the apparent paradox of abundant foramol facies in low latitudes their environmental significance and effect on platform development recent studies of low latitude carbonates from se asia and australia have revealed deposits and platform types that do not fit easily into the perceived view of warm water tropical carbonates the sediments are classified as a foramol or foralgal facies and are dominated by non framework building light dependent biota such as perforate larger benthic foraminifera coralline algae and sometimes halimeda although ancient foramol deposits are often interpreted as an indication of non tropical conditions foramol facies have long been known from the tropics yet the environmental conditions in which they form are poorly known the aims of this paper are to evaluate how these deposits are best classified and more importantly whether there are any overriding controls that influence their common development in low latitude areas and if resultant platform development is affected the deposits are all best grouped within the photozoan association james n p 1997 the cool water depositional realm in james n p clarke j a d eds cool water carbonates sepm special publication 56 1 20 an assignment synonymous with warm water conditions the facies were also found to develop in conditions of limited light or oligophoty all the platforms described are characterised by having extensive areas of oligophotic facies generally in water depths extending down from 20 m but sometimes in shallower waters platforms types vary from land attached rimmed shelves homoclinal and distally steepened ramps to isolated banks and platforms but all have extensive moderate to deep photic zone development of oligophotic facies some workers might assign these platforms to the incipiently drowned platform type read j f 1985 carbonate platform facies models american association of petroleum geologists bulletin 69 1 21 however a morphological assignment is preferred here to signify that many of the deposits did not or may not form through drowning limited light in moderate to deeper depths of the photic zone often in regions with a reduced depth euphotic high light zone is seen as key to the extensive development of these deposits and the resultant platforms these conditions are often developed in humid low latitude areas of high terrestrial runoff and therefore regions of reduced water salinity or in areas of nutrient upwelling reduced water clarity due to nutrients plankton blooms suspended particulate matter upwelling and strong currents all promote the development of oligophotic conditions and facies on a regional scale time periods of relative rises in sealevel can also promote oligophotic facies development the findings of this study have broader implications for the development of low latitude platforms beyond australasia and during time periods when non framework building oligophotic biota are dominant sediment contributors the reconstruction of past climatic conditions based on study of foramol grain assemblages requires careful study to distinguish warm water deposits from those formed in cool waters

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

northwest basin and range tectonic deformation observed with the global positioning system 1999 2003 1 we use geodetic velocities obtained with the global positioning system gps to quantify tectonic deformation of the northwest basin and range province of the western united states the results are based on gps data collected in 1999 and 2003 across five new quasi linear networks in northern nevada northeast california and southeast oregon the velocities show 3 mm yr westward movement of northern nevada with respect to stable north america west of longitude 119 w the velocities increase and turn northwest parallel to sierra nevada great valley microplate motion and similar to velocities previously obtained to the south the observations are explained by a kinematic model with three domains that rotate around euler poles in eastern oregon and western idaho northeast california experiences internal dextral shear deformation 11 2 3 6 nstrain yr subparallel to pacific north america motion relative motions of the domains imply 2 5 mm yr approximately east west extension in northwest nevada and 1 4 mm yr approximately north south contraction near the california oregon border the northward decreasing approximately east west extension in northwest nevada is consistent with the northern termination of basin and range deformation faulting and characteristic topography no significant extension is detected in the oregon basin and range the oregon cascade arc moves north at 3 5 mm yr and is possibly influenced by the approximately eastward motion of the juan de fuca plate these results disagree with secular northwest trenchward motion of the oregon forearc inferred from paleomagnetic rotations south of latitude 43 however trenchward motion exists and is consistent with block rotations approximately east west basin and range extension and northwest sierra nevada translation

seasonal and interannual variability of the mid holocene east asian monsoon in coral δ18o records from the south china sea understanding the full range of past monsoon variability with reference to specific monsoon seasons is essential to test coupled climate models and improve their predictive capabilities we present a 54 year long high resolution skeletal oxygen isotope 6180 record extracted from a well preserved massive porites sp coral at hainan island south china sea to investigate east asian monsoon variability during summer and winter similar to 4400 calendar yr ago analysis of modem coral 6180 confirms that porites from hainan island are well positioned to record winter monsoon forcing of sea surface temperature sst as well as the influence of summer monsoon rainfall on sea surface salinity sss the coral record for similar to 4400 yr ago shows similar to 9 amplification of the annual cycle of 6 delta o 18 in good agreement with coupled ocean atmosphere models showing higher summer rainfall lower coral 618 0 and cooler winter ssts higher coral delta o 18 in response to greater northern hemisphere insolation seasonality during the middle holocene mean ssts in the south china sea during the mid holocene were within 0 5 degrees c of modem values yet the mean 6 180 for the fossil coral is similar to 0 6 parts per thousand higher than that for the modem coral suggesting that the 6 180 of surface seawater was higher by at least similar to 0 5 parts per thousand relative to modem values the o 18 enrichment is likely to be driven by greater advection of moisture towards the asian landmass enhanced monsoon wind induced evaporation and vertical mixing and or invigorated advection of saltier o 18 enriched pacific water into the relatively fresh south china sea the o 18 enrichment of the northern south china sea similar to 4400 yr ago contributes to mounting evidence for recent freshening of the tropical western pacific today winter sst and summer sss variability in the south china sea reflect the interannual influence of enso and the biennial variability inherent to monsoon precipitation spectral analysis of winter ssts similar to 4400 yr ago reveals a strong enso cycle at 6 7 y which is significantly longer than the average 3 6 y cycle observed since 1970 the results suggest that the influence of enso on winter ssts in the south china sea was well established by similar to 4400 yr ago however spectral analysis of summer sss similar to 4400 yr ago shows no significant enso cycle suggesting that teleconnections between enso and summer monsoon rainfall were restricted taken together the results indicate marked differences in enso monsoon interactions during the winter and summer monsoon seasons in the past the fossil coral delta o 18 record also shows that the amplitude of interannual sst and sss variability was stronger similar to 4400 yr ago despite enso variability being significantly weaker in the pacific region thus it appears that the strengthened mid holocene monsoon was sensitive to forces other than enso that acted as alternative drivers of interannual monsoon variability if this is the case greater interannual climate variability could accompany the strengthening of the asian monsoon predicted to occur during the 21st century as transient greenhouse warming preferentially warms eurasia even if enso perturbations remain relatively stable c 2005 elsevier b v all rights reserved

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

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

ice age winds an aquaplanet model abstract factors controlling the position and strength of the surface winds during the last glacial maximum lgm are examined using a global multilevel moist atmospheric model the idealized aquaplanet model is bounded below by a prescribed axisymmetric temperature distribution that corresponds to an ocean covered surface various forms of this distribution are used to examine the influence of changes in the surface cooling and baroclinicity rates the model omits seasonal variations increasing the cooling lowers the tropopause and greatly reduces the moist convection in the tropics thereby causing a weakening and equatorward contraction of the hadley cell such a cooling also weakens the surface westerlies and shifts the peak westerly stress equatorward an extra surface baroclinicity in midlatitudes implicitly associated with an increase in the polar sea ice also shifts the peak westerly stress equatorward but strengthens the surface westerlies thus calculations with combined surface cooling an

stable warm tropical climate through the eocene epoch earth s climate cooled from a period of extreme warmth in the early eocene epoch ca 50 ma to the early oligocene ca 33 ma when a large ice cap first appeared on antarctica evidence from the planktonic foraminifer oxygen isotope record in deep sea cores has suggested that tropical sea surface temperatures declined by 5 10 degrees over this interval eventually becoming much cooler than modern temperatures here we present paleotemperature estimates from foraminifer isotopes and the membrane lipids of marine crenarcheota from new drill cores in tanzania that indicate a warm and generally stable tropical climate over this period we reinterpret the previously published isotope records in the light of comparative textural analysis of the deep sea foraminifer shells which shows that in contrast to the tanzanian material they have been diagenetically recrystallized we suggest that increasingly severe alteration of the deep sea plankton shells through the eocene produced a diagenetic overprint on their oxygen isotope ratios that imparts the false appearance of a tropical sea surface cooling trend this implies that the long term eocene climatic cooling trend occurred mainly at the poles and had little effect at lower latitudes

the late ordovician diamictic formations from southwestern europe north gondwana glaciomarine deposits abstract this paper presents a review and a discussion of the clast bearing formations diamicts which occur in the uppermost part of the ordovician succession of numerous regions in southwestern europe lithology facies stratigraphic setting and age are discussed the strong similarities of these formations with contemporaneous glaciomarine deposits of the northern and northwestern margins of the african ice sheet and the late ordovician paleogeographic pattern based upon paleomagnetic data lead to the conclusion that the south european clast bearing formations are glaciomarine deposits supplied with clasts by floating icebergs released from the african ice sheet the proposed paleogeographic model confirms the close connection between southwestern europe and gondwana during early palaeozoic times

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

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

deconvolving the δ18o seawater component from subseasonal coral δ18o and sr ca at rarotonga in the southwestern subtropical pacific for the period 1726 to 1997 to reconstruct oceanographic variations in the subtropical south pacific 271 year long subseasonal time series of sr ca and delta 18 o were generated from a coral growing at rarotonga 21 5degreess 159 5degreesw in this case coral sr ca appears to be an excellent proxy for sea surface temperature sst and coral delta 18 o is a function of both sst and seawater delta 18 o composition delta 18 o sw here we focus on extracting the delta 18 o sw signal from these proxy records a method is presented assuming that coral sr ca is solely a function of sst and that coral delta 18 o is a function of both sst and delta 18 o sw this method separates the effects of delta 18 o sw from sst by breaking the instantaneous changes of coral delta 18 o into separate contributions by instantaneous sst and delta 18 o sw changes respectively the results show that on average delta 18 o sw at rarotonga explains similar to39 of the variance in delta 18 o and that variations in sst explains the remaining similar to61 of delta 18 o variance reconstructed delta 18 o sw shows systematic increases in summer months december february consistent with the regional pattern of variations in precipitation and evaporation the delta 18 o sw also shows a positive linear correlation with satellite derived estimated salinity for the period 1980 to 1997 r 0 72 this linear correlation between reconstructed delta 18 o sw and salinity makes it possible to use the reconstructed delta 18 o sw to estimate the past interannual and decadal salinity changes in this region comparisons of coral delta 18 o and delta 18 o sw at rarotonga with the pacific decadal oscillation index suggest that the decadal and interdecadal salinity and sst variability at rarotonga appears to be related to basin scale decadal variability in the pacific copyright c 2002 elsevier science ltd

gps derived strain in northwestern california termination of the san andreas fault system and convergence of the sierra nevada great valley block contribute to southern cascadia forearc contraction abstract gps derived velocities 1993 2002 in northwestern california show that processes other than subduction are in part accountable for observed upper plate contraction north of the mendocino triple junction mtj region after removing the component of elastic strain accumulation due to the cascadia subduction zone from the station velocities two additional processes account for accumulated strain in northern california the first is the westward convergence of the sierra nevada great valley sngv block toward the coast and the second is the north northwest impingement of the san andreas fault system from the south on the northern california coastal region in the vicinity of humboldt bay sierra nevada great valley block motion is northwest toward the coast convergent with the more northerly north northwest san andreas transform fault parallel motion in addition to the westward converging sierra nevada great valley block san andreas transform parallel shortening also occurs in the humboldt bay region approximately 22 mm yr of distributed pacific sngv motion is observed inland of cape mendocino across the northern projections of the maacama and bartlett springs fault zones but station velocities decrease rapidly north of cape mendocino the resultant 6 10 mm yr of san andreas fault parallel shortening occurs above the southern edge of the subducted gorda plate and at the latitude of humboldt bay part of the san andreas fault parallel shortening may be due to the viscous coupling of the southern edge of the gorda plate to overlying north american plate we conclude that significant portions of the upper plate contraction observed north of the mtj region are not solely a result of subduction of the gorda plate but also a consequence of impingement of the western edge of the sierra nevada great valley block and growth of the northernmost segments of the san andreas fault system

variability of the intertropical convergence zone recorded in coral isotopic records from the central indian ocean chagos archipelago we have analyzed the stable oxygen isotopic composition of two porites corals from the chagos archipelago which is situated in the geographical center of the indian ocean coral 18o at this site reliably records temporal variations in precipitation associated with the intertropical convergence zone itcz precipitation maxima occur in boreal winter when the itcz forms a narrow band across the indian ocean the chagos then lies within the center of the itcz and rainfall is strongly depleted in 18o a 120 yr coral isotopic record indicates an alternation of wet and dry intervals lasting 15 to 20 yr the most recent 2 decades are dominated by interannual variability which is tightly coupled to the el nino southern oscillation enso this is unprecedented in the 120 yr of coral record as the itcz is governed by atmospheric dynamics this provides evidence of a major change in the coupled enso monsoon system

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

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

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

a 10 year record of the weathering rates of surficial pebbles in karkevagge swedish lapland abstract between 1999 and 2004 nylon mesh bags containing 6 2 mm diameter fragments of crushed dolomite or granite were exposed to weathering on the surfaces of birch dryas heath heath meadow solifluction meadow and willow vegetation communities in karkevagge a glaciated trough in swedish lapland the material in the bags had previously been in the same locations from either 1994 or 1995 when it had been placed in the field as freshly crushed pebbles results of the mass losses resulting from chemical weathering during the first 4 or 5 year period were reported in dixon et al dixon j c thorn c e darmody r g schlyter p 2001 weathering rates of fine pebbles at the soil surface in karkevagge swedish lapland catena 45 273 286 in catena the spatial results of the second 5 year period confirm those determined during the first period in general wetter more acidic sites promoted greater losses from the dolomite but the losses from the granite were too small to reveal distinctive spatial patterns dolomite consistently weathered faster than granite in at a site comparisons with total mass losses between 1999 and 2004 ranging from 1 90 to 9 98 for dolomite and 0 07 to 4 02 for granite spatial distinctions were blurred during the second period due to significant losses of bags at dry sites from atmospheric exposure however at a site comparisons between the two study periods revealed statistically significantly greater losses during the second period when compared to the first in the absence of appropriate ground climate data it is worth noting that air climates were both warmer and wetter during both study periods when compared to the 1961 1990 averages and the second 5 year study period was also warmer and wetter than the first

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

snowline instability in a general circulation model application to carboniferous glaciation for over twenty years it has been known that energy balance models ebms with snow albedo feedback are characterized by unstable behavior in some areas of parameter space this behavior leads to rapid changes in snow area due to small changes in forcing and has been termed the small ice cap instability sici it has never been clarified whether this behaviour reflects a real feature of the climate system or a limitation in ebms in this study we demonstrate that evidence for similar unstable behavior can also be found in an atmospheric general circulation model gcm using a realistic set of boundary conditions for the carboniferous 300 ma one of the most extensive periods of glaciation in earth history when solar luminosity is sequentially lowered to near values appropriate for the carboniferous there is a discontinuous increase in summer snow area the instability occurs in approximately the same area of parameter space as one previously found in an ebm analysis of selected fields indicates that the circulation is primarily affected in the area of snow increase far field effects are minimal there is good agreement between model generated summer snowcover and one reconstruction of carboniferous ice cover although more work is required on this topic our results provide increased support for the possibility that the snowline instability represents a real feature of the climate system and that it may help explain some cases of glacial inception and abrupt transitions in earth history

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

channel width response to differential uplift 1 the role of channel width and slope adjustments to differential uplift in rivers within actively deforming terrains remains contentious here high resolution topographic surveying of formerly antecedent outwash channels demonstrates marked changes in channel width as a primary response to differential uplift for five late quaternary alluvial paleochannels crossing small folds along the active ostler fault zone of southern new zealand nearly continuous measurements of paleochannel width and concomitant incision reveal abrupt narrowing of widths toward minimum values at channel positions coincident with the initial uplift when the magnitude of differential uplift is sufficiently small narrowing alone permits these channels to remain antecedent in the context of a unit stream power model for fluvial erosion observed limits on the magnitude of channel narrowing suggest that above some threshold amount of differential uplift continued incision requires concomitant changes in channel gradient thus when crossing small growing folds alluvial rivers simply narrow their channels whereas larger folds that demand greater incision prompt an initial narrowing followed by channel steepening

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

a pre hirnantian late ordovician interval of global cooling the boda event re assessed facies analysis suggests that the early mid ashgill late ordovician boda event of fortey and cocks fortey r a cocks l r m 2005 late ordovician global warming the boda event geology 33 5 405 408 was an episode of global cooling not of warming as they proposed limestones are widely developed at this level from mid latitude to high latitude commonly interrupting thick ordovician successions of fine clastic sediments bryozoan carbonate mud mounds in a laterally extensive cool water carbonate belt along the high palaeolatitude margin of w gondwana are associated with glacial diamictites and grew during glacial lowstand with sea ice prior to the hirnantian glaciation from high to mid palaeolatitudes contemporaneous cool water carbonates with mud mounds including stromatactis both in gondwana and in baltica developed in mid outer ramp settings during regression in the dominantly clastic welsh basin of avalonia phosphatic facies on an iapetus facing carbonate ramp indicate upwelling cold waters down ramp yet up ramp coralliferous algal limestones suggest warm surface waters we propose that the late ordovician cool water carbonates developed in response to global cooling and expansion of gondwanan terrestrial and sea ice that lowered eustatic sea level and on the w gondwanan shelf reduced or shut down fluvial clastic input previously poorly circulated salinity stratified oceans that had restricted benthic dispersal in mid low latitudes were replaced by thermohaline ocean circulation that oxygenated bottom waters and dispersed benthic faunas an earlier caradoc carbonate interval in low mid palaeolatitudes that shares many facies characteristics with the ashgill cool water limestones and correlates with a widely recognised isotope excursion is interpreted as indicating an earlier global cooling episode

current sierra nevada north america motion from very long baseline interferometry implications for the kinematics of the western united states we use geodetic measurements from very long baseline interferometry to estimate the motion of the sierra nevadan microplate which is composed of the sierra nevada and the great valley the motion of the sierra nevadan microplate relative to the north american plate is described by a right handed rotation of 0 61 m y about lat 32 n long 128 w this euler pole lies only 10 southwest of the sierra nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis it further predicts a velocity of the eastern edge of the sierra nevada at 38 0 n 119 3 w relative to stable north america of 11 1 mm yr toward n36 3 w quoted uncertainties are plus or minus one standard error which accounts for about one fourth of the velocity between the pacific and north american plates and is 25 clockwise of many prior estimates the velocity nearly parallels the boundary between the sierra nevada and the great basin which implies that current motion within the great basin results in a rotational noncoaxial deformation we use this velocity to estimate how motion is distributed across the broad deforming zone taking up pacific north america plate motion we find that the vector sum of strike slip along the san andreas fault and motion of the sierra nevada relative to stable north america taken up by deformation within the great basin differs little from the pacific north america plate velocity the difference can be described at 36 n along the san andreas fault by a vector of 6 mm yr directed toward n20 w this vector resolves into components of 5 mm yr parallel to the fault and 2 mm yr perpendicular to the fault with 95 confidence intervals of 0 to 10 mm yr and 1 to 5 mm yr respectively the component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault perpendicular shortening we conclude that motion previously inferred to be taken up by deformation other than strike slip along the san andreas fault or deformation within the great basin is much smaller than previously thought

sensitivity of the atlantic intertropical convergence zone to last glacial maximum boundary conditions 1 recent paleoproxy records suggest that the mean latitude of the atlantic intertropical convergence zone itcz varied synchronously with north atlantic climate over a range of timescales throughout the holocene and last glacial maximum we show that the present day meridional mode of atmosphere ocean variability in the tropical atlantic is a potentially useful model for understanding these paleoclimate changes the tropical atlantic in a coupled atmospheric general circulation and slab ocean model responds to last glacial maximum conditions with a southward displacement of the itcz this response arises primarily through the land ice sheet that forces increased north atlantic trades analogous to the forcing on the present day meridional mode changes to sea ice coverage and to ocean heat transport associated with a weakened atlantic thermohaline circulation also cause a meridional mode response though through different mechanisms our results highlight the potential for tropical atlantic paleoclimate to be driven from the high latitude influences in particular land ice on glacial interglacial timescales

geochemistry of brachiopods oxygen and carbon isotopic records of paleozoic oceans abstract combined trace element and isotope studies of 319 brachiopods covering the ordovician to permian time span show that 13c and 18o in well preserved specimens varied during the paleozoic the overall 13c secular trend is in accord with the previously published observations but its details are obscured by vital isotopic fractionation effects at generic level nonetheless the results suggest that the negative correlation between marine 13ccarbonate and 34ssulphate deteriorates at time scales of 106 years due to the long residence time and thus slow response of so42 in the ocean for oxygen isotopes all devonian and older specimens have 18o of 4 while the well preserved permian samples have near present day 18o of about 1 pdb this isotopic dichotomy is probably not due to post depositional phenomena salinity or biogenic fractionation effects this leaves open the perennial arguments for a change in 18 o 16 o of sea water versus warmer ancient oceans the present data are difficult to explain solely by the temperature alternative the coincidence of the proposed shift in 18o with the large late paleozoic changes in marine 87 sr 86 sr 13 c 12 c 34 s 32 s and sea level stands argues for a tectonic cause and for a change in 18 o 16 o of sea water although such explanation is difficult to reconcile with global balance considerations and with isotopic patterns observed in alteration products of ancient basalts and ophiolites whatever the precise cause or combination of causes the implications for tectonism and or paleoclimatology are of first order significance

seismogenic deformation between the sierran microplate and oregon coast block california usa the sierran microplate is a northwest translating block entrained in distributed motion east of the pacific plate to the north the oregon coast block ocb moves northward within the hanging wall of the cascadia subduction zone above the obliquely converging juan de fuca plate analysis of gps velocity data indicates that relative motion between the rigid sierran and ocb microplates is characterized by several millimeters per year of dextral shear directed n70 w which is distinct from and counterclockwise to macroscopic dextral shear in the walker lane east of the sierran microplate we present a new analysis of focal mechanisms from small earthquakes in an 80 km wide zone that spans the geodetically defined sierran ocb boundary to evaluate patterns of distributed deformation we find that the direction of macroscopic dextral shear in this region is parallel to sierran ocb motion derived from gps data the seismogenic deformation is consistent with postulated dextral shear within an incompletely studied west northwest trending zone of faults and lineaments that traverses the northern sierra nevada the faults and lineaments terminate westward against quaternary folds in the northern sacramento valley active deformation at the sierran ocb boundary accommodates the relative motion of the bounding microplates and probably does not represent discrete transfer of walker lane motion to the cascadia subduction zone in a restraining left step across the northern sierra and sacramento valley

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

late ordovician carbon isotope trend in estonia its significance in stratigraphy and environmental analysis abstract carbon isotope changes during most of late ordovician time from the mid caradoc kinnekulle k bentonite until the beginning of the silurian were investigated as the corresponding sequence of rocks is stratigraphically nearly complete in estonia an attempt was made to use it to elaborate the general pattern of carbon isotope changes in the late ordovician complications were caused by several local or regional hiatuses in the middle and late caradoc and hirnantian a total of 385 whole rock samples were studied from eight drill cores in northern and central estonia the following positive carbon isotope events were observed 1 the mid caradoc excursion peak 13c value 2 2 in the uppermost part of the keila stage also known in sweden 2 the first late caradoc excursion 1 9 in the lower part of the rakvere stage 3 the second late caradoc excursion 2 4 in the upper part of the nabala stage 4 the early ashgill excursion 2 5 in the lowermost part of the pirgu stage 5 the widely known large hirnantian excursion in estonia the peak value reaches 6 7 in the porkuni stage the study interval comprises a long 10 ma period characterized by low magnitude carbon isotope changes and a following brief 2 ma interval with large changes no obvious lithological preference for hosting the positive shifts was recorded in principle the 13c values exceeding the background values may occur in all types of rocks present in a sedimentary basin several 13c positive excursions values 1 5 to 3 in the mohawkian of north america are evidence that the minor caradoc and early ashgill 13c positive shifts in baltoscandia may have counterparts in laurentia if correctly correlated these shifts may have global significance the hirnantian excursion is usually linked to a major glacial event even if some carbon cycling mechanisms are not completely understood the environmental causes suggested for the earlier minor shifts range from global climatic and glacial events to very local changes in basin regime and sea level our study supports the primary role of climatic or climatically triggered oceanic processes

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

obliquity forcing with 8 12 times preindustrial levels of atmospheric pco2 during the late ordovician glaciation results from coupled ice sheet and atmospheric general circulation models show that the waxing and waning of ice sheets during the late ordovician were very sensitive to changes in atmospheric p co 2 and orbital forcing at the obliquity time scale 30 40 k y without orbital forcing ice sheets can grow with p co 2 level as high as 10 times preindustrial atmospheric level pal however with orbital forcing ice sheets can grow only with p co 2 levels of 8 times pal or lower these results indicate that the threshold of p co 2 for the initiation of glaciation is on the lower end of previously published estimates of 8 20 times pal the ice sheet model results further indicate that during exceptionally long periods of low summer insolation and low p co 2 levels 8 10 times pal large ice sheets could have formed that were able to sustain permanent glaciation under subsequently higher p co 2 values this finding suggests that in order to end the late ordovician glaciation with a rise in p co 2 atmospheric p co 2 must have risen to at least 12 times pal ice sheets therefore introduce nonlinearities and hysteresis effects to the ordovician climate system these nonlinearities might have also played a role in the initiation and termination of other glaciations in earth history

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

a 20 000 year record of ocean circulation and climate change from the santa barbara basin much of the evidence for climate driven fluctuations in ocean circulation during the past 20 000 years has come from studies of the north atlantic region1 6 the extent to which such interactions have occurred in other ocean basins and any associated teleconnec tions between basins is poorly understood here we present high resolution palaeoclimate and palaeoceanographic records from a 20 000 year sedimentary sequence from the santa barbara basin on the eastern margin of the north pacific ocean the sequence shows oscillations of the benthic environment between low oxygen conditions laminated sediments during periods of warm climate and higher oxygen conditions non laminated bioturbated sediments during cool intervals age differences between coexisting benthic and planktonic foraminifers indicate climate related changes in the age and source and hence oxygen content of basin bottom waters relatively young bottom waters are associated with the cooler intervals and are considered to reflect high proportions of intermediate waters derived from proximal sources conversely older bottom waters are associated with the warmer intervals and were derived from more distal sources these climate driven variations in ocean circulation appear to be synchronous with the main ocean climate fluctuations in the north atlantic region1 6 suggesting that a tight coupling mechanism operates between the two basins

post oligocene tectonic rotation of the oregon western cascade range and the klamath mountains paleomagnetic data indicate that the western cascade range from northern california to central washington has undergone about 27 of clockwise rotation since 25 m y b p all the available regional paleo magnetic data fit a two phase rotation model the first phase being an eocene rotation that accompanied the accretion of the oregon coast range to the continent and the second phase being a rotation associated with post early miocene extension in the basin and range province

present tectonic motion across the coast ranges and san andreas fault system in central california geodetic results from very long baseline interferometry vlbi satellite laser ranging slr and the global positioning system gps are used to estimate angular velocities between the sierran microplate pacific plate and north american plate the sierra pacific pole of rotation lies nearer to the san andreas fault than does the pacific north america pole of rotation and leads to different tectonic implications than if the latter is used the angular velocities show that the san andreas fault system and central california coast ranges accommodate motion of 39 2 mm yr mainly by strike slip faulting all confidence limits following signs in this paper are 95 confidence limits fault normal motion is small is mainly convergent at rates up to 3 3 1 0 mm yr and varies along the coast but is divergent at 2 6 1 2 mm yr across san pablo bay and associated topographic lows across which the sierran and central valley watershed drains to the pacific ocean the mountain ranges tend to be larger where the fault normal convergence rates are larger the low convergence rate 0 5 1 8 mm yr normal to the san andreas fault in the carrizo plain differs sharply from that previously inferred 8 2 1 2 mm yr and 4 9 1 6 mm yr by feigl et al 1993 the difference is due to differences between their and our elastic strain accumulation models and between how their and our pacific plate reference frames are defined the ranges in most places require a minimum of 4 2 1 m y of fault normal convergence at the present rate to attain their present cross sectional area if erosion is neglected more if it is not the amount of convergence previously estimated from a balanced cross section across the diablo range in central california requires 10 8 3 m y of convergence at the present rate the former is consistent with widely held views about the onset of the coast range orogeny but the latter is not both are consistent however with the recent plate reconstructions by s cande j stock and colleagues which indicate that pacific plate motion relative to north america changed to a more convergent direction 20 25 clockwise of its prior direction at ca 8 to 6 ma and not at 3 5 ma as had been previously inferred the inferred change in direction of plate motion is large compared with the present angle of convergence across the straight and narrow segment of the san andreas fault of 0 7 4 7 from which we infer that the sierran microplate changed motion relative to north america at the same time ca 8 to 6 ma as did the pacific plate we further infer that the motion accommodated across the great basin must also have changed at the same time we also examine the hypothesis that stable sliding occurs along the san andreas fault and other northwest striking strike slip faults in central california where the fault normal convergence rate is low or negative and that these faults are unstable where the fault normal convergence rate is high such a relationship appears to hold in general but fails in detail in particular there are substantial sections of fault with small inferred rates of fault normal convergence across which the san andreas fault is locked moreover the creeping section of the san andreas fault i e the section between parkfield and the calaveras junction is the locus of greater fault normal convergence 3 2 1 4 mm yr than is the locked part of the fault 0 5 1 8 mm yr south of parkfield thus this hypothesis is at best a partial explanation for the observed distribution of locked and nonlocked sections of the fault

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

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

changes in marine isotopic composition and the late ordovician glaciation well preserved brachiopod shells and marine cements from limestone coquinas which cap carbonate mudmounds in the siljan area of central sweden have the heaviest stable oxygen and carbon isotope values yet reported for lower palaeozoic marine sediments the coquinas formed during the eustatic regression at the onset of the late ordovician glaciation the isotopic compositions reflect simultaneous shifts in both carbon and oxygen more than 5 13 c and up to 2o 18 o away from more normal lower palaeozoic values in similar carbonates from within the mounds oxygen isotope data are consistent with a change in the isotopic composition of the sea water probably accompanied by a decrease in temperature the changes in carbon values suggest enhanced deposition of organic carbon a process which would have decreased p co 2 in the ocean and the atmosphere and thus contributed to rapid global cooling