic cap carbonate the superposition of carbonate on glacial facies in distal sections evidently records condensation in the absence of siliciclastic sediment rather than abrupt shifts between glacial and tropical conditions facies lithologically similar to cap carbonates may be less obvious in phanerozoic successions because of a secular change in carbonate composition to reefal and deep sea pelagic deposits

continental flood basalt weathering as a trigger for neoproterozoic snowball earth abstract atmospheric co2 levels and global climate are regulated on geological timescales by the silicate weathering feedback however this thermostat has failed multiple times in earth s history most spectacularly during the cryogenian c 720 635 ma snowball earth episodes the unique middle neoproterozoic paleogeography of a rifting low latitude supercontinent likely favored a globally cool climate due to the influence of the silicate weathering feedback and planetary albedo under these primed conditions the emplacement and weathering of extensive continental flood basalt provinces may have provided the final trigger for runaway global glaciation weathering of continental flood basalts may have also contributed to the characteristically high carbon isotope ratios 13 c of neoproterozoic seawater due to their elevated p contents in order to test these hypotheses we have compiled new and previously published neoproterozoic nd isotope data from mudstones in northern rodinia north america australia svalbard and south china and sr isotope data from carbonate rocks the nd isotope data are used to model the mafic detrital input into sedimentary basins in northern rodinia the results reveal a dominant contribution from continental flood basalt weathering during the ca 130 m y preceding the onset of cryogenian glaciation followed by a precipitous decline afterwards these data are mirrored by the sr isotope record which reflects the importance of chemical weathering of continental flood basalts on solute fluxes to the early middle neoproterozoic ocean including a pulse of unradiogenic sr input into the oceans just prior to the onset of cyrogenian glaciation hence our new data support the hypotheses that elevated rates of flood basalt weathering contributed to both the high average 13 c of seawater in the neoproterozoic and to the initiation of the first sturtian snowball earth

magellan observations of alpha regio implications for formation of complex ridged terrains on venus magellan images of alpha regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain we examine the complex ridged terrain of alpha regio using morphology and crosscutting relationships between structures to derive a sequence of tectonic events structures include broad 10 20 km wide linear and arcuate ridges fine scale 3 km wide ridges linear disruption zones ldzs up to several kilometers wide and numerous grabens 5 km wide and associated scarps and troughs based on their morphology we interpret the broad and fine scale ridges as compressional structures possibly folds ldzs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric graben and associated structures are interpreted as extensional features they crosscut ridges and ldzs and thus appear to be the youngest structures in alpha regio this sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain and highlands on venus in general a hotspot model and a coldspot model the presence of compressional features along and parallel to the margins of alpha regio and the lack of any high elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling mantle downwelling appears to be the most likely mode of formation of the upland of alpha regio and is likely to be important in other highland regions such as ovda and thetis regiones which are also dominated by complex ridged terrain

comparative hypsometric analysis of earth and venus the comparison between earth and venus global hypsometric distributions is reinvestigated on the basis of the high resolution data provided by the recent magellan global topographic coverage of venus surface and the detailed etopo 5 topographic database for the earth s surface the study of the cumulative hypsometric curve for both planets reveals that there exists a domain of elevation for which the relationship between the elevation and the square root of cumulative area percentage is linear this domain covers about 80 of the mapped venusian surface and nearly all the terrestrial oceanic surface 55 of the earth s surface and corresponds to an elevation range of about 2000 m for both planets proposing the upper threshold elevation of this domain as a common geophysical reference level it is shown that the terrestrial oceanic peak and the venusian peak coincide very closely in addition the terrestrial and venusian hypsometric distributions for elevations lower than this reference level are quite similar in their amplitude modal distribution and pronounced symmetry the correspondence of the venusian peak with the terrestrial oceanic peak is a striking feature but it does not necessarily imply the occurrence of plate tectonics on venus the present observation is consistent with the existence of a similar thermal isostasy acting at the planetary scale it places an upper bound at 90 10 km for the modal thermal lithosphcric thickness of venus if the venus mantle thermal conditions are earth like this estimate is reduced if hotter conditions occur

styles of tectonic deformations on venus analysis of venera 15 and 16 data analysis of the radar images obtained by veneras 15 and 16 leads to the conclusion that the ridge and groove structures on the surface of venus are the result of tectonic deformation although the mechanism of such deformation cannot yet be unequivocally deduced several styles of deformation can be described areal deformation occurs where horizontal stresses have operated over large areas shear deformation appears in bands showing differential longitudinal deformation transversal stresses operating over long and relatively narrow areas have produced belt deformation circular deformation is related to a specific locus of the stresses the absence of densely cratered areas indicates that the terrains were deformed after the period of heavy bombardment the origin of the stresses could be drag of the lithosphere by asthenospheric currents or gravity induced spreading of surface material over upwellings a general conclusion is that in the surveyed area of venus neither terrestrial plate tectonics nor lunar highland type terrain exists

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

on the anisotropy of secular variation deduced from paleomagnetic volcanic data we seek to establish whether or not secular variation the rate at which the magnetic field is changing in time is a function of field direction is secular variation anisotropic from a large number of paleomagnetic lava data many of which record polarity transitions we find that vectors from stratigraphically adjacent flows are most least correlated during nontransitional transitional periods since volcanic activity is unrelated to and therefore uncorrelated with magnetic secular variation this relationship indicates that secular variation is enhanced during transitions perhaps by a factor of 2 thus secular variation is anisotropic being a function of the deviation of field direction from an axial dipole despite this spatial dependence we find little convincing evidence that transitional fields exhibit persistent recurring quasi stationary states which manifest themselves in terms of midlatitude virtual geomagnetic poles vgps as has sometimes been hypothesized on the other hand we identify tentatively some relative quiescence in secular variation during midtransition when vgps fall near the equator numerous subsets of the database are examined for consistency and statistical errors are estimated by bootstrap analyses statistical models of secular variation need to incorporate the information content in serial correlations like those analyzed here from stratigraphie sections if the lava data are to be fully exploited

spatial and temporal relations between coronae and extensional belts northern lada terra venus preliminary studies of the distribution of coronae and volcanic rises on venus show that many of these features tend to cluster along zones of rifting and extension the plains north of lada terra are crossed by two such extensional belts each belt is composed of grabens ridges faults volcanic flows coronae and coronalike features the longer and more prominent belt is the nw trending alpha lada extensional belt which is over 6000 km long and 50 200 km wide and includes the coronae eve tamfana carpo selu derceto otygen and an unnamed corona south of otygen the second belt is the nne trending derceto quetzalpetlatl extensional belt which is about 2000 km long and in places over 300 km wide and includes the coronae sarpanitum eithinoha and quetzalpetlatl the two belts intersect at the 1600 600 km wide derceto volcanic plateau it is apparent that deformation along the two belts overlapped in time though deformation along the alpha lada extensional belt probably continued after the deformation along the derceto quetzalpetlatl extensional belt terminated in certain areas volcanism originated in grabens within the extensional belts whereas in other areas such as in eve selu derceto and quetzalpetlatl volcanism originated in the coronae and flowed into the lower parts of the extensional belts regional extension has affected the evolution of all the coronae at some stage of their development regional deformation occurred before the initiation of derceto and eithinoha and after the initiation of carpo tamfana otygen and sarpanitum it is thus unlikely that coronae formation along the belts is solely a consequence of the regional extension and it is also unlikely that regional extension has been caused solely by the coronae no corona along the belts was formed subsequent to the cessation of the regional extension we therefore suggest that the regional extension and the coronae are interrelated some of the coronae may have determined the location of the surface expression of the regional extension whereas the locations of other coronae may have been influenced by the concentration of regional extensional stresses

hot spot heat transfer its application to venus and implications to venus and earth gross similarities between earth and venus suggest that both planets might be expected to lose heat by the same mechanism available data do not resolve plate tectonics on venus however and other heat loss mechanisms such as hot spot heat loss have been suggested using a model which gives a relationship among surface elevation lithospheric thickness and heat flux we test the hot spot heat loss mechanism for venus and determine that it can easily explain the predicted heat loss of the planet with a modest number of hot spots of the order of 35 approximately 93 of the mapped topography of venus can be explained solely on the basis of lithospheric thickness variations additional compensation is required for topography above a radius of 6053 km and this can be effected by incorporating a variable thickness crust into the model if crust is assumed to be generated on the crests of the hot spots probably by processes associated with volcanism the model is consistent with almost 99 of the mapped venus topography the model is also basically consistent with available gravity data and interpretations which indicate compensated topography and great depths of compensation 100 1000 km for the mid latitudes of the planet the remaining approximately 1 of the topography not explained by hot spot crustal generation is thought to be compensated at a shallower depth primarily by variations in crustal thickness which are not directly related to hot spot volcanism the models relating surface elevation to lithospheric thickness and heat flow are grossly consistent with either a plate tectonic or hot spot heat loss mechanism for venus the range of lithospheric thicknesses implied by the range of elevations on venus implies that if plate tectonics is the dominant heat loss mechanism on venus the mean plate velocities are slower than on earth slow spreading ridges should be resolvable by the available venus topographic data but are not apparent comparison and analysis of the hypsometric curves for earth and venus suggest basic differences between the curves the skewed terrestrial ocean floor curve being consistent with plate tectonics but the relatively symmetrical venusian curve indicates that either plate tectonics differs on venus from the terrestrial analogue or that plate tectonics does not dominate the topographic expression of venus tectonism the venusian curve is consistent with a hot spot heat loss mechanism a hot spot crustal genesis mechanism is proposed which would probably result in crust very similar to the granite greenstone terranes of many terrestrial archaean cratons we speculate that hot spot heat loss may have preceded plate tectonics as the dominant terrestrial heat loss mechanism

δ13c depth profiles from paleosols across the permian triassic boundary evidence for methane release stable carbon isotopic analyses of organic carbon 13 c in individual paleosol profiles from permian triassic sequences of antarctica reveal systematic isotopic variations with profile depth these variations are in many cases analogous to those in modern soils which are functions of redox conditions soil development and degree and type of microbial decay in modern soils these isotopic depth functions develop independently from vegetation changes c 3 versus c 4 vegetation and can be diagnostic of soil orders this study shows that soil intrinsic functions can be preserved in the 13 c values of paleosols as old as 260 ma and constitute valuable data for paleoecological interpretations a large carbon isotopic offset of as much as 10 in whole paleosol profiles across the permian triassic boundary indicates significant changes in the soil biogeochemistry and the soil atmosphere system early triassic paleosols are distinctive in their extremely low 13 c values to 42 and often show an anomalous 13 c depth distribution compared to both permian paleosols and modern soils highly depleted 13 c values as the ones in early triassic paleosols are suggested to be associated with microbial methane oxidation methanotrophy this hypothesis implies increased methane concentrations in the early triassic soil atmosphere system increased atmospheric methane was probably partly responsible for the global carbon isotopic shift documented in marine and terrestrial sediments across the permian triassic boundary

alteration in the irdp drill hole compared with other drill holes in iceland the overall alteration pattern in the drill hole at reydarfjordur is very similar to alteration patterns observed in icelandic geothermal areas and in low grade metamorphosed basalts in deep crustal sections elsewhere in iceland however more detail is obtained by the study of the irdp drill core than by study of drill cuttings sampled in previous drill holes in iceland a comparatively high fossil thermal gradient is obtained at reydarfjordur by a combination of mineral stability data and the observed occurence of secondary minerals this high gradient is consistent with the measured dike dilation at the drill site and the location of the drill site adjacent to a central volcano

geochemical evidence for suppression of pelagic marine productivity at the cretaceous tertiary boundary the normal biologically productive ocean is characterized by a gradient of the 13c 12c ratio from surface to deep waters here we present stable isotope data from planktonic and benthic micro fossils across the cretaceous tertiary boundary in the north pacific which reveal a rapid and complete breakdown in this biologically mediated gradient the fluxes of barium a proxy for organic carbon and caco3 also decrease significantly at the time of the major marine plankton extinctions the implied substantial reduction in oceanic primary productivity persisted for 0 5 myr before the carbon isotope gradient was gradually re established in addition the stable isotope and preservational data indicate that environmental change including cooling began at least 200 kyr before the cretaceous tertiary boundary and a peak warming of 3 c occurred 600 kyr after the boundary event

variable dispersion of neogene geomagnetic field directions in iceland abstract analysis of the primary paleomagnetic directions in 4000 lava flows in iceland reveals that the magnitude of total virtual geomagnetic pole dispersion has decreased during the last 15 ma this dispersion which is independent of the polarity of the field is in part caused by the occurrence of erratically varying paleofield directions during intermittent periods possibly as long as 100 ka

problems in applying preservation facies and sequence models to sinian neoproterozoic glacial sequences in australia and asia abstract the sinian 800 530 ma is an extraordinary period of earth history characterized by the transition from simple to complex multicellular organisms and by extreme variations in climate eustasy and possibly tectonics understanding sinian stratigraphy may help interpret the co evolution of life and environment during the period though sinian successions are poorly dated and difficult to correlate preservation facies and sequence models based on mesozoic recent successions may be used to interpret them preservation models based on basin subsidence permit estimates of how thick and complete successions can be in different tectonic settings and how long it took for them to be deposited facies models for rift craton shelf slope and basin environments help in identification of these different tectonic settings sequence models facilitate interpretation of the interplay of tectonic subsidence eustasy rates of sedimentation and climate such models applied to the sinian successions of australia and asia indicate the following great shallowing upwards megacycles occur in sinian successions and each ends with regional glaciations deep rift basins formed around 850 800 ma preserve the oldest megacycles but these basins soon were filled with rapidly deposited glaciogenic sediments relatively complete sequences are found only in deep basins subject to repeated rifting cratons and shallow shelves preserve only the youngest megacycles older deposits have been removed by younger glacial advances deep shelves and slopes preserve one to three megacycles depending on the timing of the start of passive margin subsidence megasequences can be correlated based on limited radiometric dating and inferred regional distribution of unique rock types but these sequences are not developed to the same extent in all blocks problems in correlating the four great megacycles recognized elsewhere with those of asia will persist until the requisite facies analysis and u pb dating of interbedded volcanics are done

terrestrial record of methane hydrate dissociation in the early cretaceous reconstruction of changing c isotopic composition of early cretaceous atmospheric co2 from fossilized c3 vascular land plant tissue revealed a brief and striking negative excursion d 25 in atmospheric d 13 c followed by a rapid positive compensation d 15 during the aptian ca 117 ma mass balance calculations show that dissociation of a small amount of methane gas hydrate is the most tenable cause of the negative excursion this would also result in an increased co2 o2 mixing ratio as o2 is consumed during ch4 oxidation to co2 spurring the exponential phase of angiosperm biogeographic expansion

magma flow directions inferred from field evidence and magnetic fabric studies of the streitishvarf composite dike in east iceland abstract anisotropy of magnetic susceptibility ams and rock magnetic studies have been made on three outcrops separated by 12 km along strike nne ssw on the streitishvarf composite dike in east iceland samples for this study have been collected from the inner quartz porphyry part of the dike which show clear field evidence of a lateral flow component from north to south at one of the sites this flow component is consistent with margin ams results from all three sites the quartz porphyry has a substantial bulk magnetic susceptibility 10 2 si mainly carried by magnetically soft titanium poor titanomagnetite mdf 15 mt the ferrimagnetic grains yield a characteristic remanent magnetization in all three sites which gives a virtual geomagnetic pole at latitude 52 6 s and longitude 319 6 e the degree of anisotropy is low p j 1 033 and the magnetic fabrics shifts from oblate to prolate shapes depending on dike margin and outcrop the magnetic fabric has been interpreted according to the imbrication model using the minor susceptibility axis as shear plane indicator the absolute directions given by the minor susceptibility are then quantified using vector algebra the magma flow is indicated as an upward directed flow flowing from north to south with an inclination between 30 and 64 with a 95 confidence ellipse of 3 9 a model for the intrusion of the streitishvarf dike has been constructed where a magma pocket with felsic magma is punctured by a mafic dike enabling the felsic magma to rise and extend to the south within the pathway created the results of this study confirm the applicability of ams in studies of magma flow directions in igneous dikes of felsic composition

crustal drift in iceland summary there are indications that tertiary and post tertiary volcanism in iceland has been confined to the present main belt of active volcanism the geological structure of the tertiary lava piles in the east and west of iceland and results of geophysical studies may be explained by invoking an appreciable amount of crustal drift since the beginning of the tertiary the rocks in the extreme east and west of the island may have been carried apart by 400 km or more the crustal drift is believed to result mainly from crustal extension through the injection of dykes the structure of iceland and possibly that of other flood basalt areas is believed to be closely related to the world wide rift system

the mode of emplacement of neogene flood basalts in eastern iceland facies architecture and structure of the holmar and grjota olivine basalt groups abstract holmar and grjota are two stratigraphically distinct transitional alkaline olivine basalt lava groups within the westward dipping neogene flood basalts of eastern iceland the holmar olivine basalt group separated from the overlying grjota olivine basalt group by only a few tholeiite flows can be traced over 80 km north south with thicknesses varying from 250 m where thickest to 30 m where thinnest the grjota group can be traced over 50 km also north south reaching thicknesses of 250 m and thinning down dip to 10 m in contrast to other groups in eastern iceland that thicken down dip the studied olivine basalt groups thicken up dip the groups filled topographic confinements and formed aprons around central volcanoes we have estimated the minimum volumes to be 119 km 3 for holmar and 86 km 3 for grjota scoria cones are found in the holmar group and two thick olivine dolerite sills cross cut the holmar group and probably belong to the plumbing system that fed the grjota group the architecture of the lava groups are near identical the architecture is compound with lobes stacked horizontally and vertically varying from 1 15 m thick and 2 200 m long but do also encompass a number of thicker 15 20 m and more extensive 1 km long lava lobe in the stacks filled lava tubes are commonly observed within the lava flows the constituent lobes of the flows are often directly emplaced or welded together suggesting rapid buildup but are also found interbedded with redbeds and thicker tuff deposits and occasionally preserve tree molds the internal structure follows the characteristics for lava lobe morphology in general with an upper vesicular crust forming half to one third of the total thickness a massive core with abundant vesicle cylinders and a thin basal vesicular crust flow tops are of the pahoehoe type seldom found with scoria or clinker inflation structures such as tumuli and inflation clefts were identified in the flows the eruptions were dominantly fissure fed with a few vents evolving to short lived point source volcanism the pre existing topography exerted control on the advancement of the flows and influenced the final architecture of the groups at each location it is evident that the large and dense spatial distribution of these groups including numerous thick lava units suggests large volcanic episodes together the groups imply a period with higher production of olivine basalts in iceland and form well defined stratigraphic markers the transitional alkaline olivine basalt groups are interpreted to delineate a fossil southward propagating rift flank volcanic zone located east of a now extinct and buried mature rift zone

orbitally forced ice sheet fluctuations during the marinoan snowball earth glaciation the presence of dynamic ice sheets during snowball earth glaciations is controversial geological evidence and ice sheet modelling suggest that ice sheets may have responded to orbital forcing when pco2 reached a certain threshold

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

precise dating of the holmatindur cooling event in eastern iceland evidence for mid miocene bipolar glaciation a succession of basaltic lavas volcaniclastic sediments and lignite beds in the holmatindur region of eastern iceland provides the means for wing magnetic reversal stratigraphy and a record of major north atlantic cooling to a precise radiometric timescale a prominent part of the section is the holmatindur clastic bed which is up to 66 m thick and can be traced 80 km along strike the geological significance of this unit lies in its composition and associated thin lignite seams the volcaniclastic composition is basaltic in contrast to the common rhyolitic dacitic composition of tuff units in the area and hyaloclastic material is dominant hydroexplosive volcanism most likely produced the hyaloclastite through subglacial eruptions within the paleovolcanic zone pollen assemblages from the lignite beds indicate a dramatic climatic deterioration from subtropical to cool temperate conditions through these sediments feldspar crystals separated from the holmatindur clastic bed were determined to be from 10 72 0 16 ma from 4oar 39ar incremental heating experiments we correlate the holmatindur cooling event with a relative maximum in benthic foraminiferal bso zone mi6 of miller et al 1991 which has been interpreted as an episode of ice accumulation within the mid miocene cooling period as well as with pulses in ice rafted debris supplied to deep water sites in the north atlantic ocean a global sea level drop of 70 m and submarine canyon cutting all of which occurred within the early part of magnetic anomaly 5 normal c5n while strong evidence exists for mid miocene ice sheets in antarctica northern hemisphere glaciation is thought to have started much later the iceland sites studied here demonstrate that significant ice accumulation occurred albeit intermittently as early as late middle miocene time in the north atlantic ocean from radiometric dating of tertiary lavas that lie below and above the well established lower boundary c5n we conclude that the onset of this long normal polarity period occurred at 10 94 0 16 ma the new data are compatible with the cande and kent 1995 interpolated estimate for this boundary 10 95 ma but are significantly older or more precise than previous direct dating by conventional k ar methods

stratigraphy and paleomagnetism of a 2 9 km composite lava section in eyjafjordur northern iceland a reconnaissance study eight mountainside profiles in lava flows south of the fjord eyjafjordur northern iceland were sampled for paleomagnetic studies the sampling was concurrent with pilot stratigraphic mapping of the lava sequences in these and several supplementary profiles the eight profiles are correlated with minor overlaps so that they form a composite section of 2 9 km thickness estimated to cover the age range between about 9 and 5 million years ago hardarson et al 1999 paleomagnetic measurements made on 319 lavas generally yield primary remanence directions of high stability and within unit consistency evidence for at least 17 reversals of the geomagnetic field is seen as well as numerous field excursions frequent clustering of directions in successive lavas indicates that the volcanism was episodic

influence of rift jump and excess loading on the structural evolution of northern iceland 1 new structural data combined with published structural and geochronological data allow reconstruction of the structural evolution that followed the last rift jump across northern iceland tertiary lava flows erupted along the skagafjordur paleo rift have been down bent under the weight of and in the direction of plio pleistocene lava flows emitted from the northern volcanic zone and the central part of iceland this down bending process involved development of local flexure zones and a flexural extension along the resulting monoclines this structural reorganization explains the existence of the hunafloi skagi synform without need for a paleo rift axis along it in agreement with previous radiometric dating the large amount of plio pleistocene lava flows erupted in central iceland may have been enhanced by ice cap loading

venus topography and kilometer scale slopes during the first 8 months of the magellan mission the radar altimeter has made some three million measurements of the surface of venus covering the latitude range from 85 n to 80 s methods involving range correlation doppler filtering multi burst summation and range migration are used to focus the observations and to achieve high surface resolution results are presented as maps of the global distribution of topography meter scale roughness and power reflection coefficient the results are similar to those reported in previous experiments surface heights exhibit a unimodal distribution with more than 80 of the surface lying within 1 km of the 6051 84 km mean radius but the higher resolution of the magellan altimeter has disclosed several surprisingly steep features e g the southwest face of the maxwell montes the southern face of the danu montes and the chasmata to the east of thetis regio where average kilometer scale slopes of greater than 30 are not uncommon this conclusion is corroborated by close inspection of synthetic aperture radar imagery

tale of three cratons tectonostratigraphic anatomy of the damara orogen in northwestern namibia and the assembly of gondwana comment and reply

eocene plutonic rocks in south central idaho large areas previously thought to be underlain by cretaceous granodiorite of the idaho batholith are now known to be underlain by plutonic rocks of the eocene challis magmatic episode 50 44 ma recent mapping conducted in south central idaho as part of the u s geological survey s conterminous united states mineral assessment program cusmap indicates that roughly 30 of the exposed plutonic rocks are part of this younger event the eocene plutonic rocks are subdivided into a pink granite suite consisting of varieties of biotite granite and a quartz monzodiorite suite which includes hornblende biotite granite granodiorite quartz monzodiorite diorite and gabbro the members of each suite form discrete epizonal stocks and some exposures of pink granite are of batholithic size silica content of the quartz monzodiorite suite ranges from 48 to 70 the rocks vary considerably in texture and mineralogy the most mafic rocks of the quartz monzodiorite suite are cumulates these are layered intrusive phases containing pyroxene olivine and plagioclase other mafic phases include hornblende and pyroxene rich diorites which contain plagioclase feldspar as an interstitial phase and are cumulates in the sense that they probably represent rocks that were enriched in early formed mafic minerals by some fractionation process stocks of the pink granite suite are more compositionally restricted and range from 70 to 77 in sio2 content the quartz monzodiorite suite appears to be older than the pink granite suite but age data are limited field relations initial 87sr86sr ratios cao content and mgo mgo feo ratios demonstrate that the two suites are not related by fractional crystallization rather the respective magmas probably originated from different sources large ranges in mgo sc cr and co content in the quartz monzodiorite suite can be explained by crystal liquid fractionation of olivine pyroxene and hornblende but variations in composition of parental magmas are also likely large variations of ba content in the pink granite suite are probably a result of removal or accumulation of variable amounts of k feldspar mixing between the two eocene suites has not been extensive but some varieties of hybrid pink granite were generated that contain mafic inclusions and have less than 72 sio2 the pink granite has geochemical affinities with a type granite but unlike typical a type granite it is not uniformly enriched in the highly charged cations such as zr nb y and ce by analogy to other a type granites the pink granite suite probably originated from partial melting of lower crustal rocks under high temperature vapor absent conditions

the mode of emplacement of neogene flood basalts in eastern iceland facies architecture and structure of simple aphyric basalt groups simple flows tabular in the neogene flood basalt sections of iceland are described and their mode of emplacement assessed the flows belong to three aphyric basalt groups the kumlafell group the holmatindur group and the hjalmadalur group the groups can be traced over 50 km and originate in the brei dalur thingmuli volcanic zone the groups have flow fields that display mixed volcanic facies architecture and can be classified after dominating type morphology the kumlafell and the holmatindur groups have predominantly simple flows of p hoehoe and rubbly p hoehoe morphologies with minor compound or lobate p hoehoe flows the hjalmadalur group has simple flows of rubbly p hoehoe but also includes minor compound or lobate flows of rubble and a simple flows are most common in the distal and medial areas from the vents while more lobate flows in proximal areas the simple flows are formed by extensive sheet lobes that are several kilometers long with plane parallel contacts some reaching thicknesses of 40 m aspect ratios 0 01 they have overlapping contacts and are free of tubes and inflation structures their internal structure consists generally of a simple upper vesicular crust a dense core and a thin basal vesicular zone the brecciated flow top is formed by clinker and crustal rubble the clinker often welded or agglutinated the simple flows erupted from seemingly short lived fissures and have the characteristics of cooling limited flows we estimate the effusion rates to be 105 m3 s for the simple flows of the kumlafell and holmatindur groups and 104 m3 s for the hjalmadalur group the longest flows advanced 15 20 km from the fissures with lava streams of fast propagating flows inducing tearing and brecciation of the chilled crust compound or lobate areas appear to reflect areas of low effusion rates or the interaction of the lava with topographic barriers or wetlands resulting in chaotic flowage slowing lobes with brecciated flow tops developed into a flows the groups interdigitated with lava groups from the rey arfjor ur volcanic zone to the east and the exhumed brei dalur thingmuli volcanic zone which appear to have formed as a flank lineament parallel the main rift zone flood basalt volcanism in flank areas may support a mantle anomaly more pronounced and or perhaps more widespread in the neogene of iceland than today eruptions of simple flows have not been observed in modern times and are significant for models of crustal accretion in iceland and other large igneous provinces

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

some aspects of the late tertiary geomagnetic field in iceland summary in 1972 78 the late n d watkins and others carried out ajoint field programme of geological mapping in the mio pliocene flood basalts of iceland including sampling for k ar dating and palaeomagnetic research the major part of the palaeomagnetic sampling is represented by 2462 lavas in five long composite sections through the lava pile this paper deals with various statistical properties of this data set it is concluded that geomagnetic reversals occur more frequently than is assumed in the current ocean floor polarity time scale there is no evidence for significant asymmetries between normal and reverse polarity states of the field neither as regards chron lengths secular variation or virtual dipole moment magnitude intensities of remanence in these lavas are shown to be well approximated in terms of a hyperbolic distribution the latitude distribution of virtual magnetic poles can be fitted with a bingham function having k 4 5 and low latitude poles do not occur preferentially in any particular longitude interval

large coherent submarine landslide associated with pan african foreland flexure the ombonde detachment is a primary low angle normal fault that developed in an undeformed neoproterozoic carbonate shelf succession as it entered a west dipping pan african subduction zone the fault is mappable from the top of the shelf succession to the granitic basement surface at a paleodepth of 1 5 km and the hanging wall has not been significantly deformed the primary fault geometry is well constrained by stratigraphic cutoff relationships irrespective of secondary rotations the dip direction of the fault was 270 and its horizontal separation was 15 18 km the fault plane is composed of two ramps separated by a long flat segment at a paleodepth of 0 55 km the ramps are inclined 8 14 relative to the carbonate strata which underwent little or no compaction and the mean cutoff angle overall is 1 3 given constraints on the contemporaneous tectonic setting the primary fault dips must equal the stratigraphic cutoff angles augmented by a taper angle for lithospheric flexure of not more than 4 primary mean dips of 200 m which would have significantly reduced the water load and thereby the normal stress on a subhorizontal plane possibly leading to excess pore fluid pressures this scenario is consistent with the virtual absence of macroscopic shear deformation adjacent to the fault plane

facies dependent δ13c variation from a cryogenian platform margin south australia evidence for stratified neoproterozoic oceans abstract analysis of a cryogenian interglacial platform margin in the adelaide geosyncline reveals a strong carbonate 13 c facies relationship detailed chronostratigraphic correlation between sections ranging from shallow platform facies to deep basinal facies indicates the presence of a carbon isotopic gradient of between 8 and 11 in time equivalent strata shallow water back reef facies have 13 c values up to 8 2 while equivalent basinal sediments have 13 c values between 3 and 0 allochthonous blocks that have been transported from the platform margin into basinal environments retain their heavy 13 c values 4 9 and are surrounded by basinal calcareous shales with light 13 c values ave 0 8 the regional and stratigraphic consistency of these 13 c trends suggests a primary marine origin we interpret this 13 c facies correlation to be the result of ocean stratification stagnation that significantly reduced the rate of deep ocean ventilation and produced a large deep water organically derived carbon reservoir we suggest that stratification may have been a persistent feature of the global ocean throughout much of neoproterozoic time ocean stratification may explain many of the unusual features that characterise the sedimentary record of this era including large scale 13 c variation extreme climatic fluctuations and the presence of cap carbonates a highly variable climatic regime would be expected with the development of a large deep water carbon reservoir small changes in ocean circulation could rapidly transfer or remove large volumes of carbon to and from the surface ocean and atmospheric reservoirs leading to intense greenhouse or icehouse conditions respectively

benthic phosphorus regeneration net primary production and ocean anoxia a model of the coupled marine biogeochemical cycles of carbon and phosphorus we examine the relationships between ocean ventilation primary production water column anoxia and benthic regeneration of phosphorus using a mass balance model of the coupled marine biogeochemical cycles of carbon c and phosphorus p the elemental cycles are coupled via the redfield c p ratio of marine phytoplankton and the c p ratio of organic matter preserved in marine sediments the model assumes that on geologic timescales net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone the model incorporates the dependence on bottom water oxygenation of the regeneration of nutrient phosphorus from particulate matter deposited at the water sediment interface evidence from marine and lacustrine settings modern and ancient demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia dysoxia expands steady state simulations show that a reduction in the rate of thermohaline circulation or a decrease of the oxygen content of downwelling water masses intensifies water column anoxia dysoxia and at the same time increases surface water productivity the first effect reflects the declining supply of oxygen to the deeper parts of the ocean the second effect is caused by the enhanced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides sedimentary burial of organic carbon and authigenic calcium phosphate mineral francolite on the other hand is promoted by reduced ocean ventilation according to the model global scale anoxia dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system consequently higher rates of primary production and organic carbon burial can be achieved even when the continental supply of reactive phosphorus to the oceans remains unchanged

the source and fate of massive carbon input during the latest paleocene thermal maximum lithologic faunal seismic and isotopic evidence from the blake nose subtropical western north atlantic links a massive release of biogenic methane 55 5 million years ago to a warming of deep ocean and high latitude surface waters a large perturbation in the combined ocean atmosphere carbon cycle the largest of the past 90 million years a mass extinction event in benthic faunas and a radiation of mammalian orders the deposition of a mud clast interval and seismic evidence for slope disturbance are associated with intermediate water warming massive carbon input to the global exogenic carbon cycle pelagic carbonate dissolution a decrease in dissolved oxygen and a benthic foraminiferal extinction event these events provide evidence to confirm the gas hydrate dissociation hypothesis and identify the blake nose as a site of methane release

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

large scale spatial patterns in topography at alpha regio venus hypsometric topographic analysis of magellan altimetry data at alpha regio results in a multimodal distribution planimetric display of the spatial distribution for altitudes close to each mode reveals large scale ring like and crescentic patterns the latter result from the intersection of horizontal altitude ranges with sloping patterns that are quasi circular in shape the spatial patterns of topographic features near alpha regio have the following attributes 1 quasi circular shapes of individual elements 2 a hierarchy of distinctive sizes for those elements with diameters of various elements at approximately 2800 1500 500 700 and 200 km and 3 a large scale arrangement or pattern in which the 1500 km highland plateau of alpha regio is surrounded by a ring of 500 km structures all of which constitute a 2800 km megastructure we hypothesize that the intricate patterns of hierarchical scaling for large scale spatial elements of venusian topography result from corresponding complexity in venusian mantle convection

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

venus tectonics initial analysis from magellan radar imaging and altimetry data from the magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the venus surface the plains record a superposition of different episodes of deformation and volcanism strain is both areally distributed and concentrated into zones of extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression venus displays two geometrical patterns of concentrated lithospheric extension quasi circular coronae and broad rises with linear rift zones both are sites of significant volcanism no long large offset strike slip faults have been observed although limited local horizontal shear is accommodated across many zones of crustal shortening in general tectonic features on venus are unlike those in earth s oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide and separated by stronger and less deformed blocks hundreds of kilometers in width as in actively deforming continental regions on earth

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

evidence for late jurassic release of methane from gas hydrate four late jurassic carbonate successions deposited in the tethys atlantic ocean record a negative carbon isotope excursion of at least 2 the excursion is present in both organic and carbonate carbon records and is comparable in magnitude and duration to isotopic changes during the late paleocene thermal maximum our results indicate that during the late jurassic long considered a warm greenhouse time additional greenhouse gas was input to the atmosphere by a sudden release of methane from buried gas hydrate a potential triggering mechanism may have been the opening of an oceanic gateway through the early atlantic between the ancient tethys and pacific oceans