slow release of fossil carbon during the palaeocene eocene thermal maximum a brief period of warming 55 9 myr ago has been attributed to the release of massive amounts of carbon geochemical and model data suggest the peak rate of carbon emission during this interval was relatively slow and significantly lower than present day levels of carbon emissions to the atmosphere

variability in fault size scaling due to rock strength heterogeneity a finite element investigation abstract practical application of fault size scaling relationships usually involves extrapolation of a limited data set over a scale range beyond that observed or into adjacent unstudied areas here we investigate the validity of such extrapolations by considering the variability in the size frequency distributions of fault populations that develop under identical tectonic conditions using a numerical model to generate conjugate normal fault populations in cross section the deforming material is modelled using a strain softening von mises rheology with gaussian heterogeneity in yield strength distributed randomly throughout the mesh we present eight deformation experiments that differ only in the random spatial pattern of yield strengths we observe power law size frequency scaling i e n ax c where n is the cumulative number of faults and x is a measure of fault size but with a range of values of c the ensemble average value of c decreases with increasing percentage extension however for individual model runs the dependence of c on total strain shows significant variability that we can relate to small but important differences in fault growth and strain localisation at any particular strain the range of values of c is 10 times greater than the error estimate derived from least squares regression of the cumulative frequency data our results suggest therefore that large uncertainties should be associated with extrapolating fault population data from one scale or region to another even if the lithology and tectonic history are similar

the evolution of seismicity at parkfield observation experiment and a fracture mechanical interpretation we compare observations of local seismicity sequences with acoustic emissions during rock fracture and find similarities in the distribution of event magnitudes which lead us to consider a model of seismicity which is characterized by fracture growth laws derived from the field of fracture mechanics and which include both negative and positive feedback processes we find that the model predicts the observed negative correlation between the seismic b value and the fractal dimension of the earthquake epicentres the simple short range interactions described in the model give rise to a system with a long range order similar to current models of critical processes

silica burp in the eocene ocean the eocene was a time of greatly increased silica accumulation in the ocean and the peak was in the early middle eocene at about 50 ma the responsible geohistorical configuration included the following elements extensive volcanism about 4 m y earlier as part of the chron 24 plate reorganization early eocene warming with deep weathering to high latitudes and accumulation of the released silica in a sluggish ocean and sharp cooling in the earliest middle eocene stimulating oceanic upwelling and biosilicification it is possible on the evidence of carbon and oxygen isotopic patterns that the trigger for the exhalation of silica was a reverse greenhouse effect

rapid and sustained surface ocean acidification during the paleocene eocene thermal maximum the paleocene eocene thermal maximum petm has been associated with the release of several thousands of petagrams of carbon pg c as methane and or carbon dioxide into the ocean atmosphere system within 10 kyr on the basis of the co occurrence of a carbon isotope excursion cie widespread dissolution of deep sea carbonates and global warming in theory this rapid carbon release should have severely acidified the surface ocean though no geochemical evidence has yet been presented using boron based proxies for surface ocean carbonate chemistry we present the first observational evidence for a drop in the ph of surface and thermocline seawater during the petm planktic foraminifers from a drill site in the north pacific ocean drilling program site 1209 show a 0 8 decrease in boron isotopic composition 11b at the onset of the event along with a 30 40 reduction in shell b ca similar trends in 11b are present in two lower resolution records from the south atlantic and equatorial pacific these observations are consistent with significant global acidification of the surface ocean lasting at least 70 kyr and requiring sustained carbon release the anomalies in the b records are consistent with an initial surface ph drop of 0 3 units at the upper range of model based estimates of acidification

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

andean sponges reveal long term benthic ecosystem shifts following the end triassic mass extinction abstract thick cherts and cherty dolomites in the basal jurassic aramachay formation of peru preserve a thriving continental shelf community dominated by siliceous sponges that followed the end triassic collapse of metazoan rich carbonate accumulation similar hettangian and sineumurian deposits from nevada u s a austria and morocco suggest that an early jurassic siliceous sponge takeover was a widespread phenomenon that persisted for 2 m y until metazoan driven carbonate sedimentation recovered the post extinction dominance of siliceous sponges likely resulted from the confluence of metazoan carbonate reef collapse removal of incumbents and geochemical conditions that fostered the success of the siliceous sponge dominated ecosystem simple mass balance calculations suggest the siliceous sponge takeover was likely permitted by an increased silica flux as a consequence of weathering central atlantic magmatic province camp basalts the camp basalts alone could supply all the silica needed to sustain the sponge takeover although contributions were also likely from increased hot climate weathering of other silicates and possible reductions in dissolved silica demand by radiolarians detailed sedimentological fossil and microfacies analyses were conducted at six field sites across a shallow shelf system recorded in the central peruvian andes yauli dome focusing on the metazoan contribution to sedimentation sedimentary structures at all six sites demonstrated on shelf deposition similar to the underlying upper triassic chambara formation in contrast to the black shale rich facies of the aramachay formation in other areas of peru examination of up to 147 m of cherty dolomite from the aramachay formation revealed a siliceous sponge dominated ecosystem including sponge body fossils compressed in situ sponge materials and abundant transported spiculite sediments siliceous sponges mostly demosponges and rare hexactinellids account for the chert lithology and apparently dominated the local ecology for approximately two million years the role of metazoan biocalcifiers in sediment production and ecological structure was profoundly reduced compared to the under and overlying formations representing a clear ecological state shift from pre extinction carbonate to post extinction siliceous dominated ecosystems before the carbonate system recovered 2 m y after the extinction

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

radiolarian palaeoecology and radiolarites is the present the key to the past abstract radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint evolutionary selection pressure among silica secreting marine plankton both radiolarians and diatoms has led toward more economic usage of rapidly shrinking nutrient resources including dissolved silica of the photic zone in the late cenozoic oceans and in particular a substantial modification of oceanic cycle by the diatom explosive radiation even if there is a proved link between biomineralization and dissolved silica loading among the phytoplankton only the relative independence of modern siliceous planktic biotas from the available silica pool reflects mainly their progressive physiological specialisation during evolutionary history oceanic chemistry and productivity as well as patterns of circulation upwelling have changed radically during the phanerozoic radiolarites apparently represent an anachronistic facies as exemplified by their long lived and ocean wide distribution in palaeo pacific and hitherto highlighted actualistic models of localized intra oceanic wind driven upwelling loci are of largely questionable applicability in addition to plate drift hypersiliceous domains and intervals are explainable mostly by a large scale volcano hydrothermal activity during major plate boundary reconfigurations which in many ways favoured siliceous biotas acme and their skeletal remains accumulation and preservation factors tied to rapid voluminous submarine eruptions such as thermal buoyant megaplumes and basin overturns offer a viable alternative for traditional climatic circulation scenarios in case of hypersiliceous high productivity events irrelevant to greenhouse to icehouse climatic change the evolving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering volcano hydrothermal and tectonic uplift events related mostly to extensive rifting and or accelerated oceanic spreading were the endogenous driving force that created this perturbation of the exogenous system the present biogeochemical cycle is representative only for the overall silica depleted post eocene oceanic ecosystems which broadly correlates with a major expansion of diatoms groups extremely efficient in silica removal and closely linking the silica budget with phosphorus and nitrogen cycles thus an orthodox uniformitarian approach to biosiliceous sedimentation based on a silica starved vigorous ocean is of limited significance when applied to the pre neogene settings especially in the peculiar planktic habitats of epeiric seas as well as during biotic crises marked by strong geotectonic overprint the major turnovers in marine siliceous biota composition in particular after the end permian radiolarite gap may have been coupled with discernible changes in an increasing biological control on the long term oceanic silica cycling punctuated equilibrium the evolutionary turnovers have induced a stepdown decrease of dissolved silica levels through the phanerozoic contemporaneously with the general secular trend of upward scaling of nutrient related ecological processes and increased effectiveness of resource utilization

modeling the merensky reef bushveld complex republic of south africa the merensky pegmatoid normal reef in the western bushveld complex is commonly characterized as a pyroxene rich pegmatoidal unit with a base that is enriched in chromite and platinum group element bearing sulfides overlying a leuconorite footwall models for its formation have ranged from those that view it as entirely a magmatic cumulate succession to those that have suggested that it is a zone of volatile induced remelting the consequences of the latter interpretation are investigated using the numerical modeling program iridium which links diffusive and advective mass and heat transport with a phase equilibration routine based on the melts program the initial system consists of a simple stratigraphic succession of a partially molten leuconorite overlain by a partially molten pyroxenite both initially at 1 190 c and 2 kbar 2 wt of a volatile fluid composed of 75 mol h2o 20 mol co2 and 5 mol h2s is then added to the lower 20 cm of the pyroxenite the system is then allowed to evolve under conditions of chemical diffusion in the liquid the addition of the volatile components results in a modest increase in the amount of melt in the pyroxenite however chemical diffusion across the leuconorite pyroxenite boundary leads to more extensive melting at and below the boundary with preferential loss of opx from the underlying leuconorite preferential re precipitation of sulfide and chromite and concentration of the pge at this boundary these results mimic actual mineral and compositional profiles across the merensky pegmatoid and illustrate that long term diffusion process can effectively produce mineralogical and compositional layering not present in the original assemblage

reversed deep sea carbonate ion basin gradient during paleocene eocene thermal maximum 1 the paleocene eocene thermal maximum petm 55 ma ago was marked by widespread caco3 dissolution in deep sea sediments a process that has been attributed to massive release of carbon into the ocean atmosphere system the pattern of carbonate dissolution is key to reconstructing changes in deep sea carbonate chemistry and ultimately the rate magnitude and location of carbon input here we show that during the petm the deep sea undersaturation was not homogeneous among the different ocean basins application of a sediment model to a suite of data records from different sites and ocean basins shows that a globally uniform decrease in deep sea carbonate ion concentration co32 is inconsistent with the data rather we demonstrate that deep sea co32 increased from the atlantic through the southern ocean into the pacific our results show that the petm deep sea co32 basin gradient during dissolution was reversed relative to the modern

an osmium isotope excursion associated with the late paleocene thermal maximum evidence of intensified chemical weathering in the latest paleocene an abrupt shift to more negative 13c values has been documented at numerous marine and terrestrial sites bralower et al 1997 cramer et al 1999 kaiho et al 1996 kennett and stott 1991 koch et al 1992 stott et al 1996 thomas and shackleton 1996 zachos et al 1993 this carbon isotope event cie is coincident with oxygen isotope data that indicate warming of surface waters at high latitudes of nearly 4 6 c kennett and stott 1991 and more moderate warming in the subtropics thomas et al 1999 here we report 187os 188os isotope records from the north atlantic and indian oceans which demonstrate a 10 increase in the 187os 188os ratio of seawater coincident with the late paleocene cie this excursion to higher 187os 188os ratios is consistent with a global increase in weathering rates the inference of increased chemical weathering during this interval of unusual warmth is significant because it provides empirical evidence supporting the operation of a feedback between chemical weathering rates and warm global climate which acts to stabilize earth s climate walker et al 1981 estimates of the duration of late paleocene cie bains et al 1999 bralower et al 1997 norris and rohl 1999 rohl et al 2000 in conjunction with the os isotope data imply that intensified chemical weathering in response to warm humid climates can occur on timescales of 104 105 years this interpretation requires that the late paleocene thermal maximum os isotope excursion be produced mainly by increased os flux to the ocean rather than a transient excursion to higher 187os 188os ratios in river runoff although we argue that the former is more likely than the latter we cannot rule out significant changes in the 187os 188os ratio of rivers

controls on strain localization in a two dimensional elastoplastic layer insights into size frequency scaling of extensional fault populations 1 we use a two dimensional 2 d finite element code to investigate how mechanical properties and boundary conditions influence progressive strain localization during extension of a 2 d elastoplastic layer the deforming medium is modeled using a strain softening von mises rheology with a gaussian heterogeneity in yield strength distributed randomly in space specifically we examine the effects of changing 1 the thickness of the deforming layer 2 the basal boundary condition 3 the range of strengths in the deforming layer 4 the strength loss on failure and 5 the total accommodated strain discrete zones of plastic shear strain are observed to nucleate and grow progressively during each experiment in order to quantify and thus discriminate between the deformation patterns produced under differing conditions we calculate the size frequency distributions of the shear band populations size is defined as the total plastic strain represented by each discrete structure a wide range of size frequency distributions is observed including both power law and exponential as well as distributions that show breaks in scaling with a transition from power law at small sizes to exponential at large sizes we show that these variations in population statistics are directly reflecting a change in strain localization in space and time caused by changing model parameters furthermore by making an analogy between the model shear bands and tectonic faults we provide insights into key features of fault size frequency distributions that have been measured in continental and oceanic extensional settings and also observed in analogue experiments

the paleocene eocene thermal maximum as recorded by tethyan planktonic foraminifera in the forada section northern italy abstract the forada section in the venetian pre alps of northern italy represents an expanded record of the paleocene eocene thermal maximum petm at a depositional paleodepth of about 1 km 0 5 km high resolution planktonic foraminiferal analysis of this section in a time interval of approximately 1 3 myr across the paleocene eocene boundary reveals striking faunal changes that allow the identification of eight phases a h the late paleocene was represented by stable warm and oligotrophic surface water conditions phase a unstable environmental conditions start well before the onset of petm ca 150 kyr phase b and involved a change towards eutrophy as marked by the increase of subbotina and the concomitant decrease of morozovella this step is also characterized by enhanced fragmentation and dissolution the interval corresponding to the main body of the carbon isotope excursion cie is characterized by a marked increase of acarinina though with some differences in the species composition and relative abundance both in high and low latitudes particularly in the tethyan area forada is no exception to this pattern however at forada two prominent peaks in abundance of acarininids are recorded ca 30 kyr prior to the onset of the cie thus suggesting an increase in temperature heralding the onset of the petm phase c interestingly the lower peak in abundance of acarinina just precedes the 1 carbon isotope negative shift occurring below the onset of the main cie the basalmost eocene corresponding to the lower part of cie curve is represented by intense planktonic foraminiferal dissolution implying an extraordinary rise of the ccd this interval has an estimated duration of about 16 kyr phase d the dominance of acarininids in the lower part of the cie phase e f ca 14 and 22 5 kyr is interpreted as a consequence of the extreme warmth coupled with eutrophic conditions characterizing the forada depositional environment at that time these acarininids include at forada also the temporally constrained acarinina sibaiyaensis and a africana the morphological similarity between these peculiar species with the radially elongated chambered forms characterizing the cretaceous anoxic events suggests the hypothesis that depletion of oxygen in the upper water column might have been one of the factors causing their conspicuous occurrence at the petm the recovery in abundance of the specialized morozovellids and of other planktonic foraminiferal groups e g biserials globanomalinids igorinids planorotalids and pseudohastigerinids occurring in the middle part of the cie ca 30 kyr after the onset of the petm indicates an initial environmental recovery phase g a new stable state is definitely reached in the upper part of the forada section where the relative proportions of the main component of planktonic foraminiferal assemblages move towards values similar to those of the late paleocene conditions phase h however the perturbation during the petm produced significant changes in the ocean geochemistry that endured after the petm event as testified by the prominent high carbonate dissolution characterizing the marly levels and the large variability in relative abundance among different components of the planktonic foraminiferal assemblages these striking oscillations were not present in the latest paleocene

experimental analysis of crack propagation in granite an experimental approach is used to demonstrate the concept of an effective crack length with double edge notched specimens of charcoal and rockville granite crack propagation in rock is characterized by microcracking around the crack tip and interlocking along a portion of the crack this region is called the fracture process zone and together with the traction free length defines the effective crack length twenty closed loop strain controlled fracture tests were conducted on charcoal and rockville granite crack growth was monitored with a travelling optical microscope 100 x magnification by comparing the post peak behaviour of both granites it is suggested that the process zone is larger for the larger grain sized rock rockville conventional linear elastic fracture mechanics techniques are used to calculate the apparent fracture toughness of charcoal granite at various crack lengths in addition a j integral expression is derived for the double edged notched geometry in terms of the area under the load displacement record the shortcomings of both analyses are discussed an attempt is made to explain the inadequacies by including the process zone in the calculations of the fracture toughness author trrl

the fracture process zone in granite evidence and effect abstract evidence of the fracture process zone and its effect on fracture toughness were examined for charcoal and rockville granite average grain sizes of 1 and 10mm fracture tests were conducted on wedge loaded double cantilever beam specimens 500 mm long 200 mm wide 40 mm thick the seismic techniques of ultrasonic probing and acoustic emission were utilized to measure the inelastic region the beginning of the process zone was interpreted by a three fold increase in transmission of ultrasound from an open crack to a partially closed crack the end of the damage zone was found by comparing surface wave amplitudes before and after fracture following these procedures the inelastic region in charcoal granite was about 40 mm whereas in rockville granite a region over 90 mm long was estimated the major acoustic activity was located within the macrocrack being released from unbroken or interlocked crystals the experimental evidence indicated an effective crack to be composed of a traction free length and a ligament process zone which was observed to form a single multiconnected region within the macrocrack the effective crack was used for an r curve calculation so that the energy consumed in the process zone was included in an approximate manner by elongating the crack length a model of crack propagation in rock consistent with measurements of the inelastic region describes the effect of a process zone on fracture toughness testing

could changing ocean circulation have destabilized methane hydrate at the paleocene eocene boundary 1 during the paleocene eocene thermal maximum petm 55 ma marine and terrestrial carbon isotope values exhibit a negative shift of at least 2 5 indicative of massive destabilization of marine methane hydrates releasing 2000 gigatons of methane carbon the cause of the hydrate destabilization is unknown but has been speculated to be warming due to a change from high latitude to low latitude deepwater formation here we present results from a numerical ocean model indicating that a sudden switch of deepwater formation from southern to northern high latitudes caused middepth and deep ocean warming of 3 5 c the switch is caused by a slow increase in the intensity of the atmospheric hydrologic cycle as expected under increasing temperatures and consistent with petm sedimentary evidence deepened subtropical subduction prior to the thermohaline circulation switch causes warming of 1 4 c in limited areas at thermocline through upper intermediate depths which could destabilize methane hydrates gradually and at progressively greater depths warming accompanying a south to north switch in deepwater formation would produce sufficient warming to destabilize seafloor gas hydrates over most of the world ocean to a water depth of at least 1900 m

silicate weathering and north atlantic silica burial during the paleocene eocene thermal maximum during the paleocene eocene thermal maximum petm ca 56 ma thousands of gigatons of carbon were released into the ocean and atmosphere over several thousand years offering the opportunity to study the response of ocean biogeochemistry to a carbon cycle perturbation of a similar magnitude to projected anthropogenic co 2 release petm scenarios typically invoke accelerated chemical weathering of terrestrial silicate rocks as a significant negative feedback driving the recovery and termination of the event however the implications of this mechanism for the geochemical cycling of silica during the petm have received little attention i use back of the envelope calculations and a simple two box geochemical model of the marine silica cycle to demonstrate that the sequestration of thousands of gigatons of carbon by enhanced silicate weathering during the petm would have dramatically increased the riverine supply of dissolved silica h 4 sio 4 to the oceans this would have elevated seawater h 4 sio 4 encouraging both increased opal sio 2 production by siliceous organisms and enhanced preservation of sio 2 in the water column and sediments both of these factors would have promoted a prompt due to the relatively short oceanic residence time of silica increase in sedimentary opal burial thus balancing the marine silica budget several recently recovered deep sea sedimentary records from the central north atlantic demonstrate elevated sio 2 content across the paleocene eocene boundary which i argue is the result of enhanced production and or preservation of sio 2 in response to elevated h 4 sio 4 in the north atlantic representing the ultimate fate of excess si weathered from the continents during the petm

rapid carbon sequestration at the termination of the palaeocene eocene thermal maximum the palaeocene eocene thermal maximum warm event about 56 million years ago was caused by the release of large amounts of carbon to the ocean and atmosphere estimates of the rate of recovery from the event suggest that about 2 000 pg of the carbon released was sequestered as organic carbon