propagation of rifting along the arabia somalia plate boundary into afar it is generally accepted that the aden ridge has propagated westward from 58 e to the western tip of the gulf of aden tadjoura at the edge of afar here we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the republic of djibouti we show that it veers northward forming a narrow zone of dense faulting along the northeastern edge of the afar depression the zone includes two volcanic rifts asal ghoubbet and manda inakir connected to one another and to the submarine part of the ridge by transfer zones both rifts are composite divided into two or three disconnected parallel nw se striking subrifts all of which appear to have propagated northwestward in asal ghoubbet as in manda inakir the subrifts appear to have formed in succession through north directed jumps from subrifts more farther south at present the northernmost subrifts manda and dirko koma of the manda inakir rift form the current tip of the northward propagating arabia somalia plate boundary in afar we account for most observations by a mechanical model similar to that previously inferred for the gulf of aden in which propagation is governed by the intensity and direction of the minimum horizontal principal stress 3 we interpret the northward propagation on land almost orthogonal to that in the gulf to be related to necking of the central afar lithosphere where it is thinnest such necking may be a consequence of differential magmatic thickening greater in the center of the afar depression where the ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression the model explains why the aden ridge foregoes its wsw propagation direction constant from 58 e to asal ghoubbet at a smaller scale individual rifts and subrifts keep opening perpendicular to the arabia somalia or danakil somalia motion vector and propagate northwestward concurrently such lithospheric cracks are forced to jump northward such that the plate boundary remains inside the regional n s necking zone changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns a small angle promoting long en echelon subrifts and a high angle smaller nested subrifts within subrifts the propagation mechanism is thus similar whether in oceanic or continental lithosphere the principal change being the overall propagation path here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the aden ridge finally because the same mechanism has led rifting along the red sea to propagate southward and jump to the western edge of afar the arabia somalia and arabia nubia plate boundaries tips have missed each other and keep overlapping further leading to strain transfer by large scale bookshelf faulting

insar observations of post rifting deformation around the dabbahu rift segment afar ethiopia summary increased displacement rates have been observed following manylarge earthquakes and mag matic events although an order of magnitude smaller than the displacements associated with the main event the post seismic or post rifting deformation may continue for years to decades after the initial earthquake or dyke intrusion due to the rare occurrence of subaerial rifting events thereareveryfewobservationstoconstrainmodelsofpost riftingdeformation in2005 september a 60 km long dyke was intruded along the dabbahu segment of the nubia arabia plate boundary afar ethiopia marking the beginning of an ongoing rifting episode con tinued activity has been monitored using satellite radar interferometry and data from global positioning system instruments deployed around the rift in response to the initial intrusion using multiple satellite passes we are able to separate the rift perpendicular and vertical displacement fields around the dabbahu segment rift perpendicular and vertical rates of up to 180 and 240 mm yr 1 respectively here we show that models of viscoelastic relaxation alone are insufficient to reproduce the observed deformation field and that a large portion of the observed signal is related to the movement of magma within the rift segment our models suggest upper mantle viscosities of 10 18 19 pas overlain by an elastic crust of between 15 and 30km to fit the observations inflation and deflation of magma chambers in the centre of the rift and to the south east of the rift axis is required at rates of 0 13 and 0 08km 3 yr 1

time dependent dike propagation from joint inversion of seismicity and deformation data dike intrusions both deform earth s surface and induce propagating earthquake swarms we develop methods to utilize both deformation and seismicity from brittle volcano tectonic earthquakes to image time dependent dike propagation dieterich s 1994 seismicity rate theory is used to relate dike induced stress changes to seismicity rate and is combined with elastic green s functions relating dike opening to deformation different space time patterns of seismicity develop if earthquakes occur at the same depth as the dike compared to above below the dike in the former seismicity initiates near the dike s leading edges but shuts off as the dike tips pass and seismogenic volumes fall into stress shadows in the latter seismicity continues at a decaying rate after the tips pass we focus on lateral propagation and develop a nonlinear inversion method that estimates dike length and pressure as a function of time the method is applied to the 2007 father s day intrusion in kilauea volcano seismicity is concentrated at 3 km depth comparable to geodetic estimates of dike depth and decays rapidly in time with lateral propagation only and a vertical dike tip line it is difficult to fit both gps data and the rapid down rift jump in seismicity suggesting significant vertical propagation for the events to have occurred below the dike requires a very short aftershock decay time hence unreasonably high background stressing rate the rapid decay is better explained if the dike extends somewhat below the seismicity we suggest that joint inversion is useful for studying the diking process and may allow for improved short term eruption forecasts

sedimentary iron phosphorus cycling under contrasting redox conditions in a eutrophic estuary abstract phosphorus p is often a limiting nutrient within freshwater and estuarine systems thus excess inputs of p from anthropogenic activities dominantly agriculture can induce eutrophication in receiving water bodies the sequestration of p within estuarine sediments is controlled by sorption and precipitation processes which are impacted by local redox conditions and burial environment despite the global spread of oxygen depletion in coastal marine systems p burial under reducing conditions remains poorly understood we investigated p cycling in relation to iron fe redox chemistry in sediments from the eutrophic peel harvey estuary in western australia using a combination of porewater analysis sequential chemical p extractions synchrotron based micro scale x ray fluorescence mapping and fe k edge x ray absorption spectroscopy and po43 sorption experiments the sediments represented redox regimes varying from strongly reducing organic rich sediments with little or no reactive fe iii oxyhydr oxides to oxygenated sediments that were enriched in reactive fe iii phases organic p and fe associated p were the main p burial phases and the latter was quantitatively important even in sediments with an overall strongly reducing character we attribute this to adsorption of p onto micro scale fe iii oxyhydroxide enrichments and or fe bearing clay minerals the organic rich sediments showed a strong decline in p contents with depth p was released from organic matter and fe phases but apatite precipitation was apparently inhibited in these sediments despite greater and stronger po43 sorption capacity the oxic sediments contained relatively little p due to a lack of the primary p source in marine sediments organic matter our results provide detailed insight into p burial in dynamic estuarine sediments and show that micro scale spectroscopic analyses greatly advance our understanding of p sequestration processes

tectonic implications of the 1989 afar earthquake sequence in august 1989 a sequence of m b 5 6 normal faulting earthquakes occurred in the area between the ghoubbet asal manda inakir and manda haroro abhe bad rifts in central afar relocation of the events suggest they occurred on major faults in the dobi graben located midway between the principal rift zones the earthquakes are evidence that extensional strain is still occurring in the area the bookshelf tectonic model for central and southern afar proposed by tapponnier et al 1990 is modified to account for ongoing extension in the area between the principal rifts as well as distributed shear

measuring the sediment thickness in urban areas using revised h v spectral ratio method abstract for the sake of safety it is crucial for civil engineers to determine the thickness of the sediment layers in the urban areas however the noisy detection environment in a city presents a significant challenge to the traditional artificial source geophysical methods which are used to estimate and monitor of sediment thickness given this otherwise challenging detection environment the h v spectral ratio method provides an effective and cheap means of detecting sediment thickness it exploits microtremor signals to obtain the resonant frequency of the surface layer which is then used to determine site sediment thickness with an empirical equation using the data from a single observation station in this study we collected microtremor data at 109 locations in the aoshan bay area of the shandong peninsula and determined the resonant frequency and analysed the morphological features of the h v spectral ratio curves in order to calibrate the frequency bedrock depth equation accurately to the local conditions vs profile estimation is used to fine tune the frequency bedrock depth equation by comparing the s wave velocity structure obtained using the microtremor spac method with that found from borehole samples we were able to verify the reliability and accuracy of application of the microtremor spac method by using the h v spectral ratio method and the modified equation that transforms resonance frequencies to sediment thickness we generated a thickness map of the sedimentary layers in the aoshan bay area of the shandong peninsula the map provides useful information for ensuring the safe construction of the city s infrastructure we conclude that the use of microtremor signals can provide important geological information as a cheaper and quick way for the construction and development of urban infrastructure

capturing magma intrusion and faulting processes during continental rupture seismicity of the dabbahu afar rift continental rupture models emphasize the role of faults in extensional strain accommodation extension by dyke intrusion is commonly overlooked a major rifting episode that began in 2005 september in the afar depression of ethiopia provides an opportunity to examine strain accommodation in a zone of incipient plate rupture earthquakes recorded on a temporary seismic array 2005 october to 2006 april direct observation of fault patterns and geodetic data document ongoing strain and continued dyke intrusion along the 60 km long dabbahu rift segment defined in earlier remote sensing studies epicentral locations lie along a 3 km wide 50 km long swath that curves into the se flank of dabbahu volcano a second strand continues to the north toward gab ho volcano considering the 8 m of opening in the september crisis we interpret the depth distribution of microseismicity as the dyke intrusion zone the dykes rise from 10 km to the near surface along the 60 km long length of the tectono magmatic segment focal mechanisms indicate slip along nnw striking normal faults perpendicular to the arabia nubia plate opening vector the seismicity insar continuous gps and structural patterns all suggest that magma injection from lower or subcrustal magma reservoirs continued at least 3 months after the main episode persistent earthquake swarms at two sites on dabbahu volcano coincide with areas of deformation identified in the insar data 1 an elliptical northwestward dipping zone of seismicity and subsidence interpreted as a magma conduit and 2 a more diffuse 8 km radius zone of shallow seismicity 2 km above a shadow zone interpreted as a magma chamber between 2 5 and 6 km subsurface insar and continuous gps data show uplift above a shallow source in zone 2 and uplift above the largely aseismic gab ho volcano the patterns of seismicity provide a 3 d perspective of magma feeding systems maintaining the along axis segmentation of this incipient seafloor spreading segment

seismotectonics in the republic of djibouti linking the afar depression and the gulf of aden abstract the arta seismological array around the gulf of tadjurah monitored earthquake activity for over 15 years epicentral locations and focal mechanisms are compared with tectonic evidence from surface breaks traces of active faults and the spatial distribution of swarms of aftershocks strike slip motion on near vertical faults predominates in the gulf left lateral strike slip on n150 e oriented faults is suggested at variance with the emphasis in previous models on either 1 a continuous plate boundary with spreading centres connected by ene trending right lateral transform faults 2 a propagating rift oriented approximately east west or 3 a few broad zones of extension between n150 e normal faults left lateral strike slip motion occurring on a series of parallel faults results in clockwise rotation of the blocks between the faults the type of deformation has similarities to that further west but is not obviously controlled by a distinct pair of overlapping rifts as proposed inland by tapponnier et al 1990 it is suggested that the stresses induced by the northeasterly drift of arabia alone may deform the southern part of the more easterly oriented zone of lithosphere derived of plume and ridge which separates arabia from africa

strain transfer between disconnected propagating rifts in afar we showed before that both the aden and red sea plate boundaries are currently rifting and propagating along two distinct paths into afar through the opening of a series of disconnected propagating rifts here we use new geochronological tectonic and paleomagnetic data that we acquired mostly in the southeastern part of afar to examine the geometry kinematics and time space evolution of faulting related to strain transfer processes it appears that transfer of strain is accommodated by a bookshelf faulting mechanism wherever rifts or plate boundaries happen to overlap without being connected this mechanism implies the rotation about a vertical axis of small rigid blocks along rift parallel faults that are shown to slip with a left lateral component which is as important as their normal component of slip rates of 2 3 mm yr by contrast where rifts do not overlap either a classic transform fault maskali or an oblique transfer zone mak arrasou kinematically connects them the length of the aden red sea overlap has increased in the last 0 9 myr as the aden plate boundary propagated northward into afar as a consequence the first order blocks that we identify within the overlap did not all rotate during the same time span nor by the same amounts similarly the major faults that bound them did not necessarily initiate and grow as their neighboring faults did despite these variations in strain distribution and kinematics the overlap kept accommodating a constant amount of strain 7 to 15 of the extension amount imposed by plate driving forces which remained distributed on a limited number seven or eight of major faults each one having slipped at constant rates 3 and 2 mm yr for vertical and lateral rates respectively the fault propagation rates and the block rotation rates that we either measure or deduce are so fast 30 130 mm yr and 15 38 myr respectively that they imply that strain transfer processes are transient as has been shown to be the case for the processes of tearing rift propagation and strain jumps in afar

mediterranean climate and oceanography and the periodic development of anoxic events sapropels mediterranean sapropels are layers with elevated organic carbon concentrations that contrast with surrounding sediments which are organic poor sapropels occur quasi periodically in sedimentary sequences of the last 13 5 million years and exist both in the eastern and western mediterranean sub basins they have been the subject of extensive study based on records from both short conventional and long ocean drilling program sediment cores and from a wide variety of uplifted marine sediment sequences on the basin margins and islands previous syntheses in the 1990s and 2000s have discussed how the formation of sapropels is commonly ascribed to deep sea anoxia enhanced export productivity or a combination of these effects however a wealth of new evidence and insights has emerged during the past 1 2 decades based on traditional and novel proxy data as well as modelling which has revealed intriguing new aspects and nuances to the reconstructed conditions hence it is timely to present a new synthesis of current understanding of the processes behind the formation of sapropels which have over the past decade also become a matter of commercial interest in sub salt hydrocarbon exploration in this review we present a context of modern mediterranean climate and oceanography followed by an integrated assessment of the growing understanding of climatological and ocean circulation changes that were associated with sapropel deposition we find that sapropels predominantly formed during astronomically timed episodes when climatic and oceanographic conditions and ecological responses broadly preconditioned the basin for sapropel deposition there is strong correspondence with times of monsoon intensification fuelling runoff from north africa into the mediterranean sea while preconditioning due to sea level rise and regional precipitation and runoff may have contributed as well within these broad episodes of surface buoyancy gain and resultant decline in deep water ventilation specific deposition under dysoxic anoxic or even euxinic conditions occurred within a clearly dynamic system that was characterised by complex spatial and depth dependent patterns gradients with distinct temporal variability on at least decadal to centennial millennial timescales in the final section we evaluate the implications of different modes of deep water removal from silled basins to investigate why sapropels are more frequently and often more intensely developed in the eastern mediterranean than in the western mediterranean

current plate boundary deformation of the afar rift from a 3 d velocity field inversion of insar and gps extension faulting and magmatism are the main controls on the magnitude and localization of strain at mid ocean ridges however the temporal and spatial patterns of such processes are not clear since the strain distribution has not been resolved in the past at sufficient spatial resolution and over extended areas interferometric synthetic aperture radar insar and gps data with unprecedented resolution are now available to us from the afar rift of ethiopia here we use a velocity field method to combine insar and gps to form the first high resolution continuous three dimensional velocity field of afar we study an area that is 500 km wide and 700 km long covering three branches of the afar continental rift and their triple junctions our velocity field shows that plate spreading is currently achieved in afar in contrasting modes a transient postdiking deformation is focused at the dabbahu rift segment while in central afar spreading is distributed over several overlapping segments and southern afar exhibits an interdiking deformation pattern focused at the asal ghoubbet segment we find that current spreading rates at dabbahu following the 2005 2010 intrusions are up to 110 mm yr 6 times larger than the long term plate divergence a segment centered uplift of up to 80 mm yr also occurs indicating that magma flow is still a primary mechanism of deformation during postdiking on the other hand no vertical displacements are currently observed in central and southern afar suggesting lack of significant magmatic activity at shallow levels

bookshelf faulting and horizontal block rotations between overlapping rifts in southern afar lateral slip on initially rift parallel normal faults may be a particularly efficient mechanism to accomodate strain between overlapping oceanic rifts it occurs in southern afar where clockwise block rotations result from distributed dextral shear between the overlapping ghoubbet asal manda inakir and manda hararo abhe bad rifts faulting observed during the 1969 serdo earthquakes and on spot images is consistent with the shear being taken up by left lateral slip on steep nw se striking faults which formed as normal faults before extensional strain became localized in the two rifts this bookshelf faulting accounts quantitatively for the 14 5 7 5 rotation documented by paleomagnetism in the 1 8 0 4 ma old afar stratoid basalts given the 17 5 5 mm yr rate of separation between arabia and somalia

authigenesis of vivianite as influenced by methane induced sulfidization in cold seep sediments off southwestern taiwan abstract authigenesis of iron rich phosphate nodules occurs in iron rich cold seep sediments md052911 core at yung an ridge offshore southwestern taiwan raman ftir and quantitative x ray energy dispersive spectroscopic analyses indicate that the phosphate mineral is vivianite or baricite and shows fe mg molar ratios spanning from ca 0 6 to 4 0 and a general down core trend of increasing fe mg ratios the formation of vivianite is limited to a depth interval of 13 17 mbsf meters below seafloor and is most prominent at 16 mbsf in association with high dissolved iron concentrations and depleted dissolved sulfide below a peak sulfidization zone enriched in mackinawite and greigite alternate growths of vivianite and iron monosulfides and compositional zoning with mg enriched towards the peripheries of individual nodules occur in the transition from the zone of vivianite mineralization to the sulfidization zone the crystallization of vivianite below the sulfidization front could have been favored by scavenging of downward diffusive dissolved sulfide from pore waters in the sulfidization zone alternate growths and overlapping of the zones of iron monosulfides and vivianite can be attributed to fluctuations of the sulfidization front and methane flux the discovery of vivianite in the yung an ridge sediments implies that authigenic vivianite can be an important sink for phosphorus burial in cold seep sediments that have high reactive iron contents and high sedimentation rates

evidence from the ad 2000 izu islands earthquake swarm that stressing rate governs seismicity magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits that cannot be associated with the diffusion of pore fluids or heat such swarm seismicity also migrates with time and often exhibits a dog bone shaped distribution the largest earthquakes in swarms produce aftershocks that obey an omori type exponential temporal decay but the duration of the aftershock sequences is drastically reduced relative to normal earthquake activity here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion a 1 000 fold increase in seismicity rate and a 1 000 fold decrease in aftershock duration occurred during the two month long dyke intrusion we find that the seismicity rate is proportional to the calculated stressing rate and that the duration of aftershock sequences is inversely proportional to the stressing rate this behaviour is in accord with a laboratory based rate state constitutive law suggesting an explanation for the occurrence of earthquake swarms any sustained increase in stressing rate whether due to an intrusion extrusion or creep event should produce such seismological behaviour

comparison of dike intrusions in an incipient seafloor spreading segment in afar ethiopia seismicity perspectives 1 oceanic crust is accreted through the emplacement of dikes at spreading ridges but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood between 2005 and 2009 the 70 km long dabbahu manda hararo rift segment in ethiopia has experienced 14 large volume dike intrusions 9 of which were recorded on temporary seismic arrays a detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems all of the migrating swarms of earthquakes started from a 80 of energy is released during the propagation phase with minimal seismic energy release after the dike propagation ceases we interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip coincident with the onset of low frequency earthquakes dike lengths show no systematic reduction in length with time suggesting that topographic loading and stress barriers influence dike length as well as changes in tectonic stress the propagation velocities of all the dikes follow a decaying exponential northward propagating dikes had faster average velocities than those that propagated southward suggesting preconditioning by the 2005 megadike or ongoing heating from a subcrustal magma source north of the midsegment

earthquake distribution patterns in africa their relationship to variations in lithospheric and geological structure and their rheological implications we use teleseismic waveform inversion along with depth phase analysis to constrain the centroid depths and source parameters of large african earthquakes the majority of seismic activity is concentrated along the east african rift system with additional active regions along stretches of the continental margins in north and east africa and in the congo basin we examine variations in the seismogenic thickness across africa based on a total of 227 well determined earthquake depths 112 of which are new to this study seismogenic thickness varies in correspondence with lithospheric thickness as determined from surface wave tomography with regions of thick lithosphere being associated with seismogenic thicknesses of up to 40 km in regions of thin lithosphere the seismogenic thickness is typically limited to 20 km larger seismogenic thicknesses also correlate with regions that have dominant tectonothermal ages of 1500 ma where the east african rift passes around the archean cratons of africa through the older proterozoic mobile belts these correlations are likely to be related to the production affected by method and age of basement formation and preservation affected by lithospheric thickness of a strong anhydrous lower crust the congo basin contains the only compressional earthquakes in the continental interior simple modelling of the forces induced by convective support of the african plate based on long wavelength free air gravity anomalies indicates that epeirogenic effects are sufficient to account for the localization and occurrence of both extensional and compressional deformation in africa seismicity along the margins of africa reflects a mixture between oceanic and continental seismogenic characteristics with earthquakes in places extending to 40 km depth

xanes spectroscopy as a tool to trace phosphorus transformation during soil genesis and mountain ecosystem development from lake sediments the aim of this study is to investigate phosphorus p species modifications triggered by soil genesis and mountain ecosystem development after glacial retreat using a lake sediment archive lake anterne north french alps five lake sediment samples representative of different stages of soil and ecosystem development were selected for p speciation analyses furthermore a sequence of current soils from the catchment was analyzed to better constrain our interpretations of the lacustrine archive synchrotron techniques x ray fluorescence xrf mapping and p k edge x ray absorption near edge structure xanes spectroscopy were applied to lake sediments soils and standards mineral and organic to distinguish between different p species the results show that soil development during the first millennia of the holocene triggered increased p species diversity at the onset of the holocene p was present as apatite when rocks and leptosols dominated the catchment pedogenic processes then led to apatite dissolution and the formation of large amounts of p on metal clay organic complexes p geochemistry during the main step of soil genesis early leptosols dominated by apatite low weathered cambisols with p mainly adsorbed on iron oxides highly weathered podzols with large amounts of p on al fe clay organic complexes is thus clearly recorded in lake sediments p k edge xanes spectroscopy is particularly relevant as qualitative method to study p species in soils and lake sediments at high spatial resolution such resolution is needed to reveal the diversity of small p particles and like this better characterize the p cycle and improve our understanding of ecosystem evolution

block rotation and continental extension in afar a comparison to oceanic microplate systems the reorganization of oceanic spreading centers separating major plates often appears to occur by a process in which discrete microplates form and evolve by rift propagation to see whether such microplate behavior has implications for continental rifting we investigate the application of a microplate model to the afar region at the nubia somalia arabia triple junction studies of marine magnetic anomalies volcanic ages bathymetry and seismicity suggest that the westward propagating gulf of aden spreading center has propagated into eastern afar within the past 2 m y causing rifting and extension within the continent we derive constraints on the extension history from the geometry and timing of rift formation and from paleomagnetic data indicating that pliocene to pleistocene age rocks have undergone a clockwise rotation of 11 we suggest that the history of rifting the rotation and several other features of the regional geology can be described by combining features of an oceanic microplate model and the concept of rift localization previously proposed for afar in this scenario motion occurring on several rifts within an extensional zone preceding the propagating spreading center is gradually transferred to a single rift while motion is transferred the overlap region between the growing and dying rifts acts as one or more microplates or blocks that rotate relative to the surrounding major plates the rifting history and rotations in eastern afar are thus related to the rift propagation and localization that occurs as the plate boundary evolves provided the constraints we use are appropriate our model better describes the regional kinematics than alternative block models including one based on bookshelf faulting if the tectonics of afar are typical for continental breakup they have interesting implications for the geometry of passive margins in particular asymmetric rifted margins can be produced if the final location of the rift axis is not at the center of the zone of initially disrupted lithosphere additionally if the rate of rift propagation and the rate and location of rift localization are not uniform then along axis structural variations will result

a new particulate mn fe p shuttle at the redoxcline of anoxic basins abstract pelagic redoxclines of anoxic basins and deeps form the suboxic transition between oxygenated surface and anoxic or even sulfidic bottom waters intense element cycling favoured by elevated microbial activity causes steep gradients of physico chemical parameters nutrients and redox sensitive trace metals this study presents a conceptual model for authigenic particle formation at pelagic redoxclines which is based on the tight coupling of mn fe and p cycles besides the well known occurrence of mn oxides textural sem edx and geochemical icp oes icp ms analyses of particles from the redoxclines of the black sea and the baltic sea gotland basin landsort deep evidence the existence of earlier postulated fe oxyhydroxo phosphates and emphasize mixed phases consisting of mn oxides and fe oxyhydroxo phosphates as a new solid species most of the analyzed particles are star shaped of about 5 m in size and occur as single particles or aggregates without any morphological differences between mn oxides fe oxyhydroxo phosphates and mixed phases throughout the redoxcline these minerals show a general succession with maximum abundance of mn oxides above the redoxcline followed by mixed phases and almost pure fe phosphates within and below the redoxcline respectively molar fe p ratios of single particles argue against the formation of known pure fe phosphates like vivianite or strengite at the lower end of the redox transition zone but are consistent with recent experimental findings for colloidal p bearing hydrous ferric oxides moreover morphological similarities suggest the formation of irregular fe oxyhydroxo coatings due to oxidation of upward diffusing fe 2 by oxygen and stepwise replacement of mn iv by fe iii on sinking mno x particles followed by immediate adsorption or even co precipitation of phosphate batch type experiments using biogenic mno x particles demonstrate the efficient potential of fe 2 oxidation by sinking mno x particles when entering sulfidic waters mno x particles are progressively reduced leading to an increasing relative abundance of fe and p rich particles in deeper parts of the water column these particles are also reductively dissolved thereby releasing fe 2 and phosphate to the water column this mn fe p shuttle likely affects phosphate transport throughout the water column and thus impacts primary production at least over longer time scales furthermore the particulate mn fe p shuttle must have played an important role for the cycling of p and certain trace metals in ancient ocean basins e g during certain periods of cretaceous black shale formation and should be considered in future mass balances and modeling approaches dealing with oxic anoxic interfaces of aquatic ecosystems

redox sensitivity of p cycling during marine black shale formation dynamics of sulfidic and anoxic non sulfidic bottom waters abstract a high resolution geochemical record of a 120 cm black shale interval deposited during the coniacian santonian oceanic anoxic event 3 odp leg 207 site 1261 demerara rise has been constructed to provide detailed insight into rapid changes in deep ocean and sediment paleo redox conditions high contents of organic matter sulfur and redox sensitive trace metals cd mo v zn as well as continuous lamination point to deposition under consistently oxygen free and largely sulfidic bottom water conditions however rapid and cyclic changes in deep ocean redox are documented by short term 15 20 ka intervals with decreased total organic carbon toc s and redox sensitive trace metal contents and in particular pronounced phosphorus peaks up to 2 5 wt p associated with elevated fe oxide contents sequential iron and phosphate extractions confirm that p is dominantly bound to iron oxides and incorporated into authigenic apatite preservation of this fe p coupling in an otherwise sulfidic depositional environment as indicated by fe speciation and high amounts of sulfurized organic matter may be unexpected and provides evidence for temporarily non sulfidic bottom waters however there is no evidence for deposition under oxic conditions instead sulfidic conditions were punctuated by periods of anoxic non sulfidic bottom waters during these periods phosphate was effectively scavenged during precipitation of iron oxyhydr oxides in the upper water column and was subsequently deposited and largely preserved at the sea floor after 15 25 ka sulfidic bottom water conditions were re established leading to the initial precipitation of cds zns and pyrite subsequently increasing concentrations of h2s in the water column led to extensive formation of sulfurized organic matter which effectively scavenged particle reactive mo complexes thiomolybdates at site 1261 sulfidic bottom waters lasted for 90 100 ka followed by another period of anoxic non sulfidic conditions lasting for 15 20 ka the observed cyclicity at the lower end of the redox scale may have been triggered by repeated incursions of more oxygenated surface to mid waters from the south atlantic resulting in a lowering of the oxic anoxic chemocline in the water column alternatively sea water sulfate might have been stripped by long lasting high rates of sulfate reduction removing the ultimate source for hs production

phosphorus burial in sediments of the sulfidic deep black sea key roles for adsorption by calcium carbonate and apatite authigenesis abstract sedimentary burial of the essential nutrient phosphorus p under anoxic and sulfidic conditions is incompletely understood here we use chemical and micro scale spectroscopic methods to characterize sedimentary p burial along a water column redox transect six stations 78 2107 m water depth in the black sea from the shelf with its oxygenated waters to the anoxic and sulfidic deep basin organic p is an important p pool under all redox regimes accounting for up to 60 of p burial we find a general down core increase in the relative importance of organic p especially on the shelf where p bound to iron fe and manganese mn oxyhydr oxides is abundant in the uppermost sediment but rapidly declines in concentration with sediment depth our chemical and spectroscopic data indicate that the carbonate rich sediments unit i 3000 years 0 30 cm depth of the sulfidic deep black sea contain three major p pools calcium phosphate apatite organic p and p that is strongly associated with caco3 and possibly clay surfaces apatite concentrations increase from 5 to 25 of total p in the uppermost centimeters of the deep basin sediments highlighting the importance of apatite formation for long term p burial iron ii associated p ludlamite was detected with x ray absorption spectroscopy but was shown to be a minor p pool 5 indicating that lateral fe p transport from the shelf shuttling likely occurs but does not impact the p burial budget of the deep black sea the caco3 p pool was relatively constant throughout the unit i sediment interval and accounted for up to 55 of total p our results highlight that carbonate bound p can be an important sink for p in caco3 rich sediments of anoxic sulfidic basins and should also be considered as a potential p sink and p source in case of caco3 dissolution when reconstructing past ocean p dynamics from geological records

strike slip tectonics during rift linkage the kinematics of rift segment linkage in magmatic rifts remain debated strain patterns from afar provide tests of current models of how segmented rifts grow in areas of incipient oceanic spreading here we present a combined analysis of seismicity interferometric synthetic aperture radar insar and gps derived strain rate maps to reveal that the plate boundary linkage between the red sea and gulf of aden rifts of afar is accommodated primarily by distributed extensional faulting large rotations about vertical axes predicted by bookshelf faulting models are not detected additionally models of stress changes and seismicity induced by recent dikes provide poor fits to the observed time space patterns of strike slip earthquakes instead we explain these features as resulting from rift perpendicular shearing at the tips of spreading rifts where extension terminates against less stretched lithosphere our results demonstrate that distributed extension drives rift perpendicular shearing achieving plate boundary linkage during incipient seafloor spreading

dynamic source parameters of the march may 1969 serdo earthquake sequence in central afar ethiopia deduced from teleseismic body waves long period teleseismic p and s waveforms for the largest four earthquakes in the serdo sequence are compared with synthetic seismograms to infer mechanisms focal depths source time functions and dynamic energy release for all four events studied the mechanisms are predominantly strike slip depth estimates obtained for the four earthquakes by matching synthetic seismograms to the observations are found to lie between 5 and 8 km the azimuthal variation of observed body wave duration for the main event indicates that the rupture propagated from ne to sw average seismic moments obtained from body wave synthetics vary from 4 6 1017 to 2 5 1018 n m average final displacements stress drops and lengths of the fault are found to lie between 0 16 and 0 56 m 0 45 and 1 56 mpa and 10 and 16 km respectively the low stress drop value obtained may indicate the presence of softer materials near the source the energy release shows that the mode of energy release during the sequence took place in two steps mainly by the two largest shocks

a 225 kyr record of dust supply paleoproductivity and the oxygen minimum zone from the murray ridge northern arabian sea abstract analysis of a 15 m long sediment core recovered from the murray ridge northern arabian sea reveals a 225 000 year record of climate induced variations in surface water productivity mid water ventilation and dust input in the northern arabian sea productivity and dust input records closely resemble those from the owen ridge and oman margin also the phase lag of 10 kyr between maximum productivity and precession minimum is in phase with the summer monsoon upwelling record from the owen ridge and the oman margin this implies that the productivity variations in the northern arabian sea during the last 225 kyr are primarily controlled by changes in the indian ocean summer monsoon the winter monsoon obviously played a minor role in the productivity history of the northern arabian sea another result from this study is that the mg al record from the murray ridge nicely documents the post glacial flooding of the persian gulf which is the main glacial source for dolomite dust in this region finally it is concluded that the intensity of the omz co varies with surface water productivity being weak during periods of reduced summer monsoon especially during glacial periods

sulfur biogeochemical cycling and novel fe s mineralization pathways in a tidally re flooded wetland abstract sulfur biogeochemical cycling and associated fe s mineralization processes exert a major influence over acidity dynamics electron flow and contaminant mobility in wetlands benthic sediments and groundwater systems while s biogeochemical cycling has been studied intensively in many environmental settings relatively little direct information exists on s cycling in formerly drained wetlands that have been remediated via tidal re flooding this study focuses on a tidal wetland that was drained in the 1970s causing severe soil and water acidification and subsequently remediated by controlled re flooding in 2002 we examine so 4 2 reduction rates and fe s mineralization at the tidal fringe 7 years after the commencement of re flooding the initial drainage of the wetland examined here caused in situ pyrite fes2 oxidation resulting in the drained soil layers being highly acidic and rich in so 4 2 bearing fe iii minerals including jarosite kfe3 so4 2 oh 6 tidal re flooding has neutralized much of the previous acidity with the pore water ph now mostly spanning ph 5 7 the fastest rates of in situ so 4 2 reduction up to 300 nmol cm 3 day 1 occur within the inter tidal zone in the near surface soil layers to 60 cm below ground surface the so 4 2 reduction rates correlate with pore water dissolved organic c concentrations thereby suggesting that electron donor supply was the predominant rate determining factor elemental s was a major short term product of so 4 2 reduction comprising up to 69 of reduced inorganic s in the near surface soil layers this enrichment in elemental s can be partly attributed to interactions between biogenic h2s and jarosite a process that also contributed to enrichment in pore water fe2 up to 55 mm and so 4 2 up to 50 mm the iron sulfide thiospinel greigite fe3s4 was abundant in near surface soil layers within the inter to sub tidal zone where tidal water level fluctuations created oscillatory redox conditions there was evidence for relatively rapid pyrite re formation within the re flooded soil layers however the results indicate that pyrite re formation has occurred mainly in the lower formerly drained soil layers whereas the accumulation of elemental s and greigite has been confined towards the soil surface the discovery that pyrite formation was spatially decoupled from that of elemental s and greigite challenges the concept that greigite is an essential precursor required for sedimentary pyrite formation in fact the results suggest that greigite and pyrite may represent distinct end points of divergent fe s mineralization pathways overall this study highlights novel aspects of fe s mineralization within tidal wetlands that have been drained and re flooded in contrast to normal undisturbed tidal wetlands as such the long term biogeochemical trajectory of drained and acidified wetlands that are remediated by tidal re flooding cannot be predicted from the well studied behaviour of normal tidal wetlands

constraints on magma chamber geometry at sierra negra volcano galapagos islands based on insar observations abstract we investigate the problem of estimating magma chamber geometry using insar observations of sierra negra volcano galapagos ascending and descending interferograms are combined to determine vertical and one horizontal component of displacement the ratio of maximum horizontal to vertical displacement suggests a sill like source spherical or stock like bodies are inconsistent with the data we estimate the geometry of the sill assuming a horizontal uniformly pressurized crack with unknown periphery and depth the sill is discretized into small elements that are either open and are subjected to the pressure boundary condition or remain closed we find the best fitting sill to be located beneath sierra negra s inner caldera at a depth of about 2 km using boundary element calculations we show that any magma chamber with a flat top coincident with the sill model fits the data equally well the data are insensitive to the sides and bottom of the magma chamber

pyrite oxidation during sample storage determines phosphorus fractionation in carbonate poor anoxic sediments abstract we investigated the phosphorus p and iron fe fractionation in four cores with anoxic sediments deposited during the mid cretaceous oceanic anoxic event 2 94 ma and the paleocene eocene thermal maximum 55 ma that were exposed to oxygen after core recovery surprisingly p associated with iron oxyhydroxides fe bound p was a major p phase in these laminated sediments deposited under euxinic conditions a significant fraction of total fe was present as poorly crystalline ferric fe this fraction increased with increasing storage time of the investigated cores in carbonate poor samples fe bound p accounted for up to 99 of total p and its abundance correlated with pyrite contents in samples with higher caco3 contents 5 wt in the investigated samples p was mostly present in authigenic ca p minerals irrespective of pyrite contents we conclude that the p fractionation in anoxic carbonate poor sediments is strongly affected by pyrite oxidation that occurs when these sediments are exposed to oxygen pyrite oxidation produces sulfuric acid and iron oxyhydroxides the abundance of poorly crystalline fe oxyhydroxides provides further evidence that these were indeed formed through recent post recovery oxidation rather than in situ tens of millions of years ago the acid dissolves apatite and the released phosphate is subsequently bound in the freshly formed iron oxyhydroxides pyrite oxidation thus leads to a conversion of authigenic ca p to fe bound p in more calcareous samples caco3 can act as an effective buffer against acidic dissolution of ca p minerals the results indicate that shielding of sediments from atmospheric oxygen is vital to preserve the in situ p fractionation and to enable a valid reconstruction of marine phosphorus cycling based on sediment records

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

vivianite is a major sink for phosphorus in methanogenic coastal surface sediments abstract studies of authigenic phosphorus p minerals in marine sediments typically focus on authigenic carbonate fluorapatite which is considered to be the major sink for p in marine sediments and can easily be semi quantitatively extracted with the sedex sequential extraction method the role of other potentially important authigenic p phases such as the reduced iron fe phosphate mineral vivianite fe ii 3 po4 8h2o has so far largely been ignored in marine systems this is in part likely due to the fact that the sedex method does not distinguish between vivianite and p associated with fe oxides here we show that vivianite can be quantified in marine sediments by combining the sedex method with microscopic and spectroscopic techniques such as micro x ray fluorescence xrf elemental mapping of resin embedded sediments as well as scanning electron microscope energy dispersive spectroscopy sem eds and powder x ray diffraction xrd we further demonstrate that resin embedding of vertically intact sediment sub cores enables the use of synchrotron based microanalysis x ray absorption near edge structure xanes spectroscopy to differentiate between different p burial phases in aquatic sediments our results reveal that vivianite represents a major burial sink for p below a shallow sulfate methane transition zone in bothnian sea sediments accounting for 40 50 of total p burial we further show that anaerobic oxidation of methane aom drives a sink switching from fe oxide bound p to vivianite by driving the release of both phosphate aom with sulfate and fe oxides and ferrous fe aom with fe oxides to the pore water allowing supersaturation with respect to vivianite to be reached the vivianite in the sediment contains significant amounts of manganese 4 8 wt similar to vivianite obtained from freshwater sediments our results indicate that methane dynamics play a key role in providing conditions that allow for vivianite authigenesis in coastal surface sediments we suggest that vivianite may act as an important burial sink for p in brackish coastal environments worldwide

satellite geodetic imaging reveals internal deformation of western tibet 1 it is often assumed that the majority of the interseismic strain accumulates around the major mapped geological block boundaries however numerous recent earthquakes in the continents have occurred on faults that were previously unidentified existing deformation data from tibet are insufficiently dense to map the distribution of interseismic strain here we combine 265 interferograms formed from 166 radar images with gps data to constrain a high resolution velocity field covering 200 000 km2 of western tibet we confirm that the slip rate of the karakoram fault is low 6 mm yr but show that areas of focused strain do not coincide with the major fault structures some of this strain is due to postseismic relaxation after a mw 6 8 earthquake that occurred in 1996 on a structure that is difficult to identify in satellite imagery models of seismic hazard that rely on imperfect knowledge of the boundaries of crustal blocks can therefore underestimate hazard from unknown faults