tectonic patterns of the panama block deduced from seismicity gravitational data and earthquake mechanisms implications to the seismic hazard abstract an integration of seismicity parameters gravitational data focal mechanisms other geophysical data and geological information is used to determine boundaries of the panama block pb and tectonic regimes which control them the panama fracture zone represents a clear boundary between the nazca and cocos plate and is undergoing subduction beneath the pb its interaction with the cocos ridge at the western boundary of the block from the panamanian shelf break and inland adds a higher degree of tectonic complexity to this area in the north the boundary with the caribbean plate lies on the panama deformed belt along the caribbean margin of the isthmus of panama the eastern boundary of the pb is the panama south america suture zone in which the shear motion between the pb and the nazca plate is coupled with major compressive forces associated with the convergence of the nazca and south american plates the southern boundary consists of a combination of a left lateral transform fault south of the gulf of panama and a broad left lateral shear zone between the coiba ridge and the azuero peninsula the values of the cumulative seismic hazard index are greater in zones with more complex tectonics

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

a new look at evidence for a wadati benioff zone and active convergence at the north panama deformed belt the nature of the northern boundary of the panama microplate often referred to as the north panama deformed belt npdb has been the subject of much speculation previous studies have used a variety of data including teleseismic gravity bathymetric marine magnetic and field studies of uplift and tsunami deposits as well as modified mercalli intensity distributions from historic earthquakes to reveal the nature of the npdb data have been collected for over 30 yrs and yet the character of the npdb remains unclear current analyses and interpretation provide a number of mutually exclusive options and much controversy in this article we examine local and regional seismicity combined with teleseismic observations and historic earthquake data to present an alternative analysis for the npdb using small earthquakes recorded by a local network we image a well defined wadati benioff zone dipping southward beneath the panama microplate these data provide new evidence regarding the nature of panama microplate caribbean plate boundary kinematics and demonstrate the existence of an active subduction zone a more complete understanding of the nature of the seismicity and plate interactions along the npdb offshore panama including a subduction zone capable of producing earthquakes of m 7 should be included in future earthquake hazard assessments

age and origin of earliest adakitic like magmatism in panama implications for the tectonic evolution of the panamanian magmatic arc system abstract 40 20 ma marks a fundamental interval in the evolution of the 70 0 ma panamanian magmatic arc system during this period there is no evidence of panamanian magmatic arc activity to the east of the panama canal basin while to the west and in localized regions to the east of the panama canal basin a phase of intrusive only activity is recorded fundamentally geochemical and geochronological evidence presented herein indicate that this intrusive activity was predominantly adakitic like and becomes younger from west to east along an approximately w e striking lineament granodiorites of the petaquilla batholith western panama yield lam icp ms 206 pb 238 u zircon ages of 29 0 0 7 0 6 ma 28 5 0 7 0 5 ma 28 3 0 5 0 4 ma and 26 2 0 5 0 9 ma to the east of the panama canal basin zircons from a hypabyssal diorite of the mainly intermediate maje subvolcanic suite cedes a mean 206 pb 238 u age of 18 9 0 4 ma relative to other 70 5 ma panamanian magmatic arc lavas and intrusives maje and petaquilla intrusives yield adakitic like major and trace element abundances e g 15 wt al 2 o3 generally 3 5 wt na 2 o 400 ppm sr 120 and strongly fractionated hree patterns these 30 26 ma petaquilla and 19 ma maje suites are also compositionally similar to a subvolcanic suite of rare circa 25 ma adakitic like andesitic intrusives which occur within the panama canal basin midway between petaquilla and maje and at the same approximate latitude as petaquilla and maje collectively the geochemical and geochronological data for the adakitic like intrusives arc consistent with formation via partial melting of lowermost mafic crust above a sub horizontal slab tear that propagated from the west petaquilla to the east maje between 30 and 19 ma our new tectonic model postulates that collision between the panamanian magmatic arc system and an indentor e g a tract of thickened buoyant oceanic crust or plateau occurred at about 40 ma a time of which coincides with the initiation of left lateral offset of the early i e 70 40 ma arc system this collision resulted in the shutdown of the early arc system possible steepening of the subducting farallon slab and ultimately slab break off and the phase of mainly adakitic like intrusive activity subsequent to slab removal by 20 ma ne dipping subduction jumped to the south and initiated production of the later i e post 20 ma arc system soon thereafter

seismicity and tectonics of southern central america and adjacent regions with special attention to the surroundings of panama abstract ten new focal mechanisms are derived for earthquakes in southern central america and its adjacent regions these are combined with a study of seismicity and data of previous workers to delineate the position and nature of the plate boundaries in this complex region the middle america subduction zone may be divided into four or five distinct seismic segments the plate boundary between north america and the caribbean near the trench might be located more towards the south than previously suspected subduction has basically stopped south of the underthrusting cocos ridge there is not much evidence for a seismically active strike slip fault south of panama but its existence cannot be ruled out more activity reveals the zone north of panama which is identified as a subduction zone with normal fault events shallow seismicity induced by the interaction of the nazca plate extends from the colombia panama border south along the pacific coast to meet a high angle continental thrust fault system subduction with a pronounced slab starts only south of that point near a hot region which offsets the seismic trend at the trench the carnegie ridge and or the change of direction of subduction in ecuador produce a highly active zone of seismicity mainly at the depth of 200 km the area in the pacific displays a termination of activity at a propagating rift west of the galapagos islands the main eastern boundary of the cocos plate the panama fracture zone is offset towards the west at the southern end of the malpelo ridge its northern end consists of two active branches as defined by large earthquakes a strike slip mechanism near the southeastern flank of the cocos ridge was previously believed to be the site of an extended fracture zone this paper proposes submarine volcanic activity as an alternative explanation

variation in deformation of the south panama accretionary prism response to oblique subduction and trench sediment variation migrated single channel seismic lines and sea marc ii side scan and bathymetry data document an active accretionary prism along the obliquely convergent margin of south panama a region previously believed to be a transform margin the eastern flank of the coiba ridge is being subducted in this region creating the requisite geometry for along strike variation in trench sediment thickness and type the regional east dip of the downgoing plate causes the depth of the oceanic crust along the trench to drop approximately 1800 m from west to east in the western region of the study area the incoming sedimentary section consists of approximately 600 m of pelagic and hemipelagic sediments a wedge of trench turbidites that overlies the incoming hemipelagic sediments thickens from less than 100 m in the western region to more than 900 m in the eastern region of the study area the eastward increase in sediment thickness correlates with the following changes in the accretionary prism 1 decrease in initial surface slope 2 broadening of the inner trench slope 3 increase in thrust spacing 4 steepening of frontal thrusts and 5 fold development each of the responses reflects in varying degrees the increase in volume and changing physical properties of the accreted material as well as the changing shear strength of the decollement the increase in overburden pressure and compaction in the deeper sediments together with the change in lithology from oceanic to trenchfill sediments should produce an eastward increase in sediment shear strength both within the wedge and along its base because the decollement will tend to form where the pore fluid pressure overburden ratio is at a maximum the shear strength within the wedge should increase relative to basal shear strength decreased surface slopes will result from the increasing contrast in shear strength between the wedge and its base

long term earthquake prediction in central america and caribbean sea based on the time and magnitude predictable model abstract instrumental and historical information on strong mainshocks for 12 seismogenic sources in central america and caribbean sea has been used to show that the interevent time t t in years between two strong earthquakes and the magnitude m f of the following mainshock are given by the relations log t t 0 30 m m i n 0 48 m p 0 69 log m 0 13 70 m f 0 98 m m i n 0 31 m p 0 28 log m 0 4 55 where m min is the surface wave magnitude of the smallest mainshock considered m p the magnitude of the preceding mainshock and m 0 the moment rate per year in each source on the basis of these relations the probability of the occurrence of a mainshock during the decade 1993 to 2002 as well as the magnitude of this expected mainshock in each seismogenic source has been calculated the highest probability p 10 0 67 was estimated for the seismogenic source of el salvador a 1 for the occurrence of a large earthquake with expected magnitude m f 7 5 and high probabilities p 10 0 55 were estimated for the seismogenic sources of jamaica c 3 and puerto rico c 5 for earthquakes with expected magnitudes m f 7 5 in both sources

distinct patterns of chemical weathering in the drainage basins of the huanghe and xijiang river china evidence from chemical and sr isotopic compositions abstract aquatic geochemistry on watershed background helps elucidate the influence of natural processes and anthropogenic activities to the environment the 87sr 86sr ratios and dissolved loads were measured in 35 water samples from the huanghe and xijiang river drainage basins the dissolved loads and 87sr 86sr ratios differed considerably between the two drainage basins the 87sr 86sr ratios of the xijiang river are between 0 70856 and 0 70936 with an average of 0 70894 reflecting weathering of carbonate minerals as the dominant source and anthropogenic pollution as the secondary one especially in the upper reach tributaries while the sr isotopic ratios in the huanghe river range from 0 70986 to 0 71139 averaging 0 71118 that results primarily from evaporite dissolution precipitation contributes to 2 8 and 5 7 to the chemistry of the xijiang and huanghe river respectively evaporite dissolution and silicate weathering account for 79 4 and 11 3 respectively in the huanghe river while carbonate dissolution and silicate weathering account for 75 4 and 14 6 respectively in the xijiang river the chemical weathering rate is 107 6 t km2 yr in the xijiang river drainage basin and 34 2 t km2 yr in the huanghe river drainage basin the co2 consumption budget is 305 2 109 mol yr in the xijiang river drainage basin and 147 9 109 mol yr in the huanghe river drainage basin

occurrence and origin of minerals in a chamosite bearing coal of late permian age zhaotong yunnan china the minerals found in the no 5 coal late permian from the zhaotong coalfield yunnan province southwestern china have been examined and found to consist mainly of kaolinite pyrite chamosite quartz and calcite with trace amounts of illite and mixed layer illite smectite the proportion of chamosite in clay minerals ranges from 32 to 56 wt with an average of 46 wt chamosite is distributed not only in collodetrinite but also occurs as cell fillings in fusinite semifusinite and telinite the high content and mode of occurrence of chamosite in this mine indicate its formation by interaction of kaolinite with fe mg rich fluids during early diagenesis except for a minor amount of terrigenous quartz most quartz is of authigenic origin and formed from kaolinite desilication the calcite content of the no 5 coal is 1 4 6 3 with an average of 3 and is distributed in collodetrinite and as cell fillings of coal forming plants calcite originated from seawater invasion during peat accumulation pyrite occurs in several ways as massive framboidal isolated enhedral anhedral and euhedral forms in addition the presence of a large amount of pyritized red algae provides strong evidence of seawater invasion during peat accumulation the red algae may have played an important role in the enrichment of sulfur in the coal the characteristic assemblage of minerals in this mine resulted from a unique basinal environment in which the mineral matter was derived from a basaltic source region volcanic activity and seawater transgression during coal formation

plate motions in the north andean region repeated geodetic measurements with the global positioning system gps provide direct measurements of displacements due to plate motions and active crustal deformation in central america and northern south america an area of complex interaction of the nazca cocos caribbean and south american plates the displacement rates for the period 1988 1991 obtained from the results of the first three central and south america casa gps campaigns are in general agreement with the predictions of the nuvel 1 plate motion model but there are differences in detail between the observations and the model the nazca north andes convergence rate vector measured by gps is different from the nuvel 1 vector at 95 confidence the difference implies that the north andes are moving northward relative to south america the measured convergence between the caribbean plate and the north andes suggests that the southern margin of the caribbean plate is located in the south caribbean deformed belt the april 1991 costa rica earthquake and the cocos caribbean convergence rate determined by gps suggest the possibility of significant ongoing deformation between central america and the stable interior of the caribbean plate our gps results are consistent with deformation of the overriding plates at the convergent margins of central and south america and confirm that active convergence is occurring around much of the southern margin of the caribbean plate from colombia west to costa rica costa rica and panama are not part of the stable caribbean plate instead the south caribbean deformed belt and the north panama fold belt probably represent the southern margin of the caribbean plate

rare earth elements ree of dissolved and suspended loads in the xijiang river south china abstract the xijiang river is the main channel of the zhujiang pearl river the second largest river in china in terms of water discharge and flows through one of the largest carbonate provinces in the world the rare earth element ree concentrations of the dissolved load and the suspended particulate matter spm load were measured in the xijiang river system during the high flow season the low dissolved ree concentration in the xijiang river is attributed to the interaction of high ph and low doc concentration the paas normalized ree patterns for the dissolved load show some common features negative ce anomaly progressively heavy ree hree enrichment relative to light ree lree similar to the world s major rivers the absolute concentration of the dissolved ree in the xijiang river are mainly ph controlled the degree of ree partitioning between the dissolved load and spm load is also strongly ph dependent the negative ce anomaly is progressively developed with increasing ph being consistent with the oxidation of ce iii to ce iv in the alkaline river waters and the lack of ce anomalies in several doc rich waters is presumably due to both ce iii and ce iv being strongly bound by organic matter the paas normalized ree patterns for the dissolved load and the spm load in rivers draining the carbonate rock area exhibit middle ree mree enrichment and a distinct maximum at eu indicating the preferential dissolution of phosphatic minerals during weathering of host lithologies compared to the xijiang river waters the mree enrichment with a maximum at eu disappeared and light ree were more depleted in the south china sea scs waters suggesting that the ree sourced from the xijiang river must be further fractionated and modified on entering the scs the river fluxes of individual dissolved ree introduced by the xijiang river into the scs vary from 0 04 to 4 36 10 4 mol a 1

earthquake focal mechanisms and tectonic processes along the southern boundary of the caribbean plate the focal mechanisms of five small earthquakes 4 9 mb 5 6 that occurred in northwestern venezuela have been determined using teleseismically recorded rayleigh waves the rayleigh wave data were also used to constrain the depths of the five events studied these results suggest that the southern boundary of the caribbean plate is presently characterized by a broad zone of internal deformation where the eastward displacement of the caribbean plate with respect to the south american plate is being accommodated along variously oriented strike slip faults this strike slip faulting appears to occur along pre existing zones of weakness such as the bocono fault and the earthquakes studied here are all consistent with an east west trending compressive stress field being released along these zones of weakness these focal mechanisms therefore reflect the tectonic stress field generated by the eastward motion of the caribbean plate as opposed to relative plate motion per se thus the southern boundary of this plate cannot be classified as a simple plate boundary i e transform fault spreading center or subduction zone instead relative plate motion in this region appears to be accommodated by internal deformation within a broad zone extending from the interior of the caribbean plate into northern south america

latest pleistocene holocene tectonics of the ecuadorian andes abstract a structural study was carried out in the most tectonically active areas of the ecuadorian andes including part of the interandean valley the cordillera real and the subandean zone since the pleistocene the region has undergone complex wrench tectonics which are still active apparently two different stress states have coexisted within different parts of the andean range during the latest pleistocene and holocene 1 1 the external part of the chain including the cordillera real and the westernmost part of the subandean zone has been subject to a east northeast west southwest direction of greatest principal stress associated motions are partitioned between right lateral oblique thrusting and pure right lateral strike slip faulting along north south to north northeast trending structures as a whole these define a right lateral transpressive shear zone 2 2 in the interandean valley which lies above a major tertiary suture zone deformations are consistent with a north south direction of greatest horizontal principal stress associated with recent development of a left lateral transtensional shear zone in the first zone fault displacements are at least an order of magnitude larger than in the second zone the present tectonics of the area is explained by a kinematic model in which the wedge shaped cordillera real crustal block bounded to the east by a right lateral shear zone and to the west by a left lateral shear zone is being uplifted and extruded northward with respect to the cordillera occidental and to the stable south american foreland this motion is the result of oblique convergence between the nazca and south american plates

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

seismicity of the panama block ii spatial attenuation of seismic intensities abstract on the basis of a general consideration of seismicity and tectonics the panama block is divided into four seismic source zones the attenuation of observed modified mercalli mm intensity from 15 earthquakes with epicentral intensity i 0 larger than iv was studied fitting the observational data to different equations published in the seismological literature the effect of subdividing the total macroseismic area is discussed using the root mean square values and the coefficients in each regression formula an attempt is made to determine the rate of absorption of the seismic energy released in each seismic source zone as well as the influence of the seismotectonic characteristics on the propagation of the seismic hazard at different distances from the epicentre a possible explanation for the differences in spatial attenuation of the mm intensity within the panama block is that the younger structure of the crust in the panama fracture zone pfz and the panama deformed belt pdb contributes substantially to the absorption of seismic waves

seismicity of the caribbean nazca boundary constraints on microplate tectonics of the panama region there are at least three kinds of models that have been used to explain plate motions and boundaries in the panama region these models suggest that the major caribbean nazca boundary is 1 a thrust fault along which the panama deformed belt has developed 2 a left lateral strike slip fault along the southern margin of panama or 3 a diffuse array of mostly thrust faults which cause internal deformation to occur throughout panama in this paper we use joint epicenter location methods to obtain improved locations for earthquakes that occurred in this region between 1964 and 1985 and we use waveform modeling to obtain more precise depths for five events hypocenters of 13 events beneath the panama deformed belt suggest that overthrusting of panama upon the caribbean plate causes active deformation which supports model 1 but does not rule out models 2 and 3 the nature of seismic faulting along the panama deformed belt is complex and supports the premise that slow north south convergence loads the cold thick caribbean lithosphere and causes it to fail seismically epicenters of 64 events along the panama fracture zone indicate that this north south trending rightlateral transform fault is splayed north of 6 n overall the seismicity throughout suggests that the panama arc can be treated as a microplate in accordance with models 1 or 2 similarly a possible second microplate may exist south of panama but if so it does not affect regional plate motions significantly the strongest evidence for model 3 occurs in the easternmost and westernmost portions of panama where the diffuse seismicity prevents us from resolving specific microplate triple junction boundaries

the impact of igneous bedrock weathering on the mo isotopic composition of stream waters natural samples and laboratory experiments abstract river waters have been shown to be systematically enriched in the heavy molybdenum mo isotopes when compared to typical granites and basalts which generally possess mo isotopic compositions 98 95 mo of around 0 this inconsistency has been used to argue against weathering of crustal rocks as the cause for heavy riverine 98 95 mo signatures incongruent dissolution of primary bedrock however may be an important process by which the anomalous mo signatures of the river dissolved load are produced this study therefore investigates the effect of igneous crustal rock weathering on the aquatic 98 95 mo signal by comparing stream water and bedrock mo isotope data to results of bulk rock leach experiments for this purpose stream water and bedrock orthogneiss granite basalt as well as soil and vegetation samples were collected in a small catchment in the french massif central in accordance with the results of earlier studies on riverine mo both streams are isotopically heavier 98 95 mo 0 5 1 1 than the typical granites and basalts the excellent agreement of these data with those of mo released during experimental leaching of the basalt bedrock 0 6 1 0 identifies a predominance of basalt weathering over the stream water mo geochemistry while other processes i e soil formation secondary mineral precipitation and adsorption are subordinate in this catchment given that the basalt bulk rock 98 95 mo reflects a value typical for crustal magmatic rocks ca 0 1 mo isotope fractionation during the incongruent dissolution of basalt can explain the observed isotopically heavy aquatic mo signatures laser ablation analyses demonstrate that the volumetrically minor magmatic sulfides can be highly enriched in mo and mass balance calculations identify the sulfide melt inclusions as the principal mo source for the leach solutions these data suggest that the magmatic sulfides possess a distinctly heavier 98 95 mo signature than the coexisting silicate melt in this case mo would behave like fe by showing a detectable isotope fractionation at magmatic temperatures incongruent crustal bedrock weathering may thus cause a preferential release of heavy mo isotopes this effect however is highly dependent on the primary bedrock mineralogy consequently the average continental runoff may have been significantly affected by incongruent weathering during periods when the earth system was exceptionally far from steady state e g large glaciations with enhanced physical weathering or large subaerial basalt eruptions such as the deccan and the siberian plateau

silicon isotopic composition of dissolved silicon and suspended particulate matter in the yellow river china with implications for the global silicon cycle abstract the silicon isotopic composition of dissolved silicon and suspended particulate matter spm were systematically investigated in water samples from the mainstem of the yellow river and 4 major tributaries the spm content of the yellow river varied from 1 4 to 38 560 mg l averaging 3568 mg l and the 30 si of suspended particulate matter 30 si spm varied from 0 3 to 0 4 averaging 0 02 the major factors affecting the spm content and the 30 si spm values in the yellow river were inferred to be the mineralogical chemical and isotopic characteristics of the sediments from the loess plateau and a combination of the climate and the flow discharge of the river the major ions in the yellow river water were na ca 2 mg 2 hco 3 so 4 2 and cl high salt concentration was observed in samples from the middle and lower reaches likely reflecting the effects of evaporation and irrigation because the na mg 2 so 4 2 and k concentrations were correlated with the cl concentration the dissolved si concentration d si increased downstream varying from 0 016 to 0 323 mm the 30 si of dissolved si 30 si diss varied from 0 4 to 2 5 averaging 1 28 the major processes controlling the d si and 30 si diss of the yellow river are a the weathering of silicate rocks b the formation of phytoliths in plants c the evaporation of water from and the addition of meteoric water to the river system which only affects concentrations d the adsorption and desorption of aqueous monosilicic acid on iron oxide and e the dissolution of phytoliths in soils the d si and 30 si diss values of global rivers vary spatially and temporally in response to changes in climate chemical weathering intensity and biological activity the moderately positive 30 si diss values observed in the yellow river may be attributed to the higher rates of chemical weathering and biological activities that have been observed in this catchment in comparison with those of other previously studied catchments excluding the yangtze river human activities may also potentially influence chemical weathering and biological activities and affect the d si and 30 si diss values of the major rivers of the world further river studies should be performed to gain a better understanding of the global si isotope budget

determination of molybdenum isotope fractionation by double spike multicollector inductively coupled plasma mass spectrometry 1 molybdenum isotopic compositions are precisely determined by mc icp ms measurements using a mo double spike the double spike is added prior to chemical purification so that laboratory and instrumental mass fractionations are separated from natural mass dependent fractionation fractionation is determined on four mo mass ratios providing an internal consistency check the external standard reproducibility is at 0 06 per mil on the 98mo 95mo ratio 2 standard deviation using a normal microconcentric nebuliser with a cyclonic spray chamber the minimum quantity of mo is 1 g for high precision results a hydrothermal molybdenite shows fractionation of 0 3 per mil on the 98mo 95mo ratio relative to our standard johnson matthey 1000 g ml 0 3 icp standard solution lot 602332b fine grained sediments show fractionation of 0 1 and 0 3 per mil on the 98mo 95mo ratio the observed mo isotope fractionation is small but resolvable with the presented high resolution technique

recent large earthquakes along the middle american trench and their implications for the subduction process several large shallow earthquakes ms 7 0 have occurred along the middle american trench since the installation of the wwssn network included are the 1965 1968 and 1978 oaxaca events the 1970 chiapas event the 1973 colima event and the 1979 petatlan event these earthquakes have been studied in an attempt to identify similarities and differences between them that may lead to a better understanding of fracture and subduction processes the events have seismic moments ranging from 1 0 1027 dyne cm for the 1968 event to 3 2 1027 dyne cm for the 1978 event all events are of predominantly thrust type consistent with subduction to the northeast of the cocos plate body waves for the 1965 1968 1978 and 1979 events along the trench indicate rather simple faulting processes these events all had focal depths of 15 to 20 km and stress drops of the order of 10 bars the 1970 and 1973 events the easternmost and westernmost respectively of the events studied here are located close to triple junctions for the cocos north america caribbean plates 1970 and the cocos north america rivera plates 1973 these two events generated more complex body waves than the oaxaca and petatlan quakes this source complexity may have been related to the more complicated tectonics near the 1970 and 1973 epicenters this may offer a new perspective to viewing source complexity in relation to subduction zones

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

empirical relationships among the convergence rate of plates rollback rate of trench axis and island arc tectonics laws of convergence rate of plates abstract the data on 27 recent arcs which were subducted by normal oceanic lithosphere were compiled among the compression extension rates for island arcs v a compression is defined as positive and the kinematic components of absolute plate motions parallel to the direction of backarc compression extension v on oceanward velocity of the overriding plate v sn continentward velocity of the subducting plate and v tn rollback rate of the trench axis the equations v a v on v sn 7 2 cm y 1 v tn 7 2 cm y v sn 2 are found to hold as for the convergent zones associated with a wadati benioff zone deeper than 200 km equation 1 suggests that backarc spreading and crustal shortening play the role of compensating the deviation of the convergence rate from the critical value of 7 2 cm y the equation 2 means that any subducting oceanic lithosphere has the potentiality of making the trench axis rollback at the rate of 7 2 cm y and that the virtual rollback rate v tn is determined by the balance between the potential rollback rate 7 2 cm y and v sn the present author calls the kinematic relationships represented by equations 1 and 2 laws of convergence rate of plates the convergent zones of plates associated with the wadati benioff zone shallower than 200 km also show a tendency that the greater and smaller values of v on v sn correspond to the backarc compression and extension however the critical value is 3 4 cm y instead of 7 2 cm y

seismic cycle and plate margin deformation in costa rica gps observations from 1994 to 1997 global positioning system gps observations in costa rica from 1994 to 1997 reveal a complex pattern of motion consistent with the superposition of seismic cycle and secular plate margin deformation in the south velocity vectors are consistent with motion of the panama block plus postseismic deformation following the 1991 limon earthquake and interseismic strain due to partial locking of the middle america trench mat thrust in the northwest sites west of the volcanic arc are moving to the nw as a forearc sliver superimposed on this sliver motion are vertical and horizontal interseismic deformations from the adjacent nicoya segment of the mat we apply two different inverse methods to understand the source of the seismic strain in nw costa rica we compare fault locking models derived using a singular value decomposition inversion with that of a simulated annealing global optimization approach both methods yield similar models for partial locking of the thrust interface beneath the nicoya peninsula our results define an area of nearly fully locked fault beneath the outer coast of the southern portion of the peninsula with somewhat lower coupling beneath the northern half and with low coupling elsewhere these initial results show the promise for detailed imaging of the locked portion of a thrust interface responsible for future large subduction zone earthquakes

the molybdenum isotopic composition in river water constraints from small catchments we report molybdenum isotope compositions and concentrations in water samples from a variety of river catchment profiles in order to investigate the influence of anthropogenic contamination catchment geology within river precipitation and seasonal river flow variations on riverine molybdenum our results show that the observed variations in 98 95mo from 0 to 1 9 are primarily controlled by catchment lithology particularly by weathering of sulfates and sulfides erosion in catchments dominated by wet based glaciers leads to very high dissolved molybdenum concentrations in contrast anthropogenic inputs affect neither the concentration nor the isotopic composition of dissolved molybdenum in the rivers studied here seasonal variations are also quite muted the finding that catchment geology exerts the primary control on the delivery of molybdenum to seawater indicates that the flux and isotope composition of molybdenum to seawater has likely varied in the geologic past

chemical weathering controls on variations in the molybdenum isotopic composition of river water evidence from large rivers in china abstract mo isotopic composition in large rivers is very important for understanding global mo cycle at present temporal variation signatures in the 98 95 mo in large rivers have not been investigated which hinder a comprehensive understanding on the mechanism for the variations of mo isotopic compositions in river water in this study we report a one year long time series march 2010 to march 2011 of the 98 95 mo of both the water and suspended particulates collected at guiping from the middle reaches of the xijiang river xjr and of the water 98 95 mo from the lower reaches of the huanghe river hhr or yellow river collected at lijin china the results indicate that the temporal variations in the concentration and 98 95 mo of dissolved mo in the xjr range from 4 32 to 10 5 nmol l mean 7 31 nmol l and 1 04 to 1 31 relative to nist 3134 mean 1 20 respectively but that the particulates have a lower 98 95 mo 0 18 to 0 58 analysis of the suspended particulates and other chemical parameters of the river water suggests that the weathering of silicates and sulfides is the main contributor to the dissolved mo content in the xjr subsequently the highly efficient preferential trapping of lighter 98 95 mo by weathering products such as clay minerals fe mn oxides and organic materials in soils and saprolites which are abundant in the tropical catchment is the key to the heavy mo isotope signatures in the xjr furthermore it seems that neither mo scavenging by suspended particulates during riverine transportation nor the weathering of sulfates significantly influences the 98 95 mo of the xjr water cross comparison among the 98 95 mo in the waters of the three largest rivers in china the changjiang cjr hhr and xjr indicates that the hhr has the highest mo concentrations but the lightest 98 95 mo the xjr has the lowest mo concentrations but the heaviest 98 95 mo and the values for the cjr are intermediate between those for the other two rivers this supports the conclusion that the efficiency of the selective trapping of mo by soils and saprolites is the main factor controlling 98 95 mo in these large rivers the tropical subtropical xjr catchment experiences intense chemical weathering the semi arid temperate hhr catchment experiences very little and the cjr catchment falls somewhere between the two such a relationship between water mo concentrations and 98 95 mo agrees with previous observations from many large rivers worldwide it is therefore suggested that chemical weathering on continents is the key to variations in the isotopic composition of mo in the waters of large rivers

the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake and its tectonic implications a broadband seismic study the rapture process of a large back arc thrusting earthquake the april 22 1991 valle de la estrella costa rica mw 7 7 earthquake is investigated using broadband body waves and long period surface waves we find the source process to be relatively simple with the source models separately obtained from body and surface waves being very consistent the event occurred on a shallow southwest dipping rapture plane on which most energy is released updip of the hypocentral location 10 20 km deep high frequency radiation appears to have been released over a relatively small source area our preferred model has a focal mechanism with strike 102 10 dip 17 14 and rake 63 17 a seismic moment of 3 8 1 5 1020 n m and a total rupture duration of 40 6 s the earthquake appears to be associated with the north panama deformed belt npdb a thrust and fold complex that has accommodated the oroclinal deformation of panama this event along with previous large events north of panama in 1882 and 1916 indicates that there is substantial convergence along the npdb marking the npdb as a probable emerging plate boundary it remains difficult to gauge the earthquake hazard in the region because of the tectonic complexity

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

rhenium in the black sea comparison with molybdenum and uranium rhenium concentrations were measured in the black sea to determine the behavior of this redox sensitive element in an anoxic basin under reducing conditions re is removed from solution leading to its depletion from black sea deep waters compared with uranium water column re and mo profiles show evidence of removal from mid depths suggesting that these elements are scavenged either in the water column or slope sediments to a greater degree than u black sea surface waters are enriched in re reflecting re in rivers draining into the northern black sea that are enhanced up to 80 fold over other large world rivers this enrichment is most likely anthropogenic and may be due to coal burning in the region a simple two box model is used to demonstrate that re concentrations in the black sea are not in steady state with respect to current riverine inputs and authigenic fluxes riverine inputs of u and mo are not as significant perhaps reflecting the fractionation of re from the other elements during a high temperature combustion process re2o7 is volatile above 270 c

the new central american seismic hazard zonation mutual consensus based on up to day seismotectonic framework abstract central america is one of the most active seismic zones in the world due to the interaction of five tectonic plates north america caribbean coco nazca and south america and its internal deformation which generates almost one destructive earthquakes 5 4 mw 8 1 every year a new seismological zonation for central america is proposed based on seismotectonic framework a geological context tectonic and geological maps geophysical and geodetic evidence gravimetric maps magnetometric gps observations and previous works as a main source of data a depurated earthquake catalog was collected covering the period from 1522 to 2015 this catalog was homogenized to a moment magnitude scale mw after a careful analysis of all the integrated geological and seismological information the seismogenic zones were established into seismic areas defined by similar patterns of faulting seismicity and rupture mechanism the tectonic environment has required considering seismic zones in two particular seismological regimes a crustal faulting including local faults major fracture zones of plate boundary limits and thrust fault of deformed belts and b subduction taking into account the change in the subduction angle along the trench and the type and location of the rupture the seismicity in the subduction zone is divided into interplate and intraplate inslab seismicity the regional seismic zonation proposed for the whole of central america include local seismic zonations avoiding discontinuities at the national boundaries because of a consensus between the 7 countries based on the cooperative work of specialists on central american seismotectonics and related topics

chemical weathering in the upper reaches of xijiang river draining the yunnan guizhou plateau southwest china abstract chemical and strontium isotopic compositions of nanpanjiang river and beipanjiang river in the upper reaches of xijiang river draining the yunnan guizhou plateau southwest china were measured with a main purpose to understand the rock weathering and associated co 2 consumption rates in the karstic landform dominated area the chemical ion composition of the river water is characterized by dominance of ca 2 mg 2 and hco 3 and so 4 2 which account for more than 90 of total ion concentration 87 sr 86 sr ratios of nanpanjiang river water range between 0 7080 and 0 7140 while those of most beipanjiang river water vary within a limited range from 0 7075 to 0 7085 weathering of carbonate rocks dominate chemistry of major ions especially in the water of beipanjiang river whereas weathering of silicate minerals in the upper reaches of nanpanjiang river basin is obvious in addition to carbonate weathering mainly according to the sr isotopic composition of the river water analyses of the stoichiometry of the river water show that the water chemistry is controlled by carbonate dissolution not only by carbonic but also by sulfuric acid and indicate that sulfuric acid plays an important role in carbonate weathering the sulfate in river water is mainly from atmospheric input by coal combustion industries and from oxidation of sulfide minerals during weathering of coal containing strata and coal mining in the catchment the chemical weathering rates of silicate and carbonate and associated co 2 consumption rates by both carbonic and sulfuric acid and by only carbonic acid are respectively estimated the results show that chemical weathering rates of carbonate and silicate weathering in beipanjiang river basin are higher than those of nanpanjiang river basin showing linkages of geology and geography in the catchment to crustal weathering the involvement of sulfuric acid in carbonate weathering greatly enhances the carbonate weathering but lowers the co 2 consumption rates indicating that sulfide acid is an important agent to rock weathering and the role of sulfide acid should further be clarified in view of the regional and global budget of co 2

chemical weathering and co2 consumption in the xijiang river basin south china monthly samples of riverine water were collected and analyzed for the concentrations of major ions ca2 mg2 k na hco3 so42 cl no3 dissolved silicon and total dissolved solids tds at wuzhou hydrological station located between the middle and lower reaches of the xijiang river xjr from march 2005 to april 2006 more frequent sampling and analysis were carried out during the catastrophic flooding in june 2005 stoichiometric analysis was applied for tracing sources of major ions and estimating co2 consumption from the chemical weathering of rocks the results demonstrate that the chemical weathering of carbonate and silicate rocks within the drainage basin is the main source of the dissolved chemical substances in the xjr some 81 20 of the riverine cations originated from the chemical weathering processes induced by carbonic acid 11 32 by sulfuric acid and the other 7 48 from the dissolution of gypsum and precipitates of sea salts within the drainage basin the co2 flux consumed by the rock chemical weathering within the xjr basin is 2 37 x 10 11 mol y 1 of which 0 64 x 10 11 mol y 1 results from silicate rock chemical weathering and 1 73 x 10 11 mol y 1 results from carbonate rock chemical weathering the co2 consumption comprises 0 38 x 10 11 mol during the 9 d catastrophic flooding the co2 consumption from rock chemical weathering in humid subtropical zones regulates atmospheric co2 level and constitutes a significant part of the global carbon budget the carbon sink potential of rock chemical weathering processes in the humid subtropical zones deserves extra attention c 2008 elsevier b v all rights reserved

an experiment in systematic study of global seismicity centroid moment tensor solutions for 201 moderate and large earthquakes of 1981 data from the global digital seismograph network were used to obtain centroid moment tensor solutions using the method of dziewonski et al 1981 results were obtained for 201 earthquakes ranging in seismic moment from 7 1023 to 3 1027 dyne cm the wide dynamic range of the sro asro stations allows us to investigate using the same algorithm series of events among which the smallest and the largest may differ in moment by a factor as large as 1000 among the events studied is a particularly interesting series in january 1981 off the island of honshu the main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis there is an indication that the source mechanism changes with the size of the event statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1 6 10 8 m01 3 the shifts in origin time for deep focus earthquakes are indicative of the general complexity of these events which depends only weakly on the seismic moment the extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth while for shallow earthquakes these departures decrease from a maximum at about 1025 dyne cm with increasing moment the reverse appears to be true with respect to deep focus earthquakes shallow earthquakes north of new guinea and along the solomon islands show systematic and substantial departures from the double couple mechanism

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

arc continent collision and orocline formation closing of the central american seaway closure of the central american seaway was a local tectonic event with potentially global biotic and environmental repercussions we report geochronological six u pb la icp ms zircon ages and geochemical 19 xrf and icp ms analyses data from the isthmus of panama that allow definition of a distinctive succession of plateau sequences to subduction related protoarc to arc volcaniclastic rocks intruded by late cretaceous to middle eocene intermediate plutonic rocks 67 6 1 4 ma to 41 1 0 7 ma paleomagnetic analyses 24 sites 192 cores in this same belt reveal large counterclockwise vertical axis rotations 70 9 6 7 and moderate clockwise rotations between 40 4 1 and 56 2 11 1 on either side of an east west trending fault at the apex of the isthmus rio gatun fault consistent with isthmus curvature an oligocene miocene arc crosscuts the older deformed and segmented arc sequences and shows no significant vertical axis rotation or deformation there are three main stages of deformation 1 left lateral strike slip offset of the arc 100 km and counterclockwise vertical axis rotation of western arc segments between 38 and 28 ma 2 clockwise rotation of central arc segments between 28 and 25 ma and 3 orocline tightening after 25 ma when this reconstruction is placed in a global plate tectonic framework and published exhumation data is added the central american seaway disappears at 15 ma suggesting that by the time of northern hemisphere glaciation deep water circulation had long been severed in central america

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

geological and geochemical characteristics of high arsenic coals from endemic arsenosis areas in southwestern guizhou province china southwest guizhou province is one of the most important areas of disseminated sediment hosted type au deposits in china and is an important area of coal production the chemistry of most of the coals in sw guizhou is similar to those in other parts of china their as content is near the chinese coal average but some local small coal mines contain high as coals the highest as content is up to 3 5 wt in the coal the use of high as coals has caused in excess of 3000 cases of as poisoning in several villages the high as coals are in the longtan formation which is an alternating marine facies and terrestrial facies the coals are distributed on both sides of faults that parallel the regional anticlinal axis the as content of coal is higher closer to the fault plane the as content of coal changes greatly in different coal beds and different locations of the same bed geological structures such as anticlines faults and sedimentary strata control the distribution of high as coals small au deposits as well as sb hg and th mineralization are found near the high as coals although some as bearing minerals such as pyrite arsenopyrite realgar as bearing sulfate as bearing clays and phosphate are found in the high as coals their contents cannot account for the abundance of as in some coals analysis of the coal indicates that as mainly exists in the form of as5 and as3 perhaps combined with organic compounds the occurrence of such exceptionally high as contents in coal and the fact that the as is dominantly organically associated are unique observations

trace element geochemistry in the upper amazon drainage basin bolivia abstract the distribution of trace elements sr mo ba rb u mn ni cu zn and cd was investigated in surface waters and associated particulates in two upper amazonian basins mamore and beni dissolved sr ba mo rb u and to a lesser extent zn and cd correlate with major ions and appear to be predominantly derived from soluble rocks carbonates evaporites or sulfides essentially contained in shales these elements are conservative in waters except in highly alkaline or sulfate rich waters where calcite and barite can precipitate the concentrations of transition element are generally not correlated with major ion chemistry their behaviour can be largely explained by equilibrium reactions in solution and solid liquid exchanges additionally the distribution of mn and cu reflects also the lithological differences between the mamore and beni basins the occurrence of mineralisation in the beni basin explains the high zn cd as well as the concentrations of cu in some rivers of the beni basin the higher content of mn in the mamore basin may reflects the predominance of carbonate rocks the fractionation of trace elements in spm has been attributed to the sorting of feldspars during transport in water and to the mixing between clay minerals and quartz these minerals refractory to weathering and introduced in water rivers by mechanical erosion have not contributed to the dissolved load

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day