what is diatomite different types of biogenic remains ranging from siliceous algae to carbonate precipitates accumulate in the sediments of lakes and other aquatic ecosystems unicellular algae called diatoms which form a siliceous test or frustule are an ecologically and biogeochemically important group of organisms in aquatic environments and are often preserved in lake or marine sediments when diatoms accumulate in large numbers in sediments the fossilized remains can form diatomite in sedimentological literature diatomite is defined as a friable light coloured sedimentary rock with a diatom content of at least 50 however in the quaternary science literature diatomite is commonly used as a description of a sediment type that contains a large quantity of diatom frustules without a precise description of diatom abundance here we pose the question what is diatomite what quantity of diatoms define a sediment as diatomite is it an uncompacted sediment or a compacted sediment we provide a short overview of prior practices and suggest that sediment with more than 50 of sediment weight comprised of diatom sio2 and having high 70 porosity is diatomaceous ooze if unconsolidated and diatomite if consolidated greater burial depth and higher temperatures result in porosity loss and recrystallization into porcelanite chert and pure quartz less

geometry and physical properties of the socorro new mexico magma bodies analysis of multichannel seismic reflection profiles gathered by the consortium for continental reflection profiling cocorp constrains the geometry and material properties of a proposed magma body located within the rio grande rift a new spectral ratio method is used to find the reflectivities of the proposed midcrustal intrusion large and spatially variable reflectivities support the interpretation of the reflector as an intrusive molten magma complex modeling of the reflectivity suggests that in one location the intrusion took the form of several thin 30 to 40 m thick layers seismic wave attenuation within the rift s midcrust is low

on recent seismic tomography for the western united states 1 in recent years a number of relatively high resolution seismic tomography models for the uppermost mantle underneath the western united states have been published and vigorous debate has ensued about their tectonic interpretation i present a straightforward yet quantitative comparison between models in order to help establish a framework for geodynamic interpretation and to help judge the role of tomographic theory vs data selection mapped s and p wave anomalies are found to be remarkably consistent between models which implies that seismologists are beginning to narrow down the structure underneath active continental margins to scales of 200 km large discrepancies between published anomaly amplitudes exist however and the models on the high end of the spectrum raise questions as to how they are to be interpreted in terms of temperature composition and melting

application of fourier transform infrared spectroscopy to silica diagenesis the opal a to opal ct transformation abstract an important goal in silica diagenesis research is to understand the kinetics of opal transformation from noncrystalline opal a to the disordered silica polymorph opal ct because the conventional technique for monitoring the transformation powder x ray diffraction xrd is applicable only to phases with long range order we used fourier transform infrared spectroscopy ftir to monitor the transformation we applied this technique combined with xrd and tem to experimental run products and natural opals from the monterey formation and from siliceous deposits in the western pacific ocean using a ratio of two infrared absorption intensities i472 cm 1 i500 cm 1 the relative prop rtions of opal a and opal ct can be determined the progress of the transformation is marked by changes in slope of vs depth or time when a sufficient stratigraphic profile is available there are three stages in the opal a to opal ct reaction 1 opal a dissolution 2 opal ct precipitation whose end point is marked by completion of opal a dissolution and 3 opal ct ordering during which tridymite stacking is eliminated in favor of cristobalite stacking

the occurrence and significance of biogenic opal in the regolith abstract biogenic opal produced by vascular plants diatoms and siliceous sponges have been found in soils and terrestrial sediments of all continents except antarctica since the middle of the 19th century the opal particles range in size from fine silt to fine sand almost all soils contain detectable opal up to levels of 2 3 and a significant number contain values in excess of 5 even higher values have been found from soils and sediments of all continents in a wide range of soil types the most important factor is poor soil drainage and seasonal to permanent water logging this encourages the proliferation of silica producing organisms such conditions have been found in the soils and aquatic sediments of the monsoonal tropics tropical rain forests temperate forests tropical savanna tropical islands semi arid grasslands and savanna and temperate woodland and grassland the presence of a volcanic substrate also appears favourable in some cases but is not necessary biogenic opal preferentially collects in the a horizon of soils and to a lesser extent in the b horizon this preferential distribution facilitates identification of palaeosols in stacked soil sequences biogenic opal is also a component of windblown dust even in arid environments biogenic opal is significant to regolith processes in a number of ways firstly as in the case in marine environments it is likely to be important in silica cycling and storage because of its greater lability compared to quartz secondly dissolution and reprecipitation of opal a as opal ct or micro quartz may play a role in cementation and silicification of regolith to form silica hardpans and silcrete thirdly the organisms that form biogenic opal can have considerable palaeoenvironmental significance and be valuable in reconstructing regolith evolution finally some forms of biogenic silica in particular sponge spicules can present a health hazard their high abundance in some soils and sediments needs to be considered when assessing the health implications of airborne dust

localized imaging of the uppermost mantle with usarray pn data 1 usarray has now provided several years of high quality seismic data and improved ray coverage for much of the western united states this allows increased resolution for regional studies of the lithosphere and deeper structure of the north american continent in this study we use pn phases in the usarray data set to solve for velocity structure in the uppermost mantle in the western united states this article focuses on localized imaging techniques that complement traditional pn tomography analysis we apply waveform cross correlation to obtain inter station travel times between the closely and uniformly spaced usarray stations this allows us to use traces without phase picks and reduces errors associated with the picking we obtain differential times that can directly be used to fit locally for slowness and depending on the approach for the direction and curvature of the incoming wavefront the various measurements of incoming wavefronts at different sub arrays provide constraints on azimuthal variations in velocity the traditional tomography approach and the local fitting method reveal similar large scale features no regularization is applied with the local method and the resulting velocity maps reveal smaller scale structures than the tomographic images

anoxic events during sedimentation of a palaeogene diatomite in denmark a diatomite about 60 m thick of late palaeocene early eocene age crops out in northern jutland denmark the diatomite is locally termed moler frustules of marine diatoms constitute c 65 by weight of the diatomite and clay minerals chiefly montmorillonite make up the remainder slight variations in the relative supply of diatom frustules and clay minerals are preserved undisturbed in laminated diatomite while lamination is partly destroyed by burrowing organisms in weakly laminated diatomite and obliterated by total bioturbation in structureless diatomite these three facies alternate throughout the sequence the presence or absence of infaunal burrowing organisms is interpreted as a record of the content of dissolved oxygen in the water above the sediment water interface and hence of the position of the redox potential discontinuity interspaced in the diatomite are 179 identifiable layers of volcanic ash these ash layers provide a means of precise lateral correlation they show that levels of laminated diatomite may be followed throughout the basin and therefore that changes between anoxic and oxic conditions occurred simultaneously across the area the laminated diatomites may consequently be interpreted as representing short term anoxic events of which twelve have been recognized

seismic imaging of the crust and upper mantle using regularized joint receiver functions frequency wave number filtering and multimode kirchhoff migration 1 receiver functions provide an indispensable tool for producing discontinuity images of the crust and upper mantle from teleseismic earthquake arrivals however image quality can be significantly compromised by instabilities inherent in the deconvolution process and by irregularities in data coverage driven by source and or receiver geometries we present receiver function estimation and prestack migration techniques which both reduce receiver function deconvolution instability and produce regularized multimode receiver function images the receiver function estimation technique exploits frequency wave number filtering methods using frequency pseudowave number domain filtering of gathers to enhance arrivals with consistent moveout characteristics synthetic tests confirm robust recovery of features in the presence of high noise levels to process regularized receiver functions into accurate images of the subsurface seismic scattering potential we employ a regularized kirchhoff migration that migrates both direct and reverberated p to s converted modes to their correct subsurface locations the subsequent images can be stacked to mute mismigrated modes synthetic studies and model resolution tests demonstrate that this methodology is well suited to irregularly spaced stations and uneven data coverage and does not require the recording of a single earthquake at multiple stations

high resolution receiver function imaging reveals colorado plateau lithospheric architecture and mantle supported topography 1 after maintaining elevations near sea level for over 500 million years the colorado plateau cp has a present average elevation of 2 km we compute new receiver function images from the first dense seismic transect to cross the plateau that reveal a central cp crustal thickness of 42 50 km thinning to 30 35 km at the cp margins isostatic calculations show that only approximately 20 of central cp elevations can be explained by thickened crust alone with the cp edges requiring nearly total mantle compensation we calculate an uplift budget showing that cp buoyancy arises from a combination of crustal thickening heating and alteration of the lithospheric root dynamic support from mantle upwelling and significant buoyant edge effects produced by small scale convecting asthenosphere at its margins

evolutionary aspects of lithosphere discontinuity structure in the western u s 1 we have produced common conversion point ccp stacked ps and sp receiver function image volumes of the moho and lithosphere asthenosphere boundary lab beneath the western united states using transportable array data the large image volumes and the diversity of tectonic environments they encompass allow us to investigate evolution of these structural discontinuities the moho is a nearly continuous topographic surface whereas the lab is not and the seismic images show a more complex expression the first order change in lab depth in the western u s occurs along the cordilleran hingeline the former laurasian passive margin along the southwestern precambrian north american terranes the lab is about 50 deeper to the east of the hingeline than to the west with most of the increase in lab thickness being in the mantle lithosphere we infer that the moho and the lab are late mesozoic or cenozoic everywhere west of the hingeline modified during farallon subduction and its aftermath between the hingeline and the rocky mountain front the lab and to a lesser extent the moho have been partially reset during the cenozoic by processes that continue today seismicity and recent volcanism in the interior of the western u s are concentrated along gradients in crustal and or lithospheric thickness for example the hingeline and the eastern edge of the coastal volcanic magmatic terranes to us this suggests that lateral gradients in integrated lithospheric strength focus deformation similarly areas conjectured to be the sites of convective downwellings and associated volcanism are located along gradients in regional lithosphere thickness

a precisely dated lake level rise marked by diatomite formation in northeastern ireland the intercorrelation of palaeoclimate events from various studies is often hindered by a lack of precise chronological control tephra isochrons can overcome this problem by providing direct site linkages this paper outlines a study of holocene peat and diatomite deposits that accumulated within the floodplain of lough neagh northern ireland the icelandic hekla 4 tephra has been identified at the base of diatomite deposits at a number of sites and provides firm dating evidence for a widespread flooding event in the area at ca 2300 bc the evidence is consistent with other studies in ireland and elsewhere for increased wetness at this time the results demonstrate that the terrestrial deposits around lough neagh contain an important record of holocene lake level change dendrochronological evidence from the lough neagh area provides additional information about lake level fluctuations over the past two millennia copyright 2004 john wiley sons ltd

formation of cratonic mantle keels by arc accretion evidence from s receiver functions 1 delineating mantle interfaces can provide important clues for understanding the formation of continents we use s wave receiver functions to investigate lithospheric structure along a transect extending from vancouver island to baffin island observed sp converted waves allow for interpretation of boundaries in the depth range expected for tectonic plates receiver functions show a distinct negative amplitude feature interpreted as the lithosphere astbenosphere boundary at shallow depths beneath british columbia 85km deepening abruptly at the eastern edge of the cordillera to 260km beneath the canadian shield dipping mid lithospheric discontinuities extend beneath several giant ca 1 8 ga epicontinental magmatic arcs consistent with formation of cratonic lithosphere by arc accretion this model provides a plausible explanation for global mid lithospheric discontinuities within cratons and aids in understanding their formation

io volcanism seen by new horizons a major eruption of the tvashtar volcano jupiter s moon io is known to host active volcanoes in february and march 2007 the new horizons spacecraft obtained a global snapshot of io s volcanism a 350 kilometer high volcanic plume was seen to emanate from the tvashtar volcano 62 n 122 w and its motion was observed the plume s morphology and dynamics support nonballistic models of large io plumes and also suggest that most visible plume particles condensed within the plume rather than being ejected from the source in images taken in jupiter eclipse nonthermal visible wavelength emission was seen from individual volcanoes near io s sub jupiter and anti jupiter points near infrared emission from the brightest volcanoes indicates minimum magma temperatures in the 1150 to 1335 kelvin range consistent with basaltic composition

crustal structure and evolution beneath the colorado plateau and the southern basin and range province results from receiver function and gravity studies over the past several decades contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long term stability of the colorado plateau cp and crustal thinning beneath the basin and range province brp in the southwestern united states here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15 500 p to s receiver functions recorded at 72 usarray and other broadband seismic stations on the southwestern cp and the southern brp our results reveal that the brp is characterized by a thin crust 28 2 0 5 km a mean vp vs of 1 761 0 014 and a mean amplitude r of p to s converted wave relative to that of the direct p wave of 0 181 0 014 that are similar to a typical continental crust consistent with the model that the thin crust was the consequence of lithospheric stretching during the cenozoic the cp is characterized by the thickest crust 42 3 0 8 km largest vp vs 1 825 0 009 and smallest r 0 105 0 007 values in the study area in addition many stations on the cp exhibit a clear arrival before the p to s converted phase from the moho corresponding to a lower crustal layer of about 12 km thick with a mafic composition we hypothesize that the lower crustal layer which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines contributed to the long term stability and preuplift low elevation of the colorado plateau

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

carbon cycling within an east african lake revealed by the carbon isotope composition of diatom silica a 25 ka record from lake challa mt kilimanjaro the carbon cycle of a lake is a balance between supply from the atmosphere and catchment and the net demand exerted by primary producers minus losses back to the atmosphere and to sediment storage evaluating the sum of these processes and reconstructing them from sediment records of lake history requires a range of methods and a multi proxy approach one promising technique is to explore the carbon isotope composition 13cdiatom of organic matter incorporated within the silica frustules of diatom algae here we present a 25 000 year record of 13cdiatom from the sediments of crater lake challa on the eastern flank of mt kilimanjaro and along with other proxy data we make inferences about the three major phases in the history of the lake s carbon cycle from 25 ka to 15 8 ka years bp 13cdiatom is positively correlated with the 13c of bulk sediment organic matter 13cbulk indicating that high diatom productivity as recorded by high biogenic silica at this time was preferentially removing 12c and enriching the 13c of lake water dissolved inorganic carbon from 15 8 to 5 5 ka the correlation between 13cdiatom and 13cbulk breaks down suggesting carbon supply to the lake satisfied or exceeded the demand from productivity from 5 5 ka bp the positive correlation resumes indicating an increase in the internal demand for carbon relative to external supply diatom frustule bound carbon isotopes offer an original tool in examining long term fluctuations in a lake s carbon budget and how the balance between supply and demand has changed through time

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

tomography of crustal p and s travel times across the western united states 1 data from the usarray combined with local regional and national networks provide an unprecedented opportunity for imaging the crust of the western united states we employ a simple tomography scheme to invert pg and sg travel times for velocity structure the approach is analogous to pn tomography we assume a great circle arc between source and receiver and solve for station and event terms along with 2 d velocity structure pg velocities are high in the snake river plain western washington and the columbia plateau and in central california the colorado plateau falls midrange while the basin and range is slow as are the geysers region north of the san francisco bay and the salton trough for sg the snake river plain is fast as is the entire cordillera from the southern sierra nevada up through the cascades both the colorado plateau and most of the rocky mountains are also fast at the highest point of the rocky mountains the aspen anomaly is a low velocity region the basin and range is a composite of slower to the west abutting the sierra nevada and faster in eastern nevada coastal california is slow with the exception of the san francisco bay region and far northern california the continental crust of the north american craton appears slightly fast for both pg and sg our observations and their trends compare well with middle to upper crustal velocity estimates from previous studies they also compare reasonably well with average crustal velocities from the same studies

lithification of biogenic siliceous sediments evidence from neogene diatomaceous sequences of northeast japan abstract biogenic siliceous deposits are a significant component of neogene stratigraphic sequences of the northeast japan arc field observations demonstrate that the siliceous rocks are classified into five lithologic types diatomite opaline porcelanite opaline chert quartzose porcelanite and black shale stratigraphic data suggest that the siliceous formations were deposited continuously from the middle miocene to early pliocene diatomite occurs throughout the sedimentary sequences and changes laterally into chert and porcelanite multivariate statistical analysis of geochemical data indicates that lithification and mineralogical transformation of silica rich deposits was governed not only by compaction and cementation during burial but also by sorting of grains by biological sediment seawater mixing diatomite is composed of frustules without fine ornamentations and delicate structures fine and delicate tests were more susceptible to dissolution than coarse and solid ones and dissolved in the course of biological sediment disturbance ubiquitous occuerences of diatomite in the stratigraphic sequences are attributed to residual frustules which were more resistant to the diagenetic dissolution precipitation reaction of silica biogenic silica is the main source of silica to form chert diagenetic precipitation of silica in diatomaceous beds through progressive sediment burial formed porcelanitic rocks with opaline porcelanite being the precursor of quartzose porcelanite chert was not subjected to biological sediment reworking and fragile shells were preserved during accumulation silicification of precursor deposits of chert was promoted by intraformational dissolution precipitation reaction of silica with increasing sediment compaction and dehydration

classification of deep sea fine grained sediments abstract most deep sea sediments contain one or more biogenic components and one dominant nonbiogenic component usually clay or silty clay we present a descriptive classification scheme in which deep sea fine grained sediments are placed within a three component system of calcareous biogenic siliceous biogenic and nonbiogenic components in a three step procedure the user assesses whether the dominant component is biogenic or nonbiogenic whether the dominant biogenic component is siliceous or calcareous and what the relative abundances of the biogenic components are within limits of 10 25 and 50 the terminology proposed is that commonly used by many sedimentologists with some refinements and greater precision in the use of terms

mantle convection and the recent evolution of the colorado plateau and the rio grande rift valley the colorado plateau contains late cretaceous marine strata that are at a mean elevation of 2 km the timing and amount of uplift since the cretaceous has generated considerable debate with the exception of a few studies topography supported by vertical stresses generated by viscous fl ow in the mantle has not been explicitly considered to contribute to the elevation of this region herein we compute the viscous fl ow beneath north america that is driven by density anomalies inferred from joint seismic geodynamic modeling we fithat the colorado plateau overlies a strong mantle upwelling that is coupled to the sinking farallon slab currently beneath the eastern united states consequently the colorado plateau is currently a focused dynamic topography high within the western u s cordillera moreover this strong upwelling impacts the base of the lithosphere at an oblique angle east of the plateau directly below the rio grande rift we attribute this fl ow as being responsible for some of the recent magmatic activity along the jemez lineament as well as contributing to the recent rifting process in the rio grande rift valley

upper mantle structure beneath the eastern colorado plateau and rio grande rift revealed by bouguer gravity seismic velocities and xenolith data 1 the lithospheric architecture of the colorado plateau rio grande rift transition zone reflects a narrow corridor of active tectonism and magmatism between the tectonically stable colorado plateau and the stable interior of north america in this paper we analyze bouguer gravity data crustal thickness variations xenolith data and seismic wave speed structure to argue that the rio grande rift and southeastern colorado plateau is underlain by a low density upper mantle province that does not trend along upper crustal tectonic boundaries but rather is correlated with regions of middle to late tertiary magmatism we model uppermost mantle density contrasts using bouguer gravity along five parallel profiles across the colorado plateau and validate our models by converting modeled density contrasts to seismic velocity variations for comparison with regional seismic data as well as comparing modeled densities to calculated densities from xenoliths we find that upper crustal features and crustal thickness variations do not explain long wavelength 200 km variations in bouguer gravity across the colorado plateau rio grande rift a large 200 440 km wide region of low uppermost mantle density is modeled beneath the eastern colorado plateau margin jemez lineament rio grande rift and parts of the westernmost great plains this region has a density contrast of 90 to 30 kg m3 relative to the adjacent colorado plateau and great plains provinces and coincides with previously imaged regions of low upper mantle seismic velocities in situ densities inferred from xenolith data in the colorado plateau support the modeled upper mantle density variations and support the idea that the eastern and western margins of the plateau have been significantly modified by metasomatism we suggest that the region of low upper mantle density likely reflects some combination of temperature variations partial melt contents and compositional variations related to tertiary magmatic activity and or thinned lithosphere the relationship of this zone of modified upper mantle to extension in the rio grande rift remains enigmatic however particularly given the lack of spatial correlation between the trends of upper mantle structures and surficial extensional structures

3 d structure of the rio grande rift from 1 d constrained joint inversion of receiver functions and surface wave dispersion the southern terminus of the rio grande rift region has been poorly defined in the geologic record with few seismic studies that provide information on the deeper rift structure in consequence important questions related to tectonic and lithospheric activity of the rio grande rift remain unresolved to address some of these geological questions we collect and analyze seismic data from 147 earthscope transportable array usarray and other seismic stations in the region to develop a 3 d crust and upper mantle velocity model we apply a constrained optimization approach for joint inversion of surface wave and receiver functions using seismic s wave velocities as a model parameter in particular we compute receiver functions stacks based on ray parameter and invert them jointly with collected surface wave group velocity dispersion observations the inversions estimate 1 d seismic s wave velocity profiles to 300 km depth which are then interpolated to a 3 d velocity model using a bayesian kriging scheme our 3 d models show a thin lower velocity crust anomaly along the southeastern rio grande rift a persistent low velocity anomaly underneath the colorado plateau and basin and range province and another one at depth beneath the jemez lineament and the southern rgr

time corrections to teleseismic p delays derived from sks splitting parameters and implications for western u s p wave tomography 1 upper mantle anisotropy will affect teleseismic p wave arrival times and create artifacts in isotropic travel time tomography compiled sks splitting results indicate variations in the strength and orientation of azimuthal anisotropy beneath the western u s we use sks splitting parameters and a hexagonally anisotropic elastic tensor fast symmetry axis to estimate azimuthal anisotropy contributions to p wave delay times and evaluate the effects on isotropic p wave tomography estimated anisotropy correction times have a root mean square rms value of 0 16 s which is 37 of p wave delay time rms the magnitude of azimuthal anisotropy rather than its azimuth has the strongest effect on p delay times however if the anisotropy symmetry axis has a plunge p wave travel times may be strongly affected by both magnitude and azimuth applying azimuthal anisotropy corrections significantly increases tomographic p wave velocity beneath the high lava plains and central california coast and decreases velocity beneath the great basin

geophysical constraints on the crustal structure of the southern rio grande rift abstract evidence of geologic activity still occurring in the rio grande rift rgr includes quaternary faulting seismicity and widening at a small rate we map the crustal thickness and seismic velocity ratio to generate constrained crustal model cross sections that highlight the regional extension of the southern rgr srgr specifically we computed receiver functions and receiver function stacks for 147 stations from earthscope usarray transportable array and previously collected data in the southwestern united states and interpolated the crustal and velocity results using a kriging interpolation scheme we include a new optimized gravity inversion approach using the receiver function results as a constraint for inverted density profiles we produce constrained crustal models that characterize the srgr as showing a shallower moho 30 km in the region of the srgr which is thicker than previously suggested the crust appears to be delaminated west of the rgr in the basin and range province and thicker east of the rgr toward the great plains with a denser lower crustal layer we also find that the delaware basin shows a significant geophysical anomaly likely due to thick saturated sediments given that there is no deep mantle anomaly determined from other studies in this region we suggest that rifting could be the result of small scale convection of the uppermost mantle and we conclude that the rgr is at a minimum the boundary between the basin and range province and the great plains perhaps reflecting the terminus of flat slab subduction

present day motion and deformation of the colorado plateau 1 we analyze data from continuous gps stations in the southwestern united states and present the first observations of a systematic motion of the colorado plateau region relative to a north america fixed reference frame westward velocities increase rapidly across the wasatch fault system which comprises the plateau s northwestern margin but increase more gradually between the great plains and the mojave desert province we detect no significant extension across most of the rio grande rift but extension could be as high as 0 5 mm yr 1 along its southernmost portion we suggest that the motion between the plateau and stable continent may partly be accommodated instead across the jemez lineament only when we consider gps velocities within the plateau s center do the data support a rigid body rotation of 0 103 ma 1 around a pole in the northern rocky mountains we conclude that active extension has encroached into the plateau from both the east and west

the steeply subducting edge of the cocos ridge evidence from receiver functions beneath the northern talamanca range south central costa rica the deep structure of the south central costa rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible we present a receiver function study along a transect of broadband seismometers through the northern flank of the cordillera de talamanca south costa rica below moho depths the receiver functions image a dipping positive conversion signal this is interpreted as the subducting cocos plate slab compatible with the conversions in the individual receiver functions in finite difference modeling a dipping signal such as the one imaged can only be reproduced by a steeply 80 dipping structure present at least until a depth of about 70 100 km below this depth the length of the slab cannot be determined because of possible scattering effects the proposed position of the slab agrees with previous results from local seismicity local earthquake tomography and active seismic studies while extending the slab location to greater depths and steeper dip angle along the trench no marked change is observed in the receiver functions suggesting that the steeply dipping slab continues until the northern flank of the cordillera de talamanca in the transition region between the incoming seamount segment and cocos ridge considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision the southern costa rican subduction zone may at present be undergoing a reconfiguration of subduction style where the transition to shallow underthrusting may be underway but still incomplete

quantifying the impact of freshwater diatom productivity on silicon isotopes and silicon fluxes lake myvatn iceland diatom productivity in the oceans plays a crucial role in the carbon cycle but is strongly dependent upon the continental silicon supply however the relative influence of weathering and biological processes on continental si fluxes remains poorly constrained this study aims to quantify the impact of terrestrial diatom productivity on si fluxes to the ocean lake myvatn in north iceland is one of the most productive lakes in the northern hemisphere with nutrient rich waters almost uniquely sourced by groundwater the primary production is mainly controlled by diatom growth but also by cyanobacteria and the lake output is via a single river thereby providing a relatively simple natural laboratory to quantify the impact of diatom growth on the chemistry and si budget of lake waters silicon stable isotopes o 30si provide a tracer of this biocycling and have been measured in groundwater inputs to the lake and in time series monitoring of waters at the lake outlet the o 30si values at the outlet range from 0 70 0 08 to 1 42 0 06 which is significantly heavier than the groundwater input average cold and hot springs 0 50 0 17 2sd and consistent with the preferential uptake of light si isotopes by diatoms the o 30si value at the outlet increases by up to 0 9 in spring and autumn relative to the si isotope composition of the inflow these seasonal diatom blooms can be modeled by an open system of si uptake and affect si fluxes at the outlet of the lake by up to 79 or 53 integrated over the year in the summer a shift to lighter o 30si values is correlated with a higher ph which results in dissolution of diatoms releasing light si isotopes from mass balance this seasonal diatom dissolution affects si fluxes by up to 33 but is limited to 3 7 integrated over the year these results clearly illustrate that biological activity can have a significant impact on both isotope composition and elemental abundance of continental derived si they also demonstrate the ph dependency of diatom dissolution and or preservation which is likely to affect not only the continental si fluxes to the ocean but also the si recycling in the oceans themselves