petrochemical evolution of the white mfolozi granite pluton evidence for a late palaeoarchaean a type granite from the se kaapvaal craton south africa abstract one of the major limitations in understanding the geochemical evolution of the kaapvaal craton south africa is the scarcity of whole rock trace element data of the granitoid and other rocks compared to the vastness of this cratonic block here we present new xrf major oxide and icp ms trace element analyses of the white mfolozi granitoid wmg pluton se kaapvaal craton which suggest that the 3 25 ga u pb zircon age old wmg pluton is a peraluminous a type granite and could be equivalent to the intrusive potassic granite phase of the anhalt granitoid suite occurring to the north of the wmg pluton the pluton was generated by batch partial melting of a pre existing ttg source in two major phases under relatively anhydrous conditions and the heat of partial melting could have been provided by a voluminous mantle derived mafic magma which intruded into mid crustal levels c 17 km perhaps during a period of crustal extension the estimated pressure and temperature of generation of the wmg parent magma with average molar or or ab 0 48 could be 500 mpa and close to 1000 c respectively when compared with the results of experimental petrology interstitial occurrence of relatively iron rich biotite mg mg fe 0 41 0 45 suggests that the final temperature of crystallization of the pluton was close to 800 c an important magmatic event following the main phase of partial melting was limited mixing between the intrusive mafic magma and co existing newly generated granitic melt this magma mixing resulted in distinct variations in sio 2 and a low initial sr isotopic ratio 0 7013 of the wmg pluton although both the models of partial melting of quartzo feldspathic sources and fractional crystallization of basaltic magmas with or without crustal assimilation have been proposed for the origin of a type granites the model of magmatic evolution of the wmg pluton presented here can also be an alternative model for the generation of a type granites in this model post partial melting magma mixing is perhaps critical in explaining the daly gap in composition and extreme variations in chemical e g sio 2 and isotopic compositions observed in many bimodal a type granite suites the emplacement of the oldest known a type granitoid suite in the kaapvaal craton the wmg pluton marks a period of stabilization of the craton before erosion and deposition of the overlying volcano sedimentary succession of the pongola supergroup

did a katian large igneous province trigger the late ordovician glaciation a hypothesis tested with a carbon cycle model abstract the ordovician is known as a period with high temperatures and high sea levels and a cooling event at the end of the period leading to the hirnantian glaciation and the first of the big five mass extinctions of marine life the cause of this cooling event remains unclear several authors correlate it with a drop in atmospheric p co 2 to a threshold permitting the ice accumulation on the gondwana supercontinent however what are the reasons of the atmospheric p co 2 decrease here we follow the hypothesis initiated by barnes 2004 that an ordovician superplume event occurred during the middle to late ordovician such an event would not only have a large impact on the ordovician biodiversification barnes 2004 but it would also be responsible for the climatic upheaval during the late ordovician by the emplacement of a low latitudinal continental basaltic province that had an impact on the atmospheric p co 2 there is no direct evidence of a superplume event or of basaltic traps and the present study is therefore a hypothetical modelling approach where we demonstrate with a numerical box model including carbon alkalinity and phosphorus cycles coupled with a 1d climate model energy balance model ebm francois and walker 1992 that such a scenario allows to explain both the mid ashgill katian global warming event known as the boda event fortey and cocks 2005 and the subsequent late ordovician hirnantian glaciation because silicate weathering is enhanced upon warm and wet climate we try to constrain the size and the latitudinal emplacement of the basaltic province leading to a sufficient consumption of atmospheric p co 2 to the threshold proposed by herrmann et al 2004 a b to initiate a glaciation on gondwana

the influence of debris cover and glacial lakes on the recession of glaciers in sikkim himalaya india we investigated area changes in glaciers covering an area of similar to 200 km 2 in the tista basin sikkim eastern indian himalaya between similar to 1990 and 2010 using landsat thematic mapper tm and indian remote sensing satellite irs images and related the changes to debris cover supraglacial lakes and moraine dam lakes the glaciers lost an area of 3 3 0 8 between 1989 90 and 2010 more detailed analysis revealed an area loss of 2 00 0 82 2 56 0 61 and 2 28 2 01 km 2 for the periods 1989 97 1997 2004 05 and 2004 2009 10 respectively this indicates an accelerated retreat of glaciers after 1997 on further analysis we observed 1 the formation and expansion of supraglacial lakes on many debris covered glaciers and 2 the merging of these lakes over time leading to the development of large moraine dam lakes we also observed that debris covered glaciers with lakes lose a greater area than debris covered glaciers without lakes and debris free glaciers the climatic data for 24 years 1987 2011 measured at the gangtok meteorological station 1812 m a s l showed that the region experienced a 1 0 degrees c rise in the summer minimum temperature and a 2 0 degrees c rise in the winter minimum temperature indicating hotter summers and warmer winters there was no significant trend in the total annual precipitation we find that glacier retreat is caused mainly by a temperature increase and that debris covered glaciers can retreat at a faster rate than debris free glaciers if associated with lakes

limitation of fixed nitrogen and deepening of the carbonate compensation depth through the hirnantian at dob s linn scotland abstract the late ordovician is characterized by dramatic changes in global climate concurrent with a major mass extinction and possible changes in ocean redox to further refine our understanding of these events we present nitrogen and carbon isotope and abundance data from the ordovician silurian o s global boundary stratotype section and point at dob s linn scotland we show that this section experienced post depositional ammonium migration from the organic rich to the organic poor horizons however our data suggest that isotopic fractionations from ammonium substitution into illitic clay minerals are small and can be corrected reconstructed primary nitrogen isotope ratios indicate that unlike in tropical continental shelf sections that were transiently enriched in nitrate during the hirnantian glaciation the sub tropical continental slope setting at dob s linn experienced persistent limitation of fixed nitrogen across the o s boundary shallow subpolar settings appear to be the only environment that shows persistent nitrate availability at that time this pattern suggests that spatial trends in marine nitrate concentrations which are observed in the modern ocean as a result of latitudinal temperature gradients were already established during the paleozoic while the average marine o2 chemocline depth may have deepened during the hirnantian glaciation it probably did not lead to global ventilation of the deep ocean which may have been delayed until the carboniferous furthermore carbonate data from this and other sections suggest a deepening of the carbonate compensation depth ccd during the hirnantian this observation indicates that pacific style responses of the ccd to glacial interglacial periods were operational across the o s boundary and that the expansion of abiotic carbonate deposition and preservation beyond the shelf break could have in part mediated changes to surface co2 during these extreme changes in climate

mineralogical and stable isotope studies of gold arsenic mineralisation in the sams creek peralkaline porphyritic granite south island new zealand at sams creek a gold bearing peralkaline granite porphyry dyke which has a 7 km strike length and is up to 60 m in thickness intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the late ordovician wangapeka formation the lamprophyre dykes occur as thin 3 m slivers along the contacts of the granite dyke 18omagma values 5 to 8 vsmow of the a type granite suggest derivation from a primitive source with an insignificant mature crustal contribution hydrothermal gold sulphide mineralisation is confined to the granite and adjacent lamprophyre metapelite country rocks have only weak hydrothermal alteration three stages of hydrothermal alteration have been identified in the granite stage i alteration high fo2 consisting of magnetite siderite biotite stage ii consisting of thin quartz pyrite veinlets and stage iii low fo2 consisting of sulphides quartz and siderite veins and pervasive silicification the lamprophyre is altered to an ankerite chlorite sericite assemblage stage iii sulphide veins are composed of arsenopyrite pyrite galena sphalerite gold chalcopyrite pyrrhotite rutile graphite three phases of deformation have affected the area and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding the youngest of which is early cretaceous locally preserved early formed fluid inclusions are either carbonic showing two or three phases at room temperature liquid co2 ch4 liquid h2o co2 vapour or two phase liquid rich aqueous inclusions some of which contain clathrates salinities of the aqueous inclusions are in the range of 1 4 to 7 6 wt nacl equiv final homogenisation temperatures th of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355 c which are not too different from vein formation temperatures of 340 380 c estimated from quartz albite stable isotope thermometry 18o values of stage ii and iii vein quartz range from 12 and 17 and have a bimodal distribution 14 5 and 16 with stage ii vein quartz accounting for the lower values siderite in stage iii veins have 18o 12 to 16 and 13c values 5 relative to vpdb unlike those from wangapeka formation metasediments 13cbulk carbon values of 24 to 19 and underlying arthur marble marine carbonates 18o 25 and 13c 0 calculated 18owater 8 to 11 at 340 c and delta 13 text c rm co 2 5 values from vein quartz and siderite are consistent with a magmatic hydrothermal source but a metamorphic hydrothermal origin cannot be excluded 34s values of sulphides range from 5 to 10 relative to cdt and also have a bimodal distribution modes at 6 and 9 correlated with stage ii and stage iii mineralisation respectively the 34s values of pyrite from the arthur marble marine carbonates range from 3 to 13 and wangapeka formation range from 4 to 9 5 indicate that they are potential sources of sulphur for sulphides in the sams creek veins another possible source of the sulphur is the lithospheric mantle which has positive values up to 14 ages of the granite lamprophyre alteration mineralisation and deformation in the region are not well constrained which makes it difficult to identify sources of mineralisation with respect to timing our mineralogical and stable isotope data does not exclude a metamorphic source but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source assuming that the hydrothermal fluids were sourced from crystallisation of the sams creek granite or an underlying magma chamber then the sams creek gold deposit appears to be a hybrid between those described as reduced granite au bi deposits and alkaline intrusive hosted au mo cu deposits

distal orogenic effects on peripheral bulge sedimentation middle and upper ordovician of the nashville dome abstract a major switch in depositional style in the ordovician carbonates of the nashville dome corresponds closely with the onset of the late middle ordovician taconic orogeny this time marks a shift from tropical type to temperate type carbonates the initiation of widespread major phosphate deposition the introduction of large amounts of terrigenous silt and clay the occurrence of widespread seismically induced soft sediment deformation and a change from a low energy flat topped carbonate shelf to a high energy doubly dipping carbonate ramp soft sediment deformation and the introduction of siliciclastics are direct effects of the taconic orogeny the switch from tropical type to temperate type carbonates the initiation of phosphate deposition and the switch in carbonate ramp are lar ely oceanographic effects triggered by the orogeny in particular phosphate deposition and the switch to temperate type limestones appears to have been driven by upwelling along the eastern side of the nashville dome within the newly deepened taconic foreland basin a fourfold decrease in the rate of relative sea level rise occurred on the nashville dome nearly 3 m y following the onset of thrusting and foreland basin initiation subsidence rates were constant before and after this decrease and no evidence of a change in subsidence rates is seen to coincide with the onset of thrusting the slowing of subsidence may reflect viscoelastic uplift of the nashville dome but the abrupt change from one constant subsidence rate to another is not predicted by existing foreland basin models

the missing silica sink revisiting the marine sedimentary si cycle using cosmogenic 32si burial of biogenic silica bsitotal in high sedimentation rate continental margins remains highly uncertain cosmogenic 32si t1 2 140 years can be used to trace the fates of bsitotal postdeposition including as opal bsiopal and diagenetically altered opal bsialtered the latter dominantly authigenic clay bsiclay to determine the magnitude and form of bsitotal storage in coastal sediments conventional operational leaches targeting bsiopal and bsialtered including bsiclay were modified for large scale samples necessary for measurement of 32si the 32si activity was used to estimate total biogenic silica burial bsitotal bsiopal bsialtered in several depositional settings gulf of papua gulf of mexico long island sound and in the previously studied amazon guianas deltaic system in subtropical and temperate regions 32si was detected in both traditional biogenic silica leaches bsiopal and residual authigenic clays traditional bsiopal and modified operational leaches designed to target the most reactive authigenic silicates bsialtered consistently underestimate authigenic clay formation bsiclay and thus the magnitude of bsitotal burial in temperate coastal zones and subtropical deltas by 2 4 fold in tropical deltas 32si activities in the residual fraction after removal of bsiopal demonstrate rapid and almost complete alteration of initial bsiopal to new forms most likely bsiclay globally 4 5 4 9 tmol yr si may be trapped in marine nearshore deposits as rapidly formed clay bsiclay 100 of the missing silica sink in the marine silica budget

reconciling late ordovician 440 ma glaciation with very high 14x co2 levels geochemical data and models suggest a positive correlation between carbon dioxide changes and climate during the last 540 m y the most dramatic exception to this correlation involves the late ordovician 440 ma glaciation which occurred at a time when co2 levels may have been much greater than present 14 16x since decreased solar luminosity at that time only partially offset increased radiative forcing from co2 some other factor needs to be considered to explain the glaciation prior work with energy balance models ebms suggested that the unique geographic configuration of gondwanaland at that time may have resulted in a small area of parameter space permitting permanent snow cover and higher co2 levels however the crude snow and sea ice parameterizations in the ebm left these conclusions open to further scrutiny herein we present results from four experiments with the genesis general circulation model with co2 levels 14x greater than present solar luminosity reduced 4 5 and an orbital configuration set for minimum summer insolation receipt we examined the effects of different combinations of ocean heat transport and topography on high latitude snow cover on gondwanaland for the no elevation simulations we failed to simulate permanent summer snow cover however for the slightly elevated topography cases 300 500 m permanent summer snow cover occurs where geological data indicate the ordovician ice sheet was present these results support the hypothesis based on ebm studies further results indicate that although average runoff per grid point increases substantially for the ordovician runs the decreased land area results in global runoff 10 30 less than present with largest runoff reductions for flat topography this response has implications for co2 runoff weathering parameterizations in geochemical models finally simulated tropical sea surface temperatures ssts are the same or only marginally warmer than present this result is consistent with evidence from other warm time intervals indicating small changes in tropical ssts during time of high co2

widespread prolonged late middle to late ordovician upwelling in north america a proxy record of glaciation late middle to late ordovician subtidal ramp carbonates of new mexico texas and oklahoma contain abundant spiculitic chert to 70 chert by volume and phosphate 1 5 wt indicating that these rocks formed in an extensive upwelling zone upwelling began in the late middle ordovician ca 454 ma and persisted until the end of the ordovician late ordovician cherty carbonates also occur along the u s cordilleran margin in board of organic rich graptolitic shale and chert the widespread occurrence of late ordovician cherty and phosphatic carbonates on southern and western laurentia in addition to phosphate rich cool water carbonates over much of the north american midcontinent suggests vigorous thermohaline circulation related to prolonged 10 14 ma gondwana glaciation

response of the inter tropical convergence zone to southern hemisphere cooling during upper ordovician glaciation stable isotope ratios of whole rock carbonates and faunas from three low latitude upper ordovician sections demonstrate a coherent pattern of shifting subtropical and tropical water masses and associated climate belts we suggest that tropical water beneath the inter tropical convergence zone 18o 10 to 15 was isotopically light reflecting high sea surface temperatures sst and reduced sea surface salinity sss subtropical water in both the northern and southern hemispheres is characterised by 18o values from 4 to 6 our interpretation is consistent with the published results from physical chemical modeling of ordovician seawater composition and climate models deriving absolute temperature and salinity values from ordovician 18ocarb data is problematic as they could reflect sst sss ice volume and potentially a variable diagenetic overprint and currently no independent constraints exist for any of these variables the itcz was positioned at 10 n of the equator during the early katian consistent with the likely position of the region of peak sea surface temperature sst during the late katian boda event the itcz moved across the equator to 10 s by analogy with the pliocene this resulted in a symmetrical distribution of sea surface currents about the equator this led to a net increase in heat flow towards the poles and lower latitudinal temperature gradients potentially explaining the migration of warm water faunas to higher southern latitudes during the boda event during the hirnantian the itcz shifted north of the equator resulting in a reduced poleward heat transport itcz position has been proposed as a likely forcing mechanism of neogene glaciation and the evidence from the ordovician suggests a discrete palaeo itcz whose position was controlled by solar insolation and latitudinal temperature gradients linked to ice volume changes and hadley cell dynamics we hypothesize that the position of the itcz was sensitive to the growing hirnantian ice sheet during the late ordovician we propose that latitudinal movement of the itcz during the late ordovician resulted in a major atmosphere ocean circulation re organization higher pco2 estimates for the ordovician caused more extreme changes in this circulation that were the likely primary cause of biotic mass extinction if the monsoon is considered an off equator itcz then we provide direct evidence of an ordovician monsoonal climate

paleozoic laurentia gondwana interaction and the origin of the appalachian andean mountain system laurentia the rift bounded precambrian nucleus of north america may have broken out from a neoproterozoic supercontinent between east and west gondwana several lines of evidence suggest that the appalachian margin of laurentia subsequently collided with the proto andean margin of the amalgamated gondwana supercontinent in different relative positions during early and mid paleozoic time in route to final docking against northwest africa to complete the assembly of pangea hence the appalachian and andean orogens may have originated as a single mountain system the overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the paleozoic era laurentia gondwana collisions may help to explain contemporaneous unconformities in the paleozoic sedimentary cover of the laurentian gondwanan and baltic cratons

pb isotopes in ascension island rocks oceanic origin for the gabbroic to granitic plutonic xenoliths abstract the pb isotopic compositions and u and pb concentrations of the lava series alkali basalt to comendite and of their plutonic xenoliths gabbro to alkaline granite of ascension island are reported the data are used to evaluate the source of the xenoliths which formed two differentiation suites the acidic and intermediate xenoliths together with most of the lavas on the one hand and the gabbroic xenoliths and a basaltic tuff on the other hand the pb isotopic compositions imply a mantle origin for the source magmas of the xenoliths and confirm the possibility of generating granitic rocks in an oceanic environment by fractional crystallization of a mantle derived magma whose geochemical and isotopic characteristics are comparable to the source magmas of oceanic island basalts

oxygen and neodymium isotope evidence for recycling of juvenile crust in northeast china it has been long recognized from nd and sr isotopes that depleted mantle sources consist of recycled oceanic materials but difficulty was encountered in identifying this signature by means of oxygen isotopes because of significant postemplacement hydrother mal alteration zircon is expected to preserve this signature because it is resistant to high temperature hydrothermal alteration this effect is illustrated by a combined sm nd and oxygen isotope study of whole rock and mineral samples from a mesozoic a type granite at nianzishan in northeastern china the sm nd isotope results show positive nd t val ues of 10 86 to 14 27 with young nd model ages of 569 846 ma manifesting a significant input of newly mantle derived material the zircon d 18 o values of 3 12 4 19 are significantly lower than the d 18 o value of 5 3 6 0 3 for the normal mantle zircon and thus appear to require remelting of hydrothermally altered oceanic crust the combined nd o isotope studies not only provide compelling evidence for geochemical recycling of young juvenile crust by plate subduction but also demonstrate that the granitic magmas can result from partial melting of mantle derived rocks that were subjected to seawater hydrothermal alteration before magma generation disequilibrium oxygen isotope frac tionations are observed between common rock forming minerals with significantly lower d 18 o values for alkali feldspar than seawater corresponding to meteoric hydrothermal alteration after magma crystallization

hydrogen and oxygen isotope geochemistry of ascension island lavas and granites variation with crystal fractionation and interaction with sea water lavas and pyroclastics on ascension island contain plutonic blocks that include fluid inclusion bearing peralkaline granite 18o 16o ratios f and cl have been analysed on whole rocks and or minerals for lavas and granites and d h ratios and h2o for comenditic obsidians and granites whole rock 18o 16o ratios of fresh alkali basalt hawaiite trachyandesite trachyte and comendite range from 6 0 to 6 9 with 18o tending to increase with increase in sio2 the 18o values of the granites are from 0 0 to 0 3 depleted in 18o relative to the comendites comenditic obsidians have d 80 4 and h2o 0 3 wt while amphiboles from the granites have d 56 2 the o isotope trend of the lavas is consistent with a crystal fractionation model fresh igneous rocks with 18o values greater than 7 involve processes in addition to crystal fractionation of a basaltic magma the d h ratios and cl contents 3 000 ppm of the h2o poor comenditic obsidians represent undegassed primary magmatic values the h isotope compositions and low h2o and cl 167 ppm contents of the granites are consistent with the major degassing loss of 90 of initial h2o of an h2osaturated magma derived from the interaction of sea or possibly meteoric water with the h2o undersaturated comenditic melt it is proposed that associated with caldera subsidence and stoping water was sucked in around the residual magma before the system had time to be sealed up the h2o undersaturated magma consumed this h2o with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks at a pressure of about 1 5 kb the granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites granites from oceanic islands may in general be a result of generating an h2o saturated acid melt by such direct or indirect crustal water magma interaction processes

late ordovician global warming the boda event there is substantial evidence for mid ashgillian global warming before the latest ordovician hirnantian glaciation as shown by the movement of previously lower latitude benthic faunas such as trilobites and brachiopods to progressively higher latitudes and by an increase in endemic faunas at low latitudes this is shown by faunal dynamics on several paleocontinents and endorsed by published evidence of warmer water sediments from gondwana and elsewhere we term this global warming the boda event

hydrogen and oxygen isotope geochemistry of a type granites in the continental margins of eastern china hydrogen and oxygen isotope studies of both whole rock and mineral separates were carried out for five representative atype granites in eastern china from north to south the whole rock d d and d 18 o values vary from 2145 to 282 and from 21 2 to 5 0 for nianzishan pluton from 2135 to 2110 and from 1 6 to 6 7 for shanhaiguan pluton and from 2145 to 2111 and from 1 5 to 9 1 for laoshan massif respectively the highest whole rock dd and d 18 o values are observed in suzhou pluton and range from 281 to 259 and from 3 5 to 9 2 respectively homogeneous d d and d 18 o values are found in kuiqi massif ranging from 2113 to 299 and from 5 6 to 8 5 respectively the results show that the large variabilities in the hydrogen and oxygen isotope ratios are mainly related to the diverse processes of magma evolution such as magma degassing fractional crystallization as well as post magmatic alteration by local meteoric water under subsolidus conditions the initial d d and d 18 o values of the a type granites in eastern china are constrained to be about 250 5 and 7 5 1 0 respectively this is consistent with mantle like protoliths for granitoid rocks and thus precludes the possibility that the a type granites is derived from the anatexis of the 18 o rich upper crust however recycling of crustal materials by earlier subduction cannot be excluded in the processes of generating the a type granitic magmas the variation in the ddepletion of the granites with latitude implies that the paleogeographic localities of the a type granites have not changed significantly since the mesozoic q 2000 elsevier science b v all rights reserved

past climate and the role of ocean heat transport model simulations for the cretaceous a series of general circulation model experiments using global environmental and ecological simulation of interactive systems genesis were executed to evaluate the sensitivity of simulated mid cretaceous climate to small perturbations in ocean heat transport three experiments were performed 1 mixed layer ocean with no ocean heat transport zeroq 2 ocean heat transport specified as required for genesis to best match modern observations oneq and 3 doubled ocean heat transport twoq the oneq experiment represents an ocean heat transport which is actually about 15 of the values given by carissimo et al 1985 from modern observations as a sensitivity experiment these model simulations represent a doubling of the role of the ocean however relative to the observations they represent small perturbations to the total poleward heat transport in the model with the exception of the tropics no major changes in the structure of the general circulation of the atmosphere resulted from the modification of the ocean heat transport however relatively small increases in ocean heat transport resulted in a number of significant differences between simulations including tropical cooling polar warming weakened equator to pole surface temperature gradients weakened midlatitude jets decreased land sea pressure contrast and decreased midlatitude storminess the experiments indicate that changes in ocean heat transport which are well within the realm of possibility for earth history can have significant climatic impact although ocean heat transport may be a significant factor in explaining cretaceous polar warmth the changes specified in these experiments alone are not sufficient to explain the polar warmth of the cretaceous

al2o3 tio2 ratio of the clay fraction of late ordovician silurian carbonate rocks as an indicator of paleoclimate of the fennoscandian shield abstract in the fennoscandian shield the fluctuations in arid humid climate conditions are inferred from the al2o3 tio2 ratio of the clay fraction of estonian carbonate rocks of the baltoscandian sedimentary basin the late ordovician climate of the fennoscandian shield was mainly semi humid with several humid pulses in the sandbian mid katian and hirnantian the post hirnantian climate was fluctuating between humid and arid until stable humid conditions were established in the middle and early late llandovery starting from the latest llandovery the climate turned semi arid and arid the ordovician silurian long term trend toward aridity was caused by the equator ward drift of the baltica plate shorter term fluctuations of al2o3 tio2 reflect climate changes of the global extent

visions of ice sheets in the early ordovician greenhouse world evidence from the peninsula formation cape peninsula south africa abstract the depositional architecture within the early ordovician peninsula formation south africa indicates a three tier stratal sequence hierarchy comprising likely 4th order autogenic 3rd order allogenic and 2nd order regressive transgressive sequence stacking pattern based on correlation with the global eustatic sea level curve we interpret the 3rd order sequences comprising predominantly lowstand braided fluvial deposits with thin early transgressive system tract marginally marine sandstones as orbitally induced glacioeustacy eustacy affected sand delivery base level changes and discharge patterns the cape peninsula lay at 30 s during the early ordovician yet the primary control on sedimentation was the waxing and waning of ice sheets in the source area we hypothesize the presence of an early ordovician ice sheet of 8 12 10 6 km 3 and a polar front at 40 s

a record of astronomically forced climate change in a late ordovician sandbian deep marine sequence ordos basin north china abstract the late ordovician pingliang formation on the southwestern margin of the ordos basin north china consists of rhythmic alternations of shale limestone and siliceous beds to explore the possible astronomical forcing preserved in this lithological record continuous lithological rank and magnetic susceptibility ms stratigraphic series were obtained from a 34 m thick section of the pingliang formation at guanzhuang power spectral analysis of the ms and rank series reveal 85 5 cm to 124 cm 23 cm to 38 cm and 15 cm to 27 cm thick sedimentary cycles that in ratio match that of late ordovician short eccentricity obliquity and precession astronomical cycles the power spectrum of the ms time series calibrated to interpreted short orbital eccentricity cycles aligns with spectral peaks to astronomical parameters including 95 kyr short orbital eccentricity 35 3 kyr and 30 6 kyr obliquity and 19 6 kyr and 16 3 kyr precession cycles the 15 cm to 27 cm thick limestone shale couplets mainly represent precession cycles and siliceous bed deposition may be related to both precession and obliquity forcing we propose that precession forced sea level fluctuations mainly controlled production of lime mud in a shallow marine environment and transport to the basin precession and obliquity controlled biogenic silica productivity and temperature dependent preservation of silica may have been influenced by obliquity forcing

upper ordovician facies in the lac saint jean outlier quebec eastern canada palaeoenvironmental significance for late ordovician oceanography upper ordovician caradocian carbonates of eastern north america were deposited along the iapetus continental margin and record a transition from warm to cool water settings despite this margin having been within the southern hemisphere tropical belt this event has been documented from virginia usa to southern quebec canada although not previously from areas close to the palaeoequator field petrographic and major element geochemistry data have been gathered from the poorly known upper ordovician carbonate succession outcropping in the lac saint jean outlier in central quebec the succession consists of a lower siliciclastic formation tremblay overlain by three limestone formations simard shipshaw and galets and capped by shales pointe bleue shale from macro and microfaunal evidence carbonate sedimentation occurred during the late caradoc and is younger than the early to mid caradoc carbonate succession present farther south relative sea level fluctuations recorded in the sediments suggest an overall sea level rise briefly halted by a minor end caradocian sea level fall the lower limestone formation simard consists of muddy sediments with algal coral stromatoporoid boundstones green algae are abundant this unit reflects low energy sedimentation on a shallow warm water carbonate ramp colonized by a diverse chlorozoan fauna the upper limestone formation galets is typified by coarse grained bioclastic sediments punctuated by numerous phosphate rich hardgrounds with evidence for high energy shallow marine conditions faunas were dominated by crinoids and bryozoans this unit represents high energy sedimentation on a cool shallow water carbonate ramp colonized by a brynoderm faunal association between both units a deeper marine outer shelf limestone formation shipshaw was developed in the lac saint jean area a transition from warm to cool water carbonate ramps occurred in latest caradoc times and is litho and biofacies wise similar to what is documented for lower caradocian limestones present farther south upwelling of nutrient rich cool bottom oceanic waters was a probable cause for this transition

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

isotopic study on dokriani bamak glacier central himalaya implications for climatic changes and ice dynamics measurements of natural and artificial radioisotopes 32 si 210 pb and 137 cs and oxygen isotopes 18 o have been carried out on surface snow and ice shallow snow pits and an ice core collected from dokriani bamak glacier central himalaya to study the dynamics of glacier ice and short term climatic changes based on the 32 si and 210 pb activities in the meltwaters the age of the snout ice is 400 years and the flow rate of ice along the glacier length is 14 m a 1 the specific activity of 137 cs corresponding to 1963 fallout in the surface ice at the equilibrium line yields a flow rate of 32 m a a factor of two higher than that derived for the snout ice the depth variation of 137 cs concentration in a shallow ice core yields a mean accumulation rate of 0 43 m a 1 for the glacier ice over the past decade the 18 o of snout ice 13 4 is significantly depleted compared to the average value of 9 2 in the shallow ice core indicating that cooler climatic conditions prevailed around ad 1600 based on the oxygen isotopic ratios in the shallow pits an altitude effect of 0 9 per 100 m in 18 o variation is documented for this glacier

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

oxygen and carbon isotopic composition of ordovician brachiopods implications for coeval seawater one hundred and two articulate brachiopods and twenty one marine cements from ordovician strata were analyzed for oxygen and carbon isotopes these together with 200 previously published analyses of ordovician brachiopods mostly of upper ordovician age define a general trend of increasing 18o values with decreasing age from a minimum of 10 5 in the earliest ordovician to a maximum of 1 5 at the ordovician silurian boundary the 13c values show a general positive correlation with 18o and increase from 2 5 in the earliest ordovician to as high as 7 at the ordovician silurian boundary the magnitudes of ordovician excursions for both 18o and 13c are similar to those reported previously for the entire phanerozoic different genera of brachiopods from the same stratigraphie level did not yield any large differences in their isotopic compositions suggesting that vital effects did not influence decisively the isotope values of the studied skeletal parts petrographic examination and trace element analyses indicate that brachiopod shells from llandeilo to ashgill strata are well preserved and thus may have retained their primary isotopic signatures the same criteria for the tremadoc arenig and some llanvirn brachiopods suggest partial diagenetic alteration of 18o values in these samples but this does not preclude their utility as a recorder of a primary 13c signal the observed 18o trend for the well preserved portion of the record may reflect progressive cooling during ordovician perhaps complemented by a changing 18o values of seawater the latter could have been a consequence of a net increase in the rate of high temperature water rock interaction relative to its low temperature counterpart as indicated also by the large coeval decrease of marine 87sr86sr superimposed on the above long term 18o trend is a positive excursion of about 2 at the end of the ordovician probably reflecting a large expansion of polar ice caps the magnitude of this excursion indicates a degree of glaciation comparable to that at the height of the quaternary glacial episodes the secular 13c trend may reflect a progressive increase in marine organic productivity and or enhanced organic deposition in the ordovician oceans particularly noticeable at the time of the terminal ordovician glaciation

co2 forced climate thresholds during the phanerozoic abstract the correspondence between atmospheric co2 concentrations and globally averaged surface temperatures in the recent past suggests that this coupling may be of great antiquity here i compare 490 published proxy records of co2 spanning the ordovician to neogene with records of global cool events to evaluate the strength of co2 temperature coupling over the phanerozoic last 542 my for periods with sufficient co2 coverage all cool events are associated with co2 levels below 1000 ppm a co2 threshold of below 500 ppm is suggested for the initiation of widespread continental glaciations although this threshold was likely higher during the paleozoic due to a lower solar luminosity at that time also based on data from the jurassic and cretaceous a co2 threshold of below 1000 ppm is proposed for the initiation of cool non glacial conditions a pervasive tight correlation between co2 and temperature is found both at coarse 10 my timescales and fine resolutions up to the temporal limits of the data set million year timescales indicating that co2 operating in combination with many other factors such as solar luminosity and paleogeography has imparted strong control over global temperatures for much of the phanerozoic

accumulation and flow rates of ice on chhota shigri glacier central himalaya using radio active and stable isotopes systematic isotopic studies based on natural and artificial radio isotopes 32 si 137 cs stable isotopes 18 o and total activity measurements have been carried out on chhota shigri glacier himachal pradesh central himalaya to study the dynamics of the ice meltwater composition and to identify the deposition of the chernobyl fall out in the himalayan region using 32 si concentrations the snout ice has been dated at 250 years based on which the past average surface ice flow rate has been estimated as 28 m year 1 based on 18 o variations in a shallow ice core the accumulation rate of the ice has been estimated at 520kgm 2 year 1 32 si measurements of snout ice and englacial meltwaters indicate that at least 55 of the snow meltwater mixed with 45 of the old ice melt water that emerged from englacial streams in the month of august 1987 deposition of the artificial radionuclide 137 cs and the very high total activity observed in snow samples on chhota shigri glacier give the first evidence of chernobyl fall out deposition in the indian himalaya

the impact of paleogeography pco2 poleward ocean heat transport and sea level change on global cooling during the late ordovician abstract we performed sensitivity experiments with the global climate model genesis on two stages of the upper ordovician caradocian 454 ma ashgillian 446 ma under a range of atmospheric p co 2 values 8 18 pal pre industrial atmospheric level high and low sea level and two values of poleward ocean heat transport in order to determine the importance of these variables on global cooling we then coupled a three dimensional ice sheet model to the global climate model in order to investigate the necessary boundary conditions for ice sheet formation all simulations with a high sea level and normal heat transport remain free of ice sheets even with p co 2 levels as low as 8 pal in the caradocian simulations ice sheets form in three scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport 2 with p co 2 of 8 pal and a high sea level and reduced 50 of normal poleward ocean heat transport and 3 with p co 2 of 15 pal and a low sea level and reduced poleward ocean heat transport in the ashgillian simulations ice sheets form in only two scenarios 1 with p co 2 of 8 pal and a low sea level and normal poleward ocean heat transport or 2 with p co 2 of 8 pal and a high sea level and reduced poleward ocean heat transport the ice sheets in the ashgillian experiments are larger and thicker than the ice sheets in the caradocian simulations because the southward movement of gondwana increased land area in the higher southern latitudes where ice sheets could grow the threshold for glaciation under ashgillian paleogeography is 8 pal and either a low sea level exposed shelves or a reduced poleward ocean heat transport while the paleogeographic evolution and a drop in p co 2 during the late ordovician cooled the global climate changes in additional factors were required to initiate ice sheet formation such as a drop in sea level a reduction in poleward ocean heat transport or a combination of both

bentonite geochronology marine geochemistry and the great ordovician biodiversification event gobe abstract attribution of ordovician climate forcing to explosive volcanism and the potential global importance of volcanism in ordovician biodiversification suggest the necessity of evaluating the relationships between k bentonite deposition and increasingly high resolution records of marine biogeochemical change globally ordovician strata preserve an extensive record of explosive volcanism including the widely recognized lower to middle ordovician famatina k bentonite suite in argentina and the upper ordovician millbrig deicke kinnekulle suite of north america and europe here we present high resolution id tims u pb zircon ages of k bentonites from measured sections of the san juan formation talacasto and cerro la chilca section of the argentine precordillera k bentonites from the argentine precordillera provide stratigraphically consistent i e younging upward ages that range from 473 45 0 70 ma to 469 53 0 62 ma and constrain the age of a low magnitude 2 globally recorded negative carbon isotope excursion evaluation of the timing of k bentonite deposition in the argentina precordillera relative to marine biostratigraphic and biogeochemical records provides insight into relationships between explosive volcanism and regional to global environmental change from a regional standpoint these ages provide critical direct evidence for a dapingian to earliest darriwilian age of the upper san juan formation at sampled localities these ages are consistent with carbon isotope data suggesting that the san juan formation in the region of its type section is coeval with only the base of the often correlated table head group of western newfoundland this data thus highlights the difficulties in using regional biostratigraphic data particularly within erosionally truncated or otherwise diachronous units to define the timeframe of carbon isotope chemostratigraphy new geochronological data also indicate that a discrete negative carbon isotope excursion within the san juan and table head formations is correlative to a globally recognized pre mdice negative excursion and indicates that this aspect of the marine carbon isotope record can be used as a discrete chronologic marker san juan formation bentonites however cannot be discretely correlated with observed environmentally significant changes in the middle ordovician marine geochemical records of carbon sulfur strontium or sea surface temperature these results suggest that the extent of volcanism represented by the famatina bentonite suite was insufficient to affect global surface environments and that the relationship between explosive volcanism and environmental change may not be straightforward as previously suggested

toward reconciliation of late ordovician 440 ma glaciation with very high co2 levels although phanerozoic glaciations usually coincided with times of estimated low atmospheric co2 the late ordovician 440 ma glaciation is a significant exception co2 levels during that time may have been as much as 10 times greater than present in an earlier paper we suggested that the unique geographic configuration of gondwanaland may explain such a response as the edge of the supercontinent was essentially tangent to the south pole and the moderating effect of the nearby ocean may have suppressed the magnitude of summer warming on the landmass thereby allowing glacial inception one limitation to the earlier study was that it used a linear energy balance model ebm in this paper we further test the above hypothesis in a suite of experiments with a nonlinear ebm that allows for snow albedo feedback we also utilize updated estimates for co2 levels decreases in solar luminosity and variations in orbital forcing baseline experiments with no changes in luminosity or co2 resulted in an ice covered area of 6 3 106 km2 53 of the estimated area covered by the ordovician ice sheet additional experiments for different combinations of orbital forcing 7x 13x co2 and 3 5 5 0 luminosity yielded 0 35 of the estimated ice area in the late ordovician a crude estimate of possible topographic influences increased these numbers to7 47 of total estimated ice area additional factors related to ice sheet growth should increase these values somewhat these results provide additional support for the high co2 glaciation explanation with the caveat that even the partial success occurs only when parameters are at the extreme end of their permissible range the estimated duration of ordovician glaciation is also consistent with the migration of gondwanaland across the south pole with a centrally located pole yielding ice free conditions in the summer thus identical levels of external forcing yield either glaciated or ice free conditions with the solution dependent on location of the landmass although more work is required on this topic our experiments suggest that there may be a relatively parsimonious explanation for this perplexing paleoclimate paradox the results lend further support to the proposition that paleogeography significantly modifies the role of co2 in the long term evolution of climate

geosecs pacific and indian ocean 32si profiles abstract results of measurements of twelve 32 si vertical profiles nine from the pacific ocean at latitudes 45 n 58 s and three from the indian ocean between the equator and 38 s are presented the amounts of in situ extracted sio 2 range from 1 to 25 g the volumes of water from which dissolved silicon was extracted range from 200 to 9 10 5 kg the net 32 p activities range from 0 7 to 3 8 cph it is possible to measure accurately 32 si 32 p activities as low as 2 10 2 dpm from 25 g sio 2 with the present techniques the 32 si concentrations in water range from 0 1 dpm 10 6 kg seawater to 178 dpm 10 6 kg seawater the overall pattern of 32 si increase with depth in the oceans resembles that of si but the two differ appreciably the enrichment of the former is controlled by its relatively short half life the 32 si sio 2 ratios vary from 1 in deeper waters 1000 m to 81 5 dpm kg sio 2 in the surface waters three depth variation patterns are observed in the pacific and indian oceans i a low ratio 10 dpm kg sio 2 varying within a factor of two between the surface and 5000 m depth ii a monotonic increase from a high surface value 10 dpm kg sio 2 to a low value 5 dpm kg sio 2 at 3000 m beyond which the value either remains constant or increases slightly with depth and iii in half of the cases the ratios increase with depth in the depth interval 1 3 km the above patterns do not show any latitudinal variation the column inventories of 32 si and si in the north pacific show a rather similar latitudinal variation which is a coincidence the column inventories of 32 si show a pronounced peak at 35 n s in the pacific and indian oceans that must be attributed to an increased tropospheric fallout in the spring injections in mid latitudes this is also expected in view of the short half life of 32 si a similar feature was observed in the south atlantic ocean now that the 32 si measurements are available for the three major oceans the oceanic budget of 32 si can be estimated there are 920 10 14 dpm of 32 si in the major oceans which corresponds to a global average production rate of 4 3 10 4 atoms 32 si cm 2 s furthermore since the atmospheric fall out of 32 si has been determined we can obtain a fairly accurate half life of 32 si the geochemical half life is estimated to be 120 yrs in good agreement with the best value of 140 yrs based on other recent estimates the lower limit reflects on the present uncertainty in the size and lifetime of the transient silicon pool at the water sediment interface from the observed decrease in the water column averaged 32 si sio 2 ratios from the circumpolar atlantic waters to their pacific counterparts we deduce an apparent velocity of 55 km yr for the circumpolar current using a one dimensional model the vertical advection velocity w and turbulent diffusion coefficient k have been deduced using both the 32 si and 14 c data the values of w range from 1 3 to 15 m yr and of k from 0 5 to 8 cm 2 s except for the somali basin where we obtain w 53m yr and k 16cm 2 s the 32 si based values are about 3 times higher than those based on 14 c

late middle ordovician environmental change and extinction harbinger of the late ordovician or continuation of cambrian patterns positive excursions in carbon isotope compositions of carbonate 3 and organic carbon 4 6 from the late middle ordovician middle caradocian of the midcontinent and the eastern united states indicate widespread increases in productivity and rates of organic carbon burial that may have drawn down atmospheric p co 2 precipitating global cooling although not necessarily ice sheet formation these climatic changes were associated with regional orogenic uplift a relative rise in sea level changes in epeiric sea circulation patterns and carbonate platform destruction that led to regional extinction of marine benthos the combination of sea level rise changing ocean circulation and extinction in the middle caradocian is similar to the suite of environmental changes described for cambrian biomere boundaries suggesting shared causes for these events in contrast middle caradocian environmental changes are markedly different from the environmental patterns associated with the late ordovician mass extinction despite the evidence for long term cooling from the middle to the late ordovician

preliminary investigations of 18 o 16 o and d h compositions in rhyo ignimbrites in the in hihaou in zize magmatic center central ahaggar algeria the 620 m y old in hihaou in zize magmatic complex located at the north western boundary of the archaean in ouzzal block western ahaggar is composed of massive alkaline rhyo ignimbrites and rhyolitic domes which are intruded by a granophyric and granitic body the whole is preserved in a cauldron structure extrusive rocks are strongly 18o depleted with values as low as 1 5 smow while granophyres are less depleted minimum 18o value 2 0 smow the granite has values around 6 smow d h compositions are rather low with d 90 to 110 smow isotopic zoning of quartz phenocrysts 18o 16o fractionation among coexisting phases and heterogeneity of the whole rock 18o values suggest that the volcanic rocks have interacted with meteoric water after the eruption several mechanisms of isotopic alteration are discussed the hydrothermal alteration does not seem to have been controlled by the granitic intrusion but rather seems to have followed the deposition of thick pyroclastic deposits on permeable arkosic sandstones and fluvio glacial conglomerates pervasive circulation of water through the cooling volcanic deposits could have produced the observed 18o depletion

macroevolutionary transition in crinoids following the late ordovician extinction event ordovician to early silurian abstract the end katian late ordovician crinoid mass extinction triggered the change from the early to the middle paleozoic crinoid evolutionary faunas cefs this was a change from diplobathrid camerate disparid hybocrinid dominated faunas to faunas dominated by monobathrid camerate cladid and flexible crinoids all clades suffered extinctions at the end katian event but diplobathrid camerates disparids and hybocrinids suffered higher rates of extinction the primary amount of diversification occurred in clades that would become dominant during the silurian however the formation of the characteristic middle paleozoic cef was protracted beyond the late ordovician extinction event monobathrid camerates and flexibles diversified through the llandovery but both dendrocrinid and cyathocrinid cladids did not diversify until later monobathrid camerate genera and families diversified the flexible diversification was largely at the genus level cyathocrine diversification was largely among families and dendrocrinids did not diversify significantly until after the llandovery overall disparity decreased during the end katian extinction by reducing the disparity within each clade disparity remained fairly constant during the hirnantian but increased significantly during the llandovery by both increasing disparity within clades and expanding the morphospace of the disparids due to the radiation of families with new morphologies north america was the biogeographic center of origination for the families that survived to become dominant silurian clades

the great ordovician biodiversification event gobe the palaeoecological dimension abstract the great ordovician biodiversification event gobe saw a spectacular increase in marine biodiversity at all taxonomic levels largely within the phyla established much earlier during the so called cambrian explosion the diversification was probably the result of a combination of several geological and biological processes and the positive feedbacks resulting from them the present paper reviews the palaeoecological dimension of the gobe it involved major increases in s and biodiversity largely associated with the rise of the paleozoic evolutionary fauna dominated by suspension feeders and involving a greater occupation of ecospace and more complex ecological structures in the ecological evolutionary units p1 and p2 in the benthos these include more complex food webs than those of the cambrian greater tiering especially above the sediment water interface and the development of guild structures indicating increased competition between taxa for particular resources the ordovician is characterized by a profound change in reef composition with a switch from microbial dominated reefs in the early and middle ordovician to metazoan dominated reefs in the late ordovician increases in complexity of deep water trace fossil assemblages began in the early ordovician and mark the increasing exploitation in that environment and the development of hardgrounds permitted bioerosion and encrusting strategies together with the appearance of cryptic communities within the water column the gobe involved a significant increase in the diversity of the phytoplankton and the development of a diverse zooplankton including planktotrophic larvae from a range of invertebrate clades this revolution in the plankton enabled the establishment of a diverse fauna of pelagic vertebrates molluscs and arthropods and promoted the rise of suspension feeders in the benthos an escalation amongst predators and thus community evolution may also have been a major driver of biodiversification

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

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

radiometric chronology of neh nar glacier kashmir surface and core samples of neh nar glacier in the kashmir valley have been analysed for the radionuclides 32si 2lopb 40k and 137cs the lateral and vertical profiles at an altitude of about 4 140 m reveal i 32si activity decreasing slowly from the accumulation zone to 4 050 m altitude and then abruptly towards the snout 2 five zones of alternating high and low 2lopb activity in the surface samples 3 an horizon at between 2 and 3 m depth containing 210pb activity above natural levels this horizon is also associated with 1l7cs and a maximum in total activity the ice sam les have been dated on the basis of a simplified two component model the fresh contribution determined by 21opb and the old component by 32si the following conclusions can be drawn from these observations i the model age of the snout ice is c 850 years 2 the average rate of ice movement in the lower glacier is about 2 m year which compares well with the annual movement rate of 2 65 m year observed since 1974

comparative lower middle ordovician conodont oxygen isotope palaeothermometry of the argentine precordillera and laurentian margins abstract stratigraphic palaeontologic and palaeomagnetic data support a hypothesis that argues for the argentine precordillera rifting from the southwestern margin of laurentia in low latitudes during the cambrian migrating across the iapetus ocean colliding with the gondwanan margin in the late middle ordovician and receiving glaciogenic sediments in the late ordovician an alternative model proposes that the precordillera originated as a low latitude segment of gondwana migrated southward through major transform faulting toward high latitudes in the late middle ordovician to reach its present position in the devonian new conodont oxygen isotope compositions 18ophos have been determined by ion microprobe shrimp ii using samples from both the precordillera and laurentia marathon area of texas wilcox pass in alberta and western newfoundland significantly the 18o values of conodonts from all four widely separated areas show a consistent pattern of a cyclic but overall increasing trend in 18o ca 16 to 18 hence ocean cooling through the early and middle ordovician an apparent change occurs at the basal late ordovician where 18o values obtained from conodonts in the uppermost sample from the precordillera are significantly higher 1 5 than those from laurentia albeit from a single sample this higher value implies significantly cooler conditions as would be anticipated with a southerly poleward migration of the precordillera irrespective of either hypothesis the virtual absence of conodont bearing carbonates in most of the precordilleran upper ordovician precluded analysis of younger samples when combined with existing macrofaunal and palaeomagnetic data the oxygen isotope data would tend to favour the model of a drift of the precordillera from tropical to higher latitudes during the ordovician however further studies are needed to determine unequivocally whether the precordillera originated from southern laurentia ouachita embayment these new oxygen isotope values provide the best and regionally most consistent data through the early middle ordovician

oxygen isotopic composition of alkaline anorogenic granites as a clue to their origin the problem of crustal oxygen abstract the primary isotopic characteristics of alkaline granites are often obscured by secondary processes enhanced by their unusual chemical compositions this is true for radiogenic as well as for stable isotopes for example the 87 sr 86 sr ratios can vary drastically in closed systems because of very high rb sr ratios and can also be easily modified by direct or indirect interaction with continental crust because of low sr concentrations moreover the frequent occurrence of the granitic massifs as hypovolcanic complexes increases the probability of interaction with meteoric waters which is a common source of important isotopic variability the investigation of oxygen isotope systematics in alkaline acidic rocks from various environments shows the 18 o content of their quartz to be highly invariable and the 18 o values to be close to the mantle range of values this is due to the resistance of quartz to isotopic exchange which makes it a good tracer of primary isotopic composition if we eliminate the quartz 18 o values for which interaction with meteoric water is well documented five samples the total range of variation seventeen samples is from 6 0 to 7 3 relative to smow the values can easily be accounted for by and correspond to equilibrium with mantle type material in a temperature range of 1200 800 c if we consider possible effects of fractional crystallization this temperature range can probably be reduced to its lower limit which is much more likely for rocks of acidic composition the present oxygen isotope study strongly supports an origin for alkaline anorogenic granites from mantle dominated sources

changes in marine productivity and redox conditions during the late ordovician hirnantian glaciation abstract changes in marine productivity and redox conditions during the end ordovician hirnantian glaciation and ordovician silurian transition were investigated through mo isotope and major and trace element analyses of the wangjiawan hubei province and nanbazi guizhou province sections from the yangtze platform of south china katian shales of the wufeng formation which yield the graptolites dicellograptus complanatus dicellograptus complexus and paraorthograptus pacificus were deposited under euxinic conditions at both localities as shown by high mo ef u ef and 98 mo values a major sea level regression during the hirnantian glaciation resulted in shallowing and a shift toward better oxygenated conditions within the yangtze sea as well as deposition of thin bedded siliceous sediments calcirudite debris flows and limestone turbidites of the kuanyinchiao formation the base of which correlates with the first phase of the end ordovician mass extinction the termination of the hirnantian glaciation at the top of the kuanyinchiao formation was associated with a major sea level transgression a rapid expansion of euxinia in the yangtze sea as documented by a return of high mo ef u ef and 98 mo values and the second phase of the mass extinction during which the cool adapted hirnantian fauna went extinct the long term cooling trend of the middle and late ordovician which culminated in the hirnantian glaciation was driven by enhanced burial of organic carbon as documented by 13 c carb 13 c org and 34 s py records increased organic carbon burial was linked to high rates of marine productivity as shown by high toc and biogenic ba concentrations especially at the deeper less restricted wangjiawan locale producing the hice hirnantian isotopic curve excursion 13 c carb excursion the locus of organic carbon burial during the hirnantian crisis shifted to deeper water environments that were located outside the study region the relatively rapid onset and termination of the hirnantian glaciation were probably due to crossing of tipping points in the late ordovician climatic oceanic system

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

late ordovician cooling event evidence from the siberian craton abstract the upper ordovician of the tungus basin on the siberian platform is represented by cool water carbonates onset of cool water conditions starts at the darriwilian and it is associated with widespread phosphatization especially in the volginian kirensko kudrinian and chertovskian regional stages mid darriwilian lower sandbian the shift from tropical type to temperate type carbonates was preceded by destruction of the warm water carbonate factory and a large input of siliciclastic material baykit sandstone during the vikhorevian and mukteian regional stages lower to mid darriwilian wide distribution of temperate type carbonates across the siberian platform located in the low latitudes in the ordovician time can be explained by upwelling of cold oceanic waters and their penetration into epicontinental seas the same situation was recorded for the upper ordovician of the north american platform newly discovered k bentonite beds in the uppermost sandbian katian of the tungus basin are also associated with cool water carbonates the k bentonite beds point to extensive volcanism on or near the western in present day orientation margin of the siberian craton in the late ordovician time timing of the volcanism is surprisingly close to the period of volcanic activity of the taconic arc near the margin of laurentia both upwelling and volcanism seem to be activated by the same plate tectonic reorganization and both these processes seem to play an important role in a global later ordovician cooling terminated by the hirnantian glaciation

radiometric chronology of changme khangpu glacier sikkim the 32 si concentration in a sample of surface ice from the snout of changme khangpu glacier is 0 36 disintegrations per minute tonne compared to the fall out value of 0 7 d p m tonne if this decrease is assumed to be solely due to decay of 32 si an age off c 100 years is estimated for the surface ice of the snout leading to an average flow velocity of c 40 m year for the past century a vertical profile of 210 pb in a core taken at an altitude of 5040 m shows two horizons where this isotope is enriched one between 3 and 4 m and another between 11 and 12 m indicating that the primary concentration of 210 pb can change by physico chemical processes like adsorption on dust none the less a longitudinal profile along the glacier shows a systematic decrease of 210 pb activity with decreasing altitude the surface ice of the snout giving a value of 0 2 d p m l corresponding to an age of 100 years which is concordant with the 32 si age this surface flow rate of the glacier is much larger than the average contemporary flow rate c 13m year the difference can be understood in terms of the past history of advance and recession of the glacier as revealed by the geomorphic evidence

mass balance of glaciers other than the ice sheets small glaciers appear to have been at equilibrium or shrinking very slightly during 1961 90 according to analysis of an essentially compl ete set of published measurements simple calculations give an average annual mass balance of 195 59 mm a 1 water equivalent but this is too low because of systema tic errors eglect of internal accumulation is respon sible for some tens of millime ters of under esti mate uneven spatial coverage with fewer measurements where mass balances are less negative accounts for about 50 mm a 1 of underestimate this figure derives from spatial interpolati on based on global data on ice extent and on an analysis of correlations between balance time series the eorrelogram shows exponential decay the scale length being about 600 km the largest bias is due to a newly detec ted dependence of mass balance on glacier size among the 231 measured glaciers many arc small and belong to a restricted size range in which balance is negativ e but much of the small glacier extent is accounted for by larger glac iers in a size range where balance is indisti ngui shable from zero correct ing for this size bias increases the average balance to 35 89 mm a i inspection of time series for 1940 95 251 glaciers shows that mass balance was least negative during the 1960s and has varied in broad agreementvith northern hemisphere temperature anomalies smaller small glaciers a 16 km2 appear to be more sensitive than larger small glaciers to changes in thermal forci ng the small glaci er contribution to sea level rise implied by this asse ssment is only 0 06 0 32 mm a i consistent with glaciers in general mak i ng little or no contri bution to sea level change during 1961 90

nd and pb isotope evidence from the seychelles granites and their xenoliths mantle origin with slight upper crust interaction for alkaline anorogenic complexes abstract due to different initial pb incorporation the pb pb whole rock data of the seychelles anorogenic complex of late precambrian age form linear arrays on 24 samples with no time significance the existence of two groups of rocks in this complex with different initial pb and nd isotopic characteristics has been evidenced 1 1 the mahe group comprising the biotite hornblende granite the weakly alkaline granite and their melanocratic dioritic to gabbroic xenoliths yields nd be 705 ma 3 1 their pb isotopic compositions in k feldspar are only slightly more radiogenic than those of mantle derived rocks of the same age 2 2 the praslin la digue felicite group comprising the biotite granite and its leucocratic xenoliths yields nd be 705 ma 1 9 k feldspar pb ratios are significantly more radiogenic than those of mahe rocks thus the parental basic magma of this alkaline anorogenic complex appears to originate from a slightly depleted mantle source underlying the african continental crust this magma has been contaminated by the high level part of this crust to different degrees in the two groups the praslin one having been more affected particularly by late magmatic fluids

global carbon isotopic events associated with mass extinction and glaciation in the late ordovician mass extinctions and glacioeustatic sea level changes in the lower part of the hirnantian final stage of the ashgill are accompanied by shifts in marine stable isotope compositions previously published stable isotope changes have been used to identify the onset and demise of the gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans however the existing isotopic data set had limitations because it was derived from ordovician low latitude settings and from carbonates or organic carbon in separate areas we report new data from ordovician high latitude carbonates and demonstrate parallel shifts in organic and carbonate 13c from baltica brachiopod shells from a high palaeolatitude periglacial setting in argentina have elevated 13c values similar to those described previously from low latitude sites the new data demonstrate that the positive hirnantian 13c excursion previously only recognised from low palaeolatitude areas was widespread and probably global in extent the poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material preliminary carbon isotopic data from thermally immature organic matter from estonia provide the first indication of a synchronous shift in organic and inorganic 13c in sediments from the same basin this work provides new data of critical importance for constraining models of end ordovician palaeoceanography and climate change

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

enclaves from the a type granite of the megantic complex white mountain magma series clues to granite magmagenesis the subsolvus biotite hornblende granite at megantic contains mafic to leucocratic enclaves the groundmass of leucocratic enclaves is microgranitic that of mafic enclaves is meladioritic apatite is commonly acicular in the enclaves suggesting quenching of mafic magmas commingled with the granitic host mafic enclaves contain mafic clots hornblende cores biotite rims some with relic cores of vial cr rich diopsidic augite similar to phenocrysts in associated basaltic dykes the clots are interpreted to be pseudomorphs after mafic phenocrysts the enclaves also contain granitic microxenoliths xenocrysts of quartz with hornblende reaction rims partially resorbed oligoclase and perthite megacrysts and biotite xenocrysts the xenocrysts record hybridization between a mafic enclave magma and a granitic magma and or rock enclaves define mixing lines on many variation diagrams however rb sr ba zr nb y ree zn pb and f do not fit this pattern and appear to have been redistributed by residual granitic melts or exsolved volatiles the data suggest that the granite is anatectic heat being provided by crystallization of associated basalts mixing calculations suggest that as little as 5 hybridization with basalt or hawaiite can account for the sr isotopic heterogeneity of the granites

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

the timedjealen alkaline ring complex and related n 1bs dyke swarms adrar des iforas mali a pb 1bsr 1bo isotopic study abstract the iforas alkaline province is composed of spectacular n s trending acid dyke swarms plateaux of ignimbritic and rhyolitic lavas and silica oversaturated ring complexes this province was emplaced just after the pan african collision which occurred between the tuareg shield and the west african craton 600 ma ago it is devoid of sn mineralization in comparison with similar paleozoic mesozoic complexes in niger and nigeria the pb 1bsr 1bo data on the timedjelalen ring complex and on contemporaneous n s dykes demonstrate the cogenetic mantle source for their parental magma with a contaimination by the lower continental crust from 10 to 50 on the basis of pb isotopes there is no evidence for incorporation of old upper continental crust these alkaline rocks show little alteration except for hydrothermal activity at ring or dyke country rock contacts as shown by lower 18o values the hydrothermal water involved appears isotopically heavy in the range 0 to 3 and may have originated in a residual sea or lake lack of mineralization can be explained by the fact that the primary source is economically barren mantle and lower crust and that the interaction with country rocks during the intrusion process was weak and mainly with barren mantle derived pan african rocks

organic carbon deposition and coastal upwelling at mid latitude during the upper ordovician late katian a case study from the welsh basin uk a lack of stratigraphical sedimentological and geochemical data for sediment accumulation rates and indicators of productivity and anoxia means that causative models for ancient black shales are largely inferred from modern settings coastal upwelling has been suggested as a general hypothesis for ordovician black shale deposition within the iapetus ocean but has not been directly tested despite anchizone metamorphism we utilize a suite of geological and geochemical environmental proxy data toc wt 13corg ba al p to elucidate the mechanism for the origin of a single grey black shale cycle within the upper katian succession of the welsh basin here we interpret organic carbon oc rich deposition to be suggestive of a period of high photic productivity higher toc wt with 12c and ba enrichment comparable to high productivity events in modern coastal upwelling systems productivity proxy values are consistent with those from margins where sedimentation rates are high e g gulf of california inter bedded grey shales have low toc wt more positive 13corg and marginally lower ba and are interpreted as low productivity events thalassinoides planolites and chondrites ichnofacies indicate changing seafloor oxygen levels these show predominantly dysoxic conditions even during the deposition of oc poor grey shales during an oc rich laminated hemipelagite event oxygen levels declined at the sea floor before the deposition of the oc rich layers full anoxia was established early on in the deposition of oc rich layers and the return to more oxic conditions was rapid though fluctuating and coincident with the return of grey shale deposition this pattern suggests oc accumulation at the seafloor resulted from a complex interaction of productivity and preservation none of the widely used trace element redox proxies are reliable in anchizone metamorphic rocks climate sensitive detrital proxies k al and ti al indicate arid temperate conditions in the basin hinterland during the deposition of the oc rich layers the welsh basin was situated on the southern margin of the iapetus ocean in 30 s beneath the prevailing se trade winds we interpret the occurrence of oc rich laminated hemipelagite events to represent the intensification of upwelling prior to the hirnantian glaciation possibly having resulted from a strengthening of the trade winds associated with stepped changes in ice volume and a more arid local climate

the half life of 32si a new estimate based on varved lake sediments abstract cosmic ray produced 32 si has found several applications as a tracer in aqueous geochemical processes these applications require precise knowledge of the half life of 32 si the reported values show a wide range that varies from 101 18 to 330 40 years thus leaving the question of the half life of 32 si still wide open in this study we have determined the half life of 32 si to be 178 10 years by measuring the decrease of its activity with depth in an accurately dated varved sediment core from the kassjon lake north sweden this result reinforces the use of 32 si as a geochronometer to study earth surface processes that have occurred during the last millennium

a guide to phanerozoic cold polar climates from high latitude ice rafting in the cretaceous the high latitude extent of warm climate indicators at certain times in earth history has been considered as evidence that the globe was ice free for long intervals despite theoretical considerations and results from numerical modelling experiments1 2 indicating that this was unlikely one of the warmest periods the cretaceous displays faunal and floral evidence for cool temperate to sub tropical conditions very near to the poles however our studies of lower cretaceous mudstones of central australia that contain outsized exotic blocks have led to the conclusion that the blocks were emplaced by ice rafting implying that high latitude ice was present at sea level strata of a similar origin of mid jurassic to mid cretaceous age occur on other continents that were positioned between 65 and 78 palaeolatitude indeed there is a record of high latitude ice rafting throughout the phanerozoic suggesting that ice was present on earth for much of its history and that ice free conditions could have been at most only episodic over the past 600 myr

high frequency eustatic sea level changes during the middle to early late ordovician of southern jordan indirect evidence for a darriwilian ice age in gondwana abstract the middle to early late ordovician hiswah and dubaydib formations in southern jordan provide a well constrained sedimentary record of 4th and 3rd order relative sea level change along the northern tectonically quiescent subpolar gondwana continental margin cyclic deposition occurred on a wave and storm dominated microtidal shelf dominated by shoreface and offshore transition zone facies cycles are commonly bounded by amalgamated bounding surfaces sb ts or sb mfs and are characterised by stacked transgressive systems tracts tsts the small facies dislocation across key sequence bounding surfaces indicates a 15 50 m change in relative sea level fourth order sequences are hypothesized to be paced by the long eccentricity 405 kyr cycle and consequently 3rd order sequences are calculated to be 1 2 m y and 2 4 m y long these correlate with the global eustatic sea level curve in comparison with mesozoic and cenozoic sequence stratigraphical data we conclude that the 1 2 m y cycles correspond with long obliquity cycles predominant in icehouse conditions and the 2 4 m y cycles with the long eccentricity cycle predominant during greenhouse conditions we propose a darriwilian ice age during which orbitally induced cold snaps caused the expansion and amalgamation of small medium scale ice sheets the reduced size of the correlative positive carbon isotope excursion compared to that in the hirnantian may indicate that the darriwilian ice sheets only reached the sea in a few places the absence of glacio terrestrial deposits of middle ordovician age is most likely due to subsequent glacial erosion during repeated ice advances up to and including those in the hirnantian the existence of darriwilian ice sheets is consistent with zooplankton province distributions that indicate a polar front at 40 s during the ice age it is likely that polar ice existed throughout the ordovician

zircon oxygen isotopic constraint on the sources of late mesozoic a type granites in eastern china abstract a type granites are usually derived from partial melting of the lower crust and or the upper mantle within extensional settings but their exact sources for specific plutons are required to unravel a systematic laser fluorination analysis of oxygen isotopes was carried out for five well documented late mesozoic a type granites in eastern china zircon from 41 samples was separated and 109 aliquots of fresh zircon were analyzed along with whole rock powder and quartz from the same samples hydrothermally altered whole rock samples show scattered 18 o values of 2 3 to 7 4 with a total variation up to 9 7 in contrast zircon 18 o values vary only from 3 1 to 5 4 with a range of 2 3 most available zircon 18 o values of a type granites are lower than normal mantle zircon values of 5 3 0 6 2sd and high temperature equilibrium fractionations are well preserved between quartz and zircon in most cases the above lines of evidence suggest that the low 18 o zircons in a type granites were crystallized from low 18 o magmas because low 18 o magmas cannot be derived entirely from the mantle reworking and or recycling of crustal igneous rocks are involved for their petrogenesis the low 18 o zircons in this study thus offer evidence for a dominantly crustal origin of the late mesozoic a type granites in eastern china

did the taconic appalachians continue into southern south america the appalachian mountains now terminating abruptly at the gulf of mexico coastal plain may have formerly continued into southern south america rocks forming the basement of the argentine andes can be interpreted as remnants of an early paleozoic orogen the famatinian belt not unlike the taconic appalachians both orogens are bordered to the west present coordinates by lower paleozoic carbonate platforms bearing the olenellid trilobite fauna that is characteristic of laurentia paleomagnetic and geologic data indicate that they could have formed as one continuous mountain chain possibly extending into antarctica during ordovician closure of an ocean basin southern iapetus between laurentia and gondwana the taconic and famatinian segments of the chain may have been truncated during late ordovician separation of laurentia and gondwana along the preexisting late neoproterozoic to cambrian rift system that initiated formation of the ouachita embayment and the southern margin of north america

ordovician to devonian sea level changes in western and northern canada lower paleozoic sediments of central cordilleran and arctic canada are dominated by two megafacies supratidal to shallow subtidal carbonates and minor evaporites of the platformal cratonic are

polar front shift and atmospheric co2 during the glacial maximum of the early paleozoic icehouse our new data address the paradox of late ordovician glaciation under supposedly high pco2 8 to 22 pal preindustrial atmospheric level the paleobiogeographical distribution of chitinozoan mixed layer marine zooplankton biotopes for the hirnantian glacial maximum 440 ma are reconstructed and compared to those from the sandbian 460 ma they demonstrate a steeper latitudinal temperature gradient and an equatorwards shift of the polar front through time from 55 70 s to 40 s these changes are comparable to those during pleistocene interglacial glacial cycles in comparison with the pleistocene we hypothesize a significant decline in mean global temperature from the sandbian to hirnantian proportional with a fall in pco2 from a modeled sandbian level of 8 pal to 5 pal during the hirnantian our data suggest that a compression of midlatitudinal biotopes and ecospace in response to the developing glaciation was a likely cause of the end ordovician mass extinction

pan african volcano sedimentary formations in the adrar des iforas mali abstract late syn and post orogenic sedimentary rocks of vendian and cambrian age of the adrar des iforas are examined in an attempt to reconstruct paleogeographic evolution during the late pan african orogeny the distribution of sedimentary facies from west to cast involving deep trough island arc marginal cordillera shallow sea and continental sedimentation is consistent with the model of oceanic closure and collision between the west african craton and the tuareg shield there appears to be a continuum between flysch type deposits and molassic rocks located along shear belts in the highly mobile western margin of the tuareg shield but pre ordovician red beds in the central and western parts of the tuareg shield may be slightly younger

oxygen isotope geochemistry of the mesozoic anorogenic complexes of damaraland northwest namibia evidence for crustal contamination and its effect on silica saturation fifty six new oxygen isotope analyses of minerals separated from nine mesozoic anorogenic complexes of damaraland in northwest namibia have been used to estimate the 18o values of the original magmas magma these complexes range in composition from nepheline syenite to alkaline and peraluminous granites often with a variety of rock types present at a single centre the silica undersaturated rock types show a relatively small spread of values of magma from 6 0 to 7 0 mean 6 6 which is consistent with their derivation from the mantle with little or no subsequent crustal contamination the silica oversaturated rocks show a wide range of magma values from 4 9 to 12 0 with a mean value of 8 3 the high values of magma in the silica oversaturated rocks can only be explained with considerable involvement of the continental crust in their petrogenesis and those rocks with magma 10 are interpreted as essentially crustal melts it is generally accepted that the damaraland complexes were generated as a result of rifting across the tristan plume with the plume providing both mantle derived magma and with it the heat required for crustal melting in addition to their mantle like oxygen isotope ratios the undersaturated rocks of the damaraland complexes possess the trace element characteristics e g low zr nb ratios of ocean island basalt suggesting that their parental magmas were produced from the plume itself in contrast the oversaturated complexes generally have higher zr nb ratios that are consistent with a larger crustal input the highest values of magma in the damaraland complexes are found in granitic rocks that intrude the central zone of the pan african damara orogen where presumably there is a substantial component of sedimentary origin in the lower to middle crust

generation of high silica rhyolite a nd sr and o isotopic study of sierra la primavera mexican neovolcanic belt high silica rhyolites of the sierra la primavera a late pleistocene center near guadalajara are extremely sr poor 0 3 1 3 ppm yet with one exception values of 87sr 86sri are relatively low at 0 7041 0 7048 values of 143nd 144nd for all the rhyolites are within errors identical to a basalt at 0 5129 these surprisingly primitive values along with feldspar 18o of 6 6 are consistent with an origin by fractional crystallization of mantle derived basalt however absence of the large volume of associated intermediate rocks that would be expected if the 40 km3 of erupted rhyolite were produced mainly by fractional crystallization suggests alternative processes involving partial melting of mesozoic or tertiary mafic intrusive rocks or lower crustal metamorphic equivalents the latter interpretation is preferred especially in light of comparative data for other north american cenozoic high silica rhyolites isotopic compositions correlate with basement age but generally lie between values for associated basalts and the underlying crust nearly all can be interpreted as containing both a young mantle derived component and a crustal component probably derived by partial melting at intermediate to deep levels of the crust no matter what the proportions of mantle and crust derived material in parental magmas the extremely low concentrations of sr and ba in the high silica rhyolites require extensive fractional crystallization of feldspar rich assemblages after parental liquids attain rhyolitic compositions

a late ordovician high energy temperate water carbonate ramp southern quebec canada implications for late ordovician oceanography the trenton group late ordovician the youngest carbonate unit in the taconic foreland basin of southern quebec is a tripartite unit with a distinctive coarse grained middle part the deschambault formation lithofacies of the deschambault formation are dominated by coarse grained bioclastic intraclastic limestones finer grained lithofacies are ubiquitous but subordinate the complete spectrum of lithofacies indicates sedimentation ranging from above fairweather to below storm wave base skeletal components are indicative of the modern temperate water bryomol association non skeletal elements are represented by peloids and intraclasts accretion rates from areas of continuous sedimentation were low 14 cm 103 years from sedimentological and faunal evidence it is proposed that the late ordovician deschambault ramp was bathed by temperate waters the model compares favourably with modern cool water shelves rimming the southern edge of the australian continent palaeomagnetic data locate southern quebec in a low latitudinal setting during the late ordovician upper ordovician facies distribution in eastern canada and progressive disappearance of some faunal provinces through late ordovician time are used to conclude that the initiation of the late ordovician glaciation that covered most of gondwana was instrumental in easing northward movement of cold oceanic currents this resulted in the rapid contraction of the southern hemisphere warm water tropical belt from a 30 latitudinal wide zone in the early caradoc to a 15 zone in the late caradoc

an atmospheric pco2 threshold for glaciation in the late ordovician the late ordovician glaciation differs from other major phanerozoic glaciations because of its short duration 1 m y a satisfactory explanation for this glaciation must be able to account for its sudden initiation short duration and rapid termination mechanisms operating on tectonic time scales appear to be precluded given recent studies of a major perturbation in the carbon cycle during the glaciation a climate model investigation of the sensitivity of the late ordovician climate to changes in atmospheric p co2 was undertaken under the condition of a 4 5 reduction in solar luminosity permanent snow cover taken as a key indicator of potential for glaciation is dramatically different between five experiments the range of 18x present atmospheric level co2 ice free to 8x runaway icehouse lies within the uncertainty of previous geochemical estimates of late ordovician atmospheric p co2 the strong sensitivity to the modest direct forcing from p co2 changes is due to the ice albedo feedback a plausible increase in organic carbon burial could have drawn down enough atmospheric co2 to have briefly lowered the climate system below a critical glacial inception threshold at the end of the ordovician conversely a high p co2 is required for the rest of the early paleozoic which was essentially ice free

a sulfidic driver for the end ordovician mass extinction the end ordovician extinction consisted of two discrete pulses both linked in various ways to glaciation at the south pole the first phase starting just below the normalograptus extraordinarius zone particularly affected nektonic and planktonic species while the second pulse associated with the normalograptus persculptus zone was less selective glacially induced cooling and oxygenation are two of many suggested kill mechanisms for the end ordovician extinction but a general consensus is lacking we have used geochemical redox indicators such as iron speciation molybdenum concentrations pyrite framboid size distribution and sulfur isotopes to analyze the geochemistry in three key hirnantian sections these indicators reveal that reducing conditions were occasionally present at all three sites before the first pulse of the end ordovician extinction and that these conditions expanded towards the second pulse even though the n extraordinarius zone appears to have been a time of oxygenated deposition pyrite is significantly enriched in 34s in our sections as well as in sections reported from south china this suggests a widespread reduction in marine sulfate concentrations which we attribute to an increase in pyrite burial during the early hirnantian the s isotope excursion coincides with a major positive carbon isotope excursion indicating elevated rates of organic carbon burial as well we argue that euxinic conditions prevailed and intensified in the early hirnantian oceans and that a concomitant global sea level lowering pushed the chemocline deeper than the depositional setting of our sites in the n persculptus zone an interval associated with a major sea level rise our redox indicators suggests that euxinic conditions and ferruginous in some places encroached onto the continental shelves in our model the expansion of euxinic conditions during the n extraordinarius zone was generated by a reorganization of nutrient cycling during sea level fall and we argue overall that these dynamics in ocean chemistry played an important role for the end ordovician mass extinction during the first pulse of the extinction euxinia and a steepened oxygen gradient in the water column caused habitat loss for deep water benthic and nektonic organisms during the second pulse the transgression of anoxic water onto the continental shelves caused extinction in shallower habitats

ordovician and silurian sea water chemistry sea level and climate a synopsis following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content less

pre quaternary sea level changes while the paramount importance of global sea level changes has been widely accepted by quaternary geologists throughout this century it is only within the last decade that such changes in the rest of the phanerozoic have received much attention there are two principal reasons for this arousal of interest first increased exploration of the stratigraphic record across the earth both on land and under the sea and significant improvement in biostratigraphic correlation have drawn attention to what appear to have been globally synchronous events second oceanographic research has demonstrated the importance of tectonically controlled changes in the cubic capacity of the ocean basins notably variations in ridge volume which provide a ready mechanism for sea level changes even at nonglacial times the adjective eustatic was first proposed for global changes of sea level by suess 1906 in his great treatise the face of the earth the corresponding noun is eustasy not eustacy as it is commonly misspelt the unconvinced reader need only think of ecstasy eustatic sea level changes can be inferred as suess recognized by a plotting the temporal spread of marine sediments over the continents and b estimating the depositional depth variations in marine stratal sequences which can be correlated across and between continents to these methods can be added seismic stratigraphy in which biostratigraphically age determined seismic sequences on the con tinental margins are analyzed in terms of cycles of coastal onlap and offiap when the results of these methods are broadly in accord confidence in a eustatic interpretation is increased but the range of confidence can vary considerably it is widely accepted that there was an exceptionally high sea

modeling the early paleozoic long term climatic trend the early paleozoic climate has been described as warm and equable however recent data based on conodont oxygen isotopic composition reveal a large long cooling trend through the ordovician followed by an abrupt cooling during the late ordovician glaciation this long term climate change is associated with a major radiation in the earth life history nonetheless the driving mechanisms for this cooling trend remain unknown carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change however the impact of the plate motion context has not been explored yet although it might have a major impact on atmospheric co 2 levels simulations with a climate model coupled to a biogeochemical model geoclim show that the atmospheric co 2 decreased from more than 20 pal 5600 ppmv in the furongian down to approximately 10 pal 2800 ppmv in the llandovery before rising again in the early devonian we suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large co 2 drawdown that led to the onset of cooler to glacial conditions from the middle ordovician to the llandovery the weathering of fresh volcanic rocks is itself responsible for 33 of the late ordovician atmospheric co 2 decrease the rest being related to the continent motion through the intertropical convergence zone itcz mean annual continental temperature falls by 3 c in the early ordovician reaching 13 5 c during the glacial interval and rises to 16 c in the early devonian

biotic response to a middle ordovician paleoceanographic event in eastern north america global paleontological events such as mass extinctions have been the focus of attention in recent years yet regional biotic events account for a greater proportion of extinction in the fossil record and should receive greater attention if we are to fully understand extinction processes over the phanerozoic we use a sequence stratigraphic framework to document a major extinction and migration episode that occurred in the middle and late ordovician of north america this event is characterized in eastern north america by coordinated changes in faunal assemblages carbonate lithologies siliciclastic influx and phosphatization we ascribe these changes to a regional paleoceanographic event brought on by thrust driven flexural subsidence and sea level fluctuations that lowered water temperature and increased turbidity and nutrient input

general circulation model sensitivity experiments with pole centered supercontinents although high co2 levels appear to be required to explain most times of ice free climates in earth history previous calculations with a two dimensional energy balance model ebm have suggested that the low thermal inertia associated with pole centered supercontinents might allow seasonally snow free conditions to develop in the absence of high co2 this conclusion has been challenged by other ebm calculations with lower geographic resolution but more explicit model physics to address this problem more directly we have undertaken a series of general circulation model gcm simulations with a new model genesis developed at the national center for atmospheric research for idealized pole centered supercontinents we obtain mid summer south polar temperatures of 22 c for the present orbital configuration and 15 c with a modified orbital configuration favorable to cool summers these results support the two dimensional ebm calculations however their application to real world situations of the paleozoic is hampered by the fact that due to extremely high sea levels and decreased solar luminosity the best time periods for applying this concept 420 360 ma have supercontinent areas too small to yield an ice free climate for all orbital configurations snow free conditions with summer temperatures at the pole of 25 c are simulated for a hot orbital configuration for one realistic geography configuration carboniferous 300 ma these results indicate that while the basic idea of snow free pole centered supercontinents is still theoretically possible the real world application to specific past time periods requires some modifications our reframed hypothesis states that for realistic geography and high co2 levels pole edged supercontinents could have permanent snow cover but pole centered supercontinents would be snow free in summer

glacial onset predated late ordovician climate cooling the ordovician glaciation represents the acme of one of only three major icehouse periods in earth s phanerozoic history and is notorious for setting the scene for one of the big five mass extinction events nevertheless the mechanisms that drove ice sheet growth remain poorly understood and the final extent of the ice sheet crudely constrained here using an earth system model with an innovative coupling method between ocean atmosphere and land ice accounting for climate and ice sheet feedback processes we report simulations portraying for the first time the detailed evolution of the ordovician ice sheet we show that the emergence of the ice sheet happened in two discrete phases in a counterintuitive sequence of events the continental ice sheet appeared suddenly in a warm climate only during the second act and set against a background of decreasing atmospheric co2 followed steeply dropping temperatures and extending sea ice the comparison with abundant sedimentological geochemical and micropaleontological data suggests that glacial onset may have occurred as early as the middle ordovician darriwilian in agreement with recent studies reporting third order glacioeustatic cycles during the same period the second step in ice sheet growth typified by a sudden drop in tropical sea surface temperatures by approximate to 8 degrees c and the further extension of a single continental scale ice sheet over gondwana marked the onset of the hirnantian glacial maximum by suggesting the presence of an ice sheet over gondwana throughout most of the middle and late ordovician our models embrace the emerging paradigm of an early paleozoic ice age

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

early silurian carbonate platform ostracods from iran a peri gondwanan fauna with strong laurentian affinities abstract early silurian aeronian carbonate platform deposits of the lower niur formation of east central iran yield an ostracod fauna of fourteen species dominated by podocopes with rare palaeocopes and binodicopes the podocope rich fauna is unusual in that early palaeozoic carbonate platforms are typically dominated by palaeocopes despite its mid palaeolatitude peri gondwanan setting and isolation from low palaeolatitudes the ostracod assemblage of the lower niur formation shows striking affinities to species level with late ordovician and early silurian ostracod faunas of palaeo equatorial laurentia biogeographically the lower niur formation ostracod fauna also shows species and genus links with late ordovician and early silurian faunas of low palaeolatitude 0 30 s baltica and mid palaeolatitude peri gondwanan uzbekistan dispelling the notion of strict endemism for early silurian ostracods two new species bulbosclerites jamshidis sp nov and steusloffina fravashi sp nov are described

oxygen and hydrogen isotopes in the volcano sedimentary complex of huelva iberian pyrite belt example of water circulation through a volcano sedimentary sequence abstract the minerals of basic and acidic rocks from the volcano sedimentary sequence in the huelva area spain iberian pyrite belt display an extended 18o enrichment quartz 18o values from quartz keratophyres vary from 10 5 to 17 0 and feldspar 18o values from 14 4 to 16 0 for the spilite or spilitized dolerites 18o values vary from 9 9 to 13 4 for feldspar from 6 4 to 9 8 for chlorite from 3 7 to 4 3 for ilmenite and from 13 6 to 14 0 for quartz but pyroxene exhibits magmatic values from 5 3 to 6 1 with an exception at 7 5 the chlorite d values vary from 34 to 43 this is attributed to hydrothermal alteration with seawater enriched in 18o by circulation through sediments the temperatures of interaction determined from isotopic fractionations between minerals range from 400 to 520 c calculated d and 18o values for water in equilibrium with the minerals at isotopic temperatures range from 16 to 5 and from 8 3 to 12 8 respectively a model of circulation of seawater through a pile of sedimentary rocks and then through basaltic rocks is proposed to explain the high18o compositions of the rocks from the huelva district water rock mass ratios calculated from this model range between 0 3 and 0 7 for the determined range of temperatures

grenville magmatism in west texas petrology and geochemistry of the red bluff granitic suite zone of regional extension as suggested by recent geophysical and the red bluff granitic suite rbgs underlies part of the franklin petrological studies mountains in west texas it was emplaced into a middle proterozoic shelf sequence at 1120 35 ma the suite is predominantly metaluminous it consists of early granitic to quartz syenitic sills a main mass of alkali feldspar granite small bodies of alkali feldspar syenite leucogranitic dikes and late stage peralkaline