use of a conceptual hydrogeological model and a time variant water budget analysis to determine controls on salinity in lake burrumbeet in southeast australia lake burrumbeet which is located in the basalt plains of western victoria australia has a much lower salinity than other lakes in the region to determine the factors causing the freshness of this water body an assessment was carried out using existing geological information to reconstruct the palaeotopography beneath basalt flows and using hydrogeological data bore hydrographs and hydrological data together with a time variant lake water balance analysis calibrated from 1998 to 2008 the calibrated water balance of lake burrumbeet shows that groundwater outflow has a major influence on lake levels 31 of water losses the lake water budget analysis confirms that the interaction of lake burrumbeet with the surrounding groundwater system is volumetrically significant geological re construction of the course of the buried river valley sediments deep lead underlying the basalt shows that they run directly beneath lake burrumbeet groundwater leakage into these sediments is sufficient to transfer enough salt from the lake to maintain a low to moderate lake salinity the conceptual hydrogeological model of lake burrumbeet coupled with the time variant water budget analysis show that the difference in salinity among the lakes in the region can be explained by variations in the configuration and hydrogeological setting of the lakes and this hypothesis provides new understanding of the relative freshness of the lake

tropical sea surface temperatures during the last glacial period a view based on alkenones in indian ocean sediments abstract the quantification of tropical temperatures during the last glacial cycle 0 150 kyr bp is a controversial issue since different proxies seem to provide conflicting informations to obtain a complementary point of view we use the alkenone method to estimate sea surface temperatures and focus our work on deep sea sediments recovered from the tropical indian ocean we present alkenone data obtained in two cores which cover in detail the last deglaciation and in about twenty cores distributed between 20 s and 20 n that were chosen to evaluate the temperature contrast of the last glacial interglacial transition our results indicate that indian ocean tropical temperatures remained an average within 1 5 2 5 c of their present values during the last glaciation at 10 n the last deglaciation is characterized by two warming steps which is similar to the classical deglacial chronology observed in the north atlantic area at 20 s the deglacial warming occurred at ca 15 cal kyr bp lagging significantly 5 4 kyr behind the antarctic warming but in phase with northern hemisphere time series

shoreline fluctuations of lake hayk northern ethiopia during the last 3500years geomorphological sedimentary and isotope records abstract this paper presents a stratigraphical and geomorphological analysis supported with geochronological dating aiming to reconstruct the late holocene lake level fluctuations and paleohydrological history of lake hayk this lake 23 km 2 in surface is located in the eastern side of the northern ethiopia highlands at an altitude of around 2000 m above sea level along the northern margin of the lake which is mainly characterized by steep rocky slopes the studied succession consists of colluvial and stromatolitic deposits along the southern margin which is characterized by a wide coastal plain drained by the main watercourse feeding the lake the ankarka river the study deposits are mainly palustrine deltaic and fluvial in origin geomorphological and sedimentological data integrated with a radiocarbon chronology highlighted that during the last 3500 years the lake was affected by three main highstand phases which occurred at about 3250 3000 2600 950 and 650 160 cal yrs bp respectively these highstands are documented by aggradation of siliciclastic systems both along the southern and northern margins whereas isotope data highlight that most of the stromatolitic deposits developed during phases of intense evaporation this peculiar feature could be differently explained i stromatolite growth was triggered by rapid lacustrine rises but mainly occurred during the long lasting lacustrine falls ii stromatolite developed during tectonic induced transgressions which occurred during phases of intense evaporation despite the possible effects of local tectonics the lake hayk record shows a good correlation with those of several late holocene lakes of east africa lakes naivasha tanganyika victoria turkana abhe ziway shala bosumtwi and abyiata highlighting that it can be considered as a suitable climatic proxy for late holocene time span in particular by both siliciclastic and stromatolitic deposits occurring in the uppermost part of the sedimentary succession record high frequency oscillations which have been ascribed to three main droughts that occurred during the little ice age these droughts were previously documented only in lake naivasha and lake victoria

late ordovician glaciation under high atmospheric co2 a coupled model analysis the analysis of the geologic record has revealed a question concerning how the late ordovician glaciation could have occurred simultaneously with high co2 levels 10 18x sensitivity studies using a coupled atmosphere ocean sea ice model show that it is possible to maintain a permanent snow cover which corresponds to 60 of all the glacial deposits found on gondwana under 10x co2 levels warm fall cool spring orbital parameters a 4 5 reduction in solar luminosity a length of day of 21 5 hours and an enhanced snow sea ice albedo of 0 3 a cold summer orbit experiment with 10x co2 and a reduced snow sea ice albedo of 0 1 also sustains a permanent albeit less extensive snow cover the geographic configuration of the late ordovician results in an up to 42 increase in the global ocean poleward heat transport in the southern hemisphere relative to present day and a significant asymmetry relative to the equator

sponge dominated offshore benthic ecosystems across south china in the aftermath of the end ordovician mass extinction abstract offshore benthic communities were sparse after the end ordovician mass extinction the recent discovery of diverse abundant sponges in south eastern china anhui and zhejiang provinces that flourished during the hirnantian late ordovician post extinction interval raises questions over the extent and ecological significance of the sponge community at that time this paper dramatically expands the scale of the hirnantian sponge fauna within south china with seven new occurrences across 2000 km these occurrences cross the ordovician silurian boundary interval in the marginal and submerged platform areas of the yangtze sea in jiangsu hubei hunan chongqing and sichuan provinces in condensed sections platform margin facies the occurrences are normally represented by extensive spiculite layers but several sites yield entire exceptionally preserved body fossils at ganggangshan near nanjing jiangsu province such preservation is abundant the new occurrences are combined with palaeogeographic and tectonic reconstructions to produce a model for the distribution and preservation of the sponge fauna we argue that the exceptional preservation was due largely to nepheloid layers that were generated during rapid post glacial transgression that flooded the weathered land surface of the uplifted cathaysia block nepheloid layer collapse led to rapid deposition of suspended sediment within the deeper parts of the intra plate restricted basin and on adjacent platform margins and slopes the suspended sediment influx also introduced abundant nutrients to the water column as indicated by a significant 13c excursion during the sponge bearing mudstone interval at ganggangshan the combined sponge assemblage was dominant over large areas of the hirnantian offshore sea floor across south china we predict that similar occurrences with exceptional preservation will also be found in other continental blocks abundant articulated sponges may be encountered particularly in restricted basin successions containing rapidly deposited graptolitic mudstones near the ordovician silurian boundary

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

elves lightning induced transient luminous events in the lower ionosphere observations of optical phenomena at high alti tude a bove thunderstorms using a multichannel high speed photometer and image intensified ccd cameras were carried out at yucca ridge field station 40040 n 104o 56 w colorado as part of the sprites 95 campaign from 15 june to august 6 1995 these newneasurements indicate that diffuse optical flashes with a duration of i ms and a hori zontal scale of 100 300 km occur at 75 105 km altitude in the lower ionosphere just after the onset of cloud to ground lightning discharges but preceding the onset of sprites here we designate these events as alves to distinguish them from i ed sprites this finding is consistent with the production of diffuse optical emissions due to the heating of the lower ionosphere by electromagnetic pulses generated by lightning discharges as suggested by several authors

vlf signatures of ionospheric disturbances associated with sprites vlf perturbations on signals propagating along great circle paths gcp through electrically active midwest thunderstorms are associated with luminous high altitude glows referred to as sprites observed from aircraft or ground the data constitutes the first evidence that the physical processes leading to sprites also alter the conductivity of the lower ionosphere

new perspectives on graptolite distributions and their use as indicators of platform margin dynamics graptolite distributions in ordovician shelf slope and basinal facies in the great basin indicate that graptolites were scarce in open oceanic waters oceanward of the cordilleran shelf margin and that they thrived in waters above the margin this pattern is consistent with that of most zooplankton in modern oceans it follows from these observations that the depositional setting of typical graptolitic shale was the area of the sea floor under continental margin upwelling zones where graptolites flourished and within the oxygen minimum zone where their rhabdosomes were preserved with changes over time in relative sea level deep oceanic circulation and wind driven surface circulation the upwelling and oxygen minimum zones may have thickened or thinned migrated landward or oceanward and expanded laterally contracted or even disappeared the observed graptolite occurrences suggest that the primary graptolite biotope that is the habitat of diverse and abundant faunas was a relatively narrow belt of upwelling waters along and extending somewhat open oceanward from the continental margin provinces were maintained only to the extent that species could disperse along continental margins distribution of typical graptolitic strata may be used to interpret development of continental margins because such distribution incorporates a signal of sea level rise or fall oceanographic changes especially upwelling and tectonic events that led to creation and deterioration of upwelling conditions in which graptolites flourished

early slow events a new category of vlf perturbations observed in relation with sprites 1 analysis of subionospheric vlf transmissions observed in relation with sprites has led to the identification of a new category of vlf perturbations caused by the direct effects of tropospheric lightning on the overlying lower ionosphere they constitute a large subset of the so called early fast events where now the term fast which implies rapid onset durations less than 20 ms does not apply in contrast with early fast the perturbations have a gradual growth and thus slow onset durations ranging from about 0 5 to 2 5 s thus these events are labeled herein as early slow they are indicative of a new physical process at work which following a sprite causative cloud to ground discharge leads to a gradual buildup of conductivity changes in the lower ionosphere which must be responsible for the long onset durations of the observed perturbations analysis of broadband vlf sferic recordings made with a two channel receiver near the sprite producing storms shows that the growth phase of an early slow event coincides with the occurrence of complex and dynamic lightning action this is composed of a few sequential cloud to ground lightning strokes and clusters bursts of sferics which are attributable to intracloud lightning we postulate that the long onset durations are due to secondary ionization buildup in the upper d region below the nighttime vlf reflection heights caused mainly by the impact on sprite produced electrons of sequential electromagnetic pulses radiated upward from horizontal in cloud discharges

the early middle ordovician graptolite genus azygograptus in south china new material and paleogeographic implications abstract this study presents a new analysis of three species of the ordovician graptolite genus azygograptus nicholson lapworth from the wenxiakou section in the zhongxiang area of hubei province the paleogeographic occurrences of the genus within south china were plotted on an up to date paleogeographic map based on a revised data set of graptolite occurrences from 68 localities compiled by the online geobiodiversity database gbdb the paleogeographic distribution of azygograptus follows a nearshore offshore trend and the taxonomic diversity of azygograptus is higher in nearshore high energy environments than in offshore low energy environments however graptolites also gathered in deeper water environments surrounding oldlands partly due to the high nutrient inputs and rapid deposition azygograptus is found not only in high latitude cold water regions as endemic taxa but also in low latitude regions oceanic circulation may account for the rapid dispersal of azygograptus from high latitude regions such as britain and scandinavia to low latitude regions such as south china eventually resulting in their observed paleogeographic distribution

comprehensive hydrological and hydrogeological study of topographically closed lakes in highland ethiopia the case of hayq and ardibo a comprehensive conventional hydrological and hydrogeological study was conducted in two topographically closed crater lakes in the northeastern highlands of ethiopia particular emphasis is given to the estimation of the water balance and assessment of groundwater and surface water interactions the detailed bathymetric map of the lakes has also been established for the first time the result revealed the importance of both groundwater and surface water components in the water balance of the lakes field hydrogeological observations and the conventional water balance indicate outflow and inflow of groundwater from both lakes the lakes leak groundwater through large faults which also control the occurrence and distribution of springs the leakage of the lakes through faults is independently validated by hydrochemical analysis indicating lower ionic concentration and freshness of both lakes in contrast with closed terminal lakes which are commonly characterized by higher salinity limited environmental isotopes together with major ion analysis revealed that shallow fresh groundwater recharged from highland rainfall is one of the main driving forces of the hydrology of the lakes and their catchments there is no indication of deep groundwater with high ionic concentration and residence time that feeds the lakes from erratic lake level record and bathymetric survey it was found that lake hayq has declined in level over the last 60 years but recently due to human influences return flow from irrigation the level has shown slight rise at the expense of the leaking lake ardibo located at relatively higher topographic position these recent anthropogenic influences demand careful assessment of the complex nature of the hydrology and the water balance in this regard this study is believed to have laid the foundation for future detailed work that may help for sustainable water management practise

biostratigraphy and palaeoecology of lower ordovician graptolites from the fezouata shale moroccan anti atlas abstract the fezouata shale fezouata formation has produced the most complete lower ordovician graptolite succession known from the african continent which was studied from 32 sections located along the western central and eastern moroccan anti atlas a composite biostratigraphic and chronostratigraphic scheme is presented for the entire formation that in ascending order comprises the anisograptus matanensis rhabdinopora flabelliformis anglica zones uppermost lower tremadocian adelograptus tenellus and aorograptus victoriae zones middle tremadocian araneograptus murrayi and hunnegraptus copiosus zones upper tremadocian cymatograptus protobalticus zone lower floian baltograptus jacksoni zone middle floian and the baltograptus minutus zone plus an azygograptus interval upper floian most of these zones or biozones are recorded for the first time in africa together with important graptolite species such as ancoragraptus bulmani spjeldnaes cym protobalticus monsen b jacksoni rushton clonograptus multiplex nicholson b minutus tornquist and azygograptus eivionicus elles a correlation with the graptolite sucession from the algerian sahara is also suggested reinforced by the common record of the minute anisograptid choristograptus louhai legrand providing its second world occurrence in the tremadocian of morocco the prior late tremadocian dating of the fezouata lagerstatte is here extended to the middle floian as revealed by the graptolite biochronology the repeated record along the lower and middle parts of the fezouata formation of deep water mesopelagic species of the genera araneograptus with mass occurrences of conical colonies together with large horizontal rhabdosomes of paratemnograptus paradelograptus clonograptus tetragraptus and holograptus schizograptus do not fit the sedimentological interpretation of deposition of these strata in much shallower inshore to mid shelf environments this inconsistency has implications when inferring the living conditions of the fezouata soft bodied assemblages

the geochemistry of lake bosumtwi a hydrologically closed basin in the humid zone of tropical ghana lake bosumtwi lies in an isolated hydrologically closed basin formed by a meteor impact 1 x 10 6 yr ago comparison of lake water samples taken in 1993 with one from 1934 shows a decrease in the concentrations of dissolved solutes most likely owing to an estimated 13 increase in lake volume a mass balance calculation that uses an estimate of the average chloride composition of the rainfall shows that the most recent overflow of the lake occurred within the last 3 000 yr evaluation of the sources and sinks of major solutes suggests that deposition of harmattan dust may be an important component of the influx of ions into the lake although the lake is homogeneous with respect to major ion composition it is stratified into an upper oxygenated layer and a lower anoxic layer and there is an abrupt change in the concentration profiles of ph nh 4 po 4 3 no 3 and mn at a depth of 16 m this chemical stratification is apparently caused by photosynthesis in the upper layer and respiration and reducing conditions below the stable isotopic composition of the lake water is reasonably well simulated by a simple isotope balance model for the lake the model is used to show that the 18 o composition of the lake is relatively insensitive to changes in the water balance but is highly sensitive to relative humidity and the isotopic composition of atmospheric moisture

on the relationship between lightning peak current and early vlf perturbations 1 lightning strokes are known to cause direct heating and ionization of the d region some of which are detected via scattering of vlf transmitter signals and are known as early vlf events the disturbed ionosphere typically recovers in many tens of seconds new experimental evidence is presented demonstrating that the scattering pattern and onset amplitude of early vlf events are strongly related to both the magnitude and polarity of causative lightning peak current observations of early vlf events at nine stanford vlf receiver sites across the continental united states are combined with lightning geolocation data from the national lightning detection network nldn during january and march 2011 nldn recorded 7769 intense lightning discharges with high peak currents 100 ka generating 1250 detected early vlf events we show that the size of the scattered field due to the ionospheric disturbance increases with the peak current intensity of the causative lightning discharge the most intense peak currents of 200 and 250 ka disturb vlf transmitter signals as far as 400 km away from the lightning stroke early vlf event detection probability also increases rapidly with peak current intensity on the other hand the observed vlf amplitude change is not significantly dependent on the peak current intensity stroke polarity is also important with positive strokes being 5 times more likely to generate early vlf disturbances than negative strokes of the same intensity intense positive cloud to ground lightning discharges especially when occurring over the sea are also more likely to produce early vlf events with long recovery many minutes

reconstructing past sea surface temperatures correcting for diagenesis of bulk marine carbonate abstract a numerical model which describes oxygen isotope exchange during burial and recrystallization of deep sea carbonate is used to obtain information on how sea surface temperatures have varied in the past by correcting measured 18o values of bulk carbonate for diagenetic overprinting comparison of bulk carbonate and planktonic foraminiferal 18o records from odp site 677a indicates that the oxygen isotopic composition of bulk carbonate does reflect changes in sea surface temperature and 18o at odp site 690 we calculate that diagenetic effects are small and that both bulk carbonate and planktonic foraminiferal 18o records accurately reflect paleogene warming of high latitude surface oceans biased from diagenesis by no more than 1 c the same is likely to be true for other high latitude sites where sedimentation rates are low at dsdp sites 516 and 525 the effects of diagenesis are more significant measured 18o values of eocene bulk carbonates are more than 2 lower at deeply buried site 516 than at site 525 consistent with the model prediction that the effects of diagenesis should be proportional to sedimentation rate model corrections reconcile the differences in the data between the two sites the resulting paleotemperature reconstruction indicates a 4 c cooling of mid latitude surface oceans since the eocene at low latitudes the contrast in temperature between the ocean surface and bottom makes the carbonate 180 values particularly sensitive to diagenetic effects most of the observed variations in measured 18o values are accounted for by diagenetic effects rather than by sea surface temperature variations we show that the data are consistent with constant equatorial sea surface temperatures through most of the cenozoic with the possible exception of the early eocene when slightly higher temperatures are indicated we suggest that the lower equatorial sea surface temperatures for the eocene and oligocene reported in other oxygen isotope studies are artifacts of diagenetic recrystallization and that it is impossible to reconstruct accurately equatorial sea surface temperatures without explicitly accounting for diagenetic overprinting

recovery signatures and occurrence properties of lightning associated subionospheric vlf perturbations subionospheric vlf signatures of lightning associated ionospheric disturbances observed at multiple sites in the central united states are used to investigate the occurrence properties and recovery signatures of characteristic perturbations exhibiting rapid onset followed by slow recoveries the two different types of events so called early fast and lightning induced electron precipitation lep inan et al 1996a are distinguished by the lack of presence of a few hundred millisecond delay between vlf event onsets and causative lightning discharges respectively analysis of recovery signatures of the two types of events indicate subtle but distinct differences in the recovery rates a majority of early fast events were found to exhibit a more rapid initial recovery to preevent levels during the first 20 s of recovery when compared to lep events this experimental evidence indicates that the physical nature of the ionospheric disturbance involved in the two classes of events are different consistent with a recent theoretical suggestion inan et al 1996c the occurrence properties of early fast events observed by the holographic array for ionospheric lighting hail receivers indicate that ionospheric disturbances act primarily as forward scatterers

late quaternary climate change in the north eastern highlands of ethiopia a high resolution 15 600 year diatom and pigment record from lake hayk abstract multi proxy analyses of an 8 m sediment core from lake hayk a closed freshwater lake in the north central highlands of ethiopia provide a record of changing lake level and inferred regional climatic change for the last 15 6 cal ka years between ca 15 6 15 2 cal ka bp a lowstand was synchronous with heinrich event 1 and an intense drought across eastern africa at ca 15 2 cal ka bp a lake began to develop at the core site in response to wetter conditions at the onset of the african humid period ahp however in contrast to other lakes in eastern africa hayk lake level fell around ca 14 8 cal ka bp indicating a climate shift towards aridity the lake began filling again at ca 12 3 cal ka bp and reached maximum water depth between ca 12 0 10 0 cal ka bp lake level declined slowly during the holocene culminating in the termination of the ahp at hayk between ca 5 2 4 6 cal ka bp in the late holocene ca 2 2 1 3 cal ka bp lake hayk was again deep and fresh with some evidence of short term lake level variability the palaeo record from lake hayk indicates that while it experienced to a broad degree the same glacial interglacial dynamics and sub millennial shifts in climate found in other palaeolimnological records from eastern africa there are offsets in timing and rate of response these differences reflect chronological discrepancies between records as well as the varying climate sensitivities and site specific factors of individual lake basins this record highlights the different responses by lakes in a climatically vulnerable area of ethiopia

ground truthing late ordovician climate models using the paleobiogeography of graptolites 1 the 18o records obtained from conodonts suggest that during the mid ordovician equatorial temperatures stabilized at close to the present day a hypothesis broadly supported by published climate models however the degree to which equatorial temperatures represent global climate state and varied between different climatic modes greenhouse icehouse interglacial glacial and to what extent ordovician 18o and climate models truly reflect the global climate remain to be tested here we present paleobiogeographical data for the planktonic graptolites from the gracilis time slice i e early sandbian stage that just postdates the postulated onset of climate stabilization twinspan analysis and constrained seriation provide robust ecological groupings in paleobiogeographical presence absence data the highest level groups reflect tropical subtropical assemblages and a high southerly latitude assemblage constrained seriation defines latitude distinct biotopes that are considered equivalent to modern zooplanktonic provinces the distribution pattern of graptolite biotopes in the gracilis time slice show 1 that models explaining local graptolite ecological assemblages using lateral differentiation e g upwelling are to be favored above those using depth stratification and 2 a steep faunal gradient from equator to pole which is typical of a cool nongreenhouse world and comparable to the modern situation we therefore broadly support the climate stabilization hypothesis paradoxically the climate of the early late ordovician appears similar to that of the present day despite the higher atmospheric pco2 levels graptolite species were indicative of temperature controlled ocean water masses in much the same way as the modern zooplankton

lower palaeozoic stratigraphy and palaeontology karadere zirze area pontus mountains northern turkey lower palaeozoic rocks in the karadere zirze area east of safranbolu pontides northern turkey range from early ordovician to silurian overlying the probably tremadoc bakacak formation are aydos formation quartzites followed conformably by the karadere formation dated as early arenig to early llanvirn by means of graptolites which are assigned to seventeen genera and include three new forms eoglyptograptus bouceki prolasiograptus haplus praecursor and undulograptus mui late arenig trilobites from the karadere formation include bergamia cyclopyge dionidella leioshumardia and seleneceme in the limestone member of the overlying ketencikdere formation uncommon trilobites suggest only a mid to late ordovician age but con odonts with colour alteration index 5 6 indicate the amorphognathus tvaerensis biozone early caradoc macrofossils are rare in the siltstone member but conodonts from the middle of the unit suggest the highest subzone of the a tvaerensis biozone the youngest visible strata are on acritarch evidence at least as high as caradoc but the ashgill is not confirmed and the contact with overlying silurian rocks is unexposed the findikli formation comprises a lower member black argillites with llandovery graptolites and acritarchs and an upper member grey shales with late wenlock graptolites overlain unconformably by devonian rocks the succession differs significantly from contemporaneous deposits in southern turkey and its affinities lie with western europe including the welsh basin

nighttime d region electron density profiles and variabilities inferred from broadband measurements using vlf radio emissions from lightning 1 lightning discharges radiate most of their electromagnetic energy in the very low frequency vlf 3 30 khz and extremely low frequency elf 3 3000 hz bands and are consequently an effective tool for remotely sensing the nighttime ionospheric d region electron density profile using broadband lightning generated vlf signals we derived the night to night variations of the midlatitude ionospheric d region electron density profiles for 16 nights between 1 july and 4 august 2004 we examined measured vlf data from lightning occurring near the united states east coast 530 860 km away from duke university and received at duke university from 0400 to 0600 ut from these observed vlf radio atmospherics sferics we extracted the nighttime d region electron density profiles covering the range of electron densities from 100 to 103 cm 3 in the altitude range of approximately 70 95 km using a two dimensional laterally homogenous model of vlf propagation in the earth ionosphere waveguide the inferred electron density profile variabilities were in good agreement with those from past nighttime rocket experiments at similar latitudes using the rocket measured profiles in our propagation simulations we determined that the two parameter exponential d region electron density profiles we inferred were the best exponential fit in the electron density range of 3 to 500 cm 3 to the rocket measured d region electron density profiles in an initial effort to determine the sources of the observed variabilities we compared the sampex precipitating electron measurements to the electron density profiles inferred during july 2000 the results indicate that high energy electron precipitation might account for at least part of the night to night variations of the d region electron densities at the midlatitudes

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

long lived glaciation in the late ordovician isotopic and sequence stratigraphic evidence from western laurentia the timing and causes of the transition to an icehouse climate in the late ordovician are controversial results of an integrated 13c and sequence stratigraphic analysis in nevada show that in the late ordovician chatfieldian stage mid caradoc a positive 13c excursion in the upper part of the copenhagen formation was closely followed by a regressive event evidenced within the prominent eureka quartzite the chatfieldian 13c excursion is known globally and interpreted to record enhanced organic carbon burial which lowered atmospheric p co2 to levels near the threshold for ice buildup in the ordovician greenhouse climate the subsequent regressive event in central nevada previously interpreted as part of a regional tectonic adjustment is here attributed in part to sea level drawdown from the initiation of continental glaciation on gondwana this drop in sea level which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of p co2 by suppressing shelf carbonate production signals the transition to a late ordovician icehouse climate 10 m y before the widespread hirnantian glacial maximum at the end of the ordovician

high temporal and spatial resolution detection of d layer fluctuations by using time domain lightning waveforms 1 this paper presents a new method for probing ionospheric d layer fluctuations with time domain very low and low frequency vlf lf lightning waveforms detected several hundred kilometers away from lightning storms the technique compares the amplitude and the time delay between the direct ground wave and the first hop ionospheric reflection of the lightning signal to measure the apparent d layer reflectivity and height this time domain technique allows a higher time and spatial resolution measurement of the d layer fluctuations compared to previously reported frequency domain techniques for a region near a nighttime thunderstorm results demonstrate that the apparent reflectivity and height exhibit significant variation on spatial scales of tens of kilometers and over time periods of hours the range of the reflectivity variation was observed as large as 100 away from the averaged reflectivity for some localized regions and the height varies by as much as 5 4 km the time scales and propagation velocities of the fluctuations appear to be consistent with signatures of atmospheric gravity waves at d layer altitudes and the direction of the fluctuation propagation suggests that the gravity waves are originated from the storm superimposed on the fluctuations a general decreasing trend by 4 8 km in reflection height over the nighttime is observed in some localized ionosphere regions apparent splitting of the d layer by 2 4 km is observed to last a short time period of about 10 min

analysis of the hydrological response of a tropical terminal lake lake abiyata main ethiopian rift valley to changes in climate and human activities integrated dynamic water and chloride balance models with a catchment scale hydrological model prms are used to investigate the response of a terminal tropical lake lake abiyata to climate variability and water use practices in its catchment the hydrological model is used to investigate the response of the catchment to different climate and land use change scenarios that are incorporated into the lake model lake depth area volume relationships were established from lake bathymetries missing data in the time series were filled using statistical regression techniques based on mean monthly data the lake water balance model produced a good agreement between the simulated and observed levels of lake abiyata for the period 1968 83 from 1984 onwards the simulated lake level is overestimated with respect to the observed one while the chloride concentration is largely underestimated this discrepancy is attributed to human use of water from the influent rivers or directly from the lake the simulated lake level and chloride concentration are in better agreement with observed values r2 0 96 when human water use for irrigation and salt exploitation are included in the model a comparison of the simulation with and without human consumption indicates that climate variability controls the interannual fluctuations and that the human water use affects the equilibrium of the system by strongly reducing the lake level sensitivity analysis based on a mean climatic year showed that after prolonged mean climatic conditions lake abiyata reacts more rapidly to an abrupt shift to wetter conditions than to dry conditions this study shows the significant sensitivity of the level and salinity of the terminal lake abiyata to small changes in climate or land use making it a very good recorder of environmental changes that may occur in the catchment at different time scales copyright 2004 john wiley sons ltd

response of late ordovician paleoceanography to changes in sea level continental drift and atmospheric pco2 potential causes for long term cooling and glaciation abstract we performed sensitivity experiments using an ocean general circulation model at two stages of the late ordovician caradoc 454 ma ashgill 446 ma under a range of atmospheric pco2 values 8 18 pal pre industrial atmospheric level at high and low sea level the model results indicate that the long term cooling trend during the late ordovician can be explained by progressive cooling of the global ocean in response to falling levels of atmospheric pco2 sea level change and paleogeographic change these results also explain the occurrence of low latitude cool water carbonates in north america in all simulations a drop in sea level led to a reduction in poleward ocean heat transport this indicates a possible positive feedback that could have enhanced global cooling in response to sea level drop during the late ordovician alterations in poleward ocean heat transport linked to changes of atmospheric pco2 also indicate that there is a threshold of 10 pal above which ocean current change cannot be responsible for glaciation in the late ordovician continental drift could explain the observed global cooling trend in the late ordovician through a combined poleward ocean heat transport feedback and increased ice albedo effect if atmospheric pco2 was low during the entire late ordovician the model results further indicate that the response of meridional overturning to changes in paleogeography atmospheric pco2 and sea level is stronger than the response of surface circulation to these perturbations because the overturning circulation is so strong meridional overturning was the dominant mechanism for described changes in heat transport in the late ordovician

implications of ordovician 460 myr marine cement for constraining seawater temperature and atmospheric pco2 abstract the marine diagenetic history of the effna buildups virginia lower caradocian 460 myr is atypical of phanerozoic carbonates and reflects variable redox conditions in the semi restricted appalachian foreland basin fibrous marine cements are the focus of this study least altered fibrous calcite from the effna formation has a translucent appearance in transmitted light and a 2 3 mol mgco3 composition the latter is similar to values 2 5 mol from translucent fibrous calcite in other caradocian units holston tennessee and kullsberg sweden formations some fibrous calcite is overlain by hardgrounds that in places have an irregular micro topography with sharp overhangs indicative of syn depositional dissolution geochemical evidence supporting the marine dissolution includes 18o values from altered fibrous calcite that are more positive up to 3 9 pdb than coeval least altered fibrous calcite from holston buildups in tennessee up to 4 8 to 5 8 pdb syn depositional dissolution was likely associated with sulfide oxidation along an oxic anoxic interface and not due to upwelling of cold bottom water in the appalachian foredeep faunal and lithologic evidence suggests that oxic surface waters had a normal salinity and a water temperature conducive for metazoan growth 14 present atmospheric level during the early caradocian

graptolite biozone correlation charts charts are presented showing the correlation of graptolite biozonations for the lower ordovician to lower devonian i e for the entire stratigraphical range of the planktonic graptoloid graptolites regions chosen are those for which the most detailed biostratigraphical studies have been undertaken for baltica average graptolite zone chron duration appears to vary from c 300000 years ludlow epoch to 2 4 ma lochkovian epoch

stratigraphical and palaeoecological importance of caradoc upper ordovician graptolites from the cardigan area southwest wales graptolites from more than 60 horizons in the basinal caradoc succession of southwest wales between fishguard and cardigan allow recognition of the multidens clingani and linearis biozones the biostratigraphy permits recognition of major differences in the sedimentary rock sequence north and south of structures associated with the fishguard cardigan fault belt the penyraber mudstone formation disconformably overlying the fishguard volcanic group llanvirn is partly of multidens biozone age it is succeeded south of the newport sands fault by the cwm yr eglwys mudstone formation of clingani to linearis biozones age north of the fault the cwm yr eglwys mudstone formation is replaced laterally by the northwards thickening sandstone turbidite dominated dinas island formation clingani and linearis biozones graptolite stratigraphical distribution indicates that dicranograptus clingani occurs only rarely within the caudatus subzone of the clingani biozone and that climacograptus antiquus s l also does not range above the lower part of the clingani biozone the first occurrence of dicellograptus morrisi within the upper clingani biozone confirms its value as a marker for the morrisi subzone and is associated with the first occurrences of diplacanthograptus dorotheus and normalograptus minimus dicellograptus flexuosus used to indicate the morrisi subzone elsewhere occurs throughout the clingani biozone in the cardigan area the linearis biozone is recognized by climacograptus tubuliferus oxic bottom conditions in early and early mid caradoc times largely precluded the influx of or preservation of graptolite faunas in the penyraber mudstone formation anoxic mudstones of the cwm yr eglwys mudstone and dinas island formations preserve graptolite assemblages of 21 and 26 species signalling strong open marine influences which persisted in this area until late caradoc times this contrasts with the shelfal faunas in the whitland area south pembrokeshire where the late caradoc is dominated by low diversity normalograptus dominated assemblages