paleoclimatic and paleomagnetic constraints on the paleozoic reconstructions of south china north china and tarim abstract paleomagnetic and paleoclimatic data provide the most useful latitudinal constraints for plate reconstructions distributions through the paleozoic of five types of climatically sensitive sediments coals evaporites reefs dolomites and limestones for south china north china and tarim are shown on 15 maps that include 1578 reliable data points these paleoclimatic data agree reasonably well with available paleomagnetic directions although significant divergence between the two exists for the early paleozoic these data indicate the following 1 1 south china was in low latitudes during the entire paleozoic with a subtropical position in the cambrian 2 2 north china also remained near the equator in the early and late paleozoic except for the ordovian and the late permian when extensive evaporites suggest slightly higher latitudinal positions while its middle paleozoic position is uncertain due to the missing stratigraphie record 3 3 in south china local tectonics appears to have played a dominant role in determining paleogeography and therefore marine sedimentation especially after the late ordovician early silurian because the areal coverage of marine sediments through time is distinctly different from what would be expected from published global sea level curves 4 4 paleoclimatic and paleomagnetic data are compatible with biogeographic data which suggest that south china was part of eastern gondwana in the early paleozoic but was widely separated from gondwana in the late paleozoic and the split between the two probably happened in the devonian giving rise to a major break up unconformity in central south china

mass balance of two high arctic plateau ice caps mass balance measurements have been renewed on two small ice caps on north eastern ellesmere island original stake networks were established in 1972 and 1976 since then both ice caps have experienced significant mass losses averaging 70 to 140 kg m2 a i they have also decreased in area the equilibrium line in this area has averaged around 1150 m for the last decade or so the ice caps are remnants of former climatic conditions and are out of equilibrium with contemporary climate

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

easter microplate evolution geochemical evidence confirms that there is current active spreading on both the east and west rift zones of the easter microplate and northwards propagation of the east rift with the active tip at 25 s the east rift is influenced by lateral flow of plume derived material from a hotspot located beneath either sala y gomez or easter island spreading rate variation does not seem to be influential in controlling the composition of erupted basalts

late ordovician and early silurian rugose coral biogeography and world reconstruction of palaeocontinents abstract two epochs of early palaeozoic rugose coral diversification caradocian ashgillian and middle llandoverian early wenlockian are chosen for biogeographical analysis at the generic level following an overall revised classification of the rugosa wang et al 1989a world reconstruction maps are based on palaeomagnetic palaeotectonic and palaeoblogeographic data and were constructed through an autograph programme on an ibm pc at microcomputer the caradocian and ashgillian rugose corals are in 16 families and 71 genera including 15 cystiphyllida 40 streptelasmatida and 44 columnariida in addition to the zaphrentoidid genus protozaphrentis two biogeographical realms the west prototethyan and the east prototethyan may be discerned the west prototethyan realm includes three provinces the siberian the north american and the kazakhstan hinganian the east prototethyan realm also includes three provinces the southeast asian yangtzean the east australian and the north european the third area the gondwanan is devoid of corals in middle and late llandoverian and early wenlockian the rugose corals have 27 families and 130 genera of which 26 are cystiphyllida 32 are streptelasmatida and 41 are columariida in addition to the zaphrentoidid genus duncanella the west prototethyan realm contains three provinces the siberia mongolian the north european and the north american the east prototethyan realm consists of two provinces the southeast asian yangtzean and the kazakhatan sinokorean both bearing ceriaster and amplexoides

mesozoic aseismic ridges on the farallon plate and southward migration of shallow subduction during the laramide orogeny we propose that an aseismic ridge of late jurassic and early cretaceous age on the farallon plate was subducted beneath the unites states cordillera during the laramide orogeny the relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70 40 ma in the existing near trench magmatic arc and laramide uplift and faulting of crystalline basement 1000 1500 km inland from the trench the timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the laramide orogeny and the magmatic lull in the western cordillera because the north northeast trending aseismic ridge was moving northeast relative to the north american plate the point of collision of the aseismic ridge with north america and therefore the locus of shallow subduction migrated southward with time causing the well documented southward migration of the magmatic lull

the rates of plate creation and consumption summary an alternative vector derivation of the expression for the rates of plate creation and consumption is given from which it can be seen that the rates are proportional to the scalar product of the edges of the geohedron and plate polyhedron corresponding to the same boundary the plate polyhedron is defined by the set of triple junction vectors this is used together with a recent determination of the angular velocity vectors and a number of auxiliary plate boundary vectors to revise the rates of plate creation and consumption along sections of plate boundaries all plates which have a substantial proportion of their boundary connected to a downgoing slab are found to be decreasing in area the total rate of plate generation is used in estimates of the area of the ocean floor and the total heat loss from the oceans the rate of consumption within a relatively small area around new guinea implies a large mass flux away from this region knowledge of the rates of plate creation and consumption is used in a number of different areas calculating the contribution of plate creation to the total heat loss of the earth calculating geochemical fluxes out of the mantle such as for the rare gases and other volatiles and return fluxes back into the mantle resulting from reactions between sea water and the ocean crust during hydrothermal circulation calculating the area of the ocean floor as a function of age mass fluxes related to the distribution of the rates of creation and destruction along the plate boundaries are important boundary conditions on the flow pattern in the mantle previous estimates of these rates deffeyes 1970 chase 1972 garfunkel 1975 have applied a simple expression derived for spreading ridges deffeyes 1970 using angular distances from the poles of rotation measured graphically on a globe an alternative derivation of the expression for rates of plate creation and consumption which is independent of assumptions about the nature of the boundary is given below in terms of the framework used to describe plate motions by mckenzie parker 1967 1974 this derivation emphasizes that the rates depend on two sets of vectors one the instantaneous rotation vectors defining the relative plate motions the other a set of triple junction and auxiliary vectors defining the plate boundaries

error analysis of paleosalinity calculations a detailed error analysis is performed for the calculation of open ocean salinity changes from carbonate oxygen 18 proxy data measurement errors are generally negligible however errors due to the goodness of fit of various regressions used in the calculation are significant depending on the method used and on the proxies available error bars 1 for the salinity range from 0 6 to 1 8 in particular expected errors for the midlatitude and tropical salinity differences at the last glacial maximum are around 1 1 to 2 5 the largest errors occur because of the differences between temporal and spatial seawater oxygen 18 salinity relationships and through the use of nonconcurrent temperature measurements methods of improving paleosalinity estimates should focus on eliminating these large sources of uncertainty

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

mass balance of white glacier axel heiberg island n w t canada 1960 91 white glacier is a valley glacier at 79 5 n with an area of 38 7 km 2 its mass balance has been measured over 32 years with a 3 year gap by standard techniques using the stratigraphic system with a stake density of the order of one stake per km 2 errors in stake mass balance are about 200 250 mm due largely to the local unrepresentativeness of measurements errors in the whole glacier mass balance b are of the same order as single slake errors however the lag 1 autocorrelation in the time series of b is effectively zero so it consists of independent random samples and the error in the long term balance normal b is noticeably less b is 100 48 mm the equilibrium line altitude ela averages 970 m with a range of 470 1400 m mass balance is well correlated with ela but detailed modelling shows that the equilibrium line is undetectable on visible band satellite images a reduced network of a few stakes could give acceptable but less accurate estimates of the mass balance as could estimates based on data from a weather station 120 km away there is no evidence of a trend in the mass balance of white glacier to detect a climatologically plausible trend will require a ten fold reduction of measurement error a conclusion which may well apply to most estimates of mass balance based on similar stake densities

early palaeozoic palaeobiogeography of the variscan regions abstract the palaeobiogeographic method appears to be an attractive alternative for reconstruction of the palaeogeography of the variscan regions during the palaeozoic the evolution of faunal assemblages and climatic characteristics from the cambrian up to the end of the devonian demonstrates that baltica and north gondwana represent two major and independent palaeogeographic entities separated by a mid european rheic ocean which opened as early as the cambrian and closed progressively from the devonian the south armorican ocean whose closure generated the so called ligerian orogen is regarded as a branch of the rheic ocean both a tornquist s sea separating baltica from its western extension avalonia during the ordovician and a proto tethys ocean extending between southwestern europe and north africa during the devonian are excluded from our reconstructions

mesozoic paleomagnetism and northward translation of the baja california peninsula paleomagnetic data from cretaceous igneous and sedimentary rocks of the peninsular ranges and continental borderland of the baja california peninsula yield an average pole position of 83 n 344 e a95 3 4 suggesting that the peninsula as a whole has moved northward 11 and rotated 32 clockwise with respect to the north american craton since cretaceous time only a small portion of this movement is due to the neogene opening of the gulf of california results from stratified rocks giving both positive and negative fold tests are included in the average cretaceous pole the rocks that fail fold tests have been deformed and remagnetized by nearby intrusives that are only slightly younger unconnected directions from these sites closely match corrected directions from sites where the fold tests are positive oceanic rocks were also sampled in the continental borderland from upper triassic and upper jurassic tectonostratigraphic terranes which are apparently allochthonous with respect to the western baja peninsula and each other these oceanic rocks show magnetizations indicating low paleolatitudes although the hemisphere of origin is presently unknown the cretaceous rocks of the baja california peninsula may have originally formed adjacent to the now truncated continental margin along the northern middle america trench northward transport of the peninsula probably occurred by transcurrent faulting associated with oblique subduction of the farallon plate beneath western north america between late cretaceous and late miocene time

additional paleomagnetic results from the sierra nevada further constraints on basin and range extension and northward displacement in the western united states modal analysis of new paleomagnetic results that have a skewed distribution from four plutons in the kings canyon national park area central sierra nevada produces a pole position concordant with the results of frei and others 1984 from the yosemite area these studies suggest a that stable magnetization resides in the more mafic plutons and inclusions in the sierra nevada and b that the amount of internal deformation within the central sierra nevada does not exceed a few degrees at the 95 confidence level the mean pole of both studies limits the relative displacement between the sierra nevada and cratonal north america to 6 8 of clockwise rotation and 6 6 of poleward displacement the clockwise rotation agrees well with the model of frei and others 1984 for basin and range extension which predicts 2 of clockwise rotation for the sierra nevada the poleward displacement suggests right lateral shear of 600 km within the basin and range between 90 and 35 ma

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

a new type of active margin the convergent extensional margin as exemplified by the middle america trench off guatemala the results of dsdp legs 67 and 84 during which a transect of holes was drilled across the middle america trench off guatemala show no accretion at the front of that subduction zone since early eocene time the tectonic evolution of the trench includes extensional structure on the landward as well as the seaward trench slopes the guatemalan margin is proposed as the model of a new type of active margin the convergent extensional active margin ce margin

holocene variations in asian monsoon moisture a bidecadal sediment record from the south china sea the east asian monsoon system involves extensive transport of sensible latent heat between land and sea and from low to high latitudes our high resolution bidecadal marine records present a first detailed history of monsoon climate change over the holocene the high amplitude perturbation in monsoon moisture centered at 8 150 years ago and the monsoon maximum in the early holocene show inter hemispheric teleconnections to both a cool episode in greenland and to the indian monsoon monitored in the arabian sea periodicities of 84 102 and especially near 775 years in monsoon variation suggest a climatic forcing both by long term oscillations in thermohaline circulation and possibly solar activity cycles

silurian and devonian biogeography of china integration of newly available chinese data for the biogeography of the silurian and devonian makes possible a number of globally sigificant advances the early silurian the llandoverian can now be shown to have been moderately provincial rather than cosmopolitan as considered earlier the uralian cordilleran region defined previously for the late silurian can now be shown with chinese information to have been in existence with a co occurring north atlantic region since the beginnings of the silurian and to have been somewhat more provincial than the late silurian the view of late silurian provincialism with two major regions is further substantiated with the help of chinese data for the early devonian it is now possible to define a south china region that includes the bulk of the yangtze platform region of south china plus adjacent tonkin in north vietnam the boundary of the south china region is present in central yunnan and extends in an approximately north south direction the south china region is still prominent during the lower eifelian half of the middle devonian relatively cosmopolitan devonian conditions affect china by the latter givetian half of the middle devonian as is also the case in most other parts of the world except for eastern north america and the southern hemisphere malvinokaffric realm the chinese upper devonian is cosmopolitan in a manner similar to that of the rest of the world it is notable during the lower devonian that the nonmarine vertebrate fauna is unusually provincial and very distinct from faunas known in europe and north america as well as from those present in northern asia it is suggested that previous distinctions between lower devonian uralian and mongolo okhotsk regions are more likely provincial or even subprovincial the presence of the atrypacean genus tuvaella in the silurian of east central asia is shown to represent the restricted occurrence of a single genus rather than being indicative of a major biogeographic unit however it may indicate the presence of a subprovincial unit or of an endemic center

late cretaceous and paleogene tectonic evolution of the north pacific ocean the late cretaceous history of the northern pacific ocean has not been adequately deciphered largely because a major plate reorganization occurred during the cretaceous magnetic quiet interval using primary data to reconstruct plate motions from fracture zone trends and late cretaceous seafloor spreading magnetic anomalies allows formulation of a reasonable sequence of events that accounts for all the geologic features of that region especially the emperor and chinook troughs the primary event in our reconstruction is the subduction of the old northwest pacific triple junction new relative plate motions imposed by formation of convergent boundaries along both the northern pacific and farallon plates caused the farallon plate to crack this subdivision occurred 82 m y ago and resulted in the formation of the kula and chinook plates the chinook plate was bounded on the north by the chinook kula ridge the western arm of the great magnetic bight on the west by the southern emperor trough a slowly spreading rift valley on the south by the mendocino transform and on the east by the chinook farallon ridge the northern emperor trough formed the initial western boundary of the kula plate from 82 m y ago to 50 m y ago this configuration was stable the chinook plate expanded the chinook kula ridge migrated north and the kula plate was subducted with the subduction of this ridge the chinook plate adhered to the pacific plate and the pacific farallon ridge became the north south trending feature reflected by anomalies 22 and younger this reconstruction obviates the awkward late cretaceous shifting of triple junctions found in other proposed histories and provides reasonable explanations for the several structural features of the region to achieve closure of vector triangles representing the relative late cretaceous plate motions some extension must occur presumably across the eastern mendocino fracture zone between the northern and southern portions of the farallon plate these two plates apparently were independent units between 105 and 50 m y ago the initiation and cessation of rapid north farallon north american convergence 82 and 50 m y ago may have determined the nature and duration of the laramide orogeny

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

southward movement of the pacific intertropical convergence zone ad 1400 1850 tropical rainfall patterns control the subsistence lifestyle of more than one billion people seasonal changes in these rainfall patterns are associated with changes in the position of the intertropical convergence zone which is characterized by deep convection causing heavy rainfall near 10 n in boreal summer and 3 n in boreal winter dynamic controls on the position of the intertropical convergence zone are debated but palaeoclimatic evidence from continental asia africa and the americas suggests that it has shifted substantially during the past millennium reaching its southernmost position some time during the little ice age ad1400 1850 however without records from the meteorological core of the intertropical convergence zone in the pacific ocean quantitative constraints on its position are lacking here we report microbiological molecular and hydrogen isotopic evidence from lake sediments in the northern line islands galapagos and palau indicating that the pacific intertropical convergence zone was south of its modern position for most of the past millennium by as much as 500km during the little ice age a colder northern hemisphere at that time possibly resulting from lower solar irradiance may have driven the intertropical convergence zone south we conclude that small changes in earth s radiation budget may profoundly affect tropical rainfall

palaeozoic strike slip rift pulses and palaeogeography in the circum mediterranean tethyan realm abstract five main rifting cycles supported by different types of field evidence including magmatism sedimentation and tectonics have been recognized in the circum mediterranean area western tethys realm before the opening of the jurassic tethys proper they occurred roughly in the late cambrian early ordovician early silurian late devonian dinantian early mid permian and mid triassic times each of these rifting cycles began with intracontinental mainly shallow marine conditions evolving usually to basinal environments of quite different bathymetry the palaeozoic ccd has been proved to be much less deep than the post liassic one minor suboceanic seaways cut across and fragmented the gondwana south european plate following the early palaeozoic rifting cycles the long mid palaeozoic devono dinantian rifting cycle migh possibly have resulted in a wider system of suboceanic sea ways connecting a palaeo tethys ocean to the east of gondwana with the rheic ocean to the west of it the birth of palaeo tethys or of its westward extension would have been directly linked with the gradual death of the devono mississippian rheic ocean a complex large scale dextral transform system is inferred to have connected the two respectively expanding and contracting oceans fragmentation and mutual displacement of microblocks in the connection area the circum mediterranea area and southern europe resulted from strike slip oblique rift pulses with coeval local contractional and extensional processes a major change of the gondwana rotational vector is believed to have produced the mid late carboniferous transpressional strike slip hercynian orogen this is a good explanation of the peculiar thermal regime of the hercynian diastrophism all over europe there is no support for the hypothesis of a very wide more than 3000 km palaeozoic ocean separating northern europe from southern europe or gondwanaland this idea was mainly based on poor palaeomagnetic data and is heavily contradicted by palaeobiogeographical and geological data furthermore it ignores the dramatic shortening of the hercynian orogen all over europe the early palaeozoic to early mesozoic history of the circum mediterranean area involves the cyclic recurrence of oblique rifts culminating in the early jurassic strike slip rift and subsequent limited opening of the tethys proper to avoid confusion in terminology one should use prefixes referring to age bathymetry physiography and geotectonic setting to qualifying the general term tethys e g shallow cretaceous tethys basinal intracontinental devonian tethys oceanic jurassic tethys etc

a paleozoic pangaea paleozoic paleogeographies should be consistent with all available reliable data however comparison of three different devonian paleogeographies that are based largely or wholly on the data of remanent magnetism show them to be inconsistent in many regards when these three paleogeographies are provided with possible ocean surface current circulation patterns and have added to them lithofacies and biogeographic data they also are shown to be inconsistent with such data a pangaeic reconstruction positioned in the southern hemisphere permits the lithofacies and biogeographical data to be reconciled in a plausible manner

bathymetry and paleoecology of silurian bethic assemblages late llandoverian new york state late llandoverian marine shales in new york state are highly fossiliferous and provide and opportunity to examine gradients of biofacies the williamson and willowvale formations llandoverian c6 were deposited during a major transgressive episodes in and adjacent to the axis of a narrow ne sw trending central appalachian foreland basin the williamson shale is a black to dark green graptolite bearing unit of the upper clinton group of western new york state it grades eastward into gray fossiliferous shales and coquinoidlimestones of the willowvale formation orientation analysis of graptolites monograptus clintonensis indicates paleoccurents dominantly from the northeast to east the presence of rippled cross laminated sandstone beds tempestites within dark gray graptolitic facies of the lower williamson indicates that the darl shale facies was deposited in dysaerobic to marginally aerobic water slightly below storm wave base poorly fossiliferous green shales higher in the willimason appear to slow deposition in dysaerobic water and suggest a gradual deepening graded brachipod coquinoid limestones interpreted as storm layers occur near the middle and top of the williamson and in the willovale shale and indicate deposition of these sediments above maximum storm wave base the faunas of the williamson and willowvale shales are subdivisible into five vertically and laterally adjacent associations as follows a ichnofossil chondrites b monograptid chonetes cornutus c eopletodonta atrypta d palaeocyclus bryozoan and e eocoelia chonetes cornutus associations this spectrum of fossil associations appears to parallel the increasing frequency of bottom disturbance by waves and currents from deeper basinal areas to higher energy outer and low energy inner shallow shelf settings the fossil associations of the williamson a c probably belong to the deeper benthic assemblages 4 5 but developed in environments within to slightly below storm wave base about 50 80 m deep the willowvale fauna c d representing benthic assemblages 3 4 inhabited somewhat shallower better oxygenated environments of the photic zone probably at depths of approximately 20 50 m the easternmost to slightly southeastward equivalent of williamson willowvale fauna e evidently represents shallow water benthic assemblage 2 these conclusions indicate the need for a critical reevaluation of the bathymetric relations of benthic assemblages 3 5 a new interpretations of approximate depths for this range is presented herein

stratigraphic and oxygen isotope evidence for my scale glaciation driving eustasy in the early middle devonian greenhouse world article i nfo million year scale 3rd order depositional sequences few 10s to 100 m thick are common in the devonian marine record and are correlated between sedimentary basins and across widely separated continents implying a eustatic origin tectonically driven changes in mid ocean ridge spreading rates lengths are too slow to account for the calculated 3rd order rise fall rates and typical orbital frequencies 20 400 ky are too fast we analyzed oxygen isotopes from conodont apatite to evaluate if early middle devonian 3rd order sea level changes were controlled by paleoclimatically driven ice volume glacio eustasy and seawater temperature change thermo eustasy two successive depositional sequences in the western u s nevada and czech republic prague basin were sampled the 18 o values range from 17 8

geology of the early devonian oolitic iron ore of the gara djebilet field saharan platform algeria abstract the oolitic iron ore of the gara djebilet field occurs within the early devonian sediments of the tindouf basin algerian sahara particularly in the upper djebilet formation of pragian age three large lenses form three individual deposits extending e w for about 60 km namely gara west gara center and gara east the mineralization is interbedded with argillaceous to sandy sediments and it can be related to a barrier island palaeoenvironment bordered by an inner lagoon or shallow embayment and an epicontinental sea trapped by palaeozoic shoals the oolitic sediments show a mineralogy marked mainly by magnetite hematite goethite maghemite chamosite bavalite siderite apatite and quartz three paragenetic associations present a vertical distribution with a lower non magnetitic ore a magnetitic ore and an upper non magnetitic ore three petrographical facies types have been defined a cemented facies foc a detrital facies fod and a non detrital facies fond chemical data for the whole field show a difference between the lower non magnetitic ore fe 54 6 the magnetitic ore fe 57 8 and the upper non magnetitic ore fe 53 the magnetitic ore which corresponds mainly to the workable ore cutoff grade at 57 has the following composition sio2 4 9 al2o3 4 2 fe2o3 61 43 feo 19 2 and p2o5 1 8 the corresponding calculated economical ore reserves are 985 106t with 57 8 fe regarding the genesis of the oolitic iron ore a southern source is suggested for the iron with deposition taking place in a quiet environment there the ooids developed by an intrasedimentary accretion mechanism around detrital grain within an iron rich mud the gara djebilet field is an important occurrence of the north african palaeozoic ironstone belt extending from the zemmour to libya which also includes ironstones of ordovician silurian and devonian age

timing of late cenozoic volcanic and tectonic events along the western margin of the north american plate a number of well to relatively well dated significant volcanic and tectonic events along the western margin of the north american plate began ended or increased intensity at 16 1 m y b p 10 2 m y b p and 5 1 m y b p continental events at 16 1 m y b p were related to the upwelling of an elongated mantle diapir which created a widespread thermal disturbance east of the subducting farallon plate oceanic events at 16 1 m y b p were largely related to the approach of the east pacific rise toward the north american plate continental and oceanic events at 10 2 m y b p were related to the clockwise rotation of the pacific plate spreading direction and to the increased rate of relative motion between the pacific and north american plates most events at 5 1 m y b p were related to the opening of the gulf of california and to the oceanic plate reorganizations of which the gulf opening was a part different styles of response reflect differences between intraplate volcanic and tectonic subdomains pulses of volcanic activity do not seem to be directly or solely related to changes of plate motion increased volcanic activity at arcs and some intraplate volcanic settings may be indicators or precursors of significant tectonic transition intervals if so we may presently be in the midst of such a transition interval

core top calibration of mg ca in tropical foraminifera refining paleotemperature estimation 1 optimal use of mg ca as a paleotemperature proxy requires establishing calibrations for different species of foraminifera and quantifying the influence of dissolution to achieve this goal we have measured mg ca and 18o in a series of tropical and subtropical core tops including four depth transects the ceara rise the sierra leone rise and the rio grande plateau in the atlantic and the ontong java plateau in the pacific focusing on spinose mixed layer dwelling species globigerinoides ruber and globigerinoides sacculifer and nonspinose thermocline dwelling neogloboquadrina dutertrei shell mg ca in g sacculifer is 5 15 lower than in g ruber while n dutertrei mg ca is 49 55 lower than in g ruber this statistically significant offset has allowed us to establish different calibrations for each species multilinear regression analysis was used to develop calibration equations that include a correction term for the dissolution effect on mg ca in foraminiferal calcite presented in this paper are two sets of calibrations one set using core depth as a dissolution correction and another using co32 as a dissolution parameter the calibrations suggest that g ruber is the most accurate recorder of surface temperature while g sacculifer records temperatures below the surface at 20 30 m the depth habitat of n dutertrei is more uncertain owing to the wide range in habitat depths depending on hydrographic conditions but on average mg ca and 18o data suggest it is at 50 m of the three species n dutertrei is the most sensitive to dissolution up to 23 decrease in shell mg ca per km while g sacculifer is the most resistant

salinity records for the 1997 98 el nino from western pacific corals 1 seasonal sea surface salinity sss records can be of great value in reconstructing el nino southern oscillation enso variability in the equatorial western and central pacific a region where enso related evaporation and precipitation e p changes are dramatic the 18o values of coral skeleton 18ocoral reflect seawater 18o 18oseawater changes which in tropical oceans are also generally influenced by e p changes therefore 18ocoral is a good indicator of rainfall and enso variability we present biweekly data from palau in the northern part of the western pacific warm pool for the period 1998 2000 which indicate a strong quantitative relationship between 18ocoral sea surface temperature 18oseawater and sss the coral skeletal 18o values documented the sss changes after the 1997 98 el nino therefore 18ocoral can be used to estimate paleosalinity changes however the slope of the 18oseawater sss relationship cannot be assumed to be constant throughout the tropics making site specific calibrations is essential

chronostratigraphy and geomorphology of washover fans in the exmouth gulf nw australia a record of tropical cyclone activity during the late holocene abstract washover fans typically form due to barrier overwash or breaching and coastal inundation and generally represent geomorphological and depositional evidence of intense storms few studies have investigated the chronostratigraphy of washover fans in order to infer magnitude frequency patterns of extreme wave events over longer time scales here we present new data on the chronostratigraphy of late holocene washover fans in the exmouth gulf western australia by using ground penetrating radar and unmanned aerial vehicle uav survey techniques as well as geomorphological sedimentological and chronological investigations this study aims to i provide a detailed characterization of the washover fans geomorphology and stratigraphical architecture ii document depositional processes involved in their formation iii establish a chronostratigraphy based on optically stimulated luminescence osl and iv understand the significance of the washover fans for recording past tropical cyclone tc activity the fans consist of multiple sequences of sand shell debris and coral rubble comprising depositional units related to tc induced inundation the units are separated by palaeosurfaces with incipient soil formation formed during periods of reduced depositional activity in combination with the interpretation of a uav based high resolution digital surface model multiple phases of reactivation are inferred osl results allow the establishment of a local long term tc record and suggest storm induced deposition at 170 360 850 and 1300 years ago further units were dated to 1950 2300 and 2850 years ago the chronology of tc events is consistent with other work relating tc activity with el nino southern oscillation enso and sea surface temperature sst patterns corroborating the regional palaeotempestological relevance of this unique geomorphological record

assessing millennial scale variability during the holocene a perspective from the western tropical pacific we investigate the relationship between tropical pacific and southern ocean variability during the holocene using the stable oxygen isotope and magnesium calcium records of cooccurring planktonic and benthic foraminifera from a marine sediment core collected in the western equatorial pacific the planktonic record exhibits millennial scale sea surface temperature sst oscillations over the holocene of 0 5 c while the benthic 18oc document 0 10 millennial scale changes of upper circumpolar deep water ucdw a water mass which outcrops in the southern ocean solar forcing as an explanation for millennial scale sst variability requires 1 a large climate sensitivity and 2 a long 400 year delayed response suggesting that if solar forcing is the cause of the variability it would need to be considerably amplified by processes within the climate system at least at the core location we also explore the possibility that sst variability arose from volcanic forcing using a simple red noise model our best estimates of volcanic forcing falls short of reproducing the amplitude of observed sst variations although it produces power at low frequency similar to that observed in the md81 record although we cannot totally discount the volcanic and solar forcing hypotheses we are left to consider that the most plausible source for holocene millennial scale variability lies within the climate system itself in particular ucdw variability coincided with deep north atlantic changes indicating a role for the deep ocean in holocene millennial scale variability

cycloid kinematics of relative plate motion the trajectory of a point on one plate as observed from another plate is generally a complex curve and not a small circle around a single axis of relative motion as is commonly assumed the shape of the relative motion path is given the general name spherical cycloid because of its morphological similarity to cycloid planetary trajectories described by early astronomers the cycloid relative motion model predicts that the following phenomena occur during finite displacements 1 the relative velocity and the curvature of the trajectory of a point on one plate relative to another plate varies systematically 2 plates wobble relative to one another and 3 the angle of convergence and or divergence varies systematically along the length of any given transform fault the small circle relative motion model whereby transform faults have been considered lines of pure slip along which crust is conserved is not generally valid for finite relative displacements

a simple model of whole mantle convection isotopic evidence of the chemical variability of basalts implies the existence of distinct long lived chemical reservoirs in the mantle this has commonly been interpreted as evidence that mantle convection is layered since known models of whole mantle convection have too much shearing to allow preservation of these reservoirs however the geochemical evidence requires at least three reservoirs and models of layered convection have dynamical difficulties casting serious doubt on the viability of these models this difficulty has been resolved in the present paper by the development of a simple model of whole mantle convection which allows the preservation of chemically distinct reservoirs in this model heating of the mantle from below produces axisymmetric plumes which carry heat upward through the mantle without causing overturning hence the lower mantle is much less sheared than in the usual two dimensional model mantle convection is driven by the penetration of cold lithospheric slabs from above it is argued that the pattern of convection is governed primarily by the dependence of viscosity on temperature and this dependence drives the mantle toward an isoviscous state these ideas are quantified in a simple one dimensional model of mantle convection in which the slab mass flux as a function of radius is governed by the requirement that the temperature profile remains isoviscous the resulting convection cycles material through the upper mantle more rapidly than through the lower mantle this provides a mechanism for an initially homogeneous mantle to become chemically stratified with a depleted upper mantle and primitive lower mantle

obduction related ophiolitic polymict breccias covering the ophiolites of antalya southwestern turkey the ophiolites of the antalya nappes cropping out in the area of kemer southwestern turkey are capped by a spectacular detrital sequence composed of various ophiolitic breccias whose total thickness reaches 1000 m four main formations have been distinguished within the breccias the basal formations are in depositional contact over the previously tectonized and eroded deepest part of the ophiolites the upper breccias are dated as middle maastrichtian which is the age of the deposition of synchronous serpentinitic olistostromes onto the continental margin these data suggest that the breccias formed as debris slides from bathymetric highs on the ocean crust as a result of renewed tectonic activity accompanying ob duction onto the continental margin

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

seasonal change in foraminiferal production in the western equatorial pacific warm pool evidence from sediment trap experiments seasonal changes in the flux of individual planktonic foraminifera in the deep ocean have been studied in the western equatorial pacific warm pool using material collected by sediment traps over a period of 1 yr 1991 1992 mean total foraminiferal fluxes tffs were 171 and 97 shells m 2 day 1 at sites n1 2 59 8 n 135 01 5 e and n2 4 07 5 n 136 16 6 e respectively the seasonal change in tffs was much higher than for organic matter om and carbonate fluxes the major species of planktonic foraminifera were pulleniatina obliquiloculata neogloboquadrina dutertrei globigerinoides sacculifer globigerinella aequilateralis and globigerinoides ruber symbiont bearing species and n dutertrei were predominant in the western equatorial pacific warm pool comparison between the warm pool and upwelling deep sea research ii 43 1996 1257 mar micropaleontology 33 1998 157 indicates that the foraminiferal assemblage trapped at the western equatorial pacific warm pool was comparable with the south equatorial and subtropical assemblages during a plankton tow study conducted at 140 w in 1992 it suggests that fairly oligotrophic conditions prevailed in the western equatorial pacific warm pool in association with terrestrial nutrient inputs or temporary mixing by wind bursts the om flux was more positively correlated with the 63 m carbonate fraction mainly composed of coccoliths r 0 87 and biogenic opal which suggests that om fluxes were more affected by phytoplankton productivity although food availability is generally important for foraminiferal production tffs were poorly correlated with om fluxes at sites n1 and n2 which is partly attributed to the significant influence of symbiont photosynthesis the 18o values of p obliquiloculata and n dutertrei showed maxima in association with high om fluxes during period 2 from august to october and period 4 from late december to april at site n1 reflecting thickening of the mixed layer following wind bursts g sacculifer showed a lunar reproduction cycle when between july and october carbonate production was more predominant than biogenic opal production a similar profile was evident for g ruber orbulina universa and g aequilateralis these four species are generally predominant at low latitudes therefore the lunar reproduction cycle may significantly affect the fluctuation in carbonate production in the equatorial ocean

changes in the indonesian throughflow during the past 2000 yr the indonesian throughflow itf is the only low latitude conduit for exchange of surface and thermocline waters between the indian and pacific oceans 80 of the itf waters pass through the makassar strait the flux of heat and salt through the strait is in part a function of the density difference between its southern and northern ends here the mg ca and stable oxygen isotope compositions of the planktonic foraminifer globigerinoides ruber sampled from two sediment cores collected at the northern and southern ends of the makassar strait are used to reconstruct surface water temperature sst sea surface salinity sss and seawater density variability over the past 2000 yr maximum sst and sss occurred at both sites between 850 and 700 yr ago coinciding with the medieval solar maximum and medieval warm period ca 1000 700 yr ago sst and sss declined at both locations after 700 yr ago and reached minimum values during the little ice age between 300 and 100 yr ago however the magnitude of sst and sss change over the past 2000 yr was distinctly different between the two sites especially during the period between 2000 and 900 yr ago this led to changes in the south to north density gradient within the makassar strait specifically the time interval from ca 2000 to 800 yr ago was marked predominantly by higher densities at the northern end relative to the southern end of the strait suggesting reduced heat flux from the pacific to the indian ocean this is consistent with previous work showing that this time interval was also marked by frequent el nino conditions conversely the period from ca 800 to 300 yr ago including the lia was marked by frequent periods when surface density was higher at the southern end of the strait indicative of enhanced heat flux to the indian ocean

a kinematic model for the gorda deformation zone as a diffuse southern boundary of the juan de fuca plate several lines of evidence indicate that the space problem caused by southeastward motion of the juan de fuca plate toward the east west mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform a kinematic model for this deformation based on the assumption of nondivergent flow conservation of area within the area known as the gorda plate and on the assumption of no subduction along the mendocino transform successfully matches the isochron pattern inferred from magnetic anomalies the model assumes strain rate decreasing to zero approaching the rigid juan de fuca plate with no sharp boundary or separate rigid area that would imply a discrete gorda plate this interpretation allows magnetic anomalies from south of the blanco fracture zone to be used to constrain the kinematics of the juan de fuca plate these anomalies imply a substantial change in the distance to the rotation pole at roughly 3 ma in addition to the clockwise shift in motion direction at 5 ma

devonian palaeontological data and the armorica problem abstract various palaeogeographic ideas about the relation between gondwana and euramerica during the devonian period are formalised into four competing hypotheses involving three areas euramerica gondwana armorica separated by two marine faunal barriers palaeontological evidence from devonian vertebrates is summarised to show support for the hypothesis that gondwana and euramerica were contiguous by the middle devonian and that the intervening ocean had been occluded by the late devonian palaeontological data provide no support for the existence of a wide devonian ocean separating euramerica from gondwana nor for a discrete continental block armorica which may have traversed this ocean during the devonian the armorican region of southern europe was apparently part of gondwana during the early devonian biostratigraphic evidence of discrepancies in the first appearance of various invertebrate taxa between southern europe north africa and north america are consistent with a biotic dispersal model involving shallow marine invertebrates and vertebrates during the early middle devonian and continental faunas and floras in the late devonian the latter event may have occurred at or near the frasnian famennian boundary

the iapetus model a plate tectonic concept for the variscan belt of europe abstract a plate tectonic model is presented explaining the evolution of the variscan belt of europe it regards the iapetus ocean and its branch the tornquist sea as the only ocean involved in the caledonian variscan and alleghenian mega orogen the iapetus and its marginal basins disappeared through oblique subduction and collision in the following sequence 1 1 northeastern iapetus including the tornquist sea caledonian orogeny 2 2 central iapetus acadian orogeny 3 3 adjacent marginal basins variscan orogeny and 4 4 southwestern iapetus alleghenian orogeny the area of the variscides was part of the continental margin of gondwana made up of ridges and ensialic basins no ocean lay within the variscan area during the palaeozoic although patches of oceanic crust probably existed in rift basins e g the lizard ophiolite after the silurian early devonian closure of northwestern and central iapetus the variscan area was surrounded by cratons on three sides in a manner similar to that around the modern mediterranean sea the subsequent variscan orogeny was essentially an ensialic thrusting and folding induced by a further approach of gondwana toward laurentia after the variscan collision gondwana rotated clockwise to the west by moving along a strike slip zone this rotation during the carboniferous opened the palaeo tethys in the east and caused the alleghenian collision in the west the middle ordovician to permian palaeogeography is presented on a series of maps modelling tectonic displacement and plate motions avalonia is interpreted as the ordovician volcanic island arc of gondwana separated from mainland gondwana by an oceanic gulf or a back arc basin the iberian arc may have originated by a continental escape of iberia from the center of collision into the oceanic gulf the variscan austro alpine and southern alpine units are today located near their earliest early carboniferous position

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

variations in sea surface hydrology in the southern makassar strait over the past 26 kyr we present centennial scale records of sea surface temperature and oxygen isotopes in a sediment core from mandar bay offshore sulawesi in the southern makassar strait which provide new insights into the variability of indonesian climate over the past 26 kyr the age model for the core is constrained by 17 ams radiocarbon ages with a surface ocean reservoir age correction based on paired wood and foraminiferal samples small holocene reservoir ages of 105 180 years point to intense surface ocean atmosphere interchange linked to increased monsoonal precipitation whereas last glacial maximum and deglacial reservoir ages are significantly higher mg ca derived sea surface temperature reconstructions based on globigerinoides ruber s s white exhibit an extended plateau during the antarctic cold reversal suggesting an atmospheric connection to high latitude southern hemisphere climate and a seasonal bias on g ruber this is in agreement with southern hemisphere sites along the track of the indonesian throughflow and in contrast to northern hemisphere records from the south china sea sulu sea and western pacific off mindanao which exhibit warming during the bolling allerod ice volume corrected delta o 18 seawater delta o 18 sw increased during heinrich stadial 1 and the younger dryas whereas the bolling allerod is characterized by low delta o 18 sw we attribute delta o 18 sw variability in the southern makassar strait during the last glacial maximum and glacial termination to changes in provenance and seasonality of precipitation rather than to variability in the amount of local precipitation and runoff c 2016 elsevier ltd all rights reserved

ostracodes as indicators of brackish water environments in the catskill magnafacies devonian of new york state abstract leperditiid and beyrichiid ostracodes in marine margin deposits of the devonian catskill magnafacies of new york state document brackish conditions in what were previously considered freshwater floodplain deposits the very low diversity ostracode fauna two species includes an abundance of anomalously thin shelled individuals that belong to the genera leperditia and welleria this ostracode fauna is one of the oldest reported from brackish water in the stratigraphic record and it documents the early ecological radiation of ostracodes into waters of reduced salinity associated fossils including the bivalve archanodon and the conchostracan euestheria have been considered to be freshwater forms partly because of their occurrence in these rocks that were assumed to be fluvial in origin nearly everywhere however the ostracodes are associated with cosmine bearing fish fragments lingulid brachiopods and the feeding trace spirophyton which have a marine or brackish water affiliation the presence of open carapaces on bedding planes as well as adults and several instar stages of each ostracode species indicate lack of postmortem transport at least one ostracode bearing mudstone contains the mineral amesite associated calcareous mudrocks interbedded with these units contain symmetrical ripple marks mudcracks and locally intense manganese oxide staining collectively these features indicate extreme environmental conditions and suggest highly variable salinities in shallow pools or embayments along the paleoshoreline the presence of brackish water faunas perhaps estuarine in origin indicates an aspect of the diversity of devonian paleoenvironments along this portion of the catskill deltaic complex not previously recognized the deposits are widespread although not ubiquitous within thick sequences of fine grained flood basin deposits because most previous paleoecological studies of fossils from these rocks assumed the sediments to be of freshwater origin the new interpretation of brackish water communities presented here could have important implications for understanding the evolution of early terrestrial organisms for example some animals may have evolved in brackish estuarine environments rather than in freshwater rivers as assumed in previous studies recognition of this brackish facies elsewhere in the catskill succession will not only aid future identification of paleoshoreline position but may also help clarify whether early terrestrial organisms made the transition to land via brackish or freshwater environments

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the andes

environment of the early paleozoic vertebrates abstract a comprehensive review of the known pre devonian vertebrate localities suggests that the non marine environment was not invaded until about wenlock early mid silurian time known well dated late cambrian ordovician and early silurian vertebrate localities are all in the marine environment considerations of paleogeography for the pre wenlock indicate that most of the vertebrate localities are too distant from shorelines to provide any basis for transport from the non marine environment to a marine depositional site as a favored explanation the articulated nature of the vertebrates from some of these pre wenlock occurrences also supports this interpretation reconsideration of both the physiological and embryological evidence relative to living vertebrates and some related chordates is also consistent with the view that the vertebrates originated in the marine environment

prolonged monsoon droughts and links to indo pacific warm pool a holocene record from lonar lake central india abstract concerns about the regional impact of global climate change in a warming scenario have highlighted the gaps in our understanding of the indian summer monsoon ism also referred to as the indian ocean summer monsoon and the absence of long term palaeoclimate data from the central indian core monsoon zone cmz here we present the first high resolution well dated multiproxy reconstruction of holocene palaeoclimate from a 10 m long sediment core raised from the lonar lake in central india we show that while the early holocene onset of intensified monsoon in the cmz is similar to that reported from other ism records the lonar data shows two prolonged droughts pd multidecadal to centennial periods of weaker monsoon between 4 6 3 9 and 2 0 6 cal ka a comparison of our record with available data from other ism influenced sites shows that the impact of these pd was observed in varying degrees throughout the ism realm and coincides with intervals of higher solar irradiance we demonstrate that i the regional warming in the indo pacific warm pool ipwp plays an important role in causing ism pd through changes in meridional overturning circulation and position of the anomalous walker cell ii the long term influence of conditions like el nino southern oscillation enso on the ism began only ca 2 cal ka bp and is coincident with the warming of the southern ipwp iii the first settlements in central india coincided with the onset of the first pd and agricultural populations flourished between the two pd highlighting the significance of natural climate variability and pd as major environmental factors affecting human settlements

sea surface temperatures for the last 7200 years from the eastern sunda shelf south china sea climatic inferences from planktonic foraminiferal mg ca ratios abstract to test whether low latitude shallow shelf deposits can provide high resolution paleoclimatic records we utilized two cores from the holocene sedimentary fill of incised valleys on the sunda shelf off sarawak malaysia we developed a new sea surface temperature sst record based on planktonic foraminiferal mg ca for the last 7200 years this record reveals several significant shifts between warmer and colder conditions temperatures averaged 27 5 c ca 7200 cal years bp then climbed to 28 2 c from 6500 to 5500 cal years bp at 5500 4500 cal years bp we identified the coldest period 26 8 c of the analyzed period for the last 4500 years sst again averaged 27 5 c but the profile is rather variable the last ca 1000 years recorded the warmest sst averaging 28 5 c we record for the first time in this region a cool interval ca 1000 years in duration centered on 5000 cal years bp concomitant with a wet period recorded in borneo the record also reflects a warm interval from ca 1000 to 500 cal years bp that may represent the medieval climate anomaly variations in the east asian monsoon eam and solar activity are considered as potential drivers of sst trends however hydrology changes related to the el nino southern oscillation enso variability shifts of the western pacific warm pool and migration of the intertropical convergence zone are more likely to have impacted our sst temporal trend our findings indicate that climatic patterns in the region might be in phase with enso and out of phase with the eam

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

comparing radiolarian and diatom diversity and abundance from the northeast pacific abstract particle trap samples vertex from 4 locations in the northeast pacific were used to compare centric and pennate diatoms with nassellarian and spumellarian radiolarians the proportion of pennates and nassellarians were highest at vertex 3 from the northeast tropical oceans off the west coast of mexico colonial radiolarians were characteristic of warm high saline waters from the southern edge of the north pacific central gyre north of hawaii vertex 4 while pennate diatoms were abundant the subtropic oceanic pacific vertex 5a had more pennate diatoms while vertex 5c in a coastal cyclonic eddy located west of california had an even proportion of centric and pennate diatoms comparison of major faunas and flora at various oceanic sites of present siliceous sedimentation in conjunction with atmospheric hydrographic and chemical data can be helpful in reconstructing paleoceanographic and paleoenvironmental conditions of the past oceans throughout the phanerozoic

weaker gulf stream in the florida straits during the last glacial maximum as it passes through the florida straits the gulf stream consists of two main components the western boundary flow of the wind driven subtropical gyre and the northward flowing surface and intermediate waters which are part of the global conveyor belt compensating for the deep water that is exported from the north atlantic ocean1 the mean flow through the straits is largely in geostrophic balance and is thus reflected in the contrast in seawater density across the straits2 here we use oxygen isotope ratios of benthic foraminifera which lived along the ocean margins on the boundaries of the florida current during the last glacial maximum to determine the density structure in the water and thereby reconstruct transport through the straits using the geostrophic method a technique which has been used successfully for estimating present day flow3 our data suggest that during the last glacial maximum the density contrast across the florida straits was reduced with the geostrophic flow referenced to the bottom of the channel at only about two thirds of the modern value if the wind driven western boundary flow was not lower during the last glacial maximum than today these results indicate a significantly weaker conveyor belt component of the gulf stream compared to present day values whereas previous studies based on tracers suggested that deep waters of north atlantic origin were not widespread during glacial times indicating either a relatively weak or a shallow overturning cell our results provide evidence that the overturning cell was indeed weaker during glacial times

a 900 year 600 to 1500 a d record of the indian summer monsoon precipitation from the core monsoon zone of india 1 we present a near annually resolved record of the indian summer monsoon ism rainfall variations for the core monsoon region of india that spans from 600 to 1500 a d from a 230th dated stalagmite oxygen isotope record from dandak cave our rainfall reconstruction which spans the medieval warm period mwp and the earliest portion of the little ice age lia indicates that the short instrumental record of ism underestimates the magnitude of monsoon rainfall variability periods of severe drought lasting decades occurred during the 14th and mid 15th centuries and coincided with several of india s most devastating famines

mid devonian to mid permian floral and faunal regions and provinces summary the middle to late palaeozoic biota in seas adjacent to the caledonide appalachian orogen included an abundant generally cosmopolitan benthos in both carbonate and clastic environments where low clastic sediment input permitted organic reefs and build ups were characteristic of this tropical area early devonian biogeographic provinces did not persist into later times the pelagic fauna dominated by ammonoidea bivalvia conodonts and vertebrates was rich rapidly evolving and again cosmopolitan vertebrates and conodonts in particular were wide ranging among the land plants heterospory and arborescence evolved during the middle and late devonian in a number of classes particularly the lycopsida and progymnospermopsida genera were largely cosmopolitan during this time and only one floristic region can be recognized worldwide although there is some evidence for slight provincial floral variation outside laurussia during the early carboniferous arborescent lycopsida were abundant three floristic regions can be identified worldwide of which one is the circum tethyan region including laurussia kazakhstania and china two floral provinces are recognized within the laurussian part of this region in late carboniferous and early permian times when gymnosperms replaced the pteridophytes as the dominant forms there is a more clear cut separation of floral regions and by early permian two floral regions are recognized within laurussia which are clearly different from those away from this land mass non marine bivalvia and vertebrates were highly cosmopolitan from mid devonian time onwards with amphibia appearing in greenland in the late devonian and reptiles appearing in eastern canada and western europe in westphalian a time

the geologic history of the caribbean cocos plate boundary with special reference to the nicoya ophiolite complex costa rica and d s d p results legs 67 and 84 off guatemala a synthesis abstract the pre upper senonian basement of costa rica crops out in the santa elena and nicoya peninsulas from south to north and from base to top the basement includes the esperanza matapalo and santa elena units the esperanza unit is albian santonian in age and consists mainly of pillow basalt and massive basalt flows the matapalo unit includes callovian to cenomanian radiolarite and includes massive basalt flows basalt and dolerite basement the santa elena unit contains ultramafic and mafic rocks in which harzburgite is the major component the most important tectonic features of the nicoya complex are the large santa elena and matapalo nappes nappe emplacement was from north to south during upper santonian time the sedimentary cover of the nicoya complex comprises 1 1 the campanian el viejo formation that consists of shallow water sediments in the north santa elena peninsula and the campanian maastrichtian sabana grande formation of deep water origin in the south nicoya peninsula 2 2 paleocene strata indicating deposition in a deep water environment comprises the rivas las palmas and samara formations 3 3 a post upper eocene sequence that consists of the shallow water barra honda and montezuma formations two unconformities are significant geological features of the upper senonian to tertiary history of costa rica the lower one is at the base of the sabana grande formation and marks a major change in the geologic conditions basalt is scarce in the campanian tertiary series the upper unconformity at the base of the barra honda and montezuma formations is not as major as the lower one during post campanian time normal faulting occurred in two stages separated by a strong erosional phase the geology of the landward slope of the adjacent middle america trench is outlined by interpreting multifold seismic reflection records off the west coast of costa rica and the dsdp legs 67 and 84 transects off guatemala the western caribbean plate boundary may have been under extensional stress for the last 75 m y the strong landward dipping reflectors of the middle america trench landward slope off guatemala could be equivalent to the on land pre campanian overthrusts of costa rica the available data are consistent with the convergent extensional margin concept

flow models for back arc spreading abstract a secondary flow model for back arc spreading is developed in this paper that shows some of the characteristics of observed back arc spreading back arc spreading has formed marginal seas around the west and southwest rim of the pacific the episodic spreading and different directions of opening are not completely understood however there does appear to be a limited lifetime

paleomagnetic results from the silurian of the yangtze paraplatform gpdyke n d huang k xu g zhang w y and kent d v 1987 paleomagnetic results from the silurian of the yangtze paraplatform in d v kent and m km editors laurasian paleoma etism and tectonics teetonophysics 139 123 132 samples have been collected from sites from three localities located in sichuan and in yunnan provinces the samples were taken from limestones and red siltstones covering most of the silurian of the south china block the results from the limestone sites were disappointing and all the sites that yielded successful results were from red siltstones characteristic high temperature components were successfully isolated from fifteen of these sites the sites from xiushan county sichuan 28 48o n 108 99 0 e successfully pass the fold test at the 99 level indicating that the magnetization was acquired before folding in the mesozoic the direction obtained from this locality is directed toward the east with shallow positive inclinations d 85 6 o i 12 1 cuss 5 33 the results obtained from a single red bed locality from qijiang county sichuan 28 58 n 106 85 e yield a similar direction the results from the upper silurian of yunnan 25 5 n 103 73o e were obtained from only two sites and the isolated directions were towards the west with shallow inclinations this direction is opposed to the characteristic directions obtained from sichuan and are interpreted as a reversed silurian direction the paleomagnetic pole position determined from all sites given equal weight n 15 falls at 4 9 n lat 194 7 e long with ass 5 61 o

global tectonics since the breakup of pangea 180 million years ago evolution maps and lithospheric budget abstract pangea the earth s youngest single supercontinent broke up 180 million years ago tectonic plates were subsequently formed by dispersal of the continental fragments and accretion of new oceanic lithosphere the configurations of all the major lithospheric plates at 0 20 65 95 140 180 ma bp are reconstructed on six globes of the earth each with a radius of 10 cm it appears that plate boundaries maintain a remarkably close fit on model globes of constant radii if the reconstructions include the recovery of subducted spreading patterns this is illustrated with maps in equatorial orthographic oblique orthographic and transverse hammer aitoff projections the snug fit of the tectonic plates at every tested time since the breakup of pangea 180 ma bp is consistent with the theory of plate tectonics on a non expanding earth and contradicts rapidly expanding earth models the areas of oceanic lithosphere produced and consumed during the past 180 ma bp are estimated from surface measurements of the globes reconstructed on the basis of particular assumptions these measurements suggest a consistent increase in the production rate of oceanic lithosphere during the past 140 ma it was decided to revise the assumptions and see if alternative reconstructions of the ancient spreading patterns on the floors of the tethys and eo pacific oceans could avoid implying an increase of lithospheric production rates with time this appeared to be possible the revised maps suggest that ophiolites older than 180 ma bp may have been obducted in cenozoic collision zones of the himalayas andes rockies and the western part of the banda arc timor new guinea estimates of the ocean floor production and consumption budget appear to be quite similar for both map series and only the possible ranges are summarized here time averaged for the past 180 ma world wide production and consumption of oceanic lithosphere appears to have varied between 2 6 and 3 5 km2 a 1 at most the mean of world wide spreading velocities ranges between 2 4 and 3 5 cm a 1 and world wide means of subduction velocities lie between 4 2 and 6 3 cm a 1 more accurate estimates are possible on the basis of particular assumptions and have been specified for the various time intervals distinguished i e the neogene 0 20 ma bp palaeogene 20 65 ma bp upper cretaceous 65 95 ma bp lower cretaceous 95 140 ma bp and middle and upper jurassic 140 180 ma bp

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

holocene changes in eastern tropical pacific climate inferred from a galapagos lake sediment record abstract paleoclimate records from the tropical pacific suggest the early to mid holocene was a period of reduced el nino southern oscillation enso variability with a transition to modern increased enso frequency occurring some time in the last few thousand years however the nature and timing of this shift remains uncertain due to the discontinuous nature and or coarse resolution of many enso proxies as well as a lack of agreement between previously published records a new continuous climate record from el junco crater lake in the galapagos islands reveals several abrupt changes in lake level and precipitation through the holocene hydroclimatic model simulations suggest that el junco lake level responds sensitively to increases in precipitation associated with el nino events rising during wet el nino events and falling during the intervening dry periods grain size data from el junco sediment cores indicate past lake level variability likely associated with changing seasonal precipitation and enso frequency the grain size data suggest increased precipitation intensity prior to 9000 120 cal years bp and after 4200 130 cal years bp as well as a two step increase in precipitation at 3200 160 and 2000 100 cal years bp maximum holocene precipitation and inferred enso variability occurred between 2000 100 and 1500 70 cal years bp during the same period that six other independent proxy records suggest higher enso frequency and longer stronger el nino events decreasing sediment carbon nitrogen c n ratios in el junco sediments indicate rising lake levels from the early holocene to present corroborating the grain size data the inferred increase in precipitation at 4200 130 cal years bp and at 2000 100 cal years bp coincides with decreasing southwest asian and east asian monsoon intensity suggesting tropical pacific climate and the asian monsoon were interconnected systems at centennial to millennial timescales during the holocene a weakening trend in the asian monsoon and the trend toward wetter conditions at el junco also coincide with a trend toward cooler and drier conditions inferred from cariaco basin sediment proxies from the mid holocene to present suggesting the migration of the intertropical convergence zone itcz likely influenced hydrological changes in both the eastern tropical pacific and the asian monsoon region during the holocene

climate and hydrographic variability in the indo pacific warm pool during the last millennium planktonic foraminiferal mg ca and delta o 18 derived sea surface temperature and salinity records from the makassar strait indonesia show a long term cooling and freshening trend as well as considerable centennial scale variability during the last millennium the warmest temperatures and highest salinities occurred during the medieval warm period mwp while the coolest temperatures and lowest salinities occurred during the little ice age lia these changes in the western pacific along with observations from other high resolution records indicate a regionally coherent southern displacement of the inter tropical convergence zone during the lia with more arid conditions in the northern tropics and wetter conditions in the southern tropics

a revision of the eurasian african plate boundary in the western mediterranean the current picture of the plate configuration in the western mediterranean suggests that africa and europe are separated by a transcurrent fault striking east west through the strait of gibraltar and the alboran sea such a fault has been generally assumed because the azores transform fault approaches the horseshoe seamounts north of madeira and therefore could possibly extend through the strait of gibraltar however detailed geological studies in the arc of gibraltar and the alboran sea reveal no major east west fault transecting the crust there this finding is used here to revise the boundary between the african and eurasian plates it is suggested that the northern boundary of the african plate in the western mediterranean is formed by the hayes atlas fault instead of the azores transform

convection experiments in high prandtl number silicones part 2 deformation displacement and mixing in the earth s mantle the evolution of structures in the earth s convecting mantle is studied in a simple but carefully scaled laboratory flow the geometrical scaling is constrained by the various geometries of convection cells possible in the earth s interior two layered mantle convection with a superposed small scale circulation in the top is favoured here the model flow has a rayleigh number 104 a prandtl number 108 and approximately axially symmetric cells with aspect ratio of unity the rheology and boundary conditions are aimed to be dynamically similar to that occurring in the mantle the flow is analysed on the basis on snapshots showing the progressive deformation and displacement of passive flow markers which allow the reconstruction of velocity vectors strain rate and vorticity inferences can be made about 1 1 the progressive deformation in individual rock masses and 2 2 the nature of the large scale mixing process caused by a combination of deformation and displacement in a convecting mantle this is discussed in terms of the kinematic vorticity number both geological and fluid mechanical studies of low inertia flows may benefit from the new experimental technique and analyses presented here until now it has been impossible to visualize and quantify the finite strains and rotation components of deformation in flowing fluids by other than numerical methods the laboratory method advocated here is quick and involves equipment which is relatively cheap if compared with those required for numerical analyses basic understanding of elementary fluid dynamics is sufficient to perform innovative investigations in this new field

early middle devonian brachiopod provincialism and bioregionalization at high latitudes a case study from southwestern gondwana the paleobiogeography of early middle devonian pragian eifelian brachiopods from west gondwana was assessed to determine any potential controls regional climatic differences or global eustasy that may have driven bioregionalization the pragian eifelian interval of west gondwana was examined because work by previous authors suggested that this was a period when regionally extensive areas of provincialism among marine invertebrates were present and most pronounced factors of particular interest in this study were the controls over brachiopod bioregionalization at high 60 90 southern latitudes which the malvinokaffric realm is thought to have entirely occupied a large presence absence data matrix was compiled consisting of the occurrences of 206 genera from 17 localities across west gondwana and an array of multivariate methods cluster analysis nonmetric multidimensional scaling and network analysis was employed to assess regional bioregionalization trends the results of our study suggest that regional climatic differences brought on by latitudinal effects were the determining driver for bioregionalism of brachiopods during the pragian eifelian and these trends were coincident with a global cooling period during the early middle devonian our study further suggests that of the three regional paleobiogeographic realms thought to be present in west gondwana during the early middle devonian eastern americas old world and malvinokaffric only the malvinokaffric realm is valid as a single biogeographic area its area however is reduced it is interpreted to have been a second order biogeographic area and not a first order area as suggested by previous authors given these factors we suggest a new demonym for this area the malvinoxhosan given the racially charged connotations of malvinokaffric we present a new biogeographic framework for west gondwana that is free of preconceived biogeographic area and rank biases with the understanding that a more globally expansive study should be undertaken to elucidate these areas and their rank within the correct hierarchy two first order paleobiogeographic areas are recognized in west gondwana and are named according to their latitudinal extent namely the high latitude 60 s 90 s and temperate latitude 30 s 60 s bioregions the temperate latitude bioregion consists of a single second order paleobiogeographic area the colombian west african bioregion two second order bioregions are present in the high latitude bioregion namely the amazonian and malvinoxhosan bioregions here the amazonian 50 s 70 s compositionally was an intermediate region between the colombian west african 30 s 50 s and malvinoxhosan 70 s 90 s end members latitudinal effects may also have been responsible for dividing the malvinoxhosan bioregion into two tentative third order paleobiogeographic areas namely the andeo south african 70 s 80 s and the parana 80 s 90 s bioregions

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin