sedimentology of quaternary perched springline and paludal tufas criteria for recognition with examples from guadalajara province spain outcrop scale geometries and bed relationships of ambient temperature freshwater carbonates are poorly understood because many described tufas have been dismantled by erosion and present only part of a particular depositional model at the field scale four end member models encompass the tufa continuum 1 perched springline 2 paludal 3 fluvial and 4 lacustrine tufas individual bed types can occur with variable dominance within several of these models but one or more beds are characteristically dominant only within a single tufa model so it can be differentiated from relatively isolated outcrop fragments two models perched springline tufas and paludal tufas are known in outline only in the literature despite being present within the quaternary deposits of most karstic regions perched springline tufas generally form lobate fan shaped mound morphologies on hillslopes and develop from single or multiple spring resurgences mature deposits show a subhorizontal top and a steep face on the downflow side the steep outer zones of tufa mounds may be developed into cascades with moss curtains or can be dominated by shallow rimstone pools according to face angle tufa deposits lying downslope of the mounds are usually detrital in nature especially if some dismantling of the mound has occurred the relatively thin subhorizontal lobe top deposits commonly contain organic rich deposits paludal tufas develop predominantly in waterlogged valley bottom situations where line sourced waters emerge from valley side and bottom aquifers lime mud precipitation predominates in these sites mud is deposited as subhorizontal laminites that thin towards the valley axis and downstream of resurgences tufa spring mounds may form where lesser volumes of water are involved individual tussocks phytoherm cushions of grasses and rushes are the most diagnostic feature of the model but sapropels and peats may be intercalated diagenesis in both models is rapid lithification of individual beds is virtually instantaneous and always occurs before the decay of the associated living vegetation the resulting highly porous and permeable fabrics remain fresh in holocene tufas but aggrading neomorphism and partial spar infill of vegetation moulds is common in older deposits dissolution in many perched springline tufas is small scale many large cavities are primary but with later coatings of speleothems early removal of organics from paludal tufas is responsible for autobrecciation and differential compaction

new stratigraphic and geochemical data on the holocene environment and climate from a tufa deposit at direndall mamer valley luxembourg tufa deposits are proven to record high quality palaeoenvironmental and palaeoclimatic information they also appear to be influenced by human activities as indicated by the late holocene tufa decline observed in europe from the atlantic optimum when tufa should be well developed regarding climatic conditions the studied tufa comes from the mamer valley in luxembourg direndall as it covers a broad part of the holocene this deposit offered a great opportunity to discuss modalities and timing of the tufa decline and the human impact on fluvial formations since the early holocene recent fieldwork at direndall allowed the collection of new stratigraphic data and samples for a geochemical study we investigate calcite stable isotopes as records of past temperature and humidity conditions comparisons with the environmental reconstructions from a previous malacological study show strong consistency with the 13c record and suggest humidity might have been the main factor controlling environment evolutio

the rotation period direction of the north pole and geodetic control network of venus before the magellan mission the best estimates of the rotation rate and pole direction of venus were derived from earth based radar measurements a new determination of these rotational parameters has now been made from an analysis of magellan radar images control points were selected from the north polar region and measured on full resolution radar strips the measurements were entered into a least squares adjustment to solve for the pole direction and rotation rate of venus as well as the coordinates of the control points themselves the current data set contains 3893 measurements of 571 points on 560 radar strips spacecraft ephemeris errors dominate the measurement errors one technique used to remove ephemeris errors is to adjust the averaged orbital inclination and argument of periapsis for each orbit in a more precise technique used for selected blocks of orbits an improved spacecraft ephemeris is computed by optimally fitting measurements of additional points at all latitudes of the radar strips together with earth based spacecraft radiometric tracking measurements in a separate experiment measurements were made of a few points common to both venera 15 16 and magellan images the long time baseline between the images should have led to an accurate determination of the rotation period and pole direction of venus however the measurement residuals were unexpectedly large and these solutions are not currently considered reliable a rotation period estimate of 243 0185 0 0001 days was determined via the ephemeris improvement technique applied simultaneously to two overlapping orbit blocks with many common points and separated by two full venus rotations using this period value the control network computations estimated the north pole direction as right ascenson 272 76 0 02 and declination 67 16 0 01 in the j2000 frame

rift flank uplift at the gulf of california no requirement for asthenospheric upwelling continental rifts are commonly flanked by zones of high elevation but the cause of uplift remains controversial proposed uplift mechanisms include active and induced asthenospheric upwelling and isostatically driven lithospheric flexure discrimination between these hypotheses requires close constraint of the timing of rift flank uplift and crustal extension here we focus on the well preserved neogene gulf of california rift the western rift margin is characterized by a prominent east facing kilometer scale escarpment which bounds a west tilted topographically asymmetric rift flank we exploit west draining canyons incised into the rift flank to constrain the timing of uplift to between ca 5 6 and 3 2 ma using 40ar 39ar dating of lavas which show cut and fill relationships to the canyons rift flank uplift closely followed the onset of slip on the principal fault of the loreto rift segment at ca 8 6 ma the age of which we obtain from apatite u th he and fission track thermochronologic analysis of rift escarpment exhumation uplift was therefore coeval with lithospheric rupture and the onset of oceanic spreading between ca 6 and 3 ma but post dates a proposed asthenospheric upwelling event by 8 10 ma the timing of uplift is inconsistent with either active or induced upwelling as uplift mechanisms and we conclude that rift flank uplift was driven by the flexural response to lithospheric unloading

large topographic rises on venus implications for mantle upwelling topographic rises on venus have been identified that are interpreted to be the surface manifestation of mantle upwellings these features are classified into groups based on their dominant morphology atla and beta regiones are classified as rift dominated dione western eistla bell and imdr regiones as volcano dominated and themis eastern eistla and central eistla regiones as corona dominated at several topographic rises geologic indicators were identified that may provide evidence of uplifted topography e g volcanic flow features trending upslope we assessed the minimum contribution of volcanic construction to the topography of each rise which in general represents less than 5 of the volume of the rise similar to the volumes of edifices at terrestrial hotspot swells the total melt volume at each rise is approximated to be 104 106 km3 the variations in morphology topography and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling instead the morphologic variations between the three classes of topographic rises are interpreted to indicate the varying influences of lithospheric structure plume characteristics and regional tectonic environment within each class variations in topography gravity and amount of volcanism may be indicative of differing stages of evolution the similarity between swell and volcanic volumes for terrestrial and venusian hotspots implies comparable time integrated plume strengths for individual upwellings on the two planets

magellan mission summary magellan started mapping the planet venus on september 15 1990 and after one cycle one venus day or 243 earth days had mapped 84 of the planet s surface this returned an image data volume greater than all past planetary missions combined spacecraft problems were experienced in flight changes in operational procedures and reprogramming of onboard computers minimized the amount of mapping data lost magellan data processing is the largest planetary image processing challenge to date compilation of global maps of tectonic and volcanic features as well as impact craters and related phenomena and surface processes related to wind weathering and mass wasting has begun the magellan project is now in an extended mission phase with plans for additional cycles out to 1995 the magellan project will fill in mapping gaps obtain a global gravity data set between mid september 1992 and may 1993 acquire images at different view angles and look for changes on the surface from one cycle to another caused by surface activity such as volcanism faulting or wind activity

how does venus lose heat the tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior on the earth about 70 of the heat flow through the mantle is attributed to the subduction of cold lithosphere in order to understand the tectonic and volcanic processes on venus it is necessary to understand how heat is transported through its mantle in this paper three alternative end member hypotheses are considered the first is the steady state loss of heat through the mantle to the surface in analogy to the earth however without plate tectonics and subduction on venus a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination the required plume flux would be equivalent to about 80 plumes with the strength of the hawaiian plume the required delamination flux implies a 50 delamination of the entire venus lithosphere every 10 m y neither appears possible so that it is concluded that venus cannot transport heat through its mantle to its surface on a steady state basis the second hypothesis is that there has been a strong upward concentration of the heat producing elements into the crust of venus the heat generated is then lost by conduction surface measurements of the concentrations of the heat producing elements place constraints on this model if everything is favorable this hypothesis might be marginally acceptable but it is considered to be highly unlikely the third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence the near random distribution of craters suggests that the last subduction event occurred about 500 ma this model implies a thick thermal lithosphere 300 km at the present time which is consistent with a variety of surface observations lava lakes on the earth are considered as analogies to plate tectonics they also exhibit episodic subduction events

oxygen and carbon isotopes in modern fresh water ostracod valves assessing vital offsets and autecological effects of interest for palaeoclimate studies abstract the isotopic composition of modern benthic ostracod and mollusc carbonate the oxygen isotope composition of lake water 18ow the carbon isotope composition of dissolved inorganic carbon 13cdic and water temperatures were monitored during a year at different water depths in two lakes ammersee and starnberger see southern germany the aim was to test whether the valves are built in isotopic equilibrium and how seasonal water temperatures are recorded in the shells we find systematic offsets between 18o of valve carbonate compared to 18o expected for a theoretical calcite formed in isotopic equilibrium offsets are higher than 2 for different species of the candona group 1 5 for cytherissa lacustris and around 0 8 for limnocythere inopinata and darwinula stevensoni they seem to be constant for all instars of a species and temperature independent mean water temperatures of the calcification period are readily reflected in the valve 18o if corrected for the offsets 13c of valve carbonate is mainly controlled by 13c of the dissolved carbonate in ambient lake water but seems to be biased by micro habitat effects and vital offsets the study provides information on the seasonality of valve formation thus potentially allowing the quantification of temporal and spatial water temperature gradients from specific 18o records

climatic variations in mis 11 recorded by stable isotopes and trace elements in a french tufa la celle seine valley marine isotope stage mis 11 palaeoclimate has so far been documented in marine and ice sheet isotopic records however excepting some lacustrine pollen records very little is known about palaeoclimatic conditions in continental areas this study uses geochemical records in calcareous tufa deposits from rivers as a basis for reconstructing temperate palaeoclimatic conditions tufa deposits are now proven to record high quality palaeoclimatic information in recent to holocene deposits work on older interglacial tufas is just starting and in this paper we present the first comprehensive results from a mis 11 tufa the tufa comes from the seine valley la celle northern france geochemical data in the tufa calcite are interpreted to record primarily air temperature do and humidity dc and mg ca and sr ca the combined data identify a warm and wet climatic optimum followed by two temperature decreases associated with oscillations in humidity these marked climatic variations recorded through the la celle profile are strongly coherent with the palaeoenvironmental reconstructions from malacological data the abrupt climatic and environmental events recorded could be related to short term degradation of vegetation cover in europe which is itself controlled by global palaeoclimatic events

new evidence for complex climate change in mis 11 from hoxne suffolk uk the climatic signal of marine isotope stage mis 11 is well documented in marine and ice sheet isotopic records and is known to comprise at least two major warm episodes with an intervening cool phase terrestrial records of mis 11 though of high resolution are often fragmentary and their chronology is poorly constrained however some notable exceptions include sequences from the maar lakes in france and tenaghi philippon in greece in the uk the hoxnian interglacial has been considered to correlate with mis 11 new investigations at hoxne suffolk provide an opportunity to re evaluate the terrestrial record of mis 11 at hoxne the type hoxnian interglacial sediments are overlain by a post hoxnian cold temperate sequence the interglacial sediments and the later temperate phase are separated by the so called arctic bed from which cold climate macroscopic plant and beetle remains have been recovered the later temperate phase was deposited during an episode of boreal woodland and is associated with the artefacts a diverse vertebrate fauna and molluscs new amino acid geochronological data and biostratigraphical considerations suggest that the post hoxnian sequence correlates with late substages of mis 11 the paper further investigates the correlation of the sequence at hoxne with the palynological sequences found elsewhere in europe and adjacent offshore areas

hydrological factors in the interpretation of stable isotopic proxy data present and past a european perspective in the context of this paper stable isotopic proxies are those preserving directly or via known or calculable fractionation factors a record of 18o 16o and or 2h 1h in the water which contributed to their growth suitable proxies include amongst others tree ring cellulose bone phosphate lake sediment minerals and speleothem fluid inclusions a principal aim of studying isotopic proxies is to gain more information about past climates through a knowledge of how rainfall isotope compositions have changed over time however converting back from proxy to rainfall is not necessarily straightforward because however well the isotopic fractionation due to proxy growth or formation is understood a variety of modifying processes may have intervened between this and the actual rainfall events three main reservoirs of environmental water may contribute to the isotopic compositions of terrestrial proxy indicators surface waters soil moisture and groundwater each of these can in principle be isotopically modified compared to rainfall in the locality this review examines these effects and how they may change under different european climate types at the present day it is concluded that while groundwater isotopic compositions typically represent closely the bulk composition of rainfall under a variety of climatic regimes soil and particularly lake waters require detailed study if their isotope systematics are to be adequately understood the review also finds that to a large extent the present is the key to the past in terms of recharge processes during the late pleistocene though glaciation may have had profound isotopic consequences in certain areas while a more general aridity led to the apparent absence of recharge in most areas for much of the late devensian

the global distribution of giant radiating dike swarms on venus implications for the global stress state magellan radar data of venus reveal 163 large radial lineament systems composed of graben fissure and fracture elements on the basis of their structure plan view geometry and volcanic associations at least 72 are interpreted to have formed primarily through subsurface dike swarm eraplacement the remainder through uplift or a combination of these two mechanisms the population of swarms is used to determine regional and global stress orientation the stress configura tion recorded from 330 210oe aphrodite terra is best ex plained by isostatic compensation of existing long wave length topography or coupling between mantle flow and the lithosphere the rest are correlated with concentrations of rifting and volcanism in the beta atla themis region the global stress field on venus is different than that of earth where plate boundary forces dominate from their surroundings where domical topography occurs the ra dius of the associated lineament system is on average 2 5 times greater than that of the dome some 53 of the radiat ing systems are associated with concentric structures ranging from central depression bounding scarps 25 km wide to tec tonic rings 575 km in diameter in 51 of these cases the ra dial pattern originates within but also extends significantly beyond the annulus only 9 of the radiating systems are confined completely leaving 40 located outside but focused upon a central annular structure finally the radial geometry is usually quite pronounced near the center of each structure 52 of the time radial lineaments occupy 270 o of azimuth and 80 exceed 180 o figure 2c in their distal regions how ever only 72 of the radiating systems retain a purely radial geometry as the remaining 28 gradually develop a non radial unidirectional lineament configuration all but seven of the radiating systems exhibit volcanism figure 2d lobate flows emerging from individual linea ments or many such flows strongly correlated with multiple radial lineaments occur for 45 of the systems clusters of small shields occur 72 of the time and in 75 of these in stances distinctive alignment of small shields and or pits along radiating lineaments is seen finally 65 are associ ated with some other form of centralized volcanism including multiple forms of edifice construction limited extrusions and sheet like flows which extend into the surrounding terrain

lessons from venus for understanding mantle plumes on earth abstract mantle plumes are important in the magmatic and tectonic history for both earth and venus the expression of plumes is distinctive on venus and complementary to that on earth therefore a cross comparison is useful for better understanding plume magmatism on both planets in contrast to the earth venus has no observed record of plate tectonics a low degree of surface erosion and an apparently short duration for the formation of the present planetary surface the absence of plate tectonics indicates that all magmatism is intraplate and is generated beneath a stagnant lithospheric lid a low degree of surface erosion preserves the surface structures and short wavelength topography the short duration of preserved magmatic activity suggests a global resurfacing event magmatic elements include a individual volcanoes with diameters ranging up to 1000 km which represent hotspots b annular structures termed coronae with diameters averaging 300 km but ranging up to 2600 km and which appear to lack terrestrial i e earth analogues c radiating graben fissure systems extending up to 2000 km in radius some of which are purely uplift related while others mark the plumbing system dyke swarms of volcanic systems d lava flow fields of scale comparable to terrestrial flood basalts large igneous provinces lips and e regions of small shield volcanoes representing shallow source melting there are several hierarchies of magmatic events on venus ordered in terms of increasing scale and significance 1 isolated coronae volcanoes flow fields and radiating graben systems 2a individual and small clusters of volcanoes and coronae associated with topographic swells geoid highs and triple junction rifting these are most clearly indicative of terrestrial type plumes originating from the deep mantle 2b coronae distributed along rifts chasmata these are the clearest examples of melt generation associated with rifting 3 regional concentration of activity in the beta atla themis bat region this is the closest example of a plume cluster event sometimes termed a superplume event and 4 global volcanic resurfacing of the volcanic plains no terrestrial analogue is confirmed although the global burst of terrestrial plume activity in the neoarchean is a possible analogue

does climate control the morphological fabric of freshwater carbonates a comparative study of holocene barrage tufas from spain and britain abstract calcium carbonate production in most freshwater tufa systems is controlled by a combination of physico chemical precipitation and biomediation associated with procaryote microphyte biofilms inorganic precipitation is dominant under turbulent flow regimes whereas biomediation necessitates sluggish flow or static water we show here that temperature is also likely to affect tufa formation rates and architecture leading to the development of a clearly definable range of fabrics two contrasting models are proposed warm semi arid tufas well seen in the ruidera pools national park albacete province central spain where tufa barrages consist of vertical narrow rimmed arcuate structures frequently these are bounded both upstream and downstream by deep lakes laterally accreting mammilate stromatolites dominate the upper two thirds of lake margin subaqueous overhangs whereas conical growth forms dominate shady basal parts of subaqueous overhangs dense reed stands grow in thin peats on top of the vertical lake margin walls resistivity survey and percussion augering show up to 20 m of earlier lake fill which is banded into dark and pale beds the upper 10 m accumulated in 4600 years there is little or no preservation of organic material within these deposits cool temperate tufas typified by slowly accumulating barrage tufas at caerwys n wales and alport n derbyshire uk transverse barriers across valleys are dominated by arcuate buttress developments built up of downstream facing tabular carbonate sheets pools upstream of the barriers are shallow dominated by clotted lime mud and organic sapropels in their depocentres and stromatolite dominated laminar to low domes or oncoidal facies in broad marginal zones marginal stands of semi aquatic vegetation are rooted in thick peats but frequently are also encrusted around their exposed stems by oncoidal carbonate fabric preservation is good for the carbonate material however marginal peats rapidly oxidise and stem encrustations collapse to produce cylindrical oncoid beds depocentre sapropels are always preserved and contain important pollen records otherwise rare in carbonate regions carbonates in both models accumulate relatively rapidly they have a moderate preservation potential within late cenozoic terrestrial successions the distinctly different characteristics of their preserved fabrics should allow recognition of two contrasting climatic models for fluvial barrage tufas in ancient deposits

radiating graben fissure systems in the ulfrun regio area venus abstract radiating graben fissure systems are common on venus most are thought to be underlain by mafic dykes fed by centrally located magmatic centres from previous work it has been shown that these magmatic plumbing systems can extend out up to 2000 km or more and that interaction between neighbouring systems can provide insight into the relative chronology of their magmatic centres systematic mapping of graben fissure systems has potential as a tool for regional magmatic chronology and correlation on venus this methodology is applied to the ulfrun regio area 200 240 e 0 25 n where we mapped 47 000 graben and fissures from these 66 radiating systems comprised of 13 000 individual graben and fissures and having radii of up to 2000 km have been identified and are interpreted to be underlain by dyke swarms focussed on magmatic centres cross cutting relationships among these systems and with the hecate chasma rift zone have been examined to provide a relative chronology for the magmatic centres two trends emerged a an apparent younging from the southwest to northeast of the study area and b a cluster of older ages in the southwest linked to the atla regio mantle plume

sedimentary patterns in perched spring travertines near granada spain as indicators of the paleohydrological and paleoclimatological evolution of a karst massif perched spring travertines of the granada basin south spain constitute a perched system with four well defined steps which are formed by several facies associations deposited in different sub environments travertine pools dams and cascades these perched travertines are considered as a freshwater reef system with a facies zonation and stratigraphic architecture closely resembling that of marine reef terraces and prograding carbonate platforms the travertine deposits have been dated by 230 th 234 u and 14 c methods as in other mediterranean areas the travertine deposition occurred episodically during warm and wet interglacial periods coinciding with isotopic stages 9 7 and 5 and with the transition between isotopic stages 2 1 during these periods underground dissolution large outflow in the springs and subsequent calcium carbonate precipitation occurred in the same way that evolution of reef systems indicates sea level changes the geomorphology age and architecture of perched spring travertine systems may be used to interpret former climatically controlled changes in outflow in base level marked by the altitude of springs and in the chemistry of spring waters thus aggradation or climbing progradation may indicate an increase of outflow at the spring progradation with toplap is due to a stable base level and conversely dowlapping progradation may signify that the base level was gradually dropping therefore the travertines can be considered semiquantitative indicators of the paleohydrological evolution of karstic massifs and used as an important terrestrial proxy climate record d 2003 elsevier science b v all rights reserved

regional topographic rises on venus geology of western eistla regio and comparison to beta regio and atla regio regional topographic rises in the equatorial region of venus western eistla regio beta regio and atla regio form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes the geologically least complex of these regions is western eistla regio an elongated domical rise that contains the volcanoes sif and gula montes sif mons is a large volcano diameter 100 km whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing widespread outpouring of lava additional fracturing along its northern flank and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion sif contains a 40 km diameter summit caldera within which are nested smaller calderas and pit structures theoretical considerations suggest multiple levels of magma emplacement within the volcano with the primary reservoir residing in the crust below the edifice gula mons is made up of multiple volcanic source regions and is associated with guor linea a zone of lithospheric stretching faulting and rifting the geologically complex highland of beta regio is a tectonic junction with three arms of devana chasma intersecting at theia mons magellan data reveal 1 the details of rifting and volcanism concentrated at theia mons 2 that rhea mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question 3 that tesserae are a major unit arrayed in a pattern around theia from east to west starting on the eastern flank of beta and continuing westward to the northern part of asteria regio 4 that the northern periphery of the highland contains coronae connected by fractures forming chains and 5 the presence of a deformed crater within devana chasma split and extended in an east west direction indicating that part of the rift has undergone as much as 20 30 extension like beta atla regio is a complex area of converging rifts five separate arms that are centered on the volcano ozza mons the magellan images show that volcanism and rifting have occurred contemporaneously at ozza with lava flows both covering and being crosscut by faults a second volcano maat mons contributes to the infilling of the rift dali chasma and does not appear to be deformed by faulting two of the rifts at atla contain chains of coronae suggesting that upwelling along linear zones of extension has occurred beta atla and western eistla are similar in that they are broad regional rises are associated with major rift zones have large apparent depths of isostatic compensation and contain large volcanoes their associations with features mapped as coronae and tesserae are quite variable suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment

holocene palaeoenvironments from the direndall tufa luxembourg reconstructed from the molluscan succession and stable isotope records this paper reports the results of new malacological analyses from a thick tufa sequence at direndall luxembourg the study is temporally contextualised with radiocarbon dates and an age depth mod

stable isotope record of palaeoclimatic change in a british holocene tufa a 9 5 m long continuous core through holocene tufa deposits in derbyshire uk was analysed for oxygen and carbon stable isotope variations with a view to palaeoclimatic and palaeoenvir onmental interpretation the depositional environment of the tufa is well understood and dated using 14c and biostratigraphy it is argued that smooth variations of up to 1 in 18o of the tufa calcites are caused by changes in the isotopic composition of regional rainfall which in turn were caused mainly by changes in air temperature palaeoclimate in general lower 13c values for calcite correspond with higher 18o values suggesting a stronger component of soil derived carbon during warmer climatic phases a stable isotope approach to reading terrestrial palaeoclimatic records is of widespread application to holocene and older tufa deposits which are common in carbonate bedrock regions

tufas and travertines of the mediterranean region a testing ground for freshwater carbonate concepts and developments actively forming travertine and tufa sites are widespread in temperate and warm climates such as the mediterranean region but also develop in other climatic regimes from the arctic to equatorial latitudes the terms travertine and tufa are sometimes applied indiscriminately today however each has a specific definition most studied deposits are active or of quaternary especially holocene age there are fewer tertiary and even less late mesozoic examples on record in part this is because of a recognition problem as these deposits commonly preserve as fragmentary erosional remnants physico chemical precipitation of calcium carbonate is commonly considered to be the main mechanism contributing to tufa and travertine deposition rapid cooling of waters further aids the process in thermal travertine sites however biomediation of calcium carbonate associated with prokaryote microphyte biofilms is now recognized to be equally important in slow flowing and near static sites much of this precipitation occurs in close association with cyanobacterial heterotrophic bacterial and diatoms the mediterranean region figures significantly in the development of travertine and tufa research consequently this article employs key literature on mediterranean examples to illustrate developments and concepts in freshwater carbonate research modern work started in the 1950s and defined many previously unknown outcrops many such sites became the targets for early radiocarbon dating studies since the late 1970s a number of laboratory based and field based classification schemes have been proposed some schemes were based on associated macrovegetation types others were based on petrological characteristics or on geometry and sedimentary facies characteristics of late the sedimentological modelling has benefited greatly from the use of shallow geophysical techniques in order to better define the internal geometries of the deposits two distinct research strands were developed during the 1990s both aimed at a better understanding of environmental change the first considered biocomponents the research was multifaceted and ranged from exploring the role of micro organisms in cement and lime mud precipitation to biostratigraphic analysis of contained pollen and invertebrate remains these provided proxy environmental information which has helped to define past climate variability in the region the second research strand focussed on geochemical signatures as a proxy tool for defining environmental change this definition was made possible by new petrological studies which have greatly aided the recognition of primary depositional fabrics this improved knowledge has reduced the sampling dangers of diagenetic contamination inherent in earlier geochemical studies lithofacies modelling has also brought to light important early post depositional neomorphic changes which obscure primary fabrics and complex diagenetic histories have been revealed stable isotope signatures of carbon and oxygen have also been used to define palaeo water temperature fine tuning of depositional time frames has also been achieved from absolute dating techniques employing radiocarbon and uranium thorium

controls on trace element sr mg compositions of carbonate cave waters implications for speleothem climatic records abstract at two caves clamouse s france and ernesto ne italy cave drip and pool waters were collected and sampled at intervals over a 2 3 year period mg ca and sr ca concentration ratios corrected for marine aerosols are compared with those of bedrocks and in some cases aqueous leachates of soils and weathered bedrocks cave waters do not lie along mixing lines between calcite and dolomite of bedrock carbonate but typically show enhanced and covarying mg ca and sr ca four factors are considered as controlling processes 1 the much faster dissolution rate of calcite than dolomite allows for the possibility of increase of mg ca if water rock contact times are increased during drier conditions a theoretical model is shown to be comparable to experimental leachates 2 prior calcite precipitation along a flow path is a powerful mechanism for generating enhanced and covarying mg ca and sr ca ratios this mechanism requires the solution to lose co 2 into pores or caverns 3 incongruent dolomite dissolution has only limited potential and is best regarded as two separate processes of dolomite dissolution and calcite precipitation 4 selective leaching of mg and sr with respect to ca is shown to be important in leachates from ernesto where it appears to be a phenomenon of calcite dissolution in general selective leaching can occur whenever ca is sequestered into precipitates due to freezing or drying of soils or if there is derivation of excess sr and mg from non carbonate species the ernesto cave has abundant water supply which in the main chamber is derived from a reservoir with year round constant p co 2 of around 10 2 4 and no evidence of calcite precipitation in the karst above the cave two distinct but overlying trends of enhanced and covarying mg ca and sr ca away from the locus of bedrock compositions are due to calcite precipitation within the cave and at a variable drip site due to enhanced selective leaching at slow drip rates mg enhancement in the first chamber is due to a more dolomitic bedrock and longer residence times the clamouse site has a less abundant water supply and presents geochemical evidence of prior calcite precipitation both in the cave and in overlying porous dolomite dedolomitized limestone bedrock initial p co 2 values as high as 10 1 are inferred experimental incubations of clamouse soils which generated enhanced p co 2 and precipitated caco 3 had compositions similar to the karst waters calcite precipitation is inferred to be enhanced in drier conditions hydrological controls on cave water chemistry imply that the trace element chemistry of speleothems may be interpretable in palaeohydrological terms drier conditions tends to promote not only longer mean residence times enhancing dolomite dissolution and hence mg ca but also enhances degassing and calcite precipitation leading to increased mg ca and sr ca

evidence for triple junction rifting focussed on local magmatic centres along parga chasma venus abstract parga chasma is a discontinuous rift system marking the southern boundary of the beta atla themis bat region on venus along a 1500 km section of parga chasma detailed mapping of magellan synthetic aperture radar images has revealed 5 coronae 11 local rift zones distinct from a regional extension pattern and 47 graben fissure systems with radiating 28 linear 12 and circumferential 7 geometries the magmatic centres of these graben fissure systems typically coincide with coronae or large volcanoes although a few lack any central magmatic or tectonic feature i e are cryptic some of the magmatic centres are interpreted as the foci of triple junction rifting that form the 11 local rift zones cross cutting relationships between graben fissure systems and local rift faults reveal synchronous formation implying a genetic association additionally cross cutting relationships show that local rifting events postdate the regional extension along parga chasma further indicating multiple stages of rifting evidence for multiple centres of younger magmatism and local rifting against a background of regional extension provides an explanation for the discontinuous morphology of parga chasma examination of the atlantic rift system prior to ocean opening on earth provides an analogue to the rift morphologies observed on venus

graben fissure systems in guinevere planitia and beta regio 264 312 e 24 60 n venus and implications for regional stratigraphy and mantle plumes detailed mapping in a 14 000 000 km 2 area of northwestern guinevere planitia and northern beta regio bounded by 264 312 e 24 60 n has revealed thousands of long extensional lineaments graben fissures and related fractures these can be grouped into radiating circumferential and linear systems thirty four radiating systems have been identified of which 16 have radii greater than 300 km and eight have radii greater than 1000 km twenty six linear straight systems with a length greater than 300 km have been distinguished of which six have a length greater than 1000 km linear systems are generally associated with rifts although some may represent distal portions of radiating systems in addition 19 circumferential systems some associated with coronae have been identified the distribution of each system is compared with the host geology in order to place the graben fissure systems in a regional stratigraphic framework the majority of systems are 1 younger than tesserae ridge belts and densely fractured plains 2 coeval with and in many cases define fracture belts 3 partially flooded by wrinkle ridged plains units and 4 older than smooth and lobate plains units and young rifts the inventory of radiating graben fissure systems that we catalogue represents a database of tectono magmatic centers that complements the centers defined using other criteria e g large volcanoes coronae and shield fields we have attempted to identify those systems that are underlain by dike swarms in order to evaluate their relationship to mantle plumes at least 11 of the radiating systems extend well beyond any central topographic uplift and are therefore interpreted to be underlain by dike swarms

the geomorphology and evolution of hecate chasma venus abstract hecate chasma is a discontinuous trough and fracture system extending from atla regio through asteria regio a detailed examination of hecate chasma using magellan image and altimetry data reveals morphologic features such as normal faults including graben compressional ridges a variety of volcanic edifices and coronae there is no evidence for a systematic age progression among coronae in chains thus a single stationary thermal anomaly underlying a moving lithosphere is an unlikely origin for the chains a comparison of the observed features with two possible models of evolution subduction delamination or lithospheric extension with limited rifting shows some similarities between the predicted morphologies of each model and hecate chasma the dominance of features related to upwelling and extensional tectonism strongly favor an extensional origin for this zone the corona chain may have been formed by upwellings related to rayleigh taylor instabilities similar to those observed at terrestrial mid ocean ridges but in an environment of limited spreading

topographic analysis of devana chasma venus implications for rift system segmentation and propagation abstract devana chasma is a rift system on venus formed in association with the beta regio and phoebe regio volcanic highlands which are interpreted as mantle plumes we present a new analysis of a 2500 km long segment of devana based on the rift topography the horizontal extension across the rift boundary faults is 3 9 km this is a lower bound on the total rift extension because the altimetry does not resolve the topographic relief across the numerous faults that are visible in radar images of the rift floor the total extension across devana is approximately 20 km similar in magnitude to continental rift systems on earth rift flank elevations are up to 3 1 km in the regions nearest the mantle plumes and decay strongly with increasing distance from the plumes indicating a strong thermal component to the rift flank topography unlike the situation usually reported for terrestrial rifts as on earth there is also a flexural uplift component to the flank topography rift depths are up to 2 5 km below the surrounding plains with considerable along strike variability there is a 600 km lateral offset along devana chasma near the mid point between the two mantle plumes devana most likely formed as two distinct rifts due to the horizontal stresses created by outflow from the upwelling plumes the offset zone formed as a result of the interaction between the two rift tips which requires that upwelling at the two mantle plumes overlapped in time

the late holocene tufa decline in europe over much of europe it has been postulated that in the late holocene since c 2500 bp there was a marked decline in the deposition of tufa there has been considerable debate about the causes of this phenomenon with some authors postulating the importance of natural climatic changes and others asserting that miscellaneous human influences have been crucial this article discusses the role that various changes both natural and anthropogenic may have played and suggests ways in which the large number of possible hypotheses can be tested

petrography and environmental interpretation of tufa mounds and carbonate beds in the jurassic navajo sandstone of southeastern utah u s a abstract carbonate deposits in the jurassic navajo sandstone of utah reflect deposition in interdune lakes and springs interdune lake deposits consist of flat lying carbonate units springs formed tufa mounds that are interpreted as subaerial ambient temperature artesian spring deposits in the first systematic study of the petrography of the carbonate deposits in the navajo sandstone eleven facies were identified in several flat lying carbonate deposits and two tufa mounds fenestral mudstone and peloidal facies dominate the lacustrine deposits whereas thrombolitic mudstone characterizes the mounds the biota consists of ostracodes charophytes fish mollusks a possible freshwater sponge trace fossils and fragments of vascular plants features resulting from penecontemporaneous weathering provide evidence of episodic exposure of the lacustrine carbonate beds during deposition and formation of the tufa mounds under subaerial conditions although carbonate deposits are not rare in eolian systems few have been studied in detail comparisons between these and the navajo sandstone carbonate deposits reveal some characteristics that may be unique to the latter

molluscan successions from the holocene tufa of st germain le vasson normandy france and their biogeographical significance a detailed molluscan succession from a 10 m thick deposit of holocene tufa at st germain le vasson normandy provides the most complete record from northern france and has shed new light on the historical biogeography of several species of land snail the succession has been reconstructed from four profiles and a chronology provided by accelerator mass spectrometry radiocarbon dating of charcoal wood and shell the onset of tufa formation occurred after 9700 90 yr bp and persisted until 4213 77 yr bp the tufa appears to have accumulated at a remarkably constant rate 14 4 16 5 cm 100 yr 1 except for the upper levels where the rate increases fourfold the succession has been divided into six local molluscan zones an early assemblage consisting of ecologically tolerant species and those indicative of marshy grassland is replaced by a sequence of shade demanding taxa reflecting the encroachment of woodland shaded conditions persist until the end of the sequence but the most hygrophilous elements decline after 5422 60 yr bp zone 5 a change also reflected in the tufa lithology by the development of silty grey horizons notable species recovered from the tufa include acicula fusca vertigo substriata v alpestris v moulinsiana all rare or unknown living in northern france hygromia limbata a twentieth century introduction to britain was previously thought to be a relatively recent arrival in northern france but its record at st germain shows that it has been present in normandy since 6500 yr bp azeca goodalli another shade demanding species appeared at st germain much later just after 4420 65 yr bp several other species present in the tufa such as pomatias elegans no longer live on the site adding to the evidence for a distributional decline in normandy and elsewhere perhaps the most noteworthy record is that of leiostyla anglica between about 8500 yr bp and 5000 yr bp because this constitutes its only holocene occurrence from mainland europe following this discovery it seems likely that further holocene sites with l anglica may be found along the western seaboard of europe connecting populations in iberia with those of the british isles copyright 2004 john wiley sons ltd

global tectonic evolution of venus from exogenic to endogenic over time and implications for early earth processes venus provides a rich arena in which to stretch one s tectonic imagination with respect to non plate tectonic processes of heat transfer on an earth like planet venus is similar to earth in density size inferred composition and heat budget however venus lack of plate tectonics and terrestrial surficial processes results in the preservation of a unique surface geologic record of non plate tectonomagmatic processes in this paper i explore three global tectonic domains that represent changes in global conditions and tectonic regimes through time divided respectively into temporal eras impactors played a prominent role in the ancient era characterized by thin global lithosphere the artemis superstructure era highlights sublithospheric flow processes related to a uniquely large super plume the fracture zone complex era marked by broad zones of tectonomagmatic activity witnessed coupled spreading and underthrusting since arrested these three tectonic regimes provide possible analogue models for terrestrial archaean craton formation continent formation without plate tectonics and mechanisms underlying the emergence of plate tectonics a bolide impact model for craton formation addresses the apparent paradox of both undepleted mantle and growth of archaean crust and recycling of significant archaean crust to the mantle this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

stable isotopic compositions of recent freshwater cyanobacterial carbonates from the british isles local and regional environmental controls recent 50 years old freshwater cyanobacterial carbonates from diverse environments streams lakes waterfalls throughout britain and ireland were analysed for their stable carbon and oxygen isotope compositions the mean 18o value of 5 9 pdb for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central britain 7 5 smow assuming a mean water temperature of 9 c the mean 18o of lake data 4 5 pdb is statistically different reflecting the effects of residence time and or variations in the oxygen isotopic composition of rainfall carbon isotopes have wide variations in both fluviatile and lake data sets 3 to 12 pdb these variations are principally controlled in the fluviatile samples by contribution of isotopically light soil zone carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution lake samples have the heaviest carbon isotope values reflecting a trend toward isotopic equilibrium between atmospheric co2 and aqueous hco 3 we infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information although the effects of stabilization and diagenesis on primary 18o values will need careful consideration primary carbon isotope compositions should be well preserved although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate

classification and environmental models of cool freshwater tufas abstract tufa deposits are common in the quaternary record facies developments are dependent upon localised carbonate precipitation generally these take the form of phytoherm build ups together with associated detrital deposits derived from disagragation of the former phytoherm cements differ from speleothems and floe calcite accumulations in being biologically mediated or modified this paper recognises the following five types perched spring line cascade fluviatile paludal and lacustrine facies each type is composed of distinctive constructional and detrital sediment associations these unique sediment combinations are the product of an interplay between angle of slope water discharge rate standing water bodies phytal and microbial colonization and ambient temperature the proposed models will be of value in the interpretation of ancient and partially exposed tufas

pleistocene fluvial terraces from northern france seine yonne somme synthesis and new results from interglacial deposits the seine yonne and somme are the main rivers draining northwestern france and flowing into the channel during pleistocene cold stages they were tributaries of the fleuve manche they are characterised by well developed stepped terrace systems showing up to 10 incision steps for the last 1 ma after 15 years of research and the synthesis of stratigraphical sedimentological bioclimatical data and numerical dating these terrace systems are interpreted as the response of the fluvial environments to climatic cyclicity superimposed on a background of slow tectonic uplift the comparison of these three terrace systems shows that the incision budget is similar within the studied area about 55 65 m 1 ma implying relative homogeneity of uplift in the main part of the area beyond marine influence the analysis of each stepped alluvial formation shows regular sediment balance generally composed of coarse periglacial gravels pleniglacial covered by fine grained sediments locally overlain by calcareous tufa interglacial however in the lower seine valley close to the present coast the low terrace includes two estuarine interglacial units interbedded within periglacial gravel units nevertheless until recently evidence of full interglacial conditions remained elusive in fluvial contexts especially in the somme to address these issues a new research programme has been launched on tufa deposits that are the best candidate to register the climatic optimum initial results from la celle seine saint acheul and caours somme allow palaeoenvironmental reconstructions of mis 11 and 5e and thus confirm the importance of calcareous tufa to define a reference record of pleistocene interglacials for northern france moreover it has been previously demonstrated that the main incision leading to terrace formation can be attributed to the transition between interglacial and glacial during early glacial phases the new results from interglacial records have important implications concerning the precise occurrence of the major incision phases within the climatic cycle as they provide the starting point immediately preceding downcutting

the magellan venus radar mapping mission the nasa magellan venus radar mapper spacecraft was launched into an interplanetary transfer trajectory to venus on may 4 1989 and will be placed into orbit around venus on august 10 1990 the orbiter carries a 12 cm wavelength multimode radar system in the synthetic aperture mode it is capable of imaging most of the venus surface at a resolution of better than 300 m approaching 120 m over more than half the planet in the altimeter mode it will determine topographic relief to a vertical accuracy of better than 50 m averaged over a surface resolution cell approximately 10 km in diameter where the surface relief is not too extreme in the radiometer mode the radar receiver can determine the surface radio emission brightness temperature with an absolute accuracy of 20 k at a resolution of 2 k tracking of the orbiter s coherent radio telemetry transmitter will permit observations of small accelerations related to gravitational inhomogeneities in the planet s interior objectives of the mission include deducing the geological history of the surface and the geophysical state of the interior specific attention focuses on the origin present distribution and activity of four geological processes that modify the surface 1 volcanic and tectonic 2 impact 3 erosional depositional and chemical and 4 isostatic and convective data products resulting from the mission will be made available to members of the scientific community through nasa s planetary data system

holocene environmental changes in lake bangong basin western tibet part 1 chronology and stable isotopes of carbonates of a holocene lacustrine core abstract a 12 4 m core collected from the eastern part of lake bangong is analyzed for mineralogy radiometric chronology and stable isotope contents of authigenic inorganic carbonate isotopic compositions of the modern lake waters and of its major tributaries are first presented radiocarbon activity of these waters show that equilibrium with the atmospheric co2 is not reached because of admixture of dead carbon in solution in the system a model is thus developed to correct the radiocarbon ages of fossil material of aquatic origin for this ageing the core provides a continuous holocene record changes in mineralogy of bulk sediments and in 13c and 18o contents of authigenic carbonates reveal extremely large changes in environmental conditions induced by climate changes and or local hydrological factors e g closure opening of the lake system controlled by the altitude of the outflow sill the eastern basin of lake bangong was closed before 9 6 and after 1 2 ka b p in response to aridity on the catchment wet warm conditions attributed to the monsoon influence were established suddenly at 9 6 ka b p and maintained until 6 2 ka b p although minor reversal events occurred between 8 6 and 7 7 ka b p the return towards aridity is non linear dry spells at 6 2 3 9 3 2 and 1 3 ka b p are alternated with short term periods with positive precipitation evaporation balance

palaeoenvironmental records in holocene spanish tufas a stable isotope approach in search of reliable climatic archives tufa deposits are potential terrestrial archives of palaeoenvironmental and palaeoclimatic information this study assesses the potential of stable isotopic archives from two closely juxtaposed holocene tufa sites in se spain the ruidera site contains deep water lacustrine micrites and tufas whereas the nearby alcaraz site represents a shallow barrage tufa understanding site characteristics is critical to interpreting the stable isotopic variations these holocene lacustrine micrites have isotopic compositions consistent with modern european lake shore microbial carbonates where the isotopic chemistry is strongly influenced by hydrological and residence time effects all the lacustrine micrite d13c values were influenced by microenvironmental microbial effects to some degree because of these effects stable isotope data from lacustrine microbial micrites and tufas will not normally yield precise information on the isotopic composition of palaeoprecipitation temperature or vegetation composition of an area in contrast holocene tufas that formed in shallow fast flowing riverine settings may contain valuable palaeoclimatic archives the tufa deposits must be largely autochthonous as at alcaraz where in situ reed stem encrustations are present records of relative change in air temperature and changes in the source of airmasses are potentially resolvable in the d18o data these interpretations can be verified by other independent climatic data where chronology is constrained variations in riverine tufa d13c values probably record changes in local vegetation and or soil respiration covariation between d18o and d13c values may be intrinsically linked to climatic factors such as aridity tentative palaeoclimatic interpretations for the middle holocene at alcaraz based on the isotope data suggest warming or increasing influence of mediterranean sourced precipitation between approximately 5000 3000 radiocarbon years bp accompanied by increased aridity these interpretations are consistent with the sparse independent palaeoclimatic data and climate modelling results for the holocene of se spain this study supports the growing evidence that well chosen tufa sites could yield valuable palaeoclimatic information

a complete holocene climate and environment record for the western carpathians slovakia derived from a tufa deposit calcareous tufas are great archives of geochemical information for the reconstruction of past climate their importance increases in the regions where other proxies are rare such as western carpathians here we present the first whole holocene palaeoclimatic reconstruction for this region based on geochemical proxies we analysed o 18 c 13 and mg ca ratio on tufa calcite of the mituchovci site white carpathian mountains w slovakia and compared these with pollen plant macrofossil and mollusc data according to o 18 two distinct thermal maxima occurred in the region around 11 4 ka and between 7 3 and 6 9 ka bp according to c 13 and mg ca ratio a steep increase in moisture and rainfall took place around 8 5 ka cal bp preceded by a cold and dry event these events are well reflected in the biotic proxies and are suggested also by other palaeoenvironmental studies from the carpathians we found some later fluctuations with dry and warm bronze age ca 3 6 ka cal bp cold and wet urnfield period ca 2 8 ka cal bp warm roman period ca 2 ka cal bp and cold but humid migration period ca 1 5 ka cal bp we observed extreme abruptness and amplitude of the variation in all geochemical proxies in the last 500 years when biotic proxies imply a very intense human deforestation land use changes may have altered both the temperature regime in the studied fen and the carbon cycle in the recharge area

volcanism and tectonics on venus we review recent developments in the study of volcanism and tectonics on venus venus s crust is basaltic dry and probably about 30 km thick the mantle convects giving rise to plumes and has a similar composition and mean temperature 1300 c but a higher viscosity 10 20 pa s than that of the earth inferred melt generation rates constrain the lithospheric thickness to between 80 and 200 km the elastic thickness of the lithosphere is about 30 km on average the present day lack of plate tectonics may be due to strong faults and the high viscosity of the mantle most of the differences between earth and venus processes can be explained by the absence of water venus underwent a global resurfacing event 300 600 ma ago the cause and nature of which remains uncertain the present day surface heat flux on venus is about half the likely radiogenic heat generation rate which suggests that venus has been heating up since the resurfacing event

climate induced changes in holocene calcareous tufa formations bohemian karst czech republic abstract about 70 localities where holocene calcareous tufa is formed recently and or was formed in the past are known in the bohemian karst a small karst area located sw of prague all known tufa accumulations display a very similar pattern of lithological and biostratigraphic evolution reflecting climatic changes erosion events and biota succession a 17 m thick tufa accumulation at svat jan pod skalou was selected and thoroughly studied as a typical well developed representative of local tufa deposits tufa formation started at about 9500 bp on a flat fluvial gravel terrace of late glacial early holocene age deposition of lithologically uniform pure hard porous tufa continued until about 6500 bp from that time a more unstable climate with several dry periods and erosion events produced a lithologically varied complex of loose tufa alternating with embedded soils and scree layers termination of the tufa deposition occurred about 2500 bp and was followed by partial erosion connected with relocation of the spring below the tufa body holocene climatic changes were recorded in lithology molluscan assemblages and oscillations of oxygen and carbon stable isotope ratios in carbonate the observed patterns are in good agreement with the evolution of calcareous tufa deposits throughout central europe

multidecadal variability of the overturning circulation in presence of eddy turbulence at low resolution idealized ocean circulation models forced by prescribed differential surface heat fluxes show spontaneous multidecadal variability depending critically on eddy diffusivity coefficients the existence of this critical threshold in the range of observational estimates legitimates some doubt on the relevance of such intrinsic oscillations in the real ocean through a series of numerical simulations with increasing resolution up to eddy resolving ones 10 km and various diapycnal diffusivities this multidecadal variability proves a generic ubiquitous feature at least in model versions with a flat bottom the mean circulation largely changes in the process of refining the horizontal grid along with the associated implicit viscosity and diffusivity and the spatial structure of the variability is largely modified but there is no clear influence of the resolution on the main oscillation period the interdecadal variability appears even more robust to low vertical diffusivity and overturning when mesoscale eddies are resolved the mechanism previously proposed for these oscillations involving westward propagating baroclinically unstable rossby waves in the subpolar region and its feedback on the mean circulation appears unaffected by mesoscale turbulence and is simply displaced following the polar front

the quaternary travertine deposits of europe and asia minor abstract a summary is provided of the published information relating to all aspects of quaternary travertine formation in europe west of the ural mountains the deposits have been divided into two broad groups the meteogene travertines which result primarily from the degassing of soil borne aqueous co 2 and thermogene deposits resulting from the degassing of thermally generated co 2 meteogene deposits are rare above latitude 58 n and in regions where the mean annual air temperature is below 5 c a significant positive correlation exists between mean air temperature and travertine deposit thickness the combined effects of temperature and rainfall are used to provide a zoned map showing the travertine forming potential of limestones within the region information from 14 c dating indicates that deposition reached a maximum in the period 5 10 ka bp and is currently limited by land and water management practices in the populated areas thermogene deposits occur in regions of high co 2 discharge resulting from tectonic activity such as italy and turkey where there is much vulcanism these travertines are frequently more massive and less readily weathered than meteogene deposits fully referenced information is provided for 320 important mostly well studied sites 227 meteogene 93 thermogene of which 156 are currently active

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

the distribution of volcanism in the beta atla themis region of venus its relationship to rifting and implications for global tectonic regimes a new analysis of the spatial relationships between volcanic features and rifts on venus provides new constraints on models of planetary evolution we developed a new database of volcanic features for the beta atla themis bat region and used nearest neighbour measurements to determine relationships between different types of volcanic features and the rifts nearest neighbour analysis shows that all the dome type and corona type sub populations tend to cluster rift associations were inferred from the deviation of a feature s population distribution as a function of distance from rift from that of a random population dome type features in general have no discernible relationship with rifts most corona type features have a strong association with rifts with intermediate and large volcanoes also tending to occur close to or on rifts shield fields on the other hand tend to occur away from rifts our new evidence supports classifications of rifts on venus into different types possibly by age with a shift from globally dispersed more uniform volcanism towards the more rift focused distribution which suggests a shift in tectonic regime our observations are consistent with recent models proposing the evolution of venus from a stagnant lid regime to a subcrustal spreading regime we also present evidence for a failed rift on venus and note that this process may be analogous albeit on a larger scale to a proposed model for the evolution of the east african rift system

climatic change recorded by stable isotopes and trace elements in a british holocene tufa combined stable isotope d18o and d13c and trace element mg sr geochemistry from bulk tufa calcite and ostracod shell calcite from an early holocene british tufa reveal clear records of holocene palaeoclimatic change variation in d18o is caused principally by change in the isotopic composition of holocene rainfall recharge itself caused mainly by change in air temperature the d13c variability through much of the deposit reflects increasing influence of soil zone co2 owing to progressive woodland soil development bulk tufa mg ca and sr ca are controlled by their concentrations in the spring water importantly mg ca ratios are not related to d18o values and thus show no temperature dependence first order sympathetic relationships between d13c values and mg ca and sr ca are controlled by aquifer processes residence times co2 degassing and calcite dissolution reprecipitation and probably record intensity of palaeorainfall recharge effects stable isotope records from ostracod shells show evidence of vital effects relative to bulk tufa data the ostracod isotopic records are markedly spiky because the ostracods record snapshots of relatively short duration years whereas the bulk tufa samples record averages of longer time periods probably decades the d18o record appears to show early holocene warming a thermal maximum at ca 8900 cal yr bp and the global 8200 yr bp cold event combined d13c mg ca and sr ca data suggest that early holocene warming was accompanied by decreasing rainfall intensity the mg ca data suggest that the 8200 yr bp cold event was also dry warmer and wetter conditions were re established after the 8200 yr bp cold event until the top of the preserved tufa sequence at ca 7100 cal yr bp

terrestrial environments during mis 11 evidence from the palaeolithic site at west stow suffolk uk abstract multidisciplinary investigations of the sequence at beeches pit west stow suffolk uk have a direct bearing the age of the hoxnian interglacial and its correlation with the continental holsteinian and with the global marine record at this site glacial deposits till and outwash gravels referable to the anglian lowestoft formation fill a subglacial channel cut in chalk bedrock above these glacial deposits a series of interglacial sediments occurs consisting of limnic tufaceous and colluvial silts lacking pollen but rich in shells ostracods and vertebrates lower palaeolithic flint artefacts of acheulian character have also been recovered including refitting examples charred material is abundant at certain horizons and many of the bones have been burned several discrete areas of burnt sediment are interpreted as hearths the molluscan fauna comprises some 78 taxa and includes species of considerable zoogeographical and biostratigraphical importance the land snail assemblage from the tufa consists of woodland taxa with no modern analogue including species that are either extinct e g zonitoides sepultus or which no longer live in britain e g platyla polita p similis neniatlanta pauli this is also the type locality of retinella lyrodiscus skertchlyi which belongs to a subgenus of zonitid land snail now living only on the canary islands there are indications from this fauna the lyrodiscus biome that the climate was wetter and perhaps warmer than the present day the vertebrate fauna is also noteworthy with species of open habitats such as rabbit oryctolagus cf cuniculus and of closed forest such as squirrel sciurus sp and garden dormouse eliomys quercinus present at different times the occurrence of southern thermophiles such as aesculapian snake zamenis longissimus indicates temperatures warmer than those of eastern england today the upper levels include much material reworked from the interglacial sediments although there is clear faunal evidence for climatic deterioration both the molluscan and vertebrate faunas suggest correlation of the interglacial sediments with the hoxnian uranium series dates from the tufa 455 ka bp tl dates from burnt flints 414 30 ka bp and a range of amino acid racemization data all support correlation of this interglacial with mis 11 however four osl dates from sand beneath the interglacial sequence yield a mean age of 261 31 ka bp far younger than all other age determinations and far younger than implied by the biostratigraphy archaeologically the site is unusual in showing prolonged human occupation within closed deciduous forest and evidence for controlled use of fire in a lower palaeolithic context biostratigraphical correlations with other lower palaeolithic sites support the suggestion that acheulian and clactonian industries both occurred in southern britain during the same substage of the hoxnian although not necessarily at precisely the same time the characteristics of the mis 11 interglacial in britain are discussed in the light of evidence from beeches pit and elsewhere

a review of the tagus river tufa deposits central spain age and palaeoenvironmental record here we determined the aminostratigraphy and aminochronology of tufa deposits located in central spain associated w i t h the tagus river and some o f its tributaries the henares dulce cifuentes ruguilla trabaque escabas and guadiela rivers w e used aspartic acid and glutamic acid racemization ratios obtained from the ostracod herpetocypris reptans tufa accumulations were found to be of different origins those in t h e henares cifuentes and ruguilla rivers are of paludal origin w h i l e those in t h e dulce and tagus rivers are o f fluvial origin a generally g o o d correspondence was found b e t w e e n the age o f the deposits and the position of the terraces above the current thalweg however the geomorphological evolution of the henares cifuentes and ruguilla rivers infilling of pre existing valleys has produced deposits of distinct ages at the same elevation above the current river thalweg and sometimes older tufas are located b e l ow younger ones we distinguished eight main tufa deposition episodes these occurred predominantly during even marine isotopic stages mis at 4 0 6 90 mis 11 2 6 4 68 mis 7e 189 4 0 mis 7a 130 27 mis 6 5e 101 2 5 mis 5c 3 2 10 mis 3 1 4 4 mis 1 a n d 6 2 mis l k a these results are in agreement w i t h the dating of similar deposits from nearby areas and other zones of s p a i n a n d europe the tufa stable isotope compositions were similar to other examples in central and southern spain a n d their plot falls in the same field as other lowland european stream tufas o x y g e n stable isotopes were influenced mainly b y temperature a n d rainfall the 51 3c values indicated a major effect o f soil derived carbon rather t h a n carbon from the catchment area but moderated in each tributary b y evaporation flow regime and biological effects photosynthesis

high potential of calcareous tufas for integrative multidisciplinary studies and prospects for archaeology in europe calcareous tufas are continental carbonates from open air conditions specific to wet and warm periods they contain abundant remains of fauna and flora fossilised in situ and may accumulate regularly over thousands of years offering high stratigraphic resolution for palaeoenvironmental reconstructions as they are mainly comprised of calcite tufas allow direct and precise dating as well as geochemical reconstructions of past climates additionally recent investigations have highlighted their strong potential for archaeology as several studied sequences provided high quality record of in situ prehistoric levels tufas are thus a unique archive in continental areas for development of synergic multidisciplinary investigations of past human societies and associated environmental and climate evolution we emphasise that calcareous tufa are key deposits to investigate human environment climate interactions during interglacial periods from lower palaeolithic to antiquity in europe

coronae of parga chasma venus 1 parga chasma is a 10 000 km long fracture and trough system in the southern hemisphere of venus in this study we analyze coronae in the parga region addressing the relationship between corona and rift formation the relationship of volcanism to rifting and coronae and the overall evolution of the rift system our observations are compared with various models with the aim of determining any causative relationships between corona formation volcanism and rifting our statistical analysis shows that coronae are randomly located within this region indicating that corona locations are not significantly controlled by rifting or by lithospheric loading caused by the earlier formation of other coronae our observations suggest that the two processes of corona and rift formation were occurring more or less synchronously with no specific control on each other but that coronae and rifting in this region are genetically related we find a wide variation in the amount of volcanism associated with coronae across the parga region and no evidence for a deep seated plume or multiple plumes beneath parga chasma we suggest that the concentration of volcanic features in the beta atla themis region may actually result from three independent but overlapping broad zones of deformation corona formation and enhanced volcanism associated with the three rifts parga hecate and devana chasmata rather than one large zone of activity covering the whole of the region simultaneously

isolating the role of mesoscale eddies in mixing of a passive tracer in an eddy resolving model 1 this study examines the role of mesoscale eddies in distribution of chlorofluorocarbon cfc in a numerical model of the north atlantic our main conclusions are based on a comparison of two eddy resolving experiments one with and one without eddy advection of cfc the cfc distributions in these two experiments exhibit substantial differences within the subpolar gyre in particular in the presence of eddies the horizontal distribution of cfc in the upper 1000 m is more uniform and the cfc concentration is lower in the lower 1500 m our analysis demonstrates that these basin wide differences are caused by the interactions of eddy advection with convection and mean advection in several isolated locations the corresponding eddy flux divergence terms are spatially non uniform in magnitude their effects are often nonlocal and sometimes work against the spreading of cfc being carried out by the mean advection these conclusions are further confirmed by a third experiment in which the eddy advection was replaced by a down gradient diffusive parameterization our results demonstrate that advection by mesoscale eddies is crucial to the mixing of atmospheric gasses into the ocean and outline difficulties in representing eddies by the down gradient diffusion

small mantle upwellings are pervasive on venus and earth observations of venus and africa an analogous terrestrial plate indicate that upwellings a few hundred kilometers across are pervasive in the ambient upper mantle of both venus and earth because of their small size when they reach the lithosphere the upwellings affect the surface only under special circumstances on venus they form volcano tectonic structures called coronae where the lithosphere has been thinned by rifting on earth the small upwellings produce topographic swells where they impinge on a plate stationary with respect to the hot spot reference frame as is africa the small upwellings presence throughout the mantle explains both the corona paradox in which coronae are associated with rifts on venus but do not appear to drive rifting and the basin and swell topography of the african plate the morphology of coronae requires that the small upwellings have some diapiric properties their pervasiveness transient nature and generally small size make them fundamentally different from traditional hawaiian style plumes and they may be detached diapirs within a mantle convecting in the hard turbulence regime

sedimentation of holocene tufa influenced by the neolithic man an example from the sąspowska valley southern poland abstract a complex of inactive tufas the thickness of which reaches 3 5 m was studied in the lowermost segment of the s spowska valley in the krakow upland ojcow national park poland five fluvial tufa barrages were recognized they are composed of moss tufa and stromatolitic tufa accompanied with oncoidal rudstone and detrital tufa interbarrage ponded areas were filled with detrital tufa lutite and subordinately oncoidal rudstone limestone gravel and peat like deposit radiocarbon dates suggest that the tufa formed during subboreal boreal and atlantic time the main difference between the tufa in the s spowska valley and contemporaneous tufas in other valleys of the krakow upland is the higher amount of non carbonate fraction in the former other lower and middle holocene tufas of the krakow upland are composed mainly or exclusively of carbonate fraction the non carbonate fraction in fluviatile tufas in the s spowska valley resulted from erosion of loess cover in the upper part of the catchment the erosion was related to local activity of neolithic flint miners who cleared forest at a local scale dug shafts in loess cover and exploited flints from underlying weathered residuum of jurassic limestone consequently they made copious amounts of loose material available for transport down the valley and subsequent trapping within the tufa depositional system

parameter sensitivity of primitive equation ocean general circulation models abstract experiments with a low resolution primitive equation ocean general circulation model with idealized basin geometry and surface forcing have been carried out in order to identify the processes controlling the climatically important aspects of the circulation emphasis was placed on the sensitivity of the model solutions to the magnitude of the vertical diffusivity scaling arguments suggest and the numerical experiments confirm that the solutions are most sensitive to the magnitudes of the wind stress curl and the vertical diffusivity for small vertical diffusivity wind forcing dominates the solution the vertical scale of the thermocline is set by the strength of the ekman pumping and there is a multiple gyre circulation in the upper layers for large vertical diffusivity diabatic surface forcing dominates the solution vertical diffusion controls the vertical scale of the thermocline and there is a single large anticyclonic gyre in the upper layers both the meridionally and zonally inte