cretaceous pco2 estimation from stomatal frequency analysis of gymnosperm leaves of patagonia argentina abstract stomatal frequency of fossil conifers and ginkgoals cuticles were considered as potential indicators of the p co 2 values in the patagonian palaeoatmosphere during the cretaceous samples are from continental deposits of early upper aptian age anfiteatro de tico and punta del barco formations and upper albian lower cenomanian age kachaike formation both outcropping in andean and extra andean portions of santa cruz province argentina it can be inferred that the co 2 atmospheric content was similar during both stages or was slightly higher toward the late albian early cenomanian the co 2 content estimated from the conifers is between 700 and 1400 ppmv or lower from the mid aptian ginkgoals this co 2 range is consistent with those predicted worldwide by previous studies stomatal indices biogeochemical models and carbon isotopes and implies a co 2 content 2 3 4 0 times higher than present the high atmospheric co 2 content during the middle aptian and late albian early cenomanian was probably linked to a warming process at high latitudes during these times this is consistent with the floristic development recorded during both stages in patagonia

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

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

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

three dimensional kinematic history at an oblique ramp leamington zone sevier belt utah abstract differences in kinematics in adjoining salients in fold thrust belts are typically accommodated along bounding transverse zones the leamington zone is an oblique ramp that accommodates slip between structures in the provo salient and the central utah segment of the sevier fold thrust belt the leamington zone consists of the leamington canyon thrust associated second order asymmetric folds and an out of syncline reverse fault the leamington canyon thrust is a rotated se dipping thrust fault with top to southeast motion it represents an oblique ramp of the folded canyon range thrust fold tightening of the syncline se of the folded leamington canyon thrust trace during later emplacement of underlying structures caused out of syncline reverse faulting in the fold core analysis of the finite strain minor fault populations and conjugate fracture sets were used in interpreting the 3 d kinematic history along the leamington zone restored maximum stretching directions of finite strain ellipsoids trend eastward overall inferred motion planes from populations of slickenlines on minor faults are consistent with overall southeastward motion acute bisectors of conjugate fracture sets may reflect more southward directions later in the deformation history these superimposed deformations with local relative chronologies of plastic and brittle structures using cross cutting relationships indicate that the leamington zone area shows overall clockwise changes in kinematic directions from e to ese to sse during successive pulses of deformation the temporal changes in kinematic directions along the leamington zone most likely reflect variations in kinematics over the oblique ramp as the overall easterly displaced 3 d fold thrust belt wedge interacted with the pre existing oblique ramp structure with small magnitudes of superimposed local vertical axis rotations during later folding and fold tightening of the leamington canyon thrust

axial obliquity control on the greenhouse carbon budget through middle to high latitude reservoirs carbon sources and sinks are key components of the climate feedback system yet their response to external forcing remains poorly constrained particularly for past greenhouse climates carbon isotope data indicate systematic million year scale transfers of carbon between surface reservoirs during and immediately after the late cretaceous thermal maximum peaking in the cenomanian turonian circa 97 91 million years myr ago here we calibrate albian to campanian 108 72 myr ago high resolution carbon isotope records with a refined chronology and demonstrate how net transfers between reservoirs are plausibly controlled by 1 myr changes in the amplitude of axial obliquity the amplitude modulating terms are absent from the frequency domain representation of insolation series and require a nonlinear cumulative mechanism to become expressed in power spectra of isotope time series mass balance modeling suggests that the residence time of carbon in the ocean atmosphere system is by itself insufficient to explain the myr scale variability it is proposed that the astronomical control was imparted by a transient storage of organic matter or methane in quasi stable reservoirs wetlands soils marginal zones of marine euxinic strata and potentially permafrost that responded nonlinearly to obliquity driven changes in high latitude insolation and or meridional insolation gradients while these reservoirs are probably underrepresented in the geological record due to their quasi stable character they might have provided an important control on the dynamics and stability of the greenhouse climate

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

the plio pleistocene 405 kyr climate cycles abstract the 405 kyr cycles in proxy records for the east asian summer monsoon and global ice volume intensified and became phase locked to eccentricity forcing at 4 5 ma we revised previous forcing mechanism hypothesis for pliocene intensification of the 405 kyr cycles in paleoclimatic records and proposed that the establishment of salinity difference between the pacific ocean and the caribbean sea at 4 5 ma triggered an ocean air positive feedback mechanism this feedback mechanism caused simultaneous climate cooling and east asian summer monsoon intensification and made the two different climate components have a clipped response to insolation forcing and thus intensified sensitivity to eccentricity variations weakening of the 405 kyr cycles in monsoon and ice volume records between 2 and 1 ma mid pleistocene transition was likely to be attributed to frequency modulation that transfer power of the 405 kyr band to the 100 kyr band which was triggered by changes in the oceanic carbon storage of the southern ocean

the heartbeat of the oligocene climate system a 13 million year continuous record of oligocene climate from the equatorial pacific reveals a pronounced heartbeat in the global carbon cycle and periodicity of glaciations this heartbeat consists of 405 000 127 000 and 96 000 year eccentricity cycles and 1 2 million year obliquity cycles in periodically recurring glacial and carbon cycle events that climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle solar forcing and glacial events box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass the pronounced 405 000 year eccentricity cycle is amplified by the long residence time of carbon in the oceans

coccolith chemistry reveals secular variations in the global ocean carbon cycle the mismatch between the 100 and 400 k y components of pleistocene climate and the relative power of those terms from the eccentricity of the earth s orbit remains a challenge to the milankovitch hypothesis coccolithophores have the potential to respond to parameters of orbital forcing other than insolation and as a critical component of the ocean carbon cycle can act to modify the climate response the first direct comparison of coccolith fraction sr ca alkenone abundance and automated coccolithophore counts shows that cf sr ca is largely driven by changing production of bloom species with unusually high sr ca ratios the periods of high cf sr ca and high bloom production mark periods of high global coccolithophore production which correlate inversely with the low amplitude 100 and higher amplitude 400 k y eccentricity orbital frequency 400 k y cycles of coccolithophore bloom production correspond to periods of enhanced carbonate accumulation in some parts of the ocean deep ocean dissolution in others positive shifts in global ocean 13c and acmes of gephyrocapsa caribbeanica and emiliania huxleyi the link between production of coccolithophore blooms and eccentricity may be due to orbital control of silica leakage from the southern ocean to the orbitally defined inverse correlation between insolation and growing season length and the asymptotic growth response to these parameters or to changes in nutrient input from weathering during the pleistocene the eccentricity induced coccolithophore acmes have no apparent influence on atmospheric carbon dioxide pco2 due to the shift towards small bloom coccolithophores or to coupling with increased diatom productivity or the ballast effect of the calcium carbonate rain such that pleistocene climate has no significant variance at the largest amplitude eccentricity forcing of 400 k y coccolithophores and their influence on the carbon cycle may act as a filter between the incident orbital forcing and resultant climate

fluvial landsculpting and the fractal dimension of topography abstract quantitative models of landform development can help us to understand the evolution of mountains and regional topography and the effects of tectonic motions and climate on landscape including its fractal geometry this paper presents a general and powerful three dimensional model of fluvial erosion and deposition at hill to mountain range scale the model works by accumulating the effects of randomly seeded storms or floods precipitons that cause diffusional smoothing then move downslope on digital topography grids that erode portions of elevation differences that transport a slope limited amount of eroded material and that deposit alluvium when their sediment carrying capacity is exceeded the iteration of these simple and almost linear rules produce very complicated simulated landscapes demonstrating that complex landscapes do not require complex laws each process implemented in the model is affected differently by changes in horizontal scale erosion a scale free process roughens topography at all wavelengths this roughening is balanced by diffusive processes scaling as 1 l 2 at short wavelengths and deposition scaling scaling as 1 l 2 at long wavelenghts such a mixture of scale free and scale dependent processes can produce multifractal behavior in the models the fractal dimension of the model topography is much more sensitive to climatic variables than to tectonic uplift landscape evolution may be fractal but it does not seem to be chaotic analysis of topography of areas in southern arizona using variograms shows approximately fractal behavior with mean fractal dimension around 2 2 2 3 departures from an exact fractal relatioship imply that the topography is in detail multifractal the fractal dimension at short wavelengths is less than that at long wavelengths this variation could either be caused by the relative strengths of diffusive and erosional processes shaping the topography or a result of changes in climatic or tectonic conditions still preserved in the landscape

scaling and elevation in river networks the analysis of large river networks obtained from digital elevation models has given insight into the variation of channel slope with scale investigators have recently suggested that channel slopes are self similar with magnitude or area as a scaling parameter our data indicates otherwise in particular the variance of channel slope is larger than that predicted by simple self similarity this suggests multiscaling the scaling exponent for the standard deviation is approximately half the corresponding exponent in the relationship of the slope mean to magnitude or area a model for channel slopes based on a point process of elevation drops along the channel is presented the model reproduces observed multiscaling properties when the density of elevation increments is related to area or magnitude as a

alkenone and boron based pliocene pco2 records abstract the pliocene period is the most recent time when the earth was globally significantly 3 c warmer than today however the existing pco2 data for the pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo pco2 this disagreement coupled with the general low temporal resolution of the published records does not allow a robust assessment of the role of declining pco2 in the intensification of the northern hemisphere glaciation inhg and a direct comparison to other proxy records are lacking for the first time we use a combination of foraminiferal 11b and organic biomarker alkenone derived carbon isotopes proxies to determine the concentration of atmospheric co2 over the past 5 ma both proxy records show that during the warm pliocene pco2 was between 330 and 400 ppm i e similar to today the decrease to values similar to pre industrial times 275 285 ppm occurred between 3 2 ma and 2 8 ma coincident with the inhg and affirming the link between global climate the cryosphere and pco2

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

self organization and scaling relationships of evolving river networks the power spectra s of linear transects of earth s topography is often observed to be a power law function of wave number k with exponent close to 2 s k k 2 in addition river networks are fractal trees that satisfy several power law relationships between their morphologic components a model equation for the evolution of earth s topography by transport limited erosional processes which produces fractal topography and fractal river networks is presented and its solutions are compared in detail to real topography the model is the diffusion equation for sediment transport on hillslopes and channels with the diffusivity constant on hillslopes and proportional to the three halves power of discharge in channels the dependence of diffusivity on discharge is consistent with sediment rating curves we study the model in two ways in the first analysis the diffusivity is parameterized as a function of elevation and a taylor expansion procedure is carried out to obtain a differential equation for the landform elevation which includes the spatially variable diffusivity to first order in the elevation the solution to this equation is a self affine or fractal surface with linear transects that have power spectra s k k 1 8 independent of the age of the topography consistent with observations of real topography the hypsometry produced by the model equation is skewed such that lowlands make up a larger fraction of the total area than highlands as observed in real topography in the second analysis we include river networks explicitly in a numerical simulation by calculating the discharge at every point we characterize the morphology of real river basins with five independent scaling relations between six morphometric variables scaling exponents are calculated for seven river networks from a variety of tectonic environments using high quality digital elevation models river networks formed in the model match the observed scaling laws and satisfy tokunaga side branching statistics

how flat is tibet high resolution digital topography three arc second grid for most of tibet provides new information to characterize the relief of the highest and largest plateau on earth the arid to semiarid central and northern part of the plateau interior has low relief average slopes of over 250 m windows and a mean elevation of 5023 m above sea level at moderate wavelengths of m relief is or less for most of tibet as opposed to the much higher relief of up to 6 km on the plateau edges where glacial and fluvial dissection is greater because of higher levels of precipitation the only faults manifesting significant topographic relief are the relatively small scale generally north trending graben systems primarily in southern tibet and several large scale fault systems near the edges of tibet the flatness of tibet implies that 1 there has been little deformation especially shortening of the uppermost crust north of the graben systems during the late cenozoic and 2 shallow crustal isostatic compensation has been acting to level the surface of the plateau

co2 forced climate thresholds during the phanerozoic abstract the correspondence between atmospheric co2 concentrations and globally averaged surface temperatures in the recent past suggests that this coupling may be of great antiquity here i compare 490 published proxy records of co2 spanning the ordovician to neogene with records of global cool events to evaluate the strength of co2 temperature coupling over the phanerozoic last 542 my for periods with sufficient co2 coverage all cool events are associated with co2 levels below 1000 ppm a co2 threshold of below 500 ppm is suggested for the initiation of widespread continental glaciations although this threshold was likely higher during the paleozoic due to a lower solar luminosity at that time also based on data from the jurassic and cretaceous a co2 threshold of below 1000 ppm is proposed for the initiation of cool non glacial conditions a pervasive tight correlation between co2 and temperature is found both at coarse 10 my timescales and fine resolutions up to the temporal limits of the data set million year timescales indicating that co2 operating in combination with many other factors such as solar luminosity and paleogeography has imparted strong control over global temperatures for much of the phanerozoic

estimation of landform life expectancy landscapes change as episodic natural events such as climatic extremes earthquakes and volcanic eruptions create and destroy landforms and this change can be modeled statistically as a renewal process events or attacks on the landscape result in erosion and deposition which cumulatively create or destroy parts of it leaving an array of landscape components of different origins sizes ages and states of preservation at steady state or equilibrium the average number of components in the population is a constant and destructions balance renewals probability distributions can be derived for the ages and life spans in such a population the model is applied to geomorphic systems involving steady state and unsteady component creation and loss and uses examples are landslides and a continuous sedimentary sequence in addition to conceptual benefit to mechanistic descriptions of landscape evolution the model allows the quantitative measure of the tempo of geomorphic activity and an accounting of the destruction of evidence with time

an invasion percolation model of drainage network evolution stream networks evolve by headward growth and branching away from escarpments such as rift margins the structure of these networks and their topographic relief are known to be fractal1 3 but no model so far has been able to generate the observed scaling properties here i present a statistical model of network growth in which stream heads branch and propagate at a rate that depends only on the local strength of the substrate this model corresponds to the process of invasion percolation4 with the added requirement of self avoidance it is a self organized critical system5 with properties similar to those of standard percolation models6 a description based on self avoiding invasion percolation reproduces the known scaling behaviour of stream networks and may provide a valuable tool for delineation of drainage patterns from digital topographic data sets7 8

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

fish tooth δ18o revising late cretaceous meridional upper ocean water temperature gradients the oxygen isotope composition of fossil fi sh teeth a paleo upper ocean temperature proxy exceptionally resistant to diagenetic alteration provides new insight on the evolution of the low to middlelatitude thermal gradient between the middle cretaceous climatic optimum and the cooler latest cretaceous period the new middle cretaceous low to middle latitude thermal gradient agrees with that previously inferred from planktonic foraminifera 18o recovered from deep sea drilling project and ocean drilling program drilling sites although the isotopic temperatures derived from 18o of fish teeth are uniformly higher by 3 4 c in contrast our new latest cretaceous thermal gradient is markedly steeper than those previously published for this period fish tooth 18o data demonstrate that low to middle latitude thermal gradients of the middle cretaceous climatic optimum and of the cooler latest cretaceous are similar to the modern one despite a cooling of 7 c between the two periods our new results imply that no drastic changes in meridional heat transport are required to explain the late cretaceous climate based on climate models such a cooling without any change in the low to middle latitude thermal gradient supports an atmospheric co2 decrease as the primary driver of the climatic evolution recorded during the late cretaceous

are topographic data sets fractal the scale invariant properties of fractal sets make them attractive models for topographic profiles because those profiles are the end product of a complex system of physical processes operating over many spatial scales if topographic data sets are fractal their power spectra will be well represented by lines in log log space with slopess such that 3 s 1 the power spectra from a digital elevation model 30 meter sample spacing of the sierra nevada batholith and from seabeam center beam depths 425 meter sample spacing along a flowline in the south atlantic are curved straight sections in the spectra can be identified but the slopes of those sections are strongly dependent upon the particulars of the data analysis fractal geometry must be used with caution in the discussion of topographic data sets

a pedogenic goethite record of soil co2 variations as a response to soil moisture content abstract the terrestrial carbon cycle and the role of atmospheric co 2 concentrations in controlling global temperatures can be inferred from the study of ancient soils paleosols soil formed goethite and calcite have been the primary minerals used as a geochemical proxy for reconstructing atmospheric p co 2 from ancient terrestrial records in the case of goethite optimum sampling strategies for reconstructing p co 2 focus on the portion of the soil profile that displays steep gradients in both soil co 2 concentration and 13 c values of soil co 2 such that a keeling plot can be developed for a given soil and atmospheric p co 2 can be calculated from it we report data from a carboniferous paleosol that depart from the expected linear trends the results indicate that pedogenic goethite is sensitive to variations in the isotopic composition of soil co 2 over a range of timescales and can record these variations in the carbon isotope composition and mole fraction of fe co 3 oh in solid solution with goethite we explore possible environmental conditions that can drive these changes as a function of either moisture controlled variations in soil respired co 2 or in the residence time of carbon in soils the implications of this result are overestimation of paleoatmospheric p co 2 from pedogenic goethite

was the antarctic glaciation delayed by a high degassing rate during the early cenozoic abstract the cenozoic is a period of major climatic changes marked by the formation of the antarctic ice sheet at the eocene oligocene e o boundary the opening of the southern ocean seaways and the decrease in atmospheric co2 are two processes generally evoked to explain this e o cooling the debate is still ongoing but modeling studies tend to demonstrate that the decrease in atmospheric co2 is the main driver of the cooling however uncertainties persist on what drove the decrease in atmospheric co2 during the cenozoic in this study we investigate the impact of continental drift lithology distribution and volcanic degassing rates on the atmospheric carbon dioxide concentration over the cenozoic within a coupled climate carbon model geoclim in the model the continental drift results in driving low atmospheric co2 levels during the eocene and the oligocene the dispersed configuration and the location of a large continental area north africa northern south america within the inter tropical convergence zone itcz promote co2 consumption by weathering forcing co2 to remain low icehouse conditions are also promoted by the drifting of india and the weathering of the deccan basalts in the itcz during the eocene and by the weathering of the ethiopian traps during the oligocene to prevent the building up of the antarctic ice sheet at the eocene the model needs enhanced solid earth degassing flux by 50 so that atmospheric co2 levels stay above the glacial threshold 750 ppm we find that the decrease in atmospheric co2 from the eocene to the oligocene is probably due to a reduction in the source of volcanic co2 rather than an increase in silicate weathering the model results furthermore suggest that during the miocene period the northward drifting of both the african plate and india including the deccan traps might have decreased the continental surface exposed to chemical weathering therefore generating higher co2 values finally the uplift of the tibetan plateau from the miocene to the present day induces in the model an increase in silicate weathering through the intensification of the south eastern asian monsoon causing atmospheric co2 to come back to a pre industrial value

obscuring of long eccentricity cyclicity in pleistocene oceanic carbon isotope records abstract long eccentricity 400 kyr cycles in carbon isotope records from the pacific and atlantic oceans and the mediterranean sea of the past 5 0 ma are compared all records show maximum 13c values 13cmax at eccentricity minima during the pliocene but this relationship obscured in the pleistocene after 1 6 ma in particular for the open ocean deep water 13c records since a clear anti phase relationship was set up between oceanic 18o and 13c in the 100 kyr band from this time we attribute the obscured 400 kyr signal to a major change in the oceanic carbon reservoir probably associated with restructure of the southern ocean a similar change occurred in the miocene at 13 9 ma when the 400 kyr cyclicity in 13c records flattened out together with a drastic cooling and antarctic ice sheet expansion a remarkable exception is the mediterranean surface water 13c record which remained paced by the long term eccentricity cycle throughout the pliocene and pleistocene suggesting a low latitude climatic origin of the 400 kyr signal that is independent of glacial interglacial forcing since the earth is currently passing through an eccentricity minimum it is crucial to understand the nature of the 13cmax events

regional variation of mechanical and chemical denudation upper colorado river basin u s a the variation of mechanical and chemical denudation is investigated using discharge and sediment yield data from the upper colorado river system annual precipitation ranges from approximately 150 mm to 1500 mm mean specific yield ranges from 0 2 1 s km2 6 mm p a to 151 s km2 475 mm p a the hydrological geomorphological system adjusts itself to these varying climatic conditions in some areas however the effects of lithology or land use seem to override the climatic controls it is demonstrated that the increase in the absolute and particularly the relative amount of suspended sediment is closely related to a decrease in annual runoff and to an increase in the importance of high magnitude low frequency events this indicates that in areas of low annual runoff and high runoff variability soluble rocks are more resistant than in more humid areas during high magnitude low frequency events suspended sediment concentrations and loads are very high in semiarid areas due to sparse vegetation cover and dominance of direct runoff events of moderate magnitude and frequency which in more humid areas transport most of the dissolved load seldom occur the trend towards increasing mechanical denudation is even observed in areas of very low runoff 0 221 s km2 7 mm p a the peak of sediment yield in dry areas seems to approximate the point of no runoff very closely mechanical and chemical denudation are of equal importance at a runoff of about 300 mm per year

hack s relation and optimal channel networks the elongation of river basins as a consequence of energy minimization as pointed by hack 1957 river basins tend to become longer and narrower as their size increases this work shows that this property may be partially regarded as the consequence of competition and minimization of energy expenditure in river basins

large scale geomorphology classical concepts reconciled and integrated with contemporary ideas via a surface processes model linear systems analysis is used to investigate the response of a surface processes model spm to tectonic forcing the spm calculates subcontinental scale denudational landscape evolution on geological timescales 1 to hundreds of million years as the result of simultaneous hillslope transport modeled by diffusion and fluvial transport modeled by advection and reaction the tectonically forced spm accommodates the large scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification the following three model scales are considered micro meso and macroscale the concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift at the larger meso and macroscales which represent individual interfluves and landscapes including a number of drainage basins respectively the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel for these linear models the response time and the transfer function as a function of scale characterize the model behavior numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape when tectonic timescales are much longer than the landscape response time the resulting dynamic equilibrium landscapes correspond to those envisaged by hack 1960 when tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses much shorter than the response time evolving landscapes conform to the penck type 1972 and to the davis 1889 1899 and king 1953 1962 type frameworks respectively the behavior of the spm highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing finally nonlinear behavior resulting from more general uplift geometries is discussed a number of model experiments illustrate the importance of fundamental form which is an expression of the conformity of antecedent topography with the current tectonic regime lack of conformity leads to models that exhibit internal thresholds and a complex response

cascade model for fluvial geomorphology summary erosional landscapes are generally scale invariant and fractal spectral studies provide quantitative confirmation of this statement linear theories of erosion will not generate scale invariant topography in order to explain the fractal behaviour of landscapes we have introduced a modified fourier series that is the basis for a renormalization approach we have introduced a non linear dynamical model for the decay of the modified fourier series coefficients that yield a fractal spectra we argue that a physical basis for our approach is that a fractal or nearly fractal distribution of storms floods continually renews erosional features on all scales

a fractal interpretation of topography and geoid spectra on the earth moon venus and mars global spectra are available for topography and geoid on the earth venus mars and the moon if the spectral energy density has a power law dependence on wave number a fractal is defined the topography spectrum for the earth is a well defined fractal with d 1 5 this corresponds to brown noise with the amplitude proportional to the wave length although there is more scatter for the other planetary bodies the data for mars and the moon correlate well with the data for the earth venus topography also exhibits a brown noise behavior but with a smaller amplitude the power law dependence of the earth s geoid is known as kaula s law we show that uncompensated brown topography gives a geoid with a power law dependence that is in quite good agreement with kaula s law however the required amplitude is only 8 of the observed topography a similar result is found for the other bodies with the ratio of the amplitude of topography required to explain the geoid to the observed topography increasing to 72 for the moon

longitudinal profile development into bedrock an analysis of hawaiian channels analysis of topographic maps of rivers incised into dated hawaiian lava flows shows that the long term average bedrock erosion rate along certain reaches is linearly related to stream power field observations suggest that two processes may control hawaiian channel downcutting 1 stream power dependent erosion including abrasion of the channel bed by transported particles and 2 step wise lowering caused by knickpoint propagation modeling results indicate that a simple stream power dependent erosion law predicts the straight to weakly convex longitudinal profiles characteristic of kauai channels but is insufficient to predict two other characteristic features the upslope propagation of knickpoints and the straight 5 8 channel slopes below the knickpoints thus more than this single transport law is apparently required to model bedrock channel incision field surveys also indicate that significant portions of the channel lengths below the knickpoints are mantled with large boulders we propose that

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

effect of height and orientation microclimate on geomorphic degradation rates and processes late glacial terrace scarps in central idaho terrace scarps can serve as a nearly ideal natural laboratory for the study of the evolution of slopes this paper examines the effects of scarp size height and orientation microclimate by keeping constant variables such as age lithology and regional climate if a scarp degrades as a closed system and downslope movement is directly proportional to surface gradient the evolution of the scarp is modeled by the diffusion equation for a group of scarps of same age and known starting angle the diffusion equation model predicts the relation between maximum scarp angle and scarp height h late pleistocene terrace scarps now as steep as 33 25 as well as measured angles of repose for sand and gravel require a starting angle as steep as 33 5 for latest pleistocene idaho and utah scarps as h increases is gentler more degraded than modeled by the diffusion equation with a constant rate coefficient the degradation rate coefficient c increases tenfold with scarp height it should not change with scarp height if downslope movement is solely determined by surface gradient to the first power soil wash appears to be responsible for this departure from the diffusion equation model for transport rate by soil wash is a function of scarp size height south facing scarps are less vegetated and more degraded than north facing scarps for scarps 2 m high the degradation rate c on s facing scarps is 2 times that on n facing scarps for 10 m scarps it is 5 times the observed dependence of the rate coefficient c on scarp height can be removed by normalizing c to values for west facing scarps of the same height the residual c values calculated by this method correlate well with differences in incident solar radiation resulting from the different scarp orientations and maximum gradients this correlation demonstrates the importance of orientation on slope processes and their rates through the differences in freeze thaw cycles soil moisture and vegetative cover scarp morphology may be used to estimate age if one accounts for the effects of climate and for scarp height orientation and lithology for example using the dated bonneville shoreline scarps for calibration and comparing only scarps of equal height we estimate the drum mountains fault scarps to be 9 000 yr old this age is about twice that produced by previous diffusion equation calculations that have not accounted for the height as we have here but it is the same as independent geologic estimates of their age

cluster growth modeling of plateau erosion the pattern of erosion of a plateau along an escarpment may be modeled using cluster growth techniques recently popularized in models of drainage network evolution if erosion on the scarp takes place in discrete events at rates subject to local substrate strength the whole range of behavior is described by a combination of three cluster growth mechanisms invasion percolation eden growth and diffusion limited aggregation dla these model the relative importance of preexisting substrate strength background weathering and seepage weathering and erosion respectively the rate of seepage processes is determined by the efflux of groundwater at the plateau margin which in turn is determined by the pressure field in the plateau aquifer if this process acted alone it would produce erosion patterns in the form of laplacian fractals with groundwater recharge from a distant source or poissonian fractals with groundwater recharge uniform over the plateau dla is used to mimic the laplacian or poissonian potential field and the corresponding seepage growth process the scaling structure of clusters grown by pure dla invasion percolation or eden growth is well known this study presents a model which combines all three growth mechanisms for the first time mixed growth processes create clusters with different scaling properties and morphologies over distinct length scale ranges and this is demonstrable in natural examples of plateau erosion

development of longitudinal profiles of alluvial channels in response to base level lowering degradation of alluvial channels in cohesive sediments was studied in 15 m and 20 m long flumes with a slope of 0 01 cm cm degradation was initiated by lowering base level to a fixed position and the development of the longitudinal profile of the channel is analysed through a model formulated as a heat diffusion equation it is based on the equation of sediment continuity combined with an assumption regarding sediment transport namely that sediment discharge is linearly proportional to the channel slope in accordance with the boundary and initial conditions imposed by the experimental setup and procedure the basic equation is amenable to an analytical solution which defines bed elevation at any distance and time as a function of the amount of base level lowering and a diffusion coefficient additional problems arising from bank erosion and channel armouring are also treated successfully within the framework of the same model the results show that in homogeneous alluvial sediments not subject to armouring the ultimate result of base level lowering by a certain amount is degradation all along the channel by the same amount the main impact of erosion is felt in the early stages after initiation of the process and mainly near the mouth the rate of degradation at any station along the channel reaches a peak and then slowly decreases with time and the peak rate is attenuated with distance from the outlet the model permits the prediction of intermediate stages of profile development at any distance from the outlet and at different times

climatic and morphologic relationships of rivers implications of sea level fluctuations on river loads the characteristics of 279 rivers that discharge into the world oceans are analyzed in terms of their basin hydrology river discharge morphometry basin slope and area river length extension of the continental shelf seaward of the river mouth and climate precipitation statistically reliable relationships are found between discharge and basin area and between sediment load and a combined function of basin area and slope these functions are used to demonstrate how river hydrologic features would be strongly influenced by sea level fluctuations particularly under the influence of continental shelf emergence a fall in sea level toward a glacioeustatic lowstand would induce the merging of rivers on the subaerial continental shelf thereby allowing giant rivers to form for example rivers of western europe would reorganize themselves into two or three very large rivers sediment concentration carried by these mega rivers would decrease and thus the number of hyperpycnal plumes generated at river

atmospheric carbon dioxide linked with mesozoic and early cenozoic climate change carbon isotopes of fossil plants and model simulations suggest that atmospheric carbon dioxide levels were variable during the period 200 to 60 million years ago the large decreases in the partial pressure of co2 coincide with glaciations providing evidence against climate co2 decoupling during the mesozoic

potential origins of 400 500 kyr periodicities in the ocean carbon cycle a box model approach 1 cenozoic proxy records of both the isotopic composition of dissolved inorganic carbon in the oceans 13cdic and deep ocean carbonate preservation show significant periodicities in the range 400 500 kyr sensitivity analysis of the ocean carbon cycle to potential variability on this timescale in patterns of global oceanic primary productivity and or continental weathering fluxes is performed using a 7 box ocean atmosphere model the data constraints imposed by plio pleistocene proxy records of 13cdic carbonate preservation and atmospheric pco2 variability allow different scenarios to be evaluated forcing with the global oceanic primary production ratio of inorganic and organic carbon leads to the relative phases of response most consistent with the proxy data however only when changes also occur in total marine primary production can the observed relative amplitudes of 13cdic and pco2 variability also be reproduced this scenario is consistent with oscillations between a highly productive coccolithophore rich global ocean and a less productive diatom rich one such oscillations have been proposed to originate from the orbital eccentricity cycle leading to changes in seasonality and thus silica utilization in the southern ocean however the period of response in model 13cdic is always close to that of the forcing function used and thus a significant discrepancy remains between the orbital eccentricity cycle period of 413 kyr and the observed 500 kyr 13cdic periodicity seen in the pleistocene

evolution of tropical watersheds and continental hydrology during the late cretaceous greenhouse impact on marine carbon burial and possible implications for the future abstract regional climate modelling for the late cretaceous greenhouse and high resolution marine stratigraphic records from both sides of the low latitude atlantic show that tropical south american and african hydrology and watersheds had a strong effect on freshwater transfer into the equatorial atlantic and subsequently the marine carbon record this conclusion is derived from new detailed geochemical records from demerara rise off suriname drilled at ocean drilling program odp site 1261 combined with frequency analyses and climate simulations providing evidence for mainly eccentricity driven changes in carbon burial in the western tropical atlantic shorter orbital frequencies in particular precession clearly dominating black shale cycles off tropical africa odp site 959 are far less dominant at demerara rise despite comparable time resolution of the geochemical records we suggest that these different frequency patterns in carbon burial were related to the regional evolution of cretaceous watersheds and hydrology in tropical south america and africa river discharge deduced from simulations indicates higher and less variable discharge from south america compared to western africa at that time this runoff pattern would have supported more permanent anoxic conditions off south america compared to africa at least indirectly and caused the lack of strong higher frequency geochemical cycles in the western sector of the equatorial atlantic furthermore climate simulations show a general switch of primary runoff from either side of the cretaceous equatorial atlantic every half precession cycle i e every 10 kyr similarities between the developments of cretaceous and holocene hydrology in the tropical atlantic area imply that orbital scale evolution of watersheds is a robust feature through time that is independent from the mean global climate state based upon the comparison of cretaceous and holocene trends in hydrology we infer that future hydrology in the study region may develop in a comparable direction to the one observed in the cretaceous if true this suggests that the modern amazon rain forest could shrink over the next millennium due to a 30 loss of moisture while the congo rain forest in africa is likely to expand in response to a 14 gain in moisture

a diffusion limited aggregation model for the evolution of drainage networks abstract we propose a modified diffusion limited aggregation dla model for the evolution of fluvial drainage networks random walkers are introduced randomly on a grid and each two dimensional random walk proceeds until the walker finds a drainage network on which to accrete this model for headward growth of drainage networks generates drainage patterns remarkably similar to actual drainages the model also predicts statistical features which agree with actual networks including the frequency order bifurcation ratio r b 3 98 and the stream length order r r 2 09 using the definition of network fractal dimension d logr b logr r we find that our dla model gives d 1 87 near the observed range of d 1 80 1 85

is the drainage network a fractal sierpinski space the most common approach to extract the channel network from digital elevation models is to specify a threshold area s which is the minimum area required to drain to a point within which a channel forms this threshold area is usually specified arbitrarily although it is recognized that different threshold areas will result in substantially different channel networks for the same basin this paper studies the effects of s which is also the scale of observation on scaling and morphometric properties such as the number of sources and the areas of source basins and lateral basins the gardon basin located in southern france was extensively studied the results indicate that morphometric properties vary considerably with s and that the spatial distribution of source basins can be considered as a fractal sierpinski space a simple model based on the procedure similar to the sierpinski carpet construction is proposed to explain empirical relations between the number of source areas and the threshold s the model presents a new approach to estimate basin fractal properties and defines new indices to characterize the spatial distribution of first order basins this methodology is useful for hydrologists and geomorphologists dealing with river networks and spatial patterns of various basin properties such as vegetation soil soil moisture and human activities

the spectra of the topography of the earth venus and mars the global topography of the terrestrial bodies has been more or less determined from past planetary missions and from ground measurements and the behaviour of their spectra studied over the low frequencies only except the earth s case we here perform new spherical harmonic analysis of the topography of the earth venus and mars by using existing sometimes recently made available data sets to the highest possible degree and order so as to establish the decay of these spectra over a larger range of frequencies than before study the limit of validity of the used data sets and provide some new inputs to further geophysical analysis

a 100 myr history of the carbon cycle based on the 400 kyr cycle in marine δ13c benthic records documenting the past coevolution of earth temperatures and of the carbon cycle is of paramount importance for our understanding of climate dynamics atmospheric co2 is well constrained over the last million years through direct measurements in air bubbles from antarctic ice cores for older times many different and sometimes conflicting proxies have been suggested here we provide a new methodology to constrain the carbon cycle in the past based on marine benthic 13c records marine 13c data are recording a persistent 400 kyr cycle with an amplitude primarily linked to the total amount of carbon in the ocean or dissolved inorganic carbon dic by extracting this amplitude from published records we obtain a new strong constraint on the 100 myr history of earth s carbon cycle the obtained cenozoic evolution of dic is in surprisingly in a good agreement with existing reconstructions of pco2 suggesting that pco2 is mostly driven by dic changes over this period in contrast we find no strong decreasing trend in dic between the cretaceous and the cenozoic suggesting that cretaceous atmospheric pco2 levels were limited and high temperatures at this time should be explained by other mechanisms alternatively high cretaceous atmospheric pco2 could occur as a consequence of changes in oceanic chemistry but not carbon content

major pleistocene stages in a carbon perspective the south china sea record and its global comparison 1 carbon isotope sequences at ocean drilling program site 1143 south china sea reveal a long term cyclicity of 500 kyr that is superimposed on the glacial cycles and is present in long 13c sequences from all oceans the quaternary 13c record is punctuated by four 13c maximum events 13cmax i which began in marine isotope stage mis 3 around 50 60 kyr ago 13cmax ii mis 13 0 47 0 53 ma 13cmax iii mis 27 29 0 97 1 04 ma and 13cmax vi mis 53 57 1 55 1 65 ma as the same cyclicity is also found in carbonate curves the 13cmax events must denote major reorganization in the carbon reservoir of the global ocean they also are associated with major changes in glacial cyclicity such as the mid brunhes event following 13cmax ii and the mid pleistocene revolution following 13cmax iii which in turn were associated with expansion of the ice sheets from a carbon perspective therefore the quaternary period has passed through three major stages and each appears to represent a further step in ice cap development

convergent cenozoic co2 history reconstructions of atmospheric carbon dioxide concentrations over the past 65 million years are heading towards consensus it is time for systematic testing of the proxies against measurements and against each other

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

atmospheric carbon dioxide concentration across the mid pleistocene transition the dominant period of pleistocene glacial cycles changed during the mid pleistocene from 40 000 years to 100 000 years for as yet unknown reasons here we present a 2 1 million year record of sea surface partial pressure of co2 pco2 based on boron isotopes in planktic foraminifer shells which suggests that the atmospheric partial pressure of co2 pco2 was relatively stable before the mid pleistocene climate transition glacial pco2 was approximately 31 microatmospheres higher before the transition more than 1 million years ago but interglacial pco2 was similar to that of late pleistocene interglacial cycles 450 000 years ago these estimates are consistent with a close linkage between atmospheric co2 concentration and global climate but the lack of a gradual decrease in interglacial pco2 does not support the suggestion that a long term drawdown of atmospheric co2 was the main cause of the climate transition

astronomic calibration of the late oligocene through early miocene geomagnetic polarity time scale at ocean drilling program odp site 1090 subantarctic south atlantic benthic foraminiferal stable isotope data from cibicidoides and oridorsalis span the late oligocene through early miocene f24 16 ma at a temporal resolution of f5 ky over the same interval a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity time scale gpts thereby providing direct correlation of the isotope record to the gpts in an initial age model we use the newly derived age of the oligocene miocene o m boundary of 23 0 ma of shackleton et al geology 28 2000 447 revised to the new astronomical calculation la2003 of laskar et al icarus in press to recalculate the spline ages of cande and kent j geophys res 100 1995 6093 we then tune the site 1090 y 18 o record to obliquity using la2003 in this manner we are able to refine the ages of polarity chrons c7n through c5cn 1n the new age model is consistent within one obliquity cycle with previously tuned ages for polarity chrons c7n through c6bn from shackleton et al geology 28 447 450 2000 when rescaled to la2003 the results from site 1090 provide independent evidence for the revised age of the oligocene miocene boundary of 23 0 ma for early miocene polarity chrons c6aar through c5cn our obliquity scale age model is the first to allow a direct calibration to the gpts the new ages are generally within one obliquity cycle of those obtained by rescaling the cande and kent j geophys res 100 1995 6093 interpolation using the new age of the o m boundary 23 0 ma and the