pedogenic carbonates forms and formation processes abstract soils comprise the largest terrestrial carbon c pool containing both organic and inorganic c soil inorganic carbon sic was frequently disregarded because 1 it is partly heritage from soil parent material 2 it undergoes slow formation processes and 3 has very slow exchange with atmospheric co2 the global importance of sic however is reflected by the fact that sic links the long term geological c cycle with the fast biotic c cycle and this linkage is ongoing in soils furthermore the importance of sic is at least as high as that of soil organic carbon soc especially in semiarid and arid climates where sic comprises the largest c pool considering the origin formation processes and morphology carbonates in soils are categorized into three groups geogenic carbonates gc biogenic carbonates bc and pedogenic carbonates pc in this review we summarize the available data and theories on forms and formation processes of pc and relate them to environmental factors after describing the general formation principles of pc we present the specific forms and formation processes for pc features and the possibilities to use them to reconstruct soil forming factors and processes the following pc are described in detail earthworm biospheroliths rhizoliths and calcified roots hypocoatings nodules clast coatings calcretes and laminar caps the second part of the review focuses on the isotopic composition of pc 13c 14c and 18o as well as clumped 13c and 18o isotopes known as 47 the isotopic signature of pc enables reconstructing the formation environment the dominating vegetation 13c temperature 18o and 47 and the age of pc formation 14c the uncertainties in reconstructional and dating studies due to pc recrystallization after formation are discussed and simple approaches to consider recrystallization are summarized finally we suggest the most important future research directions on pc including the anthropogenic effects of fertilization and soil management in conclusion pc are an important part of sic that reflect the time periods and formation processes in soils a mechanistic understanding of pc formation is a prerequisite to predict terrestrial c stocks and changes in the global c cycle and to link the long term geological with short term biological c cycles

pedology of archaeological soils in tells of the judean foothills israel abstract tells archaeological mounds predominantly consist of poorly consolidated to unconsolidated sediments and soils that are highly anthropogenic this study examines pedogenic processes related to carbonate mobilization in tells in comparison with their peripheral soils reference soil profiles the key objective of this study is to test the hypothesis that tell deposits evolve through concurrent processes of sedimentation and pedogenesis synlithogenic pedogenesis and further explain it case studies are presented from three tells in semi arid and mediterranean climatic zones of the levant the methods applied included field survey analyses of particle size distribution ph caco3 and organic carbon content and soil micromorphology soils of the tells contain miscellaneous cultural materials that derive mainly from degraded mud bricks pottery and burnt wood chemical data show basic ph values high caco3 content and minor amounts of organic carbon field observations and lab analyses both indicate high similarity amongst the tells and their reference soil profiles buried tell soils show same characteristics as near surface soils however the reference soil profiles show incipient horizonation slightly darker colours and more developed structure micromorphology of both the tells and the reference profiles show cohesively welded peds in a vughy microstructure groundmass with an open porphyric c f related distribution and discontinuous carbonate recrystallization relative rates of soil formation in the tells can be estimated when archaeological records are established contrary to the tells the reference soil profiles show lower porosity and only minor remnants related to earth construction materials we classify the soils of the studied tells as archaeological calcareous anthraltic xerorthents the correlative wrb classification would be calcic urbic technosols archaic the anthropogenic materials are as calcareous as the natural soils but due to human action carbonates in the tells are distributed differently based on these observations ancient human actions and the dry climate have led to very little mobilization and accumulation of carbonates the information provided in this study adds to the pedological understanding of archaeological environments specifically it can be useful for the study of site formation processes of tells

viscosity in transition zone and lower mantle implications for slab penetration 1 the evolution of a subducting slab is strongly influenced by the viscosity of the mantle which it overlies however there is still no consensus about the viscosity in the transition zone and shallower lower mantle we use a numerical self consistent subduction model and run a set of experiments in order to find critical patterns of viscosity that would allow the evolution of the different subduction styles that can actually be found in nature our results show that a jump in viscosity of 5 times from transition zone to lower mantle gives the most reasonable results optimal values of viscosity in the transition zone are in the range of 3 1020 1021 pa s higher values would produce piling up of the slab and later sinking or even slab flattening lower viscosities result in velocities 30 cm yr that are too high while the new slab subducts through the upper mantle and transition zone a phenomenon that is rarely seen in nature reduction of the clapeyron slope related to the spinel perovskite transition variations in oceanic crustal thickness and in the age of the slab do not influence much the style of subduction when overriding velocity is applied to the upper plate the previously penetrating slab tends to lay at the 660 km boundary but does not substantially change the subduction velocity

biogenic caliches in texas the role of organisms and effect of climate abstract biogenic constituents are ubiquitous and abundant in the caliches of texas investigation of 51 caliche profiles on various host strata alluvium limestone igneous rocks etc across approximately 900 km of texas from subhumid east to arid west has shown that 43 of these profiles exhibit prominent biogenic constituents these profiles exhibit significant differences in thickness varying from centimeters to meters and maturity varying from i to vi all of the different caliche facies are composed of low mg calcite biogenic features generally occur in the upper part of the profiles including the uppermost portion of massive caliche horizons platy horizons laminar crusts and pisoids the main biogenic caliche facies include rhizoliths calcified root structures stromatolite like laminar crusts and coated grains compared to the abiogenic massive micritic to microsparitic calcite groundmass biogenic constituents are morphologically distinct these biogenic constituents are composed of several microscopic mineral components including calcified filaments needle fiber calcite e g single crystalline needles and needle pairs triangular crystals and polycrystalline chains of rhombohedrons spherulites micro rods and nano spheres a large number of calcified root cellular structures and micro organisms e g fungal filaments actinomycetes and rod like bacteria are also present plant roots as well as soil biota produce distinctive structures and also enhance lithification by inducing calcite precipitation in the caliches i e biologically controlled or influenced processes host strata did not significantly influence the abundance nor type of biogenic features in the caliches in contrast climate had an evident effect on the development of biogenic constituents in these caliches in terms of the amount as well as type the thickness of laminar crusts and grain coatings and the abundance of biotic constituents within those facies decrease as the climate shifts from subhumid and subarid in southeast and central areas to subarid and arid in west and northwest texas in addition root structures and micro rods diminish significantly from the subhumid east to the arid west

vertical variation in porosity of nāri calcrete on chalk galilee israel a new interpretation as a tribute to dan h yaalon abstract n ri calcrete is a calcareous formation common in the semiarid mediterranean region of the levant the profile studied in nazareth galilee is 2 75 m thick and divided into the four conventional layers i e a chalky bedrock a powdery lower n ri an indurated and platy upper n ri and a laminar crust capping the profile in this study mercury intrusion porosimetry mip is used to assess the genetic processes at work in the formation of n ri calcrete based on the former work by yaalon and singer 1974 and mip results various parameters are proposed to describe the different layers total porosity n p trapped porosity n t permeability k and strength c these parameters are compared to each other for each layer as well as with their respective calcium carbonate contents an unexpected relationship shows that the more caco 3 is present the more porous the layer of the n ri consequently the lower n ri is interpreted as a weathering layer of the parent chalk whereas the upper n ri which is the most porous and the richest in caco 3 results from in situ recrystallization the upper n ri permeability precludes it from playing the role of a plugged horizon to explain the formation of the laminar crust in conclusion the n ri calcrete in nazareth questions the conventional model of calcrete genesis as its cementation is due to in situ redistribution of caco 3 rather than its translocation these results strengthen yaalon and singer 1974 s conclusions

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

coloured mineral coatings on monument surfaces as a result of biomineralization the case of the tarragona cathedral catalonia successive coloured coatings on the taragona cathedral catalonia were analyzed using chemical microbiological and mineralogical techniques the coatings consist of mainly biogenic minerals calcite several oxalates phosphates and their fabric and composition is independent of the underlying rock the origin of the crusts or patinas is attributed to bygone sub fossil microflores which formed biofilms and microbial mats on and within the surface of the limestones and marbles of the cathedral the staining of the patina layers is not related to the mineral colours nor to anthropogenic dust or fly ashes the colours are most probably organic pigments embedded between the crystals and fe and mn organic compounds remaining from bioactivity a sequential hypothesis of the origin s of the patina layers is given

searching for travertines calcretes and speleothems in deep time processes appearances predictions and the impact of plants common models for modern calcite precipitation in and around caves soils springs and streams involve co 2 supplied by thick high p co 2 biogenic soils which were probably thin or non existent before vascular plants indeed plant influenced chemical weathering might have caused accelerated terrestrial carbonate production from the devonian onwards however terrestrial carbonates have also been documented from the archaean proterozoic cambrian ordovician and silurian mechanisms which could have caused non marine carbonates to precipitate without organic rich soils are described and some geological events likely to have influenced non marine carbonate precipitation up to the origin of vascular plants are highlighted as organisms have evolved so have the petrographic characteristics of non marine carbonates some examples of this are also given here

high frequency fall off of source spectra of deep focus earthquakes from wigner distribution estimation abstract using wigner distribution estimation the high frequency fall off of source spectra was estimated the technique was applied to the analysis of the source spectra of the vladivostok m 6 4 deep focus earthquake of 22 july 1994 and the northern japan sea m 5 7 deep focus earthquake of 22 july 1994 it is shown that the high frequency fall off of the source spectra of these two earthquakes may be characterized by two parts between the first corner frequency f1 and the second corner frequency f2 the high frequency fall off of the source spectra can be represented by f 5 above f2 the high frequency fall off can be represented by f 2

human induced calcretisation in anthropogenic soils and sediments field observations and micromorphology in a mediterranean climatic zone israel abstract for over a century the study of pedogenic calcrete and its formation focuses on natural processes disregarding the possible role of humans in its formation here we present field and micromorphological evidence from archaeological and modern sites in the eastern mediterranean region israel that indicate that some specific calcretes are human induced we demonstrate that anthropogenic affected calcrete differs from natural non human induced calcrete we show that the occurrence of the human induced calcrete is spatially limited and is associated with both macro and micro scale remnants and occasionally high content of organic material we propose an operative definition for anthropogenic calcretisation as the interference of humans with a component s that influence the formation of calcrete we assume that if present day calcretised environments will retain their calcretisation prone conditions for the next 5 10 millennia a distinct calcretic morphology could become a significant terrestrial formation this discovery can be particularly useful for evaluating past human impact on the environment adding a new archaeological set of research tools that should further be developed

three dimensional velocity field of present day crustal motion of the tibetan plateau derived from gps measurements 1 using the measurements of 750 gps stations around the tibetan plateau for over 10 years since 1999 we derived a high resolution 3 d velocity field for the present day crustal movement of the plateau the horizontal velocity field relative to stable eurasia displays in details the crustal movement and tectonic deformation features of the india eurasia continental collision zone with thrust compression lateral extrusion and clockwise rotation the vertical velocity field reveals that the tibetan plateau is continuing to rise as a whole relative to its stable north neighbor however in some subregions uplift is insignificant or even negative the main features of the vertical crustal deformation of the plateau are the following a the himalayan range is still rising at a rate of 2 mm yr the uplift rate is 6 mm yr with respect to the south foot of the himalayan range b the middle eastern plateau has a typical uplift rate between 1 and 2 mm yr and some high mountain ranges in this area like the longmen shan and gongga shan have surprising uplift rates as large as 2 3mm yr c in the middle southern plateau there is a basin and endorheic subregion with a series of ns striking normal faults showing obvious sinking with the rates between 0 and 3 mm yr d the present day rising and sinking subregions generally correspond well to the cenozoic orogenic belts and basins respectively e at the southeastern corner of the plateau there is an apparent trend that the uplift rate is gradually decreasing from between 0 8 and 2 3 mm yr in the inner plateau to between 0 5 and 1 6 mm yr outside the plateau with the decrease of terrain height

biogenic β fabric features in the hard laminated crusts of the mersin and adana regions southern turkey and the role of soil organisms in the formation of the calcrete profiles abstract this paper describes fabric features and their distribution in the calcretes and emphasizes the role of soil organisms in the formation of the calcrete profiles quaternary calcretes are common in the mersin and adana provinces occurring as powdery nodular tubular fracture infill and various forms of crusts the stable isotope values indicate calcrete development under semi arid climatic conditions and a pedogenic origin biogenic or fabric constituents are abundant in the hard laminated crust these are the rhizoliths alveolar septal structures needle fiber calcites calcified filaments spherical calcite bodies microcodium and vadose calcrete pisoliths rhizoliths have mainly formed by the petrification of roots and often contain cellular cortex structures the alveolar septal structure comprises root voids separated by a micritic wall or a bundle of tangentially oriented calcite needles needle fiber calcite is common in the hard laminated crust as monocrystalline rods and polycrystalline needles monocrystalline rods are smooth and small

raising the colorado plateau shallow marine rocks exposed on the 2 km high 45 km thick colorado plateau in the western united states indicate that it was near sea level during much of the phanerozoic isostatic calculations however illuminate the difficulty in maintaining a 45 km thick crust at or near sea level we propose that an isostatically balanced 30 km thick proto colorado plateau crust was thickened during the late cretaceous to early tertiary by intracrustal flow out of an overthickened sevier orogenic hinterland this plateau would have been supported by a thick 70 km crustal root which is proposed to have been the source region for hot and weak mid crustal material that flowed eastward from the plateau toward the low elevation proto colorado plateau

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

fan surface dynamics and biogenic calcrete development interactions during ultimate phases of fan evolution in the semiarid se spain murcia abstract pleistocene alluvial fan surfaces of the campo de cartagena mar menor basin murcia se spain are capped by thick mature calcretes calcrete profiles consist mainly of six different horizons prismatic chalky nodular massive laminar and coated gravels petrographic study of the calcretes has shown the occurrence of features such as alveolar septal structures calcified filaments coated grains spherulites calcified root cells and calcispheres that indicate the biogenic origin of the calcretes mainly induced by plant root related microbial activity the calcretes studied were formed initially in the soil and represented the k horizon development of the calcrete profiles took place in six main stages and was driven by multiple phases of soil formation erosion and reworking the relationships between these processes caused the formation of different calcrete profiles in proximal and distal fan areas in the distal areas which are controlled by limited distal fan aggradation episodic sediment input buried previously developed calcretes and generated new space for calcrete growth by plants growing in the overlying unconsolidated materials this allowed the renewal of calcrete formation and it led to the development of complex composite profiles which are thicker than in proximal areas where surface stabilisation and or dissection enabled calcrete reworking and brecciation these processes of erosion sedimentation reworking and renewed calcrete formation initiated by vegetation were repeated through time they explain the complex macro and microstructures of these calcretes and indicate that calcrete development even reaching mature stages can start before the fan surface is completely abandoned but it requires episodic sedimentation eventually distal fan aggradation and continuous calcrete development throughout the entire fan surface led to the ultimate fan surface induration controlling subsequent landscape evolution so fan surface calcretes cannot be envisaged as simple top surface carbonate accumulations but as complex feedback systems in which pedogenic biogenic and sedimentary processes interact in response to the evolving fan surface dynamics during the terminal phases of fan development in semiarid environments

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

needle fiber calcite a critical review and a proposed classification abstract from a review of the recent literature and our own observations the different morphologies of needle fiber calcite are classified they can be characterized as monocrystalline rods with or without serrated edges or as polycrystalline chains the simplest monocrystalline habit of needle fiber calcite is the individual micro rod m larger monocrystalline rods ma exist in multiples of two more complex forms are found as paired monocrystalline rods with serrated plates along their length mb this group includes three habits which can be distinguished by the number of plates and their orientation relative to the rod finally there is a monocrystalline form mab intermediate between the categories ma and mb which shows the transition from the simple coupled rods to rods with errated plates although polycrystalline needles p have rarely been reported in the literature three types of morphologies have been identified each type of crystal has a specific origin the m micro rods are physicochemical nuclei or calcified bacteria they do not seem to evolve towards larger or more complex morphologies the ma rods are related to biomineralization inside the fungal sheaths the mb rods postdate liberation of ma rods from mycelian bundles the serrated plates are secondary syntactic crystallizations of physicochemical origin whose growth can be inhibited for various reasons the origin of p chains is less well understood but they seem to be physicochemical crystallizations related to evaporation and desiccation therefore these three groups m ma mb and p have similar morphologies but do not seem to be related genetically the only known link is between the ma and mb rods there is no evolutionary s quence of needle fiber calcite habits but a convergence of forms between crystals whose origins are different and independent structures described in the literature as convoluted random cutane and nest like are the result of either evolution of the mycelian bundles or movement of material and organisms in the soil in the case of m micro rods the microfabric is related to the random precipitation around nuclei in pores filled with a highly supersaturated solution initially the p chains also precipitate without preferential organization and later on the growth seems to preferentially constitute chains although the mechanisms are still not understood moreover needle fiber calcites do not indicate a particular climatic environment

orocline and syntaxes formation during subduction and collision 1 the present work investigates the formation of curved ranges and syntaxes with scaled laboratory experiments we simulated subduction and collision processes comparable to india asia configuration involving a continental upper plate and a subducting plate composed of an oceanic lithosphere and a continental indenter the experiments reveal that the shape of the mountain range concave straight or convex and the development of syntaxes are controlled by the subduction interface the buoyancy number fb of the upper plate i e thickness and viscosity and the boundary conditions four end members regimes of indentation can be defined depending on the range shape and dynamics of the upper plate the curvature of the range is convex toward the subducting plate with syntaxes for a weak subduction fault and concave without syntaxes for a strong subduction fault convex curvature and syntaxes form by overthrusting of upper plate material on the subducting plate which is faster at the center than at the extremities they are associated with a rather flat slab underthrusting during continental collision low fb experiments show less pronounced curvatures associated to thickening comparable to the early stages of the india asia collision in contrast a thick and weak upper plate high fb leads to gravity collapse that increases the amplitude of the curvature and lateral escape similar to the late evolution of the himalaya tibet system important lateral decoupling on the sides of the indenter enhances the indentation and produces sharper syntaxes

experimental observations on fungal diagenesis of carbonate substrates 1 carbonate substrates dolomites and limestones are susceptible to fungal attack that results in significant microbial diagenesis of these substrates in a 15 day experimental study fungi growing in petri dishes from airborne spores attacked petrographic thin sections and chips prepared from the dolomites of terwagne formation visean bocahut quarry at avesnes sur helpe northern france and limestones of the morrone di pacentro formation lower cretaceous italy the analyses of the fungal material samples of mycelia thin sections and chips under optical microscopy scanning electron microscope sem energy dispersive x ray edx x ray diffraction xrd raman spectroscopy and stable isotopes c and o revealed an extensive fungally induced diagenesis the results indicate strong diagenesis and biomineral neomorphism neo dolomite glushinskite weddellite whewellite and possibly struvite as well as intense substrate de micritization and micritization with oxalates grain bridging and cementation open space filling formation of intergranular and intragranular porosity and permeability enhancement advanced stages of diagenesis were characterized by dissolution and replacement of original minerals by new substrates produced by fungal biomineralization the formation of new substrates on the original attacked surfaces produced microscale stratification stable isotope analysis of fungal biomineralized material and of attacked and unattacked chip surfaces revealed marked differences in their isotopic signatures the c and o isotopes of biomineralized material within the fungal mass were fractionated differently as compared to the signature measured in the original and unattacked surfaces in sedimentary cycles such microbially modified isotopic signature of carbonate substrates may be used to define microbial events and consequently whether certain types of diagenesis were produced by microbial interaction the finding of neo dolomite formed during fungi dolomite substrate interaction suggests the possibility of sedimentary dolomite recycling in a fungal microenvironment the results of this experimental study confirm the significant role of fungi in reshaping carbonate substrates and forming new biominerals in the natural environment

collision of continental corner from 3 d numerical modeling abstract continental collision has been extensively investigated with 2 d numerical models assuming infinitely wide plates or insignificant along strike deformation in the third dimension however the corners of natural collision zones normally have structural characteristics that differ from linear parts of mountain belt we conducted 3 d high resolution numerical simulations to study the dynamics of a continental corner lateral continental oceanic transition zone during subduction collision the results demonstrate different modes between the oceanic subduction side continuous subduction and retreating trench and the continental collision side slab break off and topography uplift slab break off occurs at a depth 100 km to 300 km that depends on the convergence velocity the numerical models produce lateral extrusion of the overriding crust from the collisional side to the subduction side which is also a phenomenon recognized around natural collision of continental corners for instance around the western corner of the arabia asia collision zone and around the eastern corner of the india asia collision zone modeling results also indicate that extrusion tectonics may be driven both from above by the topography and gravitational potentials and from below by the trench retreat and asthenospheric mantle return flow which supports the link between deep mantle dynamics and shallower crustal deformation

dolomitized calcrete in the middle devonian winnipegosis carbonate mounds subsurface of south central saskatchewan canada dolomitized calcrete is a widespread diagenetic feature in the uppermost portions of the middle devonian winnipegosis mud mounds in the subsurface of south central saskatchewan individual calcrete profiles are composed of 2 to 5 horizons including in descending order laminar crust massive pisolitic breccia chalky and transitional horizons the calcrete profiles are interpreted to be pedogenic in origin formed by diagenetic alteration of the host carbonate deposits during subaerial exposure of the mud mounds this interpretation is based on the occurrence of an orderly set of well differentiated horizons disconformities vadose pisoids micritic stringers circumgranular cracks linkage coatings and gradational contacts between the calcrete and underlying host carbonate rocks lack of root related structures such as rhizoconcretion and alveolar septal fabric in the winnipegosis calcrete suggests a limited influence of macrophytes on the formation of calcrete the occurrence of up to three discrete pedogenic calcrete profiles in a single winnipegosis calcrete succession indicates the mounds underwent three periods of subaerial exposure that resulted from drops of water level in the elk point basin of saskatchewan during middle devonian time d 2004 elsevier b v all rights reserved

multi storey calcrete profiles developed during the initial stages of the configuration of the ebro basin s exorrheic fluvial network abstract multi storey calcrete profiles developed in the quaternary on strath terraces of the cinca and alcanadre rivers tributaries of the ebro river in ne spain two calcrete profiles tor 1 and tor 2 near the village of el tormillo show horizons with an arrangement that differs from that of commonly described calcrete profiles significant lateral changes occur in these profiles within a distance of less than 200 m reflecting their pedofacies relationship the tor 1 profile on terrace qt1 the highest and oldest consists of six horizons from bottom to top 1 coarse fluvial gravels 2 mudstones with carbonate nodules 3 a chalky horizon 4 laminar horizons including one peloidal horizon 5 a multi storey horizon formed of at least six minor sequences each of which includes a lower detrital layer a pisolithic horizon and a thin discontinuous laminar horizon these sequences indicate several cycles of brecciation and or reworking and 6 a topmost laminar and brecciated horizon also including reworked pisoliths some 200 m to the north of tor 1 horizon 5 undergoes a lateral change to channel fill deposits the infill of the channels shows a fining upwards sequence ranging from clasts of about 10 cm in diameter to red silts with sparse pebbles all the clasts come from the underlying calcrete horizons laminar horizons are interbedded with the clastic channel deposits the youngest calcrete profiles developed on terrace qt3 of the cinca river and on the qp4 and qp6 mantled pediment levels all show relatively simple profiles composed mostly of lower horizons of coated gravels with thin laminar horizons at the top most of the horizons especially the laminar ones show biogenic features such as alveolar septal structures calcified filaments biofilms spherulites micropores and needle like calcite crystals these features indicate the important role of vegetation in the formation of all the above profiles the interbedding of clastic sediments and pisolithic horizons within the tor 2 profile indicates several stages of stabilisation during profile formation these sequences are an indication of the sedimentation soil formation and reworking processes operating on the soil surface the alternation of these processes is interpreted as the result of climate vegetation changes the channel fills of tor 2 indicate erosion and reworking of the hard laminar calcrete horizon both tor 1 and tor 2 are multi storey profiles reflecting the complex sedimentation erosion pedogenesis relationships at the final stages of the development of its corresponding fluvial terrace the study of these calcretes shows that these supposedly abandoned terraces continue to be active even though the fluvial network is entrenched both the pedofacies relationships and the complexity shown by tor 1 and tor 2 reflect the complex and unstable geomorphic setting in which these profiles developed after the establishment of the exorrheic network less complex calcrete profiles were produced in the lower terraces

development of topography in 3 d continental collision models understanding the formation and evolution of high mountain belts such as the himalayas and the adjacent tibetan plateau has been the focus of many tectonic and numerical models here we employ 3 d numerical simulations to investigate the role that subduction collision and indentation play on lithosphere dynamics at convergent margins and to analyze the conditions under which large topographic plateaus can form in an integrated lithospheric and upper mantle scale model distinct dynamics are obtained for the oceanic subduction side trench retreat slab rollback and the continental collision side trench advance slab detachment topographic uplift lateral extrusion we show that slab pull alone is insufficient to generate high topography in the upper plate and that external forcing and the presence of strong blocks such as the tarim basin are necessary to create and shape anomalously high topographic fronts and plateaus moreover scaling is used to predict four different modes of surface expression in continental collision models i low amplitude homogeneous shortening ii high amplitude homogeneous shortening iii alpine type topography with topographic front and low plateau and iv tibet himalaya type topography with topographic front and high plateau results of semianalytical models suggest that the argand number governs the formation of high topographic fronts while the amplitude of plateaus is controlled by the initial buoyancy ratio of the upper plate applying these results to natural examples we show that the alps belong to regime iii the himalaya tibet to regime iv whereas the andes altiplano fall at the boundary between regimes iii and iv

carbon isotopic subsets of soil carbonate a particle size comparison of limestone and igneous parent materials abstract the c isotopic analysis of pedogenic carbonate is an important method for investigating paleoclimatic conditions and carbon sequestration however a calcic soil sample may contain multiple types of carbonate especially in soils formed in limestone parent material to evaluate isotopic values of the different soil carbonate types stable c isotopic analysis was applied to an array of particle size classes in bulk soil that was collected from soils formed in limestone parent material and neighboring soils formed in igneous parent material a pronounced trend of decreasing 13 c with progressively decreasing soil particle size was revealed in soils of both parent materials ranging from about 2 to 6 for limestone soils and 1 to 5 for igneous soils thin section microscopy and radiocarbon dating also indicated multiple subsets of calcium carbonate types in soils thin section analysis disclosed the presence of limestone detritus calcitans aggregates of carbonate crystals and calcified root hairs radiocarbon dating revealed different 13 c values of carbonate crystals of different ages ranging from about 2620 to 40 290 years for limestone soils and 1660 to 5830 years for igneous soils these results indicate 1 limestone particles in soil may contribute to a complex mixture of 13 c values that is absent from soils formed in igneous parent material and 2 it is necessary to recognize that isotopic values of bulk soil represent a mixture of different 13 c values from different size fractions and ages in both limestone and igneous soils

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

role of calcium oxalate biomineralization by fungi in the formation of calcretes a case study from nazareth israel abstract biomineralized fungal filaments often referred to as calcified filaments are of interest because of their role in the calcium cycle in calcretes the mineralogy of the crystals associated with these filaments and the evolution of their numerous crystal habits in a calcrete from nazareth israel are the subject of this study removing the be window from an energy dispersive spectrometry eds microprobe coupled with a scanning electron microscope sem permitted the measurement of the light elements c and o us well as ca comparison of crystal spectra to those of known standards demonstrated that the great majority of needles associated with the fungal filaments are calcium oxalate based on these results a model of fungal filament mineralization is proposed that describes the e olution of weddellite polyhydrate calcium oxalate into whewellite monohydrate calcium oxalate in a sequence that includes all of the crystal morphologies observed by sem followed by a less well understood diagenetic transformation of monohydrate calcium oxalate into calcium carbonate

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

microbial biscuits of vaterite in lake issyk kul republic of kyrgyzstan discussion giralt et al 2001 reported the formation of microbial biscuits of vaterite in lake issyk kul republic of kyrgyzstan although they state that the trigger mechanisms for the natural precipitation of vaterite remain unclear they propose an explanation of the origin for the vaterite biscuits involving bacterial activity our recent research seems to support their hypothesis although the way we obtain vaterite precipitation is not strictly related to lacustrine or natural aqueous environments we have used laboratory cultures to demonstrate the ability of some soil bacteria to transform an oxalate carbon source into carbonate a hypothesis supported by observations cromack et al 1977 verrecchia et al 1993 and biogeochemical studies verrecchia and dumont 1996 the bacteria used were isolated by tamer and aragno 1980 and identified as xanthobacter autotrophicus dsm 432 atcc 35674 syn corynebacterium autotrophicum previously described by wiegel et al 1978 the bacteria were grown in a b4 media boquet et al 1973 to which calcium acetate or calcium citrate was added in this experiment the only way to produce carbonate

comparisons of the kinematics and deep structures of the zagros and himalaya and of the iranian and tibetan plateaus and geodynamic implications 1 we compare the geologic histories the deep structures and the present day kinematics of deformation of the himalaya and the adjacent tibetan plateau with those of the zagros and iranian plateau to test geodynamic processes of continental collision shortly after india and arabia collided with eurasia horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from india s and arabia s underlying crystalline basement subsequently slip on thrust faults stacked slices of india s basement to build the himalaya on india s northern margin such faulting has not yet developed in the zagros where collision is more recent and arabia penetrates into eurasia more slowly than india does so that postcollision convergence with eurasia is less the greater elevation thicker crust and more marked heterogeneity of the upper mantle beneath the tibetan than beneath the iranian plateau also reflect a more advanced stage of development moreover while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus normal faulting suggesting horizontal extension and crustal thinning occurs within tibet but not in iran hence the balance of forces that built the high tibetan plateau must have changed apparently some time since 15 ma removal of tibetan mantle lithosphere could have altered that balance if mantle lithosphere beneath the iranian plateau has been removed however the change in force balance has been too small to initiate normal faulting low seismic wave speeds in the uppermost mantle just beneath the moho of both plateaus suggest to us that lithosphere beneath both is thin consistent with late cenozoic removal of it but alternative explanations might account for these low speeds despite its apparently thin and hence presumably weak mantle lithosphere much of central iran undergoes little deformation it illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small whereas differences between the two regions clearly depend on the amount that arabia and india have penetrated into eurasia which scales with both the dates of collision and rates of convergence we see no differences in the operative processes that depend on the present day rates of convergence

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

study of a modern calcrete forming in guadalajara central spain an analogue for ancient root calcretes abstract the pajares calcrete in central spain a modern calcrete consists of an accumulation of powder carbonate around the roots of living trees and bushes which penetrates miocene deposits by more than 3 m calcrete development is mostly vertical and oblique but thinner horizontal calcified root mats also occur carbonate accumulation up to 15 cm thick is made up of micrite with some etched clasts alveolar septal structures calcified rootlets coated grains and micrite grains in detail many of these features are composed of needle fibre calcite and micro rods accumulation of carbonate was produced both by roots through direct calcification of their cells and also by microbial activity within the rhizosphere this case study provides a good example of how carbonate is accumulating around roots at present and may allow better understanding of ancient calcretes especially those which formed directly around roots in direct contact with the host rock in addition the calcrete s setting and its occurrence around roots belonging to various types of plants suggest that carbonate accumulation in modern landscapes may be a relatively rapid common process even in erosive regimes and unrelated to specific vegetation types

accommodation of transpressional strain in the arabia eurasia collision zone new constraints from u th he thermochronology in the alborz mountains north iran the alborz range of n iran provides key information on the spatiotemporal evolution and characteristics of the arabia eurasia continental collision zone the southwestern alborz range constitutes a transpressional duplex which accommodates oblique shortening between central iran and the south caspian basin the duplex comprises nw striking frontal ramps that are kinematically linked to inherited e w striking right stepping lateral to obliquely oriented ramps new zircon and apatite u th he data provide a high resolution framework to unravel the evolution of collisional tectonics in this region our data record two pulses of fast cooling associated with sw directed thrusting across the frontal ramps at 18 14 and 9 5 7 5 ma resulting in the tectonic repetition of a fossil zircon partial retention zone and a cooling pattern with a half u shaped geometry uniform cooling ages of 7 6 ma along the southernmost e w striking oblique ramp and across its associated nw striking frontal ramps suggests that the ramp was reactivated as a master throughgoing n dipping thrust we interpret this major change in fault kinematics and deformation style to be related to a change in the shortening direction from ne to n nne the reduction in the obliquity of thrusting may indicate the termination of strike slip faulting and possibly thrusting across the iranian plateau which could have been triggered by an increase in elevation furthermore we suggest that 7 6 m y old s directed thrusting predated inception of the westward motion of the south caspian basin

carbonates and oxalates in sediments and landfill monitors of death and decay in natural and artificial systems subaqueous sediments and landfill waste disposal sites are both natural locations for anaerobic microbiological metabolism where sulphates are reduced to sulphide and organic matter particularly acetate is oxidized and degraded to produce ultimately co 2 and ch 4 the microbiologically mediated geochemical processes in each case are similar however natural sedimentary porewaters can be difficult to sample and monitor whereas landfill sites are relatively easily accessible yielding leachate samples that are routinely monitored over periods of time in both environments carbonate precipitation is predicted to be a normal consequence of anaerobic microbiological activity consistent with changes in leachate composition and the predictions of geochemical modelling initial investigation of scale from a landfill drainage system confirms the presence of calcite together with gypsum and iron minerals calcium oxalate minerals whewellite and weddelite are widespread in their occurrence within septarian concretions from a number of geological formations whewellite is reportedly stable up to 160 c the solubility of calcium oxalate is very low and precipitation readily takes place when oxalate oxalic acid is present derived from fungal processes in sediments and soils calcium oxalate minerals should be expected wherever vegetable matter is decomposing this link can be demonstrated for natural sediments but has yet to be proven for landfill

the origin of carbonates in termite mounds of the lubumbashi area d r congo article i nfo the origin of carbonate accumulations in termite mounds is a controversial issue this study is an attempt to elucidate the processes of carbonate precipitation in macrotermes mounds built on ferralsols in upper katanga d r congo whereby a differentiation between pedogenic and inherited carbonates is considered carbonate features were investigated for a 9 m deep termite mound profile and for an 18 m wide cross section through a termite mound and the adjacent soil using field and laboratory techniques field evidence for a pedogenic origin includes morphological type soft powdery materials nodules and coatings on ped surfaces and distribution patterns of the carbonates thin section studies reveal that the carbonates occur predominantly as impregnative orthic nodules and less commonly as coatings both clearly pedogenic calcareous pellets are interpreted as locally reworked pedogenic carbonates x ray diffraction xrd scanning electron microscopy energy dispersive x ray spectrometry sem eds and stable isotope 13 c analyses show that all isolated carbonate features consist of high mg calcite 4 9 12 3 mol mgco3 with 13

mountain building and mantle dynamics 1 mountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen a general dynamic model for this process is still lacking here we propose that mountain belts can be classified between two end members first those of slab pull type where subduction is mainly confined to the upper mantle and rollback trench motion lead to moderately thick crustal stacks such as in the mediterranean second those of slab suction type where whole mantle convection cells conveyor belts lead to the more extreme expressions of orogeny such as the largely thickened crust and high plateaus of present day tibet and the altiplano for the slab suction type deep mantle convection produces the unique conditions to drag plates toward each other irrespective of their nature and other boundary conditions we support this hypothesis by analyzing the orogenic volcanic and convective history associated with the tertiary formation of the andes after 40 ma and himalayas after collision at 55 ma based on mantle circulation modeling and tectonic reconstructions we surmise that the forces necessary to sustain slab suction mountain building in those orogens derive after transient slab ponding from the mantle drag induced upon slab penetration into the lower mantle and from an associated surge of mantle upwelling beneath africa this process started at 65 55 ma for tibet himalaya when the tethyan slab penetrated into the lower mantle and 10 myr later in the andes when the nazca slab did this surge of mantle convection drags plates against each other generating the necessary compressional forces to create and sustain these two orogenic belts if our model is correct the available geological records of orogeny can be used to decipher time dependent mantle convection with implications for the supercontinental cycle

parameters that control lithospheric scale thermal localization on terrestrial planets 1 shear heating induced localization has been suggested as a potentially important mechanism for breaking the lithosphere yet the physical parameters that control the onset of this instability remain unclear we therefore performed systematic 2 d simulations of visco elasto plastic lithospheric deformation in which we addressed the effects of various parameters and we found that a sharp transition exists between localizing and non localizing regimes in a next step we develop a semi analytical model that combines scaling laws with a 1 d lithospheric deformation model we show that the 1 d model successfully predicts the occurrence of shear localization in 2 d models if the thickness of the plastically deforming part of the lower lithosphere is employed as characteristic length scale an application of the 1 d model to the terrestrial planets shows that this type of shear localization is expected to occur most readily on earth

the association of gold with calcrete abstract the discovery that au accumulates in calcrete pedogenic carbonate or caliche was made in 1987 by csiro calcrete is a general term describing accumulation of alkaline earth metals in soils of arid and semi arid terrains around the world the principal constituent of calcrete is calcite while au is a noble metal calcrete has been a significant tool in a number of au deposit discoveries so understanding the mechanisms by which these diametrically different components come together is valuable for enhancing future discovery numerous laboratory experiments case histories and exploration models have been published most from australia yet we do not fully understand the mechanisms involved it is timely therefore twenty five years on since the first publication of this phenomenon to review this highly unusual but economically important association critical to any review on au in calcrete is to first consider calcretes themselves the nature of a particular calcrete where it has formed and mode of formation is relevant to how where and why au accumulates within it this review commences with a background nomenclature history classification and some examples of calcrete types found near au deposits how calcretes form their origins and the role of biota is considered their locations in the regolith and landscape as well as exploration models for au in calcrete are discussed a section on the chemistry of au in calcretes details what we know about possible mechanisms of formation and considers what laboratory experiments on microorganisms and abiotic experiments tell us following on is a summary of practical aspects of identifying collecting and analysing samples for exploration purposes selected mineral exploration case histories are described and how they fit into models of exploration and different regolith settings concluding sections include a summary and implications of this accumulated knowledge to discovering au deposits

detection of active oxalate carbonate pathway ecosystems in the amazon basin global implications of a natural potential c sink abstract the oxalate carbonate pathway ocp is a biogeochemical process which has been described in milicia excelsa tree ecosystems of africa this pathway involves biological and geological parameters at different scales oxalate as a by product of photosynthesis is oxidized by oxalotrophic bacteria leading to a local ph increase and eventually to carbonate accumulation through time in previously acidic and carbonate free tropical soils former studies have shown that this pedogenic process can potentially lead to the formation of an atmospheric carbon sink considering that 80 of plant species are known to produce oxalate it is reasonable to assume that m excelsa is not the only tree that can support ocp ecosystems the search for similar conditions on another continent led us to south america in an amazon forest ecosystem alto beni bolivia this area was chosen because of the absence of local inherited carbonate in the bedrock as well as its expected acidic soil conditions eleven tree species and associated soils were tested positive for the presence of carbonate with a more alkaline soil ph close to the tree than at a distance from it a detailed study of pentaplaris davidsmithii and ceiba speciosa trees showed that oxalotrophy impacted soil ph in a similar way to at african sites at least with 1 ph unit increasing african and south american sites display similar characteristics regarding the mineralogical assemblage associated with the ocp except for the absence of weddellite the amount of carbonate accumulated is 3 to 4 times lower than the values measured in african sites related to m excelsa ecosystems still these secondary carbonates remain critical for the continental carbon cycle as they are unexpected in the acidic context of amazonian soils therefore the present study demonstrates the existence of an active ocp in south america the three critical components of an operating ocp are the presence of i local alkalinization ii carbonate accumulations and iii oxalotrophic bacteria which were identified associated to the oxalogenic tree c speciosa if the question of a potential carbon sink related to oxalotrophic oxalogenic ecosystems in the amazon basin is still pending this study highlights the implication of ocp ecosystems on carbon and calcium biogeochemical coupled cycles as previously mentioned for m excelsa tree ecosystems in africa carbonate accumulations observed in the bolivian tropical forest could be extrapolated to part or the whole amazon basin and might constitute an important reservoir that must be taken into account in the global carbon balance of the tropics

what do we really know about early diagenesis of non marine carbonates abstract non marine carbonate rocks including cave spring stream calcrete and lacustrine palustrine sediments are susceptible to early diagenetic processes these can profoundly alter the carbonate fabric and affect paleoclimatic proxies this review integrates recent insights into diagenesis of non marine carbonates and in particular the variety of early diagenetic processes and presents a conceptual framework to address them with ability to study at smaller and smaller scales down to nanometers one can now observe diagenesis taking place the moment initial precipitates have formed and continuing thereafter diagenesis may affect whole rocks but it typically starts in nano and micro environments the potential for diagenetic alteration depends on the reactivity of the initial precipitate commonly being metastable phases like vaterite ca oxalates hydrous mg carbonates and aragonite with regard to the ambient fluid furthermore organic compounds commonly play a crucial role in hosting these early transformations processes like neomorphism inversion and recrystallization cementation and replacement generally result in an overall coarsening of the fabric and homogenization of the wide range of complex primary microtextures if early diagenetic modifications are completed in a short time span compared to the annual to millennial time scale of interest then recorded paleoenvironmental signals and trends could still acceptably reflect original depositional conditions however even compact non marine carbonate deposits may behave locally and temporarily as open systems to crystal fluid exchange and overprinting of one or more geochemical proxies is not unexpected looking to the future relatively few studies have examined the behaviour of promising geochemical records such as clumped isotope thermometry and non conventional stable isotopes in well constrained diagenetic settings ongoing and future in vitro and in situ experimental approaches will help to investigate and detangle sequences of intermediate diagenetic products processes and controls and to quantify rates of early diagenesis bridging a gap between nanoscale molecular lab studies and the fossil field rock record of non marine carbonates