rapid extension in an eocene volcanic arc structure and paleogeography of an intra arc half graben in central idaho a study of extension volcanism and sedimentation in the middle eocene panther creek half graben in central idaho shows that it formed rapidly during an episode of voluminous volcanism the east southeast tilted panther creek half graben developed across the northeast edge of the largest cauldron complex of the challis volcanic field and along the northeast trending trans challis fault zone two normal fault systems bound the east side of the half graben one fault system strikes northeast parallel to the trans challis fault zone and the other strikes north to northwest the geometry of the basin fill deposits shows that movement on these two normal fault systems was synchronous and that both faults controlled the development of the panther creek half graben strikes of the synextension volcanic and sedimentary rocks are similar throughout the half graben whereas dips decrease incrementally upsection from as much as 60 to less than 10 previous k ar dates and a new 40 ar 39 ar plateau date from the youngest widespread tuff in the basin suggest that most of basin formation spanned 3 m y between about 47 7 ma and 44 5 ma as much as 6 5 km of volcanic and sedimentary rocks were deposited during that time although rates of extension and subsidence were very high intense volcanic activity continually filled the basin with ash flow tuffs outpacing subsidence and sedimentation until the end of basin development after the abrupt end of challis volcanism locally derived pebble to boulder conglomerate and massive reworked ash accumulated in the half graben these sedimentary rocks make up a small part of the basin fill in the panther creek half graben and were derived mainly from proterozoic metasedimentary rocks uplifted in the footwall of the basin the east southeast tilt of the sedimentary rocks their provenance and coarse grain size and the presence of a gravity slide block derived from tilted volcanic rocks in the hanging wall attest to continued tectonism during conglomerate deposition provenance data from the sedimentary rocks imply that the highland in the footwall of the panther creek half graben was never thickly blanketed by synextension volcanic rocks despite intense volcanic activity analysis of the panther creek half graben and other intra arc rift basins supports previous interpretations that relative rates of volcanism and subsidence control the proportion of volcanic rocks deposited in intra arc rifts

bounds on the viscosity coefficient of continental lithosphere from removal of mantle lithosphere beneath the altiplano and eastern cordillera 1 the rapid rise of the central andean plateau between 10 and 6 8 ma implies that mantle lithosphere including eclogitized lower crust was removed from beneath the region in that time interval we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum rayleigh taylor instability we place bounds on the average viscosity coefficient for central andean lithosphere when compared with laboratory measurements of flow laws for olivine and eclogite the allowed range of viscosity coefficients yields bounds on the temperature of 500 800 c at the moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than 50 mpa this range of temperature is comparable with if a slightly lower than we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity the mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part

calcium isotope constraints on the marine carbon cycle and caco3 deposition during the late silurian ludfordian positive δ13c excursion abstract this study investigates calcium isotope variations 44 40 ca in late silurian marine carbonates deposited in the prague basin czech republic which records one of the largest positive carbon isotope excursion cie of the entire phanerozoic the mid ludfordian cie which is associated with major climatic changes abrupt cooling and global sea level fluctuations our results show that during the onset of the cie when 13 c increases rapidly from 0 to 8 5 44 40 ca remains constant at about 0 3 0 1 relative to nist 915a while 87sr 86sr in well preserved carbonates are consistent with a typical ludfordian seawater composition ranging from 0 70865 to 0 70875 such decoupling between 13 c and 44 40 ca trends during the onset of the cie is consistent with the expected order of magnitude difference in the residence times of ca 10 6 yr and c 10 5 yr in the open ocean suggesting that the mid ludfordian cie was caused by processes where the biogeochemical pathways of c and ca in seawater were mechanistically decoupled these processes may include i near shore methanogenesis and photosynthesis ii changes in oceanic circulation and stratification and or iii increased production and burial of organic c in the global ocean the latter however is unlikely due to the lack of geological evidence for enhanced organic c burial and also because of unrealistic parameterization of the ocean c cycle needed to generate the observed cie over the relatively short time interval in contrast higher up in the section where 13 c shifts back to pre excursion baseline values there is a correlated shift to higher 44 40 ca values such coupling of the records of ca and c isotope changes in this part of the study section is inconsistent with the abovementioned differences in oceanic ca and c residence times indicating that the record of 44 40 ca changes does not faithfully reflect the evolution of the oceanic ca reservoir but rather some local processes in the prague basin these can be related to restricted elemental sediment cycling involving mixing of isotopically distinct ca sources and carbonate polymorphs calcite vs aragonite and or possible kinetic ca isotope effects due to changes in the rate of marine carbonate formation evidence supporting the kinetic effect in the studied mid ludfordian carbonates is indicated by correlated 44 40 ca and sr concentration data r s 0 76 p 0 001 n 41 yielding a slope of 0 00097 which is indistinguishable from the kinetic slope of abiotic calcite precipitation kinetic processes are integral to the model of rapid carbonate precipitation recently proposed by kozlowski 2015 to explain the origin of the mid ludfordian cie involving intense methanogenesis photosynthesis in near shore settings coupled with rapid caco3 precipitation i e massive whitings events and eustatically controlled carbonate hypersaturation of seawater more ca isotope studies are needed to shed light on the question of whether kinetics or mineralogy controls the coupled variations in carbonate 44 40 ca and 13 c records observed in this study and other large positive cies in geological record

an alternative model for positive shifts in shallow marine carbonate 13c and 18o positive shifts in global seawater d 13 cdic are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in d 18 o here an alternative model is proposed for the formation of local positive isotope shifts in shallow marine settings the model involves geochemically altered platform top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition both mechanisms are active on modern sub tropical carbonate platforms and result in low carbonate d 13 ca ndd 18 o relative to typical oceanic values during high amplitude transgressive events the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced and 13 c depleted platform top water mixes with open oceanic water masses having higher isotope values both factors are recorded as a transient increase in carbonate 13 c and 18 o relative to low background values these processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas

aragonite calcite seas quantifying the gray area oscillations between the dominance of aragonite and calcite in abiotic marine caco3 precipitates throughout earth history are closely coupled with the evolution of earth s seawater composition and represent the environmental context in which organisms evolved their ability to biomineralize the most important factor controlling these phanerozoic oscillations in caco3 polymorph composition is the ratio of mg ca in seawater which is thought to separate aragonite and calcite precipitation along a distinct temperature controlled threshold a sharp threshold however is contradicted by overlapping aragonite and calcite precipitation fields at a range of experimental conditions here we present experimental data that enable us to quantify the proportions of caco3 polymorphs as a function of mg ca ratio and temperature this allows us to convert published mg ca ratio proxy data and models of the phanerozoic mg ca ratio into proportions of abiotic caco3 polymorphs at a given temperature and thus provides a temperature corrected view of aragonite calcite sea conditions in this revised view abiotic calcite precipitation was inhibited during aragonite sea intervals at temperatures above 20 c whereas calcite sea intervals were characterized by the co precipitation of aragonite and calcite in environments above 20 c this continuous prominence of aragonite precipitation in phanerozoic warm water environments explains the phanerozoic increase of aragonite over calcite skeletal composition in calcifying organisms

climatic and oceanic changes during the middle late ordovician transition in the tarim basin nw china and implications for the great ordovician biodiversification event abstract the middle late ordovician transition darriwilian to sandbian age witnessed a major pulse of the great ordovician biodiversification event gobe and distinctive oceanic geochemical fluctuations such as coeval negative c and sr isotope excursions in this study investigations into geochemical variations notably the hg abundance or hg toc have been carried upon the organic rich black shale of the middle upper ordovician saergan formation to unravel the causes of this pulse based on these data three phases were identified phase 1 0 to 3 m is characterized by rising hg toc up to 138 ppb wt and ti al values as well as high ciacorr corrected chemical index of alteration values 68 9 72 3 with negligible enrichment of redox sensitive elements rse and nutrient elements e g u 5 2 ppm v 153 ppm mo 1 8 ppm p2o5 0 2 suggesting intensified volcanism which could have emitted significant amounts of greenhouse gases thereby leading to climate warming in contrast phase 2 3 to 11 m is characterized by decreasing hg toc and ti al ratios relatively low though slightly fluctuating ciacorr values generally depleted in rse except moderately enriched u up to 14 6 ppm and increased p al and ba al ratios implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling phase 3 sandbian age 11 13 m witnessed continuous decrease in hg toc ratio an increase in ti al and ciacorr values fairly low values of rse enrichment and p al and ba al ratios indicating recurrent climate warming and the potential for slowed oceanic circulation and attenuated upwelling of nutrient rich deep waters onto the shallow shelf these changes could have diminished bioproductivity and organic output onto the seafloor this study offers insights into volcanic climatic oceanic interactions during a major pulse of the gobe around the middle late ordovician transition while black shales were extensively deposited

comparative sedimentology of shelf carbonates of the middle ordovician st paul group central appalachians abstract observations of present day environments are commonly used to aid in the interpretation of ancient sedimentary sequences this approach is used to determine the paleoclimate and depositional processes operating during formation of the limestones and dolostones of the lower middle ordovician st paul group row park and new market formations of maryland and southern pennsylvania a 200 m thick carbonate platform deposit within the group six facies defined by suites of sedimentary structures faunal assemblages and early diagenetic features are arranged in a large scale regressive transgressive sequence these consist of facies i lower burrowed peloidal facies facies ii burrowed skeletal facies facies iii lower laminated peloidal facies facies iv stromatolitic peloidal facies facies v upper laminated peloidal facies and facies vi upper burrowed peloidal facies a comparison of the st paul group deposits with those of the modern peritidal carbonate environments of shark bay western australia the abu dhabi and trucial coast areas of the persian gulf and andros island and the great bahama bank reveals 1 an overall similarity in features between the st paul group and the sediments of andros island and the great bahama bank and 2 a dissimilarity between those two deposits and the sediments of shark bay western australia and shoreline areas of the persian gulf for example the supratidal zones of the latter two typically contain evaporite minerals e g gypsum and anhydrite whereas the supratidal deposits of andros island and the st paul group are freshwater algal marsh and lake sediments the subtidal deposits of shark bay and the persian gulf generally are mechanically layered skeletal sand the subtidal deposits of the great bahama bank and the st paul group are bioturbated peloidal micrites the st paul group and the great bahama bank deposits are similar on a facies scale down to fine details observed microscopically bahamian modern analogs of the facies of the st paul group are for facies i shallow restricted bank and subtidal pond environments for facies ii a normal marine subtidal environment similar to the shoals which rim parts of the great bahama bank for facies iii and v tidal channel levee pond and marsh subenvironments of the channeled belt environment hardie and garrett 1977 for facies iv levee marsh and freshwater lake subenvironments of the interior of andros island and for the facies vi a semirestricted subtidal bank environment based on the features of the st paul group and by analogy with modern deposits the st paul group was deposited in a low energy rainy environment where deposition was controlled by storm processes many facies display penecontemporaneous marine cements and dissolution features

carbon isotope records in a mesoproterozoic epicratonic sea carbon cycling in a low oxygen world abstract carbon isotope profiles of sedimentary strata are widely used as both a tool for stratigraphic correlation and as a mechanism for inferring important changes in earth s biogeochemical cycles chemostratigraphic interpretations however often rely on the assumption that the isotopic composition of dissolved inorganic carbon dic in the surface oceans is broadly homogeneous at times of globally high sea level when epicratonic epeiric environments dominate our sedimentary record carbon isotope records reveal substantial lateral variability here we investigate lateral variability in the marine carbon isotope record from pericratonic to epicratonic environments of the mesoproterozoic 1 1 ga atar el mreiti groups of the taoudeni basin west africa restricted marine epicratonic environments are consistently 2 4 lighter than open marine pericratonic environments suggesting input of an isotopically light carbon source that preferentially affected epicratonic environments in contrast to epicratonic environments in the paleozoic where input of isotopically light carbon is generally attributed to input of terrestrial organic matter we suggest that in situ remineralization of organic carbon via anaerobic microbial cycling drove observed isotopic variability within epicratonic el mreiti group strata the extent of organic matter remineralization and thus degree of 13 c depletion is correlated with water depth and associated with distinct differences in both total organic carbon toc and pyrite concentration suggesting a potential linkage to both the persistence of anoxia and the availability of sulfate within epicratonic environments in such settings the isotopic effects of organic carbon remineralization are potentially enhanced by either methane oxidation following methanogenic decomposition or the intermittent oxidation of an enhanced dissolved organic carbon doc pool

the cenozoic climatic and topographic evolution of the western north american cordillera herein we present oxygen isotope records from cretaceous to recent terrestrial sediments in the western north american cordillera the purpose of this analysis is to use oxygen isotope records to understand the coupled surface elevation and climate histories of this region through the cenozoic to do this we constructed 18o maps of surface waters for time intervals that trace the development of topography of western north america these maps are based on 4861 oxygen isotope analyses from both published 4478 and new 383 data we determined the 18o values of surface waters using paleotemperatures derived previously from floral assemblages and the appropriate isotope fractionation factors these data suggest that in the late cretaceous to early eocene the sevier hinterland formed a plateau of unknown height around 50 ma a topographic wave developed in british columbia and eastern washington that swept southward reaching northeastern nevada at 40 to 38 ma and southern nevada 23 ma this southward encroachment of an eocene plateau sweep caused reorganization of drainage patterns such that the intraforeland basins of wyoming and utah drainages extended deep within the sevier hinterland as the wave swept southward the landscape within the sevier hinterland developed into a rugged and high mountain range with the hypsometric mean elevation of 4 km and relief of 1 5 km this eocene highland was bordered on the west by a high sierra nevada ramp and on the east by the intraforeland basins that captured water draining these growing highlands the spatial and temporal evolution of this highland correlates with the timing of volcanism and extension these observations support tectonic models that call for north to south removal of the farallon slab or piecemeal removal of mantle lithosphere the isotopic data show that prior to growth of this highland the north american monsoon nam penetrated much farther north in the paleocene eocene than today the combined effects of global cooling increasing latitudinal temperature gradients and the generation of the orographic barrier created by the growing north to south highland produced a southward migration of the nam front by the oligocene the hydrologic regime that we observe today was in place it has been modified since then as a result of basin and range extension and collapse of the highlands in the mid miocene this collapse allowed the nam to penetrate farther north into the great basin of nevada and utah

magnetic imaging of the vredefort impact crater south africa while most impact craters are characterised by negative magnetic anomalies over their central regions aeromagnetic surveys over the vredefort meteorite impact crater reveal multiple concentric magnetic patterns with no significant anomaly at its centre we performed ground magnetic surveys across a portion of a prominent negative magnetic anomaly that extends in a broad semicircular belt about half way into the basement floor of the crater magnetic anomalies defined by our data are most often negative and occur over a wide range of wavelengths the longest wavelength negative anomaly coincides well with aeromagnetic data we find that this feature is centred over the amphibolite to granulite metamorphic facies transition exposed in the basement floor the transition zone is analogous to the conrad discontinuity observed at depths of about 20 km elsewhere in the kaapvaal craton petrographic studies show a marked increase in the intensity of the impact related thermal and shock metamorphism at this transition which we explain by the focusing and defocusing of shock waves at a rheologic interface during impact we therefore suggest that the magnetic signature at this boundary is caused by a combination of both thermal and shock effects related to the impact event a numerical model of the long wavelength anomaly suggests that it is underlain by a body of coherently magnetised rock whose direction and intensity are similar to those found in pseudotachylites and impact melts that formed during impact on the other hand negative anomalies occurring over smaller 100 to 20 m wavelengths often do not coincide with the surface geology these features cannot be modeled using the same criteria as that for the long wavelength anomaly

shallow water carbonate record of the paleocene eocene thermal maximum from a pacific ocean guyot the paleocene eocene thermal maximum petm ca 55 8 ma is associated with a rapid and large carbon cycle perturbation transient warming and deep sea acidification although this event is widely known from pelagic hemipelagic and continental records the lack of in situ shallow water carbonate platform sections inhibits interpretations of whether the petm had a significant effect on shallow water carbonate ecosystems limalok guyot ocean drilling program site 871 in the pacific ocean comprises a volcanic pedestal topped by a paleogene carbonate platform that drowned in the middle eocene carbon isotope stratigraphy of the platform carbonate sediments is used in conjunction with existing biostratigraphy to refine the stratigraphic framework of the carbonate platform although core recovery was poor the major late paleocene middle eocene stratigraphic trends in carbon isotopes are recognized including a prominent 3 negative carbon isotope excursion recording the onset of the petm the lithological and paleontological record of the petm on limalok guyot shows no major evidence for a carbonate production crisis suggesting that the effects of any changes in temperatures or surface ocean ph were relatively short lived or relatively minor

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

stable isotopic evidence for a pre middle miocene rain shadow in the western basin and range implications for the paleotopography of the sierra nevada 1 a recently proposed method for approximating the paleorelief of mountain belts makes use of the predictable relationship between the isotopic depletion of precipitation and the net elevation of an orographic barrier over which an air mass rises this rain shadow effect often creates desert regions on the lee side of mountain belts in which precipitation is isotopically light changes in the isotopic composition of precipitation can be estimated from the isotopic composition of authigenic or pedogenic minerals which can then be used to infer both the development of relief during orogenesis as well as the stability of a rain shadow formed by developing mountains the 18o of smectites formed from the weathering of middle miocene to late pliocene volcanic ashes currently exposed in the rain shadow of the modern sierra nevada of california show no indication of large scale late cenozoic surface uplift of the sierra and corresponding regional rain shadow development rather smectite isotope data tentatively suggest that elevations may have decreased over this time by as much as 2000 m toward the southern end of the range and 700 m in regions farther north this suggests that the modern rain shadow cast over the western basin and range has been in existence since pre middle miocene and that the sierra nevada have been a prominent orographic barrier since before this time these interpretations are in accord with several recent studies also suggesting a possible cenozoic elevation loss of an already developed sierra nevada mountain range

error analysis of co2 and o2 estimates from the long term geochemical model geocarbsulf long term carbon and sulfur cycle models have helped shape our understanding of the phanerozoic history of atmospheric co 2 and o2 but error analyses have been largely limited to testing only a subset of input parameters singly as a result the full ranges of probable co2 and o 2 are not quantitatively known here we investigate how variation in all 68 input parameters of the geocarbsulf model both singly and in combination affect estimated co 2 and o 2 we improve formula tions for land area runoff and continental temperature the latter of which now excludes land area not experiencing chemical weathering we find our resampled model co 2 and o2 estimates are well bounded and provide high confidence for a double hump in co2 during the phanerozoic with high values during the early paleozoic and mesozoic and low values during the late paleozoic and late mesozoic to cenozoic our analyses also support a distinct atmospheric o2 peak during the late paleozoic 30 followed by low values near the triassic jurassic boundary 10 most of the spread in co2 is contributed by three factors climate sensitivity to co 2 doubling and the plant assisted chemical weathering factors life and gym c o2 estimates during the paleozoic to early mesozoic are highly concordant with indepen dent records from proxies but are offset to lower values during the globally warm late mesozoic to early cenozoic the model proxy mismatch for the late mesozoic can be eliminated with a change in gym within its plausible range but no change within plausible ranges can resolve the early cenozoic mismatch either the true value for one or more input parameters during this interval is outside our sampled range or the model is missing one or more key processes

assessing offsets between the δ13c of sedimentary components and the global exogenic carbon pool across early paleogene carbon cycle perturbations 1 negative stable carbon isotope excursions cies across the paleocene eocene thermal maximum petm 56 ma range between 2 and 7 even after discounting sections with truncated records individual carbon isotope records differ in shape and magnitude from variations in the global exogenic carbon cycle through changes in 1 the relative abundance of mixed components with different 13c within a measured substrate 2 isotope fractionation through physiological change and 3 the isotope composition of the carbon source all three factors likely influence many early paleogene 13c records especially across the petm and other hyperthermal events we apply these concepts to late paleocene early eocene 58 52 ma records from lomonosov ridge arctic ocean linear regression analyses show correlations between the 13c of total organic carbon toc and two proxies for the relative contribution of terrestrial organic components to sediment toc the branched and isoprenoid tetraether index and palynomorphs we use these correlations to subtract the terrestrial component from 13ctoc and calculate marine organic matter 13c the results show that the magnitude of the cie in 13ctoc across the petm is exaggerated relative to the magnitude of the cie in 13cmom by 3 due to increased contributions of terrestrial organic carbon during the event collectively all carbon isotope records across the petm and other major climate carbon cycle perturbations in earth s history are potentially biased through one or more of the above factors indeed it is highly unlikely that any 13c record shows the true shape and magnitude of the cie for the global exogenic carbon cycle for the petm we conclude that cie in the exogenic carbon cycle is likely 4 but it will take additional analyses and modeling to obtain an accurate value for this cie

the influence of air sea exchange on the isotopic composition of oceanic carbon observations and modeling although the carbon isotropic composition of ocean waters after they leave the surface ocean is determined by biological cycling air sea exchange affects the carbon isotopic composition of surface waters in two ways the equilibrium fractionation between oceanic and atmospheric carbon increases with decreasing temperature in southern ocean surface waters this isotopic equilibrium enriches delta sup 13 c relative to the delta sup 13 c expected from uptake and release of carbon by biological processes alone similarly surface waters in the subtropical gyres are depleted in delta sup 13 c due to extensive air sea exchange at warm temperatures countering the tendency toward isotopic equilibration with the atmosphere a relatively slow process are the effects of the equilibration of co sub 2 itself a much faster process in regions where there is a net transfer of isotopically light co sub 2 from the ocean to the atmosphere e g the equator surface waters become enriched in sup 13 c whereas in regions where isotopically light co sub 2 is entering the ocean e g the north atlantic surface waters become depleted in sup 13 c a compilation of high quality oceanic delta sup 13 c measurements along with experiments performed using a zonally averaged three basin dynamic ocean model are used to explore these processes 40 refs 14more figs 1 tab less

paired isotope records of carbonate and organic matter from the middle ordovician of argentina intrabasinal variation and effects of the marine chemocline abstract we investigate the expression of the middle darriwilian isotope carbon excursion mdice across marine shelf environments in the argentine precordillera previous work identified the mdice in the las chacritas formation in argentina but did not recognize the expression of the mdice in time equivalent strata of the deeper water las aguaditas formation albanesi et al 2013 palaeogeography palaeoclimatology palaeoecology v 398 p 48 66 recent biostratigraphic investigations of these units have for the first time provided the opportunity for high resolution correlation which suggest that the mdice or at least the initiation of the mdice should be observed in both the las chacritas and las aguaditas formations here we present new paired carbon isotope data of carbonate and organic carbon from the las chacritas and las aguaditas formations we identify a 2 positive shift in the isotopic composition of marine carbonate in the las chacritas formation whereas values abruptly fall to

lithospheric topography tilted plumes and the track of the snake river yellowstone hot spot 1 the trace of the snake river yellowstone hot spot is the world s best example of a mantle plume that has been overridden by continental lithosphere the standard model calls for the plume head to rise under northern nevada and be forced northward to form basalts of the columbia plateau subsequent movement of north america to the southwest over the plume tail created a hot spot trace on the surface we present a new conceptual model for the origin of this feature that resolves inconsistencies in the current standard model and explains the recent documentation of a thermal anomaly in the mantle below yellowstone today that plunges 65 wnw our model implies that the plume tail was forced beneath thinned cratonic lithosphere to the se along with part of the plume head and has remained in this orientation for the last 12 ma we infer that almost all of the volcanism in se oregon and sw idaho prior to 12 ma results from overriding the southern extension of the plume head not the plume tail and that a distinct plume tail hot spot track was not established until formation of the bruneau jarbidge eruptive center around 12 ma the plume tail track may also be controlled by a preexisting structural boundary in lithosphere that is thinner than adjacent lithosphere this model demonstrates the potential importance of lithospheric topography on controlling the surface manifestation of plume volcanism and the complexity that may arise when lithospheric thickness is nonuniform

paleo environmental changes during the middle late ordovician transition on the yangtze platform south china and their ecological implications abstract rocks of darriwilian and sandbian age middle late ordovician record the extraordinary peak of the great ordovician biodiversification event gobe and significant global environmental changes to track paleo environmental changes through this critical time we have analyzed iron speciation trace elements and total organic carbon in samples of the miaopo formation from an intra shelf basin in south china the results reveal moderate primary productivity and possible fluctuations in benthic redox upwelling during marine transgression is thought to have increased the availability of nutrients in surface waters sustaining the biological pump enhanced export of organic matter would have consumed dissolved oxygen at depth driving the system toward anoxia however it is also possible for deep water renewal events to transfer more dissolved oxygen into the bottom water those proxies suggests high primary productivity and varying sea level global and local not benthic anoxia as the driver of accumulated black shale thus the local marine ecosystem thrived in this oxic intra shelf basin challenging the preexisting paradox that the high diversity community flourished in an oxygen depleted environment

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

isotopic evidence for geochemical decoupling between ancient epeiric seas and bordering oceans implications for secular curves isotopic analysis of conodonts and their host limestones sampled between two regionally extensive altered volcanic ash layers in eastern laurentia shows that a 454 ma epeiric sea maintained large lateral differences in nd and c isotope compositions this is consistent with inferred temperature salinity defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments because the majority of old marine fossils and sedimentary rocks are known from epeiric seas some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses rather than global scale changes in the state of the earth ocean system

strontium and carbon isotope chronostratigraphy of barremian aptian shoal water carbonates northern tethyan platform drowning predates oae 1a abstract a widely accepted hypothesis proposes that the early aptian demise of carbonate platforms in the northern tethyan realm reflects the impact of environmental changes that eventually led to the deposition of organic rich basinal sediments during oceanic anoxic event oae 1a in fact the temporal correlation of oceanic anoxia and platform drowning has not been rigorously tested and evidence indicating causality is debated the present paper provides a high resolution carbon and strontium isotope chronostratigraphy combined with detailed sedimentological analysis applied to the barremian aptian urgonian carbonate platform development at the northern rim of the tethys subalpine chains haute savoie ese france a characteristic barremian aptian carbon and strontium isotope pattern below the onset of oae 1a interval permits precise platform to basin correlation with the barremian stratotype locality of angles se france this chemostratigraphic pattern equally allows for a correlation with well studied shoal water and pelagic records of the northern and central tethys strontium isotope stratigraphy provides a high resolution numerical age data set indicating an earliest aptian age

the response of two tethyan carbonate platforms to the early toarcian jurassic oceanic anoxic event environmental change and differential subsidence chemostratigraphic analyses 87 sr 86 sr d 13 ccarb of limestones from two jurassic platform carbonate sequences in italy trento and campania lucania platforms illustrate previously established trends found in pelagic sediments and skeletal carbonates from biostratigraphically well calibrated sections elsewhere in europe chemostratigraphic correlations between the platform carbonate successions and appropriate intervals from well dated reference sections allow the application of high resolution stratigraphy to these shallow water peritidal carbonates and furthermore elucidate the facies response to the early toarcian oceanic anoxic event oae lower jurassic toarcian levels of the western trento platform southern alps northern italy contain spiculitic cherts that appear where rising carbon isotope values characterize the onset of the oae a palaeoceanographic phenomenon interpreted as driven by increased nutrient levels in near surface waters there is a facies change to more clay rich facies at the level of the abrupt negative carbon isotope excursion also characteristic of the oae higher in the section the campania lucania platform southern apennines southern italy records a change to more clay rich facies where carbon isotope values begin to rise at the beginning of the oae but the negative excursion higher in the section occurs within oolitic facies although in both examples the early toarcian oae can be recognized by a change to more clay rich lithologies this facies development is diachronous and in neither case did the platform drown although the trento platform in the south west sector studied here was adversely affected by the oae it did not drown definitively until late aalenian time the campania lucania platform persisted throughout the jurassic and cretaceous differential subsidence rates which can be calculated using comparative chemostratigraphy are identified as a crucial factor in the divergent behaviour of these two carbonate platforms relatively fast in the case of the trento platform relatively slow in the case of the campania lucania platform it is proposed that where water depths remained as shallow as a few metres during the oae campania lucania platform dissolved oxygen levels remained high nutrient levels relatively low and conditions for carbonate secretion and precipitation remained relatively favourable whereas more poorly ventilated and or more nutrient rich waters trento platform adversely

carbon isotope variability across the ediacaran yangtze platform in south china implications for a large surface to deep ocean δ13c gradient new isotope data obtained from relatively conformable carbonate rich strata of the ediacaran yangtze platform in south china reveal substantial 13 c variability in platform sections four negative 13 c anomalies with a nadir down to 8 pdb are present in the interval between the cap carbonate level 635 ma and the precambrian cambrian boundary 542 ma while in slope and basinal sections 13 c values are negative through the entire doushantuo formation 635 551 ma if these 13 c values are close to their primary seawater signature they imply a strong 10 surface to deep ocean 13 c gradient that is consistent with long term deep ocean anoxia and the presence of a large dissolved organic carbon doc reservoir the two prominent negative 13 c excursions within the doushantuo formation above the cap carbonate level are associated with shoaling and local exposure of the platform the anomalies may thus record remineralization of a large oceanic doc pool via sulfate reduction that transferred 13 c depleted carbon from the oceanic doc reservoir to the surface ocean during regression inconsistencies in ediacaran 13 c profiles globally and variations in south china in particular highlight the need for further evaluation of local departures in 13 c from an inferred average seawater signature

carbon isotope stratigraphy and correlation of depositional sequences in the upper ordovician ely springs dolostone eastern great basin usa abstract shallow water carbonate shelf strata of the ely springs dolostone accumulated on the western laurentian margin as relative sea level fluctuated during the late ordovician icehouse the ely springs dolostone is divided into four upper katian depositional sequences and one hirnantian sequence here we present new carbon isotope data from the ely springs in the context of the sequence stratigraphic architecture these data derive from a shelf transect of five stratigraphic sections exposed in the eastern great basin usa and provide records of carbon cycling on the laurentian margin prior to and during the hirnantian glacial maximum the 13 c values range between 2 and 4 at each section and chemostratigraphic tie points occur in all sections of the transect intrabasinal correlations based on the 13 c records are consistent with correlations derived from sequence stratigraphy and conodont biostratigraphy the ely springs dolostone carbon isotope curve can also be correlated with the late katian cincinnatian composite derived from epicontinental deposits of the laurentian midcontinent the globally documented hirnantian positive carbon isotope excursion commonly referred to as hice occurs within the uppermost depositional sequence immediately below a regional unconformity coinciding with peak glacial conditions on gondwana the late katian elkhorn and waynseville 13 c excursions are also present in the ely springs dolostone carbon isotope record the peak value of the hirnantian carbon excursion increases from 2 6 to 3 8 across the shallow shelf and reaches 7 3 in coeval limestones of the hanson creek formation deposited in a deep dysoxic setting this lateral gradient is not the result of changes to the 13 c of weathered carbon supplied by rivers as previously proposed it may be instead related to authigenic carbonate cement precipitation during early diagenesis of the hanson creek strata that these pervasively dolomitized shallow water carbonates retain 13 c signatures useful for regional and global stratigraphic correlation indicates that 13 c chemostratigraphy may be an appropriate correlation tool for dolostones with few biostratigraphically useful fossils

middle upper ordovician darriwilian sandbian paired carbon and sulfur isotope stratigraphy from the appalachian basin usa implications for dynamic redox conditions spanning the peak of the great ordovician biodiversification event abstract the evans ferry section from the appalachian basin of eastern north america has been analyzed for chemostratigraphic trends to elucidate possible causal mechanisms facilitating the great ordovician biodiversification event gobe paired stable isotope 13c and 34s analyses were used in this carbonate dominated locality from the appalachian basin to reconstruct the marine redox states during this key period of rapid biodiversification this succession is one of the most expanded sandbian stage upper ordovician deposits known from north america and it allows new high resolution reconstructions of long term global carbon and sulfur cycle fluctuations the integrated geochemical and sequence stratigraphic investigation presented here from the laurentian epeiric seaway allows for possible identification of the contraction and expansion of reducing water masses during the middle late ordovician that may have been linked to global marine paleoredox dynamics utilizing new conodont based 87sr 86sr isotope stratigraphy along with previous conodont biostratigraphy we can confidently correlate our stable isotope profiles to other carbonate successions globally carbonate facies indicate that this area experienced partial restriction from open marine conditions during certain intervals in the sandbian these semi restricted environments record carbon 13ccarb isotope trends that are secular in nature as they can be correlated to other successions across laurentia and baltica we identify several intervals when 13ccarb and 34scas carbonate associated sulfate trends are decoupled these inverse stratigraphic trends are at times followed by parallel positive shifts in 13ccarb and 34scas causal mechanisms for the observed decoupled 13c and 34s trends may include a wide variety of factors such as more closed system local biogeochemical processes associated in part with diagenesis or they could instead reflect variations in the global fluxes of organic matter and pyrite burial linked to changing marine paleoredox conditions the positive covariation in trends presented here likely represents transient increases in organic carbon and pyrite burial in response to expansion of reducing marine environments

tectonic control on southern sierra nevada topography california in this study we integrate the apatite u th he thermochronometric technique with geomorphic structural and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late cenozoic faulting in the southern sierra nevada the geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low relief landscape little affected by internal deformation to a more complex landscape affected by numerous faults regionally the relict landscape surface is readily resolved by age elevation relationships of apatite he ages coupled to geomorphology these relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting on the basis of 63 new apatite he ages and stratigraphic data from proximal parts of the san joaquin basin we resolve two sets of normal faults oriented approximately n s and approximately nw quaternary west side up normal faulting along the n s breckenridge kern canyon zone has resulted in a southwest step over from the owens valley system in the controlling structure on the regional west tilt of sierran basement this zone has also served as a transfer structure partitioning neogene quaternary extension resulting from normal displacements on the nw fault set this fault system for the most part nucleated along late cretaceous structures with late cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region

the paleocene eocene thermal maximum petm in shallow marine successions of the adriatic carbonate platform sw slovenia the paleocene eocene thermal maximum represents one of the most rapid and extreme warming events in the cenozoic shallow water stratigraphic sections from the adriatic carbonate platform offer a rare opportunity to learn about the nature of paleocene eocene thermal maximum and the effects on shallow water ecosystems we use carbon and oxygen isotope stratigraphy in conjunction with detailed larger benthic foraminiferal biostratigraphy to establish a high resolution paleoclimatic record for the paleocene eocene thermal maximum a prominent negative excursion in 13 c curves of bulk rock 1 3 matrix 4 and foraminifera 6 is interpreted as the carbon isotope excursion during the paleocene eocene thermal maximum the strongly 13 c depleted 13 c record of our shallow marine carbonates compared to open marine records could result from organic matter oxidation suggesting intensified weathering runoff and organic matter flux the ilerdian larger benthic foraminiferal turnover is documented in detail based on high resolution correlation with the carbon isotopic excursion the turnover is described as a two step process with the first step early ilerdian marked by a rapid diversification of small alveolinids and nummulitids with weak adult dimorphism possibly as adaptations to fluctuating paleocene eocene thermal maximum nutrient levels and a second step middle ilerdian characterized by a further specific diversification increase of shell size and well developed adult dimorphism within an evolutionary scheme controlled by long term biological processes we argue that high seawater temperatures could have stimulated the early ilerdian rapid specific diversification together these data help elucidate the effects of global warming and associated feedbacks in shallow water ecosystems and by inference could serve as an assessment analog for future changes

marine carbon reservoir corg ccarb coupling and the evolution of the proterozoic carbon cycle the post 2 0 ga proterozoic c isotope record reveals two distinct yet interrelated trends a stepwise increase in average 13c from 0 calculated with respect to the peedee belemnite isotope standard prior to ca 1 3 ga to 5 in the neoproterozoic and a concomitant increase in the magnitude of isotopic excursions steady state and non steady state models suggest that these fundamental changes are best explained by a combination of evolving burial fluxes and a secular decrease in the size of the marine dissolved inorganic carbon dic reservoir the dic reservoir size affects the sensitivity of the isotopic system to biogeochemical perturbation major rearrangements of carbon cycling during the proterozoic in part related to the evolving marine carbon reservoir permit elevated 13c values to be sustained for geologically long time spans recognition of this dependence on dic reservoir size provides for the first time a direct link between changing carbonate precipitation styles and the marine c isotope record and may help constrain estimates of proterozoic p co2

depletion of 13c in seawater σc02 on modern carbonate platforms significance for the carbon isotopic record of carbonates seawater co2 from modern carbonate platforms bahama banks and florida is depleted in 13c by as much as 4 relative to open ocean water depletion in 13c is caused by isotopically light co2 input from respiration of marine and terrestrial organic matter during water mass residence on the bank as such depletion in 13c is related to changes in water chemistry driven by evaporation freshwater discharge and caco3 withdrawal deviation of modern platform seawater 13c values away from those of surface seawater suggests that 13c values of ancient cratonic carbonate must be cautiously interpreted initial depositional environment as well as alteration during diagenesis must be considered in extracting valid secular trends for oceanic 13c variation

high resolution event stratigraphy hires and the quantification of stratigraphic uncertainty silurian examples of the quest for precision in stratigraphy abstract the ability to resolve the stratigraphic record of earth history is critical to understanding past global change events and the range of natural variability within the global paleoclimate the geosphere hydrosphere atmosphere biosphere system operates on geologically very short time scales and we must be able to resolve the stratigraphic record of past events on similar time scales if we are to understand the cause and effect relationships operating within the system global resolution 100 kyr is commonly available when evaluating events from the cenozoic era but such resolution is less common for older portions of earth history the introduction of high resolution event stratigraphy hires more than two decades ago revolutionized the ability to resolve the mesozoic and cenozoic stratigraphic records of global change and a similar revolution is currently underway for the paleozoic as well here we review the application of the principles of hires to the paleozoic stratigraphic record with examples from the silurian system discuss the quantification of chronostratigraphic uncertainty and highlight the current deep time earth history revolution that has the potential to open the entire phanerozoic record to the evaluation of short time scale global climate dynamics

techniques for assessing spatial heterogeneity of carbonate δ13c values implications for craton wide isotope gradients the sedimentary record of carbonate carbon isotopes 13ccarb provides one of the best methods for correlating marine strata and understanding the long term evolution of the global carbon cycle this work focuses on the late ordovician guttenberg isotopic carbon excursion a ca 2 5 positive 13ccarb excursion that is found in strata globally substantial variability in the apparent magnitude and stratigraphic morphology of the guttenberg excursion at different localities has hampered high resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle this work investigates the magnitude spatial scale and sources of isotopic variability of the guttenberg excursion in two sections from missouri usa centimetre scale isotope transects revealed variations in 13ccarb and 18ocarb greater than 2 across individual beds linear 13ccarb to 18ocarb mixing lines together with petrographic and elemental abundance data demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components these patterns facilitated objective sample screening to determine the least altered data a 18ocarb filter based on empirical 18ocarb values of well preserved carbonate mudstones allowed further sample discrimination the resulting least altered 13ccarb profile improves the understanding of regional as well as continental scale stratigraphic relations in this interval correlations with other laurentian sections strongly suggest that i small scale variability in guttenberg excursion 13ccarb values may result in part from local diagenetic overprinting ii peak guttenberg excursion 13ccarb values of the midcontinent are not distinct from their taconic equivalents and iii no primary continental scale spatial gradient in 13ccarb for example arising from chemically distinct aquafacies is required during guttenberg excursion time this study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for 13ccarb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry

did early land plants produce a stepwise change in atmospheric oxygen during the late ordovician sandbian 458 ma abstract a stepwise change in atmospheric oxygen o2 levels during the ordovician has been attributed to the emergence of land plants this phenomenon is tied to a major baseline shift in the stable carbon isotope 13c curve and inferred increase in nutrient delivery and enhanced primary productivity in nearshore settings which led to high organic carbon burial the timing and magnitude of this baseline shift however is still elusive in part because of the lack of high resolution 13c data that span this period much of the existing ordovician 13c literature is focused on isotopic excursions with less emphasis on identifying long term shifts in baseline pre and post excursion values this study presents new high resolution 13c data from stratigraphic sections at germany valley west virginia and union furnace pennsylvania in the central appalachian basin these sections span the entire sandbian stage and continue into the lower katian stage the 13c data from both sections are characterized by relative stability carbon isotope values mean 0 61 in the lower sandbian followed by a 1 2 shift in the upper sandbian mean 0 62 herein we propose that the positive shift represents a long term global shift in baseline 13c values of dissolved inorganic carbon the timing of this positive shift coinciding with the diversification of early land plants i e bryophytes supports earlier models that suggested enhanced organic carbon burial rates served as a mechanism for the stepwise oxygenation of the atmosphere during the late ordovician

timescales of migmatization melt crystallization and cooling in a cordilleran gneiss dome valhalla complex southeastern british columbia 1 integration of in situ and conventional ion microprobe u th pb zircon and monazite geochronology with 40ar 39ar hornblende biotite and muscovite thermochronology provides a temporal record of metamorphism partial melting fluids and cooling of exhumed midcrustal to lower crustal rocks the valhalla complex southeastern british columbia is one of a series of elongate gneiss domes within the omineca belt of the canadian cordillera we have performed u th pb and 40ar 39ar age measurements on migmatitic gneisses from a range of structural levels and settings within the valhalla complex in an effort to better document the timing of migmatization and exhumation stromatic migmatites and leucosomes crystallized in boudin necks yield concordant u pb zircon ages that cluster near 60 ma and th pb monazite ages of circa 60 50 ma in situ analyses of monazite in samples that contain 60 50 ma however the combined data also suggest that melt crystallization circa 60 ma in the valhalla complex occurred rapidly and before the other domes within the omineca belt cooling at 49 ma throughout the omineca is associated with regional extension and exhumation of migmatites in the domes the presence of abundant migmatites and the similarity in timing of melt crystallization and cooling in domes along the 400 km long omineca belt is consistent with localization of exhumation at the boundary between the flowing crust of the hinterland and the rigid colder crust of the rocky mountain foreland

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

spatial variation of carbon isotopic compositions of carbonate and organic matter from the late ordovician sedimentary succession in the yangtze platform south china implications for sea level eustacy and shoaling of marine chemocline abstract spatial correlation of isotopic chemostratigraphy across ancient epeiric basins is of particular interest in the field of geology this study examines the carbon isotopic compositions of carbonate and organic matter from the late ordovician sedimentary succession in the yangtze platform including the linxiang lx formation deposited in the bottom and the tiezufeike tzfk daduhe ddh wufeng wf formations in the top paired 13ccarb and 13corg analyses of the upper ordovician strata on the yangtze platform were recorded and the results show that the rocks in the lx formation have high 13ccarb and 13corg values relative to those in the ddf tzfk wf formations additionally a variation of 2 was found between shallow and deep water settings during the late ordovician heavier 13ccarb and 13corg values are observed at the wukemuchang wkmc and wanhe wh sections representing the shallow regions of the yangtze platform relative to the tianjiawan tjw and tianba tb sections representing the deeper margin of the yangtze platform we suggest that the variations of 13c in carbonate and organic matter are related to the sea level eustacy and shoaling of marine chemocline 1 sea level rise driven decrease in local scale carbonate weathering 2 decreased decomposition of organic matter under anoxic conditions and 3 incorporation of non photosynthetic chemoautorophic bacteria living in deeper waters the 13c 13c 13ccarb 13corg values in both the wh and wkmc sections display an increasing trend upward from the lx formation to the overlying strata varying from 27 8 to 31 2 and from 28 1 to 31 1 respectively these increased 13c values are related to the enhanced nutrients and correspondingly high primary productivity and thus that the anoxic water conditions and high primary productivity may be the significant factors in the deposition of late ordovician black shales

mio pliocene seasonality on the snake river plain comparison of faunal and oxygen isotopic evidence smith g r and patterson w p 1994 mio pliocene seasonality on the snake river plain comparison of faunal and oxygen isotopic evidence palaeogeogr pataeoctimatol palaeoecol 107 291 302 analysis of fish faunas and oxygen isotopic composition of a fish otolith from lacustrine deposits of southwestern idaho provide a means of evaluating regional miocene and pliocene climates a disharmonious assemblage consisting of coldwater salmon and trout and warmwater sunfish and catfish from the chalk hills formation of the snake river plain indicates that the climate of the late miocene was warm and moist with cool summers and mild winters colonization of the lake by deepwater sculpins and whitefish in the pliocene indicates that the climate was moist and equable but with summers cooler than either the miocene or quaternary oxygen isotopic variation among seasonal growth rings in an aragonitic otolith of a pliocene littoral sunfish suggests a seasonal range of temperatures locally more equable than at present extremely depleted values of 6180 in carbonates suggest that the lake was maintained by tributaries from high elevation watersheds with locally low evaporation rather than high precipitation

did changes in atmospheric co2 coincide with latest ordovician glacial interglacial cycles article i nfo the late ordovician hirnantian stage 444 million years ago was one of three time periods during the past half billion years in which large continental glaciers formed over earth s polar regions the effects of this glaciation were far reaching and coincided with one of the largest marine mass extinction events in earth history the cause of this ice age is uncertain and a paradoxical association with evidence for high atmospheric co2 levels has been debated precise linkages between sea level ice volume and carbon isotope 13 ccarb and 13 corg proxy records of pco2 have been poorly understood due in part to uncertainties in stratigraphic correlation and the interpretation of globally important sections although correlation difficulties remain recent hirnantian biostratigraphic studies now allow for improved correlations here we show that consistent trends in both 13 ccarb and 13 corg from two well dated stratigraphic sequences in estonia and anticosti island canada coincide with changes in late ordovician hirnantian climate as inferred from sea level and the extent of ice sheets the integrated datasets are consistent with increasing pco2 levels in response to ice sheet expansion that reduced silicate weathering ultimately the time period of elevated pco2 levels is followed by geologic evidence of deglaciation

evolution of the marine stable carbon isotope record during the early cretaceous a focus on the late hauterivian and barremian in the tethyan realm abstract in order to improve our understanding of the relationships between the late hauterivian oceanic anoxic faraoni event contemporaneous platform drowning along the northern tethyan margin and global environmental change in general we established high resolution 13 c and 18 o curves for the late hauterivian and the entire barremian stage these data were obtained from whole rock carbonate samples from the veveyse de ch tel saint denis section switzerland the fiume bosso section and the nearby gorgo a cerbara section central italy and the angles section barremian stratotype france we observe an increase of 0 3 in mean 13 c values within sediments from the middle hauterivian subsaynella sayni ammonite zone to the hauterivian barremian boundary 13 c values remain essentially stable during the early barremian during the latest early barremian and most of the late barremian 13 c values increase slowly until the imerites giraudi zone and the latest barremian is characterized by a negative trend in 13 c values with minimal values at the barremian aptian boundary during the earliest aptian 13 c mean values start to rise again and attain 2 25 we interpret the evolution of the 13 c record as resulting from the interaction between changes in the carbon cycle in the tethyan basin and the adjacent platforms and continents in particular changes towards warmer and more humid conditions on the continent and coeval phases of platform drowning along the northern tethyan margin may have contributed to enhance the oceanic dissolved inorganic carbon dic reservoir which may have pushed the 13 c record towards more negative values and exerted a general attenuation on the 13 c record from this may have come the general change from a heterozoan to a photozoan carbonate platform community which influenced the evolution in 13 c values by increasing the export of aragonite and diminishing export of dissolved organic carbon into the basins

calibration of a conodont apatite based ordovician 87sr 86sr curve to biostratigraphy and geochronology implications for stratigraphic resolution the ordovician 87 sr 86 sr isotope seawater curve is well established and shows a decreasing trend until the mid katian however uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 sr 86 sr decrease may have varied which has implications for both the stratigraphic resolution possible using sr isotope stratigraphy and efforts to model the effects of ordovician geologic events we measured 87 sr 86 sr in conodont apatite in north american ordovician sections that are well studied for conodont biostratigraphy primarily in nevada oklahoma the appalachian region and ohio valley our results indicate that conodont apatite may provide an accurate medium for sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 sr 86 sr during the middle ordovician darriwilian stage our 87 sr 86 sr results suggest that sr isotope stratigraphy will be most useful as a high resolution tool for global correlation in the mid darriwilian to mid sandbian when the maximum rate of fall in sr

chemostratigraphy of the late mesoproterozoic atar group taoudeni basin mauritania muted isotopic variability facies correlation and global isotopic trends a b s t r a c t sedimentary strata of the atar el mreiti groups taoudeni basin mauritania are characterized by car bon isotope values that fall largely between 2 5 and 4 0 and contain stratigraphic trends that are strikingly similar to those recorded worldwide in strata with depositional ages from 1 2 ga to 1 1 ga chemostratigraphic results are consistent with recent re os dates of 1 1 ga for atar el mreiti group strata rooney et al 2010 and support an emerging dataset that identifies the late mesoproterozoic as an isotopically distinctive interval in earth history the relatively low amplitude carbon isotope variabil ity limits the use of chemostratigraphy in detailed correlation during this time interval in this study we combine chemostratigraphic data with key chronostratigraphic horizons and detailed analysis of deposi tional facies to construct a rigorous and testable model for the intrabasinal correlation of atar and el mreiti group strata data support a depositional model that results in broadly uniform stratigraphic thicknesses across the west african craton with predominantly stromatolitic facies of the atar group mauritania and the hank and dar cheikh groups algeria representing deposition on shallow water craton margins and clayey carbonate and shale facies of the el mreiti group mauritania representing deposition on a broad shallow water epicratonic platform within this stratigraphic framework regional differences in carbon isotope composition are interpreted to reflect spatial differences in water chemistry likely asso ciated with local carbon cycle dynamics in cratonic environments that have limited exchange with the open ocean similarly distinct differences in trace element composition recorded in multilayered marine cements within deeper water stromatolitic reef facies are interpreted to reflect vertical differences in water chemistry associated with redox gradients in the water column we use this multifaceted approach to test a recent hypothesis that suggests correlation of atar el mreiti group strata with glaciogenic strata of the vazante group sao francisco craton critically preservation of shallow water carbonate and evaporite strata across more than 1200 km of the predominantly epicra tonic taoudeni basin with minimal evidence for high amplitude sea level change is consistent with inferences of late mesoproterozoic highstand conditions in a global greenhouse climate such charac teristics are inconsistent with suggestions of a time equivalency with glaciogenic strata of the vazante group sao francisco craton despite broad similarities in re os geochronology and carbon isotope com position together these results emphasize both the utility and limitations of carbon isotope stratigraphy to inform hypotheses regarding global correlation and biospheric evolution 2012 elsevier b v all rights reserved

gradients in the carbon isotopic composition of ordovician shallow water carbonates a potential pitfall in estimates of ancient co2 and o2 abstract the carbon isotopic composition of the global dissolved inorganic carbon dic reservoir is best estimated from open ocean pelagic carbonate sediments 13 c carb however this is not practical for most of geologic time because seafloor subduction has removed the pre jurassic record and these time periods may have lacked planktonic calcifying organisms and therefore shallow water carbonate platform or periplatform sediments are utilized shallow water deposits are susceptible to a wide range of post depositional alteration processes and syn depositional controls on 13 c carb that include carbonate mineralogy water mass restriction and a host of related variables e g ph temperature organic decomposition evaporation and co 2 solubility that can produce local gradients in dic the degree to which shallow water 13 c curves diverge from open marine deposits may be critical to understanding how well global carbon cycle isotope mass balance models can predict organic carbon burial rates but documentation of such divergence is often hindered by factors that limit chronostratigraphic correlation in restricted water masses e g endemic faunas here we integrate strontium isotope 87 sr 86 sr stratigraphy and biostratigraphy to compare 13 c curves in a case study along a depth transect in middle late ordovician carbonate platform settings the restricted tidal flat and more open marine deposits are offset by a maximum of 2 during sea level drop and 0 during highstand flooding of the platform global carbon cycle models such as geocarbsulf use published 13 c carb curves to drive organic carbon burial rates under the assumption that 13 c carb reflects a global seawater signal we show here the potential pitfalls of using a published 13 c carb curve that violates this global assumption for the 460 million year middle late ordovician time bin in geocarbsulf improper usage of our locally depleted 13 c curve to drive global organic carbon burial would result in erroneous atmospheric o 2 10 8 o 2 equal to about a 30 reduction from what it should be at 15 2 o 2 using global 13 c and co 2 400 ppm too high equal to about a 13 increase from what it should be at 2570 ppm co 2 with detailed sedimentologic analysis it may be possible to identify and exclude 13 c samples that record the influences of local carbon cycling from global carbon cycle models such as geocarbsulf

global change in the late devonian modelling the frasnian famennian short term carbon isotope excursions a model of the global biogeochemical cycles coupled to a energy balance climate model the combine model is used to identify the causes of two large n 13 c value excursions across the frasnian famennian f f boundary we test a scenario that links the sea level rise to stratification of the proto tethys ocean through the formation of warm saline deep waters in extended epicontinental seas even though this scenario can produce dysoxia below 100 m depth it fails to increase the global burial flux of organic carbon and thus seawater n 13 c values since stratification of the ocean leads to decreased productivity in surface waters several scenarios postulating a continental origin of the perturbations in the late devonian biogeochemical cycles are then tested we found that weathering of platform carbonates exposed during the early famennian sea level fall can account for a maximum positive shift in n 13 c value of 0 7x at the end of the sea level fall episode another 1 0x increase in n 13 c might originate from rapid spreading of vascular land plants near the f f boundary postulating that higher plants globally increased the weatherability of continental surface and that colonized continental area increased by 30 across the f f boundary finally the n 13 c excursion observed at the base of upper rhenana zone and the rapid increase of the carbon isotope ratios at the f f boundary require an increase of phosphorus delivery to the ocean by 40 coeval with the sea level rises once the calculated n 13 c values are in agreement with the measured data the combine model calculates a decrease in atmospheric pco2 from pre perturbation 2925 ppmv in the lower rhenana conodont zone to 1560 ppmv in the upper triangularis zone this decrease in pco2 is due to the increase in burial of organic matter during the kellwasser events and increased continental weatherability triggered by the spreading of continental vascular plants these changes occur within 4 million years the corresponding global climatic cooling reaches 4 4 c at the pole and 2 1 c at the equator f 2003 elsevier b v all rights reserved

carbon isotope stratigraphy of cenomanian turonian platform carbonates from the southern apennines italy a chemostratigraphic approach to the problem of correlation between shallow water and deep water successions the carbon isotope record of three sections of shallow water carbonates from the upper cretaceous sequences of the southern apennines italy shows a pronounced positive excursion of about 4 5 using the upper cenomanian cisalveolina fraasi level as a biostratigraphic tie point we correlate this excursion to the oae 2 isotopic event comparison with the standard reference 13 c curve of eastbourne england shows that the overall character of the excursion is clearly reproduced in the shallow water curves chemostratigraphic correlation allows definition of the age of some important shallow water biostratigraphic events to the precision of an ammonite zone using the well established bio chronostratigraphic framework defined in the eastbourne section the onset of the cenomanian turonian isotopic excursion coincides with a transgressive trend recorded by the occurrence of more open marine facies and culminating with an incipient drowning in one of the sections studied our data suggest that the open ocean isotopic signal is most faithfully recorded and preserved in shallow water platform carbonates during transgressive periods when the effects of meteoric diagenesis and seawater ageing are less severe

record of albian to early cenomanian environmental perturbation in the eastern sub equatorial pacific the present paper documents and discusses a new albian early cenomanian carbon isotope 13ccarb and 13corg curve from the subequatorial eastern pacific in peru chemostratigraphic evidences for the expression of the oae1b set and for oae1c and oae1d are presented this dataset is relevant inasmuch as previous work is strongly biased towards study sites in north america western interior basin in europe tethys and the pacific realm a comparison of the carbon isotope stratigraphy obtained in peru with published sections from the central and western pacific the western atlantic and northern and western tethys reveals an overall good agreement supporting the global nature of the isotope patterns described here the 13c from peru record is constrained by biostratigraphic evidence and 87sr 86sr isotope stratigraphy using well preserved oyster shells furthermore we document the development of a heterozoan epeiric neritic mixed carbonate siliciclastic ramp in the western platform of peru and its corresponding sedimentary facies associations this dataset was used to elucidate the complex interplay of climatic changes nutrient supply and platform drowning leading to the following conclusions i an upper aptian lower albian major change from siliciclastic dominated to carbonate sedimentation coincided with the impact of the kilian level ii a lower albian incipient platform drowning linked to the impact of the paquier level iii a lower middle albian major demise of neritic carbonate production that coincides with the leenhardt level followed by middle albian condensed sedimentation that reports prominent negative values in 13ccarb prior to the onset of oae1c and iv finally renewed carbonate ramp production during the upper albian lower cenomanian the data shown here represent the foundation for future work documenting the mid cretaceous of peru and its implications for the palaeoceanography of the se subequatorial pacific

carbon cycle dynamics following the end triassic mass extinction constraints from paired δ13ccarb and δ13corg records 1 constraining the carbon isotopic changes associated with the end triassic mass extinction is key to understanding the causes of the extinction and dynamics of recovery from it yet the pattern and timing of 13c variation surrounding the extinction remain poorly constrained here we present close to 1000 new 13c measurements from six newly sampled sections in italy we observe a sharp negative excursion in 13ccarb coincident with the disappearance of the triassic fauna and two positive excursions above it the negative 13ccarb excursion in these sections does not occur in 13corg suggesting a possible diagenetic origin in contrast the interval of elevated 13c occurs in both carbonate and organic carbon suggesting that it is likely to be a primary feature the positive excursions in the lombardy basin southern alps and mt cefalo southern apennines appear to be time correlative on the basis of their position above the disappearance of characteristically triassic biota however it is less certain that they are time correlative with positive excursions in other sections worldwide as few options exist that honor both bio and chemostratigraphy nonetheless similarity to other events that are interpreted as global as well as carbon cycle considerations suggest that the isotopic enrichment is best interpreted to reflect a shift in the isotope composition of the global surface carbon reservoir our data indicate that perturbation of the global carbon cycle was not confined to the immediate vicinity of the extinction interval but rather persisted for substantial length of geologic time afterwards

geocarbsulf a combined model for phanerozoic atmospheric o2 and co2 abstract a model for the combined long term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the geocarb iii model berner r a kothavala z 2001 geocarb iii a revised model of atmospheric co2 over phanerozoic time am j sci 301 182 204 for co2 with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for o2 berner r a 2001 modeling atmospheric o2 over phanerozoic time geochim cosmochim acta 65 685 694 new isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models sensitivity analysis is done by determining 1 the effect on weathering rates of using rapid recycling rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks 2 the effect on o2 of using different initial starting conditions 3 the effect on o2 of using different data for carbon isotope fractionation during photosynthesis and alternative values of oceanic 13c for the past 200 million years 4 the effect on sulfur isotope fractionation and on o2 of varying the size of o2 feedback during sedimentary pyrite formation 5 the effect on o2 of varying the dependence of organic matter and pyrite weathering on tectonic uplift plus erosion and the degree of exposure of coastal lands by sea level change 6 the effect on co2 of adding the variability of volcanic rock weathering over time berner r a 2006 inclusion of the weathering of volcanic rocks in the geocarbsulf model am j sci 306 in press results show a similar trend of atmospheric co2 over the phanerozoic to the results of geocarb iii but with some differences during the early paleozoic and for variable volcanic rock weathering lower co2 values during the mesozoic atmospheric oxygen shows a major broad late paleozoic peak with a maximum value of about 30 o2 in the permian a secondary less broad peak centered near the silurian devonian boundary variation between 15 and 20 o2 during the cambrian and ordovician a very sharp drop from 30 to 15 o2 at the permo triassic boundary and a more or less continuous rise in o2 from the late triassic to the present

interactions between environmental change and shallow water carbonate buildup along the northern tethyan margin and their impact on the early cretaceous carbon isotope record 1 the evolution of the early cretaceous northern tethyan carbonate platform was not only influenced by changes in sea level detrital influx and surface water temperature but also by changes in trophic levels we distinguish between phases of carbonate production dominated by oligotrophic photozoan communities and by mesotrophic and eventually colder water heterozoan communities superimposed on this bimodal trend in platform evolution were phases of platform demise for which we provide improved age control based on ammonite biostratigraphy the initial phase of these episodes of platform demise corresponds in time to episodes of oceanic anoxic events and environmental change in general on the basis of a comparison between the temporal changes in an early cretaceous ammonite calibrated d 13 c record from southeastern france and coeval changes in the platform record we suggest that the history of carbon fractionation along the northern tethyan margin was not only influenced by changes in the oceanic carbon cycle such as in the rate of production and preservation of organic and carbonate carbon and in the size of the oceanic dissolved inorganic carbon reservoir but it was also influenced by the above mentioned changes in the ecology and geometry of the adjacent carbonate platform phases of photozoan carbonate production induced positive trends in the hemipelagic carbonate d 13 c record phases of heterozoan carbonate production pushed the d 13 c system toward more negative values platform drowning episodes implied an initial increase in d 13 c values followed by a longerterm decrease in d 13 c values

middle late ordovician darriwilian sandbian decoupling of global sulfur and carbon cycles isotopic evidence from eastern and southern laurentia abstract middle late ordovician sequences from the appalachian basin and arbuckle mountain regions of north america were analyzed for carbonate associated sulfate 34 s cas and pyrite 34 s pyr paired with carbonate 13 c carb and organic matter 13 c org chemostratigraphy two major negative drops in 34 s cas 12 excursions are recognized the older decline in 34 s cas occurs within the histiodella holodentata phragmodus polonicus conodont zones and the younger drop is within the cahabagnathus sweeti amorphognathus tvaerensis baltoniodus gerdae subzone zones these overall these negative shifts in 34 s cas have an antithetical relationship with positive shifts in 34 s pyr 10 and 13 c carb 2 recorded in the same successions the older negative 34 s cas shift is coincident with the widely documented mid darriwilian 13 c excursion mdice and the younger negative 34 s cas shift is coincident with another positive 13 c carb shift in the early sandbian geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative 34 s cas trends additionally a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends while increased crustal weathering is broadly consistent with a sea level lowstand and the seawater 87 sr 86 sr isotope record of change in continental weathering in the late darriwilian stage of the ordovician geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in 34 s cas these antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates pulses of oceanic ventilation would have expanded habitable environments for marine organisms and thus is broadly consistent with major increases in biodiversification during this period of the ordovician

rain shadow development during the growth of mountain ranges an atmospheric dynamics perspective 1 an idealized atmospheric model is used to explore the links between climate and topography in the development of orographic rain shadows during orogenesis the atmospheric dynamics theory of density stratified fluid flow over topography is used to interpret the results the controlling nondimensional parameter is nh u where n is the brunt vaisala frequency a measure of atmospheric stability h is the terrain relief and u is the initial horizontal wind speed rain shadow development is found to be a nonlinear and nonunique function of both topography and atmospheric state indicating that geological records of orographic aridity cannot be interpreted in terms of relief alone when upstream topography exceeds nh u 1 during surface uplift downstream orographic precipitation vanishes and downstream orographic cloud mass decreases by as much as 90 upstream blocking of air flow can generate a forward projecting rain shadow in which a relatively low ridge nh u 1 may be decoupled from the atmospheric flow by a zone of flow stagnation extending upstream of the high terrain such an effect may occur if the valley separating the two ranges is narrower than the length scale of flow stagnation in the model configuration used here lateral widening of a relatively low nh u 1 range increases downstream cloud mass by up to a factor of 3 these results help to refine interpretations of climate tectonic interactions in shaping the geological record of the sierra nevada and andes

geochemical evidence for expansion of marine euxinia during an early silurian llandovery wenlock boundary mass extinction abstract repeated biotic crises have become the hallmark for the silurian with the three most significant marine turnover events being related to dramatic global environmental perturbations causal mechanisms linking these marine extinction events with positive carbon isotope 13 c excursions paleoceanographic change and climate remain poorly constrained here we examine the positive 13 c excursion across the llandovery wenlock boundary and the associated ireviken extinction event this positive 13 c excursion has been interpreted to reflect a major change in global oceanographic climate state and enhanced organic carbon burial new geochemical data from two paleocean basins have been analyzed to determine local and global redox conditions using carbon and sulfur isotopes 34 s and iodine i ca mg proxies the high resolution 13 c and 34 s data from both sections show positive excursions indicative of global perturbations to each of these elemental cycles with minimal temporal offsets between the two systems numerical box modeling of 13 c and 34 s data indicates that these isotopic shifts can be generated by significant increases in the burial of organic carbon and pyrite which are most likely due to enhanced burial under euxinic anoxic and sulfidic conditions independently i ca mg values point to locally anoxic bottom waters in the distal and deeper basinal setting in nevada before during and after the ireviken positive 13 c excursion i ca mg values in the proximal shelf setting in tennessee show relatively oxic waters during the onset of peak 13 c values after which bottom water oxygen concentrations dropped throughout the remainder of the excursion this multiproxy paleoredox dataset provides the first direct evidence for local and global expansion of reducing marine conditions coincident with the silurian biotic event and positive 13 c excursion integration of these geochemical data for local and global scale changes in marine redox conditions with the paleontological data and evidence for eustatic sea level rise points toward a shoaling of anoxic and or euxinic waters onto the shelf as a driver for the ireviken extinction event

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

sensitivity of the epeiric sea carbon isotope record to local scale carbon cycle processes tales from the mohawkian sea ancient marine carbon isotope records derived from epeiric sea carbonates can bear a strong influence of local scale carbon cycling processes across the late ordovician mohawkian sea of eastern laurentia the carbon isotope compositions d 13 c of contemporaneous sediments range from 2x in the interior of the sea to 2 5x along the margin here we use a box model to explore the relative importance of carbon fluxes within the mohawkian sea from a mass balance perspective in the model the mohawkian sea carbon reservoir is coupled to but distinct from the ocean and atmosphere carbon reservoirs at initial steady state conditions the residence time of carbon in the mohawkian sea is less than 10 years we find that to produce model d 13 c values representative of the range in d 13 c across the mohawkian sea we must divide the sea into an inner region and an outer region and limit dissolved inorganic carbon dic exchange between the two to 15 of the initial steady state unrestricted exchange rate it is also necessary to supply low d 13 c carbon 29x to the inner region of the sea at a rate of 17 4 10 12 mol c yr 1 the required flux of low d 13 c carbon could not have been sustained over the long term by remineralization of organic carbon stored in sediments of the inner region of the mohawkian sea and the geographic d 13 c pattern is unlikely to have been generated by upwelling along the sebree trough rather the low d 13 c flux might have been derived from bryophytedominated terrestrial ecosystems suggesting an important role for terrestrial carbon cycling prior to the advent of vascular plants aninterestingfeatureofthemodelisthattheisotopiccompositionsofboththeepeiric seaandthelargerocean atmospheresystem are sensitive to changes in exchange rates within the epeiric sea this has important implications for interpreting global d 13 c