paleohydrology of the upper miocene bicorb lake eastern spain as inferred from stable isotopic data from inorganic carbonates abstract the hydrological evolution of the upper miocene bicorb lake eastern spain has been established based on sedimentology carbonate mineralogy and stable isotopic data from inorganic carbonates from the upper lacustrine unit three main subunits have been recognized in subunit a the sedimentological features suggest a hydrologically closed lake although the isotopic data are not conclusive the isotopic compositions of primary carbonates from subunit b display a distribution typical for a hydrologically closed lake and allow the differentiation of four developmental stages the first stage was defined by the deposition of gypsum lmc dolomite and aragonite indicating large variations in the water mg ca ratio during the second and third stages only lmc was deposited suggesting only minor mg ca fluctuations the 13c and 18o pattern of the second stage shows a well defined covariance r 0 8 while in the third stage the isotopic data display a negative covariant relationship r 0 8 changes in dissolved inorganic carbon composition are considered the main factor to explain the variations from positive to negative covariance in the fourth stage the lake waters underwent large mg ca fluctuations as indicated by the deposition of lmc hmc and aragonite the isotopic trend is also covariant finally subunit c is characterized by lmc and minor amounts of hmc suggesting minor changes in mg ca ratio the isotopic features of this subunit are more congruent with a hydrologically open lake the lowest 13c and 18o values are very similar in the three subunits reflecting the most dilute conditions of the lake water and a uniform isotopic composition of the local meteoric water during these periods the changes in water balance and solute inputs recorded in the mineralogy and isotopic signatures of carbonates were controlled by the diapiric evolution and changes in the e p balance

recent progress in snow and ice research a review of snow and ice research in 1987 1990 is presented focusing on the effects of layers in seasonal snow covers ice mechanics on fresh water and sea ice and remote sensig of polar ice sheets these topics provide useful examples of general needs in snow and ice research applicable to most areas such as better representation in models of detailed processes controlled laboratory experiments to quantify processes and field studies to provide the appropriate context for interpretation of processes from remote sensing

a giant late archean lake system the meentheena member tumbiana formation fortescue group western australia abstract the 2720 ma old meentheena member of the tumbiana formation fortescue group is a 30 50 m thick succession of alternating siliciclastic and limestone beds that crop out intermittently over a distance of 680 km across the pilbara craton western australia limestones of the member consist of flat pebble intraformational conglomerate oolite grainstone calcisiltite and calcilutite siliciclastics consist of shale siltstone and sandstone and many of these are tuffaceous sedimentary structures include abundant symmetrical ripples ripple cross stratification desiccation cracks teepee structures and planar to wavy lamination the member is superficially similar in appearance wherever it crops out however the correlation of individual beds and units over distances greater than 15 km is problematic there are no regional marker beds radiometric ages from tuffaceous units young to the west by a few million years four lithofacies are defined a flat pebble conglomerate b ripple cross laminated oolite grainstone c planar to wavy laminated calcilutite calcisiltite and d shale siltstone stromatolites are found in all three of the limestone facies evaporites are rare and include halite pseudomorphs the four lithofacies are generally found in a vertical succession that characterizes a lithofacies association and reflects a deepening upward succession associations may not have all four lithofacies but do reflect the same deepening upward pattern the uppermost portions of shale siltstone facies commonly contain symmetrical ripples and desiccation cracks indicating subaerial exposure measured sections contain as many as 25 of these parasequences indicating fluctuating water levels limestone lithology and facies commonly change abruptly laterally at the meter and greater scale and vertically centimeter to meter scale the sedimentary structures differences and abrupt changes in lithofacies patterns the inability to correlate confidently between regions and the differences in age from east to west suggest a complex diachronous depositional system of fluctuating water levels in separate bodies of water the limestone is dominated by sedimentary structures indicative of shallow water within wave base however it is devoid of evidence of any tidal influence possibilities for depositional environments include a tideless sea or a lacustrine system 87sr 86sr values from limestone differ from known archean marine carbonates ree patterns are decidedly non marine as well a lacustrine system is favored for the meentheena member based on sedimentary structures the abruptness of lithologic and facies changes the facies association lateral gradation into fluvial deposits the inability to correlate many units and beds for more than a few to several kilometers geochemical data stromatolites and comparisons with younger lacustrine basins as well as marine successions there is no single line of evidence that unequivocally establishes a lacustrine origin however multiple lines of evidence are collectively consistent with a lacustrine origin the meentheena member would therefore represent the oldest known carbonate rich lacustrine system which we call the meentheena lakes system

an abrupt change in the african monsoon at the end of the younger dryas 1 high resolution studies of variations in the elemental and stable carbon and nitrogen isotope composition of organic matter in cores from lakes malawi tanganyika and bosumtwi tropical africa indicate an abrupt change in the wind driven circulation of these lakes that within the limits of available chronologies was contemporaneous with the end of the younger dryas in the northern hemisphere the change was also coincident with shifts in surface winds recorded in cores from off the west and northeast coasts of africa a range of other proxies indicate that these changes in wind regime were accompanied by a marked increase in precipitation in the northern tropics africa south of 5 10 s on the other hand initially suffered drought conditions together the evidence suggests an abrupt northward translation of the african monsoon system at circa 11 5 0 25 ka b p the data assembled here contribute to a growing body of work showing that the younger dryas was a major climatic excursion in tropical africa furthermore they add substance to recent suggestions that climatic events in the southern hemisphere may have played a significant role in the abrupt demise of the younger dryas

late quaternary relative humidity changes from mt kilimanjaro based on a coupled 2 h 18 o biomarker paleohygrometer approach abstract our understanding of african paleoclimate hydrological history is mainly based on lake level and lake sediment studies it improved during the last decade thanks to emerging stable isotope techniques such as compound specific deuterium analysis of sedimentary leaf wax biomarkers 2hleaf wax here we present the results from a multi proxy biomarker study carried out on a 100 ka loess like paleosol sequence preserved in the maundi crater at 2780 m a s l on the southeastern slopes of mt kilimanjaro in equatorial east africa the maundi stable isotope records established for hemicellulose derived sugars lignin and pectin derived methoxyl groups leaf wax derived fatty acid and n alkane biomarkers 18osugars 2hmethoxyl groups 2hfatty acids and 2hn alkanes respectively reveal similar patterns but also some distinct differences are obvious the periods from 70 to 60 ka the last glacial maximum lgm and the younger dryas yd are characterized by more positive values whereas during the holocene and around 30 39 and 56 ka bp more negative values are determined the application of a coupled 2hn alkane 18osugar paleohygrometer approach allows us to derive information about late quaternary changes of air relative humidity at the maundi study site reconstructed changes of mean day time relative humidity rhd are in good agreement with pollen results from the study area apart from the overall regional moisture availability the intensification versus weakening of the trade wind inversion which affects the diurnal montane atmospheric circulation on the slopes of mt kilimanjaro is suggested as a local factor which may contribute to the observed variability of rhd at maundi study site the combined usage of 2hn alkanes and 18osugars allowed us to reconstruct 2h 18o of source water utilized by plants in the study area which is directly linked to local precipitation the results of this reconstruction caution against a straightforward interpretation of 2hleaf wax and 18osugars records as proxies for isotopic composition of local precipitation because variable and primarily rh dependent isotopic evaporative enrichment of leaf water can mask changes of 2hprec 18oprec in the past the biomarker based 2h 18osource water records derived for the maundi site revealed a discernible link with the reconstructed rhd record lower rhd values were generally observed during periods characterized by more negative 2h 18osource water values indicating a reverse relationship with the expected precipitation amount this indicates that the empirical relationship between amount of rainfall and its isotopic composition observed nowadays on monthly timescale in the east african region might not be valid on millennial time scale

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

late quaternary vegetation dynamics in a biodiversity hotspot the uluguru mountains of tanzania abstract late quaternary vegetation history and environmental changes in a biodiverse tropical ecosystem are inferred from pollen charcoal and carbon isotope evidence derived from a 48 000 yr sedimentary record from the uluguru mountains a component of the eastern arc mountains of kenya and tanzania results indicate that eastern arc forest composition has remained relatively stable during the past 48 000 yr long term environmental stability of the eastern arc forests has been proposed as a mechanism for the accumulation and persistence of species during glacial periods thus resulting in the diverse forests observed today the pollen and isotope data presented here indicate some marked changes in abundance but no significant loss in moist forest taxa through the last glacial maximum thereby providing support for the long term environmental stability of the eastern arc anthropogenic activities including burning and forest clearance were found to play a moderate role in shaping the mosaic of forest patches and high altitude grasslands that characterise the site today however this influence was tempered by the inaccessibility of the mountain

assessing the extent of carbonate deposition in early rift settings select lacustrine and marine depositional settings show a spectrum of styles of carbonate deposition and illustrate the types of carbonates with an emphasis on microbialites and tufa to be expected in early rift settings early rift lake examples examined in this review article are all from east africa lakes turkana bogoria natron and magadi manyara and tanganyika other lake examples include four from the western united states great salt lake and high lake level lake bonneville mono lake and high lake level russell lake pyramid lake and high lake level lake lahontan and searles lake and two from australia lakes clifton and thetis marine basin examples are the hamelin pool part of shark bay from australia marginal marine and the red sea marine rift landsat images and digital elevation models for each example are used to delineate present and past lake basin margins based on published lake level elevations and for some examples the shorelines representing different lake levels can be compared to evaluate how changes in size shape and lake configuration might have impacted carbonate development the early rift lakes show a range of characteristics to be expected in lacustrine settings during the earliest stages of continental extension and rifting whereas the red sea shows well advanced rifting with marine incursion and reef skeletal sand development collectively the lacustrine examples show a wide range of sizes with several of them being large enough that they could produce carbonate deposits of potential economic interest three of the areas great salt lake and high lake level lake bonneville pyramid lake and high lake level lake lahontan and the red sea are exceedingly complex in that they illustrate a large degree of potential depositional facies heterogeneity because of their size irregular pattern and connectivity of subbasins within the overall basin outline

nearshore carbonate deposits in lake tanganyika an exceptionally wide variety of carbonate fades dominated by high magnesian calcite occurs along the littoral and shallow sublittoral zones 50 m of lake tanganyika in central africa these facies include exposed and submerged calcite cemented ridges of nearshore terrigenous sand ooid sand shoals and lithified oolite ridges chara meadows of bioturbated calcareous silts gastropod shell blankets and related coquinas and extensive thrombolitic microbial reefs though texturally and compositionally similar to many modern and ancient shallow water marine facies these deposits record carbonate deposition and cementation in a large tropical deep water lake of tectonic origin lithofacies along lake tanganyika represent the broadest spectrum of carbonate deposits yet reported from any modern lake and serve as important analogues for lacustrine carbonate sequences in the stratigraphic record

isotopic spikes from tropical cyclones in surface waters opportunities in hydrology and paleoclimatology abstract the stable isotopic ratio of precipitation from tropical cyclones is very low compared to that of normal summer precipitation lawrence and gedzelman 1996 tropical storms allison and chantal made landfall in southeast texas in the summer of 1989 and dumped large amounts of rain isotope ratios of surface waters measured at five different sites decreased markedly after the passage and remained low for a few weeks the return of isotope values to normalcy following these storms varied from site to site reflecting differences among the hydrological environments during other summers from 1985 to 1993 isotope ratios of rains were higher and in the normal range calculations using air temperatures and oxygen isotope measurements of the surface waters show that organically precipitated fresh water carbonate should have recorded the two oxygen isotopic spikes produced by allison and chantal at all five sites this suggests that isotopic analysis of ancient fresh water fossil carbonate shells fossil mammal teeth or tree rings might be used to document past tropical cyclone activity

a high resolution geochemical record from lake edward uganda congo and the timing and causes of tropical african drought during the late holocene abstract high resolution analyses of the elemental composition of calcite and biogenic silica bsi content in piston cores from lake edward equatorial africa document complex interactions between climate variability and lacustrine geochemistry over the past 5400 years correlation of these records from lake edward to other climatically forced geochemical and lake level records from lakes naivasha tanganyika and turkana allows us to develop a chronology of drought events in equatorial east africa during the late holocene major drought events of at least century scale duration are recorded in lacustrine records at about 850 1500 2000 and 4100 cal year bp of these the most severe event occurred between about 2050 and 1850 cal year bp during which time lake edward stood about 15 m below its present level numerous additional droughts of less intensity and or duration are present in the lake edward record some of which may be correlated to other lacustrine climate records from equatorial east africa these events are superimposed on a long term trend of increasingly arid conditions from 5400 to about 2000 cal year bp followed by a shift toward wetter climates that may have resulted from an intensification of the winter indian monsoon although the causes of decade to century scale climate variability in the east african tropics remain obscure time series spectral analysis suggests no direct linkage between solar output and regional rainfall rather significant periods of 725 125 63 72 31 25 and 19 16 years suggest a tight linkage between the indian ocean and african rainfall and could result from coupled ocean atmosphere variability inherent to the tropical monsoon system

composition structure and evolution of a lacustrine carbonate margin dominated by microbialites case study from the green river formation eocene wyoming usa abstract during mid eocene times the northwestern margin of the gosiute lake wyoming has been characterised by the extensive development of a microbialite dominated carbonate sequence corresponding to the laney member of the green river formation thus providing a potential analogue for petroleum carbonate reservoirs developed in similar marginal lacustrine settings a detailed sedimentological study carried out on the little mesa plateau near labarge sw wyoming allows the reconstruction of 1 lateral and vertical facies variations of the carbonate sequence across the margin 2 development patterns of complex microbial bioconstructions and 3 the evolution of the margin through time the carbonate sequence up to 18 m thick starts with the deposition of sandy limestones which fill channels in the underlying siliciclastic unit of the cathedral bluff formation the increasing microbial contribution in the upper part of the sequence results in the development of large insect microbial build ups which represent up to 80 of the total volume of the carbonate deposits and which aggregated to form reef like geometries up to several tens of metres in diameter including bioherms spur and groove alignments and reef flat structures the initiation and development of the lacustrine carbonate platform was controlled both by the general lake expansion associated to the eocene climatic optimum and by local drainage diversions responsible for a new water inflow in the basin from the northern volcanic provinces short term lake level changes driven by high frequency climate fluctuations which are superimposed to this general transgressive trend may explain the discontinuous and polygenic growth of the individual insect microbial build ups the basin infilling by prograding alluvial volcaniclastic deposits originated from the northwestern volcanic province caused the rapid turn off of the marginal lacustrine carbonate factory during mid eocene time

isotopic composition of waters from ethiopia and kenya insights into moisture sources for eastern africa 1 oxygen and deuterium isotopic values of meteoric waters from ethiopia are unusually high when compared to waters from other high elevation settings in africa and worldwide these high values are well documented however the climatic processes responsible for the isotopic anomalies in ethiopian waters have not been thoroughly investigated we use isotopic data from waters and remote data products to demonstrate how different moisture sources affect the distribution of stable isotopes in waters from eastern africa oxygen and deuterium stable isotopic data from 349 surface and near surface groundwaters indicate isotopic distinctions between waters in ethiopia and kenya and confirm the anomalous nature of ethiopian waters remote data products from the tropical rainfall measuring mission trmm and national centers for environmental prediction ncep reanalysis project show strong westerly and southwesterly components to low level winds during precipitation events in western and central ethiopia this is in contrast to the easterly and southeasterly winds that bring rainfall to kenya and southeastern ethiopia large regions of high equivalent potential temperatures e at low levels over the sudd and the congo basin demonstrate the potential for these areas as sources of moisture and convective instability the combination of wind direction data from ethiopia and e distribution in africa indicates that transpired moisture from the sudd and the congo basin is likely responsible for the high isotopic values of rainfall in ethiopia

use of charophyte carbonates as proxy indicators of subtle hydrological and chemical changes in marl lakes example from the miocene bicorb basin eastern spain abstract the lower alluvial unit in the miocene bicorb basin contains several metric scale limestone intervals which record episodic shallow lacustrine environments in an alluvial setting developed during the early stage of the basin s evolution five main carbonate facies have been differentiated in the lacustrine limestones although calcite charophyte incrustations predominate and constitute the most striking features of these deposits the thinnest limestone intervals correspond to deposits from charophyte meadows in ponded shallow depressions in floodplains the thickest limestone intervals are mainly formed by banded limestones and usually correspond to diverse types of regressive sequences that have been interpreted as resulting from the infill of shallow lakes the sedimentological features and sequences show noticeable differences in the gradient of the littoral zones and the amount of palustrine deposits with models proposed for marl lakes charophytic carbonates from the best preserved facies show similar microtextures to those from recent charophyte incrustations the variations in stable isotopes 13 c 18 o for these primary carbonates occur in parallel with luminescence variations and correspond to hydrological changes and variations in solute composition and eh ph status in the lake waters the carbonates that display moderate to strong diagenetic modifications show a diverse degree of compaction aggrading neomorphism strong cementation and nodulization the isotopic values for these are arranged in diverse clusters there is a correlation between the degree of luminescence and the 13 c this suggests that hydrological and hydrochemical variations both in the lacustrine and diagenetic environments are being recorded in parallel we emphasize the need for further comparative studies between recent and ancient charophytic carbonates as these carbonates have been used in palaeoenvironmental reconstructions special attention must be paid to the diagenetic changes in ancient charophytic marls

late holocene hydrological history of lake tanganyika east africa from isotopic data on fossil stromatolites abstract lake tanganyika situated in the western branch of the east african rift system shows a slightly positive water budget and generates a small outflow through the lukuga river dependent mainly on the ruzizi river inflow from lake kivu in the northern part of the basin conditions more arid than those presently observed closed the kivu basin from 3800 to 1300 yr b p previous studies esimated that the closure of the corresponding tanganyika basin lowered the lake level by 75 150 m in this paper we present the results of isotopic and morphologic analyses of fossil stromatolites the stromatolites formed as thick incrustations on hard substrates and are found at depths between 6 and 60 m below the modern lake level their isotopic composition 18o 13c 14c very precisely document late holocene water levels and hydrologic fluctuations in the lake tanganyika basin with reference to the actual situation a major low stand at 10 m below present level is identified from 3400 to 1900 yr b p with a rapid recession and gradual rise to modern levels maximum 18o values vs pdb in the stromatolites rose from ca 3 5 at 3400 yr b p to 6 at 2500 yr b p then decreased progressively to 1 6 at 1300 yr b p these values are interpreted as a maximum aridity episode culminating at ca 2500 yr b p at which time a negative precipitation evaporation p e ratio resulted in a much higher residence time of paleolake waters and thus a noticeable enrichment in 18o the observed decoupling of the isotopic ratio 18o 16o and water volume responses is thought to be linked to the inertia of the large rift aquifers which controlled the paleolake level

african hydroclimatic variability during the last 2000 years abstract the african continent is characterised by a wide range of hydroclimate regimes ranging from humid equatorial west africa to the arid deserts in the northern and southern subtropics the livelihoods of much of its population are also vulnerable to future climate change mainly through variability in rainfall affecting water resource availability a growing number of data sources indicate that such hydroclimatic variability is an intrinsic component of africa s natural environment this paper co authored by members of the pages africa 2k working group presents an extensive assessment and discussion of proxy historical and instrumental evidence for hydroclimatic variability across the african continent spanning the last two millennia while the african palaeoenvironmental record is characterised by spatially disjunctive datasets with often less than optimal temporal resolution and chronological control the available evidence allows the assessment of prominent spatial patterns of palaeomoisture variability through time in this study we focus sequentially on data for six major time windows the first millennium ce the medieval climate anomaly 900 1250 ce the little ice age 1250 1750 ce the end of the lia 1750 1850 ce the early modern period 1850 1950 and the period of recent warming 1950 onwards this results in a continent wide synthesis of regional moisture balance trends through history allowing consideration of possible driving mechanisms and suggestions for future research

trace metal enrichments in lake tanganyika sediments controls on trace metal burial in lacustrine systems abstract we investigate the distributions of several key diagenetic reactants c s fe and redox sensitive trace metals mo cd re u in sediments from lake tanganyika east africa this study includes modern sediments from a chemocline transect which spans oxygenated shallow waters to sulfidic conditions at depth as well as ancient sediments from a longer core 2 m taken at 900 m water depth modern sediments from depths spanning 70 335 m are generally characterized by increasing enrichments of c s mo cd and u with increasing water depth but static fe distributions it appears that the sedimentary enrichments of these elements are to varying degrees influenced by a combination of organic carbon cycling and sulfur cycling these modern lake characteristics contrast with a period of high total organic carbon corg total sulfur stot and trace metal concentrations observed in the 900 m core a period which follows the most recent deglaciation 18 11 ky this interval is followed abruptly by an interval 11 6 ky that is characterized by lower c s u and mo consistent with other work we suspect that the low concentrations of s mo and u may indicate a period of intense lake mixing during which time the lake may have been less productive and less reducing as compared to the present an alternative but not mutually exclusive hypothesis is that changes in the lake s chemical inventory driven by significant hydrological changes could be influencing the distribution of sedimentary trace elements through time

lithofacies stable isotopic composition and stratigraphic evolution of microbial and associated carbonates green river formation eocene piceance basin colorado lacustrine carbonates of the eocene green river formation crop out on the western margin of the piceance basin and the eastern margin of the uinta basin in western colorado this area allows tracing of vertical and horizontal facies variation over hundreds of meters limestone beds consist of littoral to sublittoral lithofacies bioclastic and oolitic grainstones oolitic wackestone intraclastic rudstone stromatolites and thrombolites facies form upward deepening cycles that start with sharp based grainstones and packstones followed by stromatolites or thrombolites and capped by fine grained stromatolites and or oil shale deposits the vertical succession of carbonate deposits correlates with evolutionary lake stages the succession starts with grainstone deposits rich in ostracods and gastropods that correspond to an initial freshwater lake thrombolites capped by laminated stromatolites or coarse agglutinated stromatolites correlate with a higher salinity transitional lake deepening upward cycles as much as 5 m 16 ft thick of thrombolites agglutinated stromatolites and fine grained stromatolites occur in the highly fluctuating lake the upper section is dominated by laminated stromatolites that correspond to a rising lake stable isotope 18o and 13c values covary and range from 8 to 0 8 and 3 to 5 respectively the 18o values indicate carbonate precipitating water evolved from fresh to saline and became less saline in the upper green river negative excursions of 13c values correspond to lake level rises and positive excursions of 13c values occur during lake level falls syndepositional to burial diagenesis modified carbonate porosity early dissolution is followed by burial compaction and fracturing compaction and late calcite cements occluded primary and secondary porosity

paleolimnological evidence for the onset and termination of glacial aridity from lake tanganyika tropical east africa geochemical and sedimentological data in a continuous 60 000 year sediment core record from the kalya horst region of central lake tanganyika provide a detailed history of paleoclimate mediated weathering and overflow events from upstream lake kivu univariate elemental profiles bivariate elemental ratios and multivariate analyses of chemical trends show variations between the dry late pleistocene 32 18 ka cal yr bp and the wetter conditions that both preceded and post date that interval this record places important new constraints on the timing of aridity in east africa during the high latitude last glacial maximum lgm based on significant decreases in magnetic susceptibility and soluble cation concentrations coinciding with biogenic silica elemental indicatorsintheoldestportionofthesedimentaryrecord 60 50kacalyrbp characterizethisintervalasacomparatively wetperiod similar to modern conditions our record demonstrates that the ensuing transition toward arid conditions in tropical africa during high latitude glaciation was a two staged event with intermediate levels of aridity occurring from 50 32 ka cal yr bp followed by intense aridity from 32 18 ka cal yr bp the initiation of inflow from upstream lake kivu into lake tanganyika is evidenced at 10 6 ka cal yr bp through its influence on both elemental profiles mg ca and through its effect on 87 sr 86 sr increases in elemental mg ca sr concentrations coincide with the timing of the lake kivu overflow metal geochemistry suggests that the overflow from lake kivu into lake tanganyika may have ceased between 8 and 6 ka cal yr bp suggesting a period of middle holocene aridity in east africa 2007 elsevier b v all rights reserved

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

late syn rift sequence architecture and sedimentary evolution of a continental rift basin a case study from fulongquan depression of the songliao basin northeast china abstract as an important stage in continental rift basin evolution late syn rift stage is characterized by the combination of broad crustal subsidence and episodic pulses of extension sequence architecture and sedimentary filling process during this period present a certain particularity due to distinctive tectonic subsidence and variable influencing factors this paper analyzes sequence stratigraphy and sedimentary facies development and their evolution in the fulongquan depression a continental rift basin of the songliao basin and discusses the determinants influencing the sequence architecture and depositional systems evolution during late syn rift stage a major transition in the depositional systems from predominantly lacustrine and correlated depositional systems to fluvial floodplain settings was recorded in the evolution of basin fill this gradual and slow peneplanation is the final response to the decrease of differential subsidence the spatial and temporal variability of sedimentary evolution are mainly associated with specific growth history of the major boundary faults sequence stratigraphic units recognized in the fulongquan depression are likely corresponded to different episodic pulses of extension respectively each sequence can be further divided into a lowstand systems tract lst transgressive systems tract tst and highstand systems tract hst maximum differential subsidence and water deepening mainly occur in the lst the expanded tst and hst of each sequence tend to be developed in a broader thermal subsidence regime the depositional systems transition and detrital provenance changes usually occur in the hst throughout the late syn rift stage in addition to major changes of provenance systems tectonic movement is likely to be the main factor in controlling the variations of sediment supply with the decrease of tectonic activity the impact of climate changes and lake level fluctuations on basin filling tends to be increased

cyclic variations of uranium concentrations and oxygen isotopes in tufa from the middle miocene barstow formation mojave desert california uranium concentrations of 42 to 169 ppm in fibrous calcite from spring deposited tufa in the middle miocene barstow formation california are among the highest reported for calcite fission track maps of multiple bands of uniformly dull luminescent fibrous calcite show that the concentration of u increases in the outward growth direction of the calcite of each individual band homogeneous dull luminescence in the fibrous calcite indicates no change in redox conditions of the fluid from which the calcite was precipitated it is proposed that the cyclic pattern of increase in u concentration reflects a cyclical change in the u ca ratio in the fluid episodic mixing between ca rich spring water with a low u ca ratio and ca poor saline alkaline lake water with a high u ca ratio could produce the episodes of formation of fibrous calcite with increasing u concentrations the spring water supplies the ca for calcite precipitation and the u concentrations increase with the decreasing fraction of spring water and increasing fraction of lake water these cycles reflect a variable recharge of groundwater into the lake by springs the large variation in u concentrations suggests mixing of 0 to 45 spring water with the lake while the narrow range in 18 o of 6 37 to 6 87 limits the variation of the proportions of spring and lake water to 5

oligo miocene lacustrine microbial and metazoan buildups from the limagne basin french massif central the limagne basin french massif central is an extensive continental lacustrine system accommodating microbial and metazoan buildups from chattian to aquitanian age a description of these buildups and their associated biotic components in grand gandaillat and crechy quarries provides insights into their spatio temporal distribution patterns flats cauliflowers domes cones and coalescent columnar morphologies have been identified with a main laminated mesofabric and laminated columnar filamentous and caddisfly coated microfabrics two low gradient margin models emerged based on the changes in the distribution morphology and size of the microbial and metazoan rich deposits through time the first model applies to the chattian deposits in the grand gandaillat quarry and the second to the aquitanian deposits in the crechy quarry this study emphasizes the heterogeneity of marginal lacustrine carbonate siliciclastic cycles the lacustrine palustrine cycles of the limagne basin record i lacustrine deposits composed of microbial and metazoan buildups and organic matter rich marls indicating periods of high accommodation and ii palustrine deposits composed of mudstone and clayey paleosoils indicative of periods of low accommodation the cycles differ with thin and symmetrical deposits that dominate the chattian cycles whereas thicker and asymmetrical deposits mark the aquitanian cycles in addition the chattian buildups are exclusively microbial and only a few centimeters thick whereas the aquitanian ones are multiple meters in height and are composed of both microbes and metazoans climatic tectonism volcanic and local parameters are involved in the deposition of buildups and their organization in each different cycle climate may control the long term arid humid sedimentary succession tectonic may explain the difference in the deposit thickness between two cycles of different ages and localizations as well as the specific distribution of the buildups throughout the basin volcanism may control the chemical lacustrine conditions and local physiography may impact the soil removal ability of the lake margin

lake level history of lake tanganyika east africa for the past 2500 years based on ostracode inferred water depth reconstruction assemblages of ostracodes from sediment cores illuminate lake level history at decadal to centennial timescales during the late holocene at lake tanganyika east africa the ostracode based lake level curves for several cores resemble both each other and the only previously published lake level record of comparable resolution for lake tanganyika during this interval successfully reconstructing known highstands improving the chronology of known lowstands and contributing new information on late holocene lake level variability at this important tropical african location in agreement with other late holocene records from east africa the surface level at lake tanganyika reflects predominantly arid conditions throughout this interval interrupted by relatively brief episodes of higher precipitation and lake level the most pronounced lowstand in the record occurs at v200 0 bc with other significant lowstands dating to the intervals v200 500 ad v700 850 ad the medieval warmperiod mwp v1050 1250 ad at lake tanganyika and the latter part of the little ice age lia v1550 1850 ad the most important wet intervals in the lake level record are centered on v500 ad v1500 ad and v1870 ad the highstands and lowstands reported here for lake tanganyika appear to be fairly coherent with other records of rainfall throughout east africa during the mwp and the lia prior to the mwp paleoclimate records are apparently less coherent although this may be a reflection of the resolution and abundance of recent paleoclimatic data available for this climatically complex region

early eocene carbon isotope excursions and landscape destabilization at eccentricity minima green river formation of wyoming repeated global reorganizations of carbon cycling and biotic oceanic and terrestrial processes occurred during the early eocene and appear to have been paced by cyclic variations in the eccentricity of the earth s orbit the phase relationship s between insolation variation terrestrial paleoclimate and atmospheric pco2 during these events remains enigmatic however due to their poorly constrained timing relative to specific orbital configurations here we use tiered interpolation between radioisotopic ages and paleomagnetic polarity chrons to compare high resolution 13c and lithofacies records from the wilkins peak member of the green river formation of western north america to a fe intensity xrf record from the western atlantic ocean and to numerical solutions for earth s orbital configuration wilkins peak member lithofacies stacking patterns record cyclic geomorphic responses to insolation and climate fluctuations spanning an interval of 1 8 ma previous macrostratigraphic analyses using 40ar 39ar and u pb ash bed ages indicate that these cycles reflect long and short eccentricity modulation of precession hydrologic variance appears to have occurred inversely with intervals of maximum sediment advection with carbonate and evaporite dominated lacustrine modes during eccentricity maxima and siliciclastic dominated alluvial modes during eccentricity minima stable carbon isotope analyses of 126 meters of wilkins peak member strata reveal a regular 5 per mil oscillation between high 13c lacustrine modes and low 13c alluvial modes tiered interpolation between paleomagnetically characterized terrestrial ash beds facilitates the integration of 11 radioisotopic ages with the geomagnetic polarity timescale resulting in significant expansion of chron c23 and shortening of chron c22 relative to timescales based on seafloor magnetic anomaly profiles the new proposed timescale permits direct comparison of terrestrial and marine climate proxy records from the early eocene climatic optimum at ca 250 kyr 250 kyr resolution and reveals prominent 100 kyr and 405 kyr scale oscillations within both records wilkins peak member 13c minima which occurred during low eccentricity alluvial modes likely coincided with global 13c minima scenario 1 but may alternatively reflect productivity driven local effects within lake gosiute scenario 2 if scenario 1 proves accurate early eocene negative 13c hyperthermal excursions occurred during eccentricity minima rather than maxima as formerly believed

sources and accumulation rates of organic carbon in an equatorial peat bog burundi east africa during the holocene carbon isotope constraints the relationship between organic carbon accumulation rates and 13c 12c ratios of total organic carbon toc was investigated in an highland peat bog core ru 3 from equatorial africa this core yielded a sequence spanning the last 14 kyr and was analysed with a 100 300 yr resolution for toc 13c values the holocene section shows contrasted toc accumulation regimes and toc 13c varying between 28 5 and 19 5 with a few very short isotopic excursions dated at ca 9 3 7 5 4 2 ka b p the organic carbon accumulation rates range from 2 to 20 mg c cm 2 yr 1 they increase when toc becomes more depleted in 13c notably between 12 and 9 8 ka b p 8 5 and 7 8 ka b p and after 1 6 ka b p periods of restricted carbon storage correspond to heavier toc accumulation at 9 3 and between 7 5 and 1 6 ka b p at the study site the variations can be related to variable c4 plant inputs and possibly to changes in the fractionation between co2 and the organic carbon in c3 vascular plants the ru 3 record indicates restricted carbon storage during the periods of increased contribution from c4 plants and or of decreased fractionation between co2 and organic carbon in c3 plants changes in toc 13c values in core ru 3 seem to match fluctuations of east equatorial african lakes high lake stands correspond to low 13c intervals and vice versa this points to indirect climatic forcing of 13c changes in intertropical peats

the late pleistocene and holocene stratigraphy and paleolimnology of lakes kivu and tanganyika abstract evidence from diatoms chemistry and sedimentology shows that lakes kivu and tanganyika experienced late pleistocene low stands 86 to 300 m for kivu and 250 to 350 m for tanganyika both lakes had higher 2 10 fold sedimentation rates prior to 11 500 yr b p about 9900 yr b p upwelling became important in southern lake tanganyika which had been deepening for at least several millenia by 9400 yr b p kivu had become deep and dilute with overflow via the ruzizi river to lake tanganyika southern lake tanganyika became more stably stratified about 5200 yr b p volcanism and hydrothermal activity in kivu began about 5000 yr b p and the salinity reached 3 ppt and the lake became strongly stratified the ruzizi river outflow was interrupted from 3500 yr b p until recently the sedimentary record implies that sudden hydrothermal activity and sublacustrine volcanism in kivu has probably been the cause of its impoverished fauna these results show that high lake levels in the eastern rift of africa from 10 000 to 7000 yr b p were paralleled in the western rift given the great volumes of these lakes climate has not caused great variations in salinity the diatom floras however have been remarkably sensitive to changes in stratification and nutrient supply volcanism has been a major force in the limnological and biological evolution of lake kivu while both kivu and tanganyika have responded to changes in precipitation and wind stress

carbonate microbialites and hardgrounds from manito lake an alkaline hypersaline lake in the northern great plains of canada abstract manito lake is a large perennial na so 4 dominated saline to hypersaline lake located in the northern great plains of western canada significant water level decrease over the past several decades has led to reduction in volume and surface area as well as an increase in salinity the salinity has increased from 10 ppt to about 50 ppt tds this decrease in water level has exposed large areas of nearshore microbialites these organogenic structures range in size from several cm to over a meter and often form large bioherms several meters high they have various external morphologies vary in mineralogical composition and show a variety of internal fabrics from finely laminated to massive in addition to microbiolities and bioherms the littoral zone of manito lake contains a variety of carbonate hardgrounds pavements and cemented clastic sediments dolomite and aragonite are the most common minerals found in these shoreline structures however calcite after ikaite monohydrocalcite magnesian calcite and hydromagnesite are also present the dolomite is nonstoichiometric and calcium rich the magnesian calcite has about 17 mol mgco 3 ams radiocarbon dating of paired organic matter and endogenic carbonate material confirms little or no reservoir affect although there is abundant evidence for modern carbonate mineral precipitation and microbialite formation most of the larger microbialites formed between about 2300 and 1000 cal bp whereas the hardgrounds cements and laminated crusts formed about 1000 500 cal bp

estimating the age of formation of lakes an example from lake tanganyika east african rift system age estimates for ancient lakes are important for determining their histories and their rates of biotic and tectonic evolution in the absence of dated core material from the lake s sedimentary basement several techniques have been used to generate such age estimates the most common of these herein called the reflection seismic radiocarbon method rsrm combines estimates of short term sediment accumulation rates derived from radiocarbon dated cores and depth to basement estimates derived from reflection seismic data at or near the same locality to estimate an age to basement age estimates form the rsrm suggest that the structural basins of central lake tanganyika began to form between 9 and 12 ma estimates for the northern and southern basins are younger 7 to 8 ma and 2 to 4 ma respectively the diachroneity of estimates for different segments of the lake is equivocal and may be due to erosional loss of record in the northern and southern structural basins or to progressive opening of the rift the rsrm age estimates for lake tanganyika are considerably younger than most prior estimates and clarify the extensional history of the western branch of the east african rift system 31 refs 3 figs 1 tab

use of contrasting d h ratios of snows and groundwaters of eastern new york state in watershed evaluation the d h ratios of snow on the ground and groundwaters in eastern new york state in early spring differ by 50 notation this is large in comparison to the analytical precision of 1 and permits estimation of the input of snowmelt directly to runoff versus groundwater input to runoff resulting from increased hydraulic head as the snow melts and percolates into the ground in 1978 the high overflow resulting from snowmelt contained only 25 meltwater the rest was provided by an increased groundwater exfiltration

the rise and fall of stromatolites in shallow marine environments stromatolites are abundant in shallow marine sediments deposited before the evolution of animals but in the modern ocean they are restricted to locations where the activity of animals is limited overall decline in the abundance of stromatolites has therefore been attributed to the evolution of substrate modifying metazoans with phanerozoic stromatolite resurgences attributed to the aftermaths of mass extinctions here we use a comprehensive stratigraphic database the published literature and a machine reading system to show that the rock record normalized occurrence of stromatolites in marine environments in north america exhibits three phases an initial paleoproterozoic ca 2500 ma increase a sustained interval of dominance during the proterozoic 2500 800 ma and a late neoproterozoic 700 541 ma decline to lower mean prevalence during the phanerozoic 541 0 ma stromatolites continued to exhibit large changes in prevalence after the evolution of metazoans and they transiently achieved proterozoic like prevalence during the paleozoic the aftermaths of major mass extinctions are not well correlated with stromatolite resurgence instead stromatolite occurrence is well predicted by the prevalence of dolomite a shift in carbonate mineralogy that is sensitive to changes in water column and pore water chemistry occurring during continent scale marine transgressive regressive cycles

geochemical evaluation of fenghuoshan group lacustrine carbonates north central tibet implications for the paleoaltimetry of the eocene tibetan plateau abstract a sedimentologic petrologic and geochemical evaluation of lacustrine carbonates from the eocene oligocene fenghuoshan group of north central tibet provides insight into the paleoenvironmental and paleolimnological setting of the hoh xil basin during the mid tertiary fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones these carbonates are generally less than 3 m in thickness and are intercalated with siliciclastic lacustrine and fluvial alluvial plain deposits individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward fenghuoshan carbonates also contain abundant ostracode bivalve and charaphyte fossils sedimentologic evidence suggests that carbonate deposition occurred in shallow relatively short lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems stable c and o isotopic analysis of fenghuoshan carbonates show 18o values ranging from 11 7 to 10 3 vienna pee de

a late holocene paleoclimatic history of lake tanganyika east africa abstract a nearshore core lt03 05 from the north basin of lake tanganyika provides diatom pollen and sedimentary time series covering the last ca 3800 yr at 15 36 yr resolution a chronology supported by 21 ams dates on terrestrial and lacustrine materials allows us to account for ancient carbon effects on 14c ages and to propose refinements of the region s climatic history conditions drier than those of today were followed after ca 3 30 ka by an overall wetting trend several century scale climate variations were superimposed upon that trend with exceptionally rainy conditions occurring 1 70 1 40 ka 1 15 0 90 ka 0 70 0 55 ka and 0 35 0 20 ka around 0 55 0 35 ka during the sporer sunspot minimum drier conditions developed in the northern tanganyika basin while more humid conditions were registered at lakes victoria and naivasha this indicates significant variability in the nature and distribution of near equatorial rainfall anomalies during much of the little ice age

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

13c palynological evidence of differential residence times of organic carbon prior to its sedimentation in east african rift lakes and peat bogs abstract most terrestrial plants producing large amounts of organic matter in the east african rift follow the calvin c3 photosynthetic pathway their end products have 13c values of ca 27 2 vs pdb on the contrary most cyperaceae notably cyperus papyrus and c latifolius are characterized by higher 13c contents 13 c 10 5 1 in relation to their hatch and slack c4 photosynthetic cycle in consequence 13c values in total organic matter tom from peat bog or lake cores essentially responded to the proportion of detritus from c4 cyperaceae immediate evidence of the development or disappearance of cyperaceae around lake margins or in peat bogs can be found in pollen assemblages lag times between pollen signals and correlative 13c shifts in tom from cores are therefore indicative of the residence time of organic matter prior to its sedimentation delayed sedimentation of tom will result in 14c anomalies which depend on several parameters most of them being site specific as shown by examples from a peat bog in burundi and from southern lake tanganyika an independent assessment of the chronology by high resolution paleomagnetic correlations indicates a ca 1 5 ka apparent 14c age of tom in lake tanganyika at the pleistocene holocene transition

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

three dimensional stratigraphic sedimentological forward modeling of an aptian carbonate reservoir deposited during the sag stage in the santos basin brazil abstract stratigraphic sedimentological forward modeling is a useful technique to test the role of parameters that may act in the depositional processes of sedimentary rocks the technique was applied to the santos basin brazil using the software program dionisos to test possible depositional scenarios for an aptian section of a lacustrine carbonate platform developed during the late sag stage uppermost pre salt section of the basin different parameters were tested in a set of numerical simulations with the aim of reproducing the carbonate facies spatial distribution and the overall stratigraphic stacking pattern of the platform as observed in the available well data several models were simulated and four of them are discussed in this paper based on the available descriptive data from 10 wells model 3 is considered to produce the best fit to explain the development of the 100 m thick aptian carbonate platform occurrences of grainstones and stromatolites as proximal and intermediate facies respectively and the overall carbonate stacking pattern thus based on 3d stratigraphic sedimentological forward modeling the best set of environmental conditions able to explain the pattern of carbonate deposition observed in the study area are 1 a slow carbonate depositional rate 0 08 mm y over an interval of 2 4 my which is indicative of a larger microbial contribution and 2 lake level oscillations which were essentially induced by arid climatic conditions these results also enable new perspectives for the application of stratigraphic sedimentological forward modeling as a predictive tool for hydrocarbon exploration in carbonates and for the development of depositional models of other sag carbonate platform settings

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

the paleoenvironmental distribution of phanerozoic wrinkle structures abstract wrinkle structures are microbially induced sedimentary structures that are known to have formed in ancient marine siliciclastic environments this study reviews the sedimentary context and paleoenvironmental distribution of these sedimentary structures throughout three distinct intervals of the phanerozoic the cambrian the post cambrian paleozoic and the lower triassic during these times wrinkle structures are found primarily within two sedimentary environments storm dominated subtidal environments and the intertidal zone subtidal occurrences of wrinkle structures during the post ordovician phanerozoic only appear to occur during intervals of environmental stress while earlier examples from the cambrian and lowermost ordovician likely formed prior to significant increases in extent and depth of bioturbation that hallmark most of the phanerozoic intertidal examples occur regardless of severe environmental stress and may have formed under low levels of bioturbation due to inherent vagaries of a marginal marine setting these environmental preferences appear to be conservative throughout the phanerozoic and may very well extend throughout earth s history