exceptionally fast growth rate of 100 yr old tufa big soda lake nevada implications for using tufa as a paleoclimate proxy large tufa mounds 3 m tall with a basal circumference of 5 m have been discovered on the margin of big soda lake nevada usa these tufa mounds are rooted at a maximum of 4 m below the current lake surface and are actively forming from groundwater seepage which can be seen emanating from the top of the tufa mounds big soda lake is a volcanic crater lake whose water level is maintained exclusively by groundwater the age of the tufa mounds is well constrained because prior to the development of the newlands irrigation project in 1907 the water level was 18 m lower than the current lake level the vertical columnar nature of the tufa mounds indicates that they formed under the lake and not subaerially thus the tufa mounds are 100 yr old and have grown at a rate 30 mm yr stable oxygen and carbon isotope analyses of tufa carbonate compared to isotopic analyses of groundwater and lake water and hydrochemical data indicate that the fluids responsible for their precipitation are a simple mixture of modern groundwater and lake water and do not reflect a recent climate signature the exceptionally fast growth of the tufa mounds indicates that large tufa deposits may form almost instantaneously in geologic time given this potential for rapid growth and the fact that variations in isotopic compositions of tufa deposits have been interpreted in terms of changes in paleoclimate and changes in the composition of recharge water over thousands of years care should be taken when trying to determine the significance of variations in isotopic or chemical compositions of tufas that may have been caused by mixing with groundwater

aquaplanet experiments using cam s variable resolution dynamical core abstracta variable resolution option has been added within the spectral element se dynamical core of the u s department of energy doe ncar community atmosphere model cam cam se allows for static refinement via conforming quadrilateral meshes on the cubed sphere this paper investigates the effect of mesh refinement in a climate model by running variable resolution var res simulations on an aquaplanet the variable resolution grid is a 2 222 km grid with a refined patch of 0 25 28 km resolution centered at the equator climatology statistics from these simulations are compared to globally uniform runs of 2 and 0 25 a significant resolution dependence exists when using the cam version 4 cam4 subgrid physical parameterization package across scales global cloud fraction decreases and equatorial precipitation increases with finer horizontal resolution resulting in drastically different climates between the uniform grid runs and a physics induced grid imprinting in the var res simulation

growth rate and stable isotopic character of modern stromatolites from san salvador bahamas abstract 14 c 18 o and 13 c stratigraphies were measured on the successive carbonate crusts within large mushroom shaped stromatolitic heads from hypersaline storrs lake san salvador bahamas and also on the finely laminated soft sediment that enclose them 14 c dates show that the carbonate layers in these stromatolites range in age from 2310 70 years b p at the base to modern at the surface this suggests that the heads develop at an overall rate of 16 cm 1000 yr because there are 1 5 macroscopic laminae per cm less than one crustose layer is preserved in the heads per decade thin section examination of the crusts reveals numerous discontinuous micro unconformities which suggest multiple intervals of accretion and erosion these factor complicate any history of slow growth that may occur values of 18 o in the carbonates suggest that the heads were formed in waters with isotopic and temperature conditions similar to those that presently exist in the lake however carbonate 13 c values from the heads and sediments vary over 7 5 1 2 0 pdb and are significantly out of equilibrium with atmospheric co 2 the fluctuations in the 13 c values within the heads could not be correlated between adjacent heads nor with adjacent sediment on the basis of 14 c ages thus the sampled macroscopic layers mm cm in these calcified stromatolites do not appear to be recording simultaneous basin wide events but rather local diagenetic effects and micro environmental conditions in or under the microbial mats these data do not encourage basin wide interpretations of isotopic stratigraphies observed within ancient stromatolites where independent chronologies like 14 c are not available

a balanced tropical cyclone test case for agcms with background vertical wind shear abstractthis paper presents a balanced tropical cyclone tc test case designed to improve current understanding of how atmospheric general circulation model agcm configurations affect simulated tc development and behavior it consists of an analytic initial condition comprising two independently balanced components the first provides a vortical tc seed while the second adds a planetary scale zonal flow with height dependent velocity and imposes background vertical wind shear vws on the tc seed the environmental flow satisfies the steady state hydrostatic primitive equations in spherical coordinates and is in balance with other background field variables e g temperature surface geopotential the evolution of idealized tcs in the test case framework is illustrated in 10 day simulations performed with the community atmosphere model version 5 1 1 cam 5 1 1 environmental wind profiles with different magnitudes directions and vertical inflection points are applied to ensure that the technique

fault valve behaviour in optimally oriented shear zones an example at the revenge gold mine kambalda western australia abstract quartz vein systems developed in and adjacent to shear zones host major gold deposits in the kambalda region of the norseman wiluna greenstone belt at the revenge mine two groups of mineralised reverse shear zones formed as conjugate near optimally oriented sets during ese subhorizontal shortening adjacent to a major transpressional shear system the shear zones developed at temperatures of about 400 c in a transitional brittle ductile regime deformation was associated with high fluid fluxes and involved fault valve behaviour at transiently near lithostatic fluid pressures during progressive evolution of the shear system early brittle and ductile deformation was overprinted by predominantly brittle deformation brittle shear failure was associated with fault dilation and the formation of fault fill veins particularly at fault bends and jogs a transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses the formation of subhorizontal stylolites locally subvertical extension veins and minor normal faults in association with thrust faulting indicates episodic or transient reorientation of the near field maximum principal stress from a subhorizontal to a near vertical attitude during some fault valve cycles local stress re orientation is interpreted as resulting from near total shear stress release and overshoot during some rupture events previously described fault valve systems have formed predominantly in severely misoriented faults the shear systems at revenge mine indicate that fault valve action and associated fluctuations in shear stress and fluid pressure can influence the mechanical behaviour of optimally oriented faults

an analytic vortex initialization technique for idealized tropical cyclone studies in agcms abstract the paper discusses the design of idealized tropical cyclone experiments in atmospheric general circulation models agcms the evolution of an initially weak warm core vortex is investigated over a 10 day period with varying initial conditions that include variations of the maximum wind speed and radius of maximum wind the initialization of the vortex is built upon prescribed 3d moisture pressure temperature and velocity fields that are embedded into tropical environmental conditions the initial fields are in exact hydrostatic and gradient wind balance in an axisymmetric form the formulation is then generalized to provide analytic initial conditions for an approximately balanced vortex in agcms with height based vertical coordinates an extension for global models with pressure based vertical coordinates is presented the analytic initialization technique can easily be implemented on any agcm computational grid the characteristics of the idealized tropical cyclone experiments are illustr

vein mineralization at the damang gold mine ghana controls on mineralization abstract two distinct styles of au mineralization occur at the damang gold mine palaeoproterozoic sediments of the tarkwaian group host both one style of mineralization is stratabound within quartz lithic conglomerates of the banket series the second style of mineralization is associated with an extensive low displacement fault fracture mesh that formed in a compressional stress regime late in the deformational history and after the peak metamorphism regional deformation within the tarkwaian involved initial nw se directed shortening d 1 a major nne trending f 1 anticline hosts the damang orebody broadly n s shortening during d 2 resulted in the formation of e w trending thrusts with small displacements the d 3 shortening direction was similar to that of d 1 steep d 1 faults were reactivated and a new set of low angle thrusts and associated flat lying extension veins were formed the bulk of the mineralization observed at damang is associated with the low displacement d 3 fault fracture mesh the presence of flat lying extensional veins and the reactivation of some misoriented d 1 structures is indicative of periodic episodes of supralithostatic fluid pressures low differential stress and fault valve behavior towards the end of the deformation history

sensitivity of equatorial convection to horizontal resolution in aquaplanet simulations with a variable resolution gcm abstract climate simulations using idealized zonally symmetric boundary conditions an aquaplanet are performed in order to highlight possible deviations from zonality introduced by the spatial variations in resolution caused by the variable resolution technique simulations at three different uniform resolutions with spectral truncations t21 t63 and t106 are used as control simulations and analyzed in order to describe the zonal climate simulated by this version of the arpege climat general circulation model and establish its sensitivity to varying spatial resolution the model simulates a single equatorial precipitation maximum in which the activity of the convective cells is modulated by westward and eastward propagating equatorial waves depending on the resolution a simulation with the variable resolution technique using a truncation t63 and a stretching factor of 3 with the maximum resolution pole placed at the equator is then compared to the uniform resolution simulations deviations from the

enhanced development of lacustrine microbialites on gravity flow deposits great salt lake utah usa the great salt lake utah usa is a shallow hypersaline intracontinental lake hosting extensive microbial deposits at a large spatial scale the distribution of these deposits is driven by environmental and geodynamical factors i e water level fluctuations and a fault related framework a detailed mapping of the buffalo point area in the north western part of antelope island indicates the presence of an anomalous concentration of microbial deposits dated ca 5 8 ka bp and distributed along a lobe shaped geometry this uncommon microbial deposit geometry results from an extensive colonization of a conglomerate substrate exhibiting an accumulation of m sized rounded cambrian quartzite boulders we suggest that this conglomerate substrate provides a stable nucleation point that promotes the development and preservation of the lobe shaped microbial deposits microbial deposits may also have protected the conglomerate substrate from erosional processes and thereby increased the preservation potential of the lobe shaped structure based on the characteristics of the conglomerate e g grain size texture and its location i e 200 m beyond the average shoreline this lobe shaped structure likely results from subaqueous debris or a hyperconcentrated density flow that transports sedimentary material from the buffalo point slopes downward to the shore we estimate the age of the conglomerate deposition to be between 21 and 12 ka bp the initiation of the flow may have been triggered by various mechanisms but the existence of a major active normal fault in the vicinity of these deposits suggests that an earthquake could have destabilized the accumulated sediments and resulted in conglomerate emplacement the catastrophic 15 ka bp bonneville flood which led to a drop in the lake level approximately 110 m may also provide an explanation for the initiation of the flow

external controls on the distribution fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon cayo coco cuba active carbonate mineralizing microbial mats flourish in a tropical highly evaporative marine fed lagoonal network to the south of cayo coco island cuba hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies a variable intensity of mineralization and a potential for preservation in this study the role of intrinsic i e microbial and extrinsic i e physicochemical controls on microbial mat development mineralization and preservation was investigated the network consists of lagoons forming in the interdune depressions of a pleistocene aeolian substratum they developed due to a progressive increase in sea level since the holocene the hydrological budget in the cayo coco lagoonal network changes from west to east increasing the salinity this change progressively excludes grazers and increases the saturation index of carbonate minerals favouring the development and mineralization of microbial mats in the easternmost lagoons detailed mapping of the easternmost lagoon shows four zones with different flooding regimes the microbial activity in the mats was recorded using light dark shifts in conjunction with microelectrode o2 and hs profiles high rates of o2 production and consumption in addition to substantial amounts of exopolymeric substances are indicative of a potentially strong intrinsic control on mineralization seasonal climate driven water fluctuations are key for mat development mineralization morphology and distribution microbial mats show no mineralization in the permanently submersed zone and moderate mineralization in zones with alternating immersion and exposure it is suggested that mineralization is also driven by water level fluctuations and evaporation mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation the macrofabrics of the mats evolve from early colonizing flat mats to complex cerebroid or terrace structures the macrofabrics are influenced by the hydrodynamic regime wind driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon other external drivers include i storm events that either promote for example by bioclasts covering or prevent for example by causing erosion microbial mat preservation and ii subsurface degassing through mangrove roots and desiccation cracks covered by flat mats i e forming hemispheroids and cerebroidal structures these findings provide in depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings

fine grained agglutinated elongate columnar stromatolites tieling formation ca 1420 ma north china the mesoproterozoic tieling formation near jixian northern china contains thick beds of vertically branched laterally elongate columnar stromatolites carbonate mud is the primary component of both the stromatolites and their intervening matrix its abundance is attributed to water column whiting precipitation stimulated by cyanobacterial photosynthesis neomorphic microspar gives the stromatolites a streaky microfabric and small mud flakes are common in the matrix the columns consist of low relief mainly non enveloping laminae that show erosive truncation and well defined repetitive lamination in plan view the columns form disjunct elongate ridges 10 cm wide separated by narrow matrix filled runnels the stromatolite surfaces were initially cohesive rather than rigid and prone to scour and are interpreted as current aligned microbial mats that trapped carbonate mud the pervasive ridge runnel system suggests scale dependent biophysical feedback between i carbonate mud supply ii current duration strength and direction and iii growth and trapping by prolific mat growth together these factors determined the size morphology and arrangement of the stromatolite columns and their laminae as well as their branching patterns alignment and ridge runnel spacing ridge runnel surfaces resemble ripple mark patterns but whether currents were parallel and or normal to stromatolite alignment remains unclear the formation and preservation of tieling columns required plentiful supply of carbonate mud mat building microbes well adapted to cope with this abundant sediment and absence of both significant early lithification and bioturbation these factors were time limited and tieling stromatolites closely resemble coeval examples in the belt purcell supergroup of laurentia the dynamic interactions between mat growth currents and sediment supply that determined the shape of tieling columns contributed to the morphotypical diversity that characterizes mid late proterozoic branched stromatolites this article is protected by copyright all rights reserved

using variable resolution meshes to model tropical cyclones in the community atmosphere model abstracta statically nested variable mesh option has recently been introduced into the community atmosphere model s cam s spectral element se dynamical core that has become the default in cam version 5 3 this paper presents a series of tests of increasing complexity that highlight the use of variable resolution grids in cam se to improve tropical cyclone representation by dynamically resolving storms without requiring the computational demand of a global high resolution grid as a simplified initial test a dry vortex is advected through grid transition regions in variable resolution meshes on an irrotational planet with the cam subgrid parameterization package turned off vortex structure and intensity is only affected by grid resolution and no spurious artifacts are observed cam se model simulations using an idealized tropical cyclone test case on an aquaplanet show no numerical distortion or wave reflection when the cyclone interacts with an abrupt transition region using the same test case th

structural controls on granitoid hosted gold mineralization and paleostress history of the edikan gold deposits kumasi basin southwestern ghana the 9 moz total aggregate gold endowment at the edikan mine kumasi basin ghana is contained within a cluster of orogenic gold deposits located along the akropong fault zone the granitoid hosted orebodies at edikan e g ag2 ag3 fobinso esuajah essentially an interconnected mesh of gold bearing quartz veins formed during deformation event d3edk which postdates the penetrative regional d2edk deformation the gold bearing quartz veins developed in and adjacent to n s and nw se trending low angle thrust faults that crosscut lithological contacts and earlier formed steeply dipping d2edk faults our paleostress analysis shows that the d3edk deformation during which the mineralized fault system developed was characterized by a wnw ese hybrid compression that evolved to a strike slip regime this progressive deformation is best described with the following stress regimes wnw ese transpression pure compression t1 associated with low angle thrusting subsequent transpression strike slip t2 and later strike slip transtension t3 associated with steeply dipping strike slip faulting the bulk of the granitoid hosted gold mineralization at edikan is associated with two principal sets of gold bearing quartz veins including low angle fault fill veins controlled by thrusts and shallow dipping oblique extension veins that developed during t1 the activation of the reverse and sinistral strike slip faults led to the development of restraining jogs characterized by abundant shallow and steeply dipping gold quartz veins with moderately ne plunging ore shoots the geometry of the mineralized fault fracture meshes is consistent with fault valve behavior in a horizontal compressive stress regime under sustained conditions of supralithostatic fluid pressures at low differential stress

how fast does tufa grow very high resolution measurement of the tufa growth rate on artificial substrates by the development of a contactless image based modelling device

carbon isotope record of late cambrian to early ordovician carbonates of the argentine precordillera a more than 2000 m thick cambrian ordovician carbonate platform succession developed on the exotic argentine precordillera terrane during rifting from laurentia and drifting towards gondwana on base of these carbonates a carbon isotope curve could be developed for the cambrian ordovician we measured 13ccarb and 13corg values on bulk rocks selected components and diagenetic cements whereas the carbon isotope signals of intertidal and supratidal rocks are altered by diagenesis most subtidal carbonates exhibit primary marine values we can report reliable curves for the middle late cambrian transition and for the latest cambrian to earliest middle ordovician the 13c curve matches well with the published data from global cambrian ordovician boundary sections excursions and shifts in the carbon isotope curve correspond to events in sequence stratigraphy this indicates the interdependence on sea level which rules the productivity and or preservation of organic carbon and therefore the partition between corg and ccarb burial

track deflection of typhoon nesat 2017 as realized by multiresolution simulations of a global model abstracttyphoon nesat 2017 headed west northwestward toward taiwan but took a relatively larger northward deflection about 300 km away and then a leftward deflection after landfall at northern ta

habit of bacterially induced precipitates of calcium carbonate and the influence of medium viscosity on mineralogy abstract marine bacteria can and do induce the precipitation of calcium carbonate in the laboratory and in nature which results in single crystals and aggregates of crystals crystal bundles the end member forms of the crystal bundles observed are rods and spheres with numerous variations in between these forms the most common of which is the dumbbell rounded dumbbells and single crystals resembling brushes appear to be unique to bacterially induced precipitates and consequently may serve to identify bacterially induced precipitates in the rock record viscosity of the growth medium is the single most important control over the mineral precipitated in the experiments conducted the medium viscosity controls the ion diffusion rate and thus the rate of precipitation and consequently the mineral precipitated in a liquid medium circulation and the ion diffusion rate are both high precipitation is rapid and aragonite forms in a gelatinous medium there is little circulation the ion diffusion rate is slow the rate of precipitation is slow and calcite is precipitated the overall form of the resultant precipitate rods or spheres was the same regardless of the mineral composition

influences of north pacific ocean domain extent on the western u s winter hydroclimatology in variable resolution cesm author s rhoades am jones ad o brien ta o brien jp ullrich pa zarzycki cm abstract 2020 american geophysical union all rights reserved variable resolution global climate models vrgcms are a dynamical downscaling method that can reach spatiotemporal scales needed for regional climate assessments over the years several users of vrgcms have assumed where the location and extent of the refinement domain should be based on knowledge of the prevailing storm tracks and resolution dependence of important regional climate processes e g atmospheric rivers ars and orographic uplift but the effect of high resolution domain size and extent on the simulation of downstream hydroclimatic phenomena has not been systematically evaluated here we use variable resolution in the community earth system model vr cesm to perform such a test to do this three vr cesm grids were generated that span the entire two thirds and one third of the north pacific and evaluated for a 30 year climatology using atmospheric model intercomparison project protocols simulations are compared with reanalysis products offshore fifth generation of the european centre for medium range weather forecasts atmospheric reanalysis era5 and onshore livneh 2015 https doi org 10 1038 sdata 2015 42 and parameter elevation regressions on independent slopes model prism of the western united states the westward expansion of refinement domain influenced integrated vapor transport ivt which was generally high biased but minimally impacted ar characteristics due to slight differences in landfalling ar counts in the western united states california winter precipitation generally improved with westward expansion of the refinement domain western u s mountain snowpack and surface temperatures were insensitive to refinement domain size and were more influenced by changes in topographic resolution and or land surface model version given minimal dependence of simulated western u s hydroclimate on refinement domain size over the north pacific we advise future vr cesm studies to focus grid resolution on better resolving land surface heterogeneity

transtensional origin of the ne sw simitli basin along the strouma strymon lineament sw bulgaria several depocentres have developed along the nnw sse striking strouma strymon lineament since the miocene forming the strouma strymon graben system some of these basins including the dzherman and simitli strike at high angles to the lineament the rhomboid shaped simitli basin is limited in the south by the ne sw striking seismically active kroupnik fault and contains terrestrial clastic sediments which have been deposited since the sarmatian its formation is attributed to wrench dominated transtension d b event which activated the basin9s ne sw striking boundary faults as left lateral strike slip structures during the early middle miocene this deformation was associated with nne ssw contraction and wnw ese extension and succeeded the nne ssw contraction caused by pure shear dominated transpression d a event which governed the region in late oligocene early miocene times during the middle late miocene the d b event was followed by wnw ese pure shear dominated extension d 1 event which caused further widening of the basin the transtensional origin of the simitli basin is explained by lateral extrusion of the crustal mass squeezed between the apulian adriatic microplate and the european foreland towards the north aegean sea the subduction retreat of the hellenic orogen possibly enhanced this lateral extrusion and from the late miocene onwards the ne sw oriented back arc extension balanced the continuing retreat

study of an ordovician carbonate with alternating dolomite calcite laminations and its implication for catalytic effects of microbes on the formation of sedimentary dolomite

geology geodynamics and orogenic gold prospectivity modelling of the paleoproterozoic kumasi basin ghana west africa abstract this paper describes the geology and tectonics of the paleoproterozoic kumasi basin ghana west africa as applied to predictive mapping of prospectivity for orogenic gold mineral systems within the basin the main objective of the study was to identify the most prospective ground for orogenic gold deposits within the paleoproterozoic kumasi basin a knowledge driven two stage fuzzy inference system fis was used for prospectivity modelling the spatial proxies that served as input to the fis were derived based on a conceptual model of gold mineral systems in the kumasi basin as a first step key components of the mineral system were predictively modelled using a mamdani type fis the second step involved combining the individual fis outputs using a conjunction product operator to produce a continuous scale prospectivity map using a cumulative area fuzzy favourability caff curve approach this map was reclassified into a ternary prospectivity map divided into high prospectivity moderate prospectivity and low prospectivity areas respectively the spatial distribution of the known gold deposits within the study area relative to that of the prospective and non prospective areas served as a means for evaluating the capture efficiency of our model approximately 99 of the known gold deposits and occurrences fall within high and moderate prospectivity areas that occupy 31 of the total study area the high and moderate prospectivity areas illustrated by the prospectivity map are elongate features that are spatially coincident with areas of structural complexity along and reactivation during d4 of ne sw striking d2 thrust faults and subsidiary structures implying a strong structural control on gold mineralization in the kumasi basin in conclusion our fis approach to mapping gold prospectivity which was based entirely on the conceptual reasoning of expert geologists and ignored the spatial distribution of known gold deposits for prospectivity estimation effectively captured the main mineralized trends as such this study also demonstrates the effectiveness of fis in capturing the linguistic reasoning of expert knowledge by exploration geologists in spite of using a large number of variables the curse of dimensionality was precluded because no training data are required for parameter estimation

extreme 13c depletions in seawater derived brines and their implications for the past geochemical carbon cycle extreme depletions in the 13c content of the total dissolved inorganic carbon ct were found in brines overlying microbial mat communities total alkalinity at and ct in the brines suggest that intense photosynthetic activity of the microbial mat communities depletes the ct from the brine we suggest that this depletion drives a large kinetic negative fractionation of carbon isotopes similar to that observed in highly alkaline solutions in brines of extreme salinity where microbial mat communities no longer exist the 13c content of the ct increases probably because photosynthesis no longer dominates the gas exchange processes this mechanism explains light carbon isotope compositions of carbonate rocks from evaporitic sections and bears on the interpretation of 13c values in bedded stromatolitic limestones that are ca 3 5 b y old