strike slip tectonics in the sw barents sea during north atlantic rifting swaen graben northern norway this study uses high quality three dimensional 3 d seismic data to investigate the occurrence of strike slip faults in the swaen graben sw barents sea the swaen graben is divided into two principal sub basins ssb1 and ssb2 the along strike and along dip displacement variations and scale relationships are analyzed for forty two 42 faults the displacement profiles for these faults are complex in the swaen graben showing clear evidence for polycyclic fault growth and marked syn sedimentary activity the observed variations in the displacement profiles indicate complex along strike segmentation linkage and mechanical interactions at distinct structural levels along dip displacement minima indicate fault reactivation by dip linkage importantly geometric evidence for strike slip faulting in the swaen graben includes the presence of extensional horsetail splay faults positive flower structures and minor transfer faults this study shows that the faults in the swaen graben developed under extensional regimes during late jurassic to early cretaceous rifting and were reactivated by regional stresses during the late cretaceous the two principal strike slip faults in the swaen graben reveal sinistral movement and are linked at a shallow depth by minor transfer faults at a relay zone our work further demonstrates the occurrence of late mesozoic strike slip movements in the sw barents sea which were induced by regional tectonics halokinesis and fault block rotation importantly strike slip faulting in the region extends perhaps into the cenozoic interacting with extension during the north atlantic rifting

fault slip bed length and area variations in experimental rollover anticlines over listric normal faults influence in extension and depth to detachment estimations abstract to check the behaviour of physical experiments of simple rollover anticlines over listric normal faults several fault slip bed length and area parameters are measured for different beds in two stages of evolution of two physical experiments given a certain amount of extension the heave and dip of the displacement vary for different beds whereas the throw is approximately constant for small amounts of extension and the displacement remains constant for high amounts of extension neither bed lengths nor areas beneath beds remain constant with increasing extension a number of techniques allow amounts of extension and detachment depth to be estimated given one or more marker horizons the portion of the fault between hanging wall and footwall cut off points the depth to detachment and or the shear angle these techniques are applied to the physical experiments of listric normal faults analysed since many of these techniques rely on parameters measured on the physical experiments such as fault slip bed length and area the influence of these parameters on the different magnitudes of extension and detachment depths estimated using different techniques is discussed and the accuracy of the results is compared

fault block tilting the gebel zeit example gulf of suez abstract the gebel zeit consists of the eroded crest of a tilted block in the southern part of the suez rift the zeit block displays a typical asymmetrical geometry it is bordered to the east by a 35 45 e dipping normal fault with kilometric throw and has a 30 sw dipping homoclinal flank part of the pre rift sedimentary sequence has been preserved on this flank and is unconformably covered by upper burdigalian nn4 globigerina marls in the southern part of the block crest the complete pre rift series has been eroded and evaporites of langhian age nn5 rest directly on the precambrian basement field evidence indicates alternations from erosion to sedimentation at the crest of the zeit block in an attempt to characterize its tilting subsidence curves were computed along a cross section with the backstripping method results indicate three stages in the evolution of the block 1 a rapid subsidence between 22 and 16 my 2 a pause between 16 and 10 my and 3 a slow subsidence until present time during the tectonic quiescence the sedimentary loading effect alone produced an increase of 8 in the tilt angle a simple kinematic model of tilting along a circular fault is proposed to quantify the zeit rotation depth of the brittle ductile transition is estimated at 10 km to explain the tilting strong driving of the tilting by the listric fault induces conjugate movements between the crest and trough of the block and explains the discrepancy between the zeit and regional tectonic subsidence

magnesium isotope composition of sabkha porewater and related sub recent stoichiometric dolomites abu dhabi uae abstract earliest marine diagenetic sabkha type dolomites are widespread in earth s geological record potentially these carbonates may act as archives of past seawater magnesium isotope 26mg ratios at present however the fractionation of magnesium isotopes 26mg between seawater here evaporated marine porewater and sabkha dolomite is not constrained in order to explore 26mgdol mg porewater we make use of actualistic sabkha type dolomite precipitation in the gulf region trucial coast united arab emirates this paper documents and discusses the first detailed sabkha 26mg data set of mg bearing solids including stoichiometric dolomites degree of ordering 0 9 mean 26mgdol 0 79 0 41 2 n 17 and related marine pore waters the presence of dolomite crystals with a broad range of ordering in shallow cores is documented using x ray diffraction and scanning electron microscopy but individual crystals are too small

extensive carbonate algal bioherms in upper pleistocene saline lakes of the central altiplano of bolivia during the upper pleistocene the central altiplano of bolivia was repeatedly flooded by deep and extensive saline lakes in response to climatic fluctuations development of carbonate algal bioherms took place during at least three major periods of lacustrine highstands discontinuously covering the 300 km long and 100 km wide lacustrine slopes and terraces up to an elevation of 100 m above the surface of the modern halite crust of uyuni distribution size and shape of the bioherms are diverse due to various factors e g the nature and morphology of the substrate and the hydrodynamic conditions that prevailed during growth on larger palaeoterraces the build ups coalesced to form platform like carbonate accumulations although the morphologies closely resemble those induced by cyanobacteria they were predominantly constructed by other plant communities probably dominated by filamentous green algae cyanobacterial communities flourished in association with these plants but they did not contribute significantly to the architecture of the bioherms they participated to encrust the plant stems and algal bushes or to form thin laminated layers covering the build ups a prominent feature of some bioherms is their composite structure due to repeated algal growth during successive lacustrine episodes that were separated by subaerial exposures with moderate erosional effects the build ups located between 3660 and 3680 m elevation display up to three major parts 1 a massive inner core formed during an early minchin highstand before 40 ka 2 a large peripheral envelope deposited at about 40 ka late minchin and 3 a thinner outermost crust formed during a late glacial event lake level dropped during interlacustrine stages sometimes leading to desiccation and deposition of salt layers in the deepest parts of the system i e the present day salar of uyuni

petrographic and trace element analysis of uranium rich tufa calcite middle miocene barstow formation california usa an integrated petrographic and spectroscopic x ray diffraction phosphor imaging and synchrotron x ray absorption spectroscopy study of tufas from the miocene barstow formation california relates sample morphology mineralogy and geochemical composition the tufas composed mainly of calcite that formed at the interface between an ancient alkaline lake and ground or spring waters have textures similar to those of microbially mediated terrestrial stromatolites and travertines the tufas have elevated concentrations of a number of trace elements including mn fe sr and u synchrotron x ray fluorescence analyses show that u concentrations can exceed 500 p p m x ray absorption spectroscopy indicates that the u in these samples is incorporated as u iv it is suggested here that alkaline lake waters had a high u ca ratio and tufa calcite formed where groundwater or possibly epithermal springs brought in ca and trace elements such as sr the rapid and possibly microbially mediated precipitation of calcite allowed for incorporation of high concentrations of trace metals as either structural substitutes or extremely fine scale inclusions

abiotically formed primary dolomite in the mid eocene lacustrine succession at gebel el goza el hamra ne egypt an approach to the role of smectitic clays abstract this study discusses the role of smectitic clays in the formation of an abiotic physio chemical primary dolomite within an evaporative alkaline saline marginal lake system in the absence of carbonate precursor and microbes the present work has been achieved in terms of textural mineralogical and geochemical characteristics of dolostones in the mid eocene bartonian lacustrine succession cropping out at gebel el goza el hamra shabrawet area ne egypt this lacustrine succession is 15 16 m thick and made up of alternating horizontal beds of dolostone marlstone and mudrock that show some pedogenic and subaerial exposure features the dolostones are composed mainly of dolomite 60 90 smectite 20 30 and quartz grains 5 10 the dolomite comprises fine crystalline rhombs to micro spherical crystals with no obvious relics of microbial activity and or carbonate precursor it is ordered nearly stoichiometric with 46 50 mole of mgco 3 and has 18 o and 13 c values ranging from 0 44 to 2 96 vpdb and 0 93 to 8 95 vpdb respectively the smectite occurs as thin mats that are commonly intergrown and associated with dolomite mineralogical textural and stable isotopic results of the dolomite indicated that the dolomite was formed as an abiotic primary precipitate in alkaline saline lacustrine systems in this respect the gel like highly viscous smectitic medium plus progressive co 2 degassing elevated evaporation low sedimentation rate low sulphates level and alkaline soil solution lowered the kinetic barriers of dolomite precipitation from solution and promoted the incorporation of mg 2 in the structure of dolomite consequently the presence of smectitic clays in evaporative saline lakes is significant for dolomite formation because they can generate a gel like highly viscous medium and provide mg 2 that can facilitate the physcio chemical precipitation of primary dolomite from solution at ambient temperatures however more work is needed to better characterize the role of clays during dolomite formation in alkaline lacustrine environments at ambient temperatures

methane derived authigenic carbonates mdac in northern central adriatic sea relationships between reservoir and methane seepages abstract in the adriatic basin gas venting characterized by pockmarks gas bubbling and other fluid escape related features is diffused and has been linked or to shallow gases trapped in the late pleistocene and below the holocene maximum flooding surface or to leakage from more deeply buried pliocene to pleistocene foredeep succession some seepages are associated with carbonate crusts but a possible correlation between methane leakage and carbonate precipitation in this basin never reached a clear definition carbonate concretions recovered by different areas of the adriatic sea northern central and southern at different depths from ca 30 to ca 300 m have been analysed different fabrics petrography and mineralogy can be evidenced for the different sets of samples all of them show a carbonate content mainly represented by high mg calcite up to 66 in weight of total carbonates for the samples from central adriatic close to bonaccia gas field offshore ancona and 79 to 96 of the total carbonate weight for the other samples that are mainly biogenic these differences are also reflected by the 13 c values ranging between 19 and 29 vpdb for the samples collected in the bonaccia area and moderately positive values for all the other samples bonaccia samples contain carbonate cements that record the contribution of methane derived carbon and have been interpreted as formed below the sea floor in correspondence of the sulphate methane transition zone carbonates recovered from the southern and northern adriatic basin do not show clear evidence of methane derived carbon and are mainly made up by organisms typically recovered in the adriatic coralligenous assemblage nonetheless micro morphological evidences suggest influence of bacterial sulphate reduction and methanogenesis in precipitation of some of these carbonates

the role of sulphate reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the coorong region south australia abstract the dolomitic distal ephemeral lakes of the coorong region of south australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input during late evaporation active bacterial sulphate reduction occurs in uppermost anoxic lake sediments as rising salinity and then h2s toxicity kills off the macrobiota these hypersaline lakewaters are characterised by high ph and elevated concentrations of carbonate and sulphate ions magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly electrolytic alkaline lake brines from lake margin stromatolites and mats during desiccation and may explain the remarkable increase in mg levels recorded at this time continued evaporation of the lakewaters produces a halite crust overlying the dolomite sulphate free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate reducing bacteria dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters and both the morphology and size distribution of dolomite grains is identical to bacterial cells it is not proven exactly when dolomite precipitation occurs but the submicron sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate which is known to inhibit dolomite formation bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation in adjacent non dolomitic lakes sulphate is present throughout evaporation so that other minerals principally aragonite and hydromagnesite are precipitated

microbial control on the precipitation of modern ferrihydrite and carbonate deposits from the cezallier hydrothermal springs massif central france abstract actively precipitating hydrous iron oxides and carbonates are commonly associated with cool co2 rich spring waters in the massif central france chemical isotopic and mineralogical analyses and thermodynamic calculations were undertaken to elucidate the processes involved in the formation of these deposits hydrous iron oxide particles rich in al si ca and as are the first to form in the springs accompanying the iron oxides are layered carbonate deposits composed mainly of well crystallised calcite sem examination of both oxides and carbonates sampled at the outlet of the springs shows that they basically consist of grape shaped agglomerates of small 300 nm to 1 m anhedral crystals light acid etching of these agglomerates reveals colonies of spherical nannobacteria water trace partitioning between oxide and carbonate favours the hydrous oxide with as pb zn th concentration the microscopic features of both types of deposit indicate that microbial mediation probably contributes to their formation

microbial dolomite in fresh water carbonate deposits this article reports evidence for biologically mediated precipitation of dolomite in a fresh water spring in the south east of spain build ups in the spring consisted of calcite minor dolomite barite and gypsum crystals formed by the mineralisation of microbial biofilms and bryophytes a detailed microscopic study coupled with geochemical analysis of the dolomite precipitating biofilms showed that dolomite crystals occurred in association with living coccoid micro organisms providing strong evidence that micro organisms play a fundamental role in the precipitation of this mineral many of the calcite crystals observed were associated with extracellular polymeric substances moreover microbial involvement in the precipitation of calcite was further supported by the presence of living cyanobacteria within the calcite crystals two different types of build up were found in the spring soft spongy moss tufa and laminated crusts two types of crusts were identified as follows simple pink crusts and laminated crusts the latter forming in both sub aquatic and sub aerial environments sub aquatic accumulation of simple detached pink crust fragments occurred sporadically both tufa and the carbonate crusts hosted cyanobacterial dominated biofilms with different bacteria and diatoms and their extracellular polymeric substances the 13c values of calcite showed some biogenic involvement in the origin of this mineral the 18o values were similar in the tufa and carbonate crusts and corresponded to current climatological characteristics the occurrence of different groups of minerals carbonates and sulphates in close spatial association with micro organisms suggests the presence of different macro environmental and micro environmental conditions that facilitate mineral precipitation within biofilms these findings extend the known range of aerobic microbial dolomites to include springs and show that the precipitation of both dolomite and calcite carbonates occurs while organisms are alive

plio pleistocene lacustrine stromatolites from lake turkana kenya morphology stratigraphy and stable isotopes abell p i awramik s m osborne r h and tomellini s 1982 plio pleistocene lacustrine stromatolites from lake turkana kenya morphology stratigraphy and stable isotopes sediment geol 32 1 26 a sequence of fossil stromatolites from lake turkana in kenya was examined for 51so and 513c content these stromatolites ranging in age from holocene 10 000 yrs b p to middle pliocene 3 m y showed a variety of growth forms from oncolitic columnar layered to bulbous heads the stromatolites used in our study contain filamentous blue green algae of one morphological type and rare coccoids thus the stromatolites are considered biogenic the stable isotope ratios for oxygen and carbon indicate changing climatic conditions ranging from a cool wet climate prior to ca 1 9 m y to much drier warmer conditions around 1 4 m y followed in turn by a somewhat cooler and wetter climate at the end of the pleistocene

the relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls abstract we derive an analytical expression that relates the shape of a fault in cross section to the shape of the bedding horizons in its hanging wall block the expression assumes that the hanging wall deforms by simple shear and that the footwall remains undeformed throughout although this paper concentrates on normal faults the expression is equally valid and applicable to thrust faults the direction of simple shear in the hanging wall block is arbitrary and has a dramatic effect on the predicted fault or bedding geometry there is no reason to believe that the simple shear occurs on vertical planes as is commonly assumed in graphical approaches to this problem and ignoring the presence of inclined simple shear is likely to lead to considerable underestimates of the amount of extension across normal faults and in the amount of shortening across thrusts similar though more complicated expressions can be obtained when compaction within the hanging wall block is taken into account for a planar normal fault such compaction may result in the development of a hanging wall syncline

the large thaumasia graben on mars is it a rift 1 we investigate the morphology and topography of one of the largest fault bounded tectonic structures on mars a complex approximately n s trending system of troughs and scarps at the western border of the thaumasia plateau in the claritas region hereinafter referred to as thaumasia graben or tg it is located between 15 s and 38 s latitude and at 255 e longitude no detailed investigation of its morphotectonic setting has been performed yet the region is a complexly fractured area with a number of different fault sets including simple and complex graben the tg extends over more than 1000 km along its trend averaging 100 km in width and 1 6 km in depth crustal extension is accommodated by the formation of a system of asymmetric graben or halfgraben on the basis of fault orientation and trough depth the tg can be subdivided in a north south direction into three segments except for the northernmost segment the predominant master fault system is located along the eastern flank of the tg highlighting the overall asymmetric architecture fault length segments vary from 50 to 90 km with observable displacements of 1 3 2 2 km crustal extension inferred from gridded mola topography across scarp offsets varies along trend between 0 5 km and 4 km assuming a fault dip of 60 this is relatively moderate extension if compared to terrestrial continental rifts but consistent with extension measured across the tempe rift on mars we find that the thaumasia graben displays some characteristics which are common to terrestrial continental rifts whereas other properties are distinctively not rift like

microbially mediated carbonates in the holocene deposits from sarlieve a small ancient lake of the french massif central testify to the evolution of a restricted environment both the mineralogy and facies of lacustrine bio induced carbonates are controlled largely by hydrological factors that are highly dependent upon climatic influence as such they are useful tools in characterizing ancient lake environments in this way the study of the sedimentary record from the small ancient sarlieve lake limagne massif central france aims to reconstruct the hydrological evolution during the holocene using petrographical mineralogical and geochemical analyses the fine grained marls mainly calcitic display numerous layers rich in pristine ca dolomite with small amounts of aragonite which are clearly autochthonous as these minerals are rather unusual in the temperate climatic context of western europe the question arises about their forming conditions and therefore that of the lacustrine environment ca dolomite prevails at the base of the sequence as a massive dolomicrite layer and in the middle part it builds up most of the numerous laminae closely associated with organic matter scanning electron microscope observations reveal the abundance of tiny crystals tens to hundreds of nanometres mainly organized as microspheres looking like cocci or bacilli such a facies is interpreted as resulting from the fossilization of benthic microbial communities by dolomite precipitation following organic matter consumption and extracellular polymeric substance degradation these microbial dolomites were precipitated in a saline environment as a consequence of excess evaporation from the system as is also suggested by their positive 18o values the facies sequence expresses the following evolution i saline pan i e endorheic stage with a perennial lowstand in lake level boreal to early atlantic periods ii large fluctuations in lake level with sporadic freshening of the system atlantic iii open lake stage sub boreal and iv anthropogenic drainage sub atlantic

isotopic and geochemical evidence of palaeoclimate changes in salton basin california during the past 20 kyr 2 87sr 86sr ratio in lake tufa as an indicator of connection between colorado river and salton basin abstract as a part of the colorado river drainage system southern california s salton basin has two major tributaries colorado river and whitewater river interpretations on palaeoclimatic records from the basin strongly depend on the colorado river inflow to the basin in this study we have used the 87sr 86sr ratio in lacustrine deposits to identify the time and duration of the colorado river input our measurements of 87sr 86sr ratios made on water samples collected from salton basin indicate that the 87sr 86sr ratio 0 710169 of the colorado river input is identical to that of salton sea 0 710105 implying little change in the lake water ratio since 1905 when salton sea formed and began receiving water mostly from colorado river and the 87sr 86sr ratio of colorado river is much lower than that of whitewater river 0 715960 these results enable us to trace past changes in the colorado river input using 87sr 86sr in lake carbonates using a two end member mixing model we discuss the major factors that control the 87sr 86sr variation of lake cahuilla which was an ancient lake in the salton basin between 400 and 20 500 yr bp thirty measurements of 87sr 86sr were made on ams 14c dated tufa carbonates in two slabs from lake cahuilla the results show relatively constant 87sr 86sr ratios throughout the past 20 kyr ranging from 0 709944 to 0 710140 the average of these 87sr 86sr ratios 0 710060 0 000049 n 30 is very close to that of the colorado river input our results indicate that 1 the colorado river input was the dominant water source for lake cahuilla at least during 800 20 500 yr bp when the tufa grew in the basin and 2 the provenance of run off to the colorado river did not change significantly during 800 20 500 yr bp hence the palaeoclimatic proxies retrieved from the lake cahuilla tufas in the salton basin can decipher the discharge and flood history of the colorado river under the influence of climate variability in the colorado river drainage basin

microbial primary dolomite from a norian carbonate platform northern calabria southern italy the origin of fine grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the norian dolomia principale of northern calabria southern italy microbial carbonates there consist of stromatolites thrombolites and aphanitic dolomites high relief thrombolites and stromatolites characterize sub tidal facies and low relief and planar stromatolites with local oncoids typify the inter supratidal facies skeletal remains are very rare in the latter whereas a relatively rich biota of skeletal cyanophycea red algae and foraminifera is present in the sub tidal facies some 75 of the succession consists of fabric preserving dolomite especially within the microbial facies whereas the rest is composed of coarse dolomite with little fabric preservation three end members of dolomite replacement fabric are distinguished type 1 and type 2 fabric retentive with crystal size 5 and 5 60 m respectively and type 3 fabric destructive with larger crystals from 60 to several hundred microns in addition there are dolomite cements precipitated in the central parts of primary cavities during later diagenesis microbialite textures in stromatolites are generally composed of thin dark micritic laminae of type 1 dolomite alternating with thicker lighter coloured laminae of the coarser type 2 dolomite thrombolites are composed of dark micritic clotted fabrics with peloids composed of type 1 dolomite surrounded by coarser type 2 dolomite marine fibrous cement crusts are also present now composed of type 2 dolomite scanning electron microscope observations of the organic rich micritic laminae and clots of the inter supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains these probably derived from the fossilization of micron sized coccoid bacteria and spheroidal ovoidal nanometre scale dwarf type bacterial forms furthermore there are traces of degraded organic matter probably also of bacterial origin the microbial dolomites were precipitated in a hypersaline environment most likely through evaporative dolomitization as suggested by the excess ca in the dolomites the small crystal size and the positive 18o values the occurrence of fossilized bacteria and organic matter in the fabric preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn sedimentary dolomite

microbially mediated carbonate precipitation in a hypersaline lake big pond eleuthera bahamas microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability this can result in the precipitation of carbonate minerals and thus the formation of microbialites here the microbial metabolic activity the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake big pond bahamas were determined to identify the driving forces in microbialite formation a conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented geochemical modelling indicated an oversaturation with respect to carbonates including calcite aragonite and dolomite but these minerals were never observed to precipitate at the mat water interface this failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium a layer of high mg calcite was present 4 to 6 mm below the surface of the mat just beneath the horizons of maximum photosynthesis and aerobic respiration this carbonate layer was associated with the zone of maximum sulphate reduction it is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface i e the cyanobacterial layer of the mat bind calcium both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances each process accounting for approximately half of the total consumption and low molecular weight organic carbon liberating calcium and producing inorganic carbon the combination of these geochemical changes can increase the carbonate saturation index which may result in carbonate precipitation in conclusion the formation and degradation of extracellular polymeric substances as well as sulphate reduction may play a pivotal role in the formation of microbialites both in marine and hypersaline environments

aerobic microbial dolomite at the nanometer scale implications for the geologic record microbial experiments are the only proven approach to produce experimental dolomite under earth s surface conditions although microbial metabolisms are known to induce dolomite precipitation by favoring dolomite growth kinetics the involvement of microbes in the dolomite nucleation process is poorly understood in particular the nucleation of microbially mediated dolomite remains a matter for investigation because the metabolic diversity involved in this process has not been fully explored herein we demonstrate that halomonas meridiana and virgibacillus marismortui two moderately halophilic aerobic bacteria mediate primary precipitation of dolomite at low temperatures 25 35 c this report emphasizes the biomineralogical implications for dolomite formation at the nanometer scale we describe nucleation of dolomite on nanoglobules in intimate association with the bacterial cell surface a combination of both laboratory culture experiments and natural samples reveals that these nanoglobule structures may be 1 the initial step for dolomite nucleation 2 preserved in the geologic record and 3 used as microbial tracers through time and or as a proxy for ancient microbial dolomite as well as other carbonate minerals

decimeter scale mapping of carbonate controlled trace element distribution in neoarchean cuspate stromatolites abstract ancient stromatolites can provide key insights into the early evolution of life on earth neoarchean fenestrate stromatolites from the 2520 ma upper nauga formation transvaal supergroup south africa preserve cuspate morphologies they possess clearly delineated support and drape structures interpreted as dolomitized microbial mat material petrographic observations show that the biogenic structures are composed of planar s to non planar ferroan dolomite encased in ferroan calcite including herringbone calcite textures the cuspate stromatolites were analyzed using synchrotron rapid scanning x ray fluorescence srs xrf and more conventional techniques to determine i whether element distributions could be distinguished in ancient stromatolites at both cm to dm scales ii whether element distributions show variation between biogenic and abiogenic textures and iii the sample s paragenesis the distributions of ca fe mn pb cu as br al si p and s directly correspond to dolomitized stromatolitic structures and show trace element distributions are principally controlled by calcite and dolomite occurrence dolomite formation was mainly driven by seawater derived fluids given the high concentrations of fe and retention of marine shale normalized rare earth element and yttrium reysn patterns however the spatial association of dolomite to stromatolite structures may reflect microbially influenced mineral nucleation given the complexity of this sample s paragenetic evolution trace metal distributions cannot be conclusively tied to specific metabolic pathways bioaccumulation or passive binding however the results show srs xrf can be used for quantifiable spatial in situ investigation of ancient microbialites

cretaceous and tertiary inversion in the bjornoyrenna fault complex south western barents sea a system of half flower like structures folds reverse faults and footwall cut offs is observed adjacent to the bjornoyrenna fault complex in the south western barents sea although the bjornoyrenna fault complex itself is principally an extensional feature with its period of maximum extensional activity in the early cretaceous these structures suggest that the subsidence along the fault complex was interrupted in the hauterivian aptian by left lateral transtension and in the late cretaceous early tertiary by head on inversion both these episodes caused relatively intense deformation and seem to have regional significance consequently the events need to be seen in relation to the plate tectonic development of the south western barents sea

reassessment of carbonate ages by dating both carbonate and organic material from an etosha pan namibia stromatolite evidence of humid phases during the last 20ka abstract previous research on lacustrine stromatolites from etosha pan in namibia obtained ages on carbonate close to or beyond the limits of radiocarbon dating these ages suggested that the basin was likely not subject to extensive flooding during the last ca 40 ka this study shows that ams radiocarbon ages for the carbonate of a stromatolite from poacher s point are 15 21 ka older than ages for organic material in the stromatolite structure calibrated ages range from 30 to 40 ka for carbonate and 3 19 ka for the organic residue the new ages together with petrographic and isotopic data for the stromatolite have provided important new information on past flooding of etosha pan including evidence of prolonged lacustrine conditions during the holocene climatic optimum

biotic and abiotic processes in the formation and diagenesis of permian dolomitic stromatolites zechstein group ne england abstract the crinkly bed of the upper permian zechstein group succession roker formation hauptdolomit z2c in northeast england is a distinctive 1 4 meter thick unit that has been interpreted as a stromatolite of microbial origin or alternatively as the result of purely abiotic precipitation close examination of this stratigraphically significant unit shows that it is the result of both physical and microbial processes and was deposited in hypersaline shallow water immediately following lowstand evaporite deposition within the zechstein basin the internal structure of the crinkly bed consists of fine millimeter scale laminae of alternating clotted peloidal or aphanitic micritic layers and granular laminae composed of silt size peloids with rare hollow spheroids the lamination is extremely regular in thickness and laterally persistent for up to 4 meters a microbial community composed of coccoid and filamentous microorganisms inferred from fossil evidence colonized the original sediment surface it is suggested that this biofilm was responsible for the precipitation of both the micrite laminae and most of the peloids of the granular laminae the peloids are consequently interpreted as autochthonous this interpretation of a microbial origin is consistent with the microstructure and geochemical signatures of the micritic dolomite which constitutes both the continuous micritic laminae and the peloids themselves primary dolomite precipitation is hypothesized to have taken place from marine to slightly hypersaline waters mediated by bacterial metabolism similar to the process that operates in modern intertidal microbial mats the crinkly bed shows macro structures that vary along a continuum from symmetrical to asymmetrical centimeter scale ripple like structures some showing interference patterns through to domical and conical structures more typical of stromatolites the more ripple like structures have a lamination of microbial origin identical to that within the domal conical stromatolites there is no internal cross lamination their origin is equivocal they could be the result of physical current or wave induced deformation of a surficial biofilm or their form could be inherited from true mechanically deposited ripples not observed that provided a template for biofilm growth changes in the nature and strength of the wave and or current regime and possibly in the community structure of the microbial mat produced this array of microbially induced macrostructures miss the latter plus the absence of desiccation features indicative of intertidal to supratidal deposition suggest subtidal sedimentation at a depth of several meters intergranular gypsum cement precipitated during early diagenesis and ferroan sparry dolomite precipitated during burial tertiary uplift and the consequent exposure to meteoric waters resulted in the precipitation of calcite cements that replaced gypsum and pre existing dolomite probably via the biological mediation of heterotrophic bacteria the interpretation of this distinctive stratigraphic unit illustrates the difficulty in distinguishing between biotic and abiotic processes in the sedimentary record

microbial precipitation of dolomite in methanogenic groundwater we report low temperature microbial precipitation of dolomite in dilute natural waters from both field and laboratory experiments in a freshwater aquifer microorganisms colonize basalt and nucleate nonstoichiometric dolomite on cell walls in the laboratory ordered dolomite formed at near equilibrium conditions from groundwater with molar mg ca ratios of 1 dolomite was absent in sterile experiments geochemical and microbiological data suggest that methanogens are the dominant metabolic guild in this system and are integral to dolomite precipitation we hypothesize that the attached microbial consortium reacts with the basalt surface releasing mg and ca into solution which drives dolomite precipitation via nucleation on the cell wall these findings provide insight into the long standing dolomite problem and suggest a fundamental role for microbial processes in the formation of dolomite across a wide range of environmental conditions

structural development of coprates chasma and western ophir planum valles marineris rift mars a portion of valles marineris was mapped in detail in order to clarify the dominant processes responsible for the formation of coprates chasma new crater counts indicate that the caprock on western ophir planum plateau has a late hesperian crater age whereas trough floor preserved in western coprates chasma has an early to late hesperian crater age caprock on western ophir planum correlates in relative age with the syria planum formation and the caprock may overlie lower hesperian ridged plains material the conspicuous absence of wrinkle ridges on ophir planum may reflect burial by this later material trough floor material in coprates chasma correlates in relative age with ridged plains material on the adjacent lunae planum and coprates plateaus and represents a structurally coherent block displaced downward by normal faulting the crater counts and detailed structural relationships demonstrate the commonly accepted view that coprates chasma occupies a graben the floor of this graben is only thinly mantled by intratrough deposits and is relatively intact the trough shows no evidence for a chaotically broken and jumbled floor as required by genesis models of collapse and subsurface drainage formation of coprates chasma by keystone collapse of locally elevated topography is not supported by available topographic data but a general association between the trough and volcanotectonic activity in the tharsis region is considered to be a likely explanation for the trough forming stresses subtle asymmetries in topography and boundary fault development across coprates chasma may imply a gentle down to the north half graben geometry for the trough normal faults that define the northern margin of western coprates chasma and those that deform the adjacent ophir planum plateau change trend systematically with position implying a 30 rotation of the local horizontal principal stresses in this region during trough growth faulting on ophir planum appears related to the faulting that created the coprates chasma trough the geology and structure of coprates chasma are comparable to those of other troughs such as melas ius and perhaps candor chasmata suggesting that these troughs may also have formed as grabens the revised crater age for western ophir planum indicates that the initiation of trough faulting and subsidence in the valles marineris system is early late hesperian and may have been synchronous across the system relative to the prevailing cratering rate

dolomite formation in the shallow seas of the alpine triassic the alpine triassic units of switzerland northern italy and western austria offer an extensive geological archive in which the enigmatic process of dolomite formation can be studied in a palaeoenvironmental context recent studies clearly demonstrate that large amounts of the alpine triassic dolomites are late diagenetic or hydrothermal nevertheless as part of multiple generations of diagenetic overprint some generations of fine crystalline ca rich dolomite appear strictly confined to their depositional facies and show signs of very early formation at surface temperatures in specific ancient depositional environments in this review three cases of alpine triassic dolomites are discussed where dolomite rocks may have formed during or soon after sedimentation the sedimentary facies indicate contrasting palaeoenvironmental conditions and hence document three different possible processes of dolomite formation i in the dolomite mountains northern italy dolomite beds of the partly isolated middle triassic anisian ladinian latemar platform are confined to the very top of shallowing upward lagoonal facies cycles ii dolomite beds of the san giorgio basin southern switzerland an intra platform basin that opened during the anisian ladinian transition are associated with organic carbon rich shales which were deposited in a deeper water environment under anoxic conditions iii in the entirely dolomitized platform facies of the dolomia principale hauptdolomit formation a very early generation of fine crystalline dolomite occurs in the shallowest part of evaporative peritidal cycles this platform extended over thousands of square kilometres along the tethys margin during the late triassic carnian and norian and large amounts of carbonate were deposited under hypersaline sabkha like conditions representing three distinct depositional environments these three different triassic systems show features in common with several dolomitization models developed from the study of modern dolomite forming environments for example the sabkha model the evaporative lagoon lake model the organogenic model and the microbial model although these actualistic models may be applicable to reconstruct the palaeoenvironmental conditions during dolomite formation dolomite forming processes during the triassic were apparently quite different from the modern world in terms of distribution and scale recent developments in stable isotope geochemistry and high resolution geochemical probing offer the possibility to make better reconstructions of triassic palaeoceanographic conditions and suggest a non actualistic approach to better understand dolomite formation during the triassic

isotopic hydrology and paleohydrology of the madagi kenya natron tanzania basin during the late quaternary abstract three generations of stromatolites mark the recent extension of paleolakes in the magadi natron basin on the border between tanzania and kenya the last two generations are observed some 50 m above the modern lakes level they are respectively attributed to the beginning of the last interglacial and to the pleistocene holocene transition analysis of the morphology and texture of the stromatolites reveals an ecological zonation between 0 and 10 14 m below the shoreline their relatively high 18o and 13c values reflect a long residence time for the paleolake water which would have been favorable to the establishment of an isotopic equilibrium between atmospheric co2 and the dissolved inorganic carbon despite deep carbon inputs in relation to carbonititic volcanism a study of the 18o 2h and 13c contents in the hydrothermal springs perennial rivers and lacustrine brines indicates that during the last period of significant renewal of the deep saline groundwater precipitation was depleted in heavy isotopes when compared to modern precipitation this difference corresponds to a drop of about 600 1000 m in altitudinal hydroclimatic zones finally a comparison between the modern and paleohydrological data leads us to believe that the stabilization of the paleolake shorelines at the same altitude can be attributed to control by the water table level in the aquifer of the basaltic plateaus east of the basin

quaternary sedimentology of the rose creek fan delta walker lake nevada usa and implications to fan delta facies models the 3 2 km long rose creek fan delta of west central nevada is prograding from an active rift margin into the 32 m deep walker lake a case study of the forms processes and facies of this fan delta reveals that the proximal and medial zones mainly are of sub aerial origin and the distal zone is of lacustrine origin pebbly to bouldery rock avalanche mounds 100 m thick facies a and muddy to bouldery debris flow levees 0 5 to 2 0 m thick facies b dominate the proximal zone whereas mostly matrix supported cobbly pebbly debris flow lobes 0 1 to 1 0 m thick facies c typify the medial zone surficial pebble lags and gully fills facies d are widespread in both zones but in exposures comprise only partings or lenticles between debris flow units the distal fan delta mainly consists of lakeshore to lake bottom tracts formed by extensive wave reworking of debris flow facies nearshore deposits include erosional cobbly boulder lag beaches facies e pebbly constructional beaches attached at headcuts or on barrier spits facies f pebbly upper shoreface facies g and sandy lower shoreface facies h tracts positioned lakeward of the beach and pebbly landward dipping foresets facies i and backshore pond sand and mud facies j present landward of the spits erosional lag beaches fringe the windward north side of the fan delta front attached constructional beaches characterize the central zone and southward elongating barrier spits typify the leeward south side extending from the zone of greatest projection of the fan delta into the lake shoreline facies asymmetry results from largely unidirectional longshore drift caused by high fetch to the north and minimal fetch to the south combined with the arcuate shape of the fan delta front the spits overlie a platform deposited below common wave base consisting of south east trending cones of pebbly gilbert foresets facies k and sandy toesets facies l typically slumped silt and mud facies m fringe both this platform and lower shoreface sand in deeper water this case demonstrates facies types and patterns that are inconsistent with the widely promoted fan delta facies model having a front consisting of an apron of radially directed gilbert foresets deposited where sub aerial flows enter the lake the rose creek fan delta front instead features a sharp contact between sub aerial and lakeshore facies formed where waves erode sort and redistribute heterogeneous debris flow sediment into the various shallow to deep lake facies gilbert foresets are present only in the lee of the fan delta where sediment moving by longshore drift reaches the brink of the spit front this facies scenario results from the infrequency of fan building events versus nearly constant wind induced waves a scenario that in contrast to the popular gilbert model probably is the norm for fan deltas the level of walker lake and thus the position of wave reworking on the rose creek fan delta fluctuated over a range of 157 m during the last 18 kyr producing complex interfingering between sub aerial and lakeshore facies across a 1700 m wide radial belt typifying a wave modified freestand lacustrine fan delta

microbial dolomite precipitation using sulfate reducing and halophilic bacteria results from qinghai lake tibetan plateau nw china past studies have revealed the important roles of certain types of microbes in precipitating dolomite in coastal hypersaline lakes and lagoons and marine sediments however it is still poorly known if other types of microbes can precipitate dolomite in low salinity lakes without any marine influence the objective of this study was therefore to expand the microbial dolomite precipitation model by investigating the origin of dolomite in the sediments of qinghai lake an inland slightly saline lake salinity 12 5 g l on the tibetan plateau nw china this objective was accomplished via an integrated approach combining geochemical investigations of natural dolomite present in the lake sediments with laboratory precipitation experiments a 5 m sediment core was collected and pore water chemistry was analyzed calculation of saturation index indicated that dolomite was oversaturated in sediment pore water sub spherical grains of dolomite with abundant pores and microstructures similar to microbially precipitated dolomite were found in the sediments suggesting a biogenic origin dolomite precipitation experiments were set up in the laboratory to simulate the natural conditions of the qinghai lake dolomite was precipitated using sulfate reducing bacterium srb desulfotomaculum ruminis and aerobic halophilic bacterium halomonas marina present in the qinghai lake sediments aqueous geochemistry and mineral characterization data collectively suggest that dolomite precipitation was accomplished via increase of ph and alkalinity to overcome the kinetic inhibitors and exopolymeric substances eps and cell walls as possible nucleation sites these results expand the current microbial dolomite precipitation model and have implications for dolomite formation in slightly saline inland lake environments

single and double exhumation of fault blocks in the internal sesia lanzo zone and the ivrea verbano zone biella italy the combination of magmatic structural and fission track ft data is used to unravel oligocene miocene near surface tectonics in the internal western alps this includes reburial of parts of the already exhumed sesia lanzo zone and their subsequent re exhumation we define blocks mainly on the base of their oligocene miocene cooling history ft data and on published paleomagnetic data the preservation of a paleosurface allows a detailed reconstruction of the exhumation burial and re exhumation of different tectonic blocks near surface rigid block rotation is responsible for the reburial of the lower oligocene paleosurface in part of the sesia lanzo zone the cervo block and for the conjugate uplift of deeper portions of the ivrea verbano zone the sessera ossola block this block rotation around the same horizontal axes produces in the currently exposed portions of the two blocks quite different temperature time paths while the surface of the cervo block is buried the lower part of the sessera ossola block is uplifted the rotation is constrained between the age of emplacement of the biella volcanic suite on top of the sesia lanzo zone 32 5 ma and the intrusion of the valle del cervo pluton 30 5 ma after this relative fast movements the concerned blocks remained in or underneath the partial annealing zone of zircon until in aquitanian times they were rapidly uplifted into the partial annealing zone of apatite the further stage of exhumation out of the partial annealing zone of apatite extends over the entire miocene at that time units of the external western alps underwent fast exhumation external brianconnais valais in addition to the well known post collisional deformation in the axial and external western alps the internal units i e the upper plate hold an apparent stable position in terms of exhumation

bacterially induced dolomite precipitation in anoxic culture experiments to study the process of microbial mediated dolomite formation growth experiments were carried out with selected bacterial cultures under anoxic environmental conditions simulating those found in lagoa vermelha a hypersaline lagoon in brazil where dolomite precipitation occurs specifically we report the isolation of a particular strain of sulfate reducing bacteria lvform6 from lagoa vermelha sediment which apparently promotes the formation of nonstoichiometric dolomite sulfate reducing bacteria grown in a synthetic liquid medium produced dolomite during 30 days incubation at 30 c the precipitates have morphologies similar to those observed in lagoa vermelha sediment our results demonstrate that sulfate reducing bacteria can influence dolomite precipitation under controlled lowtemperature anoxic conditions and imply that anaerobic microorganisms can play an important role in carbonate sedimentation they may have been particularly significant in earth s earliest history when a more reducing atmosphere existed

biogenic magnetite in stromatolites i occurrence in modern sedimentary environments abstract in this paper we report the occurrence of biogenic ultra fine grained single domain magnetite in both marine and non marine modern stromatolitic environments magnetotactic bacteria were found associated with the microbial communities involved in the deposition of laminated sediments at laguna figueroa baja california mexico and carbonate stromatolitic nodules at sugarloaf key fl u s a these bacteria and the ultra fine grained magnetite they produce have a profound effect on the magnetic properties of the sediments the presence of this single domain magnetite was detected using rock magnetic methods while the morphology was identified by transmission electron microscopy examination by rock magnetic methods of microbial mats and laminated sediments from solar lake sinai guerrero negro baja california mexico and shark bay australia as well as stromatolites from shark bay and clifton lake australia bacalar lake and cuatro cienegas mexico and walker lake u s a indicate these localities contain single domain magnetite biogenic magnetites of presumed bacterial origin have been extracted from shark bay mats walker lake stromatolites and bahama carbonates these findings suggest that bacterial magnetites can be recognized as the tiniest trace fossil and may be found preserved in fossil stromatolites and laminated sediments

structural stratigraphic models for extensional basins of half graben type abstract the equations describing subsidence in isostatically compensated basins formed by lithospheric stretching can be modified to define a notional depth to decollement which relates subsidence to the stretching factor in much the same way as the depth to a physical sole fault relates subsidence to extension in uncompensated thin skinned basins for a constant density basin fill this notional depth to decollement corresponds to the level of no vertical motion within the lithosphere in general the physical sole to a system of detached normal faults will not lie at the notional depth to decollement and must move vertically during extension this vertical movement must be implicitly corrected for before area bed length balancing is used to determine the depth to a physical sole fault the notional depth to decollement has been calculated numerically for an extensional basin loaded by sediments which follow a north sea shaly sand compaction law and increases with from 5 to 9 km a set of model half grabens has been constructed by combining subsidence curves generated by this shaly sand model with geometric relationships inherent in the rotated planar domino mechanism of upper crustal faulting for comparison a similar set of half grabens was constructed for an uncompensated basin with a sole fault at 7 5 km depth in both cases large fault to fault spacings give rise to large basement relief and consequent footwall uplift and erosion small fault spacings give rise to simple listric growth faults intermediate values generate more complex uplift subsidence histories which are capable of giving rise to such commonly observed features as rounded eroded fault block shoulders local regression followed by onlap or exponentially increasing subsidence at the crest of the fault block

carbonates associated with alluvial fans an example from the messinian colombacci formation of the pietrarubbia basin northern marche italy abstract the carbonates known as colombacci of a terrestrial series deposited within an upper messinian basin in marche italy have been examined by means of stable isotope analyses aas analyses study of thin sections and sem observations the results of the analyses attest to the in situ production of the carbonates in shallow ponds induced by biological activity as well as to the considerable influence of meteoric waters on late messinian sedimentation the carbonates were compared to coeval limestones from neighbouring basins the composition of the latter is indicative of precipitation out of meteoric water as well minor differences reflect the distinct settings of the separate marginal basins of the messinian mediterranean

shear angle and amount of extension calculations for normal faults emanating from a detachment implications on mechanisms to generate rollovers abstract the reconstruction restoration modelling of normal faults both listric and planar emanating from a detachment at depth and their associated rollover folds using the vertical or inclined shear method is widely utilized because its simplicity and the information it can provide however it has a rather serious issue derived from the uncertainty about the shear angle the type of shear and the amount of extension that should be employed in each situation here we describe a new methodology that using easily acquired input data allows estimation of whether the shear was vertical antithetic or synthetic and the values for both the shear dip and the amount of extension these calculations rely on the use of graphs of throw versus heave for different horizons affected by the normal fault and the associated rollover and are checked using an area based method which permits the determination of whether these values are correct these graphs may be used as a predictive tool or as a guide to show how the assumptions deviate such as distinguishing quickly whether other mechanisms apart from vertical inclined shear took place the effects of syn extension sedimentation and reverse fault reactivation on the proposed method are also examined the analysis of experimental and natural examples shows that the initiation of some rollovers with a component of fault propagation and or drag folding and or development of a crestal collapse graben cause the estimated shear dips to be smaller than the actual values and the amounts of extension to be greater in addition these analyses show that the shear dip may increase with increasing extension

reverse drag revisited why footwall deformation may be the key to inferring listric fault geometry abstract although reverse drag the down warping of hanging wall strata toward a normal fault is widely accepted as an indicator of listric fault geometry previous studies have shown that similar folding may form in response to slip on faults of finite vertical extent with listric or planar geometry in this study we therefore seek more general criteria for inferring subsurface fault geometry from observations of near surface deformation by directly comparing patterns of displacement stress and strain around planar and listric faults as predicted by elastic boundary element models in agreement with previous work we find that models with finite planar planar detached and listric detached faults all develop hanging wall reverse drag folds all of these model geometries also predict a region of tension and elevated maximum coulomb stress in the hanging wall suggesting that the distribution and orientation of near surface joints and secondary faults may also be of limited utility in predicting subsurface fault geometry the most notable difference between the three models however is the magnitude of footwall uplift footwall uplift decreases slightly with introduction of a detachment and more significantly with the addition of a listric fault shape a parametric investigation of faults with constant slip ranging from nearly planar to strongly listric over depths from 1 to 15 km reveals that footwall fold width is sensitive to fault geometry while hanging wall fold width largely reflects fault depth application of a graphical approach based on these results as well as more complete inverse modeling illustrates how patterns of combined hanging wall and footwall deformation may be used to constrain subsurface fault geometry

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

fluid inclusion analysis of twinned selenite gypsum beds from the miocene of the madrid basin spain implication on dolomite bioformation this research work is centred on continental lacustrine gypsum deposits of miocene age cropping out in the easternmost part of the madrid basin these gypsum deposits accumulated in a continental saline lake are characterized by a spectacular distinctive christmas tree morphology and a peculiar dolomite replacement a combination of microscopic petrography and scanning electron microscopy and analytical techniques fluid inclusion microthermometry x ray energy dispersive spectroscopy and x ray diffractometry was used in order to study the crystallographic distribution and the composition of the fluid inclusions within the gypsum the objectives were to characterize the continental brine from which the mineral precipitated and to detect mineral and element traces that could indicate early diagenetic processes altering the gypsum deposits data from primary fluid inclusions indicated that gypsum precipitated from an aqueous fluid lake water of low to moderate total salinity between 20 and 90 g l nacl secondary fluid inclusions represent interstitial lake brine in contact with gypsum slightly enriched in total salt content as crystal formation proceeded textural ultrastructural and microanalytical analysis indicate that the presence of dolomite precipitates inside the gypsum layers is related to the microbial colonization of the gypsum deposits and the biomineralization of the cell walls and extracellular polymeric substances around the cells our investigation emphasizes necessity of a multidisciplinary approach to assess geobiological processes

lithifying microbial mats in lagoa vermelha brazil modern precambrian relics abstract bacterial populations microbial and mineral forming processes and their products were analyzed in lagoa vermelha brazil the microbial mat and underlying sediment were studied as a unique system to define the boundary conditions responsible for high mg calcite and dolomite formation in the uppermost layers of the microbial mat oxygenic photosynthesis and aerobic respiration resulted in calcite precipitation whereas in the underlying anoxic layers of the mat sulfide oxidation and sulfate reduction induced formation of a range of carbonate minerals with increasing mg concentrations the chemical mineralogical and biological conditions presently found in lagoa vermelha may have been more common in the precambrian the microorganisms performing the metabolic processes related to carbonate mineral formation within lagoa vermelha s hypersaline microbial mat may have already been present in the precambrian thus microbial carbonate as a biomineral could be a record of metabolism throughout geological time

theoretical model of block rotation along circular faults abstract the overall structure of rifts is often controlled by normal faults and tilted blocks in several rifts the crests of the blocks have been strongly eroded while the associated half graben has undergone large subsidence the relative movement of the crest of a block can be analysed assuming simple circular geometry of its bounding fault in such a geometric model subsidence of the half graben and relative uplift of the crest are connected and this coupling can be quantified as a function of the fault shape and of the block width knowing the geometry of a fault we can calculate the movements of a rigid block rotating along this fault inversely by knowing the tilting of a block it should be possible to estimate the geometry of the guiding fault i e its detachment depth if we assume the fault to be listric numerical results show that a large block can undergo an absolute uplift the direction of the movement depends on the initial width of the block and is quite independent of the tilt angle except for very low angle faults

stromatolite lamination frequency walker lake nevada implications for stromatolites as biosignatures lamination in stromatolites considered some of the oldest fossils on earth is commonly interpreted to record the periodic response of a microbial community to daily seasonal or perhaps yearly environmental forcing but the inability to date ancient stromatolites precludes an understanding of the lamination formation processes we use high resolution 14c dating of holocene stromatolites from walker lake nevada united states to construct a record of lamination rate over the course of accretion laminae formed with a period of 5 6 1 6 yr lamination at the base of the structure 1 6 2 8 1 9 yr lamination in the middle and 4 5 0 8 yr lamination at the top of the laminated portion the predominant 4 6 yr periodicity indicates that lamination formation is likely more closely related to regional climate forcing e g el nino southern oscillation versus the typical diurnal or seasonal changes in microbial mats traditionally assumed for most ancient stromatolites thus generalizations regarding the influence of microbial mats on stromatolite lamination and the use of stromatolites as biosignatures need careful consideration

fluvial plain lacustrine facies transition in the cretaceous sindong group south coast of korea abstract along the south coast of korea a 250 m stratigraphic interval corresponding to the uppermost hasandong and lowermost jinju formations of the cretaceous sindong group has been studied in terms of facies transition the interval displays a passage from a fluvial plain facies association consisting of floodplain levee crevasse splay and channel facies to a lacustrine facies association composed of crevasse splay channel overbank sheet splay marginal lake open lake and stream mouth bar facies between these two associations a transitional facies association occurs in which some facies of both associations alternate in the lacustrine association shallow marginal lake deposits including channel facies are gradually replaced upwards by deeper open lake deposits made up of open lake and stream mouth bar facies during the hasandong jinju period small channels which originated in the uplifted areas beyond the fault s bounded western margin of the gyeongsang basin flowed through alluvial fans developed along the fault s and a fluvial plain to a lake which occupied the central part of the basin as a result the lake shoreline was non coincident with the basin margin sinuosity of the streams increased downstream reaching a maximum near the shoreline but decreased in the marginal lake areas the fluvial plain was progressively submerged by the transgression of a non deltaic shoreline resulting in a deepening of the depositional sites from a fluvial plain through a marginal lake to an open lake setting intermittent channel overflows deposited levee and crevasse splay facies in the fluvial plain and crevasse splay and overbank sheet splay facies in the marginal lake stream mouth bars were developed at the ends of the sublacustrine channels by hyperpycnal density flows operated mainly during floods the non deltaic shoreline was formed due to 1 maintenance of shallow and wide marginal lake areas 2 dispersal of sediments by small channels 3 continuous shifting of the channels and their mouths and 4 direct transport of coarser sediments to the open lake and their subsequent wide dispersions

cross section restoration and balancing as aid to seismic interpretation in extensional terranes structural cross sections in extensional terranes can be improved significantly by restoration and balancing principles originally developed for fold and thrust belts in rift basins and passive continental margins use of restoration techniques in analysis of growth faults elongate salt swells and other extensional structures results in geologically valid seismic interpretations sections must be in depth as opposed to two way time and oriented in the direction of material transport and decompaction must accompany restoration hanging wall deformation is characterized by different processes depending on the mechanical properties of the rocks being extended and can be approximated by various geometric and kinematic models antithetic faulting is analogous to antithetic shear combined antithetic and synthetic faulting is approximated by vertical shear domino style extension corresponds to rigid body rotation and flexural slip flow is modeled by bed length balance methods selection of the appropriate algorithm is crucial for proper restoration and balancing applications of these techniques illustrated here using seismic data from the gulf coast and north sea include 1 evaluating and adjusting seismic interpretations 2 projecting listric fault trajectories to depth 3 delineating more accurately the geometry and extent of hydrocarbon reservoirs 4 identifying new exploration leads 5 determining the deformation history of an area and 6 constraining the timing and direction of hydrocarbon migration and entrapment

dolomite mountains and the origin of the dolomite rock of which they mainly consist historical developments and new perspectives beginning in the late 18th century the dolomite mountains in northern italy have been the location for major sedimentological developments from the discovery of the mineral dolomite to the formulation of the coral reef hypothesis to explain the origin of the massive dolomite structures that define the splendid scenery of the region further the dolomite mountains have inspired voluminous research into the origin of dolomite questioning whether dolomite is a primary precipitate or a secondary replacement product recently with the recognition that microbes can mediate dolomite precipitation a new geomicrobiological approach combining the study of modern natural environments with bacterial culture experiments is now being used to calibrate or interpret microbial evidence derived from the dolomite rock record this three pronged methodology applied to the study of dolomite formation holds great promise for future research into the dolomite problem and provides a new impetus to revisit the dolomite mountains in the 21st century

microbial mediated pre salt carbonate deposition during the messinian salinity crisis calcare di base fm southern italy abstract a multi scale analysis of sedimentary carbonate facies and post sedimentary diagenetic features of the calcare di base formation the precursor to evaporites in upper messinian successions of northern calabria and central sicily has revealed their microbial bio mediated origin massive to laminated microbial boundstones represent the most common sedimentary facies forming flat to low relief cm to m scale stromatolitic and thrombolitic bodies the fabric of the micrite varies from peloidal to aphanitic and almost always preserves filamentous bacteria which characterized the original microbial mat the mat was dominated by sulphur oxidizing bacteria belonging to the thiotrichaceae but there is evidence for a more complex community with sulphate and or nitrate reducing bacteria all being responsible for the mediation of the carbonate precipitation microbial boundstones are rich in pseudomorphs of ca sulphate and halite which formed during the deposition of the microbial carbonate layers of primary gypsum are interbedded locally with carbonates suggesting the presence of restricted marine conditions the stable o and c isotopic composition of the carbonates that vary from dolomite to aragonite and calcite suggests a complex interplay between arid to humid climatic changes expressed cyclic interbedding of the carbonate with marl marlstone later diagenetic events mainly consist of phreatic meteoric recrystallization and cementation although considered as diachronous the microbial carbonates can be mapped out over a distance of more than 500 km across southern italy this indicates near constant environmental conditions upon the central mediterranean shelf at the beginning of the salinity crisis deposition of the extensive subaqueous microbial deposit that largely comprises the calcare di base is envisaged to have taken place across a shallow to moderate depth platform with local slopes into deeper water areas where some resedimentation occurred

nucleation of calcium carbonate on bacterial nanoglobules nucleation of calcium carbonate on microbial cell material may have been the dominant mode of microbial carbonate formation during most of earth s history current knowledge predicts that nucleation takes place on the cell surface or on extracellular polymeric substances however the initial nucleation steps have not been described in detail and the process remains elusive here we describe the bacterial nucleation of calcium carbonate at the nanometer scale in our precipitation experiment with sulfate reducing bacteria srb the bulk of calcium carbonate precipitates on hundreds of individual globules 60 200 nm in diameter globules originate from the srb cell surface but calcify significantly only when released to the culture medium similar globules have been observed albeit at a much larger scale in other bacterial precipitation experiments and in many natural microbial carbonates suggesting that the process we describe could be an important step in microbial calcification

bacterial fossils and microbial dolomite in triassic stromatolites triassic stromatolitic dolomite from italy preserves mineralized bacterial remains one of the first unequivocal identifications of such structures in the geological record they consist of empty spheroids 1 0 m diameter resembling coccoid bacteria and smaller 150 400 nm objects interpreted as dwarf bacterial forms occurring within and between syn sedimentary dolomite crystals moreover gently folded sheets 100 200 nm thick and several micrometers long form a sub polygonal network reminiscent of eps extracellular polymeric substance their granular textured surfaces suggest bacterial degradation of original organic matter these features confirm a biological origin for the stromatolites as in modern microbial mats and the preserved original geochemical signatures indicate early precipitation of mg carbonates induced through microbial sulfate reducing metabolic activities

polyphasic carbonate precipitation in the shallow subsurface insights from microbially formed authigenic carbonate beds in upper miocene sediments of the tertiary piedmont basin nw italy article i nfo authigenic methane derived carbonates hosted in upper miocene slope sediments of the tertiary piedmont basin nw italy are studied by a multidisciplinary approach including petrography stable oxygen and car bon isotopes of carbonates as well as lipid biomarkers in order to explore the relationship between microbial activity and carbonate precipitation in the shallow subsurface the studied rocks show a bed parallel geom etry and are characterized by dolomitic intergranular cement which is typified by positive 13 c values as high as 6 2 vpdb a striking feature of some dolomite beds is an intricate network of septarian like cracks filled with both injected sediments and polyphasic carbonate cements prokaryotic molecular fossils in the dolomite beds comprise archaeol 13 c 40 vpdb and various bacterial dialkyl glycerol diethers dages 13 c 30 vpdb strongly suggesting that dolomite precipitation took place at the interface of the zones of archaeal methanogenesis and bacterial sulphate reduction in contrast extremely negative 13 c values of carbonate cements as low as 56 3 vpdb and various archaeal and bacterial molecular fossils e g pentamethylicosane pmi 106 vpdb are recorded in the crack filling carbonate cements these cements precipitated due to anaerobic oxidation of methane coupled to sulphate reduction we pro pose a scenario for the formation of the diagenetic beds suggesting that carbonate precipitation was the re sult of three microbially driven processes sulphate reduction methanogenesis and finally anaerobic oxidation of methane this unusual sequence was a consequence of a dynamic change of environmental geo chemical conditions and fluid circulation patterns that prevailed in the ancient subseafloor during early dia genesis of the unconsolidated sediments anaerobic oxidation of methane which usually predates methanogenesis during increasing burial postdates methanogenesis in case of the septarian like beds after the beds were affected by crack formation induced by overcritical pore pressure allowing the ingress of sulphate rich water from above and methane rich water from below

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

biocalcification of sand through ureolysis biological processes may provide great and previously unexplored opportunities for cost effective in situ improvement of the engineering properties of soil a laboratory study was conducted to evaluate the changes in geomechanical properties of sand attributable to the formation of calcium precipitates induced through ureolysis catalyzed by sporosarcina pasteurii s pasteurii specifically direct shear and california bearing ratio cbr tests were conducted on sand specimens subjected to treatment by growing resting and dead s pasteurii cells in completely stirred tank reactors and completely mixed biofilm reactors respectively scanning electron microscopy analyses were also conducted to evaluate microbially induced precipitation the results of the study show that the bacterial cells effectively improved the geomechanical properties of the sand growing cells improved the sand properties owing to microbially induced precipitation and related pore volume changes whereas dead and resting cells ge

trends in oolite dolomitization across the neoproterozoic cambrian boundary a case study from death valley california abstract changes in dolomite fabric within the neoproterozoic cambrian stratigraphic succession in death valley california preserve evidence for a first order biogeochemical change in early diagenetic environments through the neoproterozoic cambrian transition oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time two key generalizations can be made 1 late neoproterozoic dolomitization in death valley occurred very early in the paragenetic sequence was volumetrically significant and preserved fine detail of the original aragonite or calcite precursor and 2 early cambrian dolomitization in death valley occurred late in the paragenetic sequence with burial was commonly incomplete and obliterated details of the calcite or aragonite precursor early diagenetic dolomitization of any kind was absent the dolomitization fabrics in death valley parallel the presumed pattern of seawater mg ca with respect to mimetic dolomitization in the paleozoic low mg ca little no mimetic dolomitization and cenozoic higher mg ca more common mimetic dolomitization however the volume of neoproterozoic mimetic dolomitization is immense compared to the cenozoic examples we speculate that a fundamental shift in the early diagenetic environment occurred in late neoproterozoic time perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment capping microbial mats an oscillatory trend in the fabric and volume of dolomite through the phanerozoic has been suggested but the termination of volumetrically significant mimetic dolomitization across the neoproterozoic cambrian transition a critical time in earth history for many reasons dwarfs the oscillatory trends noted in the phanerozoic

area length and thickness conservation dogma or reality abstract the basic assumption of quantitative structural geology is the preservation of material during deformation however the hypothesis of volume conservation alone does not help to predict past or future geometries and so this assumption is usually translated into bed length in 2d or area in 3d and thickness conservation when subsurface data are missing geologists may extrapolate surface data to depth using the kink band approach these extrapolations preserving both thicknesses and dips lead to geometries which are restorable but often erroneous due to both disharmonic deformation and internal deformation of layers first the bolivian sub andean zone case is presented to highlight the evolution of the concepts on which balancing is based and the important role played by a decoupling level in enhancing disharmony second analogue models are analyzed to test the validity of the balancing techniques chamberlin s excess area approach is shown to be on average valid however neither the length nor the thicknesses are preserved we propose that in real cases the length preservation hypothesis during shortening could also be a wrong assumption if the data are good enough to image the decollement level the chamberlin excess area method could be used to compute the bed length changes

miocene lacustrine algal reefs southwestern snake river plain idaho straccia f g wilkinson b h and smith g r 1990 miocene lacustrine algal reefs southwestern snake river plain idaho sediment geol 67 7 23 the hot spring limestone is a shallow water algal carbonate within a late tertiary transgressive lacustrine sequence exposed in the southwestern snake river plain this 5 m thick lensoid sequence crops out over an 80 km 2 area that closely approximates original areal extent of nearshore carbonate accumulation reefal bodies consist of closely packed algal cylinders several decimeters in height each of which includes a dense laminated carbonate wall surrounding porous digitate carbonate that radiates outward and upward from one or more hollow tubes these coalesce upsection into separate vertical columns several meters in diameter moderately well sorted terrigenous and molluscan debris deposited between columns during growth indicates these structures were resistant to wave erosion and therefore were true reefs thick rings of littoral carbonate surrounding the upper walls of each column record the final stages of reef development structural attributes exhibited by these miocene carbonate bodies are also common to a number of tertiary and quaternary algal buildups reported from other lacustrine settings although features within the hot spring limestone are complex in gross morphology and structural detail both columnar reefs and algal cylinders display little variation in size shape or internal structure between areas of varying water depth and wave energy thus reflecting the importance of biological processes as well as physical processes during reef development

ca carbonates precipitation and limestone genesis the microbiogeologist point of view abstract experiments show that the production of carbonate particles by heterotrophic bacteria follows different ways in heterotrophy the passive carbonatogenesis is generated by modifications of the medium that lead to the accumulation of carbonate and bicarbonate ions and to the precipitation of solid particles it is induced by several metabolic pathways of the nitrogen cycle ammonification of amino acids degradation of urea and uric acid dissimilatory reduction of nitrates and of the sulphur cycle dissimilatory reduction of sulphates the active carbonatogenesis is independent of the mentioned metabolic pathways the carbonate particles are produced by ionic exchanges through the cell membrane following still poorly known mechanisms in autotrophy non methylotrophic methanogenesis and cyanobacterial photosynthesis also may contribute to the precipitation of carbonates autotrophic carbonates as carbonatogenesis is neither restricted to particular taxonomic groups of bacteria nor to specific environments it has been an ubiquitous phenomenon since precambrian times carbonatogenesis is the response of heterotrophic bacterial communities to an enrichment of the milieu in organic matter after a phase of latency there is an exponential increase of bacterial numbers together with the accumulation of metabolic end products these induce a ph increase and an accumulation of carbonate and hydrogenocarbonate ions in the medium this phase ends into a steady state when most part of the initial enrichment is consumed and there is a balance between death and growth in bacterial populations particulate carbonatogenesis occurs during the exponential phase and ends more or less after the beginning of the steady state the active carbonatogenesis seems to start first and to be followed by the passive one which induces the growth of initially produced particles in eutrophic conditions the first solid products are patches that appear on the surface of the bacterial bodies and coalesce until forming a rigid coating and or particles excreted from the cell all these tiny particles assemble into biomineral aggregates which often display precrystalline structures these aggregates grow and form biocrystalline build ups which progressively display more crystalline structures with growth in oligotrophic conditions the primary solid products are rapidly smoothed in the crystalline structure and leave no trace in present aqueous environments apart from deep ocean the potential efficiency of heterotrophic bacterial carbonatogenesis in ca carbonate sedimentation is much higher than autotrophic or abiotic processes it much more likely accounts for extensive apparently abiotic limestone formation than any of the latter as far as biodetrital particles are concerned it may be observed that the shells and tests of organisms are built from the activity of cellular organites which are nowadays considered by a number of biologists as endosymbiotic bacteria thus apart from probably mythical purely evaporitic and autotrophic ones most limestones must be considered as principally of heterotrophic bacterial origin as the carbon of limestones is issued from organic matter bacterial heterotrophic carbonatogenesis appears as a fundamental phenomenon in the relationships between atmosphere and lithosphere during the biogeological evolution of the earth

microbial fossilization in carbonate sediments a result of the bacterial surface involvement in dolomite precipitation recent dolomitic sediment samples from lagoa vermelha brazil were examined microscopically to study the process of bacterial fossilization in carbonate sediments bacteria like bodies were intimately associated with carbonate mineral surfaces and coatings on the former demonstrate the calcification of single bacterial cells the bacterial fossilization process in lagoa vermelha sediments was simulated in the laboratory by cultivation of mixed and pure cultures of sulphate reducing bacteria which were isolated from the lagoa vermelha sediments these cultures produced carbonate minerals that were studied to provide insight into the initiation of the fossilization process in mixed culture experiments bacterial colonies became calcified whereas in pure culture experiments single bacterial cells were associated with dolomite surfaces dolomite nucleated exclusively in bacterial colonies intimately associated with extracellular organic matter and bacterial cells electrophoretic mobility measurements of the bacterial cells in electrolyte solutions demonstrated the specific adsorption of ca 2 and mg 2 onto the cell surfaces indicating the role of the bacterial surface in carbonate nucleation and bacterial fossilization the affinity of the cells for mg 2 was related to the capability of the strains to mediate dolomite formation combined with sulphate uptake which dissociates the mgso 4 o ion pair and increases the mg 2 availability the concentration of mg 2 ions in the microenvironment around the cells where the conditions are favourable for dolomite precipitation may be the key to overcome the kinetic barrier to dolomite formation these results demonstrate that bacterial fossilization is a consequence of the cell surface involvement in carbonate precipitation implying that fossilized bacterial bodies can be used as a tool to recognize microbially mediated carbonates

kalkowsky s stromatolites revisited lower triassic buntsandstein harz mountains germany abstract the term stromatolite was proposed by kalkowsky 1908 z dt geol ges 60 68 125 for structures he discovered in the carbonates of the lower triassic buntsandstein of the harz mountains in central germany we have re examined these localities to clarify kalkowsky s descriptions and to deduce the environmental conditions of stromatolitic growth the lower buntsandstein was deposited in a closed lacustrine basin the central european permo triassic basin the depositional environment was a playa lake with variable salinity in the centre of the basin oolite beds are intercalated in the mainly siliciclastic succession which is composed of 20 fining upward cycles thought to have been produced by climatic variations stromatolites occur almost exclusively on the surface of oolite beds stromatolitic structures consist of millimetre thick laminated crusts and centimetre high branching columnar forms smaller and bigger 2 m high domes are composed of secondary order columns and intercalated crusts occasionally upper surfaces of the domes are pitted due to syndepositional dissolution despite of the overall intense sparitization two groups of stromatolitic microstructures can be distinguished spongy fenestrate and fan like fabrics the spongy fenestrate fabric is similar to that one caused by the recent cyanobacteria phormidium calothrix and dichothrix and the fan like fabric to rivularia laminated crusts called stromatoid by kalkowsky are interpreted as being of cement origin and formed by quick precipitation the change from columnar to domical forms may reflect changes of hydrodynamic conditions clear photoautotrophism suggests that cyanobacteria or eukaryotes were the dominant organisms involved in the formation of kalkowsky s stromatolites influx of ooids or siliciclastic mud interrupted or terminated the stromatolitic growth the morphology growth pattern and fabrics of stromatolites were most likely controlled by ecological factors which all seem to be related to the general hydro climatic evolution of the lower buntsandstein

alternative origins for nannobacteria like objects in calcite more than 40 calcite precipitation experiments were performed under sterile conditions in order to investigate the origins of 25 300 nm spherical rod and ovoid shaped objects that have been widely interpreted as evidence of nanometer scale life i e nannobacteria individual experiments included the addition of soluble organic compounds common species of eubacteria or phage induced eubacterial lysates these experiments indicate that many of the nanometer scale objects have inorganic or nonnannobacterial origins in the precipitation experiments calcite formed euhedral crystals 50 800 nm in diameter and smaller 50 nm anhedral or rounded particles or protocrystals the small anhedral or rounded solids resembled nannobacteria the relative amount of anhedral or rounded calcite was greatest in experiments with a dissolved organic component these controlled experiments are in accord with observations that rounded nanometer scale objects are more common in minerals formed in organic rich environments bacterial fragments occur as rounded to irregularly shaped particles that included cell wall fragments expulsed cytoplasm and relict capsules that also closely resembled nannobacteria acid etching of the large euhedral crystals produced in the precipitation experiments also resulted in the formation of nanometer scale features that resembled nannobacteria in natural carbonates the shapes of the etching artifacts vary as a function of the strength of the acid and the duration of etching much caution is advisable in interpreting the origin of rounded features 50 nm

sedimentological and palynological evidence of regional climatic changes in the campanian to paleocene sediments of the rocky mountain foothills canada abstract evidence of paleoclimatic variations in the upper campanian to lower paleocene post wapiabi sequence of strata is visible both laterally between the central and southern foothills and vertically in the stratigraphic record these differences are expressed by the distribution of climatically sensitive sediments i e coal and caliche and associated palynological assemblages within floodplain facies as well as by changes in the style of fluvial channels the interpretation of a semi arid environment for mature caliche paleosols is supported by the impoverished character of the associated palynological assemblages both in terms of diversity and the number of specimens recovered and by the conspicuous presence of classopollis the lateral extent of the semiarid floodplain facies and associated broad and mobile channel deposits is limited to the southern part of the basin no signs of a caliche facies have been found in the post wapiabi strata of the central foothills instead some of the floodplain deposits associated with meandering streams in this part of the basin contain numerous thick coal bearing intervals the relative climatic differences between the more humid central part and the drier southern part of the basin prevailed throughout the entire post wapiabi interval as this cannot be satisfactorily explained by the position of the sea shore or by orographic influences alone there was probably another external factor this could have been the pattern of atmospheric circulation such as that responsible for the present day climatic differences existing between the southern and central foothills semiarid floodplain facies occur at two levels in the stratigraphic column of the southern foothills in the late campanian belly river formation and in the upper maastrichtian to lower paleocene willow creek formation they correspond to regressive episodes of the epicontinental seaway intervening between these semiarid floodplain facies are the marine shales of the bearpaw formation and both overlying and underlying them thin coal beds these represent recurrent periods of swamp growth and peat accumulation in back barrier environments lagoon fill and supratidal marsh respectively because of the proximity of these swamps to the sea shore it is difficult to assess the influence of climate on the relative humidity of these settings on strictly sedimentological grounds however the terrestrial components of the palynological assemblages recovered from these marginal marine settings and correlative lacustrine sediments containing fresh water stromatolites but no coal in the central foothills are in sharp contrast with the impoverished assemblages known from the caliche facies the former are prolific diverse and contain a number of triprojectate pollen species which indicate a relatively humid climate during the latest campanian time

tension fractures and extensional tectonics a characteristic pattern of normal faults that we have studied in several regions illustrates successive geometries relating to increasing amounts of extension from 10 50 gulf of suez egypt to 50 100 western gulf of california mexico up to 200 southern basin and range usa tension fractures formed at an early stage of extension have an essential role the resulting structure is described as a multiple tilted packs of cards model in which rotational deformation within the major blocks is accommodated by normal faulting on previously formed tension fractures thus defining smaller blocks of second and third generation

modern lacustrine stromatolites walker lake nevada abstract the walker river drainage basin occupies about 10 000 km2 in western nevada and parts of california and is essentially a closed hydrologic system which drains from the crest of the sierra nevada in california and terminates in walker lake nevada walker lake trends north and is about 27 4 km long and 8 km wide with water depths exceeding 30 5 m the lake is situated in an asymmetric basin with steep alluvial fans flanking the western shoreline wassuk range and more gentle but areally more extensive alluvial fans flanking the eastern shoreline gillis range exposed lake terraces and the present shoreline of walker lake record a sequence of pleistocene and holocene stromatolitic and tufaceous carbonate deposits small generalized and columnar stromatolites frequently encrusted on exposed coarse grained clasts or bedrock are present along parts of the nearshore margin of walker lake and at elevated lake stands columnar stromatolites as much as 4 cm high are subcylindrical to club shaped discrete and laterally linked at the base with local branching these digitate stromatolites start as wavy generalized stromatolites which are vertically transitional to small laterally linked cabbage heads with laminae which thicken over the crests although algal structures are not well preserved in the older stromatolites recent precipitation of low magnesium calcite occurs as smooth encrustations and as tiny mounds which are consistently associated with a diverse seasonally variable green and blue green algal community including cladophora glomerata ulothrix cf aequalis gongrosira schizothrix amphithrix janthina calothrix homeothrix spirulina anabaena lyngbya and entophysalis cladophora glomerata and a species of ulothrix which are the two most abundant algae within the walker lake stromatolite community are known to condition semi alkaline lake water by the removal of co2 from bicarbonate during photosynthesis such conditioning results in the precipitation of calcium carbonate which is trapped and bound by an understory of green and blue green algae the occurrence of stromatolites in highly siliciclastic lakes seems to be restricted to shoreline and nearshore environments and can be used to locate ancient lake margins

growth rate observation from the moss built checa travertine terrace central spain in situ observations of fossil and living specimens of the calcicolous mosses bryum pseudotriquetrum cratoneuron commutatum and catoscopium nigritum revealed very fast calcite depositional rates rhythmic layering in the fossil mosses corresponding with the seasonal climatic cycle suggests that the moss curtain occupied by these three mosses maintains the deposition of spongeous travertine layers at a mean rate of 4 cm a 1 a mean depositional rate of 4 2 cm a 1 may be calculated from measurements of the loss of bicarbonate from the springwater after it percolated through the moss curtain these rates suggest that the 8 m high travertine terrace of checa with a surface area of 800 m 2 did not exist two millennia ago mosses could be put to man s use for creating natural overgrowths on artificial surfaces an idea based on an allusion by wallner he observed that the thread forming blue green algae vaucheria builds travertine deposits at an annual rate of 0 7 1 4 cm we observed that the mosses cratoneuron commutatum and bryum pseudotriquetrum may form spongeous travertine layers at respective maximum rates of 11 and 14 cm a 1 this would reduce the time required to build natural overgrowths on artificial objects to a practical period of months

lake level and paleoenvironmental history of lake tanganyika africa as inferred from late holocene and modern stromatolites fossil and living stromatolites are abundant around the margins of lake tanganyika africa and provide a wealth of paleolimnologic and paleoclimatic information for the late holocene six lines of evidence show that stromatolites and cements are precipitating in the lake today 1 carbonate saturation state calculations 2 documentation of living stromatolites and their depth distribution 3 new stable isotope data showing the lake s present mixing state and ancient evaporation and inflow balance 4 new radiocarbon data and a reevaluation of apparent 14 c ages derived from lake tanganyika carbonates 5 the presence of modern mg calcite cements derived from lake waters and 6 the presence of modern biologically mediated mg calcite precipitates in the lake lake tanganyika s lake levels have been remarkably stable over the past 2800 yr fluctuating around the marginally open to marginally closed level through most of this time period lake lowstands and high 18 o values from the ninth century b c to the early fifth century a d indicate that the lake basin was comparatively dry during this time however the period prior to the most recent opening of lake kivu into the lake tanganyika basin ca a d 550 was not marked by major lake lowstands nor was this opening accompanied by a dramatic lakelevel rise the kivu opening was roughly coincident with a significant shift toward isotopically lighter 18 o and 13 c lake water which persists today the lake remained close to its outlet level between the sixth and thirteenth centuries a d lake levels rose between the fourteenth and sixteenth centuries at some time between the late sixteenth and early nineteenth centuries lake level fell to perhaps its lowest level in the past 2800 yr by the early nineteenth century lake level had begun to rise to the overflow level apparently the result of a regional increase in precipitation evaporation ratios weak 18 o 13 c covariance for late holocene carbonates suggests that the surface elevation of the lake has remained near the outlet level with only occasional periods of closure however there is no simple relationship between solute input from lake kivu isotope input from lake kivu and lake levels in lake tanganyika lake kivu waters are the primary source of major ions in lake tanganyika but are much less important in controlling the 18 o and the lake level of lake tanganyika because the ruzizi river s discharge into lake tanganyika is largely derived from sources other than lake kivu the overflow events in the two lakes have been uncoupled during the late holocene

an organodiagenetic model for marinoan age cap carbonates abstract the nuccaleena formation cap carbonate is the global stratotype for the marinoan glaciation and ediacaran gssp a designation that emphasises the importance of cap carbonates to our current understanding of global neoproterozoic biogeochemistry sedimentology and stratigraphy however to date there is no agreed depositional model for cap carbonates and there remains minimal detailed paragenetic data for developing process based depositional models here an early diagenetic organodiagenetic dolomite cementation model for marinoan cap carbonate formation is hypothesised and explored it is demonstrated how this process based model can explain the three main atypical sedimentary structures that are the basis for correlation of marinoan age cap carbonates giant tepee like structures sheet veins and tubestones these features are interpreted to be products of fluid overpressure deformation induced by organodiagenetic expansive cementation giant tepee like structures the hallmark of marinoan cap carbonates conform to a simple structural analysis that is consistent with the predicted stress field induced by expansive cementation driven fluid overpressure the undeformed to highly deformed gradient of deformation features in cap carbonates is consistent with the range of rheologies in a shallowly buried cementing carbonate modification of matrix textures and bedding surfaces also conforms to the range of expected textures within this model overall it is hypothesised that the aggradational condensed section architecture of cap carbonates was the primary control over the generation of these atypical sedimentary features the available paragenetic analysis suggests that the geochemical data particularly the c isotopic data can be reinterpreted as supportive of the organodiagenetic model and that the neoproterozoic ocean was perhaps similar to that of today the proposed model indicates that the current criteria for the placement of the ediacaran gssp are non unique and potentially non isochronous perhaps the most important aspect of this model is that it is testable and is a call for focused research on the much overlooked paragenesis of cap carbonates

dolomite silica stromatolites in miocene lacustrine deposits from the duero basin spain the role of organotemplates in the precipitation of dolomite this research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms and describes the involvement of highly structured extracellular polymeric secretion eps templates in dolomite nucleation the structure of eps is shown to match the hexagonal trigonal lattice geometry of dolomite which favoured the epitaxial crystallization of dolomite on the organic substrate this structure of eps also matches the arrangement of silica nanospheres in opal which further accounts for the organically templated formation of opal enabling the non replacive co existence of dolomite and silica the study is focused on a 50 m thick dolomite succession that is exposed in central areas of the tertiary duero basin and was deposited in a mudflat saline lake sedimentary complex during the middle to late miocene 9 to 15 ma in the intermediate intervals of the succession poorly indurated dolomite beds pass gradually into silica beds on the basis of sedimentological compositional geochemical and petrographic data silica and dolomite beds have been interpreted as mineralized microbial mats the silica beds formed in marginal areas of the lake in response to intense evaporative concentrations this resulted in the rapid and early precipitation of opal silicification accounted for the exceptional preservation of the microbial mat structure including biofilms filamentous and coccoid microbes and eps extracellular polymeric secretions have a layered structure each layer being composed of fibres which are arranged in accordance with a reticular pattern with frequent intersection angles at 120 and 60 therefore the structure of eps matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal additionally eps binds different elements with preference to si and mg the concurrence of suitable composition and surface lattice morphologies in the eps favoured the crystallization of dolomite on the substrate in some cases dolomite nucleation took place epicellularly on coccoid micro organisms which gave way to spheroid crystals organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation most of the microbial mats probably developed on the lake floor under sub aqueous conditions where the decomposition of organic matter took place the subsequent formation of openly packed dolomite crystals with inter related si enriched fibrils throughout is evidence for the pre existence of fibrillar structures in the mats miocene dolomite crystals are poorly ordered and non stoichiometric with a slight ca excess up to 5 which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure this confirms that microbial imprints can be preserved in the geological record and validates their use as biosignatures

geomicrobiology its significance for geology abstract some microorganisms including various kinds of bacteria fungi algae and protozoa serve as geochemical agents in the uppermost lithosphere and in the hydrosphere some promote rock weathering by mobilizing mineral constituents with inorganic or organic acids or ligands that they excrete others promote rock weathering by redox attack of mineral constituents such as fe and mn still others cause active or passive mineral formation by precipitation and subsequent nucleation of crystal formation on or in the cell or in the bulk phase they play a role in some phases of fossil fuel formation and accumulation they modulate the terrestrial and marine cycles of c n s and p and some other elements and they influence the composition of the atmosphere in respect to o 2 co 2 and ch 4 some discriminate between stable isotopes of h c o n and s

microbial biomass a catalyst for caco3 precipitation in advection dominated transport regimes microorganisms have long been thought to impact caco3 precipitation but determining the extent of their influence on sediment formation has been hampered by our inability to obtain direct experimental evidence about mineral formation processes in natural environments we address this problem by conducting kinetic experiments within a modern terrestrial carbonate spring to determine aragonite precipitation rates and to quantify the relative influences of aragonite saturation state a microbial biomass concentration and microbial viability on caco3 mineralization in advection dominated transport regimes at an a value consistent with modern seawater 3 63 0 09 our controlled in situ kinetic experiments show that 1 the natural steady state aragonite precipitation rate is more than twice that determined when microbial biomass on the aragonite mineral surface is depleted by 0 2 m filtration and 2 inhibiting microbial viability with ultraviolet uv irradiation has no significant effect on the mean precipitation rate furthermore our modeling of the caco3 precipitation process which uses the empirical crystal growth rate expression and additional in situ kinetic measurements reveals that reducing biomass concentrations by 45 can decrease the empirical rate constant by more than an order of magnitude these findings strongly suggest that microorganisms catalyze caco3 precipitation in advection dominated systems and imply that changes in carbonate mineralization rates may have resulted from changes in local microbial biomass concentration throughout geologic history