arenigian early ordovician sea level history and the response of conodont communities western newfoundland four cluster analyses were performed which recognized 17 conodont communities in the arenigian lower ordovician of western newfoundland the analyses include 69 598 identifiable conodont specimens recovered from 153 conodont bearing samples from four stratigraphical sections representing the environmental settings of the platform upper proximal slope lower proximal slope and distal slope the distribution of conodont communities along the platform to slope environmental gradient shows that sea level changes simultaneously affected the development and replacement of the conodont communities in the different facies the pattern of change in conodont communities allows an interpretation of sea level change that is correlated precisely into the detailed graptolite biozonation a gradual transgression lasted most of tetragraptus approximatus zone time which was followed by a brief regression a transgression regression cycle occurred in the t akzharensis zone time a major transgression caused a highst

conflicting isotopic and biotic evidence for tropical sea surface temperatures during the tertiary abstract the biotic evidence from a varied group of shallow water organisms including mangroves zooxanthellate corals mainly reefal and larger foraminifera suggests that sea surface temperatures at low latitudes during the tertiary from at least the early eocene to the middle miocene were subtropical to tropical in nature and similar to temperatures prevailing at low latitudes at the present day 20 28 c in a recent paper shackleton 1984 the suggestion is made on the basis of oxygen isotope studies of foraminiferal shells that low latitude sea surface temperatures during the palaeogene and much of the neogene were invariably lower than those in similar regions today with a stable temperature of around 18 c this paper discusses the conflicting evidence for sea surface temperatures obtained from palaeontological and isotopic sources and suggests that palaeotemperatures derived from some isotopic studies are too low to account for the distribution and diversity of many tertiary tropical and subtropical biotas

acritarch marine microphytoplankton diversity in an early ordovician deep water setting the skiddaw group northern england implications for the relationship between sea level change and phytoplankton diversity early ordovician acritarch microphytoplankton assemblages from an outer shelf to slope facies in northern england show changes in diversity that parallel changes in sea level with high diversity occurring during lowstands and low diversity during highstands the pattern is the reverse of that recorded in the literature in which high acritarch diversity in continental platform successions has been related to periods of transgression and highstand and low diversity to periods of regression and lowstand the apparent contradiction can be explained if high diversity assemblages move further onto continental platforms during transgression and highstand but towards basins during regression and lowstand the hypothesis that high diversity acritarch assemblages track onshore and offshore during episodes of sea level change implies that there is a direct causal link between sea level and microphytoplankton diversity at least in so far as changes in sea level influence the location of high diversity the direct physical influence of sea level change on phytoplankton diversity might have been restricted to relatively small scales however its most obvious effect being perhaps to shift the locus of maximum diversity rather than affect the overall level of diversity at larger temporal and spatial scales the direct influence of sea level change on acritarch diversity is less certain parallel large scale changes in sea level and phytoplankton diversity have been documented for the phanerozoic as a whole but possibly reflect either independent responses to other factors such as the effects of tectonic cycles and changes in palaeogeography ocean circulation and climate or else indicate an indirect influence in the sense that sea level rise created conditions under which phytoplankton diversity could increase in contrast to the results discussed in this paper other analyses of ordovician acritarch diversity at larger spatial scales than those considered here have concluded that there is no evidence for any causal link between sea level change and diversity if however the controls on diversity exerted directly by changes in sea level are scale dependent and only apparent at relatively small scales analyses at larger spatial scales that compile measures of diversity across a range of depositional environments i e that sample both onshore and offshore environments and combine the results might not evince a correlation between changes in sea level and diversity in addition to producing shifts in the location of high diversity assemblages sea level changes also appear to be accompanied by at least local turnover in species composition during the early ordovician in northern england the high diversity assemblages associated with lowstands are very different in composition elsewhere in cratonic settings maximum flooding intervals within successive early palaeozoic stratigraphic sequences are characterised by markedly different acritarch assemblages hence changes in sea level appear to be associated with local extinctions and originations as well as changes in the locus of high diversity possibly as a result of ecological stresses arising from changes in the location of acritarch assemblages

equable climates during earth history eocene and cretaceous climate model experiments demonstrate that regardless of conditions of warm polar oceans differences in pole to equator surface temperature gradient or topography above freezing surface temperatures in winter for continental interiors at middle to high latitudes cannot be maintained continental interiors will have cold winter temperatures even during globally warm periods unless currently unrecognized forcing factors influenced past climates if model simulations of eocene and cretaceous climates are accurate the term equable as it applies to paleoclimate should be reconsidered

ganderia laurentia collision in the caledonides of great britain and ireland during terrane convergence an influx of clastic sediment from an upper plate onto a lower plate is an early indication of terrane juxtaposition in the caledonides of great britain and ireland units accreted to laurentia during the early palaeozoic era include peri gondwanan terrane assemblages that earlier separated from west gondwana however the southern uplands terrane contains detrital zircon populations apparently derived entirely from laurentia characterized by a large asymmetric mesoproterozoic peak and a scarcity of zircon at 600 ma and 2 1 ga in contrast cambrian and ordovician rocks from the lake district and the leinster massif of ireland show abundant grains with these ages together with a range of mesoproterozoic zircon these characteristics are shared with the monian terrane of anglesey and with ganderia in the appalachians indicating probable derivation from amazonia in west gondwana silurian sandstones from the lake district show an influx of laurentia derived zircon and lack the peri gondwanan signal this indicates that in the caledonides ganderia was not accreted to the laurentian margin until c 430 ma in contrast to the ordovician accretion of ganderian fragments recorded in the appalachians suggesting that the configuration of the closing iapetus ocean varied significantly along the strike of the orogen supplementary material details of sample coordinates analytical procedure and u pb detrital zircon analytical data are available at www geolsoc org uk sup18739

marginal faunal belts and their structural implications with examples from the lower palaeozoic fossils provide an independent criterion for testing structural hypotheses generated from field evidence or geochemistry as well as functioning in their traditional role in the dating of rocks or determining biogeographic realms on a global scale the identification of exterior oceanic biofacies may be of use in discriminating the course of former continental margins the isograptid graptolite biofacies early to middle ordovician can be distinguished from contemporary epicratonic graptolite biofacies its worldwide occurrence corresponds with continent margins reconstructed from other criteria hence a new isograptid record from irian jaya e new guinea although based on a small sample suggests that there may have been an ordovician oceanic site there on a continental scale the tectonic juxtaposition of different biofacies or of different stratigraphical packages contributes to an understanding of tectonics and can serve to quantify displacement the accretionary prism hypothesis for the southern uplands of scotland is supported and contrasted with the rearrangement of biofacies in the cambro ordovician cow head group of western newfoundland which reveals not only the original shelf margin slope topography now vanished but also provides an estimate of the movement involved in the emplacement of one taconic allochthon on a more local scale faunal evidence is used in assessing the original position of 9suspect9 marginal areas using the simplest possible model this shows for example that anglesey was not greatly separated from gondwana in the early ordovician but strike slip movement cannot be disproved local changes in relative sea level are determinable from sediments and faunas in marginal belts these may differ from the global eustatic curve and when this happens it is a measure of local tectonic activity particular sea level curves may serve to define structurally coherent units within mobile belts new sea level curves are contrasted for a segment of n wales north of the bala fault with that for central sw wales arenig llandovery the s welsh area mostly responded passively to the eustatic curve which can adequately account for shifts in facies and faunas while n wales had a curve often at odds with eustasy and reflecting its structural history

did south american mixed paleofloras evolve under thermal equability or in the absence of an effective andean barrier during the cenozoic abstract two hypotheses have attempted to explain the development of mixed paleofloras during the cenozoic in south america one of them postulates changes in the climatic tolerances of its component taxa and the other calls for the establishment of atmospheric thermal stability equable climate at the time these floras came into existence we set out to test these hypotheses using leaf physiognomical analyses for eight mixed fossil floras of the cenozoic of chile and argentina in addition to physiognomic analyses of the complete dicotyledonous flora as known from each site we also evaluated the physiognomic significance of fossil nothofagus leaves in relation to current chilean species of the genus our results suggest that fossil nothofagus leaves have a broader climatic envelope relative to the extant species this divergence could be accounted by differences in their respective climatic tolerances we postulate that the mixed paleoflora evolved in a subtropical climate that extended as far south as 40 s with relatively warm temperatures and high annual rainfall with little seasonal variation this climatic scenario is possible only with a much reduced andean massif which would have permitted the spillover of moisture of easterly origin to the pacific andean slopes

an interpretation of cretaceous and tertiary biota in polar regions abstract rich fossil floras and vertebrate faunas lived within the arctic and atarctic circles during the cretaceous and paleogene these occurrences seem explicable by the nature of the thermal environments in polar regions and by the physiological responses of plants and animals allied to those that lived there there seems no need to invoke a lower tilt to the earth s axis to provide greater warmth and light in polar regions to account for these fossil records

palaeoenvironmental interpretation of microphytoplankton diversity trends in the cambrian ordovician of the northern sahara platform abstract analysis of vertical fluctuations in assemblage diversity and relative abundances of microphytoplankton morphological classes permits palaeoenvironmental reconstructions of the cambrian ordovician stratal sequences in the hassi r mel area north central algeria and northern rhadames basin southern tunisia abundance fluctuations of acritarch morphological classes and assemblage diversity appear to be related to changes in depositional facies the present palaeoenvironmental interpretation is primarily based on a comparison with microphytoplankton distributional trends in modern depositional environments where possible the observations are tested against palaeoenvironmental information derived independently from previous lithostratigraphical macropalaeontological and petrographical studies of the formations investigated the present data support the following palaeoenvironmental scenario during late cambrian times relatively shallow water neritic conditions prevailed throughout the study area with evidence for a more proximal environment in southern tunisia in the early tremadoc open marine offshore conditions characterized the central algerian region but a shallower water setting is indicated for southern tunisia during the late arenig llanvirn interval an open marine offshore setting is envisaged for the southern tunisian region with more proximal conditions prevailing in the algerian hassi r mel area during the entire ashgill an essentially restricted marine environment was established throughout the study area some stratigraphically important acritarch species e g acanthodiacrodium angustum appear to be facies sensitive the recognition of palaeoecological control on microphytoplankton distribution has important consequences on the biostratigraphic application of these microfossils

the messaoudensis trifidum acritarch assemblage and correlation of the base of ordovician stage 2 floian the global stratotype section and point gssp for stage 2 of the ordovician system now the floian stage and approximately equivalent to the lower and middle arenig of england and wales is defined by the first appearance datum fad of the graptolite tetragraptus approximatus in the diabasbrottet quarry section at mount hunneberg sweden one of the issues this raises is how to correlate the base of stage 2 at the gssp with areas and successions that do not contain a correlative graptolite fauna the distinctive cymatiogalea messaoudensis stelliferidium trifidum acritarch assemblage is present in the upper tremadocian araneograptus murrayi graptolite biozone of nw england and ranges across the tremadocian stage 2 boundary there the tremadoc arenig boundary of anglo welsh nomenclature it also occurs widely at other high southern ordovician palaeolatitudes around the margin of gondwana being reported from ireland wales the isle of man belgium germany spain and turkey and may also be present in bohemia and argentina eastern cordillera it therefore has the potential to contribute towards the recognition and correlation of the base of stage 2 in those areas of particular interest are the first appearance datums of various taxa within the stratigraphical range of the messaoudensis trifidum assemblage notably that of aureotesta clathrata simplex which is considered to be close to the base of stage 2 in nw england elements of the messaoudensis trifidum assemblage also occur in baltica the palaeoplate on which the gssp for the base of stage 2 is located however many of the taxa used to subdivide the messaoudensis trifidum assemblage around gondwana have not been recorded from baltica and may be restricted palaeobiogeographically to the gondwanan margin furthermore acritarch microfloras have not been reported from the diabasbrottet quarry section itself and there are hiatuses across the base of stage 2 in the two sections from baltica considered in this paper hence direct correlation of the base of stage 2 between the gssp and other sections using acritarchs is not yet possible nevertheless some taxa for example the genera peteinosphaeridium and rhopaliophora are shown to have fads at similar stratigraphical levels in the late tremadocian stage of both baltica and gondwana and therefore have the potential to correlate time slices in the late tremadocian stage between palaeoplates

climate change on tectonic time scales abstract observational and modeling studies relevant to climate change on tectonic time scales are reviewed probably the most significant climate result from this time scale involves inferences about nearly ice free conditions for some intervals of earth history some additional results suggest relatively rapid transitions between climate states modeling studies indicate that although tectonic changes significantly modify the atmosphere and ocean circulation in most cases such modifications do not appear to be sufficient to explain ice free climates increased atmospheric co2 levels 5 10 presentvalues may be needed to completely reconcile models and data although the co2 paradigm has received widespread attention in the last few years the results summarized herein indicate that co2 is only one of several important factors influencing climate i believe the time is appropriate for contemplating climate change on tectonic time scales in terms of a post co2 synthesis in which direct tectonic factors are approximately equal in importance to co2 fluctuations in the last few years modeling studies have progressed from a state of virtually no work on the topic to one where the models are in some respect ahead of the observations in the sense that data have not been compiled to test even some of the most basic model predictions in particular information is needed on past atmospheric co2 levels tropical sea surface temperatures seasonality of continental interiors and locations and production rates of deep waters initial steps are being taken to integrate observational and modeling activities on tectonic time scales and some of these shortcomings may be rectified over the next few years

self organization mechanisms for the formation of nearshore crescentic and transverse sand bars the formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow water equations and on a simplified sediment transport parameterization by assuming normally approaching waves and a saturated surf zone rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed two types of bedforms appear one is a crescentic bar pattern centred around the breakpoint and the other herein modelled for the first time is a transverse bar pattern the feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations basically the waves stir up the sediment and keep it in suspension with a certain cross shore distribution of depth averaged concentration then a current flowing with against the gradient of sediment concentration produces erosion deposition it is shown that inside the surf zone these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro morphodynamic input conditions to which of the patterns develops it is suggested that crescentic bar growth would be favoured by high energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment in intermediate conditions mixed patterns may occur

the spatial nearshore offshore distribution of latest permian phytoplankton from the yangtze block south china abstract in order to analyze the spatial paleoecological distribution of the organic walled microphytoplankton in the late permian five sections from the yangtze block south china displaying different sediment facies types from neritic to offshore paleoenvironments including basinal facies have been investigated palynologically based on the diversity and relative abundance of acritarch species and genera the new data from the permian provide similar patterns as those described from other geological periods 1 low diversities with 2 to 4 acritarch species occur in nearshore environments whereas the higher diversities more than ten acritarch species appear in the offshore environments 2 at the generic level the genera leiosphaeridia reduviasporonites and micrhystridium are distributed widely from nearshore facies corresponding to shallow water environments to offshore facies corresponding to deeper water settings whereas some genera such as dictyotidium and veryhachium have a narrower distribution occurring on the continental shelf and towards the basin indicating open marine environments 3 the genus schizosporis only occurs around the permian triassic boundary ptb when the sea level declined probably indicating nearshore environments with shallow water settings 4 at the specific level the species micrhystridium breve displaying short spines and leiosphaeridia minutissima are indicative of neritic facies whereas the other species of micrhystridium and veryhachium with longer spines e g micrhystridium stellatum and veryhachium hyalodermum and leiosphaeridia microgranifera indicate more open marine environments big spherical acritarch species over 80 m in diameter such as dictyotidium reticulatum indicate shallow water environments and 5 in the ptb strata the relative abundance of the enigmatic reduviasporonites interpreted by some authors as a fungal spore is not higher than 14 in every sample indicating that a spike of reduviasporonites does not occur in the yangtze area reduviasporonites chalastus 40 m in length obviously dominates in shelf environments of shallow water whereas the smaller reduviasporonites catenulatus 15 m in length are more common in deeper water

global mass wasting during the middle ordovician meteoritic trigger or plate tectonic environment abstract mass wasting at continental margins on a global scale during the middle ordovician has recently been related to high meteorite influx although a high meteorite influx during the ordovician should not be neglected we challenge the idea that mass wasting was mainly produced by meteorite impacts over a period of almost 10 ma having strong arguments against the impact related hypothesis we propose an alternative explanation which is based on a re evaluation of the mass wasting sites considering their plate tectonic distribution and the global sea level curve a striking and important feature is the distribution of most of the mass wasting sites along continental margins characterised by periods of magmatism terrane accretion and continental or back arc rifting respectively related to subduction of oceanic lithosphere such processes are commonly connected with seismic activity causing earthquakes which can cause downslope movement of sediment and rock considering all that it seems more likely that most of this mass wasting was triggered by earthquakes related to plate tectonic processes which caused destabilisation of continental margins resulting in megabreccias and debris flows moreover the period of mass wasting coincides with sea level drops during global sea level lowstand in some cases sea level drops can release pore water overpressure reducing sediment strength and hence promoting instability of sediment at continental margins reduced pore water overpressure can also destabilise gas hydrate bearing sediment causing slope failure and thus resulting in submarine mass wasting overall the global mass wasting during the middle ordovician does not need meteoritic trigger

the palaeogeographic position of the lake district in the early ordovician the early ordovician was a time of maximum continental separation and hence a time when faunal evidence can be used to assess palaeogeography in a critical way we summarize the known trilobite occurrences 18 genera from the arenig llanvirn of the lake district and record some genera for the first time maps of the distribution of some of these forms are given all genera except cyclopyge were confined to the gondwana continent at the time and some are known from many localities and two species are widespread in england wales france iberia and bohemia the fauna is entirely distinct from those of scandinavia and north america all the palaeontological evidence points to the lake district being adjacent to ordovician gondwana in the earlier ordovician it is not reasonable to suggest that the iapetus ocean lay to the south of the lake district as did allen 1987

concepts in gravel beach dynamics abstract the dominant processes in gravel beach dynamics are reviewed highlighting some common themes which unify the various components of the gravel beach system the repercussions of which impart on how gravel beach dynamics might be understood conceptually in particular gravel beach dynamics are thought to be highly dependent on the temporal and spatial variation in grain size and the continual adjustments made by an active beach step both of which act not only as the expression of changing morphodynamic conditions but also as a controlling influence morphodynamics the notion that the exchanges on beaches between the hydrodynamics sediment transport and morphological change takes the form of reciprocal relationships which are mediated through feedback mechanisms in such a way that they cannot be thought of or studied independently is not a new one yet it appears that for the gravel beach morphodynamics must be re defined to describe conditions where variations in sediment size are thought to deserve parity rather than as merely a sequent entity or boundary condition morpho sedimentary dynamics is a phrase coined to intuit such cause and effect detailing the co evolution of morphology hydro hydraulics and sediment properties whilst acknowledging causative pluralism feedbacks and multiplier effects this is the recommended conceptual framework within which to crystallise thought and organise further research for the gravel beach essentially it increases the minimum number of parameters needed to describe the state of the gravel beach as a physical system therefore it is advised that simplicity will be most expedient in our future modelling efforts if complexity is to be adequately encapsulated

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

what is an equable climate abstract the ideal theoretical equable climate has a mean annual temperature of 14 c and a mean annual range of 0 c since 14 c is and has been the average mean temperature of the earth s surface any departure from 14 c and 0 c results in a climate of lower temperateness or equability therefore reference to an equable paleoclimate is meaningless unless quantified by a centrum from which equability is measured climates of low seasonality are not necessarily equable because such conditions occur from the inner wet tropics into glacial border environments freezing often occurs in climates of moderate to high equability equable climates may support taxa from different vegetation zones a relation also evident in the paleobotanical record

the middle rhuddanian lower silurian hot shale of north africa and arabia an atypical hydrocarbon source rock an important hydrocarbon source rock a hot shale has been identified from the middle rhuddanian llandovery lower silurian of a number of localities in north africa and arabia it is unusual in having been deposited during a eustatic fall in sea level evidence for this sea level fall in libya and jordan is manifested in a wide variety of palynological proxy data including reduced acritarch and chitinozoan total abundance reduced acritarch diversity and changes in the relative abundances of various environmentally sensitive acritarch morphogroups and from the presence of a minor positive 13corg excursion at the same stratigraphical level as the hot shale the environment of deposition was anoxic before during and after deposition of the hot shale except for some very brief incursions of more oxygenated water that enabled the development of a very limited burrowing benthos and graptolite preservation as three dimensional pyrite internal moulds these incursions occurred just before and during deposition of the hot shale and are interpreted as resulting from the influx of sediment laden oxygenated currents from shallower water environments enhanced organic matter preservation within the hot shale is attributed to greater productivity resulting from increased nutrient input particularly from wind blown dust more rapid descent through the water column of organic matter in post bloom marine snow macroaggregates development of geochemically sealing microbial mats at the sea floor and more rapid burial of organic material than had occurred earlier in the rhuddanian

phytoplankton dynamics across the ordovician silurian boundary at low palaeolatitudes correlations with carbon isotopic and glacial events the late ordovician culminated in a major glacial period that has been related to one of the strongest mass extinctions recorded during the phanerozoic during this interval anticosti island quebec eastern canada was located at low to intermediate palaeolatitudes 15 30 s on the eastern margin of laurentia it displays a relatively complete section across the ordovician silurian o s boundary upper ordovician to lower silurian strata of anticosti studied here comprise the vaureal katian ellis bay hirnantian and becscie latest hirnantian rhuddanian formations phytoplankton dynamics inferred from the acritarch content of these strata are locally compared with new geochemical environmental proxies 13c with recent palynological data from baltica and also with phytoplankton dynamics as described from near polar high latitude localities of the gondwana region two positive excursions of the carbon isotope record have been identified in the lower and upper ellis bay formation overall phytoplankton trends are as follows 1 as with chitinozoans conodonts brachiopods and corals acritarchs from anticosti display a major turn over during the early hirnantian uppermost vaureal formation lower ellis bay formation with the appearance of taxa that exhibit silurian affinities e g tylotopalla sp ammonidium sp oppilatala sp evittia sp dilatisphaera spp high polymorphism for some common taxa is observed in the interval corresponding to the first 13c positive excursion using sequence stratigraphy this period may be related to a first glacially driven sea level drawdown corresponding to terrestrial ice sheet growth on gondwana such as recorded in morocco 2 subsequently during the late hirnantian of anticosti uppermost ellis bay formation phytoplanktic communities suffered a crisis exemplified by the disappearance of several typical large late ordovician taxa i e orthosphaeridium spp peteinosphaeridium spp large baltisphaeridum spp sacculidium sp the lower becscie strata corresponding to the time of deglaciation at the o s boundary interval only contain a low diversity assemblage dominated by large acritarchs of the genus hoegklintia and other morphologically related forms and a few tolerant and long ranging taxa e g evittia remota micrhystridium spp leiospheres despite a possible taphonomic bias in the uppermost ellis bay formation the major crisis in the late hirnantian coincides with the second 13c excursion and a second stronger glacially driven sea level fall corresponding to the time of the maximum extend of the ice sheets on gondwana 3 such phytoplankton events have also been detected in gondwanan ice center area but with typical ordovician taxa crises occurring apparently later when plotted against positive carbon isotope excursions as also observed in baltica this time lag may be linked to a basin configuration of the studied areas where environmental stress consequently differed or linked to the presence of omission surfaces coupled with preservation bias of acritarch assemblages in the uppermost ordovician strata of anticosti in addition to their biostratigraphic and palaeobiogeographic utility acritarchs turn out to be more specific in highlighting palaeoenvironmental conditions

investigation of a self organization model for beach cusp formation and development a recent numerical investigation of self organization werner and fink 1993 suggests that the feedback process between currents and sediment response can result in self organized patterns and can be used to predict beach cusp formation and spacing a similar model based on self organization is tested here in order to understand the processes occurring during beach cusp formation and development to evaluate the sensitivity toward the parameters used and to examine how the model might relate to field observations results obtained confirm the validity of the self organization approach and its capacity to predict beach cusp spacing with values in fair agreement with the available field measurements with most of the input parameters primarily affecting the rate of the process rather than the final spacing however changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior cusp spacing tends to change with time and cusp regularity shows large long term variations cusps are found to be accretionary in the swash zone and in agreement with most observations mean flows are horn divergent over developed topography simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed results indicate that preexisting large amplitude cusps are destroyed if their spacing is substantially different from that expected under self organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach these findings support the hypothesis that self organization is a robust mechanism for beach cusp formation

morphometric and sediment analysis of beach cusp in correlation to rip currents a case study from tropical coast west coast of india the presence of rhythmic beach cusps in foreshore regions are of persistent research interest to coastal researchers engineers with respect to their formation in beaches morphometric and sediment analysis were used to study beach cusp morphology at vengurla beach a gentle slope beach characterized by medium sand the cusp dimensions such as spacing elevation amplitude and depth were measured for detailed analysis the distinctive sediment characteristics between the horn and the bays of the cusp are different and cusp measurements show a high level of irregularity horns and bays have medium sand but sorting is varying sorting variations clearly demonstrated the difference in energy condition between horns and bay negative skewness of few horn samples has been correlated with erosional trend based on the grain size analysis result it was observed that vengurla beach cusps follow the regular grain size distribution pattern additionally the low intensity rip currents were also noticed in the study area the rate of occurrence and strength of rip currents are decided by the nearshore bathymetry and play an imperative role in rip current formation the present study is based upon the field measurements along vengurla coast and contributes to morphometric analysis of beach cusps regarding beach cusps there are no documented databases along the indian coast understanding the beach cusp formation and its seasonal changes along the beaches will be useful for effective coastal management

biostratigraphy taxonomic diversity and patterns of morphological evolution of ordovician acritarchs organic walled microphytoplankton from the northern gondwana margin in relation to palaeoclimatic and palaeogeographic changes abstract acritarchs the fossilizable resting cysts of phytoplanktonic algal protists are the dominant component of marine organic walled microfossils in the palaeozoic the majority of acritarchs show strong similarities with dinoflagellate cysts in morphological and biogeochemical features as well as distributional patterns in the sediments the production of these organic walled microfossils and their distribution and survivorship in the sediments were controlled by differences in ecological tolerances and life cycle autecology of the planktonic parent organisms calculation of evolutionary rates and development of a detailed diversity curve at specific level form the basis for discussing the influence of global palaeoenvironmental perturbations on the evolution of organic walled microphytoplankton in northern gondwana during latest cambrian through ordovician times the potential of acritarchs for biostratigraphic correlation at the regional scale northern gondwana domain is much improved by our detailed revision of distributional patterns of 245 acritarch taxa the most important cambro ordovician acritarch bio events are short periods of diversification which also correspond to introduction of morphological innovations observed in latest cambrian and earliest tremadoc late tremadoc early arenig basal llanvirn and latest ashgill and an important extinction phase in the early caradoc overall acritarch diversity increased from the basal ordovician up to the middle llanvirn then declined in the early and middle caradoc during ashgill times the assemblages are poorly diversified at the generic level as a result of a combined effect of sea level drawdown and onset of glacial conditions but no major extinction event is observed in connection with the end ordovician biotic crisis the peak in acritarch diversity during middle ordovician times appears to be correlated to maximum spread of palaeogeographical assembly acritarch dynamics appear largely uncorrelated to second order sea level oscillations the primary abiotic controls on acritarch evolution were palaeogeographical and the associated palaeoceanographic changes especially during middle ordovician and the end ordovician palaeoclimatic shift the acritarch fossil record provides important information on the evolution of oceanic primary producers however the relationships between acritarch diversity oceanic productivity and evolution of invertebrate animals are proving much more complex than previously thought in particular the hypothesis of a causal relationship between changes in acritarch diversity and metazoan evolution in the palaeozoic is not supported by our data

marine biodiversification in response to evolving phytoplankton stoichiometry abstract diversification of the marine biosphere is intimately linked to the evolution of the biogeochemical cycles of carbon nutrients and primary productivity a meta analysis of the ratio of carbon to phosphorus buried in sedimentary rocks during the past 3 billion years indicates that both food quantity and critically food quality increased through time as a result of the evolving stoichiometry nutrient content of eukaryotic phytoplankton evolving food quantity and quality was primarily a function of broad tectonic cycles that influenced not just carbon burial but also nutrient availability and primary productivity increasing nutrient availability during the middle to late proterozoic culminated in the production of food phytoplankton biomass and fresh dead organic matter with c p redfield ratios sufficient to finally promote geologically rapid biodiversification during the proterozoic phanerozoic transition this resulted in further massive nutrient sequestration into biomass that triggered positive feedback via nutrient recycling bioturbation mesozooplankton grazing on phytoplankton productivity increasing rates and depths of bioturbation through the phanerozoic suggest that nutrient recycling continued to increase increasing bioturbation and nutrient cycling appear to have been necessary to sustain the primary productivity and energetics biomass metabolic rates and physical activity such as predation of the marine biosphere because of the geologically slow input of macronutrients like phosphorus from land and the continued sequestration of nutrients into marine and terrestrial biomass

biodiversity patterns of early middle ordovician marine microphytoplankton in south china based on new materials from six sections and all available literature data new diversity curves are presented for the phytoplankton acritarchs from south china covering the early middle ordovician interval when the great ordovician biodiversification event took place the total diversity curve and the origination data imply that a major radiation of the phytoplankton occurred during the analysed interval a peak of the total acritarch diversity curve appears in the a suecicus graptolite biozone the diversity changes vary in the different parts of the investigated area most probably depending on the position of the analysed sections on the carbonate shelf or the slope reflecting diversity differences due to the position on an inshore offshore transect the early middle ordovician diversity pattern of the phytoplankton is compared with those of several marine invertebrate groups compared with the diversity curve peak of the acritarchs the conodonts and brachiopods reached their highest diversities before the acritarchs while the highest diversity of the chitinozoans appears slightly later the graptolites show two peaks during the early middle ordovician while the trilobites diversity curve shows a peak only in the sandbian the different fossil groups such as chitinozoans conodonts graptolites brachiopods and trilobites show therefore different evolutionary patterns to that of the acritarchs that are not yet fully understood and correlations are so far difficult the acritarch diversity changes can partly be compared to the local sea level changes from four sections in south china at a larger scale the acritarch radiation coincides with a general transgression at a regional or local scale correlations are not straightforward pointing out that more detailed data based on both acritarch studies and more precise sea level investigations are necessary