sources sinks and storage of river sediment in the atlantic drainage of the united states the history of sediment and its movement in the atlantic drainage demonstrate some of the difficulties of modeling sediment on a river basin scale soil erosion was accelerated by a factor of at least 10 when european settlers cleared forests and planted crops although increasing soil conservation practice and decreasing crop farming have since reduced the rates of erosion large quantities of eroded material are still stored on hillslopes and in stream valleys where they continue to augment the sediment loads of the rivers the sediment from this episode of erosion that is largely past can be expected to emerge from storage for many decades and perhaps even several centuries to come the reservoirs that have been built on many of the major rivers trap significant portions of the moving sediment which in some places may be remobilized by large floods essentially all the river sediment that reaches the atlantic coastal zone is trapped in estuaries and coastal marshlands probably less than 5 is deposi

first results from a pre operational system for automatic detection and recognition of seismic signals associated with avalanches a system for automatic reliable semi instantaneous estimation of avalanche activity is presented in an attempt to check deterministic models and improve the surveillance of risk zones the principle is the seismic detection of avalanches two experimental sites equipped with standard seismological equipment are described avalanche seismic signals are recorded as well as many extraneous signals of natural or human origin after several years of a posteriori identification of the signals we are now able to set up criteria for the automatic recognition of non avalanche signals we have therefore developed an automatic analysis decision system to discriminate betweenn avalanche signals and others this system worked satisfactorily in pre operational conditions during the winter of 1995 96 the results are presented and compared with other data related to avalanche activity although there is still room for improvement our system seems to be able to estimate avalanche activity better than human visual observations

temporal trends in avalanche activity in the french alps and subregions from occurrences and runout altitudes to unsteady return periods we present an analysis of temporal trends in 55 000 avalanches recorded between 1946 and 2010 in the french alps and two north south subregions first bayesian hierarchical modelling is used to isolate low intermediate and high frequency trends in the mean avalanche occurrence and runout altitude per year winter variables are then combined to investigate their correlation and the recent evolution of large avalanches comparisons are also made to climatic and flow regime covariates the results are important for risk assessment and the development of new high altitude climate proxies at the entire french alps scale a major change point exists in 1978 at the heart of a 10 year period of high occurrences and low runout altitudes corresponding to colder and snowier winters the differences between this change point and the beginning end of the study period are 0 1 avalanche occurrences per winter and per path and 55 m in runout altitude trends before after are well correlated leading to enhanced minimal altitudes for large avalanches at this time a marked upslope retreat 80 m for the 10 year return period runout altitude accompanied by a 12 decrease in the proportion of powder snow avalanches has occurred since then interrupted from about 2000 the snow depth and temperature control on these patterns seems significant r 0 4 0 6 but is stronger at high frequencies for occurrences and at lower frequencies for runout altitudes occurrences between the northern and southern french alps are partially coupled r 0 4 higher at low frequencies in the north the main change point was an earlier shift in 1977 and winter snow depth seems to be the main control parameter in the south the main change point occurred later 1979 84 was more gradual and trends are more strongly correlated with winter temperature

an oceanic model for lithological and faunal changes tested on the silurian record a model for oceanic cycles is presented the cycles are expressed as changes between more humid low latitude and cooler high latitude climates p episodes and dryer low latitude and warmer high latitude climates s episodes the cyclicity may be self regulating through changes in the co 2 storage capacity of the deep ocean due to temperature changes and through changes in the oceanic input and output of dissolved carbonate the model permits over 30 predictions regarding observable changes in both deep water and shelf lithologies and variations in the abundance diversity and extinction rate of planktic and benthic faunas sedimentary changes involve fluctuations in the rate of clay deposition and carbonate production the advance and retreat of carbonate deposition on the shelf edge the timing of oolite reef and black shale formation and changes between oxic and anoxic deep water faunal effects include both iterative evolution and intermittently widespread taxa lazarus taxa these predictions agree well with changes observed in well known sequence of wenlock and ludlow age as well as changes seen in selected older and younger sequences

early devonian kess kess carbonate mud mounds of the eastern anti atlas morocco and their relation to submarine hydrothermal venting abstract spectacular conical early devonian carbonate buildups up to 55 m high that crop out in the eastern anti atlas of southern morocco are microbially mediated carbonate mud mounds that were surfaced by soft bottom communities dominated by small tabulate corals they formed on the hamar laghdad elevation which was created by a submarine volcanic eruption and were associated with a network of synsedimentary radial and tangential faults that originated by uplift of the intrusive laccolithic body underlying the kess kess formation these faults served as conduits for the migration of hydrothermal fluids to the sea floor most mounds developed over cross points of radial and tangential faults vents were episodically active until the famennian but extensive vent carbonate production occurred only during the emsian preliminary geochemical results document that mud mound carbonates and calcite cements in neptunian dikes precipitated from brines comprising a mixture of hydrothermal fluids and seawater in addition carbon isotope compositions 13c as low as 18o oo pdb suggest a contribution from thermogenic methane derived presumably from underlying basaltic intrusives aerobic bacterial oxidation of methane is favored as the main process driving carbonate precipitation in and rapid lithification of the mounds

snow avalanche activity in żleb żandarmerii in a time of climate change tatra mts poland abstract this paper reports from a survey of the occurrence of large avalanches in leb andarmerii this couloir is known to be one of the most hazardous avalanche paths in the tatra mountains and has one of the longest histories of avalanche observation this survey looked at the runout distance return period dynamics and geoecological implications of avalanches in the context of current climate change the study took advantage of the longest record of meteorological data available in the tatra mountains as well as archival avalanche observations topographical maps orthophotomaps and a high resolution digital terrain model avalanche data were obtained using geomorphological and dendrogeomorphic methods and through modelling with the ramms numerical avalanche dynamics simulation software the largest avalanches reach the foot of its counter slope their length release volume flow velocity and pressure can exceed respectively 1000 m 80 000 m 3 45 m s and 600 kpa the results of our study suggest that current climate warming has been accompanied by thinning and shortening of the duration of snow cover as well as by an upward expansion of the timberline including in the large avalanche runout zones of up to 80 m since the mid 1920s no distinct temporal trend was identified in the large avalanche return period since 1909 but their mass and intensity have declined forests and timberline expansion were found to have no influence on the extent of the avalanches in our study while ground relief could determine both their downward extent and lateral expansion

impact of a climate change on avalanche hazard the safran crocus mepra software is used to assess the climatology of the avalanche hazard and its sensitivity to climate change a natural avalanche hazard index based on mepra analysis is defined and validated against natural avalanche observations triggered avalanches are not taken into account a 15 year climatology then allows a comparison of avalanche hazard in the different french massifs finally a simple climate scenario with a general increase of precipitation and temperature shows that avalanche hazard may decrease slightly in winter mainly february and more significantly in may june the relative proportion of wet snow avalanches increases

interpreting benthic oxygen levels in mudrocks a new approach quantified paleoecology and gamma ray spectrometry have been applied in the analysis of the kimmeridge clay a highly organic rich british jurassic mudrock decreasing benthic oxygen trends are reflected in decreasing species richness and dominance diversity values similarly the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen the calculation of authigenic uranium values from data gathered by gamma ray spectrometry shows enrichment in more oxygen deficient environments the good correlation between the independently derived paleoecological and authigenic u data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks

climatic geomorphic and archaeological implications of a late quaternary alluvial chronology for the lower salt river arizona usa abstract recent archaeological excavations along the lower salt river arizona resulted in the unexpected discovery of buried late pleistocene soils and cultural features dating 5800 7100 cal ybp early archaic the latter representing the earliest evidence of human activity in the lower salt river floodplain thus far identified because the lower salt river floodplain has been heavily impacted by recent agriculture and urbanization and contains few stratigraphic exposures our understanding of the river s geological history is limited here we present a late quaternary alluvial chronology for a segment of the lower salt river based on 19 accelerator mass spectrometry 14 c and four optically stimulated luminescence ages obtained during two previous geoarchaeological investigations deposits are organized into allostratigraphic units and reveal a buried late pleistocene terrace inset into middle to late pleistocene terrace deposits holocene terrace fill deposits unconformably cap the late pleistocene terrace tread in the site area and the lower portion of this fill contains the early archaic archaeological features channel entrenchment and widening 900 cal ybp eroded much of the older terrace deposits leaving only a remnant of fill containing the buried latest pleistocene and middle to late holocene deposits preserved in the site area subsequent overbank deposition and channel filling associated with a braided channel system resulted in the burial of the site by a thin layer of flood sediments our study confirms that the lower salt river is a complex mosaic of late quaternary alluvium formed through vertical and lateral accretion with isolated patches of buried soils preserved through channel avulsion although channel avulsion is linked to changes in sediment load and discharge and may have climatic linkages intrinsic geomorphic and local base level controls limit direct correlations of lower salt river stratigraphy to other large rivers in the north american southwest

carbonate mud sedimentation on a temperate shelf bass basin southeastern australia bass basin 66 000 km2 is unusual as a carbonate dominated basin because calcitic carbonate muds are accumulating in relatively shallow 70 85 m water depths in the central part of the basin 20 000 km2 the carbonate muds are produced by the primary accumulation and disintegration of nannoplankton as well as through the biodegradation of skeletal carbonate grains accumulating on the sea floor the muds are transported to the south of the basin to the end of tidal current transport paths where they accumulate in the lowest energy environment available in enclosed bass basin although they are still subject to periodic reworking by storm energy the basin margins consist of coarser partially palimpsest carbonate sands whereas the central muds overlie a late pleistocene disequilibrium surface in cores these muds are up to 1 m thick and thin away from the centre towards the margins bioturbation is important in modifying the textural character of the muds by increasing the grain size as faecal pellets and therefore creating a sea floor not in equilibrium with modern hydrodynamic conditions radiocarbon dates for an early post glacial marine transgression embayment facies in the basin gave ages of 10 290 250 to 11 660 300 years b p an age of 8700 710 years b p was obtained for the base of the modern strait facies these ages and facies thicknesses were used to establish holocene sedimentation rates of 12 cm 1000 y for the basin centre falling to 6 cm 1000 y towards the margins

active channel width as a proxy of sediment supply from mining sites in new caledonia although the channel morphology of upland fluvial systems is known to be strongly controlled by sediment supply from hillslopes it is still difficult to isolate this effect from the other controlling factors of channel forms such as the sediment transport capacity depending notably on the size of the catchment and local conditions e g confinement riparian vegetation valley floor slope the rivers in new caledonia offer an interesting field laboratory to isolate the morphological effect of contrasted sediment supply conditions some of these rivers are known to be highly impacted by the coarse sediment waves induced by the mining of nickel deposits that started in the early 1870s which was particularly intensive between the 1940s and 1970s the propagation of the sediment pulses from the mining sites can be traced by the presence of wide and aggraded active channels along the stream network of nickel rich peridotite massifs a first set of 63 undisturbed catchments in peridotite massifs distributed across the grande terre was used to fit a classic scaling law between active channel width and drainage area a second set of 86 impacted sites where the presence of sediment waves was clearly attested by recent aerial imagery showed systematically wider active channels with a width ratio around 5 established from the intercept ratio of width area power laws more importantly this second set of disturbed sites confirmed that the residual of active channel widths computed from the scaling law of undisturbed sites is statistically positively related to the catchment scale relative area of major mining sediment sources it is therefore confirmed that the characterization of sediment supply conditions is crucial for the understanding of spatial patterns of active channel width and this should be more thoroughly considered in morphological studies of rivers draining environments with contrasted geomorphic activities on hillslopes

carbon isotope stratigraphy across the silurian devonian transition in podolia ukraine evidence for a global biogeochemical perturbation the carbon and oxygen isotope composition of marine carbonates 13 c and 18 o respectively are studied in the fossiliferous stratigraphically well constrained and remarkably expanded successions of podolia sw ukraine spanning the silurian devonian transition significant isotopic shifts are directly comparable to previously published global secular trends in well preserved brachiopod calcite isotopic ratios from this region and therefore may be taken as a reliable primary record of seawater 13 c changes the sections reveal a major positive 13 c excursion with an amplitude above 6 beginning in the upper pridoli and reaching peak values as heavy as 4 2 in the lowermost lochkovian this turnover in carbon cycling is followed by a general trend toward more negative 13 c values in the upper lochkovian the podolian isotopic signals provide strong support for the previously inferred global biogeochemical perturbation across the silurian devonian transition reflecting a complex combination of palaeogeographical biogeochemical and evolutionary processes in the late caledonian geodynamic setting with a likely undervalued role of the expanding vegetation in vast near coastal shallows and deltas

the sediment delivery problem abstract the linking of on site rates of erosion and soil loss within a drainage basin to the sediment yield at the basin outlet and improved knowledge and representation of the associated processes of sediment delivery represent a major research need within the field of erosion and sedimentation and also an important scale problem in drainage basin studies this paper reviews the limitations of the sediment delivery ratio concept by considering the problems of temporal and spatial lumping and its blackbox nature some recent advances in our understanding of the sediment delivery system and its modelling are described and the lack of detailed empirical investigations is highlighted the significance of recent concern for the role of sediments in the transport of nutrients and contaminants to sediment delivery studies is introduced and the need for further work in this field is emphasised

a computer based system simulating snowpack structures as a tool for regional avalanche forecasting this paper describes the current state of a complete automatic system of three numerical models that simulate snow cover stratigraphy and avalanche risks for operational avalanche forecasting the first model safran estimates relevant meteorological parameters affecting snowpack evolution the second crocus is a snow numerical model which simulates the physical processes inside the snowpack and its stratigraphy the last model mepra is an expert system based on an assessment of snowpack stability it deduces natural and accidental avalanche risks to describe the great variability of the snowpack and the associated avalanche risks this automatic system simulates snow cover evolution and its stability for many typical slopes elevations and aspects representative of the different french massifs to achieve this result different kinds of validations have been carried out since winter 1981 they are mainly based on comparisons with different sets of measurements and on the opinion of users although the routinely obtained results do not yet take into account all small scale effects such as wind transport they have been considered as valuable information by avalanche forecasters since 1992 93 and used operationally since then

pre hirnantian latest ordovician benthic community assemblages controls and replacements in a siliciclastic dominated platform of the eastern anti atlas morocco lower middle ashgill sedimentary strata from the mediterranean region have recorded a key episode of temperate to cold water carbonate productivity predating the onset of the hirnantian glaciation the latitudinal position of the moroccan margin of north gondwana during ashgill should have been adequate for the recorded development of carbonate factories however carbonate productivity was neither homogeneous nor laterally persistent as documented in the eastern anti atlas whereas in the erfoud area the bryozoan dominated limestones of the ashgill khabt el hajar formation indicate the intensive activity of carbonate factories these were dramatically reduced in the tiskaouine and gaiz jbels alnif area ca 100 km to the wsw of erfoud in alnif the caradoc ashgill transition lies within the ktaoua group which represents a siliciclastic dominated platform partly truncated by the hirnantian glaciogenic unconformity there three distinct carbonate factories are recognized across the caradoc ashgill transition and early mid ashgill times primarily controlled by accommodation space distance from source areas and subsequent siliciclastic input and benthic community replacements i quiescent episodes of prograding sandy shoals are indicated by a distinct assemblage of epibenthic to quasi endobenthic brachiopods heterorthids drabovids and rafinesquinids and non pelmatozoan echinoderms sphaeronitids and aristocystitids that colonized inclined foresets ii bryozoan thickets rich in robust and delicate branching and encrusting growth forms flourished on carbonate substrates of the distal offshore parts of the platform during major flooding episodes and iii stratigraphically condensed densely fossiliferous phosphatized coquinas display peaks in biodiversity patterns as they are rich in epibenthic rhynchonellid heterorthid and rafinesquinid brachiopods calymenid dalmanitid cheirurid and illaenid trilobites sinuitid eotomariid and holopeid gastropods and hyolithids other episodic and environmentally restricted carbonate factories developed on the alnif mixed platform but are incompletely constrained due to the allochthonous character of their benthic pelagic assemblages which are ecologically mixed due to the effects of continuous shallow water erosive reworking and taphonomically complex histories these shelly carbonate factories allow recognition of the dominant benthic community assemblages that predated the onset of the hirnantian glaciation on this part of the moroccan margin of north gondwana

sequestration of 12c in the deep ocean during the early wenlock silurian positive carbon isotope excursion abstract marine carbonates from three biostratigraphically well dated silurian sections from the mid continent of north america were analyzed for carbon isotopic 13 c carb study the early wenlock positive carbon isotope excursion ireviken excursion is recorded in three sections from tennessee iowa and ohio with values of 4 5 this global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event ireviken event that spanned the llandovery wenlock boundary although several oceanographic models have been developed to account for the coincident changes in lithology biology and carbon isotope stratigraphy during this interval of the silurian their proposed causal connections among glaciation oceanography primary productivity and the global carbon cycle remain a matter of debate this investigation provides carbon isotopic and lithologic evidence that the ireviken excursion occurred during a time of relatively oligotrophic conditions a third order transgressive regressive cycle and increased carbonate production throughout the mid continent of north america the ireviken excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections it is proposed that the ireviken excursion was the product of sequestration and burial of 12 c in deep water as a result of altered deep ocean circulation

bioerosion and carbonate mud production on high latitude shelves low latitude carbonate muds often are composed either of entire units of skeletons e g algal muds or of precipitates whereas high latitude carbonate muds are bioerosional or result from maceration bioerosion at high latitudes is most intense in the photic zone particularly down to 25 m depth shelly substrata may be crushed bitten drilled bored or scraped clionid sponges endolithic algae acmaeid gastropods and regular echinoids are the most significant agents clionids produce distinctive facetted carbonate silt chips when boring which have been described from both high and low latitudes faecal pellets break down to yield mud sized carbonate particles that are more irregular than those produced by maceration exhumed infaunal bivalves are often preferred to epifaunal organisms as substrata bioerosion occurs very rapidly shells may be totally infested with boring algae in three months a moth eaten appearance therefore does not denote a relict grain reliable rates of fine sediment production are not yet available the mud fraction of northwest european shelf sediment generally contains 10 20 caco3 though an inshore and offshore belt with higher values may be identified some holocene supratidal mud flats exceed 50 caco3 much of the shelf represents a modern day equivalent of the calcareous shale facies common in the geological record instances of synsedimentary cementation are not uncommon particularly in association with heavily burrowed muds

the response of braided planform configuration to flow variations bed reworking and vegetation the case of the tagliamento river italy morphological features of braided rivers bars channels and pools experience major changes in area shape and spatial distribution as a response to i the pulsation of discharge during a flood and ii the bed evolution induced by floods in this work at a station relationships between water level and planform configuration were investigated on the tagliamento river a large gravel bed braided river in northeast italy over a 2 year study period comprising three bankfull events and several small to medium floods the analysis was performed on two 1 km long reaches characterized by different riparian vegetation cover ground based images with an hourly temporal resolution were acquired using software controlled digital cameras bars channels pools and vegetated patches were manually digitized on more than 100 rectified images sequences of constant level images spanning the study period were used to quantify the impact of floods on the stability of at a station relationships and on the turnover rate of water bodies the analysis shows that wetted area increased almost linearly with water level in both reaches the average number of branches per cross section peaked at intermediate flow levels increasing from 2 at low flow up to 6 7 the number of branches displayed the largest fluctuations over time with significant changes produced also by moderate floods turnover rates were high in both reaches with more than 30 of wetted areas at low flow converting into bare gravel in less than 2 months vegetation colonization was found to limit the mobility of the low flow channels over time by concentrating the flow in fewer deeper anabranches the number of channels per cross section was 30 40 less in the vegetated reach and the proportion of low flow water bodies in the same position after 12 months increased from 3 to 14 copyright 2012 john wiley sons ltd

changes in braided river morphology resulting from aggradation and degradation forty three experiments were completed in a large flume 1 8 x 18 3 m to examine the effects of sediment load sediment size and channel gradient on braided river morphology the laboratory results were supported by field research and analogs from the published literature subaerially exposed braid bars were formed by the dissection and further accretion of stalled linguoid dunes which are actively migrating bedforms bar forming processes and channel dynamics were the same in both sand and gravel bed channels however linguoid dunes stalled and were transformed into braid bars more frequently in gravel bed channels and therefore gravel bed channels were more intensely braided aggradation resulted in the development of more bars rather than in an increase in the size of preexisting braid bars the number of actively migrating linguoid dunes also increased with sediment load in the highly mobile sand bed channels conversely in degrading channels the number of braid bars decreased and average bar

channel morphology and bedload pulses in braided rivers a laboratory study bedload pulses in gravel bed rivers have been widely reported in recent years and attempts have been made to relate them to channel morphology bedload transport and channel morphology were measured in a small scale generic model of braided gravel bed streams two experiments are described in which braided channels developed in a 14 m 3 m sand tray total bedload output from the tray was weighed every 15 minutes stream bed geometry was surveyed every four hours pulses were observed in the bedload output time series and were qualitatively related to the channel morphology immediately upstream of the measuring section the bagnold 1980 bedload equation generally overpredicts measured bedload transport rates when applied to channels that were in equilibrium or aggrading underprediction occurred when applied to degrading channels aggradation was associated with channel multiplication and bar deposition channel pattern simplification occurred when degradation took place and bars emerged from the water flow development of phases of aggradation and degradation is dependent upon the three dimensional geometry of the stream beds spatial and temporal feedback loops can be identified enabling links between channel morphology and bedload transport rate to be directly identified

controls of mud mound formation the early devonian kess kess carbonates of the hamar laghdad antiatlas morocco the origin and development of early devonian late pragian to late zlichovian predominantly uppermost zlichovian as indicated by conodont faunas mud mounds of the hamar laghdad area in the eastern antiatlas morocco are controlled by extrinsic and intrinsic factors extrinsic factors include the existence of a paleohigh lochkovian volcaniclastics unidirectional currents and repeated storm events as well as sea level fluctuations intrinsic biologically induced factors are the preferred growth of organisms on the top and the flanks of the mounds because of more favourable ecological conditions and a rapid synsedimentary lithification of the steep mound flanks by interskeletal cementation of auloporid tabulate corals the mounds developed in an epicontinental basin below the wave base but within the range of storms the formation of the mounds started within the uppermost part of the bedded crinoid facies of the kess kess formation with the hydrological accumulation of a bioclastic pile this elevation became settled by crinoids and high diverse tabulate corals producing bioclastic sediment binding activities of the organisms were missing calcareous algae and stromatoporoids are completely absent baffling by thamnoporid tabulate corals might have occurred locally but was not important for the development of the mounds steep slopes to the north and less steep slopes to the south may be the result of north northwest to south southeast trending currents derived from orientation patterns of orthocone nautiloids in the uppermost beds of the kess kess formation a synsedimentary cementation of the flanks possibly triggered by submarine interskeletal cementation of patchily distributed auloporid colonies protected the bioclastic sediment against redistribution by frequent storms indicated by densely spaced eventstone intervals and partly also by the common stromatactis like structures within the mound facies accumulation of more parautochthonous bioclastic sediment within the mounds as compared to the intermound area therefore is caused by a self sustaining system of hydrologic piling of sediment triggered by storms preferred settlement of organisms upon these piles producing bioclastic sediment and coeval biocementation of the growing mound flanks this model differs from existing mud mound models in the lack or only minor significance of binding and baffling in the lack of mound facies sequences and in the greater importance of extrinsic control factors

origin of red pelagic carbonates as an interplay of global climate and local basin factors insight from the lower devonian of the prague basin czech republic abstract red pelagic sediments are relatively common in the phanerozoic they are often interpreted as products of sea bottom oxidation during greenhouse climate showing a conspicuous alternation with black shales and thus carrying important palaeoceanographic information the lower devonian pragian carbonate strata of the prague basin czech republic praha formation contain a marked band of red pelagic carbonate up to 15 m thick which can be correlated for several tens of km we investigated seven sections 17 to 255 m thick of the prague basin using the methods of facies analysis outcrop gamma ray logging diffuse reflectance spectroscopy optical microscopy element geochemistry magneto mineralogy and electron microprobe analysis the aim was to find the mineral carriers of the red colour investigate the stratigraphic context of the red carbonates and evaluate the local and global prerequisites for their formation the red pigmentation represents enrichment by hematite with respect to goethite approximately 31 of the total reflectance falling in the red colour band represents a threshold for red coloration the red pigmentation is carried by submicronic hematite dispersed in argillaceous pelagic calcilutite and or inside skeletal allochems gamma ray log correlation indicates that the red carbonate band developed in stratigraphic levels with low sedimentation rates typically from 1 to 7 1 mm kyr which are comparable to the mesozoic rosso ammonitico facies the red beds and the whole praha formation pragian to early emsian are characterized by low toc values

formation of braided river floodplains waimakariri river new zealand floodplains along the braided gravel bed waimakariri river are discontinuous although generally extensive landforms composed predominantly of gravel bars capped with vertically accreted fines in order of importance three mechanisms lead to floodplain formation river bed abandonment by lateral migration of the braid train initiates the formation of the largest floodpiains which usually occur downstream of tributary fans and valley bedrock spurs in the headwater reaches localized river bed aggradation during high magnitude events leads to floodplain formation by producing a surface resistant to erosion by lesser events due to either the coarse nature of the deposit or by its elevation over the river bed or both the least important mechanism initiating floodplain development is localized river or channel incision a six stage model is proposed for the sequential development of floodplains on the waimakariri river from i active river bed ii stabilizing bar iii incipient floodplain iv established floodplain v mature floodplain and to vi terrace two mechanisms lateral migration by the braid train and reactivation of abandoned channels within floodplains operating separately or in combination are responsible for floodplain reworking and their relatively young age 250 years clearly braided rivers can construct substantial areas of well developed floodplain

palaeozoic oolitic ironstones on the north american craton abstract more than 40 mostly minor palaeozoic oolitic ironstones oi accumulated on low latitude cratonic north america almost entirely in usa a few middle cambrian oi among the oldest anywhere were deposited on the western cratonic shelf of usa widely scattered late cambrian ones developed on the southwestern southeastern and northeastern flanks of the transcontinental arch middle ordovician oi were deposited on the cratonic interior and on the southern and southeastern cratonic margins in latest ordovician time neda oi spread across north central usa east of the arch and south of the laurentian upland minor ones developed in the southern part of the taconian foreland basin after major orogeny early and middle silurian clinton oi flourished throughout the foreland basin in eastern usa producing the largest oi deposit on the north american craton minor early late silurian oi accumulated in the central part of the basin middle and late devonian oi in the acadian foreland basin in northeastern cratonic usa developed progressively westward of the encroaching catskill delta an isolated middle devonian one accumulated in southwestern cratonic usa and latest devonian ones in north central usa east of the arch during this palaeozoic episode sites of oi deposition shifted eastward across the craton most of the oi were deposited during a hiatus in normal sedimentation accumulating in the upper part of shoaling upward sequences some early palaeozoic ones occur in glauconitic siliciclastic carbonate facies and contain mostly spherical hematitic ooids these suggest derivation from iron rich soils developed on glauconitic deposits oi in wholly siliciclastic facies containing distorted chamositic ooids with cores of mud peloids were common in later palaeozoic time these oi like many other phanerozoic ones suggest a synsedimentary to early diagenetic origin

an introductory perspective on non tropical shelf carbonates abstract this brief account introduces the topic of non tropical shelf carbonate deposits both modern and ancient specific aspects of which are described and discussed in the 22 accompanying papers in this special issue of sedimentary geology non tropical shelves are loosely defined as lying poleward of the present limit of development of hermatypic coral reefs or beyond the mean annual surface water isotherm of about 20 c typically near 30 latitude most geoscientists have traditionally regarded shallow water carbonate formation as a low latitude phenomenon however wherever the rate and extent of terrigenous sediment dilution is small the potential exists for extensive deposits of skeletal carbonate to accumulate on shelves beyond the sub tropics the facies attributes of the modern non tropical carbonates are distinctive and contrast significantly with the structural lithological faunal floral mineralogical and diagenetic character of their tropical counterparts in the rock record non tropical limestones probably occur throughout the phanerozoic but generally remain to be identified and or interpreted as such they are presently described mainly from the ordovician permian and tertiary quaternary periods of major continental glaciations and accentuated latitudinal thermal gradients a uniformitarian approach to the recognition and interpretation of ancient shallow marine limestones requires acceptance of both tropical and non tropical carbonate facies models

late early silurian wenlockian general circulation model generated upwelling graptolitic black shales and organic rich source rocks an accident of plate tectonics through an evolutionary process of plate tectonics the wenlockian physical world was composed of an oceanic northern hemisphere and a mainly continental southern hemisphere dominated by the giant continent of gondwana the high latitude position of gondwana placed much of its extensive margin in the middle latitudes laurentia and baltica occupied an equatorial position siberia and kazakh were to the north the global paleoclimate was created and driven by the diverse paleogeography of the two hemispheres in the model the northern hemisphere is dominated by strong zonality in all seasons in contrast the continental southern hemisphere reacts to wide temperature ranges of summer heating and winter cooling of gondwana the extensively flooded 28000 km margin of gondwana occupied the zone of the southern hemisphere westerlies these westerlies produced intense surface upwelling and by extension of present analogies high primary biologic productivity gondwana s northern margin extended far enough toward the equator that significant winter sea ice did not form in the southern hemisphere this allowed a vertically stratified sea to develop as well as probable dysaerobic and anoxic conditions a high organic flux of marine plankton accumulated on the sea floor where it was subsequently buried and preserved a strong positive correlation exists between upwelling generated by a general circulation model gcm on gondwana s northern margin and the distribution of southern hemisphere basins with source rocks containing types i and ii kerogens

shell directions as a tool in palaeocurrent analysis abstract conical shells mostly orthoconic nautiloids locally gastropods and rugose corals were used to determine current directions in ludlovian to upper famennian cephalopod limestones in the eastern anti atlas of morocco ougarta aulacogen and ahnet basin both algeria data plots established on 50 413 measurements from 217 localities document rather consistent current patterns which show only minor variations through subsequent intervals a conspicuous feature are currents derived from pelagic platforms and directed towards adjacent basins shell accumulations decrease markedly towards platform margins yielding less distinctive information on current directions which due to lack of shells cannot be established in the basins proper orientation patterns of styliolinids show such a puzzling variation in adjacent samples that their use for current analysis is doubtful the same is true for the presumed down stream position of goniatite apertures which shows a highly variable pattern which is rarely consistent with that of concomitant orthoconic nautiloids the direction of orthocones in cephalopod limestones onlapping lower givetian mud mounds and ridges in the ahnet basin of algeria shows a radial pattern which is the result of a mere gravitational deposition of shells on the steep slopes of these buildups apart from this exception the applicability of conical shells for current analysis is confirmed

assessing mechanisms of environmental change palynological signals across the late ludlow silurian positive isotope excursion δ13c δ18o on gotland sweden abstract the silurian is characterised by strong environmental changes as indicated by several pronounced positive 13c and 18o excursions the mechanisms responsible for these isotopic shifts are a matter of much debate the purpose of the present study is a quantitative high resolution evaluation of the palynomorph distribution across the late ludlow ludfordian isotope excursion on gotland marine and terrestrial palynomorphs have been chosen as palaeoenvironmental indicators because they are widely distributed and exceptionally well preserved moreover the combined analysis of marine and terrestrial palynomorphs allows the reconstruction of contemporaneous palaeoenvironmental change in the marine and terrestrial realms to correlate our results with the carbon isotope stratigraphy we have sampled only localities with published isotope data from diagenetically unaltered brachiopod shells our results show that fluctuations in the composition of the acritarch assemblages are closely correlated with the stable isotope development low abundances of acritarchs occur in times of high stable isotope values and vice versa indicating that an increase in marine productivity cannot have been the reason for the positive 13c excursion the results are in good agreement with climatic models for the silurian assuming alternating humid and arid climatic conditions in low latitudes times of high stable isotope values correspond to arid climatic conditions in low latitudes with low input of terrestrial nutrients resulting in impoverished acritarch and conodont communities whereas times of low stable isotope values correlate with humid climate high nutrient input and abundant and diverse acritarch communities the major change in acritarch communities took place during the increase of the stable isotope values after hemipelagic planktonic or nektonic organisms graptolites conodonts have been affected this indicates that environmental changes connected with the stable isotope excursion first affected deeper water settings and later the photic zone the absolute abundance of terrestrial spores closely mirrors the marine 13c and 18o development showing that both the marine and the terrestrial realm were synchronously affected by the climatic changes the high abundance of spores during the isotope excursion might be explained by increased aeolian input

analysis and forecast of extreme new snow avalanches a numerical study of the avalanche cycles of february 1999 in france snow and weather conditions typical of exceptional cycles of fresh snow avalanches in the northern alps are investigated using the numerical avalanche hazard forecasting procedure of meteo france sensitivity tests are performed on the events of february 1999 in the chamonix france region and resulting snowpack instability modeled at the massif scale is compared using adapted new indices and maps our results complete conclusions of earlier observation based studies by providing new insights into the snow and weather conditions of february 1999 the large avalanches mainly resulted from large and very unstable fresh snow accumulations moreover the snowpack instability was increased locally by wind transport of light and fresh snow in february the mechanical weaknesses resulting from the weather conditions prior to february were a key factor in explaining the unusual volumes of these avalanches this study suggests that the operational numerical safran crocus mepra scm chain provides reliable forecasts of extreme new snow avalanche situations at the massif scale but that local scale simulations are still needed to improve the efficiency of risk mitigation and civil protection policies

fluctuations in epeiric sea carbonate production during silurian positive carbon isotope excursions a review of proposed paleoceanographic models positive excursions in the carbon isotopic ratio of marine carbonates 13ccarb are commonly attributed to an increase in the burial ratio of organic versus carbonate carbon in the global oceans regardless of whether this increased deposition of organic carbon was oceanographically or biologically induced the fact that high carbon isotope values are most often attributed to the sequestration and burial of organic carbon remains the notion that high 13ccarb values should be the result of organic rich deposition in epeiric sea settings is not surprising considering that the overwhelming majority of organic carbon burial in the modern oceans occurs in continental shelf and margin settings what is surprising is that major positive carbon isotope excursions during the silurian are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas in fact silurian 13ccarb excursions occurred during intervals of prolific reef development and an overall expansion of carbonate platform environments globally this discrepancy has sparked considerable debate among silurian carbon isotope specialists but has offered a valuable opportunity to re examine the intricate connections among nutrients primary productivity carbonate sedimentology and the global carbon cycle in the lower paleozoic here we consider several sedimentological and geochemical features of one of these silurian positive carbon isotope 13ccarb excursions and discuss the broader implications of current silurian 13ccarb models to the interpretation of lower paleozoic carbonate sequences

from black and white to colour in the silurian abstract ironstones and iron rich limestones regularly occur as components of time specific intervals of the palaeozoic as well as in younger times brett et al 2012 this issue silurian sediments deposited at high latitudes along the peri gondwana border are characterized by black and white limestone and graptolitic shale sequences those in the carnic alps southern austria additionally contain colourful pink to red limestones and ironstones laminated structures such as the ferruginous coatings around skeletal fragments mostly trilobites and some cephalopods and echinoderms and stromatolitic features along discontinuity surfaces display dark red green white and brownish colours due to the presence of goethite magnetite hematite chamosite calcite and subordinate apatite confocal laser raman microscopy and complementary microscopic analysis of these ferruginous laminated structures document the presence of carbonaceous matter associated with fossilized microbial structures in the form of stromatolites filaments and coccoids suggesting a microbial role in the colouring of the silurian world of the carnic alps iron concentrations up to 30 that of matrix and surrounding non ferruginous rocks suggest blooms of iron microbe activity in response to the time specific occurrence of chemically charged sea water during global biotic events

palaeozoic low oxygen high latitude carbonates silurian and lower devonian nautiloid and scyphocrinoid limestones of the anti atlas morocco abstract high latitude silurian and lowermost devonian basinal limestones of northern gondwana are characterised by mass accumulations of orthocone nautiloids bivalves and planktonic crinoids scyphocrinoids extensive exposures in the anti atlas of morocco reveal this basinal facies passes up gradient into an iron rich bioclastic facies associated with distal shelf siliciclastics and into crinoid brachiopod dominated bioclastic blankets and shoals the limestones formed in a cool water setting in which upwelling of nutrient rich waters during sea level lowstands is interpreted to have triggered the abundance of nautiloids and scyphocrinoids preservation of organic matter in under and overlying graptolite shales as well as iron precipitation in the neighbouring shallower water sub oxic zone the nautiloid limestones proved to be good correlation markers on a regional to global scale contemporaneous siliciclastic supply into the neighbouring facies belt as well as new biostratigraphic data are taken as evidence that these limestones can be correlated to 2nd order eustatic sea level lowstands and the following transgression the low oxygen basinal facies disappeared more or less abruptly during the early devonian pragian and was replaced by a well oxygenated basinal lowstand facies suggesting the end of upwelling at the same time the first reefal build ups coral biostromes and mud mounds appear in north africa indicating a significant rise in water temperatures the change from low oxygen lowstand mass accumulations to well oxygenated basinal facies with a concomitant rise in water temperature is in accordance with recent models of a cool silurian to earliest devonian lochkovian ocean with oscillating oxic to anoxic ventilated to poorly circulated oceanic states and a warm ocean with anoxic to euxinic poorly ventilated to stagnant ocean waters from the beginning of the pragian onwards

assessment of intermediate fine sediment storage in a braided river reach southern french prealps this paper presents a field investigation on river channel storage of fine sediments in an unglaciated braided river the bes river located in a mountainous region in the southern french prealps braided rivers transport a very large quantity of bedload and suspended sediment load because they are generally located in the vicinity of highly erosive hillslopes consequently these rivers play an important role because they supply and control the sediment load of the entire downstream fluvial network field measurements and aerial photograph analyses were considered together to evaluate the variability of fine sediment quantity stored in a 2 5 km long river reach this study found very large quantities of fine sediment stored in this reach 1100 t per unit depth 1 dm given that this reach accounts for 17 of the braided channel surface area of the river basin the quantities of fine sediment stored in the river network were found to be approximately 80 of the mean annual suspended sediment yields ssys 66 200 t year 1 comparable to the ssys at the flood event scale from 1000 t to 12 000 t depending on the flood event magnitude these results could explain the clockwise hysteretic relationships between suspended sediment concentrations and discharges for 80 of floods this pattern is associated with the rapid availability of the fine sediments stored in the river channel this study shows the need to focus on not only the mechanisms of fine sediment production from hillslope erosion but also the spatiotemporal dynamics of fine sediment transfer in braided rivers copyright 2010 john wiley sons ltd

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

riparian vegetation and island formation along the gravel bed fiume tagliamento italy after more than 300 years of river management scientific knowledge of european river systems has evolved with limited empirical knowledge of truly natural systems in particular little is known of the mechanisms supporting the evolution and maintenance of islands and secondary channels the dynamic gravel bed fiume tagliamento italy provides an opportunity to acquire baseline data from a river where the level of direct engineering intervention along the main stem is remarkably small against a background of a strong alpine to mediterranean climatic and hydrological gradient this paper explores relationships between topography sediment and vegetation at eight sites along the active zone of the tagliamento a conceptual model of island development is proposed which integrates the interactions between large woody debris and vegetation geomorphic features sediment calibre and hydrological regime islands may develop on bare gravel sites or be dissected from the floodplain by channel avulsion depositional and erosional processes result in different island types and developmental stages differences in the apparent trajectories of island development are identified for each of the eight study sites along the river the management implications of the model and associated observations of the role of riparian vegetation in island development are considered in particular the potential impacts of woody debris removal riparian tree management regulation of river flow and sediment regimes and changes in riparian tree species distribution are discussed

fluvial islands first stage of development from nonmigrating forced bars and woody vegetation interactions abstract fluvial islands can develop from the channel bed by interactions between pioneer trees and bars although vegetation recruitment and survival is possible on all bar types it is easier for trees to survive on nonmigrating bars developed from a change in channel geometry or to the presence of a steady perturbation this field study details the first stages of development of a vegetated mid channel nonmigrating or forced bar and its evolution toward an island form over six years analysis of bed topographical changes vegetation density and roughness scour and fill depths sediment grain size and architecture and excess bed shear stress highlighted a specific signature of trees on topography and grain size segregation two depositional processes combining the formation of obstacle marks and upstream shifting deposition of sediments led to the vertical accretion of the vegetated bar during the first stage of the bar accretion bedload sediment supply coming from surrounding channels during floods was identified as a key process modulated by the presence of woody vegetation and a deflection effect induced by the preexisting topography grain size segregation between vegetated and bare areas was also highlighted and interpreted as an important process affecting the development of surrounding channels and the degree of disconnection and hence the speed of development of a growing island the heterogeneity of bedload supply can explain why sediment deposition and density of trees are not strictly related a general conceptual model detailing the first stages of evolution from a bar to an established island is proposed for relatively large lowland rivers

carbon isotope stratigraphy of lochkovian to eifelian limestones from the devonian of central and southern europe lower to middle devonian carbonates of the prague syncline the carnic alps the montagne noire and the cantabrian mountains were investigated for 13ccarb and 13corg these values were measured on bulk rocks selected components and cements many carbonates exhibit primary marine values but some are altered by diagenesis a 13c curve can be presented for the latest pridolian to emsian time interval several sharp or broad positive excursions are obvious in the woschmidti postwoschmidti sulcatus kitabicus late serotinus and kockelianus conodont zones the excursion at the silurian devonian boundary is known worldwide and therefore considered global in nature some of the others are described for the first time from central and southern europe and their global nature has to be verified by further investigations in other regions most excursions relate to and or started during major regressions whereas sea level highstands correspond to minimal 13c values similar relationships between sea level changes and 13c have been observed from other early palaeozoic intervals the transgressive chotec and kacak events are marked by positive isotope excursions this type of combination is usually observed in late palaeozoic to cenozoic black shale events

mixed siliciclastic carbonate shelf sedimentation lower devonian sequences of the sw anti atlas morocco abstract in the south western anti atlas of morocco dra plain a continuous exposure of lower devonian sedimentary successions more than 400 km long provides an example of mixed siliciclastic carbonate shelf sequences the carbonates were deposited adjacent and down dip to large lobes of storm dominated deltaic complexes during sea level lowstands and subsequently transgressed over the 200 m thick prograding siliciclastic wedges during sea level rise depositing limestone units between 5 and 20 m thick the deltaic lobes switched through time and caused alternating siliciclastic supply along the shoreline oblique parallel transect condensed limestone successions formed in clastic sediment starved areas due to delta complex abandonment down gradient in outer ramp environments the direct juxtaposition of carbonate and siliciclastic lithofacies associations comprising rhythmic lime mudstones marls and massive shelf sandstones provides the opportunity for mutual interpretation of their mode of deposition and bathymetry this lateral relationship suggests that significant amalgamation took place in falling stage systems tracts to produce laterally extensive shelf sandstone sheets on the outer ramp two types of stacking patterns are observed in prograding siliciclastic wedges a normal progradational shoreline pattern with well developed parasequence sets in the proximal ramp and an abrupt regressive succession in which the amalgamated shelf sandstones rest more or less directly on offshore siltstones in the distal ramp in the latter case the falling stage systems tract sandstones are overlain by a relatively thick cyclic alternation with brachiopod rich storm beds below a marked transgressive surface at the base of the capping transgressive limestones the cyclic alternation is interpreted as lowstand systems tract deposits marked flooding surfaces on parasequence sets are fe enriched and contain abundant articulated brachiopods indicating breaks in sedimentation and an overall deepening upward facies from the underlying maximum regression and sequence boundary these aggradational to retrogradational stacked lowstand systems tracts are thus rather transgressive not regressive in character as is commonly described in sequence stratigraphic ramp models

floodplain stages in the braided ngaruroro river new zealand this study investigates floodplain deposits in the braided ngaruroro river new zealand to characterize and quantify the spatial extent of deposits at different stages of development using a sequence of aerial photography depositional surfaces were mapped and then assigned a floodplain development stage based on age the discharge causing inundation and field observations of sediment and vegetation characteristics the ensemble of floodplain deposits makes up 40 of the river valley the remainder being the active river bed stabilizing bars and mature floodplains dominate the floodplain ensemble while the existence of stabilizing bars points to a recent shift or fluctuation in controlling conditions the extent and location of mature units suggest an earlier change in hydrologic and sediment supply conditions that limited braidtrain mobility mosaics of different aged deposits range from simple two stage mosaics to more complex arrangements of three and four stages in the rapid evolution from stabilized bars to mature floodplains a large amount of fine grained sediment is stored in the ngaruroro river valley highlighting the possibility of finding extensive fine grained facies in the ancient sedimentary record of braided systems understanding floodplain ensembles and their controls will contribute to effective restoration of braided rivers

relationship between sediment supply and avulsion frequency in braided rivers the interplay between sediment supply s s sedimentation rate s r and the frequency of channel avulsion a f exerts a primary control on alluvial architecture in order to investigate the effect of sediment supply on avulsion frequency four froude scale model experiments of an aggrading braided river were undertaken in which the magnitude of s s was progressively increased over an eightfold range the value of a f increases at a rate slower than the increase in s s contrary to the trend previously reported by bryant falk and paola in their experimental study on alluvial fan dynamics these results suggest that the relationship between a f and s s is dependent upon bed slope and that the response of a f to an increase in s s in unconfined braided rivers may be different than that on steep alluvial fans

a global biogeochemical perturbation across the silurian devonian boundary ocean continent biosphere feedbacks abstract the large scale global biogeochemical perturbation across the silurian devonian s d boundary recorded in the major positive excursion of the 13c time curve klonk event of an amplitude of 2 5 to 3 0 max 4 0 in europe and up to 5 0 in north america reflects a unique combination of palaeogeographic biogeochemical and evolutionary processes in the late caledonian geodynamic setting the steady sulfur isotopic ratios show an overall stability of the s d oceanic geochemical system as a whole and do not indicate any synchronous changes in anoxic deep oceanic and sediment processes therefore this led us to a hypothesis that the crucial changes that contributed to the recorded carbon cycling turnover are related to rapidly evolving ocean continent biosphere feedback coastal zones of the latest silurian epicontinental seas accumulated considerable quantities of organic carbon from early vascular vegetation which explosively expanded to inhabit vast near coastal shallows and deltas large primary production of these early terrestrial plants and rapidly enhanced sedimentary burial of organic carbon were also responsible for co2 drawdown which have resulted in reversed greenhouse effect and a global climatic cooling tendency this feedback was blocked when the sea level gradually dropped and led to shrinking of the silurian epicontinental seas and the growing climatic deterioration during the s d transition limited primary production furthermore continued processes of regressive abrasion and erosion limited the storage of organic carbon as well as the efficiency of the carbonate factory during the following early devonian greenhouse interval marine regression and active latest caledonian tectonism promoted progressive weathering of the sedimentary organic matter the oxidation of c rich deposits caused the subsequent growth of co2 levels in the early devonian culminating in the warming of global climate

silurian δ13c stratigraphy a view from north america three positive shifts in 13 c are recognized in well dated carbonate successions in north america and can be confidently correlated with shifts in the baltic region by using conodont and graptolite zonations the 13 c excursions reach distinct peaks of 3 to 4 at the ends of the amorphognathoides and siluricus conodont zones and within the ludensis graptolite zone the three events are associated with the most significant extinction horizons in the silurian and occur near clastic carbonate transitions that mark eustatic or oceanographic changes the magnitudes of the two oldest 13 c peaks compare well with the baltic region however the end siluricus excursion reaches far greater values of as much as 11 in sweden likely reflecting local modification of the global seawater signature

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events

stratigraphic significance and resolution of spectral reflectance logs in lower devonian carbonates of the barrandian area czech republic a correlation with magnetic susceptibility and gamma ray logs abstract spectral reflectance sr is a relatively novel technique which is used as a proxy for organic carbon and siliciclastic input in carbonate sediments in particular in core research this study summarizes results from high resolution outcrop sr logging of a lower devonian distal ramp carbonate section of the barrandian area czech republic the sr data were correlated with parallel petrophysical and geochemical data with an aim to improving their cyclostratigraphic and event stratigraphic interpretation potential and the gaps sr data can fill transgressive regressive trends indicated by facies stacking patterns caco 3 data and spectral gamma ray grs logs are well correlated in the regional context and point to third order eustatic sea level fluctuations peak regressions of the tr cycles are sensitively marked by cie l a b parameters and percent reflectance in colour bands the sr and magnetic susceptibility ms data are markedly cyclic on a more detailed scale their inferred periods vary between 49 2 and 205 kyr indicating that they may have been forced by orbital cycles in the milankovitch band the sr and ms cycles are either in phase or out of phase the latter cycles point to a strong diagenetic forcing of the colour cyclicity hematite concentration cycles inferred from sr signal in the pragian red hemipelagic carbonates are interpreted as reflecting changing bottom oxygenation and subsurface redox gradients during early diagenesis the cie l parameter was found to be related to the presence of diagenetic hematite which indicates that cyclostratigraphic analysis of simple greyscale data can be affected by considerable error due to chromacity although the cie l parameter is partly correlated with caco 3 concentrations the ms data are not which suggests that the ms signal can be significantly influenced by diagenetic minerals and its cyclostratigraphic interpretation can be potentially misleading the sr data backed up by grs ms and caco 3 data indicate that the red colour in the pragian hemipelagic limestones originated from early diagenetic hematite precipitation under the conditions of an oxygenated ocean floor this suggests that deep sea bottom oxygentation may have been in operation during the early devonian which is traditionally perceived as a typical greenhouse period the sr data can be obscured in several m thick zones of elevated cie l and other colour changes leaching zones related to postdepositional alteration along subvertical faults however this fault related telogenic alteration has little influence on the ms signal the combined use of sr and ms has a synergic effect and can significantly improve the interpretation of their respective cyclic signals particularly in relation to the diagenetic overprint and possible carriers of the ms

oxygen and carbon stable isotope records of the lochkovian pragian boundary interval from the prague basin lower devonian czech republic abstract climate changes close to the lochkovian pragian event are still widely discussed carbonate 18o and 13c and conodont apatite 18o from medial to distal carbonate ramp sediments were analysed to provide further stable isotope data from the stratotype area in the prague basin the uppermost silurian to lower emsian 18o trends are put into an updated conodont biostratigraphy framework carbonate 18o and 13c are discussed in the context of facies dependent diagenesis the 13c pattern measured from different sections enables the correlation of intervals with development of different facies inside the prague basin positive 13c shifts are often coupled with trends of decreasing computed gamma ray cgr values interpreted as regressions whereas negative 13c shifts commonly coincide with opposite trends in cgr the lochkovian pragian boundary interval coincided with an increase in carbonate 13c and an increase in the 18o of conodont apatite and carbonate which we interpret as consequence of climate cooling generally the lochkovian was warmer than the pragian and earliest emsian