scaling gridded river networks for macroscale hydrology development analysis and control of error a simple and robust river network scaling algorithm nsa is presented to rescale fine resolution networks to any coarser resolution the algorithm was tested over the danube river basin and the european continent coarse resolution networks at 2 5 5 10 and 30 min resolutions were derived from higher resolution gridded networks using nsa and geomorphometric attributes such as river order shape index and width function these parameters were calculated and compared at each resolution simple scaling relationships were found to predict decreasing river lengths with coarser resolution data this relationship can be used to correct river length as a function of grid resolution the length corrected width functions of the major river basins in europe were compared at different resolutions to assess river network performance the discretization error in representing basin area and river lengths at coarser resolutions were analyzed and simple relationships were found to calculate the minimum number of grid cells needed to maintain the catchment area and length within a desired level of accuracy this relationship among geomorphological characteristics such as shape index and width function derived from gridded networks at different resolutions suggests that a minimum of 200 300 grid cells is necessary to maintain the geomorphological characteristics of the river networks with sufficient accuracy

controls of climate topography vegetation and lithology on drainage density extracted from high resolution topography data summary mark melton in 1957 found that climate basin morphometry and surficial variables control drainage density d d but differences observed between field surveyed channels and those mapped on topographic contours or blue lines left doubts on these results later several landscape evolution model and observational studies analyzed the behavior of d d however only a few studies have been performed over a large number of landscapes of different characteristics and have relied on high resolution topography data we revisit melton s hypothesis by using meter resolution digital terrain models dtms in 101 subbasins in the usa we first propose a dimensionless drainage density d dd metric based on the ratio of likely channelized pixels to total number of basin pixels which has the advantage of eliminating the computation of the channel network our analysis shows that d dd is a weak scaling function of the input dtm resolution compared to the classic dimensional d d metric ratio of total channel length to total basin area we analyze the correlation of d dd and mean annual precipitation map with a gaussian mixture model which identifies two sub groups displaying different correlation negative in arid and semi arid environments and positive in humid environments the transition in correlation is around 1100 100 mm yr of map and is accompanied by the occurrence of thick soil layers and high available water capacity that promote dense vegetation cover v cov and low d dd while small variation in d dd is observed across vegetation types increasing v cov corresponds to decreasing d dd we also explore the relationship between d dd and relief r and d dd and lithology d dd and r are weakly correlated in arid and semi arid environments while they have strong positive correlation in humid environments no significant correlation is found between d dd and lithology although the results are likely affected by our sample choice

high resolution 87sr 86sr chemostratigraphy of the silurian implications for event correlation and strontium flux analyses of 87sr 86sr in silurian conodonts recovered from localities in north america and europe representing 13 of the 14 defined silurian conodont zones provide a high resolution record of seawater chemistry for the silurian period these data which are characterized by little or no scatter depict several high frequency cycles superimposed on a gradual longer term rise in 87sr 86sr for the silurian high frequency cycles have a duration of about one conodont zone and many correlate with sequence boundaries recognized around the world these data provide a much higher resolution image of secular changes in 87sr 86sr during the silurian and may require a rethinking of models of strontium isotope flux in marine basins

automated upscaling of river networks for macroscale hydrological modeling 1 we developed a hierarchical dominant river tracing drt algorithm for automated extraction and spatial upscaling of basin flow directions and river networks using fine scale hydrography inputs e g flow direction river networks and flow accumulation in contrast with previous upscaling methods the drt algorithm utilizes information on global and local drainage patterns from baseline fine scale hydrography to determine upscaled flow directions and other critical variables including upscaled basin area basin shape and river lengths the drt algorithm preserves the original baseline hierarchical drainage structure by tracing each entire flow path from headwater to river mouth at fine scale while prioritizing successively higher order basins and rivers for tracing we applied the algorithm to produce a series of global hydrography data sets from 1 16 to 2 spatial scales in two geographic projections wgs84 and lambert azimuthal equal area the drt results were evaluated against other alternative upscaling methods and hydrography data sets for continental u s and global domains these results show favorable drt upscaling performance in preserving baseline fine scale river network information including 1 improved automated extraction of flow directions and river networks at any spatial scale without the need for manual correction 2 consistency of river network basin shape basin area river length and basin internal drainage structure between upscaled and baseline fine scale hydrography and 3 performance largely independent of spatial scale geographic region and projection the results of this study include an initial set of drt upscaled global hydrography maps derived from hydro1k baseline fine scale hydrography inputs these digital data are available online for public access at ftp ftp ntsg umt edu pub data drt

nonmonotonic variation of seawater 87 sr 86 sr across the ivorian chadian boundary mississippian osagean evidence from marine cements within the irish waulsortian limestone abstract detailed analysis of compositionally unaltered marine fibrous cements mfc from a single core through the mississippian irish waulsortian limestone indicates that the variation of seawater 87sr 86sr is nonmonotonic across the ivorian chadian boundary this nonmonotonic variation has not been recognized by previous studies furthermore marine cements yielded 87sr 86sr ratios lower than previously reported values for the ivorian chadian osagean marine fibrous cements are interpreted to be compositionally unaltered on the basis of nonluminescent character and stable isotope c o composition comparable to previous estimates of mississippian marine calcite the isotope chemistry c o sr and cathodoluminescent character of the marine fibrous cements therefore remained intact during their conversion from high mg calcite to low mg calcite microdolomite a conversion that probably took place in marine water during precipitation of zone 1 calcite cement the oldest non mfc cement high stratigraphic resolution was obtained by restricting the sample set to a single core 429 m long thereby eliminating chronostratigraphic correlation errors the mfc cements are pre arundian based on previous study and are interpreted to be in stratigraphic order and to be approximately the same age as the host limestones the core is estimated to represent about 9 8 million years of waulsortian limestone deposition 87sr 86sr ratios of pristine mfc samples range from a high of 0 707908 in the early ivorian to a low of 0 707650 in the late ivorian and mid chadian with an early chadian maximum at 0 707800 the maximum rate of change in seawater 87sr 86sr is 0 00012 ma comparable in magnitude to tertiary values our data document the presence of fine scale seawater 87sr 86sr modulations for the ivorian chadian in contrast to the previously published monotonic seawater 87sr 86sr curve for this interval and emphasize the importance of well characterized intraformational isotopic baselines

construction of the silurian and devonian seawater 87sr 86sr curve the estimated path of seawater 87sr 86sr during the silurian and devonian is based on 86 analyses 84 whole rocks presented here and in other published results the seawater 87sr 86sr rose steadily throughout the silurian from a poorly constrained value near 0 70793 sw 114 at the ordovician silurian boundary the 87sr 86sr value at the silurian devonian boundary is near 0 70871 sw 36 based on an earliest devonian limestone suite from new york the 87sr 86sr falls through the early devonian to reach a low near 0 70772 sw 135 in the eifelian and givetian and rises to near 0 70803 sw 104 in famennian time the seawater 87sr 86sr at the devonian mississippian boundary is estimated to be near 0 70805 sw 102 the middle paleozoic sample suite is judged less suitable for defining seawater 87sr 86sr than those from other paleozoic periods the middle paleozoic seawater 87sr 86sr variation indicates domination of continental sr contributions during the silurian and later devonian the high ratio continental sr contribution is controlled by the climate and the area age and composition of exposed crystalline rocks the dominance of the oceanic low ratio sr during the early and part of middle devonian is coincident with a reported increase in continental volcanism the estimated rise and fall of sealevel leading to flooding or exposure of the continents does not appear to control seawater 87sr 86sr during the middle paleozoic

deriving river networks and catchments at the european scale from medium resolution digital elevation data abstract the extraction of drainage networks and catchment boundaries from digital elevation models dems has received considerable attention in recent years and is recognized as a viable alternative to traditional surveys and the manual evaluation of topographic maps however most studies have covered limited areas due to the lack of detailed information and or the lack of highly efficient algorithms in this paper we present an application that delineates river networks and catchment boundaries across the european continent from a medium resolution 250 m dem we exploit novel algorithms based on the concepts of mathematical morphology and implement a landscape stratification for drainage density a flow direction grid is computed using an efficient algorithm for the removal of spurious pits river networks are then derived by imposing a variable threshold for the minimum contributing area needed to form and maintain a channel this is achieved through a landscape stratification that reflects the ability of the terrain to develop different drainage densities it is shown that the analysis of environmental characteristics coupled with the analysis of local slope versus contributing area enables river network mapping with a spatially varying drainage density the result has been validated by comparing the derived data to digital river and catchment data from other sources and with varying scales of observation

late ordovician mass extinction in the selwyn basin northwestern canada geochemical sedimentological and paleontological evidence we present a detailed study of the trace element and stable isotope geochemistry sedimentology and fossil distributions in two avalanche lake av4b av1 ordovician silurian boundary sections in the selwyn basin trilobites and conodonts indicate a profound extinction at the end of the ordovician which is constrained stratigraphically within a 3 in magnitude is recorded in the extinction interval iridium abundances 0 051 ppb in the extinction interval are low and fail to provide evidence for an impact the highest ir abundance is found to be associated with reduced sedimentation in a condensed horizon cerium anomalies indicate a s

towards a general model of quick clay development abstract within the clear definition of a quick clay as being a soil which has a sensitivity greater than 30 and a remoulded strength less than 0 5 kpa a general model for quick clay development is presented based on the influences of various material properties and natural processes on the sensitivity and remoulded strength the influences are separated according to depositional requirements and post depositional factors and according to whether the influence is to increase the undisturbed strength or to decrease the remoulded strength depositional factors are the requirements for a flocculated structure and the dominance of low activity minerals post depositional factors are cementation and slow load increase which increase the undisturbed strength and little consolidation leaching and dispersants which decrease the remoulded strength the application of the model to marine brackish water and freshwater sediments is discussed

validation of a new global 30 min drainage direction map abstract digital drainage direction maps are a prerequisite for analyzing the flow of water on the land surface of the earth for continental or global studies the most appropriate and most frequently used resolution for such data sets appears to be 30 longitude by latitude in this paper we present the new global drainage direction map ddm30 a 30 raster map of surface drainage directions which organizes the earth s land area into drainage basins and provides the river network topology ddm30 was generated by first upscaling two drainage direction maps ddms at higher resolutions the resulting map was then extensively corrected in an iterative manner by comparison against vectorized high resolution river maps and other geographic information finally it was co referenced to the locations of 935 gauging stations provided by the global runoff data centre grdc which again involved manual corrections ddm30 was validated against drainage basin areas from the literature against the given upstream areas of the grdc stations and most importantly against information from hydro1k a data set based on a hydrologically corrected 1 km digital elevation model which is thought to afford the best information on surface drainage currently available at the global scale in the course of the validation the quality of ddm30 was compared to three other 30 ddms the validation results show that ddm30 provides a more accurate representation of drainage directions and river network topology than the other 30 ddms

criteria for the retention of original seawater 87sr86sr in ancient shelf limestones abstract a suite of 40 pennsylvanian and permian shelf limestones from kansas and oklahoma has been analyzed for major and trace elements and for c o and sr isotopes the chemistry and c and o isotope ratios were compared to the difference between the measured 87 sr 86 sr and original seawater ratio concentrations of mn sr and fe varied most systematically with the change in the seawater sr ratio all samples containing 700 ppm mn yielded altered ratios fe and sr contents were also found to have a less well defined correlation with the retention of the original sr ratio all limestones with 900 ppm sr and 20 of 22 samples with a sr mn 2 0 yielded correct ratios the geochemical guidelines were applied to the chemical and isotopic results from 76 shelf limestones mostly of pennsylvanian age from texas and oklahoma more than 92 of the samples that contain sr mn 2 0 yielded correct seawater ratios every sample with sr mn 2 0 retained the original seawater ratio

δ18o values 87sr86sr and sr mg ratios of late devonian abiotic marine calcite implications for the composition of ancient seawater abstract late devonian frasnian abiotic marine calcite has been microsampled and analyzed for 87 sr 86 sr ratios 18o and 13c values and minor element concentrations portions of marine cement crystals from the alberta and canning basins have escaped diagenetic alteration and preserve original marine 18o values 4 8 0 5 pdb 13c values 2 0 to 3 0 pdb 87 sr 86 sr ratios 0 70805 3 and sr mg weight ratios 0 04 to 0 05 marine 87 sr 86 sr ratios are globally consistent and can be correlated within the alberta basin and among the alberta canning and williston basins correlation of isotopic and chemical data strengthen the conclusion that marine cements from the leduc formation preserve original marine 18o values which are 3 to 4 lower than those of modern marine cements these low 18o values are best explained by precipitation from 18o depleted seawater and not by elevated seawater temperature or diagenetic alteration for comparison with devonian data analogous data were collected from holocene mg calcite and aragonite marine cements from enewetak atoll marshall islands mg calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater and mg calcite cements are homogeneous with respect to sr and mg contents empirically derived homogeneous distribution coefficients for mg and sr in modern abiotic mg calcite from enewetak atoll are 0 034 and 0 15 respectively an equation describing the dependence of dsr on mg content was based on a compilation of sr and mg data from holocene abiotic marine calcite dsr 3 52 10 6 ppm mg 6 20 10 3 unlike that derived from experimental data this sr mg relation is consistent over a range of 4 to 20 mol mgco3 and may represent precipitation phenomena which are minimally controlled by kinetic effects comparison of sr and mg contents of analogous devonian and holocene marine cements suggests that the mg ca ratio of late devonian seawater was significantly lower and that the sr ca ratio was significantly higher than that of modern seawater

strontium isotopic variations in jurassic and cretaceous seawater a high resolution seawater strontium isotope curve has been generated through the analysis of well dated and well preserved belemnites and oysters from the middle and upper jurassic and the lower cretaceous of great britain analysis of fe and mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered the strontium isotope curve remains relatively flat through the aalenian and early bajocian rapidly descends through the late bajocian and bathonian and reaches a minimum in the callovian and oxfordian it then begins a rapid increase in the kimmeridgian and portlandian that continues through much of the early cretaceous the curve levels off in the barremian suddenly dips downwards in the aptian and recovers gradually through the albian the strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of great britain using the techniques of strontium isotope stratigraphy model results indicate that much of the variation in seawater 87sr86sr between 120 and 40 ma can be explained by changing the intensity of mid ocean ridge hydrothermal fluxes proportionally to estimated mid ocean ridge crustal generation rates it is also possible that the variations during the rest of the mesozoic and the permian are primarily reflections of changing hydrothermal inputs the model results have several important implications first they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs second they suggest that from at least the aptian through the eocene variations in continental weathering were minimal this heightens the importance of the rapid rise in seawater 87sr86sr beginning 40 ma as a significant transition to an extended period of increasing fluvial 87sr fluxes continuing to the present finally the results suggest that several documented short term excursions towards lower 87sr86sr in the latest triassic pliensbachian toarcian callovian oxfordian aptian albian and cenomanian turonian are interpretable as pulses of seafloor hydrothermal activity if so the strontium isotope record offers a means of constraining the timing duration and magnitude of known or proposed hydrothermal events in the geological record

87sr 86sr ratios in permo carboniferous sea water from the analyses of well preserved brachiopod shells abstract sr isotopic analyses of well preserved portions of permo carboniferous brachiopods distributed globally confirm the general shape of the sr isotope age curve established by previous workers for this time interval there is little variation between the sr isotopic composition of unaltered portions of brachiopods and that of portions of the same shell interpreted to be diagenetically altered based on cathodoluminescence elemental and stable isotopic data however the sr isotopic composition in diagenetically altered micritic matrix adjacent to the shell is more radiogenic the sr isotopic composition in the unaltered portions of calcitic megafossils has potential as a stratigraphie tool

a new procedure for gridding elevation and stream line data with automatic removal of spurious pits abstract a morphological approach to the interpolation of regular grid digital elevation models dems from surface specific elevation data points and selected stream lines is described the approach has given rise to a computationally efficient interpolation procedure which couples the minimization of a terrain specific roughness penalty with an automatic drainage enforcement algorithm the drainage enforcement algorithm removes spurious sinks or pits yielding dems which may be used to advantage in hydrological process studies the drainage enforcement algorithm has also been found to significantly increase the accuracy of dems interpolated from sparse but well chosen surface specific elevation data moreover it facilitates the detection of errors in elevation data that would not be detected by more conventional statistical means and forms a sound physical basis for cartographic generalization

which dem is best for analyzing fluvial landscape development in mountainous terrains abstract regional studies of fluvial landforms and long term quaternary landscape development in remote mountain landscapes routinely use satellite derived dem data sets the srtm and aster dems are the most commonly utilised because of their longer availability free cost and ease of access however rapid technological developments mean that newer and higher resolution dem data sets such as alos world 3d aw3d and tandem x are being released to the scientific community geomorphologists are thus faced with an increasingly problematic challenge of selecting an appropriate dem for their landscape analyses here we test the application of four medium resolution dem products 30 m srtm aster aw3d 12 m tandem x for qualitative and quantitative analysis of a fluvial mountain landscape using the dades river catchment high atlas mountains morocco this landscape comprises significant dem remote sensing challenges notably a high mountain relief steep slopes and a deeply incised high sinuosity drainage network with narrow canyon gorge reaches our goal was to see which dem produced the most representative best fit drainage network and meaningful quantification to achieve this we used arcgis and stream profiler platforms to generate catchment hillshade and slope rasters and to extract drainage network channel long profile and channel slope and area data tandem x produces the clearest landscape representation but with channel routing errors in localised high relief areas thirty metre dems are smoother and less detailed but the aw3d shows the closest fit to the real drainage network configuration the tandem x elevation values are the closest to field derived gps measurements long profiles exhibit similar shapes but with minor differences in length elevation and the degree of noise smoothing with aw3d producing the best representation slope area plots display similarly positioned slope break knickpoints with modest differences in steepness and concavity indices but again best represented by aw3d collectively our study shows that despite the higher effective resolution of tandem x 12 m the aw3d 30 m data performs strongly across all analyses suggesting that it currently offers the greatest potential for regional mountain geomorphological analyses

periodicity in the early palaeozoic marine realm evidence from the british isles suggests that during 3 intervals in the early palaeozoic conditions approximating to those of a non uniformitarian greenhouse g state earth were realized middle early upper cambrian st davids merioneth ages 30 ma mid ordovician llandeilo caradoc ages 32 ma and early silurian llandovery early wenlock ages 12 ma during these times sea levels were high and ocean waters below the zone of surface overturning poorly aerated and at times euxinic pelagic biotas graptoloids were diverse and in aerated portions of the highly productive shelf seas benthic biotas brachiopods flourished opposing conditions marking a well oxygenated o state earth occurred to varying degrees in the intervening intervals early cambrian comley age 20 ma late cambrian mid ordovician tremadoc late llandeilo ages 46 ma late ordovician ashgill age 4 ma and mid to late silurian post early wenlock age 15 ma within the constraints of the present data perturbations in the early palaeozoic marine realm do not appear to follow the same regular periodicity as in the mesozoic and caenozoic among a number of likely causes for this the greater tendency for the early palaeozoic seas to become anoxic seems the most important

seawater sr isotope variations through time a procedure for constructing a reference curve to date and correlate marine sedimentary rocks a new curve of seawater 87 sr 86 sr variation through the past 450 m y enables dating and correlation of marine sedimentary rocks with quantifiable uncertainties we discuss the method by which this curve was constructed strictly standardized 87 sr 86 sr ratios and biostrat igraphic data for 1300 87 sr 86 sr analyses of marine minerals were input into a data base objective criteria were used to assign each data point a reliability score these scores were used as initial weights in a robust nonparametric regression lowess to obtain a curve of seawater 87 sr 86 sr variation with rigorous 95 confidence intervals for prediction of age uncertainty precise dating and stratigraphic correlation are feasible for much of the phanerozoic the main problem with future sr isotope stratigraphy is likely to be sample quality we discuss guidelines for sample selection

strontium isotopes in early jurassic seawater abstract the analysis of well preserved well dated belemnites and oysters from the jurassic of great britain has resulted in a well constrained detailed seawater strontium isotope curve for the early jurassic the preservation of fossil low mg calcite was monitored using mn fe 13 c and 18 o iron was the most useful indicator with about 75 of the samples containing more than 150 ppm fe showing 87 sr 86 sr ratios elevated relative to adjacent points on the curve high mn concentrations less often correlated with elevated 87 sr 86 sr ratios however low mn concentrations 87 sr 86 sr 13 c and 18 o proved to be insensitive to diagenesis as it affects 87 sr 86 sr the principal features of the strontium isotope curve include a rise to about 0 70772 in the latest triassic and earliest jurassic hettangian from the hettangian the curve begins a roughly linear descent through the sinemurian and pliensbachian following a small levelling off and increase in the late pliensbachian the curve falls rapidly to its early jurassic minimum of 0 70706 it then gently increases through the toarcian until the falciferum zone where it shows an apparently abrupt increase to 0 70719 before continuing its slow increase to 0 70728 in the aalenian bajocian middle jurassic this reversal of the downwards trend established in the sinemurian and pliensbachian had not been previously identified the sinemurian and pliensbachian section of the curve potentially allows correlation and dating to within 1 or 2 ammonite subzones 0 5 to 1 ma

construction of the mississippian pennsylvanian and permian seawater 87sr86sr curve abstract the estimated path of seawater 87 sr 86 sr during the mississippian pennsylvanian and permian is based on 228 analyses the samples are largely from the southern interior of the u s a where rocks of this age are widely exposed multiple analyses from single rock units are used to define the seawater ratio for different time periods the seawater 87 sr 86 sr declines from 0 70812 sw 95 mississippian to an estimated low in the early meramecian of near 0 70755 sw 152 the value rises to near 0 70812 sw 95 in latest mississippian the ratio rises to 0 70830 sw 77 in mid pennsylvanian and decreases to near 0 70809 sw 98 during virgilian time the earliest permian value is near 0 70793 sw 114 after rising to 0 70804 sw 103 in the early leonardian the seawater 87 sr 86 sr falls sharply to reach a low of 0 70672 sw 235 in ochoan time the end of the paleozoic is estimated to be near 0 70707 sw 200 based on samples from china analysis of the paleozoic curve shape shows an alternating dominance of oceanic and continental sr contributions the extended and precipitous decline in the 87 sr 86 sr during mid permian is unmatched during the phanerozoic the contrast of the paleozoic seawater sr curve shape to that in the mesozoic and cenozoic indicates a potentially fundamental change in the balance of oceanic and continental sr contributions at the end of the permian

the strontium isotopic composition of brigantian late dinantian seawater abstract strontium isotopic determinations of different fossiliferous biominerals from brigantian late dinantian carbonates in northern england reveal that many are affected by diagenetic overprinting of their original 87 sr 86 sr ratios only pristine non luminescent articulate brachiopod shell material can be used to gain reliable estimates of carboniferous and by deduction palaeozoic ocean chemistries conodonts are found to be the only type of phosphatic biomineral capable of preserving original palaeoseawater chemistries however this only applies to specimens subjected to minimal burial temperatures other phosphatic biominerals are conclusively shown to be readily susceptible to diagenetic overprinting of original sr isotopic ratios a brigantian strontium curve compiled from pristine brachiopod calcite shows a relatively complex evolution with time but it can be resolved into two separate components operating on different time scales a cyclic sinusoidally shaped component with a frequency of 1 ma appears to fluctuate about a longer term more gradual linear increase in the brigantian seawater isotopic composition

remagnetization of the tonoloway formation and the helderberg group in the central appalachians testing the origin of syntilting magnetizations summary palaeomagnetic rock magnetic geochemical and strain studies of the helderberg group and the tonoloway formation in the valley and ridge province of west virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization crm acquisition the test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse grained thickly bedded helderberg and the thinly bedded and finer grained tonoloway standard tilt tests as well as optimal differential untilting from two transects on the wills mountain anticline indicate that the helderberg contains a pervasive late syntilting 30 per cent untilting characteristic remanent magnetization chrm whereas the tonoloway contains a well defined early syntilting chrm 1 76 per cent untilting as well as a chrm 2 which is similar to the chrm in the helderberg the chrms reside in magnetite the rock magnetic results from the two units are similar although the helderberg samples have a finer apparent grain size perhaps as a result of magnetic hardening strain data reveal an apparent correlation between the syntilting magnetizations and types degrees of pressure solution strain pressure solution strain is higher in the helderberg and lower in the tonoloway strain may have caused rotation of magnetic minerals but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results several other viable hypotheses were also investigated one hypothesis is a strain enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding a preliminary test of this hypothesis produced inconclusive results another hypothesis consistent with some of the characteristics of the magnetizations in the helderberg and tonoloway units is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process

sr and nd isotopes at the prmian triassic boundary a record of climate change abstract we present a detailed curve of seawater 87sr 86sr for the middle permian to triassic based on analyses of conodonts from overlapping sections in the u s a pakistan correlated using conodont biostratigraphy the isotope ratio decreased in the middle permian at an average rate of 0 000062 ma 1 reached a minimum in the capitanian 257 258 ma and increased in the late permian at an average rate of 0 000097 ma 1 the late permian rate of increase was roughly two and a half times greater than the average increase over the past 40 ma and approximately equal to the highest cenozoic rates which occurred over much shorter time intervals modeling results suggest that decreasing middle permian 87sr 86sr ratios were driven by changes in the riverine sr flux to the oceans while increasing ratios in the late permian triassic are attributed to both increased riverine 87sr 86sr and flux the reduced middle permian riverine flux coincides with extreme continental aridity associated with the formation of pangea and recorded by massive evaporite deposits in addition mountains in the equatorial region of pangea may have created a rain shadow thereby minimizing precipitation in regions that currently contribute the bulk of chemical weathering products to the ocean increasing riverine 87sr 86sr in the late permian is suggested by the observation that 143nd 144nd values decrease at the same time however the source of radiogenic sr is not known frequently cited mechanisms for increasing 87sr 86sr in runoff such as glaciations and continent to continent collisions coincide instead with decreasing seawater 87sr 86sr in the middle permian one possible source may have been deep erosion into older orogens associated with a dramatic increase in chemical weathering in the late permian the cause of enhanced weathering appears to have been increased levels of atmospheric co2 with associated global warming and increased humidity proposed sources of co2 include dissociation of gas hydrates and oxidation of organic matter during extreme sea level regression as well as volcanic emissions from siberian traps eruptions continental floral and faunal distributions are consistent with this interpretation as are oceanic 13c patterns and variations in shallow water sediment lithologies

detrital ooids of holocene age in glaciomarine champlain sea sediments gatineau quebec canada detrital ooids from 0 1 to 0 5 mm diameter almost completely dominate two 0 3 m thick layers between one and two metres depth at a site in clay size rich isostatically uplifted glaciomarine champlain sea sediments north of gatineau quebec the ooids composed of layers of tangentially oriented platy particles are physical aggregations of glacially ground rock flour with the identical mineral suite that constitutes the regional champlain sea sediments the ooids are not cemented and there is no evidence of diagenetic formation of minerals a hydrodynamic hail stone model which requires a somewhat turbulent sedimentary environment within the water column probably created by wind wave action that kept ooid nuclei in suspension while the flocculating effect of brackish to marine conditions induced particle accretion onto their surface is proposed for their formation

strontium isotope stratigraphy of the middle devonian brachiopods and conodonts a set of 145 strontium isotope ratios based on skeletal components of emsian to frasnian age reveals a detailed structure of the seawater 87sr 86sr curve that can ultimately serve as a tool for high resolution isotope stratigraphy in the devonian brachiopod and conodont samples were collected in the eifel region of germany the area around the global stratotype for the middle devonian with an apparent average temporal resolution in the 105 y range preservation of the brachiopod shell material has been assessed by optical microscopy sem and cathodoluminescence and only the better preserved internal secondary layer of the shell has been utilized for strontium isotope measurements the 87sr 86sr curve shows a short decline from 0 7081 to 0 7078 through the late emsian followed by short term fluctuations with an amplitude of up to 10 4 around a mean of 0 7078 during the eifelian and givetian and a rise from late givetian into the late devonian the above high frequency oscillations and spikes are likely a reflection of condensed sedimentary records due to erosional and nondepositional events but a direct confirmation for such a scenario can be claimed only for the latest eifelian otomari event insufficient stratigraphic resolution precludes testing for the other events and isotopic spikes conodont skeletal elements even if well preserved at cai 2 5 yielded results that are mostly enriched in radiogenic 87sr if compared to coeval brachiopods although this enrichment is usually within the 10 5 range conodonts despite their superior stratigraphic resolution should be utilized only as a material of second choice in cases where brachiopods are rare or absent

the temperatures and oxygen isotope composition of early devonian oceans the oxygen isotope composition of pre carboniferous 360 myr old marine carbonates cherts and phosphates has been reported1 8 to be lower than that of later post devonian samples according to one explanation1 3 this is a reflection of warmer generally 40 c pre carboniferous oceans relative to modern oceans alternatively it has been proposed5 7 that the pre carboniferous oceans were depleted in 18o by 2 smow relative to modern oceans here i report high 18o values 1 9 to 2 9 pdb in normal shallow marine limestones and three brachiopod shells from the lower devonian haragan and bois d arc formations in south central oklahoma i interpret these values as near primary and therefore constraining the temperature and oxygen isotope composition of early devonian sea water to 25 7 c and 0 1 smow respectively in conjunction with similarly high 18o values obtained from older ordovician and silurian samples8 these results imply that the temperature and oxygen isotope composition of pre carboniferous oceans may at least during some time intervals have been similar to those of modern oceans

assessment of drainage network extractions in a low relief area of the cuvelai basin namibia from multiple sources lidar topographic maps and digital aerial orthophotographs abstract accurate delineation of drainage networks dns is crucial for hydrological or hydraulic modelling and the comprehension of fluvial processes this task presents challenging aspects in complex lowland terrains with subtle relief and particularly for data poor areas like the cuvelai river basin crb namibia where the present study takes place in the crb standard methods of drainage network extraction from low resolution gridded digital elevation models dems are unsuitable hence airborne light detection and ranging lidar solutions have been utilized however lidar also presents challenges to large areal applications especially with a surface roughness exceeding the capacity of numerous algorithms indeed lidar based dems 2 and 50 m resolutions need to be hydrologically corrected and smoothed to enable the extraction of scale relevant geomorphologic features such as dns in the present contribution channels from topographic maps blue lines were compared to those from hydrologically corrected and uncorrected lidar dems heads up digitized channels from high resolution digital aerial orthophotographs field mapped channels and auxiliary data the maximum gradient deterministic eight d8 gis algorithm was applied to the corrected and uncorrected lidar dems using two network extraction methods area threshold support and curvature drop analysis different progressive flow accumulation threshold values 12 were used to delineate channels with these methods validation was performed between the field mapped channels the modelled channels and those derived from multiple sources additionally spatial and quantitative analyses were performed on geomorphologic parameters and indices the results have shown that hydrologically corrected lidar dems offer useful details for identifying low order stream segments in headwaters while blue lines derived from the national hydrography datasets for watersheds located in elevated and low lying areas of the study area underestimated total stream lengths for field mapped channels by 15 3 and 88 5 respectively this study also confirmed that dns can be extracted from complex low terrain areas with standard gis algorithms and accurate field data the results will aid national mapping agencies in data poor regions to modernize their national hydrography datasets and to account for changing land surface conditions that can affect channel spatial arrangements over time