the ordovician silurian boundary on cornwallis and truro islands arctic canada preliminary data four sections of the lower part of the cape phillips formation two outcrops on northeastern cornwallis island and one outcrop and one drill core from truro island northwest territories canada provide significant new data on the ordovician silurian boundary they show evidence of continuous sedimentation through the boundary interval and a continuous sequence of graptolite zones including the bohemicus and persculptus zones which have not been previously found in arctic canada strata yield graptolites including uncompressed specimens and conodonts through most of the sections the ordovicicus conodont zone occurs within the pacificus to lower persculptus graptolite zones the nathani conodont zone contains a transitional fauna a mixing of species typical of the preceeding ordovicicus zone and those generally regarded as silurian indicators this conodont zone ranges from the middle of the persculptus graptolite zone into the lower acuminatus graptolite zone and thus spans the ordovician silur

post hunnebergian ordovician carbon isotope trend in baltoscandia its environmental implications and some similarities with that of nevada abstract carbon isotope changes during post hunnebergian ordovician time were investigated as the corresponding sequence of rocks is stratigraphically nearly complete in baltoscandia an attempt was made to use it to elaborate the general pattern of carbon isotope changes in most of the ordovician complications caused by several local hiatuses condensed sections and facies changes were mitigated by the study of overlapping sections a total of 326 new whole rock analyses and published data from 18 drill cores and 4 outcrops in estonia latvia and sweden were considered the following positive carbon isotope events were observed 1 the mid darriwilian excursion peak 13c value 1 9 in the aseri stage 2 the mid caradoc excursion 2 2 in the uppermost part of the keila stage 3 the first late caradoc excursion 2 3 in the lower part of the rakvere stage 4 the second late caradoc excursion 2 4 in the upper part of the nabala stage 5 the early ashgill excursion 2 5 in the lowermost part of the pirgu stage 6 the mid ashgill excursion 2 0 in the upper part of the pirgu stage 7 the widely known large hirnantian excursion in estonia the peak value reaches 6 7 in the porkuni stage the study interval comprises a long 19 ma period characterized by low magnitude carbon isotope changes and a following much briefer 12 ma interval with often medium scale shifts and a major event at the end the latter interval reflects onset in the mid caradoc of a period of variable climate alternation of arid and humid episodes which ended with a serious cooling and glaciation the baltoscandian and north american nevada 13c trends are in general lines very similar providing good prospects for global chemostratigraphical correlation and understanding of role of oceanic processes in carbon cycling

early silurian paired δ13ccarb and δ13corg analyses from the midcontinent of north america implications for paleoceanography and paleoclimate abstract paired 13ccarb and 13corg samples from well preserved marine carbonates of the wayne formation in central tennessee were analyzed to test a previously proposed model of relative changes in atmospheric pco2 during the well known ireviken early wenlock positive carbon isotope 13ccarb excursion our investigation provides the first detailed 13corg stratigraphy through this interval and documents a negative excursion in 13corg associated with the positive 13ccarb excursion this is consistent with the notion that the ireviken positive 13ccarb excursion was associated with elevated global temperatures that began during an icehouse greenhouse transition although our data are consistent with predictions made by the silurian oceanographic model jeppsson l 1990 an oceanic model for lithological and faunal changes tested on the silurian record j geol soc lond 147 663 674 a link to global changes in atmospheric pco2 remains tenuous until it can be shown that the 13corg trends shown herein can be reproduced globally

palynology organic geochemistry and carbon isotope analysis of a latest ordovician through silurian clastic succession from borehole tt1 ghadamis basin southern tunisia north africa palaeoenvironmental interpretation abstract palynological and palynofacies analyses combined with organic carbon isotope measurements have been performed on terminal ordovician through silurian clastic sediments from the north african margin of gondwana southern tunisia a first carbon isotopic signal 13corg from gondwanan silurian sedimentary sequences is presented showing interesting correlation with existing coeval isotopic curves from other areas changes in lithology palynofacies characteristics palynomorph diversity carbon isotope developments and organic geochemistry parameters appear to be all causally linked and to reflect changes in palaeoceanographic conditions in particular the detailed chronostratigraphic correlation and the observed similarities in patterns of carbon isotopic developments and palyno and litho facies changes through the study section has permitted to identify the supposedly global earliest wenlock ireviken event and late ludlow lau event isotopic excursions for the first time in high latitude gondwana this confirms that these excursions were linked to global changes in the oceanic system the present results suggest that from rhuddanian to early wenlock times an extended period of black shale deposition occurred over the north african gondwanan margin progressively transgressing from basin palaeodepressions to basin palaeohighs palynofacies and organic geochemistry support a coastal upwelling promoted productivity increase during this interval associated to a decrease in diversity of the marine microplanktonic communities the earliest wenlock strong positive isotopic shift appears associated with this protracted period of massive black shale deposition and thus of organic carbon burial on continental platforms located in high latitude settings this could well explain the apparent paradox between excessive carbonate deposition simultaneous to a carbon isotopic shift towards lighter values observed in low palaeolatitude localities laurentia and baltica the strong distinct isotope excursion occurring in late ludlow times is possibly linked to the well know lau event evidence for extensive organic carbon burial is lacking to explain this strong excursion some significant changes in the marine palynomorph communities are recorded in connection to the increase in stable isotope values 13corg additionally a strong correlation between abundance of terrestrially derived palynomorphs miospores and 13corg development is recorded which seems to support increased clastic input from a terrestrial source

the slow boiling of magma chambers and the dynamics of explosive eruptions abstract explosive eruptions are fed by shallow magma chambers and are mainly characterized by a water saturated magma a temporal increasing eruption rate and deep seismicity following the later stages of eruption we develop two simple physical models based on the decompression of a water saturated magma and the non equilibrium growth of bubbles that drives the magma out of the chamber the eruption ends when sufficient decompression is attained to cause magma chamber collapse and restoration of the initial pressure conditions the first model considers a single step decompression the second one a multi step decompression the models predict that the duration of the eruptive phase is mainly controlled by the bubble number densities and the initial decompression pressure decrease of the order 0 1 1 mpa results in eruption lasting days hours the erupted volume magnitude and eruption rate intensity are mainly controlled by the volume of the magma chamber by the strength of the rocks surrounding the reservoir and the physical properties of the magma water diffusivity and bubble density we simulated the eruptions at mt st helens 1980 and vesuvius 79 ad the models correctly predict the duration and evolution of the eruptions with the initial phases characterized by a slow eruption rate with the formation of a plinian column and the subsequent increase resulting eventually in pyroclastic flow activity

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

maximum extent of ice sheets in morocco during the late ordovician glaciation new field data demonstrate that during the late ordovician hirnantian glaciation an ice sheet expanding northwestwards over the anti atlas range reached into the southern meseta of northern morocco its growth to a glacial maximum position resulted in extensive subglacial erosion and deformation including the development of soft sediment striated surfaces and streamlined subglacial bedforms preserved between the high atlas of marrakech and rehamna these features imply that this ice mass extended 200 km further than previously thought and increase its size by at least ca 190 000 km2 comparable in area to the uk correlation between a measured section in the high atlas of marrakech and that of the southern meseta identifies sedimentary evolution within an ice contact system common to both these findings imply that the west african craton and northern morocco were in full glaciological communication during the latest ordovician palaeogeographic reconstruction shows that beyond the ice sheet south and southeastward palaeoslopes persisted on the shelf a palaeohigh beyond the main ice sheet was a major source for sand feeding delta systems that grew along the shelf as far as the shelf break this palaeohigh probably formed as a result of rift shoulder uplift and supported a satellite ice mass in the eastern meseta a thick 350 m underflow dominated deep marine fan was fed both from this shelf delta system and from glaciogenic debris derived from the main ice sheet the occurrence of this unexpected deep marine area in northern morocco implies that continued northward advance of the ice sheet was hampered by a dramatic break in bathymetry two depositional units are recognised across the meseta containing four distinct sedimentary cycles each recognised as a glacioeustatic response to the waxing and waning of ice masses elsewhere in west gondwana

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

the end ordovician glaciation and the hirnantian stage a global review and questions about late ordovician event stratigraphy abstract this paper proposes a global review of hirnantian event stratigraphy the hirnantian gssp in south china is tentatively correlated with latest ordovician strata from the peri gondwanan glacial regions problems of biostratigraphical correlation are highlighted at a worldwide scale the major biostratigraphically useful fossil groups graptolites chitinozoans brachiopods conodonts acritarchs are analysed and their limits for global correlation of the uppermost ordovician are discussed palaeobiogeographical disparities are invoked as the primary cause of the difficulty in establishing an effective late ordovician global biostratigraphical scheme as an alternative correlative tool the hice hirnantian isotopic curve excursion event is often put forward in the literature however carbon isotope chemostratigraphy shows like biostratigraphy some limits to the present state of knowledge no good independent biostratigraphical control of the hice exists in both shallow carbonate deposits and deeper shaly ones recent studies have also demonstrated inconsistencies between carbon isotopic signals obtained from organic 13corg and inorganic 13ccarb carbon species further complicating the use of the hice as an isochronous benchmark all of these difficulties for hirnantian event stratigraphy are discussed in detail in order to enable them to be overcome in the future precise late ordovician and early silurian event stratigraphies are essential for the understanding of the mechanisms linked to the first of the big five extinctions

origin sequence stratigraphy and depositional environment of an upper ordovician hirnantian deglacial black shale jordan the upper ordovician succession of jordan was located 60 s less than 100 km from the hirnantian ice sheet margin new graptolite dates indicate glaciation ended in jordan in the late hirnantian persculptus biozone the succession records two glacial advances within the ammar formation and the subsequent deglaciations organic rich black shales batra formation form part of the final deglacial transgressive succession that in filled an existing low stand glacial continental shelf topography the base of the black shale is coincident with the maximum flooding surface during transgression interfluves and sub basin margins were breached and black shale deposition expanded rapidly across the region the top of the black shales coincides with peak highstand the expanding puddle model sensu wignall for black shale deposition adapted for the peri glacial setting provides the best explanation for this sequence of events we propose a hypothesis in which anoxic conditions were initiated beneath the halocline in a salinity stratified water column a fresher surface layer resulted from ice meltwater generated during early deglaciation during the initial stages of marine incursion nutrients in the monimolimnion were isolated from the euphotic zone by the halocline increasing total organic carbon toc and 13corg up section indicates the organic carbon content of the shales was controlled mainly by increasing bioproductivity in the mixolimnion the strakhov model mixolimnion nutrient levels were sustained by a continual and increasing supply of meltwater derived nutrients modulated by obliquity changes in high latitude insolation anoxia was sustained over tens to hundreds of thousands of years the formation of black shales on the north gondwana shelf was little different to those observed in modern black shale environments suggesting that it was the nature of the ordovician seas that pre disposed them to anoxia

the origins of glacially related soft sediment deformation structures in upper ordovician glaciogenic rocks implication for ice sheet dynamics in the upper ordovician glaciogenic rocks of west gondwana many soft sediment deformation structures are attributed to glaciotectonic processes however little is known about the subglacial and ice marginal processes associated with this ice sheet a systematic description and interpretation of these soft sediment deformation structures is supplemented by a comparison with late quaternary and modern glacial settings to provide qualitative constraints on the character of ancient subglacial thermal regimes soft sediment striations were formed by discontinuous shear induced deformation beneath an ice sheet that oscillated between warm and cold based thermal regimes beneath warm based ice coupling across the ice bed interface was poor with sliding and loading occurring at this surface loading of soft waterlogged sediments allowed the development of fluted surfaces which are also associated with the formation of truncated folded zones during times of cold thermal regimes coupling across the ice bed interface was enhanced by freezing which caused changes in the position of the freezing front these discontinuous changes coupled with sustained subglacial shearing resulted in the development of stacked soft sediment striated surfaces in areas where the subglacial substrate had an irregular topography complex shear zones formed that comprised stacked duplex systems interlayered with highly strained units preserving pervasive folded lineations at the ice front large glaciotectonic deformation structures comparable with push moraines developed during sustained periods of re advance that created composite thrust and fold systems deformation was transmitted for up to 1 km from the ice front and reduced in intensity with distance other deformation structures include dome like folds that consist of central anticlines 30 to 70 m in width with tight rim synclines that formed by the load induced diapiric intrusion of muds an ice sheet surging mechanism is inferred for the development of composite thrust and fold systems and the coeval sediment diapirs these surges potentially caused the widespread reorganisation of a glacial load resulting in the pro to subglacial extrusion of soft sediments

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

reefs during the multiple crises towards the ordovician silurian boundary anticosti island eastern canada and worldwide multiple latest ordovician rawtheyan hirnantian glaciations in central africa with concomitant global sea level lowstands and cooler restricted equatorial carbonate shelves and ramps interrupted by warmer interstadial highstands had a dramatic global impact on the tropical shallow water reef ecosystem and carbonate production with the ordovician silurian boundary strata on anticosti island as a global standard for a carbonate shelf ramp setting the latest ordovician and earliest silurian reveal three reef phases ended by three extinctions the first extinction towards the end of the rawtheyan affected the last richmondian type reefs vaureal formation mill bay member the second extinction was less pronounced ending with reefs at the base of the prinsta member ellis bay formation interpreted as the top of the normalograptus extraordinarius graptolite subzone the third and most severe extinction phase capped the laframboise patch reef complex ellis bay formation at the top of the no

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

paired δ13ccarb and δ13corg records of upper ordovician sandbian katian carbonates in north america and china implications for paleoceanographic change late sandbian early katian marine carbonates from two sections in north america and one in south china were analyzed for paired 13ccarb and 13corg and revealed similar 13ccarb but varying trends in 13corg stratigraphy while all sections recorded a well known positive 13ccarb shift these 13corg records are the first through the guttenberg 13ccarb excursion gice in china oklahoma and west virginia in south china a positive excursion in 13corg is associated with the latter part of the positive 13ccarb excursion and closely resembles trends from iowa and pennsylvania that have been interpreted to reflect a lowering of atmospheric pco2 levels the 13corg trends from oklahoma and west virginia differ significantly from what is observed in china this likely indicates that local changes in nutrient cycling and phytoplankton growth rates were the dominant control on 12c fractionation in oklahoma and west virginia masking a possible global signal of lowered atmospheric pco2 levels observed in china

oxygen and carbon isotopes and event stratigraphy near the ordovician silurian boundary anticosti island quebec the growth and destruction of continental ice sheets in north africa and adjacent parts of gondwana during the hirnantian stage had profound influence on the sedimentary style of mixed carbonate siliciclastic sequences on anticosti island quebec canada these strata deposited in a tropical mid shelf setting show direct evidence of local sea level changes of up to 40 m after correction for tectonic and load induced subsidence this indicates maximum global glacio eustatic changes of 32 to 36 m this implies maximum ice volume of about 12 to 14 106 km3 and related changes in the oxygen isotopic composition of global sea water amounting to as little as 0 5 to 0 6 given that the oxygen isotope ratios in syn glacial samples from the anticosti sequence range from 8 9 to 2 8 18o it is not possible to clearly distinguish the effects of ice cap development from temperature and diagenetic controlled variation the strong negative shift in 13c values at the top of the ellis bay formation may indicate a pronounced increase in atmospheric co2 coincident with destruction of the late ordovician ice caps

stratigraphy of the upper ordovician upper vaureal and ellis bay formations eastern anticosti island quebec marked facies changes occur in late ordovician strata assigned to the uppermost vaureal and ellis bay formations ashgill rawtheyan hirnantian on anticosti island quebec western anticosti features shales and carbonates whereas outcrops along the eastern coast contain prominent discontinuous mixed siliciclastic carbonate units detailed section measurement along the northeast coast allows for the first time accurate definition of seven new members within this uninterrupted sequence sands present in the upper vaureal and lower ellis bay formations in the east appear to have deterred the growth of muddy bottom brachiopod communities comparable to those in the western and central regions of anticosti sand units within the upper vaureal formation contain 1 m diameter colonies of paleofavosites coeval small coral patch reefs are found in the central part of the island where sands are absent the uppermost ellis bay formation of northeast anticosti is marked by a shallow subtidal coral algal onco

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

preliminary report on the late ordovician graptolite extinction in the yangtze region abstract the present work is mainly based on five continuous ashgillian to earliest llandovery sections together with data from more than 30 other published sections from south china rarefaction analysis indicates a sudden drop in diversity from the upper part of the paraorthograptus pacificus zone to the akidograptus ascensus zone we combined the species range data from some of these sections into a single graptolite composite standard sequence css using graphic correlation technique a temporal scale for graptolite zones in the ordovician and silurian was adopted and the css ranges were plotted against this temporal scale the new css reveals that the mass extinction was gradual or stepwise and began with a major extinction spanned an interval from near the top of the tangyagraptus typicus subzone to the middle of the normalograptus extraordinarius zone a secondary more minor pulse of extinction minor extinction took place late in the interval of the upper normalograptus persculptus zone using temporally scaled range data species diversities can be calculated more precisely according to the species area relationship we can calculate that reduction in the area of the yangtze sea only led to less than 21 5 decrease in graptolite diversity at least 41 decline of the species diversity was due to the destruction of the favorite habitat of graptolites such as a sudden change from dysoxic to fully oxygenated waters decreasing of water temperature reduction of primary producers and interruption of food web

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

milankovitch band cyclicity in bedded halite deposits contemporaneous with late ordovician early silurian glaciation canning basin western australia abstract the mallowa salt of the early palaeozoic carribuddy group canning basin western australia is a halite mudstone anhydrite dolomite evaporite sequence of late ordovician to early silurian age deposited in barred marginal marine to ephemeral saltpan and saline mudflat environments bhp utah minerals potash exploration well gingerah hill no 1 obtained drill core of a 477 m stratigraphic interval of the mallowa salt which permitted the study of geochemical variations in a thick halite sequence contemporaneous with late ordovician early silurian glaciation on other continents a strip of uniform width was continuously ground from the core and the powdered rock bagged at 1 m intervals for geochemical assay the resulting geochemical stratigraphic series display conspicuous cyclicity several orders of cycles ranging from 3 m to100 m stratigraphic thickness are discernible in the plots of minor constituents br mgo and k2o in extracted soluble salts and also in the plots of na2o and total salts content fast fourier transform fft of the br mgo and k2o series gives principal spectral peaks in the 0 00 0 10 cycles m frequency band at 250 113 35 4 22 1 and 19 7 m as well as peaks at 65 m and between 2 8 and 13 5 m fft of the na2o and total salts series shows dominant spectral peaks at 220 259 and 109 114 m if the strongest spectral peak at 113 m is taken to represent the relatively stable eccentricity period of 100 ka and a constant net rate of accretion assumed for long sections the other main periods in the mallowa salt as exemplified by the br spectrum would be31 3 3 0 19 6 1 1 and17 4 1 1 ka these figures are consistent with predicted late ordovician early silurian 440 ma periods for obliquity 30 5 ka and precession 19 3 and 16 4 ka based on evolutionary change in the earth moon system additional spectral peaks identified with implied periods of approximately 206 233 57 5 8 12 and 2 5 3 5 ka also may be of palaeoclimatic relevance the relative amplitudes and structure of spectral peaks in the 0 00 0 10 cycles m frequency band support the identification of climatic oscillations forced by orbital cycles the data indicate a precession eccentricity dominated pattern which accords with the know low palaeolatitude

δ18o and δ13c records from a cenozoic sedimentary sequence in the lanzhou basin northwestern china implications for palaeoenvironmental and palaeoecological changes abstract in northwestern china carbonate 18 o variation has been closely associated with evaporation and precipitation whereas the variation of carbonate 13 c generally reflects patterns of palaeovegetation located within the transitional zone between the chinese loess plateau and the tibetan plateau the lanzhou basin has developed a continuous sequence of cenozoic sediments which have been subjected to detailed sedimentological and high resolution magnetostratigraphic analyses in the present study pedogenic carbonate o and c isotopic analyses were obtained throughout the entire cenozoic sequence the 18 o record exhibits a general positive trend with several abrupt changes a dramatic positive shift in the 18 o record at 33 ma indicates the initiation of the aridification process within the basin which was likely associated with the late eocene westward retreat of the tethys sea and global cooling two significant positive shifts in the 18 o record at 22 ma and 3 5 ma are synchronous with major increases in aeolian dust deposition on the chinese loess plateau and in the north pacific ocean suggesting the intensified aridity of the asian interior which is likely related to the stepwise uplift of the tibetan plateau via the blocking of water vapour pathways the 13 c values exhibit a weak positive trend with a remarkable shift at 3 5 ma this trend is likely related to a decrease in vegetation density in response to the ongoing cenozoic aridification whereas the shift at 3 5 ma may reflect the large scale expansion of c 4 plants